RU2401703C2 - Method of solidifying substance by uv radiation and device to this end - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention can be used for UV solidification of colour printer inks, polymer coats, vanishes etc. Proposed method comprises subjecting the substance containing photo initiators to radiation of UV-LEDs. Radiation spectrum of all said UV-LEADs is identical and corresponds to maximum sensibilisation of substance photo initiators. Intensity of UV radiation is controlled by feeding to UV-LEDs current pulses with frequency varying from 1 kHz to 10 MHz, as well as by frequency, current strength and current pulse on-off time ratio so that average dissipated power of said LEDs equal or approximates to maximum value, not exceeding critical value that causes LED failure. Proposed device allows implemented the claimed method.

EFFECT: invention can be used in piezo-jet fill-colour printers with various printing heads.

5 cl, 5 dwg, 2 ex

Description

The invention relates to UV curing of a substance and can be used for UV curing of inks, polymeric coatings, varnishes, in particular, for applying an image or text by the method of full-color piezo-ink printing and subsequent fixing of the print by photopolymerization of a binder when irradiated with UV radiation in a narrow spectral range. The present invention allows to obtain full-color text, graphic and line art on flat surfaces of various materials: flexible and sheet polymers, glass, metal, ceramic and wooden products, etc.

Known as the closest analogue is a method of curing a substance with UV radiation according to US patent No. 7175712, priority date 01/09/2003, in which a substance including photoinitiators is exposed to radiation from UV LEDs and fluorescent lamps in a wide range of wavelengths, with UV intensity -radiation is controlled depending on the properties of the curable substance and the conditions of curing, maintaining a constant temperature of UV LEDs. The method is used for UV curing of ink, polymer coatings, varnishes.

Known as the closest analogue is a device for curing a substance by UV radiation according to US Pat. No. 7,175,712, priority dated January 9, 2003, which can be used, for example, in inkjet printers. The device contains a UV radiation source, including a primary UV radiation source in the form of rows (rulers) of UV-LEDs connected in series with different wavelengths and a secondary UV radiation source, which is one or more fluorescent lamps. The rows of primary source LEDs are mounted on a substrate that is mounted on an air-cooled radiator. The temperature sensor is located on the radiator and is connected to the UV LED control unit. When using several rows of LEDs, the space between adjacent rows is shifted by 1 / x, where x is the number of rows, or the LEDs are staggered. To protect the UV LEDs from UV ink or other substances, a transparent plastic protective sheet is used, which must be replaced periodically.

When ink is applied quickly, it may spread and leak. In this case, the partial or complete curing of the ink is carried out by exposure to UV radiation from the primary radiation source to fix and partially polymerize and / or gel the ink droplets in order to prevent ink from spreading and smudging. It does not take much time to fix the ink. The curing of the ink is completed by exposure to a primary source or a secondary source of UV radiation, which is one or more fluorescent lamps. Between UV-approved inks, coatings, varnishes and UV sources, it is possible to create an oxygen-free zone from an inert gas, such as helium, which is an anaerobic region to increase the performance of UV photoinitiators.

The primary source of UV radiation includes several rows of UV LEDs, while adjacent LEDs have different wavelengths in at least two different ranges. UV LEDs with different wavelengths are arranged in random, mixed or sequential order. To achieve a greater variety of wavelengths, a secondary source of UV radiation is used, including one or more fluorescent lamps, whose phosphors are designed to intensify radiation with a given wavelength. For example, the 2011C type of fluorescent lamp emits a wavelength of 351 nm, type 2052-371 nm, type 2092 - 433 nm, and type 2162 - 420 nm. UV LEDs with a wavelength of more than 400 nm are also used, since the efficiency of LEDs increases with increasing wavelength, which allows you to effectively increase the power of UV radiation. Preferably, the UV LEDs are alternated in rows so that the radiation of the LEDs with different wavelengths is in the range between 180 nm and 420 nm. To cure large-thickness polymers, UV radiation with a longer wavelength is required. Surface curing requires a shorter wavelength of UV radiation. Pigment coatings are better cured by UV radiation with a wavelength different from the wavelength absorbed by the pigments. This also applies to the absorption properties of resins and additives in ink, coatings, or adhesives. In addition, part of the UV LEDs can emit light in the visible part of the spectrum so that the user can visually observe whether the device is operating. An air cooling system is provided in order to ensure the desired temperature of the UV LEDs at the required radiation intensity. The air cooling system includes a radiator on which UV LEDs are mounted on the substrate and a radiator blower (fan) in order to maintain a constant temperature of the UV LEDs. The substrate temperature or radiation intensity is monitored and used to control the current strength or voltage supplied to the working fan of the substrate cooling system. Improving the cooling of the substrate, the substrate temperature is maintained at a constant level, thereby stabilizing a constant radiation intensity, since heating of UV LEDs can lead to a decrease in the radiation intensity.

