Compression and Transfer Molding

Both compression molding and transfer molding are still widely used in the industry worldwide, even though injection molding has a number of advantages. This is often because the fabricator has an existing press in good working order. When starting from scratch both economic and technical factors have to be considered [1]. The main economic factor is that the capital cost of machines and molds increases in the order compression molding, through transfer molding, to injection molding. A production volume may be too small to justify a high initial capital cost.

In compression molding, Fig. 9.1, the weight, dimensions, and positioning of the charge have to be closely controlled, or the dimensions of the product can vary widely. This is often due to variation in the amount of material lost from the cavity as flash. This can be controlled, rather than be eliminated, by employing a shallow plunger, which, because of close clearances, means that excess rubber can only escape when high pressure is applied, after the mold is completely filled.

In transfer molding, Fig. 9.2, rubber flows from a separate reservoir through a narrow flow channel, called a sprue, into the mold. There are three advantages over compression molding.
1. As the mold is closed before the rubber charge is forced into it, closer dimensional control is achievable.
2. In the transfer process, fresh rubber surfaces are produced, and this allows development of a strong rubber-metal bond with any insert in the mold.
3. Unit production costs are lower due to shorter cure times as a result of heating the rubber due to flow through sprue, runner, and gates and shorter downtime between runs, as only one charge blank is necessary, even if a multicavity mold is used.

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