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New Zinc-Aluminum Molding Alloy

A new metal molding alloy creates new mechanical design options for light-emitting diodes, shielded housings, heatsinks and other applications.

Called Xyloy, the material combines zinc and aluminum in an alloy that allows processing in a conventional injection molding machine, allowing significant cost savings compared to die cast parts.

"Die casting requires heating and keeping a large amount of metal in a molten state," says Jim Miller, product manager for Xyloy at Cool Polymers Inc., Warwick, RI. "That's a large draw from an energy standpoint. With injection molding, you're only heating a small amount of metal at any one time." It's an increasingly important factor as hydrocarbon prices soar.

The newly developed material is also cost competitive with plastics and offers important mechanical advantages, as well as shorter cycle times. Key mechanical properties are:  ultimate tensile strength, 325 MPa (47 kpsi); yield strength, 250 MPa (36 kpsi); elongation at break, 2 percent and thermal conductivity, 110W/mK (764 BTUin/hrft²F). Mold shrinkage is just 0.5 percent.

The trick in the technology is developing alloys with a broad enough temperature range that their viscosity can be controlled in an injection molding machine. "To work in a conventional injection molding machine, you want the material to behave like a polymer does," says Miller. "You want a viscosity that isn't too high or too low over a wide range of temperatures."  That transition for metals generally is extremely sharp.

The melting point for zinc is 787F and aluminum, 1,221F.  Special injection molding machine barrels can handle thermal loads up to around 1,100F.  Cool Polymers is working on the development of other lightweight, nonferrous alloys. Steel is out of the question because of its high melting point. Stainless steel, for example, has a melting point of 2,550F.

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