k. Stripping. After the plate making fails or the printing is finished, in order to make the screen regenerate, the template needs to be removed from the web. This film removal work is called a taxation film.
Figure 2-119 Screen frame with snap-in clips attached
When removing the film, the remaining ink on the printing plate should be completely removed. Otherwise, it is very difficult to strip the film. Therefore, the residual ink must be completely removed with a solvent or a descaler. If the ink on the plate is hardened, removal is more difficult. Film remover is commonly used bleach, sodium hypochlorite, hydrogen peroxide, caustic soda, ammonia, potassium permanganate plus oxalic acid.
Sodium hypochlorite is sold on the market with 10% chlorine solution. This kind of diluent can be used to wash dishes. In addition to the membrane, 20-50% of water should be used. Chlorine smells slightly.
3% hydrogen peroxide (hydrogen peroxide), diluted to about 15% can be used. When using, pay special attention to the concentration. In use, the hydrogen peroxide solution will gradually become thinner and less stable.
When applying hydrogen peroxide to the screen, the net should be facing up, and white newspapers should be laid on it. After being coated for 5 to 10 minutes, the film surface should be whitened and then rinsed with water.
Stripping requirements are quick, simple and safe, especially for screens.
The release agent for the release film varies depending on the type of the photosensitive adhesive and the screen type. Usually, the commercial photosensitive adhesive is provided with a corresponding special release agent.
Gelatin system template stripping methods are: 1 hot water swelling method: the plate immersed in 43 ~ 46 °C hot water until the gelatin expansion, brushing and strong water rinse. 2 Oxidizing with bleaching agent; usually add 10 times of bleaching agent as the release liquid. During the operation, first swell the template in the heat, and then use a brush to stick the stripping solution to brush the two sides of the printing plate. When it stays a little, use high-pressure water. Elimination. (3) Dissolving with enzyme preparations: Enzyme preparations are available in powder and liquid form. They can hydrolyze peptide bonds in gelatin polymers. During the operation, wet the two sides of the printing plate, sprinkle the enzyme powder or spray the enzyme solution, and then cover the both sides with a film to prevent the evaporation of the volatile components, and keep it for 10 to 30 minutes before the water is flushed. Crush the knot and call it with 5% acetic acid or white vinegar. The enzymatic release method is very safe and can be used for silk screens. 4 The use of photosensitive adhesive manufacturers supporting the release agent.
Non-gelatin system stencil release method. Such as PVA, PVA + PVAC and other photosensitive adhesive, available methods are: 1 oxidation with potassium permanganate; Preparation of 6% potassium permanganate solution, with a wire mesh filter undissolved wafers, to prevent cutting the screen when the film is removed. With a good solution for long-term use. Apply the solution on both sides of the printing plate during the operation. After 4 minutes, the solution can be washed. For dyed silk screens, it can't be too long, otherwise there is a danger of discoloration. 2 Use sodium hypochlorite or hydrogen peroxide: coat or soak the plate with 4-5% sodium chlorate solution or 3% hydrogen peroxide solution, and rinse with hot water after a few minutes.
For fresh stencils that have not been used for two days, they can be boiled off with boiling water. It usually takes 10-15 minutes without beating or scalding. For gelatin-based stencils, steam blanching is more preferable and water rinsing can be performed. Caustic water or potassium hydroxide solution can also be used for corrosion, but concentration and time should be controlled and rubber gloves should be worn. A certain concentration of strong people have corrosion patterns, but the synthetic mesh is damaged, so it should not be used seriously.
Hardened stencils, with some of the above methods for demoulding, are not effective. Some use strong corrosive agents to destroy their films, but they are very unsafe for screens and should be tested for them.
Some solvent-based stencils use the corresponding solvents and release agents for film stripping, such as hand-mosquito shell mold stripping with ethanol, and domestic SA-II special stripping agent for nylon stencils.
It must be noted that if there are other residues (such as ink, sealant, and oil stains) on the stencil, it will be difficult to remove the film. Therefore, remove the residue before applying the release agent. If the ink is cleaned immediately after printing and the film is removed before it is dried, the best cleaning effect can be obtained.
Some stencils that synthesize polymers are invaded by certain vinyl inks, so after the normal release process, some residual film (called stiff film) will still appear. This residual film needs to be removed with a solvent.
