Ghosting is one of the most vexing issues in printing. For a small web offset press with a short ink path, ghosting is mainly manifested in graphics where there should be no graphics, irregularities, sometimes on printed pages. No footprints. There are several types of ghosts that usually appear in large areas. On the ground or a small area of ​​blank area. The type of ghost can be divided into two types of ghosts and mechanical ghosts according to the principle of ghosting. (1) Process ghosting Process ghosting is mainly caused by chemical changes in the drying process of the ink, which occurs when glossy ink is printed on matte paper or matte ink is printed on glossy paper. The appearance of such ghosts is irregular and difficult to control and avoid. Occasionally occurs after the job has been printed and the ink has dried. It is almost impossible to completely eliminate this kind of process ghosting, but the printing factory can take corresponding measures to avoid ghosting, such as improving the ventilation of the delivery section and keeping the temperature in the processing workshop and storage workshop constant. If ghosts still appear, you can also polish them in graphic parts. (2) Mechanical ghosting Mechanical ghosting is the most common ghosting and is primarily caused by the mechanical causes of the press. I remember once I heard people say that “the last printing roller has ink collection effectâ€. At that time, I was skeptical about this, and carefully thought that the ink given by the printing roller was integral, but after the selective transfer of printing plates, There should be the remaining amount of ink being returned to the ink path. The ink in the ink path is continuously supplying ink to the printing plate. The supply, transfer, and return of the ink volume are inconsistent. By analyzing the ink The relationship between supply, supply and pressure, and characteristics of elastic rubber, It can be concluded comprehensively that the direct cause of the ghost image is the ink supply to the printing plate by the printing roller, the selective transfer of the printing plate to the ink, the backflow of the excess ink volume, and the contradiction in the amount of ink in the ink path. The reason is that there are two inherent flaws in the characteristics of pressurized ink supply and transmission media. The purpose of this paper is to examine the root causes of ghosting in the actual production process and to reveal the reasons for the existence of secrets and more phenomena in the ink road. In the ink path, the ink transfer is largely dependent on the pressure, and the pressure determines how much ink is delivered. But is it just pressure to decide on ink? No, the change in the ink transfer amount largely changes the value of the pressure (in this case, the ink change change pressure does not mean that the amount of ink is changed depending on the actual condition of the printed sheet but the pressure is changed), and the two influence each other. What is the specific situation? Let us assume that there are two rubber rollers, and the two are squeezed to produce a deformation, and the pressure = R roller 1 + R roller 2 - both center distances. If the two rubber rollers are rotated towards each other to allow the ink to pass into it, the pressure = R roller 1 + R roller 2+ ink layer thickness - both center distances. This shows that the pressure has become greater. Is the pressure consistent with the overall pressure on the ink road? No, the reason is that the ink has an ink transfer rate during the transfer process. After the ink enters the ink path, the amount of ink passing through each roller is different, and the amount of ink is getting smaller and smaller. Therefore, it is concluded that we are When the pressure in the way is adjusted, using the same plug to control the pressure is not completely correct. The result of this is actually to make the pressure inconsistent, and the ink transfer rate is also different when the ink passes through the ink roller. . Therefore, during the transfer of ink, the pressure and the amount of ink actually affect each other. In the ink path, the relationship between the ink transfer and the pressure can be pushed to the ink path to the ink. We know that ink fountains are partitioned for ink, and partitions provide different ink levels for the ink path. After passing through the three-stage ink roller in the ink path, the ink layer should be as thick, but the fact is that it corresponds to the ink area. The amount of ink is the same, and the amount of local ink on the print is different. In this regard, we can make the following assumptions: different settings for the ink zone, so that the ink zone area to the ink is different, when the ink roller to these areas ranging from the ink to the uniform roller, because the two roller center distance, The thickness of the ink layer passing through different regions is different, so that the pressure of the uniform ink roller in the axially received region is not equal, and the axial region of the uniform ink roller is not deformed in the same direction. If different ink zones are considered to be different channels, the largest amount of ink is assumed to be point A, and the deformation corresponding to A is the largest, and the ink channel formed by A is corresponding to the deformation of the ink roller is the largest. This channel and A The amount of flow is appropriate, while the throughput of other channels is also It is less than A's. The lower part of the uniform ink roller is connected with the ink roller, and the ink roller performs the axial movement. At this time, the condition of the uniform ink roller should be that the ink layer on the roller surface is flat, but the ink amount in each zone is not equal, and the deformation is not the same. Because the center distance of the two rollers is equal, and the thickness of the ink layer in each zone is not equal, the axial region of the ink roll is not uniformly deformed. However, the movement of the ink roller is not only the axial rotation but also the circumferential rotation. Different from the case of uniform ink roller deformation, the deformation trajectory is not a straight line but a curved line. Since there are no relevant ink roller diameters, data on the tumbling distance of the ink roller, the number of revolutions of one revolution, the jerk time, etc., it is impossible to use