Development trend of ink curing and drying technology
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At Drupa last year, the maximum speed of sheetfed offset presses has reached 20,000 sheets per hour, and the maximum speed of rotary newspaper presses has reached 100,000 copies per hour. The customers who prefer these high-speed machines are China. Printing industry, especially cigarette packs, newspaper printing plants. It can be said that increasing the printing speed has great potential for the printing machine itself, but the drying speed of the ink is a bottleneck for development. Therefore, research on UV curing, thermal drying and quick-drying inks of inks has been active in recent years. This paper starts from the drying principle of ink, and discusses the latest development of UV curing and heat drying light source and device design.
Principle of ink drying and curing
Drying of printing inks mainly includes drying of oxidized conjunctiva, osmotic drying, volatile drying and radiation drying.
1. Oxide conjunctiva drying
The ink contains a dry oil, and the unsaturated fatty acid in the molecular structure of the oil chemically reacts with oxygen in the air, and is polymerized into a macromolecular network structure by a linear structure of a small molecule to form a solid substance. This chemical reaction process is called The oxidized conjunctiva is dry.
2. Osmosis drying
The ink binder contains more mineral oil, such as gasoline, high-boiling kerosene, etc. When the mineral oil in the ink penetrates into the porous paper fiber, the pigment is fixed on the surface of the paper to form a solid film layer, which is called osmotic drying.
3. Volatile drying
The ink contains a large amount of volatile substances such as aliphatic hydrocarbon rosin esters, aromatic hydrocarbon solvents, alcohols, ketones, ester solvents and the like. After the ink is transferred to the surface of the printing substrate, the solvent evaporates and the pigment is fixed to form a solid film layer, which is called volatilization drying.
4. Radiation curing
Radiation curing is a chemical reaction in which a chemical monomer is crosslinked into a polymer network to form a solid, and the curing speed is fast, and the formed film layer is hard and bright, but the adhesion is poor. There are currently three types of radiation curing methods commonly used.
(1) Infrared radiation drying. The ink contains a chelating agent, and can be irradiated by medium-wave infrared light having a wavelength of 3 to 30 μm to accelerate the polymerization reaction, and the generated heat promotes solvent evaporation and accelerates ink drying. Short-wave, long-wave infrared is not conducive to the drying of the ink.
(2) UV (UV) radiation curing. The ink contains a photoinitiator such as benzophenone (absorption wavelength 240-340 nm), 2-thioxanthone (absorption wavelength 200-400 nm) and other derivatives, and the photoinitiator is in the range of 250-420 nm. Under the irradiation of UV light, the unsaturated double bond-initiating resin can be quickly opened in less than 0.1 second, and the crosslinking, such as polyurethane or epoxy acrylate, is solidified and dried. The UV curable ink is substantially free of solvents.
(3) Electron beam (EB) radiation curing. The thermionic cathode tube emits a high-energy radiant electron beam, and the wavelength of the radiation is selected by adjusting the cathode tube emitter, so that there is no visible light and infrared ray accompanying radiation, and it is completely a cold light source. The electron beam has a high effective radiation intensity and can directly cause the resin to polymerize into a solid. Therefore, a photoinitiator can be omitted in the EB ink. The photoinitiator in the UV ink is removed or added in a small amount to serve as an electron beam curing ink.
Usually, the drying of the printing ink is not a single drying method, but several dryings occur simultaneously. For example, the drying of offset ink is oxidized conjunctiva, infiltration, volatilization, etc., which are mainly caused by oxidized conjunctiva drying; newspaper printing ink is mainly osmotic drying; gravure ink is mainly volatile and dry.
Common ink drying and curing equipment
Ink drying is often compound drying, so what drying equipment is used, how to set the parameters, and the best drying efficiency while saving energy is the most concerned issue for printers.
1. Infrared drying equipment
(1) Light source. For ink drying, the effect of medium-wave infrared is the best, and there are three kinds of infrared light sources.
The first type is a heating wire radiation source, and a common type is a non-closed resistance wire-coated quartz lamp. The heat radiation intensity of the lamp can not be controlled with the distribution of the wavelength of the light, the efficiency is low, the price is low, and the service life is about 2000 to 3000 hours.
The second kind of light source is to place the resistance wire in the closed quartz tube, and can charge different gases in the tube to change the wavelength distribution characteristics of the radiation light, and at the same time isolate the filament from the oxygen in the air, and the radiation efficiency is high. At present, most imported printing machines are equipped with such a light source, and the service life is 5,000 to 10,000 hours. However, a large part of the radiant energy is distributed in the visible light and far infrared rays, and the medium wave infrared rays are not very rich.
