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FLEXO Magazine : January 2012
where the laydown of the ink, the color density, the traps, and any process screening are in balance. If a set of process inks has always dropped into density range on previous jobs with the same rolls and plates, the crews shouldn’t overreact to the first pull. Experience indicates that the job should settle in. Colors with small takeoff will get hotter and hotter, driving off the faster components of solvent-based inks or the ammo- nia amines in water-based products. Operators can “tap out” the sump ink versus fresh ink to determine just how much the dry rate has changed. Alternatively, operators could test the pH of water inks to see if it has changed. Low pH water inks increases viscosity. Operators must bring pH within range before adding water to neutralize the solution. Some presses utilize temperature control systems on the ink sumps helping to ease this process. AUTOMATED ERRORS? Automated press settings, where print job parameters are pre-entered prior to job start-up or saved after a success- ful run, are a major selling feature of current presses. This is especially useful the next time that copy comes up. Print sequence, anilox specifications, plate-to-print impression and anilox-to-plate impression, web tension, oven settings and, if equipped with automated controls, ink viscosity are saved with little to no crew involvement. If it’s a repeat job, you call up the job-specific parameters previously put into the press, and you’re ready to run, right? What happens when something is different such as when copy is changed, even a little? Could it be a screen or vignette trapping over a color previously not trapped, or just bleed- ing over one-sixteenth more than before? Then those “dialed in” automated press settings may not apply. The press goes down and efficiencies go with it. ScEnARiOS TO cOnSiDER Let’s say you usually print a combo black, including UPC code, in the last deck on a Central Impression (CI) press. This way, the dry rate of the black can be slow—really slow, if nec- essary—to cleanly print highlight dots. Say for instance, the last down color, with no worry about the next color tracking it. But now an upstream color has changed from a reflex blue to yellow and the black is picking it up in spots and tracking it visibly where the reflex wasn’t noticeable. The answer can be as simple as speeding up the dry rate on that upstream color. Unlike the job sheet or automated settings, the experienced operator will understand these complexities. Or, for some reason, this job is overprinted and the black is moved up a deck where the anilox has 50 percent more volume. This is a two-fold problem because when slow black is applied to thicker ink film, the OPV lifts and tracks terribly. Now, the press goes down in order to change the anilox roll, which nobody wants because of the time involved. The roll is covered in black so it must be cleaned and the OPV has to be pumped out and replenished. What a mess! Yet, this 10 FLEXO january 2012 www.flexography.org