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Jun 1, 2009 12:00 AM
It's essential to understand your UV system's potential as well as proper component configuration. It really comes down to flexibility. A packaging printer running paperboard has different requirements vs. a commercial printer running a mix of specialty nonporous substrates and paper stock.
Suppose you are printing on a clear substrate. You'll want to apply opaque white first, with additional inks on top of it. Applying UV opaque white in the first two print units, curing each application via UV interdeck modules located after each printing unit supports the application of additional inks over a fully cured UV opaque white (dry trap).
On a lenticular plastic job, process color typically is put down first, with the double UV opaque white on the last two print units. In both cases, a variety of factors determine the number of UV curing stations between the units, including ink (hybrid vs. UV); dry trap requirements; amount of opaques or metallics used and in what print unit(s); and coverage, substrates and target press running speed.
In the early days, for conventional printers' convenience, hybrid inks combined conventional pigments and UV curable components — the ink was compatible with conventional rollers, plates, blankets and chemistry.
Hybrid presses progressed from end-of-press UV curing to adding one interdeck UV module after the last print unit. Current hybrid machines typically have three or more interdeck UV modules in addition to the end of press UV lamps.
Pure UV inks are the clear choice for printers that specialize in non porous substrates such as plastics, synthetics, Mylar, special foils and metalized stock. Virtually all of these printers opt for a full UV machine: one interdeck UV module after every print unit in addition to end of press UV curing. Rollers and blankets are EPDM (UV only) compound. Pure UV inks contain materials that that aid in potential adhesion related issues on these challenging substrates.
Hybrid UV Inks are designed to run with conventional roller and blanket compounds (although a UV combi roller is recommended). Therfore, inks are formulated to minimize damage and swelling to blankets and rollers.
As originally conceived, hybrid UV inks had a mix of conventional pigments and UV curable components. But ink suppliers don't have a uniform “hybrid”definition. Some inks contain refined pure UV components that work well with some conventional and most combi roller compounds. Other ink makers mix UV and conventional technology and still others label their products “hybrid” regardless of the content. It's a frustrating situation, one that has prompted some printers to stick with full-time UV rather than switching between conventional and hybrid inks.
Ink and water balance typically are more critical on hybrid and pure UV vs. conventional inks. Neither pure nor hybrid UV process inks are as transparent as their conventional counterparts. Yellow is the worst offender. Rather than the standard KCMY color sequence, some printers run YMCK or YCMK, an important consideration if switching between conventional and hybrid/pure UV.
Beware of using pure UV solvents used on non EPDM rollers — it can lead to swollen rollers and blankets as well as poor print quality. This is the No. 1 complaint we hear from printers switching between conventional and hybrid inks.
Bill Bonallo is vice president of IST America Corp.'s (Bolingbrook, IL)sheetfed group. Contact him at firstname.lastname@example.org.