Patent Publication Number: US-8985736-B2

Title: Image forming apparatus having ink jet head and method for performing maintenance of the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-060923, filed on Mar. 16, 2012, the entire contents of all of which are incorporated herein by reference. 
     FIELD 
     The present embodiments relate an image forming apparatus and a method for performing maintenance of the ink jet head of the image forming apparatus. 
     BACKGROUND 
     An image forming apparatus such as an ink jet printer ejects ink from the nozzle provided on the ink jet head to form an image. The ink may be coagulated or caked when viscosity thereof is increased because of evaporation of the liquid component and the volatile component of the ink. If ink is caked within the nozzle, it may course failure of printing. 
     To restrain the failure of printing with the caked ink, it is well known, for example, that pressure applied to the ink in the ink jet head is varied to flow ink nearby the nozzle. The ink is stirred with the flow of ink and thus the coagulation of the ink is restrained. 
    
    
     
       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 only provided to illustrate embodiments of the invention and not limited to the scope of the invention, wherein: 
         FIG. 1  is a block diagram roughly illustrating construction of a printing apparatus according to a first embodiment; 
         FIG. 2  is an exploded perspective view illustrating an ink jet head according to the first embodiment; 
         FIG. 3  is a sectional view illustrating the ink jet head, taken along the lines F 3 -F 3  of  FIG. 2 ; 
         FIG. 4  is a flow chart showing one example of a method for performing maintenance of the printing apparatus shown in  FIG. 1 ; 
         FIG. 5  is a block diagram roughly showing the printing apparatus at a state that a meniscus is pulled back; 
         FIG. 6  is a sectional view illustrating the pressure chamber and the nozzle of the ink jet head; 
         FIG. 7  is a sectional view illustrating a state in which the meniscus is withdrawn from the nozzle in the ink jet head; 
         FIG. 8  is a sectional view illustrating a state in which the meniscus is further withdrawn from the state shown in  FIG. 7  in the ink jet head; 
         FIG. 9  is a sectional view illustrating a state in which the meniscus is sill further withdrawn from the state shown in  FIG. 8  in the ink jet head; 
         FIG. 10  is a sectional view illustrating a state in which the meniscus is further withdrawn from the state shown in  FIG. 9  in the ink jet head; 
         FIG. 11  is a sectional view illustrating the empty pressure chamber and nozzle of the ink jet head; 
         FIG. 12  is a sectional view illustrating a state in which ink is supplied to the pressure chamber in the ink jet head; 
         FIG. 13  is a sectional view illustrating a state in which ink is further supplied to the pressure chamber from the state shown in  FIG. 12  in the ink jet head; 
         FIG. 14  is a sectional view illustrating a state in which ink is charged in the pressure chamber in the ink jet head; 
         FIG. 15  is a sectional view illustrating the ink jet head in which a purge operation is performed; 
         FIG. 16  is a flow chart illustrating another example of the maintenance method in the printing apparatus; and 
         FIG. 17  is a block diagram roughly illustrating construction of a printing apparatus according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, it is to provide an image forming apparatus including an ink jet head, having a pressure chamber in which ink is filled, a flow path fluidly communicating with the pressure chamber and a nozzle open to the pressure chamber, which is configured to eject the ink in the pressure chamber through the nozzle, a first tank connected to the flow path of the head to store ink, a supply unit configured to supply the ink in the first tank to the pressure chamber of the head through the flow path of the head, a second tank connected to the flow path of the head to store ink, a discharge unit configured to move the ink in the pressure chamber of the head to the second tank through the flow path of the head, and a control section configured to activate the supply unit and the discharge unit so that the ink in the pressure chamber is discharged to the second tank by the discharge unit, the ink in the first tank is charged to the pressure chamber by the supply unit and the liquid surface of the ink supplied to the pressure chamber is projected from the nozzle of the head, 
     Embodiments 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 descriptions thereof are not repeated. 
     First Embodiment 
     A first embodiment will be described with reference to the  FIG. 1  to the  FIG. 16 .  FIG. 1  is a diagram roughly illustrating construction of an ink jet printing apparatus (hereinafter referred to as a printing apparatus)  10 . The printing apparatus  10  is an example of an image forming apparatus. 
     The printing apparatus  10  includes an ink jet head  11 , a T-shaped junction  13 , an upper-stream side open/close valve  15 , a pressure regulation tank  17 , a first filter  19 , a first disposable filter  21 , an upper-stream side ink tank  23 , an upper-stream side air pump  25  and a second disposable filter  27 . The printing apparatus  10  further includes a lower-stream side open/close valve  29 , a lower-stream side ink tank  31 , a third disposable filter  33 , a lower-stream side suction pump  35 , an upper-stream side open/close valve  37 , a lower-stream side open/close valve  39 , a tank moving mechanism  41  and a control section  43 . 
