The reason of porosity shrinkage of casting parts
There is only one reason that is after metal melt filling, there must be a phase-change contraction when it changes from liquid phase to solid phase. Due to the solidification characteristics of die casting pieces (cooling) is from outside to inside. When the wall thickness of the casting is large, the problem of porosity shrinkage is inevitable.
Therefore, in the case of die casting pieces, especially the large all thickness, there must be have the problem of porosity shrinkage and cannot be solved.
The only way to solve the defect of porosity shrinkage of casting parts
The problem of porosity shrinkage of casting parts cannot be solved completely from the die casting process itself. To solve this problem completely, we can only go beyond the process, or seek solutions from out of the system.
Then what is this method?
According to the process principle, the solution to solve the defect of porosity shrinkage of casting can only be carried out according to the process idea of supplementary shrinkage. The phase change shrinkage of casting solidification is a natural physical phenomenon. We can not reverse the law of this natural phenomenon, but only follow its law to solve this problem.
Two ways of feeding
There are two ways of casting feeding, one is the natural feeding, the other is forced feeding.
In order to realize the natural feeding, we need to have the technological measures of "sequential solidification" in our casting process system. Many people intuitively think that porosity shrinkage can be solved in low-pressure casting method, but that’s not the case. Applying the low pressure casting process is not equivalent to solving the porosity shrinkage. If the low-pressure casting process system does not have the technical measures for feeding, then the blank produced by this low-pressure casting method may also be 100 percent defective in porosity shrinkage.
As the characteristics of the die casting process, it is difficult and complicated to set up the natural process measures of "sequential solidification". The most fundamental reason may be that the process measures of "sequential solidification" always require long casting solidification time , which is somewhat contradictory with the die casting process itself.
The biggest characteristic of forced feeding is its short solidification time, usually only a quarter or less than the time of “sequential solidification”, therefore, on the basis of die casting process system, the addition of compulsory of feeding process measure is in line with the characteristic of die casting process, which can solve the problem of porosity shrinkage.
It is an effective way to solve the defect of porosity shrinkage of casting by using the technology of "first die casting for filling mould, then die forging for feeding".