UV curing ink curing problem

In the packaging field, UV printing inks are favored more and more because of their good decorative effect, non-volatile harmful substances, and fast fixation of ink on non-absorbent substrates. However, UV ink curing is a very important issue when printing. If the ink cannot be firmly fixed to the substrate after the printing is completed, the printed product cannot meet the packaging quality requirements. Therefore, the UV curing must pay attention to the curing of the ink. This is particularly important for UV offset printing. Because the UV offset press is a high-speed printer, there are water in the UV offset press, so there is a problem of ink balance, ink emulsification problems, the existence of these problems will affect the ink curing speed, but also the choice of UV curing device, ink formulation choice, The requirements for the suitability of printing materials, the control of ink and water balance, etc. put forward higher requirements. The following explains:

1, UV curing device

Modern UV offset printing is mostly high-speed multi-color printing. After printing is completed, the ink must be completely cured, and the printed products can meet the quality standards. This poses high requirements for the selection of UV curing devices. UV offset curing device is mainly composed of four parts, namely lamp, reflector, control system and cooling system. Here mainly introduces the lamp and the reflector.

1) Lamp. UV offset inks and curing mechanisms are photosensitizers (photoinitiators) in the ink that are excited to generate free radicals after absorbing certain wavelengths of ultraviolet light, causing ice to cause photopolymerization cross-linking reactions of unsaturated acrylic oligomers and reactive monomers. A solidified layer is formed. That is, the wavelength of UV light required for UV offset ink curing is mainly determined by the wavelength of the photoinitiator in the ink. The wavelength of ultraviolet light absorbed by most photoinitiators in UV offset inks is in the range of 200 nm to 400 nm, so UV curing inks require a lamp with better photochemical matching performance. Photochemical matching means that the spectral output distribution of the lamp used should be matched with the spectral sensitivity of the photosensitive material, that is, the photosensitive material absorbs ultraviolet light and the chemical reaction occurs, and the wavelength range of the absorbed light is just the output range of the light source's emission spectrum. The maximum absorption peak of the material happens to be at the peak of the output of the light source, so that the light energy emitted by the light source is absorbed by the photosensitive material as much as possible to produce a chemical reaction. At present, the commonly used lamps in the UV offset printing ink curing device are mostly high-pressure mercury lamps. The ultraviolet rays emitted by the lamp have a wide range of the spectral line length and overlap, and from the perspective of the emission spectrum, the ultraviolet rays mainly occur in the ultraviolet region. There is strong radiation at 365 nm, followed by strong light that is also emitted in the ultraviolet region, and spectral bands having wavelengths of 366.6 nm, 313 nm, 303 nm, and 253.7 nm, respectively. These happen to be within the range of the main wavelength of ultraviolet light detected by the UV offset ink photosensitizer. An important indicator of a high-pressure mercury lamp is the energy output per unit length. The power of the weighed wire, expressed in W/cm, is directly related to the intensity of the light. When the ink is cured, in order to increase the curing speed, it is most effective to increase the irradiation light intensity and increase the line power. According to the high-speed, multi-color characteristics of UV offset printing, the UV lamp used must have a minimum line power of 80 W/cm or even 120 W/cm, and it is better to have a curing lamp behind each printing color group; if it is applied online, After the glazing, there must be at least three lamps in the delivery section to ensure that both the varnish and the color ink are completely cured.

