Patent Publication Number: US-7717547-B2

Title: Inkjet head

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priorities from Japanese Patent Application No. 2005-81914 filed on Mar. 22, 2005 and Japanese Patent Application No. 2005-324919 filed on Nov. 9, 2005, the entire contents of which are incorporated herein by reference. 
     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. Specifically, US 2005/0083379 A1 discloses an inkjet head having: 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 path each of which communicates with the common ink chamber and extends to a nozzle through a pressure chamber. The reservoir unit is formed with a reservoir for supplying stored ink to the common ink chamber. The reservoir unit is joined to the flow path unit. The actuator unit imparts ejection energy to the ink in the flow path unit. A plurality of ink supply ports are formed in the flow path unit. A plurality of tributary flow paths, which communicate with the common ink chamber through the respective ink supply ports, are formed in the reservoir. The ink stored in the reservoir is supplied to the common ink chamber through the respective tributary flow paths and the corresponding ink supply ports, which communicate with the respective tributary flow paths (see FIGS. 4 and 5 of US 2005/0083379 A1). 
     SUMMARY OF THE INVENTION 
     In the inkjet head of US 2005/0083379 A1, in the process of initially introducing the ink into the inkjet head, ink that has flown into one tributary flow path flows into the common ink chamber through a corresponding ink supply port, and the ink that has flown into the common ink chamber sometimes reaches another ink supply port to which ink from another tributary flow path has not yet reached. At this time, the other ink supply port are blocked by the ink in the common ink chamber, and therefore air accumulation is formed in the tributary flow path communicating with the other ink supply port. When air accumulation is formed in a tributary flow path, the ink flow in the tributary flow path is disturbed. In order to discharge air accumulation from tributary flow paths, a large amount of ink must be supplied to the reservoir. 
     The invention provides an inkjet head in which, in the process of initially introducing an ink, air accumulation is hardly formed in a tributary flow path. 
     According to one aspect of the invention, an inkjet head includes a flow path unit and a reservoir unit. The flow path unit includes a plurality of ink supply ports, a common ink chamber and a plurality of individual ink flow paths. Ink flowing from the ink supply ports is supplied into the common ink chamber. Each of the individual ink flow paths extends from an outlet of the common ink chamber to a nozzle through a pressure chamber. The reservoir unit stores the ink. The reservoir unit is joined to the flow path unit so that ink stored in the reservoir unit is supplied to the common ink chamber of the flow path unit through the ink supply ports. The reservoir unit includes an ink inflow path, a reservoir flow path and an ink drop flow path. The ink inflow path is formed with an ink inflow port into which ink flows. The reservoir flow path includes a plurality of ink outflow ports communicating with the ink supply ports. The ink drop flow path is disposed between the ink inflow path and the reservoir flow path. The reservoir flow path includes a main flow path and a plurality of tributary flow paths. The main flow path elongates in a longitudinal direction of the reservoir unit. The main flow path is formed with a plurality of tributary communication ports. Each of the tributary flow paths is formed between a corresponding tributary communication port and a corresponding ink outflow port. A section area of the main flow path taken along a width direction of the reservoir unit is larger than each of section areas of the tributary flow paths taken along a direction perpendicular to a flow direction of ink. The ink drop flow path drops ink flowing from the ink inflow path onto a substantially center of the main flow path as viewed in a plan view. The tributary communication ports are substantially equal to each other in an opening area. 
     According to this configuration, in a process of initially introducing the ink, the ink, which is dropped from the ink drop flow path onto the center of the main flow path forms flow of ink, which flows from the center of the main flow path toward the both ends, and then flows into the tributary flow paths through the tributary communication ports. At this time, since the tributary communication ports have the same opening area, a substantially same amount of ink flows at a substantially same speed into all of the tributary flow paths through the tributary communication ports. Among all the tributary flow paths, therefore, the difference in time when the ink, which has flown into the tributary flow paths, reaches the common ink chamber through the respective ink supply ports is reduced. Consequently, air accumulation is hardly formed in the tributary flow paths. 
    
    
     
       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. 
         FIG. 4  is an exploded plan view of the reservoir unit shown in  FIG. 3 . 
         FIG. 5  is a partial enlarged view of a vicinity of one end of a reservoir flow path shown in  FIG. 4F . 
