Rapid Prototyping: An update on RP applications, technology improvements, and developments in the industry

Terry T. Wohlers
Copyright 1991 by Wohlers Associates

Companies that use CAD/CAM, model-making techniques, and product prototyping are experiencing exciting times. They are gaining access to a new breed of technology that enables them to build physical models from CAD systems like never before. The technology, becoming known as rapid prototyping (RP), has improved dramatically over the past year. Parts are being produced faster and more accurately and in a wider range of materials. Announcements of new developments continue to dazzle the industry.

RP systems build parts using a layer building process. This means thin layers of material, as thin as 0.002 inch, are fabricated, intricately, one on top of another. The layer information comes from horizontal cross sections of a 3D CAD model. The layering process repeats, from bottom to top, until the part is complete. The technique permits the construction of literally any shape that can be modeled on a CAD system, even shapes that cannot be formed using conventional fabrication tools and techniques.

RP systems link to popular DOS and Unix-based CAD products. Approximately 40 CAD software products support RP systems with their capability of translating the CAD model into an STL file format. The STL file is a de facto standard format created by 3D Systems, the first developer to commercially introduce an RP system. Most RP systems available today optionally accept or require the STL format as the standard link to CAD.

Although there are now more than 12 companies making rapid prototyping systems, most RP sales and service activity in the industry continues to be centered around the StereoLithography Apparatus (SLA) from 3D Systems, which uses an ultraviolet laser to solidify thin layers of UV-sensitive photopolymers. Of the estimated 335 total rapid prototyping systems installed at customer sites, approximately 270 (82 percent) of them are SLA products from 3D Systems. Most of the 275 are the SLA-250 model, the company's mid-range system priced at $187,000.

3D Systems' 1987 introduction of the SLA beat other RP developers to market by approximately three years, which accounts for the SLA's dominance. Other companies are now shipping systems or are on the verge of shipping a competing product. Several of these companies are currently testing their systems at beta customer sites. Three Japanese developers offer systems similar to the SLA, but have not yet sold units in the U.S. due to patents by 3D Systems.

RP Applications

The excitement of RP comes from the dramatic affect it can have on shortening design and manufacturing cycles, as well as improving quality of a design. Also, RP users are discovering not just one, but several areas in design and manufacturing in which RP can be applied.

The most natural application of RP is the creation of concept models. Concept models provide CAD-equipped design engineers the opportunity to touch the design, examine it more completely, and easily transport the design to others for review. RP, therefore, becomes instrumental in facilitating concurrent engineering. RP users can share the design with other groups, such as marketing, purchasing, and manufacturing engineering, early in the design when changes can be made inexpensively in the computer.

Manufacturers are also beginning to send RP models to subcontractors who build parts for them. This makes it easier for subcontractors to develop accurate quotes, since the models leave less room for ambiguity than 2D or even 3D drawings.

Soft Tooling

During product prototyping, manufacturers will produce not just one or two prototypes, but as many as 25-100 of them. Speed and accuracy improvements in the technology make RP an excellent choice for producing master patterns for molded tooling. The RP part (master) serves as the pattern around which material is formed to create a molded tool. Soft molded tooling, involving epoxy, urethane, silicon rubber, or plaster, is used for short run production during product prototyping. Spray metal tooling is also gaining acceptance, but it is more costly. Regardless, patterns made from RP systems for soft tooling can play a strategic role in the design and manufacture of a new product by shortening development cycles.

The Low Voltage Breaker Division of Westinghouse STC (Pittsburgh, PA) uses the SLA-250 to create patterns for silicon rubber molds and spray metal molds. The division designs and manufacturers electrical circuit breakers, ranging from the 5 amp version we have in our homes to the 2000 amp breakers that protect office buildings throughout the world. As the speed of RP technology improves, parts being molded today may be mass produced directly, bypassing the need for soft tooling.

Investment Casting

Manufacturers, as well as medical firms, are using RP to produce expendable patterns for investments castings. Investment casting involves a process of melting or burning out a pattern, forming a hollow cavity the same shape as the pattern. The most popular is the lost wax process which has been in use for decades. Material is formed completely around an RP pattern made out of wax. A small opening from the outside to the pattern allows the wax pattern to be melted out when heat or steam is presented, forming a cavity in which molten metal is later poured. Certain plastics can also be used, but they require a high temperature burn-out technique over an extended period of time.

DePuy (Warsaw, IN), a leading manufacturer of orthopedic implants, uses parts made from their SLA-250 in a lost wax investment casting process. When DePuy makes a hip implant, an epoxy mold is created using an SLA part as the master. The SLA master is the same shape and size as the final implant. The mold is used to create a wax pattern which is used to create a second mold. The wax pattern is melted out and replaced with molten metal which becomes the hip implant. Dave Trimmer, director of Engineering Services at DePuy claims RP technology enables their company to supply metal implants to surgeons within 16 days and the implants meet the expectations of surgeons and the various departments at DePuy. Previously, the implants would take eight weeks.

