Application of the most popular direct photosensit

2022-07-31
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The application of direct photosensitive plate making in silk printing there are many methods for printing plates, among which photosensitive plate making is the most commonly used and the best quality. In the photosensitive plate making method, it can be divided into direct method, indirect method and direct method according to the different process methods of coating photosensitive adhesive or film. But in essence, the technical and quality requirements of these three methods are basically the same, and the difference is only the difference in process and procedure

I. process and technical requirements for the direct photosensitive method

the direct method is to directly coat the photosensitive solution on the stretched plate to form a photosensitive film. After drying, the positive film is pasted on the plate coated with photosensitive adhesive for exposure, so that the transparent part of the positive film is photosensitive and hardened, and the image and text part is not transparent, and the photosensitive adhesive is not hardened. The development shows that with the increase of the content of hydrophilic monomer Tris in inp4h-hema-tris system, Its oxygen permeability increases, forming holes after shadow and making printing plates. The main process of direct plate making is to apply photosensitive adhesive before drying. The film thickness can be adjusted by coating times. The film is firmly combined with silk, with good mechanical resistance and high printing resistance (up to 50000 ~ 100000 prints). In the photosensitive plate making, the direct photosensitive method (also known as the direct method) is the earliest used plate making technology, and it is also the most widely used method to achieve the limit load of the machine up to 800 kg. It has the characteristics of low cost of photosensitive materials and simple operation process. However, coating and drying need to be repeated, so it is time-consuming. In addition, the image definition and resolution of the plate made by the direct method are not very high. When the film is thick, the fine line definition is easy to be affected, so it is more suitable for the printing of thick lines and uneven surface substrates

the technological process of direct plate making is as follows: pretreatment → coating photosensitive adhesive → drying → plate printing → developing and washing → finishing

the operation methods and technical requirements of each process mainly include the following aspects

1. pretreatment. Before coating photosensitive adhesive, the newly purchased or regenerated version shall be decontaminated and degreased first. There are many kinds of detergents used for degreasing, with uneven performance and quality. The commonly used 20% caustic soda aqueous solution has a good degreasing and decontamination effect. Normally, neutral or weakly alkaline household washing powder can also be used as a film remover. However, if conditions permit, it is best to use a film remover. It has good degreasing effect, can improve the lubricity of the photosensitive solution to the silk, improve the bonding degree between the photosensitive adhesive and the silk, and can coat the film very evenly. The degreased and decontaminated version shall be dried in a constant temperature oven at 50 ~ 60 ℃. The common treatment steps are cleaning with detergent → washing with water → dehydration → drying. The cleaning operations include manual operation and automatic cleaning machine (the most commonly used is spray gun, but also ultrasonic cleaning). In addition, the pretreatment sometimes uses physical methods to roughen the silk surface, so as to improve the adhesion of the silk to the photosensitive film

2. apply photosensitive adhesive. First, select the appropriate photosensitive adhesive according to the photosensitive time of the photosensitive adhesive, the formula of the photosensitive solution, the resolving power, the solvent resistance, the water resistance, etc. At present, diazo sensitizer is widely used in China. It is made by adding some additives in polyvinyl alcohol and a small amount of vinyl acetate emulsion. When using, according to the requirements of each emulsion, add a certain concentration of diazo saline solution to the emulsion, fully mix and stir, and use it after being placed in the dark for 8h. The best thing is to put it in the refrigerator

the most commonly used methods for coating photoresist are rotation and scraping. Rotary coating is to fix the plate on the applicator to make it rotate, pour the photosensitive liquid in the center of the plate, and the fallen liquid is evenly coated around due to the action of centrifugal force. It has the advantages of simple operation and uniform coating thickness, but it is only suitable for small pieces (50 × Within 50cm). Scraping method is the most commonly used gluing method for direct printing. The blade edge where the scraping bucket contacts the wire must have a high flatness, and there must be no bruise marks, otherwise there will be streaks or uneven film thickness, which will cause problems in the printing quality. During the scraping operation, the adjusted photosensitive glue shall be poured into the scraping bucket, and the frame shall be placed against the glue coating rack. The plate is at an included angle of about 15 ° with the glue rack. Hold both ends of the scraper with both hands and lean it against the bottom edge of the plate to scrape smoothly and evenly from bottom to top at a certain angle. When the scraper moves 2cm away from the top, make the scraper tilt down (about more than 100 ° included angle) and stop slightly, move the scraper left and right and continuously scrape upward to make the glue on the plate flow back into the bucket; Take away the scraper and finish one coating

the main factors affecting the coating quality of the scraping method are the flatness of the edge of the scraping groove, the angle, strength, speed and times of coating. The coating angle is generally 70 ~ 80 °. If the angle is too large or too small, the thickness of the front and rear release film will be uneven. The silk has a certain elastic tension. During coating, the left and right hands obviously need to produce a C-shaped frame through other processes. The same strength and speed are used for smooth and uniform coating. If the speed is too fast, it is easy to produce bubbles and form pinholes; Too slow will cause lines in the coating layer, so you need to have some experience in coating. The number of gluing depends on the required template thickness. In order to make the coating surface smooth (with good printing quality), drying and coating shall be conducted alternately (the thickness of the photosensitive film coated at one time shall be 1.2 ~ 1.6 in the completely dry state μ M is good) until the requirements are met. Pay attention to sufficient drying after coating. If hot air is used for drying, the appropriate temperature shall be mastered. In addition, the concave surface of the plate can be coated back and forth for 1 ~ 2 times to reduce the wear of the scraper on the wire and improve the printing plate resistance. Nowadays, this kind of coating operation has been gradually replaced by the automatic coater, but it still needs the technical level of operators to coat smoothly and evenly

