The influence of solids in TXM

We make daiquiris," joked Thixomat's Stephen L. LeBeau to the Molding 2001 audience in New Orleans, LA. Owing to the location, LeBeau's was an apt metaphor as he described the TXM process. Like a frozen daiquiri, a semisolid TXM slurry consists of spherical solid microstructures in a liquid matrix. Solids content provides a number of advantages over superheated molten metals, such as those used in diecasting, including less shrink and better part mechanical properties. 

In TXM a thixotropic alloy like magnesium is conveyed through a controlled multitemperature heating zone and into a shot accumulator via a nonreturn valve, all the while under a blanket of deoxidizing argon gas. Temperature profiling, around 1110F, heats the magnesium just enough to put it somewhere between a liquid and a solid. 

Meanwhile, screw rotation provides the shear force necessary to uproot dendrites from solid particles and round them off. The semisolid slurry is injected into the 400 to 450F mold in a matter of milliseconds, some three to four times faster than a typical plastics thin-wall molding application. 

The low-viscosity nature of the slurry helps produce laminar flow, drastically reducing the kind of inmold turbulence found in superheated molten metal processes. Such turbulence traps gas and causes part porosity, creep, and other residual stress-induced dimensional and aesthetic problems. Experienced TXM molders can produce parts over a wide range of solids content—up to 40 percent or more—allowing them, in a sense, to dial in a part's mechanical properties. 

Shrinkage has been held to within ±.005 in/in for some parts, and others have been precision molded with .35-mm-thick walls. LeBeau also told the crowd in New Orleans that of the 40-plus molders Thixomat had at that time licensed around the world, all but three were plastics molders, people who obviously know how to handle a shot.