Loss of Efficiency

Q: We used to send our product out to have it coated by a customer coater but we built our own powder system a year ago and began to do our own coating in-house. We powder coat automotive springs and we have just one color, a black epoxy. The system worked pretty well when we started out a year ago but we have gradually declined in efficiency and productivity. We are seeing a lot of light coating and inconsistent film build. We have increased the flow rate of the automatic powder guns a lot and that helps a little, but our material use is going up and we do not understand what has changed since startup. Our application efficiency has gradually declined to the point that we have to spray a lot more powder to get the same coverage that we used to get. Our film thickness is inconsistent and our cost of material is much higher. We have consulted with our application equipment vendor and he says there is nothing wrong with our powder guns. He did tell us that our racks are too dirty and we have a grounding problem so we are working on that and seeing some moderate improvement, but things are still not where we need them to be or consistent with the way the system worked when it was new. Can you offer any advice on what to do to get back to where we were when we started?

A: The key to getting you back to where you were is to figure out what has changed and restore it to the original condition. Your racking has been used and the suggestion to keep the rack contact points clean is an excellent idea. Earth grounding is essential for film build control. Dirty hooks can cause a loss of ground that will result in inconsistent deposition of powder and poor efficiency.

Since you have just one color, it is likely that you use a cartridge module reclaim system for your powder booth. The cartridge module is noted for efficiency collection of powder overspray and will collect 100% of your overspray for reuse. If you have been running at higher flow rates with poor efficiency you have probably built up more fine powder particles in your reclaim system. Larger particles have a more cumulative charge and are more likely to build on a poorly grounded part. Finer particles are less likely to build on any surface and poor grounding reduces the deposition of finer particles. Fines normally accumulate in a cartridge system and if transfer efficiency is low they will accumulate even faster. The result of a higher percentage of fines is poor transfer efficiency, more orange peel, uneven and low film build and poor coverage in general. You probably need to remove the powder from the module and scrap it all. You can have a particles size analysis done before you throw it away but chances are it will only confirm that the powder is too loaded with fines to be effective.

You definitely want to maintain good earth ground at all times. Get your racks clean and measure the earth ground with an ohmmeter on a regular basis. Resistance to ground should be below 1 mega-ohm and should not be compromised ever. Measure your particle size in the collector periodically and compare it to fresh powder material. If the particle size distribution for the powder in the collector has more than 25% under 10 microns, you should throw it away or pump it out and slowly blend it back in with virgin material.

Your guns need regular maintenance. Having the vendor in once a year is not nearly enough to ensure good operation. Establish a regular time to thoroughly clean and inspect the guns and make sure all wear parts are in good condition. Have the vendor send in a service person twice a year to check the controllers and everything in the system to make sure it is all working correctly.

If the guns are in good working condition, the ground is good and the powder is not loaded with fines, you should be able to get your settings back down to more reasonable flow rates and get good coverage. In the future, keep your guns in good repair, maintain clean hooks and consume your reclaim powder fast enough to avoid an accumulation of fines.