Analysis of the factors affecting the shallower im

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Analysis of factors affecting the shallowness of imprint in offset printing process

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. This paper analyzes the factors that make the imprint lighter in the printing process

I 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 oil should also include multi-dimensional structural adjustments such as enterprise system, property rights diversification, laws and regulations, and the viscosity of ink is lower than the limit of paper roughing, 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 another 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 reducing the amount of water in the printing process, while ignoring the impact of the above stacking effect on the shallower impression, which aggravates the stacking phenomenon and continues to make the impression shallower

to sum up, during the offset printing process, the printing quality changes significantly due to the stacking effect - the imprint becomes 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

II In addition to the accumulation reasons, there are several situations that can also cause the imprint to become shallower

1. Generally, when the amount of fountain solution increases, the ink layer thickness will decrease, which can be found and adjusted by the operator in time. However, some inks have a large amount of water absorption, but the situation is opposite when printing. When using this kind of ink for printing, the operator often reduces the amount of fountain solution according to the conventional practice, and wants to make the imprint deeper, but the imprint continues to become shallower

2. Too much dry oil is added to the ink during offset printing. After shutdown, the ink layer on the ink roller and ink bucket roller dries and solidifies quickly, which weakens its ability to absorb ink and ink transfer performance. When printing again, the amount of ink transfer becomes less, and the imprint suddenly becomes shallow. Sometimes, in the middle of printing, the ink layer on the ink roller will dry and solidify gradually, which makes the imprint shallow quickly (this phenomenon is easy to happen in visual debugging printing)

3. Due to not observing the ink bucket frequently, it was not found in time that the ink level in the ink bucket decreased too much, or the ink thixotropy of the ink bucket was too large and the fluidity was too poor, which reduced the amount of ink and made the impression shallow

4. During the printing process of the whole batch of prints, the smoothness and ink absorption performance of some papers suddenly deteriorate, which will also cause the impression to become shallow

5. During the printing process, under the repeated friction of the water roller, inking roller, rubber roller and paper powder and paper wool falling off the paper, the photosensitive layer is gradually worn and destroyed, the lipophilicity is weakened, and the hydrophilicity is enhanced. Sometimes the Jinan assay testing machine of the printing plate guarantees the quality, and the image and text part will gradually become poorly inked, Make the imprint lighter (the anti-wear ability of the photosensitive layer can be enhanced by baking the plate, which can prolong the imprint shallowing time)

6. The cylinder diameter of the high-speed offset press is small, and the width of the imprint during the rolling process is also reduced accordingly. If the printing speed is suddenly increased during the printing process, but the printing pressure is not increased correspondingly, the imprint will also become shallow. This is because when the printing speed is increased, the printing plate and blanket will inevitably be, The contact time of the points corresponding to each other between the blanket and the printing sheet is correspondingly reduced, which increases the possibility of incomplete contact between the printing surfaces, resulting in incomplete graphic transfer, false dots, and light ink

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