– Various attributes of Anilox Roller and the parts they play in printing with the Flexographic process.
Flexography Printing is a roll-fed web printing process. The Anilox Roller is the heart of the Flexography printing press. An engraved metal or ceramic roller is used in Flexographic printing press to transfer ink from the fountain roller to the printing plate. A Flexographic Inking System is also known as Anilox System. The function of an Anilox Roller is to pick up ink from a fountain roller and deliver a predetermined, metered, uniform amount of ink to the rubber printing plate. The Anilox Roller is also known as a Metering Roller, a Knurled Roller or an Engraved Roller.
Anilox Line Per Inch (LPI)
Line count or a screen count refers to the number of cells per inch or centimetre as measured along the engraving angle (see Anilox Line Per Inch Figure). The lines per centimetre (lpcm) and lines per inch (lpi) refers to the number of cells per inch or cm. Line count is the number of cells per linear inch, at the angle of engraving, typically 30°, 45° or 60° about the axis of the Anilox Roll.
Anilox Cell Structures
The Anilox Rollers are engraved with varying cell structures such as Trihelical, Pyramid, Quadrangular, Hexagonal or Hexagonal Channel Screen. A Trihelical screen is a line engraved at an angle of 45-degree. A Pyramid cell is virtually an inverted pyramid. They are mainly used in metered roll systems which do not require a doctor blade. The ink transfer is an important function in the printing process. Quadrangular engraving is very similar to pyramid engraving and is more versatile with positive release characteristics. This engraving type commonly appears with a doctor blade system. The Hexagonal cells have superb release qualities and can be used in both chrome and ceramic engravings. Each cell is linked together to be a shallow channel.
Cell Volume
Cell Volume is the capacity of the engraved surface in a square inch, expressed in Billions of Cubic Microns (BCM). It is the amount of ink held in each cell.
Higher volume translates to higher solid ink density, more colour, or a heavier coating thickness (see Anilox Cell Volume Figure). Lower volumes apply thinner ink films directly associated with higher print quality and process efficiency. Anilox cell volume changes over time, due to wear, plugging and cleaning procedures. Dirty or plugged cells are temporary and can be corrected to restore cell volume and colour. Wear is irreversible and permanent, resulting in weaker or less colour due to lower volume.
Anilox Screen Angle
Varying screen angles are used depending upon the transfer and application. The 60° hex pattern is recommended for most Flexography printing applications because the cells are fixed more tightly together, allowing approximately 15% more cells in each area. Cells can be shallower while still releasing the same amount of ink because more cells fit into a square inch area. Channeling is avoided (compared to the 30°) because the straight side of the cell wall is not positioned horizontally.
Too finer anilox screen rulings result in cell clogging. As engraving line screen increases, the average cell depth to opening ratio also increases. The line screen distribution of Anilox Rollers is engraved at angle of 60° for the level of clogging, so as the level of pollution increases, the average line screen also increases.
The roughness of a printed ink film depends on the volume of ink carried by the Anilox Roller. The cell pitch of the Anilox Roller does not affect the printed ink roughness. If cell volume changes, the anilox engraving with the highest cell volume produces the highest optical density. The major defect of the Anilox Roller is the cell wear. The surface engraving cell of an Anilox Roller determines the ink film thickness. The depth and angle (volume) of each cell determine the amount of ink transfer, cell wear makes the walls lower, and the amount of ink carried by the worn spots will increase reducing the uniformity and quality of print. The spikes spread out with cell wear. Anilox volume and printing force have a dominant effect on the printed line width.
The choice of the correct Anilox Roller is of central importance for an optimal printing result in Flexography Printing. Anilox rollers allow to print packaging-film materials even with water-based coloured inks, thereby fulfilling the current tendency to use environment-friendly packaging materials. The number, size, and geometry of the anilox cells vary and will determine the amount of ink that the anilox roll delivers to the plate to achieve a better quality of print.
