Compression Rubber Molding

Hawthorne Rubber Mfg. Corp. uses compression, transfer and injection molds. On our quotations, we may propose more than one type of mold. To clarify these different molding options, their advantages and disadvantages, we have put together three education/information web pages on molds. The following covers the most basic type of mold, compression.

Diagram of how the compression molding process works.This is an example of a basic two plate single cavity compression mold, however, you are not limited to two plates or one cavity. The mold does not require heater elements or temperature controllers. The molding temperature is fully controlled by the press it is running in.

Molding Procedure: The mold is removed from the press and opened. A precut or shaped "slug " of uncured rubber, at a set weight, is placed into the cavity. When bonding to a metal insert, it is also placed into the cavity at this time. The loaded mold is then placed back into the press and the press is then closed. The pressure forcing the plates together forces the uncured rubber to flow into the shape of the cavity in the mold. A slight excess of material flows out of the cavity along the gates and vents allowing the mold to close. The semi-positive design (extension of top plate into cavity) is used with some parts that require a ram-like additional push on the material during the final travel of the closing plates. The mold remains closed until the rubber is cured completing the cycle.

Photo of compression molding press.General Comments: Compression molds do not consume additional rubber when filling the cavity like the "runner" of an injection mold or the "pot" of a transfer mold. However, while being conservative on the amount of rubber used to make the part, it requires a greater amount of effort to weigh and prepare each individual slug of rubber. Compression molds take longer to cure the rubber and each slug of rubber is hand loaded into each cavity. However, due to the simplicity of the mold, it is the most economical mold to buy.

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