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Compatibility of Tribo Application with Coronal Application

Why won't the powder penetrate into Faraday areas?
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Q. We have an automatic powder system with 12 corona guns. We are powder coating various kinds of light fixtures, frames and hanging hardware in several colors. We have had some trouble getting the powder to penetrate into Faraday areas and so we decided to buy a Tribo gun to see if it would help. We were told that the Tribo gun would penetrate better.

Now when the operator uses the Tribo gun, all the powder is repelling and falling from the edges of the part. This new problem is worse than the old one and we do not know why the powder will not stick anymore. We believe it might be caused by the effect of the corona guns. What are we doing wrong?—A.R.

A. The corona gun charges by adding electrons to the powder particles to produce a negative charge. The powder is attracted to earth ground to bleed electrons back into the earth ground. The tribo gun charges by removing electrons from the powder particle to produce a positive charge. When you spray powder with a positive charge onto a surface that has a high negative potential (from the corona charging process) you neutralize the charge effect of both and that causes powder to fall off the surface.

 Faraday cage is caused by natural electrostatic resistance created by the inside recesses on your parts. For Faraday cage effect you should work on lower voltage and the correct spray pattern. Pre-touch-up is also a possible way to help with the Faraday areas because the resistance is lower in the Faraday area prior to application with the 12 automatic guns. Excess fine particles in your powder may be contributing to your difficulty with the Faraday areas also. If you reclaim powder you may be accumulating fine particles in the reclaimed powder.

Try using a manual corona gun with virgin powder (no reclaim in the touch up material) and use lower voltage. To recap, use lower voltage or limit current, avoid high velocity from the powder gun, pre-touch-up if possible and maintain a consistent powder particle size in the medium to high range.

Originally published in the July 2015 issue.

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