Patent Publication Number: US-2007115311-A1

Title: Liquid ejection apparatus and liquid agitation method

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a liquid ejection apparatus and a liquid agitation method, and more particularly, to a liquid ejection apparatus which ejects liquid toward a prescribed medium and a liquid agitation method for agitating the liquid.  
      2. Description of the Related Art  
      There is a liquid ejection apparatus which ejects dispersion liquid in which dispersed micro-particles are suspended. Examples of material of the micro-particles include, for instance, pigment, high-polymer resin, metal, glass, or oxide or compound of these. Generally, the micro-particles tend to aggregate and settle with the passage of time. When the liquid in which the micro-particles have aggregated and settled is ejected, then there is deterioration of quality in the ejection results, namely, density non-uniformities or distortions, poor color reproduction, non-uniform density of the micro-particles, and the like. Therefore, technology for agitating the dispersion liquid has been proposed.  
      Japanese Patent Application Publication No. 6-87220 discloses that piezoelectric elements are used as energy converting elements for ejecting ink, and are also driven to agitate the ink under a condition where ejection of the ink does not occur, thereby re-dispersing solids in the ink that have aggregated and settled.  
      Japanese Patent Application Publication No. 2002-96484 discloses that the piezoelectric elements are driven in a state where the nozzle openings are sealed with a sealing cap, thereby agitating pigment-based ink inside pressure chambers and a reservoir. Japanese Patent Application Publication No. 2002-96484 also discloses changing the voltage value of the input signal of the piezoelectric element, increasing the drive time of the input signal of the piezoelectric element, and increasing the number of drive operations of the piezoelectric elements, in accordance with the idle period of the piezoelectric elements.  
      Japanese Patent Application Publication No. 2003-72104 discloses that a manifold for guiding ink to nozzles is provided with piezoelectric elements for agitating the ink inside the manifold, in such a manner that the ink inside the manifold is agitated at all times by means of the piezoelectric elements.  
      In particular, in a liquid ejection apparatus having a liquid ejection head in which the liquid ejection face is situated in a bottommost position, nozzle blockages are liable to occur due to sedimented micro-particles in the nozzles. In the case of a so-called shuttle head structure in which the liquid ejection head performs a reciprocal back and forth movement, the liquid inside the liquid ejection head is agitated by the reciprocal motion of the liquid ejection head, but in the case of a line head structure where the liquid ejection head does not perform reciprocal movement, the liquid is not agitated usually.  
      Methods for agitating the liquid have been proposed, but it is difficult to agitate the liquid with good efficiency.  
      Although Japanese Patent Application Publication No. 6-87220 discloses that the piezoelectric elements for ejecting liquid are driven under a condition where liquid ejection does not occur, it does not teach or suggest concretely how to appropriately set the frequency, waveform and voltage of the input signal applied to the piezoelectric elements, in order to effectively re-disperse the solids in the ink.  
      The voltage of the input signal applied to the piezoelectric elements must be set within a range that does not cause liquid ejection, and therefore, it cannot be set to a high voltage, and there are limits on the voltage.  
      Moreover, the state of aggregation and the state of sedimentation of the micro-particles vary in accordance with the conditions of liquid ejection (for example, liquid type, micro-particles type, the temperature, and so on), and therefore, in practice, it is difficult to set an appropriate input signal for the piezoelectric elements. The aforementioned Japanese Patent Application Publication Nos. 6-87220, 2002-96484 and 2003-72104 do not teach or suggest any solutions for these problems.  
      For example, Japanese Patent Application Publication No. 2002-96484 discloses that the application duration of the input signals to the piezoelectric elements is lengthened or the number of drive operations is increased, in accordance with the idle period of the piezoelectric elements; however, it is difficult to agitate the liquid in accordance with the state of aggregation or sedimentation of the micro-particles, which varies depending on liquid type, micro-particles type, or the temperature.  
     SUMMARY OF THE INVENTION  
      The present invention has been contrived in view of the foregoing circumstances, an object thereof being to provide a liquid ejection apparatus and a liquid agitation method whereby it is possible to efficiently agitate the dispersion liquid even in the case of various states of the micro-particles.  
      In order to attain the aforementioned object, the present invention is directed to a liquid ejection apparatus, comprising: a liquid ejection head which has an ejection port ejecting liquid, and an energy application element applying energy to the liquid to be ejected from the ejection port; and a driving device which applies a drive signal having a frequency changing with time to the energy application element so as to agitate the liquid in the liquid ejection head.  
      According to this aspect of the present invention, it is possible to efficiently agitate the liquid, even in the case of various states of the micro-particles (for example, cases where the liquid to be ejected differs between apparatuses, cases where a plurality of different types of liquids are to be ejected from one apparatus, cases where micro-particles of a plurality of different types are contained in the liquid, cases where liquid is ejected in an environment of changing temperature, and the like).  
      Preferably, the driving device causes the frequency of the drive signal to continuously change from a first frequency to a second frequency different from the first frequency.  
      According to this aspect of the present invention, even in the case of various states of the micro-particles, it is possible to use a single standard waveform for the drive signal used for liquid agitation, and hence the circuit composition is simplified and manufacturing costs are reduced.  
      Preferably, a coloring material is dispersed in the liquid; and the liquid ejected from the ejection port is deposited on a prescribed recording medium to form an image on the recording medium, whereby the liquid ejection apparatus serves as an image forming apparatus.  
      In order to attain the aforementioned object, the present invention is also directed to a liquid agitation method of agitating liquid in a liquid ejection head which has an ejection port ejecting the liquid, and an energy application element applying energy to the liquid to be ejected from the ejection port, the method comprising the step of: applying a drive signal having a frequency changing with time to the energy application element so as to agitate the liquid in the liquid ejection head.  
      According to this aspect of the present invention, it is possible to efficiently agitate the liquid even if the state of the micro-particles varies depending on the type of liquid, the type of micro-particles, the temperature, or the like, and hence deterioration of the liquid as a result of aggregation or sedimentation of the micro-particles in the liquid is prevented, and the liquid can be ejected stably. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein:  
       FIG. 1  is a plan view perspective diagram showing an approximate view of the general structure of a liquid ejection head according to an embodiment of the present invention;  
       FIG. 2  is a cross-sectional diagram along line  2 - 2  in  FIG. 1 ;  
       FIG. 3  is a diagram showing the general functional composition of an image forming apparatus according to an embodiment of the present invention;  
       FIG. 4  is a plan diagram showing the principal part of an image forming system of the image forming apparatus;  
       FIG. 5  is a schematic drawing showing the principal part of a liquid flow system of an image forming apparatus according to an embodiment of the present invention;  
       FIG. 6  is a schematic drawing showing the principal part of a liquid flow system of an image forming apparatus according to another embodiment of the present invention;  
       FIG. 7  is a plan diagram showing a liquid receptacle according to an embodiment of the present invention;  
       FIG. 8  is a cross-sectional diagram along line  8 - 8  in  FIG. 7 ;  
       FIG. 9  is a cross-sectional diagram showing a state where the belt has been rotated through a ¼ turn in the liquid receptacle shown in  FIG. 8 ;  
       FIG. 10  is a developed view of a belt according to an embodiment of the present invention;  
       FIG. 11  is a developed view of a belt according to another embodiment of the present invention;  
       FIG. 12  is a cross-sectional diagram along line  12 - 12  in  FIG. 11 ;  
       FIG. 13  is a schematic drawing showing a liquid pool;  
       FIG. 14  is a block diagram showing the general composition of the image forming apparatus;  
       FIGS. 15A  to  15 E are diagrams showing a maintenance process using a liquid receptacle;  
       FIG. 16  is an approximate flowchart showing an embodiment of a sequence of a liquid agitation process performed by withdrawing the free surface of the liquid;  
       FIGS. 17A  to  17 C are schematic drawings showing the free surface position;  
       FIG. 18  is a waveform diagram showing an embodiment of an actuator drive signal for liquid agitation;  
       FIG. 19  is an approximate flowchart showing an embodiment of a sequence of a liquid agitation process performed by forming a liquid pool;  
       FIG. 20  is a schematic diagram showing a state of liquid agitation using a liquid pool;  
       FIG. 21  is a flowchart showing an embodiment of a processing sequence when the power supply is turned off, in a liquid agitation process performed by returning all of the liquid inside the apparatus; and  
       FIG. 22  is a flowchart showing an embodiment of a processing sequence when the power supply is turned on, in a liquid agitation process performed by returning all of the liquid inside the apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Liquid Ejection Head  
       FIG. 1  is a plan diagram showing the general structure of a liquid ejection head in a liquid ejection device according to an embodiment of the present invention, giving a perspective view of the left-hand half in the diagram.  
      The liquid ejection head  50  shown in  FIG. 1  is a so-called full line head, having a structure in which a plurality of liquid ejection ports or nozzles  51 , which eject liquid toward an ejection receiving medium or a recording medium  116 , are arranged through a length corresponding to a width Wm of the recording medium  116  in a main scanning direction indicated by arrow M in  FIG. 1  perpendicular to a sub-scanning direction indicated by arrow S in  FIG. 1 , which is a conveyance direction of the recording medium  116 .  
      More specifically, the liquid ejection head  50  has a composition in which a plurality of pressure chamber units  54 , each having the nozzle  51 , a pressure chamber  52  connected to the nozzle  51 , and an opening section serving as a liquid supply port  53  to supply the liquid to the pressure chamber  52 , are arranged two-dimensionally along two directions, namely, the main scanning direction, and an oblique direction forming a prescribed acute angle θ (where 0°&lt;θ&lt;90°) with respect to the main scanning direction. In  FIG. 1 , in order to simplify the drawing, some of the pressure chamber units  54  are omitted from the drawing.  
      More specifically, by arranging the nozzles  51  at a uniform pitch of d in the direction forming the acute angle of θ with respect to the main scanning direction, it is possible to treat the nozzles  51  as being equivalent to an arrangement of nozzles at a prescribed pitch (d×cos θ) in a straight line in the main scanning direction. According to this nozzle arrangement, for example, it is possible to achieve a composition substantially equivalent to a high-density nozzle arrangement reaching 4800 nozzles per inch in the main scanning direction. In other words, the effective nozzle pitch (projected nozzle pitch) obtained by projecting the nozzles to a straight line aligned with the lengthwise direction of the liquid ejection head  50  (the main scanning direction) can be reduced, and high image resolution can be achieved.  
