Manufacture and application of flexographic ink
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Flexographic printing is a mainstream printing method of the future, based on the various cutting-edge technologies used by flexographic printing manufacturers, such as UV (ultraviolet) curing, EB (electron beam) curing and ink painting technology.
First, flexographic printing and ink review
Flexographic printing refers to a printing method in which ink is transferred by an anilox roller using a flexographic plate. The layout of the plate is the same as that of the letterpress plate, and the image is partially raised above the non-image portion. Moreover, the flexographic printing plate is also a direct printing method using a rubber plate or a photosensitive resin plate and a quick-drying liquid ink. It can be seen that flexographic printing is a branch of letterpress printing, which differs from conventional letterpress printing in the composition of the printing plate, the composition of the ink and the embossing process.
The transfer of images during printing is done by a photo-chemically reactive printing plate or a patterned rubber band. The ink is conveyed to the printing plate by an amount of ink through an anilox roller in an ink reservoir. The reticulated transfer roller is composed of a chrome alloy and is coated with a ceramic wear layer or an ion sprayed ceramic wear layer. The amount of ink delivered is determined by the number of lines on the surface of the transfer roller. The surface of the thick anilox roller has about 120 lines per inch, and the transfer roller for printing fine patterns can reach 500 to 900 lines per inch.
Flexo inks have a wide range of applications and a wide variety of substrates, so there are many types of flexo inks that match them. Can be roughly divided into the following four categories:
1. Solvents: Types including various mixed solvents.
2. Alcohols: including various alcohols.
3. Aqueous: The solvent used is water rather than organic solvent.
4, UV curing (UV) class: Unlike ordinary ink, it does not contain solvents, only contains some reactive diluent. The respective characteristics are shown in the figure below.
As far as the main components of the flexographic ink are concerned, they are composed of a toner (pigment or dye), a resin (linking material), a solvent, an auxiliary agent and the like. Most of them are liquid inks mainly based on volatilization and drying, but there are also a few inks which are dried in the form of oxidized conjunctiva, osmosis and the like. Since the flexographic printing press uses an anilox roller to transfer ink, the viscosity of the flexographic printing ink should not be too high, generally between 1 and 100 poise.
Flexographic inks are typical solvent inks, namely liquid inks, which are dried by evaporation of solvents. This ink is transferred to the flexographic plate by an anilox roller (a dent on the ink roller). According to its color body, it can be divided into the following three categories, namely: dye type, pigment type, dye pigment type. Generally, the dye type has a lower viscosity and the pigment type has a higher viscosity to facilitate dispersion and storage (preventing sedimentation). Due to the different materials of the substrate, the resin and solvent in the ink formulation are also various, but the ink must have suitable viscosity and fluidity, dryness is also required, and there is no residual odor after drying. The adhesion of the object is good and does not work on the plate.
The development of flexographic printing is getting faster and faster. More and more ink manufacturers are developing high-quality and low-cost inks, creating more opportunities for flexographic printing. The prospect of water-based inks is the most promising. We will focus on the latter. Introduce this type of ink.
Second, solvent-based flexographic ink and alcohol-based flexographic ink
Solvent-based flexographic inks are generally used in the printing of non-absorbent substrates, and the prints are dried by evaporation of the solvent in the ink.
According to the selected colorant, it can be further divided into dye ink and pigment ink. The dye ink has good transparency and is printed on the surface of a metal substrate such as aluminized paper, which can exhibit a special gloss effect. Pigment ink, mainly used for nitrocellulose (NC) resin as the main material for the printing on the paper; 50% olefin resin and acrylic resin as the main material for printing on the surface of the plastic film; used on the surface of aluminum foil The printing ink can be selected from dye ink and transparent pigment ink. The pigment ink can be selected from pigment color ink, which makes the ink have high gloss, good transparency and resistance performance better than light-resistant dye ink.
The alcohol type ink is obtained by dissolving an alcohol-soluble resin in an alcohol-based solvent, and further adding an ester solvent and a cellosolve to the colorant. Most of the flexographic printing of various plastic films for packaging uses this type of ink. The resin used for the binder is mainly maleic acid resin, polyamide resin, acrylic resin, epoxy resin, alkyd resin, nitrocellulose, ethyl cellulose, and the like. Commonly used solvents are ethanol, n-propanol, isopropanol, n-butanol, isobutanol, etc., and sometimes a small amount of aromatic hydrocarbons and ester solvents are added. The ink is generally not used in a single solvent, but is mixed with several solvents to improve solubility and drying properties.
