Patent Publication Number: US-8113631-B2

Title: Liquid ejecting head

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2008-245456, which was filed on Sep. 25, 2008, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a liquid ejecting head for ejecting a liquid therefrom. 
     2. Description of the Related Art 
     The viscosity of a liquid such as ink ejected from a liquid ejecting head varies depending upon the temperature of the liquid. In general, the viscosity of the liquid is increased under a low temperature condition. Accordingly, under the low temperature condition, a resistance against a flow of the liquid at a time when the liquid flows into a pressure chamber becomes large, so that it is difficult to obtain a satisfactory ejection effect even if a drive frequency is increased. Further, in order to obtain, under the low temperature condition, the same ejection characteristic (including the ejection amount and the ejection speed) as obtained under an ordinary temperature condition, it is needed to give, to the liquid in the pressure chamber, ejection energy larger than that given under the ordinary temperature condition, by increasing a drive voltage. In this instance, however, it is needed to increase a withstand voltage of an actuator configured to give the ejection energy to the liquid in the head and a withstand voltage of a driver IC configured to drive the actuator. Thus, ejection of the liquid having high viscosity entails some difficulty. 
     In view of the above, the following Patent Document 1 discloses an ink-jet recording apparatus in which a sub tank, an ink supply pipe connecting the sub tank and a head chip, and a flow-passage substrate provided on the head chip are provided with respective heating devices, for the purpose of lowering the viscosity of the ink under the low temperature condition.
     Patent Document 1: JP-A-2002-264362   

     SUMMARY OF THE INVENTION 
     The ink-jet recording apparatus disclosed in the above-indicated Patent Document 1 is provided with the three heating devices, and one of the three heating devices is disposed outside the head chip, rendering the structure of the apparatus complicated. Further, even though the heating device is disposed on the upper surface of the flow-passage substrate, it is impossible to effectively warm the ink in the head chip, so that the viscosity of the ink in the head cannot be sufficiently lowered. 
     A need has arisen for a liquid ejecting head capable of effectively warming a liquid that flows thereinto. 
     According to one embodiment herein, a liquid ejecting head for ejecting a liquid from a plurality of ejection holes may comprise: a first flow-passage member in which is formed a first liquid-supply passage to which the liquid is supplied from an exterior of the liquid ejecting head; a second flow-passage member in which is formed a second liquid-supply passage connected to the first liquid-supply passage and which has a plurality of outflow ports for dispensing the liquid from the second liquid-supply passage; a third flow-passage member in which are formed (a) at least one common liquid passage each communicating with at least one of the plurality of outflow ports of the second flow-passage member and (b) a plurality of individual liquid passages which are provided so as to respectively correspond to the plurality of ejection holes, each of which is connected to any one of the at least one common liquid passage, and which respectively have pressure chambers formed therein, each of the plurality of individual liquid passages introducing the liquid to a corresponding one of the plurality of ejection holes via a corresponding one of the pressure chambers; and at least one energy giving member configured to give ejection energy to the liquid in each of the pressure chambers that are formed respectively in the plurality of individual liquid passages, wherein the first flow-passage member, the second flow-passage member, and the third flow-passage member are superposed in this order on each other, and wherein the liquid ejecting head further comprises a heater disposed between one surface of the first flow-passage member that faces the second flow-passage member and facing surface of the second flow-passage member as one surface thereof that faces the first flow-passage member. 
     In the liquid ejecting head described above, the liquid in the head can be effectively warmed by the heater disposed between the one surface of the first flow-passage member and the facing surface of the second flow-passage member which faces that one surface, whereby the viscosity of the liquid in the head can be sufficiently lowered. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, advantages and technical and industrial significance of the present invention will be better understood by reading the following detailed description of a preferred embodiment of the invention, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is a vertical cross sectional view showing an internal structure of an ink-jet printer including an ink-jet head according to one embodiment of the invention; 
         FIG. 2  is an exploded perspective view of the ink-jet head of  FIG. 1 ; 
         FIG. 3  is a plan view of a part of a plurality of plates constituting the ink-jet head of  FIG. 1 ; 
         FIG. 4  is a plan view of a part of the plurality of plates constituting the ink-jet head of  FIG. 1 ; 
         FIG. 5  is a plan view of a part of the plurality of plates constituting the ink-jet head of  FIG. 1 ; 
         FIG. 6  is a cross sectional view of a filter support member included in the ink-jet head; 
         FIG. 7  is a schematic cross sectional view of the ink-jet head in its longitudinal direction; 
         FIG. 8  is an enlarged plan view of a part of a flow-passage unit included in the ink-jet head; 
         FIG. 9  is a cross sectional view taken along line IX-IX in  FIG. 8 ; and 
         FIG. 10A  is an enlarged cross sectional view of the actuator unit and  FIG. 10B  is a plan view of an individual electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Referring to the drawings, there will be explained an embodiment of the present invention. 
