Patent Publication Number: US-10759166-B2

Title: Liquid ejection head and liquid ejection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2018-062797 filed on Mar. 28, 2018, the content of which is incorporated herein by reference in its entirety. 
     FIELD OF DISCLOSURE 
     Aspects disclosed herein relate to a liquid ejection head and a liquid ejection apparatus including a liquid ejection head. 
     BACKGROUND 
     A known liquid ejection apparatus includes a liquid ejection head configured to eject liquid, such as ink, to a recording medium. The liquid ejection head includes a manifold for storing liquid therein, and ejection channels to which liquid is supplied from the manifold. Each ejection channel includes a pressure chamber and a nozzle. Upon application of pressure to liquid in the pressure chamber, liquid is ejected from the nozzle. 
     The ejection channels are arranged in an array. A dummy channel is provided next to an ejection channel at an end of the array. This allows the ejection channels in the middle of the array and the ejection channel at the end of the array to have a similar surrounding structure to each other, and thus makes the ejection characteristics uniform among the ejection channels. 
     SUMMARY 
     It is conceivable to provide a nozzle for the dummy channel in addition to the nozzles of the ejection channels so that, through all theses nozzles located downstream, liquid in an upstream tank connected to the manifold is drawn into and spread throughout the manifold. In this case, liquid in the dummy channel near the nozzle is exposed to the atmosphere and may change in quality over time. 
     It may be beneficial for a liquid ejection head and a liquid ejection apparatus to include a dummy channel which has a nozzle and is configured to prevent or suppress deterioration in quality of liquid therein. 
     According to one or more aspect of the disclosure, a liquid ejection head comprises a manifold configured to store liquid therein, a plurality of ejection channels, and a dummy channel. Each ejection channel communicates with the manifold and is configured to receive liquid from the manifold and eject liquid through a corresponding nozzle thereof open to a nozzle surface. The dummy channel communicates with the manifold and includes a dummy nozzle open to the nozzle surface. The dummy channel further includes a pressure chamber, an actuator configured to apply pressure to liquid in the pressure chamber, a communication passage connecting the manifold to the pressure chamber, and a circulation passage through which the pressure chamber communicates with the manifold. The circulation passage is different from the communication passage and located between the dummy nozzle and the manifold. 
     According to one or more aspect of the disclosure, a liquid ejection apparatus comprises the above-described liquid ejection head and a driver integrated circuit. The driver integrated circuit is configured to output a drive signal for driving the actuator. The drive signal is a pulse signal changing between a first potential v1 and a second potential v2. The pressure chamber of the dummy channel is configured to have a first volume V1 when the drive signal applied to the actuator is at a first potential v1, and have a second volume V2 less than the first volume V1 when the drive signal applied to the actuator is at the second potential v2. An inertance of a path located toward the circulation passage relative to the pressure chamber is different from an inertance of a path toward the communication passage relative to the pressure chamber. The driver integrated circuit is configured to output to the actuator a drive signal which takes a first time period t12 to change from the first potential v1 to the second potential and takes a second time period t21 to change from the second potential v2 to the first potential v1. The first time period t12 is different from the second time period t21. 
     According to one or more aspect of the disclosure, a liquid ejection head comprises a plurality of nozzles open to a nozzle surface, a manifold communicating with the plurality of nozzles, a plurality of pressure chambers, a plurality of channels, and a plurality of actuators. Each channel includes a communication passage which connects a corresponding outlet of the manifold to a corresponding pressure chamber, and each channel extends from the corresponding outlet of the manifold, via the communication passage, to a corresponding nozzle. Each actuator partially defines a wall of a corresponding pressure chamber and is configured to change a volume of the corresponding pressure chamber. The plurality of channels are arranged in an array, and a channel located at an end of the array is a dummy channel. The dummy channel defines a path extending from a corresponding outlet of the manifold to a corresponding nozzle and includes, in addition to a corresponding communication passage, a circulation passage which is located between the corresponding nozzle and the manifold and through which a corresponding pressure chamber communicates with the manifold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the disclosure are illustrated by way of example and not by limitation in the accompanying figures in which like reference characters indicate similar elements. 
