Patent Publication Number: US-11046077-B2

Title: Liquid ejection head

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2019-103662 filed on Jun. 3, 2019, the content of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Aspects of the disclosure relate to a liquid ejection head. 
     BACKGROUND 
     A known liquid ejection head includes a stack structure including a plurality of stacked plates. The stack structure includes ejection holes for liquid ejection, pressure chambers respectively connected to the ejection holes, narrow individual channels respectively connected to the pressure chambers, and dummy pressure chambers. 
     SUMMARY 
     Such a stack structure is formed, as an example, by stacking and compressing a plurality of plates with an adhesive agent. In this case, an adhesive agent overflowing from a bonding zone of the plates may enter and fill a dummy pressure chamber. The adhesive agent may further enter some of the narrow individual channels connected to the respective dummy pressure chambers, causing clogging of the individual channels. 
     Aspects of the disclosure provide a liquid ejection head configured to reduce clogging of individual channels. 
     According to one or more aspects of the disclosure, a liquid ejection head includes a stack structure including a plurality of plates stacked and bonded at facing surfaces of adjacent plates with an adhesive agent, a plurality of individual channels formed in the stack structure, a plurality of dummy channels formed in the stack structure separately from the plurality of individual channels, and a first relief groove formed in the stack structure separately from the plurality of individual channels and configured to trap therein an excessive adhesive agent between the adjacent plates. Each of the individual channels includes a pressure chamber, a supply throttle channel, and a return throttle channel. An ejection pressure is applied to the pressure chamber for liquid ejection from a nozzle. The supply throttle channel is connected to the pressure chamber and to a supply manifold having a supply opening through which liquid is supplied. The supply throttle channel has a smaller cross-sectional area than the pressure chamber. The return throttle channel communicates with the pressure chamber and is connected to a return manifold having a return opening through which liquid is discharged. The return throttle channel has a smaller cross-sectional area than the pressure chamber. The dummy channels include dummy chambers arranged laterally to an array of the pressure chambers arranged in an array direction. The first relief groove is connected to the dummy channels. 
    
    
     
       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  is a schematic diagram of a liquid ejection apparatus including a liquid ejection head according to a first illustrative embodiment. 
         FIG. 2  is a cross-sectional view of the liquid ejection head of  FIG. 1  taken along a line orthogonal to an array direction. 
         FIG. 3  is a partial view of a lower surface of a first channel plate of the liquid ejection head. 
         FIG. 4  is a cross-sectional view of a liquid ejection head taken along a line orthogonal to an array direction, according to a second illustrative embodiment. 
         FIG. 5  is a cross-sectional view of a liquid ejection head taken along a line orthogonal to an array direction, according to a third illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Illustrative embodiments of the disclosure will be described with reference to the drawings. 
     First Illustrative Embodiment 
     A liquid ejection apparatus  10  including a liquid ejection head  20  (hereinafter referred to as a “head”) according to a first illustrative embodiment is configured to eject liquid. Hereinafter, the liquid ejection apparatus  10  will be described by way of example as applied to, but not limited to, an inkjet printer. 
     &lt;Structure of Liquid Ejection Apparatus&gt; 
     As shown in  FIG. 1 , the liquid ejection apparatus  10  employs a line head type and includes a platen  11 , a transport unit, a head unit  16 , tanks  12 , and a controller  13 . The liquid ejection apparatus  10  may employ a serial head type or other types than the line head type. 
     The platen  11  is a flat plate member to receive thereon a sheet  14  and adjust a distance between the sheet  14  and the head unit  16 . Herein, one side of the platen  11  toward the head unit  16  is referred to as an upper side, and the other side of the platen  11  away from the head unit  16  is referred to as a lower side. However, the liquid ejection apparatus  10  may be positioned in other orientations. 
     The transport unit may include two transport rollers  15  and a transport motor (not shown). The two transport rollers  15  are disposed parallel to each other while interposing the platen  11  therebetween in a transport direction, and are connected to the transport motor. When the transport motor is driven, the transport rollers  15  rotate to transport the sheet  14  on the platen  11  in the transport direction. 