       FIG. 1  shows an internal structure of an ink-jet printer including an ink-jet head as a liquid ejecting head according to one embodiment of the invention. As shown in  FIG. 1 , the ink-jet printer generally indicated at  101  in  FIG. 1  has a casing  101   a  having a rectangular parallelepiped shape. In the casing  101   a , there are disposed: four ink-jet heads  1  which respectively eject magenta ink, cyan ink, yellow ink, and black ink; and a sheet conveying mechanism  16 . On the inner surface of the top plate of the casing  101   a , a control portion  100  for controlling operations of the ink-jet heads  1  and the sheet conveying mechanism  16  is attached. A sheet-supply unit  101   b  is disposed below the sheet conveying mechanism  16 . The sheet-supply unit  101   b  is removably attached to the casing  101   a . Below the sheet-supply unit  101   b , an ink tank unit  101   c  is disposed so as to be detachable from the casing  101   a.    
     In the ink-jet printer  101 , there is formed a sheet delivery path through which a sheet P is delivered along solid arrows in  FIG. 1  from the sheet-supply unit  101   b  toward a sheet receiving recessed portion  15 . The sheet-supply unit  101   b  includes: a sheet tray  11  having a box-like shape opening upward and accommodating a stack of the sheets P; and a sheet-supply roller  12  configured to supply an uppermost one of the sheets P accommodated in the sheet tray  11 . The sheet P supplied from the sheet tray  11  by the sheet-supply roller  12  is delivered to the sheet conveying mechanism  16  while being guided by sheet guides  13   a ,  13   b  and nipped by rollers of a feed roller pair  14 . 
     The sheet conveying mechanism  16  includes: two belt rollers  6 ,  7 ; an endless sheet conveyor belt  8  wound around the two rollers  6 ,  7  so as to be stretched therebetween; a tension roller  10  which is in contact with the inner circumferential surface of the sheet conveyor belt  8  at the lower half portion of the loop of the sheet conveyor belt  8  while being biased downwardly, thereby applying tension to the sheet conveyor belt  8 ; and a platen  18  which is disposed in a region enclosed by the sheet conveyor belt  8 . The platen  18  supports, at a position where the platen  18  is opposed to the ink-jet heads  1 , the sheet conveyor belt  8  so as to prevent the sheet conveyor belt  8  from sagging downward. The belt roller  7  is a drive roller configured to be rotated clockwise in  FIG. 1  by a drive force given to its shaft from a sheet delivery motor  19 . The belt roller  6  is a driven roller configured to be rotated clockwise in  FIG. 1  by the movement of the sheet conveyor belt  8  in accordance with rotation of the belt roller  7 . The drive force of the sheet delivery motor  19  is transmitted to the belt roller  7  through a plurality of gears. 
     The outer circumferential surface  8   a  of the sheet conveyor belt  8  is silicone-treated so as to have adhesion property. A nip roller  4  is disposed at a position on the sheet delivery path at which the nip roller  4  faces the belt roller  6  with the sheet conveyor belt  8  interposed therebetween. The nip roller  4  is configured to press the sheet P supplied from the sheet-supply unit  101   b  onto the outer circumferential surface  8   a  of the sheet conveyor belt  8 . The sheet P pressed onto the outer circumferential surface  8   a  of the sheet conveyor belt  8  is conveyed in a sheet conveyance direction, namely, in a sub scanning direction, (in the rightward direction in  FIG. 1 ) while being held on the outer circumferential surface  8   s  of the sheet conveyor belt  8  owing to its adhesion property. 
     A separation plate  5  is disposed at a position on the sheet delivery path where the separation plate  5  faces the belt roller  7 . The separation plate  5  separates the sheet P held on the outer circumferential surface  8   a  of the sheet conveyor belt  8  therefrom. The separated sheet P is delivered upward while being guided by sheet guides  29   a ,  29   b  and nipped by rollers of each of two feed roller pairs  28 . Subsequently, the sheep P is ejected from an outlet  30  formed at the upper portion of the casing  101   a  to the sheet receiving recessed portion  15  formed on the upper surface of the casing  101   a.    
     The four ink-jet heads  1  respectively eject inks of the mutually different colors, i.e., magenta, yellow, cyan, and black. Each ink-jet head  1  has a generally rectangular parallepiped shape having a longer dimension in a main scanning direction that is perpendicular to the sub scanning direction. The dimension of each head  1  as measured in the main scanning direction is larger than the width of the sheet. The four ink-jet heads  1  are arranged side by side in the sheet conveyance direction and immovable in the main scanning direction. That is, the ink jet printer  101  is a printer of a line type. 