         FIG. 1  shows a schematic diagram showing an overall structure of a liquid ejection apparatus in a first embodiment according to one or more aspects of the disclosure. 
         FIG. 2  is a block diagram showing a functional structure of the liquid ejection apparatus. 
         FIG. 3  is an enlarged plan view of a part of a liquid ejection head in the first embodiment. 
         FIG. 4A  is an enlarged cross-sectional view of an ejection channel of the liquid ejection head, cut along line IVa-IVa in  FIG. 3 , and  FIG. 4B  is an enlarged cross-sectional view of a dummy channel of the liquid ejection head, cut along line IVb-IVb in  FIG. 3 . 
         FIGS. 5A and 5B  are diagrams each showing the change with time in potential of a drive signal applied to an actuator of the dummy channel. 
         FIG. 6  is an enlarged plan view of a part of a liquid ejection head in a second embodiment according to one or more aspects of the disclosure. 
         FIG. 7  is an enlarged plan view of a part of a liquid ejection head in a third embodiment according to one or more aspects of the disclosure. 
         FIG. 8A  is an enlarged cross-sectional view of an ejection channel of a liquid ejection head in a fourth embodiment according to one or more aspects of the disclosure, and  FIG. 8B  is an enlarged cross-sectional view of a dummy channel of the liquid ejection head. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A liquid ejection head and a liquid ejection apparatus according to embodiments will be described with reference to the drawings. An example of a liquid ejection apparatus is configured to eject ink onto a recording sheet, as will be described hereinafter. However, the liquid ejection apparatus may eject liquid other than ink, and ejected liquid may be adhered onto a medium other than a sheet-like medium. 
     [Structure of Liquid Ejection Apparatus] 
     As shown in  FIG. 1 , a liquid ejection apparatus  1  includes a feed tray  10 , a platen  11 , and a carriage  12  which are assembled from below in this order. The feed tray  10  stores a plurality of recording sheets P therein. The platen  11  is elongate in a right-left direction and is disposed above the feed tray  10 . The platen  11  is a flat plate and supports from below a recording sheet being conveyed. The carriage  12  is disposed above the platen  11 . The carriage  12  is movable reciprocally in the right-left direction and supports a liquid ejection head  13 . A discharge tray  14  is disposed further toward the front than the platen  11  and receives a recording sheet P having an image recorded thereon. 
     A sheet conveying path  20  is defined to extend rearward from the feed tray  10  to the discharge tray  14 . The sheet conveying path  20  includes a curved path portion  21 , straight path portion  22 , and an end path portion  23 . The curved path portion  21  is curved upward from the feed tray  10  and extends to a position near the rear of the platen  11 . The straight path portion  22  extends from an end of the curved path portion  21  to a position near the front of the platen  11 . The end path  23  extends from an end of the straight path portion  22  to the discharge tray  14 . 
     The liquid ejection apparatus  1  includes, as a sheet conveying mechanism for conveying a recording sheet P, a feed roller  30 , conveying roller  30 , and a discharge roller  34 . The sheet conveying mechanism conveys a recording sheet P from the feed tray  10  to the discharge tray  14  along the sheet conveying path  20 . 
     Specifically, the feed roller  30  is disposed directly above the feed tray  10  and contacts an uppermost sheet P. A conveying roller pair  33 , including the conveying roller  31  and a pinch roller  32 , is disposed near a downstream end of the curved path portion  21 . The conveying roller pair  33  is disposed between the curved path portion  21  and the straight path portion  22 . A discharge roller pair  34 , including the discharge roller  34  and a spur roller  32 , is disposed near a downstream end of the straight path portion  22 . The discharge roller pair  36  is disposed between the straight path portion  22  and the end path portion  23 . 
