Composites featured in deep-draw sheet for city buses

Composites specialist Amtech LLC has developed a large (5 by 4 by 3 ft) luggage cover for aluminum wheelhouses, made via deep-draw vacuum-forming with a 40-inch draw.

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Composites specialist Amtech LLC (Wapata, Wash., USA) reports that it has molded a luggage cover for aluminum wheelhouses, designed for durability, performance, safety and appearance.

The cover is made for a major manufacturer of mass transit buses and used in vehicles throughout the United States. The dimensions of the angled structure are 5 by 4 by 3 ft (1.5 by 1.2 by 0.9m). It is fabricated in a deep-draw vacuum-forming process that achieves a notable 40-inch (1,016 mm) draw. The interior layer is a glass-reinforced epoxy vinyl ester resin; the core is a proprietary bonding material and the top layer is 0.180-inch (4.5 mm) gauge thermoplastic sheet from Boltaron of Newcomerstown, Ohio, USA.

The Boltaron sheet, Grade 4800, is the key component of the structure. Bill Bushbaum, senior vice president of business development at Amtech, says the Boltaron sheet was specified in place of a composite laminate initially used for the part. The original laminate schedule had a gel coat surface that didn’t withstand the harsh conditions and occasional abuse the luggage covers receive in service and during the assembly process. “The finish didn’t hold up well in the field and needed numerous repairs,” he says.

The Boltaron sheet, in contrast, has impregnated color—in this case gray and black—so part-to-part aesthetics are consistent and the part achieves a cosmetic finish. Bushbaum says the sheet processes better in the deep-draw vacuum-forming process, achieves greater surface texture resolution, and fewer surface contaminants migrate during forming.

The material also deters “taggers”—graffiti vandals—due to the sand finish that Amtech specifies. “The graffiti from felt pens and other markers does not adhere well to the Boltaron sheet, and is easy to clean,” Bushbaum says.

Grade 4800 is a proprietary, fire retardant, extruded sheet designed specifically for mass transit interior applications. Boltaron 4800 meets ASTM E-162 and ASTM E-662 stringent requirements for flammability and low smoke emissions and FTA, UMTA and Docket 90A guidelines. The material offers good impact strength, abrasion resistance, stain and chemical resistance and thermoformability. It comes in custom colors and a range of textures, and maintains lot-to-lot consistency. The sheet is available in 0.040 to 0.250 inch (1.02 to 6.35 mm) gauges, widths to 60 inches/1524 mm and lengths to 120 inches/3048 mm.

Amtech uses a conventional vacuum-forming machine to make the wheelhouse cover. Bushbaum declines to reveal process details but notes that fabrication involves multiple steps. After forming, a part goes to Amtech’s composites shop where a proprietary bonding material is applied to the interior surface. Following this, a layer of proprietary glass-reinforced resin is applied.

The laminate is what makes the part both structurally sound and economical for the end-user. “If we had built the part entirely of Boltaron sheet, the thickness needed to meet the structural requirement would have made the cover cost-ineffective,” Bushbaum says.