Patent Publication Number: US-10780692-B2

Title: Droplet discharging apparatus and maintenance method for droplet discharging apparatus

Description:
The present application is based on, and claims priority from JP Application Serial Number 2018-130461, filed Jul. 10, 2018, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a droplet discharging apparatus such as an ink jet printer and a maintenance method for a droplet discharging apparatus. 
     2. Related Art 
     In JP-A-2004-276544, a droplet discharging apparatus that performs a flushing operation of preliminarily discharging droplets from a nozzle to suppress an increase in viscosity of liquid is described. 
     In the droplet discharging apparatus described in JP-A-2004-276544, the flushing operation is regularly performed as nozzle maintenance. Therefore, the amount of liquid consumed for maintenance is large. 
     SUMMARY 
     According to an aspect of the disclosure, there is provided a droplet discharging apparatus including: a droplet discharger including a common liquid chamber to which liquid is supplied from a liquid supply source via a liquid supply flow path, a plurality of pressure chambers communicating with the common liquid chamber, actuators provided respectively corresponding to the plurality of pressure chambers, nozzles provided respectively corresponding to the plurality of pressure chambers, and a discharge flow path coupled to the pressure chambers such that the liquid in the pressure chambers are discharged to an outside, the droplet discharger performing a recording process with respect to a recording medium by driving the actuators such that the liquid in the pressure chambers are discharged from the nozzles in the form of droplets; a return flow path coupled to the discharge flow path and forming a circulation path for circulation of the liquid together with the liquid supply flow path; and a controller performs, as a maintenance operation for the droplet discharger, a first discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path when no droplets are discharged from the nozzles during the recording process. 
     According to another aspect of the disclosure, there is provided a maintenance method for a droplet discharging apparatus which includes: a droplet discharger including a common liquid chamber to which liquid is supplied from a liquid supply source via a liquid supply flow path, a plurality of pressure chambers communicating with the common liquid chamber, actuators provided respectively corresponding to the plurality of pressure chambers, nozzles provided respectively corresponding to the plurality of pressure chambers, and a discharge flow path coupled to the pressure chambers such that the liquid in the pressure chambers are discharged to an outside, the droplet discharger performing a recording process with respect to a recording medium by driving the actuators such that the liquid in the pressure chambers are discharged from the nozzles in the form of droplets; and a return flow path coupled to the discharge flow path and forming a circulation path for circulation of the liquid together with the liquid supply flow path, the method including performing, as a maintenance operation for the droplet discharger, a first discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path when no droplets are discharged from the nozzles during the recording process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view schematically illustrating a droplet discharging apparatus. 
         FIG. 2  is a plan view schematically illustrating an internal structure of the droplet discharging apparatus. 
         FIG. 3  is a side view of a wiping mechanism. 
         FIG. 4  is a sectional view schematically illustrating a pressure adjustment mechanism and a droplet discharger with an on-off valve closed. 
         FIG. 5  is a sectional view taken along line V-V in  FIG. 4 . 
         FIG. 6  is a sectional view schematically illustrating a plurality of pressure adjustment mechanisms and a pressure adjustment unit. 
         FIG. 7  is a block diagram illustrating an electrical configuration of the droplet discharging apparatus. 
         FIG. 8  is a diagram showing a simple harmonic motion calculation model made in consideration of residual vibration of a vibration plate. 
         FIG. 9  is a diagram for describing a relationship between an increase in viscosity of liquid and a residual vibration waveform. 
         FIG. 10  is a diagram for describing a relationship between air bubble intrusion and the residual vibration waveform. 
         FIG. 11  is a flowchart illustrating an example of a maintenance process. 
         FIG. 12  is a flowchart illustrating an example of a cleaning process. 
         FIG. 13  is a sectional view schematically illustrating the pressure adjustment mechanism and the droplet discharger with the on-off valve opened. 
         FIG. 14  is a sectional view schematically illustrating the pressure adjustment mechanism and the droplet discharger in the middle of a pressure reducing operation. 
         FIG. 15  is a sectional view schematically illustrating the pressure adjustment mechanism and the droplet discharger in the middle of a finishing wiping operation. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, an embodiment of a droplet discharging apparatus will be described with reference to drawings. The droplet discharging apparatus is an ink jet printer which records an image such as a character or a photograph by discharging ink, which is an example of liquid, to a recording medium such as a paper sheet. 
     As illustrated in  FIG. 1 , a droplet discharging apparatus  11  is provided with droplet dischargers  12  that discharge droplets, a supporting table  112  that supports a recording medium  113 , and a transporter  114  that transports the recording medium  113  in a transportation direction Y. The droplet dischargers  12  discharge liquid, which is supplied from a liquid supply source  13 , to the recording medium  113  in the form of droplets. The droplet dischargers  12  discharge droplets from a plurality of nozzles  19  formed in nozzle surfaces  18 . 
     The droplet discharging apparatus  11  is provided with a guide shaft  122  and a guide shaft  123  that extend along a scanning axis X and a carriage  124  that is supported by the guide shaft  122  and the guide shaft  123 . The droplet discharging apparatus  11  is provided with a carriage motor  125  that moves the carriage  124  along the guide shaft  122  and the guide shaft  123 . The scanning axis X is an axis not parallel to the transportation direction Y and a vertical direction Z. The carriage  124  reciprocates along the guide shaft  122 , the guide shaft  123 , and the scanning axis X when the carriage motor  125  is driven. 
     The droplet dischargers  12  are installed in the carriage  124 . The droplet dischargers  12  are attached to a lower end portion of the carriage  124  which is an end portion in the vertical direction Z. In the present embodiment, two droplet dischargers  12  are attached to the carriage  124 . The two droplet dischargers  12  are, at the lower end portion of the carriage  124 , disposed to be separated from each other in the scanning direction X by a predetermined distance and to be offset from each other in the transportation direction Y by a predetermined distance. 
     The droplet discharging apparatus  11  is configured as a serial type apparatus in which the droplet dischargers  12  reciprocate along the scanning axis X. The droplet discharging apparatus  11  may be configured as a line type apparatus in which the droplet dischargers  12  are provided to be elongated along the scanning axis X. 
     The supporting table  112  is disposed to face the droplet dischargers  12 . The supporting table  112  is provided to extend along the scanning axis X. The supporting table  112 , the transporter  114 , the guide shaft  122 , and the guide shaft  123  are assembled into a main body  116  that is configured of a housing, a frame, and the like. The main body  116  is provided with a cover  117  configured to be opened and closed. 
     The transporter  114  includes a pair of transportation rollers  118  that is positioned upstream of the supporting table  112  in the transportation direction Y and a pair of transportation rollers  119  that is positioned downstream of the supporting table  112  in the transportation direction Y. The transporter  114  includes a guide plate  120  that is positioned downstream of the pair of transportation rollers  119  in the transportation direction Y and that guides the recording medium  113 . The transporter  114  includes a transportation motor  121  that causes the pair of transportation rollers  118  and the pair of transportation rollers  119  to rotate. The pair of transportation rollers  118  and the pair of transportation rollers  119  transport the recording medium  113  when being rotated with the transportation motor  121  being driven in a state where the recording medium  113  is interposed therebetween. At this time, the recording medium  113  is transported along a surface of the supporting table  112  and a surface of the guide plate  120  while being supported by the supporting table  112  and the guide plate  120 . The transportation direction Y in the present embodiment is a direction in which the recording medium  113  on the supporting table  112  is transported. 
     As illustrated in  FIG. 2 , the droplet discharging apparatus  11  may be provided with a flushing mechanism  130 , a wiping mechanism  140 , and a capping mechanism  150 . In the present embodiment, the flushing mechanism  130 , the wiping mechanism  140 , and the capping mechanism  150  are provided in a non-recording region in the droplet discharging apparatus  11 , the non-recording region being a region in which no droplets are discharged to the recording medium  113 . The non-recording region in the present embodiment is a region in which the droplet dischargers  12  do not face the recording medium  113  in the middle of transportation, that is, a region adjacent to the supporting table  112  in a direction along the scanning axis X. 
     The flushing mechanism  130  includes a liquid receiver  131  receiving liquid that is discharged from the nozzles  19  of the droplet dischargers  12  due to a flushing operation. The flushing operation is an operation of discharging droplets not related to recording from the nozzles  19  in the purpose of preventing or resolving clogging or the like in the nozzles  19 . The liquid receiver  131  is formed in a box shape. The liquid receiver  131  is provided with an opening  132  that is open toward a moving region of the carriage  124 . The droplet dischargers  12  discharge droplets toward the opening  132  of the  131  at the time of the flushing operation. 
     As illustrated in  FIG. 3 , the wiping mechanism  140  includes a casing  141 , a feed roller  142 , a winding roller  143 , and an intermediate roller  144 . An upper portion of the casing  141  is provided with an opening  141   a . The feed roller  142  is positioned upstream in the transportation direction Y in the casing  141 . The winding roller  143  is positioned downstream in the transportation direction Y in the casing  141 . The intermediate roller  144  is positioned in the casing  141  such that the intermediate roller  144  is exposed through the opening  141   a.    
     The wiping mechanism  140  includes a pressing member  145 , a first wiper driving unit  146 , and a second wiper driving unit  147 . The pressing member  145  presses the intermediate roller  144  toward the outside of the casing  141 . When the first wiper driving unit  146  is driven, the casing  141  moves in the transportation direction Y. When the second wiper driving unit  147  is driven, the casing  141  moves in the vertical direction Z. When the second wiper driving unit  147  moves the casing  141  in the vertical direction Z, an interval between the casing  141  and the nozzle surfaces  18  in the vertical direction Z is adjusted. 
     The feed roller  142 , the winding roller  143 , and the intermediate roller  144  are configured to rotate and are supported by the casing  141  such that axial directions thereof become parallel to each other. A fabric wiper  148  configured to absorb liquid is wound onto the feed roller  142  in a roll shape. When the feed roller  142  rotates, the fabric wiper  148  is fed from the feed roller  142 . The fabric wiper  148  fed from the feed roller  142  is wound onto the intermediate roller  144  and wound onto the winding roller  143 . When the winding roller  143  rotates, the fabric wiper  148  is wound onto the winding roller  143 . 
     The wiping mechanism  140  is configured to wipe the nozzle surfaces  18 . A wiping operation is an operation of wiping the nozzle surfaces  18  to remove foreign substances such as liquid and dust adhering to the nozzle surfaces  18 . The wiping mechanism  140  wipes the nozzle surfaces  18  with a wiping portion  149 , which is a portion of the fabric wiper  148  that is wound onto the intermediate roller  144 . 
     The wiping mechanism  140  wipes the nozzle surfaces  18  in a state where the droplet dischargers  12  are positioned above the wiping mechanism  140 . In the case of the wiping mechanism  140  according to the present embodiment, when the wiping operation is performed, first, the casing  141  moves with the second wiper driving unit  147  being driven and thus the wiping portion  149  comes into contact with the nozzle surfaces  18 . Thereafter, the casing  141  moves with the first wiper driving unit  146  being driven and thus the wiping portion  149  wipes the nozzle surfaces  18 . In this manner, the wiping mechanism  140  wipes the nozzle surfaces  18 . 
     When the wiping mechanism  140  wipes the nozzle surfaces  18 , the droplet dischargers  12  may move relative to the wiping mechanism  140  and both of the wiping mechanism  140  and the droplet dischargers  12  may move. When the wiping mechanism  140  wipes the nozzle surfaces  18 , the wiping mechanism  140  and the droplet dischargers  12  move relative to each other. 
     When the winding roller  143  is rotated after liquid is absorbed by the wiping portion  149  due to the wiping operation, a portion of the fabric wiper  148  that has absorbed liquid is wound. Accordingly, a portion serving as the wiping portion  149  is changed from a portion of the fabric wiper  148  that has absorbed the liquid to a portion of the fabric wiper  148  that has not absorbed liquid. 
     As illustrated in  FIG. 2 , the capping mechanism  150  includes caps  151  that are configured to cap the nozzle surfaces  18  and a cap driving unit  152  that lifts and lowers the caps  151 . A capping operation is an operation of causing the caps  151  to come into contact with the droplet dischargers  12  such that a space into which the nozzles  19  are open is formed. The caps  151  cap the nozzle surfaces  18  to cover openings of the nozzles  19 . Accordingly, it is possible to suppress an increase in viscosity of liquid in the nozzles  19  which occurs when the liquid is dried. 
     The caps  151  may be configured to form closed spaces such that no fluid such as air or liquid enters or exits the caps  151  in a state where the nozzle surfaces  18  are capped. In this case, it is possible to further suppress the drying of liquid in the nozzles  19  by means of the capping operation. 
     The capping mechanism  150  includes a plurality of caps  151  corresponding to the number of droplet dischargers  12 . In the present embodiment, the capping mechanism  150  includes two caps  151 . The capping mechanism  150  caps the nozzle surfaces  18  of the two droplet dischargers  12  in a state where the two droplet dischargers  12  face the two caps  151  respectively. 
     In the case of the capping mechanism  150  according to the present embodiment, when the capping operation is performed, the cap driving unit  152  drives the two caps  151  such that the two caps  151  are lifted. Therefore, the two caps  151  come into contact with the nozzle surfaces  18  of the two droplet dischargers  12  such that the caps  151  cover the openings of all of the nozzles  19 . As a result, the nozzle surfaces  18  of the droplet dischargers  12  are capped by the caps  151 . That is, each cap  151  is configured to cap a region including all of the nozzles  19  in the nozzle surface  18  of each droplet discharger  12 . 
