A piece of uncured rubber of the correct size is placed between two halves of a heated mould. The mould is closed in a press under a pressure of around one ton/sq in and the rubber is forced into the exact shape of the cavity. The rubber gains heat by conduction from the mould surfaces and “cures”. When the rubber has had sufficient time to cure, the mould can be opened and the part removed.
Compression moulding is a relatively simple process and is often used for components required in fairly low quantities. It is also the most economic method for parts with simple shapes.
Parts moulded by this method will always have some flash because the mould surfaces are held apart by the necessary excess rubber in the “blank”.
The heated mould is closed in a press and the rubber injected by a hydraulic cylinder through a feed hole in the cavity. The cylinder can either be incorporated in the press or sometimes in the mould.
Provision must be made for air to escape from the cavity as the rubber enters, and the feeding method chosen to suit the operational requirements of the part.
This method of moulding can produce high-precision parts in moderate quantities without high tooling costs. In the simplest case, the mould can be the same as a compression mould with the addition of a feed hole. Maximum weights and number of cavities are governed by the capacity of the transfer cylinder and the clamp pressure.