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FLEXO Magazine : May 2012
Plants & Processes Maintaining Optimum Coverage Viscosity controller can result in substantial savings By Mark st Georges Pressroom supervisors, plant managers and quality con- trol managers of modern flexographic printing opera- tions are increasingly responsible for saving time and ensuring proven performance on all flexographic printing jobs. Adhering inks to a wide variety of substrates with no streaks and attaining the proper color density are critical. Whether a firm produces boxboard corrugate, flexible packaging, folding cartons, preprint linerboard or labels, requires reliable viscos- ity and pH control to assure the uniform application of inks. Viscosity, or a liquid’s ability to resist flow, strongly affects how ink behaves on press and is ultimately transferred to the substrate. Inks have thixotropic properties—they become less viscous when subjected to an applied stress. While this phenom- enon can be easily seen, it can negatively impact print quality. A problem with ink film that’s too light is easily seen, but the eye cannot always detect ink film that’s too heavy. Once mass-tone is achieved, ink film does not change as it becomes thicker. To the unaided eye the top of a .001-in. thick film will appear little different from a film of the same ink that is .0001-in. thick. It is possible to waste a lot of expensive ink without sophis- ticated viscosity control. Automatic control systems continuously sense ink viscosity and transmit a signal when ad- ditional solvent is required. This prevents wasteful ink build-up. Control of the slightest change in viscosity can make all the difference in successful ink adhe- sion—particularly in the low viscosity region of the thin inks currently in use. IMPORTANT VARIABLE Ink viscosity is perhaps the most significant variable on the press. Unless it is controlled continuously and closely, perfect color, uniform ink coverage and accurate color match can- not be achieved. Given today ’s higher costs for solvent- and water-based inks, large, medium and even small printing op- erations can save significant costs by controlling ink viscosity. There are other variables that will affect the quality of the finished product, but none change as rapidly, or are as diffi- cult to spot as viscosity. It is the prime parameter that controls ink transfer to the substrate after all other mechanical and setup aspects of the printing press—like nip, speed, and selection of paper—have been established. Today ’s complex ink products have been formulated in response to the many requirements of modern printing. The end-use of the printed substrate determines the properties that must be built into ink, whether it’s water-resistant inks for outdoor displays, scuff-scratch-resistant inks for shipping containers or special odorless inks for food packaging. Opti- mum press performance, in general, results from rigorously complying with the ink manufacturers’ specifications. PRODUCTION PROFITS Numerous articles have affirmed that a one-second increase in viscosity can lead up to a 25 percent increase in ink consumption. What should be kept in mind are the other aspects of the operation, which, when properly controlled, increase production profits, namely: • Reduction of set-up waste • Ink and solvent economy • Short set-up times • Elimination of rejected product Also, standardizing the viscometer unit, regardless of the wide variety of inks used by the printer, has numerous built-in cost efficiencies including: • Uniform application of ink and less ink waste • Less operator downtime • Easier cleanup REAL-WORLD APPLICATIONS Recently, a printing operation specializing in lottery and gaming tickets for governments, corporations and retail MEASURE VISCOSITY & SAVE • Studies have proven that even a one-second increase in viscosity can lead up to a 25 percent increase in ink consumption • Given today’s higher costs for solvent- and water- based inks, printing operations can save significant costs by controlling viscosity • Viscometers maintain ink formulations at predeter- mined levels to ensure economical usage 56 FLEXO May 2012 www.flexography.org