Flexo printing ink transfer (3)

3. The number of lines of the anilox roller. The number of ink holes per unit length in the linear direction of the anilox roller surface, that is, the number of anilox roller lines (also called network line density). It determines the uniformity and the amount of ink transmitted by the anilox roller. The higher the number of lines of the grain roller, the closer the ink layer on the anilox roller is to the "continuous" state.

In the case of printing halftone or color image prints, the bright or high light area of ​​the printing plate, the size of the dot is very small. If the low line number of the anilox roller is used to transmit ink, the opening area of ​​the ink hole is larger than the area of ​​some small dots. When these small area dots exactly oppose the ink hole of the anilox roller, the dot dot dips into ink because there is no partition wall support. In the hole, not only the surface of the dot is inked, but also the side wall of the dot is inked. Therefore, the dots printed on the substrate have a serious increase in the periphery, and even the adjacent dots are connected together, and the tone and color are changed. Therefore, when performing halftone or color image printing, the number of lines of the anilox roller is generally high to ensure that the opening area of ​​the ink injecting hole is smaller than the area of ​​the smallest dot on the printing plate. However, the higher the line number of the anilox roller is, the smaller the ink transfer amount is. Therefore, the line number of the anilox roller is not as high as possible, and it is necessary to comprehensively consider the situation of the printing plate dot and the requirement for the ink of the printed material.

In lithographic and gravure printing, the misalignment of the screen angles of the various printing plates results in the appearance of "rhizome" that impedes the beauty of the image. In flexographic printing, in addition to the moiré caused by the improper relationship between the angles of the screens of various colors, the relationship between the angle and the number of lines of the anilox roll and the printing plate may also cause moire.

The ink reticles on the anilox roller are generally sculpted at an angle of 45° with the axis, which is the same as the angle of the main color plate in the ordinary color image printing, such as magenta or cyan, and the anilox roller is connected with the ink hole. The network cable interfered with the magenta or cyan network cable to form a moire. Therefore, in flexo color printing, the printing plate should avoid the use of a 45° screen angle. The commonly used angles are 15°, 30°, 60°, 75°, and 90°.

In the long-term development of flexographic printing, people have found that the main cause of moiré is the mismatch between the number of anilox roller lines and the number of plate lines. Over the years, in order to find the right number of matching lines, based on the wire mesh interferograms first proposed by American Nathan, an arachnid wire number-one-plate version of the wire number matching map, also called Nasentu, was obtained. As shown in Figure 6-21.

In the figure, the abscissa indicates the number of lines of the anilox roller, and the ordinate indicates the number of screen printing lines. The four slashes in the figure indicate the optimum matching zone between the number of anilox roller lines and the number of screen printing screen lines. The number matching corresponds to the oblique line, and the generated moiré is actually not perceived by the human eye. The farther away from the oblique line, the more disturbing the formation of interference fringes.

The number of matching anilox roller lines can be obtained from the given number of printing plate screen lines by using the Nass plot, and the number of screen printing screen lines can also be obtained from the given number of anilox roller lines. For example, printing uses 39 lines/cm plate, in order to avoid the occurrence of moire, anilox roller line number 110 lines / cm is the most ideal, and for example, an anilox roller equipped with a flexographic printing machine, the number of screen lines is 118 lines/cm, the number of y-axis printing plates on the intersection of A, B, and C oblique lines is 41 lines/cm, 32 lines/cm, and 28 lines/cm, respectively. If the quality of printed products is high, A 41 line/cm screen printing plate is most suitable; if the printing paper has a small smoothness and is very rough, a 28 line/cm screen printing plate can be used. Conventionally, for general printing products, the number of screen printing screen lines and the number of lines of anilox rollers are often matched by 1:4. However, for fine print products with high quality requirements, the number of printing plate screen lines and anilox roller lines is still It should be matched with the Nassen diagram.

4. Anilox roller maintenance and maintenance. Anilox rollers must be carefully maintained and maintained due to their special surface structure and their important role in ink transfer.

The anilox roller should be stored in a special packaging box, so that it is in a vertical position, so as to avoid the surface being injured by foreign objects, or bending deformation due to gravity, in the handling and use on the printing press, to prevent foreign matter and anilox roller collision.

In flexographic printing presses, the angle of the tangent to the pressed contact of the anilox roll is typically 28-35° when the doctor blade is in the working position. Under the premise of ensuring the effective control of the amount of ink, the contact pressure between the squeegee and the anilox roller should be as small as possible. After measurement, the line pressure of the squeegee on the anilox roller is generally between 2.8 and 5.6. g/cm. Under normal pressure, there will be a thin layer of ink between the squeegee and the surface of the anilox roller. This layer of ink has the function of a lubricant, which can reduce the abrasion of the squeegee to the anilox roller. In order to maintain the ink lubrication effect on the anilox roller wear protection, it is absolutely not allowed to rub between the anilox roller and the doctor blade in the dry state. The pressure between the anilox roller and the squeegee blade should be as low as possible. If the pressure is too high, the anilox roller wear can be exacerbated.

After each printing, the anilox roll must be carefully cleaned, otherwise the pigment in the ink will dry at the bottom of the anilox roll ink influencing hole, which will affect the ink quality of the anilox roll.

For cleaning the anilox roller, scrubbing and scrubbing methods can be used. Generally, the scrubbing effect is better, but the cleaning agent and the appropriate brush matching the ink are selected. Such as brushing chrome-plated roller, you can choose the bristle diameter suitable for the size of the ink hole brush; brushing ceramic anilox roller should use hard nylon brush, if brushed with copper brush, high hardness of the ceramic surface will be grinding from the copper brush Tiny copper material will block the ink hole.