Third, water-based flexographic ink
For ink manufacturers, ink in the flexo market is a profitable market. It is also a potential market worth developing. In the end consumer market, ink is used primarily in the printing of paper, cartons, corrugated paper, household paper and pre-printed applications. In cardboard printing, flexo inks are increasingly used. In addition, the further development of ink technology has greatly improved the solubility of ink.
The flexographic ink refers to an ink which uses water, alcohol, cellosolve or the like as a solvent, and is mainly used for printing of a highly absorbent substrate such as paper, cardboard, corrugated paper, cement bag, and the like. Depending on the type of resin used, the ink is mainly composed of an alkali-soluble type, an emulsion type, and a water-soluble type. The ink has the characteristics of not containing excessive volatile organic solvent, is non-flammable, does not damage the health of the ink manufacturer and the printing operator, and has no pollution to the atmospheric environment; ink as a new printing ink, toxic and harmful substances remain in the printed matter. The amount is small, the pollution to the packaged goods is small; the overall environmental quality is improved.
A high-quality flexographic ink should have the following properties: no excessive odor, good flow properties and viscosity, fast and proper drying, no influence on plates and printing presses, and easy printing on the press It should have good storage stability and long-term storage of precipitates. It should be easy to use after stirring.
Fourth, the composition of the flexible version of ink
Colorants are generally made of alkali-resistant pigments, including organic and inorganic pigments, which require good dispersion in aqueous systems, including organic and inorganic pigments, which are essentially the same as colorants in solvent inks. At present, in order to meet the decorative needs of packaging materials, pigments with bright colors and strong tinting strength are often used. Dyes are also sometimes used to increase the color strength of the ink and to improve the viscosity. However, no matter which kind of pigment is used, it is preferably treated with an anionic or nonionic surfactant, because the surface tension and polarity of water in the ink are relatively large, and the dispersion of the colorant is relatively difficult, such as untreated. It will result in uneven distribution.
The binder of the flexographic ink is an important factor affecting the quality of ink, especially high-precision ink. There are many types, which can be selected according to different occasions and uses. It is not an exaggeration to say that the current research and development of ink in domestic ink companies focuses on the research of binders. The following is a brief introduction to the properties of commonly used binders.
(1) water-soluble binders, including polyvinyl alcohol, hydroxyethyl cellulose, and polyvinylpyrrolidone, etc., such a binder can be permanently dissolved by water, and thus the ink used therein can be subjected to a certain range of use. Restrictions can only be applied to products where the ink film is not exposed to water.
(2) Alkali-soluble binders Currently, acrylic resins are commonly used as binders at home and abroad.
In fact, acrylic resins come in three forms: alkali soluble, nonionic polymeric and acid soluble polymeric. In the ink, the alkali-soluble type is mainly used (because the non-ionic film is relatively hydrophilic and has poor water resistance; the acid-soluble type has no suitable volatile acid and is odorous). In the ink preparation process, ammonia water, amines or a mixture thereof is usually used to make the resin a water-soluble amine salt. After being printed on the substrate, the ammonia film is evaporated, and after the ink film is dried, it has a certain degree of gloss, water resistance and abrasion resistance.
(3) The diffusion type binder is a fine resin particle suspended in water, and is usually called a latex. Propylene, ethylene or styrene-butadiene polymers are typically included in such binders. Latex has more significant advantages than dissolved resin: First, latex can have high density and low viscosity, which can produce high quality thin coating; second, latex can contain considerable polymer molecules, macromolecules They are more resistant to abrasion and heat than small molecules, and their adhesion is good. Although macromolecules usually cause the viscosity of the solution to be too high, latex does not have this problem, which is a characteristic of latex inks. The biggest problem with latex inks is that printing is difficult and difficult to clean, because once the latex has set, it becomes an insoluble material, so latex is often used as a coating ink. In addition, the latex can also be mixed with an alkaline binder, which retains some of the properties unique to the latex and has the printability of the alkali-soluble binder.
Fifth, the application points of ink printing on the machine
Water-based ink is one of the main factors affecting the quality of flexographic printing. Especially in cigarette packaging, it is a factor that cannot be ignored. The following should introduce the matters that should be paid attention to when using flexo-printed spot color ink in cigarette package printing.
Precautions before printing:
(1) The ink should be sealed and stored at a temperature of 15 to 25 ° C and a relative humidity of 65% to 70%. In addition, according to the shelf life of ink, reasonable purchase and storage of ink. If you do not have the above storage conditions, you can also use the method of short-term purchase and storage, and place the ink next to the printing machine three days before use to adapt to the scene.
