Patent Publication Number: US-10315433-B2

Title: Inkjet head and inkjet recording device

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
     This is the U.S. national stage of application No. PCT/JP2016/051161, filed Jan. 15, 2016. Priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) is claimed from Japanese Application No. 2015-006633, filed Jan. 16, 2015, the disclosures of which are also incorporated herein by reference. 
     TECHNOLOGICAL FIELD 
     The present invention relates to an inkjet head and an inkjet recording device. 
     BACKGROUND ART 
     Conventionally, there is a known inkjet recording device which ejects drops of liquid ink from a plurality of nozzles provided in an inkjet head to form an image on a recording medium. 
     In conventional inkjet heads, there are problems such as nozzles clogging, failure of ejecting, etc. due to air bubbles caused in the inkjet head or foreign substances being mixed in the inkjet head. 
     Depending on the type of ink, if the inkjet head is not used for a long period of time, the ink viscosity near the nozzle increases due to the ink particles settling, and it becomes difficult for the ink to be ejected stably. 
     In view of the above, there is an inkjet recording device which is provided with an ink circulation flow path in the head chip of the inkjet head so that the air bubbles, etc. in the head can flow to the circulation flow path with the ink (for example, patent documents 1 and 2). 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent No. 5385975 
     Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2005-119287 
     SUMMARY 
     Problems to be Solved by the Invention 
     However, the circulation flow path as described in patent document 1 is formed so that the impedance of the circulation flow path is two to ten times higher than the impedance of the nozzle. The ink can be circulated at an ink circulation speed of about 1/100 compared to the maximum ejecting point (circulation amount 1500 (pL/s) for ejecting amount 150000 (pL/s)). However, such flow speed is too slow, and it may become difficult to effectively discharge the air bubbles, foreign substances, and the like. 
     The circulation flow path as described in patent document 2 is provided with a valve which opens and closes with air pressure. When printing is not performed, the valve is opened to apply pressure to the supply flow path and to reduce the pressure in the circulation flow path. With this, the air bubbles in the pressure chamber can be effectively discharged to the circulation flow path. However, the valve needs to be closed during printing (ink ejecting) and the ink cannot be circulated. Therefore, the air bubbles which suddenly occur while the ink is ejected cannot be discharged. 
     Further, when a circulation flow path is made in the channel in a conventional method using a Helmholtz resonance method (vent method or push method), the pressure escapes to the circulation flow path. Therefore, pressure efficiency decreases and the ejecting performance decreases. 
     In order to prevent the decrease of the ejecting performance, the circulation flow path may be made thinner to prevent the pressure from escaping to the circulation flow path. However, if the circulation flow path is made thinner, the circulation speed decreases. Therefore, it becomes difficult to effectively discharge the air bubbles, foreign substances, etc. 
     Alternatively, for example, the pressure of the circulation flow path can be raised using a pump, etc. and the circulation speed can be accelerated without changing the circulation flow path. However, such measures become a burden to the device. Moreover, there is a possibility that the meniscus of the nozzle breaks and the ink may leak from the nozzle. 
     The present invention is conceived in view of the above problems, and provides an inkjet head and an inkjet head recording device in which the reduction of ejecting properties of ink is suppressed to a minimum by providing a circulation flow path and which can effectively discharge air bubbles, etc. near the nozzle without providing a burden on the apparatus. 
     Means for Solving the Problem 
     To achieve at least one of the above-mentioned objects, according to an aspect of the present invention, an inkjet head reflecting one aspect of the present invention includes, a plurality of nozzles which eject ink; a pressure chamber which is individually communicated with each nozzle and which is filled with ink inside; a pressure generator which is a driving source to eject ink by applying pressure to the pressure chamber; an inlet which includes a narrow portion with a flow path narrower than the pressure chamber and which supplies ink to the pressure chamber; and a circulation flow path which is able to discharge ink in the pressure chamber from near the nozzle, wherein, viscosity resistance of the circulation flow path is smaller than viscosity resistance of the nozzle, and impedance of the circulation flow path is equal to or more than 0.5 times of impedance of the inlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given herein below and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention. 
         FIG. 1  is a perspective diagram showing a schematic configuration of an inkjet recording device. 
         FIG. 2  is an exploded perspective diagram of an inkjet head. 
         FIG. 3  is a cross-sectional diagram dividing a portion along line (III)-(III) in  FIG. 2 . 
         FIG. 4  is a planar diagram of a head chip. 
         FIG. 5  is a cross-sectional diagram dividing a portion along line (V)-(V) in  FIG. 4 . 
         FIG. 6  is a cross-sectional diagram dividing a portion along line (VI)-(VI) in  FIG. 2 . 
         FIG. 7 . is a schematic diagram describing a configuration of an ink circulation mechanism. 
