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In printing, anilox is a method used to provide a measured amount of ink to a flexo printing plate An anilox roll is a hard cylinder, usually constructed of a steel or aluminum core which is coated by an industrial ceramic whose surface contains millions of very fine dimples, known as cells.
Depending on the detail of the images to be printed, the press operator will select an anilox roll with a higher or lower line screen. Low line screen rolls are used where a heavy layer of ink is desired, such as in heavy block lettering. Higher line screens produce finer details and are used in four-color process work such as reproducing photographs. Often a job will require a different line screen for each color to be printed. Experienced press operators are skilled at determining the appropriate rolls for a given print job. Depending on the design of the printing press, the anilox roll is either semi-submerged in the ink fountain, or comes into contact with a so-called metering roller, which is semi-submerged in the ink fountain. In either instance, a thick layer of typically viscous ink is deposited on the roll. A doctor blade is used to scrape excess ink from the surface leaving just the measured amount of ink in the cells. The roll then rotates to contact with the flexographic printing plate which receives the ink from the cells for transfer to the printed material.
Roll manufacture and specification
Anilox rolls were originally made using a diamond pecking machine, but modern rolls are laser engraved. The characteristics of an anilox roll determine the amount of ink that will be transferred to the plate: angle of the cells, cell volume, and line screen. A 60 percent angle ensures maximum density in a given space. Lower volume makes for less ink. Low line numbers will allow for a heavy layer of ink to be printed, whereas high line numbers will permit finer detail in printing. Both cell volume and line screen are closely correlated.
The rolls are often specified by their line screen, which is the number of cells per linear inch. These often range from around 250 to upwards of 2000, though the precise numbers vary by manufacturer. Most rolls sold are within 800 LPI, although a spike in demand for those with 800–1200 lines has been seen. Anilox rolls are almost always designed to be removed from the press for cleaning and for swapping out with different line screen rolls.
Handling and maintenance
Though large wide-web flexo rolls are only maneuverable by overhead crane, on smaller presses anilox rolls are often handled directly by operators. Extreme caution must be taken when handling these pieces of hardware as a single bump against a hard surface or sharp corner can destroy the delicate cell structure on the surface and render a roller completely useless, at a cost of around US$5000 for even small narrow-web rollers. Nicks and scratches add up quickly, so fine brushes (never brass brushes) are used for cleaning the anilox roll.
Anilox rollers that are used with water, solvent and oil based inks, which dry when left sitting out and unagitated, must be cleaned immediately after use or a problem known as plugging occurs, where minuscule amounts of ink dry in the cells. This leaves tiny, but unacceptable, pinholes in anything printed from the roll in the future.