Flexographic printing is a mainstream printing method in the future. The reason is that various cutting-edge technologies used by flexographic printing manufacturers, such as UV (ultraviolet light) curing, EB (electron beam) curing, and ink-jet technology, can be seen.
1. Overview of flexo printing and ink
Flexographic printing refers to a printing method that uses a flexographic plate to transfer ink through an anilox roller. The layout structure of the printing plate is the same as that of the letterpress printing plate, and the protrusion of the graphic part is higher than that of the non-graphic part. Moreover, the flexographic printing plate is also a direct printing method using an offset printing plate or a photosensitive resin printing plate and quick-drying liquid ink. It can be seen that flexographic printing is a branch of letterpress printing, but it differs from traditional letterpress printing in the composition and imprinting process of printing plates and inks.
The image transmission in the printing process is accomplished by a flexible photosensitive resin plate (PrintingPlate) obtained by a photochemical reaction or a patterned rubber stick. The ink is in an ink storage tank, and a certain volume of ink is transferred to the printing plate through the anilox roller. The anilox roller is composed of chromium alloy, and the surface is coated with ceramic wear-resistant layer or ion-sprayed ceramic wear-resistant layer. The amount of ink transferred is determined by the number of lines on the surface of the transfer roller. The surface of the thick anilox transfer roller has about 120 lines per inch, while the transfer roller used for printing fine patterns can reach 500 to 900 lines per inch.
The application range of flexo ink is very wide, and its printing materials are also diverse, so there are many types of flexo ink that match it. It can be roughly divided into the following four categories:
1. Solvents: including various mixed solvents.
2. Alcohols: including various alcohols.
3. Water-based: The solvent used is water instead of organic solvents.
4. Ultraviolet curing (UV): Unlike ordinary inks, it contains no solvents and only contains some active diluents. The characteristics of each are shown in the figure below.
As far as the main components of flexographic inks are concerned, they are all composed of colorants (pigments or dyes), resins (linking materials), solvents and additives. Most of the liquid inks are mainly volatile drying, but there are also a few inks that are dried in the form of oxidized conjunctiva, penetration drying, etc. Because the flexographic printing machine uses anilox rollers to transfer ink, the viscosity of the flexographic printing ink cannot be too high, generally between 1 and 100 poises.
Flexographic ink is a typical solvent ink, that is: liquid ink, dried by the evaporation of the solvent. This ink is transferred to the flexographic plate through the anilox roller (the dent on the roller). According to its colored 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 (prevention of settling). Due to the different materials of the printing materials, the resins and solvents in the ink formulations are also various, but the ink must have suitable viscosity and fluidity, and the dryness needs to be good. There is no residual odor after drying. The adhesion of the object is better, and it has no effect on the printing plate.
The development of flexographic printing is getting faster and faster. More and more ink manufacturers are developing high-quality and inexpensive inks to create more opportunities for flexographic printing. Among them, the prospect of water-based inks is the most promising. Introduce this type of ink.
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2. Solvent-based flexographic ink and alcohol-based flexographic ink
When printing non-absorbent printing materials, solvent-based flexographic ink is generally used, and the printed matter is dried by the evaporation of the solvent in the ink.
According to the selected colorant, it can be divided into dye ink and pigment ink. The dye ink has good transparency, and it can be printed on the surface of metal substrates such as aluminized paper, which can show a special gloss effect. Pigment ink is mainly used on paper with nitrocellulose (NC) resin as the main connecting material; for printing on plastic film surface, it is based on 50% vinyl resin and acrylic resin as the main connecting material; used on the surface of aluminum foil Dye ink and pigment ink with good transparency can be used for printing ink. Pigment ink can use ink prepared with pigment flake, which makes the ink with high gloss, good transparency and resistance performance better than light-resistant dye ink.
Alcohol ink is made by dissolving alcohol-soluble resin in alcohol-based solvent, and then adding ester solvent, cellosolve and colorant. Most of the flexographic printing of various plastic films for packaging uses this type of ink. The resins used in the connecting material mainly include maleic resin, polyamide resin, acrylic resin, epoxy resin, alkyd resin, nitrocellulose and ethyl cellulose. Commonly used solvents are ethanol, n-propanol, isopropanol, n-butanol, isobutanol, etc. Sometimes aromatic hydrocarbons and ester solvents are added in small amounts. Generally, a single solvent is not used in the ink, but several solvents are used together to improve the dissolution performance and drying performance.
