Rapid prototyping: rapidly getting stronger

Automotive Design & Production, Nov, 2003 by Lawrence S. Gould

When it comes to rapid prototyping (RP), there are a variety of available systems, but one statement holds true across them: "Materials are critical," says Mervyn Rudgley, senior director for product development at 3D Systems (Valencia, CA). And when it comes to equipment, size matters, and for some, smaller is better.

MATERIALS ONLY

ProtoComposites, a family of rapid prototyping (RP) materials recently released by DSM Somos (New Castle, DE), a third-party supplier of RP materials, are mixtures of solids and liquids optimized for RP. Explains Michelle Wyatt, DSM account manager, "These are complex materials in which two or more distinct, complementary substances--especially metals, ceramics, glasses or polymers--combine to produce functional properties not possible using individual components." For example, Somos ProtoTool from DSM, the first in the company's line of commercially available ProtoComposite materials, is a silica-based resin. Its heat deflection temperature is greater than 517[degree]F at 66 psi. Another new ProtoComposite, Somos SolidCast, is a hollow-spherically filled material with low-density investment casting pattern properties--a density that's about half that of conventional resins. Large parts from SolidCast are lighter than those produced with conventional resins. Parts made from SolidCast have a wax-like appearance, a very low heavy-metals content (O.015%), and they require no internal drainage, holes to seal, nor pressure or vacuum testing of patterns.

These materials are in addition to the popular DSM Somos 9100. Parts made from this material replicate polypropylene tensile strength and elongation at yield, and have good mechanical memory. This high-speed liquid photopolymer is a good, all-purpose material for prototyping interior automotive parts. The General Motors Technical Center uses 9100 exclusively. Chrysler uses that material, too. But note this: Compared to Somos 9100 series resins. DSM ProtoTool 20L has six times the flexural strength, 2.5 times the tensile strength, and up to 3.5 times the heat deflection temperature. This is one durable resin--one well suited for functional prototypes like impellers, pump housings, headlight reflectors, and wind tunnel test models.

Unlike DSM, 3D Systems is an RP equipment supplier, too. 3D also has a broad selection of RP materials. For laser sintering RP, 3D has long offered a metal powder called LaserForm ST-100. Its green strength--the strength of material straight out of the machine before curing--was extremely low, explains 3D's Mervyn Rudgley. You had to use a fine-haired bristle brush to clear the powder around a prototype made of this material. In December 2002, 3D announced LaserForm ST-200, which has about three times the green strength of ST-100. ST-200 has the characteristics of P20 steel. It is comprised of 420 stainless steel, which is sintered in the selective laser sintering (SLS) system. The sintered part is then infiltrated with bronze in an oven to produce a dense part or tool with complex geometries and intricate feature detail. Or tens of thousands of parts, even with aggressive injection molding materials. "Engineers will be able to design without the necessity to adapt their designs to the limitations of traditional manufacturing methods," claims Rudgley. 3D is about to release another material for its Vanguard SLS system: LaserForm A6 steel. This material will be stronger still--hardness in the Rockwell C range. (The hardness of ST100 and ST200 are in the Rockwell B range.) A6 material is primarily for injection mold tooling applications.

For stereolithography, 3D released last year its Accura line of materials. Accura SI 10 is a general-purpose material with a long vat life, high green strength, and high humidity resistance. SI 10 yields parts with a glossy top finish, and is well suited for thin-wall parts and for master patterns. SI 20 is a durable white material ideal for snap-fit testing and room temperature vulcanization (RTV) applications. SI 30 is a durable low-viscosity material with a fast photo speed. Last, SI 40 resists high temperatures and its toughness is akin to Nylon 6. Parts out of SI 40 feature optical clarity, high flexural modulus, and moderate elongation to break, with a high heat deflection temperature. "This material is ideal for automotive applications, including under-the-hood applications, wind tunnel testing, and flow analysis," says Rudgley.

Another material suitable for under-the-hood automotive applications comes from the other giant in RP: Stratasys (Eden Prairie, MN). PPSF (polyphenylsulfone) is extremely durable and remains strong at high temperatures. Its heat-deflection temperature of PPSF is 417[degree]F at 66 psi and 405[degree]F at 264 psi. This FDM material resists chemicals, acids, and petroleum products. PPSF is used in Stratasys FDM Titan, which can also use ABS plastic and polycarbonate.

SMALLER IS POPULAR

Here's a tidbit from Rudgley: Laser sintering machines are lying dormant. In fact, the use of RP machines across the board has gone down. This has nothing to do with RP technology and everything to do with the current economy. Several RP companies are targeting small prototyping applications, such as design shops and collaborative office situations--places where a communications tool is needed, says Rudgley, so that "instead of everybody staring at a complex drawing for 10 to 15 minutes trying to work out what they're looking at, they can look at a 3D model." The result is a rash of RP machines that are basically 3D versions of a standard 2D printer. These RP "printers" sit in an office, require no special training beyond what's in the user's manual, and they come with print drivers to load into any standard Windows-based computer.