Patent Publication Number: US-7712873-B2

Title: Inkjet head

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the divisional application of U.S. patent application Ser. No. 11/387,855, filed Mar. 24, 2006, which claims the benefit of Japanese Patent Application No. 2005-085798, filed Mar. 24, 2005, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an inkjet head, which ejects ink to a recording medium. 
     2. Description of the Related Art 
     US 2005/0083379 A1 discloses an inkjet head, which ejects ink from nozzles to a recording medium such as a printing sheet. The inkjet head has a flow path unit, a reservoir unit and an actuator unit. The flow path unit is formed with a common ink chamber and a plurality of individual ink flow paths that communicate with the common ink chamber while reaching nozzles via respective pressure chambers. The reservoir unit has a reservoir for supplying a stored ink to the common ink chamber. The reservoir unit is joined to the flow path unit. The actuator unit applies an ejection energy to the ink in the flow path unit. A filter for removing dust or the like staying in the ink is also placed in the reservoir. 
     SUMMARY OF THE INVENTION 
     However, dust or the like, which passes through the filter placed in the reservoir, may enter the individual ink flow paths that are minute flow paths. Complicated and minute flow paths are formed in the flow path unit. Therefore, it is relatively difficult to place in the flow path unit a filter for preventing dust from entering into the individual ink flow paths. 
     The invention provides an inkjet head in which entering of dust or the like into individual ink flow paths can be suppressed by a simple configuration. 
     According to one embodiment of the invention, an inkjet head includes a flow path unit, an actuator unit, a plurality of filters, a reservoir unit, a flexible flat cable, a cover member and a sealant. The flow path unit includes a plurality of ink inflow ports, a common ink chamber and a plurality of individual ink flow paths. Ink flowing into the ink inflow ports is supplied to the common ink chamber. Each of individual ink flow paths extends from an outlet of the common ink chamber to a nozzle through a pressure chamber. The actuator unit applies an ejection energy to the ink in the pressure chambers. The actuator unit is joined to an inflow-port face of the flow path unit in which the ink inflow ports are formed. The filters are joined to the inflow-port face of the flow path unit. The filters cover the ink inflow ports. The reservoir unit is formed with an ink reservoir that stores the ink. The reservoir unit includes a first face, a second face opposite to the first face and a side face connecting the first face and the second face. The second face includes a first region and a second region. The first region at least partially faces the actuator unit with a gap therebetween. The second region at least partially abuts against the filters. The side face defines a first recess and a second recess. 
     The first recess reaches the first region of the second face. The second recess reaches the second region of the second face between adjacent two filters. The reservoir unit supplies the ink in the ink reservoir into the flow path unit through the filters. The flat flexible cable includes a fixed portion and a extending portion. The fixed portion is fixed to the actuator unit. The extending portion is withdrawn from the fixed portion and extends in a direction away from the flow path unit. The cover member includes an end face and an accommodation region. The end face abuts against the first face of the reservoir unit. The accommodation region is accommodated in the first recess. The extending portion of the flat flexible cable is interposed between the first recess and the accommodation region. The sealant that is applied to a gap between side faces of the two adjacent filters on the inflow-port face of the flow path unit and applied to the second recess. 
     According to this configuration, entering of dust or the like into the individual ink flow paths can be suppressed by the simple configuration in which the filters are placed between the flow path unit and the reservoir unit. Since the second recess is formed, the sealant for preventing the ink from passing through the gap between two adjacent filters and reaching the actuator unit can be easily applied to the gap between the side faces of the two filters. Thereby, it is possible to prevent ink mist, that is, tiny drops of ink from entering through the gap between two adjacent filters into the inkjet head to damage the actuator unit. Since the cover member partly covers the side face of the reservoir unit, the inkjet head can be miniaturized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an inkjet head according to one embodiment of the invention. 
         FIG. 2  is a section view of the inkjet head taken along a line II-II of  FIG. 1 . 
         FIG. 3  is a section view of a reservoir unit and a head body, which are shown in  FIG. 1 , taken along a main scanning direction. 
         FIGS. 4A to 4H  are exploded plan views of the reservoir unit shown in  FIG. 3 . 
         FIG. 5  is a partial plan view of a lower face of a plate shown in  FIG. 4H . 
         FIG. 6  is a plan view of the head body shown in  FIG. 1 . 
         FIG. 7  is an enlarged view of a region enclosed by a one-dot chain line in  FIG. 6 . 
         FIG. 8  is a partial section view taken along a line VIII-VIII in  FIG. 7 . 
         FIG. 9  is a partial exploded perspective view of the head body shown in  FIG. 1 . 
         FIG. 10A  is an enlarged section view of an actuator unit shown in  FIG. 9 , and  FIG. 10B  is a plan view showing an individual electrode placed on a surface of the actuator unit in  FIG. 10A . 
         FIG. 11  is a partial side view of the inkjet head shown in  FIG. 1 . 
         FIG. 12  is a plan view of a head body according to another embodiment, and corresponds to  FIG. 6 . 
         FIG. 13  is a partial side view of an inkjet head according to the another embodiment, and corresponds to  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Hereinafter, embodiments of the invention will be described with reference to the accompanying drawings. 
       FIG. 1  is an external perspective view of an inkjet head  1 , which is used in an inkjet printer.  FIG. 2  is a section view taken along a line II-II shown in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , the inkjet head  1  has a shape elongating in a main scanning direction. The inkjet head  1  has a head body  1   a , a reservoir unit  70 , two thin film filters  54   a  and four thin film filters  54   b,  and a control section  80  for controlling driving of the head body  1   a  in order from its bottom. Hereinafter, the components of the inkjet head  1  will be described. 
     The control section  80  has: a main board  82 ; sub-boards  81 , which are placed on the both sides of the main board  82 ; and driver ICs  83 , which are fixed to side faces of the sub-boards  81  opposed to the main board  82 . The driver ICs  83  generate a signal for driving actuator units  21 , which are included in the head body  1   a.    