         FIG. 8A  is a graph showing a relation between a ratio (Zc/Zi) of an impedance of a circulation flow path and an inlet and a driving voltage (V) of an actuator when ejecting liquid drop amount of an ink liquid drop is 3.5 pL and ejecting speed is 7 m/s. 
         FIG. 8B  is a graph showing a relation between the ratio (Zc/Zi) of the impedance of the circulation flow path and the inlet and ejecting negative pressure (kPa) when the ejecting liquid drop amount of the ink liquid drop is 3.5 pL and the ejecting speed is 7 m/s. 
         FIG. 9A  is a graph showing a relation between the ratio (Zc/Zi) of the impedance of the circulation flow path and the inlet and the driving voltage (V) of the actuator when the ejecting liquid drop amount of the ink liquid drop is 1.0 pL and the ejecting speed is 7 m/s. 
         FIG. 9B  is a graph showing a relation between the ratio (Zc/Zi) of the impedance of the circulation flow path and the inlet and the ejecting negative pressure (kPa) when the ejecting liquid drop amount of the ink liquid drop is 1.0 pL and the ejecting speed is 7 m/s. 
         FIG. 10  is a planar diagram of a head chip according to a modification. 
         FIG. 11  is a cross-sectional diagram dividing a portion along line (XI)-(XI) in  FIG. 10 . 
     
    
    
     EMBODIMENT FOR CARRYING OUT THE INVENTION 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     A preferable embodiment of the present invention is described with reference to the drawings. The scope of the present invention is not limited to the illustrated examples. In the description below, the same reference numerals are applied to the same functions and configurations, and the description is omitted. 
     According to the description below, the embodiment described uses a one pass drawing method which draws by only conveying the recording medium using the line head. Alternatively, other drawing methods can be suitably applied, for example, a drawing method using a scanning method or a drum method can be applied. 
     According to the description below, a conveying direction of a recording medium K is a front and back direction, a direction orthogonal to the conveying direction on a conveying surface of the recording medium K is to be a left and right direction, and a direction perpendicular to the front and back direction and the left and right direction is to be an up and down direction. 
     Although embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and not limitation, the scope of the present invention should be interpreted by terms of the appended claims. 
     [Overview of Inkjet Recording Device] 
     An inkjet recording device  100  includes a platen  101 , conveying rollers  102 , line heads  103 ,  104 ,  105 , and  106 , an ink circulation mechanism and the like (see  FIG. 1  and  FIG. 7 ). 
     The platen  101  supports the recording medium K with the upper surface, and when the conveying roller  102  is driven, the recording medium K is conveyed in the conveying direction (front and back direction). 
     The line heads  103 ,  104 ,  105 , and  106  are provided aligned in a width direction (left and right direction) perpendicular to the conveying direction from an upstream side of the conveying direction (front and back direction) of the recording medium K to the downstream side. At least one later-described inkjet head  1  is provided inside the line heads  103 ,  104 ,  105 , and  106 , and ejects ink with colors such as cyan (C), magenta (M), yellow (Y), and black (K) to the recording medium K. 
     An ink circulation mechanism is described later (see  FIG. 7 ). 
     [Overview of Configuration of Inkjet Head] 
     An inkjet head  1  includes a head chip  2 , a holding plate  3 , a connecting member  4 , an ink flow path member  5 , and the like (see  FIG. 2  and  FIG. 3 ). 
     A head chip  2  is a plurality of layered substrates, and a nozzle  211  to eject ink is provided in the most bottom layer. The upper surface of the head chip  2  is provided with a piezoelectric element  24  as a pressure generating unit. The piezoelectric element  24  is displaced, pressure is applied to the ink filled in a pressure chamber  231  inside the head chip  2 , and the ink liquid drops are ejected from the nozzle  211 . 
     The holding plate  3  is attached to the upper surface of the head chip  2  using an adhesive to maintain strength of the head chip  2 . The holding plate  3  includes an opening  31  in the center so that the piezoelectric element  24  in the upper surface of the head chip  2  is stored inside the opening  31 . 
     The connecting member  4  includes lines such as FPC. The width direction of the connecting member  4  is connected to a portion near the back side of the upper surface of the holding plate  3  along the left and right direction of the holding plate  3 . The connecting member  4  is electrically connected to the piezoelectric element  24  through a bonding wire  41  passing through the opening  31  provided in the center of the holding plate  3 . The connecting member  4  is connected to the driving unit (not illustrated) and electricity can be provided from the driving unit to the piezoelectric element  24  through the connecting member  4  and the bonding wire  41 . 
     One ink flow path member  5  is attached to each edge of the upper surface of the holding plate  3  in the left and right direction. Each ink flow path member  5  is provided with one ink supplying flow path  501  and  502  used to supply the ink to the inside of the head chip  2  and one ink circulation flow path  503  and  504  used to discharge ink from inside the head chip  2 . 
