In offset printing, the pH of the paper and fountain solution can directly affect the ink balance, ink drying performance, and ink layer quality of the printing process. Therefore, when controlling and adjusting the balance of water and ink, it is necessary to understand the acidity and alkalinity of the paper and dampening solution and understand its influence on the printing process.

1. Paper pH

Due to the increasing market demand for printing quality and the advancement of printing technology, the influence of paper properties and printability on print quality has begun to receive attention, and paper pH value should be a key factor to control.

If the pH of printing paper is neutral, printing quality can be greatly improved. However, due to factors such as raw materials and processing processes in the production of paper, absolute neutral paper with a pH of 7 is not available, and the paper surface or coating layer often has a certain degree of acidity or alkalinity. When the pH value is 7, the paper The surface is alkaline. At present, China's papermaking industry mainly adopts alkaline pulping and uses NaOH as a cooking agent, so most papers are slightly alkaline. If the alkalinity is too large, the acidity of the fountain solution will change significantly and the acidity of the hydrophilic adhesive layer of the non-graphical part of the printing plate will be weakened. If it cannot be adjusted in time, the hydrophilicity of the non-graphical part will inevitably decrease, and the ink and ink imbalance will occur. The coloring power of the ink is reduced, resulting in a dark color of the printed graphic and text, and even the phenomenon of de-inking of the print plate, the paste plate, and the ink roller. At the same time, those weak alkaline fibers that have fallen from the paper coating will be transferred to the blanket and transferred to the printing plate via the blanket, causing the printing plate to be dirty and the resistance force to fall.

When the pH value of the paper is too low, the acidity of the fountain solution is significantly increased, so that the oxidative polymerization reaction and the drying speed of the ink are suppressed, resulting in poor drying of the printed matter.

2. Fountain solution pH

Traditional offset printing materials rely on the principle of oil-water repulsion to achieve ink image transfer. The fountain solution is indispensable, and its acidity and alkalinity have a significant impact on the printing plate and ink balance. Alumina film layer formed by offset printing plate base treatment is amphoteric oxide. If the dampening solution acidity is too strong, it will increase the corrosion of the printing plate sand, destroy the blank part of the printing plate, and cause the printing plate to be dirty; and alkali If you are too strong, you will not be able to protect the blank area and it will corrode ink and paper. Therefore, the offset printing fountain solution is generally weakly acidic, and the ideal pH value is between 5 and 6. This ensures the clear printing dots and the hydrophilicity of the non-graphical part of the printing plate is stable, which helps to prolong the life of the printing plate, and can also suppress the printing plate life. The fountain solution is invaded by bacteria and other impurities to better maintain the ink balance.

The composition of the fountain solution is mainly buffering agent, preservative, printing plate protection agent, desiccant algae inhibitor, etc., wherein the buffer agent has the greatest influence on the pH value of the fountain solution. The role of the buffer is to stabilize the fountain solution because the tap water is usually neutral. In order to achieve the pH (5-6) required for normal printing, an acid is added to the fountain solution to maintain the pH of the fountain solution. It is weakly acidic, and the carbonate in the water reacts with the added acid to increase the pH. A buffer is added to maintain the pH of the fountain solution at a stable level. At the same time, the buffering agent has a significant effect on the impact of cleaning agents and other factors on the fountain solution. However, with the change in the amount of buffer added, the pH of the fountain solution will fluctuate more. The inappropriate addition will also affect the service life of the printing machine and will also hinder ink drying, especially for inks containing metallic pigments.

Water also has a great influence on the pH of the fountain solution. The water quality of tap water is divided into soft and hard. The water with high metal cation content is hard water, and the water with high anion content is soft water. High water hardness will increase the pH of the fountain solution. When formulating the fountain solution, the hardness of the water should be adjusted according to actual needs, preferably 10° dH. It is also important to note that changes in water temperature affect the concentration of ions in the water. Therefore, the hardness and temperature of the water can be controlled to achieve the best effect of the fountain solution.

To ensure that the pH of the fountain solution is relatively stable, an advanced method is to monitor the conductivity of the fountain solution while controlling the amount of buffer added and the hardness of the water.

Because the conductivity of the fountain solution will increase and the pH value will decrease as the concentration of the fountain solution increases, the hardness of the water, and the temperature of the water increase. Conversely, if alcohol is added to the fountain solution, the fountain solution conducts conductivity. The rate decreases and the pH increases. The conductivity of the fountain solution should generally be controlled at (1400±200)μs/cm, but due to the influence of dust, ink, and some other impurities, the conductivity is also allowed within 800μs/cm~2000μs/cm, but when the dampening When the fluid appears to be significantly contaminated, it must be reconstituted.

In summary, we can adopt more precise methods before printing production, such as measuring the pH value of the paper surface with a surface pH meter, measuring the pH and conductivity of the fountain solution with a liquid pH meter and a conductivity meter, and Control in a more reasonable range, effectively avoid the occurrence of pH-related problems in the printing process, improve the quality of printed matter, and reduce printing consumables. This is very important for the printing factory.

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