Causes of paper printing deformation
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During the printing process, the paper is deformed by the printing pressure and the peeling tension. After the paper with large plastic deformation, after leaving the embossing area, the deformation amount cannot be recovered, resulting in a large overprinting error.
1. Printing pressure and paper deformation
Paper is a material with plastic deformation, and the plastic deformation characteristics of different papers are also different. Under the action of large printing pressure, the width and the length of the two corners of the paper may be slightly expanded, resulting in inaccurate overprinting. Especially in the tip of the paper is more serious.
The greater the printing pressure, the more obvious the deformation of the paper. Therefore, when printing the same batch of products, the pressure of each color plate should be as consistent as possible, but sometimes the pressure should be adjusted according to the specific conditions, for example, the printing pressure of the multi-color offset printing machine. It is often a random group decrement.
The deformation of the paper is closely related to the silk thread of the paper. Considering the general offset printing machine, the overprinting error caused by the deformation of the paper under the printing pressure is difficult to adjust in the axial direction than in the radial direction, so when printing, the sheet is single. The paper should make the longitudinal direction of the paper parallel to the axial direction of the drum as much as possible, and the same batch of products, the direction of the silk paper must be the same in order to ensure the accuracy of overprinting, and never allow the same batch of products to be printed on different silk paper.
Under the action of printing pressure, due to the phenomenon of fan-shaped deformation, the image that has high requirements for overprinting accuracy or the part that plays an important role in the theme should be arranged near the cornice during pre-press processing and plate making. Because of the small deformation of the paper near the mouth when printing, it is easy to overprint accurately.
2. Paper peeling tension and paper deformation
The paper after printing through the embossed area is caught by the roll gripper, forcing it to detach from the blanket, and the ink on the plate acts as a medium to adhere the sheet to the surface of the blanket with a strong adhesive force. At this time, the fibrous structure on the printed sheet is subjected to the force in two directions, one is the force of the gripper against the paper, and the other is the adhesion of the ink to the paper. The two forces are opposite in direction. When the printed sheet is peeled off from the blanket, the trailing edge of the paper is easily curled and does not fit against the impression cylinder to form a folding angle, which is called a peeling angle. If the adhesion of the ink is large, the longer the sheet is peeled off by the gripper, the larger the peel angle, the greater the peeling tension, the more the peeling deformation of the paper is increased, and the paper is elongated along the radial direction of the drum, overprinting The accuracy is reduced.
During the printing process, if the gripper of the impression cylinder has sufficient force to ensure that the paper does not shift, the amount of peeling tension becomes the main factor affecting the deformation of the paper. The amount of deformation depends on the following aspect:
(1) The nature of the paper. Paper with large plastic deformation has large peeling deformation, and vice versa.
(2) Printing pressure and time when the rollers are mutually imprinted. The greater the printing pressure, the longer the embossing time, the longer the paper is in contact with the ink on the blanket, and the greater the peeling deformation.
(3) The ink receiving area of the paper surface. The larger the ink area, the thicker the ink layer and the greater the peeling deformation.
(4) Viscosity and adhesion of the ink. The greater the viscosity and adhesion of the ink, the greater the peeling deformation, and vice versa.