3. Water-based flexible printing 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 mainly used in the printing of paper, carton, corrugated paper, household paper and pre-printing applications. In cardboard printing, flexographic ink is increasingly used. In addition, the further development of ink technology has also greatly improved the solubility of ink.
Flexographic ink refers to ink that uses water, alcohol, cellosolve, etc. as a solvent, and is mainly used for printing of highly absorbing printing materials, such as paper, cardboard, corrugated paper, and cement bags. Depending on the type of resin used, there are three main types of ink: alkali-soluble, emulsion, and water-soluble. Ink ink does not contain excessive volatile organic solvents, is not flammable, does not harm the health of ink manufacturers and printing operators, and does not pollute the atmospheric environment; as a new type of printing ink, ink ink has toxic and harmful substances remaining in the printed matter Less quantity, less pollution to packaged goods; improved overall environmental quality.
A high-quality flexographic ink should have the following properties: no strong odor, good flow properties and viscosity, fast and appropriate drying, no impact on the printing plate and printing machine, the printing machine is easy to clean 1. It should have good storage stability, after long-term storage, it should be easy to use after stirring.
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Fourth, the composition of the flexible version of ink
Colorants usually use strong alkali-resistant pigments, including organic pigments and inorganic pigments, which require good dispersibility in aqueous systems, including organic pigments 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 coloring power are mostly used. Sometimes dyes are used to increase the color strength of the ink and improve the viscosity. However, no matter what kind of pigment is used, it is best to use anionic and nonionic surfactants before use, because the surface tension and polarity of the water in the ink are relatively large, and the dispersion of the colorant is relatively difficult, such as untreated It will cause uneven distribution.
The linking material of flexographic ink is an important factor that affects the quality of ink, especially high-precision ink. There are many types, which can be selected according to different occasions and uses. It is no exaggeration to say that at present, domestic ink companies focus on the research and development of ink and ink on the research of connecting materials. The following is a brief introduction to the performance of commonly used connecting materials.
(1) Water-soluble binders, including polyvinyl alcohol, hydroxyethyl cellulose and polyvinylpyrrolidone, etc. Such binders can be permanently dissolved by water, so the use range of inks configured with it will be affected to a certain extent Restrictions can only be applied to products where the ink film is formed without contact with water.
(2) Alkali-soluble linking materials, acrylic resins are commonly used as linking materials at home and abroad.
In fact, acrylic resins come in three forms: alkali-soluble, nonionic polymer, and acid-soluble polymer. In ink painting, the alkali-soluble type is mainly used (because the non-ionic film is relatively hydrophilic after film formation and has poor water resistance; the acid-soluble type has no suitable volatile acid and has an odor). In the preparation process of ink, ammonia, amines or their mixed form is usually used to make the resin into an amine salt soluble in water. After printing on the substrate, with the volatilization of ammonia, the ink film has a certain gloss, water resistance and abrasion resistance after drying.
(3) Diffusion type connecting material is fine resin particles suspended in water, usually called latex. Propylene, ethylene or styrene butadiene polymers are usually contained in this type of binder. Latex has more significant advantages than dissolved resins: first, latex can be very dense and low in viscosity, which can produce high-quality thin coatings; second, latex can contain quite large polymer molecules, which are generally large They are more resistant to wear and heat than small molecules, and have better adhesion. Although large molecules usually cause the viscosity of the solution to be too high, latex does not have this problem. This is a unique performance of latex inks. The biggest problem with latex ink is that it is difficult to print and difficult to clean, because once the latex coagulates, it will become an insoluble substance, so latex is usually used as a coating ink. In addition, latex can also be used in combination with alkaline binders, which not only retains some of the unique properties of latex, but also has the printability of alkali-soluble binders.
5. The main points of application of ink-on-machine printing
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 introduces the matters that should be paid attention to when using flexo spot color ink in cigarette pack printing.
Precautions before printing:
(1) Ink and ink should be stored in a sealed state with a storage 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 short-term purchase storage method, and place the ink next to the printing machine three days before use to adapt to the scene.
