Liam O’Hara, Bobby Congdon & Brad Gasque
Expanded gamut (EG) offers printers the opportunity to reduce their reliance on spot color formulations to achieve the brand color demands of CPCs. Today’s packaging often employs 4-color process printing for photographic elements in a package’s design and the use of spot colors for high chroma, high impact brand colors for logos and brand recognition. Three to four spot colors are not unusual.
While some spot colors can be replicated with traditional 4-color process, the chroma that can be achieved for many hues is limited, and subsequently 4-color builds often fail to achieve the desired vividness that is required. The Pantone Color Book is a common reference for spot colors; approximately 40 percent of the Pantone book can be replicated using the GRACoL2006Coated1v2 printing gamut (Furr, 2014). Pantone claims 90 percent of its book can be replicated using CMYK supplemented with orange, green and violet EG (Gundlach, 2015). In testing previously performed at Clemson University, a CMYKOGV ink set yielded 72 percent of the Pantone library with an average Delta E 2000 of 1.39 (Furr, 2014).
The use of EG enables printers to dramatically reduce makeready and tooling costs, particularly where jobs can be ganged together (Furr, 2014). These economic drivers have bolstered the adoption of EG in packaging, and with this proliferation, the need for standardization becomes increasingly important to brand owners to ensure consistent results from printers. Toward this end, FTA has specified pigments and hue angles for CMYKOGV ink sets (depending on whether the formulations are water based, solvent or UV inks) in its Flexographic Image Reproduction Specifications & Tolerances (FIRST) methodology. However, the question remains as to what sequence these inks should be incorporated into a CMYK process.
Studies Say…
Prior research into EG print sequences for flexography is fairly limited. A study at Western Michigan University (Sheth, G., et al., 2013) compared YMCKOGV to YOMGCVK, with single pigmented and two pigmented EG inks. The study found enhanced gamut with single pigmented inks, and the YOMGCVK sequence provided a greater gamut (a comparison which held true with both single and two pigment ink sets). The authors state the YOMGCVK sequence was determined based on the transparency of the overprints. The gamut increases were 5.2 percent for the single pigmented inks and 4.9 percent for the two pigment ink sets.
Another print sequence study employing flexographic printing was conducted at Rochester Institute of Technology on 4-color process inks (Patel, 2009). The RIT study attempted five print sequences (YMCK, MYCK, CMYK, KYMC and KCMY) and found that while certain sectors (red, green, blue) were enhanced by various sequences, no overall superior gamut was determined, although the greatest black point density was achieved by KYMC.
Looking beyond flexo, there are a number of studies on print sequence for offset lithography, but the wet on wet trap of litho plays a significant role in this, and is not applicable for flexography, which is a dry trap printing method. An optimization of the gravure printing process (Chung and Hsu, 2006) was performed that employed ink sequencing (along with pigment concentration and gamma adjustments). Researchers found they had improved the gamut by printing MYCK rather than the traditional KCMY generally used in gravure printing.
The study discussed in this article was undertaken to provide a set of reference data for an optimized print sequence for EG in flexographic printing, focusing on where best to place OGV in relation to their analogous process pairs (for example, green in relation to cyan and yellow: GCY, CYG, CGY or YCG). After the completion of the test trials, we set about finding a predictive model to allow one to optimize the sequence without the time and expense of multiple press trials.
Methodology
Print trials were conducted on an Omet Varyflex 530 flexo press at Clemson University. A set of characterization plates was made using 0.067-in. DuPont Cyrel DPR photopolymer plates, utilizing Esko Full HD screening on 4,000 dpi RIP with an Esko CDI Spark UV2. This technology creates plates with flat top dot profiles by employing a high intensity bank of UV lights that overwhelms the oxygen inhibition to create the necessary features.
The plates were imaged at 175 lpi with circular dots and a relief of 0.020-in. A randomized IT8.7/4 characterization target was used to create profiles. The IT8.7/4 was reproduced twice on each plate, with the target in different orientations in order to capture the press variation across the substrate. In addition to the characterization chart, a color bar with solid patches of the seven primaries, as well as the various 2- and 3-color overprints, was included. The overprints also overprinted black, in order to provide a basis for opacity measurements. Figure 1 shows the overprint target, which was used to acquire opacity data.
The print trials were conducted using CMYKOGV single pigmented UV inks from Siegwerk (L39 series) that conformed to FIRST standards for EG printing, which are specified by hue angle. Table 1 shows the FIRST specifications.
WestRock 10-pt. Printcote paperboard was used for all trials. The press was run at 300 fpm for each of the print sequences tested. The modular nature of the press was ideal for a study of this nature—the individual print decks can be removed and repositioned to create the various ink sequences, so it was not necessary to clean the stations between sequence trials. Furthermore, the same anilox and blade were always used with each color, so there was minimal variation between the various sequences; the anilox roll, doctor blade and anilox to plate settings remained constant, and only the plate to substrate impression had to be reset. The anilox rolls were ceramic rolls with a 60 degree angle, and Flexo Concepts composite blades were employed on each station. The cpi/bcm specifications for the anilox rolls at each station were:
- K: 800/2.0
- CMY: 1200/1.8
- OGV: 900/2.2
Lastly, UV inks provide great stability, as there are no amines or solvents to monitor and replenish. The solid ink specs between pressruns were maintained within 1.0 Delta Eab. Once impressions were set and solid ink colors confirmed to match between runs, the press was run at 300 fpm for a minimum of two minutes.
The study employed the Esko Equinox strategy of running four sets of characterization targets—CMYK, CGYK, OMYK and CMVK—rather than a single characterization. In this way, each of the EG primaries is only sampled with its analogous process colors—for instance, orange is only useful in the red sector, so it is only sampled with yellow and magenta for hue and chroma, with black providing tone. Equinox employs a 100 percent GCR strategy.
Because of the constraints of web width and repeat length (20-in. wide and 18-in. repeat), it was not possible to compile all four characterization targets on a single set of plates. So, the OGV plates were made identical to their complementary process colors, and once a print sequence was set up, the various print stations were turned on and off to create the four sets of characterization targets necessary to create the profile. Figure 2 illustrates how the full 7-color target was “broken down” to create the four sets of targets.
Three print sequences were tested with the aim of placing the OGV primaries before, between and after their analogous process colors: KCMYOGV, KOGVCMY and KCGVMOY. It is standard practice at Clemson to print KCMY, and we elected to restrict the print sequence study to the placement of OGV within that sequence, a decision motivated to limit the number of pressruns for the experiment. A fourth sequence, KYOMGVC, was added to investigate the “between” strategy with the process colors reversed, to provide data on MOY and YOM (for example).
Ten samples were pulled from each of the pressruns, and from these, three sets were measured using an X-Rite i1iO table via MeasureTool 5.0.8. The three measurements were then averaged, and the averaged data was used to create EG profiles via the Equinox Profile Creator software. Color Engine Pilot was used to compare the resulting gamut volumes.