Among the four printing methods of flat concave and soft hole, only one method is obviously different from the other three printing methods on the principle of ink transfer. This is lithography, which is what we usually call offset printing.
The most basic printing principle in offset printing is to use the two rules of incompatibility of oil and water, and selective adsorption of the printing plate to keep the ink and water balanced on the printing plate to achieve the transfer of the dots, and to achieve clear images and full colors Effect. This is where the term ink balance is derived.
1. Understanding the balance of ink and wash
The ink and water on the printing plate must exist and maintain balance at the same time.The purpose is to maintain the maximum ink load of the graphic printing area, make the ink color bright, saturated, and the dots clear and smooth, but also keep the non-printing area highly clean and tidy. The balance between water supply and ink supply is called water-ink balance.
Offset water-ink balance refers to: under a certain printing speed and printing pressure, adjust the supply of fountain solution to make the emulsified ink contain less than 26% fountain solution, forming a slight W / O emulsion ink 3. Use the minimum liquid supply to compete with the ink on the printing plate. Offset printing is to use the natural law of incompatibility of oil and water to form graphics and blank parts on the printing plate. It supplies both ink and water to the printing plate. It absorbs oil and resists water, and the blank part absorbs water and resists oil. print. If in the ideal water-ink balance state, the blank part of the printing plate is attached with fountain solution, and the graphic part is attached with 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 ink maintain relative balance on the interface and do not wet each other. In this case, the printing effect is ideal. However, in actual printing, the water-ink balance is a dynamic balance, which is constantly changing. If the amount of ink supplied to the plate exceeds the balance, the ink will expand toward the blank portion after being strongly squeezed, invading the blank surface and causing stickiness. Conversely, if the amount of water supplied to the layout exceeds the equilibrium value, when the water roller passes through the surface of the graphic, more water will be left on the surface of the graphic, and then it will be strongly squeezed by the water roller and the ink roller and adhere to the graphic The ink layer is deeply emulsified, resulting in a dull imprint. And when the water supply volume is too large, the water will enter the ink fountain along the inking roller and the inking roller, causing a wide range of ink emulsification, and eventually printing will not be able to continue.
The balance of water and ink is the basis of lithography. In the offset printing process, whether the water-ink balance is just right has a very close relationship with 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. Whether or not the ink-ink balance can be properly controlled and controlled is the key to ensuring stable print quality.
Second, the conditions for generating ink and ink balance
During the printing process, even if the water conveying device is adjusted and operated correctly, due to the impact of the shutdown, the temperature and humidity changes in the printing workshop, the operating speed of the printing machine, the type of ink, paper, and the blank space of the layout increase with the number of printed copies Restrictions such as weakening the hydrophilic property will also directly affect the water-ink balance.
1 The printing plate must have a solid graphic and blank foundation to keep the lipophilic and hydrophilic relative stable. If a certain condition is not met, the ink and wash will be out of balance.
2 The thickness of the sand should be controlled well. The grit on the surface of the printing plate must be kept even and fine, with a certain physical strength, so that the graphics and blank parts have a solid attachment foundation and good water storage conditions.
3 The temperature and humidity of the workshop should be constant. In principle, the workshop is required to have conditions of constant temperature and constant humidity, because changes in temperature will cause changes in the viscosity and fluidity of the ink, and the increase in temperature will evaporate water, which will make the water and ink lose balance. Especially resin ink is very sensitive to temperature. It is best to install an air conditioner to control the temperature within the range of 20 ℃ ～ 25 ℃.
4 The machine should run at a constant speed. In production, the machine must maintain normal speed and uniform speed. The machine will quickly and slowly, and it is easy to lose control of the water and ink balance.
5 To grasp the characteristics of different types of paper, ink and other raw materials. The performance of ink and paper used in printing differs greatly. The use of different papers and inks has different requirements for water-ink balance. For example, paper has a close texture and loose texture, and a moisture content; ink has viscosity, fluidity, and pigment particle size. Under the same conditions of printing plate, temperature, humidity, etc., we must grasp the changes of their properties.
6 The amount of drying oil should be moderate. Putting dry oil in the ink promotes the drying of the ink, but the dry oil promotes the stickiness of the ink. Therefore, the unsuitable amount of dry oil is the main factor that makes the balance of the water and ink out of control and the dirty version.
7 The pH value of dampening dilution should not fluctuate. The PH value of fountain solution is also an important condition that affects the balance of water and ink. It must be made constant to avoid large-scale fluctuations.
8 Adjust the pressure correctly. The printing press must have precise cylinder pressure, ink roller pressure, and water roller pressure. These three pressures directly determine the water-ink balance.
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