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2022-07-24
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Material factors affecting the overprint accuracy of color newspapers

China's newspaper publishing has made great progress in recent years, and the proportion of color newspapers has increased significantly every year. At present, most color newspaper printing adopts imported rotary printing machines, which greatly improves the printing quality of color newspaper. However, relatively speaking, in rotary offset printing, there are many and complex factors affecting the accuracy of color newspaper overprint, including machinery, technology and materials. This paper only discusses the influence of common paper, printing plate and blanket in printing materials on the overprint accuracy of color newspapers

I. printing plate

in lithography, after processing the image and text of the original, the graphic information of the original is transferred to the PS plate, and the batch copy of the original is completed through printing. PS version is generally based on industrial pure aluminum with a thickness of 0.3mm. Its technical indicators directly affect the accuracy of overprint

The plane geometric dimension of the

version

ps version should be perforated and positioned before drying, so its overall dimension is particularly important. PS plate should be cut into rectangles to ensure that the two diagonals are equal in length, so that the punching, bending and machine operation can maintain accurate positioning before drying. If the printing plate is cut irregularly, it will lead to deviation when loading the plate on the machine, which will inevitably lead to inaccurate overprint. It should be emphasized here that the positioning parts of the plate printer, bending machine and plate cylinder must be matched, and the matching accuracy between them must be ensured to ensure accurate positioning and eliminate the impact caused by incorrect plate cutting

plate thickness

ps plate is usually rectangular in cross-section geometry. When it is bent and wrapped on the drum, its cross-section changes from rectangular to fan-shaped. At this time, the front face of the printing plate and the plate face correspond to the same wrapping angle. However, due to the certain thickness of the printing plate, the length of the front face and the back face of the printing plate changes. The length of the front face of the printing plate is greater than the length of the back face, indicating that the front face of the printing plate has a certain elongation, This phenomenon is called metal cracking. When the printing plate is installed on the cylinder, due to the uneven effect of the tension and internal stress, the size of the text and text on the plate will elongate with the cracking of the plate base surface, as shown in Figure 1. The elongation (△ L) of the printing plate can be calculated by the following formula:

this elongation is slight, but for products with high requirements for overprint accuracy, it must be taken into account and corrected accordingly

The overprint error varies with the thickness of the plate base. After the thick plate base is wrapped on the drum, the friction force is large after its back is attached to the drum surface, and the plate drawing force is only concentrated at the two ends of the plate mouth. The whole printing plate is not completely attached to the drum, and the stress distribution of each particle is uneven. Except for small natural cracking, the tensile deformation is small, and it is a rigid body during the printing process, with almost no extrusion deformation, so its surface Hafei Dong'an and other large-scale industrial manufacturing enterprises basically maintain a stable size when making mechanical component models; For the thin plate base, after the printing plate with backing paper is installed on the drum, due to the isolation of the backing paper, the corresponding friction becomes smaller. Under the action of tension, the stress distribution of each particle is more uniform, and the printing plate is well attached to the surface of the drum, so its tensile deformation is large, and the overprint error is large

based on the above analysis, plates with the same thickness should be used for the same print. If plates with different thickness are used, they should be decreased according to the color sequence to reduce the overprint error

II. Paper

in color newspaper printing, the substrate is mainly paper, and its printability directly affects the accuracy of overprint

1. Dimensional stability

the surface of the paper is porous, hydrophilic and hygroscopic. It contains a certain amount of water. When the temperature and humidity of the environment change, the water content of the paper also changes: it increases with the increase of relative humidity and decreases with the increase of temperature. The result is the change of paper size. Generally, the temperature of the paper warehouse shall be 18 ~ 25 ℃, and the relative humidity shall be controlled at 60 ~ 65%. Generally speaking, when the relative humidity changes by 10%, the water content changes by 1%; On the premise of a certain relative humidity, the moisture content of the paper changes by ± 0.15% for every ± 5% change in temperature. In multi-color overprint, if the moisture content of the paper changes by more than ± 0.1%, the overprint accuracy will be affected. In the printing process of color newspaper, the paper has to contact with water for many times, resulting in the change of water content, which leads to the change of dimensional stability, and affects the overprint accuracy by investing in this field. Therefore, during the production, transportation and storage of paper, the factors affecting the change of paper moisture content should be controlled as much as possible to maintain a stable moisture content and avoid unnecessary printing failures

