Application of UV Curing Technology in Flexo Printing

Flexo printing machine has the function of printing and post-press processing. It can add offset printing, gravure printing and silk screen printing units to the entire unit to complete the printing, drying, hot stamping, die-cutting and curing processes. The purpose of this paper is to introduce the role of ultraviolet curing in flexographic printing and some basic knowledge of its application. Because UV curing plays a significant role in flexographic printing, it directly affects the quality of printing.

First, the application of ultraviolet light

UV is the abbreviation of English Ultraviolet Rays, which is ultraviolet light. Ultraviolet light is invisible light, and it is an electromagnetic wave other than visible violet light. The wavelength is in the range of 10 nm to 400 nm. Ultraviolet light is usually finely divided by several segments depending on its wavelength:
a. Vacuum UV, with a wavelength of 10 nm to 200 nm;
b. Short-wave ultraviolet (UV-C) wavelengths from 200 nm to 290 nm;
c. Ultraviolet-B (UV-B) wavelengths from 290 nm to 320 nm;
d. Long-wave ultraviolet (UV-A) wavelengths of 320 nm to 400 nm;
e. Visible light, with a wavelength of 400 nm to 760 nm;
Ultraviolet light is used in industrial production, and generally used is long-wave UV (UV-A).
Nm is called "nano", is the English namometer abbreviations nm translation, is a measure unit. Nanometers are one-millionths of a millimeter, which is also one-billionth of a meter. It is about the length of 45 atoms strung together.
Commonly used ultraviolet light spectrum diagram, see Figure 2, Figure 3, Figure 4.
Mercury lamp: The main peak is 365 nm. Mainly used for surface curing, such as printing, furniture spraying and other post-cure light source.
Xenon lamp: The main peak is 420 nm. Mainly used in screen printing, exposure, photo etching and other processes.
Iron lamp: The crest is a continuous crest. Because it has various peaks, the spectrum between 365 nm and 380 nm is enhanced, so this lamp can be used for both exposure and solidification. Most of the curing lamps used in high-speed flexographic printing presses are UV lamps for this spectrum. Under the same conditions, their UV energy is about 30% higher than that of UV high-pressure mercury lamps, which solves the problem of deep curing.

The latter two lamps have different influences and changes on the lamp's spectrum after the halide has been added to the lamp. Due to the differences in the plate material used in the flexographic plate making, the cold light source lamp (fluorescent lamp) has a peak between 350 nm and 450 nm, and the main peak of the cured ink is between 350 nm and 400 nm.

Second, ultraviolet curing lamp supporting equipment

1. The UV lamps need matching matching transformers, capacitors, ballasts, triggers and other related power accessories, the lights can be lit (1 KW, 2 KW matching ballasts, triggers; more than 2 KW matching transformers, capacitance). High-grade UV devices use electronic power can be steplessly adjusted from 50% to 100% UV intensity, with rewind UV curing.
2. UV lamps need to be equipped with reflectors. On UV high-pressure mercury lamps, the function of the reflector is to gather light, reflect the UV light emitted by the UV lamp to the surface of the product to be cured, and concentrate on a line or a point (ie, on the surface of the cured object). Form a focus).
3. Lamp power density = lamp power ÷ lamp pole pitch.
Different UV coatings have different power density requirements for lamps, such as 80 w/cm, 120 w/cm, and 240 w/cm.
4. UV energy = UV intensity × time.
In a certain range of UV intensity, the accumulation of time reaches full cure.
5. The use and characteristics of lampshade:

The lampshade that is used today for the soft-groove printing is mainly used not only as a condenser, but also as a shutter. It can be controlled online. When the printer stops, the shutter closes and the lamp power is reduced to a minimum level to prevent the printed material from catching heat. If it is a heat-sensitive material, it is required to be equipped with a quartz heat insulation sheet (the quartz reflector reflects ultraviolet, transmission infrared, reflection infrared, transmission enhanced UV). As shown in Figure 6.

Third, the UV coating curing process

After the UV coating is irradiated with ultraviolet light, the photoinitiator is initiated to generate a ionic group or ion. These radicals or ions crosslink with the double bond in the unsaturated monomer of the oligomer to form a monomer gene, thereby initiating polymerization. , cross-linking and grafting reactions to convert resins (UV coatings, inks, adhesives, etc.) from liquid to solid within seconds (this variation is called “UV cure”), a complete cure The process is over.

UV flexo ink has excellent physical and chemical properties. The process of UV curing and drying is the photochemical reaction of UV ink, that is, it changes from a linear structure to a network structure, which has many excellent physical and chemical properties such as water resistance, alcohol resistance, abrasion resistance, and aging resistance. This is the other types of inks. Unavailable. UV flexo printing ink has good applicability, high printing quality, no change in physical properties during printing process, no volatile solvent, stable viscosity, insusceptible plate stencil printing, strong printing force, high dot definition, good reproducibility of steps, and bright ink Bright, firm, suitable for fine product printing.

From the current point of view, UV printing technology has a process of promotion and application in China. On the one hand, it is a further understanding and understanding of UV flexo printing technology; on the other hand, it is a collaborative improvement of equipment, light sources, supporting equipment, and process technologies. perfect. This requires equipment manufacturers, ink companies and light source companies must cooperate in order to achieve a perfect print curing effect. Such as UV light source: UV flexo printing Every color group needs to be equipped with UV light source. Different color UV flexo printing inks have different UV absorption capabilities, such as strong yellow and magenta absorption ability, easier to cure and dry, and blue and black The ability to absorb ultraviolet light is weak, it is not easy to dry, it is cured at the back and the power is increased. The power of UV lamps can be assigned according to different colors, so as to ensure reasonable drying of various colors of UV inks, and also to save energy. Pay attention to the aging of UV lamps and replace the UV lamps in time to prevent incomplete curing and adhesion.
The lifetime of the UV lamp is 1 000 hours. Frequent start-up will significantly shorten the life of the lamp.

Fourth, on the UV curing device in the flexible

Some Suggestions for Printing in Printing
1. The entire UV curing device should be divided into three parts: light box, power control box and cooling device.
2. The appearance of the light box section should be compact and complete, and ensure that the cooling wind can flow convectively.
3. The power of the light source is high, and the power can be converted to full power, half power or 75% power.
4. The fan switch is controlled in the electrical box, and there is a delay of 6 to 8 minutes after the lamp is turned off to cool the lamp.
5. The main switch is equipped with indicator lights and there are protective measures for the main electrical components.
6. The shade part can be equipped with quartz heat insulation sheet for curing the heat sensitive material.

At present, flexographic presses and web machines equipped with UV ink curing systems are increasing in foreign countries. At the same time, existing self-adhesive machines are constantly being converted to UV ink curing systems. Replacing water-based inks and solvent-based inks with UV inks has become a development trend, and has been widely used in soft-convex machines. There is no doubt that the development prospect of UV flexo printing technology is very good. The promotion and application of domestic UV flexo technology needs to be based on our national conditions and the needs of products, as well as domestic equipment and equipment manufacturers to be able to support and improve, to improve the product's cost performance and equipment stability and durability. With increasingly stringent environmental protection requirements and quality requirements, it is believed that in the near future, domestic UV device technology will certainly have a larger development. ※