Ch.5

A STUDY OF PROCESS CONTROL IN EXPANDED GAMUT PRINTING

Teresa Clancy

Introduction

In recent years, an upward trend in the variety of jobs and number of short-run jobs has impacted the graphics industry in many ways. The need for efficiently producing short-run jobs has produced many solutions from enhancements in automation to color management. One method of enhancing efficiency is utilizing expanded gamut technologies.

Brand colors, which are often featured on printed products, have been historically printed with spot colors as opposed to a typical four-color build. Many jobs are printed with CMYK, plus one or two spot colors depending on the job. By adding additional process colors beyond CMYK, the gamut of a press can be expanded to more accurately reproduce brand colors. Depending on one’s tolerance for color shift, expanded gamut methods can reduce or even eliminate the need for spot colors. Eliminating spot colors via ECG can lead to a reduction in downtime on press, makeready time, and inventory (Furr, 2015).

Achieving the benefits of ECG does involve some risk. A company may have to overhaul their entire workflow to gain the full advantages the method offers. The shift from a four-color process to a seven-color process requires changes in almost every step of a job’s workflow from pre-press to production.

Beyond the implications of application, few standards exist for expanded gamut printing beyond CMYK-only expanded print conditions such as Idealliance’s XCMYK and ICC’s CRPC-7. For individual ink channels, use of the ISO 20654 standard is on the rise. This standard serves to calibrate and predict spot color tonality using colorimetric tone value, similar to the G7 approach (Ellis 2017). However, standards specific to the ECG production have yet to be published.

By increasing the number of ink stations creating builds, concerns regarding the importance of process control often arise. While builds are still typically made of no more than three inks, the additional stations require a watchful eye on the process: “printers utilizing this technology must have in place superb process documentation and control systems to be successful” (FIRST 6.0). While some research has shown less color shift in seven-color builds compared to four-color builds (Furr, 2015), the extent to which process variation effects ECG color shifts has not been fully analyzed.

A spot’s color depends primarily on the ink composition and the substrate reflectance it is filtering. Hence, assuming a white substrate, most color shifts of spot colors occur along the L axis. ECG builds, on the other hand, are often made up of up to three colors. Hence, there is more potential for color shift in the direction of multiple axes compared to that of a typical spot color print.

Objective

This study was designed to observe color shifts in ECG-built swatches and determine the correlation between process control and color variation in ECG color reproduction.

Materials

Plates were made using MacDermid’s ITP 60 plate material. Laser imaging was done by the Esko CDI Crystal and exposure was done using the Esko XPS Crystal. Plates were processed using the Mekrom Evo3 Solvent Plate Processor. The plate was mounted using the Camis Irismal S600 Sleeve Mounter and 3M™ Cushion-Mount™ E1320 mounting tape. Anilox rolls for black, yellow, and magenta were 1200 cpi with 1.8 bcm. The anilox roll for orange had 1200 cpi and 2.2bcm. Substrate used was mactac 60# semi-gloss adhesive paper stock. UVH inks were used.

Methodology

Part I: Building the Target

The target was built to include single-color control patches and patches made up of three-color builds of magenta, yellow, black, or orange. There was a color bar on each side of the web. Nineteen control patches were included on each side of the web, and the remaining patches were builds. The control patches included solid patches for each color and 12 single-color tint patches at 66%, 33%, and 10%. The control patches also included two-color overprints MY, OY, and OM. On the 18-inch repeat, an inch clearance was included on the top and bottom of the target, allowing 81 patches to fit on each side. Hence, there were 162 patches total, including 124 patches of 3-color builds. These builds were combinations of 5%, 33%, 66%, and 100% tints of each included color. Some unlikely builds were eliminated from the target, such as builds of three 100% colors, builds featuring 100% K, and some combinations of 100%, 100%, and 5%. In pre-press the print sequence was set to KYOM, an order recommended for the largest gamut increase (O’Hara, Congdon, Gasque, 2016). Techkon’s SpectroVision inline spectrophotometer measurement system was used to collect data during the press run. The SpectroVision uses ChromaQA software to load the artwork, detect the graphics, and collect data. After creating the color bars, ChromaQA was manipulated to measure the designed target.

