by David Parr,Technical Sales Manager, Pamarco Global Graphics
Since the creation of Flexo printing, Anilox have been at the core of the flexo printing process and remain at the heart of the process today. Every major technology advance with flexo, from the introduction of reverse angle doctor blades and laser engraved ceramic anilox, through to the recent developments of H.D. (High Definition) flexo, printers have needed to consider, what changes in anilox are necessary, to get the best out of the process. For flexo printers, the choice of anilox has never been greater. Not only can they choose from many different anilox suppliers, but the developments in laser engraving technology, mean that each anilox supplier, can produce and promote a range of different cell shapes & profiles, EFlo, IPro, GTT, to name a few. However, this can sometimes lead to confusion about what is best for YOUR specific printing.
Optimize your Anilox selection
Standardising on one specific cell profile can sometimes be successful, but often, what works for solid and tone printing, doesn’t always give the best results for process colours. In trying to establish a single, universal, the so called “one size fits all” anilox for your range, of flexo printing, does it mean that you have to compromise on quality or performance. Unfortunately the market is littered with expensive casualties, where this has been attempted. We offer some ideas as a solution.
Let’s keep it simple and consider the fundamental task of the anilox. The anilox’s sole function is to transfer a consistent film of ink on to the flexo printing plate. Nothing more. Nothing less. Too little ink can cause printing defects such as low print density & pin holing. Too much ink can cause problems such as dot gain, dot bridging and dirty print. Flexo is generally a forgiving print process, allowing printers to do things in a variety of ways, therefore having the wrong anilox in your press, isn’t always immediately obvious. However as the demands of the market mean that you have to print higher quality, print faster, change jobs more frequently, reduce your downtime and costs, just to remain competitive, it is vital for flexo printers to understand and control every part of the flexo process, to suit the exact needs of their customers. The means correct anilox selection, remains fundamental to the flexo process. The first two questions are still the same. Which line Screen?
Which Cell Volume?
So what are the steps to correct anilox selection, what controls the ink film thickness delivered by the anilox roll and which cell shapes give the best ink transfer? To use an analogy, let’s take a coffee break and imagine the anilox cells as the cup and saucer of a large cappuccino. If both cup and saucer were filled to the top with coffee, which would be easier to carry to your table without spilling? For sure, the cup. You would only have to tip the saucer a couple of degrees, for a significant amount of coffee to spill. So by definition which anilox cell shape, cup or saucer, would have the ability to quickly release or transfer the most amount of fluid. Logically, the wide, shallow saucer shape. Secondly, if you wanted a large coffee, would you have it dispensed into an espresso or cappuccino cup. Of course, you would select the cappuccino cup. Selection of Anilox is exactly the same. Bigger cells carry more ink. Wide shallow shaped cells, transfer more ink, but are more difficult to control. This is critical to understand to try and establish the correct ink film thickness.
Like coffee cups, big anilox cells, carry more liquid and wide shallow cells release greater amount of fluid. Unlike coffee cups, a high percentage of fluid stays in the anilox cells during use
One other important factor is how much liquid is actually transferred from the anilox during the printing process, the “ink transfer”. Many people imagine that, just like with a coffee, you can empty the anilox cups, leaving just a small liquid residue in the bottom. This myth is sometimes magnified by enthusiastic, but totally false claims that anilox can release 70- 90% of the ink from the cells. The reality is very different, with ink transfer measurement, demonstrating that, depending upon anilox cell shape, ink viscosity and press speed, ink transfer rates in flexo printing are typically between 25-50%. This unfortunately means that 50-75% of the ink can remain in the cell; little wonder that anilox cleaning and maintenance is a continuous fact of life, and significant cost factor, in flexo printing.
