Patent Publication Number: US-10766270-B2

Title: Liquid supply unit and liquid injection device

Description:
INCORPORATION BY REFERENCE 
     This application is based on Japanese Patent Application No. 2018-57665 filed with the Japan Patent Office on Mar. 26, 2018, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to a liquid supply unit for supplying liquid stored in a liquid storage container to a liquid injection head and a liquid injection device to which the liquid supply unit is applied. 
     For example, in an ink jet printer, a liquid injection head for injecting a tiny amount of ink (liquid) to a print object is used. Ink is supplied to this liquid injection head from an ink cartridge (liquid storage container) storing the ink through a predetermined supply passage. Conventionally, a liquid injection device is known in which a liquid supply unit (valve unit) including a pressure chamber for setting a discharge hole of a liquid injection head to a negative pressure is arranged in a supply passage in the case of supplying ink from an ink cartridge to the liquid injection 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. 
     The above liquid supply unit adopts such a structure that a part of a pressure chamber set to a negative pressure is defined by a flexible film and a pressing plate (pressure receiving plate) attached to this flexible film directly presses a movable valve. The movable valve is biased in a direction opposite to a direction of the pressing by a biasing member. If a negative pressure degree of the pressure chamber increases due to the suction of ink by the liquid injection head, the movable valve is pressed against the pressing plate to move according to a displacement of the flexible film, an ink supply passage into the pressure chamber is opened and the ink flows into the pressure chamber. If the negative pressure degree of the pressure chamber decreases due to this inflow of the ink, the movable valve is moved in a reverse direction by a biasing force of the biasing member and the pressure chamber returns to a sealed state. 
     SUMMARY 
     A liquid supply unit according to one aspect of the present disclosure is a liquid supply unit for supplying predetermined liquid from a liquid storage container storing the liquid to a liquid injection head for injecting the liquid. The liquid supply unit includes a pressure chamber capable of storing the liquid, a first supply passage allowing communication between the liquid storage container and the pressure chamber, a second supply passage allowing communication between the liquid injection head and the pressure chamber, and a valve mechanism configured to operate according to a pressure state of the pressure chamber. The pressure chamber and the second supply passage are maintained at a predetermined supply negative pressure when the liquid is supplied to the liquid injection head. A part of a wall portion defining the pressure chamber is formed by a flexible film member. 
     The valve mechanism prohibits the inflow of the liquid from the first supply passage in a first state where the pressure chamber is within the range of the supply negative pressure, allows the inflow of the liquid from the first supply passage in a second state where the pressure chamber is at a pressure lower than the supply negative pressure, and releases the pressure in the pressure chamber in a third state where the pressure chamber is at a pressure higher than the supply negative pressure by a predetermined value. 
     A liquid injection device according to another aspect of the present disclosure includes a liquid injection head configured to inject predetermined liquid and the above liquid supply unit configured to supply the liquid from a liquid storage container storing the liquid to the liquid injection head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the external appearance of an ink jet printer to which the present disclosure is applied, 
         FIG. 2  is a sectional view along line II-II of  FIG. 1 , 
         FIG. 3  is a front view of the ink jet printer with an outer cover removed, 
         FIG. 4  is an overall perspective view of a carriage mounted in the ink jet printer, 
         FIG. 5  is a perspective view showing one liquid supply unit and one head unit, 
         FIG. 6  is a block diagram showing a liquid supply system in an embodiment showing a state where a print mode is being performed, 
         FIG. 7A  is a diagram showing a state where a pressurized purge mode is being performed and  FIG. 7B  is a diagram showing a state where a decompression mode is being performed, 
         FIG. 8A  is a front view of the liquid supply unit,  FIG. 8B  is a side view thereof and  FIG. 8C  is a top view thereof, 
         FIG. 9  is a perspective view showing an internal structure of the liquid supply unit, 
         FIG. 10  is a perspective view showing the internal structure of the liquid supply unit, 
         FIG. 11A  is an exploded perspective view of the liquid supply unit and  FIG. 11B  is an exploded perspective view of the liquid supply unit obliquely viewed in a different direction, 
         FIG. 12A  is a perspective view of a pressing member and  FIG. 12B  is a perspective view of the pressing member obliquely 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. 8  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 along line XV-XV of  FIG. 8  showing the state where the on-off valve is in the closing posture and  FIG. 15B  is an enlarged view of a part A 2  of  FIG. 15A , 
         FIG. 16A  is a sectional view, corresponding to  FIG. 14A , showing a state where the on-off valve is in an opening posture and  FIG. 16B  is an enlarged view of a part A 3  of  FIG. 16A , 
         FIG. 17  is a sectional view, corresponding to  FIG. 15B , showing the state where the on-off valve is in the opening posture, 
         FIGS. 18A and 18B  are diagrams showing the operation of the pressing member utilizing a leverage ratio, 
         FIG. 19A  is an exploded perspective view of an air vent mechanism of the liquid supply unit and  FIGS. 19B and 19C  are perspective views of a lever member, 
         FIG. 20A  is a sectional view showing a state before the lever member is operated and  FIG. 20B  is a sectional view showing a state where air is vented by the operation of the lever member, 
         FIG. 21  is an enlarged view of a part A 4  of  FIG. 20B , 
         FIG. 22  is an exploded perspective view of a backflow prevention mechanism of the liquid supply unit, 
         FIG. 23A  is a perspective view of the backflow prevention mechanism showing a state where a spherical body opens a valve conduit,  FIG. 23B  is a view showing a state where the spherical body closes the valve conduit and  FIG. 23C  is a perspective view of a branched head portion, 
         FIG. 24A  is a sectional view showing a state of the backflow prevention mechanism in a print mode and  FIG. 24B  is an enlarged view of a part A 5  of  FIG. 24A , 
         FIG. 25A  is a sectional view showing a state of the backflow prevention mechanism in a pressurized purge mode and  FIG. 25B  is an enlarged view of a part A 6  of  FIG. 25A , 
         FIG. 26A  is a sectional view showing a state where an umbrella valve is sealing a communication opening and  FIG. 26B  is a sectional view showing a state where the umbrella valve is opening the communication opening, and 
         FIG. 27  is a diagram showing a modification of a location to which an umbrella valve is applied. 
     
    
    
     DETAILED DESCRIPTION 
     [Overall Configuration of Printer] 
     Hereinafter, one embodiment of the present disclosure is described with reference to the drawings. First, an ink jet printer to which a liquid supply unit or a liquid injection device according to the present disclosure is applied is described.  FIG. 1  is a perspective view showing the external appearance of an ink jet printer  1  according to the embodiment,  FIG. 2  is a sectional view along line II-II of  FIG. 1 , and  FIG. 3  is a front view of the printer  1  with an outer cover  102  removed. Note that front-rear, lateral and vertical directions are indicated in  FIGS. 1 to 3  and figures described later, but this is only for the convenience of description and not intended to limit directions at all. 
     The printer  1  is a printer for performing a printing process of printing characters and images on various works W such as paper sheets, resin sheets or cloth fabrics, and particularly a printer suitable for a printing process on large-size and long works. The printer  1  includes a base frame  101  with casters and an apparatus body  11  placed on the base frame  101  and configured to perform the printing process. 
     The apparatus body  11  includes a work conveyance path  12 , a conveyor roller  13 , pinch roller units  14  and a carriage  2 . The work conveyance path  12  is a conveyance path extending in a front-rear direction for loading a work W, to which the printing process is applied, into the apparatus body  11  from a rear side and unloading the work W from a front side. The conveyor roller  13  is a roller extending in a lateral direction and configured to generate a drive force for intermittently feeding the work W along the work conveyance path  12 . The pinch roller unit  14  is arranged to face the conveyor roller  13  from above and includes a pinch roller which forms a conveyance nip together with the conveyor roller  13 . A plurality of the pinch roller units  14  are arranged at predetermined intervals in the lateral direction. 
     The carriage  2  is a movable body on which units for performing the printing process on the work W are mounted and which can reciprocate along the lateral direction on the base frame  101 . A carriage guide  15  with a guide rail for guiding reciprocal movements of the carriage  2  stands to extend in the lateral direction on a rear side of the base frame  101 . A timing belt  16  is so assembled with the carriage guide  15  as to be able to circulate in the lateral direction. The carriage  2  includes a fixing portion for the timing belt  16 , and moves in the lateral direction while being guided by the guide rail as the timing belt  16  circulates in a forward or reverse direction. 
