In gravure printing, the transfer rate of ink from the screen hole on the plate to the substrate surface is usually about 50% - 70%, and about 30% - 50% of the ink is always left in the screen hole. If the residual rate of ink is always maintained at a certain value, of course, there will be no problem, but in the printing process, due to some reason, the ink residue rate in the screen hole will increase, resulting in blocking. After blocking, the printing pattern and text will be blurred, the color of printed matter will change, and the printing cannot be continued seriously. Shallow version of the most prone to block. Blocking plate is mainly caused by the ink solidification, high viscosity substances or foreign matters and other insoluble substances blocking the mesh hole. 1、 The reasons for blocking: 1. The ink is dry and solid in the plate, and the ink remaining in the screen cavity will be affected by some factors (such as the increase of ink viscosity), and part of the ink will dry. At this time, when the printing plate cylinder enters the ink groove again, the solidified ink in the screen hole is difficult to completely dissolve, and the ink transfer rate is also reduced. As time goes by, the net hole is blocked more and more shallow, the ink transfer rate is getting lower and lower, and finally causes blocking fault. This kind of blocking fault is related to the type of ink, viscosity, solvent, drying speed, printing speed and other factors, and also has a great relationship with the structure of the printing machine. (1) Solvent drying is too fast. In order to meet the needs of printing process, it is necessary to adjust the drying speed of ink. When the ink dries too fast. Before the ink is transferred to the printing substrate, there will be a part of the ink film. The poor re solubility of ink film will lead to poor ink transfer, after a period of time accumulation will cause plate blocking. When the ink circulation is poor, the ink in the ink hopper is easy to crust, which will also cause blocking. Of course, the printing workshop temperature is too high, too low humidity is easy to cause the above failures. (2) There are some problems in the structure of gravure press, such as the position of the scraper is far away from the plate cylinder, the diameter of the cylinder is too large, and the plate cylinder is very shallow immersed in the ink groove, which may lead to the hot air leaking from the drying box to blow the plate directly, thus causing the ink to dry in the mesh. The printing speed is too low to make the ink dry and fixed in the mesh and block. When it is difficult to improve the structure of gravure press, it is necessary to increase the printing speed as much as possible, shorten the distance between the scraper and the imprint line, or use a slow drying agent. 2. The intaglio printing speed is relatively fast when impurities are mixed into the ink. The printing substrate (film) often generates static electricity during high-speed operation. Due to the adsorption of static electricity, the dust in the surrounding environment and the debris off the printing substrate will be brought into the ink. These mechanical impurities are the direct cause of the printing plate mesh plug. In order to prevent this kind of accident, an 80-120 mesh metal mesh can be installed in front of the suction nozzle of the circulating pump to filter the ink to remove the insoluble impurities. (1) At present, the 175 L / in gravure plate which is often used in gravure printing has the maximum mesh hole of 130 μ m in longitudinal and transverse direction and about 35 μ m in depth. It can be blocked by coarse particles, insoluble substances, precipitates or dust mixed in the printing process. (2) Ink itself separation, precipitation and pigment re coagulation and other changes, due to the low Re solubility will also cause blocking. Therefore, should try to avoid the proportion of significant difference between the ink used together. 3. Plate making quality is low, gravure plate making must go through surface corrosion or engraving, aluminum plating, grinding and other processing procedures. In these processes, due to corrosion or engraving caused by the inner wall of the mesh hole is not smooth, grinding burr and Ω type mesh hole, etc., all have a certain impact on the transfer rate of the ink. When the idle condition is serious, the ink transfer rate will be reduced, resulting in blocking. According to the introduction of relevant information, the regular quadrilateral mesh holes are changed into concave arc mesh holes in plate making by using corrosion process, which can greatly reduce the phenomenon of missing points and help to reduce the blocking fault in plastic gravure printing. 4. Poor ink dissolution (deterioration) due to poor ink flow, turbidity precipitation, or due to the parking cleaning is not thorough and mixed with different ink, and misuse of other bad solvents, resulting in poor ink dissolution, transfer rate reduction, also can cause blocking. (1) The deterioration of ink and the change of solvent balance will lead to the decrease of the solubility of ink, resulting in the separation of ink itself, precipitation and pigment re coagulation. (2) In the printing process, if water is mixed into the ink, the fluidity of the ink will be affected, resulting in the recondensation of the pigment.(3) When changing the ink, if the printing machine cleaning is not complete and the new ink is mixed with different kinds of ink, or due to misuse and mixing with inappropriate solvents, it will directly lead to poor ink dissolution, resulting in blocking. (4) When adding solvent (especially more solvent) to the ink tank and ink barrel during printing, if the adding speed is too fast, the solubility may be poor due to solvent impact. Therefore, the solvent should be added slowly. 5. Chemical change of ink, two-liquid ink or reactive ink will gradually cross-linked and other chemical reactions with the continuation of the printing process, resulting in poor ink liquidity, increased viscosity, and reduced transfer rate, which may be the cause of blocking. Therefore, in the use of two-liquid ink or reactive ink, after shutdown to wipe the plate, and careful use of expired ink and residual ink. In addition, different types of ink mixed use may cause chemical reactions, resulting in blocking. Therefore, it is necessary to confirm whether different types of ink can be mixed through experiments. 2、 Solutions to the problem of blocking plate according to the above analysis, we can summarize the solution to the problem of blocking plate in plastic gravure printing. 1. When plastic gravure block occurs, solvent or special cleaning agent should be used for cleaning, and the causes of blocking should be solved respectively. 2. The temperature and humidity of printing workshop should be appropriate, and the drying speed of solvent must be compatible with printing speed and printing environment. 3. The distance between the scraper and the embossing cylinder should be as short as possible to prevent the hot air leaking from the drying oven from directly blowing the layout. 4. Mixed use of slow drying solvent to increase printing speed. 5. In case of blocking caused by ink performance defects, the ink should be replaced in time (or contact the ink supplier to solve the problem), and try to avoid the combination of inks with significant difference in specific gravity. 6. The ink in the ink tank should be stirred frequently to make the ink in each part in the flow state. Should also be timely added new ink or replace new ink, in order to avoid ink film. 7. In case of blocking caused by the quality problem of the inner wall of the printing plate mesh hole, the plate should be re plated or re made. 8. Before starting or just starting, wipe the plate cylinder with solvent. During the printing process, try to avoid the midway stop. If you stop for a long time, you must clean the plate first, or immerse the printing plate cylinder into the ink groove to continuously idle. 9. In the fresh ink, due to temperature difference and other reasons, the additives and waxes in the ink will crystallize out. At this time, try to heat the ink before use (40 ~ 50 ℃) to dissolve it. 10. Solvent misuse, ink deterioration and mixing with different inks will significantly affect the re solubility of ink. Therefore, we should use regular special diluting solvent. When changing the ink, first wash the ink trough and circulating pump thoroughly, and then pour the new ink.
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