This article highlights the striking facts about rapid prototyping; a process that fabricates physical objects directly from computer-aided design sources. The use of rapid prototyping as a replacement for injection molding is still the overwhelming exception and may always be limited to a very narrow niche. Three-dimensional printing has also seen the introduction of materials that improve the durability and appearance of conceptual prototype parts. Z Corp. of Burlington is incorporating new pigments into its binders for its starch- and plaster-based materials. The pigments result in truer and brighter colors and replace the dyes that were previously incorporated into the liquid binders. The company has recently introduced a urethane infiltrant that increases part strength significantly and allows parts with delicate geometries to be.

## Article

One of the most striking facts about rapid prototyping, a process that fabricates physical objects directly from computer-aided design sources, is the broadening reach in how it is used.

Since the late 1980s, when stereolithography first opened the possibility of creating solid objects directly from computer models, rapid prototyping has evolved quickly in terms of process technology and new applications. In the roughly 15 years since it first hit the market, stereolithography has been joined by a host of other prototyping techniques, each carving out a niche with varying degrees of success. The evolution of these techniques as a means of producing tooling, concept model parts, and functional prototypes has been impressive.

Lately, rapid prototyping has been bridging the gap to rapid manufacturing. “This is one of the more interesting trends,” noted Terry Wohlers, president of Wohlers Associates, an industry consultant based in Fort Collins, Colo., who said that a handful of companies, for example, have been using rapid prototyping techniques to produce parts that would normally be injection molded. The use of rapid prototyping as a replacement for injection molding is still the overwhelming exception and may always be limited to a very narrow niche. Yet it speaks volumes about a range of advances in rapid prototyping materials and processes to make products with improved mechanical properties, accuracy, and aesthetics.

While material advancements are most evident in plastics, research is also taking place in metals and ceramics. One new process currently under development is based on photoreactive pastes to produce various composites for rapid prototyping.

## Emulating Thermoplastics

David Rosen, an associate professor of mechanical engineering at Georgia Institute of Technology in Atlanta and chair of the Computers in Information and Engineering Division of ASME, has seen a big push for plastics and metals that better mimic the properties of production materials. That’s a significant challenge for all of the processes.

One process that has seen developments in this area is stereolithography. In stereolithography, parts are built from a photosensitive polymer fluid that cures under exposure to a laser beam.

The resins used in stereolithography are photosensitive thermosets that crosslink during the curing process, and are fundamentally different from the thermoplastics used in injection molding that they are designed to emulate.

“Usually, these materials are good at matching a couple of mechanical properties, such as elastic modulus and yield strength,” said Chuck Hull, chief technology officer of 3D Systems in Valencia, Calif., a supplier of stereolithography machines, selective laser sintering systems, and three-dimensional printers. He said that the industry has had some success in making resins that mimic polypropylene, a widely used thermoplastic. He also expects stereolithography resin suppliers to continue to make progress in creating materials that have selected thermoplastic properties, which will drive specific applications.

Mahesh Kotnis, technical marketing manager of Van-tico Inc. in East Lansing, Mich., a major stereolithography resin supplier, said that, over the last two years, better stereolithography materials have yielded functional prototype parts that mimic the thermoplastic properties of final parts. Vantico is currently marketing polypropylene-like stereolithography resins, and plans to follow that with the introduction of a resin that mimics acrylonitrile-butadiene-styrene, or ABS, later this year.

Kotnis acknowledges the challenges of approximating the properties of thermoplastics, particularly impact strength and tensile elongation, which measure the ability of a material to resist shock. Improvements in toughness and rigidity usually reduce a material’s heat resistance, and vice versa. The polypropylene-like grade of stereolithography resin has a flexural modulus of 180,000 psi, notched izod impact strength of 0.8 ft.-lbs./in., and heat deflection temperature of about 180°F. According to Kotnis, these properties still fall short of matching the properties of polypropylene, but stereolithography resins have come a long way and development continues.

Kotnis said that stereolithography resins are, in a few cases, being used as end-use products. One example of where this is happening is in the medical device industry, where stereolithography resins are being used to produce hearing aid shells, he said. Widex, a hearing aid manufacturer based in Vaerloese, Denmark, developed a process to digitize the ear canal and create the stereolithography part directly from the CAD data. This eliminates the laborious process of creating a wax pattern from an impression of the ear canal, which is used to make the silicone mold to shoot the part.

Kotnis sees medical instrument applications as a big growth area for stereolithography. The company markets a line of Stereocol medical-grade resins, which pass USP Class 6 tests—a standard to measure the biological response to plastic materials. The resins stand up to standard sterilization techniques.

