Q. We've powder coated automotive parts with a single-color, black epoxy for around a year. At first, we had few rejects and good transfer efficiency. However, our application efficiency gradually declined so we spray more powder to get the same coverage that we used to get at lower flow rates. Our film build is less consistent, and our material usage per part has increased. Our application equipment vendor says there is nothing wrong with our guns but suggested we keep our racks cleaner for better ground, and we began cleaning the racks more often. This helped a little, but we're still not getting the coverage when the system was new. What changed and how can we regain that initial efficiency?
A. Your system is no longer brand new, so it is logical to look hard at maintenance. The answers can be found by reviewing how the system has changed since startup.
The suggestion to clean the racks is an excellent start. Good earth ground helps to ensure good transfer efficiency and uniform film build and wrap. Poor ground affects the way charged particles will transfer to the substrate. Larger particles hold more electrostatic charge for a longer period of time, therefore they can transfer to the substrate easier than a smaller powder particle. If you have poor grounding, your system will start building up fines (an excessive amount of reclaimed finer particles) that will cause orange peel, as well as uneven, low film build and coverage in critical areas such as Faraday areas (inside corners and recesses).
Make sure your loading contact points that attach to the conveyor are clean. Get the resistance checked between the conveyor chain and a loaded part. The resistance should be below 1 mega-ohm. Check the wear parts (hoses, venturi inserts, connectors) in the powder guns. Check that operators are keeping the powder path inside the delivery system clean. Make sure your regulators work properly. Test your gun tips and make sure they get the proper voltage.
I also suggest you get a powder particle size analysis from your supplier to determine if your powder is accumulating finer particles. Check the virgin particle size against the reclaim particle size and see how much it has changed. You may find that you have broken down the powder and you now have too many smaller particles (less than 20 microns). Try cleaning out the hopper module and introducing all virgin powder to the system.
Once the system is in good condition, record your settings. Use that to measure changes over time and set up maintenance practices to keep the system within a good range for application.
Originally published in the February 2017 issue.