The third type is an electrothermal semiconductor light-emitting tube. The shape is a red or gray matt semiconductor material coated on the outside of the ceramic tube, and the inner wall of the tube is wound around the resistance wire. The electric radiation is used to induce the radiation of the semiconductor material, and the light is dark red or black when the lamp is ignited. The wavelength distribution of the radiant energy can be changed by selecting the semiconductor material, and the radiation efficiency is extremely high.
(2) Infrared lamp reflector. At present, there are two kinds of common ones, one is a gold-plated reflector, which is coated with a metal film around 180 °, the metal film reflects infrared efficiency up to 90%; the other is an elliptical or figure-shaped aluminum or aluminum plate Reflector, as shown in Figure 2, the closer the lamp is to the lampshade, the higher the reflection efficiency, the highest reflectivity can reach 85%.
(3) Heat dissipation of infrared lamps. The surface of the infrared lamp can be cooled by the blowing method, and the air volume does not affect the infrared power output of the lamp. Since only about 1% of the energy radiated by the infrared lamp is absorbed by the paper and the ink, and the rest of the energy is completely converted into heat, causing the temperature to rise around the machine, it needs to be exhausted to the outside or recycled.
(4) Scope of application. Infrared lamps are used for infrared ink drying. The drying efficiency of non-special infrared inks is very low, and it does not even work for some inks. Infrared has penetrating power, can simultaneously dry the upper and lower layers of the ink film, accelerate the penetration of ink, and promote the oxidation of the conjunctiva, especially in the application of thick ink layer printing and varnish drying. The drying effect of offset, embossing, screen printing, printing ink, flexographic printing ink and water-based varnish with low solvent content is very good.
2. Hot air drying equipment
(1) Heat source. There are three kinds of heat sources for generating hot air: electric hot air ducts, such as stainless steel, quartz glass, and hot air ducts of ceramic shells; high temperature oil pipes, high temperature oils heat the air through pipes to generate hot air; high temperature steam pipes generate high temperature steam through boilers to generate hot air. .
(2) Hot air device. The air around the heat source is heated by a fan to be blown onto the surface of the ink, and the angle between the direction of the hot air and the running of the paper is adjusted to improve the efficiency of hot air drying.
The higher the speed of hot air relative to the paper, the better it is to reduce the vapor pressure on the surface of the paper and increase the evaporation rate of the solvent; adjusting the wind direction makes the wind direction add effect to the running speed of the paper, which is more conducive to drying. The wind speed of the hot air duct should not be lower than the required speed. Otherwise, the heat dissipation on the surface of the duct will burn out the tube. The wind speed should not be too high, because some energy will be lost during the wind cycle, and the optimal drying effect can be achieved by appropriately selecting the fan pressure and air volume.
(3) Scope of application. It is suitable for the drying of plastic gravure and flexographic printing inks and solvent-based varnishes, and can also accelerate the drying of water-based inks and varnishes.
3.UV curing equipment
(1) Light source. Commonly used high-pressure mercury lamps and metal halide lamps. High-pressure mercury lamps are excited by mercury atoms to form a continuous spectrum, which regulates the mercury content in the lamp tube so that the radiation wavelength is 254-420 nm in the curing section, and the ultraviolet rays are rich. The metal halide lamp is a metal halide such as gallium or iron added to the mercury lamp, and the radiation is excited under the excitation condition of the mercury. The different metals can be added according to the difference of the absorption spectrum of the ink, so that the UV lamp absorbs more ink and absorbs it. Light waves improve curing efficiency.
Metal halide lamps have extremely low impurity content and high purity of electrode materials. Therefore, the manufacturing process is complicated, difficult and expensive. At present, high-quality, high-power metal halide lamps cannot be produced in China. The existing ones in the market are American Fusion, Germany IST, Products of companies such as Yanqi, Japan. The lamp with a power density of 160 W/cm has a life of up to 2000 hours and the lamp head is not black. It can still reach 50% radiance after 4000 hours.
(2) Cold UV light source. When the UV lamp is ignited, its internal temperature should reach 600~1000 °C. If the temperature inside the lamp is too low or the wind speed is too high, the lamp will go out, so the "cold light source" does not actually exist, but the lamp cooling system can be used.
(3) Microwave UV lamps. This is a new product of Fusion Company. The lamp is 6 inches long and has no metal electrodes at both ends. The microwave stimulates the high-speed movement of the molecules that absorb microwaves in the lamp to generate heat, and then excites mercury, metal halides and other substances to emit light. Aluminum is a good material for reflecting microwaves. The microwave source emits microwaves onto an elliptical aluminum lampshade, focusing on the induction of mercury or metal halide luminescence in an electrodeless lamp.
The characteristics of microwave lamps are:
1 Adjustable radiation intensity with wavelength distribution characteristics, select the optimal radiation wavelength, so that the ink achieves the desired curing effect.
