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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