## See-Through Resin

Other developments in stereolithography resins are adding to the fit and function capabilities of prototype parts, said Jim Reitz, business director of DSM Somos, a unit of DSM Desotech. DSM Somos, based in New Castle, Del., recently introduced a line of WaterClear resins for building transparent prototype parts.

According to Reitz, potential applications include fluid flow analysis, in which researchers can see how gases mix in a manifold prototype, or pump housings that allow viewing of how internal assemblies work together.

Rosen of Georgia Tech said that clear resins open up new opportunities for prototyping. For example, they may allow soft drink suppliers to design prototype bottles without investing in molds. If the materials can be made truly clear, they might even be suitable for lenses, he said.

Reitz added that the WaterClear resins have a fast photo speed, allowing parts to be formed quickly, and low viscosity for easy cleanup. The stiffness and toughness of the material allow parts to be tapped and drilled. The clarity of a prototype is limited to the flat surfaces; sidewalls must be finished to allow for internal viewing, Reitz said.

The company’s newest stereolithography resin is Raven, introduced last December. It is not a transparent, but a general-purpose grade, marketed for a range of applications, from conceptual models to functional prototypes to patterns for molds, Reitz said. Although it is not a “super fast” curing material, it is set at a lower price—around $180 per kilogram versus$225 to \$235/kg for the company’s other general-purpose products. The material, which is clear as a liquid in the vat, cures to a dark color as the build takes place. This allows the customer to view the prototype as it is being formed, he said.

Last month, DSM Somos introduced a photosensitive polymer called Somos 11120 Watershed, which resists humidity. High humidity can degrade the mechanical properties of stereolithography resins. The company is targeting markets in humid climates such as the Asia/ Pacific region.

The company is also developing an elevated-temperature resin, which is expected to retain its useful mechanical strength at temperatures to 250°F without growing brittle, Reitz said.

## Not Just Parts

According to Kotnis, the use of stereolithography resins to create master patterns for tooling for a secondary process, such as plastic injection molding or rubber molding, is the original and still dominant market application of the process. He said that stereolithography resins were originally used to make master patterns for silicone tooling, which was then used to mold polyurethane parts, and that this is still an important application.

Hull said that rapid prototyping techniques can be used to create forms that are used in casting. “We have three different approaches that can help investment casters, and this has become a significant focus of what we do,” said Hull. The company said that stereolithography casting patterns have been used successfully in shell investment casting, sand casting, die casting, and other techniques. Three-dimensional printing has also been used to build models in a material similar to casting wax, although with less accuracy than stereolithography.

Also, a significant part of the selective laser sintering business, which 3D Systems acquired last year, was used to create patterns from a polystyrene material, Hull said. Selective laser sintering spreads a thermoplastic powder layer. The part of it exposed to a laser beam melts and bonds to form the structure. The process has also been applied to ceramics and metals.

3D Systems’ laser sintering process can also be used to form metal parts, Hull said. The system is being used to form green metal tools, which are partly sintered, and then infiltrated with bronze to get full density, Hull said. He said the process has been used to create injectionmolding tools.

Hull said that advanced stereolithography materials and improvements in the laser sintering process are leading to some crossover in applications between the two processes.

## Rapid Composites

One new process now under development may bring rapid prototyping into the realm of composites. In December 2001, 3D Systems formed a joint venture with DSM Desotech called OptoForm LLC to develop a rapid prototyping process, called direct composite manufacturing, which uses photosensitive paste. The technology was originally developed by a French company, OptoForm SARL, which was acquired by 3D Systems last year. The joint venture is now refining the process and materials in evaluation testing with a few customers.

Chuck Hull of 3D Systems, said that direct composite manufacturing brings rapid prototyping and rapid manufacturing into the composites arena. “You get to work with higher-viscosity toughening agents and other things to get better physical properties than you might get with a liquid material,” he said. Although Hull said it is too early in research to predict the market for the technology, he sees potential in prototyping and in manufacturing applications.

Although it uses a stereolithography-like technique, direct composite manufacturing differs in some key aspects from conventional stereolithography systems. For one thing, the equipment is vatless; because it uses a viscous paste, there is no liquid resin in which to form the part. Instead, the paste is pushed up through a cylinder, where a special coating system smooths out the paste to a solid layer.

Mirrors, driven by a computer, direct a laser beam to build the pattern, explained Reitz of DSM Desotech. Because there is no liquid resin or waiting for the liquid resin in the vat to settle before the build is dipped in it to form the next layer, direct composite manufacturing is a very quick process, he said.

