Flexographic printing has unique advantages in production process and environmental protection, and has been increasingly used in food packaging, pharmaceutical packaging and other fields. However, there are many factors that affect flexographic printing, and its quality improvement faces certain challenges. To obtain good printing quality, it must be considered in many ways. This article mainly discusses how to improve the quality of flexo printing from the influencing factors of flexo printing dot blocking problem.
1. Add network cable
In flexographic printing, the number of screen lines should be selected carefully. The higher the number of screen lines, the higher the possibility of causing blockage faults during the printing process. Blocking is mainly manifested as the adjacent small dots are connected together during the transfer process, and the dots overlap. In flexo printing, the two areas where blocking is most likely to occur are mid-tone areas around 40% and highlight areas below 10%. Compared with offset printing, the flexible printing plate has a certain elasticity, and the dots generated during the printing process increase to a large extent, about 15%, and sometimes up to 20%.
On the one hand, the dot diameter of the highlight area below 10% is small, then the choice of anilox roller has a direct impact on the occurrence of blocking. Because the opening of the anilox roller cell has a certain mathematical correspondence, often a specific dot has one or several ink holes on the anilox roller to give ink, so that one or several dots do not fall into an ink hole. Once this relationship is improperly controlled, for example, if the minimum dot diameter is smaller than the opening of the anilox roll cell, the dot may fall into the cell, which will increase the probability of blocking failure. Therefore, in order to ensure the accurate reproduction of the halftone dots, each halftone dot on the printing plate must be accurately inked, and the number of halftones of the printing plate must be considered when selecting the number of anilox rollers. For example: the number of screen lines is 120lpi, the diameter of 2% circular dots is about 33μm, and the diameter of 1% dots is about 24μm. If you choose a 600lpi anilox roller, the cell opening is about 34μm; while a 700lpi anilox roller has a cell opening of about 30μm. It is not difficult to see that the possibility of blocking the plate with 700lpi anilox roller is lower than that of 600lpi anilox roller. It can be seen that choosing the correct number of screen lines and anilox roller lines is of great significance for improving the quality of flexo printing.
On the other hand, in the mid-tone region, as the dots increase, they are close to the critical point of 70% of the flexographic dots. The probability of dot overlap is high, and the adjacent dots are stuck, resulting in blockage failure. Therefore, in the printing of the screen version, through pre-press compensation, try to pull the middle tone level apart to obtain a richer level effect.
2. Printing mechanism and printing pressure
Flexographic printing uses light pressure printing, and the adjustment of printing pressure is one of the key factors affecting the quality of flexo printing. The inking mechanism of flexo printing machine is mainly divided into: rubber roller-anilox roller inking system, doctor blade-anilox roller inking system and closed blade inking system. No matter what kind of inking mechanism, its printing pressure mainly includes the following three.
â‘´Inking pressure
That is the pressure between rubber roller-anilox roller or doctor blade-anilox roller. It mainly controls the amount of ink applied to printing. If the pressure of inking is large, the amount of ink on the anilox roller will be low; otherwise, it will be high. If the inking pressure is too low, it will cause difficulty in controlling the amount of ink when printing, and the inking will be uneven. Eventually, the ink film of the print will be too thick and cause dot clogging, sticky ink stains, poor clarity, line spreading, and illegible writing. Disadvantages. If the inking pressure is too high, the amount of ink on the anilox roller will be relatively reduced, the dots can be clearly restored, the possibility of blocking is also reduced, but it will reduce the gloss of the printed product. At the same time, due to excessive pressure, the two ends of the anilox roller and the rubber roller may be bent, the printer gear may jump, the teeth may be broken, or the middle portion of the doctor blade may be arched, which reduces the life of the doctor blade. It can be seen that the adjustment principle of the inking pressure should be determined according to the characteristics of the product. The pressure between the two rollers of the network cable product can be relatively large; the pressure of the small text and line products can also be slightly larger; the pressure of the large characters and the field products can be relatively small ; For products with a larger printing area, the pressure can be lighter to increase the amount of ink, thereby improving the density and vividness of the field, and brightness; for products with a smaller area or more delicate, the pressure should be slightly larger to reduce the amount of ink and increase Print clarity. This pressure must be horizontally tangent to both ends of the anilox roller without deviation, otherwise it will affect the clarity and printing effect of the printed matter.
