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FLEXO Magazine : May 2012
High-speed presses shear the inks more severely by exac- erbating the volatility of the solvents, and it is critical that the ratio of these ink blends stay the same during the pressrun. If the ink solvency changes over time during the pressrun, then the inks are not stable and equally capable of causing defects even with a slower dry ink blend. POOR INK RESOLUBILITY When defects occur as ink dries on plates and anilox rolls, it is because the ink resolubility is simply not good enough. Resolubility refers to the balance required between the ink resins and the solvent blend of the ink formulation. If the ink has reduced resolubility, the resin cannot re-dissolve the partially dry ink on the anilox roll/plate, allowing it to transfer cleanly for the next impression. Ink has to resolubilize in the microseconds that it comes in contact with the anilox in the doctor blade chamber and the plate comes in contact with the ink in the cells of the anilox. Resins will tend to retain or hold onto the solvents they are more soluble in. Figure 2 shows a headspace vial with an evaporated solvent blend (Solvent A & B) above high-speed ink with a general resin chemistry of rosin ester vs. polyamide. There is a different ratio of solvents (a blend of alcohol and acetate) in the headspace composition above the ink than the solvent ratio in the ink. Polyamide selectively absorbs alcohol and releases more acetate into the headspace and is less tolerant of acetate compared to rosin ester. Polyamide should be more readily resolubilized in a blend containing a higher level of alcohol. A higher proportion of acetate should favor resolubility of rosin ester. Solubility Parameter Mapping is a way of predicting if one material will dissolve in another and form a solution. This mapping is based on the idea that like dissolves like where one molecule is defined as being similar to another if it bonds to itself in a similar way. To reach optimum resolubility, the solubility parameters (δ) of polymer and solvent need to match in order to improve high-speed flexo print quality. (See Figure 3) Dynamic resolubilization of ink resins on the anilox de- pends upon print speed and solubility parameters of polymer and solvent. Poor match ups will contribute to dirty printing due to excessive drying in the anilox or poor bond strength for lamination to undesired retention of solvents. The solubil- ity parameter is related to the cohesive energy density and ultimately dispersive, polar, and hydrogen bonding forces. This means that reformulated inks for high-speed printing must have a wider solubility parameter window, so that resin solubility is not as sensitive to changes in the solvency and better balance can be achieved inside the solubility parame- www.flexography.org may 2012 FLEXO 23