Ink and ink balance control of lithographic offset printing (2)
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Third, the principle of ensuring ink and water balance
To maintain the balance of ink and water, you should first manage the water. A deep understanding of the nature and role of water is the basis for managing good water. Water is widely distributed in nature, he is a colorless and odorless transparent liquid. Water is a dipolar substance that dissolves with many other substances. But there are also substances that are incompatible with it. For example, he is incompatible with oils, and the substances that are compatible with them are also intimate, and can be seen from the order of the hydrophilic properties of the metals.
The following arrangement is the order of hydrophilicity from strong to weak: potassium, calcium, sodium, magnesium, aluminum, zinc, chromium, iron, nickel, tin, lead, etc., the plate machine we use is better hydrophilic. Made of aluminum metal.
Offset printing inks generally use inks with good water resistance, bright colors, transparency, saturation and purity. Oil is a non-polar substance that is immiscible with water under normal conditions. Nothing in the world is a purely single elemental substance, as is the water repellency of inks. From the molecular structure of oil, all kinds of fats in oil molecules are mainly composed of two parts: one part is a long hydrocarbon chain, which has water-repellent and lipophilic properties, called water-repellent group; The oil is hydrophilic and is called a hydrophilic group. These two groups are related to each other and contradict each other. The completely opposite group exists in one molecule. The former is a hydrophobic and oleophilic water-repellent group, which is non-polar, and the latter is an oil-repellent hydrophilic group with polarity. .
That is to say, the oil has duality, which is both non-polar and polar, and there is a possibility that the oil and water are immiscible and that the oil and water are miscible. However, since the polar carboxyl group of the hydrophilic oil-repellent oil is extremely rare in the entire oil molecule, it is only subordinate in the oil molecule, and the non-polar water-repellent lipophilic group accounts for an absolute majority in the entire oil molecule. It takes a dominant position, so the oil is characterized by oil and water being immiscible under normal temperature and pressure. According to the above analysis and discussion, it is impossible for the offset printing process to make the ink not emulsified at all. The key is to master properly and achieve ink and water balance.
Fourth, measures and control methods to ensure ink and water balance
1 During the printing process, the plate must have a solid graphic foundation and a blank foundation to maintain oleophilic and hydrophilic stability.
2 Under the premise of ensuring that the printing plate is not dirty, the water supply quantity should be controlled to the smallest possible range (the water supply of the layout should be 26%), and the water supply quantity and the ink quantity should be relatively stable, so as to ensure the printed matter. The ink color is consistent before and after and the printing operation is stable.
3 Master the principle of less water and less ink. The amount of water referred to here is based on the premise that the blank part of the layout is not dirty. The so-called thick ink is also based on the lack of water. The water is large and the ink is emulsified. The ink layer cannot be thick. It can be seen from the inkjet ink transfer process that in a single water supply and ink supply, a total of three ink mixing and emulsification occurs. It is impossible to maintain a strict boundary between water and oil. . Therefore, the ink-and-wash balance in offset printing can only be a relative concept, and the perfect ideal ink-and-wash balance does not exist. As long as the best balance is achieved, the ideal print can be printed.
4 According to the material type of the printing plate, the size of the ink can be selected. The water volume of the PS plate can be appropriately smaller. The water volume of the PVA plate can be slightly larger; the smooth paper can be slightly smaller, the rough paper can be slightly larger, and the machine runs faster. The amount of water can be slightly smaller, and it can be larger at low speeds.
5 Environmental conditions and temperature and humidity can not be ignored. Since the moisture of the layout is distributed in both direct and indirect forms, the moisture of the layout meets the needs of ink balance during printing, and most of it is distributed to the space. The higher the ambient temperature, the faster the emission.
6 The pH of the dampening solution must be controlled (generally controlled from 4.5 to 5.5). In addition, since the pH of the surface of the offset printing paper has a large influence on the pH of the dampening solution, it is preferable to measure the pH of the paper. If the pH of the paper is too low, the pH of the fountain solution should be slightly increased. Conversely, if the pH of the paper is too high, the pH should be appropriately lowered to neutralize the OH- of the paper, thereby buffering The pH of the dampening solution is excessively elevated. According to the conclusions of the relevant data and practice, when the paper PH value is 9, the pH value of the dampening solution is 4; when the PH value of the paper is 8, the pH value of the dampening solution is 5 is better.
7 Scientific instrumentation is used to control ink and water balance. Because the printing obtained under the state of ink balance is inevitably thick and the density is uniform, the change of the ink balance in the printing process can be detected by continuously measuring the density. When the density value reaches the standard value range, it can be inferred that the ink balance state is normal.
In addition, in the work, in addition to considering the offset transfer process of water and ink, the operator should also consider the different types of raw and auxiliary materials (paper, ink, plate, blanket, etc.) used and the differences in working environment. In order to achieve or achieve a balance that is suitable for the printing process.