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Three Tips to Solve Spot Coating Issues

Members of the Foil & Specialty Effects Association can submit questions through HelpLinks and receive feedback from industry peers. This question regarding spot coating issues and the responses were sent through HelpLinks in February 2013.

Question:
We have a situation with coating on our Sakurai. When we are spot coating a job, we test the coating with an MEK rub, a scratch test and a tape test. Everything passes. When our customer receives the job and diecuts it, it flakes off on the scores or scratches off. Why does the coating pass our tests, but fail in diecutting? We do know that our UV lights are ok, because we’ve tested them and run other jobs without issues.

Tip #1:
If the ink is not completely dry before the UV coating is applied, it will not fully cure under the coating. This leads the coating to soften again until the ink fully cures. To help with this, we have used a hybrid UV ink – the Sun Chemical Suncure series. The drier we add is Sun Chemical Photo Initiator. We only use it in white ink, however, when two hits of opaque white are layed down on a dark stock and overprinted so the 4-color still pops.

Tip #2:
It is more than likely a wet ink problem, but the printers will never be convinced. Inks can be dried by running them under the UV lamps, but on most jobs that isn’t an option due to time. Also, most printers won’t pay you to do it.

The flaking or scratching is not happening right off press because most UV coatings have a post-cure time. During this post-cure, the inks are losing adhesion. We have changed our procedure and switched to a UV ink with more flexibility and now are curing the UV slightly. This seems to help the ink keep its adhesion properties. It is a very fine line between over-curing and under-curing. We also have spent time trying to educate our customers on the difference between a true UV primer and the work-and-turn aqueous they want to run. There is a huge difference. The primers allow the inks to breathe and the UV to penetrate the aqueous, giving a much better bond to the entire surface.

Tip #3:
As an offline coater, we don’t have the option of hitting the inks with the lamps before coating in the same pass. Sometimes a little under-cure can help, but this is a slippery slope. You’re causing other potential problems trying to fix a problem you didn’t cause. If we are doing the converting in-house, usually a day of rest after coating solves the problem when turn-time allows.

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