Pinholes in Epoxy

Q. We are dealing with epoxy powder for insulating coating. The process is like this:

1. Cleaning of bars done by sanding and buffing

2. Pretreatment of the bars using a three-tank process

3. Preheat or dry-off oven to remove moisture

4. Automatic electrostatic spray booth

5. Curing oven

After the bars come out of the cure oven, they are tested for pinholes in the coating. We do a flash test using 2.5 kV. Since last month, we have started getting more pinholes in our bars (ranging from 2 - 10 percent). This level of pinholes is very bad news for us. I need your suggestions for what we can do to overcome this. —​G.H.

A. Pinholes could be caused by substrate contamination, outgassing or substrate temperature. Make sure the part is clean and dry before coating. Make sure your rinse water is clean and that there are no drips leaving salt mineral deposits on the part. Check for outgassing, which could be caused by brazing material, plating, or bends in the part. You also may be able to get some relief by preheating the part to thermally remove contamination and relieve trapped air pockets. If the metal is too cold or too hot it can also cause pin-holes. Finally, a cure temperature that is too hot can cause pin-holes.

The fact that the pinholes are mostly along the lower edge suggests that something is running down the bar during pretreatment and building up along that edge. I would review the pretreatment process and try to improve your cleaning and rinsing. If you are using a poor quality rinse water, replace it with deionized (DI) water or water filtered by reverse osmosis (RO). DI or RO water will help ensure removal of all residual minerals at the end of the washer.