Influence factors of precision casting

In general, the dimensional accuracy of precision casting parts is affected by many factors, such as casting structure, casting material, mold making, shell making, roasting, pouring and so on. Unreasonable setting and operation of any one of them will change the shrinkage rate of the casting, resulting in deviation between the dimensional accuracy of the casting and the requirements.  The following are the factors that can cause dimensional accuracy defects of precision castings:  

(1) The effect of casting structure: a. Casting wall thickness, large shrinkage, casting wall thin, small shrinkage.  B. Large free shrinkage rate, small obstruction shrinkage rate.  

(2) The influence of casting material: a. The higher the carbon content in the material, the smaller the line shrinkage rate; the lower the carbon content, the greater the line shrinkage rate.  B. The casting shrinkage rate of common materials is as follows: casting shrinkage rate K=(LM-LJ)/LJ×100%, LM is cavity size, LJ is 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 of casting line: a. The influence of wax injection temperature, injection pressure and holding time on the investment mold size is the most obvious, followed by the injection pressure, holding time has little influence on the final investment mold size after ensuring the investment mold forming.  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 the shrinkage value is about 10% of the total shrinkage, but after 12 hours of storage, the investment mold size is basically stable.  D. The radial shrinkage rate of wax mold is only 30-40% of the shrinkage rate in the length direction. The influence of wax injection temperature on the free shrinkage rate is far greater than that on the blocked shrinkage rate (the best wax injection temperature is 57-59℃, and the higher the temperature, the greater the shrinkage).  

(4) Influence of shell making materials: zirconium sand, zirconium powder, upper sand and upper powder are used. Due to their small coefficient of expansion, only 4.6×10-6/℃, they can be ignored.  

(5) Influence of shell roasting: Due to the small expansion coefficient of shell, when shell temperature is 1150℃, it is only 0.053%, so it can be ignored.  

(6) The influence of casting temperature: the higher the casting temperature, the greater the shrinkage rate, the lower the casting temperature, the smaller the shrinkage rate, so the casting temperature should be appropriate.