     The main board  82  and the sub-boards  81  have a rectangular planes elongating in the main scanning direction, and are upright in parallel to each other. The main board  82  is fixed to the upper face of the reservoir unit  70 . The sub-boards  81  are above the reservoir unit  70  and are placed on the both sides of the main board  82  with being separated from the main board  82  by the same distance. The main board  82  and the sub-boards  81  are electrically connected to each other. Heat sinks  84  are fixed to faces of the driver ICs  83  opposed to the sub-boards  81 . Specifically, the heat sinks  84  are formed on the both side faces of the sub-boards  81 , and the driver ICs  83  are thermally coupled to the heat sinks  84  via thermal conduction sheets  85 . 
     Each of FPCs (Flexible Printed Circuits)  50  function as a power supplying member. One end of each FPC  50 , which functions as a fixed portion, horizontally extends along a plane of a flow path unit  4 . The fixed portions are fixed and connected to the actuator units  21 . Extending portions, which are withdrawn from the fixed portions of the FPCs  50 , are bent and extend in a direction (the upward direction in  FIG. 2 ) away from the head body  1   a . At his time, parts of the extending portions are accommodated in recesses  53  (functioning as first recesses), which are formed in side faces of the reservoir unit  70 . The other ends of the FPCs  50  are connected to the sub-boards  81 . The FPCs  50  are connected also to the driver ICs  83  on the way from the actuator units  21  to the sub-boards  81 . Namely, the FPCs  50  are electrically connected to the sub-boards  81  and the driver ICs  83  to transmit signals output from the sub-boards  81  to the driver ICs  83 , and supply driving signals output from the driver ICs  83  to the actuator units  21 . 
     The inkjet head  1  is further provided with an upper cover  51 , which covers the control section  80 , and a lower cover  52  (functioning as a cover member), which covers a lower portion of the head  1 . The covers  51 ,  52  prevent inks scattering in the printing process from adhering to the control section  80 , etc. In  FIG. 1 , the upper cover  51  is omitted so that the control section  80  can be seen. 
     As shown in  FIG. 2 , the upper cover  51  has an arched ceiling, and covers the control section  80 . The lower cover  52  has a substantially rectangular cylindrical shape, which is open upward and downward. The lower cover  52  covers a lower portion of the main board  82 . In an upper portion of the lower cover  52 , upper walls  52   b,  which projects inward from the upper end of the sidewall of the lower cover  52 , is formed. The lower end of the upper cover  51  is placed on a portion where the upper wall  52   b  is connected to the sidewall. The lower cover  52  and the upper cover  51  have a substantially same width as that of the head body  1   a.    
     In the lower end of each of the both sidewalls (only one of the sidewalls is shown in  FIG. 1 ) of the lower cover  52 , two projections  52   a  (functioning as accommodation regions) projecting downward are arranged in the longitudinal direction of the lower cover. The projections  52   a  are placed in the recesses  53  while covering the extending portions of the FPCs  50  accommodated in the recesses  53 . Namely, the projections  52   a  face the side faces of the reservoir unit  70  with a gap therebetween. The lower end faces of the sidewalls other than the projections  52   a  abut against the upper face of the reservoir unit  70  (functioning as a first face of the reservoir unit  70 ). The tip end faces of the projections  52   a  face the flow path unit  4  of the head body  1   a  while forming a gap therebetween for absorbing a production error. A sealant (not shown) is applied between (i) all of the end face of the lower cover  52  and (ii) the reservoir unit  70  and the flow path unit  4 . In this embodiment, a sealant made of a soft material is used, and specifically a silicon resin is used for sealing. 
     Next, the reservoir unit  70  will be described with further reference to  FIGS. 3 and 4 .  FIG. 3  is a section view of the reservoir unit  70  and the head body  1   a  taken along the main scanning direction.  FIG. 4  is an exploded plan view of the reservoir unit  70 . In FIG.  3 , for the sake of convenience in description, the scale in the vertical direction is expanded, and an ink flow path of the reservoir unit  70 , which is not usually shown in a section taken along the same line, is shown desirably. 
     The reservoir unit  70  temporarily stores ink, and supplies the stored ink to the flow path unit  4  of the head body  1   a . As shown in  FIG. 4 , the reservoir unit  70  has a stacked layer structure in which seven plates  71 ,  73 ,  74 ,  75 ,  76 ,  77 , and  78  that have a rectangular plane elongating in the main scanning direction (see  FIG. 1 ), and one damper sheet  72  are stacked. The seven plates  71 ,  73  to  78  are plates of a metal such as stainless steel. 
     In the uppermost first plate  71 , as shown in  FIGS. 3 and 4A , circular holes  71   a ,  71   b  are formed in the vicinities of one and other ends of the first plate  71  in the longitudinal direction, respectively. The circular holes  71   a ,  71   b  are placed in positions, which are shifted from the center of the first plate  71  in the width direction toward the one and other width ends. An oval recess  71   c , which elongates in the longitudinal direction of the first plate  71 , is formed in the lower face (the face on the side of the damper sheet  72 ) of the first plate  71 . The oval recess  71  c is positioned between the center of the first plate  71  in the longitudinal direction and the circular hole  71   b . A circular hole  71   d  is formed in the center of the bottom of the oval recess  71   c.  The oval recess  71   c  and the damper sheet  72 , which will be described below, constitute a damper chamber. 
     The damper sheet  72 , which is the second layer from the top, is made of a flexible thin film member. As shown in  FIGS. 3 and 4B , circular holes  72   a,    72   b  corresponding to the circular holes  71   a ,  71   b  formed in the first plate  71  are formed in the damper sheet  72 . The material of the flexible thin film member may be a metal, a resin, or the like, and is not limited those examples so long as it can easily bend in accordance with pressure variation in the ink. In this embodiment, used is a composite resin film in which a gas barrier film is added to a PET (polyethylene terephtalate) resin that originally has an excellent gas barrier property. According to this configuration, transmission of air or steam through the flexible thin film member is very suppressed, and the member functions also as an excellent damper against pressure variation in the ink. 
