Dots Do It Right

Getting the right dot structure for its flexo plates was critical for CSW to support both its label and corrugated clientele.

In general, advances in –digital platemaking techniques in flexographic printing have been driven by the desire to achieve improved print quality, a streamlined workflow, and more sustainable manufacturing processes. The first generation of flexo platesetters used round-tipped dots rather than a traditional flat-tipped structure, a change that presented challenges for corrugated converters. These round-tipped dots resulted when the dot surface lost its sharpness and strength due to the presence of oxygen during the plate exposure process. On a –corrugated press, this can result in a loss of highlight detail, unpredictable dot gain, increased fluting, and decreased plate longevity.

Waiting for the right time

Until January 2009, CSW (Ludlow, Mass.) followed these developments closely and conducted research into several digital platemaking technologies, but held off from committing to invest in any one system.

"Our highly diversified clientele is about 50 percent corrugated, 45 percent wide- and narrow-web, and 5 percent other," explains Marek Skrzynski, director of graphics, R&D. "For us to have completed the back end of our digital workflow before the right technology became available would have meant that half of our business would remain unsatisfied. It was on that –basis that we were unwilling to make an early investment."

The technical hurdles challenged CSW's entire R&D team. "Having earned a reputation among our corrugated customers for high-quality analog plates, rendered at very high resolutions (170 lpi with 660 aniloxes), we didn't want to lose that by going digital," notes Skrzynski. CSW knew it needed to find a better way to prevent oxygen interaction with the photopolymer during UV exposure in order to maintain equivalent print quality.

From the beginning, carbon-coated digital plates prevented platemakers from using a vacuum to remove oxygen from their frame exposure units. This inability to inhibit oxygen led to the partial polymerization of the dots, which gives them desirably steep shoulders, but, for corrugated converters, a less-than-desirable round-tipped profile.

On a corrugated press, these dots often needed to be over-impressed due to the irregular surface. Flute tops were printing darker, especially in quarter to mid-tones. This phenomenon is known in the corrugated world as “fluting,” and the elimination or visual reduction of this effect is considered by many to be the “Holy Grail” of corrugated printing. “The right dot structure can significantly help reduce the appearance of fluting, but to eliminate it entirely will take some effort from the corrugated manufacturers to produce a better quality board,” Skrzynski said.

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