Ultraviolet fluorescent inks can be printed with silk screen, gravure, embossing, offset printing, flexographic 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. Screen-printed fluorescent ink prints can form a uniform ink film layer, which is necessary for good gloss, stability and weathering resistance. Because the particle size of photoluminescent pigments in fluorescent inks is larger than that of traditional pigments, fluorescent inks generally use lower mesh (usually 80 to 150 mesh) screens in screen printing, and are only used for larger meshes. 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 is fully capable of printing fine screen printing products. The rheological characteristics of most fluorescent inks are imitating plastic flow and low viscosity. This characteristic determines that the printing process is very different from the printing process of ordinary inks. In screen printing, the tension of the screen should be high, and an appropriate squeegee should be selected to increase the thickness of the ink layer, thereby improving the luminous effect of the ink. Since inorganic phosphors emit light from crystals, if the pressure is too high, the crystals will be broken and the brightness of the light will be reduced. Therefore, in addition to the characteristics of viscosity, bonding material, and dryness during screen printing, attention should also be paid to the printing pressure. The adjustment should not make the printing pressure too large and affect the printing effect. In the fluorescent ink printing, we should pay attention to the following issues.
1. Before using fluorescent ink, the ink stick, printing plate and other related parts must be thoroughly cleaned to avoid mixing other colors. Of course, it is better to use a new ink stick.
2. Colorless red ink has the best fluorescent effect when matching the color of fluorescent ink and ordinary colored ink. Colorless red printing blue ink or colorless blue printing red ink has poor fluorescence effect. The depth of the background color is different, which has a greater impact on the fluorescent brightness. The darker the ink background color, the worse the fluorescent effect. In addition, it should be noted that fluorescent ink can not be mixed with a large number of ordinary colored inks, and try not to mix with ordinary inks, otherwise the fluorescent effect will be lost.
3. Because the fluorescent ink has poor light resistance, it is not suitable for use on printed matter that is 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 the fluorescent ink to improve the fluorescent effect. It is not advisable to use desiccant for fluorescent ink, and it is better not to add overcoat varnish or use clear varnish with better transparency.
4. In order to improve the printing effect, it is necessary to arrange the printing color sequence reasonably. When printing on paper, fluorescent ink is generally used as the final color, otherwise it will be covered by other inks and affect the luminous effect. When printing on transparent plastic, the situation is different. Fluorescent ink is used as the last color for external printing; for internal printing, it is used as the first color. When the fluorescent ink is used for the background color, if the same color can be used for printing, the color saturation can be increased, and the light resistance can also be improved.
5. Fluorescent ink is not absolutely colorless or absolutely no imprinting. On pure white paper, even if it is printed with completely transparent, five-color varnish, there will be visual differences due to the difference in refraction and reflection coefficients of the paper and varnish, resulting in the difference between oil stains and the brightness of the paper surface. Colorless fluorescent inks can only reduce marks or be basically invisible when the patterns or text change the color blocks to lines, paper with good absorption (such as special invoice paper, etc.) and where the background color can cover the fluorescent ink traces.
6. pay attention to the factors that affect the brightness of the fluorescence. For paper, you should choose bond paper or paper without fluorescent whitening agent. In this way, the fluorescence effect is ideal. When printing on plain paper (paper with fluorescent brightener), the fluorescent effect will be significantly reduced. The more the brightener content of the paper, the weaker the fluorescence effect. Therefore, when using ordinary 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 at the same time the more obvious the imprinting.
7. pay attention to control the printing pressure. Since inorganic phosphors emit light from crystals, if the pressure is too high, the crystals will be broken and the brightness of the light will be reduced, so relief printing is generally not used. When performing screen printing, gravure printing, etc., in addition to paying attention to its viscosity, binder, dryness and other characteristics, it is also necessary to pay attention to the adjustment of the printing pressure. It is not advisable to make the printing pressure too large and affect the printing effect.
8. Control the printing speed. Due to the high fluidity of the fluorescent ink, the drying is relatively slow. If the ink layer is thick, it is easy to spread due to the incomplete drying of the ink during fast printing and cause a paste. 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 the excitation, but its half-life is about a few milliseconds, which is invisible to 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 period of time, and the emitted light is the same as the color of the body. different. The color of phosphorescent ink is zinc cadmium sulfide containing a small amount of impurities. The binder is transparent and can transmit ultraviolet rays. Because the color is not acid and alkali resistant, the resin used should be neutral. Do not add desiccant, drying oil, etc. to the phosphorescent ink, so as not to react with the color material and affect 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 a dark environment after the light is removed, while fluorescence can only emit light in the presence of light after absorbing light. Fluorescent ink can also emit light in the presence of "luminous" or purple light. However, it does not emit light in the dark and without light. Fluorescent ink can not only 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 does not match the absorption wavelength of the visible light pigment. This ink is especially suitable for fast and reliable automatic inspection of securities and quality inspection during printing due to its very good durability. The fluorescent ink used for anti-counterfeiting packaging is best prepared by the user, because different formulations can obtain different fluorescent effects, which is difficult to imitate. In addition, it is possible to combine the characteristics of phosphorescent inks and mix fluorescent inks with phosphorescent inks to obtain better anti-counterfeiting effects.