Patent Publication Number: US-8523336-B2

Title: Ink supply apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-230048, filed on Oct. 19, 2011, the entire contents all of which are incorporated herein by reference. 
     FIELD 
     Embodiment described herein relates generally to an ink supply apparatus which is applied to an image forming apparatus, such as, e.g., an ink jet printer. 
     BACKGROUND 
     An ink jet printer is equipped with a recording head for ejecting ink, and an ink supply apparatus for supplying ink to the recording head. In such a printer, a wiping, for example, is executed to clean the recording head which has an orifice plate on which a plurality of nozzles are formed. When the wiping is executed, ink is projected from the surface of the orifice plate through the nozzles to wet the surface of the orifice plate with ink. 
     In one of the examples of the conventional ink supply apparatus, a circulation flow path is composed of a first flow path which connects the inflow side of the recording head with the ink storage section and a second flow path which connects the outflow side of the recording head with the ink storage section. In addition, the first and second flow paths of the circulation flow path are connected with a bypass flow path, and a relief valve is provided in the bypass flow path. When the wiping is performed, a part of ink that flows from the first flow path toward the recording head flows to the second flow path through the bypass flow path. At this moment, the pressure of ink flowing from the ink storage section to the recording head is controlled by regulating the opening degree of the relief valve. 
     On the other hand, in case in which an image is formed with ink, it is desired to circulate ink between the recording head and the ink supply source. In the above-described ink supply apparatus, it is difficult to eject ink from the nozzle while ink is circulated when an image is formed. This is because that the pressure of ink in the recording head is maintained at a positive pressure when wiping is executed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of this disclosure will become apparent upon reading the following detailed description and upon reference to the accompanying drawings. The description and the associated drawings are provided to illustrate embodiments of the invention and not limited to the scope of the invention, wherein: 
         FIG. 1  is a schematic view illustrating a constitution of an ink supply apparatus according to one embodiment; 
         FIG. 2  is a view illustrating a state in which the ink supply apparatus shown in  FIG. 1  is in an ink circulation mode; 
         FIG. 3  is a view illustrating a state in which the ink supply apparatus shown in  FIG. 1  is in an ink projection mode; 
         FIG. 4  is a block diagram illustrating a constitution of the controller of the ink supply apparatus; and 
         FIG. 5  is a flow chart showing an ink supply process of the ink supply apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, it is to provide an ink supply apparatus which supplies ink to a recording head. The ink supply apparatus comprises a pump, a first tank having a first inflow port connected to the discharge port of the pump and a first outflow port connected to the ink supply opening of the recording head, a second tank including a second inflow port connected to the ink recovery opening of the recording head and a second outflow port connected to the intake port of the pump, a third tank, including an ink storage part open to air, which is supplied with ink from an ink supply source, a first relay pipe connected between the first tank and the third tank, a first pressure adjusting mechanism, arranged at the first relay pipe, that is configured to open if the pressure in the first tank is greater than a first given value to discharge ink in the first tank to the third tank, a second relay pipe which connects the ink storage part of the third tank and the second tank, and a second pressure adjusting mechanism, arranged at the second relay pipe, that is configured to open if the pressure in the third tank is lower than a second given value smaller than the first given value to draw ink in the third tank into the second tank. 
     Embodiment will now be described in more detail with reference to the accompanying drawings. However, the same numerals are applied to the similar elements in the drawings, and therefore, the detailed description thereof are not repeated. 
     (Embodiment) 
     According to one embodiment, in an ink supply apparatus which supplies ink to a recording head, an image can be formed by ejecting ink from the recording head while ink is circulated. 
     Hereafter, an ink supply apparatus according to the present embodiment will be described with reference to FIGS.  1 □ 5 . 
       FIG. 1  is a schematic view showing an ink supply apparatus  20  for supplying ink to the recording head (ink jet head)  10  of an ink jet printer as an example of an image forming apparatus. The ink supply apparatus includes one function in which ink is supplied from an ink cartridge  21  serving as an ink supply source to the recording head and another function in which ink is circulated. 
