Patent Publication Number: US-11020981-B2

Title: Liquid ejection device

Description:
INCORPORATION BY REFERENCE 
     This application is based on Japanese Patent Application No. 2019-56671 filed with the Japan Patent Office on Mar. 25, 2019, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Field of the Invention 
     The present disclosure relates to a liquid ejection device with a liquid ejection head and a liquid supply unit for supplying liquid stored in a liquid storage container to the liquid ejection head. 
     Related Art 
     For example, in an ink jet printer (liquid ejection device), a liquid ejection head for injecting a tiny amount of ink (liquid) to a print object is used. Ink is supplied to this liquid ejection head from an ink cartridge (liquid storage container) storing the ink through a predetermined supply passage. Conventionally, a liquid ejection device is known in which a liquid supply unit (valve unit) including a pressure chamber for setting a discharge hole of a liquid ejection head to a negative pressure is arranged in a supply passage in the case of supplying ink from an ink cartridge to the liquid ejection head by a water head difference. By disposing the liquid supply unit for generating the negative pressure, unlimited dripping of the ink from the discharge hole is suppressed even if the ink is supplied by the water head difference. 
     In the above liquid ejection device, the ink is supplied to the liquid ejection head via the pressure chamber for generating a negative pressure. Accordingly, a predetermined amount of the ink needs to be initially filled into the pressure chamber during initial usage, after maintenance and the like. At this time, it is necessary to vent air in the pressure chamber. Further, the ink stored in the pressure chamber may generate air bubbles by heating or the like associated with the operation of the liquid ejection device. Also in this case, air in the pressure chamber needs to be vented. 
     SUMMARY 
     A liquid ejection device according to one aspect of the present disclosure includes a liquid ejection head configured to inject liquid and a liquid supply unit configured to supply the liquid from a liquid storage container storing the liquid to the liquid ejection head. 
     The liquid ejection head includes a plurality of liquid discharge holes, individual passages configured to individually supply the liquid to the respective liquid discharge holes, and a common passage configured to supply the liquid to the individual passages. The liquid supply unit includes a pressure chamber capable of storing the liquid, an exhaust valve configured to release or close the pressure chamber to or from outside air, a liquid passage, valve bodies, a pump mechanism and a controller. 
     The liquid passage includes a first supply passage allowing communication between the liquid storage container and the pressure chamber, a second supply passage allowing communication between an upstream side of the common passage and the pressure chamber, a return passage allowing communication between a downstream side of the common passage and the pressure chamber, and a short-circuit passage configured to short-circuit the second supply passage and the return passage. One end of the short-circuit passage is connected to the return passage to form a first branch portion and the other end is connected to the second supply passage to form a second branch portion. The valve bodies include a first valve body configured to open and close the first supply passage, a second valve body configured to open and close the second supply passage on a side closer to the common passage than the second branch portion, a third valve body configured to open and close the return passage on a side closer to the pressure chamber than the first branch portion and a fourth valve body configured to open and close the short-circuit passage. The pump mechanism is capable of feeding the liquid to the second supply passage. The controller controls operations of the valve bodies and the pump mechanism. 
     The controller executes an ejection control, a first circulation control and a second circulation control. The ejection control is a control for supplying the liquid from the pressure chamber to the liquid ejection head through the second supply passage by closing the third and fourth valve bodies while opening the first and second valve bodies, and setting the pump mechanism in a non-operative state with the exhaust valve closed. The first circulation control is a control for circulating the liquid through the second supply passage, the common passage and the return passage by opening the second and third valve bodies while closing the first and fourth valve bodies, and operating the pump mechanism with the exhaust valve closed. The second circulation control is a control for circulating the liquid through the second supply passage, the short-circuit passage and the return passage by closing the second valve body while opening the first, third and fourth valve bodies, and operating the pump mechanism with the exhaust valve opened. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an overall configuration of a liquid ejection device according to one embodiment of the present disclosure, 
         FIGS. 2A and 2B  are diagrams schematically showing a cross-section of the head unit in a front-rear direction, wherein  FIG. 2A  shows a state where ink is being discharged from a head unit and  FIG. 2B  shows a state where the ink is being circulated through the head unit, 
         FIG. 3  is a block diagram of a liquid supply system using the liquid ejection device of the embodiment showing a state where a print mode is being performed, 
         FIG. 4  is a block diagram showing a state where a first circulation mode is being performed, 
         FIG. 5  is a block diagram showing a state where a second circulation mode is being performed, 
         FIG. 6  is a block diagram showing a state where a pressurized purge mode is being performed, 
         FIG. 7  is a block diagram showing a state where a decompression mode is being performed, 
         FIGS. 8A and 8B  are perspective views of the liquid supply unit, wherein  FIG. 8A  is a perspective view viewed from the side of a first chamber and  FIG. 8B  is a perspective view viewed from the side of a second chamber, 
         FIG. 9  is a perspective view of the liquid supply unit with a sealing film on the side of the first chamber removed, 
         FIG. 10A to 10C  are perspective views of the liquid supply unit with an atmospheric pressure detection film on the side of the second chamber removed, 
         FIG. 11  is an exploded perspective view of the liquid supply unit, 
         FIG. 12A  is a perspective view of a pressing member and  FIG. 12B  is a perspective view of the pressing member viewed in a different direction, 
         FIG. 13A  is a perspective view of an on-off valve and  FIG. 13B  is an exploded perspective view of the on-off valve, 
         FIG. 14A  is a sectional view along line XIV-XIV of  FIG. 8A  showing a state where the on-off valve is in a closing posture and  FIG. 14B  is an enlarged view of a part A 1  of  FIG. 14A , 
         FIG. 15A  is a sectional view, corresponding to  FIG. 14A , showing a state where the on-off valve is in an opening posture and  FIG. 15B  is an enlarged view of a part A 2  of  FIG. 15A , 
         FIGS. 16A and 16B  are perspective views of a lever member and  FIG. 16C  is an exploded perspective view of the lever member, 
         FIGS. 17A and 17B  are perspective views of the pressing member, the on-off valve and the lever member, 
         FIG. 18A  is a sectional view showing a state before the lever member is operated and  FIG. 18B  is a sectional view showing a state where air is vented by the operation of the lever member, 
         FIG. 19  is a sectional view of the liquid supply unit in the front-rear direction, 
         FIG. 20  is an exploded perspective view of a backflow prevention mechanism, 
         FIG. 21A  is a perspective view of the backflow prevention mechanism showing a state where a spherical body opens a valve conduit,  FIG. 21B  is a view showing a state where the spherical body closes the valve conduit and  FIG. 21C  is a perspective view of a branched head portion, 
         FIG. 22A  is a sectional view showing a state of the backflow prevention mechanism in the print mode and  FIG. 22B  is a sectional view showing a state of the backflow prevention mechanism in the pressurized purge mode, 
         FIG. 23A  is a sectional view showing a state where an umbrella valve seals a communication opening and  FIG. 23B  is a sectional view showing a state where the umbrella valve releases the communication opening, 
         FIG. 24  is a block diagram showing an electrical configuration of the liquid ejection device, 
         FIG. 25  is a perspective view showing a flow of the ink in the print mode, 
         FIG. 26  is a perspective view showing a flow of the ink in the pressurized purge mode, 
         FIG. 27  is a perspective view showing a flow of the ink in the first circulation mode, and 
         FIG. 28  is a perspective view showing a flow of the ink in the second circulation mode. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, one embodiment of the present disclosure is described with reference to the drawings. A liquid ejection device according to the present disclosure can be applied to apparatuses for various uses. For example, the device according to the present disclosure can be applied to apparatuses for ejecting or spraying water, chemical, aqueous solution, fuel or the like. Above all, this device can be suitably applied to an ink jet printer. Thus, a liquid supply device for supplying ink to an ink ejection head of an ink jet type and a liquid ejection device using the same are illustrated in this embodiment. 
     [External Configuration of Liquid Ejection Device] 
       FIG. 1  is a perspective view showing an overall configuration of a liquid ejection device  1  according to a first embodiment of the present disclosure. The liquid ejection device  1  includes a head unit  21  (liquid ejection head) for ejecting ink (liquid) to various works such as paper sheets and resin sheets of various sizes and cloth fabric, and liquid supply units  3  each for supplying the ink from an ink cartridge IC storing the ink to the head unit  21 . The head unit  21  and the liquid supply units  3  are mounted on an unillustrated carriage and move in a main scanning direction of the works. Four liquid supply units  3  are equipped for each head unit  21  to supply respective inks of cyan, magenta, yellow and black. In  FIG. 1 , out of four liquid supply units  3 , only one is shown to simplify graphical representation. 
     The liquid supply unit  3  includes a body portion  30  with a tank portion  31  and a pump portion  32 . Further, the liquid supply unit  3  includes an upstream pipe  33  (part of a first supply passage) arranged on an upstream side of the body portion  30  in an ink supply direction (liquid supply direction), a downstream pipe  34  (part of a second supply passage) arranged on a downstream side of the body portion  30 , a return pipe  35  (return passage) serving as a path for returning the ink from the side of the head unit  21  to the side of the liquid supply unit  3 , a short-circuit pipe RP (short-circuit passage) short-circuiting the downstream pipe  34  and the return pipe  35 , a monitor pipe  36  and a bypass pipe  32 P (bypass supply passage). 
     The tank portion  31  is a region forming a space for temporarily storing the ink to be supplied to the head unit  21  under a negative pressure environment. The pump portion  32  is a region for housing a pump  9  (pump mechanism;  FIGS. 3 to 7 and 19 ) to be operated at the time of the discharge of a preservation solution filled in the head unit  21  during initial usage, a decompression process for forming the negative pressure environment, a pressurized purge process for cleaning the head unit  21  (ink ejecting portion  22 ), the circulation of the ink between the head unit  21  and the liquid supply unit  3  and the circulation of the ink in a short-circuit manner using the short-circuit pipe RP and the return pipe  35 . 
     The upstream pipe  33  is a supply pipe allowing communication between the tank portion  31  (second chamber  42 ) and an ink cartridge IC (liquid storage container). An upstream end  331  of the upstream pipe  33  is connected to a terminal end part of a tube  330  (part of the first supply passage) extending from the ink cartridge IC, and a downstream end  332  is connected to an inlet part of the tank portion  31 . A first valve body  33 V functioning to open and close the upstream pipe  33  is mounted in the tube  330 . If the first valve body  33 V is opened, the ink can be supplied from the ink cartridge IC to the tank portion  31 . If the first valve body  33 V is closed, the supply cannot be made. 
     The downstream pipe  34  is a supply pipe allowing communication between the tank portion  31  (second chamber  42 ) and the head unit  21  (upstream side of a common passage  27  to be described later). An upstream end  341  of the downstream pipe  34  is connected to an outlet part of the tank portion  31  via a backflow prevention mechanism  38  to be described later and a downstream end  342  is connected to the head unit  21 . A second valve body  34 V for opening and closing the downstream pipe  34  is mounted in this downstream pipe  34 . 
     The return pipe  35  is a pipe allowing communication between the head unit  21  (downstream side of the common passage  27  to be described later) and the tank portion  31  (second chamber  42 ). An upstream end  351  of the return pipe  35  is connected to the head unit  21 , and a downstream end  352  is connected to the tank portion  31 . A third valve body  35 V for opening and closing the return pipe  35  is mounted in this return pipe  35 . 
     The short-circuit pipe RP is a pipe short-circuiting the head unit  21  and the downstream pipe  34 . The short-circuit pipe RP allows communication between a part of the downstream pipe  34  upstream of an arrangement position of the second valve body  34 V and a part of the return pipe  35  upstream of (below) an arrangement position of the third valve body  35 V. One end side of the short-circuit pipe RP is connected to the return pipe  35  to form a first T-branch portion Ra (first branch portion), and the other end side is connected to the downstream pipe  34  to form a second T-branch portion Rb (second branch portion). A fourth valve body RPV for opening and closing the short-circuit pipe RP is mounted in this short-circuit pipe RP. Note that the second valve body  34 V is arranged in the downstream pipe  34  on a side closer to the head unit  21  (side of the common passage  27  to be described later) than the second T-branch portion Rb. Further, the third valve body  35 V is arranged in the return pipe  35  on a side closer to the tank portion  31  (side of the second chamber  42 ) than the first T-branch portion Ra. 
     Further, the third valve body  35 V may be arranged in the return pipe  35  on a side closer to the head unit  21  than the first T-branch portion Ra. A second embodiment having such an arrangement can be basically used similarly to the first embodiment. The embodiments are basically described for the first embodiment and the description of the embodiment is limited to the second embodiment for points of difference of the second embodiment from the first embodiment. 
     The monitor pipe  36  is a pipe for indicating an ink level in the tank portion  31 . The bypass pipe  32 P is a conduit for feeding the ink to the downstream pipe  34  without via the negative pressure environment (second chamber  42 ) of the tank portion  31 . The bypass pipe  32 P includes an upstream bypass pipe BP 1  arranged upstream of the pump portion  32  and a downstream bypass pipe BP 2  arranged downstream of the pump portion  32 . 
