Control of dot increase in the most popular flexog

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Control of the increase of flexographic printing point

although flexographic printing develops rapidly, it is not an omnipotent printing method. In the actual printing samples, the missing points in the highlight and the solid printing in the dark tone are the prominent problems of flexographic printing at present. This is because the dot increase in flexo printing is more serious. The so-called dot increase refers to that the dot area on the substrate is larger than that of the corresponding part on the printing plate. In theory, point increase includes mechanical point increase (also known as physical point increase) and optical point increase. In practice, it usually refers to the increase of mechanical points

why does flexo printing have a serious dot increase? Because flexographic plates are soft and elastic, under the action of printing pressure, the image and text size on the printing plate will become larger, that is, the dot area will become larger. Because the flexographic plate is a relief plate, when printing with low viscosity water-based ink, as long as the pressure between the plate surface and the substrate surface is slightly larger, the ink on the plate will spread around, making the point form an enlarged point with thin intermediate ink layer, thick edge ink layer, similar to hollow, which is the increase of mechanical points in flexible printing

there are many factors to increase the starting point. Improper control of any link in technology, process, equipment and materials may cause point increase. Now it introduces how to control point increase from five aspects:

(I) printing pressure: with high printing pressure, there are more inks transferred from the printing plate to the substrate, and the adjustment clarity is enhanced, but too high pressure will lead to point increase, resulting in plate pasting or ghosting. The pressure control is generally expressed by the compression of the printing plate, which should be strictly controlled within 0.1mm. The pressure of fine line printing should be relatively small; When printing on the spot, the pressure is relatively higher

(II) printing plate: hardness is a key index of flexo performance. The contact between the printing plate with low hardness and the substrate is more uniform, and the ink color printed is also more uniform, but the hierarchical reproducibility of the image is poor, and the point increase is more significant. Therefore, when printing the substrate with smooth surface, the hardness of the printing plate is required to be high, while when printing the substrate with rough surface, the hardness of the printing plate is required to be low

The consistency of the thickness of the printing plate can ensure the minimum increase rate of the midpoint of the eye tone printing, and improve the tone reproduction ability, so as to improve the reproducibility of the highlights and dark tone details

for the Saili version, for the thin version with a thickness less than 3.18mm, the allowable thickness error between each version is ± 0.025mm, while the thickness error of each part of the same version is ± 0.013mm. For thick plates with a thickness greater than 3.94mm, the allowable thickness error between each plate is ± 0.046mm, and the thickness error between parts of the same plate is ± 0.025mm

(III) corrugation roller: the corrugation roller with high line number can form a thinner and more uniform ink layer, which can meet the printing requirements of prints with rich layers, especially the needs of highlight parts, and can reduce the dot increase during printing and maintain a uniform and constant ink transfer

but it's not that the higher the number of lines of the corrugated roller, the better. First, the corrugated roller takes a long time to process and the price is high; Second, it will cause insufficient ink during printing

ink distribution system. In flexographic printing, the chemical industry in the United States usually launches a double reverse investigation on our tires. There are three kinds of ink metering roller arrangement methods:

ink metering system for rubber roller scraping: This is an ink metering system that controls the ink metering by changing the relative speed of rotation between the rubber roller and the roller and the pressure between the two rollers

this method can only achieve limited control of ink metering, and with the increase of printing speed, more ink is transferred to the stripper roll, which not only increases the ink density value, but also it is difficult to control the accuracy of ink metering under high-speed operation, and the point increase also increases

2. Ink metering system with positive doctor blade: the doctor blade can scrape away the excess ink on the surface of the roller and only store the ink in the hole, so the ink metering hardly depends on the printing speed. When the printing speed is 200~300 M/min and China will host this year's APEC summit, the ink volume can be well controlled, but when the printing speed is more than 300 m/min, the ink cannot be well controlled

3. Ink metering system with reverse doctor blade: this device can still control the ink well when the printing speed reaches 500 m/min. Therefore, many printing plants use this device for high-quality dot printing

(V) ink: dot increase is related to many factors and properties in the ink, such as pigment, viscosity, viscosity and drying speed. The chemical and raw material zone is crowded with audiences looking for new material technology. With high coloring power, the ink layer during printing can be thinner, which can reduce the dot increase. Viscosity is the most important control index in the application of flexo ink, which directly affects the transfer performance of ink and the quality of flexo products. If the viscosity of the ink is high, the dot increase will be smaller, but if the viscosity is too high, it will affect the transfer performance of the wrinkle roller, resulting in ink unevenness, paste, poor drying and other ills

reprinted from: paper industry information

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