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Photopolymers build a solid future.

Currently, the leader of the free-form fabrication field is 3D Systems Inc. Founded in 1986 by Raymond Freed and Charles Hull, 3D Systems engineered the ultraviolet laser/photo-polymer-based stereolithography process technology during 1987. Shipments of the company's SLA line of systems began the next year, and by 1990, revenues had reached $30 million. Ciba-Geigy Ltd. (Basel, Switzerland), a chemical company that is a major supplier of photopolymer resins, owns 38 percent of 3D Systems,

To date, about 300 SLA systems have been ordered by businesses in the automotive, aerospace, medical, and electronics industries, said Dennis Medler, vice president for sales and marketing. Some 250 of these systems have been delivered to their buyers and are up and running.

The midrange SLA-250 model accounts for approximately 235 of the 250 installed systems. It constructs parts up to 10 x 10 x 10 inches with a 16-milliwatt helium-cadmium laser and costs $185,000.

About 12 high-end SLA-500 systems, which was introduced in May 1990, are now operating. The $385,000 SLA-500 system features a 200-milliwatt argon-ion laser that builds parts up to 20 x 20 x 24 inches with high throughput rates.

First brought to market last July, the entry-level SLA-190 system has been installed at five sites. The $95,000 SLA-190, which features a user-friendly computer interface, uses a 7.5-milliwatt helium-cadmium laser to produce models up to 7.5 x 7.5 x 9.8 inches.

Stereolithography starts with a solid or surface CAD model of an object that is downloaded to a slicing algorithm, which cuts the CAD model into many thin layers. The part itself is built in a vat of photocurable liquid acrylate resin that hardens when laser light hits it. An elevator table in the polymer vat rests just below the liquid surface. An ultraviolet laser beam is deflected in the x and y axes by galvanometer-driven mirrors so that it moves across the surface of the resin to produce a solid pattern. Part borders are scribed with the laser beam and the layer's surface is formed with x-y cross-hatching passes.

After a layer is built, the elevator drops a programmed distance so that a new coating of liquid resin covers the solidified layer. A wiper balde then sizes the coating depth. Next, the laser draws a new layer on top of the first one, so the model is built layer by layer from the bottom up. When the part is completed, it is placed in a curing oven, which floods the object with ultraviolet light to complete the polymer solidification process. The part is then ready for painting and sanding. Part accuracies are [+ or -] 0.005 inch.

Over the last year, researchers at 3D Systems developed a new part-building technique called Weave, which improves dimensional tolerances. Typical x-y cross-hatching methods produce a rather fragile matrix of thin-walled chambers that trap liquid or semicured resin inside like water is trapped in partially frozen ice cubes in a freezer tray. Often, only 50 percent of the acrylate material is polymerized. These standard techniques, which minimize the amount of time the laser is operating, rely on the post-curing oven to achieve full solidification.

Experience has shown that postcure warpage is reduced and surface finish improves as a greater portion of the liquid resin is cured in the vat. Post-cure distortion decreases, for example, because there is less overall post-cure shrinkage. But curl distortion, in which layers separate from the structure, increases dramatically as the beam cross-hatching is tightened up and the cure fraction approaches 100 percent, due to the buildup of internal stress.

Researchers in 3D Systems' process development group evaluated alternate part-building schemes to determine whether they could achieve a high cure percentage while maintaining low curl distortion. They found that the most efficient building method is Weave, which is characterized by a closely spaced (0.001-inch) x-y beam hatching that is exposed so that the cure depth is slightly less than the polymer layer thickness. Thus, the x-axis hatch doesn't adhere to the lay below or to itself. When the y-axis hatching is drawn over the x-axis hatching, the additional light exposure leaves little liquid resin remaining in the part, resulting in 96 to 98 percent polymerization. In addition, the double exposures at the hatching intersections tack the layers together. The minimum overcuring produced by Weave limits curt distortion because the polymer structure has some give in it that allows it to accommodate internal stresses. Even though more cross hatches are used in the Weave method than with previous hatching techniques, each individual cross hatch has less light exposure, so the total building time is not substantially increased. 3D Systems recommends that SLA owners equip their systems with the company's 80386-based control processor upgrade to use Weave, which speeds up the loading of hatching commands.