Another manufacturer of orthopedic implants, Biomet (also located Warsaw, IN) uses the Stratasys 3D-Modeler system to build patterns directly out of investment casting wax. The company believes the system trims model-making by two to six times.

Fit & Function Testing

Manufacturers are increasingly using RP parts for fit and functional testing of new designs. This has been a limitation of RP parts in the past. Companies have not been able to construct parts that would withstand fit and function requirements. For example, mating parts would crack when trying to snap them together. Parts would break if they were bumped or dropped. The weak material would prevent parts from being fastened tightly to others parts.

But the situation is beginning to improve considerably. In the past few months, several new resin materials from chemical suppliers have been introduced. For instance, the new XB 5143 resin from Ciba-Geigy can be milled, drilled, and even tapped with screw threads. This is the kind of material users have been requesting since the SLA was first introduced in 1987. It permits them to use the RP parts for fit and functional testing that was previously impossible. DuPont is also making its tough resin materials -- currently being beta tested -- available to users of the SLA and other photopolymer-based RP systems.

Eventually, companies will be able to produce prototype parts using the same material in which the part is later manufactured. This will allow engineers to more accurately test the functionality of the prototype part because the prototype is made of the same material as the finished part.

Improvements in StereoLithography

Feedback from the growing list of SLA users have been getting attention from 3D Systems. In addition to making available better materials, 3D Systems is improving accuracy, surface quality, and speed of the process.

Without good accuracy, the RP system becomes of little use to companies that want to create models for fit and functional testing and patterns for molded tooling and investment casting. Chris Franks, president of Solutions in 3D, Inc. (St. Louis, MO), says accuracy has been a problem in the past. Franks operates a service bureau that offers custom-built parts from his SLA-250. Previously, parts containing a combination of thick and thin walls would shrink inconsistently. Thick walls would shrink more than thins ones, and there was no way to compensate for this non-uniformity.

3D Systems, therefore, has been devoting a great deal of research and development in this area. In fact, 3D Systems' vice president Dennis Medler claims that models made today with the SLA are many times more accurate than parts made when the SLA was first introduced. The improved accuracy, in large part, comes from an approximately six-fold reduction in non-uniform shrinkage.

Much of the shrinkage reduction and improved accuracy comes from years of refining materials, resin recoating and leveling techniques, laser exposure and draw procedures, machine calibration, part cleaning, and post curing.

Franks and other SLA users agree that significant progress has been made by 3D Systems in the last year. Contributing greatly to the reduction in non-uniform shrinkage and post-cure distortion are the new layer building techniques called Weave and Star-Weave. Weave cures the resin to 96 percent, and Star-Weave to 99 percent, during the laser drawing process. Previously, the SLA would trap uncured resin in honeycomb-like cavities and later solidified in a UV oven. Most of the non-uniform shrinkage and distortion would occur during this post cure solidification process.

Accuracy currently stands in the +/- 0.002" to +/- 0.005" per inch range, but accuracy can vary widely depending on the part geometry and the skill of the user. New users without a good grasp on the many operating parameters (e.g. an understanding of laser intensity, beam width, cure depth, and draw speed) of an RP system typically do not make parts as accurate as experienced users. Also note that while certain dimensions of a part may fall well within a +/- 0.005" or less per inch, other dimensions on the same part may be much worse.

3D Systems is also reducing surface roughness of SLA parts. Poor surfaces are usually the result of the stair step effect caused by the individual layers. Surface quality is approximately four times better now than in 1987. This reduction comes mainly from being able to create thinner layers. The surface of a part made with 0.003 inch layers is twice as smooth as a part made with 0.006 inch layers. As the 3D Systems continues to fine-tune the system, they expect to reduce surface roughness even more, according to Medler, without paying performance penalties.

In the meantime, parts used for certain applications, such as molded tooling, usually require considerable hand work, such as sanding and polishing. Frost Prioleau, president of Plynetics Corporation (Emeryville, CA) says he dedicates two full-time employees just to finish the parts which come off his SLA-250 machine. Plynetics is a service bureau with years of experience in molded tooling and investment casting.

Faster Prototyping

3D Systems is also improving the speed of the SLA process. As a result, company president Charles Hull says the SLA is approximately 25 times faster today than in 1987. The current version of the SLA-250 is five times faster than the original SLA-1 beta system, and the large SLA-500 is five times faster than the SLA-250, according to Hull. The increased speed of the SLA allows users to produce prototype parts much faster than before, often reducing the time from more than a day to a few hours.

