Machine Design

Hungry for large parts? Try a composite sandwich

A new composite used for making large structural parts including vehicle hard tops, tonneaus, and other exterior body panels, gives automakers a lightweight, durable, and low-cost alternative to sheet-molding compound and other body-panel materials.

Developed by Advance USA, Old Lyme, Conn., STRATA technology is a multistep sandwich composite made by combining three primary processes -- thermoforming, reinforced fiber preforming, and polyurethane foam molding. Complex, three-dimensional parts as large as 25 to 250 ft2, in production volumes beginning at 10,000 units, are possible.

The new composite, weighing 40% less than traditional systems, features a high strength-to-weight ratio, good fatigue and impact resistance, and good sound and temperature insulation properties.

Here's how the process works. Two engineering thermoplastic sheets are thermoformed into three-dimensional skins, producing both an inner and outer skin. Reinforcement fibers are then attached to the inner surface of each skin.

These formed skins are then transferred to a matched die mold, where they are held in place. When the die is closed, a cavity is created between the two surfaces. Polyurethane resin is then dispensed into the cavity, creating a low-density foamed system, used to create a core structure as well as a matrix to consolidate the reinforcement fibers.

When released from the mold, the completed part consists of a durable, stiff, dimensionally stable composite structure with a finished, attractive surface. A wide variety of colors and textures can be incorporated into STRATA's thermoformed plastic surface.

The compound is suitable for automotive and light truck applications, as well as recreational vehicle tops, folding trailers, SUV hardtops, sleeper bunks and sidewalls in RV and heavy trucks, vehicular roof systems, golf carts, and agricultural and construction equipment.

1 A thermoplastic sheet is thermoformed, heated, then placed in an open mold and vacuum formed to its finished shape. Reinforcement fibers are applied to the inner and outer plastic skins. The reinforced fiber-backed skins are placed in the liquid molding press.

2 The press is closed and urethane foam is injected.

3 The finished part shows a cross section of the integrated composite sandwich structure.

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