     The bottom surface of each ink-jet head  1  is made as an ejection surface  2   a  in which are formed a plurality of ejection holes  108  (FIG.  9 ) through which the ink is ejected. When the sheet P being conveyed passes right below the four ink-jet heads  1 , the inks of the different colors are ejected from the ejection holes  108  toward the upper surface of the sheet P, whereby an intended color image is formed on the upper surface, i.e., on the print surface, of the sheet P. 
     The four ink-jet heads  1  are connected respectively to four ink tanks  17  disposed in the ink tank unit  101   c . The inks of the mutually different four colors are stored in the respective four ink tanks  17 . The inks are supplied from the ink tanks  17  to the respective ink-jet heads  1  via respective tubes. 
       FIG. 2  is an exploded perspective view of the ink-jet head  1 . As shown in  FIG. 2 , the ink-jet head  1  includes: a base plate  31 ; a reservoir unit  32  that includes a first flow-passage member and a second flow-passage member; a head main body  33  that includes a flow-passage unit  9  as a third flow-passage member; and two sheet-like heaters  34 ,  35 .  FIGS. 3-5  are plan views showing a plurality of components constituting the head  1 , except for the base plate  31  and a COF  51  that will be explained. As shown in  FIGS. 2-5 , the reservoir unit  32  is constituted by: a laminar body  37  including six plates  42 - 47  and a small-plate group  48 ; and a filter support member  41  that is fixed to the upper surface of the laminar body  37 . The small-plate group  48  consists of eight inner small plates  48   a  and two outer small plates  48   b.    
     Referring to the cross sectional view of  FIG. 6 , the filter support member  41  as the first flow-passage member will be explained. The filter support member  41  is formed by integral molding of a resin material. In the filter support member  41 , there is formed a first liquid-supply passage to which the ink is supplied from the ink tank  17 . Two cylindrical projections  70   a ,  70   b  project upward from an upper surface  70   f  of the filter support member  41 . A vertically extending inlet  71  is formed in the cylindrical projection  70   a . To the cylindrical projection  70   a , a flexible tube is attached, and the ink in the ink tank  17  as an ink supply source is introduced into the filter support member  41  from the inlet  71  via the tube. 
     There is formed, in the filter support member  41 , an ink flow passage  73  as the first liquid-supply passage that includes the vertically extending inlet  71  in which an ink inlet opening is formed and two vertically extending outlets  72   a ,  72   b  in each of which an ink outlet opening is formed. The ink flow passage  73  includes an intermediate portion  93  between the inlet  71  and the two outlets  72   a ,  72   b . In the intermediate portion  93 , there is formed an elongate, rectangular opening  74   a  opening downward. 
     To the filter support member  41 , there is attached a filter  79  in which a plurality of minute through-holes are formed for filtering the ink. The filter  79  divides the intermediate portion  93  into: a first space  74  which is held in communication with the inlet  71  and which is defined by the rectangular opening  74   a ; and a second space  75  which is held in communication with the outlets  72   a ,  72   b . A region of the second space  75  which does not face the filter  79 , i.e., a non-facing region  76 , horizontally extends at a height level that is slightly higher than a height level of a region of the second space  75  which faces the filter  79 . The two outlets  72   a ,  72   b  extend from the non-facing region  76  in the vertically downward direction so as to open to a lower surface  70   e  of the filter support member  41 . 
     The first space  74  has an elongate, rectangular shape. The opening  74   a  is sealed by a damper film  78  as a seal member. The damper film  78  has generally the same shape as the opening  74   a  in plan view. Thus, the damper film  78  cooperates with the filter support member  41  to define the ink flow passage  73 . A peripheral wall  74   b  that defines the opening  74   a  extends downward to a predetermined height level throughout its periphery, so that the damper film  78  fixed to the lower end of the peripheral wall  74   b  extends horizontally. 
     In the second space  75 , a downward opening  75   a  is defined by a recess. The opening  75   a  faces a part of the damper film  78  that extends from a position on a right side of the center of the damper film  78  to the right-side end of the same  78 . The opening  75   a  has a shape, in plan view, that tapers in both of a direction of the ink flow and a direction opposite to the ink flow direction. The filter  79  has a shape substantially similar to that of the opening  75   a  and has a size in plan view somewhat larger than the opening  75   a . The filter  79  is fixed in the first space  74  so as to cover the opening  75   a . In other words, the filter  79  is fixed to the filter support member  41  so as to be opposed to the opening  74   a  and the damper film  78 . 
     The ink introduced from the inlet  71  initially flows substantially horizontally in the first space  74  from the left to the right in  FIG. 6 , then reaches the region of the first space facing the filter  79 , and flows upward through the filter  79 . Subsequently, the ink flows into the second space  75  through the filter  79 . In this occasion, foreign substances present in the ink flowed from the first space  74  are caught by the filter  79 , and the ink from which the foreign substances have been removed by the filter  79  flows in the second space  75 . After the ink has flowed in the non-facing region  76  of the second space  75 , the ink flows downward through the outlets  72   a ,  72   b  and is finally discharged into the plate  42 . 
