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INSIGHT February 10, 2002 - Metalcasting processes can be the most effective methods for producing metal parts in terms of material use and labor cost. However, the long lead-time between purchasing and product approval can offset these advantages: a result of the need for a pattern to create the mold. The pattern design process often requires iterative trialand-error prototyping since gating design, usually described as an art, is often based on empirical, mostly undocumented, experience. Rush to Market Timing is essential-a delay can result in lost market share and may cause a potentially successful product to fail. Companies introducing new cast parts must achieve a combination of efficient design, concurrent engineering, and just-in-time (JIT) production. Once a casting prototype has been tested, the race is on to deliver it to the market before the competition. Computers play a major role in expediting time-to-market for cast parts. Lasers and Computer Technologies known as rapid prototyping (RP), solid free-form fabrication, and rapid manufacturing, allow foundrymen to fabricate 3- D models, prototypes, patterns, tooling, and production parts directly from CAD data in a fraction of the total time and cost of conventional methods. Practical systems are commercially available. Rapid prototyping systems in commercial use include: Fused Deposition Modeling (FDM), Stratasys Inc., Eden Prairie, MN; [no longer in business] Stereolithography (SL) and Selective Laser Sintering (SLS), 3D Systems, Valencia, CA; 3D Printing (3DP) from Z Corp., Burlington, MA; and Laminated Object Manufacturing (LOM), Cubic Technologies, Carson, CA.. Each technology shares the same basic approach: a computer analyzes a CAD solid model that defines the object to be fabricated and "slices" the object into thin cross sections. The cross sections are then systematically recreated and combined to form a 3D object. SL recreates the object by sequentially solidifying layers of photoactive liquid polymer by exposing the liquid to ultraviolet light. In SLS, a layer of powder is deposited in a build chamber, and a laser sinters the material to form the first cross section of the object. Another layer of powder is deposited, sintered, and bonded to the previous layer. This process is repeated until the model is complete. The unused powder remains in the container during the process, serving as a natural support for the model; it is discarded when the process is finished. LOM builds 3-D objects from thin layers of sheet materials, and the FDM process extrudes a heated plastic filament to conform to the desired shape. Benefits include greatly reduced prototyping cost and design time, and the ability to achieve part geometry in one operation that would otherwise require multiple steps to produce. Rapid prototyping allows participants in simultaneous engineering projects to evaluate designs at an early stage. Engineers can make changes to correct design errors, improve manufacturability, cut cost, and optimize performance. The Great Payoff * Speed: Methods of rapid prototyping, in some cases, can produce a model in a matter of hours, thereby shortening the design cycle. The techniques are inherently fast and can potentially be scaled up to produce objects with dimensions of up to about two feet. * Cost factors: Lower costs associated with product design are another advantage offered by RP methods. The cost of a simple model created by such techniques is frequently a fraction of the cost of a model created by conventional means. * Materials: The ability to use a multiplicity of materials is the strong suit claimed by developers of some of the rapid prototyping technologies. Nylon, ABS and some of the newer photopolymers permit functional durability testing. Investment casting wax and and other foundry-friendly materials are also commercially available. * Expendable Patterns: Efforts to use rapid prototypes as expendable patterns failed in the past because of the high thermal expansion characteristics of the material. Patterns made using standard rapid prototyping techniques caused cracked or broken molds during burnout. Material developments and altered building techniques can produce patterns that bum out without cracking most shells. Eliminating the preliminary tooling stage in wax pattern production translates into numerous benefits for investment casters. The process minimizes up-front tooling costs, saves a significant amount of time, and is capable of producing complex shapes-including those with hollowed areas, undercuts, and other difficult-to-tool areas and surfaces. The ability to obtain expendable patterns before tool construction allows foundries to reduce lead times dramatically on sample castings. These patterns are best for short run applications where tooling is time or cost prohibitive. Foundries also can use these expendable patterns to confirm gating techniques and shrink factors early in the process development cycle. * Marketing Tools: Rapidly produced patterns and models can also serve as effective, hands-on marketing tools and sales samples. Rapid prototyping processes can produce a sample quickly and inexpensively enough to accompany an estimate or quote. * Gating and Tool Design: Physical models obtained before bidding new jobs or while tooling is being constructed can be invaluable tools for developing gating and tree design. A physical prototype can allow tool designs to be optimized quickly so that mistakes and rework are minimized. Patterns for sand casting are another potential application of these laser-based technologies. * Masters for "Soft Tooling:" Parts made by rapid prototyping technologies can be used as masters for soft tooling (silicone rubber, epoxy, plaster, and spray metal) from which short runs of wax patterns can be made. Sheet Materials