To obtain high-quality prints for lithography, it is very important to master the water supply of the printing plate. For lithography, water hinders the transfer of ink, but it cannot be printed without water. Therefore, fundamentally speaking, according to the principle of water-ink balance, the amount of water supplied to the printing plate should be controlled to the minimum at which the blank portion of the printing plate starts to get dirty.
1. Water consumption of the layout
The print on the printed matter is formed by the transfer of ink to the substrate. Lithography uses a rubber blanket with good lipophilicity and hydrophobicity to transfer ink. Of course, the consumption of ink is greater than the consumption of water. However, in production, the amount of water used by the offset press exceeds the amount of ink used, sometimes even several times or tens of times. After experiments and analysis, water consumption is mainly through the following methods.
The emulsification of water in the ink is the main way of water consumption on the printing plate. After emulsification of the water in the ink, the water is transferred to the substrate (such as paper) with the ink, and then consumed through penetration and evaporation. Production practice shows that the larger the graphic area and the thicker the ink layer, the greater the water supply required by the printing plate. Figure 5-18 is the relationship between the amount of water transferred from the graphic and blank parts of the printing plate to the paper and the running time of the offset press after the offset press is started. The amount of water transferred from the graphic part to the paper reaches a stable value after the offset press is started for 10 minutes, and the blank part only takes 7.5 minutes to reach the stable value. It can also be seen from the figure that the graphic part is transferred to the paper than the blank part There is much more water on it.
Moisture, water rollers, ink rollers, printing plates, blankets and other surfaces of the offset press will evaporate into the surrounding space every time during the printing process, especially when the ambient temperature is high, and the printing speed of the machine Soon, the more water is consumed by evaporation.
When printing on paper with loose texture and low sizing degree, the moisture in the blank part of the printing plate is transferred to the paper through the blanket during the imprinting process and is directly absorbed by the paper. The consumption of this part of water decreases with the increase of the hydrophobicity of the blanket and the increase of the sizing degree and tightness of the paper.
In addition, when the water supply of the printing plate is too large, due to the hydrophobicity of the blanket, a part of the water is squeezed against each other by the roller, and is driven to the trailing tip of the blanket or drip down along both ends of the roller. The other part of the water flows in the ink fountain in the opposite direction of the cylinder running of the offset press. In severe cases, it will cause the ink roller to deink.
The above water consumption methods make the water balance of lithography more complicated, and increase the difficulty of water control.
Figure 5-18 Relationship between the amount of water transferred and time ▲ -Image part × -Non-image part [next]
2. Factors affecting the amount of wetting fluid
The supply amount of the dampening fluid on the printing plate can be changed within a certain range. The range should be between the beginning of the blank part of the printing plate (dirty version) and the printed matter starting to have water marks (printing flower) This range is called the practical range of the supply amount of the wetting liquid (referred to as the practical range of the liquid supply amount). Obviously, the larger the practical range of liquid supply, the easier it is to control the supply of wetting fluid.
The type and composition of the wetting fluid, the temperature of the printing press, the water absorption of the ink, the thickness of the ink layer, the performance of the paper and other factors all have an impact on the practical range of the amount of liquid supplied.
1. The effect of the type of wetting fluid on the practical range of the amount of liquid supplied. Figure 5-19 plots the practical range of liquid supply when printing with the same ink using three different wetting fluids at a constant temperature. Among them, A is a common wetting fluid, B is a wetting fluid configured with water and alcohol, and C is a wetting fluid configured with water, alcohol, and some chemical components such as a buffer to stabilize pH, hydrophilic gum arabic, etc.
Figure 5-19 Practical range of wetting fluid supply
It can be seen from Figures 5-19 that the practical range of the liquid supply amount of the wetting liquid A and the wetting liquid B is basically the same, but the practical range of the total liquid supply amount of the wetting liquid B has moved downward, indicating that the specific wetting liquid A small amount of water can control the occurrence of dirty version. The lower limit of the practical range of the supply volume of the wetting fluid C is the same as that of the wetting liquid B, but the upper limit of the range moves upwards, that is, the practical range of the total supply volume is significantly expanded, so that even if more The amount of water will not appear on the printed matter. Therefore, printing with wetting fluid C, the amount of water is easy to control, and the water-ink balance is also relatively stable.
