The design and manufacture of molds is an important issue in the selection of mold materials, and it directly affects the service life of molds, mold manufacturing processes, mold processing costs, and the quality of plastic parts.
In order to extend the service life of molds, a lot of research and engineering practice has been carried out on the surface enhancement treatment of mold forming parts, and good results have been achieved. The working conditions of plastic standard mould parts are affected by both temperature and pressure, and they generally work at 150-200℃. The basic performance requirements of steel for plastic molding molds are summarized as follows:
After heat treatment, the mold should have sufficient surface hardness. The hardness of plastic molds is usually below 50-60HRC to ensure sufficient rigidity. Molds must withstand high compressive stress and frictional stress, and due to the filling and flow of plastics, this requires stable dimensional accuracy and shape accuracy of molds to ensure sufficient service life of the molds. Increasing the hardness of molds is beneficial to improving their wear resistance.
Steel grades that are easy to cut and can obtain high-precision parts after processing should be selected. Therefore, medium carbon steel and medium carbon alloy steel are most commonly used. This is particularly important for large molds. For parts that require electric discharge machining, the burn hardening layer of the steel should be thin.
High-quality plastic products require a small cavity surface roughness. Therefore, the working surface of injection standard mould parts needs to be polished into a mirror finish. If the surface is too hard, polishing is difficult. Therefore, the selected steel should have a uniform and dense microstructure, low impurities, no fiber directionality, and no defects such as pitting or orange peel during polishing.
Plastic injection molds should select materials with good thermal stability as much as possible, because the parts are often complex in shape and difficult to machine after quenching. In order to ensure the dimensional accuracy and surface roughness requirements of the molds, the deformation after heat treatment should be small, the size change rate caused by temperature difference should be small, the molding piece linear expansion coefficient should be small, the metallographic structure and mold size should be stable, which can reduce or stop processing.
The cavity of injection mould base parts is not only affected by the high-pressure plastic melt flushing, but also by the temperature stress of cold and hot alternation. Generally, high-carbon alloy steel can obtain high hardness after heat treatment, but it has poor toughness and is prone to surface cracking, and is not suitable for use. The selected steel should reduce the number of polishing and repair cycles for injection molds and maintain the dimensional accuracy of the cavity for a long time to achieve the service life of mass production.
For some plastic varieties, such as PVC and flame-retardant plastics, corrosion-resistant steel should be considered.
In addition, when selecting materials for injection mould base parts, it is necessary to prevent scratches and adhesion. If there is relative movement between two surfaces, materials with the same structure should be avoided as much as possible.