In the process of plastic gravure printing, the phenomenon of knife wire often occurs. The reason for its occurrence is very complicated, involving many aspects such as plate-making, ink, doctor blade and environment. It is now analyzed and discussed for reference by peers.
1 Plate aspect
The surface finish and hardness of the chromium-plated layer on the printing plate will have an important influence on the production of the wire. The pigment of the ink used in plastic gravure is generally dispersed in the connecting material in the form of particles. During the printing process, after the ink is covered with the mesh, the excess ink is scraped off by the scraper, while constantly rubbing the surface of the printing plate. If the hardness of the chromium plating layer on the surface of the printing plate is not enough, it is easy to be scratched by coarse particle ink and produce a knife wire.
On the other hand, if the plate cylinder is poorly polished during plate making, the surface of the plate is finely thrown or the plate pattern is too deep. During the printing process, the ink will remain here and cannot be scraped off by the scraper, and transferred to the printing material. Will produce irregularly thin filaments. Therefore, special polishing equipment and special polishing sandpaper should be used for polishing.
In addition, the processing accuracy and installation accuracy of the cylinder should be strictly controlled to prevent the eccentricity from exceeding the standard, so as to avoid the impact on the doctor blade during high-speed printing and the generation of wire.
2 Ink
If the ink used in plastic gravure is not fine enough, the pigment particles are too large, or if particulate impurities are mixed, it will lead to the appearance of knives. Therefore, inks with better quality should be selected, and the pigment particles should be less than 10 μm. A filtering device should be added to the inking system, and it should be checked frequently to prevent coarse particles in the ink from damaging the printing plate and the doctor blade.
If the viscosity of the ink is too high and the drying is too fast, it will increase the relative friction between the plate cylinder and the doctor blade, making it difficult for the doctor blade to scrape off excess ink on the plate surface to form a knife wire. Therefore, the viscosity of the ink should be well controlled. For ink that dries too quickly, a slow-drying solvent can be used or the ink can be diluted.
In addition, if the ink dilution is not standardized, the use of incompatible solvents will cause the pigment in the ink to precipitate out, forming large particles, which will produce knives. Therefore, the solvent should be selected to match the gravure ink. If the antistatic performance of the ink is poor, or the lubrication performance is poor, it will also lead to the phenomenon of knife wire, so the quality of the ink should be strictly controlled and analyzed.
3 Squeegee
Squeegee is an important part of plastic gravure printing. Self-grinding squeegee with tight material and moderate hardness should be used. In general, a hard squeegee produces more wire than a soft squeegee. When the blade of the squeegee is damaged and a gap occurs, it will produce regular ink traces on the printing material. The fine gap of the doctor blade can be polished with fine sandpaper until it is smooth and smooth. If there is a large gap, it needs to be replaced. Therefore, it must be smooth when installing the squeegee. Before installing the knife, carefully check whether there is a gap in the squeegee. Grind carefully after installing the knife. If there is a wire phenomenon during the printing process, the machine must be stopped for grinding. In the ink.
In addition, during the printing process, the angle and pressure between the squeegee and the printing plate must be mastered to reduce the relative friction of the squeegee against the printing plate. Generally speaking, the angle between the doctor blade and the plate cylinder is 70 ° to 75 °, the inclination is upward, and the doctor blade pressure is controlled between 200 kPa and 400 kPa.
4 Environmental aspects
The temperature, humidity and air cleanliness of the printing workshop will affect the appearance of the knife wire. If the temperature is too high, the solvent volatilizes too quickly and the ink dries too quickly; if the humidity is too high, the ink dissolves poorly, the viscosity becomes large, and it is difficult to scrape the ink. Both of these situations are likely to cause the appearance of wire. Therefore, in the printing process, the temperature should be controlled at 18 ℃ ~ 23 ℃, the humidity is controlled at 50% ~ 60%. If there is a lot of dust in the air, the plastic film will be electrostatically adsorbed during printing. These dusts will be transferred from the film to the plate cylinder during printing. If they gather in the ink, they will stick to the doctor blade and produce a knife. wire. For this purpose, a static eliminator should be installed, and an antistatic wire mesh or antistatic plant belt should be suspended above the plastic substrate film. The production environment of gravure printing should be kept as close as possible, and air filters should be installed and the dust content should be measured to maintain the cleanliness of the air and avoid the appearance of cutting wires.
In short, in the production process of plastic gravure printing, there are many reasons for the phenomenon of knife wire, and they often affect each other. It should be inspected and analyzed from the printing plate, ink, scraper and environmental aspects, and excluded one by one.
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