Patent Publication Number: US-10786998-B2

Title: Inkjet recording device having controller for bubble discharging operation

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This Application is a 371 of PCT/JP2017/017969 filed on May 12, 2017 which, in turn, claimed the priority of Japanese Patent Application No. 2016-111957 filed on Jun. 3, 2016, both applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an inkjet recording device. 
     BACKGROUND ART 
     There has been an inkjet recording device which records an image on a recording medium by ejecting ink from a nozzle and by landing the ink on the recording medium. In the inkjet recording device, ejection failure of ink from the nozzle results in deterioration of the recorded image. Therefore, there are various techniques for inspecting whether or not the ink is ejected in a proper state and performing a maintenance operation according to the inspection result. 
     An example of the main cause of the ejection failure of ink from the nozzle is contamination and air bubbles in the ink. In order to prevent contamination, a filter is conventionally provided in the ink flow path from an ink tank to each nozzle in the inkjet recording device. Further, for the purpose of discharging bubbles and the like, a technique is known in which a common flow path for feeding ink separately into individual flow paths communicating with a plurality of nozzles is provided so that the ink in the common flow path is circulated and returned to an ink tank. In such a technique, in a state where ink hardly leaks by causing a negative ink pressure in the nozzle, a driving operation is performed to cause minute vibrations at the liquid surface of the ink such that the ink is not ejected from the nozzle. By circulating the ink after the driving operation, bubbles in the individual flow paths and the nozzles are easily detached from the wall surfaces, so that air bubbles and foreign matter are removed (for example, Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     [Patent Document 1] Japanese Patent Application Laid Open Publication No. 2015-071231 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in discharging the bubbles in the ink flow path, there is a problem that it is difficult and takes time to return the bubbles once entered into a thin individual flow path or a nozzle to the upstream side for discharging. 
     The object of the present invention is to provide an inkjet recording device which can more easily and reliably discharge bubbles in an ink flow path. 
     Solution to Problem 
     In order to achieve at least one of the above-described objects, according to one aspect of the invention, an inkjet recording device includes: 
     an inkjet head which is provided with one or more nozzles which perform ejection of ink; 
     an ink supplier which supplies ink to the inkjet head; 
     a driver which performs a driving operation to cause a pressure variation of ink in the nozzles regarding the ejection; and 
     a controller which controls an operation of the ink supplier and an operation of the driver, 
     wherein the inkjet head includes: 
     a common flow path in which supplied ink to the inkjet head flows; 
     a filter which is provided in the common flow path and through which supplied ink passes; 
     one or more individual flow paths which respectively feed ink having has passed through the filter from the common flow path to each of the nozzles; and 
     a first discharge port from which ink having passed through the filter and being in the common flow path is discharged; 
     wherein, in a bubble discharging operation of ink in the inkjet head, the controller causes the ink supplier to supply ink with a pressure which allows ink to leak out from the nozzles while the controller causes the driver to perform a predetermined driving operation, so that ink is discharged from the first discharge port. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention. 
         FIG. 1  is a schematic diagram showing configuration of an inkjet recording device. 
         FIG. 2  is a diagram which describes configuration of an ink flow path of an inkjet recording device. 
         FIG. 3  is a cross-sectional view of an ink flow path in an inkjet head from the front side. 
         FIG. 4  is a block diagram showing functional components of the inkjet recording device. 
         FIG. 5  is a flowchart of control procedure of bubble discharging process. 
         FIG. 6A  is a diagram showing a modified example of the inkjet recording device. 
         FIG. 6B  is a diagram showing a modified example of the inkjet recording device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     An embodiment of the present invention is described with reference to the diagrams. 
       FIG. 1  is a schematic diagram showing configuration of an inkjet recording device  100  of an embodiment of the present invention. 
       FIG. 1  indicates the inkjet recording device  100  when it is seen from the front. 
     The inkjet recording device  100  is a printer including a line head and employing a one-pass method in which a color image is formed by ejecting ink of four colors at appropriate timings while moving a recording medium relative to the line head. 
     The inkjet recording device  100  includes a medium supplier  10 , a medium receiving unit  15 , an image forming body  20 , an ink supplier  30  (see  FIG. 2 ), a controller  40  (see  FIG. 4 ), and the like. In this inkjet recording device  100 , on the basis of the control by the controller  40 , the recording medium P stored in the medium supplier  10  is conveyed to the image forming body  20  and discharged to the medium receiving unit  15  after image formation. 
     The medium supplier  10  conveys the recording medium P stored inside to the image forming body  20  one by one. 
     As the recording medium P, various objects are used, such as printing paper of various thickness, cell, film, and cloth, which can be curved around and held on the outer periphery surface of the image forming drum  21 . 
     The medium supplier  10  includes a paper feeding tray  11  which stores the recording medium P, a feeder board  12  which conveys the recording medium P from the paper feeding tray  11  to the image forming body section  20 . The paper feeding tray  11  is a plate-like member which is made mountable for one or more recording medium P. The paper feeding tray  11  is provided to move vertically according to the amount of the recording medium P mounted on the paper feeding tray  11 , and for the vertical move, the topmost of the recording medium P is kept in a position where it is able to be conveyed by the feeder board  12 . 