Machine Tool Accuracy

Industrial-strength stereolithography is the best description of the approach taken by Cubital Ltd., an Israeli/German company formed in 1987 that has 35 employees based at Herzlia Industrial Park, Israel. Cubital's room-sized Solider 5600 system weighs several tons and costs $490,000, but its high throughput and robust design make it well suited to high-volume use in large manufacturing companies and service bureaus.

"Our philosophy is that the Solider system is not a peripheral to a CAD system, but a machine tool that produces parts with machine-tool accuracy," said Haim Levi, chief operating officer of Cubital America Inc., Cubital's Warren, Mich.-based U.S. subsidiary.

Three prototype Solider 5600 systems have been built. Each consists of a DEC Vax workstation, slicing and control software, the model-building machine, and a cleaning station. The developmental version is at Cubital's Israeli headquarters, while beta systems operate at demonstration/service centers in Warren, Mich., and near Zurich, Switzerland.

The Solider 5600, which cost $7 million to develop, uses light-curable acrylate photopolymers and a photomasking technique similar to the process used to manufacture printed circuit boards. Models as large as 20 x 20 x 14 inches can be built. According to Levi, the machine produces models with 0.1 percent dimensional accuracy in the x, y, and z directions. Cubital's CAD interface reads both the industry-standard .STL files as well as Structural Dynamics Research Corp. (SDRC) Universal files, which allow precise curve-fitting techniques to be used.

Shipments of the production version of Solider are expected by the third quarter of 1991. "We're now adapting the prototype to the production environment, in part by exploiting the factory-floor expertise of Maho AG, a German machine-tool builder that owns a 12 1/2 percent interest in Cubital," Levi said.

Like all rapid prototyping processes, the Solider 5600 starts with a solid or surface CAD model that is sliced into thin cross sections. A slice is then transferred from the computer to the mask generator, which operates something like a photocopier: a negative image of the cross section is produced on a glass mask plate by charging portions of the surface and "developing" the electrostatic image with toner powder.

In the meantime, a thin layer of liquid photopolymer is spread across the surface of the workbench. The mask plate with the negative image of the cross-sectional slice is then positioned over the workbench. A shutter above both mask and workbench opens for 2 seconds, allowing strong ultraviolet light from a 2-kilowatt lamp to expose and solidify the photopolymer layer all at once. Areas external to the model are left in liquid form.

The exposed mask is then physically wiped down and electrostatically discharged, erasing the mask plate and making it ready for the next negative cross-section image. At the same time, the uncured polymer is removed from the workbench by the combination of forced air and vacuum pressure and is collected for reuse. The workbench moves to the next station, where hot wax is laid down to fill the cavities left by the uncured polymer. At the next station, a cooling plate is applied to solidify the wax, which acts as a support structure to reduce distortion due to gravitational or shrinkage effects. Finally, the surface of the entire polymer/wax layer is milled with a cutter to the desired thickness, which makes the workpiece surface ready to accept the next polymer layer.

The steps are repeated until the part is completed. After the model is constructed, the supporting wax is removed with microwave energy, hot air from a blower, and a rinse with solvent. Because each layer is fully cured, no post-curing is required, Levi stressed. Each cycle currently takes about 110 seconds. Levi predicted that the cycle time for producing a layer will drop in the production version of the system.

"In the future," Levi said, "we intend to enlarge our line, though we're not sure whether the next system will be larger or smaller. In addition, we want to customize the system to specific applications. We are also looking into low-volume part manufacturing for series-production situations in which the designs change from day to day." Researchers at Cubital are said to be incorporating more sophisticated, though as yet unspecified, imaging technologies into the newer machines.

Design Differences

A recent entry into the rapid prototyping race is Quadrax Laser Technologies (Portsmouth, R.I.), a company best known for its work in advanced composites. Quadrax's photopolymer-based Mark 1000 Laser Modeling System is outwardly similar to 3D Systems' SLA units, but there are design differences.

Larry Girouard, Quadrax's vice president for marketing and sales, said that the company acquired the basic model-building technology for the Mark 1000 from Laser Fare Inc. After further engineering, the Mark 1000 system was introduced in February 1990. A turnkey system, including the modeling unit, curing oven, CAD software interface, and initial supply of resin, costs $195,000. To date, two Mark 1000 systems have been shipped.

