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FLEXO Magazine : December 2012
true red or by replacing yellow with an orange, etc. The report indicated that adding one pigment could result in a gain in gamut of up to 40 percent over CMYK alone. Figure 2 shows the expected gamut increases in the region of CIELAB space near to the added primary, as reported by the Esko software. The report from Clemson recommends adding an ink con- taining pigment Green-7, an ink containing pigment Violet-23 and an ink containing an orange pigment. But the report was unable to make a specific recommendation on a single orange pigment. Of the five submitted orange inks, the study was able to reduce the number to three colorants but was not able to distinguish between, pigments Orange-5, Orange-34 and Orange-64. However, Orange-5 is a difficult pigment to manu- facture and its shade and undertone vary dramatically from batch-to-batch and would thus not be a suitable candidate for a fixed process set. Orange-64 is a high performance reddish orange with better light and chemical fastness than Orange-34. Due to its improved stability O-64 is most often used in high performance inks and coatings, as a result it generally has a higher cost per pound than O-34. RECOMMENDATIONS Using the theoretical, ideal color aims illustrated above, the committee developed practical aims for a 1.5 to 2 BCM ink transfer from an 800 to 1000-line anilox that represented a rea- sonable flexographic process print. The aims were given to a small number of converters who requested ink sets or received ink sets from inkmakers that are close to these aims. Note that there are two BLUE pigments; both are used in flexible packaging, though the redder shade blue is not available as a mono-pigmented ink. In fact, there are not many really good red-shade blue inks, capable of being printed in a process set, so this ink needed to be produced by blending two colorants. The inks were printed on typical job stocks (coated paper, clear film, opaque film, foils, board) using an IT8/7.4 target to capture the fingerprint of the inks (CMYK in one run and then substitute OCMK, YGCK and finally CMVK), as demonstrated in the Esko report. An experienced inkmaker determined whether a specific pigment could be dispersed into a flexo sol- vent or water born vehicle in a way that resulted in an ink that printed a strong or bright color using low volume aniloxes and still remained transparent enough for overprinting. To fully characterize the increase in gamut during a press- run, the converter obtained a known, reliable CMYKOGV profile target and processed those targets into plates. Such targets are available from high end profiling packages, such as X-Rite's ProfileMaker for Packaging. There may also be third party forms available from some inkmakers or from some prepress providers. Label printers chose a substrate that was appropriate to their business and wide web similarly to their business. Thus, the committee received prints on both paper and flexible film substrates. The printed substrate was cut and sampled as per ANSI/CGATS Recommended Industry Practice, Color characterization dataset development -- press- run guidelines.11 These guidelines describe how many charts to read and how to select those charts from the full finger printrun. A seven-color test chart contained a very large number of patches and it was impractical to try to read those patches manually using a handheld spectrodensitometer. Fortunately, X-Rite has included the EyeOne and iSIS in the new XRGA12 standardization scale, so that readings from either of these instruments were comparable to the scales obtained from the handhelds. The printed sheets were collected and provided to two or three locations for color characterization using a modern digital spectrocolorimeter, such as the EyeOne-IO, the EyeOne-iSIS or a similar X-Y scanning system, following the guidelines of ANSI/CGATS Recommended Industry Practice, Color Characterization Dataset Development - Procedures for Color Measurement System Process Control and for Inter-lab Coordination.13 From the measurement data and profiles cre- ated, the gamut was plotted and the volume of the gamut solid computed, as well as the overlap or lack of overlap between the four-color gamut and the seven-color gamut. FIELD TRIALS To validate the reports from the literature, a call was sent out to flexographic converters who would be interested in investigating, or were in the process of investigating, the use of extended gamut process colors in the printing plant. About 10 converters answered the call and volunteered to assist in this study. There are currently no standard characterization targets for extended color gamut printing. In addition, some of the commercial ICC profile analysis software, designed for n-color applications, does not compute the n-color gamut TABLE III --- RECOMMENDED EXTENDED GAMUT INK SET FOR FLEXOGRAPHY Color Index ID Common Name† Available For Use in Ink Chemisty Solvent Energy Cure Water ††Pigment Black 7 Carbon Black X X X ††Pigment Blue 15:3 Phthalo Blue B X X X ††Pigment Red 57:1 Lithol Rubine X X X ††Pigment Yellow 14 Diarylide Yellow AAOT X X X Pigment Orange 34 Diarylide Orange X X X Pigment Green 7 Phthalo Green (Cl) X X X Pigment Violet 23 Carbazole Violet X X X †Names from NPIRI Raw Materials Data Handbook, Volume 4 -- Pigments ††Pigments recommended in the FIRST Guide 18 FLEXO DECEMBER 2012 www.flexography.org