The plate-making process of the positive figure PS plate (5)
3. Coated with black ink and glue
1. In addition to developing black ink. The developed black ink is a black ink prepared by blending oily substances and water-based substances, and is an emulsified ink emulsified by oil phase and water phase substances. The developed black ink is coated on the surface of the printing plate, the oily substance is absorbed by the graphic part, and the aqueous substance is absorbed by the blank part, which not only improves the lipophilicity of the graphic part, but also increases the stability of the blank part. Therefore, whether it is a roasted printing plate or a printing plate that has only been developed and decontaminated without baking, it is necessary to apply developed black ink. In addition, the development and development of black ink can darken the graphics and text, and it is also convenient to check the quality of printing.
The oily substance in the developed black ink is composed of terpene resin, turpentine, carbon black and oil-soluble emulsifier, the water-based substance is composed of gum arabic (or carboxymethyl cellulose), water-soluble emulsifier and water. There are Span-60 and Tween-60.
Terpene resin is a linking agent in developing black ink and is an unsaturated terpene. It is polymerized under the action of a catalyst and can be dissolved in turpentine, kerosene, gasoline and other mineral oils. It has excellent lipophilic properties. It is used to formulate developing black ink, which has good affinity for the photosensitive layer and good inking properties. When using, generally choose a terpene resin with a higher boiling point, because the terpene resin with a lower boiling point is easily volatile when heated.
Turpentine is a good solvent for terpene resins. It is composed of hydrocarbons with a ring structure. It is volatile and flammable. It has a strong affinity with lipophilic substances.
Carbon black is a pigment in developing black ink, which is an amorphous powdery substance. It is insoluble in any solvent and has poor dispersion in the binder. When preparing developing black ink, carbon black must be mixed into terpene resin After being pressed and ground by the grinding roller of the grinding machine for many times, it can be dispersed evenly.
Span-60 is also called emulsifier-S or emulsifier S-60. It is the trade name of sorbitan stearate. It is formed by the condensation of sorbitan and higher fatty acids. Its molecular formula is . Span-60 is a non-ionic surfactant. It is a high-quality water-in-oil (W / O) emulsifier with good emulsification, dispersion and wetting properties. It is soluble in water and used with Tween-60. , The emulsification effect is more obvious.
Tween-60 is formed by the condensation of Span-60 and ethylene oxide. The molecular formula is It is a high-quality oil-in-water (O / W) emulsifier with good emulsification, dispersion and wetting properties, and is soluble in warm water. It is often used together with Span-60.
Gum Arabic is a hydrophilic colloid, used in developing black ink, the function is to enhance the hydrophilicity of the blank part. The price of gum arabic is relatively high, and it can be replaced by hydrophilic colloid CMC (powder methyl cellulose).
There are many formulas for developing black ink, one of which is listed below for reference.
Developing black ink formula:
100g of preene resin,
Turpentine 200ml,
Carbon black 50g,
Spanner 60 3 g,
Tween one 60 l4g,
Arabic gum 8 g,
1000ml of water.
The steps to prepare the developed black ink are as follows:
(1) Dissolve the terpene resin in turpentine oil at a temperature of 50 to 60 ° C according to the ratio of the formula, put the carbon black into it, and stir it evenly; then put it in a grinder and grind until the carbon black particle size reaches 4 ～ At 6 μm, when the carbon black is evenly dispersed in the terpene resin, grinding is stopped and a resin black ink is prepared.
(2) Dissolve Span-60 in 60 ° C water according to the ratio of the formula, and then mix the obtained solution with the resin black ink to prepare an oil phase substance.
(3) Dissolve Tween-60 and gum arabic in water at 60 ℃ according to the ratio of the formula, then slowly add phosphoric acid dropwise, adjust the pH value of the solution to about 5.5, and prepare an aqueous phase substance.
(4) Slowly inject the water phase material into the oil phase material while stirring. At first, less water phase material was added, forming a water-in-oil emulsion; as the amount of water phase material added increased, the emulsion The viscosity of the emulsion rises. When the amount of water-phase substances in the emulsion reaches a certain amount, the viscosity of the emulsion drops sharply, and the water-in-oil type is converted to the oil-in-water type. The result is a good performance black ink.
The developed black ink is prepared by the above method. The emulsification speed is fast, the particles are fine, the water phase and the oil phase are evenly dispersed, and it is not easy to separate and layer. The performance of the developed black ink is good and stable.
The operation of applying and developing black ink is relatively simple. For the roasted printing plate, first apply a layer of gum arabic solution on the surface of the printing plate to make the plate surface in a wet state, and then apply the developed black ink. The unbaked printing plate can be coated with developed black ink as long as the water left on the surface of the plate after development and water washing is removed with a blade. To coat the black ink, use a slightly damp sponge to gently wipe the circle. When the black ink is not dry, wipe the layout with the sponge while rinsing with water to remove all non-graphic ink. After the printing plate is glued, it can be printed on the machine or stored for future use.
2. Glue. Gluing is the last processing step before printing on the printing plate, that is, coating a layer of Arabic glue on the surface of the printing plate to make the non-image blank portion more stable in hydrophilicity and protect the plate surface from being stained.
Arabic gum is a natural polymer compound, the main component is arabic acid, the structural formula of arabic acid is
writing The carboxyl group (-COOH) in the molecule is usually adsorbed on the metal surface directionally, and on the outer surface of the arabic gum layer attached to the dry metal, a large amount of one OH (hydroxyl group) is maintained as shown in Figure 9
Fig. 9 Adsorption of gum arabic on the surface of PS plate
Gum arabic is a reversible colloid, which can absorb a large amount of water and swell until it is completely dissolved, or it can gradually evaporate water and change from a viscous to a solid. Therefore, gum arabic can be formulated into various concentrations of glue waves to coat the plate surface, and after the water evaporates, it will be transformed into a solid again, forming a strong cover film on the plate surface to prevent the oxidation of the plate surface and the dirt on the plate surface from being dirty . The concentration of the glue solution is generally 5 ° Be ', which is prepared by dissolving 100g gum arabic in 1000ml of water. The gum arabic market price is relatively high, and sometimes chemical compounds such as CMC can be used instead. However, if the printed plates need to be printed on the machine every few days, gum Arabic must be used.
The plate after gluing should be thoroughly dried before it can be used for printing. If the plate with poor drying is directly used for printing on the machine, when the water roller of the offset press supplies water to the plate, the glue on the plate will be used. Washed out, resulting in the disadvantage of poor hydrophilicity of the printing plate.
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