In general, the precision of precision casting specifications is affected by the casting structure, casting materials, mould making, shell making, firing, pouring and other factors, in any one of the stages set, the actual operation is not scientific will make the shrinkage rate of the casting caused by the transformation, resulting in the precision of the casting specifications and the provisions of the error. The following are the elements that can lead to high precision casting specifications precision problems.
(1) the impact of casting structure: a. Casting wall thickness, shrinkage rate is large, casting wall thin, shrinkage rate is small. b. Random shrinkage rate is large, blocking shrinkage rate is small.
(2) The influence of casting material: a. The higher the carbon content in the raw material, the smaller the line shrinkage, the lower the carbon content, the larger the line shrinkage. b. The casting shrinkage of universal materials is shown below: Casting shrinkage K = (LM-LJ)/LJ x 1 00%, LM is the concave mould specification, LJ is the casting size. k is influenced by the following reasons: wax mould K1, casting construction K2, aluminium alloy type K3, the pouring temperature K4.
(3) The influence of mould making on the line shrinkage rate of the casting: a. The influence of injection wax temperature, injection wax working pressure and test press length on the specification of the molten mould is most significant for injection wax temperature, followed by injection wax working pressure, and test press length has little influence on the final specification of the molten mould after ensuring that the molten mould is formed. b. The line shrinkage rate of the wax (mould) material is approximately 0.9-1.1%. c. When the molten mould is stored and placed, it will further cause a closing, the The shrinkage value is about 10% of the total shrinkage, but when stored for 12 hours, the molten mould specification is basically smooth. d. The tangential shrinkage of the wax mould is only 30-40% of the lengthwise shrinkage, and the influence of the injection wax temperature on the random shrinkage rate far exceeds the influence on the shrinkage rate of the encounter (the best injection wax temperature is 57-59°C, the higher the temperature the greater the shrinkage).
(4) The influence of raw materials for shell making: zirconia sand, zirconia powder, Shangdian sand and Shangdian powder are chosen because their expansion coefficient is small, only 4.6×10-6/°C, and thus can be ignored.