     The feed roller  30  feeds a recording sheet P along the curved path portion  21  toward the conveying roller pair  33 . The conveying roller pair  33  conveys the recording sheet P along the straight path portion  22  to the discharge roller pair  36 . The liquid ejection head  13  ejects ink onto the recording sheet P supported on the platen  11  and conveyed along the straight path portion  22 , thereby recording an image on the recording sheet P. The discharge roller pair  36  conveys the recording sheet P having the image recorded thereon to the discharge tray  14 . 
     As shown  FIG. 2 , a controller  40  of the liquid ejection apparatus  1  includes a first substrate and a second substrate. A central processing unit (CPU)  41 , a read-only memory (ROM)  42 , a random-access memory (RAM)  43 , and an electrically erasable programmable ROM (EEPROM)  44  are mounted on the first substrate. An application specific integrated circuit (ASIC)  45  is mounted on the second substrate. Two motor driver integrated circuits (ICs)  46 ,  47  and a head driver integrated circuit (IC)  48  are connected to the ASIC  45 . The motor driver IC  46  drives a conveying motor  50 , and the motor driver IC  47  drives a carriage motor  51 . The head driver IC  48  drives actuators  71 ,  81  (to be described later) of the liquid ejection head  13 . 
     When the liquid ejection apparatus  1  receives an input of a print job from a user or a communication device, the CPU  41  outputs to the ASIC  45  a command for executing the print job based on a program stored in the ROM  42 . The ASIC  45  controls the driver ICs  46 - 48  based on this command. Consequently, a recording sheet P is fed, and recording is executed by ink ejection onto the recording sheet P in synchronization with conveyance of the recording sheet P. 
     Specifically, the motor driver IC  46  drives the conveying motor  50  to rotate the feed roller  30 , the conveying roller  34 , and the discharge roller  34 . The motor driver IC  47  drives the carriage motor  51  to reciprocate the carriage  12  in the right-left direction (e.g., in a main scanning direction). The head driver IC  48  selectively drives the actuators  71 ,  81  to cause vibration of menisci and ink ejection. 
     The head driver IC  48  outputs a drive signal which is a pulse signal changing between a first potential v1 and a second potential v2 different from the first potential v1. When the drive signal is of the first potential v1, a pressure chamber has a first volume V1. When the drive signal is of the second potential v2, the pressure chamber has a second volume V2 which is less than the first volume V1 (V1&gt;V2). As described above, upon application of a drive signal to an actuator, the actuator deforms thereby changing the volume of a pressure chamber. 
     The liquid ejection apparatus  1  further includes various sensors, such as a sheet edge sensor for detecting the position of a recording sheet, and an encoder for detecting the position of the carriage. The controller  40  controls the driver ICs  46 - 48 , based on signals from the various sensors, for image forming on a recording sheet P. 
     [Structure of Liquid Ejection Head] 
     As shown in  FIG. 3 , the liquid ejection head  13  includes a manifold  60  for temporally storing ink, and ejection channels  70  and a dummy channel  80  to which ink is distributed from the manifold  60 . The manifold  60  is elongate in a front and rear direction (e.g., in a sub-scanning direction) and defines a rectangular parallelepiped space. 
     The manifold  60  is greater in size than other channel elements located downstream of the manifold  60  in an ink flow direction toward each nozzle. Thus, the manifold  60  has a less ink flow resistance than the other channel elements, such as a pressure chamber  73 , a descender  74 , and an ejection nozzle  75 . A supply passage extending from a liquid tank (not shown) is connected to the rear (upstream end) of the manifold  60 . 
     The liquid ejection head  13  includes the ejection channels  70  and the dummy channel  80  arranged as shown in  FIG. 3 . Each channel  70 ,  80  partially overlaps with the manifold  60  in a top-bottom direction and is disposed on one of opposite sides (e.g., on the right side in  FIG. 3 ) of the manifold  60  in the right-left direction. The channels  70 ,  80  are equally spaced with each other in the front-rear direction and arranged along the manifold  60  to form an array of channels. The dummy channel  80  is located at an end of the array of channels. 
       FIGS. 4A and 4B  each also show a cross-sectional view of the manifold  60  cut in its longitudinal direction. 