     The head chip  2 , the holding plate  3 , and the ink flow path member  5  are described in detail below. 
     For the purpose of description, the structure inside the head chip  2  in  FIG. 4  is shown with a broken line. The ink flow path from a common supply flow path  25  to a communicating hole (intermediate communicating path)  221 , . . . and the like is shown with dots. 
     [Head Chip] 
     The upper surface of the head chip  2  is provided with piezoelectric elements  24  provided aligned in one line along the left and right direction, ink supply openings  201  and  202  to supply ink inside the head chip  2  from the ink flow path member  5 , ink circulation openings  203  and  204  to discharge ink to the ink flow path member  5  from inside the head chip  2  (see  FIG. 4 , etc.), and the like. 
     In the embodiment below, a circulation flow path  213  is formed in a nozzle plate  21 . The circulation flow path  213  is to be positioned to the nozzle side than a body plate  23  in which the pressure chamber  231  is formed and alternatively, for example, the circulation flow path  213  can be provided in an intermediate plate  22 . Therefore, near the nozzle means toward the nozzle side than a body plate  23  in which the pressure chamber  231  is formed. As described above, the problem of ejecting trouble due to air bubbles and foreign substances can be suppressed by providing the circulation flow path  213  near the nozzle but from the viewpoint of removing the air bubbles and the foreign substances nearer to the position of the nozzle  211  which more easily leads to ejecting trouble, preferably, the circulation flow path  213  is provided in the nozzle plate  21 . Therefore, in the description below, providing the circulation flow path  213  in the nozzle plate  21  is described in detail. 
     In the head chip  2 , the following three substrates are layered as one from the bottom in the order of a nozzle plate  21 , an intermediate plate  22 , and a body plate  23  ( FIG. 5 ). 
     The nozzle plate  21  is a substrate positioned in the lowest layer of the head chip  2 , and for example, includes a SOI wafer including the following three layers, a nozzle layer  21   a , a binding layer  21   b , and a nozzle supporting layer  21   c.    
     The nozzle layer  21   a  is a layer in which the nozzle  211  to eject the ink liquid drops is formed, and includes a Si substrate with a thickness such as 10 to 20 μm. An ink repelling layer (not illustrated) is formed in the nozzle surface  214  on the bottom surface of the nozzle layer  21   a.    
     For example, the binding layer  21   b  includes a SiO 2  substrate with a thickness such as 0.3 to 1.0 μm. 
     The nozzle supporting layer  21   c  includes a Si substrate with a thickness such as 100 to 300 μm formed with a large size portion (nozzle communicating path)  212  with a diameter larger than the nozzle  211  to communicate with the nozzle  211 , and the circulation flow path  213  to communicate with the large size portion  212  and used to circulate the ink. 
     Here, since the nozzle layer  21   a  and the nozzle supporting layer  21   c  each include the Si substrate, the nozzle layer  21   a  and the nozzle supporting layer  21   c  can be easily processed by dry etching or wet etching. Moreover, since the binding layer  21   b  includes the SiO 2  substrate which is thinner and has a much lower etching rate than the Si substrate, when the nozzle layer  21   a  and the nozzle supporting layer  21   c  are processed toward the binding layer  21   b , even if there is a variation in the processing of the nozzle layer  21   a  and the nozzle supporting layer  21   c , the processing can be controlled with the binding layer  21   b.    
     Since the circulation flow path  213  is formed with a space facing the binding layer  21   b,  the circulation flow path  213  is made with fine accuracy. Alternatively, the circulation flow path  213  can be formed with a space facing the nozzle layer  21   a  by removing the binding layer  21   b  by the wet etching process using the buffered hydrofluoric acid (BHF), etc. after forming the space facing the binding layer  21   b.    
     For example, the intermediate plate  22  includes, for example, a glass substrate which is about 100 to 300 μm. A communicating hole (intermediate communicating path)  221  which is to be an ink flow path when the ink is ejected is formed in a position corresponding to the large size portion  212  of the nozzle plate  21  so as to penetrate the intermediate plate  22 . 
     The communicating hole  221  adjusts the shape of the ink flow path such as forming a shape to reduce the diameter of the ink passing flow path and adjusts movement energy applied to the ink when the ink is omitted. 
     Preferably, borosilicate glass (for example, Tempax glass) is used as the glass substrate of the intermediate plate  22 . 
     The body plate  23  includes a pressure chamber layer  23   a  and a vibrating layer  23   b.    
