Patent Publication Number: US-7708383-B2

Title: Ink jet head and ink jet recording apparatus

Description:
This application claims priority on Japanese patent application No. 2004-070564, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an ink jet head with which ink droplets are ejected from ejection ports formed in a substrate toward a recording medium, and an ink jet recording apparatus using the ink jet head. 
     An ink jet recording apparatus ejects ink through ejection ports to record an image corresponding to image data on a recording medium. Examples of known ink jet recording apparatuses include an electrostatic type, thermal type, and piezoelectric type ink jet recording apparatuses which are classified depending on differences of means for controlling ejection of ink. 
     Hereinafter, the electrostatic ink jet recording apparatus will be described as an example. In the electrostatic ink jet recording apparatus, ink containing charged colorant particles (charged color particles) is used, and predetermined voltages are respectively applied to ejection portions of an ink jet head in accordance with image data, whereby ejection of the ink from the ink jet head is controlled by utilizing electrostatic forces to record an image corresponding to the image data on a recording medium. Known as an example of the electrostatic ink jet recording apparatus is an ink jet recording apparatus disclosed in JP 10-138493 A. 
       FIG. 8  is a schematic view showing a construction of an example of an ink jet head of the electrostatic ink jet recording apparatus disclosed in JP 10-138493 A. In an ink jet head  100  shown in  FIG. 8 , only one ejection portion of the ink jet head disclosed in JP 10-138493 A is conceptually shown. The ink jet head  100  includes a head substrate  102 , an ink guide  104 , an insulating substrate  106 , a control electrode  108 , a counter electrode  110 , a D.C. bias voltage source  112 , and a pulse voltage source  114 . 
     Here, the ink guide  104  is disposed on the head substrate  102 , and a through hole (ejection port)  116  is bored through the insulating substrate  106  so as to correspond in position to the ink guide  104 . The ink guide  104  extends through the through hole  116 , and its projecting tip portion  104   a  projects upwardly and beyond a surface of the insulating substrate  106  on a side of a recording medium P. In addition, the head substrate  102  is disposed at a predetermined distance from the insulating substrate  106 . Thus, a passage  118  of ink Q is defined between the head substrate  102  and the insulating substrate  106 . 
     The control electrode  108  is provided in a ring-like shape on the surface of the insulating substrate  106  on the side of the recording medium P so as to surround the through hole  116  of every ejection portion. In addition, the control electrode  108  is connected to the pulse voltage source  114  for generating a pulse voltage in accordance with image data. The pulse voltage source  114  is grounded through the D.C. bias voltage source  112 . 
     In addition, the counter electrode  110  is disposed at a predetermined distance from the ink guide  104  so as to face the tip portion  104   a  of the ink guide  104  and is grounded. The recording medium P is disposed on a surface of the counter electrode  110  on a side of the ink guide  104 . That is to say, the counter electrode  110  functions as a platen for supporting the recording medium P. 
     During the recording, the ink Q containing colorant particles which are charged in the same polarity as that of a voltage applied to the control electrode  108  is circulated through the ink passage  118  from the right-hand side to the left-hand side in  FIG. 8  by a circulation mechanism for ink (not shown). In addition, a high voltage of 1.5 kV for example is continuously applied to the control electrode  108  by the D.C. bias voltage source  112 . At this time, the Coulomb attraction between the bias voltage applied to the counter electrode  110  and the electric charges of the colorant particles in the ink, the viscosity of the ink (dispersion medium), the surface tension, the repulsion among the charged particles, the fluid pressure when the ink is supplied, and the like operate in conjunction with one another. Thus, the balance is kept in a meniscus shape as shown in  FIG. 8  in which the ink slightly rises from the ejection port (nozzle)  116 . 
     In addition, the colorant particles migrate to move to the meniscus surface due to the Coulomb attraction or the like. In other words, the ink Q is concentrated on the meniscus surface. 
     If a pulse voltage of for example 0 V is applied from the pulse voltage source  114  to the control electrode  108  biased at 1.5 kV by the bias voltage source  112 , then a voltage of 1.5 kV obtained by superposing both the voltages on each other is applied to the control electrode  108 . In this state, an electric field strength in the vicinity of the tip portion  104   a  of the ink guide  104  is relatively low, and hence the ink Q that contains the colorant particles concentrated at the tip portion  104   a  of the ink guide  104  does not fly out from the tip portion  104   a  of the ink guide  104 . 
     On the other hand, if a pulse voltage of for example 500 V is applied from the pulse voltage source  114  to the control electrode  108  biased at 1.5 kV, then a voltage of 2 kV obtained by superposing both the voltages on each other is applied to the control electrode  108 . As a result, the ink Q containing the colorant particles which are concentrated at the tip portion  104   a  of the ink guide  104  flies out in the form of ink droplets R from the tip portion  104   a  of the ink guide  104  by the electrostatic force, is electrostatically attracted by the grounded counter electrode  110  and adheres to the recording medium P to form thereon a dot of the colorant particles. 
     In such a manner, recording is carried out with the dots of the colorant particles while the ink jet head  100  and the recording medium P supported on the counter electrode  110  are relatively moved to thereby record an image corresponding to the image data on the recording medium P. 
     Here, in the image recording with the ink jet head for ejecting ink droplets from the ejection ports, the meniscus needs to be stably formed in order to stably eject the ink droplets. 
     However, in the case of the ink jet recording apparatus disclosed in JP 10-138493 A, since the holding property of the formed meniscus is poor and thus the meniscus shape is not stabilized, the ejection performance fluctuates. For this reason, there is encountered a problem in that the drawing cannot be satisfactorily carried out. 
     In addition, when the holding property of the meniscus is poor, the formed meniscus is broken, and thus the ink overflows from the ejection port. As a result, there is also encountered a problem in that since the surface of the ejection port substrate gets dirty, the cleaning and maintenance for the surface of the ejection port substrate need to be carried out. 
     SUMMARY OF THE INVENTION 
     In light of the foregoing, the present invention has been made to solve the problems associated with the prior art. It is, therefore, an object of the present invention to provide an ink jet head which has a high meniscus holding property and which is capable of stably forming a meniscus and stably drawing image dots each having a desired size. 
     Another object of the present invention is to provide an ink jet recording apparatus using the ink jet head. 
     In order to solve the above-mentioned object, the present invention provides an ink jet head including a plate-like substrate having an ejection port bored through the plate-like substrate; and ejection means for ejecting ink droplets from the ejection port, wherein at least a part of a periphery of the ejection port convexly projects along an ejection direction of the ink droplets. 
     Here, an angle between a surface parallel with the ejection direction and an uppermost surface of a convexly-projecting portion of the periphery of the ejection port that convexly projects along the ejection direction is preferably an acute angle. 
     In addition, a tip portion of a convexly-projecting portion of the periphery of the ejection port that convexly projects along the ejection direction preferably has an acute angle. 
     Also, the ink droplets are preferably ejected from the ejection port by causing an electrostatic force to act on ink. 
     In addition, the present invention provides an ink jet head for ejecting ink droplets from an ejection port by causing an electrostatic force to act on ink containing charged colorant particles, comprising: an ejection port substrate having an ejection port bored through the ejection port substrate, the ink droplets being adapted to be ejected from the ejection port; a head substrate disposed at a predetermined distance apart from the ejection port substrate to form an ink flow path between the ejection port substrate and the head substrate; an ink guide provided in the head substrate in a position corresponding to the ejection port of the ejection port substrate, a tip portion of the ink guide penetrating through the ejection port; and an ejection electrode formed in correspondence to the ejection port for ejecting the ink droplets from the ejection port by causing the electrostatic force to act on the ink, wherein at least a part of a periphery of the ejection port convexly projects along an ejection direction of the ink droplets. 
     Here, an angle between a surface parallel with the ejection direction and an uppermost surface of a convexly-projecting portion of the periphery of the ejection port that convexly projects along the ejection direction is preferably an acute angle. 
     In addition, a tip portion of a convexly-projecting portion of the periphery of the ejection port that convexly projects along the ejection direction preferably has an acute angle. 
     Here, a height of the projecting portion is preferably a range of 10 μm or more, and 500 μm or less. 
     In addition, the present invention provides an ink jet recording apparatus for recording an image corresponding to image data on a recording medium using any one of the above ink jet heads. 
     According to the present invention, the meniscus holding property in the ejection port can be enhanced, and thus the ink can be prevented from overflowing from the ejection port. As a result, the maintenance property can be improved. Moreover, the meniscus shape is stabilized, so the image dot drawing performance is also stabilized, and thus the image dots having a uniform dot diameter can be consistently drawn. 
    
    
     
       BRIEF DESCRIOTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1A  is a schematic cross-sectional view of an ink jet head according to an embodiment of the present invention; 
         FIG. 1B  is an enlarged view showing the periphery of an ejection port of the ink jet head shown in  FIG. 1A ; 
         FIGS. 2A and 2B  are conceptual views illustrating the ink jet head shown in  FIG. 1A ; 
         FIG. 3  is a perspective view showing the shape of a projection in the ink jet head shown in  FIG. 1A ; 
         FIG. 4  is a perspective view showing another example of the shape of the projection; 
         FIG. 5A  is a schematic cross-sectional view of the ink jet head according to another embodiment of the present invention; 
         FIG. 5B  is an enlarged view showing the periphery of an ejection port of the ink jet head shown in  FIG. 5A ; 
         FIG. 6A  is a schematic cross-sectional view of the ink jet head according to still another embodiment of the present invention; 
         FIG. 6B  is an enlarged view showing the periphery of an ejection port of the ink jet head shown in  FIG. 6A ; 
         FIG. 7A  is a schematic cross-sectional view of an ink jet recording apparatus according to an embodiment of the present invention; 
         FIG. 7B  is a perspective view schematically illustrating a head unit and conveyance means for conveying a recording medium provided on the periphery of the head unit; and 
         FIG. 8  is a schematic cross-sectional view of a conventional ink jet head. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An ink jet head and an ink jet recording apparatus using the ink jet head according to the present invention will hereinafter be described in detail based on preferred embodiments illustrated in the accompanying drawings. 