      A common liquid chamber  55  supplying the liquid or ink to the pressure chambers  52  is formed in a common liquid chamber forming plate  506  as a flow channel that occupies a single space covering all of the pressure chambers  52 . An opening formed at an end of the common liquid chamber  55  serves as a liquid inlet port  553 , through which the ink is introduced into the common liquid chamber  55  from the outside of the liquid ejection head  50  (more specifically, from a sub-tank  61  described later with reference to  FIGS. 5 and 6 ).  
      In the present embodiment, the common liquid chamber  55  is formed by etching a metal plate (more specifically, the common liquid chamber forming plate  506 ), and the rigidity of the common liquid chamber  55  is ensured.  
       FIG. 2  shows a cross-sectional view along line  2 - 2  in  FIG. 1 . As shown in  FIG. 2 , the liquid ejection head  50  has a laminated structure of a plurality of plates including a nozzle forming plate  501 , a pressure chamber forming plate  502 , a diaphragm  503 , actuator protection plates  504  and  505 , the common liquid chamber forming plate  506 , and a sealing plate  507 .  
      The nozzles  51  ejecting the liquid are formed in a two-dimensional matrix fashion in the nozzle forming plate  501 .  
      The pressure chambers  52  connected to the nozzles  51  are formed in the pressure chamber forming plate  502  bonded on the nozzle forming plate  501 .  
      The diaphragm  503 , on which actuators  58  are arranged, is bonded on the pressure chamber forming plate  502 , and constitutes one face (a vibrating face) of each pressure chamber  52 .  
      Each actuator  58  has a laminated structure of the diaphragm  503 , a piezoelectric body  580  for generating pressure, and an individual electrode  57 , such that the piezoelectric body  580  is arranged between the diaphragm  503  and the individual electrode  57 . The piezoelectric body  580  is made of piezoelectric material such as PZT (lead zirconate titanate), and the diaphragm  503  and the individual electrode  57  are made of conductive material.  
      The actuators  58  are arranged on the diaphragm  503  at positions corresponding to the pressure chambers  52 , and each actuator  58  functions as a pressure generating device causing the pressure inside the pressure chamber  52  to change by changing the volume of the pressure chamber  52 .  
      The diaphragm  503  is grounded, and constitutes a common electrode for the actuators  58 . The other electrodes for the actuators  58  are the individual electrodes  57 , from which electrical wires (drive wires)  59  for driving the actuators  58  extend.  
      The liquid supply ports  53  shown in  FIG. 1  are formed in the diaphragm  503 .  
      The actuator protection plates  504  and  505  are bonded on the diaphragm  503 , and protect the whole actuators  58  while preventing any obstruction of the operation of the actuators  58  by forming spaces  581  around the actuators  58 .  
      The common liquid chamber forming plate  506  is bonded on the actuator protection plate  505  on the side reverse to the side where the actuator protection plate  504 , the diaphragm  503 , and the pressure chamber forming plate  502  are arranged. The common liquid chamber  55  supplying the liquid to the pressure chambers  52  is formed in the common liquid chamber forming plate  506 .  
      The sealing plate  507  constituting a ceiling of the common liquid chamber  55  is arranged on the common liquid chamber forming plate  506 . The space between the actuator protection plate  505  and the sealing plate  507  constitutes the common liquid chamber  55 , in which the liquid or ink is filled.  
      When viewed with the nozzles  51  positioned below the pressure chambers  52 , the common liquid chamber  55  is arranged over the pressure chambers  52  and is connected to the pressure chambers  52  through liquid supply flow channels  531  extending from connecting ports  530 , which are opening sections formed in the base of the common liquid chamber  55 , passing through the actuator protection plates  504  and  505 , to the liquid supply ports  53  formed in the diaphragm  503 . In other words, the ink inside the common liquid chamber  55  flows directly to the pressure chambers  52  situated under the common liquid chamber  55  through the liquid supply flow channels  531 , and good refilling characteristics are hence achieved in the supply of ink to the pressure chambers  52 .  
      The drive wires  59  for the actuators  58  are arranged on the actuator protection plates  504  and  505  in the horizontal direction parallel to the plane on which the actuators  58  are arranged.  
      There are no particular restrictions on arrangement of the drive wires  59  for the actuators  58 . For example, it is possible to arrange the drive wires  59  to pass through the common liquid chamber forming plate  506  in the vertical direction inside partitions defining the liquid chamber  55 .  
      When a drive signal is applied to the individual electrode  57  of the actuator  58  through the drive wire, the piezoelectric body  580  of the actuator  58  is displaced, and the volume of the pressure chamber  52  is changed through the diaphragm  503 . Accordingly, the liquid in the pressure chamber  52  is ejected from the nozzle  51  connected to the pressure chamber  52 .  
      The actuator protection plates  504  and  505  are formed with a recess section  545  (recess section for heat transmission), from the common liquid chamber  55  side, passing through the actuator protection plates  505  and  504  in the thickness direction thereof, to the diaphragm  503 , in such a manner that the liquid in the common liquid chamber  55  makes direct contact with the diaphragm  503 . By adopting the structure in which the recess section  545  is provided in this way, the heat generated by the actuators  58  is transmitted through the diaphragm  503  to the liquid inside the common liquid chamber  55 , at the position of the recess section  545 . Thus, a temperature differential is generated in the liquid inside the common liquid chamber  55 , and the liquid inside the common liquid chamber  55  is thereby made to circulate in the common liquid chamber  55 . In other words, the liquid inside the common liquid chamber  55  is agitated by the thermal energy produced by the driving of the actuators  58 , and no heat-generating element is necessary other than the actuators  58  in the common liquid chamber  55 .  
      Furthermore, since the common liquid chamber  55  is disposed above the diaphragm  503 , then the length of nozzle flow channels  511  from the pressure chambers  52  to the nozzles  51  is short, and it becomes possible to eject ink of high viscosity (for example, approximately 10 cP to 50 cP).  
      In the present embodiment, the common liquid chamber  55  is formed in the common liquid chamber forming plate  506  as the flow channel that occupies the single space covering all of the pressure chambers  52 . It is thereby possible to increase the size of the common liquid chamber  55  and to reduce the flow channel resistance inside the common liquid chamber  55 , and hence the present embodiment is suitable for the ejection of high-viscosity liquid. In implementing the present invention, the structure of the common liquid chamber  55  is not limited in particular to the above-described embodiment. For example, it is also possible to form the common liquid chamber  55  in the common liquid chamber forming plate  506  to include a main channel and distributary channels branching from the main channel.  
      In implementing the present invention, the arrangement structure of the nozzles  51 , and the like, is not limited in particular to the embodiment shown in  FIGS. 1 and 2 . For example, it is also possible to compose a full line liquid ejection head by adopting a staggered arrangement of a plurality of short liquid ejection head blocks each comprising a plurality of nozzles  51  arranged two-dimensionally, thus achieving a long head by joining these liquid ejection head blocks together.  
      General Composition of Image Forming Apparatus  
       FIG. 3  is a schematic drawing showing a general view of an image forming apparatus  110  according to an embodiment of the present invention. The image forming apparatus  110  comprises a plurality of the liquid ejection heads  50  shown in  FIGS. 1 and 2 , and these heads are denoted in  FIG. 3  with reference numerals “ 112 ” appended with letters indicating the colors of ink ejected (K: black, C: cyan, M: magenta, and Y: yellow).  
      More specifically, the image forming apparatus  110  comprises: a liquid ejection unit  112  having the liquid ejection heads  112 K,  112 C,  112 M and  112 Y for respective ink colors; an ink storing and loading unit  114 , which stores the inks to be supplied to the liquid ejection heads  112 K,  112 C,  112 M and  112 Y; a paper supply unit  118 , which supplies a recording medium  116 , such as paper; a decurling unit  120 , which removes curl in the recording medium  116 ; a belt conveyance unit  122 , which is disposed facing the nozzle face of the liquid ejection unit  112  and conveys the recording medium  116  while keeping the recording medium  116  flat; a print determination unit  124 , which reads the ejection result (liquid droplet deposition state) produced by the liquid ejection unit  112 ; and a paper output unit  126 , which outputs printed recording medium to the exterior.  
      By depositing liquids (inks) containing coloring agents (also referred to as coloring material) on the recording medium  116  from the liquid ejection heads  112 K,  112 C,  112 M and  112 Y, an image is formed on the recording medium  116 .  
      The ink contains an insoluble or slightly water-soluble coloring material dispersed in water, and examples of the coloring material include, for instance, a dispersive dye, a metal complex dye, a pigment, or the like. Examples of dispersing agents for the coloring material in the ink dispersion, it is possible to use a so-called dispersant, surfactant, a resin, or the like. Examples of the dispersant or surfactant include anionic or nonionic materials, and examples of the resin dispersant include styrene or derivatives, vinylnaphthalene or derivatives, acrylic acid or derivatives, and the like. Desirably, the resin dispersant is alkali-soluble resin, which can be dissolved in an aqueous solution containing a basic material. The pigment may be an organic pigment or an inorganic pigment, but it is not limited to these. Pigment-based inks have excellent resistance to light and water; however, they tend to sediment more readily than dye-based inks.  
      In  FIG. 3 , a supply of rolled paper (continuous paper) is displayed as one embodiment of the paper supply unit  118 , but it is also possible to use a supply unit which supplies cut paper that has been cut previously into sheets. In a case where rolled paper is used, a cutter  128  is provided. The recording medium  116  delivered from the paper supply unit  118  generally retains curl. In order to remove this curl, heat is applied to the recording medium  116  in the decurling unit  120  by a heating drum  130  in the direction opposite to the direction of the curl. After decurling in the decurling unit  24 , the cut recording medium  116  is delivered to the belt conveyance unit  122 .  
      The belt conveyance unit  122  has a configuration in which an endless belt  133  is set around rollers  131  and  132  so that the portion of the endless belt  33  facing at least the nozzle face of the liquid ejection unit  112  and the sensor face of the ejection determination unit  124  forms a horizontal plane. The belt  133  has a width that is greater than the width of the recording medium  116 , and a plurality of suction apertures are formed on the belt surface. A suction chamber  134  is disposed in a position facing the sensor surface of the ejection determination unit  124  and the nozzle surface of the liquid ejection unit  112  on the interior side of the belt  133 , which is set around the rollers  131  and  132 , as shown in  FIG. 3 ; and this suction chamber  134  provides suction with a fan  135  to generate a negative pressure, thereby holding the recording medium  116  onto the belt  133  by suction. The belt  133  is driven in the clockwise direction in  FIG. 3  by the motive force of a motor (not shown) being transmitted to at least one of the rollers  131  and  132 , which the belt  133  is set around, and the recording medium  116  held on the belt  133  is conveyed from left to right in  FIG. 3 . Since ink adheres to the belt  133  when a marginless print or the like is formed, a belt cleaning unit  136  is disposed in a predetermined position on the exterior side of the belt  133 . A heating fan  140  is provided on the upstream side of the liquid ejection unit  112  in the paper conveyance path formed by the belt conveyance unit  122 . This heating fan  140  blows heated air onto the recording medium  116  before printing, and thereby heats up the recording medium  116 . Heating the recording medium  116  immediately before printing has the effect of making the ink dry more readily after landing on the paper.  
