Several key issues in UV printing

In recent years, with the appearance of new packaging materials such as aluminum foil composite paper, transfer paper and polyester composite paper, UV curing technology has been widely used in the printing industry. Printing manufacturers have carried out UV transformations on traditional offset, flexo, and screen printing equipment to suit this trend. The performance of UV printing process itself is also very good, mainly in: 1 environmental protection, does not contain organic volatiles; 2 has a good printability, downtime does not dry, see UV light curing instantly, without spray, dot reproduction High "sharpness", color stability; 3UV coating has good surface properties after curing, high gloss, strong adhesion, abrasion resistance, acid and alkali resistance.
First, the UV ink curing 1. UV Ink Curing Mechanism UV inks have two types of curing, one is radical polymerization reaction, in which the photoinitiator in the ink generates free radicals after absorbing UV light, activates monomers, oligomers, and polymerizes instantaneously. reaction. Currently used for labels, folding cartons. The other is a cationic polymerization, and the cation acts as a "catalyst" in the polymerization reaction. Different from radical polymerization reaction, cationic polymerization has hysteresis, not in the instant polymerization, and does not produce odor when cured. The adhesion of ink layer is stronger than that of free radical polymerization. It is mostly used in the printing of food plastic packaging and metal packaging.
2. The curing of UV inks and curing of UV inks by UV lamps are usually performed using two types of UV lamps, one is a high-pressure mercury lamp and the other is a metal halide lamp. The wavelengths of UV light emitted from the two lamps are shown in FIGS. 1 and 2 .

The high-pressure mercury lamp has the highest curing efficiency in the wavelength range of 200 to 230 nm, and the highest efficiency range of the metal halide lamp is in the wavelength of 300 to 400 nm. Since the pigment in the ink also absorbs UV light, metal halide lamps are often used for UV ink curing, and high-pressure mercury lamps are used for UV varnish curing. In addition, it is also important that the absorption wavelength of the photoinitiator in the ink be matched with the UV lamp. If a photoinitiator can be excited at the wavelength of 250nm, and the UV lamp can not release the UV light of this wavelength, the ink will not be cured; if the UV lamp can release the UV light of this wavelength, the ink The pigment can absorb UV light of this wavelength, and the UV ink will not be completely cured. Therefore, the ink manufacturer must choose the pigment as much as possible to avoid the competition between the selected pigment and the photoinitiator for UV light. Among the pigments used in UV inks, black pigments absorb light of all wavelengths, so black inks are hardest to cure, followed by blue inks, and yellow inks and magenta inks are relatively easy to cure. Therefore, the printing sequence of UV inks is usually black, cyan, magenta, and yellow.
UV lamp power is usually divided into high, medium and low gears. Choosing the right power can, on the one hand, prevent the incomplete curing due to the increased printing speed, and on the other hand can prevent excessive curing and cause ink layer embrittlement.
Second, the UV curing heat management In UV curing, people often refer to cold UV technology, that is, heat removal technology. When the UV ink and UV varnish are cured, the amount of heat released by them is very small, and most of the heat is caused by the large amount of infrared rays released by the UV lamp. UV lamp temperature is too high, on the one hand will reduce its own emission power, on the other hand, heat reaches the printing material, will lead to shrinkage and deformation of the printing material, resulting in overprinting is not allowed. Figure 3 shows several common methods of heat reduction (flat mirrors and lenses are made of quartz material).

In order to better solve the problem of heat emission, people now use double-color mirror method, cold air circulation method and chill roll method to achieve heat transfer, see Figure 4 and Figure 5.

