Influencing factors and precautions of precision casting dimensional accuracy defects

Under normal circumstances, the dimensional accuracy of precision casting is affected by many factors such as the structure of the casting, the material of the casting, the molding, the shell, the roasting, and the casting. Any setting or unreasonable operation will change the shrinkage of the casting. The dimensional accuracy of the casting is deviated from the requirements. The following are factors that can cause defects in the precision of precision castings: (1) Influence of casting structure: a. The wall thickness of the casting is large, the shrinkage rate is large, the casting wall is thin, and the shrinkage rate is small. b. The free shrinkage rate is large, which hinders the shrinkage rate. (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 larger 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, casting temperature K4. (3) The influence of mold making on the shrinkage rate of castings: a. The effect of wax temperature, wax pressure and dwell time on the size of the investment is obvious. The temperature of the wax is obvious, followed by the pressure of the wax, and the pressure holding time is guaranteed. After the investment molding, the influence on the final size of the investment mold is small. b. The linear shrinkage of the wax (molding) material is about 0.9-1.1%. c. When the investment mold is stored, further shrinkage will occur, and the shrinkage value is about 10% of the total shrinkage amount, but after 12 hours of storage, the investment size is substantially stable. d. The radial shrinkage of the wax mold is only 30-40% of the shrinkage in the longitudinal direction, and the effect of the wax temperature on the free shrinkage is far greater than the effect on the resistive shrinkage (the preferred wax temperature is 57-59 ° C, The higher the temperature, the greater the shrinkage). (4) Influence of shell material: Zircon sand, zircon powder, Shangdian sand, Shangdian powder, because of its small expansion coefficient, is only 4.6×10-6/°C, so it can be ignored. (5) Effect of shell roasting: Since the coefficient of expansion of the shell is small, when the shell temperature is 1150 ° C, it is only 0.053%, so it can be neglected. (6) Effect of casting temperature: The higher the pouring temperature, the larger the shrinkage rate, the lower the casting temperature and the smaller the shrinkage rate, so the pouring temperature should be appropriate.

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