(2) Since spot color inks are prepared from two or more primary color inks, each spot color ink is required to be prepared according to the standard sample in advance, 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) The ink needs to be fully stirred two hours before the machine is started, at least half an hour, and then measure the PH value and viscosity of the ink. According to the printing environment and other factors, adjust the PH value and viscosity to make the ink performance stable and the printing effect achieved optimal.
Precautions during printing: (1) When inking each shift, the newly prepared ink should be used. About one hour later, that is, after the performance is relatively stable, the remaining ink can be added the previous time.
(2) Due to the large thixotropy of water ink, it is necessary to add appropriate additives (stabilizers, retardants, PH value adjustment fluids and other additives) for adjustment at intervals. The specific interval time and dosage should be determined according to the specific situation, control the PH value and viscosity of the ink in the printing process, so as to maintain the best performance.
(3) Since the spot color ink will change its hue during the long-term printing process, the operator is required to fine-tune the ink based on the color difference to ensure that the color difference is within the allowable range.
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6. Ink and wash control index
The control indexes of water ink mainly include viscosity, PH value and dryness.
1. Master the viscosity of ink. Ink viscosity refers to the ability of ink to hinder the movement of other molecules under the influence of its own internal force. Generally use a certain amount of ink, flow through the capillary at a fixed temperature (generally 20 ℃), record the time required to determine the viscosity.
The viscosity of the ink is the main factor that determines the transfer performance of the ink, the fastness of the imprint, the amount of penetration and the gloss. The viscosity of water ink is too high, the color becomes darker, the consumption of ink increases, and the drying speed slows down; the viscosity is too low, the color changes, and the dots expand, resulting in a decline 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 roller, image conditions, and other factors. The actual viscosity should be able to be easily circulated to the doctor blade by the pump, and can be transferred to the printing plate and the substrate in equal amounts, so that the density of the printed product and the image dots do not change significantly, and the printing quality must be maintained. For consistency, it is necessary to maintain the consistency of ink viscosity.
When printing screen version and field version, the ink viscosity should be different. In general, when printing a screen version, the viscosity should be slightly lower, and the printing speed of the printer also has an effect on the ink viscosity; when the printing speed is high, the ink viscosity is lower, and when the printing speed is low, the ink viscosity is higher. Generally, after the anilox roller and the ink transfer roller are added with a doctor blade, if the stripes of ink can be observed, it means that the viscosity is appropriate; if the droplet-shaped ink dots appear, it means that the viscosity of the ink is too large; if the ideas are not seen at all , Indicating that the viscosity is too small.
2. Adjustment of ink viscosity. The adjustment and control of ink viscosity should start with the control of ink temperature and the amount of thinner. In order to ensure the consistency of printing quality, the consistency of ink viscosity must be maintained. The following table gives the amount of thinner:
It can be seen from the table that temperature has a greater influence on the amount of diluent added. Dilute ink has a great influence on the final density of the printed matter. For example, the temperature of the ink before printing is 10 ° C and the viscosity is 25S. After a short period of operation, the temperature of the ink increases to 20 ° C and the viscosity decreases to 18S. As a result, the ink layer is diluted and the density decreases. Therefore, in actual production, the temperature of the ink used should be stabilized at the temperature of the printing workshop. For the increase in ink temperature and the change in viscosity due to friction during printing, cooling can be considered to solve it.
Ink is particularly sensitive to the addition of thinner, so you must be careful and precise in adjusting the viscosity. Avoid excessive (not more than 5%) to prevent the ink viscosity is too low. Bubbles are formed due to surface tension, and generally hard water produces more foam than soft water. For reducing bubbles, defoamers need to be added. Do not use paraffin-based defoamers, otherwise the printing durability of the plate will be affected. Silicone-based defoamers must be diluted with water and added to the ink under continuous stirring. Silicone-based defoamers with high undiluted concentration will be added to the ink, which will form pinholes in the printed matter. Deinking has occurred.
