How to prevent streaks on aluminum extrusion sizing belts?

The die-swell streaks are like many fine lines that are continuously drawn parallel to the extrusion direction with a needle. Sometimes it is difficult to distinguish only by extrusion, but a gloss can be seen when viewed from a certain angle. It can be distinguished through corrosion or anodizing treatment.

It is not caused by scratches on the surface of the die-swell belt of the aluminum profile mold, but because the coating of the die-swell belt is unstable and the flow is uneven during the extrusion of aluminum alloy.

Methods to eliminate die-swell streaks: The nitriding treatment on the surface of the die-swell belt should be uniform, and the shape of the die-swell belt should be machined appropriately.