Voids in Coating

Q. We have a lot of tight recesses on our parts where it is hard to get good powder coverage due to Faraday cage effect. We have worked with different voltages and gun setups but still struggle to get the inside corners covered consistently. In some cases, we can see that the small recesses are covered after they exit the booth, but the same spot has a void when it comes out of the oven. What can we do to get better coverage on the small recesses? L.T.

A. Faraday cage effect is a reference to the electrostatic resistance created by a recessed area on the part. The current flowing from the electrode and the powder that is charged in the corona field are strongly attracted to earth ground. The powder will typically follow the path of least resistance, so the more prominent surfaces get covered quickly, while the deeper inset areas do not get covered as well.

In addition to electrical resistance, the powder may be influenced by aerodynamic forces such as gravity, the natural resistance of the air and the force of the delivery pressure. These aerodynamic forces can also limit coverage in the Faraday areas. The net result is that the Faraday areas get a thinner film with weaker attraction to the part. This can lead to thin coating or even voids in the areas with resistance. And in some cases, like the one you describe, the part looks covered but comes out of the oven with a void. This can occur when the powder is not thick enough to flow out and cover a small area or the force of the air velocity in the oven disturbs the powder in the recess.

So, what to do about Faraday cage effect? First, make sure that the gun settings limit the amount of current to some optimum number, usually as low as 20 to 25 micro-amps. Second, work on the optimum position of the gun. If you are too close (especially if you have high-delivery pressure), you will pull too much current or blow powder with too much force to stick. If you are too far away, the powder will coat the easier surfaces. Next, make sure you are delivering a decent amount of powder with as low a pressure as possible to avoid disturbing the powder in the recess.

The powder can be a factor, too. Many lower-cost powders have more filler and are less effective in Faraday areas. Racking is another critical factor. You must have consistent racking that holds parts in a firm position with good access to the Faraday area and maintains good ground. Finally, make sure that the oven does not have too much turbulence. If necessary, add an infrared zone or a quiet zone at the oven entrance to avoid disturbing powder that is weakly attached in the Faraday area.