Summary & Conclusions
The soaking procedure clearly illustrated that the degree of thinning and hardening of finished flexo plates is directly related to the duration that they are in contact with common solvent-based ink solvents—longer contact has greater thinning and hardening effects on the plates. Interestingly, the 1 x 24 soaked plates experienced greater degrees of thinning and hardening compared to the 3 x 8 soaked plates, despite the soak time being equivalent. This points to a conclusion that longer runs cause more change in plate properties compared to multiple short runs, but more exploration here would be needed to confirm.
Some difference could be seen in the solid ink density and printed dot area of the control plates versus the soaked plates, with the control plates producing slightly higher values in both categories. But in the opinion of the project team, the degree of differences was within the range of normal and expected print variation. In a real production environment, these differences would not require a job ran with similar plates to be pulled off press.
It is likely these differences are due to the increased hardness of the soaked plates, as harder plates are expected to print with less dot gain, but typically at the expense of solid ink density. One unexplained exception is that there was a substantial drop in printed dot area in the 2 percent field with the 1 x 4 variants of both Plates A and B compared to the control (-7.2 percent), but this could be related to other factors (different print sleeve, different print deck, etc.)
The presence and severity of observed pinholing and TEV were the clearest examples of change in the print performance of the variants compared to the control plates. The results point to a correlation that longer solvent ink exposure equates to increased plate thinning and hardening, as well as increased pinholing and TEV. However, in a real-world production environment, steps would be taken to try to overcome the pinholing and TEV via a variety of relatively simple ways – ink adjustments, impression settings, different tape modulus, etc. It was not known if changes such as these to correct the pinholing and TEV would have had an adverse effect on solid ink density or printed dot area, as this was not part of the experiment plan.
Based on this newly discovered unknown, as well as the lack of significant correlation between the measured plate properties versus solid ink density and printed dot area, the project team decided to alter the plan for the second phase of the project. Rather than verifying the results of the Phase I experiment through recruitment of flexible packaging printers to track gauge and hardness of previously printed plates as an indicator versus performance, we instead conducted a controlled verification experiment to learn how correcting for pinholing and TEV on press would impact solid ink density and printed dot area.
Phase II: Verification
For the verification experiment, the goal was to determine if correcting the pinholing and TEV of the variant plates via normal means on press would have a negative effect on solid ink density and printed dot area.
To test this, the control plates, as well as plates that had the same physical attributes as those that printed with pinholing and TEV in the Phase I experiment, needed to be reprinted in a similar environment.
Once ideal impression settings were achieved, the press operator would then adjust other press variables in a preferred order to resolve the subsequent pinholing and TEV. Print samples would be taken and the effect of these adjustments on solid ink density and printed dot area would be measured. In addition, extra time and wasted materials required to resolve the pinholing and TEV would be documented.
The project team opted to reprint the Plate A and B control plates, as well as the 1 x 4 and 1 x 14 variants. As these plates had only been on press and exposed to ink solvents in the Phase I trial for approximately 5 minutes, the need to remake and repeat the soak procedure was unnecessary.
The 1 x 14 variants were chosen instead of the 1 x 24 variants because, while TEV showed an increase in severity with increased soak duration with both Plates A and B (Figure 10), pinholing was slightly reduced with the 1 x 24 compared to the 1 x 14 variant of Plate A (Figures 9 & 10). While this is likely an anomaly, we could not rule out a deeper, yet unexplained cause. The control plates, 1 x 4 variants, and 1 x 14 variants showed a smooth progression of severity of pinholing and TEV with both Plate A and B, and for this reason, were chosen for the Phase II experiment.
Pressrun
Plates were printed at Plastic Packaging Technologies, LLC (now a part of PPC Flexible Packaging, LLC) in Kansas City, KS. The plates were run on press in a similar way as the Phase I experiment, whereby corresponding A and B Plates were mounted in different decks with the same ink, and printed side-by-side on the substrate. In addition, the following press parameters were noted:
- Run Speed: 1,200 fpm (W&H MiraFlex CM)
- Substrate: 48 ga PET
- Cylinder: 26.389-in. CO
- Cushion Tape: 3M 1320 Medium-Soft
- Ink: Sun Chemical solvent-based
- Anilox: Both Cyan = 1200 CPI / 2.2 BCM; White = 220 CPI / 9.0 BCM
- Sequence: Cyan in Deck #3 (Plate A); Cyan in Deck #4 (Plate B); Flood White in Deck #10
- Ink Viscosity: Deck #3 = 28 sec; Deck #4 = 25 sec; Deck #10 = 30 sec (measured using #2 Zahn Cup)
Surprisingly, the plates printed with lower levels of severity of pinholing and TEV compared to the Phase I experiment. The press operator easily adjusted impression settings in order to resolve what little pinholing and TEV occurred with the 1 x 4 and 1 x 14 plates. No loss of time or additional substrate waste was required.
