CompositesWorld got the unique chance in March to visit BMW Group’s manufacturing and assembly facility in Leipzig, Germany, where its i3 all-electric, four-door passenger car now rolls out at a rate of 100 vehicles per day. Located just northwest of the city in an open, sprawling industrial park, the BMW campus hosts 10 buildings and is bordered on one side by four 2.5-MW wind turbines, which provide some of the facility’s electric power. An imposing three-story, vaulted glass-and-concrete structure that serves as the main entrance and architectural focal point of the complex is made all the more striking by an overhead conveyor system that moves BMW’s production bodies-in-white from an adjacent assembly plant to paint booths elsewhere. Four of the 10 buildings house i3 production, including all aspects of molding and assembling its carbon fiber composite Life Module (passenger cell).
Manufacture of every carbon fiber part produced for the i3 actually begins in Otake, Japan, where a joint venture of Mitsubishi Rayon Co. Ltd. (Otake) and carbon fiber maker SGL Group (Wiesbaden, Germany), called MRC SGL Precursor Co. Ltd. (MSP), produces the PAN precursor required for the manufacture of the carbon fiber used in the car. The precursor is delivered to another joint venture, this one between BMW and SGL, called SGL Automotive Carbon Fibers (SGL ACF). Based in Moses Lake, Wash., to take advantage of relatively inexpensive hydroelectric power there, the SGL ACF facility is dedicated exclusively to supplying BMW, and it produces 50K tow on two production lines with a current total nameplate capacity of 3,000-metric tonnes/6.613 million lb (see related news story). From Moses Lake, carbon fiber roving is sent to a second SGL ACF facility in Wackersdorf, Germany, for conversion into fabrics that are cut, kitted and stacked in preparation for molding. Parts for the i3 Life Module are molded at two sites in Germany: Leipzig and Landshut (northwest of Munich). BMW Leipzig, which opened in 2005, is the principal i3 manufacturing hub. It molds all part numbers for the i3 and also houses assembly operations for the Life Modules and the subsequent integration of Life Modules and Drive Modules. The Landshut plant provides supplementary production of certain components, using identical processes and equipment.
When kitted stacks arrive from Wackersdorf, they are delivered to the CFRP Building. There, plies in the stacks are stapled together and then placed in a heated preforming mold in one of three Fill Gesellschaft mbH (Gurten, Austria) Preformliner presses, where a thermoplastic binder is applied to the fabric. The presses then preform the stacks to the shape of the mold. Preforms are transferred to an ultrasonic cutting station, where they are trimmed to near-net shape. (BMW minimizes touch labor, relying instead on robotics wherever possible.) After they are cut, preforms are high-pressure resin transfer molded (HP-RTM). The Leipzig plant is equipped with seven 3,000-ton hydraulic presses from Schuler Pressen GmbH (Waghäusel, Germany). Each is serviced by two Krauss-Maffei (Munich, Germany) injection units. The resin matrix, Huntsman Advanced Materials’ (The Woodlands, Texas and Basel, Switzerland) Araldite LY 3585 epoxy with Hardener XB 3458, is injected at pressures greater than 40 bar/580 psi. Parts cure in less than 10 minutes.
In another building, the Body Shop, the Life Module’s 16 RTM’d parts are bonded in a fastener-free process, using BETAFORCE polyurethane-based adhesive supplied by Dow Automotive (Auburn Hills, Mich., and Schwalbach, Germany). Adhesive application and part manipulation are automated — the Body Shop is equipped with 173 robots, supplied by ABB Robotics (Zurich, Switzerland). The Life Module is subsequently outfitted with metallic hardware, roof panels, glass and other components and then joined to the i3’s Drive Module, which comprises the car’s battery pack and drive train.
Read a much expanded version of this story in HPC’s sister publication Composites Technology (CT June 2014, p. 24, or visit short.compositesworld.com/BMWLeipzig.)