Thickness
Thinner films of wet ink are easier to control, reduce dot gain and increase graphic capability. You can use a lower-volume anilox roller with a 60-degree cell because more cells fit into a square inch in this configuration. However, a cell with too small of a volume will cause ink to dry before it can be transferred, unless there are enough drying inhibitors utilized.
The goal is to specify an anilox roller that will deliver the color strength and densities needed with the thinnest ink film possible. Determining the best option is usually done with a banded roll trial.
Scoring
The geometry chosen for the anilox will not have much effect on the resistance to scoring. It is the polishing of the roll after the engraving process that gives it a smooth finish for the doctor blade to ride on that provides some resistance to scoring. In almost all cases, anilox scoring is preventable with good manufacturing practices.
Score lines are caused by something rubbing against the surface of the anilox—usually it is a fragment of metal from a doctor blade. Fragments of metal can be removed from the ink system by installing filters and magnets inline to capture the impurities. Typically, the scale of the scoring size is multiple cells and dwarfs the contribution a cell wall can make in stopping scoring, regardless of angle or thickness.
Uniformity
The 60-degree hex pattern is the most easily reproduceable from a manufacturing standpoint. This leads to greater engraving consistency within a roll and from roll to roll. The 60-degree hex also has a large volume range—from less than 1 bcm up to 40 bcm. Of all patterns, it is geometrically neutral; no grain means uniformity of ink film.
Foaming
Channel engravings have beneficial impacts on inks or coatings susceptible to foaming. The longer, shallower cell configuration allows for disbursement of microbubbles of air suspended in the ink or coating. Trihelical engravings, because of the open nature of the cell, greatly reduce shear that would otherwise create microbubble formation. The more vertical the channel, the less shear and microbubble formation.
Spitting
Higher-viscosity coatings and UV inks tend to “spit.” Use of open-cell or channeled engravings to reduce hydraulic pressure buildup from the accumulation of particles at the blade/anilox contact point has shown great promise at eliminating ink spitting—even without other changes to the inking process.
Misting
Today’s higher press speeds sometimes lead to ink misting, so ink manufacturers usually reformulate inks to ensure they will resolubilize and not “mist” off the anilox roller.
Elongated closed cells will help with the cell replenishment and to a degree serve to mitigate the misting issue. Sometimes, it is necessary to go to a shallower cell because of lower volume demands. This can aggravate the problem.
Modern presses, capable of speeds in excess of 2,000 fpm, are designed with larger circumference aniloxes to assist in re-solubility by slowing the RPM of the anilox surface.
Linescreen
The geometry that has been the proven workhorse for flexographic applications is the 60-degree hex. Some extended hexagonal structures allow for greater volumes in process and combination printing as well. The “rule of thumb” is you want your anilox cell count to be six times the plate linescreen (lpi) to give good dot support for screened areas. This means if you want to use printing plates with 120 lpi, you should use a minimum of 720 cells per inch (cpi) anilox rolls. Usually, we round this up and use an 800 cpi roll, depending on what the minimum dot size is on the graphics.
Standardization
Good color management practices allow for standardization in the pressroom. The ability to standardize a pressroom on a set of anilox rollers that will reproduce the line colors, combination printing, process colors and opacity needs will lead to improved efficiencies, repeatability, productivity and lower costs.
Standardization has the benefit of allowing you to move work around to various presses and balance capacity requirements.
It is not always easy to determine the right geometry, especially when taking only one shot at the target. What is the best way to determine the appropriate geometry to optimize for a particular application?
The tried-and-true method is a banded roll trial after a good study of the technical data sheets for physical properties of the inks or coatings. A banded roll incorporates various geometries to help you evaluate different options. Work with your anilox supplier and get input from the ink or coating supplier to develop a layout for a banded roll. The layout should include a variety of geometries, cell counts and volumes with the goal of providing the correct, most consistent and, from a print standpoint, sustainable quality.
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