Causes of insufficient precision of precision castings

Update:10 Feb 2020
Summary:

1. Influence of precision casting structure: casting wa […]

1. Influence of precision casting structure: casting wall thickness, large shrinkage, thin casting wall, small shrinkage. The free shrinkage rate is large, and the hindrance shrinkage rate is small. Impact of precision casting material: The higher the carbon content in the material, the smaller the linear shrinkage, and the lower the carbon content, the greater the linear shrinkage. 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.

2. The influence of mold making on the shrinkage rate of precision castings: The effects of wax injection temperature, wax injection pressure, and holding time on the specifications of the investment mold are the most significant. After molding, it has little effect on the final specifications of the investment mold. The linear shrinkage of wax (mold) is about 0.9-1.1%. 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. 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 is far more than the impact on the hindered shrinkage. The best wax injection temperature is 57-59 ° C. The higher the temperature, the larger the shrinkage.

3. 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. The influence of shell firing: Due to the small expansion coefficient of the shell, when the shell temperature is 1150 ° C, it is only 0.053%, so it can be ignored. Influence of casting temperature: The higher the casting temperature of precision castings, the larger the shrinkage rate, the lower the casting temperature, and the smaller the shrinkage rate, so the casting temperature should be appropriate.