INCONSISTENT FINISH

Q. We run a job shop where we coat a large variety of parts of all sizes and shapes. We recently took on a job to powder coat subway acoustic panels with a copper metallic powder. We coated the first lot of parts, and the color and appearance on the parts is widely different from one part to another and even from one area of the part to another. It is lighter or darker, brighter or duller and we cannot seem to get a consistent look from the powder. We contacted the powder supplier and they told us it was because of the perforations in the panels and the electrostatic behavior of metallic powders. They said that our manual applicators were not putting the coating on right.

We need some help. A.B.

 

A. The powder supplier is correct in pointing out that the perforated panel will be hard to coat with a metallic powder using manual application equipment. The electrostatic behavior on the perforated surface can cause some variance in the way the metallic flakes distribute within the film and their position within the film.

In order to prevent wider variances in appearance, you need better-than-average control over the application process. First of all, be aware that some powders have a bonded metallic pigment and some are blended. Either one can work, but a bonded metallic is typically easier to apply on challenging surfaces and easier to reclaim. If you do reclaim the overspray, be sure to maintain a consistent blend of reclaim to virgin material. Use a feed hopper instead of a box-feed system to prevent segregation of the metallic pigment.

Avoid high pressure for fluidization because it can cause some segregation of the pigments in the feed hopper. If you need to get more air in the fluid hopper, add an additional air inlet into the plenum instead of turning up the pressure. Make sure that the parts are earth grounded with less than 1 meg-ohm of resistance. Use a fan spray tip with consistent output and very steady operating parameters (voltage, amperage, flow, atomizing air, stroke speed and gun-to-target distance). Use a very consistent stroke and pattern for application.

One final idea—find out if the material absolutely has to be a metallic color. It would be much easier to apply a material that did not have metallic content over a perforated surface. Ask your customer about the possibility of using a similar color without the metallic content.