Causes of insufficient precision of precision casting parts:
1. The influence of the structure: the thickness of the casting wall, the shrinkage rate is large, the casting wall is thin, the shrinkage rate is small; the free shrinkage rate is large, the hindered shrinkage rate is small. Effect of material: the higher the carbon content in the material, the smaller the linear shrinkage, the lower the carbon content, and the larger the linear shrinkage; casting shrinkage of common materials: casting shrinkage k = (lm-lj) / lj100%, LM is the cavity specification, LJ is the casting specification. K is affected by the following reasons: wax mold K1, casting structure K2, alloy type K3, pouring temperature K4.
2. The influence of mold making on the linear shrinkage of precision casting parts: the wax injection temperature, wax injection pressure and holding time have a significant influence on the investment mold specification, in addition to the wax injection pressure, the holding time has little influence on the final specification of investment mold after the investment mold is formed; the linear shrinkage of wax (mold) material is about 0.9-1.1%; when the investment mold is stored, further shrinkage will occur, and its shrinkage value is about the total 10% of the shrinkage, but when stored for 12 hours, the mold specification is generally stable; the radial shrinkage of wax mold is only 30-40% of the shrinkage in the length direction, and the effect of wax injection temperature on the free shrinkage is far more than the effect on the blocked shrinkage. The better wax injection temperature is 57-59 ℃, the higher the temperature, the greater the shrinkage.
3. The influence of shell making materials: Zircon sand, zircon powder, Shangdian sand and Shangdian powder are used, because their expansion coefficient is only 4.610-6 / ℃, it can be ignored; the influence of shell baking: because the expansion coefficient of shell is small, when the shell temperature is 1150 ℃, it is only 0.053%, so it can also be ignored; the influence of casting temperature of precision casting parts: the higher the pouring temperature, the shrinkage The higher the ratio, the lower the pouring temperature and the smaller the shrinkage, so the pouring temperature should be appropriate.