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Stereolithography Delivers Multiple Rapid Prototyping Options

Improved resins make stereolithography suitable for presentation models of all sizes and shapes.

By David Gaines
Design-2-Part Magazine

Rapid PrototypingPrior to the late '80s, prototyping was expensive and time-intensive. Confirming form, fit, and function for a given product was usually fraught with a great deal of trial and error, as parts were worked and reworked before a decent prototype could be produced. Tooling had to be made, liquid metal poured or workpieces machined, and a multitude of finishing processes was always necessary. In addition, design changes were prevalent and costly, metal and plastics were often scrapped, and designs frequently required re-engineering.

After several years of research and development, a new, rapid prototyping technology—called stereolithography—was introduced in 1987. With the advent of stereolithography, a prototype could be manufactured inexpensively and quickly — usually in a day or two — from concept to final part. The technology did not, however, become commercially competitive until versatile epoxies began to be used in 1994.

One well known Northeastern job shop is a pioneer in this unique field. ProtoCAM, of Northampton, Penn. (near Allentown), is a fully integrated, rapid prototyping service provider that offers more than just finished prototypes. Also available are engineering and design services, 3D modeling, finite element analysis, mold testing, prototype manufacturing, and medium-volume production capabilities in several technologies. The company has design and manufacturing engineers and modelmakers on staff.

"We were one of the first companies on the east coast to start doing rapid prototyping with stereolithography," recalls Ray Biery, a managing partner of ProtoCAM. "We can do very large parts, like scale models for wind tunnels, or very small parts like electronic connectors. We've done parts 40 inches x 100 inches, and as small as 0.020 inch in diameter."

Solidifying Light-Sensitive Liquids

Stereolithography is initiated by using a 3D computer model to simulate a part's geometry. The prototyping machine digitally cuts the 3D model into thin slices and then duplicates them—one layer at a time, in thicknesses of 0.002 inch to 0.005 inch—with a liquid epoxy polymer. The liquid is sensitive to ultra-violet light, and solidifies wherever the light hits it. A heat process is often used to increase the heat deflection temperature of the model. If needed, finishing is performed to create smooth surfaces on the final model. Parts are dimensionally accurate to within 0.005 inch. One benefit of the process is that several design changes or modifications can be made easily before going into minimal production or full production with soft aluminum or hard steel tooling.

A rapid prototype can be used as a pattern to make a sand or investment casting, an RTV (room temperature vulcanization) part, or an injection molded part. For RTV, a manufacturer pours silicone rubber around a stereolithography part to form a mold. Then the stereolithography piece is extracted out of it, leaving a two-part mold capable of being used to form fine details in urethane plastics or epoxy resins for about 25 pieces.

Most customers want rapid prototypes for upcoming trade shows or the development of new parts. Biery says that a customer might need several parts for a show—for example, three parts in plastic and three in aluminum—and may want them in different colors to see what reaction they get from show participants.

A long-time customer came to ProtoCAM a while back with an engineering problem. The OEM had a complex design for a rubber bellows on a piece of industrial machinery, and needed to take the part directly from an IGES file to final product installed on machines in the field. ProtoCAM's materials and molding experts worked with the customer and quickly came up with a process for creating the rubber bellows that fulfilled the customer's criteria for time, material, and quantity.

"Not too many rapid prototyping companies are able to make collapsible-cored parts," Biery insists. "But we've developed a proprietary process to make these specialized parts, like for a bellows or a shifter boot. It's very costly to make a prototype for these parts because of the complex geometry, but we can mold these parts with minimal tooling."

Better Epoxy Resins

The trend in the stereolithography sector of the rapid prototyping industry is towards more and better epoxy resins. One thing that makes ProtoCAM unique, says Biery, is the company's extensive use of these photosensitive materials. The firm has experimented with a multitude of different resins for many different uses over the years. Among its offerings are a clear resin that can be used for lenses, a flexible resin for snap fits, and one with engineering properties that resemble nylon.

There are also thousands of different types of urethanes available for RTV molding or production casting. ProtoCAM first queries a client about what mechanical properties are desired for a part, and then matches the urethane to those criteria. In addition, the contract manufacturer's foundries can use prototypes to mold almost any ferrous or non-ferrous metal by investment casting, plaster mold casting, or sand casting.