     The head unit  16  has a length greater than or equal to the length of the sheet  14  in a direction (an orthogonal direction) orthogonal to the transport direction of the sheet  14 . The head unit  16  includes a plurality of heads  20 . 
     Each head  20  includes a channel unit and a volume changer. The channel unit includes liquid channels formed therein and a plurality of nozzle holes  21   a  open on a lower surface (an ejection surface  40   a ). The volume changer is driven to change the volume of a liquid channel. In this case, a meniscus in a nozzle hole  21   a  vibrates and liquid is ejected from the nozzle hole  21   a . The head  20  will be described in detail later. 
     Separate tanks  12  are provided for different kinds of inks. For example, each of four tanks  12  stores therein a corresponding one of black, yellow, cyan, and magenta inks. Inks of the tanks  12  are supplied through corresponding liquid channels to corresponding nozzle holes  21   a.    
     The controller  13  includes a processor such as a central processing unit (CPU), memories such as a random access memory (RAM) and a read only memory (ROM), and driver integrated circuits (ICs) such as an application specific integrated circuit (ASIC). In the controller  13 , upon receipt of various requests and detection signals from sensors, the CPU causes the RAM to store various data and outputs various execution commands to the ASIC based on programs stored in the ROM. The ASIC controls the driver ICs based on the commands to execute required operation. The transport motor and the volume changer are thereby driven. 
     Specifically, the controller  13  executes ejection from the head unit  16 , and transport of sheets  14 . The head unit  16  is controlled to eject ink from the nozzle holes  21   a . A sheet  14  is transported in the transport direction intermittently by a predetermined amount. Printing progresses by execution of ink ejection and sheet transport. 
     &lt;Structure of Head&gt; 
     As described above, each head  20  includes the channel unit and the volume changer. As shown in  FIGS. 2 and 3 , the channel unit includes a stack structure  25  of a plurality of plates, and the volume changer includes a vibration plate  55  and piezoelectric elements  60 . 
     The plurality of plates include a nozzle plate  40 , a first channel plate  41 , a second channel plate  42 , a third channel plate  43 , a fourth channel plate  44 , a fifth channel plate  45 , a sixth channel plate  46 , a seventh channel plate  47 , an eighth channel plate  48 , a ninth channel plate  49 , and a 10th channel plate  50 . These plates are stacked in this order in a stacking direction. The plurality of plates may include the vibration plate  55 . 
     Each plate has holes and grooves of various sizes. A combination of holes and grooves in the stacked plates of the channel unit define liquid channels such as a plurality of nozzles  21 , a plurality of individual channels  30 , a plurality of dummy channels  70 , a first relief groove  80 , a second relief groove  90 , a supply manifold  22 , and a return manifold  23 . The dummy channels  70 , the first relief groove  80 , and the second relief groove  90  are provided separately from the individual channels  30 . These elements will be described in detail later. 
     The nozzles  21  are formed to penetrate the nozzle plate  40  in the stacking direction. Each nozzle  21  extends in the stacking direction and has a distal-end opening (a nozzle hole  21   a ) and a base-end opening opposite to the distal-end opening. For example, each nozzle  21  has a shape of a cone without a tip, and the area of the base-end opening is greater than that of the nozzle hole  21   a . The nozzle holes  21   a  are arranged, as a nozzle array, in an array direction on the ejection surface  40   a  of the nozzle plate  40 . 
     The array direction is orthogonal to the stacking direction and may be parallel or inclined relative to the orthogonal direction shown in  FIG. 1 . A lateral direction is a direction orthogonal to the stacking direction and crossing (e.g., orthogonal to) the array direction, and may be parallel or inclined relative to the transport direction. 
