The working conditions of plastic molds are different from those of cold stamping dies. Generally, they must work at 150°C-200°C. In addition to a certain pressure, they must also be affected by temperature. According to the different use conditions and processing methods of plastic molding molds, the basic performance requirements of steel for plastic molds are roughly summarized as follows:
1. Sufficient surface hardness and wear resistance
The hardness of the plastic mold is usually below 50-60HRC, and the heat-treated mold should have sufficient surface hardness to ensure that the mold has sufficient rigidity. Due to the filling and flow of plastic, the mold is required to bear large compressive stress and friction during work, and the mold is required to maintain the stability of shape accuracy and dimensional accuracy to ensure that the mold has a sufficient service life. The wear resistance of the mold depends on the chemical composition of the steel and the hardness of heat treatment, so increasing the hardness of the mold is beneficial to improve its wear resistance.
2. Excellent machinability
In addition to EMD processing, most plastic molding dies also require certain cutting processing and fitter repair. In order to prolong the service life of the cutting tool, improve the cutting performance, and reduce the surface roughness, the hardness of the steel used for plastic molds is necessary.
3. Good polishing performance
For high-quality plastic products, the roughness value of the cavity surface is required to be small. For example, the surface roughness value of the injection mold cavity is required to be less than Ra0.1~0.25, and the optical surface is required to be Ra<0.01nm. The cavity must be polished to reduce the surface roughness value. The steel selected for this purpose requires less material impurities, fine and uniform structure, no fiber orientation, and no pitting or orange peel defects during polishing.
4. Good thermal stability
The shape of the parts of the plastic injection mold is often complicated, and it is difficult to process after quenching. Therefore, it should be selected as far as possible with good thermal stability. When the mold is formed and processed after heat treatment, the linear expansion coefficient is small, the heat treatment deformation is small, and the dimensional change caused by temperature difference. The rate is small, the metallographic structure and the mold size are stable, and the processing can be reduced or no longer required to ensure the mold size accuracy and surface roughness requirements.