The Quadrax system uses a 5-watt visible light, argon-ion laser to cure photopolymer resin in a one-cubic-foot tank, Girouard said. "Visible light lasers are more powerful and last longer than the ultraviolet lasers that have been used in a number of photopolymer systems. In addition, the viscosity of visible light resins is a lot lower than UV resins, Girouard said. "This means our resins settle more quickly when a new layer is applied so there's less delay."

Quadrax's laser scanning system employs three galvanometers rather than two. "Two galvanometers drive the mirrors that deflect the beam in the x and y directions, while a third galvanometer moves a flat-field beam-correction lens back and forth to adjust the beam's focal length in real time," Girouard said. "This dynamic focus system allows the operator to enlarge the beam diameter without having to slow down the scanning speed. Essentially, it gives users different-sized paintbrushes. For example, in fine mode, with the spot size at 0.0035 inch, the laser runs at 125 milliwatts. When using the broadest brush 0.125 inch), the laser is pumped to 5 watts." Most stereolithography systems use an elevator to position parts for application of the next layer by lowering and raising the bed. Instead, the Mark 1000 system pumps in additional resin from above. "We use no elevator because we wanted to avoid the distortion that can be caused when fragile, partially cured walls move up and down through viscous photopolymer liquid," Girouard said. In the Quadrax design, the resin is pumped up to an applicator device from which it drops down to coat the part. A wiper blade then normalizes the surface and the laser starts scanning. "The key is that the part doesn't move, the optics do," according to Girouard.

Girouard claimed that the Quadrax system cures a greater percentage of the resin than do ultraviolet laser systems. "This means that encapsulation of liquid polymer within thin, semicured walls is not needed. It leads to more homogeneous structures that shrink less," he said. Post-curing with ultraviolet lamps is still required.

The Mark 1000 is equipped with an 80486-based central processing unit. Its CAD interface accepts both .STL and SDRC Universal files. To produce precisely modeled parts, users can control the combination of process parameters such as laser power, beam spot size, layer thickness, and beam scanning speed. Dimensional tolerances of [+ or -] 0.005 inch can be attained. Layer thicknesses can be varied between 0.002 and 0.015 inch.

Software features of the Quadrax system include laser beam offsetting, which avoids the creation of surface bumps caused by the double exposure that occurs when the beam rescans an area. For example, the end points of the surface hatching passes stop one beam radius from the outer perimeter of a surface to avoid overcure.

Quadrax engineers are six months away from adding software algorithms that provide laser-depth off-setting, a feature that would avoid overcuring on overhang features caused by the laser's constant cure depth. Girouard foresees the development of expert systems to control the model-building process parameters automatically.

Light Sculpting

One of the pioneers of the rapid prototyping business is Light Sculpting Inc. (Milwaukee), which developed its proprietary model-building process in 1986. Since then, the small firm has operated as a fast-prototyping service bureau while the technology was developed further.

According to Light Sculpting's president and driving force, Efrem V. Fudim, the company's layer-building process is comparatively fast and accurate because "it solidifies liquid photopolymers a layer at a time rather than a point at a time as laser-based systems do."

In the LSI-0609MA system, a 140-watt ultraviolet lamp irradiates an entire layer of liquid acrylate photopolymer through a photomask. In a process akin to printed circuit board fabrication, the mask, which is a negative image of a slice or cross section, is produced by using a photo-plotter to transfer the inverse image from a solid or surface CAD model to the glass masking plate. Transparent and opaque patterns on the photomask correspond to the hollow portions in the slice through the object.

"To prevent warpage and shrinkage distortion the irradiated surface is constrained and supported from above by a rigid glass plate positioned below the mask," Fudim said. A key to the Light Sculpting technology is a proprietary material on the bottom surface of the glass plate. When this plate is placed in contact with the unexposed liquid surface of the photopolymer, the proprietary material preserves the cross-linking capability of the surface polymer so that the subsequently deposited polymer layer adheres to the irradiated surface. The special material also assures that the glass can be removed without distorting the solidified slice, he said.