     The feeder board  12  includes a conveyance mechanism which conveys the recording medium P on the belt  123  by driving the ring-shaped belt  123  which is held by a plurality of (for example, two) rollers  121  and  122  from inside, and a supplier which delivers the topmost recording medium P mounted on the paper feeding tray  11  to the belt  123 . The feeder board  12  conveys along the belt  123  the recording medium P which has been delivered from the supplier to the belt  123 . 
     The image forming body  20  includes a first delivery unit  27 , an image forming drum  281 , a head unit  282 , an irradiator  283 , and a second delivery unit  29 , and the like. 
     The reception unit  27  receives a recording medium P from the medium supplier  10  and then conveys it to the image forming drum  281 . The reception unit  27  includes a swing arm  271  which holds one end of the recording medium P conveyed on the feeding board  12  and a cylindrical reception drum  272  which conveys the recording medium P carried on the swing arm  271  to the image forming drum  281 . The swing arm  271  receives the recording medium P on the feeding board  12  and then conveys it to the reception drum  272 . This allows the reception unit  27  to guide the recording medium P along the outer periphery of the image forming drum  281  and then convey it to the image forming drum  281 . 
     The image forming drum  281  has a cylindrical outer shape, carries three recording medium P at the maximum on the cylindrical outer peripheral surface, and carries out a conveyance operation of conveying the recording medium P according to the rotation around the central axis of the cylinder. Beside the outer peripheral surface of the image forming drum  281 , a drum heater  2811  for heating the outer peripheral surface and the recording medium P is provided. Here, the drum heater  2811  is provided from the delivery position, where the first delivery unit  27  delivers the recording medium P to the image forming drum  281 , to the image recording position, where the head unit  282  records an image on the recording medium P, on the rotation direction of the image forming drum  281 . The duration and intensity of a heating operation by the drum heater  2811  are controlled so that the recording medium P to be held is at an appropriate temperature, on the basis of the temperature of the outer circumferential surface of the image forming drum  281  measured by a temperature measuring unit (not shown). As a result, an appropriate curing rate of the ink landed on the recording medium P is maintained, so that high quality images are stably formed. In the drum heater  2811 , for example, an infrared heater or a heating wire that generates heat by energization is used. The drum heater  2811  may be provided inside of the image forming drum  281  and heat the outer peripheral surface by thermal conduction. 
     The head unit  282  ejects ink droplets toward one recording target surface of the recording medium P which moves according to the rotation of the image forming drum  281 . The ink droplets are ejected at appropriate timings from a plurality of nozzle openings provided on a surface (nozzle surface) of the head unit  282  facing the recording target surface of the recording medium P, and landed on the recording target surface of the recording medium P. An image is thereby formed. The head unit  282  includes one or more inkjet heads  24  provided with a plurality of nozzles (see  FIG. 2 ). In the inkjet recording device  100  according to the present embodiment, four head units  282  are arranged at predetermined intervals in the conveying direction of the recording medium P, each corresponding to inks of multiple colors, four in this case. The four head units  282  each eject inks of C (cyan), M (magenta), Y (yellow), and K (black). Here, inks cured by being irradiated with ultraviolet rays are used. Further, the inks are heated by an ink heater  2822  (see  FIG. 4 ) inside and/or outside the head unit  282  so as to be maintained at an appropriate temperature. 
     Each of the head units  282  here has a plurality of nozzle openings arranged in the width direction, which is perpendicular to the conveying direction of the recording medium P conveyed on the image forming drum  281 , over the width of image formation on the recording medium P. That is, each of the head units has a line head capable of forming an image by one-pass method by ejecting ink from the nozzle openings onto the recording medium P moving in the conveying direction. The head units  282  are each attached to a support portion (carriage) (not shown). By operating a motor and a brake for position adjustment via a unit position adjustment driver  58 , the relative positions of the head units  282  with respect to the image forming drum  281 , in particular, the distances from the outer peripheral surface of the image forming drum  281  to the head units  282 , can be changed. 
     The irradiator  283  emits an energy ray (electromagnetic wave) of a predetermined wavelength, an ultraviolet ray in a near ultraviolet region of a wavelength of about 400 nm in this case, so that the ink (that is, an image formed by the ink) ejected from the head unit  282  and landed on the recording medium P is cured and fixed. The irradiator  283  has, for example, a light emitting diode (LED) for emitting ultraviolet rays. The LED emits ultraviolet rays when a current flows by voltage application to the LED in the driving operation of an irradiator driver  51  (see  FIG. 4 ). The irradiator  283  is provided to irradiate the recording medium P conveyed by the rotation of the image forming drum  281  with ultraviolet rays, after the ink is ejected from the head unit  282  onto the recording medium P and before the recording medium P is delivered to the second delivery unit  29 . In order to reduce the leakage of ultraviolet rays outside the setting range for ultraviolet irradiation in the recording medium P, the irradiator  283  has a light shielding plate  283   a  which covers the LED and the setting range. 