     The ink cartridge  21  is changeable and stores ink  22  in a sealed housing thereof. An ink supply port  21   a  and an air-open port  21   b  are provided in the housing of the ink cartridge  21 . In  FIG. 1 , an up and down direction (height direction) is indicated by an arrow Y. 
     The ink supply apparatus  20  is provided with a connection section  25  connecting with the ink cartridge  21 . The connection section  25  includes an ink-supply-connection port  25   a  which connects with the ink supply port  21   a  of the cartridge  21  and an air-open-connection port  25   b  which connects with the air-open port  21   b . The ink-supply-connection port  25   a  is connected to an ink supply pipe  31  and the air-open-connection port  25   b  is connected with a through pipe  32 . The ink supply pipe  31  and the through pipe  32  respectively extend from the connection section  25  in a lower direction. 
     The recording head  10  includes an orifice plate  41  on which a nozzle  40  that selectively ejects ink is formed, a pressure chamber  42  which is fluidly communicated with the nozzle  40 , an actuator  43  which makes ink in the pressure chamber  42  eject through the nozzle  40 , an ink supply port  45  and an ink recovery port  46  which are fluidly communicated with the pressure chamber  42 , respectively. The pressure chamber  42  is deformed directly or indirectly by using a piezoelectric element such as a PZT as an actuator  43 . Other examples of the recording head  10  are: one which drives a diaphragm with static electricity, one which heats ink by a heater to produce bubble and generates pressure by bubble produced, and further one which moves ink with static electricity, directly. Any one of the examples is usable. In a practical manner, the nozzle  40 , the pressure chamber  42  and the actuator  43  of the above-described recording head  10  are provided in plural although each one thereof is illustrated in  FIG. 1 . 
     Further, the ink supply apparatus  20  includes a first tank  51 , a second tank  52 , a third tank  53 , a waste ink tank  54 , a circulation pump  55 , an open/close valve  56 , a liquid surface sensor  57  and a controller  58  all of which are arranged under the ink cartridge  21 . 
     In the present embodiment, the ink cartridge  21  is arranged at a position higher than the first tank  51 . The second tank  52  is arranged at a position lower than the first tank  51 . The third tank  53  is arranged at a position in height between the first and second tanks  51  and  52 . The waste ink tank  54  is arranged at a position lower than the third tank  53 . A connection port  54   a  and an air-open port  54   b  are provided at an upper part of the waste ink tank  54 . Relative positions in height of the recording head  10 , the circulation pump  55  and the controller  58  may be set freely. 
     The circulation pump  55  is driven by a motor (not shown) and is controlled by the controller  58  as described later. The circulation pump  55  includes a discharge port  55   a  for discharging ink and an intake port  55   b  for drawing ink. 
     The first tank  51  is arranged at an upstream side to the recording head  10  in the ink supply direction. The interior of the first tank  51  is sealed to store ink  22 . The first tank  51  is provided with a first inflow port  51   a , a first outflow port  51   b , a filter  60 , a first relief valve  61  as an example of a first pressure adjusting mechanism and a first elastic member  62  as an example of a first pressure follow member. 
     The first inflow port  51   a  of the first tank  51  is connected to the discharge port  55   a  of the circulation pump  55  through the discharge pipe  70 . Ink which is discharged from the discharge port  55   a  of the circulation pump  55  and flows toward the first tank  51  through the discharge pipe  70  passes the first inflow port  51   a  and the filter  60  and flows into the first tank  51 . An example of the filter  60  is a fine mesh member attached to a frame. 
     The first outflow port  51   b  of the first tank  51  is arranged at a height the same as that of the bottom of the first tank  51 . The first outflow port  51   b  is connected to the ink supply port  45  of the recording head  10  through a supply pipe  71 . The reason why the outflow port  51   b  of the first tank  51  is provided at the bottom of the first tank Si is that it prevents air in the first tank  51  from being sent to the recording head  10  through the supply pipe  71 . 