       FIG. 1A  is a schematic cross-sectional view of an ink jet head according to an embodiment of the present invention, and  FIG. 1B  is an enlarged view showing the periphery of an ejection port of the ink jet head shown in  FIG. 1A .  FIGS. 2A and 2B  are cross sectional views taken along the line A-A and the line B-B of  FIG. 1A .  FIG. 3  is a perspective view of an ejection port substrate. 
     An electrostatic ink jet head  10  shown in these drawings includes a head substrate  12 , ink guides  14 , an ejection port substrate  16  having ejection ports  28 , ejection electrodes  18  formed on the periphery of the ejection ports  28  in the ejection port substrate  16 , and a guard electrode  20  provided on the upper side in  FIG. 1A  of the ejection electrodes  18  inside the ejection port substrate  16 . As will be described later in detail, an insulating substrate  32  is covered with a first insulating layer  34   a , which in turn is covered with a second insulating layer  34   b  to thereby form the ejection port substrate  16 . Further, a projection  38  is formed on the second insulating layer  34   b  of the ejection port substrate  16  at a position where the layer  34   b  surrounds the ejection port  28 . 
     The head substrate  12  and the ejection port substrate  16  are disposed apart from each other by a predetermined distance, and the gap defined by those substrates  12 ,  16  forms an main ink flow path  30  for supplying ink to each ejection port  28 . The main ink flow path  30  and each ejection port  28  extending to the opening end on the ejection side form an ink flow path. 
     A counter electrode  24  which supports a recording medium P and a charge unit  26  for the recording medium P are disposed so as to be opposed to the ejection portions of the ink jet head  10  (more specifically, ejection ports (nozzles)  28 , ink guides  14  and ejection electrodes  18 ). 
     The ink jet head  10  described above ejects ink Q prepared by dispersing charged fine particles containing a pigment or other colorant component (hereinafter referred to as colorant particles) into an insulating liquid (carrier liquid) under an electrostatic force. The drive voltage to be applied to the ejection electrode  18  for ejection ON/OFF is controlled in accordance with image data, whereby ink droplets are modulated in accordance with the image data and ejected to record an image on the recording medium P. 
     As shown in  FIGS. 2A and 2B , the ink jet head  10  has a multi channel structure where the ejection portions are arranged two-dimensionally for high density image recording. However, in order to clearly represent the structure,  FIG. 1A  shows only one ejection portion. 
     In the ink jet head  10  according to the present invention, it is possible to freely choose the number of the ejection electrodes  18  and the physical arrangement thereof. For example, the structure may be the multi channel structure of the embodiment shown in  FIG. 2A  or  2 B or a structure having only one line of the ejection portions. The ink jet head  10  may be a so-called (full-)line head having lines of ejection portions corresponding to the whole area of the recording medium P or a so-called serial head (shuttle type head) which performs scanning in a direction perpendicular to the nozzle row direction. The ink jet head  10  of the present invention can cope with a monochrome recording apparatus and a color recording apparatus. 
     In the illustrated ink jet head  10 , the ink guide  14  is formed of a ceramic flat plate with a predetermined thickness having a convex tip end portion  14   a , and disposed on the head substrate  12  for each ejection port  28  (ejection portion). 
     Through-holes serving as the ejection ports  28  for ejecting ink droplets R are formed in the ejection port substrate  16  to be described later. The ink guides  14  are disposed in the respective ejection ports  28  (ejection portions) and their tip end portions  14   a  project from the surface of the ejection port substrate  16  on the recording medium P side. For the sake of convenience, the surfaces of the insulating layer  34   b  on the upper side and the lower side in  FIG. 1A  are hereinafter referred to as the upper and lower surfaces, respectively. Note that a slit functioning as an ink guide groove for guiding the ink Q to the tip end portion  14   a  through the capillary phenomenon may be formed in the top-bottom direction on the paper plane of  FIG. 1A  in a center portion of the ink guide  14 . 
     In the illustrated case, the ink guide  14  on the tip end portion  14   a  side is processed to be upwardly tapered and to have a substantially triangular shape (or a trapezoidal shape). The shape of the ink guide  14  is not particularly limited as long as the ink Q, more specifically, the charged fine particle component in the ink Q is allowed to pass through the ejection port  28  of the ejection port substrate  16  and to be concentrated at the tip end portion  14   a . For example, the tip end portion  14   a  is not necessarily convex but the shape may be appropriately changed, and a known shape can be used as well. 
     A metal is preferably vapor-deposited onto a distal end portion of the ink guide  14 . With the vapor-deposition of the metal, the tip end portion  14   a  of the ink guide  14  has practically large permittivity to facilitate generation of an intense electric field, thereby improving ink ejection properties. 
     As described above, the head substrate  12  and the ejection port substrate  16  are disposed apart from each other by a predetermined distance, and the gap defined by those substrates  12 ,  16  forms the main ink flow path  30  which functions as an ink reservoir (ink chamber) for supplying the ink Q to each ejection port  28  (ink guide  14 ). 
     During image recording, the ink Q is circulated by an ink circulating mechanism (not shown) in a predetermined direction (the ink is circulated in the main ink flow path  30  from the right to the left in the illustrated case) at a predetermined speed (for example, at an ink flow rate of 200 mm/s). 
     The ejection port substrate  16  includes the insulating substrate  32 , the first insulating layer  34   a  having the ejection electrodes  18  formed on its lower side, and the second insulating layer  34   b  having the guard electrode  20  formed on its lower side and the projection  38  formed on its upper side. The ejection port  28  for ejecting the ink droplets R is formed so as to perfectly extend through the ejection port substrate  16 . The ink guide  14  is inserted through each ejection port  28  so as for its tip end portion to project upwardly. Here, the ejection electrodes  18  and the projections  38  are formed in the ejection ports  28  of the ejection port substrate  16 . Moreover, the guard electrode  20  is formed above the ejection electrodes  18  and between the ejection electrodes  18 . 
     In the illustrated ink jet head  10 , the ejection port substrate  16  has such a construction that the ejection electrodes  18  are formed on the upper surface of the insulating substrate  32  made of an insulating material, the first insulating layer  34   a  is formed so as to cover the whole upper surface of the insulating substrate  32 , the guard electrode  20  is formed on the upper surface of the first insulating layer  34   a , the insulating layer  34   b  is formed so as to cover the whole upper surface of the first insulating layer  34   a , and thereafter, an area of the insulating substrate  32  corresponding to the ejection electrodes  18  is removed by utilizing a known etching technique for example. 
     Consequently, in the illustrated ink jet head  10 , the lower surfaces of the ejection electrodes  18  are exposed to the main ink flow path  30  constituted by the gap between the head substrate  12  and the ejection port substrate  16  with the ejection electrodes  18  being buried in the first insulating layer  34   a  on its lower surface side. That is, the lower surfaces of the ejection electrodes  18  contact the ink Q in the main ink flow path  30 . 
     In addition, as described above, the ejection port substrate  16  has the projection  38  on the upper surface of the second insulating layer  34   b  on the periphery of each ejection port  28 . As shown in  FIG. 3 , the projection  38  has a shape which is obtained by removing a portion corresponding to the ejection port  28  from a cone having the base on the upper surface of the second insulating layer  34   b  and having a vertex on the central axis of the ejection port  28 . That is, as shown in  FIG. 1B , the cross-sectional shape of the projection  38  is a triangular shape in which the lower surface of the projection  38  contacting the second insulating layer  34   b  corresponds to a base  38   b , the side surface of the projection  38  as a part of the ejection port  28  corresponds to an opposite side  38   c , and the upper surface of the projection  38  having an inclination which gets closer to the recording medium P as the distance from the ejection port  28  is decreased corresponds to a hypotenuse  38   d.    
     A tip portion  38   a  of the projection  38  has a sharp shape. That is, the angle Φ at a joining point between the opposite side  38   c  and the hypotenuse  38   d  of the projection  38  is an acute angle. The projection  38  has a predetermined height. Note that the height of the projection  38  means a height h from the upper surface of the second insulating layer  34   b  as a portion in which the ejection port substrate  16  does not project to the tip portion  38   a  of the projection  38 . 
     In the present invention, as described above, the periphery of the ejection port  28  has the projection  38  in which the tip portion  38   a  on the recording medium P side has an acute angle, whereby the meniscus holding property is enhanced, and the stable ejection of the ink droplets is greatly enhanced. This point will be described in detail later together with an operation of the ejection of the ink droplets. 
     As shown in this embodiment, a part of the periphery of the ejection port  28  on the lower surface side of the ejection port substrate  16  is preferably removed by a predetermined thickness. In this way, the length of the ejection port  28  can be shortened, the resistance between the ink Q and the inner wall of the ejection port  28  is reduced, and the ink Q can be speedily ejected from the ejection port  28 . The ejection port substrate  16  preferably has the shape in which a part of the periphery of the ejection port  28  corresponding to the insulating substrate  32  is removed as in the illustrated case. However, the ejection port substrate  16  may also have a shape in which a part of the ejection port substrate  16  is not removed. 