     The pressure chamber layer  23   a  includes, for example, a Si substrate which is about 100 to 300 μm. The pressure chamber layer  23   a  is formed with a plurality of pressure chambers  231  which are a substantial circle shape in a planar view and communicate with the communicating hole  221  of the intermediate plate  22 , a common supplying flow path  25  which commonly supplies ink to the plurality of pressure chambers  231 , and an inlet  232  which individually communicates the common supply flow path  25  with each pressure chamber  231  to supply the ink in the common supply flow path to the pressure chamber  231 . The inlet  232  includes a narrow portion which is a flow path narrower than the pressure chamber  231 , and it becomes difficult for the pressure applied to the pressure chamber  231  to escape from the inlet  232  side. The narrow portion is to have a flow path narrower than the pressure chamber  231  and the shape can be suitably changed. 
     The vibrating layer  23   b  is, for example, a thin Si substrate which is about 20 to 30 μm and can be elastically deformed. The vibrating layer  23   b  is layered on the upper surface of the pressure chamber layer  23   a . In the vibrating layer  23   b , the upper surface of the pressure chamber  231  functions as a vibrating plate  233 . The vibrating plate  233  vibrates according to the operation of the piezoelectric elements  24  provided on the upper surface of the vibrating plate  233 . With this, pressure can be applied to the ink in the pressure chamber  231 . 
     A common circulation flow path  26  is provided in the intermediate plate  22  and the pressure chamber layer  23   a . The common circulation flow path  26  is where the ink flowing from the plurality of circulation flow paths  213  formed in the nozzle supporting layer  21   c  join. 
     The vibrating layer  23   b  includes a damper  234  formed on an upper surface of the common supply flow path  25 , and a damper  235  formed on the upper surface of the common circulation flow path  26 . The dampers  234  and  235  are able to slightly deform flexibly when, for example, pressure is applied at once to the pressure chamber  231  and the ink flows at once to the common circulation flow path  26 . With this, it is possible to prevent drastic change of pressure in the ink flow path. 
     Next, the ink circulation path is described. The ink is supplied from the ink supply openings  201  and  202  to the common supply flow path  25 . Next, the ink separates from the common supply flow path  25  and flows to each nozzle  211 , . . . , corresponding inlet  232 , . . . , pressure chamber  231 , . . . , communicating hole  221 , . . . , large size portion  212 , . . . , and circulation flow path  213 , . . . Next, the ink from each circulation flow path  213 , . . . meet at the common circulation flow path  26 , the ink is discharged from the ink circulation openings  203  and  204 , and the ink passes the ink circulation flow path  504  to return to a circulation sub-tank  63  (see  FIG. 4 ,  FIG. 5 , and  FIG. 7 ). 
     [Holding Plate] 
     The holding plate  3  is attached to the upper surface of the head chip  2  with adhesive. For example, the holding plate  3  is a substrate including a Si substrate or a glass substrate with a thickness of about 0.5 mm to 3.0 mm. Moreover, by using the Si substrate or the glass substrate as the holding plate  3 , the linear expansion coefficient becomes close to the substrate included in the head chip  2 . Therefore, even if the holding plate  3  is attached to the head chip  2  by a method using heat such as using thermosetting adhesive as the adhesive, the bend between the holding plate  3  and the head chip  2  can be suppressed. 
     The shape of the holding plate  3  from a planar view is formed larger than the head chip  2  in both the front and back direction and the left and right direction. Specifically, both edges of the holding plate  3  in the left and right direction are largely outside the head chip  2 . 
     An opening  31  is formed through the center of the holding plate  3  in a size which can surround all piezoelectric elements  24  aligned on the upper surface of the head chip  2  when the head chip  2  is attached to the holding plate  3 . 
     The opening  31  is formed in a rectangular shape extending along the left and right direction. The inside of the opening  31  is formed in a size which is able to surround all of the piezoelectric elements  24  on the upper surface of the head chip  2 , but does not reach the position of the ink supply openings  201  and  202  and the ink circulation openings  203  and  204  provided on both edges of the upper surface of the head chip  2 . When the holding plate  3  is viewed from a planar view, each nozzle  211  formed in the nozzle plate  21  is positioned in the region in the front and back direction and the left and right direction in which the opening  31  is provided. 
     The bottom side of the opening  31  of the holding plate  3  is formed so that the space is larger than the upper side and the region of the opening  31  is formed to be a convex shape pointing up. The bottom side of the opening  31  is formed in a size so as to be able to include the piezoelectric element  24 , the common supply flow path  25  provided in the front and back direction of the piezoelectric element  24  and the common circulation flow path  26  when the holding plate  3  is attached to the head chip  2 . 
     Through holes  301 ,  302 ,  303 , and  304  are formed near both edges of the holding plate  3  in the left and right direction in a size which can surround each one of the ink supply openings  201  and  202  and the ink circulation openings  203  and  204  provided on the upper surface of the head chip  2 . The through holes  301 ,  302 ,  303 , and  304  are used as ink flow paths to establish communication between the ink flow path member  5  and the head chip  2 . 