     The first relief valve  61  is provided at an uppermost part of the first tank  51 . The first relief valve  61  is fluidly communicated with an air part  74  in the third tank  53  through a first relay pipe  73 . Under the air part  74 , an ink storage part  75  for storing ink exists. The reason why the first relief valve  61  is provided at the uppermost part of the first tank is that air in the first tank  51  is released completely. 
     An example of the first relief valve  61  is a one way valve comprising a valve body  61   a  and a spring  61   b . When the pressure in the first tank  51  increases greater than a first given value, the valve body  61   a  is opened against the elastic force of the spring  61   b  and thus ink in which bubble may be mixed is supplied to the third tank  53  through the first relay pipe  73 . That is, the first relief valve  61  functions as the first pressure adjusting mechanism which maintains the pressure in the first tank  51  at the first given value (positive pressure). Besides, instead of the valve body  61   a  and the spring  61   b , combination of a pressure sensor and an electromagnetic valve may be employed. In this case, when the pressure in the first tank  51  reaches at the first given value, the electromagnetic valve is open to discharge ink. 
     Further, a first elastic member  62 , an example of the first pressure follow member, is provided at the first tank  51 . The first elastic member  62  is a diaphragm made of a material having elasticity such as a rubber, but other member may be used such as a balloon in which a gas, e.g., air, is sealed. Since the first elastic member  62  is in contact with ink  22  in the first tank  51 , the first elastic member  62  is deformed elastically such that it is expanded toward the outside of the first tank  51  when the pressure of ink  22  in the first tank  51  increases. The first tank  51  is hard to be deformed because of its rigidity and thus the first elastic member  62  is elastically deformed in response to the pressure variation in the first tank  51 . That is, the first elastic member  62  acts as a pressure storing means to maintain the inside of the first tank  51  at a positive pressure. 
     The second tank  52  is arranged at a downstream side to the recording head  10  in the ink discharging direction. The inside of the second tank  52  is also sealed as the same as the first tank  51 . A second inflow port  52   a  is provided at the bottom of the second tank  52  such that the height of the second inflow port  52   a  is the same as that of the bottom of the second tank  52 . The second inflow port  52   a  is connected to the ink recovery port  46  of the recording head  10  through a return pipe  81 . 
     The open/close valve  56  is provided at the halfway of the return pipe  81 . An example of the open/close valve  56  is a normally open valve which usually opens (at electricity being off) and closes when an instruction is output from the controller  58  (at electricity being on). Thus the open/close valve  56  is controlled by the controller  58  as described later. 
     A second outflow port  52   b  is provided at an upper part of the second tank  52 . The second outflow port  52   b  is connected to the intake port  55   b  of the circulation pump  55  through a suction pipe  85 . The second outflow port  52   b  is arranged at near the uppermost part of the second tank  52 . The reason that the second outflow port  52   b  is arranged at the uppermost part of the second tank  52  is that air in the second tank  52  is completely sucked out to fill the second tank  52  with ink. 
     A second relief valve  90 , an example of the second pressure adjusting mechanism, is provided at the second tank  52 . The second relief valve  90  is arranged near the uppermost part of the second tank  52 . The inflow side of the second relief valve  90  is fluidly communicated with the third tank  53  through the outflow side connection port  53   a  and the second relay pipe  95  which are arranged at the same height as the bottom of the third tank  53 . 
     An example of the second relief valve  90  is a one way valve comprised of a valve body  90   a  and a spring  90   b . When the pressure in the second tank  52  decreases smaller than a second given value, the valve body  90   a  is opened against the elastic force of the spring  90   b  and thus ink in the third tank  53  is drawn into the second tank  52 . That is, the second relief valve  90  acts as the second pressure adjusting mechanism which maintains the pressure in the second tank  52  at the second given value smaller than the first given value. Besides, instead of the relief valve  90  comprising the valve body  90   a  and the spring  90   b , a combination of a pressure sensor and an electromagnetic valve may be employed. In this case, when the pressure in the second tank  52  decreases below the second given value, the electromagnetic valve is open. 
