Correcting Back-Ionization Starring

We recently converted to powder coating from our old liquid operation for application on office furniture. The powder coating has been a success for the most part and our first run quality has improved. One part that is giving us trouble is drawer fronts. The parts pass through the automatic spray guns and then we attempt to fill in the drawer pull with manual application. We are getting a very rough, textured look around the pull and nothing we have tried so far has worked. Any suggestions?

Your problem is commonly referred to as back-ionization, an excess of negative ions in a location on the part that causes an electrical reaction under the applied powder film and creates orange peel and an appearance sometimes called "starring." First, make sure that your part is grounded properly with resistance below 1 megaohm. Poor ground will magnify the problem. With good ground, there are several techniques that can be employed to reduce or eliminate back-ionization, anything that will reduce the amount of free ions on the part surface. You may be able to just reduce the voltage or limit the current with your gun controller. Another possibility is the use of an ion-collector, a device that attracts a large number of free ions and prevents them from being attracted to the grounded part. Another possibility is applying the powder into the pull before the automatic coverage. This can be impossible if the booth is not set up for it, but it does help to reduce the ionization on the part surface if the manual touchup is done before the automatic application.

Curing Urethane Polyester

We recently switched to a urethane polyester powder material and we are having trouble with our cure oven. Our lighter colors are yellowing in the cure oven and we have a lot of smoke and odor coming from the oven. Our supplier tells us we need more air turnover in the oven. What should we do?

The exhaust rate in the oven is designed to relieve the by-products of combustion and volatile content from the coating. Most powder coating materials have very small amounts of volatile content and require a limited amount of exhaust. However, if the rate of exhaust is too low to adequately ventilate the oven interior and gases from the cure process begin to accumulate, it may cause oven fouling and discoloration of the curing powder. Urethane polyester powders have a higher percentage of volatiles than other powders and are more likely to cause this problem. You need to increase the exhaust volume to keep up with the amount of volatiles being given off by the curing powder. Keep in mind that if you increase the exhaust volume you will put additional load on the burner and you may need to increase the burner Btu if it was not sized with sufficient capacity to handle the higher exhaust rates. As far as smoke and odor is concerned, the entrance and exit of the oven should have longer vestibules or a heat relief hood to prevent excessive rollout into the plant. You can find information on these issues in the Powder Coating book published by Chemical Coaters Association International (CCAI). Visit www.ccai to check out their manuals.