Q: Our current product, which is an aluminum sand casting, is powder coated to many of our customers’ color choices. We find that we have many more problems with outgassing or bubbling on colors we consider as higher gloss than other colors. This may just be a coincidence, but we were wondering if the gloss levels can affect the susceptibility to outgassing because of a faster or slower set or something else we are unaware of.
A: This is an interesting observation. This situation requires one to slip on their “correlation vs. causation” helmet and sort through the facts. It sounds like you have established the correlation factor here. Nice work. Root cause analysis is a mightier task, but I will provide some insight to help you in understanding the outgassing phenomenon and how it relates to powder coatings.
Fundamentally, as you know, the porosity in castings causes the expulsion of entrapped gas at elevated temperatures. Powder coatings typically require 10-20 minutes at 350-400°F to cure. These baking conditions will cause pores to expel gas. If the gas is emitted during a critical stage in the powder coating crosslinking reaction, the escaping gas creates an ugly crater-like defect.
We powder formulators have developed formulation schemes that can alleviate the formation of these kinds of defects. We basically incorporate an additive that allows the gas to pass through the film and then reflow into the void created by the expulsion. Some powder suppliers call these products off-gassing resistant (OGR) or out-gassing friendly (OGF). I’m sure other monikers exist. These products work pretty well, but this formulating trick is not available in every product; hence, the probable disparity in performance you are observing.
A solution to this problem could lie in a process alteration. Preheating the casting before powder coating usually eliminates the defects caused by the outgassing of castings. Perhaps you could segregate the special, high-gloss powder coating runs and preheat the parts prior to applying the powder. Alternately, you could bug your powder coating supplier to modify the high-gloss products with their secret sauce.