Powder Coating Q&A: Recoating Adhesion Problems
How to recoat a part without textured film.
Q. We occasionally need to apply a second coat of powder due to minor defects such as light coating or dirt. When we apply the second coat, we often have problems with poor appearance and a rough, textured film, especially around the edges of the part. We also have problems with inner-coat adhesion. How can we recoat a part without causing these problems?
A. In any discussion of a problem, I like to start with the root problem. In this case, defects and dirt. Put some time into evaluating the causes of the defects and see if you can reduce them. Light coats and other appearance defects are frequently related to a lack of process controls and operator training. Accurate and consistent gun settings, a controlled environment, good maintenance and understanding electrostatic application can help reduce defects and requiring less recoating.
When you do have parts to be recoated, consider the powder material and test it to find out how well the two coats will adhere. Many powders can be applied directly to the first coat with no issue. Some powders need to be lightly sanded to create a rougher surface for better bond of the second coat. If you are running the parts in the oven for a time above the recommended cure, you will have more trouble with inner-coat adhesion. Do some testing to figure out the proper cure time and make sure the two coats will bond.
For application of a second coat, make a few simple adjustments to the spray guns for good results. Recoated parts must have 100 percent coverage to avoid a “dry-spray” look. Parts coated once will be more resistant to electrostatic attraction, so the electrostatic load line should be different. The ionization from the gun will rapidly build on the surface because there is no simple path to earth ground. As the powder begins to build on the surface, the excess ionization will interfere with electrostatic attraction and generate more texture and inconsistent appearance. Powder will have difficulty building near edges and “stars” may appear in the coated surface as the free ions build at the part surface. This can lead to severe back-ionization and unacceptable appearance of the powder film. To overcome back-ionization, the current levels must be limited to avoid excess ions from building on the surface.
Typical adjustments for recoating include a reduction of current levels, a slightly farther gun-to-target distance, and a slight increase in flow rates. This will work much better if the recoats are done in batches rather than being mixed in with raw parts. If you run the parts alongside raw parts, you can still reduce the micro-amps a little and make it work better. Also be sure of the presence of a good ground connection. It is always important to have good ground to the parts, but it is even more critical for recoats.
Originally published in the October 2015 issue.
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