Abstract:
A printhead module includes a plurality of rows of printhead nozzles, at least some of the rows including at least one displaced row portion, the displacement of the row portion including a component in a direction normal to that of a pagewidth to be printed, wherein the displaced row portions of at least some of the rows are different in length than the displaced row portions of at least some of the other rows.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a division of U.S. patent application Ser. No. 14/707,536, filed May 8, 2015, which is a division of U.S. patent application Ser. No. 14/185,262, filed Feb. 20, 2014, which is a division of U.S. patent application Ser. No. 13/346,325, filed Jan. 9, 2012, which is a division of U.S. patent application Ser. No. 12/289,959, filed Nov. 7, 2008, which is a divisional of U.S. patent application Ser. No. 11/125,098, filed May 10, 2005, which is a division of U.S. patent application Ser. No. 10/368,351, filed Feb. 20, 2003, which is a Continuation-in-Part of U.S. patent application Ser. No. 10/305,979, filed Nov. 29, 2002, the disclosures of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The invention relates to an ink jet head for printing by ejecting ink onto a print medium, and to an ink jet printer having the ink-jet head. 
         [0004]    2. Description of Related Art 
         [0005]    In an ink-jet printer, an ink-jet head distributes ink supplied from an ink tank to pressure chambers. The ink-jet head selectively applies pressure to each pressure chamber to eject ink through a nozzle. As a means for selectively applying pressure to the pressure chambers, an actuator unit may be used in which ceramic piezoelectric sheets are laminated. 
         [0006]    As an example, a generally known ink-jet head has one actuator unit in which continuous flat piezoelectric sheets extending over a plurality of pressure chambers are laminated. At least one of the piezoelectric sheets is sandwiched by a common electrode which is common to many pressure chambers and is being kept at the ground potential, and many individual electrodes, i.e., driving electrodes, disposed at positions corresponding to the respective pressure chambers. When a individual electrode on one face of the sheet is set at a potential different from that of the common electrode on the other face, the part of piezoelectric sheet being sandwiched by the individual and common electrodes and polarized in its thickness, is expanded or contracted in its thickness direction as an active layer by the so-called longitudinal piezoelectric effect. This causes the volume of the corresponding pressure chamber to change, so that the ink can be ejected toward a print medium through a nozzle communicating with the pressure chamber. 
         [0007]    In the above-described ink-jet head, to ensure good ink ejection performance, the actuator unit must be accurately positioned to a passage unit so that the individual electrodes must be at predetermined positions corresponding to the respective pressure chambers in a plan view. 
         [0008]    Generally, in an ink-jet head such as the one described above, the passage unit in which ink passages including pressure chambers have been formed is manufactured separately from the actuator unit. The passage unit is then bonded with an adhesive to the actuator unit so that the pressure chambers are close to the actuator unit. This bonding process is done by matching a mark formed on the passage unit against a mark formed on the actuator unit. 
         [0009]    Generally, the piezoelectric sheets of the actuator unit are manufactured through a sintering process while the passage unit is laminated with metallic sheets. Therefore, as the size of the piezoelectric sheets increases, the positional accuracy of the electrodes decreases. Thus, the longer the head is, the more difficult the positioning process is between the pressure chambers in the passage unit and the individual electrodes in the actuator unit. As a result, the manufacturing yield for the printer heads is reduced. 
         [0010]    Furthermore, because the actuator unit it is made of ceramic, it is an expensive and very brittle component. In particular, in the actuator unit having a polygonal shape, the corners can easily brake. The breakage loss causes the manufacture cost to increase. Further, the actuator unit requires very delicate handling to ensure that a corner does not collide against another component. This makes the ink-jet head assembling difficult. 
       SUMMARY OF THE INVENTION 
       [0011]    An objective of the invention is to provide an ink-jet head in which an actuator unit has been accurately positioned relative to a passage unit. 
         [0012]    Another objective of the invention is to provide an ink-jet head having an actuator unit that is difficult to brake. 
         [0013]    According to one aspect of the invention, a printhead module includes a plurality of rows of printhead nozzles, at least some of the rows including at least one displaced row portion, the displacement of the row portion including a component in a direction normal to that of a pagewidth to be printed, wherein the displaced row portions of at least some of the rows are different in length than the displaced row portions of at least some of the other rows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Various exemplary embodiments of the invention will be described in detail with reference to the following figures, in which: 
           [0015]      FIG. 1  is a general view of an ink-jet printer including ink-jet heads according to a first exemplary embodiment of the invention; 
           [0016]      FIG. 2  is a perspective view of an ink-jet head according to a first embodiment of the invention; 
           [0017]      FIG. 3  is a sectional view taken along line in  FIG. 2 ; 
           [0018]      FIG. 4  is a plan view of a head main body included in the ink-jet head of  FIG. 2 ; 
           [0019]      FIG. 5  is an enlarged view of the region enclosed with an alternate long and short dash line in  FIG. 4 ; 
           [0020]      FIG. 6  is an enlarged view of the region enclosed with an alternate long and short dash line in  FIG. 5 ; 
           [0021]      FIG. 7  is a partial sectional view of the head main body of  FIG. 4 ; 
           [0022]      FIG. 8  is an enlarged view of the region enclosed with an alternate long and two short dashes line in  FIG. 5 ; 
           [0023]      FIG. 9  is a partial exploded view of the head main body of  FIG. 4 ; 
           [0024]      FIG. 10  is an enlarged sectional view when laterally viewing the region enclosed with an alternate long and short dash line in  FIG. 7 ; 
           [0025]      FIG. 11  is a plan view of a head main body included in an ink-jet head according to a second exemplary embodiment of the invention; 
           [0026]      FIG. 12  is a bottom view of the head main body of  FIG. 11 ; 
           [0027]      FIG. 13  is a cross-sectional view of the head main body of  FIG. 11 ; 
           [0028]      FIG. 14  is an enlarged view of the region Q enclosed with an alternate long and short dash line in  FIG. 13 ; 
           [0029]      FIG. 15  is a partial sectional view of the head main body of  FIG. 11 ; 
           [0030]      FIG. 16  is an enlarged sectional view illustrating the detailed construction of an actuator unit in the head main body of  FIG. 11 ; 
           [0031]      FIG. 17  is an enlarged plan view of an actuator unit in the head main body of  FIG. 11 ; 
           [0032]      FIG. 18  is an enlarged plan view showing a seam portion between two actuator units of  FIG. 17 ; 
           [0033]      FIG. 19  is an enlarged plan view of an actuator unit according to a modification of a second exemplary embodiment of the invention; 
           [0034]      FIG. 20  is an enlarged plan view showing a seam portion between two actuator units of  FIG. 19 ; 
           [0035]      FIG. 21A  is a plan view of a head main body included in an ink-jet head according to a modification of the invention, in which four actuator units are arranged; 
           [0036]      FIG. 21B  is a plan view of a head main body included in an ink-jet head according to another modification of the invention, in which four actuator units are arranged; 
           [0037]      FIG. 22  is a plan view of a head main body included in an ink-jet head according to a third exemplary embodiment of the invention; 
           [0038]      FIG. 23  is a bottom view of the head main body of  FIG. 22 ; 
           [0039]      FIG. 24  is a cross-sectional view of the head main body of  FIG. 22 ; 
           [0040]      FIG. 25  is an enlarged view of the region E enclosed with an alternate long and short dash line in  FIG. 24 ; 
           [0041]      FIG. 26  is a partial sectional view of the head main body of  FIG. 22 ; 
           [0042]      FIG. 27  is an enlarged sectional view illustrating the detailed construction of an actuator unit in the head main body of  FIG. 22 ; 
           [0043]      FIG. 28A  is a schematic view illustrating the profile of an actuator unit included in the head main body of  FIG. 22 ; 
           [0044]      FIG. 28B  is a schematic view illustrating the profile of an actuator unit as a modification; 
           [0045]      FIG. 29A  is a plan view of a modification of the head main body of  FIG. 22 , which includes heptagonal actuator units; 
           [0046]      FIG. 29B  is a plan view of an actuator unit included in the head main body of  FIG. 29A ; 
           [0047]      FIG. 30A  is a plan view of another modification of the head main body of  FIG. 22 , which includes octagonal actuator units; 
           [0048]      FIG. 30B  is a plan view of an actuator unit included in the head main body of  FIG. 30A ; 
           [0049]      FIG. 31A  is a plan view of still another modification of the head main body of  FIG. 22 , which includes partially rounded actuator units; 
           [0050]      FIG. 31B  is a plan view of an actuator unit included in the head main body of  FIG. 31A ; and 
           [0051]      FIG. 32  is a schematic view of a principal part of an ink-jet printer according to the fourth exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0052]    With reference to  FIGS. 1 to 10 , an ink-jet head will be described as a reference for understanding ink-jet heads according to various exemplary embodiments of the invention.  FIG. 1  is a general view of an ink-jet printer having ink-jet heads according to a first exemplary embodiment of the invention. The ink-jet printer  101  shown in  FIG. 1  is a color ink-jet printer having four ink-jet heads  1 . In this printer  101 , an image recording medium feed unit  111  and an image recording medium discharge unit  112  are disposed in left and right portions of  FIG. 1 , respectively. 
         [0053]    In the printer  101 , an image recording medium transfer path is provided extending from the image recording medium feed unit  111  to the image recording medium discharge unit  112 . A pair of feed rollers  105   a  and  105   b  is disposed immediately downstream of the image recording medium feed unit  111  for pinching and advancing an image record medium sheet, such as a paper. In various exemplary embodiments, the image recording medium includes, for example, a sheet of paper, card stock, photo paper, a transparency, or the like. 
         [0054]    The image recording medium is transferred by the pair of feed rollers  105   a  and  105   b  from the left to the right in  FIG. 1 . In the middle of the image recording medium transfer path, two belt rollers  106  and  107  and an endless transfer belt  108  are disposed. The transfer belt  108  is wound on the belt rollers  106  and  107  to extend between them. The outer face, i.e., the transfer face, of the transfer belt  108  has been treated with silicone or like material. Thus, an image recording medium fed through the pair of feed rollers  105   a  and  105   b  can be held on the transfer face of the transfer belt  108  by the adhesion of the silicone treated face. In this state, the image recording medium is transferred downstream (rightward) by driving one belt roller  106  to rotate clockwise in  FIG. 1  (the direction indicated by an arrow  104 ). 
         [0055]    Pressing members  109   a  and  109   b  are disposed at positions for feeding an image recording medium onto the belt roller  107  and taking out the image recording medium from the belt roller  106 , respectively. Either of the pressing members  109   a  and  109   b  can be for pressing the image recording medium onto the transfer face of the transfer belt  108  so as to prevent the image recording medium from separating from the transfer face of the transfer belt  108 . Thus, the image recording medium securely adheres to the transfer face. 
         [0056]    A peeling device  110  is provided immediately downstream of the transfer belt  108  along the image recording medium transfer path. The peeling device  110  peels off the image recording medium, which has adhered to the transfer face of the transfer belt  108 , from the transfer face to transfer the image recording medium toward the rightward image recording medium discharge unit  112 . 
         [0057]    Each of the four ink-jet heads  1  has, at its lower end, a head main body  1   a . Each head main body  1   a  has a rectangular section. The head main bodies  1   a  are arranged close to each other with the longitudinal axis of each head main body  1   a  being perpendicular to the image recording medium transfer direction (perpendicular to  FIG. 1 ). That is, this printer  101  is a line type printer. The bottom of each of the four head main bodies  1   a  faces the image recording medium transfer path. In the bottom of each head main body  1   a , a number of nozzles are provided, each having a small-diameter ink ejection port. The four head main bodies  1   a  eject ink of magenta, yellow, cyan, and black, respectively. However, various other embodiments of the invention are not limited by the above described colors or order. 
         [0058]    The head main bodies  1   a  are disposed such that a narrow clearance must be formed between the lower face of each head main body  1   a  and the transfer face of the transfer belt  108 . The image recording medium transfer path is formed within the narrow clearance. In this construction, while an image recording medium that is being transferred by the transfer belt  108  passes immediately below the four head main bodies  1   a  in order, the inks are ejected through the corresponding nozzles toward the upper face, i.e., the print face, of the image recording medium to form a desired color image on the image recording medium. 
