Generally speaking, the dimensional accuracy of precision castings is affected by many factors such as casting structure, casting material, mold making, shell making, roasting, pouring, etc. Unreasonable settings and operations in any of these links will cause changes in the shrinkage rate of the castings. As a result, the dimensional accuracy of the castings deviates from the requirements.

The following are factors that can cause dimensional accuracy defects in precision castings:

1. Influence of the casting structure:
a. The wall thickness of the casting has a large shrinkage rate; the casting wall is thin and the shrinkage rate is small. b. The free shrinkage rate is large and the hindered shrinkage rate is small.

Influence 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 of common materials is as follows: Casting shrinkage K=(LM-LJ)/LJ×100%, LM is the cavity size, and LJ is the casting size.
K is affected by the following factors: wax pattern K1, casting structure K2, alloy type K3, and pouring temperature K4.

The influence of molding on the linear shrinkage of castings:

a. The influence of wax injection temperature, wax injection pressure, and holding time on the size of the investment mold is most obvious with the wax injection temperature, followed by the wax injection pressure, and the holding time is guaranteed The impact on the final size of the investment mold after investment molding is very small.
b. The linear shrinkage rate of wax (mold) material is about 0.9-1.1%.
c. When the investment mold is stored, further shrinkage will occur, and the shrinkage value is approximately 10% of the total shrinkage. However, after 12 hours of storage, the size of the investment mold is basically stable.
d. The radial shrinkage rate of the wax pattern is only 30-40% of the lengthwise shrinkage rate. The impact of the wax injection temperature on the free shrinkage rate is far greater than the impact on the hindered shrinkage rate (the optimal wax injection temperature is 57-59℃, The higher the temperature, the greater the shrinkage).

Influence of shell-making materials:

zircon sand, zircon powder, Shangdian sand, and Shangdian powder are used. Because their expansion coefficient is small, only 4.6×10-6/℃, it can be ignored.

Influence of mold shell baking:

Since the expansion coefficient of the mold shell is small, when the mold shell temperature is 1150°C, it is only 0.053%, so it can be ignored.

Effect of casting temperature:

The higher the pouring temperature, the greater the shrinkage rate; the lower the pouring temperature, the smaller the shrinkage rate, so the pouring temperature should be appropriate.

Eternal Bliss Alloy Casting & Forging Co, Ltd.

Caesar

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