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FLEXO Magazine : June 2011
Technologies & Techniques properly cleaned. If you are having trouble cleaning these cells out, contact your anilox supplier for help. Primary Causes Most anilox scoring occurs when contamination is intro- duced to the anilox surface through the inking system, doctor blade chambers and ink hoses. There are several ways this can happen. Doctor blade tip slivers. Large pieces of steel blades or slivers created by improper doctor blade use are the primary cause of anilox scoring. (See Figure 6.) These large steel par- ticles break off of overloaded doctor blades as they wear. Improper doctor blade use. In general, normal blade shavings of quality steel do not damage a roll because they are too small to get lodged between the blade edge and anilox cell walls. But when there is too much pressure, the stress causes the doctor blade to bend back, which increases the footprint of the contact area. The resulting increase in friction creates much larger particles. As the tip of the blade wears through, the tip breaks away in the form of a long metal sliver. (See Figure 7.) Too much pressure. Ideally, only the very tip of the doctor blade would touch the anilox, as shown in Figure 8. Correct positioning of the doctor blade provides the best ink meter- ing and print quality while minimizing blade wear. However, improperly aligned chambers, incorrect end seal sizing (and the pressure compensation it requires) and rippled doctor blades from warped or over-tight clamps can create too much pressure. This will result in large slivers and pieces of metal that contaminate the ink system and threaten the integrity of the engraving. There are several ways to ensure the correct pressure on doctor blades. First, when installing new doctor blades, always back off all mechanical settings and reset the blade pressure. The scenario illustrated in Figure 7 is typical of what occurs when blade settings are not reset after a new doctor blade is installed. An over-impressed blade will yield a thicker ink film and give more color. Although it is tempting to over-impress blades instead of replacing the anilox roller when you need more color, this can send a lot of slivers from the blade into the ink system. Second, make sure the blade chamber is aligned properly and receives adequate maintenance. Both horizontal and vertical alignment of the chamber are critical determinants of anilox wear. Older chambers warp and pose serious problems. Third, maintain clean, properly sized end seals. End seals that do not function properly promote doctor-blade over-im- pression. If the settings are right but it takes additional pres- sure to prevent end seal leaks, consider modifying the end seals. (See sidebar, “Ink Chamber Maintenance Checklist,” on this page for quick tips on maintaining ink chambers.) Your doctor blade supplier can help you evaluate doctor blade wear, verify the blade contact angle, analyze the chamber Figure 6: Metal debris caused by over-impressed blades. Figure 7: over-impressed scenario. Figure 8: Proper blade pressure and alignment. 70 FLeXO June 2011 www.flexography.org ink Chamber maintenanCe CheCklist • Never overpressure doctor blades to get more color. • Keep chambers and blades aligned. • Check vertical and horizontal alignment at least once a month. • Use a .005-inch plastic feeler gauge to test contact between the upper and lower blades and the anilox. • Bring the chamber (without end seals installed) in toward the anilox, while checking the upper and lower blade until you feel drag from the feeler gauge. The blades should contact the anilox at the same time, top and bottom, side to side. If not, the chamber is not installed correctly, or the bracket is not properly set. • If stops are available, make sure to use them and have the chamber calibrated, so it cannot touch the anilox. • Look for signs of wear on blade clamps and check blade clamps for scratches or grooves, which indi- cate contact with the anilox.
Sustainable Spring 2011