So after the coffee break, let’s get back to flexo printing. If you want to a large ink film, for higher density and better coverage, you need to either have bigger cells, or have cell shapes that transfer a high amount of the ink. In contrast, if you have too much ink, causing defects like dot bridging and dirty print, you need to reduce ink film by reducing the size of the cells. A simple concept, but increasingly complicated by the fact that the demand for higher resolution printing, means that flexo plates dots are getting smaller and smaller. For example, flexo plate screens of 150 l/in (59 l/cm) have highlight dot diameter of 19 um. By a general rule of matching anilox cell opening to plate dot diameter would mean the selection of an anilox of 1120 l/in (440 l/cm).
Matching anilox cell size to suit the smallest highlight dot on the plate is one method of anilox selection.
Of course, the finer the anilox screen, the smaller the anilox cell opening. However with the reduced cell opening, there is generally also a proportionate decrease in cell depth, with the resulting reduction in Cell volume range (shown in the chart). Even with latest multi-hit anilox laser engraving technologies, there is a logical limit to the range of cell volumes, that can be achieved with each anilox screen. Most high quality anilox manufacturers have their own anilox screen selection charts, each giving a range of cell volumes for every anilox screen count. The cell volume ranges are generally specified to ensure that the anilox Depth:Opening ratios (depth of cell/opening at top of cell measured in microns) remain in the region of 25-30%. This ensures good ink transfer rates from the cells, but what happens, if you need a higher cell volume, a common occurrence when using low strength inks or high absorbency Paper or board substrates which requires more ink? For example, can you get a 1120 (440 l/cm) screen anilox with a cell volume of 5 or 6 cm3/m2? It is tempting to push the boundaries of what cell volume is possible with a particular anilox screen. The drawback, however, is that as the cell depth is increased, in an effort to achieve higher cell volume, the anilox Depth:Opening ratios, also increases. It is already a highly proven fact that maintaining good Depth:Opening ratios is essential to good ink transfer and with every increase in Depth:Opening, there is a proportionate reduction in ink transfer from the cells. Result, more ink remaining in the cells, faster plugging of cells, quicker loss of print density and leading to defects, downtime and cost.
Pamarco’s solution to this problem has been the development of extended cell technology, aptly named EFlo. The EFlo cells, have double length cells in the direction of printing, which offer several advantages for flexo printing. First the elongated cells of EFlo can carry and deliver to the printing plate, a much larger amount of ink, than with conventional 60 deg hexagon cells. Secondly, the cells are engraved at a consistent angle of 75 degrees to ensures no moire clash with flexo plate screens. Finally and most significantly for the printer, the “saucer” shape of the EFlo cell opening in the direction of printing, gives a substantial increase in ink transfer to the printing plate. This means that screen counts can be increased, without compromising Depth:Opening ratios, maintaining good support to flexo plate dots and providing excellent ink film thicknesses on to the printing plate. It also helps to keep anilox cells cleaner for longer, helping to reduce downtime, loss of print density and quality.
Extended anilox cells (EFlo) are able to carry and transfer more ink, but still maintain good support for the highlight dots.
Since EFlo’s introduction in 2010, extended anilox cell technology has been extensively proven in all flexo printing disciplines and is now widely adopted as an industry standard for a substantial number of high quality flexo printing applications. This includes both brown box and high graphic post print corrugated and HD flexo, currently being used extensively in flexible film and paper printing. Most recently, Pamarco’s European team, in collaboration with a major global flexible packaging company, conducted a recent study to optimise print quality and performance, together with driving down the cost of their products. The project, in addition to helping to establish the client’s vision for future print, had targets to utilise Pamarco’s unique Eflo technology, to reduce the number of different anilox screens being used for short run printing and to take “right first time graphics”, to a new level.
The client conducted a series of print trials, utilising 2 banded anilox test rolls from Pamarco, one for improving dot gains and solid densities on high graphic flexo process work up to 200 l/in (80 l/cm) using HD flexo plates, the other for optimising combination solid, fine line and mid-tone vignettes printing, for the clients full range of packaging work. The EFlo anilox screens ranged from 360 to 1200 l/in (140 to 390 l/cm)
One of the stand-out results of the trials was the ability of EFlo, to release high solid densities of ink, on to flexible film, even at anilox screens up to 700 l/in (275 l/cm). The increased advantage of Plate cell patterning also gave some startling results, with solid printed areas being completely free from pin-holes and having densities of at least 30% higher than conventional 60 degree anilox and non-patterned printing plates.