     As shown in  FIGS. 4A and 4B , the ejection channels  70  and the dummy channel  80  share the manifold  60 . Each channel  70 ,  80  has a common structure where an outlet of the manifold  60  is fluidly connected to a communication passage, a pressure chamber, a descender, and a nozzle sequentially. As another common structure, a wall of each pressure chamber  70 ,  80  is partially defined by an actuator  71 ,  81 . 
     Each actuator  71 ,  81  includes a piezoelectric element and a vibration plate which are stacked one on another. The piezoelectric element includes a piezoelectric layer and electrodes (an individual electrode and a common electrode) laminated on the top and bottom of the piezoelectric layer. Upon application of a drive voltage to the piezoelectric element, the piezoelectric element expands and contracts in its surface direction (e.g., in a direction orthogonal to the top-bottom direction). The vibration plate does not deform by itself, and the actuator shifts toward the pressure chamber. 
     As shown in  FIG. 4A , the ejection channel  70  defines a passage extending from an outlet of the manifold  60 , via a communication passage  72 , a pressure chamber  73 , and a descender  74 , to an ejection nozzle  75 . The communication passage  72  connects the manifold  60  to the pressure chamber  73 . The descender  74  connects the pressure chamber  73  to the ejection nozzle  75 . As described above, a vibration plate  71   a  of an actuator  71  partially defines a wall of the pressure chamber  73 . Upon application of a drive voltage to the actuator  71 , the vibration plate  71   a  is deformed by the actuator  71 , thereby ejecting ink from the pressure chamber  73 . 
     More specifically, the communication passage  72  is a crank-shaped narrow passage and has a relatively high ink flow resistance. The communication passage  72  directly connects an upper portion of the manifold  60  to a lower portion of the pressure chamber  73 . One end  72   a  of the communication passage  72  corresponds to the outlet of the manifold  60  and is connected, from above, to a left end of the manifold  60 . The communication passage  72  extends upward from one end  72   a  and is bent to the right. Then, the communication passage  72  is bent upward at a position near a right end of the manifold  60  and extends to the other end  72   b . The other end  72   b  of the communication passage  72  is connected, from below, to one end  73   a  of the pressure chamber  73 . 
     The pressure chamber  73  is elongate in the right-left direction and defines a space extending rightward from the one end  73   a  to the other end  73   b . The pressure chamber  73  is open upward and, as described above, is sealed with the vibration plate  71   a.    
     The descender  74  is a substantially straight passage and connects a lower portion of the pressure chamber  73  to the ejection nozzle  75 . One end  74   a  of the descender  74  is connected, from below, to the other end  73   b  of the pressure chamber  73 . The descender  74  extends downward from the one end  74   a  to the other end  74   b . The other end  74   b  is connected to the ejection nozzle  75 . The descender  74  allows the manifold  60  to be relatively deep and have a relatively low ink flow resistance. 
     The actuator  71 , when driven, pressurizes ink in the pressure chamber  73 , thereby ejecting ink from the ejection nozzle  75 . 
     As shown in  FIG. 4B , the dummy channel  80  extends from an outlet of the manifold  60 , via a communication passage  82 , a pressure chamber  83 , and a descender  84 , to a dummy nozzle  85 . Channel elements of the dummy channel  80 , including an actuator  81  and the communication passage  82 , are similar in disposition to those of the ejection channel  70 . Similarly to the actuator  71 , a vibration plate  81   a  of the actuator  81  partially defines a wall of the pressure chamber  83 . The actuator  81 , when energized, deforms to change the volume of the pressure chamber  83 . 
     Further, the dummy channel  80  is characterized by a circulation passage  86 . The pressure chamber  83  is in fluid communication with the manifold  60  through the circulation passage  86 . The circulation passage  86  is located between the dummy nozzle  85  and the manifold  60 . The circulation passage  86  is closer to the dummy nozzle  85  than to the actuator  81 . Specifically, the circulation passage  86  connects the descender  84  to the manifold  60 . Note that the communication passage  82  is connected to one end of the pressure chamber  83 , the one end being opposite from the dummy nozzle  85  relative to the actuator  81 . 
