Analysis and Discussion on Offset Printing Water-ink Balance
In the process of offset printing, the supply of water and ink is a very critical link. Lithographic offset printing is based on the principle that the pattern part of the printing plate is lipophilic and water resistant, the blank part is hydrophilic and oil resistant, and oil and water are not mixed with each other.
Printing principle Printing worker warned us: "Only when the water film and graphic part of the ink film printed blank portion there is a very strict boundaries, and do not water infiltration, in order to achieve a balance offset printing ink.
The water-ink balance is the basis of lithography. In the offset printing process, whether the water-ink balance is just right is closely related to the normal transfer of the blot, the shade of the ink color, the accuracy of the overprint, and the dry state of the printed matter. Therefore, whether the ink-ink balance can be correctly mastered and controlled is the key to ensure the stability of printed products.
1. Analysis of the phenomenon of imbalance of ink and ink
How to achieve ink-and-ink balance, the operator must not only understand and study the organic relationship between ink and ink, the constraints between them and the rules of subtle changes between them, but also analyze and discuss the undesirable consequences caused by the imbalance of ink and ink.
(1) Analysis of the phenomenon that there is little water and little ink: In the printing process, there is a certain water film in the blank part of the printing plate. When the water film and the oil can contend, it will not be stained by the ink on the ink roller. If the moisture is too small, the amount of water layer can not resist the adsorption of ink on the blank part, the blank part will adhere to the ink, resulting in hanging dirt, and the amount of ink supply will make the printed matter dull, light gray, and imprinted. In fact, the blots are covered with snowflake-like white dots. In this state, although the "water-ink balance" is also reached, this "water-ink balance" is not the "water-ink balance" we want.
(2) Analysis of the phenomenon of less water and more ink: At this time, the ink color of the printed product is most likely to be uneven, hang dirty, and a part or a large section of the layout causes paste and paste characters due to lack of water. At the same time, the printed ink color of the printed product is also compared Deep, making the printed product become black and sloppy, the dots are not clear, especially for the reproduction of fine dots, and the image is not clear.
(3) Analysis of the phenomenon of less water and ink: If the water content of the plate is too large, it is gradually spread to all the ink roller surfaces to form a water layer of a certain thickness, which hinders the normal transfer of ink. The speed of ink emulsification is accelerated, and the color of the printed ink is gradually not. Saturated, the text becomes lighter, the handwriting is dull, gray, hairy, flowery, dull and dull. There is a halo around the imprint, the image is not clear, and there are no layers.
(4) Analysis of the phenomenon of large amount of water and ink: when the water content of the plate is excessive, the ink color will become lighter, and it will often blindly think that the amount of ink supply is small, so the amount of ink is continuously increased. , Resulting in a vicious cycle of water and ink imbalance, resulting in serious emulsification of the ink and accumulation on the surface of the ink roller, making printing impossible.
Second, the principle of ensuring ink-water balance
To maintain the balance of water and ink, we should first manage the water. A deep understanding of the nature and function of water is the foundation for good water management. Water is widely distributed in nature, he is a colorless and odorless transparent liquid. Water is a dipolar substance and can be dissolved with many other substances. But there are also substances that are incompatible with it. For example, it is incompatible with oils, and it is also incompatible with substances that are compatible. It can be seen from the order of the hydrophilic properties of metals. The following arrangement is the order of the hydrophilic properties from strong to weak: potassium, calcium, sodium, magnesium, aluminum, zinc, chromium, iron, nickel, tin, lead, etc. The plate machine we use is the one with better hydrophilic properties 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 and is immiscible with water under normal conditions. Nothing in the world is a pure single-element substance, so is the water repellency of ink. From the molecular structure of oil, all kinds of fats in oil molecules are mainly composed of two parts: one part is a very long hydrocarbon chain, which has water-repellency and lipophilicity, and is called a water-repellent group; Oil hydrophilicity, called hydrophilic group. These two groups are interconnected and contradict each other. Groups with completely opposite properties exist in a molecule. The former is a hydrophobic and lipophilic water-repellent group, which is non-polar, and the latter is an oil-repellent hydrophilic group, which is polar. . In other words, oil has duality, both non-polar and polar components, there is both oil-water immiscibility, and the possibility of oil-water miscibility. However, since the polar carboxyl groups of the hydrophilic oil repellent are very few in the entire oil molecule, they are only subordinated in the oil molecule, while the non-polar water-repellent lipophilic groups account for the absolute majority in the entire oil molecule. Because of its dominant position, oil is immiscible in oil and water at normal temperature and pressure. Through the above analysis and discussion, we can see that 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-ink balance.
3. Measures and control methods to ensure water-ink balance
(1) During the printing process, the printing plate must have a solid graphic and blank foundation to maintain lipophilic and hydrophilic stability.
(2) On the premise of ensuring that the printing plate is not stained, the water supply volume should be controlled as small as possible (the water supply of the plate should be 26%), and the water supply volume and the ink volume should be in a relatively stable state, so that Ensure that the ink color of the printed matter is consistent before and after and the printing operation is stable.
(3) Master the principle of less water and thick ink. The less water here refers to the premise that the blank part of the layout is not stained. The so-called thick ink is also based on less water. Ink is emulsified due to large water, and the ink layer cannot be thick. From the transfer process of offset ink, it can be seen that in a water supply and ink supply, a total of three times the mixing and emulsification of ink and water, it is impossible to maintain a strict boundary between water and oil. . Therefore, the ink-water balance in offset printing can only be a relative concept, and a perfectly ideal ink-water balance does not exist. As long as the optimal balance is reached, the ideal print can be printed.
(4) Choose the size of the ink according to the material type of the printing plate. The water volume of the PS version can be appropriately smaller, the water volume of the PVA version can be slightly larger; the water volume of the smooth paper can be smaller, the rough paper can be larger, the machine runs The speed is fast, the water volume can be slightly smaller, and it can be larger at low speed.
(5) Environmental conditions, temperature and humidity cannot be ignored either. Since the moisture on the layout is distributed in two forms, direct and indirect, while the moisture on the layout meets the needs of water-ink balance during printing, most of it is distributed to the space.
(6) PH value of fountain solution must be controlled (generally controlled at about 4.5 to 5.5). In addition, since the PH value of the offset printing paper surface has a great influence on the PH value of the fountain solution, it is best to measure the PH value of the paper. If the PH value of the paper is too low, the PH value of the fountain solution should be increased slightly; on the contrary, if the PH value of the paper is too high, the PH value should be appropriately lowered so that it can neutralize the OH- of the paper, thereby buffering Excessive PH value of fountain solution. According to the conclusions stated in relevant materials and practice, when the paper PH value is 9, the fountain solution PH value is 4; when the paper PH value is 8, the fountain solution PH value is 5 is better.
(7) Use the method of scientific instrument detection to control the water-ink balance. Because the ink obtained in the state of water-ink balance must have thick ink color and consistent density, the change of water-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 concluded that the water-ink balance is normal.
In the work, in addition to the offset printing process of water and ink, the operator should also consider the different types of raw and auxiliary materials used (paper, ink, plates, blankets, etc.) and the differences in working environment, and other related factors. In order to make the ink reach or basically reach the balance suitable for the printing process requirements.
Bamboo Sushi Series,Bamboo Sushi,Sushi Bamboo,Bamboo Sushi Mat
Nanchang Sou Bamboo Network Technology Co., Ltd. , https://www.168bamboo.com