2. The effect of temperature on the practical range of liquid supply. The heat source of the offset press is mainly the cross-inking rollers which move in series. With the passage of time, the heat of the cross-inking rollers is slowly transferred to other rollers. As the temperature increases, not only the wetting fluid on the printing plate evaporates much, but also more wetting fluid is dispersed into the ink through the inking roller, so that during continuous printing, the wetting fluid on the printing plate is continuously reduced.
Figure 5-20 The effect of temperature on the practical range of liquid supply [next]
The effect of temperature on the practical range of liquid supply is shown in Figure 5-20. As the temperature of the inking roller increases, the upper limit of the practical range of liquid supply (printing begins to bloom) has not changed, and the amount of liquid supply The lower limit of the practical range (the printing plate starts to be dirty) will continue to move to the upper limit as the temperature of the inking roller increases, and the practical range of the liquid supply amount will become smaller and smaller.
As far as the actual printing is concerned, if the paper used and the ink match, the dampening fluid used can just control the dirty version. Due to the gradual increase in the temperature of the machine, the lower limit of the practical range of the supply volume of the dirty version gradually increases If the upper limit of the flower movement is moved, if the water supply is not adjusted, a dirty version will suddenly appear.
When an alcohol wetting system equipped with a cooling circulation device is attached to the offset press, the temperature of the wetting liquid can be maintained at 5 to 10 ° C, which is undoubtedly advantageous for the control of the amount of water.
3. The relationship between the amount of wetting fluid and the thickness of the ink layer.
The relationship between the wetting fluid and the thickness of the ink layer is more complicated, and it first depends on the water absorption performance of the ink. This relationship is described below.
Print with two inks with different water absorption, A and B, of which A ink has low water absorption, and the effect of the amount of wetting liquid on the thickness of the ink layer is shown in Figure 5-21. B ink has high water absorption, and the effect of the amount of wetting liquid on the thickness of the ink layer is shown in Figure 5-22.
It can be seen from Figures 5-21 that when the amount of wetting fluid is reduced, the thickness of the ink layer increases, and when the amount of wetting fluid is increased, the thickness of the ink layer decreases. However, it can be seen from Figures 5-22 that as the amount of wetting fluid increases, the thickness of the ink layer increases, and as the amount of wetting fluid decreases. The thickness of the ink layer is reduced.
The reason for the difference in the above experimental results is that the ink with low water absorption, the wetting liquid present in the ink roller, part of the ink is emulsified and dispersed in the ink; part of it floats on the ink in the form of small water droplets, There is a lot of water on the surface, which results in poor ink transfer of the ink roller, and the ink layer on the printing plate decreases accordingly.
Ink with large water absorption can disperse the increased amount of wetting fluid in the ink in a large amount without affecting the transfer of the ink. Therefore, the thickness of the ink layer of ink with different water absorption has different responses to the change in the amount of wetting fluid.
Figure 5-21 The relationship between the thickness of the ink layer with a small amount of water absorption and the amount of wetting liquid
Figure 5-22 The relationship between the thickness of the ink layer with a large amount of water absorption and the amount of wetting liquid
Figure 5-23 Practical range of liquid supply for inks with different water absorption [next]
As shown in Figure 5-21 and Figure 5-22, the two inks A and B are printed with ordinary wetting liquid under the same ink layer thickness and constant temperature. The practical range of the amount of wetting liquid supply is shown in Figure 5- 23 is shown. Obviously, when printing with inks with large water absorption, the practical range of the amount of supply of the wetting liquid is much larger than that of inks with a small amount of water absorption.