     When the caps  151  cap the droplet dischargers  12 , the droplet dischargers  12  may move relative to the capping mechanism  150  and both of the cap  151  and the droplet dischargers  12  may move. When the caps  151  cap the droplet dischargers  12 , the cap  151  and the droplet dischargers  12  move relative to each other. Each of the caps  151  may include an atmosphere opening valve. The atmosphere opening valve is a valve that can cause the inside of the cap  151  and the atmosphere outside the cap  151  to communicate with each other in a state where the nozzle surface  18  is capped by the cap  151 . Therefore, when the atmosphere opening valve is opened, a space inside the cap  151  is opened to the atmosphere. 
     As illustrated in  FIG. 4 , the droplet discharging apparatus  11  is provided with a liquid supply flow path  27  through which liquid is supplied from the liquid supply source  13  to the droplet discharger  12  and a return flow path  28  through which liquid returns to the liquid supply flow path  27  from the droplet discharger  12 . The liquid supply flow path  27  is coupled to the liquid supply source  13  and the droplet discharger  12 . The liquid supply flow path  27  is a flow path through which liquid is supplied from the liquid supply source  13 , which is disposed upstream in a supply direction A of liquid, to the droplet discharger  12 , which is disposed downstream in the supply direction A. 
     The return flow path  28  is coupled to the droplet discharger  12  and the liquid supply flow path  27 . The return flow path  28  is coupled to an intermediate portion of the liquid supply flow path  27 . The return flow path  28  forms a circulation path  30  for circulation of liquid together with the liquid supply flow path  27 . That is, the circulation path  30  is configured to include the liquid supply flow path  27  and the return flow path  28 . Liquid flowing through the circulation path  30  circulates through the droplet discharger  12 , the liquid supply flow path  27 , and the return flow path  28 . The return flow path  28  is provided with circulation pumps  29  that circulate liquid. The circulation pumps  29  cause liquid to flow in a circulation direction B. 
     The liquid supply source  13  is, for example, a container configured to accommodate liquid. The liquid supply source  13  may be a replaceable cartridge or a tank to which liquid can be supplied. A plurality of the liquid supply sources  13 , a plurality of the liquid supply flow paths  27 , and a plurality of the return flow paths  28  are provided corresponding to the number of kinds of liquid discharged from the droplet dischargers  12 . In the present embodiment, four liquid supply sources  13 , four liquid supply flow paths  27 , and four return flow paths  28  are provided. The droplet discharging apparatus  11  may be provided with a mounting portion  26  into which the liquid supply source  13  is mounted. 
     As illustrated in  FIGS. 4 and 5 , the droplet discharger  12  is provided with a common liquid chamber  17  into which liquid is supplied. Liquid is supplied to the common liquid chamber  17  from the liquid supply source  13  via the liquid supply flow path  27 . The liquid supply flow path  27  is coupled to the common liquid chamber  17 . The common liquid chamber  17  may be provided with a filter  16  that captures air bubbles, foreign substances or the like in liquid supplied to the common liquid chamber  17 . The common liquid chamber  17  stores liquid passing through the filter  16 . 
     The droplet discharger  12  is provided with a plurality of pressure chambers  20  communicating with the common liquid chamber  17 . The nozzles  19  are provided corresponding to the plurality of pressure chambers  20 . The pressure chambers  20  communicate with the common liquid chamber  17  and the nozzles  19 . A portion of a wall surface of the pressure chamber  20  is formed by a vibration plate  21 . The common liquid chamber  17  and the pressure chambers  20  communicate with each other via a supply side communication path  22 . 
     The droplet discharger  12  is provided with a plurality of actuators  24  provided corresponding to the plurality of pressure chambers  20 . The actuators  24  are provided on a surface of the vibration plate  21  that is opposite to a portion facing the pressure chambers  20 . Each actuator  24  is accommodated in an accommodation chamber  23  disposed at a different position from that of the common liquid chamber  17 . The droplet discharger  12  discharges liquid in the pressure chambers  20  from the nozzles  19  in the form of droplets when the actuators  24  are driven. The droplet discharger  12  performs a recording process on the recording medium  113  by discharging droplets to the recording medium  113  from the nozzles  19 . 
     In the present embodiment, a piezoelectric element which shrinks when a drive voltage is applied thereto constitutes each actuator  24 . When application of a drive voltage to the actuators  24  is stopped after the vibration plate  21  is deformed due to the actuators  24  shrinking attributable to the drive voltage application, liquid in the pressure chambers  20  changed in volume is discharged from the nozzles  19  in the form of droplets. 
     The droplet discharger  12  includes a discharge flow path  80  through which liquid in the droplet discharger  12  is discharged to the outside without passing through the nozzles  19 . The discharge flow path  80  is provided with a first discharge flow path  81  that is coupled to the pressure chambers  20  such that liquid in the pressure chambers  20  is discharged to the outside. Liquid flowing through the first discharge flow path  81  is discharged to the outsides of the pressure chambers  20  from the pressure chambers  20  without passing through the nozzles  19 . 
     The droplet discharger  12  may include a discharge liquid chamber  83  communicating with the plurality of pressure chambers  20  and the first discharge flow path  81 . In this case, the first discharge flow path  81  communicate with the plurality of pressure chambers  20  via the discharge liquid chamber  83 . That is, the first discharge flow path  81  is indirectly coupled to the pressure chambers  20 . The pressure chambers  20  and the discharge liquid chamber  83  communicate with each other via a discharge side communication path  84 . Since the discharge liquid chamber  83  is provided, it is sufficient that one first discharge flow path  81  is provided for the plurality of pressure chambers  20 . That is, since the discharge liquid chamber  83  is provided, it is not necessary to provide the first discharge flow path  81  for each pressure chamber  20 . Therefore, it is possible to simplify the configuration of the droplet discharger  12 . The droplet discharger  12  may be provided with a plurality of the first discharge flow paths  81  corresponding to the plurality of pressure chambers  20 . 
     The droplet discharger  12  may include a second discharge flow path  82  that is coupled to the common liquid chamber  17  and the return flow path  28  such that liquid in the common liquid chamber  17  is discharged to the outside without passing through the pressure chambers  20 . In this case, the discharge flow path  80  is provided with the first discharge flow path  81  and the second discharge flow path  82 . That is, the droplet discharger  12  includes the first discharge flow path  81  and the second discharge flow path  82 . The first discharge flow path  81  is the discharge flow path  80  coupled to the pressure chambers  20 . The second discharge flow path  82  is the discharge flow path  80  coupled to the common liquid chamber  17 . 
     The return flow path  28  may be provided with a first return flow path  281  coupled to the first discharge flow path  81  and a second return flow path  282  coupled to the second discharge flow path  82 . The return flow path  28  in the present embodiment is configured such that the first return flow path  281  and the second return flow path  282  join each other. The return flow path  28  may be configured such that the first return flow path  281  and the second return flow path  282  do not join each other and may be configured such that the first return flow path  281  and the second return flow path  282  are coupled to the liquid supply flow path  27 . 
     In the present embodiment, the circulation pump  29  is provided for each of the first return flow path  281  and the second return flow path  282 . The first return flow path  281  is provided with a first circulation pump  291  as the circulation pump  29 . The second return flow path  282  is provided with a second circulation pump  292  as the circulation pump  29 . 
     The first return flow path  281  may be provided with a first on-off valve  283 . In the first return flow path  281 , the first on-off valve  283  is positioned between the first circulation pump  291  and the droplet discharger  12 . When the first circulation pump  291  is driven with the first on-off valve  283  opened, liquid flows from the pressure chambers  20  to the liquid supply flow path  27  through the discharge liquid chamber  83  and the first return flow path  281 . 
     The second return flow path  282  may be provided with a second on-off valve  284 . In the second return flow path  282 , the second on-off valve  284  is positioned between the second circulation pump  292  and the droplet discharger  12 . When the second circulation pump  292  is driven with the second on-off valve  284  opened, liquid flows from the common liquid chamber  17  to the liquid supply flow path  27  through the second return flow path  282 . 
     Only one circulation pump  29  may be provided in the first return flow path  281  and the second return flow path  282 . In this case, the circulation pump  29  is disposed between a portion of the return flow path  28  at which the first return flow path  281  and the second return flow path  282  join each other and a portion of the return flow path  28  at which the return flow path  28  is connected to the liquid supply flow path  27 . In this case, it is possible to cause liquid to flow through any of the first return flow path  281  and the second return flow path  282  by controlling the first on-off valve  283  and the second on-off valve  284 . 
     In the first return flow path  281 , a first damper  285  may be provided between the droplet discharger  12  and the first on-off valve  283 . The first damper  285  is configured to store liquid. For example, one surface of the first damper  285  is formed as a flexible film and the first damper  285  is configured such that the volume of liquid stored in the first damper  285  can be changed. In the second return flow path  282 , a second damper  286  having the same configuration as the first damper  285  may be provided between the droplet discharger  12  and the second on-off valve  284 . In this case, it is possible to suppress, by means of a change in volume of the first damper  285  and the second damper  286 , a fluctuation in pressure in the droplet discharger  12  which occurs when liquid flows through the first return flow path  281  and the second return flow path  282 . 
     As illustrated in  FIG. 4 , the liquid supply flow path  27  is provided with a pressurizing mechanism  31 , a filter unit  32 , a static mixer  33 , a liquid storing unit  34 , a degasification mechanism  46 , and a pressure adjustment device  47 . In the liquid supply flow path  27 , the pressurizing mechanism  31 , the filter unit  32 , the static mixer  33 , the liquid storing unit  34 , the degasification mechanism  46 , and the pressure adjustment device  47  are disposed in this order in a direction from the liquid supply source  13  side which is positioned upstream to the droplet discharger  12  side which is positioned downstream. 
     The pressurizing mechanism  31  is positioned in the liquid supply flow path  27  while being positioned closer to the liquid supply source  13  side than a position at which the return flow path  28  is coupled to the liquid supply flow path  27 . The filter unit  32 , the static mixer  33 , the liquid storing unit  34 , the degasification mechanism  46 , and the pressure adjustment device  47  are positioned in the liquid supply flow path  27  while being positioned closer to the droplet discharger  12  side than a position at which the return flow path  28  is coupled to the liquid supply flow path  27 . 
     The pressurizing mechanism  31  causes liquid to flow in the supply direction A from the liquid supply source  13  such that the liquid is supplied to the droplet discharger  12 . The pressurizing mechanism  31  includes a volume pump  38 , an one-way valve  39 , and an one-way valve  40 . The volume pump  38  is configured to pressurize a predetermined amount of liquid by reciprocating a flexible member  37  which is flexible. 
     The volume pump  38  includes a pump chamber  41  and a negative pressure chamber  42 . The volume pump  38  is partitioned into the pump chamber  41  and the negative pressure chamber  42  by the flexible member  37 . Furthermore, the volume pump  38  includes a pressure reduction unit  43  that reduces the pressure in the negative pressure chamber  42  and a pressing member  44  that is provided in the negative pressure chamber  42  and urges the flexible member  37  toward the pump chamber  41  side. 
     The one-way valve  39  is positioned upstream of the volume pump  38  in the liquid supply flow path  27 . The one-way valve  40  is positioned downstream of the volume pump  38  in the liquid supply flow path  27 . The one-way valve  39  and the one-way valve  40  are configured to allow liquid to flow to downstream from upstream in the liquid supply flow path  27  and to inhibit liquid from flowing to the upstream from the downstream. That is, the pressurizing mechanism  31  can pressurize liquid to be supplied to the pressure adjustment device  47  with the pressing member  44  pressing liquid in the pump chamber  41  via the flexible member  37 . Accordingly, a pressurizing force at which the pressurizing mechanism  31  pressurizes the liquid is set by means of a pressing force of the pressing member  44 . In this regard, it can be said that the pressurizing mechanism  31  can pressurize liquid in the liquid supply flow path  27  in the present embodiment. 
     The filter unit  32  is configured to capture air bubbles and foreign substances in liquid. The filter unit  32  is provided to be replaceable. The static mixer  33  is configured to cause changes such as direction reversal or division in the flow of the liquid and reduce concentration bias in the liquid. The liquid storing unit  34  is configured to store liquid in a space with variable volume that is pressed by a spring  45  and alleviate a fluctuation in pressure of the liquid. 
     The degasification mechanism  46  includes a degasification chamber  461  in which liquid is temporarily stored, a pressure reduction chamber  463  that is separated from the degasification chamber  461  by a degasification film  462 , a pressure reduction flow path  464  connected to the pressure reduction chamber  463 , and a pump  465 . The degasification film  462  has a property of allowing a gas to pass through the degasification film  462  and prevent liquid from passing through the degasification film  462 . The degasification mechanism  46  decreases, by driving the pump  465 , the pressure in the pressure reduction chamber  463  through the pressure reduction flow path  464  such that air bubbles, a resolved gas, and the like mixed in liquid stored in the degasification chamber  461  are removed. The degasification mechanism  46  may be configured to increase the pressure in the degasification chamber  461  such that air bubbles, a resolved gas, and the like mixed in liquid stored in the degasification chamber  461  are removed. 
     Next, the pressure adjustment device  47  will be described. 
     The pressure adjustment device  47  includes a pressure adjustment mechanism  35  that constitutes a portion of the liquid supply flow path  27  and a pressing mechanism  48  that presses the pressure adjustment mechanism  35 . The pressure adjustment mechanism  35  includes a main body portion  52 , in which a liquid inflow portion  50  into which liquid that is supplied from the liquid supply source  13  via the liquid supply flow path  27  flows and a liquid outflow portion  51  that can accommodate the liquid are formed. 
