Uniform Coverage

Q. We are powder coating fencing components and we have trouble getting uniform film thickness. Our vertical fence posts are made of 1-inch square aluminum and range in length from 3–6 feet. We changed racks recently and the new rack holds seven parts on each side in a vertical position with a slight stagger to open up the backside of the part.

This helped a little, but we are still are finding a variance of over 2 mils from one side to the other of the fence post. Obviously, we need good coverage on all four sides of the part, so we are forced to recoat parts that do not meet our minimum film build of 2 mils and we are getting as much as 6 mils on some surfaces. We currently re-coat around 10% of our parts, and the ones we are accepting have film thickness ranging from our minimum to as much as 8 mils, if it is a recoat.
We need 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 post. H.K.

 

A. The staggered position may have given you more parts per linear foot of conveyor, but it is not the best way to control film thickness.

Gun-to-target distance is a very important variable for film build control. With a staggered pattern, you have differences in the distance between the gun tip and the part that will affect film build because of the forward velocity and the electrostatic impact.
Consider a rack that holds the parts in a single file. Good profiles provide the best possible film build control. The distance between parts has to be enough to allow throw along the depth of the part but they can be close together as long as they do not touch. You may have to drop a couple of parts going to a single row, but your first pass yield will be much better and you will use a lot less powder. You can use lower overall flow rates, too, so you will use less compressed air and you will not need to replace the wear parts as often. Consistent racking in neat profiles will lead to a higher overall yield per day and a reduced cost per part.