Rapid prototyping: a young technology evolves.

Technological advancements and a growing understanding of its metalcasting possibilities is moving rapid prototyping toward more widespread foundry utilization.

By its most basic definition, rapid prototyping is the use of various additive processes to produce a solid, three dimensional physical model directly from computer aided design (CAD) data. For the purposes of this article, this precludes computer numerical cutting (CNC), which is a subtractive process subject to its own set of parameters (and worthy of its own article).

The first rapid prototyping system, called stereolithography, was patented in 1986 by Charles Hull. The first commercialized unit, the Stereolithography Apparatus (SLA-1) was sold in 1987. Since then, a number of competing methods and machines have entered the market.

Though only a decade old, rapid prototyping is undergoing a transformation in application, as well as evolving in process. For foundries, this transformation means the technology is coming to have a more direct usefulness in slashing lead times and part development costs. This article details advances in the use of rapid prototyping for pattern and foundry tooling, and lists specific attributes of some of the major systems.

Evolving Applications

In its original conception, rapid prototyping existed to provide engineers with models for visual inspection and form, fit and function testing, without having to wait the considerable length of time for prototype parts to be conventionally fabricated. As such a design tool, rapid prototyping proved a great success.

However, the main thrust of rapid prototyping may be changing. Graham Thomas, Rover Group, and David Wimpenny, Warwick Manufacturing group, wrote in U.K.-based Rapid News Magazine that improvements in the on-screen capabilities of CAD packages will soon be reducing the need for physical model production. "It is now widely recognized throughout industry," they said, "that the future of rapid prototyping lies in the area of rapid tooling."

This assessment may be good news for foundries, who stand to benefit even more from rapid tooling than the production of a simple model. "Rapid prototyping needs to go further toward actual tool building," said Paul Carey, Ashland Chemical Co. "Foundrymen don't want models, they want hard tools you can cast from."

Rapid prototyping's role in manufacturing has already been expanded by a fuller understanding of its potential in the various phases of casting production. Take patternmaking, for example.

Traditionally, the patternmaker has used two-dimensional prints with cross-sectional views to help define the component and determine transitions, variable wall thicknesses and internal passages. This manual process is not only time consuming, but can lead to discrepancies from the patternmaker's use of "liberties" or "fudge factors."

With the use of CAD and any of the additive processes, the guesswork of patternmaking is no longer an issue because rapid prototyping will build the model to net shape. Ronald Gustafson, Clinkenbeard & Associates, Rockford, Illinois--a 23-employee rapid prototyping service bureau with a full traditional pattern shop--said, "Traditional pattern makers--who are still extremely skilled and knowledgeable--find they are now doing more of the finishing work on patterns."

Both the CAD data and the rapid prototyping model are valuable in the front end phase of casting development. "We're finding more and more people willing to build rapid prototype mock-ups," Gustalson said. "In the case where the foundry doesn't see the CAD file, the mock-up is helpful, allowing the foundryman to see it for gating--and the same with the traditional patternmaker."

Larry Andre, Solidiform, Inc., Fort Worth, Texas, said that in that foundry's extensive experience with the technology, "Rapid prototyping allows an engineer to maximize his capabilities by maturing the part on the front end to get the most accurate tooling available in the shortest amount of time. And with tooling, there are three things to keep in mind: accuracy, accuracy, accuracy!"

With the CAD file, the foundry can create the master pattern, cope and drag, and define the parting lines directly on the CAD model. The foundry engineer can then use various mold fill and flow analysis programs to determine the gating, risers and metal flow. This information is then added to the CAD model prior to rapid prototype simulation.

"When coupled with solidification and heat transfer modeling," said Joe Santner, AFS, Urapid prototyping lets you ensure quality. OEMs are using rapid prototyping more than foundries because they know how the part is going to be used, and know exactly where the stresses are. Once you determine the heat transfer, you can begin predicting metallurgy, which transfers into mechanical properties. That tells you the hot spots--the stresses. They then can tell the foundry how it should be cast. We're really talking about concurrent engineering in its highest form."

