What causes the precision defects of precision castings


Precision castings have high dimensional accuracy and s […]

Precision castings have high dimensional accuracy and small surface roughness. Suitable for castings with complex structure and shape. The dimensional accuracy of precision castings is affected by many factors such as casting structure, casting material, mold making, shell making, roasting, pouring, etc. Any setting or unreasonable operation of any link will change the shrinkage rate of the casting, resulting in the dimensional accuracy of the casting. The requirements are biased. The following are the factors that can cause defects in the dimensional accuracy of precision castings:

(1) Influence of casting structure:

a. The casting wall is thick and the shrinkage rate is large, and 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.

(2) Influence of casting material:

a. The higher the carbon content in the material, the smaller the linear shrinkage rate, and 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 size of the cavity, and LJ is the size of the casting. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, pouring temperature K4.

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

a. The influence of wax injection temperature, wax injection pressure, and pressure holding time on the investment size is the most obvious in the wax injection temperature, followed by the wax injection pressure. The pressure holding time has little effect on the final size of the investment after the investment is formed. .

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

c. When the investment mold is stored, there will be further shrinkage, and its shrinkage value is about 10% of the total shrinkage, but when stored for 12 hours, the investment mold size is basically stable.

d. The radial shrinkage rate of the wax mold is only 30-40% of the lengthwise shrinkage rate. The wax injection temperature has a far greater influence on the free shrinkage rate than the hindered shrinkage rate (the best wax injection temperature is 57-59℃, The higher the temperature, the greater the shrinkage).

(4) The influence of shell-making materials: zircon sand, zircon powder. Because of their small expansion coefficient, only 4.6×10-6/℃, they can be ignored.

(5) The effect of shell baking: Because the expansion coefficient of the shell is small, when the shell temperature is 1150℃, it is only 0.053%, so it can be ignored.

(6) The influence of casting temperature: the higher the casting temperature, the greater the shrinkage rate, and the lower the casting temperature, the smaller the shrinkage rate, so the casting temperature should be appropriate.




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