     The ink jet head  11  is an example of a head. The pressure regulation tank  17  is an example of a first tank. The upper-stream side air pump  25  is an example of a supply unit. The lower-stream side ink tank  31  is an example of a second tank. The lower-stream side suction pump  35  is an example of a pump. The tank moving mechanism  41  is an example of a discharge unit. 
       FIG. 2  is an exploded perspective view illustrating an ink jet head  11 .  FIG. 3  is a sectional view illustrating the ink jet head, taken along the lines F 3 -F 3  of the  FIG. 2 . 
     As shown in the  FIG. 2 , the ink jet head  11  is a so-called shear-mode type ink jet head. The ink jet head  11  includes a head body  50 , a frame member  51 , a cover member  52 , a nozzle plate  53  and a circuit base plate  54 . 
     The head body  50  includes a base member  61  and a piezoelectric member  62 . The base member  61  is formed in a rectangular plate shape and is provided with a cut portion  64  and a plurality of grooves  65 . The plurality of grooves  65  are formed in parallel with one another. The plurality of grooves  65  open at the upper surface  61   a  of the base member  61  and the cut portion  64 , respectively. 
     The piezoelectric member  62  is formed such that two piezoelectric plates made of lead zinconate titanate are stuck with one another. The two piezoelectric plates are polarized in an opposite direction. The piezoelectric member  62  is deformed by a voltage applied. The piezoelectric member  62  is mounted on the cut portion  64  of the base plate  61 . 
     A plurality of pressure chambers  67  are formed on the piezoelectric member  62 . The plurality of pressure chambers  67  are respectively formed in a groove shape and provided in parallel with one another. The plurality of pressure chambers  67  fluidly communicate with the plurality of grooves  65  of the base member  61  one to one manner. Each pressure chamber  67  opens at the upper surface  62   a  and the front surface  62   b  of the piezoelectric member  62 . 
     A pole portion  68  is formed between pressure chambers  67 , A plurality of pole portions  68  respectively partition the plurality of pressure chambers  67  and form the side surface of each pressure chamber  67 . 
     As shown in the  FIG. 3 , a plurality of electrodes  71  are respectively provided on the piezoelectric member  62  and the base member  61 . The plurality of electrodes  71  respectively cover the side surface and the bottom surface of the plurality of pressure chambers  67 . The plurality of electrodes  71  are continuously extended from the pressure chamber  67  to the groove  65 . The electrode  71  is formed with a thin film of nickel, for example. However, it is not limited to this and the electrode  71  may be made of gold or cupper. Pole portions  68  on which the electrode  71  is formed on the both sides thereof act as an actuator. 
     On the other hand, as shown in the  FIG. 2 , a plurality of wiring patterns  73  are formed on the upper surface  61   a  of the base member  61 . The plurality of wiring patterns  73  are formed, for example, such that a thin film of nickel on the upper surface  61   a  of the base member  61  is processed with patterning by a laser. The plurality of wiring patters  73  respectively extend from the rear end of the upper surface  61   a  of the base member  61 . Each one of the end portions of the wiring pattern  73  is connected to the electrode  71 . 
     The circuit base plate  54  is arranged at the other end portion of the wiring pattern  73 . The circuit base plate  54  is, for example, a film carrier package including a plastic film on which a plurality of conductor patterns are formed, and an IC (Integrated Circuit) Device connected to the plurality of conductor patterns. The plurality of conductor patterns are electrically connected to the other ends of the plurality of wiring patterns, respectively. 
     The frame member  51  is mounted on the head body  50  with glue. The cover member  52  is mounted on the frame member  51 . An ink supply opening  81  is provided on the cover member  52 . Such combined frame member  51  and cover member  52  cover the plurality of pressure chambers  67  from the upper surface  61   a  of the base member  61 . 
     As shown in the  FIG. 3 , an ink chamber  82  to which ink is supplied is formed in the inside of the combined frame member  51  and cover member  52 . The ink chamber  82  is an example of the flow path. The cover member  52  covers the ink chamber  82  by attaching on the frame member  51 . The ink supply opening  81  is formed on the cover member  52  and is fluidly communicated with the ink chamber  82 . The ink chamber  82  is fluidly communicated with the plurality of pressure chambers  67 . Ink supplied to the ink chamber  82  through the ink supply opening  81  is further supplied to each pressure chamber  67 . 
     The nozzle plate  53  is a rectangular shaped film made of polyimide plastic. However, the nozzle plate  53  is not limited to be made of polyimide but may be made of steel such as a stainless steel or other material which is able to be fine-processed by a laser. The nozzle plate  53  is attached on the head body  50  and frame member  51 . The nozzle plate  53  covers the plurality of pressure chambers  67  at the side of the front surface  62   b  of the piezoelectric member  62 . 