     As shown in  FIGS. 3 and 4C , circular holes  73   a,    73   b  corresponding to the circular holes  71   a ,  71   b  formed in the first plate  71 ; and an oval hole  73   c  corresponding to the oval recess  71   c  formed in the first plate  71  passes through the third plate  73 , which is the third layer from the top. 
     In the fourth plate  74 , which is the fourth layer from the top, as shown in  FIGS. 3 and 4D , thin recesses  74   a,    74   b  are formed so as to obliquely elongate toward the center of the fourth plate  74  in the short side direction from regions corresponding to the circular holes  71   a ,  71   b  formed in the first plate  71 . Furthermore, an oval hole  74   c , which elongates to the center of the fourth plate  74  while communicating with the thin recess  74   a,  is formed in the fourth plate  74 . Two step faces  74   d,    74   e,  which have different heights, are formed in the peripheral portion of the oval hole  74   c.  A reservoir filter  74   g,  which removes dust and the like in the ink, is placed on the step face  74   e , which is lower than the step face  74   d.  Furthermore, an oval recess  74   f,  which elongates to the center of the fourth plate  74  while communicating with the thin recess  74   b,  is formed in the fourth plate  74 . The oval recess  74   f,  which is concaved, has a shape and size, which are substantially identical with those of the oval hole  73   c  of the third plate  73 . The oval recess  74   f  is open on the side of the third plate  73 . The bottom faces of the thin recesses  74   a,    74   b;  those of the step face  74   d;  and the oval recess  74   f  are formed on the same plane. A damper communication port  74   h  is formed in a sidewall in the vicinity of the center of the fourth plate  74 . The oval hole  74   c  and the oval recess  74   f  communicate with each other through the damper communication port  74   h.  The thin recess  74   a,  and the portion of the oval hole  74   c  on the side of the plate  73  with respect to the step face  74   e  form an upstream ink reservoir  61   a . The oval recess  74   f  and the thin recess  74   b  form a damper flow path  62 . 
     As shown in  FIGS. 3 and 4E , a circular hole  75   a  is formed in the center of the fifth plate  75 , which is the fifth layer from the top. The circular hole  75   a  forms a drop flow path  63 . The fifth plate  75  is stacked from the lower side so that the circular hole  75   a  communicates with the through hole  74   c  of the fourth plate  74 . The circular hole  75   a  faces an acute angle portion of the through hole  74   c,  which is on the side of the center of the fourth plate  74 . 
     As shown in  FIGS. 3 and 4F , a through hole  76   a  is formed in the sixth plate  76 , which is the sixth layer from the top. The plan shape of the through hole  76   a  elongates so as to be bent and tapered along the main scanning direction, and symmetric about its center. Specifically, the through hole  76   a  includes a main flow path  76   b,  which elongates in the main scanning direction, and tributary flow paths  76   c,  which diverge from the main flow path  76   b.  The tributary flow paths  76   c  have a flow path width that is smaller than that of the main flow path  76   b.  Each two tributary flow paths  76   c,  which elongate in the same direction, are paired. Two pairs of tributary flow paths  76   c,  which elongate in different directions, elongate from each end of the main flow path  76   b  in the width direction while separating from each other in the longitudinal direction of the main flow path  76   b.  The four pairs of tributary flow paths  76   c  are arranged in a staggered pattern. The portion of the oval hole  74   c  of the fourth plate  74  on the side of the plate  75  with respect to the step face  74   e,  the circular  75   a  of the fifth plate  75 , and the through hole  76   a  form a downstream ink reservoir  61   b.  The both ends of the main flow path  76   b  in the longitudinal direction are shifted toward the side opposite to the region corresponding to the circular holes  71   a ,  71   b  of the first plate  71  with respect to the width direction of the sixth plate  76 . According to this configuration, the strength of rigidity of the whole reservoir unit  70  is not deviated. 
     In the seventh plate  77 , which is the seventh layer from the top, as shown in  FIGS. 3 and 4G , a total of ten circular holes  77   a  are formed in positions corresponding to the both ends of the main flow path  76   b  formed in the sixth plate  76  in the longitudinal direction, and tip end portions of the tributary flow paths  76   c.  Five of the circular holes  77   a  are arranged in the longitudinal direction in the vicinity of each end of the seventh plate  77  in the width direction. Specifically, one, two, and two holes  77   a  are arranged in the one width end in order from one end side (the left side of  FIG. 4G ) in the longitudinal direction and, one, two, and two holes  77   a  are arranged in the other width end in order from the other end side (the right side of  FIG. 4G ) in the longitudinal direction, so as to be separated from each other in a staggered manner to avoid notches  53   f,  which will be described later. The circular holes  77   a  are arranged symmetrically about the center of the plate  77 . 
     In the eighth plate  78 , which is the lowest layer, as shown in  FIGS. 3 and 4H , circular holes  78   a  corresponding to the circular holes  77   a  formed in the seventh plate  77  are formed. In the lower face (the face, which is closer to the head body  1   a ) of the eighth plate  78 , peripheral portions (portions enclosed by broken lines in the figure) of the circular holes  78   a  project downward. Openings of the circular holes  78   a  in the lower face of the eighth plate  78  function as ink supply ports  59  for supplying the ink to the flow path unit  4 . 
