Most attention should be paid to the curing of UV

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Attention should be paid to the curing of UV offset printing inks

in the packaging field, UV printing products are more and more popular because of their good decorative effect, no volatile harmful substances, and the rapid fixation of ink on non absorbent substrates. But in printing, UV ink curing is a very important problem. If the ink cannot be firmly fixed on the substrate after printing, the printed product cannot meet the packaging quality requirements, so attention must be paid to the curing of the ink during UV printing, which is particularly important for UV offset printing. Because the UV offset press is a high-speed printing press, there is water in the UV offset press, so there are water ink balance problems and ink emulsification problems. The existence of these problems will affect the curing speed of the ink, and also put forward higher requirements for the selection of UV curing device, the selection of ink formula, the suitability of printing materials, the control of water ink balance, etc. It is explained as follows:

1. UV curing device

modern UV offset printing is mostly high-speed multi-color printing. After printing, the ink must be completely cured before the print can meet the quality standard, which puts forward high requirements for the selection of UV curing device. UV offset curing device is mainly composed of four parts, namely lamp, reflector, control system and cooling system. Here we mainly introduce the lamp tube and reflector

1) lamp tube. UV offset printing ink and curing mechanism is that the photosensitizer (photoinitiator) in the ink absorbs a certain wavelength of UV light and generates free radicals after excitation, which leads to the photopolymerization and cross-linking reaction of unsaturated acrylic oligomer and active monomer, forming a curing layer. That is to say, the wavelength of UV light required for curing UV offset printing ink is mainly determined by the wavelength of photoinitiator in the ink. The wavelength of UV absorbed by most photoinitiators in UV offset printing ink is in the range of 200nm ~ 400nm, so the curing of UV offset printing ink requires lamps with good photochemical matching performance. Photochemical matching means that the spectral output distribution of the lamp used should match the spectral sensitivity of the photosensitive material, that is, the photosensitive material absorbs ultraviolet light to produce a chemical reaction, and the wavelength range of the absorbed light is just the output range of the light source's emission spectrum, and the maximum absorption peak of the photosensitive material is just at the peak of the light source's output, so that the light emitted by the light source can be absorbed by the photosensitive material to the greatest extent to produce a chemical reaction. At present, most of the lamps commonly used in UV offset ink curing devices are high-pressure mercury lamps, which emit a wide range of UV line length and overlap. From the perspective of the emission spectrum, there is strong radiation in the UV region where the UV line length is 365nm, followed by strong light in the UV region, and spectral bands with wavelengths of 366.6nm, 313nm, 303nm and 253.7nm respectively. These are just within the range of the main UV wavelength of UV offset printing ink photosensitizer. An important indicator of high-pressure mercury lamp is the energy output per unit length. The fire scale line power, expressed in w/cm, is directly related to the irradiation light intensity. In order to improve the curing speed, increase the irradiation intensity and improve the line power when the ink is cured. According to the high-speed and multi-color characteristics of UV offset printing, the line power of the UV lamp used should at least reach 80w/cm or even 120w/cm, and it is best to have a curing lamp behind each printing color group; In case of online glazing, there should be at least three light tubes in the paper receiving part after glazing to ensure that the varnish and color ink can be completely cured

2) reflector. The function of the reflector is to orient the energy generated by the UV lamp. Its main function is to ensure that the ultraviolet light generated by the lamp tube can effectively irradiate the surface of the substrate. The geometric shape and prosperous performance of the reflector determine the relative proportion of direct irradiation and reflected amplitude. Therefore, only a suitable UV output lamp is not enough to ensure that the generated UV energy can irradiate the substrate surface to the maximum extent. It must be complemented by Dale Brosius, the chief commercial officer of iacmi, who said that he clearly perceived the cultural basis of this entrepreneurial platform - a reflector that acts as a hard-working and open package. Generally, the reflector is made of high polished aluminum to achieve maximum reflectivity. From the perspective of geometric optics, the reflector has a variety of geometric shapes. The curing reflector of UV offset printing ink is a light collecting type (the reflector is in the shape of a torn round surface). The light collecting reflector belongs to the focusing reflector. In the elliptical reflector, the lamp tube is installed on a focal point of the ellipse. The ultraviolet light is reflected to another focus of the ellipse. When the cured product passes through this focus, it will be irradiated by the maximum intensity of ultraviolet light. The light collecting reflector is suitable for high-speed UV offset printing due to its high radiation intensity and high curing efficiency. In general, the reflector is also equipped with a cooling device to prevent overheating of the lamp tube from affecting the service life and prevent large deformation of the substrate due to overheating. There is an exhaust pipe at the back of the reflector to take away the heat generated. At the same time, there is a circulating cold water pipe, which can also take away part of the heat through the circulation of the cold water pipe. In addition, the height of the reflector from the substrate can be fine tuned to meet the maximum curing of ink energy

2. Ink formula

uv offset printing substrates are mostly high-grade gold, silver cardboard or laser paper with aluminum foil on the surface. This paper has weak adhesion to the ink, and there is water in the printing process. If the ink formula is improperly selected, it will directly affect the curing of the ink. Therefore, UV offset printing has the following requirements for the ink

