As we all know, the problem of inaccurate overprinting has always been a big problem in the industry. In the printing process, it not only affects the production schedule, but also wastes materials very seriously. This is more obvious in high-speed printing web printing machines. The author will do the following analysis on the possible causes of inaccurate overprint.
The faults of the overprinting system are divided into horizontal overprinting, vertical overprinting, and overprinting caused by other aspects.
1. Horizontal overprint is not allowed
The reasons for the inaccurate horizontal overprinting include the wrong position of the printing plate, the adjustment of the tension of paper, ink and paper tape.
(1) The position of the printing 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 cylinder, which makes the position of the printing plate incorrect. The solution is to adjust the axial position of the printing plate cylinder. If the adjustment is not enough, you can only reinstall the plate.
(2) Paper
The main reason is that the transverse humidity and thickness of the paper are inconsistent, which makes the paper wrinkle; the paper roll has large and small heads, which makes the paper belt run off. The solutions are: do not open the package before starting the machine to prevent the paper roll from being damp and avoid longitudinal ripples on the paper tape; increase the paper path from the paper feeding mechanism to the first printing unit, or increase the left and right screw rollers to make the paper tape Flatten out.
(3) Ink
The viscosity of the ink is too high, which causes the solid part of the rubber cylinder to stick and pull the paper tape, causing excessive local instantaneous tension and local stretching of the paper tape, 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 that of the paper feeding mechanism, or the tension between the paper feeding mechanism and the first printing unit is too large. After printing the first color, the tension of the paper belt decreases, causing the paper belt to become wider. The solution is to increase the tension between the printing units or increase the paper guide roller of the paper feeding part or lengthen the paper path, extend the relaxation time of the paper tape, and make the change amount of the paper tape as small as possible after entering the printing unit.
2. Vertical overprint is not allowed
The reasons for the inaccuracy of longitudinal overprinting are: stretching deformation of the printing plate, tension adjustment, installation of the printing plate, roller lining, etc.
(1) Stretching deformation of printing plate
The printing length is inconsistent due to the skewness of the printing plate, improper position of the printing plate cylinder, inconsistent printing length of the printing plate, etc., resulting in inaccurate overprinting. In addition, the plate material of the printing plate is mostly a metal plate, which has a certain degree of extensibility, and tensile deformation will occur under the action of tensile force. The tensile deformation of the zinc plate is particularly obvious. Under normal circumstances, when printing is found to be inaccurate, most of them are adjusted by stretching the printing plate. When the plate surface is stretched by an external force, the plate closely adheres to the surface of the drum, and at the same time, tensile deformation is unavoidable. The greater the tensile force, the greater the tensile deformation of the plate and the uneven distribution of the tensile deformation. The force is the largest on the grip edge and trailing edge of the printing plate, and the tensile deformation is also large. The force on the printing plate is small. The extension deformation is also small.
(2) Tension adjustment
During the printing process, the tape tension must be kept constant, and the size is appropriate. If the tape tension is too small or too large, it will cause the tape tension to be unstable and inaccurate overprinting, especially when the printing speed changes greatly, such as starting , Brake, paper change and other stages, causing tension instability and affecting the accuracy of overprinting.
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: aging and agglomeration of the magnetic powder, leakage of the cooling water joint, and water in the magnetic powder wall, causing the magnetic powder brake The torque is unstable or the torque drops too much, or even stuck, and the connection between the shaft head of the magnetic powder brake and the paper roll shaft is aroused, making the connection between the two is not good, and the magnetic powder brake cannot effectively brake the paper roll, resulting in unstable tension ; For the tension system, due to the small amount of springs in the floating roller and the tension sensing roller in the system, it cannot be balanced with the tension of the paper belt, so that the floating roller and the tension sensing roller frequently jump and swing, causing tension instability.
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Solution: repair the magnetic powder brake, check the water inlet pipe, and weld it firmly; check and replace the connecting key, adjust the spring force of the floating roller and the paper tension sensor roller.
3. Overprinting caused by other aspects is not allowed
(1) Photoelectric scanning head
The photoelectric scanning head is the key part to detect whether the printing plate overprint is accurate. The signal it scans is directly transmitted to the main console, so the stability of its signal plays a vital role in overprinting. In order to prevent the scanning head from providing false signals, the photoelectric scanning head must be kept clean in daily work. The main operator should regularly check the scanning head to ensure that the detection head is transparent and clear to prevent dust, paper wool, paper scraps and ink particles Debris enters the photocell and interferes with the scanning signal.
(2) Install version
In the process of plate installation, the problem of inaccurate overprint will also occur. When installing the plate, the excessive force should not be used, which shall be subject to the tightening. When the printing plate is shifted, the corresponding plate-drawing screws should be loosened to minimize the friction between the back of the plate and the surface of the cylinder. At this time, the plate will be displaced under the action of tension and the plate will be stretched. The deformation is minimal.
The rule line should be aligned when installing the plate. In the process of multi-color overprinting, the position of the rule line should be marked on the plate cylinder before the first color is printed and removed. When installing the second color and later When printing plate, align with the rule line of the first color. This can not only increase the speed of the plate installation, but also reduce the number of plate pulls, which can avoid abnormal deformation of the printing plate.
(3) Plate thickness
Different thickness of the printing plate will also lead to inaccurate overprint. Because the graphic of the offset printing is copied on the surface of the metal plate, when the printing plate is tightened on the cylinder, the metal plate is bent, the inner part of the plate is compressed, and the outer part is stretched, making the figure stretch. Therefore, the thicker the metal plate, the larger the graphic size extension. In the plate making, try to use the plates from the same manufacturer, the same model or the same box.
(4) Blanket
The blanket is stretchable, the tighter the blanket is stretched on the roller, the smaller the elongation, the smaller the change in graphics; the blanket with small elongation should be selected; the vertical, horizontal and vertical directions of the blanket should be paid attention to during installation It should be perpendicular to the axis of the cylinder to reduce the elongation. Secondly, during the overprinting process, the tension of the blanket should be kept as constant as possible. In addition, if there is a slight amount of overprint in the width direction, the rubber should be adjusted appropriately. The cloth is tight and should not be adjusted by the pull plate. The cutting, punching of the rubber cloth, loosening of individual screws, tearing of the rubber layout part, partial unevenness of the cushion, etc., will cause the rubber layout part to be loose and uneven, resulting in partial overprinting.
In short, in addition to mechanical, adjustment, paper and other problems, overprinting should also be noted that overprinting caused by some minor changes is not allowed. Because overprint inaccuracy is usually generated at the moment of high-speed printing, it is formed by the accumulation of slight changes, and it is difficult to directly observe. Therefore, as long as we pay full attention to printing, analyze the reasons, and take the most appropriate measures, we can avoid or reduce the occurrence of overprinting.
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