Patent Publication Number: US-2009225138-A1

Title: Liquid ejection head, liquid ejection apparatus

Description:
The entire disclosure of Japanese Patent Application No. 2008-045312, filed Feb. 26, 2008 is incorporated by reference herein. And the entire disclosure of Japanese Patent Application No. 2009-042326, filed Feb. 25, 2009 is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid ejection head and a liquid ejection apparatus that eject liquid from nozzle orifices, and more specifically, it is useful when applied to an ink jet recording head and an ink jet recording apparatus that eject ink as liquid. 
     2. Description of the Related Art 
     An ink jet recording head is an example of a liquid ejection head. Some ink jet recording heads include an actuator unit provided with piezoelectric elements and pressure generating chambers, a nozzle plate provided with nozzle orifices that communicate with the pressure generating chambers and eject ink, and a passage unit provided with a reservoir serving as a common ink chamber of the pressure generating chambers. 
     A known reservoir of such an ink jet recording head is configured to have a width that decreases with distance from a liquid inlet disposed in the central part thereof (JP-A-2002-292868). Another known reservoir is configured to branch. The directions of branch ports correspond to the flows. The passage resistances of branch passages are thereby made uniform (JP-A-2006-297897). The former is devised to increase the flow rate in an area prone to accumulation of bubbles to prevent the accumulation of bubbles. The latter is devised to prevent bubbles from remaining in the reservoir by filling the pressure generating chambers with ink at the same time. 
     However, the above-described reservoir structures cannot deal with the increase in length of an ink jet recording head. Hitherto, ink has been introduced into a reservoir through a liquid inlet disposed in the central part of the reservoir. However, in the case of a long-sized ink jet recording head with a length exceeding, for example, one inch, the reservoir is also long-sized in proportion thereto. As a result, the reservoir has a high pressure loss. To eliminate the effect of such a high pressure loss and to ensure ink supply performance, two or more liquid inlets need to be provided. However, in this case, a new problem arises that, in the ink confluence area, the flows stagnate, and the discharge of bubbles is difficult. As described above, the techniques disclosed in the Patent Documents 1 and 2 cannot deal with the new problem of the worsening of bubble discharge performance caused by stagnation of ink in the case where a plurality of liquid inlets are provided. The reason is that both techniques are premised on the case where one liquid inlet is provided to one reservoir. 
     Such a problem exists not only in an ink jet recording head but also in liquid ejection heads that eject liquid other than ink. 
     SUMMARY OF THE INVENTION 
     In view of the problem with the above known techniques, an object of the invention is to provide a liquid ejection head and a liquid ejection apparatus that can improve the bubble discharge performance in a reservoir without generating stagnation in a confluence area of liquid in a case where a plurality of liquid inlets are provided. 
     To solve the above problem, in an embodiment of the invention, a liquid ejection head includes pressure generating chambers that are provided side by side in a first substrate so as to be made to eject liquid through nozzle orifices by pressure variation, and a reservoir that is provided in a second substrate so as to supply the liquid to the pressure generating chambers and to constitute a common liquid chamber provided in the direction in which the pressure generating chambers are provided side by side. The reservoir is supplied with the liquid through a plurality of liquid inlets, and the cross-sectional area of the reservoir in a plane perpendicular to the line joining the liquid inlets in a confluence area of the liquid supplied through the liquid inlets is smaller than the cross-sectional area of the reservoir in a plane perpendicular to the line joining the liquid inlets in a predetermined area other than the confluence area. The first substrate and the second substrate may be the same substrate or different substrates. 
     According to this embodiment, stagnation in a confluence area of liquid supplied through a plurality of liquid inlets can be eliminated by a small width portion in the confluence area. Therefore, bubbles accumulating due to stagnation can be favorably discharged. In addition, the flow of liquid supplied through the reservoir to the pressure generating chambers can be made closer to being parallel to the longitudinal direction of the pressure generating chambers. Therefore, also in this regard, favorable bubble discharge performance can be ensured. 
     When a plurality of heads or a plurality of reservoirs are simply arranged side by side to increase the length, variation in structural strength of the heads or reservoirs, variation in static pressure of the reservoirs, variation in compliance of the reservoirs, and so forth cause cross talk. However, in this embodiment, even when the length of a head is increased, a common reservoir can be easily used. Therefore, the structural strength of the heads or reservoirs, the static pressure of the reservoirs, the compliance of the reservoirs, and so forth can be made uniform to prevent cross talk, and the bubble discharge performance can be made sufficiently favorable. 