     The printing process is performed by intermittently feeding the work W by the conveyor roller  13  and the pinch roller units  14  and moving the carriage  2  in the lateral direction while the work W is stopped to print and scan the work W. Note that, in the work conveyance path  12 , a platen  121  (see  FIG. 2 ) additionally provided with a function of sucking the work W is arranged below a passage path of the carriage  2 . During the printing process, the carriage  2  performs printing and scanning with the work W sucked to the platen  121 . 
     The apparatus body  11  is covered by an outer cover  102 . A side station  103  is arranged in a region to the right of the outer cover  102 . An immovable ink cartridge shelf  17  for holding ink cartridges IC ( FIGS. 5 and 6 ) for storing ink (predetermined liquid) for the printing process is housed in the side station  103 . 
     A carriage retraction area  104  serving as a retraction space for the carriage  2  is present in a front part of the side station  103 . As shown in  FIG. 3 , a left frame  105  and a right frame  106  stand on the base frame  101  while being spaced apart in the lateral direction by a distance corresponding to the work conveyance path  12 . An area between these left and right frames  105 ,  106  serves as a printing area where the printing process can be performed. The carriage guide  15  has a lateral width longer than the printing area, and the carriage  2  is movable to a right outer side of the printing area. When the printing process is not performed, the carriage  2  is retracted to the carriage retraction area  104 . Further, a pressurized purge process to be described later is also performed in this carriage retraction area  104 . 
     A feeding unit  107  housing a feed roll Wa, which is a winding body of the work W having the printing process applied thereto, is provided on a rear side of the base frame  101 . Further, a winding unit  108  housing a winding roll Wb, which is a winding body of the work W after the printing process, is provided on a front side of the base frame  101 . The winding unit  108  includes an unillustrated drive source for rotationally driving a winding shaft of the winding roll Wb, and winds the work W while applying predetermined tension to the work W by a tension roller  109 . 
     [Configuration of Carriage] 
       FIG. 4  is an overall perspective view of the carriage  2 . Head units  21  (liquid injection heads) for injecting the ink (liquid) to the work W and liquid supply units  3  for supplying the ink from the ink cartridges IC to the head units  21  are mounted on the carriage  2 .  FIG. 4  shows an example in which two head units  21  and eight liquid supply units  3  are mounted on the carriage  2 . Specifically, four liquid supply units  3  are equipped for each head unit  21  to supply respective inks of cyan, magenta, yellow and black. Note that the ink of a different color is filled into each liquid supply unit  3 , and inks of at most eight colors may be injected from the two head units  21 . 
     The carriage  2  includes the head units  21  and a carriage frame  20  for holding the head units  21 . The carriage frame  20  includes a lower frame  201  located at a lowermost position, an upper frame  202  arranged above and at a distance from the lower frame  201 , a rack  203  mounted on the upper surface of the upper frame  202  and a back surface frame  204  mounted on the rear surface of the upper frame  202 . The lower frame  201  and the upper frame  202  are coupled by coupling support columns  205  extending in the vertical direction. An unillustrated ball screw mechanism is mounted on the back surface frame  204 , and a nut portion driven by that ball screw is mounted on the lower frame  201 . Further, the back surface frame  204  is provided with guiding support columns  206  extending in the vertical direction. By the drive of the ball screw mechanism, a coupled body of the lower frame  201  and the upper frame  202  can move in the vertical direction while being guided by the guiding support columns  206 . That is, a body part of the carriage  2  is movable in the vertical direction with respect to the back surface frame  204 . 
     The head units  21  are mounted on the lower frame  201 . Since the body part of the carriage  2  is movable in the vertical direction as described above, vertical height positions of the head units  21  with respect to the work W are adjustable. The liquid supply units  3  are mounted on the upper frame  202 . The eight liquid supply units  3  are supported on the upper frame  202  while being aligned in the lateral direction in the rack  203 . A guided portion to be guided by the guide rail of the carriage guide  15 , a fixing portion to the timing belt  16  and the like are provided on the back surface frame  204 . 
       FIG. 5  is a perspective view showing one liquid supply unit  3  and one head unit  21 . The liquid supply unit  3  includes a body portion  30  with a tank portion  31  and a pump portion  32 , an upstream pipe  33  (part of 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 second supply passage) arranged on a downstream side of the body portion  30 , and a bypass pipe  35  (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  (pressurizing mechanism;  FIG. 6 ) to be operated during a decompression process for forming the negative pressure environment and a pressurized purge process for cleaning the head unit  21  (ink discharging portion  22 ). 
     The upstream pipe  33  is a supply pipe allowing communication between the tank portion  31  and the 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 (not shown) extending from the ink cartridge IC, and a downstream end  332  is connected to an inlet part of the tank portion  31 . The downstream pipe  34  is a supply pipe allowing communication between the tank portion  31  and the head unit  21 . An upstream end  341  of the downstream pipe  34  is connected to an outlet part of the tank portion  31  and a downstream end  342  is connected to the head unit  21 . The bypass pipe  35  is a conduit for feeding the ink to the downstream pipe  34  without via the negative pressure environment (second chamber  42  to be described later) of the tank portion  31 . 
     The head unit  21  includes the ink discharging portion  22 , a control unit  23 , an end tube  24  and a discharge tube  25 . The ink discharging portion  22  is a nozzle part for discharging ink droplets toward the 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 discharging portion  22 . The control unit  23  includes a control board for controlling the piezo element or the heating element provided in the ink discharging portion  22  and controls an operation of discharging ink droplets from the ink discharging portion  22 . 
     The end tube  24  is a tube linking the downstream end  342  of the downstream pipe  34  and the ink discharging 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 discharge tube  25  is a tube for discharging preservation solution sealed in the liquid supply unit  3  during initial usage. During initial usage, the downstream end  342  of the downstream pipe  34  is attached to the upper end fitting part of the end tube  24  and a separate tube is connected to the discharge tube  25  to open a storage space for the preservation solution, whereby an operation of discharging the preservation solution is performed. 
     [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 discharged from the ink discharging 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 path and set the ink discharging 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. 6  is a block diagram schematically showing the liquid supply system adopted in the carriage  2  of this embodiment. The ink cartridge IC is arranged at a position higher than the ink discharging portion  22  by a height h. This height h serves as the 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 path 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  (part of the 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 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 ρdenotes 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 discharging portion  22  via the downstream pipe  34 . 
     An on-off valve  6  (valve mechanism/single valve body) coupled to a pressing member  5  is arranged on a wall surface partitioning between the first chamber  41  and the second chamber  42 . Further, a wall portion defining the second chamber  42  is partially 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 on-off valve  6  coupled to the pressing member  5  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 this, the bypass pipe  35  for short-circuiting the first chamber  41  and the downstream pipe  34  without via the second chamber  42  is provided. The pump  9  capable of rotating in forward and reverse rotation directions is arranged in the bypass pipe  35 . 
       FIG. 6  is also a diagram showing a state where the liquid supply system is performing a print mode (during normal liquid supply) for performing the printing process. In the print mode, a predetermined amount of the ink is filled in each of the first and second chambers  41 ,  42  and the second chamber  42  is set to a predetermined negative pressure. The pressure in the first chamber  41  is the atmospheric pressure+ρgh [Pa] due to the water head difference as described above and the ink can be supplied from the ink cartridge IC by the water head difference any time. As basic setting of the print mode, the on-off valve  6  is set in the closing posture and the first and second chambers  41 ,  42  are separated. The pump  9  is in a stopped state. Although described later, the pump  9  is a tube pump and the bypass pipe  35  is in a closed state when the pump  9  is stopped. Thus, the downstream pipe  34  and the ink discharging portion  22  are also maintained at the negative pressure. 
     To smoothly fill the ink into the second chamber  42 , an air vent mechanism  37  is attached to the second chamber  42 . 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 discharging 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 discharging 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. 
