Introduction to the pre press process flow and law

2022-08-18
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Introduction to the prepress process flow and rules of flexo

from the perspective of original design and plate making process, the flexo printing process is self-contained and unique, and its plate making process flow is basically as follows: electronic color separation of the original or photographic positive and negative image back exposure, main exposure, development, development, drying, exposure and plate pasting after treatment. Compared with offset plate making, there are the following differences:

1. Reproducible color value range. Offset printing is 1% - 99% (or 2% - 98% of all experimental data and curves are automatically stored), and flexo printing is 3% - 95%

2. point enlargement (50%). Offset printing is 15% - 20%, flexo printing is 30% - 40%

3. The number of lines added. Offset printing generally has a maximum of 175 lines/inch, and the number of lines added in flexo printing generally does not exceed 150 lines/inch

dimensional deformation of color separations

the most obvious feature of the flexo is its elasticity. When the flexo is installed on the cylindrical drum, the printing plate bends and deforms along the surface of the drum. This deformation affects the patterns and words on the surface of the printing plate, so that the printed pictures and texts cannot be reproduced correctly, and even serious deformation occurs. In order to compensate for the deformation of the printed image, it is necessary to reduce the size of the corresponding image and text on the print negative. When designing the original or color separation before plate making, the elongation of the printing plate should be considered, and the corresponding value should be subtracted from the circumferential length in the original as compensation, so that the printed product can meet the size requirements

dot transfer law of flexographic plate

1. Reasons for dot expansion

dot expansion in printing is inevitable, and there are two main reasons for dot expansion: one is physical reasons. At the moment of imprinting, the ink on the printing plate dot deforms due to extrusion, resulting in dot expansion; Another reason is optical. Light is scattered at the edge of the dot ink film, resulting in considerable dot expansion visually. Optical dot enlargement depends on the transparency of the ink and the smoothness and absorption performance of the paper

2. Dot transfer law of flexographic printing

in flexographic printing, because the elasticity of the photosensitive resin plate usually requires high tensile properties of materials, such as heat shrinkable film and tensile film, it is necessary to apply a certain amount of printing pressure in the printing process, which will lead to the expansion of image dots and the elongation of images on the printed matter, causing changes in color and level reproduction. In practice, by measuring and drawing the corresponding flexographic printing characteristic curve, it can be seen that the point expansion in the flexographic printing process is very serious, and the points below 10% are difficult to control. Therefore, the highlight area should be specially treated and flattened as much as possible

3. Influencing factors

(1) the higher the number of lines, the more serious the point expansion

(2) the size of printing pressure. The larger the printing pressure is, the more serious the dot expansion is. Try to maintain "zero pressure" in flexographic printing

(3) point shape. Circular points, square points, chain points and elliptical points have different point expansion under different tones. In flexographic printing, it often has excellent thermal aging performance, and uses chain dots; For small dots in the highlight area, the effect of using frequency modulation point (FM) is the best

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