On the Fence about Film Thickness

Q. We are powder coating fence rails and we have trouble getting uniform film thickness on all surfaces. We changed our racking system recently and the new rack holds eleven parts on each side with a slight stagger to open up the backside of each part. The parts are racked in the vertical position and they slide down onto a peg. Our old rack had 12 parts per side arranged back-to-back. The new rack helped a little but we are still finding a variance of over 1.5 mils from one side of the part to the other. Any suggestions on how to set up our racks and how to adjust our spray guns to get more uniform coverage on all four sides of the fence post?

A. Powder film build control works best when parts are presented in neat profiles. A single row of parts will coat more effectively than working with a double row the way you describe. In an effort to get more parts per lineal foot of conveyor we sometimes compromise the ideal position for coating. Racking should be a very important part of the operation. You can help yourself a lot by paying close attention to proper racking technique. Parts should be held in a stable position with good spacing for application and dense racking for good throughput.

A single profile will make it much easier to get uniform coverage. You may be able to rack the parts in the horizontal position on very close centers if you may contact on both ends, or position the parts so that the corner of the fence post faces the guns to open up all four sides of the post. You may fit more parts per foot than you think this way, and your material use and rejects will be much better. Imagine near 100-percent first pass yield with a much tighter film thickness variance. Definitely worth a trial run.