     The lower face of the eighth plate  78  (functioning as a second face of the reservoir unit  70 ) will be described with reference to  FIG. 5 .  FIG. 5  is a partial plan view of the lower face of the eighth plate  78 . In  FIG. 5 , a region against which the thin film filters  54   a,    54   b  abut is indicated by the one-dot chain line. In the lower face of the eighth plate  78 , as shown in  FIG. 5 , surfaces of the downward projecting portions function as second regions  57  at least part of which the thin film filters  54   a,    54   b  abut against and are joined to by an adhesive agent. The surface other than the downward projecting portions function as a first region  58 , which at least partially faces the actuator units  21  with a gap therebetween (see  FIGS. 2 and 7 ). Each of the second regions  57  includes a groove region  57   a  where lattice-like grooves are formed, and a flat non-groove region  57   b  where the lattice-like grooves are not formed. The lower-face openings (the ink supply ports  59 ) of the circular holes  78   a  are formed in the groove region  57   a.  The thin film filters  54   a,    54   b  are placed so as to abut against the groove region  57   a  while covering the ink supply ports  59 . At this time, the whole circumferences of the outer edges of the thin film filters  54   a,    54   b  abut against the non-groove region  57   b.  That is, the non-groove region  57   b  has an annular shape along the outer edges of the thin film filters  54   a,    54   b.    
     As shown in  FIG. 3 , the reservoir unit  70  of this embodiment is configured so that the seven plates  71 ,  73  to  78  and the one damper sheet  72  are stacked and fixed to each other while being positioned. The side faces of the reservoir unit  70  connect its upper face (first face) and its lower face (second face). As seen from  FIG. 4 , the three plates  71 ,  73 ,  74  are longer in the longitudinal direction than the remaining plates  75  to  78 . The inkjet head  1  can be fixed to a fixing portion (not shown) of the printer with using the both end portions of the three plates  71 ,  73 ,  74 , i.e., the portions which further extend toward the both sides in the longitudinal direction as compared with the plates  75  to  78 . 
     In the both ends of each of the plates  71 ,  73  to  78  of the width direction, as shown in  FIGS. 4A to 4H , two and two or a total of four rectangular notches  53   a  to  53   g  are formed in the longitudinal direction in a staggered pattern. As result of vertically positioning the plates  71 ,  73  to  78  and the damper sheet  72  with each other, the recesses  53 , which elongate from the upper face of the reservoir unit  70  to the first region  58  to penetrate the reservoir unit  70  in the stack direction, are formed by the notches  53   a  to  53   g  (see  FIGS. 1 ,  2 , and  7 ). The width of the reservoir unit  70  except the regions where the recesses  53  are formed is substantially identical with that of the flow path unit  4 . In the second regions  57  of the eighth plate  78 , in order to prevent the ink from leaking from the ink supply ports  59 , a region having a predetermined area is required in the peripheries of the ink supply ports  59 . This region is a factor of determining the width of the flow path unit  4 . On the other hand, the FPCs  50 , which are withdrawn from the actuator units  21 , and the projection regions  52   a  of the lower cover  52 , which cover FPCs  50 , are accommodated in the recesses  53 . Therefore, the width of the inkjet head  1  can be reduced to that of the flow path unit  4 . Namely, the formation of the recesses  53  enables the inkjet head  1  to be miniaturized. As seen also from  FIG. 5 , a recess formed by the first region  58  is continuous with the recesses  53  (the notches  53   g ). In this embodiment, the frontages (lengths of the openings in the longitudinal direction) of the recesses  53  are wider than those of openings formed by the first region  58 . Since the openings of the recesses  53  are equal to or larger than the openings of the first region  58 , the extended portions of the FPCs  50 , which are withdrawn from the side of the first region  58  can easily extend upward through the recesses  53 . 
     In each of the ends of the plates  76  to  78 , as shown in  FIGS. 4F to 4H , each of the rectangular notches  55   a  to  55   c  is formed in a region corresponding to a region between the thin film filter  54   a  and the thin film filter  54   b,  which is closest to the filter  54   a . 
     When the plates  76  to  78  are vertically positioned to each other, the notches  55   a  to  55   c  form a recess  55  (functioning as a second recess), which extends from the lower face of the plate  75  to reach the second regions  57  of the plate  78  (see  FIGS. 1 and 7 ). 
     Next, the ink flow in the reservoir unit  70  when the ink is supplied will be described. 
     As shown in  FIG. 3 , a supply joint  91  and a discharge joint  92  are fixed to the positions of the upper face of the first plate  71  where the circular holes  71   a ,  71   b  are formed. The joints  91 ,  92  are cylindrical members, which have base ends  91   b,    92   b  having a slightly larger outer diameter. Openings of cylindrical spaces  91   a ,  92   a  in the lower faces of the base ends  91   b,    92   b  are placed on the upper face of the first plate  71  so as to coincide with the openings of the circular holes  71   a ,  71   b  of the first plate  71 , respectively. Hereinafter, the flow (indicated by the solid arrows in  FIG. 3 ) of the ink, which is supplied through the supply joint  91  into the reservoir unit  70 , will be described. 
     As indicated by the solid arrows in  FIG. 3 , the ink, which has flown into the circular holes  71   a  through the cylindrical space  91   a  of the supply joint  91 , flows into the upstream ink reservoir  61   a  through the circular holes  72   a,    73   a.  The ink, which has flown into the upstream ink reservoir  61   a , flows into the damper flow path  62  through the damper communication port  74   h,  and passes through the reservoir filter  74   g  and flows into the downstream ink reservoir  61   b.  In the downstream ink reservoir  61   b,  the flow-in ink is caused by the circular hole  75   a  of the fifth plate  75  to drop onto a substantially center of the main flow path  76   b  of the sixth plate  76 . As indicated by the arrows in  FIG. 4F , thereafter, the ink is directed from the substantially center of the main flow path  76   b  to the both ends of the main flow path  76   b  in the longitudinal direction, and also to the tip ends of the tributary flow paths  76   c.  The ink, which has reached the both ends of the main flow path  76   b  in the longitudinal direction and the tip ends of the tributary flow paths  76   c,  flows into ink inflow ports  5   b  (see  FIG. 6 ), which are open in the upper face of the flow path unit  4 , from the ink supply ports  59  through the circular holes  77   a,    78   a.  In this way, the ink is temporarily stored in the upstream ink reservoir  61   a  and the downstream ink reservoir  61   b . In the initial process of introducing the ink, the ink, which flows into the damper flow path  62 , is discharged to the outside from the discharge joint  92 , whereby air bubbles existing in the upstream ink reservoir  61  a and the damper flow path  62  can be easily discharged. Namely, the space on the upstream side of the reservoir filter  74   g  is filled with the ink in a state where there is no residual air bubble. 
