Analysis of the problem of flexographic dot blocking
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Flexographic printing has unique advantages in production technology and environmental protection, and has been increasingly used in food packaging, pharmaceutical packaging and other fields. However, there are many factors affecting flexographic printing, and the improvement of its quality faces certain challenges. To obtain good printing quality, it must be considered in many aspects. This paper mainly discusses how to improve the quality of flexo printing from the influencing factors of flexographic dot-blocking problems.
1. Screening line
In flexographic printing, the choice of the number of screen lines should be cautious. The higher the number of screen lines, the higher the probability of causing a jamming failure during the printing process. Blocking is mainly manifested by the adjacent small dots being connected together during the transfer process, and the dots are bridged. In flexo printing, the two areas that are most prone to blockage are about 40% of the midtones and 10% of the highlights. Compared with offset printing, the flexographic printing plate has a certain elasticity, and the dot size generated during the printing process is increased by about 15%, sometimes as high as 20%.
On the one hand, the dot diameter of the highlight region below 10% is small, and the choice of the anilox roller has a direct influence on the occurrence of the jam. Because the opening degree of the anilox roll mesh has a certain mathematical correspondence, often a specific dot has one or several ink marks on the anilox roller to give ink, and one or several dots are not trapped in one ink hole. Once this relationship is improperly controlled, such as the minimum dot diameter is smaller than the anilox roll cell opening degree, the dot may fall into the cell, which increases the probability of jamming failure. Therefore, in order to ensure accurate reproduction of the dots, each dot on the plate must be accurately inked. When selecting the number of wires of the anilox roller, the number of wires of the plate must be considered. For example, the number of screen lines is 120 lpi, the diameter of a 2% circular dot is about 33 μm, and the diameter of a 1% dot is about 24 μm. If a 600 lpi anilox roll is selected, the cell opening degree is about 34 μm; and the 700 lpi anilox roll has a cell opening of about 30 μm. It is not difficult to see that the 700lpi anilox roll is less likely to block the plate than the 600lpi anilox roll. It can be seen that choosing the correct number of screen lines and the number of anilox lines is of great significance for improving the quality of flexo printing.
On the other hand, the intermediate adjustment area is close to the critical point of 70% of the flexographic dot point due to the increase of the dot size, and the probability of occurrence of dot overlap is high, and adhesion of adjacent dots occurs, resulting in jamming failure. Therefore, in the printing of the screen version, through the pre-press compensation, try to pull the level of the middle adjustment as much as possible to obtain a richer layer effect.
2. Printing mechanism and printing pressure
Flexographic printing uses light pressure printing, and adjustment of printing pressure is one of the key factors affecting the quality of flexo printing. The inking mechanism of the flexo printing machine is mainly divided into: a rubber roller-anilox roller inking system, a doctor blade-anilox roller inking system and a closed blade inking system. Regardless of the type of inking mechanism, the printing pressure mainly includes the following three types.
(1) Inking pressure
That is, the pressure between the rubber roller - anilox roller or the doctor blade - anilox roller. It mainly controls the amount of ink on the printing, the ink pressure is large, and the amount of ink on the anilox roller is small; If the ink pressure is too small, it will cause the ink volume to be difficult to control during printing, and the ink is not evenly distributed. Eventually, the ink film of the printed product will be too thick to cause dot jam, drag ink stain, poor definition, line spreading, handwriting, etc. Disadvantages. If the inking pressure is too large, the amount of ink on the anilox roller is relatively reduced, the dot can be clearly restored, and the possibility of blocking is reduced, but the gloss of the printed product is lowered. At the same time, due to excessive pressure, the ends of the anilox roller and the rubber roller may be bent, the gear of the printing machine may be beaten, the teeth may be broken, or the middle portion of the scraping blade may be arched to reduce the life of the doctor blade. It can be seen that the adjustment principle of the ink pressure should be determined according to the characteristics of the product. The pressure between the two rollers of the cable product can be relatively large; the pressure of the small text and line products can be slightly larger; the pressure of the large characters and the solid products can be relatively small. For products with a large printing area, the pressure can be lighter to increase the amount of ink, thereby increasing the density and vividness of the field, and the brightness; the product with smaller or more delicate area, the pressure is slightly larger to reduce the amount of ink and improve The clarity of printing. This pressure must be horizontally tangent to both ends of the anilox roll, and there should be no deviation, otherwise it will affect the sharpness and printing effect of the printed matter.
