Application of water-based ink in flexographic pri

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The application of water-based ink in flexographic printing

the federal air cleanliness regulation of the United States has confirmed that the solvent used in flexographic printing ink binder will produce air pollution, cause respiratory diseases, and endanger people's health. Regulations controlling the discharge of hazardous wastes, such as the regulations on resource protection and recycling, require that all wastes discharged from flammable inks must be treated to a certain extent, and the wastes must be classified into different levels according to the type of solvent contained in the wastes and the degree of harm

about 60% - 80% by weight of inks commonly used for film printing are solvents such as ethanol, n-propanol, isopropanol, and n-propanol acetate. These solvents are volatile organic compounds and flammable substances. Some inks and thinners also contain butanone, isoacetone, toluene and ethylbenzene acetate, which are classified as F-grade polluting wastes

the use of water-based ink provides a way to reduce environmental pollution, and reduces many restrictions on printers

water is an acceptable control technology

the United States Environmental Protection Agency has formulated and issued guidelines for national and regional prevention and control agencies of air pollution, so that they can determine which technology can reduce the impact of pollution in the production process. These guidelines are called control technology guidelines (CTGs). The control technical guidelines set the minimum allowable limits of air pollution for specific industries or specific production processes. The basic control technology guide for flexographic printing was issued in 1978, which provides three feasible control technologies:

1. Addition control technology: control by adding catalyst, thermal oxide, carbon absorption technology or solvent recovery. Pollution can be reduced by 60% by adding control technology. This is the preferred method among the effective control methods for large amount of waste discharge. The additive control technology requires that the additive must play at least 90% of the role of degrading pollution, so that the efficiency of degrading pollution in the whole production control process can reach 67%

2. high solid content ink control technology: the volume ratio of solvent in high solid content ink is generally not more than 40%. Compared with the control technical standards of environmental protection agencies, in fact, there is no really high solid content ink using solvents

3. water based ink control technology: the proportion of water in water-based ink should occupy at least 75% of the liquid part of the ink, which is a volume percentage, not a weight percentage

market status of water-based ink application

the application degree of water-based ink depends on the type of substrate being printed and the end use of the printed matter. Paper and low board are easy to accept water-based ink and water-based paint. When water-based ink is applied to other non absorbent substrates, such as film and metal foil, these substrates need special treatment to make their surfaces well accept water-based ink or water-based paint

as early as a few years ago, the printing technology of water-based ink on paper and paperboard has been quite perfect, and it is not difficult to achieve high-quality printing. However, with the emergence of film printing, using water-based ink for film printing, there are still some insurmountable problems in some aspects. However, with the continuous development of new resins in inks and the use of other advanced technologies, film printing with water-based inks has achieved considerable success

printers should note that the reliability of water-based inks has greatly exceeded that of a few years ago. In the past few years, there have been many technological achievements in water-based inks, which are enough to meet the current challenges. In the past month, ink manufacturers have brought us many new technological improvements to further meet the requirements of customers to reduce volatile organic compounds. Similarly, changing the surface characteristics of the substrate, using new resins and changing the additives in the film will also help the substrate accept water-based inks, including other commonly used products that were not easy to accept water-based inks before

in practical application, the key to determine whether water-based ink can be used is after the water-based ink is put into use. However, if the printer really wants to use water-based ink for printing, it still needs to make some necessary improvements and train the printing operators, so that the water-based ink can be well used in printing

obstacles to the application of water-based ink in printing

since the invention of flexographic printing technology, water-based ink has been used in flexographic printing machines for paper and Paperboard Printing. Due to the good absorption characteristics of paper fibers, water-based inks have been successfully printed on paper and paperboard. Some inks and solvents (including water) are absorbed by paper and paperboard to heat up, and the other part evaporates into the surrounding air. But if the aqueous solvent is placed on the surface of the film or metal foil, it is like dropping water on the freshly waxed car body - the water will turn into droplets and roll around on the car surface. The reason is:

1. The surface tension between ink and substrate is different - especially the difference between water-based ink and solvent based ink, which requires that the surface tension characteristics of substrate can be changed in combination with local advantageous resources, so that water-based ink can be transferred and adhered to the surface of substrate more easily

2. Lubricating additives or dehydrating agents are often added to film substrates, which will complicate the application of water-based inks. Lubricating additives are added to the film so that the plastic bag can be easily opened during the packaging and filling operation. However, when water-based ink printing is carried out on the surface of the film, lubricating additives will leach to the surface of the film, forming an obstacle to the adhesion of water-based ink