     Further, a second elastic member  92 , an example of the second pressure follow member, is provided at the second tank  51 . The second elastic member  92  is a diaphragm made of a material having elasticity such as a rubber, but other member may be used. Since the second elastic member  92  is in contact with ink  22  in the second tank  51 , the second elastic member  62  is deformed elastically in a concaved fashion toward the inside of the second tank  52  when the pressure in the second tank  52  decreases. The second tank  52  is hard to be deformed because of its rigidity and thus the second elastic member  92  is elastically deformed in response to the pressure variation in the second tank  52 . That is, the second elastic member  92  acts as a negative pressure maintaining means to maintain the pressure in the second tank  51  lower than that in the first tank. 
     The third tank  53  includes the outflow side connection port  53   a  arranged at near the bottom of the thirst tank  53 , a connection port  53   b  connected with the ink supply pipe  31 , a connection port  53   c  connected with the through pipe  32 , a connection port  53   d  connected with the first relay pipe  73  and an air through port  53   e  that is open at a position higher than the upper limit level of the liquid surface of ink  22  in the third tank  53 . The liquid surface sensor  57  is arranged at the third tank  53  to detect the liquid surface of ink below a prescribed value that indicates empty of the third tank  53 , substantially. 
     The outflow side connection port  53   a  of the third tank  53  is arranged at a position in height equal to that of the second relief valve  90 . The outflow side connection port  53   a  is connected to the inflow side of the second relief valve  90  through the second relay pipe  95  which is arranged at a position in height equal to that of the second relief valve  90 . In this way, an influence caused by the water head difference between the liquid surface of ink  22  in the third tank  54  and the second relief valve  90  can be minimized. 
     The connection port  53   b  connected to the ink supply pipe  31  is inserted into the third tank  53  from the upper side of the third tank  53 . The lower end of the connection port  53   b  opens below the liquid surface of ink  22  in the third tank  53 . The connection port  53   c  connected to the through pipe  32  is also inserted into the third tank  53  from the upper side of the third tank  53 . The lower end of the connection port  53   c  opens at a position corresponding to a predetermined liquid surface level of ink  22  in the third tank  53 . 
     The connection port  53   d  connected to the first relay pipe  73  is inserted into the third tank  53  from the upper side of the tank  53 . The lower end of the connection port  53   d  opens at a position that is apart from the liquid surface of ink in the third tank  53  by a distance H 1  (shown in  FIG. 1 ) in the upper direction. Therefore, ink reaching to the connection port  53   d  from the first relief valve  61  through the first relay pipe  73  drops into the third tank  53  through air in the third tank  53 . 
     The air through port  53   e  arranged near the upper part of the third tank  53  opens at a position higher than the upper limit level of the liquid surface of ink in the third tank  53 . The air through port  53   e  is connected to the connection port  54   a  of the waste ink tank  54  through an air flow pipe  96 . Thus, ink in the third tank  53  is discharged to the waste ink tank  54  through the air flow pipe  96  when the liquid surface of ink  22  in the third tank  53  increases higher than the air through port  53   e.    
     Since the air open port  54   b  of the waste ink tank  54  always opens to air, the air part  74  in the third tank  53  is fluidly communicated with air through the air through port  53   e , the air flow pipe  96 , the waste ink tank  54  and the air open port  54   b . In case in which ink is water-soluble, ink evaporated in the waste ink tank  54  enters into the air part  74  in the third tank  53  through the air flow pipe  96  and the air through port  53   e . Thus, the inside of the third tank  53  is maintained at a given saturated vapor amount. 