     The reservoir may supply the liquid to the pressure generating chambers through liquid supply ports in the direction along the surface of the second substrate, and an inner wall of the reservoir that faces the liquid supply ports in the confluence area may project toward the liquid supply ports. In this case, the above-described effect can be achieved in a liquid ejection head in which the liquid is supplied to the pressure generating chambers through liquid supply ports from the direction parallel to the surface direction of the substrate. The reservoir may supply the liquid to the pressure generating chambers through liquid supply ports in the direction of the thickness of the second substrate, and inner walls of the reservoir that extend with the liquid supply ports therebetween in the direction in which the pressure generating chambers are provided side by side, may project toward each other in the confluence area. In this case, the above-described effect can be achieved in a liquid ejection head in which the liquid is supplied to the pressure generating chambers through liquid supply ports from the direction parallel to the thickness direction of the substrate. It is preferable that both of the inner walls of the reservoir that extend with the liquid supply ports therebetween in the direction in which the pressure generating chambers are provided side by side, be distant from the liquid supply ports by a distance larger than the maximum diameter of bubbles that can spontaneously disappear, in the predetermined area. The reason is that the smaller the distance, the more easily harmful bubbles grow. 
     Reducing the width as described above can be easily achieved, for example, by forming at least one narrowed portion in the confluence area. It is preferable that the at least one narrowed portion have such a shape that the width gradually decreases in the direction in which the pressure generating chambers are provided side by side, from the sides of the liquid inlets toward the intermediate part between adjacent liquid inlets. The reason is that by making the liquid flow along the narrowed portion, bubbles can be effectively discharged. It is preferable that the width of the at least one narrowed portion gradually decrease along the flow line of the liquid. The reason is that the flow of the liquid is the most smooth, and therefore bubbles can be favorably discharged. 
     The at least one narrowed portion may include a plurality of narrowed portions provided in the reservoir. In this case, by forming a plurality of fluid confluence areas, stagnation of fluid in each confluence area can be eliminated. Therefore, this is useful particularly in the case where the size of a reservoir in the longitudinal direction is large. 
     In another embodiment of the invention, a liquid ejection apparatus has the above-described liquid ejection head. 
     According to this embodiment, high-speed printing can be achieved using a long-sized head, and in addition, the printing quality can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a liquid ejection head according to a first embodiment of the invention. 
         FIG. 2  is a plan view showing the reservoir portion of  FIG. 1 . 
         FIG. 3  is a plan view showing a case where the reservoir shown in  FIG. 2  has no narrowed portion. 
         FIG. 4  is a sectional view of a liquid ejection head according to a second embodiment of the invention. 
         FIG. 5  is a plan view showing the reservoir portion of  FIG. 4 . 
         FIG. 6  is a schematic diagram of an ink jet recording apparatus according to an embodiment of the invention. 
     
    
    
     
         
           10 ,  110 : ink jet recording head 
           11 ,  121 : pressure generating chamber 
           12 ,  122 : passage forming substrate 
           13 ,  134 : nozzle orifice 
           14 ,  135 : nozzle plate 
           15 ,  123 : vibrating plate 
           16 ,  130 : passage unit 
           17 ,  140 : piezoelectric element 
           18 : piezoelectric element unit 
           19 : housing portion 
           20 : case head 
           21 ,  138 : ink inlet 
           22 ,  132 : reservoir 
           22   a ,  132   a : narrowed portion 
       
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the invention will now be described in detail with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a sectional view of an ink jet recording head that is an example of a liquid ejection head according to a first embodiment of the invention. As shown in the figure, the ink jet recording head  10  has a passage unit  16 . The passage unit  16  has a passage forming substrate  12  having a plurality of pressure generating chambers  11 , a nozzle plate  14  in which are formed a plurality of nozzle orifices  13  communicating with the pressure generating chambers  11 , and a vibrating plate  15  provided on the side of the passage forming substrate  12  opposite to the nozzle plate  14 . In addition, the ink jet recording head  10  has a piezoelectric element unit  18  that has piezoelectric elements  17  provided in areas on the vibrating plate  15  corresponding to the pressure generating chambers  11 , and a case head  20  that is fixed to the vibrating plate  15  and has a housing portion  19  in which the piezoelectric element unit  18  is housed. 
     In the surface layer part on one side of the passage forming substrate  12 , a plurality of pressure generating chambers  11  are formed by partition walls and are provided side by side in the width direction thereof. For example, in this embodiment, in the passage forming substrate  12 , a plurality of pressure generating chambers  11  are provided side by side. On the outer side of the row of the pressure generating chambers  11 , a reservoir  22  to which ink is supplied through ink inlets  21  communicating with an ink supply means (not shown) outside the case head  20 , is provided through the passage forming substrate  12  in the thickness direction. The reservoir  22  communicates with the pressure generating chambers  11  through ink supply ports  23 . The pressure generating chambers  11  are supplied with ink from the ink supply means through the ink inlets  21  and the reservoir  22 . The ink supply ports  23  have a width smaller than the width of the pressure generating chambers  11  and maintain constant passage resistance of ink that flows from the reservoir  22  into the pressure generating chambers  11 . In addition, at the end of each pressure generating chamber  11  opposite to the reservoir  22 , a nozzle communication hole  24  is formed through the passage forming substrate  12 . 