The conclusion of the trial was to reduce the number of different anilox specifications to just 3, for all the clients short run flexible packaging work, as follows:
Printing requirement Eflo HD Anilox specification
4 col Process up to 200 l/in: 1200 l/in (475 l/cm) x 3.8 cm3/m3
Light solid, half tone and fine lines: 700 l/in (275 l/cm) x 6.5 cm3/m3
Heavy solid & line: 360 l/in (140 l/cm) x 9.5 cm3/m2
Of no surprise to anyone, was the fact that the three different plate/cushion tape packages, used in the trial, had a significantly greater impact on variations in density and dot gains, than any of the differences in the anilox screens and cell volumes. This is an important fact to remember, that it is useful to optimise and standardise on your printing plate package, before making any significant changes to your anilox inventory.
Even with the outstanding success of EFlo, Pamarco refuse to stand still and are constantly challenging the industry perceptions, to establish the real facts about what is the best anilox to suit YOUR printing. For example, it sounds counter-intuitive to consider lowering your anilox line screen to improve the performance of your Flexo printing process. For years, anilox producers have been recommending that printers increase the anilox line screen to provide adequate support for smaller and smaller flexo plate dots. One of the down-sides of smaller anilox cells, can be anilox durability, with the smaller cell walls, providing less resistance to scratching, scoring, and premature wear. The experiences gained with EFlo, have demonstrated, that is not essential to support 100% of the dots on the plate, in order to print clean vignettes and process colours. Why, therefore, increase anilox screen counts, when actually what your print needs, is just a lower ink film thickness.
To prove this concept, recently Pamarco collaborated with a major flexible packaging printer in North America, to make print trials on a range of different plate screens using conventional 60 deg hexagon anilox screens engraved on a banded test roll. The customer’s existing anilox rolls were 600 l/in (240 l/cm) x 6.5cm3/m2, for combination half tone/line/solid printing with solid density 1.65 to 1.7 and a 900 l/in (360 l/cm) x 4.0cm3/m2, for process print with a solid density 1.4 to 1.45. The target was to establish if it was possible to utilize a more durable, lower screen count anilox, without sacrificing print cleanliness.
The end result was, that by reducing both line screen and volume, it was possible to produce nearly identical density and dot gain results to their existing rolls, but using lower screen count and cell volumes This clearly demonstrated that the reduced Depth:Opening advantage of utilizing the lower screen and cell volume, resulted in improved ink transfer, acceptable dot gains, clean print and good solid densities, even when using a lower screens and cell volume.
Printing trials using different plate and anilox screens, demonstrate it is possible to reduce anilox screen and still maintain good density and dot gains
In conclusion, some final advice for anilox screen selection as follows:
- Review your existing anilox inventory to establish what Cell volume are giving you the correct, defect free solid densities using your chosen plate, inks, tapes and substrate
- If you don’t have any existing references, consider a banded roll trial to establish them.
- Chose the lowest possible line screen for your selected Cell volume
- If you need to increase screen count for finer print, consider EFlo extended cells as a way to increase cell volumes and maintain good ink transfer
The anilox’s sole function is to transfer a precise, predictable, and consistent ink film to the printing plate. The ink film is substantially determined by the cell volume, not the line screen, but lowering the screen count, could help to improve ink transfer rates. It may be time to re-think and optimize the specifications of this essential part of your printing equipment, for greater long-term printing performance, consistency and durability.
About the Author:
After training as a mechanical Engineer, David Parr has worked in the European and North American flexo printing markets, since the mid 1980s, having 28 years working with companies for the production and sales of anilox rolls and laser imaging systems for flexo.