The printed graphic part of the printing plate has the characteristics of being lipophilic and water-repellent; on the contrary, the non-graphic part is hydrophilic and oil-repellent, so the quality of the plate-making plate will directly affect the quality of the offset printing product. The metal oxide Al2O3 and its crystal water belong to a high-energy surface (0.7J /?). It can be well wetted by the fountain solution during printing. It usually reacts with O2, N2, CO2 in the air, etc. Point, the surface energy is reduced, the wettability is deteriorated, and it absorbs impurities, oil stains, etc., and it is difficult to clean during printing and cause dirt. Therefore, it is necessary to protect the blank part, use the film-forming properties of the glue (the low-energy surface of the polymer compound) to isolate the plate from the air, and prevent the plate from being scratched, protect it from light, ensure the hydrophilicity of the blank part and confer The blank part has better hydrophilic properties. This article provides some reference for plate making by introducing the method and precautions of using protective glue.
1. Composition and function of protective glue
1. Composition
A kind of transparent glue composed of natural glue or synthetic glue, which can protect the graphic and non-graphic parts of the printing plate and prevent the plate from being oxidized and dirty. Commonly used protective glues include gum arabic and new polymer glues (the protective glue used by the author is gum arabic, and this article takes it as an example).
2. Features
â‘ Hydrophilic. The molecular structure of gum arabic consists of -COOH and -OH linked to the same hydrocarbon bond. These two functional groups are hydrophilic groups, and the hydroxyl group (-OH) is easier to accept moisture than the carboxyl group (-COOH). Therefore, when gum arabic is dissolved in water, it will dissolve in a process of diffusion-absorption-swelling for about 72 hours to form a molecular solution of a dispersed system, in which the hydroxyl groups in each molecule are extremely hydrophilic and can be in water The automatic dissolution is a single-phase stable system, which will neither condense nor precipitate, and can guarantee the stability of the dispersion system for a long time.
â‘¡ Weakly acidic. Peach gum contains arabic acid (XCOOH), and its aqueous solution is weakly acidic. It neutralizes with alkali and reacts with metals or metal oxides to form arabinate. It releases hydrogen gas and has good adsorption.
③ Easy to corrupt. In a hot environment (using gum arabic at a temperature of -5 ° C to 45 ° C), yeasts can easily propagate and ferment in gum arabic fluid, freeing arabic acid and changing the gum. This is the generality of sugars, and it is also the cause of rancidity of gum arabic and long-term use, and as the temperature increases, the acidity increases, the corrosion of the metal printing plate is enhanced, and the plate is easy to drop Al2O3 falls off. This point should be paid special attention in summer to keep the protective glue away from high temperature.
3. Function
Improve the hydrophilicity of non-graphical parts and prevent printing from getting dirty. Protect the layout and prevent scratches.
Second, the use of protective glue
Glue by hand: use collagen solution and water to dilute 1: 1. Pour an appropriate amount of glue (1 ~ about 45ml) on the surface of the plate, and then use a cellulose sponge to coat it evenly, wipe off the excess glue with a dry cloth, and then dry it.
Machine glue application: Use collagen liquid and water to dilute 1: 3, and the drying temperature should be controlled below 70 ℃, generally 60 ℃ ～ 65 ℃.
Note: The glue machine without supplementary structure should be replaced every three days. The glue machine with supplementary structure should be replaced once a week. The lower the concentration of the protective glue, the better the inking property when printing, but it is easy to be scratched and dirty. Conversely, if the concentration of the protective glue is too high, it is easy to cause poor ink.
3. Matters needing attention in the use and management of protective glue
When the concentration of the protective rubber is normal, the PH value of the protective rubber is weakly acidic.
The developer reversed flow, causing a sudden rise in pH.
The reverse flow of water reflects the decrease of specific gravity and conductivity, and the increase of PH value.
Prevent the concentration of protective glue: Because the moisture in the protective glue is volatile, the concentration of the glue will become larger, so the protective glue should be diluted or replaced in time.
Using modified protective glue, it is difficult to remove during printing, and reduce the adhesion of the photosensitive layer on the PS plate, the plate is dropped, and the oxide layer falls off in severe cases. For the large-scale graphic and large-area layout glue, the concentration of the protective glue can be appropriately reduced.
When the protective glue is not dry, do not touch the layout in the printing area with your hands to prevent fingerprints from being printed on the printed matter. The protective glue is harmful to the human body. Pay attention to safety when using it. Do not let the protective glue touch the skin and clothing. Inside.
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