     The supply manifold  22  and the return manifold  23  extend long in the array direction and are connected to the individual channels  30 . The supply manifold  22  has a supply opening  22   a  at an end in its longitudinal direction, and the return manifold  23  has a return opening  23   a  at an end in its longitudinal direction. The supply manifold  22  is stacked on the return manifold  23  to overlap the return manifold  23  in the stacking direction. 
     The cross-sectional area defined by the supply manifold  22  to face the array direction is equal to the cross-sectional area defined by the return manifold  23  to face the array direction. For example, the supply manifold  22  and the return manifold  23  may be the same in size and shape in the lateral direction and in the stacking direction. The return manifold  23  may be longer than the supply manifold  22  in the array direction. 
     The supply manifold  22  is formed by through-holes penetrating in the stacking direction the sixth channel plate  46  and the seventh channel plate  47 , and a recess recessed from a lower surface of the eighth channel plate  48 . The recess overlaps the through-holes in the stacking direction. A lower end of the supply manifold  22  is covered by the fifth channel plate  45 , and an upper end of the supply manifold  22  is covered by an upper portion of the eighth channel plate  48 . 
     The return manifold  23  is formed by through-holes penetrating in the stacking direction the second channel plate  42  and the third channel plate  43 , and a recess recessed from a lower surface of the fourth channel plate  44 . The recess overlaps the through-holes in the stacking direction. A lower end of the return manifold  23  is covered by the first channel plate  41 , and an upper end of the return manifold  23  is covered by an upper portion of the fourth channel plate  44 . 
     The supply manifold  22  and the return manifold  23  define a buffer space  24  therebetween. The buffer space  24  is formed by a recess recessed from a lower surface of the fifth channel plate  45 . In the stacking direction, the supply manifold  22  and the buffer space  24  are adjacent to each other via an upper portion of the fifth channel plate  45 , and the return manifold  23  and the buffer space  24  are adjacent to each other via an upper portion of the fourth channel plate  44 . The buffer space  23  sandwiched between the supply manifold  22  and the return manifold  23  may reduce interaction between the liquid flow pressure in the supply manifold  22  and the liquid flow pressure in the return manifold  23 . 
     The plurality of individual channels  30  are branched from the supply manifold  22  and merge into the return manifold  23 . Each individual channel  30  is connected, at its upstream end, to the supply manifold  22 , connected, at its downstream end, to the return manifold  23 , and connected, at its midstream, to a base end of a corresponding nozzle  21 . Each individual channel  30  includes a first hole  31 , a supply throttle channel  32 , a second hole  33 , a pressure chamber  34 , a descender  35 , a return throttle channel  36 , and a third hole  37 , which are arranged in this order. 
     The first hole  31  is connected, at its lower end, to an upper end of the supply manifold  22 , and extends upward from the supply manifold  22  in the stacking direction to penetrate an upper portion of the eighth channel plate  48  in the stacking direction. The first hole  31  is offset to one side (a first side) from a center of the supply manifold  22  in the lateral direction. The cross-sectional area defined by the first hole  31  to be orthogonal to the stacking direction is less than the cross-sectional area defined by the supply manifold  22  to be orthogonal to the array direction. 
     The supply throttle channel  32  is connected, at its first-side end, to an upper end of the first hole  31  and extends toward a second side in the lateral direction. The supply throttle channel  32  is formed by a groove recessed from a lower surface of the ninth channel plate  49 . The cross-sectional area defined by the supply throttle channel  32  to be orthogonal to the lateral direction is less than the cross-sectional area defined by the first hole  31  to be orthogonal to the stacking direction. 
     The second hole  33  is connected, at its lower end, to a second-side end of the supply throttle channel  32  and extends from the supply throttle channel  32  upward in the stacking direction to penetrate an upper portion of the ninth channel plate  49  in the stacking direction. The second hole  33  is offset to the other side (a second side) from the center of the supply manifold  22  in the lateral direction. The cross-sectional area defined by the second hole  33  to be orthogonal to the stacking direction is greater than the cross-sectional area defined by the supply throttle channel  32  to be orthogonal to the lateral direction. 
