Molding i3 body panels

Volume production of a carbon fiber composite passenger cell is not the only unusual aspect of the BMW i3’s story. The car also features doors, door inners, fenders, bumpers, a hood and a rear spoiler made from an injection molded thermoplastic.

The parts are molded at Munich, Germany-based BMW AG’s i3 production facilities in Leipzig, on six massive injection molding machines, which range from 2,000 to 4,000 tons in clamp tonnage — two from Krauss-Maffei (Munich, Germany) and four from Engel (Schwertberg, Austria). Like other processes at the Leipzig plant, all of the injection molding operations are robot-attended and highly automated. Mold-loading and all part transfer and postmold extraction functions are handled by 6-axis robots built by ABB Robotics (Zurich, Switzerland).

Arrayed together on the factory floor, the machines produce parts from a polypropylene/ethylene-propylene-diene monomer (PP/EPDM), a thermoplastic vulcanizite that features fully cured, rubber-like EPDM particles in the PP to enhance the compound’s toughness. BMW is using virgin PP/EPDM for parts that have surfaces visible to the exterior, and PP/EPDM recyclate for parts (e.g., door liners) that are not visible to the exterior.

The most impressive of the machines is a large Krauss-Maffei unit that features two rotating platens. It molds a two-component door structure for the i3. During the process, the door inner is molded on one mold. When that mold opens, the platen carriage rotates 90° to present the molded panel to a six-axis robot, which removes it and places it in the second mold. There, the door inner is enclosed and the exterior door panel is molded over it. When the mold opens again, the overmolded outer and inner, now a finished unit, is removed by a second robot and placed on a cooling rack.After they cool, body panels are transferred immediately from the molding floor up to an overhead conveyor system that delivers parts to the paint booth. BMW says its painting process for the i3 consumes 70 percent less water and 50 percent less energy than painting systems for steel body panels.