⑵ Ink transfer pressure
It is usually called side pressure, that is, the pressure between the anilox roller and the plate cylinder. The correctness of the lateral pressure adjustment has a great influence on the clarity of the printed dots. If the side pressure is adjusted too much, the pressure of the anilox roller on the flexible printing plate is too large, which makes the dot expansion serious. The outlets in the high-profile area below 10% are seriously expanded, which is very easy to cause blockage. The middle-tone outlet jumps up, the tone level becomes darker and darker, and the dark tone level outlets are combined. 70% of the outlets are almost on the ground. Tonal gradation loss, poor image clarity, and poor tone reproduction. In addition, excessive pressure will also reduce the printing plate's durability. If the side pressure is too light, the ink on the printing plate is insufficient, the ink layer is too thin, and it is difficult to obtain the desired printing effect. The adjustment principle of side pressure should make the pressure on both ends of the anilox roller and the plate cylinder be the same, and try to make the ink layer on the anilox roller contact with the round cut of the plate surface horizontally.
⑶ imprinting pressure
Sometimes called positive pressure, the pressure between the plate cylinder and the impression cylinder. Positive pressure is a key factor to ensure that the ink layer obtained on the printing plate is correctly transferred to the printing material, which directly affects the quality of flexo printing. When screen printing, it is not difficult to find under the magnifying glass. Sometimes the printed dots are spread out, and there are small burrs around the dots. In the printing direction, the round dots are deformed into ellipses, and the image gradation is serious. It can be judged that the positive pressure may be too large at this time. Sometimes the shape of the small dots is well restored, but the color in the middle is lighter and the surroundings are darker, which is usually called "hollow dots". Sometimes, the image may not be printed. At this time, it can be judged that the positive printing pressure may be too light. Correctly adjust the pressure at both ends of the impression cylinder and the plate cylinder to keep the size of the two consistent and make a round cut horizontal contact. And in the printing process, according to the specific conditions of printing and adjust it at any time to keep it in the best state, it is of great significance to improve the quality of flexo printing.
3. Ink factor
The ink factors that cause flexographic dot blocking are mainly in the following aspects.
â‘´Ink surface tension
Taking water-based ink as an example, the ink first contacts the flexible printing plate, and then transfers from the flexible printing plate to the substrate. In general, the surface tension of water-based inks, flexible printing plates, and substrates increases in sequence. Among them, the surface tension of water-based ink and flexible printing plate is closer. The greater the difference between the surface tension of the water-based ink and the surface tension of the flexible printing plate, the better the wettability on the flexible printing plate and the easier it is to spread. However, when the difference is too large, a blockage fault will occur. Similarly, the greater the difference in surface tension between the water-based ink and the substrate, the more ink transferred to the substrate, and the more saturated the ink film obtained. Therefore, in the printing process, effectively controlling the surface tension between the three is conducive to improving the quality of flexo printing. In actual production, the surface tension of flexible printing plates and substrates is basically a fixed value, and the water-based ink should be appropriately adjusted according to the actual situation to obtain the best combination of three surface tensions.
⑵Viscosity and drying speed of ink
If the viscosity of the ink is too high or the drying speed of the ink is too fast, it may cause blockage failure.
The viscosity of the ink is too high, which is directly manifested as too much ink on the anilox roller or printing plate, which can be adjusted by adding solvents such as stabilizers.
The factors that cause the ink to dry too quickly include: improper use of solvents in the ink; hot air flowing next to the printing plate and the platen roller, the drying temperature is too high; there is dried ink on the printing plate at the beginning of printing; the pH value is too low.
4. Other factors
â‘´Wear of anilox roller
During the printing process, the doctor blade and the anilox roller are in contact with each other, which will cause wear due to friction and change the shape of the mesh, especially the pyramid and square mesh. Because there is a certain slope from the opening to the bottom surface, according to geometric analysis, the upper ink storage volume is larger than the lower part, if the depth of mesh wear is 20% of the original depth, the ink storage capacity of the mesh will be reduced by more than 50%. It can be seen that the actual ink transfer volume will be significantly reduced due to the wear of the anilox roller. In response to this situation, printing operators should adjust the ink formulation. If the ink is too thick, you can use a new filler to reduce the thickness to compensate for the wear of the anilox roller. If the method of increasing the thickness of the ink is adopted, it will cause blockage failure. Different inking methods, the anilox roller suffers different wear in use. The reverse scraper-type ink infusion method should use a prism-shaped anilox roller due to the application of a certain pressure to increase the wear of the anilox roller; while the double-roller and forward scraper-type ink infusion method, due to the existence of the ink accumulation area, Pyramid anilox roller can be used for less wear.
⑵The surface of the printing material is fluffed and powdered
(3) Irregular network shape
â‘· Printing speed is too low
⑸ Improper adjustment of the angle of the doctor blade
⑹The anilox roller is not clear
⑺Flexible printing plate is uneven
essential oil
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