Factors Affecting Dimensional Accuracy in Investment Casting

2021-11-16

Factors Affecting Dimensional Accuracy in Investment casting

Under normal circumstances, the dimensional accuracy of precision castings is affected by many factors such as casting structure, casting material grade, construction of casting mold, shell making, roasting, melting, pouring, etc. The setting and operation of any production step involved will change the shrinkage rate of the casting, leading to the deviation of casting dimensional accuracy and requirements. The following are factors that can cause dimensional accuracy defects in precision castings:

(1) The influence of the structure of the casting:

a. Typically, when the wall thickness of the casting is thick, the shrinkage rate is bigger, and when the wall thickness of the casting is thin, the shrinkage rate is smaller.

b. The free shrinkage rate is large, and the hindered shrinkage rate is small.

(2) Influence of casting material grade:

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 mold K1, casting structure K2, alloy type K3, pouring temperature K4.

(3) The influence of mold making on the linear shrinkage rate of castings:

a. The wax injection temperature, wax injection pressure, and pressure holding time have the most obvious influence on the size of the investment mold. The wax injection temperature is the most obvious, followed by the wax injection pressure, and the pressure holding time is guaranteed. After the investment is formed, the effect on the final size of the investment is very small.

b. The linear shrinkage of wax (mold) material is about 0.9-1.1%.

c. When the investment mold is stored, it will further shrink, and the shrinkage value is about 10% of the total shrinkage, but after 12 hours of storage, the size of the investment mold is basically stable. d. The radial shrinkage rate of the wax mold is only 30-40% of the shrinkage rate in the length direction, and the influence of the 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℃, The higher the temperature, the greater the shrinkage).

(4) Influence of shell-making materials: The use of zircon sand, zircon powder, Shangdian sand, and Shangdian powder has a small expansion coefficient of only 4.6×10-6/℃, so it can be ignored.

(5) The influence of shell roasting: 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) Effect of pouring 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.