Precision casting---factors affecting dimensional accuracy of precision casting parts
Factors affecting dimensional accuracy of precision casting parts:
Under normal circumstances, dimensional accuracy of precision casting parts is affected by multiple factors including casting structure, casting texture, molding, shell, roasting and pouring. Unreasonable setup and operation of any link would bring to changes of shrinking percentage of casting and hence lead to deviation of dimensional precision from what is required. Factors probably giving rise to defects of dimensional accuracy are listed below:
(1) Influence of casting structure: a. casting section thickness leads to high shrinking percentage; thin casting section leads to low shrinking percentage; b. High free shrinking percentage leads to low hindrance shrinking percentage.
(2) Influence of casting texture: a. High content of carbon in materials lead to low linear shrinking percentage; low content of carbon in materials lead to high linear shrinking percentage; b. Casting shrinkage ratios of commonly-seen textures are: casting shrinkage ratio K= K=(LM-LJ)/LJ×100%. LM is size of mold cavity, LJ is casting size. K is affected by the following factors: wax pattern K1, casting structure K2, variety of alloy K3 and pouring temperature K4.
(3) Influence of moudling on casting linear shrinkage ratio: a. Among wax pattern making temperature, was pattern making pressure and holdup time, wax pattern making temperature exerts the most evident influence on size of fire mold followed by wax pattern making pressure. Holdup time exerts little affect on the final size of fired mold after ensuring fired mold molding. B. Linear shrinkage rate of wax (reclaim wax) is around 0.0-1.1%. c. When fired mold is stored, shrinkage is further intensified and the shrinkage is around 10% of total shrinkage amount, but after it is stored for 12 hours, size of the fired mold is basically stable. d. Radial shrinkage ratio of fired mold is 30-40% and influence on free shrinkage ratio by wax pattern making temperature is far higher than its influence on hindrance shrinkage rate (best wax pattern making temperature is 57-59℃ and higher temperature leads to bigger shrinkage).
(4) Influence of shell materials: it adopts zircon sand, n. zircon powder, precision casting sand and precision casting powder. It can be neglected as its swelling coefficient is low at 4.6×10-6/℃.
(5) Influence of shell roasting: as the swelling coefficient of shell is low and it is only 0.053% when the shell temperature reaches 1150℃, so it can be neglected.
(6) Influence of casting temperature: higher casting temperature leads to higher shrinkage rate; low casting temperature leads to low shrinkage rate, so casting temperature should be appropriate.