| Rubber is a very versatile material, it is favoured for applications where flexability, high heat or chemical resistance is important. It is available is a wide range of hardness and several colours.Rubber can be used to make components through compression moulding, transfer moulding, and injection moulding. Choosing the correct process depends on quantity, complexity, and accuracy required.
Compression Moulding A piece of uncured rubber the correct size is placed between two halves of a heated mould. The mould is closed and placed under pressure, the rubber is forced into the exact shape of the cavity. The mould is heated and the rubber cures, at this stage the part can be removed and the process repeated. Compression moulding is a relatively simple process and is often only used for components required in low quantities. It is the most economic method for parts with simple geometry. A negative of this process is that the components will always have some degree of ‘flash’ from the excess rubber used to fill the mould. Transfer Moulding The heated mould is closed in a press and the rubber is injected by a hydraulic cylinder through a feed hole in the cavity. Provision must be made for the 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 feedhole. The main advantage of this process of compression moulding is the components have no flash and the cycle time is slightly faster. Injection Moulding This is the most accurate and cost effective way of producing large volumes of rubber components. A screw injection system delivers a metered quantity of rubber into the closed mould. The injection dose of rubber is fed from a reservoir of uncured rubber. This process is generally used for multi-cavity moulds and can produces hundreds of components per cycle. Large moulds require complex feed systems to balance the pressure in each cavity. Generally the ‘runners’ are in the heated top half of the mould and cure at the same time as the component. Rubber is a thermoset material so the cured runners are cannot be reused and the additional waste has to be included in the component cost. In very high quantity productions the addition of a cold runner system can save material costs. |