Reasons and solutions for ghosting in offset printing

In the process of offset printing of printed newspapers and other products, due to the tightness of the blanket, the lack of tension during plate loading, the wear of the plate cylinder, the blanket cylinder, the impression cylinder gear and bearings, and excessive printing pressure, etc. For reasons, it is easy for offsets and double-images to be offset when they are printed. Newspapers and magazines appear to be seriously affected, which will seriously affect the quality of printed products and the reading of readers. Whenever you see the products of ghosting, there is a feeling of dazzling eyes, numbness, and inability to read.

When ghosting occurs during offset printing, there are problems with poor mechanical accuracy, factors of paper, improper adjustment of various components, and problems encountered in ghosting at work. Comprehensive analysis is necessary to identify the root cause of the problem and solve the problem. problem.

Ghosting can be further divided into vertical and horizontal double shadows. 1 Ghosting faults and vertical lines on the axis of the drum are called vertical ghosting; 2 graphics, lines, and dots have ghosting. Roller axis lines are parallel, or exhibit a smaller angle parallel, it is called horizontal ghost. When encountering tiny dots, patterns, and lines, you can use a 5- to 10-fold magnifying glass to observe. To eliminate the problem of ghosting, it is first necessary to figure out whether it is a longitudinal ghost or a horizontal ghost, determining the type and orientation of ghosts. Area, come up with a solution to the problem.

One, longitudinal ghosting problems and solutions

1. Longitudinal ghosting appears to be a major cause of failure when the blanket is installed. Offset differs from other printing methods in that offset printing utilizes the indirect printing of a rubber roller intermediate elastomer. There are two cases where the blanket is not tightly stretched. First, the entire rubber bandage is not compact, so that the blanket cylinder and the plate cylinder and the impression cylinder can not be reset in time under pressure due to the printing pressure, resulting in a large area. Second, when the blanket was cut, it was skewed or the iron plywood was inaccurate. It was not in place, resulting in partial tension and causing partial ghosting.

Solution: When cutting the blanket, the radial direction must be used as the force direction of the blanket. That is, the radial direction of the blanket should be wrapped around the drum. The radial direction of the blanket should be perpendicular to the axial direction of the roller. . When cutting, the four sides of the blanket should be squared and the diagonal lines should be equal. In this way, the force of the blanket on the roller can be equal and the product quality can be guaranteed. If the cutting is not correct, there will be skew, it will make the blanket force is not balanced, the appearance of a strip of uneven stripe marks on the surface of the blanket, resulting in the network point is not clear, the local outlets elongated deformation and other issues.

When installing the blanket, prepare the work and then put the tray to the nip of the rubber cylinder. Put the iron cloth on the blanket and put it tightly. Put it tightly and put it in the liner to make the disc drive to the tip of the rubber. The cloth dragging tip iron plate is installed, and the tip iron clamp is tightened to completely tighten the blanket. Note that when the blanket is installed, the liner must not be wrinkled. When the blanket is stretched, it is required that the two sides of the mouth should be pulled at the same time. Do not apply tension on one side. The new rubber is flexible and stretchable. The stretched blanket should be followed. Progressively and gradually. Tighten once, wait for the offset press to run at a low speed for 3 to 5 minutes, and then find that there is no tension and then continue to tighten. In addition, the blanket material requires a smooth and tidy service.

2. Plates do not strain, causing ghosting. Due to inaccurate and paralyzing work, there is no strain on the printing plate. Features are large areas or local longitudinal ghosting, double shadows, with the wrench fastening screws used to pull the plate, the tension should be consistent and balanced, to avoid the pressure plate under the pressure of the press, a small amount of convex pressure slip, Printed product ghosting, in addition, pull the plate when the pulling force is too large, the plates at both ends of the pliers pull to break, or the base becomes thin, resulting in open after the fracture, partial ghosting.

