For two-color or multi-color offset presses, the overprinting method is wet stacking wet, which is different from the wet stacking dry printing method of monochrome machines. Under normal circumstances, the drying speed of the ink is more than ten minutes, and when the two-color or multi-color offset press is overprinting, the gap time between the two colors overprinted is only about 0.5 seconds. In such a short time, it is impossible to completely fix the ink on the paper surface (especially the solid part), which is the most fundamental reason for the color mixing phenomenon of the two-color or multi-color offset press. The main factors affecting the color mixing phenomenon of the offset press are the following.
1 Ink viscosity
The printing color sequence of general multi-color machines is black, cyan, red, and yellow, that is, black is printed first and yellow is printed. Absorb the previous color ink. But sometimes the manuscript requires that the color of the whole page be biased to the color of some ink, such as black, then black must be printed in the last group (generally black is the full color). At this time, considering that the black ink has a high viscosity, a certain amount of viscosity reducer should be added to the black ink to reduce its viscosity, so as to reduce the occurrence of color mixing.
2 Ink coloring rate
In actual production, most of the multi-color machines use domestic inks, plus printing aids (mainly tackifiers), so that the coloring rate is lower than the standard coloring rate index. In order to achieve the same solid density value during printing, it is necessary to increase the amount of ink, that is, increase the thickness of the ink layer. However, when the ink is not dried, the thicker the ink layer, the easier it is to be damaged by external forces (printing pressure, peeling tension), and it is easy to produce color mixing.
3 Drying speed of ink
In actual production, the operation of the machine causes the rubber roller to gradually heat up, especially in summer, the viscosity of the ink on the rubber roller gradually increases. For example, the two-color machine prints normally when printing magenta and cyan, but when printing to thousands of sheets, the magenta ink has a tendency to gradually darken. At this time, the magenta rubber rollers can be completely cleaned with gasoline, and then re-inked for printing, but pay attention to the control color must be consistent with the original color.
4 Ink balance control
The quality of offset printing products has a great relationship with the fountain solution. In actual operation, we often control the "water" to be large so that the printed products do not paste, so that the amount of ink must be increased accordingly, that is, we often The "water" that is said is big, and this is the taboo of offset printing. Because this increases the ink staying on the rubber roller, which is equivalent to increasing the diameter of the ink roller and increasing the pressure between the rollers, making the ink roller hotter and hotter during operation, the easier the ink is to dry, causing a series of vicious cycles and easy Produce color mixing.
5 Clean the PS version
During printing, the pressure of the water and ink roller should be leveled, and the pH of the fountain solution should be paid attention to to prevent the dirty version of the unused parts of the printing plate (on both sides and slightly). If the blank part is dirty, it will be transferred to the blanket and impression cylinder, and then transferred to another group of ink rollers, which will also cause color mixing. In addition, the color code during plate making and the signal bar during printing (mainly used to measure the solid density value during proofing) are also exposed on the plate, and this part needs to be repaired during printing to prevent color mixing.
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