Preventing Paint Blisters
We have an old gas catalytic infrared oven that we use for drying air-dry, water-based liquid paint applied to wooden doors. Sometimes we have trouble getting the material to dry enough before we have to stack our parts. We slow the line down to get more time in the oven but then we sometimes get too hot in some areas and the paint blisters. Is there a way to get better control over the oven? Is there a better type of oven that we can use for our operation?You will probably be able to improve your results by upgrading your oven with new or refurbished catalytic emitters and perhaps adding some zoning.
Gas catalytic emitters rely on a platinum catalyst to oxidize the gas. After a period of years, the catalyst can migrate causing hot spots and cold spots across the face of the emitter. Because these "flameless" emitters do not glow, this problem cannot be detected by the naked eye. You can use a hand held infrared thermometer to take temperature measurements across the face or use a thermal-imaging camera to detect this problem. If you find that your emitters are no longer uniform, you can replace them with new units or have them rebuilt.
Having more than one zone in the direction of travel can make a big difference in fine tuning any process, and this is especially true for drying paint. In the first zone, where the product is at its lowest temperature, you can set your emitters at a high output to quickly ramp up the temperature of the product. The next zone(s) can be set at a lower output to hold the product at the required temperature. If the entire oven is only a single zone, then the ramping occurs all the way through the oven and it is harder to make subtle changes and harder to make it repeatable.
In addition, you may be able to benefit from vertical zoning. With only one zone in the vertical direction, the top of the part can get overheated. By adding vertical zoning, you can add as much heat as needed to the bottom, without overheating the top.
Blistering occurs when the outside surface of the paint forms a dry skin while the inside layer of paint is still too wet. As heat is conducted to the inside layer of paint, the evaporating water or solvent blisters up through the dried skin usually popping and leaving a crater with a pin hole. This commonly occurs when trying to push product through an oven too quickly.
In a convection oven, the air directly heats the outside surface of the paint. The inside layers of the paint are then heated by conduction through the layers of paint. It is the rate of conductive heat transfer through the paint that limits this process. If the air temperature is too high, and the rate of heat transfer to the outside layer of paint greatly exceeds the rate of conductive heat transfer through the paint, then the dry skin forms. In a convection oven, the only solution is to use lower air temperature and a longer dwell time.
As you discovered, even an infrared oven can create blistering, but in an infrared oven, some of the radiant energy heats the surface of the paint, and some of the radiant energy is transmitted deeper into the paint. This means the drying process is not totally dependent on conduction through the paint and therefore not as limited by it. The infrared penetration can help to dry the inner layers before the surface skin forms.
The amount of energy that penetrates through the coating is dependent on the wavelengths emitted and the absorption characteristics of the paint. In general, as the emitter temperature increases, the amount of penetration increases. The lower the emitter temperature gets, the more it behaves like a convection oven. Having a higher temperature ramp zone at the oven entrance can help drive out water from the inside layers faster and sometimes minimize blistering.
In any type of oven, a safe way to address blistering is to slow down the drying process and be more patient. But with a multi-zone infrared oven, you can also experiment with higher temperatures in the first zone to address the problem.
