Factors That Affect The Accuracy Of Castings in Precision Castings

Precision casting is a special casting method compared to the traditional casting process. It can obtain relatively accurate shape and high casting accuracy, in addition, the product is precise, complex, close to the final shape of the part, can be used directly without processing or very little processing, is an advanced process in the casting industry that is similar to molding.

Precision casting process:

Wax pressing----- wax trimming----- wax inspection----- group tree----- shell making----- dewaxing (steam dewaxing), ----- mold shell roasting---- laboratory (spectral assay chemical composition) - pouring (pouring molten steel in the mold shell), ---- vibration shelling, shelling----- casting and pouring rod cutting and separation---- grinding, gate --- heat treatment---- shot blasting---- blank inspection---- machining----- surface treatment---- finished product inspection---- warehousing

Factors affecting the accuracy of castings:

Under normal circumstances, the dimensional accuracy of precision castings is affected by many factors such as casting structure, casting material, molding, shell making, roasting, pouring, etc., and any of the links are set and operated unreasonably will change the shrinkage rate of the casting, resulting in a deviation between the dimensional accuracy of the casting and the requirements. The following are the factors that can cause dimensional accuracy defects in precision castings:

(1) The influence of the casting structure: a. The wall thickness of the casting is large, and the wall of the casting is thin, and the shrinkage is small. b. The free shrinkage rate is large, and the hindered shrinkage rate is small.

(2) The 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, LJ is the casting size. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, pouring temperature K4.
(3) The influence of mold making on the shrinkage rate of the casting line: a. The influence of wax injection temperature, wax injection pressure and holding time on the size of the investment mold is the most obvious with the wax injection temperature, followed by the wax injection pressure, and the holding time has little influence on the final size of the investment mold after ensuring the investment molding. b. The linear shrinkage of wax (mold) material 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, but when it is stored for 12 hours, 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, and the influence of the wax injection temperature on the free shrinkage is much greater than the influence on the hindered shrinkage (the wax injection temperature is 57-59 °C, the higher the temperature, the greater the shrinkage).

(4) The influence of shell-making materials: the use of zircon sand, zircon powder, Shangdian sand, Shangdian powder, because of its small expansion coefficient, only 4.6×10-6/°C, so it can be ignored.

(5) The influence of shell roasting: due to the small expansion coefficient of the shell, when the shell temperature is 1150 °C, it is only 0.053%, so it can also 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