Uniform Film Thickness

Q. We are powder coating fence rails and have trouble getting uniform film thickness. We recently changed racks to add parts, and the new rack holds seven parts on each side with a slight stagger to open up the backside of the part. The old rack held only six parts. The new design has helped a little, but we are still finding a variance of more than 1.5 mils from one side to the other. How can we set up our racks and adjust our spray guns to get more uniform coverage? S.V.

A. Film build control starts with the rack design. Parts should be profiled as much as possible to avoid having to throw powder around one surface to get to another. When you stagger parts, you will inevitably get some variance from inside to outside. If at all possible, line the parts up straight across. You do not need much gap between them. For example, if you are coating 1-inch square pickets you can space them with only about 1 ft between parts. Of course, that means that they have to be held rigidly so they do not touch, but that will also help with coverage. You may need contacts at both ends to hold the part steady. If you can, point the corner to the guns to get even more uniform coverage. Avoid inconsistent racking, and keep the racks in good repair. Always fill the rack up, if possible, and fix bad contacts. Avoid the stagger if you can.

As for gun setup, arrange the guns in consistent, evenly spaced patterns. Use moderate flow rates. Keep the guns clean at all times to avoid spits. Optimize the electrostatic setup and avoid high current levels (keep micro-amps less than 50). Maintain your powder in a controlled atmosphere for consistency and optimum fluidization and deposition. Avoid spraying excess amounts of reclaimed powder by blending it in consistently and consuming it regularly so that it does not accumulate fines. Follow these guidelines, and you should see better results.