Double shadow is also called double shadow. Refers to the light lines that appear next to printed lines or the silhouettes that appear next to printed dots.
The appearance of ghosting makes the thin lines thick, and the original dots become one and a half or two (as shown in Figure 8-2). The graphics of the entire printed matter are blurred, and the sharpness is obviously reduced.
Figure ghost type
The generation and type of a ghost
When printing with an offset printing machine, there is a slight displacement between the cylinders during each rolling of the cylinder, and the corresponding particles do not coincide. The ink layer left on the blanket is not completely overlapped with the ink layer transferred this time. In addition to the ink layer transferred this time, a light imprint of the remaining ink layer is also displayed at the same time. As shown in Figure 8-2, the dark and light ink overlapping with the ghost image is also displayed on the substrate at the same time, and a ghost failure is formed.
Ghosting failures mostly occur when printing starts, when the cylinder is pressed together and when the cylinder is depressurized; when the printing surface of the plate cylinder and the blanket cylinder are in contact and separated, the printing surface of the blanket cylinder and the impression cylinder are in contact and separated ; When the plate cylinder and the transfer cylinder transfer paper; the inking roller starts to contact the plate or release the plate and when the printing speed changes.
The type of ghosting is divided into three types: vertical ghosting, horizontal ghosting, and A and B ghosting according to the direction of ghosting.
Vertical ghosting, also called vertical ghosting. The ghosts of the dots are fixed at the upper and lower ends of the original dots and in the direction perpendicular to the axis of the drum, as shown in the figure on the upper left.
Horizontal ghosting, also called ghosting. The ghosts of the dots are on the left and right sides of the original dots, parallel to the axis of the drum or at a certain angle. The ghosts are light and heavy, and their positions are fixed, as shown in the figure on the upper right of Figure 8-2.
A and B ghost images, ghost images of dots, are generally on the upper and lower sides of the original dots, and some are on the left and right sides of the original dots, and their positions are not fixed.
2. The cause and elimination of ghosting
There are many reasons for ghosting. You must analyze the cause according to the type of ghosting and find out the elimination method.
(1) Vertical ghosting
1. The cooperation gap between the cylinder journal and the sleeve, the eccentric sleeve and the wallboard hole is too large, especially the cooperation gap between the printing plate cylinder is large, which causes the cylinder to vibrate during operation, the rotation angle speed of the cylinder is uneven, the printing plate cylinder and the rubber The cloth cylinder cannot be contacted at the same position every time, and the ink on the printing plate is transferred to different positions of the blanket cylinder for two consecutive times, and ghosting occurs. Most of this type of ghosting occurs at the mouth of the print. It is generally believed that the matching clearance of the roller sleeve is preferably 0.01 to 0.02 mm, and maintenance is required when it exceeds 0.07 to 0.08 mm to ensure the printing accuracy of the printing machine.
2. The backlash and radial runout of the roller gear are too large, the service life of the offset press is too long, the gear wears, the tooth thickness decreases, and the backlash of the meshing teeth increases, resulting in the rollers not operating at a uniform speed, resulting in instantaneous relative between rollers Displacement, ghosting appears. In conditional factories, the gears or bearings with severe wear can be exchanged; the center distance of the drum can be appropriately reduced to keep the side clearance within a certain range; the pad of the rubber cloth can be trapezoidal with liner paper to reduce The vibration caused by the impact of the small gripper can overcome the ghosting caused by the excessive backlash of the drum gear and the excessive radial runout to varying degrees.
3. Improper lining of the rollers, excessive pressure, and inconsistent surface linear velocity, causing excessive squeezing force between the rollers and changing the squeezing value of the blanket. Due to the large amount of deformation and displacement of the blanket, the blanket cannot be completely restored to the state before compression, resulting in ghosting during ink transfer.
During the rolling process of the three cylinders of the offset press, it must be a reasonable rolling, so that the three cylinders can obtain a pure rolling without sliding under the condition of the same angular velocity. In fact, a small amount of slip always exists, but only on 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 liner should be adopted to avoid ghosting caused by improper liner.
4. The units of the multi-color offset printing machine are not allowed to hand over each other. The ink of the multi-color offset printing machine is overprinted under the condition of wet pressing, so the ink imprint printed first is easily transferred to the surface of the rubber blanket after the color. When the positioning and handover between the units are inaccurate, the first printed ink will be displaced, so that the transfer marks of each color cannot overlap and cause ghosting. During printing, the paper has to pass through many parts from the feed to the take-up table. The more times the paper is transferred, the greater the cumulative overprint error and the more chances of displacement, that is, the greater the possibility of ghosting. Therefore, it is required that the positioning and transfer parts between the rollers are accurate, and cannot be loose or worn, so as to ensure that the rollers transfer the paper at the cutting point.
In addition, inadequate pressure on the rollers, excessive pressure on the rollers, poor quality of the blanket, excessive peeling force of the paper, incomplete tension of the printing plate, or cracking of the printing plate at the nip will cause longitudinal ghosting failure .
[Case 1]
After printing a thick paper with a basis weight of 300g /? On a four-color, four-open imported offset press of a certain factory, and then printing a cigarette label with a basis weight of 80g /? On a single-sided coated paper, the large-area longitudinal ghost of the second color appeared on the printed product (From the mouth to the center line of the rule), the distance between the light blot and the deep coin is 1.5mm.
[case study]
The main reason for the above failures is: in the process of printing 300g /? Thick paper, the impression cylinder gripper teeth work for a long time under high load, resulting in fatigue failure of some gripper teeth compression springs, the gripping force drops, gripping The paper is unstable, causing ghosting.
[Approach]
(1) Carry out a comprehensive inspection of the gripper teeth of the impression cylinder, and replace the small compression springs with partial fatigue failure to ensure that the gripper teeth have the opening angle in place, the tooth length is consistent, and the grip force is consistent.