       FIG. 4  is a principal plan diagram showing the liquid ejection unit  112  of the image forming apparatus  110 , and the peripheral region of the liquid ejection unit  112 .  
      In  FIG. 4 , the liquid ejection heads  112 K,  112 C,  112 M and  112 Y constituting the liquid ejection unit  112  are arranged following a direction perpendicular to the medium conveyance direction (sub-scanning direction) (in other words, they are arranged in the main scanning direction), and they are full line heads having the nozzles (ejection ports) arranged through a length exceeding at least one edge of the maximum-size recording medium  116  that can be used in the image forming apparatus  110 .  
      The liquid ejection heads  112 K,  112 C,  112 M and  112 Y corresponding to the respective ink colors are disposed in the order, black (K), cyan (C), magenta (M) and yellow (Y), from the upstream side (left-hand side in  FIG. 4 ), following the direction of conveyance of the recording medium  116  (the sub-scanning direction). A color image can be formed on the recording medium  116  by ejecting the inks including coloring material from the print heads  112 K,  112 C,  112 M and  112 Y, respectively, toward the recording medium  116  while conveying the recording medium  116 .  
      The liquid ejection unit  112 , in which the full-line heads are thus provided for the respective ink colors, can record an image over the entire surface of the recording medium  116  by moving the recording medium  116  and the liquid ejection unit  112  relatively to each other in the medium conveyance direction (sub-scanning direction) just once (in other words, by means of a single sub-scanning action). Higher-speed printing is thereby made possible and productivity can be improved in comparison with a shuttle type head which moves reciprocally back and forth in the main scanning direction.  
      The terms “main scanning direction” and “sub-scanning direction” are used in the following senses. In a full-line head comprising rows of nozzles that have a length corresponding to the entire width of the recording medium, “main scanning” is defined as printing one line (a line formed of a row of dots, or a line formed of a plurality of rows of dots) in the breadthways direction of the recording medium (the direction perpendicular to the conveyance direction of the recording medium) by driving the nozzles in one of the following ways: (1) simultaneously driving all the nozzles; (2) sequentially driving the nozzles from one side toward the other; and (3) dividing the nozzles into blocks and sequentially driving the nozzle from one side toward the other in each of the blocks. The direction indicated by one line recorded by a main scanning action (the lengthwise direction of the band-shaped region thus recorded) is called the “main scanning direction”.  
      On the other hand, sub-scanning is defined as printing the line (a line constituted by a single dot array or a line constituted by a plurality of dot arrays) formed by the main scanning described above repeatedly by moving the full line head and recording medium relative to each other as described above. The direction in which this sub-scanning is performed is known as the sub-scanning direction. Consequently, the recording medium conveyance direction is the sub-scanning direction, and the direction perpendicular to the sub-scanning direction is the main scanning direction.  
      Although a configuration with the four standard colors, K, C, M and Y, is described in the present embodiment, the combinations of the ink colors and the number of colors are not limited to those of the present embodiment, and light and/or dark inks can be added as required. For example, a configuration is possible in which liquid ejection heads for ejecting light-colored inks such as light cyan and light magenta are added.  
      As shown in  FIG. 3 , the ink storing and loading unit  114  has ink tanks for storing the inks of the colors corresponding to the liquid ejection heads  112 K,  112 C,  112 M and  112 Y, and the ink tanks are connected to the liquid ejection heads  112 K,  112 C,  112 M and  112 Y through channels (not shown).  
      The ejection determination unit  124  has an image sensor (line sensor, or the like) for capturing an image of the ejection result of the liquid ejection unit  112 , and functions as a device to check for ejection defects such as blockages of the nozzles in the liquid ejection unit  12  on the basis of the image read in by the image sensor.  
      A post-drying unit  142  is provided at a downstream stage from the ejection determination unit  124 . The post-drying unit  142  is a device for drying the printed image surface, and it may comprise a heating fan, for example. A heating and pressurizing unit  144  is provided at a stage following the post-drying unit  142 . The heating and pressurizing unit  144  is a device which serves to control the luster of the image surface, and it applies pressure and heat to the image surface by means of pressure rollers  145  having prescribed surface undulations. Accordingly, an undulating form is transferred to the image surface.  
      The printed object generated in this manner is output via the paper output unit  126 . In the image forming apparatus  110 , a sorting device (not shown) is provided for switching the outputting pathway in order to sort the printed matter with the target print and the printed matter with the test print, and to send them to output units  126 A and  126 B, respectively. If the main image and the test print are formed simultaneously in a parallel fashion, on a large piece of printing paper, then the portion corresponding to the test print is cut off by means of the cutter (second cutter)  140 . The cutter  140  is disposed immediately in front of the paper output section  126 , and serves to cut and separate the main image from the test print portion, in cases where a test image is printed onto the white margin of the image. Moreover, although omitted from the drawing, a sorter for collating and stacking the images according to job orders is provided in the paper output section  126 A corresponding to the main images.  
      Liquid Flow System  
       FIG. 5  is a schematic drawing showing a liquid flow system in the image forming apparatus  110  according to an embodiment of the present invention. In  FIG. 5 , the liquid ejection head is denoted with the reference numeral  50 .  
      In  FIG. 5 , the main tank  60  stores liquid to be supplied to the liquid ejection head  50 , and it corresponds to the ink storing and loading unit  114  in  FIG. 3 .  
      A stirrer  32 , in which a metal or magnet member is embedded, is arranged in the main tank  60 . A stirrer drive unit  224  having a magnetic member  34  made of magnet or metal (according to the embedded member of the stirrer  34 ) is arranged on the outer side of the main tank  60 , and the stirrer drive unit  224  rotates the stirrer  32  in the main tank  60  by means of a magnetic force without making contact with the stirrer  32 , thus causing the stirrer  32  to agitate the liquid inside the main tank  60 .  
      The stirrer  32  in the present embodiment is disposed on the bottom of the main tank  60 , and the stirrer  32  agitates the liquid inside the main tank  60  by performing a rotational movement in a plane parallel to the bottom face of the main tank  60  (in other words, a plane parallel to the free surface of the liquid in the main tank  60 ), taking as the center of rotation an axis following a substantially perpendicular direction with respect to the bottom face of the main tank  60 .  
      It is also possible that the stirrer  32  is arranged at a position other than the bottom of the main tank  60 , for example, on the side wall of the main tank  60 , and it is further possible that the stirrer  32  performs a rotational movement within a plane other than a horizontal plane, for example, a vertical plane (in other words, a plane perpendicular to the free surface of the liquid in the main tank  60 ).  
      When the image forming apparatus  110  is in a power-on state, in other words, when there is a supply of power from a main power source  240  to the respective units of the image forming apparatus  110 , the stirrer drive unit  24  rotates the stirrer  32  at prescribed time intervals through an electrical power supplied from the main power source  240 , according to the present embodiment. On the other hand, when the image forming apparatus  110  is in a power-off state, the stirrer drive unit  24  rotates the stirrer  32  at prescribed time intervals through an electrical power supplied from a standby power source  242 .  
      The standby power source  242  is constituted by a rechargeable battery or a non-rechargeable battery, or the like.  
      In the composition in which the stirrer  32  can be driven by means of the standby power source  242  in this way, even if the power to the image forming apparatus  110  is switched off and the image forming apparatus  110  remains in an idle state for a long period of time, the liquid inside the main tank  60  is still agitated during the idle period, and the micro-particles in the liquid are thereby prevented from aggregating and sedimenting.  
      Although the compositional embodiment is described above in which the stirrer drive unit  224  is not directly coupled to the stirrer  32 , it is also possible to adopt a composition in which the stirrer drive unit  224  is directly coupled to the stirrer (for example, a rotating blade).  
      A sub-tank  61  is provided between the main tank  60  and the liquid ejection head  50 , and the liquid supplied from the main tank  60  is stored temporarily in the sub-tank  61  before being supplied to the liquid ejection head  50 .  
      A pump  62  (referred to as a “liquid supply pump”) that drives the liquid from the main tank  60  to the sub-tank  61  is provided in a flow channel  600  (referred to as a “first liquid supply flow channel”) connecting the main tank  60  and the sub-tank  61 . A flow channel  630  (referred to as a “second liquid supply flow channel”) connects the sub-tank  61  and the liquid ejection head  50 .  
      An opening section  619  (referred to as an “atmosphere connection port”) connecting to the atmosphere is formed in the ceiling of the sub-tank  61 . By allowing the air to move into and out of the sub-tank  61  through the atmosphere connection port  619 , the pressure inside the sub-tank  61  is kept to the atmospheric pressure.  
      The internal pressure of the liquid ejection head  50  is adjusted to a prescribed negative pressure by means of the height differential (head differential) between the free surface of the liquid in the sub-tank  61 , to which the liquid is supplied with the liquid supply pump  62 , and the nozzle face  510  of the liquid ejection head  50 . Here, the prescribed negative pressure is a pressure below the atmospheric pressure, and is the pressure that causes the free surface of the liquid (the liquid-atmosphere interface, which is also commonly called “meniscus”) in the nozzles  51  to be in the vicinity of the nozzle face  510 , in preparation for ejection of the liquid.  
      A first electromagnetic valve  41  is provided in a flow channel  610  (referred to as a “first liquid returning flow channel”) connecting the main tank  60  with an opening section  611  formed in the bottom face of the sub-tank  61 , and the first electromagnetic valve  41  opens and closes the flow channel  610 . A second electromagnetic valve  42  is provided in a flow channel  620  (referred to as a “second liquid returning flow channel”) connecting the main tank  60  with an opening section  612  formed in a side wall of the sub-tank  61 , and the second electromagnetic valve  42  opens and closes the flow channel  620 .  