     An operation of the ink supply apparatus  20  will be described below. Ink  22  in the ink cartridge  21  is supplied to the third tank  53  through the ink supply connection port  25   a  of the connecting section  25 , the ink supply pipe  31  and the connection port  53   b . When the liquid surface of ink  22  in the third tank  53  increases to reach at the lower end of the connection port  53   c , the liquid surface of ink  22  in the third tank  53  does not increase thereafter. Instead of this, ink  22  flows from the connection port  53   c  to the through pipe  32  and then goes up in the through pipe  32 . Ink in the through pipe  32  goes up to the same height H 2  (shown in  FIG. 1 ) as the liquid surface of ink in the ink cartridge  21 . 
     On the other hand, when the liquid surface of ink  22  in the third tank  53  goes down, ink  22  in the ink cartridge  21  flows to the third tank  53  through the ink supply pipe  31  because of the connection port  53  being open to air. Or, ink staying in the through pipe  32  drops toward the third tank  53  and thus the liquid surface of ink  22  in the third tank  53  goes up to a height at which it covers the lower end (opening) of the connection port  53   c . Therefore, the liquid surface of ink  22  in the third tank  53  is always maintained at the height of the lower end (opening) of the connection port  53   c . This height is the predetermined liquid surface level. 
     In case in which the position of the second relief valve  90  is a height reference, the pressure occurred by the water head difference between the liquid surface of ink  22  in the third tank  53  and the ink in the second relief valve  90  influences the open pressure of the second relief valve  90 , if the liquid surface level in the third tank  53  is set to excessively high. Thus, the liquid surface level in the third tank  53  is set to be a height at which air in the third tank  53  does not go toward the second relief valve  90 . That is, the position of the lower end of the connection port  53   c  should be set to be higher than that of the outflow side connection port  53   a.    
     If the ink cartridge  21  becomes empty, the liquid surface level of ink  22  in the third tank  53  decreases and the liquid surface sensor  57  is finally on. This is because that the liquid surface of ink in the third tank  53  can not maintain the predetermined liquid surface level. When the liquid surface sensor  57  is on, the controller  58  outputs a signal that informs a user or an upper level controller that the ink cartridge is empty. 
       FIG. 2  shows a state in which the ink supply apparatus  20  supplies ink to the recording head  10  as the apparatus  20  circulates ink (ink circulation mode). At the image forming operation, ink is consumed by ejecting ink  22 ′ from the nozzle  40  of the recording head  10 . In case in which ink is supplied while ink is circulated, an appropriate value in the ink pressure Pn near the nozzle  40  of the recording head exists. The pressure value Pn is a range between 0 (zero) Pa and −3000 Pa, for example. 
     To maintain the pressure Pn near the nozzle  40  at an appropriate pressure value, a first “energy par unit volume” P 1  is managed by the first relief valve  61  and a second “energy par unit volume” P 2  is also managed by the second relief valve  90  to satisfy the following formula (1):
 
 P 2={( R 1 +R 2) / R 1}× Pn −( R 2 /R 1)× P 1   (1)
         Wherein:
           P 1  (Pa): the energy per unit volume near the outflow port  51   b  of the first tank  51 ,   R 1  (Pa·sec/m 3 ): the flow pass resistance of the supply pipe  71  from the outflow port  51   b  to the nozzle  40  of the recording head  10 ,   P 2  (Pa): the energy per unit volume near the inflow port  52   a  of the second tank  52 , and   R 2  (Pa·sec/m 3 ): the flow pass resistance of the return pipe  81  from the inflow port  52   a  to the nozzle  40  of the recording head  10 .   