     A plurality of nozzles  85  are provided on the nozzle plate  53 . The nozzles  85  are formed by a laser processing, respectively. As shown in the  FIG. 3 , the plurality of nozzles  85  open to the plurality of pressure chambers  67 , respectively. 
     In such an ink jet head as described above, the circuit base plate  54  applies a voltage to the electrode  71  through the wiring pattern  73  based on a print signal input from the control section  43 . The pole portion  68  to which the voltage is applied performs a shear-mode deformation in response to the print signal. Ink to which pressure is increased is ejected from the corresponding nozzle  85 . 
     The ink jet head  11  is loaded in the printing apparatus  10  such that it is supported by a head support mechanism. As shown in the  FIG. 1 , one of the ends of a pipe member  101  is connected to the ink supply opening  81  of the ink jet head  11  and the other end thereof is connected to the T-shaped junction  13 . 
     The pressure regulation tank  17  is connected to the upper-stream side of the ink jet head  11  through the T-shaped junction  13  and a pipe member  102 . In other words, the pressure regulation tank  17  is connected to the ink chamber  82  of the ink jet head  11 . 
     The upper-stream side open/close valve  15  is arranged at the pipe member  102  between the T-shaped junction  13  and the pressure regulation tank  17 . The upper-stream side open/close valve  15  is selectively switched between an open position at which flow of ink from the pressure regulation tank  17  to the ink jet head  11  is allowed and a closed position at which flow of ink between the pressure regulation tank  17  and the ink jet head is shut. 
     Ink supplied from the upper-stream side ink tank  23  is stored in the pressure regulation tank  17 , temporarily. One of the ends of the pipe member  102  is positioned within the ink stored in the pressure regulation tank  17  such that it is slightly apart from the bottom surface of the pressure regulation tank  17 . A first hydraulic head sensor  104  is arranged in the pressure regulation tank  17 . The first hydraulic head sensor  104  varies its output in response to the position of the gas-liquid boundary surface of ink and air, i.e., surface of ink, stored in the pressure regulation tank  17 . The first hydraulic head sensor  104  is a float type level sensor, for example, However, the first hydraulic head sensor  104  may be other sensor such as a photo-sensor. 
     A pipe member  106  is connected to the pressure regulating tank  17 . One of the ends of the pipe member  106  is positioned within the ink stored in the pressure regulation tank  17  such that it is slightly apart from the bottom surface of the pressure regulation tank  17 , and the other end thereof is connected to the first filter  19 . 
     A pipe member  107  is connected to the first filter  19 . The first filter  19  removes a foreign substance contained in the ink flowing from the pipe member  107  located at the upper-stream side of the first filter  19  to the pipe member  106  located at the lower-stream side of the first filter  19  thereby providing ink from which a foreign substance is removed to the pressure regulation tank  17 . 
     The first filter  19  is connected to the upper-stream side ink tank  23  through the pipe member  107 . One end of the pipe member  107  is positioned within the ink stored in the upper-stream side ink tank  23  such that it is slightly apart from the bottom surface of the upper-stream side ink tank  23 . 
     A pipe member  109  is connected to the pressure regulation tank  17 . One of the ends of the pipe member  109  is positioned within the air in the pressure regulation tank  17  such that it is slightly apart from the gas-liquid boundary surface (ink surface) between the ink and the air in the pressure regulation tank  17 . The other end of the pipe member  109  is connected to the first disposable filter  21 . The first disposable filter  21  removes a foreign substance contained in the air flowing into the pressure regulation tank  17  through the pipe member  109 . The upper-stream side open/close valve  37  is provided at the pipe member  109 . 
     The upper-stream side ink tank  23  is connected to the upper-stream side air pump  25  through a pipe member  111 . The upper-stream side ink tank  23  stores ink supplied to the ink jet head  11 . The upper-stream side ink tank  23  is able to be supplemented with ink. 
     One of the ends of the pipe member  111  is positioned within the air in the upper-stream side ink tank  23  such that it is slightly apart from the gas-liquid boundary surface between the air and the ink. The other end of the pipe member  111  is connected to the second disposable filter  27 . The second disposable filter  27  removes a foreign substance contained in the air flowing into the upper-stream side ink tank  23  through the pipe member  111  thereby providing air from which a foreign substance is removed to the upper-stream side ink tank  23 . 