         [0059]    The ink-jet printer  101  is provided with a maintenance unit  117  for automatically carrying out maintenance of the ink jet heads  1 . The maintenance unit  117  includes four caps  116  for covering the lower faces of the four head main bodies  1   a , and a purge system (not shown). 
         [0060]    During ink-jet printer  101  operation, the maintenance unit  117  is at a position immediately below the image recording medium feed unit  117  (withdrawal position). When a predetermined condition is satisfied after finishing the printing operation (for example, when a state in which no printing operation is performed continues for a predetermined time period or when the printer  101  is powered off), the maintenance unit  117  moves to a position (cap position) immediately below the four head main bodies  1   a . At this cap position, the maintenance unit  117  covers the lower faces of the head main bodies  1   a  with the respective caps  116  to prevent ink in the nozzles from becoming dry. 
         [0061]    The belt rollers  106  and  107  and the transfer belt  108  are supported by a chassis  113 . The chassis  113  is put on a cylindrical member  115  disposed under the chassis  113 . The cylindrical member  115  is rotatable around a shaft  114  provided at an off center position of the cylindrical member  115 . Thus, by rotating the shaft  114 , the level of the uppermost portion of the cylindrical member  115  can be changed to move up or down the chassis  113  accordingly. When the maintenance unit  117  is moved from the withdrawal position to the cap position, the cylindrical member  115  must have been rotated at a predetermined angle in advance so as to move down the transfer belt  108  and the belt rollers  106  and  107  by an applicable distance from the position illustrated in  FIG. 1 . A space for the movement of the maintenance unit  117  is thereby ensured. 
         [0062]    In the region surrounded by the transfer belt  108 , a nearly rectangular global change guide  121  (having its width substantially equal to that of the transfer belt  108 ) is disposed at an opposite position to the ink-jet heads  1 . The guide  121  is in contact with the lower face of the upper part of the transfer belt  108  to support the upper part of the transfer belt  108  from the inside. 
         [0063]    With reference to  FIGS. 2 and 3 , the construction of each ink jet head  1  according to this embodiment will be described in more detail. The ink-jet head  1  according to this embodiment includes a head main body  1   a  having a rectangular shape in a plan view and extending in a main scanning direction, and a base portion  131  for supporting the head main body  1   a . The base portion  131  further supports driver ICs  132  for supplying driving signals to individual electrodes  35   a  and  35   b  (shown in  FIG. 6  and  FIG. 10 ), and substrates  133 . 
         [0064]    Referring to  FIG. 2 , the base portion  131  includes a base block  138  partially bonded to the upper face of the head main body  1   a  to support the head main body  1   a , and a holder  139  bonded to the upper face of the base block  138  to support the base block  138 . The base block  138  is a nearly rectangular member having substantially the same length of the head main body  1   a . The base block  138  is made of metal material such as stainless steel and functions as a light structure for reinforcing the holder  139 . The holder  139  includes a holder main body  141  disposed near the head main body  1   a , and a pair of holder support portions  142  each extending on the opposite side of the holder main body  141  to the head main body  1   a . Each holder support portion  142  is configured as a flat member. The holder support portions  142  extend along the longitudinal direction of the holder main body  141  and are disposed in parallel with each other at a predetermined interval. 
         [0065]    Skirt portions  141   a  in a pair, protruding downward, are provided in both end portions of the holder main body  141   a  in a direction perpendicular to the main scanning direction. Each skirt portion  141   a  is formed through the length of the holder main body  141 . As a result, in the lower portion of the holder main body  141 , a nearly rectangular groove  141   b  is defined by the pair of skirt portions  141   a . The base block  138  is received in the groove  141   b . The upper surface of the base block  138  is bonded to the bottom of the groove  141   b  of the holder main body  141  with an adhesive. The thickness of the base block  138  is slightly larger than the depth of the groove  141   b  of the holder main body  141 . As a result, the lower end of the base block  138  protrudes downward beyond the skirt portions  141   a.    
         [0066]    Within the base block  138 , as a passage for ink to be supplied to the head main body  1   a , an ink reservoir  3  is formed as a nearly rectangular space or hollow region extending along the longitudinal direction of the base block  138 . Openings  3   b  (see  FIG. 4 ) are formed in the lower face  145  of the base block  138 , each communicating with the ink reservoir  3 . The ink reservoir  3  is connected with a not-illustrated main ink tank or ink supply source through a supply tube (not shown) within the printer main body. Thus, the ink reservoir  3  is appropriately supplied with ink from the main ink tank. 
         [0067]    In the lower face  145  of the base block  138 , the surrounding of each opening  3   b  protrudes downward from the surrounding portion. The base block  138  is in contact with a passage unit  4  (see  FIG. 3 ) of the head main body  1   a  at the only vicinity portion  145   a  of each opening  3   b  of the lower face  145 . Thus, the region of the lower face  145  of the base block  138  other than the vicinity portion  145   a  of each opening  3   b  is distant from the head main body  1   a . Actuator units  21  are disposed within the distance. 
         [0068]    To the outer side face of each holder support portion  142  of the holder  139 , a driver IC  132  is attached with an elastic member  137  such as a sponge being interposed between them. A heat sink  134  is disposed in close contact with the outer side face of the driver IC  132 . The heat sink  134  is made of a nearly rectangular member for efficiently radiating heat generated in the driver IC  132 . A flexible printed circuit (FPC)  136 , which acts as a power supply member, is connected to the driver IC  132 . The FPC  136  connected with the driver IC  132  is bonded to, and electrically connected with, the corresponding substrate  133  and the head main body  1   a  by soldering. The substrate  133  is disposed outside the FPC  136  above the driver IC  132  and the heat sink  134 . The upper face of the heat sink  134  is bonded to the substrate  133  with a seal member  149 . The lower face of the heat sink  134  is also bonded to the FPC  136  with a seal member  149 . 
         [0069]    A seal member  150  is disposed between the lower face of each skirt portion  141   a  of the holder main body  141  and the upper face of the passage unit  4 , to sandwich the FPC  136 . The FPC  136  is fixed to the passage unit  4  and the holder main body  141  by the seal member  150 . Therefore, even if the head main body  1   a  is elongated, the head main body  1   a  can be prevented from bending, the interconnecting portion between each actuator unit and the FPC  136  can be prevented from being stressed, and the FPC  136  can be securely held in place. 
         [0070]    Referring to  FIG. 2 , near each lower corner of the ink-jet head  1  along the main scanning direction, six protruding portions  30   a  are disposed at regular intervals along the corresponding side wall of the ink-jet head  1 . These protruding portions  30   a  are provided at both ends in the sub scanning direction of a nozzle plate  30  in the lowermost layer of the head main body  1   a  (see  FIGS. 7A and 7B ). The nozzle plate  30  is bent by about 90 degrees along the boundary line between each protruding portion  30   a  and the other portion. The protruding portions  30   a  are provided at positions corresponding to the vicinities of both ends of various image recording mediums to be used for printing. Each bent portion of the nozzle plate  30  has a shape not right-angled but rounded. This configuration makes it difficult for an image recording medium to jam, which typically occurs in known devices because the leading edge of the image recording medium, which has been transferred to approach the head  1 , is stopped by the side face of the head  1 . 
         [0071]      FIG. 4  is a schematic plan view of the head main body  1   a . In  FIG. 4 , an ink reservoir  3  formed in the base block  138  is conceptually illustrated with a broken line. Referring to  FIG. 4 , the head main body  1   a  has a rectangular shape in the plan view extending in the main scanning direction. The head main body  1   a  includes a passage unit  4  in which a large number of pressure chambers  10  and a large number of ink ejection ports  8  at the front ends of nozzles (see  FIGS. 5, 6, and 7 ), are formed as described later. Trapezoidal actuator units  21  arranged in two lines in a crisscross manner are bonded onto the upper face of the passage unit  4 . Each actuator unit  21  is disposed such that its parallel opposed sides (upper and lower sides) extend along the longitudinal direction of the passage unit  4 . The oblique sides of each neighboring actuator units  21  overlap each other in the lateral direction of the passage unit  4 . 
         [0072]    The lower face of the passage unit  4  corresponding to the bonded region of each actuator unit  4  is made into an ink ejection region. In the surface of each ink ejection region, a large number of ink ejection ports  8  are arranged in a matrix, as described later. In the base block  138  disposed above the passage unit  4 , an ink reservoir  3  is formed along the longitudinal direction of the base block  138 . The ink reservoir  3  communicates with an ink tank (not shown) through an opening  3   a  provided at one end of the ink reservoir  3 , so that the ink reservoir  3  is always filled up with ink. In the ink reservoir  3 , pairs of openings  3   b  are provided in regions where no actuator unit  21  is present, so as to be arranged in a crisscross manner along the longitudinal direction of the ink reservoir  3 . 
         [0073]      FIG. 5  is an enlarged view of the region enclosed with an alternate long and short dash line in  FIG. 4 . Referring to  FIGS. 4 and 5 , the ink reservoir  3  communicates through each opening  3   b  with a manifold channel  5  disposed under the opening  3   b . Each opening  3   b  is provided with a filter (not shown) for catching dust and dirt contained in ink. The front end portion of each manifold channel  5  branches into two sub-manifold channels  5   a . Below each single actuator unit  21 , two sub-manifold channels  5   a  extend from each of the two openings  3   b  on both sides of the actuator unit  21  in the longitudinal direction of the ink-jet head  1 . That is, below the single actuator unit  21 , four sub-manifold channels  5   a  in total extend along the longitudinal direction of the ink-jet head  1 . Each sub-manifold channel  5   a  is filled up with ink supplied from the ink reservoir  3 . 
         [0074]      FIG. 6  is an enlarged view of the region enclosed with an alternate long and short dash line in  FIG. 5 . Referring to  FIGS. 5 and 6 , on the upper face of each actuator unit  21 , individual electrodes  35   a , each having a nearly rhombic shape in a plan view, are regularly arranged in a matrix. In addition, individual electrodes  35   b  having the same shape as the individual electrodes  35   a  are disposed in the actuator unit  21  to vertically overlap the respective individual electrodes  35   a . A large number of ink ejection ports  8  are regularly arranged in a matrix in the surface of the ink ejection region corresponding to the actuator unit  21  of the passage unit  4 . In the passage unit  4 , pressure chambers (cavities)  10 , each having a nearly rhombic shape in a plan view but somewhat larger than that of the individual electrodes  35   a  and  35   b , are regularly arranged in a matrix. In the passage unit  4 , apertures  12  are also regularly arranged in a matrix. These pressure chambers  10  and apertures  12  communicate with the corresponding ink ejection ports  8 . The pressure chambers  10  are provided at positions corresponding to the respective individual electrodes  35   a  and  35   b . In a plan view, the large part of the individual electrode  35   a  and  35   b  is included in a region of the corresponding pressure chamber  10 . In  FIGS. 5 and 6 , for ease of understanding, the pressure chambers  10 , the apertures  12 , etc., are illustrated with solid lines, although they should be illustrated with broken lines because they are within the actuator unit  21  or the passage unit  4 . 
         [0075]      FIG. 7  is a partial sectional view of the head main body  1   a  of  FIG. 4  along the longitudinal direction of a pressure chamber. As shown in  FIG. 7 , each ink ejection port  8  is formed at the front end of a tapered nozzle. Each ink ejection port  8  communicates with a sub-manifold channel  5   a  through a pressure chamber  10  (length: 900 μm, width: 350 μm) and an aperture  12 . Thus, within the ink-jet head  1 , ink passages  32 , each extending from an ink tank to an ink ejection port  8  through an ink reservoir  3 , a manifold channel  5 , a sub-manifold channel  5   a , an aperture  12 , and a pressure chamber  10  are formed. 
         [0076]    Referring to  FIG. 7 , the pressure chamber  10  and the aperture  12  are provided at different levels. Therefore, in the portion of the passage unit  4  corresponding to the ink ejection region under an actuator unit  21 , an aperture  12  communicating with one pressure chamber  10  can be disposed within the same portion in plan view as a pressure chamber  10  neighboring the pressure chamber  10  communicating with the aperture  12 . As a result, because the pressure chambers  10  can be arranged close to each other at a high density, high resolution image printing can be achieved with an ink-jet head  1  having a relatively small work area. 