     As shown in  FIG. 3 , the third plate  73  serves as a flow path wall, which defines the damper flow path  62 . The opening of the oval hole  73   c,  which is formed in the flow path wall, is covered by the damper sheet  72 . The region of the damper sheet  72 , which covers the opening of the oval hole  73   c , faces the oval recess  71   c  of the first plate  71 . 
     The space, which is defined by the damper sheet  72  and the oval recess  71   c,  forms a damper chamber. The damper chamber communicates with the atmosphere through the circular hole  71   d . Namely, the damper sheet  72  is interposed between the ink in the damper flow path  62  and the atmosphere. Even when pressure variation occurs in the ink filling the reservoir unit  70 , therefore, the pressure variation can be attenuated by vibration of the damper sheet  72 . Furthermore, excess displacement of the damper sheet  72  toward the oval recess  71   c  is restricted by the bottom of the oval recess  71   c.    
     Therefore, the damper sheet  72  is prevented from being damaged. The bottom of the oval recess  71   c  prevents an external force, which may break the damper sheet  72 , from being applied to the sheet. 
     Next, the thin film filters  54   a,    54   b  and the head body  1   a  will be described with reference to  FIG. 6 .  FIG. 6  is a plan view of the head body  1   a  to which the thin film filters  54   a,    54   b  are joined. As shown in  FIG. 6 , the head body  1   a  includes the flow path unit  4  and the four actuator units  21 , which are fixed to the upper face of the flow path unit  4 . 
     The flow path unit  4  has a substantially rectangular parallelepiped external shape, which has an approximately same width as the reservoir unit  70 , and which has a length in the main scanning direction substantially equal to the length of a stack structure formed by the fifth to eighth plates  75  to  78  of the reservoir unit  70 . As described later, the flow path unit  4  is formed with a manifold flow path  5  and many individual ink flow paths  32 , which communicate with the manifold flow path  5 , and each of which includes a pressure chamber  10  and a nozzle  8  (see  FIG. 8 ). The upper face of the flow path unit  4  functions as an inflow-port face  4   a  in which ten ink inflow ports  5   b  communicating with the manifold flow path  5  are formed. The ink inflow ports  5   b  are placed so as to correspond to the ink supply ports  59  of the circular holes  78   a  formed in the eighth plate  78 . Namely, five ink inflow ports  5   b  are arranged in the longitudinal direction in the vicinity of each of the width ends of the flow path unit  4 . Specifically, one, two, and two ink flow ports  5   b  are arranged in the one width end in order from one end side (the upper side of  FIG. 6 ) in the longitudinal direction, and one, two, and two ink flow ports  5   b  are arranged in the other width end in order from the other end side (the lower side of  FIG. 6 ) in the longitudinal direction, so as to be separated from each other in a staggered manner. 
     The actuator units  21  have a function of selectively applying an ejection energy to the ink in the pressure chambers  10  formed in the flow path unit  4 , and have a trapezoidal plan shape. In the inflow-port face  4   a  of the flow path unit  4 , the four actuator units  21  are placed in a staggered pattern so as to avoid the ink inflow ports  5   b.    
     In each of the actuator units  21 , the parallel opposing sides extend along the longitudinal direction of the flow path unit  4 . Oblique sides of adjacent actuator units  21  overlap with each other with respect to the width direction of the flow path unit  4 . The four actuator units  21  have a relative positional relationship in which the actuator units  21  are separated by the same distance from the center of the flow path unit  4  in the width direction toward the opposite sides. The actuator units  21  are placed in a region, which faces the first region  58  of the reservoir unit  70 . The FPCs  50  connected to the actuator units  21  are withdrawn from the longer ones of the parallel opposing sides of the actuator unit  21 . 
     The thin film filters  54   a,    54   b  are thin films having: an ink not-passing region, which does not allow the ink to pass therethrough; and an ink passing region, which allows the ink to pass therethrough while filtering dust and the like in the ink. The thin film filters  54   a,    54   b  are joined by an adhesive agent to the second regions  57  of the reservoir unit  70  and to the inflow-port face  4   a  of the flow path unit. At this time, the ink passing regions of the thin film filters  54   a,    54   b  are sandwiched between the ink supply ports  59  opening in the second regions  57  and the corresponding ink inflow ports  5   b  opening in the inflow-port face  4   a  of the flow path unit  4 . 
     The thin film filters  54   a  are placed to correspond to the ink inflow ports  5   b  respectively formed in the vicinities of the ends of the flow path unit  4  in the longitudinal direction. The think film filters  54   a  extend in a band-like manner over the whole region in the short side direction of the flow path unit  4 . Each of the thin film filters  54   b  is placed between the thin film filters  54   a  so as to cover two of the ink inflow ports  5   b , which are arranged in a staggered pattern. At this time, no actuator unit  21  is located between a certain thin film filters  54   a  and a thin film filters  54   b  closest to the certain thin film filter  54   a.  An actuator unit  21  is present between a certain thin film filter  54   a  and a thin film filter  54   b  other than the thin film filter  54   b  closest to the certain thin film filter  54   a.  An actuator unit  21  is present between the thin film filters  54   b.    
     Next, the flow path unit  4  and the actuator units  21  will be described in detail with further reference to  FIGS. 7 to 10 .  FIG. 7  is an enlarged view of the region enclosed by the one-dot chain line in  FIG. 6 . In  FIG. 7 , for the sake of convenience in description, the nozzles  8 , pressure chambers  10 , and apertures  12 , which are placed below the actuator units  21 , and which are to be drawn by broken lines, are drawn by solid lines. 