The device also uses an individual selection of UV LEDs according to their characteristics in order to ensure the same voltage drop across each UV LED and, accordingly, to achieve the same current and radiation in each UV LED from the group connected in parallel. Reducing the current in the forward direction between the UV LEDs varies between 5-10%, thereby minimizing losses in individual UV LEDs.

The distance between the source of UV radiation and the cured substance is selected from the condition of achieving the same radiation intensity at all points of the irradiated surface of the substance.

The UV LED control unit is designed to turn on and off the UV LEDs and serves to stabilize the radiation intensity of UV LEDs located on a substrate mounted on a radiator by monitoring the intensity of UV radiation using a radiation intensity sensor and monitoring the current and voltage in the system air cooling, which acts on the radiator depending on the intensity of UV radiation. Intensifying the cooling of UV LEDs, thereby stabilizing the set temperature, which ensures constant radiation intensity of UV LEDs, which is controlled by a UV radiation sensor.

To prevent overheating of the UV LEDs, the power is periodically turned on and off at a relatively high frequency. The period depends on the intensity of UV radiation.

The above method of curing a substance by UV radiation and a device for its implementation have several significant disadvantages.

Firstly, the known method of curing a substance by UV radiation and a device for its implementation do not allow to abandon the use of fluorescent lamps and therefore have all the disadvantages inherent in fluorescent lamps, such as low efficiency, high operating temperature, low time resource, low environmental friendliness for ozone emissions, high power consumption. The above-described drawbacks limit the scope of the known method for curing the substance with UV radiation and a device for its implementation due to the impossibility of using, for example, in piezo-jet full-color printers with various types of printheads to obtain a full-color image on surfaces of various materials, such as flexible and sheet polymers, glass, metal, ceramics, wood, etc., while ensuring a high curing speed of a sufficiently thick layer of UV cure th substance. The use of fluorescent lamps and UV LEDs simultaneously as radiation sources, in addition, complicates the device and increases the cost of curing the substance with UV radiation. The complexity of creating an anaerobic region from an inert gas, which is necessary to increase the productivity of UV photoinitiators, also contributes to the increase in the cost of curing a substance by UV radiation.

Secondly, the range of UV radiation is too wide, but it does not take into account that the photoinitiator has maximum sensitization in a narrow spectrum, i.e. its physicochemical properties are not taken into account. Since well-known photoinitiators have maximum sensitization in the wavelength range of not more than 365 nm, the use of LEDs with radiation with a longer wavelength is not effective. The use of UV radiation for curing a substance with a wide spectrum of radiation is not effective, since it leads to a decrease in the intensity of UV radiation in that part of the spectrum in which the photoinitiators that make up the substance have maximum sensitization. By reducing the number of LEDs that the photoinitiator reacts to and installing LEDs with a longer wavelength, for example 400 nm, in the end, we obtain a decrease in the effective radiation power of the wavelength for which the photoinitiator is designed. The use of LEDs with radiation in the visible spectrum to control the health of the emitter also helps to reduce the radiation power of the wavelength at which the photoinitiator has the greatest sensitization. In addition, the location of the emitter is such that the emitted and irradiated surfaces are practically invisible during operation of the emitter.

Thirdly, monitoring the radiation intensity of LEDs according to the readings of the radiation intensity sensor, as well as using feedback and temperature stabilization, is not effective. In LEDs, as the temperature rises, the crystal degrades, and the higher the temperature, the greater the degradation. Due to the degradation of the crystal, the radiation intensity of the LED decreases. When the light sensor detects a decrease in the intensity of UV radiation of a degrading crystal, the cooling system tries to cool the radiator more strongly, thereby lowering the temperature of the LEDs and increasing the radiation intensity of the LEDs. Since there is no increase in the intensity of UV radiation, the cooling system will work at maximum performance. Such a system for controlling the radiation intensity is not effective, since it does not prevent the degradation of LED crystals, since they are not located on the radiator, but on a substrate mounted on the radiator. As a result, the radiation intensity of UV LEDs is reduced.