     The component for emitting ultraviolet rays in the irradiator  283  is not limited to an LED. The irradiator  283  may include, for example, a mercury lamp. If the ink has a property of being cured upon reception of energy rays other than ultraviolet rays, instead of the above-described component for emitting ultraviolet rays, a well-known light source which emits energy rays of a wavelength to cure the ink is provided. 
     The second delivery unit  29  conveys the recording medium P from the image forming drum  21  to the medium receiving unit  15  after formation of the image and curing of the landed ink. The second delivery unit  29  includes a cylindrical delivery roller  291 , a plurality of (for example, two) rollers  292  and  293 , and a circular belt  294  inside of which is supported by rollers  292  and  293 . The delivery roller  291  guides the recording medium P from the image forming drum  281  onto the belt  294 . After conveying the recording medium P from the delivery roller  291  onto the belt  294 , which circles around the rollers  292  and  293  in accordance with the rotation thereof, the second delivery unit  29  moves the recording medium P on the belt  294  to the medium receiving unit  15 . 
     The medium receiving unit  15  stores the recording medium P conveyed from the image forming body  20  via the second delivery unit  29  until a user picks it up. The medium receiving unit  15  has a plate-shaped copy receiving tray  16  on which the recording medium P is mounted after image formation. 
     The controller  40  controls operations of the medium supplier  10 , the image forming body  20 , the ink supplier  30 , and the medium receiving unit  15 , and forms an image on the recording medium P according to the data of the image to be formed by an image formation command (job) and settings regarding image formation. 
     Among the above configurations, a conveyance section is constituted by the medium supplier  10 , the image forming drum  281  in the image forming body  20 , the first delivery unit  27 , the second delivery unit  29 , and the medium receiving unit  15 . 
     The ink supplier  30  stores ink of each color used for image recording and supplies the ink to the inkjet head  24 . Each component of the ink supplier  30  is arranged in a dedicated rack or the like, and is connected to the image forming body  20  via a pipe such as a tube. 
     Next, the configuration relating to the flow of ink from the ink supplier  30  to the image forming body  20  in the inkjet recording device  100  of this embodiment will be described. 
       FIG. 2  is a diagram which describes a configuration of an ink flow path of the inkjet recording device  100  of the present embodiment. 
     The ink supplier  30  includes a main tank  31 , a filter  311 , a supplying pump  32 , a supplying valve  33 , and the like. 
     The ink in the main tank  31  is fed to the first sub tank  21  of the image forming body  20  via the supplying valve  33  by the operation of the supplying pump  32 . The filter  311  prevents foreign substances and contaminants such as waste and dust from being mixed in the main tank  31  which is open to the atmosphere. The supplying valve  33  determines whether or not ink is supplied from the main tank  31  to the first sub tank  21 . The supplying valve  33  is an electromagnetic valve which is opened and closed under the control operation of the controller  40 , however, it may be possible to switch the opening and closing manually at the time of refilling the ink to the main tank  31 , replacing the main tank  31 , and the like. Further, the supplying valve  33  may be provided not in the ink supplier  30  but in the image forming body  20 . 
     The image forming body  20  includes a first sub tank  21  (ink storage), a liquid feeding pump  22  (ink supplier), a second sub tank  23  (pressure adjuster), an inkjet head  24 , a reflux unit  25 , an ink discharge unit  26 , and the like. They are provided for each of the multiple inks and for each of the multiple inkjet heads  24  forming a line head described above, and receive ink supplied from a common main tank  31  corresponding to the kind of the ink. 
     The ink supplied from the main tank  31  to the first sub tank  21  of the image forming body  20  by the supplying pump  32  is fed to the inkjet head  24 . The ink which is not ejected or leaked from the inkjet head  24  is returned to the first sub tank  21  through the reflux unit  25 . 
     Here, the first sub tank  21  is an ink tank having a smaller capacity than the main tank  31 . The first sub tank  21  is provided with a first liquid level sensor  21  which detects the amount of ink in the first sub tank  21  and outputs a detected signal(s) to the controller  40  (see  FIG. 4 ). The first liquid level sensor  211  may simply detect whether or not the ink amount is below the predetermined lowest reference value and output a detected signal(s) to the controller  40 . The first sub tank  21  further stores the ink returned from the inkjet head  24 . The operation of the supplying pump  32  is switched depending on the detected amount of the ink in the first sub tank  21 , and the amount of ink in the first sub tank  21  is maintained appropriately. 
     The liquid feeding pump  22  feeds ink from the first sub tank  21  to the second sub tank  23 . Conventionally well-known pumps can be used as the liquid feeding pump  22 . When the second sub tank  23  is not communicated with the atmosphere or the air tank  234 , the ink pressurized by the liquid feeding operation of the liquid feeding pump  22  is supplied to the inkjet head  24  via the second sub tank  23 . 
     A second liquid level sensor  231  is provided in the second sub tank  23 . The second liquid level sensor  231  performs the same operation as the first liquid level sensor  211  in the first sub tank  21 , regarding the amount of the ink in the second sub tank  23 . 
