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FLEXO Magazine : November 2009
www.flexography.org NOVEMBER 2009 FLEXO 45 trolled and can easily be in the 90° range, especially in the southern part of the U.S. during the summer. This environ- mental heat warms ink like a can of soda left outside and can be one of the leading sources of heat in ink. The second source of environmental heat is from the dryers used between decks. These dryers, which can evaporate water or solvent from a web traveling at speeds far greater than 1,000fpm, generate many kilowatts per deck. All this heat has to go somewhere, and either passes into the room or migrates into the press. Smaller sources include the power it takes to run the press, which is dissipated through motors and other electro-mechanical items. Mechanical friction heat is cause by movement, and there is plenty of movement on a modern printing press. From our experience, the rubbing of doctor blades on anilox rolls appears to be a major contributor. With the rolls turning at high speeds, the pressure of the blades generates heat right in the ink train. The anilox transfers this energy to the ink in the chamber or pan, where it gets re-circulated back to the bucket or tank. In addition to the anilox, some heat comes from the pump, though not for the reason usually cited. A common misperception is that a hot motor warms up the ink; however tests have shown no consistent or significant differ- ence between an electric motor, which is warm to the touch, and an air motor, which is cool to the touch. Instead, the heat comes from friction in the ink lines---a hose that is too narrow and too long, small inlets, partial-port fittings, 90° bends, and the spinning of the impeller. The hydrodynamic efficiency of the pump also plays a role, with efficient pumps imparting less heat. So what can easily be done to reduce the temperature of the ink? A new HVAC system might be nice but it doesn't fall under the category of a quick fix. And dryers aren't optional. Instead, there are some cheap and easy fixes: • Make sure your doctor blades are in good shape and that the chamber is properly aligned. Chamber misalign- ment and poor or worn blades are usually compensated for by increasing the pressure of the blades on the anilox, greatly increasing friction. • Upgrade your ink lines to a larger diameter, but always use a wider tube on the return than the feed. For wide- web presses, a 1in. tube on the feed and 1.25in. tube on the return is a must. • Eliminate excess tube length and any fittings, especially 90° elbows or partial-port fittings, that aren't essential. • Take into consideration hydrodynamic efficiency of the pump. Is the ink flow smooth, or are there sharp bends and abrupt diameter changes? • Consider an inverter drive for your pumps, so that you can run them at the proper speed to provide the flow you need, rather than running them full-out. • Examine the use of water-jacket tanks or in-line heat exchangers. THE EFFECT OF TEMPERATURE ON PRINTING The following section summarizes findings from the tests run by the EPA, et. al., to quantify the effects of temperature on printing. The test was run on a 42in. press, printing 1.25mil polyethylene film at 300fpm. Cyan and magenta ink, thinned with a 50/50 blend of n-propyl acetate and n-propyl alcohol, was tested at 66°F, 79°F, and 92°F. Viscosity was measured and adjusted every five minutes. Graymills, in conjunction with several industry partners, will be running similar tests with water- and solvent-based flexo inks. In reviewing the results of this test, there are three areas to cover: ink and solvent consumption, print quality, and the economics of ink temperature. INK AND SOLVENT CONSUMPTION Simply put, it costs more to print with hot ink than cool ink. Heat means wasted ink and solvent. Traditionally, we have acted on the assumption that a constant viscosity means a constant color. However, this is not true, because viscosity is temperature-dependent. Ink thins out naturally with temperature and will read a lower viscos- ity, even if the solvent concentration is held constant. The outcome is that, if the viscosity control does not correctly take temperature into account, the ratio of solvent to ink will de- cline; less solvent is needed to thin the ink out to the viscosity setpoint. This causes an increased consumption of ink---the higher concentration of pigment results in a thicker layer on the substrate. Indeed, testing showed a steady increase in ink consumption with temperature. Remember that all other vari- ables, including press speed, were held constant. See Figure 1 for findings on ink consumption at the test temperatures. Solvent consumption also rises with temperature. This might seem to contradict the point I just made, where higher 66F 79F 92F 0 2 4 6 8 10 12 3.5 4.0 11.8 4.1 4.0 7.2 Figure 2 Solvent Consumption (lb/hr) Magenta Cyan TECHNOLOGIES & TECHNIQUES
Sustainable Fall 2009