Steven Ashley, "Rapid Prototyping is Coming of Age," Mechanical Engineering, vol. 117, no. 7, July 1995, pp. 62-68.

Copyright © 1995, Mechanical Engineering.

Rapid prototyping is coming of age

By accelerating the design process and speeding tooling development, rapid prototyping technology helps manufacturers cut new product cycle times and costs.

By Steven Ashley, Associate Editor

During the last decade, a class of technologies has emerged by which a computer-aided design file of an object can be converted into a physical model through special sintering, layering, or deposition techniques. Called rapid prototyping (RP), or solid free-form fabrication, the major application for this technology has been early verification of product designs and quick production of prototypes for testing. Multiple prototypes can now be reproduced more economically by using the RP master as a pattern for creating molded or soft tooling. Interest in desktop (or more accurately, office) rapid prototyping for visualization and design verification is growing, but it is not yet easy to cost-justify.

Recently, the fabrication of patterns for limited-run production tooling has become more common. Today, the most popular rapid tooling options are silicone rubber (RTV) molding and epoxy and spray metal tooling. For metal part production, the choices are investment, plaster, spin, and sand casting (see "Prototyping With Advanced Tools," June 1994).

Results from one of the world's foremost RP practitioners--Pratt & Whitney's Rapid Prototyping Casting Laboratory, in East Hartford, Conn.--indicate just how useful RP technology can be for investment casting. "Last year, we made 2000 RP castings at a cost between $600,000 to $700,000," said John Marcin, methods engineering specialist. "Using previous methods, the cost of those castings would have been about $7 million. That's an order of magnitude reduction. In addition, we've realized time savings of 70 to 90 percent."

A Growth Market

These trends are starting to take hold throughout the U.S. manufacturing industry. In a recent survey of 312 American RP users conducted by Denton and Co., of Dearborn, Mich., ground transportation companies account for 24 percent of users, aerospace firms 23 percent, consumer electronics manufacturers 14 percent, and business machine makers 11 percent.

"For the RP industry, 1994 was a pivotal year," said Terry Wohlers, president of Wohlers Associates, an engineering automation consulting group in Fort Collins, Colo. "More systems were sold than in the previous two years, and tens of thousands of RP jobs were processed--more than ever before. We estimate that it's a $200 million industry, today. If you add secondary tooling and duplicate parts, the industry mushrooms to $280 million." During the same two-year period, the number of service bureaus grew from 105 to 155, and existing service providers posted record revenues and growth.

Wohlers said 336 rapid prototyping units were sold in 1994, bringing the installed RP base to 1000 worldwide, with 3D Systems Inc., in Valencia, Calif., accounting for over half of the systems now operating. Other major RP vendors in the United States include Helisys Inc., in Torrance, Calif. (115 machines); Stratasys Inc., of Eden Prairie, Minn. (82 units); DTM Corp., in Austin Tex. (68); the Israeli firm Cubital America Inc., in Troy, Mich. (24); and newcomer Sanders Prototype Inc., of Wilton, N.H (40 machines).

Wohlers Associates predicts the 1995 RP market will exceed $318 million and grow to $475 million in 1996. RP unit sales should reach 1500 by the end of 1995, he predicted.

RP software also made gains. Imageware, in Ann Arbor, Mich.; Pogo International, of College Station, Tex.; and XOX, of Minneapolis, each introduced an .STL part file viewing and editing product. In the meantime, Solid Concepts, in Valencia, updated its popular Bridgeworks support structure software for .STL.

All is not entirely upbeat in the RP business, however, according to Wohlers. RP machine prices are still too high, he said, and fights rage among competitors over intellectual property. The RP industry is still on a steep learning curve as well, and technical shortcomings linger. Vendors are reluctant to quote accuracy and precision figures because there are few agreed-upon standards and procedures regarding dimensional measurement of parts. Limited z-axis (vertical) accuracy and surface quality are problems that still pervade the industry.

Meanwhile, the RP industry is beginning to consolidate and mature. Stratasys, for example, purchased IBM's developmental RP technology and its applications expertise. And the American Society for Testing and Materials (ASTM), in Philadelphia, established a new subcommittee, E28.16 on Rapid Prototyping, part of the ASTM Committee E-28 on Mechanical Testing, highlighting the need for consensus standards for RP.