               

     The energy per unit volumes P 1  and P 2  are derived from Bernoulli&#39;s principle, and are the total of a static pressure, a dynamic pressure and a potential pressure. If the dynamic pressure can be ignored, the energy per unit volumes P 1  and P 2  are expressed as following formulas (2) and (3):
 
 P 1= Ps 1−ρ· g·H 3   (2)
 
 P 2= Ps 2−ρ· g·H 4   (3)
         Wherein:
           Ps 1  (Pa): the static pressure near the outflow port  51   b  of the first tank  51 ,   Ps 2  (Pa): the static pressure near the inflow port  52   a  of the second tank  52 ,   H 3  (m): the distance from the orifice plate  41  to the outflow port  51   b  of the first tank  51 ,   H 4  (m): the distance from the orifice plate  41  to the inflow port  52   a  of the second tank  52 ,   ρ(kg/m 2 ): the density of ink  22 , and   g (m/s 2 ): the acceleration of gravity.   
               

     In the present embodiment, as stated below, the management of the static pressures Ps 1  and Ps 2  of the above formulas (2) and (3) are realized with the first relief valve  61 , the second relief valve  90 , the first elastic member  62  and the second elastic member  92 . The item of the potential pressure is determined by H 3  and H 4 , exclusively. Therefore, the energy per unit volumes P 1  and P 2  can also be managed. 
     If the instruction of commencement of an ink circulation is input from a user or an upper level controller, the controller  58  drives the circulation pump  55 . When the circulation pump  55  is driven, ink  22  in the second tank  52  is drawn into the circulation pump  55  from the intake port  55   b  and then the ink is discharged from the discharge port  55   a  of the circulation pump  55  toward the first tank  51 . 
     At this moment, the first elastic member  62  is elastically deformed with the pressure of ink supplied to the first tank  51  and the pressure in the first tank  51  increases. When the internal pressure of the first tank  51  increases above the first given value, the first relief valve  61  opens and ink  22  in the first tank  51  is supplied to the third tank  53  through the first relief valve  61 . The first given value of the internal pressure of the first tank  51  is a value at which the pressure near the outflow port  51   b  of the first tank  51  becomes the pressure Ps 1 . 
     If the first relief valve  61  opens, the inside of the first tank  51  opens to air for a moment. However, at this moment, the first elastic member  62  generates an elastic force which causes the elastic member  62  to return to the original state and thus the pressure in the first tank  51  is maintained at the first given value. Therefore, if the first relief valve  61  is closed while the first elastic member  62  generates the elastically restitutive force, the pressure in the first tank  51  can be maintained at the first given value. When the first relief valve  61  opens, ink  22  in the first tank  51  flows into the third tank  53  through the first relief valve  61  and the first relay pipe  73 . 
     On the other hand, since ink  22  in the second tank  52  is drawn out by the circulation pump  55 , the pressure in the second tank  52  decreases. Therefore, the second elastic member  92  is elastically deformed to the inside of the second tank  52  and thus the pressure in the second tank  52  decreases. If the pressure in the second tank  52  decreases below the second given value, the second relief valve  90  opens and ink  22  in the third tank  53  is drawn into the second tank  52  through the second relay pipe  95 . That is, ink  22  in the ink storage part  75  flows into the second tank  52  through the second relay pipe  95  and the second relief valve  90 . It should be noted that the second given value of the internal pressure of the second tank  52  is a value at which the pressure near the inflow port  51   a  of the second tank  51  becomes the pressure Ps 2 . 
     If the second relief valve  90  opens, the inside of the second tank  52  opens to air for a moment. However, at this moment, the second elastic member  92  generates an elastic force which causes the elastic member  92  to return to the original state and thus the pressure in the second tank  52  is maintained at the second given value. Therefore, if the second relief valve  90  is closed while the second elastic member  92  generates the elastically restitutive force, the pressure in the second tank  52  can be maintained at the second given value. 
     Assuming that the first relief valve  61  and the second relief valve  90  are directly connected with each other through a pipe, ink and air in the first tank  51  go directly toward the second relief valve  90  through the first relief valve  61  and thus air returns to the second tank  52 . Moreover, the pressure that causes the first relief valve  61  to open acts on the second relief valve  90 . There is a possibility that each relief valve  61 ,  90  does not open at each given pressure by the mutual action between the first relief valve  61  and the second relief valve  90 . 
