It's the ultimate in touchy-feely - a molded-in covering on cell phones, laptops, a computer mouse and other thin-wall consumer electronics.
``The intense competition has driven product differentiation from the original equipment manufacturers or OEMs,'' said Brent Salamon of Dow Chemical Co. By offering more choices than a hard-plastic surface, electronics makers can drive sales, he said.
During a presentation at Antec 2007 in Cincinnati, Salamon described a new patented process to apply the fabric, with a second step of an overmolded plastic trim edge to keep it in place. He is senior research specialist at Dow's Inclosia Solutions Group in Midland, Mich.
Overmolding fabric seems fairly straightforward, but it's a challenge, he said. The fabric laminate must be durable. It cannot separate from the plastic substrate over the entire life of the product, to keep the fabric from fraying or peeling away.
First, the fabric is cut to size. For complex, contoured parts, such as the computer mouse, the material must be preformed by thermoforming.
Next, the preform is placed into the injection mold. The substrate resin is molded behind the fabric. The most common substrate resins are polycarbonate, ABS, polypropylene, nylons and PC/ ABS blends.
A dual-barrel injection molding press with a rotating mold first injects the substrate. After the resin has solidified, the mold is opened and the core side with the part is rotated 180 degrees, and the tool then is closed.
A second material is injected into a mold cavity that distributes the melt to cover the fabric edges. According to Salamon, thermoplastic polyurethane is the most desirable resin for the edge.
Mold design is key, because the second cavity must prevent the second shot of resin from flashing out of the flow challenge, ruining the fabric.
``Also, during injection, the fabric can be compressed by as much as 50 percent,'' he said.
An alternative way to mold the parts, by transferring them between two separate injection presses, is not as good as the single-press method, according to Salamon. But for mass production, both of those techniques are better than the labor-intensive methods of hand-cutting and application of the fabric, and using adhesives to bond the fabric to the molded part, he said.