     The liquid supply system of this embodiment is capable of performing the pressurized purge mode and a decompression mode in addition to the above print mode. The pressurized purge mode is a mode for supplying high-pressure ink to the ink discharging portion  22  and causing the ink discharging portion  22  to discharge the ink in order to recover or prevent ink clogging. The decompression mode is a mode for setting the second chamber  42  at normal pressure to the predetermined negative pressure during initial usage, after maintenance and the like. 
       FIG. 7A  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. 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  35  while bypassing the second chamber  42 . That is, the ink pressurized in the pump  9  is supplied to the ink discharging portion  22 . In this way, the ink is forcibly discharged from the ink discharging portion  22  to clean the ink discharging portion  22 . Note that an operation similar to that in the pressurized purge mode is also performed when the preservation solution sealed in the liquid supply unit  3  is discharged during initial usage. 
     A 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 a joint part a of the downstream pipe  34  and a downstream end of the bypass pipe  35 . Since a 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  35  flows toward the ink discharging portion  22 . Thus, the breakage of the atmospheric pressure detection film  7  defining the second chamber  42  is prevented. 
       FIG. 7B  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. When the pump  9  is driven in the reverse rotation direction, the ink discharging portion  22  and the second chamber  42  are decompressed through the downstream pipe  34  and the bypass pipe  35 . The ink discharging 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 from the ink discharging portion  22  even if the ink is supplied by the water head difference, by this decompression mode. Note that if the ink discharging portion  22  is set to an excessive negative pressure, ink discharge by the drive of the piezo element or the like in the ink discharging portion  22  may be impeded. Thus, the ink discharging 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. 
     [Overall Structure of Liquid Supply Unit] 
     Next, the structure of the liquid supply unit  3  according to this embodiment which enables the execution of each mode of the liquid supply system described above is described in detail.  FIG. 8A  is a front view of the liquid supply unit  3 ,  FIG. 8B  is a side view thereof and  FIG. 8C  is a top view thereof.  FIGS. 9 and 10  are perspective views showing an internal structure of the liquid supply unit  3  on the side of the first chamber  41  and on the side of the second chamber  42 .  FIGS. 11A and 11B  are exploded perspective views of the liquid supply unit  3  viewed from the side of the second chamber  42  and from the side of the first chamber  41 . 
     As preliminarily described on the basis of  FIGS. 5 to 7B , 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 bypass pipe  35 , 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 a monitor pipe  36  for monitoring an ink liquid surface in the second chamber  42 , a communication pipe  32 P allowing communication between the pump portion  32  and the first chamber  41  and a 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  (see also  FIGS. 9, 10 and 22 ) 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 portion defining the pressure chamber) 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 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 roughly L-shaped in a plan view. The first chamber  41  is defined by a first partition wall  411  projecting leftward from the tank portion base plate  310 . An inflow opening  412  for the ink is perforated in an uppermost part of the first partition wall  411 . An inflow port  417  ( FIG. 22 ) formed of a receiving plug stands on an outer side surface of the first partition wall  411  in correspondence with the inflow opening  412  for the ink. The downstream end  332  of the upstream pipe  33  is inserted and connected to this inflow port  417 . That is, the inflow opening  412  is an opening allowing communication between the ink cartridge IC and the first chamber  41 , and the ink flows into the first chamber  41  through this inflow opening  412  by the water head difference. 
     A bottom wall portion  413  of the first partition wall  411  is located on the lower end of the tank portion base plate  310 . A purge port  414  is provided in a rear side wall of the first partition wall  411  near the bottom wall portion  413 . An upstream end of the communication pipe  32 P is connected to this purge port  414 . A spring seat  415  formed of a hollow cylindrical cavity projects near a vertical center of the first chamber  41 . The spring seat  415  is a cavity for housing a biasing spring  45  to be described later, and open toward the second chamber  42 . 
     The communication opening  43  is located above the spring seat  415  in the first chamber  41 . As already described, the first chamber  41  is a chamber in which the 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. When the ink flows through the inflow opening  412 , the ink starts being pooled from the bottom wall portion  413 . When an ink liquid level exceeds the communication opening  43 , the ink can be supplied into the second chamber  42  through this communication opening  43 . Further, when the pump  9  is operated, the ink stored in the first chamber  41  is sucked through the purge port  414  and the communication pipe  32 P and the pressurized ink is supplied to the head unit  21  through the bypass pipe  35  and the downstream pipe  34 . 
     With reference to  FIGS. 10 and 22 , the second chamber  42  roughly has a circular shape in a plan view. 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  includes a hollow cylindrical wall  422  having a hollow cylindrical shape and an upper wall  423  formed of a rectangular part projecting further upward than the hollow cylindrical wall  422 . The aforementioned spring seat  415  is recessed in the tank portion base plate  310  at a center position of a region surrounded by the hollow cylindrical wall  422 , i.e. at a position concentric with the hollow cylindrical wall  422 . The communication opening  43  is arranged on the spring seat  415  on a vertical line passing through a center point of the spring seat  415 . 
     A communication chamber  44  (part of the second supply passage) is connected to the lower end of the second chamber  42 . The communication chamber  44  is a rectangular space elongated in the front-rear direction and extends straight forward from the lower end of the hollow cylindrical wall  422 . The communication chamber  44  is defined by a wall portion  441 . A lower passage  424  allowing communication between the second chamber  42  and the communication chamber  44  is provided on the lower end of the hollow cylindrical wall  422 . The wall portion  441  is linked to the hollow cylindrical wall  422  at the position of the lower passage  424 . The communication chamber  44  is a space linking the second chamber  42  and the downstream pipe  34  and set to a negative pressure, and substantially constitutes a part of the second chamber  42 . 
     In a region surrounded by the upper wall  423  of the second chamber  42 , a pair of front and rear supporting plates  425  project rightward from the tank portion base plate  310 . Each of the pair of supporting plates  425  includes a pivotally supporting portion  426  for pivotally supporting the pressing member  5  to be described later. A boss portion  427  and an upper monitor port  428  project upward on a top wall  423 A constituting an uppermost part of the upper wall  423  (defining a top wall of the second chamber  42 ). The boss portion  427  internally includes a boss hole  42 A ( FIG. 19A ), which is an opening allowing the second chamber  42  to communicate with the atmosphere. This boss portion  427  constitutes a part of the air vent mechanism  37 , and a lever member  46  and a return spring  47  ( FIG. 19A ) to be described later are assembled therewith. 
     On the top wall  423 A, an upper monitor hole  42 B is perforated in front of the boss hole  42 A. Further, a top wall  442  of the wall portion  441  defining the communication chamber  44  is perforated with a lower monitor hole  444 . The upper monitor port  428  stands on the top wall  423 A in correspondence with the upper monitor hole  42 B. A lower monitor port  445  stands on the top wall  442  in correspondence with the lower monitor hole  444 . The upper end of the monitor pipe  36  is connected to the upper monitor port  428 , and the lower end thereof is connected to the lower monitor port  445 . That is, the monitor pipe  36  communicates with upper and lower end sides of the second chamber  42  and the ink liquid level in the monitor pipe  36  is linked with that in the second chamber  42 . 
     In this embodiment, the monitor pipe  36  is formed of a transparent resin tube. Accordingly, a user can know the ink liquid level in the second chamber  42  by seeing the monitor pipe  36 . In this embodiment, as shown in  FIG. 4 , the plurality of liquid supply units  3  are arranged in parallel in the lateral direction in the carriage  2 . Thus, even if a transparent film is used as the atmospheric pressure detection film  7  located on the right side surface, the liquid supply units  3  other than the one in a rightmost part cannot allow the ink liquid level in the second chamber  42  to be seen. However, in this embodiment, the monitor pipe  36  stands in front of the liquid supply unit  3 . Thus, the user can know the ink liquid level in each second chamber  42  by seeing the monitor pipe  36  of each liquid supply unit  3  from the front of the carriage  2 . 
     The backflow prevention mechanism  38  is installed on the top wall  442  near the front end of the communication chamber  44 . The top wall  442  is perforated with a supply hole  443  in correspondence with the backflow prevention mechanism  38 . The upstream end  341  of the downstream pipe  34  is connected to the backflow prevention mechanism  38 . The ink stored in the second chamber  42  is supplied to the downstream pipe  34  through the support hole  443  and the backflow prevention mechanism  38  by being sucked by the ink discharging portion  22 . The backflow prevention mechanism  38  is described in detail later. 