(2) Since the spot color inks are all prepared by mixing two or more primary color inks, it is required that each spot color ink is prepared in advance according to the standard, and the primary color ink and the prepared spot color ink are measured. PH value and viscosity are used as technical parameters for analysis.
(3) Ink should be fully stirred two hours before the machine is on, at least for half an hour, then measure the PH value and viscosity of the ink, adjust the PH value and viscosity according to the printing environment and other factors, so that the ink performance is stable and the printing effect is achieved. optimal.
Precautions in printing: (1) When the ink is applied every shift, the newly blended ink should be used. After about one hour, that is, the performance is relatively stable, the previous remaining ink can be added.
(2) Due to the large thixotropy of the ink, it is necessary to add appropriate additives (stabilizers, retarders, pH adjustment liquids, etc.) at intervals to adjust. The specific interval time and dosage should be determined according to the specific conditions to control the PH value and viscosity of the ink in the printing process to maintain the best performance.
(3) Since the color of the spot color ink will change somewhat during the long-time printing process, the operator is required to fine-tune the ink according to the color difference to ensure that the color difference is within the allowable range.
Sixth, the control index of ink
Ink control indicators mainly include viscosity, pH and dryness.
1. Mastery of ink viscosity. Ink viscosity refers to the ability of the ink to hinder the movement of other molecules under the influence of its internal internal forces. A certain amount of ink is generally used to flow through the capillary at a fixed temperature (typically 20 ° C), and the time required for recording is measured as a viscosity.
The viscosity of the ink is the main factor determining the transfer performance of the ink, the firmness of the print, the amount of penetration and the gloss. The viscosity of the ink is too high, the color is darkened, the ink consumption is increased, the drying speed is slowed down; the viscosity is too low, the color changes, and the dot size is enlarged to cause a decrease in printing quality.
In flexographic printing, the viscosity of the ink is not determined solely by the viscosity of the ink itself, but by all factors that affect the viscosity of the ink, such as the anilox roll, image condition, and other factors. The viscosity actually used should be easily circulated to the squeegee by the pump, and can be transferred to the printing plate and the substrate in the same amount, so that the density of the printed matter and the image dot are not changed significantly, and the printing quality is maintained. The consistency of the ink must be consistent.
The ink viscosity should be different when printing the screen version and the field version. Generally, when the screen is printed, the viscosity should be slightly lower, and the printing speed of the printing machine also affects the viscosity of the ink; when the printing speed is high, the viscosity of the ink is lower, and when the printing speed is low, the viscosity of the ink is higher. Usually, after the anilox roller and the transfer roller are added with the squeegee, if the striped ink is observed, the viscosity is suitable; if the ink droplet appears, the viscosity of the ink is too large; if the dot is not visible at all , indicating that the viscosity is too small.
2. Adjustment of ink viscosity. The adjustment and control of the viscosity of the ink should be controlled by controlling the temperature of the ink and the amount of the diluent. In order to ensure the consistency of the printing quality, the consistency of the ink viscosity must be maintained. The following table gives the amount of diluent:
As can be seen from the table, the temperature has a large influence on the amount of diluent increase. Dilution ink has a large effect on the final density of the print. For example, the temperature of the ink before printing is 10 ° C, the viscosity is 25 S, and after a short period of operation, the temperature of the ink is raised to 20 ° C, and the viscosity is lowered to 18 S. As a result, the ink layer is diluted and the density is lowered. Therefore, in actual production, the temperature of the ink used should be stabilized at the temperature of the printing shop. For the temperature of the ink due to friction during printing, the viscosity changes, and it can be considered to be solved by cooling.
Ink is particularly sensitive to the addition of diluents, so care must be taken to ensure viscosity. Avoid excessive (not more than 5%) to prevent the ink viscosity from being too low. Because surface tension will form bubbles, generally hard water will produce more foam than soft water. For example, to reduce bubbles, it is necessary to add antifoaming agent. Do not use paraffin-based defoaming agent, otherwise it will affect the printing durability of the printing plate. The defoaming agent containing a silicon-based component must be diluted with water and added to the ink while continuously stirring, and a silicon-based defoaming agent having a high concentration of undiluted is added to the ink to form pinholes in the printed matter. Deinking occurs.
3. The pH of the ink. Ink generally controls a pH of about 9 and is slightly alkaline. If the pH value is too large, it will cause the ink to be denatured (meaning that the ink becomes thin when it is subjected to an external force, and a property that returns to its original state after standing) changes to make the ink thick. The ink will flow into the inside of the scraper or rubber roller, causing the ink to dry. The amount of ink on the anilox roller is insufficient, and a white leak or the like is generated during printing. The pH value is too high, the viscosity is lowered, and the drying is slow. The method of simply adding a small amount of water to reduce the viscosity of the ink to adjust the fluidity of the ink is not satisfactory. For this reason, the alkaline substance should be appropriately added to adjust the ink. PH value.
