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FLEXO Magazine : June 2011
Technologies & Techniques Anilox Scoring Prevention Guide All You need to Know to Avoid Those ominous lines By Bill Poulson Anilox scoring is an expensive and frustrating problem. Anilox ceramic is a metal oxide and although it is extremely wear-resistant, it does not possess impact- resistance or ductility. When any hard particle becomes trapped between the blade and the anilox, there is a potential for the particle to destroy the cell wall structure on the surface of the anilox roller. If the particle is not consumed, once trapped it will ride in that location and destroy rows of cells. These rows of damaged cells appear as thin bands traveling around the circumference of an anilox and are commonly referred to as score lines. ScorinG TyPeS There are three types of score lines. First, light polishing score lines appear as light streaks in the printed image. (See Figure 1.) Second, deep gouging score lines will initially appear as dark streaks in the printed image. (See Figure 2.) Gouging score lines are caused by large particles lodged between the tip of the doctor blade and the surface of the roll. The larger the particle, the less likely it will be deposited on the substrate or consumed before damaging the engraving. The dam- aged area will typically be several cells wide and run around the entire circumference of the roll. A deep gouge can cause ink to spit or sling. As the doctor blade wears into the score line, a lighter streak will rapidly ap- pear. If this occurs, look for uneven wear on the doctor blade. In Figure 3, the “polished looking” surface of the land area is a heavy score line. The isolated pressure from contamina- tion that was trapped under the doctor blade polishes the land area, creating a wear pattern. These score lines reduce the ink delivery volume of the anilox roll just enough to show as a continuous, lighter line in the graphic image. Third, the non-scored printed line is a lightly colored line, resulting from a line of cells around the anilox that cannot be fully filled with ink. (See Figures 4 and 5.) Before trying to treat the problem, be certain what you have is a true score line. A continu- ous, lighter line within a graphic image is not necessarily caused by scoring, so a printed sample will not provide enough evidence to make a final verdict. Notice how the cell walls in Figures 4 and 5 are not enlarged. The damage shown in these figures is because of over-impression from debris trapped between the doctor blade and the anilox surface. Plugged cells can also create lines on graphics. Particles of metal or dried ink actually get pressed into the anilox roll cells, filling them in. This is usually caused by back-doctoring build-up on plas- tic containment blades. Plugged cells result in a loss of ink transfer and will appear as a score line until the roll has been Photo credits: Courtesy of Harper Corporation of America. Figure 1: light polishing score lines. Figure 2: Deep gouging score lines. Figure 3: “Polished” heavy score line. Figure 4: non-scored print line. Figure 5: non-scored print line. www.flexography.org June 2011 FLeXO 69 Kicking Contamination • Anilox score lines can appear as either light or dark streaks, depending on the cause. • Most scoring comes from hard particles trapped be- tween the blade and the anilox, or the improper use of doctor blades. • Shop towels, rags, roll covers, cleaning brushes, work gloves—anything that comes into contact with the anilox surface—can transfer contaminants that create scoring. • Using rare-earth magnets in tandem with ink filters with 40-60 mesh screens will trap and contain metal fragments.
Sustainable Spring 2011