Part II: Press Run

The press run was performed on the OMET VaryFlex 530 Press. The press was run at 100fpm, which yielded about 67 repeats per minute. After completing makeready operations and achieving consistency on press, the status of the press at that time served as the baseline against which subsequent measurements would be compared. Impression was increased and returned to baseline on each plate in the print order, KYOM.

When controlling impression, the amount of impression increase was recorded and the dependent variable, TVI and its resulting impact on color data, was recorded via the SpectroVision. The impression was increased in two intervals, held at each interval for approximately three minutes and then returned to baseline state.

Data

To observe the impact that process variation has on a color built using the ECG process, patches built of KYOM were measured during each induced variation. Hence, the impact of each variation could be measured in each patch. Because spot colors are made of one ink, while ECG builds of such spot colors are created with multiple inks, the expectation was that the variation would create a larger color shift in these patches than it would in a typical spot color.

Build % vs Color Variation

The charts in Figure 1 show the ∆E_ab of each patch as a function of the patch’s dot percentage of each ink color. This represents the potential correlation between a single ink station’s variation and a build’s color. These graphs show the ∆E_ab measurement of each patch throughout the entire press run, including the intervals of increased impression. The black chart has fewer data points, as many builds featuring black were eliminated, as stated previously. Aside from yellow, each graph has a slightly positive skew. This suggests that the higher the percentage of these colors is present, the more stable the built color is.

Color Variation

Table 1 displays the ∆E_ab data, divided by condition. Total ∆E_ab represents the average ∆E_ab for each measurement of each patch. The baseline column limits the ∆E_ab data to just baseline measurements. The black column limits the ∆E_ab data to only measurements within the condition of increased black impression. The yellow, orange, and magenta columns are divided in that same fashion.

By comparing the total ∆E_ab data to that of baseline ∆E_ab, it appears that the impression conditions had no significantly negative impact on the color variation. In fact, the total ∆E_ab appears to have a stronger process control compared to the baseline. However, looking at the ∆E_ab data of each impression condition signifies that this is mainly due to the data within the condition of orange impression increase. Still, this data may signify that the ECG method can stay within an acceptable tolerance, even with process variation.

Patch Number x ∆E_ab

Figure 2 provides a visual display of the color builds under the x axis. The ∆E_ab of each patch is documented on the y-axis. This graph shows that less color shift occurs at the solid control patches. The ∆E_ab within these areas is not only lower, but fall within a more condensed area, meaning they have a smaller range. This suggests that the single-color patches are more stable than the three-color builds. Though not a direct comparison between a spot and an ECG build, this result could support the use of spot colors for jobs that require staying within strict tolerances.

Conclusion

This data suggests that ECG colors may be more resilient to process variation than originally thought. Despite significant increase to impression, most ∆Eab measurements did not fall far from the industry standard of 2.0.

Considerations for Further Research

The data collected in this study did not align with expectations. While this study analyzed certain aspects of process variation on press, there are many other aspects that can impact a run beyond impression. Further, while this examined the stability of ECG builds with process control variation, the builds were not tested directly against spot colors. If this research continues, it would be beneficial to compare spot colors and ECG reproductions side by side, on the same press run. While the study may be limited in terms of patch quantity, it would be beneficial to compare the stability of the colors directly.

Acknowledgments

This paper was adapted from a group project at the undergraduate Graphic Communications program at Clemson University. Group members included Elizabeth Wassynger, Kailey Arnold, Mkyya Johnson, and Anthony Fattibene. Special thanks to Kenny Tucker at The Sonoco Institute of Packaging Design and Graphics for his help with our press runs and data collection. Thank you to John Seymour for helping us organize and analyze the color data. Also, thank you to Steve Rankin for introducing us to Techkon’s SpectroVision system and assisting us with the ChromaQA software.

References

1. O’Hara, L., Congdon, B., and Gasque, B. Optimizing the Print Sequence for Expanded Gamut, 2016 TAGA Proceedings. (2016).
2. Ellis, Ron. Techkon. The Printer’s Guide to Expanded Gamut: Understanding the Technology landscape and implementation approach. (February 2017).
3. FIRST 6.0, Flexographic Image Reproduction Specifications and Tolerances, 5.0, Flexographic Technical Association, 2017.
4. Furr, Matthew. The Effect of Press Variation on Color Stability on 7-Color and 4-Color Process. 2015 TAGA Proceedings. (2015)