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FLEXO Magazine : November 2009
46 FLEXO NOVEMBER 2009 www.flexography.org 66F 79F 92F 1.50% 1.60% 1.70% 1.80% 1.90% 2.00% 2.10% 2.20% 2.05% 1.98% 1.70% Figure 3 Reflection Density (%) temperature makes the viscosity read lower than it truly is, reducing solvent additions done to keep viscosity constant. However, higher temperatures cause solvent to evaporate faster, especially if the ink tank is not securely covered or is the press uses open ink pans. Faster evaporation means that more solvent must be replaced. Solvent consumption nearly tripled on the magenta test deck when the ink went from 79°F to 92°F; see Figure 2 for findings on solvent consumption. Solvent evaporation also creates other issues beyond the cost of the material. Many common solvents are volatile or- ganic compounds (VOCs). At the national, regional, and local level, more and more regulations are being written that aim to reduce VOC emissions. Pollution control systems are an expensive solution and do not always capture all the fumes. Compliance costs and penalties for failure can easily add up. Additionally, there are concerns about the effects of VOCs on workers. All of these problems can be minimized by the rela- tively inexpensive step of controlling ink temperature. Imagine the effect of reducing solvent usage by half of more on com- pliance costs, something not even covered in this study. PRINT QUALITY The print quality issues of ink temperature fall into two relat- ed categories---reflection density and dot/solid structure. Given that we have shown that ink consumption rises with temperature, it might be expected that reflection density would also rise. Strangely, this is not the case. Reflection density actually declines; see Figure 3 for findings on reflec- tion density. This seems counter-intuitive; wouldn't more ink print with a higher reflection density? Actually, no. Warm inks evaporate faster, and once the temperature gets high enough, they dry before properly spreading. This leads to a smaller printed area covered by a thicker layer of ink. Dots and solids are also affected by ink temperature. As temperature rises, dots turn into "donuts," rings with an unprinted center. Two issues cause this. First, because of their lower viscosities, warm inks tend to hold more air in each anilox cell and potentially on the printing plate as well; the surface of the ink looks con- cave. As this ink is deposited, the air forms a bubble, which in turn creates the middle of the "donut." Second, the faster drying speed of the hotter ink results in cell lines showing up on printed solids. There is still debate as to what degree flexo printing is susceptible to the "donut" problem, as opposed to gravure, where the ink is directly deposited on the substrate; this will be examined in further research and testing. ECONOMICS Even ignoring all the potential print qual- ity problems and the regulatory and health issues, it pays to keep your ink temperature in check. The EPA study took a look at two scenarios using the same setup: a 23in. press with three decks running 300fpm and 7,200 hours per year. This is obviously not a state-of-the-art press in terms of speed or number of decks, so it should be viewed as a very conservative number. Ink and solvent consumption figures were averaged from the data collected. Solvent was assumed to cost $0.50/pound and ink $4.00/pound. These numbers are all 2001 prices, from back when gasoline was $1.20 per gallon, so again I think the study understates the potential savings. The first scenario assumed that the printers implemented temperature control and kept the ink at 79°F year-round. The second scenario assumed ink of 79°F for nine months of the year and 92°F for three months; in other words, three-quarters of the time, even this press would be running with cool ink, something that is not likely in the real world. The EPA study found that controlling temperature, even for only three months a year, reduced solvent usage from 113,400 to 86,400 pounds, or 24 percent. Ink consumption declined from 97,200 to 86,400 pounds, or 11 percent. Combining these two reductions led to a savings of $56,700. Remember, this is for a small press running at a low speed with ink that is only warm one quarter of the time! CONCLUSION Far from a solution looking for a problem, keeping ink temperature in check can result in higher quality, reduced regulatory burden, and quantifiable savings that will go right to the bottom line. More research is being done in this area, especially as it relates to print quality issue in flexography and how temperature affects water-based ink. However, the potential savings are large enough to make it worth your time to investigate how to minimize the heat your system gener- ates and how to mitigate the heat that remains. If you keep your cool, you just might make some money. And, unlike most schemes that promise big pay days, this is no scam! ABOUT THE AUTHOR: Craig Shields is president, Graymills Corp. TECHNOLOGIES & TECHNIQUES
Sustainable Fall 2009