     Each of the ejection electrodes  18  is disposed as a ring-like circular electrode on the lower surface of the first insulating layer  34   a  (the surface on the head substrate  12  side) and on the upper side of the insulating substrate  32  in  FIG. 1A  (the surface on the recording medium P side) so as to surround the ejection port  28  extending through the ejection port substrate  16 . The ejection electrode  18  is connected to a signal voltage source  33  for generating a drive voltage (e.g., a pulse voltage) having a predetermined electric potential and corresponding to ejection data (ejection signal) such as image data or printing data. 
     Since, as described above, the illustrated embodiment has a multi-channel structure in which the ejection ports  28  are two-dimensionally disposed, the ejection electrodes  18 , as a matter of course, are two-dimensionally disposed in the ejection ports  28  as shown in  FIG. 2B . 
     Here, the ejection electrode  18  contacts the ink Q. Thus, when a voltage is applied to the ejection electrode  18 , a part of the electric charge supplied to the ejection electrode  18  is injected into the ink Q to increase the conductivity of the ink Q in the vicinity of the ejection electrode  18 . As a result, only when the voltage is applied to the ejection electrode  18 , the ink Q becomes a state in which the ink droplets are remarkably easy to eject (the ejection property is enhanced). 
     Thus, while the ejection electrode  18  preferably contacts the ink Q, the present invention is not limited thereto. The ejection electrode  18  may also be disposed in a position where the ejection electrode  18  does not contact the ink Q, e.g., inside the ejection port substrate  16 . 
     It should be noted that the ejection electrode  18  is not limited to the ring-like circular electrode, and thus various shapes can be utilized for the ejection electrode  18 . A preferable example thereof is an enclosing electrode disposed so as to surround the ejection port  28  (a part thereof may be cut). The ejection electrode  18  is more preferably a nearly circular electrode and is much more preferably a circular electrode. 
     In this embodiment, the ejection electrode  18  is disposed on the lower surface of the first insulating layer  34   a , but the position of the ejection electrode  18  is not especially limited. The ejection electrode  18  may be disposed within the ejection port substrate  16 . Moreover, the position of the ejection electrode  18  is not limited to the periphery of the ejection port substrate  16 . The ejection electrode  18  may also be disposed on the head substrate  12 , within the head substrate  12 , or the like. 
     Furthermore, while one ejection electrode  18  is disposed for every ejection portion, the present invention is not limited thereto. A multi-layer electrode structure may also be adopted in which ejection electrodes are disposed for every ejection portion. 
     The guard electrode  20  is formed on the upper surface of the first insulating layer  34   a , and its surface is covered with the second insulating layer  34   b . As shown in  FIG. 2A , the guard electrode  20  is a sheet-like electrode which is made of a metallic plate and which is common to the ejection electrodes  18 . Openings  36  are bored through the guard electrode  20  and correspond to the ejection electrodes  18  which are respectively formed on the peripheries of the ejection ports  28  two-dimensionally disposed. Each opening  36  has a diameter larger than that of each ejection electrode  18 . 
     The guard electrode  20  shields against electric lines of force between the adjacent ejection electrodes  18  to suppress the electric field interference between the adjacent ejection electrodes  18 . Thus, a predetermined voltage (including the grounding voltage, i.e., 0 V) is applied to the guard electrode  20 . In the illustrated embodiment, the guard electrode  20  is grounded, and hence a voltage thereof is 0 V. 
     In the illustrated embodiment, preferably, the guard electrode  20  is formed in the layer different from that containing the ejection electrodes  18 , and moreover, its whole surface is covered with the second insulating layer  34   b.    
     The ink jet head  10  has the guard electrode  20 , whereby the electric field interference between the adjacent ejection electrodes  18  can be suitably prevented, and the colorant particles of the ink Q can be prevented from being deposited to cause the discharge between the ejection electrodes  18  and the guard electrode  20 . 
     Here, the guard electrode  20  needs to be provided so as to shield against the electric lines of force directed from other ejection ports  28  (hereinafter referred to as “other channels” for the sake of convenience) and the electric lines of force directed to the other ejection ports  28  while ensuring the electric lines of force acting on the corresponding ejection port  28  (hereinafter referred to as “own channel” for convenience) among the electric lines of force generated from the ejection electrodes  18 . 
     If there is no guard electrode  20 , the electric lines of force generated from the inner peripheral portion of the ejection electrode  18  converge into the inner side of the ejection electrode  18  to act on the own channel, thereby generating the necessary electric field. On the other hand, the electric lines of force generated from the outer peripheral portion of the ejection electrode  18  diverge to the outer side of the ejection electrode  18  to influence the other channels to cause the electric field interference. 
     If the above points are taken into consideration, the diameter of the opening  36  of the guard electrode  20 , when the substrate plane is viewed from above, is preferably made larger than the internal diameter of the ejection electrode  18  of the own channel to avoid shielding against the electric lines of force directed to the own channel. Specifically, the end portion of the guard electrode  20  on the ejection port  28  side (hereinafter, an ejection port side end portion and an opposite side end portion of each member are referred to as “an inner edge portion” and “an outer edge portion”, respectively) is preferably more spaced apart from the ejection port  28  than the inner edge portion of the ejection electrode  18  of the own channel. According to the studies made by the inventor of the present invention, the distance between the inner edge portion of the ejection electrode  18  and the inner edge portion of the guard electrode  20  is preferably equal to or larger than 10 μm. 
     In addition, for the efficient shielding against the electric lines of force directed to the other channels, the diameter of the opening  36  of the guard electrode  20 , when the substrate plane is viewed from above, is preferably made smaller than the outer diameter of the ejection electrode  18  of the own channel. Specifically, the inner edge portion of the guard electrode  20  is preferably closer to the ejection port  28  than the outer edge portion of the ejection electrode  18  of the own channel. According to the studies made by the inventor of the present invention, the distance between the outer edge portion of the ejection electrode  18  and the inner edge portion of the guard electrode  20  is preferably equal to or larger than 5 μm, more preferably equal to or larger than 10 μm. 
     With the above construction, the stable ejection of the ink droplets from the ejection port  28  is ensured, variations in the ink adhering position due to the electric field interference between the adjacent channels can be suitably suppressed, and thus a high-quality image can be consistently recorded. 
     In the above embodiment, the ejection electrode  18  has been described as the circular electrode. However, when the ejection electrode  18  is not the circular electrode, an effective diameter such as an average diameter that can be substantially regarded as a diameter has to be taken into consideration in accordance with the shape of the ejection electrode  18 . Alternatively, the guard electrode  20  may also be provided so that the shape of the opening  36  of the guard electrode  20  is made substantially similar to the inner peripheral shape or the outer peripheral shape of the ejection electrode  18 , and in each peripheral position of the ejection electrode  18 , the inner edge portion of the guard electrode  20  is more spaced apart from the ejection port  28  than the inner edge portion of the ejection electrode  18  of the own channel and is closer to the ejection port  28  than the outer edge portion thereof. 
     In the above embodiment, the guard electrode  20  is a sheet-like electrode, but the present invention is not limited thereto. Any electrode may be used as the guard electrode  20  as long as this electrode is provided so as to shield against the electric lines of force directed from the other channels among the ejection portions. For example, the guard electrode  20  may be provided in a mesh-like structure among the ejection portions. Alternatively, the guard electrode  20  may be provided not between the ejection portions which are so distant from one another as not to cause the electric field interference, but only between the ejection portions close to each other. 
     In this case as well, the guard electrode  20  may be formed so that its inner edge portion is more spaced apart from the ejection port  28  than the inner edge portion of the ejection electrode  18  of the own channel, and is closer to the ejection port  28  than the outer edge portion of the ejection electrode  18  of the own channel. 
     As described above, in  FIG. 1A , the counter electrode  24  is disposed so as to face the surface of the ink jet head  10  from which the ink droplets R are to be ejected. 
     The counter electrode  24  is disposed so as to face the tip end portion  14   a  of the ink guide  14 , and includes an electrode substrate  24   a  which is grounded and the insulating sheet  24   b  which is disposed on a lower surface of the electrode substrate  24   a  in  FIG. 1A , that is, on a surface of the electrode substrate  24   a  on the side of the ink jet head  10 . 
     The recording medium P is supported on the lower surface of the counter electrode  24  in  FIG. 1A , that is, on the surface of the insulating sheet  24   b  by electrostatic attraction for example. The counter electrode  24  (the insulating sheet  24   b ) functions as a platen for the recording medium P. 
     At least during recording, the recording medium P held on the insulating sheet  24   b  of the counter electrode  24  is charged by the charge unit  26  to a predetermined negative high voltage opposite in polarity to that of the drive voltage (for example, the pulse voltage) applied to the ejection electrode  18 , e.g., −1.5 kV. 
     As a result, the recording medium P is charged negative to be biased to the negative high voltage to function as the substantial counter electrode to the ejection electrode  18 , and is electrostatically attracted to the insulating sheet  24   b  of the counter electrode  24 . 
     The charge unit  26  includes a scorotron charger  26   a  for charging the recording medium P to a negative high voltage, and a bias voltage source  26   b  for supplying a negative high voltage to the scorotron charger  26   a . Note that the charge means of the charge unit  26  used in the present invention is not limited to the scorotron charger  26   a , and hence various discharge means such as a corotron charger, a solid-state charger and an electrostatic discharge needle can be used. 
