Powder Coating Q&A: Coating Wire Welded Mesh Areas
How to improve penetration into welded areas.
Q. We manufacture a line of high quality outdoor furniture and a lot of our chairs and lounges have wire welded mesh seats and backs. The points on the parts where the wires cross always give us trouble with coverage. A year ago, we started to use an e-coat primer on everything to reduce failures from rust at the welds and the edges of our parts. This has helped add substantial corrosion resistance, but we still have some trouble with coverage in the weld area. If the topcoat is black, it does not show as much but many of our colors have visible misses in this area and we have to recoat them. We use tribo guns in an all-manual operation. Is this the correct gun for this type of product? Will automatic spray guns do a better job in the weld area? Is there a way to improve the penetration into the welded areas?
A. This is difficult to answer because of there are so many application issues that affect coverage in these areas. The difficulty begins with the geometry of the affected area. When a part has recesses in the surface, they can be hard to cover due to electrical resistance. This is referred to as the Faraday Cage Effect. The wire weld areas have significant electrostatic resistance in the Faraday areas as well as aerodynamic issues that can occur as the powder and air travel around the wire and intersections. The e-coat provides some insulation effect, which will magnify the Faraday Effect.
The theoretical advantage of the tribo guns is the minimal amount of external charging field the guns generate. The lack of strong lines of force from the gun can help with penetration. While good in theory, it is not very effective in practice. Tribo charging relies on frictional charging inside the gun barrel. The velocity of the powder has to be sufficient to do a good job of imparting a positive charge to the powder. Particle size and chemistry are critical to success. Tribo guns often have relatively poor transfer efficiency and other issues. I would not recommend them for your manual coating operation. Rather, I would test a corona charging gun. Newer models have excellent control over the micro-amp levels and they can be very effective at coating Faraday areas when used correctly. They charge more effectively and can be used at lower flow rates to help avoid the aerodynamic issues of higher velocity. Run some trials and see if a newer gun design will help. Be sure to limit the current draw and the powder velocity.
Originally published in the October 2015 issue.
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