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FLEXO Magazine : January 2014
balance on press throughout a production run is critical for a successful and reproducible outcome. Ink chemistry balance determines color, ink transfer and ink lay characteristics, as well as drying, trapping and end use performance. The establishment of “standards” for process ink formulation for flexo has remained an evolving process. The Flexo Quality Consortium (FQC) arm of FTA has conducted a great deal of research and study on this topic over the past couple years and has recently unveiled a general implication that for all printing processes, the standards should be the same for hue angle and the variation will be in chroma. This recommended direction will aid printers in being able to more closely align with the standards outlined in ISO 12647-2 and 12647-6 . As stated in FIRST 4.1 , “When optimizing the color gamut, the goal is to increase chroma while maintaining hue angle and lightness. In practice, all three components change at different rates as the ink strength is increased... It is better to operate at an ink strength where normal variations result in minor hue angle changes. Additionally, it is important to eval- uate the overprint colors (red, blue and green) to minimize changes that may occur in the overprints while the individual process colors perform acceptably. ” The FQC study also recommends that, in the application of an ECG ink set, an important initial step in the process control implementation is for the printer to work with its ink supplier in the selection of monopigmented formulations that will result in the closest match to the recommended OGV/RGB base colors. The printer may choose to use a commercial system with a pre- defined ink set, or create a custom ink set. The goal, obviously, lies in determining the formulations that will provide for the widest gamut possible under the given print conditions. OPTIMIZATION STEPS Once ink formulation has been established, the key ingredients in optimizing process color print capability lie in determining the anilox engraving specification and plate and mounting tape materials that will deliver the highest possible densities, while minimizing dot gain and maintaining gray balance. Color is controlled by the amount of ink film that is transferred to the substrate. It has been stated often that the anilox roll remains the heart of the flexo reproduction process. Any anilox roll man- ufacturer will support the statement that the implementation of a successful process color control program involves the establishment of a standardized anilox inventory, followed by well defined and documented cleaning and maintenance procedures and including regular inspection and audit of cell volume and wall condition. Partnering with the roll manufac- turer in the establishment of audits, banded anilox trials and ongoing cleaning and maintenance procedures remains a vital link in controlling color on a consistent basis. Effective process control with the ink metering process continues with ink viscosity/pH and temperature control systems. Controlled viscosity has a direct impact on color hue and strength throughout a production run. Establishing a regular program for the replacement of end seals and doctor blades also aids in providing more consistent and repeatable print results. Equally important, however, is establishing the correla- tion in color consistency from the ink lab to the press or, in simpler terms, “color matching.” Once a standardized anilox inventory has been established at press, it becomes a much more simplified process in predicting the performance of ink formulation in the lab. There exist several different methods in proofing of wet ink samples, but the most accurate and consistent results have been demonstrated when press anilox volumes can be repli- cated in the ink lab. Proofing systems are available today that utilize ceramic anilox rolls engraved to match the exact vol- ume used on press, as well as implementing doctor blades, printing plates, mounting tapes and substrate in the proofing process that replicate the correlating print condition—the proofer, in effect, becomes a “miniature printing press.” Establishing and documenting acceptable “pass/fail” delta error tolerances for wet ink samples aids considerably in ongoing process control functionality and leads to more con- sistent and repeatable results. In the case of process ink sets, in particular, the implementation of a Certificate of Assurance (COA) program with every batch of ink delivered from the manufacturer is an important step in maintaining ongoing process control. Cleaning anilox rolls to maintain consistent cell volume is a key step in maintaining process control specifications. When dried ink plugs anilox cells, the ink carrying volume of the roll decreases erratically. (Photo credit: FIRST 4.1) 56 FLEXO JANUARY 2014 www.flexography.org