Features: in the thick section of the casting, hot spot or axis and other places of the final solidification, rough holes are formed on the surface, and more or less with dendrite crystal. The large and concentrated holes are called shrinkage cavities, and the small and scattered holes are called shrinkage cavities. Shrinkage cavity and shrinkage porosity are mainly caused by the liquid shrinkage and solidification shrinkage of liquid metal during cooling and solidification, which are much larger than that of solid metal, and no supplement of liquid metal can be obtained at the final solidification place of casting.

Precautions:

(1) According to the thickness of the casting, the proper chemical composition should be selected. The residual amount of magnesium should be strictly controlled to reduce the pouring temperature as much as possible,

(2) The riser and gating system should be designed reasonably to make the casting fully fed. If necessary, chill or inner chill should be set at the thick section of the casting.

(3) In general, the amount of primary inoculant is 0.4% – 0.6%, and the amount of instantaneous flow inoculation is 0.05% – 0.15%.

(4) To prevent the oxidation of molten iron, the iron oxide content of cupola slag should be less than 5% (preferably less than 3%), and the molten iron of electric furnace should not be kept in high temperature for a long time.

(5) Try to improve the mold rigidity.