Problems caused by plates and solutions

The most common causes of failures in lithographic printing plates are: pasting and non-inking (blind). When the non-graphic area begins to ink, there will be a paste phenomenon; when the graphics area can not be attached to the ink will appear white phenomenon. Pasteplates When some parts of the non-graphical area of ​​the plate begin to print ink, stencil printing occurs. "Non-text" refers not only to large areas of blank space without graphics on the printing plate, but also includes blank areas between halftone dots. Therefore, when the halftone dots begin to fill up with ink, we call it a pasteboard. The mechanism of the stencil problem is as follows: In the process of making the printing plate, the non-graphic area is to be made insensitive to ink, that is, to be made hydrophilic and ink-repellent. This is usually achieved by applying a layer of hydrophilic gum, such as gum arabic. When the plate is cleaned, the hydrophilic adhesive layer may gradually fall off, but it will be replaced by the rubber grease in the dampening solution. When for some reason this kind of substitution does not work properly, the ink may infiltrate the non-graphic area of ​​the printing plate. The result is a blurring of the printing plate. The composition of the ink and paper may also be the cause of blurred printing plates. Paste Prevention: Because aluminum is a very easy to make hydrophilic metal, so in the past plate-making often face such problems. But now people use the anodized aluminum plate to prepare the printing plate, the vagueness of the printing plate is not the main problem, but the stencil is still one of the problems that people encounter. To prevent the occurrence of stencils, the printing plate must be subjected to a suitable hydrophilic-hydrophilic treatment during the process of making a printing plate, and at the beginning of the printing, it must be contacted with water and wetted. If the plate is stopped for more than one hour, its non-graphic area should be protected with a hydrophilic adhesive. If the above process is used, the printing plate should be dampened at the time of printing unless a particularly oily ink is used. If the plate begins to appear blurred, immediate remedial action should be taken. Once the ink is strongly adhered to the non-graphic area of ​​the printing plate, it will be very difficult for the part to be restored to its original shape and become hydrophilic and ink-repellent. The plate should be treated with special hydrophilic degreasing or conditioning agents. For non-graphic areas that have been inked, relatively large pressures are needed to remove the ink and replace it with a hydrophilic coating. Finishing agents should eventually remain on the dried plates, as any repellent will only get better after drying. This treatment method is preferably carried out at least twice before considering the addition of a repellent to the fountain solution for the printing plate. Paste printing on aluminum printing plates: Sometimes aluminum printing plates produce a special stencil phenomenon in lithographic printing called "dyeing paste". This paste consists of tens of thousands of tiny ink dots. The areas between ink dots still have good hydrophilicity. The reason for the "dye paste" is due to the presence of corroded pits on the aluminum plate. When the aluminum plate is corroded, many small dots are produced, and these small dots eventually become many pits. When the hydrophilic adhesive layer above the pit is etched away, they will be inked, resulting in a paste. The reason for this phenomenon usually occurs when the surface of the printing plate is covered with water and the moisture on the plate slowly evaporates. On the plate facing the water roller, the stencil is often distributed in strips. If the "dye paste" is not too severe, it can be treated with a solution containing phosphoric acid and gum arabic. White spots (prints do not ink): If the printing plate can not print graphics, we call it blind version. This may be due to corrosion of the graphic area of ​​the printing plate, or it may be due to the coating area being covered with a layer of hydrophilic rubber or other hydrophilic material. If the graphic area is worn away from the plate, we can simply call the plate wear, and the blind plate is created by the wear of the graphic area. Blind version usually refers to the fact that there is no ink attached to the plate area. When the graphic area cannot be pasted by the ink roller as in the past, then blinding occurs. With the advancement of plate making technology, the phenomenon of blind printing has been greatly reduced, and the service life of printing plates has been correspondingly extended. Graphic area wear: If some parts of the graphic area are worn away, this area will become inaccessible again. The blinding phenomenon caused by the wear of the printing plate is the worst case, because there is little room for salvage. It is usually caused by friction with a hard, improperly installed roller; it may also be caused by excessive pressure between the plate cylinder and the blanket cylinder; it may also be due to friction in the ink or paper. Particle friction caused. Sometimes the pigment particles on the paper surface are very hard and abrasive. Paper fibers also have a certain degree of abrasion. When newsprint is used for lithographic printing, a large amount of fiber is often accumulated on the blanket cylinder. This fiber accumulation (also called paper fluffing) causes the printing plate to wear out, which often results in the inability of ink on the graphic area. Another cause of wear is the smearing of the paper. Sometimes the substrate will be sprayed with a color, and the second color will be printed on the printer. At this time, the dry particles of the first color will wear on the printing plate of the second printing press. The expansion of the blanket causes its thickness to increase, which can cause an increase in the pressure between the printing plate and the blanket cylinder. Eventually it will lead to plate wear and blinding. The adhesion of the graphic area to the metal substrate is unfavorable: sometimes a printing plate starts to print well, but there is no ink in the printing process. One of the reasons for this situation is that the adhesion of the graphic area to the metal substrate is not reliable. The coating layer hardened by light irradiation must be firmly bonded to the metal substrate; if painting is used, the painting substrate must be firmly bonded to the coating layer; the ink must be combined with the painting. If the physicochemical properties of viscose are not good, it will also cause the image area to be partially inaccessible. Sometimes the printer must re-make a printing plate, make some changes in the production process or the use of raw materials to prevent the same problem from happening again. The graphic area is partially hydrophilic: In the process of printing plate development, the lithographic printing plate is treated with a water-soluble glue solution. Gum arabic or other hydrophilic gums are also often used in this water-containing solution. Hydrophilic glue sometimes adheres to the graphic area of ​​the printing plate. In this case, this part of the graphic area will be accompanied by water rather than ink, the result is that the printing plate graphic part can not be inked, that is, this part of the image can not be printed. In the printing process, there are many situations that cause the hydrophilic adhesive to adhere to the graphic part and cause the printing plate to fail to ink. These conditions include: • The ink layer is too thin when printed. Too thin a layer of ink will make it very easy for a hydrophilic adhesive to penetrate the ink layer and combine with the underlying material. If you want to make the printing color lighter, it is best to add a transparent thick layer above the ink layer, so that the ink layer can be thickened during printing. · There is too much water in the ink. If the ink contains too much water, the ink in the plate graphic area is easily replaced by the hydrophilic glue. • The dampening solution contains too much hydrophilic glue. The more hydrophilic glue contained in the wetting solution, the more easily the hydrophilic glue emulsifies in the ink. This makes it easier for the hydrophilic glue to stick to the graphic area of ​​the printing plate. • The dampening solution contains too much acidic material. The acid will make the hydrophilic glue more hydrophilic, so it will adhere to the plate more easily if there is a chance. There is also a reason why the printing plate area can not be inked, that is, the problem of dry printing ink plate. If the ink is dry on the plate, this part of the plate is usually not easy to re-ink when printing again. Therefore, when the plate needs to be placed overnight or temporarily stored, it is best to wipe it with a treatment agent before turning off the printing press. Ink Solvents and Plate Cleaners: Most coating and coating layers that harden on light on a printing plate are extremely resistant to printing inks or plate cleaners. Do not use plate cleaners on positive presensitized plates that have not been plated. If these plates have been exposed to alkaline plate cleaners, they may attack the graphic portion of the plate, resulting in a blind print. The photosensitive material in UV inks is polar and most easily attacks the coating layer of some printing plates. Therefore, diazo pre-printed printing plates and sensitized printing plates cannot use UV inks, and many photosensitive resin printing plates use UV inks better.