Powder Coating Q&A: Reducing Outgassing
Our outgassing blemishes are costing us time and money.
Q. I am facing outgassing problems on zinc die casting components after powder coating. Our parts are highly aesthetic and outgassing blemishes are costing us time and money.
A. Outgassing will occur with any cast material. When the molten metal hardens after casting, the core of the part has pockets of porosity, trapped air and impurities that form pockets at or near the cast surface. Porosity in the cast material can erupt during the cure cycle and leave blemishes in the coating. The severity of the outgassing depends on the quality of the casting and on other steps taken to minimize the issue. You can X-ray a casting to determine how bad the porosity is before you coat it, which can be useful if you are comparing two potential sources or if you want a process control measure from batch to batch.
The best way to deal with outgassing is to impregnate the casting with a resin material, which seals the casting and nearly eliminates the problem. Dry vacuum pressure impregnation uses two process tanks. During this process, gases are pulled from the pores of the casting and no liquid is present during the initial vacuum stage. This process can be used to remove moisture entrapped in pores. Wet vacuum pressure impregnation requires only one process tank. With this process, parts are submerged in the sealant and the vacuum is applied to the parts and sealant simultaneously, followed by pressurization with air.
With each method, a sealant cures inside the part, preventing leakage of gases and fluids. Residual sealant on the part’s surface is carefully cleaned before further processing. This can be a very effective way to eliminate troubles with casting porosity, but the equipment is expensive and few companies are willing to invest in the process. You can have it done as a service, but that is also costly.
Other options that can limit the impact of casting porosity include using powder designed for castings and preheating the casting before coating.
Powder can be formulated to enable more flow before hardening, giving the trapped air and gas time to evolve through the film before it gels, limiting craters in the film. A powder designed for this purpose can be very effective.
A casting can be heated up to a temperature around 50°F above the subsequent cure temperature, which will help to release some of the air pockets before the coating is applied. Before the coating is applied, the part should be cooled. A lower temperature cure powder provides less stress on the casting during the cure cycle.
These methods can help to limit the issue, though not completely eliminate it unless the casting is impregnated.
Originally published in the May 2016 issue.
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