What is Vulcanization of Rubber – How to vulcanize rubber

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 –

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