     With reference to  FIGS. 11A and 11B , 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 matching a wall shape of the first partition wall  411  viewed from left. A peripheral edge part of the sealing film  7 A is welded or adhered to an end surface of the first partition wall  411 , whereby the sealing film  7 A seals the opening of the first chamber  41 . 
     An opening in a right surface side of the second chamber  42  is sealed by the atmospheric pressure detection film  7  made of a flexible resin film member. The atmospheric pressure detection film  7  has an outer shape matching a wall shape of an integral assembly of the second partition wall  421  of the second chamber  42  and the wall portion  441  of the communication chamber  44 . Specifically, the atmospheric pressure detection film  7  includes a body portion  71  corresponding to the hollow cylindrical wall  422  of the second chamber  42 , an upper extended portion  72  corresponding to the rectangular upper wall  423  and a lower extending portion  73  corresponding to the wall portion  441  of the communication chamber  44 . The atmospheric pressure detection film  7  seals the openings of the second chamber  42  and the communication chamber  44  by welding or adhering a peripheral edge part of the body portion  71  to an end surface of the hollow cylindrical wall  422 , a peripheral edge part of the upper extending portion  72  to an end surface of the upper wall  423  and a peripheral edge part of the lower extending portion  73  to an end surface of the wall portion  441 . Note that the atmospheric pressure detection film  7  is welded or adhered without particular tension being applied thereto. 
     The pump portion  32  is arranged behind 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 a cam shaft  93  ( FIG. 4 ) for pivotally supporting an eccentric cam  91  ( FIG. 24A ) of the pump  9  is inserted. The pump cavity  321  is a hollow cylindrical cavity arranged at a center position of the pump portion housing  320  in the front-rear and vertical directions. 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 just described, in this embodiment, the pump cavity  321  is integrally provided to the tank portion base plate  310  serving as the base board of the tank portion  31 , 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  2  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 are summarily described above on the basis of  FIG. 6  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 that displacement force 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. 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 outward from an upper end side (one end side) of the disk portion  51 , pivot portions  53  (pivot point) provided on extending tip parts of the respective arm portions  52  and a pair of link bosses  54  (pressing portion). The pair of pivot portions  53  are pivotally supported by the pivotally supporting portions  426  ( FIGS. 10 and 22 ) of the pair of supporting plates  425  arranged in the second chamber  42 . In this way, the disk portion  51  is rotatable about an axis of the pivot portions  53 . 
     The disk portion  51  is a disk having a diameter, which is about half the inner diameter of the hollow cylindrical wall  422  defining most of the second chamber  42 . The hollow cylindrical wall  422  and the disk portion  51  in a state pivotally supported by the pivotally supporting portions  426  are substantially concentrically arranged. 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 . 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 into this spring fitting projection  511 . Note that a region of the spring fitting projection  511  is formed into 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 a region of a peripheral edge part of the spring fitting projection  511  on the first surface  51 A of the disk portion  51 . 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 a naturally hanging 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 . 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 axis of pivot portions  53  and is inclined leftward from the hanging state. 
     Lower end parts  521  of the pair of arm portions  52  are respectively located on both lateral parts of the spring fitting projection  511 , whereby the spring fitting projection  511  is positioned to be sandwiched by a pair of the lower end parts  521 . The pair of arm portions  52  extend straight upward from the respective lower end parts  521 . A cutout portion  512  cut along a radial direction is provided in the disk portion  51  between the pair of arm portions  52 . The pair of arm portions  52  extend in parallel from the disk portion  51  with this cutout portion  512  therebetween. 
     Rectangular thick portions  522  are provided at vertical intermediate positions of the respective arm portions  52 . The thick portions  522  are arranged near the upper end of the disk portion  51  and lateral to the cutout portion  512 . That is, a pair of the thick portions  522  face each other in the front-rear direction across the cutout portion  512 . The pivot portion  53  projects in the front-rear direction from a tip part  523 , which is an extending end, of each arm portion  52 . In particular, the pivot portions  53  project in directions separating from each other such that the pivot portion  53  projects forward from the front surface of the front tip part  523  and the pivot portion  53  projects rearward from the rear surface of the rear tip part  523 . The pivot portions  53  are fit into the pivotally supporting portions  426  of the pivot portions  425 . It contributes to increasing a leverage ratio to be described later to provide the pivot portions  53  on the extending tip parts of the arm portions  52 . 
     The pair of pivot portions  53  are arranged on an axis of rotation  5 AX extending in the front-rear direction. The front and rear pivot portions  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 equivalent to a central region in a plane direction of the disk portion  51 . The distance D can be set to about 40% to 80% of a diameter of the disk portion  51 . In this way, pivot points formed by the pair of pivot portions  53  are pivot points spaced wide apart to sandwich the central region of the disk portion  51 . Thus, the disk portion  51  rotating about the pivot points is less likely to be twisted about an axis perpendicular to the axis of rotation  5 AX. Therefore, the rotating operation 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 of the disk portion  51 . In particular, the link bosses  54  formed of rectangular flat plates respectively stand from end edges of the pair of thick portions  522  facing the cutout portion  512 . Accordingly, the pair of link bosses  54  are located inwardly of the pair of pivot portions  53  in the central region of the disk portion  51 . Each link boss  54  includes a link hole  541 . This link hole  541  is used to link and connect the pressing member  5  and the on-off valve  6 . By this link connection, opening and closing operations of the on-off valve  6  are linked with the rotating operation of the pressing member  5 . 
     In other words, the link bosses  54  serve as pressing portions for pressing and moving the on-off valve  6  in the lateral direction according to the rotating operation of the pressing member  5  rotating about the axis of the pivot portions  53 . In a relationship of the pressure receiving portion  5 A (point of force application) and the pivot portions  53  (fulcrum), the link bosses  54  (point of action) are set between the pressure receiving portion  5 A and the pivot portions  53 . That is, the pressure receiving portion  5 A, the pivot portions  53  and the link bosses  54  are set to satisfy a positional relationship of a second class lever. Thus, a pressing force can be applied to the on-off valve  6  from the link bosses  54  by increasing the displacement force of the atmospheric pressure detection film  7  received by the pressure receiving portion  5 A by the leverage ratio. 
     &lt;On-Off Valve&gt; 
     Next, the on-off valve  6  (valve mechanism/single valve body) is described. As shown in  FIGS. 11A and 11B , the on-off valve  6  is arranged in the communication opening  43  allowing communication between the first chamber  41  and the second chamber  42 . The on-off valve  6  opens and closes the communication opening  43  by moving in the lateral direction in the communication opening  43 , following the rotating operation of the pressing member  5 . The on-off valve  6  is link-connected to the link bosses  54  (pressing portions) of the disk portion  51  to follow the above rotating operation. 
       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. 8  and  FIG. 14B  is an enlarged view of a part A 1  of  FIG. 14A .  FIG. 15A  is a sectional view along line XV-XV of  FIG. 8  and  FIG. 15B  is an enlarged view of a part A 2  of  FIG. 15A . The on-off valve  6  is an assembly of a valve holder  61  and an umbrella valve  66  held by the valve holder  61 . The communication opening  43  is an opening having a circular cross-sectional shape 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 including 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 tube 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 tube 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 tube portion  62  is a tubular part having a largest outer diameter in the valve holder  61 . The tube portion  62  includes a guide surface  62 S, which is the outer peripheral surface of the tube portion  62 , a flow passage cutout  621  formed by cutting a part of the tube portion  62  in a circumferential direction, and a holding groove  622  annularly recessed on an inner peripheral side of the tube portion  62 . The tube portion  62  is housed into 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 holding a locking spherical portion  663  of the umbrella valve  66 . 
