Analysis of the causes of offset printing and paste
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The so-called missing version means that the image on the printing plate is degraded, which makes the ink retention of the plate weakened, and the area of the dot becomes smaller or even lost. The paste plate refers to the enlarged deformation of the area of the layout dot, and the ink is adhered to the blank area to make the mark thick. Adhered to the blank portion, the printed image is blurred. Looking at the lithographic printing process, the reasons for the loss of plate and paste are various. It must be carefully analyzed and necessary precautions should be taken in production to reduce or avoid the occurrence of plate and paste, and ensure the quality of product printing.
1 over pressure
1) The plate cylinder and blanket cylinder are overstressed. Pressure is a necessary condition for the transfer of the imprint, and only moderate pressure can effectively ensure the transfer effect of the imprint. If the pressure is too light, the transfer of the print is uneven and incomplete, which may cause the ink color of the printed product to be unclear. Conversely, if the pressure is too heavy, the friction of the blanket on the surface of the printing plate will increase, and the imprint will inevitably increase the printing. The wear of the ink-repellent film layer on the surface of the plate is prone to the phenomenon of plate-off, which makes the blot lighter, and even makes the graphic printing unclear or the phenomenon of lack of pen break. In addition, if the pressure between the plate cylinder and the blanket cylinder is too large, it is easy to cause the label to be enlarged and denatured due to the extrusion phenomenon during the transfer process, resulting in the occurrence of paste defects.
2) The impression cylinder and the blanket cylinder are overstressed. When the pressure between the impression cylinder and the blanket cylinder is too heavy, the imprint on the surface of the blanket is bound to be squeezed and spread, and the paste phenomenon is easily formed.
3) Contact pressure between the inking roller and the water roller and the printing plate. If the pressure of the ink roller or the water roller and the printing plate are too heavy, excessive pressing friction will be generated on the surface of the printing plate, so that the ink-repellent and hydrophilic base of the printing plate is damaged, so that the plate-off or paste-proof phenomenon occurs.
2 Ink roller and water roller
1) The inking roller and the water roller roller shaft are worn or deformed or the bearing is damaged. When the shaft of the inking roller or the water roller is worn due to poor lubrication and the bearing is damaged, on the one hand, due to the occurrence of bounce and sliding, the plate is subjected to abnormal friction, thereby destroying the ink and hydrophilic foundation of the plate. On the other hand, due to the uneven rotation of the roller, it affects the uniform and sufficient supply of ink and water supply to the layout, and it is also easy to cause the phenomenon of plate-off and paste.
2) The ink roller is hard or aged. If the ink roller colloid is hard or the surface is cracked and aging, one will increase the friction on the plate; the other is that the sliding phenomenon is easy to occur during the inking process; the third is that the adsorption performance of the ink is degraded and cannot be maintained well. The inking effect will inevitably affect the uniform and normal ink supply of the layout, thus increasing the chance of the plate being lost.
3) There is eccentricity in the inking roller or the watering roller. When the eccentricity phenomenon occurs in the roller body of the inking roller or the watering roller, the frictional force of the roller having a large radius facing the printing plate surface is increased during the rolling process, so that the printing plate layer is damaged, and the plate is easily broken. The paste phenomenon occurs.
4) Water roller aging. When the water roller is worn or the pile fluff is worn, and the water roller is attached with ink, impurities, etc., it will affect the normal water absorption and water transfer performance of the water roller. Then, in the case where the water supply is not normal, the hydrophilic film layer of the layout easily adsorbs the ink, and the ink on the edges of the characters, lines or dots is expanded to form a paste phenomenon.
3 Paper quality
Paper as a printing material, its quality directly affects the printing quality and the printing durability of the printing plate. If the paper quality is not good, the surface strength of the paper is poor, and it is easy to fall off the powder and pull the hair during the printing process. After the paper powder and paper wool adhere to the ink roller or the water roller, on the one hand, it affects the normal ink absorption, ink absorption, and water absorption. And the water transfer effect, on the other hand, the impurities attached to the ink roller or the water roller will increase the friction against the printing plate, and even adhere to the printing plate, resulting in the occurrence of the missing plate and the paste.
4 equipment aging
When the equipment is used aging, some key components are poorly lubricated, causing wear and looseness, such as roller shaft heads, bearing bushes, gear parts, eccentric sleeves, and the like. Then, during the pressing process, the drum is prone to a slight amount of chattering or sliding phenomenon, thereby aggravating the friction and wear on the printing plate surface, so that the extrusion deformation during the transfer process indirectly causes the phenomenon of plate-breaking and paste-preserving.
5 blanket
1) The blanket is not flat. When the blanket of the blanket is not flat or the case is not tight, the surface of the blanket is embossed, and abnormal friction is generated during the embossing to damage the printing plate layer, thereby causing the occurrence of plate-breaking and paste-preserving. At the same time, due to the looseness of the blanket, it is easy to cause a printing ghosting phenomenon that resembles a paste.
2) The blanket is hardened by aging. When the blanket is used for a long time, the material will be hardened, and the frictional force on the plate will be increased when the embossing is caused, so that the plate layer is damaged, and the plate-disapping and paste-preserving phenomenon may occur.
3) The direction of the crepe wire is wrong. The transverse thread of the blanket is easily stretched and deformed. If it is used for the circumferential direction of the roller body, it is easy to be loose due to the extrusion elongation during the printing process. The embossed portion of the blanket is not in close contact with the roller, and the sliding friction and the squeezing phenomenon are easily generated when the embossing is caused, resulting in the occurrence of the smear and the ghosting failure.
6 dampening solution adjustment, improper control
1) The acidity and alkalinity of the dampening solution is not properly adjusted. If the acidity of the fountain solution is too weak, sufficient inorganic salts cannot be formed to supplement the hydrophilic base layer whose layout is constantly being worn out, so that the oil resistance of the blank portion of the layout will gradually weaken, so that the dots of the graphic portion are outward. Expand to form a paste phenomenon. On the other hand, when the acidity of the fountain solution is too strong, it is easy to corrode the graphic part of the printing plate and the surface of the plate, so that the ink absorption of the dot and the image is weakened, the lipophilic ability is lowered, and the lipophilic base is destroyed, resulting in generation. Off-page phenomenon.
2) The moisture level of the layout is not properly controlled. If the moisture of the layout is too large, the layout and the ink roller will have a thick water film, which will affect the normal uniform transfer and coating of the ink, which will easily cause the phenomenon of missing the plate; and if the moisture of the layout is too large, the printing ink will be printed. If it is blind, if you increase the amount of ink to increase the ink density, it will cause excessive emulsification due to large water. If the moisture content of the layout is too small, it will not be able to compete with the ink, so that the layout ink layer will spread, which will also cause the paste phenomenon.