3. PH value of ink. Ink and ink generally control PH value around 9, slightly alkaline. If the PH value floats down too much, it will cause the ink erosion (a property that the ink becomes thin when it is subjected to external force and returns to its original state after standing), making the ink thicker. Ink will flow into the inside of the scraper or rubber roller, resulting in dry ink. Insufficient amount of ink on the anilox roller, such as white leakage during printing. If the PH value is too high, the viscosity drops, 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 will not be satisfactory. For this reason, alkaline substances should be added appropriately to adjust the ink PH value.
4. Dry. The drying of ink and wash has always been troublesome. To solve the drying of ink, comprehensive adjustment is needed. For example, the comonomer composition, neutralizing agent, co-solvent and auxiliary agent of water ink are screened and improved in many aspects. 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 gather all the previous views on the use of ink and wash, but nothing more than the following aspects: shake well before using or sampling, and confirm the viscosity under the same test conditions; according to the design status of the printing plate and the printing Speed, determine the machine viscosity of the water-based ink, when the viscosity is high, it should be appropriately diluted with water or a special solvent, especially pay attention to the viscosity of the water-based ink when it is diluted easily.
If the viscosity is too low, it is best to use the original ink to adjust, and use as little or no thickener as possible. When the color density is high, you should first use a special diluent to properly rinse and then adjust the viscosity; during the printing process, pay attention to the change of viscosity, check the viscosity and make a record; during use, a small amount of foam can be added The 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 sealed and cool place; restart It is best to use a 90-mesh or above net to filter before use.
In recent years, traditional inks have the disadvantages of slow drying, poor gloss, and easy paper stretching. With the improvement of the application of new materials and related conditions of printing equipment, there have been obvious improvements. The application of the domestically produced high-precision flexographic ink shows that the drying speed can reach 200m / min and the resolution has exceeded 150 lines.
7. Analysis of key points of flexo spot color ink
The preparation of spot color ink refers to the process of mixing one or more inks together and adding certain auxiliary materials to adapt it to printing needs. The ink preparation mainly includes two aspects: on the one hand, it refers to the adjustment of the ink printing suitability, including the viscosity, adhesion, fluidity, and drying properties of the ink; on the other hand, it refers to the color adjustment of the ink. Color adjustment of flexo ink
The following basic operations should be paid attention to in the flexo spot color matching: the ink must be adjusted under the standard light source D65. If it is daytime, it can be done in the north window with sunlight. According to the analysis of standard color samples provided by customers, there are several kinds of hue and which are spot colors. With the help of spot color chromatography, determine the number of anilox roller lines to be selected when printing spot color samples, which primary color inks the spot color sample is composed of, and what are the primary and secondary colors. According to the quality requirements of the printed products, choose the primary and secondary inks suitable for the special color ink. Prepare a small amount of spot color ink for proofing, and note the proportion of each component. Use a color difference densitometer to measure the color difference between the proof sheet and the standard sample, and make the final adjustment. According to the final adjusted ratio, a certain amount of printing ink is formulated, and after applying varnish, ensure that the hue and brightness are as consistent as possible with the standard sample. Do a good job of recording and write down the proportion and viscosity of this spot color ink to adjust for future reference.
There are different requirements for different shades of spot color ink. The ink prepared by adding a certain proportion of diluent or white ink to the primary color, intermediate color and multi-color ink is collectively called light color ink. The preparation process of light color ink is as follows: analyze and judge the light spot color in the standard sample Determine the line number of the anilox roller during printing, the ratio of primary color ink to diluent (or white ink). Add a small amount of primary ink to the diluent gradually. If there is more than one primary color ink, the large amount is added first, and then the small amount is added. Measure the viscosity of the ink sample at this time to see if it is suitable for printing. If improper, use ink additives to adjust. Use a flexo wiper (to keep the line number of the printing anilox roller) to scrape the sample, and compare it with the standard color sample until the hue and brightness meet the requirements. Finally, we must do a good job of recording this color ink mix.
The deployment of flexo dark ink should also be noted. The ink prepared with three primary color inks in different proportions without any diluent is called dark ink, and the preparation process is as follows: the dark spot colors in the standard samples are analyzed and judged to determine the printing network. The number of lines of the anilox roller 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 primary color ink in sequence, adding the large amount first and then the small amount. Measure the viscosity of the ink sample at this time to see if it is suitable for printing. If not, it can be adjusted with ink additives. Use a flexo wiper (to keep the line number of the anilox roller consistent) to scrape the sample and compare it with the standard color sample until the hue and lightness meet the requirements. For the same requirements, we must finally make a record of this special color ink blending.