     The intermediate portion  64  is a tubular part having a smaller outer diameter than the tube portion  62 . The intermediate portion  64  includes an open portion  641 , which is an open 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 also guided by the inner surface of the small-diameter portion  43 B. On a boundary part between the tube portion  62  and the intermediate portion  64 , an annular contact portion  62 A formed by a step based on an outer diameter difference between the both is present. 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 pin  65  projects in the vertical direction from each of the pair of flat surfaces. These link pins  65  are fit into the link holes  541  provided in the link bosses  54  of the pressing member  5  as shown in  FIG. 15B . By this fitting, the pressing member  5  and the on-off valve  6  can be link-connected and translate a rotational motion of the pressing member  5  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 spherical portion  663  integrally provided to the pin portion  662 . The umbrella portion  661  has an umbrella diameter larger than an 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  416 , which is a wall surface around the communication opening  43  (closing posture/first state). On the other hand, if the sealing surface  67  is separated from the sealing wall surface  416 , the sealed state is released (opening posture/second state). Note that the umbrella shape of the umbrella portion  661  is inverted ( FIGS. 26A and 26B /third state) if a predetermined pressure is applied to the right surface side of the umbrella portion  661 . 
     The pin portion  662  is a rod-like part extending in the lateral direction and serving as a support column for the umbrella portion  661 . The pin portion  662  is inserted into the tube portion  62  of the valve holder  61  and the pin housing portion  642  of the intermediate portion  64 . That is, the umbrella portion  661  can come into contact with the first end part  661  of the valve holder  61 , whereas the pin portion  662  can be fit into an inner tube portion of the valve holder  61 . The locking spherical portion  663  is a part formed by spherically bulging a part of the pin portion  662  near a left end and to be fit into the holding groove  622 . By fitting the locking spherical portion  663  into the holding groove  622 , the umbrella valve  66  is held in the valve holder  61  with lateral movements restricted. Specifically, the umbrella valve  66  moves in the lateral direction integrally with the valve holder  61 . 
     &lt;Biasing Spring&gt; 
     The biasing spring  45  is a coil spring interposed between the second surface  51 B of the disk portion  51  and the tank portion base plate  310  and supporting (biasing) 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 thereof is housed in the spring seat  415  recessed in the tank portion base plate  310 . When the pressure receiving portion  5 A of the disk portion  51  receives a leftward displacement force acting against a rightward biasing force of the biasing spring  45 , the disk portion  51  rotates leftward about the axis of the pivot portions  53 . Unless receiving the above displacement force, the disk portion  51  is maintained in a hanging posture by the biasing force. 
     &lt;Operation of On-Off Valve&gt; 
     Next, the opening and closing operations of the on-off valve  6  are described.  FIGS. 14A to 15B  show a state where the on-off valve  6  is in the closing posture. This state is a state where the atmospheric pressure detection film  7  is not generating such a displacement force as to rotate the pressing member  5  (disk portion  51 ), i.e. a state where the sum of a spring pressure (biasing force) of the biasing spring  45  and an inner pressure of the second chamber  42  is larger than the atmospheric pressure. Although the second chamber  42  is set to the negative pressure, the biasing spring  45  biases the biased portion  5 B of the disk portion  51  by a biasing force exceeding a displacement force of the atmospheric pressure detection film  7  caused by the negative pressure (state where the second chamber  42  is maintained within a predetermined supply negative pressure; first state). Thus, the disk portion  51  does not rotate about the axis of the pivot portions  53  and is maintained in the aforementioned hanging posture. 
     In this case, the on-off valve  6  link-connected to the pressing member  5  by the link bosses  54  is in the closing posture 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  416 . Therefore, the communication opening  43  is sealed by the umbrella valve  66 . The biasing spring  45  can be said to bias the on-off valve  6  in the direction toward the closing posture, utilizing a lever force, by biasing the disk portion  51  rightward. 
       FIG. 16A  is a sectional view, corresponding to  FIG. 14A , showing the state where the on-off valve  6  is in the opening posture and  FIG. 16B  is an enlarged view of a part A 3  of  FIG. 16A .  FIG. 17  is a sectional view, corresponding to  FIG. 15B , showing the state where the on-off valve is in the opening posture. As the ink discharging portion  22  continues the operation of discharging ink droplets from the state of  FIGS. 14 to 15B , the negative pressure degree of the second chamber  42 , which is a sealed space, gradually increases as the ink decreases. Eventually, when the second chamber  42  reaches a negative pressure exceeding the predetermined threshold value (second state where the second chamber is at a pressure lower than the supply negative pressure), the atmospheric pressure detection film  7  applies a pressing force acting against the biasing force of the biasing spring  45  to the pressure receiving portion  5 A of the disk portion  51 . Specifically, a state is entered where the sum of the spring pressure of the biasing spring  45  and the inner pressure of the second chamber  42  is less than the atmospheric pressure. 
     In this case, the disk portion  51  rotates leftward about the axis of the pivot portions  53  against the biasing force of the biasing spring  45 . By this rotation, the link bosses  54  generate a pressing force to move the on-off valve  6  leftward and changes 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  416  to form a gap G. Thus, the sealing of the communication opening  43  by the umbrella valve  66  is released. 
     When the on-off valve  6  reaches the opening posture, the ink flows from the first chamber  41  into the second chamber  42  due to a pressure difference between the first chamber  41  set to the pressure, which is the sum of the atmospheric pressure and ρgh, and the second chamber  42  with a progressed negative pressure degree as indicated by an arrow F in  FIG. 17 . 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  416 , the flow passage cutout  621  prepared in the tube portion  62  of the valve holder  61  and the open portion  641  prepared in the intermediate portion  64 . 
     As the ink flows into the second chamber  42 , the negative pressure degree of the second chamber  42  is gradually alleviated. Eventually, when the sum of the spring pressure of the biasing spring  45  and the inner pressure of the second chamber  42  becomes more than the atmospheric pressure, the disk portion  51  is pushed back rightward by the biasing force of the biasing spring  45 . Specifically, when the second chamber  42  reaches a negative pressure below the predetermined threshold value, the disk portion  51  rotates rightward about the axis of the pivot portions  53  by being pressed by the biasing force of the biasing spring  45 . In this way, the on-off valve  6  also linearly moves rightward by being pulled by the link bosses  54 . At some 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  416 . Thus, the on-off valve  6  returns to the closing posture. 
     &lt;Functions and Effects of Negative Pressure Supply Mechanism&gt; 
     Functions and effects of the negative pressure supply mechanism of this embodiment having the above configuration are described using diagrams of  FIGS. 18A and 18B .  FIG. 18A  shows a state where the pressing member  5  (disk portion  51 ) is in the hanging posture and the on-off valve  6  is in the closing posture, and  FIG. 18B  shows a state where the pressing member  5  is rotated to reach an oblique posture and the on-off valve  6  is in the opening posture. 
     First, the pressing member  5  has pivot points, which are the pivot portions  53 , and are pivotally supported by the supporting plates  425  disposed in the second chamber  42 . Thus, if the pressure receiving portion  5 A receives a displacement force of the atmospheric pressure detection film  7 , the pressing member  5  rotates about the axis of the pivot portions  53 . That is, an unstable moving force, which is a displacement of the atmospheric pressure detection film  7 , can be translated into a stable moving force, which is rotation about the axis of the pivot portions  53 . Thus, the displacement force of the atmospheric pressure detection film  7  can be efficiently transmitted to the on-off valve  6  through the link bosses  54  (pressing portions). For example, if a pressing member for the on-off valve  6  does not have any pivot point, such as by being attached to the atmospheric pressure detection film  7 , such a behavior becomes unstable and a pressing force is unstably transmitted to the on-off valve  6 . However, since the pressing member  5  can generate a stable pressing force according to this embodiment, the posture of the on-off valve  6  can be changed between the closing posture and the opening posture at a desired timing and the ink can be stably supplied to the head unit  21 . 
     Further, the pressing member  5  can cause the link bosses  54  to generate a large pressing force, utilizing a lever force. Specifically, the link bosses  54  for pressing the on-off valve  6  are arranged between the pressure receiving portion  5 A and the pivot portions  53 . That is, the pressing member  5  realizes a pressing structure for the on-off valve  6  utilizing the principle of leverage with the pivot points by the pivot portions  53  serving as a fulcrum P 1 , the pressure receiving portion  5 A serving as a point of force application P 2  and the link bosses  54  serving as a point of action P 3 . Accordingly, a pressing force applied to the pressure receiving portion  5 A by a displacement force of the atmospheric pressure detection film  7  can be applied from the link bosses  54  to the on-off valve  6  while being increased by the leverage ratio. Thus, the link bosses  54  can be caused to press the on-off valve  6  by a large pressing force and a sufficient pressing force for timely moving the on-off valve  6  can be ensured. 