         FIG. 6  is a partial section view of a plate shown in  FIG. 4F , taken along a chain line VI-VI in  FIG. 5 . 
         FIG. 7  is a plan view of the head body shown in  FIG. 1 . 
         FIG. 8  is an enlarged view of a region enclosed by a one-dot chain line in  FIG. 7 . 
         FIG. 9  is a partial section view taken along a line IX-IX in  FIG. 6 . 
         FIG. 10  is a partial exploded perspective view of the head body shown in  FIG. 1 . 
         FIG. 11A  is an enlarged section view of an actuator unit shown in  FIG. 9 , and  FIG. 11B  is a plan view showing an individual electrode placed on a surface of the actuator unit in  FIG. 11A . 
         FIG. 12  is a plan view of a sixth plate constituting a part of an inkjet head according to another embodiment of the invention. 
         FIG. 13  is an enlarged plan view of the sixth plate shown in  FIG. 12 . 
         FIG. 14  is a partial section view of the sixth plate shown in  FIG. 12 , taken along a chain line XIV-XIV in  FIG. 13 . 
     
    
    
     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 .  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  elongates in a main scanning direction, and has a head body  1   a , a reservoir unit  70 , and a control section  80 , which controls driving of the head body  1   a , in order from its bottom. Hereinafter, the components of the inkjet head  1  will be sequentially described with in order from its top. 
     The control section  80  has a main board  82 , sub-boards  81  and driver ICs  83 . The sub-boards  81  are placed on the both sides of the main board  82 . The driver ICs  83  are fixed to side faces of the sub-boards  81  opposed to the main board  82 . The driver ICs  83  generate signals 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 plane 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 placed on the both sides of the main board  82  with being separated from the main board  82  by the same distance and being upwardly separated from the reservoir unit  70 . 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 . 
     FPCs (Flexible Printed Circuits)  50 , which function as power supplying members, are upwardly withdrawn from a lower portion of the head  1 . One end of each FPC  50  is connected to the actuator units  21 , and the other end of each FPC  50  is connected to one of 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 the driving signals output from the driver ICs  83  to the actuator units  21 . 
     The inkjet head  1 , furthermore, has an upper cover  51 , which covers the control section  80 ; and a lower cover  52 , 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, and covers a lower portion of the main board  82 . An upper portion of the lower cover  52  has an upper wall  52   b , which inwardly projects from the upper end of the sidewall. 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 sidewalls (only one of the sidewalls is shown in  FIG. 1 ) of the lower cover  52 , two projections  52   a , which downwardly project, are formed so as to be arranged in the longitudinal direction of the lower cover  52 . The projections  52   a  are accommodated in recesses  53  of the reservoir unit  70 . Also, the projections  52   a  cover the portions of the FPCs  50  placed in the recesses  53 . Namely, when the projections  52   a  are accommodated in the recesses  53 , gaps through which the FPCs  50  can be passed are formed therebetween. The lower ends of the sidewalls of the lower cover  52  other than the projections  52   a  are contacted with the upper face of the reservoir unit  70 . The tip ends of the projections  52   a  are opposed to the flow path unit  4  of the head body  1   a  with a gap therebetween for absorbing a production error. 
     The vicinities of one-ends of the FPCs  50  connected to the actuator units  21  horizontally elongate along the face of the flow path unit  4 . The FPCs  50  are passed through the recesses  53  of the reservoir unit  70 , and are upwardly withdrawn out while forming bent portions. 
     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 in the reservoir unit  70 , which is not usually shown in a section taken along the same line, is shown arbitrarily. 
     The reservoir unit  70  temporarily stores ink, and supplies the ink to the flow path unit  4  of the head body  1   a . As shown in  FIG. 4 , the reservoir unit  70  has a laminated 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 in the longitudinal direction of the first plate  71 , respectively. The circular holes  71   a ,  71   b  are placed in positions, which are deviated from the center in the width direction of the first plate  71  toward the one and other ends in the width direction. 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 in the longitudinal direction of the first plate  71  and the circular hole  71   b . Furthermore, a circular hole  71   d  is formed in the center of the bottom of the oval recess  71   c . The oval recess  71   c  cooperates with the damper sheet  72 , which will be described below, to 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 . A material of the flexible thin film member may be a metal, a resin, or the like, and is not restricted 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 terephthalate) 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 flexible thin film 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  pass through the third plate  73 , which is the third layer from the top. The oval recess  71   c  and the oval hole  73   c  have a substantially identical shape in a plan view, and a substantially same size. 