     At the lower-stream side of the ink jet head  11 , the lower-stream side ink tank  31  in which ink is stored is connected through the T-shaped junction  13  and a pipe member  113 . In more detail, the lower-stream side ink tank  31  is connected to the ink chamber  82  of the ink jet head  11 . One end of the pipe member  113  is positioned within the ink in the lower-stream side ink tank  31  such that it is slightly apart from the bottom surface of the lower-stream side ink tank  31 . 
     The lower-stream side open/close valve  29  is provided at the pipe member  113 , as shown in the  FIG. 1 . The lower-stream side open/close valve  29  is selectively switched between an open position at which the ink flow from the ink jet head  11  to the lower-stream side ink tank  31  is allowed and a shut down position at which the ink flow between the ink jet head and the lower-stream side ink tank  31  is inhibited. 
     A pipe member  115  is connected to the lower-stream side ink tank  31 . One of the ends of the pipe member  115  is positioned within the air in the lower-stream side ink tank  31  such that it is slightly apart from the gas-liquid boundary surface, i.e., ink surface, between the air and the ink in the lower-stream side ink tank  31 . The other end of the pipe member  115  is connected to the third disposable filter  33 . The third disposable filter  33  removes a foreign substance contained in the air flowing into the lower-stream side ink tank  31  through the pipe member  115 . The pipe member  115  is provided with the lower-stream side ink tank  39 . 
     The lower-stream side ink tank  31  is provided with a second hydraulic head sensor  119 . The second hydraulic head sensor  119  varies its output in response to the position of the gas-liquid boundary surface of the ink and the air, i.e., surface of ink, stored in the lower-stream side ink tank  31 . The second hydraulic head sensor  119  is a float type level sensor, for example, however, the second hydraulic head sensor  119  may be other sensor such as a photo-sensor. 
     The tank moving mechanism  41  moves the lower-stream side ink tank  31  in a vertical direction, for example. The tank moving mechanism  41  comprises a gear, a piston and other mechanism to move the lower-stream side ink tank  31 . 
     The control section  43  functions as hardware elements such as an IC, a memory, a circuit board and others to control various hardware components of the printing apparatus  10 . For example, control section  43  issues a print command to the ink jet head  11  responding to the operation by a user. The control section  43  makes the upper-stream side open/close valve  15 , the lower-stream side open/close valve  29 , the upper-stream side open/close valve  37  and the lower-stream side open/close valve  39  open or close. The control section  43  also makes the upper-stream side air pump  25  and the lower-stream side suction pump  35  start or stop. The control section  43  moves the lower-stream side ink tank  31  through the ink moving mechanism  41 . 
     In the following, a method for filling or charging ink in the ink jet head  11  will be described with reference to the  FIG. 1 . 
     Firstly, the upper-stream side open/close valve  37  is set to the shut down position and the upper-stream side open/close valve  15  and the lower-stream side open/close valve  29  are set to the open position respectively. Then, the upper-stream side air pump  25  is activated to feed air into the upper-stream side ink tank  23 . The internal pressure of the upper-stream side ink tank  23  is increased with the air fed to the ink tank  23 . When the internal pressure of the upper-stream side ink tank  23  is increased to a given value, the ink stored in the upper-stream side ink tank  23  is pushed out to the pipe member  107 , as shown by an arrow in the  FIG. 1 , with the increased internal pressure of the upper-stream side ink tank  23 . 
     The ink pushed out of the upper-stream side ink tank  23  is conveyed to the first filter  19  through the pipe member  107 . In the ink conveyed to the first filter  19 , a foreign substance contained in the ink is removed while the ink passes through the first filter  19  and then the ink is further conveyed to the pressure regulation tank  17 . 
     When ink is filled in the pressure regulation tank  17  by a given amount, the air in the pressure regulation tank  17  is compressed. When the pressure in the pressure regulation tank  17  reaches at a prescribed level, the ink in the pressure regulation tank  17  is pushed out to the pipe member  102  with the increased internal pressure in the pressure regulation tank  17 , as shown by the arrow in the  FIG. 1 . 
     The ink pushed out of the pressure regulation tank  17  is fed to the ink supply opening  81  of the ink jet head  11  through the pipe member  102  and the T-shaped junction  13 . Then, as shown in the  FIG. 3 , the ink from the ink supply opening  81  is charged to the plurality of pressure chamber  67  through the ink chamber  82 , respectively. The ink charged to the pressure chamber  67  finally reaches at the nozzle  85 . 
     In explaining the above description with other expression, the upper-stream side air pump  25  supplies ink in the pressure regulation tank  17  to the pressure chamber  67  through the ink chamber  82  shown in the  FIG. 3 . Furthermore, in other words, the pressure regulation tank  17 , the upper-stream side ink tank  23  and the upper-stream side air pump  25  supply ink to the pressure chamber  67 . The upper-stream side ink tank  23  and the upper-stream side air pump  25  are an example of element which functions as a supply device (first device). 