Investment casting has been the most successful application for rapid prototypes as tooling. Several of the methods produce "sacrificial" parts that can be used to form investment casting shells. Several systems have the capability to fabricate the part out of investment casting wax or other materials that behave similarly within the parameters of a typical investment casting operation. One, Direct Shell Production Casting (DSPC), goes so far as to build an investment casting shell out of colloidal silica, directly from the CAD file so that molten metal can be poured right into the shell and a metal casting is the direct product.

But what of rapid tooling for sand casting? For sand casters, maximizing the technology means receiving the CAD file, making the appropriate modifications, sending it to the rapid prototyping machine and in a matter of hours having a pattern for use directly against sand. Because shrinkage factors for both the tooling material and the cast metal can be incorporated within the CAD file and gating can be modeled in, in many cases rapid prototype tools can be coated with a release agent and used as patterns for limited production runs. The amount of castings needed determines how the rapid prototype pattern is used--whether used loose for A few parts or mounted to a matchplate for medium runs. For high-volume runs, a rapid prototyping master can be used to cast metal tooling.

In this way, it is also possible to be simultaneously casting from a temporary prototype while a traditional foundry pattern is being constructed for permanent use.

Solidiform's Experience

Solidiform, Inc., Fort Worth, Texas, is a 40-employee aluminum foundry using chemically bonded nobake sand molding and investment casting for small to medium-sized runs, characterizing itself as an aerospace and commercial production foundry. The foundry uses rapid prototyping--its own SLA 250/40 machine and a variety of other processes provided by outside service bureaus--to make production tooling for its investment and sand casting operations.

While Solidiform's rapid prototyping experience began using: 3-D's QuickCast for investment casting, "sand is coming on stronger," vice president Larry Andre said, "because the pattern isn't disposable. Increasingly, customers are coming to us and asking which process will best serve their needs."

In the four years since its first venture, the foundry has used rapid tooling to produce more than 2500 investment casting: parts in 450 different configurations, and 3500 sand casting in 100 different configurations. "Since we started with rapid prototype tooling, we never use conventional patterns," he said. "It's all near-net shape with the right part. And you can do things like model in cores. This is where the patternmaker of the future is."

For an average sand cast part at Solidiform, the process begins using Pro-Esoftware to model the part completely, followed by a Procast model of the master pattern. That data then goes to the SLA machine where the part is fabricated. "The only thing missing is the aluminum plates to mount the pattern on," Andre said. Release agent is applied to the SLA pattern, and it is used directly against the nobake sand.

"The SLA pattern wouldn't hold up on a jolt squeeze machine, but for nobake it's perfect. We've made up to 750 molds with these patterns and they show no signs of abrasion or degradation. And they provide' 125-150 RMS of surface finish--we're getting journey-man-like quality from these patterns."

The use of rapid prototype tooling has profoundly affected the does business. The firm treats rapid prototyping as its own market segment, and has moved into doing prototype work. Andre said: "We can provide anywhere from 1-5000 parts. We look to make money on a single part, instead of passing that up for the volume work." The ability to do that comes with providing what Andre calls "time compression."

"Customers want the same quality, but are looking for rapid turnaround," Andre said, "and our customers are purchasing time compression. We quote for one, three or five weeks and the price depends on which they choose. At least 95% of our customers opt for the fastest turnaround." Solidiform's manufacturing philosophy is a departure from the traditional. As Andre said: "We're delivering castings, but we're selling time."

Asked why more foundries aren't taking Solidiform's path, Andre responded: "Sand casters haven't been awakened to the potential. You can't wait for the machine manufacturers to develop applications for you, you have to come up with your own answers."

For its part, Solidiform has created its own answers. The firm has an address on the Internet, providing easy access for its increasingly global customers. "We're very excited about this," Andre said. "It's the best thing that ever happened to us."