         [0077]    In the plane of  FIGS. 5 and 6 , pressure chambers  10  are arranged within an ink ejection region in two directions, i.e., a direction along the longitudinal direction of the ink-jet head  1  (first arrangement direction) and a direction somewhat inclining from the lateral direction of the ink-jet head  1  (second arrangement direction). The first and second arrangement directions form an angle theta θ somewhat smaller than the right angle. The ink ejection ports  8  are arranged at 50 dpi in the first arrangement direction. On the other hand, the pressure chambers  10  are arranged in the second arrangement direction such that the ink ejection region corresponding to one actuator unit  21  include twelve pressure chambers  10 . Therefore, within the whole width of the ink-jet head  1 , in a region of the interval between two ink ejection ports  8  neighboring each other in the first arrangement direction, there are twelve ink ejection ports  8 . At both ends of each ink ejection region in the first arrangement direction (corresponding to an oblique side of the actuator unit  21 ), the above condition is satisfied by making a compensation relation to the ink ejection region corresponding to the opposite actuator unit  21  in the lateral direction of the ink-jet head  1 . Therefore, in the ink-jet head  1 , by ejecting ink droplets in order through a large number of ink ejection ports  8  arranged in the first and second directions with relative movement of an image recording medium along the lateral direction of the ink-jet head  1 , printing at 600 dpi in the main scanning direction can be performed. 
         [0078]    Next, the construction of the passage unit  4  will be described in more detail with reference to  FIG. 8 .  FIG. 8  is a schematic view showing the positional relation among each pressure chamber  10 , each ink ejection port  8 , and each aperture (restricted passage)  12 . Referring to  FIG. 8 , pressure chambers  10  are arranged in lines in the first arrangement direction at predetermined intervals at 50 dpi. Twelve lines of pressure chambers  10  are arranged in the second arrangement direction. As the whole, the pressure chambers  10  are two-dimensionally arranged in the ink ejection region corresponding to one actuator unit  21 . 
         [0079]    The pressure chambers  10  are classified into two types, i.e., pressure chambers  10   a , in each of which a nozzle is connected with the upper acute portion in  FIG. 8 , and pressure chambers  10   b , in each of which a nozzle is connected with the lower acute portion. Pressure chambers  10   a  and  10   b  are arranged in the first arrangement direction to form pressure chamber lines  11   a  and  11   b , respectively. Referring to  FIG. 8 , in the ink ejection region corresponding to one actuator unit  21 , from the lower side of  FIG. 8 , there are disposed two pressure chamber lines  11   a  and two pressure chamber lines  11   b  neighboring the upper side of the pressure chamber lines  11   a . The four pressure chamber lines of the two pressure chamber lines  11   a  and the two pressure chamber lines  11   b  constitute a set of pressure chamber lines. Such a set of pressure chamber lines is repeatedly disposed three times from the lower side in the ink ejection region corresponding to one actuator unit  21 . A straight line extending through the upper acute portion of each pressure chamber in each pressure chamber lines  11   a  and  11   b  crosses the lower oblique side of each pressure chamber in the pressure chamber line neighboring the upper side of that pressure chamber line. 
         [0080]    As described above, when viewing perpendicularly to  FIG. 8 , two first pressure chamber lines  11   a  and two pressure chamber lines  11   b , in which nozzles connected with pressure chambers  10  are disposed at different positions, are arranged alternately to neighbor each other. Consequently, as the whole, the pressure chambers  10  are arranged regularly. On the other hand, nozzles are arranged in a concentrated manner in a central region of each set of pressure chamber lines constituted by the above four pressure chamber lines. Therefore, in case that each four pressure chamber lines constitute a set of pressure chamber lines and such a set of pressure chamber lines is repeatedly disposed three times from the lower side as described above, there is formed a region where no nozzle exists, in the vicinity of the boundary between each neighboring sets of pressure chamber lines, i.e., on both sides of each set of pressure chamber lines constituted by four pressure chamber lines. Wide sub-manifold channels  5   a  extend there for supplying ink to the corresponding pressure chambers  10 . In this ink-jet head, in the ink ejection region corresponding to one actuator unit  21 , four wide sub-manifold channels  5   a  in total are arranged in the first arrangement direction, i.e., one on the lower side of  FIG. 8 , one between the lowermost set of pressure chamber lines and the second lowermost set of pressure chamber lines, and two on both sides of the uppermost set of pressure chamber lines. 
         [0081]    Referring to  FIG. 8 , nozzles communicating with ink ejection ports  8  for ejecting ink are arranged in the first arrangement direction at regular intervals at 50 dpi to correspond to the respective pressure chambers  10  regularly arranged in the first arrangement direction. On the other hand, while twelve pressure chambers  10  are regularly arranged also in the second arrangement direction forming an angle θ with the first arrangement direction, twelve nozzles corresponding to the twelve pressure chambers  10  include ones each communicating with the upper acute portion of the corresponding pressure chamber  10  and ones each communicating with the lower acute portion of the corresponding pressure chamber  10 , as a result, they are not regularly arranged in the second arrangement direction at regular intervals. 
         [0082]    If all nozzles communicate with the same-side acute portions of the respective pressure chambers  10 , the nozzles are regularly arranged also in the second arrangement direction at regular intervals. In this case, nozzles are arranged so as to shift in the first arrangement direction by a distance corresponding to 600 dpi printing resolution per pressure chamber line from the lower side to the upper side of  FIG. 8 . Contrastively in this ink-jet head, because four pressure chamber lines of two pressure chamber lines  11   a  and two pressure chamber lines  11   b  constitute a set of pressure chamber lines and such a set of pressure chamber lines is repeatedly disposed three times from the lower side, the shift of nozzle position in the first arrangement direction per pressure chamber line from the lower side to the upper side of  FIG. 8  is not always the same. 
         [0083]    In the ink-jet head  1 , a band region R will be discussed that has a width (about 508.0 μm) corresponding to 50 dpi in the first arrangement direction and extends perpendicularly to the first arrangement direction. In this band region R, any of twelve pressure chamber lines includes only one nozzle. That is, when such a band region R is defined at an optional position in the ink ejection region corresponding to one actuator unit  21 , twelve nozzles are always distributed in the band region R. The positions of points respectively obtained by projecting the twelve nozzles onto a straight line extending in the first arrangement direction are distant from each other by a distance corresponding to a 600 dpi printing resolution. 
         [0084]    When the twelve nozzles included in one band region R are denoted by ( 1 ) to ( 12 ) in order from one whose projected image onto a straight line extending in the first arrangement direction is the leftmost, the twelve nozzles are arranged in the order of ( 1 ), ( 7 ), ( 2 ), ( 8 ), ( 5 ), ( 11 ), ( 6 ), ( 12 ), ( 9 ), ( 3 ), ( 10 ), and ( 4 ) from the lower side. 
         [0085]    In the thus-constructed ink-jet head  1 , by properly driving active layers in the actuator unit  21 , a character, an figure, or the like, having a resolution of 600 dpi can be formed. That is, by selectively driving active layers corresponding to the twelve pressure chamber lines in order in accordance with the transfer of a print medium, a specific character or figure can be printed on the image recording medium. 
         [0086]    By way of example, a case will be described wherein a straight line extending in the first arrangement direction is printed at a resolution of 600 dpi. First, a case will be briefly described wherein nozzles communicate with the same-side acute portions of pressure chambers  10 . In this case, in accordance with transfer of an image recording medium, ink ejection starts from a nozzle in the lowermost pressure chamber line in  FIG. 8 . Ink ejection is then shifted upward with selecting a nozzle belonging to the upper neighboring pressure chamber line in order. Ink dots are thereby formed in order in the first arrangement direction with neighboring each other at 600 dpi. Finally, all the ink dots form a straight line extending in the first arrangement direction at a resolution of 600 dpi. 
         [0087]    On the other hand, in this ink-jet head, ink ejection starts from a nozzle in the lowermost pressure chamber line  11   a  in  FIG. 8 , and ink ejection is then shifted upward with selecting a nozzle communicating with the upper neighboring pressure chamber line in order in accordance with transfer of a print medium. In this embodiment, however, because the positional shift of nozzles in the first arrangement direction per pressure chamber line from the lower side to the upper side is not always the same, ink dots formed in order in the first arrangement direction in accordance with the transfer of the print medium are not arranged at regular intervals at 600 dpi. 
         [0088]    More specifically, as shown in  FIG. 8 , in accordance with the transfer of the print medium, ink is first ejected through a nozzle ( 1 ) communicating with the lowermost pressure chamber line  11   a  in  FIG. 8  to form a dot row on the print medium at intervals corresponding to 50 dpi (about 508.0 μm). Next, as the print medium is transferred and the straight line formation position has reached the position of a nozzle ( 7 ) communicating with the second lowermost pressure chamber line  11   a , ink is ejected through the nozzle ( 7 ). The second ink dot is thereby formed at a position shifted from the first formed dot position in the first arrangement direction by a distance of six times the interval corresponding to 600 dpi (about 42.3 μm) (about 42.3 μm×6=about 254.0 μm). 
         [0089]    Next, as the print medium is further transferred and the straight line formation position has reached the position of a nozzle ( 2 ) communicating with the third lowermost pressure chamber line  11   b , ink is ejected through the nozzle ( 2 ). The third ink dot is thereby formed at a position shifted from the first formed dot position in the first arrangement direction by a distance of the interval corresponding to 600 dpi (about 42.3 μm). As the print medium is further transferred and the straight line formation position has reached the position of a nozzle ( 8 ) communicating with the fourth lowermost pressure chamber line  11   b , ink is ejected through the nozzle ( 8 ). The fourth ink dot is thereby formed at a position shifted from the first formed dot position in the first arrangement direction by a distance of seven times the interval corresponding to 600 dpi (about 42.3 μm) (about 42.3 μm×7=about 296.3 μm). As the print medium is further transferred and the straight line formation position has reached the position of a nozzle ( 5 ) communicating with the fifth lowermost pressure chamber line  11   a , ink is ejected through the nozzle ( 5 ). The fifth ink dot is thereby formed at a position shifted from the first formed dot position in the first arrangement direction by a distance of four times the interval corresponding to 600 dpi (about 42.3 μm) (about 42.3 μm×4=about 169.3 μm). 
         [0090]    After this, in the same manner, ink dots are formed with selecting nozzles communicating with pressure chambers  10  in order from the lower side to the upper side in  FIG. 8 . In this case, when the number of a nozzle in  FIG. 8  is N, an ink dot is formed at a position shifted from the first formed dot position in the first arrangement direction by a distance corresponding to (magnification n=N−1)×(interval corresponding to 600 dpi). When the twelve nozzles have been finally selected, the gap between the ink dots to be formed by the nozzles ( 1 ) in the lowermost pressure chamber lines  11   a  in  FIG. 8  at an interval corresponding to 50 dpi (about 508.0 μm) is filled up with eleven dots formed at intervals corresponding to 600 dpi (about 42.3 μm). Therefore, as the whole, a straight line extending in the first arrangement direction can be drawn at a resolution of 600 dpi. 
         [0091]    Next, the sectional construction of the ink-jet head  1  will be described.  FIG. 9  is a partial exploded view of the head main body  1   a  of  FIG. 4 .  FIG. 10  is an enlarged sectional view when laterally viewing the region enclosed with an alternate long and short dash line in  FIG. 7 . Referring to  FIGS. 7 and 9 , a principal portion on the bottom side of the ink-jet head  1  has a layered structure laminated with ten sheet materials in total, i.e., from the top, an actuator unit  21 , a cavity plate  22 , a base plate  23 , an aperture plate  24 , a supply plate  25 , manifold plates  26 ,  27 , and  28 , a cover plate  29 , and a nozzle plate  30 . Of them, nine plates other than the actuator unit  21  constitute a passage unit  4 . 