     [Ink Flow Path Member] 
     The ink flow path member  5  is formed with synthesized resin such as poly phenylene sulfide resin (PPS) in a box like shape with the lower surface open. One ink flow path member  5  is provided in each edge of the upper surface of the holding plate  3  in the left and right direction. 
     Since the ink flow path members  5  provided to the left and the right have similar structures, the configuration of only the right ink flow path member  5  is described, and the description of the left ink flow path member  5  is omitted. 
     The ink flow path member  5  is provided with an ink supply flow path  501  which functions as a flow path to supply ink and an ink circulation flow path  504  which functions as a flow path to discharge ink. 
     Inside the ink flow path member  5 , a filter  51  is provided for each of the ink supply flow path  501  and the ink circulation flow path  504  to remove impurities such as waste and air bubbles in the ink passing inside the ink flow path member  5 . For example, metal mesh such as stainless steel, etc. is used as the filter  51 , and this is attached to resin in the ink flow path member  5 . 
     [Ink Circulation Mechanism] 
     The ink circulation mechanism as the ink circulation member is described. In the ink flow path member  5 , the ink supply flow path  501  is connected to a supply sub-tank  62  through an ink flow path  72 , the ink is supplied inside the ink flow path member  5  from the supply sub-tank  62 , and the ink is supplied in the head chip  2  through the through hole  301  and the ink supply opening  201  (see  FIG. 6  and  FIG. 7 , etc.). 
     In the ink flow path member  5 , the ink circulation flow path  504  is connected to a circulation sub-tank  63  through an ink flow path  73 , and the ink discharged inside the ink flow path member  5  from the ink supply opening  201  of the head chip  2  through the through hole  304  can be discharged to the circulation sub-tank  63 . 
     The supply sub-tank  62  and the circulation sub-tank  63  are provided in a position different in the up and down direction (gravity direction) with respect to a position reference surface in which the common supply flow path  25  and the common circulation flow path  26  are provided inside the head chip  2 . With respect to the position reference surface, pressure P 1  due to the hydraulic head difference from the supply sub-tank  62  and pressure P 2  due to the hydraulic head difference from the circulation sub-tank  63  allows the ink inside the head chip  2  to circulate. 
     The supply sub-tank  62  is connected to the circulation sub-tank  63  by the ink flow path  74 . The ink can be returned from the circulation sub-tank  63  to the supply sub-tank  62  using the pump  82 . 
     The supply sub-tank  62  is connected to the main tank  61  by the ink flow path  71 . The ink can be supplied from the main tank  61  to the supply sub-tank  62  using the pump  81 . 
     Therefore, by suitably adjusting the hydraulic head difference of the supply sub-tank  62  and the circulation sub-tank  63 , and the position of each sub-tank in the up and down direction (gravity direction), the pressure P 1  and the pressure P 2  are adjusted, and the ink inside the head chip  2  can be circulated at a suitable circulation flow rate. 
     [Filling Ink Inside the Head Chip] 
     A configuration similar to the above-described right ink flow path member  5  may be provided in the left side, and the left ink flow path member  5  can be configured as described below so that even pressure can be applied to the nozzles  211  and ink can be filled stably when the ink is filled inside the head chip  2 . 
     In the left ink flow member  5 , the ink supply flow path  502  and the ink circulation flow path  503  are connected with a tube through a valve (illustration not shown). When the ink is filled inside the head chip  2 , the valve is opened, and pressure is applied from the ink supply flow path  501  of the right ink flow path member  5  toward the ink circulation flow path  504 . With this, the ink can be filled in the common supply flow path  25  of the head chip  2 . 
     Next, the valve between the ink supply flow path  502  and the ink circulation flow path  503  is closed, and pressure is further applied from the ink supply flow path  501 , and the ink filled in the common supply flow path  25  is filled from each inlet  232 , to near each nozzle  211 . Further, the ink can be flown from each circulation flow path  213 , to the common circulation flow path  26 . 
     Viscosity resistance of the circulation flow path  213  is made sufficiently lower than the viscosity resistance of the nozzle  211 . Therefore, the ink can be filled in the nozzle without breaking the meniscus of the nozzle  211  and throwing away the ink. 
     After the ink is filled, the ink can be circulated by suitably adjusting the above-described pressure P 1  and P 2  so that the pressure near the nozzle  211  and the speed of the circulation flow rate becomes a predetermined value. 
     [Ink Ejecting Properties] 
     As described above, the ink can be ejected from the nozzle  211  by applying pressure to the pressure chamber  231  with the piezoelectric element  24 . Here, the ink is supplied to the pressure chamber  231  from the inlet  232 , and the ink circulation flow path  213  is provided near the nozzle  211 . 