      When forming an image, in a state where the first electromagnetic valve  41  is closed and the second electromagnetic valve  42  is opened, the liquid is supplied to the sub-tank  61  from the main tank  60  by driving the liquid supply pump  62  forwards, from a prescribed time period before the start of ejection from the liquid ejection head  50 . The liquid is thereby supplied from the sub-tank  61  to the liquid ejection head  50  through the second liquid supply flow channel  630 , and the surplus liquid in the sub-tank  61  is returned to the main tank  60  through the opening section  612  in the side wall of the sub-tank  61  and the second liquid returning flow channel  620 . Hence, the supply of liquid from the main tank  60  to the sub-tank  61  is stabilized, and the free surface of the liquid in the sub-tank  61  is kept at a uniform height. The pressure inside the liquid ejection head  50  is maintained to the prescribed negative pressure and the free surface of the liquid in the nozzles  51  is positioned, by means of the height differential between the nozzle face  510  of the liquid ejection head  50  and the free surface of the liquid in the sub-tank  61 , which free surface is maintained as described above. When a prescribed time period has elapsed after the end of the ejection operation, the driving of the liquid supply pump  62  is halted.  
      On the other hand, when the first electromagnetic valve  41  and the second electromagnetic valve  42  are both opened, the liquid inside the liquid ejection head  50 , the liquid inside the second liquid supply flow channel  630  connecting the sub-tank  61  with the liquid ejection head  50 , the liquid inside the sub-tank  61 , and the liquid inside the first liquid returning flow channel  610  and the second liquid returning flow channel  620  connecting the sub-tank  61  with the main tank  60 , is all returned into the main tank  60 . Moreover, by driving the liquid supply pump  62  in reverse, the liquid inside the first liquid supply flow channel  600  connecting the main tank  60  with the sub-tank  61  is also returned into the main tank  60 .  
      Further, in the present embodiment, the position of the free surface of the liquid in the nozzles  51  of the liquid ejection head  50  can be withdrawn into the pressure chambers  52  from the vicinity of the nozzle face  510 . More specifically, the first electromagnetic valve  41 , which serves as a liquid surface movement device, is set to an open state for a prescribed period of time so that the liquid of a prescribed volume corresponding to the displacement of the free surface of the liquid in the nozzles  51  is returned from the liquid ejection head  50  to the sub-tank  61  through the second liquid supply flow channel  630 , and the position of the free surface is thereby withdrawn.  
      A liquid receptacle  70  having a recessed shape receives the liquid ejected from the nozzles  51  of the liquid ejection head  50 , in a state where the liquid receptacle  70  is opposite to the nozzle face  510  of the liquid ejection head  50 . Moreover, a pump  67  (referred to as a “suction pump”) is provided in a flow channel  670  (referred to as an “expulsion flow channel”) connecting a waste liquid tank  68  with an opening section  76  (referred to as a “suction port”) formed in the bottom face of the liquid receptacle  70 . The liquid received in the liquid receptacle  70  from the nozzles  51  of the liquid ejection head  50  is expelled to the waste liquid tank  68  through the expulsion flow channel  670 .  
      A liquid receptacle movement unit  226  is capable of moving the liquid receptacle  70  in a horizontal direction in parallel with the nozzle face  510  of the liquid ejection head  50 , and also in a perpendicular direction with respect to the nozzle face  510 . The liquid receptacle movement unit  226  includes a commonly known mechanism and a motor.  
      The liquid receptacle  70  is used for various types of maintenance processes for maintaining the state of the liquid inside the liquid ejection head  50 , at the position opposite to the nozzle face  510  of the liquid ejection head  50 . Typical examples of maintenance processes using the liquid receptacle  70  are described later in detail.  
       FIG. 6  is a schematic drawing showing a liquid flow system in the image forming apparatus  110  according to another embodiment of the present invention. In  FIG. 6 , the constituent elements that are the same as the constituent elements of the liquid flow system shown in  FIG. 5  are denoted with the same reference numerals, and contents described above are omitted from the following description.  
      In the present embodiment, a third electromagnetic valve  43 , which opens and closes the second liquid supply flow channel  630 , is provided in the second liquid supply flow channel  630  supplying the liquid from the sub-tank  61  to the liquid ejection head  50 , in other words, on the upstream side of the liquid ejection head  50 . Moreover, a flow channel  640  (referred to as a “circulation flow channel”) for circulating the liquid from the liquid ejection head  50  to the sub-tank  61  is provided. Further, a fourth electromagnetic valve  44  for opening and closing the circulation flow channel  640 , and a pump  64  (referred to as a “liquid circulation pump”) for circulating the liquid from the liquid ejection head  50  to the sub-tank  61 , are provided in the circulation flow channel  640 , in other words, on the downstream side of the liquid ejection head  50 .  
      In the liquid flow system shown in  FIG. 6 , the liquid supplied from the sub-tank  61  to the liquid ejection head  50  is circulated from the liquid ejection head  50  to the sub-tank  61  by driving of the liquid circulation pump  64 , and the liquid inside the common liquid chamber  55  is thereby agitated.  
      In the present embodiment, the position of the free surface of the liquid in the nozzles  51  of the liquid ejection head  50  can be withdrawn into the pressure chambers  52  from the vicinity of the nozzle face  510 . More specifically, in a state where the third electromagnetic valve  43  is closed, the fourth electromagnetic valve  44  is set to an open state and the liquid circulation pump  64 , which serves as the liquid surface movement device, is driven for a prescribed period of time so that the liquid of a prescribed volume corresponding to the displacement of the free surface of the liquid in the nozzles  51  is returned from the liquid ejection head  50  to the sub-tank  61  through the circulation flow channel  640 , and the position of the free surface is thereby withdrawn.  
      Furthermore, by driving the liquid circulation pump  64  for a prescribed period of time, for instance, when the image forming apparatus  110  is started up (when the power is turned on), it is possible to circulate the liquid through the circulation flow channel  640 .  
      Next, the liquid receptacle  70  is described in detail.  
       FIG. 7  is a plan diagram showing one embodiment of the liquid receptacle  70 , which can be positioned oppositely to the liquid ejection head  50 , as viewed from the side of the nozzle face  510  of the liquid ejection head  50 .  FIG. 8  is a cross-sectional diagram along line  8 - 8  in  FIG. 7 .  
      The liquid receptacle  70  has a recess part  71 , and the suction port  76  is formed in the bottom face of the recess part  71  and is connected to the waste liquid tank  68  through the expulsion flow channel  670 . The liquid inside the recess part  71  flows to the waste liquid tank  68  by the gravity force or a suction force applied by the suction pump  67  in the expulsion flow channel  670 .  
      An endless belt  80  is provided in the recess part  71  of the liquid receptacle  70 . The belt  80  is suspended about four rollers  73  ( 73 - 1 ,  73 - 2 ,  73 - 3 ,  73 - 4 ), which are disposed along the lengthwise direction of the liquid ejection head  50  (i.e., the main scanning direction), and it is supported rotatably by these rollers  73 .  
      The four rollers  73  are disposed in the recess part  71  of the liquid receptacle  70  in such a manner that they form a quadrilateral shape corresponding to the shape of the recess part  71 , in a cross-section perpendicular to the nozzle face  510  of the liquid ejection head  50 . The clearance Ca between the first roller  73 - 1  and the second roller  73 - 2 , which are aligned on the side opposite to the nozzle face  510  (in other words, on the upper side), is greater than the width in the sub-scanning direction of the nozzle arrangement constituted by the two-dimensional arrangement of the plurality of nozzles  51  on the nozzle face  510 .  
      When the four rollers  73  are rotated by means of a motor  228  (belt drive unit) shown in  FIG. 7 , the belt  80  suspended about the rollers  73  moves in a plane perpendicular to the nozzle face  510  of the liquid ejection head  50 , in conjunction with the movement of the four rollers  73 .  
       FIG. 9  is a diagram showing a state where the belt  80  has been rotated from a state shown in  FIG. 8  through approximately ¼ of a turn in the direction denoted by an arrow N, in other words, the state where the belt  80  has been rotated forwards (clockwise in  FIGS. 8 and 9 ) through approximately ¼ of a turn from the state shown in  FIG. 8 . If the belt  80  is rotated through approximately ¼ of a turn in the direction denoted by an arrow R from the state shown in  FIG. 9 , in other words, if the belt  80  is rotated in reverse (counter-clockwise in  FIGS. 8 and 9 ) through approximately ¼ of a turn from the state shown in  FIG. 9 , then the state shown in  FIG. 8  is achieved. In this way, the belt  80  can be rotated freely in forward or reverse directions by means of the motor  228  through the rollers  73 .  
       FIG. 10  is a developed view of the belt  80  shown in FIGS.  7  to  9 , and shows the outer circumferential surface of the belt  80 .  
      As shown in  FIG. 10 , the belt  80  has two opening parts  81  and  82 . Moreover, a liquid-philic part  83  having liquid-philic properties with respect to the liquid ejected from the nozzles  51  is formed on the external circumferential surface of the belt  80 , and a liquid-phobic part  84  having liquid-phobic properties with respect to the liquid ejected from the nozzles  51  is formed so as to surround the liquid-philic part  83 . The contact angle of the liquid on the liquid-philic part  83  is smaller than on the liquid-phobic part  84 . Furthermore, the contact angle of the liquid on the liquid-philic part  83  is smaller than on the nozzle face  510  having liquid-philic properties.  
      In the present specification, the term “liquid-philic” means “having a strong affinity for the liquid (e.g., ink)”. For example, in the case where the liquid or the ink is an aqueous solution or water-based, the terms “liquid-philic”, “liquid-philicity”, “liquid-philize” and “liquid-philization” correspond to “hydrophilic”, “hydrophilicity”, “hydrophilize” and “hydrophilization”, respectively; and the antonymous term “liquid-phobic” and its derivatives correspond to “hydrophobic” and its derivatives. On the other hand, in the case where the liquid or the ink is an oleaginous solution or oil-based, the term “liquid-philic” and its derivatives correspond to “oleophilic” and its derivatives; and the term “liquid-phobic” and its derivatives correspond to “oleophobic” and its derivatives.  
      When the liquid is ejected from the nozzles  51  of the liquid ejection head  50  in the state where the liquid-philic part  83  is opposite to the nozzle face  510  of the liquid ejection head  50 , then as shown in  FIG. 13 , it is possible to form a liquid pool  351  between the liquid-philic part  83  of the liquid receptacle  70  and the nozzle face  510  of the liquid ejection head  50 . The liquid-philic part  83  is formed to be wider than the full range NA (nozzle range), in which the nozzles  51  are formed on the nozzle face  510 , and it is possible to form a layer-shaped liquid pool  351  over the whole of the nozzle range NA, in other words, so as to cover all of the nozzles  51 .  