     The head unit  21  includes the ink ejecting portion  22 , a control unit  23 , an end tube  24  and a recovery tube  25 . The ink ejecting portion  22  is a nozzle part for discharging ink droplets toward a work W. A piezo method using a piezo element, a thermal method using a heating element or the like can be adopted as a method for discharging ink droplets in the ink ejecting portion  22 . The control unit  23  includes a control board for controlling the piezo element or the heating element provided in the ink ejecting portion  22  and controls an operation of discharging ink droplets from the ink ejecting portion  22 . 
     The end tube  24  is a tube linking the downstream end  342  of the downstream pipe  34  and the ink ejecting portion  22 . The downstream end  342  is a cap-type socket and attachable to an upper end fitting part of the end tube  24  in a single operation. The recovery tube  25  is a tube linking the ink ejecting portion  22  and the upstream end  351  of the return pipe  35 . Note that the recovery tube  25  is used also to discharge the preservation solution sealed in the liquid supply unit  3  during initial usage. Specifically, the recovery tube  25  constitutes a part of a return path for returning the ink from the side of the head unit  21  to the side of the liquid supply unit  3 . 
       FIGS. 2A and 2B  are views schematically showing a cross-section of the head unit  21  in a front-rear direction, wherein  FIG. 2A  shows a state where the third and fourth valve bodies  35 V, RPV are closed (print mode to be described later) and  FIG. 2B  shows a state where the third valve body  35 V is opened and the fourth valve body RPV is closed (first circulation mode). Note that the second valve body  34 V is open in either state. The ink ejecting portion  22  includes a plurality of ink discharge holes  22 H (liquid discharge holes) for discharging the ink toward the work W. Individual passages  26  for individually supplying the ink to the ink discharge holes  22 H and the common passage  27  for supplying the ink to these individual passages  26  are provided inside the head unit  21 . Note that the preservation solution for preventing air from being trapped in these passages is filled in the individual passages  26  and the common passage  27  before the head unit  21  is actually used. 
     The common passage  27  is an ink passage extending in a horizontal direction. An upstream end of each individual passage  26  communicates with the common passage  27 . The downstream end  342  of the downstream pipe  34  communicates with an upstream side of the common passage  27  via the end tube  24 . The upstream end  351  of the return pipe  35  communicates with a downstream side of the common passage  27  via the recovery tube  25 . In other words, the upstream side and the downstream side of the common passage  27  communicate with the tank portion  31  (second chamber  42 ) respectively through the downstream pipe  34  and through the return pipe  35 . 
     If the ink is supplied from the downstream pipe  34  to the head unit  21  with the return pipe  35  closed by the third valve body  35 V as shown in  FIG. 2A , the ink is ejected from the ink discharge holes  22 H by way of the common passage  27  and the respective individual passages  26 . On the other hand, if the ink is supplied from the downstream pipe  34  to the head unit  21  with the third valve body  35 V released to open the return pipe  35  as shown in  FIG. 2B , the ink returns to the tank portion  31  exclusively through the return pipe  35 . In this case, if the return pipe  35  is set to a negative pressure, the ink does not leak from the ink discharge holes  22 H. 
     [Summary of Liquid Supply System] 
     In this embodiment, the device is configured such that the ink cartridge IC is arranged above the head unit  21  and the ink is supplied to the head unit  21  by a water head difference. In the case of supplying the ink by the water head difference, the ink is constantly ejected from the ink ejecting portion  22  of the head unit  21  if the ink is supplied at normal pressure. Thus, it is necessary to dispose a negative pressure generating portion for generating a negative pressure environment in the ink supply passage and set the ink ejecting portion  22  to a suitable negative pressure. The tank portion  31  of the liquid supply unit  3  functions as the above negative pressure generating portion. 
       FIG. 3  is a block diagram schematically showing a liquid supply system using the liquid ejection device  1  of this embodiment. The ink cartridge IC is arranged at a position higher than the ink ejecting portion  22  by a height h. This height h serves as a water head difference and the ink in the ink cartridge IC is supplied to the head unit  21  by this water head difference. The liquid supply unit  3  is incorporated at an intermediate position of the ink supply passage between the ink cartridge IC and the head unit  21 . 
     The tank portion  31  of the liquid supply unit  3  includes a first chamber  41  (upstream chamber/part of first supply passage) set to a pressure higher than an atmospheric pressure by receiving the water head difference and the second chamber  42  (pressure chamber) arranged downstream of the first chamber  41  in the ink supply direction and set to a negative pressure. The second chamber  42  is a chamber capable of storing the ink. The first chamber  41  is a chamber in which a negative pressure operation is not performed and to which a pressure P by the water head difference is applied in addition to the atmospheric pressure. This pressure P is expressed by P=ρgh [Pa] when p denotes water density (ink can be handled equivalent to water in density), g denotes a gravitational acceleration and h denotes the water head difference. The first chamber  41  communicates with the ink cartridge IC via the upstream pipe  33 . The second chamber  42  communicates with the ink ejecting portion  22  via the downstream pipe  34 . 
     An on-off valve  6  (opening/closing member) coupled to a pressing member  5  is arranged on a wall member partitioning between the first chamber  41  and the second chamber  42 . Further, a wall portion defining the second chamber  42  is constituted by an atmospheric pressure detection film  7  (flexible film member). When a pressure in the second chamber  42  reaches a negative pressure exceeding a predetermined threshold value, the atmospheric pressure detection film  7  detects the atmospheric pressure to be displaced. This displacement force is applied to the pressing member  5 , a posture of the coupled on-off valve  6  changes from a closing posture to an opening posture, and the first chamber  41  and the second chamber  42  are allowed to communicate. An ink supply route during a normal printing process is a route passing through the upstream pipe  33 , the first chamber  41 , the second chamber  42  and the downstream pipe  34 . 
     In addition to the above route, the liquid supply unit  3  includes the bypass pipe  32 P short-circuiting the first chamber  41  and the downstream pipe  34  without via the second chamber  42 . The upstream end of the bypass pipe  32 P is connected to the upstream pipe  33  via the first chamber  41  and the downstream end joins the downstream pipe  34  (joint part a) on a side closer to the tank portion  31  (second chamber  42 ) than the second T-branch portion Rb. The pump  9  capable of rotating in forward and reverse directions is arranged in the bypass pipe  32 P. The pump  9  can forcibly feed the ink to the downstream pipe  34 . Further, the liquid supply unit  3  includes the return pipe  35  allowing communication between the ink ejecting portion  22  and the first chamber  41  (communicating also with the second chamber  42  via the on-off valve  6 ) and including the third valve body  35 V, and the short-circuit pipe RP allowing communication between the downstream pipe  34  and the return pipe  35  and including the fourth valve body RPV. 
       FIG. 3  is also a diagram showing a state where the liquid supply system is performing the print mode (ejection control) for performing the printing process. In the print mode, the ink is supplied from the second chamber  42  to the head unit  21  through the downstream pipe  34 . In this print mode, the first valve body  33 V of the upstream pipe  33  and the second valve body  34 V of the downstream pipe  34  are opened, whereas the third valve body  35 V of the return pipe  35  and the fourth valve body RPV of the short-circuit pipe RP are closed. Further, in the print mode, a predetermined mount of the ink is filled in the first chamber  41  and the second chamber  42  and the second chamber  42  is set to a predetermined negative pressure. The pressure in the first chamber  41  is an atmospheric pressure+ρgh [Pa] by the water head difference as described above, so that the ink can be supplied from the ink cartridge IC by the water head difference any time. As basic settings of the print mode, the on-off valve  6  for setting the second chamber  42  to a negative pressure is set in the closing posture to separate the first and second chambers  41 ,  42 . The pump  9  is set in a stopped state. The pump  9  is a tube pump and thus the bypass pipe  32 P is closed when the pump  9  is stopped. Thus, the downstream pipe  34  and the ink ejecting portion  22  are also maintained at a negative pressure. 
     To smoothly fill the ink into the second chamber  42 , an air vent mechanism  37  (exhaust valve) is attached to the second chamber  42 . The air vent mechanism  37  functions as an exhaust valve for releasing or closing the second chamber  42 , which is a pressure chamber, to and from outside air. Of course, the air vent mechanism  37  is closed when the above print mode is performed. A predetermined amount of the ink needs to be initially filled into the second chamber  42  during initial usage, after maintenance and the like. The air vent mechanism  37  promotes the initial filling by allowing the second chamber  42  set in the negative pressure environment to temporarily communicate with the atmosphere (by venting air in the second chamber  42 ). Further, the ink stored in the second chamber  42  may generate air bubbles by heating. The air vent mechanism  37  is also used in removing air based on the air bubbles from the second chamber  42 . 
     When the head unit  21  operates and the ink ejecting portion  22  discharges ink droplets, the ink in the second chamber  42  is consumed and, accordingly, a degree of the negative pressure in the second chamber  42  progresses. That is, the ink ejecting portion  22  sucks the ink from the second chamber  42  in a state separated from the atmosphere and enhances a negative pressure degree of the second chamber  42  every time discharging ink droplets. When the pressure in the second chamber  42  reaches a negative pressure exceeding the predetermined threshold value as the ink in the second chamber  42  decreases, the atmospheric pressure detection film  7  detects the atmospheric pressure to be displaced as described above. By this displacement force, the posture of the on-off valve  6  changes from the closing posture to the opening posture through the pressing member  5  and the first and second chambers  41 ,  42  communicate. Thus, the ink flows from the first chamber  41  into the second chamber  42  due to a pressure difference between the both chambers. 
     As the ink flows into the second chamber  42 , the negative pressure degree of the second chamber  42  is gradually alleviated and approaches the atmospheric pressure. Simultaneously, the displacement force applied to the pressing member  5  from the atmospheric pressure detection film  7  also becomes gradually smaller. When the pressure in the second chamber  42  reaches a negative pressure below the predetermined threshold value, the posture of the on-off valve  6  returns to the closing posture and the first and second chambers  41 ,  42  are separated again. At this time, the ink is replenished into the first chamber  41  from the ink cartridge IC by the water head difference by an amount flowed into the second chamber  42  from the first chamber  41 . In the print mode, such an operation is repeated. 
     In the second embodiment, the print mode in which the ink flows as in the first embodiment is set by opening the first and second valve bodies  33 V,  34 V and closing the third and fourth valve bodies  35 V, RPV. 
     In the second embodiment, the print mode may be set by opening the first, second and fourth valve bodies  33 V,  34 V and RPV and closing the third valve body  35 V. In this case, the ink is supplied from the second chamber  42  to the head unit  21  through the downstream pipe  34 , and also passes from the tank portion  31  to the return pipe  35  until the first T-branch portion Ra and passes through the short-circuit pipe RP and is supplied to the head unit  21  from the second T-branch portion Rb through the downstream pipe  34 . However, in this case, it is necessary to also provide the second chamber  42 , the on-off valve  6 , the atmospheric pressure detection film  7  and the pressing member  5  between the tank portion  31  and the return pipe  35  and enable the ink to be supplied while keeping the head unit  21  at a negative pressure. If the print mode in which the fourth valve body RPV is closed is set, it is not necessary to provide such additional structures. Thus, it is better to set the print mode by opening the first and second valve bodies  33 V,  34 V and closing the third and fourth valve bodies  35 V, RPV in the second embodiment. 
     The liquid ejection device  1  of this embodiment is capable of performing the first circulation mode, a second circulation mode, a pressurized purge mode and a decompression mode in addition to the above print mode. The first circulation mode is a mode for recovering air trapped in the ink passage (individual passages  26 , common passage  27 ) in the head unit  21  into the liquid supply unit  3  by circulating the ink using the return pipe  35 . The second circulation mode is a mode for discharging the air recovered into the liquid supply unit  3  to outside from the air vent mechanism  37  by circulating the ink without passing through the head unit  21  using the short-circuit pipe RP and the return pipe  35 . The pressurized purge mode is a mode for supplying high-pressure ink to the ink ejecting portion  22  and causing the ink ejecting portion  22  to discharge the ink in order to remove or prevent ink clogging in the ink ejecting portion  22 . The decompression mode is a mode for setting the second chamber  42  at a constant pressure to the predetermined negative pressure during initial usage, after maintenance and the like. 
       FIG. 4  is a block diagram showing a state where the first circulation mode (first circulation control) is being performed. In the first circulation mode, the upstream pipe  33  and the short-circuit pipe RP are closed by closing the first and fourth valve bodies  33 V, RPV, whereas the downstream pipe  34  and the return pipe  35  are released by opening the second and third valve bodies  34 V,  35 V. The air vent mechanism  37  is closed and the second chamber  42  is maintained at the negative pressure. Further, the pump  9  arranged in the bypass pipe  32 P is driven in the forward rotation direction. As shown in  FIG. 2B , the upstream end  351  of the return pipe  35  communicates with the downstream end of the common passage  27  in the head unit  21 . On the other hand, the downstream end  352  of the return pipe  35  communicates with the second chamber  42  via the first chamber  41  directly communicating therewith and the on-off valve  6 . 
