A Good System Gone Bad

Q. We used to send our automotive product out to be coated, but we built our own powder system and began to do our own black epoxy coating a few months ago. The system worked pretty well when we started, but we have had problems lately with light coating. We have increased the flow rate of the powder and that helps, but our material use is worse and we still do not understand what has changed. Our application efficiency has gradually declined so that we have to spray a lot more powder to get the same coverage that we used to get. Our film thickness is inconsistent but our material cost is much higher.

Our application equipment vendor says there is nothing wrong with our powder guns but we need to keep our racks cleaner for better ground. We have started to clean the racks more often, and this seems to help a little, but we still are not getting the kind of coverage we got when the system was new. P.T.

A. You need to consider what has changed since you started. Your racking has been used, and the suggestion to keep the racks clean is an excellent start. Earth grounding is essential for uniform film build at reasonable flow rates. Dirty hooks can cause a loss of ground for deposition of powder. With only one color, I will assume that you use a cartridge module reclaim system. This module will collect 100 percent of your overspray for reuse. If you have been running at higher flow rates with poor efficiency, you have probably built up more fine powder particles in your reclaim system. Larger particles have more cumulative charge and are more likely to build on a poorly grounded part. Finer particles are less likely to build on any surface, and poor grounding reduces the deposition of finer particles. In addition, reclaimed powder ordinarily builds up a higher percentage of fines in a module collection system, resulting in poor transfer efficiency, more orange peel, uneven and low film build, and poor coverage in Faraday areas (inside recesses).

Get your racks clean and measure the earth ground with an ohmmeter. Resistance to ground should be below 1 mega-ohm. Measure your particle size in the collector and compare it to the virgin material. Chances are good that the percentage of fines in the collector is much higher than the virgin material. If the particle size distribution for the powder in the collector has more than 25 percent under 10 micron, you should throw it away or pump it out and slowly blend it back in with virgin material. If the guns are in good working condition, the ground is good and the powder is not loaded with fines, you should be able to get your settings back down to more reasonable flow rates and get good coverage.

In the future, keep your guns in good repair, maintain clean hooks and consume your reclaim powder fast enough to avoid an accumulation of fines.