In the final step—verification or when printing a customer job—image data and graphics are processed through the vendor/ICC profile to create plate separations for color-accurate printing. It is at this step that the operator can use color management software to target any required printing condition, such as a house standard or GRACoL, SWOP, CRPC, etc.
It is important to note that when printing the customer’s job, the profile is only valid, and the results will be as expected, only if the printing conditions for the final job are the same in terms of the original press optimization, curve calibration and color characterization. In other words, we have come so far, done all these pressruns, but if the customer has now changed the media type, we have to repeat all the aforementioned steps and go back to the drawing board!
Test Charts
We have noted that the color management process relies on a characterization chart as used to determine the color characteristics of the printing process. The number and choice of color patches in expanded gamut characterization test charts is a work in progress. There is no standardized IT8.7/4, IT8.7/5 or ECI 2002 as used in CMYK printing. In narrow web flexographic printing, there is also the balance between press sheet real estate versus number of patches; in other words, how many patches do I need to make an accurate color profile?
In this project, we tested proprietary charts from Esko and GMG Color, and a non-proprietary test chart from Idealliance (see Figure 1).
- Esko Equinox 4×4 Chart, EDK version – 3,872 patches
- GMG OpenColor EG Chart – 700 patches
- Idealliance EG Test Chart CMYKOGV Small v1 (2019) – 400 patches
The Esko Equinox solution has a unique approach that is different to any other solution on the market in that it has a very large number of patches, which creates separate press forms. The chart is therefore printed on different parts of the media roll and in separate pressruns that can create variation and errors. GMG OpenColor offers a test chart that is directly related to the printing process, TAC and can be scaled to the customer’s plate size. The Idealliance EG test chart is available in a small version, as used here, and a more extensive four-page version that would not fit on our 7-in. Comco Cadet label press.
Mark Samworth from Esko comments, “EDK stands for Esko Descending Black and compared to the IT8.7/4, the EDK sampling logic, with far more black sampling, is designed to produce more accurate high GCR 4C or expanded gamut 7-color printing.”
Ryerson Pressruns
In this project, flexographic plates were made using the CDI system, press optimization was carried out to establish optimum plate and anilox characteristics, ink viscosities, print densities and screen angles for CMYKOGV printing. Next, a curve calibration pressrun was done to gray balance (G7) the CMYK channels, while leaving the OGV separations linear in terms of SCTV.
The curve calibration was then used in a set of characterization pressruns with the three different 7-color characterization charts. In the verification pressrun, 30 in-gamut spot colors from the Pantone+ Solid Coated library were printed using separations generated from Esko Equinox (Curve Pilot and Color Pilot) and GMG OpenColor. The following Delta E (CIEDE2000) results were computed between the published, official L*a*b* value for the spot color vs. the spot color printed on press.
- Esko Equinox with Esko Equinox test chart – 2.45 Delta E
- Esko Equinox with Idealliance test chart – 3.49 Delta E
- GMG OpenColor with OpenColor test chart – 1.55 Delta E
- GMG OpenColor with Idealliance test chart – 2.94 Delta E
In interpreting this data, Delta E refers to a color difference, and a lower Delta E number is better. In package printing, a general rule of thumb is that < 2 Delta E is acceptable for most customer specifications. We should note that there is a margin of error in these results due to normal variations in printing, inks, media and measurement. However, these variations are normal and are not expected to significantly alter the conclusions.
The results suggest that both systems perform better with their own proprietary chart, and that neither system was able to produce acceptable results with the very small number of patches in the Idealliance test chart (only 400 patches). Birgit Plautz of GMG Americas suggests, “These results show that EG printing is very viable today and the customer could have printed 30 different spot color jobs with a single press setup.”
Get on Board
The immediate benefits of printing with a CMYKOGV ink set include the ability to reproduce a large number of spot colors without spot color inks—the ability to gang jobs, less ink changes and washups. In addition, the intelligent use of inks—using orange and yellow ink to make a yellow color, not cyan—leads to better accuracy on press that minimizes the visual effect of press drift during long pressruns. Finally, the extra colorants do generally extend the color gamut of the printing process.
About the Authors
Contact the authors for a copy of the full report for spot color accuracy in flexographic EG. They can also present their findings to your company and help you convert to EG printing today.