     When ink is filled in the pressure chamber  67 , the ink pushed out of the pressure regulation tank  17  is supplied to the lower-stream side ink tank  31  through the pipe member  113  and the lower-stream side open/close valve  29 . 
     In a state that the ink jet head  11  does not eject ink, a proper pressure (Pn) in the nozzle  85  is set to be −1 (kPa), for example, with the hydraulic head. That is, the height difference (h 1 ) between the nozzle  85  of the ink jet head  11  and the gas-liquid boundary surface of the ink in the pressure regulation tank  17  is set to be a prescribed amount so that a formula (h 1 =−1(kPa)/ρg) is satisfied, wherein, ρ is the density of ink and g is the acceleration of gravity. Besides, the height difference (h 1 ) is detected by the first hydraulic head sensor  104  arranged in the pressure regulation tank  17 . 
     A height difference (h 2 ) between the nozzle  85  of the ink jet head  11  and the gas-liquid boundary surface of the ink in the lower-stream side ink tank  31  is set to be equal to the height difference (h 1 ). However, the height difference (h 2 ) may be set to be greater than the height difference (h 1 ). In other words, the lower-stream side ink tank  13  is arranged at a height the same as that of the pressure regulation tank  17  or lower than that of the pressure regulation tank  17 . 
     Next, an example of the maintenance method of the printing apparatus  10  including the above-described construction will be described with reference to the  FIGS. 4 and 5 . For example, the control section  43  executes the maintenance of the printing apparatus  10  in case that the printing apparatus  10  is not used for a long time or the print-standby state thereof is continued for a relatively long time. 
       FIG. 4  is a flow chart illustrating an example of the maintenance method of the printing apparatus  10 .  FIG. 5  is a block diagram roughly illustrating the printing apparatus  10  when the meniscus is pulled back. 
     As shown in the  FIG. 4 , in case in which the printing apparatus  10  is not used for a long time or the print-standby state thereof is continued for a relatively long time (Act S 1 ), the proper pressure (Pn) of the nozzle  85  is controlled by a well known negative pressure generation means. A meniscus of the ink formed in the nozzle  85  is pulled back by setting the pressure of the nozzle  85  at a negative pressure value greater than the normal negative pressure value (−1(kPa)). 
     In more detail, firstly, the upper-stream side open/close valve  15  positioned between the pressure regulation tank  17  and the ink jet head  11  is set to the shut down position at which the flow of ink is prevented (ACT S 2 ). 
     Next, the lower-stream side ink tank  31  is moved downward by the tank moving mechanism  41  (ACT S 3 ). As shown in the  FIG. 5 , the height difference (h 2 ) between the nozzle  85  of the ink jet head  11  and the gas-liquid boundary surface of the ink in the lower-stream side ink tank  31  is increased when the lower-stream side ink tank  31  is moved downward. 
     The height difference (h 2 ) reaches, for example, at four times the height difference (h 1 ) between the nozzle  85  and the gas-liquid boundary surface of the ink in the pressure regulation tank  17  (YES in ACT S 4 ), the tank moving mechanism  41  is stopped (ACT S 5 ). When the height difference (h 2 ) reaches at four times the height difference (h 1 ), the proper pressure (Pn) of the nozzle  85  is to be −4(kPa). That is, when the lower-stream side ink tank  31  is moved, the negative pressure in the nozzle  85  is increased from the proper pressure (Pn: −1(kPa)) therein at the standby state. The height difference (h 2 ) is detected by the second hydraulic head sensor  119 . 
     By increasing the negative pressure, the ink in the ink jet head  11  flows toward the lower-stream side ink tank  31  through the pipe member  101 , the T-shaped junction  13  and the pipe member  113 , as shown by the arrow in the  FIG. 5 . In other words, ink charged in the pressure chamber  67  of the ink jet head  11  is sucked into the lower-stream side ink tank  31 . 
     As described above, the tank moving mechanism  41  increases the height difference (h 2 ) between the nozzle  85  and the ink surface in the lower-stream side ink tank  31  by moving the lower-stream side ink tank  31 . When the height difference (h 2 ) is increased, the pressure in the nozzle  85  is decreased and thus the ink in the pressure chamber  67  of the ink jet head  11  moves to the lower-stream side ink tank  31  through the ink chamber  82 . In other words, the lower-stream side ink tank  31  and the tank moving mechanism  41  suck ink charged in the pressure chamber  67 . The lower-stream side ink tank  31  and the tank moving mechanism  41  are one example of the element which functions as a second unit. 
