Episode 1: Shrinking Cell Volumes
The shade of red your press is putting down on the 28,000-foot run of potato chip bags looks a little light. So you pull out the spectrophotometer and learn that the color is off by more than three Delta E. Not good. But, you think, the press is set up exactly as it was when you ran this 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 decrease of even just 0.25 BCM in cell volume 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 installation, process control, or anilox roll manufacturing.
The Devil is in the Details
“For example, when printing on film the volume of a cell may range from 1.6 to 2.2 BCM. But if you lose a quarter of a BCM —a tiny amount of ink— it is still more than 10% of the ink in a cell,” describes Randy Carter, Technical Sales Representative at Provident Group. “Multiplying that across many cells that each have a tiny bit of dried ink in them has a direct effect on the amount of ink that makes it to the substrate. This probably wouldn’t matter much on a corrugated substrate, but it can cause a job on a non-absorbent film to print incorrectly.”
“Something that happens with inadequate cleaning is that cells on one section of an anilox roll may be delivering less ink than required. This can cause color accuracy to vary across the web,” continues Carter. “When this happens there is little you can do except shut down the press, clean, adjust, and restart.”
Carter also recounts instances of press operators wiping anilox rolls with used rags that leave ink, chemicals and other materials on the surface of the roll. “The devil really is in the details. Any extra materials on the roll can have an effect on the inks, so it’s essential to make sure rolls are well cared for and as clean as possible.”
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.
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 of the roll 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 1/1000 of an inch difference across the web can cause an inking problem that shows up in color accuracy and print quality, notes Carter.
Even harder to identify is a defective anilox roll. Carter recalls a customer who was baffled by what appeared to be a significant variance in ink volume provided by a new roll. Testing with a Troika Anicam —a device that measures anilox cell capacity— showed the roll to have a 20% variance, well beyond the acceptable variance of about 5%.
Being able to check rolls is a relatively new capability for converters and it 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. “Wash the roll completely and measure cell capacity all along its length,” recommends Carter. “Then rotate it 180 degrees and measure it all again. Take an average of the cell capacities and tag or mark that roll so you know its present capabilities.”
And, those capacities can change over time, so you have to stay on top of it. Carter cites a larger converter who operates 15 ten-color, 18-inch flexo presses. Some 550 anilox rolls are ready for action. The practice at this shop is to power wash each anilox roll after each job, measure cell capacity using an Anicam, then tag the roll with its current capacity. This enables a press operator to select a roll based on its known ink volume, and if there is a color discrepancy when running a job he knows it is the plate or the ink. Although time-consuming, the process helps the jobs come up to color faster, helps ensure color quality and repeatability, and sharply reduces costly press downtime. “If this lets all 15 presses each get up to color 30 minutes faster that saves seven and a half hours a day,” notes Carter. “At $500 an hour for press time, that adds up quickly.“