     The second sub tank  23  communicates with the atmosphere when the air releasing valve  232  is opened, and communicates with the air tank  234  when the air releasing valve  233  is opened. When the air releasing valve  233  is opened, the ink does not normally leak out from the nozzle because a pressure difference from the ink pressure on the nozzle face of the inkjet head  24  is caused due to the air pressure (negative pressure) in the air tank  234 . When the pressure difference changes due to ejection of ink and the like, the ink pressure is adjusted so that ink corresponding to the pressure difference is supplied to the inkjet head  24 . Alternatively, when the air releasing valves  232  and  233  are closed, ink which is pressurized according to the liquid feeding operation by the liquid feeding pump  22  is fed to the inkjet head  24  via the second sub tank  23 , as described above. 
     The pressure in the air tank  234  may be adjustable as appropriate. 
     The inkjet head  24  allows ink to flow in from the inlet  241 , distributes the ink to the individual flow paths  2471  (see  FIG. 3 ) each communicating with the nozzles for ejecting ink, and allows ink that has not been ejected from the outlet  242  or  243  to flow out. The inlet  241  is connected to the second sub tank  23 , and the outlets  242  and  243  are connected to the first sub tank  21  via the reflux unit  25 . The reflux unit  25  includes an ink flow path(s) (circulation flow path(s)) which individually connects the outlets  242  and  243  with the first sub tank  21 . A first reflux valve  251  (first discharge valve) and a second reflux valve  252  (second discharge valve) are provided in the respective circulation flow paths. Thereby the ink circulation availability (discharge ability) can be switched. 
     All of the air releasing valve  233 , the first reflux valve  251 , and the second reflux valve  252  are electromagnetic valves and are electromagnetically opened and closed on the basis of the control by the controller  40 . 
     The ink receiving unit  26  receives ink in a maintenance operation and the like, where ink is ejected from the nozzle opening of the inkjet head  24  to a portion other than the recording medium or in an operation where ink leaks. An ink tray  261  is a tray for receiving ink from these nozzles. The waste liquid tank  262  stores the ink received by the ink tray  261 . Although the ink stored in the waste liquid tank  262  is discarded here, it may be stored for each of the inks to be reused. Here, the ink tray  261  is configured to be movable to a position facing the nozzle openings in a state where the distance between the head unit  282  and the conveying surface is enlarged. Alternatively, the ink tray  261  may be provided at a predetermined maintenance position, and the head unit  282  may be moved as needed to arrange the nozzle surface of the inkjet head  24  at a position facing the ink tray  261 . 
       FIG. 3  is a cross-sectional view of the ink flow path in an inkjet head  24  from the front side. 
     The plurality of inkjet heads  24  attached to the head unit  282  are oriented as in this front view when viewed from the conveying direction. 
     The ink flow path in the inkjet head  24  includes a common ink chamber  245  (common flow path) to which the inlet  241  and the outlets  242  and  243  are connected, and an ink ejector  247  (a head chip) for ejecting ink from each nozzle. 
     The ink flowing in from the inlet  241  is fed to the common ink chamber  245 . In the common ink chamber  245 , there is provided a filter  246  with one side (the upstream ink chamber  2451 ) of which the inlet  241  communicates. The outlet  243  (second discharge port) is provided on the same side as the inlet  241  (upstream ink chamber  2451 ) with respect to the filter  246 . Further, the outlet  242  (first discharge port) is provided on the side opposite to the inlet  241  (downstream ink chamber  2452 ). The filter  246  is interposed between the inlet  241  and the outlet  242 . 
     The filter  246  prevents passage of contaminants through the ink. Further, the filter  246  suppresses passage of bubbles. In the inkjet recording device  100 , the inkjet head  24  is provided such that the filter  246  is substantially horizontal. As a result, when air bubbles flow in from the inlet  241 , they normally gather at the ceiling side of the upstream ink chamber  2451 , and do not easily come into contact with the filter  246  or pass through the filter  246 . Through holes  2452   a  each communicating with a nozzle of the ink ejector  247  are provided on the bottom surface of the downstream ink chamber  2452 . 
     The ink ejector  247  includes a plurality of individual flow paths  2471  and nozzles  2472  respectively corresponding to the plurality of individual flow paths  2471 , and ejects ink from openings of these nozzles  2472 . The positions of the attached individual flow paths  2471  are the same as the positions of the through holes  2452   a  of the downstream ink chamber  2452 . The ink in the common ink chamber  245  is distributed to the respective nozzles  2472 . 
     The openings of the plurality of nozzles  2472  are provided on the nozzle surface of each inkjet head  24  at a predetermined interval (pitch) in the width direction. The arrangement pattern of the nozzle openings is not particularly limited, and may be a simple one-dimensional arrangement, a staggered lattice arrangement having a plurality of rows in the conveying direction, or the like. It is preferable that the nozzle openings provided in each of the inkjet heads  24  whose positions in the width direction are adjacent to each other partially overlap with each other in the width direction, so that ink is reliably ejected over the entire width of the recording medium. 
     An actuator (not shown) such as a piezoelectric element is provided in contact with a wall surface of the individual flow path  2471 . The ink droplets are ejected from the openings of the nozzles  2472  at an appropriate liquid amount, droplet shape, and speed, by changing the pressure of ink in the individual flow path  2471  by an operation (driving operation) of the actuator according to a drive signal output from a head driver  54  (see  FIG. 4 ). 
