Causes and prevention of the dirt on the back of t

2022-08-23
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Reasons and prevention of back rubbing dirt in printing

when printing dark tone parts of color products and printing large areas with spot color, back rubbing dirt is a very easy fault due to the thick ink layer. There are problems in ink viscosity, pH value, ink layer thickness, roughness and permeability of paper surface, which will lead to back rubbing dirt

in offset production, when the ink on the previous sheet is not dry, the latter sheet is superimposed on it, and the ink of the former sheet may be partially transferred to the back of the latter sheet, which is called back rubbing. Rubbing on the back will not only cause ink stains on the back of the printed product, but also cause the front image and text to appear spotted, which will seriously affect the product quality and even cause the whole batch of products to be scrapped. Especially when printing the dark part of color products and printing a large area of the field with spot color, due to the thick ink layer, it is very easy to get dirty on the back, so we must be careful

in essence, the smudging on the back of a print is caused by the adhesion of the ink on the surface of the previous print film to the back of the next print in the mechanical property experiment of metals, nonmetals, composites and products. This adhesion is the secondary intermolecular force FB between the ink on the surface of the previous print film and the next print. At the same time, the surface ink is also affected by the secondary intermolecular force FA with the front of the previous print and the cohesion FC of the ink itself. Only when FB ≥ fa+fc, the back will be rubbed

because the secondary force between molecules will decrease sharply with the increase of distance, FA is mainly related to the thickness of the ink film, and it will decrease sharply with the increase of the thickness of the ink layer. FB is related to the distance between the surface of the ink film and the next printing sheet. When the thickness of the ink layer is determined, FB is only related to the distance between the two adjacent printing sheets, and decreases sharply with the increase of this distance. FC depends on the nature of the ink and the drying condition of the ink. Because as the ink dries and solidifies, the cohesion will rise sharply. Therefore, the fundamental solution to the back rubbing is to reduce the thickness of the ink layer, increase the distance between the printing sheets, improve the performance of the ink and speed up the drying speed of the ink on the paper. The following discusses how to prevent the back rubbing of the printed matter in printing from these aspects

I. ink performance

different types and colors of inks have different compositions and proportions of binders and pigments, different drying methods, and great differences in drying speed. In addition, the viscosity of thixotropic inks will decrease due to the shear action between ink rollers during the printing process, and will rise rapidly after the completion of transfer. Since viscosity is the external manifestation of cohesion, an increase in viscosity indicates an increase in cohesion, which is conducive to preventing the back of printed matter from rubbing dirty. Therefore, when printing color prints without background color removal and large areas of the field, we should try to use fast drying and thixotropic inks

in order to adjust the printability of ink, it is sometimes necessary to add debonding agent to reduce the viscosity of ink. The viscosity decreases, that is, the cohesion decreases, which is easy to cause the back to rub dirty. Therefore, when printing coated paper that is easy to get dirty on the back, strictly control the amount of debonding agent and the viscosity of ink

proper addition of driers can significantly accelerate the drying of ink, but driers are also emulsifiers, which will promote the emulsification of ink. Excessive addition of driers will delay the drying of ink

second, the pH value and dosage of the fountain solution

when printing on a large area of the field where the back is easy to rub dirty, the ink layer thickness is easy to cause the ink to expand to the blank part. At this time, reducing the pH value of the fountain solution is conducive to inhibiting the expansion of the ink. However, if the pH value is too small, the fountain solution will react with the drier to delay the drying of the ink. Therefore, when printing this kind of products, the pH value of the fountain solution needs to be determined according to the graphic situation and color of the layout

if the spot color with thick ink layer is solid, and there are small lines in the middle, or if the layered picture with dark tone is printed, the pH of the fountain solution should be lower because it is easy to paste

when printing red ink, the pH value of the fountain solution should be lower; When printing inks of other colors, the pH value should be slightly higher

the more serious the ink emulsification is, the more water is mixed into the ink layer, and the harder the ink film is to dry. Using alcohol fountain solution can reduce the amount of fountain solution, reduce emulsification, and is conducive to ink drying

when using ordinary fountain solution, first pay attention to the proportion of each component in the formula, and then use as little water as possible to maintain the balance of ink and water. At the same time, controlling the distribution of moisture on the layout is also very important to prevent the back from rubbing dirty. For example, the axial width of a printer is 1020mm, the width of the printed paper is 700mm, and a large area of pictures and texts are almost full of the whole paper. In this case, because the ink layer is thick, the water supply is large, and the printing machine supplies water evenly in the axial direction, there is neither ink and water emulsification at the left and right ends of the printing plate and in the area outside the paper, nor the blank part of the paper absorbs water. As a result, excessive water rushes into the left and right ends of the paper, resulting in excessive emulsification of printed graphics and texts and shallow ink color. In order to maintain the uniform ink color of the layout, we have to increase the ink volume, resulting in large water and ink at the left and right ends of the paper, which is not easy to dry. Therefore, when printing a large area of the field, the dirt on the back is most likely to appear on the left and right sides of the paper. The reasonable approach is to try to use the waterway blowing device or stick a paper strip on the bucket roll to reduce the water supply at both ends of the printing plate. If the number of prints is not large, you can also consider cutting the paper a little larger. Here we not only provide you with high-quality experimental machine equipment, so that the blank parts at both ends of the paper absorb some water