         [0092]    As described later in detail, the actuator unit  21  is laminated with five piezoelectric sheets  41  to  45  (see  FIG. 10 ) and is provided with electrodes so that only the uppermost layer and the second layer neighboring the uppermost layer include portions to be active when an electric field is applied (hereinafter, simply referred to as “layer including active layers (active portions)”) and the remaining three layers are inactive. The cavity plate  22  is made of metal, in which a large number of substantially rhombic openings are formed corresponding to the respective pressure chambers  10 . The base plate  23  is made of metal, in which a communication hole between each pressure chamber  10  of the cavity plate  22  and the corresponding aperture  12 , and a communication hole between the pressure chamber  10  and the corresponding ink ejection port  8  are formed. The aperture plate  24  is made of metal, in which, in addition to apertures  12 , communication holes are formed for connecting each pressure chamber  10  of the cavity plate  22  with the corresponding ink ejection port  8 . The supply plate  25  is made of metal, in which communication holes between each aperture  12  and the corresponding sub-manifold channel  5   a  and communication holes for connecting each pressure chamber  10  of the cavity plate  22  with the corresponding ink ejection port  8  are formed. Each of the manifold plates  26 ,  27 , and  28  is made of metal, which defines an upper portion of each sub-manifold channel  5   a  and in which communication holes are formed for connecting each pressure chamber  10  of the cavity plate  22  with the corresponding ink ejection port  8 . The cover plate  29  is made of metal, in which communication holes are formed for connecting each pressure chamber  10  of the cavity plate  22  with the corresponding ink ejection port  8 . The nozzle plate  30  is made of metal, in which tapered ink ejection ports  8  each functioning as a nozzle are formed for the respective pressure chambers  10  of the cavity plate  22 . 
         [0093]    Sheets  21  to  30  are positioned in layers with each other to form such an ink passage  32  as illustrated in  FIG. 6 . The ink passage  32  first extends upward from the sub-manifold channel  5   a , then extends horizontally in the aperture  12 , then further extends upward, then again extends horizontally in the pressure chamber  10 , then extends obliquely downward in a certain length away from the aperture  12 , and then extends vertically downward toward the ink ejection port  8 . 
         [0094]    Referring to  FIG. 10 , the actuator unit  21  includes five piezoelectric sheets  41 ,  42 ,  43 ,  44 , and  45  having the same thickness of about 15 μm. These piezoelectric sheets  41  to  45  are made into a continuous layered flat plate (continuous flat layers) that is disposed so as to extend over many pressure chambers  10  formed within one ink ejection region in the ink-jet head  1 . Because the piezoelectric sheets  41  to  45  are disposed so as to extend over many pressure chambers  10  as continuous flat layers, the individual electrodes  35   a  and  35   b  can also be arranged at a high density by using, e.g., a screen printing technique. Therefore, the pressure chambers  10  formed at positions corresponding to the individual electrodes  35   a  and  35   b  can also be arranged at a high density. This makes it possible to print a high-resolution image. In this embodiment, each of the piezoelectric sheets  41  to  45  is made of a lead zirconate titanate (PZT)-base ceramic material having ferroelectricity. 
         [0095]    Between the uppermost piezoelectric sheet  41  and the piezoelectric sheet  42  neighboring downward the piezoelectric sheet  41 , an about 2 micron-thick common electrode  34   a  is interposed formed on the whole of the lower and upper faces of the piezoelectric sheets. Also, between the piezoelectric sheet  43  neighboring downward the piezoelectric sheet  42  and the piezoelectric sheet  44  neighboring downward the piezoelectric sheet  43 , an about 2 μm-thick common electrode  34   b  is interposed formed like the common electrode  34   a . On the upper face of the piezoelectric sheet  41 , an about 1 μm-thick individual electrode  35   a  is formed to correspond to each pressure chamber  10  (see  FIG. 6 ). The individual electrode  35   a  has a similar shape (length: 850 μm, width: 250 μm) to that of the pressure chamber  10  in a plan view, so that a projection image of the individual electrode  35   a  projected along the thickness direction of the individual electrode  35   a  is included in the corresponding pressure chamber  10 . Further, between the piezoelectric sheets  42  and  43 , an about 2 micron-thick individual electrode  35   b  is interposed formed like the individual electrode  35   a . No electrode is provided between the piezoelectric sheet  44  neighboring downward the piezoelectric sheet  43  and the piezoelectric sheet  45  neighboring downward the piezoelectric sheet  44 , and on the lower face of the piezoelectric sheet  45 . Each of the electrodes  34   a ,  34   b ,  35   a , and  35   b  is made of, e.g., a silver-palladium (Ag—Pd)-base metallic material. 
         [0096]    The common electrodes  34   a  and  34   b  are grounded in a region (not shown). Thus, the common electrodes  34   a  and  34   b  are kept at the ground potential at a region corresponding to any pressure chamber  10 . The individual electrodes  35   a  and  35   b  in each pair corresponding to a pressure chamber  10  are in contact with leads (not shown) wired within the FPC  136  independently of another pair of individual electrodes so that the potential of each pair of individual electrodes can be controlled independently of that of another pair. The individual electrodes  35   a  and  35   b  are connected to the driver IC  132  through the leads. In this case, the individual electrodes  35   a  and  35   b  in each pair vertically arranged may be connected to the driver IC  132  through the same lead. In a modification, many pairs of common electrodes  34   a  and  34   b  each having a shape larger than that of a pressure chamber  10  so that the projection image of each common electrode projected along the thickness direction of the common electrode may include the pressure chamber, may be provided for each pressure chamber  10 . In another modification, many pairs of common electrodes  34   a  and  34   b  each having a shape somewhat smaller than that of a pressure chamber  10  so that the projection image of each common electrode projected along the thickness direction of the common electrode may be included in the pressure chamber, may be provided for each pressure chamber  10 . Thus, the common electrode  34   a  or  34   b  may not always be a single conductive sheet formed on the whole of the face of a piezoelectric sheet. In the above modifications, however, all the common electrodes must be electrically connected with one another so that the portion corresponding to any pressure chamber  10  may be at the same potential. 
         [0097]    In the ink-jet head  1 , the piezoelectric sheets  41  to  45  are polarized in their thickness direction. That is, the actuator unit  21  has a so-called unimorph structure in which the upper (i.e., distant from the pressure chamber  10 ) three piezoelectric sheets  41  to  43  are layers wherein active layers are present, and the lower (i.e., near the pressure chamber  10 ) two piezoelectric sheets  44  and  45  are made into inactive layers. Therefore, when the individual electrodes  35   a  and  35   b  in a pair are set at a positive or negative predetermined potential, if the polarization is in the same direction as the electric field for example, the electric field-applied portion in the piezoelectric sheets  41  to  43  sandwiched by the common and individual electrodes works as an active layer (pressure generation portion) and contracts perpendicularly to the polarization by the transversal piezoelectric effect. On the other hand, because the piezoelectric sheets  44  and  45  are not influenced by an electric field, they do not contract in themselves. Thus, a difference in strain perpendicular to the polarization is produced between the upper piezoelectric sheets  41  to  43  and the lower piezoelectric sheets  44  and  45 . As a result, the whole of the piezoelectric sheets  41  to  45  is ready to deform into a convex shape toward the inactive side (unimorph deformation). At this time, as illustrated in  FIG. 10 , the lowermost face of the piezoelectric sheets  41  to  45  is fixed to the upper face of the partition (the cavity plate)  22  partitioning pressure chambers, as a result, the piezoelectric sheets  41  to  45  deform into a convex shape toward the pressure chamber side. Therefore, the volume of the pressure chamber  10  is decreased to raise the pressure of ink. The ink is thereby ejected through the ink ejection port  8 . After this, when the individual electrodes  35   a  and  35   b  are returned to the same potential as that of the common electrodes  34   a  and  34   b , the piezoelectric sheets  41  to  45  return to the original shape and the pressure chamber  10  also returns to its original volume. Thus, the pressure chamber  10  draws ink through the manifold channel  5 . 
         [0098]    In another driving method, all the individual electrodes  35   a  and  35   h  are set in advance at a different potential from that of the common electrodes  34   a  and  34   b . When an ejecting request is issued, the corresponding pair of individual electrodes  35   a  and  35   b  is once set at the same potential as that of the common electrodes  34   a  and  34   b . After this, at a predetermined timing, the pair of individual electrodes  35   a  and  35   b  is again set at a potential different from that of the common electrodes  34   a  and  34   b . In this case, at the timing when the pair of individual electrodes  35   a  and  35   b  is set at the same potential as that of the common electrodes  34   a  and  34   b , the piezoelectric sheets  41  to  45  return to their original shapes. The corresponding pressure chamber  10  is thereby increased in volume from its initial state (the state that the potentials of both electrodes differ from each other), to draw ink from the manifold channel  5  into the pressure chamber  10 . After this, at the timing when the pair of individual electrodes  35   a  and  35   b  is again set at the different potential from that of the common electrodes  34   a  and  34   b , the piezoelectric sheets  41  to  45  deform into a convex shape toward the pressure chamber  10 . The volume of the pressure chamber  10  is thereby decreased and the pressure of ink in the pressure chamber  10  increases to eject the ink. 
         [0099]    On the other hand, in case where the polarization occurs in the reverse direction to the electric field applied to the piezoelectric sheets  41  to  43 , the active layers in the piezoelectric sheets  41  and  42  sandwiched by the individual electrodes  35   a  and  35   h  and the common electrodes  34   a  and  34   b  are ready to elongate perpendicularly to the polarization by the transversal piezoelectric effect. As a result, the piezoelectric sheets  41  to  45  deform into a concave shape toward the pressure chamber  10 . Therefore, the volume of the pressure chamber  10  is increased to draw ink from the manifold channel  5 . After this, when the individual electrodes  35   a  and  35   b  return to their original potential, the piezoelectric sheets  41  to  45  also return to their original flat shape. The pressure chamber  10  thereby returns to its original volume to eject the ink through the ink ejection port  8 . 
         [0100]    Next, a manufacturing method of the ink-jet head  1  will be described. 
         [0101]    To manufacture the ink-jet head  1 , the passage unit  4  and each of the actuator units  21  are separately manufactured and then both are bonded to each other. To manufacture the passage unit  4 , each plate  22  to  30  forming the passage unit  4  is subjected to etching using a patterned photoresist as a mask, to form openings illustrated in  FIGS. 7 and 9  in the respective plates  22  to  30 . Next, the nine plates  22  to  30  are placed in layers with adhesives being interposed so as to form therein ink passages  32 . The nine plates  22  to  30  are thereby bonded to each other to form a passage unit  4 . 
         [0102]    To manufacture each actuator unit  21 , a conductive paste to be individual electrodes  35   b  is first printed in a pattern on a ceramic green sheet to be a piezoelectric sheet  43 . In parallel with this, conductive pastes to be common electrodes  34   a  and  34   b  are printed in a pattern on ceramic green sheets to be piezoelectric sheets  42  and  44 . After this, five green sheets to be piezoelectric sheets  41  to  45  are positioned in layers with a jig. The layered structure obtained is then baked at a predetermined temperature. After this, individual electrodes  35   a  are formed on the piezoelectric sheet  41  of the baked layered structure. For example, the individual electrodes  35   a  may be formed in the manner that a conductive film is plated on the whole of one surface of the piezoelectric sheet  41  and then unnecessary portions of the conductive film are removed by laser patterning. Alternatively, the individual electrodes  35   a  may be formed by depositing a conductive film on the piezoelectric sheet  41  by PVD (Physical Vapor Deposition) using a mask having openings at portions corresponding to the respective individual electrodes  35   a . To this process, the manufacture of the actuator unit  21  is completed. 
         [0103]    Next, the actuator unit  21  manufactured as described above is bonded to the passage unit  4  with an adhesive so that the piezoelectric sheet  45  may be in contact with the cavity plate  22 . At this time, both are bonded to each other based of positioning marks formed on the surface of the cavity plate  22  of the passage unit  4  and the surface of the piezoelectric sheet  41 , respectively. 
         [0104]    After this, through-holes used for connecting vertically arranged corresponding individual electrodes  35   a  and  35   b  with each other are formed. The through-holes are then filled up with a conductive material. After this, an FPC  136 , used for supplying electric signals to the individual electrodes  35   a  and  35   b  and the common electrodes  34   a  and  34   b , is bonded onto and electrically connected with bonding positions corresponding to the respective electrodes on the actuator unit  21  by soldering. Further, through a predetermined process, the manufacture of the ink-jet head  1  is completed. 
