1. Dirty plates are printed with a thin layer of resin (resin entrained in the resin) on the non-printing surface of the cylinder in plastic gravure printing, that is, there is often a layer of ink on the non-printing surface of the printing plate cylinder , And transferred to the substrate, so that the printed matter appears sheet-like or line-shaped dirt, this is the dirty version (also known as fog version) failure. The causes of the dirty plate problem are mainly related to the viscosity of the ink in addition to the blade (including the hardness of the blade, the pressure of the blade, and the contact angle of the blade), the quality of the printing plate, and the ink formulation. If the viscosity of the ink is too large, the viscosity of the ink will increase, and the fluidity of resins, pigments and other substances will be poor. The pigment in the ink is not evenly distributed in the ink system, or the resin in the ink fails to wrap the pigment particles well, so that the pigment particles will re-aggregate and form a larger pigment group (that is, the phenomenon of coarseness). Under the relatively high-speed movement of the doctor blade and the plate cylinder, the doctor blade and the plate cylinder are easily damaged. These larger particles are very likely to form linear or strip-shaped knife lines during printing, and linear stains occur. In addition, because the resin is also in a supersaturated state, a large amount of resin coalesces together, and the affinity of the resin to the roller. Under the pressure of the scraper, the resin will stick tightly to the roller (the resin is entrained with pigment), so that sheet-like stains will appear. Under the high-speed movement of the printing plate, the ink will also produce a large impact force against the doctor blade, so that it is difficult for the doctor blade to scrape off the non-graphic part of the ink smoothly to produce a dirty version.
In general, the lower the viscosity of the ink, the less likely it is to cause dirty plate problems. However, if the viscosity of the ink is too small, it means that the content of the organic solvent in the ink is large, and the components such as resin and pigment are relatively small, so that it cannot form a smooth film when dry coal, so it is white and lacks gloss. If the printed product needs to have a good gloss, generally consider using a larger ink viscosity (13 ~ 19 seconds, Zahn Cup 3), that is, the resin and pigment components are relatively more to meet the needs of the printed product . But pay special attention to the dirty version caused by the high viscosity. In addition, when the ink is printed on the plastic and becomes an ink film after drying, it should be able to firmly attach the wrapped pigment to the surface of the plastic, and resin is the main material for the plastic to adhere firmly. Therefore, the firmness of the pigment is also related to the viscosity of the ink, which means that the ratio of the resin and the pigment in the ink cannot be too large or too small, but should be within the normal range, and the firmness of the pigment can only meet the requirements. Under the condition of maintaining normal environmental humidity, the viscosity of the ink is above 16 seconds (Cahn Cup No. 3), and the static phenomenon generally does not occur. The viscosity of the ink is less than 16 seconds (Zahn Cup No. 3), as the viscosity becomes smaller, electrostatic phenomena such as whiskers, stripes, edge repulsion, flying ink, poor transfer, and extremely irregular water stains will follow. It happens and gets worse. Therefore, in the actual printing process, special attention should be paid to the changes in the viscosity of the printing ink, and the viscosity should be adjusted in time to improve the quality of the printing.
2. Flying ink When high-speed gravure printing (speed exceeds 20 m / min), if the gravure ink uses domestically produced chlorinated polylactene resin with a wide molecular weight distribution, the viscosity of the printing ink is too low, it may be The problem of flying ink. The reason for this is that the viscosity of the printing ink is too low. The cohesion of the ink is greatly reduced. When the ink is transferred to the printed film, the low molecular weight resin is easily separated, and small ink dots can be seen in the blank part of the printed product. Solve the problem of flying ink, mainly from two aspects: one is to use chlorinated polypropylene resin with a narrow molecular weight distribution; the other is to appropriately increase the viscosity of the ink according to the printing situation.
3. Influence on color, transfer rate and color saturation Gravure ink is transferred to the printing substrate by the dot ink indented by the gravure, relatively speaking, the volume of the printed dot is basically fixed. As the viscosity of the ink increases, the viscosity of the ink also increases, and the more ink transferred from the dots, the higher the color density and saturation of the printed matter, but there are certain limits. When the viscosity of the ink continues to rise to a certain level, the transfer rate of the ink also increases synchronously. If the viscosity of the ink continues to rise, the viscosity of the ink becomes stronger and stronger, and it becomes difficult for the ink to enter a small Dots, the ink in the dots is getting less and less, and the transfer rate of the ink starts to decrease gradually. The color density of the printed matter also began to decline. Therefore, in the actual printing process, in order to achieve a good printing transfer effect, the viscosity should be selected very carefully. Generally speaking, the viscosity of gravure ink can be considered controlled in l3 ~ l9 seconds (No. 3 Zahn Cup).
4. Impact on printing speed According to the data, the printing speed of domestic gravure printing machines currently ranges from 30 to 30,000 meters / minute, and the printing speed of general printing machines is between 80 and 200 meters / minute. In actual printing, it is necessary to choose according to the printing speed of the printing machine. When the printing speed increases, its viscosity value will decrease. Of course, the most suitable viscosity is also affected by ink formulation, temperature and humidity, printing plate and other factors. Even at the same printing speed, due to differences in ink formulation design. Its viscosity will also be different. In the actual printing process, it must be adjusted according to the effect of printing. Viscosity is also affected by seasonal temperature. The viscosity of the same printing ink in summer and winter is also different. Although ink manufacturers have been working hard to reduce the effect of temperature on ink viscosity, this effect is still relatively common. In addition, the ink viscosity is also related to the printing plate, so the most suitable ink viscosity should be selected according to the actual conditions of the printing plate cylinder.
5. The effect on the gloss of the printed matter The lower the viscosity of the gravure ink (that is, the more solvent is added to the original ink), the content of resin and pigment in the printing ink is reduced, and the ink is not easy to form after transferring to the printing substrate The smooth ink layer, and the white flower phenomenon, directly leads to the lack of gloss of the printed matter. Therefore, in order to obtain excellent gloss, it is necessary to ensure that the printing ink has a suitable resin and pigment content. In summary, viscosity is an important parameter of ink, and many factors affect this parameter. Ink viscosity has a great influence on printing quality. Different printing equipment, materials, substrates, environmental factors, etc. have different requirements for ink viscosity. Therefore, in the printing of plastic film, according to the characteristics of the substrate, the printing equipment and the environment, comprehensive consideration and reasonable control of the viscosity of the printing ink to ensure that beautiful printed products are printed
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