The process was used by Chi-cago White Metal Casting Inc. to make the housing for a new fiber optic analyzer designed by Tektronix (Beaverton, OR). The materials marriage provides better impact strength and dent resistance than plastic without a large increase in weight.
According to Jim McGrath, a senior manufacturing engineer at Tektronix, magnesium beat out thermoplastics on four crucial counts. First, it provides ruggedness for the analyzer to survive a battery of tough physical tests, including a 48-inch drop to concrete. "Many of the plastics used for such housings would disintegrate," he says.
Magnesium's torsional rigidity protects the unit's glass touchscreen display, and the metal also does a better job meeting thermal requirements than an all-plastic case. Finally, magnesium provides electromagnetic and radio fre-quency inference shielding without expensive coatings or gaskets.
Although the project was driven by functionality rather than cost, using magnesium for the case proved to be cost-effective. Thanks to part consolidation and process simplification, the design cost 20 percent less than a comparable plastic design, despite the cost of separate casting and molding tooling.
Intricate features are designed into the die cast housing components, and the overmold allowed integration of many additional cost-saving details, such as weather seals, door hinges, bumpers and other mechanical support features. Overmolding also enhanced the grip and feel of the unit.
The rear case is cast with 45 tiny "gripper" teeth and 55 holes--39 to size and 16 tapped. Fifteen holes serve as molding shutoffs, with machined surfaces parallel to 0.002 inch.
For more information on magnesium die-casting and overmolding, call Chicago White Metal Die Casting at 630-595-4424 or visit www.cwmdiecast.com.