     The presence of the descender  84  enables increase in volume of the manifold  60 , reliable dilution of ink flowing into the circulation passage  86 , and supply of fresh ink into the communication passage  82 . 
     One end  86   a  of the circulation passage  86  is connected to a portion of the descender  84 , the portion being near the other end  84   b  of the descender  84 . The circulation passage  86  extends leftward from the one end  86   a  to the other end  86   b . The other end  86   b  is connected to a lower right end of the manifold  60 . 
     A laminate body  13   a  is formed by laminating a plurality of plates having through-holes, dents, and grooves. The laminate body  13   a  defines therein channel elements such as the manifold  60 , the communication channels  72 ,  82 , and the pressure chambers  73 ,  83 . A lower surface  13   b  of the laminate body  13   a  is a nozzle surface  13   b  to which the nozzles  75 ,  85  are open. The boundaries (bonded surfaces) of the plates forming the laminate body are omitted from  FIGS. 4A and 4B . 
     The ejection channels  70  in the channel array are same in size of the communication passage  72 , the pressure chamber  73 , the descender  74 , and the ejection nozzle  75 . These elements of each ejection channel  70  are almost same in size as corresponding elements of the dummy channels  80 . 
     As described above, the dummy channel  80  is disposed adjacent to an ejection channel  70  located at a far end of the channel array. This structure enables unifying the ejection characteristics among the ejection channel  70  at the far end and the other ejection channels  70 . 
     Ink in the dummy channel  80  is exposed to the atmosphere through the dummy nozzle  85  and may change in quality (e.g., in viscosity) with time. In this embodiment, however, the circulation passage  86  allows ink in the dummy channel  80  to flow back into the manifold  60  when the actuator  81  is driven to such extent as not to cause ink ejection. Ink in the dummy channel  80  is maintained as fresh as ink in the manifold  60 . This prevents thickened ink in the dummy channel  80  from affecting the ejection characteristics of the adjacent ejection channel  70 . 
     In the liquid ejection head  13  in this embodiment, the manifold  60  has opposite ends (left and right ends) in a cross section (shown in  FIG. 4B ) intersecting the longitudinal direction (the front-rear direction) of the manifold  60 . In the dummy channel  80 , the other end  86   b  of the circulation passage  86  is connected to the right end of the manifold  60 , and the one end  82   a  of the communication passage  82  is connected to the left end of the manifold  60 . 
     The dummy channel  80  is connected to the manifold  60  at two different positions, namely at the left and right ends of the cross section of the manifold  60 . This prevents ink returning into the manifold  60  from flowing immediately to the circulation passage  86  of the dummy channel  80 . Thus, deterioration in quality of ink in the dummy channel  80  is prevented or suppressed. 
     [Structure Unique to Dummy Channel] 
     As described above, the elements of the dummy channel  80  in this embodiment are almost same in size as the corresponding elements of the ejection channel  70 . However, the dummy channel  80  has a unique structure different from that of the ejection channel  70 , as described in detail below. 
     In the dummy channel  80 , a liquid inertance Mc of the circulation passage  86  is set to be less than a liquid inertance Mn of the dummy nozzle  85 . A liquid inertance of a passage is expressed by the following equation (1):
 
 M=ρ*l/S   (1)
 
where M is the inertance, ρ is the density of liquid, l is the length of a passage, and S is the cross-sectional area of the passage.
 
     In the dummy channel  80 , Mc is set to be less than Mn (Mn&gt;Mc). Because of this setting, ink near the dummy nozzle  85  is more likely to move to the circulation passage  86  than to the dummy nozzle  85  when ink flows from the pressure chamber  83  toward the dummy nozzle  85 . Ink ejection from the dummy nozzle  85  is restricted in this way. As apparent from the equation (1), the inertance is adjustable by appropriately setting the diameter and the length of a target passage. 