     In addition, in the illustrated embodiment, the counter electrode  24  includes the electrode substrate  24   a  and the insulating sheet  24   b , and the charge unit  26  is used to charge the recording medium P to a negative high voltage to apply a bias voltage to the medium P so that the medium P functions as the counter electrode and is electrostatically attracted to the surface of the insulating sheet  24   b . However, this is not the sole case of the present invention and another configuration is also possible in which the counter electrode  24  is constituted only by the electrode substrate  24   a , and the counter electrode  24  (the electrode substrate  24   a ) is connected to a bias voltage source for supplying a negative high voltage and is always biased to the negative high voltage so that the recording medium P is electrostatically attracted to the surface of the counter electrode  24 . 
     Further, the electrostatic attraction of the recording medium P to the counter electrode  24 , the charge of the recording medium P to the negative high voltage, and the application of the negative high bias voltage to the counter electrode  24  may be performed using separate negative high voltage sources. Also, the support of the recording medium P by the counter electrode  24  is not limited to the utilization of the electrostatic attraction of the recording medium P, and hence any other supporting method or supporting means may be used for the support of the recording medium P by the counter electrode  24 . 
     Hereinafter, the present invention will be described in greater detail by reference to the ejection operation for the ink droplets R in the ink jet head  10 . 
     As illustrated in  FIG. 1A , upon recording, the ink Q containing colorant particles charged in the same polarity as that of the voltage to be applied to the ejection electrode  18 , for example positively charged colorant particles is circulated by the ink circulating mechanism including a pump (not shown) in a direction shown by an arrow (from the right to the left in  FIG. 1A ) in the main ink flow path  30  of the ink jet head  10 . 
     On the other hand, the recording medium P on which an image is to be recorded is charged to have the polarity opposite to that of the colorant particles, that is, a negative high voltage (for example, −1500 V) by the charge unit  26 . While being charged to the bias voltage, the recording medium P is electrostatically attracted to the counter electrode  24 . 
     In this state, the recording medium P (counter electrode  24 ) and the ink jet head  10  are moved relatively while the signal voltage source  33  applies a drive voltage (pulse voltage) to each ejection electrode  18  in accordance with supplied image data. Ejection ON/OFF is controlled depending on whether or not the drive voltage is applied, whereby the ink droplets R are modulated in accordance with the image data and ejected to record an image on the recording medium P. 
     Here, when the drive voltage is not applied to the ejection electrode  18  (or the applied voltage is at a low voltage level), i.e., in a state where the bias voltage is only applied, Coulomb attraction between the bias voltage and the charges of the colorant particles (charged particles) of the ink Q, Coulomb repulsion among the colorant particles, viscosity of the carrier liquid, surface tension, and dielectric polarization force, and the like act on the ink Q, and these factors operate in conjunction with one another to move the colorant particles and the carrier liquid. Thus, the balance is kept in a meniscus shape as conceptually shown in  FIG. 1A  in which the ink Q slightly rises from the ejection port  28 . 
     In addition, the Coulomb attraction and the like allow the colorant particles to move toward the recording medium P charged to the bias voltage through a so-called electrophoresis process. That is, the ink Q is concentrated at the meniscus in the ejection port  28 . 
     From this state, the drive voltage is applied to the ejection electrode  18 . As a result, the drive voltage is superposed on the bias voltage, and hence the motion occurs in which the previous conjunction motion operates in conjunction with the superposition of the drive voltage. Thus, the colorant particles and the carrier liquid are attracted toward the bias voltage side (the counter electrode side), i.e., the recording medium P side by the electrostatic force. The meniscus grows to form a nearly conical ink liquid column, i.e., the so-called Taylor cone from the tip portion of the meniscus. In addition, similarly to the foregoing, the colorant particles are moved to the meniscus surface through the electrophoresis process so that the ink Q at the meniscus is concentrated and has a large number of colorant particles at a nearly uniform high concentration. 
     When a finite period of time further elapses after the start of the application of the drive voltage, the balance mainly between the Coulomb attraction acting on the colorant particles and the surface tension of the carrier liquid is broken at the tip portion of the meniscus having the high electric field strength applied thereto due to the movement of the colorant particles or the like. As a result, the meniscus abruptly grows to form a slender ink liquid column called the thread having about several μm to several tens of μm in diameter. 
     When a finite period of time further elapses, the thread grows, and is divided into small portions due to the interaction resulting from the growth of the thread, the vibrations generated due to the Rayleigh/Weber instability, the ununiformity in distribution of the colorant particles within the meniscus, the ununiformity in distribution of the electrostatic field applied to the meniscus, and the like. The divided thread is then ejected and flown in the form of the ink droplets R and is attracted by the bias voltage as well to adhere to the recording medium P. The growth of the thread and its division, and moreover the movement of the colorant particles to the meniscus and/or the thread are continuously generated while the drive voltage is applied to the control electrode. 
     After the end of the application of the drive voltage (ejection is OFF), the meniscus returns to the above-mentioned state where only the bias voltage is applied. 
     Here, as described above, in the ink jet head  10  for ejecting the ink droplets from the ejection ports, it is required for the ink jet heads using other systems as well as for the electrostatic ink jet head that the ink meniscus is stably formed in order to stabilize the ejection of the ink droplets, and moreover the property of holding the ink meniscus is enhanced in order to prevent the ink from leaking from the ejection ports. 
     As a result of intensive studies about this point, the inventor of the present invention has found out that irrespective of the system used in the ink jet head, the property of holding the ink meniscus changes depending on the shape of the ejection port substrate at a point at which the surface of the ink meniscus contacts the ejection port substrate (this point is hereinafter referred to as “a contact point”). 
     Furthermore, the inventor of the present invention has also found out that by increasing the angle θ at the contact point between the ejection port substrate and the ink meniscus surface, the property of holding the ink meniscus can be enhanced and thus the ink meniscus can be stably formed. Here, the angle θ at the contact point means an angle between the ink meniscus surface at the contact point and the surface of the ejection port substrate  16  outside the contact point (the hypotenuse  38   d  of the projection  38  in  FIG. 1B ). 
     Here, in this specification, a portion in which the surface of the projection or a protrusion to be described later having an inclination becoming nearer the recording medium P in a position nearer the ejection port contacts the surface of the ink meniscus is defined as an uppermost surface. In addition, the surface which passes through the end portion, on the recording medium P side, of the surface of the projection or a protrusion to be described later whose inclination becomes nearer the recording medium P in a position nearer the ejection port, and which is parallel with the ejection direction is defined as a reference surface. 
     In the ink jet head  10  of the illustrated embodiment, as described above, the projection  38  is formed on the upper surface of the ejection port substrate  16  (on the recording medium P side) so as to surround the ejection port  28 . At this time, the cross-sectional shape of the projection  38  has the tip portion  38   a  on a line extending from the periphery of the ejection port  28  toward the recording medium P. 
     Since the projection  38  is formed on the periphery of the ejection port  28 , the tip portion  38   a  of the projection  38  becomes the contact point. Here, the tip portion  38   a  of the projection  38  is sharp. Specifically, the angle between the surface (reference surface) which passes through the end portion of the upper surface of the projection  38  on the recording medium P side, and which is parallel with the ejection direction, and the portion (uppermost surface) formed by the surface of the projection  38  having the inclination becoming nearer the recording medium P in a position nearer the ejection port, and the surface of the ink meniscus, i.e., the angle Φ between the side surface (the opposite side  38   c ) of the projection  38  and the upper surface (the hypotenuse  38   d ) of the projection  38  is an acute angle. 
     As a result, the angle θ at the contact point (the tip portion  38   a ) between the upper surface  38   b  and the surface of the ink meniscus becomes larger than the case where the meniscus is formed in the ejection port bored through the plate-like ejection port substrate of the conventional ink jet head, i.e., the case where as shown in  FIG. 8  in which the meniscus is formed in the ejection port  116  in which the angle between the surface of the ejection port substrate  106  on the recording medium P side and the surface (reference surface) passing through an end portion of the ejection port substrate  106  on the ejection port  116  side and being parallel with the ejection direction is a right angle. 
     The angle between the uppermost surface and the reference surface is made acute, whereby the angle between the uppermost surface and the surface of the ink meniscus becomes larger, and hence the property of holding the ink meniscus in the ejection port is enhanced. Moreover, since the angle Φ of the tip portion of the projection is also an acute angle, the property of holding the ink meniscus in the ejection port is further enhanced. As a result, the ink meniscus is stably formed, whereby the ejection responsivity to the drive voltage becomes constant, the ejection of the ink droplets is stabilized and hence a high-quality image can be formed. 
     While in an example shown in  FIG. 3 , the projection  38  is provided so as to wholly surround the ejection port  28 , the present invention is not limited thereto. For example, as shown in  FIG. 4 , a projection  39  having a shape in which portions having a predetermined width are removed on an ink inflow side and an ink outflow side of the ejection port  28  may also be used. 
     The projection is provided in at least a part of the circumference of the ejection port, whereby the property of holding the ink meniscus can be enhanced. Here, the projection having a size equal to or larger than the width of the surface of the ink guide parallel with an ink flow direction is preferably provided in the portion of the ejection port orthogonally intersecting the ink flow direction, and is more preferably provided throughout the entire periphery of the ejection port. 
     In addition, while the upper surface  38   d  of the projection  38  of this embodiment forms a straight line in cross section, the present invention is not limited thereto. Alternatively, the upper surface  38   d  of the projection  38  may be curved. Hence, when the upper surface  38   d  of the projection  38  is curved, an end portion of the upper surface of the projection on the recording medium P side becomes the uppermost surface. Thus, the angle between the tangent of the uppermost surface and the reference surface has to be an acute angle. 
