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Could we possibly see this technology on a future Corvette? 
Much has been said -- and done -- about the "pops" or craters in the painted surfaces of compression molded composite exterior automotive body panels that, until recently, raised doubts about the utility of sheet molding compound (SMC) in applications requiring Class A finishes. New SMC resins, improved compounds and the use of special sealers and primers have significantly reduced these defects to within industry acceptable reject rates (see CT February 2004, p. 16). But automaker smart Gmbh (Böblingen, Germany, a business unit of DaimlerChrysler) had a better idea. Why not eliminate the painting process altogether?
The smart roadster's "glass-optics" roof surface provides a glossy "wet" look without paint. Source: GE Advanced MaterialsFor the smart roadster and several other models, the company challenged ArvinMeritor (Troy, Mich., U.S.A.) to develop a paintless roof module with a durable, glass-like surface, recalls Stefan Ricker, ArvinMeritor's roof modules program manager, "because they intended to build an entire car, which needn't be painted in the plant." ArvinMeritor operates more than 150 manufacturing sites worldwide, including six that, together, produce more than 2 million automotive roof systems each year, using a variety of fabrication techniques. The company's Gifhorn, Germany facility responded to smart's challenge by developing an innovative long-fiber injection molding process that enables comolding of a "paintless" exterior shell made from a thermoformable surface film with a glass-reinforced polyurethane understructure and a finished fabric headliner. The result is the smart roadster's removable "hardtop," which is produced in two halves, weighing only 5 kg/11 lb each, which can be stowed in the car's rear luggage compartment. The roof weighs 20 percent less than a comparable steel roof, thus improving fuel economy and reducing rollover risk by lowering the .roadster's center of gravity. Arvin Meritor's one-piece roof modules for other smart cars have similar weight savings. The fully integrated structural composite sandwich also includes thermoformed water channels and all hardware and electrical system inserts. Additionally, the module offers more than double the stiffness of a comparable aluminum or glass/thermoset roof, eliminating the need for structural bows (beams) while offering greater resistance to side-impact crashes.
Pushing The Envelope
Ironically, the module's original design involved not a glossy finish but rather a textured grain. ArvinMeritor engineers initially investigated structural reaction injection molding (SRIM) because smart had already designated SRIM for other external smart car components. "But within the course of development," reports engineering director Hubert Bachmann, "we found that [SRIM] resulted in imprints on the surface from the glass fiber mats." While ArvinMeritor researched fabrication methods to overcome the print-through problem, smart's engineers upped the ante, asking for the glossy, "wet" look of a "glass-optics" finish. Bachmann recalls, "We decided to develop this technology for series-production readiness because we recognized a big potential, not only in the roof area, but also for the complete vehicle design."
This stress analysis, showing intrusion of a side-impact crash, demonstrates the improved safety of the glass/polyurethane roof module -- more than 15 percent better than conventional construction that features bows (cross beams). Source: ArvinMeritorFabricating The Finish First
To achieve the glass-optics finish, the company turned to GE Advanced Materials' (Pittsfield, Mass., U.S.A.) LEXAN SLX multilayer thermoplastic film, a weatherable polymer resin system designed to replace painted and clearcoated surfaces in molded plastics. The material reportedly performs as well as or better than exterior paint systems specified by most automotive OEMs, retaining 95 percent of its gloss after the equivalent of 10 years of Florida sun exposure.
The original design of the smart roadster roof module called for a textured finish (left), here compared to the subsequent LEXAN film finish. Source: ArvinMeritorThe 1.3-mm/0.05-inch thick LEXAN SLX film consists of three co-extruded layers. The outer layer is a transparent, proprietary polyester carbonate that provides the film's weather-, chemical- and scratch-resistance. A center layer of LEXAN polycarbonate holds the color effectively and provides heat resistance. The inner layer, GE Advanced Materials' CYCOLOY material, is a blend of polycarbonate and ABS (acrylonitrile butadiene styrene), which adheres well to the polyurethane in the roof module's composite inner structure. ArvinMeritor also has qualified a film product from Senoplast Klepsch GmbH & Co. KG (Piesendorf, Austria) to serve as the roof module's outer, protective shell.
