Mechanical Engineering Series Frederick F. Ling Editor-in-Chief
For further volumes: http://www.springer.com/series/1161
Mechanical Engineering Series Frederick F. Ling Editor-in-Chief
The Mechanical Engineering Series features graduate texts and research monographs to address the need for information in contemporary mechanical engineering, including areas of concentration of applied mechanics, biomechanics, computational mechanics, dynamical systems and control, energetics, mechanics of materials, processing, production systems, thermal science, and tribology.
Advisory Board/Series Editors Applied Mechanics
F.A. Leckie University of California, Santa Barbara D. Gross Technical University of Darmstadt
Biomechanics
V.C. Mow Columbia University
Computational Mechanics
H.T. Yang University of California, Santa Barbara
Dynamic Systems and Control/ Mechatronics
D. Bryant University of Texas at Austin
Energetics
J.R. Welty University of Oregon, Eugene
Mechanics of Materials
I. Finnie University of California, Berkeley
Processing
K.K. Wang Cornell University
Production Systems
G.-A. Klutke Texas A&M University
Thermal Science
A.E. Bergles Rensselaer Polytechnic Institute
Tribology
W.O. Winer Georgia Institute of Technology
Wanlong Wang • Henry W. Stoll James G. Conley
Rapid Tooling Guidelines For Sand Casting
ABC
Wanlong Wang Wang Consulting International 1415 Bellevue Avenue Unit 6 Burlingame, CA 94010 USA
[email protected]
Henry W. Stoll Department of Mechanical Engineering Northwestern University 2145 Sheridan Road Evanston, IL 60208-3111 USA
James G. Conley Kellogg School of Management and McCormick School of Engineering Northwestern University 2001 Sheridan Road Evanston, IL 60208 USA
ISSN 0941-5122 ISBN 978-1-4419-5730-6 e-ISBN 978-1-4419-5731-3 DOI 10.1007/978-1-4419-5731-3 Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2010920244 © Springer Science+Business Media, LLC 2010 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)
Series Preface
Mechanical engineering, an engineering discipline forged and shaped by the needs of the industrial revolution, is once again asked to do its substantial share in the call for industrial renewal. The general call is urgent as we face profound issues of productivity and competitiveness that require engineering solutions among others. The Mechanical Engineering Series features graduate texts and research monographs intended to address the need for information in contemporary areas of mechanical engineering. The series is conceived as a comprehensive one that covers a broad range of concentrations important to mechanical engineering graduate education and research. We are fortunate to have a distinguished roster of consulting editors on the advisory board, each an expert in one of the areas of concentration. The names of the consulting editors are listed on the facing page of this volume. The areas of concentration are applied mechanics, biomechanics, computational mechanics, dynamic systems and control, energetics, mechanics of materials, processing, production systems, thermal science, and tribology. Austin, Texas
Frederick F. Ling
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Preface Sand casting is a versatile manufacturing process with a history that dates back thousands of years. As automation and production technologies have evolved with time, sand casting methods have advanced to realize improved adaptability and efficiency. The process is now used for both large and small dimensioned products in both high volume and small batch production lot sizes. Many metals and alloys that can be poured in a foundry can use the sand casting process. A chronic challenge in managing sand casting production is the efficient (cost, time) fabrication and maintenance of the elements that form the sand molds, specifically patterns and core tooling. Traditionally, sand casting patterns and tooling are produced manually from wood or dimensionally stable synthetic materials. The advance of numerical controls and computer numerically controlled (CNC) machining dramatically improves the efficiency and reliability of this form of tooling production. With the invention and commercialization of the stereo lithography process in 1986, a variety of new and improved layered manufacturing processes were introduced to the marketplace. The working materials of these processes evolved from laser cured epoxy to wood like layered paper, engineering plastics and other forms. With the appropriate part data, rapid prototyping processes could produce patterns and core box tooling directly, hence the dawn of rapid tooling. While CNC fabrication processes are accurate, they consume a considerable amount of working material to get to the net tooling shape. They are also time consuming, often a critical variable in a world of short supply chains. In this environment of dynamic technological change, the use of either CNC or emergent rapid tooling techniques to produce parts that have uncertain demand curves, like military hardware, becomes an important question for procurement professionals. In an effort to inform such decision, the authors undertook an analysis of how tools and sand castings may be produced for a variety of part geometries. With the support of the Defense Logistics Agency, Clinkenbeard, and Lufkin, the authors developed best practice guidelines for rapid tooling of sand castings. The methodology used included: • •
Identification and selection of components (w/ solid model) suitable for sand casting; Evaluate tooling error sources for sand casting; vii
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• • • •
Produce the rapid tooling (by either CNC, RT or a combination) and create castings; Express tool path selection framework for sand casting; Dimensional metrology investigation of rapid tooling patterns, molds and resultant castings using best practices; Analysis of tooling related costs and lead times.
The results of these investigations provide the research foundation for this book.
Acknowledgement
The authors are grateful to Dan Gearing of the Defense Logistics Agency, Joseph Santner of the American Foundry Society and Ron Gustafson of Clinkenbeard for their support of the research that provides the foundation for this book. James Conley would also like to acknowledge the early career support of Prof. Morris E. Fine of Northwestern University and Dr. Akio Urakami of the Ryobi Group of Companies. Both positively influenced the author’s logic for addressing the practical challenges described in this manuscript. The authors also want to express their appreciations to Alex Greene, Editorial Director, and Ciara.Vincent from Springer US for their support in realizing the publication of this work.
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Table of Contents Series Preface........................................................................................................ v Preface................................................................................................................. vii Acknowledgement ................................................................................................ix Chapter 1 Sand Casting Processes...................................................................... 1 1.1 Basic Steps in Making Sand Castings.......................................................... 1 1.1.1 Patternmaking ...................................................................................... 2 1.1.2 Coremaking .......................................................................................... 3 1.1.3 Molding................................................................................................ 3 1.1.4 Melting and Pouring............................................................................. 3 1.1.5 Cleaning ............................................................................................... 4 1.2 Mold Making Processes............................................................................... 4 1.2.1 Green-sand molding ............................................................................. 4 1.2.2 Dry-sand molding................................................................................. 7 1.2.3 Core-sand molding ............................................................................... 8 1.2.4 Shell molding ....................................................................................... 8 Chapter 2 Tool Design and Construction for Sand Casting ........................... 11 2.1 Sand-Casting Tool Design and Construction............................................. 11 2.2 Pattern Type............................................................................................... 15 2.2.1 Loose Patterns .................................................................................... 16 2.2.2 Gated Patterns .................................................................................... 17 2.2.3 Match-Plate Patterns .......................................................................... 17 2.2.4 Cope & Drag Patterns ........................................................................ 19 2.3 Pattern Design............................................................................................ 20 2.3.1 Pattern and Core Box Materials ......................................................... 20 2.3.2 Pattern Allowances............................................................................. 23 2.3.3 Draft ................................................................................................... 27 2.3.4 Parting Line........................................................................................ 28 xi
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2.3.5 Geometry Considerations................................................................... 28 2.4 Master Pattern Fabrication.........................................................................30 2.4.1 Manual Fabrication.............................................................................31 2.4.2 Computer Numerical Control Machining...........................................32 2.4.3 Fast Freeform Fabrication ..................................................................33 2.5 Core and Core Box Fabrication .................................................................34 2.5.1 Core Fabrication Process....................................................................34 2.5.2 Core Boxes .........................................................................................35 Chapter 3 Fast Freeform Fabrication Methods and Processes ......................37 3.1 Fast Freeform Fabrication (FFF) Processes ...............................................38 3.1.1 Stereolithography Apparatus (SLA)...................................................38 3.1.2 Laminated Object Manufacturing (LOM) ..........................................39 3.1.3 Fused Deposition Modeling (FDM) ...................................................41 3.1.4 Selective Laser Sintering (SLS) .........................................................42 3.1.5 Solid Ground Curing (SGC)...............................................................44 3.1.6 Three Dimensional Printing (3DP).....................................................45 3.1.7 Sanders Prototype (SP).......................................................................46 3.2 FFF Model Building Procedure .................................................................48 3.2.1 CAD Modeling and Data Preparation ................................................48 3.2.2 Machine Setup....................................................................................49 3.2.3 Building..............................................................................................50 3.2.4 Post Processing...................................................................................50 Chapter 4 Rapid Tooling Processes ..................................................................53 4.1 CNC Based Tooling Process......................................................................54 4.2 Rapid Tooling Processes Based on Fast Freeform Fabrication..................55 4.2.1 Stereolithography Based RT Methods................................................56 4.2.2 LOM based RT methods ....................................................................59 4.2.3 Selective laser sintering (SLS) Based RT Methods............................63 4.2.4 Fused Deposition Modeling (FDM) Based RT Methods....................66 4.2.5 Solid Ground Curing (SGC) Based RT Methods ...............................66 4.2.6 Other Tooling Processes.....................................................................66 Chapter 5 Sand Casting Dimensional Control.................................................69 5.1 Error Sources .............................................................................................69 5.1.1 Sand Casting Process..........................................................................70
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5.1.2 Rapid Tooling Process........................................................................73 5.1.3 Shrinkage ........................................................................................... 77 5.2 Geometric Dimensions and Tolerances ..................................................... 81 5.2.1 Dimensions......................................................................................... 82 5.2.2 Tolerances .......................................................................................... 83 5.2.3 Allowances......................................................................................... 84 5.2.4 Feature Control................................................................................... 84 5.2.5 Standards on Casting Dimensional Tolerances .................................. 87 5.3 Dimensional Metrology ............................................................................. 88 5.3.1 Basic Definitions................................................................................ 88 5.3.2 Types of Dimensional Error ............................................................... 89 5.3.3 Process Capability .............................................................................. 92 5.4 Measurement System Analysis .................................................................. 95 5.4.1 Procedures for Gage R & R Test........................................................ 95 5.4.2 Performing Gage R & R on Different Pieces of Equipment .............. 96 Chapter 6 Evaluating Tooling Alternatives ..................................................... 99 6.1 Decision Variables..................................................................................... 99 6.1.1 Fabrication Method ............................................................................ 99 6.1.2 Tool Material.................................................................................... 101 6.1.3 Tooling Approach ............................................................................ 101 6.2 Decision Factors ...................................................................................... 101 6.2.1 Data Status ....................................................................................... 102 6.2.2 Production Volume .......................................................................... 104 6.2.3 Prototype or Production?.................................................................. 105 6.2.4 Who Will Use the Tooling? ............................................................. 105 6.2.5 Part Geometry .................................................................................. 105 6.2.6 What Geometrical Features are Important?...................................... 106 6.2.7 Pattern Shop Capability.................................................................... 106 6.2.8 Tool Cost.......................................................................................... 107 6.2.9 Lead-Time ........................................................................................ 107 6.2.10 Required Accuracy......................................................................... 107 6.2.11 Tool Durability............................................................................... 107 6.3 Decision Structure ................................................................................... 107 6.3.1 Universe of Possible Tooling Alternatives....................................... 108 6.3.2 Decision Constraints ........................................................................ 110
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6.3.3 Cost, Time, and Quality Tradeoffs................................................... 110 6.3.4 Geometry Considerations .................................................................111 6.4 Decision Process ......................................................................................113 6.4.1 What is the Data Status?...................................................................113 6.4.2 Geometry Independent Requirements ..............................................114 6.4.3 Geometry Analysis ...........................................................................114 6.4.4 Develop Alternative Tooling Proposals ...........................................114 6.4.5 Negotiate the Final Selection............................................................115 Chapter 7 Tooling Alternative Selection Case Studies..................................117 7.1 Tooling Path Reviewing Form.................................................................117 7.2 Case Studies.............................................................................................117 7.2.1 Sensor Box .......................................................................................117 7.2.2 Hitch Housing ..................................................................................121 7.2.3 Yoke .................................................................................................123 7.2.4 Carrier Housing 1 .............................................................................125 7.2.5 Inlet Boost Pump ..............................................................................127 7.2.6 Manifold ...........................................................................................128 7.2.7 Turbine Blade Machining Fixture ....................................................129 7.2.8 Upper Arm........................................................................................131 7.2.9 Rigid Connecting Link .....................................................................132 7.2.10 25mm Projectile .............................................................................134 7.2.11 Carrier Houseing 2 .........................................................................135 7.2.12 Muzzle Brake .................................................................................137 7.2.13 Ice Cleat..........................................................................................138 7.2.14 Roller Bracket ................................................................................140 Appendix Glossary of Terms ............................................................................143 References ..........................................................................................................157 Index ...................................................................................................................161
Chapter 1 Sand Casting Processes Sand casting is a mold based net shape manufacturing process in which metal parts are molded by pouring molten metal into a cavity. The mold cavity is created by withdrawing a pattern from sand that has been packed around it. Since the pattern imprint forms the cavity, the pattern creates the external shape of the cast part. If the part has undercuts or hollow internal regions, these can be formed by sand cores that are fabricated separately and then placed in the mold cavity. The cores are supported by core prints, and/or chaplets that allow the molten metal to flow between the core and the mold wall. In addition, cores may be necessary to produce a desired “zero” draft external surface, depending on the parting line selected. The parting line is formed by the interface between the cope (upper portion) and drag (lower portion) of the mold. The separate cope and drag are necessary to allow the pattern to be withdrawn from the sand and to allow the cores to be properly positioned within the mold. 1.1 Basic Steps in Making Sand Castings
In general, the sand casting process consists of five basic steps: 1. 2. 3. 4. 5.
Patternmaking (including core boxes); Coremaking; Molding; Melting and pouring; Cleaning.
As shown in Figure 1.1, these steps are implemented in a series of foundry operations. Typically, the detail technical processes involved in each of these operations are the primary concern of the foundry.
W. Wang et al., Rapid Tooling Guidelines For Sand Casting, Mechanical Engineering Series, DOI 10.1007/978-1-4419-5731-3_1, © Springer Science+Business Media, LLC 2010
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Fabricate Core Boxes
Fabricate Pattern
Core Mat’l
Mold Core
Sand
Prepare Sand
Build Mold
Raw Metal
Melting Metal
Pour Casting
Break Out Raw Casting
Degate, Clean & Inspection
Solidification and Cooling
Finished Casting
Figure 1.1 The manufacturing steps for the sand casting process. 1.1.1 Patternmaking
Patterns are required to make sand molds. Therefore, before a sand casting can be produced, necessary patterns must be fabricated in the pattern shop or by a toolmaker. The pattern, which is made of wood, metal, plastic or a variety of other materials, has the external shape of the cast part but is enlarged by a shrinkage allowance to compensate for contraction of liquid metal during solidification and cooling to room temperature. Also, an allowance, called taper or draft, is provided on the vertical sides of the pattern to facilitate removal from the sand. The pattern may also include core prints, gates and risers, although, in some cases these features can be cut into the mold sand by hand or can be made by placing loose pattern
Chapter 1 Sand Casting Processes
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pieces in the mold. The gating system is a channel, or network of channels, through which the molten metal flows into the cavity. It is typically composed of a pouring cup, sprue, and runner. The riser is a reservoir in the mold that serves as a source of liquid metal to feed the casting and compensate for shrinkage during solidification. 1.1.2 Coremaking
If the casting is to be hollow, as in the case of a pipe fitting, additional patterns, referred to as core boxes (negative patterns), are used to form the sand cores that are used to create these cavities. Sand cores are molded in core boxes to the same standards of accuracy, and with the same problems of service life under abrasion, as patterns. After being molded in core boxes, the cores are placed in the mold cavity to form the interior surfaces of castings. Cores are ordinarily made separately from molds in a designated area of the foundry referred to as a core room. They are then transported to the molding area to be positioned in the sand molds. Like patterns, core boxes must be fabricated by the toolmaker or pattern shop prior to production of the sand casting. 1.1.3 Molding
Molding consists of all of the operations necessary to prepare a mold for receiving molten metal. This typically involves packing sand or other molding aggregate around the pattern which is held within a supporting frame, or flask, withdrawing the pattern to form the mold cavity, setting the cores in the mold cavity, and assembling the cope and drag halves to close the mold. Once assembled, the mold is ready to receive the molten metal. 1.1.4 Melting and Pouring
The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, where melt chemistry, gas content, and other metallurgical factors are monitored and controlled before pouring. When the melt is ready, it is poured into the mold.
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1.1.5 Cleaning
Cleaning includes all operations necessary to remove sand, scale, and excess metal from the casting. The casting is separated from the molding sand and transported to the cleaning department. Burned-on sand and scale are removed from the casting surface to improve the surface appearance of the casting. Excess metal, in the form of fins, wires, parting-line fins, and gates, are cut off by grinding. Defective castings may be salvaged by welding or other repair. Inspection of the casting for defects and general quality follows. The casting is then ready for shipment and/or further processing such as heattreatment, surface treatment, or machining. 1.2 Mold Making Processes
Sand casting is an “expendable mold” casting process since the mold is destroyed during removal of the solidified casting. Therefore, a new mold must be made for each sand casting produced. From the point of view of dimensional accuracy, mold making is the key step in the sand casting process. Quality castings cannot be produced without quality molds. “Mold making process” refers to the method of making the mold. The differences among sand molding processes are primarily in the method of forming the mold and in bonding of the granular refractory. Exemplary mold making processes for sand casting include the following: • • • •
Green-sand molding Dry-sand molding Core-sand or core molding Shell molding
1.2.1 Green-sand molding
Green-sand molding utilizes a mold made of compressed or compacted moist sand. The sand is called “green” because of the moisture present. The mold is not baked or dried. The mold materials consist of silica sand mixed with a suitable bonding agent (moist clay). To produce the mold, a metal or wood frame (flask) is
Chapter 1 Sand Casting Processes
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placed over the pattern to produce a cavity representing one half of the casting. Compaction or ramming of the sand is achieved by either jolting or squeezing the mold. The opposite half of the mold is produced in a similar manner. The two flasks are positioned to form the complete mold. Figure 1.2 illustrates the physical process of the green-sand molding.
Iron casting to be produced.
1. Bottom half of the pattern is placed on the mold board and surrounded by the drag half (bottom) of the flask.
2. Molding sand is rammed 3. After the bottom half of around the pattern in multiple the mold is filled, it is rotated steps to provide uniform sand upright and the top half of the density. pattern and flask are put in place to complete the mold.
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Rapid Tooling Guidelines For Sand Casting
4. This section view shows the competed mold with pattern still in place and the sprue hole formed for entrance of molten metal.
5. The cope and drag halves of the mold are separated so that the pattern can be removed. The gate channel is then cut from the sprue to the mold cavity.
6. The core is placed in core prints located in the drag half of the mold. The space between the core and mold wall form the internal passages of the casting.
7. After placing the core in the mold, the mold is closed and clamped to resist pressure exerted by the molten metal when it is poured in the mold.
Figure 1.2 The green sand molding process (Courtesy: AFS, 1989)
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Green sand molding is most adaptable to light, bench molding for medium-sized castings or for use with production molding machines. Advantages. Most metals can be cast by this method. Pattern cost and material cost are relatively low. The method is adaptable to large or small quantities. Disadvantages. There are practical limits to complexity of design. Machining is often required to achieve the finished product. Dimensional accuracy cannot be controlled as well as with other molding processes, although good standards are possible with quality pattern equipment, modern process controls, and high density molding. 1.2.2 Dry-sand molding
Dry-sand molding is similar to green sand molding in that moist sand is used to initially form the mold. The green sand mold is then dried by baking it at 400 to 6000F (or 204 to 3160C ) to develop good strength and other properties. Generally the mold cavity surface is coated or sprayed with a mixture that increases the hardness of the mold walls when dry. Some foundries use dry sand molds to produce intricate parts that are difficult to cast to exact size and dimensions. Castings of large or medium size, and of complex configuration such as frames, engine cylinders, rolls, large gears and housing are often made by utilizing the dry sand technique. Both ferrous and nonferrous metals can be cast using this type of process. Advantages. Dry sand molds are generally stronger than green sand molds and can therefore withstand additional handling. Castings produced using dry sand molds usually possess improved dimensional consistency compared to green sand. Also, the improved quality of the sand mixture due to the removal of moisture generally results in a smoother surface finish. Disadvantage. This type of molding is more expensive than green sand molding and is not well suited for high-production. Optimal baking (drying) time is essential.
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1.2.3 Core-sand molding
Core-sand molding is achieved by fitting cores together to form the mold. In this approach, core boxes are used in place of a pattern for making all parts of the mold. Core sands usually consists of mixture of sand grains and organic binders which develop great strength after baking. These strengths make it possible to cast metal around thin sand projections without having them break or erode because of the hot-metal erosion. Core sand molds are also sometimes made with dry molding cement-bonded sands. 1.2.4 Shell molding
Shell molding is a semi-precision method for producing castings repetitively in high volume. It is probably used more for making cores than molds. The sand used for shell molding consists of fine dry sand grains and a phenolic resin. This is mixed dry or in the presence of alcohol. No water is used. The phenolic resin is a thermosetting plastic which is either mixed with the sand as a powder or applied as a liquid and then dried on the sand grains. To be suitable for molding, the mixture must be dry and free-flowing. The mold is formed by using a machined metal pattern, usually made of gray iron, aluminum, or brass. The pattern is heated to 450 to 5500F (232 to 3880C) and then the sand-resin mixture is dumped or blown on the surface of the pattern. Upon contact with the hot pattern, the resin melts, hardens, and strongly bonds the sand grains together. After a carefully planned dwell time (usually 20 to 30 seconds) the pattern and sand are inverted to remove excess unbonded sand from the layer or shell of bonded sand that adheres to the pattern. The thickness of the shell can be accurately controlled by the length of the dwell time, which is tailored to provide the required strength and rigidity for the weight of metal to be poured into the mold. The usual thickness is ¼ inch (6.35mm) or more. The uncured shell is then moved into an oven where it is cured for 15 to 120 seconds. After curing, the shell is rigid and can be stripped from the pattern by means of ejector pins. Once cured, the shell is readily stored and handled.
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The two halves of the mold are then assembled with the aid of alignment pins. The shells may also be joined together by pasting and curing and/or by clips, clamps, or other fastening techniques. In some cases, the shells are placed on end and are put in a flask with sand or shot around them. This backing resists the hydrostatic pressure of the metal, prevents distortion of the shell, and promotes close dimension control of the casting. If the casting has no heavy or thick wall sections, it is frequently possible to prepare the shell for pouring in the horizontal position by including a pouring basin in the mold. By controlling the dwell time and the thickness of the pattern, it is often possible to produce a shell of sufficient thickness to withstand the hydrostatic pressure without the use of backing. This greatly simplifies the process. Figure 1.3 further illustrates the shell molding process. In production, the shell-making operations take place within a machine or set of machines with very little attention from the operator except to remove the shells from the machine. The resulting automation contributes to superior dimensional consistency. Advantages. Molds may be stored for extended periods of time. Good casting detail and dimensional accuracy may be realized. Mold is light in weight. Disadvantages. High pattern cost, high pattern wear, high energy costs, higher materials cost compared to green sand molding.
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Figure 1.3 Steps in the shell molding process. (1) Equipment for heating and measuring temperature of shell-molding pattern. (2) Dump box in position for dumping sand on pattern. (3) Shell on pattern. (4) Stripping shell from pattern. (5) Shell-mold halves. (Courtesy: Heine et al, Principles of Metal Casting, 1967, reproduced with permission of The McGraw-Hill Companies)
Chapter 2 Tool Design and Construction for Sand Casting 2.1 Sand-Casting Tool Design and Construction
Typically, the sand-casting tool design and fabrication process begins when the tool builder receives the part design from the client or design engineer. In general, the part design can be communicated in variety of ways such as by a physical part, a 2D engineering drawing, a 3D computer generated solid model, or other means that sufficiently conveys the design intent needed for tooling design and fabrication. Given this initial part design data set and a general understanding of the cost, lead-time, accuracy, and production volume requirements of the casting, the tool builder must decide on the best approach for constructing the tooling (pattern, core boxes, etc.) needed to produce the sand casting. This requires a deep understanding of the sand casting process and of foundry operations. In designing and building a pattern, the tool builder must consider a large range of often conflicting considerations and requirements and then design the pattern to achieve the best possible balance of cost, quality, and time. Primary considerations include: • • • • • • •
The functionality of the casting. How to best achieve the specified external and internal shape of the casting. How molten metal will flow during filling and feed during solidification of the casting. How the casting will shrink during solidification and cooling to room temperature. Where the parting line should be located to best facilitate foundry operations. Where and how much draft should be applied. Where and how much extra material should be provided for machining.
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• • •
What type of pattern should be used. What fabrication process should be used to construct the pattern. What pattern material should be used.
Traditional pattern making is performed in a woodworking shop equipped with a compete line of woodworking equipment (band saw, table saw, jointer, planer, drill press, disc sander, spindle sander, and layout tables). Manually building a pattern requires producing a three dimensional shape from a two-dimensional drawing. The pattern shape is usually constructed from a variety of separate pieces that are glued and/or fastened together. Draft is added to facilitate withdrawal of the pattern from the sand and dimensions are modified to provide extra metal where the casting will be finish machined. After the pattern is assembled, fillets are added by gluing in leather fillet material or using plastic material specially formulated for this purpose. A key factor that influences every stage of the pattern design and construction is the requirement that the pattern be able to withstand the repeated fractions of the molding process in a foundry environment. The patternmaker must also decide on the number and shape of the cores required and then design and build the associated core boxes. To do this, the patternmaker must visualize what the negative shape looks like and how it will be assembled in the mold to produce the desired internal geometry. It is important to note that not all pattermakers are proficient at designing cores. For example, if a particular pattern shop or foundry does a lot of automotive or aircraft engine work, where internal complexity is high, it is likely that they will be proficient at core fabrication. Patternmakers who work in shops where simpler shapes such as frames are produced, however, may not be as experienced with cores. Designing and building sand casting tooling that includes complex arrangements of cores that will run successfully in the foundry benefits from experience and skill. Each core must have a shape that can be easily produced by the core maker and handled by the foundry. It must also be easily assembled into the mold without interfering with other cores or mold parts. One of the critical issues is that the cores must be correctly located and positively held in place to assure consistency from casting to casting.