     Therefore, the ink ejecting properties can be determined by the impedance Zn of the nozzle  211 , the impedance Zi of the inlet  232 , and the impedance Zc of the circulation flow path  213 . 
     The impedances Z are values which can be determined by the viscosity resistance R and the inertance M of the flow path. As described below, the value can be calculated as an electric equivalent circuit constant. With the electric circuit simulator, the resonance frequency of the pressure chamber, and ejecting properties such as liquid drop speed, ejecting negative pressure, and driving voltage can be calculated. 
     Specifically, in the inlet  232  and the circulation flow path  213 , when the shape of the flow path is a rectangular solid, and when a flow path width (front and back direction) is w (μm), a flow path height (up and down direction) is h (μm), a flow path length (left and right direction) is l (μm), an ink fluid viscosity is η (Pa/s), an ink density is ρ (kg/m 3 ), and a driving pulse frequency (an inverse of driving pulse length) is f (Hz), the following calculations are possible, the inertance M=ρl/hw, the viscosity resistance R=8ηl(h+w) 2 /(hw) 3 , the impedance Z=(R 2 +2πfM 2 ) 1/2 . 
     In the nozzle  211 , when the shape of the nozzle  211  is a cylinder, and when a flow path diameter is d (μm), a flow path height (up and down direction) is l (μm), an ink fluid viscosity is η (Pa/s), an ink density is ρ (kg/m 3 ), and a driving pulse frequency (an inverse of driving pulse length) is f (Hz), the following calculations are possible, the inertance M=4ρl/πd 4 , the viscosity resistance R=128ηl/πd 4 , the impedance Z=(R 2 +2πfM 2 ) 1/2 . 
     The rectangular solid shape and the cylinder shape are described above. Alternatively, when other shapes are employed, such as a tapered shape, calculations are possible by segmenting the tapered shape as the rectangular solid shape in the length direction and integrating. 
     A setting value of impedance Zc of the circulation flow path  213  according to the present invention is described using experimental values shown in  FIG. 8A ,  FIG. 8B ,  FIG. 9A  and  FIG. 9B . 
       FIG. 8A  and  FIG. 8B  show a state when the ink liquid drop is ejected with the ejecting liquid drop amount as 3.5 pL and the ejecting speed as 7 m/s.  FIG. 8A  shows the relation of the values when the ratio (Zc/Zi) between the impedance Zc of the circulation flow path  213  and the impedance Zi of the inlet  232  is shown in the horizontal axis and the driving voltage (V) of the piezoelectric element  24  is shown in the vertical axis.  FIG. 8B  shows the relation of the values when the ratio (Zc/Zi) between the impedance Zc of the circulation flow path  213  and the impedance Zi of the inlet  232  is shown in the horizontal axis and the ink ejecting negative pressure (kPa) is shown in the vertical axis. Here, the ejecting negative pressure is pressure near the nozzle caused at the time of ejecting, and when this value becomes too low, air bubbles are generated by cavitation. Therefore, it is necessary to maintain the ejecting negative pressure to a predetermined value or more. 
       FIG. 9A  and  FIG. 9B  show a state when the ink liquid drop is ejected with the ejecting liquid drop amount as 1.0 pL and the ejecting speed as 7 m/s. Similar to  FIG. 8A  and  FIG. 8B ,  FIG. 9A  shows the relation of the values when the impedance ratio (Zc/Zi) is shown in the horizontal axis and the driving voltage (V) of the piezoelectric element  24  is shown in the vertical axis, and  FIG. 9B  shows the relation of the values when the impedance ratio (Zc/Zi) is shown in the horizontal axis and the ink ejecting negative pressure (kPa) is shown in the vertical axis. 
     As shown in  FIG. 8A ,  FIG. 8B ,  FIG. 9A , and  FIG. 9B , regardless of whether the ejecting liquid drop amount is 3.5 pL ( FIG. 8A  and  FIG. 8B ) or the ejecting liquid drop amount is 1.0 pL ( FIG. 9A  and  FIG. 9B ), the driving voltage (V) of the piezoelectric element  24  is almost constant when Zc/Zi is 0.5 or more, and the inclination of the increase of the ejecting negative pressure (kPa) becomes moderate at 0.5 or more. 
     As described above, when the ratio (Zc/Zi) between the impedance Zc of the circulation flow path  213  and the impedance Zi of the inlet  232  is 0.5 or more, the rise of the driving voltage (V) of the piezoelectric element  24  and the ejecting negative pressure can be suppressed. With this, the air bubbles being generated can be suppressed. Therefore, it can be said that the ink ejecting performance is high. The ratio (Zc/Zi) of the impedance Zi can be set without upper limits from the view point of ejecting performance. However, if the ratio (Zc/Zi) of the impedance Zi rises, the viscosity resistance also rises at the same time. Therefore, the value is set to a range of 0.5 or higher and lower than the nozzle viscosity resistance. 
