Deep draw: Unique laminate architecture enables bus part

Interior parts for public buses are designed for durability, fire safety and appearance, to exacting requirements, with little room for trade-offs. One such part is a luggage cover for aluminum wheelhouses on buses, produced by composites specialist Amtech LLC (Wapato, Wash.) for a major mass-transit manufacturer — a component now installed on buses in cities throughout the U.S.

The large, angular 5-ft by 4-ft by 3-ft (1.5m by 1.2m by 0.9m) laminated structure met the threefold design goals despite the use of a vacuum-forming process with a remarkably deep draw of 40 inches (1,016 mm). Success was due, in large part, to a laminate architecture that includes a top layer of 0.180-inch/4.5-mm gauge Grade 4800 thermoplastic sheet, supplied by Boltaron Performance Products LLC (Newcomerstown, Ohio), backed with a structural interior layer of glass-reinforced epoxy vinyl ester resin and bonded together by an adhesive core.

Bill Bushbaum, Amtech’s senior VP of business development, says the Boltaron sheet was specified in place of a composite laminate that had a gel coat surface that couldn’t withstand the harsh conditions and occasional abuse during assembly and in service. “The finish didn’t hold up well in the field and needed numerous repairs,” he says.

The Boltaron sheet, in contrast, has an integral color (for the luggage cover, both gray and black) added when the multilayer sheet is extruded, so the part-to-part aesthetics are consistent. Bushbaum says the sheet processes better than the original composite laminate did in the deep-draw process, and surface contaminant migration is minimal during forming. It also achieves greater surface texture resolution in the sand finish that Amtech specifies — a deterrent to taggers. “The graffiti from felt pens and other markers does not adhere well to the Boltaron sheet and is easy to clean,” explains Bushbaum.

Designed for mass-transit interiors, the proprietary, fire-retardant, extruded Grade 4800 sheet meets ASTM E-162 and E-662 requirements for flammability and low smoke emissions and FTA, UMTA and Docket 90A guidelines. The material is said to offer excellent impact strength, thermoformability, and abrasion-, stain- and chemical-resistance. It comes in custom colors and a range of textures and reportedly maintains lot-to-lot consistency. The sheet comes in 0.040 to 0.250 inch (1.02 to 6.35 mm) gauges, widths to 60 inches/1,524 mm and lengths to 120 inches/3,048 mm.