4. Dry. The drying of ink has always been confusing. To solve the problem of drying ink, comprehensive adjustment is needed. For example, the comonomer composition of the ink, the neutralizing agent, the co-solvent and the auxiliary agent are screened and improved in various directions. The drying speed of the ink will be greatly improved to meet the requirements of the existing production speed. If the drying system of the existing equipment can be adjusted and improved accordingly, the drying of the ink will be more ideal.
5. We will concentrate all the previous opinions on the use of ink, except for the following aspects: shake well before use or sampling, confirm the viscosity under the same test conditions; according to the design condition of the plate and the printing Speed, determine the viscosity of the water-based ink. When the viscosity is high, it should be diluted with water or a special solvent. Pay special attention to the viscosity of the water-based ink when it is diluted.
If the viscosity is too low, it is best to adjust with the original ink and use as little or no thickener as possible. When the color concentration is high, it should be properly rinsed with a special dilution agent to adjust the viscosity; the viscosity change is often paid attention to during the printing process, and the viscosity is recorded. In the process of use, when there is more foam, a small amount can be added. Defoamer (usually controlled within 0.2%); at the end of printing, the relevant mechanical equipment should be cleaned as soon as possible; unfinished ink should be returned to the barrel (but not mixed with the original ink), kept in a cool and cool place; restart When it is better to filter with a mesh of 90 mesh or above, use it first.
In recent years, the conventional ink has the disadvantages of slow drying, poor gloss, and easy paper shrinkage. With the application of new materials and the improvement of printing equipment and other related conditions, significant improvements have been made. The application of the domestically produced flexographic high-precision ink shows that the drying speed can reach 200m/min and the resolution has exceeded 150 lines.
Seventh, the analysis of the key points of flexo printing spot color ink
The formulation of spot color inks refers to the mixing of one or more inks and the addition of certain auxiliary materials to adapt to the entire process of printing. The formulation of the ink mainly includes two aspects: on the one hand, the adjustment of the ink printing suitability, including the viscosity, adhesion, fluidity, dryness, etc. of the ink; on the other hand, the color matching of the ink, the main discussion in this section is Color matching of flexo inks
The following basic operations should be noted for the flexo-tone color matching: the ink preparation must be carried out under the standard light source D65. If it is during the day, it can be carried out in the sunny north window. According to the standard color sample analysis provided by the customer, there are several kinds of hue and which are spot colors. With the use of spot color chromatography, the number of anilox lines to be selected for the printing of the spot color samples is determined, and which kinds of primary color inks are used for the spot color samples, and what are the main colors and the auxiliary colors. According to the quality requirements of the printed products, the main and auxiliary color inks suitable for the color ink are selected. Dispense a small amount of spot color ink for proofing and note the proportion of each ingredient. The color difference between the proof and the standard was measured by a color difference meter and the final adjustment was made. According to the proportion of the final adjustment, a certain amount of printing ink is prepared. After the varnish is applied, the hue and brightness are as close as possible to the standard. Do a good job of recording, and record the proportion and viscosity of the final adjustment of this spot color ink for future reference.
There are different requirements for spot colors of different shades. Any ink that is blended with a certain proportion of dilute agent or white ink in the primary color, intermediate color, and complex color ink is collectively referred to as a light-colored ink. The mixing process of the light-colored ink is as follows: the light-colored spot color in the standard sample is analyzed and judged. Determine the number of anilox lines in the printing, the ratio of the primary ink to the thinner (or white ink). A small amount of primary color ink is gradually added to the thinner. If the primary color ink is more than one type, the amount of the primary color is increased first, and the amount added is small. Measure the viscosity of the ink at this time to see if it is suitable for printing. If not, adjust it with ink additives. Use a flexo wiper (consistent with the number of printed anilox lines) to scrape the sample and compare it to the standard color sample until the hue and brightness are met. Finally, be sure to do a good job of recording this spot color ink.
The preparation of flexo-printed dark ink should also be noted. The inks blended with the three primary colors in different proportions without any thinning agent are called dark inks, and the mixing process is as follows: the dark spot color in the standard sample is analyzed and judged, and the net at the time of printing is determined. The number of lines and the approximate proportion of each color ink. Observe the brightness of the complex color with reference to the gray scale. Add the secondary color ink to the main color ink in turn, first add a large amount, and then add a small amount. Measure the viscosity of the ink at this time to see if it is suitable for printing. If not, it can be adjusted with ink additives. Use a flexo wiper (consistent with the number of anilox rolls) to scrape the sample and compare it to the standard color sample until the hue and brightness are met. The same requirements, in the end must be done in this special color ink deployment record.
