Do we print with anilox line count, the number of cells per linear inch? Or do we print with anilox cell volume and the resulting ink film thickness that transfers to the plate and then to the substrate?
A typical press room scenario:
A repeat process color job comes to press, so the printer, helper or setup person prepares four anilox rolls appropriate for process printing. A quick check of the job history shows that the last time the job was run, four 800 line count anilox rolls were used for cyan, magenta, yellow and black. No problem. There are six 800 line count anilox rolls available on the anilox roll storage rack. The printer installs four anilox rolls, selected randomly, pours some ink, installs the plates and proceeds to set impression.
With impression set properly, the printer stops to measure for specified solid ink densities utilizing running targets on the trim area of the web and a densitometer.
With the same stock, ink, plates and mounting tape the printer is not getting the same color as the last run.
Anilox rolls installed are all 800s, just like the last run, right?
Yes, no and maybe.
They are 800s – that is stamped clearly on the side of the roll by the manufacturer – and line count cannot change, unless the roller is re-engraved.
So what's different, and why does it take wasted stock, ink and production time to see the difference?
Out of the box from the anilox roll supplier, specified to the same volume, anilox rolls will print the same, within manufacturing tolerances. Over time, however, anilox rolls that are specified the same will print differently as a result of cleaning procedures, damage and wear.
So with six anilox rolls to choose from, what are the chances of selecting four that will print the same as the last run based on anilox line count alone? Not too good.
The six anilox rolls vary in age from six months to four years. The older rolls were subjected to improperly used off-press cleaning equipment and damaged over time.
Above is a numerical and graphic representation comparing the measured results of dot gain (150 dpi) from printed samples utilizing anilox line counts of 1,500, 1,000 and 700 cells per inch at the same specified cell volume of 2.5 BCM. Comparisons between the three line counts show that at the same volume-measured dot gain was essentially the same utilizing identical ink, plate material, mounting tape, press and printing conditions. (Test data and charts are provided by Harper Corporation of America's GraphicSolutions Division.
Regular anilox roll audits, effective cleaning procedures, appropriate inspection capability and proper identification of roll to job history may have minimized the waste incurred.
A better way to minimize or eliminate waste is to identify the anilox rolls by serial number and volume. Knowing which anilox rolls have the higher volumes allows the printer to apply them accordingly. Given a choice, a printer will use stronger rolls to apply black or use the weakest anilox roll for yellow.
Clearly this is a question of anilox volume, not line count, because the line count was the same.
How much time and material is lost in identifying the cause of this potential problem that is avoidable with a different way of thinking and communicating?
Density and color
Flexographic printing is all about solid ink density and color. Density and color come from ink pigments formulated by ink manufacturers. Pigments are delivered with vehicle systems and additives providing desired performance characteristics after printing, such as scratch, scuff, fade or moisture resistance.
Ink systems are formulated to the lowest anilox volume possible to achieve solid ink density or color match with the thinnest ink film.
Anilox roll suppliers provide cell volumes required within manufacturing parameters that provide accuracy, consistency, optimum performance and reproducibility.
The purpose of this article is to break this paradigm paralysis and help printers and suppliers alike to understand exactly what is more critical in the flexo printing process and why: anilox roll line count or engraved cell volume.
Anilox Line Count
Line count is the number of cells per linear inch, at the angle of engraving, typically 30°, 45° or 60° in reference to the axis of the anilox roll. Once engraved, anilox cell count does not change.
Lower anilox line counts accommodate higher volumes for solid coverage and coatings. A volume of 8.0 BCM is engraved within a range of 165 to 300 cells per inch.
Higher line counts are better suited to lower cell volumes. A 2.0 BCM volume for process or combination printing is engraved between 700 and 1,200 cells per inch.
Laser engraving parameters dictate that in order to achieve accurate reproducible cell volumes and cell structure with thin, smooth cell walls, an engraving cannot be too shallow or too deep.
Historically, shallow engravings are specified for viscous coating applications where optimum release and subsequent cleaning are of concern. To achieve the same volume at different cell counts, cell depth will be deeper the higher the cell count.
Volumes engraved at higher line counts tend to transfer a smoother ink film.
Cell counts increased by 200 cells per inch increase the number of cells in a square inch exponentially by 40,000 cells, providing more control of the ink film transferred.
Line count accommodates cell volume and has little to no effect on print quality, despite conventional specifications.
Anilox Cell Volume
Cell volume is the capacity of the engraved surface in a square inch, expressed in billions of cubic microns (BCM).
Higher volume translates to higher solid ink density, more color or a heavier coating thickness. Lower volumes apply thinner ink films directly associated with higher print quality and process efficiency.
