Humidity Levels for Powder Application

Q. We have had some trouble lately with powder application, especially metallic colors and textures. The attraction to the part is not as good, even though we have good earth ground. We need higher flow rates to achieve the same film build. We get uneven film build, and some variation in color and gloss. One of our powder suppliers suggested that the reason for our problems is the extremely low humidity levels in our area during the winter months. Is this possible? What can do about it? What is the proper level of humidity for ease of powder application? A.L.

A. Humidity can have a profound impact on powder coating application. If the humidity is too high, the powder will absorb moisture and agglomerate, making it harder to fluidize and more resistant to flow. If the humidity is too low, the normal charging characteristics are going to be influenced by the excess static in the atmosphere. The atmosphere is changed, and the behavior of the negatively charged powder particles is not as likely to work effectively. The part can build a repelling charge more rapidly, and current is more likely to arc form part to part or part to rack.

The ideal range of humidity is between 40 and 60 percent. Below 40 percent, you could easily see the problems that you describe. If you have an enclosed room around your powder booth, you probably have an HVAC that reduces humidity, but it is more unusual to have a system that can add humidity. The ideal method for addition of humidity is a misting system that is part of the air delivery into the room, but it is not a simple thing to add. Room humidifiers can be added. You should look into different systems for addition of humidity and try to get the humidity up to around 50 percent or so for better results.