QUICK-TURN CASTINGS
Rapid manufacturing bridges the gap from prototype to production and opens doors to design flexibility.
O DENISE KAPEL, SENIOR EDITOR
riginally developed to produce models in the product design and development stage, rapid prototyping (RP) and rapid manufacturing techniques offer a fast and economical solution to reduce lead times on cast products. RP is highly suited to casting orders of one or
two pieces, and it provides designers with a flexible option for testing a cast part before production, allowing them to make any required
changes before the final tooling and molds are created. Te process has evolved from prototyping to the manufacture of tooling used in
full-run production for larger volumes of cast parts. Sand molds and cores can be produced from RP patterns as well as “printed” directly on the latest RP equipment. Rapid prototypes often are used for die cast tooling to get a leg up on production while the hard tooling is being created. In the investment casting process, rapid prototypes are used as patterns to produce castings. Machining is another RP option, for both metal tooling and sand molds and cores. Te common denominator among the many RP methods is the 3D CAD data used
to fabricate the parts. In traditional RP, that data is converted into an .STL file and the pattern is “printed” on a machine that employs stereolithography (SLA), selective laser sintering (SLS) or fused deposition modeling (FDM) to build a plastic prototype in layers. Depending on the method, photopolymer, thermopolymer, polystyrene or another material is used. Metalcasters, design engineers and equipment suppliers are continuously developing new RP processes, enhancing the capabilities and potential of the available technology to evolve beyond just the plastic “prototypes.” RP material costs are low due to the elimination of hard tooling. Plastic patternmak- ing is used mainly for smaller cast parts, within one cubic foot. Some niche suppliers can take RP patterns to a larger size, at a greater cost, but RP printers generally have a limited size capacity. Te protoypes can be assembled into a larger pattern for casting. Frequently, multiple small prototype parts are built, side by side, at the same time.
Investment Casting Options For the investment casting process, RP patterns often are produced as plastic
models on SLA, SLS and FDM equipment. 26 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2012
RP prototypes produced on SLA or SLS equipment can be attached to a gating system and run through the typi- cal investment casting process. Plastic tooling offers a significant cost savings and reduces the setup required to pro- duce shells for the final cast metal parts. Te equipment used for SLA builds
layers on a platform that is submerged in a vat of liquid polymer, employing laser technology to cure one layer at a time. Vertical support is needed for products that incorporate horizontal overhangs. Te “steps” between layers are smoothed, and any details are added in post-cure processing. Of the three methods above, SLA is considered the fastest and offers the best surface finish. Coming in at a close second in total speed of production, SLS offers more flexibility as it incorporates powders cured with a laser to sinter the thin lay- ers that compose a finished pattern. Its surface finish is not as smooth as what can be achieved with SLA, and both methods have limitations in terms of the prototype piece’s durability over time. Te FDM method applies a continu- ous filament of thermoplastic polymer or wax through an extruding head, con- structing the pattern layer by layer with no required supporting structure for hor- izontal overhangs. Te result is a stronger pattern than those created through other
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