Powder Coating Q&A: Application of Platinum over Black Coating
How do you coat platinum over black coating to eliminate rust issues?
Q. We are having a difficult time trying to powder coat chair frames purchased from China. They are shipped with a black coating, which is supposed to eliminate rusting issues, but we know the steel quality is poor. We use freshly burned-off racks so ground doesn't appear to be the problem. The powder we are having trouble with is platinum—the chairs we paint black over black turn out fine.
We typically apply powder on recoats at 60 kV so this is where we started with the platinum over the black chair. Our automatic equipment is about 12 years old, so we are not able to adjust the micro-amps unless we paint the chairs with hand guns. We have mixed results regardless of which equipment we use. Most days, we have up to a 50 percent rejection rate with our auto-coater. We have tried kV settings from 50 to 80 and various gun distances. The biggest issue appears to be back-ionization and what appears to be light paint, regardless of the gun settings. Our wash tank system has an alkaline cleaner in stages one and three; stage six has a “dry in place” chemical, which I don’t believe is great for recoats. Do you have any suggestions?
A. It sounds like you have a highly conductive powder that you are trying to apply over a surface that is largely insulated. Not a good combination for bleeding off electrons from the corona field. Back-ionization can happen instantly.
The best solution would be to remove the black coating. The normal approach over a coating would be move the guns back a little, increase the flow a little and control the micro-amps to around 20. That does not seem to have helped you much with your automatic guns. In your case, the powder is clearly a major variable, since black-over-black seems to work well. It should be easier based on the difference in charging characteristics of the black powder when compared with any metallic powder. One possible way to improve deposition is to experiment with changes in the powder. A reduction in metallic content will make it easier to apply, but will also change the color. Different silica in the powder could have a profound effect without change of color. Talk to your powder vendor and ask them to supply some silica for post-add that you can experiment with. Try at least two different silica materials. The silica is hard to blend, so use small amounts in small amounts of powder. This could move the polarity of your powder substantially. If it works, they should be able to change it during production.
Another approach would be to preheat the part. But this is probably not practical for you, especially in a production run.
A conductive prep-coat may also help. The old industry standard for conductive materials was Rans-Prep, designed for electrostatic applications over non-conductive surfaces, but it is no longer offered by Ransburg (the original source). Variations are available on the market, but if adhesion is still a problem, you would need to roughen the black prior to applying a prep-coat.
Finally, consider newer spray guns with better control over the micro-amps. That may help with all of your powder materials and provide some return by reducing overspray.
Originally published in the December 2015 issue.
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