Outgassing is a process by which the trapped gases are released through a powder coating in the curing process. The passing of gas through this coating can result in the formation of small holes or pinholes. The occurrence of such pinholes can cause moisture and corrosive substances to come in contact with the surface thus causing numerous surface defects of the product. It can lead to the coating to become ineffective thus making it useless in offering protection to the casted part.
Causes
Cast Metal Surface – Applying powder coating on a cast metal surface such as iron, brass, and aluminum can result in outgassing. During sand and die casting, the gases can get trapped in the casting material that is either from the outside or those produced when the molten metal cools itself. The amount of gas entrapped is dependent entirely on the metal quality and the necessary precautions taken while it is poured.
Surface Contaminants – Substrates having any contaminants on its surface like oil, mold release or grease can cause outgassing when it is applied with powder coating. These contaminants can vaporize in the powder coating while curing thus leading to outgassing. It usually occurs due to improper cleaning of the substrate and the defect caused is the same.
Galvanized Substrates – Powder coating when applied to a zinc coated surface like galvanized steel it can lead to outgassing. Besides, applying zinc to the steel substrate can lead to the formation of gasses that get trapped in the surface coating.
Prevention
Preheating – During this process, the part to be coated is preheated above the cure temperatures. This heating will take the same amount of time used for curing the powder for releasing the entrapped gases. This method is incapable of clearing outgassing fully, especially if the coated part contains several pockets of trapped gases.
Changing Powder Formula – Changing the formulation of the powder can create more flow characteristics can also aid in preventing outgassing. This ensures that the powder will remain in its liquid state for longer periods during the curing process. Trapped gases in the substrate can escape due to the liquid state of the powder, thus producing a smooth surface.
Sealing Part Surface – This method involves the material to be applied in pressurized conditions that are used for sealing the trapped gases. This can clear the issue of outgassing from the substrate completely and is an effective method used by most die cast companies.