A bit of glue helps elastomer overmolding stickA bit of glue helps elastomer overmolding stick

Minnetonka, MN-An overmolded elastomer might seem like a good way to improve the ruggedness and tactile qualities of a cold, hard magnesium electronics enclosure. But these chemically disparate materials normally peel apart as easily as a cold candy bar and its wrapper. Rockwell Automation's Data Myte division recently solved the elastomer-to-metal bonding problem by adding an adhesive coating step to the overmolding process.

The company's latest handheld data collection device, the Data Myte 4000 Industrial Digital Assistant, features a diecast magnesium front bezel overmolded with a melt processable rubber from Advanced Polymer Alloys (Wilmington, DE). According to Data Myte mechanical engineer Eric Krause, the elastomer covering plays both functional and aesthetic roles. It helps the device, which weighs about 4 lbs, survive a 4-ft drop test onto the faces and edges surrounding its large, color touchscreen. Krause estimates that the elastomer-covered enclosure has at least 30% more impact resistance than a plain magnesium enclosure. The elastomer also protects the device from a harsh operating environment. "It has to resist industrial chemicals, rain, dust, sand, and temperature extremes," Krause reports. Finally, the elastomer imparts that "soft-touch" look and feel that's become so important for handheld devices.

To create a reliable bond between the magnesium substrate and the elastomer covering, Data Myte and its suppliers enhanced the overmolding process: Valmont Applied Coating Technology (Mendota Heights, WI) first applies a proprietary heat-cure adhesive to the entire surface of the diecast magnesium bezel. Only then does Contour Plastics Inc. (Baldwin, WI) injection mold the elastomer on top.

The use of an adhesive coating sets this project apart from previous efforts to combine magnesium and overmolded elastomer, according to Tom Houle, Contour's engineering manger. "In the past, there would be only a mechanical lock between the two materials," he says. The Data Myte bezel does make some use of these mechanical locks. "But the main bond here is adhesive," he says. "And we have full adhesion between the elastomer and the magnesium."

For more information about elastomers from Advanced Polymer Alloys:Enter 536