     As described above, in this embodiment, ink is made to flow from the reservoir  22  through the ink supply ports  23  in the surface direction of the passage forming substrate  12 , and thereby ink is supplied to the pressure generating chambers  11 . That is, the passage forming substrate  12  is provided with pressure generating chambers  11 , a reservoir  22 , ink supply ports  23 , and nozzle communication holes  24 . Such a passage forming substrate  12  is formed of a silicon single crystal substrate. The above-described pressure generating chambers  11  and so forth provided in the passage forming substrate  12  are formed by etching the passage forming substrate  12 . 
     To one side of the passage forming substrate  12 , a nozzle plate  14  in which nozzle orifices  13  are formed is joined with adhesive  50 . The nozzle orifices  13  communicate with the pressure generating chambers  11  through nozzle communication holes  24  provided in the passage forming substrate  12 . On the other hand, to the other side of the passage forming substrate  12 , that is, the side on which the pressure generating chambers  11  open, the vibrating plate  15  is joined. The pressure generating chambers  11  are sealed by this vibrating plate  15 . The vibrating plate  15  is formed of a composite plate including an elastic film  25  that is formed of an elastic member, for example, a resin film, and a support plate  26  that supports this elastic film  25  and that is formed, for example, of a metallic material. The elastic film  25  side is joined to the passage forming substrate  12 . In areas in the vibrating plate  15  facing the pressure generating chambers  11 , islands  27  with which the distal ends of the piezoelectric elements  17  are in contact are provided. The distal end faces of these piezoelectric elements  17  are joined to the islands  27  with adhesive  28 . In addition, in an area of the vibrating plate  15  facing the reservoir  22 , a compliance portion  29  is provided. In the compliance portion  29 , the support plate  26  is removed by etching, and therefore the compliance portion  29  consists substantially only of the elastic film  25 . In this compliance portion  29 , when a pressure change occurs in the reservoir  22 , the elastic film  25  of this compliance portion  29  is deformed and thereby absorbs the pressure change and maintains a constant pressure in the reservoir  22 . In addition, the vibrating plate  15  is provided with openings  30  so that the ink inlets  21  communicate with the reservoir  22 . This vibrating plate  15  is joined to the passage forming substrate  12  with adhesive  51 . 
     The piezoelectric elements  17  are integrally formed in a single piezoelectric element unit  18 . Specifically, a piezoelectric material  31  is sandwiched between electrode forming materials  32  and  33 , and thereby a piezoelectric element forming member  34  is formed. This piezoelectric element forming member  34  is cut into a comb-like shape so that the teeth correspond to the pressure generating chambers  11 , and thereby the piezoelectric elements  17  are formed. Inactive areas of the piezoelectric elements  17  (the piezoelectric element forming member  34 ) that do not contribute to vibration, that is, the base ends of the piezoelectric elements  17  are fixed to a fixing substrate  35 . In this embodiment, the piezoelectric elements  17  (the piezoelectric element forming member  34 ) and the fixing substrate  35  constitute the piezoelectric element unit  18 . Near the base ends of the piezoelectric elements  17 , and to the side opposite to the fixing substrate  35 , a circuit board  37  that has wirings  36  supplying signals for driving the piezoelectric elements  17  is connected. 
     Such a piezoelectric element unit  18  is fixed with the distal ends of the piezoelectric elements  17  in contact with the islands  27  of the vibrating plate  15  as described above. For example, in this embodiment, a case head  20  is fixed on the top of the vibrating plate  15  as described above, the piezoelectric element unit  18  is housed in the housing portion  19  of this case head  20 , and the side of the fixing substrate  35  opposite to the side to which the piezoelectric elements  17  are fixed is fixed to the case head  20 . Specifically, in the housing portion  19  of the case head  20 , a step portion  38  is provided. The fixing substrate  35  is joined to the step portion  38  of the case head  20  with adhesive  39 . 
     In addition, to the top of the case head  20  is fixed a wiring board  41  provided with a plurality of conductive pads to which the wirings  36  of the circuit board  37  are connected. The housing portion  19  of the case head  20  is substantially closed by this wiring board  41 . In an area of the wiring board  41  facing the housing portion  19  of the case head  20 , a slit-like opening  42  is formed. The circuit board  37  is pulled out of the housing portion  19  through the opening  42  of the wiring board  41 . 