3-D Printing

Research also continues on improved RP processes. Three-dimensional printing, developed at the Massachusetts Institute of Technology, in Cambridge, Mass., is progressing both in-house and in the hands of licensees, said Emmanuel Sachs, associate professor of mechanical engineering. Three-dimensional printing uses electrostatic ink jets to spray computer-defined patterns of liquid binder onto layers of precursor powders. As the printing process is repeated to form subsequent layers, Sachs explained, a green, or preconsolidated, part eventually emerges ready for post-processing. Using an eight-nozzle print head and steerable droplets, the MIT team has produced relatively complex tools with conformal cooling channels of stainless steel-bronze that exhibit properties akin to wrought 316 aluminum. Sachs said that the MIT group is also continuing its work on an Advanced Research Projects Agency (ARPA)/Office of Naval Research (ONR) program to produce structural ceramic green parts using RP techniques. One focus at MIT is functionally gradient and microcomposite materials. The researchers, he said, are currently working with zirconia-toughened alumina.

The three licensees of the 3D-printing technology are also making progress. One licensee, Soligen Technologies Inc., in Northridge, Calif., has established its Parts Now unit in which the company's direct shell production casting (DSPC) technology, used to produce ceramic molds and cores for investment castings, has been integrated with a captive foundry. Companies such as Caterpillar Inc., in Peoria, Ill., and several automotive firms have been able to turnaround functional investment-cast parts in record time, boosting the users' time-to-market performance. Yehoram Uziel, Soligen's president and chief executive officer, said that the latest DSPC machines now have enlarged print heads with 1280 jets.

Though one of the 3D-printing licensees remains anonymous, the third, Therics Inc., of New York City, has revealed that it is developing the MIT process for medical applications, including medical devices, implantable tissue healing matrices, and drug delivery systems, said Tucker Swan, vice president of Therics' new business development unit. Using the technology's ability to alter an object's microarchitecture and composition as it is built, the small start-up company is pursuing such products as pills that release measured drug doses at specified times during the day. Therics is building a new 3D-printing machine to manufacture these and other devices, Swan said.

Stereolithography

With nearly 500 SLA-190, -250, and -500 stereolithography machines sold since 1988, 3D Systems is the clear RP industry leader. SLA stereolithography machines, which cost from $110,000 to $450,000 each, convert three-dimensional CAD data of physical objects into vertical stacks of slices. A low-power ultraviolet laser beam is then carefully traced across a vat of photocurable liquid polymer, producing a single layer of solidified resin--the first slice of the object under construction. The initial layer is then lowered incrementally by the height of the next slice, whereupon the layer is recoated with resin and another is traced on top of it. This procedure is repeated until the entire part is fabricated. Last year's big product introduction, the QuickCast build geometry, which produces parts with a skinned hollow-lattice structure for rapid investment casting production, "is coming of age," said Paul F. Jacobs, 3D Systems' director of research and development.

There were various shortcomings to QuickCast 1.0 that needed to be addressed, Jacobs said. Pinholes frequently formed when supports were removed from downward-facing surfaces, which led to the ceramic slurry entering the casting pattern's interior. Upward-facing and downward-facing surface quality was less than desired, Jacobs said, with upfacing surfaces often sagging and downfacing surfaces marred by print-through that looked like a waffle iron. In addition, QuickCast 1.0 parts often exhibited drainage and void-ratio problems, especially in thin curved sections. Finally, the less-than-optimal 80-percent yield of aerospace industry-acceptable castings attainable with QuickCast 1.0 was found to be caused primarily by shell cracking due to solid, incompletely drained patterns with low void ratios.

QuickCast 1.1 avoids these problems, Jacobs said. The build style now features triple upfacing and downfacing skins 27 times stronger than they were previously, which eliminate pinholes and sag. Triple skins, he continued, also improve the surface finish from 100 to 150 microinches root mean square (RMS) to 15 to 20 microinches RMS. The replacement of small triangular cells (hatch pattern) with larger square cells has led to better pattern drainage (due to much less meniscus-caused resin retention) and a boost in void ratio from 66 percent to as high as 83 percent. These enhancements have in turn produced lower expansion stresses on ceramic investment casting shells, bringing casting yields up to 95 percent.

