Molding Procedure: The mold is removed from the press and opened. A precut or shaped “slug” of uncured rubber is placed into the cavity at a set weight. 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 causes 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 with the gates and vents, closing the mold. The semi-positive design (extension of the top plate into the 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.
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 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.