Minimizing Defects in Polyester Wrinkle Powder Coatings

Michael Withers

Q. I really like the appearance and toughness of polyester wrinkle powder coatings but they seem to be very prone to defects – particularly a starring defect. How can I minimize/eliminate these?

I can speak from personal experience with polyester wrinkles. Yes, spraying them can be very different in general. I’ve found that some can be easily charged electrostatically, so if you use the same gun settings for these that you do with smooth polyester powder coatings, then you might see significantly higher film builds and if the gun tip is too close to the part, then it is very susceptible to back ionization.

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It is a best practice to document any defects that occur. You may well find that the rate of defects with a polyester wrinkle exceed those for smooth polyesters. Here are some of the questions that should be answered to resolve a reoccurring defect.

• Do they occur on all parts or just some?
• Are they on one side of the part or all sides?
• What is the gun-to-part distance where the defect is occurring?
• Is the distance different for areas without a defect?
• What is the film thickness of the coating where the defect occurs?
• Is it different than areas with no defects?  

In general, you should reduce the powder feed rate so that only enough powder is coming out of the gun to coat the part to the required film build. Next, gun-to-part distance would be 8" - 12". Manual pre-touch up, if required, could help reduce the amount of kV buildup on the part. Remember to only coat the areas that the automatics aren’t going to cover. There is no need to coat the entire part in pre-touch up when it will go through the automatics.

Additionally, reduce the microamps on the gun to a low setting, perhaps as low as 15 uA. You’ll have to test what works best for your application, but that’s a good starting point. You may have to go up or down based on your part geometry, gun manufacturer and powder coating supplier.

Some powder manufacturers have developed a newer generation of polyester wrinkles that are formulated to be less susceptible to the starring defect that you specifically mentioned. If all the above fails to eliminate the issue, then you might try another powder coating. As always, when you are struggling with issues with any powder coating, your best source for solutions is your powder coating manufacturer. They want to help you use their coatings without issues.

There are also situations where wrinkles are applied over ecoat. In this case, controlling film build is even more important as high film builds will reduce the mechanical properties of the topcoat significantly. One of the issues that can occur is the cured coating can chip off when bumped against another part or some mechanical force is applied to it.

Managing the film build is critical to produce a coated product that will perform in the long run.