Chapter 2 Tool Design and Construction for Sand Casting
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In some cases a template or fixture is used to assure proper core alignment. Engineering Drawing or Other Input Data Set
Manual Fabrication
Computer-Aided Fabrication
Create 3D Solid Model
• Select Parting Line • Add Shrinkage • Add Draft
CNC Machining Fabrication
Fast Freeform Fabrication
Finished Pattern
Figure 2.1 Tool fabrication process for sand casting As shown in Figure 2.1, there are essentially three ways to fabricate tooling for sand-casting: manual, CNC machining, and fast freeform fabrication. The traditional approach is to manually fabricate the tooling from hand crafted prototypes of clay, wood, plastic or other materials by skilled workers. In the CNC (Computer
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Rapid Tooling Guidelines For Sand Casting
Numerical Control) machining approach, the tooling is produced by removing material from a workpiece using the machining process (subtractive process). In the fast freeform fabrication method, tooling is created by building up one layer at a time (additive process). The CNC machining and fast freeform fabrication approaches are also known as computerized patternmaking or rapid tooling because they utilize the computer to control the fabrication process. Consequently, a 3D solid model representation of the part is required to use these fabrication methods. When 3D data or a solid model is available, the pattern and cores can be fabricated by utilizing rapid tooling methods. In the case of simple shapes like a frame, these fabrication alternatives may not offer much advantage over traditional manual methods. However, when the configuration is more complex (e.g., blends, sweeping shapes, bosses in various attitudes), very dramatic savings in time and improvement in accuracy is often possible with these newer methods. Substantial cost savings are also usually possible since the casting will typically get through first article or volume production ready inspection in the foundry with less iteration. It should be noted that even with relatively complicated part geometry, manual pattern fabrication is often the fastest and most economical fabrication method because of the complex preprocessing and set-up activities required by rapid tooling processes. However, computerized patternmaking is being greatly facilitated by the introduction of dedicated mold and pattern manufacturing software packages. These programs work with solid models to rapidly create parting lines, introduce allowances, add draft, generate mold and core shapes, and create other tool engineering features. Ultimately, the choice of tooling fabrication method depends on tradeoffs between lead-time, cost and accuracy. It is also dependent on the availability of a solid model representation of the finished casting geometry. If a solid model of the finished casting is not available or cannot be created, then manual fabrication is the only alternative. The availability of a solid model can also effect the number of iterations required to achieve first article acceptance in
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the foundry. For example, shrinkage generally occurs in the direction of the local center of mass, but not the geometric center of the part. As a result, when shrink factors are applied globally, which is often the case in manual fabrication, it is frequently necessary to re-work the tooling to achieve an acceptable casting. With the combination of a CAD solid model and rapid tooling methods, shrinkage factors can be applied locally rather than globally, thereby possibly avoiding rework of the tooling. In all cases, however, the exact amount of shrinkage and draft that is applied, either locally or globally, is determined by tooling standards and/or experience. Hence, some uncertainty in the tooling design is unavoidable regardless of the fabrication method used. There are a number of documented cases in which complex castings (e.g., a pump housing with fifteen cores) were successfully produced on the first attempt with zero deviations from design intent. In the great majority of these cases, the tooling was produced using CNC machining by a skilled tool builder having extensive experience in the design and construction of complex tooling. In all cases, the key success factor was early identification of the tool builder and foundry having the right level of capability and experience for the particular casting in question. With the right tool builder and foundry, the need to add a cost margin for uncertity into the price of the tooling either by the tool builder or the foundry is reduced. Ultimately, the end user gets a viable part in less time and for less cost. 2.2 Pattern Type
There are several types of patterns used in foundry practice. The pattern type can be categorized as follows: • • • • •
single or loose pattern gated pattern match-plate pattern cope and drag pattern special patterns and device
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Each of these pattern types has advantages and limitations. Hence, the selection of pattern type will generally depend on the requirements associated with a particular casting.
Figure 2.2 A loose pattern for molding a rocker arm (Courtesy: Heine et al, 1967) 2.2.1 Loose Patterns
A loose pattern is a copy of the part to be cast that also incorporates shrinkage allowance, core prints, and other features required by the casting process. Typically, this pattern type is made of wood, but it can also be made of metal, plaster, plastics, wax, various fast freeform fabrication materials, or other suitable materials. Loose patterns are frequently used to produce prototype castings or for other low quantity applications because they can usually be fabricated quickly and for low cost. However, because of the large amount of manual effort required to produce the actual casting, loose patterns are not usually suitable for applications where large quantities of castings are required. Manual operations include forming the parting surface by hand, hand cutting the gating system, and manually drawing the pattern from the sand, after rapping and loosening it from the sand. Because of the manual operations,
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casting dimensions are also likely to vary from casting to casting. Figure 2.2 shows a loose pattern for molding a rocker arm. 2.2.2 Gated Patterns
A gated pattern is a loose pattern that has the gating system included as a part of the pattern. This eliminates the time and inconsistency associated with hand-cutting the gates and runners. Also, since the gating system is designed and fabricated as part of the pattern, the consistency of molten metal flow into the casting and feeding of the casting during solidification is improved. Gated patterns are appropriate for pouring small quantities of castings when quick turn around and low cost are important. Figure 2.3 shows an example of gated pattern for molding a rocker arm. 2.2.3 Match-Plate Patterns
A match-plate pattern consists of a wood or metal plate with the cope portion of the pattern mounted on one side and the drag portion of the pattern mounted on the other side. Hence, the match plate pattern incorporates the parting line into the pattern and also includes the gating system as an integral part of the pattern. Matchplates may be assembled by the tool builder or they may be integrally cast as one piece in sand or plaster molds. Match-plate patterns are suitable for large quantity production. Figure 2.4 shows a metal match-plate pattern for use in sand-casting a rocker arm. Match plates are generally used with some type of molding machine in order to maximize the speed of molding and minimize the cycle time to produce a casting. The improved production rate possible with this pattern serves to compensate for their increased cost. Match-plates also increase the dimensional accuracy of the casting because of the built in registration between the cope and drag portions of the pattern. The size of the casting produced by using a match-plate pattern is limited by the weight of mold and flask that can be handled by the foundry. Therefore, match-plate patterns are best suited for mass production of small castings.
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Rapid Tooling Guidelines For Sand Casting
Figure 2.3 A gated pattern for molding a rocker arm. (Courtesy: Heine et al, 1967)
Figure 2.4 A match-plate pattern for molding a rocker arm. (Courtesy: Heine et al, 1967)
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2.2.4 Cope & Drag Patterns
In a cope and drag pattern, the cope and drag halves of the pattern are each mounted on a separate plate. Separate plates enable significant cycle time reduction because they allow the cope and drag halves of the mold to be made simultaneously by workers using different molding machines. Additionally, cope and drag patterns are well suited for use with high speed mechanized and/or automated molding equipment. For these reasons, this pattern type is often the pattern of choice for high volume production applications. The cope and drag pattern is also a good choice for molding medium and large castings on molding machines because each half of the mold is handled separately so weight is greatly reduced. Separate pattern plates are more costly, however and they require accurate alignment of the two mold halves by means of guide pins and bushings on the flasks to insure that the upper and lower halves of the casting match accurately. Figure 2.5 shows an example of cope and drag pattern plates.
Figure 2.5 Separate cope and drag pattern plates for molding a rocker arm (Courtesy: Heine et al, 1967)
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2.3 Pattern Design
Pattern design includes all of the factors that must be considered by the tool builder when creating the pattern and core boxes for a given sand-casting application. The master pattern is the original pattern that is designed and fabricated by the tool builder. In some situations, production patterns may be generated from the master pattern. For example, several copies of a master pattern might be cast and mounted on a match plate for high volume production. In other situations, the master pattern may be directly used by the foundry to produce castings. The discussion that follows applies equally to both to the master pattern and production pattern if they are not the same. 2.3.1 Pattern and Core Box Materials
Several materials are used for master pattern construction. Each has its advantages in different applications. Wood Wood materials are widely used in the foundry industry. The most frequently used types of wood include pine, poplar, mahogany, and cherry. Because it is soft, white pine is usually the least costly and easiest to work with. However, pine has poor wear resistance and is prone to warpage due to changes in moisture content. Poplar is a hardwood with good strength and abrasion resistance, however, like pine it tends to warp. If properly laminated or segmented and coated, warpage can be minimized. Honduran mahogany is hard and fairly stable and, for these reasons, it is usually the wood of choice for high quality production wood patterns. Cherry is also good for production wood patterns; it is harder than mahogany and is easy to work. Mounting a wood pattern helps to maintain its shape. All wood patterns should be properly segmented to offset the effects of grain orientation. Each segment should be glued and secured with screws. Nailed pattern construction should be avoided wherever possible. To increase the life of a wood pattern, coreprints and other areas prone to wear can be made of hard maple or birch. Metal
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Metal patterns are particularly well suited for long production runs. Compared to wood, metal is more abrasion resistant and less subject to warpage. Tighter dimensional tolerances can be held with metal tooling compared to wood. Metal patterns are usually made of cast aluminum or cast iron, but steel and other metals are also occasionally used. Stainless steel is increasing in popularity as a pattern material because it is easier to weld and does not corrode or rust. Metal patterns can be cast to size with little or no machining, or they may be fully machined. In either case, the finishing of the pattern requires considerable polishing. In fact, finishing operations may represent 50% or more of the total pattern construction time. Some areas of a pattern or core box may be more prone to wear or require tighter tolerances. Examples include locating points, fixture locations, core prints, etc. Hardened steel or polyurethane inserts are occasionally used at these critical points for improved wear and stability. Because of the high temperatures needed for thermally setting resins, the shell molding and core making process necessitates the use of metal equipment. Since aluminum tends to gall and wear rapidly at high temperature, cast iron or certain copper alloys, such as bronze or chrome-copper, are recommended. Plastic Plastic materials are being increasingly used in modern foundry practice. Epoxy resins, which are bonded to reinforcing materials have high strength and have proven to be very acceptable as pattern materials. Epoxy resin patterns have many advantages including good dimensional stability, high compressive strength, good abrasion resistance, resistance to chemical attack, high bending strength, and easy release from the molding sand. Unreinfored epoxy patterns are satisfactory for small pieces and limited production. An iron or carbide-filled epoxy surface coat backed by fiberglass or epoxy resin with wood filler can be used for higher production patterns. This type of construction can be an ideal alternative to an expensive large metal cope and drag pattern. Reworking or repairing an epoxy pattern such as this however may be quite difficult.
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Polyurethane elastomer is also extensively used. In addition to having excellent wear qualities this material is less brittle than epoxy materials and has greater resistance to chipping. Polyurethane is used for complete patterns, wear-resistant inserts, and loose pieces. Core boxes made with a urethane elastomer must be reinforced with wood or aluminum. Another advantage of using plastics (either urethane or epoxy) is that complete patterns and core boxes can be inexpensively duplicated from an existing wood master. Styrofoam Styrofoam patterns are most commonly used in the full mold casting process. In this process, the polystyrene pattern is left in the sand mold during the pouring of the molten metal. The heat causes the pattern to decompose into liquid and then gas. Special glues and coatings are used in the construction of foam patterns for this process. Polystyrene is a hydrocarbon existing as a long chain polymer which consists of 92% carbon and 8% hydrogen by weight. Styrofoam patterns can be fabricated from 4 by 8 foot polystyrene sheets with thickness which range from 1 to 12 inches (25.4 to 304.8 mm). Not all styrofoam can be used as pattern material. The recommended density for pattern material is 1 to 1.25 lb/ft3 (16 to 20 kg/m3). Less dense polystyrene is not strong enough while heavier polystyrene is unsuitable because it leaves behind excessive contaminating residues in the full mold process. The primary advantages of styrofoam patterns and the full mold process is the reduced cost compared to the traditional sand-casting process. Mold fabrication time is less and frequently cores may be eliminated. When high production runs are required, the styrofoam patterns can be produced by injection molding. Fast Freeform Fabrication Materials Fast freeform fabrication materials are diverse because of the many different fast freeform processes that are commonly used. Fast
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freeform processes that have found wide use in foundry tooling production include the following: • •
•
•
Stereolithography (SLA): this process builds the model one layer at a time by exposing ultra violet cured epoxy resin to laser light. SLA models look like unreinforced epoxy resin. Laminated Object Manufacturing (LOM): this process builds the model by gluing sheets of paper together in a stack, one layer at a time after each sheet has been cut out by a laser. A completed LOM model looks like it is made out of wood. Selective Laser Sintering (SLS): this process builds the model up by depositing and sintering layers of powder. A variety of powders can be used such as metal powder, ABS plastics, etc. Fused Deposition Modeling (FDM): in this process, the model is built by extruding thermal plastic or wax to form each layer.
Models produced by fast freeform fabrication processes can be used as a one-time throw away pattern or core or they can be used as a master pattern or core box, either to produce short run castings or to make plastic tooling as discussed above. 2.3.2 Pattern Allowances
When designing a pattern, it is often necessary to modify the dimensions of the pattern to account for particular characteristics or requirements of the casting process. Allowances that are commonly applied as part of the pattern design process include shrinkage allowance, distortion allowance, machining allowance, and finish allowance. Shrinkage allowance (Blair and Stevens, 1995) Shrinkage allowance is a correction factor built into the pattern that compensates for dimensional changes that occur as the casting solidifies and cools to room temperature. Since metal contracts as it cools, the pattern is typically made larger than finished part dimensions so that when the casting has cooled to room temperature, its dimensions are at design intent (i.e., the same as those shown on the data representation or drawing within acceptable tolerances).
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This allowance for contraction is often called “patternmaker’s allowance.” The amount of shrinkage allowance that must be applied depends on the following factors: • • •
Casting alloy Resistance of the mold to contraction of the casting Foundry practice
Since different metals contract differently as they cool, the casting alloy determines the starting value for the shrinkage allowance. For example, most aluminum casting alloys have a shrinkage allowance that ranges from 3/32 inch/foot to 5/32 inch/foot (7.8 mm/m to 13.0 mm/m). For carbon and low-alloy steels, the shrinkage allowance is 3/16 inch/foot while 5/16 inch/foot (15.6 mm/m to 26.0 mm/m) is used for austenitic alloys. Because each particular casting alloy shrinks differently, the pattern must be designed for a specific alloy and once designed, the pattern is generally not suitable for use in casting a different alloy. The resistance of the mold to contraction of the casting, which is influenced by the casting shape and the mold material, is also an important consideration. Consider, for example, the casting shown in Figure 2.6. For this casting, length dimension “X” has virtually no constraint on its ability to contract since the casting will pull away from the mold walls as it shrinks. Hence, the full value of patternmaker’s allowance would be applied to dimension “X.” Contraction of diameter “Y”, on the other hand, is moderately to severely constrained depending on the stiffness (i.e., rigidity) of the core used to form the hole. Therefore, little or no patternmaker allowance would be applied to this dimension. It is clear from this example that application of patternmaker’s allowance is highly dependent on judgement and experience. Also, shape of the casting can strongly influence the amount of local shrinkage that occurs. Other mold features, such as mold rigidity, risers, gates, and so forth can also constrain casting shrinkage. When close tolerances are required on a particular dimension, a larger-than-normal shrinkage allowance may be initially applied. The pattern is then corrected
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iteratively by measuring the precise amount of shrinkage that actually occurs and machining the pattern accordingly. Y
X
Figure 2.6 Two contraction situations. Local foundry practices can also influence the amount of shrinkage that occurs. For example, the stiffness of the mold depends on moisture content and other characteristics of the sand. In one foundry, these characteristics may be strongly affected by a variety of practices employed by the foundry. In another foundry that does not follow these same practices, the molding sand characteristics will be different and therefore the shrinkage allowance that should be applied will be different. Consequently, it is important to recognize that unique shrinkage allowances may be required for each foundry because the specific sand management and environmental factors will vary. To avoid “surprises,” it is strongly suggested that buyers of castings have their patterns made at the foundry, or under the direction of the foundry where the castings are to be produced. Direct involvement of the foundry in designing the pattern and other tooling will minimize difficulties that arise from a lack of knowledge regarding the dependency of shrinkage allowance on foundry practice. Distortion allowances Casting distortion can occur when adjacent sections of the casting have significantly different shapes or dimensions and therefore cool at different rates. Distortion allowances seek to adjust the pattern in
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such a way that any distortion that occurs will act to cause the casting to be of the proper shape and size when cool. Distortion allowances are often applied to large, flat areas, U-shaped sections, circular sections, and other areas that tend to distort during solidification and cooling of the casting. A review of experience with similar castings can help in predicting restraint and warpage and is often a good place to start if distortion is a concern. If appropriate, this experience can be applied to the design of the new pattern. Frequently, distortion is discovered to be a problem during try-out of the tooling. In some cases, minor distortions found in castings can be corrected by mechanical means. If the distortion is consistent or prominent, it may be possible to make changes to the pattern dimensions and shape, to counteract these distortions. When patterns are to be changed, it is important to involve the design engineer, the buyer, the foundry engineer, and other interested parties to insure that all ramifications of the change are properly considered. These discussions should consider future production, dimensional tolerances, and other factors that may influence the cost of pattern equipment or casting production. Machining allowances Machining allowance is the amount by which dimensions on a casting are made oversize to provide stock for machining. Dimensions on a pattern drawing include machining allowance. The amount of metal left for machining must be no more than necessary but enough to assure that cutters can get an ample bite beneath and completely remove the hard scale and surface skin of the casting. The necessary allowance depends on a variety of considerations including the size and shape of the casting, the surface to be machined, the pouring position, the tendency to warp, the machining set-up and fixturing, and so forth. Casting finish allowance When large quantities of castings are required, patterns may be deliberately made oversize by applying a finish allowance. The prototype or first castings produced are then measured. Based on
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these results, the patterns may or may not be machined to the final size to achieve the desired casting dimensions. 2.3.3 Draft
Draft is the angle or taper that is applied to vertical faces of a pattern to permit easy removal from the sand mold as shown in Figure 2.7. Draft must be applied regardless of the type of pattern used and should be specified on the casting drawing. As a general rule, the draft angle should be as generous as possible to minimize mold build cycle time. In cases where the amount of draft may affect the subsequent functionality of the casting, the drawing should specify whether draft is to be added to or subtracted from the part dimensions.
Figure 2.7 Draft illustration (Courtesy: Blair & Stevens, 1995) The amount of draft depends upon the size of the casting, the method of production, and whether the molding is done by hand or by machine. Machine molding requires a smaller amount of draft than hand molding because withdrawal forces are more uniformly and consistently applied. Interior surfaces in green sand molding usually require more draft than exterior surfaces. Smaller vertical dimensions need more draft angle than larger surfaces. The amount of draft recommended under normal condition is about 3/16 inch/foot (16 mm/m) or approximately 1.5%. This allowance can be
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eliminated, in some cases, by using cores and/or separate pieces. These situations should be discussed with the foundry engineer. 2.3.4 Parting Line
In sand casting, the mold is divided into two halves (cope and drag) so that the pattern can be withdrawn. When the cope and drag halves are assembled, cavities in each half meet, producing the air space into which the molten casting alloy flows. From the inside, the mating junction between the mold halves appears as a line. This line also appears on the casting and is called the parting line or parting plane. Parting in one plane is preferred since it facilitates the design and fabrication of the pattern as well as the construction and assembly of the mold. The cost of patterns with straight parting lines is also lower, in general, compared to those that have irregular or offset parting lines. Casting designs that are symmetrical about a centerline or plane readily suggest the position of the parting line. Patterns should be made as “split patterns” to reduce labor and handwork in the mold. Except in the case of machine molding, offset parting lines require costly pattern equipment and skilled molding technique. For some cases, the need for an offset or irregular parting line can be avoided by modifying the casting geometry. 2.3.5 Geometry Considerations
The geometry of the casting is often a key factor in many tooling approaches and decisions. It is strongly recommended that a concurrent engineering approach in which the design engineer and foundry engineer work closely together as a team during the design of the casting be used whenever possible. By using a concurrent engineering approach, the geometry of the casting can be optimized for both functionality and ease of tooling and production. This can often result in considerable cost and lead time savings. A summary of key geometry considerations follows. Fillets
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A fillet is a rounded filling along the convergence of two surfaces of a pattern. The rounded corner thus produced on the casting is also called a fillet. Fillets may be carved in wood patterns but are usually made more inexpensively of wax, plastic, wood coving, or leather. Fillet radii typically vary in size from 1/8 inch to 1 inch (3.175 mm to 25.4 mm) depending on the size, shape, and material of the casting. Fillets smooth sharp angles and corners and thus strengthen both patterns and castings. They provide for easier removal of the pattern from the sand, a cleaner mold, freer flow of metal through the mold, less washing of the sand in the mold, and fewer shrinking strains and hot tears between sections as the casting cools. Undercuts and loose piece Any feature that prevents the pattern from being withdrawn is referred to as an undercut. In sand casting molds, manually placed loose pieces and sand cores are used to facilitate undercuts. Loose pieces and cores are expensive, they add tolerance stacks to the mold build, and they introduce complexity and quality risk into the process. Hence, loose pieces should be avoided whenever possible. Good casting design eliminates the need to use loose pieces by designing out areas that will not draw naturally. Loose pieces should be considered only for very low quantity runs. As illustrated by the example in Figure 2.8, undercuts can often be eliminated by careful attention to casting geometry. The result is a lower cost casting that is much simpler to make and more consistent from part-to-part.
Figure 2.8 Attention to casting geometry eliminates undercuts (Courtesy: Blair & Stevens, 1995)
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Unnecessary ribs, lugs, and bosses that require cores also increase cost. Engineers often place circular ribs around the outside of a casting to add strength and maintain a lightweight part as is shown in Figure 2.9(A). If ribs of this nature can be modified as shown in Figure 2.9(B), substantial cost saving can be achieved. The same practice applys to bosses as well. They should be designed to avoid the use of cores or loose pieces.
Figure 2.9 Casting geometry determines the need for cores (Courtesy: Blair & Stevens, 1995) 2.4 Master Pattern Fabrication
As discussed previously, there are three different approaches for fabricating a master pattern. • • •
Manual fabrication CNC machining Fast freeform fabrication
Each of these approaches is now widely used in the foundry industry and each offers advantages and disadvantages that make them more or less suitable for a given situation.
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Figure 2.10 A master pattern made of mahogany (Courtesy: Clinkenbeard & Associates) 2.4.1 Manual Fabrication
Manual fabrication is the traditional and most widely used method for making master patterns for sand casting. In this approach, the patternmaker fabricates the pattern manually using traditional wood working tools and pattern materials. Figure 2.10 shows a manually fabricated master pattern. To manually fabricate a master pattern, the patternmaker must do the following: • • • • • •
interpret the 2D engineering drawing select an appropriate pattern material select the parting plane and appropriate datum points use traditional wood or metal patternmaking tools and equipment to cut out and shape individual pieces of the pattern join the individual pieces together using glue and screws finish the completed pattern to meet accuracy and tool life requirements
The key to effective manual pattern fabrication is the availability of craftsman who have the detail knowledge and skill required to carry out this type of work. The primary advantage of manual
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fabrication is its adaptability to almost any situation. The primary disadvantage is the inability of the craftsman to manually produce complex surfaces and geometry quickly and to a high degree of precision. Therefore, manual fabrication is best suited for simple parts that do not have excessive geometric complexity and/or high accuracy requirements. Although more labor intensive that the other alternatives, manual fabrication does not require the time consuming and costly up-front planning and setup activities required by computer aided rapid tooling methods. Consequently, for simple parts that do not required highly accurate features, manual fabrication is often the fastest and lowest cost pattern fabrication alternative. Also, manual fabrication is the only alternative that does not require a 3D CAD solid model representation of the part geometry. 2.4.2 Computer Numerical Control Machining
Computer numerical control machining, also called CNC machining, is a computer based approach that utilizes CNC machining processes such as milling and drilling to fabricate the pattern. In this method, a 3D CAD solid model of the part is used to drive the various CNC machining processes to machine the pattern out of a solid workpiece made of the selected pattern material. After receiving a 3D CAD solid model from the client, the tool builder modifies the file by adding necessary allowances, deciding on a parting line, dividing the model into cope and drag portions and separate pieces as required, and adding draft as necessary. The resulting solid model or solid models are then used to computer generate the tool path and machining code required by the CNC machining processes to fabricate the pattern or pattern pieces. The appropriate CNC machines are then set-up and the CNC programs executed to make the pattern pieces, which are assembled and finished to complete the pattern. The CNC machines typically used in master pattern production range from 3 to 5 axis. Figure 2.11 shows a CNC milling machine in the process of machining a master pattern. CNC machining is the most accurate and the fastest rapid tooling method available. Because of its inherent accuracy, CNC machining is often the method of choice for complex geometry and high
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precision applications. Also, because it is suitable for making metal patterns and can provide a high quality surface finish, CNC machining is widely used to fabricate tooling for large volume production castings.
Figure 2.11 CNC machining process (Courtesy: Clinkenbeard & Associates) 2.4.3 Fast Freeform Fabrication
In fast freeform fabrication, the pattern or pattern pieces are fabricated by successively adding particles or layers of raw material to create a solid volume of the desired shape. Typically, a laser or other device is driven by computer control to perform this process. Consequently, as with CNC machining, fast freeform fabrication can only be used when a 3D CAD solid model of the part has been created. A variety of fast freeform fabrication machines or processes are commercially available. These include Laminated Object Manufacturing (LOM), Stereolithography (SLA), Selective Laser Sintering (SLS), Fused Deposit Modeling (FDM), and Solid Ground Curing (SGC). Each of these processes operate in different ways and are capable of utilizing different materials. For example, the LOM process constructs the model out of sheets of paper or plastic, the SLS process fuses various powdered materials together, and the FDM process builds the model by extruding a polymer along a controlled path. Because of the diversity of processes and materials,
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each fast freeform method has its own set of advantages and disadvantages. In selecting the best method for a given application, four factors need to be evaluated: accuracy, build time, strength, and post finishing. 2.5 Core and Core Box Fabrication
Cores are separate pieces that are used to shape interior surfaces and other areas of a casting that cannot be shaped by the pattern. Cores are typically made of sand that is mixed with organic binders such as linseed oil or cereal. Organic binders are preferred over clay because they break down under the heat of the molten alloy, which makes them easy to remove during shakedown. The raw sand mixture is typically molded in a core box to form the core. After molding, the core is baked in an oven to give it the strength it needs to be handled during placement in the mold and to withstand the fluid flow and buoyancy forces that may develop during filling and solidification of the casting. Ideally, the core should have enough strength to maintain integrity throughout the casting process and also collapse and revert to sand after solidification so that it is easy to remove. 2.5.1 Core Fabrication Process
Figure 2.12 illustrates the steps involved in making a typical core. Cores are made by ramming or blowing the raw sand mixture into a core box. Metal rods are sometimes used to strengthen the core. If the core is made in parts, the parts are glued together after baking and are then briefly rebaked with circulating air at about 4500F (or 2300C) until the core reaches a nut-brown color. Oxidation, condensation, polymerization, and drying develops the desired strength in the core according to its original composition. It is very important that the proper baking time and temperature be used for a given binder and core size. A properly baked core does not produce harmful gases, has adequate strength, and collapses at the right time after the casting is poured.
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Figure 2.12 Making a core. (a) ramming core sand, (b) drawing the core box, (c) baking the core half (in the dielectric oven), (d) gluing the core halves, (e) “washing” the core with refractry slurry to improve casting surface finish. (Courtesy: Taylor et al, 1959) With the development of rapid tooling technology, cores can now be directly produced using computer numerical control (CNC) machining and/or fast freeform fabrication (FFF). With the combination of the CAD/CAM techniques and rapid tooling, core design and fabrication can be performed very quickly and with high quality. Figure 2.13 shows a master core plug being produced by CNC machining. DTM Corporation has developed the SandFormTM Zr & Si materials and a Sinterstation® System to fabricate both cores and molds using a fast freeform fabrication process. Similarly, the KelToolTM process has been developed by 3D Systems Inc. 2.5.2 Core Boxes
A core box is a device for molding a core. Core boxes may be made of wood or metal, depending on the number of cores to be produced. A metal box is generally made by casting it from a wood master core box. The added cost of making and finishing a metal box is justified when a wood box would not outlast the job. Metal core boxes are usually made of aluminum and have steel strips
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inserted in areas that are exposed to excessive wear. Rapid tooling processes may also be used to fabricate a master core box. Figure 2.14 shows a core box made using the layered object manufacturing (LOM) process.