     If the pump  9  is driven in the forward rotation direction in the first circulation mode, the ink is circulated through a circulation path composed of the downstream bypass pipe BP 2 , a part of the downstream pipe  34  downstream of the joint part a, the common passage  27  in the head unit  21 , the return pipe  35  and the upstream bypass pipe BP 1 . At this time, since the first valve body  33 V is closed, the return pipe  35  and the common passage  27  are set to a negative pressure by an ink sucking operation of the pump  9 . Accordingly, the ink does not leak from the ink discharge holes  22 H. By performing the first circulation mode, air taken into the head unit  21  can be recovered into the liquid supply unit  3  (first chamber  41 ). In this way, air can be prevented from staying in the individual passages  26  and the ink discharge holes  22 H and an ink discharge failure can be suppressed. 
     Also in the second embodiment, the first circulation mode in which the ink flows as in the first embodiment is set by opening the second and third valve bodies  34 V,  35 V while closing the first and fourth valve bodies  33 V, RPV as in the first embodiment. 
       FIG. 5  is a block diagram showing a state where the second circulation mode (first circulation control) is being performed. In the second circulation mode, the first, third and fourth valve bodies  33 V,  35 V and RPV are opened, whereas the second valve body  34 V is closed. In this way, a part of the downstream pipe  34  downstream of the second T-branch portion Rb is closed, whereas the ink can be supplied from the ink cartridge IC and the short-circuit pipe RP and the return pipe  35  are opened. Further, the air vent mechanism  37  is released, the pressure in the second chamber  42  is set to the atmospheric pressure, the on-off valve  6  is set in the opening posture by a mechanism ( FIG. 18 ) to be described later, and the first and second chambers  41 ,  42  communicate. 
     If the pump  9  is driven in the forward rotation direction in the second circulation mode, the ink is circulated through a circulation path composed of the downstream bypass pipe BP 2 , the part of the downstream pipe  34  downstream of the joint part a, the short-circuit pipe RP, the return pipe  35  and the upstream bypass pipe BP 1 . Accordingly, the ink can be circulated through the first and second chambers  41 ,  42  communicating with the atmosphere in the circulation path not passing through the head unit  21  while being supplied from the ink cartridge IC to the first chamber  41 . By this circulation, the second chamber  42  is gradually filled with the ink by the inflow of the ink from the first chamber  41 , and the air recovered into the first chamber  41  in the first circulation mode can be expelled to outside through the air vent mechanism  37  in the released state. Specifically, the air having entered the head unit  21  and the liquid supply unit  3  can be easily and reliably discharged without being accompanied by the removal of the supply passage. 
     In the second embodiment, the second circulation mode is set by closing the second valve body  34 V while opening the first and fourth valve bodies  33 V, RPV. In the second circulation mode of the second embodiment, the third valve body  35 V may be open or may be closed. 
     In the second circulation mode of the second embodiment, the ink flows as in the second circulation mode of the first embodiment in the case of opening the third valve body  35 V. 
     In the case of closing the third valve body  35 V, the second circulation mode of the second embodiment differs from the second circulation mode of the first embodiment in that the ink does not flow from the first T-branch portion Ra to the head unit  21  since the third valve body  35 V is closed. Also in the second circulation mode of the first embodiment, since the ink more easily flows toward the second chamber  42  than toward the head unit  21 , the ink basically does not flow to the head unit  21 . Since the ink is stopped at the third valve body  35 V in the case of closing the third valve body  35 V in the second circulation mode of the second embodiment, there is no likelihood that the ink flows to the head unit  21  and ink droplets leak down from the ink ejecting portion  22 , for example, even if the ink is excessively supplied from the pump  9 . 
       FIG. 6  is a diagram showing a state where the pressurized purge mode is being performed. In the pressurized purge mode, the pump  9  is driven in the forward rotation direction. The first and second valve bodies  33 V,  34 V are opened, whereas the third and fourth valve bodies  35 V, RPV are closed. By the forward drive of the pump  9 , the ink directly moves from the upstream pipe  33  toward the downstream pipe  34  via the first chamber  41  and the bypass pipe  32 P while bypassing the second chamber  42 . That is, the ink pressurized in the pump  9  is supplied to the ink ejecting portion  22 . In this way, the ink is forcibly ejected from the ink ejecting portion  22  to clean the ink ejecting portion  22 . 
     The backflow prevention mechanism  38  is provided to prevent the pressurized ink from flowing back to the second chamber  42  through the downstream pipe  34  when the pressurized purge mode is performed. The backflow prevention mechanism  38  is arranged in the downstream pipe  34  on a side upstream of the joint part a of the downstream pipe  34  and the downstream end of the bypass pipe  32 P. Since the side of the downstream pipe  34  upstream of the joint part a is closed by the backflow prevention mechanism  38 , all the high-pressure ink generated in the bypass pipe  32 P flows toward the ink ejecting portion  22 . Thus, the breakage of the atmospheric pressure detection film  7  defining the second chamber  42  is prevented. 
     Also in the second embodiment, the pressurized purge mode in which the ink flows as in the first embodiment is set by closing the third and fourth valve bodies  35 V, RPV while opening the first and second valve bodies  33 V,  34 V as in the first embodiment. 
       FIG. 7  is a diagram showing a state where the decompression mode is being performed. In the decompression mode, the pump  9  is driven in the reverse rotation direction. The second valve body  34 V is opened, whereas the first, third and fourth valve bodies  33 V,  35 V and RPV are closed. If the pump  9  is driven in the reverse rotation direction, the ink ejecting portion  22  and the second chamber  42  are decompressed through the downstream pipe  34  and the bypass pipe  32 P. The ink ejecting portion  22  and the second chamber  42  are set to a predetermined negative pressure, i.e. a negative pressure at which ink droplets do not leak down from the ink ejecting portion  22  even if the ink is supplied by the water head difference, by this decompression mode. Note that if the ink ejecting portion  22  is set to an excessive negative pressure, ink discharge by the drive of the piezo element or the like in the ink ejecting portion  22  may be impeded. Thus, the ink ejecting portion  22  and the second chamber  42  are desirably set, for example, to a weak negative pressure of about −0.2 to −0.7 kPa. 
     Also in the second embodiment, the decompression mode in which the ink flows as in the first embodiment is set by closing the first, third and fourth valve bodies  33 V,  35 V and RPV while opening the second valve body  34 V as in the first embodiment. 
     [Overall Structure of Liquid Supply Unit] 
     Next, the structure of the liquid supply unit  3  according to this embodiment that enables the execution of each mode of the liquid ejection device  1  described above is described in detail.  FIGS. 8A and 8B  are perspective views of the liquid supply unit  3 , wherein  FIG. 8A  is a perspective view viewed from the side of the first chamber  41  and  FIG. 8B  is a perspective view viewed from the side of the second chamber  42 .  FIG. 9  is a perspective view of the liquid supply unit  3  with a sealing film  7 A on the side of the first chamber  41  removed, and  FIG. 10A to 10C  are perspective views of the liquid supply unit  3  with the atmospheric pressure detection film  7  on the side of the second chamber  42  removed.  FIG. 11  is an exploded perspective view of the liquid supply unit  3 . 
     As preliminarily described on the basis of  FIGS. 3 to 7 , the liquid supply unit  3  includes the body portion  30  having the tank portion  31  and the pump portion  32 , the upstream pipe  33 , the downstream pipe  34 , the return pipe  35 , the bypass pipe  32 P, the short-circuit pipe RP, the air vent mechanism  37 , the backflow prevention mechanism  38 , the pressing member  5 , the on-off valve  6  and the atmospheric pressure detection film  7 . Besides these, the liquid supply unit  3  includes the monitor pipe  36  for monitoring an ink liquid surface of the second chamber  42  and the sealing film  7 A constituting a part of a wall surface defining the first chamber  41 . 
     The body portion  30  includes a base board  300  formed of a flat plate extending in the front-rear direction. A front side of the base board  300  is a tank portion base plate  310  (wall member) serving as a board of the tank portion  31  and a rear side thereof is a pump portion housing  320  forming a housing structure in the pump portion  32 . The first chamber  41  is arranged on a left surface side of the tank portion base plate  310 , and the second chamber  42  is arranged on a right surface side thereof. The first and second chambers  41 ,  42  are spaces capable of storing the ink. The tank portion base plate  310  is perforated to form a communication opening  43  allowing communication between the first chamber  41  and the second chamber  42 . The aforementioned on-off valve  6  is arranged in this communication opening  43 . 
     As shown in  FIG. 9 , the first chamber  41  is a narrow space roughly U-shaped when viewed from left. The first chamber  41  is defined by a first partition wall  411  projecting leftward from the tank portion base plate  310 . The first partition wall  411  is composed of a pair of wall pieces facing each other at a predetermined distance. An inflow portion  412 , which is an upstream side of the first chamber  41 , communicates with a filter chamber  44 . The ink supplied from the upstream pipe  33  to the tank portion  31  flows into the first chamber  41  from the inflow portion  412  via the filter chamber  44 . 
     The first chamber  41  is shaped to extend forward in the horizontal direction from the inflow portion  412  and be then curved downward. A bypass communication chamber  413  and a return communication chamber  414  are Y-branched and connected to a downstream end of the first chamber  41 . The bypass communication chamber  413  is a section for linking the first chamber  41  and the upstream bypass pipe BP 1 . An upstream end of the upstream bypass pipe BP 1  is connected to a wall portion defining near the lower end of the bypass communication chamber  413 . The return communication chamber  414  is a section for linking the first chamber  41  and the return pipe  35 . The downstream end  352  of the return pipe  35  is connected to a wall portion defining near the front end of the return communication chamber  414 . Note that the return communication chamber  414  is shown as a part of the return pipe  35  in  FIGS. 3 to 5 . 
     A lower monitor communication chamber  415  is arranged above the return communication chamber  414 , and an upper monitor communication chamber  416  is arranged above a horizontal part of the first chamber  41 . An upstream end  361  of the monitor pipe  36  communicates with the lower monitor communication chamber  415 , and a downstream end  362  of the monitor pipe  36  communicates with the upper monitor communication chamber  416 . Also with reference to  FIG. 10 , the tank portion base plate  310  is perforated with a lower communication hole  41 A and an upper communication hole  41 B arranged above the lower communication hole  41 A. The lower monitor communication chamber  415  communicates with the second chamber  42  via the lower communication hole  41 A, and the upper monitor communication chamber  416  communicates with the second chamber  42  via the upper communication hole  41 B. That is, the monitor pipe  36  communicates with an upper end side and a lower end side of the second chamber  42 , and an ink level in the monitor pipe  36  is linked with an ink level in the second chamber  42 . 
     The monitor pipe  36  is formed of a transparent resin tube. Accordingly, a user can know the ink level in the second chamber  42  by visually confirming the monitor pipe  36 . In this embodiment, the monitor pipe  36  stands on a front side of the liquid supply unit  3 . Thus, the user can know the ink level in each second chamber  42  by visually confirming the monitor pipe  36  of each liquid supply unit  3  from a side forward of the unillustrated carriage on which the plurality of liquid supply units  3  are arranged in parallel in the lateral direction. 
     A spring seat  417  formed of a cylindrical cavity projects leftward near a vertical center of the first chamber  41 . The spring seat  417  is a cavity for housing a biasing spring  45  to be described later, and open toward the second chamber  42 . The first chamber  41  is set to surround a substantially half of an outer peripheral wall of this spring seat  417 . A spacer chamber  418  is provided behind the spring seat  417 . The spacer chamber  418  is provided to make a volume of the first chamber  41  as small as possible. If the volume of the first chamber  41  increases, the amount of the stored ink increases. A swinging force is applied to the liquid supply unit  3  when the carriage carrying this liquid supply unit  3  moves. If the weight of the ink increases, the atmospheric pressure detection film  7  and the sealing film  7 A may be peeled or broken by an inertial force. Note that if there is no such concern, the spacer chamber  418  may be omitted and, for example, the first chamber  41  may surround the spring seat  417 . 
     The communication opening  43  is arranged at a position above the spring seat  417  in the first chamber  41 . A hollow cylindrical boss portion  419  projects leftward from the tank portion base plate  310  in the first chamber  41 . The communication opening  43  is provided to penetrate through this boss portion  419  in the lateral direction. The first chamber  41  is a chamber in which a decompression process and the like are not performed and to which the pressure P=ρgh by the water head difference is applied in addition to the atmospheric pressure. If the ink flows into the first chamber  41  from the inflow portion  412 , the ink starts being successively pooled in the bypass communication chamber  413  and the return communication chamber  414 . If the ink level exceeds the communication opening  43 , the ink can be supplied to the second chamber  42  through the communication opening  43 . Further, if the pump  9  is operated, the ink stored in the first chamber  41  is sucked through the upstream bypass pipe BP 1  and the high-pressure ink is supplied toward the head unit  21  through the downstream bypass pipe BP 2  and the downstream pipe  34 . 
     Mainly with reference to  FIGS. 10A to 10C and 11 , the second chamber  42  has a circular shape when viewed from right. The pressing member  5  and the on-off valve  6  described above and the biasing spring  45  and a lever member  46  (operating member) to be described later are assembled with this second chamber  42 .  FIG. 10A  shows a state where these four members are assembled with the second chamber  42 ,  FIG. 10B  is a state where the pressing member  5  is removed, and  FIG. 10C  shows a state where the on-off valve  6  and the biasing spring  45  are further removed. 
