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Why does the mold surface produce periodic temperature changes?


The mold should be preheated to a certain temperature before production, otherwise, chilling will occur when the high temperature molten metal is filled, which will cause the temperature gradient of the inner and outer layers of the mold to increase, forming thermal stress, and causing the mold surface to crack or even crack. In the production process, the mold temperature continues to rise. When the mold temperature is overheated, it is easy to produce mold sticking, and the moving parts fail to cause damage to the mold surface. A cooling temperature control system should be installed to keep the mold working temperature within a certain range. The molten metal is filled with high pressure and high speed, which will inevitably produce fierce impact and erosion on the mold, thus generating mechanical stress and thermal stress. During the impact process of automobile mold processing, molten metal, impurities, and gas will also produce complex chemical effects with the mold surface, and accelerate the generation of corrosion and cracks. When molten metal is surrounded by gas, it will expand first in the low-pressure area in the cavity. When the gas pressure increases, inward blasting occurs, pulling out the metal particles on the surface of the cavity and causing damage and cracks due to cavitation. In the process of core pulling and mold opening, mechanical stress will also be generated when some components are deformed. In the production process of each die-casting part of the automobile mold, due to the heat exchange between the mold and the molten metal, the surface of the mold produces periodic temperature changes, causing periodic thermal expansion and contraction, and generating periodic thermal stress. For example, the surface of the mold is subjected to compressive stress due to the heating during pouring, and after the mold is opened to eject the casting, the surface of the mold is subjected to tensile stress due to the cooling. When this alternating stress is repeatedly cycled, the stress accumulated inside the mold becomes larger and larger. When the stress exceeds the fatigue limit of the material, cracks occur on the surface of the mold.
Therefore, we must take active and effective measures to reduce the damage of the mold. The wall thickness of the castings is as uniform as possible to avoid hot spots. Automobile mold processing can reduce the thermal fatigue caused by the local heat concentration of the mold. The corners of the castings should have appropriate casting fillets to avoid stress caused by sharp corners on the mold.

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