Precision factors for precision casting:
In general, the dimensional accuracy of precision castings is affected by many factors such as the structure of the casting, the material of the casting, mold making, shell making, firing, pouring, etc., any of these links will be changed due to the unreasonable setting and operation. As a result, the casting dimensional accuracy deviates from the requirements. The following factors can cause defects in dimensional accuracy of precision castings:
(1) Influence of casting structure: a. Thickness of casting wall, large shrinkage, thin wall of casting, small shrinkage. b. The free shrinkage is large and the hindrance shrinkage is small.
(2) Influence of casting material: a. The higher the carbon content in the material, the smaller the linear shrinkage, and the lower the carbon content, the greater the linear shrinkage. 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 size of the casting. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, and pouring temperature K4.
(3) The influence of mold making on the linear shrinkage of castings: a. The impact of wax injection temperature, wax injection pressure, and holding time on the size of the investment mold is the most obvious, and the second is injection wax pressure. The holding time is guaranteed. The effect of the investment molding on the final size of the investment is small. b. The linear shrinkage of wax (mold) is about 0.9-1.1%. c. 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 size of the investment mold is basically stable. d. 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 much greater than the impact on the hindered shrinkage (the optimal wax injection temperature is 57-59 ℃ The higher the temperature, the greater the shrinkage).
(4) Impact of shell making materials: Zircon sand, zircon powder, Shangdian sand, Shangdian powder are used, because their expansion coefficient is small, only 4.6 × 10-6 / ℃, so it can be ignored.
(5) Effect of roasting of the shell: Because the expansion coefficient of the shell is small, when the shell temperature is 1150 ° C, it is only 0.053%, so it can be ignored.
(6) Influence of casting temperature: The higher the pouring temperature, the larger the shrinkage, the lower the pouring temperature, and the smaller the shrinkage, so the pouring temperature should be appropriate.
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