Iscar Metals
Plastics: The Return of Body Panels?

A problem with plastics and steel structures is a difference in the coefficient of expansion. Some material modifications may ameliorate that problem.

Although there might be some people who think that in the light of the fact of the relative lack of success of the SMART vehicle lineup and the fact that the company that made plastic cars comparatively cool, Saturn, is moving toward steel with alacrity, don’t count plastics out for body applications, not by a long shot. In fact, according to Derek Buckmaster, global market director, Exterior Body Panels & Glazing, GE Plastics (www.geplastics.com; Bergen op Zoom, The Netherlands), new materials developments are showing solutions vis-à-vis one of the key problems that plastics seem to present, which is the coefficient of thermal expansion (CTE). Buckmaster explains that the problem is a simple one. That is, most body structures are steel. Steel has a CTE of around 1.9. Most plastics, on the other hand, he says, have a CTE between 5 and 10. “If you’ve got plastic exterior panels and it is a hot day, the gaps may no longer exist, and that’s not a good thing,” he says with understatement. “That has been known to happen. I know of an OEM in Japan that built a prototype vehicle with plastic doors, and they literally couldn’t get in on hot days.”

 So the issue is one of lowering the CTE for plastics. And Buckmaster says that they’re working on two different materials for body panels that have reduced CTE. One of them is what they’re calling “HMD” or “high modulus ductile.” He explains, “Say you have a polycarbonate—one of the best materials in terms of impact resistance, or ductility—and you want it to be stiffer. You would typically put glass fibers in it. But the glass fibers cause it to become brittle. So you always have this trade-off between a stiff material and an impact resistant material.” Enter HMD technology. Rather than adding glass, they are adding a nano material. He says that exactly what the material is is proprietary, but he does admit that it is not a nano-clay, which has been used in some automotive applications (e.g., the 2002 GMC Safari and Chevy Astro vans had a running board with a nano-talc material included in the TPO base; see: http://www.autofieldguide.com/columns/1001mat.html). This material, Buckmaster says, “sets up a secondary structure within the plastic itself and builds a nano-scale skeleton that stiffens the material without losing the impact resistance. It is very different to how you would use glass fibers to stiffen a polymer matrix.” Depending on the resin used, Buckmaster advises that the CTE can be reduced by as much as 50%.

This development is moving toward production, he says. “We’re launching a PCPBT [polycarbonate polybutylene terephthalate] material, Xenoy, with the HMD technology. We’ve already done trials with Krauss-Maffei”—a machine manufacturer—“and BMW and have seen some terrific results. There are a couple of car manufacturers looking at it.” He predicts that within the next 24 months there will be parts produced with the HMD technology. “It’s precisely about reducing the gaps and improving the coefficient of thermal expansion on the vehicle.”

While the HMD technology is for injection molded parts and is suitable for components including door skins and fenders, there is another material technology that lends itself to the production of horizontal panels including hoods and roofs. This is a “high performance thermoplastic composite,” of “HPPC.” Ordinarily, composite materials are glass or carbon fibers in a matrix of a thermoset, an epoxy or a polyester material. The materials are combined, then put in an autoclave, where they are subjected to high temperatures and high pressures. While the part produced tends to have excellent engineering properties, as a practical manner, the cycle times are such that production part applications are exceedingly limited. However, according to Buckmaster, because the HPPC uses a thermoplastic base rather than a thermoset, with continuous glass fibers as the stiffening material, part production is expedited because what’s involved is melting the thermoplastic, molding, and cooling it down again. The cycle time is much shorter than is the case with conventional composites. “Because HPPC is glass-filled and the fibers are continuous, you don’t get that sagging you used to get in the past,” Buckmaster says, referring to the fact that historically large horizontal plastic panels have either been avoided or ribbed on the underside because of this tendency to sag. “We tend to have the same stiffness as a piece of aluminum sheet.”

Buckmaster suggests that within the next year there will be production parts produced with HPPC. But does this mean that there could actually be a return to plastic skins on cars? Probably not on a wholesale basis. Buckmaster admits that one of the issues that they’re faced with when dealing with OEMs is that of an installed base of metal stamping machines: How can plastic parts production be justified if the machinery sits idle?

Perhaps part of the answer to that question simply goes to the design advantages that Buckmaster says plastics can provide. He explains that typically, if OEM designers want to save weight they consider aluminum, but then pay a cost penalty. What’s more, he says, “Typically, because aluminum’s ductility is not as good as steel, they’re limited in the depth of draw that they can pull on a sheet metal pressing. So that means you have to go to more expensive tooling and be gentler on the aluminum sheet or”—and here’s where the other shoe drops, as you might expect—“you can move to a polymer material.” He provides a solid example of a case where aluminum is used—but so is plastic for the fundamental formability: The BMW 6 Series. “Look at the body-in-white of that vehicle,” he says. Although the majority of body panels are aluminum, the front fenders of the vehicle are made of Noryl GTX. “They would have done it with aluminum if they could have,” Buckmaster claims, “but with the styling design—the curvature and the depth of draw—it wouldn’t be possible.” (The decklid is an SMC composite.)

Another example that he cites regarding the importance of formability is one that is probably more unexpected than that of a sleek coupe: The fenders on the 2006 HUMMER H3. “They’re a complex, deep-drawn shape that are not possible to make with steel or aluminum,” Buckmaster suggests. The fenders, produced with Noryl GTX resin, are said to be the first thermoplastic fenders on a North American truck platform.