2. The influence of paper expansion and contraction on overprint accuracy

paper forms two thread directions in the manufacturing process, i.e. transverse and longitudinal. After one fiber of the paper is completely wetted, its diameter can be increased by 30%, its longitudinal direction can only be increased by 1% ~ 2%, and the horizontal expansion and contraction amount is 2 ~ 3 times of the longitudinal expansion and contraction amount, which has a great impact on the overprint accuracy of the color newspaper. The maximum change is about 0.15%, and the maximum overprint error can reach more than 0.3mm, resulting in that only one side of the paper can be aligned. It can be seen that the water absorption and expansion of the paper have a great impact on the registration accuracy of the color newspaper, especially in the horizontal direction of the paper, the registration accuracy is more obviously affected by the water absorption and expansion

in order to ensure that the paper tape has sufficient tension during the printing process of color newspaper, the longitudinal thread direction must be taken as the paper direction. It is difficult to control the dimensional stability in the transverse direction. The fan-shaped expansion and the fan-shaped contraction after drying of the water absorption part of the paper can be controlled by the expansion of the fan-shaped expansion roller and the borrowing of lines during the assembly; The longitudinal thread direction can be eliminated by adjusting the registration adjusting roller 7 and the refrigerant high-pressure protection switch between the units: protecting the compactor and extending the service life of the tensile testing machine (for the printing machine with satellite structure, the paper path between the color groups is very short, and the expansion amount of the paper after absorbing water is small, which generally has no impact on the registration). However, no matter which equipment is used in the printing process of the color newspaper, it is important to control the water consumption

at present, there are many kinds of paper used in printing plants, and their moisture absorption and dimensional stability are different. At the same time, the design of color newspaper layout is increasingly diversified, and most of the rotary machines have huge structures and long paper paths, so higher requirements are put forward for overprint accuracy. Therefore, different paper should not be used for the same print, and the expansion rate of paper should be taken as an important parameter to determine the deformation degree of paper during printing. Before printing, appropriate paper should be selected according to the overprint requirements of printing products, At the same time, the temperature and humidity of the printing workshop shall be kept constant

III. blanket

in offset lithography, blanket, as an intermediate medium for image and text transfer, will have a direct impact on the overprint accuracy of color newspaper due to its elongation and tightness after being wrapped on the drum

1. Elongation

elongation is a performance index to characterize the elongation of rubber blanket under a certain tensile force, which mainly depends on the number of layers and strength of its backing fabric. It is required that the backing fabric should be fine and uniform, smooth and firm, with small elasticity, good overlapping and adhesion with the adhesive layer, not easy to crack, vertical and horizontal directions should be absolutely vertical, and the filament direction should follow the radial direction. The elongation of the blanket should be as small as possible to ensure accurate overprint, complete dots and clear graphics. If the rubber blanket is easy to be stretched, the adhesive layer will be thinner, and the elasticity will be reduced. It is certain that the starting point will increase and deform, and the sleeve drawing will not be accurate

2. Tightness

different tightness of the blanket will also cause inaccurate overprint. After the blanket is tensioned on the drum, the degree of tension, the relative friction with the surface of the drum, and the size changes caused by the blanket extrusion deformation and elongation are closely related to the registration error. The pushing deformation of the blanket depends on the direction and size of the friction force and the elongation of the blanket, and also depends on the tightness of the blanket on the drum. That is, under the action of the same direction and the same size of friction force, the elongation is different, the deformation degree is also different. The tighter the blanket is, the smaller the elongation is in the printing process, and the smaller the change in the size of the picture and text is. But the friction of the layout is increased; On the contrary, large elongation reduces the friction of the layout. Therefore, to avoid inaccurate overprinting caused by tightness, the rubber blanket with small elongation should be selected. The tightness should be kept constant. At the same time, the rubber blanket should be installed correctly and avoid skew

information source: Printing Technology

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