Not satisfied with the speed and accuracy of current rapid-tooling techniques, Dynamic Tooling, an injection molder in Fresno, Calif., recently devised two new ways to make tooling in a hurry. First, the "PolySteel" process produces mold inserts from a semi-solid epoxy/steel compound that is pressure formed directly over a stereolithography or other rapid-prototype master. The inserts are more than 90% steel, providing good thermal conductivity. According to company president Paul Vawter, PolySteel tooling is 300-400% stronger than conventional aluminum-filled epoxy and more abrasion resistant than 6061 T6 aluminum.
Compared with other rapid-tooling methods, PolySteel is more accurate, Vawter says. He cites accuracy of [+ or -]0.025-0.050 mm per 25 mm (including pattern error) and repeatability of [+ or -]0.0013 mm. Vawter adds that epoxy/steel can provide a mirror-like finish of 2 RMS. And it doesn’t have the size limitations of some rapid-tooling processes.
Besides making core and cavity inserts, PolySteel can produce complete injection molds, including the ejector sleeves, cooling lines, and sprue hole. "You don’t even need a mold base," says Vawter. From the time the rapid-prototype model is in hand, complete injection molds reportedly can be produced in as little as 24 hr.
Dynamic Tooling has used epoxy/steel cavity inserts for its own custom molding jobs. One eight-cavity tool for ABS sprinkler heads has survived more than 18,000 shots. PolySteel inserts have also produced several hundred shots of a 30% glass-filled nylon automotive fuse cover that is molded at temperatures above 550 F.
Epoxy/steel gets noticed
Though just a few months old, the epoxy/steel method has attracted interest from major manufacturers. Eastman Kodak is evaluating it, as is General Motors’ Delphi Automotive & Lighting Systems subsidiary. According to Delphi engineer Tom Greaves, Delphi built a PolySteel tool with fragile internal details and ran 250 parts of PP, ABS, and 30% glass-filled nylon at temperatures up to 575 F and injection pressures to 1000 psi. "We did not see any erosion in the gate after the tryout was complete," Greaves reports.
Dynamic Tooling will license its process and provide training.
Another method creates mold cores and cavities from powdered steel faster than traditional machining. The powder-metal (P/M) forging process uses a rapid-prototyped part model, ceramic punches, and a hot press to transform powdered H13 or P20 steel into production-quality mold inserts.
The punches, which carry the surface detail for the insert, are made by casting a ceramic slurry into an RTV impression of the part model. Alternatively, the slurry can be cast directly into a rapid-prototyped mold.
Punches go into a hot press filled with powdered steel. At around 2300 F and 5000 psi, the press densifies the powder while transferring surface detail from the punches. Vawter says resulting parts have tensile strength comparable to wrought steel. Because the steel never melts, shrinkage is minimal, yielding accuracy of about 0.001 in./in. Hardness of P/M forged steel is about 45 Rockwell C, though heat treating can raise it to about 68 RC. Inserts can be finish-machined and polished.
In a test performed with Ford Motor Co.’s Research Laboratory, Dynamic Tooling created a P/M tool for an ABS wingnut in 70 hr. Ford researchers estimate it would take 96 hr to create the same tool by EDM. Tony Anderson of Ford’s rapid-tooling team expects the P/M tool to last for 250,000 shots.
P/M forging currently makes parts up to 4-in. diam., though Vawter is having a system built to handle a 12-in. cube. Dynamic Tooling plans to open a P/M service bureau and later to sell the forging machines and technology.
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