Again, a very slight drop in SID could be seen with the soaked plates compared to the control, but well within normal accepted variation (Figure 12). Of note, the printed solid ink densities of the comparable variants of Plates A and B were much more similar than the results found in the Phase I experiment. A reason for this may be due to different performance of the surface screening patterns in each print scenario.
Most importantly, there appeared to be no effect on printed solid ink density resulting from the change in impression required to fully eliminate TEV and pinholing on press.
The impression settings to overcome pinholing and TEV did not have a significant impact on printed dot area of the A or B Plates. The printed dot area between the variants and controls was remarkably similar (Figures 13 & 14).
Figure 15 summarizes the amount of change measured or observed between the variants and the control plates. For this, the average of the A and B plate measurements and observations were used.
Results of the Phase II verification experiment can be boiled down to the following:
- Despite similar print setups and utilizing the exact same plates, the problems of pinholing and TEV at the Phase II print site were less severe than at the Phase I test site
- Problems of pinholing and TEV in the Phase II trial were easily solved by basic press adjustments to no detriment of the solid ink density nor printed dot area
- It is not known if more severe pinholing and TEV could be overcome with similar press adjustments without causing solid ink density or printed dot area problems
As such, the Phase II verification experiment confirmed something very important in regard to the printing of wide web flexible packaging, and perhaps flexo in general: the cumulative effect of the many variables and options in the printing process, makes it difficult, if not impossible, to predict when any one particular printer will experience a very specific problem.
Conclusions
The goal of this project was to try to find an easily and inexpensively measured property of surface screened finished plates that could indicate if challenging print issues would result from its use on press during the printing of wide web flexible packaging with solvent-based ink.
If a clear correlation could be found and thresholds identified, then the measured feature could be used by wide web flexible packaging printers to identify plates that should be remade and not reprinted, therefore saving valuable downtime and press materials wasted in an effort to overcome the print problems.
In the Phase I experiment, we found that a measurable increase of thinning and hardening of plates contributed to the print defects of pinholing and TEV with increasing severity but had little effect on printed solid ink density and printed dot area. Furthermore, as pinholing and TEV are not easily quantified, the severity is relative to the observer and impossible to link to a measurable threshold of thinning or hardening of the plates.
In the Phase II experiment, we found that a different but similar printing environment using the same plates had profoundly less pinholing and TEV, and simple adjustments to impression solved these problems without negatively affecting solid ink density and printed dot area or increasing setup time and waste.
Taking the results of both experiments into consideration, the project team concludes that using an easily measurable indicator such as flexo plate thinning or hardening is neither reliable nor robust enough to predict not-easily-solved print problems throughout the broader wide web flexible packaging printing community. Attempting to create industry-wide thresholds for plate thinning or hardening could have the opposite intended effect, and many printers may dispose of their plates too early.
However, on a case-by-case basis, individual wide web flexible packaging printers—in particular those where pinholing and TEV is especially problematic—may find that tracking the thickness and hardness of their plates throughout their life is useful in helping to determine an “in house” threshold unique to their own operation and personal judgement of pinholing or TEV severity.
By further correlating the hours on press in the presence of solvent-based ink to the threshold at which the change in plate thickness and hardness yields unacceptable pinholing or TEV to the individual printer, it may be possible to abandon measuring the plates all together and instead use printing time as the metric for when to replace plates.
The project team would also like to stress that the life of a plate can be influenced by many other factors,. For instance, an improperly cleaned plate can result in broken dots which render the plate damaged and useless prior to ever printing on press.
While this study focused on exploring a very particular “natural” and normal cause of changes in plate properties which may necessitate decommissioning, it bears mentioning that printers may be better served by focusing on the basics of proper plate making, care, storage and handling to improve the longevity of their plates.
About the Authors
The pair chaired the Flexo Quality Consortium’s Plate Life Indicator Project and issued a final report on findings, summarized in this article, in February 2023. The research team’s conclusions and experiment parameters were briefed at FTA’s FORUM 2023 in Columbus, OH.
Project Team
We would like to express our sincere gratitude toward the FTA and the FTA FQC Executive Committee for sponsoring this project and for the guidance that was offered to us throughout its duration.
Most importantly, we thank the following project team members for dedicating their time, resources, and experience toward making this study relevant, accurate, and thought provoking:
- Alix Guyot, tesa tape Inc
- Tim Reece, All Printing Resources Inc
- Cori Devlin, DuPont Cyrel
- Joe Riccardella, ABX Innovative Packaging Solutions
- Bob Coomes, Plastic Packaging Technologies LLC