Rubber Mouldings Rubber Mouldings

Rubber Mouldings

Rubber Mouldings Rubber compounds are complex, chemically active, viscoelastic materials and undergo both transient and permanent changes of properties during flow in practical processes. Many are blends of two or more elastomers and virtually all contain particulate fillers (carbon black, silica, china clay, calcium carbonate etc.) and a chemical crosslinking system, usually based on sulphur. In addition, lubricants, plasticisers and organic process aids are used to modify properties. A typical rubber compound is a microcomposite with, on average, twelve components. It is these characteristics which make the modelling of both material behaviour and rubber processes such difficult tasks and hamper the diagnosis of processing problems.

Rubber versus Thermoplastics

Moving downstream, a similar picture appears for extrusion and moulding processes. Both are highly evolved, but it is only recently that effective quantitative methods for design and simulation have begun to emerge. In extrusion, it is possible to borrow methodology from thermoplastics extrusion. Rubber extrusion is, very approximately, equivalent to thermoplastics melt extrusion. Hence the basic models for screw design are similar. However, the detailed behaviour of a rubber compound in an extruder screw is well removed from that of a thermoplastic melt. Viscosity is much higher and it is more elastic sometimes exhibiting substantial wall slip and thixotropy, particularly in the feed zone. In addition, the complex geometries needed to introduce a mixing action in rubber extruders, for effective heat transfer and minimisation of hot spots, create a further challenge for the mathematical modeller.

Compression Moulding is the simplest form of rubber moulding. The uncured rubber is placed in each hot cavity of the mould tool. The mould is closed by a hydraulic ram on the press, forcing the rubber into the shape of the cavity and maintaining pressure on the tool. After the cure cycle is completed the pressure is released and the part is removed from the mould.

This type of Moulding is used for prototype moulding, low volume production and where there is a constraint on the investment available for tooling. The simple construction normally means that a tool is a relatively cost effective solution.

Compression moulding is the slowest moulding method, as the material has to be heated by the mould to reach its cure temperature. The technique is also not suitable for complex shapes and where rubber is to be bonded on to metal or plastic inserts.

Transfer Moulding is a simple form of injection moulding through ports. Rubber is placed in a "pot" and, as the mould closes, a piston pushes the rubber through transfer holes into the cavity. Heat is generated in the compound during transfer that can shorten the total cure time, as the material is nearer its curing temperature as it enters the cavity.

Transfer moulding allows complicated shapes and bonded components to be manufactured.
Rubberatkins can also offer other moulding processes if our clients need to create Dip Moulded, Polyurethane or Plastic parts.