Figure 2.13 A master core plug being made by a CNC machine (Courtesy: Clinkenbeard & Associates)
Figure 2.14 A master core box produced by the LOM process
Chapter 3 Fast Freeform Fabrication Methods and Processes Fast freeform fabrication, also called rapid prototyping, is the automatic manufacturing techniques using slicing and additive processes. The first techniques for fast freeform fabrication became available in the late 1980s and were used to produce models and prototype parts. With the fast freeform fabrication method, the machine reads in data from a CAD drawing, slicing the CAD model into a thin, virtual, horizontal cross-sections and lays down successive layers of liquid, powder, or sheet material, and in this way builds up the model from a series of cross sections. These layers, which correspond to the virtual cross section from the CAD model, are joined together or fused automatically to create the final shape. Figure 3.1 shows the principles of this slicing and additive manufacturing technology. The primary advantage to this slicing and additive fabrication is its ability to create almost any shape or geometric feature. 3D CAD Model Z Direction Slicing Set Manufacturing Info
Discretizing CAD model & info processing in PC
Auto Layered Building Post-Processing
Fast freeform fabrication in the machine
Finished Model Figure 3.1 The principle of fast freeform fabrication W. Wang et al., Rapid Tooling Guidelines For Sand Casting, Mechanical Engineering Series, DOI 10.1007/978-1-4419-5731-3_3, © Springer Science+Business Media, LLC 2010
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The standard data interface between CAD software and the machines is the STL file format. An STL file approximates the shape of a part or assembly using triangular facets. Smaller facets produce a higher quality surface. Currently, the fast freeform fabrication has been widely used in rapid tooling, rapid manufacturing, medical industry, arts and other applications. 3.1 Fast Freeform Fabrication (FFF) Processes
Many FFF processes have been invented and commercialized in recent years. The more successful of these technologies are briefly discussed as follows. 3.1.1 Stereolithography Apparatus (SLA)
This process involves selective curing of a photo-curable liquid polymer, using a laser beam directed by a computer in accordance with a CAD model. Figure 3.2 illustrates the process.
Figure 3.2 Stereolithography process
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Process 1. A control unit directs a laser beam on to the surface of a vat of the liquid polymer. The polymer is photo-curable. 2. The laser cures a thin layer of polymer on the surface, according to the cross section traced out by the laser as it is guided by the CAD system. 3. A movable table just beneath the surface moves downward by one layer. 4. Steps 1 through 3 are repeated until the part is completed. Materials Liquid photopolymer, such as SL 5149, SL 5154, SL 5170, SL 5180, SL 5190, SL 5195, SL 5410. Characteristics The average layer thickness in this process is 0.02 inch (0.51 mm). Because of shrinkage and warping of the model, additional support for the model must usually be provided. Machine cost ranges from $95,000 to $400,000 depending on the envelope size of the part it can make. 3.1.2 Laminated Object Manufacturing (LOM )
In this process, the model is generated by stacking successive layers of paper. Each layer is laser cut to match the corresponding cross-section of the model. Figure 3.3 illustrates the process. The LOM process was developed and commercialized by Helisys, Inc. The company was out of business several years ago. Process 1. The paper sheet material is fed from a supply roll into position over the previous layer. 2. The paper layer, which is backed with a thin layer of glue, is bonded to the previous layer by passing a laminating roller over it. The roller bonds by simultaneously compressing and heating the laminate. After rolling over the laminated area, the roller retracts into standby position. 3. A CO2 laser beam, which is tuned to one lamination thickness, traverses over the bonded laminate to cut the external and
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internal profiles to match the corresponding cross-section of the CAD model. 4. After completion of the cutting action, the “Z” axis table lowers slightly, the paper sheet material is fed forward, the “Z” axis returns to the working position, and the process repeated. 5. The waste material, which has been cut into small blocks, must be removed in a “decubing” operation. Decubing is usually a manual process.
Laser Optics X-Y positioning device
Laminating Roll
Layer outline and crosshatch
Sheet material
Part block Platform
Take-up roll
Material supply roll
Figure 3.3 Laminated Object Manufacturing Materials Plastic, paper, foil, glass or fiber composite. The mostly frequently used materials are LOMPapersTM such as LPH 042 (0.0042” thick), LPH 080 (0.008” thick) and LPS 038 (0.0038” thick). Characteristics Accuracy up to 0.005 inch (0.127 mm) is claimed. Layers can be 0.002 to 0.01 inch (0.051 to 0.254 mm) thick. Typical models are less fragile and more precise than other FFF processes such as SLA.
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Materials are cheaper than those used in other processes such as SLA and SLS. Because decubing is a manual process, it adds time and some degree of imprecision to the process. Also, waste material removal from complex internal voids can be difficult. Another disadvantage is that waste material can not be reused. 3.1.3 Fused Deposition Modeling (FDM)
In this process, which was invented and commercialized by Scott Crump, president of Stratasys Inc., filament is heated and deposited to form successive layers. Figure 3.4 illustrates the process. Process 1. A spool of filament is fed into the unit’s heated extruding head like wire feeds into an automatic welder. 2. Inside the flying extruder head, the filament is melted to liquid by a resistance heater. 3. As the head moves in the X and Y direction according to the control path generated by CAD model, the material is extruded out of the head and deposited on the 2-D work surface. 4. An elevator controls motion in Z direction. After extrusion, the new layer is wiped and sheared, on the fly. It is then lowered by the elevator so that the next layer can be deposited. 5. The process continues until the model is complete. Materials Proprietary thermoplastic material and wax. Characteristics Layer thickness varies from 0.001 to 0.05 inch (0.025 to 1.27 mm). Accuracy of the model is claimed to be 0.005 inch (0.127 mm). The system costs $178,000 for 12 × 12 × 12 inches (305 × 305 × 305 mm) envelop size.
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Extrusion head
Green Part
Support material
Building material
Support Platform Z Axis
Figure 3.4 Fused Deposition Manufacturing 3.1.4 Selective Laser Sintering (SLS)
The process is developed by Carl Deckard and Professor Beaman at the University of Texas, Austin and is commercialized by DTM corporation. Figure 3.5 illustrates the basic steps of the process: Process 1. Layer of heat-fusible powder is deposited in the part build chamber. 2. An initial cross-section of the object under fabrication is ‘drawn’ on the layer of powder by a heat generating CO2 laser. The interaction of the laser beam with the powder elevates the powder temperature to the point of melting, causing the particles to melt and fuse together to form a solid mass. The intensity of the laser beam is modulated to melt the powder only in areas defined by the object’s design geometry. 3. An additional layer of powder is deposited via a roller mechanism on top of the previously fused layer. 4. The process is repeated, with each layer fusing to the layer below it. The process continues until the part is complete. 5. The part is removed from the build chamber and the loose powder fall away.
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Laser
Add powder
Sinter layer
Lower platform
Repeat cycle
Last layer sintered
Finished Part
Figure 3.5 Selective Layer Sintering Materials Any material that softens and has decreased viscosity upon heating can be used, such as ABS, PVC, wax, and nylon. DTM supplies a variety of materials specifically formulated for the process. These materials include DTM Polycarbonate, DTM DuraForm Polyamide, SOMOS 201 Elastomer, TrueForm Polymer investment casting material, SandForm Zr & Si sand casting materials, and RapidSteel metal for tooling applications. Characteristics Accuracy of 0.002 to 0.01 inch (0.051 to 0.254 mm) is commonly achieved. Due to an absence of chemical processes in SLS process, cycle time is generally less than competing FFF processes.
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3.1.5 Solid Ground Curing (SGC)
Photo cured polymer models are built in a solid wax environment, eliminating curling, warping, support structure, and any need for final curing. Figure 3.6 illustrates the process.
Complete Part 2. Spreading 3. Exposing
1. Leveling
4. Wiping 6.Milling 5. Filling
Figure 3.6 Solid Ground Curing Process 1. A computer analyzes a CAD file and renders the object as a stack of “slices”. 2. An negative image of the working slice is “printed” on a glass photo-mask using an electro-static process similar to laser printing. Because the image is a negative, areas of the “slice” representing solid material are transparent. 3. A thin layer of photo-reactive polymer is laid on the work surface and spread evenly. 4. The photo-mask is positioned above the newly spread layer of liquid polymer. An ultra-violet floodlight is projected onto the photo-mask curing and solidifying those regions of the polymer layer where ultra-violet light shows through. 5. The unexposed resin, which remains a liquid, is then vacuumed off. 6. Liquid wax is spread across the work area, filling the cavities previously occupied by the unexposed liquid polymer resin.
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7. A chilling plate hardens the wax. The entire layer, wax and polymer, is now solid. 8. The layer is milled to the correct thickness. 9. The process is repeated for the next slice with each layer adhering to the previous one. The process is repeated until the object is finished. 10. The wax is removed by melting or rinsing, revealing the finished prototype. Materials Liquid photo-reactive polymer. Characteristics Because the whole layer is exposed simultaneously, the Solid Ground Curing process has 10 to 15 times the throughput of other rapid prototyping methods based on photo-reactive polymer. Any geometric shape can be created in any orientation. The use of wax means that supports do not have to be added to support overhangs and other fragile regions. 3.1.6 Three Dimensional Printing (3DP)
Three-dimensional printing is a FFF process developed at the Massachusetts Institute of Technology. Figure 3.7 illustrates the process. Process 1. A thin distribution of powder is spread over the surface of a powder bed. 2. A slicing algorithm computes information for the layer from the computer model of the desired part. 3. Using a technology similar to the ink-jet printing, a binder material is deposited on the layer of powder. The binder joins the powder particles together in regions where the object is to be formed. 4. A piston then lowers the build so that the next powder layer can be spread and selectively joined. 5. This layer by layer process repeats until the part is completed. 6. Unbounded powder is removed, leaving the fabricated part.
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Spread powder
Print layer
Drop piston
Repeat cycle
Intermediate stage
Last layer printed
Finished part
Figure 3.7 Three Dimensional Printing Materials 3DP can form any powder material. Ceramic powders, such as alumina, silica, zirconium and silicon carbide, are the most commonly used materials. Characteristics The distance between the intersecting planes is 0.0375 inches (0.9525 mm). As the same as SLS, the 3DP process is less time consuming than some other processes. The current machine build volume is about 3 cubic inches (442,450 cubic millimeters). 3.1.7 Sanders Prototype (SP)
This process was developed by SPI (Sander Prototyping Inc., now called SolidShape). It involves a liquid to solid inkjet plotter with separate z-axis input. Figure 3.8 illustrates the process.
Chapter 3 Fast Freeform Fabrication Methods and Processes CAD Interfaces, STL, DXF & HPGL Files
Control software
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Control electronics X-Y motion Drop-on Demand jets
Planar mechanism
Overhead support
Model Build substrate Build table
Z motion
Figure 3.8 Sanders Prototype method Process 1. The dual inkjet subsystem rides on a precision x/y carriage and deposits both thermoplastic and wax materials on the build substrate under program control, according to the path generated by CAD model. Utilizing patented technology, these droplets may be placed at any desired location upon the build substrate within 0.00025 inches (0.007 mm) in the X and Y directions. The droplets adhere to each other during the liquid-to-solid phase transition to form a uniform mass. The drying process is fast enough to allow milling of the layers immediately following the deposition cycle. 2. The x/y drive carriage also energizes a flatbed milling subsystem for maintaining precise z-axis dimensioning of the model by milling the excess vertical height of the current build layer. Thus, subsequent layers will have a known surface reference layer to build upon. 3. The build substrate moves downward by one layer. 4. All these steps are repeated until the part is completed. Materials The ProtoBuild material is a Sanders proprietary thermoplastic material. This build material is claimed to be compatible with investment casting and other mold making processes and has
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excellent burn-out characteristics. The ProtoSupport material is also a Sander Prototyping. Inc. proprietary wax material designed to be removed with a solvent without damage to the build material. Characteristics Nominal z-axis slice thickness is 0.002 to 0.003 inches (0.050 to 0.075 mm) per slice. The x and y micro-droplet placement can be specified to within 0.00025 inches (0.007 mm) under program control. The support material is used to support overhangs and cavities in the model during the model build sequence. 3.2 FFF Model Building Procedure
The procedure for making a fast freeform model can be divided into four major steps: (1) modeling and data preparation, (2) machine setup, (3) build, and (4) post processing. Although details of individual steps may differ depending on the particular method, the overall procedure is essentially the same for all currently available fast freeform fabrication methods. In the following, the detailed steps required to create a sand-casting master pattern using the LOM process is described. 3.2.1 CAD Modeling and Data Preparation
Drawing Interpretation A 3D CAD solid model of the part to be built is necessary as the starting point for all fast freeform fabrication processes. If the initial data is a 2D engineering drawing, then the tool builder must create a solid model of the part using a commercial CAD package. This requires that the tool builder properly interpret the 2D drawing to understand the dimensions, tolerances, and importance of all the geometrical features that constitute the part. 3D Modeling After the 3D CAD solid model has been received from the customer or created from a 2D drawing by the tool builder, the tool builder must select an appropriate parting plane, divide the model into pieces as appropriate, add draft and allowances, and specify datum points as necessary to facilitate construction of the pattern. Tessellation
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The solid model of the pattern pieces must be tessellated to be suitable for driving the LOM process. The tessellation process results in an .STL file, which is a de-facto standard used in rapid prototyping. In the .STL format, curved surfaces are approximated by triangular facets that are stored as a set of X, Y, and Z coordinates for each of the three vertices. A unit normal vector with X, Y and Z values is also stored to indicate the positive direction of the triangular facet (outside surface of the part). Most 3D CAD/CAM software packages will generate an .STL file on demand based on the solid model of the part. 3.2.2 Machine Setup
Machine setup involves the preparation activities required to prepare the LOM machine prior to building the model. It includes several important steps: (1) checking the machine, (2) selecting & loading material, (3) determining and setting parameters, and (4) selecting the build direction and creating the base. Check the Machine This step involves checking and verifying the status of the machine to assure the quality of the LOM model. For example, as part of this step, the alignment of the platform and roller pressure are carefully checked to insure they are within specification. Select and Load Material In selecting the material, the tool builder must consider the purpose of the LOM model, the build time and accuracy, and the strength requirement for the model. After the material to be used has been selected, a roll of the material is loaded on the LOM machine. Determine and Set Parameters Parameters can be divided into two basic categories: machine parameters and material/model parameters. Machine parameters include the location of the model on the platform (X and Y coordinates), system speed, and various mechanical movements. Material/part parameters include bonding (temperature) parameters and laser parameters. Different materials have different parameter values. The LOM machine manufacturer provides a set of recommended parameter values for each material. The user may
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then adjust each parameter to better meet the specific requirements of the model being built. Once the parameter values have been chosen and set, they should not be changed until the model is finished. Select the Build Direction and Create the Base In this step, the most appropriate build direction for the part is selected. The goal is to minimize material consumption and build time while ensuring the highest fidelity reproduction of the CAD solid model. A base for the model, usually consisting of 20 or more laminated layers, is then built. The base provides support for the model, strengthens the connection between the model and the platform, and facilitates removal of the model after completion. 3.2.3 Building
After the proper preparation of CAD data and the LOM machine, the build process begins. This process is automated and typically requires no involvement from the operator unless a new roll of material must be loaded or the process must be interrupted for other reasons. During the build process, the computer screen displays the part image, build time, cross-section information, and a progress report. When the build is finished, the computer flashes a completion message. 3.2.4 Post Processing
The LOM process involves building the model up out of layers of paper or other material. As each layer of paper is added, a laser cuts out the shape of the part cross section. A “cross hatch” pattern of laser cuts is made in all regions of the paper layer that are not part of the solid volume of the part. After the build is finished, the model and base are removed from the machine platform. The material that is not a part of the solid volume of the model must then be removed in a “decubing” process and the surface of the model sanded and sealed to provide a smooth finish. These steps are described as follows: Decubing This process involves removing all of the material from internal voids and other areas that are not a part of the model itself. The
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decubing process should be performed immediately after model completion to make the separation process easier. Decubing is usually performed in the following sequence: remove the base from the bottom of the model block by prying off the base tiles, remove the outer wall of the model block, remove the crosshatch material from internal cavities, and lastly remove the crosshatch material from the outer features of the part. Model Repair Sometimes the model may be damaged during the decubing process. When this happens, the model must be repaired to insure high fidelity. Typically, dents and gouges are repaired by applying wood filler and sanding the filler smooth with 180-grit sand paper. In doing this, the operator must take care to only sand the filler and not the model. Cyanoacrylate “super” glue is used to repair small lifted and torn layers while two-part epoxy glue is used for larger regions where layer separation has occurred. Surface Finish The model is finished by spraying it with lacquer sanding sealer, sanding the dried lacquer surface smooth, and then carefully brushing off all sanding residue. For a sand casting finish, only one coat of lacquer is used. For the standard “satin” finish, four coats of lacquer are typically used. A show-quality glossy finish is achieved by very fine sanding and polishing of the model. It is important to note that surface finishing of the model is a labor intensive and timeconsuming process that can also effect accuracy of the model since it is manually performed. Strengthening Thin Walls To ensure structural integrity, it may be necessary to strengthen thin walls and other fragile features. When this is the case, a base coat of two-part epoxy may be applied and then sanded when dry. As in the finishing process, care must be taken to ensure that the model features and dimensions are not changed appreciably by the addition and subsequent sanding of the epoxy.
Chapter 4 Rapid Tooling Processes Sand casting is one of several near net shape manufacturing processes. In near net shape manufacturing, multiple copies of the product are produced by imprinting the shape of the tool or die on a suitable working material. Typically, the working material starts out as a liquid, powder, or pliable material that is eventually solidified or hardened after being formed by the tool. Net shape manufacturing processes include other casting processes such as die casting and investment casting, polymer molding processes such as injection molding, blow molding, and thermoforming, bulk deformation processes such as extrusion and forging, and sheet metal forming processes such as bending and deep drawing. Design and construction of tooling for near net shape manufacturing processes has traditionally been time consuming and costly. With the advent of modern computer-aided machining and fast freeform fabrication (FFF) technologies, many new and innovative approaches to making near net shape tooling have been developed. This has led to the rapidly expanding new field known as “rapid tooling.” Rapid tooling (RT) processes utilize computer modeling and advanced manufacturing technologies to produce tooling for net shape manufacturing processes such as sand casting. Two basic approaches are used: 1) additive processes in which fast freeform fabrication (FFF) technologies are used to build up a desired object layer by layer, and 2) subtractive processes in which computer numerical control (CNC) machining technology is used to create a desired object by removing material from a work piece. Both approaches are driven by a digital database and utilize modern CAD/CAM techniques to quickly and accurately fabricate the tooling. There are a variety of ways in which CNC and FFF technologies can be used to create tooling. These include at least the following:
W. Wang et al., Rapid Tooling Guidelines For Sand Casting, Mechanical Engineering Series, DOI 10.1007/978-1-4419-5731-3_4, © Springer Science+Business Media, LLC 2010
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• • •
•
Fabricate the pattern or mold directly by CNC machining it from a material that is appropriate for the process and production quantity required. Generate the pattern or mold directly using an appropriate FFF process. Fabricate a mold that can then be used to cast the production pattern or mold. This requires translating the desired geometry into a negative representation that may also include draft and shrinkage allowance as necessary. Fabricate a positive pattern on which the mold is cast. Use the mold to produce the production tooling. Apply draft and shrinkage allowance to the positive pattern as necessary.
Different types of tooling result depending on the approach used. If the tooling material is an FFF material such as paper or photopolymer, it can only be used to make a few production copies before it is destroyed. Such tooling is often referred to as soft tooling because of its poor durability. Hard tooling, on the other hand, is production tooling that is made of a durable material such as mahogany or steel that can be used to produce thousands of copies. A third type of tooling, called bridge tooling, is soft or hard tooling that is used as an alternative to typical production tooling in order to produce prototype parts or a short run of production parts quickly. 4.1 CNC Based Tooling Process
The CNC machining process uses state-of-the-art CAD/CAM software and advanced CNC equipment to produce high-quality soft tooling, hard tooling, and bridge tooling from 3D CAD data in a short time. The manufacturing process is shown in Figure 4.1. CNC machining offers several important advantages. CNC machining is the most accurate of all the RT processes. All of the materials typically used to make tooling for sand casting, such as REN Board, polycarbonates, glass-filled nylon, ABS plastic, aluminum, steel, graphite, copper, and wood, are readily machined using the same CNC equipment and fixturing. CNC equipment is technologically mature, moderate in cost, and readily available.
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Additionally, CNC machining offers high quality surface finish and true 3D surfacing. The disadvantages of CNC machining include additional time and cost for fixturing and machining of internal features. Object to be Machined (sketch, drawing, model)
Create 3D Soid Model (CAD) Generate Cutting Tool Path (CAM) CNC Machine Object from Raw Stock
Finished Object
Figure 4.1 Object creation using the CNC machining process 4.2 Rapid Tooling Processes Based on Fast Freeform Fabrication
FFF based rapid tooling techniques can be used to produce pattern, core and cavity inserts. Since there are several FFF processes, there are also several FFF based rapid tooling processes that utilize these technologies. Examples include silicone rubber tooling (RTV), aluminum-filled epoxy tooling, Direct AIMTM, 3D KeltoolTM from 3D Systems, RapidToolTM from DTM Corporation, LOM based tool fabrication, and FDM based tool fabrication. As
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illustrated by Table 1 (next page), these RT technologies have wide application in casting and net shape manufacturing in general. 4.2.1 Stereolithography Based RT Methods
Stereolithography apparatus (SLA) is commonly used to fabricate soft tooling, bridge tooling, and hard tooling. Each of these applications is briefly described as follows. 4.2.1.1 Room Temperature Vulcanizing (RTV) Process (Soft Tooling) A widely used method to obtain 3-dimensional parts is the process known as RTV or Rubber Tooling. In this process, a master SLA pattern is used to form the cavity in a room temperature vulcanizing (RTV) rubber mold. The process utilizes a master SLA pattern in the "positive" form for producing the final part. The SLA master must be fully sanded and polished since the surface finish is critical when making the RTV mold. The mold will reproduce any and all surface defects left on the master. In turn it will transfer any imperfections onto the finished part. Even fingerprints will be picked up on an RTV mold and ultimately appear on each of the parts produced from the mold. 4.2.1.2 KelToolTM Process (Hard Tooling) 3D KeltoolTM is a unique, commercially proven tool-making solution that creates production inserts in a prototype timeframe. 3D KeltoolTM produces tool inserts ideal for injection molding and die casting in 8 calendar days. 3D KeltoolTM is a process that is complementary to CNC and EDM machining. It's a tool in the toolmaker's toolbox providing a flexible solution that is used to produce prototype, bridge and production tooling.
Pattern, spray
Solid ground curing Pattern (SGC) Fused deposition Pattern modeling (FDM) 3D printing (3DP) Shell, pattern
Laminated object manufacturing (LOM)
Rapid Tooling 3D KelToolTM, Qucik/ CastTM, pattern, cold spray Selective laser sintering Sinterstation, shell, (SLS) pattern, core
FFFF Process Stereolithography (SL)
Application Injection molding, investment casting, spin casting, spray metal mold Injection molding or die casting, investment casting, sand casting Investment casting, sand casting, spray metal mold, silicone rubber molding Investment casting, sand casting Investment casting, sand casting Injection molding
Table 4.1 Typical applications of FFF processes in rapid tooling.
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The 3D KeltoolTM process starts with the cavity and core designed in 3D CAD. Once the tool design has been finalized, .STL files are generated from the CAD files. The stereolithography apparatus (SLA) quickly and accurately produces “Master Patterns” using the .STL files. The stereolithography patterns are called "Master Patterns" because the 3D KeltoolTM process uses them to produce silicone rubber transfer molds. The tool-maker determines the quality of the tooling by finishing the stereolithography Master Patterns. Because 3D KeltoolTM replicates the tool geometry, the quality of the stereolithography Master Patterns determines the quality of the 3D KeltoolTM mold inserts. Using the SLA Master Pattern, a RTV Transfer Mold is produced. The mold is then filled with a thoroughly mixed "slurry" of 70% A6 tool steel powder, tungsten carbide powder, and 30% epoxy binder which is used to bind the two powders together. Once the slurry has cured in the mold, the "green part" is de-molded and placed into a graphite furnace boat, which is then loaded into a hydrogenreduction furnace and sintered. During sintering the binder material is burned off resulting in a "brown part" that is 70% A6 steel and tungsten carbide, and 30% void (air). The final step is to infiltrate the open spaces in the brown (sintered) part with copper. The resulting part is a fully dense, production tool-making insert comprised of 70% A6 tool steel and tungsten carbide, and 30% copper. The SLA master pattern must be made slightly oversize to compensate for the finite shrinkage that occurs in the SLA process, the KeltoolTM process (0.6%) and the near net shape manufacturing process. 4.2.1.3 Direct AIM™ and QuickCastTM (Bridge Tooling) Direct AIM™ is a new and important "bridge tooling" process that quickly and inexpensively produces functional prototype parts that can be made of a variety of engineering thermoplastics in a very short time (as short as two weeks) without the need for production tooling. The Direct AIM™ process utilizes the SLA FFF process to fabricate core and cavity mold halves. In order to facilitate production of the SLA cavity and core, a CAD solid model of the part is required. The process also requires generous draft on the part
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(1.0 degree or more) to ensure reliable part release from the mold. In general, increased draft results in lower cycle times and longer tool life. As a rule of thumb, it is recommended that draft be as severe as possible without degrading the part's usability. Drafts as high as 5.0 degrees can significantly reduce the chance that the mold will tear in deeper cavity formations. In the QuickCastTM process, developed by 3Dsystems, Inc., a positive SLA model of the part is used to directly product a functional metal prototype. In essence, the QuickCastTM process uses specially built stereolithography models (drained, sealed and leak checked) as patterns for investment casting, completely eliminating the time and cost of tooling. Experience has shown that the QuickCastTM SLA patterns are more durable than traditional wax patterns and provide comparable production yields with standard foundry procedures. 4.2.2 LOM based RT methods
In principle, any production process that uses wooden or plastic tooling can be duplicated using LOM masters. The stability, accuracy and cost-effectiveness of LOM-produced patterns results in substantial time and cost savings for production of many components regardless of their size or geometry. The following are techniques that produce metal and plastic components using LOM masters. 4.2.2.1 Metal Prototypes Lost Paper Process (Direct Investment Casting) To avoid the need to fabricate molds for injection molding the wax patterns used in traditional investment casting, foundries are converting to a "lost paper" (e.g. direct) investment casting process that uses LOM masters for low volume production of metal components. This new process uses the LOM master as the pattern on which the ceramic shell is built. Once the casing is dry, it is placed in a high temperature furnace to harden the shell and burn out the LOM master. The stability and lack of expansion of the LOM pattern during the burn cycle is critical in preventing cracks in the ceramic shell. After removing the remaining ash, the ceramic casing
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is reheated and metal is poured into the shell. Once the metal is cool, the ceramic shell is chipped away to expose a single metal component. This direct investment casting technique is very cost effective, accurate, and timely for the production of highly complex components or two-part negative tooling. Figure 4.2 shows the direct investment casting tooling.
Figure 4.2 Direct investment cast tooling (Courtesy: Helisys Inc.)
Figure 4.3 Indirect investment cast tooling (Courtesy: Helisys Inc.)