     The second chamber  42  is defined by a second partition wall  421  projecting rightward from the tank portion base plate  310 . The second partition wall  421  is a wall having a hollow cylindrical shape. The second chamber  42  is in such a positional relationship as to face the first chamber  41  located on the left side across the tank portion base plate  310 . The aforementioned spring seat  417  is provided by recessing the tank portion base plate  310  at a center position of a region surrounded by the hollow cylindrical second partition wall  421 , i.e. at a position concentric with the second partition wall  421 . The biasing spring  45  is housed in a recess of this spring seat  417 . The communication opening  43  is arranged on the spring seat  417  on a vertical line passing through a center point of the spring seat  417 . 
     The lever member  46  for venting air in the second chamber  42  is arranged on an upper end part  422  of the second chamber  42 . The second partition wall  421  is perforated with a supply hole  42 H in a lower end part  423  (lowermost part of the second chamber  42 ). The upstream end  341  of the downstream pipe  34  communicates with this supply hole  42 H via the backflow prevention mechanism  38 . The second chamber  42 , the backflow prevention mechanism  38  and the downstream pipe  34  are so arranged in the vertical direction that the backflow prevention mechanism  38  is located below the second chamber  42  to correspond to the supply hole  42 H and the joint part a of the downstream pipe  34  and the downstream end of the bypass pipe  32 P (downstream bypass pipe BP 2 ) is located below the backflow prevention mechanism  38 . The ink stored in the second chamber  42  is supplied to the downstream pipe  34  through the supply hole  42 H and the backflow prevention mechanism  38  while being sucked by the ink ejecting portion  22 . The backflow prevention mechanism  38  is described in detail later. 
     A pair of front and rear supporting plates  424  project rightward from the tank portion base plate  310  near the lower end part  423 . Each of the pair of supporting plates  424  includes a pivotally supporting portion  425  for pivotally supporting the pressing member  5  to be described later. The aforementioned lower communication hole  41 A is perforated in the tank portion base plate  310  at a position in front of and adjacent to the front supporting plate  424 . Further, the upper communication hole  41 B is perforated in the tank portion base plate  310  near the upper end part  422 . 
     A boss portion  426  and holding frames  427  project upward on the upper end part  422  of the second chamber  42 . The boss portion  426  is a tubular body extending vertically upward and internally provided with a boss hole  42 A ( FIG. 18 ), which is an opening allowing the second chamber  42  to communicate with the atmosphere. The holding frames  427  are composed of a pair of frame pieces arranged to sandwich the boss portion  426  in the front-rear direction. Locking claws  428  bent in directions to face each other are provided on the upper ends of the respective holding frames  427 . The boss portion  426  and the holding frames  427  constitute a part of the air vent mechanism  37 , and the lever member  46  ( FIG. 16 ) to be described in detail later is assembled with these. 
     With reference to  FIG. 9 , the filter chamber  44  is arranged on a side upstream of the first chamber  41  in the ink supply direction. The filter chamber  44  constitutes a path for supplying the ink from the ink cartridge IC to the first chamber  41  together with the upstream pipe  33 . The filter chamber  44  has an inner wall surface  441  defining a rectangular tubular space having a rectangular cross-section in the lateral direction and extending in the ink supply direction. The filter chamber  44  houses a filter member  442  for removing foreign substances in the ink, a holding member  443  for holding this filter member  442  and a coil spring  446  for fixing the filter member  442 . An inflow opening for the ink is perforated in a ceiling wall of the filter chamber  44 . The downstream end  332  of the upstream pipe  33  is connected to a receiving plug standing on the ceiling wall to correspond to this inflow opening. The ink flows into the filter chamber  44  and, after having foreign substances removed by the filter member  442 , flows into the first chamber  41  through the inflow portion  412 . 
     With reference to  FIGS. 8 and 11 , an opening in a left surface side of the first chamber  41  is sealed by the sealing film  7 A made of resin. The sealing film  7 A has an outer shape capable of covering not only the first chamber  41 , but also the bypass communication chamber  413 , the return communication chamber  414 , the lower monitor communication chamber  415 , the upper monitor communication chamber  416  and the filter chamber  44 . A peripheral edge part of the sealing film  7 A is welded or bonded to opening end surfaces of the first partition wall  411  and other walls, whereby the sealing film  7 A seals the openings of the respective chambers. 
     An opening in a right surface side of the second chamber  42  is sealed by the atmospheric pressure detection film  7  formed of a film member made of flexible resin. The atmospheric pressure detection film  7  has a circular outer shape matching a wall shape of the second partition wall  421  of the second chamber  42  when viewed from right. A peripheral edge part of the atmospheric pressure detection film  7  is welded or bonded to an opening end surface of the second partition wall  421  to seal the opening of the second chamber  42 . Note that the atmospheric pressure detection film  7  is welded or bonded without particular tension being applied thereto. 
     The pump portion  32  is arranged behind, oblique below and adjacent to the tank portion  31  and includes a pump cavity  321  for housing the pump  9  and a cam shaft insertion hole  322  into which an unillustrated cam shaft for pivotally supporting an eccentric cam  91  ( FIG. 19 ) of the pump  9  is inserted. The pump cavity  321  is a hollow cylindrical cavity arranged in the pump portion housing  320 . The cam shaft insertion hole  322  is a boss hole provided at a position concentric with the pump cavity  321 . An opening in a right surface side of the pump cavity  321  is sealed by a pump cover  323  as shown in  FIG. 8B . Two positioning pins  391  project on the rear surface of the pump portion housing  320  and a rib  392  projects on the lower surface thereof. These positioning pins  391  and rib  392  function as a positioning member in mounting the liquid supply unit  3  on the carriage. 
     The liquid supply unit  3  of this embodiment is integrally formed with the tank portion  31  and the pump portion  32 . Specifically, the tank portion base plate  310  serving as the board of the tank portion  31  and the pump portion housing  320  with the pump cavity  321  are integrated, and the pump  9  for pressurized purging is mounted in the liquid supply unit  3  itself. In this way, the device configuration of the carriage can be made compact and simple. 
     [Details of Negative Pressure Supply Mechanism] 
     Next, a negative pressure supply mechanism for supplying the ink from the first chamber  41  to the second chamber  42  as the ink in the second chamber  42  decreases is described in detail. The negative pressure supply mechanism includes the pressing member  5 , the on-off valve  6  and the atmospheric pressure detection film  7 , whose operations were outlined on the basis of  FIG. 3  above, and further includes the biasing spring  45  (biasing member). The on-off valve  6  is arranged in the communication opening  43  and the posture thereof changes between the closing posture for closing the communication opening  43  and the opening posture for opening the communication opening  43 . The biasing spring  45  biases the on-off valve  6  in a direction toward the closing posture. The pressing member  5  can press the on-off valve  6  in a direction toward the opening posture. The atmospheric pressure detection film  7  is displaced based on a negative pressure generated as the ink in the second chamber  42  decreases, and transmits a displacement force thereof to the pressing member  5 . 
     &lt;Pressing Member&gt; 
       FIGS. 12A and 12B  are perspective views of the pressing member  5  viewed in different directions, and the on-off valve  6  is also shown therein. The pressing member  5  is a member rotatably arranged in the second chamber  42 . The pressing member  5  includes a disk portion  51  (flat plate portion) formed of a circular flat plate, a pair of arm portions  52  extending downward from a lower end side  5 C of the disk portion  51 , pivot portions  53  provided on extending end parts of the respective arm portions  52 , a pair of link bosses  54  (pressing portion) arranged on an upper end side  5 D of the disk portion  51  and receiving slopes  55  (operated portion) configured to interfere with the lever member  46  and receive an operating pressing force. The pair of pivot portions  53  are pivotally supported on the pivotally supporting portions  425  ( FIG. 10 ) of the pair of supporting plates  424  arranged in the second chamber  42 . In this way, the disk portion  51  is rotatable about axes of the pivot portions  53 . 
     The disk portion  51  is a disk having a diameter, which is about ½ of an inner diameter of the hollow cylindrical second partition wall  421  defining the second chamber  42 . The disk portion  51  pivotally supported by the pivotally supporting portions  425  is arranged to be substantially concentric with the second partition wall  421 . The disk portion  51  has a first surface  51 A facing the atmospheric pressure detection film  7  and a second surface  51 B facing the on-off valve  6  (facing the tank portion base plate  310 ). A spring fitting projection  511  is provided to project from the second surface  51 B in a radial center of the disk portion  51 . A right end part of the biasing spring  45  formed of a coil spring is fit to a part of the spring fitting projection  511  on the side of the second surface  51 B. Note that a region of the spring fitting projection  511  is a cylindrical recess on the side of the first surface  51 A. 
     The disk portion  51  includes a pressure receiving portion  5 A for receiving a displacement force from the atmospheric pressure detection film  7  and a biased portion  5 B for receiving a biasing force from the biasing spring  45 . The pressure receiving portion  5 A is set at a predetermined position of the first surface  51 A of the disk portion  51 . In this embodiment, the pressure receiving portion  5 A is a region of a peripheral edge part of the spring fitting projection  511  on the first surface  51 A. The biased portion  5 B is a region of the spring fitting projection  511 , to which the biasing spring  45  is fit, on the side of the second surface  51 B. Specifically, the biased portion  5 B is set at a position corresponding to the pressure receiving portion  5 A. 
     If the pressure receiving portion  5 A receives no displacement force from the atmospheric pressure detection film  7 , the disk portion  51  is in a state close to an upright state. However, the right end of the biasing spring  45  is in contact with the biased portion  5 B and the first surface  51 A is in contact with the inner surface of the atmospheric pressure detection film  7  by a biasing force of the biasing spring  45 . On the other hand, if the pressure receiving portion  5 A receives a displacement force equal to or larger than the biasing force of the biasing spring  45  from the atmospheric pressure detection film  7 , the disk portion  51  rotates leftward about the axes of the pivot portions  53  to be inclined leftward from the upright state. 
     The pair of arm portions  52  are arranged apart from each other in the front-rear direction on the lower end side  5 C of the disk portion  51 . Upper end parts  521  of the pair of arm portions  52  extend further upward than the lower end side  5 C of the disk portion  51  and are located below both side parts of the spring fitting projection  511 . Tip parts  522  of the pair of arm portions  52  respectively extend straight downward from the lower end side  5 C. The pivot portions  53  respectively project from the tip parts  522  in the front-rear direction. In particular, the pivot portion  53  projects forward from the front surface of the front tip part  522  and the pivot portion  53  projects from the rear surface of the rear tip part  523 , i.e. the pivot portions  53  project in directions separating from each other. The pivot portions  53  are fit into the pivotally supporting portions  425  of the supporting plates  424 . The provision of the pivot portions  53  on the tip parts  522  of the arm portions  52  contributes to an increase of a swing width of the upper end side  5 D of the disk portion  51  when the pressing member  5  rotates about the pivot portions  53 . 
     The pair of pivot portions  53  are arranged on an axis of rotation 5AX extending in the front-rear direction. The front pivot portion  53  and the rear pivot portion  53  are arranged at a predetermined distance D from each other. That is, the pair of pivot portions  53  are arranged apart from each other across a part corresponding to a central region of the disk portion  51  in a plane direction. The distance D can be set to about 40% to 90% of the diameter of the disk portion  51 . In this way, pivot fulcrums formed by the pair of pivot portions  53  are spaced widely from each other across the central region of the disk portion  51 . Thus, the disk portion  51  rotating about the pivot fulcrums is unlikely to be twisted about an axis perpendicular to the axis of rotation 5AX. Therefore, a rotational movement of the disk portion  51  can be stabilized. 
     The pair of link bosses  54  project leftward from the second surface  51 B near the upper end side  5 D of the disk portion  51 . In particular, the disk portion  51  is provided with a cutout  512  extending radially inward with the upper end side  5 D serving as an opening edge. The link bosses  54  formed of rectangular flat plates respectively stand on front and rear end edges facing a space of the cutout  512 . Each link boss  54  includes a link hole  541 . This link hole  541  is used to link the pressing member  5  and the on-off valve  6 . By this linkage, an opening/closing operation of the on-off valve  6  is linked with a rotational movement of the pressing member  5 . 
     In other words, the link bosses  54  serve as pressing member for pressing the on-off valve  6  to move in the lateral direction according to a rotational movement of the pressing member  5  rotating about the pivot portions  53 . The pair of link bosses  54  are arranged on the upper end side  5 D separated from the pair of pivot portions  53  arranged on the lower end side  5 C by a predetermined distance. That is, the link bosses  54  serving as the pressing member are arranged at counter positions on the disk portion  51  with respect to the pivot portions  53  forming the pivot fulcrums. Thus, movement amounts of the link bosses  54  during the rotation of the pressing member  5  and a movement amount of the on-off valve  6  linked to the link bosses  54  can be increased. 
     In a relationship of the pressure receiving portion  5 A or the biased portion  5 B (point of force) and the pivot portions  53  (fulcrum), the link bosses  54  (point of action) are arranged at positions more distant from the pivot portions  53  than the pressure receiving portion  5 A and the biased portion  5 B. In other words, the link bosses  54  are arranged on the upper end side  5 D of the disk portion  51  to face the pivot portions  53  across the pressure receiving portion  5 A and the biased portion  5 B. By adopting such an arrangement, a movement force received by the pressure receiving portion  5 A or the biased portion  5 B can be given to the link bosses  54  while being amplified by a separating distance from these. 