For the matching of the color code of the flexo screen, when the color sample provided by the customer is a color sample which is superimposed with different dots of the three primary colors and at the same time, it is required to print the effect in the field, it is necessary to involve the deployment of the network color sample. . The color depth of the screened prints is adjusted by the percentage of the unit area of the internet point, so the percentage of the dot combination of the colors presented by the different color blocks on the chromatogram can be used for the ink blending.
Precautions in flexo spot color ink blending: The compatibility of the binder properties, pigments and binders determines the stability of the ink, so inks produced by different manufacturers have their own shelf life. Before adjusting the ink, first pay attention to whether the ink has expired and deteriorated. It is best to use a single color ink produced by the same manufacturer. The type of primary color ink should be as small as possible during the mixing process, otherwise the brightness of the required ink will be lowered, the saturation of the color will be affected, and the ink color will be gray and unclear. During the preparation and printing process, the viscosity should be ensured to be appropriate and stable, because too much viscosity change will cause the ink to change in depth. During the printing process, it is also necessary to check the change of the color of the substrate. For example, if the whiteness of the paper changes, the brightness of the spot color will change.
Eighth, an overview of the development direction of flexo ink
For ink manufacturers, ink in the flexo market is a lucrative market and a potential market worth developing. In the end consumer market, ink is primarily used in the printing of paper, cartons, corrugated and preprinted applications. In cardboard printing, flexo inks are increasingly used. In addition, the further development of ink technology has improved the solubility of ink. The study of new surfactants and the use of cleaner solvents have improved the printing of ink laminates. Narrow web reel printers have been using ink for a long time. Based on environmental protection and the health of printers, wide-format web-based rotary printers are now also interested in ink. But for the foreseeable future, the web-based rotary printing market still has demand for solvent-based inks. At least in our opinion, inkjet is their ultimate choice in order to follow the V0C (Organic Volatile) regulations and invest a lot of money in equipment manufacturers. From the perspective of a wide web rotary press, a significant trend in wide web rotary film printing is the shift to ink printing. Today's inks can be applied to a number of substrates, mainly polyester films and laminate films.
Ink manufacturers regard environmental issues as a driving factor in shifting to production ink. Ink and wash have obvious advantages in terms of V0C emissions, environmental safety, health and safety of printers, and reduced storage and transportation costs as non-combustible materials. With the perfection of resin and additive technology, the functional properties of ink formulations will have a powerful impact on solvent-based inks and energy-curing inks.
The future trend is that film printing will increasingly use inkjet flexo instead of solvent-based ink flexo. People are looking forward to the emergence of better quality water-based film printing inks. Although the benefits of ink are environmentally friendly, the success of ink does not mean that solvent-based inks will exit the flexo ink market. The biggest advantage of ink is that V0C volatiles are rare, but the person in charge of research and development in some well-known foreign companies believes that the use of solvent-based inks has not been reduced for this reason.
Whether to use solvent-based ink or ink, one is the cost issue. For some printing companies, the use of ink for environmental reasons increases production costs. Another aspect is to look at local environmental protection regulations for V0C emissions. Solvent-based inks are slightly less expensive if you don't have to deal with volatiles.
UV (ultraviolet) and EB (electron beam) curing technologies continue to be the direction of the ink industry. The flexo plate continues to favor radiation-curable inks. Of course, UV/EB technology is also the most concerned issue for flexo packers. Narrow web web label printers have successfully used cured inks and gained a favorable position in the printing market. For the rest of the printing market, printers are waiting for radiation-curable inks. Undoubtedly, from the perspective of print quality, flexo printing using UV inks is the best printing method of all current flexographic printing techniques, and it is superior to flexo printing of solvent-based inks and inks.
Like other printing methods, UV/EB inks face some obstacles. The UV/EB technology in the packaging and printing market faces several challenges. The biggest obstacle is that UV/EB technology cannot adapt to traditional printing techniques. Before adopting UV/EB technology. Printing companies must install curing equipment, and although their prices are declining, they are still a big investment. In addition, the cost issue is also one of the main reasons that have plagued the further development of radiation-curable inks. Raw materials and equipment are still expensive. According to experts' estimates, if the cost of ink can be close to other types of inks, then the speed of development will be several times faster than other inks.