     The pressing member  5  includes the arm portions  52  extending upward from the upper end side of the disk portion  51 , and the pivot portions  53  serving as the pivot points are provided on the extending tip parts  523  of the arm portions  52 . This configuration contributes to extending a distance between the pressure receiving portion  5 A (point of force application P 2 ) and the link bosses  54  (point of action P 3 ) and increasing the leverage ratio. Thus, the pressing force generated by the pressing member  5  can be made even larger. 
     Further, an advantage brought by the link connection of the on-off valve  6  to the pressing member  5  can be cited as an advantage of another perspective. In particular, the link connection is formed by the link pins  65  disposed near the right end (second end part  612 ) of the on-off valve  6  and the link holes  541  of the link bosses  54 . The biasing spring  45  biases the on-off valve  6  in the direction toward the closing posture by pressing the biased portion  5 B of the disk portion  51 . Thus, the disk portion  51  rotates about the axis of the pivot portions  53  to be inclined, but the on-off valve  6  can be prevented from being inclined, following the inclining movement of the disk portion  51 , by the link connection. Therefore, the on-off valve  6  can be linearly moved in the lateral direction in the communication opening  43  and the on-off valve  6  can be stably operated between the closing posture and the opening posture. 
     Here, a biasing member equivalent to the biasing spring  45  may be structured to bias the on-off valve  6  directly rightward (direction toward the closing posture) as a modification. However, in this embodiment, the biasing spring  45  presses the disk portion  51  and indirectly biases the on-off valve  6  in the direction toward the closing posture. Thus, a degree of freedom of the biasing structure for the on-off valve  6  can be enhanced as compared to the case where the biasing structure is provided near the communication opening  43 . Further, the biased portion  5 B for receiving the biasing force from the biasing spring  45  is set at the position corresponding to the pressure receiving portion  5 A. Thus, an efficient biasing structure is realized, utilizing the principle of leverage, also in biasing the on-off valve  6  via the disk portion  51  by the biasing spring  45 . 
     [Air Vent Mechanism of Second Chamber] 
     Next, the air vent mechanism  37  attached to the second chamber  42  is described in detail.  FIG. 19A  is an exploded perspective view of the liquid supply unit  3  including the air vent mechanism  37  and  FIGS. 19B and 19C  are perspective views of the lever member  46 . As described above, the air vent mechanism  37  is used in venting air and deaerating air bubbles generated from the ink when the ink is initially filled into the second chamber  42  during initial usage, after maintenance and the like. 
     The air vent mechanism  37  includes the lever member  46 , a sealing ring  46 C and the return spring  47  in addition to the aforementioned boss portion  427  projecting on the second partition wall  421  defining the second chamber  42 . The boss portion  427  projects on the top wall  423 A defining the top surface of the second chamber  42  and includes an opening allowing the second chamber  42  to communicate with the atmosphere, i.e. the boss hole  42 A serving as an air vent hole. The second chamber  42  can be reliably deaerated by providing the boss hole  42 A in the top wall  423 A located at the uppermost position of the second chamber  42 . 
     The lever member  46  includes a rod-like member  461  to be partially inserted into the boss hole  42 A and a pressing piece  464  connected below the rod-like member  361 , and has a shovel-like shape. 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 an opening posture for opening the boss hole  42 A. In this embodiment, a posture changing operation of the lever member  46  is linked with that of the on-off valve  6  via the pressing member  5 . Specifically, the on-off valve  6  is allowed to be in the closing posture when the lever member  46  is in the sealing posture, and the posture of the on-off valve  6  is changed from the closing posture to the opening posture when the lever member  46  is in 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 includes an upper end part  462  and a lower end part  463 . The upper end part  462  serves as an input portion for receiving an operational pressing force for pressing the lever member  46  downward from a user. The lower end part  463  is linked to the pressing piece  464 . The pressing piece  464  functions as a transmitting portion for transmitting the operational pressing force applied to the upper end part  462  to the pressing member  5  (receiving slopes  55 ). 
     The upper surface of the pressing piece  464  to which the lower end part  463  of the rod-like member  462  is linked is a flange surface  464 F larger than the hole diameter of the boss hole  42 A. The flange surface  464 F is a rectangular plane perpendicular to an axis of the rod-like member  461  and, with the rod-like member  461  inserted in the boss hole  42 A, faces the inner surface of the top wall  423 A. The pressing piece  464  is shaped to be trapezoidal when viewed in the front-rear direction and substantially square when viewed in the lateral direction, and includes a pair of pressing slopes  465  inclined with respect to the axis of the rod-like member  461  and a lower end edge  466  extending in the front-rear direction on the lowermost end. The pair of pressing slopes  465  are respectively slopes (oblique sides) extending upward with end parts of the lower end edge  466  in the front-rear direction as starting points. 
     The pressing slopes  465  and the lower end edge  466  interfere with the pressing member  5  when the lever member  46  receives the operational pressing force. With reference to  FIGS. 12A and 12B , the pressing member  5  is provided with a pair of receiving slopes  55  on the first surface  51 A on a side below the pivot portions  53  and facing the atmospheric pressure detection film  7 . The receiving slopes  55  are arranged between the link bosses  54  and the arm portions  52  on the upper end of the disk portion  51 . An interval between the pair of receiving slopes  55  is set to match an interval between the pair of pressing slopes  465 . The pressing slopes  465  and the lower end edge  466  come into contact with the receiving slopes  55  and transmit the operational pressing force to the pressing member  5  when the user applies the operational pressing force. In this way, the pressing member  5  rotates leftward about the axis of the pivot portions  53  to change the posture of the on-off valve  6  from the closing posture to the opening posture. 
     An engaging groove  467  is formed near the upper end part  462  of the rod-like member  461 . A washer  47 W for locking the upper end of the return spring  47  is fit into the engaging groove  467 . The flange surface  464 F of the pressing piece  464  is formed with a sealing groove  468  into which the sealing ring  46 C is fit. The return spring  47  is a coil spring having an inner diameter larger than the outer diameter of the boss portion  427  and a spring length longer than a vertical length of the boss portion  427 , and is externally fit to the boss portion  427 . The sealing ring  46 C is an O-ring having an inner diameter somewhat larger than the rod-like member  461 . The sealing ring  46 C is fit from the upper end part  462  of the rod-like member  461  and mounted into the sealing groove  468 . Note that the sealing groove  468  may be omitted. 
     Next, the operation of the lever member  46  is described.  FIGS. 20A and 20B  are sectional views respectively showing a state before the lever member  46  is operated and a state where air is vented by the operation of the lever member  46 .  FIG. 21  is an enlarged view of a part A 4  of  FIG. 20B .  FIG. 20A  shows a state where the upper end part  462  of the lever member  46  is not pressed down, i.e. the sealing posture in which the lever member  46  seals the boss hole  42 A. On the other hand,  FIG. 20B  shows a state where the upper end part  462  is pressed downward to apply an operational pressing force, i.e. the opening posture in which the lever member  46  opens the boss hole  42 A. 
     The sealing posture is maintained by a biasing force of the return spring  47 . The return spring  47  generates a force for lifting the lever member  46  upward via the washer  47 W. That is, the return spring  47  biases the lever member  46  toward the sealing posture. In this way, the sealing ring  46 C held on the flange surface  464 F comes into contact with the top wall  423 A on the peripheral edge of the boss hole  42 A. Accordingly, the boss hole  42 A is sealed. A state at this time is the same as the previously mentioned state shown in  FIGS. 14A and 14B . The pressing piece  464  (pressing slopes  465  and lower end edge  466 ) of the lever member  46  is separated from the receiving slopes  55  of the pressing member  5  and applies no 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  receives an operational pressing force to be lowered and assume the opening posture, the flange surface  464 F is also lowered and, accordingly, the sealing ring  46 C is separated from the top wall  423 A. Thus, the boss hole  42 A is opened. Specifically, the second chamber  42  and outside air communicate through a clearance between the inner surface of the boss hole  42 A and the outer peripheral surface of the rod-like member  461 . Thus, a state is set in which air staying in the second chamber  42  can be exhausted to outside through the boss hole  42 A. 
