Recently, our US client commented on our latest shipment of CNC milling aluminum anodizing parts. All the part profile dimensions are okay, except that the surface of the part was blotchy and not as shiny and sharp as before. Our client tried to wipe a part with alcohol to see if the surface appearance improved, but it did not work. The photo below shows the part surface.
Blotchy Spots on Anodized CNC Aluminum Part Surface
How the blotchy spots happened?
According to our production experience in the past, blotchy spots randomly occurred in various batches of aluminum raw material. From raw material to machining procedures, the blotchy spot was not observable till anodizing.
The dendrite-like or grain-boundary-like blotchy spots might have happened due to uneven current density across the surface of the part. The blotchy spots had a higher current density than the surrounding area. High current density raised the temperature locally and degraded the surface finish. Chemical reaction during anodizing could also create this kind of blotchy spots.
Uneven Anodizing CNC Machining Aluminum Parts
How to avoid the blotchy and uneven anodizing?
To avoid this situation from happening, we will ask the supplier to pre-anodize the aluminum raw material. Then, they will provide us with batches of material free from blotchy marks after anodizing. You MUST inform us if your workpieces are for customer-facing use or have cosmetic surface requirements.
Moreover, you can take an aluminum alloy panel and cut it into several pieces. Then, respectively blast, buff, and polish one piece and leave one with nothing done to it. Black hard anodize all the pieces at the same time and note that all the pieces above will come out differently due to the finishes. Sometimes, the pre-treatments above work and can improve the surface well.
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