Flexo Printing and Platemaking (I)

Flexo printing, offset printing, gravure printing and screen printing are the four most popular printing methods in the world. They have their own characteristics. Because flexographic printing has unique flexibility and economy, and it is beneficial to the protection of the environment, it meets the hygienic standards for food packaging prints, which is one of the reasons for the rapid development of flexographic printing in foreign countries. However, judging from the current situation in China, offset printing is more popular, and gravure printing has already occupied a large market in the packaging industry. However, flexo printing is relatively late for this technology, although it has indeed been achieved in recent years. Great progress, but there is still a big gap with the international advanced technology.
First, the flexographic plate process flow:
From the point of view 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:
Manuscripts - electronic color separation or photography - positive chart - back exposure - main exposure - development rinse - drying - post-processing - post-exposure - compared with the offset plate offset printing plate, the following are the main Differences:
1. Reproducible range of color values. Offset is 1% to 99% (or 2% to 98%), and flexo is 3% to 95%.
2. Expanded outlets (50%). Offset is 15% to 20%, flexo is 30% to 40%.
3, the number of screen lines. Offsets are generally up to 175 lines per inch, while flexo screens generally do not exceed 150 lines per inch.
Second, the size of the color separation sheet deformation The most obvious feature of the flexographic plate is elastic, when the flexographic plate is mounted on a cylindrical roller, the plate produces a curved deformation along the surface of the cylinder, and this deformation affects the pattern on the surface of the printing plate. With text, the printed artwork is not a correct reproduction of the design manuscript, even a serious distortion. Such static deformation (tensile deformation) generated in the circumferential direction of the drum after the flexographic plate is mounted on the drum is always unavoidable. 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 color separation, the amount of elongation of the plate should be taken into account. The corresponding value should be subtracted from the circumferential length of the manuscript for compensation so that the printed product meets the dimensional requirements.
In addition to the reduction ratio, it is related to the radius of the platen roller and the thickness of the double-sided tape, and also related to the thickness of the plate. When the plate is exposed to a flexographic plate, the following formula is generally used to calculate the reduction ratio of the color separation plate:
Reducing ratio (percentage) = K/R × 100%
Among them, R is the printing length of the plate cylinder, K is the coefficient, it depends on the thickness of the plate used, for example, when the thickness of the plate is 1.70mm, the K value is 9.89mm; and when the thickness of the plate is At 2.29 mm, the K value is 13.56 mm.
Third, the flexible version of the network transmission law 1, the reasons for the expansion of the network points in the print is not inevitable exemption, resulting in the expansion of the network there are two main reasons: one is a physical reason, or mechanical reasons, in the imprinted one In an instant, the ink on the printing plate dot will have a certain deformation due to the effect of the extrusion, which will cause the dot to enlarge; the other reason is the optical reason, that is, the dot enlargement is caused by the reflection of the light. Scattering occurs at the edge of the ink film at the dots, thereby creating a considerable dot enlargement in the vision. 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 flexo printing, because the elasticity of the photosensitive resin plate used is relatively large, and a certain printing pressure needs to be applied during the printing process, even though light pressure printing is used in the flexographic printing, it still leads to image dot on the printed matter. The extended image expands and causes changes in color and gradation replication. In practice, by measuring and plotting the corresponding flexographic characteristics, 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 performed for the high gloss region. Should try to flatten the net.
Compensation for dot gain can be done at the scanner side or on the imagesetter, preferably on an imagesetter. The edges of the small dots generated after exposure are relatively neat and of good quality.
3. Influencing factors (1) The number of screen lines increases. The higher the number of screen lines, 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 vice versa, the smaller the dot expansion. Therefore, you should try to maintain "zero pressure" in 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. (To be continued)