Ultraviolet fluorescent ink can be printed by screen printing, gravure printing, relief printing, offset printing, flexo printing and other methods. With the development of screen printing technology and equipment, screen printing has become an ideal printing method for fluorescent inks in recent years. Fluorescent inks printed by screen printing can form a uniform ink film layer, which is necessary for good gloss, stability and weather resistance. Since the particle size of the photoluminescent pigment in the fluorescent ink is coarser than the particle size of the traditional pigment, the fluorescent ink generally uses a screen with a lower mesh number (usually 80 to 150 mesh) in screen printing and is only used for Rough printing. However, with the continuous emergence of new fluorescent ink products, the particle size of its pigments is getting finer and finer, and now it can fully print fine screen printing products. The rheological characteristics of most fluorescent inks are pseudoplastic flow, and the viscosity is small. This characteristic determines that the printing process is very different from that of ordinary inks. In screen printing, the tension of the screen should be high, and an appropriate doctor blade should be selected to increase the thickness of the ink layer, thereby improving the luminous effect of the ink. Since the inorganic phosphor emits light from the crystal, if the pressure is too large, the crystal will be broken, which will reduce the brightness of the light. The adjustment cannot make the printing pressure too high and affect the printing effect. In the fluorescent ink printing, we must pay attention to the following issues.
1. Before using fluorescent ink, you must thoroughly wash the ink stick, printing plate and other related parts to avoid mixing with other colors. Of course, it is best to use a new ink stick.
Second, when matching the color of fluorescent ink and ordinary colored ink, the best fluorescent effect is with colorless red printing ink. Colorless red ink blue ink or colorless blue ink red ink has poor fluorescence effect. The different shades of the background color have a greater impact on the fluorescence brightness. The darker the background color of the ink, the worse the fluorescence effect. In addition, it should be noted that fluorescent inks cannot be mixed with a large number of ordinary colored inks, and try not to use them with ordinary inks, otherwise the fluorescent effect will be lost.
3. Due to the poor light resistance of fluorescent inks, it is not suitable for use on printed products that have been used outdoors for a long time. In addition, when the substrate is a transparent object, it is best to print a layer of white ink before printing fluorescent ink, which can improve the fluorescent effect. Fluorescent ink should not use desiccant. It is better not to add varnish or use bright oil with better transparency.
Fourth, in order to improve the printing effect, the printing color sequence should be arranged reasonably. When printing on paper, fluorescent ink is generally used as the final color printing, otherwise it will be covered by other inks, affecting the luminous effect. The situation is different when printing on transparent plastics. Fluorescent ink is used as the last color when printing externally; as the first color when printing internally. When fluorescent ink is used as the base color, if it can be printed with similar colors, the color saturation can be improved and the light resistance is also improved.
Fifth, the fluorescent ink is not absolutely colorless or absolutely imprinted. On white paper, even if completely transparent, five-color varnish is printed, there will be visual differences due to the different refraction and reflection coefficients of the paper and varnish, resulting in oil stains and differences in the surface brightness of the paper. Colorless fluorescent ink can only reduce the imprint or basic invisibility when the color block of the pattern or text is changed into a line, and the paper with better absorption (such as special paper for invoices, etc.) and where the background color can cover the trace of the fluorescent ink.
Sixth, pay attention to the factors that affect the fluorescence brightness. Such as paper, you should choose bond paper or paper without fluorescent whitening agent, so that the effect of fluorescence is more ideal. When printing on plain paper (paper with optical brightener), the fluorescent effect will be significantly weakened. The more the whitening agent content of the paper, the weaker the fluorescent effect. Therefore, when using plain paper, use paper with low whitening agent as much as possible. In addition, the thinner the ink layer of the printed product, the weaker the fluorescent effect; the thicker the ink layer, the stronger the fluorescent effect, but also the more obvious the imprint.
7. Pay attention to control printing pressure. Since inorganic phosphors and the like emit light from crystals, if the pressure is too high, the crystals will break, thereby reducing the brightness of light emission, so relief printing is generally not used. When performing screen printing, gravure printing, etc., in addition to paying attention to its viscosity, connecting materials, drying properties and other characteristics, we must also pay attention to the adjustment of printing pressure, it is not appropriate to make the printing pressure too large, affecting the printing effect.
8. Control printing speed. Due to the high fluidity of fluorescent ink, the drying is slow. If the ink layer is thick, it is easy to spread due to the incomplete drying of the ink during rapid printing. So the printing speed cannot be too fast.
Finally, pay attention to the difference with phosphorescent ink. The difference between phosphorescent ink and fluorescent ink is that phosphorescent ink continues to emit light after excitation, but its half-life is about several milliseconds, which cannot be seen by the naked eye. Phosphorescent ink is a crystal with phosphorescent substance added to the ink. Under the excitation of ultraviolet light, this kind of substance absorbs the light projected on it, and then emits it at a certain wavelength within a certain time, and the emitted light and the color of the body different. The color material of phosphorescent ink is zinc cadmium sulfide containing a small amount of impurities. The connecting material is transparent and can transmit ultraviolet rays. Because the color material is not resistant to acids and alkalis, the resin used should be neutral. Phosphorescent ink can not add desiccant, dry oil, etc., so as not to react with the pigment, affecting the printing and luminous effect. Phosphorescence emits light in a dark environment after absorbing light, while fluorescence absorbs light of different colors and then emits light. That is, phosphorescence absorbs natural light or artificial light and emits light in the dark environment after the light is removed, and fluorescence must emit light in the presence of light after absorbing the light. Fluorescent ink can also emit light in the presence of "luminous" or purple light. But it does not glow in the dark and dark. Fluorescent ink not only can show its own color, but also has the ability to absorb light and change the light intensity, so that the color it shows has a stronger light intensity, but the hue (or color) does not change. The phosphorescent color can be combined with the hue of visible light, provided that the emission wavelength of the luminescent pigment and the absorption wavelength of the visible light pigment cannot match. This ink is particularly suitable for fast and reliable automatic inspection of securities and quality inspection during printing due to its very good durability. The fluorescent ink used in anti-counterfeiting packaging is best prepared by the user, because different formulas can get different fluorescent effects, which is more difficult to imitate. In addition, you can also combine the characteristics of phosphorescent ink, mixing fluorescent ink with phosphorescent ink, will get better anti-counterfeiting effect.
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