      The nozzle face  510  of the liquid ejection head  50  generally has a liquid-phobic surface having liquid-phobic properties. If the liquid-phobic part  84  is positioned oppositely to the liquid-phobic nozzle face  510  when forming the liquid pool  351 , then since both of the opposite surfaces have liquid-phobic properties, the liquid pool is destabilized, and hence the liquid moves between the nozzle face  510  and the belt  80  and is liable to spill out. In order to eliminate this problem, even in a case in which the nozzle face  510  has liquid-phobic properties, it is possible to stably make the liquid pool  351  of a small quantity of liquid by positioning the liquid-philic part  83  oppositely to the nozzle face  510  when forming the liquid pool  351 , and hence the liquid is not liable to spill out from the space between the nozzle face  510  and the belt  80 .  
      The cross-sectional area of each of the opening parts  81  and  82  in the belt  80  is greater than the full range NA (nozzle range) in which the nozzles  51  are formed on the nozzle face  510  of the liquid ejection head  50 , and even in cases where the liquid is ejected from all of the nozzles  51 , all of the ejected liquid is able to pass through the opening parts  81  and  82 .  
      The belt  80  used is manufactured by impregnating a base material made of weaved fibers with a rubber material, such as silicone. In this case, preferably, a liquid-philic rubber material and a liquid-phobic rubber material are used selectively to form the liquid-philic part  83  and the liquid-phobic part  84  on the external circumferential surface of the belt  80 .  
      It is also possible to manufacture the belt  80  by carrying out a surface treatment to form the liquid-philic part  83  and the liquid-phobic part  84  on a base material made of metal. This method is preferable in that the liquid-philic part  83  and the liquid-phobic part  84  can be formed readily and there is little stretching of the belt  80 .  
      The liquid-philization treatment is a treatment carried out on the belt  80  so that the contact angle of the liquid on the treated surface is smaller than the prescribed angle (for example, 45 degrees or lower). In a case where the contact angle of the liquid on the external circumferential surface of the belt  80  is originally a prescribed angle which shows liquid-philic properties, it is not necessary to carry out liquid-philization treatment.  
      The liquid-phobization treatment is a treatment carried out on the belt  80  so that the contact angle of the liquid on the treated surface is greater than the prescribed angle (for example, 50 degrees or greater). In a case where the contact angle of the liquid on the external circumferential surface of the belt  80  is originally a prescribed angle which shows liquid-phobic properties, it is not necessary to carry out liquid-phobization treatment.  
      Furthermore, if the stretching of the belt  80  exceeds a tolerable level, then it is preferable to adopt a composition including a belt tension mechanism in the liquid receptacle  70 .  
      As shown in  FIG. 7 , the liquid receptacle  70  has sealing members  74  formed in a ring shape on a rim part  72 , which surrounds the recess part  71 . The sealing members  74  are made of an elastic member, and the cross-sectional shape thereof perpendicular to the nozzle face  510  is a projecting shape as shown in  FIG. 8 . In a state where the liquid receptacle  70  is pressed against the nozzle face  510  of the liquid ejection head  50 , the sealing members  74  are in contact with the nozzle face  510  tightly due to their elastic force, in other words, the sealing members  74  hermetically seal the nozzle face  510  of the liquid ejection head  50  and shut off all of the nozzles  51  from the atmosphere. Consequently, evaporation of the liquid from the liquid ejection head  50  is prevented.  
      In a case where the distance between the belt  80  and the nozzle face  510  is set to approximately 1 mm, the sealing members  74  have, for example, a height of approximately 2 mm in a free length, and when the sealing members  74  are compressed and bended when pressed, a distance of approximately 1 mm is kept between the belt  80  and the nozzle face  510 .  
      As shown in  FIG. 7 , wipers  75  are formed on the rim part  72  of the liquid receptacle  70  along the main scanning direction M, in other words, following a direction substantially perpendicular to the sub-scanning direction S (medium conveyance direction), in which the liquid receptacle  70  is moved horizontally with respect to the liquid ejection head  50 . The wipers  75  are made of an elastic member, and the cross-sectional shape thereof perpendicular to the nozzle face  510  is a projecting shape as shown in  FIG. 8 . When the liquid receptacle  70  is moved horizontally with respect to the liquid ejection head  50  in the sub-scanning direction S, the wipers  75  of the liquid receptacle  70  slide over the nozzle face  510  of the liquid ejection head  50  in the sub-scanning direction S, thereby wiping away the liquid, and the like, on the nozzle face  510 .  
      The liquid wiped away from the nozzle face  510  flows toward the bottom of the recess part  71 , either directly or through a liquid guiding channel  77  connecting the rim part  72  of the liquid receptacle  70  with the side wall of the recess part  71 .  
      The wipers are not limited in particular to the composition where they are formed on the rim part  72  of the liquid receptacle  70 , and it is also possible to form the wipers on the belt  80 .  
       FIG. 11  is a developed view of a belt  800  provided with wipers  85 . In  FIG. 11 , the constituent elements that are the same as the constituent elements of the belt  80  shown in  FIG. 10  are denoted with the same reference numerals as  FIG. 10 , and contents described above are omitted from the following description.  FIG. 12  is a cross-sectional diagram along line  12 - 12  in  FIG. 11 .  
      In the present embodiment, the projection-shaped wipers  85  are formed along the main scanning direction M on the liquid-phobic part  84  of the belt  800 .  
      By adopting the composition in which the wipers  85  are provided on the belt  800  in this way, the wiping of the nozzle face  510  is carried out by means of the rotation of the belt  800 , and the mechanism can be simplified in comparison with the composition in which the wipers  75  are provided on the rim part  72  of the liquid receptacle  70  as shown in  FIG. 10 . More specifically, in the composition in which the wipers  75  are formed on the rim part  72  of the liquid receptacle  70  as shown in  FIGS. 7 and 8 , it is necessary to provide the mechanism for moving the liquid receptacle  70  and/or the liquid ejection head  50  precisely in parallel relatively to each other, in such a manner that the liquid does not drop out from the liquid receptacle  70 . On the other hand, in the case of the composition where the wipers  85  are formed on the external circumferential surface of the belt  800  as shown in  FIGS. 11 and 12 , it is sufficient to rotate the belt  800  through the rollers  73  by means of the motor  228 . Moreover, in the composition where the wipers  75  are disposed on the rim part  72  of the liquid receptacle  70  as described above, in order to prevent the liquid from dropping out from the liquid receptacle  70 , essentially, a unidirectional sweeping motion is adopted. On the other hand, according to the composition of the present embodiment in which the wipers  85  are disposed on the belt  800 , it is possible to move the wipers  85  back and forth reciprocally over the nozzle face  510  through forward rotation and reverse rotation of the belt  800 . In other words, the freedom of the sweeping direction is increased, and hence wiping can be carried out efficiently.  
      System Composition of Image Forming Apparatus  
       FIG. 14  is a block diagram showing one embodiment of the system composition of the image forming apparatus  110 .  
      As shown in  FIG. 14 , the image forming apparatus  110  principally includes: the stirrer  32  in the main tank  60  as shown in  FIG. 5  or  6 ; the liquid ejection head  50  as shown in  FIGS. 1 and 2 ; the liquid receptacle  70  as shown in FIGS.  7  to  9 ; a communication interface  210 , which performs communications with a host computer  300 ; a system controller  212 , which performs overall control of the image forming apparatus  110 ; memories  214  and  252 ; the conveyance unit  222 , which conveys an ejection receiving medium; the stirrer drive unit  224 , which drives the stirrer  32 ; the liquid receptacle movement unit  226 , which moves the liquid receptacle  70 ; the belt drive unit  228 , which drives the belt  80  in the liquid receptacle  70 ; a liquid flow unit  230 , which drives the flow of the liquid; a head controller  250 , which performs control relating to the liquid ejection head  50 ; and an actuator drive unit  254 , which drives the actuators  58  of the liquid ejection head  50 .  
      In  FIG. 12 , the second memory  252  is depicted as being appended to the head controller  250 ; however, it can be combined with the first memory  214 . Also possible is a mode in which the head controller  250  and the system controller  212  are integrated to form a single micro-processor.  
      The image forming apparatus  110  has the plurality of liquid ejection heads  50 , which constitute the liquid ejection unit  112  shown in  FIG. 3  and respectively eject inks of the colors of black (K), cyan (C), magenta (M) and yellow (Y).  
      The communication interface  210  is an image data input device for receiving image data transmitted by a host computer  300 . For the communication interface  210 , a wired or wireless interface, such as a USB (Universal Serial Bus), IEEE 1394, or the like, can be used. The image data acquired by the image forming apparatus  110  through the communication interface  210  is stored temporarily in a first memory  214  for storing image data.  
      The system controller  212  is constituted by a microcomputer and peripheral circuits thereof, and the like, and it forms a main control device which controls the whole of the image forming apparatus  110  in accordance with a prescribed program. More specifically, the system controller  212  controls units of the communication interface  210 , the conveyance unit  222 , the stirrer drive unit  224 , the liquid receptacle movement unit  226 , the belt drive unit  228 , the liquid flow unit  230 , the head controller  250 , and the like.  
      The conveyance unit  222  comprises a conveyance motor and driver circuit for same, and it conveys the recording medium  116  by using the rollers  131  and  132  and the belt  133  shown in  FIG. 3 , under the control of the system controller  212 . In other words, by means of the conveyance unit  222 , the liquid ejection heads  50  and the recording medium  116  move relatively to each other.  
      The stirrer drive unit  224  drives and rotates the stirrer  32 , which serves as the liquid agitating device, in the main tank  60  under the control of the system controller  212 , thus agitating the liquid in the main tank  60 . The stirrer drive unit  224  has a function for changing the direction of rotation of the stirrer  32  and the speed of rotation of same, with time, under the control of the system controller  212 .  
      The liquid receptacle movement unit  226  is constituted by a mechanism and a circuit which perform two-directional movement, namely, horizontal movement, in which the liquid receptacle  70  is moved in the medium conveyance direction (sub-scanning direction), and vertical movement, in which the liquid receptacle  70  is moved perpendicularly with respect to the nozzle face  510  of the liquid ejection head  50 , under the control of the system controller  212 .  
      The belt drive unit  228  is constituted by a mechanism and a circuit which move the belt  80  in the liquid receptacle  70 , under the control of the system controller  212 . The belt drive unit  228  causes the belt  80  to rotate under the control of the system controller  212 , thereby switching between a state where the opening section  81  of the belt  80  is opposite to the nozzle face  510  of the liquid ejection head  50  and a state where the liquid-philic part  83  of the belt  80  is opposite to the nozzle face  510  of the liquid ejection head  50 .  