(2) Clean and lubricate the original gripper.
(3) The machine is left idle for about half an hour, and then try printing again.
[Expert Tips]
There are many machines that print thick paper first, and then thin paper again. At first, there are ghost images, but after printing to 1000 to 2000 sheets, the ghost images will disappear naturally. This shows that some machines can recover the force of paper teeth.
(2) Horizontal ghosting
1. The axial movement of the gripper shaft is installed on the gripper shaft. The crosstalk of the gripper shaft causes inaccurate paper transfer and displacement of the paper, resulting in ghosting. Generally, the gap between the two ends of the roller gripper shaft and the end surface of the bushing can be adjusted to overcome the ghost caused by the axial movement of the gripper teeth.
2. Loosening of the thrust bearing screw of the cylinder or wear of the thrust bearing or the nut tightening the thrust bearing. The looseness of these components causes the axial displacement of the plate cylinder, blanket cylinder and impression cylinder during high-speed operation, causing ink marks. The transfer does not coincide, resulting in ghosting. It is generally required that the axial series movement of the machine does not exceed 0.03mm, so find out the series of rollers and tighten the nut of the thrust bearing to eliminate this ghosting.
In addition, the water and ink rollers are axially oscillating; the "lotus edge" of the paper; the improper adjustment of the front and side stop gauges causes excessive pressure on the edge of the paper to cause curling; and unstable paper feed will cause lateral ghosting.
[NextPage]
(3). A, B ghosting
A and B ghosting are typical faults of multi-color offset presses, which directly affect the tone and color reproduction of printed products, reduce image resolution, reduce image gradation, and blur text.
1. The basic concept of A and B ghosting
Most modern multi-color offset presses use double-diameter impression cylinders and paper transfer cylinders. There are two sets of grippers on the cylinder. These two pairs of grippers are called A gripper (axis) and B gripper (axis). (See Figure 8-3). If the manufacturing and installation accuracy of the gripper shaft of the machine is very high, the ideal state is reached, ie aa = 0 bb = 0 cc = 0
There will be no ghosting. On the contrary, when these three sets of data are greater than zero or less than zero, the dots on the paper groups of A and B cannot overlap completely on the post-printed rubber, which will inevitably produce ghosting. The ghost image caused by the double-diameter roller gripping teeth is called "A, B" ghost image.
2. A and B ghost images are ghost theaters appearing in the continuous operation of multi-color offset presses with double-diameter cylinders in wet overprinting. Such ghost images are regular. It refers to the failure of a set of grippers on the double-diameter roller of the previous unit, which caused the paper to slip during the printing process or the handover of the paper is inaccurate. The imprints of the printed products of the previous unit will not overlap on the latter groups of blankets , A and B ghosts are produced.
For example, the gripper A of the impression cylinder of the first unit works normally, and the mark left on the blanket of the second unit is point A, while the gripper of the first unit B has a malfunction, which causes the paper to slip backward during printing. Moving, the mark left on the blanket of the second unit is below point A, called A, at this time, a ghost image of point A appears above A. When the third sheet of paper is another set of gripper paper, the print left on the second set of blankets coincides with point A, so that under point A, the print left by A on the blanket coincides with point A In this way, A appears below point A, and the transfer mark left on the blanket is repeated like this, and ghost images appear continuously. Generally printing thick paper is not obvious, and printing thin paper (100g /? Below)-easy to see.
The main reason for the A and B ghosting theaters is that regular printing of different thickness papers wears the dental pads, or because the speed of the machine is inconsistent, the lubrication is not in place, and the precision of the dental shaft wear is reduced.
According to the above reasoning, a pair of gripper devices corresponding to the ghost above the normal footprint (trailer direction) is faulty, and a pair of gripper working devices corresponding to the ghost below the normal blot (direction of the gripper) work normally .
Figure A, B gripper axis and gripper accuracy diagram
AB ghost detection and elimination
Under the condition of high-speed operation, how to judge which of the two gear grippers of AB is out of order?
The first method is a group-by-group inspection method, which is to print with the first and second units first. The purpose is to check whether the marks on the paper of the first unit can overlap on the blanket of the second unit. Imprinting and transfer cylinder gripper devices work properly; if they do not overlap, it proves that the first unit's impression and transfer cylinder gripper devices are faulty and need to be eliminated. Then use the second and third units to print to determine whether the second unit gripper is working properly, and so on.
The second method is that four units print at the same time, observe the color scale, and set the printing color sequence to black, cyan, magenta, and yellow (ie, B, C, M, and Y). M and Y color marks are clear, then prove that the imprints of BK and C cannot overlap on the blankets of M and Y, and it can be judged that there is a set of gripper device of the imprinting or transfer roller of group C, and the color group M The two sets of gripping devices work normally, because the M color mark imprint can be completely overlapped on the blanket of the Y color group, so the M and Y color marks are clear.
According to the above introduction, we have been able to quickly determine which unit and which gear gripper device is faulty, and we can troubleshoot which set of gripper device.
The method of elimination is as follows:
(1) First check whether the tooth opening roller is excessively worn;
(2) Check whether the gripper shaft bearing and the shaft seat are worn;
(3) Perform cleaning work and grip force adjustment on gripping teeth, dental slices, and dental pads. Komori, KBA, Mitsubishi, and Roland all have gripping teeth closing devices, first adjust the opening and closing tooth roller to the small surface, and then one by one Loosen the dental slices and clean the dental slices and the dental pads, then close the dental slices on the dental pads, and finally adjust the opening roller to the large surface (forced) to close the teeth and adjust the gripping force and the opening and closing. During the above operations, the power supply must be cut off to avoid damage to the equipment due to misuse.
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