For the wet-press printing method of four-color offset printing press, the ink of the latter color can be better transferred to the ink layer of the previous color printing material, and the adhesiveness of the ink should be reduced along with the printing color sequence, otherwise, it will produce Reverse overprinting phenomenon.
Group-type lithographic printing machine (such as domestic four-color machine, Heidelberg four-color machine, etc.), the ink on each color group is directly transferred to the surface of the paper at the time of printing, and the low-viscosity component and the solvent in the ink are instantaneously fixed. Infiltrate into the gap of the paper, leaving high viscosity components and synthetic grease enveloping the pigment in a gelled state and fixing it on the surface of the paper. In this process, the viscosity of the ink rises sharply, so that the viscosity of the latter color ink is sticky even with the previous color ink. Values ​​that are the same or slightly larger than the viscosity of the previous color ink do not produce an inverse overprint. However, the viscosity of the ink is also affected by many factors, such as the thickness of the printed ink layer, the printing speed, the temperature, the degree of ink emulsification, the amount of ink accessories added, etc., which are likely to break the original normal ink transfer relationship.
In order to ensure the normal transfer of ink, following the printing sequence, the viscosity of the ink should be reduced in turn. For a satellite lithographic printing machine, since each color of ink is transferred to the blanket cylinder after printing, the ink is transferred to the paper, and the color ink of the preceding color does not increase in viscosity. This type of printing machine has a structural deficiency. Easy to produce anti-stack phenomenon. Therefore, for this type of printing press, printing must be strictly followed in accordance with the principle of decreasing ink viscosity in the printing sequence, and between the front and rear colors, the tack value of the ink should be significantly different.
In addition, when printing smooth, low-absorbency paper, consider the effect of ink stickiness on ink transfer. For the ink viscosity control, generally speaking, the single-long offset press has a speed of 7 to 13 at a speed of about 5000 to 10000 rph. The stickiness is 4 to 5 per hour when the speed is around 20000rph, and the stickiness is 3 to 5 when the speed of the web offset printing machine is over 2000rph. After testing, the general order of the stickiness of four-color printing inks is from black to blue to magenta to yellow.
Ink is a special fluid. Because the same kind of color printing inks use the same kind of linking material, the dispersion state of each color pigment in the linking material is extremely similar. In the case of the ink ink property unchanged, it can be roughly assumed that the ink viscosity increases with the increase of the ink viscosity. (Actually, the viscosity and viscosity of an ink are two different physical quantities of the ink and cannot be mistaken for two different ways of expressing the same property of the ink). Therefore, in actual printing, the printing color sequence is often determined according to the size of the ink viscosity. For example, the viscosity of the Shanghai Mudan brand four-color version 05 ink is:
BK118.82Pa·S
C 48,85Pa·S
M 47.71Pa·S
Y14.70Pa·S