     When the viscosity resistance Rc of the circulation flow path  213  becomes larger than the viscosity resistance Rn of the nozzle  211 , the meniscus of the nozzle  211  may break when the ink is filled or the ink is circulated. Therefore, the viscosity resistance Rc of the circulation flow path  213  needs to be made smaller than the viscosity resistance Rn of the nozzle  211 . 
     According to the configuration of the present invention, as a result of electric circuit simulation, the ink circulation flow rate can be made equal to or faster than the point of maximum ejecting (ejecting liquid drop amount (pL) x ejecting frequency (Hz)), and foreign substances such as air bubbles can be effectively removed. The electric circuit simulation is obtained from the pressure of in and out and replacing the flow path with the electric equivalence circuit similar to the above-described ejecting performance. 
     Preferably, in order to make the circulation flow rate faster, the total viscosity resistance Rs (Rs=Ri+Rc) of the viscosity resistance Ri of the inlet  232  and the viscosity resistance Rc of the circulation flow path  213  can be made smaller than the viscosity resistance Rn of the nozzle  211 . 
     The value of the impedance Zc of the circulation flow path  213  and the value of the impedance Zi of the inlet  232  can be set suitably. If the impedance Zc of the circulation flow path  213  is set to a large value, the pressure loss to the circulation flow path  213  becomes small. Therefore, the ejecting performance becomes closer to a configuration which does not include the circulation flow path  213 . 
     [Modification] 
     The modification of the head chip  2  provided with the circulation flow path  213  is described with reference to  FIG. 10  and  FIG. 11 . 
     For the purpose of ease of description,  FIG. 10  shows the configuration inside the head chip  2  with broken lines. The ink flow path from the common supply flow path  25  to the communicating hole  221 , . . . is shown with dots. 
     The description of the configuration similar to the above-described embodiment is omitted. 
     On the upper surface, the head chip  2  includes a piezoelectric element  24  provided aligned in two lines so as to be in a staggered arrangement along a left and right direction, ink supply openings  201  and  202  which supply ink from the ink flow path member  5  to inside the head chip  2 , ink circulation openings  203  and  204  which discharge ink from inside the head chip  2  to the ink flow path member  5 , and the like ( FIG. 10 ). 
     Inside the head chip  2 , the following three substrates are layered as one from the bottom in order, the nozzle plate  21 , the intermediate plate  22 , and the body plate  23  ( FIG. 11 ). The pressure chamber  231  is formed to correspond to each piezoelectric element  244  on the pressure chamber layer  23   a  in the lower side of the piezoelectric element  24  provided aligned in two lines so as to be in the staggered arrangement. 
     The common supply flow path  25  is formed only on the pressure chamber layer  23   a  of the body plate  23 . The common supply flow path  25  is provided in two lines along the left and right direction near the front side and the back side of the head chip  2  with the position in which the piezoelectric elements  24  are aligned in between. 
     The vibrating layer  23   b  which can be slightly elastically deformed is formed on the upper surface of the pressure chamber layer  23   a . The vibrating layer  23   b  on the upper surface of the common supply flow path  25  functions as the damper  234 . 
     The common circulation flow path  26  is formed on only the intermediate plate layer  22   a  of the intermediate plate  22  so as to be positioned on the lower side of the body plate  23  in which the common supply flow path  25  is formed. 
     The vibrating layer  22   b  which can be slightly elastically deformed is formed on the upper surface of the intermediate plate layer  22   a  of the intermediate plate  22 , and the vibrating layer  22   b  on the upper surface of the common circulation flow path  26  functions as the damper  236 . 
     Regarding the ink circulation flow path, first the ink is supplied from the ink supply openings  201  and  202  to the common supply flow path  25  formed aligned near the front side and the back side of the head chip  2 . Next, the ink is supplied to each pressure chamber  231 , . . . provided on the lower side of the piezoelectric element  24  aligned in a staggered state from the common supply flow path  25  on the front side or the back side, which has the short distance, through the inlets  232 , . . . . Next, the ink is flown in the following order, the communicating holes  221 , . . . , the large size portion  212 , . . . , and the circulation flow path  213 , . . . . Next, the ink from each circulation flow path  213 , . . . meets the common circulation flow path  26  on the front side or the back side. The ink is discharged from the ink circulation openings  203  and  204 , and the ink flows through the ink circulation flow path  504  and returns to the circulation sub-tank  63  (see  FIG. 7 ,  FIG. 10 , and  FIG. 11 ). 
