Working principle and process of screen printing

Screen printing has five elements: ink, screen scraper, screen plate, substrate and printing table.The basic principle is: the use of a graphic part of the screen plate can be through the ink, and not with the graphic part of the mesh is not through the ink.The squeegees move on the screen plate and extrude the ink to the substrate through part of the screen. At the same time, the printing ink can be fixed within a certain range due to the viscosity of the ink itself.In the process of printing, the scraper is in linear contact with the screen plate, and the screen plate is in linear contact with the substrate, and the two linear contact is identical, the contact line is not fixed, and it moves with the scraper.Before printing, due to a certain gap between the substrate and the screen printing plate, when printing, the scraper moves, the wire mesh plate due to the pressure of the scraper produces a slight elastic deformation, thus generating a certain tension itself, and there is a reaction force on the scraper, that is, the resilience of the wire mesh plate.Under the action of resilience, the substrate and screen plate only linear contact, and the other parts of the two appear out of the state, so that the ink and screen fracture movement, to ensure the screen printing accuracy and avoid dirty substrate.

2.1.2 Screen printing quality

The whole screen printing process is composed of the following elements: screen, paste, substrate, printing equipment, environment.All of these factors can affect the final quality of printing.The influencing parameters of these factors are as follows:

1) Printing parameters and printing equipment: printing speed, scraper pressure, screen spacing, scraper;

2) Slurry (ink): rheological property, degree of adhesion;

3) Screen version;Screen diameter and mesh, latex thickness, opening and tension;

4) Environment: humidity and temperature.

The role of the scraper is a certain Angle and speed of the paste (ink) into the screen opening, and then press on the substrate.The material selection of scraper is very important, it directly affects the quality of printing.The scraper is in the shape of a square strip. The four long edges can be used as a knife edge.Scraper material is not metal, but relatively soft material, usually polyurethane rubber, its hardness is between Shaw A60 and Shaw A90.The hardness of the scraper is selected according to the requirements of the process. For example, the lower the hardness of the scraper, the thicker the printed figure will be, but the printing edge will be more and more blurred. Therefore, improving the hardness of the scraper can improve the resolution of printing.However, if the hardness is too high, although it overcomes the shortcoming of printing ambiguity, it will produce uneven printing effect.

The Angle of the scraper refers to the Angle between the substrate plane or table plane and the side of the scraper along the printing direction.Set the size of the blade Angle is related to ink (generally for 45 degrees), the higher the viscosity of the ink, the greater the setting Angle of blade, which is due to the higher the viscosity of the ink, the liquidity is poor, therefore, increasing squeegee pressure equivalent to scraper to ink vertical downward pressure, so as to make the transfer through the wire net hole ink on the substrates.The value range of scraper Angle is 45 degrees -90 degrees.Printing pressure refers to the pressure exerted by the scraper on the screen or the force exerted by the scraper on the substrate.In the printing process, in order to compensate for the spacing of the screen, the scraper will have a certain pressure on the screen, and then the ink extrusion on the substrate.If the scraper pressure is too large, it will make the screen deformation, make the printing graphics deformation, affect the printing accuracy, but also increase the screen and scraper friction, reduce the service life of the screen.However, if the scraper pressure is too low, the ink is not enough to screen to the substrate, resulting in printing image and text blurry, resulting in grid or virtual printing.General blade pressure is controlled at 70N-120N, depending on the process.

Printing speed refers to the speed at which the scraper moves horizontally along the printing direction.The general printing speed is maintained at 120-300mm/s.The printing speed is determined by the viscosity of the graphic and ink.The more fine printing images and texts, the higher the ink viscosity, the lower the printing speed.Screen spacing is defined as the screen in the absence of external forces between the screen and substrate distance.In the printing process, if the screen spacing is too small, due to the viscosity of the ink itself makes the screen and substrate stick together, affecting the accuracy of screen printing and graphic cleanliness; if the screen spacing is too large, this requires the scraper to provide more printing pressure, resulting in the increase of screen tension, screen life is shortened.