Basics of Rapid Prototyping

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

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.

Three-Dimensional Printing (3DP)

Imagine an ink-jet printer. Now think of it producing 3D prototypes instead of printed pages. That's the 3DP technology invented at Massachusetts Institute of Technology (Cambridge, MA). A variety of RP vendors have licensed that technology to make relatively small and inexpensive RP machines that can make a quick, on-demand concept model, as well as more durable prototypes for production (as in for making dies).

3DP: Direct Shell Production Casting (DSPC)

Soligen Technologies Inc. (Northridge, CA) is a rapid-castings company. Its DSPC process produces ceramic casting molds for metal casting using a layer-by-layer printing process. The process involves a multijet print head depositing liquid binder onto a layer of ceramic powder. After a mold is "printed," it is fired to create a rigid ceramic mold. You can pour any molten metal into these molds, thereby eliminating several steps required in investment casting. Plus, these molds are more accurate than those from standard sand casting. Tolerances for lengths smaller than 1 in. are [+ or -]0.021 in.; for lengths greater than 6 in., accuracy is [+ or -]0.031 plus 0.003 in./in. over 6 in.

3DP: Photopolymer

The Objet Quadra printer from Objet Geometries (Mountainside, NJ) should be commercially available about now. The printer has 1,536 ink-jet nozzles that spit out a proprietary photopolymer that is cured under UV lamps located on the ink-jet head assembly. For models with undercuts and overhangs, the Quadra deposits support material made of a photopolymer designed for easy removal. The Quadra printer has a print resolution of 600 dpi x 300 dpi x 1270 dpi (X, Y, Z); the prototypes it creates can measure 10.6 x 11.8 x 7.8 in.

3DP: Powdered metals

The ProMetal Div. of Extrude Hone (Irwin, PA) uses an electrostatic ink-jet printing head to deposit a liquid binder material onto powder metals. The resulting metal part is then sintered in a furnace and infiltrated with secondary metal. These printers can make steel molds and parts up to 40 x 20 x 10 in. at over 250 [in.sup.3]/hr.

3DP: Starch

Z Corp. (Burlington, MA) uses a conventional print head from an ink-jet printer to produce full-color (24-bit, six million colors) models using a special--and inexpensive--starch powder for the model and binder as "ink." (You can just about eat your design failures.) Models can be as large as 8 x 10 x 8 in., with a model accuracy of [+ or -]0.005 in. in the X- and Y-axes, and [+ or -]0.010 in. in the Z-axis. Finished parts can be dipped in wax; sanded, finished, and painted; or infiltrated with another material, such as an elastomer, urethane, cyanoacrylate, or wax. Z Corp. also offers a plaster-based powder for greater part strength (10 MPa versus 4 MPa), which is particularly suited for delicate or thin-walled parts.

3DP: Thermoplastic

RP printers from Solidscape Inc. (Merrimack, NH) are best for making high-precision tooling and casting patterns, and for making tiny, intricate parts. Solidscape's RP machines have two ink-jet heads that spew out 6,000 to 12,000 droplets per second. One head deposits a non-toxic thermoplastic material similar to an investment casting wax. The other deposits a red wax that serves as a sacrificial support.

Solidscape's RP machines have a 34 x 26-in. footprint and can make models measuring 12 x 6 x 8.5 in., with accuracy [+ or -]0.001 in./in., surface finish between 32 to 63 micro-inches (RMS), and a minimum feature size of 0.010 in. (3D Systems offers a similar printer that makes models as large as 10 x 7.5 x 8 in.)

Principal Rapid Prototyping TechnologiesTechnology *               CompanyDirect Metal Deposition    POM (Precision Optical(DMD)                      Manufacturing) GroupDirect Shell Production    Soligen Technologies Inc.Casting (DSPC)Fused Deposition Modeling  Stratasys Inc.(FDM)Ink-jet printing           Extrude Hone Corp.Ink-jet printing           Objet Geometries Ltd.Ink-jet printing           Solidscape, Inc.Ink-jet printing (Multi    3D Systems Inc.-Jet Modeling, MJM)Ink-jet printing           Z Corp.Laminated object           Cubic Technologies, Inc.manufacturing (LOM)Selective Laser            DTM Corp.sintering (SLS)Solid Ground               Objet Geometries Ltd.Curing (SGC)Stereolithography          3D Systems Inc.Apparatus (SLA)Technology *               RP processDirect Metal Deposition    Laser beam bonds(DMD)                      powdered metalsDirect Shell Production    Multijet print head depositsCasting (DSPC)             liquid binder onto powder layer                           to produce ceramic casting moldsFused Deposition Modeling  Melted resin from a spool(FDM)                      extruded directly into partsInk-jet printing           Electrostatic ink-jet printer deposits                           liquid binder material onto powdersInk-jet printing           Ink-jet printer sprays layers of droplets                           of photopolymer cured by UV lightInk-jet printing           Ink-jet printer sprays layers                           of droplets of thermoplasticInk-jet printing (Multi    Ink-jet printer sprays layers-Jet Modeling, MJM)        of droplets of thermoplasticInk-jet printing           Ink-jet printing using starch- or                           plaster-based powders, plus a binderLaminated object           Laser cuts sheets of adhesive-coatedmanufacturing (LOM)        paper laminated into a single modelSelective Laser            Laser melts and fusessintering (SLS)            powdered materialsSolid Ground               UV light through a mask solidifiesCuring (SGC)               photo-sensitive liquid resinStereolithography          Laser solidifies photo-sensitiveApparatus (SLA)            liquid resin(*)(trademarked name, if applicable)

Material Issues

Just as there are various machines, there is an array of materials to select from, depending on both the machine's capability and the model's application.

Thermoplastics:

For durable plastic parts and patterns or test parts for aggressive functional testing. Resists heat and chemicals, provides an excellent surface finish, is machinable and weldable, and can be joined mechanically or with adhesives.

Elastomers:

For flexible, rubber-like prototypes and parts. Features high elongation, water impermeability, and heat, abrasion, and chemical resistance.

Photopolymeric and polypropylene-like resins:

For near end-use plastic prototypes right out of the RP device. Offer various characteristics including durability for fine features and thin walls snap-back memory, translucency, good thermal performance, and humidity resistance.

Foundry wax:

For small quantities of investment casting parts or for creating complex patterns without tooling. Foundry waxes can be infiltrated with other materials to make the resulting RP models work well with cast ferrous and non-ferrous metals, as well as autoclaves, low-temperature furnaces, and vacuum plaster casting methods.

Powered metals (infiltrated or not):

For complex metal tooling and durable metal molds for injection molding and for directly creating metals parts. Has high thermal conductivity and can be plated, textured, machined, or worked with electrical discharge machining equipment.

Polycarbonate, ABS, and polyphenylsulfone:

For durable, high-strength, functional prototypes for testing and final design verification, as well as for producing tooling patterns and masters for casting and spray-metal tooling applications. Feature high impact resistance, toughness, heat stability, rigidity, and chemical resistance to corrosive agents such as oil, gasoline, and acids. Prototypes can be machined, drilled, tapped painted, glued, and sanded.