       FIG. 8  is a partial section view taken along a line VIII-VIII shown in  FIG. 7 .  FIG. 9  is a partial exploded perspective view of the head body  1   a .  FIG. 10A  is an enlarged section view of the actuator unit  21 .  FIG. 10B  is a plan view showing an individual electrode  35  placed on the surface of the actuator unit  21  in  FIG. 10A . 
     On the lower face of the flow path unit  4 , as shown in  FIGS. 7 and 8 , ink ejection surface in which the many nozzles  8  are arranged in a matrix are formed. In a region corresponding to the ink ejection surface, also the pressure chambers  10  are arranged in a large number in a matrix in a similar manner as the nozzles  8 . In this embodiment, namely, the ink ejection surface in which the nozzles  8  are open in a matrix, and the surface in which the pressure chambers  10  are arranged in a matrix constitute a pair of opposing surfaces of the flow path unit  4 . A plurality of individual ink flow paths  32 , which will be described later, are formed in the flow path unit  4  so as to be sandwiched between the pair of faces. The actuator units  21  are fixed together with the thin film filters  54   a,    54   b  onto the surface in which the pressure chambers  10  are arranged. 
     As shown in  FIG. 9 , the flow path unit  4  is formed by nine metal plates which are a cavity plate  22 , a base plate  23 , an aperture plate  24 , a supply plate  25 , manifold plates  26 ,  27 ,  28 , a cover plate  29 , and a nozzle plate  30  in order from its top. These plates  22  to  30  have a rectangular plane, which elongate in the main scanning direction (see  FIG. 1 ). 
     In the cavity plate  22 , through holes, which correspond to the ink inflow ports  5   b  (see  FIG. 6 ), and those, which correspond to the pressure chambers  10  and have a substantially rhombus shape, are formed in a large number. In the base plate  23 , for each of the pressure chambers  10 , a communication hole between the pressure chamber  10  and the aperture  12 , and that between the pressure chamber  10  and the nozzle  8  are formed, and communication holes between the ink inflow ports  5   b  and the manifold flow path  5  are formed. In the aperture plate  24 , for each of the pressure chambers  10 , a through hole corresponding to the aperture  12 , and a communication hole between the pressure chamber  10  and the nozzle  8  are formed, and communication holes between the ink inflow ports  5   b  and the manifold flow path  5  are formed. In the supply plate  25 , for each of the pressure chambers  10 , a communication hole between the aperture  12  and a sub-manifold flow path  5   a,  and a communication hole between the pressure chamber  10  and the nozzle  8  are formed, and communication holes between the ink inflow ports  5   b  and the manifold flow path  5  are formed. In the manifold plates  26 ,  27 ,  28 , for each of the pressure chambers  10 , a communication hole between the pressure chamber  10  and the nozzle  8 , and through holes which, when the plates are stacked, communicate with each other to be formed as the manifold flow path  5  and the sub-manifold flow path  5   a  are formed. In the cover plate  29 , for each of the pressure chambers  10 , a communication hole between the pressure chamber  10  and the nozzle  8  is formed. In the nozzle plate  30 , for each of the pressure chambers  10 , a hole corresponding to the nozzle  8  is formed. 
     The nine plates  22  to  30  are stacked and fixed to each other while being positioned so that the individual ink flow paths  32  such as shown in  FIG. 8  are formed in the flow path unit  4 . 
     Inside the flow path unit  4 , the manifold flow path  5  communicating with the ink inflow ports  5   b,  and the sub-manifold flow path  5   a  branched from the manifold flow path  5  are formed. For each of the nozzles  8 , the individual ink flow path  32  such as shown in  FIG. 8 , which passes from the manifold flow path  5  through the sub-manifold flow path  5   a  and the pressure chamber  10  to reach the nozzle  8  is formed. The ink, which is supplied from the reservoir unit  70  into the flow path unit  4  through the ink inflow ports  5   b,  is branched from the manifold flow path  5  to the sub-manifold flow path  5   a,  and reaches the nozzle  8  through the aperture  12 , which functions as an orifice, and the pressure chamber  10 . 
     Each of the actuator units  21  is configured by four piezoelectric sheets  41 ,  42 ,  43 ,  44 , which are made of a ferroelectric ceramic material of lead zirconate titanate (PZT), and which have a thickness of about 15 μm (see  FIG. 10A ). The thickness of the actuator units  21  in a direction perpendicular to the inflow-port face  4   a  of the flow path unit  4  is larger than the thicknesses of the thin film filters  54   a,    54   b  (see  FIG. 11 ). The piezoelectric sheets  41  to  44  are placed over the many pressure chambers  10 , which are formed to correspond to one ink ejection surface. 
     Individual electrodes  35  are formed in positions on the uppermost piezoelectric sheet  41  and corresponding to the pressure chambers  10 . A common electrode  34 , which is over the whole sheet and has a thickness of about 2 μm, is sandwiched between the uppermost piezoelectric sheet  41  and the piezoelectric sheet  42 , which is below the piezoelectric sheet  41 . The individual electrodes  35  and the common electrode  34  are made of a metal material such as Ag-Pd. No electrode is placed between the piezoelectric sheets  42 ,  43 , and between the piezoelectric sheets  43 ,  44 . 
     Each of the individual electrodes  35  has a thickness of about 1 μm. As shown in  FIG. 10B , each of the individual electrodes  35  has a substantially rhombus plan shape, which is similar to the plan shape of the pressure chambers  10 . One of the acute angle portions of the individual electrode  35  having a substantially rhombus shape is elongated. A circular land  36 , which is electrically connected to the individual electrode  35  and has a diameter of about 160 μm, is disposed at the tip end of the elongated portion. The land  36  is made of gold, which contains, for example, a glass frit. As shown in  FIG. 10A , the land  36  is formed in a position, which is on the elongated portion of the individual electrode  35  and is opposed to the wall of the cavity plate  22  defining the pressure chamber  10  with respect to the thickness direction of the piezoelectric sheets  41  to  44 , i.e., the position, which does not overlap with the pressure chamber  10 . The land  36  is electrically joined to a contact disposed on the FPC  50  (see  FIG. 2 ). 
