Printing fairy water - fountain solution
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As we all know, the principle of offset printing is to use the characteristics of water and oil to resist each other for printing. The printed part of the printing plate has the characteristics of oleophilic and water-repellent. On the contrary, the non-printed part is hydrophilic and oil-repellent. Therefore, after the printing plate is wetted by water, the non-printed part forms a water film. When the ink passes through the printing plate, only the printed portion will be stained with ink, and a single printed piece will be printed smoothly. However, the water used to wet the plate is not ordinary water. It is actually a chemical with electrolyte added to the water; and what is added to the water is the fountain solution.
Dampening solution function
In the high-speed printing process, it is unlikely that the non-printed part will remain hydrophilic and oil-repellent for a long time, and the water film is adsorbed. In fact, the non-printing part is very attractive to water. It is limited. If it is wetted by ordinary water in the process of printing, it is not easy to form a water film in the blank part without image. In this way, we only use the dampening solution to stabilize the hydrophilic oil-repellent properties of the non-printed parts in order to obtain a good printing effect.
Why does the fountain solution print the quality of the print? Here, let's talk about the role of dampening solution in printing.
(1) Forming a water film on the non-printed portion
The formation of a water film is the primary function of the fountain solution. The fountain solution itself has a low surface tension, so it can be added to the water to spread the water on the non-printed surface. On the other hand, the electrolyte of the fountain solution will act on the metal of the plate. Chemical reaction, the production of inorganic salts, so that the water film adheres to the blank part of the printing plate, to prevent the ink from stretching to these parts, otherwise we will not be able to print the normal graphic.
(2) Supplementing the damaged hydrophilic layer
When printing, the roller and the ink roller rotate to rub the printing plate, causing the hydrophilic layer on the printing plate to be worn; at the same time, the paper has more or less paper powder and paper hair falling off during the printing process. Further deepen the wear and damage the hydrophilic layer. In this way, if no dampening solution is in the process, it will continue to chemically interact with the metal layer of the printing plate, forming a new hydrophilic layer again and again, and the offset printing cannot use the water-oil phase rejection property to print.
(3) reducing the surface temperature of the printing plate
The ink roller is operated at a high speed during printing, and the ink is drawn into a film for printing. Since the ink roller moves at a very high speed, its temperature rises, and the associated ink also changes due to temperature. At high temperatures, the viscosity of the ink is lowered and the fluidity is increased. Thus, when printing, the ink spreads rapidly, which tends to cause the dot to expand and affect the printing quality. However, in the process of printing, adding the dampening solution can effectively reduce the temperature of the printing plate and avoid the above problems.
Appropriate dampening solution dosage
How much dampening solution should I use for each printing to have the best printing results? No one can say this, because every printing factor changes every minute. No matter the printing plate, ink or paper, they are slightly changed, which is enough to affect the dosage of dampening solution. Therefore, the printing staff can only determine the correct dosage according to the printing environment at that time.
Plate
The most intimate contact with the fountain solution, of course, is the plate. There are many kinds of offset printing plates, generally there are PS version, paper version, silver salt version and so on. The structure of these plates is different, and the adsorption force of the plate on the water film is different, so the demand for dampening solution is also different. For example, the blank portion of the paper version has a stronger adsorption force than the PS plate and the silver salt plate, and it requires much less dampening solution than the other two plates. On the other hand, the picture and text type of the layout will also affect the amount and pH of the dampening solution. The printed graphics can be roughly divided into three categories, namely, real maps, texts, and dot maps. The amount of dampening solution is proportional to the amount of ink required to print the graphic, that is, when printing the map, it requires the most dampening solution, followed by the text, and the dot pattern is the least.
The pH is also related to the amount of ink used. The real map is generally printed with more ink, and the ink layer is thicker, so a dampening solution with a lower pH value is needed to prevent the ink from spreading outward from the edge of the ground. However, contrary to the real map, it is a dot pattern, because the dot area of the dot pattern is small. It is easier to emulsify and disappear, so it is not suitable for using a fountain solution with a low pH value, otherwise it will accelerate the emulsification. As for printing text, the pH of the dampening solution required is just between the real map and the dot pattern.
Ink
The second important factor is the ink. Different types of inks of different colors have different viscosity, fluidity and water resistance. Generally speaking, inks that are difficult to emulsify require a higher concentration of fountain solution, while inks that are easier to emulsify do not need to be used. The concentration of dampening solution. As for the relationship between the pH value and the concentration of the dampening solution and the ink, the factors considered are similar to the above: the ink which is not easy to be emulsified needs to be mixed with the dampening liquid which is heavier in acidity and higher in concentration; the ink which is easy to emulsify is changed. Only the fountain solution with high pH and low concentration can be used. The middle of the acid and alkali pairing is not adjustable.
Paper
As for the effect of paper on dampening solution, it is mainly from the PH value of paper texture paper. A loose paper, if its surface strength is low, then the paper and paper powder are easily adhered by the ink and piled on the printing tape, which increases the wear of the printing plate and makes it easy to get dirty. In this case, you have to increase the amount of dampening solution, or increase the concentration to reduce the accumulation Damage to the plate. In addition, different printing papers have different PH values, either acidic or alkaline. Therefore, the printer needs to adjust the pH of the fountain solution according to the acidity and alkalinity of the paper.
The dampening solution is called the printed "Shenxian Water", although it is a bit exaggerated, but the correct use of dampening solution can effectively control the ink balance and improve the quality of printing. There is still a lot of knowledge about dampening solution. The above mentioned is just one of the ones and one half of the claws. To truly understand the dampening solution, we have to work hard to study.