     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 to elongate toward the center in the short side direction (width direction) of the fourth plate  74  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 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 formed concavely, has a shape and size substantially identical with those of the oval hole  73   c  of the third plate  73 , and is open on the side of the third plate  73 . The bottom faces of the thin recesses  74   a ,  74   b , and 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  located 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 of the fourth plate  74  so that the circular hole  75   a  communicates with the through hole  74   c  of the fourth plate  74 . The circular hole  75   a  is opposed to 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 through hole  76   a  forms a reservoir flow path  94  including a main flow path  76   b , and six tributary flow paths  76   c , which communicate with the main flow path  76   b . The plan shape of the reservoir flow path  94  is symmetric about a center P of the main flow path  76   b  (the center of gravity of the through hole  76   a ). The main flow path  76   b  elongates in the longitudinal direction of the sixth plate  76  with being slightly tapered as advancing from the center P of the sixth plate  76  toward the both ends in the longitudinal direction. The center P of the main flow path  76   b  in a plan view corresponds to the circular hole  75   a  of the fifth plate  75 . Three tributary communication ports  94   a  are formed in the vicinity of each of the both ends in the longitudinal direction of the main flow path  76   b . In other words, three tributary communication ports  94  are provided on each of both sides of an imaginary line A-A, which passes through the center P of the main flow path  76   a  and is perpendicular to the longitudinal direction of the reservoir unit  70 . The tributary flow paths  76   c  communicate with the main flow path  76   b  via the tributary communication ports  94   a , respectively. 
     The main flow path  76   b  and the tributary flow paths  76   c  will be described in detail with further reference to  FIGS. 5 and 6 .  FIG. 5  is a partial enlarged view of the vicinity of one end of the reservoir flow path  94 .  FIG. 6  is a partial section view of the sixth plate  76  taken along a chain line VI-VI in  FIG. 5 .  FIG. 6  shows a state where the sixth plate  76  is cut away so that the section surface is perpendicular to the ink flow direction in each tributary communication port  94   a  and that the three tributary communication ports  94   a , which are formed in the vicinity of one end of the reservoir flow path  94 , appear in the section view. As shown in  FIGS. 5 and 6 , all the tributary communication ports  94   a  have the same opening area S 1  in the section taken along the direction perpendicular to the ink flow direction in each tributary communication port  94   a . An ink outflow port  94   b  is formed in an end portion of each of the tributary flow paths  76   c . A section area of each tributary flow path  76   c  along a direction perpendicular to a flow direction of the ink is approximately constant over a range from the corresponding tributary communication port  94   a  to the corresponding ink outflow port  94   b . In all the tributary flow paths  76   c , the length and section area along the ink flow direction are substantially identical. Therefore, the tributary flow paths  76   c  are configured so that their flow-path resistances have a substantially same value. 
     The region 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.    
     In the seventh plate  77 , which is the seventh layer from the top, as shown in  FIGS. 3 and 4G , a total of ten oval holes  77   a  are formed in positions corresponding to the ink outflow ports  94   b  of the tributary flow paths  76   c  formed in the sixth plate  76 . Five of the oval holes  77   a  are arranged in the longitudinal direction in the vicinity of each of the width ends of the seventh plate  77 . Specifically, one, two, and two holes 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 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 pattern to avoid notches  53   f , which will be described later. The one or two oval holes  77   a , which are arranged in the staggered manner as described above, correspond to one corresponding ink outflow ports  94   b . The oval holes  77   a  are arranged to be symmetric about the center of the seventh plate  77 . 
     In the eighth plate  78 , which is the lowermost layer, as shown in  FIGS. 3 and 4H , oval holes  78   a  corresponding to the oval holes  77   a  formed in the seventh plate  77  are formed. In the lower face of the eighth plate  78 , peripheral portions (portions enclosed by the broken lines in the figure) of the oval holes  78   a  project downwardly. Only the projected portions are fixed to the upper face of the flow path unit  4 , and the portion other than the projected portions is separated from the flow path unit  4  (see  FIG. 2 ). 