     The common electrode  34  is grounded in a region, which is not shown. Therefore, the common electrode  34  is equally kept to the ground potential in a region corresponding to all the pressure chambers  10 . By contrast, the individual electrodes  35  (the lands  36 ) are connected to the driver ICs  83  through the FPCs  50  including other lead lines, which are independent for the individual electrodes  35 , in order to enable their potentials to be selectively controlled (see  FIG. 2 ). 
     Hereinafter, a method of driving the actuator units  21  will be described. 
     The piezoelectric sheet  41  is polarized in the thickness direction. When one of the individual electrodes  35  is set to a potential different from that of the common electrode  34  and an electric field is applied to the piezoelectric sheet  41  in the polarization direction, a portion of the piezoelectric sheet  41  to which the electric field is applied operates as an active portion, which is distorted by the piezoelectric effect. Namely, the piezoelectric sheet  41  is extended or contracted in the thickness direction, and contracted or extended in the planar direction by the piezoelectric transverse effect. By contrast, the remaining three piezoelectric sheets  42  to  44  are inactive layers, which have no region sandwiched between the individual electrodes  35  and the common electrode  34  and thus cannot be spontaneously deformed. 
     Namely, each of the actuator units  21  is of the so-called unimorph type in which the upper one piezoelectric sheet  41  that is apart from the pressure chamber  10  is formed as a layer including the active layer, and the lower three piezoelectric sheets  42  to  44  that are close to the pressure chambers  10  are formed as the inactive layers. As shown in  FIG. 10A , the piezoelectric sheets  41  to  44  are fixed to the upper face of the cavity plate  22  defining the pressure chamber  10 . When a difference in distortion in the planar direction is produced between the electric field applied portion of the piezoelectric sheet  41  and the lower piezoelectric sheets  42  to  44 , therefore, the whole piezoelectric sheets  41  to  44  are deformed so as to be convexed toward the pressure chamber  10  (unimorph deformation). As a result, the volume of the pressure chamber  10  is reduced to increase the pressure in the pressure chamber  10 , the ink is pushed out from the pressure chamber  10  into the nozzle  8 , and the ink is ejected from the nozzle  8 . 
     When the individual electrode  35  is thereafter returned to the same potential as the common electrode  34 , the piezoelectric sheets  41  to  44  are caused to have the original flat shape, and the volume of the pressure chamber  10  is returned to the original value. In accordance with this, the ink is introduced from the manifold flow path  5  into the pressure chamber  10 , and the ink is again stored in the pressure chamber  10 . 
     Next, positional relationships among the reservoir unit  70 , the thin film filters  54   a,    54   b,  and the head body  1   a  will be described with reference to  FIG. 11 .  FIG. 11  is a partial enlarged side view of the inkjet head  1 . In  FIG. 11 , for the sake of convenience in description, the lower cover  52  is indicated by a one-dot chain line, and illustration of the FPCs  50  is omitted. As shown in  FIG. 11 , the reservoir unit  70  and the flow path unit  4  are joined together through the thin film filters  54   a,    54   b,  whereby a space S where the actuator units  21  is placed is formed between the first region  58  of the reservoir unit  70  and the inflow-port face  4   a  of the flow path unit  4  (see  FIG. 2 ). At this time, a plurality of gaps, which communicate with the space S, are formed between the thin film filters  54   a  and the thin film filters  54   b,  and between the thin film filters  54   b.  Among the gaps, gaps between the thin film filters  54   a  and the thin film filters  54   b  and between the thin film filters  54   b —where the longer parallel opposing sides of the actuator units  21  are exposed—are covered by the projections  52   a  of the lower cover  52  placed in the recesses  53  and sealed by a sealant applied between the lower end faces of the projections  52   a  and the flow path unit  4 . On the side of the shorter parallel opposing sides of the actuator units  21 , gaps between the thin film filters  54   a  and the thin film filters  54   b  closest to the thin film filters  54   a  are sealed by applying a sealant  56  made of a soft material to the recesses  55 . In this embodiment, the gaps, which are open toward the recesses  55 , between the thin film filters  54   a  and the thin film filters  54   b,  the gaps between the thin film filters  54   b,  and a portion between (i) the lower end face of the lower cover  52  and (ii) the reservoir unit  70  and the flow path unit  4  (more specifically, the portion along the one-dot chain line indicating the lower cover  52  in  FIG. 11 ) are sealed by the sealant. Particularly, all gaps, which tend to be widened, between the thin film filters  54   a  and the thin film filters  54   b,  and gaps between the thin film filters  54   b  are sealed. Hence, ink mist do not enter the space S through the gaps. According to this configuration, it is possible to prevent the actuator units  21  from being damaged by ink mist. 
     As described above, according to the inkjet head  1  of this embodiment, entering of dust or the like into the individual ink flow paths  32  can be suppressed with the simple configuration in which the thin film filters  54   a,    54   b  are placed between the flow path unit  4  and the reservoir unit  70 . Since the recess  55  is formed on the side face of the reservoir unit  70 , the sealant  56  for sealing the gaps between the thin film filters  54   a  and the thin film filters  54   b  closest to the thin film filters  54   a  can be easily applied. At this time, the sealant  56  may be applied only to a limited portion, i.e., the recess  55 . Hence, a situation where the sealant  56  flows into or protrudes into another portion does not occur. Since the gaps between the thin film filters  54   a  and the thin film filters  54   b  closest to the thin film filters  54   a  are sealed by the sealant  56 , the lower cover  52  is not necessary to cover the gaps between the thin film filters  54   a  and the thin film filters  54   b  closest to the thin film filters  54   a.  Therefore, the width of the lower cover  52  is not widened to be larger than that of the flow path unit  4 , and the inkjet head  1  can be miniaturized. Furthermore, an easily breakable part is eliminated from the projections  52   a  of the lower cover  52 . Therefore, the production yield can be improved. 