The dichroic mirror method uses a two-tone coating on the surface of the reflector, which absorbs infrared light and reflects UV light. The heat generated by the absorbed infrared rays is carried by the circulating cold air.
The chill roll method is to let the printed product pass through a cooling roller full of circulating water to achieve the purpose of cooling when curing. Printing some temperature-sensitive substrates, usually using the two methods described above in combination. In the actual production process. Under the premise that the ink layer thickness and printing speed can be guaranteed, low-power curing should be adopted as much as possible to avoid excessive temperature. What needs to be explained here is that people always hope that the UV lamp will not generate heat. In fact, a certain amount of heat is necessary for smooth UV curing.
Third, the factors that affect the UV ink curing 1. The higher the UV lamp power of the UV lamp, the faster the UV ink cures. But to avoid UV lamp tube power caused by UV ink embrittlement.
2. The thicker the ink layer thickness, the slower the curing speed. In actual production, it often happens that due to the poor surface smoothness of the printing material, the ink transfer rate decreases, and uneven printing appears. At this point, the machine operator will usually increase the amount of ink, in order to print better. This situation is most likely to lead to incomplete ink curing, poor adhesion and other issues. If the surface of the printing material encounters a poor surface smoothness, it is best to use a diluting agent and then print it.
3. The printing speed is slow, and the curing time is long, and the curing effect is naturally good. The printing speed is fast and the lighting time is short, and the curing may not be complete.
4. The number of other UV lamps. The use of time and the degree of surface cleanliness, the degree of cleanliness of the reflector, the distance from the lamp to the substrate, the excitation wavelength of the photoinitiator and the emission wavelength of the lamp also affect the degree of curing of the UV ink.
Fourth, the surface tension and UV ink adhesion In the printing of aluminum foil, polyester cardboard or PET, PVC and other substrates, often encounter poor ink adhesion, intolerance friction and other issues. This may be due to the fact that the surface tension of the substrate itself is too low, in addition to the incomplete curing of the ink. In general, the surface tension of the substrate is 1.0×10-2N/m larger than the surface tension of the ink. The ink can adhere to the surface of the printing material. Different types of ink, the surface tension requirements of the printing materials are also different. For solvent-based inks, if the surface tension of the substrate is 3.8×10-2N/m or more, the ink can adhere well: while the UV ink requires the surface tension of the substrate must reach (4.O- 4.2) × 10-2N/m or more. In the actual production, it is often encountered that the surface tension of the printing material reaches more than 4×10-2N/m, and the adhesion of the ink is not ideal. This is because, on the one hand, the surface tension of the substrate is 1.0 × 10-2 N/m larger than that of the ink (assuming 1.0 × 10-2 N/m is large enough to achieve good adhesion), which is only necessary for ink adhesion. Not a sufficient condition. In other words, the surface tension of the substrate must be 1.0×10-2N/m greater than that of the ink in order for the ink to adhere well to the surface of the substrate; and the surface tension of the substrate is 1.0×10 greater than the surface tension of the substrate. -2N/m, does not necessarily achieve good adhesion, but also depends on the polarity of the substrate. On the other hand, if you use the Dyne pen test, it may also be that the Dyne pen is contaminated and the surface tension of the substrate cannot be accurately reflected. Of course, as the storage time of the substrate (film) is extended, the slip agent may migrate to the surface, causing the surface tension of the substrate to decrease.
5. The development of UV printing is undeniable. The development of UV printing technology has been quite mature so far, but the author believes that UV ink may become a bottleneck restricting the continued development of UV printing technology. The reason is:
(1) UV ink prices are high.
(2) Due to adhesion problems, the range of substrates that UV inks can print is relatively narrow compared to solvent-based inks. Therefore, how to reduce the surface tension of the UV ink to improve its adhesion and to achieve good transfer is a major problem for ink manufacturers.
(3) In folding carton printing, the problem of electro-aluminum hot stamping on UV varnish has not yet been fundamentally solved.
(4) The number of anilox rollers used in flexographic printing is getting higher and higher, which will bring two problems: how to reduce the viscosity of ink to increase its flowability and how to achieve the required conditions under thinner ink layers? The color density,
(5) Odor problems in UV printing.
If ink manufacturers can make more efforts to solve the above problems, the development of UV printing technology will surely go further.