     Further, the ink flow resistance of the dummy channel  80  is set to be equal to the ink flow resistance of the ejection channel  70 . For example, because the circulation passage  86  additionally provided in the dummy channel  80  decreases the ink flow resistance, the cross-sectional area of the communication passage  82  or the dummy nozzle  85  may be set to be less than that of the communication passage  72  or the ejection nozzle  75  of the ejection channel  70 . 
     This setting prevents or reduces outflow of fresh ink from any nozzle having less flow resistance when ink is purged from the nozzles and when ink is initially drawn to the nozzles. Thus, wasteful ink ejection is prevented or reduced. 
     The flow resistance of the dummy channel  80  being “equal” to that of the ejection channel  70  does not necessarily mean that the flow resistance of the dummy channel  80  being “exactly equal” to that of the ejection channel  70 . Specifically, if the flow resistance of the dummy channel  80  is within plus and minus 10% of that of the average flow resistance of the ejection channels  80 , the flow resistance of the dummy channel  80  may be regarded as equal to that of the ejection channel  70 . The ejection channels  70  have a common design value in terms of the flow resistance. 
     [Liquid Circulation Direction] 
     The liquid ejection head  13  in this embodiment is configured such that ink circulates between the dummy channel  80  and the manifold  60 . Ink in the manifold  60  flows to the communication passage  82 , the pressure chamber  83 , the descender  84 , and back to the manifold  60 , as described in detail below. 
     With respect to an ink circulation direction, the dummy channel  80  may be divided into three portions, namely the pressure chamber  83 , an inflow path upstream of the pressure chamber  83 , and an outflow path downstream of the pressure chamber  83 . An inertance Mo of the outflow path is set to be less than an inertance Mi of the inflow path (Mo&lt;Mi). 
     The outflow path is a path located toward the descender  84  (and the circulation passage  86 ) relative to the pressure chamber  83 . More specifically, the outflow path extends from an outlet of the pressure chamber  83 , via the descender  84  and the circulation passage  86 , to the manifold  60 . The inflow path is a path located toward the communication passage relative to the pressure chamber  83 . More specifically, the inflow path extends from an outlet of the manifold  60 , via the communication passage  82 , to the pressure chamber  83 . 
     The head driver IC  48  is configured to output a drive signal S 1  shown in  FIG. 5A . As described above, the drive signal S 1  is a pulse signal changing between the first potential v1 and the second potential v2. The drive signal S 1  takes a time period t12 to change from the first potential v1 to the second potential v2, and takes a time period t21 to change from the second potential v2 to the first potential v1. The time period t12 is less than the time period t21 (t12&lt;t21). 
     This setting allows liquid in the dummy channel  80  to move to the pressure chamber  83 , the descender  84 , and the circulation passage  86  sequentially. Consequently, deteriorated ink near the dummy nozzle  85  is replaced with fresh ink in the manifold  60 . 
     The liquid circulation direction is determined in the above-described structure by the following fact: there is a greater difference, between the two paths having different ink inertances, in the movement of ink in response to a volume change of a pressure chamber when the pressure change due to the volume change is rapid than when moderate. 
     A difference in the outflow path having a less inertance between the ink moving amount upon a change from the first potential v1 to the second potential v2 and the ink moving amount upon a change from the second potential v2 to the first potential v1 is greater than a difference in the inflow path having a greater inertance between the ink moving amount upon a change from the first potential v1 to the second potential v2 and the ink moving amount upon a change from the second potential v2 to the first potential v1. Thus, when the drive signal changes from the first potential v1, via the second potential v2, back to the first potential v1, a relatively big ink flow occurs from the manifold  60  toward the pressure chamber  83 . As a result, in the dummy channel  80 , ink moves from the pressure chamber  83 , via the descender  84 , to the circulation passage  86  sequentially. 
     The magnitude relation between the inertances Mo and Mi, and the magnitude relation between the time periods t12 and t21 may be set reversely. 
