The water used in the printing process is not pure water in the true sense, but contains many chemical substances. It is called fountain solution in printing, but in actual printing production, printing operators are accustomed to call it water.
In order to meet the requirements of lithography, the water used in the printing process needs to have certain functions. Of course, one of the most important functions is to form a uniform water film on the blank part of the printing plate to resist the infiltration of the ink of the graphic part into the blank part of the printing plate. However, due to the friction between the rubber cylinder, the water roller, the ink roller and the printing plate, it will cause the abrasion of the printing plate; and under the action of the fountain solution, the paper powder and paper wool that have fallen on the paper are exacerbated This process; as the number of printing increases, the hydrophilic layer on the printing plate will be damaged by a certain degree of wear and tear. For this reason, the second function of the fountain solution is to require a chemical reaction with the base metal in the blank portion of the printing plate during the printing process to form a new hydrophilic layer, thereby maintaining the wettability of the blank portion of the printing plate. Because it is required to react with the printing plate metal, it is required to have a certain pH value, that is, pH value.
To ensure that the blank part of the printing plate can be fully wetted, it is necessary to ensure that the fountain solution has good wetting performance, that is, the fountain solution has a low surface tension, and the surface tension of the fountain solution depends entirely on the damping The chemical composition of the plate solution and the amount of the chemical composition in the fountain solution can be evaluated by the electrolysis index. The surface tension of water in printing is relatively large. In order to control the water-ink balance in printing, it is generally necessary to add some substances that reduce surface tension in the water, that is, surface active substances, such as alcohol, isopropyl alcohol, etc. At present, in offset printing The alcohol fountain solution used belongs to this situation, which is called alcohol fountain solution. Since alcohol or isopropanol is a volatile substance and is greatly affected by temperature, it is necessary to control the temperature when using this fountain solution.
As can be seen from the above introduction, in order to achieve the balance of water and ink in the printing process and prolong the service life of the printing plate, it is necessary to control the indexes of the electrolytic solution, pH value, temperature, alcohol level and the like of the fountain solution.
Electrolysis degree is an indicator that reflects the concentration of fountain solution and the quality of water. Whether the value of electrolytic solution is set directly affects whether the amount of fountain solution added in printing is appropriate, which in turn affects the water-ink balance and printing quality.
If the pH is too high or too low, it will adversely affect printing. If the pH value is too high, the surface tension of the fountain solution drops, and the oleophilic layer of the plate surface is destroyed, causing the ink to emulsify seriously, and the water and ink are unbalanced. .
Temperature and alcohol level will affect the surface tension of fountain solution. In printing, the surface tension is high, the amount of liquid used in the printing process is large, and the thick water film on the plate surface will transfer too much moisture to the printed matter, which will deform, wrinkle, powder, and fuzz the paper. Printing quality brings a certain degree of harm. At the same time, too much water on the plate surface and the surface of the ink roller will also affect the effective transfer of ink, causing serious emulsification of the ink and affecting normal production. However, the use of alcohol will have a certain impact on the environment, so in actual use, the amount of alcohol should be reduced as much as possible, which is also conducive to reducing printing costs.
Therefore, in actual production, there are no absolute values ​​for the electrolytic solution, pH value, temperature, and alcohol level of the fountain solution, but it should be controlled within a certain range. The range of electrolytic degree control is relatively large, generally within 800-2000μs, the pH value is generally controlled within 4.7-5.5, the temperature is generally controlled around 9 ° C, and the alcohol level is generally controlled below 10%.
As there are many types and brands of fountain solution on the market, when choosing, in addition to considering the performance, reliability and environmental protection of fountain solution, it should also consider that different offset printing modes should use different fountain solutions. The general principle is: the selected fountain solution is easy to achieve water-ink balance; it can adapt to different printing machines, printing plates and printing inks; it does not corrode the printing machine; it does not cause excessive emulsification of the ink; the amount of alcohol used is low .
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