Consortium proves quality growth of flexo printing

Published: May 01, 1995, By Sacharow, Stanley

As a result, flexographic printing is now used to print sophisticated packaging and point-of-sale advertising, and process color is printed on a routine basis. Flexography is now the fastest growing, segment of the international printing industry.

Flexographic printing has recently taken on a new dimension with the introduction of photopolymer plates, eight-color central impression cylinder presses, aqueous-based inks, doctor blade metering systems and other recent innovations.

Introduced as a printing technique around 1890 by English paper bagmakers Bibby, Baron and Sons Ltd., the first printed materials were sticky and bled badly.

Developed to an infinite degree of sophistication by the Germans because of their technology in coal-tar dyes, the first flexo presses were introduced in the US in the mid-1920s.

The presses were used on overall design printing roll-to-roll, but by far, their largest use was as a printing unit running tandem with bagmaking machines.

For the next 70 years, little was done in evaluating flexo-print quality on a technical level. As mentioned before, developments in machinery components allowed press speeds to increase from 200 ft./mint to more than 2,000 ft./mint

As a result, flexographic printing is now used to print sophisticated packaging and point-of-sale advertising, and process color is printed on a routine basis. Flexography is now the fastest growing segment of the international printing industry.

In 1991, the Flexo Quality Consortium was formed in an attempt to provide a better understanding of the factors controlling the quality of the flexo-printed image.

Dr. Stephen Cushner, a Du Pont expert in photopolymer plates and a founder of the consortium, told me there wasn't enough emphasis on flexo-printing technology as print quality. Dr. Gwen Chen, also a consortium founder, patented the Cyrel photopolymer plate development in 1974 with Jim Brennen at Du Pont.

In the first phase, the consortium's objective was to determine quantitatively the relationship between selected press, ink, anilox, plate and substrate variables with print quality in order to improve overall quality and thereby expand flexographic printing opportunity.

The quantitative relationship was examined because, until now, the flexographic printer has had to rely on historical, anecdotal and trial-and-error data in order to achieve satisfactory printing results.

Toward this end, a statistically designed experiment was used to probe the printing process. The goal of the consortium in doing this experiment was to develop predictive equations for each measure of print quality.

Certain variables were fixed in the experimental design. Photopolymer plates were selected over rubber plates since more than 60% of all flexo plates in the US are photopolymer.

Experimenters also chose plate thickness of .107 in. over .067 in., hard mounting tape over foam tape, laser-engraved over mechanically engraved anilox rolls, aqueous over solventborne inks, stainless-steel blades over plastic doctor blades and film over paper.

These variables weren't excluded for lack of interest, but because the complexity of the resulting experiment was found to increase disproportionately with the number of variables included.

A mathematical model appeared in the TAPPI Proceedings, 1992, with subsequent publications in the Flexographic Technical Association meetings of 1993 and 1995.

In 1993, the consortium became the research search committee of the FTA. All trials were on a substrate of opaque-white polyethylene supplied by Tredegar Film Products and carried out at the Flexographic Education and Research Center of Fox Valley Technical College on a Carint 47-in. wide, pix-color central impression press.

Phase I results, as reported in the 1992 FTA Annual Forum, examined the mathematical models and yielded a set of predictive equations, one for each measure of print quality which depended on main factor effects.

The mathematical data in the Phase I experiments pointed to the use of capped plates, lower-volume anilox rolls, high-strength inks and impression control. In order to bring these results in better focus, Phase II was conducted to determine quantitatively the suitability, reproducibility and possibility of the Phase I equations and findings.

The overall results of the trials were summarized in the 1993 FTA presentations. They found there were two important factor interactions. First, the choice of the anilox is strongly influenced by the choice of the ink and plate. Second, the anilox and the plate variable affect one another.

In addition, halftone printing was found to be more difficult at higher line screens. For example, 133-line halftones had narrower tonal range, poorer contrast and higher dot gain than 120-line halftones. The same relationship holds between 120-line halftones and 100-line halftones and between 100-line halftone and 85-line halftones.

Specifically, the trials determined:

[] Printing plate: All print-quality attributes pointed to the capped plate. Capped plates gave better uniformity of solids and higher density without affecting coating weight. Halftones quality was better as well, with lower dot gain, better contrast and broader tonal range. Solid density in halftone images was higher.

[] Anilox rolls: Lower-volume anilox rolls produced smoother solids and better halftones than did higher-volume rolls but also produced inadequate color strength and density. Anilox volume was, in fact, the single largest influence on print quality of those factors tested. Control of anilox volume, both by the manufacturer and then by the converter, is vital to achieving consistent print quality.

[] Aqueous inks: Quality printing on polyolefin film was demonstrated using waterborne inks. More importantly, the key to solving the anilox dilemma, high-density or high-quality, will be the development of higher-strength inks.

[] Printing press: Control of impression is vital for high-quality printing. Many believe that impression control is mainly a function of plate variability, but it is truly a matter of system variability--substrate, plate, tape and plate cylinder. So long as one continues to gouge the plate cylinder with razor blades, which raises burrs three to four times the thickness variability in the plate, we won't control impression.

In 1994-95, there are three consortium projects scheduled for completion. The first project, which looks into the factors affecting print quality of narrow ultraviolet flexo, is under the leadership of Peter Kerschner of Mark Andy.

The second project, which probes corrugated printing, is under the leadership of John Kelley, Dusobox. The final project, which deals with color control, is under the stewardship of Iain Trevor Pike, X-Rite.

For more information about the consortium and its activities, contact: Foundation of Flexographic Technical Association, 900 Marconi Ave., Ronkonkoma, NY 11779-7212; 516/737-6026; fax, 516/737-6813.