Engineering plastics continue to break new ground in automotive applications. One good place to get a look at that progress is the annual design competition held by the Society of Plastics Engineer's Automotive Division. With parts from all over the vehicle, the competition highlighted ways to use well-established materials to lower costs or improve performance (see sidebar on page 37). This year's competition featured first-time applications for newly developed materials, including a roof module that needs no paint and an underbody shield made from a composite that offers exceptional design flexibility.
The roof is tops
As the winner of both the exterior category and the competition's grand prize, a thermoformed and reaction injection-molded roof module for the 2004 Smart Roadster came out on top twice. This targe roof marks the first automotive use of a polycarbonate film to form a paintless Class A surface capable of withstanding ten-year weathering tests as well as meeting OEM scratch- and chemical-resistance targets.
Supplied by GE Plastics (www.gep.ge.com), this film gets its weatherability from a thin layer of Lexan SLX, a new polycarbonate copolymer that generates its own UV absorbers in response to light. Venkatakrishnan Umamaheswaran, body panel industry manager for GE Plastics Automotive, describes the SLX copolymer as "self protecting," explaining that material generates its own UV shield through a photo-frieds rearrangement of the polyester carbonates in the polymer backbone. Depending on the application, the thickness of the SLX film can vary. In this application, GE supplies a film consisting of clear 0.25-mm SLX cap layer goes over a 0.75-mm layer of conventional polycarbonate.
The Lexan SLX film becomes part of the finished roof in a two-step process performed by Arvin Meritor, the system supplier. The company first thermoforms the SLX film to create a weatherable "skin" for the roof. It then applies in-mold-decorating methods to combine the skin with a glass-reinforced polyurethane substrate during a reaction injection-molding process. The resulting roof, aside from its weatherability, offers a 50% weight reduction compared to painted steel, Umamaheswaran reports. And he adds that the roof module marks the first time a thermoformed in-mold-decorating film has been able to achieve a Class A surface.
Aside from its ability to impart weatherability without paint, the SLX film provides a host of benefits in body panel applications. Lexan SLX has what Umamaheswaran calls "extremely high gloss"—greater than 100 @ 60 degrees. With a tensile strength of 75 MPa and 100% tensile elongation, it offers thermoforming draw depths as much as 200% to support a variety of body panel shapes. Its traditional polycarbonate layer offers both solid and metallic color options. It's repairable. And it's transparent to radio wave transparency, an important characteristic given the projected growth of automotive antennas. Umamaheswaran says Lexan SLX can go over a variety of substrates—including PC/PBT, SMC, glass-mat-thermoplastics, polyester/PC, and even metals. "It's the only solution for total paint elimination on both vertical and horizontal body panels," he says, revealing that four more body-panel applications will follow over the next two years.
Dial in the properties
A new glass-mat-thermoplastic (GMT) from Quadrant Plastic Composites Inc. (www.quadrant.ch) is a bit like a radio—you can tune it. Called SymaLITE, this sheet-form composite enables design engineers to vary the physical and mechanical properties of the part through variations in density and localized part thickness. The ability to tune SymaLITE, which shared top honors in the competition's materials category, starts with the way Quadrant makes the blank sheets. Using a dry process that borrows techniques from textile manufacturing, the company first creates a high-loft fleece by mixing and needling polypropylene and 80-mm glass fibers. This fleece, whose glass fibers are oriented in all three axes, then goes through a heating and consolidation process, which compresses it into a solid blank. Quadrant can alter the glass-content of these blank sheets between 20 and 60%—with higher glass levels increasing the loft, lowering the density, and ultimately contributing to thicker, stiffer parts. The company can also add functional skin layers—such as scrims, adhesives or wear layers—to the outside of SymaLITE cores.
The final tuning of these GMT parts comes into play during the low-pressure thermoforming process that transforms blank sheets into finished parts. As Quadrant Automotive Manager Marcia Kurcz explains, the SymaLITE blanks are heated prior to thermoforming, causing the glass-fibers in the fleece to return to their original orientation. "Heating causes the laminate to loft up from five to six times its consolidated thickness," she says. The low-pressure (50 psi) thermoforming process that follows uses the tooling to selectively compress some areas of the part while allowing others to retain the loft. Kurcz calls this aspect of the process "tailored consolidation" and says it can locally reduce the density of the SymaLITE part to less than one-third that of the original blank.
Together, the variable reinforcement levels in the blanks and the tailored processing allow engineers to tune part stiffness by varying its thickness—and do so without increasing its weight. With traditional compression-molded GMT, by contrast, making a thicker part simply requires more material than a thinner part, Kurcz points out. "With SymaLITE, you can get rigidity where you need it and reduce thickness in other areas without changing the weight of the part," she says. Bumping stiffness without adding material contributes to weight reduction in another way too. "You can reduce the need for ribs and structural features," which add some weight of their own, Kurcz says. BMW recently applied SymaLITE to the underbody closure system on the 2004 Series 5. Covering an area of roughly 3 m2, this thermoformed underbody shield consists of four parts. All are made in a family tool from a 1400 GSM SymaLITE core covered by 1-mm polypropylene skins that add abrasion and moisture resistance, and help the finished shields pass a battery of tough durability tests.
To tune the panel, BMW engineers left the center region about 4-mm thick "to provide stiffness and acoustic performance," Kurcz says. But the perimeter of the part, which has to act as a water seal, is consolidated very tightly, to about 1-mm thick. Areas around the parts boltholes are likewise 100% consolidated to enhance tear strength, she adds.
In the past, sheet metal, SMC, traditional compression molded GMT, and injection-molded long-fiber-reinforced thermoplastics have all been used for this type of part. And compared to these materials SymaLITE saved both weight and cost. Kurcz says the weights savings versus a traditional polypropylene GMT has been 30%, or 4 kg on the 5 Series shields. The parts, which are produced in a family tool on a low-pressure thermoforming machine, also resulted in a 50% tooling cost saving versus the four tools needed for traditional GMT shields. This BMW application represents the first commercial application of SymaLITE. But look for it in other automotive applications too. Kurcz says the material is under evaluation for load floors and interior trim components—including ones, like door panels, that can benefit from in-mold fabric application. A couple of upcoming developments could also pave the way for more exterior applications for SymaLITE. For one, Quadrant has worked out ways to combine SymaLITE with an aluminum skin for roof modules. For another, the company has launched a development effort that would pair SymaLITE panels with in-mold decorating films for exterior components that require a Class A finish.
The SPE competition also honored the following projects:
Fast load floors. A low-density, fast-reacting SMC developed for the 2003 Renault Megane's load floors shared the top award in the materials category. Developed by the floor's supplier, IncoPlastic Omnium, the SMC enables a part-to-part cycle time of just 60 seconds.
Powerful plastics. The first continuously variable intake manifold, used on BMW 4.4 L engines, won the powertrain award. To match air intake to various torque engine speed and torque requirements, the manifold features a series of internal rotors that rotate on twin worm drives.
Materials swap. An instrument panel lower carrier for the GMT 800, took the body interiors award. Delphi Safety and Interior Systems swapped the filled SMA originally used on the lower carrier with a homopolymer PP that cost 43% less.