HT200 is generally selected for the material of the appearance, and its design mainly considers two aspects of machining allowance and shrinkage rate. The iron mold sand coating process can obtain castings with high dimensional accuracy. However, due to the rapid cooling of the molten iron in the mold by the iron mold, the floating time of small inclusions in the molten iron is shortened, and these small inclusions are easy to deposit on the castings. Under the skin; at the same time, considering the deformation of the casting, the machining allowance of the casting cannot be too small. Take a four-cylinder crankshaft of about 40 kg as an example.
Generally, the radius of the spindle and connecting rod is 4-5 mm; the axial end face is 3 mm. Due to the good rigidity of the iron mold and the high strength of the sand coating, the free shrinkage of the casting in the iron mold and the expansion of graphite precipitation are prevented.
Therefore, the shrinkage rate of the iron mold sand coating process casting is relatively stable, generally 0.6% to 0.7%. Of course, the actual shrinkage of the casting is also related to the opening time of the casting. If the opening time is early, the casting will get rid of the obstacles of the mold and shrink freely under high temperature conditions.
At this time, the shrinkage of the casting will be larger; on the contrary, if the opening time is too long At the end of the day, the shrinkage of the casting in the iron mold has been blocked, and the shrinkage of the casting is smaller at this time. Therefore, in actual production, in order to ensure the dimensional accuracy of the casting, it is best to open the box when the temperature of each part of the casting is close.