The shade of red your press is putting down on that 28,000-ft. run of potato chip bags looks a little light. So, you pull out a spectrophotometer and learn the color is off by more than 3 Delta E. Not good. But, you think, the press is set up exactly as it was when you ran the same job last month, and it was fine. What’s not right?
The problem could be the capacity of the cells in the anilox roll you’re using for magenta. Perhaps surprisingly, a cell volume decrease of just 0.25 bcm can be enough to visibly change a color.
What’s more, cell capacity can vary across an anilox roll, causing colors on one side of a web to differ from those on the other side. The usual suspects are excessive ink build-up due to insufficient cleaning or poor roll care, as well as errors in press design, doctor blade selection and installation, process control or anilox roll manufacturing.
The Devil Is in the Details
[perfectpullquote align=”right” cite=”” link=”” color=”#00ffff” class=”” size=””]It is especially important to clean an entire roll immediately after use. The operative words here are “entire” and “immediately,” especially when solvent- or alcohol-based inks are used. Those inks can dry in less than a minute and are often resistant to re-wetting, resulting in less cell volume.[/perfectpullquote]
For example, when printing on film, the volume of a cell may range from 1.6 bcm to 2.2 bcm. But if you lose a quarter of a bcm—a tiny amount of ink—it is still more than 10 percent of the ink in a cell. When multiplied across many cells, each having a tiny bit of dried ink in it, that difference has a direct effect on the amount of ink that ultimately reaches the substrate. This probably may not matter much on a corrugated substrate, but it can cause a job on a non-absorbent film to print incorrectly.
Inadequate cleaning on one section of an anilox roll can result in less ink being delivered, causing color accuracy to vary across the web. When this happens, there is little you can do except shut down the press, clean, adjust and restart—and plan to clean the roll properly next time.
In many shops, press operators casually wipe anilox rolls with used rags that leave ink, chemicals and other materials on the surface. Those old agents don’t magically disappear and can have an effect on the inks being used, so it’s essential to make sure rolls are well cared for and as clean as possible. Details matter!
Still, there are times when what seem to be good cleaning practices can go awry. It is especially important to clean an entire roll immediately after use. The operative words here are “entire” and “immediately,” especially when solvent- or alcohol-based inks are used. Those inks can dry in less than a minute and are often resistant to re-wetting, resulting in less cell volume. If a roll cannot be removed from a press, be sure to use the appropriate cleaning agent and clean cloths.
Tiny Differences Matter
Yet even if an anilox roll is pristine, there can still be trouble. Especially on mid or wide web presses, an improperly seated doctor blade may not make consistent contact across the full width of a roll, removing more ink on one side than the other. This can result in flooding or allowing too much ink to reach part of the plate. On a mid or wide web press, even a 0.001-in. difference across the web can cause an inking problem that shows up in color accuracy and print quality.
[perfectpullquote align=”right” cite=”” link=”” color=”#00ffff” class=”” size=””]When printing on film, the volume of a cell may range from 1.6 bcm to 2.2 bcm. But if you lose a quarter of a bcm—a tiny amount of ink—it is still more than 10 percent of the ink in a cell. When multiplied across many cells, each having a tiny bit of dried ink in it, that difference has a direct effect on the amount of ink that ultimately reaches the substrate.[/perfectpullquote]
Even harder to identify is a defective anilox roll. It doesn’t happen often, but it’s possible for a new roll to have an ink variance well in excess of the 5 percent generally considered to be acceptable. Being able to check rolls is a relatively new capability for converters and doing so can also catch basic human errors that would otherwise not show up until a job was running.
For instance, it sometimes happens that a perfectly good roll is incorrectly tagged and shipped. Press operators normally trust the accuracy of the information on the tag attached to every new roll, but it still makes sense to verify cell capacity to avoid errors on press. One of the best practices leading converters follow is to wash a new roll completely and measure cell capacity all along its length. Then, they rotate it 180 degrees and measure it all again. Finally, they take an average of the cell capacities and tag or mark that roll so they know its present capabilities.
Be Diligent & Vigilant
Stay on top of it! Those capacities can change over time, so clean and re-measure on a schedule.
There is a large converter that operates 15 10-color, 18-in. flexo presses with about 550 anilox rolls all poised for action. This shop power washes each anilox roll after every job, measures cell capacity, then tags the roll with its current capacity. It enables a press operator to select a roll based on its known ink volume, so if there is a color discrepancy when running a job, the operator knows if it is the plate or the ink. Although a time-consuming process, this helps jobs come up to color faster, ensures color quality and repeatability, and sharply reduces costly press downtime. That boosts quality while enabling all 15 presses each to get up to color faster.
In this shop, each press getting up to color about 30 minutes faster saves seven-and-a-half hours every day. And at $500 per hour for press time, that adds up quickly.
About the Author: Randy Carter is senior technical sales representative at Provident.
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