Service Bureaus

While Solidiform is fortunate enough to have an SLA machine in-house, it still must go outside for work done in the variety of competing processes. Enter the service bureau. Service bureaus are firms that specialize in jobbing out their rapid prototyping capabilities, much like pattern shops. In fact, most service bureaus have traditional pattern shops in-house, lending foundry patternmaking experience and the ability to combine new technology with older methods.

A service bureau might have one machine or several machines handling the same process, but many have an assortment of rapid prototyping technologies under one roof. For the foundry considering getting involved in rapid prototyping, these establishments offer a cost-effective way to gauge the new technology without capital outlay.

A customer can approach a service bureau with anything from a pre-converted stl file to a set of drawings. The bureau will do all the necessary computer work to ready the data for prototyping, charging depending on how much work that is. Prototype Express Schaumburg, Illinois, a service that specializes in SLA modeling uses SDRC I-DEAS, Aries Construction, CADKey and Pro/Engineer solid modeling systems, accepts data from Catia Solids, and will work from IGES files or prints. This variety of software helps increase the chances that the foundry's or OEM's CAD files can be used to create the part.

According to Gustalson, Clinkenbeard & Assoc. never sees a drawing for 30% of its Laminated Object Manufacturing (LOM) and CNC prototype manufacture, "And we're talking about some very complex parts with 40 or 50 cores." He said that for direct tooling applications, the firm's LOM masters can be used very effectively in sand molding. But while Clinkenbeard serves foundries, the firm sees its LOM prototyping and tooling business driven by end-users. "The OEMs aren't going to wait the 11 weeks to get the prototype, so they're going on their own."

If more foundries aren't using Clinkenbeard's LOM services, Gustalson said it may simply be out of fear of what they've never done before. "Those that have used it seem to be quite satisfied with it." In the case of Clinkenbeard's business, the firm's CNC services are in competition with LOM. "CNC is holding it back because it's a `safe' technology that's been around for a number of years," he said. "We still think CNC pattern-making is viable segment and a lot of our business is in CNC."

But of the firm's LOM capabilities, Gustafson said, "We've been doing quite a bit of it, and we're slowly evolving. We believe in it 1000%.

Entering its second decade, rapid protoyping now has a proven record of benefit to manufacturing. There are countless examples of lead time and development costs for parts slashed 50% or better. For foundries, there are even further advantages.

The quick turnaround time on rapid tooling and the integration of casting simulation in the design process, "relieves the major up-front time constraints, enabling foundries to compete directly with weldments," Santner said. "Once designers understand the quality that can be obtained through casting, in a quick amount of time, they may be more reluctant to go with other processes."

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Laminated Object Manufacturing (LOM), Helisys, Inc.

In LOM, a single beam-[CO.sub.2], laser cuts the outline of a cross-section of a 3-D CAD design on a sheet of heat-sensitive paper. Another sheet then lays on top and is laminated to the previous cross-section and cut in the same way. The cutting/laminating process continues layer after layer until the model is complete.

Build Envelope 15 x 10 x 14 in. or 32 x 22 in.

Layer Thickness: Typically 0.002-0.008 in.

Materials: Rolls of thin sheet paper and composites with held-sensitive adhensives.

Software Required: Windows NT and Helisys' proprietary LOM Slice.

Post-processing: All excess surrounding material, including internal cavities, must be removed.

Pros: Completed patterns look and feel like wood and are well suited as master patterns for secondary tooling. Requires no support structures or post-curing and provides reduced warpage.

Cons: Parts will delaminate in high humidity, and are not easily machined.

Price: $99,000 or $199,000

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Direct Shell Production Casting (DSPC), Soligen

In DSPC, colloidal silica is inkjetted on ceramic alumina powder, creating the bottom cross-section and successively bonding each new layer to the one below. It produces a shell of the part, and not the part itself. That shell is then treated similarly to those used in investment casting, and molten metal is poured directly into it.

Build Envelope" 12 x 10 x 10 in.

Materials: Ceramic alumina.

Software Required: Proprietary Soligen software.

Post-processing: Excess powder is removed, the shell is immersed in colloidal solution and then fired for hardening.