The system for controlling the temperature of the substrate of UV LEDs also has a drawback. It is known that with increasing crystal temperature its resistance decreases. Since the voltage drop across the LED crystal is constant, according to Ohm's law, the current flowing through the LED grows. The increased current heats the LED crystal even more, contributing to a further increase in current and temperature, which leads to degradation and destruction of the crystal. Since it is not the crystal temperature that is controlled, but the temperature of the radiator or the substrate (board) on which the LEDs are installed, it does not take into account that there are still materials with a certain thermal resistance between the crystal and the radiator (crystal substrate, LED housing, solder mask, conductive material tracks, dielectric layer). Between the radiator and the crystal there is a temperature difference due to thermal resistance - the higher the temperature of the crystal, the greater the temperature difference between the radiator and the crystal. Since the current flowing through the crystal is not limited with increasing temperature, the temperature difference can be several tens of degrees, while the temperature control system of the LED substrate does not protect the crystal from degradation, which means that the cooling system is not effective, which reduces the intensity of the LED radiation.

Fourth, since some of the LEDs are included in parallel circuits and the LEDs have radiation with different wavelengths, the current in the chains is selected with a deviation of 5% and 10% percent. The need for such a selection leads to an increase in the cost of the UV curing process. In addition, after warming up the LEDs, their internal resistance and, accordingly, the current through the LEDs change, as a result of which the radiation intensity of the LEDs is different and decreases in individual LEDs. In addition, the use of LEDs of various types emitting light with different wavelengths and with different characteristics, complicates the control system of LEDs and reduces the intensity of their radiation.

The above disadvantages inherent in the known closest analogues lead to a limitation of the scope of the method for curing the substance with UV radiation and a device for its implementation due to the inability to use, for example, in piezo-jet full-color printers with various types of printheads to obtain a full-color image on surfaces of various materials such as flexible and sheet polymers, glass, metal, ceramics, wood, etc., due to the low intensity of UV radiation in that narrow hour and a spectrum in which the photoinitiators included in the composition of matter have maximal sensitization, and the inability to provide high speed of curing a sufficiently thick layer of UV curable substance.

The invention is aimed at solving the problem of expanding the scope of application of a method for curing a substance with UV radiation and a device for its implementation due to the possibility of using, for example, piezoelectric full-color printers with various types of printheads to obtain a full-color image on surfaces of various materials, such as flexible and sheet polymers, glass, metal, ceramics, wood, etc., while ensuring a high curing rate of a sufficiently thick layer of UV curable in a narrow range of UV radiation due to an increase in the intensity of UV radiation in that part of the spectrum in which the photoinitiators that make up the substance have maximum sensitization.

The invention is also directed to creating a device for curing a substance by UV radiation, in which the efficiency of the LED control system and the cooling system of the LEDs is increased, and also aimed at simplifying the device, reducing its weight and size and providing mounting capabilities, for example, on moving parts of the printer, reduction of cost and increase of manufacturability of UV curing of the substance, increase of environmental friendliness, reduction of energy consumption, extension of the service life due to the rejection of used fluorescent lamps and the use of LEDs with the same emission spectrum, as well as by eliminating the need to select LEDs with the same parameters and creating anaerobic areas.

The essence of the invention lies in the fact that in the method of curing the substance with UV radiation, in which the substance, including photoinitiators, is exposed to UV LEDs, while the intensity of UV radiation is controlled depending on the properties of the cured substance and the curing conditions, a radiation spectrum of all Perform UV-LEDs the same, corresponding to the spectral region in which the photoinitiators of the substance have maximum sensitization, it is also proposed to control the intensity of UV radiation, applying a sequence of current pulses to UV LEDs, the frequency of which is in the range from 1 kHz to 10 MHz, and controlling the frequency, current, and duty cycle of the current pulses so that the average power dissipation of the UV LEDs is equal to or close to the maximum, but does not exceed the critical values leading to the destruction of UV LEDs.

In this case, the frequency, magnitude of the current and the duty cycle of the current pulses can be controlled depending on the parameters from the series: the polymerization energy of the UV-curable substance and its composition; the thickness of the layer of UV-curable substances and the method of applying the layer; the duration of exposure to UV radiation on the substance;

ambient temperature and humidity; Characteristics of UV LEDs.