      The liquid flow unit  230  is constituted by the main tank  60 , the sub-tank  61 , the liquid supply pump  62 , the liquid circulation pump  64 , the suction pump  67 , the waste liquid tank  68 , the electromagnetic valves  41  to  44 , the flow channels  600 ,  610 ,  620 ,  630  and  640  between the main tank  60  and the liquid ejection head  50 , and the flow channel  670  between the liquid receptacle  70  and the waste liquid tank  68 , as shown in  FIG. 5  or  6 . The electromagnetic valves  41  to  44  and the pumps  62 ,  64  and  67 , which constitute a part of the liquid flow unit  230 , are controlled by the system controller  212  and the head controller  250 .  
      The actuator drive unit  254  applies drive signals to the actuators  58  of the liquid ejection head  50  shown in  FIG. 2 , under the control of the head controller  250 , which operates in accordance with instructions from the system controller  212 . More specifically, the actuator drive unit  254  serves as a drive device that drives the actuators of the liquid ejection head  50 , when ejecting the liquid from the nozzles of the liquid ejection head  50 , and when agitating the liquid in the liquid ejection head  50 . There are various conditions of the drive signals (drive conditions), and typical embodiments thereof are described later in detail.  
      The head controller  250  is constituted by a microcomputer and peripheral circuits thereof, and the like, and it forms a control device which controls the liquid ejection heads  50  through the actuator drive unit  254  in accordance with a prescribed program.  
      The head controller  250  generates data (dot data), which is required when forming dots on a recording medium  116  by ejecting liquid toward the recording medium  116  from the liquid ejection heads  50  on the basis of the image data input to the image forming apparatus  110 . More specifically, the head controller  250  is a control unit that functions as an image processing device carrying out various image treatment processes, corrections, and the like, in order to generate dot data from the image data stored in the first memory  214 , in accordance with the control of the system controller  212 , and the head controller  250  supplies the dot data thus generated to the actuator drive unit  254 . When the dot data is supplied to the actuator drive unit  254 , drive signals are output to the actuators  58  of the liquid ejection heads  50  from the actuator drive unit  254  according to the dot data, and liquid is ejected from the nozzles  51  of the liquid ejection heads  50  toward the recording medium  116 .  
      Furthermore, various maintenance processes for maintaining the state of the liquid inside the liquid ejection head  50  are controlled by the system controller  212  and the head controller  250 .  
      Maintenance Processes Using Liquid Receptacle  
      The image forming apparatus  110  according to the present embodiment performs various maintenance processes using the liquid receptacle  70 , under the control of the system controller  212 .  
      Firstly, the liquid receptacle  70  is used for a liquid agitation process, which agitates the liquid inside the liquid ejection head  50 .  
      The liquid receptacle  70  is located in a prescribed standby position during image formation. When agitating the liquid with the liquid receptacle  70 , then as shown in  FIG. 15A , the liquid receptacle  70  is moved horizontally from the prescribed standby position to the position opposite to the nozzle face  510  of the liquid ejection head  50  and is also moved vertically in such a manner that the liquid-philic part  83  of the liquid receptacle  70  and the nozzle face  510  form a prescribed clearance for forming a liquid pool. Moreover, the belt  80  of the liquid receptacle  70  is rotated in such a manner that the liquid-philic part  83  of same is opposite to the nozzle face  510 . Then, for example, the liquid inside all of the pressure chambers  52  is ejected from all of the nozzles  51  by driving all of the actuators  58  of the liquid ejection head  50 . A layer-shaped liquid pool  351  is thereby formed between the liquid-philic part  83  of the liquid receptacle  70  and the nozzle face  510  of the liquid ejection head  50 . The liquid pool  351  is not limited to being formed by driving all of the actuators  58 , and it can also be formed by driving a selected plurality of actuators  58 . Next, the liquid agitation process described later is carried out.  
      During agitation of the liquid, the liquid-philic part  83  is surrounded by the liquid-phobic part  84  as described above in such a manner that the liquid pool  351  formed between the belt  80  and the nozzle face  510  does not flow out from the space between the belt  80  and the nozzle face  510 , and leakage is prevented by the sealing members  74  to avoid outflow of the liquid from the liquid receptacle  70 . The amount of the liquid consumed during the liquid agitation is hence reduced.  
      The liquid agitation process may also be carried out with forming no liquid pool  351 , and a mode of this kind where no liquid pool  351  is formed is described later in detail.  
      Secondly, the liquid receptacle  70  is used in a capping process, which hermetically seals (caps) the nozzle face  510  so as to prevent evaporation of the liquid from the nozzles  51  of the liquid ejection head  50 .  
      During capping, similarly to the case of the liquid agitation using the liquid receptacle  70 , the layer-shaped liquid pool  351  is formed between the liquid-philic part  83  of the liquid receptacle  70  and the nozzle face  510  of the liquid ejection head  50  as shown in  FIG. 15B , and furthermore, the whole of the nozzle area and the whole of the liquid pool is covered by the sealing members  74 .  
      Thirdly, the liquid receptacle  70  is used in a dummy ejection process (which is also referred to as “purging”) which performs dummy ejection of the liquid from the nozzles  51  of the liquid ejection head  50 .  
      The liquid receptacle  70  is located at the prescribed standby position during image formation. When performing dummy ejection, then as shown in  FIG. 15C , the liquid receptacle  70  is moved horizontally from the prescribed standby position to the position opposite to the nozzle face  510  of the liquid ejection head  50  and the belt  80  of the liquid receptacle  70  is rotated in such a manner that one of the opening parts ( 81  or  82 ) of the belt  80  is opposite to the nozzle face  510 . Thereby, the other of the opening parts ( 82  or  81 ) of the liquid receptacle  70  is in a state where it is opposite to the bottom of the liquid receptacle  70  (in other words, a state where it is opposite to the suction port  76 ). Then, the liquid inside the pressure chambers  52  is ejected from the nozzles  51  by driving the actuators  58  of the liquid ejection head  50 . Thereupon, the liquid ejected from the nozzles  51  of the liquid ejection head  50  passes through both of the opening parts  81  and  82 , reaches the bottom of the liquid receptacle  70 , and is then sent to the waste liquid tank  68  through the suction port  76  formed in the bottom of the liquid receptacle  70 . By performing the dummy ejection in this way, the liquid of increased viscosity inside the liquid ejection head  50 , dust adhering to the nozzles  51 , and the like, are cleaned away.  
      Fourthly, the liquid receptacle  70  is used in a suction process, which suctions liquid, and the like, from the nozzles  51  of the liquid ejection head  50 .  
      The liquid receptacle  70  is located in the prescribed standby position during image formation. When a suction process is carried out using the liquid receptacle  70 , then as shown in  FIG. 15D , the liquid receptacle  70  is moved horizontally from the prescribed standby position to the position opposite to the nozzle face  510  of the liquid ejection head  50  and is also moved vertically in such a manner that the sealing members  74  of the liquid receptacle  70  hermetically seal the nozzle face  510  of the liquid ejection head  50 . Moreover, the belt  80  of the liquid receptacle  70  is rotated in such a manner that one of the opening parts ( 81  or  82 ) of the belt  80  is opposite to the nozzle face  510 . Then, the suction pump  67  is driven. By performing the suction in this way, mater that is difficult to remove by the dummy ejection described above, for example, semi-solid mater or solid mater caused by the liquid increasing in viscosity and sedimentation inside the nozzles  51 , is suctioned by the suction pump  67  through the liquid receptacle  70 , together with the liquid.  
      Fifthly, the liquid receptacle  70  is used in a wiping process, which wipes the nozzle face  510  of the liquid ejection head  50 .  
       FIG. 15E  shows a case where the wipers  75  formed on the rim part  72  of the liquid receptacle  70  shown in FIGS.  7  to  9  slide over the nozzle face  510 . More specifically, the liquid receptacle  70  is moved in the sub-scanning direction S in a state where the wipers  75  are in contact with the nozzle face  510  of the liquid ejection head  50 .  
      Although the embodiment is shown in  FIGS. 8 and 9  in which the two opening parts  81  and  82  having the same opening cross-sectional area are provided, the present invention is not limited in particular to a case of this kind, and the sizes of the two opening parts  81  and  82  may be different from each other.  
      However, it is necessary to adopt a composition where the opening part that is opposite to the nozzle face  510  during the dummy ejection has at the least an opening cross-sectional area greater than the whole region (nozzle region) in which the nozzles  51  of the nozzle face  510  are formed. Hence, since all of the liquid ejected from the nozzles  51  is able to pass through the opening section, then splashing of the liquid from the belt  80  is prevented.  
      On the other hand, the opening part that is opposite to the nozzle face  510  during the suction may have a cross-sectional area of a size corresponding to the whole of the nozzle region in the nozzle face  510 , or a cross-sectional area of a size corresponding to a portion of the nozzle region. In a case where the opening part has a size corresponding to a portion of the nozzle region, then the suction force is increased in comparison with a case where the opening part has a size corresponding to the whole of the nozzle region.  
      Liquid Agitation Processes  
      The liquid to be subject to the liquid agitation processing in the image forming apparatus  110  includes the following portions: a first portion inside the liquid ejection head  50 , a second portion inside the tank, such as the main tank  60  or the sub-tank  61 , and a third portion inside the flow channels connecting the main tank  60  with the liquid ejection head  50 .  
      There are various types of the liquid agitation processes for agitating these portions of the liquid. Modes for achieving a large liquid agitation effect are described below: a first mode where the portion of the liquid inside the liquid ejection head  50  is agitated in a state where the free surface of the liquid in the nozzles  51  has been withdrawn toward the pressure chamber  52  side; a second mode where the portion of the liquid inside the liquid ejection head  50  is agitated in a state where a liquid pool has been formed between the liquid ejection head  50  and the liquid receptacle  70 ; and a third mode where the portion inside the liquid ejection head  50 , the portion inside the sub-tank  61  and the portion inside the flow channels between the main tank  60  and the liquid ejection head  50  are all returned into the main tank  60  and the whole liquid is then agitated inside the main tank  60 .  
      First Liquid Agitation Mode: Mode Withdrawing Free Surface of Liquid  
      In the first mode, the free surface of the liquid positioned in the vicinity of the nozzle face  510  in the nozzles  51  of the liquid ejection head  50  is withdrawn toward the pressure chamber  52  side, and then the actuators  58  used for liquid ejection in the liquid ejection head  50  are driven and the diaphragm  503  is caused to vibrate, so that the liquid inside the ejection head  50  can be agitated efficiently.  
