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FLEXO Magazine : June 2011
Technologies & Techniques a minimum of 550 cell count for the anilox roll. As digital plate materials have become more common, the multiplier increased by 2 to a 6-1 anilox-plate screen. This accommo- dated for 1 percent dots at higher plate screen counts, and the newer digital dot structures (round top vs. flat top). The reasoning behind this method is to insure that there is a mini- mum of four anilox cells applying ink to each individual dot. If the dots at the highlight section of the tone range become smaller than the anilox cell, it may cause a “dipping” effect that can lead to bridging dots and dirty print. Now select a cell volume that is in the achievable list of vol- umes for that particular cell count. (See volume range Table 1). A 550 cell count can have a volume ranging from 3.0-4 .5 . TESTS & TARGETS A banded anilox roll test is highly recommended at this juncture to establish the anilox cell count and volume com- bination that will achieve the minimum target density and provide the best graphic reproduction. A banded anilox is an anilox roll that has several differ- ent combinations of engraving on a single cylinder. It can be engraved onto any existing anilox roll from inventory. A test plate is made to match the widths of the anilox engravings. The graphics in each lane will be identical to each other. This process isolates the anilox roll and allows for critical analysis of the ink film thickness delivered by the various engraving specifications. If you do not currently have established density targets, I suggest using FTA’s FIRST(Flexographic Image Reproduction Specifications and Tolerances) densities as a guide. Run each color on your banded anilox roll to determine what volume achieves the density target. PLATES & PROCESSING In conjunction with anilox roll selection, we should deter- mine what kind of printing plate materials we will be using and the ink transfer characteristics of each. There are a wide range of photopolymer materials available in a range of durometers and thicknesses. Each will affect how the ink film thickness is transferred to the substrate, thus impacting the final images. The primary selection criteria for plate materi- als are substrate and graphic requirements. Many questions arise that need to be answered, in order to select the proper photopolymer material. Namely: • Is the substrate rough or smooth? • Will the substrate quickly wear out my plates? • How does the substrate accept ink? High vs. low holdout • Graphic requirements?Not all materials can hold fine dots at high screen counts. • Thickness? What is my cylinder undercut--0 .125-in. or 0.085-in.? • Is the material I want available in the thickness needed for my undercut? Also to be considered along with photopolymer material is the processing method. Will the print job be processed using analog or digital processing? There are new methods that ex- ist for both, each offering unique advantages. Many variables are associated with plate materials and processing methods, but for the purposes of this discussion, we’ll cover the primary factors; solid ink transfer and dot structure. From the data collected during the single color banded roll test we will select the anilox cell count and volume combi- nation to have four process anilox rolls engraved to. Data collection should be done in partnership with your prepress supplier. Generally, the anilox roll that achieves target density, while providing the highest level of contrast throughout the tonal scale, is the specification selected. After we select the anilox that yielded the best results from a density and dot gain perspective we will now be able to run a four-color characterization to determine the printable color gamut. Another set of testing plates will be made and an on- press test run. (See sample characterization target, page 64). The greater the level of printable contrast for each process color, the larger the printable color gamut will be. Therefore minimizing dot gain and maximizing density is a primary goal for selecting the correct anilox roll. Keeping the tone ranges from 1percent-99 percent allows for the largest possible color space.We should be able to print a neutral gray if the CMY process colors are in balance with each other. Once we have optimized the anilox to plate relationship the process will need to be maintained. We will have to manage anilox volume loss and plate wear. As anilox rolls wear or become plugged, they will inevitably lose density. As plates wear, dots print larger. Both of these issues will throw the neutral gray out of balance. Constant monitoring and measurement of the printing process will keep this potential problem under control and manageable. In order to track changes and catch issues quickly, use some form of printed target on every job you run. This will assist and notify the printer when the system is mov- ing away from center. In order to manage the vast number of variables it is necessary to use some guidelines to bring the process more in focus. A characterization will need to be run, the process re-calibrated, and monitored regularly. 62 FLeXO June 2011 www.flexography.org
Sustainable Spring 2011