     &lt;On-Off Valve&gt; 
     Next, the on-off valve  6  is described. The on-off valve  6  is arranged in the communication opening  43  allowing communication between the first and second chambers  41 ,  42 . The on-off valve  6  opens or closes the communication opening  43  by moving in the lateral direction in the communication opening  43 , following a rotational movement of the pressing member  5  about the pivot portions  53 . To follow the rotational movement, the on-off valve  6  is linked to the link bosses  54  of the disk portion  51 . 
       FIG. 13A  is a perspective view of the on-off valve  6  and  FIG. 13B  is an exploded perspective view of the on-off valve  6 .  FIG. 14A  is a sectional view along line XIV-XIV of  FIG. 8A  and  FIG. 14B  is an enlarged view of a part A 1  of  FIG. 14A . The on-off valve  6  is an assembly composed of a valve holder  61  and an umbrella valve  66  held by this valve holder  61 . The communication opening  43  is a cylindrical hole penetrating through the tank portion base plate  310  and the boss portion  419  and includes a large-diameter portion  43 A, a small-diameter portion  43 B having a smaller inner diameter than the large-diameter portion  43 A and a step portion  43 C based on a diameter difference between the both. 
     The valve holder  61  is a semi-cylindrical member with a first end part  611  located on the side of the first chamber  41  (left side) and a second end part  612  located on the side of the second chamber  42  (right side) in a state mounted in the communication opening  43 . The valve holder  61  includes a tubular portion  62  on the side of the first end part  611 , a flat plate portion  63  on the side of the second end part  612 , an intermediate portion  64  located between the tubular portion  62  and the flat plate portion  63  and link pins  65  disposed on the flat plate portion  63 . The umbrella valve  66  is held on the side of the first end part  611  of the valve holder  61 . 
     The tubular portion  62  is a tubular part having a largest outer diameter in the valve holder  61 . The tubular portion  62  includes a guide surface  62 S, which is an outer peripheral surface of the tubular portion  62 , a flow passage cutout  621  formed by cutting a part of the tubular portion  62  in a circumferential direction, and a holding groove  622  annularly recessed in the inner periphery of the tubular portion  62 . The tubular portion  62  is housed in the large-diameter portion  43 A of the communication opening  43  and the guide surface  62 S is guided by the inner surface of the large-diameter portion  43 A when the on-off valve  6  moves in the lateral direction. The flow passage cutout  621  serves as a flow passage in which the ink flows when the on-off valve  6  is in the opening posture. The holding groove  622  is a groove for locking a locking sphere portion  663  of the umbrella valve  66 . 
     The intermediate portion  64  is a tubular part having a smaller outer diameter than the tubular portion  62 . The intermediate portion  64  includes a releasing portion  641 , which is a releasing part connected to the flow passage cutout  621 , and a pin housing portion  642  for housing a pin portion  662  of the umbrella valve  66 . The intermediate portion  64  is housed in the small-diameter portion  43 B of the communication opening  43 , and the outer peripheral surface thereof is guided by the inner surface of the small-diameter portion  43 B. An annular contact portion  62 A formed by a step based on an outer diameter difference between the tubular portion  62  and the intermediate portion  64  is present on a boundary part between the tubular portion  62  and the intermediate portion  64 . The annular contact portion  62 A faces and comes into contact with the step portion  43 C of the communication opening  43 . 
     The flat plate portion  63  is a part projecting rightward from the communication opening  43  with the on-off valve  6  mounted in the communication opening  43 . The flat plate portion  63  has a pair of front and back flat surfaces extending in the lateral direction. The link pins  65  respectively project from the pair of flat surfaces. As shown in  FIG. 12B , the link pins  65  are fit into the link holes  541  provided in the link bosses  54  of the pressing member  5 . By this fitting, the pressing member  5  and the on-off valve  6  are linked, and a rotational motion of the pressing member  5  about the pivot portions  53  can be translated into a linear motion of the on-off valve  6 . 
     The umbrella valve  66  is an article made of rubber and includes an umbrella portion  661 , the pin portion  662  extending rightward from the umbrella portion  661  and the locking sphere portion  663  integrally provided to the pin portion  662 . The umbrella portion  661  has an umbrella diameter larger than the inner diameter of the large-diameter portion  43 A of the communication opening  43 . A peripheral edge part on an inner side (right surface side) of the umbrella portion  661  is a sealing surface  67 . The sealing surface  67  can seal the communication opening  43  by coming into contact with a sealing wall surface  43 S, which is a peripheral wall surface of the communication opening  43  and a projecting end surface of the boss portion  419  (closing posture). On the other hand, if the sealing surface  67  is separated from the sealing wall surface  43 S, the sealed state is released (opening posture). Note that the umbrella shape of the umbrella portion  661  is inverted if a predetermined pressure is applied to the right surface side ( FIG. 23 ). 
     The pin portion  662  is a rod-like part extending in the lateral direction and serving as a support column of the umbrella portion  661 . The pin portion  662  is inserted into the tubular portion  62  of the valve holder  61  and the pin housing portion  642  of the intermediate portion  64 . That is, the umbrella portion  661  comes into contact with the first end part  611  of the valve holder  61 , whereas the pin portion  662  can be fit into an inner tubular portion of the valve holder  61 . The locking sphere portion  663  is a spherically bulging part near the left end of the pin portion  662  and to be fit into the holding groove  622 . By fitting the locking sphere portion  663  into the holding groove  622 , the umbrella valve  66  is held in the valve holder  61  with a lateral movement restricted. Specifically, the umbrella valve  66  moves integrally with the valve holder  61  in the lateral direction. 
     &lt;Biasing Spring&gt; 
     The biasing spring  45  is a coil spring disposed between the second surface  51 B of the disk portion  51  and the tank portion base plate  310  and configured to support (bias) the second surface  51 B. In particular, as shown in  FIG. 14B , a right end side of the biasing spring  45  is fit to the spring fitting projection  511  of the disk portion  51  and a left end side is housed in the spring seat  417  recessed on the tank portion base plate  310 . When the pressure receiving portion  5 A of the disk portion  51  receives a leftward displacement force against a rightward biasing force of the biasing spring  45 , the disk portion  51  rotates leftward about the pivot portions  53 . If the displacement force is not received, the disk portion  51  is maintained in an upright posture by the biasing force. 
     &lt;Operation of On-Off Valve&gt; 
     Next, an opening/closing operation of the on-off valve  6  is described.  FIGS. 14A and 14B  show a state where the on-off valve  6  is in the closing posture. In this state, the atmospheric pressure detection film  7  does not generate such a displacement force as to rotate the pressing member  5  (disk portion  51 ), i.e. the sum of a spring force (biasing force) of the biasing spring  45  and an internal pressure of the second chamber  42  exceeds the atmospheric pressure. Although the second chamber  42  is at a negative pressure, the biasing spring  45  biases the biased portion  5 B of the disk portion  51  rightward with a biasing force exceeding a displacement force of the atmospheric pressure detection film  7  by the negative pressure. Thus, the disk portion  51  does not rotate about the pivot portions  53  and is maintained in the aforementioned upright posture. 
     In this case, the on-off valve  6  linked to the pressing member  5  at the link bosses  54  is in the closing posture to be located on a rightmost side. Specifically, the valve holder  61  is pulled rightward via the link bosses  54  by the biasing force of the biasing spring  45 . Thus, the annular contact portion  62 A of the valve holder  61  butts against the step portion  43 C of the communication opening  43  and the sealing surface  67  of the umbrella valve  66  comes into contact with the sealing wall surface  43 S. Thus, the communication opening  43  is sealed by the umbrella valve  66 . The biasing spring  45  can be said to indirectly bias the on-off valve  6  in the direction toward the closing posture by biasing the disk portion  51  rightward. 
       FIG. 15A  is a sectional view, corresponding to  FIG. 14A , showing a state where the on-off valve  6  is in the opening posture, and  FIG. 15B  is an enlarged view of a part A 2  of  FIG. 15A . If the ink ejecting portion  22  continues an ink droplet ejecting operation from the state of  FIG. 14 , a negative pressure degree of the second chamber  42 , which is a sealed space, gradually increases as the ink decreases. If the pressure in the second chamber  42  eventually reaches a negative pressure exceeding a predetermined threshold value, the atmospheric pressure detection film  7  comes to apply a pressing force acting against the biasing force of the biasing spring  45  (pressing force exceeding the biasing force) to the pressure receiving portion  5 A of the disk portion  51 . Specifically, the sum of the spring pressure of the biasing spring  45  and the internal pressure of the second chamber  42  becomes lower than the atmospheric pressure. 
     In this case, the disk portion  51  rotates leftward about the pivot portions  53  against the biasing force of the biasing spring  45 . By this rotation, the link bosses  54  generate a pressing force PF for moving the on-off valve  6  leftward, thereby changing the posture of the on-off valve  6  to the opening posture. That is, the pressing force is transmitted from the link holes  541  of the link bosses  54  to the link pins  65  of the valve holder  61 , and the valve holder  61  linearly moves leftward while the guide surface  62 S is guided by the inner surface of the communication opening  43 . According to this movement, the umbrella valve  66  also moves leftward and the sealing surface  67  thereof is separated from the sealing wall surface  43 S. That is, a gap G is formed between the sealing surface  67  and the sealing wall surface  43 S. Thus, the sealing of the communication opening  43  by the umbrella valve  66  is released. 
     If the on-off valve  6  is set in the opening posture, the ink flows into the second chamber  42  from the first chamber  41  due to a pressure difference between the first chamber  41  having a pressure, which is the sum of the atmospheric pressure and ρgh, and the second chamber  42  having the advanced negative pressure degree as indicated by arrows F in  FIG. 15B . Specifically, the ink flows into the second chamber  42  through a flow passage composed of the gap G between the sealing surface  67  of the umbrella valve  66  and the sealing wall surface  43 S, the flow passage cutout  621  prepared in the tubular portion  62  of the valve holder  61  and the releasing portion  641  prepared in the intermediate portion  64 . 
     If the ink further flows into the second chamber  42 , the negative pressure degree of the second chamber  42  is gradually mitigated. If the sum of the spring pressure of the biasing spring  45  and the internal pressure of the second chamber  42  eventually becomes larger than the atmospheric pressure, the disk portion  51  is pushed back rightward by the biasing force of the biasing spring  45 . Specifically, if the pressure in the second chamber  42  reaches a negative pressure below the predetermined threshold value, the disk portion  51  is pressed by the biasing force of the biasing spring  45  and rotates rightward about the pivot portions  53 . In this way, the on-off valve  6  also linearly moves rightward by being pulled by the link bosses  54 . At a certain stage, the annular contact portion  62 A of the valve holder  61  butts against the step portion  43 C of the communication opening  43  and the sealing surface  67  of the umbrella valve  66  comes into contact with the sealing wall surface  43 S. Thus, the on-off valve  6  returns to the closing posture. 
     [Air Vent Mechanism for Second Chamber] 
     Next, the air vent mechanism  37  attached to the second chamber  42  is described with reference to  FIGS. 16A to 18B  in addition to  FIG. 10A  already described.  FIGS. 16A and 16B  are perspective views of the lever member  46  constituting the air vent mechanism  37  and  FIG. 16C  is an exploded perspective view of the lever member  46 .  FIGS. 17A and 17B  are perspective views showing a positional relationship of the pressing member  5 , the on-off valve  6  and the lever member  46 .  FIGS. 18A and 18B  are sectional views showing the same cross-section as  FIG. 14A  and explaining an air vent operation of the lever member  46 . As described above, the air vent mechanism  37  is used to vent air in initially filling the ink into the second chamber  42  and to remove air bubbles generated from the ink (during the execution of the second circulation mode) during initial usage, after maintenance and the like. 
     The air vent mechanism  37  includes the lever member  46 , a seal ring  46 C and a stopper  47  in addition to the already described boss portion  426  projecting on the upper end part  422  of the second chamber  42 . The boss portion  426  projects from the uppermost end of the second partition wall  421  defining the second chamber  42  as shown in  FIG. 10A  and includes an opening allowing communication between the second chamber  42  and the atmosphere, i.e. the boss hole  42 A having a circular cross-section and serving as an air vent hole. By providing the boss hole  42 A at an uppermost position of the second chamber  42 , the air in the second chamber  42  can be reliably vented. The boss portion  426  includes a large-diameter portion  426 A located right above the upper end part  422  and a small-diameter portion  426 B connected to and above the large-diameter portion  426 A. An inner diameter of the boss hole  42 A is larger in the large-diameter portion  426 A than in the small-diameter portion  426 B. 
     As shown in  FIG. 16C , the lever member  46  has a shovel-like shape with a rod-like member  461  to be partially inserted into the boss hole  42 A and a pressing piece  464  connected to and below the rod-like member  461 . The lever member  46  is one type of a valve member whose posture is changed between a sealing posture for sealing the boss hole  42 A and a releasing posture for releasing the boss hole  42 A. In this embodiment, the lever member  46  is configured such that a posture changing operation thereof is linked with the posture changing operation of the on-off valve  6  via the pressing member  5 . Specifically, with the lever member  46  held in the sealing posture, the on-off valve  6  is allowed to be set in the closing posture. With the lever member  46  held in the releasing posture, the posture of the on-off valve  6  is changed from the closing posture to the opening posture. 