     In addition, the surface (the upper surface  38   d  in this embodiment) of the projection outside the contact point with the surface of the ink meniscus, and the surface of the ejection port substrate  16  preferably have ink repellency. The ink repellency processing is carried out on the outer portion of the contact point between the meniscus and the ejection port  28 , whereby the property of holding the ink meniscus is further enhanced, and hence the ink droplets can be stably ejected. 
     Here, the ink repellency means water repellency in the case of aqueous ink, and oil repellency in the case of oily ink. 
     Examples of the method for imparting the ink repellency to the surface of the projection outside the contact point with the surface of the ink meniscus and the surface of the ejection port substrate include a method in which ink repellency processing is performed on the ejection port substrate and the inclined surface of the projection, and a method in which an ink-repellent material such as an ink-repellent film is stuck on or attached to the ejection port substrate and the inclined surface of the projection. 
     In addition, the height h of the projection  38  from the upper surface of the second insulating layer  34   b  to the tip portion  38   a  of the projection  38  is preferably in a range of 10 μm to 500 μm, and more preferably in a range of 10 μm to 200 μm, and much more preferably in a range of 10 μm to 100 μm. 
     When the height h is equal to or larger than 10 μm, the property of holding the ink meniscus can be enhanced, and when the height h is equal to or smaller than 500 μm, the length of the ejection port  28  can be reduced. Thus, the resistance between the ink Q and the inner wall of the ejection port  28  is reduced, the ejection responsivity of the ink droplets is enhanced, and the ejection of the ink droplets can follow up to an ejection frequency of 5 kHz. Here, the ejection frequency means a frequency at which the ink droplets are ejected. 
     In addition, when the height h is equal to or smaller than 300 μm, the resistance between the ink Q and the inner wall of the ejection port  28  can be further reduced and the ejection responsivity can be further enhanced Thus, the ejection of the ink droplets can follow up to the ejection frequency of 10 kHz. 
     Moreover, when the height h is equal to or smaller than 100 μm, the resistance between the ink Q and the inner wall of the ejection port  28  can be further reduced and the ejection responsivity can be further enhanced. Thus, the ejection of the ink droplets can follow up to the ejection frequency of 15 kHz. 
       FIGS. 5A and 5B  show conceptual views each showing another embodiment of the ink jet head of the present invention. 
     Note that an ink jet head  40  shown in  FIGS. 5A and 5B  is different from the ink jet head  10  shown in  FIGS. 1A and 1B  only in construction of the ejection port substrate and position of the ejection electrodes. Thus, the same members as those in the ink jet head  10  are designated with the same reference numerals and their detailed description is omitted here. A different point will hereinafter be mainly described. 
     In an ejection port substrate  44  of this embodiment, a guard electrode  20  is formed on an insulating substrate  46  and the guard electrode  20  is covered with an insulating layer  48  and ejection ports  28  are bored through the ejection port substrate  44 . The peripheral portion of each ejection port in the ejection port substrate  44  has a shape in which the peripheral portion extends upwardly to be closer to a counter electrode as the distance from the ejection port  28  is decreased. Hereinafter, a protruded portion of the ejection port substrate  44  is referred to as a protrusion  44   a.    
     The protrusion  44   a  has a tip portion  44   b  on the recording medium P side. Here, the tip portion  44   b  is a corner portion formed by a side face  44   c  of the protrusion  44   a  as a part of the ejection port  28 , and an upper surface  44   d  of the protrusion  44   a  which slopes down as the distance from the center of the ejection port  28  increases, whereby the distance between the recording medium P and the upper surface  44   d  is increased. The angle α between the surface (reference surface) which passes through the tip portion  44   b  and is parallel with the ejection direction, and the upper surface  44   d  of the protrusion  44   a  (the uppermost surface of the protrusion  44   a ) is an acute angle. 
     In this embodiment as well, the tip portion  44   b  becomes a contact point between the protrusion  44   a  and the surface of the ink meniscus. As described above, the angle α between the reference surface and the uppermost surface of the protrusion  44   a  is an acute angle. Hence, the angle between the surface of the ink meniscus and the uppermost surface of the protrusion  44   a  becomes larger than that in the conventional ink jet head, and thus the property of holding the ink meniscus is enhanced. 
     Even in the case where the protrusion is formed by processing the ejection port substrate, the property of holding the ink meniscus can also be enhanced as in the embodiment shown in  FIGS. 1A and 1B . 
     The protrusion  44   a  as in this embodiment can be formed for example by embossing a plate-like substrate. 
     In addition, in the present invention, the shape only has to be determined such that the angle formed at the contact point between the surface of the ink meniscus and the uppermost surface of the projection or the protrusion is large. Thus, in the projection or the protrusion, the surface constituting the ejection port may incline as in this embodiment as long as the angle between the uppermost surface and the reference surface is an acute angle. In addition, the projection or the protrusion may have a surface perpendicular to the ejection direction, for example, the tip portion may have a given width, if the formation of the ink meniscus on the ink guide side as viewed from the contact point cannot be impeded. 
     While in this embodiment, the ejection electrode  42  is disposed on the head substrate  12 , the ejection electrode  42  of this embodiment may be disposed on a lower surface of the ejection port substrate  44 , inside the ejection port substrate  44 , or inside the head substrate  12  as in the ink jet head  10  shown in  FIGS. 1A and 1B . In addition, in this embodiment, one ejection electrode  42  is disposed in every ejection port  28 . However, it is to be understood that even when the ejection electrode  42  is disposed on the head substrate  12 , a multi-layer electrode structure may also be adopted in which ejection electrodes are disposed in every ejection portion. 
       FIGS. 6A and 6B  are conceptual views each showing still another embodiment of the ink jet head of the present invention. 
     An ink jet head  50  shown in  FIGS. 6A and 6B  is different only in shape of the ejection port substrate from the ink jet head  40  shown in  FIGS. 5A and 5B . Thus, the same members as those in the ink jet head  40  shown in  FIGS. 5A and 5B  are designated with the same reference numerals and their detailed description is omitted here. Hereinafter, a different point will be mainly described. 
     In an ejection port substrate  52 , a guard electrode  20  is disposed on an insulating substrate  54  and the guard electrode  20  is covered with an insulating layer  56  and ejection ports are bored through the ejection port substrate  52 . In addition, the peripheral portion of each ejection port  28  in the ejection port substrate  52  has a protrusion  52   a  which protrudes toward the counter electrode as the distance from the ejection port  28  is decreased. 
     The protrusion  52   a  has a tip portion  52   b  on the record medium P side. Here, the tip portion  52   b  is a corner portion formed by a lower surface  52   c  of the protrusion  52   a  which forms the ejection port  28  and faces the ink flow path  30  side, and an upper surface  52   d  of the protrusion  52   a  which slopes down as the distance from the center of the ejection port  28  increases, whereby the distance between the recording medium P and the upper surface  52   d  is increased. Thus, the tip portion  52   b  forms a sharply pointed portion. That is, the protrusion  52   a  of this embodiment has a shape in which both the upper surface  52   d  and the lower surface  52   c  extend upwardly toward the recording medium P side and also the ejection port substrate  52  becomes thinner toward the center of the ejection port  28 , and thus the upper surface  52   d  and the lower surface  52   c  of the ejection port substrate  16  (the protrusion  52   b ) are joined to each other in the tip portion  52   d.    
     In such a shape as well, the angle γ between the surface (reference surface) which passes through the tip portion  52   b  and is parallel with the ejection direction, and the upper surface  52   d  (uppermost surface) can be an acute angle. Hence, the angle in the tip portion  52   d  between the upper surface  52   d  and the ink meniscus surface can be made larger, and thus the property of holding the ink meniscus can be enhanced. Moreover, since the angle in the tip portion  52   b  of the protrusion  52   a , i.e., the angle β between the upper surface  52   d  and the lower surface  52   c  becomes an acute angle, the property of holding the ink meniscus in the ejection port  28  is further enhanced. 
     As described above, the ink jet head of the present invention has the shape in which the portion convexly protruding along the ejection direction is provided in at least a part of the periphery of the ejection port by projecting the periphery of the ejection port, by providing the projection on the periphery of the ejection portion, or by integrally forming the projection on the periphery of the ejection portion, whereby the ink meniscus holding property can be enhanced. 
     In particular, as described above, in the ink jet head of the present invention, at least a part of the ejection port substrate serving as the contact point between the ejection port substrate and the ink meniscus surface is formed into the shape in which the tip portion of the projection or the protrusion is sharpened, i.e., into the shape in which the angle between the uppermost surface of the projection or the protrusion, and the reference surface is an acute angle. Therefore, the angle between the uppermost surface and the meniscus surface becomes large, and hence the property of holding the ink meniscus can be further enhanced. 
     Moreover, the shape in which the tip portion of the projection or the protrusion has an acute angle, i.e., the angle of the projection or the protrusion in the contact point between the projection or the protrusion, and the ink meniscus surface becomes an acute angle, enables further enhancement in the property of holding the ink meniscus. 
     It should be noted that as long as at least a part of the ejection port substrate has a portion convexly protruding along the ejection direction of the ink droplets, various constructions can be utilized for other portions. For example, the present invention can be also applied to an inkjet head including no ink guide. 
     The ink Q (ink composition) which is ejected by the ink jet head  10  is obtained by dispersing colorant particles (charged fine particles which contain colorants) in a carrier liquid. 
     The carrier liquid is preferably a dielectric liquid (non-aqueous solvent) having a high electrical resistivity (equal to or larger than 10 9  Ω·cm, and more preferably equal to or larger than 10 10  Ω·cm). If the electrical resistivity of the carrier liquid is low, the concentration of the colorant particles does not occur since the carrier liquid receives the injection of the electric charges and is charged due to a drive voltage applied to the ejection electrodes. In addition, since there is also anxiety that the carrier liquid having a low electrical resistivity causes the electrical conduction between the adjacent ejection portions, the carrier liquid having a low electrical resistivity is unsuitable for the present invention. 
