UV printing ink curing principle and application review

The use of solvent-based inks in the printing process has a long history due to the delay in the market for UV printing inks.

In general, the adhesion of the ink to the substrate is mainly based on the matching of the physical and chemical properties of the resin in the ink with the substrate. Solvent inks can be easily selected for various types of resins to achieve good adhesion. However, the types of resins for UV inks are very small, and it is difficult to match a wide variety of substrates with only one type of photosensitive resin. Ink manufacturers use a variety of additives to achieve their goals. Due to the tolerance of ink leakage of screen printing plates, it is easier to manufacture various types of UV screen printing inks. However, UV printing inks are not as simple as that. The transfer of plastic printing heads is poor, and slight differences in composition can easily cause ink transfer problems. Therefore, the vast majority of ink manufacturers can produce mature UV screen printing inks, but can not produce mature UV printing inks. UV screen printing inks cannot replace UV printing inks like solvent type screen printing inks instead of solvent type printing inks.

The drying of UV inks is different from solvent inks, which are the chemical results of the cross-linking reaction of the photosensitive resin in the ink under the irradiation of ultraviolet light. As a result, the key points of the reaction in UV technology include: photosensitive materials and additives, UV light, UV curing and other core content. First of all, for a certain type of substrate, we must find the photosensitive resin and add some additives to achieve the compatibility with the printing materials, that is, to ensure that they are tightly bonded to the surface of the printing material after reasonable UV irradiation; secondly, We are able to produce machines that produce UV light whose wavelengths in the center band correspond to the energy needed for the chemical crosslinking reaction of the photosensitive resin; we must also grasp the elements under the control of the effect of UV curing, and control them to achieve the best Curing effect.

UV printing ink curing principle

UV printing ink under UV irradiation, the photosensitizer in the system excitation is decomposed into free radicals, free radicals make the crosslinking reaction occurs rapidly. All components of the system instantly become solid, so the UV printing ink is generally a chemical process in which components of the liquid react under the action of ultraviolet light to form a macromolecular network. Compared with the drying process of solvent-based inks, the drying of UV pad printing inks produces a new type of resin. The matching of the resin and the substrate is very important. It determines the final adhesion of the ink. The drying of a solvent-based ink is basically a settling process. The solvent evaporates and the resin settles into a solid, which dries on the surface of the substrate.

Photosensitizer UV Absorption

The photosensitizer in the UV printing ink system plays a key role. For example, when benzophenone is irradiated with a wavelength of 310 to 340 nm, the benzophenone is converted into a triplet state, and the triplet captures the H atom in the active amine, resulting in two Free radicals, free radicals can initiate polymerization cross-linking reactions of photosensitive resins and reactive diluent monomers. Free radicals further promote the polymerization crosslinking reaction between the photosensitive resin and the reactive diluent monomer to form a network of high-molecular compounds, and complete the UV ink drying process.

UV curing

The UV-cured light source should be capable of emitting ultraviolet light in the 300-380 nm band. The most widely used screen printing and printing industry are mercury lamps, which can be classified into 80W/cm, 120W/cm, and 200W/cm according to the intensity of light emitted from mercury lamps. The higher the intensity, the shorter the curing time required. But consider the distance of the light source from the substrate, because the distance is too close to deformation of the temperature-sensitive substrate. Practical recommendations should ensure that the distance between the substrate and the light source is at least 0.5m. Or when designing the curing device, the heat source can be cut off quickly to prevent the temperature from affecting the substrate.

UV printing ink application knowledge

Printing materials

With the rise of environmental protection in the world, the application of UV ink has gained a relatively high degree of recognition. Whether it is the field of UV offset printing, UV flexo printing, UV gravure printing, or UV screen printing, the application of UV technology has fully matured and is moving toward the front of water-based UV inks. UV printing ink as a latecomer to the UV printing process has received widespread attention from the ink industry. Many manufacturers in China have claimed to have successfully developed UV printing inks and market them. It can be said that UV printing technology has entered a period of rapid development. Pad printing and screen printing are two special printing process technologies that are different in the process but face similar industrial environments. Therefore, we all require that the inks used have a complete solution for different substrates, such as adding What kind of additives can be used to meet the more difficult to print materials, a simple UV ink can not meet the customer's requirements whether to learn from the two-component model. From this point of view, the UV printing ink system is far more complex than other processes.

ABS

ABS material is widely used in the industrial field. It has two major advantages of excellent physical properties and good printability. Therefore, it is the most commonly used printing material for screen printing and pad printing technology, accounting for the entire ink consumption. 56% of the links. To date, UV printing inks on the ABS surface are still the most mature and stable. It does not require prepress treatment, nor does it require the addition of additives to improve fitness. Under normal circumstances, it is not necessary to add a hardener to achieve satisfactory ink layer properties.

PP, PE

It is well known that PP and PE are prepressed before printing. UV printing inks are no exception. There are three kinds of prepress processing: flame treatment, high frequency treatment and chemical treatment. Because the products involved in pad printing are usually irregular, so the flame treatment is also good, high-frequency processing is also not like a flame-retardant hollow container and thin film high-frequency processor with a relatively standard form, must be based on the product Uniquely designed fire fighting device and high frequency jet head for shape and printing position. Some automated printing machines equipped with flame processors and high-frequency processor nozzles have flexible adjustment modes to accommodate a wide variety of substrates. The so-called chemical treatment is the use of some strong oxide PP, PE material infiltration, non-polar PP, PE molecules introduced oxygen atoms to change the polarity of the surface of the method. It turns out that chemical treatment is sometimes a very effective prepress method, but how to operate continuously in large batch production is a problem, and manual wiping methods are difficult to achieve high work efficiency. The effect of the prepress treatment can be characterized by the surface tension value of the material. MARABU's test result is 48MN/M to obtain a good adhesion tension value.