Pros: An investment casting shell is directly created, so master patterns are not required.

Cons: Removal of powder from internal passages is difficult. Interior surfaces are rough.

Price: Units are not for sale. Instead, Soligen operates its own service bureau, called Parts Now.

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Solid Ground Curing (SGC), Cubital Solider

SGC is the process of exposing an entire layer of photopolymer using a strong UV light passing through a mask and fully curing all exposed areas. Unexposed resin is vacuumed off and replaced with a water soluble wax. The resin/wax layer is then machined to precise thickness and the process begins again. Once complete, the wax must be removed.

Build Envelope: 20 x 14 x 20 in.

Layer Thickness: 0.004-0.006 in.

Materials: UV liquid polymer.

Software Required: Cubital's proprietary DFE front end software.

Post-processing: Wax removed is difficult requiring scraping, chipping, heating and soaking.

Pros: No support structures, no post-curing, multiple geometries can be modeled simultaneously and models are easily machined.

Cons: Polymer is expensive and resin/wax waste is great.

Price: About $470,000.

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Fused Deposition Modeling (FDM), Stratasys, Inc.

In FDM, a thin layer of low-melting temperature thermoplastic filament is extruded one layer at a time, with each layer representing a cross-section of the CAD data. Built from the bottom up, the model rests on a styrofoam base that can be removed on completion.

Build Envelope: 9.5 x 10 x 10 in.

Layer thickness: Typical 0.010 in.

Materials: Machinable wax, investment casting wax, and a nylon-like material, all supplied in spools of solid filament similar to fishing line.

Software Required: CAMAX NC, Stratasys' proprietary ProtoSlice, and Bridge-works automated support generator.

Post-processing: Support layer removal, step removal, sanding and painting.

Pros: Fast material changeover, nontoxic, low maintenance and, since it doesn't use lasers, is suitable to office environment.

Cons: Very thin wall thickness, poor durability, prominent parting line.

Price: About $97,000

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Selective Laser Sintering (SLS), DTM Sinterstation

SLS uses a [CO.sub.2] laser to trace a cross-section of CAD model by sintering together the particles of a powdered material. After each layer, the model is lowered, powder reapplied and the process begins again.

Build Envelope: 12 in. diameter x 16 in. deep.

Layer Thickness: Typically 0.0005 in.

Materials: Investment casting wax, polycarbonate and a nylon-like material.

Post-processing: Unsintered powder removal, wax parts can be wiped with acetone to improve surface quality.

Pros: No post-curing, unsintered powder acts as support, nontoxic materials.

Cons: Rough surface finish, unpredictable material densities, build chamber must be continually flooded with inert gas to avoid explosions.

Price: About $315,000.

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Stereolithography Apparatus (SLA), 3D Systems

SLA is the process of selectively curing a photopolymer with a helium-cadmium ultra violet laser. Starting from the bottom of the part geometry, the laser traces a cross-section of the CAD data on the surface of a vat filled with UV-curable liquid photopolymer, creating a solid. As each layer of the part is finished, the support platen descends into the vat, coating the previous layer with resin, which is then cured to the completed layer. When the last layer is complete, the model ascends out of the vat ready for post-processing.

Build Envelope: 7.4 x 7.4 x 9.75 in., up to 20 x 20 24 in., depending on model.

Layer Thickness: Varies depending on the feature size and required surface finish (0.004-0.0020 in., typical).

Materials: Acrylates or epoxy-based photopolymers.

Software Required: 3D Systems proprietary viewing and slicing software and QuickCast software (for investment casting applications).

Post-Processing: Support structure and excess resin removal and additional UV curing.

Pros: Unattended operation, good accuracy and surface finish.

Cons: Requires support structures, which can be difficult to remove, post-curing needed, photopolymers may be toxic and/or hazardous.

Price: $100,000-$206,500, depending on model.

The product descriptions and parts of the text of this article were originally presented by Thomas A. Sorovetz, Chrysler Corp., at the 1995 AFS Midwest Regional Conference.