     As shown in  FIG. 3 , the seven plates  71 ,  73  to  78  and the one damper sheet  72  are stacked and fixed to each other while being positioned, to thereby configure the reservoir unit  70  according to this embodiment. As seen from  FIG. 4 , the four plates  71  to  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 plates  71  to  74 . 
     In the both ends of each of the plates  71 ,  73  to  78  in the width direction, as shown in  FIGS. 4A to 4H , two and two, that is, a total of four rectangular notches  53   a  to  53   g  are arranged 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  (see  FIG. 1 ), which pass through the reservoir unit  70  in the stack direction, are defined by the notches  53   a  to  53   g . The width of the reservoir unit  70  except the recesses  53  is substantially identical with that of the flow path unit  4 . 
     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 include 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 arranged 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 , in 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 also passes through the filter  74   g  to flow into the downstream ink reservoir  61   b . In the downstream ink reservoir  61   b , the inflow ink drop through the drop flow path  63  of the fifth plate  75  into the center P of the main flow path  76   b  of the reservoir flow path  94  of the sixth plate  76 . As indicated by the arrows in  FIG. 4F , thereafter, ink flows with directing from the substantial center of the main flow path  76   b  to the both ends in the longitudinal direction of the main flow path  76   b . As indicated by the arrows in  FIG. 5 , the ink, which has reached the vicinities of the both ends in the longitudinal direction of the main flow path  76   b , flows into the tributary flow paths  76   c  through the tributary communication ports  94   a . At this time, since all the tributary communication ports  94   a  are open in the ink flow direction and have the same opening area S 1 , the same amount of ink flows uniformly at the same speed into all of the tributary flow paths  76   c  through the tributary communication ports  94   a . The ink, which has flown into the tributary communication ports  94   a , flows into ink supply ports  5   b  (see  FIG. 7 ), which are open in the upper face of the flow path unit  4 , through the ink outflow ports  94   b  and the oval holes  77   a ,  78   a . At this time, whenever the ink passes through any one of the tributary flow paths  76   c , the resistance of the flow path extending from a substantial center of the main flow path  76   b  in a plan view to a manifold flow path  5  is substantially identical. As described later, the ink, which has flown into the flow path unit  4 , is distributed into a plurality of individual ink flow paths  32 , which communicate with the manifold flow path  5 , and then reaches nozzles  8 , which are terminal ends of the individual ink flow paths  32 , to be discharged to the outside. Namely, in the process of filling the flow paths extending from the supply joint  91  to the nozzles  8  with ink, air accumulation does not stay in the course of the flow paths. This is caused by the configuration in which the resistance of the flow path extending from a substantial center of the main flow path  76   b  to the manifold flow path  5  is substantially identical. 
     In this way, the ink is temporarily stored in the upstream ink reservoir  61   a  and the downstream ink reservoir  61   b . The opening of the circular hole  71   a  in the upper face of the first plate  71  functions as an ink inflow port of the upstream ink reservoir  61   a , and the circular holes  71   a ,  72   a ,  73   a  function as an ink inflow path. 
     Next, the flow (indicated by the open arrows in  FIG. 3 ) of the ink, which is discharged in reverse purge through the discharge joint  92 , will be described. Reverse purge is a process in which an ink or washing liquid is injected under pressure from the nozzles  8 , supplied along a flow path in a direction opposite to the ink flow path in the normal printing operation, and then discharged from the inkjet head  1 . By this reverse purge, the interior of the inkjet head  1  can be washed away (that is, foreign substances such as dust, air bubbles, and the like staying in the inkjet head  1  can be removed away). 
     In the reverse purge, the washing liquid flows into the reservoir unit  70  through the ink supply ports  5   b  of the flow path unit  4 . The washing liquid flowing into the reservoir unit  70  reaches the downstream ink reservoir  61   b  through the oval holes  78   a ,  77   a , passes through the filter  74   g , and flows into the upstream ink reservoir  61   a . As indicated by the open arrows in the figure, the washing liquid flowing into the upstream ink reservoir  61   a  is discharged from the discharge joint  92  through the damper flow path  62  and the circular holes  73   b ,  72   b ,  71   b . At this time, an ink existing in the flow path unit  4  and the reservoir unit  70  is pushed by the washing liquid and discharged together with the washing liquid. Also, foreign substances caught by the filter  74   g  are discharged, and therefore cleaning of the flow path and recovery of the filter performance are achieved. 
