KraussMaffei, which took fourth place in the best of K for its work on the BMW i3, last Friday disclosed technical details about the project for the first time.
Body panels made of thermoplastics and structural components made of fiber-reinforced plastic are produced in KM injection molding and reaction equipment. It’s described as the first time that carbon fiber-reinforced plastics (CRP) are used in series production. GM, however, may claim bragging rights for use of carbon composite hoods in the new
Corvette, which went into production in January of last year.
Germans and Americans disagreeing over who invented what first–What else is new?
Equipment supplied for the BMW project includes two double swivel plate machines which are fully automated with two industrial robots. Weighing 400 metric tons, each MX 4000-17200/12000/750 WL is 24 meters long, 9 meters wide and 7 meters high. The thermoplastic outer shells of the BMW i3 are produced on these machines, which have a clamping force of 4,000 metric tons, at the BMW plant in Leipzig, Germany.
Using the “joining-in-injection-molding” process, the outer door shell and its substructure are injected in a single pass, and then joined as the two swivel plates turn and are bonded with a third plastic component. The process is said to ensure high dimensional accuracy, one of the issues with thermoplastic panels in the past. See Bob Lutz and the GM Saturn.
In addition to the sidewall panels and the rear bumper, the hood of the BMW i3 is molded on what KraussMaffei describes as the most complex injection molding machines it has ever developed.
Twenty machines for high-pressure resin transfer molding (HP-RTM) supply reactive resin components for the supporting structures, for example the side frame, at the BMW plants in Leipzig and Landshut. HP-RTM is used to attain short cycle times.
High-pressure injection produces a high degree of fiber wetting. A 10-person team from the KraussMaffei Reaction Process Machinery Segment worked to improve metering technology and provide mixing heads with the possibility of adding an internal separating agent. The result is also said to be a first for series production.
In order to produce fiber-reinforced parts with an epoxy matrix in larger series, KraussMaffei says it enhanced high-pressure resin transfer molding (HP-RTM) for series use. In this process a self-cleaning, high-pressure mixing head injects the resin into the closed cavity and saturates the fibers inside under high pressure with careful control of timing and temperature so that the resin and hardener are fully networked. Compared with the previous autoclave process or vacuum infusion, the advantage of automated production is that the cycle times are reduced from up to 24 hours to minutes depending on the complexity and size of the component.
Another advantage: This type of HP-RTM process is also suitable for the use of polyurethane instead of epoxy resin as a matrix material. In addition to easier handling of the generally lower raw material price, another advantage of polyurethane is the reduced processing temperature.
The word “series” is used in Germany to describe high-volume production, or at least something beyond the prototype or novelty car level. Projected production volume of the i3 has not been disclosed, but it would be surprising if more than 25,000 units are sold globally this year. That qualifies as a volume production car–barely—but hardly at the level of global small cars like the VW Polo, which had sales above 680,000 last year.