Several factors account for the speed enhancements. In 1989, a recoater blade was added to spread a precise thickness of fresh resin for each layer being produced. Also in 1989, a Silicon Graphics Personal Iris workstation was offered for faster layer slicing. A faster control personal computer was added in 1990. The much improved Weave and Star-Weave layer building techniques were introduced in 1991. Also in 1991, improved merge and control software were added. This, alone, cut part building time in half. The availability of lower viscosity resins also contribute to increased speed because layers can be leveled more quickly.

Hull anticipates future speed improvements from enhancements in software and computers. Eventually, the company will unbundle the SLA software so users and third-party developers can optimize the software to the specific needs of the parts being produced. There is also room for improving the ease of learning and using the system. Hull believes the equipment should be easy to use, with few operator decisions, but with enough flexibility for advanced users.

Other Developments

But even as 3D Systems is working to improve its RP systems, other vendors are working hard to improve their own technologies. Four RP system developers are currently testing their systems at customer sites. In addition, most U.S. RP developers that have working systems in-house produce parts for customers on a contract basis. This is providing additional feedback to the industry. Companies that offer part building as a service include DTM, DuPont, Helisys, Quadrax, Cubital, Light Sculpting, and Stratasys. Operating an in-house service bureau allows developers the opportunity to generate cash while fine-tuning their technology prior to building productions units.

DTM's Sinterstation 2000, based on Selective Laser Sintering (SLS), is being tested this winter at five beta sites. DTM was the second U.S. company after 3D Systems to announce the development of an RP system (1988). The sintering process uses powdered materials and a laser to solidify CAD-generated models into solid objects. The powder environment solves the problem of supporting overhanging geometry. Liquid-based RP systems, such as the SLA, require support structures which are later removed. The SLS system uses a wide variety of materials including polycarbonate and investment casting wax. By the end of 1991, the company will introduce ABS and nylon. Commercial shipments will begin by mid-1992 at a cost of $397,000 per unit. A Sinterstation 2000 system configured for use with four materials is priced at $427,000.

Also in 1988, Michael Feygin of Helisys (then called Hydronetics) announced his plans for a system based on Laminated Object Manufacturing (LOM). LOM involves the lamination of sheet material and the laser cutting of each layer based on cross sections from a CAD model. The current LOM system bonds polyethelene-coated paper using a heated roller mechanism. The paper is the same that butchers use to wrap meat. Parts made from paper laminations resemble wood and, therefore, are easily hand sanded and finished. Parts made from other RP systems can also be hand sanded and finished. Helisys currently builds two versions of the system, priced at $75,000 and $110,000. The company has sold its first two beta systems to service bureaus in Detroit.

DuPont was next to announce the development of an RP system (1989), a technology similar to the SLA. DuPont, however, does not plan to build and sell systems. Instead, DuPont is finalizing a licensing agreement with another company that will manufacture some version of their Somos 1000 stereolithography sysem. DuPont wants to stay focused on the development of materials for liquid based systems, such as their Somos system and 3D Systems' SLA. Presently, the company offers two resin materials and they are developing a third for investment casting applications. The materials will sell for $550 per gallon, which is approximately $200 - $250 more than other stereolithography resins.

Quadrax Laser Technologies offers a stereolithography system using a visible light laser instead of the invisible UV light used in the SLA. Prior to recent speed improvements in the SLA, the Quadrax visible light technique was faster and cured the parts more fully in the vat. A more fully cured part minimizes post cure distortion, thus creating a more accurate part. Some of the technology, such as the ability to vary the width of the laser beam, was originally developed at Laser Fare Ltd., which is currently working on a second-generation system. Quadrax has installed three of their Mark 1000 systems at customer sites. Units sell for $195,000.

Cubital has sold eight of its large Solider 5600 systems: four production systems in the U.S. and four in Europe. Baxter Healthcare will be the first of the U.S. customers to install a production system. Baxter was also the first company to install an SLA from 3D Systems. Cubital's $490,000, 10,000 lb. machine involves a relatively complex series of processes that builds parts surrounded by water-soluble wax. Like DTM's powder process, the wax material supports overhanging geometry. The system automatically creates masks of a CAD model's cross sections through which UV light shines to solidify an entire layer at once. This makes the process fast, especially when building parts containing wide areas.

Light Sculpting uses a similar technique of solidifying a whole layer at once. However, their system is not fully automated. It uses a Gerber photoplotter to create masks that are manually positioned prior to focusing a flood of light on the surface of the liquid polymer. Light Sculpting has yet to sell a system, even though the technology may have accuracy advantages over other RP technologies. Two versions of the technology are currently available for $129,700 and $159,500. The photoplotter and post cure device together sell for $35,000.

