In the four printing methods of plain and concave soft holes, only one way is obviously different from the other three printing methods in the principle of ink transfer. This is lithography, which is what we usually call offset printing.

The basic printing principle in offset printing is to use the two laws of oil and water immiscibility and selective plate adsorption to make ink and water balance each other on the printing plate to realize the transfer of dots, so as to achieve a clear and full color image of the printed matter. Effect. The word "ink balance" comes from this.

I. Understanding of the Ink Balance

The ink and water on the printing plate must be present and balanced at the same time. The purpose is to maintain the maximum amount of ink loaded in the graphic printing area, so that the ink is bright, saturated, and the dots are clear and clean, while keeping the non-printing area highly clean and tidy. The balance between water supply and ink supply is called ink-water balance.

Offset ink balance refers to adjusting the amount of fountain solution at a certain printing speed and printing pressure so that the emulsified ink contains less than 26% of fountain solution, forming a slight W/O emulsion ink. Use a minimum amount of liquid to compete with the ink on the plate. Offset printing is the use of natural laws in which oil and water are incompatible with each other, forming graphic and blank parts on the printing plate. It supplies both ink and water to the printing plate. It absorbs water and resists water while the blank part absorbs water and resists oil. print. If the ideal balance of ink and water, the blank part of the printing plate adheres the fountain solution, and the graphic part is attached with the ink. Only when the surface tension of the fountain solution and the surface tension of the ink are equal, the diffusion pressure at the interface between the two is Zero, the fountain solution and the ink remain relatively balanced at the interface, do not infiltrate each other, in this case the printing effect is ideal. However, in actual printing, the balance of ink and water is a dynamic balance and it is changing all the time. If the amount of ink applied to the layout exceeds equilibrium, the ink will expand toward the blank after being strongly squeezed, and will infiltrate the blank surface causing stickiness. On the other hand, if the water supply to the layout exceeds the equilibrium value, when the water roller passes through the surface of the graphic, it will leave more water on the surface of the graphic, and then it will be strongly pressed by the water roller and the ink roller and attached to the graphic part. The deep emulsification of the ink layer causes the imprint to be dull. When the amount of water supplied is too large, the water enters the ink fountain along the ink fountain roller and the ink fountain roller, causing a large range of ink emulsification, and eventually the printing cannot be continued.

The ink balance is the basis for lithography. In the offset printing process, the balance of ink and ink is just right, and there is a very close relationship with the normal transfer of imprinting, the depth of ink, the accuracy of overprinting, and the dryness of the printed matter. Whether it is possible to correctly control and control the ink-and-wash balance is the key to ensuring the quality of printed matter.

Second, the conditions to produce ink and water balance

In the printing process, even if the water delivery device is adjusted and operated correctly, it is affected by the shutdown, the temperature and humidity of the printing shop, the running speed of the printing machine, the type of ink, the type of paper, and the blank area of ​​the layout with the increase in the number of printed copies. The restriction of the hydrophilic properties, etc. will directly affect the ink-water balance.

1. The printing plate must have a solid graphic foundation and a blank basis to keep the oleophilic and hydrophilic waters relatively stable. If a certain condition is not met, ink will lose its balance.

2. The thickness of the sand should be well controlled. The grain of the surface of the printing plate must be kept uniform and dense, with a certain physical strength, so that the graphic and blank parts have a firm attachment base and have good water storage conditions.

3. The workshop temperature and humidity should be constant. In principle, the workshop is required to have a constant temperature and constant humidity, because the change of the temperature will cause the viscosity and fluidity of the ink to change, and the temperature rise will cause the water to evaporate, thus causing the ink to lose its balance. Especially resin inks are sensitive to temperature. It is best to install an air conditioner and control the temperature within the range of 20°C to 25°C.

4. The machine should run at a constant speed. In the production process, the machine must run at a normal speed and at a constant speed. The machine is suddenly fast and slow, and it is easy for the ink and ink balance to lose control.

To master the characteristics of different types of paper, ink and other raw materials. The performance of printing inks and papers varies greatly. Different papers and inks are used, which have different requirements for the ink-water balance. If the paper has a close and loose texture, the water content is divided; the ink has the viscosity, fluidity, and the size of the pigment particles. In the same conditions as the printing plate, temperature, humidity, etc., it is necessary to grasp the changes in their properties.

Drying oil should be used in moderation. Putting dry oil in the ink promotes the drying of the ink, but the drying oil promotes ink stickiness. Therefore, the amount of dry oil is not the main factor that causes the ink balance to lose control and become dirty.

