Since traditional offset printing relies on the principle of ink-water repellence, the use and control of fountain solution in offset printing is a key part of printing. Although waterless offset printing has been developed to a great extent, but waterless printing is still not widely popularized today, how to control the indicators of fountain solution in offset printing and use fountain solution more reasonably, It is still an important topic that we need to discuss.
PH value of fountain solution
In the offset printing process, due to the printing pressure of the printing process and some other abrasion reasons, it is easy to destroy the hydrophilic oxide layer on the surface of the PS plate, so the fountain solution needs to ensure a certain acidity in order to form a new hydrophilic salt Layer, repair the damaged hydrophilic layer in time. In addition, in the printing process to ensure that the fountain solution has a certain weak acidity, it can also play a role in removing the oil on the blank part of the plate. Practice has proved that for the PS version, the PH value range of the fountain solution that best meets the printing technical requirements is 5 to 6.
When the PH value of the solution is too low, it will not only play a role in repairing the printing plate, but will corrode the printing plate in a vicious manner, and cannot form a stable hydrophilic salt layer, the combination of the graphic part of the printing plate and the metal plate base It may also be damaged, thereby reducing the printing durability of the printing plate. In addition, the fountain solution will react with the drier in the ink, thereby delaying the drying of the ink during the printing process, causing the back of the printed product to smudge, and affecting the overprint rate of the ink.
If the PH value of the solution is too large, the ink-receptive layer of the graphic part will be dissolved first, resulting in a decrease in the printing durability of the printing plate; in addition, the basic groups in the solution will react with the fatty acids present in the ink, resulting in very active Strong hydrophilic groups, such a large increase in hydrophilic groups will cause serious emulsification of the ink during the printing process.
The PH range given above is not an absolute range. The situations encountered in actual production are varied, and the PH value of the fountain solution is not constant. That is to say, in the printing process, we should make some appropriate adjustments to the PH value of the fountain solution according to the actual situation.
1. The effect of paper acidity and alkalinity on fountain solution. The PH value of the paper is preferably neutral, but in fact, due to different papermaking processes, the acidity and alkalinity of various papers are also different. Generally speaking, the pH value of non-coated paper such as offset paper is 4.5-7.0, which is weakly acidic; the pH value of coated paper is 7.0-9.0, which is weakly alkaline. The printing worker can adjust the dampening according to the nature of the paper Adjust the fluid accordingly. When the paper is acidic, the PH value of the fountain solution can be higher (when printing offset paper, the PH value can be controlled between 5.5 and 6.5 according to the situation), and when the paper is alkaline, the PH value of the fountain solution can be smaller ( Generally, when printing on coated paper, the pH value of fountain solution can be controlled at 4.8 ～ 5.0, and it can be reduced appropriately according to the situation).
2. When the thickness of the ink layer increases, the ink in the graphic part will inevitably spread to the blank part, which is prone to "dirty version"; when the amount of desiccant in the ink increases, the viscosity of the ink increases and the particles become thicker The adhesion is increased, and dirty plates are prone to occur; when printing products with low surface strength, due to the serious phenomenon of powder and fluff, the hydrophilic salt layer of the plate surface is easy to be worn; when the ambient temperature is high, More free fatty acids will be separated from the ink, which will react with the OHˉ in the fountain solution, causing serious emulsification of the ink. In the above situations, we should pay attention to lower the pH value of fountain solution to increase the acidity of the solution.
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