Geometry, according to Webster’s dictionary, is a “branch of mathematics that deals with the measurement, properties and relationships of points, lines, angles, surfaces and solids.”
In other words, it is the shape and relative arrangement of the parts of something.
How fluids (and the potential solids they contain) flow in, around and through those structures is paramount in making an anilox geometry recommendation. We often hear about volume in terms of anilox rollers, but it also is the selection and preservation of the geometry itself. Choosing the right geometry and keeping it whole will deliver the ink or coating you need without losing the consistency of transfer.
There is a myriad of considerations when trying to select the proper geometry. Circumstances of a particular ink or coating’s nature prevail in the selection. Counted as contributing factors are: viscosity, density, particulate matter, opacity, scoring, uniformity, thickness, foaming, spitting, misting, linescreen and standardization. Each is discussed here.
Viscosity
Is the ink or coating thin or thick (viscous)? The 60-degree hex and other closed-cell geometry work best with thin inks. Since the amount of ink that is transferred from a cell depends on the carrying capacity of the cell, you will want to print with the thinnest film of ink as possible. A thinner deposit of ink is easier to control with a closed cell because there is less ink available to transfer and retention of ink at the surface is paramount to deposit efficiency.
Dot gain is decreased using closed cells for thin inks, as the control is for both vertical release and stabilization of the ink film. Thin inks tend to sag in a channel, where a closed cell remains fully charged.
For thicker inks or coatings, a channeled cell geometry may be preferable. Channeled geometries allow the more viscous fluids to move and replenish before dwell time renders that activity difficult or impossible. Always consider the drying/curing capacity of the press. You may be able to put down a heavier volume of ink or coating than you have the capability to dry. This can lead to winding up rolls with ink or coating that is not dried or cured, which causes it to transfer to the backside of the substrate. Some applications require out-of-the-box thinking in terms of drying to accomplish desired results. Tunnel dryers, dryer placement and press speed will all factor in the final capability.
Density
While depth alone is not a geometric feature in laser-engraved cells, it does play a significant role in the deposit efficiency of the cells. Are you trying to achieve specific ink densities? Targeting Flexographic Image Reproduction Specifications & Tolerances (FIRST) standards?
Higher ink densities lead to an expansion of the color gamut and improved graphic capabilities. Increasing cell volume may increase density to a degree. There is a point of diminishing returns, though. Deeper cells do not always mean more ink transferred. Choosing a cell configuration that is too deep can lead to difficulties in keeping the roll clean.
Depth-to-opening ratio is also important. For proper ink transfer characteristics, Harper Corporation of America recommends working with ink suppliers to determine volume requirements. In terms of depth to opening, 23 percent and 33 percent of the cell’s opening were traditional ratios. It is common to see the depth-to-opening ratios in the high 30s and low 40s for some applications as increased density is required.
Particulates
A good rule of thumb is the minimum cell opening should be three times the size of the largest particle in the coating or ink. Trihelical geometries work well with transferring particulates. Open channels work well for reducing shear in particulate mixtures, especially when blading off the excess on the surface of the anilox.
The unending channel around the circumference of the roll replenishes the inks or coatings on each revolution of the roll, permitting the chemistry to run cleaner. Trihelicals look similar to a “screw thread” engraving and come in 45-degree, 60-degree, 75-degree and 89-degree configurations.
Opacity
The need to “hide” the product with a strong white in reverse print applications in flexible packaging is always in demand. The ability to go from two bumps of white to one bump helps lower manufacturing costs and improves graphics. Channeled engravings work well with opaque whites, as well as some masking applications.
In many cases, it depends on the viscosity of the white ink itself and its ability to flow. For a thin viscosity white, a closed-cell geometry is preferred to maintain ink at the surface of the cells. Incorporate plate technology, such as surface structures and cells in the solids for volumes less than 10 bcm, to further enhance the ink film transfer.