Application of a Second Coat of Powder
Why am I not getting good attraction to the surface or a consistent second coat?
Q. I am trying to put a second coat of powder onto a part that is already powder coated. I am having a lot of trouble getting good attraction to the surface and getting a good looking and consistent second coat. Can you tell me why, and what I can do to improve it?—J.B.
A. Powder is attracted to the grounded part surface by electrostatic attraction. The powder particle is loaded with electrons that are seeking earth ground to bleed back into atmosphere. When they come in contact with the bare metal the electrical charge will begin to dissipate from the powder to ground.
All of the charge does not immediately go to ground. If it did, the powder would fall off of the part. Instead, the charge slowly bleeds off through the grounded part, hanger, and conveyor and eventually to earth. As the layer builds, the surface becomes somewhat insulated and the attraction gradually grows. Once the powder has been cured, the surface is insulated and application is gradually reduced. So the application efficiency during a recoat is lower due to resistance from the first coating. The surface of the part is covered and it will reach a point where the charged powder on the part is at, or near, the same potential as the powder being applied, thereby creating back ionization or rejection.
The gun settings for application of a second coat should be modified to compensate for the resistance of the first coat. The electrostatic charge should be reduced (either by lowering voltage or limiting current). The gun-to-part distance should be increased slightly to soften the forward velocity of the pattern and further reduce current draw to the part.
You should also need to adjust the powder delivery settings to increase the output by 10-20 percent. Be aware that the application will require more time and transfer efficiency will decrease when doing re-coats but application is possible. The reduction in charge, combined with a slight increase in flow rate and keeping the gun farther from the target should provide the coverage needed. Be sure to cover the entire part to avoid a dry-spray appearance.
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