Molding
Rubber Molding is a process to shape polymer products to their final form. Injection, Compression and Transfer are the three basic types of molding. There are other forms of molding within these three basic types of molding. This is a more appropriate process for shaping polymer when the finished item needs to be conjoined and isn’t going to be a continuous form.
Injection Molding
This is the most complex rubber molding process out of all three basic types. This biggest difference with this process compared to the others is that the halves of the mold are fastened together initially so the uncured rubber is forced into the joined mold where it reaches the appropriate curing temperature and ultimately hardens to the form of the mold cavity.
Within this type of molding, there are several variations, but most of the processes are encompassed in the injection molding cycle. This sequence of events is as follows:
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The mold closes
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The polymer is injected into the mold cavity
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A holding pressure is preserved to offset shrinkage
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A screw-type plunger is used to incrementally feed the uncured rubber into the mold cavity
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Full cure temperature is reached, and the mold opens
Compression Molding
This process involved forcing two mold plates together. A surplus amount of the preheated rubber is put into an open mold cavity (heated). Once the material is in place, the mold is forcefully closed, and intense pressure is used as the material cures. Any excess material is trimmed off.
This is a high-volume, high-pressure method of molding. Determining the appropriate amount of material, proper temperatures, accurate timing, and required pressure are all very important factors to consider with compression molding.
Transfer Molding
This process is similar to compression molding in that the material is measured, heated and then added to the pot. From the pot the heated material is forced by a plunger through a sprue into the mold cavity. The mold cavity remains closed until the curing process is complete.