The essence of the invention lies in the fact that in the device for curing the substance by UV radiation, containing a source of UV radiation in the form of rulers (rows) of UV-LEDs connected in series, a radiator for cooling the UV LEDs, a temperature sensor connected to the unit, located on the radiator control of UV LEDs, it is proposed to provide a source of UV radiation with an optical focusing system, and UV LEDs with the same emission spectrum, the control unit of the UV LEDs should be made containing the controller, connected to external computing devices and connected to the control controller by its first and second information inputs, power modules for controlling UV LEDs, each of which is connected to the corresponding line of LEDs with its power terminals, while the temperature sensor is located directly on the radiator and is connected to its output with the corresponding information input of the controller, and each of the power modules is made in the form of a pulsed controlled current stabilizer, providing first supply of current pulses to the corresponding line of UV-LEDs in the frequency range from 1 kHz to 10 MHz, while the frequency, current value and duty cycle of the current pulses are set depending on the properties of the cured substance and the curing conditions.

UV LEDs can be mounted directly on the radiator, mainly by soldering. The radiator can be made in the form of a liquid heat exchanger.

The invention is also illustrated by the following reasoning. Currently, the most common source of UV radiation is a fluorescent mercury lamp, which has the following disadvantages: high heat generation, low efficiency in the UV radiation range, wide non-uniform emission spectrum in the short, medium and long wavelength range, short life. The biggest advantage of a fluorescent lamp is its low cost. However, due to the improvement of the manufacturing technology of LEDs, reducing their cost per unit of power and increasing efficiency, LEDs become the most promising sources of UV radiation.

The UV radiation range is divided into three sub-ranges - short-wave with a wavelength of 200 to 280 nm, medium-wave with a wavelength of 280 to 315 nm and long-wave with a wavelength of 315 to 380 nm. Short-wave UV radiation is not very suitable for curing a substance, such as ink in printers, due to the fact that radiation with a wavelength of less than 280 nm causes the formation of ozone and is harmful to human health. According to the results of medical research, medium-wave UV radiation is harmful to health, as it causes incurable diseases for humans, such as cataracts and skin cancer. Another obstacle to the use of the short-wave and medium-wave ranges is that LEDs in this range have a very high cost with an efficiency of less than 1%.

Closest to natural radiation is the long-wave UV radiation, it is the least harmful to health. However, its use is fraught with a number of difficulties. The first problem is the fact that in UV-curable substances the largest number (several tens) of photoinitiators has maximum sensitization at a wavelength of 300-330 nm. With maximum sensitization at a wavelength of 365 nm, there are only a few photoinitiators, and for a wavelength of 395-400 nm they are completely absent. The second problem is the fact that with decreasing wavelength, the cost increases and the efficiency of LEDs decreases. The cost of a powerful emitter with a wavelength of 300-350 nm is very high, which is not economically feasible, and at a wavelength of 375-405, the cost will be low, but there are no photoinitiators whose maximum sensitization corresponds to this range. The most promising is the range of 350-375 nm, since the cost of LEDs is not too high, the efficiency is not too low, and there are photoinitiators with maximum sensitization in this range.

As an ink for printers, a composition is used that includes:

photopolymer, photoinitiator and solid insoluble pigment resistant to UV radiation (does not fade when exposed to UV radiation, for example, black soot). Under the influence of UV radiation, the photoinitiator breaks the internal bonds. Substances resulting from decay enter into a chemical reaction with a photopolymer as a result of this reaction, a polymer (plastic) is formed. The main problem is that the pigment delays UV radiation, 90% of the radiation is delayed by 1/8 of the top coat of paint, as a result of which the chemical reaction proceeds slowly. A twofold increase in the radiation power can increase the reaction rate by several times, based on studies on the current density in a crystal carried out at the Ioffe Institute in St. Petersburg, the essence of which is that, with appropriate cooling of the crystal, the current density can be 5-7 times higher than the nominal value. Therefore, to achieve a high emitter power, an effective cooling system is required, which will allow the crystal to be effectively cooled with increasing power and to prevent degradation (destruction) of the crystal and a decrease in the radiation intensity on the LED. The most effective and inexpensive cooling system is a water cooling system.

In order for the UV radiation to penetrate as deep as possible into the paint layer, it is better to use a powerful short pulse than long-term irradiation of the surface with low-power radiation. It is optimal to use short and powerful pulses that allow the radiation to penetrate deep into a sufficiently thick layer of paint, which at the same time has a large duty cycle in order to allow the crystal to cool in the LEDs between pulses.

In order to accurately know (calculate) the power of the pulse and prevent overheating of the crystal, a current stabilizer is used. The current stabilizer is designed to stabilize the current flowing through the LEDs, which allows, knowing the voltage drop on the LED, to accurately calculate the pulse power and limit the current through the LED, preventing its destruction.

