The use of additional inks to augment the reproducible color gamut of CMYK process color sets has been employed for decades now, but the movement towards standardized processes for expanded color gamut (ECG) has only recently gained momentum. To this end, the Flexographic Image Reproduction Specifications and Tolerances document, FIRST 5.0, has established a base set of pigments for Orange, Violet and Green inks, which were employed in this study. However, the issue of print sequence has yet to be addressed in an official manner. In this paper, the authors investigate the print sequence for a FIRST-compliant ECG UV ink set with the goal of achieving the greatest possible gamut of reproducible colors.
This work was conducted on a seven-color OMET Varyflex flexographic printing press using single-pigment UV inks on coated paper. The modular nature of the press allowed the researchers to move entire printing decks around within the press, thus ensuring minimal variation in the production other than ink sequence. Four sequences (KCMYOGV, KOVGCMY, KCGVMOY, and KYOMGVC) were conducted to insert the OGV inks either before, in between, or after their process ink components. Overall gamut volumes were measured, and the two-color overprints were examined with respect to their chroma, opacity, hue angle and lightness. From this analysis, a model for optimizing ink sequence is proposed. This study has bearing on four-color process printing as well as expanded color gamut--gamut expansions of 106% (4/c process comparisons) and 107% (ECG comparisons) were obtained through adjusting print sequence.