Each layer is formed in seconds. A new coat of polymer is then applied and the next masking plate is positioned above the workpiece ready for ultraviolet irradiation. The total cycle time for each layer is less than one minute.

Parts up to 6 x 6 x 9 inches can be manufactured in the LSI-0609MA. Dimensional resolution of the process is from 0.00025 to 0.0015 inch, depending on the accuracy of the photoplotter that is used. Layer thicknesses vary from 0.001 to 0.050 inch. A beta-test version of the LSI-0609MA is $75,000. A production unit costs $100,000.

In future LSI systems, according to Fudim, photoplotter masks will not be used. Negative slice images will be transmitted directly to the polymer surface using a flat array Of backlighted liquid crystals under individual control. He predicted that a working prototype would be ready by midyear.

Brute-force Imaging

As an early entrant into the rapid prototyping field, du Pont/Somos Venture (New Castle, Del.) was expected to make its weight felt in no uncertain terms. The company was formed by du Pont Imaging Systems (New Castle), which is a subsidiary of the chemical giant E.I. du Pont de Nemours & Co. (Wilmington, Del.). When it unveiled the Somos 1000 Solid Imager, several industry experts said the unit was one of the more sophisticated model-building systems yet produced. Add du Pont's unquestioned expertise in developing polymer resin systems, and it's little wonder that much was expected from Somos. Unfortunately, to date du Pont's foray into rapid prototyping has remained mostly on hold because of the company's reluctance to sell equipment and because the photopolymer market remains relatively small.

"We have decided not to enter the equipment market and are currently in negotiations with potent licenses to transfer the Somos solid imaging technology to others," said Daniel J. Mickish, senior research associate and Somos coordinator. "We are also giving serious consideration to remaining in the solid-imaging materials business, provided that satisfactory working relationships can be formed with key solid-imaging-equipment manufacturers.

"Our philosophy in developing the System 1000 is what some might call the brute-force method of solid imaging," Mickish said. The Somos 1000 Solid Imager, which operates in a manner similar to the 3D Systems' SLA machines, uses a 500-milliwatt argon-ion ultraviolet laser with a high-precision scanning system and high-speed beam modulation. Quoted accuracies for the System 1000 are: x and y axes, 0.002 inch/inch; z axis, 0.006 inch/inch;. and layer thickness, 0.005 to 0.020 inch.

The Somos 1000 uses a Unix workstation to process CAD data and a Windows-based user interface with extensive graphics capability. The system accepts .STL files.

Models are built in a one-cubic-foot vat that is equipped with a high-speed resin coating system to lay down new layers of liquid photopolymer. "With its partial beam overlap, the laser has sufficient power to fully cure the Somos 2100 photopolymer as each layer is formed," Mickish said. For increased strength, a post-process heat treatment causes additional polymer cross linkage.

"The Somos 2100 photopolymer is substantially different from other photopolymers because acrylates make up only a minor part of its composition," he said. A du Pont technical paper on the 2100 formulation claimed high photospeed, low shrinkage and warpage, wide exposure latitude, flexible and homogeneous photoformed parts, and good layer-to-layer adhesion.

Another technical paper indicated that du Pont polymer scientists are working on a variety of photocurable materials for different applications, some flexible, some rigid, some transparent, others suitable for investment casting use.

Currently, eight Somos 1000 systems are running in-house at various du Pont facilities. "We continue to maintain a Somos service bureau operation to demonstrate our materials capability and maintain customer contact in the solid image/rapid prototyping market," Mickish added.

Other Efforts

Word has come from Japan that several Japanese companies have entered the rapid prototyping field. Sony Corp. (Tokyo) has reportedly built a free-form fabrication system based on lasers and photopolymers said to be similar to 3D Systems' SLA machines. Parts come from the vat fully cured. The system is said to incorporate relatively sophisticated build techniques that provide improved structural properties. Sony has reportedly built four machines and has 10 more on order. Company management is said to foresee the need for about 100 such systems for use in house.

Another Japanese firm, Mitsubishi Corp. (Tokyo), has reportedly sold about 15 photopolymer-based systems in Japan.