     The circuit board  37  constituting the piezoelectric element unit  18  is formed, for example, in this embodiment, of a chip on film (COF) on which a driving IC (not shown) for driving the piezoelectric elements  17  is mounted. The base ends of the wirings  36  of the circuit board  37  are connected, for example, with solder or an anisotropic conductive material to the electrode forming materials  32  and  33  constituting the piezoelectric elements  17 . On the other hand, the distal ends of the wirings  36  are connected to the conductive pads  40  of the wiring board  41 . Specifically, the distal end of the circuit board  37  pulled out of the housing portion  19  through the opening  42  of the wiring board  41  is folded along the surface of the wiring board  41 , and the wirings  36  are connected to the conductive pads  40  of the wiring board  41 . 
       FIG. 2  illustrates the planar shapes of various types of reservoirs according to this embodiment. Although four types of  FIGS. 2  ( a ) to  2  ( d ) are shown, the invention is not limited to these. A first common characteristic of the reservoirs  22 ,  72 ,  82 , and  92  in this embodiment is that the reservoirs communicate with a plurality of (two or three in the figure) ink inlets ( 21   a  and  21   b ), ( 71   a  to  71   c ), ( 81   a  to  81   c ), ( 91   a  and  91   b ). A second common characteristic is that, in confluence areas of ink supplied from the ink inlets ( 21   a  and  21   b ), ( 71   a  to  71   c ), ( 81   a  to  81   c ), ( 91   a  and  91   b ), the wall (the upper wall in the figure) opposite to the pressure generating chambers  11  (see  FIG. 1 ) is made to project, and narrowed portions  22   a , ( 72   a  and  72   b ), ( 82   a  and  82   b ),  92   a  are formed so that the width, the size in a direction (the vertical direction in the figure) perpendicular to the longitudinal direction (the horizontal direction in the figure), of these portions is smaller than the width of the other portions. That is, to reduce the pressure loss of ink owing to the increase in length of the reservoir  22 , first, the number of the ink inlets ( 21   a  and  21   b ), ( 71   a  to  71   c ), ( 81   a  to  81   c ), ( 91   a  and  91   b ) is determined. Then, in each case, to eliminate the stagnation of ink in the reservoir  22 , narrowed portions  22   a , ( 72   a ,  72   b ), ( 82   a ,  82   b ),  92   a  are formed in the above confluence areas. 
       FIG. 2  ( a ) shows a case where a reservoir  22  communicates with two ink inlets  21   a  and  21   b  at both ends thereof in the longitudinal direction. The flow lines in this case are shown by arrows in the figure. In a confluence area where the front ends of the arrows meet, a narrowed portion  22   a  is formed. Thus, ink can be prevented from stagnating in the confluence part. As a result, bubbles accumulating in the stagnant part can be effectively eliminated, and bubble discharge performance can be improved. The flow lines in this case are closer to being parallel to the axis lines (the vertical direction in the figure) of the pressure generating chambers  11  formed in the lower part the figure. Therefore, also due to this, bubbles can be favorably discharged. 
       FIG. 2  ( b ) shows a case where a reservoir  72  communicates with two ink inlets  71   a  and  71   b  at both ends thereof in the longitudinal direction, and communicates with one ink inlet  71   c  in the central part thereof. That is, a reservoir  72  communicates with three ink inlets  71   a  to  71   c . In confluence areas where flows of ink introduced through the ink inlets  71   a  to  71   c  merge, narrowed portions  72   a  and  72   b  are formed.  FIG. 2  ( c ) shows a case where a reservoir  82  is divided into three blocks, and the central parts of the blocks communicate with ink inlets  81   a ,  81   b , and  81   c . In this case, to prevent a decrease of the flow rate at the left end or the right end of the blocks at both ends of the reservoir, both ends of the reservoir  82  are narrowed and relatively small width portions  82   c  and  82   d  are formed, in addition to providing narrowed portions  82   a  and  82   b  in confluence areas where flows of ink merge. Thus, the bubble discharge function of the narrowed portions  82   a  and  82   b  and the bubble discharge function of the small width portions  82   c  and  82   d  combine, and bubbles can be favorably discharged. 
       FIG. 2  ( d ) shows a case where a reservoir  92  communicates with two ink inlets  91   a  and  91   b  at both ends thereof in the longitudinal direction. In this respect, this case is the same as the case shown in  FIG. 2  ( a ). However, in the case shown in  FIG. 2  ( d ), the shape of the reservoir  92  itself is formed in such a manner that the width (the size in the vertical direction in the figure) decreases gradually from the ink inlets  91   a  and  91   b  toward the confluence area along the longitudinal direction. Therefore, in this case, by the change of the width of the reservoir  92  itself, the flow of ink can be made smooth. However, the passage resistance increases, and therefore the rate of change of width needs to be adjusted with the pressure loss due to this passage resistance in mind. 
