Detailed offset printing faults - ghosting failure
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The ghost image is also called double shadow. Refers to the shallow lines appearing beside the printed lines or the side shadows appearing next to the printed dots.
The appearance of the ghost image makes the thin lines thicker, and the original dot becomes one and a half or two. The graphic of the entire print is blurred and the sharpness is obviously reduced.
Graph ghost type
The generation and type of a ghost
Printing with an offset printing machine, in each rolling of the drum, a slight displacement occurs between the rollers, the corresponding dots do not coincide, and the remaining ink layer on the blanket is not completely coincident with the ink layer transferred this time. In addition to the ink layer transferred this time, the light blot of the remaining ink layer is also presented, as shown in Fig. 8-2. The ghost image is superimposed on the substrate, and a ghosting fault is formed.
The ghosting failure occurs mostly when the printing starts, when the drum is pressed and the roller is pressed; when the printing cylinder is in contact with and separated from the printing surface of the blanket cylinder, when the printing surface of the blanket cylinder and the impression cylinder are in contact and phase separation When the plate cylinder and the paper feed roller transfer paper; when the ink roller starts to contact the plate or when the plate is released and the printing speed is changed.
The type of ghosting is divided into vertical ghosting, horizontal ghosting, and A and B ghosting according to the direction of ghosting.
Vertical ghosting, also called ghosting. The virtual shadow of the dot is fixed at the upper and lower ends of the original dot in the direction perpendicular to the axial direction of the drum, as shown in the upper left figure.
Horizontal ghosting, also called to go to the ghost. The virtual shadow of the dot is on the left and right sides of the original dot, parallel to the axis direction of the roller or at a certain angle, and the virtual shadow is light and heavy, and its position is fixed, as shown in the right figure of Figure 8-2.
A, B ghosting, the virtual shadow of the dot, generally on the upper and lower sides of the original dot, some in the left and right of the original dot, its position is not fixed.
2. Reasons for and causes of ghosting
There are many reasons for ghosting. You must analyze the cause according to the type of ghosting and find out the troubleshooting method.
(1) Vertical ghosting
1. The roller journal and the bushing, the matching clearance between the eccentric bushing and the wall plate hole is too large, especially the matching clearance of the plate cylinder, so that the drum vibrates during the running process, the angular velocity of the drum is not uniform, the plate cylinder and the rubber The cloth roller can not be in contact at the same position at a time, and the ink on the printing plate is transferred to the different positions of the blanket cylinder twice in succession, and a ghost image appears. Most of these ghosts occur in the mouth of the print. It is generally believed that the matching clearance of the roller bushing is preferably 0.01 to 0.02 mm, and maintenance is required when the roller bushing exceeds 0.07 to 0.08 mm to ensure the printing precision of the printing machine.
2. The backlash and radial runout of the drum gear are too large. The offset machine has a long service life, the gear wears, the tooth thickness decreases, and the backlash of the meshing teeth increases, so that the rollers cannot run at a constant speed, resulting in instantaneous relative between the rollers. Displacement, ghosting occurs. In a conditional factory, the gears or bearings with severe wear can be exchanged; the center distance of the drum is appropriately reduced to keep the side gap within a certain range; the gasket is padded into a trapezoidal shape with a liner paper to reduce The vibration caused by the impact of the small mouth can overcome the ghost caused by the excessive backlash of the drum gear and the excessive radial runout.
3, the drum is not properly lining, the pressure is too large, the surface line speed is inconsistent, causing excessive pressing force between the rollers, changing the extrusion value of the blanket. Due to the large displacement of the blanket extrusion deformation, it cannot be completely restored to the state before compression, causing ghosting when the ink is transferred.
During the rolling process, the three rollers of the offset printing machine must be properly rolled so that the three rollers can obtain pure rolling rolling without slipping under the same angular velocity. In fact, a small amount of slip is always present, but in the condition of synchronous rolling, the amount of slip is not required to exceed a certain amount. Therefore, according to the requirements of the machine manual, a reasonable lining should be taken to avoid the ghosting caused by improper lining.
4. The multi-color offset printing machines are not allowed to cross each other. The ink of the multi-color offset printing machine is overprinted under the condition of wet and wet, so the ink imprint of the first printing can be easily transferred to the surface of the back-colored blanket. When the positioning between the units is not accurate, the first printed ink will be displaced, so that the imprints of each color transfer cannot overlap and cause ghosting. When printing, the paper passes through many parts from the paper feeding to the delivery table. The more times the paper is handed over, the greater the accumulated overprinting error, and the more chances of displacement, the greater the possibility of ghosting. Therefore, the position of the transfer joint between the rollers is required to be accurate and cannot be loosened or worn to ensure that the roller delivers the paper at the tangential point.
In addition, the roller is not in place, the roller is over-compressed, the quality of the blanket is not good, the peeling force of the paper is too large, the plate is not pulled tightly, or the plate is broken at the plate, which may cause vertical ghost failure. .
[Case 1]
After printing a thick paper of 300g/m2 by a four-color four-open imported offset printing machine in a factory, immediately after printing a single-sided coated paper cigarette with a quantitative basis of 80g/m2, the printed product has a large vertical shadow of the second color. (From the mouth of the mouth to the position of the ruled midline), the distance between the light blot and the deep money track is 1.5mm.
[case study]
The main reason for the above failure is that during the printing of 300g/m2 thick paper, the impression cylinder is working under high load for a long time, which causes the fatigue of some of the crepe small compression springs to fail and the force drop. The paper is unstable, causing a ghost.
[Approach]
(1) Carry out a comprehensive inspection of the impression cylinder gripper and replace some of the fatigue springs to ensure that the angles of the grippers are in place, the length of the flaps is consistent, and the force is consistent.
