Tech Briefs
Clear lens reflector optic headlamps

The outer lenses of the 1999 Dodge Ram's redesigned headlamps and fog lamps are formed from Apec DP9-9353, a high-heat polycarbonate from Bayer Corp.'s Plastics Division.

More and more, lighting systems are taking on an increasingly important role in automotive styling, and the redesigned front end of the Sport version of DaimlerChrysler's 1999 Dodge Ram pickup truck is no exception. In designing the forward lighting system, Federal-Mogul Lighting Products improved not only the system's appearance but also its performance by introducing the first headlamp with clear lens reflector optics for use on a domestically produced light-duty truck. The material forming the outer lenses of the redesigned headlamps had to possess a combination of high-heat resistance, clarity, durability, and processibility. A high-heat polycarbonate from Bayer Corp.'s Plastic Division, Apec DP9-9353, was the choice for meeting the performance criteria the application demanded.

The headlamp lenses, which measure 14 cm (5.5 in) by 23 cm (9 in) with a lens thickness of 0.25 mm (0.1 in), are injection molded at Federal-Mogul's Sevierville, TN, plant. After molding, a UV hard coating is applied, and the lenses are shipped to the company's Hampton, VA, plant, where the headlamp assemblies are manufactured. The lenses are then adhesively bonded to the headlamp assemblies, and the units are shipped to DaimlerChrysler for final assembly into the vehicle.

The headlamp optical unit is an advanced, high-performance, reflector-based quad headlamp system. Because the quad system utilizes two dual-filament halogen capsules, as opposed to the two single-filament bulbs used in traditional dual-headlamp designs, the lens material had to withstand the higher operating temperatures generated by both dual-filament capsules being energized at the same time in high-beam mode, according to Jerry Anway, Director of OEM Sales for Federal-Mogul Lighting Products. This posed no problem for Bayer's polycarbonate, which exhibits heat performance of up to 171°C (340°F), compared with approximately 129°C (265°F) for standard polycarbonates, according to Roy Palasek, Market Development, Bayer Automotive Plastics Center. Also, because clarity was another important consideration, an optically clear lens and reflector were used in the headlamp assembly. "This lighting system utilizes an optically clean Apec lamp lens that reveals the brightwork of the inner lamp system, a critical styling goal for DaimlerChrysler," said Palasek.

"The redesigned headlamps achieved better optical illumination performance over the previous model's design," Anway said. He explained this was determined by night-drive comparisons, which evaluate the beam pattern on the road, as well as photometric evaluation that measures light output of both low and high beams.

The new headlamp system also utilizes the same assembly package size as the former model, so no expensive front-end sheet metal retooling was required. "Had this resin not been available, glass was an alternative," said Anway. "However, the thickness of the lens would have to have been increased, resulting in a heavier lens and higher costs because the headlamp assembly would have needed to be redesigned."

Apec is also used for the truck's fog lamp lenses. The lenses, which measure 8.9 cm (3.5 in) in diameter with a 0.33-cm (0.13-in) lens thickness, are injection molded, then spin-welded onto the fog lamp reflector. By employing an optically clear lens and reflector, the redesigned fog lamp, which uses a 37.5 W bulb, is able to deliver an optical performance equal to that of a more expensive 55 W fog lamp system in a smaller package size, according to Anway.

The redesigned headlamps and fog lamps went from concept to reality in just 18 months, something Anway described as "unheard of in the industry."

Jean L. Broge