Packaging offset printing is used by more and more customers due to its high speed, high efficiency, good dot reducibility, strong pattern texture, and wide range of printing capabilities. For some non-absorbable substrates such as aluminum cardboard and plastic sheets, the offset printing is generally a UV printing process. Because UV offset printing has the following advantages over traditional offset printing:
1. Fast curing
Ordinary offset printing takes several hours or more to fully dry the ink and go to the next processing step. UV offset printing can be completely cured in 1 second to several seconds, so it is especially suitable for printing some non-absorbent substrates.
2. Strong wear resistance
After the UV ink is cured and formed into a film, the friction resistance thereof is good, and the loss caused by the friction during the use and transportation of the packaging product is reduced.
3. Green printing
UV inks do not contain solvents, there is no evaporation of solvents, will not cause pollution to the atmosphere, are green printing.
4. Powder free
Because the ultraviolet rays can be solidified and dried in time, no dusting is required, which greatly improves the working environment. At the same time, it is also conducive to post-press processing, such as laminating and polishing.
5. High efficiency and good quality
UV offset printing production cycle is short, the production capacity is high, and the printing outlets are clear and of good quality. However, because the UV offset printing process is more complex and the application time is not long, it needs to be continuously summarized in the application of the printing process. To see the deficiencies in UV offset printing, continuous improvements are made. I would like to talk about my own opinion on the issues that should be noted in packaging UV offset printing:
1. UV curing device
Since the curing of the UV ink is based on absorption of ultraviolet light by the photoinitiator, polymerization and cross-linking reaction of the monomer and the prepolymer are initiated to cause curing. So offset UV must install UV lamp. General offset presses need to install 3 to 6 UV lamps and exhaust pipes. The UV UV lamp must use a suitable wavelength, typically 200 to 450 nm, and control the distance from the illuminated surface. Also have a spotlight cover. Under the same lamp tube, the efficiency of the lampshade will be very different. Under the irradiation of ultraviolet light, ozone is generated in the air, so ventilators such as exhaust pipes are required. After a certain period of time, the UV lamp will be aged and the energy will be reduced. Its service life is generally around 1 000 hours. Therefore, in the usual printing, attention should be paid to the UV lamp's spotlight, and dust must be wiped clean. The use of UV lamps should also be remembered. More than the use of time should always pay attention to the detection of UV ink curing fastness, in order to replace the UV lamp. UV light has some damage to the human body, so it is necessary to take measures to block light.
2. Use of UV ink
Some components of UV inks come into contact with human skin to produce certain irritating effects and may even cause skin allergic reactions. Therefore, avoid skin contact with ink during use, such as wearing gloves or washing hands frequently. UV inks and ordinary inks If you exchange printing on one machine, then we must clean the ink roller and ink fountain. If the ordinary ink is not clean, it will affect the curing and drying of the UV ink. The auxiliary material of the ink cannot be used casually, and it is necessary to use a dedicated auxiliary material that matches the UV ink.
Plastic sheets, aluminum foil and other substrates have poor permeability and weak ink adsorption. In order to improve the adhesion of UV inks to these non-absorbent materials, UV offset printing requires the surface energy of the substrate to be 40 dyn or more. Before printing, we must check the surface energy of the substrate. One of the surface energy treatment methods is corona treatment. It has timeliness. Normal corona treatment will automatically degenerate after one week. If re-use must be further corona treatment, it will affect the curing drying and ink layer fastness.
4. Ink emulsification and ink balance
Offset emphasizes ink balance and control of emulsification value. If the UV ink emulsion causes the UV ink ink film to be opaque, the pass rate of ultraviolet radiation energy is affected, thereby slowing the film drying time. Even though the ink is cured, there is still moisture in the ink film. Once the pattern is rubbed or adhered with an adhesive tape, the ink layer is pulled off.
5. Control the temperature and humidity
If the temperature is low and the humidity is high, the photo-curing reaction of the UV ink is slow, and the binding ability between the ink and the substrate is reduced. Therefore, the ambient temperature and humidity of the workshop must be controlled during UV printing. The general temperature of 18 Â°C ~ 25 Â°C, relative humidity of about 50% ~ 65%.
6. Control the printing speed
The speed of printing affects the curing and drying of UV inks. The speed is fast, the UV irradiation time is relatively short, and the curing and drying conditions are poor, so we must control the printing speed. The printing speed is generally controlled from 6 000 to 7 000 pieces/hour.
7. Printing color sequence
The arrangement of the color sequence is very important. Careful consideration must be given to the characteristics of the printed product. The thickness of the ink on the substrate affects the quality of the dot, and it also affects the curing speed of the ink. This is mainly an ink overprinting problem. Therefore, when printing, it is necessary to print a small area with a small amount of ink. On the one hand, it can reach the highest and full color of the ink, on the other hand it will not affect the curing and drying.
8. Print registration
Accurate print overprinting is very important. UV lamp irradiation will bring about a certain degree of substrate deformation, so generally take the UV lamp after the last color is printed. If there are special circumstances, the UV lamp should be irradiated in the middle. The recommended irradiation time is as short as possible.
In short, environmentally friendly offset UV printing is the trend of future printing. It can achieve 100% curing, solvent-free volatilization, no dusting, instant drying, ink stability, overprinting accuracy, convenient processing after printing, ink layer has a high wear resistance and high strength gloss, especially suitable for non-absorption Printing of printing materials.
As we continue to explore the process of UV offset printing, as well as the continuous development of new processes and materials in the future, the level of UV offset printing will go even higher.
Reprinted from: Print World
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