Two-Part Negative Tooling (Indirect Investment Casting) LOM parts are also ideal for creating two-part negative molds for wax injection. After the LOM mold is sealed with a protective coating, it can be used for up to 100 wax injections without degrading tolerances. If higher production runs are required, the LOM master can be used to create inexpensive, metal-filled epoxy tooling which is capable of accurate production of at least 1,000 parts. Figure 4.3 shows the indirect investment casting tooling. Sand Casting Sand casting is a high volume production technology that requires stable foundry patterns, cores and core boxes. The LOM process is well suited for the creation of the often large, bulky patterns and cores used in sand casting. When up to 100 components are needed, LOM parts can be finished, sealed, painted and used directly to create impressions in the sand. If thousands of components are needed, LOM patterns and cores can be converted to hard plastic patterns via a rubber molding process, or aluminum or steel patterns through the investment casting process. For example, high temperature and high pressure thermoplastic sand injection requires
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that LOM patterns and cores be converted to aluminum or steel patterns which can be achieved through direct investment casting. Figure 4.4 shows a sand casting pattern and part.
Figure 4.4 LOM pattern and resulting sand cast part (Courtesy: Helisys Inc.) Rubber Plaster Casting Rubber plaster casting is used to manufacture highly complex aluminum components that require a better surface finish than sand casting can provide. Rubber Plaster casting is also well suited for making prototype components whose surface finish must closely simulate that of die cast parts. In this process, a rubber sheet is wrapped around a LOM master pattern and packed with plaster to form a mold cavity. After the plaster is dry, the pattern and rubber sheet are removed. Metal is then poured into the plaster cavity, which is later broken away to expose the metal part. 4.2.2.2. Plastic Prototypes Silicone Rubber Molding Silicone rubber molding is a quick and inexpensive process used to create prototype plastic components. Silicone rubber is poured around a LOM master to produce a mold. After the rubber sets around the LOM master, it is cut in half and separated. The LOM master is then removed, and the rubber mold is rejoined except for an opening at one end in which plastic urethane or epoxy is poured or injected. Once hardened, the positive plastic part is removed to reveal the finished product. It is claimed that at least 50 components per mold can be created with this technique.
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Vacuum Molding Vacuum molding is similar to silicone rubber molding, but differs in that the silicone is cast after a vacuum has been drawn in the mold. Vacuum molding results in better surface and feature definition along with denser components. Various materials can be used in this process to cast functional components, including those that simulate hard plastic, flexible rubber, and high temperature plastic. LOM parts work well in this process since they do not react with the silicone and their distortion-free behavior produces highly accurate parts. Sheet Vacuum Forming Sheet vacuum forming is commonly used to create thin plastic packaging as well as other prototypes. In a typical sheet vacuum forming operation, LOM patterns can be used to create prototypes and provide low- to medium-volume production runs for a wide variety of plastic materials, including polycarbonate, ABS, and polypropylene, etc. A vacuum formed piece is created by heating a sheet of plastic and mechanically stretching it over the LOM core by using vacuum suction. The strength of LOM objects makes them ideal for withstanding the high compression forces and temperatures encountered during this process. Spray Metal Tooling Spray metal tooling is used to create tooling for low volume prototype injection molding. The LOM master is mounted to a wood or metal base and parting lines are created. Generally, the model is split into two halves (unless it was originally created in a cope and drag form) by a parting board made of wood or clay. The LOM master is then coated with a thin high temperature barrier such as stove paint and a release agent such as polyvinyl alcohol (PVA). Next, metal spray is applied to one half of the LOM master. Once this is complete, the shell is framed which establishes an outer boundary into which epoxy is poured. Within the frame's boundaries, cooling lines are strategically placed along the shell to ensure that it remains the proper temperature so that it does not crack. The shell is backed with epoxy fill, after which the entire apparatus is turned over and the parting board and LOM master pattern are removed. The process is repeated for the other half of the
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model. Once complete, the two halves create a two-part mold that is capable of producing up to 1,000 injection molded parts using almost any thermoplastic material. 4.2.3 Selective laser sintering (SLS) Based RT Methods
Selective laser sintering (SLS) was developed by the University of Texas at Austin and commercialized by DTM Corporation. Its RapidToolTM process tooling applications include metal molds for injection molding or die casting, cores and molds for sand casting, and pattern masters for investment casting. 4.2.3.1. Injection Molding Or Die Casting The RapidToolTM process takes advantage of the multi-material capability of DTM’s SLS® Selective Laser Sintering technology. In the RapidTool process, the material used is DTM’s RapidSteel powder, which is carbon steel pellets of 55 micron average size, coated with a thermal plastic binder. Using a CAD file as input, DTM’s SLS process fuses, or sinters, the powder to form the mold geometry. Figure 4.5 gives the injection mold produced by the SLS process.
Figure 4.5 A plastic injection mold made using the SLS process (Courtesy: DTM Corp.)
Figure 4.6 Investment pattern and resulting casting (Courtesy: DTM Corp.)
The sintered part consists of metal particles bound by polymer necks. This is called a “green part”. Its strength is approximately 400
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PSI so it can withstand normal handling, including the building and cleaning of walls as thin as 0.040 inch (1.016 mm). A single sintering cycle turns the green part into a fully dense mold. The part is set on an alumina plate within a graphite crucible and surrounded by copper ingots. At 300° C, the plastic polymer burns off while the friction between the metal particles helps the part holds its shape. At 700° C, the iron powder begins to sinter, forming a porous “brown” part. At 1120° C, the copper melts and infiltrates the part through capillary action. The un-infiltrated copper doesn’t adhere to the alumina plate, so the part is easily removed after cooling. For maximum dimensional accuracy, the RapidToolTM process calls for a sintering furnace with accurate temperature and atmosphere control. An appropriate furnace is available through DTM Corporation, although other controlled-atmosphere furnaces with similar specifications can be used. To improve the dimensional accuracy, some refinement methods have been adopted in this tooling process: • •
•
The polymer level in the RapidSteel powder has been carefully formulated so that it is low enough to minimize shrinkage and yet high enough to hold the part together. Before sintering, the green part is soaked in an acqueous acrylic emulsion bath which contains a crosslinker, then dried in an oven at 50° C. The cross linked coating prevents creep distortion after the polymer burns off. The alumina plate, which mates with the flat surface of the mold is machined to a tolerance of ±0.005 inch (±0.127 mm) to eliminate any microscopic sagging during polymer burnoff.
The copper infiltration step is also key. In principle the RapidSteel powder could be fully densified through sintering alone, but at the cost of large shrinkage. Instead, the part is densified through copper infiltration, holding shrinkage to a known factor that is less than
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2.5%. This shrinkage is easily compensated for during the initial build in the Sinterstation system. The accuracy of the initial green part is typically ±0.005 inch (0.13 mm). The tolerance that can be achieved for the finished mold is ±0.01 inch (±0.25mm) for dimensions of less than 4 or 5 inches (101.6 or 127 mm). Final finishing corrects the most critical dimensions, such as parting lines and shutoffs. Typically, 0.02 inch (0.51mm) of additional material is added to these surfaces in the CAD file to allow for finish machining. The initial surface finish is 10 microns Ra, which can be improved to 0.1 to 1 micron through polishing. The resulting molds are 60% iron and 40% copper, and have properties similar to P-20 steel or 7075 aluminum. The molds can be drilled, tapped, welded and plated just like conventional molds. They can be fine-machined and polished with conventional finishing equipment including grinding wheels and discs, files and silicon carbide stones. Fixturing for integrating the cores and cavities with the mold base can either be machined or built into the original .STL file. Depending on the type of plastic and the force and temperature of injection molding, core and cavity sets created through the RapidTool process can produce in excess of 100,000 parts. In die casting applications, tools made with this process can produce several hundred die cast parts. 4.2.3.2 Sand Casting The SLS process is particularly well suited for quickly making sand cores, which can be produced in two days or less. New sand casting materials have been developed for rapid tooling by DTM Corp. These include SandForm Zr material, which is polymer-coated zircon and SandForm Si, which is polymer-coated silica. Both are carefully sifted to provide optimal distribution with an AFS (American Foundry Society) grain fineness number of 101. To make a sand core, the desired shape is input in the form of a CAD file. One or more cores are then created directly in the build chamber of a Sinterstation system. The cores are then removed and cured in a conventional core curing oven. Sand cores produced using the SLS
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process are of equivalent accuracy and have properties that are virtually identical to cores created with conventional methods. 4.2.3.3 Investment Casting TrueForm polymer was developed by DTMTM especially for use in the Sinterstation® System to make patterns for investment casting. It is claimed that the TrueForm polymer has an exceptionally low shrink rate and excellent dimensional accuracy. When used in conjunction with DTM’s patented process, it gives the most accurate first-time builds in the industry. Figure 4.6 shows the investment pattern created by the SLS process and the investment cast part. 4.2.4 Fused Deposition Modeling (FDM) Based RT Methods
FDM models can be used as patterns for investment casting. Stratasys developed the first commercially available FFF system with the ability to generate an investment casting wax master that could be used with standard foundry processes. Stratasys offers a modeling wax with a very low ash content. In addition, Stratasys models can be used as direct output masters for all forms of casting. 4.2.5 Solid Ground Curing (SGC) Based RT Methods
Cubital developed the SoliCast® process to use its solid ground curing (SGC) models to make investment cast metal parts. In this process, the polymer resins that make up the SGC models can be completely consumed leaving a clear cavity for the metal casting. Since the SGC is a highly accurate FFF process, this process is well suited for part geometry with thin wall feature. 4.2.6 Other Tooling Processes
There are many other fast freeform fabrication based rapid tooling processes. Some of these are briefly discussed as follows: 4.2.6.1 ProMETAL Rapid Tooling System ExtrudeHone developed the ProMETAL rapid tooling system (RTS-300) which is an FFF machine capable of creating steel molds and parts up to 10 by 12 inches (254 by 305 mm) in size. The ProMETAL process is based on the three-dimensional printing (3DP) technology development by Professor Emanuel Sachs at
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Massachusetts Institute of Technology. ExtrudeHone took an exclusive license to use 3DP to make metal parts and tooling. ProMETAL uses an electrostatic ink-jet printing head to deposit a liquid binder onto powders, selectively hardening slices of an object layer by layer. Successive layers of powder are spread on top of the growing prototype until the object is complete. The unbound powder supports the object as it is constructed. The result is a green metal part that is then sintered in a furnace and infiltrated with secondary metal. ProMETAL works with a variety of materials including stainless steel and tool steel. Applications include tooling for injection molding, vacuum forming, blow molding, lost-foam patterns, and direct fabrication of powder metal components. Prototype plastic injection molds have shown that ProMETAL tooling is capable of holding fine detail at injection pressures up to 30,000 pounds per square inch (20,700 newtons/cm2), and can survive well over 100,000 shots of glass-filled nylon. It is claimed the part shrinkage is 1.7% during sintering but grows 0.2% during infiltration. Final part accuracy is said to be ± 0.002 inch (0.051 mm). 4.2.6.2 Nickel-Ceramic Composite (NCC) Tooling The nickel-ceramic composite (NCC) tooling process from CEMCOM Corp. is aimed at fabricating “bridge” and short-run production tooling for plastic injection molding and compression molding. This process uses plastic FFF models as patterns for the fabrication of nickel-ceramic composite tooling that can withstand tens of thousands of cycles. The nickel shells are coupled with a standard mold frame using CEMCOM’s patented chemical bonded ceramic (CBC), which does not shrink, is very rigid, and has the same coefficient of thermal expansion as both nickel and steel. The NCC process is particularly suited for large components (i.e., larger than 10 by 10 inches, or 254 by 254 mm). Accuracy is said to be on a par with that of the FFF process used to make the pattern. 4.2.6.3 Polysteel Polysteel, developed by Dynamic Tooling, is a process to produce mold inserts that are 90% steel and 10% epoxy using an FFF model as a pattern. It is claimed that the molds are several times stronger
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than aluminum. The molds work well for prototyping glass-filled nylons, ABS, and wax patterns for investment casting. The process captures the undercuts and can replicate small details. Polysteel has very good thermal conductivity. Injection-molding cycle times are similar to those for a machined steel mold. It is said that accuracy is ± 0.001 to 0.002 inch per inch (1 to 2 mm/m), depending on the pattern and repeatability is ± 0.0005. 4.2.6.4 ExpressTool ExpressTool is working on an RT process called electroforming that facilitates the use of conformal cooling channels, a previously difficult-to-accomplish technique that can help to avoid hot spots in the mold. The process produces 1 to 2-mm-thick layers of nickel on a CNC machined graphite mandrel. The process is capable of making accurate core and cavity mold inserts because there is little or no shrinkage. With conformal cooling lines in place, the nickel shell is backed with aluminum-filled epoxy. The high aluminum fraction helps thermal conductivity. An advantage of this process is its ability to produce large parts. A disadvantage is that deep holes do not electroform well, but machined inserts can be welded to the shell. Mold life has been shown to be in excess of 20, 000 shots with thermoplastics. 4.2.6.5 Cold Gas Dynamic Spray This new spray deposition process works by accelerating metal particles to high speeds with gas pressure and then spraying them on a target surface. The process is being studied by a research consortium that includes the National Center for Manufacturing Sciences in Ann Arbor, Mich., Ford Motor Company, General Motors, Pratt & Whitney, GE Aircraft Engines, TubalCain Co., and Flame Spray Industries. The cold gas dynamic spray process produces a dense, cold worked metal deposit with low porosity, high hardness, and little oxidation. In the process, the energy for softening and deformation comes from the kinetic energy of the particle, which can reach velocities ranging from 300 to 1200 meters per second. Deposition efficiencies of up to 70 to 80 percent have been attained. It is said that the process can develop high residual compressive stress at the substrate surface and that Young’s Modulus can reach 85% of the handbook value.
Chapter 5 Sand Casting Dimensional Control Dimensional accuracy and variability are critical factors in the casting process that must be considered at each stage of the process. Dimensional accuracy is an indication of how close a casting dimension is to design intent (the actual target value). Dimensional accuracy is often referred as a system error. The main causes for poor dimensional accuracy in sand casting are pattern equipment errors, pattern wear, and casting contraction uncertainty. When properly understood and controlled, system error can often be corrected before production runs. Dimensional variability is the variation of individual casting dimensions about their mean. Dimensional variability is often referred to as random error. Many foundry process variables such as placement of cores and sand consistency contribute to dimensional variability. Accuracy requirements, which typically depend on the form, fit, and function of the casting, are established as part of the casting design and are specified as dimensional and form tolerances on the casting drawing (or as part of the engineering release package). To achieve required accuracy, each step of the casting process, including tooling design and fabrication, mold building, pouring and solidification, and post processing of the cast part, must be monitored and the variability controlled. This chapter provides a basic background needed to understand dimensional accuracy and variability issues in sand casting. 5.1 Error Sources
Every stage in the casting development process is a potential error source. Likewise, every step in the casting production process contains numerous potential error sources. Error sources are development and processing activities that impact dimensional accuracy of the casting and/or introduce dimensional variability into the process.
W. Wang et al., Rapid Tooling Guidelines For Sand Casting, Mechanical Engineering Series, DOI 10.1007/978-1-4419-5731-3_5, © Springer Science+Business Media, LLC 2010
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5.1.1 Sand Casting Process
The many individual processing steps needed to produce a sand casting are the most obvious sources of error. These steps include sand preparation, core-making, mold-making, mold and core assembly, and finishing. Each of these steps, as well as other factors such as pattern wear and material shrinkage will contribute to the overall dimensional accuracy and variability of casting features. As shown in Figure 5.1, molding methods, molding equipment, metal conditions, molding materials, and casting geometry are the major sources of error in the sand casting process. These major factors can be further expanded as shown in Figures 5.2 through 5.6 to delineate the detailed variables that influence dimensional accuracy and variability.
Molding Methods
Molding Equipment
Metal Conditions Core boxes
Mold Making Core Making
Patterns
Dimensional Variability Overall casting Geometry
Core Type(s)
Feature Geometry
Molding Sand
Molding Materials
Geometry
Figure 5.1 Fishbone diagrams of error sources associated with the sand casting process (Courtesy: Peters, et al, AFS Transaction v104)
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Patterns
Molding Equipment
Pattern Materials Pattern Set Up
Condition
Dimensional Variability Type of Core Box Condition
Flask Pin Alignment
CoreBox Materials
Core Boxes
Figure 5.2 Error sources associated with the molding equipment (Courtesy: Peters, et al, AFS Transaction v104)
Numbers of Castings Per Mold
Molding Methods
Mold Compaction Method
Flask Type
Height of Cope
Mold Wash Use of Jackets
Core Setting Method
Mold Size
Dimensional Variability Core Wash
Core Box Filling Method
Core Making
Figure 5.3 Error sources associated with the molding methods (Courtesy: Peters, et al, AFS Transaction v104)
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Molding Sand Binder Percentage
Molding Materials
Reclaimed Sand Binder Type Additives
Dimensional Variability
Additives
Binder Level
Core Type(s)
Figure 5.4 Error sources associated with the molding materials (Courtesy: Peters, et al, AFS Transaction v104)
Overall Casting Geometry Projected
Casting Size
Geometry
Area of Casting on Parting Line
Pour Weight
Volume of Internal Coring
Finished Weight
Number of Cores
Relation of Feature to Mold Wall and Cores
Dimensional Variability Feature Length Draft
Feature Geometry
Orientation of Feature Relative to Parting Line
Figure 5.5 Error sources associated with the casting geometry (Courtesy: Peters, et al, AFS Transaction v104)
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Metal Conditions
Alloy Type
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Dimensional Variability
Pouring Temperature
Figure 5.6 Error sources associated with the metal conditions (Courtesy: Peters, et al, AFS Transaction v104) 5.1.2 Rapid Tooling Process
When rapid tooling (RT) processes are used, they often introduce additional sources of error in the process. To illustrate error sources that are present in rapid tooling processes, consider the LOM (layered object manufacturing) process. This process begins with a 3D solid model of the casting. This model is then tessellated into triangular facets and sliced into layers each having the thickness of a sheet of paper before being used to drive the LOM fabrication process. The LOM model is constructed by laser cutting and gluing sheets of paper together. Surrounding material and material in regions of the part that are hollow are removed in a “decubing” process. The surface of the resulting LOM model must then be sanded to smooth the “stair step” texture created by the layers of paper. Finally, the surface must be sealed with lacquer or an equivalent sealant to keep moisture from being absorbed into the model. Each step in this process introduces error as shown in Figure 5.7. A detailed discussion of each error source is given below.
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Building Process Orientation Parameters
Distortion in Decubing Pattern Production
Model Tessellation Sanding and Spray
Shrinkage and Expansion
Figure 5.7 Fishbone diagrams of error sources associated with the LOM process 2D to 3D Conversion Error In many practical situations, it may be necessary to create a 3D solid model by interpreting an existing 2D drawing of the part. This can be a major source of error because of the assumptions or interpretation decisions that may be necessary. The error introduced in this process is very dependent on the particular geometry involved and the detail and accuracy of the part print. Tessellation Error Solid models of the tooling or part to be created by the LOM process must be rendered through tessellation. The tessellation process results in an .STL file, which is a de-facto standard in the rapid prototyping area. In the .STL format, triangular facets are stored as a set of X, Y, and Z coordinates for each of the three vertices. A unit normal vector with X, Y and Z is also stored to indicate the positive direction of the triangular facet (outside surface of the part). Most 3D CAD/CAM software packages generate the .STL file on demand from the solid model of the part. Figure 5.8 gives a STL representation of a 3D solid CAD model.
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Figure 5.8 .STL representation of a 3D solid CAD model Approximating a curved surface by triangular polygons introduces dimensional error. In general, the primary parameter that controls tessellation accuracy is chord height. Chord height specifies the maximum distance between a chord and a surface (see Figure 5.9). To minimize tessellation error, the chord height should be made as small a possible. The penalty for a small chord height is a larger .STL file (i.e., more triangular facets and correspondent X, Y, and Z coordinates) and a longer build time since more steps are required during the laser cutting process. Experiments have shown, however, that the difference in build time between large and small tessellation error is relatively minor. Therefore, the greatly improved accuracy afforded by small chord heights is well worth the slightly increased build time.
Chord height
Part surface
Tessellated surface
Figure 5.9 Chord height for surface tessellation.
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Orientation Error Orientation of the part relative to the build direction can have a significant effect on dimensional accuracy, especially for an inclined plane, cylindrical surface, or free form surface. Due to the layered nature of the LOM process, staircase surface features (because the paper thickness is constant) are unavoidable. Also, thin wall sections may sag due to the effect of gravity. Generally, the part orientation must be considered from four aspects: accuracy, building time, surface finish and strength (maximum lamination area). There is an existing body of research devoted to identifying the optimal orientation of the part relative to the build direction. It is very difficult to generalize these results because of the geometry and feature specific nature of the problem. Each part has a preferred orientation that results in the best combination of good accuracy, short building time and sufficient strength. In most practical situations, decisions regarding build orientation are based on customer requirements and the operator’s experience and judgment. Process Parameter Error There are several process parameters that effect dimensional accuracy. These include laser power, cutting speed, laser beam diameter, paper thickness, and offset distances at object boundaries. Different combinations of laser power and cutting speed will effect cutting width and thickness. Also, laser power must be adjusted to avoid scorching the cut edge of the paper. The choice of paper thickness may affect the building time, cost, and resolution. Thick paper requires less build time since a fewer number of layers is required to achieve a given Z height. However, for surfaces which are neither parallel or perpendicular to the build direction, thick paper increases stair step effects and dimensional error. Decubing Error Residual stress can develop in the LOM build as the result of temperature change. During the building process, the temperature of the workpiece will rise because of heat used to improve the adhesive bonding between paper layers. After the building process is finished, residual stress due to volume shrinkage develops within the model as it cools to room temperature. When the model is “decubed”,
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constraints are released and the residual stresses are redistributed. This can result in shape distortion of the model and in some cases, delamination of layers. As with other sources of error in the LOM process, the dimensional error that may occur depends on the detailed geometry of the part, the part orientation with respect to build direction, and other process factors. In general, distortion is greatest in thin wall parts. Post-Finishing Error After decubing, the model is usually sanded and sealed. Sanding is necessary to remove “stair step” irregularities and to smooth part features. The sprayed lacquer coating seals the surface and strengthens the model. The lacquer coating is also sanded to smooth the surface finish of the model. These operations are usually performed manually by experienced operators. Error introduced during these manual procedures depends on operator skill and specific part geometry. Because of the number of hard to control factors involved, post finishing error is likely vary significantly from build to build. Time Dependent Error LOM patterns may shrink or expand depending on environmental conditions. As discussed previously, shrinkage may occur due to the thermal effects. Likewise, expansion may occur if the LOM model absorbs water from the environment. For repeated use of a LOM pattern, wear and delamination are also factors to be considered. 5.1.3 Shrinkage
Shrinkage is a leading error source in sand casting. Different materials may have different contraction behaviors and thus need different shrinkage compensation factors. Most foundries use an average shrinkage compensation calculation that is based on experimental data and industry practice. To improve the accuracy of shrinkage compensation, it is necessary to comprehend all of the sources of contraction and theoretically understand the shrinkage behavior of each. Sand casting starts with a liquid metal heated well above its solidification temperature to render it fluid and castable, and ends
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with a solid. In this process, the metal experiences phase conversions (from liquid to solid) and volume changes. Major sources of shrinkage or contraction include the following: • Contraction in volume of the liquid metal as it cools to solidification temperature. • Shrinkage or decrease in volume as the liquid metal changes state and becomes a solid. • Contraction of the solid metal as it cools further to room temperature. • Metallurgical changes in the solid state that may be accomplished by volume changes, e.g., austenite to pearlite transformation in ferrous castings. • Volume and dimensional changes that may occur if the casting is heat-treated. • Auxiliary changes. Examples include: − Changes in volume of the mold material as it becomes heated by the molten metal. − Volume changes resulting from the pressure of molten metal on the mold cavity, coupled with a decrease in strength of the mold due to heat. This strength decrease causes what is popularly known as "mold wall movement". Each of these sources of shrinkage are briefly discussed as follows. Volume contraction of liquid metal This is of no practical significance because castings are made with risers and gating systems which contain a reservoir of molten metal that enters the mold cavity to compensate for liquid contraction. Solidification Shrinkage The change (decrease) in volume that occurs when passing from the liquid phase to the solid phase typically can not be avoided for most casting materials. The amount of shrinkage that occurs varies according to the type of metal. For example, solidification shrinkage of some types of gray cast iron is often negligible because it is
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almost completely offset by an accompanying expansion that results from graphite being percipitated from solution. In the case of metals like steel, white iron, and aluminum, however, solidification shrinkage may be as high as 6% by volume. Shrinkage cavities and/or porous areas will form due to solidification shrinkage if an adequate supply of liquid feed metal is not available. Ensuring an adequate supply of feed metal is the job of both the casting designer and the foundryman. The casting designer can help by providing geometrical features that help control the solidification rate and direct the solidification front toward the supply of liquid metal. The foundryman uses a combination of proper gating, properly located and sized risers, and properly located and sized chills and other means to control and direct solidification so that the last area to solidify is located in the riser or feeder head. Ultimately, for complex casting geometry, achieving a sound, accurate, high integrity casting requires close cooperation and coordination between the design engineer, the purchasing agent, and the foundry. Solid Metal Contraction due to Cooling This contraction is what the foundryman allows for when he uses a shrinkage rule for sizing the pattern. Solid shrinkage of different metals varies and may be as high as ¼ inch per foot (20.83 mm/m) to as low as 1/10 inch per foot (8.33 mm/m). Typically, the same shrinkage rule will be applied to all of the pattern dimensions. For example, if a 12" (305 mm) long casting is to be cast in a metal with a 1/8 inch per foot (10.417 mm/m) shrinkage factor, the pattern will actually measure 12 1/8" (307.975 mm). Unfortunately, solid metal contraction is seldom completely uniform in all directions. Consequently, a long thin casting may tend to shrink more in terms of length than it will in terms of width or thickness. The most practical way to deal with this problem is to provide generous machining allowances and/or to relax tolerances where possible. When it is absolutely imperative that high accuracy be achieved in the final casting, prototypes must be made and the pattern modified until a uniform shrinkage is achieved in all directions.
