Atmospheric Control of the Powder Spray Area

Q. I have been told that I need to maintain 45 to 60% relative humidity for ideal powder coating conditions. Is this true? Also, do I have to control the temperature around the spray area? –D.J.

A. The relative humidity of 40% to 60% can be very helpful for powder application.  First, you need to consider how the corona system works. A high voltage discharge from the gun breaks down air molecules and generates a large number of free ions as a result. This corona field of charge at the front provides a charge to the powder particles as they are delivered through the voltage field. Some moisture in the air is necessary to facilitate the process of transferring free ions to the powder particles. If the moisture content is too low, the corona field is not as effective and results in lower transfer efficiency of the powder to the grounded part. In addition, if the humidity is too low there is a tendency for the material to create a frictional charge that strips away negative ions and reduces the effective charge to the powder.

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The relative humidity of the surrounding environment is also important because if it is too humid the powder will not fluidize as well, it will resist flow and it can generate more impact fusion inside the delivery system. Excess moisture in the air will cause the material to agglomerate and make it hard to deliver. Therefore, maintaining 40 to 60% relative humidity will provide consistent behavior of the powder and improved transfer efficiency.  It can also help to reduce rejects and costs associated with rework and scrap by providing process control over atmospheric influences. Consistent conditions improve the quality of the finish.

As far as the temperature is concerned, a good plan is to control the temperature between 60° and 80°F. This will make the material behave in a consistent and predictable way and make it easier to establish standard gun settings. The bottom line is, temperature and humidity control make the material more predictable and consistent and that saves money.

Control of the atmosphere means putting an enclosure around the spray area. A powder room like this can help with dirt and provide a controlled area for powder storage. The standard answer to the powder room question is that it is not absolutely necessary but it is a way to gain much better control of the application process and often worth the investment.