Even with prepress, the adhesion of UV printing inks on the surface of PP and PE materials is not impeccable, because no manufacturer uses only pure materials. There is another situation where the customer has a particularly high requirement on the chemical and physical properties of the ink layer. Adding a suitable hardener at this time will bring obvious results. The UV ink is a UV system, and the hardener is a type of two-component ink in a solvent ink system. The combination of the two solves the fragility of the UV pad printing ink in transferability and adaptability to some extent.

metal

The more mature metal printing solution is a two-component ink. By adding a hardener, the resin in the ink system reacts chemically to form a tough network that firmly grips the metal surface. UV pad printing ink is basically a mixture of resin and photosensitive resin used for the metal ink. The drying process of UV metal pad printing ink is accompanied by two processes of UV curing drying and two-component chemical reaction. Part of the heat of UV curing also promotes the two-component chemical reaction process. Of course, UV curing is the main process in this process, and excessive addition of hardener should be avoided.
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UV printing process features

The printing process requires that the ink be dried as slowly as possible on the surface of the printing plate, so that the text on the printing plate is not easily blocked by the ink, but the faster the drying is, the better after the transfer to the surface of the printing material, so that even the same type of There may also be differences in materials, and preliminary testing is critical.

Fitness stability

Pursuing the stability of the ink compatibility is basically the goal of all printing processes, but due to factors such as ink type, environmental conditions and the like, the change of the ink adaptability is inevitable, especially the solvent type ink, and the adaptability change means to take necessary Measures to adjust, UV printing inks and other UV inks, the drying process is affected by the environment is very small, the change in fitness is also very limited, which brings convenience for continuous and durable printing.

Instant drying

UV inks are dried under UV light irradiation with high efficiency. As long as the UV lamp power and the UV printing ink reach a perfect match, the cured product will be basically delivered instantaneously. This is undoubtedly for the company's market viability. It is a help. The use of pad printing in the industrial sector requires high hiding power, but the indirect transfer characteristics of pad printing make its hiding power significantly constrained. This is a contradictory factor, and the volatilization of solvents in solvent-based ink further aggravates the hiding power. loss. UV printing inks do not have this disadvantage.

Facilitate four-color printing

Printing involving typographic printing is a challenging task because of the wet and wet overprinting between the inks, ensuring that the same curing conditions for the four inks will significantly reduce the color difference after ink curing. The transparency of the four primary color inks provides the conditions for the primary curing, and the drying between the colors is bound to bring about the disadvantages of the adhesion between the ink layers and even the color decay. Now the domestic design of the four-color machine, whether it is oil pad printing machine or ink pad printing machine is for solvent-based inks, UV pad printing ink basically can not be used. If you use UV pad printing ink, it is limited to four-tone transfer printing, and then one-time UV curing; if it is multi-color pad printing of opaque UV pad printing ink, you should use monochrome pad printing machine to print in one color and one color. Between the drying.

Excellent ink layer

The physical properties and chemical properties of UV printing inks are the second most favored factor for UV inks. High gloss, high abrasion resistance and excellent environmental resistance all dwarf solvent-based inks. The ink ink layer can show excellent anti-friction ability, it can not only resist the erosion of most chemicals, oil, grease and solvent, but also can resist the corrosion of perspiration. Pad printing inks usually require high hiding power, which requires high color density and fineness of the pigments. However, due to the inherent defects in the transfer effect of the pad printing process, high solid content will affect the performance of hiding power. This is pad printing. The reason why ink is difficult to make. So far, domestic printing inks have not been able to obtain satisfactory hiding power. UV pad printing inks are actually more difficult to achieve with high hiding power and high curing speed. High curing speed means high transparency and a relatively thin ink layer, but this is not allowed by high hiding power. MARABU's UV pad printing ink adopts a variety of ink mixing technologies. It is more like adding a variety of ink manufacturing processes to UV ink systems, such as two-component or solvent inks, to achieve a relative balance of these properties. .

UV curing conditions

The drying speed of the UV pad printing ink on the substrate depends on the intensity of UV light per unit area. When UV light strikes the surface of the ink, a portion of the light is reflected and the other portion of light is absorbed to form refracted light. The reflected light mainly affects the drying of the ink surface, and the refracted light mainly affects the deep drying of the ink. The more transparent the ink is, the stronger the refracted light is and the easier it is to dry the ink. UV ink requirements are essentially transparent to make way for dry conditions, but this will sacrifice their hiding power. The actual situation requires that we balance the transparency, hiding power, and even the composition of the ink to achieve the effect and speed of UV curing.

Hue of ink

The hue of the ink is the type of color of the pigment used in the ink. Because the pigment itself comes from natural or artificial synthesis, there is a natural difference in the absorption and reflection of light. Different ink pigments therefore have different projection and reflection of UV light, and thus affect the curing speed of the ink. UV pad printing inks are even finer than UV offset inks, mainly due to the balance of hiding power and UV curing speed achieved on a variety of substrates.

UV lamp power

Although the printing ink layer of the UV pad printing is thin, the UV pad printing ink is not a simple photosensitive material matching ratio of other UV inks for the hiding power and adaptability to the requirements of the complex substrate properties, and many functional auxiliary agents. The addition requires higher curing power. Follow the universal