The pulse repetition rate is calculated from the following conditions. Since the carriage moves over the material at a speed of 1.5 m / s, and the radiation must penetrate at every point on the surface, taking into account the width of an individual LED and emitter, we can calculate the frequency of operation of the emitters. So, for example, in 1 second the carriage of the printer moves at a maximum speed of 1500 mm, at a frequency of 1000 hertz between two pulses, the carriage will move by 1.5 mm, at a frequency of 10000 hertz, the carriage of the printer will move by 0.15 mm. For deeper penetration of the applied paint into the layer and acceleration of the polymerization process (software), increase the current value by a predetermined number of times with a corresponding increase in duty cycle to ensure cooling of the LED crystal, while the effective instantaneous power increases by a corresponding number of times.

The frequency, duty cycle and magnitude of the current pulses through the UV LEDs depend on many factors, such as the energy of the polymerization of the paint (sensitivity depends on the composition and properties of the paint or UV-cured substance); applied photopolymer; applied pigments with different ability to absorb or reflect UV radiation and particle size of pigments; used photoinitiator and the percentage of its content; various additives; external factors affecting the polymerization process; curable layer thickness; droplet size of a curable material, such as ink (depends on the print head used); number of heads (colors applied in one pass of the head); print head resolution (number of nozzles per inch); head operation mode; emitter speed relative to UV curable; frequency of operation of the heads; focused UV beam size; the distance from the UV radiation source to the surface of the UV-curable substance; UV curable temperature ambient temperature and humidity; radiation source power.

A device for curing a substance of UV radiation can be used in various fields of technology where exposure to UV radiation is necessary, for curing polymer adhesives, coatings, inks, for example, in large format printers. Due to the fact that the used ink differs in characteristics (polymerization energy, particle size and its absorption spectrum, used photopolymers and photoinitiators, the presence of additives, the thickness of the cured layer), the UV radiation power and emitter characteristics are selected based on the characteristics of the ink and the heads used . The pulse repetition rate of the control pulses of the UV LEDs is calculated from the following conditions. Since the carriage with the UV radiation source moves over the material onto which UV-curable ink is applied, at a speed of 1.5 m / s, and the UV radiation must penetrate at every point on the surface of the material, given the width of each individual UV LED and source UV radiation, you can calculate the frequency of the UV source. So, for example, in 1 second the carriage of the printer on the XAAR 126 printheads moves at a maximum speed of 1,500 mm, at a frequency of 10,000 hertz between the two pulses the carriage has time to move by 0.15 mm.

Since there are no exact calculation formulas that take into account all of the above factors, the operating mode of the device is determined empirically. Below are examples of SUN inks, XAAR 128 printheads, a drop of 40 picalitres, 6 colors.

Example 1. Printing mode 180 dpi, carriage speed 1.2 m / s, current 1 ampere, frequency 10 kilohertz, duty cycle 5.

Example 2. Printing mode 400 dpi, carriage speed 0.8 m / s, current 2 amperes, frequency 8 kilohertz, duty cycle 10.

Figure 1 shows a block diagram of a device for curing a substance with UV radiation. Figure 2 shows the functional diagram of the control unit UV LEDs. Figure 3 shows a timing diagram of current pulses on UV LEDs. Figure 4 shows the block diagram of the power module. Figure 5 shows a design diagram of a UV radiation source in the form of lines of UV LEDs with a radiator in the form of a liquid heat exchanger for cooling UV LEDs, a temperature sensor and an optical focusing system.

The device for curing the substance by UV radiation, shown in figure 1, contains a source of UV radiation in the form of rulers (rows) 2 sequentially connected UV LEDs with the same emission spectrum corresponding to the region of the spectrum in which the photoinitiators of the cured substance have maximum sensitization. Rulers 2 UV-LEDs are located on the radiator 3, made in the form of a water heat exchanger for efficient cooling of UV-LEDs. The temperature sensor 4 is located directly on the radiator 3 and serves to control the temperature of the UV LEDs. UV radiation source 1 is provided with a system 5 for optical focusing of radiation made on lenses, as shown in FIG. The UV LED control unit 6 is designed to generate control pulses of the UV LEDs of the lines 2 and contains a controller 7 and a power module unit 8. The temperature sensor 4 is connected to the control input of the control unit 6, which is the control input of the controller 7. The UV LED lines 2 are fixed directly on the radiator 3, for example, by soldering. The heat from the UV LEDs is removed to a radiator 3, which is effectively cooled using a water flow cooling system (not shown). All UV LEDs of the rulers 2 are located in the same plane on one face of the radiator 3. The UV LEDs used in the device have a high power consumption of more than 1 watt per crystal and are installed on the radiator 3 with a high density at a minimum distance between the cases. The surface of the UV LEDs is protected from damage by the optical radiation focusing system 5, which is a lens system for increasing the optical power per unit surface, made of materials that transmit UV radiation well, which is directed in one direction.

