mass reductions grew, plastics were used in automobile lighting fixtures. Changes in the design of fixtures enabled by the flexibility in molding plastic shapes have led to significant reductions in component mass in recent models. Ford initiated work on plastic headlamp lenses and parabolic housings. The 1984 Mark IV headlamp system was a breakthrough in this area. It had five plastic parts; the lens and body were PC; the replaceable bulb base, the electric connector, and locking retainer ring were also made of plastic. This system saved 0.9 kg (2 lb) over a conventional two-lamp system and 1.4 kg (3 lb) over a four-lamp system done in glass . Additional mass savings resulted from the elimination of a separate trim piece.
Plastics are now used extensively in exterior lighting to reduce mass. Lenses are made from acrylic and polycarbonate. Housings are made from many materials including PC, PP, polyetherimide (PEI), polyetherimide plus polyethylene terephthalate (PEI/PETP), bulk molding compound (BMC), and nylon. The PC and acrylic are much lighter weight than glass and are also tougher.
The brackets that hold the lighting systems in place have also gone from steel to plastic. Typical materials for this application include glass-filled nylon, PC, glass-filled PET, and sheet molding compound (SMC). The plastic brackets are much lighter in weight than the metal part they replaced.
Pickup Beds Both SMC and RIM have been used for pickup beds and demonstrated to yield at least a 20% mass reduction over steel. They also eliminate the need for a separate bedliner, thus reducing raw material needs. The 2000 Chevrolet Silverado offered an optional composite pickup box (Fig. 17.7). The bed and tailgate were structural RIM while the fenders and tailgate outer were reinforced RIM. The tailgate was 6.8 kg (15 lb) lighter, which not only helps fuel economy but also requires less effort to close. The overall system was 23 kg (50 lb) lighter than the original steel system.
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