On Ghosting in Printing Industry

      The appearance of shadow makes the thin line thick, and the original dot becomes one and a half or two (as shown in Figure 8-25). The picture and text of the whole print is blurred and the definition is significantly reduced.On Ghosting in Printing Industry

1、 Generation and category of ghosting

       When printing with a lithographic press, in each rolling of the rollers, there is a small displacement between the rollers, and the corresponding particles do not coincide. The remaining ink layer of the blanket transferred last time cannot completely coincide with the ink layer transferred this time. On Ghosting in Printing Industry

       In addition to the ink layer transferred this time, it also presents the light imprint of the remaining ink layer, and the dark and light ink overlapped with wood on the substrate at the same time, A ghosting fault is formed.On Ghosting in Printing Industry

      Ghosting failures often occur at the beginning of printing; When the drum is pressed in and off; When the printing plate cylinder is in contact with and separated from the printing surface of the rubber cylinder; When the printing surface of the rubber drum and the embossing drum are in contact and separated; On Ghosting in Printing Industry

      When the printing plate cylinder and the paper transfer cylinder transfer paper; When the plate roller starts to contact or disengage the printing plate and the printing speed changes.On Ghosting in Printing Industry

      According to the direction of ghosting, there are three types of ghosting: longitudinal ghosting, transverse ghosting and a and B ghosting.On Ghosting in Printing Industry

       Vertical ghosting, also known as up and down 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 of figure 8-25.On Ghosting in Printing Industry

      Horizontal ghosting, also known as come and go ghosting. The virtual shadow of the dot is on the left and right sides of the original dot, parallel to or at a certain angle to the axis direction of the drum. The virtual shadow is light and heavy, and its position is fixed, as shown in the right figure on figure 8-25.On Ghosting in Printing Industry

      A. B ghosting, the virtual shadow of the dot, some on the upper and lower sides of the original dot, some on the left and right sides of the original dot, and its position is not fixed, as shown in the figure 8-25 below. A. B double images are sometimes found on each printed sheet, sometimes only on one or several separate sheets.On Ghosting in Printing Industry

2、 Causes and troubleshooting of ghosting

There are many reasons for ghosting. We must analyze the reasons according to the type of ghosting and find out the troubleshooting methods.On Ghosting in Printing Industry

1. Longitudinal ghosting.

        The matching clearance between the cylinder journal and the shaft sleeve, the eccentric sleeve and the wallboard hole is too large, especially the matching clearance between the printing plate cylinder, which causes the cylinder to vibrate during operation, the rotation angular speed of the cylinder is uneven, the printing plate cylinder and the rubber cylinder cannot contact at the same position each time.On Ghosting in Printing Industry

       And the ink on the printing plate is transferred to different positions of the rubber cylinder for two consecutive times, resulting in ghosting. This kind of ghosting mostly occurs in the mouth of printed matter. It is generally believed that the fit clearance of the drum shaft sleeve is preferably 0.01-0.02MM, and maintenance is required when it exceeds 0.07-0.08mm to ensure the printing accuracy of the printer.On Ghosting in Printing Industry

        The side clearance and radial runout of drum gear are too large. The service life of offset press is too long, the gear is worn, the tooth thickness is reduced, and the backlash of Whistler teeth is increased, so that each drum can not run at a uniform speed, resulting in instantaneous relative displacement between drums and ghosting. On Ghosting in Printing Industry

       In qualified factories, gears or bearings with severe wear can be replaced; Properly reduce the center distance of the drum to keep the side clearance within a certain range without jacking teeth (e.g. 0.1mm); On Ghosting in Printing Industry

     pad the mouth of the blanket into a trapezoid with backing paper to reduce the vibration caused by the impact of the mouth, which can overcome the double shadow caused by excessive side clearance of the drum gear and excessive radial jump to varying degrees.On Ghosting in Printing Industry

        Improper drum lining, excessive pressure and inconsistent surface linear speed cause excessive extrusion pressure between rollers and change the extrusion value of blanket. Due to the large extrusion deformation displacement of the rubber market, it can not completely return to the state before compression, resulting in ghosting during ink transfer.On Ghosting in Printing Industry

       The three rollers of offset press must be rolled reasonably in the rolling process, so that the three rollers can obtain pure rolling without sliding under the condition of consistent angular velocity. In fact, a small amount of slip always exists.On Ghosting in Printing Industry

