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Printing technology in anti-counterfeiting packaging printing (II)

II. Anti counterfeiting ink in anti-counterfeiting packaging printing

at present, there are many kinds of anti-counterfeiting inks developed, but according to the characteristics of printing, the most suitable inks for printing are as follows:

1. Liquid crystal ink

liquid crystal ink is a new special ink originated in the United States in the 1970s, which refers to the addition of compounds with crystalline properties to the ink. From the perspective of manufacturing method, it also belongs to the type of microcapsule structure ink. From the perspective of application, it belongs to thermochromic ink with heat sensitivity. However, from the perspective of the characteristics of liquid crystal ink, it mainly uses the characteristics of liquid crystal thermochromic ink. Liquid crystal ink is mainly composed of water-soluble resin, liquid crystal capsule, additives, binder and defoamer. It is prepared by dispersing liquid crystal and additives enclosed in microcapsules in binder. Liquid crystal ink is not composed of color pictures and texts with ink layer pigments, but uses the liquid crystal temperature sensitivity in the ink layer to cause the change of molecular direction, so as to selectively reflect the visible light of specific wavelength and absorb the optical characteristics of light of other wavelengths, so as to show color changes. The color forming mechanism of liquid crystal is formed by its selective reflection of specific wavelength light. Liquid crystal must be printed on the black or dark background color. Because the molecular arrangement of liquid crystal is not as firm as the crystal structure, it is easy to be affected by the external environment to change its optical properties. Liquid crystal ink makes use of this characteristic of liquid crystal, so that the lattice of liquid crystal changes under the influence of weak current and temperature, and shows light and dark patterns and colors

the printing method of liquid crystal ink is silk printing, which makes the printed matter have reversible reaction or irreversible reaction. The key of liquid crystal printing technology is to display bright colored liquid crystals at different temperatures, and form a series of color temperature range values. The manufacturing technology of liquid crystal microcapsules is also the key. Microcapsules should be small and uniform, with transparent and thin capsule walls, and mixed with ink. It is required to be solvent resistant, stable, reliable, and long service life; Ensure that the liquid crystal microcapsules are not crushed during the printing process; The surface is covered with protective film, etc. The amount of liquid crystal added into the ink is 40% of the total amount of ink, and the diameter of the capsule is 10 ~ 3 μ M is appropriate. The silk used for liquid crystal ink printing generally adopts nylon silk or polyester silk. If the printing plate is required to upload a large amount of ink, it can be made of stainless steel silk. According to the requirements of ink microcapsule diameter and ink layer thickness, 100 ~ 175 mesh is generally selected. Because silk liquid crystal microcapsule ink is water-soluble, photosensitive adhesive with strong water resistance should be used in plate making

the following problems should be paid attention to when printing liquid crystal ink:

① in each printing, there should be enough ink on the printing plate, and the ink should be given evenly. It is best not to supplement the ink in the middle of the process to prevent foaming

② microcapsule ink is easy to block the version. If blocking and foaming are found, stop the machine immediately and thoroughly clean it with alcohol water

③ the printing pressure should be moderate. Low pressure, insufficient ink on the substrate, affecting the color effect; The pressure is too high, causing the liquid crystal capsule to rupture, thereby reducing the color effect

④ in order to make the hair color of the printing part obvious, the printing background color should be black or dark tone. The ground color can be water-soluble ink or organic solvent ink, and the printing method can be printing, offset printing, gravure printing. When using organic solvent ink, it must be fully dried after printing to avoid residual organic solvent in the ink film, which will react with liquid crystal and affect the chromogenic effect

⑤ pay attention to the smooth and bright ink layer during the printing process, and the thickness should be controlled within 15 ~ 35 μ m。

⑥ reasonably arrange the printing color sequence. You can use black (or dark) ink to print the background color first, and then use liquid crystal ink to print the desired pattern; You can also print the base with liquid crystal ink first, and then print the required negative pattern with water-soluble black (or dark color) ink to cover it

⑦ it is best to dry the printed matter naturally, or dry it with hot air at about 40 ℃, and avoid rapid drying with accelerated and heated high temperature

in addition, do not press the printed matter heavily when stacking, and try not to stack and overlap too much after drying, and do not add too much local pressure when cutting, so as to prevent the liquid crystal microcapsule from being crushed. When using liquid crystal ink, the discoloration temperature range should also be determined according to the purpose of use, and the durability of liquid crystal should be considered according to the purpose. In order to improve the wear resistance, temperature resistance, gas insulation and maintain its gloss of the ink layer surface, the surface can be coated with gloss paint or pasted with protective film

2. Magnetic ink

the anti-counterfeiting principle of magnetic ink is to use magnetic pigments, such as iron oxide or cobalt and other chemical substances added to iron oxide, and use a magnetic detector to detect the magnetic signal and decode it. Magnetism is produced by the magnetic pigments contained in the ink. The best magnetic pigments are Fe3O4 (black) and Fe2O3 (brown). Most of these pigments are needle shaped crystals smaller than microns. Such particle size and shape make it easy for them to be evenly arranged in the magnetic field and obtain relatively high residual magnetism. Symbols and numbers with this residual magnetism realize the recognition function through the friction in the automatic processing device. The binder of magnetic ink is usually alkyd resin. At the same time, other pigments can be added to get different colors of ink. The most prominent features of magnetic ink are deep color appearance and simple detection instrument