(2) Clean and lubricate the original crepe paper.
(3) The machine is running for about half an hour, and then try again.
[Expert Tips]
There are many machines that print thick paper first, then print thin paper, and there is a ghost image at first, but after printing to 1000-2000 sheets, the ghost image disappears naturally. This shows that some machine crepe tooth spring force can be restored.
(2) Horizontal ghosting
1. The axial movement of the gripper shaft is carried out on the gripper shaft. The pick-up of the gripper shaft causes the paper to be misaligned and the paper is displaced, resulting in ghosting. Generally, the gap between the two tight rings of the roller gripper shaft and the end face of the sleeve is adjusted, and the ghost caused by the axial movement of the gripper can be overcome.
2. The thrust bearing of the drum is loose or the bearing of the thrust bearing is worn or the nut of the thrust bearing is tightened. The loosening of these components causes the plate cylinder, the blanket cylinder and the impression cylinder to axially displace during high-speed operation, resulting in ink. The transitions do not coincide, resulting in ghosting. It is generally required that the axial movement amount of the machine does not exceed 0.03 mm, so it is possible to eliminate the ghost by finding the roller that is strung and tightening the nut of the thrust bearing.
In addition, the string of water, the ink roller in the axial direction; paper "lobe edge"; improper adjustment of the front and side gauges, so that the paper edge pressure is too large to produce curl; paper feeding is not stable, etc. will cause horizontal ghosting.
(3). A, B ghost
A, B ghosting is a typical fault common to multi-color offset printing presses. It directly affects the reproduction of tone and color of printed products, reduces image resolution, reduces image levels, and blurs text.
1, the basic concept of A, B ghosting
Most of the modern multi-color offset presses use a double-diameter impression cylinder and a transfer cylinder. There are two sets of cavities on the drum. These two cavities are called A cavities (shafts) and B cavities (shafts). (See Figure 8-3). If the manufacturing and installation accuracy of the machine shaft is very high, the ideal state is reached, ie a-a=0 b-b=0 c-c=0
Then there will be no ghosting. On the contrary, when the three sets of data are greater than zero or less than zero, the dots on the papers of the A and B groups cannot be completely overlapped on the post-printing eraser, which inevitably produces ghost images. The double image caused by the double-diameter roller dent is called the "A, B" ghost.
2, A, B ghost is a multi-color offset press with double-diameter roller in wet overprinting. In the case of continuous operation, the ghost theater appears. This kind of ghosting is regular. It refers to the failure of one set of double-diameter rollers of the previous unit, which causes the paper to slip during the printing process or the paper is not delivered correctly. The imprint of the previous unit will not coincide on the latter groups of blankets. , A, B ghost image is produced.
For example, the first unit of impression cylinder A set of caulking works normally, and the imprint left on the second unit blanket is point A, and the first unit B set of caulking device is faulty, causing the paper to slide backward in printing. Move, the mark left on the second unit blanket is below the point A, called A, and the shadow of point A appears above A. When the third sheet of paper is again a set of crepe paper, the mark left on the second set of blankets coincides with point A, so that under the point A, the mark of A remaining on the blanket coincides with point A. Thus, A appears below the A point, and the transfer print is left on the blanket, so that the ghost appears continuously and continuously. Generally, printing thick paper is not so obvious, and printing thin paper (100g/m2 or less) is easy to see.
The reason for causing the A and B ghost theaters is that the papers with different thicknesses are often printed to make the pads wear, or because the machine speeds are inconsistent, the lubrication is not in place, and the wear accuracy of the teeth is reduced.
According to the above reasoning, a pair of gingival devices corresponding to the ghost image above the normal blot (the direction of the tip) is faulty, and a pair of dentition working devices corresponding to the ghost under the normal blot (the direction of the mouth) work normally. .
AB ghost detection and elimination
In the case of high-speed operation, how to judge which of the two files of AB is faulty?
The first method is a group-by-group test, that is, printing with the first and second units first, in order to check whether the marks on the paper of the first unit can be overlapped on the second unit blanket, and if the coincidence, the first unit is proved. The embossing and transfer roller tampering device works normally; if it does not overlap, it proves that the first unit embossing and the transfer roller damper device are faulty and need to be eliminated. Then use the second and third units to print to determine whether the second unit's tooth decay device is working properly, and so on.
The second method is to print four units at the same time, observe the color code, and set the printing color order to black, cyan, magenta, yellow (ie B, C, M, Y), such as BK, C color standard at the same time ghosting The M and Y color standards are clear, so it is proved that the BK and C imprints cannot overlap on the M and Y blankets, and it can be judged that the C group embossing or the paper transfer roller has a set of tooth decay devices, and the M color group The two sets of tooth decay devices work normally, because the M color mark imprints can completely coincide with the blanket of the Y color set, so the M and Y color standards are clear.
According to the above introduction, we have been able to quickly determine which unit, which one of the tooth decay devices has failed, can be used to troubleshoot which set of dental devices.
The method of exclusion is as follows:
(1) First check whether the open roller is excessively worn;
(2) Check the jaw shaft bearing, whether the shaft seat is worn or not;
(3) For the cleaning work and force adjustment of the caries, teeth and pads, Komori, KBA, Mitsubishi and Roland all have the device for clamping the teeth, first turn the roller to the facet, then one by one. Loosen the tooth piece and clean the tooth piece and the tooth pad, then close the tooth piece to the tooth pad, and finally adjust the tooth opening roller to the large face (forced) to close the tooth, and adjust the force and the proper closing. In the above operations, the power must be turned off to avoid erroneous operation and damage to the equipment.