The reason for non-metallic pressing into the aluminum plate may be

The process equipment of impurities is not clean enough, and dirty things fall onto the aluminum plate during processing, and are pressed into the material after rolling.

The pressure of the process lubricant injection is too low, and non-metallic dirty things adhere to the surface of the aluminum plate and are not washed off.

The emulsion is not replaced in time, the aluminum powder is not rinsed cleanly, and the emulsion tank is not clean enough.

There are scratches on the surface of the aluminum billet.