     The drive signal to be used is not particularly limited, and may have a voltage waveform (waveform for ejection) including successive trapezoidal waveforms each representing output of the lower voltage side (negative voltage side) and output of higher voltage side, with respect to the reference voltage (such as ground voltage). Here, when the higher voltage is applied, the actuator compresses the individual flow path  2471  (pressure chamber) and raises the ink pressure. When the lower voltage is applied, the actuator expands the individual flow path  2471  and decreases the ink pressure. That is, the actuator once lowers the ink pressure in order to draw the ink to the back side of the nozzle, and then raises the ink pressure to eject the ink from the nozzle (an ejection driving operation). In addition, the head driver  54  outputs a driving voltage pattern having a minute vibration waveform in which potential differences (that is, amplitudes) from the reference voltage to the high voltage and to the low voltage are smaller than those at the time of ink ejection, thereby reducing the pressure variation of the ink. This enables a minute driving operation for vibrating ink in the nozzle without actually ejecting the ink from the nozzle. This prevents thickening of ink due to evaporation or the like near the nozzle opening by stirring the ink in the nozzle when ink is not ejected for a predetermined time or more. 
       FIG. 4  is a block diagram showing a functional configuration of the inkjet recording apparatus  100  according to the present embodiment. 
     As described above, the inkjet recording device  100  includes the supplying pump  32 , the supplying valve  33 , the liquid feeding pump  22 , the first liquid level sensor  211 , the second liquid level sensor  231 , the air releasing valves  232  and  233 , the first reflux valve  251 , the second reflux valve  252 , the drum heater  2811 , and the like. Also, the inkjet recording device  100  includes the controller  40 , the unit position adjustment driver  58 , the ink heater  2822 , a conveyance driver  52 , a cleaner driver  53 , the head driver  54  (driving unit), a tray driver  55 , the irradiator driver  51 , an operation display unit  56 , a communication unit  57 , a bus  59 , and the like. 
     The controller  40  comprehensively controls the inkjet printing apparatus  100  and includes a central processing unit (CPU)  41 , a random access memory (RAM) device  42 , ROM (Read Only Memory)  43 , and a memory  44 . 
     The CPU  41  performs various calculations to control conveyance of a recording medium, ejection of ink, the maintenance operation, and the like in the inkjet recording device  100 . This maintenance operation includes bubble discharge process for discharging air bubbles in the ink flow path. Further, the CPU  41  performs various processes regarding image recording based on image data, a status signal and a clock signal of each part, and the like according to the programs which are read out from the ROM  43 . 
     The RAM  42  provides a working memory space for the CPU  41  and stores temporary data. 
     The ROM  43  stores control programs and initial setting information. The control programs include programs regarding the bubble discharge process described above. The ROM  43  includes an overwritable updatable nonvolatile memory to store data which is set and maintained at any time, such as setting data. The memory  44  includes a RAM for temporarily storing the image data to be recorded. 
     The conveyance driver  52  generates and outputs a drive signal for respectively rotating, in an appropriate direction and rate, the motors such as a rotary motor of the image forming drum  281 , a motor for rotating each of the feeder board  12  and the second delivery unit  29 . The conveyance driver  52  outputs the drive signal according to the rotation direction and the rotation rate of each of these motors on the basis of the control signal from the controller  40 . 
     The cleaner driver  53  causes a wiper and the like (not shown) to perform wiping and removing operations of ink and ink mist adhering to the nozzle surface and the like. A winding type nonwoven fabric, a sponge material, a blade member, or the like can be used as the wiper depending on the shape and material of the nozzle surface. The cleaner driver  53  may have a configuration for applying a cleaning liquid to these wipers. The cleaner driver  53  may further have a configuration of wiping the nozzle surface and the like using a cleaning liquid and further wiping the cleaning liquid with a dried nonwoven fabric or the like. 
     The head driver  54  generates and outputs a drive voltage signal for deforming the pressure chamber (piezoelectric element) (for performing the driving operation) so that the ink ejector  247  ejects ink properly. Under the control signal from the controller  40 , the head driver  54  selects a voltage waveform pattern stored in advance and generates the drive voltage signal which is amplified in power, and switches according to the image data input from the memory  44  whether or not the drive voltage signal for each piezoelectric element can be output. 
     The wires related to the head driver  54  may be collectively formed together with the ink flow path inside the inkjet head  24  or may be formed separately in part. 
     In accordance with the control signal from the controller  40 , the irradiator driver  51  applies a predetermined voltage to the LED of the irradiator  283  for supplying an electric current, and causes the LED to emit ultraviolet rays. 
     The unit position adjustment driver  58  outputs the drive signal to the motor and/or the brake for position adjustment in accordance with a control signal from the controller  40 , so that the head unit  282  is moved to a desired position and fixed. 
     The tray driver  55  moves the ink tray  261  according to a control signal from the controller  40 . Various well-known techniques such as a gear train and an actuator driven by a motor can be used for the movement operation of the ink tray  261 . In this case, the positional relationship between the ink tray  261  and the head unit  282  (inkjet head  24 ) need not to be strictly determined as in image recording, as long as ink ejected from the inkjet head  24  does not come off the ink tray  261 . Further, there may be provided a collision prevention mechanism, or the controller  40  may perform collision prevention control, so that the ink tray  261  does not move to a position facing the inkjet head  24  as long as the head unit  282  is not separated from the conveying surface by a predetermined height or more, and/or so that the head unit  282  does not approach within a predetermined height from the conveying surface as long as the ink tray  261  is not retracted. 
