How to Get Powder into Recessed Areas

Q. We manufacture metal office furniture. Our pedestal file cabinets have drawer pulls formed into the front of the drawers as recessed areas that are about ¾" wide by 1" deep. We use automatic spray guns to coat the outer surface and then manual spray to fill in the drawer pull area. The line speed is 18 to 20 fpm. We have trouble covering inside the pull area without getting roughness and heavy coating around the perimeter of the pull. How can we get the coverage inside without the orange peel? 

A. The drawer pull area that you describe is a classic example of the dreaded Faraday Cage area. The recessed area creates resistance to electrostatic charge, and powder is not strongly attracted to that deep area like it is to the external surfaces and flat spaces. It is partly related to the electrostatic behavior and partly related to the aerodynamics of the compressed air that delivers the powder. The electrostatic issue reduces the attraction and velocity of the air pressure, and can cause the powder to blow out of the recess. The coverage can be improved by understanding these forces and adjusting to compensate for those impacts.

The electrostatic charging with a corona-style gun is typically related to the discharge of high voltage from the gun tip that can be as high as 100 kV. Current is usually limited to a number below 100 micro-amps. The voltage and amperage are closely related in a normal circuit: when one goes up, the other goes down. The movement of voltage and amperage is related to the resistance between the gun tip and the grounded part. Obviously, good earth ground is essential.

Higher voltage is usually beneficial to higher charging efficiency. Current should be reasonably low for efficiency and uniform film build. If the current draw is too high, the surface will look rougher, and application inside Faraday areas will be more difficult. The first trick is to find the optimum level of voltage and amperage. Assume that lower is generally better, and try a range between 20 and 40 micro-amps. This will help with the deposition of powder where attraction is low. Many spray guns now have a current-limiting feature for Faraday areas.

Next, work on the right powder volume and air velocity. A very slight increase in powder volume with a decrease in total air velocity is probably the best direction. Experiment on a test part to get the right setup.

Gun-to-part distance and technique are also part of the challenge. Steady and repeated strokes are helpful. The gun has to be close to get the powder into the slot, but if it is too close, it will cause back-ionization, a phenomenon that occurs with too much current draw. Again, practicing on sample parts is advised to develop the right technique. Lower amperage, lower velocity and skilled technique are the answers to better coverage in Faraday areas.

The right combination of electrostatic settings and aerodynamic settings is the best solution. Another consideration is application into the drawer pull before you expose the rest of the part to the automatic guns. This will provide a better chance of overcoming the resistance in the recess, since the rest of the part is still bare and not insulated. Pre-touchup can work well if you have the ability to do it in your line. It requires a higher skill level because the operators need to know exactly what to cover when the whole part is bare, but it can reduce the impact of the Faraday Cage effect.  

 

 

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Originally published in the February 2017 issue.