Discussion on flexographic printing technology of

2022-08-22
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Discussion on flexographic printing technology of coated paper wine label (II)

IV. problems in post-processing

online follow-up processing of flexographic wine label generally includes sheet breaking and die cutting. When cutting sheets online, semi-finished products sometimes stick together and are difficult to separate, which is mainly caused by excessive static electricity. The general solution is to install more Static Eliminators in the second half of the paper path, or add humidifiers in the subsequent units of printing. As flexographic printing is a new printing method in China, it is not standardized in many aspects. Therefore, many people always require flexographic printing based on the standards of other printing methods. This is a misunderstanding, because the standards of other printing methods are not universally applicable. Compared with other printing methods, flexographic printing has both advantages and disadvantages. Therefore, we must pay attention to giving full play to the advantages of flexographic printing and abandon or overcome its disadvantages

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1 About the printing equipment

when printing the wine label of coated paper with the unit flexographic printing machine, due to the long paper path, it is very easy to appear the phenomenon of inaccurate overprint after multiple infrared drying, resulting in some unnecessary waste when printing multi-color printing, especially when printing paper with water-soluble ink. The control measure of this phenomenon is to install humidifier to reduce the pumping range of printing as much as possible

when using satellite flexographic printing machine for printing, the embossing cylinder will be heated and expanded to increase the pressure, which will affect the printing quality and damage the printing plate. Therefore, it is generally necessary to install a cooling device in the embossing cylinder. In addition, due to the short distance between the printing cylinders in the satellite flexographic press, it is difficult to dry, so it is necessary to install an efficient drying device in the satellite flexographic press

2. About printing ink

in the printing process, the viscosity of the ink will directly affect the transfer characteristics of the ink and the stability of the ink transfer volume in the ink transfer system. Generally speaking, when the temperature rises, the viscosity of the ink will decrease, and the stability of the ink transfer is easy to be guaranteed

at present, the inks used in flexographic printing mainly include water-soluble inks, UV inks, solvent based inks, etc. here is a brief introduction to the use and preservation of water-soluble inks and UV inks

(1) water soluble inks

water soluble inks are mainly composed of pigments, binders, defoamers and antifoam agents, which are generally alkaline

when using water-soluble ink: before printing, when adding ink to the ink tank, the ink should be diluted to the best viscosity, and the thixotropic ink should be fully stirred; In the printing process, the pH value of the ink should be kept at 8.5 ~ 9.5. Ensuring that the ink has a suitable pH value is conducive to maintaining the appropriate viscosity of the ink, and the drying speed of the ink tends to be reasonable, so as to reduce the paste failure

the auxiliary agent for adjusting the pH value of ink is pH stabilizer. Add an appropriate amount of stabilizer. Excessive addition will affect the hue of prints and reduce the friction resistance of graphic parts of prints

the preservation of water-soluble inks is also crucial. Reasonable preservation will create good conditions for the reuse of inks. Water soluble inks contain ammonia and amines that keep the ink alkaline, as well as auxiliary solvents such as ethanol or isopropanol, which are highly volatile substances. Therefore, water-soluble inks must be kept sealed. Because the used ink contains some additives that adjust the performance of the ink, if it is directly injected into the original ink, it will inevitably cause the original ink to deteriorate. Therefore, the used ink should be stored separately and used up as soon as possible to reduce losses. Before using the remaining ink, a certain amount of raw ink should be added and adjusted appropriately. In addition, some dark or interchromatic inks (such as black ink, green ink, etc.) have high thixotropy, especially at low temperature. Therefore, the dimensional tolerance requirements of water-soluble inks are very low, and the temperature should be controlled at 20 ~ 25 ℃

(2) UV ink

when using and storing UV ink, the following points should be achieved:

① UV ink should be strictly stored away from light and heat, and the storage temperature should be controlled at 5 ~ 30 ℃, which affects some vertical installations. The shelf life is generally half a year

② it needs to be stirred evenly before printing on the machine. Special active diluent or UV varnish and a small amount of ethanol can be used for dilution

③ special UV cleaner or ethanol can be used for cleaning

④ we must pay attention to the luminous intensity of the light source, and the aging UV lamp should be replaced in time to prevent the printing ink layer from being completely cured, resulting in print adhesion. The service life of UV lamp is generally 800 ~ 1000 hours. Note that frequent startup will significantly shorten the service life of the lamp

⑤ try to avoid direct contact between the ink and the skin to prevent irritation and allergy, and wash it in time after contact

3. About substrate materials

flexo printing is suitable for a wide range of substrate materials, generally paper, film, self-adhesive, etc. Here, we will only take the printing of paper wine labels as an example. Generally, when printing paper, especially when using water-soluble ink for printing, it is very easy to have inaccurate overprint. The main reason is that the paper is an absorbent material, and after the water-soluble ink is printed on the paper, the water in it will be absorbed by the paper, and then dried by infrared. Such repeated times will inevitably cause inaccurate overprint due to the pumping up of the paper. The way to solve this kind of fault is to humidify the printing locally and reduce the gear of infrared drying as much as possible. In addition, appropriately increasing the machine speed can also alleviate this kind of problem to a certain extent

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