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Re-curing powder coat
We powdercoat aluminum parts and we have discovered a potential undercuring problem (fairly bad chipping on contact with hard objects and unsatisfactory cross hatch adhesion). While we are also exploring the possibility of contamination, does anyone know the protocol on "re-curing"? Can I send the parts back through the oven for additional cure time and achieve the same results as a complete 1st cure? I have done studies and found the cross hatch adhesion and impact resistance has greatly improved with re-curing, but I just wanted to make sure I wasn't "missing" something.
Roger Downscoater - Akron, Ohio
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Thermo-setting powder is the complete opposite of liquid paint when considering the baking cycle. Under cure of powder will be apparent by a brittle film where as a liquid paint film will be soft and pliable. A powder film will withstand over baking much more than a wet film. The following simple coin check will show this ª on completion of the baking cycle find an area of coating that cannot be seen when in situ. Grasp a coin and drag it across this surface (may require some pressure) suffice to cause some damage.
An under-cured powder film the gouged line will be jagged and smooth in the case of a liquid film. Sufficient cure has been achieved in the case of a powder film when a smooth groove is apparent. Over cure is rarely seen in powder coating and this is noticeable by a colour change where as even slight over cure with liquids will result in brittle films.
I would suggest in your case a 10-15% rise in temperature would overcome your problems and don't forget the more mass you put through your oven then longer or higher stoving schedules will be required.
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Terry Hickling Martex Paints Ltd  Birmingham, United Kingdom |
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Roger, yes, if the paint is undercured you can put it through the oven and cure it again to obtain a full cure, most powder paints can handle a double full cure without the ovebakeing causing cosmetic problems.
But the results of your test show that you have been undur-curing your paint all along, and things are getting worse. You need to put a temp tracker through your oven(s) immediately to find out if you have an equipment or process problem. Your paint supplier should have one(if you don't) and will run the test for you.
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Sheldon Taylor supply chain electronics Wake Forest, North Carolina |