Wide web flexible packaging printing of films is now predominantly done with flat top dot flexographic plates that have a finely patterned texture imparted into the plate surface via the imaging process.
Such digital texturization is typically known as “surface patterning” or “surface screening” and it is a very powerful tool.
It delivers high solid ink density and smooth, pinhole free solids that are a tremendous improvement over solids printed by smooth-surfaced plates on films. These microscopic textures are extremely fine—typically between 1400 and 2400 LPI for process inks—and subsequently can be applied to all the graphics of a plate, including dots—with typical spacing of 120 to 175 LPI in this print application.
As a beneficial byproduct of the plate surface texture doing the bulk of the work in delivering high quality solids to the print result, the burden of the traditional methods of improving solids—more anilox volume, higher impression settings, harder cushion tape, softer plates, etc.—has been eased. It is common to see printers who have implemented surface patterning onto their plates also find that they can use lower volume aniloxes, as well as softer and fewer tapes, which then improves the ability to print more subtle highlights.
There are several commercial offerings in the market to enable plate makers to apply these surface patterns onto their plates. Since the ink, anilox volume, and substrate can have a large influence on the performance of any particular surface pattern to deliver optimal solids, each commercial offering has a library of different patterns, or ways to manipulate a pattern via the imaging in order to best accommodate any given set of press variables.
Typically, a “surface patterning optimization test” is performed whereby various patterns from the library are tested on press with the printer’s other chosen press variables (anilox, ink, etc.), and the best performing surface pattern is selected for that application. If the press variables change, then it may be necessary to also change to a different surface pattern on the plate to achieve optimal results.
Another commonality of wide web flexible packaging of films is that solvent-based ink is predominantly used. Certain solvents commonly used in solvent-based ink (for example, esters like normal propyl acetate) penetrate the plate and cause it to swell and soften over the course of a printrun. The longer the printrun and contact with the ink, the more swelling and softening of the plate occurs. This is a well-known effect and ink suppliers are conscious to limit the amount of the most plate-aggressive solvents in the ink, however some is nearly always necessary.
A known effect of plate swelling from ink solvents is that once the solvents are completely dried from the plate after printing, the plate is often slightly thinner and harder than before printing, and frequently curled inward to some degree. This is because there are certain components of properly made, press-ready plates that are not completely crosslinked and are susceptible to being solubilized by the solvent absorbed from the ink and subsequently extracted from the plate during use and drying in storage.
This has the effect of contracting the photopolymer layer—which makes it thinner and harder (Figure 1)—and since the polyester backing of the plate remains stable, the plate has a tendency to curl inward (Figure 2). The more the plate is swelled by the ink solvents, the more extraction of components takes place, and the thinner and harder it becomes in storage. Furthermore, the effect can be cumulative in that a plate printed several times will become thinner and harder after each use.
Wide web flexible packaging film printers reusing surface textured plates have often noted that while other press variables are kept constant, reprinted plates may no longer transfer ink as they did when new. But, when new, identical replacement plates are used, performance is restored. So, there is reason to believe that a change in the plate has occurred during and due to the process of the ink solvents swelling the plate and subsequently evaporating in storage. This change has introduced a variation (or variations) in the plate that renders it no longer optimal in combination with the other press variables for the application.
Purpose & Scope
It was the purpose of this project to measure the variables of the surface patterned plate that are changing from its initial state due to the ink solvent swelling and evaporation process and determine if these correlate to the transfer of solids in the print.
The potential outcome could be an easily measured identifier that will alert the printer that the plate will no longer perform as expected and should not be put back on press but rather remade. This could be an advantageous time and cost savings for printers that struggle with repeating results on press when reprinting plates.
This project relates to wide web flexible packaging process printing of films with solvent-based inks and surface patterned flat top dot flexo plates.
It was decided to split the project into two phases. The first was an experiment to determine if there is a predictive identifier (plate property) that can be easily measured on a plate that correlates to a high likelihood of poor press performance. The second phase hinged on success of the first, whereby the discovery of an indicator would lead to it being used by volunteer printers to evaluate its effectiveness in real-world settings over time, with feedback routinely provided to the project team.
