How to Limit Outgassing on Castings

Q. I cannot find a way to powder coat aluminum castings without severe outgassing and blisters. Does the temperature have a role to play versus line speed? Will higher temperature emit more gas from the aluminum casting? D.A.

A. Outgassing is caused by trapped air contained within the pores of the cast surface. The air escapes from the cast surface when it is heated and often causes a blemish in the film during cure. The root cause is trapped porosity in the casting. Anything that can be done to improve the quality of the casting and reduce the trapped porosity will help. Impregnation of a resin into the cast surface is sometimes effective. However, in most cases the casting is what it is and impregnation is not practical.

The temperature does have some influence on the amount of air released from the cast surface. Generally, lower cure temperatures will help.

Recommendations to a coater having serious problems include working on the powder formula, preheating and lower cure temperatures. Powder can be formulated with an outgas agent that reduces the impact of the air release during cure. This is sometimes the best solution, but not always effective. Preheating to a temperature above the subsequent cure point has been tried with various degrees of success ranging from very good to not effective at all. A cure temperature in the lower range has also been helpful. You should work on all three ideas to see how successful you can be. One other possibility is to apply a prime coat and then a top coat. This is sometimes a good way to reduce the impact of outgassing, but it also adds a lot of cost.