Analysis on the direct photosensitive plate making method of screen printing
In fact, the screen direct photosensitive plate making method uses photochemical reaction of photosensitive glue, the photosensitive part of the photosensitive glue is cross-linked and hardened and firmly combined with the screen to form a plate film, and the non-photosensitive part is washed away by water or other developing solution to form a through hole to form a screen printing plate.
Photosensitive plate making method is of high quality, economical and practical, and has good effect. It is the most important plate making method in modern screen printing. Photosensitive plate making method can be divided into direct method, indirect method and direct method according to different operation modes.
Direct photosensitive plate-making method is the earliest proposed plate-making technology and the most widely used method. It is to coat the stretched screen plate with photosensitive glue, and then stick the positive film on the screen plate coated with photosensitive glue for exposure, so that the transparent part of the positive film is photosensitive and hardened, the graphic part is opaque, and the photosensitive glue is not hardened. After washing and developing, it forms a mesh to make a printing plate.
Pretreatment before coating: No matter the newly purchased screen plate or the regenerated version, the screen should be treated before coating the photosensitive adhesive, the main purpose is to prevent the shrinkage holes, sand holes, image disconnection, photosensitive adhesive falling off and so on of the photosensitive film layer caused by dirt, dust and grease. Common treatment steps are degreasing agent cleaning, flushing cleaning, dehydration, drying and so on. There are many kinds of release agents commonly used in the market, with good and bad performance. The main component is 20% caustic soda aqueous solution, which has good effect. Of course, there is another one, that is, using neutral or weakly alkaline washing powder as release agent. Under the condition of acceptable parts, it is best to use a release agent for degreasing, which can improve the lubricity of the photosensitive liquid to the screen and improve the bonding degree between the photosensitive adhesive and the screen.
Screen coating. Common coating methods of direct photosensitive plate making method include scraping bucket method and rotating method. It is pointed out here that the scraping bucket method is the most widely used due to some limitations of the rotary method. The factors affecting the coating quality of the scraping bucket method mainly include the flatness of the edge of the scraping bucket groove, coating angle, coating strength, coating speed, coating times and so on. If the edge of the scraping groove is uneven or there is a wound, the film layer will appear streaks or uneven thickness, resulting in burrs or uneven thickness of the ink layer, color cast and so on. The coating angle refers to the inclined angle of the screen placed during coating, which is generally 80 ~ 90. If the angle is too large or too small, the thickness of the release layer before and after the screen will be unequal. The silk screen has a certain elastic tension, and the strength used in coating will also cause different thickness of coating layer, so the left and right hands use the same strength to coat at a flat speed. Too fast, it is easy to produce bubbles, thus forming pinholes; Too slow will cause lines in the coating layer, so it is necessary to have some experience in coating. According to the specific conditions of the printed parts, the coating times are determined to achieve the desired effect. It can be coated on the concave surface of the screen plate once or twice, which can reduce the wear of the scraper on the screen and improve the printing resistance of the printing plate.
Screen exposure: stick the film surface of the film to the proper position of the dry screen plate coated with photosensitive film without dust, and expose it on a printing machine. Exposure is the most critical step in the process of plate making, which directly affects the quality of plate making.
The principle of exposure is the principle of photosensitive plate making. Generally speaking, the non-graphic part is hardened to form a plate film, while the graphic part does not see light, and the transparent ink is washed away by development.
Exposure conditions have a direct impact on the quality of screen printing. Exposure conditions should be determined according to the type of emulsion, the thickness of light source, the distance between the film surface of light source, exposure time, etc. In fact, only exposure time can be controlled during exposure. Therefore, in a professional printing company, it is necessary to carry out trial exposure in advance to obtain suitable data, and then control other conditions to carry out actual operations according to the data. In any case, overexposure or underexposure will cause the edge of the photoresist to be coarsened, which will affect the size and clarity of the printed piece.
Development: The photosensitive film of the picture and text part absorbs moisture and swells, and is washed with water for development. The time is too long, the film layer is wet, and the swelling seriously affects the image clarity. Therefore, the shorter the soaking time, the better, on the premise that it is transparent.
In a word, pay attention to the following formulas in the whole process of plate making: full degreasing before coating, stable coating experience, dust-free and dry film layer, appropriate exposure time, shorter soaking time, better, correction and inspection of revision.