A Quick Look at Rapid Prototyping. solid model - The technology for creating a physical model directly from a computer-aided design CAD

Automotive Manufacturing & Production, Sept, 2001 by Lawrence S. Gould

Nowadays you have a slew of machines to choose from for making physical reality out of virtual solid models.

Rapid prototyping (RP)--the technology for creating a physical model directly from a computer-aided design (CAD) solid model--has grown quite a bit since its introduction in the early 199Os. For starters, there are a lot of RP technologies. Second, there are a lot of vendors making RP machines. Third, these machines are getting smaller, while the models they can create are getting larger and more accurate.

All the RP technologies have some things in common. All are additive processes (machining is subtractive). All the RP machines "grow" models one, thin, two-dimensional layer at a time--from the bottom up. Models are grown on an elevator-Like platform, which is lowered one layer-height once that layer is completed. Each layer is a cross section of a solid model created in CAD. The thinner the layer, the smoother the finish on the completed model; however, once the model is complete (after curing and support structures removed, if required), most of them, depending on material, may be sanded, plated, painted, or finished in some way.

There are differences. RP technologies are mostly either "dry" or "wet" processes. Most of the RP machines solidify some sort of loose powder, liquid, or semi-liquid. One RP machine cuts through adhesive-coated sheets of material. RP powdered materials are either some sort of polymer, powdered metal, or wax. One company uses starch. Some of the powders require a binder. The liquid materials are photosensitive polymers that solidify when exposed to either laser or ultraviolet (UV) light. Wet RP processes generally require a curing phase.

Here's a rundown of several RP technologies.

Dry: Direct Metal Deposition (DMD)

DMD, commercialized by Precision Optical Manufacturing (Plymouth, Ml), uses a computer numerically controlled (CNC) laser to fuse layer-upon-layer of metal powder. The resulting prototypes--made from H13 tool steel, aluminum, and other metals--are finished injection and die casting molds meant to be used in production.

While the layering process is slow--for steel, the deposition rate is approximately 1-in. 3/hr--DMD has one big advantage over other RP methods: the metallic composition of the finished parts can be altered "on-the-fly" by Adding different types of metal powders to the mix (e.g., adding copper for heat sinks in tool stell molds).

Dry: Fused Deposition Modeling (FDM)

FDM from Stratasys Inc. Eden Prairie, MN) acts like a finely controlled hotmelt glue gun. But instead of glue, FDM gingerly extrudes an ultrathin layer of thermoplastic filament from a spool. Actually, two filaments are extruded: one for the model and the other for the undercut/overhang support. FDM modeling materials include ABS investment casting wax, elastomer, polycarbonate, polyphenylsulfone, and durable polyester. Stratasys makes RP machines ranging from small, networkable "office modeling systems" to large standalone machines. The office systems can make parts as large as 12 x 8 x 8 in. at a rate of 4 in./sec. accuracy is [ or -]0.013 in. The standalone machines can build models measuring 23.6 x 19.7 x 23.6 in. The accuracy of these models, when larger than 5 in., is [ or -]0.0015 in./in.

Dry: Laminated Object Manufacturing (LOM)

The LOM process from Cubic Technologies, Inc. (Carson, CA), which acquired LOM from Helisys, builds wood-like parts using a laser to cut layers of thin paper coated with heat-activated adhesive. This paper is individually cut and bonded together until the model is finished. Along the way, crosshatches are cut into the excess paper. The finished part out of the LOM system is inside a solid block of material as big as the work envelope. This excess material, along with other unwanted material within the part, is removed manually. LOM models are accurate to 0.002 in. along the Z-axis and 0.005 in. overall. Large parts-up to 22 x 32 x 20 in.-can be made at a rate of 3 to 7 hours per vertical inch. Thick- walled parts are made just as fast as thin-walled ones.

Dry: Selective Laser Sintering (SLS)

DTM Corp. (Austin, TX) uses a CNC laser to draw cross-sections in a bed of fine, heat-fusible powder. The laser raises the temperature of the powder particles momentarily to where they sinter. Hence the name SIS. ("Suntering." just as a refresher, means welding without melting.) SLS works with a broad range of materials, including rigid thermoplastics, thermoplastic elastomers, polystyrene, stainless steel powder, investment casting wax, and ceramic powder. DTM's latest SLS RP machine can create complex parts with features as thin as 0.020 to 0.025 in. in a work envelope measuring 15 x 13 x 18 in.

Wet: Stereolithography Apparatus (SLA)

The SL series of machines from 3D Systems (Valencia, CA) creates models as large as 20 x 20 x 23.75 in. having a laser "draw" cross sections of the model in a vat of liquid pho boundaries of the cross as its internal structure, are draw and cured-- by laser under separate intense UV fies the model's internal structure.