When I was first learning how to run a press, one of the biggest things I could remember thinking was, “How am I supposed to know how these different anilox rollers print?” I would watch all the veteran operators switching out anilox rollers, and they knew exactly which one to drop into press for each job. How did they learn it?
They learned from trial and error. For the development of my understanding, it was much the same. As time went on, I figured out how to use the anilox volumes to my advantage and knew that some jobs would take this anilox, and others would run better with that one. It was all part of “learning the trade.” There had to be a better way to know which one to use, and a better understanding of what it took to obtain reliable performance.
I would like to take the opportunity to discuss how to get reliable performance and a basic understanding of the anilox, while giving an in-depth look at how to get your shop’s ink department and presses correlated, anilox rollers identified, and how to keep things organized and maintained to help you reach the full potential of your anilox rollers. In turn, this will help keep your waste down, help to promote color consistency and keep production time up.
All images courtesy of Harper Corporation of America
Linescreen, Volume & Geometry
Understanding linescreen, volume and geometry, and how they work together, is the beginning step to consistent, quality printing. You may have certain artwork or a certain account whose print dynamics always seem to be a problem when running on press. The simplest solution could be that you are running a plate, anilox and ink combination that is not made up of the optimum materials for that application. Locking down your anilox selection is where to start.
How does linescreen play a role in the anilox? The linescreen provides the cell opening on the surface of the anilox (see Figure 1). The linescreen is defined by how many cells are present per linear inch on the roll (lpi). A properly selected linescreen does two things: It allows for distribution of volume without compromising cell quality, and it supports the printed dot. When running artwork like 4-color process, a high linescreen is used to help support the low dot percentages, also known as highlight screens.
Historically, because the inks are formulated to hit color with thin ink films, your volume will not need to be high. Pairing more open cells (lowering linecount) with a set volume provides a greater deposit efficiency, and puts more ink on the plate. The wider opening can be useful in specialty applications like particle transfers. Particle size would then dictate required volume.
Another way the lpi helps is by supporting the printed dot. Knowing the linescreen of your artwork and what look you are going to achieve is how you will determine the lpi of your anilox. If you have a high line of artwork, you do not want a low linescreen anilox because your dots on the plate will over-ink or under-ink instead of having ink evenly distributed to the plate dots. This will cause the bridging of dots and dirty print, since too much ink is distributed. One thing to keep in mind is that the lpi of an anilox does not change with wear. It is always consistent. The only time the linescreen will change is when you send it in to be remanufactured and request a different lpi.
How is volume a factor in anilox selection? Volume is the amount of ink contained in the cells of a defined area (see Figure 2). This is measured in bcm. A frame of reference will help you scale the concept of the micron: Take one human hair and look at a cross section and divide from one side to the other into 100 equal width pieces. Each equal width is one micron. A cell volume of 0.5-2.0 bcm is used along with a high linescreen. With a high linescreen, not too much ink is needed because you are typically printing fine screens without solids. A volume of 5.0-10.0 bcm is a deep cell, which will hold a lot of ink that will be distributed to the plate. These are typically used for varnishes, coatings and whites.
You would use these volumes with a 300 lpi to 150 lpi anilox where full solids and complete coverage are needed. In many cases, the volume will be dictated by the ink supplier because formulations are dialed in to specific volume expectations. Always consult with the ink supplier to better understand the needed volumes and why. Volume can change, because the bcm will be affected by wear and the cleanliness of cells. As an anilox wears down, the cell will get shallower and not carry as much ink as it did when it was new. Plugged cells immediately lower volume and can only be remedied by a thorough cleaning.
As you can see, the lpi and bcm of an anilox are major factors in how much ink will be distributed to the plate. Both play off each other and cannot work independently. But there is one more factor in your anilox identity—the geometry of your anilox.
We know how the lpi and bcm of an anilox play a role in print. But what is the best way to be assured that the ink is distributed from the cells? The geometry of the anilox is the answer.
There are so many ink systems in the industry now to help catch the eye of the customer that converters need to be sure they have the tools that can create the expected look. Some coatings are designed to be thin in consistency and others very viscous; there are different geometries that release ink in different ways to accommodate the viscosity. Hexagons, channels, elongated cells, trihelical designs and 45-degree quads are just a few engravings that are offered to help with converter applications (see Figure 3). Some geometries are universal in application, like the 60-degree hexagonal. Others have niche applications but are more effective. Typically, thin viscosities perform better with enclosed cells and higher viscosities perform better in channel arrangements.
For coatings and inks that have never seen a flexographic press, these often need to be tested with a banded roll to dial in the right engravings. There certainly are exceptions to every rule, so speak with your anilox supplier’s regional technical support about engravings they offer and how they can help.
You will also want to know the needs of your plate material, ink and artwork, and include their representatives in selecting the correct anilox rollers.
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