     The pressure chamber  34  is connected, at its first-side end, to an upper end of the second hole  33  and extends toward a second side in the lateral direction. The pressure chamber  34  penetrates the 10th channel plate  50  in the stacking direction. The cross-sectional area defined by the pressure chamber  34  to be orthogonal to the lateral direction is greater than or equal to the cross-sectional area defined by the second hole  33  to be orthogonal to the stacking direction. 
     The descender  35  has a columnar shape such as a cylindrical shape and is located to a second side in the lateral direction of the supply manifold  22  and the return manifold  23 . The descender  35  is formed by through-holes penetrating in the stacking direction the first channel plate  41  through the ninth channel plate  49 . The descender  35  is connected, at its upper end, to the second-side end of the pressure chamber  34  and extends from that connected portion downward in the sacking direction. The base-end opening of the nozzle  21  is connected to a center of a lower end of the descender  35 . 
     The return throttle channel  36  is connected, at its second-side end, to the lower end of the descender  35  and extends from the descender  35  toward a first side in the lateral direction. The return throttle channel  36  is formed by a groove recessed from a lower surface of the first channel plate  41 . The cross-sectional area defined by the return throttle channel  36  to be orthogonal to the lateral direction is less than the cross-sectional area defined by the descender  35  to be orthogonal to the stacking direction. 
     The third hole  37  is connected, at its lower end, to a first-side end of the return throttle channel  36  and extends from the return throttle channel  36  upward in the stacking direction to penetrate an upper portion of the first channel plate  41 . The third hole  37  is connected, at its upper end, to a lower end of the return manifold  23 . The third hole  37  is offset to a second side from a center of the return manifold  23  in the lateral direction. The cross-sectional area defined by the third hole  37  to be orthogonal to the stacking direction is greater than the cross-sectional area defined by the return throttle channel  36  to be orthogonal to the array direction. 
     The vibration plate  55  is stacked on the 10th channel plate  50  to cover upper openings of the pressure chambers  34 . The vibration plate  55  may be integral with the 10th channel plate  50 . In this case, each pressure chamber  34  is recessed from a lower surface of the 10th channel plate  50 . An upper portion of the 10th channel plate  50 , which is above each pressure chamber  34 , functions as the vibration plate  55 . 
     Each piezoelectric element  60  includes a common electrode  61 , a piezoelectric layer  62 , and an individual electrode  63 , which are arranged in this order. The common electrode  61  entirely covers the vibration plate  55  via the insulating film  56 . Each piezoelectric layer  62  is provided for a corresponding pressure chamber  34  and is located on the common electrode  61 . Each individual electrode  63  is located on a corresponding piezoelectric layer  62  to overlap a corresponding pressure chamber  34 . In this case, a piezoelectric element  60  is formed by an active portion of a piezoelectric layer  62 , which is sandwiched by an individual electrode  63  and the common electrode  61 . 
     Each individual electrode  63  is electrically connected to a driver IC. The driver IC receives control signals from the controller  13  ( FIG. 1 ) and generates drive signals (voltage signals) selectively to the individual electrodes  63 . In contrast, the common electrode  61  is constantly maintained at a ground potential. 
     In response to a drive signal, an active portion of each selected piezoelectric layer  62  expands and contracts in a surface direction, together with the two electrodes  61  and  63 . Accordingly, the vibration plate  55  corporates to deform to increase and decrease the volume of a corresponding pressure chamber  34 . This applies a pressure to the corresponding pressure chamber  34  which in turn ejects liquid from a nozzle  21 . 
     &lt;Liquid Flow&gt; 
     For example, the supply opening  22   a  of the supply manifold  22  is connected, via a supply conduit, to a subtank, and the return opening  23   a  of the return manifold  23  is connected, via a return conduit, to the subtank. When a pressure pump in the supply conduit and a negative-pressure pump in the return conduit are driven, liquid from the subtank passes through the supply conduit to flow into the supply manifold  22  where liquid flows in the array direction. 