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Another problem caused by solid contraction is residual stress that results because it is often impossible for all areas of the casting to cool and therefore shrink uniformly at the same time. Residual stress can cause the casting to distort, especially when machined, and to possibly crack or break, either during cooling or during use. To avoid these problems and to ensure acceptable accuracy, it is often necessary to stress relieve the casting by heating it and holding it at a specified temperature for a specified period of time. Also, abrupt section changes should be avoided when possible. Solid Metal Contraction due to Metallurgical Change Many metals, particularly the ferrous alloys, undergo phase changes in the solid condition which involve dimensional changes. For example, cast iron and steel will transform from austenite, which exists at higher temperatures, to pearlite or ferrite at room temperature. This type of metallurgical change also typically involves a volume change which inevitably results in additional residual stress. The best way to avoid severe or aggravated residual stress due to this type of solid contraction is to use a uniform wall through out the casting when possible and/or to have gradual changes from thin to thick sections. Abrupt section changes should be avoided whenever possible. In many cases, solid contraction due to metallurgical change will produce predictable warping and distortion of the casting that can be compensated for by making slight dimensional changes to the pattern. Stress relieving is also an option if necessary. Volume and Dimensional Changes due to Heat Treatment Heat treatment will also cause expansion and contraction that must be allowed for. Consequently, uniform sections and gradual section changes are preferred to avoid residual stress and warping. Also, when possible, it is important to avoid abrupt temperature changes by heating and cooling slowly. Changes in Mold Dimensions When a mold cavity is heated by molten metal, it stands to reason that some dimensional changes may occur. Fortunately, these are quite small and can usually be ignored. Most sand molds are made from silica, however, which undergoes a phase change at around
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1000oF that involves a slight contraction. It is possible, therefore, for certain areas of the mold to be expanded while other areas are contracted. This can set up severe strains in the mold which must be allowed for by the foundry. Ways for doing this include adjusting the degree of ramming used, varying the grain size of silica sand used, and/or by judicious use of binders that may contract and offset the silica expansion. This is a problem that seldom comes to the attention of the casting user because, if a mold is not properly balanced from the standpoint of heat stresses, it may rupture or crack. This, in turn, will lead to a casting defect that may cause the casting to be scrapped before it is sent to the customer. The phenomena of mold wall movement is of far greater consequence to the foundryman. At the temperatures associated with the casting of metal, the binders, such as clay and various carbonaceous materials, may actually become plastic, to the point where ferrostatic pressure from the liquid metal can force a change in the dimension of the mold cavity. This very often occurs at or near the solidification point and a mold which exhibits mold wall movement at this critical stage solidification will invariably lead to a casting that contains internal shrinkage cavities. The foundryman relies on judicious selection of sand binders and on rigid, firm, hardrammed molds to offset these conditions. On larger castings, the foundry will dry or bake the molds to provide added rigidity. This is why castings made in dry sand molds may often appear to be more expensive than similar castings made in green sand molds. Fortunately, modern production machines used in making green sand molds are capable of ramming molds to a high hardness and rigidity which considerably decreases the' problem of mold wall movement in smaller castings normally made in green sand. 5.2 Geometric Dimensions and Tolerances
The ANSI Y14.5M-1982 geometric dimensioning and tolerancing (GD&T) system is widely used in the casting industry. GD&T enhances the technical description of physical objects, primarily on blueprints and is an international standard for making and interpreting engineering drawings. GD&T is used to dimension and tolerance castings according to their specific geometry, their
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functional relationship to other castings, and to the function that each casting performs in the completed assembly or product. 5.2.1 Dimensions
A dimension is a numerical value that defines the size (a size dimension), the shape (a geometric characteristics), or the location (a location dimension) of a feature using an appropriate unit of measure. Dimensions may be given in inches (in.) or millimeters (mm). In both systems, a fraction of a unit is expressed as a decimal. If drawings are dimensioned using the metric system of units, a metric notation will appear in a conspicuous location on the drawing. Otherwise, the units used on the drawing are assumed to be inches. There are several types of dimensions used in engineering drawings. A basic dimension is the theoretical or nominal size, which, for practical reasons, is only approximated; or, it is the dimension that would be obtained if perfection were possible and did not result in increased manufacturing costs. A basic dimension describes exact size, shape or location, and is denoted by placing the dimension in a rectangular block. No tolerances are assigned to basic dimensions. A reference dimension is an informational dimension. Reference dimensions are used to specify length overall, operating clearance, and similar items of general information. Like the basic dimension, a reference dimension is given without tolerances. A reference dimension is denoted by placing the dimension in parentheses. A specified dimension is a “working” dimension. Tolerances are applied to specified dimensions to derive the limits of size of a feature. Limits of size are the maximum and minimum (or limit) dimensions that a feature of size may assume. For example, a height dimension is denoted as 4.15±0.02. A nominal dimension is a “naming” or “callout” dimension. These dimensions are used to refer to the specified dimensions of feature of size in a general way. It is usually used to specify the general size of
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commercial stock and threaded fasteners. This include such readymade forms as round bars, hex bars, flat plate, and the like. A nominal dimension is also called a “stock size” in some instances. Actual size indicates the produced or manufactured size of a feature or casting. Actual size is determined by physical measurement of the feature or casting. 5.2.2 Tolerances
Dimensional Tolerance Tolerances for dimensions of as-cast features are a matter for agreement between the producer and the customer of the casting. It is impossible to produce a feature of a casting that is exact or perfect. Moreover, it is very expensive to produce feature of a casting with high precision. In practice, the specified dimensions of a feature of size are permitted to vary to some degree, the amount depending upon functional requirements. This permissible variation in size is the tolerance that is placed on the specified dimension. A tolerance indicates how much a casting may deviate from its ideal or basic dimension and still function properly. It is a compromise between the part quality and machining cost. Generally, strict tolerances, specifications, and requirements are more expensive and more difficult to obtain and hold in manufacturing. The casting design engineer must consider the tradeoff of dimensional accuracy, time and cost efficiency. Tolerances on the dimensions of a feature can be expressed in three ways: either as bilateral tolerances, unilateral tolerances, or as limit dimensions. Geometric Tolerance Geometric tolerances are tolerances that involve the shape features of the part. It is a general term applied to a category of tolerances used to specifically control form, profile orientation, and location. To completely describe the shape of a component and to set tolerances on all aspects of its shape, tolerances are needed for such features as parallelism, concentricity, angles, etc.
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Weight Tolerance When weight tolerances are necessary, i.e., weight considerations are important to the customer, an allowance must be made for variations in weight from casting to casting within a lot. 5.2.3 Allowances
Allowance is the intentional difference in maximum size between mating parts, such as a hole and a shaft. An allowance can be either positive or negative. A positive allowance means there is always a space between the mating parts. It is also called clearance fit. Conversely, a negative allowance means that there is no space at all between the mating parts. That is, the shaft is larger than the hole under all conditions. This type of fit is also termed as interference fit. Allowance can be thought of as the minimum clearance or maximum interference (that is, the tightest possible fit) between two mating parts. 5.2.4 Feature Control
GD&T employs a feature control system that associates a geometric characteristic or feature with a tolerance and datum (see Figure 5.10).
Geometric characteristic symbol
Tolerance value
Datum reference
Figure 5.10 Feature control symbol (Courtesy: Kennedy et al, 1987) The datum reference indicates a reference point or plane. Datums are denoted by letters that are associated with points, lines (axis), and planes. The tolerance value expresses the amount that a dimension may vary from the intended value. Datums and tolerance values are linked to a geometric characteristic symbol in order to specify a desired relationship. The characteristic symbols include
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various descriptions and relationships relating to form, profile, orientation, location, and runout (see Figure 5.11). These geometric features are grouped into categories described as individual or related features. Individual features pertain to a feature itself. Related features involve relationships between features.
Figure 5.11 Geometric characteristic symbols (Courtesy: Kennedy et al, 1987) Geometric dimensioning and its related tolerancing abilities, like the real physical world, can be simple or complex depending on the nature of the casting part. Datum references can be single or multiple, and are arranged in order of importance if there is more than one datum. Additionally, supplementary geometric symbols add more detail to the basic list of five characteristic symbols (Figure 5.12). M
MMC
L
LMC
S
RFS Regardless of Feature Size
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P
PTS Projected Tolerance Zone Diameter
S
Spherical Diameter
R
Radius
S
Spherical Radius
()
Reference Dimension Basic Dimension
Figure 5.12 Supplementary symbols (Courtesy: Kennedy et al, 1987)
MMC Size
LMC Size
Figure 5.13 MMC and LMC size of a part (Courtesy: Kennedy et al, 1987) Two important supplementary symbols used in this system are the “MMC” and the “LMC” symbols. These stand for “Maximum Material Condition” and “Least Material Condition”, respectively.
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MMC specifies a condition in which the component feature has the maximum or most allowable amount of material (according to the size limits specified). The LMC stipulates the least material within the specified limits of size. Figure 5.13 shows an example for MMC and LMC cases. When MMC is indicated, the specified tolerance only applies at the maximum material condition and that the tolerance can be relaxed as the actual dimension moves away from the maximum material condition. A circled S, which denotes “Regardless of Feature Size,” means that the indicated tolerance value applies, no matter what the actual size of the feature is. The reader is referred to Kennedy et al, 1987 and other readily available references on GD&T for more information. 5.2.5 Standards on Casting Dimensional Tolerances
The International Standard Organization (ISO) has issued ISO 8062, which is a system of geometrical dimensional tolerance grades for castings made in all metals and their alloys. When first issued in 1989, the standard included 16 different dimensional tolerance grades for specific alloys and mold types. Revisions made in 1994 and 1995 resulted in the current version which is denoted as ISO/CD 8062-2. This version deals with the following geometrical tolerances: • • • • • • •
Straightness Circularity Flatness Parallelism Perpendicularity Coaxiality Symmetry
There are 7 casting geometrical tolerance grades (CTG) which are designated CTG 2 to 8. CTG1 is reserved for finer values that may be required in the future. The European Committee for Standardization issued “Pattern, pattern equipment and coreboxes for the production of sand moulds and cores” standard Draft prEN 190/131. This draft includes
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terminology definitions, order information and delivery conditions, classification, drawing, design and specification, manufacturing requirements, quality control, usage, and other references materials. The Steel Founders’ Society of America (SFSA) has also developed a tolerancing system for steel castings. This system is based on an analysis of actual measurements taken in foundries. The SFSA system takes into account the weight of the casting being produced. The tolerance ranges extend from T3 to T7, with T3 being the tightest. For details of this system, the reader is referred to the Steel Casting Handbook. 5.3 Dimensional Metrology
In order to compare a particular casting produced by a foundry with appropriate casting standards and specifications, the produced casting must be measured. To be sure that the measurements are correct, most quality systems require the supplier to determine the accuracy and precision of the measurement system used. For example, ANSI Q92 (ISO 9002), 4.10(d) states “(the supplier shall) ensure that the inspection, measuring and test equipment is capable of the accuracy and precision necessary.” Over and above the requirements of such standards, it is just good practice for a supplier to know his measurement capability in order to be sure the customer’s requirement can be met and that the measurement system is not obscuring this determination. 5.3.1 Basic Definitions
Accuracy is defined as the closeness of agreement between a test result and an accepted reference value. Precision is defined as the closeness of agreement between independent test results under prescribed conditions. Repeatability is the precision of independent test results that is obtained with the same method on identical test material in the same laboratory by the same operator using the same equipment within a short interval of time. Reproducibility is the precision of independent test results that is obtained with the same methods on identical test material in different laboratories with different operators using different equipment.
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5.3.2 Types of Dimensional Error
It is well known that the resultant size of each casting feature will not be the same for each casting produced. Rather, the casting feature size will be distributed over a range of values. This variation of feature dimensions from casting to castings is generally referred to as random error and is commonly expressed in terms of standard deviations about the mean. A total of six standard deviations (6σ) centered on the mean accounts for 99.7% of all observations made for normally distributed data. This means that, for any particular feature, the feature dimension will be expected to be within the 6σ range for 997 out of 1000 castings. As the process variability decreases, the probability of a random dimensional casting error also decreases. An example of dimensional variability and its relationship to the customer-specified feature (i.e., nominal dimension and tolerance limits) is shown in Figure 5.14. Normal Dimension
Random Error
Lower
Upper
Tolerance Limit
Tolerance Limit
Random Error
Figure 5.14 Distribution of feature dimensions illustrating both narrow and wide process variability with respect to the casting customer’s tolerance limits. (Courtesy: Peters, et al, AFS Transaction v104) In addition to the casting dimensional errors caused by process variability, there are also systematic errors associated with the
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design and construction of the pattern. Metal castings contract as they cool from solidification temperature to room temperature. This contraction can be predicted and is compensated for by applying a shrinkage allowance to the pattern. If the actual shrinkage of a casting feature is different from the shrinkage allowance used in the pattern design, or if the pattern feature was made incorrectly, then the resultant casting feature will not be the proper size. Either error in the pattern will cause the average value of the casting feature to be shifted from the specified nominal dimension as shown in Figure 5.15. Nominal Dimension Pattern Error Lower Tolerance Limit
Upper Tolerance Limit
Figure 5.15 Shift in mean casting size due to pattern errors. (Courtesy: Peters, et al, AFS Transaction v104) The total dimensional error seen by the casting customer is a combination of the spread of dimensions (random error) and the offset of the mean from the nominal dimension (systematic error). Figure 5.16 schematically illustrates the dimensional error that results from a combination of both error types. In Figure 5.16, the area under that part of the distribution curve that is to the left of the lower tolerance limit represents the percentage of features that are out of tolerance for this example. For a casting production run to be within dimensional tolerance, three standard deviations of random
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error caused by the foundry process variability plus the systematic error caused by shrinkage allowance and pattern inaccuracies must be less than half of the specified tolerance. If the random dimensional error due to foundry processing variables is equal to the specified tolerance, then the shrinkage allowance used and pattern construction and management must be perfect to produce intolerance parts. Clearly, the foundry and the pattern shop must share the total tolerance specified by the customer for the casting features in question. As the total tolerance specified by the customer decrease, it becomes important to control all sources of dimensional variability, both in the foundry and in the pattern shop. A third important type of error that should be regularly monitored and controlled by the pattern shop and the foundry is measurement error. Measurement error is the error that is inherent in the measurement system. Controlling measurement error is not simply a matter of insuring that measurement instruments are correctly calibrated. Measurement error also incorporates the accuracy limitations of the measurement instruments themselves, and their usage. In many cases, a feature is measured only once and the measurement is assumed to be perfect. However, a range of dimensional values is likely if the same measurement is repeated by a single inspector, or by different inspectors. Variability in these measurements can significantly add to the dimensional variability during patternmaking. These typically small errors may not be significant for castings produced to wide customer dimensional tolerances, but may be unacceptable when close tolerances are required.
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Nominal Dimension Lower Tolerance Limit
Upper Tolerance Limit
Pattern Error
Random Error
Random Error
Figure 5.16 Casting feature made with a process with relatively wide dimensional variability and an incorrect pattern results in many out-of-tolerance castings. (Courtesy: Peters, et al, AFS Transaction v104) 5.3.3 Process Capability
Process capability relates the actual behavior of a process characteristic to the tolerance requirement specified for that characteristic by the customer and is measured using three different capability indices identified as Cp, Cpk, and Cpm, respectively. Each capability index has a different meaning and a different utility. The capability index, Cp, is simply the ratio of the tolerance range of the specification to six times the standard deviation of the process characteristics, Cp =
USL − LSL 6σ
In this equation, USL and LSL are the upper and lower specification limits, respectively, and σ is the standard deviation for the process. Cp measures the ability of the process variation to fit within the tolerance range of the specification. Six times the standard deviation is the common measure used because, if the characteristic is distributed normally, 99.73% of all the measured values will lie within ±3 standard deviations, or 6 standard
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deviations of the process average. This means that if the process average is centered within the tolerance range of the specification, one would expect to have no more than 0.27% of parts out of specification; i.e., 2700 parts per million. The relationship between Cp and parts-per-million defects is shown in Figure 5.17. Cp does not consider the position of the process distribution with respect to the tolerance limits, that is, it does not take into account how centered the process is between the upper and lower bounds. Since location of the distribution is an important consideration in many cases, the capability index number, Cpk, has been developed to take location into account.
Figure 5.17 Expected nonconformities per million for various values of Cp or Cpm (Courtesy: Blair, Steves, 1995) Cpk is defined as the smallest value of the ratio of the distance of the process average ( X ) from the upper and lower specification limits to three standard deviations, i.e., ⎤ ⎡ USL − X X − LSL ⎥ ⎢ Cpk = MIN , ⎢ 3σ 3σ ⎥ ⎦ ⎣
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If the quality characteristic is normally distributed, and the average is at least 3 standard deviations away from the specification limit, Cpk will be greater than 1.0 and the tail area outside the specification limit will contain no more than 0.135% nonconforming parts. It should be noted that Cpk alone is still an inadequate measure of process centering and, for cases where the process average is not centered in the tolerance range, most practitioners require both Cp and Cpk to be used. To avoid the need to use two different indexes this difficulty and To provide a capability index that is a better indicator of centering, the index Cpm has been proposed. This index is calculated using the following formula: Cpm =
USL − LSL ⎛T − X ⎞ 6 σ +⎜ ⎟ ⎝ σ ⎠ 2
2
=
Cp ⎛T − X ⎞ 1+ ⎜ ⎟ ⎝ σ ⎠
2
where T is the target or desired value of the process (if T is taken to be the midpoint of the specification; then T = (USL - LSL)/2). The capability index Cpm has more satisfactory statistical properties than Cpk and is more easily understood in terms of its relationship to Cp. Also, when the process is centered, Cpm = Cp. As the process average departs from the target, Cpm becomes smaller and smaller, approaching zero as a limit, Cpk, on the other hand, becomes negative when the process average is outside the tolerance range of the specification, which makes it more difficult to interpret. The relationship between the three capability indices is further described in Steel Casting Handbook (Blair, Steves, 1995). Capability indices are statistical estimates obtained from sample data. Consequently, they are not exact values and have confidence intervals associated with them. Demonstrating achievement of a particular capability is more than a simple calculation because it involves statistical hypothesis testing. Also, an important assumption underlying process capability indices is that their interpretation is based on a normal distribution of the process characteristic of
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interest. If the distribution is not normal, then any conclusions concerning the percent of nonconforming parts for a particular capability index may be in serious error. 5.4 Measurement System Analysis
The methods and procedures for measurement system analysis, which are widely used in automotive and other industries, were developed by the Measurement Systems Analysis (MSA) Work Group. This effort was sanctioned by the Chrysler/Ford/General Motors Supplier Quality Requirements Task Force and developed under the auspices of the Automotive Division of the American Society for Quality Control (ASQC) and the Automotive Industry Action Group (AIAG). Measurement system analysis is focused on gage repeatability and reproducibility, which is commonly referred to as “Gage R&R”. In Gage R&R, repeatability is the variation in measurement values obtained when one appraiser repeats the same measurement several times. Reproducibility is the variation between measurement values obtained by different appraisers performing the same measurement on the same part using the same measurement apparatus. Excessive measurement error can confound measurement results. If a large percentage of the part variation is due to measurement error, the foundry is likely to reject acceptable castings or to accept unacceptable castings because it does not know what its dimensional capability is. To avoid this undesirable situation, is necessary to set limits on the percent measurement error that is allowed. The MSA manual (AIAG , 1995) recommends the following: Gage R & R 0 - 10% 10-20% 20 - 30% >30%
Recommendation preferred usable marginal unacceptable
5.4.1 Procedures for Gage R & R Test
AIAG has defined a standard procedure of Gage R&R test. The procedures are described on pages 44 through 69 of the Automotive
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Industry Action Group's (AIAG) Measurement Systems Analysis Manual (AIAG, 1995). A description of the analysis method, which is based on the Average and Range Method, is also given in the manual beginning on page 55. To be successful, it is recommended that the foundry have all inspectors that will be collecting data to participate in the gage R & R study. As a minimum, it is essential that at least two inspectors participate in the gage R&R study to ensure that both the repeatability and reproducibility components of measurement error are captured. In order to obtain useful and reliable study results, the following guidelines are recommended: Ideal Study: 3 operators, 10 castings, 3 trials This study is the most time consuming, but is usually well worth the effort. This amount of detail is not necessary, however, unless three or more inspectors regularly make these measurements. Common Study: 2 operators, 10 castings, 2 trials This study is performed quickly and is a good test for many foundry measurements. It is relatively quick and will ensure that different foundry personnel are measuring castings correctly by having two sets of results for comparison. Repeatability Studies This type of study is acceptable when it is known that only one inspector ever uses the measurement equipment being tested. Because these tests only give repeatability results, they should not be used if reproducibility is a concern. 5.4.2 Performing Gage R & R on Different Pieces of Equipment
Bob Voight (1996) offers the following recommendations and comments regarding gage R&R studies performed on common foundry measurement equipment. Caliper (digital and vernier) and Micrometer (digital and manual) Gages These are often the most common pieces of measuring equipment used in a foundry. Gage R & R studies are quickly performed and
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generally yield good results. A minimum of ten castings should be measured with a minimum of two trials apiece. Portage Machine (manual and probe-equipped) Gage R & R studies on Portage machines are generally time consuming and it is common to only perform a repeatability test (one inspector). For gage R & R on a Portage machine, it is important to follow a standard measurement procedure. When obtaining repeated measurements on the same casting, the jacks should be loosened and the casting rotated so that the casting is no longer level or square. Now use the same points and lines that scribed for the first measurement and follow the same procedure as used in the previously. Follow this same procedure for each repeated measurement. If additional appraisers perform the test, each should follow exactly the same procedure. As a final note, be aware of the type of measuring probes that are used for manual Portage machines. If a long probe is used, deflection could occur which will increase measurement error. Try a shorter probe, if necessary. Coordinate Measuring Machine (CMM) CMMs are often used to measure smaller castings. These machines are usually more automated than typical measuring devices and generally, only a repeatability test needs to be performed. However, for manual CMM’s, it is important to determine the reproducibility component since considerable operator skills can be involved. If reproducibility error is found to be a problem, then a formal training program is strongly recommended to ensure that all appraisers are using the same techniques. Figure 5.18 shows a manual CMM.
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Figure 5.18 A Manual CMM Ultrasonic Machine (UT Meter) UT meters are often used to measure wall thickness. The best strategy for performing gage R & R study is to mark a point at the same location on all of the castings to be measured. UT meters require the use of contact gel. Be sure to apply the contact gel consistently for all measurements. Also, the UT meter may need to be calibrated before measuring. For foundry program tests, the first inspector should calibrate the UT meter and randomly perform at least two trials of measurements on all ten casings. The second inspector should then recalibrate the instrument and repeat the measurements.
Chapter 6 Evaluating Tooling Alternatives Evaluation of tooling alternatives is the comprehensive mental exercise involved in selecting the most efficient way to produce a sand casting pattern. Selection of the proper tooling process can significantly effect the time, cost and accuracy of castings involved. Selection of an appropriate tooling alternative is typically performed by the tool builder. The process requires a sound understanding of the interactions between casting geometry and pattern design, required tooling properties, production volume, available shop capacity, available processes and process capability, and technical capabilities of the foundry involved. 6.1 Decision Variables
A sand casting is produced by pouring molten metal in a mold cavity. The mold cavity is created by withdrawing a pattern from sand that has been packed around it. Since the pattern imprint forms the cavity, the pattern creates the external shape of the cast part. If the part has hollow regions, these are formed by cores that are placed in the mold cavity. The cores are supported by core prints, and also chaplets in some cases, that allow the molten metal to flow between the core and the mold wall. In addition, cores may be necessary to produce a desired “zero” draft external surface, depending on the parting line selected. The tool alternative selected for making the pattern and cores is determined by a series of decisions regarding the fabrication method, material, and tooling approach to be used. 6.1.1 Fabrication Method
In traditional fabrication, patterns and core boxes are manually fabricated from handcrafted prototypes of clay, wood, plastic or other materials by skilled workers. This time consuming process can now be performed using CNC machining processes to generate the tooling by removing material from a starting workpiece (subtractive
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process) or by using one or more FFF processes to generate the tooling by building it up one layer at a time (additive process). A flow chart for creating sand casting tooling using the LOM (layered object manufacturing) process is shown in Figure 6.1. LOM is typical of the several FFF processes that are commonly used for rapid tooling. In this process, the part or tool is built up by laser cutting sheets of paper in the shape of the part cross-section and gluing the sheets together to form the three-dimensional part. For the shape of each slice to be known, FFF processes must be computer controlled using a CAD solid model representation of the part geometry.
2D Drawing & Manufacturing Requirements
3D CAD Solid Model (Add Draft & Shrinkage if Needed
Tessellation
LOM
(.STL format)
Production
Conventional Sand Casting Process
Tool Design (Pattern Design & Core Box Design)
Post-Processing (Decubing, Smooth and Seal)
First Article Parts
Figure 6.1 Rapid tooling for sand casting using the LOM process.
With the addition of rapid tooling as a pattern fabrication method, there are three basic categories of fabrication methods to consider: manual, FFF, and CNC machining. Each of these categories has a range of choices. For example, there are several FFF processes that might be used depending on the capability of the particular pattern shop involved.
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In addition to patterns, core fabrication must also be considered. Cores can be fabricated directly or molded using a core box. Typically, a core box would be used when higher production quantities are required. Like patterns, either the core itself or the core box can be fabricated using manual, CNC machining, or FFF methods. 6.1.2 Tool Material
Casting tools can generally be fabricated from a large variety of different materials. Frequently used materials include mahogany or other wood, urethane plastics, synthetic materials, metals and FFF materials. Mahogany and other woods are particularly well suited for manual fabrication because they are easy to work with. Urethane plastic offers high quality and strength. Synthetic materials such as renboard are popular because of their durability and machineability. Metals (mainly aluminum and steel) are used when long tool life is a concern. FFF materials range from polymer, paper, nylon, ABS plastic, wax, etc. depending on the particular FFF processes used. 6.1.3 Tooling Approach
The tooling approach relates to the way the mold cavity is created. Commonly used tooling approaches include single or loose patterns, gated patterns, match-plate (mounted) patterns, and cope and drag patterns. In practice, each tool build approach has its own set of advantages and disadvantages. Generally, the dimensional accuracy, reliability (i.e. tool life) and production rate increases as the selected tooling approach moves from loose patterns to cope and drag plates. But time and cost also increase. Hence, the tooling approach selected will generally depend on the production volume required in addition to dimensional accuracy and delivery. 6.2 Decision Factors
There are a variety of project requirements that influence the tool alternative selection process. These can be divided into independent and dependent factors. Independent factors are decision factors that are determined independently of the tool build alternative that is selected. The major independent factors include data status,
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production volume, type of casting (i.e., prototype or production) required, foundry that is to produce the part, part geometry, important geometrical features and pattern shop capability. Dependent factors, on the other hand, are outcomes that result based on the particular tool building alternative that is selected. These include tooling cost, lead-time, dimensional accuracy, and tool durability. Each factor is briefly discussed as follows. 6.2.1 Data Status
Data status refers to the way in which the part geometry is represented. Typically, the part geometry is represented in one of four different ways: physical part, 2D drawing or CAD file, intermediate representation, or 3D CAD solid model. Physical Part The data status may be in the form of an existing physical part. When this is the case, the part features and dimensions must be measured and then converted into a form that can be used to generate the tooling. If the traditional tool making process is to be used, then the part features and dimensions can be measured manually. If, on the other hand, a CNC machining process or FFF process is to be used, then the geometry must be digitized and formatted as an appropriate 3D CAD solid model. One way to do this is to use a coordinate measuring machine (CMM) or laser digitizing machine to generate a “point cloud” which is then processed using appropriate computer software. 2D Drawing or 2D CAD File Representation of part geometry as a 2D drawing is probably the most widespread data status. In recent years, manual creation of the 2D drawing has been largely replaced by the use of 2D CAD software that generates an electronic 2D CAD file. Such a file can be printed on paper and can also be transmitted electronically as a DFX (Drawing Interchange File) file. Generally, the 2D drawing defines the nominal dimensions, geometrical tolerances and manufacturing requirements of the part. It can be used directly by the skilled pattern maker to make the tooling using the traditional manual approach. However, if rapid tooling processes (CNC or FFF) are to be used, this data status must be converted into a 3D CAD solid model.
