Because the printing process is the use of water adsorption on the surface of the blank part of the printing plate to absorb water, forming a water film on the surface of the printing plate, using water and ink mutual exclusion insoluble theory to repel ink, so the water on the surface of the printing plate The film condition directly affects the repellent effect of the blank on the ink. The thin water layer is not enough to repel and remove the ink, and the blank part is sticky to form waste products; the water layer is thick and the ink repelling ability is strong. During the ink supply process, there is a certain pressure between the ink roller and the printing plate. Thick water film and fluid ink will inevitably occur mechanical mixing phenomenon under pressure, called emulsification in printing. It adversely affects the color, fluidity, transport, adhesion, and drying properties of the ink, and is not conducive to color reproduction and drying of the print during printing. The formation and thickness of the water film are determined by the nature of the water and the nature of the substrate. Therefore, as a printing worker, it is necessary to understand and grasp the principle, composition, physical and chemical properties in printing, and how to control the relationship between water and ink, water and product quality in practical work, and make it blank in the printing plate. Some of them are easy to form water film, and at the same time, the mechanical mixing state with the ink is the lowest, so as to achieve the purpose of improving product quality and reducing consumption. This is printability.
In the actual work, we found that ordinary water cannot absorb and form a water film on the surface of the plate base. Therefore, instead of using clean water in actual printing, we dissolved some chemicals in ordinary water to form a kind of energy. It is better to form a water film on the blank part of the printing plate surface and repel the ink while minimizing the effect on the color flow and adhesion drying properties of the ink. We call this the fountain solution.
From the above analysis, we can know that the role of fountain solution mainly includes the following two aspects: First, the fountain solution should have good adsorption and spreading ability on the surface of the printing plate, and can form a surface on the plate. Stable water film to maintain the blank part of the printing plate has the best ink repellency;
Second, the fountain solution maintains relative stability under pressure and the emulsification of the ink is minimal. In addition, the fountain solution can not harm the layout text part, and has a certain ability to wash the blank part of the adhesion of the ink. Below we analyze the above content:
1. Selective adsorption of fountain solution on the printing plate
The fountain solution is selectively adsorbed on the plate and can be analyzed using the molecular polarity theory. We know that substances in nature, except for inert gases, do not exist as single atoms, but exist in the form of molecules or crystals in which atoms are bound to each other, and each molecule has positively charged nuclei and bands. The electron charge of the negative electron charge is formed by chemical bonding. In the molecule, the number of positive and negative electrons is equal, and the entire molecule is neutral. However, for positive and negative charges, the charge center may be gathered at one point or at different points. When the positive and negative charge centers are divided at two points, the entire molecule has polarity; for example, positive and negative charge centers are At the same time, the entire molecule has no polarity. Due to the presence of polarity, the relationship between the molecules of matter can be divided into the following three situations:
(1) For polar molecules, due to the presence of electrodes, when the two molecules are close to each other, under the influence of electrostatic attraction, induced forces are generated between the molecules, and polar molecules have a strong adsorption tendency.
(2) When polar molecules and non-polar molecules are close to each other, although there is no fixed electrode attraction between molecules, since the charges in the molecules move at a high speed, the positive and negative charge centers will produce instantaneous separation, making the non-polar molecules and There is an instantaneous mutual attraction between the polarities, and there is also a weak force. External performance is mutually exclusive.
(3) Between non-polar molecules, due to the instantaneous separation of positive and negative charge centers in non-polar molecules, there is a constant mutual attraction when non-polar molecules are close to a certain distance. The role of non-polar molecules with mutual attraction between. It is precisely because there are such polar phenomena and similar attracting laws among substances. That is, polar molecules adsorb polar molecules and nonpolar molecules adsorb nonpolar molecules. The polar solution with strong polarity can be adsorbed by the plate base when passing through the plate base that is also strongly polar, and the molecules (light-sensitive compounds) in the graphic part are non-polar substances that do not absorb moisture, thereby realizing the dampening The selective adsorption of liquids.
2. The adsorption and spreading of fountain solution on the plate base
Although the fountain solution realizes selective adsorption on the printing surface, the molecules on the surface of the fountain solution are always subject to the attraction of the internal molecules and contact with it due to the attraction between the molecules and the different sizes of the different molecules. The gravity of the external air molecules causes the molecules on the fountain solution to be subjected to a vertical liquid surface and internal force. The molecules on the liquid surface always have a tendency to move to the inside, minimizing the contact surface between the liquid surface and the air. The force that shrinks the surface of the liquid is called the interfacial tension of the contact of the liquid. The same principle is also true for interfacial tension on solid surfaces. The difference is that the interfacial tension of solids does not shrink the solid surface, but rather exhibits the ability to adsorb other substances. The stronger the polarity, the larger the contact area, the greater the adsorption capacity. Strong. The existence of interfacial tension makes the fountain solution adsorbed on the plate, it always shows the tendency of minimizing the contact between the fountain solution and the plate, which is extremely disadvantageous for the fountain solution to form a film on the plate. Through experiments, we can conclude that the situation in which the fountain solution adsorbs and spreads on the plate depends on the following three forces: the interfacial tension between the fountain solution and air, the interfacial tension between the fountain solution and the plate base, and The interfacial tension of air. The following relationships exist between the three:
We use the size of α angle to measure the fountain solution on the plate. The smaller the α angle, the better the film formation.
