During the printing process, the paper is deformed by printing pressure and peeling tension. For paper with large plastic deformation, after leaving the embossing area, the amount of deformation cannot be recovered, causing a large overprint error.
1. Printing pressure and paper deformation
Paper is a material with plastic deformation, and different papers have different plastic deformation characteristics. Under the action of large printing pressure, paper with large plastic deformation will expand slightly in width and length of the two corners of the trailing edge, resulting in inaccurate overprinting. Especially in the trailing part of the paper is more serious.
The greater the printing pressure, the more obvious the deformation of the paper, so 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 situation, for example, the printing pressure of multi-color offset presses Often the random group decreases.
The deformation of the paper is closely related to the thread of the paper. Considering the general offset press, the paper produces an overprint error caused by the deformation of the paper under the action of printing pressure. It is difficult to adjust in the axial direction than in the radial direction, so when printing, the single sheet The paper should be as parallel as possible to the longitudinal direction of the paper and the axis of the cylinder, and the same batch of products, the direction of the silk of the printing paper must be the same, in order to ensure the accuracy of overprinting, the same batch of products are never allowed to use different silk paper for printing
Due to the fan-shaped deformation of the paper under the pressure of printing, the images that have high requirements for overprint accuracy or the parts that have an important role in the theme should be arranged near the mouth during pre-press processing and plate making. Because when printing, the deformation of the paper near the mouth is small, so it is easy to register accurately.
2. Paper peeling tension and paper deformation
The paper after printing in the embossing area is held by the roller gripper, forcing it to detach from the blanket, and the ink on the printing plate is used as a medium to adhere the printed sheet to the surface of the blanket with a strong adhesive force. At this time, the fibrous structure on the printed sheet is affected by the force in two directions, one is the gripping force of the gripper to the paper downward, and the other is the upward adhesion of the ink to the paper. The directions of these two forces are opposite. When the printed sheet is peeled off from the blanket, the trailing edge of the paper is easily curled and does not fit with the impression cylinder, forming a folding angle, called the peeling angle. If the adhesion of the ink is large, the longer the printed sheet is peeled down by the gripper, the greater the peeling angle, the greater the peeling tension, the greater the peeling deformation of the paper, the paper is elongated in the radial direction of the cylinder, and overprint The accuracy drops.
During the printing process, if the gripping teeth of the impression cylinder have enough gripping force to ensure that the paper does not move, the size of the peeling tension becomes the main factor affecting the peeling 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 and stretching deformation, and vice versa.
(2) The printing pressure and the time for the cylinder to imprint each other. The greater the printing pressure, the longer the embossing time, the longer the contact time between the paper and the ink on the blanket, the greater the peeling and deformation.
(3) Ink receiving area on the paper surface. The larger the inked area, the thicker the ink layer, the greater the peeling and extension deformation.
(4) Viscosity and adhesion of ink. The greater the viscosity and adhesion of the ink, the greater the peeling and stretching deformation, and vice versa.
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