     The liquid supply flow path  27  and the liquid inflow portion  50  are separated from each other by a wall  53  of the main body portion  52  and communicate with each other via through holes  54  formed in the wall  53 . The through holes  54  are covered by filter members  55 . Therefore, liquid in the liquid supply flow path  27  flows into the liquid inflow portion  50  while being filtered by the filter members  55 . 
     At least a portion of the wall portion of the liquid outflow portion  51  is configured of a diaphragm  56 . A first surface  56   a  of the diaphragm  56 , which is an inner surface of the liquid outflow portion  51 , receives the pressure of liquid in liquid outflow portion  51 . A second surface  56   b  of the diaphragm  56 , which is an outer surface of the liquid outflow portion  51 , receives atmospheric pressure. Therefore, the diaphragm  56  is displaced corresponding to the pressure in the liquid outflow portion  51 . The volume of the liquid outflow portion  51  changes when the diaphragm  56  is displaced. The liquid inflow portion  50  and the liquid outflow portion  51  communicate with each other via a communication path  57 . 
     The pressure adjustment mechanism  35  includes an on-off valve  59  that can switch between a closed state in which the liquid inflow portion  50  and the liquid outflow portion  51  do not communicate with each other via the communication path  57  and an opened state in which the liquid inflow portion  50  and the liquid outflow portion  51  communicate with each other. The one-off valve  59  shown in  FIG. 4  is in the closed state. The on-off valve  59  includes a valve portion  60  that can block the communication path  57  and a pressure receiving portion  61  that receives a pressure from the diaphragm  56 . The on-off valve  59  moves when the pressure receiving portion  61  is pressed by the diaphragm  56 . That is, the pressure receiving portion  61  also functions as a moving member that can move in a state of being in contact with the diaphragm  56  that is displaced in a direction in which the volume of the liquid outflow portion  51  is reduced. 
     An upstream pressing member  62  is provided in the liquid inflow portion  50 . A downstream pressing member  63  is provided in the liquid outflow portion  51 . The upstream pressing member  62  and the downstream pressing member  63  urge the on-off valve  59  in a direction in which the on-off valve  59  is closed. The state of the on-off valve  59  is changed to the opened state from the closed state when a pressure applied to the first surface  56   a  is lower than a pressure applied to the second surface  56   b  and a difference between the pressure applied to the first surface  56   a  and the pressure applied to the second surface  56   b  is equal to or greater than a predetermined value. The predetermined value is, for example, 1 kPa. 
     The predetermined value is a value determined corresponding to the pressing force of the upstream pressing member  62 , the pressing force of the downstream pressing member  63 , a force required to displace the diaphragm  56 , a sealing load which is a pressing force required to block the communication path  57  with the valve portion  60 , the pressure in the liquid inflow portion  50  which acts on a surface of the valve portion  60 , and the pressure in the liquid outflow portion  51 . That is, the predetermined value for switch from the closed state to the opened state increases as the pressing forces of the upstream pressing member  62  and the downstream pressing member  63  increase. 
     The pressing forces of the upstream pressing member  62  and the downstream pressing member  63  are set such that the pressure in the liquid outflow portion  51  becomes a negative pressure at which a meniscus can be formed on a gas-liquid interface in the nozzle  19 . For example, when a pressure applied to the second surface  56   b  is atmospheric pressure, the pressing forces of the upstream pressing member  62  and the downstream pressing member  63  are set such that the pressure in the liquid outflow portion  51  becomes −1 kPa. In this case, the gas-liquid interface is a boundary at which the liquid and the gas are in contact with each other and the meniscus is a curved liquid surface which is generated when liquid comes into contact with the nozzle  19 . In addition, it is preferable that a concave meniscus suitable for droplet discharge be formed in the nozzle  19 . 
     In the present embodiment, when the on-off valve  59  in the pressure adjustment mechanism  35  is in the closed state, the pressure of liquid positioned upstream of the pressure adjustment mechanism  35  generally becomes a positive pressure due to the pressurizing mechanism  31 . Specifically, when the on-off valve  59  is in the closed state, the pressure of liquid in the liquid inflow portion  50  and the pressure of liquid positioned upstream of the liquid inflow portion  50  generally become a positive pressure due to the pressurizing mechanism  31 . 
     In the present embodiment, when the on-off valve  59  in the pressure adjustment mechanism  35  is in the closed state, the pressure of liquid positioned downstream of the pressure adjustment mechanism  35  generally becomes a negative pressure due to the diaphragm  56 . Specifically, when the on-off valve  59  is in the closed state, the pressure of liquid in the liquid outflow portion  51  and the pressure of liquid positioned downstream of the liquid outflow portion  51  generally become a negative pressure due to the diaphragm  56 . 
     When the droplet discharger  12  discharges droplets, liquid accommodated in the liquid outflow portion  51  is supplied to the droplet discharger  12  via the liquid supply flow path  27 . As a result, the pressure in the liquid outflow portion  51  is reduced. When a difference between a pressure applied to the first surface  56   a  of the diaphragm  56  and a pressure applied to the second surface  56   b  becomes equal to or greater than the predetermined value due to the above-described pressure reduction, the diaphragm  56  is bent and deformed in a direction in which the volume of the liquid outflow portion  51  is reduced. When the pressure receiving portion  61  is pressed and moved in accordance with the deformation of the diaphragm  56 , the on-off valve  59  enters the opened state. 
     When the on-off valve  59  enters the opened state, since the liquid in the liquid inflow portion  50  is pressurized by the pressurizing mechanism  31 , liquid is supplied to the liquid outflow portion  51  from the liquid inflow portion  50 . As a result, the pressure in the liquid outflow portion  51  increases. When the pressure in the liquid outflow portion  51  increases, the diaphragm  56  is deformed such that the volume of the liquid outflow portion  51  increases. When the difference between the pressure applied to the first surface  56   a  of the diaphragm  56  and the pressure applied to the second surface  56   b  becomes lower than the predetermined value, the state of the on-off valve  59  changes to the closed state from the opened state. As a result, the on-off valve  59  inhibits liquid from flowing to the liquid outflow portion  51  from the liquid inflow portion  50 . 
     As described above, the pressure adjustment mechanism  35  adjusts the pressure of liquid supplied to the droplet discharger  12  by means of displacement of the diaphragm  56  in order to adjust the pressure in the droplet discharger  12  in which the nozzle  19  causes a back pressure. 
     The pressing mechanism  48  includes an expansion and contraction portion  67  that forms a pressure adjustment chamber  66  which is positioned close to the second surface  56   b  of the diaphragm  56 , a retaining member  68  that retains the expansion and contraction portion  67 , and a pressure adjustment unit  69  that can adjust the pressure in the pressure adjustment chamber  66 . The expansion and contraction portion  67  is formed of rubber or resin and is formed into a balloon-like shape. The expansion and contraction portion  67  expands or contracts in response to adjustment of the pressure in the pressure adjustment chamber  66  which is performed by the pressure adjustment unit  69 . The retaining member  68  is formed in a bottomed cylindrical shape. A portion of the expansion and contraction portion  67  is inserted into an insertion hole  70  formed in the bottom portion of the retaining member  68 . 
     An end edge portion of an inner surface of the retaining member  68  that is on an opening portion  71  side is given roundness through R-chamfering. The retaining member  68  is attached to the pressure adjustment mechanism  35  such that the opening portion  71  is blocked by the pressure adjustment mechanism  35 . Therefore, the retaining member  68  forms an air chamber  72  that covers the second surface  56   b  of the diaphragm  56 . The pressure in the air chamber  72  is set to atmospheric pressure. Therefore, the atmospheric pressure acts on the second surface  56   b  of the diaphragm  56 . 
     The pressure adjustment unit  69  causes the expansion and contraction portion  67  to expand by adjusting the pressure in the pressure adjustment chamber  66  to be higher than the atmospheric pressure which is the pressure in the air chamber  72 . The pressing mechanism  48  presses the diaphragm  56  in a direction in which the volume of the liquid outflow portion  51  is reduced with the pressure adjustment unit  69  causing the expansion and contraction portion  67  to expand. At this time, the expansion and contraction portion  67  of the pressing mechanism  48  presses a portion of the diaphragm  56  that comes into contact with the pressure receiving portion  61 . The area of the portion of the diaphragm  56  that comes into contact with the pressure receiving portion  61  is greater than the cross-sectional area of the communication path  57 . 
     As illustrated in  FIG. 6 , the pressure adjustment unit  69  includes a pressurizing pump  74  that pressurizes fluid such as air or water and a coupling path  75  that couples the pressurizing pump  74  and the expansion and contraction portions  67  to each other. The pressure adjustment unit  69  includes a pressure measurer  76  that measures the pressure of fluid in the coupling path  75  and a fluid pressure adjustment unit  77  that adjusts the pressure of fluid in the coupling path  75 . 
     The coupling path  75  branches into a plurality of flow paths and the flow paths are respectively coupled to the expansion and contraction portions  67  of a plurality of the pressure adjustment devices  47 . In the present embodiment, the  75  branches into four flow paths and the four flow paths are respectively coupled to the expansion and contraction portions  67  of four pressure adjustment devices  47 . Fluid pressurized by the pressurizing pump  74  is supplied to each of the expansion and contraction portions  67  via the coupling path  75 . A changeover valve that switches the state of a flow path between an opened state and a closed state may be provided for each of the plurality of branches of the coupling path  75 . In this case, it is possible to selectively supply the pressurized fluid to the plurality of expansion and contraction portions  67  by controlling the changeover valves. 
     The fluid pressure adjustment unit  77  is configured of, for example, a safety valve. The fluid pressure adjustment unit  77  is configured to be automatically opened when the pressure of fluid in the coupling path  75  becomes higher than a predetermined pressure. When the fluid pressure adjustment unit  77  is opened, the fluid in the coupling path  75  is discharged to the outside. In this manner, the fluid pressure adjustment unit  77  reduces the pressure of fluid in the coupling path  75 . 
     Next, the electrical configuration of the droplet discharging apparatus  11  will be described. 
     As illustrated in  FIG. 7 , the droplet discharging apparatus  11  is provided with a controller  160  that collectively controls constituent elements of the droplet discharging apparatus  11  and a detector group  170  controlled by the controller  160 . The detector group  170  includes a detector  171  that detects the state of the insides of the pressure chambers  20  by detecting the vibration waveforms of the pressure chambers  20 . The detector group  170  monitors a situation in the droplet discharging apparatus  11 . The detector group  170  outputs the result of the detection to the controller  160 . 
     The controller  160  includes an interface unit  161 , a CPU  162 , a memory  163 , a control circuit  164 , and a drive circuit  165 . The interface unit  161  transmits and receives data between a computer  180 , which is an external device, and the droplet discharging apparatus  11 . The drive circuit  165  generates a drive signal to drive the actuators  24 . 
     The CPU  162  is a calculation processing device. The memory  163  is a storing device that secures a region storing a program for the CPU  162  or a working region and includes a storing element such as a RAM, an EEPROM, or the like. The CPU  162  controls, based on a program stored in the memory  163 , the circulation pumps  29 , the pressurizing mechanism  31 , the pressure adjustment devices  47 , the transporter  114 , the wiping mechanism  140 , the capping mechanism  150 , the droplet dischargers  12 , and the like via the control circuit  164 . 
     The detector group  170  includes, for example, a linear encoder that detects the state of movement of the carriage  124 , a medium detecting sensor that detects the recording medium  113 , and the detector  171  which is a circuit detecting residual vibration of the pressure chambers  20 . The controller  160  performs nozzle inspection, which will be described later, based on the result of detection performed by the detector  171 . The detector  171  may include piezoelectric elements constituting the actuators  24 . 
     Next, the nozzle inspection will be described. 
     When voltage is applied to the actuators  24  through a signal from the drive circuit  165 , the vibration plate  21  is bent and deformed. Accordingly, there is a fluctuation in pressure in the pressure chambers  20 . Due to the fluctuation, the vibration plate  21  vibrates for a while. This vibration is called residual vibration. Detecting the state of the pressure chambers  20  and the nozzles  19  communicating with the pressure chambers  20  from the state of the residual vibration will be referred to as the nozzle inspection. 
       FIG. 8  is a diagram showing a simple harmonic motion calculation model made in consideration of the residual vibration of the vibration plate  21 . 
     When the drive circuit  165  applies a drive signal to the actuators  24 , the actuators  24  expand and contract corresponding to the voltage of the drive signal. The vibration plate  21  is bent corresponding to the expansion and contraction of the actuators  24 . Accordingly, the volume of the pressure chambers  20  is decreased after being increased. At this time, due to a pressure generated in the pressure chambers  20 , a portion of liquid filling the pressure chambers  20  is discharged from the nozzles  19  in the form of droplets. 
     At the time of the above-described series of actions of the vibration plate  21 , the vibration plate  21  free-vibrates at a natural vibration frequency which is determined by a flow path resistance r, an inertance m, and the compliance C of the vibration plate  21 . The flow path resistance r is determined by the shape of a flow path in which liquid flows, the viscosity of the liquid, and the like and the inertance m is determined by the weight of liquid in the flow path. The free vibration of the vibration plate  21  is the residual vibration of the vibration plate  21 . 
     The residual vibration calculation model of the vibration plate  21  which is shown in  FIG. 8  can be represented with a pressure P, the inertance m, the compliance C, and the flow path resistance r. When step response at a time when the pressure P is applied to a circuit in  FIG. 8  is calculated with respect to a volume velocity u, the following equations are obtained. 
     