     The relative permittivity of the dielectric liquid used as the carrier liquid is preferably equal to or smaller than 5, more preferably equal to or smaller than 4, and much more preferably equal to or smaller than 3.5. Such a range is selected for the relative permittivity, whereby the electric field effectively acts on the colorant particles contained in the carrier liquid to facilitate the electrophoresis of the colorant particles. 
     Note that the upper limit of the specific electrical resistance of the carrier liquid is desirably about 10 16  Ω·cm, and the lower limit of the relative permittivity is desirably about 1.9. The reason why the electrical resistance of the carrier liquid preferably falls within the above-mentioned range is that if the electrical resistance becomes low, then the ejection of the ink under a low electric field becomes worse. Also, the reason why the relative permittivity preferably falls within the above-mentioned range is that if the relative permittivity becomes high, then the electric field is relaxed due to the polarization of the solvent, and as a result the color of dots formed under this condition becomes light, or the bleeding occurs. 
     Preferred examples of the dielectric liquid used as the carrier liquid include straight-chain or branched aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, and the same hydrocarbons substituted with halogens. Specific examples thereof include hexane, heptane, octane, isooctane, decane, isodecane, decalin, nonane, dodecane, isododecane, cyclohexane, cyclooctane, cyclodecane, benzene, toluene, xylene, mesitylene, Isopar C, Isopar E, Isopar G, Isopar H, Isopar L, Isopar M (Isopar: a trade name of EXXON Corporation), Shellsol 70, Shellsol 71 (Shellsol: a trade name of Shell Oil Company), AMSCO OMS, AMSCO 460 Solvent, (AMSCO: a trade name of Spirits Co., Ltd.), a silicone oil (such as KF-96L, available from Shin-Etsu Chemical Co., Ltd.). The dielectric liquid may be used singly or as a mixture of two or more thereof. 
     For such colorant particles dispersed in the carrier liquid, colorants themselves may be dispersed as the colorant particles into the carrier liquid, but dispersion resin particles are preferably contained for enhancement of fixing property. In the case where the dispersion resin particles are contained in the carrier liquid, in general, there is adopted a method in which pigments are covered with the resin material of the dispersion resin particles to obtain particles covered with the resin, or the dispersion resin particles are colored with dyes to obtain the colored particles. 
     As the colorants, pigments and dyes conventionally used in ink compositions for ink jet recording, (oily) ink compositions for printing, or liquid developers for electrostatic photography may be used. 
     Pigments used as colorants may be inorganic pigments or organic pigments commonly employed in the field of printing technology. Specific examples thereof include but are not particularly limited to known pigments such as carbon black, cadmium red, molybdenum red, chrome yellow, cadmium yellow, titanium yellow, chromium oxide, viridian, cobalt green, ultramarine blue, Prussian blue, cobalt blue, azo pigments, phthalocyanine pigments, quinacridone pigments, isoindolinone pigments, dioxazine pigments, threne pigments, perylene pigments, perinone pigments, thioindigo pigments, quinophthalone pigments, and metal complex pigments. 
     Preferred examples of dyes used as colorants include oil-soluble dyes such as azo dyes, metal complex salt dyes, naphthol dyes, anthraquinone dyes, indigo dyes, carbonium dyes, quinoneimine dyes, xanthene dyes, aniline dyes, quinoline dyes, nitro dyes, nitroso dyes, benzoquinone dyes, naphthoquinone dyes, phthalocyanine dyes, and metal phthalocyanine dyes. 
     Further, examples of dispersion resin particles include rosins, rosin-modified phenol resin, alkyd resin, a (meta)acryl polymer, polyurethane, polyester, polyamide, polyethylene, polybutadiene, polystyrene, polyvinyl acetate, acetal-modified polyvinyl alcohol, and polycarbonate. 
     Of those, from the viewpoint of ease for particle formation, a polymer having a weight average molecular weight in a range of 2,000 to 1,000,000 and a polydispersity (weight average molecular weight/number average molecular weight) in a range of 1.0 to 5.0 is preferred. Moreover, from the viewpoint of ease for the fixation, a polymer in which one of a softening point, a glass transition point, and a melting point is in a range of 40° C. to 120° C. is preferred. 
     In the ink Q, the content of colorant particles (total content of colorant particles and dispersion resin particles) preferably falls within a range of 0.5 to 30.0 wt % for the overall ink, more preferably falls within a range of 1.5 to 25.0 wt %, and much more preferably falls within a range of 3.0 to 20.0 wt %. If the content of colorant particles decreases, the following problems become easy to arise. The density of the printed image is insufficient, the affinity between the ink Q and the surface of the recording medium P becomes difficult to obtain to prevent the image firmly stuck to the surface of the recording medium P from being obtained, and so forth. On the other hand, if the content of colorant particles increases, problems occur in that the uniform dispersion liquid becomes difficult to obtain, the clogging of the ink Q is easy to occur in the ink jet head or the like to make it difficult to obtain the stable ink ejection, and so forth. 
     In addition, the average particle diameter of the colorant particles dispersed in the carrier liquid preferably falls within a range of 0.1 to 5.0 μm, more preferably falls within a range of 0.2 to 1.5 μm, and much more preferably falls within a range of 0.4 to 1.0 μm. Those particle diameters are measured with CAPA-500 (a trade name of a measuring apparatus manufactured by HORIBA LTD.). 
     After the colorant particles are dispersed in the carrier liquid and optionally a dispersing agent, a charging control agent is added to the resultant carrier liquid to charge the colorant particles, and the charged colorant particles are dispersed in the resultant liquid to thereby produce the ink Q. Note that in dispersing the colorant particles in the carrier liquid, a dispersion medium may be added if necessary. 
     As the charging control agent, for example, various ones used in the electrophotographic liquid developer can be utilized. In addition, it is also possible to utilize various charging control agents described in “DEVELOPMENT AND PRACTICAL APPLICATION OF RECENT ELECTRONIC PHOTOGRAPH DEVELOPING SYSTEM AND TONER MATERIALS”, pp. 139 to 148; “ELECTROPHOTOGRAPHY-BASES AND APPLICATIONS”, edited by THE IMAGING SOCIETY OF JAPAN, and published by CORONA PUBLISHING CO. LTD., pp. 497 to 505, 1988; and “ELECTRONIC PHOTOGRAPHY” by Yuji Harasaki, 16(No. 2), p. 44, 1977. 
     Note that the colorant particles may be positively or negatively charged as long as the charged colorant particles are identical in polarity to the drive voltages applied to ejection electrodes  18 . 
     In addition, the charging amount of colorant particles is preferably in a range of 5 to 200 μC/g, more preferably in a range of 10 to 150 μC/g, and much more preferably in a range of 15 to 100 μC/g. 
     In addition, the electrical resistance of the dielectric solvent may be changed by adding the charging control agent in some cases. Thus, the distribution factor P defined below is preferably equal to or larger than 50%, more preferably equal to or larger than 60%, and much more preferably equal to or larger than 70%.
 
 P= 100×(σ1−σ2)/σ1
 
     where σ1 is an electric conductivity of the ink Q, and σ2 is an electric conductivity of a supernatant liquid which is obtained by inspecting the ink Q with a centrifugal separator. Those electric conductivities were obtained by measuring the electric conductivities of the ink Q and the supernatant liquid under a condition of an applied voltage of 5 V and a frequency of 1 kHz using an LCR meter of an AG-4311 type (manufactured by ANDO ELECTRIC CO., LTD.) and electrode for liquid of an LP-05 type (manufactured by KAWAGUCHI ELECTRIC WORKS, CO., LTD.). In addition, the centrifugation was carried out for 30 minutes under a condition of a rotational speed of 14,500 rpm and a temperature of 23° C. using a miniature high speed cooling centrifugal machine of an SRX-201 type (manufactured by TOMY SEIKO CO., LTD.). 
     The ink Q as described above is used, which results in that the colorant particles are likely to migrate and hence the colorant particles are easily concentrated. 
     The electric conductivity of the ink Q is preferably in a range of 100 to 3,000 pS/cm, more preferably in a range of 150 to 2,500 pS/cm, and much more preferably in a range of 200 to 2,000 pS/cm. The range of the electric conductivity as described above is set, resulting in that the applied voltages to the ejection electrodes are not excessively high, and also there is no anxiety to cause the electrical conduction between the adjacent ejection electrodes. 
     In addition, the surface tension of the ink Q is preferably in a range of 15 to 50 mN/m, more preferably in a range of 15.5 to 45.0 mN/m, and much more preferably in a range of 16 to 40 mN/m. The surface tension is set in this range, resulting in that the applied voltages to the ejection electrodes are not excessively high, and also the ink does not leak or spread to the periphery of the head to contaminate the head. 
     Moreover, the viscosity of the ink Q is preferably in a range of 0.5 to 5.0 mPa·sec, more preferably in a range of 0.6 to 3.0 mPa·sec, and much more preferably in a range of 0.7 to 2.0 mPa·sec. 
     The ink Q can be prepared for example by dispersing colorant particles into a carrier liquid to form particles and adding a charging control agent to the dispersion medium to allow the colorant particles to be charged. The following methods are given as the specific methods.