     The damper film  78  is a flexible resin film. Between the damper film  78  and the upper surface of the plate  42 , there is formed a clearance that allows deflection of the damper film  78  in accordance with vibration of the ink. According to the structure described above, the damper film  78  is deflected in the substantially vertical direction in accordance with the vibration of the ink, whereby the vibration of the ink can be absorbed and damped. 
     An opening is formed in an upper surface  70   f  of the filter support member  41  to define the non-facing region  76 . The opening is sealed by a film  76   a  having flexibility, and the film  76   a  is deflected in accordance with the vibration of the ink, whereby the vibration of the ink is absorbed and damped. 
     In the filter support member  41 , there is further formed a discharge passage connecting the first space  74  and an outlet opening of the cylindrical projection  70   b . The discharge passage initially extends below the non-facing region  76  in the width direction of the filter support member  41 , then extends in the longitudinal direction of the filter support member  41  after having extended upward to the same height level as the non-facing region  76 , and finally communicates with the cylindrical projection  70   b  on the downstream side of a position at which the discharge passage comes down to a height level lower than the filter  79 . A region  77  having the same height level as the non-facing region  76  is defined by sealing an opening formed in the upper surface  70   f  of the filter support member  41  with a film  76   b . The discharge passage is utilized for discharging air bubbles staying in a portion of the filter support member  41  located on the upstream side of the filter  79 . 
     The laminar body  37  including the plates  42 - 47  and the small-plate group  48  constitutes the second flow-passage member. Each of the plates of the laminar body  37  is formed of a metal material having a higher degree of heat conductivity than the resin material of the filter support member  41 . In the plates of the laminar body  37 , there are formed through-holes, openings, and a recess which provide the second liquid-supply passage and eighteen outflow ports described below. 
     More specifically, two through-holes  42   a ,  42   b  are formed through the thickness of the plate  42  in the vicinity of the central portion of the same  42 , so as to be opposed to the inlets  72   a ,  72   b , respectively. The two through-holes  42   a ,  42   b  are connected to the ink flow passage  73  as the first liquid-supply passage. The upper surface of the plate  42  faces the lower surface  70   e  of the filter support member  41 . In the following description, the upper surface of the plate  42  is referred to as a “facing surface”. 
     Two openings  43   a ,  43   b  are formed through the thickness of the plate  43 . The opening  43   a  extends from the vicinity of the central portion of the plate  43  to one of longitudinal ends of the same  43  while the opening  43   a  extends from the vicinity of the central portion of the plate  43  to the other of the longitudinal ends of the same  43 . Each opening  43   a ,  43   b  has a tapered section that tapers in a direction toward the central portion of the plate  43 . The openings  43   a ,  43   b  are opposed, around ends of the respective tapered sections, to the through-holes  42   a ,  42   b , respectively. Two through-holes  44   a ,  44   b  are formed through the thickness of the plate  44  so as to be located at respective longitudinal end portions of the plate  44 . The through-holes  44   a ,  44   b  are respectively opposed to outer ends of the respective openings  43   a ,  43   b.    
     An elongate, rectangular opening  45   a  is formed through the thickness of the plate  45  so as to extend from one of longitudinal end portions of the plate  45  to the other of the longitudinal end portions thereof. The opening  45   a  is opposed, at its longitudinally opposite ends, to the respective through-holes  44   a ,  44   b . A circular through-hole  46   a  is formed through the thickness of the plate  46  around the central portion of the same  46 . The through-hole  46  has a diameter slightly smaller than the width of the opening  45   a  and is opposed to the central portion of the opening  45   a.    
     An elongate recess  47   a  is formed in the plate  47  so as to extend from one of longitudinal end portions of the plate  47  to the other of the longitudinal end portions of the same  47 . The central portion of the recess  47   a  is opposed to the circular opening  46   a . The recess  47   a  is formed by etching a substantially upper half portion of the plate  47  in its thickness direction. 
     In addition to the recess  47   a , eighteen through-holes  47   b  are formed through the thickness of the plate  47  so as to be located within the recess  47   a . More specifically, the eighteen through-holes  47   b  are located so as to be contiguous to the periphery of the recess  47   a  and are arranged, along the longitudinal direction of the plate  47 , in two rows each consisting of nine through-holes  47   b . The nine through-holes  47   b  in each of the two rows are disposed such that eight through-holes  47   b  except for the outermost one of the through-holes  47   b  form four pairs. Each pair consists of two through-holes  47   a  that are located adjacent to each other. Further, the eighteen through-holes  47   b  are disposed so as to have point symmetry with respect to the center of the plate  47 . 