       
         
           
             
               
                 
                   u 
                   = 
                   
                     
                       P 
                       
                         ω 
                         · 
                         m 
                       
                     
                     ⁢ 
                     
                       
                         e 
                         
                           
                             - 
                             ω 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           t 
                         
                       
                       · 
                       sin 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     ω 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     t 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   ω 
                   = 
                   
                     
                       
                         1 
                         
                           m 
                           · 
                           C 
                         
                       
                       - 
                       
                         a 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   α 
                   = 
                   
                     r 
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
       FIG. 9  is a diagram for describing a relationship between an increase in viscosity of liquid and a residual vibration waveform. The horizontal axis in  FIG. 9  represents time and the vertical axis represents the magnitude of residual vibration. For example, when liquid near the nozzle  19  is dried, the viscosity of the liquid is increased. When the viscosity of the liquid is increased, the flow path resistance r increases and thus the vibration cycle and attenuation of residual vibration become great. 
       FIG. 10  is a diagram for describing a relationship between air bubble intrusion and the residual vibration waveform. The horizontal axis in  FIG. 10  represents time and the vertical axis represents the magnitude of residual vibration. For example, when air bubbles intrude into a liquid flow path or a tip end of the nozzle  19 , the inertance m, which is the weight of liquid, decreases corresponding to the air bubble intrusion in comparison with a case where the nozzle  19  is in a normal state. As the inertance m decreases, an angular velocity co increases as understood from Equation (2) and thus the vibration cycle becomes short. That is, the vibration frequency becomes great. 
     In addition, it is considered that the amount of liquid in the pressure chambers  20  and the amount of liquid corresponding to seepage are increased in comparison with a normal state as seen from the vibration plate  21  such that the inertance m is increased when foreign substances such as paper dust adheres to the vicinity of openings of the nozzles  19 . It is considered that the flow path resistance r is increased due to fibers of the paper dust adhering to the vicinity of outlets of the nozzles  19 . Therefore, when paper dust adheres to the vicinity of the openings of the nozzles  19 , a frequency becomes lower in comparison with a case where liquid is discharged normally and becomes higher in comparison with a case where the viscosity of the liquid is increased. 
     When an increase in viscosity of liquid, intrusion of air bubbles, adhesion of foreign substances, or the like occurs, the state of the insides of the nozzles  19  and the state of the insides of the pressure chambers  20  become abnormal and thus liquid becomes not able to be discharged from the nozzles  19  in a typical manner. Therefore, dot omission on an image recorded onto the recording medium  113  occurs. Even if droplets are discharged from the nozzles  19 , the amounts of droplets may be small or the droplets may not be landed on target positions due to flying direction deviation of the droplets. The nozzle  19  with such a discharge failure will be referred to as an abnormal nozzle. 
     As described above, the residual vibration of the pressure chamber  20  communicating with an abnormal nozzle is different from the residual vibration of the pressure chamber  20  communicating with the nozzle  19  in a normal state. Therefore, the detector  171  detects the state of the inside of the pressure chamber  20  by detecting the vibration waveform of the pressure chamber  20 . The controller  160  performs inspection of the nozzle  19  based on the result of the detection performed by the detector  171 . 
     The controller  160  may estimate whether the state of the inside of the pressure chamber  20  is normal or abnormal based on the vibration waveform of the pressure chamber  20 , which is the result of the detection performed by the detector  171 . When the state of the inside of the pressure chamber  20  is abnormal, the nozzle  19  communicating with the pressure chamber  20  is estimated as an abnormal nozzle. The controller  160  may estimate, based on the vibration waveform of the pressure chamber  20 , whether the state of the inside of the pressure chamber  20  is abnormal due to air bubbles present therein or the state of the inside of the pressure chamber  20  is abnormal due to an increase in viscosity of liquid. The controller  160  may estimate, based on the vibration waveform of the pressure chamber  20 , the total volume of air bubbles present in the pressure chamber  20  and the nozzle  19  communicating with the pressure chamber  20  and the degree to which liquid in the pressure chamber  20  and the nozzle  19  communicating with the pressure chamber  20  is increased in viscosity. 
     The frequency of a vibration waveform that is detected in a state where air bubbles are present in the pressure chamber  20  and the nozzle  19  filled with liquid is higher than the frequency of a vibration waveform that is detected in a state where air bubbles are not present in the pressure chamber  20  and the nozzle  19  filled with liquid. The frequency of a vibration waveform that is detected in a state where the pressure chamber  20  and the nozzle  19  are filled with air is higher than the frequency of a vibration waveform that is detected in a state where air bubbles are present in the pressure chamber  20  and the nozzle  19  filled with liquid. The larger the air bubbles present in the pressure chamber  20  and the nozzle  19  filled with liquid, the higher the frequency of the vibration waveform is. 
     When liquid becomes stagnant in the droplet discharging apparatus  11 , the liquid becomes likely to be increased in viscosity or air bubbles become likely to be accumulated. In this case, there is a high possibility of an abnormal nozzle. That is, the state of the insides of the pressure chambers  20  is likely to be abnormal. Therefore, the droplet discharging apparatus  11  is configured to perform a maintenance operation of performing maintenance of the droplet discharger  12  in order to suppress an increase in viscosity of liquid or discharge air bubbles. The droplet discharging apparatus  11  in the present embodiment is configured to perform a first discharge operation, a second discharge operation, a third discharge operation, a fourth discharge operation, and a fifth discharge operation as the maintenance operation for the droplet discharger  12 . 
     The droplet discharging apparatus  11  performs, as the maintenance operation for the droplet discharger  12 , the first discharge operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  when no droplets are discharged from the nozzles  19  during a recording process. The first discharge operation is an operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the first discharge flow path  81 . 
     A time when no droplets are discharged from the nozzles  19  during the recording process is, for example, a returning time of the carriage  124  or an inter-page time of the recording medium  113 . The returning time of the carriage  124  is a time at which the carriage  124  moves to return to a home position. The inter-page time of the recording medium  113  is a time between when an image is recorded on the recording medium  113  and when the next recording medium  113  reaches a position facing the droplet dischargers  12 . The droplet discharging apparatus  11  performs the first discharge operation at such a time. 
     In the droplet discharger  12  in the middle of the recording process, the nozzles  19  used for recording and the nozzles  19  not used for the recording are present. In the nozzles  19  used for the recording and the pressure chambers  20  communicating with the nozzles  19 , liquid is less likely to be increased in viscosity since the liquid is discharged from the nozzles  19 . In the nozzles  19  not used for the recording and the pressure chambers  20  communicating with the nozzles  19 , liquid becomes stagnant and is likely to be increased in viscosity since the liquid is not discharged from the nozzles  19 . 
     In order to suppress an increase in viscosity of liquid, generally, the flushing operation is performed. If the flushing operation is performed at a time when no droplets are discharged from the nozzles  19  during the recording process, that is, at the returning time of the carriage  124  or the inter-page time of the recording medium  113 , an increase in viscosity of liquid in the droplet discharger  12  can be suppressed. When the flushing operation is performed, droplets are discharged from the nozzles  19  and thus liquid is consumed. When the flushing operation is performed for each time the recording process is performed in order to suppress an increase in viscosity of liquid, the amount of liquid consumed becomes large. 
     When the droplet discharging apparatus  11  performs the first discharge operation, liquid discharged from the pressure chambers  20  to the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  flows in the circulation path  30 . Since the liquid flows, an increase in viscosity of the liquid is suppressed. Therefore, by using the first discharge operation, it is possible to suppress an increase in viscosity of liquid without discharging droplets from the nozzles  19 . Therefore, it is possible to reduce the amount of liquid consumed for maintenance. 
     In the first discharge operation, the droplet discharging apparatus  11  may cause liquid to be discharged toward the return flow path  28  with the liquid in the pressure chambers  20  sucked from the discharge flow path  80  side such that meniscuses on gas-liquid interfaces in the nozzles  19  are maintained. The droplet discharging apparatus  11  in the present embodiment performs the first discharge operation by driving the circulation pumps  29 . When the first discharge operation is performed with the liquid in the pressure chambers  20  sucked from the discharge flow path  80  side, the meniscuses on the gas-liquid interfaces in the nozzles  19  are moved toward the pressure chambers  20 . That is, liquid in the nozzles  19  flows. Therefore, an increase in viscosity of the liquid in the nozzles  19  can be suppressed. 
     The droplet discharging apparatus  11  may be configured to cause liquid in the pressure chambers  20  to be discharged toward the return flow path  28  by pressurizing the liquid in the pressure chambers  20  from the liquid supply flow path  27  side. In this case, the liquid may be pressurized at such a pressure that the liquid does not flow out through the nozzles  19 . 
     The droplet discharging apparatus  11  may perform the first discharge operation when it is estimated, based on the result of the detection performed by the detector  171 , that the state of the insides of the pressure chambers  20  is abnormal since the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  is equal to or greater than a set value. The set value is stored in the memory  163  of the controller  160 . The memory  163  stores the vibration waveform that is detected by the detector  171  when the volume of air bubbles present in the pressure chamber  20  and the nozzle  19  is equal to the set value. 
     When the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  is small, the air bubbles may be eliminated by being dissolved in liquid with time. When the volume of the air bubbles is small, it is possible to remove the air bubbles from the pressure chambers  20  and the nozzles  19  without performing the first discharge operation by, for example, waiting for a predetermined time. On the contrary, when the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  is large, the air bubbles may grow with time. Therefore, the set value is a value that indicates the minimum volume of air bubbles estimated not to be eliminated with time. 
     The droplet discharging apparatus  11  performs the first discharge operation when the air bubbles are not estimated to be eliminated with time. In this case, it is not necessary to perform the first discharge operation when the air bubbles are estimated to be eliminated with time. Therefore, it is possible to decrease a frequency at which the first discharge operation is performed. 
     When the first discharge operation is not performed since the air bubbles are estimated to be eliminated, the nozzle  19  in an abnormal state caused by the air bubbles may not be able to be used for the recording until the air bubbles are eliminated. Therefore, when the recording process is continued without performing the first discharge operation, a complementary recording operation of compensating for droplets to be discharged from the nozzle  19  in an abnormal state by means of droplets discharged from the nozzle  19  in a normal state may be performed. 
     For example, when one of the plurality of nozzles  19  discharging the same kind of droplet is in an abnormal state, droplets larger than droplets to be discharged from the nozzle  19  in the abnormal state are discharged from the nozzle  19  in the normal state that is positioned near the nozzle  19  in the abnormal state such that dot omission is compensated. For example, when the nozzle  19  discharging black ink is in an abnormal state, yellow, cyan, and magenta droplets are discharged in a superimposed manner to a position to which droplets to be discharged from the nozzle  19  is to be landed such that dot omission of black ink is compensated. 
     The droplet discharging apparatus  11  may estimate whether the state of the insides of the pressure chambers  20  is improved or not by comparing the vibration waveforms of the pressure chambers  20  that are detected by the detector  171  at intervals and when it is estimated that the state of the insides of the pressure chambers  20  is not improved, the droplet discharging apparatus  11  may perform, as the maintenance operation for the droplet discharger  12 , the second discharge operation of causing liquid in the pressure chambers  20  to be discharged to the outside from the nozzles  19 . The second discharge operation is the flushing operation. 
     For example, when the state of the insides of the pressure chambers  20  is not improved even after the first discharge operation is performed, the droplet discharging apparatus  11  performs the second discharge operation of causing liquid in the pressure chambers  20  to be discharged to the outside from the nozzles  19 . In this case, the droplet discharging apparatus  11  detects the state of the insides of the pressure chambers  20  again with the detector  171  after the first discharge operation is performed based on the result of the detection performed by the detector  171 . At this time, when it is estimated, based on the vibration waveforms of the pressure chambers  20 , that the volume of air bubbles in the pressure chambers  20  and the nozzles  19  is large or an increase in viscosity of liquid is in progress, the droplet discharging apparatus  11  determines that the state of the insides of the pressure chambers  20  is not improved and performs the second discharge operation. 
     Since the second discharge operation is an operation of causing the liquid in the pressure chambers  20  to be discharged to the outside from the nozzles  19 , the second discharge operation is an operation that has a higher maintenance effect with respect to the droplet discharger  12  than the first discharge operation of discharging liquid in the pressure chambers  20  to the return flow path  28  via the discharge flow path  80 . In this manner, by performing the second discharge operation when the state of the inside of the pressure chamber  20  is not improved with the first discharge operation, it is possible to appropriately perform maintenance of the droplet discharger  12 . The droplet discharging apparatus  11  may perform the second discharge operation when the first discharge operation is not performed since the volume of air bubbles present in the pressure chamber  20  and the nozzle  19  is smaller than the set value but the state of the inside of the pressure chamber  20  is not improved even after a time estimated to be taken for the air bubbles to be eliminated elapses. 
     When the number of pressure chambers  20  estimated as the pressure chamber  20  of which the inside is in an abnormal state due to air bubbles present in the pressure chamber  20  and the nozzle  19  based on the result of the detection performed by the detector  171  is equal to or larger than a set number, the droplet discharging apparatus  11  may perform, as the maintenance operation for the droplet discharger  12 , the third discharge operation of causing liquid in the common liquid chamber  17  to be discharged toward the return flow path  28  via the discharge flow path  80  coupled to the common liquid chamber  17  before the first discharge operation is performed. The third discharge operation is operation of causing liquid in the common liquid chamber  17  to be discharged toward the return flow path  28  via the second discharge flow path  82 . The set number is stored in the memory  163  of the controller  160 . 
     When the number of pressure chambers  20  estimated as the pressure chamber  20  of which the inside is in an abnormal state due to air bubbles present in the pressure chamber  20  and the nozzle  19  is equal to or larger than the set number, it is considered that air bubbles are present in the common liquid chamber  17  communicating with the plurality of pressure chambers  20 . In this case, there is a possibility that consecutive nozzles in the nozzle surface  18  are in an abnormal state and thus it is difficult to perform the complementary recording operation. Therefore, when the number of pressure chambers  20  estimated as the pressure chamber  20  of which the inside is in an abnormal state due to air bubbles present in the pressure chamber  20  and the nozzle  19  is equal to or larger than the set number, the third discharge operation is performed as the maintenance operation for the droplet discharger  12 . Accordingly, it is possible to discharge liquid in the common liquid chamber  17  in which air bubbles are expected to be present. In the present embodiment, air bubbles in liquid discharged from the droplet discharger  12  is removed by the degasification mechanism  46  when being circulated in the circulation path  30 . 
     The droplet discharging apparatus  11  may perform, as the maintenance operation for the droplet discharger  12 , the fourth discharge operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  at a flow rate lower than the first discharge operation when droplets are discharged from the nozzles  19  during the recording process. The fourth discharge operation is an operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the first discharge flow path  81  at a flow rate lower than the first discharge operation. 