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Intermediate Representation An intermediate representation is a special purpose file that has been created by the CAD software to facilitate file transfer or to drive another computer-based process. Examples include the STL (Stereolithography) file, IGES (Initial Graphics Exchange Specification) file, STEP (Standard for the Exchange of Product Mode Data) file, VRML (Virtual Reality Modeling Language) file, and so forth.
The STL file is typically used to drive most FFF processes. Assuming the pattern maker is familiar with the part geometry, the STL file is all that is needed to make the tooling using a FFF process. STL files come in two versions, binary or ASCII. Of the two, the binary version is more popular because the ASCII version (known as STLA) requires a large file size. The advantage of the ASCII version is its very simple data structure. In most cases, however, STL files cannot be used to drive a CNC process. IGES is an American National Standard that defines a neutral data format for the digital exchange of information among computeraided design (CAD) systems and other applications that utilize graphic (vector) information. STEP is a set of standards that provide for the exchange of engineering product data. It was created specifically for exchanging solid models, which is essential for the widespread use of solid model representations. VRML is a subset of the Silicon Graphics Open Inventor file format intended for use in Internet applications. This particular format has become the standard internet modeling language format for the World Wide Web. 3D CAD Solid Model File The 3D CAD solid model representation exactly defines the geometry of the part. For this reason, it is the ideal data status for rapid tooling. The 3D CAD solid model can be used by the tool maker to directly generate the NC code required for CNC machining or the STL file and slicing data needed for FFF processes. Data Status Interpretation
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The amount of interpretation required is different for each data status. Generally, interpretation difficulties (freedoms) decrease as the data status goes from a physical part (i.e., no data) to a fully specified 3D CAD solid model (Figure 6.2). The “physical part” data status has the most freedoms because there are many judgements that must be made and error sources that may affect the final tool accuracy. These error sources include erroneous deduction of design intent, measurement error, fitting error and representation (approximation) errors. 2D drawings must also be interpreted, but to a lesser extent since design intent is usually communicated by tolerances and notes. Intermediate representations also lack design intent and can be very difficult to use unless the toolmaker has a model of the part or a 2D drawing. Because the 3D CAD solid model representation is exact, it requires the least interpretation and can be directly used to generate the tooling using rapid tooling processes.
Interpretation Required
No data 2D file
Intermediate Representative
3D CAD
Figure 6.2 Amount of interpretation required for different data status. 6.2.2 Production Volume
The tooling material selected and the tooling approach used depends to a large extent on the production volume, which is the number of castings to be produced over the lifetime of the tooling. If a large number of castings are to be produced, then the tooling
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material and approach is likely to differ from that used for a short production run. For example, if only a few castings are to be poured (<10), then a loose pattern may be acceptable. If moderate production volumes (10 to 300) are anticipated, then the pattern must be mounted to ensure consistent quality. For large production volumes (>300), the use of highly durable materials and a cope and drag tooling approach becomes essential. 6.2.3 Prototype or Production?
Prototype and production impose different demands on the tooling requirements and quality. For a prototype part, the ability to modify the tooling to accommodate anticipated and unanticipated design changes is important. Also, since production quantities are likely to be low, short cuts such as the use of loose patterns or the use of a pattern made of a FFF material may be an option. For production parts, on the other hand, the production volume is usually the key decision factor. 6.2.4 Who Will Use the Tooling?
Often it is important to know which foundry will be pouring the casting. For example, foundry experience with handling loose patterns or using tooling made of FFF materials can be a key consideration in the decision process. 6.2.5 Part Geometry
Part geometry is an over-riding constraint that governs the tooling approach that is eventually selected. Some geometrical features may be very difficult or expensive to make using a particular rapid tooling method. Also, part geometry determines the number of cores that are required as well as their complexity. In addition, it effects the choice of parting plane and pattern complexity. Cores Cores are used to form internal features of the casting. Cores can be made using manual, CNC, or FFF methods. They can also be molded using tooling (core boxes) that is made using these methods.
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Therefore, like the pattern, a tooling alternative for each core must be selected. If there are many cores, the lead-time may be so long that rapid tooling becomes an option, even when lead-time is not a controlling factor. On the other hand, a few simple cores may be easier and more cost efficient to make manually. Parting Line Complexity The parting line is a very important tooling decision since it determines the complexity of the pattern features that must be generated. Complexity of the pattern will often strongly effect the tooling alternative that is selected. For example, a particular pattern geometry that is very difficult or costly to produce using CNC machining may be easily produced using a FFF process. The parting line will also effect the number of cores required. Draft To facilitate pattern withdrawal, part surfaces that are perpendicular to the parting plane must be drafted. If the geometry requirement of a particular surface is such that it cannot be drafted, then a core must be used or different parting line must be selected. These tooling decisions drive pattern complexity and the number and complexity of the cores, which in turn influences the tooling alternative that is selected. 6.2.6 What Geometrical Features are Important?
Geometrical features play an important role in the selection of an appropriate tooling alternative. Therefore, when a variety of features are involved, it is often very important to know which features and relationships between features are most important. This is because some tool making processes are better suited for accurately producing a particular feature than others. 6.2.7 Pattern Shop Capability
The capability of the pattern shop can be a major tooling alternative selection factor, depending on who is making the selection. If the design or manufacturing engineer is making the decision, he or she has the option of going to a toolmaker that has
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the required capability. If, on the other hand, the tool builder is making the selection, then shop capability is clearly a factor. If the shop used by the tool builder does not have a FFF capability, this option will not be considered. 6.2.8 Tool Cost
Tool cost is the result of the particular tooling alternative that is selected. When more than one tooling alternative is feasible, cost of the tool can be a major determining factor in the final selection. 6.2.9 Lead-Time
Lead-time is the time required to fabricate the tooling and produce a first article casting. Lead-time is directly affected by the tooling alternative that is selected and is therefore a major consideration in the tooling decision process. It is also closely associated with cost and quality. In general, if lead-time is a high priority, rapid tooling must be used. 6.2.10 Required Accuracy
Each tooling alternative is capable of producing a different level of accuracy. If accuracy is not particularly important, manual, CNC or FFF may be equally viable. If accuracy is very important, then CNC may be the only acceptable choice. 6.2.11 Tool Durability
Tool durability is typically a function of the tooling material selected. If high production quantities are required, then a durable tool material must be selected. This, in turn, can directly effect the choice of fabrication method and thereby indirectly effect tool cost and lead-time. 6.3 Decision Structure
In this section, we seek to explore and characterize the decision space that must be navigated in evaluating tooling alternatives for sand casting. More specifically, we are interested in defining the set
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of possible decision combinations that are possible and in understanding what constitutes a “best” tooling selection. 6.3.1 Universe of Possible Tooling Alternatives
From Section 6.2, we found that tooling alternative selection involves three major decision variables: fabrication method, tool material, and tooling approach. If we view each of these decision variables as a dimension of the tooling evaluation process, we can envision a decision space as shown in Figure 6.3. If we further assume that there are three types of fabrication methods to choose from (i.e., manual, CNC, FFF), five classes of materials (i.e., wood, urethane plastic, synthetic, metal, FFF materials), and three tooling approaches (i.e., loose pattern, match-plate pattern, cope and drag pattern), then there are 45 possible tooling alternative selection decision variable combinations. This is shown graphically by the morphological box depicted in Figure 6.3. To keep this discussion simple, we have limited the number of combinations depicted in Figure 6.4 by confining many of the decision variable choices to a particular “type” or “class.” In reality, the actual number of possible combinations may be much greater since there may be several alternative choices within each type or class. For example, one could select LOM, stereolithography, or some other FFF process. Similarly, there are typically a variety of different materials associated with each particular material class.
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Tooling Approach
Tool Material
Fabrication Method
Figure 6.3 Decision space for tooling alternative selection.
FFFF Mat抣
Cope & Drag
Metal
MatchPlate
Synthetic Urethane
Loose Wood Manual
CNC
FFFF
Figure 6.4 Morphological box for tooling alternative selection. Each cell represents a different combination of decision variables.
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6.3.2 Decision Constraints
Each of the tooling alternative selection factors discussed in Section 6.2 imposes constraints on the selection process by eliminating certain decision variable combinations from further consideration. Independent decision factors typically impose rigid constraints that must be satisfied. A production quantity of 30 units is a rigid constraint, the tool must be capable of producing 30 acceptable castings without breaking or wearing out. This requirement automatically eliminates many decision variable combinations from further consideration. Dependent decision factors, on the other hand, typically impose constraints that are negotiable to some extent. Any tooling alternative selection that is capable of producing 30 or more castings is acceptable. Similarly, any tooling alternative that exceeds a required “threshold” level of accuracy is acceptable. 6.3.3 Cost, Time, and Quality Tradeoffs
The dependent decision factors (cost, time, accuracy, and durability) are negotiable because of the complex couplings and tradeoffs that exist between these factors. For example, there are many tooling alternatives that are capable of producing 30 castings. The tooling alternative that is eventually selected will typically result in the most desirable tradeoff between cost, time, and quality. This, in turn, is likely to depend on the relative importance of the dependent decision factors. To illustrate, suppose that a certain dimensional accuracy is required for the 30 castings and that lead-time is critical. Given this scenario, of all the tooling alternatives that are capable of producing 30 good castings, only those that can achieve the required accuracy would be considered. Assuming that several tooling alternatives are capable of the required accuracy, the alternative offering the shortest lead-time would be selected. If two or more tooling alternatives have essentially the same lead-time, then the alternative offering the lowest cost would be selected. Now, suppose that low cost is more important than lead time. Given this set of relative importances, we see that a totally different tooling alternative is likely to be selected since cost would drive the decision rather than lead-time.
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Each decision variable choice carries with it a certain set of time, cost, and quality outcomes (see Figures 6.5, 6.6, and 6.7). Because the tooling alternative selection is a combination of these choices, it is safe to assume that each possible tooling alternative selection will also have a particular time, cost, and quality outcome. Therefore, the “best” choice will be the combination of decision variable choices that satisfies all of the independent decision factors and also yields the most desirable cost, time, and quality tradeoff for a given production volume. 6.3.4 Geometry Considerations
Part geometry constrains and underlies the tooling alternative decision. It constrains because only those tooling alternatives that are capable of generating the desired geometry can be considered. It underlies the decision because each tooling alternative that is capable of producing the desired geometry will also be more or less suited for producing particular features or aspects of the geometry. Deciding on the right tooling alternative for a given part geometry is therefore very dependent on the decision-maker’s experience and judgment. For example, most experienced tool builders can look at a part and immediately determine that CNC is the best fabrication method or that LOM would not work well. The tool builder does this by noting specific features of the part geometry and then mentally filtering the possibilities based on learned experience. This mental evaluation is the “essence” of the tooling alternative selection process.
Manual (Time & Cost Intensive)
FFFF (Time & Cost Efficient)
Increasing Accuracy
CNC (Time & Cost Efficient)
Figure 6.5 Tool fabrication method tradeoffs.
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Rapid Tooling Guidelines For Sand Casting
FFFF M aterial W ood Urethane
Increasing D urability
Synthetic M etal Figure 6.6 Tool material tradeoffs.
Loose Pattern Increasing Accuracy, Reliability, & Production Rate
Gated Pattern
Increasing Time & Cost
Match-Plate Pattern Cope & Drag Pattern
Figure 6.7 Tooling approach tradeoffs
In making the evaluation, the decision-maker generally considers several aspects of the part geometry. These include: • • • • •
Wall thickness Aspect ratio Type of undercuts and coring complexity Cross-section geometry Surface complexity
Thin walls and severe aspect ratios can be a problem for some FFF processes such as LOM. Conversely, certain types of undercuts may be very difficult to create using CNC machining, but are easily
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obtained using the LOM process. Cross-section geometry such as fillets, rounds and transitions between features can be difficult for some FFF processes if they are in the Z-axis direction (layering direction). Sculpted surfaces and radical splines may be difficult for manual processes. Similarly, a moving parting line may pose problems for CNC machining. 6.4 Decision Process
The tooling alternative selection process is a comprehensive mental exercise that is generally undertaken by an experienced tool builder. In general, the procedure followed by each expert tool builder will depend upon both the decision-makers’ knowledge and experience and upon the pattern shop capabilities available. To validate the theoretical understanding developed in Section 6.3, we investigated the tooling alternative decision process used at Clinkenbeard and Associates, Inc., an experienced tool builder for the casting industry located in Rockford, Illinois. The general flow of Clinkenbeard’s tooling alternative decision process is shown in Figure 6.8. It is important to note that, although the process appears to be linear, in reality, it is highly iterative and non-linear and may involve several conversations with the customer. Typically, the final tooling alternative decision emerges as the customer and tool builder work together to evolve an acceptable approach. It is also important to note that the Clinkenbeard shop capability includes extensive manual, CNC, and FFF facilities and that Clinkenbeard is very experienced in building tools for complex aerospace castings such as valve bodies and pump housings. In the following, we briefly review the considerations that are evaluated during each step of the procedure. 6.4.1 What is the Data Status?
Many tooling options depend on the data status. This is because a solid model of the part geometry is required if rapid tooling options are to be considered. If a solid model doesn’t exist and the customer is unwilling or unable to create one, then manual construction is the
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only feasible tool fabrication method. This automatically eliminates many rapid tooling options that would be possible if a solid model was available. Lead-time and cost also automatically increase for 2D geometry representations because added customer interaction is required to fully understand design intent and/or time and effort are required to create the solid model from existing 2D data. 6.4.2 Geometry Independent Requirements
After understanding the customer’s design intent, the next important step is to understand the geometry independent requirements of the project. These include product volume, leadtime, choice of foundry, and type of part (i.e., prototype or production). As previously discussed, this information will immediately eliminate a variety of tooling alternatives from further consideration. 6.4.3 Geometry Analysis
The part geometry is studied to determine what geometrical constraints exist and to evaluate these against possible tooling alternatives. Often, this step will initiate additional conversations with the customer to clarify the importance of various geometrical features and tolerances. Also, Clinkenbeard may make suggestions at this point regarding ways to simplify the design to make it easier to tool. Must this surface have “zero” draft? Can this undercut be eliminated? Would it be possible to modify this feature slightly so that a moving parting line or extra core could be avoided? These questions and their answers illustrate the iterative and non-linear process that is sometimes followed as the most appropriate tooling alternative is evolved. 6.4.4 Develop Alternative Tooling Proposals
Once Clinkenbeard has developed a comprehensive understanding of the design requirements, one or more tooling alternatives are developed and proposed to the customer. All of the decision
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variables and decision factors discussed previously are simultaneously considered in the mental evaluation performed at this stage. In addition, in considering time and cost of the various alternatives, Clinkenbeard will factor in the current shop workload and the availability of particular machines and expertise. Data Status: • •
Solid Model If no, can part be modeled
Geometry Independent Requirements: • • • • •
Prototype or production? If prototype, will part ever go into production? If production, what is the volume? Lead-time? Foundry?
Geometry Analysis: • • • •
Critical features? Critical tolerances? Parting lines? Draft?
Develop alternative tool path proposals Customer Selects
Figure 6.8 Clinkenbeard tooling alternative selection process. 6.4.5 Negotiate the Final Selection
The advantages and disadvantages of each alternative are discussed with the customer. In some cases, this discussion may lead to other options, which must be investigated and discussed in a
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Rapid Tooling Guidelines For Sand Casting
follow-on meeting. In other cases, the customer selects the tooling alternative to be used from those proposed.
Chapter 7 Tooling Alternative Selection Case Studies 7.1 Tooling Path Reviewing Form
The tool path evaluation form shown in Table 7.1 is a simple form that allows a sand casting tool design professional to quickly estimate the best tooling alternative selection for a given casting geometry as a function of production volume. The form also allows the tooling professional to document the key considerations driving the selection. It should be noted that the “key tool considerations and issues” include the decision factors discussed in Chapter 6. 7.2 Case Studies
The following case studies illustrate how casting geometry, production volume, and other key tooling considerations and issues drive tooling alternative selection for a variety of different parts that are to be produced as sand castings. The tooling alternative evaluations presented were performed with the assistance of Clinkenbeard & Associates, Rockford, Illinois. 7.2.1 Sensor Box
PART DESCRIPTION AND VIEWS Envelop Dimensions: 15.25x19.65x11.45 inches Materials: Aluminum
W. Wang et al., Rapid Tooling Guidelines For Sand Casting, Mechanical Engineering Series, DOI 10.1007/978-1-4419-5731-3_7, © Springer Science+Business Media, LLC 2010
117
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Rapid Tooling Guidelines For Sand Casting
Figure 7.1 The 3D CAD view of the sensor box
Notes: RECOMMENDATION:
TRADE-OFF TABLE: Production Tool Volume Fabrication 3 30 Manual 300 3 30 LOM 300 3 30 CNC 300 Time (days)
Table 7.1 Tool path evaluation form.
Geometry Drivers
KEY TOOL CONSIDERATIONS AND ISSUES Envelop Dimensions (inches): Material: Parting Line: Draft: Cores: Special Features:
Cost ($)
Quality
Chapter 7 Tooling Alternative Selection Case Studies 119
Wall is too thin for LOM. Also, ribs are too small.
Ribs on interior surface
Geometry Drivers
6
too long
Time (days) 5 6
3k 5.3k 6.0k
3k 5.3k
Cost ($) 3k 5.3k
[1]
[1]
Quality
Notes: [1] Wall thickness may vary up to 0.045” depending on accuracy of core and accuracy of core placement. Recommendation: Use CNC tooling fabrication. Table 7.2 Tool path evaluation for the sensor box
Trade-off Table: Volume Tooling Option 3 Manual 30 300 3 LOM 30 300 3 CNC 30 300
Key Tooling Considerations and Issues: (1) What surfaces should be drafted and by how much? (2) Assume interior is cored using a two-part core that includes holes and ribs. (3) For more than 300 parts, a cope & drag pattern is required, but core box remains the same. (4) Dry sand recommended for improved accuracy.
120
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Chapter 7 Tooling Alternative Selection Case Studies
121
7.2.2 Hitch Housing
PART DESCRIPTION AND VIEWS Envelop Dimensions: 17.74 x 19.0x3.88 inches Materials: Aluminum
Figure 7.2 The 3D CAD view of the hitch housing Key Tooling Considerations and Issues: (1) Draft on holes? Draft on surfaces perpendicular to parting plane; (2) If no draft, use CNC machining; (3) Radii in corners.
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Rapid Tooling Guidelines For Sand Casting
Trade-Off Table: Tooling Geometry Time Cost ($) Volume Option Drivers (days) No critical 4 [1], [4] 2.3k 3 features 8 Manual 30 300 3.0k 4 [4] 3 5.3k 8 LOM 30 8 300 2.3k 4 [4] 3 4.3k [5] CNC 8 30 4.3k 8 300
Quality [2] [6] [3] [6] [2] [6]
Notes: [1] May vary depending on shop workload. [2] Renboard most stable; [3] LOM materials less stable, some warpage expected ( ± 0.030" possible). [4] Assumes proper drawing is available. [5] Foundry should reduce piece price, enhance reliability; [6] Use match plate for higher production quantity to give more consistent gating and dimensions. Recommendation CNC maching recommended. Table 7.3 Tool path evaluation for the hitch housing
Chapter 7 Tooling Alternative Selection Case Studies
7.2.3 Yoke
PART DESCRIPTION AND VIEWS Envelop Dimensions: 17.80 x 24.16 x 19.71 inches Materials: Aluminum
Figure 7.3 The 3D CAD view of the yoke
123
Key Tooling Consideration and Issues (1) 3 cores with parting line down plane of sights. (2) Radii should be optimized. (3) Undercuts make it difficult to master. (4) Machine master, pour core boxes except with undercuts, machining undercuts & pour around it for core boxes; (5) 30 parts, big boxes w/ gating. Trade-off Table: Volume Tooling Geometry driver Time Cost Quality Option (days) ($) 23 $12k 3 Pine; loose; split with 3 core boxes Manual 30 300 Part is too tall, no cost saving; might 14 $14k 3 save time; LOM is possible [1] LOM 30 300 $12k 16 [2] 3 CNC Machining a master pine 20 [3] $14.5k 30 24 [3] 300 Notes: [1] Quality suffers because of warpage and Z-axis variation Recommendation: All possibilities depend on capabilities of specific shop. Table 7.4 Tool path evaluation for the yoke
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Chapter 7 Tooling Alternative Selection Case Studies
7.2.4 Carrier Housing 1
PART DESCRIPTION AND VIEWS Envelop Dimensions: 18.27 x 7.97 x 8.13 inches Materials: Aluminum
Figure 7.4 The 3D CAD view of the carrier housing
125
LOM okay for exterior, CNC interior cores. Loose pattern
Long hole, undercuts, parting line
Geometry driver
13
10
15
15
Time (days)
8.3k 10.3k
8.3k
Cost ($) 8.3k
Notes: Recommendation:CNC machine. Table 7.5 Tool path evaluation for the carrier housing 1
Trade-off Table Volume Tooling Option 3 Manual 30 300 3 LOM 30 300 3 CNC 30 300
Key Tooling Consideration and Issues: (1) Loose pine pattern plus 3 core boxes. (2) Draft requirement? (3) LOM exterior & CNC interior cores. (4) Mount pattern. (5) Dry sand, better exterior for mass production.
Quality
126 Rapid Tooling Guidelines For Sand Casting
Chapter 7 Tooling Alternative Selection Case Studies
127
7.2.5 Inlet Boost Pump
PART DESCRIPTION AND VIEWS Envelop Dimensions: 7.30 x 4.78 x 4.06 inches Materials: Aluminum
Figure 7.5 The 3D CAD view of the inlet boost pump Key Tooling Consideration and Issues: (1) Flange features must be created by coring to meet zero draft requirement. Trade-off Table: Volume Tooling Geometry Driver Time Cost ($) Quality Option (days) 10 10k Thin wall & 3 Manual flange & zero 30 ± 0.03” draft on flange 300 surface, blends Not suitable for 3 too long LOM LOM due to 30 warpage 300 3 CNC 7 [1] 10k 30 ±0.05” 300
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Rapid Tooling Guidelines For Sand Casting
Notes: [1] Renboard for low quantity, urethane for higher quantity. Urethane could either be machined or cast using a renboard master. Recommendation: CNC machine. Table 7.6 Tool path evaluation for the inlet boost pump
7.2.6 Manifold
PART DESCRIPTION AND VIEWS Envelop Dimensions: 20 x 5 x 4 inches Materials: Iron
Figure 7.6 The physical part of manifold Key Tooling Consideration and Issues: (1) Straight, planar parting plane with slight offset on one end. (2) One large interior core, 2 small exterior cores to create undercut on flange. (3) No tight tolerance. (4) No zero draft surfaces. (5) Compound angles on flange require special setups.
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129
Trade-off Table: Volume Tooling Geometry Driver Time Cost ($) Quality Option (days) 30 23k No special 3 33 [2] 28k Manual features that favor 30 ± 0.03" manual, 300 compound angles increase difficulty 17k This geometry is 3 18 LOM favorable for the 30 ± 0.03" LOM process: 300 25 ± 3 No features that 25k 3 CNC make CNC 30 ± 0.005" preferrable. 30k 300 28 ± 3 [2] [1]
Notes: [1] Requires cope and drag pattern. [2] For manual & CNC, high volume requires more durable material. Table 7.7 Tool path evaluation for the manifold
7.2.7 Turbine Blade Machining Fixture
PART DESCRIPTION AND VIEWS Envelop Dimensions: 11 x 11 x 2.5 inches Materials: Cast Iron
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Rapid Tooling Guidelines For Sand Casting
Figure 7.7 The physical part of the turbine blade fixture Key Tooling Consideration and Issues (1) Flat, one side straight parting plane. (2) No coring required. (3) No tight or challenging tolerances. (4) No zero draft surfaces, generous draft everywhere. (5) Special features: one side has the blade profile. Trade-off Table: Volume Tooling Geometry Driver Time Option (days) 16 Blade surface is 3 20 Manual not easy to create 30 24 by hand. 300 8 Blade surface is 3 12 LOM easily created by 30 16 LOM process 300 16 Blade surface is 3 20 CNC not easy to create 30 24 by hand. 300
Cost ($)
Quality
2.5 k 3.5k 3.9 k 1.7k 2.8 k 3.4 k 2.5 k 3.5k 3.9 k
Notes: This part geometry is not critical. Any process would produce an acceptable pattern. The advantage CNC has is for 300 parts, the
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131
pattern can be produced using a durable material without reversing the LOM into a urethane pattern. Recommendation: Use LOM since the user will never require more than a few castings. Table 7.8 Tool path evaluation for the turbine blade machining fixture
7.2.8 Upper Arm
PART DESCRIPTION AND VIEWS Envelop Dimensions: 25 x 6 x 4.5 inches Materials: Cast Iron
Figure 7.8 The physical part of the upper arm Key Tooling Consideration and Issues: (1) Straight, planar parting plane split through center. (2) 2 cores required. (3) No tight or challenging tolerances. (4) No zero draft surfaces, generous draft everywhere. (5) No special features.
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Trade-off Table Volume Tooling Geometry Driver Time Option (days) 10 No special 3 Manual geometry features, 12 30 12 suitable for all 300 kinds of tooling processes 10 3 14 LOM 30 18 300 3 10 CNC 30 14 300 14
Cost ($)
Quality
3.5k 4.5k 5.0k
OK
3.5k 4.5k 5.0k 3.0k 4.0k 4.5k
Good Best
Notes: This iron part is finish machined. No critical features, so pattern accuracy is not important. For 3 pieces LOM or CNC are the least expensive. For 300 pieces, the LOM would have to be reversed into urethane, which adds time and cost. Recommendation: Use LOM or CNC. For 3 pieces, use loose pattern with 2 cores; for 30 or more pieces, use matchplate pattern including gating. Table 7.9 Tool path evaluation for the upper arm
7.2.9 Rigid Connecting Link
PART DESCRIPTION AND VIEWS Envelop Dimensions: 19.5x11.25x7.5 inches Materials: Aluminum
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133
Figure 7.9 The 3D CAD view of the rigid connecting link Key Tooling Consideration and Issues: (1) Parting line: (2) No zero draft surfaces. (3) All cores are perpendicular to parting line (4 cores). (4) No special features. (5) Part is finish machined. Trade-off Table: Volume Tooling Option 3 Manual 30 300 3 LOM 30 300 3 CNC 30 300
Geometry Driver Easy to do manually.
Time Cost ($) Quality (days) 10.3k [4] 4-5 11.8k 4-5 ±0.025 12.8k [5] 5-6 [1] 10.4k Acceptable for 2.0-3.5 12k LOM 2.0-3.5 ±0.020 12k 4 1.5-2.5 [2] 10.2k 11.7k Acceptable ±0.010 13k 2.5-3.5 [3]
Notes: [1] Use more durable material.
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Rapid Tooling Guidelines For Sand Casting
[2] [3] [4] [5]
Use Renboard. Use urethane board. Loose pattern. Mounted and gated pattern.
Recommendation: CNC if time is critical, manual is okay if time is not a factor. Table 7.10 Tool path evaluation for the rigid connecting link
7.2.10 25mm Projectile
PART DESCRIPTION AND VIEWS Envelop Dimensions: 1.38x1.38x3.30 inches Materials: Cast Iron
Figure 7.10 The 3D CAD view of the 25mm projectile Key Tooling Consideration and Issues: (1) Simple planer parting line. (2) Normal draft. (3) 1 core.
