Multicolor offset press mixing causes
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For two-color or multi-color offset presses, the overprinting method is wet-stacked, unlike the wet-stack printing method of a monochrome machine. In general, the drying speed of the ink is ten minutes, and when the two-color or multi-color offset printing machine is overprinted, the gap time between the two colors printed is only about 0.5 second. In such a short period of time, it is impossible to completely fix the ink (especially the solid block portion) on the paper surface, which is the most fundamental reason for the color mixing phenomenon of the two-color or multi-color offset printing machine. The main factors affecting the color mixing phenomenon of the offset printing machine are as follows.
1 ink viscosity
Generally, the printing color sequence of multi-color machine is black, cyan, red and yellow, that is, the first black is printed first, and the color sequence is arranged according to the principle of the viscosity of the ink from high to low to prevent the latter color ink. Adsorb the previous color ink. But sometimes the manuscript requires that the full-page color be biased toward the color of a certain ink, such as black, then the black must be placed in the last set of prints (usually black is full-color). At this time, considering that the viscosity of the black ink is large, a certain amount of the viscosity reducing agent should be added to the black ink to lower the viscosity, thereby reducing the occurrence of color mixing.
2 ink coloring rate
In actual production, most of the multi-color machines use domestic inks, and in addition, printing aids (mainly viscosity reducing agents) are placed, 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, to increase the thickness of the ink layer. However, when the ink is not dried, the thicker the ink layer, the more easily it is destroyed by an external force (printing pressure, peeling tension), and color mixing is liable to occur.
3 ink drying speed
In actual production, the operation of the machine causes the rubber roller to gradually heat up, especially in the summer, the viscosity of the ink on the rubber roller gradually increases. For example, when the two-color machine prints red and blue, the printing is normal, but when printed to several thousand sheets, the magenta ink has a tendency to gradually darken. At this point, the magenta rubber roller can be completely cleaned with gasoline, and then re-inked, but the color should 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 printing water to make the printing of the water more compact, so that the amount of ink must be increased accordingly, that is, we often Said "water" big ink, this is more taboo. Because this increases the ink staying on the rubber roller, which is equivalent to increasing the diameter of the ink roller, increasing the pressure between the rollers, making the ink roller hotter and hotter during operation, and the ink is more dry, resulting in a series of vicious cycles. Produce color mixing.
5 Do a good job of cleaning the PS version
When printing, it is necessary to level the pressure of the water and the ink roller, pay attention to the pH value of the fountain solution, and prevent the dirty parts of the parts (both sides and dragging) that are not used by the printing plate. If the blank part is dirty, it will be transferred to the blanket and the impression cylinder, and then transferred to the ink roller of the other group, which will also cause color mixing. In addition, the color code at the time of plate making and the signal strip at the time of 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.