     The time when droplets are discharged from the nozzles  19  during the recording process is, for example, a time when an image is recorded on the recording medium  113 . When liquid in the pressure chambers  20  is discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  in order to suppress an increase in viscosity of liquid, the pressure in the pressure chambers  20  is likely to become unstable due to the flow of liquid. If the pressure in the pressure chambers  20  becomes unstable when droplets are discharged from the nozzles  19  during the recording process, the discharge accuracy of the nozzles  19  discharging droplets is decreased. Therefore, when droplets are discharged from the nozzles  19  during the recording process, the fourth discharge operation is performed as the maintenance operation for the droplet discharger  12 . 
     In the fourth discharge operation, the pressure in the pressure chambers  20  does not significantly fluctuate since liquid flows from the pressure chambers  20  to the return flow path  28  at a low flow rate in comparison with the first discharge operation. That is, the pressure in the pressure chambers  20  is less likely to be unstable. By performing the fourth discharge operation, it is possible to suppress an increase in viscosity of liquid while suppressing a fluctuation in pressure in the pressure chambers  20  even when droplets are discharged from the nozzles  19  during the recording process. The fourth discharge operation is particularly effective in suppressing an increase in viscosity of liquid in the nozzles  19  not used for the recording during the recording process and the pressure chambers  20  communicating with the nozzles  19 . The flow rate of liquid is the volume of liquid flowing per unit time. 
     In  FIG. 5 , the position of a normal meniscus that is formed when the liquid in the pressure chambers  20  does not flow is represented with a meniscus E, the position of a meniscus that is formed when the fourth discharge operation is performed is represented with a meniscus F, and the position of a meniscus that is formed when the first discharge operation is performed is represented with a meniscus G. When the first discharge operation or the fourth discharge operation is performed, a meniscus on the gas-liquid interface in the nozzle  19  is moved toward the pressure chamber  20  side. Therefore, the meniscus E is positioned closer to the nozzle surface  18  than the meniscus F and the meniscus G in the nozzle  19 . 
     In the case of the fourth discharge operation, the amount of movement of a meniscus in the nozzle  19  is small since liquid flows at a lower flow rate than the first discharge operation. Therefore, the meniscus F is positioned between the meniscus E and the meniscus G in the nozzle  19 . 
     The droplet discharging apparatus  11  may perform, as the maintenance operation for the droplet discharger  12 , the fifth discharge operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  at a flow rate higher than the first discharge operation in a state where the nozzle surface  18  is capped by the cap  151  when the recording process is not performed. The fifth discharge operation is an operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the first discharge flow path  81  at a flow rate higher than the first discharge operation in a state where the nozzle surface  18  is capped by the cap  151  when the recording process is not performed. 
     When a flow rate at which liquid flows from the pressure chambers  20  toward the return flow path  28  is made higher with the liquid sucked from the discharge flow path  80  side, there is a possibility that the outside air is drawn into the pressure chambers  20  through the nozzles  19 . However, if the nozzle surface  18  is capped by the cap  151  when liquid in the pressure chambers  20  is discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20 , a possibility that the outside air enters the pressure chambers  20  through the nozzles  19  is decreased. 
     When a flow rate at which liquid flows from the pressure chambers  20  toward the return flow path  28  is made higher with the liquid pressurized from the liquid supply flow path  27  side, there is a possibility that the liquid flows out through the nozzles  19 . However, if the nozzle surface  18  is capped by the cap  151  when liquid in the pressure chambers  20  is discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20 , a possibility that the liquid flows out through the nozzles  19  is decreased. 
     Due to the above-described reasons, in a state where the nozzle surface  18  is capped by the cap  151 , it is possible to make a flow rate at which liquid is discharged from the insides of the pressure chambers  20  toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  higher. The higher the flow rate at which liquid is discharged from the insides of the pressure chambers  20  to the return flow path  28 , the greater the maintenance effect with respect to the droplet discharger  12 . By performing the fifth discharge operation with the nozzle surface capped, it is possible to effectively perform maintenance of the droplet discharger  12 . When the cap  151  is provided with the atmosphere opening valve, the fifth discharge operation is performed with the atmosphere opening valve closed. 
     Next, as a maintenance method for the droplet discharging apparatus  11 , an example of a maintenance process for performing the maintenance operation of the droplet discharger  12  will be described. The maintenance process is repeatedly performed while the droplet discharger  12  is performing the recording process. 
     As illustrated in  FIG. 11 , the controller  160  that performs the maintenance process detects the state of the insides of the pressure chambers  20  with the detector  171  in Step S 21 . The controller  160  detects the state of the insides of all of the pressure chambers  20  by performing the nozzle inspection with respect to all of the nozzles  19  in Step S 21 . The vibration waveforms of the pressure chambers  20  detected by the detector  171  in Step S 21  may be vibration waveforms attributable to the actuators  24  driven to discharge droplets or vibration waveforms attributable to the actuators  24  driven to such an extent that droplets are not discharged. 
     In Step S 22 , the controller  160  determines whether a current time is the returning time of the carriage  124  or the inter-page time of the recording medium  113  or not. In other words, in Step S 22 , the controller  160  determines whether a current time is a time when droplets are discharged from the nozzles  19  or not. The controller  160  transitions into a process in Step S 31  when it is determined that the current time is not the returning time of the carriage  124  or the inter-page time of the recording medium  113  in Step S 22 . The controller  160  transitions into a process in Step S 23  when it is determined that the current time is the returning time of the carriage  124  or the inter-page time of the recording medium  113  in Step S 22 . 
     In Step S 23 , the controller  160  determines whether an abnormal nozzle is present or not. In Step S 23 , the controller  160  determines whether an abnormal nozzle is present or not based on the result of the nozzle inspection performed in Step S 21 . In other words, in Step S 23 , the controller  160  estimates whether the state of the insides of the pressure chambers  20  is abnormal or not. The controller  160  transitions into a process in Step S 24  when it is determined that an abnormal nozzle is present in Step S 23 . The controller  160  terminates the maintenance process when it is determined that an abnormal nozzle is not present in Step S 23 . When the maintenance process is terminated while the droplet discharger  12  is performing the recording process, the controller  160  restarts the maintenance process. 
     In Step S 24 , the controller  160  determines whether an abnormal nozzle caused by air bubbles is present or not. In Step S 24 , the controller  160  estimates whether a cause of the abnormal nozzle is air bubbles or not based on the vibration waveforms of the pressure chambers  20  detected in Step S 21 . In other words, in Step S 24 , the controller  160  estimates whether a cause of the abnormality in the pressure chamber  20  is air bubbles or not. The controller  160  transitions into a process in Step S 25  when it is determined that a cause of the abnormal nozzle is air bubbles in Step S 24 . The controller  160  transitions into a process in Step S 41  when it is determined that a cause of the abnormal nozzle is not air bubbles in Step S 24 . 
     In Step S 25 , the controller  160  determines whether the number of abnormal nozzles caused by air bubbles is equal to or greater than the set number or not. In Step S 25 , the controller  160  estimates whether the number of abnormal nozzles caused by air bubbles is equal to or greater than the set number or not based on the vibration waveforms of the pressure chambers  20  detected in Step S 21 . In other words, in Step S 25 , the controller  160  estimates whether the number of pressure chambers  20  in an abnormal state caused by air bubbles is equal to or greater than the set number or not. The controller  160  transitions into a process in Step S 26  when it is determined that the number of abnormal nozzles caused by air bubbles is equal to or greater than the set number in Step S 25 . The controller  160  transitions into a process in Step S 51  when it is determined that the number of abnormal nozzles caused by air bubbles is smaller than the set number in Step S 25 . 
     In Step S 26 , the controller  160  performs the third discharge operation. In Step S 26 , since the number of abnormal nozzles caused by air bubbles is equal to or greater than the set number, it is considered that air bubbles are present in the common liquid chamber  17 . Therefore, the third discharge operation is performed such that the air bubbles are discharged from the common liquid chamber  17 . The controller  160  performs the third discharge operation for a predetermined time in Step S 26 . 
     In Step S 27 , the controller  160  performs the first discharge operation. It is considered that air bubbles are present in the pressure chambers  20  when a process in Step S 27  is reached after the process in Step S 26  is performed. Therefore, the controller  160  performs the first discharge operation in Step S 27  after the process in Step S 26  is finished such that the air bubbles are discharged from the pressure chambers  20 . In Step S 27 , the controller  160  performs the first discharge operation for a predetermined time. 
     In Step S 28 , the controller  160  detects the state of the insides of the pressure chambers  20 . In Step S 28 , the controller  160  performs the same process as in Step S 21 . 
     In Step S 29 , the controller  160  determines whether the state of the insides of the pressure chambers  20  is improved or not due to the maintenance operation. That is, in Step S 29 , the controller  160  estimates whether the state of the insides of the pressure chambers  20  is improved or not by comparing the vibration waveforms of the pressure chambers  20  detected at intervals in Step S 21  and Step S 28 . The controller  160  terminates the maintenance process when it is determined that the state of the insides of the pressure chambers  20  is improved in Step S 29 . The controller  160  transitions into a process in Step S 61  when it is determined that the state of the insides of the pressure chambers  20  is not improved in Step S 29 . 
     In Step S 61 , the controller  160  performs the second discharge operation. In Step S 61 , since the state of the insides of the pressure chambers  20  is not improved with the first discharge operation performed in Step S 27 , a discharge operation having a higher maintenance effect than the first discharge operation is performed. Therefore, in Step S 61 , the controller  160  performs the second discharge operation having a high maintenance effect such that the state of the insides of the pressure chambers  20  is improved. The controller  160  terminates the maintenance process after the second discharge operation is performed. 
     When it is determined in Step S 22  that the current time is not the returning time of the carriage  124  or the inter-page time of the recording medium  113 , the controller  160  performs the fourth discharge operation in Step S 31 . In Step S 31 , since an image is being recorded on the recording medium  113 , a great fluctuation in pressure in the pressure chambers  20  is not preferable. Therefore, in Step S 31 , the controller  160  performs the fourth discharge operation in which liquid flows at a flow rate lower than the first discharge operation. In Step S 31 , the controller  160  terminates the maintenance process after performing the fourth discharge operation for a predetermined time. 
     When it is determined in Step S 24  that a cause of the abnormal nozzle is not air bubbles, the controller  160  determines whether an abnormal nozzle caused by an increase in viscosity of liquid is present or not in Step S 41 . In Step S 41 , the controller  160  estimates whether a cause of the abnormal nozzle is an increase in viscosity of liquid or not based on the vibration waveforms of the pressure chambers  20  detected in Step S 21 . In other words, in Step S 41 , the controller  160  estimates whether a cause of the abnormality in the pressure chamber  20  is an increase in viscosity of liquid or not. The controller  160  transitions into a process in Step S 27  when it is determined that a cause of the abnormal nozzle is an increase in viscosity of liquid in Step S 41 . The controller  160  terminates the maintenance process when it is determined that a cause of the abnormal nozzle is not an increase in viscosity of liquid in Step S 41 . 
     It is considered that there is an increase in viscosity liquid in the pressure chambers  20  when the process in Step S 27  is reached after the process in Step S 41  is performed. Therefore, in Step S 27 , the controller  160  performs the first discharge operation after the process in Step S 41  is finished such that the liquid increased in viscosity is discharged from the pressure chambers  20 . 
     When it is determined in Step S 25  that the number of abnormal nozzles caused by air bubbles is smaller than the set number, the controller  160  determines whether the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  communicating with the pressure chambers  20  is equal to or greater than the set value or not in Step S 51 . The controller  160  transitions into a process in Step S 27  when it is determined that the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  communicating with the pressure chambers  20  is equal to or greater than the set value in Step S 51 . 
     It is considered that air bubbles are present in the pressure chambers  20  when the process in Step S 27  is reached after the process in Step S 51  is performed. Therefore, in Step S 27 , the controller  160  performs the first discharge operation after the process in Step S 51  is finished such that the air bubbles are discharged from the pressure chambers  20 . In Step S 27 , the controller  160  performs the first discharge operation for a predetermined time. 
     When it is determined in Step S 51  that the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  communicating with the pressure chambers  20  is smaller than the set value, the controller  160  terminates the maintenance process. When it is determined in Step S 51  that the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  communicating with the pressure chambers  20  is smaller than the set value, it is estimated that the air bubbles will be eliminated with time. Therefore, in this case, the controller  160  does not perform the first discharge operation. When the recording process is continued after the process in Step S 51  is finished, the controller  160  may perform the above-described complementary recording operation. The controller  160  may wait for a time estimated to be taken for the air bubbles to be eliminated after the process in Step S 51  is finished. 
     Next, a cleaning operation of the droplet discharger  12  will be described. 
     The droplet discharging apparatus  11  is configured to perform the cleaning operation of causing liquid to be forcibly discharged from the nozzles  19  of the droplet discharger  12 . The cleaning operation is an operation which has a higher maintenance effect with respect to the droplet discharger  12  than the discharge operation. 
     In the present embodiment, the controller  160  performs the cleaning operation of causing liquid to be discharged from the nozzles  19  of the droplet discharger  12  by causing the pressurizing mechanism  31  to pressurize the inside of the droplet discharger  12  such that pressure in the droplet discharger  12  is made higher than the pressure of the outside of the droplet discharger  12 . That is, the controller  160  performs pressurization cleaning as the cleaning operation by causing the pressurizing mechanism  31  to pressurize the inside of the droplet discharger  12 . The droplet discharging apparatus  11  may be configured to perform suction cleaning as the cleaning operation, the suction cleaning being an operation of forcibly discharging liquid from the nozzles  19  by sucking air in the cap  151  in a state where the nozzle surface  18  is capped. 
     That is, when performing the cleaning operation, the controller  160  causes the pressing mechanism  48  to press the diaphragm  56  such that the on-off valve  59  is opened. The controller  160  drives the pressurizing mechanism  31  with the on-off valve  59  opened such that liquid is supplied to the pressure adjustment mechanism  35  and the droplet discharger  12 . In this manner, the controller  160  causes the pressurizing mechanism  31  to pressurize the inside of the droplet discharger  12 . In this manner, the cleaning operation is performed. 
     The controller  160  drives the pressurizing pump  74  when opening the on-off valve  59  such that pressurized liquid is supplied to the expansion and contraction portion  67 . The expansion and contraction portion  67  expands due to the supplied liquid and thus the diaphragm  56  is displaced in a direction in which the volume of the liquid outflow portion  51  is reduced. Therefore, the on-off valve  59  enters the opened state. The controller  160  controls the pressure adjustment unit  69  when closing the on-off valve  59  such that fluid supplied to the expansion and contraction portion  67  is discharged to the outside. As described above, the controller  160  opens or close the on-off valve  59  based on the driving of the pressing mechanism  48 . 
     The pressure in the droplet discharger  12  after the cleaning operation is likely to be higher than the pressure in the droplet discharger  12  at the time of the recording process. Specifically, the pressure in the droplet discharger  12  becomes a negative pressure at the time of the recording process but the pressure in the droplet discharger  12  is likely to become a positive pressure higher than the atmospheric pressure after the cleaning operation. Therefore, when the recording process is performed after the cleaning operation is performed, droplets may be unstably discharged from the nozzles  19 . For example, the size of a droplet discharged from the nozzle  19  of the droplet discharger  12  may not be a desired size or droplets may not be discharged at a time when the droplets need to be discharged. 
