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July 20, 1999

4 Min Read
Manufacturing Polycarbonate Cover Lenses

The Hella group is a multinational supplier to the automotive industry. Based in Lippstadt, Germany, it has licensees throughout Europe, North America, and Asia. The company's 17,000 employees, 3,000 of whom are based at the Lippstadt II works, notch up sales of almost DM 2 billion (US$ 1.3 billion). The most important products are automotive lights, locking systems, and components for engine electronics.

In Europe, Hella's reputation for automotive lights stretches back over decades. Diffusers for cover lenses are now made increasingly of polycarbonate, which in many cases has supplanted glass (Figure 1). Hella has production capacity for polycarbonate cover lenses at its sites in England, Slovenia, and Germany. At the Lippstadt works (which also conducts research and development), IMI saw how polycarbonate cover lenses are made—from delivery of the raw polymer to decorating the finished parts.

Material Delivery

Hella's injection moulding plant for cover lenses works five days a week in three shifts. It goes through 70 tons of polycarbonate a month. The raw polymer is stored in silos with capacity for 35 tons. The resin is delivered by freight vehicles, which Hella unloads itself via its own hoses, couplings, etc. to completely rule out any contamination of material handling systems with foreign materials. The polycarbonates comprise Lexan (from GE Plastics, Bergen op Zoom, The Netherlands) and Makrolon (Bayer, Leverkusen, Germany) in equal proportions.

Production takes place in several buildings and production lines. The machines are fed separately by independent drying and conveying lines manufactured by Motan, Isny, Germany. Since polycarbonate must be dried intensively before processing, it remains for approximately 4 hours in contact with air dried to a dew point of approximately –30 to –40°C. After exposure at 120°C, the residual moisture content has been reduced to a maximum of .02 percent.

The material is then conveyed to the machines by vacuum conveyors. It is distributed to the various machines via a central routing system. The air used for conveying is also dried. After each conveying operation, the material lines are emptied completely so that there is no risk of material cooling in the lines and thereby absorbing moisture. All conveyor parts coming into contact with the product are made of stainless steel. The drying hoppers used to be fed from octabins, but are currently being converted for feed from silos.

The silos have weighing units, so that the instantaneous level can be read off at any time. The air escaping during filling of the silos is discharged to the atmosphere via a central venting filter. The air required for conveying is filtered. On average, each vehicle delivers 20 tons and a delivery must therefore be received roughly every week.

Machine Technology

Hella uses many types of injection moulding machines, but cover lenses are manufactured only on Mannesmann Demag and Krauss-Maffei machines (Figure 2). The current machines are in the 6,500 to 8,000 kN clamping force range, and smaller machines of 5,000 kN will shortly be added. All machines—the Demag D-Series and Krauss-Maffei C-Series—are standard, with no special outfitting. The plastication unit has a standard screw, like that used for polyolefins, with a length of 23D and a diameter of 80 mm.

Hella has developed its own monitoring system for individual process parameters, which must lie within tolerances. These process parameters were determined separately for all seven diffuser systems in the production program. If one of the process parameters strays outside tolerances, the finished part must be tested manually.

Mould Technology

A pair of cover lenses is produced in each mould. The two-cavity mould, for a total shot weight of 900g, is gated via a heated sprue bushing. The two cavities are fed via a simple cold runner bridge. Following robotic removal, an ionized air jet completely removes all dust particles (Figure 3). If all the process parameters monitored were within tolerances, the two halves of the moulded part are then separated by cutting the solidified runners. Otherwise, the two parts are left on the sprue, thus identifying them as bad parts for removal from the good parts.

Production Stoppages and Mould CleaningThe injection moulds are cleaned regularly after 15,000 cycles. Hella has a fixed procedure for production stoppages, as explained by Manfred Goldstein, head of plastics processing:

  • The screw motor is stopped and the plastication unit cooled to 180°C (the injection temperature is usually 270 to 280°C).

  • The mould is exchanged, but the plastication unit is not cleaned.

  • The plastication unit is heated up to 270°C again.

  • A few cycles after the restart of production, the moulded parts are free of black spots again.

There is virtually no loss of production in the event of stoppages with this procedure. Mould maintenance is carried out in the mouldmaking department. There is no separate maintenance department.

Coating the Parts

Cover lenses are at risk from flying stones, other mechanical loads, and weathering. There is no way of preventing scratches on pure polycarbonate, so coating is essential. There are various techniques for doing this. Hella has opted for spraying. The parts are passed to the cleanroom coating line, where they are coated by an industrial robot in individual cycles (Figure 4). After being coated, they are annealed at 125°C in order to reduce the internal stress that was introduced during moulding and coating. The UV-curable coating forms a 10- to 12-µm layer on the surface of the lamp.

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