Comprehensive analysis of overprinting problems during printing
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As we all know, the problem of overprinting is always a big problem that plagues the industry. In the printing process, it not only affects the production schedule, but also wastes the material. This is more obvious in high-speed printing web presses. Below, the author will analyze the reasons why the overprint may be inaccurate.
The failure of the overprinting system is divided into lateral overprinting, vertical overprinting, and misregistration caused by other aspects.
1. Horizontal overprinting is not allowed
The reasons for inaccurate lateral overprinting are problems with incorrect plate position, paper, ink and tape tension adjustment.
(1) The position of the plate is wrong.
The main reason is that the center line of the printing plate is not aligned with the engraved line in the middle of the roller, so that the position of the printing plate is not correct. The solution is to adjust the axial position of the printing plate cylinder. If the adjustment amount is not enough, the plate can only be reinstalled.
(2) Paper
Mainly because the horizontal humidity and thickness of the paper are inconsistent, the paper is wrinkled; the paper roll has a large size head, which makes the paper strip run off. The solution is as follows: do not open the package before the machine to prevent the paper roll from getting wet, avoid the longitudinal corrugation of the paper strip; increase the paper path from the paper feeding mechanism to the first printing unit, or increase the left and right spinning thread rollers to make the paper tape Flattening.
(3) Ink
The viscosity of the ink is too large, causing the solid part of the blanket cylinder to stick to the paper strip, causing excessive local tension, causing the paper strip to partially stretch, resulting in inaccurate overprinting. The solution is to reduce the ink viscosity or reduce the printing speed.
(4) Paper tape tension
The tension between the printing units is lower than the paper feeding mechanism, or the tension between the paper feeding mechanism and the first printing unit is too large, and the tension of the paper strip is reduced after the printing of the first color, resulting in a widening of the paper tape. The solution is to increase the tension between the printing units or to increase the paper guide roller or the lengthening paper path of the paper feeding portion, thereby prolonging the slackening time of the paper strip, so that the amount of change after the paper strip enters the printing unit is as small as possible.
2. Vertical overprinting is not allowed
The reasons for longitudinal overprinting are: plate tensile deformation, tension adjustment, plate installation, roller lining, etc.
(1) Plate tensile deformation
The printing length is inconsistent due to the skew of the printing plate, the improper position of the plate cylinder, and the inconsistent printing length of the printing plate, which causes the overprinting to be inaccurate. In addition, the plate of the printing plate is mostly a metal plate, which has a certain elongation, and tensile deformation occurs under the action of the pulling force, and the tensile deformation of the zinc plate is particularly obvious. Under normal circumstances, printing is found to be inaccurate, mostly by stretching the plate to adjust. When the plate is stretched by an external force, the plate is tightly attached to the surface of the drum, and at the same time, tensile deformation is inevitably generated. The greater the pulling force, the greater the tensile deformation of the plate, and the uneven distribution of the tensile deformation. The force on the edge of the plate and the tip of the printing plate is the largest, and the tensile deformation is also large, and the force inside the plate is small. The deformation is also small.
(2) Tension adjustment
During the printing process, the tension of the paper tape must be kept constant, and the size is appropriate. If the tension of the paper tape is too small or too large, the tension of the paper tape will be unstable, and the overprinting will be inaccurate, especially when the printing speed changes greatly, such as starting. During the stages of braking, paper changing, etc., the tension is unstable and the overprinting accuracy is affected.
However, the root cause of this problem in printing is the failure of the magnetic powder brake and the tension control system. The problems of the magnetic powder brake itself are: magnetic powder aging, agglomeration, water leakage in the cooling water joint, water in the magnetic powder wall, causing the magnetic powder brake The torque is unstable or the torque drops too much, even stuck, and the magnetic powder brake's shaft head and the paper roll connection are swayed, which makes the connection of the two is not good. The magnetic powder brake can not effectively brake the paper roll, resulting in unstable tension. For the tension system, since the spring amount of the floating roller and the tension induction roller in the system is small, it cannot balance with the tension of the paper strip, so that the floating roller and the tension sensing roller frequently jump and swing, causing the tension to be unstable. Therefore, the upper part must be cleaned with the cone and the tapered hole to ensure that the plate cylinder is tightened.
4. Improper adjustment of the guide roller causes misregistration
The non-parallel adjustment of the guide rollers will affect the smoothness of the paper feed, so that the conveying speed of the two sides of the aluminum foil is not synchronized, and thus the overprinting is not accurate. Therefore, the guide rollers on the press should be adjusted less after leveling.
5. Unstable tension causes misregistration
In the process of gravure printing of the aluminum foil cap, the diameter of the discharge roll is gradually reduced, and the diameter of the take-up roll is continuously increased, so the printing tension is constantly changing. The change in tension directly affects the elongation of the aluminum foil. Therefore, if the front and rear tensions change frequently and irregularly during printing, the overprinting may be inaccurate. Therefore, in addition to reasonable adjustment of the tension of the receiving and unwinding according to the actual situation, to avoid excessive or too small tension, the tension should be kept as constant as possible to avoid large changes.
6. Failure of automatic registration system causes overprinting
Problems in any part of the automatic registration system directly affect the normal operation of the entire system, resulting in the overprinting error not being corrected in time, resulting in inaccurate overprinting. Therefore, if the automatic registration system fails, it is necessary to check each component of the system one by one, find out the problem and eliminate it, and if necessary, ask the professional maintenance personnel to further inspect and repair.
7. Improper arrangement of the plate cylinder causes overprinting
Aluminum foil is a soft metal material that gradually elongates during printing, which adversely affects overprinting. In order to compensate for the resulting overprinting error, the plate cylinder diameter of each color group during plate making must be incremented by color with a certain coefficient. The plate cylinder diameter increment coefficient should meet the requirements of the printing process, otherwise some color groups will be overprinted. At the same time, it is necessary to look at the color sequence when loading the plate. If the color sequence is wrong, the order of the plate cylinders is not correct, which will also result in inaccurate overprinting.
In addition, the arrangement of color sequence and the processing of color registration during plate making will also have a certain influence on printing overprinting.