In the United States, the company that developed the technology used in Quadrax's Mark 1000 system, Laser Fare, is working on a new laser-based design that uses nonlinear optics. "With nonlinear optics, you can take one input wavelength out with a broad band of many wave-lengths," said Laser Fare president Terry Feeley. "The variety of wave-lengths allows you to build models with several materials besides acrylate photopolymers." This approach, which is to employ a 50-watt laser, should make it easier to produce parts with engineering properties, Feeley claimed. He expects the new design to be completed in the first half of 1992. Laser Fare plans to incense the new technology to other firms.

Rapid Prototyping Technology (All Methods)

The following is a partial list of rapid prototyping technology developers and the equipment they make. For each system, information in the second column indicates model name, process, material used, energy source, maximum part size, equipment price, and availability.

Cubital America inc. 23467 Ryan Rd. Warren, MI 48091 (313) 754-7557

Soldier 5600; solid base curing; acrylate liquid photopolymers; 2-kW ultraviolet lamp; 20 x 20 x 14 in.; $490,000; shipping 3d quarter 1991

DTM Corp. 8716 Mopac N. Suite 200 Austin, TX 78759 (512) 339-2922

SLS Model 125; selective laser sintering; polycarbonate, polyvinyl chloride, and investment casting wax powders; 23-W carbon dioxide laser; 12 in. dia. x 15 in.; $300,000-$400,000; shipping 1st quarter 1992

du Pont/Somos Venture New Castle Corporate Commons Two Penns Way, Suite 401 New Castle, DE 19720 (302) 328-5435

Somos 1000 Solid imager; solid imaging; acrylate liquid photopolymers; 500-mW argon-ion ultraviolet laser; 12 x 12 x 12 in.; technology licensing available

Helisys inc. 2301 205th St. Suite 107 Torrance, CA 90501 (213) 782-1949

LOM; laminated object manufacturing; plastic, foil, paper, and composite sheet; 40-W carbon dioxide laser; 15 x 10 x 15 in.; $100,000; beta systems available

Light Sculpting Inc. 4815 N. Marlborough Dr. Milwaukee, Wi 53217 (414) 964-9860

LSI-0609 MA; layer-at-a-time fabrication; acrylate liquid photopolymers; 140-W ultraviolet lamp; 6 x 6 x 9 in.; $100,000; shipping beta systems

Massachusetts Institute of Technology 77 Massachusetts Ave. Cambridge, MA 02139 (617) 253-5381

No product, three-dimensional printing; ceramic powders and colloidaloxide binders; 3 x 3 x 3 in.; no price; system under development

Perception Systems Inc. Box 8002 1110 Powdersville Rd. Easley, SC 29640 (803) 859-7518

No name yet; ballistic particle manufacturing; organic wax; resistance-heating element; 12 x 12 x 12 in.; about $50,000; system under development

Quadrax Laser Technologies Inc. 300 High Point Ave. Portsmouth, Ri 02871 (401) 683-6600

Mark 1000 Laser Modeling System; laser modeling; acrylate liquid photopolymers; 5-W argon-ion visible light laser; 12 x 12 x 12 in.; $195,000; shipping

Stratasys Inc. 7411 Washington Ave. S. Minneapolis, MN 55439 (612) 941-5607

3D Modeler; fused depositon modeling; nylon-like thermoplastic, machinable wax, investment casting wax filaments; resistance-heating element; 12 x 12 x 12 in.; $178,000; shipping beta systems

3D Systems Inc. 26081 Avenue Hall Valencia, CA 91355 (805) 295-5600

SLA-190; stereolithography; acrylate liquid photopolymers; 7.5-mW helium-cadmium ultraviolet laser; 7.5 x 7.5 x 9.8 in.; $95,000; shipping

SLA-250; stereolithography; acrylate liquid photopolymers; 16.0-mW helium-cadmium ultraviolet laser; 10 x 10 x 10 in.; $185,000; shipping

SLA-500; stereolithography; acrylate liquid photopolymers; 200-mW argon-ion ultraviolet laser; 20 x 20 x 24 in.; $385,000; shipping
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Title Annotation:use in manufacture of engineering prototypes
Author:Ashley, Steven
Publication:Mechanical Engineering-CIME
Date:Apr 1, 1991
Words:3698
Previous Article:Rapid prototyping systems.
Next Article:Making models one layer at a time.
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