     The rod-like member  461  of the lever member  46  is a cylindrical body having an outer diameter smaller than a hole diameter of the boss hole  42 A and has an upper end part  462  and a lower end part  463 . The upper end part  462  serves as an input portion for receiving an operating pressing force for pressing the lever member  46  downward from a user. The lower end part  463  is linked to the pressing piece  464 . As shown in  FIGS. 17A and 17B , the pressing piece  464  functions as a transmitting portion for transmitting the operating pressing force given to the upper end part  462  to the receiving slopes  55  of the pressing member  5 . An intermittent projection portion  463 A including a plurality of small projections annularly arranged in a circumferential direction of the rod-like member  461  is provided at a position somewhat above the lower end part  463 . 
     The pressing piece  464  has a pressing slope  465  inclined with respect to an axis of the rod-like member  461  and a lower end edge  466  extending in the front-rear direction on a lowermost end. The pressing slope  465  is a slope extending upward with the lower end edge  466  as a start point. The pressing slope  465  and the lower end edge  466  serve as parts which interfere with the pair of front and rear receiving slopes  55  of the pressing member  5  when the lever member  46  receives the operating pressing force. A width of the pressing slope  465  in the front-rear direction is set longer than an interval between the pair of receiving slopes  55 . The pressing slope  465  and the lower end edge  466  come into contact with the receiving slopes  55  to transmit the operating pressing force to the pressing member  5 , whereby the pressing member  5  rotates leftward about the pivot portions  53  and changes the posture of the on-off valve  6  from the closing posture to the opening posture. 
     An upper engaging groove  467 A and a lower engaging groove  467 B arranged at a distance in the vertical direction are formed near the upper end part  462  of the rod-like member  461 . An upper washer  46 A is fit into the upper engaging groove  467 A, and a lower washer  46 B is fit into the lower engaging groove  467 B. Further, a sealing groove  468  is provided near the lower end part  463 . An outer diameter of the lower end part  463  is set larger than those of other parts of the rod-like member  461 , and a space between the lower end part  463  and the intermittent projection portion  463 A serves as the sealing groove  468 . Further, air vent longitudinal grooves  461 A formed by recessed grooves are provided over the entire length of the rod-like member  461  in the front-rear direction. The positions of these air vent longitudinal grooves  461 A are aligned with those of valley parts of the intermittent projection portion  463 A in the circumferential direction. 
     The seal ring  46 C and the stopper  47  are mounted on the rod-like member  461 . The seal ring  46 C is an O-ring having an inner diameter somewhat larger than the diameter of the rod-like member  461 . The seal ring  46 C is fit on the rod-like member  461  and fit into the sealing groove  468 . The outer peripheral surface of the seal ring  46 C slides in contact with an inner peripheral surface IS of the large-diameter portion  426 A of the boss portion  426  with the seal ring  46 C mounted in the sealing groove  468 . The stopper  47  is a substantially rectangular plate member and includes a rotation hole  47 H into which the rod-like member  461  is inserted. The stopper  47  is mounted at a position near the upper end part  462  and between the upper and lower engaging grooves  467 A and  467 B. The upper and lower washers  46 A,  46 B are respectively fit into the upper and lower engaging grooves  467 A,  467 B to sandwich the stopper  47  and restrict a movement of the stopper  47  in an axial direction. 
     The stopper  47  is rotatable about the axis of the rod-like member  461  while being sandwiched by the upper and lower washers  46 A,  46 B. The stopper  47  is a member planned to come into contact with upper surfaces  428 A or lower surfaces  428 B ( FIGS. 18A and 18B ) of the pair of locking claws  428  of the holding frames  427  according to a vertical movement of the lever member  46 . During the above vertical movement, the stopper  47  is so rotated that a longitudinal direction is aligned with the lateral direction and passes through a clearance between the pair of locking claws  428 . The stopper  47  is formed with a pin hole  471  and a locking recess  472 . At least when the stopper  47  comes into contact with the upper surfaces  428 A, a pin member  48  in the form of a split pin is fit into the pin hole  471  and the locking recess  472  as shown in  FIG. 10A , the rotation of the stopper  47  is stopped and the stopper  47  is retained, i.e. the stopper  47  is fixed. The stopper  47 , the pin member  48  and the pair of locking claws  428  function as a fixing mechanism for fixing the posture of the lever member  46 . 
     Next, the operation of the lever member  46  is described.  FIG. 18A  is a sectional view showing a state before the lever member  46  is operated and  FIG. 18B  is a sectional view showing a state where the air in the second chamber  42  is vented by the operation of the lever member  46 .  FIG. 18A  shows a state where the upper end part  462  of the lever member  46  is receiving no operating pressing force, i.e. a state where the lever member  46  is in the sealing posture for sealing the boss hole  42 A. On the other hand,  FIG. 18B  shows a state where the upper end part  462  is pressed downward to apply an operating pressing force, i.e. a state where the lever member  46  is in the releasing posture for releasing the boss hole  42 A. 
     The sealing posture is set by fixing the stopper  47  and the locking claws  428  by the pin member  48  with the stopper  47  held in contact with the upper surfaces  428 A of the locking claws  428 . By this fixing, the lever member  46  is lifted upward. In this state, the intermittent projection portion  463 A and the lower end part  463  of the rod-like member  461  are housed in the large-diameter portion  426 A of the boss portion  426 . That is, the outer peripheral surface of the sealing ring  46 C is in contact with the inner peripheral surface IS of the large-diameter portion  426 A. Thus, the boss hole  42 A is sealed. The pressing piece  464  (pressing slope  465  and lower end edge  466 ) of the lever member  46  are separated from the receiving slopes  55  of the pressing member  5  and is not applying any force to the pressing member  5 . Thus, the on-off valve  6  is maintained in the closing posture. 
     On the other hand, if the lever member  46  is lowered by receiving the operating pressing force and set in the opening posture, the seal ring  46 C is separated from the inner peripheral surface IS as the intermittent projection portion  463 A and the lower end part  463  are also lowered. In this way, air passages formed by the valley parts of the intermittent projection portion  463 A and the air vent longitudinal grooves  461 A of the rod-like member  461  communicate with the space in the second chamber  42 . That is, the boss hole  42 A is released and the second chamber  42  communicates with outside air. Thus, the air staying in the second chamber  42  can be exhausted to outside through the boss hole  42 A. 
     Further, if the lever member  46  is set in the releasing posture, the operating pressing force is transmitted to the pressing member  5 . As shown in  FIG. 18B , the pressing slope  465  and the lower end edge  466  press the receiving slopes  55 . If the receiving slopes  55  are pressed, the pressing member  5  (disk portion  51 ) rotates leftward about the axes of the pivot portions  53 . As described above, if the pressing member  5  rotates leftward, the on-off valve  6  is pressed leftward via the link bosses  54  and the posture of the on-off valve  6  is changed from the closing posture to the opening posture. In this way, the sealing of the communication opening  43  is released and the first and second chambers  41 ,  42  communicate. 
     The releasing posture is set by the stopper  47  being pressed against the lower surfaces  428 B of the locking claws  428 . Specifically, in setting the releasing posture, the stopper  47  is pushed down to slip under the locking claws  428 . Since the pressing member  5  is rotated against the biasing force of the biasing spring  45  by the pressing piece  464  pressing the receiving slopes  55 , the biasing force of the biasing spring  45  is applied to the pressing piece  464 . That is, a biasing force acts on the lever member  46  to lift the lever member  46  upward. The stopper  47  is pressed against the lower surfaces  428 B of the locking claws  428  by this biasing force and the releasing posture is maintained. 
     As just described, if the lever member  46  is set in the releasing posture, an inlet for fluid (communication opening  43 ) and an outlet for fluid (boss hole  42 A) for the second chamber  42  are secured. Accordingly, an operation of filling the ink into the second chamber  42  from the first chamber  41  through the communication opening  43  while the air in the second chamber  42  is vented through the boss hole  42 A can be smoothly performed utilizing water head difference supply during initial usage. Further, if the amount of air in the second chamber  42  increases such as due to the generation of air bubbles from the ink (can be confirmed in the monitor pipe  36  due to a drop of the ink level in the second chamber  42 ), the air in the second chamber  42  can be easily vented by performing the ink circulation in the second circulation mode and setting the lever member  46  in the releasing posture. 
     In this embodiment, the posture of the on-off valve  6  is changed to the opening posture as the lever member  46  is set in the releasing posture, utilizing the pressing member  5  with the pressure receiving portion  5 A for receiving a displacement force from the atmospheric pressure detection film  7  and the link bosses  54  for pressing the on-off valve  6  by the displacement force received by the pressure receiving portion  5 A. That is, the inlet and outlet for fluid for the second chamber  42  can be secured by a one-touch operation of the lever member  46 . Thus, the user can easily perform the air vent operation of the second chamber  42 , for example, in the above second circulation mode. 
     [Backflow Prevention Mechanism] 
     Next, the configuration of the backflow prevention mechanism  38  for preventing a backflow of the ink pressurized by the pump  9  to the second chamber  42  in performing the pressurized purge mode described on the basis of  FIG. 6  is described.  FIG. 19  is a sectional view of the liquid supply unit  3  in the front-rear direction including a cross-section of the backflow prevention mechanism  38 ,  FIG. 20  is an exploded perspective view of the backflow prevention mechanism  38 , and  FIGS. 21A to 21C  are perspective views of the backflow prevention mechanism  38 .  FIGS. 22A and 22B  are enlarged views of a part A 3  of  FIG. 19 , wherein  FIG. 22A  is a sectional view showing a state of the backflow prevention mechanism  38  in the print mode and  FIG. 22B  is a sectional view showing a state of the backflow prevention mechanism  38  in the pressurized purge mode. 
     The backflow prevention mechanism  38  includes a valve conduit  81 , a branched head portion  82 , a spherical body  83 , a sealing member  84 , a coil spring  85  and an O-ring  86 . The valve conduit  81  is a member integral with the lower end part  423  of the second chamber  42  and the other components are assembled with the valve conduit  81 .  FIGS. 21A and 21B  are perspective views of the backflow prevention mechanism  38  excluding the valve conduit  81 , and  FIG. 21C  is a perspective view of the branched head portion  82  viewed from below. 
     The valve conduit  81  is a conduit extending vertically downward from the supply hole  42 H perforated in the lower end part  423  of the second chamber  42 , and integrated with the second partition wall  421 . The valve conduit  81  provides an ink flow passage linking the second chamber  42  and the downstream pipe  34  and constitutes a part of the ink supply passage from the second chamber  42  to the ink ejecting portion  22 . To lock the branched head portion  82 , locking pieces  811  project on the outer peripheral surface of the valve conduit  81  and a fitting annular projection  812  projects on the inner peripheral surface thereof. 
     The branched head portion  82  is a member for forming the joint part a described above on the basis of  FIGS. 3 to 7 . The branched head portion  82  includes a first inlet port  821 , a second inlet port  822 , an outlet port  823 , trunk portions  824 , locking windows  825 , cutouts  826  and fitting claws  827 . The first inlet port  821  is a port connected to the second chamber  42  and, in this embodiment, communicates with the second chamber  42  via the valve conduit  81 . The second inlet port  822  is a port connected to the downstream end of the bypass pipe  32 P (downstream bypass pipe BP 2 ). The outlet port  823  is a port connected to the upstream end  341  of the downstream pipe  34 . 
     The branched head portion  82  is a T-shaped pipe including a vertical portion  82 A extending vertically downward from a lower end side of the valve conduit  81  and a horizontal portion  82 B joining an intermediate part of the vertical portion  82 A in the horizontal direction. The upper end of the vertical portion  82 A is the first inlet port  821 , and a lower end side thereof is the outlet port  823 . The tip of the horizontal portion  82 B is the second inlet port  822 . In the aforementioned print mode, the ink is supplied to the downstream pipe  34  through the first inlet port  821 . On the other hand, in the pressurized purge mode, the ink is supplied to the downstream pipe  34  through the second inlet port  822 . 
     The trunk portions  824  are composed of a pair of arcuate pieces arranged to face each other on an outer side of the first inlet port  821  facing downward. The valve conduit  81  is inserted into a clearance between the pair of trunk portions  824  and the first inlet port  821 . The locking windows  825  are openings which are provided in the pair of trunk portions  824  and with which the locking pieces  811  of the valve conduit  81  are engaged. The cutouts  826  are parts formed by cutting parts of the peripheral wall of the tubular first inlet port  821  and securing the ink flow passage. The fitting claws  827  are hook-shaped parts projecting upward from the upper end of the first inlet port  821 , and engaged with the fitting annular projection  812  of the valve conduit  81 . That is, the branched head portion  82  is fixed to the valve conduit  81  by the engagement of the locking pieces  811  and the locking windows  825  on the inner periphery of the valve conduit  81  and the engagement of the fitting annular projection  812  and the fitting claws  827  on the outer periphery of the valve conduit  81 . An upper end edge  828  of the first inlet port  821  serves as a sphere receiving portion for receiving the spherical body  83  to be described next. 