Since the goal of the project was to identify a measurable change in a plate that could be used to predict performance reduction in printed solids, the surface patterning used, plate type, inks, etc., were considered irrelevant. Some scenarios may show change more quickly based on these differences, but change was all that is being sought, not longevity or superiority of one set of test variables versus another.
Phase I: Experiment
An experiment was carried out to determine if there is a correlation between measured changes in properties of used plates due to the ink solvent swelling and evaporation process and the quality of printed solids of process inks on press when reprinted.
A method was developed to simulate the plates being exposed to solvent ink for the same durations, but without actually printing on press:
- Plates with surface textures were made and soaked in typical ink solvents for varying lengths of time to simulate pressruns and allowed to fully dry
- Various plate properties of all the plates were measured before and after
- Plates were printed for a short length of time (simulating startup) in a typical wide web flexible packaging application
- Print results were measured and compared. Correlation of any differences in print to the plate measurements made prior to printing were evaluated
Plates were soaked for the following times in 80 percent nP Alcohol: 20 percent nP Acetate to simulate ink contact over varying typical run lengths. This solvent blend was chosen as it generally mimics the solvent composition of solvent-based inks and is also at the maximum ester level recommended by plate manufacturers:
Plates were properly stored for a minimum of one week to allow to dry fully before measurement and printing.
Plates were measured after soaking and prior to printing, focusing on the following attributes:
- Shore A Hardness
- Thickness
For the sake of plate neutrality and to evaluate the key surface patterning options in the market, two plates from different manufacturers and two different commercially available surface patterning technologies were used:
- Plate A: 70 Shore A 0.067-in. plates with Surface Screening Technology 1
- Plate B: 67 Shore A 0.06-in. plates with Surface Screening Technology 2
Plates were approximately 20-in. across by 25-in. around and while the focus was on detecting any change in the way solids print. Other typical flexo targets were present in the graphics.
Due to differences in surface patterning technology, the graphics were somewhat different but still similar between plates A and B (Figure 3), and care was taken in the design to not unintentionally influence the print results of one or the other.
Pressrun
Plates were printed at ABX Innovative Packaging Solutions in Macedon, NY, with the following usual production conditions and startup procedures (clean plates with alcohol prior to printing):
- Press Speed: 1,400 fpm (W&H VistaFlex)
- Substrate: 42-in. wide opaque PE substrate (1.5)
- Cylinder: 25-in. repeat
- Cushion tape: 3M 1320 Medium-Soft
- Ink: Sun Chemical Solvent-based
- Anilox: 900 CPI / 2.2 BCM
- Ink viscosity: 24 sec, #2
Plates from the Plate A group printed in a different deck and left/right position than the plates from Plate B group. This enabled efficient use of the substrate while allowing each plate to be mounted singularly on a cylinder, without fear that differences in thicknesses between gang mounted plates could lead to impression set difficulties.
Since the problem reported by printers occurs at startup, once impression was set for each plate, the run length of each was limited to a few minutes.
Sample Evaluation
From each print sample, the following was measured and/or observed:
- Solid ink density (SID)
- Printed dot area
- Mottle/pinholing of solids
- Presence of Trail Edge Void (TEV)
Print measurement results were then compared to the plate measurements to identify areas of correlation to be considered as predictive identifiers for the second phase of the project.
Test plates were soaked and measured as previously described. The thickness and hardness of the plates showed expected changes based on soak duration—decrease in thickness and increase in hardness as soak time lengthens (Figures 4 & 5).
Both control plates printed with slightly higher solid ink density (SID) than the comparative soaked plates, but within 0.10 difference, which is generally accepted as insignificant change. No further correlation between SID and soak length was observed (Figure 6).
Dot area measurements were taken at the printed 2 percent, 20 percent, 50 percent, and 70 percent values in the print. In all but the 2 percent values, a very slight reduction in dot area could be observed as soak length increased. In the opinion of the project team, the difference in dot area from plate to plate would be acceptable in a production environment and not considered excessive (Figures 7 & 8).
Soaked Plate A variants showed an increase in pinholing in the printed cyan solids with the exception of the 3 x 8 plate, which printed much like the control (Figure 9).
The soaked Plate A and especially Plate B variants showed an increase in trail edge void (TEV) in the printed cyan solids. The 3 x 8 plates showed less TEV severity than the 1 x 24 plates, despite equivalent total soak times (Figure 10).
Figure 11 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.