     The ink heater  2822  heats the ejected ink and keeps it at an appropriate temperature in the head unit  282 , thereby maintaining the viscosity and the like of the ink in an appropriate state. If a type of ink which becomes a gel at room temperature or low temperature is used, the ink heater  2822  has a configuration to heat the ink in the entire ink flow path described above. The temperature of the ink is estimated from the temperature measured by a thermometer (not shown) near the nozzle of the inkjet head  24 . The operation state of the ink heater  2822  is controlled on the basis of the temperature of the ink. 
     The communication unit  57  is a communication interface for controlling a communication operation with an external device. The communication interface includes one or more communication interfaces (e.g. a LAN board and a LAN card) corresponding to various communication protocols. The communication unit  57  may acquire image data to be recorded and setting data (job data) image recording from an external device under the control of the controller  40 , and may transmit the status information and the like to the external device. 
     The operation display unit  56  displays the status of the inkjet recording device  100 , an operation menu, and the like according to the control signal from the controller  40 , accepts user&#39;s operation, and outputs the received operation to the controller  40 . The operation display unit  56  includes a liquid crystal display unit on which a touch sensor is provided as operation accepting means, for example. The controller  40  causes the liquid crystal display unit to display statuses and various menus for accepting commands via the touch sensor. The controller  40  performs a control operation to cause each part of the inkjet recording device  100  to perform a process corresponding to the user&#39;s touch operation detected by the touch sensor and information on content and/or position of the displayed menu. The controller  40  performs a control operation for causing each part of the inkjet recording device  100  to perform processes depending on the information on the content and position of the displayed menu and the user&#39;s touch operation detected by the touch sensor. 
     The bus  59  is a path for connecting the above components electrically and exchanging signals between them. 
     In addition to these configurations, the inkjet recording device  100  may include a notification unit such as an LED lamp and/or a beep sound generator used for a notification operation, a reading unit such as a line sensor for detecting an image quality abnormality (defect) of an image formed on a recording medium and a placement abnormality detection sensor for detecting that the supplied recording medium is not normally placed on the conveying surface. 
     Next, a bubble discharging operation in the inkjet recording device  100  of the present embodiment will be described. 
     In the inkjet recording device  100  according to the present embodiment, a bubble discharging process (bubble discharging operation) is performed at the time of filling (refilling) the ink flow path with ink or during maintenance when a predetermined recording failure is detected in image recording. This bubble discharging process can be automatically executed in conjunction with the above-described conditions. Alternatively, the bubble discharging process may be started on the basis of a predetermined input operation via the operation display  56  by the user. 
     In the inkjet recording device  100 , air bubbles in the common ink chamber  245  are discharged from the common ink chamber  245  to the first sub tank  21  in the bubble discharging process, by operating the liquid feeding pump  22  to return the ink in the common ink chamber  245  to the first sub tank  21  while the first reflux valve  251  (and the second reflux valve  252 , if necessary) is opened. Further, at the same time, ink is allowed to leak out from the nozzle opening portion in parallel to discharge bubbles entering the nozzle from the nozzle opening portion by a small pressure variation of the ink in individual flow paths  2471  depending on the drive voltage pattern of minute vibration waveform and applying the ink pressure by the liquid feeding operation by the liquid feeding pump  22 . The ink pressure at this time needs to be a pressure at which ink reliably leaks from all the nozzles. The ink pressure may be set to a level at which ink leaks continuously from all the nozzles regardless of the phase of the drive voltage pattern of the minute vibration waveform or the like. 
       FIG. 5  is a flowchart showing a control procedure by the controller  40  in the bubble discharging process executed in the inkjet recording device  100 . 
     When the bubble discharging process is started, the controller  40  (CPU  41 ) outputs a control signal to the unit position adjustment driver  58  as needed to increase the distance from the conveying surface of the head unit  282  and outputs a control signal to the tray driver  55  so that the ink tray  261  faces the nozzle surface. In addition, the controller  40  causes the air releasing valve  233  to be closed and the second sub tank  23  to be disconnected from the air tank  234  (step S 101 ). 
     The controller  40  causes the first reflux valve  251  to be opened so that the downstream ink chamber  2452  communicates with the first sub tank  21  (step S 102 ). At this time, the controller  40  may cause the second reflux valve  252  to be opened. The controller  40  causes the liquid feeding pump  22  to perform pressurizing and feeding of ink so that pressurization supply of ink to the inkjet head  24  starts via the second sub tank  23 . Further, the controller  40  outputs a control signal to the head driver  54  so as to cause the actuator corresponding to each individual flow path  2471  to output a driving voltage signal related to the minute vibration waveform (step S 103 ) and to perform a driving operation (a minute driving operation). There is no problem even if the pressurization supply of ink and the output of the driving voltage signal start at slightly different timings, however, it is preferred that they are performed basically at the same time. 
