Editor's note: Offset product offsets are easier to identify. If the product is in text form, strokes are thicker and the text appears double strokes. If the image is blurred, the color and level will change, resulting in a decrease in product quality. The ghosting makes reading difficult for readers. This phenomenon is common in the printing process. There are many causes of ghosting. Solid issues in the printing industry have to be faced by relevant persons in the industry.
A four-color offset printing press was overhauled due to the abrasion of the impression cylinder (double diameter). The entire assembly of the cylinder was repaired after spraying and repairing. During the trial printing process, ghosts were found in the first color group. The more serious the tow is, and the one on the drive side is more severe than on the other side, and it is a circumferential ghost image. If only a single color is printed in this color group, no ghost image will appear.
According to this symptom, there are two reasons for the initial judgment: First, the paper transfer time, and second, the pressure of the grippers of each gripper is not adjusted.
For these two reasons, re-commissioning the point of view of the transfer, the pressure of the grippers, dental pieces, tooth pads one by one to adjust each other's position, to replace the suspected tooth pads, so that the entire row of tooth pads level, position consistent. As a result, the problem of ghosting could not be solved. In the process of adjusting the grippers, there was a change in the position of the ghost image. The final conclusion was that the reason for the tooth chip and the dental pad was to replace all the new ones and the tooth pads; for the sake of caution, Ask the maintenance personnel to diagnose and check with the dial indicator that no excessive processing error was found on the surface of the impression cylinder, which is still a problem with the grippers. Again, the teeth adjustment and gripper teeth still failed to solve the problem; the maintenance personnel renewed Measuring, adjusting the ink stick, blanket cylinder pad, plate cylinder, blanket cylinder, impression cylinder pressure, re-cleaning the dental plaque, gum pad and adjusting the gripper pressure, still failed to lift the ghost image, and then directly transferred the ink To the surface of the impression cylinder, observe the ink receiving condition on the surface of the impression cylinder, and see that the ink is not uniform. The conclusion is that the surface of the cylinder cannot be processed, and the size of the cylinder is small (taper); finally, the engineers and technicians make diagnosis and listen carefully to the front personnel. After the efforts made, the four groups of impression cylinders were measured again, and the gear meshing conditions and assembly conditions of the transmission were measured, and the processing opinions were proposed:
1. Basically rule out the problem of grippers, so there is no ghosting failure before the machine is overhauled. Previously, no adjustments were made by many people.
2, the use of dial indicator on the surface of the roller, the data show that the error (≤ 0.02mm) is not enough to lead to such a serious ghost, the impression roller itself can be basically excluded;
3, through the dial indicator and other side of the instrument, found that the gear assembly error of 0.6mm, (because the helical gear is connected to the drum through the six bolts) In theory, the two gears meshing its pitch circle for pure rolling its transmission is stable, and The root cause of ghosting is the occurrence of ghosting due to gear assembly errors that cause the impression cylinder to run. Then why is it not noticeable when measuring the surface error of the cylinder? In fact, it is a matter of static and dynamic. Using the above processing advice, the assembly error of all gears was finally corrected, and the clearance of the tapered bearing on the axis of the impression cylinder was adjusted. Ghosting was basically eliminated.
In summary, there are many reasons for ghosting in printing. How to look through the phenomenon to see the nature, analyze the cause of the failure, and shorten the maintenance time is very important, otherwise it will affect the production and delay the loss caused by the delivery period is not Estimated. In the case of a sheet-fed multi-color printing press, ghosting causes a shift in the transfer process of the mechanically imprinted (net) dots, and secondly, there is a displacement during the paper transfer process. For this reason, the conditions that do not produce ghosts must be smooth for all cylinders (plate cylinders, blanket cylinders, impression cylinders, transfer cylinders) and the geometrical dimensions meet the requirements; the paper from the transfer cylinder to the final delivery row The time and position of the entire route are accurate, stable, and the line speed is constant. If there is a change in one part of the process, it will cause ghosting.
Excluding ghosting can be roughly determined as follows:
1 The printing machine with a short service life is an adjustable ghost image. This kind of printing machine parts are generally not heavily worn, often due to changes in the force of the individual grippers during use, causing the paper feed teeth row and the impression roller teeth. Unevenness caused by uneven drainage force can generally be relieved by adjusting the pressure of the gripper.
2 The printing machine with a relatively long service life is a wear ghosting. There is a process of this type of ghosting. The ghosting is from small to large. Generally, there are tooth pads and tooth chips that cause changes in the strength and size of the jaws. Axial bearing wear, the gap becomes larger; open ball, cam, roller bearing wear, resulting in displacement; if there is a partial ghost, there will often be some teeth, tooth pads wear or part of the "dead tooth" caused; if the whole page Ghosting, mainly check the open ball or each roller bearing wear, whether the clearance increases and the radial and axial runout caused by the result, often determine the group and then by looking at the blanket impression of the roller to find out from which a roller .
3 sudden ghosting, this type of ghosting is usually caused by damage or failure of a bearing or a part, and at the same time the printing press is replaced by parts, maintenance, etc. The example is this type, which needs to be analyzed according to the specific situation.