In color packaging printing, the attachment of the latter color ink on the previous color ink film is called ink overprinting. The color effect of printed matter is not only related to the coloring force of ink, the color difference of ink, but also related to the overprint rate of ink. The overprint rate of the ink is also called the amount of ink received by the ink, which refers to the percentage of the main density of the previous color ink layer transferred to the overlapping color block.
 
At present, many printing presses used by carton printing enterprises are multi-color water-based printing slotting machines, which often encounter the problem of low overprint rate. The color after printing can not completely cover the previous color, and there are pockmarks, which seriously affect the printing effect and cause waste of cardboard. This phenomenon is particularly obvious in large area field printing. How to improve the overprint rate of water-based ink?
 
Speed up the drying of the first color ink on the substrate.
 
1) When printing with a four-color printing slotting machine, the two inks that need to overlap are set respectively in the first color group and the last color group, which can appropriately increase the drying time of the first color ink layer, which is conducive to the improvement of the overprint rate.
 
2) When printing the first color, try to use fast drying ink, which can speed up the penetration and fixation of the first color ink. Of course, it can not dry too fast, otherwise the ink will dry on the roller, affecting the transfer of ink.
 
3) Pay attention to the arrangement of printing color sequence. Try to print the color with the smaller graphic area first, and then print the color with the larger graphic area. The advantage of this is that the first color printing has a small graphic area and dries faster, which is conducive to the overprinting of ink; At the same time, due to the small graphic area, it can also reduce the probability of the last few color groups of the cardboard in the transmission process of the ink layer.
 
4) Under the premise that the color saturation meets the requirements, the thickness of the first color ink layer is reduced as much as possible, which is conducive to the drying of the first color and can also improve the overprint rate of the ink.
 
Adjust the relationship between the fluidity and viscosity of the two inks.
 
Although water-based ink is soluble in water, in order to reduce the saturation of the ink or increase its fluidity during printing, it can be diluted with water, but water dilution is not the best method. For carton printing with bright color requirements, it is best to use a stabilizer to make the ink color lighter, which is conducive to the drying of the ink, and the additive can improve the gloss of the ink layer. However, after diluting with water, when the ink is attached to the cardboard, it cannot be dried instantaneously, and must be dried for a period of time, which will reduce the holder strength and compressive strength of the cardboard.
 
In addition, the relationship between the first color and the viscosity of the color after printing is adjusted. For multi-color printing, the viscosity of the first color water-based ink should be properly reduced, so as to appropriately increase the viscosity of the second color ink to improve the overprint rate.

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