     The spherical body  83  is housed into the valve conduit  81  movably in the ink supply direction and works as a valve. An outer diameter of the spherical body  83  is smaller than an inner diameter of the valve conduit  81  and even smaller than an inner diameter of the coil spring  85 . Various materials can be used as a material for forming the spherical body  83 , but the spherical body  83  is preferably made of a material having a specific weight equal to or less than twice the specific weight of the ink, particularly in a range of 1.1-fold to 1.5-fold of the specific weight of the ink. If a material in this range is used, the specific weight of the spherical body  83  is larger than that of the ink. Thus, the spherical body  83  can easily descend by its own weight in the valve conduit  81 , whereas the spherical body  83  can quickly ascend in the valve conduit  81  during pressurized purging since the specific weight of the spherical body  83  is close to that of the ink. 
     Generally, ink used in an ink jet printer is water-soluble solution and has a specific weight equal to or near 1. Thus, it is desirable to select a material having a specific weight less than 2 as the material of the spherical body  83 . Further, the above material desirably has properties such as chemical resistance and wear resistance not to be deteriorated even if the material is constantly in contact with the ink. From these perspectives, it is particularly preferable to use polyacetal (specific weight=1.42), polybutylene terephthalate (specific weight=1.31 to 1.38), polyvinyl chloride (specific weight=1.35 to 1.45) or polyethylene terephthalate (specific weight=1.34 to 1.39) as the material of the spherical body  83 . 
     The sealing member  84  is a ring-shaped sealing component to be seated on a seat portion  813  provided above the spherical body  83  and on an upper end side of the valve conduit  81  as shown in  FIGS. 22A and 22B . A ring inner diameter (through hole) of the sealing member  84  is set smaller than the outer diameter of the spherical body  83 . When the spherical body  83  is separated downward from this sealing member  84  as shown in  FIG. 22A , the valve conduit  81  is opened. On the other hand, when the spherical body  83  contacts the sealing member  84  as shown in  FIG. 22B , the valve conduit  81  is closed. 
     The coil spring  85  is a compression spring mounted in the valve conduit  81  such that an upper end part thereof comes into contact with the sealing member  84  and a lower end part comes into contact with the upper end edge  828  of the first inlet port  821  of the branched head portion  82 . The coil spring  85  biases the sealing member  84  toward the seat portion  813 , whereby the sealing member  84  is constantly pressed into contact with the seat portion  813 . Further, the spherical body  83  is housed inside the coil spring  85  and the coil spring  85  also functions to guide a movement of the spherical body  83  in the ink supply direction. Thus, a loose movement of the spherical body  83  in the valve conduit  81  can be restricted and a valve structure realized by movements of the spherical body  83  toward and away from the sealing member  84  can be stabilized. 
     The O-ring  86  seals butting parts of the valve conduit  81  and the branched head portion  82 . The O-ring  86  is fit on the outer peripheral surface of the first inlet port  821  and in contact with a projecting base portion  829  of the first inlet port  821 . 
       FIG. 19  shows the pump  9  housed in the pump portion  32 . The pump  9  is arranged in the bypass pipe  32 P and pressurizes the ink flowing in the bypass pipe  32 P. The pump  9  can feed the ink from the ink cartridge IC to the head unit  21  through the upstream pipe  33  and the downstream pipe  34 . In this embodiment, a tube pump including the eccentric cam  91  and a squeeze tube  92  is illustrated as the pump  9 . A rotational drive force is applied to this eccentric cam  91  through unillustrated drive gear and cam shaft. The squeeze tube  92  is arranged on the peripheral surface of the eccentric cam  91  and squeezed by the rotation of the eccentric cam  91  around a cam shaft  93  to feed the liquid (ink) in the tube from one end side toward the other end side. In this embodiment, the squeeze tube  92  is a tube integral with the bypass pipe  32 P. Specifically, one end side of the squeeze tube  92  serves as the upstream bypass pipe BP 1  communicating with the bypass communication chamber  413  of the first chamber  41 , the other end side serves as the downstream bypass pipe BP 2  communicating with the second inlet port  822  of the branched head portion  82 , and a central part serves as a squeezing portion arranged on the peripheral surface of the eccentric cam  91 . 
     As described above, the pump  9  is stopped in the print mode shown in  FIG. 3 . In this case, the eccentric cam  91  is stopped while squeezing the squeeze tube  92 , wherefore the ink supply passage passing through the bypass pipe  32 P is closed. On the other hand, the pump  9  is driven in the forward rotation direction in the first circulation mode shown in  FIG. 4 , the second circulation mode shown in  FIG. 5  and the pressurized purge mode shown in  FIG. 6 . In  FIG. 19 , a forward rotation direction of the eccentric cam  91  is a counterclockwise direction. By this forward drive of the pump  9 , the ink is sucked from the first chamber  41  through the upstream bypass pipe BP 1  and flows toward the backflow prevention mechanism  38 , which is the joint part a, from the downstream bypass pipe BP 2 . Note that if the pump  9  is driven in the reverse rotation direction, the second chamber  42  and the downstream pipe  34  are set to a negative pressure through the bypass pipe  32 P and the branched head portion  82  as in the decompression mode shown in  FIG. 7 . 
     Next, the operation of the backflow prevention mechanism  38  is described. In the print mode, the ink is supplied to the head unit  21  from the second chamber  42  along a supply route passing through the backflow prevention mechanism  38  and the downstream pipe  34 . In such a print mode, the spherical body  83  is separated downward from the sealing member  84  and seated on the upper end edge  828  of the branched head portion  82  as shown in  FIG. 22A . This relies on the fact that the specific weight of the spherical body  83  is larger than that of the ink and the spherical body  83  descends by its own weight. Further, it also contributes to the spherical body  83  being kept seated on the upper end edge  828  that, in the print mode, the supply route from the second chamber  42  to the downstream pipe  34  is maintained at the negative pressure and the ink present in the supply route is sucked every time the ink ejecting portion  22  of the head unit  21  discharges ink droplets. 
     Since the spherical body  83  is separated from the sealing member  84 , the supply hole  42 H is opened. Further, since the upper end edge  828  of the first inlet port  821  on which the spherical body  83  is seated is provided with the cutouts  826 , the ink passage is secured. Thus, the ink in the second chamber  42  can flow toward the downstream pipe  34  from the second chamber  42  through the branched head portion  82  as indicated by an arrow F 1  in  FIG. 22A . 
       FIG. 22B  is a sectional view showing a state of the backflow prevention mechanism  38  in the pressurized purge mode (and the first and second circulation modes). In the pressurized purge mode, the ink pressurized through the bypass pipe  32 P is supplied to the second inlet port  822  (joint part a) of the branched head portion  82  by the forward drive of the pump  9 . Thus, the pressurized ink is present inside the bypass pipe  32 P and a part of the downstream pipe  34  located downstream of the joint part a. In this case, the ink is pressurized to a high pressure exceeding 100 kPa. If such a high pressure is applied to the second chamber  42 , the atmospheric pressure detection film  7  defining a part of the second chamber  42  may be torn or an attached part to the second partition wall  421  may be peeled. 
     However, in this embodiment, the spherical body  83  is pressed to ascend by a pressure force applied to the joint part a and comes to contact the sealing member  84 . Specifically, the spherical body  83  is lifted up by being pressed, and fit into a ring of the sealing member  84 . By the contact of the spherical body  83  with the sealing member  84  pressed against the seat portion  813  by the coil spring  85 , the supply hole  42 H is closed. Specifically, out of the ink supply passage in the print mode, a part located upstream of the joint part a and the second chamber  42  are blocked from pressurization by the pressurized ink. Thus, the breakage of the atmospheric pressure detection film  7  and the like can be prevented. 
     Further, this embodiment also has an advantage that the ink trapping air is less likely to be supplied to the head unit  21 . If air dissolved into the ink and air mixed into the ink when the ink liquid is filled into the liquid supply unit  3  enter the head unit  21  while being trapped in the ink and further enter the individual passages  26  and the common passage  27  ( FIG. 2 ), the air may not be easily vented and may not be eliminated even if pressurized purging is performed. In this case, the ejection of the ink from the ink discharge holes  22 H is impeded. However, in this embodiment, the second chamber  42 , the backflow prevention mechanism  38  and the downstream pipe  34  are successively arranged from top to down in this order. Thus, air generated from the ink stored in the second chamber  42  or air mixed into the second chamber  42  does not move toward the backflow prevention mechanism  38  and the downstream pipe  34  located below. Therefore, the ink trapping air can be prevented from flowing to the head unit  21  and an ejection failure of the head unit  21  can be prevented. 
     Further, even if air enters the branched head portion  82  or the downstream pipe  34 , the air can be allowed to escape into the second chamber  42  from the vertical portion  82 A through the valve conduit  81  and the supply hole  42 H by the floating of air bubbles. Note that the above air can be discharged from the second chamber  42  by the air vent mechanism  37 . Thus, it can be prevented that an internal volume of the second chamber  42  is excessively occupied by the air. 
     [Double Protection Mechanism by Umbrella Valve] 
     As described above, in this embodiment, a backflow of the pressurized ink to the second chamber  42  in the pressurized purge mode is prevented by providing the backflow prevention mechanism  38 . However, a pressure force possibly acts on the second chamber  42  due to a certain trouble of the backflow prevention mechanism  38 , e.g. an operation failure of the spherical body  83 . In view of this point, a double protection mechanism, i.e. a mechanism for causing the on-off valve  6  to release a pressure, is provided in this embodiment. That is, the on-off valve  6  includes a pressure release mechanism for releasing the pressure from the second chamber  42  to the first chamber  41  if a pressure relationship that the second chamber  42  is at a negative pressure and the first chamber  41  is at an atmospheric pressure+ρgh at normal time is reversed and the pressure in the second chamber  42  becomes higher than that in the first chamber  41 . 
     The umbrella valve  66  of the on-off valve  6  functions as the above pressure release mechanism. As described on the basis of  FIGS. 14 to 15 , the umbrella valve  66  is configured such that the sealing surface  67  comes into contact with the sealing wall surface  43 S to seal the communication opening  43  if the second chamber  42  is at a negative pressure below the predetermined threshold value. In this way, the inflow of the ink from the first chamber  41  into the second chamber  42  is prohibited. On the other hand, if the pressure in the second chamber  42  reaches a negative pressure exceeding the predetermined threshold value, the umbrella valve  66  moves leftward together with the valve holder  61  linked to the pressing member  5  and the sealing surface  67  is separated from the sealing wall surface  43 S to release the communication opening  43  (release of sealing). In this way, the inflow of the ink from the first chamber  41  to the second chamber  42  is allowed. 
     In addition to this, the umbrella valve  66  singly releases the communication opening  43  if the pressure relationship of the second chamber  42  and the first chamber  41  is reversed due to a factor such as the application of the pressure of the pressurized ink to the second chamber  42  in the pressurized purge mode. That is, the umbrella valve  66  releases the sealed state of the communication opening  43  and releases the pressure in the second chamber  42  to the first chamber  41  without any assistance of being pressed by the pressing member  5 . Specifically, the umbrella shape of the umbrella portion  661  (sealing surface  67 ) of the umbrella valve  66  is inverted if a predetermined pressure is applied to the right surface side of the umbrella portion  661 . 
       FIG. 23A  is a sectional view showing a state where the umbrella valve  66  seals the communication opening  43  and  FIG. 23B  is a sectional view showing a state where the umbrella valve  66  releases the communication opening  43 . The state of  FIG. 23A  is equal to the state of  FIG. 14B  described above. The umbrella portion  661  has an umbrella shape convex leftward. Further, the valve holder  61  is located at a rightmost position by the biasing force of the biasing spring  45  and the annular contact portion  62 A thereof is stopped in contact with the step portion  43 C of the communication opening  43 . Therefore, the sealing surface  67  is in contact with the sealing wall surface  43 S. 
     The state of  FIG. 23B  shows a state where the umbrella shape of the umbrella portion  661  of the umbrella valve  66  is inverted by a pressure given from the side of the second chamber  42 . That is, the umbrella portion  661  is deformed to have an umbrella shape convex rightward. This inverted state is reached when the pressure in the second chamber  42  becomes higher than the pressure in the first chamber  41  by a predetermined value. In this embodiment, a case is assumed in which a high positive pressure by pressurized purging is applied to the second chamber  42  and, as a result, the pressure in the second chamber  42  becomes higher than the pressure in the first chamber  41  having the atmospheric pressure+ρgh. The predetermined value depends on an inversion pressure of the umbrella portion  661 . This inversion pressure is set at a value lower than burst strength of the atmospheric pressure detection film  7  or attachment strength of the atmospheric pressure detection film  7  to the second partition wall  421 . 
     If the second chamber  42  is pressurized, the pressing member  5  does not rotate leftward. That is, the pressing member  5  does not generate a pressing force for pressing the on-off valve  6  leftward. This is because the atmospheric pressure detection film  7  is displaced to bulge rightward due to a high pressure in the second chamber  42  and does not give a displacement force to the pressure receiving portion  5 A. Therefore, a state where the valve holder  61  is located at the rightmost position is maintained by the biasing force of the biasing spring  45 . 