     It should be noted that the height difference (h 2 ) at a position that the tank moving mechanism  41  stops is not limited to the above (h 2  is four times h 1 ). The height difference (h 2 ) may be a given degree that the ink in the ink jet head  11  is smoothly sucked into the lower-stream side ink tank  31 . 
       FIG. 6  is a sectional view illustrating the pressure chamber  67  and the nozzle  85  of the ink jet head  11 .  FIG. 7  is a sectional view illustrating a state in which the meniscus M is pulled back from the nozzle  85  of the ink jet head  11 .  FIG. 8  is a sectional view illustrating a state in which the meniscus M is further pulled back from the state shown in the  FIG. 7 .  FIG. 9  is a sectional view illustrating a state in which the meniscus M is still further pulled back from the state shown in the  FIG. 8 .  FIG. 10  is a sectional view illustrating a state in which the meniscus M is pulled back from the state shown in the  FIG. 9 . 
     As shown in the  FIG. 6 , the bubble B and the caked ink SL are contained in the ink charged in the pressure chamber  67 . If the bubble B and the caked ink SL are present in the nozzle  85 , it may cause failure of ink ejection by the ink jet head  11 . 
     As shown in the  FIGS. 6 to 10 , when the ink charged in the pressure chamber  67  is sucked, the meniscus M (liquid surface) charged in the pressure chamber  67  is moved to the inside of the pressure chamber  67  from the nozzle  85 . The bubble B and the caked ink SL presented in the nozzle  85  are flown together with the ink because of moving the meniscus M. The bubble B and the caked ink SL are conveyed with the flow of the ink from the nozzle  85  to the lower-stream side ink tank  31 . 
     With the elapse of the time, the ink charged in the pressure chamber  67  is discharged to the lower-stream side ink tank  31  and thus the pressure chamber  67  becomes empty. In other words, ink is discharged from the pressure chamber  67  by the lower-stream side ink tank  31  and the tank moving mechanism  41 . Note that a certain amount of ink may be left in the pressure chamber  31 . 
     As shown in the  FIG. 4 , the control section  43  determines whether or not the gas-liquid boundary surface of the ink, i.e., ink surface, in the lower-stream side ink tank  31  raises by a predetermined height (ACT S 7 ) after a prescribed time elapses from the moving of the lower-stream side ink tank  31 . (YES in ACT S 6 ). The predetermined time is a time required to discharge all of the ink charged in the plurality of pressure chambers  67  to the lower-stream side ink tank  31 , for example. The predetermined height is a height to which the ink surface raises when all of the ink charged in the plurality of pressure chambers  67  is conveyed to the lower-stream side ink tank  31 , for example. In other words, the control section  43  judges whether or not the pressure chamber  67  is empty. The increasing amount of the gas-liquid boundary surface of the ink in the lower-stream side ink tank  31  is detected by the second hydraulic head sensor  119 . 
     The control section  43  further moves downward the lower-stream side ink tank  31  by the tank moving mechanism  41  (ACT S 8 ), if the ink surface in the lower-stream side ink tank  31  does not rise by a prescribed height (NO in ACT S 7 ). For example, the control section  43  drives the tank moving mechanism  41  so that the height difference h 2  shown in the  FIG. 5  becomes five times the height difference h 1 . 
     When the height difference h 2  reaches at five times the height difference h 1 , the proper pressure value Pn of the nozzle  85  becomes −5 (kPa). That is, the negative pressure in the nozzle  85  further increases and thus the ink remaining in the pressure chamber  67  is discharged. 
     On the other hand, if the ink surface in the lower-stream side ink tank  31  rises at a prescribed height (YES in ACT S 7 ) or a given time elapses after the height difference h 2  comes five times the height difference h 1  (YES in ACT S 9 ), the control section  43  returns the lower-stream side ink tank  31  to the standby position by the tank moving mechanism  41  (ACT S 10 ). In other words, the tank moving mechanism  41  moves the lower-stream side ink tank  31  so that the height difference h 2  comes to be equal to the height difference h 1 . 
     Next, the upper-stream side open/close valve  15  is set at the open position (ACT S 11 ), and ink is charged to the pressure chamber  67  of the ink jet head  11 . As described above, the ink in the upper-stream side ink tank  23  is fed to the pressure regulation tank  17  and thus the ink in the pressure regulation tank  17  is conveyed to the ink jet head  11  by activating the upper-stream side air pump  25  (ACT S 12 ). 
       FIG. 11  is a sectional view illustrating a state that the pressure chamber  11  and the nozzle  85  of the ink jet head  11  are empty.  FIG. 12  is a sectional view illustrating a state in which ink is supplied to the pressure chamber  67 .  FIG. 13  is a sectional view illustrating a state in which ink is further supplied to the pressure chamber  67 .  FIG. 14  is a sectional view illustrating a state that ink is filled in the pressure chamber  67 .  FIG. 15  is a sectional view illustrating the ink jet head  11  which performs the purge action. 
