Cure Oven Contamination

Question:

We have a powder coat system. For some time we have been fighting contamination in our high gloss black (we also paint white, blue, green and beige). I believe the contamination may be coming from ash getting airborne in the cure oven. The burn-off oven is a great distance from the booth, so I do not believe that the contamination comes from that source. I believe that the carriers, hooks and chains may be losing ash in the cure oven and possibly contaminating the part. Do you know of anyone who sells a product similar to a tack cloth that I could run through the cure oven to pick up the particles? If not, do you have any ideas of what I could run through the cure oven to pick up the airborne ash? S.K.

Answer:

Before I start talking about corrective actions you can take, I want to talk about the causes of the problem. Let’s start with whether the contamination is actually ash or dirt from the oven or dirt from another source. I tell my clients that troubleshooting contamination starts with close examination of the defect on the part. Using a magnifier, determine if the contaminant is under or on top of the coating. If it is under the coating, then the contamination was introduced before or during the application of the coating. If the contaminant is on top of the coating, then it was introduced after the application of the coating. If the contaminant is easily removed from the coating by brushing the surface, then the contaminant was introduced after the coating has set-up, which means after the coating was cured. You can’t fix the problem without doing this investigation first. How else can you determine which part of the process needs corrective action?

Since you said that this contaminant is on top of the coating, we can ignore the spraying and cleaning operations of the process. So let’s focus on the other areas of the process that would cause this contaminant. Contaminants for the conveyor carriers, chain, etc. can be removed using conveyor cleaning systems available from several manufacturers (look here: conveyor lubricators and cleaners). These devices should be located before the conveyor lubrication system and immediately ahead of the pretreatment system to remove any debris that falls onto the parts. While on the subject of conveyor lubrication, you must use a high-temperature lubricant to ensure that this material doesn’t burn in the cure oven and add to your contamination problem. Select a lubricant that will withstand 500F or higher, if required.

Ash on the parts is caused when powder coatings are blown from the parts and hangers into the oven airstream or by contaminants entering the oven from the plant airspace that have catalyzed or burned. Adjusting the velocity of the air seals or circulation air at the oven entrance will reduce the problems of blowing the powder coating from the parts and/or the hangers. A little known cause of this phenomenon is poor part grounding. If the part is improperly grounded, then the electrostatic force that holds the powder onto the part’s surface is lower. The lower the electrostatic force, the more likely the powder coating will be displaced by the oven airstream. Frequent hanger cleaning and conveyor maintenance will improve the ground (less than 1 megohm resistance) and eliminate this as a cause to your contamination problem.

If you are sucking airborne contaminants into your oven from the plant airspace, then you must isolate their sources or use filters to remove them. Filtration is available to clean air that is introduced by the combustion blower in convection ovens. These filters are located outside the oven and do not have to be high-temperature resistant. Isolating the cure oven from your plant airstream may be required to prevent airborne contaminants from entering the part entrance/exit of the oven. Walling off the oven and providing an outside air supply will sufficiently isolate your oven from the dirty plant airspace. However, isolating the specific plant area that is causing the contamination may be easier than isolating the cure oven.

Last, check your oven exhaust turnover rate. It should be between 4-12 air changes per hour. Adjusting this is tricky, since too little air will allow more time for the contaminants to catalyze and too much air will introduce more airborne plant contaminants. Furthermore, the limits on oven exhaust are regulated by NFPA codes. Always consult your oven manufacturer to determine these limits before making any changes.

Enough about the causes of contamination, let’s talk about the corrective action necessary to remove contamination in the cure oven. Normal oven maintenance requires monthly cure oven cleaning. Cleaning your oven more frequently may be required if you cannot get control of the contaminants or have a higher than average surface quality requirement. Cleaning is accomplished by vacuuming all surfaces clean and following with a damp mop. Be sure to clean all surfaces including inside the ductwork, if possible. Do not use water to clean the oven unless your oven was designed to be watertight and has a floor drain. A mop dampened in water will not cause any problems, as long as you wring it out good and clean it frequently. Using tack rags or other impregnated devices can add to your contamination problem, since they leave traces of the impregnated material on the oven surfaces. These materials will eventually catalyze and end up as ash on your parts.

Remember, it is always best to stop the sources of the contamination than to implement plans to collect the debris after it has entered the cure oven. Also, remember that you must perform monthly cure oven cleaning as part of a preventive maintenance program.