     However, in the present embodiment, the outflow side of the first relief valve  61  is fluidly communicated with the air part  74  in the third tank  53  through the first relay pipe  73  and the inflow side of the second relief valve  90  is fluidly communicated with the ink storage part  75  in the third tank  53  through the second relay pipe  95 . Moreover, the liquid surface level of ink  22  in the third tank  53  is maintained constant according to the position of the lower end opening of the connection port  53   b . Therefore, ink flowing into the third tank  53  from the first tank  51  through the first relief valve  61  and the first relay pipe  73  drops into the air part  74  from the lower end opening of the connection port  53   d  and further drops into the ink storage part  75  in the third tank  53  through the air part  74 . 
     As forming the construction described above, even if air is mixed in ink in the first tank  51 , ink and air are finally separated in the third tank  53  while ink including air in the first tank  51  is supplied to the third tank  53  from the first relief valve  61  through the first relay pipe  73 . Therefore, only ink  22  in the third tank  53  returns to the second tank  52  through the second relay pipe  95  and the second relief valve  90  which is opened. It can prevent air from returning to the second tank  52 . 
     When the ink cartridge  21  is empty, the liquid surface of ink  22  in the third tank  53  is lowered. If the liquid surface of ink  22  in the third tank  53  reaches at the position of the outflow side connection port  53   a , air in the third tank  53  is fed to the second tank  52  through the second relief valve  90 . However, since the liquid surface sensor  57  is arranged at a position higher than the outflow side connection port  53   a , the liquid surface sensor  57  operates before air in the third tank  53  goes toward the second relief valve  90 . Therefore, the controller  58  can inform a user or an upper level controller that the ink cartridge  21  is empty. 
     Arrows A and B in  FIG. 2  respectively indicate directions in which ink circulates in the ink supply apparatus  20 . As shown in  FIG. 2 , ink circulates the supply pipe  71  and the return pipe  81 . And, at the image forming operation, ink  22 ′ is ejected from the nozzle  40  of the recording head  10 . In this circulation, since the pressure Pn near the nozzle  40  is set to satisfy the above-described formula (1), bubble is discharged to the second tank  52  through the ink recovery port  46  of the recording head  10  and the return pipe  81  even if the bubble is produced near the nozzle  40 . The bubble discharged to the second tank  52  stays at the upper part of the second tank  52  as air, and is drawn out with the intake port  55   b  of the circulation pump  55  through the suction pipe  85  together with ink from the outflow port  52   b  of the second tank  52  when amount of the air exceeds a constant value. The air drawn out with the intake port  55   b  is discharged toward the inflow port  51   a  of the first tank  51  together with the ink through the discharge pipe  70  from the discharge port  55   a  of the circulation pump  55 . 
     Ink including bubble supplied to the inflow port  51   a  of the first tank  51  enters into the first tank  51  through the filter  60  and the bubble in the ink stays at the upper part of the first tank  51  as air. When amount of the air exceeds a constant value, the air is discharged to the third tank  53  through the first relief valve  61  and the first relay pipe  73 . 
     The ink supply apparatus  20  of the present embodiment properly maintains the pressure Ps 1  near the outflow port  51   b  of the first tank  51  and the pressure Ps 2  near the inflow port  52   a  of the second tank  52  by the first relief valve  61  acting as the first pressure adjusting mechanism and the second relief valve  90  acting as the second pressure adjusting mechanism. Thus, the pressure Pn near the nozzle  40  is also maintained properly. Therefore, the ink supply apparatus  20  removes bubble that may be mingled with ink during the image forming operation by circulating ink under a proper pressure and can charge ink in the circulation path. Since ink is supplied to the recording head  10  while ink is circulated, a foreign matter that is developed near the nozzle  40  of the recording head  10  returns to the second tank  52  through the ink recovery port  46  of the recording head  10 , the return pipe  81  and the inflow port  52   a  of the second tank  52 . 