     In the present embodiment, when the cleaning operation is performed, the controller  160  performs a pressure reducing operation after performing a cleaning stopping operation of stopping the cleaning operation. The pressure reducing operation is an operation of reducing the pressure in the droplet discharger  12  and a portion of the liquid supply flow path  27  that is positioned downstream of the pressure adjustment mechanism  35 . 
     The controller  160  performs a finishing wiping operation of wiping the nozzle surface  18  of the droplet discharger  12  in a state where the pressure in the droplet discharger  12  is reduced due to the pressure reducing operation. In this case, the pressure in the droplet discharger  12  becomes an appropriate pressure before the recording process is performed and meniscuses suitable for droplet discharge are formed in the nozzles  19  of the droplet discharger  12 . In the pressure reducing operation, the pressure in the droplet discharger  12  is reduced such that the meniscuses formed in the nozzles  19  are positioned in the nozzles  19 . 
     In addition, when the cleaning operation is performed for a long period of time, the amount of liquid consumed by being discharged from the nozzles  19  of the droplet discharger  12  may become excessively large with respect to the amount of liquid that the pressurizing mechanism  31  supplies to the droplet discharger  12 . In this case, the flow speed of liquid flowing in the liquid supply flow path  27  gradually decreases. When the flow speed of liquid flowing in the liquid supply flow path  27  is decreased, it may not be possible to effectively discharge foreign substances such as air bubbles present in the droplet discharger  12  and the liquid supply flow path  27 . 
     In the present embodiment, the controller  160  repeatedly performs the cleaning operation and the cleaning stopping operation of stopping the cleaning operation to be performed at short intervals. Accordingly, a gradual decrease in flow speed of liquid flowing in the liquid supply flow path  27  is suppressed. An effect of discharging foreign substances such as air bubbles present in the liquid supply flow path  27  becoming weak is suppressed. 
     Next, an example of a cleaning process performed by the controller  160  in the present embodiment will be described with reference to a flowchart in  FIG. 12 . The cleaning process is a process including the cleaning operation. The cleaning process may be performed for each predetermined control cycle, may be performed only when it is expected that there is droplet discharge failure in the nozzles  19 . The cleaning process may be performed manually by a user or an operator of the droplet discharging apparatus  11 . 
     As illustrated in  FIG. 12 , the controller  160  resets a counter Cnt, which is a variable for counting, in Step S 11 . That is, the controller  160  resets the counter Cnt to “0” in Step S 11 . 
     In Step S 12 , the controller  160  performs the cleaning operation. In Step S 12 , the controller  160  controls the driving of the pressing mechanism  48  such that the diaphragm  56  is displaced in a direction in which the volume of the liquid outflow portion  51  is reduced. In this manner, the controller  160  causes the on-off valve  59  to enter the opened state. When the on-off valve  59  enters the opened state, pressurized liquid flows into the liquid outflow portion  51 , the liquid supply flow path  27 , the common liquid chamber  17 , the pressure chambers  20 , and the nozzles  19 . As a result, the liquid is discharged from the nozzles  19 . In Step S 12 , the controller  160  performs the cleaning operation for the predetermined time. 
     In Step S 13 , the controller  160  performs the cleaning stopping operation to stop the cleaning operation. In Step S 13 , the controller  160  controls the driving of the pressing mechanism  48  such that the diaphragm  56  is displaced in a direction in which the volume of the liquid outflow portion  51  increases. In this manner, the controller  160  causes the on-off valve  59  to enter the closed state. When the on-off valve  59  enters the closed state, pressurized liquid is not supplied downstream of the pressure adjustment mechanism  35 . As a result, the cleaning operation is stopped. A period of time between the start of the cleaning operation and the start of the cleaning stopping operation may be, for example, a period of time of about 0.1 seconds to 1.0 second. 
     In Step S 14 , the controller  160  increments the counter Cnt by “1”. 
     In Step S 15 , the controller  160  determines whether the counter Cnt is equal to or greater than a determination number CntTh. The determination number CntTh is a determination value for determining the number of times the cleaning operation and the cleaning stopping operation are repeatedly performed. Therefore, the determination number CntTh may be determined based on the specifications of the droplet discharging apparatus  11  or set by the user. Note that, when the nozzle inspection is performed for all of the nozzles  19  of the droplet discharger  12 , the determination number CntTh may be determined corresponding to the number of abnormal nozzles in each of which a droplet discharge failure occurs. 
     The controller  160  transitions into a process in Step S 12  when it is determined that the counter Cnt is smaller than the determination number CntTh in Step S 15 . The controller  160  transitions into a process in Step S 16  when it is determined that the counter Cnt is equal to or greater than the determination number CntTh in Step S 15 . 
     In Step S 16 , the controller  160  performs the pressure reducing operation. In the present embodiment, the pressure reducing operation is a wiping operation of wiping the nozzle surface  18  by using the wiping mechanism  140 . Hereinafter, the wiping operation is referred to as a preceding wiping operation. As a result of the preceding wiping operation, the wiping portion  149  comes into contact with gas-liquid interfaces positioned outside the nozzles  19  or in the vicinity of the openings of the nozzles  19 , so that pressurized liquid leaks out from the nozzles  19 . Accordingly, the pressure in the droplet discharger  12  is reduced. 
     Immediately after the last cleaning stopping operation is performed in the cleaning process, the liquid may continue to leak out from the nozzles  19  of the droplet discharger  12  due to the cleaning operation performed immediately before the cleaning stopping operation. Therefore, it is preferable that the preceding wiping operation be performed after the liquid stops to leak out due to the cleaning operation. In the present embodiment, since the pressure reducing operation is performed when the counter Cnt is equal to or greater than the determination number CntTh, the pressure reducing operation is an operation that is performed after the last discharge stopping operation is performed. 
     In Step S 17 , the controller  160  performs a finishing wiping operation. The finishing wiping operation is a wiping operation of wiping the nozzle surface  18  by using the wiping mechanism  140 . Therefore, in the present embodiment, the controller  160  performs the wiping operations in both of Step S 16  and Step S 17 . As a result of the finishing wiping operation, liquid or foreign substances adhering to the nozzle surface  18  is removed and meniscuses suitable for droplet discharge are formed in the nozzles  19 . The controller  160  temporarily terminates the cleaning process after the process in Step S 17  is finished. 
     The cleaning process in the present embodiment is a process including the cleaning operation, the cleaning stopping operation, the preceding wiping operation which is the pressure reducing operation, and the finishing wiping operation. The cleaning process in the present embodiment is an operation for recovering the droplet discharge performance of the droplet discharger  12 . The cleaning process may be performed, for example, when it is expected that the droplet discharge performance of the droplet discharger  12  is not recovered in the maintenance process in which the discharge operation is performed. The cleaning process may be performed, for example, when the state of the insides of the pressure chambers  20  is not improved continuously. 
     Next, the effect when the droplet discharging apparatus  11  performs the cleaning process will be described. 
     When the droplet discharging apparatus  11  performs the recording process, a portion of the plurality of nozzles  19  provided in the droplet discharger  12  may become abnormal nozzles in which a droplet discharge failure occurs. In this case, the cleaning process may be performed to recover the defective nozzles from the droplet discharge failure. 
     As illustrated in  FIG. 13 , when the cleaning process is performed, the pressurizing pump  74  is driven such that pressurized fluid is supplied to the expansion and contraction portion  67 . Then, the expansion and contraction portion  67  supplied with the fluid expands and presses a region of the diaphragm  56  that comes into contact with the pressure receiving portion  61  such that the on-off valve  59  enters the opened state. 
     The pressing mechanism  48  moves the pressure receiving portion  61  against pressing forces of the upstream pressing member  62  and the downstream pressing member  63  such that the on-off valve  59  enters the opened state. In this case, since the pressure adjustment unit  69  is coupled to the expansion and contraction portions  67  of the plurality of pressure adjustment devices  47 , all of the on-off valves  59  in the pressure adjustment devices  47  enter the opened state. 
     When the on-off valve  59  is caused to enter the opened state, the diaphragm  56  is displaced in a direction in which the volume of the liquid outflow portion  51  is reduced. Therefore, liquid accommodated in the liquid outflow portion  51  is pressed out toward the droplet discharger  12  side. That is, a pressure with which the diaphragm  56  presses the liquid outflow portion  51  is transmitted to the droplet discharger  12  and thus the meniscuses collapse and liquid flows out from the nozzles  19 . The pressing mechanism  48  presses the diaphragm  56  such that the pressure in the liquid outflow portion  51  becomes higher than a pressure at which at least one meniscus collapses. The pressing mechanism  48  presses the diaphragm  56  such that, for example, a liquid side pressure becomes 3 kPa higher than an air side pressure for each of the gas-liquid interfaces in the nozzles  19 . 
     The pressing mechanism  48  presses the diaphragm  56  such that the on-off valve  59  enters the opened state regardless of the pressure in the liquid inflow portion  50 . In this case, the pressing mechanism  48  presses the diaphragm  56  with a pressing force that is greater than a pressing force that is generated when a pressure, which is obtained by adding the above-described predetermined value to a pressure at which the pressurizing mechanism  31  pressurizes liquid, is applied to the diaphragm  56 . 
     The pressure reduction unit  43  is periodically driven in a state where the on-off valve  59  is in the opened state and thus the liquid pressurized by the pressurizing mechanism  31  is supplied to the droplet discharger  12 . That is, when the pressure reduction unit  43  is driven and the pressure in the negative pressure chamber  42  is reduced, the flexible member  37  moves in a direction in which the volume of the pump chamber  41  increases. 
     When the flexible member  37  moves in a direction in which the volume of the pump chamber  41  increases, liquid from the liquid supply source  13  flows into the pump chamber  41 . When the pressure reduction performed by the pressure reduction unit  43  is stopped, the flexible member  37  is pressed by the pressing force of the pressing member  44  in a direction in which the volume of the pump chamber  41  is reduced. That is, liquid in the pump chamber  41  is pressurized by the pressing force of the pressing member  44  via the flexible member  37 . The liquid in the pump chamber  41  is supplied to the downstream of the liquid supply flow path  27  while passing through the one-way valve  40  positioned downstream of the pump chamber  41 . 
     While the pressing mechanism  48  presses the diaphragm  56 , the opened state of the on-off valve  59  is maintained. Therefore, if the pressurizing mechanism  31  pressurizes liquid a state where the opened state of the on-off valve  59  is maintained, the pressurizing force is transmitted to the droplet discharger  12  via the liquid inflow portion  50 , the communication path  57 , and the liquid outflow portion  51 . Accordingly, the pressurization cleaning, which is the cleaning operation in which liquid is discharged from the nozzles  19  is performed. As illustrated in  FIG. 13 , when the cleaning operation is performed, the carriage  124  may be moved such that the droplet discharger  12  faces the liquid receiver  131  and the liquid receiver  131  receives liquid discharged from the nozzles  19 . 
     After the cleaning operation is performed, the cleaning stopping operation of stopping the cleaning operation is performed. In the cleaning stopping operation, the pressing mechanism  48  is caused to stop to press the diaphragm  56  such that the on-off valve  59  enters the closed state. Accordingly, the upstream and the downstream of the pressure adjustment mechanism  35  are blocked and pressurized liquid is not supplied from the liquid supply source  13  to the droplet discharger  12 . 
     In the present embodiment, the cleaning operation and the cleaning stopping operation are repeatedly performed at short intervals. Accordingly, a decrease in flow speed of liquid flowing in the liquid supply flow path  27  and the droplet discharger  12  during the cleaning operation is suppressed and it becomes easy to remove foreign substances such as air bubbles from the liquid supply flow path  27  and the droplet discharger  12 . 
     The pressure in the droplet discharger  12  disposed downstream of the pressure adjustment mechanism  35  becomes high immediately after the cleaning stopping operation is performed. That is, immediately after the cleaning stopping operation is performed, the state of the inside of the droplet discharger  12  becomes not suitable for the recording process. Therefore, after the cleaning stopping operation is performed, the preceding wiping operation is performed as the pressure reducing operation to reduce the pressure in the droplet discharger  12 . 
     Immediately after the cleaning stopping operation is performed, liquid continues to drop from the nozzles  19 . That is, immediately after the cleaning stopping operation is performed, a state in which liquid is discharged from the nozzles  19  continues. The liquid continues to be discharged from the nozzles  19  until the pressure in droplet discharger  12  is reduced and meniscuses are formed in the nozzles  19 . At this time, each of the meniscuses that are formed in the nozzles  19  or in the vicinity of the openings of the nozzles  19  is a meniscus that is curved toward the outside of the nozzle  19  from the nozzle opening or the vicinity of the opening of the nozzle  19  instead of a meniscus that is formed in the nozzle  19  in a case where the recording process is performed and that is curved toward the inside of the nozzle  19 . 
     As illustrated in  FIG. 14 , in the preceding wiping operation, the carriage  124  is moved such that the droplet discharger  12  faces the wiping mechanism  140  and the wiping mechanism  140  wipes the droplet discharger  12 . Therefore, the pressure in the droplet discharger  12  becomes a positive pressure, the gas-liquid interfaces swelling toward the outside of the nozzles  19  come into contact with the wiping portion  149  of the fabric wiper  148 , and liquid leaks out from the droplet discharger  12 . 
     The purpose of the preceding wiping operation is to reduce the pressure in the droplet discharger  12  by causing liquid to leak out from the nozzles  19 . Therefore, in the preceding wiping operation, the wiping operation may be performed in a state where the gas-liquid interfaces swelling from the nozzles  19  are in contact with the wiping portion  149  while the nozzle surface  18  of the droplet discharger  12  is not in contact with the wiping portion  149  as illustrated in  FIG. 14 . In the preceding wiping operation, the wiping operation may be performed in a state where the nozzle surface  18  of the droplet discharger  12  is in contact with the wiping portion  149 . 
     When the cleaning process is performed, air bubbles may not be fully discharged from droplet discharger  12  and the liquid supply flow path  27  and the air bubbles may remain in the droplet discharger  12  and the liquid supply flow path  27 . In the cleaning operation, since the pressure of liquid is high, the volume of air bubbles in the liquid is small. After the cleaning stopping operation, the pressure of liquid is reduced and thus the volume of air bubbles becomes large. Therefore, the volume of air bubbles is changed in the cleaning operation and the cleaning stopping operation. Due to the change in volume of air bubbles, the pressure in the droplet discharger  12  and the liquid supply flow path  27  when the meniscuses are formed in the nozzles  19  may become higher. 
     When the wiping operation is performed in a state where the pressure in the droplet discharger  12  and the liquid supply flow path  27  is made higher, the wiping portion  149  may break unstable convex meniscuses swelling from the nozzle openings while coming into contact with the meniscuses and thus liquid may spread over the nozzle surface  18 . That is, when the wiping operation is performed, the meniscuses formed in the nozzles  19  may become unstable. Therefore, a state where the pressure in the droplet discharger  12  and a portion of the liquid supply flow path  27  that is positioned downstream of the pressure adjustment device  47  is stable is a state where the pressure in the droplet discharger  12  and the liquid supply flow path  27  becomes a negative pressure to such an extent that meniscuses are formed in the nozzles  19 . 
     When the preceding wiping operation is finished, the pressure in the droplet discharger  12  and the portion of the liquid supply flow path  27  that is positioned downstream of the pressure adjustment device  47  becomes stable. Thereafter, the finishing wiping operation is performed. 
     As illustrated in  FIG. 15 , in the finishing wiping operation, wiping is performed in a state where the wiping portion  149  of the fabric wiper  148  is in contact with the nozzle surface  18  of the droplet discharger  12 . In this manner, liquid adhering to the nozzle surface  18  of the droplet discharger  12  is removed and normal meniscuses are formed in the nozzles  19  of the droplet discharger  12 . 
     Next, a method of manufacturing the pressure adjustment device  47  according to the present embodiment will be described. 
     First, the main body portion  52  in the present embodiment is formed of a light absorbing resin which generates heat when absorbing laser light, or a resin colored with a dye which absorbs light. The light absorbing resin is, for example, polypropylene or polybutylene terephthalate. 
