Powder Coating Q&A: Micro-Porosity in Cured Coating
Q. We have had an ongoing problem with micro-pores for the last several years that our chemical supplier, our powder gun manufacturer and about six powder reps have not been able to shed any light on. When the parts are inspected it looks like thousands of very small pin pricks.
We blast 99 percent of our product with 80-grit aluminum oxide, which imparts a manageable profile without leaving ferrous residue. We have a reclaim unit that filters the media of any dust and debris. Over this time period, we have used three suppliers with no change. Our air system has an air dryer that gets to at least 38° dew point and a three filter arrangement to remove any aerosol contaminants. The powder gun has a dedicated earth ground through the concrete only a few feet away and the clean hangers (one use only) also use this ground.
We use polyester for the majority of our work. Zinc rich epoxy is used as a primer on occasion. We use a box feed system that is blown out with compressed air after every color change. We only apply the manufacturer’s recommended thickness which typically is 2–4 mils.
To reduce possible back ionization, we currently are applying powder at two micro amps and the parts typically draw 20–50 KV when the gun is 6 to 8 inches from the part. We have tried different powder flow rates and air flows.
We have used at least six different powders on aluminum, steel, stainless steel and even purchased test panels with no difference. We suspect the problem is back ionization, but we’re unsure how to eliminate it.
A. I have seen some coatings that would fit the description you provided. It seems to occur more often in polyester powder, though it may also occur in some other resin system, frequently more often in higher gloss powder and metallic colors. If the material is the issue, the cause may be some missive component that evolves from the film during cure and leaves the small pin-pricks you describe. If this is occurring in several colors and different resin systems, the powder is not the culprit.
Normally, micro-porosity in the coating that is not related to the material is caused by one of two things: contamination or application.
There are a few subheadings there that relate to a particular type of contamination or other issue.
It is most likely not the substrate, the powder, the preparation or the compressed air, although it is hard to rule anything out completely without a design of experiment (DOE) to be certain. A good DOE would lead to a strategic method of finding an issue unlike an intuitive or irregular experimentation process.
That leaves a few things to consider:
- Hopper contamination
- Powder feed system contamination
- Airborne contamination in shop
- Airborne contamination in oven
- Excess film thickness or ionization
I would also include the compressed air and equipment in the test process. The process to work on is elimination of these items one at a time; use a different hopper, use a different feed system and gun, use a different location, use a different oven or have your parts and powder used in a completely different location with different equipment. If the micro-pores do not occur in another location then you know there is contamination in the system. Make sure the sample size is large enough to give you adequate information.
If it is back-ionization, it is caused by excess negative ions in the applied film, though this is unlikely because the level of charging you are using is extremely low. If you want to test this, you can adjust these items one at a time or in combination and measure the impact on the problem: micro-amperage, voltage, gun-to-target distance, flow rate and film build.
If you try everything listed you should find the issue. Be sure to isolate one idea at a time and carefully measure the results. You can also use a microscope to look at the surface and see if magnification helps to show anything that could help.
Originally published in the June 2016 issue.
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