Rapid Metalcasting Methods O
Rapid manufacturing bridges the gap from prototype to production and opens doors to design flexibility.
riginally developed to pro- duce models in the product design and development stage, rapid prototyping (RP) and rapid manufactur-
ing techniques utilized in metalcasting offer a fast and economical solution to reduce lead times on cast products. RP is highly suited to short-run
casting orders, 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 pro-
totyping 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 cast- ing process, rapid prototypes are used as patterns to produce castings. Machining is another RP option for metal or plastic tooling, foam patterns and sand molds and cores. Te common denominator among the many RP methods is the 3-D
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, enhanc- ing 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 pat- ternmaking 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. Fre- quently, multiple small prototype parts are built, side by side, at the same time.
Investment Casting Options For the investment casting pro-
cess, RP patterns often are produced
as plastic models on SLA, SLS and FDM equipment. RP prototypes produced on SLA or SLS equipment can be attached to a gating system and run through the typical investment casting process. Plastic tooling offers a significant cost savings and reduces the setup required to produce 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
continuous filament of thermoplastic polymer or wax through an extruding head, constructing the pattern layer by layer with no required supporting structure for horizontal overhangs. Te result is a stronger pattern than those created through other RP methods. But as the steps between wider layers are larger, the finish quality is not as high. FDM machines are quiet and offer an environmentally friendly option, without the volatile organic compound emissions present in the resins used for other methods.
Sand Casting Solutions For sand casting, technology ex-
The CAD file of a pattern is programmed into a machining center. The patterns then are machined in layers to suit the lengths of the cutters.
12 METAL CASTING DESIGN & PURCHASING
ists to “print” sand molds and cores directly, eliminating the patternmaking step. Plastic protoypes created using additive methods of rapid manufactur- ing can be used as tooling to produce molds and cores, as well. Machining is another commonly used RP method for sand tooling, molds and cores. RP equipment manufacturers have
2016 CASTING SOURCE DIRECTORY
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