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Surface defects on powder coated panels

I am having production problems on my powder painting line for the last 4 to 5 months. I have been unable to "fix" the problem.

My company fabricates powder painted panels fabricated from two formed sheets of galvanized steel having a paper honeycomb core glued to the sheets. The sheets and honeycomb are held with corner welded interlocking galvanized sheet steel strips forming of about one inch wide. The sheets have lengths and widths ranging from 3" to 86" to 60" X 75", Within the last few months I have been experiencing "fish eyes" areas of lengths of anywhere from 1/2" to as much as 6" in lengths. These defects are always found at the joints between the interlocking strips with the sheets. These assemblies are processed as follows: Parts are hung on a paint line and passed thru a tunnel where a water/phosphate solution fog wash is applied as the parts exit the tunnel they pass thru a section where excess water is blown-off with compressed air. The parts are then passed thru a dry-off oven maintained at 320 to 365 deg F. (duration of about 6 to 7 minutes)Finally, the parts are painted on a powder spray booth and cured in an gas infra red oven.

These defects do not occur if the parts are allowed to go thru the phosphate solution washing line once, twice thru the dry-off and twice thru the infra-red oven. The first time thru the infra-red oven the parts are not painted. Any idea what can be causing the problem and more importantly any idea how to "fix it"?

Thank You.

Maximo A. Moore PE
General Manager - Deer Park, New York
2004

I think you have answered your own question. The problem appears to be trapped water/phosphate which requires two passes through the blow off & infra red stages. Answer: cut out phosphate use a spray water based primer (anti corrosive type) - apply primer pass through warm air drier - powder coat - bake. Solved the problem - and saved money!

Terry Hickling
Birmingham, United Kingdom
2004

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2004

Dear Maximo A. Moore PE

These problem may be due to 1- the dryer after the treatment tunnel may be case a contamination on the surface of steel so you must clean it

2-the galvanized steel must be free from any brightener which evaporate at hight temperature in the infra-red oven and do not mack any defect with powder coating

3- may be there are some water in the surface of the steel so try increase the temperature and time for dryer

Aly Gomaa
- Cairo, Egypt

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2004

You first have to be systematic on your approach in finding the problem. What may be different or what has changed from the time you were getting good product and now. Painting galvanized metal can have many pit falls. I have seen this problem before and here are some of the things I had to do in the past.

1. The PH of my phosphate tank needed to be in the 4.4 to 4.3 range if I recall correctly (powder coated for 15 years but it been a few)

2. I always made sure that my dry off oven was set to a higher temp than my bake off when I had parts that had seams that could trap water to make sure that it would not be forced out in baking.

3. I also ran into problems with different suppliers of galvanized sheet, some would have out gassing problems that caused pinholes in the paint and others did not (this was also helped by increasing the dry off temp).

Martin Borho
- Ticonderoga, New York