Tech gyaan on anilox roller

Anilox roller, aptly described as the heart of flexography, is the ink-metering roller with a uniform recessed cellular surface, which transfers a pre-determined quantity of ink from the fountain to the flexographic plate.

13 Sep 2012 | By Rushikesh Aravkar

The quantity of ink that is to be transferred is decided by the characteristics of the anilox roller: cell angle, cell volume, and screening. Flexographic printing unit has two types of ink feed systems: two roll system (fountain roller) and enclosed doctor blade system. The characteristics of anilox roller also, depends on the ink feed system in use.

Anilox roller screening plays a major role to determine the fineness of ink layer deposited onto the plate. It is measured as cells per inch or cells per centimetre. Today, the rollers ranging from 250 cells/inch up to 2000 cells/inch are used in the industry for various applications. Higher the screening number, greater is the number of cells per unit and finer is the deposited ink layer. It has to be understood that ratio of anilox screening to the screening of plate has to be atleast 3:1 for proper reproduction of halftone dots.

With the developments in the anilox roller technology, the attempts have been to reduce the hydraulic force that acts when doctor blade or roller wipes the anilox roller. Multiple options have been developed for the shape of an individual cell or the cell configuration each having its own pros and cons. Commonly used cells configurations include inverted pyramid, quadrangular, hexagonal and conical.

The inverted pyramid cell which is a four-sided cavity with a pointed bottom is designated by letter P as a suffix. This configuration is typically used with  fountain roller inking system. Due to the pointed bottom, a specific amount of ink always remains in the cells that mean it has low release factor, not all the ink, which it picks up from the fountain, is transferred to the plate.

The quadrangular cells or quad cell is an inverted pyramid with truncated base. Because of truncated base, the hydraulic force is reduced during the wiping of roller or doctor blade and also during the ink transfer. This increases the release factor and therefore more ink is transferred to the plate. Quad cells are typically used with doctor blade systems and are designated by letter Q.

Again hexagonal cells can be with pointed base (designated as HP) or truncated base (designated as HT). Hex cells can accommodate more volume of ink as compared to similar pyramid or quad cells. In addition, hex cells have longer life than quad or pyramid cells because of the two more sides. Designated as C or R, conical cell has a round opening, tapering sides, and curved bottom. They have less volume than the other cells. Some cell configurations have channels connecting the cells grooved around the circumference of the anilox roller. Such a design helps to reduce the hydraulic force.

Cell angle or engraving angle
Cell angle is measured from the axis of the cylinder. It describes the manner in which the cells in adjoining rows are arranged. The square cells can be aligned at 90o or 45angle. When hexagonal cells come in to the picture there is extra scope for the arrangement of cells in a more compact structure. Hexagonal cells can be arranged at 30or 60o in a compact manner to accommodate more cells in a given area. 60o is the most widely used angle.

The anilox roller is a copper or mild steel roller on the surface of which the cells are engraved either mechanically using steel milling machine or electro-mechanically using a diamond stylus or by chemical etching or more commonly by laser. The cell walls undergo wear and tear due to the wiping action of doctor blade. This leads to decrease in the cell volume over a period of time. To protect against this, the surface of anilox roller is covered with a layer ofhard material such as chrome or ceramic. 

The plasma coating of ceramic is applied by flame spraying which leaves a rough coating of ceramic on the metal surface as compared to the uniform and smooth chrome coating. The major advantage of ceramic roller over chrome coated roller is that it has longer life due to greater hardness. But this greater hardness of ceramic roller results in wear of fountain rollers, doctor blades and printing plates.

The anilox roller providers in India are Royal Coat (Zecher), Praxair Surface Technologies, Harper Corporation and Multitec.