3. drying. The coating and drying of the photosensitive film shall be carried out under the yellow light, and the temperature shall be well controlled. The coated plate can be dried in an oven at 40 ~ 45 ℃ for about 5 ~ 8min. When checking whether it is completely dry, you can turn the side of the plate to the light to observe whether there is a feeling of moisture. You can also touch the corners with your hands to make sure that your hands are free of moisture and stickiness. In order to keep the glue surface flat, you should bake the plate film face up. The photosensitive emulsion has low sensitivity in the liquid phase. It will rise with the drying of the coating film. The specified sensitivity can be reached only after it is completely dried. Therefore, it should be fully dried before printing to avoid printing failure due to insufficient drying. In addition, after drying, the plate should be printed in a short time to avoid pinholes caused by dust on the film surface during drying

4. print. The film surface of the positive film is closely attached to a dry, dust-free plate coated with a photosensitive film, and exposed on a silk plate printer. The non image and text parts harden to form a plate film. No light is found in the image and text parts, and the ink is developed and washed off. Before printing, check whether there are stains or dust on the positive film, the silk film surface and the printing glass surface. Then put the positive film and the stretched frame plate into the printing frame for sealing, and then check from the glass surface. If the positive image is placed in the correct position of the frame, it can be electrified for exposure. The exposure conditions shall be determined according to the type of emulsion, coating thickness, type of light source, distance between the light source and the film surface, etc. in fact, the exposure time shall be controlled during the exposure process. Before the formal exposure, the silk plate ladder shall be used for testing to determine the best exposure time. Usually, in professional printing companies and batch production, trial drying is carried out in advance to obtain appropriate data, and then other conditions are controlled for operation. The most important step in exposure is to make the wire frame and the base plate closely attached

5. development. Put the exposed version into the water tank and soak it for about 10s, then immerse it in water for 1 ~ 2min, and constantly shake the frame. After the non photosensitive part absorbs water and swelling, it can be developed by washing with water. The development should be completed in a short time as far as possible. After the image appears, the printing surface of the frame can be washed with urgent water. The photosensitive film of the image and text part expands due to absorption of water, which is washed away by water, and the exposed part is preserved due to hardening. The difficulty of development is closely related to the type of photosensitive solution. Different development methods can be considered for different types, such as vinyl alcohol emulsion film with high polymerization degree and solvent resistant emulsion film, which can be developed with warm water; Nylon photosensitive film can be developed with industrial alcohol. In short, it must be ensured that the non photosensitive part is completely dissolved. The control principle of development degree is that the shorter the time, the better under the premise of transparency. If the time is too long, the wet expansion of the film is serious, which affects the clarity of the image; If the time is too short and the development is not complete, there will be obscuration and blocking holes, resulting in waste version. The haze is a very thin residual film of photosensitive adhesive, which is easy to appear in the details of the image. It is highly transparent, difficult to distinguish from the water film, and is often mistaken for transparent. In order to facilitate observation, the ordinary protective shadow water tank of light display 2 and spring fatigue testing machine can be used, and the self-made light observation platform can also be used. When the dots and lines in the picture and text are thin, you can use an 8 ~ 10 times magnifying glass to check whether the subtle parts are completely empty. On the premise of transparency, the shorter the soaking time, the better. In addition, it should be noted that the glue layer must not be flushed during flushing, so as not to break the glue film surface, damage the pattern, and cause hairy teeth during printing. In order not to damage the adhesive layer, it is best to spray with an air gun. If the layout is treated with film hardener, it can be carried out after washing, and pay attention to uniform distribution

6. finishing. Use a sponge or hair dryer to quickly remove moisture from the developed version. Then put the plate in a dust-free drying oven at about 45 ℃. After the plate is dried, it can be checked on the light transmission inspection bench to see if there are bubbles, sand holes and other pollution marks. The edge of the drawing line can be repaired with glue. Before or after plate drying, in order to strengthen the plate film and improve water resistance and solvent resistance, anhydrous dilute chromic acid can be coated. However, from the point of view of preventing public hazards, chromic acid drugs should not be used. In order to block the needle holes or the unnecessary openings on the plate, you can use blocking liquid or photosensitive liquid for plate making. In order to protect the cleanness of the frame bonding surface, facilitate the regeneration of the frame, and prevent the ink solvent from eroding and affecting the bonding force, seal (block the blank hole along the edge of the adhesive layer film), that is, seal the inner side and bonding surface of the frame with adhesive tape, or seal the place with solvent resistant paint. Mark the date and other memo records. If you want to increase the printing resistance, it is best to re expose the sorted version, that is, place the version on the plate printing table and re expose it for about 10min without inhaling, so as to enhance the firmness of the film

$page break $2. Version inspection and quality control

version inspection is the final process of the whole plate making work, and it is also an extremely important process. Minor defects after development can be corrected by revision. If major defects occur, they must be remade. The quality inspection of version mainly includes the following contents

1. whether the exposure time is correct. In addition to testing the hardening degree of the adhesive film with a density ladder, you can also see how well the fine lines of the bottom plate reproduce on the silk. You should also observe whether the lines are complete, whether the edges are clear, whether the sawtooth is serious, etc

2. check the quality of the version. Including whether all the pictures and texts are developed, and whether the picture and text points and lines have burrs, deformities, broken pens and sealed holes. If the above situation is found, various methods shall be taken to remedy it in time, and plate making can be considered again to ensure the printing quality. If it is found that the transparent hole on the plate is still closed by the adhesive film, it should be developed again. If there is still no effect, it should be remade

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