Only when the displacement of mold wall caused by molding sand is enough to increase the section size of critical section (section easy to form coarse grains), mold is a factor to cause the coarse grains defects. Because the mold wall at the thick section may move the most, this kind of defect is possible to occur. At this time, the coarse defect of the product grain is related to sand expansion.
The use of oil sand cores that are not completely baked or air hardened should be avoided in production, because such cores may produce exothermic reactions, which may cause excessive heat collection. This occurs either in large castings or in thick large cores with exothermic adhesives. In a sense, the core acts as an efficient insulator and slows down the cooling rate of molten metal to a dangerous level.
(1) In the case of thick casting section, the cooling rate of casting is related to the rate of heat release through molding sand. Sufficient exhaust gas will help the water gas to be discharged quickly, thus producing a cooling effect.
(2) The absence of chill nails or irons is usually due to carelessness.