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FLEXO Magazine : June 2011
Technologies & Techniques Tips on Tips simple Doctor Blade spec Changes Have impact By Bobby Furr Flexography has been resilient throughout the tough economic times of recent years. The printing industry is thriving and the picture on a label or package is still a determining factor in whether a consumer walking down a gro- cery store aisle takes a product off the shelf. The old phrase, “a picture is worth a thousand words’’ remains true today. Flexo has made tremendous progress in the last 30 years. Today we have the ability to duplicate offset and gravure, whereas back in the early 1980s it felt good just to print a halfway decent vignette. Back then, line screens were no more than 360 line count and many systems used rubber roll metering. The doctor blade did not apply or factor into many processes. In 1981, the industry saw the first laser-engraved ceramic anilox roll, which was developed with 45-degree engraving. The ability to increase the line screen on the anilox allowed flexographers to improve graphics and put a much more attractive package on the shelf. This application required a steel doctor blade, which went on to play an increasingly important role in enhancing the look of a package. In the late 1980s, 60 -degree laser-engraved ceramic anilox rolls entered the market. An anilox roll with an 800- line screen, 60 -degree hexagonal engraving served as a strong workhorse. It made many converters a lot of money by enabling them to produce graphics similar in quality to those printed on offset or gravure presses. The steel doctor blade held its position as the blade of choice for these rollers. Many converters still use the same anilox rolls to print quality graphics. Today, we see laser-engraved anilox rolls with screens up to 2,000 lpi print on plate screens over 200 lpi while attempt- ing to hold a 1-percent dot. The quality of flexo printing has caught up to, and in many cases surpassed, that of offset and gravure. Through it all, doctor blade manufacturers have kept pace with the changes in plate, sleeve and anilox technology as well as new flexo press technology. WIDE WEB In the mid-1990s, many of the doctor blades used in wide web flexo were steel with rounded edges. These blades, touted as safe, could be installed either side up because both sides were rounded and they metered well on the anilox rolls used at the time. Some printers could still achieve a decent print with plastic blades, however. Through the years, doctor blade technology has had to keep up with an increase in graphics, anilox line screens and press speeds. Nowadays, many printers stay away from doc- tor blades with rounded edges, instead opting for blades with beveled edges. With the plastic blades, it is extremely difficult to meter a uniform film of ink and hold a good dot. The wrong doctor blade can affect many aspects of print- ing. Chatter, dot-gain, anilox scoring, UV spitting, streaking, Photo credits: Daetwyler Corp. FIgurE 1. Press contact zone components. 54 FLeXO June 2011 www.flexography.org To the Point • Ink matters when choosing a doctor blade. Blades that work well with one type of ink may not work well with others. • Of the four critical components in the contact zone of the press—anilox roll, doctor blade, ink and photo- polymer plate—the doctor blade is the least expen- sive. • Custom modification can raise the price of the doctor blade, but the price is offset by the savings gener- ated by uninterrupted runs. • A .015-in. or .020-in. polyester containment blade on the backside of a central impression press will easily eliminate ink “icicles” caused by back doctoring.
Sustainable Spring 2011