Automakers have long had a love-hate relationship with paint. On the one hand, paint provides a time-tested way to achieve glossy, weather-resistant cosmetic surfaces. Yet, it also represents a substantial chunk of piece-part costs—as much as 30% for some components.
So automakers have over the years developed a keen interest in paint alternatives that can preserve cosmetic quality while doing away with some of the cost. Two such technologies, both of which involve the use of colored films over thermoplastic substrates, turned up as finalists in the recent Innovation Awards sponsored by the Society of Plastics Engineer's Automotive Division.
Paintless fascias. Adding a decorative surface to plastic parts by injection molding a thermoplastic part onto the back of a film skin has taken place for years. And in many ways, the front fascia for DaimlerChrysler 2002 Dodge Neon builds on this technology. Instead of the painted TPO used for previous fascias, the Neon part consists of a colored film over a polypropylene copolymer substrate. The company makes the fascias by first vacuum forming the colored film to the shape of the mold cavity. It then injection molds the polypropylene substrate onto the back of the film.
That process may sound simple enough. Yet the Neon fascia does feature a few twists on in-mold film technology. One relates to the size of the parts. Fascia-sized parts required the development of complex robotic and tooling systems to get the vacuum-formed film into the tool without wrinkling it, according to Mike McMain, senior engineer for exterior systems. Another twist involves materials, which were supplied by A. Schulman Inc. (Akron, OH) and ExxonMobil Chemical (Houston, TX). Neon engineers picked a new film whose extruded-in color has a Class A finish, one that matches the metallic colors and gloss levels of painted body panels. The film also stands up to a battery of quality, durability, and chemical tests. "Our criteria was that the unpainted fascias perform as good or better than painted fascias," says McMain. DaimlerChrysler gets that performance from a four-layer film: A clear Schulman ionmer top layer provides scratch- and UV-resistance. A second layer of ionmer contains the extruded-in color. A third adhesive layer ties the ionmer layers to a polypropylene backing layer—which, in turn, forms a chemical bond with the ExxonMobil polypropylene substrate during the molding process.
The resulting fascia offers some compelling advantages for DaimlerChrysler. With the elimination of fascia painting and the associated in-process costs, each in-mold-film saves from $5 to $15 per fascia, depending on the site of the part, McMain reports. Further cost saving of $3-12 per fascia may result from reduced shipping costs—since the elimination of a paint line allows fascias to be made adjacent to the car assembly plant. The paintless fascias also produce a physical properties bump up, particularly with cold impact performance. "The film remains ductile in conditions where the paint becomes brittle," McMain explains, adding that DaimlerChrysler tests show improved scratch and mar resistance for the film. Finally, the polypropylene fascia bumpers can be reground and recycled in the same stream as other olefins.
It's worth noting that dry paint films can go through a similar in-mold film process and can match the metallic paints of adjacent body panels, but the engineering team rejected them in this application. "Dry-paint films are expensive and have difficulties with complex surfaces such as a fascia," McMain explains. Molded-in color, a seemingly less complex option, was also ruled out because it can not yet provide a close enough match with metallic body paints, McMain adds. In-mold-film, by contrast, only stands to grow in use at DaimlerChrysler. "The technology can be transferred to other vehicle lines and exterior components," McMain says, citing rocker panels, body-side moldings, mirror housing and grills as a few examples. "With additional development, the film could even replace paint on body panels," he says.
Thermoformed moldings. Injection molding isn't the only way to apply decorative films to automotive exteriors as the rocker moldings for General Motor's 2003 Chevrolet Northface Trailblazer show. Like the painted, injection molded parts they replace, these moldings feature a high-gloss, body-matched Class A surface. Unlike injection-molded parts, GM makes the rocker moldings from multi-layered thermoformed sheet with an integrated dry-paint film. Aside from the colored layer, the film's remaining functional layers include a TPO substrate, a tie-coat and primer layer, and a fluoropolymer clear coat. Total film thickness tops out at roughly 5 mm. Following the thermoforming process, the rocker moldings go through just a CNC routing step before heading off to assembly.
Used in this instance for a low build-volume specialty vehicle, the thermoformed parts don't represent the first use of dry paint films. "Dry paint film has been inserted into injection molds for a long time," acknowledges Ed Crowe, technical director for the molding manufacturer, Carlisle Engineered Products (Crestline, OH). But GM did use this proven technology in unconventional ways. At 0.02 to 0.05 mm thick, the paint film layer has a heavier gauge than injection molding applications, Crowe says. And it plays a crucial color-matching role in what may be the first thermoformed part of its kind. "This is the first application of a thermoformed, high-gloss, body-color component on a production vehicle," Crowe says.
When considering any paint-alternative technology, automakers have so far been unwilling to give up cosmetic quality or durability. This job was no exception. Crowe reports that GM tests show that the thermoformed panels beat painted thermoplastic parts when it comes to chip resistance, weatherability, and chemical resistance. The panels also up the ante on low temperature impact compared to injection molding grades, he adds.
Then there's cost reduction. According to Crowe, the thermoformed panels achieve a 7 to 10% cost out per part, mostly through the elimination of painting costs. Thermoforming also has another expense reduction: In this application, its tools cost 80 to 85% less than injection molds, resulting in a $7.00/part cost advantage, Crowe reports. "And we believe that's a conservative estimate," he says.
Thermoforming also allowed a speedier launch. GM got its prototypes in less than four weeks and production tools in eight weeks, compared to about 24 weeks to procure a comparable injection mold. Thermoforming also helped keep pace with engineering changes. "Tooling changes are faster and less expensive," Crowe notes. And CNC trimming process can be modified with a quick software change.
Taken together, these advantages could drive thermoforming into other low-volume specialty vehicles. "It's an excellent technology for rockers in general," says Crowe. "But we believe we can expand the technology." Carlisle has plans, for example, to prototype a thermoformed fascia.