Reasons why castings turn black after shot blasting

　　After the casting is shot peened, if the overall surface turns black or there are obvious black marks or black spots in some parts, some of them can be thrown away, while others have been immersed in the casting matrix, and the area and position are not fixed.

　　Cause Analysis:

　　1. Defects that existed before shot blasting brought from the die-casting process:

　　(1) Too much black oil is used in die casting;

　　(2) The punch oil splashes when opening the mold;

　　(3) Paint splashing during die casting;

　　2. The product has been left for too long or the temperature is humid, and the surface is severely corroded, moldy or dusty.

　　3. The dust removal device of the shot blasting machine fails, and the steel shot contains a lot of dust.

　　4. The operator did not wear gloves as required and directly touched the surface of the shot-peened casting with his hands, causing fingerprints.

　　5. After being left for a long time after shot blasting, there will be dust on the surface or water and oil splashed on it, and the environment will be humid and oxidized.

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Reasons why castings turn black after shot blasting