A flexo press does not meter ink with a duct and a train of rollers the way an offset press does. It uses one roll with hundreds of thousands of tiny cells cut into its surface, and the size of those cells decides how much ink reaches the plate. That roll is the anilox.
What the roll is
An anilox starts as a steel or aluminium cylinder. The surface is coated with ceramic, usually chromium oxide, and then polished. The cells are burned into the ceramic by a laser, one pass per cell pattern, and the finished roll is polished again to remove the ridge left around each cell.
The ceramic is what makes the roll last. A bare steel roll would wear at the doctor blade within weeks and the cell volume would change. Ceramic holds its shape for years, and when it does wear out the coating can be stripped and reapplied.
How ink is metered
A chambered doctor blade sits against the roll. Ink is pumped through the chamber under pressure, fills the cells as the roll turns, and two blades wipe the surface clean. One blade does the metering, the other closes the chamber. What leaves the chamber is a roll whose surface is dry and whose cells are full.
From there the ink goes to the plate cylinder. The plate touches the anilox, the cells release their ink onto the raised image area, and the plate transfers it to the substrate. There is no fountain solution and no ink train, which is why a flexo press can run at speed with very little ink on the floor.
Line screen and volume
Two numbers describe an anilox. The line screen is the number of cells per inch or per centimetre, and it sets how fine the ink pattern is. The cell volume is how much ink the cells hold, measured in cubic centimetres per square metre. Volume is the number that matters for colour.
The two numbers are related but not the same. A 400 line roll and an 800 line roll can hold the same volume if the cells are cut deeper. That is why a printer who buys a roll by line screen alone can end up with a roll that prints too dark or too light.
Typical volumes run from about 2 cm³ per square metre for fine process work up to 12 or more for a heavy white or a coating. A roll for four-colour process on film might sit at 3 to 4. A roll for a solid white on a dark substrate needs far more.
Cell shape
The usual pattern is hexagonal, with cells set at 60 degrees to each other. The walls between cells are thin, which gives a high open volume and good ink release. A channeled cell has a small groove connecting neighbours, which helps release at high speed but can leave a visible pattern in a light tint.
The cell shape also controls how the ink leaves. A cell with a narrow neck holds ink until the plate pulls it out. A wide, shallow cell releases easily but empties unevenly at speed.
Wear and clogging
An anilox does not fail suddenly. It loses volume. Dried ink left in the cells cuts the volume a little each time, and a roll that is cleaned with the wrong tool loses its polish. A wire brush or a sharp scraper takes ceramic with it.
The safe cleaning methods are a dedicated anilox cleaner, an ultrasonic bath for a roll taken off the press, or a gentle soda blast. On press, the chamber should be flushed before the ink dries, not after.
Wear shows as a drop in density that the operator compensates with more ink pressure, which drives ink onto the plate and fills the shadow dots. The job then looks flat and the plate wears faster.
Checking a roll
A banded test print, run at a known speed and pressure, shows banding that points to a clogged area. A microscope with a measuring reticle shows cell depth and wall thickness. A volume check with a draw-down on a known substrate gives a number the shop can log.
A roll with a log is easier to trust than a roll with a label. The number on the label is the volume when it was new. The number in the log is what the press is getting now.
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