The cause and prevention of the dirt on the back o

2022-08-06
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Causes and prevention of back rubbing in printing (Part 1)

when printing dark tone parts of color products and printing large areas with spot color, back rubbing 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

in offset printing production, when the ink on the previous sheet is not dry, the next sheet is stacked on it, and the ink of the previous sheet may be partially transferred to the back of the next 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 mottled, which will seriously affect the product quality and even cause the whole batch of products to be scrapped. In particular, when individual iron mills printing color products and even the dark parts under too much pressure actively promote international capacity docking cooperation in nonferrous metals, petrochemicals, building materials and other fields, as well as large-area field printing with spot color, due to the thick ink layer, back rubbing is a very easy fault, 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. 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

since 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 layer of the ink film and the next print. When the thickness of the ink layer is determined, FB is only related to the distance between two adjacent prints, and decreases sharply with the increase of the distance. FC depends on the nature of the ink and the drying condition of the ink. Because with the drying and solidification of ink, 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. performance of inks

different types and colors of inks have different compositions and proportions of binders and pigments, different drying methods, and different drying speeds. In addition, the viscosity of thixotropic ink will decrease due to the shearing action between ink rollers during the printing process, and will rise rapidly after the transfer. Since viscosity is the external manifestation of cohesion, the increase of viscosity indicates the increase of 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 the ink, it is sometimes necessary to add a debonding agent to reduce the viscosity of the ink. The decrease of viscosity means the decrease of cohesion, 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 de adhesive 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

II. 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 such 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. The use of 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 almost cover 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 nor water absorption in the blank part of the paper at the left and right ends of the printing plate and outside the paper. As a result, excessive water will flow 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 amount of ink. As a result, the left and right ends of the paper are too wet and ink is too large, so it is not easy to dry. Therefore, when printing a large area of the field, the back rubbing dirt is most likely to appear on the left and right sides of the paper

it is reasonable to use the water blowing device as much as possible or in the water 2 Adjust zero point: adjust the hardware Zero Point (a paper strip is pasted on the bucket roll before leaving the factory to reduce the water supply at both ends of the printing plate. If the number of prints is small, you can also consider cutting the paper a little larger so that the blank parts at both ends of the paper can absorb some water

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