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FLEXO Magazine : June 2012
blade, be sure that all of the components are clean and free of any damage. Inspect the components and repair any dam- age found if possible. Minor dings can usually be corrected by filing the damaged area smooth. If the component is beyond repair, purchase a replacement part and take the damaged part out of service so that it can’t be accidently used. When installing the blade, be sure the back side of the blade is firmly resting on the ledge or pins that are built into the chamber. This will ensure that the doctor blade is installed parallel to the chamber. When the blade is not installed parallel to the blade holder, metering quality will likely suffer. Finally, after the blades are installed, look down the length of the blade to see if it is wrinkle free. If wrinkles are found, reinstall the blade after fixing the condition that caused the wrinkles. ALIGNMENT TO ANILOX When installing the chamber on the press, be sure to take the time to make sure the chamber is in alignment with the anilox roller. It is important that the chamber’s centerline is aligned with the anilox roll centerline and that it is not skewed to the anilox roll. Be sure that both blades touch the anilox roll at the same time. Any misalignment between the doctor blade and the anilox roll will reduce the quality of the wipe that is delivered by the blade (Figure 2). One way to check alignment is to use a plastic feeler gage. Remove the end seals from the chamber and move the chamber so that the blades are close to the anilox roll but not touching it. Slowly move the chamber closer to the anilox roll while sliding the feeler gage between the doctor blade and anilox roll on one end. Set the chamber position so that the feeler gage will slip be- tween the blade and anilox roll with slight tension. Now check the gap between the rest of the blade and anilox roll. It should be the same everywhere on the blade and the same on the containment blade. Adjust the chambers position as neces- sary until there is uniform gap on both blades. Be sure to follow any and all plant safety rules while check- ing the chamber alignment. END SEAL EFFECTS End seals that are not matched to the application can be a source of doctor blade problems because a poor fitting end seal or the wrong material will require extra applied pressure to stop leaks. End seals have one function in the flexo inking system and that is to prevent the ink from flow- ing out of the chamber. Most end seals accomplish this by friction between the anilox and end seal and, depending on material, will rub the anilox until it is worn beyond contact to the anilox. At that point the result is leakage, slinging ink, wasted substrate etc. The only corrective action is to replace the end seal with a new one and this can happen after only a few hours of pro- duction time. On most presses with chambers and end seals, the doc- tor blade is also replaced with the end seal, even though the doctor blade may have many hours of life remaining. To extend end seal and blade life, the press operator must use the least possible force when setting the blade for printing. He must also understand and control one of the most common overlooked items in every pressroom, the ink pump and ink flow rate. As press speed is increased, the ink force will add outward pressure to the end seal. To relieve some of this pressure, know your ink flow rate. Most chambers today use very little ink to fill the cavity, some as little as two pints. Measure the amount of ink required to fill your chamber and set your pump flow rate at slightly more than your chamber volume per min- ute. If you only need pints per minute and you are pumping gallons per minute, the end seal and doctor blade perfor- mance will likely degrade quickly and need to be replaced several times before the run is over. Many flexo printers are moving away from diaphragm and centrifugal high flow pumps and utilizing dual feed peristal- tic pumps that relieve the cavity pressure on end seals and blades. The dual feed pump can run the chamber at negative pressures thereby extending the life of doctor blades and end seals for longer production runs without changing either item. APPLICATION PRESSURE Application pressure has been mentioned a few times in this article. That is because excessive application pressure is a significant cause of inconsistent blade metering. Excessive pressure will over deflect the doctor blade, flatten the doctor blade contact angle, and prevent uniform blade metering. Extreme pressures can deflect the blade to the extent that the blade tip will lift. A lifted blade tip can trap hard particles under it and lead to anilox scoring (Figure 3). At the very least, 80 FLEXO june 2012 www.flexography.org