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Processing of Thermoset Molding Compounds

Molding compounds are processed to molded parts through the combined forces of pressure and temperature. The following methods are used in processing:

    Compression Molding

    The molding compound, fed volumetrically or by weight, is filled into the (typically) electrically heated mold, either loosely or pelletized, preheated or preplasticized. Upon closing the mold, the molding compound softens and fills the mold. The molding can be removed after a sufficient curing period, which will depend on the reactivity of the molding compound, its prior treatment, the mold temperature and the wall thickness of the mold profile.

    Advantages:

    • Minor processing and post shrinkage
    • Minor warpage of molded part
    • Minor damage of reinforcing filters

    Disadvantage:

    • Cycle times depend on maximum wall thickness

    Transfer Molding

    Technically, transfer molding is a process between the compression molding and the injection molding process. Here, the pelletized and preheated or preplasticized molding compound is pushed by a plunger from an antechamber in the mold via a runner and gate system into the cavity where the material cures. Three plate molds with the plunger working from the top can be built into conventional presses. From the perspective of process technology, single split molds are easier – the plunger works from below or horizontally from the side and requires a press with two hydraulic plungers.

    Advantages:

    • Shorter cycle times, compared with compression molding
    • Minimal flash since form filling occurs in the closed mold

    Disadvantage:

    • More warpage, compared with compression molding (due to anisotropy)

    Transfer Systems with Screw Preplastification and Plunger Feeding

    These systems are an obvious choice for manufacturing items that are subjected to stress and good dimensional tolerances. Using this method, a more reactive molding compound can be used in suitable applications. The benefit is relatively short cycle times and accurate shot weights.

    Injection Molding

    Due to its high efficiency, injection molding is the most widely used processing method today. Here, one unit automatically performs all the required operational steps. The screw feeds and homogenizes the molding compound – having melted it through friction and heat conduction – and acts as a plunger during the injection process. Because of high material inlet temperature, very little energy needs to be introduced through heat conduction for curing the molded part, resulting in short cycle times.

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