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FLEXO Magazine : June 2014
Diverse, Yet Compatible Containment Blades, Doctor Blades & Back Doctoring Paul Sharkey H ave you ever asked yourself, “Should I be using the same blade in both posi- tions?” The short answer is—no! Here’s why. Fundamentally the purpose of the containment blade is very different than that of the doctor blade. The doctor blade meters ink and the containment blade, well, contains ink in the chamber. The containment blade is positioned in the chamber opposite the doctor blade; one upper and one lower. Together with end seals, they contain ink within the chamber against the anilox. Ideally, ink leaves the chamber by two ways: within the anilox cell and by return line from the chamber to the ink reservoir or pail. On a press with a central impression (CI) drum, their positions reverse depending on if the chamber is on the up or down side of the CI drum. On the upside, the anilox is turning up, requiring the doctor blade to be positioned at the top of the chamber to meter ink. On the downside, the doctor blade is positioned at the bottom of the chamber to meter the down turning anilox. Again, the containment blade is positioned opposite the doctor blade. BACK DOCTORING The biggest difference between these two blades is that unlike the doctor blade, the containment blade engages the anilox in a wiping or positive position. In this position, the blade intentionally is subject to hydraulic lift, allowing ink to pass between the anilox and blade to reenter the chamber. At least that’s the idea. If the containment blade is too stiff or ridged or at the wrong angle, it meters the returning ink, preventing it from reentry and causes back doctoring. While back doctoring can occur when the containment blade is in the bottom or top position in the chamber, it is more pronounced when the containment blade is at the bottom. The returning “metered” ink travels down the blade and drips into the drip pan. Back doctoring is a major industry problem. Not only is it a waste of ink, it causes ink slinging, print defects and makes a housekeeping mess. PREVENTING BACK DOCTORING We’ve established the necessity of making sure the containment blade is not the same as the doctor blade, but there are other factors to consider: • Material Because the containment blade is employed in a wiping position and has a wider contact area allowing more ink to pass between blade and anilox, the confrontation with the anilox is reduced. With this in mind, the containment material does not have to be of the same material or dimension as the doctor blade 70 FLEXO | JUNE 2014 TECHNOLOGY & TECHNIQUES THIRD INA SERIES Anilox Roll, Doctor Blade, Ink Selection Guide