Composite body panels, hold the paint

Glass-mat-thermoplastics (GMT) have been around for years now, finding a home in automotive interiors among other applications. But these materials, which consist of short-glass-fiber mat in various thermoplastic base resins, have not made any headway in exterior automotive components where cosmetic, structural, and production concerns have limited their appeal.

That situation may soon change. Engineers for GE Advanced Materials (Pittsfield, MA), one of the suppliers behind the Azdel Inc. GMT materials, have been working on ways to extend GMT materials into automotive body panels as well as other demanding large-part applications such as recreational vehicles. In the United Kingdom, GE and BI Composites recently created sports car hood from Azdel Superlite, a polypropylene-based material with a glass content of 42 to 55 percent. This composite hood, which replaced steel and goes into production in 2004, meets stringent Class A finish requirements the old-fashioned way-with a polypropylene-friendly primer and a coat of paint. Still, Azdel offered some important advantages in this application. The thermoplastic composite hood has about the same stiffness as the steel it replaces but weighs about 50% less. It promises a cost edge too, since GMT materials can be formed on lower-pressures than steel. "Low-pressure forming reduces the size and cost of processing equipment." and slashes tooling costs, explains Luca Saggese, a project engineer in GE's

Large Part Group.
Looking further down the road, GE has even more ambitious plans for GMT. In the company's Polymer Processing Development Center in Pittsfield, engineers in the company's Large Part Group have developed ways to use compression molded GMT in conjunction with a thermoformed skin made from the company's weatherable SLX film. The resulting automotive body panels, or other large parts, would offer a Class A finish without paint, according to Tom Dunton, GE's who leads GE's large part processing efforts. This technology targets not only vertical panels but also horizontal panels, which have more demanding thermal and mechanical requirements. "We're looking at a total body solution," Dunton says. 

GE engineers have successfully produced a variety of test parts on equipment installed in their development center. But Dunton says there's still more work to be done on materials and process development before this paintless in-mold-decorated composite technology is ready to roll.

EVOLVING MATERIALS
In some ways, the materials system itself doesn't pose much of a challenge for body panels. SLX, a material based on polycarbonate, has been developed specifically to offer the weatherability, high-gloss levels, chemical resistance, surface finish, and scratch resistance needed for exterior automotive parts.  It recently chalked up an exterior application on the roof module of the SMART roadster.  And with its adjustable glass content, GMT can attain the stiffness, impact, strength, and CTE properties needed to meet typical automotive structural requirements. "Most of these properties depend on the glass," Dunton notes.

But the materials system still has two hurdles to clear. The first has to do with adhesion between the film and GMT layer. Automotive adhesion tests designed for paint may not shed much light on film's capabilities. "Paint and IMD film are very different," says Saggese, who adds that paints tends to fail in a flaky, brittle manner while films fail by delaminating. At this point, it's unclear just how strong the bond between film and substrate needs to be. For now, GE has worked out its own adhesion values to use for the purposes of application development.

New materials will doubtless help with adhesion. GMT has traditionally been based on polypropylene, but grades based on polycarbonate, PBT, and combinations of the two are also in various stages of development, Dunton reports. In addition to boosting the structural properties above those of polypropylene-based GMT, these other base resins inherently offer a diffusion bond with SLX resin used in the film. "SLX and these other resins have similar chemistries, so they adhere quite well," says Dunton. "We've been getting excellent adhesion."

UNDER PRESSURE?
Then there's the cosmetics issue. Freshly extruded SLX film can achieve something that closely matches a painted Class A finish, but that film can easily develop defects as it goes through subsequent forming steps. "The real trick is maintaining the surface through the entire process," says Saggese.

Sophisticated thermoforming capabilities turn out to be one part of this trick. GE forms the decorative films only male tools-so that the show surface doesn't come in contact with the tool surface. And it must use only the thermoforming tools with critical surface quality characteristics to avoid the possibility of read-through. The company also installed a high-end Geiss thermoforming machine whose advanced film handling and high-frequency heating system play a role in maintaining the surface quality of the decorative films. Not every thermoforming house has the kind of machines needed to make cosmetic skins that are up to automotive standards, Dunton says, "but these capabilities are becoming more widespread."

The second part of the manufacturing process, the compression molding, has proven to be more problematic than the thermoforming.  So far, GE has been forced to make these large in-mold-decorated GMT parts on a compression-molding machine so large that it takes up the space of a small office building.  A 40 x 60-inch part, for example, has requires molding pressures of roughly 4,000 tons. Why so high? As Dunton explains, high pressure contributes to adhesion between the film and substrate. The high pressure also helps with the surface quality by smoothing out any minor defects in the film. Other kinds of GMT parts are already compression molded at pressures this high, but GE engineers want to drive down the pressures as a way to make this manufacturing method more accessible. "Our challenge is getting a good surface and adhesion with much lower pressures," he says.

Dunton believes that with optimized materials and more processing know-how, GE engineers can reduce the required molding pressures by "a factor of 10 or more."  And that's where the real cost savings could come in. From a capital equipment standpoint, that reduction would allow smaller, less expensive compression molding machines. Ultimately, Dunton says, the company would like to develop a process that enables these GMT parts to be made even less expensively on modified thermoforming machines, rather than on compression molding presses. Low pressures also could reduce tooling costs substantially, by allowing the use of aluminum rather than steel tools or the use of thermoforming tooling rather than pricier compression molds.

GE also has to work out a few manufacturing strategies, like how to best automate the film placement in tools.  And it still has some design details to work out, such as the best way to encapsulate or hide the edge of the films. But Dunton sounds confident that the company will overcome all the remaining barriers. "We've already made some parts that would have scared me early on," he says.