Raw rubbers can’t be used in their natural state. The raw rubber mixed with the right type of additives will get us to the optimum properties of rubber products, rubber seal rings, Rubber O rings, Rubber gasket seals etc… Rubber processing and compounding has utmost importance.

The additives in a rubber processing and compounding may vary from application to application. The additives are added by weight and will include some or all of the following:

Curatives

Sulphur and peroxide curing are used as cross linking curatives for rubber chains and best known curatives in rubber processing and compounding. They are first to get discovered and most commonly used.

The constituents are weighed out and combined by a mixing process which must blend the ingredients thoroughly in a repeatable way. This is achieved either by an internal mixer, where the compound is mixed by two meshing rotors in an enclosed case; or by open mill mixing, adding the ingredients carefully into the “nip” between two steel rollers, typically of 30″ diameter.

The result of either process is a batch of uncured rubber compound. This is allowed to settle for a time before undergoing Quality Assurance tests. Once passed, it can be formed into suitable shapes for moulding.

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”.

A screw injection system delivers a metered quantity of rubber into the closed mould. The injection unit is fed from a continuous strip or a reservoir of uncured rubber and is cooled to avoid premature curing.

This process is generally used for multi-cavity moulds and can produce hundreds of components per press cycle. Because of the amount of rubber in the system, it is inadvisable to change materials frequently

Large moulds require complex feed systems to balance the pressures in each cavity. Generally these are in the heated top half of the mould and cure at the same time as the components. Unlike thermoplastics, cured thermoset rubber cannot be reground or reused and the additional waste has to be included in the material usage per piece. Where very large volumes of mouldings are required, cold runner systems should be considered. These are justified by material savings over £10,000 pa.

This process lends itself to relatively large quantities, a large number of cavities and infrequent changes of materials or moulds. Parts are repeatable and can be made to a high level of precision.

Compression moulded and some injection moulded parts require deflashing. This is done in various ways depending upon the shape and size of the component and the type of rubber used.

Sub-Zero Finishing

The most modern and efficient method of finishing uses cryogenics. Parts are frozen to temperatures as low as -120°C and then tumbled and/or bead-blasted while cold to remove the brittle flash. The machines are individually programmed with the optimum temperature and running times for each particular type and number of components, which are tested and proven during the development stages.