Do Plastic Body Panels Have A Future?

The 2005 Saturn Relay minivan set to debut this fall may be most notable for something it doesn’t have–plastic body panels. Though Saturn long championed polymer panels while everybody else stuck with traditional stamped steel, the once-independent division is getting drawn more tightly back into the GM fold of shared products and that means a lot more steel use. An upcoming large SUV and a roadster based on the new Kappa platform will most certainly have steel body panels (though officially Saturn is mum on the subject). Which leads us to wonder: If the strongest proponent of polymer is moving away from its use, do plastic body panels have a future?

The 2005 Saturn Relay
Plastics apostate. Saturn has produced over three million vehicles with plastic body panels, but the Relay will be its first using all steel, and there are more to come.

Yes, but… The answer to that question from plastic material suppliers is an enthusiastic “Yes!” In fact, they are probably more bullish about plastics’ prospects today than in the past because they see a collection of market, cost and technical factors coalescing to create an environment where plastic can beat out steel. At least in limited applications. The big hitters that everyone is focusing on are fenders and side panels. Bayer Polymers (Pittsburgh, PA) projects that by 2010 the demand for thermoplastic fender material alone will double in both the U.S. and Europe. And Mutsuo Aoki, global programs director, body panels and glazing, GE Advanced Materials (Pittsfield, MA), says that starting around 2007, “We will see explosive growth.” Why? According to Aoki, automakers, especially those based in high fuel cost areas like Europe and Asia, are becoming increasingly interested in plastic body panels, which can weigh only about half as much as comparable steel parts, as a way to reduce weight and improve fuel economy. Also, Aoki says, demand for more-aggressive styling in Europe coupled with pedestrian safety legislation gives plastics the edge there, since it offers a greater range of design freedom and is more forgiving than steel during collision impacts.

In North America, the home of cheap fuel and conservative styling, other factors are at the forefront of plastic adoption. Chief among them is the trend toward smaller production runs and greater vehicle differentiation. Plastic suppliers readily admit that at high annual production volumes they cannot compete with steel, since the cost of multiple molds combined with injection molding’s higher cycle times push the piece cost above that of stampings. But at up to 80,000 units/year molding plastic panels is cheaper for automakers than investing in a costly set of progressive stamping dies. And as the number of vehicles in that production range increases, so should the demand for plastic body panels. Of course, the fact that both steel and gasoline prices are spiking in the U.S. doesn’t hurt plastic’s case, as automakers try to divine where those commodities’ prices will end up in a few years when the vehicles they are designing now reach production.

Taking the Heat. Another source of optimism for plastic suppliers is that technical improvements in their materials have made them much more appealing to automakers. For years, the Achilles’ heel of plastic body panels has been their inability to endure the heat of manufacturers’ paint ovens without deforming. Saturn gets around this problem by attaching its plastic panels to a buck and sending them through a lower-temperature e-coat process prior to assembling them to the body-in-white, but this so-called “in-line” process requires extra space and equipment and must be designed into a plant from the get-go–something other makers have been unwilling to do. Another solution is off-line painting either in-house or at suppliers’ plants, but this approach is risky because of potential color match discrepancies. “OEMs have historically said, ‘Give us on-line paintability or give us nothing,'” says David Loren, exterior segment leader, Bayer Polymers. Now plastics suppliers have finally achieved that goal, turning out a variety of materials that can withstand the approximately 200º C necessary for on-line painting. Among them are Rhodia Engineering Plastics’ Nylon 6,6 mineral-filled Technyl; Bayer Polymers’ polyamide/ABS blend Triax TP 3161; and GE Advanced Materials’ PPO/PA alloy Noryl GTX. Gary Kachin, account manager, Rhodia Engineering Plastics (Farmington Hills, MI), offers this assessment: “The ability to paint on-line will re-invigorate the plastic body panel market.” Loren is more blunt: “It has blown the door wide open.”

BMW's front fenders on its new 6-Series
Plastics material suppliers expect plastics use in fenders to explode in the next few years. But BMW didn’t wait, it speced the front fenders on its new 6-Series out of GE Advanced Materials’ Noryl GTX resin.

Expansion and Contraction. Plastics suppliers have had less success in overcoming another traditional problem–the pronounced expansion and contraction plastic panels undergo during temperature extremes. Plastics have a much higher coefficient of linear thermal expansion (CLTE) than steel, and therefore require more space to grow and shrink. This leads to wider panel gaps than most automakers, driven by quality survey results to eliminate fit-and-finish concerns, are comfortable with. The problem can be ameliorated on fenders and side panels by firmly securing them where they meet the door and allowing them to expand in only one direction. But that doesn’t work with doors, since they have to be free to expand in two directions. Plastics CLTE has been reduced in the newest formulations by tweaking blends and adding mineral fillers, but it is still nowhere close to steel and no one really expects it to get there. GE Advanced Materials’ Aoki says that gaps can be successfully managed through clever design, and he points to his company’s extensive design work on smart’s plastic-paneled lineup as proof. But like Saturn, smart’s vehicles are designed from the beginning for plastic use, and it’s unlikely that automakers will be willing to upend their design processes in order to accommodate the vagaries of polymer doors. Indeed, fenders and side panels are seen as a growth market for plastics precisely because special handling has been eliminated.

As for horizontal panels like hoods, roofs and decklids, it looks like steel will continue to rule there for the foreseeable future. These large panels require a great deal of stiffness, show minor deforms easily, and have to sustain high heat loads, none of which are thermoplastic strengths. Bayer’s Loren notes that some thermoset materials like SMC are “starting to creep in,” but clearly no one sees this as a large potential market in the short-term. So the best answer to our original question about the future of plastics in body panels seems to be: bright but limited.