     After a predetermined time has elapsed, the controller  40  causes the liquid feeding pump  22  to stop the operation and causes output of the driving voltage of the minute vibration waveform to be paused (step S 104 ). The controller  40  causes the first reflux valve  251  to be closed (step S 105 ). When the second reflux valve  252  is opened, the controller  40  also causes the second reflux valve  252  to be closed. 
     The controller  40  performs an operation to recover the ink level (meniscus) in the nozzle  2472  as necessary (step S 106 ). The controller  40  outputs a control signal to the head driver  54  to output a waveform for ejection, and causes each nozzle to eject ink. 
     The controller  40  causes the ink tray  261  to be retracted from the nozzle surface and the air releasing valve  233  of the second sub tank  23  to opened (step S 107 ). In addition, at this time, the controller  40  can output a control signal to the cleaner driver  53  for performing a cleaning operation of nozzle surface and the like. Then, the controller  40  finishes the bubble discharge process. 
     Modified Example 
       FIGS. 6A and 6B  are diagrams showing modified examples of the inkjet recording device  100  of the present embodiment. 
     As shown in  FIG. 6A , in the reflux unit  25   a , the circulation flow paths communicating with the outlets  242  and  243  may be joined and connected to the first sub tank  21  via the first reflux valve  251 . In this case, so that ink does not flow from the outlet  243  to the outlet  242 , the flow path resistance at the connected portion is sufficiently larger than the flow path resistance of the filter  246 . 
     Further, as shown in  FIG. 6B , the outlet  243  for discharging ink from the upstream ink chamber  2451  may not be provided. In this case, the ink necessarily passes through the filter  246  and then is ejected (or leaks) from the nozzles  2472  or is discharged from the outlet  242 . 
     As described above, the inkjet recording device  100  according to the present embodiment includes the inkjet head  24  provided with the nozzles  2472  for ejecting ink, the liquid feeding pump  22  for supplying ink to the inkjet head  24 , the head driver  54  for performing a driving operation to cause a pressure variation regarding ejection of ink in the nozzles, and the controller  40  for controlling the operation of the liquid feeding pump  22  and the head driver  54 . The inkjet head  24  includes the common ink chamber  245  in which ink supplied to the inkjet head  24  flows, a filter  246  provided in the common ink chamber  245  for passing the supplied ink, the individual flow paths  2471  for feeding the ink having passed through the filter  246  in the common ink chamber  245  to each of the nozzles  2472 . In the bubble discharging operation of the ink in the inkjet head  24  the controller  40  causes the liquid feeding pump  22  to supply ink with the pressure at which ink leaks from the nozzles  2472 , while causing the head driver  54  to perform the predetermined driving operation, and causes the ink to be discharged from the outlet  242 . 
     Thus, in the bubble discharging operation, the bubbles can be easily and collectively discharged from various portions in the ink flow path of the inkjet head  24  in a single operation by causing the liquid feeding pump  22  to perform the liquid feeding operations simultaneously: the operation for leaking ink in the individual flow paths  2471  and the nozzles  2472  from the nozzles  2472 ; and the operation for leaking ink in the common ink chamber  245  from the outlet  242 . Further, by applying a driving voltage with a predetermined driving waveform pattern to the actuator at this time, air bubbles on the wall surfaces of the individual flow paths  2471  and the nozzles  2472  are effectively detached, and can be discharged from the nozzle openings effectively and reliably. 
     The inkjet recording device  100  according to the present embodiment includes a first sub tank  21  which stores ink to be supplied to the common ink chamber  245 , and a reflux unit  25  which returns ink discharged from the outlet  242  to the first sub tank  21 . As a result, most of the ink fed by the liquid feeding pump  22  for discharging air bubbles can be returned to the first sub tank  21  to be reused, so that the amount of wasted ink can be reduced. 
     The inkjet recording device  100  of the present embodiment is provided with a first reflux valve  251  for switching whether to discharge ink from the outlet  242 . The controller  40  causes the first reflux valve  251  to be closed when performing a normal ejection operation related to image recording, and causes the first reflux valve  251  to be opened when performing the bubble discharging operation. 
     As a result, it is possible to appropriately control and adjust the ink flow and the ink pressure in the inkjet head  24 . 
     The inkjet recording device  100  of the present embodiment is further provided with an outlet  243  for discharging ink not having passed through the filter  246  and being in the common ink chamber  245 . The controller  40  causes the supplied ink to be discharged from at least the outlet  242  when executing the bubble discharging operation. 
     That is, if necessary, it is possible to discharge ink, that is, air bubbles and the like, from the inkjet head  24  without passing through the filter  246  and to promptly discharge large bubbles and the like without passing through the filter  246 . It is possible to appropriately select whether or not to perform the discharge from the outlet  243 . 
     The inkjet recording device  100  of the present embodiment is provided with a second reflux valve  252  for switching whether to perform the discharge from the outlet  243 . Since the controller  40  causes the second reflux valve  252  to be closed when executing a normal ejection operation related to at least image recording, it is possible to appropriately control and adjust the ink flow and the ink pressure in the inkjet head  24 . 