      One embodiment of the liquid agitation process carried out by withdrawing the free surface of the liquid in this way is described.  
      After completing liquid ejection, the free surface of the liquid in the nozzles  51  of the liquid ejection head  50  is in a state where it is positioned in the vicinity of the nozzle face  510 , as shown in  FIG. 17A . In this state, the liquid agitation process shown in the flowchart shown in  FIG. 16  is started.  
      In  FIG. 16 , firstly, the free surface of the liquid having been positioned in the vicinity of the nozzle face  510  inside the nozzle  51  of the liquid ejection head  50  is withdrawn toward the pressure chamber  52  side (step S 12 ).  
      More specifically, by adjusting the amount of the liquid inside the liquid ejection head  50 , the free surface of the liquid having been positioned in the vicinity of the nozzle face  510  as shown in  FIG. 17A  is withdrawn to a position within an ejection flow channel  521  between the pressure chamber  52  and the nozzle  51  as shown in  FIG. 17B . It is also preferable that the free surface of the liquid is withdrawn to the boundary between the pressure chamber  52  and the ejection flow channel  521  (in other words, to a connection port  5210 , which is the inlet port to the ejection flow channel  521  from the pressure chamber  52 ) as shown in  FIG. 17C .  
      In the liquid flow system shown in  FIG. 5 , the first electromagnetic valve  41  in the first liquid returning flow channel  610  connecting the opening section  611  in the bottom of the sub-tank  61  to the main tank  60  is opened for a prescribed period of time so as to move the liquid in the sub-tank  61  to the main tank  60  by a prescribed amount in accordance with the amount of withdrawal of the free surface of the liquid in the liquid ejection head  50 , and the liquid is thereby made to flow from the liquid ejection head  50  to the sub-tank  61 , thus withdrawing the free surface of the liquid. In other words, the free surface of the liquid is withdrawn through the so-called siphon effect caused by the differential (head differential) between the height of the nozzle face  510  and the height of the free surface of the liquid in the sub-tank  61 , with reference to the highest point of the flow channel  630  between the liquid ejection head  50  and the sub-tank  61 .  
      Alternatively, in the liquid flow system shown in  FIG. 6 , the third electromagnetic valve  43  in the second liquid supply flow channel  630 , which supplies the liquid from the sub-tank  61  to the liquid ejection head  50 , is closed and the fourth electromagnetic valve  44  in the circulation flow channel  640 , which returns the liquid from the liquid ejection head  50  to the sub-tank  61 , is opened, then the liquid circulation pump  64  is driven in a direction for removing the liquid from the liquid ejection head  50  to the sub-tank  61  for a prescribed period of time so as to move the liquid in the liquid ejection head  50  to the sub-tank  61  by a prescribed amount in accordance with the amount of withdrawal of the free surface of the liquid in the liquid ejection head  50 . The free surface of the liquid is thus withdrawn, and the fourth electromagnetic valve  44  is then closed. By adopting the method in which the free surface of the liquid is withdrawn with the pump, it is possible to control the displacement of the free surface of the liquid precisely by finely controlling the driving of the pump.  
      In the state where the free surface of the liquid in the nozzles  51  has been withdrawn as described above, the actuators  58  of the liquid ejection head  50  are driven so as to vibrate the liquid inside the pressure chambers  52  through the diaphragm  503 , and the liquid inside the liquid ejection head  50  is thereby agitated (step S 14 ).  
      Thereupon, the free surface of the liquid in the nozzles  51  that has been withdrawn is returned to its original position, in other words, to the position in the vicinity of the nozzle face  510  in the nozzles  51  (step S 16 ).  
      In the liquid flow system shown in  FIG. 5 , the free surface of the liquid in the nozzles  51  is returned to its original position by causing the liquid to flow from the sub-tank  61  into the liquid ejection head  50 , by driving the liquid supply pump  62  for a prescribed period of time and thereby supplying a prescribed amount of the liquid corresponding to the return amount of the free surface in the nozzles  51 , from the main tank  60  to the sub-tank  61 .  
      In the liquid flow system shown in  FIG. 6 , it is also possible to return the free surface of the liquid in the nozzles  51  to its original position by opening the third electromagnetic valve  43  and driving the liquid circulation pump  64  for a prescribed period of time in a direction for supplying the liquid from the sub-tank  61  to the liquid ejection head  50 , thereby causing a prescribed amount of the liquid corresponding to the return amount of the free surface in the nozzles  51  to flow from the sub-tank  61  into the liquid ejection head  50 . Moreover, similarly to the liquid flow system shown in  FIG. 5 , it is also possible to return the free surface in the nozzles  51  to its original position by using the liquid supply pump  62 .  
      According to the above-described liquid agitation process performed by withdrawing the free surface of the liquid in the nozzles  51 , the amplitude of the drive signal applied to the actuators  58  can be raised, the displacement of the liquid inside the pressure chambers  52  can be increased, and hence the liquid inside the pressure chambers  52  can be agitated with good efficiency in a short period of time, compared to a liquid agitation method of the related art in which the free surface of the liquid is vibrated slightly in a state where it is positioned in the vicinity of the nozzle face  510  in the nozzles  51 .  
      There are various conditions of the drive signal (drive conditions) applied to the actuators  58  when agitating the liquid inside the liquid ejection head  50 , and two typical embodiments of same (drive condition  1  and drive condition  2 ) are described below.  
      Drive Condition 1  
      As a drive signal for liquid agitation, a drive signal having substantially the same frequency and amplitude as the drive signal applied to the actuators  58  for the liquid ejection, in other words, a drive signal having substantially the same waveform as during the liquid ejection, is applied to the actuators  58 .  
      In the liquid agitation process according to the present embodiment, since the actuators  58  are driven after the free surface of the liquid having been positioned in the vicinity of the nozzle face  510  in the nozzles  51  of the liquid ejection head  50  is withdrawn toward the pressure chamber  52  side as described above, then it is possible to agitate the liquid efficiently without causing the liquid ejection from the nozzles  51 , even if the drive signal of substantially the same waveform as the drive signal for the liquid ejection is applied to the actuators  58 .  
      In a case where the drive signal of substantially the same waveform as the drive signal for the liquid ejection is used, it is not necessary to provide a drive signal having a new waveform, and hence the composition of the drive circuit can be simplified.  
      In order to improve the liquid agitation efficiency yet further, it is possible to drive the actuators  58  under a drive condition where the displacement of the free surface of the liquid in the nozzles  51  and the movement of the micro-particles in the liquid are greater than during the liquid ejection. Moreover, since the resonance frequency of the liquid in the region composed of the pressure chamber  52 , the supply side, and the ejection side, is altered by the withdrawal of the free surface of the liquid in the nozzles  51 , then the actuators  58  can be driven under a drive condition where the displacement is greater than during the liquid ejection and no liquid ejection occurs.  
      Drive Condition 2  
      A drive signal having a sweeping frequency is applied to the actuators  58  as a drive signal for the liquid agitation.  
      In this case, a simple waveform, such as a sinusoidal wave or a rectangular wave can be used, so that the composition of the drive circuit can be simplified, and hence costs can be restricted. For example, the rectangular drive signal shown in  FIG. 18  is applied to the actuators  58 . The drive signal shown in  FIG. 18  is subjected to a frequency sweep as steadily changing frequency over time from a low frequency to a high frequency. In other words, the cycles of the signal (the time intervals between pulses in  FIG. 18 ) are steadily changed with time from a long cycle to a short cycle.  
      In the frequency sweep, the frequency is changed continuously, or in steps (for example, by intervals of several kilohertzs (kHz)), over a broad frequency range (for example, a frequency range from several kilohertzs to several tens kilohertzs).  
      As described above, the drive signal having the sweeping frequency is applied to the actuators  58 , in other words, the vibration with sweeping the frequency is applied to the liquid. Therefore, although the effective frequency for aggregated material, which is generally formed by micro-particles in the liquid (for example, aggregated material caused by aggregation and sedimentation of the coloring material in the ink), varies depending on the size and aggregation conditions, it is possible to obtain effective agitation effects while using the standardized waveform for the drive signal.  
      Second Liquid Agitation Mode: Mode Forming Liquid Pool  
      In the second mode, the liquid pool is formed between the nozzle face  510  of the liquid ejection head  50  and the belt  80  of the liquid receptacle  70 , and then the actuators  58  used for liquid ejection in the liquid ejection head  50  are driven and the diaphragm  503  is caused to vibrate, so that the liquid inside the liquid ejection head  50  is agitated.  
      One embodiment of the liquid agitation process carried out by forming the liquid pool in this way is described.  
      The liquid agitation process shown in  FIG. 19  is started in a state where the free surface of the liquid in the nozzles  51  of the liquid ejection head  50  is positioned in the vicinity of the nozzle face  510 , as shown in  FIG. 17A .  
      As shown in  FIG. 19 , firstly, the liquid receptacle  70  is placed in the proximity of the nozzle face  510  of the liquid ejection head  50  (step S 202 ).  
      More specifically, the liquid receptacle  70 , which has been positioned in a prescribed standby position, is moved in the sub-scanning direction to a maintenance position directly below the liquid ejection head  50 , and furthermore, the belt  80  of the liquid receptacle  70  is rotated and the liquid-philic part  83  of the belt  80  in the liquid receptacle  70  is made opposite to the nozzle face  510  of the liquid ejection head  50 . The clearance between the liquid-philic part  83  of the belt  80  of the liquid receptacle  70  and the nozzle face  510  is set to a distance where the liquid pool can be maintained by means of the interfacial tension of the liquid.  
      Thereupon, a drive signal for liquid pool creation is applied to the actuators  58  of the liquid ejection head  50  to drive the actuators  58 , whereby the liquid is ejected from the nozzles  51  of the liquid ejection head  50  toward the belt  80  of the liquid receptacle  70 , thus forming the liquid pool  351  between the nozzle face  510  of the liquid ejection head  50  and the liquid-philic part  83  of the belt  80 , as shown in  FIG. 13  (step S 204 ).  
      Thereupon, a drive signal for liquid agitation is applied to the actuators  58  of the liquid ejection head  50  to drive the actuators  58 , and the liquid inside the liquid ejection head  50  is agitated (step S 206 ). Here, the drive condition of the actuators  58  is set to either the drive condition  1  or the drive condition  2  described above.  
