Analysis of two major reasons for the shallowness

Analysis of two major reasons for shallower imprinting

in the offset printing process, the interaction between paper, ink and fountain solution produces variables, just like machine parts and printing materials, these variables not only affect the offset printing process, but also affect the printing quality. Therefore, we must constantly observe and analyze in order to make the printing process normal. In the first half of this year, Asian ethylene supply will be relatively tight. This paper analyzes the organic combination of the two materials based on the factors that make the imprint lighter in the printing process. Accumulation of paper and ink residues on the blanket

in the offset printing process, there are many reasons for the shallower impression, but the main reason is the accumulation of paper and ink residues on the blanket. Generally speaking, the washed blanket will accumulate after it starts printing to 1500 sheets, and its accumulation effect is mainly affected by the following aspects

1. The influence of ink viscosity on accumulation

the viscosity of ink is divided into dry viscosity and wet viscosity. Dry viscosity can be understood as the viscosity of ink when it does not contain fountain solution components; Wet viscosity can be understood as the viscosity of ink containing fountain solution during printing. Viscosity changes in the process of batch printing. When the viscosity of ink is lower than the limit of paper plate hair loss, the imprint is neither satisfied nor cut in the whole printing, and it is not easy to produce paper wool accumulation; When the viscosity of the ink is higher than the hair loss limit of the paper, the imprinting will become more and more prominent in 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 hairs to accumulate on the blanket. In addition, there are a few loose small particles (paper powder) on the surface of the paper, and the small particles are also easy to separate from the paper and adhere to the inked part of the blanket to form a pile

at the beginning of printing, that is, shortly after the blanket is washed, under the normal amount of fountain solution, the ink volume of the graphic part (dot, line, field) is 100% transferred to the paper. When printing to 1500 sheets or when the lubricating fluid is abnormal, the ink on the blanket will accumulate, which is more obvious through high-power micrograph observation. The diameter of the point transferred to the paper is damaged, and there are small white dots without ink in the middle of the point. Therefore, the point appears light, which indicates that the amount of ink transferred to the paper is reduced. If you don't wash the blanket frequently or adjust the amount of fountain solution and ink correctly, there will be more and more paper wool, paper powder and ink residues accumulated on the blanket, and the imprint will continue to become shallow. The ink-free part printed in the middle of the point on the paper will be further expanded, among which the trend of lightening of eye-catching prints is more obvious

2. The effect of the amount of fountain solution on the accumulation phenomenon

(1) the effect of the amount of fountain solution on the viscosity of ink

from the above, we can see that the viscosity of ink is divided into dry viscosity and wet viscosity. Wet viscosity is lower than dry viscosity because water reduces the viscosity of the ink. Therefore, the amount of fountain solution has a direct impact on the viscosity of ink, that is, it has a direct impact on the accumulation phenomenon. Generally, the viscosity of ink decreases with the increase of the amount of fountain solution, but affected by many factors such as the type of ink, the model and the temperature of the ink roller, other experiments show that the wet viscosity of ink does not always decrease with the increase of the amount of fountain solution. Therefore, we must pay attention to observation and adjustment

(2) in addition to the influence of the amount of fountain solution on the paper powder and ink accumulated on the blanket, the paper powder and paper coating can also be adhered and dissolved by the fountain solution, so as to accumulate on the blanket. This kind of stacking phenomenon mostly occurs on the paper with high stackability, and accumulates on the non graphic part of the blanket surface. When the stacking amount reaches a certain degree, it will expand to the imprinted part. Under the condition that the ink layer thickness, ink viscosity, temperature, humidity and field density remain unchanged, as long as the fountain solution is sufficient, there will be no such deposits on the blanket. Of course, what we are talking about here is to give enough fountain solution, on the premise that even if we use more fountain solution for printing, there will be no water marks on the printed products and the quality of the printed products will be guaranteed

(3) some offset printers dogmatically pursue the balance of water and ink, constantly reduce the amount of water in the printing process, and ignore the impact of the above stacking effect on the shallower impression, which aggravates the stacking phenomenon. At present, some screw rods of tension machines on the market use T-shaped ordinary screw rods to continuously shallow the impression

to sum up, in the offset printing process, because the indirect measurement method can only confirm the total value accumulation effect of the wear on each friction surface, the printing quality changes significantly, and the imprint continues to become shallower. This kind of accumulation is inevitable. In addition to frequently washing the rubber cloth to ensure the printing quality, it is also necessary to adjust the viscosity of the ink. In addition, when the amount of fountain solution used in the whole batch of prints exceeds the specific accumulation limit value of the printing station, the interval of washing the rubber cloth will be greatly extended. At this time, it will play a great role in eliminating the fault of shallowing the impression and ensuring the consistency of the depth of the whole batch of prints