     As shown in  FIG. 3 , the third plate  73  serves as a flow path wall, which defines the damper flow path  62 , and 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 , is opposed to 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, and the damper chamber communicates with the atmosphere via 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 , and therefore the damper sheet  72  is prevented from being damaged. The bottom of the oval recess  71   c  prevent an external force, which may break the damper sheet  72 , from being applied to the damper sheet  72 . 
     Next, the head body  1   a  will be described with reference to  FIGS. 7 to 11 .  FIG. 7  is a plan view of the head body  1   a .  FIG. 8  is an enlarged view of a region enclosed by a one-dot chain line in  FIG. 7 . In  FIG. 8 , for the sake of convenience in description, pressure chambers  10  and apertures  12  which are located below the actuator units  21 , and which are to be drawn by broken lines are drawn by solid lines.  FIG. 9  is a partial section view taken along a line IX-IX shown in  FIG. 8 .  FIG. 10  is a partial exploded perspective view of the head body  1   a .  FIG. 11A  is an enlarged section view of the actuator unit  21 , and  FIG. 11B  is a plan view showing an individual electrode disposed on the surface of the actuator unit  21  in  FIG. 11A . 
     As shown in  FIG. 7 , the head body  1   a  includes the flow path unit  4  and the four actuator units  21  fixed to the upper face of the flow path unit  4 . The actuator units  21  have a function of selectively applying an ejection energy to the inks in the pressure chambers  10  formed in 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 the length in main scanning direction slightly shorter than that of the reservoir unit  70 . On the lower face of the flow path unit  4 , as shown in  FIGS. 8 and 9 , ink ejection faces in each of which many nozzles  8  are arranged in a matrix are formed. In each of the fixing faces between the flow path unit  4  and the actuator units  21 , also the pressure chambers  10  are arranged in a large number in a matrix in a similar manner to the nozzles  8 . 
     As shown in  FIG. 10 , the flow path unit  4  is configured 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 elongates in the main scanning direction (see  FIG. 1 ). 
     In the cavity plate  22 , through holes which correspond to the ink supply ports  5   b  (see  FIG. 7 ), and rhombic through holes which correspond to the pressure chambers  10  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 supply 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 supply 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 supply 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 , communication holes 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. 9  are formed in the flow path unit  4 . 
     As shown in  FIG. 7 , a total of ten ink supply ports  5   b  are open in positions corresponding to the oval holes  78   a  (see  FIG. 4H ) of the reservoir unit  70  in the upper face of the flow path unit  4 . Inside the flow path unit  4 , the manifold flow path  5  communicating with the ink supply 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 , the outlet of 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 supply ports  5   b , is branched from the manifold flow path  5  to the sub-manifold flow path  5   a , and reaches the nozzle  8  via the aperture  12 , which functions as an orifice, and the pressure chamber  10 . 
     As shown in  FIG. 7 , the four actuator units  21  have a trapezoidal plan shape, and placed in a staggered pattern so as to avoid the ink supply ports  5   b  opened in the upper face of the flow path unit  4 . The above-mentioned ink ejection faces correspond to regions of the lower face of the flow path unit  4  corresponding to bonding regions of the actuator units  21 . In this embodiment, namely, the ink ejection face in which the nozzles  8  are open in the matrix, and the face in which the pressure chambers  10  are arranged in the matrix constitute a pair of opposing faces of the flow path unit  4 . The plurality of individual ink flow paths  32  are formed in the flow path unit  4  so as to be interposed between the pair of faces. The parallel opposing edges of each actuator unit  21  elongate along the longitudinal direction of the flow path unit  4 . Oblique edges 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 fixed to portions of the upper face of the flow path unit  4  opposed to and separated from the lower face of the reservoir unit  70  (see  FIG. 2 ). The FPCs  50  are fixed onto the actuator units  21 , but are not in contact with the lower face of the reservoir unit  70 . 
     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. 11A ). The piezoelectric sheets  41  to  44  are arranged over the many pressure chambers  10 , which are formed correspondingly with one ink ejection face. 
     Individual electrodes  35  are formed in positions corresponding to the pressure chambers  10 , on the uppermost piezoelectric sheet  41 . A common electrode  34  which is formed over the whole sheet and which has a thickness of about 2 μm is interposed 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 the piezoelectric sheets  43 ,  44 . 
