After shot blasting, the whole surface of the casting is blackened or there are obvious black marks and spots locally. Some of them can be thrown away, while others have invaded into the casting matrix. The area and location are not fixed for the following reasons:

Defects before shot peening brought by die casting process:

1. Too much black oil is used in die casting

2. Punch oil splashing during mold opening

3. Paint splashing during die casting

The storage time or temperature of the product is humid, and the surface is seriously corroded, moldy or dusty;

The dust removal device of shot peening machine is invalid, and there is a lot of dust in the steel shot;grinding steel shot;steel cut wire shot;

The operator did not wear gloves according to the requirements and directly contacted the surface of the shot peened castings with his hands, resulting in fingerprints;

After shot peening, it will be stored for a long time, with dust or splashing water and oil on the surface, and the environment will be humid and oxidized.

In case of the above situation, corresponding measures should be taken:

1. Defects do not cover the whole casting surface. Strengthen control and management by die casting process

2. It’s black oil, the color is black

3. It is punch oil with dark red color;

4. It is the light color on the surface of castings with different colors

After shot peening, the surface trace is shallow and cannot be removed because it has invaded into the casting matrix. Strong products should not be placed too long. If it needs to be put in time, it must be covered and protected, and placed in a suitable environment.

The surface color of the whole casting turns black and dark. Restore dust removal or replace steel shot;

The operator must be required to operate according to the operation instruction and must wear gloves.

Shot peening should be carried out as soon as possible for final inspection, packing and storage. If it needs to be stored for a period of time, strict protection must be carried out.