3. thermochromic ink

thermochromic ink, also known as temperature sensitive ink, is a special ink that can change color with the change of temperature. The anti-counterfeiting principle of thermal ink is that the color of the pigment changes with temperature. In recent years, some domestic and foreign scientific research departments and manufacturers have successfully developed some thermochromic inks with different characteristics. Now there are three types of color reversible, color irreversible and memory. Reversible type, that is, when the heat source is removed, the ink color can be restored to its original state once it is removed. Its color change types can be divided into sublimation, melting, thermal decomposition, oxidation and curing reactions, mainly due to the transformation of crystal form after the loss of crystal water or the color change due to the change of pH value. Therefore, when the heat source is removed, it can be restored to its original state. Irreversible type refers to the physical or chemical changes that occur when the color changing pigment in the printing ink is heated, which changes the original physical and chemical properties, resulting in color changes. Therefore, when the heat source is removed, the ink color cannot return to the original color. Memory type has memory function

thermal color changing ink can be used for printing, gravure printing and flexo printing, especially silk printing. Silk printing is suitable for substrate materials of any shape, especially for substrates with small batch but special shape. When printing heat sensitive color changing ink, we should pay attention to the relationship between the thickness of the ink layer and the temperature sensitive color changing effect, and also consider the viscosity and volatility of the ink, because these characteristics not only affect the printing process, but also affect the color rendering effect; The drying of printed matter should be careful, and it is best not to use heating and drying. In addition, thermal color changing ink is not suitable for direct printing on aluminum foil materials

4. photochromic ink

photochromic ink, also known as photochromic ink, is the addition of photochromic or light activated compounds to the ink. Under the sun, the ink can change from colorless to colored, or from orange to black, etc., which can generally be divided into ordinary type and UV type. Ordinary type refers to that under the action of ordinary sunlight and strong light, the ink will change in color depth and return to its original state after leaving the light source. It seems to change color in the sun, but it is actually changed by ultraviolet radiation. The color changing wavelength is 400 ~ 800nm. The application of this kind of ink in packaging and decoration can not only improve the grade of goods, increase interest, but also highlight the anti-counterfeiting effect. UV ink is colorless or does not change color under the irradiation of ordinary light source, but it will show bright red, orange red, yellow green and green fluorescence under the irradiation of UV light (ultraviolet light). General large denomination banknotes are printed with this ink, and the authenticity can be found under the action of the banknote detector

in silk printing, thermochromic and photochromic inks can be transferred to different substrates through silk printing, such as paper, cloth, nylon, plastic, metal, glass, ceramics, etc. It has the characteristics of sensitive response to external conditions, obvious changes and rapid response. It is an ideal material for anti-counterfeiting technology and interesting gifts at present, and has broad development and application value

5. Fluorescent ink

fluorescent ink is made by dissolving fluorescent materials in corresponding resins. The particles of fluorescent pigments are generally coarse, and there are many types of binders. Different binders will produce different fluorescence. When using fluorescent ink, it should be ensured that it has sufficient concentration and the ink layer on the print is thicker, so as to ensure satisfactory fluorescence effect. After printing, the fluorescent ink has bright colors and good decoration effect. Under the action of visible light and ultraviolet light, it can emit glittering fluorescence. Fluorescent ink can be mixed by itself. Different formulas can get different fluorescent effects, and its anti-counterfeiting effect is better. However, fluorescent ink has poor light resistance, and the effect of outdoor printing is not good

6. light storage and phosphorescence inks

light storage pigments are prepared by using some zinc sulfide/copper phosphors and calcium sulfide/bismuth phosphors with phosphorescence effect in inorganic phosphors. Inorganic fluorescents emit light by crystals. If they are pressurized, the crystals will break and the luminosity will be reduced. Therefore, it is generally appropriate to use silk printing. The carrier of ink and fluorescents are mixed during use. The characteristic of phosphorescent ink is that it can absorb light and emit light with a certain wavelength within a certain period of time. The pigment of this ink is zinc sulfide (such as zinc cadmium sulfide) containing a small amount of impurities. It is afraid of acid and alkali. Therefore, neutral resins such as cyclohexanone tree ester, fiber ester and ether are required. In addition, heavy metals, desiccants and dry oils should not be added to the ink to avoid damaging pigment substances. The binder used should be transparent and can pass through ultraviolet light. This kind of ink is mainly used for printing advertisements and signs, but if phosphorescent ink and fluorescent ink are mixed for anti-counterfeiting packaging and printing, good packaging decoration and anti-counterfeiting effects can be achieved

7. Special printing ink

printing ink is mainly composed of pigments, fillers, binders, etc. the composition of pigments is an important link to determine the hue and brightness of prints. Many manufacturers and ink factories have customized the special printing ink for each product, and the matching party is kept confidential by the ink factory. For example, the ink used in the "Zhonghua" and "Peony" cigarette packages produced by Shanghai cigarette factory is the special ink specially customized in Shanghai ink factory. This kind of ink is difficult to adjust without knowing the detailed formula, so it has a good anti-counterfeiting effect

8. composite ink

after mixing two or more anti-counterfeiting inks in a certain proportion, a composite ink with new performance can be developed. If the ink formula is not fully mastered, it will be difficult to copy

9. other anti-counterfeiting inks

plastic anti-counterfeiting inks. It is suitable for printing PP, PVC, PE, PS, ABS, PC and other plastic products. With 150 ~ 200 mesh polyester, 20 ~ 30m2 per kilogram of ink can be printed. It can be dried naturally or at 130 ℃ for 3min. Metal, glass, ceramic anti-counterfeiting ink. It is suitable for printing metal temperature signs, color changing glass water utensils, color changing milk cups and ceramic tea sets after the sample is broken or damaged. It can be printed with 70 ~ 100 mesh polyester, 10 ~ 20m2 per kilogram of ink, and the drying conditions are 25 ℃, 48h or 120 ℃, 45min. Glass and ceramic decal transfer paper is used to prevent low precision fake special ink. apply