Why weld metal stampings together when you can join them in an injection mold instead? The plastics engineers at Bayer Corp. (www.bayer.com) asked themselves that very question recently and came up with an in-mold-assembly method based on the company's patented plastic-metal hybrid (PMH) technology.
PMH structural components combine thin-wall metal stampings— usually steel—with a network of thermoplastic ribs—usually glass-filled nylon 6. The metal and plastic come together inside the injection mold. The stamping first goes into the mold as an insert. During the filling cycle, the plastic flows through perforated openings in the metal stamping and encapsulates its edges, creating a mechanical interlock between the two materials.
"You get the best from both materials," says Norm Brozenick, a program manager in Bayer's Industry Innovations Group. He explains that the plastic ribs provide enough support to keep the thin-wall stampings from buckling, increasing bending and axial load bearing capabilities and boosting torsional stiffness. At the same time, plastic helps save weight by reducing the thickness and size of the stamping needed to meet a given load requirement. (Bayer has a paper at http://bayerplastics.com/pdf/512.pdf that details some PMH load cases.)
Over the past decade, these principles have gained a growing acceptance by automakers. Mark Witman, technology director in the Industry Innovations Group, reports that Bayer customers already have 12 PMH programs in production, mostly front ends such as the well-publicized grill opening reinforcement for the Ford Focus and a front end for the Audi A6. "Another ten programs look like they'll take off," Witman adds. Over the past year, Bayer has started exploring other kinds of structural applications. In car interiors, for example, seats are one area of interest. So are cross-car beams. Outside the automotive world, structural parts for appliance and business equipment have also peaked OEM interest, according to Brozenick.
Now for assembly
In-mold-assembly represents the most recent evolution of PMH and could help it push into applications with complex structures that would normally have to be welded from multiple stampings. This assembly method works just like the usual PMH process, except that multiple stampings overlap in the tool. During filling, the plastic flows through perforations in the adjacent stampings, joining them together. Think of the plastic buttons as taking the place of spot welds.
Traditionally, by contrast, PMH started out with single stampings. One classic configuration, for example, would be a U-shaped metal profile filled with the ribbing. In the case of some automotive front ends, two stampings have been used, but they tended to be separated by segments of pure plastic.
Weld No More: Bayer's
plastic-metal-hybrid technology has evolved into a method for joining
steel stampings that would otherwise have to be welded.
Brozenick notes that Bayer has new patents pending on in-mold-assembly. Some of the technical issues revolve around the geometry of the buttons and the design of the joint—how much overlap between stampings, for example. "The placement of the buttons is also critical," he says. With the proper design, though, Brozenick estimates that resulting joints are "at least 95%" as strong as their spot welded counterparts. "And you save the expense of a separate welding step," he says.