The deployment of flexographic screen color samples, when the color samples provided by the customer are color samples printed with multiple primary dots and different dots in multiple layers, and at the same time require the effect to be printed on the spot, the preparation of the screen color samples is involved . The color depth of the screen printed matter is adjusted by the percentage of dots per unit area. Therefore, the percentage of the dot combination of colors represented by different color blocks on the chromatogram can be used for the preparation of ink.
Matters needing attention in the preparation of flexo spot color ink: the performance of the binder, the compatibility of the pigment and the binder determine the stability of the ink, so the 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 each primary color ink produced by the same manufacturer to prepare a spot color ink. During the blending process, the types of primary color inks should be as few as possible, otherwise the brightness of the required ink will be reduced, the saturation of the color will be affected, and the ink color will be dark and unclear. In the preparation and printing process, it should be ensured that the viscosity is appropriate and stable, because too much change in viscosity will cause changes in ink depth. During the printing process, we must also pay attention to check the color change of the substrate, such as the change of the whiteness of the paper will cause the change of the brightness of the spot color ink.
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8. Overview of the development direction of flexo ink
For ink manufacturers, the ink in the flexo market is a profitable market and a potential market worth developing. In the end consumer market, ink is mainly used in the printing of paper, carton, corrugated paper and pre-printing applications. In cardboard printing, flexographic ink is increasingly used. In addition, the further development of ink technology improves the solubility of ink. The study of new surfactants and the use of cleaner solvents have improved the printing effect of ink lamination. Narrow-web web printers have been using ink for a long time. Based on environmental protection and the health of printing workers, wide web web printers are now also interested in ink. But in the foreseeable future, the web-fed web printing market still has demand for solvent-based inks. At least in our opinion, in order to comply with the VOC (Organic Volatile) regulations and invest a lot of money for equipment printers, ink and wash is their ultimate choice. From the perspective of the wide-web web rotary printing machine, in the wide-web web rotary film printing, an obvious trend is to turn to ink printing. The current ink can be applied to some substrates, mainly polyester film and laminated film.
Ink manufacturers regard environmental protection issues as a driving reason for shifting to the production of ink. Ink and ink have obvious advantages in terms of compliance with V0C emissions, environmental safety, the health and safety of printing workers and the reduction of storage and transportation costs as non-combustibles. With the improvement of resin and additive technology, the functional characteristics of water ink formulations will launch a powerful impact on solvent-based inks and energy-curable inks.
The future trend is that film printing will increasingly use water-based ink flexo to replace solvent-based ink flexo. People look forward to the emergence of better quality water-based film printing inks. Although the benefit of ink is environmental protection, the success of ink does not mean that solvent-based inks will exit the flexo ink market. The biggest advantage of ink is that there are very few V0C volatiles, but the person in charge of research and development of 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 inks or water-based inks, one is cost. For some printing companies, the use of ink for environmental reasons will increase production costs. Another aspect is to look at local environmental regulations on V0C emissions. If you don't have to deal with volatiles, using solvent-based inks is slightly cheaper.
UV (ultraviolet) and EB (electron beam) curing technology is still the development direction of the ink industry. Flexographic plates continue to favor radiation-curable inks. Of course, UV / EB technology is also the most concerned issue of flexo packaging manufacturers. Narrow web rotary label printers have successfully used cured inks and have gained a favorable position in the printing market competition. For the rest of the printing market, printers are waiting for radiation-curable inks. Needless to say, from the perspective of printing quality, flexo printing using UV inks is the best printing method of all flexo technologies at present, and it is superior to solvent-based inks and water-based flexo printing.
Like other printing methods, UV / EB ink also faces 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 technology. Before adopting UV / EB technology. Printing companies must install curing equipment. Although their prices are declining, they are still a significant investment. In addition, the cost problem is also one of the main reasons for the further development of the radiation-curable ink Xu materials. Raw materials and equipment are still very expensive. According to expert estimates, if the cost of ink can be close to other types of ink, then its development rate will be several times that of other inks.
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