Anti-counterfeit printing is an important market for current printing, and it is widely used in many fields such as banknotes, securities, and commodity packaging, and plays a very important role in safeguarding goods. Anti-counterfeit printing mainly uses materials (inks, paper, etc.) for anti-counterfeiting and printing technology for anti-counterfeiting. Most of the printing technologies used in security printing are the continuous progress of screen printing technology and the unique advantages of screen printing. It has also begun to occupy a place in the field of security printing, especially in card printing, lottery printing, and commodities. The anti-counterfeit packaging printing has fully exhibited satisfactory results.

Second, security printing anti-counterfeiting printing screen printing ink

At present, there are many kinds of anti-counterfeiting inks developed, but according to the characteristics of screen printing, the most suitable inks for screen printing are the following:

1. LCD ink

Liquid crystal ink is a new type of special ink originated from the United States in the 1970s. It refers to the addition of a compound with crystalline properties to the ink. From the production method, it also belongs to the type of microcapsule structure ink, from the application point of view, is a thermochromic temperature-induced color ink, but from the perspective of the characteristics of liquid crystal ink, mainly the use of liquid crystal thermochromic characteristics. The liquid crystal ink is mainly composed of a water-soluble resin, a liquid crystal capsule, an auxiliary agent, a binder, and an antifoaming agent. The liquid crystal ink is prepared by dispersing the liquid crystals, additives and the like enclosed in the microcapsules in a binder. The liquid crystal ink does not constitute a color image with the ink layer pigment, but utilizes the temperature change of the liquid crystal in the ink layer to cause a change in the molecular direction, thereby selectively reflecting visible light of a specific wavelength, absorbing optical characteristics of other wavelength light, thereby rendering color Variety. The color forming mechanism of the liquid crystal is formed by selective reflection of light of a specific wavelength, and the liquid crystal must be printed on a black or dark background. Since the molecular arrangement of the liquid crystal is not as strong as the crystal structure, it is easily influenced by the external environment and its optical properties are changed. The liquid crystal ink utilizes this property of the liquid crystal, so that under the influence of the weak current and the temperature of the liquid crystal, the crystal lattice changes to show bright and dark patterns and colors.

The liquid crystal ink printing method is preferably screen printing, so that the printed product has a reversible reaction or an irreversible reaction printing method. The key to liquid crystal printing technology is the ability to display brightly colored liquid crystals at different temperatures, and to make series in the color temperature range. The production technology of liquid crystal microcapsules is also critical. The microcapsules have to be small and uniform, and the capsule wall is Transparent and thin. The ink is incorporated and requires solvent resistance, stability, reliability, and long life; and the liquid crystal microcapsules are not to be protected during the printing process. Crush; surface covered with protective film. The amount of liquid crystal added to the ink is 40% of the total amount of ink, and the diameter of the capsule is preferably 10 to 3 μm. The screen used for the printing of liquid crystal ink is generally a nylon screen or a polyester screen. If a large amount of ink is required for the printing plate, a stainless steel screen plate may be used. According to the requirements of ink microcapsule diameter and ink layer thickness, generally choose 100~175 mesh. Since the screen liquid crystal microcapsule ink is water-soluble, the photosensitive resist which has strong water resistance should be used when making the plate.

Please pay attention to the following issues when printing screen printing inks:

1 In each printing, there must be enough ink on the plate to give the ink even, and it is best not to fill the ink halfway to prevent foaming.

2 The microcapsule ink tends to clog the screen plate. If a plugging or blistering phenomenon is found, the machine should be shut down immediately and thoroughly cleaned with alcohol.

3 The printing pressure should be moderate. The pressure is low and the amount of ink on the substrate is insufficient, which affects the color effect. When the pressure is too high, the liquid crystal capsules are broken, thereby reducing the color effect.

4 In order to make the color of the printing part obvious, the black color or dark tone should be used for the printing background color. Background color can be water-soluble ink or organic solvent ink, printing methods available screen printing, offset printing, gravure. When organic solvent inks are used, they must be fully dried after printing to avoid the organic solvent remaining in the ink film and react with the liquid crystals to affect the coloring effect.

5 The printing process should pay attention to the smoothness of the ink layer, and the thickness is controlled at 15~35μm.

6 Reasonably arrange the printing color sequence. Firstly, the background color can be printed with black (or dark) ink, and then the desired pattern can be printed with liquid crystal ink; or the desired negative pattern can be printed with water-soluble black (or dark color) ink after printing the substrate first with the liquid crystal ink. cover.

7 The printed matter is preferably dried naturally. It can also be dried with hot air at about 40°C. Avoid rapid heating and high temperature drying.

In addition, when the printed matter is not stacked, it should not be heavily pressed. After drying, it should not be piled up as much as possible. When cutting, it is also not appropriate to add excessive partial pressure to prevent the liquid crystal microcapsules from being crushed. When using liquid crystal inks, the color temperature range should also be determined according to the purpose of use, and the durability of the liquid crystals should be taken into consideration according to the use. In order to improve the surface wear resistance, temperature resistance, gas barrier and maintain its gloss, the surface can be coated with varnish or protective film.

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