Over 50 percent of 3D Systems users are using the relatively new Cibatool epoxy build material instead of the earlier acrylate photopolymers, according to Richard P. Fedchenko, vice president for strategy and market development at 3D Systems. Fedchenko cited last year's introduction of the SLA-500/30H machine, which has a new Coherent 328 laser with twice the power of the laser on the SLA-500/30 model. The new laser provides twice the drawing speed, accelerating the build process.

3D Systems is working on an office concept modeler, which Fedchenko termed a networked solid-object printer to serve the CAD engineer. Designed to be easy to use and nonhazardous, prototype units of the modeler will be evaluated during the remainder of 1995. Production office RP units will appear in 1996.

Jacobs also pointed to two tooling applications he views as very promising: ACES for Wax and Keltool. In the first, the ACES build style is used for limited production tooling using investment casting wax. ACES for Wax fits a niche in which the customer wants 10 to a couple of hundred parts, said Bob Dzugan, metal casting specialist at the Edison Materials Technology Center (EMTEC), a technology-transfer consortium in Dayton, Ohio. "Since it uses casting wax, it also has the big advantage of familiarity for the founders."

EMTEC consortium member Bastech Engineering Services, in Dayton, developed the ACES for Wax process. First, the ACES epoxy build style is used to build a negative image (die halves) of the final part. The RP model is then used as a die for injecting investment casting wax to produce casting cores and cavities. Dzugan warned that the technique requires mold-building expertise to create usable dies. One example of EMTEC's early success is development of a golf putter head made in six parts.

The second highly promising tooling application Jacobs cited was developed at Keltool Inc., in St. Paul, Minn. Keltool, a metal-sintering technology invented by 3M Co., has been adapted and refined by Keltool's current president, Wayne Duescher, and his coworkers to rapidly and economically create tools for small metal parts (no larger than 6 inches) that could be reproduced in quantities from several hundred to millions.

An .STL master, Duescher explained, is used to make long-life production tooling from Stellite, A-6 tool steel, and a composite material of A-6 tool steel, tungsten carbide, and a copper alloy. In the composite material, the dispersion of tungsten carbide enhances wear resistance, while copper improves thermal conductivity by about 30 percent.

In the Keltool process, a stereolithography part designed by a tool and mold specialist is made as a negative of the final shape. The negative pattern is placed in a box filled with RTV rubber, which cures and hardens. Ultrafine metal powder (Stellite, A-6 tool steel particles) is then poured into the rubber positive. The powder has some thermoplastic binder in it, as well as a varied size distribution (from 0.5 to 20 microns) for tight packing and high fill ratio. The molded powder form is heated in a low-temperature oven (100 C) to create a green part. The green part is then demolded and fired in a furnace at 1300 C, which burns out the binder and sinters the part together. The sintered parts can then be infiltrated with copper to produce a tool. The resulting small parts have dimensional accuracy to 0.001 inch per linear inch. Keltool is looking into licensing the technology for in-house and service bureau use, Duescher said.

Selective Laser Sintering

DTM's Sinterstation 2000, which sells for about $340,000, uses a rapid prototyping technology called selective laser sintering (SLS). In the SLS process, a modulated laser beam follows the shape of a slice of a CAD-generated object; it traces the object across a bin of special heat-fusable powders, heating the particles so they fuse or sinter together. After the initial layer is formed, powder is reapplied, and the laser processes the next layer.

Last year, DTM introduced LaserWrite LNC-7000 composite nylon for the Sinterstation 2000. "Glass-filled composite nylon addresses customer requests for build materials with enhanced stiffness and heat resistance," said Michael A. Ervin, vice president of engineering and development and manufacturing for DTM. Tensile modulus for the new material is 408,000 pounds per square inch; flexural modulus is 625,000 lbs./sq. in. The material is said to be easier to form because the presence of spherical glass reinforcement smooths out the laser's thermal effects, eliminating thermally induced curl.

DTM user groups also wanted faster processing to speed build preparation, so the company has made the Intel Pentium processor available on the Sinterstation 2000. "For a large part file, the Pentium can complete the required processing 10 times as fast," Ervin said.

