Q: We have been experiencing a gloss issue on our beige powder coat. The gloss range we shoot for is 77-87.
The problem occurs when we run the parts like we have for two-plus years on our line. They will come out good one day and then the next may have five of the 15 parts come out with low gloss (59-66 range). Then the gloss starts to climb up through the run.
We have tested film thickness, MEK resistance, adhesion, and ran a Datapaq™ oven reader – nothing is out of control. In fact, when we have had one good part and one bad part, the MEK, thickness and adhesion all come out the same.
The oven temperatures have not changed for these parts in quite some time, but this issue has just appeared. We thought it was something with the powder, but we still have some good parts and some bad without changing batch numbers on the powder.
A: What a problem! It sounds like you have investigated the usual suspects. I have a couple thoughts.
First, has the pretreatment changed at all? Are the rinses all running in specification? Could there be a residue that is volatilizing and causing micro-pinholing? Have you looked at the lower gloss surface under magnification? Does this tell you anything about the surface of the coating, such as micro-wrinkling or a high density of pinholes? Is the gloss consistent across the part or blotchy?
Another remote possibility is that the powder supplier has blended two powders to “work off” a batch slightly out of specification (usually for color). Blending two batches of powder can cause a gloss drop. I have seen this phenomenon only once or twice in the field, but it definitely caused the inconsistency that you are describing.
Magnification could help discern this possibility. If there appears to be two different colors under the microscope, that could support this theory.
Another thing to try is to deliberately vary the cure by spraying test panels offline. You could conduct a bake ladder of undercure, standard cure, and overbake.