Flexographic Expanded Gamut Printing with Proprietary and Nonproprietary Characterization Charts
Dr. Martin Habekost, Dr. Reem El Asaleh and Abhay Sharma, Ryerson University
Expanded gamut printing is a relatively new concept for spot color printing in packaging and commercial print production. Instead of only using the four process colors cyan, magenta, yellow and black and spot colors the number of process colors is expanded by the three colors orange, green and violet to create many spot colors with the new fixed CMYK-OGV ink set. The benefit of using expanded gamut printing is that the same inks can stay in the ink fountains of a printing press and only the printing plates need to be changed from job to job.
The authors have conducted earlier studies with expanded gamut printing in digital printing, inkjet printing and offset printing at the School of Graphic Communications Management and the results of these studies were presented at the 2019 and 2020 TAGA conference and published in Color Research and Application. In response to interest from the community we now evaluate flexographic printing and also the new Idealliance ECG small v1 (2019) chart.
The study was carried out on a 7-color 7-inch wide Comco Cadet narrow web flexographic label press located at the School of Graphic Communications Management at Ryerson University. The water-washable printing plates from Toyobo were exposed on a ESKO CDI 2530 and processed on an OrbitalX plate processor. The substrate used in this study was a 40 lb commercial coated label stock.
For this study two expanded gamut software solutions were used. One was Esko Equinox and the other was GMG OpenColor. Each of these software solutions use their own proprietary characterization test target to characterize the color behaviour of the printing process and make a spot colorbuild of the test colorsusing the CMYKOGV inks. Idealliance has also published a test target for expanded gamut printing -the Idealliance ECG test chart v1 (2019) with 4340 patches and the Idealliance ECG small v1 (2019) with 400 patches. The reason for this initiative from Idealliance is to establish a standard test target , like the IT8.7/4 is for four-color offset printing, for expanded gamut printing. Due to the restrictions of the width of substrate that can be fed through the press used in this study the Idealliance ECG small test target was used.
Before the characterization test charts could be printed it was necessary to conduct an optimization press run to determine the ideal combination of plate lpi, anilox lpi, anilox BCM, ink viscosity and printed ink density. Once these parameters were established a curve calibration press run was required to determine the cutback curves that were needed for each color so that good tone value increase numbers were measured. The CMYK colors were tone balanced to approximate G7 grey balance while the OGV colors were linear in SCTV.
The data gathered from the characterization run allowed us to determine the color gamut of the ink set used in this study. The color gamut of the ink set was compared to the Pantone+ Solid Coated library. This was done so in-gamut spot colors could be selected. Overall 30 test colors were selected. These test colors were spread as evenly as possible over the 360º hue angle chart. Three Pantone colors that were very close to the orange, green and violet inks were also selected with four-step tint ramps. These selected colors were incorporated into a custom test chart with 50 test patches.
Figure 1: Custom test chart used for the verification run
In the next step of this study the custom test chart was processed using the characterization data from the proprietary and nonproprietary test charts. This was done to see if the data from these characterization charts changes the build of the test colors.
The Idealliance small chart was processed by both software solutions and the build of the selected Pantone colors was analyzed. CIEDE2000 was calculated between the M1 L*a*b* of the Pantone+ Solid Coated library and printed and measured 7-color builds of these spot colors.
The main outcome from this study was that we conducted expanded gamut printing on a completely manual 7” narrow web label press and went through the step of optimization, curve calibration, characterization and verification. The average DE2000 for the tested Pantone colours using the proprietarycharacterization charts was a DE2000 of 2 and 3.2 with the Idealliance small chart respectively. Both software solutions did better in regards to color accuracy with their proprietary characterization targets than using the data gathered from the Idealliance ECG small chart.
Figure 2: Visualization of the press runs that were done for this project