How to Address Declining Powder Coating Coverage Over Time

Q: We manufacture automotive parts, including springs and other parts that are used under-body. A year ago, we built our own powder system to do our own coating in-house. Everything is coated with just one color, a black epoxy. At first, it was working well, and we were happy about having control of the process here at our own plant. Over the course of our first year of operation, parts started to come out with thinner film build and misses in some places.

Our operation is automatic, so it is not a case of bad operators. We have increased the flow rate of the powder, which helped a little, but our powder material use is worse, and we still get light coats. Our application efficiency has gradually declined so 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 material cost is higher. Our application equipment vendor checked the powder guns, and there is nothing wrong. We tried cleaning the racks more often for better ground, and it helps but the coverage still is not as good as it was when the system was new. How do we get back to optimal performance?

A: You are right to ask about what has changed since you started. Clearly, the system is not in the same condition it once was. Your racking may be a part of the issue. Since it has been used, it probably has some damaged or missing hooks. The suggestion to keep the racks clean is an excellent start, but also be sure they are in good repair with no missing or bent hooks. Earth grounding is essential for uniform film build at reasonable flow rates. Dirty hooks can cause a loss of ground and poor deposition of powder.

As you’re only using one color, I will assume that you use a cartridge module reclaim system. The module system will collect 100% of your overspray in the fluid bed at the base. You need to understand that the oversprayed powder is not the same as the fresh material from the box. The distribution of particle sizes will change. Specifically, more fine particles are in the recovered powder by percentage than what comes from the factory. If you have been running at higher flow rates with poor efficiency, then you have probably built up more fine powder particles in your reclaim system than you can readily consume. Larger particles have 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. The result of a higher percentage of fine particles is poor transfer efficiency, more orange peel, uneven and low film build and poor coverage in Faraday areas (inside recesses). More fines will create more rejects.

Get your racks clean and measure the earth ground with an ohmmeter. Resistance to ground should be below 1 megaohm. Measure your particle size in the collector and compare it to the virgin material. Chances are good that the percentage of fines in the collector is much higher than the virgin 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.

Also, check out your guns to make sure all wear parts are in good condition and the guns are delivering the proper voltage. If the guns are in good working condition, the ground is good and the powder is not loaded with fines, then 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.

---

Originally published in the July 2020 issue of Products Finishing.