How to prevent the phenomenon of cracks in Precision Casting parts?
In fact, the Precision Casting parts in the latter part of the basic process of the phenomenon of cracks can be said to be a common phenomenon, but the emergence of cracks will not only affect the use of Precision Casting parts, and will affect the cost of Precision Casting and quality, , Then in our lives how to prevent the phenomenon of cracks in Precision Casting it?
In the casting of Precision Casting when the attention to improve the sand and sand core of the concession. In the production process to strictly control the charge and molten steel in the P, S content. Precision Casting parts of the wall thickness as much as possible to avoid a sudden change in wall thickness, if the conditions permit, then you can set the time to strengthen the ribs, cross-section of the two sections as far as possible using rounded corners to minimize stress concentration. Pay attention to adjust the cooling parts of the Precision Casting parts to avoid the local overheating of steel parts, in the thick section or hot section placed cold iron, the appropriate dispersion of the runner, which can ensure that the Precision Casting parts of the temperature trend of heat Evenly. Casting after pouring attention to open type can not be too early, it is best to use water blast sand casting process, in the process of attention to master the time and temperature.
In general, the precision precision of cast parts is affected by the casting structure, casting material, mold making, shelling, roasting, pouring and other factors, any one of the links set, the operation will cause the casting shrinkage will change, Resulting in casting dimensional accuracy and requirements are biased. The following are the factors that can cause defects in the accuracy of Precision Castings:
(1) the impact of casting structure: a. Casting wall thickness, shrinkage, casting wall thin, small shrinkage. B. Free shrinkage rate, hinder the shrinkage rate is small.
(2) the impact of Precision Casting material: a. The higher the carbon content of the material, the smaller the line shrinkage, the lower the carbon content, the greater the line shrinkage. B. Common material casting shrinkage rate is as follows: casting shrinkage K = (LM-LJ) / LJ × 100%, LM for the cavity size, LJ for the casting size. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, casting temperature K4.
(3) the impact of molding on the shrinkage of the casting line: a. Wax temperature, wax pressure, holding time on the size of the melt to the temperature of the most obvious wax, followed by wax pressure, Precision Casting holding time in the guarantee The effect of melt-molding on the final size of the melt is very small. B. The line shrinkage of the wax (mold) material is about 0.9-1.1%. C. When the mold is stored, shrinkage will be further generated, the shrinkage value is about 10% of the total shrinkage, but after 12 hours of storage, the die size is basically stable. D. The radial shrinkage of wax mold is only 30-40% of the shrinkage rate in the longitudinal direction. The effect of wax wax temperature on the free shrinkage is much greater than that on the shrinkage shrinkage (the best wax temperature is 57-59 ℃, The higher the temperature the greater the contraction).
(4) the impact of shell material: the use of zircon sand, zircon powder, on the shop sand, shop powder, because of its small expansion coefficient, only 4.6 × 10-6 / ℃, so negligible.
(5) the effect of shell Precision Casting: the expansion coefficient of the shell is small, when the shell temperature is 1150 ℃, only 0.053%, so it can be ignored.
(6) the impact of Precision Casting temperature: the higher the pouring temperature, the greater the shrinkage, pouring temperature is low, the smaller the shrinkage, so the pouring temperature should be appropriate.