The Cause of Craters in Powder Coating

Q: We have been having trouble with craters and small defects in our zinc die-cast components after powder coating. Appearance on our parts is critical and the blemishes are costing a lot of time and money. Our powder supplier says the blemishes are being caused by outgassing and we should turn down the oven temperature. We have tried this, and the parts are a little better, but we still have some defects. Our cure is good enough but not great. Any ideas of the root cause and what to do to improve it?

A: It certainly does sound like classic outgassing, which can occur with any cast material. When the molten metal hardens after it is cast, the core of the part has pockets of porosity, trapped air and impurities that form pockets at or near the cast surface. These pockets of porosity in the cast material can erupt during the cure cycle and leave blemishes in the coating that can be depressions like craters or small bumps. The severity of the outgassing depends partly on the quality of the casting and partly on other steps taken to minimize the issue.

You can X-ray a casting to determine how bad the porosity is before coating it. This can be useful when comparing two potential sources or for a process control measure from batch to batch. It can help identify a change in quality from the supplier.

The best way to deal with outgassing is to impregnate the casting with a resin material after casting. This seals the casting and almost completely eliminates the problem. This can be effective, but it requires special equipment in-house or working with an outside contractor. It is more efficient with small parts.

Other options that can help limit the impact of casting porosity include use of a powder designed for use with castings and preheating the casting before coating.

Powder can be formulated to enable more flow before hardening. That gives the trapped air and “gas” more time to evolve through the film before it gels, limiting the craters in the film. A powder designed for this purpose can be effective in some cases. It is not likely to completely eliminate the problem but it can radically reduce it.

A casting can be heated up to a temperature that is around 50°F above the subsequent cure temperature. That will help to release some of the air pockets before the coating is applied. The part should be cooled before the powder coating is applied. A lower temperature cure powder is a good idea to provide less stress on the casting during the cure cycle. Lower cure temperature helps, as you have already discovered. Polyester powders typically need minimum temperatures of around 325°F, and low-temperature epoxy powders need at least 250°F to reach cure.

These methods can help to limit the issue. It is hard to completely eliminate it unless the casting is impregnated.