     In  FIGS. 2  ( a ) to  2  ( d ), every one of the narrowed portion  22   a  and so forth has such a shape that the width gradually decreases in a predetermined confluence area from both ends toward the central part along the longitudinal direction of the reservoir  22  and so forth, and the width gradually decreases in a curve. However, the invention is not limited to this. The width of the narrowed portion  22  and so forth may gradually decrease linearly. When the width of the narrowed portion  22  and so forth gradually decreases along the flow lines of ink, ink can be made to flow the most smoothly, and the bubble discharge performance is best. 
     If a long-sized reservoir communicating with a plurality of ink inlets does not have any one of the narrowed portion  22   a  and so forth shown in  FIG. 2 , a problem shown in  FIGS. 3  ( a ) and  3  ( b ) occurs. That is, in a reservoir  102 , a stagnation area  105  such as that shown in  FIG. 3  ( b ) is formed in a confluence area of ink supplied through ink inlets  101   a  and  101   b . Due to this, as shown in  FIG. 3  ( a ), bubbles  104  accumulate in the stagnation area  105  of  FIG. 3  ( b ) and are not discharged and worsen the printing performance. 
     According to this embodiment described above, by varying the volumes of the pressure generating chambers  11  by the deformation of the piezoelectric elements  17  and the vibrating plate  15 , ink droplets can be ejected. Specifically, after ink is supplied from an ink cartridge (not shown) through a plurality of ink inlets  21  to the reservoir  22 , ink is distributed through the ink supply ports  23  to the pressure generating chambers  11 . By applying a voltage to one of the piezoelectric elements  17 , the piezoelectric element  17  is contracted. Thereby, the vibrating plate  15  is deformed together with the piezoelectric element  17 , the volume of a corresponding one of the pressure generating chambers  11  is increased, and ink is drawn into the pressure generating chambers  11 . After the inside is filled with ink to a corresponding one of the nozzle orifices  13 , the voltage applied to the electrode forming materials  32  and  33  of the piezoelectric element  17  is removed according to a recording signal supplied through the wiring board. Thereby, the piezoelectric element  17  is expanded and returns to its original state, and the vibrating plate  15  is displaced and also returns to its original state. As a result, the volume of the pressure generating chamber  11  decreases, the pressure in the pressure generating chamber  11  increases, and ink is ejected from the nozzle orifice  13 . 
     At the time of such ink ejection, ink in the reservoir  22  is guided to the narrowed portion  22   a  (see  FIG. 2  ( a )) as described above and favorably flows into the pressure generating chamber  11 . As a result, ink can be prevented from stagnating in the confluence area, and favorable bubble discharge performance can be obtained. 
     Second Embodiment 
       FIG. 4  is a sectional view of an ink jet recording head that is an example of a liquid ejection head according to a second embodiment of the invention. As shown in the figure, the ink jet recording head  110  according to this embodiment includes an actuator unit  120  and a passage unit  130  to which this actuator unit  120  is fixed. 
     The actuator unit  120  is an actuator unit having piezoelectric elements  140 , and has a passage forming substrate  122  having pressure generating chambers  121  formed therein, a vibrating plate  123  provided on one side of the passage forming substrate  122 , and a pressure generating chamber bottom plate  124  provided on the other side of the passage forming substrate  122 . 
     The passage forming substrate  122  is formed, for example, of a plate about 150 μm thick of ceramic, such as alumina (Al2O3) or zirconia (ZrO2). In this embodiment, a plurality of pressure generating chambers  121  are provided side by side along the width direction thereof. On one side of this passage forming substrate  122 , a vibrating plate  123  formed, for example, of a thin stainless-steel (SUS) plate 10 to 12 μm thick is fixed. One side of each pressure generating chamber  121  is sealed by this vibrating plate  123 . 
     The pressure generating chamber bottom plate  124  is fixed to the other side of the passage forming substrate  122  and seals the other side of each pressure generating chamber  121 . The pressure generating chamber bottom plate  124  has supply communication holes  125  and nozzle communication holes  126 . Each supply communication hole  125  is provided near one end in the longitudinal direction of a corresponding one of the pressure generating chambers  121  and connects the pressure generating chamber  121  and a reservoir described below. Each nozzle communication hole  126  is provided near the other end in the longitudinal direction of a corresponding one of the pressure generating chamber  121  and communicates with a nozzle orifice  134  described below. 
     The piezoelectric elements  140  are provided in areas on the vibrating plate  123  facing the pressure generating chambers  121 . 
