Briefly explain the reasons for the shallowing of the offset printing process
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During the offset process, the interaction between paper, ink and fountain solution produces variables, just like machine parts and printed materials. These variables not only affect the offset process, but also the print quality. Therefore, the analysis must be continuously observed in order for the printing process to proceed normally. This paper analyzes the factors that cause the printing process to be shallow.
First. Stacking of paper and ink residue on the blanket
There are many reasons why the print is lightened during the offset process, but the main reason is the accumulation of paper and ink residue on the blanket. Generally speaking, the washed blanket will accumulate after about 1500 sheets are printed, and the stacking effect is mainly affected by the following aspects.
1. The effect of ink viscosity on the accumulation
The viscosity of the ink is dry and viscous, and the dry viscosity is understood to be the viscosity of the ink without the dampening solution. The wet viscosity can be understood as the ink during the printing process. Viscosity exhibited by the dampening solution. The viscosity is changed during the batch printing process. When the viscosity of the ink is lower than the limit of the paper, the imprint is neither full nor flawy in the whole printing, and it is not easy to produce paper pile accumulation; when the ink is sticky When the hysteresis is higher than the paper drop limit, the print will become more and more sloppy during the printing process. Due to the high viscosity of the ink, many small paper hairs are pulled off the surface of the paper, causing the paper to accumulate on the blanket. In addition, the surface of the paper has a small amount of loose small particles (paper powder), and the small particles are also easily separated from the paper and adhered to the inked portion of the blanket to form a pile.
At the beginning of printing, that is, after the blanket was washed, the amount of ink in the graphic part (mesh, line, and field) was transferred to the paper by 100% under the normal amount of dampening liquid. When printing to 1500 sheets or when the lubricating fluid is not normal, the ink on the blanket will accumulate, which is more obvious by high-power micrographs. The dots that are transferred to the paper are damaged in diameter, and there is no white dots in the middle of the dots. Therefore, the dots appear light, which indicates that the amount of ink transferred to the paper is reduced. If you do not wash the blanket or adjust the amount of the fountain solution and the amount of ink properly, the paper, paper powder and ink residue accumulated on the blanket will be more and more, the imprint will continue to be lighter and printed on the paper. The part with no ink in the middle of the dot should be further expanded, and the trend of lightening the mesh is more obvious.
2. The effect of the amount of dampening solution on the accumulation phenomenon
(1) The effect of the amount of dampening solution on the viscosity of the ink
As can be seen from the above, the ink viscosity is divided into two types: dry viscosity and wet viscosity. Wet viscosity is lower than dry viscosity because moisture reduces the viscosity of the ink. It can be seen that the amount of dampening solution has a direct influence on the viscosity of the ink, that is, it has a direct influence on the accumulation phenomenon. In general, the viscosity of the ink decreases with the increase of the amount of dampening solution, but it is affected by many factors such as the type of ink, the model and the temperature of the ink roller, and other experimental conditions indicate that the ink is wet viscous. As the amount of dampening solution increases, it does not always decrease. Therefore we must pay attention to observation and adjustment.
(2) The amount of dampening solution In addition to the influence of the paper powder and ink deposited on the blanket, the paper powder and paper coating can also be adhered and dissolved by the dampening solution, thereby accumulating on the blanket. This accumulation phenomenon occurs mostly on paper that is highly stackable and accumulates on the non-image portion of the surface of the blanket. When the amount of accumulation reaches a certain level, it will expand to the imprinted portion. In the case of the thickness of the ink layer, the viscosity of the ink, the temperature, the humidity, and the density of the solid, the deposit is not present as long as the dampening solution is given to the foot. Of course, the dampening solution mentioned here is given to some extent, even if it is printed with some dampening solution, it will not cause water marks on the printed products, and the quality of the printed products will be guaranteed.
(3) Some offset printing workers pursue the “ink and water balance”, and continuously reduce the amount of water in the printing process, while ignoring the influence of the above-mentioned accumulation effect on the lightening of the blotting. As a result, the accumulation phenomenon is aggravated and the blotting continues to become shallow.