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FLEXO Magazine : June 2011
Technologies & Techniques Right Blade/Right Roll/How to Tell? it’s not easy! By Paul sharkey As the quality of the flexographic printed image has improved, many key process elements have become more complex. One of the most important of these evolved process elements relates to ink transfer--the process of metering and transferring the correct amount of ink from the anilox roll’s surface to the image carrier and ultimately to the substrate. Technical gains in this critical process area have, perhaps more than any other, enabled flexography to achieve its current stature. While many pressrooms have kept up with the technical evolution of anilox rolls, a surprising number have not taken advantage of the significant advancements in doctor blade technology. They continue to run the same blade today that they ran 15 years ago in an entirely different process environment. Why? Aside from price and supplier performance claims, it’s not easy to “see” the differences that exist from one blade to the next. On top of that, it’s a challenge for pressroom man- agers to sort through the many doctor blade choices offered by so many doctor blade suppliers. Understandably it’s easier to stay with what they know or think they know. Finding the doctor blade most compatible with today ’s technically advanced anilox roll surface can be challenging. To understand where blade technology is today, it helps to understand what has driven the changes that have occurred. HISTORICAL PERSPECTIVE As recently as the 1970s, excess ink was metered or removed from an anilox roll’s engraved- and chrome-plated surface using a rubber metering or wipe roller. At that point in flexo’s evolution, anilox line counts ranged from 160 to 360 cells per inch. (360 was the maximum possible for mechani- cal engraving). At the time, most flexo printing consisted of solids, reverses, large type and some screens. When process print was attempted, it was coarse and looked best from across the grocer ’s isle. Fine screens and process print were not possible because anilox ink films were too thick. Rubber roll metering further compounded the problem because the rolls were hydraulically sensitive to speed. As speed increased, ink pushed the rubber roll away from the anilox, making the ink film even thicker. In an attempt to meter ink more consistently, some flexo presses were modified to replace the rubber wipe roll with a steel doctor blade. While this move improved metering, blade materials available at the time were coarsely structured strip steel. They were very wearing to the engraved- and chrome- plated surface. And, the mechanical engraving process was still limited to a maximum anilox line count of 360, meaning the surface ink film was still thick. In 1981, everything changed with the introduction by Praxair Surface Technologies of laser-engraved ceramic anilox rolls. While this new surface was first introduced because of its durability, the real advantage quickly became apparent. It became possible to “burn” a much higher line anilox sur- face; one better able to be tightly metered to achieve a thin- ner ink film. Ink could be transferred under more control from more cells that were deeper with wider openings. Reduced anilox surface ink films made it possible to print fine process dots for longer periods with less dot gain. Fast forward to 2011..The ink transfer process changes described did not happen overnight, nor did they happen alone. Every flexographic process element changed in step with one another to bring flexography to this moment when print quality and production efficiency is as formidable as that of litho-offset and rotogravure. IDEAL ANILOX ROLL Today, an ideal anilox roll / sleeve meets very specific verifi- able tolerance specifications. The following are major points to consider when establishing specs: • Dimensionally, the entire roller must be perfect, or as close to perfect, as possible. • The ceramic’s hardness and density needs to be consis- tent across and around the roll. • Cell dimensions (opening, depth and wall thickness) must be within agreed tolerances and uniform across the surface. • Post laser-engraving polishing to reduce surface friction is essential. The more the better. Don’t be afraid to ask for a wider, flatter wall. • Know how the surface is protected, over time from sub- surface corrosion. Bottom line, the performance of an anilox roll speaks for itself in terms of ink transfer and wear-ability. 42 FLeXO June 2011 www.flexography.org Best Blades For most flexo printers an ideal doctor blade is one that: • Meters an anilox surface uniformly and precisely. • Wears slowly and uniformly. • Produces minimum and uniform sized debris as it wears. • Produces the lowest amount of friction against the anilox roll’s surface. • When needed, can last long enough to finish a job without stopping. • Is made from highly refined strip steel. • Its chemistry reduces wear without increasing hard- ness.
Sustainable Spring 2011