The process transforms elastomers with weak thermoplastic properties into a strong, elastic and tough substance with useful properties and introduces a network of cross links into the elastomers. Tensile strength is maximum at optimum cure. Beyond this point, the stock is said to be over cured. It gets stiffer and harder but weaker and less extensible.
Techniques of Vulcanization
Vulcanisation can be carried out using several techniques.
- Compression Moulding – This is the most common method used in the industry. Here the blank is placed in a two piece mould, one part of which is movable and the other stationary. The mould is closed, then heat and pressure are applied so that the material flows to fill the mould. A little excess of the material is used to ensure that the mould is completely filled.
- Transfer Moulding – This technique is a variation of compression moulding. The blank is placed in a separate chamber called as pot, preheated to a temperature below cure temperature and transferred into the heated mould where cure takes place. Although in this technique, moulds are more expensive, the process takes shorter cure time due to use of higher temperature and there is better heat transfer.
- Injection Moulding – Injection moulding is used mostly for plastics. However rubber articles can also be moulded using this technique. The polymer is preheated in a cylindrical chamber, a temperature at which it will flow and is then forced into a comparatively cold, closed mould cavity by means of high pressure. A great advantage of the process is the speed with which the articles can be produced in seconds.
Change in Properties of Rubber after Vulcanization – The table below illustrates the salient changes brought by vulcanization:
Changes After Vulcanization
|Before Vulcanization||After Vulcanization|
|Low viscosity||High viscosity|
|Low modulus||High modulus|
|Low tensile strength||High tensile strength|
|Low elongation||High elongation|
|Low hardness||High hardness|
|High compression set||Low compression set|
|High solvent swell||Low solvent swell|
Curing temperature of 150-160C is normally used. After curing, the seals are trimmed to remove the mould flash and inspected for dimension.