Reducing Production Time and Saving Money

A standard scenario at ProtoCAM would be for a customer to come in and request a rapid prototype, either with a loose hand drawing or a tight 3D computer file. The company can handle every step in the production sequence, from checking the first prototype to final, medium production runs. A client usually gives the firm several modifications after the first prototype. If the second prototype doesn't require further changes, ProtoCAM will produce 25 RTV parts, often in several different colors.

"We can make an RTV mold in about one week, and then do about six parts per day," Biery maintains. "If they're happy with their design, they will decide to have us build tooling and then turn out several thousand parts by injection molding or casting, up to about 5000 parts. We can make 2000 to 4000 injection molded parts for them in two to four weeks."

One project had ProtoCAM working on an aerospace part that formerly had been produced by CNC machining, which turned out to be very expensive and time consuming for this application. The OEM needed to reduce the cost of the part and the lead time, but was not familiar with rapid prototyping or casting processes. ProtoCAM decided that the aerospace part could be made with a high-quality investment casting. After prototyping and casting, ProtoCAM was able to reduce the 15-week lead time down to 9 weeks, and to save the customer $250,000 on the overall cost of the final parts.

Stereolithography—still the most common rapid prototyping technology—was researched and developed by Charles Hull, who founded 3D Systems (Valencia, Calif.) in 1986. From the time the company shipped its first six stereolithography apparatuses (SLAs) in 1988, 3D Systems has been the leading producer of rapid prototyping equipment.

"Our stereolithography machinery is designed and manufactured by 3D Systems," says Biery. "We have five of their SLAs, equipment that runs from $200,000 to $800,000. And we have access to three SLS (selective laser sintering) machines, which are 3D Systems Sinterstations."

The SLSs use powdered thermoplastic, instead of epoxy liquid, that's laid on a platen for rapid processing. A laser beam melts the particulate matter and the part grows layer by layer, similar to an SLA part. ProtoCAM can hold +/- 0.005 inch with an SLA machine, and +/- 0.010 inch with the SLS machines.

Several factors are to be considered when determining which prototyping process a client would want to use. The SLS process offers many more plastic materials, so is preferable for companies that have many diverse parts, but the SLA process gives a better surface finish, more detail, and is much more accurate. The prototyping material used is also the determining factor for surface texture. If RTV molding is to be used for the next step, a particular surface finish can be obtained from a particular mold.

"The processes we use all depend on lead time, cost, and the level of quality a customer wants," Biery explains. "The tooling we make on-site is made with milling machines and a plunge EDM machine, and our model shop does the finishing work, like sanding, polishing, deburring, and boring, for the SLA parts."

Ray Biery and Ron Belknap, the company's managing partners, started ProtoCAM in 1993, and now lease a 10,000-sq-ft building with 20 staff members on two shifts. Their diverse markets bring anyone who develops new products into their shop. Surprisingly, 95% of their business comes from the internet; however, personal contact remains a priority for ProtoCAM. Much of the firm's work is consumer products, like hair brushes and toys, as well as aerospace parts, like wind tunnel scale models, and scale models of buildings. But they also form automotive parts, medical components like surgical and hip replacement parts, and electronic connectors.

Many Production Options Are Evident

Depending on run volume and part complexity, ProtoCAM offers a number of different production options with the prototype. One short run option is RTV molding, which is less expensive than injection molding because expensive tooling is not required. One disadvantage is that only 25 pieces can be obtained from each mold. And a customer will pay more per piece, since it is more labor intensive than injection molding. ProtoCAM's RTV equipment also allows the firm to make investment casting wax patterns.

Rapid prototyped parts work well for investment casting. The image is first built by rapid prototyping; the mold shape is then used for a production part. The mold shape takes on the role of the wax model used in investment casting. It is coated with a ceramic material and the prototype is removed from the ceramic mold as wax would be. The hollow ceramic mold will now accept molten metal from the casting furnace to create a high quality mold for part production.

Rapid tooling is part of the rapid prototyping equation, whether handled in-house or by one of the company's vendors. Besides standard soft aluminum and hard steel tooling for casting and injection molding, ProtoCAM also offers Keltool™ tooling. Utilizing the Keltool™ process, the job shop can fit inserts into a mold base in as little as four to six weeks. The inserts are alloyed with mostly A6 tool steel; therefore, high production quantities can be expected from these molds.

Injection molds can also be created with the use of spray metal tooling. In this process, a thin metal shell is sprayed over a rapid prototype model. The sprayed model is then surrounded with epoxy and mounted in a frame for added strength during injection molding. Plaster casting and sand casting are also possible with a rapid prototype.

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