Solution: When there is longitudinal ghosting, check whether the printing plate is stretched and bite the mouth, whether the two ends of the drawing are pulled too much and broken. If there is no tensioned plate to be tightened, if the bite is broken, it must be reprinted. When printing plates and liners are to be tiled, the clamping screws must be tightened and the tension screws must be evenly pulled to avoid ghosting in the longitudinal slide.

3. The gravitational effect caused by the wear of the roller gears and bearings. The offset machine is equipped with a helical gear with high precision on the journal shaft of the drum shaft, transmits more than 80% of the power, and has a high level of accuracy. It not only transmits power but also has precise positioning.

Due to the normal operation of the offset press at 6000 sheets/h, the machine speed is high and the working time is long, the wear will be severe, and the gap between gears may be too large when the gears are engaged, causing mutual slippage between the longitudinal drums, gears and gears. Indirect micro-dislocation, resulting in ghosting phenomenon.

Solution: If there is serious abrasion between the rollers in the offset press, under normal circumstances, the machine should be overhauled, and the gears and bearings should be replaced; the center distance of the rollers should be reduced to make the gap smaller, and the gear meshing backlash should be controlled within 0.1mm.

4. Ghosting caused by excessive pressure between the rollers. When the pressure between the rollers is too large, especially when the pressure between the blanket cylinder and the impression cylinder is excessively adjusted, the longitudinal large-area ghost image cannot be completely reset due to the large deformation displacement of the blanket due to compression deformation of the blanket. .

Solution: When most of the layout ghosts appear, check whether the printing pressure between the rollers is too large. The pressure between the three cylinders of the plate cylinder, the blanket cylinder, and the impression cylinder must be adjusted properly. Between the plate cylinder and the blanket cylinder The pressure is adjusted to 0.1-0.15mm, and the pressure between blanket cylinder and impression cylinder is adjusted to 0.2-0.25mm.

5. The partial and large areas of ghosting caused by the ups and downs of the paper, the unevenness of the arches, and the stretching and deformation of the paper are mainly the inherent quality of the paper itself, which is caused by improper handling of the paper. Due to the problem of paper, the paper bite during offset printing. The teeth, side gauges, and paperboards are difficult to adjust. Papers that are uneven and uneven in moisture will not be spread out. They are often prone to ghosting.

Solution: Rehanging the air conditioning and conditioning paper, offset printing requires uniform moisture content, good flatness, high smoothness, small paper stretchability, strong water resistance, no nap, no powder removal, tight texture, and proper ink absorption.

Second, horizontal ghosting failures and solutions

1. The horizontal ghosting is mainly caused by the lateral motion of the drum. The specific feature is that the direction of ghosting is parallel to the axis of the drum, and the axial end of the drum has serious wear of the sliding bearing, an excessive clearance, or is caused by the loosening of the nut bearing the radial bearing.

Solution: Swap the worn out sliding bearing, find out the rotating roller, tighten the nut of the sliding bearing, this kind of ghost can be eliminated.

2. Both vertical and horizontal ghosts are closely related to the quality of the paper itself and the poor handling of the paper. If the paper is bowed and wrinkled, it will cause ghosting, especially when the paper is pulled. "Leaf edge" bow wrinkles, resulting in lateral ghosting at both ends of the site.

Solution: Paper re-hanging and conditioning treatment, so that the water content of the paper can be printed on the machine. Paper can be printed normally and must meet two conditions:

1. The water content of the paper should be uniform before printing, and it is compatible with the ambient temperature and humidity of the printing shop.
2. In the printing process, the ambient temperature and humidity should be stable, and the impact of other factors on the moisture content of the paper should be reduced. The windows are not opened during production.

Causes of paper bow wrinkle: First, the uneven moisture content of the paper; Second, the printing press failure, biting paper and cardboard, side regulation is not adjusted properly.

The paper presents the problem of bowing and bowing ("foliage"). On the one hand, the ink plate of the impression cylinder is printed, and the impression cylinder must be cleaned in time. In addition, one aspect is the cause of uneven water content at the tip of the paper.
Solution: Resuspend the air conditioning solution, remove moisture, blow air, and make the water in the middle and all around the paper uniform to reduce ghosting.