Generally speaking, the dimensional accuracy of Dongguan precision castings is affected by many factors such as casting structure, casting materials, mold manufacturing, shell making, baking and pouring. The unreasonable setting and operation of any link will change the shrinkage rate of the casting and cause the deviation between the dimensional accuracy of the casting and the requirements. The following factors can cause dimensional accuracy defects of precision castings:
(1) The influence of the pouring structure: a. The pouring wall is thick and the shrinkage is large, while the pouring wall is thin and the shrinkage is small. b. The free shrinkage rate is large, and the hindered shrinkage rate is small.
(2) Influence of casting materials: 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 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 model K1, casting structure K2, alloy type K3 and pouring temperature K4.
(3) The influence of molding on the line shrinkage of castings: a. The wax injection temperature, wax injection pressure and holding time have the most obvious influence on the investment size, followed by the wax injection pressure to ensure that the holding time after the investment is formed has the effect on the final size of the investment. Not big. b. The linear shrinkage rate of wax (mold) material is about 0.9-1.1%. c. The investment mold will shrink further when stored, the shrinkage value is about 10% of the total shrinkage, but the investment mold size is basically stable after 12 hours of storage. d. The radial shrinkage of the wax mold is only 30-40% of the longitudinal shrinkage. The influence of wax injection temperature on free shrinkage is much greater than that on hindered shrinkage (the best wax injection temperature is 57-59°C, the higher the temperature, the greater the shrinkage ).
(4) The influence of shell-making materials: zircon sand, zircon powder, Shangdian sand, and Shangdian powder are used, and the expansion coefficient is only 4.6×10-6/℃, which can be ignored.
(5) The effect of grilled shell: Due to the small expansion coefficient of shell, when the shell temperature is 1150℃, it is only 0.053%, which can be ignored.
(6) The influence of pouring temperature: the higher the pouring temperature, the greater the shrinkage, and the lower the pouring temperature, the smaller the shrinkage, so the pouring temperature should be appropriate.