SLS, SL, and FDMallow designers to produce complex three- dimensional models from a CAD model without any tooling. In fact, these processes can manufacture parts that are impossible to produce using conventional machining techniques. Also, if the designer does not like a prototype, it is easy to alter a CAD model and build a new prototype. Direct Shell Production Casting (DSPC) The DSPC machine is not available for purchase, although Soligen, Northridge, CA, uses it to produce metal castings for its customers. This process is one in which the ceramic shell-complete with integral coresis manufactured automatically and directly from a CAD file without tooling or patterns. It is a turnkey process that allows parts designed on a computer to be fabricated by simply pouring molten metal into a ceramic shell produced by the DSPC process. It provides a complete solution, from customer's design data acceptance through mold design and production. Because it eliminates the need for a wax pattern and since there is no need for tooling or setup for creating the ceramic molds, DSPC combines the advantages of casting and computerized numerical control (CNC) machining in a unique process for fabricating metal parts. The SLS process is compatible for use with actual investment casting wax. Many manufacturing companies have used SLS to produce polystyrene casting patterns for countless metal castings. An RP technology that has direct metalcasting applications is astereolithography building process called QuickCast from 3D Systems. The process uses an epoxy resin and a special build style that works well as patterns for investment castings. Instead of building a part that is solid plastic, a part constructed by the QuickCast method is two-thirds hollow internal lattice structure, which permits the plastic to collapse upon itself during pattern burnout instead of expanding and cracking the shell mold. FMT RP's Benefits It is important to remember that improving and shortening product development are not the only benefits to be gained by using RP early on in the production process. To fully reap the benefits, foundries should consider the following points. * Every prototype should be aimed at a specific question that needs answering: When RP was expensive and slow, engineers could only afford to use prototypes that tested several ideas at once. Now they can afford to test ideas individually, then mix and match later. Consequently, to assimilate this new behavior, explicitly plan to use each prototype to test only one idea or assumption. It's good scientific practice to test hypotheses independently, and now engineers can afford to do it. *Prototypes should be only elaborate enough (strength, surface finish, etc.) to answer the question: In later stages of development, where models have traditionally been cost justified, designs were mature and well refined, so refined models seemed appropriate. When models are used early in the product-development process, such refinement is wasteful. Engineers should build prototypes with just enough detail to answer the specific question at hand. When you have this answer, toss the prototype away and move on. This prevents money and time from being wasted on polishing models that won't move the project forward. * If you think of multiple alternatives, build multiple prototype\s in parallel: Everyone is aware of the speed advantages gained by proceeding on multiple activities in parallel. Now it can be done affordably with models. If designers or engineers think of alternative ways to solve a design problem, they can make prototypes of each option rather than presupposing the best solution and modeling only one. With prototypes of the alternatives, new combinations may become more apparent. * Make decisions as questions are answered. Don't wait until the final prototype appears: This is probably the most difficult habit to change, but it is also the most crucial. This change will affect others in the organization, such as marketers, people on the shop floor, and top management. * The trick to progressing quickly with RP is to move forward incrementally with small but sound steps. But if the final decision makers continue to wait until the final "perfect" prototype appears before making any commitments, there will still be a lot of second guessing, indecision, and rework. And there will be no real improvements in the process. * The faster you can make prototypes, the faster you can develop the product: Making the best use of RP is predicated on making small but sound steps quickly. Thus, the process accelerates to the extent technology will let us make and assess models more quickly. But it won't work if a foundry still makes conceptual prototypes by sending them out for processing at service bureaus, still ships prototypes to decision makers the same way they used to, or if management still takes as much time to make decisions as before. Success depends on the ability to shorten the iterative product-development loop. FOUNDRY MANAGEMENT & TECHNOLOGY Articles with More Information About Rapid Prototyping * "Optimizing Casting Simulation," David C. Schmidt, December 2001, p. 34. * "Reaping RP's Benefits," Virginia D. Cahill, July 2001, p. 20. * "Building Better Bones," August 2000, p. 82. * "Aluminum Diesel Vies to Power Drone Aircraft," May 2000, p. 40. * "Software Savvy," April 2000, p. 20. Edited by Terry Wohlers, Wohlers Associates Inc., Fort Collins, CO. Wohlers is an industry consultant, author, and analyst in addition to being president of the independent consulting firm he founded in 1986. He can be reached at twohlers @compuserve.com or visit his website at www wohlersassociates.com. Did you find this material interesting? Do you want more information of this type? Comment via FEEDBACK
Source: Foundry Management & Technology
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