     However, even if the valve holder  61  does not move, the sealing surface  67  is separated from the sealing wall surface  43 S and a gap g is formed between the both due to the inversion of the umbrella shape of the umbrella portion  661 . Thus, the communication opening  43  is released. In this way, the pressurized ink (pressure) in the second chamber  42  is allowed to escape (released) toward the first chamber  41  through the communication opening  43 . Therefore, it can be prevented that an excessive force acts on the atmospheric pressure detection film  7  itself or the attached part of the atmospheric pressure detection film  7 , whereby the breakage of the atmospheric pressure detection film  7  can be prevented. 
     [Control Examples of Liquid Ejection Device] 
       FIG. 24  is a block diagram showing an electrical configuration of the liquid ejection device  1 . The liquid ejection device  1  includes a controller  10  for integrally controlling the operation of this liquid ejection device  1 . The controller  10  controls to drive (ON) and stop (OFF) the pump  9  and controls to open and close the first, second, third and fourth valve bodies  33 V,  34 V,  35 V and RPV, which are, for example, constituted by electromagnetic valves. 
     The controller  10  operates the liquid ejection device  1  at least in the print mode, the pressurized purge mode, the first circulation mode and the second circulation mode. As described above, the print mode is a mode for applying the printing process to a predetermined work by causing the ink to be ejected from the ink ejecting portion  22  of the head unit  2 . The pressurized purge mode is a mode for supplying the high-pressure ink to the ink ejecting portion  22  and causing the ink ejecting portion  22  to discharge the ink to remove or prevent ink clogging in the ink ejecting portion  22 . The first circulation mode is a mode for circulating the ink between the head unit  21  and the liquid supply unit  3  and recovering the air on the side of the head unit  21  to the side of the liquid supply unit  3 . The second circulation mode is a mode for circulating the ink using the short-circuit pipe RP to vent the air staying in the second chamber  42 . 
       FIG. 24  shows an “ON” or “OFF” state of the pump  9  and the “open” or “closed” state of the first, second, third and fourth valve bodies  33 V,  34 V,  35 V and RPV in each mode. A flow of the ink in each mode is described below.  FIGS. 25, 26, 27 and 28  are perspective views respectively showing the flow of the ink in the print mode (ejection control), in the pressurized purge mode, in the first circulation mode (first circulation control) and in the second circulation mode (second circulation control). 
     &lt;Print Mode&gt; 
     In the print mode ( FIG. 25 ), since the ink is supplied by the water head difference, the controller  10  sets the pump  9  in an non-operation state (OFF). Further, since the ink is not circulated using the return pipe  35  and the short-circuit pipe RP, the controller  10  sets the third and fourth valve bodies  35 V, RPV in the “closed” state while setting the first valve body  33 V ( FIG. 3 ) and the second valve body  34 V in the “open” state. Of course, the air vent mechanism  37  is set in the “closed” state and the second chamber  42  is separated from the atmosphere. 
     As indicated by an arrow F 11  of  FIG. 25 , the ink discharged from the ink cartridge IC enters the filter chamber  44  through the upstream pipe  33  by the water head difference. Solid foreign substances contained in the ink are removed when passing through the filter member  442  ( FIG. 9 ) in this filter chamber  44 . Thereafter, the ink enters the first chamber  41 . 
     If the on-off valve  6  is opened by the operation of the aforementioned negative pressure supply mechanism, directly by the operation of the pressing member  5 , the ink is stored into the second chamber  42  from the first chamber  41  through the communication opening  43  as indicated by an arrow F 12 . The ink in the second chamber  42  is sucked and enters the downstream pipe  34  through the backflow prevention mechanism  38  by the ink discharging operation in the ink ejecting portion  22 . Thereafter, the ink enters the common passage  27  ( FIG. 2 ) of the head unit  21  by way of the end tube  24  as indicated by an arrow F 13 . Then, the ink is ejected from the respective ink discharge hole  22 H through the individual passages  26  (arrows F 14 ). 
     &lt;Pressurized Purge Mode&gt; 
     In the pressurized purge mode ( FIG. 26 ), the pump  9  is set in a drive state (ON) since the ink is forcibly fed to the downstream pipe  34 . As in the print mode, since the ink is not circulated using the return pipe  35  and the short-circuit pipe RP, the controller  10  sets the third and fourth valve bodies  35 V, RPV in the “closed” state while setting the first and second valve bodies  33 V,  34 V in the “open” state. The air vent mechanism  37  is also set in the “closed” state. 
     In this pressurized purge mode, the pump  9  is operated in the forward rotation direction, and the ink is forcibly supplied to the head unit  21  without depending on the water head difference. If the pump  9  is operated, the ink in the first chamber  41  is consumed. Thus, the ink is discharged from the ink cartridge IC. As indicated by an arrow F 21 , the ink enters the filter chamber  44  through the upstream pipe  33  and further enters the first chamber  41 . Then, as indicated by an arrow F 22 , the ink enters the upstream bypass pipe BP 1  by way of the bypass communication chamber  413  without flowing toward the second chamber  42 . 
     The ink is pressurized by the squeezing operation of the pump  9  and fed toward the downstream side. Specifically, as indicated by an arrow F 23 , the ink is fed from the downstream bypass pipe BP 2  to the downstream pipe  34 . As described above, since the backflow prevention mechanism  38  is provided in the joint part a of the downstream bypass pipe BP 2  to the downstream pipe  34 , the ink does not flow back toward the second chamber  42 . Thereafter, as indicated by an arrow F 24 , the ink enters the common passage  27  ( FIG. 2 ) of the head unit  21  by way of the end tube  24 . Then, the ink is discharged at a high pressure from the respective ink discharge holes  22 H through the individual passages  26  (arrow F 25 ). In this way, foreign substances causing clogging in the ink ejecting portion  22 , the air staying in the individual passages  26  and the like are removed. 
     &lt;First Circulation Mode&gt; 
     In the first circulation mode ( FIG. 27 ), the controller  10  sets the pump  9  in a forward drive state (ON) since the ink is forcibly fed to the downstream pipe  34 . Further, since the ink is circulated using the downstream pipe  34  and the return pipe  35 , the controller  10  sets the fourth valve body RPV in the “closed” state to close the short-circuit pipe RP while setting the second and third valve bodies  34 V,  35 V in the “open” state. Further, since the ink is circulated between the liquid supply unit  3  and the head unit  21 , the first valve body  33 V is also set in the “closed” state. In this way, a closed ink circulation path composed of the bypass pipe  32 P, the downstream pipe  34 , the common passage  27  of the head unit  21 , the return pipe  35  and the return communication chamber  414  and the bypass communication chamber  413  is formed. The air vent mechanism  37  is kept in the “closed” state. 
     If the pump  9  is operated, the circulation of the ink in the above ink circulation path is started. Specifically, by the operation of the pump  9 , the ink is sucked into the upstream bypass pipe BP 1  from the bypass communication chamber  413  as indicated by an arrow F 31  and, subsequently, fed to the downstream bypass pipe BP 2  as indicated by an arrow F 32 . Thereafter, the ink flows into the head unit  21  (arrow F 33 ) by way of the joint part a, the downstream pipe  34  and the end tube  24 , passes through the common passage  27  in the head unit  21  and enters the recovery tube  25  (arrow F 34 ). Then, as indicated by an arrow F 35 , the ink returns from the recovery tube  25  to the bypass communication chamber  413  successively by way of the return pipe  35 , the return communication chamber  414  and the joint part b. At this time, since the first valve body  33 V is closed, the return pipe  35  and the common passage  27 , from which the ink is sucked by the pump  9 , are set to a negative pressure. Therefore, the ink does not leak from the ink discharge holes  22 H during ink circulation. 
     If the first circulation mode is performed, the ink can be circulated in the ink circulation path as described above. In other words, the ink temporarily fed toward the head unit  21  can be returned toward the liquid supply unit  3  using the return pipe  35 . Thus, even if air enters the head unit  21  such as due to the feed of the ink containing air, the air can be recovered into the liquid supply unit  3  together with the ink by the above circulation. The air (air bubbles) recovered into the liquid supply unit  3  enters the first chamber  41  located above from the return communication chamber  414  by buoyancy and stays near the communication opening  43  arranged near an uppermost part of the first chamber  41 . 
     &lt;Second Circulation Mode&gt; 
     Also in the second circulation mode ( FIG. 28 ), the controller  10  sets the pump  9  in the forward drive state (ON) since the ink is forcibly fed to the downstream pipe  34 . Further, since the ink is circulated using the short-circuit pipe RP and a part of the return pipe  35  downstream of the first T-branch portion Ra, the controller  10  sets the second valve body  34 V in the “closed” state to close the part of the downstream pipe  34  downstream of the first T-branch portion Ra while setting the third and fourth valve bodies  35 V, RPV in the “open” state. Further, the first valve body  33 V is set in the “open” state to expel the air from the second chamber  42  by filling the ink, and the ink can be supplied from the ink cartridge IC. The air vent mechanism  37  is set in the “open” state to set the second chamber  42  at a constant pressure. 
     By the above valve operation, an ink circulation path composed of the bypass pipe  32 P, a part of the downstream pipe  34  upstream of the second T-branch portion Rb, the short-circuit pipe RP, the part of the return pipe  35  downstream of the first T-branch portion Ra, the return communication chamber  414  and the bypass communication chamber  413  is formed. The ink can flow into the first chamber  41  from the ink cartridge IC. Thus, if the pump  9  is operated, the ink is sucked into the first chamber  41  as indicated by an arrow F 41  and flows into the upstream bypass pipe BP 1  from the bypass communication chamber  413  (arrow F 42 ). Subsequently, the ink is fed from the downstream bypass pipe BP 2  to the downstream pipe  34  as indicated by an arrow F 43 . Then, the ink enters the short-circuit pipe RP from the second T-branch portion Rb, enters the return pipe  35  from the first T-branch portion Ra and returns to the bypass communication chamber  413  successively by way of the return communication chamber  414  and the joint part b as indicated by an arrow F 44 . 
     In the second circulation mode, the air vent mechanism  37  is set in the “open” state. That is, as shown in  FIG. 18B , the lever member  46  is pushed down to forcibly operate the pressing member  5 , the on-off valve  6  moves to reach the opening posture, and the first and second chambers  41 ,  42  communicate through the communication opening  43 . By the operation of the pump  9 , the ink is circulated by way of the short-circuit pipe RP while being supplied to the first chamber  41  from the ink cartridge IC. By this operation, the first chamber  41  is gradually filled with the ink and the air recovered into the first chamber  41  in the first circulation mode and the ink flow into the second chamber  42  through the communication opening  43 . Eventually, the second chamber  42  is gradually filled with the ink by the above inflow, the recovered air is driven upwardly of the second chamber  42  and expelled to outside through the air vent mechanism  37  in the released state. That is, the air having entered the head unit  21  and the liquid supply unit  3  can be easily and reliably discharged without being accompanied by the removal of the supply passage. 
     As described above, the air can be prevented from staying near the individual passages  26  and the ink discharge holes  22 H of the head unit  21  by the execution of the first and the second circulation modes. The air having entered the head unit  21  can be removed also by the pressurized purge mode. However, the air once having entered the head unit  21  is not easily vented and pressurized purging of discharging a considerable amount of the ink needs to be performed. Thus, there is a problem that a large amount of the ink is consumed only to vent air from the head unit  21 . However, according to the first and second circulation modes, since air is recovered into the liquid supply unit  3  by circulating the ink, the ink is not consumed. Further, in the first and second circulation modes, it is sufficient to circulate the ink in the ink circulation path and the ink needs not be pressurized unlike in the pressurized purge mode. Thus, it is sufficient to operate the pump  9  at a low speed. Therefore, the application of a large pressure load to the liquid supply unit  3  can be avoided and the breakage of the atmospheric pressure detection film  7  and the sealing film  7 A can be prevented. 
     Besides, the controller  10  performs a liquid extraction mode for discharging the preservation solution stored in the head unit  21  during initial usage. In this liquid extraction mode, the second and fourth valve bodies  34 V, RPV are set in the “open” state to circulate the ink using the downstream pipe  34  and the short-circuit pipe RP. Since the preservation solution in the head unit  21  is pushed out by the ink, the first valve body  33 V is also set in the “open” state. On the other hand, the third valve body  35 V is set in the “closed” state. In this state, the pump  9  is operated and the ink is supplied to the head unit  21  in two routes of the downstream pipe  34  and the short-circuit pipe RP to successively push out the preservation solution in the head unit  21 . 
     [Modifications] 
     Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to these and various modifications can be employed. For example, in the above embodiments, the liquid supply unit  3  according to the present disclosure is illustrated to supply the ink to the head unit  21  of the ink jet printer. The liquid stored in and supplied by the liquid supply unit  3  is not limited to the ink, and various liquids can be used. For example, water, various types of solutions, chemicals, industrial chemical liquids and the like can be stored in and supplied by the liquid supply unit  3 . 
     According to the present disclosure described above, it is possible to provide a liquid ejection device capable of easily and reliably venting air in a liquid supply unit. 
     Although the present disclosure has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present disclosure hereinafter defined, they should be construed as being included therein.