The Anilox roller
Anilox rollers are one of the most important elements of the flexo process. The full surface of the anilox roller is engraved with recessed cells and each cell holds a specified volume of ink thus ensuring that a consistent ink film is delivered to the printing plate.
Each individual anilox roller is made up of a steel shaft with end bearings which hold a copper sleeve.The cells are engraved onto the copper surface and the cells can be varied in both depth and size dependent on the specification required. The ‘cell volume’ is the measure of the ink capacity of the cell and this is determined by the width and depth of the cell. The depth and shape of the cells is a key factor in the efficient delivery of a uniform ink film to the printing plate and allows the operator to vary the volume of the ink film by changing the anilox roller to the achieve the correct color.
The number of cells on an anilox roller are measured in cells per linear inch (CPI) or the cells per centimeter (CPC). (See Figure 5.6). As cell count increases, ink delivered to the plate decreases, but as the line screen resolution of the image increases, the CPI should also increase. After the roller has been engraved, a ceramic coating is applied to it giving it a very hard and durable surface finish and extending the life of the roller.
The screen angle of the anilox cell is very important. Print faults can occur that are caused by incorrect angles of the engraved cells. This can create a ‘moiré’ pattern which is a result of a screen clash between the anilox cell angle and the angle used for imaging the dot on the printing plate. Anilox cells are normally engraved at an angle of 45% – 60% from the center of the anilox roller in the horizontal position. Thus, anilox roll cells are engraved at one of three angles: 30°, 45° or 60°.
Careful monitoring of the ink densities delivered by each anilox roller is important and is strongly recommended. Each roller should be inspected on a regular basis for any damage or reduction in the cell depth and a record of the ink volume of each anilox should be kept to ensure that the anilox specification is correct for the next print run.
ENGRAVING THE ANILOX ROLLER
The engraving of the cells on a modern anilox roller is now predominantly carried out using a laser engraving system. Previously the majority of anilox rollers would be engraved using mechanical engraving and this method of engraving is still in use today. The cell formation for this type of engraving differs in shape from the laser engraved cell.
Mechanical engraving can produce a Pyramid cell (See Figure 5.8) which is a fully inverted pyramid shape. The other mechanical cell is the quadrangular cell, (See Figure 5.9) which is a truncated pyramid shaped cell offering a higher ink release factor and good uniformity over the face of the roller.
This article is part of our knowledge series covering the basics of pressure-sensitive labels. Further parts of the article series “The flexograpic printing process” will follow.
This series of articles is provided by Label Academy, a global training program for the label and package printing industry. More details find here: http://www.label-academy.com/