Molding the future of auto exteriors

Within the pervasive hustle and bustle of the automotive industry lie the seeds of dramatic change. Some industry insiders believe it is only a matter of time before a number of vehicles begin sporting molded plastic bodies. For example, at a recent press conference held during SAE '99, Tom Bouchard, general manager, GE Plastics Automotive, predicted passenger vehicles will contain 600 lb of plastic by 2007. To reach this or even more conservative goals, plastics must replace large body panels currently made from steel or aluminum.

Today's market environment provides fertile ground for such an idea to take hold. Fueling the metal-to-plastics transition are several issues near to OEMs' hearts-from fuel economy requirements and modular assembly practices to lower production costs and better impact and corrosion resistance. Even styling demands for sleeker, more aerodynamic exteriors are propelling the trend toward plastic body panels.

Moreover, designers themselves have developed a greater understanding of engineering thermoplastic properties and capabilities in the past decade. While resins are still newer than metals, they are no longer aliens in the engineering realm, in part because testing efforts, CAE software, and resulting analyses have compiled reliable data. Materials and processing technology have also come a long way toward meeting automotive needs.

Both of these factors-industry trends as well as progress in the triad of design, material, and process technologies-have exerted a cumulative effect on the automotive industry, one that could first be seen when Saturn chose injection molded PC/ABS doors ten years ago. In Europe, the exuberant Smart Car shows its colors and thermoplastic panels with pride. More proof can be seen on U.S. roads today in the form of fascia, fenders, and trim.

Chrysler Takes the Lead
Of the Big Three automakers, DaimlerChrysler has made the firmest commitment to designing and building plastic-bodied passenger cars. Larry Oswald, executive engineer, body engineering, confirms the company is continuing development efforts that began in 1994 with the CCV (Composite Concept Vehicle). "The injection molding process uses far less labor and produces a very good surface finish. Thermoplastics save 20 to 50 percent in weight and 50 to 70 percent in tooling," he explains.

Oswald confirms budgets set to develop this technology range in the tens of millions. "DaimlerChrysler invested early in this technology because, culturally, they are more apt to take a risk and because their supplier relationships are based on partnerships," he says. In fact, 80 engineers at various suppliers and 20 at DaimlerChrysler are devoted to the development effort. The company will lease a Manufacturing Development Center in Novi, MI, built and owned by partner Husky, where teams will begin low volume production of body panels for the CCV in July of this year. Decoma, a division of Magna, will run the molding operations using Husky presses.

One of the main challenges presented by the CCV's in-mold-colored body panels is surface appearance. Lack of gloss and luster make the unpainted panels unsuited to U.S. or European markets. "We are currently trying several in-mold surface coating solutions," says Oswald. In addition, materials technology continues to aim at improved unpainted surface quality. For example, the 2000 Dodge Neon line will debut glossier bumper fascia thanks to a new material grade called Surlyn Reflection series, a high-gloss alloy based on ionomer and polyamide resins (DuPont).

Another issue for plastics-intensive vehicles appears to be headed for a solution. To bolster the CCV's crashworthiness, designers now include a steel bow in the roof and door beams. In addition, the underlying steel frame handles suspension point loads and front end collisions. To date, prototype CCVs have passed several federal crashworthiness tests from the front, rear, and sides.

Material choices are constrained by a simple goal: the cost must be less than $1/lb. "Although we're not there yet," says Oswald, "we have two partners working on a solution-Ticona on PET material and Montell with Hivalloy PP/PS. We've set up a competition of sorts to spur creativity."

One of the factors motivating DaimlerChrysler's efforts is a reduction in the time required to assemble a vehicle, according to Oswald. "While today's conventional cars spend five to 10 hours on an assembly line, a CCV-type vehicle may spend only one to two hours there."

Whatever the outcome, the CCV itself will most likely remain a development tool rather than a production vehicle. It appears DaimlerChrysler will use the knowledge it gains to produce plastic bodies for other vehicles, such as the Pronto Spyder or Intrepid ESX2, which are currently in the concept phase. The ESX2 is the company's PNGV (partnership for a new generation vehicle), one of the entrants in a government-sponsored competition among automakers to produce a vehicle with 80-mpg fuel consumption by 2003. It features a hybrid powertrain, part electric and part internal combustion, which weighs more than current engines. "The plastic body helps offset the powertrain's additional mass and cost," Oswald explains.