     (1) A method including: previously mixing (kneading) a colorant and/or dispersion resin particles; dispersing the resultant mixture into a carrier liquid using a dispersing agent when necessary; and adding the charging control agent thereto.   (2) A method including: adding a colorant and/or dispersion resin particles and a dispersing agent into a carrier liquid at the same time for dispersion; and adding the charging control agent thereto.   (3) A method including adding a colorant and the charging control agent and/or the dispersion resin particles and the dispersing agent into a carrier liquid at the same time for dispersion.   

       FIG. 7A  is a conceptual diagram of an embodiment of an ink jet recording apparatus of the present invention which utilizes the ink jet head of the present invention. 
     An ink jet recording apparatus  60  (hereinafter, referred to as printer  60 ) illustrated in  FIG. 7A  is an apparatus for performing four-color one-side printing on the recording medium P. The printer  60  includes conveyor means for the recording medium P, image recording means, and solvent collecting means, all of which are accommodated in a casing  61 . 
     The conveyor means includes a feed roller pair  62 , a guide  64 , rollers  66  ( 66   a ,  66   b , and  66   c ), a conveyor belt  68 , conveyor belt position detecting means  69 , electrostatic attraction means  70 , discharge means  72 , peeling means  74 , fixation/conveyance means  76 , and a guide  78 . The image recording means includes a head unit  80 , an ink circulating system  82 , a head driver  84  and recording medium position detecting means  46 . The solvent collecting means includes a discharge fan  90 , and a solvent collecting device  92 . 
     In the conveyor means for the recording medium P, the feed roller pair  62  is a conveyance roller pair disposed in the vicinity of a feeding port  61   a  provided on a side surface of the casing  61 . The feed roller pair  62  feeds the recording medium P fed from a paper cassette (not shown) to the conveyor belt  68  (a portion supported by the roller  66   a  in  FIG. 7A ). The guide  64  is disposed between the feed roller pair  62  and the roller  66   a  for supporting the conveyor belt  68  and guides the recording medium P fed by the feed roller pair  62  to the conveyor belt  68 . 
     Foreign matter removal means for removing foreign matter such as dust or paper powder adhered to the recording medium P is preferably disposed in the vicinity of the feed roller pair  62 . 
     As the foreign matter removal means, one or more of known methods including non-contact removal methods such as suction removal, blowing removal and electrostatic removal, and contact removal methods such as removal using a blush, a roller, etc., may be used in combination. It is also possible that the feed roller pair  62  is composed of a slightly adhesive roller, a cleaner is prepared for the feed roller pair  62 , and foreign matter such as dust or paper powder is removed when the feed roller pair  62  feeds the recording medium P. 
     The conveyor belt  68  is an endless belt extended over the three rollers  66  ( 66   a ,  66   b , and  66   c ). At least one of the rollers  66   a ,  66   b , and  66   c  is connected to a drive source (not shown) to rotate the conveyor belt  68 . 
     At the time of image recording by the head unit  80 , the conveyor belt  68  functions as scanning conveyor means for the recording medium P and also as a platen for holding the recording medium P. After the end of image recording, the conveyor belt  68  further conveys the recording medium P to the fixation/conveyance means  76 . Therefore, the conveyor belt  68  is preferably made of a material which is excellent in dimension stability and has durability. The conveyor belt  68  is for example made of a metal, a polyimide resin, a fluororesin, another resin, or a complex thereof. 
     In the illustrated embodiment, the recording medium P is held on the conveyor belt  68  under electrostatic attraction. In correspondence with this, the conveyor belt  68  has insulating properties on a side on which the recording medium P is held (front face), and conductive properties on the other side on which the belt  68  contacts the rollers  66  (rear face). Further, in the illustrated embodiment, the roller  66   a  is a conductive roller, and the rear face of the conveyor belt  68  is grounded via the roller  66   a.    
     In other words, the conveyor belt  68  also functions as the counter electrode  24  including the electrode substrate  24   a  and the insulating sheet  24   b  shown in  FIG. 1A  when the conveyor belt  68  holds the recording medium P. 
     A belt having a metal layer and an insulating material layer manufactured by a variety of methods, such as a metal belt coated with a resin material, for example, fluororesin on the front face, a belt obtained by bonding a resin sheet to a metal belt with an adhesive or the like, and a belt obtained by vapor-depositing a metal on the rear face of a belt made of the above-mentioned resin may be used as the conveyor belt  68 . 
     The conveyor belt  68  preferably has the flat front face contacting the recording medium P, whereby satisfactory attraction properties of the recording medium P can be obtained. 
     Meandering of the conveyor belt  68  is preferably suppressed by a known method. An example of a meandering suppression method is that the roller  66   c  is composed of a tension roller, a shaft of the roller  66   c  is inclined with respect to shafts of the rollers  66   a  and  66   b  in response to an output of the conveyor belt position detecting means  69 , that is, a position of the conveyor belt  68  detected in a width direction, thereby changing a tension at both ends of the conveyor belt in the width direction to suppress the meandering. The rollers  66  may have a taper shape, a crown shape, or another shape to suppress the meandering. 
     The conveyor belt position detecting means  69  suppresses the meandering of the conveyor belt etc. in the above manner and detects the position of the conveyor belt  68  in the width direction to regulate the recording medium P to situate at a predetermined position in the scanning/conveyance direction at the time of image recording. Known detecting means such as a photo sensor may be used. 
     The electrostatic attraction means  70  charges the recording medium P to a predetermined bias voltage with respect to the head unit  80  (ink jet head of the present invention), and charges the recording medium P to have a predetermined potential such that the recording medium P is attracted to and held on the conveyor belt  68  under an electrostatic force. 
     In the illustrated embodiment, the electrostatic attraction means  70  includes a scorotron charger  70   a  for charging the recording medium P and a negative high voltage power source  70   b  connected to the scorotron charger  70   a . While being conveyed by the feed roller pair  62  and the conveyor belt  68 , the recording medium P is charged to a negative bias voltage by the scorotron charger  70   a  connected to the negative high voltage power source  70   b  and attracted to the insulating layer of the conveyor belt  68 . 
     Note that the conveying speed of the conveyor belt  68  when charging the recording medium P may be in a range where the charging is performed with stability, so the speed may be the same as, or different from, the conveying speed at the time of image recording. Also, the electrostatic attraction means may act on the same recording medium P several times by circulating the recording medium P several times on the conveyor belt  68  for uniform charging. 
     In the illustrated embodiment, the electrostatic attraction and the charging for the recording medium P are performed in the electrostatic attraction means  70 , but the electrostatic attraction means and the charging means may be provided separately. 
     The electrostatic attraction means is not limited to the scorotron charger  70   a  of the illustrated embodiment; a corotron charger, a solid-state charger, an electrostatic discharge needle, and various means and methods can be employed. As will be described in detail later, at least one of the rollers  66  is composed of a conductive roller, or a conductive platen is disposed on the rear side of the conveyor belt  68  in a recording position for the recording medium P (side opposite to the recording medium P). Then, the conductive roller or the conductive platen is connected to the negative high voltage power source, thereby forming the electrostatic attraction means  70 . Alternatively, it is also possible that the conveyor belt  68  is composed of an insulating belt and the conductive roller is grounded to connect the conductive platen to the negative high voltage power source. 
     The conveyor belt  68  conveys the recording medium P charged by the electrostatic attraction means  70  to the position where the head unit  80  to be described later is located. 
     The head unit  80  uses the ink jet head of the present invention to eject ink droplets in accordance with image data to thereby record an image on the recording medium P. As described above, the ink jet head of the present invention uses a charge potential of the recording medium P for the bias voltage and applies a drive voltage to the ejection electrodes  18 , whereby the drive voltage is superposed on the bias voltage and the ink droplets R are ejected to record an image on the recording medium P. At this time, the conveyor belt  68  is provided with heating means to increase the temperature of the recording medium P, thus promoting fixation of the ink droplets R on the recording medium P and further suppressing ink bleeding, which leads to improvement in image quality. 
     Image recording using the head unit  80  and the like will be described in detail below. 
     The recording medium P on which the image is formed is discharged by the discharge means  72  and peeled off the conveyor belt  68  by the peeling means  74  before being conveyed to the fixation/conveyance means  76 . 
     In the illustrated embodiment, the discharge means  72  is a so-called AC corotron discharger, which includes a corotron discharger  72   a , an AC power source  72   b , and a DC high voltage power source  72   c  with one end grounded. In addition thereto, various means and methods, for example, a scorotron discharger, a solid-state charger, and an electrostatic discharge needle can be used for discharge. Also, as in the electrostatic attraction means  70  described above, a structure using a conductive roller or a conductive platen can also be preferably utilized. 
     A known technique using a peeling blade, a counter-rotating roller, an air knife or the like is applicable to the peeling means  74 . 
     The recording medium P peeled off the conveyor belt  68  is sent to the fixation/conveyance means  76  where the image formed by means of the ink jet recording is fixed. A pair of rollers composed of a heat roller  76   a  and a conveying roller  76   b  is used as the fixation/conveyance means  76  to heat and fix the recorded image while nipping and conveying the recording medium P. 
     The recording medium P on which the image is fixed is guided by the guide  78  and delivered to a delivered paper tray (not shown). 
     In addition to the heat roll fixation described above, examples of the heat fixation means include irradiation with infrared rays or using a halogen lamp or a xenon flash lamp, and general heat fixation such as hot air fixation using a heater. Further, in the fixation/conveyance means  76 , it is also possible that the heating means is used only for heating, and the conveyance means and the heat fixation means are provided separately. 