From the formula, we can see that there are three ways to improve the film formation of the fountain solution on the plate:
1 Reduce the interfacial tension between fountain solution and air;
2 reduce the tension between the fountain solution and the plate;
3 Increase the interfacial tension between the substrate and air (the ability to adsorb external substances).
In practical work, on the one hand, we improved the film formation of the fountain solution on the plate by covering the plate with inorganic salt layer, oxide film layer to enhance polarity, attach hydrophilic colloids, and increase plate area. On the other hand, the same objective can be achieved by reducing the surface tension of the fountain solution.
3 The effects of various components of the fountain solution, various physical and chemical indicators, and the effects on the ink
Typical fountain solutions consist of water, acids, inorganic salts, hydrocolloids and surface-active substances. The largest proportion of water in the fountain solution, as the carrier of various chemicals in the fountain solution, is the main body of the fountain solution. Hydrocolloids commonly used in fountain solutions are gum arabic and carboxymethyl cellulose (CMC), gum arabic (or CMC) and the acid in the fountain solution displacement reaction, free a lot of arabinic acid (or CMC acid), can It is firmly adsorbed on the surface of the printing plate, adding a certain polar group in the fountain solution, and increasing the hydrophilicity of the plate surface.
In addition to this, hydrocolloids can form a dense protective layer on the surface of the printing plate to prevent the plate from contacting other materials and prevent the plate from being damaged.
In addition to the substitution reaction between the acid and the hydrophilic colloid in the fountain solution, it is also possible to use its acidic grease-removing property to eliminate the oily surface of the printing plate. Another important role of acid is to chemically react with the surface of the printing plate to form a layer of inorganic salt on the surface of the plate to enhance the hydrophilic properties of the plate. Therefore, we must strictly control the acidity of fountain solution.
In general, we require that the pH of the liquid is appropriate, the pH is too high, the acidity is weak, the removal capacity of the layout is poor, the pH is too high, the fountain solution is alkaline, and the photosensitive film of the graphic part will undergo chemistry Reacts, erodes the network part of the graphic part and destroys the integrity of the graphic and text; if the pH value is too low, the acidity is too strong, the plate base will be seriously corroded, the surface of the printing plate will be destroyed, and the graphic part will fall off, which will affect the normal printing process. . In printing, we believe that it is appropriate to keep the pH of the fountain solution between 4.5 and 5.5.
In the printing process, to maintain the acidity of the fountain solution, that is to maintain the pH of the solution. However, due to the contact between the fountain solution and various printing materials during printing, such as paper, blanket, ink, etc., these substances are brought into dampening through the constant movement of water and ink rollers during printing. It will affect some inorganic salts, such as phosphates, nitrates, etc. in the dampening version. It will also be used to maintain the content of the dampening liquid. The purpose of the pH.
Any substance that can significantly reduce the surface tension of a liquid is a surface active material. There are many types of surface-active substances. According to the structure and properties of the surface-active substances, they can be classified into general surface-active substances and surfactant groups. According to whether the active group ionized by the surfactant in the fountain solution is charged or not, the surfactant can be divided into two types: ionic and non-ionic. According to the charge nature of the active group, the ionic type can be divided into three types: cationic, anionic and zwitterionic. Ionic surfactants are generally unsuited to fountain solutions.
The surface active material can greatly reduce the surface tension of the fountain solution and make the surface of the printing plate easy to form a water film. The lower surface tension can also cause the fountain solution to emulsify with the ink under pressure and cause poor printing. Because of this, it is not possible to reduce the surface tension of fountain solution without limit.
Although the polar water that makes up the bulk of the fountain solution is theoretically immiscible with the non-polar binder of the constituent ink body, we know that because the constituent dampening solution dissolves a certain amount of inorganic salt, the surface Active agents and hydrocolloids have a sharp drop in surface tension, while inks are not completely composed of non-polar oily linking materials, as well as pigments, fillers, and driers, which are not simply between water and oil. The relationship between the two, so when printing, between the pressure between the ink roller and the plate, the pressure between the printing and the heat generated by the pressure during the printing process, the two will be mixed to some extent, so that the fountain solution The fine moisture is dispersed into the ink, forming a temporary mix, which is emulsification. The presence of emulsification inevitably affects the viscosity, color, drying, etc. of the ink and hinders the normal printing process. The above reasons require us to control the surface tension of the fountain solution and the degree of emulsification of the ink.
Since the surface tension of the ink is 30 to 36 dyn∕cm, the ideal surface tension of the fountain solution should also be controlled to the same degree, and the two are balanced and stable. However, since the fountain solution will slightly decrease in printing due to other factors, the surface tension of the fountain solution is controlled at 35 to 42 dyn∕cm, which is slightly higher than the surface tension of the ink. Although we can control the surface tension of the fountain solution to a certain extent, ink emulsification is unavoidable due to the existence of surface active substances in the fountain solution and the effect of printing pressure, but it must be controlled within a certain range. At work, we often adjust the fountain solution
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