We recently switched to a powder coat with Teflon® in it. The color is a bronze with gloss. Our associates generally shut down the line during lunch; parts remain in the oven during this time, approximately 30 minutes. We have some issues with unexpected color change—some of the parts have a green tint and some are much darker. It’s assumed this is due to us shutting down the line during lunch and leaving parts in the oven. L.N.
Yep, you are absolutely right. You are seeing what over-baking your powder coating looks like first hand. A thermoset powder coating typically has 100% over-bake resistance built into the formulation. This means that the coating can remain in the oven for twice the time at the prescribed cure temperature. For instance, if your powder can cure in 10 min at 400°F, then the powder will be fine at 20 min at that temperature. However, cure time and oven dwell time are completely different animals.
Oven dwell time is determined to be the time it takes your parts to achieve the cure temperature for the powder coating plus the cure time at that temperature. For instance, if it takes 15 min for your parts to reach 400°F and your powder cures at 10 min at that temperature, then the oven dwell time is 15+10=25 minutes.
Now lets look at your problem. Lets say your operators stop the conveyor for a 30-minute lunch break and you have a 25-minute oven dwell time (as described above). Some parts can be in the oven for 55 minutes or more. Using the above example, that would mean that the parts were over-baked for three times (300%) the over-bake resistance of the powder. As Martha Stewart said entering the jailhouse: "This is a bad thing."
I don’t recommend that you turn your oven temperature down during lunch breaks, since the oven temperature will take some time to cool and you will end up with the same problem. What I do recommend is that you put a break in the conveyor at lunch to ensure that the oven is empty of parts during lunch. This is how a staggered shift works. Either that or have a relief crew available to run the line during lunch to avoid stopping the conveyor at all.