The causes of insufficient precision of precision castings are as follows:

(1) The influence of the structure of precision casting: a. the wall thickness of the casting, the shrinkage rate is large, the wall thickness of the casting is thin, and the shrinkage rate is small. b. The free shrinkage rate is large, and the hindered shrinkage rate is small.

(2) The influence of the material of precision casting: a. the higher the carbon content in the material, the smaller the linear shrinkage rate; the lower the carbon content, the greater the linear shrinkage rate. b. The casting shrinkage of common materials is as follows: casting shrinkage k = (lm-lj) / lj100%, LM is cavity specification, LJ is casting specification. K is affected by the following reasons: wax pattern K1, casting structure K2, alloy type K3, pouring temperature K4.

(3) The effect of mold making on linear shrinkage of precision casting: a. the wax injection temperature, wax injection pressure and holding time have the most significant effect on the investment mold specification, and the wax injection pressure is the most significant one. The pressure holding time has little effect on the final investment specification after the investment molding. b. The linear shrinkage of wax is about 0.9-1.1%. c. The shrinkage value is about 10% of the total shrinkage. However, after 12 hours of storage, the mold specifications are stable. d. The radial shrinkage of wax pattern is only 30-40% of that in the length direction, and the effect of wax injection temperature on free shrinkage is far more than that on hindered shrinkage (the best wax shooting temperature is 57-59 ℃), and the higher the temperature, the greater the shrinkage.

(4) The influence of shell material: using zircon sand, zircon powder, Shangdian sand and Shangdian powder can be ignored because of its small expansion coefficient, which is only 4.610-6 / ℃.

(5) The effect of shell roasting: because the expansion coefficient of shell is small, when the shell temperature is 1150 ℃, it is only 0.053%, so it can be ignored.

(6) The effect of casting temperature: the higher the pouring temperature is, the greater the shrinkage rate is, and the lower the pouring temperature is, the smaller the shrinkage rate is. Therefore, the pouring temperature should be appropriate.