Technical information re 'Gassing Out'problems

Due to the unique nature of the processing of castings, the finished castings contain a certain level of porosity. Some castings when cross-sectioned look like a sponge. The level/amount of porosity depends on the casting method (mold type/materials, etc.) and control of the casting process.

In the process of making a casting, molten metal is poured into a mold cavity. To effectively remove the part from some types of molds, release agents are required. If too large of an amount of mold release is used, the excess mold release can become entrapped in the holes of the sponge-like metal during the casting process. This is the one of the steps in the process where contaminants are introduced into the product/part. In addition to the above, when the custom coater/OEM receives the casting, the casting may go through several machining steps before the casting becomes a finished part (e.g., removal of flashing or parting line). Forming/cutting fluids can also become entrapped during machining of the castings. Note: Machining, even sanding, usually makes the problem worse even if cutting fluids are not used because it seals off the porous surface. This provides another step in the process where contaminants can be introduced to the product/part.

Other problems that may cause gassing is, Corrosion, where corrosion has been allowed to fester under paint work and absorbed into the casting, Oil or chemical contaminates.

What is required to remove these soils/contaminants is a several step process. Most finishers send their parts through a pretreatment system (typically liquid power spray cleaning) prior to powder coating. The purpose of the pretreatment system is to remove soils (organic and inorganic) from the surface of the part and, typically, to provide a protective coating/adhesion promoter just prior to powder coating the part. Note: Unfortunately, it is very difficult, if not impossible, for a pretreatment system to remove entrapped contaminants (pockets of contaminants created during the casting or machining processes). In addition, some contaminants may not actually be removed from the surface of the castings if the proper pretreatment chemicals are not matched to the soils that need to be removed. The pretreatment system can also redeposit contaminants if the final rinse stages are not properly maintained. This provides another step in the process where contaminants can be introduced to the product/part.

When the casting enters the oven to cure the powder coating, the part begins to rise in temperature and eventually the contaminants entrapped in the pockets volatilize and expand until the pressure is too great causing the pore to burst open releasing the volatiles into the coating. The out-gassing of volatiles may create defects in the coating and these defects are often referred to as craters, pinholes, and/or fisheyes. There are a couple of methods to reduce the defects caused by out-gassing.
One method to help eliminate defects caused by out-gassing is to volatize the contaminants before the part is powder coated. To achieve this, the part can undergo an“out-gassing cycle.” An out-gassing cycle is the preheating of the part prior to powder coating at a temperature higher and/or dwell time longer than what is required to cure the powder coating. This can take from 1 hour up to one week and in some cases will not solve the problem.

Some metal substrates that have hot-dipped (e.g., galvalume) coatings can also cause out-gassing. Depending on the quality of the hot-dipping process, the substrates can have contaminates or cracks in the hot-dipped coating which may cause out-gassing defects. Note: The thickness of the hot-dipped coating and post-forming of the hot-dipped substrate can further contribute to the out-gassing problem. In addition to the above remedies, please consult with the metal supplier to help alleviate this problem.
In cases where gassing cannot be controlled the only alternative is to us high quality wet paint system and the obvious drawbacks to this is it is costly and less effective in its protection properties.