     Each piezoelectric element  140  includes a lower electrode film  141  provided on the vibrating plate  123 , a piezoelectric body layer  142  provided independently to each pressure generating chamber  11 , and an upper electrode film  143  provided on each piezoelectric body layer  142 . The piezoelectric body layer  142  is formed by attaching or printing a green sheet made of a piezoelectric material. The lower electrode film  141  is provided so as to cover the piezoelectric body layers  142  provided side by side, serves as a common electrode of the piezoelectric elements  140 , and functions as a part of the vibrating plate. Of course, one lower electrode film  141  may be provided to each piezoelectric body layer  142 . 
     The passage forming substrate  122 , the vibrating plate  123 , and the pressure generating chamber bottom plate  124  constituting layers of the actuator unit  120  are integrated without requiring adhesive by shaping a clay-like ceramic material called green sheet into a predetermined thickness, forming, for example, the pressure generating chambers  121 , and thereafter laminating and firing. After that, the piezoelectric elements  140  are formed on the top of the vibrating plate  123 . 
     On the other hand, the passage unit  130  includes a liquid supply port forming substrate  131  that is joined to the pressure generating chamber bottom plate  124  of the actuator unit  120 , a reservoir forming substrate  133  in which a reservoir  132  serving as a common ink chamber of a plurality of pressure generating chambers  121  is formed, a compliance substrate  150  that is provided on the side of the reservoir forming substrate  133  opposite to the liquid supply port forming substrate  131 , and a nozzle plate  135  in which nozzle orifices  134  are formed. 
     The liquid supply port forming substrate  131  is formed of a thin stainless steel (SUS) plate 60 μm thick and is provided with nozzle communication holes  136 , ink supply ports  137 , and ink inlets  138 . The nozzle communication holes  136  connect the nozzle orifices  134  and the pressure generating chambers  121 . The ink supply ports  137  connect the reservoir  132  and the pressure generating chambers  121  together with the supply communicating holes  125 . The ink inlets  138  communicate with each reservoir  132  and supply ink from an external ink tank. The number of the ink supply ports  137  is the same as the number of the pressure generating chambers  121 . The ink supply ports  137  are provided at the same pitch as that of the pressure generating chambers  121 . The number of the ink inlets  138  is determined according to the size of the reservoir  132  in the longitudinal direction. Therefore, flows of ink from a plurality of places into the reservoir  132  merge in an intermediate area between adjacent ink inlets  138 . That is, in the reservoir  132 , a confluence area of ink is formed in an intermediate area between adjacent ink inlets  138 . 
     The reservoir forming substrate  133  is formed of a corrosion-resistant plate material, for example, a stainless steel plate 150 μm thick, suitable to form ink passages. The reservoir forming substrate  133  has a reservoir  132  and nozzle communication holes  139 . The reservoir  132  is supplied with ink from an external ink tank (not shown) and supplies ink to the pressure generating chambers  121 . The nozzle communication holes  139  connect the pressure generating chambers  121  and the nozzle orifices  134 . 
     The reservoir  132  is provided so as to cover a plurality of pressure generating chambers  121 , that is, in a direction in which the pressure generating chambers  121  are arranged side by side. In addition, the reservoir  132  is configured such that the width between reservoir inner walls that face each other across the ink supply ports  137  in the above-described ink confluence area is smaller than the width in the other areas. In this embodiment, narrowed portions  132   a  and  132   b  are formed in the inner walls of the reservoir  132  that face each other in the confluence area of ink, and the width of the reservoir  132  in the ink confluence area is reduced. For this point, a detailed description will be given below with reference to  FIG. 5 . 
     The compliance substrate  150  is joined to the side of the reservoir forming substrate  133  opposite to the liquid supply port forming substrate  131  and seals the bottom surface of the reservoir  132 . An area of the compliance substrate  150  facing the reservoir  132  has a thickness smaller than the thickness of the other area and serves as a compliance portion  151  that is deformed by the pressure change in the reservoir  132 . The compliance substrate  150  is formed, for example, of metal such as stainless steel, or ceramic. Of course, the material of the compliance substrate  150  is not limited to this. The compliance substrate  150  may be formed, for example, of an elastic film that constitutes the compliance portion  151  and a support substrate having a through hole in the thickness direction. 
     In addition, the compliance substrate  150  is provided with nozzle communication holes  152  that connect the nozzle orifices  134  and the nozzle communication holes  139  formed through the reservoir forming substrate  133  in the thickness direction. That is, ink from the pressure generating chambers  121  flows through the nozzle communication holes  136 ,  139 , and  152  provided in the liquid supply port forming substrate  131 , the reservoir forming substrate  133 , and the compliance substrate  150 , respectively, and is then ejected from the nozzle orifices  134 . 