     It should be noted that in the case of heat fixation, when a sheet of coated paper or laminated paper is used as the recording medium P, there is a possibility of causing a phenomenon called “blister” in which irregularities are formed on the sheet surface since moisture inside the sheet abruptly evaporates due to rapid temperature increase. To avoid this, it is preferable that a plurality of fixing devices be arranged, and at least one of power supply to the respective fixing devices and a distance from the respective fixing devices to the recording medium P be changed such that the temperature of the recording medium P gradually increases. 
     The printer  60  is preferably constructed such that no components will contact the image recording surface of the recording medium P at least during a time from the image recording with the head unit  80  until the completion of fixation with the fixation/conveyance means  76 . 
     Further, the movement speed of the recording medium P at the time of fixation with the fixation/conveyance means  76  is not particularly limited, which may be the same as, or different from, the conveying speed by the conveyor belt  68  at the time of image formation. When the movement speed is different from the conveying speed at the time of image formation, it is also preferable to provide a speed buffer for the recording medium P immediately before the fixation/conveyance means  76 . 
     Image recording using the printer  60  will be described below in detail. 
     As described above, the image recording means of the printer  60  includes the head unit  80  for ejecting ink, the ink circulation system  82  that supplies the ink Q to the head unit  80  and recovers the ink Q from the head unit  80 , the head driver  84  that drives the head unit  80  based on an output image signal from a not-shown external apparatus such as a computer or a raster image processor (RIP), and the recording medium position detection means  86  for detecting the recording medium P in order to determine an image recording position on the recording medium P. 
       FIG. 7B  is a schematic perspective view showing the head unit  80  and the conveyor means for the recording medium P on the periphery thereof. 
     The head unit  80  includes four ink jet heads  80   a  for four colors of cyan (C), magenta (M), yellow (Y), and black (K) for recording a full-color image, and records an image on the recording medium P transported by the conveyor belt  68  at a predetermined speed by ejecting the ink Q supplied by the ink circulation system  82  as ink droplets R in accordance with signals from the head driver  84  to which image data was supplied. The ink jet heads  80   a  for the respective colors are arranged along a traveling direction of the conveyor belt  68 . 
     Note that the ink jet head  80   a  for each color in the head unit  80  is the ink jet head of the present invention. 
     In the illustrated embodiment, each of the ink jet heads  80   a  is a line head including ejection ports  28  disposed in the entire area in the width direction of the recording medium P. The ink jet head  80   a  is preferably a multi-channel head as shown in  FIG. 2A , which has multiple nozzle lines, arranged in a staggered shape. 
     Therefore, in the illustrated embodiment, while the recording medium P is held on the conveyor belt  68 , the recording medium P is conveyed to pass over the head unit  80  once. In other words, scanning and conveyance are performed only once for the head unit  80 . Then, an image is formed on the entire surface of the recording medium P. Therefore, image recording (drawing) at a higher speed is possible compared to serial scanning of the ejection head. 
     Note that the ink jet head of the present invention is also applicable to a so-called serial head (shuttle type head), and therefore the printer  60  may take this configuration. 
     In this case, the head unit  80  is structured such that a line (which may have a single line or multi channel structure) of the ejection ports  28  for each ink jet head agrees with the conveying direction of the conveyor belt  68 , and the head unit  80  is provided with known scanning means which scans the head unit  80  in a direction perpendicular to the direction in which the recording medium P is conveyed. 
     Image recording may be performed as in a usual shuttle type ink jet printer. In accordance with the length of the line of the ejection ports  28 , the recording medium P is conveyed intermittently by the conveyor belt  68 , and in synchronization with this intermittent conveying, the head unit  80  is scanned when the recording medium is at rest, whereby an image is formed on the entire surface of the recording medium P. 
     As described above, the image formed by the head unit  80  on the entire surface of the recording medium P is then fixed by the fixation/conveyance means  76  while the recording medium P is nipped and conveyed by the fixation/conveyance means  76 . 
     The head driver  84  receives image data from a system control portion (not shown) that receives image data from an external apparatus and performs various processing on the image data, and drives the head unit  80  based on the image data. 
     The system control portion color-separates the image data received from the external apparatus such as a computer, an RIP, an image scanner, a magnetic disk apparatus, or an image data transmission apparatus. The system control portion then performs division computation into an appropriate number of pixels and an appropriate number of gradations to generate image data with which the head driver  84  can drive the head unit  80  (ink jet head). Also, the system control portion controls timings of ink ejection by the head unit  80  in accordance with conveyance timings of the recording medium P by the conveyor belt  68 . The ejection timings are controlled using an output from the recording medium position detection means  86  or an output signal from an encoder arranged for the conveyor belt  68  or a drive means of the conveyor belt  68 . 
     The recording medium position detecting means  86  detects the recording medium P being fed to a position at which an ink droplet is ejected onto the medium P from the head unit  80 , and known detecting means such as photo sensor can be used. 
     Here, when the number of the ejection portions to be controlled (the number of channels) is large as in the case where a line head is used, the head driver  84  may separate rendering to employ a known method such as resistance matrix type drive method or resistance diode matrix type drive method. Thus, it is possible to reduce the number of ICs used in the head driver  84  and suppress the size of a control circuit while lowering costs. 
     The ink circulating system  82  allows each ink Q to flow in the main ink flow path  30  (see  FIG. 1A ) of the corresponding ink jet head  80   a  of the head unit  80 . For each of the ink of the four colors (C, M, Y, K), the ink circulating system  82  includes: an ink circulating device  82   a  having an ink tank, a pump, a replenishment ink tank (not shown), etc.; an ink supply system  82   b  for supplying the ink Q of each color from the ink tank of the ink circulating device  82   a  to the main ink flow path  30  of each ink jet head  80   a  of the head unit  80 ; and an ink recovery system  82   c  for recovering the ink Q from the main ink flow path  30  of each ink jet head  80   a  of the head unit  80  into the ink circulating device  82   a.    
     An arbitrary system may be used for the ink circulating system  82  as long as this system supplies the ink Q of a color corresponding to each ink jet head  80   a  from the ink tank to the head unit  80  through the ink supply system  82   b  and recovers the ink from each ink jet head  80   a  to the ink tank through the ink recovery system  82   c  to allow ink circulation in a path for returning the ink into the corresponding ink tank. 
     Each ink tank contains the ink Q of the corresponding color and the ink Q is supplied to the head unit  80  by means of a pump. Ejection of the ink from the head unit  80  lowers the concentration of ink circulating in the ink circulating system  82 . Therefore, it is preferable in the ink circulating system  82  that the ink concentration be detected by an ink concentration detecting device and the ink tank be replenished as appropriate with ink from the replenishment ink tank to keep the ink concentration in a predetermined range. 
     Moreover, the ink tank is preferably provided with an agitator for suppressing precipitation/aggregation of solid components of the ink and an ink temperature control device for suppressing ink temperature change. When the ink temperature changes due to ambient temperature change or the like, physical properties of the ink are changed, which causes the dot diameter change. As a result, a high quality image may not be recorded with stability. 
     A rotary blade, an ultrasonic transducer, a circulation pump, or the like may be used for the agitator. 
     The head unit  80 , the ink tank, an ink supply line and other components are provided with a heating element such as a heater or a cooling element such as Peltier element as the ink temperature control device, and any known method, for example, a method in which control is performed with a temperature sensor like a thermostat can be used. When arranged inside the ink tank, the temperature control device is preferably arranged with the agitator such that temperature distribution is kept constant. Then, the agitator for keeping the concentration distribution in the tank constant may double as the agitator for suppressing the precipitation/aggregation of solid components of the ink. 
     As described above, the printer  60  includes solvent collecting means composed of the discharge fan  90  and the solvent collecting device  92 . The solvent collecting means collects the carrier liquid evaporated from the ink droplets ejected on the recording medium P from the head unit  80 , in particular, the carrier liquid evaporated from the recording medium P at the time of fixing the image formed of the ink droplets. 
     The discharge fan  90  sucks air inside the casing  61  of the printer  60  to blow the air to the solvent collecting device  92 . 
     The solvent collecting device  92  is provided with a solvent vapor absorber. This solvent vapor absorber absorbs solvent components of gas containing solvent vapor sucked by the discharge fan  90 , and exhausts the gas whose solvent has been absorbed and collected, to the outside of the casing  61  of the printer  60 . Various active carbons are preferably used as the solvent vapor absorber. 
     While the electrostatic ink jet recording apparatus for recording a color image using the ink of four colors including C, M, Y, and K has been described, the present invention should not be construed restrictively; the apparatus may be a recording apparatus for a monochrome image or an apparatus for recording an image using an arbitrary number of other colors such as pale color ink and special color ink, for example. In such a case, the head units  80  and the ink circulating systems  82  whose number corresponds to the number of ink colors are used. 
     Furthermore, in the above embodiments, the ink jet recording in which the ink droplets R are ejected by positively charging the colorant particles in the ink and charging the recording medium P or the counter electrode on the rear side of the recording medium P to the negative high voltage has been described. However, the present invention is not limited to this. The ink jet image recording may be performed by negatively charging the colorant particles in the ink and charging the recording medium or the counter electrode to the positive high voltage. When the charged color particles have the polarity opposite to that in the above-mentioned case, the applied voltage to the electrostatic attraction means, the counter electrode, the drive electrode of the ink jet head, or the like is changed to have the polarity opposite to that in the above-mentioned case. 
     The ink jet head and the ink jet recording apparatus according to the present invention are not limited to the electrostatic type but can be applied to various ink jet heads and ink jet recording apparatuses of thermal type, piezoelectric type or the like. 
     The ink jet head and the ink jet recording apparatus using the ink jet head according to the present invention have been described in detail, but the present invention is not limited to the above embodiments. It will be obvious that various modifications and changes can be made without departing from the scope of the present invention.