The ghosting fault of the offset printing press is actually a type of overprint fault. The lighter ones have the dot distortion, the lines become thicker, the color of the printed product is unstable, and the heavy lines appear double lines and the writing is blurred.
In monochromatic printing, if there is an axial or circumferential displacement between the plate Cylinder and the blanket cylinder, the impression of each time the plate cylinder is transferred to the blanket cylinder changes, forming a monochromatic ghost.
In the case of multi-color printing using a multi-color machine, since wet-wet printing is used, the first color (or the previous color) of the paper is printed on the rear rubber rollers, and the prints on the rubber cylinder are printed again. Once again transferred to a subsequent printed sheet, if these blots do not completely coincide with the imprints on subsequent sheets, ghosting will occur. Strictly speaking, if a magnifier is used to inspect the outlets of printed products, a considerable part of the printed products will have ghost images. The phenomenon of ghosting of printed materials is quite common for those equipment that is not repaired. Many manufacturers only think about solving the ghosting problem when the ghosting phenomenon is very serious.
Let us analyze the factors causing the ghosting of printed products.
One, rubber roller and plate cylinder
1. The plate cylinder or rubber cylinder bearing is damaged. The clearance between the roller bearings of the plate cylinder or the rubber roller shaft end is too large, the rotation process of the roller is unstable, and the roller is likely to slip and form a circumferential ghost. When the axial end thrust bearing is damaged, the axial turbulence of the drum is generated and an axial ghost is formed.
2. The rubber roller sleeve wears on the outside. Heidelberg offset press uses a three-point suspension clutch pressure system. Due to frequent pressure build-up, some rubber roller sleeves wear out at the pressure fulcrum position. When the pressure is applied, the final position of the sleeve is unstable or after the pressure is applied, causing a jump and forming a heavy load. shadow. Especially when printing more than a dozen prints before, the ghosting is more obvious.
3. When the blanket is too loose or the plate is not tightened, when the plate cylinder and the blanket cylinder are rolled against each other, a slip occurs between the plate and the eraser, causing ghost images.
Second, the paper section
The failure of the paper feeding section is one of the main factors that cause the rejection of prints and the wrinkling of printed products. It is also one of the main reasons for the “A and B ghosting†of printed products.
1. When the paper board is not flat or the spacing between the paper pad is unreasonable, the delivery paper is uneven, the paper is deformed axially during printing, and the deformation of the paper tail is more serious, resulting in "A, B ghosting. ".
2. When the individual front pressure gauge paper tongue is too low, the paper will be partially on the top of the front gauge when the paper is fed. When paper is handed, the paper will easily tear or cause the paper to be deformed to form a ghost image.
3. Due to wear, improper adjustment, and installation problems, the process of transferring paper to the paper handler is not stable. When the paper is handed or when the paper is transferred to the first group of impression rollers, the paper has already undergone bending deformation or even wrinkles. In this way, when the paper is printed in the latter colors, when these deformations (or wrinkles) are partially rolled off, the first color print (especially the tail) will be axially larger than the latter color. As a result, the first color of the paper tail has a severely deformed, unclear handwriting and double lines.
Third, drum teeth
1. After the teeth of the drum teeth are damaged, it will cause the failure of the transfer and form a ghost image. When a row of teeth in a row of double-pass rollers is broken, the ghost image is in the form of a ghost image and a ghost image.
2. When the dead teeth appear on the impression roller's row, the pick-up force at the dead teeth is not enough. When the paper and the blanket are peeled off, the paper is loose at the back of the mouth and the print appears ghost at the mouth. If there are multiple dead teeth in a row of teeth, there will be a misalignment in the circumferential direction, that is, a circumferential ghost.
3. Improper adjustment of the transfer time of the roller tooth row, the long delivery time, too short or inaccurate delivery point, resulting in overprinting failure and ghosting.
4. When the jaws are creped, some tusks do not pick up paper, causing the paper to come loose from here and form ghost images. Or some cavities close their teeth too early and scratch the paper, causing overprinting failures and ghosting.
5. The teeth of the row shaft or the row of teeth are seriously worn. When the teeth are closed, there is a large gap between the teeth, the time for closing the teeth is not stable, and the printed product produces ghost images.
6. The needle end of the row of teeth on the open ball side is damaged, the clearance between the shaft and the bearing is too large, and when the row of teeth with a high point of closing teeth is closed, the shaft will produce a certain deformation on the side of the open ball. , so that the lack of force in the teeth, closed teeth instability, the formation of ghosting.
7. Two rows of teeth are installed on each large transfer cylinder. If the open and close teeth of two rows of teeth have different time or the installation position is inconsistent, the position of the rear row of jaws will change, and the prints will be separated by one at a time. Zhang appeared once again.
8. If the squab seat of the small transfer drum row is improperly installed or broken, when it is transferred to other rollers, the pad gap is unreasonable or not fixed, which can also cause overprint failure and form ghost images.
The cylinder body may sometimes be deformed. At this time, the printing pressure of various parts of the printing product is inconsistent, so that the paper is locally deformed when the color is printed, and the printing does not completely coincide with the printing plate to form a ghost image. When the cylinder body is deformed more severely, wrinkles may appear when the tissue paper is printed, resulting in serious ghosting.
Normally, the rollers of Heidelberg offset press are pressed in neutral position. When a failure occurs, the blanket cylinder and the impression cylinder are pressed together in the second half of the cylinder. This is the problem of premature pressure.
At the time of starting up, the operator is customarily using advanced paper and post-compression. The pressure point of the impression cylinder and the blanket cylinder is at the rear half of the paper. The front half of this paper is not completely attached to the surface of the drum. Zhang Yinpin will appear semi-white half-color phenomenon, and the first paper tail of the imprinting is delayed than the normal situation, the first dozen prints will appear more serious ghosting.