7. The dampening solution pH should not fluctuate. The pH of the fountain solution is also an important condition that affects the ink-water balance. It must be made constant to avoid large-scale fluctuations.

8. Adjust pressure properly. The press must have precise roller pressure, ink roller pressure, and water roller pressure. These three pressures directly determine the ink balance.

The principle of ensuring ink and water balance

To maintain the balance between water and ink, we must 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, odorless transparent liquid. Water is a dipole substance that dissolves with many other substances. But there are substances that are incompatible with it. For example, he is immiscible with oils. The substances that are compatible with them are also indistinguishable from each other, and 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 printing machine we use is hydrophilic. Made of aluminum.

Offset printing inks generally use inks with good water resistance, color, transparency, saturation, and purity. Oils are non-polar substances that are immiscible with water under normal conditions. Nothing in the world is purely a single element of matter, and so is the water repellency of ink. From the perspective of the molecular structure of oil, various types of fat in oil molecules are mainly composed of two parts:

One part is a very long hydrocarbon chain, which has water repellent and lipophilic properties and is called a water-repellent group. Another part of it has an oil-repellent and hydrophilic property, which is called a hydrophilic group. These two groups are related to each other and contradictory to each other. The completely opposite nature of the group exists in one molecule. The former is a hydrophobic, hydrophobic, water-repellent group that is non-polar. The latter is an oil-and water-repellent hydrophilic group. .

In other words, oil has two properties: it is both non-polar and polar. It has both oil-water incompatibility and oil-water miscibility. However, since the polar and carboxyl groups of the oil-repellent and oil-repellent groups are very few in the entire oil molecule, they are only subordinate to the oil molecules, while the non-polar hydrophobic and oleophilic groups account for an absolute majority of the total oil molecules. The occupies a dominant position in the oil, so the oil is immiscible with oil and water under normal temperature and pressure. Through the analysis and discussion above, we can see that it is impossible for the offset printing process to make the ink not emulsify at all. The key is to master it properly and achieve a balance between ink and water.

Measures to ensure ink balance and control methods

1. During the printing process, the printing plate must have a solid graphic foundation and a blank basis to maintain the stability of oleophilic and hydrophilic.

2. Under the premise of guaranteeing that the printing plate is not stained, control the water supply volume to be as small as possible (the water supply of the layout should be 26%), and make the amount of water supply and the amount of ink in a relatively stable state, so as to ensure The prints are consistent in depth before and after the ink and the print job is stable.

3. Master the principle of less ink thickness. There is less water here, assuming that the blank part of the layout is not stained. The so-called ink thickness is also based on less water. Due to the large amount of water caused by ink emulsification, the ink layer cannot be thick. From the process of the transfer of the offset ink, it can be seen that in a single water supply and ink supply, a total of three ink mixing and emulsification occur. It is impossible to maintain a strict boundary between water and oil. . Therefore, the balance of water and ink in offset printing can only be a relative concept, and the perfectly ideal ink-water balance does not exist. As long as the best balance is achieved, the ideal print can be printed.

4. According to the printing plate material type to select the size of the ink, the PS version of the water can be appropriately smaller, PVA version of the water can be slightly larger; smooth paper water can be slightly smaller, rough paper can be slightly larger, the speed of the machine operation Faster, the amount of water can be slightly smaller and can be larger at low speeds.

5. Environmental conditions and temperature and humidity cannot be ignored either. As the layout water is distributed in both direct and indirect forms, the layout moisture meets the needs of the ink-and-wash balance during printing, and most of them are distributed to the space. The higher the ambient temperature, the faster the emission will be.

6. It is necessary to control the pH of the fountain solution (usually controlled at around 4.5 to 5.5). In addition, since the pH of the surface of the offset printing paper has a great influence on the pH of the fountain solution, it is preferable to measure the pH of the paper. If the pH value of the paper is too low, the pH of the fountain solution should be slightly increased. On the other hand, if the pH of the paper is too high, the pH of the paper should be properly reduced so that it can neutralize the OH- of the paper. Excessive increase in pH of fountain solution. According to relevant data and conclusions drawn from practice, when the pH value of the paper is 9, the pH of the fountain solution is 4; when the pH value of the paper is 8, the pH of the fountain solution is 5 as good.

7. Use scientific instrumentation to control the ink balance. Because the ink obtained under the ink-balanced state is necessarily thick and uniform in density, the change in water-ink balance during the printing process can be detected by continuously measuring the density. When the density value reaches the standard value, it can be concluded that the ink balance is normal.

In addition, 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 used (paper, ink, plate, blanket, etc.) as well as working environment differences and other related factors. , so that the ink reaches or basically meets the balance required for the printing process.

Source: Bison