Major progress has also been made in the RapidTool iron/copper prototype tooling process. The RapidTool technology is now being released for its beta-test program, and Ervin expects it to be commercialized by this fall.

DTM engineers are working on enlarging the workspace of the Sinterstation 2000, which involves the success of ongoing technical development programs concerning beam delivery, powder delivery, and thermal control.

Solider Process

Solider RP machines from Cubital, which cost from $275,000 to $445,000, are large, tool-like units that make models using light-curable photopolymers and a photomasking technique analogous to that used to manufacture printed circuit boards. Instead of a laser solidifying a photopolymer into slices, an ultraviolet lamp hardens the material en masse.

Yehuda Baron, Cubital's president and chief executive officer, said his technical staff has redesigned the Solider production cycle to speed it up. By changing the sequence of operations so that support wax is easier to remove, the dewaxing process is about 35 percent faster, he said. Baron added that a new, more flexible acrylic build material, Solimer 7501 has been introduced, offering less shrinkage, improved accuracy, and better surface finish.

Another significant enhancement, Baron said, is the Solider's new CT 2010 ionographic cartridge, the system that creates the photomask via a photocopier-like process. According to Baron, the CT 2010 offers improved image quality--and thus final model accuracy and surface quality--by doing away with the fine saw-tooth patterns previously seen along edges perpendicular to the motion axis.

In a significant move, Cubital has also redirected the Solider process to cope with future applications, Baron said. "We are working to change the cycle completely. The idea is to open up the system to use the photopolymer as the support material and, instead of using wax, to use other materials to build parts." The new "reverse cycle" machines will construct RP parts of castable wax (estimated introduction in the third quarter of 1995), epoxy, and a new acrylic (fourth quarter 1995), and metals and ceramics (first quarter 1996), Baron said. "The engineering part of the new process, which must be compatible with the existing Solider machines, is not yet completed."

Fused Deposition Modeling

Stratasys' $50,000-to-$62,000 fused deposition modeling (FDM) machines extrude heated thermoplastic material from a nozzle positioned above an x-y table, depositing it as thin layers to build parts from the bottom up.

Over the past year, Stratasys more than doubled the size of its facility in the Minneapolis area and opened a development center in Westchester County, N.Y., that will be staffed with former IBM RP researchers, said Scott Crump, chief executive officer. Stratasys purchased IBM's extrusion technology and hired 18 of their people. The idea was to buy IBM's previously performed research in various areas of application and the sophisticated bead modeling techniques that IBM was pursuing, according to Crump.

Several new FDM materials are in the engineering-development stage, including polycarbonate, polypropylene, PMMA (polymethylmethacrylate), and various polyesters. These new materials will be commercially introduced later this year, Crump estimated.

Stratasys is also taking part in the ARPA/ONR effort to develop RP technology for complex, net-shape functional ceramics using materials such as silicon nitride and silicon carbide. Also part of the three-year, $4.2 million project are Rutgers University, AlliedSignal Ceramics Research Inc., Argonne National Laboratories, Advanced Ceramics Research, the University of Michigan, and Lone Peak Engineering Inc.

Laminated Object Manufacturing

Laminated Object Manufacturing (LOM) machines from Helisys use CAD data to position a focused laser beam above a table covered with a sheet of heat-activated adhesive coated paper. The beam cuts a two-dimensional cross-section of the object to be built by incising the slice's outline in the paper. A second layer of paper is then bonded to the top of the first, and the next cross section is cut. The result is a part that looks like laminated wood. LOM systems cost between $95,000 and $180,000.

Helisys received a comprehensive patent for the paper-based LOM technology last year, said Michael Feygin, the company's president. The company also established a business link with Toyoda Machine Tool Co., in Nagoya, Japan, to distribute LOM machines in Japan, and to work jointly on machine/process development.

Feygin said that much of last year's development effort was focused on making the process more consistent and reliable. Cutting build time was another goal. "We improved the stability of the machine's frame by adding stiffening members, so the system does not require as much adjustment as before," he said. New software that optimizes the laser's x- and y-axis movement by permitting continuous path motion has also been implemented. In addition, the z-axis elevator now has a higher-precision actuation system. Helisys engineers have also installed a beam offset feature--0.005 inch overall--which enables users to compensate for the width of the laser beam when cutting part slices. Finally, data preparation procedures have been optimized to cut processing time markedly. These changes have led to a 30-to-50 percent decrease in build time, Feygin said.

