Brief Introduction of Flexographic Prepress Process and Rules

From the perspective of the manuscript design and platemaking process, the flexographic printing process is self-contained and has its own unique features. The platemaking process flow is basically as follows: manuscript → electronic separation or photo → positive image → back exposure → main exposure → development and rinsing → Drying → Post-processing → Post exposure → Paste. Compared with offset printing plates, there are the following differences:

1. Reproducible range of color values. Offset is 1% to 99% (or 2% to 98%), and flexo is 3% to 95%.

2. Network expansion (50%). Offset is 15% to 20%, flexo is 30% to 40%

3. Number of screens. Offset is generally up to 175 lines per inch, and the number of flexo screens generally does not exceed 150 lines per inch.

Color plate deformation

The most obvious feature of the flexographic plate is its elasticity. After the flexographic plate is mounted on a cylindrical roller, the plate produces a bending deformation along the surface of the cylinder. This deformation affects the pattern and text on the surface of the printing plate, so that the printed figure can be printed. The text could not be reproduced correctly, and even severely deformed. In order to compensate for the distortion of the printed image, the size of the corresponding graphic text on the negative plate must be reduced. Before designing a manuscript or separating a plate, the amount of elongation of the plate should be taken into consideration, and the corresponding value in the circumferential length of the manuscript should be subtracted from the corresponding value in order to compensate, so that the printed product can meet the dimensional requirements.

Flexonet transmission rules

1. Reasons for network expansion

The network expansion in printing is unavoidable. There are two main reasons for the expansion of the network. One is the physical reason. At the moment of imprinting, the ink on the printing plate outlets will be deformed due to the extrusion and cause the network to expand; Another reason is that for optical reasons, light scatters at the edge of the ink film at the dot, which results in a considerable dot gain in the visual sense. Optical dot enlargement depends on the transparency of the ink, the smoothness of the paper, absorption properties, and the like.

2. Flexonet transmission rules

In the flexographic printing, because the elasticity of the photosensitive resin plate used is relatively large, a certain printing pressure is required to be applied during the printing process, which leads to enlargement of image dots on the printed matter and elongation of the image, causing changes in color and gradation reproduction. In practice, by measuring and drawing the corresponding flexographic printing characteristic curve, it can be seen that the dot enlargement 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 region and try to put Flat net.

3. Influencing factors

(1) The height of the number of screens and 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, and the “zero pressure” should be maintained in the flexographic printing.

(3) Dot shape. Circular dots, square dots, chain dots, and elliptical dots have different dot gains in different gradations. In the flexographic printing, the chain-shaped dots are commonly used; for the small dots in the high-light area, the use of FM dots (FM) works best.