Material Requirements for Plastic Moulds
The working condition of plastic die is different from that of cold punching die. Generally, it must work under 150 C - 200 C. Besides being subjected to certain pressure, it must also bear the influence of temperature. The basic performance requirements of steel for plastic moulds are summarized as follows according to the different working conditions and processing methods of plastic moulds.
1. Adequate surface hardness and wear resistance
The hardness of plastic moulds is usually below 50-60 HRC. The heat treated moulds should have sufficient surface hardness to ensure that the moulds have sufficient stiffness. Because the filling and flowing of plastics must bear large compressive stress and frictional force in the work of the die, it is required to keep the shape accuracy and dimensional accuracy of the die stable, and ensure that the die has sufficient service life. The wear resistance of the die depends on the chemical composition of the steel and the hardness of heat treatment, so improving the hardness of the die is beneficial to improving its wear resistance.
2. Excellent machinability
In most plastic moulds, besides EMD processing, certain cutting and fitter repairs are needed. In order to prolong the service life of cutting tools, improve cutting performance and reduce surface roughness, the hardness of steel for plastic die must be appropriate.
3. Good polishing performance
For high quality plastic products, the surface roughness of the cavity is required to be small. For example, the surface roughness of injection mold cavity is required to be less than Ra 0.1-0.25 level, while the optical surface is required to be Ra < 0.01 nm. The cavity must be polished to reduce the surface roughness. For this reason, the selected steel requires less impurities, uniform microstructure, no fiber orientation, and no pitting or orange peel defects in polishing.