What is Thixomolding?

Just in case you missed our first report on the Thixomolding process (October 1995 IMM), here's a refresher.

At room temperature, chips of Mg alloy are fed into the hopper of a molding machine under a blanket of Argon gas. The material is progressively heated in the machine barrel to about 40 deg C below the liquidus of the alloy. The material's solid-to-liquid phase in the barrel is tightly controlled. The Mg chips are safely converted to a uniform, semisolid "thixotropic" state with a low but stable viscosity. After the material has accumulated to a predetermined shot size, it is injected with relatively high pressure and speed into the die cavity. Flow into the die cavity is laminar, unlike the extremely turbulent flow in diecasting.

For these reasons, molds can be kept under a vacuum for the complete filling phase, which can be very beneficial with complex part designs. Porosity levels of .4 to 1 percent can be achieved, in contrast to 3.5 to 7 percent or more in diecast products, for very good stiffness, ductility, and corrosion-resistance properties in finished parts. Molds are heated to temperatures appropriate to the material's characteristics and the part design. Molded parts are removed automatically and can be trimmed prior to any required texturing. Gates and runners, like the molded Mg parts themselves, are 100 percent recyclable through regrinding.

In addition to recyclability, Mg Thixomolding offers a number of intriguing environmental, economical, and performance advantages over both metal diecasting and plastics injection molding. Foundries are eliminated. Thixomolding is a self-contained system requiring no external melting, melt handling, and losses. There is no flux, sludge, dross, or waste disposal. Operating temperatures are about 130 deg C lower than diecasting. Lower operating temperatures increase tool life by a factor in comparison to diecasting.

Mg is 30 percent lighter than Al, yet has the same yield strength as diecast Al. It has excellent damping, and with its extreme thin-wall capability, heat sink properties comparable to Al. Mg costs less than many engineering thermoplastics. It can repeatably produce parts that provide inherent EMI/RFI shielding, better strength and stiffness-to-weight ratios, and improved heat resistance when compared to plastics. It is 20 times stiffer than engineering resins. Excellent paintability is another plus.

In general, Thixomolding is an economic and flexible process solution with a relatively low initial investment, reduced energy costs, improved die life, and high yields of net-shaped parts with complex designs and molded-in features. Low mold filling temperatures result in minimal, extremely controllable shrinkage. Most parts require little or no secondary finishing. Parts can be molded with minimum draft angles or none at all. Deep holes up to .5-mm diameter and wall thickness as low as .5 mm over 150-by-100-mm surface areas can be produced with extremely good surface flatness and rigidity. Meanwhile, parts with walls up to 18 or 19 mm thick can be molded. Linear tolerance capability generally is at ±.3 percent and flatness is at .002 mm/mm.

The Dow Chemical Co. received the original patents for the process in 1987. In 1989, a consortium company called Thixomat Inc. (Ann Arbor, MI) was formed. It received the worldwide license to the patents. Thixomat is commercializing the process around the world with its partners and its shareholders. Amptech Corp. was one of the original partners in the consortium. Thixomolding licenses are available from Thixomat. Thixomolding machines are built by two machinery manufacturers: The Japan Steel Works Ltd. (Tokyo) for Asian markets and HPM Corp. (Mount Gilead, OH) for the Americas. HPM built the world's first Thixomolding machine in 1991—commercial production only began around 1994. At press time, Thixomat was evaluating several candidate machinery manufacturers to serve markets in Europe.

The machines resemble a diecasting machine cross-bred with a plastics injection molding machine. They may look a bit ugly to some, but they are computer controlled and agile—quick mold and materials changes are possible, and the machines can be used in fully automated manufacturing cells, just like molding machines that might look somewhat more familiar.