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PPCJ’s regular columnist gives advice to readers on all things powder coatings
Q. Dear Joe,
We have been powder coating for about 28 years now and a question recently came up about looking into something that will measure and/or analyse the finished surface for gloss, orange peel or other finish anomalies. We recently ran into three different batches of powder (two different colours) over a two-week period that progressively seemed to get dirtier up to the point it was more like applying a wrinkle finish. (Little exaggeration on the wrinkle finishing.)
We know something must be going on with the powder and this is being addressed, but for future reference, to eliminate at what point we wind up in a no-go situation on our finishes, it was asked about something to take and compare previous finish/surface readings to take the guess work out of the situation.
Any suggestions much appreciated.
Dale Myers, New London, Wisconsin
A. Hi Dale,
Thanks for the message and your questions. This sounds pretty frustrating. There are many actions you can take to define and mitigate problems like these. You mention that you’ve seen this with three different batches involving two different products. Are these from the same powder supplier? Regardless, here are some things to do, if you haven’t done them already:
- Isolate variables such as powder, substrate and environment (storage, application area, oven, etc.) Defects can emanate from all of these.
- If you suspect the powder is contaminated or defective, isolate it from the environment and substrate. Do you have off-line capability to spray and cure powder samples? If not, you should think about building a small lab system in your quality department to evaluate powder off-line. Alternatively, you can send the powder to an independent lab for them to spray and cure the powder independent of your production environment. In addition, you should ask your powder supplier to respray retain samples of these batches in their quality lab to demonstrate cleanliness. Proactively along these lines, you should require your powder suppliers to furnish a lab test panel of every batch that they ship to you. With each, there should be a certificate of analysis that reports colour vs. an agreed-upon standard, gloss, cleanliness, impact resistance and solvent resistance.
If you determine that the powder(s) are inherently dirty, try to isolate the extent of the problem. Are the defects relegated to a few containers of powder or the entire batch? This will help your powder supplier zero in on where the problem is and hopefully what caused it.
- Regarding substrate, you should compare production substrate/parts to lab test panels. If the defects are caused by irregular or unclean production parts, the lab test panels will exhibit good surface, colour and gloss. If the defects are inherent to the powder, both the lab test panels and the production parts will exhibit the same appearance.
If you discover that the defects are substrate related, initiate a quest to determine if the parts have serious surface issues that are difficult to clean or remove. Oxidation, smut, mill scale, weld spatter and aggressive oils (e.g. HRPO steel) can cause surface defects in a powder finish if they are not eliminated during your pretreatment process. Inspect incoming steel and fabricated parts for unusual surface quality. Inspect the surface before and after the pretreatment process to see if the surface is clean and properly pretreated. In addition, ensure that parts are thoroughly dry before they enter the powder spray area. Residual water can cause surface defects like the ones you described.
- If you want to isolate the oven atmosphere, you can cure parts in an off-line oven if one is available. Alternately, you can coat lab test panels or a small production part and enclose it into a metal can then run it through your production oven. Be sure to pop a few exhaust holes in the can before introducing it to the oven. Most powders emit a little vapour during cure. This can contaminate the surface of coated items. If you find that the oven is causing surface defects, you may want to visually inspect the oven atmosphere to determine if there is any vapour build-up in the oven. If there is, you should engage an oven expert to adjust your oven air turns and exhaust conditions.
- Another recommendation is to inspect the surface defects with a magnifier. You can use a visual or digital magnifier. 10X is usually sufficient to help define the nature of the defect (“wrinkling”, gassing, protrusion, voids, etc.) This will help you zero in on the mechanism of the defect.
I hope this helps you in your quest to eradicate these surface issues.
BTW, is Jeff Stillman still running operations at your plant? If so, tell him Joe sends his regards,
All the best,