Chapter 7 Tooling Alternative Selection Case Studies
135
(4) No special features Trade-off Table: Volume Tooling Geometry Option Driver No special 3 or Manual geometry 30 complex 300 features. 3 LOM 30 300 3 CNC 30 300
Time (days)
Cost ($)
Quality
15
8.4k
±0.020"
12 [1]
6.0k[1]
±0.015
9
6.0k
±0.005
Notes: [1] These data are actual time and cost. The production volume for this case was 400 units using a cope & drag pattern. Recommendation: LOM was used for actual production. Table 7.11 Tool path evaluation for the 2.5mm projectile
7.2.11 Carrier Houseing 2
PART DESCRIPTION AND VIEWS Envelop Dimensions: 15.56x8.78x5.86 inches Materials: Steel
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Rapid Tooling Guidelines For Sand Casting
Figure 7.11 The 3D CAD view of the carrier housing 2 Key Tooling Consideration and Issues: (1) Offset parting line. (2) Normal draft. (3) No cores. (4) No special features. Trade-off Table Volume Tooling Geometry Option Driver No special 3 Manual geometry 30 features 300 3 LOM 30 300 3 CNC 30 300 Notes:
Time (days)
Cost ($)
Quality
4.5k 5.8k
±0.04"
9
7.0k [1]
±0.03"
10
4.5k 5.8k 5.8k
±0.01"
Chapter 7 Tooling Alternative Selection Case Studies
[1] Actual cost for production run. Recommendation: LOM was used for actual production. Table 7.12 Tool path evaluation for the carrier housing 2
7.2.12 Muzzle Brake
PART DESCRIPTION AND VIEWS Envelop Dimensions: 29.25x17.50x11.88 inches Materials: Aluminum
Figure 7.12 The 3D CAD view of the muzzle brake
137
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Rapid Tooling Guidelines For Sand Casting
Key Tooling Consideration and Issues: (1) Simple planer parting line. (2) Normal draft everywhere. (3) 3 cores. (4) No special features. Trade-off Table Volume Tooling Geometry Option Driver No special 3 Manual geometry 30 features 300 3 LOM 30 300 3 CNC 30 300
Time (days) 25
9
Cost ($)
Quality
17.3k [1] 19.8k [2] ± 0.040" 21.8k 14.9k [4] ± 0.040"
10
15.0k 17.6k 19.6k
± 0.010"
Notes: [1] Loose pattern. [2] Cope and drag pattern. [3] Actual total time for cope and drag pattern. [4] Actual cost for cope and drag pattern. Recommendation: LOM was used in actual production. Table 7.13 Tool path evaluation for the muzzle brake
7.2.13 Ice Cleat
PART DESCRIPTION AND VIEWS Envelop Dimensions: 7.7 x 6.4 x 2.3 inches Materials: Steel
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139
Figure 7.13 The 3D CAD view of the ice cleat Key Tooling Consideration and Issues: (1) Simple planer parting plane. (2) No special draft requirement (3) No cores. (4) No special features. Trade-off Table: Volume Tooling Geometry Option Driver No special 3 Manual geometry or 30 complex 300 features. 3 LOM 30 300 3 CNC 30 300
Time (days) 6 10 14
Cost ($)
Quality
2.1k [1] 3.1k [2] 3.6k [3]
± 0.030
5 9 13 5 9 13
1.4k 1.4k 2.9k 1.6k 2.6k 3.1k
± 0.030 ± 0.010
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Rapid Tooling Guidelines For Sand Casting
Notes: [1] Loose pattern. [2] Gated match plate. [3] Gated cope and drag pattern. Recommendation: This is a good candidate part for the LOM process. Table 7.14 Tool path evaluation for the ice cleat
7.2.14 Roller Bracket
Part Description and Views Envelop Dimensions: 3.85 x 3.8 x 1.2 inches Materials: Steel
Figure 7.14 The 3D CAD view of the roller bracket Key Tooling Consideration and Issues: (1) Simple planer parting plane. (2) 1.0 degree of draft on all surfaces. (3) No cores. (4) No special features.
Chapter 7 Tooling Alternative Selection Case Studies
Trade-off Table Volume Tooling Geometry Option Driver No special 3 Manual geometry or 30 complex 300 features. No thin or 3 LOM fragile areas. 30 300 3 CNC 30 300
141
Time (days) 10 10 13
Cost ($)
Quality
2.1k [1] 3.1k [1] 3.3k [2]
± 0.030
8 8 10 8 8 10
1.2k 1.2k 1.9k 1.4k 1.4k 2.2k
± 0.030 ± 0.010
Notes: [1] 1-gang matchplate. [2] 2-gang matchplate. Recommendation: This part has very simple geometry, which makes it easily produced using either manual or LOM. It is recommended that LOM be used if time is a factor. Table 7.15 Tool path evaluation for the roller braket
Appendix Glossary of Terms
Baked Core: A core which has been A Abrasion:
heated for a sufficient time and temperature The
displacement
and/or
to produce the desired physical properties.
detachment of metallic particles from a
Baked Strength: Compressive, shear,
surface as a consequence of being exposed to
tensile or transverse strength of a molded
flowing solids, fluids or gases.
sand mixture when baked at a temperature above 230 oF (110 oC) and then cooled to
Aging: A change in properties of metals
room temperature.
and alloys that occurs slowly at room temperature and will proceed rapidly at
Base Plate: A plate to which the pattern
higher temperatures. The change in properties
assemblies are attached and to which a flask
is often, but not always, due to a phase
is subsequently attached to form the mold
change (precipitation), but never involves a
container. Blow Holes: 1) Holes in the head plate or
change in chemical composition of the metal
blow plate of a core-blowing machine
or alloy. metallic
through which sand is blown from the
properties and composed of two or more
reservoir into the core box. 2) Irregular
chemical elements of which at least one is
shaped cavities with smooth walls produced
metal. Usually possesses qualities different
in a casting when gas is entrapped during
from those of the components.
mold filling. The gas sources may be air,
Alloy: A substance having
binder decomposition products or gases
Angle Testing (UT): A method of ultrasonic
testing
using
shear
dissolved in the molten steel.
waves
Blow Plate: The plate containing the core
introduced from the surface of the material at
sand entrance holes or blow holes used in
approximately 45 degrees. Apparent
Contraction:
The
open-face core boxes.
net
Blower, Core or Mold: A device using air
contraction of a casting dimension due to true
pressure to fill a core box or flask with sand.
metal contraction, mold wall movement, and
Boss: A projection of circular cross-
restraint during solidification and cooling.
section on a casting. Usually intended for
As Cast: Referring to metal which has not
drilling and tapping for attaching parts.
received finishing (beyond gate removal or sandblasting), or treatment of any kind
Bracket: Strengthening strip, rib, or
including heat treatment after casting. (See
projection on a casting. Usually used to
Finishing).
prevent hot tearing. Breakoff Notch: A thinner section of a gate or riser to facilitate clean breaking-off
B Backing Board (backing plate): A second
during the cleaning process.
bottom board on which molds are opened.
143
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Rapid Tooling Guidelines For Sand Casting
Brinell Hardness: The value of hardness
Casting Layout: A check of dimensions
of a metal on an arbitrary scale representing
against
kg/mm2,
specifications.
determined
by
measuring
the
applicable
drawings
and
diameter of the impression made by a ball of
Chaplet: Metal support that holds a core
given diameter applied under a known load.
in place within a mold; molten metal
Values are expressed in Brinell Hardness
solidifies around the chaplet and fuses it into
Numbers, BHN
the finished casting.
Buckle: 1) Bulging of a large flat face of
Cheek: The intermediate section of a
a casting; in investment casting, caused by
flask that is used between the cope and the
dip coat peeling from the pattern. 2) An
drag when molding a shape requires more
indentation in a casting, resulting from
than one parting plane.
expansion of the sand, may be termed the start of an expansion defect.
Chill (External): Metal, graphite, or carbon blocks that are incorporated into the
Burnishing: Developing a smooth finish
mold or core to locally increase the rate of
on a metal by tumbling or rubbing with a
heat removal during solidification and reduce
polishing tool.
shrinkage defects. Chill (Internal): A metallic device/insert
C CAD: Computer Aided Design.
in molds or cores at the surface of a casting or
CAE: Computer Aided Engineering.
removal, induce directional solidification and
CAM: Computer Aided Manufacturing.
reduce shrinkage defects. The internal chill
CNC: Computer numerical control.
may then become a part of the casting.
within the mold to increase the rate of heat
Carbide: A compound of carbon with one or more metallic elements. Carbon: Element occurring as diamond and as graphite. Carbon reduces many metals from their oxides when heated with the latter, and small amounts of carbon greatly affect the properties of iron. Carbon
Dioxide
Cleaning: The removal of gates, runners, and risers from the rough casting, This term also involves any hand finishing such as grinding or blasting. CMM: Coordinate Measuring Machine. CNC
Machine
Tools:
Computer
Numerical Controlled Machine Tools. (Silicate
Cold-Box Process: 1) Any core binder
Process, Schmidt-Philipp Process): A process
Process
process that uses a gas or vaporized catalyst
for hardening molds or cores in which carbon
to cure a coated sand while it is in contact
dioxide gas is blown through dry clay-free
with the core box at room temperature.
silica sand to precipitate silica in the form of a gel from the sodium silicate binder.
Collapsibility: The requirement that a sand mixture break down under the pressures
Carburizing: A form of case hardening
and temperatures developed during casting, in
that produces a carbon gradient inward from
order to avoid hot tears or facilitate the
the surface, enabling the surface layer to be
separation of the sand and the casting.
hardened by either quenching directly from
Compressive
Strength
(Yield):
The
the carburizing temperature or by cooling to
maximum stress in compression that can be
room temperature, then reaustenitizing and
withstood without plastic deformation or
quenching.
failure.
Casting: The metal shape, exclusive of
Conductivity (Thermal): The quantity of
gates and risers, that is obtained as a result of
heat that flows through a material measured
pouring metal into a mold.
in heat units per unit time per unit of cross-
145
Appendix Glossary of Terms sectioned area per unit of length, (electrical)
Core Float: A casting defect caused by
the quantity of electricity that is transferred
core movement towards the cope surface of
through a material of known cross-section
the mold, as a result of core buoyancy in
and length.
liquid steel, resulting in a deviation from the
Contraction: occurring
in
solidification
The metals
and
volume and
cooling
change
alloys to
intended wall thickness.
on
Core Prints: Portions of a pattern that
room
locate and anchor the core in the proper
temperature.
position in the mold.
Convection: The motion in a fluid resulting from the differences in density. In heat transmission, this meaning has been extended to include both forced and natural motion or circulation.
Core Rod: A wire or rod of steel used to reinforce and stiffen the core. Core Setting Jig/Gage: A device used to help position a core in the mold. Core Shooter: A device using low air
Cooling Curve: A curve showing the
pressure to fluidize the sand mix which is
relationship between time and temperature
released quickly in such a way as to force it
during the solidification and cooling of a
into a core box.
metal sample. Since most phase changes
Core Vents: 1) Holes made in the core for
involve evolution or absorption of heat, there
the escape of gas. 2) A metal screen or slotted
may be abrupt changes in the slope of the
piece used to form the vent passage in the
curve.
core box employed in a core-blowing
Conduction: The transmission of heat,
machine. 3) A wax product, round or oval in
sound, etc., by the transferring of energy from
form, used to form the vent passage in a core.
one particle to another.
Corrosion:
Cope: Upper or topmost section of a flask, mold, or pattern.
electrochemical
1)
Gradual
attack
on
chemical
or
a
by
metal
atmosphere, moisture, or other agents. 2)
Coping Out: The extension of sand of the
Chemical attack of furnace linings by gases,
cope downward into the drag, where it takes
slags, ashes, or other fluxes occurring in
an impression of a pattern.
various melting practices.
Core: A separate part of the mold, made
Cover Core: A core set in place during
of sand and cured, which is used to create
the ramming of a mold to cover and complete
openings and various shaped cavities in the
a cavity partly formed by the withdrawal of a
casting.
loose part of the pattern. Also used to form
Core Assembly: Putting together a core made of a number of sections. Core
Arbor:
An
iron
part or all of the cope surface of the mold cavity. A core placed over another core to
framework
embedded in a large core to stiffen it and for convenience in handling.
create a flat parting fine. Cracking Strip: A fin of metal molded on the surface of a casting to prevent hot tearing.
Core Extruder: A special shell-core-
Critical Cooling Rate: minimum rate of
making machine that produces a continuous
continuous cooling just enough to prevent
length of cores, usually of cylindrical cross-
undesired transformations.
section.
Crystallization: The formation of crystals
Core Filler: Material used in place of
by the atoms assuming definite positions in
sand in the interiors of large cores--coke,
the crystal lattice, e.g. when a metal
cinder, sawdust, etc., usually added to aid
solidifies.
collapsibility.
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Rapid Tooling Guidelines For Sand Casting
D Datum Plane: In layout and machining operations, the reference plane from which
ways: bilateral tolerance, unilateral tolerance and limit dimensions.
dimensions are measured in the perpendicular
Direct AIM™ (ACES Injection Molding): This is a new "soft/bridge tooling"
direction.
process which quickly and inexpensively
Datum Points: In layout and machining
builds prototype parts using a variety of
operations, the reference points that define
engineering thermoplastics in a very short
the datum plane from which dimensions are
time without the need for production tooling.
measured.
The Direct AIM™ process utilizes core and
Decarburization: Loss of carbon from the surface of a ferrous alloy as a result of heating in a medium, usually oxygen, that reacts with carbon.
cavity mold halves produced using the SLA process. Direct Shell Production Casting (DSPC): DSPC produces the actual ceramic molds for
Defect: A discontinuity in the product
metal castings directly from 3-D CAD
whose severity is judged unacceptable in
designs using the Three Dimensional Printing
accordance with the applicable product
(3DP) technology. No tooling or patterns are
specification.
required.
Deformation Test: An AFS test using an
Disruptive Strength: Maximum strength
instrument such as the Dietert Universal
of a metal when subjected to three principal
Sand-Strength
tensile stresses at right angles to one another
Testing
machine
(with
deformation accessory) to determine the amount, in inches, that the sand specimen is compressed before it ruptures.
and of equal magnitude. Dowel: A wooden or metal pin used in the parting surface of patterns and core boxes to
Deoxidation: Removal of oxygen from
locate and hold them in position.
molten metal, usually accomplished by
Downsprue (Sprue, Downgate): The first
adding materials with a high affinity for
channel, usually vertical, which the molten
oxygen, the oxides of which are either
metal enters.
gaseous or readily form slags. Die
Casting:
A
rapid,
Draft: Slight taper given to pattern to watercooled
permanent mold casting process limited to non-ferrous metals. Dilatometer:
allow drawing from sand. Drag: Lower or bottom section of a mold or pattern
for
Drawback: A part of the mold, made of
measuring the length of a metal sample
An
instrument
green sand, which may be drawn back to
during heating and cooling.
clear overhanging portions of the pattern.
Dimension: A dimension is a numerical value associated with an appropriate unit of measurement. It defines the size, shape and location of features. Dimensional Tolerance: A tolerance is the permissible variation in size of a feature
Drawing: Removing the pattern from the sand. Draw Plate: A plate attached to a pattern to facilitate drawing of a pattern from the mold. Dry and Baked Compression Test: A
placed on the specified dimension. It is
sand
usually assigned to a dimension based on
compressive stress that a baked sand mixture
limitations inherent in production processes
is capable of developing.
and equipment. It can be expressed in three
test
to
determine
the
maximum
Dry Permeability: The property of a molded mass of sand bonded or unbonded,
147
Appendix Glossary of Terms dried at 220-230 oF (105~110 oC and cooled to room temperature that allows passage of gases resulting during pouring of molten metal into a mold.
F Fabrication: The joining, usually by welding, of two or more parts to produce a finished assembly The components of the
E Elastic Limit: Maximum stress that a material will withstand without permanent deformation. (See Yield Strength).
assembly may be a combination of cast and wrought materials. Fast Freeform Fabrication (FFFF): see Rapid prototyping.
Elasticity: The property of recovering
Feature: A feature is any physical portion
original shape and dimensions upon removal
of an object, such as a hole, surface, or
of a stress.
groove. It is a general term that is used to
Elongation:
Amount
of
permanent
extension in the vicinity of the fractures in the
denote or refer to any part of an object. Feeding: The
process of supplying
tensile test; usually expressed as a percentage
molten metal to compensate for volume
of original gage length.
shrinkage while a casting is solidifying.
Embrittlement: Loss of ductility of a metal due to a chemical or physical change. EPC (Expendable Pattern Casting): See Lost Foam Process. Erosion: Abrasion of metal or other material by liquid or gas. Etchant: A solution for the chemical
Fillet: A concave comer piece used on foundry patterns, a radiused joint replacing sharp inside comers. Fin: A thin projection on a casting due to an imperfect joint in the mold. Finish Allowance: The amount of stock left on the surface of a casting for machining.
etching of the polished surface of a metal
Finish Mark: A symbol (f, f1, f2, etc.)
specimen to reveal macro- or microstructures.
appearing on the line of a drawing that
Evaporative Pattern Casting/Expendable Pattern Casting/EPC: See Lost Foam
represents the edge of the surface of the
Process.
casting to be machined or otherwise finished. Finite Element Analysis (FEA): A
Epoxy: Containing oxygen attached to
computerized numerical analysis technique
two different atoms already connected to a
used for solving mechanical engineering
ring. A word often used by itself for epoxy
problems relating to stress analysis, heat
resin.
transfer, and so forth.
Epoxy Resin: A plastic resin and hardener
Flash: A thin section of metal formed at
that sets or hardens itself as room temperature
the mold, core or die joint, or parting in a
to form a new chemical, used as an adhesive
casting due to the cope and drag not
or workable material.
contacting completely, or where core and
Expended Polystyrene: A foamy plastic
core print do not contact completely.
composed of bonded beads, density between
Flask: A metal frame used for making or
1 to 1.2 lb./per ft., compressive strength
holding a sand mold. The upper part is the
13~15 psi.
cope and the bottom half is the drag.
Expended Polyurethane: A lightweight
Flask
Bar:
A
reinforcing
member
expanded plastic polymer whose foamed
attached within either half of a flask to assist
texture results from the trappings of CO2
in holding the rammed sand in position.
evolved during production. In resin from it is used for coating and adhesive.
Flask Clamp: A device for holding together the cope, drag and cheek of a flask.
148
Rapid Tooling Guidelines For Sand Casting
Flask Pins: Pins to fit corresponding
Geometry Driver: The critical features
sockets on the joint of a flask to permit
which may affect or determine the tooling
separation.
path selection.
Flask Pin Guides: Guides used to
Grain Fineness Number: A system
accurately align the match plate pattern in the
developed by AFS for expressing the average
flask and flask to flask location.
grain size of a given sand, It approximates the
Flat Back: A pattern with a flat surface at
number of meshes per inch of that sieve that
the joint of the mold. It ties wholly within the
would just pass the sample if its grains were
drag, and the cope is a plane surface.
of uniform size.
Floor Molding: Used where the pattern
Green Sand: A naturally bonded sand or
size prohibits the use of molding machines.
a compounded molding sand mixture which
The pattern is bolted to the floor and the
has been tempered with water for use while
assembled mold is moved by crane.
still in the damp or wet condition.
Fluidity: The ability of molten metal to flow. Common devices used to measure fluidity are spiral casting and the Chinese Puzzle.
Green Sand Core: One that is made of green sand but not baked. Green Strength: The strength of a tempered sand mixture at room temperature.
Foam Plastics: Resinoids in spongy form, as polystyrene. The sponge maybe flexible or rigid, the cells closed or interconnected, with
H Hardness: Resistance of a material to
a density from that of the parent resin to, in a few cases, 2 lb/ft3.
indentation as measured by such methods as
Follow Board: A board shaped to the
hardness also refers to the ability of the metal
parting line of the mold. Foundry Returns: Metal in the form of sprues, gates, runners, risers, and scrapped castings, with known chemical composition that are returned to the furnace for remelting. Sometimes referred to as "revert".
Brinell, Rockwell, and Vickers. The term to resist scratching, abrasion, or cutting. It is related to yield strength and ultimate tensile strength. High Pressure Mold: A strong highdensity mold. Hot Tear: A crack or fracture formed prior to completion of metal solidification as
G Gage Length: The original length of that
a result of hindered contraction. A hot tear is
portion of the specimen over which strain or
and is commonly associated with design
change of length is determined.
limitations.
frequently open to the surface of the casting
Gage Marks: Reference marks; in tensile
Hot Strength (Sand): (compressive, shear
testing, the marks which indicate the gage
or transverse) Strength of a sand mixture
length, used in determination of tensile
determined at any temperature above room
elongation.
temperature.
Gate: A channel through which the molten metal enters the casting cavity. Gating System: The gating system is a
I Impact
Strength: The resistance to
channel, or network of channels, through
impact loads; usually expressed as the foot
which the molten metal flows into the cavity.
pounds of energy absorbed in breaking a
It is typically composed of a pouring cup,
standard specimen.
sprue, and runner.
149
Appendix Glossary of Terms Induction Furnace: An AC melting
Keyhole Specimen: A type of notched
furnace which utilizes the heat of electrical
impact test specimen, which has a hole-and-
induction.
slot notch, shaped like a keyhole.
Induction
Hardening:
A
surface
Kissing
(Touching):
Gating
with
hardening process involving the localized use
minimum metal left at casting breakoff point,
of pulsating magnetic currents to achieve
having a gate just "kiss" the surface.
heating above the austenite transformation temperature, AC3, followed by quenching. Induction Heating: Process of heating by electrical resistance and hysteresis losses
Knockout Pins (Ejector Pins): Small diameter pins affixed to a pattern back-up plate for removing the cured mold in the shell-molding process.
induced by subjecting a metal to the varying magnetic field surrounding a coil carrying an alternating current. Ingates: See Gate.
L Layered Manufacturing (LM): see Rapid prototyping.
Insert: A part usually formed from metal,
Layout: A full size drawing of pattern
which is placed in a mold and may become an
showing its arrangement and structural
integral part of the casting.
features.
Internal Shrinkage: A void or network of
Loam Mold: A mold built up of brick,
voids within a casting caused by inadequate
covered with a loam bud, and then baked
feeding of that section during solidification.
before being poured.
Internal Stresses (or Thermal Stresses):
Loose Molding: The molding process
Generally stresses which occur during the
utilizing unmounted pattern. Gates and
cooling of a part.
runners are usually cut by hand.
Investment Casting: Casting produced in
Loose Piece: Part of a pattern that
a mold obtained by investing an expendable
remains in the mold and is taken out after the
pattern with a refractory to produce a shell.
body of the pattern is removed. It can be
The expendable pattern may consist of wax,
indicated as 1) Core box: part of the core box
plastic, or other material, and is removed
which remains embedded in the core and is
prior to filling the mold with metal.
removed after lifting off the core box. 2)
Isocyanate Resin: A basic chemical component of the urethanes.
Pattern: laterally projecting pan of a pattern so attached that it remains in the mold until the body of the pattern is drawn. Back-draft is
J Jarring Machine: A molding machine that packs the sand by jarring. Jig: A device arranged to expedite a hand or machine operation.
avoided by this means. 3) Permanent mold: part which remains on the casting and is removed after the casting is ejected from the mold. Lost Foam Process: Casting process in
Jolting Machine: See Jarring Machine.
which a foam pattern is removed from the
Jolt-Squeezer Machine: A combination
cavity by the molten metal being poured.
machine that employs a jolt action followed by a squeezing action to compact the sand around the pattern.
M Machine Allowance: Stock added to the part to permit machining to final dimensions.
K
Machine Finish: Allowance of stock on the surface of the pattern in order to permit
150
Rapid Tooling Guidelines For Sand Casting
machining of the casting to the required
compressed air; allows for faster production
dimensions.
than gravity rollover dump.
Master Pattern: An original pattern made
Mold Board (Follow Board): The board
to produce metal castings that are then used
upon which the pattern is placed to make the
for production.
mold.
Matchplate: A plate on which patterns
Mold Cavity: The space in a mold which
and gating systems, split along the parting
is filled with liquid metal to form the casting
fine, are mounted. Matchplates are mounted
upon solidification.
back to back to form an integral piece.
Mold Clamps: Devices used to hold or
Mechanical Properties: Properties of a material that reveal its strength and elastic behavior.
lock cope and drag flask parts together. Mold Coating (Mold Facing, Dressing): Coating to prevent surface defects on
Metallurgy: The science and technology
permanent mold castings and die castings. 2)
of metals. A broad field that includes but is
Coating on sand molds to prevent metal
not limited to the study of internal structures
penetration and improve metal finish.
and properties of metals and the effects on them of various processing methods. Microporosity: See Microshrinkage.
during pouring.
Microshrinkage: Very finely divided porosity
resulting
from
Mold Jacket: A wooden or metal form slipped over a mold to support the sides
interdendritic
shrinkage resolved only by use of the
Mold Shift: A casting discontinuity resulting from misalignment of the cope and drag halves.
microscope; may be visible on radiographic
Molding, Floor: Making sand molds from
films as mottling. Etching shows they occur
loose patterns of such size that they cannot be
at intersections of convergent dendritic
satisfactorily handled on a bench or molding
directions.
machine, the equipment being located on the
Misrun: Denotes an irregularity of the casting surface caused by incomplete filling of the mold. Mock-up:
floor during the entire operation of making the mold. Molding, Pit: Molding method in which
A
full-size
model
built
accurately for study, testing or display. Model: A representative of an object; miniature or full size.
die mold is made in a pit or hole in the floor. Molding Sand: A sand which binds strongly without losing its permeability to air or gases.
Modulus of Elasticity (E): In tension, it is
Mulling and Tempering: The thorough
the ratio of stress to the corresponding strain
mixing of sand with a binder, either natural or
within the limit of elasticity (Yield Point) of a
added, with lubricant of other fluid, as water,
material. For carbon and low alloy steels of any composition and treatment, the value is approximately 30,000,000 psi. Mold: A body of molding sand or other heat-resisting material containing a cavity which forms a casting when filled with molten metal. Mold Blower: Molding equipment for blowing a sand mixture onto the pattern with
N Non-destructive Testing (NDT): Testing or inspection that does not destroy the object being tested or inspected. Nucleation:
1)
(Homogeneous).
The
initiation of solid crystals from the liquid stage, or a new phase within a solid without outside
interference-rarely
occurs.
2)
(Heterogeneous). Foreign particles altering
151
Appendix Glossary of Terms the liquid-solid interface energy during phase changes usually occurs.
