With the rapid development of China's printing industry, lithographic offset printing technology has been widely adopted as a printing method with fast printing speed, good color reproduction and high economic benefits. The quality of lithographic offset printing products mainly depends on the balance of water (fluid solution) ink balance in the printing process. Lithography is the use of water and oil (ink) immiscible principle, so that water and ink in the same plane on the plate to maintain a balance between the relationship, to achieve the normal transfer of ink to achieve the purpose of color reproduction. This article will discuss the ink and water balance from the following aspects:
First, the composition and function of water
The water mentioned in lithographic offset printing is not ordinary tap water, because the tap water can wet the printing plate surface and be adsorbed on the non-image plate blank space, but its wetting performance does not meet the printing requirements, so it must be in tap water. Add some electrolyte to make it have stronger wettability. That is, the water referred to in the lithographic offset printing is what we usually say as the fountain solution. It is formed by dissolving dampening powder in a certain proportion. The main function of water is to keep the blank (non-graphic) part of the printing plate from absorbing ink, and at the same time adsorbing the ink of the graphic part of the printing plate to achieve normal ink transfer and achieve the purpose of lithographic offset printing.
Second, the main water consumption
In lithographic offset printing, there are several ways to consume water:
1. Space evaporation. In nature, there is a common conversion of the liquid and gaseous materials. The phenomenon of vaporization on the surface of liquids is called evaporation, and the evaporation of water is no exception. In the printing process, due to the surface of water rollers, ink rollers, printing plates, and blankets, etc. The presence of water exacerbates the evaporation of water. The evaporation rate of water is related to the temperature. The higher the temperature is, the faster the evaporation rate is; the higher the surface area is, the faster the evaporation is.
2. Paper absorption. In lithography, the blank part of the printing plate is hydrophilic and oleophobic. Moisture in the blank part of the printing plate is absorbed by the paper through the rubber cloth on the paper during the imprinting process. The amount of water absorbed is related to the characteristics of the paper. The more closely the structure of the paper, the greater the degree of sizing, the higher the smoothness, the smaller the amount of water absorption, and vice versa, the greater the amount of water absorption. For example: the amount of water absorbed by double-size paper is higher than that of double-coated paper.
3. Ink emulsification. Emulsification refers to the mixing of two liquids that cannot be mixed together, and one of the liquids is dispersed into fine particles and dispersed in the other liquid. The water is emulsified with the ink, transferred onto the paper together with the ink, and then consumed by evaporation, infiltration, and the like. Practice has proved that the larger the area of ​​the graphic and the thicker the ink layer, the greater the amount of water used. The smaller the picture area and the thinner the ink layer, the smaller the water consumption. The emulsification of ink is not good for printing, but it is inevitable. In the printing process, the emulsification of the ink should be minimized.
4. Squeeze and drop. In the imprinting process, excess moisture on the surface of the printing plate cannot be absorbed by the blanket in large quantities (because the surface of the blanket has oleophilic and hydrophobic properties), and it is driven by the roller to the tip of the blanket or both sides of the roller, and then dropping.
Third, control the ink balance
In lithographic offset printing, mastering the balance between water and ink is crucial. Lithography is based on the principle of oil-water immiscibility on the basis of the oil-and-water-repellent part of the plate and the hydrophilic and oleophobic parts of the blank. Whether the ink is in balance or not depends on whether the printing can be carried out normally or not. It also determines the quality of the printing. During the printing process, since there is a certain amount of water film in the blank part of the printing plate, when the blank part of the printing plate comes into contact with the ink roller, ink will not be adsorbed; if the amount of water film is insufficient, the blank part of the printing plate comes into contact with the ink roller, Can not resist the ink on the blank part of the printing plate adsorption, will make the blank part of the printing plate to absorb ink, which is commonly referred to as the blank part of the dirty. On the contrary, if the layout of water is too large, and gradually spread to all the surface of the ink roller to form a certain amount of water film, it will hinder the normal transmission of ink, affect the normal printing, affect the size of the print ink, affect the color of the print Restore, affect the quality of printed products. That is, when the moisture is too large or too small, normal printing cannot be performed. This is the ink-water balance in the printing process discussed herein.
For ink-water balance, there can be the following three kinds of equilibrium states: one kind of equilibrium state is water and large ink; one kind of equilibrium state is small water and small ink; one kind of equilibrium state is moderate ink and water. Obviously, the first two kinds of equilibrium are all incorrect. How to achieve the ideal third equilibrium? One is to adjust the amount of water supply and the amount of ink to be properly adjusted by adjusting the water and ink controllers (commonly known as the water ruler and ink ruler) and adjust the pressure of the water roller and the ink roller appropriately because of the pressure of the water roller and the ink roller. And size determines the amount of water given and the amount of ink. The second is to comprehensively analyze the factors that affect the balance of water and ink, determine the amount of water supply and the amount of ink to be given. For example: When the workshop temperature is high, the printing machine speed is fast, the paper absorbs the water strong, the ink quantity is large, must increase the water supply quantity appropriately; On the contrary, should reduce the water supply quantity appropriately. The standard of ink and water balance is: In the premise of non-dry version, use the minimum amount of water evenly, and leave a certain amount of anti-dirty water balance. The criteria for identifying the balance of ink and water with prints are: the print dot is well restored, the dots are not glued, the dots are not printed, and the color reproduction is good. In the printing process, when the ink and ink cannot maintain balance, the following effects will be brought to the printing: the excessive use of water will hinder the normal ink transfer, make the imprints empty, and a large amount of moisture will be transferred to the paper, causing the paper to be deformed and the paper will not be delivered together. Overprinting is not allowed; water consumption is too small to make the blank part sticky and dry. The use of ink in excess of the General Assembly enlarges the outlets, resulting in stencils; using too little ink will make the ink layer thinner, resulting in dull imprints and discoloration. The moisture content of the layout is mainly judged from the following aspects: water droplets on the ink fountain and ink roller, not easy to mix when adding ink, thick ink layer on the ink roller and low viscosity, softness and weakness of paper after printing, curling after printing , uneven delivery, blankets with water shadows, large color differences before and after printing interruptions, etc., indicate that the layout is too large.
After the above analysis, everyone will have a deeper understanding of the ink-wash balance. How can we master the balance of water ink? There are no special measuring instruments. We rely mainly on the experience of the operators to grasp. We just want to go through the above analysis so that we can understand the intrinsic relationship of ink and water balance in lithographic printing and learn to correctly determine the size of ink. Correctly adjust the size of the ink and make it reach the ideal ink-water balance. In order to become a good captain of a offset lithographic printing machine, it is necessary to pay attention to the regularity of the work and observe the regularity of the balance between water and ink in order to control the ink and water balance. Produce prints that meet the proofs and beautiful prints.