According to the length of mould life, plastic moulds can be classified into 5 classes.
Class 1 mould runs 1 million or more shots;
Class 2 mould runs 0.5 – 1 million shots;
Class 3 mould runs 0.3 – 0.5 million shots;
Class 4 mould runs 0.1 – 0.3 million shots;
Class 5 mould runs less than 0.1 million shots.

Class 1 and class 2 moulds require the steel materials that can be harden up, with hardness around HRC50, or the moulds will wear easily, leading to out-of-tolerance injection molded products. As a result, the selected steel materials need to possess good heat treatment properties and machinability despite the high hardness. Of course, there are some other considerations, too.
Usually, the steel choices include Swedish 8407 and S136; American 420 and H13; European 2316, 2344 and 2083 and Japanese SKD61. For strongly corrosive plastics, S136, 2316 and 420 steels are normally chosen. In addition to S136, 2316 and 420 steels, low corrosion plastics can also choose SKD61, NAK80, PAK90 and 718M steels. Product appearance also has a great influence on the mould materials. S136, 2316, 718S, NAK80, PAK90 and 420 steel materials are suitable for transparent and mirror polishing products, while highly transparent products should primarily select the S136, and secondarily the 420 steel materials.
Class 3 moulds mostly use pre-hardened steels, such as S136H, 2316H, 718H and 083H, with a hardness ranging from HB270 to 340.
Class 4 and class 5 moulds usually use the P20, 718, 738, 618, 2311 and 2711 steel materials. For moulds with extremely low requirements, S50C and 45# steels may be used, i.e. creating a cavity directly in the mold base.