     Each of the individual electrodes  35  has a thickness of about 1 μm and has, as shown in  FIG. 11B , 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 the substantially rhombus shape elongates. The elongated tip end of each individual electrode  35  is electrically connected to a circular land  36 , which has a diameter of about 160 μm. The land  36  is made of gold which contains, for example, a glass frit. As shown in  FIG. 11A , the land  36  is formed in a position, which is on the elongated portion of the individual electrode  35 , and which 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., in 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 lands  36  and the FPCs  50 , which have other independent lead lines 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 interposed between the individual electrodes  35  and the common electrode  34 , and 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  apart from the pressure chamber  10  is formed as a layer including the active layer, and the lower three piezoelectric sheets  42  to  44  close to the pressure chambers  10  are formed as the inactive layers. As shown in  FIG. 11A , 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 occurs 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 restored 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 . 
     As described above, according to the inkjet head  1  according to this embodiment, in the process of initially introducing the ink, the ink, which has dropped into the center P of the main flow path  76   b  from the drop flow path  63  forms flow of the ink, which flows from the center P of the main flow path  76   b  toward its both ends, and then flows in the vicinities of the both ends of the main flow path  76   b  into the tributary flow paths  76   c  through the tributary communication ports  94   a . At this time, since the tributary communication ports  94   a  have the same opening area S 1 , a substantially same amount of ink flows at a substantially same speed into all of the tributary flow paths  76   c  through the tributary communication ports  94   a . Moreover, the tributary communication ports  94   a  are open in the ink flow direction. Therefore, the ink flows more uniformly into all of the tributary flow paths  76   c . Consequently, the difference in time periods when the ink, which has flown into the tributary flow paths  76   c , reaches the manifold flow path  5  through the respective ink supply ports  5   b  is reduced. As a result, air accumulation is hardly formed in the tributary flow paths  76   c.    
     The section area of each tributary flow path  76   c  along the direction perpendicular to the ink flow direction is approximately constant over the range from the corresponding tributary communication port  94   a  to the corresponding ink outflow port  94   b . Moreover, the section areas of all the tributary flow paths  76   c  are substantially identical, and hence the same amount of ink flows out from the ink outflow ports  94   b  at a substantially same speed. Among all the tributary flow paths  76   c , therefore, the difference in time periods when the ink, which has flown into the tributary flow paths  76   c , reaches the manifold flow path  5  through the respective ink supply ports  5   b  is further reduced. 
     Furthermore, the number of the tributary communication ports  94   a  formed on one side of the imaginary line, which passes through the center P of the main flow path  76   b  and is perpendicular to the longitudinal direction of the reservoir unit  70 , is equal to that of the tributary communication ports  94   a  formed on the other side of the imaginary line, all the tributary flow paths  76   c  have a substantially same length, and the reservoir flow path  94  is point-symmetric in a plan view. Therefore, the same amount of ink flows at a substantially same speed into all of the tributary flow paths  76   c  through the tributary communication ports  94   a , and among all the tributary flow paths  76   c , the difference in time period when the ink which, has flown into the tributary flow paths  76   c , reaches the manifold flow path  5  through the respective ink supply ports  5   b  is approximately eliminated. 
     Whenever the ink passes through any one of the tributary flow paths  76   c , the resistance of the flow path extending from the substantial center of the main flow path  76   b  in a plan view to the manifold flow path  5  is substantially identical. In the process of initially introducing the ink, therefore, inks flow from the tributary flow paths  76   c  into the manifold flow path  5  at a substantially same timing. Consequently, it is possible to surely prevent air accumulation from being formed in the tributary flow paths  76   c.    
     Then, an inkjet head according to another embodiment will be described with reference to  FIGS. 12 to 14 .  FIG. 12  is a plan view of a sixth plate  176  constituting a part of the inkjet head according to the other embodiment of the invention. In the inkjet head according to this embodiment, the above-described sixth plate  76  is replaced with the sixth plate  176  shown in  FIG. 12 , and the other components are identical with those described above. Therefore, components identical with those of the above-described embodiment are denoted by the same reference numerals, and their description will be omitted. 