     The nozzle plate  135  is formed by forming nozzle orifices  134  in a thin plate formed, for example, of stainless steel, at the same arrangement pitch as the pressure generating chambers  121 . 
     Such a passage unit  130  is formed by fixing the liquid supply port forming substrate  131 , the reservoir forming substrate  133 , the compliance substrate  150 , and the nozzle plate  135  using adhesive or thermal welding films. Such a passage unit  130  and the actuator unit  120  are joined and fixed using adhesive or a thermal welding film. 
       FIG. 5  illustrates the planar shapes of various types of reservoirs according to this embodiment. The reservoir  132 , in particular, the narrowed portions  132   a  and  132   b  will be described in detail with reference to the figure. 
       FIG. 5  ( a ) shows a case where a reservoir  132  communicates with two ink inlets  138   a  and  138   b  at both ends thereof in the longitudinal direction. This case corresponds to the case shown in  FIG. 2  ( a ) in the first embodiment. In the case of the first embodiment shown in  FIG. 1  and  FIG. 2  ( a ), the reservoir  22  is configured to supply ink from a direction parallel to the surface direction of the passage forming substrate  12  through the ink supply ports  23  to the pressure generating chambers  11 . Therefore, only the inner wall of the reservoir  22  facing the ink supply ports  23  is made to project to form the narrowed portion  22   a.    
     On the other hand, in this embodiment, the ink supply ports  137  are formed between the inner walls of the reservoir  132  facing each other, and therefore providing a narrowed portion  132   a  to the inner wall  132   c  on the side of the ink inlets  138   a  and  138   b  is not enough. The reason is that ink flowing into the reservoir  132  through the ink inlets  138   a  and  138   b  stagnates in the confluence area on the side of the inner wall  132   d , and bubbles caused by this stagnation grow and can flow through the ink supply ports  137  into the pressure generating chambers  121 . In particular, when the distance d from the inner wall  132   d  to the ink supply ports  137  is larger than the size of small-diameter bubbles likely to spontaneously disappear or bubbles that one wants to discharge, grown bubbles are likely to flow through the ink supply ports  137  into the pressure generating chambers  121 . 
     So, in this embodiment, the inner wall  132   d  is also provided with a narrowed portion  132   b . That is, narrowed portions  132   a  and  132   b  are formed in the inner walls  132   c  and  132   d , respectively, of the reservoir  132  facing each other in the ink confluence area so as to reduce the width of the reservoir  132  in the ink confluence area. 
     In  FIG. 5  ( a ), the flow lines in this case are shown by arrows. In a confluence area where the front ends of the arrows meet, narrowed portions  132   a  and  132   b  are formed. Thus, ink can be prevented from stagnating in the confluence part. As a result, bubbles accumulating in the stagnant part can be effectively eliminated, and bubble discharge performance can be improved. 
       FIG. 5  ( b ) shows a case where a reservoir  172  communicates with two ink inlets  171   a  and  171   b  at both ends thereof in the longitudinal direction, and communicates with one ink inlet  171   c  in the central part thereof. This case corresponds to the case shown in  FIG. 2  ( b ) in the first embodiment. That is, a reservoir  72  communicates with three ink inlets  171   a  to  171   c . In confluence areas where flows of ink introduced through the ink inlets  171   a  to  171   c  merge, narrowed portions  172   a  and  172   b  are formed. In addition, narrowed portions  172   c  and  172   d  are formed across therefrom. 
       FIG. 5  ( c ) shows a case where a reservoir  182  is divided into three blocks, and the central parts of the blocks communicate with ink inlets  181   a ,  181   b , and  181   c . This corresponds to the case shown in  FIG. 2  ( c ) in the first embodiment. In this case, to prevent a decrease of the flow rate at the left end or the right end of the blocks at both ends of the reservoir, both ends of the reservoir  182  are narrowed and relatively small width portions  182   c  and  182   d  are formed, in addition to providing narrowed portions  182   a  and  182   b  in confluence areas where flows of ink merge. In addition, narrowed portions  182   e  and  182   f  are provided across from the narrowed portions  182   a  and  182   b , respectively, and small width portions  182   g  and  182   h  are provided across from the small width portions  182   c  and  182   d , respectively. 
     Thus, the bubble discharge function of the narrowed portions  182   a ,  182   b ,  182   e , and  182   f  and the bubble discharge function of the small width portions  182   c ,  182   d ,  182   g , and  182   h  combine, and bubbles can be favorably discharged. 