     Meanwhile, liquid partially flows into the individual channels  30 . In each individual channel  30 , liquid flows from the supply manifold  22 , via the first hole  31 , into the supply throttle channel  32  where liquid flows in the lateral direction. Liquid further flows from the supply throttle channel  32 , via the second hole  33 , into the pressure chamber  34  where liquid flows in the lateral direction. Then, liquid flows from an upper end to a lower end of the descender  35  in the stacking direction to enter the nozzle  21 . When the piezoelectric element  60  applies an ejection pressure to the pressure chamber  34 , liquid is ejected from a nozzle hole  21   a.    
     The remaining liquid flows from the descender  35  to the return throttle channel  36  and enters, via the third hole  37 , the return manifold  23 . Then, liquid passes the return manifold  23  in the array direction and returns through the return conduit to the subtank. Thus, liquid not having flown into the individual channels  30  circulates between the subtank and the individual channels  30 . 
     &lt;Structures of Dummy Channels, First Relief Groove, and Second Relief Groove&gt; 
     The dummy channels  70  are arranged, as an array, in the array direction, and the individual channels  30  are arranged, as an array, in the array direction. An array of dummy channels  70  is provided at each of opposite ends of the stack structure  25  in the lateral direction. A plurality of arrays of individual channels  30  are sandwiched between two arrays of dummy channels  70 . The two arrays of dummy channels  70  are symmetrical to each other relative to a cross section thereof orthogonal to the lateral direction. Hereinafter, among the two arrays of dummy channels  70 , an array of dummy channels  70  located at a second side in the lateral direction will be described. 
     Also, the two arrays of individual channels  30  are symmetrical to each other relative to a cross section thereof orthogonal to the lateral direction, and are connected to the same supply manifold  22  and return manifold  23 . The stack structure  25  includes an edge portion  26  located opposite to the arrays of individual channels  30  relative to the array of dummy channels  70  in the lateral direction. The edge portion  26  is located between the array of dummy channels  70  and an end of the stack structure  25 . 
     Each dummy channel  70  is filled with no liquid and includes a dummy chamber  71 , a dummy descender  72 , a dummy return channel  73 , and a first dummy hole  74 , which are arranged in this order. The dummy chamber  71 , the dummy descender  72 , the dummy return channel  73 , and the first dummy hole  74  may be the same in shape and size as the pressure chamber  34 , the descender  35 , the return throttle channel  36 , and the third hole  37 , respectively. 
     The dummy chamber  71  penetrates the 10th channel plate  50  in the stacking direction. A plurality of dummy chambers  71  are arranged, as an array, in the array direction. The array of dummy chambers  71  is located laterally to an array of pressure chambers  34 . The dummy chambers  71  are not connected to the supply manifold  22 . 
     The dummy descender  72  penetrates in the stacking direction the first channel plate  41  through the ninth channel plate  49 . The dummy descender  72  is connected, at its upper end in the stacking direction, to a first-side end of the dummy chamber  71 . The dummy descender  72  is not connected to a nozzle  21  nor open on the ejection surface  40   a.    
     The dummy return channel  73  is connected, at its first-side end, to a lower end of the dummy descender  72  and extends from that connected portion toward a second side in the lateral direction. The dummy return channel  73  is formed by a groove recessed from a lower surface of the first channel plate  41 . The cross-sectional area defined by the dummy return channel  73  to be orthogonal to the lateral direction is less than the cross-sectional area defined by the dummy chamber  71  to be orthogonal to the lateral direction. 
     The first dummy hole  74  is connected, at its lower end, to a second-side end of the dummy return channel  73  and extends from the dummy return channel  73  upward in the stacking direction to penetrate an upper portion of the first channel plate  41 . The cross-sectional area defined by the first dummy hole  74  to be orthogonal to the stacking direction is greater than the cross-sectional area defined by the dummy return channel  73  to be orthogonal to the lateral direction. The first dummy hole  74  is not connected to the return manifold  23 . 