     As shown in  FIG. 12 , a through hole  176   a  is formed in the sixth plate  176 , which is the sixth layer from the top of the plurality of plates constituting the reservoir unit  70 . The through hole  176   a  forms a reservoir flow path  194  including a main flow path  176   b , and ten tributary flow paths  176   c , which communicate with the main flow path  176   b . The plan shape of the reservoir flow path  194  is symmetric about the center P′ of the main flow path  176   b  (the center of gravity of the through hole  176   a ). The main flow path  176   b  elongates in the longitudinal direction of the sixth plate  176 . In the same manner as the above-described main flow path  76   b , the center P′ of the main flow path  176   b  in a plan view corresponds to the circular hole  75   a  of the fifth plate  75 . Five tributary communication ports  194   a  are formed in the vicinity of each of the both ends of the main flow path  176   b  in the elongating direction. In other words, five tributary communication ports  194   a  are provided on each of both sides of an imaginary line B-B shown in  FIG. 12 , which passes through the center P′ of the main flow path  176   b  and is perpendicular to the longitudinal direction of the reservoir unit  70 . The tributary flow paths  176   c  communicate with the main flow path  176   b  through the tributary communication ports  194   a , respectively. 
     The main flow path  176   b  and the tributary flow paths  176   c  will be described in detail with further reference to  FIGS. 13 and 14 .  FIG. 13  is an enlarged plan view of the sixth plate  176  shown in  FIG. 12 .  FIG. 14  is a partial section view of the sixth plate shown in  FIG. 12 , taken along a chain line XIV-XIV in  FIG. 13 .  FIG. 14  shows a state where the sixth plate  176  is cut away so that the section surface is perpendicular to the ink flow direction in each tributary communication port  194   a  and that the five tributary communication ports  194   a , which are formed in the vicinity of one end of the main flow path  176   b , appear. As shown in  FIGS. 13 and 14 , all the tributary communication ports  194   a  have the same opening area S 1 ′ in the section taken along the direction perpendicular to the ink flow direction in each tributary communication port  194   a . Ink outflow ports  194   b  are formed in other end portions of the tributary flow paths  176   c , which are connected to the tributary communication ports  194   a , respectively. The sixth plate  176  is formed with the ten outflow ports  194   b , which are equal in number to the ten tributary communication ports  194   a . The outflow ports  194   b  are formed correspondingly with positions, which communicate with the ten oval holes  77   a  formed in the above-mentioned seventh plate  77 . In each of the tributary flow paths  176   c , a section area of the tributary flow path  176   c  along a direction perpendicular to the ink flow direction is approximately constant over a range from the tributary communication port  194   a  to the ink outflow port  194   b . In all the tributary flow paths  176   c , also the length along the ink flow direction is substantially identical. Therefore, the tributary flow paths  176   c  are configured so that their flow-path resistances have a substantially same value. 
     The flow of the ink, which has flown into the sixth plate  176 , will be described. From the drop flow path  63  formed in the fifth plate  75 , the ink flows into the center P′ of the main flow path  176   b  of the reservoir flow path  194  of the sixth plate  176 . As indicated by the arrows in  FIG. 12 , the inflow ink forms flow of ink, which flows from the substantial center of the main flow path  176   b  toward the both ends in the longitudinal direction. As shown in  FIG. 13 , the ink, which has reached the vicinities of the both ends of the main flow path  176   b  in the longitudinal direction flows into the tributary flow paths  176   c  through the tributary communication ports  194   a . At this time, since all the tributary communication ports  194   a  have the same opening area S 1 ′, a substantially same amount of ink flows at a substantially same speed into all of the tributary flow paths  176   c  through the tributary communication ports  194   a . The ink, which has flown into the tributary communication ports  194   a , flows into the ink supply ports  5   b , which are open in the upper face of the flow path unit  4 , through the ink outflow ports  194   b  and the above-mentioned oval holes  77   a ,  78   a.    