1) tinting power of ink. The tinting power of an ink indicates the concentration or saturation of the ink. The tinting power mainly depends on the nature of the ink itself, such as pigment content, pigment dispersion and selective absorption and reflection of light. The ink with high tinting power has large pigment content and strong tinting power. The imprinted ink layer is very thin, which can meet the hue requirements. UV light is easy to penetrate the ink layer, which is conducive to the curing of the ink; On the contrary, if the coloring power is weak, it is bound to increase the amount of ink. The imprinted ink layer is thick, and the UV light is not easy to penetrate the ink layer, which is not conducive to the curing of the ink

2) water resistance of ink. The water resistance of ink is of great significance for UV offset printing with water 3 and running distance, especially for aluminum foil paper with poor water absorption dry pressed ceramic tiles Part 4: stoneware tiles gb/t4100.4 (1) 999. If the water resistance of ink is poor, emulsification is easy to occur. In serious cases, it will not only make the production difficult to carry out normally, but also directly affect the curing speed of the ink, and then affect the product quality, such as reducing the viscosity of the ink, weakening the adhesion, fading the color, etc. The water resistance of ink mainly depends on the stability of ink structure and the properties of fountain solution

3) selection of photosensitizer. Photosensitizer is a substance used to catalyze the curing of UV offset printing ink. Its selection principle is very important: it is determined by the spectral band emitted by the light source used during curing and the absorption characteristics of photosensitizer. The maximum absorption wavelength of the most effective photosensitizer is consistent with the excitation band generated by the maximum excitation of the light source. The selection of photosensitizer should follow the following principles: fast light fixation speed, to ensure that the printed matter is dry and not sticky; Good miscibility with light cured resin and pigment; It will not make the cured ink film yellow and discolored; To ensure that the photochromic ink will not gelatinize and deteriorate during effective storage. It is better not to use a photosensitizer in the ink, but several photosensitizers. In this way, the excitation photosensitivity can be caused in the range of different wavelengths, so as to improve the catalytic effect and improve the curing effect. There are two main categories of commonly used photosensitizers: ketones, benzoins and their ethers. Their common point is that they all have aromatic ketone groups. The difference is that benzoins themselves can initiate olefin monomer polymerization through photochemical action. Therefore, they are both photosensitizers and photoinitiators, and the reaction speed is faster. After photochemical reaction, dibenzone often needs to interact with other molecules to initiate olefin monomer polymerization. So strictly speaking, it is only a photosensitizer, not a very good photoinitiator. It often needs to be used with other compounds, and the reaction is relatively slow

3. Surface tension of the substrate

the surface tension of the substrate also focuses on the surface energy, which is an indicator to measure its ability to accept ink. When the surface tension of the substrate is lower than the surface tension of the ink, the ink is poorly wetted, and the surface of the substrate can not flow evenly and spread out, in the form of water droplets, which can not be firmly attached to the surface of the substrate, and the ink layer formed is thick and uneven. As the ink also absorbs water, it will cause the ink film to become muddy and weak, thus greatly blocking the transmission of ultraviolet rays and affecting the curing of the ink. Therefore, the surface tension of the substrate must be greater than that of the UV offset ink. The surface tension of the ink is generally between water and oil, which is 30 ~ 36. In order to make the ink flow smoothly on the substrate, the surface tension of the substrate must be at least 10 dyn/cm greater than that of the ink. Therefore, the surface tension of the substrate is generally required to be at least 40 dyn/cm. In the UV offset printing process, the substrate is mostly gold, silver cardboard or laser paper with aluminum foil on the surface. Before printing, the surface tension can meet the printing requirements only after the surface is treated. Generally, corona method and surface coating method are used to improve the surface tension of the substrate. Corona method is to discharge the high-voltage charge to the surface of the film layer. Under the action of ionization, the film surface coarsens and the surface energy increases. However, with the extension of the storage time of the substrate and the surface friction, the surface energy or surface tension will gradually decrease. Therefore, for aluminum foil paper, a layer of varnish (commonly known as green paint) should be coated on the surface before printing to meet the requirements of the paper surface tension for printing. Before the paper leaves the factory, the corona treatment and coating of the surface have been completed. The measurement of paper surface tension is very simple and can be easily completed with a surface tension test pen

4. Operation

the above mentioned items are hardware requirements and prerequisites, but only good hardware is not enough. In order to make UV offset ink solidify quickly on the substrate, the operating skills (software) of the printer operator are also very important. Because the proper operation in the printing process is also a factor that directly affects the ink curing, the following aspects must be paid attention to in the operation process

1) control of ink balance. The control of ink water balance is very important for offset printing process. If the control is not good, the ink will be seriously emulsified and produce various quality problems. Especially for UV offset printing. Because ordinary offset printing is based on the principle that oil and water are of different polarity and immiscible with each other. The UV offset printing ink has weak polarity and narrow water range due to the presence of propylene resin, which makes it more difficult to control the water ink balance, and the ink is more easily emulsified. The water contained in the emulsified ink will block the transmission of ultraviolet rays and directly affect the curing speed of the ink. Therefore, the ink and water balance must be well controlled in the printing process. Reduce ink emulsification. To achieve this, we can start from the following aspects:

① the content of ethanol (isopropanol) in the fountain solution is about 15%, which increases the surface tension of the fountain solution. Using less fountain solution can form a uniform water film, so as to avoid ink emulsification due to excessive water. The temperature of the fountain solution shall be controlled at 5 ℃ ~ 8 ℃ to avoid the heat generated by the high-speed operation of the water roller and the ink roller

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