Over the past 25 years, the CAE/CAD/CAM industry has developed technological advances that
have provided industrial users the ability to increase productivity and decrease the cycle time necessary
for product development. These technologies include basic wireframe and surface design, specializedapplication
software packages, finite element analysis, numerical control, solid modeling, and rapid
prototyping.
Each of these technologies plays a significant role in industry today. The Defense Systems & Electronics
Group of Texas Instruments currently uses these technologies in the mechanical design engineering
process. This paper discusses the two specific technologies of solid modeling and rapid prototyping
(specifically stereolithography), including their advantages, benefits, and practical applications within the
Texas Instruments Defense Systems & Electronics Group. This paper also discusses the use of
stereolithography (SLA) rapid~prototype parts within the solid-mold investment~casting process.Mechanical Engineerin

Baumgardner, Owen

Blake, Paul

English

Texas ScholarWorks

SOLID MODELING AND STEREOLITHOGRAPHY AS A SOLID FREEFORMFABRICATION TECHNIQUE AT TEXAS INSTRUMENTS INCORPORATEDBy Owen Baumgardner and Paul Blal<eTexas Instruments IncorporatedI. INTRODUCTIONOver the past 25 years, the CAE/CAD/CAM industry has developed technological advances thathave provided industrial users the ability to increase productivity and decrease the cycle time necessaryfor product development. These technologies include basic wireframe and surface design, specialized-application software packages, finite element analysis, numerical control, solid modeling, and rapidprototyping.Each of these technologies plays a significant role in industry today. The Defense Systems & Elec-tronics Group of Texas Instruments currently uses these technologies in the mechanical design engineer-ing process. This paper discusses the two specific technologies of solid modeling and rapid prototyping(specifically stereolithography), including their advantages, benefits, and practical applications within theTexas Instruments Defense Systems & Electronics Group. This paper also discusses the use ofstereolithography (SLA) rapid~prototype parts within the solid-mold investment~casting process.II. SOLID MODELINGSolid modeling is an advanced design capability that offers the mechanical design engineer numer-ous advantages over traditionalwireframe design methods. Visual and analytic capabilities are improvedand required inputs for rapid prototyping are provided.As its name implies, solid modeling represents an object as a solid part. These parts are typicallyrepresented as shaded images on.the computer screen (Figures I and 2). On the other hand, wireframedesign systems represent edges as linesa~dfaces as surfaces. As the complexity of a wireframe designincreases, the ability to visualizeandund~rstand the design decreases. Figure 3 demonstrates the visualambiguity that wireframe designs create; solid modeling alleviates this ambiguity. Solid modeling also isable to generate cross sections or cutaway views with minimal effort (Figure 4). These capabilities allowthe design engineer to visualize and understand what the part looks like and how it functions within anassembly by clearly displaying the critical relationship of part layouts, interferences, and clearances.The numerical and material information intrinsic to a solid model offers two unique capabilities.First, the solid model permits the engineer to derive mass property data (weight, center of gravity,moments of inertia, surface areas, etc.) of parts and assemblies (Figure 5). This eliminates the need forhand calculations to estimate the data from drawings or sketches. Second, the mechanical integrity of thedesign may be investigated using various analysis methods to determine structural, thermal, kinematic,and aerodynamic properties. These capabilities allow the engineer to design, analyze, and redesign on theCAE computer, using the same solid model. Automating the design process helps reduce the need for, butdoes not eliminate, the costly iterations of fabrication and physical testing of the hardware.Finally, solid-model data bases serve as direct input to rapid~prototyping systems. (It should benoted that surfaced-part data bases may also be used as inputs.) The input is in the form of a tessellation,or "tile" file, that is generated from the solid model or surface data base. The tessellation file is atriangular extraction of the surfaces of the solid model (Figure 6).171The build process is directed by a computer-controlled optical scanning system. It moves a laserbeam in the X and Y direction while drawing an image of the part on the photopolymer surface. Thephotopolymer solidifies wherever the laser beam strikes it. After the laser is finished drawing, a verticalelevator, moving along the Z axis, lowers the newly formed layer into the vat of liquid resin. A recoatingblade is used to establish the next layer's thickness. Succes~ive layers are built one on top of another toform a completed "green" part. Part·build time varies as the part size and complexity varies. Typically,part build time ranges from 1 to 2 hours for small and simple parts, to upwards of 60 hours for largecomplex parts.The green part is cleaned by stripping excess resin with alcohol, then hardened by exposure toultraviolet light in a postcuring apparatus. After curing, the supports are removed from the part and, ifnecessary, a light sanding is applied to improve the overall appearance. Required part cleanup time alsovaries from 1/2 hour to 2 hours per part.Part size is limited by the volume of the vat (10 X 10 X 10 inches). The