2) Reflector. The role of the reflector is to direct the energy generated by the UV lamp. Its main function is to ensure that the UV light generated by the lamp can be effectively irradiated to the surface of the substrate. The geometry and robustness of the reflectors determine the relative proportions of direct and reflected light. Therefore, only one suitable UV output lamp is not enough to guarantee that the generated UV energy can be radiated to the surface of the substrate to the maximum extent, and it must be supplemented with a matched reflector. In general, the material used for the reflector is highly polished aluminum to achieve maximum reflectivity. From the geometrical optics point of view, the reflector has a variety of geometric shapes. The curing reflector of the UV offset ink is of the concentrating type (reflection mask is a torn circular surface). The collector reflector is a focussed reflector in which the lamp is mounted over a focal point of the ellipse. Ultraviolet light is reflected to the other focus of the ellipse and when the cured product passes through the focus, the UV light is irradiated with maximum intensity. The light-collecting reflector reflects light with high irradiation intensity and high curing efficiency, and is suitable for high-speed printing of UV offset printing. The reflector is generally equipped with a cooling device to prevent the lamp from overheating and affect the service life and prevent the substrate from being deformed due to overheating. There is a draft tube on the back of the reflector to take away the heat generated. At the same time, there is a circulation of cold water pipes, and some of the heat can be taken away through the circulation of the cold water pipes. In addition, the reflector can be fine-tuned from the height of the substrate to meet the maximum curing of ink energy.

2, ink formula

UV offset printing substrates are mostly high-grade gold, silver cardboard or laser laser paper coated with aluminum foil on the surface. Such paper has weak adhesion to the ink and there is water in the printing process. If the ink formulation is not properly selected, Drop will directly affect the curing of ink, so UV offset printing ink has the following requirements.

1) The tinting strength of the ink. The coloring power of an ink indicates the concentration or saturation of the ink. The strength of the coloring depends on the nature of the ink itself, such as the pigment content, the degree of pigment dispersion, and the selective absorption and reflection of light. The pigment with high coloring power has large pigment content and strong coloring ability. The ink layer is very thin to meet the hue requirements. The UV light easily penetrates the ink layer, which is beneficial to the curing of the ink. On the contrary, the coloring power is weak and the ink must be enlarged. The amount of imprinted ink layer is thick, and UV light does not easily penetrate the ink layer, which is not conducive to ink curing.

2) The ink's water resistance. The quality of the ink's resistance to water is of great importance for UV-based offset printing, especially for poorly water-absorbing aluminum foils. If the ink has poor water resistance, emulsification is likely to occur. In severe cases, it will not only make the production difficult to perform normally, but will also directly affect the curing speed of the ink, and thus affect the product quality, such as causing the ink viscosity to decrease, the adhesion is weakened, and the color is dull. The water resistance of an ink depends mainly on the stability of the ink structure and the nature of the fountain solution.

3) Selection of photosensitizers. Photosensitizers are substances used to catalyze the curing of UV offset inks. Its selection principle is very important: It is determined by the band emitted by the light source used for curing and the absorption characteristics of the photosensitizer. The most effective photosensitizers have a maximum absorption wavelength consistent with the excitation band generated by the light source having the largest excitation effect. The selection of photosensitizers should follow the following principles: fast photo-curing speed; ensure that the printed materials are dry and non-sticky; good compatibility with photo-curable resins and pigments; no yellowing and discoloration of the cured ink film; Loss of gelation during effective storage. It is best not to use a photosensitizer in the ink, but to use several photosensitizers, so that the photosensitizing effect can be induced in different wavelength ranges to improve the catalytic promotion effect and improve the curing effect. The commonly used photosensitizers mainly have two major categories: ketones, benzoin and their ethers. Their common point is that there are aromatic ketone groups. The difference is that benzoin itself can initiate the polymerization of vinyl monomers through photochemical action. Therefore, it is both a photosensitizer and a photoinitiator, and the reaction speed is faster. After the photochemical reaction of dibuxonone, it is often necessary to react with other molecules to initiate the polymerization of vinyl monomers. Therefore, strictly speaking, it is only a photosensitizer, not a very good photoinitiator, and often needs to be used in combination with other compounds, and the reaction is relatively slow.

3, the substrate surface tension

The surface tension of the substrate is also a coke surface energy, which is an indicator to measure the ink receiving capacity. When the surface tension of the substrate is lower than the surface tension of the ink, the ink wets poorly and the surface of the substrate cannot be evenly distributed. It is in the form of droplets and does not firmly adhere to the surface of the reprinted substrate, and the formed ink layer is thick and uneven. Since the ink also absorbs water, it will cause the ink film to become swarthy and weak, which will greatly block the transmission of ultraviolet rays and affect the curing of the ink. Therefore, the surface tension of the substrate must be greater than the surface tension of the UV offset ink. The surface tension of the ink is generally between 30 and 36 in order to make the ink level on the substrate and the surface of the substrate. Tension is at least greater than the surface tension of the ink is more than 10dyn/cm, so the surface tension of the substrate is generally required to be at least 40dyn/cm. In the UV offset printing process, the substrates are mostly gold, silver cardboard or laser paper coated with aluminum foil on the surface. Before the printing, the surface tension of the substrates can meet the printing requirements after processing. Generally, the corona surface is used to drink the surface. Coating method to improve the surface tension of the substrate. The corona method discharges high-voltage charge onto the surface of the thin film layer. Under the action of ionization, the surface of the thin film is roughened and the surface energy is increased. However, the surface energy or surface tension gradually increases with the storage time of the substrate and the surface friction. Falling, so for the foil before the printing surface is painted with a layer of varnish (commonly known as blue paint), in order to meet the surface tension of the paper on the printing requirements. The corona treatment and coating of the surface were completed before the paper was shipped. Paper surface tension is simple to measure and can be easily accomplished with a surface tension test pen.

4, operation

The above mentioned items are all hardware requirements and preconditions, but only good hardware is also not acceptable. In order for UV offset inks to quickly solidify on substrates, the operation of the press operator Skills (software) are also very important. Because the proper operation in the printing process is also a direct influence on the ink curing factors, so in the operation process must pay attention to the following aspects.

1) Control of ink balance. The control of the ink balance is very important for the offset printing process. If the control is not good, the ink will emulsify seriously and produce various quality problems. Especially for UV offset printing is even more important. Because ordinary offset printing is based on the principle of different polarity of oil and water and incompatible with each other. The UV offset printing ink shows weak polarity due to the presence of propylene resin, the water is narrower, the ink balance is more difficult to control, and the ink is more easily emulsified. The water contained in the emulsified ink will block the transmission of ultraviolet rays and directly affect the curing of the ink. speed. Therefore, the ink balance must be controlled during the printing process. Reduce ink emulsification. To do this you can start with the following aspects:

1 The content of ethanol (isopropanol) in the fountain solution is about 15%, which increases the surface tension of the fountain solution. With less fountain solution, a uniform water film can be formed to prevent ink emulsification due to excessive water volume. The temperature of the fountain solution is controlled at 5°C～8°C to avoid the decrease of the plate surface water due to the heat generated by the water roller and the ink roller due to the high speed operation and the amount of the fountain solution needs to be increased.

2 Strictly follow the operating procedures to assess the pressure of the inking roller, the roller roller, the roller, and the printing roller, and constantly stir the ink in the ink fountain to maintain a stable machine running speed to maintain a stable printing pressure and a stable temperature in the workshop. Humidity (temperature 25 °C ± 1 °C, humidity 50% ~ 60%), to avoid changes in the amount of water caused by the above factors, ink, resulting in ink emulsification.

2) Arrangement of printing color sequence

In addition to the same factors as ordinary offset printing of a fish, the determination of the printing sequence of the UV offset printing ink is mainly determined by the curing properties of the ink. The curing of ordinary offset printing inks is a combination of oxidative conjunctive drinking and permeation curing, and the curing performance of each color ink is basically the same. The curing of the UV offset ink is accomplished by the irradiation of UV light. Since the degree of absorption of the UV light by each color pigment in the ink is different, the degree of curing of the ink of different colors is also different. From the perspective of the penetrating power of the UV light to the pigment, the magenta transmittance is 50% to 60%, the yellow transmittance is 20% to 30%, the cyan transmittance is 8% to 20%, and the black transmittance is It is 10% to 12%, while the white ink reflects most of the UV light buckets and the transmittance is lower. Due to the difference in the transmittance of UV light for each color ink, some UV inks can be cured at the same time, and some inks can be cured.