The position, angle and pressure of the squeegee contacting the printing plate have a great influence on the ink color of the printed matter, especially the ink color of the light mesh part. The ideal state is that the doctor blade is in the same position from beginning to end, but for some reasons, the position, angle and pressure of the doctor blade have to be adjusted during the printing process. Therefore, it is necessary to understand the influence of the change on the color of the printed matter, so that the range of color change is acceptable.
The effect of doctor blade on the color of printed matter
1. Printing distance
The contact position between the doctor blade and the plate cylinder determines the distance between the contact point of the doctor blade and the impression point. Generally speaking, the longer the distance, the easier the ink in the cell dries and the lower the transfer rate of the ink. The printing effect is equivalent to a partial reduction of the ink. On the contrary, when the distance is short, the transfer rate of ink is relatively high, but at the same time, some small lines on the plate surface are reflected on the printing substrate.
The length of the printing distance should be adjusted according to the printing speed, ink viscosity, cell depth, and plate quality. Most doctor blades are installed in the upper quarter of the plate cylinder.
2. The pressure and angle of the doctor blade
The blade contact angle refers to the angle between the tangent line at the contact point between the blade and the plate cylinder and the blade. When the squeegee scrapes the ink, it actually scrapes off part of the ink in the mesh. When the pressure of the blade is large or the angle of the blade is steep, the ink in the mesh is scraped more, the amount of ink transfer becomes smaller, and the color density is reduced. The pressure of the scraper is lower and the angle is flatter, so the ink in the mesh is scraped less, the amount of ink transfer is relatively larger, and the color density is also larger, but the corners of the text are prone to blur.
When the squeegee is assembled, it must ensure that the edge line is parallel to the generatrix of the printing plate, otherwise the pattern color at both ends of the printing plate will be inconsistent due to the difference in printing distance, angle, and pressure at both ends.
(1) Angle of doctor blade. According to experience, the contact angle of the scraper is generally 55 °. The blade is properly installed, and the blade should be bent about 5 ° under load, so the contact angle of the installation is 60 °. But in the actual production process should not stick to the rules, can adjust the angle according to the actual situation, to ensure the quality of printing.
(2) Squeegee pressure. The pressure of the scraper on the plate cylinder is set according to actual needs. In general, the pressure of the scraper is set at 20 ~ 25kg / m, so that the scraper maintains a certain elasticity when moving on the printing plate, which is beneficial to the scraper operation. Under the premise of ensuring effective ink scraping and controlling the amount of ink in the printing process, the contact pressure between the ink blade and the surface of the gravure cylinder should be as light as possible to reduce the unnecessary wear of the blade on the printing plate.
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