         [0105]    As described above, unlike the other electrodes, individual electrodes  35   a  to be the piezoelectric sheets  41  to  45  are not baked together with the ceramic materials. The reason for this is because the individual electrodes  35   a  are exposed, they are apt to evaporate at a high temperature upon baking. As a result, it is difficult to control their thickness in comparison with the other electrodes  34   a ,  34   b , and  35   b  being covered with ceramic materials. However, even the thickness of the other electrodes  34   a ,  34   b , and  35   b  may somewhat decrease upon baking. Therefore, it is difficult to form them into a small thickness if keeping the continuity after baking is taken into consideration. Contrastively, because the individual electrodes  35   a  are formed by the above-described technique after baking, they can be formed into a smaller thickness than the other electrodes  34   a ,  34   b , and  35   b . Thus, in the ink-jet head  1 , by forming the individual electrodes  35   a  in the uppermost layer to have smaller thickness than the thickness of the other electrodes  34   a ,  34   b , and  35   b , the deformation of the piezoelectric sheets  41  to  43  including active layers is difficult to be restricted by the individual electrodes  35   a . Therefore, the electrical efficiency and the area efficiency of the actuator unit  21  are improved. 
         [0106]    In the ink-jet head  1 , because the piezoelectric sheets  41  to  43  having active layers and the piezoelectric sheets  44  and  45  as the inactive layers are made of the same material, the material need not be changed in the manufacturing process. Thus, they can be manufactured through a relatively simple process, which may reduce the manufacturing cost. Furthermore, because each of the piezoelectric sheets  41  to  43  including active layers and the piezoelectric sheets  44  and  45  as the inactive layers has substantially the same thickness, a further reduction of cost can be achieved by simplifying the manufacturing process. This is because the thickness control can be more easily performed when the ceramic materials to be the piezoelectric sheets are applied to be put in layers. 
         [0107]    Furthermore, in the ink-jet head  1 , separate actuator units  21  corresponding to the respective ink ejection regions are bonded onto the passage unit  4 , and are arranged along the longitudinal direction of the passage unit  4 . Therefore, each of the actuator units  21 , which may be uneven in dimensional accuracy and in positional accuracy of the individual electrodes  35   a ,  35   b  because they are formed by sintering or the like, can be positioned to the passage unit  4  independently from another actuator unit  21 . Thus, even in case of a long head, the increase in shift of each actuator unit  21  from the accurate position on the passage unit  4  is controlled, and both can accurately be positioned to each other. Therefore, even for individual electrodes  35   a ,  35   b  that are relatively apart from a mark, the individual electrodes  35   a  and  35   b  can not be shifted considerably from the predetermined position to the corresponding pressure chamber  10 . Thus results in good ink ejection performance and an improved manufacture yield of the ink-jet heads  1 . 
         [0108]    In contrast to the above, if a long-shaped actuator unit  4  is made like the passage unit  4 , the more the individual electrodes  35   a  and  35   b  are apart from the mark, the larger the shift of the individual electrodes  35   a  and  35   b  is from the predetermined position on the corresponding pressure chamber  10  in a plan view when the actuator unit  21  is laid over the passage unit  4 . This causes, the ink ejection performance of a pressure chamber  10  to deteriorate, which also decreases the ink ejection performance of the ink-jet head  1 . 
         [0109]    In addition, in the ink-jet head  1  constructed as described above, by sandwiching the piezoelectric sheets  41  to  43  by the common electrodes  34   a  and  34   b  and the individual electrodes  35   a  and  35   b , the volume of each pressure chamber  10  can easily be changed by the piezoelectric effect. Further, because each of the piezoelectric sheets  41  to  43  having active layers is in a shape of a continuous flat layer, this can be easily manufactured. 
         [0110]    Furthermore, the ink-jet head  1  has the actuator units  21  each having a unimorph structure in which the piezoelectric sheets  44  and  45  near each pressure chamber  10  are inactive and the piezoelectric sheet  41  to  43  distant from each pressure chamber  10  include active layers. Therefore, the change in volume of each pressure chamber  10  can be increased by the transversal piezoelectric effect. As a result, in contrast to an ink-jet head in which a layer including active layers is provided on the pressure chamber  10  side and a inactive layer is provided on the opposite side, the voltage to be applied to the individual electrodes  35   a  and  35   b  and/or high integration of the pressure chambers  10  can be lowered. By lowering the voltage to be applied, the size of the driver for driving the individual electrodes  35   a  and  35   b  can be reduced, thus reducing costs. In addition, each pressure chamber  10  can be reduced. Furthermore, even when the pressure chambers  10  are highly packed, a sufficient amount of ink can be ejected. Thus, leads to a decrease in the size of the head  1  and a highly dense arrangement of printing dots. 
         [0111]    Further, in the ink jet head  1 , each actuator unit  21  has a substantially trapezoidal shape. The actuator units  21  are arranged in two lines in a crisscross manner so that the parallel opposed sides of each actuator unit  21  extend along the longitudinal direction of the passage unit  4 , and the oblique sides of each neighboring actuator units  21  overlap each other in the lateral direction of the passage unit  4 . Because the oblique sides of each neighboring actuator units  21  overlap each other, when the ink-jet head  1  moves along the lateral direction of the ink-jet head  1  relatively to a print medium, the pressure chambers  10  along the lateral direction of the passage unit  4  can compensate each other. As a result, high-resolution printing, can be achieved by using a small-size ink-jet head  1  with a very narrow width. 
         [0112]    Furthermore, because many pressure chambers  10  neighboring each other are arranged in a matrix in the passage unit  4 , the pressure chambers  10  can be disposed within a relatively small size at a high density. 
         [0113]    In the above-described ink-jet head  1 , trapezoidal actuator units are arranged in two lines in a crisscross manner. However, each actuator unit may not be trapezoidal. Further, actuator units may be arranged in only one line along the longitudinal direction of the passage unit. Actuator units may be arranged in three or more lines in a crisscross manner. 
         [0114]      FIG. 11  shows is a plan view of a head main body of an ink-jet head according to second exemplary embodiment of the invention. In the ink-jet head and ink-jet printer according to this second exemplary embodiment, because the parts other than the head main body are similar to those of the above-described first embodiment, the detailed description thereof will be omitted. 
         [0115]    Referring to  FIG. 11 , a head main body  201  of an ink jet head according to this embodiment has a rectangular shape in a plan view extending in a main scanning direction. The head main body  201  includes a passage unit  204  in which a large number of pressure chambers  210  and a large number of ink ejection ports  208  are formed, as will be described later. Onto the upper face of the passage unit  204 , two actuator units  221  (In  FIG. 11 , the right and left ones are denoted by reference numerals  221   a  and  221   b , respectively) are bonded so as to neighbor each other. Each actuator unit  221  is disposed so that its one side B extends along the longitudinal direction of the head main body  201 . The neighboring actuator units  221  are disposed so as to be aligned with each other along the width (shorter length) direction of the head main body  201  with their oblique sides C being close to each other. An ink supply port  202  is open in the upper face of the passage unit  204 . The ink supply port  202  is connected with an ink supply source through a passage (not shown). 
         [0116]    As shown in  FIG. 12 , which representing the head main body  201  viewed from the printing face side, two ink ejection regions R 1  are provided in the lower face of the passage unit  204  to correspond to the respective regions where the actuator units  221  are disposed. A large number of small-diameter ink ejection ports  208  are arranged in the surface of each ink ejection region R 1 . 
         [0117]    This exemplary embodiment shows a case of monochrome printing. Thus, the ink supply port  202  is supplied with a single color ink (e.g., black). To perform multicolor printing, head main bodies  201  corresponding in number to colors (for example, in case of four colors of yellow, cyan, magenta, and black, four head main bodies  201 ) are aligned along the lateral direction of the passage unit. The head main bodies  201  are supplied with color inks different from one another to print. 
         [0118]      FIG. 13  is a sectional view illustrating the internal construction of the passage unit  204 . Referring to  FIG. 13 , a manifold channel  205  is formed in the passage unit  204 . The manifold channel  205  communicates with an ink supply source through the ink supply port  202 , as a result, the manifold channel  205  is always filled up with ink. The ink supply port  202  is preferably provided with a filter for catching dust and dirt contained in ink. 
         [0119]    The manifold channel  205  is formed in the most part of passage unit  204  to extend over the two ink ejection regions R 1 . In part of the manifold channel  205  corresponding to each ink ejection region R 1 , a large number of slender island portions  205   a  are formed to be arranged at regular intervals. The length of each island portion  205   a  is along the longitudinal direction of the passage unit  204 . In this construction, ink supplied through the ink supply port  202  passes between each neighboring island portions  205   a  in the manifold channel  205 , and then it is distributed to pressure chambers  210  formed in the passage unit  204  in each ink ejection region R 1 . 
         [0120]    Referring to  FIG. 15 , each ink ejection port  208  is made into a tapered nozzle. The ink ejection port  208  communicates with a manifold channel  205  through a pressure chamber  210  having a substantially shape in a plan view and an aperture  212 . In this construction, ink is supplied from the manifold channel  205  to the pressure chamber  210  through the aperture  212 . By driving an actuator unit  221 , energy is applied to the ink in the pressure chamber  210  to eject the ink through the ink ejection port  208 . 
         [0121]      FIG. 14  illustrates a detailed construction of the region denoted by reference Q in  FIG. 13 . As shown in  FIG. 14 , in a region of the upper face of the passage unit  204  corresponding to an ink ejection region R 1 , a large number of pressure chambers  210  are arranged in a matrix adjacent to or neighboring each other. Because the pressure chambers  210  are formed at a different level than that of the apertures  212  (as illustrated in  FIG. 15 ), an arrangement such as that illustrated in  FIG. 14  is possible in which each aperture  212  connected with a pressure chamber  210  overlaps another pressure chamber  210 . As a result, a dense arrangement of the pressure chambers  210  can be closely or densely arranged, which reduces the size of the head main body  201  and increases the resolution of the formed image. 
         [0122]      FIG. 15  illustrates a specific construction of a passage from a manifold channel  205  to an ink ejection port  208 . Referring to  FIG. 15 , the passage unit  204  is laminated with nine sheet materials in total, i.e., a cavity plate  222 , a base plate  223 , an aperture plate  224 , a supply plate  225 , manifold plates  226 ,  227 , and  228 , a cover plate  229 , and a nozzle plate  230 . The above-described actuator units  221  are bonded to the upper face of the passage unit  204  to form a head main body  201 . The detailed construction of each actuator unit  221  will be described later. 
         [0123]    An opening is formed in the cavity plate  222  to form a pressure chamber  210  as described above. A tapered ink ejection port  208  is formed in the nozzle plate  230  using a press. Communication holes  251  are formed through each of the plates  223  to  229  between the plates  222  and  230 . The pressure chamber  210  communicates with the ink ejection port  208  through the communication holes  251 . An aperture  212  is formed as an elongated hole in the aperture plate  224 . One end of the aperture  212  is connected with an end portion of the pressure chamber  210  (opposite to the end portion connecting with the ink ejection port  208 ) through a communication hole  252  formed in the base plate  223 . The aperture  212  is used to properly control the amount of ink to be supplied to the pressure chamber  210  and to prevent too much or too little ink from being ejected or released through the ink ejection port  208 . A communication hole  253  is formed in the supply plate  225 . The communication hole  253  connects the other end of the aperture  212  with the manifold channel  205 . 
         [0124]    Each of the nine plates  222  to  230  forming the passage unit  204  is made of metal. The pressure chamber  210 , the aperture  212 , and the communication holes  251 ,  252 , and  253  are formed by selectively etching each metallic plate using a mask pattern. The nine plates  222  to  230  are arranged in layers and bonded to each other so that the passage as illustrated in  FIG. 15  is formed therein. 
         [0125]    Referring to  FIG. 16 , each actuator unit  221  includes five piezoelectric sheets  241  to  245  having the same thickness of about 15 microns (μm). The piezoelectric sheets  241  to  245  are made into continuous flat layers. One actuator unit  221  is disposed to extend over many pressure chambers  210  formed in one ink ejection region R 1  of the head main body  201 . This can lead to a highly dense arrangement of individual electrodes  235   a  and  235   b  in the actuator unit  221 . Each of the piezoelectric sheets  241  to  245  is made of a lead zirconate titanate (PZT)-base ceramic material having ferroelectricity. 
