Talk about ink and water balance in 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.
First, understanding of ink and water 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. A stenciled offset printing plate is divided into a hydrophilic blank portion and a lipophilic graphic portion. When printing, the ink is supplied first after the water is supplied, so that the blank portion and the graphic portion selectively adsorb water and ink to form uniform water. Film and ink film. Only when there is a strict demarcation line between the ink film and the water film of the blank part of the printing plate graphic part and the oil and water do not infiltrate each other, the ink and water balance of offset printing can be achieved. However, in the actual production process, it is impossible to maintain a strict dividing line between the aqueous phase and the oil phase. Therefore, the ideal ink-and-wash balance in this theory does not exist. Balance is only a relative concept. What we want to achieve is a relative balance of ink and wash. This balance is only a temporary unity of contradiction, and imbalances are often there. In the actual printing process, ink and wash from the imbalance to the relative balance, and then from the new imbalance to the new relative balance, this relative balance is a more complex and difficult to master the process, only in the long-term production Constantly sum up experience to master its regularity.
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 plate must have a solid graphic foundation and a blank foundation to keep the oleophilic and hydrophilic properties 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.
5 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.
6 The amount of dry oil should be moderate. 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 Fluent dilution 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.
Third, the law of ink and water balance
Photographic imprints on plates are represented by ink, and the amount of ink supplied in printing has a great influence on the quality of printed matter. If the amount of ink is too small, the printed matter will show that the dots are not full, and the ink color is dull and dull; the excessive ink will cause the paste plate, the graphic network dot to be expanded excessively, and the ink will be splashed, and the blank part of the printing will be stained on the dirty and back side. Dirty, etc., seriously affect the quality of the product. In actual production and application, there are often phenomena such as large ink water, small thick ink, large ink, small ink, and small water. In terms of ink volume control, it should be determined based on the content of the layout, proof requirements, and paper properties.
1 layout text. In actual map printing, the amount of ink can be printed thicker; when the printed dot or text line is printed, it can be printed full on a clear basis, but the field density should be within the control range.
2 ink coloring power. Some inks have poor coloring power, and the imprinted prints are dull and lifeless. They are not printed in real time, but can be adjusted to a larger amount.
3 single, double-sided printing. For single-sided printing, the printing ink layer can be thicker than the double-sided printing ink layer.
4 Paper properties. Coated paper, glass paper jam, such paper surface is smooth, absorption is slow, should make the ink smaller. However, when printing real maps, the amount of ink should not be too small. Thin paper, printing ink layer should not be printed thick, and rough absorption of large paper, the amount of ink can be larger.
The thickness of the printed ink layer on the print and the thickness of the ink on the ink roller can be used to scratch the stripe to observe the amount of ink on the ink roller, thereby estimating the amount of ink on the layout and the surface of the print.
In summary, the amount of ink supply in printing should be adjusted according to the specific conditions of the production site at the time. By analyzing the specific reasons that affect the relative balance of ink and water, we must take necessary measures to reasonably increase or decrease the ink and wash to avoid product quality accidents caused by ink and water imbalance.