Irregular Appearance

Q. We apply several medium-gloss colors on aluminum extrusions that are about 6-inches wide by 20-ft long. The parts are hung six high in the horizontal position on a monorail conveyor. They pass through a set of automatic spray guns and then we do some touchup around the edges to get powder into a small groove. We are having trouble with the consistency of the appearance. Some areas have blotches of darker color with slightly lower gloss. The pattern is not consistent and we cannot figure out what is causing it. H.P.

A. A stain of some kind on the part would be visible prior to coating, so you can check for this and most likely rule it out. It could be that your automatic spray guns are surging and delivering an inconsistent volume of powder. Check the ratio of powder flow air to total air, and make sure that it is balanced and consistent with no surges in the pattern. The most likely cause is electrostatic properties of the manual guns. When the gun gets too close to the part, it delivers higher microamps and generates a distortion in the powder that creates the dark blotches. Try turning your voltage down to around 65 kV and the set the amperage at 10 µA. Do not move the guns in too close to the parts; keep them out as far as possible to avoid excess current draw. This should take care of the problem.