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FLEXO Magazine : June 2013
per inch (dpi) specification. The rule of thumb was to multiply the dpi of the plate screen by four to determine the lines-per- inch specification of the anilox roller. A 50 dpi screen could run fairly well with a 200 or 300 lpi roller. Eighty-five dpi screens took a 360 or 400 lpi roller. Diamond or hexagonal shaped cells burned into the roller with one instantaneous hit from a laser usually provided plenty of volume, keeping ink density high. (Figure 2). These rollers are still used exten- sively because they are economical to produce and they provide a relatively long service period. However, as lpi counts increase, the size of the cell, of course needs to decrease. A smaller cell made from a single laser burn does not hold enough ink to maintain the density required in some high graphic applications. Finer halftones and gradu- ated blends with a 133 or 150 dpi plate screen may take a minimum 600 lpi roller—possibly higher. Using a lower lpi roller, with its larger cell size, can also produce greater dot gain—a condi- tion where the size of the dot printed on the substrate is slightly larger than the dot on the plate. This can cause hues in process colors to shift. So, technolo- gies were developed to produce an anilox roller that could deliver a greater volume of ink in higher line counts, and thus maintain higher ink density. ATTAINING GRAPHICS IMPACT The specific alignment, shape, width and depth of anilox roller cells determines how much ink can be transferred from that roller to the plate and in turn to your substrate. In general, the bigger and deeper these cells are, the more volume of ink they can transfer. Advanced laser engravers can now, with additional hits of the laser beam, burn away more of the ceramic coating in which each cell is formed. This gives the cell more volume by burning a deeper, broader cell. This also reduces “land area”— the unburned ceramic surface around each cell that cannot hold ink—and thus the thickness of the cell walls within the same surface area is minimized. (Figure 3). Dot gain is also reduced because the area of the group of cells supplying ink to each dot on the plate is closer in size to the actual dot, so a more precise volume of ink is transferred. (Figure 4). In general, as line counts increase, dot gain percentage decreases. This is especially important when running film or similar non-porous and extruded substrates. By manipulating the way the cell is formed, greater ink volumes can be achieved for those high graphic applications that require rollers with a higher lpi specification. The effectiveness of extreme cell depth, however, is limited by the size of the opening at the top of the cell. Ink will www.flexography.org June 2013 FLEXO 43 Anilox/ gravure cylinders Plates for wide & narrow web Plate sleeves Rotary screens Parts TALK DIRTY TO US... and we’ll clean your: Leading Cleaning Technology www.flexowashus.com • firstname.lastname@example.org 888-493-5396 • Recovers Cell Volume • Safe, Fast & Effective Figure 3