Originally Posted by
HI-TEK
jsizemore
Can I direct matters back to the start.
Flaking failures, aside from contaminants/lubes being on cast, for failures, 99% plus, is directly due to inadequate drying which also can result from using far too much coating on the first coat.
I am not including complications from possibility of coated cast not getting up to correct cure temperatures.
Explanation of what I am trying to get to as set out below.
These details are specifically applicable for the first coat, and I have to assume that coated alloy gets to 180C or above during baking.
Oven air temperature quoted, does not tell me, the temperature of coated cast load inside oven.
Evaporation of solvent, chills cast by up to 5C below ambient. This chilling, attracts moisture to the coating.
During drying, a skin forms on surface of coating which is already colder than ambient temperatures.
If too much coating is used, these are main things that happen:-
1. More solvent evaporation, and more chilling of alloy which attracts more moisture and forms thick coating.
2. Drying, results of a formation of a skin on coating surface, and traps moisture that is absorbed from ambient air.
3. Inadequate drying, can be simply exposed, the instant you put coated product into the oven.
Trapped moisture can expand over a thousand times original volume, sooooo, the steam produced, (below dry skin of coating) simply lifts coating off alloy allowing coating to cure, but coating is not bonded to alloy.
A simple test can be done to determine if coating is dry enough or not.
When you think that coating is dry, take only a few, place it into the oven, bake for say 10-12 minutes at 200C.
Don't worry about colour at this stage.
Remove the few from oven, cool and examine.
If surface is irregular and looks like orange peel with fine blisters, you had moisture in the coating and was definitely not dry.
These few will fail tests.
Rubbing with solvent will remove non bonded flaky coating, and smash test will shred coating off as flakes.
Continue drying the bulk, with warmed forced air circulation.
Re test with a few, after longer drying.
Continue drying of the bulk. . Keep testing with a few each time, at exactly same conditions, until you get a pass with tests. ONLY then, bake the bulk lot.
I hope these steps resolve the problems.