Bubbles in the Finished Product

Question:

We have been powder coating park benches, garbage cans, etc. for a few years. My issue is with bubbles in the finished product. The substrate is CRS with e-coat. We use Poly-TGIC, as well as TGIC free, several suppliers and variety of colors. My customer also has other powder coaters, who all have the same issue. The electrocoater is the only constant. I don’t know much about the e-coat process. Could that be the cause? I have seen the same type of bubbling when powder coating over aerosol spray paint, so I suspect they may be using spray paint touch-up without knowing the problems it causes.  Could it be something else? Please share your experiences. M. T.

Answer:

Powder coatings are routinely applied over e-coat materials without bubble problems. What you describe sounds more like contamination that is vaporizing in the cure oven under the powder coating. I suppose that if the e-coat is not fully cured then it can be solvent popping through the powder coating surface, but I doubt that this is your problem. However, if your customer is solvent spraying their parts after e-coat (touch-up) then you can have solvent popping through the surface when the part is subjected to heat during the powder curing process.

Without knowing anything else, it is very difficult to be confident what is the best solution. I can recommend several things for you to try, however. First try curing your powder coating at a lower temperature for a longer cycle time. This can eliminate solvent pop. Next examine your cleaning prior to powder coating. I suspect that you are not doing much cleaning now since the parts already have an e-coating on them and appear clean. You may want to look at a neutral cleaner and DI rinse before applying the powder coating. This will remove any surface contaminants that may be causing the bubbles. I am sorry that I can’t be more definite.