      But under the condition of synchronous rolling, this displacement is required not to exceed a certain amount. Therefore, correct and reasonable lining shall be adopted according to the requirements of the machine manual, so as to avoid ghosting caused by improper lining.On Ghosting in Printing Industry

      It is difficult for each unit of multi-color offset press to hand over wood to each other. The ink of multi-color offset press is overprinted under the condition of wet pressing, so the ink imprint printed first can be easily transferred to the surface of the rubber blanket after color registration. On Ghosting in Printing Industry

      When the positioning handover between units is not on time, the ink of the first printing color will be displaced, so that the transferred imprints of each color cannot coincide and cause ghosting. During printing, the paper passes through many parts from the paper feeding to the paper receiving table. On Ghosting in Printing Industry

       The more times the paper is handed over, the greater the cumulative overprint error and the more opportunities for displacement, that is, the greater the possibility of ghosting. Therefore, the positioning and handover parts between the rollers are required to be accurate and not loose or worn, so as to ensure that the rollers handover the paper at the tangent point.On Ghosting in Printing Industry

      In addition, the failure of longitudinal ghosting can be caused by insufficient drum closing pressure, excessive drum closing pressure, poor blanket quality, excessive stripping tension of ink, loose pulling of printing plate or rupture of printing plate at the clamping position.On Ghosting in Printing Industry

2. Horizontal ghosting.On Ghosting in Printing Industry

        Axial string movement of the gripper shaft. The paper gripper is installed on the paper gripper shaft. The serial movement of the paper gripper shaft causes inaccurate paper transfer and paper displacement, resulting in ghosting. On Ghosting in Printing Industry

      Generally, the double shadow caused by the axial series movement of the paper teeth can be overcome by adjusting the gap between the tightening rings at both ends of the paper teeth shaft and the end face of the shaft sleeve.On Ghosting in Printing Industry

       The screw of the thrust bearing of the drum is loose or the thrust bearing is worn or the nut fastening the thrust bearing is loose, resulting in axial displacement of the printing plate drum, rubber drum and embossing drum during high-speed operation, resulting in inconsistent ink transfer, resulting in ghosting.On Ghosting in Printing Industry

        It is generally required that the axial string momentum of the machine shall not exceed 0.03mm. Therefore, find out the string drum and tighten the nut of the thrust bearing to eliminate this ghosting.On Ghosting in Printing Industry

In addition, the water and ink roller move axially in series; Improper adjustment of paper “Ruffle edge”, front stop gauge and side stop gauge causes excessive paper edge pressure and curl; Uneven paper feeding will cause horizontal ghosting.On Ghosting in Printing Industry

       A and B ghosting. On the multi-color offset press, the ink is overprinted by wet pressing. The ideal ink transfer should be that the ink printed in the previous unit leaves the impression on the rubber market in the latter unit, which completely coincides in the same part of the paper. On Ghosting in Printing Industry

      However, in actual printing, for various reasons, the ink left by the previous paper on the later color blanket does not coincide, so a virtual shadow appears next to the dot of the later printed paper. If the dots on the third sheet of paper coincide with the dots on the first sheet of paper, a ghost opposite to the direction on the second sheet of paper will be left on the third sheet of paper to form the so-called a and B ghost.On Ghosting in Printing Industry

        A. B. the production of ghosting is due to the design and manufacturing of the machine itself, as well as the process operation.On Ghosting in Printing Industry

        The main reason for the machine is that the manufacturing and installation accuracy of the two pairs of teeth of the double diameter paper transfer drum is not enough. For example, in Akiyama four-color offset press in Japan, because the double diameter paper transfer drum has two pairs of teeth, the manufacturing and installation accuracy of these two teeth is very high. On Ghosting in Printing Industry

       The ideal state should be A-A ′ = 0, B-B ′ = 0, C-C ′ = 0, (see Figure 8-26). When these three groups of data are greater than or less than zero, the dots on the paper of groups A and B cannot completely coincide on the back printing blanket to produce a and B ghosting. “A and B ghosting” is also named after this.On Ghosting in Printing Industry

      The main problem of process operation is that the frequently used dental pads are worn and the accuracy is reduced.On Ghosting in Printing Industry

       As an operator, correct the bite force of the two paper teeth pads and paper teeth according to the requirements of the manual. Check the working condition of the gripper frequently, make the action of the gripper flexible, and replace the worn tooth pad in time to ensure the paper transfer accuracy.On Ghosting in Printing Industry