Mold heat treatment is an important process to ensure the performance of the mold. It has a direct impact on the manufacturing accuracy of the mold, the strength of the plastic mold, the working life of the mold, the manufacturing cost of the mold, etc. Mold heat treatment is to give full play to the potential of the mold material and improve the performance of the mold. The performance of the mold must meet the following requirements: high strength (including high temperature strength, resistance to cold and hot fatigue), high hardness (wear resistance) and high toughness, and also requires good machinability (including good polishing), weldability and corrosion resistance, etc. The deformation of plastic molds after heat treatment is generally considered to be caused by cooling, which is incorrect. The correctness of the processing technology often has a greater impact on the deformation of the mold, especially for complex molds. The comparison of some mold heating processes shows that the faster the heating speed, the greater the deformation. (1) Causes of deformation Any metal will expand when heated. When steel is heated, the temperature of each part in the same mold is uneven (i.e., uneven heating), which will inevitably cause the expansion of each part in the mold to be inconsistent, thereby forming internal stress due to uneven heating. At temperatures below the phase transition point of steel, uneven heating mainly produces thermal stress. When the temperature exceeds the phase transition point, uneven heating will also produce asymmetric structural transformation, which will also produce structural stress. Therefore, the faster the heating speed, the greater the temperature difference between the mold surface and the core, the greater the stress, and the greater the deformation of the mold after heat treatment.
(2) Preventive measures
When heating complex molds below the phase transition point, they should be heated slowly. Generally speaking, the deformation of the mold in vacuum heat treatment is much smaller than that in salt bath furnace heating and quenching. ? Use preheating. For low-alloy steel molds, one preheating (550-620°C) can be used; for high-alloy steel molds, two preheating (550-620°C and 800-850°C) should be used.
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