     The diaphragm  56  is formed by laminating different materials such as polypropylene and polyethylene terephthalate. The diaphragm  56  has transparency which allows laser light to pass therethrough and flexibility. 
     The retaining member  68  is formed of a light transmitting resin which transmits laser light. The light transmitting resin is, for example, polystyrene or polycarbonate. The transparency of the diaphragm  56  is greater than the transparency of the main body portion  52  and is lower than the transparency of the retaining member  68 . 
     As illustrated in  FIG. 4 , first, as an interposing step, the diaphragm  56  is interposed between the retaining member  68 , in which a portion of the expansion and contraction portion  67  has been inserted into the insertion hole  70 , and the main body portion  52 . Next, irradiation with laser light is performed via the retaining member  68  as an irradiation step. As a result, the laser light passing through the retaining member  68  is absorbed by the main body portion  52  and the main body portion  52  generates heat. The main body portion  52 , the diaphragm  56 , and the retaining member  68  are welded to each other due to the heat generated at this time. Therefore, the retaining member  68  also functions as a jig which presses the diaphragm  56  when the pressure adjustment device  47  is manufactured. 
     Next, an operation and effect of the present embodiment will be described. 
     (1) The droplet discharging apparatus  11  droplet discharging apparatus  11  performs, as the maintenance operation for the droplet discharger  12 , the first discharge operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the discharge flow path  80  when no droplets are discharged from the nozzles  19  during the recording process. As a result, the liquid discharged from the pressure chambers  20  to the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  flows in the circulation path  30 . Since the liquid flows, an increase in viscosity of the liquid is suppressed. Therefore, by using the first discharge operation, it is possible to suppress an increase in viscosity of liquid without discharging droplets from the nozzles  19 . Therefore, it is possible to reduce the amount of liquid consumed for maintenance. 
     (2) In the first discharge operation, the droplet discharging apparatus  11  causes liquid to be discharged toward the return flow path  28  with the liquid in the pressure chambers  20  sucked from the discharge flow path  80  side such that meniscuses on gas-liquid interfaces in the nozzles  19  are maintained. As a result, the meniscuses in the nozzles  19  are moved toward the pressure chambers  20  with the liquid in the pressure chambers  20  sucked from the discharge flow path  80  side. That is, liquid in the nozzles  19  flows. Therefore, an increase in viscosity of the liquid in the nozzles  19  can be suppressed. 
     (3) The droplet discharging apparatus  11  performs the first discharge operation when it is estimated, based on the result of the detection performed by the detector  171 , that the state of the insides of the pressure chambers  20  is abnormal since the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  is equal to or greater than a set value. When the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  is small, the air bubbles may be eliminated by being dissolved in liquid with time. When the volume of the air bubbles is small, it is possible to remove the air bubbles from the pressure chambers  20  and the nozzles  19  without performing the first discharge operation by, for example, waiting for a predetermined time. On the contrary, when the volume of air bubbles present in the pressure chambers  20  and the nozzles  19  is large, the air bubbles may grow with time. Therefore, the droplet discharging apparatus  11  performs the first discharge operation when the air bubbles are not estimated to be eliminated with time. It is possible to decrease a frequency at which the first discharge operation is performed since it is not necessary to perform the first discharge operation when the air bubbles are estimated to be eliminated with time. 
     (4) The droplet discharging apparatus  11  estimates whether the state of the insides of the pressure chambers  20  is improved or not by comparing the vibration waveforms of the pressure chambers  20  that are detected by the detector  171  at intervals and when it is estimated that the state of the insides of the pressure chambers  20  is not improved, the droplet discharging apparatus  11  performs, as the maintenance operation for the droplet discharger  12 , the second discharge operation of causing liquid in the pressure chambers  20  to be discharged to the outside from the nozzles  19 . That is, when the state of the insides of the pressure chambers  20  is not improved even after the first discharge operation is performed and when the state of the insides of the pressure chambers  20  is not improved after the droplet discharging apparatus  11  waits for a predetermined time, the droplet discharging apparatus  11  in the present embodiment performs the second discharge operation of causing liquid in the pressure chambers  20  to be discharged to the outside from the nozzles  19 . Since the second discharge operation is an operation of causing the liquid in the pressure chambers  20  to be discharged to the outside from the nozzles  19 , the second discharge operation is an operation that has a higher maintenance effect with respect to the droplet discharger  12  than the first discharge operation of causing liquid in the pressure chambers  20  to be discharged to the return flow path  28  via the discharge flow path  80 . In this manner, by performing the second discharge operation when the state of the inside of the pressure chamber  20  is not improved with the first discharge operation, it is possible to appropriately perform maintenance of the droplet discharger  12 . 
     (5) When the number of pressure chambers  20  estimated as the pressure chamber  20  of which the inside is in an abnormal state due to air bubbles present in the pressure chamber  20  and the nozzle  19  based on the result of the detection performed by the detector  171  is equal to or larger than a set number, the droplet discharging apparatus  11  performs, as the maintenance operation for the droplet discharger  12 , the third discharge operation of causing liquid in the common liquid chamber  17  to be discharged toward the return flow path  28  via the second discharge flow path  82  before the first discharge operation is performed. When the number of pressure chambers  20  estimated as the pressure chamber  20  of which the inside is in an abnormal state due to air bubbles present in the pressure chamber  20  and the nozzle  19  is equal to or larger than the set number, it is considered that air bubbles are present in the common liquid chamber  17  communicating with the plurality of pressure chambers  20 . Therefore, when the number of pressure chambers  20  estimated as the pressure chamber  20  of which the inside is in an abnormal state due to air bubbles present in the pressure chamber  20  and the nozzle  19  is equal to or larger than the set number, the droplet discharging apparatus  11  in the present embodiment performs the third discharge operation of causing liquid in the common liquid chamber  17  to be discharged toward the return flow path  28  via the second discharge flow path  82  coupled to the common liquid chamber  17  and the return flow path  28 . Accordingly, it is possible to discharge liquid in the common liquid chamber  17  in which air bubbles are expected to be present. 
     (6) The droplet discharging apparatus  11  performs, as the maintenance operation for the droplet discharger  12 , the fourth discharge operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the discharge flow path  80  at a flow rate lower than the first discharge operation when droplets are discharged from the nozzles  19  during the recording process. When liquid in the pressure chambers  20  is discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  in order to suppress an increase in viscosity of liquid, the pressure in the pressure chambers  20  becomes unstable due to the flow of liquid. If the pressure in the pressure chambers  20  becomes unstable when droplets are discharged from the nozzles  19  during the recording process, the discharge accuracy of the nozzles  19  discharging droplets is decreased. Therefore, when droplets are discharged from the nozzles  19  during the recording process, the droplet discharging apparatus  11  performs the fourth discharge operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  at a flow rate lower than the first discharge operation. In the fourth discharge operation, the pressure in the pressure chambers  20  does not significantly fluctuate since the flow rate is low in comparison with the first discharge operation. That is, by performing the fourth discharge operation, it is possible to suppress an increase in viscosity of liquid while suppressing a fluctuation in pressure in the pressure chambers  20  even when droplets are discharged from the nozzles  19  during the recording process. 
     (7) The droplet discharging apparatus  11  performs, as the maintenance operation for the droplet discharger  12 , the fifth discharge operation of causing liquid in the pressure chambers  20  to be discharged toward the return flow path  28  via the discharge flow path  80  at a flow rate higher than the first discharge operation in a state where the nozzle surface  18  is capped by the cap  151  when the recording process is not performed. When liquid in the pressure chambers  20  is discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20  in order to suppress an increase in viscosity of liquid, the pressure in the pressure chambers  20  fluctuates due to the flow of liquid. If a flow rate at which the liquid flows from the pressure chambers  20  to the return flow path  28  is high, the pressure in the pressure chambers  20  significantly fluctuates and thus there is a possibility that the outside air enters the pressure chambers  20  through the nozzles  19  or the liquid flows out through the nozzles  19 . However, if the nozzle surface  18  is capped by the cap  151  when liquid in the pressure chambers  20  is discharged toward the return flow path  28  via the discharge flow path  80  coupled to the pressure chambers  20 , a possibility that the outside air enters the pressure chambers  20  through the nozzles  19  or the liquid flows out from the nozzles  19  due to a fluctuation in pressure in the pressure chambers  20  is decreased. Therefore, in a state where the nozzle surface  18  is capped by the cap  151 , it is possible to make a flow rate at which liquid is discharged from the insides of the pressure chambers  20  toward the return flow path  28  via the discharge flow path  80  higher. By performing the fifth discharge operation with the nozzle surface capped, it is possible to effectively perform maintenance of the droplet discharger  12 . 
     The present embodiment can be modified as follows. The present embodiment and the following modification examples can be combined with each other unless there is a technical contradiction. 
     In the first discharge operation, the actuators  24  may be driven to such an extent that liquid is not discharged from the nozzles  19 . In this case, it becomes easy to discharge liquid in the pressure chambers  20  with the first discharge operation. In this case, all of the actuators  24  may be driven or the actuator  24  corresponding to the nozzle  19  with air bubbles detected by the detector  171  may be driven. When the actuator  24  corresponding to the nozzle  19  with air bubbles detected by the detector  171  is driven, the actuator  24  may be driven by using the frequency of a vibration waveform detected by the detector  171 . 
     At the time of the fourth discharge operation, the actuator  24  corresponding to the nozzle  19  not used for the recording process may be driven to such an extent that liquid is not discharged from the nozzle  19 . In this case, since the liquid is displaced in the nozzle  19  not used for the recording process, the viscosity of the liquid in the nozzle  19  is less likely to be increased. 
     At least a portion of the first discharge flow path  81  and at least a portion of the second discharge flow path  82  may be formed of a flexible member. In this case, it is possible to even out a fluctuation in pressure in the droplet discharger  12 , which occurs when liquid flows in the discharge flow path  80 , without providing the first damper  285  and the second damper  286 . 
     A pressure sensor may be provided in the first return flow path  281  while being positioned closer to the droplet discharger  12  side than the first on-off valve  283  and a pressure sensor may be provided in the second return flow path  282  while being positioned closer to the droplet discharger  12  side than the second on-off valve  284 . In this case, feedback control of the circulation pumps  29  may be performed based on a pressure detected by the pressure sensors. For example, opening and closing of the first on-off valve  283  and the second on-off valve  284  may be controlled to an extent that a fluctuation in pressure in the droplet discharger  12  is allowed. In this case, it is possible to suppress a significant fluctuation in pressure in the droplet discharger  12  which occurs when liquid flows through the discharge flow path  80  with the circulation pumps  29  driven. 
     The third discharge operation may be performed in the purpose of discharging air bubbles in the liquid supply flow path  27 . For example, the third discharge operation may be performed to discharge air bubbles accumulated in the pressure adjustment mechanism  35 . 
     The second return flow path  282  may be coupled t a portion of the droplet discharger  12  in which air bubbles are likely to be accumulated. For example, the second return flow path  282  may be coupled to the vicinity of the filter  16 . 
     A flow path that couples the liquid inflow portion  50  or the liquid outflow portion  51  of the pressure adjustment mechanism  35  to the liquid supply flow path  27  may be provided. In this case, it is possible to cause liquid to circulate without passing through the droplet discharger  12 . In this case, the flow path that couples the liquid inflow portion  50  or the liquid outflow portion  51  of the pressure adjustment mechanism  35  to the liquid supply flow path  27  may be provided with an on-off valve. 
     When a plurality of the droplet dischargers  12  are provided for each of the kinds of liquid, the droplet dischargers  12  may perform different discharge operations respectively. For example, the droplet discharger  12  performing the recording process may perform the fourth discharge operation and the droplet discharger  12  not performing the recording process may perform the first discharge operation. When a monochrome image is recorded, only black ink is used and thus cyan ink, magenta ink, and yellow ink are not used. When monochrome images are recorded consecutively, there is a possibility that an increase in viscosity of liquid may be prompted in the droplet dischargers  12  corresponding to cyan ink, magenta ink, and yellow ink not used for the recording process even if the first discharge operation is performed. Therefore, when monochrome images are recorded consecutively for a time equal to or longer than a predetermined time, the third discharge operation or the second discharge operation may be performed. 
     In the second discharge operation, droplets may be discharged toward the recording medium  113 . In this case, the droplets discharged during the second discharge operation may be droplets that are so fine that a user cannot visually recognize the droplets when the droplets adhere to the recording medium  113 . Droplets may be discharged such that the droplets are not noticeable from a recorded image and droplets may be discharged to an edge portion of the recording medium  113  that does not influence an image. 
     The fourth discharge operation may be continuously performed while droplets are discharged from the nozzles  19  in the recording process. 
     The first discharge operation may be continuously performed while droplets are not discharged from the nozzles  19  in the recording process like in the returning time of the carriage  124  and the inter-page time of the recording medium  113 . 
     The fourth discharge operation may be basically performed while the droplet discharging apparatus  11  is activated and the first discharge operation and the second discharge operation, and the third discharge operation may be performed based on the result of the nozzle inspection in the maintenance process, may also be adopted. 
     The droplet discharging apparatus  11  may not be provided with the detector  171 . In this case, the fourth discharge operation may be performed while droplets are discharged from the nozzles  19  in the recording process and the first discharge operation may be performed while no droplets are discharged from the nozzles  19 . 
     The pressure reducing operation performed in Step S 16  is not limited to the preceding wiping operation. The pressure reducing operation may be any operation as long as it is possible to decrease the pressure in the droplet discharger  12  by discharging pressurized liquid from the inside of the droplet discharger  12 . 
     For example, the pressure reducing operation may be an operation of displacing the vibration plate  21  by driving the actuators  24 . Specifically, the pressure reducing operation may be an operation of causing the vibration plate  21  to vibrate. In this case, it is possible to decrease the pressure in the droplet discharger  12  by discharging liquid from the nozzles  19  in a state where the pressure in the droplet discharger  12  is high and the gas-liquid interfaces in the nozzles  19  are unstable. 
     When the actuators  24  are driven as the pressure reducing operation, a low voltage may be applied to the actuators  24  such that the vibration plate  21  is vibrated weakly. In this case, unstable meniscuses formed in the nozzles  19  collapse due to vibration of the vibration plate  21 . As a result, liquid leaks out from the nozzles  19 . Vibration pertaining to a case where the vibration plate  21  is vibrated weakly means vibration of the vibration plate  21  with which liquid is not discharged from the nozzles  19  even when normal meniscuses are formed in the nozzles  19 . 
     When the actuators  24  are driven as the pressure reducing operation, a high voltage may be applied to the actuators  24  such that the vibration plate  21  is vibrated strongly. In this case, liquid is discharged from the nozzles  19  and thus it is possible to more reliably reduce the pressure in the droplet discharger  12 . Note that, vibration in a case where the vibration plate  21  is vibrated strongly means the vibration of the vibration plate  21  at a time when liquid is discharged to the recording medium  113  (for example, at time of recording process). 
     The pressure reducing operation may be a combination of the preceding wiping operation and an operation of driving the actuators  24 . 
     In the flowchart illustrated in  FIG. 12 , the controller  160  may perform the flushing as the second discharge operation after the finishing wiping operation is performed. In this case, normal meniscuses are likely to be formed in the nozzles  19  of the droplet discharger  12 . 