Powerful UV LEDs of the rulers 2 have a high heat dissipation. To ensure effective cooling, the UV-LEDs forming line 2 are soldered directly (or glued with heat-conducting adhesive) directly to the heat sink 3. The voltage to the anode and cathode of the UV LEDs is supplied by conductors isolated from the heat sink 3. To ensure active cooling, the UV-LEDs are mounted on the cooled surface of the radiator 3, made in the form of a water heat exchanger. The other side of the heat exchanger is cooled by a liquid. The water cooling system has small dimensions and allows you to effectively cool powerful UV-LEDs.

Powerful UV LEDs with a power consumption of more than 1 watt per crystal are controlled using the control unit 6. The controller 7 of unit 6 is connected via its information input to external control devices, for example, using the “start” button or a personal computer (not shown) ) The first and second inputs of block 8 of the power modules 9 are connected to the control outputs of the controller 7 — with the output of the analog signal “current setting” and with the output of the digital signal “control pulses”, respectively. The power terminals of each of the power modules 9 are connected to the corresponding line 2 of UV-LEDs (figure 2).

Each power module 9 is made in the form of a pulsed controlled current stabilizer with pulse width modulation (PWM) and provides current pulses to the corresponding line of 2 UV LEDs in the frequency range from 1 kHz to 10 MHz, with a frequency of 1 / T, the current and the duty cycle of the current pulses is set depending on the properties of the curable substance and the curing conditions. The power module 9 is made in the form of a pulsed controlled current stabilizer with a PWM (Fig. 4) and contains a master oscillator 10, RS trigger 11, a first comparison circuit 12, a key 13, an inductance 14, a current sensor 15, a reference current source 17, a second circuit 16 comparison and the reverse diode 18. Each power module 9 generates a stable predetermined current through the corresponding line 2 of UV-LEDs regardless of external factors (temperature). The set current Im is set according to the “current setting” signal from the controller 7. The power module 9 is switched on (controlled) by pulses with a variable duty cycle according to the “control pulses” signal from the controller 7. By changing the duty cycle of the control pulses and the amount of current flowing through the UV LEDs possible smooth adjustment of the power of the UV LEDs. UV-LEDs are turned on (controlled) from external control devices either programmatically (remotely) or manually from the start button (not shown).

Power module 9 operates as follows. When a control pulse arrives from controller 7, the master oscillator 10 puts the RS-trigger 11 in a charged state (a logic unit signal is set at the output of the RS-trigger 11) and two unit signals are supplied to the comparison circuit 12, as a result of which the comparison circuit 12 is generated a low level signal that opens the key 13. Current flows through the inductance 14, the UV LEDs of line 2, the current sensor 15. From a rapid increase in current, the UV LEDs of line 2 are protected by the inductance 14, in which energy is accumulated. The signal from the current sensor 15 is amplified by a current sensor amplifier (not shown) and compared in the comparison circuit 16 with the signal of the reference current source 17, which is set either by manual adjustment or from the output of the DAC of the controller 7, and compared in the comparison circuit 16. As soon as the current reaches a predetermined value, the comparison circuit 16 resets the RS-trigger 11, as a result of which the key 13 is turned off. As soon as the key 13 is turned off, the energy stored in the inductor 14 supports the current in the UV LEDs of line 2, which flows through the circuit: inductor 14, UV LEDs of the line 2, reverse diode 18. As a result, the current through the UV LEDs line 2 begins to decrease smoothly.

The next pulse from the master oscillator 10 RS-trigger 11 is placed in a charged state. At the output of the comparison circuit 12, a logical zero signal is set, since the key 13 is turned off and no current flows through the sensor 15 and the processes are repeated. When heating the UV LEDs of line 2, their internal resistance drops, as a result of which the current flowing through line 2 also increases, which can lead to an avalanche-like process. A decrease in internal resistance leads to a large amount of current, a larger current heats the crystal even more strongly, and so on until the UV LED fails. However, when using current stabilization this does not happen. The current sensor 15 allows you to hold a given amount of current and prevent overheating of the crystal. The control pulses have a programmable frequency and duty cycle. The current on the UV LEDs has the form shown in Fig.3.