         [0126]    Between the first and second piezoelectric sheets  241  and  242  from the top, an about 2 μm-thick common electrode  234   a  is interposed formed on substantially the entire of the lower and upper faces of the piezoelectric sheets. Between the third and fourth piezoelectric sheets  243  and  244 , an approximately 2 μm-thick common electrode  234   b  is also interposed. On the upper face of the first piezoelectric sheet  241 , an about 1 μm-thick individual electrode  235   a  is formed to correspond to each pressure chamber  210 . As illustrated in  FIG. 13 , the individual electrode  235   a  has a similar shape to that of the pressure chamber  210  in a plan view, although the individual electrode  235   a  is slightly smaller than the pressure chamber  210 . The individual electrode  235   a  is disposed such that the center of the individual electrode  235   a  coincides with the center of the corresponding pressure chamber  210 . Further, between the second and third piezoelectric sheets  242  and  243 , an about 2 μm-thick individual electrode  235   b  is arranged and formed like the individual electrode  235   a . The portion where the individual electrodes  235   a  and  235   b  are disposed corresponds to a pressure generation portion A for applying pressure to ink in the pressure chamber  210 . No electrode is provided between the fourth and fifth piezoelectric sheets  244  and  245 , and on the lower face of the fifth piezoelectric sheet  245 . Each of the electrodes  234   a ,  234   b ,  235   a , and  235   b  is made of, e.g., an Ag-Pd-base metallic material. 
         [0127]    The common electrodes  234   a  and  234   b  are grounded in a region (not shown). Thus, the common electrodes  234   a  and  234   b  are kept at the ground potential at a region corresponding to any pressure chamber  210 . In order that the individual electrodes  235   a  and  235   b  in each pair corresponding to a pressure chamber  210  can be controlled in potential independently of another pair, they are connected with a suitable driver IC through a lead provided separately for each pair of individual electrodes  235   a  and  235   b.    
         [0128]    In the head main body  201 , the piezoelectric sheets  241  to  245  are to be polarized in their thickness. That is, the actuator unit  221  has a so-called unimorph structure in which the upper (i.e., distant from the pressure chamber  210 ) three piezoelectric sheets  241  to  243  are layers including active layers, and the lower (i.e., near the pressure chamber  210 ) two piezoelectric sheets  244  and  245  are made into inactive layers. 
         [0129]    In this structure, when the individual electrodes  235   a  and  235   b  in a pair are set at a positive or negative predetermined potential, if the polarization is in the same direction as the electric field for example, the portion (an active layer, i.e., a pressure generation portion) in the piezoelectric sheets  241  to  243  sandwiched by the common and individual electrodes contracts perpendicularly to the polarization. On the other hand, because the inactive piezoelectric sheets  244  and  245  are affected by an electric field, they do not contract in themselves. Thus, a difference in strain is produced along the polarization between the upper piezoelectric sheets  241  to  243  and the lower piezoelectric sheets  444  and  245 . As a result, the piezoelectric sheets  241  to  245  are ready to deform into a convex shape toward the inactive side (unimorph deformation). At this time, because the lower face of the lowermost piezoelectric sheet  245  is fixed to the upper face of the partition dividing pressure chambers  210 , the pressure generation portion A of the piezoelectric sheets  241  to  245  deforms into a convex shape toward the pressure chamber  210  side to decrease the volume of the pressure chamber  210 . As a result, the pressure of ink is raised and ink is ejected through the ink ejection port  208 . After this, when a driving voltage is no longer applied to the individual electrodes  235   a  and  235   b , the piezoelectric sheets  241  to  245  return to the original shape and the pressure chamber  210  also returns to its original volume. Thus, the pressure chamber  210  draws ink therein through the manifold channel  205 . 
         [0130]    Next, the shape of the two actuator units  221   a  and  221   b  and the arrangement of individual electrodes  235   a  and  235   b , i.e., the pressure generation portions A, will be described.  FIG. 17  illustrates the shape of an actuator unit  221   a  and the arrangement of pressure generation portions.  FIG. 18  shows the relation between a seam portion between the actuator units  221   a  and  221   b  and pressure generation portions in an additional region. 
         [0131]    The head main body  201  includes two actuator units  221   a  and  221   b  as described above. The two actuator units  221   a  and  221   b  have a similar shape and arrangement for pressure generation portions A. 
         [0132]    As illustrated in  FIGS. 11 and 17 , the actuator unit  221   a  has a rectangular shape is disposed so that its side B extends in parallel with the longitudinal direction of the passage unit  204  and its other side C inclines to the longitudinal direction of the passage unit  204 . As illustrated in  FIG. 17 , in the actuator unit  221   a , two regions P 1  and P 2  are provided which are separated in the lateral direction of the passage unit  204  by a straight line along the longitudinal direction of the passage unit  204 . That is, the regions P 1  and P 2  neighbor each other in the lateral direction of the passage unit  204 . 
         [0133]    In region P 1 , a large number of pressure generation portions A 1  are arranged to neighbor each other in a matrix along the longitudinal direction of the passage unit  204  and along the other side C of the rectangle. 
         [0134]    In region P 2 , pressure generation portions A 2  are arranged to neighbor each other in a matrix only in the vicinity of a corner D of the rectangle near to the actuator unit  221   b.    
         [0135]    As shown in  FIG. 18 , when the two actuator units  221   a  and  221   b  are arranged in line along the longitudinal direction of the passage unit  204 , the pressure generation portions A 2  of the additional region P 2  provided in the actuator unit  221   a  are in a place corresponding to a region (shown as hatched region G in  FIG. 18 ) where no pressure generation portion A can be disposed in the basic region P 1  because it is in the seam between the actuator units  221   a  and  221   b . That is, the pressure generation portions A 2  of the additional region P 2  are disposed to correspond to a gap portion G between the pressure generation portions A 1  of the region P 1  provided in the actuator unit  221   a  and the pressure generation portions A 1  of the region P 1  provided in the neighboring actuator unit  221   b . Thus, although no separate actuator unit is provided for ejecting ink through the gap portion G, the head main body  201  print through the longitudinal direction of the passage unit without any breaks. 
         [0136]    In other words, because no pressure generation portion can be disposed in the region (region G) near the seam portion between the actuator units  221   a  and  221   b , no pressure chamber  210  and no ink ejection port  208  also can be disposed in that region. Therefore, if the pressure generation portions A 2  were not disposed in the additional region P 2  provided in the actuator unit  221   a , printing in the portion corresponding to the gap portion G cannot be done. As a result, a portion where ink ejection cannot occur is produced in the seam portion between the actuator units  221   a  and  221   b . However, because the pressure generation portions A 2  are disposed in the additional region P 2  provided in the actuator unit  221   a  in a portion overlapping that region G in the lateral direction of the passage unit, there is no portion where ink ejection cannot occur. As a result, an image without any breaks can be formed on an image recording medium. 
         [0137]    As described above, in this embodiment, the actuator unit  221  includes lines in each of which a large number of pressure generation portions A 1  and A 2  are arranged along the longitudinal direction of the passage unit  204 . Regarding the lengths of these lines along the longitudinal direction of the passage unit  204 , each line in the basic region P 1  is longer than each line in the additional region P 2 . Further, as for the number of lines along the lateral direction of the passage unit  204 , the number of lines in the additional region P 2  is the same as the number of lines that might exist in the length of the corresponding region G along the lateral direction of the passage unit  204 . Therefore, if an imaginary straight line is drawn to extend along the lateral direction of the passage unit  204 , the number of lines that the imaginary straight line crosses in the region where the neighboring actuator units  221   a  and  221   b  overlap each other is the same as the number of lines that the imaginary straight line crosses in the region where the neighboring actuator units  221   a  and  221   b  do not overlap each other. 
         [0138]    The above-described feature can be achieved by arranging two actuator units  221   a  and  221   b  having the same construction. Thus, the arrangement of parts can be simplified and the cost and the number of process steps necessary for designing or manufacturing the actuator units  221   a  and  221   b  can be reduced. 
         [0139]    Various exemplary arrangement of pressure generation portions A in the actuator unit  221  are described below. As shown in  FIG. 19 , an exemplary arrangement of pressure generation portions in an actuator unit  255  is provided.  FIG. 20  shows the relation between a seam portion between actuator units and pressure generation portions in an additional region in the arrangement of  FIG. 19 . 
         [0140]    The actuator unit  255   a  of  FIG. 19  is divided into three regions P 11 , P 12 , and P 13  in the lateral direction of the passage unit. The middle region P 11  in the lateral direction of the passage unit is used as a basic region and the remaining regions P 12  and P 13  are used as additional regions. 
         [0141]    In the basic region P 11 , similar to the arrangement of  FIG. 17 , a large number of pressure generation portions A 11  are arranged neighboring each other in a matrix along the longitudinal direction of the passage unit and along the other side C of the rectangle. In an additional region P 12 , pressure generation portions A 12  are arranged neighboring each other in a matrix in the vicinity of an acute corner D of the rectangle near to the actuator unit  255   b . In the other additional region P 13 , pressure generation portions A 13  are arranged neighboring each other in a matrix in the vicinity of an acute corner D of the rectangle far from the actuator unit  255   b.    
         [0142]    Therefore, as illustrated in  FIG. 20 , the pressure generation portions A 12  of the additional region P 12  of the actuator unit  255   a  and the pressure generation portions A 13  of the additional region P 13  of the actuator unit  255   b  are disposed in a gap portion G between the pressure generation portions A 11  of the basic region P 11  provided in the actuator unit  255   a  and the pressure generation portions A 11  of the basic region P 11  provided in the neighboring actuator unit  255   b . Thus, the head main body  201  can be provided such that ink can be ejected with any breaks through the longitudinal direction of the passage unit. 
         [0143]    Further, this embodiment can have the same advantages as those of the above-described first embodiment. More specifically, because the two actuator units  255   a  and  255   b  are arranged along the longitudinal direction of the passage unit  204 , even in case of a long passage unit  204 , high accuracy can be obtained in positioning of the actuator units  255   a  and  255   b  to the passage unit  204 . Therefore, good ink ejection performance can be obtained and the manufacture yield of ink-jet heads  201  can be remarkably improved. In addition, by sandwiching the piezoelectric sheets  241  to  243  between the common electrodes  234   a  and  234   b  and the individual electrodes  235   a  and  235   b , the volume of each pressure chamber  210  can easily be changed by the piezoelectric effect. Further, the piezoelectric sheets  241  to  243  having active layers are continuous flat layers that can be easily be manufactured. Further, because an actuator unit  221  of a unimorph structure is provided in which the piezoelectric sheets  244  and  245  near to each pressure chamber  210  are inactive and the piezoelectric sheets  241  to  243  far from each pressure chamber  210  are layers including active layers, the change in volume of each pressure chamber  210  can be increased by the transversal piezoelectric effect. This leads to a lower voltage that needs to be applied to the individual electrodes  235   a  and  235   b , as well as a high integration of the pressure chambers  210 . Further, in the passage unit  204 , because a large number of pressure chambers  210  neighboring each other are arranged in a matrix, the pressure chambers  210  can be disposed at a high density within a relatively small size. 
         [0144]    In this embodiment, only two actuator units are arranged. However, three or more actuator units may be arranged. Arrangement of many actuator units can bring about a long ink-jet head. Such a long ink-jet head is advantageous because it can perform printing onto even a large-size image recording medium at a high speed. 
         [0145]      FIGS. 21A and 21B  illustrate head main bodies  271  and  272  according to modifications of the invention, in which four actuator units  261   a ,  261   b ,  261   c , and  261   d  each constructed like an actuator unit  221  or  255 , are arranged in line on and bonded to passage units  274  having ink supply ports  273  near their both ends. Such an actuator units  261   a - d , like an actuator unit  221  or  255 , can be used in common to passage units different in length, e.g., from a relatively short passage unit as illustrated in  FIG. 11  to a long passage unit as illustrated in  FIG. 21A . Thus, such an actuator unit has high applicability as a component, which can reduce the manufacture cost. 
