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FLEXO Magazine : May 2012
Industry Indicators Perfect the Imperfections ink Developments for High-speed Printing By Jim Felsberg With the challenging economy, rising costs of produc- tion materials, and increasing sustainability compli- ance requirements, converters are looking for ways to create production efficiencies or cut operating costs. One method many converters are using to improve press- room efficiency is upgrading of presses for increased high- speed capabilities. Flexo press speeds continue to increase at a blistering pace—with speeds approaching, and even surpassing, 2,000 feet per minute. As press speeds increase, design parameters for the inks used also need to change. These variations ensure that inks dry at the appropriate rate in order to avoid defects in printed output. TYPICAL DEFECTS A wide range of defects can occur when inks are used without these new design parameters. One of these defects is the dirty print; where large clumps of ink build up and transfer to the print web. As shown in Figure 1, this is not the traditional dirty print where the dots are bridging, but more a clump of ink many times larger than the half tone images that transfer to the print web. The defect varies in position from proof to proof. A second defect, known as feathering, involves ink that spreads out in an irregular fashion on the edge of a solid. This defect is typically seen on the edge of the plate. Instead of a sharp break, the edge of the plate prints in an irregular pattern. The third defect, ghosting, is the presence of a faint image on the printing area. This is typically seen where the anilox is plugging and/or the ink does not resolubilize quickly enough for the speed and the doctor blade opening. Anilox plugging occurs when small amounts of ink dry in the anilox roll, effec- tively reducing the volume of ink applied. Misting is another challenge that frequently occurs with high-speed presses. This happens when the ink is transfer- ring from the anilox to the plate, and the plate to the substrate under certain conditions. During the process, a mist of ink will be transferred into the air. The inks will dry into a dust, falling onto the press parts, such as the doctor blade chambers. Since this dust does not typically transfer to the web, it wastes ink and requires extra clean up. It also represents a potential health hazard to workers. Typically, there are three main reasons why these defects occur with high-speed printing: 1. The ink drying speed is incorrect 2. The ink resolubility is reduced 3. The ink needs to be stronger in color DRYING SPEED One of the most common causes for defects is that the ink drying speed is incorrect. As press speeds increase, the ink drying speed needs to be slower. Historically, inks that are based on ethanol tend to dry too fast, so as press speeds increase, a different combination of raw materials should be considered for high-speed presses. Converters who are only moderately increasing press speeds may still use ethanol in some cases, but studies show that more are now using inks based with normal propanol, which dries slower than ethanol. Diacetone alcohol and a variety of glycol ethers are sol- vents that would help slow down the drying of ink on press even further. As more converters have invested in high-speed presses since 2007, there has been a significant shift in the solvents used for the flexo printing process. In 2007, 85 percent of converters used ethanol-based inks for their printing, while only 15 percent used glycol ether- based inks (Figure 1). The shift to high-speed printing can be seen in 2012, where 50 percent of converters now use normal propanol for their printing needs and 35 percent use glycol ethers. Only 15 percent of converters now use ethanol-based inks for the printing needs. The key challenge is to develop an ink solvent blend that meets both the needs of high-speed presses and maintains stability. It is also important to remember that inks are typi- cally not mono-solvent, but contain a mix of solvents that can include a percentage of alcohol, acetates and glycols. DETERMINING DEFECTS • A wide range of defects can occur when inks are used without new design parameters • Possible defects that accompany high-speed flexo printing include: dirty print, feathering, ghosting, and misting • The three main reasons why these defects occur are: incorrect ink speed, reduced resolubility, and ink needs to be strong in color • The correct ink means a world of difference in the world of high-speed flexo printing 22 FLEXO may 2012 www.flexography.org