The proposed method for curing the substance by UV radiation is as follows.

UV-curable material, including photoinitiators, is exposed to the radiation of UV-LEDs forming line 2, and the emission spectrum of all UV-LEDs corresponds to the region of the spectrum in which photoinitiators of the substance have maximum sensitization, for example, correspond to a wavelength of 365 nm. The intensity of the UV radiation source 1 is controlled depending on the properties of the curable substance and the curing conditions. To do this, a sequence of current pulses, the frequency of which is in the range from 1 kHz to 10 MHz, are supplied to the UV LEDs of the bars 2, and using the control unit 6, the frequency, duty cycle and current pulses are controlled so that the average power dissipation of the UV LEDs is equal to or approaching the maximum. For example, for UV LEDs manufactured by the Japanese company NICHIA, type NCCU 033 with a wavelength of 365 nm, the maximum power dissipation is 3.3 W, but does not exceed the critical value that leads to the destruction of UV LEDs (for UV LEDs such as NCCU 033 experimentally established that the critical value of power dissipation is 4.1 W). At the same time, the frequency, magnitude of the current and the duty cycle of the current pulses are controlled depending on the parameters from the series:

the polymerization energy of the UV-curable substance and its composition; the thickness of the layer of UV-curable substances and the method of applying the layer; the duration of exposure to UV radiation on the substance; ambient temperature and humidity; Characteristics of UV LEDs.

A device that implements the above method works as follows. Via the communication interface with external control devices, for example, a computer (not shown), data on the operation parameters of block 6 are transmitted to the controller 7 of the control unit 6: the frequency of the control pulses, their duty cycle, maximum operating temperature and the power of the UV LEDs. These parameters are recorded in the non-volatile memory of the controller 7. Turning on (control) of unit 6 is performed from external control devices programmatically (remotely) using the appropriate command from the computer or manually using a button (not shown). Upon the command to turn on, an analog signal appears on the analog output of the control unit 6, which corresponds to the programmed current value I _m flowing through a line of 2 UV LEDs. The digital output of the controller 7 generates a signal "control pulses" in accordance with a given frequency and duty cycle of the control pulses. The controller 7 generates a signal "control pulses" until the command to turn on is released, or until the temperature of the UV LEDs reaches the maximum set temperature. The controller 7 monitors the temperature of the UV LEDs by the signal from the temperature sensor 3 located on the radiator 4, to which the line 2 of the UV LEDs are attached. When the signal from the temperature sensor 3 reaches the set value of the maximum operating temperature, which is stored in the memory of the controller 7, the latter, in accordance with the control program, either stops the supply of control pulses or increases the duty cycle of the control pulses in order to reduce the output power of the UV LEDs, or reduces the signal level at the analog output of the controller 7 in order to reduce the current flowing through each line of 2 LEDs. The signal "current setting" from the analog output of the controller 7 simultaneously arrives at all power modules 9, setting the amount of current in the lines 2 of the UV LEDs. The digital outputs of the controller 7 are connected respectively to each power module 9, which allows each module 9 to be switched on with a delay relative to each other to reduce the peak power of the device’s power source (not shown). The power module 9 upon receipt of the control signals from the controller 7 generates current pulses in the line 2 of UV LEDs of such magnitude and duty cycle that correspond to the control signals of the controller 7. When current pulses flow through the UV LEDs of the bars 2, the latter generate UV radiation and heat. The heat generated by the UV LEDs is removed to a water-cooled radiator 3 and is dissipated there. UV radiation passing through the optical focusing system 5 is collected in a beam, i.e. focuses. Focused UV radiation is directed onto the material on which the UV curable material is applied.

Thus, the known method of curing a substance by UV radiation and a device for its implementation, firstly, make it possible to abandon the use of fluorescent lamps and therefore have a higher efficiency, a stable operating temperature, a long time resource, and increased environmental friendliness due to the elimination of ozone, less power consumption. Due to which, the scope of the proposed method for curing a substance by UV radiation and a device for its implementation are expanding. In addition, the use of only UV LEDs as a radiation source simplifies the device and reduces the cost of curing the substance with UV radiation.

The need to create an anaerobic region from an inert gas to increase the productivity of UV photoinitiators also contributes to the reduction in the cost of curing the substance by UV radiation.

