① Good machining performance. It is necessary to choose a steel grade that is easy to cut and can obtain high-precision parts after processing. For this reason, medium carbon steel and medium carbon alloy steel are the most commonly used, which is especially important for large molds. For parts that need EDM, the burn hardened layer of this steel grade is also required to be thinner.
②Good abrasion resistance and fatigue resistance. The mold cavity of the injection molding process is not only washed by the high-pressure plastic melt, but also subjected to the temperature stress of the alternating cold and heat. General high-carbon alloy steel can be heat treated to obtain high hardness, but its toughness is poor and surface cracks are easy to form, so it is not suitable for use. The selected steel grade should enable the injection mold to reduce the number of polishing and repair molds, maintain the dimensional accuracy of the cavity for a long time, and reach the service life of mass production.
③Excellent polishing performance. The working surface of the molded parts of the injection molding process usually needs to be polished to a mirror surface, Ra￡0.05mm, and the hardness of the steel is 35～40HRC. Excessive hard surface will make polishing difficult. The microstructure of the steel should be evenly dense, with less impurities and no pin points.
④It has corrosion resistance function. For some plastics, such as polyvinyl chloride and flame-retardant plastics, steel grades with corrosion resistance must be considered.
The selection of materials for forming parts requires good machinability, good wear resistance and fatigue resistance. The polishing function is excellent, and it must have the requirements of corrosion resistance.