vat contains 7.8 gallons ofliquid photopolymer resin. The laser is a helium-cadmium-powered device that generates a beam0.010 inch in diameter with a wavelength of 325 nanometers at a maximum power of 28 milliwatts. Theoptical scanning system is controlled by a DOS-based 386 computer, while the "slicing" takes place on aUNIX-based 386 computer.n: BENEFITS OF SOLID MODELING AND RAPID PROTOTYPINGUse of solid modeling and the SLA rapid-prototyping process have resulted in numerous benefitsfor the entire design engineering community at Texas Instruments. These benefits include increasedvisualization capability, decreases in cost and cycle time associated with fabrication of prototype parts,and increased ability to calculate mass properties and detect design flaws before hardware fabrication.The first benefit of these two technologies, and perhaps the most obvious one, is the enhancedvisualization capability. Engineers, designers, technicians, and managers now discuss designs by lookingat three-dimensional shaded images and full-scale plastic prototype parts, not two-dimensional drawings.The SLA prototype parts and assemblies are also used for fit-check verification (where tolerances permit),assembly methods and tooling planning, cable routing and weight reduction studies, customer communi-cation models, and vendor quoting tools.Another benefit derived from the use of rapid prototyping is the decrease of cycle time required toproduce prototype parts. The current SLA prototype turnaround cycle, defined as the time differencebetween data-base receipt to part fabrication completion, is between 2 and 10 days, depending on engi-neering requirements. This is a dramatic decrease over conventional machining methods, which typicallytake months. Therefore, the engineering team may visually, dimensionally, and functionally inspect andreview the critical and high-risk parts and assemblies earlier in the design phase of the overall designcycle. This benefit also results in the increased ability of the design engineer to explore, accept or reject,and integrate new design ideas into the existing design. Another cycle-time benefit is the increased timeavailable to explore the potential use of the less expensive casting process for part fabrication overtraditionally more expensive machine processes.Texas Instruments has experienced typical cycle-time reductions of 2 weeks for simple parts and20 weeks for complex machined or cast parts in the fabrication of prototype parts. Also, customer con-cept communication models have been fabricated for half the cost and in half the time of conventionalmethods. Specific examples of cycle-time and cost reductions are found in Figures 7 through 10.The third benefit of solid modeling and SLA rapid prototyping is the cost reduction and avoidanceassociated with eliminating design flaws early in the design cycle. The product development and fabrica-tion cycle is constantly at the mercy of continuous requirements and design changes. These continuous174Figure 6. Tesselation Tile Filechanges, coupled with the inability to easily verify overall part layout configurations, contribute to partassembly problems, s~chasinterferences and misaligned hole patterns. Often, these problems were notfound until piece parts were fabricated and assembled. Costs rose as parts were scrapped or modified andthe amount of required touch labor increased to assemble and disassemble the piece parts. Engineeringchange notices had to be incorporated into the design, significantly adding to the overall cost.Solid modeling permits engineers to visually inspect the designed piece parts and assemblies formating hole patterns and part clearances. As the design is completed and verified, the engineer may usethe SLA rapid-prototypingprocess to build a three-dimensional model as a final review before commit-ting the design to fabrication. These capabilities serve as cost-avoidance steps that help meet properfabrication and assembly requirements.The fourth benefit, specifically attributed to solid modeling, is an increased ability to calculatemechanical properties of the parts and assemblies throughout the design process. For example, weight is acritical design requirement. Solid-modeling mass-property calculations assist the engineer by calculatingpart and assembly weights. This information allows the engineer to optimize the design for weightrequirements before part fabrication. Weight calculations using solid modeling have typically been within± I percent of the actual part weight.V. STEREOLITHOGRAPHY AND INVESTMENT CASTINGA significant and immediate impact on the cost and cycle time associated with the investmentcasting process has been achieved by incorporating the SLA part into the process. The ability to use theSLA-modeled part as a wax substitute is the. key to supplying castings for low-production runs withoutthe high cost of foundry pattern tooling. In addition, providing machine shops with a near net-shapedpart reduces the number of required machining steps to produce a completed part.175MATERIAL: 6061-T6TIME REQUIRED: 104 HOURS*CYCLE TIME: 4 MONTHSFigure 7.Picture of Machined Part (Motor Housing)OUTSIDE QUOTE: $10,000 AND 2 MONTHS CYCLE TIMETIME REQUIRED: 75 HOURS*CYCLE TIME: 1 MONTHFigure 9. Picture of SLA Model (Missile)TIME REQUIRED: 54 HOURS*CYCLE TIME: 8 WEEKSLESSONS LEARNED:1. PRIMARY MIRROR HAD INTERFERENCEWITH GIMBAL IN AZIMUTH SWEEP.2. SECONDARY MIRROR WAS ST~ENGTHENED IN ITSATTACHMENT LEGS.Figure 10. Picture of SLA Volume Model(Seeker Assembly)*TIME REQUIRED IS FOR SLA RAPID-PROTOTYPE TOUCH LABOR ONLY, NOT MACHINE RUN TIME.176There are two types of investment casting processes: solid mold and shell. They differ only in themethod used to form the ceramic mold. Both processes require a pattern, gating to a central sprue,ceramic mold (either solid or shell), removal of the pattern by melting, pouring metal into the cavity leftby the melted pattern, removal of the mold material from the cast cluster, and cutting of the castings fromthe sprue.Currently, the only successful procedure to use the SLA part as a wax pattern substitute for invest-ment casting is the solid-mold process. The SLA part expands more than the traditional wax pattern as itis heated to be melted from the mold. This increased expansion causes the SLA part pattern to crack theweaker mold in the shell process, but not the metal-reinforced mold in the solid-mold process.The ceramic solid mold is created by placing the SLA part, with the gate and runner systemattached, into an open-ended metal flask. Investment slurry is poured into the flask, completely sur-rounding the SLA part with its gate and sprue system. Before the binder in the slurry solidifies, the flask isplaced under a vacuum to remove all entrapped air. When the investment is set, the SLA part pattern ismelted out of the mold and the process is completed by following the same procedures used for anynormal investment casting.There are four classes and grades for castings: Classes 1, 2, 3, and 4 and Grades A, B, C, and D. Theclass is established by the engineer according to design requirements for functionality, safety, reliability,and mechanical properties. The grade is also established by the engineer; it is used as the accept/rejectcriterion for radiographic inspections. Materials available for the investment casting process include allferrous and nonferrous alloys.Texas Instruments has been successful in pouring these nonferrous alloys to date: A356-T6,356-T6, A357-T6, 357-T6, A20l-T7, and an MMC of A357-T6 with 20-percent silicon carbide. Typi-cally, the castings have been Class 3 and Grade C with a cycle time of 4 weeks. This is a 4-month decreasein time required to fabricate a casting. The class and grade of the SLA castings meet the engineeringrequirements of Texas Instruments. See Figure 11 for examples of parts cast using this process.VI. GUIDELINES AND PRACTICAL LIMITATIONSFive practical guidelines can be applied to assist in the implementation of solid modeling and SLArapid prototyping in the industrial environment. First, there is no substitute for good engineering.CAE/CAD/CAM, computers, and rapid prototyping assist engineers in performing their tasks. Thesedesign tools cannot replace engineering judgment, discipline, design creativity, and practical design appli-cations. Solid modeling and SLA rapid prototyping are not panaceas, but are excellent tools to assistengineers in accomplishing their tasks.Figure 11. Stereolithography Parts With Casting177Second, as with all CAE/CAD tools, there is a learning curve associated with solid modeling. Toreceive maximum benefit, sufficient time should be allowed for the user to become acquainted with thecapabilities and intricacies of the software.Third, designing with a solid-modeling system is the most effective way to take advantage of rapidprototYl'ing. .lfdesigns are m.aintaiJlediJlwirefraI11el11()de and anSLA rapid prototype part is desired,parts must be remodeled or converted into an acceptable data-base format. This adds to the costcycle time required to produce a rapid-prototype part.Fourth, the solid-mold casting process will eventually be supplanted by the current traditional Shell-mold investment casting process. As the chemical, casting, and rapid-prototyping industries are success-ful developing an acceptable rapid"prototype part for the shell-mold investment process, therequirements of the solid-mold process will diminish.Finally, the rapid prototyping process does have limitations which need to be balanced againstthe part requirements. These limitations include: surface finish, tolerance, wall thickness, and featureconfigurations.• Surface finish -The general finish is acceptable for engineering evaluation requirements. How-ever, for a display model, surface-finish flaws are noticeable at 5 to 6 feet, but may be cor-rected by using a fiberglass filler, sanding, and painting.• Tolerance-Typical part dimensions can be held to ± 5 mils per inch. Tighter tolerances canbe controlled with experimentation. NOTE: Part accuracy is controlled by part configuration.• Wall thickness-As a general rule, 50 mils is the minimum wall thickness that can betively produced. Thinner sections may suffer distortion during the postcuring cycle.• Feature configuration ..... Part position and orientation should be aligned with the importantengineering design features for the best possible surface finish and accuracy. The layering, orstaircase, effect dictates thatall critical part features be built in the X and Y plane of themachine, not in the Z plane.Solid modeling and rapid prototyping offer the mechanical design engineer a more co~~t..ette(~tr\i'emeans of designing and building prototype parts than do traditional methods. Texas Instrumentsexperienced productivity increases as these processes have been used in the design and fabrication areasof the product-development life cycle. Both solid modeling and rapid prototyping will play importantroles in the future as companies strive to decrease design costs and cycle times.178

Solid Modeling and Stereolithography as a Solid Freeform Fabrication Technique at Texas Instruments Incorporated

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Solid Modeling and Stereolithography as a Solid Freeform Fabrication Technique at Texas Instruments Incorporated

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