Thixomolding promises savings.

The AFS International Cast Metals Journal in 1976 described a new metal forming process called rheocasting. Developed at MIT, it uses a phenomenon called thixotropy that involves the vigorous agitation of a semisolid metal to produce a highly fluid, diecastable semisolid slurry.

In principle, thixotropy involves the agitation, or shearing action, of the random-shaped dendrite structures forming in the cooling molten metal. This shearing action rounds and suspends the reshaped dendrites as solid spheroids in the liquid metal.

Advantages of the process were described then as providing longer die and shot chamber life, finer grained castings with fewer defects and greater economy (less loss) of metal fed to the diecasting machine. The use of nonmetallic materials to produce composites also was explained.

Now comes Thixomat, Inc., a consortium of entrepreneurs who have moved that earlier research into a current technology that offers equipment, consulting and licensing of the updated process. The consortium is concentrating on magnesium automotive castings, seeking to capitalize on federal requirements to improve vehicle mpg by reducing vehicle weight.

The organization predicts the use of more than 100 lb of Mg per automobile by the year 2000 and forecasts its Thixomolding|TM~ technology eventually will include zinc and aluminum alloys and metal matrix composite castings. All are expected to replace ferrous alloys as lightweight materials destined for future vehicle components.

Thixomolding was introduced in 1987 by Dow Chemical Co. as a direct injection molding process for thixotropic Mg alloys after the company had spent several years of proprietary research and development.

Thixomat was formed to commercialize the technology and includes Amptech Corp., Dow Chemical Co., Lindberg Corp., Comalco, USP, Inc., and George Spalding (founder of Diemakers, Inc.). It licensed HPM Corp. of Mt. Gilead, Ohio, to manufacture Thixomolding machines under Dow's North American patents.

The first Beta (production capable) site machine was approved for installation in Lindberg's Racine, Wisconsin, Thixomolding Development Center. It is being used to prove the manufacturing viability of the machine and the process, and to conduct prototype development projects for Thixomat investors and prospective licensees. The machine also will be used to carry out advanced development work to extend the range of applications.

According to Robert Killbert, venture manager of Lindberg's Engineered Products Group and head of its Advanced Development Center, nine prototype projects have been completed successfully.

One of the chief benefits of the process is that the cast parts have less porosity than conventionally diecast parts. The thixotropic material enters the die as one mass and does not entrap air, a common cause of porosity in diecast parts. Lower porosity levels allow thixocast parts to be heat treated, further enhancing the mechanical properties possible with thixocast parts. Other benefits include reduced warpage after the part has been removed from the mold and better material flow in thin-wall sections of a part.

The Thixomolding process differs from earlier rheocasting efforts in that the metal is heated only to its mushy state between the liquidus and solidus states as opposed to being fully melted and then cooled. It requires less time and energy. Machine cycle times range from 35-40 seconds up to about two minutes, depending on part size. Melt loss runs about 2%.