After the stencil is completely removed, there may be ink traces on the screen part of the screen, called "ghosting." It is the result of ink abrasion due to printing, ink intrusion and interweaving. The removal of ghosts is more difficult. The British AUTC-TYPE company's practice is: first apply a layer of appropriate solvents, then apply a strong alkaline washing pulp, and then use the brush to remove the rotating motion, which lasted 15 minutes. Rinse with water, the effect is better.
Apart from manual work, there is a special stripping machine. The most advanced stripping machine can complete the three steps of ink cleaning, stripping and degreasing. Because the machine is equipped with a temperature control system, it has a high efficiency of stripping; after washing off the material, after being filtered by the circulating system, the solvent can be recovered and reused, thus avoiding pollution and saving the solvent.
1. Platemaking aids.
Cleaner. In order to remove the land and grease attached to the screen during the plate-making, a neutral detergent such as a 5% aqueous solution of trisodium phosphate (warmed to 5°C) may be used to wash or brush, wash with water, and then dry. Ready-to-use detergents are also sold in the market.
Pretreatment agent. It is a kind of chemical liquid that roughens the surface of nylon and polyester mesh, which can improve the adhesion of screen and photosensitive film. Previously treated with water-resistant sandpaper or caustic soda solution, the abrasive is now placed on a rag, polished on the surface of a nylon mesh, or coated with a 5 to 10% solution of meta-cresol and methanol and left for several minutes. Washed. Nylon's proprietary pre-treatment fluids and pre-treated products are sold on the market. When pretreating polyester webs, a mixture of strong acids can be used and allowed to stand for 10 minutes before being washed with water. However, in the recent direct methods and straight-line methods, these pretreatments are no longer necessary due to the improvement of sensitizers and techniques, and are mainly used in the indirect method and the direct adhesive method with poor adhesion. Stainless steel wire mesh does not require pretreatment, as long as it can be washed.
Anti-halo solution. When nylon or polyester mesh is used, the mesh is generally dyed yellow to prevent blurring of the image due to halo. Anti-halation dyes are available on the market. When using, use a brush to apply 2 to 3 times on the screen, or soak for a few seconds. Diazo-based emulsions are yellow, so when using a diazo sensitizer, it is not always necessary. In addition, there are also pre-use cationic dyes in the pressure cooker to dye the screen yellow or red. When using a yellow screen, the exposure time increases by approximately 20% over a colorless screen.
Filler. It is an emulsion used to block the pores on the surface of the photosensitive film and the mesh around the frame. The sealants sold on the market are mostly used with photosensitive adhesives. For example, the DS-I type net sealant produced by Beijing Taipingqiao Printing Material Factory is a hole-filling agent used with DS-I type diazo photosensitive adhesive. At the time of coating, the use of a bristle brush tends to cause pinholes. Therefore, stainless steel coating hoppers are used, and in particular, they are often used in the indirect method and the straight-line method.
Hardener. It is an agent that is used to enhance the degree of photo-induced film, mainly in the direct legal system. There are two methods of chemical hardening and laminating in the hardening process.
In the former, chromic acid-based aqueous solutions, boron-based colorless aqueous solutions, and diazo-based emulsion-specific aqueous solutions are mainly used. After the exposure and development, the film was coated on both sides. After standing for several minutes, the image portion was sprayed with compressed air and water, followed by drying. The above hardening method is not suitable for high-precision plate making.
The latter is mostly coated with polyurethane. In the production of high-precision screen plate, after drying and flattening with this solution for drum coating to achieve the purpose of hardening.
Adhesives and adhesive tape. Secure the screen to the frame using adhesive and adhesive tape. A special quick-drying adhesive for screens is sold on the market. The adhesive tape is used to protect the adhesive surface to prevent the gel opening phenomenon caused by the immersion of lotion and solvent. The adhesive tape was coated with polyurethane on one side of brown paper to impart water resistance and solvent resistance, and the other side was coated with a resin adhesive, which was applied to the edge of the plate after plate making. The adhesive applied to tape is polypropylene or ester. Adhesive polypropylene tape is colored paper tape, which has different colors depending on the purpose of use. Polyester tape is vacuum-plated, so it is also called aluminum tape or silver paper tape, and it is extremely solvent-resistant.