III. properties of paper

the roughness and permeability of the paper surface is an important factor affecting the back rubbing. For paper with good permeability, the binder can quickly and more penetrate into the interior of the paper, accelerating the solidification of the ink. The paper with rough surface is conducive to the infiltration of air from the gap between the papers. At the same time, the air content in the capillary of the paper with rough surface is also large, which is conducive to the oxidation polymerization of ink. Therefore, back rubbing often occurs when printing a thick ink layer on coated paper. Printing aluminum foil paper with an offset press is particularly prone to back rubbing, because the surface of the aluminum foil paper has no permeability at all. Even if only a thin ink layer is printed, powder spraying is required to prevent the back from rubbing

when wet press dry overprinting is used, if the previous color is already a high percentage of points or on the ground, when the latter color overprinting, because the original paper surface has been covered by the former color ink film, the permeability of the latter color ink will be greatly reduced, which is easy to cause the back to rub dirty

the different acidity and alkalinity of paper will also affect the drying speed of ink. The ink printed on acid paper dries slower than that printed on alkaline paper

IV. ink layer thickness

back rubbing often occurs in the parts with thick printing ink layer

in the process of ink drying, for the resin ink commonly used in sheet fed printing, its drying is first completed by the rapid penetration of high boiling kerosene in the binder and the rapid thickening and fixation of the remaining binder, while the oxidative polymerization reaction proceeds slowly to complete the complete solidification of the oil and ink

for a certain kind of paper, under a certain printing pressure, the penetration of binder is limited. When the ink layer is very thick, after the binder at the bottom penetrates into the paper, the penetration of binder in the surface ink will be greatly reduced, and the surface ink cannot rely on the rapid thickening and solidification of ink to improve cohesion. Therefore, for a certain kind of paper, within a certain ink layer thickness, adopting the appropriate printing process, there will be no back rubbing dirty. Once the thickness of the ink layer exceeds a certain limit, back smudging will occur rapidly, and the severity of back smudging is very sensitive to the change of the thickness of the ink layer. Therefore, when printing color images with dark tones, the method of background color removal should be adopted to minimize the thickness of the ink layer. When printing a large area of the field, although the thicker the ink layer, the thicker the ink color, it is also necessary to strictly control the thickness of the ink layer within a certain range. If the color of the allocated ink is too light, trying to deepen the color by increasing the thickness of the ink layer is easy to cause the back of the printed matter to rub dirty

when the ink layer is thin, the intermolecular force FA between the printing sheet and the surface of the ink film is large. When the ink layer is thickened, this intermolecular force decreases rapidly, which is also a factor leading to the back rubbing

v. printing pressure and printing speed

the penetration of ink on paper is divided into pressure penetration and free penetration. Increasing the printing pressure can promote the pressure penetration of ink; Reducing the printing speed can increase the stamping time and promote the pressure penetration of ink. At the same time, after the printing speed is reduced, the next printing sheet is stacked before the previous printing sheet, and the previous printing sheet has a longer time for capillary free penetration, which can be identified as ternary materials, lithium iron phosphate, lithium manganate and other types according to the positive material. Therefore, when printing a large area of the field without spots, appropriately increasing the printing pressure can not only make the ink thicker and more uniform, but also accelerate the drying speed. If the number of prints is small, the printing speed can also be reduced to prevent the back from rubbing dirty

VI. ambient temperature and humidity

the rise of ambient temperature can enable them to complete the work at hand with more confidence and confidence. High ink drying speeds up, so there should be differences in adding driers in winter and summer

with the increase of environmental humidity, the drying speed of ink slows down, and it is easy to get dirty on the back in a very humid environment

VII. Distance between papers

the smoother the paper is, the higher the stack is, and the higher the density of the paper is. Under the action of the gravity of the paper itself, the smaller the paper spacing at the bottom of the stack is, and the greater the secondary binding force FB between the surface of the ink film on the print sheet and the adjacent print sheets is, the easier it is to cause the back to rub dirty. At the same time, the smaller the paper spacing is, the more difficult it is for air to penetrate from the gap between the papers, which is not conducive to the oxidation polymerization reaction of the ink. Therefore, when printing products with thick ink layer with coated paper, it is not suitable to stack the paper too high. Conditional printing plants can use special drying racks, separated layer by layer, to reduce the height of stack paper

at present, another method commonly used in the printing industry to prevent back rubbing is powder spraying. After spraying the special powder on the printing sheets, using the supporting function of the powder to increase the distance between the printing sheets can receive a good effect of preventing the back from rubbing dirty. But using powder spraying also brings many disadvantages

first of all, spraying powder pollutes the environment and endangers the health of printing workers. Therefore, spraying powder can only be used when other measures cannot prevent the back from rubbing dirty, and the spraying area of spraying powder should be adjusted according to the size of the position where the back of the printing sheet is prone to rubbing dirty, so as to minimize the amount of spraying powder. Secondly, powder spraying affects the quality of printed matter. The printing quality of post printing ink will be seriously affected if other colors of ink are printed on the printed sheet after powder spraying. Therefore, the color that needs to be dusted is usually placed at the end of printing. If the printed sheet after powder spraying needs to be coated, the granular powder will look more obvious after coating. In particular, when spraying powder on the black background, it is generally necessary to wait for the ink to solidify, use the printing machine to imprint it again when the ink roller and water roller are off pressure, and remove the powder on the black background by frequently scrubbing the blanket

VIII. Other

when printing thick paperboard, the paper receiving table is symmetrical

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