Coating Aluminum Castings, Part 2

Question:

I was reading the Powder Coating Clinic in the November issue of Products Finishing. In that clinic, the company is experiencing problems when coating an aluminum speaker frame. They explain that they are using a silver metallic and that they don’t have any trouble right away...it’s only after spraying for a while. I agree with you that poor powder reclaim and/or grounding could cause this kind of defect. There’s also another phenomenon I’ve seen on occasion. I’m not even sure what to call it but it’s a similar problem to what’s happening on these speaker frames. Please keep in mind, I’m only talking about metallics (blended and/or bonded). For example, I was at a facility that was having a problem with a bonded silver metallic. They would start up their automated system and start running production. After a while, the KV’s would start to drop off and eventually go down to zero. What we found was that the metallic ingredient of this powder coating was rubbing off/smearing off inside the feed tube, pump, powder hose and even the gun itself. It seemed like this was creating a conduit to ground, which was making it difficult or impossible to charge the powder particles themselves. By cleaning out the gun, hose and pump or by just changing the powder hose, the problem would be gone, temporarily, until the same thing started happening all over again. Just thought I would pass this information along. R. W.

Answer:

You are absolutely right; corona-charging guns can short out when using metallic powders. All corona guns have built-in safety current limitation circuitry to ensure that the gun cannot draw an arc if it is short circuited to ground. In this case, the voltage drops to zero and all powder charging ceases. This is why most gun manufacturers sell metallic muzzles that have their electrodes outside the powder stream. That way the powder coating cannot short circuit the gun. My Grandfather always said: “You must have the right tool for the job.” (I guess that’s why my tool box “runneth over” with every type of tool imaginable.) In this case, take my Grandfather’s advice and use a metallic nozzle on the gun and the problem you describe will simply go away.