What are the reasons for insufficient precision castings?
Reasons that can cause insufficient precision of precision casting specifications:
- (1) Influence of precision casting structure: a. Thickness of casting wall, large shrinkage, thin wall of casting, small shrinkage. b. The free shrinkage is large and the hindrance shrinkage is small.
- (2) Impact of precision casting material: a. The higher the carbon content in the material, the smaller the linear shrinkage, and the lower the carbon content, the greater the linear shrinkage. b. The casting shrinkage of common materials is as follows: Casting shrinkage K = (LM-LJ) / LJ100%, LM is the cavity specification, and LJ is the casting specification. K is affected by the following reasons: wax mold K1, casting structure K2, alloy type K3, and pouring temperature K4.
- (3) The effect of mold making on the shrinkage rate of precision castings: a. The effect of wax injection temperature, wax injection pressure, and holding time on the specifications of the investment mold is the most significant. Ensure that the final specifications of the investment mould have little effect after the investment mould is formed. b. The linear shrinkage of wax (mold) is about 0.9-1.1%. c. When the investment mold is stored, it will further shrink, and its shrinkage value is about 10% of the total shrinkage. However, after 12 hours of storage, the specifications of the investment mold are generally stable. d. The radial shrinkage of the wax mold is only 30-40% of the shrinkage in the length direction. The effect of the wax injection temperature on the free shrinkage far exceeds the effect of the hindered shrinkage. The higher the temperature, the greater the shrinkage).
- (4) The effect of shell material: Zircon sand, zircon powder, Shangdian sand, Shangdian powder are used, because their expansion coefficient is small, only 4.610-6 / ℃, so it can be ignored.
- (5) Effect of roasting of the shell: Because the expansion coefficient of the shell is small, when the shell temperature is 1150 ° C, it is only 0.053%, so it can be ignored.
- (6) Influence of casting temperature: the higher the pouring temperature, the larger the shrinkage, the lower the pouring temperature, and the smaller the shrinkage, so the pouring temperature should be appropriate.