Analysis and Discussion on Offset Ink Balance
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In the offset printing process, the supply of ink is a very important link. The lithographic offset printing is based on the principle that the printing plate is partially lipophilic and water resistant, and the blank part is hydrophilic and oil resistant, and the oil and water are not mixed with each other.
The printing principle warns our printing workers: "Only when the water film in the blank part of the printing and the ink film in the graphic part have very strict limits, the oil and water do not infiltrate each other, and the ink-and-wash balance of the offset printing can be achieved.
Ink balance is the basis of lithography. In the offset process, the ink balance is just right, and has a very close relationship with the normal transfer of the print, the depth of the ink, the accuracy of the overprint and the dry state of the print. Therefore, the correct control and control of ink and water balance is the key to ensuring the quality of printed products.
First, the phenomenon of imbalance of ink and water balance
How to achieve the balance of ink and water, the operator must not only understand and study the organic relationship between ink and wash, the constraints between each other and the law of subtle changes between them, but also analyze and discuss the adverse consequences caused by the imbalance of ink and wash.
(1) Analysis of the phenomenon that there is less water and less ink: During the printing process, the printing plate has a certain water film in the blank part. When the water film and the oil can compete, it will not be stained by the ink on the ink roller. If the moisture is too small, the amount of the water layer can not resist the adsorption of the ink on the blank part, then the blank part will be stained with the ink, resulting in hanging dirty. If the amount of ink supply is small, the printed product will be dull, light gray, and the print will not be In fact, the imprint is covered with snowflake-like white spots. In this state, although the "ink-ink balance" is reached, this "ink-ink balance" is not the "ink-ink balance" we want.
(2) Analysis of the phenomenon of less water and more ink: At this time, it is most likely to produce uneven printing ink color, hanging dirty, a part or large layout due to lack of water, resulting in paste, paste. At the same time, the printed ink color of the printed product is also compared. Deep, the printed matter becomes black and the dots are not clear, especially for the reproduction of fine dots, and the images are indistinguishable.
(3) Analysis of the phenomenon of less water and less ink: If the moisture of the layout is too large, it will spread to the surface of all the ink rollers to form a certain thickness of water layer, which hinders the normal transfer of ink and accelerates the emulsification speed of the ink. Saturated, the text becomes lighter, the writing is imaginary, gray, hairy, hairy, dull and dull. There is dizziness around the imprint, the image is not clear, no level.
(4) Analysis of the phenomenon of large water and ink: When the water content of the plate is excessive, the color of the ink will become lighter, and the amount of ink will be blindly considered to be less, so the amount of ink will be increased continuously, and the ink will be emulsified for a long time. The vicious cycle of unbalanced ink and ink causes the ink to be heavily emulsified and accumulates on the surface of the ink roller, making printing impossible.
Second, 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, both non-polar and polar components, and there is the 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. Through the above analysis and discussion, it is known that the offset printing process makes it impossible for the ink to be emulsified at all. The key is to master properly and achieve ink and water balance.
Third, measures and control methods to ensure ink and water balance
(1) During the printing process, the printing plate must have a solid graphic foundation and a blank foundation to maintain the stability of lipophilic and hydrophilic.
(2) Under the premise of ensuring that the printing plate is not dirty, the water supply quantity should be controlled as small as possible (the water supply of the layout should be 26%), and the water supply quantity and the ink quantity should be relatively stable. Ensure that the ink color is consistent before and after the ink color 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 perfectly 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 is running. The speed is fast, the water volume can be slightly smaller, and it can be larger at low speeds.
(5) Environmental conditions and temperature and humidity should 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 at around 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 value of the fountain 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) The method of scientific instrument detection is used to control the 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 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.