Pattern Draft: The taper on vertical elements in a pattern which allows easy separation of pattern from compacted sand
O Oil Sands: Sands bonded with such oils as linseed and the synthetics. One-Piece Pattern: Solid pattern, not necessarily made from one piece of material. May have one or more loose pieces. Overheated: A term applied when, after exposure to an excessively high temperature,
mixture. Pattern Layout: Full-sized drawing of a pattern showing its arrangement and structure features. Permanent Mold: A metal mold of two or more parts; not an ingot mold. It is used repeatedly for the production of many castings of the same form.
a metal develops an undesirable coarse grain
Pilot Casting: Casting produced prior to
structure, but is not necessarily damaged
the production run to verify correctness of
permanently. Unlike burned structure, the
procedures, materials, and process to be used
structure produced by overhearing can be
in production.
corrected by suitable heat treatment, by
Polymer: A compound of high molecular
mechanical work, or by a combination of the
weight, in which the molecules are packed
two.
closely, with water, alcohol and the like
Oxidation: Any reaction of an element
eliminated.
with oxygen. In a narrow sense, oxidation
Polymerization: The hardening or setting
means the taking on of oxygen by an element
of plastic materials, as epoxy, urethane, when
or compound, and on the basis of the electron
the resin and hardener are mixed together.
theory, it is a process in which an element loses electrons. Overhang: The extension on the vertical
Polystyrene: A polymer of styrene used in making molding products. In particular, used in the lost foam process.
surface of a core print in order to provide
Polyurethane: Synthetic resin polymer
clearance for closing the mold over the core,
used for pattern material, ranging from dense
also known as “shingle”.
elastomer to expanded, spongy, lightweight. Porosity: Unsoundness in cast metals due
P Parted Pattern: A pattern made in two or more parts. Parting: Joint where mold separates to permit removal of pattern. Parting Line: A line on a pattern or casting corresponding to the separation between the cope and drag portions of a sand mold.
to blow holes, shrinkage cavities and voids. Post-processing: The processing of a prototype to remove extra material, finish the surface, and apply proper protection. Pouring: Filling the mold with molten metal. Pouring Cup: The flared section of the top of the downsprue. Process Capability: The amount of
Parting Sand: A bondless sand dusted on
variation in the product of a controlled
the parting surface to prevent the parts of the
manufacturing process, the range defined by
molds from adhering to each other.
plus or minus three standard deviations.
Pattern: A form of wood, plastic, foam,
Pyrometry: A method of measuring
metal, or other material around which
temperature with any type of temperature-
molding material is placed to make a mold.
indicating instruments.
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Rapid Tooling Guidelines For Sand Casting
Q Quickcast: A rapid tooling process developed by 3D Ssytems Inc. The Quickcast
part is required and computer numerical control technology is used to produce the master pattern or mold.
process utilizes a stereolithography (SLA)
Rapping: Knocking or jarring the pattern
model with a solid outer skin and a hollow,
to loosen it from the sand in the mold before
honeycombed interior. The model is dipped
withdrawing the pattern.
in a ceramic slurry, which is allowed to
Rapping Plate: A metal plate attached to
harden. When the ceramic shell is heated, the
a pattern to prevent injury to the pattern and
thin-walled SLA model disintegrates allowing
assist in loosening it from the sand.
a metal of the customer's choice to be poured
Rechucking: Reversing a pattern upon a
into the empty mold. After solidification, the
face plate in order to permit turning the
ceramic is broken away to reveal the metal
opposite face to the required shape.
casting which is then finish machined (adding threads and removing burrs) as necessary.
Recrystallization: A process whereby the distorted grain structure of cold-worked
Quenching: Rapid cooling for hardening;
metals is replaced by a new, strain-free grain
normally achieved by immersion of the object
structure during annealing above a specific
to be hardened in water, oil, or solutions of
minimum temperature.
salt or organic compounds in water.
Resinoid: Solid materials produced by the union (polymerization) of a large number of
R Ramming: Packing sand in a mold by raising and dropping the sand, pattern, and
molecules of one or more relatively simple compounds.
Resinoids
are
classed
as
thermosetting or thermoplastic.
flask on a table. Jolt squeezers, jarring
Rigging: Gates, risers, loose pieces, etc.,
machines, and jolt rammers are machines
needed on the pattern to deliver the metal to
using this principle.
the mold cavity and produce a sound casting.
Rapid Prototyping (RP): The physical
Riser: Reservoir of molten metal from
modeling of component or tooling geometry
which casting feeds as it shrinks during
using layered manufacturing. RP technologies
solidification.
make it possible to quickly generate polymer,
Riser, Blind: A riser that does not break
wax, metal, or paper-based prototype parts
through the top of the cope and is entirely
from
model
surrounded
(CAD)
atmosphere by means of a firecracker core.
three-dimensional
computer-aided
solid
design
by
sand;
opened
to
the
representations. Parts are typically generated
Riser-Gating: Practice of running metal
by building up one layer at a time, with the
from the casting through the riser to help in
thickness of each layer determining the
directional solidification.
accuracy of the part and the time required to make it. Rapid Tooling (RT): The use of RP or CNC
machining
Riser Neck: The connecting passage between the riser and casting. Usually only
technology to
quickly
fabricate tooling for near net shape processes.
the height and width or diameter of the riser neck are reported, although the shape can be equally important.
Depending on the process and production
Riser, Open: Conventional form of riser
volume involved, the mold or die can be
usually located at the heaviest section of the
made directly or a master pattern may be
casting and extending through the entire
produced that is then used to make the mold
height of the cope.
or die. In all cases, a CAD solid model of the
153
Appendix Glossary of Terms An
Shakeout: 1) The operation of removing
enlargement of the riser neck where it joins
Riser
Pad
(Riser
Contact):
castings from the mold. 2) A mechanical unit
the casting. The purpose of the pad is to
for separating the molding materials from the
prevent the riser from breaking into the
solidified metal casting.
casting when it is struck or cut from the casting. Rollover Board: A wood or metal plate on which the pattern is laid top face
Shaw
(Osborn-Shaw)
Process:
A
precision casting technique in ceramic molds, which does not require wax or plastic investment.
downward for ramming the drag half mold,
Shear Modulus (G): In a torsion test, the
the plate and half mold being turned over
ratio of the unit shear stress to the
together before the joint is made.
displacement caused by it per unit length in
RTV: Room Temperature Vulcanizing, a
the elastic range. Units are Pa or psi.
term describing ability of silicone rubber to
Shear Strength: Maximum shear stress a
set at ambient temperatures, as in flexible,
material is capable of withstanding without
synthetic rubber molds. There are two
failure.
families, the RTV-11, 60 and 88 and a newer high-strength RTV-662. Runner: The channel through which the
Shell Molding: A process for forming a mold from resin-bonded sand mixtures brought in contact with preheated (300-500 F) metal patterns, resulting in a firm shell
molten metal is carried from the sprue to the
o
gate.
with a cavity corresponding to the outline of
Runner Extension: In a mold, that part of a runner which extends beyond the farthest ingate as a blind end. Runout: A casting defect caused by incomplete filling of the mold due to molten metal draining or leaking out of some part of the mold cavity during pouring; escape of molten metal from a furnace, mold, or melting crucible.
the pattern. Shift: A
casting defect caused by
mismatch of cope and drag, or of cores and mold. Shrinkage: The decrease in volume when molten metal solidifies. Shrinkage Hole: A cavity in a casting due to the insufficient feed metal. Silica:
Silicon
dioxide,
the
prime
ingredient of sand and acid refractories. S Sand Mulling: A method of evenly distributing the bond around the sand grain by a rubbing action.
Slicing: Cutting 3D geometry into thin slice using a planar plane vertical to building direction is called slicing. Silicon: An abundant element, chemically
Sand Reclamation: Processing of used
classed as a nonmetal, metallurgically a
foundry sand grains by thermal, attraction, or hydraulic methods so that it may be used in
metal, used extensively in ferrous and nonferrous alloys; melting point 2593.4 oF
place of new sand without substantially
(1423 oC).
changing current foundry sand practice.
Simultaneous Engineering: Refers to the
Scale Measurement: Measurements taken
process where user/designer and producer
from a scale drawing or a model to ascertain
interact to reduce lead time and improve the
the true dimensions.
manufacturing efficiency of a part design.
Segment: A section of a circle , the ends of which are radial lines.
This process is faster and more efficient than the traditional sequential process of design and manufacture.
154
Rapid Tooling Guidelines For Sand Casting
Sintering: The bonding of adjacent
Enables the data representation of a CAD
surfaces of particles of a mass of powder or a
solid model to be directly converted into a
compact by heating to a suitable temperature
photo cured plastic model of a part or tool.
and cooling. Skeleton
STL format: A data format widely used Pattern:
A
framework
in rapid prototyping industry. STL is an
representing both the exterior and interior of
abbreviation for Stereolithography and was
the casting.
first proposed by 3D Systems Inc. The .STL
Skewed Tolerances: Tolerances which are nonsymmetrically
distributed
about
the
design intent.
file is now a de-facto standard interface between
CAD
packages
and
rapid
prototyping.
Skim Core (Skimmer): A flat core or tile
Stock Core: Core of standard diameter
placed in a mold to skim a flowing stream of
usually made on a core machine and kept on
metal. Commonly used in pouring basins.
hand, cut to required length.
Skin-Drying: Drying the surface of the mold by direct application of heat.
Stress, Residual: Those stresses set up in a metal as a result of nonuniform plastic
SLA: See Stereolithography Apparatus.
deformation or the unequal cooling of a
Slurry: A term loosely applied to any
casting.
clay-like dispersion. It may be used to wash
Striping Plate: A plate, formed to the
ladles or other refractory linings to impart a
contour of the pattern, which holds the sand
smoother surface.
in place while the pattern is drawn through
Snap Flask: One that has hinges and latches so that it may be removed from the mold prior to the pouring.
the plate. Styrene: See Polystyrene. Styrofoam: A proprietary name for
Soldiers: Wooden pegs used to reinforce a body of sand.
expanded polystyrene. Synthetic Molding Sand: Any sand
Splash Core: A core of tile placed in a
compounded
from
selected
individual
mold to prevent erosion of the mold at places
materials which, when
where metal impinges with more than normal
produce a mixture of the proper physical and
force. Splash cores are commonly used at the
mechanical properties from which to make
bottom of large rammed pouring basins, at the
foundry molds.
mixed together,
bottom of long downsprues, or at the ingates of large molds. Spline: A thins strip of wood to reinforce
T Template: A thin piece of material with
butt joints. Also known as “feather” or
the edge contour in reverse to the shape to be
“tongue”.
checked.
Split Pattern: A pattern that is parted for convenience of molding. Sprue: The opening into which the metal is first poured.
withstand prior to fracture. The ultimate tensile strength is calculated from the
Squeeze Board: A board used on the cope half of a green sand mold to permit squeezing of the mold.
maximum load applied during the test divided by the original cross-sectional area. Tessellation: Representation of a smooth
Stereolithography Apparatus (SLA): Equipment used for computerized building of three-dimensional
Tensile Strength: The maximum stress in uniaxial tension testing which a material will
models
and
patterns.
curvilinear CAD solid model surface with triangular shaped facets. Most RP processes
155
Appendix Glossary of Terms require tessellated surfaces which is a part of the .STL data format.
Tooling Points: The fixed positions on the casting surfaces used for reference during
Thermal Conductivity: The property of
layout and machining.
matter by which heat energy is transmitted through particles in contact. For engineering purposes, the amount of heat conducted
U Ultrasonic Testing: A nondestructive
through refractories is usually given in Btu
method of testing metal for flaws based on
per hour for one square foot of area, for a
the fact that ultrasonic waves are reflected
temperature
and refracted at the boundaries of a solid
difference
of
one
degree
fahrenheit, and for a thickness of one inch, Btu/hr - ft2 - 0F/in.
Urea
Thermal Shock: Stress developed by rapid and uneven heating of a material. Thermal
Spalling:
Breaking
of
repeated heating and cooling cycles. Stability:
Formaldehyde
Resin:
A
thermosetting product of condensation from urea or thio-urea and formaldehyde, soluble
up
refractory caused by stresses that arise during Thermal
medium.
Resistance
in water, and used as a sand binder in core and mold compounds. Urethane
of
a
material to drastic changes in temperature.
Elastomer:
A
resilient
rubberlike pattern compound which has excellent abrasion resistance. It is a castable
Thermocouple: A device for measuring temperatures by the use of two dissimilar metals in contact; the junction of these metals
plastic requiring a hardener to complete polymerization. Urethane Foam: See Polyurethane.
gives rise to a measurable electrical potential which varies with the temperature of the junction. Thermocouples are used to operate temperature indicators or heat controls.
V Vacuum Casting: A casting process in which metal is melted and poured under very
3D KeltoolTM: A rapid tooling process
low
atmospheric
pressure;
a
form
of
developed by 3D Systems Inc. that integrates
permanent mold casting where the mold is
the use of SLA master patterns, RTV
inserted into liquid metal, vacuum is applied,
intermediate molds, 3D KeltoolTM fused
and metal drawn up into the cavity.
powder metal inserts and standard MUD
Veins: A discontinuity on the surface of a
(Master Unit Die) frames to produce cores
casting appearing as a raised, narrow, linear
and cavity inserts.
ridge that forms upon cracking of the sand
Tight Flask: A type of flask which remains on mold during pouring. Lugs are normally provided for clamping cope and drag together for pouring. Alternative
during filling of die mold with molten metal. Vent: Small opening in mold to facilitate escape of air and gases.
Tongue: See Spline. Tooling
mold or core due to expansion of the sand
Vibrator: A mechanical device used to Selection:
the
comprehensive mental exercise involved in
loosen pattern from mold. Viscosity:
The
resistance
of
fluid
selecting the most efficient way to produce
substance
sand casting tooling. It generally includes the
characteristic for an individual substance at a
choice of tool fabrication method, material
given temperature, and under other definite
selection, and choice of tooling approach.
external conditions. W
to
flowing,
quantitatively
156
Rapid Tooling Guidelines For Sand Casting
Washburn Core: A thin core which
Yield: Comparison of casting weight to
constricts the riser at the point of attachment
total weight of metal poured into mold,
to the casting. The thin core heats quickly and
expressed as a percentage.
promotes feeding of the casting. Riser removal cost is minimized.
Yield Strength: The stress at which a material
exhibits
a
specified
limiting
permanent strain. X X-Ray: Form of radiant energy with wave
Young's Modulus (E): See Modulus of Elasticity.
length shorter than that of visible light, and with the ability to penetrate materials that usually produced by bombarding a metallic
Z Zircon: The mineral zircon silicate, ZrSiO4, a very high melting point acid
target with electrons in a high vacuum. In
refractory material used as a molding material
nuclear reactions, it is customary to refer to
in steel foundries.
absorb or reflect ordinary light. X-rays are
photons originating in the nucleus as gamma
Zirconia: ZrO4, an acid refractory up to
rays, and to those originating in the
4532 oF (2500 oC) having good thermal shock
extranuclear part of the atom as x-rays.
resistance and low electrical resistivity. Zirconium:
Y
Silvery-white,
metallic
element, mp 3380 oF (1860 oC), a powerful oxidizer and aluminum stabilizer, when added to molten steel.
References AIAG (1995). Measurement System Analysis (MSA) – Reference Manual, Automotive Industry Action Group, Southfield, MI American Foundrymen’s Society, Cast Metals Institute and Modern Casting Magazine. (1989). Metalcaster’s reference & guide. American Foundrymen’s Society, Inc. Des Plaines, IL. American Foundrymen’s Society (1986). Patternmaker’s manual. American Foundrymen’s Society, Inc. Des Plaines, IL. Amstead, B. H., Ostwald, P. H., Begeman, M. (1977). Manufacturing processes (seventh edition). New York: John Wiley & Sons, Inc. Beeley, P. R. (1972). Foundry Technology. New York: Halsted Press Division, John Wiley & Sons Inc. Blair, M. and Stevens, T. L. (1995). Steel casting handbook. Steel Founder’s Society of America. Bradney D. D. (1994). The NFFS guide to aluminum casting design: sand and permanent mold. The Non-Ferrous Founder’s Society, Des Plaines, IL. Burns, M.. (1993). Automated Fabrication. Englewood Cliffs, NJ PRT Prentice Hall. Cook, G. J. (1961). Engineering casting, New York: McGrawHill Book Company, Inc. Doyle, L. E., Keyser, C. A., Leach, J. L., Schrader, G. F., and Singer, M. B. (1969). Manufacturing processes and materials for engineers (second edition). Englewood Cliffs, New Jersey: PrenticeHall, Inc. 157
158
Rapid Tooling Guidelines For Sand Casting
Er, A., Sweeney, E. T., Kondic V. (1996). "Knowledge-Based System for Casting Process Selection", Transactions of the American Foundrymen’s Society, Des Plaines, IL. p 363-370. European Standard Draft prEN 190/130 (1995). Patterns, pattern equipment and coreboxes for the production of sand moulds and cores. European Committee for Standardization. Groover, M. P. (1996). Fundamentals of Modern Manufacturing: Materials, Processes and Systems. Upper Saddle River, NJ: Prentice Hall Guida, G. and Tasso C. (1994). Design and development of knowledge based systems, New York: John Wiley & Sons, Inc. Gustafson, R. (1999). "Rapid Prototyping: A Tool for Casting Design and Verification", Modern Casting. V 89 n 3. p 44-47. Gustafson, R., Guinn, E. and Tait, D. (1995). “Rapid Prototyping for Pattern and Foundry Tooling”, Modern Casting. V 85 n 2. p 4850. Heine, R. W., Loper, C. R., Jr. and Rosenthal, P. C. (1967). Principles of Metal Casting. New York: McGraw-Hill Book Company. ISO/CD 8062-2 (1995). Castings - System of geometrical tolerances. International Standard Organization. ISO/CD 8062 (1994). Castings - System of dimensional tolerances and machining allowances. International Standard Organization. Kennedy, C. W., Hoffman, E. G. and Bond, S. D. (1987). Inspection and Gaging. New York: Industrial Press Inc. Lascoe, O. D. (1988). Handbook of fabrication process. Metal park, OH: ASM International
References
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Marek, G. T. (1950). Fundamentals in the production and design of castings. New York: John Wiley & Sons, Inc. Maus, R., and Keyes, J. (1989). Handbook of Expert Systems in Manufacturing, New York: McGraw-Hill, Inc. Moore, W. H. (1969). "Shrinkage: a real problem to the Foundryman", Casting Engineering, Oct/Nov, p27-31. Peters, F., Voigt, R. and Blair, M. (1996). Dimensional Repeatability of Investment Castings. Proceedings of 9th World Conference on Investment Casting: San Francisco, California. p 22:1~22:10 Peters, F., R. Velaga, and R. Voigt. (1996). Assessing the Dimensional Repeatability of Metal Casting Processes. AFS Transactions, v 104, Des Plaines, IL. Peters, F. E., Voigt, R. C. (1995). Assessing the capabilities of patternshop measurement systems. Transactions of the American Foundrymen’s Society, v 103, Des Plaines, IL. p 207-213. Potter, L., Voigt, R., Peters, L., Lies, J. and Chandra, M. (1996). A statistically based pattern approval process. Transactions of the American Foundrymen’s Society, v 104, Des Plaines, IL. p307-316. Schuett, T. (1999). "Rapid milling for prototypes", Rapid Prototyping and Manufacturing '99 -- Advanced Product Development Technologies in Action, April 20-22, 1999, Rosement, Illinois. p. 167-178 Stoll, H. W., Conley, J. G., Wang, W. L., and Gustafson, R. (1999). “Tooling Path Selection for Sand Casting”, Accepted by Transactions of the American Foundrymen’s Society, Des Plaines, IL. Taylor, H. F., Flemings, M. C., and Wulff, J. (1959). Foundry engineering. New York: John Wiley & Sons, Inc.
160
Rapid Tooling Guidelines For Sand Casting
Voigt, R. & Metal Casting Program (1996). Assessing and Controlling Casting Dimensional Variability. The Pennsylvania State University, Department of Industrial and Manufacturing Engineering, 1996 Wang, W., Conley, J. G. and Stoll, H. W. (1998). "Dimensional Variability Analysis In Post-Processing Of Rapid Tooling", Proceedings of the 9th Solid Freeform Fabrication Symposium held in University of Texas at Austin. Wang, W., Conley, J. G. and Stoll, H. W. (1998). “Rapid Tooling Error Analysis for Sand Casting”, Transactions of the American Foundrymen’s Society, Des Plaines, IL. pp 567-571. Wang, W., Conley, J. G. and Stoll, H. S. (1999) "Rapid tooling for sand casting using laminated object manufacturing". Accepted by Rapid Prototyping Journal, MCB University Press, England. Vol 5 No 3 pp134-140.
Index Numbers
D
25mm projectile, 133 2D drawing, 48, 74, 100, 102, 104 2D CAD file, 102 3D Modeling, 48 3DP, see three dimensional printing
Data preparation, 48 Data status, 102, 113 Datum, 84 datum reference, 85 Datum points, 31, 48, Decision factors, 101 data status, 101 physical parts, 101 intermediate representation, 101 production volume, 103 prototype, 104 production, 104 part geometry, 104 pattern shop capability, 106 tool cost, 107 lead-time, 107 required accuracy, 107 tool durability, 107 Decision process, 113 data status, 113 geometry independent requirements, 114 geometry analysis, 114 alternative tooling proposals, 114 negotiate the final selection, 115 Decision space, 107 Decision structure, 107 decision constraints, 110 tradeoffs, 110 geometry considerations, 111 Decision variables, 99 fabrication method, 99-101 tool material, 101 tooling approach, 101 Decubing, 40, 50-51, 73-74 decubing error, 76-77 Dependent factors, 101 Die casting, 53, 56, 63 Dimensions, 82
A Accuracy, 69, dimensional accuracy, 69 tessellation accuracy, 75 Actual size, 87 Allowance, 23-27, 84 pattern allowance, 23
B Bridge tooling, 58-59 Build direction, 55 Building, 55 Building procedure, 53
C CAD, 15, 32, 33, 35 CAD model, 37, 38, 40 CAD modeling, 53 CAD software, 38, 102-103 Carrier housing 1, 125-126 Carrier housing 2, 135-136 Casting, 1 sand casting, 1 Caliper, 96 Ceramic powders, 46 Cleaning, 1, 4 CNC based tooling, 54-55 Cold gas dynamic spray, 68 Coordinate measuring machine (CMM), 97 Core, 3, 8, 20, 34, 101, 105 core making, 21, 70 core fabrication, 34, 101 core fabrication process, 34 Core box, 34-35 core box fabrication, 34-35 Core sand molding, 8 161
162 basic dimension, 82 reference dimension, 82 specified dimension, 82 nominal dimension, 82 Dimensional accuracy, 4, 7, 9, 17, 64, 69 Dimensional error, 75-77, 89 Dimensional metrology, 88 Dimensional tolerance, 83, 87 Dimensional variability, 69 Direct AIM™ , 55, 58 Discretizing, 37 Draft, 27-28 Drawing interpretation, 48 Dry-sand molding, 7
E Errors, 69 conversion error, 74 decubing error, 76 orientation error, 76 post-finishing error, 77 process parameter error, 76 random error, 69, 89 system error, 69, 89 tessellation error, 74 time dependent error, 77 Error sources, 69-73 ExpressTool, 68
F Fast freeform fabrication, 33, 37-48 3DP, 45 FDM, 41 LOM, 39 materials, 39-48 methods, 39-48 processes, 39-48 SGC, 44 SLA, 38 SLS, 42 SP, 46 FDM based RT, 66 Feature control, 84 Fishbone diagrams, 70-74 Fused deposition modeling (FDM), 41
G Gage, 95 Gage R&R, 95 Geometry considerations, 28, 111
Rapid Tooling Guidelines For Sand Casting fillets, 28 undercuts, 29 loose piece, 29 ribs, 29 lugs, 29 bosses, 29 Geometric dimensions and tolerances, 81 Geometric symbols, 85 Geometric tolerance, 83 Green sand molding, 4-7
H
Hard tooling, see KelToolTM Hitch housing, 121
I Ice cleat, 138 Independent factors, 101 Injection molding, 57, 63 Inlet boost pump, 127 Investment casting, 57, 59-60, 66 ISO, 87 ISO 8062, 87 ISO/CD 8062-2, 87
J K
KelToolTM Process, 55, 56, 58
L Laminated object manufacturing (LOM ), 39 Least material condition (LMC), 86-87 Load material, 49 LOM based RT, 59 LOMPapersTM, 40 Lost paper process, 59 Lower specification limit (LSL), 92
M Manifold, 128 Master core, 35-36 Master pattern fabrication, 30 manual, 31 CNC, 32 fast freeform fabrication, 33 Machine parameters, 49 Machine setup, 49 Maximum material condition (MMC), 86
163
Index Measurement system analysis, 95-98 Melting, 1, 3 Metal prototype, 59-61 Micrometer, 96-97 Model repair, 51 Mold dimensions, 80 Molding, 1, 3 Mold making process, 4 core sand molding, 8 dry sand molding, 7 green sand molding, 4 shell molding, 8 Muzzle brake, 136
N Nickel-ceramic composite (NCC) tooling, 67
O P Paper, 23, 33, 39-41 Parting line, 1, 28 parting line complexity, 106 Parting plane, see parting line Pattern Allowance, 23 shrinkage allowance, 23 distortion allowance, 25 machining allowance, 26 Casting finish allowance, 26 Pattern, 15 pattern design, 20 pattern fabrication, 30 pattern type, 15 Patternmaking, 1, 2 Pattern design, 20 Pattern Materials, 20 wood, 20 metal, 20 plastic, 21 styrofoam, 22 fast freeform fabrication, 22 Pattern Type, 15 single pattern, 16 loose pattern, 16 gated pattern, 17 match-plate pattern, 17 cope and drag pattern, 19
special patterns and device, 19 Photo-reactive polymer, 44 Photopolymer, 39, 54 Plastic, 59 Plastic prototypes, 61 Polysteel, 67-68 Portage machine, 97 Post processing, 37, 48, 50, 100 Pouring, 1, 3 Precision, 88 Process capability, 92 ProMetal rapid tooling, 66-67 ProtoBuild, 47
Q
QuickCastTM, 58-59
R Random error, 69 Rapid prototyping (RP), see fast freeform fabrication Rapid tooling (RT), 53, 73 Repeatability, 88, 95 Reproducibility, 88 Rigid connecting link, 132 Roller bracket, 139 Room temperature vulcanizing (RTV), 56 Rubber plaster casting, 61 Rubber tooling, 56
S Sand casting, 1-10 Sand casting process, 1, 70 Sanders prototype (SP), 46 Select materials, 49 Selective laser sintering (SLS), 42-44 Sensor box, 117 Set parameters, 49 Sheet vacuum forming, 62 Shell molding, 8 Shrinkage, 77 Silicone rubber molding, 57, 61 SLS based RT, 63 Soft tooling, 54 Solid ground curing (SGC), 44 Solidification shrinkage, 78 Spray metal tooling, 62 Stereolithography, 38 Stereolithography apparatus (SLA), 38 STL, 38 Stock size, see nominal dimension
164 Surface finish, 51 System error, 69
T Tessellation, 48-49 Tessellation error, 74 Three dimensional printing (3DP), 45 Thin wall, 51 Tolerances, 81 Tool, 11 Tool design, 11 Tool construction see Tool fabrication Tool fabrication, 13 manual, 13 CNC, 14 fast freeform fabrication, 14 Tooling alternatives, 99, 108 Tooling path review form, 119 Tooling path selection process, 115 Two part negative tooling, 60 Turbine blade machining fixture, 129
U Ultrasonic machine, 98 Upper arm, 130 Upper specification limit (USL), 92
V Vacuum molding, 62 Volume contraction, 78
W Weight tolerance, 84
X Y Yoke, 123
Z
Rapid Tooling Guidelines For Sand Casting
