Overprinting Failure Analysis of Offset Printing Machine (2)

The influence of the gripping force and the transfer relationship on the overprint

In the printing process, the gripper not only conveys the paper, but also fixes the paper in a certain position, so that the graphic on the plate or blanket is transferred to the designated position. However, when imprinting, if the gripper is weak or uneven, the position of the paper will change, and misprinting will result.

1. Gripper force and overprint

The grippers mentioned here mainly refer to delivery paper and roller grippers. Imprinting cylinder grippers uneven force, can make the paper appear four kinds of deformation (see Figure 8-1), the print will inevitably produce a partial overprint is not allowed.

(Figure 8-1 Distortion of the paper due to uneven grip force)

In order to prevent misalignment of grippers from being uneven, the following points should be made.

(1) The adjustment of the force of the gripper should be carried out during the trial printing. The strength of the gripper shall not be changed during the process of printing. The strength of the gripper of the multicolor machine and the gripper on the impression cylinder shall be consistent. Grinding tooth pads must be flat to ensure uniform strength.

(2) For products with overprinting on both sides, the gripping force of the impression cylinder must be uniform.

(3) The retainer spring of the gripper shaft should have good elasticity, prevent the lack of force, and the positioning of the paper is inaccurate.

(4) Tighten the tight ring of the gripper to prevent the gripper teeth from moving in the axial direction. Use the gripper shaft or bushing with high manufacturing precision to make the gripper stable in the gripping process. The gripping distance is always Keep a constant value.

2. Handover relationship and overprint

There are problems with the transfer of paper in the entire transfer process of paper feeders, gauge parts, delivery dents, and transfer rollers. As mentioned before, the transfer relationship refers to the handover position and handover time. The handover relationship is essentially guarantee. The paper must be transported without errors and never allow the paper to run out of control. The transfer position refers to the transfer of paper at a certain point of the paper, and the transfer time is the time for the transfer machine to complete the transfer of the paper together. This time requirement must be strictly adjusted according to the regulations, neither short nor long, for example, The transfer time of the impression roller's teeth, if the transfer time is short, the overprinting must be inaccurate. If the transfer time is long, it will tear the edge of the mouth. Therefore, the adjustment of the handover relationship is a very cautious technical work. Never be careless.

The relationship between three printing pressure, paper stripping force and overprinting is not allowed

In the printing process, the paper undergoes a certain deformation due to the effects of the printing pressure and the peeling tension. Plastic deformation of the paper, leaving the embossed area, the deformation can not be restored, resulting in a larger overprint error.

1. Printing pressure and paper deformation

Paper is a material with plastic deformation. The plastic deformation characteristics of different papers are not the same. Under the effect of large printing pressure, the width of paper and the length of the two corners of the plastic film will be slightly expanded, resulting in inaccurate overprinting. Especially in the dragging 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 printing plate should be as consistent as possible, but sometimes it is necessary to adjust the pressure according to the specific circumstances, for example, the printing pressure of multicolor offset printing press It is often a random group decrement.

Deformation of the paper is closely related to the silkiness of the paper. Taking into account the general offset press, the overprint error caused by the deformation of the paper under printing pressure is difficult to adjust in the axial direction than in the radial direction, so when printing, single sheets Paper should be as long as possible so that the longitudinal direction of the paper and the roller axis parallel, and the same batch of products, printing paper, the direction of the silk must be the same in order to ensure the accuracy of overprinting, and never allow the same product with different silk paper printing .

Under the influence of the printing pressure, due to the fact that the paper has fan-shaped deformation, the image that requires high overlay precision or the part that has an important effect on the subject matter should be arranged near the mouth during prepress processing and plate making. Because when printing, the paper near the mouth is less deformed, and overprinting is more accurate.

2. Paper peel tension and paper deformation

After printing through the embossed area, the paper is caught by the roller gripper, forcing it to detach from the blanket, and the ink on the plate serves as a medium to adhere the printed sheet to the surface of the blanket with a strong adhesive force. At this point, the fibrous tissue on the printed sheet is subjected to two directions of force, one is the grip force of the gripper toward the paper, and the other is the upward adhesion of the ink to the paper. In the opposite direction of the two forces, when the sheet is peeled off from the blanket, the trailing edge of the sheet is easily curled and does not conform to the impression cylinder, forming a folding angle called the peeling angle. If the adhesiveness of the ink is large, the longer the time the sheet is peeled by the gripper, the larger the peeling angle, the greater the peeling tension, and the more the paper is stretched and deformed, and the sheet is elongated in the radial direction of the drum. The accuracy drops.

During the printing process, if the grippers of the impression cylinder have enough force to ensure that the paper does not displace, the stripping tension will become the main factor affecting the stripping deformation of the paper. The amount of deformation depends on the following aspect:

(1) The nature of the paper. Plastic deformation of the paper, stripping deformation is also large, and vice versa is small.

(2) The printing pressure and the time for the roller to imprint each other. The greater the printing pressure and the longer the imprinting time, the longer the contact time between the paper and the ink on the blanket and the greater the exfoliation deformation.

(3) The ink surface area of ​​the paper surface. The larger the ink area, the thicker the ink layer, the greater the exfoliation deformation.

(4) Viscosity and adhesion of the ink. The greater the viscosity and tackiness of the ink, the greater the exfoliation deformation, and vice versa.