     In the bubble discharging operation, the controller  40  causes the head driver  54  to perform a minute driving operation for causing a pressure variation which does not allow ink to be ejected from the nozzles  2472  during the image recording operation. 
     By using the minute driving waveform as described above, it is possible to prevent ink from ejecting from the nozzles more than necessary. In addition, by pressurizing the pressurized ink further according to the operation of the liquid feeding pump  22 , it is possible to prevent excessive increase of ejection pressure. As a result, generation of unnecessary mist and the like can be suppressed. In addition, by using the same driving waveform as usually used, it is not necessary to increase the type of driving waveform to be held and output, and the driving operation can be simplified. 
     It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. 
     For example, in the bubble discharging process of the above embodiment, the actuator is driven with the normal driving voltage pattern having a minute vibration waveform, but the present invention is not limited thereto. The actuator may be driven with a voltage of the waveform for ejection, or with a drive voltage pattern having a vibration frequency different from the normal drive waveform, for example, at a vibration frequency lower than that of a drive waveform which is output normally, so that power consumption can be reduced. 
     In the above embodiment, the inkjet head  24  is attached such that the filter  246  is substantially horizontal, but the present invention is not limited thereto. For example, the filter  246  may be obliquely inclined toward the outlets  242  and  243 . In this case, air bubbles in the upstream ink chamber  2451  and the downstream ink chamber  2452  easily move toward the outlets  242 ,  243  by buoyant force only. 
     In addition, the waveform for ejection and the minute vibration waveform may not be trapezoidal waveforms but rectangular waveforms or combinations thereof. When vibrations of multiple amplitudes or waveforms are combined, the waveform pattern for discharging bubbles may be different from normal waveforms for ejection and minute vibration waveform. 
     In the above embodiment, a piezo-type inkjet recording device in which ejection of ink and minute vibrations are performed with an actuator using a piezoelectric element has been described as an example, however, the actuator may be an element other than a piezoelectric element, for example, a magnetostrictive element. Alternatively, the present invention can be similarly applied to a thermal-type inkjet recording device. 
     Further, ink leaks to the ink tray  261  in the above embodiment, however, a sponge material which absorbs ink or simply a recording medium may be used. 
     Further, the operation to recover the meniscus need not to be performed in combination with the bubble discharging process, but may be performed only at the start of normal image recording as needed. 
     Further, the liquid feeding pump  22  is capable of pressurizing and supplying ink to the inkjet head  24  via the second sub tank  23  in the above embodiment, however, other configurations may be adopted. For example, when ink is pressurized and fed to the inkjet head  24 , a flow path bypassing the second sub tank  23  may be used. 
     Further, a one-pass type inkjet recording device including a line head is described as an example in the above embodiment, however, a scan type inkjet recording device ejecting ink onto a recording medium while moving the inkjet head, a multipass type inkjet recording device, or the like may be used. 
     According to the present invention, there is an effect of being able to discharge air bubbles in ink flow path more easily and reliably in an inkjet recording device. 
     In addition, specific details such as the configuration, arrangement, procedure of control operations, and the like shown in the above embodiment can be appropriately changed without departing from the spirit of the present invention. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to inkjet recording devices. 
     REFERENCE SIGNS LIST 
     
         
           10  Medium Supplier 
           11  Paper Feeding Tray 
           12  Feeder Board 
           121 ,  122  Roller 
           123  Belt 
           15  Medium Receiving Unit 
           16  Copy Receiving Tray 
           20  Image Forming Body 
           21  First Sub Tank 
           211  First Liquid Level Sensor 
           22  Liquid Feeding Pump 
           23  Second Sub Tank 
           231  Second Liquid Level Sensor 
           232 ,  233  Air Releasing Valve 
           234  Air Tank 
           24  Inkjet Head 
           241  Inlet 
           242 ,  243  Outlet 
           245  Common Ink Chamber 
           2451  Upstream Ink Chamber 
           2452  Downstream Ink Chamber 
           2452   a  Through Hole 
           246  Filter 
           247  Ink Ejector 
           2471  Individual Flow Path 
           2472  Nozzle 
           25 ,  25   a  Reflux Unit 
           251  First Reflux Valve 
           252  Second Reflux Valve 
           26  Ink Receiving Unit 
           261  Ink Tray 
           262  Waste Liquid Tank 
           27  First Delivery Unit 
           271  Swing Arm 
           272  Reception Drum 
           281  Image Forming Drum 
           2811  Drum Heater 
           282  Head Unit 
           2822  Ink Heater 
           283  Irradiator 
           283   a  Light Shielding Plate 
           29  Second Delivery Unit 
           291  Delivery Roller 
           292 ,  293  Roller 
           294  Belt 
           30  Ink Supplier 
           31  Main Tank 
           311  Filter 
           32  Supplying Pump 
           33  Supplying Valve 
           40  Controller 
           41  CPU 
           42  RAM 
           43  ROM 
           44  Memory 
           51  Irradiator Driver 
           52  Conveyance Driver 
           53  Cleaner Driver 
           54  Head Driver 
           55  Tray Driver 
           56  Operation Display Unit 
           57  Communication Unit 
           58  Unit Position Adjustment Driver 
           59  Bus 
           100  Inkjet Recording Device