         [0146]    In the head main bodies  201  and  271  as illustrated in  FIGS. 11 and 21A , actuator units are arranged on a passage unit in a straight line with being aligned in the lateral direction of the passage unit. However, as in a head main body  272  illustrated in  FIG. 21B  for example, actuator units  261   a ,  261   b ,  261   c , and  261   d  may be arranged in a crisscross form. However, from the viewpoint of making an ink-jet head compact, the arrangement as illustrated in  FIG. 11 or 21A  is preferable in which actuator units are arranged in a straight line along the longitudinal direction of the passage unit with being regularly aligned in the lateral direction of the passage unit. Particularly in case of the arrangement of  FIG. 11 or 21A , the width of the ink-jet head can be made small. Therefore, when two or more ink-jet heads are arranged along their width to be supplied with inks of different colors for multicolor printing, they can be disposed within a compact space. This is further advantageous because occurrence of a shear in color of an image can be lessened even when an image recording medium runs in an oblique state upon printing. 
         [0147]    Next, a third embodiment of the invention will be described.  FIG. 22  is a plan view of a head main body of an ink-jet head according to this embodiment. In the ink-jet head and ink-jet printer according to this embodiment, because the parts other than the head main body is similar to that of the above-described first embodiment, the detailed description thereof is omitted here. 
         [0148]    Referring to  FIG. 22 , a head main body  301  of an ink-jet head according to this embodiment has a rectangular shape in a plan view extending in one direction. The head main body  301  includes a passage unit  304  in which a large number of pressure chambers  310  and a large number of ink ejection ports  308  are formed as will be described later. On the upper face of the passage unit  304 , four regular-hexagonal actuator units  321  (In  FIG. 22 , they are denoted by reference numerals  321   a ,  321   b ,  321   c , and  321   d , respectively, in order from the right) are arranged in two lines in a crisscross manner and they are bonded to the upper face of the passage unit  304 . Each actuator unit  321  is disposed so that its opposed parallel sides (upper and lower sides) extend along the longitudinal direction of the head main body  301 . Each neighboring actuator units  321  are disposed so that their oblique sides is to be close to each other and have overlapping portions in the lateral direction of the passage unit. 
         [0149]    Referring to  FIG. 23 , four hexagonal ink ejection regions R 2  are provided in the lower face of the passage unit  304  to correspond to the respective regions where the actuator units  321  are disposed. A large number of small-diameter ink ejection ports  308  are arranged in the surface of each ink ejection region R 2 . A base block  302  is disposed on the upper face of the head main body  301 . A pair of ink reservoirs  303  each having a slender shape along the longitudinal direction of the head main body  301  is provided in the base block  302 . An opening  303   a  is formed in the upper face of the base block  302  at one end of each ink reservoir  303 . Each opening  303   a  is connected with a ink tank (not shown). As a result, each ink reservoir  303  is always filled up with ink. 
         [0150]      FIG. 24  is a sectional view illustrating the internal construction of the passage unit  304 . Referring to  FIG. 24 , manifold channels  305  acting as ink supply sources are formed in the passage unit  304 . Each manifold channel  305  communicates with an ink reservoir  303  through the corresponding opening  305   a  formed in the upper face of the passage unit  304 . Each opening  305   a  is preferably provided with a filter for catching dust and dirt contained in the ink. 
         [0151]    Each manifold channel  305  branches at its opening  305   a  to supply ink to a number of pressure chambers  310  as described later. When each hexagonal ink ejection region R 2  illustrated in  FIG. 23  is evenly divided vertically into two regions, one manifold channel  305  is formed so as to correspond to one of the two regions. Eight manifold channel  305  is provided and each of them is so designed in shape as to distribute and supply ink to all pressure chambers  310  included in the corresponding region. 
         [0152]    The ink ejection port  308  in one half region in the lateral direction of the passage unit communicates with one of the ink reservoirs  303  in a pair through a manifold channel  305 . The ink ejection port  308  in the other half region in the lateral direction of the ink-jet head communicates with the other ink reservoir  303 . By configuring the manifold channels  305 , the openings  305   a , and the ink reservoirs  303  in such a manner, two printing modes can be realized: (1) a mode in which the ink reservoirs  303  in the pair are supplied with ink of the same color to perform monochrome high-resolution printing; and (2) a mode in which the ink reservoirs  303  in the pair are supplied with ink of different colors to perform two-color printing with the single head main body  301 . This is a widely usable construction. 
         [0153]    Referring to  FIG. 26 , each ink ejection port  308  is made into a tapered nozzle. The ink ejection port  308  communicates with a manifold channel  305  through a pressure chamber  310  having a nearly rhombic shape in a plan view and an aperture  312 . In this construction, ink is supplied to the manifold channel  305  through the ink reservoir  303  and further supplied from the manifold channel  305  to the pressure chamber  310  through the aperture  312 . By driving an actuator unit  321  as will be described later, jet energy is applied to the ink in the pressure chamber  310  to eject the ink through the ink ejection port  308 . 
         [0154]      FIG. 25  illustrates a detailed construction of the region denoted by reference E in  FIG. 24 . As shown in  FIG. 25 , in a region of the upper face of the passage unit  304  corresponding to an ink ejection region R 2 , a large number of pressure chambers  310  are arranged in a matrix neighboring each other. Because the pressure chambers  310  are formed at a different level from that of the apertures  312  as illustrated in  FIG. 26 , an arrangement is possible in which each aperture  312  connected with a pressure chamber  310  overlaps another pressure chamber  310 . As a result, a highly dense arrangement of the pressure chambers  310  can be realized, which may reduce the size of the head main body  301  and increase the resolution of a formed image. 
         [0155]      FIG. 26  illustrates a specific construction of a passage from a manifold channel  305  to an ink ejection port  308 . Referring to  FIG. 26 , the passage unit  304  is laminated with nine sheet materials in total, i.e., a cavity plate  322 , a base plate  323 , an aperture plate  324 , a supply plate  325 , manifold plates  326 ,  327 , and  328 , a cover plate  329 , and a nozzle plate  330 . The above-described actuator units  321  are bonded to the upper face of the passage unit  304  to constitute a head main body  301 . The detailed construction of each actuator unit  321  will be described later. 
         [0156]    A rhombic opening is formed in the cavity plate  322  to form a pressure chamber  310 . A tapered ink ejection port  308  is formed in the nozzle plate  330  with a press. Communication holes  351  are formed through each of the plates  323  to  329  between the plates  322  and  330 . The pressure chamber  310  communicates with the ink ejection port  308  through the communication holes  351 . An aperture  312  as an elongated hole is formed in the aperture plate  324 . One end of the aperture  312  is connected with an end portion of the pressure chamber  310  (opposite to the end portion connecting with the ink ejection port  308 ) through a communication hole  352  formed in the base plate  323 . The aperture  312  is for properly controlling the amount of ink to be supplied to the pressure chamber  310  and preventing too much or too little ink from being ejected through the ink ejection port  308 . A communication hole  353  is formed in the supply plate  325 . The communication hole  353  connects the other end of the aperture  312  with the manifold channel  305 . 
         [0157]    Each of the nine plates  322  to  330  forming the passage unit  304  is made of metal. The above-described pressure chamber  310 , aperture  312 , and communication holes  351 ,  352 , and  353  are formed by selectively etching each metallic plate using a mask pattern. The nine plates  322  to  330  are put in layers and bonded to each other with being positioned to each other so that the passage as illustrated in  FIG. 26  is formed therein. 
         [0158]    Next, the structure of each actuator unit  321  will be described. Referring to  FIG. 27 , the actuator unit  321  includes five piezoelectric sheets  341  to  345  having the same thickness of about 15 μm. These piezoelectric sheets  341  to  345  are made into continuous flat layers. One actuator unit  321  is disposed to extend over many pressure chambers  310  formed in one ink ejection region R 2  of the head main body  301 . This can realize a highly dense arrangement of individual electrodes  335   a  and  335   b . Each of the piezoelectric sheets  341  to  345  is made of a lead zirconate titanate (PZT)-base ceramic material having ferroelectricity. 
         [0159]    Between the first and second piezoelectric sheets  341  and  342  from the top, an about 2 μm-thick common electrode  334   a  is interposed formed on substantially the whole of the lower and upper faces of the piezoelectric sheets. Also, between the third and fourth piezoelectric sheets  343  and  344 , an about 2 μm-thick common electrode  234   b  is interposed. On the upper face of the first piezoelectric sheet  341 , an about 1 μm-thick individual electrode  335   a  is formed to correspond to each pressure chamber  310 . As illustrated in FIG.  24 , the individual electrode  335   a  has a similar shape to that of the pressure chamber  310  in a plan view though the individual electrode  335   a  is somewhat smaller than the pressure chamber  310 . The individual electrode  335   a  is disposed such that the center of the individual electrode  335   a  coincides with the center of the corresponding pressure chamber  310 . Further, between the second and third piezoelectric sheets  342  and  343 , an about 2 μm-thick individual electrode  335   b  is interposed formed like the individual electrode  335   a . No electrode is provided between the fourth and fifth piezoelectric sheets  344  and  345 , and on the lower face of the fifth piezoelectric sheet  345 . Each of the electrodes  334   a ,  334   b ,  335   a , and  335   b  is made of, e.g., an Ag-Pd-base metallic material. 
         [0160]    The common electrodes  334   a  and  334   b  are grounded in a region (not shown). Thus, the common electrodes  334   a  and  334   b  are kept at the ground potential at a region corresponding to any pressure chamber  310 . In order that the individual electrodes  335   a  and  335   b  in each pair corresponding to a pressure chamber  310  can be controlled in potential independently of another pair, they are connected with a suitable driver IC (not shown) through a lead provided separately for each pair of individual electrodes  335   a  and  335   b.    
         [0161]    In the head main body  301 , the piezoelectric sheets  341  to  345  are to be polarized in their thickness. That is, the actuator unit  321  has a so-called unimorph structure in which the upper (i.e., distant from the pressure chamber  310 ) three piezoelectric sheets  341  to  343  are layers including active layers, and the lower (i.e., near the pressure chamber  310 ) two piezoelectric sheets  344  and  345  are made into inactive layers. 
         [0162]    In this structure, when the individual electrodes  335   a  and  335   b  in a pair are set at a positive or negative predetermined potential, if the polarization is in the same direction as the electric field for example, the portion (an active layer, i.e., a pressure generation portion) in the piezoelectric sheets  341  to  343  sandwiched by the common and individual electrodes contracts perpendicularly to the polarization. On the other hand, because the inactive piezoelectric sheets  344  and  345  are influenced by no electric field, they do not contract in themselves. Thus, a difference in strain perpendicular to the polarization is produced between the upper piezoelectric sheets  341  to  343  and the lower piezoelectric sheets  344  and  345 . As a result, the whole of the piezoelectric sheets  341  to  345  is ready to deform into a convex shape toward the inactive side (unimorph deformation). At this time, because the lower face of the lowermost piezoelectric sheet  345  is fixed to the upper face of the partition partitioning pressure chambers  310 , the piezoelectric sheets  341  to  345  deform into a convex shape toward the pressure chamber  310  side to decrease the volume of the pressure chamber  310 . As a result, the pressure of ink is raised and the ink is ejected through the ink ejection port  308 . After this, when application of the driving voltage to the individual electrodes  335   a  and  335   b  is stopped, the piezoelectric sheets  341  to  345  return to the original shape and the pressure chamber  310  also returns to its original volume. Thus, the pressure chamber  310  draws the ink therein through the manifold channel  305 . 
         [0163]    To manufacture each actuator unit  321 , first, ceramic green sheets to be piezoelectric sheets  341  to  345  are put in layers and then baked. At this time, a metallic material to be individual electrodes  335   a  or a common electrode  334   a  or  334   b  is printed into a pattern on each ceramic green sheet at need. After this, a metallic material to be individual electrodes  335   a  is formed by plating on the whole of the upper face of the first piezoelectric sheet  341  and then unnecessary portions of the material are removed by laser patterning. Alternatively, a metallic material to be individual electrodes  335   a  is deposited using a mask having openings at portions corresponding to the respective individual electrodes  335   a.    
         [0164]    The actuator unit  321  thus manufactured is very brittle because it is made of ceramic. In particular, because corners of the actuator unit  321  are very easily broken, very delicate handling is required upon manufacture and assembling in order that any corner must not be brought into contact with another component. 