Anilox cell volume changes over time, due to wear, plugging and cleaning procedures. Dirty or plugged cells are temporary and can be corrected to restore cell volume and color. Wear is irreversible and permanent, resulting in weaker or less color due to lower volume.
The graph above represents print contrast results between a 1,500 (27%), 1,000 (28%) and 700 (28%) line counts engraved at 2.5 BCM volume. Print contrast is calculated by measuring the ink density of a solid area and the ink density in a tint area. Print Contrast (%) = (Ds – Dt) / Ds, where Ds = Density of a solid area and Dt = Density of a tint. Minimum print contrast for acceptable process printing is 20%. The higher the percentage, the greater tonal reproduction range is possible.
- Solid coverage / coatings: 5.0 BCM or higher
- Combination printing: 2.0 to 4.0 BCM
- Process printing: 1.3 to 3.0 BCM
If referred to by volume alone, the anilox roll becomes much easier to understand and its effect on the printed results is not hindered by misconceptions relating line count to print quality. This is especially true today as anilox roll manufacturers offer increasingly more volume and line count combinations as engraving capabilities change with available laser technology.
Imagine referring to an anilox roll inventory by volumes alone with their respective applications, as follows:
- Process work: 1.5 BCM
- Combination work: 2.2 BCM
- Line work / solid coverage: 3.5 BCM
How much easier would it be for management, purchasing, supervisors and printers to manage and apply an inventory identified by volume alone?
In these times of flexography's continued growth with a limited pool of skilled printers available, print shops resort to training internal personnel and mechanically inclined applicants. What better way to eliminate a critical variable in the process and level the playing field with more experienced operators and printers?
One application: one volume. Process work: 1.5 BCM only. No choice. No opportunity to install the wrong anilox roll, sacrificing print quality, rejected material and associated down and rerun time.
And what of suppliers whose own products are affected by the selection and performance of the anilox roll? Customer says their anilox rolls are 800s.
But are they traditional 800s at 2.0 BCM, or are they progressive and employ 3.5 BCM rolls? As an ink supplier I need to know so that my ink is specified to the correct volume for accurate color reproduction.
Line count means little and is simply a means to an end for the anilox roll manufacturers.
So, which prints better, and why? A 500, 700, 1000 or a 1500 line count anilox roll?
Engraved at the same volume, they will all print relatively the same solid ink density and dot gain. According to most published engraving charts, all four line counts could be engraved at 2.5 BCM. The 500 line count would be shallow at 2.5 BCM, but it is on the charts, nonetheless.
- Same volume = same ink film thickness.
- Same ink film thickness = same solid ink density.
- Same ink film thickness = same dot gain.
What about the traditional idea of printing plate support through anilox roll screen count? At one time the ideal ratio of plate to anilox ratio was 4:1, then 6:1. Then came the comparison between the physical size of the smallest dot on the plate to the cell opening dimension of the engraving. This was a great idea, but to what end?
If the printing plate was composed of 1 percent digital dots without the support of successfully larger highlights, midtone dots, screens, and solids, then yes, this would make perfect sense. I have yet to see that plate in use on press in a production environment.
This graph shows solid ink density. Using a densitometer, printed samples (cyan ink) were measured to confirm that at the same anilox volume printed results were essentially the same for all three line counts. Though measured density is the same, higher line count anilox rolls apply a more controlled ink film transfer to the plate and substrate resulting in a smoother ink appearance.
As flexography has ascended through the ranks of traditionally higher quality printing processes, virtually all of the components of the process have evolved to allow higher quality print capabilities, increased process efficiency and entrance into previously inaccessible markets.
Contemporary printing presses and converting and finishing equipment bear little resemblance to the equipment of 10 years ago in performance and capabilities.
Printing plates changed from rubber to photopolymer materials allowing the ability to make and hold less than a 1 percent dot on screen resolution to 200 dots per inch or higher and are made in a fraction of the time.
Water based ink systems have transformed from pH sensitive, maintenance dependent formulations with color matching capabilities down to 1.0 BCM anilox volume, with little to no maintenance required. UV inks have maintained color strength while viscosities have decreased to near water based ink numbers.
Press tooling, doctor blades, mounting tapes, web inspection systems and virtually every other component of the process have improved and contributed to this industry's capabilities and growth.
Along with the improved hardware and the capabilities of the flexographic printing process, new attitudes and outlooks on the process have encouraged and embraced continuous improvements to this dynamic industry.
So after all these years and changes in technology, new ideas, understanding and appreciation of the relationships of anilox, ink, plates and print quality, why do we still refer to a critical component of the process backwards?
Call it the "heart" of the flexo printing process or whatever you want to, but the anilox roll is all about volume and ink film thickness, not line count.
Got your attention? Go to press and prove or disprove what I have suggested and see it for yourself.