     When the preceding wiping operation is performed with the wiping portion  149  coming into contact with the nozzle surface  18 , the contact force of the wiping portion  149  with respect to the nozzle surface  18  in the preceding wiping operation and the finishing wiping operation may be appropriately changed. For example, the contact force of the wiping portion  149  with respect to the nozzle surface  18  in the preceding wiping operation may be the same as that in the finishing wiping operation and may be weaker than that in the finishing wiping operation. 
     The liquid receiver  131  may be provided above the casing  141  of the wiping mechanism  140  in the vertical direction. In this case, it is possible to perform the pressure reducing operation without moving the droplet discharger  12  after the cleaning operation is performed. Therefore, it is possible to suppress pressurized liquid leaking out from the nozzles  19  of the droplet discharger  12  due to vibration acting on the droplet discharger  12  when the droplet discharger  12  moves. 
     The liquid receiver  131  may be configured of a movable belt that can receive liquid. In this case, it is preferable that a component such as a motor for driving the belt be provided such that a portion of the belt that has received liquid can be changed to a portion of the belt that has not received liquid. 
     The pressing mechanism  48  may not be provided with the expansion and contraction portion  67  and may press the diaphragm  56  by adjusting the pressure in the air chamber  72 . Specifically, the pressing mechanism  48  may displace the diaphragm  56  in a direction in which the volume of the liquid outflow portion  51  is reduced by increasing the pressure in the air chamber  72 . The pressing mechanism  48  may displace the diaphragm  56  in a direction in which the volume of the liquid outflow portion  51  is increased by reducing the pressure in the air chamber  72 . Note that, in a case where this configuration is adopted, as the pressure reducing operation, the pressure in the air chamber  72  may be reduced to a negative pressure lower than the atmospheric pressure such that the pressure in the droplet discharger  12  is reduced. 
     A buffer tank into which liquid flows and from which liquid flows out may be provided between the pressure adjustment mechanism  35  and the droplet discharger  12 . In this case, it is preferable that a portion of a wall portion of the buffer tank be an elastically deformable flexible wall and a displacement mechanism for displacing the flexible wall be provided such that the volume of the buffer tank can be changed. In this case, it is possible to perform the pressure reducing operation by increasing the volume of the buffer tank after the cleaning operation is performed in a state where the volume of the buffer tank is reduced. 
     Liquid discharged by the droplet discharger  12  is not limited to ink and may be liquid into which functional particles are dispersed or mixed. For example, the droplet discharger  12  may discharge liquid in the form of a dispersion or a solution containing a material such as an electrode material or a pixel material used for production of liquid crystal displays, electroluminescent displays, and surface emission displays. 
     Hereinafter, the technical idea and the effect thereof figured out from the above-described embodiment and the modification examples will be described. 
     A droplet discharging apparatus includes: a droplet discharger including a common liquid chamber to which liquid is supplied from a liquid supply source via a liquid supply flow path, a plurality of pressure chambers communicating with the common liquid chamber, actuators provided respectively corresponding to the plurality of pressure chambers, nozzles provided respectively corresponding to the plurality of pressure chambers, and a discharge flow path coupled to the pressure chambers such that the liquid in the pressure chambers are discharged to an outside, the droplet discharger performing a recording process with respect to a recording medium by driving the actuators such that the liquid in the pressure chambers are discharged from the nozzles in the form of droplets; and a return flow path coupled to the discharge flow path and forming a circulation path for circulation of the liquid together with the liquid supply flow path. The droplet discharging apparatus performs, as a maintenance operation for the droplet discharger, a first discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path when no droplets are discharged from the nozzles during the recording process. 
     According to this configuration, the liquid discharged from the pressure chambers to the return flow path via the discharge flow path coupled to the pressure chambers flows in the circulation path. Since the liquid flows, an increase in viscosity of the liquid is suppressed. Therefore, by using the first discharge operation, it is possible to suppress an increase in viscosity of liquid without discharging droplets from the nozzles. Therefore, it is possible to reduce the amount of liquid consumed for maintenance. 
     In the first discharge operation, the droplet discharging apparatus may cause the liquid to be discharged toward the return flow path with the liquid in the pressure chambers sucked from the discharge flow path side such that meniscuses on gas-liquid interfaces in the nozzles are maintained. 
     According to this configuration, when the liquid in the pressure chambers is sucked from the discharge flow path side, the meniscuses in the nozzles are moved toward the pressure chambers. That is, liquid in the nozzles flows. Therefore, an increase in viscosity of the liquid in the nozzles can be suppressed. 
     The droplet discharging apparatus may further include a detector configured to detect a state of insides of the pressure chambers by detecting vibration waveforms of the pressure chambers, and the droplet discharging apparatus may perform the first discharge operation when it is estimated, based on a result of the detection performed by the detector, that the state of the insides of the pressure chambers is abnormal since a volume of air bubbles present in the pressure chambers and the nozzles is equal to or greater than a set value. 
     When the volume of air bubbles present in the pressure chambers and the nozzles is small, the air bubbles may be eliminated by being dissolved in liquid with time. When the volume of the air bubbles is small, it is possible to remove the air bubbles from the pressure chambers and the nozzles without performing the first discharge operation by, for example, waiting for a predetermined time. On the contrary, when the volume of air bubbles present in the pressure chambers and the nozzles is large, the air bubbles may grow with time. According to the above-described configuration, the first discharge operation is performed when the air bubbles are not estimated to be eliminated with time. It is possible to decrease a frequency at which the first discharge operation is performed since it is not necessary to perform the first discharge operation when the air bubbles are estimated to be eliminated with time. 
     The droplet discharging apparatus may further include a detector configured to detect a state of insides of the pressure chambers by detecting vibration waveforms of the pressure chambers. The droplet discharging apparatus may estimate whether the state of the insides of the pressure chambers is improved or not by comparing the vibration waveforms of the pressure chambers that are detected by the detector at intervals and when it is estimated that the state of the insides of the pressure chambers is not improved, the droplet discharging apparatus may perform, as a maintenance operation for the droplet discharger, a second discharge operation of causing the liquid in the pressure chambers to be discharged to the outside from the nozzles. 
     According to this configuration, for example, when the state of the insides of the pressure chambers is not improved even after the first discharge operation is performed and when the state of the insides of the pressure chambers is not improved after the droplet discharging apparatus waits for a predetermined time, the second discharge operation of causing liquid in the pressure chambers to be discharged to the outside from the nozzles is performed. Since the second discharge operation is an operation of causing the liquid in the pressure chambers to be discharged to the outside from the nozzles, the second discharge operation is an operation that has a higher maintenance effect with respect to the droplet discharger than the first discharge operation of causing liquid in the pressure chambers to be discharged to the return flow path via the discharge flow path. In this manner, by performing the second discharge operation when the state of the inside of the pressure chamber is not improved with the first discharge operation, it is possible to appropriately perform maintenance of the droplet discharger. 
     The droplet discharging apparatus may further include a detector configured to detect a state of insides of the pressure chambers by detecting vibration waveforms of the pressure chambers. When the discharge flow path is a first discharge flow path, the droplet discharger may further include a second discharge flow path that is coupled to the common liquid chamber and the return flow path such that the liquid in the common liquid chamber is discharged to the outside without passing through the pressure chambers and when the number of pressure chambers estimated as the pressure chamber of which the inside is in an abnormal state due to air bubbles present in the pressure chamber and the nozzle based on the result of the detection performed by the detector is equal to or larger than a set number, the droplet discharging apparatus may perform, as a maintenance operation for the droplet discharger, a third discharge operation of causing the liquid in the common liquid chamber to be discharged toward the return flow path via the second discharge flow path before the first discharge operation is performed. 
     When the number of pressure chambers estimated as the pressure chamber of which the inside is in an abnormal state due to air bubbles present in the pressure chamber and the nozzle is equal to or larger than the set number, it is considered that air bubbles are present in the common liquid chamber communicating with the plurality of pressure chambers. Therefore, when the number of pressure chambers estimated as the pressure chamber of which the inside is in an abnormal state due to air bubbles present in the pressure chamber and the nozzle is equal to or larger than the set number, the third discharge operation of causing liquid in the common liquid chamber to be discharged toward the return flow path via the second discharge flow path coupled to the common liquid chamber and the return flow path is performed. Accordingly, it is possible to discharge liquid in the common liquid chamber in which air bubbles are expected to be present. 
     The droplet discharging apparatus may perform, as the maintenance operation for the droplet discharger, a fourth discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path at a flow rate lower than the first discharge operation when droplets are discharged from the nozzles during the recording process. 
     When liquid in the pressure chambers is discharged toward the return flow path via the discharge flow path coupled to the pressure chambers in order to suppress an increase in viscosity of liquid, the pressure in the pressure chambers becomes unstable due to the flow of liquid. If the pressure in the pressure chambers becomes unstable when droplets are discharged from the nozzles during the recording process, the discharge accuracy of the nozzles discharging droplets is decreased. Therefore, when droplets are discharged from the nozzles during the recording process, the fourth discharge operation of causing liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path coupled to the pressure chambers at a flow rate lower than the first discharge operation is performed in this configuration. In the fourth discharge operation, the pressure in the pressure chambers does not significantly fluctuate since the flow rate is low in comparison with the first discharge operation. That is, by performing the fourth discharge operation, it is possible to suppress an increase in viscosity of liquid while suppressing a fluctuation in pressure in the pressure chambers even when droplets are discharged from the nozzles during the recording process. 
     The droplet discharging apparatus may further include a cap configured to cap a nozzle surface in which the nozzles are open. The droplet discharging apparatus may perform, as a maintenance operation for the droplet discharger, a fifth discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path at a flow rate higher than the first discharge operation in a state where the nozzle surface is capped by the cap when the recording process is not performed. 
     When liquid in the pressure chambers is discharged toward the return flow path via the discharge flow path coupled to the pressure chambers in order to suppress an increase in viscosity of liquid, the pressure in the pressure chambers fluctuates due to the flow of liquid. If a flow rate at which the liquid flows from the pressure chambers to the return flow path is high, the pressure in the pressure chambers significantly fluctuates and thus there is a possibility that the outside air enters the pressure chambers through the nozzles or the liquid flows out through the nozzles. However, if the nozzle surface is capped by the cap when liquid in the pressure chambers is discharged toward the return flow path via the discharge flow path coupled to the pressure chambers, a possibility that the outside air enters the pressure chambers through the nozzles or the liquid flows out from the nozzles due to a fluctuation in pressure in the pressure chambers is decreased. Therefore, in a state where the nozzle surface is capped by the cap, it is possible to make a flow rate at which liquid is discharged from the insides of the pressure chambers toward the return flow path via the discharge flow path higher. According to the above-described configuration, by performing the fifth discharge operation with the nozzle surface capped, it is possible to effectively perform maintenance of the droplet discharger. 
     There is provided a maintenance method for a droplet discharging apparatus which includes: a droplet discharger including a common liquid chamber to which liquid is supplied from a liquid supply source via a liquid supply flow path, a plurality of pressure chambers communicating with the common liquid chamber, actuators provided respectively corresponding to the plurality of pressure chambers, nozzles provided respectively corresponding to the plurality of pressure chambers, and a discharge flow path coupled to the pressure chambers such that the liquid in the pressure chambers are discharged to an outside, the droplet discharger performing a recording process with respect to a recording medium by driving the actuators such that the liquid in the pressure chambers are discharged from the nozzles in the form of droplets; and a return flow path coupled to the discharge flow path and forming a circulation path for circulation of the liquid together with the liquid supply flow path, the method including: performing, as a maintenance operation for the droplet discharger, a first discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path when no droplets are discharged from the nozzles during the recording process. 
     According to this method, the liquid discharged from the pressure chambers to the return flow path via the discharge flow path coupled to the pressure chambers flows in the circulation path. Since the liquid flows, an increase in viscosity of the liquid is suppressed. Therefore, by using the first discharge operation, it is possible to suppress an increase in viscosity of liquid without discharging droplets from the nozzles. Therefore, it is possible to reduce the amount of liquid consumed for maintenance. 
     In the maintenance method for a droplet discharging apparatus, as the maintenance operation for the droplet discharger, a fourth discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path at a flow rate lower than the first discharge operation may be performed when droplets are discharged from the nozzles during the recording process. 
     When liquid in the pressure chambers is discharged toward the return flow path via the discharge flow path coupled to the pressure chambers in order to suppress an increase in viscosity of liquid, the pressure in the pressure chambers becomes unstable due to the flow of liquid. If the pressure in the pressure chambers becomes unstable when droplets are discharged from the nozzles during the recording process, the discharge accuracy of the nozzles discharging droplets is decreased. Therefore, when droplets are discharged from the nozzles during the recording process, the fourth discharge operation of causing liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path coupled to the pressure chambers at a flow rate lower than the first discharge operation is performed in this method. In the fourth discharge operation, the pressure in the pressure chambers does not significantly fluctuate since the flow rate is low in comparison with the first discharge operation. That is, by performing the fourth discharge operation, it is possible to suppress an increase in viscosity of liquid while suppressing a fluctuation in pressure in the pressure chambers even when droplets are discharged from the nozzles during the recording process. 
     In the maintenance method for a droplet discharging apparatus, the droplet discharging apparatus may further include a cap configured to cap a nozzle surface in which the nozzles are open, and, as a maintenance operation for the droplet discharger, a fifth discharge operation of causing the liquid in the pressure chambers to be discharged toward the return flow path via the discharge flow path at a flow rate higher than the first discharge operation may be performed in a state where the nozzle surface is capped by the cap when the recording process is not performed. 
     When liquid in the pressure chambers is discharged toward the return flow path via the discharge flow path coupled to the pressure chambers in order to suppress an increase in viscosity of liquid, the pressure in the pressure chambers fluctuates due to the flow of liquid. If a flow rate at which the liquid flows from the pressure chambers to the return flow path is high, the pressure in the pressure chambers significantly fluctuates and thus there is a possibility that the outside air enters the pressure chambers through the nozzles or the liquid flows out through the nozzles. However, if the nozzle surface is capped by the cap when liquid in the pressure chambers is discharged toward the return flow path via the discharge flow path coupled to the pressure chambers, a possibility that the outside air enters the pressure chambers through the nozzles or the liquid flows out from the nozzles due to a fluctuation in pressure in the pressure chambers is decreased. Therefore, in a state where the nozzle surface is capped by the cap, it is possible to make a flow rate at which liquid is discharged from the insides of the pressure chambers toward the return flow path via the discharge flow path higher. According to the above-described configuration, by performing the fifth discharge operation with the nozzle surface capped, it is possible to effectively perform maintenance of the droplet discharger.