This paper introduces a new space for the accurate application of flexographic printing through the introduction of flexographic printing plates in terms of chemical composition, characteristics, structure, etc.
After more than ten years of development in China, flexographic printing has begun to take shape and began to occupy a place. The knowledge of flexo printing has been greatly popularized, the threshold for flexo printing has been greatly reduced, and the economic and environmental advantages of flexo printing, we have reason to believe that the development of flexo printing will usher in a better spring. However, in actual flexographic production, the flexographic plate, which is one of the main equipments for flexographic printing, is still vague and improper in use, operation and understanding. So how can we choose the most suitable flexo model? How to judge the quality of flexo? When problems occur in the printing process, is it all attributed to flexo?
Let us try to briefly sort out the application principles of flexo.
1. The chemical composition of flexographic printing plate: Different chemical composition will affect the ink adaptability of flexographic printing plate. Different ink types (water-based ink, solvent ink and UV ink, etc.) must use the corresponding flexographic plate material. The flexo only suitable for water-based inks will be printed with UV inks by mistake, which will reduce the printing durability of the plate and the printing quality may be very bad.
2. Hardness of flexo plate: Hardness is an important indicator to distinguish the quality of flexo plate. The softer flexo plate is suitable for printing rough printing substrates and simpler designs, and the harder flexo plate is suitable for designs with rich printing levels. The hardness is usually related to the thickness. The greater the thickness, the lower the hardness. For example, the printing of corrugated boxes should use softer and thicker flexo plates. The commonly used thickness in China is 3.94mm, and the flexo plates with a hardness of about 34 Shore. The folding carton, flexible packaging, etc. should be used correspondingly harder and thinner flexo, the usual thickness is 2.30mm, 1.70mm and 1.14mm, etc., the hardness is from Shore 57 to 77 degrees.
3. The physical structure of flexo plate: it is divided into single-layer plate and multi-layer plate. Nowadays, most of them are single-layer plate. Multi-layer plate only has one more layer of elastic foam than single-layer plate. Expanded outlets. Due to the application of elastic double-sided tape, the multi-layer version is gradually being eliminated.
4. The thickness of the flexo plate: the thickness of the flexo plate is usually 3.94mm as the cut-off point for corrugated carton printing, the thick plate is higher than 3.94mm, and the thin plate is lower than 3.94mm. Most domestic corrugated box printing currently uses flexographic plates with a cut-off thickness of 3.94mm, and thin plates with thicknesses of 3.18mm and 2.84mm are rarely used. For folding cartons, plastic bags and flexible packaging flexo, of course, use a thinner flexo plate, the thickness is not greater than 2.54mm, the thinnest even reaches 0.76mm.
Compared with the thick version, the thin version technology has the following advantages: vivid reproduction of patterns and colors, expanded dot reproduction range, enhanced printing contrast, stable "relief" on the plate, making high-speed printing a reality, shortened plate making time, and less consumption of plate washing solvent. It is more environmentally friendly and more resistant to printing, reducing downtime for plate washing and printing plate thinness, which is conducive to saving the consumption of plate materials and resources.
5. Traditional flexo and digital flexo: Digital flexo does not require film when making a plate, and data information (printed graphics) is directly transferred to the flexo by laser engraving a black film on the surface of the plate, avoiding the traditional plate making process Various unfavorable factors (such as film copy errors, the influence of dust during the main exposure and light scattering, etc.) have greatly improved the quality of the plate making and the stability of the re-replicated version, and it is also easier to obtain uniform and finer dots.
The main advantages of digital flexo printing are: the quality of the reprinted version is stable, the dot expansion value is smaller, the printing contrast is greater, the printing gradient is finer, the printing quality of the highlight area is high, and even if the main exposure time is longer, the highlight part can be kept open.
6. Flexo for special applications: flexo designed for printing inks with white base, usually containing esters, ketones or alcohols, which has a strong corrosion effect on flexo. Another special application of flexo printing is water-washing flexo printing. Water is used as a washing solvent, no chemical solvent is needed, it is very environmentally friendly, plate making is very fast, the entire process takes only 45 minutes, and has very good resistance to UV inks and UV varnish But it is not suitable for water-based ink.
7. Special flexo for glazing: At present, there are basically four types of post-press glazing: offset glazing, rubber roller glazing, ordinary flexo glazing and special flexo glazing. The special flexo glazing has the best effect. It is gradually replacing the other three glazing methods and is widely used.
8. Determine the quality of the flexo plate mainly from the following aspects:
(1) Ink transfer ability of flexo plate. Flexographic ink transfer must have two sides, including both the ability to obtain ink from the anilox roller and the ability to easily release the ink onto the printing substrate. Ink transfer ability is the fundamental and prerequisite for whether flexo is excellent.
(2) Whether the flexo printing is durable. This is crucial. For flexo production with a large printing volume, the flexo printing durability has become the key to reducing production costs. Flexo printing is not durable, not only because of the small number of prints, but also because of the large difference in printing results (between the same batch of products), which causes frequent pauses in printing production and high printing scrap rates, even reaching the level of customer receipt or claims .
(3) Flexo printing, especially wide-format flexo printing, has very high requirements on the flatness of the plate material. The flatness of the plate material should be +/- 15um to be ideal. The flatness of the plate is not good, which will cause the printing pressure to not reach the best, so that high-quality and uniform products cannot be printed in the end.
(4) Exposure latitude of flexo. The exposure latitude should be greater, which is conducive to the reproduction of details and the maintenance of open shaded graphics, so as to ensure that the flexo printing is durable and not easy to dirty. At the same time, the greater exposure latitude requires lower plate-making equipment and operators, and the chance of errors will be reduced.
(5) Compatibility of outlets and field. The flexo should have the ability to take into account both the dot and the field printing. In terms of dots, it is required that the dot expansion rate of the plate material is small, and the level reproduction is good; on the ground, the ink transfer performance of the plate material is required to be good, and the ink coverage rate is good.
In summary, the types and application principles of flexo cannot be summarized one by one. In actual flexo production, the corresponding analysis should be made according to the actual conditions of the printing machine, printing ink, printing substrate and pattern design used, so as to select The most suitable flexo model to achieve the best flexo effect.
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