     In each of eight inner small plates  48   a  in the small-plate group  48 , there are formed two through-holes  49   a  which are to be opposed to corresponding two adjacent through-holes  47   b  of the plate  47 . In each of two outer small plates  49   b  between which the eight inner small plates  48   a  are disposed, one through-hole  49   b  is formed so as to be opposed to a corresponding one of the outermost through-holes  47   b  in the plate  47 . 
     In the present embodiment, the second liquid-supply passage is constituted by the through-holes  42   a ,  42   b  formed in the plate  42 ; the openings  43   a ,  43   b  formed in the plate  43 ; the through-holes  44   a ,  44   b  formed in the plate  44 ; the opening  45   a  formed in the plate  45 ; the through-hole  46   a  formed in the plate  46 ; and recess  47   a  formed in the plate  47 , which are in communication with each other. The through-holes  47   b  in the plate  47  and the through-holes  49   a ,  49   b  in the plate  48  constitute a plurality of outflow ports connected to the second liquid-supply passage. More specifically, each outflow port is constituted by a combination of the through-hole  47   b  formed in the plate  47  and the through-hole  48   a  formed in a corresponding small plate  48   a  or the through-hole  49   b  formed in a corresponding small palate  48   b . Each outflow port is connected to a corresponding manifold  105  in the flow-passage unit  9  via a corresponding ink supply hole  105   b  described below. 
     The two heaters  34 ,  35  are fixed to the facing surface  42   c  of the plate  42  so as to be in contact therewith. The length of each heater  34 , as measured in the longitudinal direction of each of the plates  42 - 47  is not larger than half the length of each of the plates  42 - 47  as measured in the same direction. Each heater  34 ,  35  has a generally rectangular shape that extends in the longitudinal direction of the reservoir unit  32 , and is disposed on the facing surface  42   c  such that the longitudinal direction of each heater  34 ,  35  coincides with the longitudinal direction of the reservoir unit  32 . A mid point between a line connecting the two heaters  34 ,  35  coincides with the center of the laminar body  37 , as the second flow-passage member, that includes the plates  42 - 47  and the small-plate group  48 . The arrangement reduces a variation in the temperature in the head  1 , thereby reducing a variation in the temperature of the ink. Accordingly, it is possible to minimize nonuniformity in the printed image. 
     The head main body  33  includes the flow-passage unit  9 , ten filters  106 , and eight actuator units  21 . The filters  106  and the actuator units  21  are fixed to the upper surface of the flow-passage unit  9 . Each filter  106  is provided for a corresponding one of the ten small plates  48   a ,  48   b , and covers one or two ink supply holes  105   b  which will be explained. 
     Each of the eight actuator units  21  includes a plurality of piezoelectric actuators for giving ejection energy to the ink in respective pressure chambers  110  ( FIG. 9 ). The COF  51  which is a flat flexible substrate is bonded to the upper surface of each actuator unit  21 . On each COF  51 , a driver IC  52  for generating drive signals to be supplied to the corresponding actuator unit  21  is mounted. In each driver IC  52 , a temperature sensor is disposed. The filter support member  41 , the laminar body  37  as the second flow-passage member including the plates  42 - 47  and the small-plate group  48 , and the flow-passage unit  9  are stacked on one another in a direction in which the ink flows from the eighteen outflow ports to the manifolds  105 , so as to provide a laminated structure. 
     A plurality of electronic components are disposed on the base plate  31  of the head  1 . The two heaters  34 ,  35  and the COFs  51  are connected to the electronic components via connectors  31   a  attached to the base plate  31 . The electronic components disposed on the base plate  31  are connected to the control portion  100  via wires not shown. The operations of the two heaters  34 ,  35  are controlled by the control portion  100 . As shown in  FIGS. 2 and 3 , in the present embodiment, the two heaters  34 ,  35  have respective heating portions each as a heat element and respective temperature sensors  34   a ,  35   a  for detecting the temperature of the corresponding heating portions. Each temperature sensor  34   a ,  35   a  is constituted by a thermister as a thermoelectric element. Only when the temperature detected by the temperature sensors  34   a ,  35   a  is lower than a prescribed temperature, the heaters  34 ,  35  are electrified. 
