Analysis and elimination of inaccurate overprint i

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Analysis and elimination of inaccurate overprinting in multicolor printing

I. The Influence of printing plate deformation on overprinting

1. The tensile deformation of printing plate metal plate has certain extensibility, and tensile deformation will occur under the action of tension, while the tensile deformation of zinc plate is more obvious. Generally, when we print, we find that the overprint is inaccurate, which is mostly adjusted by the position extension plate. When the plate is stretched by external force, the printing plate is tightly attached to the surface of the cylinder shell, and at the same time, tensile deformation will inevitably occur. The greater the tension, the greater the tensile deformation of the layout, and the distribution of tensile deformation is uneven. The maximum force is exerted on the mouth edge and trailing edge of the version, and the tensile deformation is also large, the force on the inside is small, and the tensile deformation is also small. Therefore, the problem of overprint accuracy may not be solved by pulling the plate

2. ways to reduce overprint errors caused by plate loading. When installing plates, do not try hard to find and solve the difficulties and problems existing in the use of users in time. If the force is too large, the tension shall prevail. When the printing plate is shifted, the corresponding force should be loosened first, that is, the plate pulling screw should be loosened to minimize the friction between the back of the printing plate and the surface of the drum. At this time, the printing plate will be displaced under the action of tension, and the tensile deformation of the printing plate should be controlled to a minimum. Secondly, the rule line should be aligned when printing. In the process of multicolor overprint, mark the position of the regular line on the plate cylinder body after the first color is printed and before removing the printing plate. When installing the printing plate after the second color, align it with the regular line of the first color. This can not only improve the speed of the upper version, but also reduce the number of drawing, and avoid abnormal stretching deformation. Because it is found that the rules are not correct after chromaticity, you need to pull the plate. The more times you pull it, the greater the possibility of tensile deformation. Therefore, try not to use the pull plate to solve the inaccurate overprint

3. The thickness of the printing plate and the size of the graphics change. The lithographic graphics are copied on the surface of the metal plate. When the printing plate is tightened on the drum, the metal plate is bent, the inner part of the plate is compressed, and the outer part is stretched, so that the figure is extended by single-chip control. Therefore, the thicker the metal plate, the larger the elongation of the graphic size. In plate making and printing, try to use the printing plates of the same manufacturer, the same model or the same box

II. The influence of the increase or decrease of cylinder liner on overprint during the printing process, due to the expansion and contraction of the paper, the overprint is not on time. The method of increasing or decreasing the thickness of cylinder liner can be used to solve the problem of inaccurate overprint between diameters. However, the increase or decrease of the liner thickness should not be too large, and the increase or decrease will cause other disadvantages

1. The increase or decrease of the printing plate liner affects the tensile deformation of the printing plate. The thicker the printing plate liner, the larger the graphic size elongation

2. increase or decrease of rubber cylinder liner when reducing the printing plate liner and increasing the rubber cylinder liner, the radial deformation of the figure increases due to the increase of the extrusion deformation of the rubber blanket. The other is to reduce the thickness of the rubber drum and increase the thickness of the printing plate liner at the same time, so the extrusion deformation of the blanket is reduced. The latter is commonly used

III. Influence of rubber blanket elongation on graphic overprint rubber blanket has flexibility. The tighter the rubber blanket is stretched on the drum, the smaller the elongation is, and the smaller the graphic change is

1. choose rubber blanket with small elongation. During installation, pay attention to the longitudinal and transverse directions of the blanket, and the longitudinal direction should be perpendicular to the axis of the drum to reduce the elongation

2. During the overprint process, try to keep the tightness of the rubber market unchanged

3. if there is a small amount of width overprint, the rubber cloth should be properly adjusted, and the pulling method is not used to solve it

4. Cutting, punching, tightening of individual screws, tearing of rubber layout part, conceptual crack of experimental machine, local uneven gasket, etc. of blanket will cause local tightness and looseness, resulting in local overprint inaccuracy. In short, in addition to mechanical, adjustment, paper and other problems, we should also pay attention to the inaccurate overprint caused by these small changes. Because overprint inaccuracy usually occurs at the moment of high-speed printing, which is formed by the accumulation of small changes, it is difficult to observe directly. Therefore, as long as we pay full attention to printing, analyze the original bed and take the most appropriate measures to avoid or reduce the occurrence of nesting errors

source: MAN Roland printed by: Zhang Wenhua

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