Cold Mill Banding Problem

Question:

What is "cold mill banding" in 3003 aluminum sheet? How does it affect the finishing process when the part is matte-clear anodized? I have been having a problem with parts coming back with gray-black color grain lines showing up in these parts. The lines definitely follow the grain pattern of the aluminum. The material supplier says there is nothing wrong with the material. The anodizer says it is “cold mill banding,” and is a metal problem that they can do nothing about. Before anodizing there is no evidence of this condition. It can only be seen after anodizing.

Any advice that you can give would be greatly appreciated. M.B.

Answer:

The problem could be cold mill banding (CMB), or it could be structural streaking. CMB is described as a condition caused by a lubricating (lack of) problem on the line when the sheet is rolled to final thickness. It can appear as a narrow, matte streak in the mill finish material but can be difficult to see until after anodizing. This is a condition that the anodizer has no control over.
Structural streaking is a metal condition having to do with grain structure. Specifically, the casting conditions and alloying chemistry are where it begins. It shows up as darker and rather grainy streaking in the sheet. Again, this condition is beyond the control of the anodizer.