       FIG. 7  is a schematic cross sectional view of the head  1  in its longitudinal direction, in which the base plate  31  is not illustrated. In  FIG. 7 , the aspect ratio of each component is largely changed in order that passages can be easily visible. As shown in  FIG. 7 , there is formed a clearance between the facing surface  42   c  of the plate  42  and the lower surface of the filter support member  41 , facilitating installation of the heaters  34 ,  35 . The two heaters  34 ,  35  are fixed to the facing surface  42   c  of the plate  42  so as to be located within the clearance, without contacting the lower surface of the filter support member  41 . In other words, the heaters  34 ,  35  are disposed between two components of the reservoir unit  32 , more specifically, between the facing surface  42   c  of the plate  42  and the lower surface of the filter support member  41 . According to the arrangement, a ratio of the heat that escapes to the exterior of the head  1  with respect to the heat generated by the heaters  34 ,  35  is made small, whereby the laminar body  37  including the plates  42 - 47  and the small-plate group  48  can be effectively warmed by the heat generated by the heaters  34 ,  35 . Consequently, the ink flowing in the laminar body  37  can be effectively warmed. 
     The ink that has flowed from the through-holes  42   a ,  42   b  down to the openings  43   a ,  43   b  flows in the openings  43   a ,  43   b  in mutually opposite directions toward the respective longitudinal end or outer end portions of the plate  43 . Each of the openings  43   a ,  43   b  is a first extending passage portion in the second liquid-supply passage extending along the facing surface  42   c  of the plate  42 . The opening  43   a  is opposed to the heater  34  while the opening  43   b  is opposed to the heater  35 , in the direction of lamination of the plates of the laminar body  37 . As described above, in the present embodiment, the two heaters  34 ,  35  are disposed on the facing surface  42   c  of the plate  42 , and the second liquid-supply passage has the two first extending passage portions that are opposed to the respective heaters  34 ,  35 . Accordingly, the liquid (ink) can be effectively warmed by the two heaters  34 ,  35 . Further, the openings  43   a ,  43   b  are passage portions that are the closest to the heaters  34 ,  35  in the above-indicated lamination direction, so that the ink flowing in the openings  43   a ,  43   b  can be more effectively warmed owing to the plates  42 ,  43 ,  44  that have absorbed the heat of the haters  34 ,  35 . 
     The ink that has flowed from the openings  43   a ,  43   b  down to the opening  45   a  of the plate  45  via the through-holes  44   a ,  44   b  of the plate flows in the opening  45   a  in mutually opposite directions toward the center of the plate  45 . The opening  45   a  includes two second extending passage portions one of which corresponds to a right half portion of the opening  45   a  and the other of which corresponds to a left half portion of the same  45   b , as seen in  FIG. 7 . The two second extending passage portions extend along the facing surface  42   c  of the plate  42  and respectively overlap the openings  43   a ,  43   b  each as the first extending passage portion, as viewed in the lamination direction of the plates of the laminar body  37 . The two second extending passage portions merge with each other at the upstream end of the through-hole  46   a  of the plate  46 . (The upstream end of the through-hole  46   a  will be hereinafter referred to as a “merge point” where appropriate.) The ink flowing in the two second extending passage portions can be effectively warmed owing to the plates  44 ,  45 ,  46  that have absorbed the heat of the heaters  34 ,  35 . 
     In the present embodiment, a resistance against a flow of the ink that flows from the inlet, i.e., the upstream end, of the through-hole  43   a  to the merge point (i.e., the upstream end of the through-hole  46   a ) is equal to a resistance of a flow of the ink that flows from the inlet, i.e., the upstream end, of the through-hole  43   b  to the merge point (i.e., the upstream end of the through-hole  46   a . Therefore, there is not caused a difference between the amount of ink that flows in the through-hole  43   a  and the amount of ink that flows in the through-hole  43   b , whereby it is less likely to be caused a difference in the temperature of the ink in the opening  43   a  and the temperature of the ink in the opening  43   b , which inks are to mix with each other at the merge point. As a result, the temperature of the ink after having mixed tends to be uniform, thereby reducing a variation in the temperature of the ink that flows into the respective eighteen outflow ports. Accordingly, it is possible to minimize nonuniformity in the printed image. 
     In the present embodiment, the opening  45   a  has a length about twice as large as each of the openings  43   a ,  43   b . In other words, a length of each of the two second extending passage portions from its inlet (corresponding to one longitudinal end of the opening  45   a ) to its outlet (corresponding to the central portion of the opening  45   a ) is equal to a length of the opening  43   a  or  43   b  as the first extending passage portion from its inlet (corresponding to the inner end of the opening  43   a  or  43   b ) to its outlet (corresponding to the outer end of the opening  43   a  or  43   b ). Since the second extending passage portions are long, the temperature of the ink can be easily raised by the heaters  34 ,  35 . 
     The ink flow as a result of merging of the ink flows in the respective two second extending passage portions at the merge point (i.e., the upstream end of the through-hole  46   a ) drops into the recess  47   a  of the plate  47  from the downstream end of the through-hole  46   a . Then the ink flows in the recess  47   a  and subsequently flows into the flow-passage unit  9  via the eighteen outflow ports provided by the through-holes  47   b  and the through-holes  49   a ,  49   b . In the present embodiment, in the laminar body  37 , two downstream portions which are located on the downstream side of the corresponding openings  43   a ,  43   b  merge with each other, and the eighteen outflow ports are connected to the second liquid-supply passage on the downstream side of the merge point. Accordingly, it is possible to reduce a variation in the temperature of the ink that flows into the eighteen outflow ports. 
