Ink and ink balance control of lithographic offset printing (1)
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Among the four printing methods of flat and concave soft holes, only one way is obviously different from the other three printing methods in the principle of ink transfer. This is lithographic printing, which is what we usually call offset printing.
The most basic printing principle in offset printing is to use two rules of oil-water incompatibility and selective adsorption of printing plates to maintain the balance of ink and water on the printing plate to achieve the transfer of dots, and to achieve clear and full-color print images. Effect. The word ink and water balance comes from this.
First, the 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 in the printing area, make the ink bright, saturated, clear and clean, and keep the non-printing area clean and tidy. The balance between the amount of water supplied and the amount of ink supplied is called the ink balance.
Offset ink balance refers to: adjusting the supply amount of dampening solution under a certain printing speed and printing pressure, so that the ratio of dampening liquid contained in the emulsified ink is less than 26%, forming a slight W/O type emulsion ink. With the minimum amount of liquid and the ink on the plate to compete. Offset printing is a natural law that uses oil and water to be incompatible with each other. It forms a graphic and a blank part on the printing plate. It supplies both the ink and the water supply to the printing plate, and absorbs oil against the water through the layout, while the blank part absorbs water and oil. print. If the ideal part of the ink is in equilibrium, the blank part of the printing plate adheres to the dampening solution, and the ink part is attached to the image. Only when the surface tension of the dampening solution and the surface tension of the ink are equal, the diffusion pressure at the interface between the two is Zero, dampening solution and ink remain relatively balanced on the interface, and do not infiltrate each other. In this case, the printing effect is ideal. However, in actual printing, the ink-and-wash balance is a dynamic balance that is constantly changing. If the amount of ink supplied to the layout exceeds the balance, the ink expands toward the blank portion after being strongly pressed, and invading the blank surface causes stickiness. On the other hand, if the water supply to the plate exceeds the balance value, when the water roller passes through the surface of the image, it will leave more water on the surface of the image, and then it will be strongly squeezed by the water roller and the ink roller, and attached to the graphic part. The ink layer is deeply emulsified, causing the print to be dull. Moreover, when the water supply amount is too large, the water enters the ink fountain along the inking roller and the squeezing roller, causing a large range of ink emulsification, and finally the printing cannot be carried out.
Ink balance is the basis for lithography. In the offset printing 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. The correct mastery and control of ink and water balance is the key to ensuring the quality of printed matter.
Second, the conditions for the production of ink and water balance
In the printing process, even if the water delivery device is adjusted and operated correctly, due to the impact of the shutdown, the temperature and humidity of the printing workshop, the running speed of the printing machine, the type of ink, paper, and the blank space of the layout increase with the number of printed copies. The restriction of the hydrophilic property and the like will directly affect the ink and water balance.
1 Plates must have a strong graphic basis and a blank foundation to maintain relative oleophilic and hydrophilic stability. If a certain condition is not available, the ink will be out of balance.
2 The thickness of the sand should be controlled. The sand surface of the printing plate must be kept even and fine, with a certain physical strength, so that the graphic and blank parts have a strong attachment base and good water storage conditions.
3 The temperature and humidity of the workshop should be constant. In principle, the workshop is required to have constant temperature and constant humidity conditions, because the change of temperature causes changes in the viscosity and fluidity of the ink, and the temperature rise causes the water to evaporate, thereby causing the ink to lose balance. Especially resin inks are very 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 production, the machine must be running at a normal speed and at a constant speed. The machine flies quickly and easily, and the ink balance is easily lost.
5 To grasp the characteristics of different types of raw materials such as paper and ink. The performance of inks and paper used in printing varies greatly. Different papers and inks are used, which have different requirements for ink balance. For example, if the paper has a tight texture and looseness, the water content is divided; the ink has viscosity, fluidity, and pigment particle size. Under the same conditions of printing plate, temperature, humidity, etc., it is necessary to grasp the changes in their properties.
6 The amount of dry oil should be moderate. The drying oil in the ink promotes the drying of the ink to accelerate, but the drying oil causes the ink to stick. Therefore, the discomfort of the dry oil is the main factor that makes the ink balance uncontrolled and dirty.
7 The pH of the dampening solution should not fluctuate. The pH of the fountain solution is also an important condition affecting the balance of ink and water. It must be made constant to avoid large fluctuations.
8 Correctly adjust the pressure. The press must have precise drum pressure, ink roller pressure and water roller pressure, which directly determine the ink balance.