Cylinder center distance and the formation of printing pressure
The so-called roller center distance refers to the distance between the centers of two adjacent rollers, which is the sum of the pitch radius of the two roller gears in value.
The plate cylinder and the impression cylinder are hard surfaces and are not easily deformed when pressure is applied. The printing pressure is obtained by the elastic force generated by the compression deformation of the blanket on the blanket cylinder. Therefore, during the printing process, the contact between the blanket cylinder and the other two cylinders is not a straight line, but an arc with a fixed width. The maximum compression deformation value of the blanket is usually called the printing pressure in production. In fact, the maximum compression amount (deformation value) is the sum of the radius of the two-phase pressure roller cylinder plus the difference between the thickness of the paper, pad, etc. and the actual center distance of the two rollers, as shown in Figure 6-9.
(Figure 6-9 Schematic diagram of printing pressure)
The role of printing pressure is:
(1) Maintain a rough printed surface to achieve complete contact. The surface of the paper, blanket and printing plate are rough micropores. These rough surfaces must be deformed under certain pressure to achieve reliable and complete contact and overcome the roughness and unevenness of each printing surface. So that the ink and wetting liquid can adsorb to the solid surface to achieve effective transfer.
(2) Overcoming the attraction of ink molecules and enhancing the adsorption force, there is a certain attraction between ink molecules, when the adsorption force of ink exceeds the attraction between ink molecules. The ink starts to be transferred from the middle of the fracture, but in the adsorption transfer of ink, the contact distance of each transfer surface must be close to produce adsorption. Therefore, only under a certain pressure can overcome the gravitational force between the molecules, can the ink be better absorbed and transferred.
(3) Pressure penetration. The ink is printed on the paper, and the linking material has to penetrate the paper. The penetration of the ink on the paper can be divided into two physical phenomena: pressure penetration and free penetration.
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