Stratasys has installed five of its 3D Modeler systems at beta sites, including General Motors, 3M, Texas Instruments, Pratt & Whitney, and Biomet. Advantages to Stratasys' Fused Deposition Modeling (FDM) technology is safety and simplicity. It feeds non-toxic thermoplastic filament material through a heated extrusion head that deposits the material, layer by layer, with accuracy of +/- 0.005 inch over a 12 inch part. Spools of machinable wax, investment casting wax, and a nylon-like material are currently available. The system is priced at $178,000, which includes a Silicon Graphics Personal Iris 4D/25 workstation.

Developments in Japan

An estimated 44 RP systems have been sold in Japan. Ten of these come from 3D Systems Japan, a wholly-owned subsidiary of 3D Systems here in the U.S. Approximately 14 units are the Solid Creation System (SCS) co-developed by Sony and Japan Synthetic Rubber (JSR). The SCS is based on stereolithography technology similar to the SLA. It is currently being sold by JSR's subsidiary D-MEC Ltd. in Japan. The system uses a powerful Argon-ion laser similar to 3D Systems' SLA-500, but offers a larger part building chamber. Toyota recently purchased an SCS for $500,000.

Mitsubishi's C-MET has reportedly sold 20 units. Their system, called the Solid Object Ultraviolet Laser Plotter (SOUP), is also similar to the SLA. 3D Systems is currently challenging both D-MEC and C-MET in Japanese courts for patent infringements. Mitsui Ship Building has developed a stereolithography system, called Colamm, but no sales have been reported.

Future RP Developments

Many other systems are under development in laboratories and research facilities in and outside the U.S. For example, the Massachusetts Institute of Technology (MIT) is developing a rapid prototyping system based on ink jet printing technology, which they refer to as a 3D printer. It deposits small droplets of ceramic binder onto a flat bed of ceramic powder. The powder offers support of overhanging geometry similar to DTM's powder system. The ink jet concept has been proven at MIT and is currently being refined by a team of faculty and graduate students. MIT is hoping to license the technology to an interested company.

Perception Systems has also done the ground work on a concept involving ink jet technology. It uses a process called Ballistic Particle Manufacturing (BPM) that deposits tiny droplets of molten wax material that solidify on impact. Ellison Smith of Perception Systems says the industry can expect to see accelerated development of BPM by their company. He hopes to have prototypes within a year and eventually wants to introduce a system for under $50,000.

Stratasys' Scott Crump discussed the idea of a low-cost version of his FDM technology at NCGA '91 in Chicago. Crump envisions a small-scale system that sits next to desktop CAD systems, a concept being considered by others too, such as Perception Systems and a company in Texas that must remained unnamed. RP systems available today are much to large and heavy to sit on the top of a desk or table.

Meanwhile, the U.S. Navy's David Taylor Research Center (Annapolis, MD) is developing an amazing desktop process involving an electrosetting technique. Electrodes made from a standard pen plotter or laser printer are stacked, submerged in a container of liquid resin, and subjected to an intense electrical charge. The process involves the Winslow Effect, the solidification of a fluid by imposition of a biasing electric field through the fluid, invented by Willis Winslow in 1939. Experiments indicate the process would involve only $5,000 - $10,000 worth of standard office equipment. Parts are being made from silicon, epoxy, polyurethane, and polyester.

Norman Kinzie of Landfoam Topographics has patents on a LOM process that involves the coloration of each layer. This would permit the production of multi-colored parts from a desktop output device. Currently, RP systems are able to produce single color parts only. A working prototype is not yet available.

Desktop CAD Market and RP

Hewlett-Packard and other graphics peripheral manufacturers are investigating the possibility of introducing an affordable 3D output device for the desktop. If a company like HP were to introduce such a device, it would have a significant impact on sales volume. A small, low-cost unit would have a natural connection to the mechanical CAD market because RP systems are well-suited for the production of mechanical parts and RP systems require a 3D CAD model prior to part production.

Consider, for example, a $25,000 unit price and the approximately 135,000 3D mechanical CAD users in existence today. The market would grow to $337.5 million if just one RP system would sell for every 10 3D mechanical CAD users. If 10 percent of the approximately 500,000 2D and 3D mechanical CAD users were to accept the technology, the market would grow to $1.25 billion. Peter Sferro of Ford Motor Company said at that price, he would want each of his engineers to have one.

Of course, the market extends beyond mechanical design. Architects are exploring RP for creating building models. The architectural market, alone, represents $.75 billion, using the above 1:10 illustration.

But while the concept of 3D desktop RP sounds promising, an important factor would come into play. RP systems require a complete, water-tight 3D surface or solid model and most CAD work being done today is 2D detail drafting, even among those who own 3D CAD systems. Perhaps an affordable 3D output device would give companies reason to tap the 3D functionality of their CAD system for the first time.