     Further, if the lever member  46  assumes the opening posture, the operational pressing force is transmitted to the pressing member  5 . As shown in  FIG. 21 , the pressing slopes  465  and the lower end edge  466  press the receiving slopes  55 . The receiving slopes  55  are located below the pivot portions  53  and shifted toward right (toward the atmospheric pressure detection film  7 ). Thus, if the receiving slopes  55  are pressed, the pressing member  5  (disk portion  51 ) rotates leftward about the axis 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. 
     As just described, if the lever member  46  assumes the opening posture, an inlet (communication opening  43 ) for fluid into the second chamber  42  and an outlet (boss hole  42 A) for fluid are ensured. Thus, during initial usage, the operation of filling the ink from the first chamber  41  into the second chamber  42  through the communication opening  43  while air in the second chamber  42  is vented through the boss hole  42 A can be smoothly performed, utilizing the supply by the water head difference. Further, if the amount of air in the second chamber  42  increases (confirmed by the monitor pipe  36  since the ink liquid level in the second chamber  42  drops) such as due to the generation of air bubbles from the ink, air can be easily vented from the second chamber  42  by setting the lever member  46  to the opening posture. 
     In the above embodiment, the posture of the on-off valve  6  is changed to the opening posture in conjunction with the lever member  46  assuming the opening posture, utilizing the pressing member  5  including 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 into and from the second chamber  42  can be ensured in a single operation of the lever member  46 . Accordingly, the user can easily perform the operation of venting air in the second chamber  42 . Further, since the air vent mechanism  37  is arranged on the upper surface of the tank portion  31 , the user can perform the air venting operation for each liquid supply unit  3  by accessing from the front of the carriage  2  even with the plurality of liquid supply units  3  mounted in the carriage  2  as shown in  FIG. 4 . 
     [Backflow Prevention Mechanism] 
     Next, the configuration of the backflow prevention mechanism  38  for preventing the pressurized ink from flowing back to the second chamber  42  when the pressurized purge mode described on the basis of  FIG. 7A  is performed is described.  FIG. 22  is a perspective view of the base board  300  of the liquid supply unit  3  including an exploded perspective view of the backflow prevention mechanism  38 . 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 top wall  442  of the communication chamber  44  and the other components are mounted into the valve conduit  81 .  FIGS. 23A and 23B  are perspective views of the backflow prevention mechanism  38  excluding the valve conduit  81 , and  FIG. 23C  is a perspective view of the branched head portion  82  viewed from below. 
     The valve conduit  81  is a conduit extending in the vertical direction from the upper surface of the top wall  442 . The valve conduit  81  provides an ink flow passage linking the communication chamber  44  and the downstream pipe  34  and constitutes a part of an ink supply passage from the second chamber  42  to the ink discharging portion  22 . A locking piece  811  projects on the outer peripheral surface of the valve conduit  81  and a fitting annular projection  812  projects on the inner peripheral surface of the valve conduit  81  to lock the branched head portion  82 . 
     The branched head portion  82  is a member for forming the joint part a described above on the basis of  FIGS. 6 to 7B . The branched head portion  82  includes a first inlet port  821 , a second inlet port  822 , an outlet port  823 , trunk portions  824 , a locking window  825 , a cutout portion  826  and fitting claws  827 . The first inlet port  821  is a port connected to the downstream end of the second chamber  42  and, in this embodiment, communicates with the second chamber  42  via the valve conduit  81  and the communication chamber  44 . The second inlet port  822  is a port connected to the downstream end of the bypass pipe  35 . The outlet port  823  is a port connected to the upstream end  341  of the downstream pipe  34 . 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 outside the first inlet port  821  facing downward. The valve conduit  81  enters a clearance between a pair of the trunk portions  824  and the first inlet port  821 . The locking window  825  is an opening which is provided in the pair of trunk portions  824  and with which the locking piece  811  of the valve conduit  81  is engaged. The cutout portion  826  is a part formed by partially cutting a peripheral wall of the tubular first inlet port  821  and a part for securing the ink flow passage. The fitting claws  827  are hook-shaped parts projecting downward from the lower end of the first inlet port  821 , and engage 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 piece  811  and the locking window  825  on the inner periphery of the valve conduit  81  and by the engagement of the fitting annular projection  812  and the fitting claws  827  on the outer periphery of the valve conduit  81 . 
     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 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 formed of a material having a specific weight equal to or less than twice the specific weight of the ink. The spherical body  83  is immersed in the ink in the valve conduit  81 . By approximating the specific weight of the spherical body  83  to that of the ink, an operating pressure of the spherical body  83  in the ink supply direction (vertical direction here) can be made smaller. 
     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 resin (specific weight≈1.5) as the material of the spherical body  83 . 
     The sealing member  84  is a sealing component having a ring shape and to be seated on a seat portion  813  below the spherical body  83  and on a bottom wall of the valve conduit  81  (upper surface of the top wall  442 ), for example, as shown in  FIG. 24B . A ring inner diameter (through hole) of the sealing member  84  is set smaller than the outer diameter of the spherical body  83 , but larger than the supply hole  443  perforated in the top wall  442 . When the spherical body  83  is separated from this sealing member  84  as shown in  FIG. 23A , the valve conduit  81  is opened. On the other hand, when the spherical body  83  contacts the sealing member  84  as shown in  FIG. 23B , the valve conduit  81  is closed. 
     The coil spring  85  is a compression spring mounted in the valve conduit  81  such that a lower end part thereof comes into contact with the sealing member  84  and an upper end part thereof comes into contact with a lower 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. 24A  is a sectional view showing a state of the backflow prevention mechanism  38  in the print mode, and  FIG. 24B  is an enlarged view of a part A 5  of  FIG. 24A .  FIG. 24A  shows the pump  9  housed in the pump portion  32 . The pump  9  is a tube pump including the eccentric cam  91  and a squeeze tube  92 . The cam shaft  93  ( FIG. 4 ) serving as an axis of rotation of the eccentric cam  91  is inserted into a shaft hole  91 A of the eccentric cam  91 . A rotational drive force is applied to this eccentric cam  91  from an unillustrated drive gear. 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 the 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 communication pipe  32 P and the bypass pipe  35 . Specifically, one end side of the squeeze tube  92  communicates with the bottom wall portion  413  of the first chamber  41  (communication pipe  32 P), the other end side communicates with the second inlet port  822  of the branched head portion  82  (bypass pipe  35 ) 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. 6 . In this case, the eccentric cam  91  is stopped by squeezing the squeeze tube  92 , wherefore the ink supply passage passing through the bypass pipe  35  is closed. On the other hand, the pump  9  is driven in the forward rotation direction in the pressurized purge mode shown in  FIG. 7A . In  FIG. 24A , the 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 communication pipe  32 P and flows toward the backflow prevention mechanism  38 , which is the joint part a, from the bypass pipe  35 . Note that when the pump  9  is driven in the reverse rotation direction, the communication chamber  44 , the second chamber  42  and the downstream pipe  34  are set to the negative pressure through the bypass pipe  35  and the branched head portion  82  as shown in  FIG. 7B . 
     Next, the operation of the backflow prevention mechanism  38  is described. In the print mode, the ink is supplied to the head unit  21  along a supply route passing through the communication chamber  44 , the backflow prevention mechanism  38  and the downstream pipe  34  from the second chamber  42 . In such a print mode, the spherical body  83  is separated from the sealing member  84  and lifted upward as shown in  FIG. 24B . This relies on the fact that the supply route from the second chamber  42  to the downstream pipe  34  is maintained at the negative pressure in the print mode. Coupled with the suction of the ink present in the supply route by the ink discharging portion  22  of the head unit  21  every time ink droplets are discharged, a force acts on the spherical body  83  in the ink supply direction and the spherical body  83  is lifted from the sealing member  84  in the liquid ink. 
