In general, the dimensional accuracy of precision castings is influenced by various factors such as casting structure, casting material, mold making, shell making, roasting, pouring, etc. Any unreasonable setting or operation of any link can cause changes in the shrinkage rate of the castings, leading to deviations from the requirements for dimensional accuracy of the castings. The following are the factors that can cause dimensional accuracy defects in precision castings:
(1) The impact of casting structure: a The wall thickness of the casting has a high shrinkage rate, while the wall thickness of the casting is thin with a low shrinkage rate. b. The free shrinkage rate is high, while the hindrance shrinkage rate is low.
(2) The impact of casting material: 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 rate of common materials is as follows: casting shrinkage rate K=(LM LJ)/LJ × 100%, LM is the cavity size, and LJ is the casting size. K is influenced by the following factors: 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: a. The influence of wax injection temperature, wax injection pressure, and holding time on the investment mold size is obvious in wax injection temperature, followed by wax injection pressure, and holding time has little effect on the investment mold size after ensuring investment molding. b. The linear shrinkage rate of wax (mold) material is about 0.9-1.1%. c. When the 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 mold is basically stable. d. The radial shrinkage rate of wax molds is only 30-40% of the shrinkage rate in the length direction. The influence of wax injection temperature on the free shrinkage rate is much greater than that on the hindered shrinkage rate (the optimal wax injection temperature is 57-59 ℃, and the higher the temperature, the greater the shrinkage).
(4) The impact of shell making materials: Using zircon sand, zircon powder, Shangdian sand, and Shangdian powder, due to their small expansion coefficient, it is only 4.6 × 10-6/℃, therefore it can be ignored.
(5) The effect of shell baking: Due to the small expansion coefficient of the shell, when the shell temperature is 1150 ℃, it is only 0.053%, so it can also be ignored.
(6) The impact of pouring temperature: The higher the pouring temperature, the greater the shrinkage rate. The lower the pouring temperature, the smaller the shrinkage rate. Therefore, the pouring temperature should be appropriate.