      By driving the actuators  58 , the liquid inside the pressure chambers  52  is vibrated and agitated through the diaphragm  503 , and the liquid in the region over the ejection flow channels  521  and the liquid-philic part  83  of the belt  80  is also vibrated and agitated, as shown in  FIG. 20 . Moreover, by means of the recess section  545  formed in the actuator protection plates  504  and  505  shown in  FIG. 2 , the heat generated by the driving of the actuators  58  is transmitted to the liquid inside the common liquid chamber  55  through the diaphragm  503 , thereby agitating the liquid inside the common liquid chamber  55  as well.  
      Then, after applying the drive signal for the liquid agitation to the actuators  58  for a prescribed period of time, the belt  80  in the liquid receptacle  70  is rotated, and the opening section  81  of the belt  80  in the liquid receptacle  70  is made opposite to the nozzle face  510  of the liquid ejection head  50 . Moreover, the clearance between the nozzle face  510  and the liquid-philic part  83  of the belt  80  in the liquid receptacle  70  is set to a distance where the wipers  75  arranged on the liquid receptacle  70  come in contact with the nozzle face  510  when sliding the wipers  75  over the nozzle face  510  of the liquid ejection head  50  (step S 208 ).  
      Next, an operation (wiping operation) for sweeping the wipers  75  over the nozzle face  510  of the liquid ejection head  50  is carried out (step S 210 ).  
      Thereupon, the liquid inside the liquid receptacle  70  is suctioned with the suction pump  67 , and expelled to the waste liquid tank  68  (step S 212 ).  
      The wiping operation (step S 210 ) and the expulsion of the liquid (step S 212 ) may be performed in reverse sequence or simultaneously.  
      By carrying out the liquid agitation in the state where the liquid pool has been formed in this way, it is possible to efficiently agitate the liquid containing aggregated and sedimented micro-particles in the vicinity of the nozzles  51 . It is also possible to remove semi-solid material of increased viscosity, solid material, dust, and the like, that have adhered to the nozzles  51 . Furthermore, no excessive load is applied to the actuators  58 .  
      Third Liquid Agitation Mode: Mode Returning all Liquid into Main Tank  
      In this mode, the liquid inside the liquid ejection head  50 , the sub-tank  61 , and all of the flow channels from the main tank  60  to the liquid ejection head  50  is completely returned into the main tank  60 , and the returned liquid is then agitated inside the main tank  60 .  
      One embodiment of the liquid agitation process that is carried out by returning all of the liquid inside the image forming apparatus  110  into the main tank  60  in this way is described below.  
      Firstly, the liquid agitation process that is performed when the power supply of the image forming apparatus  110  is turned off is described.  
      The liquid agitation process shown in  FIG. 21  starts when the free surface of the liquid inside the nozzles  51  of the liquid ejection head  50  is positioned in the vicinity of the nozzle face  510  and the power supply of the image forming apparatus  110  is turned off.  
      In  FIG. 21 , the liquid inside the liquid ejection head  50 , the sub-tank  61 , and the flow channels between the main tank  60  and the liquid ejection head  50  is all returned into the main tank  60  (step S 310 ).  
      Here, in the liquid flow system shown in  FIG. 5 , the first electromagnetic valve  41  and the second electromagnetic valve  42  are opened, and the liquid supply pump  62  is driven in reverse for a prescribed period of time. On the other hand, in the liquid flow system shown in  FIG. 6 , the first electromagnetic valve  41 , the second electromagnetic valve  42  and the third electromagnetic valve  43  are opened, the liquid supply pump  62  is then driven in reverse for a prescribed period of time, and the fourth electromagnetic valve  44  is opened and the pump  64  is then driven in reverse for a prescribed period of time.  
      After the liquid is all returned into the main tank  60 , the stirrer  32  in the main tank  60  is rotated by driving the stirrer drive unit  224 , thereby starting agitation of the liquid inside the main tank  60  (step S 312 ). Then, it is judged whether or not a prescribed time period T 1  has elapsed (step S 314 ), and when the prescribed time period T 1  has elapsed, then the stirrer  32  is halted (step S 316 ).  
      Thereupon, the supply of electrical power from the main power source  240  of the image forming apparatus  110  to the respective units is halted (step S 318 ). In other words, the image forming apparatus  110  is turned to the power-off state.  
      While the image forming apparatus  110  is in the power-off state, in other words, while the power supply from the main power source  240  is halted, then after waiting for a prescribed period of time with the stirrer  32  in the halted state (step S 320 ), the stirrer drive unit  224  is driven and the stirrer  32  in the main tank  60  is rotated by supplying power from the standby power source  242  (step S 322 ). It is then judged whether or not the prescribed time period T 1  has elapsed (step S 324 ), and when the prescribed time period T 1  has elapsed, then the stirrer  32  is halted (step S 356 ).  
      Thereafter, the liquid inside the main tank  60  is agitated by supplying power from the standby power source  242  at prescribed time intervals. Accordingly, it is possible to prevent aggregation and sedimentation of the micro-particles in the liquid, even if the image forming apparatus  110  remains in the power-off state for a long period of time.  
      Rather than setting the direction of rotation of the stirrer  32  to one direction only, it is preferable to perform forward rotation and reverse rotation, in alternating fashion. It is also possible to repeat the sequence of: forward rotation for the prescribed period of time→leave in idle state for the prescribed period of time→reverse rotation for the prescribed period of time→leave for the prescribed period of time→forward rotation for the prescribed period of time→, and so on.  
      Furthermore, it is preferable that the rotational speed is increased continuously (or in steps) from a low speed to a high speed, and then decreased continuously (or in steps) from the high speed to the low speed, the stirrer is halted, and the direction of rotation is then reversed. Thereby, it is possible to further increase the agitation effect.  
      In order to avoid blockages due to aggregation and/or sedimentation of the micro-particles in the liquid, at all places in the image forming apparatus, it is important to adopt the mode that agitates all of the liquid inside the liquid ejection head  50 , the main tank  60 , the sub-tank  61  and the flow channels. Here, there is a mode that simultaneously agitates the liquid at all places in the liquid ejection head  50 , the main tank  60 , the sub-tank  61  and the flow channels; however, if the liquid is agitated at all places in the image forming apparatus  110 , then the power consumption inevitably increases. Therefore, if it is necessary to agitate the whole liquid, then a preferable mode is one in which the liquid at all places is first returned into the main tank  60  and the whole liquid is then agitated inside the main tank  60 .  
      Next, the liquid agitation process when the power of the image forming apparatus  110  is turned on is described below with reference to the flowchart in  FIG. 22 .  
      In  FIG. 22 , in a state where the liquid has all been returned to the main tank  60 , the stirrer  32  in the main tank  60  is rotated by driving the stirrer drive unit  224 , thereby starting agitation of the liquid in the main tank  60  (step S 352 ). It is then judged whether or not the prescribed time period T 1  has elapsed (step S 354 ), and when the prescribed time period T 1  has elapsed, then the stirrer  32  is halted (step S 356 ).  
      Moreover, in the liquid flow system shown in  FIG. 5 , the first electromagnetic valve  41  and the second electromagnetic valve  42  are closed (step S 358 ), the liquid supply pump  62  is driven (step S 360 ), and it is then judged whether or not the prescribed time period T 2  has elapsed (step S 362 ). On the other hand, in the liquid flow system shown in  FIG. 6 , the first electromagnetic valve  41  and the second electromagnetic valve  42  are closed and the third electromagnetic valve  43  and the fourth electromagnetic valve  44  are opened (step S 358 ), the liquid supply pump  62  is driven (step S 360 ), and then it is judged whether or not the prescribed time period T 2  has elapsed (step S 362 ).  
      When the prescribed time period T 2  has elapsed, then in the liquid flow system shown in  FIG. 5 , the second electromagnetic valve  42  is opened (step S 364 ), and the liquid supply pump  62  is halted (step S 366 ). In the liquid flow system shown in  FIG. 6 , the second electromagnetic valve  42  is opened, the fourth electromagnetic valve  44  is closed (step S 364 ), and the liquid supply pump  62  is halted (step S 366 ).  
      Thereupon, the liquid receptacle  70  is made opposite to the liquid ejection head  50 , and the nozzle face  510  of the liquid ejection head  50  is wiped (step S 368 ).  
      It is not necessary to carry out all of the first, second and third liquid agitation modes described above. Moreover, it is also possible to perform the liquid agitation by means of a mode other than the first, second and third liquid agitation modes.  
      Preferably, the liquid agitation process is selected in accordance with the circumstances of the image forming apparatus  110 . For example, in the power-off state or when the power is turned on, the third liquid agitation mode is adopted, in other words, substantially all of the liquid inside the image forming apparatus  110  including the liquid inside the liquid ejection head  50  is returned into the main tank  60  and the liquid is then agitated inside the main tank  60 . On the other hand, when the power is turned on, if it is judged that the image forming apparatus  110  has been in the standby state for a prescribed time period or above (a prolonged standby state), then the second liquid agitation mode is adopted, in other words, the liquid inside the liquid ejection head  50  is agitated by forming the liquid pool using the liquid receptacle  70 , and the liquid inside the main tank  60  is agitated by using the stirrer  32 . If it is judged that the image forming apparatus  110  has been in the standby state for a period less than the prescribed time period (a short standby state) when the power is turned on, then the first liquid agitation mode is adopted, in other words, the free surface of the liquid in the nozzles  51  are withdrawn and the liquid only inside the liquid ejection head  50  is then agitated. Alternatively, in the case of the short standby state, it is also possible to carry out slight vibration of the free surface of the liquid in the nozzles  51  by driving the actuators  58  to an extent that does not cause the liquid to be ejected, while the free surface of the liquid in the nozzles  51  is positioned in the vicinity of the nozzle face  510 , without withdrawing the free surface of the liquid, forming a liquid pool, and returning the liquid into the main tank  60 . In this slight vibration of the free surface of the liquid, it is desirable to sweep the frequency of the drive signal applied to the actuators  58 . In the image forming apparatus  110  shown in  FIG. 14 , the liquid agitation process is selected in accordance with the circumstances of the image forming apparatus  110  by the system controller  212 .  
      The common liquid chamber  55  is situated on the opposite side of the actuators  58  from the pressure chambers  52  in the above-described embodiments as shown in  FIGS. 1 and 2 , but the present invention may also be applied to a composition where the common liquid chamber is situated on the same side of the actuators as the pressure chambers, as long as the direction of liquid ejection is a downward direction.  
      The liquid ejected from the liquid ejection head  50  is ink in the above-described embodiments, but the present invention may also be applied to a conductive liquid ejected toward a substrate when forming conductive wires on the substrate, or a liquid ejected toward an optical material during manufacture of a color filter, or the like.  
      It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.