3. The ink used in the printing of flexible packaging bags must be resistant to roughness and abrasion. The form of flexible packaging can play the display characteristics that other paper packaging does not have. The packaging of frozen food, agricultural products, ice cream and other hard items is required to be able to withstand the impact, so the packaging bag must be made of these resistant films. During the packaging process of the products in the packaging workshop, chemical reactions will also occur to confirm whether the notch processing of the tested impact sample is qualified, such as ammonia, water vapor, etc., so the printing ink must be able to resist the influence of chemical reactions

overcoming the above obstacles is the main challenge that packaging printers must face when choosing water-based ink for printing

basic conditions for printing with water-based ink

using water-based ink technology will cause some problems in printing machines and printing operators. This requires a comprehensive understanding of the functional components of flexographic printing press and corresponding improvements. When printing with water-based ink, the following equipment and settings need to be properly improved

1. ink transfer device: the ink transfer device is composed of ink bucket roller, ink metering roller and grain transfer roller. It should be noted that when printing with water-based ink, ensure that the metering function and structural performance of the ink metering roller are compatible with water-based ink. Because water-based inks (some similar to high solid content inks) use less solvents (i.e. water) and are easy to dry, it is necessary to make some changes to the ink roller and pay close attention to the possible corrosion and wear of the ink roller material. When printing with water-based ink, the metering roller must rotate constantly, because the water-based ink is very hard to dry, and it is very difficult to re wet each ink roller, which is important when the printing machine breaks down. At the same time, measures must be taken to automatically separate the printing plate cylinder and the grain transfer ink roller after the printing machine stops, otherwise the dry water-based ink will produce scratches on the ink roller and the printing plate

the old models generally do not have the above functions, so they must be improved during use, so that when the machine stops for any reason, the ink bucket roller and the ink transfer roller can rotate continuously and be out of pressure with the plate cylinder, otherwise the water-based ink will dry in the inking hole of the ink roll, and it is very difficult to remove, which will inevitably damage the ink transfer performance of the ink roll

in addition, the ink bucket roller and the ink transfer roller in the ink transfer device must also be replaced with new ink rollers that are conducive to the application of water-based ink. The soft rubber ink bucket roller often transfers too much ink, which cannot scrape the excess ink on the grain transfer roller, and is not conducive to printing. Therefore:

because the function of the ink bucket roller is to transfer the ink in the ink bucket to the metering roller, the harder the rubber used by the ink bucket roller, the less ink transferred. The standard hardness of ink bucket roller image glue with solvent based ink is shore hardness 65-75a, while the rubber hardness of water-based ink is required to be shore hardness 85-90a

inking holes for measuring and transferring ink are carved on the ink transfer roller. The thinner the line (the number of inking holes per inch), the shallower the inking holes carved, the less ink transferred. For example, a 360 line ink transfer roller will transfer thinner ink layers and less ink and solvent than a 200 line ink transfer roller. The ink transfer roller with the same number of lines but smaller ink hole volume will reduce the ink transfer volume. Using high-quality water-based inks requires less ink transfer than alcohol based inks, so it is necessary to improve the inking hole volume of the ink transfer roller to reduce the ink transfer

2. drying device: in order to better control drying and obtain a higher waste discharge rate, the original drying device must be improved or re equipped. Those drying devices suitable for solvent based ink drying are not suitable for water-based ink drying at all. The usual flexographic printing drying oven is to introduce air into the strong pressure area and blow it on the printing plate through slits and holes. This drying device is suitable for solvent based inks that volatilize rapidly and can be carried away by a slightly higher exhaust gas stream

but the use of water-based ink brings two problems. First, the actual amount of steam evaporated by heating water-based solvent in water-based ink is 10 times that of solvent based ink in the same volume

the second problem comes from the amine required to keep the resin and pigment in suspension in the preparation of water-based ink. If amines are completely transferred by scraping, they will remain in the ink layer, thereby reducing the adhesion of the subsequently overprinted ink layer

a more scientific method is adopted for the drying nozzle of the new generation drying oven and the type of air flow generated. Its latest design idea is to seek to make the air flow remove amine and water in a scraping action, and can remove the evaporated water vapor at a high speed

dryness is also a major factor affecting the printing speed. Low speed printers may not need a large amount of dry air flow, but on high-speed printers, if the flow of dry air is not appropriate, the printing speed will be limited

routine investigation results show that on the existing high-speed printing machine, using water-based ink for printing but only using ordinary drying device will reduce the printing speed by 20% - 30%, which is mainly due to the additional downtime required to dry the water-based ink

3. Substrate surface treatment: the film surface must be effectively treated before machine printing to improve its surface tension. Prepress treatment device (discharge or flame treatment) uses high-speed particles to bombard the film surface, making it easier to accept water-based ink. Discharge treatment can not only increase the surface tension of the film, but also make the film easier to accept ink wetting and adhere to ink. The principle is to prevent the lubricating additive from dialysis to the film surface temporarily

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