by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
FLEXO Magazine : August 2014
Tip Tips Pondering What Edge to Use—Radius, Beveled or Lamella Paul Sharkey T he fundamental purpose of the flexog- raphic doctor blade is to remove excess ink from the anilox roll surface. The remain- ing ink film on the anilox, combined with ink in the anilox cells, needs to be enough to achieve color density on the printed substrate. If the blade fails to remove enough ink, what remains on the anilox surface transfers more than is necessary to plate and substrate. The excess ink requires more dry time and energy consumption. In addi- tion, the thicker ink film more rapidly overwhelms the plate’s raised image resulting in dot gain, dirty print and mid run press stops to clean the plate. The converse is not always true. While it is possible to remove too much surface ink from the anilox, especially when metering a low line count surface used in heavy transfer applications, it rarely happens with higher line counts. In fact, flexo made its biggest print quality gains by moving to higher line anilox and all but eliminating the surface ink film. Today’s HD flexo relies on a precise volume of ink from cells, not surface ink films. The importance of the blade contact area in the metering process can’t be overstated. From the beginning, it was understood that the width of the blade at the point it contacts the anilox surface should be as narrow as possible. The wider the contact area, the greater the ink’s hydraulic impact. As press speeds increase, hydraulic pressure lifts the blade, allowing more ink to pass and resulting in the list of negatives already mentioned. CONTACT AREA MANIPULATION Flexo adopted the use of doctor blades to meter ink from the rotogra- vure print process. In roto, the blade is used in a wiping position. The dynamics of “wiping” a gravure cylinder, especially in the area that is not engraved, to achieve and maintain optimum performance, are very different than in the sheering or scraping action of the doctor blade in flexo. The rotogravure press operator purposely manipulates the blade’s con- tact area to achieve more or less wipe of the area that is not engraved. A very small amount of ink is allowed to pass beneath the blade to lower friction and wear of both the chrome cylinder and the blade. Both the beveled tip and a lamella tip were developed to aid in this process. Part of this manipulation also involves the use of more or less pressure to adjust the blade’s flex. In the flexo process, the challenge is to set a blade to the anilox at roughly 30 degrees to 35 degrees and keep it there. Most doctor blade metering systems are preset to position the blade at the optimum contact angle. It is much easier to maintain the angle in a single blade system like those common to the label printing industry. However, chambered blade metering systems are a challenge. Here’s why: During a pressrun, anilox cells trap an increasing amount of air, thereby reducing the amount of ink they hold. To overcome this starva- tion, the pump’s pressure can be increased to try to force more ink into the cells. Increased pressure within the chamber causes end seals to fail. To stop the end seals from leaking, the chamber and seals are pressed tighter to the anilox. Increased pressure causes blades to flex, bending “The wider the contact area, the greater the ink’s hydraulic impact. ” 38 FLEXO | AUGUST 2014 TECHNOLOGY & TECHNIQUES FOURTH INA SERIES