Saturn Doors Revisited
This July, Saturn will celebrate its tenth anniversary in Spring Hill, TN. Randy Scott, market manager for exteriors, Dow Automotive, reports Saturn's original decision to use molded PC/ ABS door panels has not waivered since the 1989 launch. "During market research prior to the recent redesign of their vehicles, Saturn found that customers really liked the plastic doors," Scott says. "In fact, they helped to give the cars brand definition. Also cost and weight benefits are still meeting design requirements."

An upfront decision to use thermoplastics was the key to the success of this program, he believes. "The plant was designed for plastics from the start with a separate painting line for the doors and captive molding operations. There were no retrofit issues," adds Scott. "Saturn chose a space frame design with thermoplastic panels, so the structural demands were minimized. They wanted a simple process that added value to the vehicle."

For the redesign, Saturn engineers asked Dow for higher heat deflection temperatures on the customized Pulse PC/ABS resin used to mold the doors. "They needed a wider window with better dimensional stability for the paint line," he says, "with the same impact strength and processability but about 20 deg F more HDT." Paint line temperatures reach 240F ±10 deg F, and the new material, Pulse B-270, has an HDT of 270F.

Saturn is using the new material now, and it appears on several new vehicles, including its SC sport model with a third door. The newly designed '99 Saturn LS will also use the higher-temperature resin for the doors, as well as PC/PBT front fenders (Xenoy from GE Plastics).

What about the notorious fit problems experienced with plastic body panels in the early years? These have been overcome by designing a gap between door panels and fender quarter panels to allow for thermal expansion. During the attachment process, oval slots and special rubber grommets are used instead of circular holes on the space frame. "Basically, the panels are allowed to slip minimally on the space frame, which, in turn, manages thermal stress," says Scott.

Getting Smart
You may never see the cuddly and colorful Smart Car in your neighborhood, but be assured technology developed to produce the exterior body panels will be crossing the Atlantic.

At K'98, plastics engineer Bernhard Sax of GE Plastics told IMM the beauty of this project stemmed from MCC's willingness to start from scratch. "All of the suppliers-ourselves, Dynamit Nobel, Krauss-Maffei-were involved at the concept phase, and the entire body panel system was designed for the use of plastics," he adds.

The team spent 31/2 years developing the panels from concept through production. One initial challenge involved the PC/PBT panels' compatibility with a PU-based clearcoat for chemical adhesion, an issue solved by modifications to the Xenoy grade. Design work was followed by moldfilling simulations for tool layouts, gating, and runners.

Next came sampling on prototype tooling. The team performed several DOEs (design of experiments) for gating, wall thickness, and filling optimization. At one point, the design was adjusted to avoid early freeze-off. Long flow lengths also contributed to color matching challenges. Krauss-Maffei collaborated on screw design, backflow nozzles, and other features of the ten machines-ranging from 1300 to 4000 metric tons-dedicated to molding the panels.

GE Plastics' V. Umamaheswaran, market development manager for automotive exteriors, offers additional input on the future of thermoplastic body panels. "There is a strong interest among U.S. and transnational automakers to make the switch," he says, "and discussions have accelerated since the Smart Car was introduced. Molded-in-color panels are especially attractive for OEMs putting in new plants because they can avoid a $350 million investment for a new paint line."

Certain applications such as fascia, exterior trim, cladding, and mirror housings will probably be the first to be produced as molded-in-color parts. Umamaheswaran sees the first iteration likely being accent color matches rather than body color matches with more materials development needed for the latter.

Enabling Technologies
What makes molded plastic exteriors possible from a design standpoint? Overall, there is a concerted effort among material suppliers, molding machinery manufacturers, and software vendors that, when combined, create the engineering possibilities. Materials must be tweaked to provide an exact cost/performance balance; molding presses must be able to supply the correct cycle times, accuracy, and part quality; software systems have to allow all team members to participate in upfront design decisions.

Vincent Render, a technical specialist at Ford, believes Design for Manufacture and Assembly methodology (DFMA) is a central part of these enabling technologies. "DFMA embodies the standard engineering practices of good engineers. It improves top-down assembly and parts orientation, locating features and reducing the number of parts," he says. Although DFMA software (Boothroyd Dewhurst) doesn't promote plastics per se, its goal of integrating functions and consolidating components lends itself to replacing metals with plastics.

DFMA analysis also promotes multifunctional teams that include material suppliers, design engineers, and assembly and manufacturing groups. According to Tom Clinton, director of advanced engineering at GE Plastics Automotive, the process focuses each group and gives them direction. "It allows us to put together meetings where we can brainstorm cost savings, which would never happen if you tried to integrate product development without a rigorous evaluation process," he adds.