1. For custom molds for plastic with high hardness, high wear resistance and high toughness, carburized steel should be selected for manufacture, and carburizing, quenching and low-temperature tempering should be taken as final heat treatment.
2. For the requirement of the carburizing layer, the thickness of the carburizing layer is 0.8-1.5mm in general, 1.3-1.5mm in mould when pressing plastics with hard fillers, and 0.8-1.2mm in soft custom molds for plastic. The best carbon content of the carburized layer is 0.7%~1.0%. If carbonitriding is used, the wear resistance, corrosion resistance, oxidation resistance, and stickiness resistance will be better.
3. The carburizing temperature is generally 900-920 ℃. The small custom molds for plastic with complex cavity can be carburized at 840-860 C at medium temperature. The holding time of carburizing is 5 to 10 hours, which should be selected according to the requirement of the thickness of the carburizing layer. The optimum carburizing process is graded carburizing, i.e. the high-temperature stage (900-920 ℃) is to rapidly infiltrate the carbon into the surface of the parts; the medium temperature stage (820-840 ℃) is to increase the thickness of the carburizing layer, so that a uniform and reasonable gradient distribution of carbon concentration can be established in the carburizing layer, which is convenient for direct quenching.
4. The quenching process after carburizing is different according to the steel type. After carburizing, the following methods can be adopted: reheating quenching; direct quenching after grading carburizing (such as alloy carburizing steel); direct quenching after medium temperature carbonitriding (such as small precision die formed by cold extrusion of industrial pure iron or low carbon steel); air quenching after carburizing (such as large and medium dies made of high alloy carburizing steel).