calculation to determine the transfer of the force, ie, the curve track and the next ink roller. Does the intersection point correspond to the area? Although the intersection point of the specific force with the next ink roller is not accurately deduced, since the ink area has a certain width, and the ink roller oscillates only within 40 mm, it can be inferred that the ink roller The force A zone and the force A1 zone corresponding to the next ink roller do not necessarily overlap in the straight line direction, but must intersect each other as if a straight line narrowed. And its deformation is also due to the increase of the force area (curve force area is larger than the straight force area), which affects the ink transfer rate and becomes smaller. As a result, the ink receiving amount on the next ink roller is also different in the area and the area is not deformed in the same way. According to this, it can be deduced that the amount of ink received in the corresponding ink area on the printed sheet is different after the axial compaction of the ink roll is performed. The control of the ink road is lagging behind. The hysteresis of the control of the ink path means that when the ink area of ​​the ink path is changed, the effect of the change on the print sheet is not immediately reflected, but it is reflected after a certain period of time. The change of Jimo lags behind the change in control. The direct consequence of this control delay is to increase the number of defective products in the printing. Modern presses are pursuing high finished products, so this problem must be solved. Through observation, it was found that when the ink zone setting is changed, the time when the ink is transferred from the ink fountain to the printing plate is much shorter than the time when the printing effect changes. At the same time, the effect of the printing sheet gradually changes, and the entire ink line changes only. The variable is the thickness of the incoming ink layer, so it is believed that the cause of this hysteresis is the amount of ink and the pressure, the transmission medium. In the ink path, the transmission medium is rubber, which is based on the viscoelastic and visco-elastic properties of rubber. It is a combination of sensitive elasticity, hysteresis elasticity, and elasticity. The hysteresis elasticity causes hysteresis in the pressure change of the rubber, and the hysteresis of the ink path control is fundamental. The reason is the hysteresis of rubber. The entire process causing the control hysteresis should be such that when the ink area setting is changed to increase the amount of ink in the area A, the increase in the amount of ink causes the roll pressure to increase, and the roll pressure increases to increase the deformation of the roll, and since the ink roll is made of rubber , There is hysteresis in the change of the rubber, so the deformation of the ink roller also has hysteresis, and the ink channel formed by the deformation does not become large immediately, so although the ink is given more, it does not allow the increased ink volume to quickly pass through the ink path. It flows to the printing plate, but as the deformation gradually increases, the passage gradually becomes larger, and the amount of ink flowing to the printing plate increases. The hysteresis of the ink path is also related to the number of ink rollers. The more ink rollers, the more accumulated total lag time. Therefore, in order to solve this problem, various manufacturing companies use the method of reducing the total number of ink rollers. In order to fundamentally solve the problem of control lag, it is only to find more ideal materials or use another ink system. Therefore, mechanical ghosting is affected by factors such as the ink roller, blanket, and printing plate on the printing press. Ghosts caused by blankets and plates need to be excluded by the printer; the ghosting caused by the ink roller is directly caused by the printing roller supplying ink to the plate, and the selective transfer of printing plates. Excess ink flow backflow makes the value of the ink in the ink path contradictory, but the contradiction in the supply of ink does not produce ghost images, because the ink path has a certain coefficient of ink storage, and has the ability to regulate the amount of ink. The root cause of ghosting is that the contradiction of the ink volume causes the pressure between the rollers to change. The change in pressure causes the deformation of the roller to change. The deformation of the roller causes a change in the channel of the ink path in the ink path. The change of the channel in turn causes a change in the amount of ink passing through the channel, and this change does not change the change of the channel by changing the setting of the ink, and thus changes the effect of the printed sheet. This kind of change is caused by the contradiction of the ink volume. It is abnormal to change the channel from the transmission end to the ink supply end in the opposite direction. It is not normal, and the change of the channel will not be controlled by people. It will create contradiction with the setting channel, and the ghosting is such The result of a conflict between the reverse channel change and the set channel change. The appearance of mechanical ghosting is mainly due to lack of ink ghosts. The lack of ghosting is due to the fact that a certain part of the printing screen requires a lot of ink, and the ink roller cannot be caused by ink supply in time. For example, large bold letters of the same color are designed over a large area of ​​the field. The parts of the ink roller used for printing bold characters do not have sufficient time to ink before the printing of the back ground, so the ratio of the solid part behind the bold characters is larger. The other part is slightly shallower in the field. The lack of ink ghosting phenomenon in the individual areas of the solid border is mainly caused by the design reasons. The ink roller cannot quickly replenish enough ink to meet the needs of printing in the field. Due to the large amount of printing ink required for the actual frame, the roller is likely to suffer from ink shortage in the circumferential direction, which causes the solid frame ink printed in the circumferential direction to be lighter than the frame printed in the axial direction.
The Essence of the Formation of "Printing Ghosts" (I)