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After making slow gains for years, plastic/metal hybrid structures are finally starting to take off. Front end modules like this one are the most popular application. [Photo: Bayer Corp.]

Steeling Plastics

Having conquered interiors and trim parts, plastics are now making inroads into automotive structural components with the help of an unlikely ally–steel.

 Let’s face it, when it comes to structural components in vehicles, parts that have to stand up to the kind of forces crash tests inflict, there is a strong and understandable bias toward steel. Yet there is some concern with mass. So there is a burgeoning trend toward plastic/metal hybrids for structural applications. They are touted as a “best of both worlds” engineering solution that can enhance strength while slashing weight and adding functionality. The concept has been around for a while but is just now starting to gain real traction.

The basic idea is that parts that have traditionally been made of heavy stamped steel can be made of lighter thin-walled steel strategically reinforced with thermoplastic resins to keep them from buckling under load. According to a study by Bayer Corp. (Pittsburgh, PA), the resulting structures can have up to 13 times more torsional stiffness than steel alone and almost twice the bending and axial load capacities. Which might allay the fears of steel-centric engineers concerned that plastic parts won’t stand up to punishment. But performance numbers only get you so far—in the current automotive purchasing atmosphere, delivering cost reductions are what really sells any new approach.

Cheaper, lighter. The cost-down pitch for plastic/metal hybrids is parts integration and reduced processing time. On complex components like front-end modules and seat frames, parts count and its associated assembly time can be greatly reduced since small parts are simply molded into the overall structure. “Brackets and other weldments go away,” says Bill Burnham, automotive marketing specialist at Bayer. “Along with the eight- or nine-cent cost per spot weld and the associated risk of moving the part out of tolerance.” Painting for corrosion resistance on all of those little bits and pieces also goes away, which removes the concomitant emissions headaches.

And though cost considerations currently dwarf other factors, meeting weight reduction goals still help drive many engineering decisions, which works in the favor of hybrids. When Ford went with a hybrid grille opening reinforcement on the re-design of the Focus it cut component weight by 40% compared to an all-metal structure. “That level of weight savings would be true for most cars with closed front architectures,” says Burnham. With an open architecture that would accommodate an independent front module, Burnham notes that the savings could climb to 50% or more.

Different Approaches. Within the small community of plastic/metal hybrid practitioners there is disagreement on the best way to bring the two materials together. Bayer uses an in-mold assembly technique where the stamped metal parts are placed in a mold which is then shot with thermoplastic resin in a standard injection molding process. The stampings are perforated to allow the plastic to go through as well as around them interlocking the two materials into a unified component. A series of specially designed “buttons” connect the steel pieces and transfer loading from one to another. Bayer says this method not only eliminates any secondary processing steps, but the metal can be exactly positioned where it is needed to strengthen the part.

BASF Corp.’s Engineering Plastics unit (Wyandotte, MI) goes a different way with its recently developed “collar-joining” process. Instead of loading the stampings into the injection tool, it molds the plastic part separately and then cold-presses the metal and the plastic together in a secondary operation. The two are mechanically joined by small circular collars that have been punched into the metal during the stamping process and set themselves firmly in the plastic during cold-pressing. Though collar-joining seems a heavy-handed way of achieving plastic/metal hybrids, Scott Schlicker, manager of advanced development at BASF, says it offers some big advantages, “In other methods where you load the sheet metal into the injection machine, you can only inject plastic on one side of the metal, so you only get one die draw direction to put your plastic features on, because the other side is prohibited by the metal itself. With our method, you can have at least two die draw directions—the cavity and the core side. You can get much more complex shapes and stronger overall parts because we can do three dimensional ribbing whereas the other processes can only do a standard rib.” Schlicker notes that BASF is currently involved in several development programs to validate the technology and expects commercial products utilizing collar-joining to be available within about 18 months.

Future Prospects. By far the biggest progress made by plastic/metal hybrids to date is in front end modules, and everyone seems to agree that those large, complex structures will continue to be the big fish to angle for. American and Japanese automakers lag behind their European counterparts when it comes to hybrid use partly because of greater European use of modules, though there are notable exceptions like the aforementioned Ford Focus. One thing that has inhibited wider use of hybrids in front end modules is the paucity of clean-sheet re-designs. It’s hard to justify the investment needed to move to a hybrid structure unless it can be designed in from the beginning of a vehicle’s development as a truly independent module in an open architecture front end. That is, if you have to accommodate an assortment of legacy brackets and clips, why bother? But with all-new designs coming into production, the prospects for plastic/metal hybrids are turning up. Bayer’s Burnham points out that his company is working on hybrid applications for 40 different vehicles, and predicts, “By 2005, 20% of all passenger cars built in North America will have a plastic/metal hybrid front end structure.”

Trying to convert automakers to hybrids on other components has produced mixed results. For example, Bayer developed an instrument panel crossbeam for a car program that achieved stiffness equivalent to a tubular steel structure and offered a 10% cost reduction, but that still wasn’t enough to lure the maker away from an all-steel solution. On the success side, BASF recently introduced the first hybrid oil pan on DaimlerChrysler’s Actros heavy truck. Schlicker says oil pans could prove a bright spot for hybrids since with a high level of integration of parts like baffles, oil filter and oil pump overall component costs could be reduced to half of what they are today–savings that OEMs would find hard to pass up.