     Specifically, the inertance Mo may be greater than the inertance Mi (Mo&gt;Mi) in the dummy channel  80 . Further, the head driver IC  48  may be controlled to output a drive signal S 2  shown in  FIG. 5B . The drive signal S 2  may change in potential over a time period t12 and a time period t21 which is less than the time period t12 (t12&gt;t21). 
     Such settings also allow liquid in the dummy channel  80  to move from the pressure chamber  83 , via the descender  84 , to the circulation passage  86  sequentially. 
     Alternatively, the time period t12 may be greater than the time period t21 (t12&gt;t21) while the inertance Mo is less than the inertance Mi (Mo&gt;Mi). In this case, ink in the dummy channel  80  moves in a reverse direction, from the pressure chamber  83 , via the communication passage  72 , the manifold  60 , and the circulation passage  86 , to the descender  84 . 
     The above-description referring to  FIG. 5  is given on the premise that a positive pressure is applied to the pressure chamber  83  during the time period t12 in which the potential changes from the first potential v1 to the second potential v2 which is greater than v1. However, the actuator  81  may be configured to apply a positive pressure to the pressure chamber  83  during the time period t21 in which the potential changes from the second potential v2 to the first potential v1. 
     In this case, when the inertance Mo is less than the inertance Mi (Mo&lt;Mi) in the dummy channel  80 , the time period t21 for application of a positive pressure should be less than the time period t12 (t12&gt;t21). Alternatively, when the inertance Mo is greater than the inertance Mi (Mo&gt;Mi), the time period t21 should be greater than the time period t12 (t12&lt;t21). In either case, ink in the dummy channel  80 , ink in the dummy channel  80  moves from the pressure chamber  80 , via the descender  84 , to the circulation passage  86  sequentially. 
     Second Embodiment 
     A liquid ejection head  131  in a second embodiment shown in  FIG. 6  has substantially the same structure as the liquid ejection head  13  in the first embodiment shown in  FIG. 3 , except for a few points described below. 
     As shown in  FIG. 6 , an end of a communication passage  721  of each ejection channel  701  is connected to a right end of a manifold  60 . Similarly, an end  821   a  of a communication passage  821  of a dummy channel  801  is connected to the right end of the manifold  60 . 
     The dummy channel  801  is distinctive in the connecting position of an end  861   a  of a circulation passage  861  and the manifold  60 . One or more connecting positions of ejection channels  701  (ends  721   a  of communication passages  721 ) to the manifold  60  are positioned between the connecting position of the end  821   a  of the communication passage  821  of the dummy channel  801  to the manifold  60 , and the connecting position of the end  861   a  of the circulation passage  861  of the dummy channel  801  to the manifold  60 . As shown in  FIG. 6 , two ends  721   a  of two communication passages  721  of two ejection channels  701  are positioned between the end  821   a  of the communication passage  821  and the end  861   a  of the circulation passage  861  of the dummy channel  801 . In this case, the presence of a descender is not essential. 
     In the dummy channel  801 , the connecting position of the circulation passage  861  to the manifold  60  is away, in a longitudinal direction of the manifold  60 , from the connecting position of the communication passage  821  to the manifold  60 . 
     Ink returned from the dummy channel  801  to the manifold  60  is readily diluted with ink in the manifold  60 , and is prevented from flowing immediately back to the dummy channel  801 . Further, when the ejection channels  701  positioned between the two connecting positions are driven, the returned ink is partially used for image forming. Thus, ink in the dummy channel  801  is reliably prevented from thickening. 
     Third Embodiment 
     A liquid ejection head  132  in a third embodiment shown in  FIG. 7  has substantially the same structure as the liquid ejection head  13  in the first embodiment shown in  FIG. 3 , except for a few points described below. 
     As shown in  FIG. 7 , an end of a communication passage  722  of each ejection channel  702  is connected to a right end of a manifold  60 . Similarly, an end  822   a  of a communication passage  822  of a dummy channel  802  is connected to the right end of the manifold  60 . 
