Rubber Compression Moulding

High volume manufacture

Rubber can be formed into components by several processes, including extrusion, calendaring, coating onto fabric and moulding. Harboro’s knowledge and expertise is broad and can offer components across the full range of processes. Harboro’s key area of specialism is moulding, operating a wide variety of high tech machinery. We utilise the full range of moulding techniques from compression moulding to cold runner fully automated injection moulding.

The choice of moulding method will depend upon factors such as the required material, tolerances, finish desired, the number of components, and should be discussed in detail during the initial design review.

Compression Moulding

This is a forming process where a piece of uncured rubber of the correct geometry and volume is placed into a heated mould. The mould is closed under controlled pressure and speed, multiple movements and closure cycles may be employed to displace any air to fill the cavity form before the onset of cure. 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 mid to low quantities. Mould costs are generally lower than transfer or injection. Parts moulded by this method will always have some flash because the mould surfaces are held apart by the overspill. Dependent on the product this flash evidence may need to be removed.

Harboro’s range of machines including vacuum chamber is from 1 to 500 tons and can accommodate micro to very large thick section components.

Transfer Injection Moulding

The heated mould is closed in a press and the rubber injected by a hydraulic cylinder through a feed gate and into 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 cavity feed gate. Maximum weights and number of cavities are governed by the capacity of the transfer cylinder and the clamp pressure.

Injection Moulding

Injection moulding including variations on the material presentation is probably the most widely used for high volume manufacture. The application of controlled chamber or Cold runner systems helps to eliminate waste and enhance the moulded product. Rapid cycle times with minimal / no flash can be achieved providing the opportunity for full automation and low unit cost supply. Most Natural and synthetic rubbers can be injection moulded with each enquiry presented to the technical team being evaluated for the selection of the most appropriate manufacturing method. With machines ranging from 1 ton to 500 tons the annual volume of parts to justify the injection moulding approach is not critical. Low volume parts can be injection moulded without concern.

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