Secondly, the use of UV LEDs with the same range of UV radiation ensures its full compliance with the wavelength at which the photoinitiator has maximum sensitization, which increases the curing efficiency of the substance.

Thirdly, control of the radiation intensity of the LEDs according to the readings of the current sensor of the UV LEDs, and not by the radiation intensity, as well as using feedback and temperature stabilization, can reduce crystal degradation and increase the radiation intensity of the UV LEDs. The effectiveness of the radiation intensity control system is also increased due to the fact that the UV-LEDs are located directly on the radiator, and not on the substrate mounted on the radiator. Reducing the degradation of the crystal also contributes to the fact that the temperature sensor is located directly on the radiator, and not on the substrate.

Fourthly, since all UV LEDs are connected in series and all UV LEDs have the same wavelength, there is no need to select current in the LED lines, which reduces the cost of the UV curing process. In addition, this increases the stability of the current through the UV LEDs, and hence the stability of the radiation intensity of the LEDs. In addition, the use of LEDs of the same type, emitting light with the same wavelength and having the same characteristics, leads to a simplification of the control system of LEDs and increase the intensity of their radiation.

Therefore, the present invention provides a device for curing the substance by UV radiation, in which the efficiency of the LED control system and the cooling system of the LEDs is improved by reducing the degree of degradation of the LED crystals, and it also simplifies the device, reduces its weight and size and allows mounting, for example, on moving parts of the printer, reducing costs and improving the processability of UV curing, increasing environmental friendliness spine, reduced power consumption, prolongation of life by avoiding use of fluorescent lamps and use of LEDs with the same radiation spectrum, as well as by eliminating the necessity of selection of LEDs with the same parameters and creation of anaerobic areas.

As a result, the present invention allows to expand the scope of the method of curing the substance by UV radiation and a device for its implementation due to the possibility of using, for example, in piezo-jet full-color printers with various types of printheads to obtain a full-color image on surfaces of various materials, such as flexible and sheet polymers, glass, metal, ceramics, wood, etc., while ensuring a high curing rate of a sufficiently thick layer of UV-curable material Islands in a narrow range of UV radiation.

Claims (5)

1. A method of curing a substance with UV radiation, in which a substance including photoinitiators is exposed to UV LEDs, the intensity of UV radiation being controlled depending on the properties of the cured substance and the curing conditions, characterized in that the substance including photoinitiators, they are exposed to UV radiation with the same emission spectrum of all UV LEDs, corresponding to the region of the spectrum in which the photoinitiators of the substance have maximum sensitization, while the UV LEDs are fed a sequence of current pulses, the frequency of which is in the range from 1 kHz to 10 MHz, and control the intensity of UV radiation by controlling the frequency, current magnitude and duty cycle of the current pulses, so that the average power dissipation of the UV LEDs is equal to or close to the maximum, but did not exceed the critical value leading to the destruction of UV LEDs. 2. The method of curing the substance by UV radiation according to claim 1, characterized in that the frequency, magnitude of the current and the duty cycle of the current pulses are controlled depending on the parameters of the series: the polymerization energy of the UV-curable substance and its composition; the thickness of the layer of UV-curable substances and the method of applying the layer; the duration of exposure to UV radiation on the substance;
ambient temperature and humidity;
UV LED characteristics. 3. A device for curing the substance by UV radiation, containing a source of UV radiation in the form of lines (rows) of UV-LEDs connected in series, a radiator for cooling UV-LEDs, a temperature sensor associated with the UV-LED control unit, characterized in that the source UV radiation is equipped with an optical focusing system, and UV LEDs are made with the same emission spectrum, the UV LED control unit contains a control controller connected to external computing devices and connected to the control With their first and second information inputs, the power control modules for UV LEDs, each of which is connected to the corresponding line of LEDs with its power terminals, the temperature sensor is located directly on the radiator and connected to the corresponding information input of the controller by its output, and each of the power modules made in the form of a pulsed controlled current stabilizer, providing current pulses to the corresponding line of UV LEDs in the hour range frequencies from 1 kHz to 10 MHz, while the frequency, current magnitude and duty cycle of current pulses is set depending on the properties of the cured substance and the conditions of curing. 4. The device for curing the substance with UV radiation according to claim 3, characterized in that the UV LEDs are mounted directly on the radiator mainly by soldering. 5. The device for curing the substance with UV radiation according to claim 3, characterized in that the radiator is made in the form of a liquid heat exchanger.

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