     A circulation passage  862  of the dummy channel  802  is connected to a left end of the manifold  60 . In this embodiment, the circulation passage  862  extends along the front of the manifold  60  and around a corner to reach the left of the manifold  60 . Alternatively, the circulation passage  862  may extend below the manifold  60  and around a corner to reach the left of the manifold  60 . In this case, the presence of a descender is not essential. 
     In the dummy channel  802 , the connecting position of the circulation passage  862  to the manifold  60  is away, in the right-left direction orthogonal to a longitudinal direction of the manifold  60 , from the connecting position of the communication passage  822  to the manifold  60 . 
     Ink returned from the dummy channel  802  to the manifold  60  is readily diluted with ink in the manifold  60 , and is prevented from flowing immediately back to the dummy channel  802 . Thus, ink in the dummy channel  802  is prevented from thickening. 
     Fourth Embodiment 
       FIGS. 8A and 8B  are each an enlarged cross-sectional view of a part of a liquid ejection head in a fourth embodiment. A laminate body including a plurality of plates, which define a path of an ejection channel  703  and a path of a dummy channel  803 , is omitted from  FIGS. 8A and 8B . In the fourth embodiment, none of the ejection channel  703  and the dummy channel  803  has a descender. 
     As shown in  FIG. 8A , the ejection channel  703  defines a path extending from an outlet of a manifold  60 , via a communication passage  723  and a pressure chamber  733 , to an ejection nozzle  753 . A vibration plate of an actuator  713  in  FIG. 8A  partially defines a wall of the pressure chamber  733 . Upon application of a drive voltage to the actuator  713 , the vibration plate deforms to change the capacity of the pressure chamber  733 . The communication passage  723  connects the manifold  60  to the pressure chamber  733 . An ejection nozzle  753  is directly connected to the pressure chamber  733 . 
     As shown in  FIG. 8B , the dummy channel  803  defines a path extending from an outlet of a manifold  60 , via a communication passage  823  and a pressure chamber  833 , to a dummy nozzle  853 . Channel elements of the dummy channel  803 , including an actuator  813  and the communication passage  823 , are disposed similarly to the above-described ejection channel  703 . A vibration plate of the actuator  813  partially defines a wall of the pressure chamber  833 , similarly to the actuator  713 . 
     The dummy channel  803  is characterized by a circulation passage  863 . The pressure chamber  833  is in fluid communication with the manifold  60  through the circulation passage  863 . The circulation passage  863  is located between the dummy nozzle  853  and the manifold  60 . The circulation passage  863  is closer to the dummy nozzle  863  than to the actuator  813 . Because of the lack of a descender, the circulation passage  863  connects a bottom portion of the pressure chamber  833  to the manifold  60 . The communication passage  823  connects a top portion of the pressure chamber  833  to the manifold  60 . 
     The liquid ejection head thus structured makes the ejection characteristics uniform among the ejection channels  703 . When the actuator  813  is driven to such extent as not to cause ink ejection, ink in the dummy channel  803  flows back into the manifold  60  though the circulation passage  863 . This prevents thickened ink in the dummy channel  803  from affecting the ejection characteristics of the ejection channel  703  next to the dummy channel  803 . 
     An ink circulation direction in the dummy channel  803  may be set as in the dummy channel  80 , by setting a driving signal and inertances Mo and Mi as described with reference to  FIG. 5 . 
     In the liquid ejection head in the fourth embodiment, the circulation passage  863  and the manifold  60  may be connected as described in the second embodiment shown in  FIG. 6  or in the third embodiment shown in  FIG. 7 . In such cases, as advantageously as described above, a distance between the connecting position of the communication passage  823  to the manifold  60 , and the connecting position of the circulation passage  863  to the manifold  60  is increased, thereby achieving dilution of thickened ink and supply of fresh ink in the dummy channel  803 , and accordingly unifying the ejection characteristics among the ejection channels  703 . 
     While the disclosure has been described in detail with reference to the specific embodiments, various changes, arrangements and modifications may be applied therein without departing from the spirit and scope of the disclosure.