Production of high-quality components using specialty injection molding methods
Injection compression molding
Specialty injection molding methods are used for specific tasks. Depending on the application, processes used with other thermoplastics are also used for the processing of polycarbonate. Injection-compression molding, for example is used for both large-area components, such as polycarbonate automotive glazing, and thick-walled components such as focusing lenses.
When processing amorphous thermoplastics, it is particularly important to ensure that the injection molded components have low internal stresses to prevent stress cracking in subsequent practical use. Injection-compression molding is a process technology that enables gentle, low-stress shaping.
With injection-compression molding, the melt is first injected into a cavity whose die gap at the time of injection, larger than the target thickness of the molding. Subsequent closing of the mold compresses the melt to the desired thickness thereby reducing the required filling pressure and lessening the levels of molding stress and flow orientation. The motion of the mold also applies holding pressure over a large area to uniformly compensate for shrinkage. With the help of this process/mold technology, glazing modules with surface areas of roughly 1.5 to 2 m² can be produced with low internal stresses.
There are essentially three variants of this method:
- Parallel injection-compression molding, in which the melt is first injected into a larger cavity and the mold halves pressed together to achieve the target thickness.
- Expansion injection-compression molding, in which the cavity is initially enlarged during the melt injection phase by the cavity pressure generated or by controlled closing-side motion and then reduced to the target thickness.
- Wedge injection-compression molding, in which there is initially a wedge-shaped opening of the compression gap, which is then “collapsed down” to the target thickness simultaneously with the injection of the melt.
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