Deep-Draw Sheet Yields Structural Part for City Buses
WAPATO, WA Interior parts for public buses are designed for durability, performance, safety and appearance, an exacting balance that leaves no margin for tradeoffs. One such part, produced by composites specialist Amtech LLC of Wapato, WA, is a luggage cover for aluminum wheelhouses that combines all of these features in an outsized laminated structure.
Large part: Luggage cover for mass transit bus wheelhouse is fabricated in a deep-draw vacuum-forming process that achieves a remarkable 40 in. (1,016 mm) draw.
The wheelhouse cover is molded in a conventional vacuum-forming press using a special process.
The laminate is comprised of an outer layer of Boltaron 4800 sheet, an inner layer of proprietary glass-reinforced resin, and a core of proprietary bonding material.
Amtech workers prep and trim the part.
Quality control inspection: The deep-draw process and material selection achieve excellent surface texture resolution, consistent part-to-part aesthetics, and cosmetic finish.
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 0.9 m). It is fabricated in a deep-draw vacuum-forming process that achieves a remarkable 40 in. (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 in. (4.5 mm) gauge thermoplastic sheet from Boltaron of Newcomerstown, OH.
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.
Formable, aesthetic sheet
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 excellent 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 in. (1.02 to 6.35 mm) gauges, widths to 60 in. (1524 mm) and lengths to 120 in. (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.
Laminate meets cost requirement
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.
The part is unusual for its size. "Most fabricators don't have the capability to form a part this large," Bushbaum says. "We have a tremendous amount of experience in large-part fabrication."
Amtech designs, prototypes and fabricates components for a number of markets. Among these are cooling tower, wind energy, construction, food service, recreational vehicle, mass transit, rail and military markets.
In addition to deep-draw and conventional vacuum forming, process capabilities include resin transfer molding (RTM), vacuum-assist RTM (VARTM), thermoforming, finishing, "value added" assembly and custom packaging. Operations take place in a 220,000 sq ft (20,440 sq m) facility in central Washington, which employs approximately 200.