Q: We have an automatic spray booth, with eight automatic and two manual (touch-up) guns. When is it best to use manual touch-up: before the automatic guns or after?
Also, we heard that the charging is negative. Why? Would it be better if it was positive? What is the difference?
A: One of the challenges with electrostatically applying powder coatings is penetrating tight inside corners and other hard-to-reach areas. The electrostatic field created by the ions generated in a powder gun creates a Faraday Cage that repels the approaching powder particles. This is exacerbated by the insulative effect of powder already deposited on the part.
From my experience, it is much easier to coat the tight inside corners and recesses on a part before applying powder on the easy-to-coat areas. Hence, it is better to manually touch-up these tough-to-coat areas before triggering the automatic spray guns.
As for negative vs. positive charge, creating negative charge is easier and more efficient than generating positive charge. Negative charge consists of electrons, whereas positive charge relies on protons. Protons are 2,000 times heavier than electrons. In addition, generating electrons is quite easy and controllable using the well-known corona discharge technique.
That said, triboelectric charging technology creates a positively charged powder particle by frictional charging. This involves conveying a powder coating through a specially fabricated spray gun with a PTFE lining that robs the powder particles of their electrons, thus resulting in a net-positive charge on the particle.
The output of these guns is somewhat less than the delivery of a conventional corona discharge gun, but the charging efficiency is greater with fewer free ions generated. This technology was developed in Sweden in the 1980s, and it is still quite commonly used in Scandinavian powder application systems.