When pressing plastics, they can be divided into two types of matrix, one is thermosetting plastics, the other is thermoplastic plastics.

Thermosetting plastics, such as bakelite powder, are pressed and permanently formed under heating and pressure. The bakelite mould is subjected to pressure periodically and is continuously heated at temperatures of 150-200 C.

Thermoplastics, such as polyvinyl chloride, are usually produced by injection moulding. Plastics are heated in an independent heating chamber and then injected into a cooler moulding in a softening state to exert pressure so as to make it hard and cold.

The normal operating temperature of injection moulds ranges from 120 180 C to 260 C. However, when working, the cavity is usually water-cooled, so the degree of heat, stress and wear is relatively light. Only some varieties, such as plastics containing chlorine and fluorine, emit harmful gases during pressing, which has a greater erosion effect on the cavity.

According to the working conditions and characteristics of precision plastic moulds, the following requirements are put forward for steel used in plastic moulds:

(1) Steel material needs to be very pure in order to obtain a high glossy machined surface. For this reason, less inclusions, less segregation, compact structure and high surface hardness are required.

(2) The surface should be wear-resistant and corrosion-resistant, requiring a certain surface hardening layer, which should generally be above 45 hrc, and the surface finish should remain unchanged for a long time.

Thirdly, it has enough strength and toughness to withstand load without deformation or damage.

(4) The heat treatment deformation is small to ensure interchangeability and precision;

_Good electrical processing ability.