     In the present embodiment, the laminar body  37  is formed of the material having heat conductivity higher than that of the material of the filter support member  41 , and the two heaters  34 ,  35  are fixed so as to be in contact with the facing surface  42   c  of the plate  42 . Accordingly, the heat generated by the two heaters  34 ,  35  can be efficiently transmitted to the ink. Moreover, the temperature sensors  34   a ,  35   a  are integrally disposed on the respective heaters  34 ,  35 , so that it is possible to directly detect, without delay, changes in the temperature of the plates  42  and so on that are caused by the heat generated by the heaters  34 ,  35 . 
     Referring next to  FIGS. 8 ,  9 ,  10 A, and  10 B, the head main body  33  will be explained in detail.  FIG. 8  is a plan view showing a part of two adjacent actuator units  21 .  FIG. 9  is a partial cross sectional view of the flow-passage unit  9  along line IX-IX in  FIG. 8 .  FIG. 10A  is an enlarged cross sectional view of an area enclosed by the dashed line in  FIG. 9  and FIG.  10 B is a plan view of an individual electrode. In  FIG. 8 , apertures  112  that should be indicated by a broken line are indicated by a solid line for easier understanding. 
     As shown in  FIG. 8 , a plurality of pressure chambers  110  each having a generally rhombic shape are regularly disposed in a matrix on the upper surface of the flow-passage unit  9 . Each actuator unit  21  includes a plurality of individual electrodes  135  ( FIG. 10A ) disposed so as to be respectively opposed to the plurality of pressure chambers  110  formed in the flow-passage unit  9 . The actuator unit  21  has a function of selectively giving ejection energy to the ink in the pressure chambers  110 . 
     The ink supply holes  105   b  ( FIG. 5 ) are open to the upper surface of the flow-passage unit  9  so as to respectively correspond to the eighteen outflow ports of the reservoir unit  32 . The ink supply holes  105   b  are covered with corresponding filters  106  each having a smaller mesh size than the filter  79 . In the flow-passage unit  9 , there are formed: a plurality of manifolds  105  each extending from a corresponding one of the ink supply holes  105   b ; and a plurality of sub manifolds  105   a , each as a common liquid passage, which are branched from corresponding manifolds  105 . On the lower surface of the flow-passage unit  9 , the ejection surfaces  2   a  are arranged in each of which a plurality of ejection holes  108 , each as a nozzle opening, are regularly arranged in matrix. 
     As shown in  FIG. 9 , the flow-passage unit  9  is constituted by nine metal plates including a cavity plate  122 , a base plate  123 , an aperture plate  124 , a supply plate  125 , three manifold plates  126 ,  127 ,  128 , a cover plate  129 , and a nozzle plate  130 , which are arranged in this order from the top of the flow-passage unit  9 . Each of the nine plates  122 - 130  has a rectangular shape in plan view which is long in the main scanning direction. 
     The nine plates  122 - 130  are positioned with and stacked on each other, whereby a plurality of individual ink passages  132  as a plurality of individual liquid passages are defined in the flow-passage unit  9  each of which extends from an outlet of a corresponding one of the sub manifolds  105   a  to a corresponding one of the ejection holes  108  via a corresponding one of the pressure chambers  110 . The ink which has supplied from the reservoir unit  32  to the flow-passage unit  9  via the ink supply holes  105   b  flows into the sub manifolds  105   a  from the manifolds  105 . The ink in the sub manifolds  105   a  flows into the individual ink passages  132  and reaches nozzle ejection holes  108  via the apertures  112  each functioning as an orifice and the pressure chambers  110 . 
     The actuator unit  21  will be explained. As shown in  FIG. 5 , the eight actuator units  21  each having a trapezoidal shape in plan view are arranged in a zigzag fashion in the longitudinal direction of the flow-passage unit  9  so as to avoid the ink supply holes  105   b . Parallel facing sides (short and long sides) of each actuator unit  21  are parallel to the longitudinal direction of the flow-passage unit  9 , and oblique sides of neighboring two actuator units  21  partially overlap as viewed in the longitudinal direction of the flow-passage unit  9 , namely, in the main scanning direction, as shown in  FIG. 8 . 
