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FLEXO Magazine : May 2012
The type of blade chosen, and pressure settings, used will determine the achievable level of ink metering. Different doc- tor blades can give an operator on-press options to achieve different print results with the same anilox roller. This is beneficial because sometimes it is easier to change a doctor blade than an anilox roll. With so many adjustments available, measurement methods are needed to verify print consistency. CHAMBER BLADE SYSTEM The chamber blade system is an ideal metering system for ink control. As seen in Figure 2, an enclosed chamber replaces the rubber roller. A chamber has an extra blade to contain the ink and the chamber ends are sealed with end dams or end seals. This configuration allows for best ink control because blade pressure settings are not too operator sensitive and the ink is efficiently circulated through a pump- ing system. Any of the previous metering systems can use a pump, but the large volume of ink in the pan requires mixing. The chamber blade system has very little ink in the cham- ber, so it does not take long for ink adjustments to balance throughout the system. Additionally, a chamber system is not as susceptible to heat and airflow because the ink is enclosed until it is metered onto the anilox roll. It is easily seen why there can be so many significant dif- ferences when printing on the same anilox roll with the same printing plate on the same substrate. The method ink is ap- plied to, and metered off the anilox roller, plays a major role in consistency. To complicate the situation further, the physical size of the anilox roll and the repeat of the printing plate can influence the behavior of the ink by how long it is exposed to the air before rewetting. Therefore, trying to predict what anilox roller will work with a given substrate is nothing more than a guess. Until an optimization test is run—with the spe- cific conditions—a printer will always be wondering if he is achieving full graphics potential. Here is a general guideline one can use to understand the relationships among substrate, ink metering and graphics: • Poly film or coated paper = high ink hold out or low ink absorption surface • High ink hold out = ink sits on top of substrate • Ink sits on top of substrate = don’t want a lot of ink • Don’t want a lot of ink = low volume anilox roller • Low volume anilox roller = thin ink film thickness • Thin ink film thickness = less dot gain • Less dot gain = higher LPI graphics • Higher LPI graphics = smaller size images • Smaller size images = closer viewing distance because people by nature look closer • Closer viewing distance = needs better registration Printing a thin ink film thickness means less ink needs to dry and, therefore, a fast press speed can be achieved. A thin ink film might also require a stronger pigment load to achieve color (See Figure 3). Figure 3: Low BCM anilox (1.5 BCM) allows for good ink coverage and clean halftone screens. Figure 4: Mid BCM anilox (3.0 BCM) allows for more ink that equates to more dot gain. Notice the “dirty” print beginning to show at higher graphics LPI. Figure 5: Notice ink coverage at 100 percent. The low BCM anilox (1.5 BCM) does not give enough ink coverage. 50 FLEXO May 2012 www.flexography.org