     The thickness of the actuator units  21  in the direction perpendicular to the inflow-port face  4   a  of the flow path unit  4  is larger than the thicknesses of the thin film filters  54   a,    54   b.  Even after the actuator unit  21  and the thin film filters  54   a,    54   b  are fixed to the inflow-port face  4   a  of the flow path unit  4 , therefore, the individual electrodes  35  and the lands  36  can be easily formed on the actuator unit  21 . Irrespective of such thickness relationships among the actuator units  21  and the thin film filters  54   a,    54   b,  the configuration in which the thin film filters  54   a,    54   b  are placed on the inflow-port face  4   a  can prevent dust, dirt, a foreign material, or the like, which may be produced when the individual electrodes  35  and the land  36  are formed on the actuator units  21 , from entering the flow path unit  4 . 
     In the second regions  57  of the reservoir unit  70 , the whole circumferences of the outer edges of the thin film filters  54   a,    54   b,  which cover the ink supply ports  59  of the circular holes  78   a,  abut against the non-groove region  57   b.  Therefore, the outer edges of the thin film filters  54   a,    54   b  are in close contact with the non-groove region  57   b.    
     According to this configuration, ink mist entering between the flow path unit  4  and the reservoir unit  70  do not reach the actuator unit  21  through the lattice-like grooves of the groove region  57   a.    
     In the above, one embodiment of the invention has been described. However, the invention is not limited to the above-described embodiment, and the design may be variously modified within the scope of the claims. For example, the above embodiment is configured so that the thickness of the actuator units  21  in the direction perpendicular to the inflow-port face  4   a  of the flow path unit  4  is larger than the thicknesses of the thin film filters  54   a,    54   b.  Alternatively, the thickness of the actuator units  21  may be equal to the thicknesses of the thin film filters  54   a,    54   b,  or smaller than the thicknesses of the thin film filters  54   a,    54   b.    
     In the above-described embodiment, the whole circumferences of the outer edges of the thin film filters  54   a,    54   b  abut against the non-groove region  57   b  in the second regions  57  of the reservoir unit  70 . Alternatively, only parts of the outer edges of the thin film filters  54   a,    54   b  may abut against the non-groove region  57   b.  From a viewpoint that entering of splashes or mist of ink from the outside is prevented from occurring, the outer edges of the thin film filters  54   a,    54   b  may abut against the non-groove region  57   b  in the vicinities of the width ends of the flow path unit  4 . From another viewpoint that both ink from the outside and ink from the ink inflow ports  5   b  or the ink supply ports  59  are prevented from entering the actuator units  21  in which electrical connecting portions exist, the outer edges of the thin film filters  54   a,    54   b  may abut against the non-groove region  57   b  so as to have an approximately C-like shape, which surrounds the ink inflow ports  5   b  or the ink supply ports  59  from portions adjacent to the width ends of the flow path unit  4 . 
     The whole circumferences of the outer edges of the thin film filters  54   a,    54   b  may not abut against the non-groove region  57   b.  According to this configuration, the degree of freedom of the regions where the thin film filters  54   a,    54   b  are to be placed is enhanced, and the thin film filters  54   a,    54   b  can be easily placed. 
     In the above-described embodiment, the recesses  55  are formed in the side faces of the reservoir unit  70 , and (i) the gap between each filter  54   a  and the filter  54   b  closest to each filter  54   a  and (ii) the recesses  55  are sealed with the sealant  56 . However, the invention is not limited to this configuration. In another embodiment, in place of each filter  54   a  and the filter  54   b  closest to each filter  54   a,  an integrated filter  54   c  may be used as shown in  FIG. 12 . As shown in  FIG. 12 , the actuator units  21  are arranged on the inflow-port face  4   a  of the flow path unit  4  in a row in the longitudinal direction of the flow path unit  4 . The filters  54   b  are disposed between the actuator units  21 . The filters  54   c  are disposed outside the row of the actuator units  21 . Specifically, each filter  54   c  extends along two adjacent sides of the actuator unit  21 , which is located at a corresponding end of the row of the actuator units ( 21 ). 
     Although the filter  54   a  and the filter  54   b  closest to the filter  54   a  are separate from each other and the gap is formed therebetween in the above-described embodiment, each filter  54   c  is a single part in the another embodiment. Therefore, as shown in  FIG. 13 , the reservoir unit  70  of this embodiment is not formed with the recess  55 . 
     The gaps between the ends of the projections  52   a  of the cover member  52  and the inflow-port face  4   a  of the flow path unit  4  are sealed with the sealant. Thus, a combination of the filters  54   b,    54   c  and the sealant surrounds the row of the actuator units  21  (i.e., a circumference of a group of the four actuator units  21 ). 
     Since each integrated filter  54   c  is the single part, it is not necessary to seal the gap between each filter  54   a  and the corresponding filter  54   b  closets to the filter  54   a  with the sealant. Furthermore, it is not necessary to form the recesses  55  in the side faces of the reservoir unit  70 . 
     According to the another embodiment, the reservoir unit  70 , which has a simpler configuration (that is, has no recess  55 ), can prevent ink mist from entering the space S. Therefore, it is possible to prevent the actuator units  21  from being damaged by ink mist. 
     The inkjet head of the invention is not limited to the piezoelectric type inkjet head having the actuator units  21 , and may be a thermal type inkjet head, or an electrostatic type inkjet head. 
     The application of the inkjet head of the invention is not limited to a printer, and the inkjet head may be applied to an inkjet facsimile apparatus or copier.