GE Advanced Materials delivers LEXAN SLX film to ArvinMeritor in flat sheets, cut to size. To make the roof module shells, ArvinMeritor uses an automated process that feeds sheets through a cleaning process and then positions them over the male thermoforming tools for each roof half or a single tool for one-piece versions of the roof module (the latter is detailed in the photo series on p. 2). Each sheet then is heated to forming temperature, then drawn down over the tool and conformed to its surface by vacuum. LEXAN SLX has a "high draw ratio," which means that it can be stretched (at forming temperature) beyond 100 percent. That's enough to meet the deep-draw that is required for the roof module -- yet after thermoforming, it also retains sufficient stiffness to maintain its shape.
But ArvinMeritor also needed a deep-draw tool technology that was not yet available on the world market. As a result, the company developed a proprietary technique for optimizing the temperature profile for the part's geometry, which involved modification of both the thermoforming tool's heating system and its heating cycles.
"The temperature distribution was a critical factor for thermoforming," notes ArvinMeritor's process development manager Frank Niebuhr.
Additionally, a proprietary coating was added to each tool's surface to eliminate surface defects on the thermoformed film. As a result, the thermoforming tools now successfully shape unprecedented detail into the modules, i.e., the roof water channels, and produces a net-shape roof shell.
Roof Module Reflects Well On Paintless Composites - October 2004
Unique long-fiber injection process yields a paintless high-gloss finish, 20 percent weight reduction and improved safety -- all in one shot.
By: Karen Fisher Mason, Composites World Magazine, October 2004
Much has been said -- and done -- about the "pops" or craters in the painted surfaces of compression molded composite exterior automotive body panels that, until recently, raised doubts about the utility of sheet molding compound (SMC) in applications requiring Class A finishes. New SMC resins, improved compounds and the use of special sealers and primers have significantly reduced these defects to within industry acceptable reject rates (see CT February 2004, p. 16). But automaker smart Gmbh (Böblingen, Germany, a business unit of DaimlerChrysler) had a better idea. Why not eliminate the painting process altogether?
Pushing The Envelope
Ironically, the module's original design involved not a glossy finish but rather a textured grain. ArvinMeritor engineers initially investigated structural reaction injection molding (SRIM) because smart had already designated SRIM for other external smart car components. "But within the course of development," reports engineering director Hubert Bachmann, "we found that [SRIM] resulted in imprints on the surface from the glass fiber mats." While ArvinMeritor researched fabrication methods to overcome the print-through problem, smart's engineers upped the ante, asking for the glossy, "wet" look of a "glass-optics" finish. Bachmann recalls, "We decided to develop this technology for series-production readiness because we recognized a big potential, not only in the roof area, but also for the complete vehicle design."
To achieve the glass-optics finish, the company turned to GE Advanced Materials' (Pittsfield, Mass., U.S.A.) LEXAN SLX multilayer thermoplastic film, a weatherable polymer resin system designed to replace painted and clearcoated surfaces in molded plastics. The material reportedly performs as well as or better than exterior paint systems specified by most automotive OEMs, retaining 95 percent of its gloss after the equivalent of 10 years of Florida sun exposure.
GE Advanced Materials delivers LEXAN SLX film to ArvinMeritor in flat sheets, cut to size. To make the roof module shells, ArvinMeritor uses an automated process that feeds sheets through a cleaning process and then positions them over the male thermoforming tools for each roof half or a single tool for one-piece versions of the roof module (the latter is detailed in the photo series on p. 2). Each sheet then is heated to forming temperature, then drawn down over the tool and conformed to its surface by vacuum. LEXAN SLX has a "high draw ratio," which means that it can be stretched (at forming temperature) beyond 100 percent. That's enough to meet the deep-draw that is required for the roof module -- yet after thermoforming, it also retains sufficient stiffness to maintain its shape.
But ArvinMeritor also needed a deep-draw tool technology that was not yet available on the world market. As a result, the company developed a proprietary technique for optimizing the temperature profile for the part's geometry, which involved modification of both the thermoforming tool's heating system and its heating cycles.
"The temperature distribution was a critical factor for thermoforming," notes ArvinMeritor's process development manager Frank Niebuhr.
Additionally, a proprietary coating was added to each tool's surface to eliminate surface defects on the thermoformed film. As a result, the thermoforming tools now successfully shape unprecedented detail into the modules, i.e., the roof water channels, and produces a net-shape roof shell.