## Thermoplastic Progress

Stratasys of Eden Prairie, Minn., a supplier of fused deposition modeling machines, is extending the range of thermoplastics used in its systems. A widely used rapid prototyping technology, fused deposition modeling, is based on a thermoplastic filament that is extruded from a nozzle that moves over a platform to build the part by depositing the plastic in the required geometry.

Stereolithography Cuts Its Teeth

A Stereolithography Application that has made the transition to rapid manufacturing is the Invisalign process, developed by Align Technology of Santa Clara, Calif., to manufacture teeth aligners—a clear plastic replacement for wire braces.

The process is an example of stereolithography used for mass customization. The company worked with 3D Systems of Valencia, Calif., which supplied the high-end SLA-7000 solid imaging machines to create the thermoforming tools on which the plastic aligners are formed.

Because each patient’s teeth are unique, the process starts with a set of dental impressions, explained Len Hedge, vice president of manufacturing at Align Technology. Plastic is poured into the impressions to create a representation of the patient’s teeth. That physical model is scanned and converted into a digital file. Then a suite of software tools, developed in-house, calculates the orthodontic treatment, which consists of the tooth movements that a series of aligners will produce over time.

Once the digital representation of the treatment is done, stereolithography takes it back to the physical world. Hedge said that the process required a rapid prototyping technique that was capable of high throughput and high accuracy, and selected the SLA-7000 machine, which had just been introduced. The SLA-7000 has dual beam capability: a 10-mil-diameter laser beam for detailed components and, to speed the process, a 30-mil-diameter beam for cross sections that do not require as much accuracy, Hedge said.

The machine has a large platform that can hold 90 aligner patterns—about two and a half patients’ worth. The standard for accuracy of a build is within 1.5 thousandths of an inch.

After the teeth reproductions are formed, they are brought to a thermoforming machine and used as tools to form the plastic aligners. The aligners are pressure-formed in the thermoforming machine, which uses air pressure to slide the heated plastic over the mold. The aligners are clear, made from a blend of polycarbonate and polyurethane to impart the desired mechanical properties and tooth movements. The thermoforming mold of stereolithography resin has to withstand the heat, temperatures, and pressures of the thermoforming process. The plastic used for the aligners, which are 30 to 40 thousandths of an inch thick, has a melting point of 425°F. The stereolithography resin also has small shrinkage and a fast build time, Hedge said.

Depending on the length of an individual’s treatment, the patient is supplied with a series of 12 to 48 aligners. Each aligner is worn for about six weeks, correcting the teeth in progressive stages.

Align Technology has ordered 39 SLA-7000 systems from 3D Systems, and currently operates 16 at its Santa Clara location. Last year, the company produced 1.1 million molds, and expects to manufacture 4 million molds this year, said Hedge.

Jon Cobb, vice president of marketing and customer service, said the company supplies two main types of materials: ABS and polycarbonate. Because the process builds prototypes from thermoplastics, the prototypes closely replicate the actual injection-molded parts. Typically, ABS parts are 80 to 90 percent of the strength of the injection-molded part, he said.

Stratasys plans to introduce a polyphenyl sulfone resin for its machines this summer. PPS is a high-performance thermoplastic that can be autoclaved, has high chemical resistance, and high heat deflection temperature.

The company also plans to introduce a fine feature detail capability on its FDM Maxum, a high-speed, large-envelope machine, which will be capable of producing high-detail parts, Cobb said. The company is also working on a project to use the FDM process to produce small, finely detailed components in disposable cameras. Cobb added that Stratasys is also working on using the process to produce hearing aid housings.

## Colorful Concepts

Three-dimensional printing has also seen the introduction of materials that improve the durability and appearance of conceptual prototype parts.

Z Corp. of Burlington, Mass., is incorporating new pigments into its binders for its starch- and plaster-based materials, according to the company’s CEO, Marina Hatsopou-los. The pigments result in truer and brighter colors, and replace the dyes that were previously incorporated into the liquid binders, said Hat-sopoulos, an ASME member. She believes that color is an important aspect of concept modeling, to give a clearer idea of what the final product will look like. But it also has other uses. It can reproduce an FEA pattern on an actual model of a soft drink container to locate stresses, for example.

Z Corp. is also developing materials to produce stronger parts. The company recently introduced a large format machine, producing parts as large as 16 x 20 x 24 inches. Often, larger parts have more complex geometries and higher strength-to-weight requirements. The company is working with Vantico on infiltrants—liquids that can be absorbed into the porous material to increase strength. Z Corp. recently introduced a urethane infiltrant that increases part strength significantly, and allows parts with delicate geometries to be handled, Hatsopoulos said.