About the author: Wang Shouhong, graduated from Wuhan University of Surveying and Mapping in 1998 with a bachelor's degree in engineering. In 2001, he graduated from the Department of Printing and Communication of Wuhan University with a master's degree and is currently working at Guangdong Light Industry Vocational and Technical College.
Energy curing ink concept
Various technologies are emerging in the printing industry, one of the fastest growing is energy curing technology. It means that the ink or glazing paint is fixed or hardened on the substrate under the irradiation of radiant energy, thereby obtaining a dry imprint. UV light and accelerated high-energy electron beam (EB) are two energy forms of energy curing technology. Accordingly, inks cured with UV light and EB energy are called UV inks and EB inks, respectively.
Now, UV inks and EB inks are valued by many packaging and printing manufacturers, and the cost has been significantly reduced. The UV ink and EB ink printing processes are increasingly worth studying.
Composition of energy-curable ink
The main chemical components of UV ink and EB ink include:
â— Monomer (active diluent): a chemical substance with low molecular weight, which determines the surface characteristics of the ink after drying, such as gloss, hardening degree and elasticity. Other monomers are the source of the harmfulness of energy curing inks.
â— Resin (prepolymer): refers to the chemical skeleton of the ink, which will affect the consistency, wetting ability and cross-linking characteristics of the ink.
â— Photoinitiator: It is a chemical substance that the ink becomes a high-energy state under UV light irradiation and starts the curing process. EB ink does not require a photoinitiator because the high-energy electron beam has enough energy to initiate the polymerization reaction.
â— Additives: including waxes, wetting agents, ink adjusters that change the characteristics of the ink, etc., to improve the physical and chemical properties of the ink.
â— Color: The particle size and concentration of the color will affect the curing rate of the UV ink. Therefore, not only the color of the color, but also the wettability of the color and the ability to receive UV light should be considered when selecting the color. Y and M inks are easier to cure, followed by C and K inks. EB inks do not need to consider this problem when choosing color materials, because the energy of the high-energy electron beam is enough to cure the thick ink film layer.
UV / EB ink curing principle
The drying of the ink film layer of ordinary ink after printing is that the connecting material (solvent, grease) in the ink is absorbed and volatilized by the substrate, and the solid substances (colorant, resin, wax, desiccant, etc.) remain on the substrate surface to form A thin layer of ink. The difference between the drying of general ink and the fixation of energy curing ink is that the chemical composition of the ink / glazing coating is different, and the second is whether the printing workshop needs to be equipped with energy curing equipment.
Figure 1, drying of common ink; Figure 2, curing of UV ink; Figure 3, curing of EB ink
So it is called energy curable ink because it contains some chemical substances. Under the action of UV light or high-energy electron beam, they generate free radicals or ionic groups. The free radicals or ionic groups are then cross-linked with other substances to form a network polymer. . The curing process of UV ink is as follows:
Figure 1: UV ink before curing; Figure 2: UV light excites the photoinitiator to generate free radicals; Figure 3: Polymer bonds begin to grow, and the resin is attracted to free radicals to form new radicals; Figure 4: The reaction continues, which is the polymerization reaction; Figure 5: After the ink is completely cured, a strong three-dimensional network structure ink film layer is formed, so the UV ink has excellent resistance to resistance.
Advantages and disadvantages of UV / EB ink
The curing time of the first UV / EB ink is very short, generally only 1/10 second can be completely dried and fixed on the surface of the substrate. For traditional thermosetting inks and sheet-fed offset inks, the drying time may take several minutes. It takes only a few days to dry completely. Therefore, energy-curable inks are used to print offset printing jobs. The delivery time is short, which is comparable to flexo and gravure printing, and greatly improves the productivity of offset printing.
Secondly, after UV ink printing, UV glazing and continuous machining are performed to obtain a printed product with high glossiness and smooth and uniform surface. After the other machine is polished, the packaging and processing technologies such as die cutting, creasing, gluing and bronzing can be completed immediately.
Thirdly, the printed matter printed with energy curing ink has strong resistance. Because the chemical substances in the ink form a polymer three-dimensional network polymer through the cross-linking reaction, the surface of the ink has strong chemical resistance and friction resistance.
The solid content of the fourth UV / EB ink after curing is close to 100%, and usually does not contain VOC (organic volatiles), which strongly shows that the UV / EB ink system is more environmentally friendly than the ordinary ink system. For example, the chemical composition of UV / EB ink is properly prepared, and the odor is much smaller than many common inks with dry oxide conjunctiva, and the surface of the printed product is clean and free of dirt.
The fifth energy curing ink has less and less irritation to human skin, similar to the irritation of ordinary ink containing gasoline fraction.
Of course, energy-curable inks / glazing paints also have some disadvantages. Some require NBR rubber rollers or EPDM rubber rollers. For offset printing, a special blanket may also be required, and the blanket may also need to be cleaned with a special cleaning agent to prevent the blanket from shrinking excessively, swelling, or degrading quality.
Secondly, its cost is higher than traditional ink. However, the high cost is worthwhile compared to the performance of the printed product finally obtained.
In addition, a one-time investment in energy curing equipment requires certain funds. The prices of current UV curing equipment, UV lamps, and EB curing equipment have been steadily declining, especially the price of EB curing equipment has been the most obvious. Compared with ten years ago, the cost of EB curing equipment has decreased by four times. In the past few years, the price of UV light curing equipment has also declined steadily. There are enough reasons to believe that the price of energy curing equipment will continue to decline in the next few years.
The latest technological progress of UV / EB ink
Although mixed ink products of different manufacturers have some differences in performance, they should generally have the following characteristics:
â— Can be applied to various types of ordinary printing machines equipped with on-line curing unit
â— There is no need to replace the various chemicals used in rubber rollers, blankets, printing plates or printing machines
â— In the continuous printing process, you can get a higher gloss of the printed matter
â— Can clean rubber rollers, blankets, etc. with conventional cleaning agents
â— The suitability of the printing process on the printing machine is comparable to that of traditional grease-based inks
Looking forward to the application prospects of energy-curable inks in the printing industry, it still depends on whether the cost of inks can be reduced to levels close to ordinary inks.
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