Screen printing has five elements: ink, screen printing squeegee, screen printing plate, substrate and printing table. The basic principle is that the screen printing plate with the graphic part can penetrate the ink, and the mesh without the graphic part can not penetrate the ink. The squeegee moves on the screen printing plate, and squeezes the ink onto the substrate through the mesh of the graphic part, and at the same time, the imprint can be fixed in a certain range due to the viscous effect of the ink itself. During the printing process, the squeegee is in linear contact with the screen printing plate, and the screen printing plate and the substrate are also in linear contact, and the two linear contacts are coincident, and the contact line is not fixed. move. Before printing, a certain gap is maintained between the substrate and the screen printing plate, so when printing, the scraper moves, and the screen plate is slightly elastically deformed due to the pressure of the scraper, which generates a certain tension by itself, and has a negative effect on the scraper. The effect of force is the resilience of the screen. Under the action of the rebound force, the substrate and the screen printing plate are only in linear contact, and the other parts of the two are in a state of separation, so that the ink and the screen are broken, which ensures the accuracy of screen printing and avoids the need for Dirty substrates.
2.1.2 Screen Printing Quality
The entire screen printing process is composed of the following elements: screen, paste, substrate, printing equipment, environment. These elements can affect the final quality of printing. The influencing parameters of these elements are:
(1) Printing parameters and printing equipment: printing speed, squeegee pressure, screen spacing, squeegee;
(2) Slurry (ink): rheology, adhesion;
(3) Screen: wire diameter and mesh, latex thickness, opening and tension;
(4) Environment: humidity and temperature.
The function of the scraper is to press the slurry (ink) into the opening of the screen at a certain angle and speed, and then press it onto the substrate. The selection of scraper is very important, which directly affects the quality of printing. The scraper is in the shape of a square strip, and all four long edges can be used as a knife edge. The material of the scraper is not metal, but a relatively soft material, generally polyurethane rubber, whose hardness is between Shore A60 and Shore A90. The hardness of the squeegee is selected according to the needs of the process. For example, the lower the hardness of the squeegee, the thicker the printed graphics, but the printing edge will become more and more blurred, so increasing the hardness of the squeegee can improve the printing resolution. However, if the hardness is too high, although the disadvantage of blurred printing is overcome, the printing effect will not be uniform enough.
The angle of the squeegee refers to the angle between the plane of the substrate or the plane of the worktable and the side of the squeegee along the printing direction. The setting of the squeegee angle is related to the ink (- is generally 45 degrees), the higher the viscosity of the ink, the larger the angle of the squeegee is set. The pressure is equivalent to increasing the vertical downward pressure of the squeegee on the ink, so that the ink is transferred to the substrate through the screen mesh. The value range of the scraper angle is 45 degrees to 90 degrees.
Printing pressure refers to the pressure exerted by the squeegee on the screen or the force exerted by the squeegee on the substrate. In the printing process, in order to compensate the screen spacing, the scraper will have a certain pressure on the screen, and then squeeze the ink on the substrate.
If the pressure of the squeegee is too large, it will deform the screen, which will deform the printing pattern and affect the printing accuracy. At the same time, it will also increase the friction between the screen and the scraper, and reduce the service life of the screen. However, if the pressure of the squeegee is too small, the ink will not be enough to print on the substrate through the screen, making the printing image blurred, resulting in broken grid or virtual printing. The general scraper pressure is controlled at 70N-120N, which is determined according to the process.
Printing speed refers to the speed at which the squeegee moves horizontally along the printing direction. The general printing speed is kept at 120-300mm/s. The printing speed is determined by the printing graphics and the viscosity of the ink. The finer the printing image, the higher the ink viscosity and the lower the printing speed.
The definition of screen spacing is the distance between the screen and the substrate under the action of no external force. In the printing process, if the screen spacing is too small, the screen will stick to the substrate due to the viscosity of the ink itself, which will affect the accuracy of screen printing and the cleanliness of the graphics; if the screen spacing is too large, a scraper will be required. Provides greater printing pressure, resulting in increased screen tension and reduced screen life.