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Jan 1, 2010 12:00 AM
Testing for proper cure | When UV ink cures, it changes chemically. Measuring and analyzing this change is possible under laboratory conditions, but out on press, it's a bit more subjective. The main objective always should be satisfying the customer, thus it might be more appropriate to say UV ink is adequately cured once it behaves as the customer wants. Some of the principal methods for testing cure are methylethylketone (MEK) rub tests, tape adhesion tests, coefficient of friction (slide angle) tests and potassium permanganate swabbing.
Testing for proper adhesion | Adhesion testing measures how much effort is required to remove an ink from the substrate (“very easy” indicates poor adhesion and “difficult” signals good adhesion). Many factors influence an ink's adhesion to the substrate: cure, brittleness, surface energy and other chemistries. Testing for proper adhesion is most commonly done via tape test.
Cure | If the ink remains wet or is easily removed, lamps might be underpowered. Increasing lamp power might increase curing.
Brittleness | High cross linking can cause higher rigidity or brittleness, perhaps from too much UV energy (overexposure).
Surface energy | If ink surface tension is not matched to the surface energy of the substrate, it might crawl or adhere poorly. Pretreating the substrate, e.g., corona treating, will solve it.
Other chemistries | After an ink is printed, adding another layer or coating can weaken the bond to the substrate.
Eliminating glossback | Glossback is the reduction in gloss of a printed UV ink 24 hours after printing, primarily caused by underlying ink not being fully dry/cured. Hybrid UV inks have eliminated most glossback concerns. If the printer tries to UV coat over wet UV litho inks and then cure them all at once, gloss will be lower vs. allowing the UV litho inks to cure first.
(Thanks to NAPL's Ray Prince and Wikoff Color Corp.'s Don P. Duncan for their insights.)