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FLEXO Magazine : June 2011
Technologies & Techniques back doctoring and excessive blade wear are just a few of the potential problems the wrong blade can create on press. The doctor blade’s “ wear-in” factor is an important consideration. As the doctor blade wears in, it changes both print and ink, requiring adjustments that can cost a printer a lot of down- time. See Table 1 for recommended doctor blades for wide web flexo. The longer a doctor blade maintains its tip and durabil- ity throughout a run, the fewer the adjustments required on press. Essentially, the conditions on the press are the same from start up to the end of the run. Keep in mind that many elements affect the wiping of the ink film on the anilox roll. Table 2 offers a general rule of thumb for anilox line screen correlated to specific blade thickness on the tip. NARROW WEB Labels, tags and envelopes comprise the bulk of the narrow web flexo market. There have been large increases in anilox line screens and graphics among label printers, pushing doc- tor blade technology toward precision blades and tips that allow printers to achieve their desired results. See Table 3 for recommended doctor blades for narrow web flexo. Ink is an important consideration when choosing a doctor blade. For example, blade recommendations will be different for UV inks and water-based inks even when a printer uses similar graphics and anilox line screens within a pressroom. A doctor blade that works well with water-based inks may cause UV spitting on a UV inking system. Envelope presses present their own set of challenges. These presses typically run fast and pump out the envelopes 24/7. Most envelope printers print simple lines (type), block solids and reverses. A good workhorse anilox for this appli- cation is a common 360-line screen. This gives the press the ability to hold type and reverses, yet allows densities in solids. For this application, it is important to avoid blades that wear out quickly, otherwise it will be necessary to stop the press for changeovers. In some cases, the nature of the anilox and the graphics in printing envelopes allows printers to get away with using a plastic blade. Table 4 can be used as a general guideline for doctor blades when printing narrow web graphics. TABlE 4. Recommended blade tip thickness for narrow web graphics. Blate Tip Thickness Process Printing with Fine Dots (1 percent) Combination Printing (Solids and Vignettes) Heavy Lines and Solids 75 mic. X 95 mic. X X 125 mic. X X 150 mic. X 200 mic. X 250 mic. X CUSTOM BlADES When problems crop up, many printers take the doctor blade for granted and instead focus on the anilox, ink or graphics. Yet a slight change in doctor blade specification can provide a solution. Remember that four critical components come in contact with each other in the contact zone of the press: the anilox roll, the doctor blade, the ink and the photopolymer plate. (See Figure 1.) First, the ink is distributed to the anilox roller. Then the doctor blade comes in contact with the anilox roll and ink, and wipes the ink to a fine linear film. The anilox roller then distributes the ink to the plate. Each component has to be right: If even one is wrong, the others will fail. The doctor blade is the least expensive of the four com- ponents. That said, in many situations the doctor blade may TABlE 3. narrow web flexo doctor blade applications. Anilox Line Screen Recommended Blade Recommended Thickness Recommended Tip Sizes Notes <500 Bevel edge . 008 in. 15-degree Good for line and block solids with line screens of 500 and lower. Provides quick, trouble-free start-up. >500 Standard lamella . 008 inc. 125 mic. lamella Very common for label printers and fits the widest application range. Helps control dot gain by reducing ink film layer on anilox roll. 56 FLeXO June 2011 www.flexography.org
Sustainable Spring 2011