In addition to the above factors that affect the practical range of the amount of wetting fluid supplied, the area and distribution of the graphics on the printing plate, the nature of the paper, the printing speed, the type of printing plate and the surface properties, etc. influences. Therefore, the amount of wetting fluid cannot be specified mechanically, and should be appropriately adjusted according to the production process conditions.
3. Control of the amount of wetting fluid
For lithographic printing, the amount of water on the plate surface is closely related to the transfer of ink, precision of overprinting, emulsification and drying of ink, printing durability, and printing gloss. The water supply of the printing plate should be determined according to the actual printing conditions. However, because there are too many variable factors, it is necessary to pay close attention to the changes in the water volume of the plate during the printing process and adjust the water supply of the plate in time. At present, in China, most of the changes in plate water volume are estimated based on experience and visual observation. Some new foreign printing machines have been equipped with plate water film measuring instruments to control the changes in plate water volume.
1. Visual method. According to some problems in printing, the amount of water is estimated by experience. In general, when the following problems occur, it can be considered that the page moisture is too large:
â‘ The ink color on the printed matter is light, even if the amount of ink is increased, the ink color will not deepen in time;
â‘¡The amount of ink accumulated on the ink roller increases, and the ink particles become thick;
â‘¢The printed sheet absorbs excessive moisture, the amount of water on the front and back sides is uneven, and the paper is curled;
â‘£ There is too much water remaining on the surface of the ink film, there are drops of water when the ink shovel scrapes the ink, the ink added to the ink fountain is not easy to stir evenly, and the transfer roller has a slip phenomenon;
⑤There is a viscous flow of water at the tip of the blanket, and drops of water are dropped at both ends of the roller;
â‘¥ There is a big gap between the shades of the printed products before and after the printing is interrupted;
⑦The layout will not dry after shutdown.
2. Water film thickness measuring instrument. Figure 5-24 is the absorption spectrum of the near-infrared wave of the aqueous solution. It can be seen from the figure that in the 2.96μm band, the aqueous solution has a significant amount of absorption, but it is hardly absorbed in the 2.55μm band. For this reason, the former is designated as the measuring section, and the latter is designated as the reference section. The water film thickness measuring instrument is made according to the difference between these two wave reflections. Using the infrared absorption spectrum, the thickness of the water film can be calculated using the following formula:
Figure 5-24 Absorption spectrum of aqueous solution in infrared band
In (Io / I) = k · b · c (5-4)
Where I0 is the incident intensity, I is the transmitted light intensity, k is a constant, b is the thickness of the water film, and c is the density of the aqueous solution. Figure 5-25 is the absorption curve of infrared spectrum of water film with different thickness. Curve A represents the dried printing plate, curve B represents the amount of water film of 0.4 g / m2, and curve C represents the amount of water film of 1.48 g / m2.
The principle of the water film thickness measuring instrument is: using a rotating filter wheel to irradiate the light with a wavelength of 2.96 μm and 2.55 μm onto the printing plate of the offset press alternately, and the two infrared rays are reflected by the plate after passing through the water film. One of the infrared rays is not weakened by the water film when it passes through the water film, while the other infrared light is strongly absorbed as the water thickens. Therefore, the infrared rays returned from the board have different intensities. Infrared rays of different intensities are detected by a photocell and converted into the thickness of a water film by a special pulse circuit.
The water film thickness measuring instrument, also called infrared photometer, is developed by the German FOGRA Printing Research Institute and provides a test tool for accurate measurement of water film. It has been installed on the Roland VLTRT machine. If different thicknesses of ink film are used to absorb infrared spectrum differently, infrared photometer can also be used to determine the thickness of the ink film in the graphic part of the printing plate.
In addition, the method of irradiating the plate surface with a laser, measuring the amount of reflection or applying an AC voltage to the water roller, measuring its negative value, and then obtaining the thickness of the water film has also been tried on offset presses. However, to achieve digital management of the water volume on the printing plate, it is also necessary to trial-produce sophisticated water volume testing instruments and improve the test method on the basis of improving theoretical research.
Figure 5-25 Infrared absorption curve of water film with different thickness
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