Colors Wrong After CuringWe recently installed a small, used powder coating line to begin a transition from liquid coating to powder. Our liquid coating supplier has color-matched several of our stock colors and we have been successful with these colors. In the past two months we began to take in work from another source and had to use the manufacturer's specified powder materials. The problem is that these colors are not coming out right. The powder supplier tells us that our cure oven is too short to provide adequate cure time. The oven cycle time is eight minutes at 400ºF. Our oven works fine on our colors but not on the customer's colors. We have asked the powder manufacturer to reformulate his material to work in our oven, but he says he cannot do that because it would be a different product than what he supplies to the manufacturer whose parts we are coating. He says we need to slow down the line or add length to our cure oven. Why does the oven work well for some colors and not for others? Is there some simple way to change the other powders to make them work in our oven?
Typical cure requirements for powder coatings are 10 minutes at 400°F substrate temperature in a convection oven. That means that once your part has reached 400°F, it needs 10 additional minutes of time in the oven to cure. Depending on part thickness, it can take quite a while for parts to reach 400°F. It is not uncommon for powder coated parts to have 30 to 45 minutes of total dwell time in the cure ovens.
It is important to note that there are powder coatings that will gel and flow very quickly. They may even look like a fully cured coating. Color, gloss, and physical properties may not be quite right. Impact resistance, adhesion, gloss and color are typically not up to specification on undercured coatings.
The powder that you are trying to use is most likely on the manufacturer's specification. It probably can not be reformulated without being submitted for reapproval, which is probably why the manufacturer was reluctant to reformulate.
You could also consider adding an IR booster to your cure oven. It may allow you to cure the parts without slowing down the line or adding length to your oven. There are issues associated with any suggestion that you may consider. It is strongly recommended that you do thorough testing prior to making any final decisions on your coating line modifications.
Corrosion of Gold Leaf
There are several things that could be causing your parts to rust. If the gold leafing was left uncoated after it was applied to the powder coating, it is possible that the corrosion you are seeing is from the gold leafing alone.
If an aggressive blast media was used, 2 mils of powder coating may not be enough to cover all of the peaks and valleys left by the blast media. There could be sharp peaks sticking through the coating that would then corrode when exposed to the environment. You would also most likely see more corrosion on the areas of the parts that have the thinnest film build.
If the parts were not sufficiently cleaned, then any soils left on the substrate could cause coating failure out in the field. When blasting is utilized, ensure that clean blast media is used to prevent the impingement of dirty blast media onto the part.
Proper film thickness is critical to powder performance-especially corrosion resistance. Be certain that you adhere to the specifications set forth in the technical data supplied by your vendor.
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