New Zinc-Aluminum Molding Alloy Targets LEDs

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

Called Xyloy, the materialcombines zinc and aluminum in an alloy that allows processing in a conventionalinjection molding machine, allowing significant cost savings compared to diecast parts.

"Die castingrequires heating and keeping a large amount of metal in a molten state," saysJim Miller, product manager for Xyloy at CoolPolymers Inc., Warwick, RI. "That's a large draw from anenergy standpoint. With injection molding, you're only heating a small amountof metal at any one time." It's an increasingly important factor as hydrocarbonprices soar.

The newly developed material is also costcompetitive with plastics and offers important mechanical advantages, as wellas 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 thermalconductivity, 110W/mK (764 BTUin/hrft²F). Mold shrinkage is just 0.5 percent.

The trick in the technology is developingalloys with a broad enough temperature range that their viscosity can becontrolled in an injection molding machine. "To work in a conventionalinjection molding machine, you want the material to behave like a polymerdoes," says Miller. "You want a viscosity that isn't too high or too low over awide range of temperatures." ?That transition for metals generally is extremely sharp.

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

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