2 Since the lamp has no electrode, the life can reach 5000 hours when the power density is 300W/cm, and the switch does not affect the lamp life.
3 Since the microwave source is easy to control, the microwave lamp can be started frequently and frequently, but the microwave also has a standby state (maintaining the working state), and only in this state can the quick start. This function is suitable for use with optical discs and self-adhesive printing presses, because screen printing discs and self-adhesive printing are mostly intermittent printing.
4 microwave lamp UV curing section is rich in spectrum, less infrared band, so the microwave radiation temperature is lower.
5 Since the microwave lamp has no electrodes, it is convenient to change the lamp tube and no wiring is required.
6 Because there is no electrode release, the ends of the lamp will never turn black.
7 Due to the limitation of microwave transmission mode, the maximum length of the microwave tube is 6 inches. The connection of the tube can increase the curing width. Since the gap of the tube is small, the docking point will not affect the curing effect.
8 microwave lamps are more expensive, a set of 6-inch curing equipment is about 50,000 yuan. At present, the domestic market is mainly used for curing UV inks matched with inkjet printing, cable, pipe, flexible packaging and other inkjet printers.
9 Microwave lamp illuminating material is the same as ordinary UV lamp, and the radiation wavelength distribution is also the same, so no special ink is needed.
(4) UV lamp reflector design. Scientifically selecting reflective materials and rationally designing the reflector structure can double the curing effect and save energy. The aluminum crystal formed by acid oxidation of pure aluminum has a reflectance of 90% to UV light, while the aluminum light without oxidized treatment reflects UV light efficiency less than 60%; the reflectance of stainless steel to UV light is less than 60%; and the reflection of UV light by semiconductor coating The rate is 94%, but the cost is higher. At the same time, infrared rays and visible light can pass through the plating layer to reach the surface of the reflector, and are discharged through the cooling device.
The development trend of reflector design is the change of opening and height, that is, the cross-sectional area becomes smaller to shorten the reflection path and reduce the loss of light intensity due to long path length. In addition, the top of the reflector and the reflector change the design of the original top vent, so that the radiant energy at the top of the tube is reflected to the paper, as shown in Figure 4. Since the cross-sectional area of the lamp cover is small, the distance from the wall of the lamp to the reflective surface is too high, so the temperature of the reflector is too high, and the wind resistance of the top reflector affects the heat dissipation of the lamp cover. The solution is to use a cold water cover or increase the amount of heat dissipation to properly design the cooling air path.
Introduction to advanced curing equipment
1. Microwave UV curing equipment
The characteristic of the device is that due to the needs of the microwave transmission channel, the lamp has a large volume and should not be installed in the delivery channel of the sheetfed offset printing machine, generally with the unit type flexographic printing machine, gravure printing machine, table and self-adhesive printing. The machine is suitable for use, and the optical disc and inkjet printing system are also suitable for the equipment, but due to the high price, the printing factory is rarely used.
2. High power density UV curing equipment
The electrode UV lamp produced in Japan has a power density of 240-300 W/cm, which can be used for the curing of the rotary printing of the UV silver paste in the form printing unit; the UV lamp power density of the American McAndy flexographic printing machine. It has reached 400W/cm and the curing speed is 150m/min; the microwave lamp power density of Fusion Company is 400W/cm.
The high power density UV curing equipment not only speeds up the curing speed, but also facilitates the complete curing of the UV ink underlayer on thick UV ink layers and rough paper. In addition, increasing the power density of the UV lamp can reduce the sensitivity of the UV ink and reduce the content of the photoinitiator, thereby improving the stability and sensitivity of the UV ink, facilitating storage and transportation, and reducing the price.
3. Water cooling UV curing system
Due to the development of the design concept of printing equipment, the structure of the printing machine is more compact and beautiful, and the larger UV curing air cooling passage can no longer meet the requirements. Therefore, such printing machines generally use water-cooled UV curing equipment. For example, the UV drying system of the imported multi-color offset printing machine is water-cooled, and the satellite self-adhesive printing machine is also equipped with a water-cooled UV system.
The author often finds in the printing factory that there are many unreasonable phenomena in the ink drying and curing process, resulting in a large waste of electric energy. For example, some curing equipments are unscientific, although the price is cheap, but the efficiency is very low. The speed of a pair of glazing machines is less than 3,000 sheets per hour. Three 10 kW UV lamps are used. A 12kW lamp can be used. For example, the power supply of the front and rear unit curing system of the unit type printing machine is unreasonable, resulting in a printing speed. And many manufacturers only consider the cost of equipment, ignoring the expenses of electricity. In response to the above problems, it is recommended that the printing company should consider many aspects when selecting curing equipment.