Thixomolding: New business opportunities await

Moldmakers today can expand their business portfolios not only by producing molds for new markets in plastics, but by entering molding markets that not only compete with plastics, but have nothing to do with plastics at all. Try this on for size: making molds for molding magnesium (Mg), a relatively new single-step process for high-volume injection molding of net-shaped Mg parts has begun to take off. It's called Thixomolding. Molders around the world are gearing up to cash in. Guess what? They're going to need molds.

Thixomolding is licensed from Thixomat Inc. (Ann Arbor, MI). Commercial production of Mg parts using the process only began two years ago. Raymond F. Decker, Thixomat's chairman, says business is picking up week by week. "There are 14 Thixomolding machines out there now, with six more to be delivered this year. Based on our projections right now, we expect there will be 25 more machines delivered next year and somewhere around 100 per year by 2000." Decker says only three of the machines were purchased by diecasters. The remaining 17 were bought by plastics injection molders.

High-performance, economical, lightweight Thixomolded parts will be moving into a number of big markets, many currently dominated by plastics and diecast metals. Not the least of these is automotive. Use of Mg in automotive applications is growing about 18 percent/year, according to Decker. Parts like steering column upper brackets, steering wheel cores, cylinder head covers, brake pedal brackets, and shift cans may soon be Mg-Thixomolded. Insiders are already talking of Thixomolding instrument panels in Mg, thanks largely to its superior energy management performance. And aluminum Thixomolding is in the wings. When that hits, fasten your seat belt.

Unlike metal injection molding (MIM), which is more for small, intricate, golf-ball-sized parts, Thixomolding can produce big parts in big molds in a single processing step. HPM specs its Thixomolding machines up to 600 tons. Decker says there are already two 850-metric-ton JSW Thixomolding machines in operation in Japan, and that 1300- and 1800-metric-tonners are already on the drawing boards. "We can't project beyond that right now," he adds.

So far, parts have been molded in diecast tooling and molds originally designed for plastics injection molding. At SPE's ANTEC '96, Frederick T. Gerson of F.T. Gerson Ltd., an engineering consultant firm, presented a paper about Thixomolding entitled, "Metal Injection Molding, An Emerging New Market For Mold Makers." Gerson says that for successful full-scale production, molds specifically designed for Thixomolding will have to be developed.

Material is partially melted in Thixomolding. Though it has a laminar flow, like plastic, the material is in a thixotropic, semisolid state—a "pasty metallic charge," as Gerson puts it. It enters the mold at a 100 to 150 deg F lower temperature and at a higher viscosity than do materials in conventional die casting.

Yet, at 1150F, the material is injected at a much higher temperature than are plastics. Molds in H-13 provide satisfactory performance, but Gerson contends that precipitation hardening steels would work better. Even though one of the primary benefits of the process is long tool life, Gerson says molds will last even longer when precipitation hardening alloys are used. He also says such materials of construction reduce surface defects caused by heat checking, and make it easier and safer to weld, should any emergency repairs or last-minute engineering changes be required.

Decker agrees with the general idea of precipitation hardening alloys, but admits, "We haven't studied this particular area too closely yet. I believe conventional alloy tool steels will be sufficient. The material goes in colder than in diecasting and it's partly solid. There's less thermal shock on the die and less abrasion at the inlet since it's colder." Standard metal cutting machine tools can be used, as well as standard tool steels.

Above all else, Decker says that Thixomolding molds should be designed more like plastic molds than diecasting dies. That's largely because of the semisolid slurry's laminar flow properties. Attention has to be paid to the material's viscosity and fluidity, which behave more like engineering resins than like diecast metals.

"There are already people working on computer flow modeling for Thixomolding," he says. He also says that he is unaware whether or not Thixomolds can go runnerless. It's a new world and it's changing fast. Still, his attitude toward what lies ahead is cautious and conservative. "I do believe it's a revolution, but material revolutions take time. Thixomolding is a new avenue, but it's something that will grow in the future."