From the perspective of manuscript design and plate-making process, the flexographic printing process is self-contained and has its own unique features. The plate-making process is basically as follows: manuscript → electronic color separation or photography → positive negative image → back exposure → main exposure → development and processing → Drying → Post-processing → Post-exposure → Plate. Compared with offset printing plate, there are the following differences:
1. Reproducible color value range. Offset printing is 1% to 99% (or 2% to 98%), flexo printing is 3% to 95%.
2. Expansion of outlets (at 50%). 15% ～ 20% for offset printing, 30% ～ 40% for flexographic printing
3. The number of network cables. Offset printing generally has a maximum of 175 lines / inch, and the number of flexo and screen lines generally does not exceed 150 lines / inch.
Distorted dimensional deformation
The most obvious feature of the flexographic plate is its elasticity. When the flexographic plate is installed on the cylindrical cylinder, the printing plate is bent and deformed along the surface of the cylinder. The text could not be reproduced correctly, or even deformed seriously. In order to compensate for the distortion of the printed image, it is necessary to reduce the size of the corresponding graphic on the negative film. 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 of the original to compensate, so that the printed product will meet the size requirements.
Flexible dot transfer rule
1. Reasons for the expansion of outlets
The expansion of dots in printing is inevitable. There are two main reasons for the expansion of dots: One is physical. At the moment of imprinting, the ink on the dots of the printing plate is deformed due to the squeezing action, causing the dots to expand; Another reason is the optical reason. The light is scattered at the edge of the dot ink film, resulting in a considerable dot expansion visually. Optical dot expansion depends on the transparency of the ink and the smoothness and absorption properties of the paper.
2. Flexible version of dot transfer law
In flexographic printing, due to the relatively high elasticity of the photosensitive resin plate used, a certain amount of printing pressure needs to be applied during the printing process, which will cause the expansion of the image dots and the extension of the image on the printed matter, causing changes in color and gradation replication. In practice, by measuring and drawing the corresponding flexographic printing characteristic curve, it can be seen that the dot expansion in the flexographic printing process is very serious, and the dots below 10% are difficult to control. Therefore, special treatment should be done for the highlight area. Flat net.
3. Influencing factors
(1) The number of screens increases, the higher the number of screens, the more serious the network expansion.
(2) The size of the printing pressure. The greater the printing pressure, the more serious the dot expansion. In the flexographic printing, try to keep "zero pressure".
(3) Dot shape. Circular dots, square dots, chain dots and elliptical dots have different expansions at different levels. Chain-shaped dots are commonly used in flexographic printing; for small dots in high-gloss areas, FM dots (FM) are the best.
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