     When ink is fed to the ink jet head  11 , the ink is supplied to the pressure chamber  67 . The ink supplied flows within the pressure chamber  67 , as shown in the  FIGS. 11 to 15 , to be filled in the pressure chamber  67 . That is, a fresh ink stored in the upper-stream side ink tank  23  is charged to the pressure chamber  67  by the upper-stream side air pump  25 , the upper-stream side ink tank  23  and the pressure regulation tank  17 . 
     As shown in the  FIG. 11 , there is a possibility that a caked ink SS smaller in size than the caked ink SL discharged remains in the pressure chamber  67 . As shown in the  FIGS. 12 and 13 , charging of ink to the pressure chamber  67  begins when ink is conveyed to the ink jet head  11 . After that, as shown in the  FIG. 14 , ink is filled in the pressure chamber  67 . While ink is filled in the pressure chamber  67 , the caked ink SS is forcibly moved toward the nozzle  85  with the ink and the meniscus M of the ink. 
     As shown in the  FIG. 4 , the height difference h 1  becomes −1 (kPa)/ρg by filling ink in the pressure chamber  11  (YES in ACT S 13  in the  FIG. 4 ), the control section  43  stops the upper-stream side air pump (ACT S 14 ). However, it is not limited to that, and the upper-stream side air pump  25  may operate continuously. 
     After ink is filled in the pressure chamber  67 , the above-described purge action is performed (ACT S 15 ). That is, the control section  43  applies voltage to the electrode  71  by the circuit board  54  of the ink jet head  11 . When the voltage is applied to the electrode  71 , the pole portion  68  is deformed in a shear mode, as shown by the two-dot chain line in the  FIG. 15 . The ink filled in the pressure chamber  67  is pressurized by the shear mode deformation of the pole portion  68 . 
     As shown by the two-dot chain line in the  FIG. 15 , the meniscus M of the ink projects from the nozzle  85  of the ink jet head  11  when the ink is pressurized. Thus, the ink and the meniscus M of the ink projected to the outside of the nozzle  85  of the ink jet head  11  discharge the caked ink SS from the nozzle  85  to the outside of the ink jet head  11 . 
     After the ink and the meniscus M thereof project to the outside of the nozzle  85  of the ink jet head  11 , the shear mode deformation of the pole portion  68  is released. As shown by the solid line in the  FIG. 15 , when the shear mode deformation of the pole portion  68  is released, the ink and the meniscus M thereof are returned to the inside of the nozzle  85  of the ink jet head  11 . In other words, the ink and the meniscus M thereof temporarily project to the outside of the nozzle  85  in the purge action. Then, the maintenance of the printing apparatus  10  is completed and the bubble B and the caked inks SL, SS which are present in the pressure chamber  67  and the nozzle  85  are finally discharged. 
     Instead of the above-described purge action, ink may be ejected from the nozzle  85 , for example. When the pole portion  68  performs the shear mode deformation, the ink droplet including the caked ink SS is ejected from the nozzle  85  and thus the caked ink SS is discharged from the pressure chamber  67  and the nozzle  85 . 
     The purge action is not limited to the above but other method may be adopted. For example, the height difference h 1  is decreased by moving the pressure regulation tank  17 , and whereby the ink charged in the pressure chamber  67  is pressurized and then the ink and the meniscus M of the ink project to the outside of the nozzle  85 . 
     It should be noted that the maintenance method of the printing apparatus  10  is not limited to the above-described method.  FIG. 16  is a flow chart showing another example of the maintenance method of the printing apparatus  10 . As shown in the  FIG. 16 , the ink in the lower-stream side ink tank  31  is discharged by activating the lower-stream side suction pump  35  (ACT S 23 ), instead of moving the lower-stream side ink tank  31  by the tank moving mechanism  41 . By discharging ink, the gas-liquid boundary surface in the lower-stream side ink tank  31  moves downward. 
     By moving the gas-liquid boundary surface downward, the height difference h 2  becomes four times the height difference h 1  and the proper pressure Pn in the nozzle  85  increases to −4 (kPa). When the negative pressure is increased, the ink in the ink jet head  11  flows as shown by the arrow in the  FIG. 5 . The meniscus M of the ink in the pressure chamber  67  is pulled back because of the flow of ink, as shown in the  FIGS. 6 to 10 . The lower-stream side suction pump  35  is stopped when the height difference h 2  becomes four times the height difference h 1  (ACT S 25 ). After that, the lower-stream side suction pump  35  is activated to further move the gas-liquid boundary surface in the lower-stream side ink tank  31  downward (ACT S 28 ) if the gas-liquid boundary surface (ink surface) in the lower-stream side ink tank  31  does not rise at a prescribed height (NO in ACT S 7 ). 