     As shown in  FIG. 10A , each actuator unit  21  includes three piezoelectric layers  141 - 143  formed of a ceramic material of lead zirconate titanate (PZT) having ferroelectricity. The individual electrodes  135  are formed on respective regions of the uppermost piezoelectric layer  141  that correspond to the pressure chambers  110 . A common electrode  134  is provided on an interface between the uppermost piezoelectric layer  141  and the piezoelectric layer  142  located under the layer  141 . As shown in  FIG. 10B , each individual electrode  135  has a generally rhombic shape in plan view similar to the pressure chamber  110 . One acute end portion of the individual electrode  135  extends beyond the pressure chamber  110 , and a circular land  136  is formed at the acute end portion for electrical connection with the individual electrode  135 . In addition to the lands  136  for the individual electrodes  135 , a land for the common electrode  134  is formed on the upper surface of the piezoelectric layer  141 . The land for the common electrode  134  is connected to the common electrode  134  via the conductive material in through-holes. 
     The common electrode  134  is kept at a ground potential as a basic potential given by the COF  51 . The individual electrodes  135  are electrically connected to terminals of the driver IC  52  via the respective lands  136  and respective internal wires of the COF  51 . A drive signal for driving the actuator unit  21  is supplied from the driver IC  52  to the individual electrodes  135  independently of each other. Accordingly, respective portions in the actuator unit  21  sandwiched by and between the individual electrodes  135  and the pressure chambers  110  function as individual actuators which are independent of each other. That is, a plurality of actuators, each as an energy giving member, are provided in the actuator unit  21  in the same number as the pressure chambers  110 . 
     There will be next explained a method of driving each actuator unit  21  to permit ink droplets to be ejected from the nozzles. The piezoelectric layer  141  is polarized in its thickness direction. When an electric field is applied to the piezoelectric layer  141  in the polarization direction with one individual electrode  135  kept at a potential different from that of the common electrode  134 , a portion of the piezoelectric layer  141  to which the electric field is applied functions as an active portion that undergoes strain owing to a piezoelectric effect. The active portion expands in a direction of thickness of the layer  141  and contracts in a direction parallel to the plane of the layer  141  (i.e., in the plane direction) when the electric field and the polarization are in the same direction. In this instance, the amount of deformation of the active portion upon expansion and contraction is larger in the plane direction than in the thickness direction. In the actuator unit  21 , the uppermost one  141  of the three piezoelectric layers that is the most distant from the pressure chambers  110  is an active layer including the active portions while the lower two piezoelectric layers  142 ,  143  nearer to the pressure chambers  110  are non-active layers. As shown in  FIG. 10A , the piezoelectric layer  143  is fixed to the upper surface of the cavity plate  122  that defines the pressure chambers  110 . Accordingly, when there is generated a difference in strain in the plane direction between the portion of the piezoelectric layer  141  to which the electric field is applied and the piezoelectric layers  142 ,  143  located under the layer  141 , the entirety of the piezoelectric layers  141 - 143  deforms into a convex shape that protrudes toward the pressure chamber  110  (unimorph deformation). Accordingly, the pressure (ejection energy) is given to the ink in the pressure chamber  110 , so that there is generated a pressure wave in the pressure chamber  110 . The generated pressure chamber propagates from the pressure chamber  110  to the ejection hole  108  of the corresponding nozzle, whereby the ink droplets are ejected from the ejection hole  108 . 
     In the illustrated embodiment, the ink that flows in the laminar body  37  can be effectively warmed by the heaters  34 ,  35  disposed between the facing surface  42   c  of the plate  42  and the lower surface of the filter support member  41 . Accordingly, the viscosity of the ink in each head  1  can be sufficiently lowered. Hence, even under the low temperature condition, the resistance against the flow of the ink at a time when the ink flows into the pressure chamber  110  does not become high, so that increasing the drive frequency becomes effective for obtaining a satisfactory ejection effect. 
     Further, the same ejection characteristic as obtained under the ordinary temperature condition can be obtained under the low temperature condition without increasing the drive voltage, so that it is not required to increase the withstand voltage of the actuator units  21  and the withstand voltage of the driver ICs  52  configured to drive the actuator units  21 . 
     It is to be understood that the invention is not limited to the details of the illustrated embodiment, but may be embodied with various changes and modifications, which may occur to those skilled in the art, without departing from the spirit and scope of the invention defined in the attached claims. For instance, the heaters  34 ,  35  may be disposed on the lower surface of the filter support member  41 . Only one heater or more than three heaters may be used. Only one through-hole may be formed in the plate  42 . Only one opening may be formed in the plate  43 . Two openings may be formed in the plate  45 . Only one common liquid passage may be formed in the flow-passage unit  9 . The passage structure in the head  1  is not limited to that in the illustrated embodiment, but may be otherwise modified. The energy giving member is not limited to the one utilizing the piezoelectric body, but the one of a thermal type may be utilized. 
     It is to be understood that the principle of the invention may be applicable not only to the head for a line printer as in the illustrated embodiment, but also to a head for a serial printer, and further to a head for ejecting a liquid other than the ink.