     [Technical Effects of the Present Invention] 
     As described above, the viscosity resistance Rc of the circulation flow path  213  is made smaller than the viscosity resistance Rn of the nozzle  211 , and the impedance Zc of the circulation flow path  213  is made 0.5 times or more than the impedance Zi of the inlet. With this, as for the inkjet head  1  of the present invention, the reduction of the ejecting performance of ink can be suppressed to a minimum and the air bubbles, etc. near the nozzle can be effectively discharged without applying a burden to the apparatus. 
     Specifically, when the circulation flow path  213  is formed, the pressure escapes to the circulation flow path  213 . Therefore, the pressure is easily lost. However, according to the configuration of the present invention, the pressure loss can be suppressed to a minimum, and the mechanism can be driven with low voltage. 
     Since the ejecting negative pressure is suppressed, the air bubbles being generated can be suppressed. 
     The ink circulation flow rate can be made the same as or faster than the point of maximum ejecting (ejecting liquid drop amount (pL)×ejecting frequency (Hz)). With this, the foreign substances such as air bubbles can be efficiently removed. 
     Since the circulation flow path  213  is formed near the nozzle  211 , the foreign substances such as air bubbles near the nozzle can be removed. 
     Since the inlet  232  includes a narrow portion with the flow path narrower than the pressure chamber  231 , the pressure of the pressure chamber  231  can be effectively raised. 
     The total viscosity resistance of the inlet  232  and the circulation flow path  213  is made smaller than the viscosity resistance of the nozzle  211 . With this, the circulation flow rate can be made faster. 
     The nozzle communicating path (large size portion  212 ) which has a larger diameter than the nozzle  211  which communicates ink from the pressure chamber  231  and the circulation flow path  213  are formed in the nozzle supporting layer  21   c  layered on the upper surface of the nozzle layer  21   a  in which the nozzle  211  is formed. With this, the circulation flow path  213  can be formed directly above the nozzle. Therefore, the air bubbles, etc. near the nozzle can be effectively removed and the clogging of the nozzle  211  can be prevented. 
     The binding layer  21   b  with a lower etching rate than the nozzle supporting layer  21   c  is provided between the nozzle layer  21   a  and the nozzle supporting layer  21   c . The nozzle supporting layer  21   c  includes a space facing the binding layer  21   b  or the nozzle layer  21   a . The circulation flow path  213  is formed by this space. Therefore, the circulation flow path  213  can be made while reducing the error in manufacturing as much as possible. 
     The common circulation flow path  26  connected to the circulation flow path  213  corresponding to each of the plurality of nozzles  211  is formed in at least one of the body plate (body layer)  23  and the intermediate plate (intermediate layer)  22 . With this, the common circulation flow path  26  can be provided stably, and the manufacturing can be performed reducing the error in manufacturing as much as possible. 
     The inkjet head  1  of the present invention can be used in an inkjet recording device  100  by separately providing an ink circulator to cause a circulation flow (ink circulation mechanism). 
     [Others] 
     The embodiments disclosed in the present application are merely examples in all points and do not limit the present invention. The scope of the present invention is shown in the following claims and is not limited by the above detailed description. The scope of the present invention includes equivalents and all modifications within the scope of the present invention. 
     For example, the ink flow path member  5  of the present invention including the ink supply flow path and the ink circulation flow path is provided so that there is one in each side in the left and right direction. Alternatively, the configuration can be suitably changed as long as the ink can be circulated, and one can be provided in one side. Alternatively, for example, only the ink supply flow path may be provided in the left ink flow path member  5  and only the ink circulation flow path may be provided in the right ink flow path member  5 . 
     As the ink circulation method to cause the circulation flow, the method of control by using the pressure due to hydraulic head difference is described. Alternatively, this can be suitably changed to any configuration which can generate the circulation flow as described in the present invention. 
     The inkjet head  1  is a configuration which ejects the liquid drop such as ink using the piezoelectric element. Alternatively, any mechanism which can discharge liquid drops can be employed, and for example, thermal units (thermal electric conversion elements) can be used. 
     Further, the scope of the present invention is not limited to the above, and various modifications and changes in design are possible without leaving the scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used in an inkjet head and an inkjet recording device including an inkjet head and an ink circulator to generate a circulation flow. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1  inkjet head 
           21  nozzle plate 
           21   a  nozzle layer 
           21   b  binding layer 
           21   c  nozzle supporting layer 
           211  nozzle 
           212  large size portion (nozzle communicating path) 
           213  circulation flow path 
           22  intermediate plate (intermediate layer) 
           221  communicating hole (intermediate communicating path) 
           23  body plate (body layer) 
           231  pressure chamber 
           232  inlet 
           24  piezoelectric element (pressure generating unit) 
           25  common supply flow path 
           26  common circulation flow path 
           100  inkjet recording device