The following are several factors that can cause dimensional accuracy defects in precision casting:
1. The influence of casting structure. Casting wall thickness, high shrinkage rate, casting wall thickness, low shrinkage rate; The free shrinkage rate is high, while the obstructed shrinkage rate is low.
2. The influence of casting material. The higher the carbon content in the material, the smaller the linear shrinkage rate, and the lower the carbon content, the larger the linear shrinkage rate; The casting shrinkage rate of common materials is K=(LM-LJ)/LJ × 100%, where LM is the cavity size, LJ is the casting size, and K is influenced by several factors such as wax mold K1, casting structure K2, alloy type K3, and pouring temperature K4.
3. The influence of mold making on the linear shrinkage rate of castings. The influence of wax injection temperature, wax injection pressure, and holding time on the size of the investment mold is most significant in wax injection temperature, followed by wax injection pressure. Holding time has little effect on the final size of the investment mold after ensuring its formation; The linear shrinkage rate of wax (mold) material is about 0.9-1.1%; When the investment mold is stored, further shrinkage will occur, with a shrinkage value of about 10% of the total shrinkage. However, after 12 hours of storage, the size of the investment mold is basically stable; The radial shrinkage rate of wax molds is only 30-40% of the longitudinal shrinkage rate, and the effect of wax injection temperature on free shrinkage rate is much greater than that on hindered shrinkage rate (wax injection temperature is 57-59 ℃, the higher the temperature, the greater the shrinkage).
4. The influence of shell materials, such as zircon sand, zircon powder, Shangdian sand, and Shangdian powder, can be ignored due to their small expansion coefficient of only 4.6 × 10-6/℃.
5. The influence of shell roasting, due to the small expansion coefficient of the shell, is only 0.053% when the shell temperature is 1150 ℃, so it can also be ignored.
6. The influence of casting temperature is that the higher the casting temperature, the greater the shrinkage rate, and the lower the casting temperature, the smaller the shrinkage rate. Therefore, the casting temperature should be appropriate.