     If the specific gravity of the foreign matter that is mingled with ink is greater than that of ink, the foreign matter is deposited at the bottom of the second tank  52  and thus it can prevent the foreign matter from moving in the circulation path together with ink. If the specific gravity of the foreign matter is smaller than that of ink, the foreign matter goes to the upper part of the second tank  52  and is drawn out from the intake port  55   b  of the circulation pump  55  through the outflow port  52   b  arranged at the upper part of the second tank  52  and the suction pipe  85  together with ink. The foreign matter is discharged to the first tank  51  together with ink from the discharge port  55   a  of the circulation pump  55  through the discharge pipe  70  and is collected with the filter  60 . 
       FIG. 3  illustrates an ink circulation pass along which ink is projected to wipe the orifice plate  41  of the recording head  10  (ink projection mode) in the ink supply apparatus. If an instruction that ink is projected is issued from a user or an upper level controller while the ink circulation is carried out, the controller  58  closes the open/close valve  56 . After a given time T1 passes, the controller  58  opens the open/close valve  56 . In this way, the pressure of ink from the first tank  51  is transmitted to the recording head  10  while the open/close valve  56  is closed. Thus, a positive pressure is applied near the nozzle  40  and ink  22 ′ projects from the surface of the orifice plate  41 . The given time T1 can be adjusted with a time at which the open/close valve  56  is closed. 
       FIG. 4  is a block diagram illustrating components of the controller  58  of the ink supply apparatus  20  according to the present embodiment. The controller  58  includes a CPU (Central Processing Unit)  100  acting as a processor. The CPU  100  is connected with a ROM (Read Only Memory)  102 , a RAM (Random Access Memory)  103 , a communication interface section  104 , a circulation pump driver  105 , an open/close valve driver  106  and a sensor input/output port  107  with a has line. 
     The ROM  102  stores programs for controlling the CPU  100  and various basic data. The RAM has a memory area to store various data which is required to eject ink and to circulate ink, for example, and provides work area in which data is temporarily stored. The communication interface section  104  controls a data communication with external devices through a communication line. The circulation pump driver  105  drives the circulation pump  55 . The open/close valve driver  106  drives the open/close valve  56 . The sensor input/output port  107  is connected with various sensors such as, e.g., liquid surface sensor. 
       FIG. 5  is a flow chart showing an example of the ink supply process by the controller  58  of the ink supply apparatus  20  according to the present embodiment. In Act A 001  in  FIG. 5 , an electric power source of the ink supply apparatus is on. In Act A 002 , it is determined whether or not the ink circulation begins. If the circulation begins, the circulation pump  55  is driven (Act A 003 ). In Act A 004 , it is judged whether or not the liquid surface sensor  57  is off. If the liquid surface sensor  57  is on (there is no ink), it is informed that the ink cartridge  21  is empty (Act A 011 ). And then, the circulation pump  55  is stopped (Act A 010 ) and the ink circulation is terminated. In Act A 004 , if the liquid surface sensor  57  is off (there is some amount of ink), it is judged whether or not the ink projection performs to execute wiping (Act A 005 ). 
     If the ink projection performs, the open/close valve  56  is closed (Act A 006 ) and ink is projected from the orifice plate  41 . In Act A 007 , a given time T1 is waited and the open/close valve  56  is being opened in Act A 008 . In Act A 009 , it is judged whether or not the ink circulation is finished. If the ink circulation is not finished, the ink circulation is continued (Act A 004 ). In Act A 009 , if the ink circulation is finished, the circulation pump  55  is stopped to finish the operation (Act A 010 ). 
     The ink supply apparatus  20  according to the present embodiment can be applied to an image forming apparatus other than the ink jet printer, and a use other than the image forming apparatus. 
     The present invention has been described with respect to specific embodiments. However, these embodiments have been presented by way of example only. Other embodiments based on the principles of the present invention should be obvious to those of ordinary skill in the art. Such embodiments are intended to be covered by the claims.