Wetting offset water-ink balance of printing plate surface
Two offset printing ink balance
Theoretically: only when there is a strict boundary between the water film in the blank part of the printing plate and the ink film in the graphic part, oil and water do not infiltrate each other, and the water-ink balance of offset printing is achieved. However, it can be seen from the offset ink and ink transfer process: in a water supply and ink supply cycle, a total of several times the mixing and emulsification of ink and water, the whole process is dynamic, it is impossible to maintain a strict boundary between the water phase and the oil phase Therefore, in the actual printing process, the ideal water-ink balance does not exist. The water-ink balance of offset printing can only be a relative concept, and its ink emulsification is inevitable.
The emulsification of offset inks is not only inevitable, but also required for the transfer of offset inks. Absolutely non-emulsified inks cannot be used for offset printing. The ink and the wetting fluid must be incompatible with each other, but to a certain extent they are miscible, that is, they must form a mixture of fine water droplets dispersed in the ink. The emulsification of water in the ink provides a way to eliminate the wetting fluid in the graphic part. Otherwise, the printed matter will appear flowering. On the contrary, the fine ink beads are not allowed to be dispersed in the water, so as to prevent the blank part from becoming dirty.
The so-called "emulsion" is a liquid in the form of fine droplets dispersed in another liquid that is immiscible with it, and "oil" and "water" are just two substances that are incompatible with each other. If oil is the disperse phase and water is the dispersing medium, the resulting emulsion is called "oil-in-water emulsion" and is represented by the symbol "O? W". That is, the oil is dispersed in water. Conversely, if the emulsion formed by dispersing water in oil is called "water-in-oil emulsion", it is represented by the symbol "W? O"
Figure 5-1 Types of emulsion
"Oil-in-water emulsion" is extremely harmful to the quality of offset printing and the normal production of offset printing. It will make the blank part of the printed matter dirty, water flushing will occur, and the ink roller will be deinked, and the ink cannot be transferred. . The microscopic "water-in-oil" emulsified ink is not only required for the transfer of offset printing ink, but also reduces the viscosity of the ink slightly, which can improve the fluidity of the ink and facilitate the transfer of the ink to the paper. However, serious "water-in-oil" emulsified ink will cause the viscosity of the ink to drop sharply, the ink filament will become shorter, and the ink transfer performance will be worse. At the same time, the wetting liquid immersed in the ink will also corrode the metal ink roller, forming a hydrophilic layer on the surface of the ink roller to repel the ink, causing the metal ink roller to deink. In the offset printing process, the blank part of the printing plate must always maintain a certain thickness of water film to make the printing proceed normally. The thickness of the water film on the printing plate is related to the water content of the ink on the printing plate. The thicker the water film, the greater the water content in the ink. In order to ensure the quality of the printed matter and the normal production, combined with the rule of water and ink transmission in the printing process, the meaning of offset ink and water balance should be:
Under a certain printing speed and printing pressure, the supply of dampening fluid and the supply of graphics and text are controlled. After emulsified ink, the volume fraction of the dampening fluid is between 15% and 26%, forming a slight "water-in-oil" emulsified ink, and the minimum liquid supply is used to compete with the ink on the printing plate.
What does it mean to compete with the ink on the printing plate with the least amount of liquid supply? This essentially means that the ratio of the thickness of the water film and the thickness of the ink film on the printing plate should be 1: 2 after infrared detection.
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