     By the above-described another maintenance method also, the ink filled in the pressure chamber  67  of the ink jet head  11  is discharged to the lower-stream side ink tank  31 . In this another maintenance method, the lower-stream side ink tank  31  is an example of a discharge unit and a combination of the lower-stream side suction pump  35  and the lower-stream side ink tank  31  is an example of an element which functions as a second unit. However, the unit which conveys the ink in the pressure chamber  67  to the lower-stream side ink tank  31  is not limited to the combination of the lower-stream side suction pump  35  and the lower-stream side ink tank  31  but several other units may be adopted. 
     According to the printing apparatus  10  having the above construction, the control section  43  discharges the ink in the pressure chamber  67  to the lower-stream side ink tank  31  and charges the ink in the pressure regulation tank  17  to the pressure chamber  67 . Furthermore the control section  43  projects the meniscus M of the ink supplied to the pressure chamber  67  from the nozzle  85  to the outside of the ink jet head  11 . Therefore, the babble B and the caked ink SL, SS existing in the nozzle  85  are able to flow and thus they are removed from the nozzle  85 . As a result, the surface of the nozzle  85  is wetted with a fresh ink when print is performed and thus, for example, clogging of the nozzle  85  with ink whose viscosity is increased, failure of the ink-ejection and the ink-ejection in an undesired direction are restrained. That is, failure of printing with the caked ink can be restrained. 
     The control section  43  charges ink in the pressure regulation tank  17  to the pressure chamber  67  of the ink jet head  11  to perform the purge action after discharging ink in the pressure chamber  67  to the lower-stream side ink tank  31 . By discharging the ink to the lower-stream side ink tank  31  beforehand, it is restrained that the nozzle  85  is clogged with the caked ink SL. Furthermore, it is also restrained that bubbles are generated in the pressure chamber  67  and the nozzle  85  because the ink-discharging which causes the suction of air from the nozzle  85  is carried out beforehand. 
     Second Embodiment 
     A second embodiment will be described with reference to the  FIG. 17 . In the following embodiment, the same reference numerals in the Figures are applied to the similar component elements in the  FIG. 17  having the same functions as the printing apparatus  10  in the first embodiment, and therefore, the detailed descriptions in whole or in part thereof are not repeated. 
       FIG. 17  is a block diagram roughly illustrating the constructions of a printing apparatus  10 A according to the second embodiment. As shown in the  FIG. 17 , the printing apparatus  10 A further includes a circulation path  121 , a second filter  122  and a circulation pump  123 . 
     The circulation path  121  is connected between the pressure regulation tank  17  and the lower-stream side ink tank  31 . One of the ends of the circulation path  121  is slightly apart from the ink surface in the pressure regulation tank  17 . The other end of the circulation path  121  is inserted into the ink in the lower-stream side ink tank  31  such that it is slightly apart from the bottom surface of the lower-stream side ink tank  31 . 
     The second filter  122  is arranged at the circulation path  121 . The second filter  122  removes a foreign substance contained in the ink flowing from the lower-stream side ink tank  31  to the pressure regulation tank  17 . Therefore, ink from which a foreign substance is removed is supplied to the pressure regulation tank  17 . 
     The circulation pump  123  is also arranged at the circulation path  121 . When the circulation pump  123  is operated, the ink stored in the lower-stream side ink tank  31  is conveyed to the pressure regulation tank  17  through the circulation path  121 . 
     The control section  43  judges whether or not an amount of the ink stored in the lower-stream side ink tank  31  is increased from the prescribed amount based on the output of the second hydraulic head sensor  119 . If the amount of the ink in the lower-stream side ink tank  31  is increased from the prescribed amount, the control section  43  activates the circulation pump  123 . 
     Then, the ink discharged to the lower-stream side ink tank  31  is returned to the pressure regulation tank  17  by the operation of the circulation pump  123 . In other words, the ink is circulated through the pressure regulation tank  17 , the ink jet head  11  and the lower-stream side ink tank  31 . 
     According to the printing apparatus  10 A having the above construction, the ink stored in the lower-stream side ink tank  31  is fed to the pressure regulation tank  17  by the circulation pump  123 . Therefore, the consumption amount of the ink is decreased. 
     According to the printing apparatus of at least one embodiment described above, the ink in the pressure chamber is discharged to the second tank by the discharging device, the ink in the first tank is charged to the pressure chamber by the supplying device, and the liquid surface of the ink supplied to the pressure chamber is located at the outside of the inkjet head from the nozzle, whereby the printing failure with the caked ink can be restrained. 
     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.