Helisys worked with Brown Bridge Kimberly-Clark, a paper products company in Troy, Ohio, to develop a new LOM paper with a proprietary adhesive that activates more quickly. The LOM material also features a new paper base that creates a strong internal bond and low internal stress in completed parts, so that flat parts have less tendency to curl.

Helisys is also working with researchers at the University of Dayton on a $3.5 million ARPA/ONR grant to develop ceramic composite technology.

Office RP Machines

One of the prominent developments in the field last year was the unveiling of two new office-type RP machines. Sanders Prototype started selling its Model Maker MM-6B in July 1994. Billed as a designer's associate, the $70,000 unit's enclosure measures 17.5 inches by 20.5 inches by 23 inches, and its workspace is a 6-inch cube. It needs no special siting requirements and uses nontoxic materials, said Sanders president Al Hastbacka. Forty units have been sold to date.

The Model Makers design was developed by Roy Sanders and colleagues at E-Systems, a defense contractor based in Dallas. Two piezoelectric ink-jet print heads, one for a proprietary thermoplastic building material, the other to deposit support wax, eject 0.003-inch diameter droplets that spread out to a 0.004-inch wide bead that is 0.0025-inch tall. The machine tests its jets repeatedly to validate proper material flow. After deposition, a cutting head mills off 0.0005 inch off the top of the bead to control z-axis accuracy. "The x-y plotter velocity is not constant, so milling off a bit controls this variation," Hastbacka said. A vacuum then removes the residue from the workspace. The Model Maker's table elevator is said to be controlled to within 0.0001 inch. Sanders claims calibrated accuracy of 0.001 inch in the x- and-y axis, 0.5 to 0.002-inch accuracy in the vertical axis, and 100 microinches RMS surface quality. The Model Maker can use .STL and Hewlett-Packard Graphic Language files. POGO International has developed a powerful Windows-based preprocessor program called Model-Win that functions as a graphical user interface, Hastbacka said.

New versions of the software provide easier viewing and editing, an automated surface model slicer, automated support structure insertion, and an automated cellular fill pattern. Droplet jetting has been accelerated to address user complaints about slow build times. The result of these changes is a threefold improvement in preparation time and build rate and a twofold cut in support wax removal time.

The Sanders unit has found a market niche in the jewelry business, "where it can take six months to do one ring by hand," Hastbacka said. The new technology is sufficiently accurate to handle the jewelry's fine detail, and it can also scale the design up or down as desired.

Company researchers are working on a variety of new build materials (including acrylanitrile butadiene styrene, or ABS) and on expanding the workspace envelope.

The other office-type RP machine is from BPM Technology Inc., in Greenville, S.C. BPM purchased the patent and intellectual property related to ballistic particle manufacturing from the technology's inventor, Bill Masters (see "Special Report: Rapid Prototyping Systems," April 1991), said Peter Petrunich, BPM's director of market development.

The Personal Modeler is intended as a mechanical design workstation, so it is office friendly, uses nontoxic materials, and requires little postprocessing, Petrunich said. The device's enclosure is 54 by 24 by 19.5 inches, and its workspace is 10 by 8 by 6 inches.

Using ballistic particle manufacturing, the Personal Modeler droplet by droplet produces relatively crude parts by "digital microsynthesis"--a single piezoelectric ink-jet head sprays out 0.003-inch diameter drops of a heated proprietary thermoplastic build material at the rate of 12,000 per second. Claimed accuracy is 0.001 inch per inch over a span of 2 inches, and 0.002 inch per inch under 2 inches. Surface quality is 250 microinches RMS. The machine also features automatic .STL file correction capability.

BPM has sold several Personal Modeler units, Petrunich said, and product is now shipping. The Model 2000 costs $29,900, and the more elaborate Model 2100 is $34,900.

Most RP industry observers report that users are still looking for multiple parts, done cheaply and quickly, in the final material of choice. The technology is not there yet, but several experts predicted that structural plastics and hard metal tooling will be produced on rapid prototyping systems by 1998.