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Luggage cover for mass-transit bus wheelhouse is fabricated in a deep-draw vacuum-forming process that achieves a remarkable 40 in.-draw. The formed sheet is then backed up with glass-reinforced thermoset vinyl ester composite.
Amtech LLC considers itself a composite specialist and engages in thermoset resin transfer molding and vacuum-assist RTM, plus finishing, assembly, and custom packaging from a 220,000 ft² plant in Wapato, Wash. Amtech (amtechcorp.com) designs, prototypes and fabricates components for a number of far-flung markets. Among these are cooling towers, wind energy, construction, food service, recreational vehicles, mass transit, rail, and military.
But it recently put its skills in deep-draw vacuum forming to work on a luggage cover for aluminum wheelhouses that combines thermoplastic sheet and thermoset composite in an outsized laminated structure for a major manufacturer of mass-transit buses. The dimensions of the angled structure are 5 × 4 × 3 ft. It is has a remarkable 40-in. depth of draw. “Most fabricators don’t have the capability to form a part this large,” says Bill Bushbaum, sr. v.p. of business development at Amtech. “We have a tremendous amount of experience in large-part fabrication.”
The interior layer is a glass-reinforced epoxy vinyl ester resin; the core is a proprietary bonding material; and the top layer is a 4.5-mm thermoplastic sheet from Boltaron, Newcomerstown, Ohio (boltaron.com).
The Boltaron sheet, Grade 4800, is the key component of the structure. It’s made from a proprietary, fire-retardant PVC/acrylic alloy designed specifically for mass-transit interior applications. Boltaron 4800 meets stringent ASTM E-162 and ASTM E-662 requirements for flammability and low smoke emissions, as well as FTA, UMTA, and Docket 90A guidelines.
Boltaron 4800 reportedly also offers excellent impact strength, abrasion resistance, stain and chemical resistance, and thermoformability.
Bushbaum says the thermoplastic sheet was specified in place of a composite laminate initially used for the part. The original laminate had a gel coat surface that didn’t withstand the harsh conditions and occasional abuse the luggage covers receive in service and during assembly. “The finish didn’t hold up well in the field and needed numerous repairs,” he says.
FORMABLE, AESTHETIC SHEET
The PVC/acrylic sheet, in contrast, has integral color—in this case gray and black—so part-to-part aesthetics are consistent and the part achieves a cosmetic finish. Compared with the earlier gel-coated laminate, Bushbaum says the sheet processes more easily in deep-draw vacuum-forming, 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 sheet and is easy to clean off,” Bushbaum says.
Amtech uses a conventional vacuum forming machine to make the wheelhouse cover. 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 thermoformed sheet, the thickness needed to meet the structural requirement would have made the cover too costly,” Bushbaum says.