The term outgassing is a reference to the release of trapped air in a
cast substrate during the cure cycle. Porosity in the casting traps air.
When the casting is heated, the air expands and blows through the
partially gelled coating, leaving a small defect like a pinhole.
Outgassing is a serious problem for powder coaters. The severity of
the problem is dependent on the type of casting and the quality of the
casting. Generally, aluminum is not too bad, while zinc die cast can be more of a problem and sand castings are the worst.
Methods for dealing with this problem are limited.
If a manufacturer has control over the casting process, they can work
to minimize the contamination in the casting process, x-ray castings
for porosity as a process control measure and use an impregnator to
fill open porosity in the cast surface. Many times these measures are
not available or not practical.
Two widely used methods of dealing with outgassing are preheating
and the use of "outgassing forgiving" powders.
Preheating means bringing the part to a temperature in excess of the
cure temperature prior to coating. For example, running the part
through the dry-off oven at 450 °F and curing at 350 °F. The theory
is that the trapped air will escape in the preheat and no more will
come out in the cure oven.
Actual field evidence is that this can be an effective way to reduce the damage of outgassing by a large margin.
Outgassing forgiving powders are designed to provide a longer flow
cycle so that the air can escape before the powder has gelled. This
also has been demonstrated to be an effective way of reducing
June 14, 2012