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May 1, 2008 12:00 AM
To understand where hybrid UV is today, you have to understand its roots. In the mid-1980s, some progressive package printers tried combining the best of the conventional and UV printing worlds with the wet trap application of UV coating over non-UV or conventional inks. Initially, things looked promising — freshly printed, coated and cured sheets looked glossy and felt dry. But the gloss level soon decreased dramatically.
The heavier the coverage, the more the gloss decreased, particularly when underprinted solids or screens were involved. Other problems included failure of the adhesion between the ink and the UV coating — in some cases the conventional ink never dried under the UV coating.
To address these issues, presses were configured with inline multiple coating units or double coaters. “The idea was that by applying a water-based primer or sealant coat over the conventional ink and then drying it, you would eliminate the intimate contact between the conventional ink and the UV coating, just as if you dry trapped it on a separate press pass,” says Bill Bonallo, director of sheetfed sales and business development, IST America (Bolingbrook IL).
But the same problems persisted until ink, primer and UV coating compatibility issues were resolved, paving the way for hybrid UV technology. Replacing some of the ink's conventional oil- and solvent-based components with UV-curable ones helped address gloss back and intercoat adhesion issues. “Conventional pigments in the formula, combined with UV vehicles and a photo initiator package, led to an ink with the running characteristics of a conventional ink but none of the previously encountered compatibility issues,” says Bonallo.
Users achieved excellent results by applying a UV coating wet trap over hybrid UV inks with no intermediate aqueous primer, then curing the hybrid UV inks and the UV coating together using a multilamp UV curing unit in the press delivery. As the process continued to be refined, it was found that by adding UV interdeck lamps prior to the UV coating being applied, thus creating a “semi dry trap” condition, gloss levels continued to increase and adhesion was further improved.
In recent years, according to Bonallo, the term hybrid has become ambiguous. “There's no agreed upon definition,” he says. “When we started with hybrid in the Co-Cure days, it referred to an ink with a certain percentage of UV and conventional components. But as the process [evolved], more often than not, hybrid ink is a refined ‘full’ UV ink.”
The issue is less about the definition and more about process consistency, he says. “Hybrid ink ‘A’ is not the same as hybrid ink ‘B.’ You put one set of inks on a press and it performs perfectly. You put another set of inks on the press, and [without knowing the formulation] you can destroy a set of rollers with little effort. The industry should get together and define what hybrid really is, so that people know what they are buying.”
Bonallo also advocates clearer guidelines for UV cleaning solvents. “It's critical to define wash-up solution [parameters],” he says. “Users must be able to match the rollers, blanket and chemistry with the right inks, coatings and solvents. Do you need a UV wash for cleaning hybrid inks from a roller?”
IST plans to address these and other UV issues at its ?“Share the Knowledge” UV seminar, which returns to Graph Expo after a one-year hiatus.