       FIG. 5  ( d ) shows a case where a reservoir  192  communicates with two ink inlets  191   a  and  191   b  at both ends thereof in the longitudinal direction. In this respect, this case is the same as the case shown in  FIG. 5  ( a ). However, in the case shown in  FIG. 5  ( d ), the shape of the reservoir  192  itself is different. The inner wall  192   c  is formed in such a manner that the width (the size in the vertical direction in the figure) decreases gradually from the ink inlets  191   a  and  191   b  toward the confluence area along the longitudinal direction. In addition, the inner wall  192   d  across therefrom is formed in a shape symmetrical thereto. Thus, narrowed portions  192   a  and  192   b  are formed in the ink confluence area. 
     Therefore, in this case, by the change of the width of the reservoir  192  itself, the flow of ink can be made smooth. However, the passage resistance increases, and therefore the rate of change of width needs to be adjusted with the pressure loss due to this passage resistance in mind. 
     In  FIGS. 5  ( a ) to  5  ( d ), every one of the narrowed portion  132   a  and so forth has such a shape that the width gradually decreases in a predetermined confluence area from both ends toward the central part along the longitudinal direction of the reservoir  132  and so forth, and the width gradually decreases in a curve. However, the invention is not limited to this. The width of the narrowed portion  132   a  and so forth may gradually decrease linearly. When the width of the narrowed portion  132   a  gradually decreases along the flow lines of ink, ink can be made to flow the most smoothly, and the bubble discharge performance is best. 
     According to this embodiment described above, ink is taken into the reservoir  132  from an ink cartridge (storage means) through a plurality of ink inlets  138 , and the insides of the ink passages from the reservoir  132  to the nozzle orifices  134  are filled with ink. Thereafter, according to a recording signal from a driving circuit (not shown), a voltage is applied to each piezoelectric element  140  corresponding to each pressure generating chamber  121 , and the vibrating plate  123  is bent together with the piezoelectric elements  140 . Thereby, the pressure in each pressure generating chamber  121  increases, and an ink droplet is ejected from each nozzle orifice  134 . 
     At the time of such ink ejection, ink in the reservoir  132  is guided to the narrowed portions  132   a  and  132   b  in the confluence area as described above, and favorably flows into the pressure generating chambers  121 . As a result, ink can be prevented from stagnating in the confluence part, and favorable bubble discharge performance can be obtained. 
     Other Embodiments 
     In the above embodiments, a description is given of an ink jet recording head having longitudinal vibration type piezoelectric elements in which piezoelectric material and electrode forming material are alternately laminated and that expand and contract in the axial direction. However, the type of ink jet recording head is not limited as long as the ink jet recording head has a reservoir. For example, an ink jet recording head having thick-film type piezoelectric elements; an ink jet recording head having thin-film type piezoelectric elements having piezoelectric material formed by, for example, a sol-gel method, a MOD method, or a sputtering method; an ink jet recording head having so-called static actuators in which a vibrating plate and an electrode are arranged with a predetermined gap therebetween and the vibration of the vibrating plate is controlled by electrostatic force; and an ink jet recording head in which a heater element is disposed in each pressure generating chamber and a bubble generated by the heat of the heater element ejects a liquid droplet from a nozzle orifice, can achieve the same effect. 
     The ink jet recording head according to any one of the above embodiments constitutes a part of a recording head unit having ink inlets communicating, for example, with an ink cartridge, and is mounted in an ink jet recording apparatus.  FIG. 6  is a schematic diagram showing an example of the ink jet recording apparatus. As shown in the figure, cartridges  2 A and  2 B constituting ink supply means are detachably provided to recording head units  1 A and  1 B, respectively, each having an ink jet recording head, and a carriage  3  on which the recording head units  1 A and  1 B are mounted is provided to a carriage shaft  5  attached to the apparatus main body  4 , movably in the axial direction. These recording head units  1 A and  1 B eject, for example, black ink composition and color ink composition, respectively. 
     The driving force of a driving motor  6  is transmitted via a plurality of gears (not shown) and a timing belt  7  to the carriage  3 . Thereby, the carriage  3  on which the recording head units  1 A and  1 B are mounted is moved along the carriage shaft  5 . On the other hand, an apparatus main body  4  is provided with a platen  8  along the carriage shaft  5 , and a recording sheet S that is a recording medium, such as paper, fed by a paper feed roller (not shown) or the like is transported on the platen  8 . 
     In the above-described embodiments, a description is given of an ink jet recording head as an example of a liquid ejection head. However, the present invention can be applied to any liquid ejection head and, of course, can also be applied to a method for inspecting a liquid ejection head that ejects liquid other than ink. Other examples of a liquid ejection head include various recording heads used in an image recording apparatus such as a printer, a color material ejection head used for manufacturing color filters for a liquid crystal display or the like, an electrode material ejection head used for forming electrodes of an organic EL display, FED (field emission display), or the like, and a bioorganic substance ejection head used for manufacturing biochips.