One method of producing plastic bodywork parts that enable color matching to neighboring metal parts is through back-injecting pre-formed films with thermoplastics, also known as film insert molding (FIM). The film is responsible for the surface quality of the component (Class A) and the back-injection material is responsible for its mechanical and thermal properties in particular, such as stiffness and thermal expansion. The FIM process offers several benefits for large components in particular, including roof and liftgate modules, glazing elements, antenna covers and spoilers. This means design freedom is greater than for equivalent components made of metal, and in some cases weight is significantly reduced. By integrating functions, substantial savings can be made in assembly, logistics and machine investment costs. “To offer our customers from the automotive industry an optimum and production-oriented development and test environment for mass production of these components, we have installed the complete process chain for film back-injection on a technical service lab scale at our site in Leverkusen,” explains Olaf Zöllner, Head of the Injection Molding Technologies Group in the Polycarbonates Business Unit.
The customized equipment ranges from units for printing and trimming films and related technology for automated transportation, cleaning and handling of components to injection molding machines including injection-compression molding technology. “Our development partners can also rely on our comprehensive know-how in design engineering, using all the associated simulation processes, and in testing,” says Zöllner.
The FIM process starts with forming the printed films. These can be cold-formed using the high-pressure forming (HPF) process patented by Bayer MaterialScience or by thermoforming. Appropriate machines are also available for trimming formed films. As well as film shrinkage, account should be taken of the fact that the edges of FIM bodywork parts may need to be protected. This can be done by wrapping the film around the edges. “By deploying our substantial experience in film processing, we can provide valuable support for component design with regard to thermal expansion in the mold or shear stress through the melt, for instance,” says Zöllner.
New handling process ensures precise positioning of film in mold
A clean room is set up for back-injection. Before this, the decorated film which has been cut to size is cleaned using brushes to remove fluff and other impurities. Correct back-injection depends on the film being positioned accurately in the mold, ensuring the polished high-gloss surface of the mold is not damaged in the long term. For this, Bayer MaterialScience has developed with its partners a handling process in which the film is electrostatically charged before being inserted. This enables it to sit tightly and accurately in the mold. “In most cases, positioning pins and slides are not needed to secure the film mechanically,” says Zöllner.
With back-injection, it is important to ensure that the films are not creased and that printing is not washed out by the hot melt. Variables that influence these defects are the number of gating points, the rate of volumetric flow for each injection point and the design of the injection points. “We have the design engineering knowledge and appropriate calculation tools to finely coordinate these variables and prevent errors,” explains Zöllner. If it is necessary to lower the melt pressure for mold filling to relieve stress on the mold and film, a system for gentler injection-compression molding is available.
For demolding and for all other steps in the FIM process installed in Leverkusen, a handling technology is used that does not damage the surface of the film component. For example, robots with rubber suckers are used to lift, transport and deposit the parts.
Three methods for color matching-compatible FIM bodywork parts
In principle, there are three methods to perform successful color matching for an FIM bodywork part. The film component can be coated off-line in the conventional manner. It is also possible to start with a laminated film that consists of a carrier film and a paint film comprising a transparent film on top of a colored film layer. A third method is to use a film that is coated with a colored base coating and a UV-curing clearcoat (Desmolux®). The clearcoat is hardened after forming and back-injection. For all these methods, Bayer MaterialScience offers customized Makrofol® and Bayfol® films in its range and these should be based on polycarbonate or ABS, depending on whether the bodywork part is horiziontal or vertical.
Instead of being back-injected with thermoplastics, films can also be backfilled with fiber-reinforced polyurethane systems. Bayer MaterialScience is also pursuing this technology and offers appropriate polyurethane systems. The long fiber injection (LFI) method is used to produce polyurethane film modules. The polyurethane reaction mixture and long glass fibers are directly injected into the mold simultaneously using a special mixing head, in which a glass cutting unit is integrated.