Images and data courtesy of Troika Systems Ltd
Most flexographic printers, whether narrow web, wide web or corrugated, have experienced the frustration of time and material wastage with prolonged press setup to achieve the correct densities required for jobs.
It appears to be standard procedure to blame, in order, the ink or what the ink technicians have done, then the doctor blade setup, then the substrate and plates, and finally, when all else fails, to consider the aniloxes.
The condition and actual volume of the anilox needs to be known to minimize press setup time and cost.
History of Anilox Volume Measurement Methods
The methods of volumetric measurement on laser-ablated ceramic rolls, as with any science, have progressively developed and evolved into something that can now be described as accurate.
Originally the measurement method was, quite naturally, copied from the mechanically engraved charts where the screen count and opening of the cell were easily established and, knowing the angles of the engraving tool, the volume was calculated using trigonometric formulae.
In the early days of anilox measurement, most engravers used a chart to give an indication of the cell volume. However, there can be a large difference in volumes due to the shape of these cells. As shown in Figure 1, if the sharp pyramid were accurately measured at a volume of 3, the flattened straight cut cell would have a volume greater than 8 and the middle flattened pyramid, which is comparable to early ceramic engraved cells, would have a volume (if measured accurately) of approximately 6. Measurements with large variations in readings were good enough at that time, simply because flexo was still in its evolutionary stages and certainly nowhere near the quality we have today.
As time progressed, laser ablating of longer-lasting ceramic aniloxes was a real step forward in their manufacture. This allowed the evolution of higher screen counts and hexagonal cell structures (30 degree and 60 degree) and, over time, the evolution of complex open-cell structures.
With laser engraving, all anilox manufacturers had to evolve from the simple chart method of volume measurement to a more sophisticated technique. This was the “liquid volume – ink drawdown” method. Many manufacturers developed their own method of liquid volume checking, however they needed a reference point to use when stating the volume of the aniloxes. If they had done their calculations on the sharp cell (as shown in Figure 2), that volume would be very different than those that made their reference from the normal cell, and certainly very different from the U-shape straight cut, wide bottom cell example. This explains why there can be a very real difference in the ink-carrying capacity of aniloxes from different manufacturers, despite their documentation stating they are of the same volume.
This difference was proven in research carried out by Troika Systems in 2007, when a banded roll with 12 engravings from 100 lpi to 1,200 lpi was taken to 15 different anilox manufacturers, in Europe and North America, to be measured using the method each was using at that time.
The results, shown in the table below, illustrate the minimum and maximum volumes stated by the manufacturers. With these very significant variations on each band, it is no wonder printer/converters have such elongated press setup times and so much waste. It is also no surprise coating companies waste so much solution when they have so little idea what the actual carrying capacity is on their rollers.
As higher anilox screens, new elongated cells and open-cell structures evolved, the need for volume measurement could not be met by conventional methods, and measurement systems had to evolve. This variation between what should have been comparable aniloxes has been very costly to all flexo printers, from the smallest narrow web to the largest corrugated sites. It was time to help them save costs and time.
Liquid Volume – Ink Drawdown Method
While the liquid volume test was the first positive step toward attempting to improve volumetric measurement, and was adapted by many anilox manufacturers, it could not claim to be accurate or consistent.
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That’s because it attempted to follow a similar process to the press by dispensing a known amount of ink, spread by a doctor blade onto the anilox. The area the ink spread over was measured in square millimeters, and using the formula of ink volume/square area (cm3/m2 or bcm/in2), a volume was calculated. While this method was a positive step forward from the original mechanical method of volumetric calculation, there are many opportunities for errors:
- If the ink density is not absolutely constant, variations in measurement will occur. Therefore, if there is a density difference between the pots of ink, or if the ink in the pots evaporates at all during use, a very real variation in measurements is seen[perfectpullquote align=”right” cite=”” link=”” color=”#00ff00″ class=”” size=””]The methods of volumetric measurement on laser-ablated ceramic rolls, as with any science, have progressively developed and evolved into something that can now be described as accurate. [/perfectpullquote]
- Studies have shown very significant variations between operators. In October 2010 at Clemson University, FTA carried out a gauge repeatability and reliability (R&R) study on the variety of rolls measured that had different circumferences and different screen counts. The study, done under controlled conditions, showed average variations of in excess of ±10 percent using the liquid volume method. This meant a 5 volume anilox was measured at anything from 4.5 to 5.5 volume—far beyond the requirements printers need to manage their aniloxes for optimal productivity and minimizing setup waste. It is interesting to note that, during subsequent tests at one global anilox manufacturer, eight of their competent and experienced liquid volume users were only able to achieve a minimum 16 percent variation in their measurements on a five-banded master roll. When similar tests were carried out at another international anilox manufacturer, using six very experienced operators on a banded roll, the best results still showed a variation of 14 percent. These variations are beyond what is acceptable today
- If the anilox has been through a cleaning bath, it needs to cool to room temperature before measurements can be taken; if it is warm, the ink used for the measurements evaporates and gives very inaccurate readings
- This method is not suitable to measure open-cell structures because it simply flows out and does not fill the cells as needed
It is not surprising that when anilox manufacturers were taking their measurements using the liquid volume test method (nicknamed the “squirt and scrape” method by some users), printers found they would get different densities from aniloxes that were stated to be of the same volume. Fortunately, very few anilox manufacturers use this method today.
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