     As described above, according to the inkjet head of this embodiment, the tributary communication ports  194   a  the number of which is equal to that of the ink supply ports  5   b  are formed in the sixth plate  176  constituting a part of the reservoir unit  70 , and the ink, which has passed through the tributary communication ports  194   a , flows into the corresponding ink supply ports  5   b . Therefore, the ink, which has once flown into one tributary communication port  194   a , flows only into one ink supply port  5   b . The tributary communication ports  194   a  are placed in a concentrated manner in the terminal portions of the main flow path  176   b  on its both sides in the longitudinal direction, and all of the ports  194   a  are directed toward the terminal portions of the main flow path  176   b . Therefore, the length of the flow line, which follows the center of gravity of the through hole  176   a , the terminal portion of the main flow path  176   b , the tributary communication port  194   a , and the ink outflow port  194   b  is substantially identical whenever the flow line passes through any one of the tributary flow paths  176   c . Moreover, the flow-path resistances of the tributary flow paths  176   c  are substantially coincident with each other. Among all the tributary flow paths  176   c , therefore, the difference in time period when the ink which has flown into the tributary flow paths  176   c  reaches the manifold flow path  5  through the respective ink supply ports  5   b  is approximately eliminated. 
     The tributary communication ports  194   a  are concentrated in both the terminals. In the process of initially introducing the ink, therefore, a difference in timing when ink flows into the tributary flow paths  176   c  is hardly produced among the tributary flow paths  176   c . Air bubbles can be discharged from the inkjet head for a short time period. 
     In the above, the embodiments of the invention have been described. However, the invention is not limited to the above-described embodiments. The design may be variously modified within the scope of claims. For example, the embodiment is configured so that the section area of the tributary flow path  76   c  along the direction perpendicular to the ink flow direction in the main flow path  76   b  is approximately constant over a range from the tributary communication port  94   a  to the ink outflow port  94   b , and the section areas of all the tributary flow paths  76   c  are substantially identical. Alternatively, the section area of each of the tributary flow paths may be changed on the way, or the tributary flow paths may have different section areas so long as in the process of initially introducing the ink, air accumulation does not stay in the flow paths. 
     In the embodiment described above, from a similar viewpoint, the lengths of all the tributary flow paths  76   c  are substantially identical. Alternatively, the tributary flow paths may have different lengths. 
     In the embodiment described above, the three tributary communication ports  94   a  are formed in the vicinity of each of the both ends of the main flow path  76   b  in the longitudinal direction. The number of the tributary communication ports formed in each of the ends may be a number other than three. The number of the tributary communication ports  94   a  formed in the vicinity of one end of the main flow path  76   b  in the longitudinal direction may be different from that of the tributary communication ports  94   a  formed in the vicinity of the other end. The reservoir flow path  94  is point-symmetric in a plan view. Alternatively, the reservoir flow path may not be point-symmetric. 
     The expressions “substantially same”, “approximately constant” and similar expressions don&#39;t require strictly same and strictly constant. Those expressions may have tolerance of size, for example, ±5%. Specifically, the tributary communication ports  94   a  may have opening areas in a range of from 95% of the average opening area to 105% of the average opening area. The tributary flow paths  76   c  may have section areas in a range of from 95% of the average section areas to 105% of the average section areas. The tributary flow paths  94   a  may have lengths in a range of 95% of the average length to 105% of the average length. Flow paths extending from the substantial center of the main flow path  76   b  as viewed in the plan view through the respective tributary flow paths  76   c  to the common ink chamber  5   a  may have resistances in a range of 95% of the average resistance to 105% of the average resistance. The section area of each tributary flow path  76   c  taken along a direction perpendicular to the flow direction of the ink may fluctuate in a range of 95% of the average section area to 105% of the average section area. 
     In the embodiment described above, whenever the ink passes through any one of the tributary flow paths  76   c , the resistance of the flow path extending from the substantial center of the main flow path  76   b  in a plan view to the flow path unit  4  is substantially identical. Alternatively, the resistances of the respective flow paths extending from the substantial center of the main flow path  76   b  in a plan view to the flow path unit  4  may be different so long as in the process of initially introducing the ink, air accumulation does not stay in the flow paths. 
     Also, for example in  FIG. 5 , the lower two of the tributary flow ports  94   a  are open toward the longitudinal direction of the reservoir unit  70  but the uppermost tributary flow port  94   a  isn&#39;t open toward the longitudinal direction of the reservoir unit  70 . Alternatively, all of the tributary flow ports  94   a  ( 194   a ) may be open toward the longitudinal direction of the reservoir unit  70 . 
     The inkjet head according to 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 according to the invention is not limited to a printer, and the inkjet head may be applied to an inkjet facsimile apparatus or copier.