About ghosting in printing

Double shadow is also called double shadow. Refers to the appearance of light lines next to the printed lines or the silhouette that appears next to the printed dot.

The appearance of ghosting makes the thin lines thick, and the original halftones become one half or two (as shown in Fig. 8-25). The graphic of the whole print is blurred and the sharpness is obviously reduced.

First, the production and classification of ghosting

Printing with a lithographic printing press produces tiny displacements between the rollers during each roller rolling. The corresponding dots do not overlap. The remaining ink layer on the blanket can not be completely overlapped with the transferred ink layer. In addition to the ink layer of the current transfer, the light-colored prints of the remaining ink layers and the dark and light inks of the woody phase coincidentally appear on the substrate at the same time, causing a ghosting failure.

Ghosting failures often occur at the beginning of printing; when the drum pressure is combined with the roller pressure separation; when the plate 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 separated; When the platen and transfer roller transfer the paper; when the platen contacts the plate or the plate is released and the printing speed changes.

The type of ghosting is classified into vertical ghosting, horizontal ghosting, and A and B ghosting, depending on the direction of ghosting.

Longitudinal ghosting, also known as upper and lower ghosting. The phantom of the dot is at the upper and lower ends of the original dot, and is perpendicular to the axial direction of the roller. Its position is fixed, as shown in the left figure on Figure 8-25.

Lateral ghosting is also called ghosting. The imaginary shadows of the dots are on the left and right sides of the original dot, parallel to the axis of the drum or at a certain angle. The imaginary shadows are light and heavy and their positions are fixed, as shown in the right figure on Figure 8-25.

A, B ghosting, the virtual point of the network, some in the original site of the upper and lower side, some in the left and right side of the original site, its position is not fixed, as shown in Figure 8-25 below. A, B ghosting is sometimes found on every printed sheet, but it is sometimes separated by one or several sheets.

Second, the cause of ghosting and troubleshooting

There are many reasons for ghosting. You must analyze the cause according to the type of ghosting and find out how to eliminate it.

1. Longitudinal ghosting.

1There is a large gap between the drum journal and shaft sleeve, eccentric sleeve and wall plate hole, especially the large clearance gap of the plate cylinder, which causes the drum to vibrate in the process of operation, the roller rotation angular velocity is not uniform, the plate cylinder and the rubber Each time the drum is not in contact with the same position, the ink on the printing plate is successively transferred to a different position on the blanket cylinder, resulting in ghosting. Most of this type of ghosting occurs at the mouth of prints. It is generally believed that the fitting clearance of the roller bushing is preferably 0.01 to 0.02 mm, and when it exceeds 0.07 to 0.08 mm, maintenance is required to ensure the printing accuracy of the printing press.

2 roller gear backlash and radial runout is too large. The service life of the offset press is too long, the gear wears, the tooth thickness decreases, and the backlash of the whistle tooth increases. As a result, the rollers cannot run at a constant speed, resulting in instantaneous relative displacement between the rollers and ghosting. Conditional factories may exchange gears or bearings with severe wear; reduce the center distance of the rollers properly so that the backlash is kept within a certain range without tipping (for example, 0.1mm); use blankets to make blankets. The gills are trapped in a trapezoidal shape to reduce the vibrations caused by the impact of the gills. The ghost caused by the excessive backlash of the roller gear and excessive radial runout can be overcome in varying degrees.

3 Roller lining is improper, the pressure is too large, the surface line speed is inconsistent, resulting in excessive extrusion force between the rollers, changing the extrusion value of the blanket. Because the rubber market has a large amount of displacement due to squeezing deformation, it cannot completely return to the pre-compressed state, resulting in ghosting when the ink is transferred.

The three rollers of the offset press must be reasonably rolled during the rolling process, so that the three rollers can obtain pure rolling rolling without slip at the same angular velocity. In fact, a slight amount of slip is always present. However, under the condition of synchronous rolling, the amount of thirst is not required to exceed a certain amount. Therefore, in accordance with the requirements of the machine instructions, proper and reasonable lining should be adopted to avoid ghosting caused by improper lining.

4 The multi-color offset printing presses each unit is difficult to cross each other. Multi-color offset printing inks are superposed under wet conditions, so the printed ink marks can be easily transferred to the back-colored blanket surface. When the positioning between the units is inaccurate, the ink color of the first printing ink will be displaced, so that the blotting of each color transfer can not coincide and cause ghosting. During printing, from the start of paper feeding to the delivery table, many parts are required. The more papers are transferred, the greater the cumulative overprinting error and the greater the chance of displacement, ie, the greater the possibility of ghosting. Therefore, it is required that the positioning and transfer components between the rollers be positioned accurately and cannot be loosened or worn to ensure that the rollers transfer the paper at the tangent point.

In addition, failure of the roller pressure, lack of pressure in the roller, poor quality of the blanket, excessive peeling force of the ink, tightness of the plate, or cracking of the plate at the platen may cause longitudinal ghosting. .

2. Horizontal ghosting.

1 Pickup shaft axial movement. The grippers are mounted on the gripper shaft and the gripper shafts are tangled. As a result, the transfer of the paper is inaccurate and the paper is displaced, resulting in ghost images. Generally adjusting the gap between the tight end of the roller gripper shaft and the end face of the sleeve can overcome the ghost caused by the axial movement of the gripper teeth.

2 Looseness of the thrust bearing bearing screw of the drum or wear of the thrust bearing or loosening of the nut of the thrust bearing causes axial displacement of the plate cylinder, the blanket cylinder and the impression cylinder during high-speed operation, resulting in non-coincidence of ink transfer. Produce ghosting. General requirements for the axial direction of the machine does not exceed 0.03mm, so find out the roller, tighten the nut of the thrust bearing, you can eliminate this ghosting.

In addition, the string of water, string ink roller axial movement; paper "foliar", the front stop and the side gauge improper adjustment, so that the paper edge pressure is too large to produce curl; paper is not smooth and so will cause horizontal ghosting.

3. A, B ghosting. On multi-color offset printing presses, the ink is superimposed wet and wet. The ideal ink transfer should be the impression that the ink printed on the previous unit is left on the rubber market of the latter unit and completely overlaps on the same part of the paper. However, in actual printing, due to various reasons, the ink on the previous color blanket left by the previous sheet of paper does not overlap, and an imaginary image appears next to the dot of the post-printed sheet. If the dots on the third sheet coincide with the dots on the first sheet of paper, ghosts in the opposite direction to the second sheet of paper are left on the third sheet, resulting in so-called A and B ghosts.

The generation of A and B ghosts, the reasons for the design and manufacture of the machine itself, and the reasons for process operations.

The reason for the machine is mainly that the manufacturing and installation accuracy of the two teeth of the double-diameter transfer roller is not enough. For example, the Japanese Akiyama four-color offset press, because the double-diameter transfer cylinder has two teeth, the production and installation accuracy of these two teether wheels are very demanding. The ideal state should be aa′=0,bb′=0,cc '=0, (see Figure 8-26). When these three sets of data are greater than zero or less than zero, the dots on the two sets of paper on A and B cannot be completely overlapped on the back print blanket, resulting in A and B ghosts. "A, B ghosting" is also named after this.

Process operation problems are mainly due to the fact that the dental pads that are used regularly are worn and the precision is reduced.

As an operator, correct the bite force of the two gripper pads and grippers as required by the instructions. Regularly check the working condition of the cavities, so that the grippers can move flexibly, and replace the worn tooth pads in time to ensure the accuracy of the transfer.





Author: FENG Rui dry
Source: "printing principle and process"