Screen printing plate classification - Photosensitive plate (1)

3. Photographic plate method.

The plate making method is of high quality, good effect, and economical and practical, so this plate making method is the most important plate making method in modern screen printing.

The photoengraving method utilizes the photochemical change of the photo-resist (film), that is, the light-receiving portion of the photo-resist (membrane) is cross-linked and hardened and forms a plate with the screen, and the unexposed portion is washed and developed with water or other developer. Through holes, made of screen printing plates.

In the specific operation, the positive plate base plate should be made first. When it is printed, it is sealed on the screen sensitive adhesive (film). The pattern part is shaded when exposed, the photosensitive adhesive (film) does not undergo chemical changes, and the transparent part is sensitive to light. The glue (film) cross-links and hardens, and is then rinsed with a developer solution, and the photosensitive resist (film) that is not used for light is washed away, and the photosensitive resist (film) that is light is retained to form a film.

The positive version of the positive and the original is the same as black and white, shading, and light and dark, and vice versa. Screen printing uses positives in principle, and negatives are sometimes used.

The plate studio is a semi-dark room, so it is possible to use amber or red light as the illumination source. The brightness of the light is suitable for clearly seeing objects.

The photolithography method is divided into direct method, indirect method, and straight type method. Essentially, the technical requirements of the above three kinds of plate making methods are the same, but the process method of coating the photosensitive adhesive or film is different because of these three methods. The method is currently the most commonly used photolithography method, for which the three methods are described separately (Figure 2-109).

Figure 2-109 Types of Photographic Methods

Direct method.

a. Features. It is the most widely used method. This method of plate making is to apply a photosensitive liquid directly on the screen to form a photosensitive film. The photosensitive material has a low cost and a simple process. This coating operation has been replaced by recent automatic applicators, but it is still inseparable from the operator's technology to smoothly and evenly apply the emulsion. In addition, the disadvantage of this method is that coating and drying need to be repeated. In order to obtain the required film thickness, a certain coating and drying time and a direct legal plate making process are required, as shown in Figure 2-110.

Figure 2-110 Direct Process Plate Process

Direct coating of photosensitive films

a) Screen treatment. In order to prevent phenomena such as shrinkage of the photosensitive film due to dirt, dust, grease, etc., chattering, image breakage, etc., it is necessary to use a detergent to fully wash the web before applying the photosensitive liquid. The detergents used at this time generally use neutral or slightly alkaline detergents for household use, but the screen cleaners sold in the market, such as 20% aqueous caustic soda, have good degreasing effect and can be improved. The wettability of the stencil to the screen can be a very uniform coating film, so the use of such screen cleaners works well. The cleaning operations range from manual operations to the use of automatic washers (most commonly spray guns and ultrasonic cleaners) in a variety of ways. No matter what kind of method, you must go through the process of degreasing, washing, dehydration, and drying of the cleaning solution.

Other pre-treatments also include physical work to roughen the surface of the screen to improve the adhesion of the screen to the photosensitive film. In addition, in order to prevent the reproducibility of the image from being reduced due to the random reflection of light on the surface of the screen, dyeing may also be performed on the surface of the screen with dyes such as yellow, red, and orange. In order to prevent reflection of light, screens that have been industrially dyed are generally used.

b) Modulation of the photosensitive liquid. There are many kinds of photosensitive adhesives used for direct plate making. Its photosensitive time, photosensitive liquid formulation, resolution, solvent resistance, water resistance are different, should choose the appropriate use.

The diazo sensitizer is commonly used in China and is made by adding a number of additives to polyvinyl alcohol and a small amount of vinyl acetate emulsion. In use, an aqueous solution of a diazonium salt in a certain concentration should be added to the emulsion in accordance with the requirements of each emulsion to make it photosensitive. After adding the diazonium photosensitizer to the agent, it should be mixed thoroughly, stirred, and placed in a cool and dark place for 8 hours before use. Due to the large amount of emulsion bubbles that have just been mixed, the mixing is insufficient, and if it is used immediately, the plate is liable to produce pinholes and the desired sensitivity cannot be achieved. Keep it in a cool and dark place when it is stored. Ideally, it should be placed in a refrigerator.

Recently, from the viewpoint of preventing pollution, diazonium salts have begun to replace hexavalent chromium (dichromate) as a photosensitive group. The diazonium salt is slightly less sensitive than chromate, but it is less toxic and less prone to dark reactions. If the diazonium salt mixture is stored in a cool dark place, it can be stored for several months. If the polyvinyl alcohol emulsion and the diazonium salt are separately stored, they can be stored for a longer period of time.