     Since the spherical body  83  is separated from the sealing member  84 , the supply hole  443  of the communication chamber  44  is opened. On the other hand, the spherical body  83  may be lifted to contact the lower end edge  828  of the first inlet port  821  by a suction force of the ink discharging portion  22 .  FIG. 23A  shows a state where the spherical body  83  is lifted to an uppermost position. Even in such a state, since the cutout portion  826  is provided in the peripheral wall of the first inlet port  821 , a passage for the ink is ensured. Thus, the ink can pass from the communication chamber  44  to the branched head portion  82 . 
       FIG. 25A  is a sectional view showing a state of the backflow prevention mechanism  38  in the pressurized purge mode and  FIG. 25B  is an enlarged view of a part A 6  of  FIG. 25A . In the pressurized purge mode, the ink pressurized through the bypass pipe  35  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 bypass pipe  35  and the downstream pipe  34  located downstream of the joint part a are pressurized by the pressurized ink. 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 broken or a part thereof attached to the second partition wall  421  may be peeled off. 
     However, in this embodiment, the spherical body  83  is pressed downward (upstream side in the ink supply direction) to contact the sealing member  84  by a pressurizing force applied to the joint part a.  FIGS. 23B and 25B  show a state where the spherical body  83  is fit into the ring-shaped sealing member  84  by being pressed. 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  443  is closed. Specifically, out of the ink supply path in the print mode, the communication chamber  44  and the second chamber  42  located upstream of the joint part a are blocked from pressurization by the pressurized ink. Thus, the breakage of the atmospheric pressure detection film  7  and the like can be prevented. 
     [Double Protection Mechanism by Umbrella Valve] 
     As described above, in this embodiment, a backflow of the ink pressurized in the pressurized purge mode to the second chamber  42  is prevented by providing the backflow prevention mechanism  38 . However, the pressurizing force may possibly act on the second chamber  42  due to a certain trouble of the backflow prevention mechanism  38  such as an operation failure of the spherical body  83 . In view of this point, a double protection mechanism for releasing the pressure to the on-off valve  6  is provided in this embodiment. That is, the on-off valve  6  has 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 set at a negative pressure and the first chamber  41  is set at the atmospheric pressure+ρgh at normal time is reversed and the second chamber  42  is set at a pressure higher than 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. 14A to 17 , the umbrella valve  66  seals the communication opening  43  by the sealing surface  67  coming into contact with the sealing wall surface  416  if the second chamber  42  is at a negative pressure below the predetermined threshold value (predetermined supply negative pressure) (first state). In this way, the inflow of the ink from the first chamber  41  to the second chamber  42  is prohibited. On the other hand, if the second chamber  42  is at a negative pressure exceeding the predetermined threshold value (second state), the umbrella valve  66  moves leftward together with the valve holder  61  link-connected to the pressing member  5  and the sealing surface  67  is separated from the sealing wall surface  416  to open the communication opening  43  (release of sealing). In this way, the inflow of the ink from the first chamber  41  into the second chamber  42  is allowed. 
     In addition, the umbrella valve  66  singly opens the communication opening  43  if the pressure relationship of the second chamber  42  and the first chamber  41  is reversed, such as due to the application of the pressure of the pressurized ink to the second chamber  42  in the pressurized purge mode (third state where the second chamber  42  is at a pressure higher than the supply negative pressure by a predetermined value). That is, the umbrella valve  66  releases the sealed state of the communication opening  43  to release the pressure in the second chamber  42  to the first chamber  41  without 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 when a predetermined pressure applied to the right surface side of the umbrella portion  661 . 
       FIGS. 26A and 26B  are sectional views respectively showing a state where the umbrella valve  66  seals the communication opening  43  and a state where the umbrella valve  66  opens the communication opening  43 . The state of  FIG. 26A  is equal to the state of  FIG. 14B  previously described. The umbrella portion  661  has the 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 . Thus, the sealing surface  67  is in contact with the sealing wall surface  416 . 
     The state of  FIG. 26B  is a state where the umbrella shape of the umbrella portion  661  of the umbrella valve  66  is inverted by the pressure applied from the side of the second chamber  42 . That is, the umbrella portion  661  is deformed into an umbrella shape convex rightward. This inverted state is obtained when the pressure in the second chamber  42  becomes higher than that in the first chamber  41  by a predetermined value. In this embodiment, a case is assumed where a high positive pressure by pressurized purge is applied to the second chamber  42  and, as a result, the second chamber  42  is set to a higher pressure than in the first chamber  41  set at the atmospheric pressure+ρgh. The predetermined value depends on an inverted pressure of the umbrella portion  661 . This inverted pressure is set at a value lower than the burst strength of the atmospheric pressure detection film  7  or the 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  generates no pressing force for pressing the on-off valve  6  leftward. This is because the atmospheric pressure detection film  7  is displaced to bulge rightward by a pressure increase of the second chamber  42  and applies no displacement force to the pressure receiving portion  5 A. Thus, the valve holder  61  is maintained at the rightmost position 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  416  to create the gap g between the both by the inversion of the umbrella shape of the umbrella portion  661 . Accordingly, the communication opening  43  is opened. In this way, the pressurized ink (pressure) in the second chamber  42  is allowed to escape (release) toward the first chamber  41  through the communication opening  43 . Thus, it can be made possible to prevent an excessive force from acting on the atmospheric pressure detection film  7  itself or the attaching part thereof and prevent breakage. 
     [Modification] 
     Although the embodiment of the present disclosure has been described above, the present disclosure is not limited to this. For example, the following modifications can be employed. 
     (1) In the above embodiment, the pressing member  5  presses the on-off valve  6 , utilizing the principle of leverage, with the pivot portions  53  serving as the fulcrum P 1 , the pressure receiving portion  5 A serving as the point of force application P 2  and the link bosses  54  serving as the point of action P 3  ( FIGS. 18A and 18B ). In the present disclosure, the set positions of the pressure receiving portion  5 A and the link bosses  54  are not limited. The positions of the pressure receiving portion  5 A and the link bosses  54  can be set according to a pressing force necessary to move the on-off valve  6 . For example, the link bosses  54  may be arranged at the same position as the pressure receiving portion  5 A on the back surface (second surface  51 B) of the disk portion  51 . 
     (2) Although the pressing member  5  and the on-off valve  6  are link-connected by the link bosses  54  and the link pins  65  in the above embodiment, the both may not be link-connected. For example, a state may be formed in which a part of the pressing member  5  and a part of the on-off valve  6  are constantly held in contact by a spring or the like and the pressing member  5  may press the on-off valve  6  through the contact parts. 
     (3) In the above embodiment, the pressing member  5  includes the pair of pivot portions  53  spaced apart in the direction of the axis of rotation. Instead of this, one long shaft extending in the direction of the axis of rotation may be used as the pivot portions  53 . Alternatively, if the rotational twist of the pressing member  5  is not problematic, one arm having pivot portions formed on tips may be used in place of the pair of arm portions  52  and the pair of pivot portions  53  of the above embodiment. Further, the arm portions  52  may be omitted and the pivot portions  53  may be provided near the upper end of the disk portion  51 . 
     (4) In the above embodiment, the single on-off valve  6  functions as the valve mechanism in the present disclosure. Instead of this, a valve body for releasing the pressure in the second chamber  42  in the third state where the second chamber  42  is set at the pressure higher than the supply negative pressure by the predetermined value may be provided separately from a valve body for allowing communication between the first and second chambers  41 ,  42 . 
       FIG. 27  is a diagram showing a modification of a location to which an umbrella valve is applied. In this modification, an umbrella valve  66 A is arranged at a location different from an on-off valve  6 A arranged in the communication opening  43 , i.e. in an opening provided in the second partition wall  421 . If the second chamber  42  reaches a high pressure exceeding a predetermined value, the umbrella valve  66 A operates to release the pressure in the second chamber  42  to the atmosphere. Unless otherwise, the umbrella valve  66 A seals the above opening of the second partition wall  421 . In this case, a sealing member for inverting the sealing surface  67  may not be applied to the on-off valve  6 A unlike in the above embodiment. Alternatively, an umbrella valve may be also mounted in the on-off valve  6 A for more protection. 
     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.