         [0165]    However, as illustrated in  FIG. 28A  that is a plan view of the actuator unit  321 , in the ink-jet head according to this embodiment, the actuator unit  321  has a substantially regular-hexagonal profile. Any of six straight portions (sides) L 1  to L 6  included in this profile is connected with a neighboring straight portion L at about 120°. As a result, because any of the six corners (portions of each neighboring straight portions L crossing each other) θ 1  to θ 6  is not sharp, it is difficult to be broken off. Therefore, the actuator unit  321  as an expensive precise component may not easily brake in the middle of manufacture process. This may contribute to a reduction of manufacture cost. 
         [0166]    The above effect is not obtained only when any of the corners θ 1  to θ 6  is formed into 120°. If a corner θn is formed into 90° or more, the corner θn is hard to be broken off. Therefore, for making any of the six corners θ 1  to θ 6  hard to be broken off, it suffices that any of the six straight portions L 1  to L 6  is connected with a neighboring straight portion L at the right angle or an obtuse angle (the minimum value of the angles θ 1  to θ 6  at the crossing portions is 90° or more). The hexagonal profile can freely be changed as far as the above condition is satisfied.  FIG. 28B  illustrates an actuator unit  355  as an example in which the above condition is satisfied. 
         [0167]    Further, this embodiment also can bring about the same advantages as those of the above-described first embodiment. More specifically, because the four actuator units  321  are arranged along the longitudinal direction of the passage unit  304 , even in case of a long passage unit  304 , high accuracy can be obtained in positioning of the actuator units  321  to the passage unit  304 . Therefore, good ink ejection performance can be obtained and the manufacture yield of ink-jet heads  301  can be remarkably improved. Furthermore, by sandwiching the piezoelectric sheets  341  to  343  between the common electrodes  334   a  and  334   b  and the individual electrodes  335   a  and  335   b , the volume of each pressure chamber  310  can easily be changed by the piezoelectric effect. Furthermore, the piezoelectric sheets  341  to  343  including active layers can easily be manufactured because they are continuous flat layers. Furthermore, because an actuator unit  321  of a unimorph structure is provided in which the piezoelectric sheets  344  and  345  near to each pressure chamber  310  are inactive and the piezoelectric sheets  341  to  343  far from each pressure chamber  310  are layers including active layers, the change in volume of each pressure chamber  310  can be increased by the transversal piezoelectric effect, and lowering the voltage to be applied to the individual electrodes  335   a  and  335   b  and/or high integration of the pressure chambers  310  can be intended. Further, in the passage unit  304 , because a large number of pressure chambers  310  neighboring each other are arranged in a matrix, the many pressure chambers  310  can be disposed at a high density within a relatively small size. 
         [0168]    In the invention, the profile of each actuator unit is not limited to a hexagon. That is, the number of straight portion L may be not six but five, seven, eight, or more. Hereinafter, modifications in profile of each actuator unit will be described with reference to  FIGS. 28 to 30 . In the below modifications, the same components as in the above-described third embodiment are denoted by the same reference numerals as in the third embodiment, respectively. 
         [0169]      FIG. 29A  is a plan view of a head main body in which each actuator unit is made into a heptagonal shape.  FIG. 29B  is a plan view of an actuator unit included in the head main body of  FIG. 29A . As apparent from  FIGS. 29A and 29B , in this modification, the components of the head main body  361  other than the actuator units  362  (In  FIG. 29A , they are denoted by reference numerals  362   a ,  362   b ,  362   c , and  362   d , respectively, in order from the right) are constructed like those of the head main body  301  of the third embodiment. 
         [0170]    Referring to  FIG. 29B , each actuator unit  362  has its profile in which one corner of a hexagon according to the above-described embodiment has been cut off along a straight line. As a result, the number of straight portion L is seven (L 8  to L 14 ), and as for the angle of each corner, θ 8  to θ 12  are about 120° and θ 13  and θ 14  are about 150°. 
         [0171]      FIG. 30A  is a plan view of a head main body in which each actuator unit is made into an octagonal shape.  FIG. 30B  is a plan view of an actuator unit included in the head main body of  FIG. 30A . As shown in  FIGS. 30A and 30B , in this modification, the components of the head main body  371  other than the actuator units  372  (In  FIG. 30A , they are denoted by reference numerals  372   a ,  372   b ,  372   c , and  372   d , respectively, in order from the right) are constructed like those of the head main body  301  of the third embodiment. 
         [0172]    Referring to  FIG. 30B , each actuator unit  372  has its profile in which two corners of a hexagon according to the above-described embodiment has been cut off along straight lines. As a result, the number of straight portion L is eight (L 15  to L 22 ), and as for the angle of each corner, θ 15 , θ 16 , θ 19 , and θ 20  are about 120° and θ 17 , θ 18 , θ 21 , and θ 22  are about 150°. In the above-described two modifications, because the angle of each corner of each cut-off portion is 150°, which is larger than that of the above-described hexagonal actuator unit  321 , the corner is harder to be broken off than that of the above-described hexagonal actuator unit  321 . 
         [0173]      FIG. 31A  is a plan view of a head main body in which two interconnecting portions of neighboring straight portions L in the actuator unit of the above-described third embodiment have been made into rounded portions F.  FIG. 31B  is a plan view of an actuator unit included in the head main body of  FIG. 31A . As shown in  FIGS. 31A and 31B , in this modification, the components of the head main body  381  other than the actuator units  382  (In  FIG. 31A , they are denoted by reference numerals  382   a ,  382   b ,  382   c , and  382   d , respectively, in order from the right) are constructed like those of the head main body  301  of the second embodiment. 
         [0174]    Referring to  FIG. 31B , each actuator unit  382  has six straight portions L 23  to L 28 . Two interconnecting portions of neighboring straight portions L (L 23  and L 28 , and L 25  and L 26 ) in the actuator unit  382  are made into rounded portions F, where neighboring straight portions L are smoothly interconnected. Each rounded portion F is very hard to be broken off. Also in this case, the angle between each neighboring straight portions L, including two straight portions on both sides of each rounded portion F, ( 023  to  027 ), is more than 90° (about 120°). 
         [0175]    Next, the fourth exemplary embodiment of the invention will be described with reference to  FIG. 32 . In the ink-jet head and ink-jet printer according to this embodiment, because the parts other than the head main body is similar to that of the above-described first embodiment, the detailed description thereof is omitted here. 
         [0176]    A head main body  401  as illustrated in  FIG. 32  includes a passage unit  404  in which a large number of pressure chambers and a large number of ink ejection ports are formed like the above-described embodiments. Onto the upper face of the passage unit  404 , two actuator units  421  (In  FIG. 32 , the right and left ones are denoted by reference numerals  421   a  and  421   b , respectively) are bonded neighboring each other. Each actuator unit  421  is disposed so that its one side B extends along the longitudinal direction of the head main body  401 . The neighboring actuator units  421  are disposed so as to be aligned with each other along the lateral direction of the head main body  401  with their oblique sides C being close to each other. Two actuator units  421  partially overlap each other along the lateral direction of the passage unit  404 . An ink supply port  402  is open in the upper face of the passage unit  404 . The ink supply port  402  is connected with an ink supply source through a passage (not shown). 
         [0177]    An FPC  436  is bonded onto the upper face of each actuator unit  421 , and is used for supplying electric signals to individual and common electrodes in the actuator unit  421 . A driver IC  432  is bonded onto each FPC  436 , and is used as a driving circuit for generating driving signals to be supplied to the individual electrodes in the corresponding actuator unit  421 . Each FPC  436  is electrically connected with a control unit  440  including CPU, RAM, and ROM. The control unit  440  supplies printing data to each driver IC  432 . Each driver IC  432  generates driving signals for individual electrodes on the basis of the printing data. 
         [0178]    Two regions P 21  and P 22  are provided in each actuator unit  421 . Of them, the basic region P 21  has a substantially rectangular shape having its sides in parallel with the respective sides of the corresponding actuator unit  421 . The basic region P 21  has its width somewhat shorter than the side B of the actuator unit  421  and its length of about ¾ the side C of the actuator unit  421 . In  FIG. 32 , the basic region P 21  is provided in an upper portion of the actuator unit  421 . The additional region P 22  has a substantially rectangular shape having its sides in parallel with the respective sides of the corresponding actuator unit  421 . The additional region P 22  has the same width as the basic region P 21  and is disposed on the lower side of the basic region P 21 . The additional region P 22  is divided into two sub-regions P 22   a  and P 22   b  each having a substantially rectangular shape having its sides in parallel with the respective sides of the actuator unit  421 . The sub-region P 22   a  has its width of about ⅕ the side B of the actuator unit  421  and its length of about ⅕ the side C of the actuator unit  421 . In  FIG. 32 , the sub-region P 22   a  is near the lower left acute portion of the actuator unit  421 . The sub-region P 22   b  has its width of about ⅗ the side B of the actuator unit  421  and its length of about ⅕ the side C of the actuator unit  421 . In  FIG. 32 , the sub-region P 22   b  is on the lower side of the basic region P 21  and on the right side of the sub-region P 22   a.    
         [0179]    In each of the basic region P 21  and the sub-regions P 22   a  and P 22   b  of the additional region P 22 , a large number of pressure generation portions are arranged with neighboring each other in a matrix along the longitudinal direction of the passage unit  404  and along the side C of the rectangle. Pressure chambers and ink passages including nozzles are formed in the passage unit  404  to correspond to the respective pressure generation portions. 
         [0180]    When the two actuator units  421   a  and  421   b  each constructed as described above are arranged in line along the longitudinal direction of the passage unit  404  as illustrated in  FIG. 32 , a region (hatched region G in  FIG. 32 ) where no pressure generation portions exist is formed near the seam portion between the actuator units  421   a  and  421   b . When the only pressure generation portions in the basic region P 11  are taken into consideration, the number of pressure generation portions along the lateral direction of the passage unit  404  in the vicinity of the seam portion is less than that in the portion other than the vicinity of the seam portion. 
         [0181]    Hence, in this embodiment, utilizing the feature that the sub-region P 22   a  of the additional region P 22  provided on the lower side of the basic region P 21  is provided to correspond to the region G where no pressure generation portions exist, near the seam portion, along the lateral direction of the passage unit  404 , the control unit  440  controls each driver IC  432  upon printing so as to drive pressure generation portions in the basic region P 21  and in the sub-region P 22   a  of the additional region P 22  and not to drive any pressure generation portion in the sub-region P 22   b  of the additional region P 22 . By this, because pressure generation portions in the actuator unit  421  are arranged in a region having substantially the same shape as in the actuator unit  221  of  FIG. 18 , the number of pressure generation portions along the passage unit  404  near the seam portion is the same as that in the other portion. That is, because the pressure generation portions of the sub-region P 22   a  of the additional region P 22  are disposed so as to correspond to the gap portion between the pressure generation portions of the basic region P 21  provided in one actuator unit  421   a  and the pressure generation portions of the basic region P 21  provided in the neighboring actuator unit  421   b , the head main body  401  is capable of printing without any breaks throughout the longitudinal direction of the passage unit, and without providing any other actuator unit for ejecting ink through the gap portion. Further, because the pressure generation portion formation region in each actuator unit  421  has a similar shape to that of the actuator unit  421 , problems of distortion, bend, or the like, of the actuator unit  421  is difficult to arise. 
         [0182]    As apparent from the above description, in this embodiment, ink passages may not be provided in the portion of the passage unit  404  corresponding to the sub-region P 22   b  of the additional region P 22 . 
         [0183]    The materials of each piezoelectric sheet and each electrode used in the above-described embodiments are not limited to the above-described ones. They can be changed to other known materials. The shapes in plan and sectional views of each pressure chamber, the arrangement of pressure chambers, the number of piezoelectric sheets including active layers, the number of inactive layers, etc., can be changed properly. Each piezoelectric sheet including active layers may differ in thickness from each inactive layer. 
         [0184]    Furthermore, in the above-described embodiments, each actuator unit is constructed in which individual and common electrodes are provided on a piezoelectric sheet. However, such an actuator unit may not always be used bonded to the passage unit. Any other actuator unit can be used if it can change the volumes of the respective pressure chambers separately. Furthermore, in the above-described embodiments, pressure chambers are arranged in a matrix. However, the pressure chambers may be arranged in a line or lines. Further, although any inactive layer is made of a piezoelectric sheet in the above-described embodiment, the inactive layer may be made of an insulating sheet other than a piezoelectric sheet. 
         [0185]    While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.