(2) Ink transfer pressure
Often referred to as side pressure, ie the pressure between the anilox roll-plate cylinder. Whether the side pressure adjustment is correct or not has a great influence on the sharpness of the printed dots. If the side pressure is adjusted too much, the pressure of the anilox roller on the flexographic printing plate is too large, which makes the dot enlargement serious. Below 10% of the high-profile areas of the network expansion is serious, it is easy to cause blockage, the middle adjustment network point jumps up, the tone level becomes darker and darker, the dark level of the network level is level, 70% of the Internet points are almost on the ground. The tone level loss, the image sharpness is poor, and the tone reproduction is not good. In addition, excessive pressure will also reduce the printing durability of the printing plate. If the side pressure is too light, the ink on the plate is insufficient, and the ink layer is too thin to obtain an ideal printing effect. The adjustment principle of the side pressure should be such that the pressure between the anilox roller and the plate cylinder is the same, and the ink layer on the anilox roller is rounded to the horizontal extent.
(3) Imprint pressure
Sometimes referred to as positive pressure, the pressure between the plate cylinder and the impression cylinder. Positive pressure is a key factor in ensuring that the ink layer obtained on the printing plate is correctly transferred onto the printing material, which directly affects the quality of the flexographic printing. When printing the network cable, it is not difficult to find under the magnifying glass. Sometimes the printed dots are spread, and there are small burrs around the dots. In the printing direction, the circular dots are elliptical, and the image levels are severely graded. When it is off, it can be judged that the positive pressure may be too large. Sometimes the shape of the small dot is well restored, but the middle color is lighter and the circumference is deeper, which is commonly referred to as the "hollow dot". Sometimes there may even be no image printed. At this time, it can be judged that the printing positive pressure may be too light. Correctly adjust the pressure at both ends of the impression cylinder and the plate cylinder so that the size of the two cylinders is consistent and round and horizontal. In the printing process, according to the specific conditions of printing, it can be adjusted at any time to keep it in the best state, which is of great significance for improving the quality of flexo printing.
3. Ink factors
The ink factors that cause the flexo dot jam are mainly the following.
(1) Ink surface tension
In the case of aqueous inks, the ink is first contacted with a flexographic printing plate and then transferred from a flexographic printing plate to a substrate. In general, the surface tension of aqueous inks, flexographic printing plates, and substrates increases in turn. Among them, the surface tension of the aqueous ink and the flexographic printing plate is closer. The greater the difference between the surface tension of the aqueous ink and the surface tension of the flexographic printing plate, the better the wettability on the flexographic printing plate and the easier the spreading. However, when the difference is too large, a blockage failure will occur. Similarly, the greater the difference in surface tension between the aqueous ink and the substrate, the more the amount of ink transferred to the substrate, the more saturated the obtained ink film. Therefore, in the printing process, the surface tension between the three is effectively controlled, which is advantageous for improving the quality of the flexo printing. In actual production, the surface tension of the flexographic printing plate and the substrate is basically constant, and the aqueous ink should be appropriately adjusted according to the actual situation to obtain the best combination of the three surface tensions.
(2) Viscosity and drying speed of the ink
The viscosity of the ink is too high or the drying speed of the ink is too fast, which may cause blockage failure.
The viscosity of the ink is too high, and it is directly expressed as too much ink on the anilox roller or the printing plate, and can be adjusted by adding a solvent such as a stabilizer.
The factors causing the ink to dry too quickly are: improper use of the solvent in the ink; hot air flow around the plate and the embossing roller, the drying temperature is too high; when printing starts, there is a dry ink on the plate; the pH is too low.
4. Other factors
(1) Wear of anilox roller
During the printing process, the squeegee and the anilox roller are in contact with each other, which causes wear due to friction, which changes the shape of the cell, especially the pyramid and the square cell. Because there is a certain slope from the opening to the bottom surface, according to geometric analysis, the upper ink storage is larger than the lower part. If the cavity wear depth 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 amount of ink transferred will be significantly reduced by the abrasion of the anilox roll. In this case, the printing operator should adjust the ink formulation. If the ink is too thick, the thickness can be reduced with a new filler bonding agent to compensate for the abrasion of the anilox roller. If the method of increasing the thickness of the ink is used, it will cause a jamming failure. Different inking methods, the anilox rolls are subject to different wear during use. The reverse scraper type inking method is due to the application of a certain pressure, which aggravates the abrasion of the anilox roller, and the prismatic anilox roller is preferred; and the double roller type and the forward scraper type inking method, due to the presence of the ink accumulation zone, the anilox roller Less wear, a pyramidal anilox roll can be used.
(2) The surface of the substrate is pulled off the powder
(3) The shape of the dot is irregular
(4) Printing speed is too low
(5) improper adjustment of the angle of the doctor blade
(6) The anilox roller is not clear
(7) The flexible printing plate surface is not flat