Packaging offset UV printing should pay attention to the problem
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Packaging offset printing is more and more used by customers because of its high speed, high efficiency, good dot reduction, strong texture and wide range of printing. For some non-absorbent substrate materials such as aluminum cardboard and plastic sheets, packaging offset printing is generally carried out by UV printing. Because UV offset has the following advantages over traditional offset printing:
1. Fast curing speed
Ordinary offset printing takes several hours or more to completely dry the ink and proceed to the next processing step. UV offset printing can be fully 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 into a film, the abrasion resistance is good, and the loss caused by friction during use and transportation of the packaged product is reduced.
3. Green printing
UV inks contain no solvents, no solvent volatilization, no pollution to the atmosphere, and are green printing.
4. Free powder spray
Since it can be solidified and dried in time after ultraviolet irradiation, it does not need to be sprayed, which greatly improves the working environment. At the same time, it is also conducive to post-press processing, such as filming, glazing and the like.
5. High efficiency and good quality
The UV offset printing cycle is short, the production capacity is high, and the printed dots are clear and the quality is good. However, because the packaging UV offset printing process is more complicated and the application time is not long, it needs to be continuously summarized in the printing process application, and the shortcomings in the UV offset printing should be seen and continuously improved. I would like to talk about my own views on the issues that should be noted in packaging UV offset printing:
1.UV curing device
Since the curing of the UV ink relies on the radiation of the ultraviolet light by the photoinitiator, the polymerization of the monomer and the prepolymer is initiated and the crosslinking reaction is initiated to cause curing. Therefore, UV printing must be installed on the offset UV. Generally, 3 to 6 UV lamps and exhaust pipes need to be installed in the offset printing machine. The UV UV lamp must be at a suitable wavelength, typically in the range of 200 to 450 nm, and the distance from the illuminated surface is controlled. Also have a spotlight cover. Under the same lamp, the concentrating of the lampshade will make the efficiency much different. Under the illumination of the ultraviolet lamp, ozone is generated in the air, so ventilation equipment such as an exhaust pipe is required. After a certain period of time, the UV lamp will age and the energy will be weakened. Its service life is generally around 1 000 hours. Therefore, in the usual printing, pay attention to the concentrating of the UV lamp, and dust should be wiped clean. The use time of the UV lamp should also be remembered. Always check the curing fastness of the UV ink after the use time, so that the UV lamp can be replaced in time. UV light has certain damage to the human body, so it is necessary to take shading measures.
2. Use of UV ink
Some components in the UV ink come into contact with human skin to produce a certain stimulating effect, and may even cause an allergic reaction to the skin. Therefore, avoid skin and ink contact when using, such as wearing gloves or washing your hands. UV ink and normal ink If you exchange printing on one machine, we must clean the ink roller and ink fountain. If the ordinary ink is not cleaned, it will affect the curing and drying of the UV ink. Insufficient materials for inks should not be used casually. Always use special auxiliary materials that match the UV inks.
3. Adhesion
Substrates such as plastic sheets and aluminum foil have poor permeability and weak adsorption to ink. In order to improve the adhesion of UV inks to these non-absorbent materials, UV offset printing requires a surface energy of 40 dyn or more. We must check the surface energy of the substrate before printing. One of the surface energy treatment methods is corona treatment. It is time-sensitive. Generally, corona treatment will degenerate automatically after one week. If it is used again, it must be corona treated, otherwise it will affect curing drying and ink layer fastness.
4. Ink emulsification and ink balance
Offset printing places great emphasis on ink balance and control of emulsification values. If the UV ink is emulsified, the UV ink film is opaque, affecting the passage rate of the ultraviolet radiant energy, thereby slowing down the film drying time. Even though the ink is solidified, there is still moisture in the ink film. Once the pattern is rubbed or adhered with an adhesive tape, the ink layer is found to be pulled off.
5. Control the temperature and humidity
If the temperature is low and the humidity is high, the photocuring reaction of the UV ink is slow, and the bonding ability of the ink and the substrate is lowered. Therefore, the environmental temperature and humidity of the workshop must be controlled during UV printing. Generally, the temperature is about 18 ° C to 25 ° C, and the relative humidity is about 50% to 65%.
6. Control the printing speed
The speed of printing can affect the curing and drying of UV inks. The speed is fast, the ultraviolet light 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 at around 6,000 to 7,000 sheets per hour.
7. Printing color sequence
The arrangement of the color sequence is very important and should be carefully considered according to the characteristics of the printed product. The thickness of the ink on the substrate affects the quality of the dots and also affects the curing speed of the ink. This is mainly a problem of the overprinting rate of the ink. Therefore, it is necessary to print a color with a small printing area and a small amount of ink at the time of printing. In this way, the highest amount of ink and the fullness of the color can be achieved, and on the other hand, the curing and drying are not affected.
8. Printing registration
The accuracy of the printed overprint is very important. UV lamp irradiation will bring about a certain degree of substrate deformation, so it is generally necessary to irradiate the UV lamp after the last color is printed. If there are special circumstances, the UV lamp should be irradiated in the middle. It is recommended that the irradiation time be as short as possible.
In short, environmentally friendly offset UV printing is the future development trend of printing. It can achieve 100% curing, solvent-free volatilization, no powder spraying, instant drying, stable ink color, accurate overprinting, convenient printing after printing, high abrasion resistance and high-strength gloss of ink layer, especially suitable for non-absorption. Printing of sexual substrates.
As we continue to explore the process of UV offset printing, and the continuous development of new processes and new materials in the future, the level of UV offset printing will be further enhanced.