     The first relief groove  80  includes a return-side relief groove  81  connected to the dummy return channels  73 . The return-side relief groove  81  is located at the edge portion  26  between the end  41   a  of the first channel plate  41  and the dummy return channels  73 , and is formed by a groove recessed from a lower surface toward an upper surface of the first channel plate  41 . 
     In other words, the first channel plate  41  is a grooved plate formed with the return-side relief groove  81  and the dummy return channels  73 . The return-side relief groove  81  and the dummy return channels  73  are open on a lower surface of the first channel plate  41  and do not penetrate through the upper and lower surfaces of the first channel plate  41 . 
     The return-side relief groove  81  includes first groove portions  81   a , second groove portions  81   b , and a third groove portion  81   c . The cross-sectional area defined by each of these groove portions to be orthogonal to its extending direction is equal to or less than the cross-sectional area defined by a corresponding dummy return channel  73  to be orthogonal to the lateral direction. 
     Each first groove portion  81   a  is connected, at its first side end, to a second-side end of a corresponding dummy return channel  73  and extends from that connected portion toward a second side. Thus, the first groove portion  81   a  and the corresponding dummy return channel  73  are located on the same straight line extending in the lateral direction. 
     Each second groove portion  81   b  extends in the array direction at a position further to the second side than the first groove portions  81   a  and is connected to corresponding at least two of the first groove portions  81   a . Each second groove portion  81   b , which extends in the array direction, is branched toward the first side in the lateral direction to extend between corresponding two dummy return channels  73  adjacent in the array direction. The branched portion is equal or substantially equal in length to the corresponding dummy return channels  73 . 
     Each second groove portion  81   b  extends in the array direction and is curved in the lateral direction to surround the second-side end of the nearest dummy return channel  73  while being spaced by a uniform distance from that second-side end. Each second groove portion  81   b  is curved such that its connected position to a corresponding first groove portion  81   a  is located further to the second side than its branched position. 
     The third groove portion  81   c  extends in the array direction and is branched at plural positions toward the first side in the lateral direction. The third groove portion  81   c  is also branched to extend toward the second side in the lateral direction and is branched in the array direction to form a meshed pattern. A second-side end of the third groove portion  81   c  is connected to a communication passage  82 . 
     The communication passage  82  is formed by through-holes penetrating in the stacking direction the first channel plate  41  through the 10th channel plate  50 . The communication passage  82  is connected, at its lower end, to the return-side relief groove  81  and has an upper-end opening open to an exterior of the stack structure  25 . For example, a lid  83  is attachable to the upper-end opening to shut the communication passage  82  from the exterior. 
     The second relief groove  90  is formed by a groove recessed from a lower surface of the first channel plate  41  and is located at a zone where the individual channels  30  are formed. The second relief groove  90  extends in the array direction at a position between the two arrays of return throttle channels  36  adjacent in the lateral direction. The second relief groove  90  is branched in the lateral direction to extend between every two return throttle channels  36  adjacent in the array direction. The second relief groove  90  is not connected to and thus is separate from the dummy channels  70  and the communication passage  82 . 
     &lt;Assembly of Stack Structure&gt; 
     The nozzle plate  40  and the first channel plate  41  through the 10th channel plate  50  are prepared by forming grooves and through-holes in each plate. An adhesive agent is applied to an upper surface of the nozzle plate  40 , to upper and lower surfaces of the first channel plate  41  through the ninth channel plate  49 , and to a lower surface of the 10th channel plate  50 . These plates are stacked one on another and compressed. The adhesive agent may be applied to either one of upper and lower facing surfaces of these plates. 
     The facing surfaces of the nozzle plate  40  and the first channel plate  41  through the 10th channel plate are bonded to each other by the adhesive agent to form the stack structure  25 . The stack structure  25  may be formed by applying the adhesive agent to one of a lower surface of the vibration plate  55  and an upper surface of the 10th channel plate  50 , by staking the vibration plate  55  on the 10th channel plate  50 , and by compressing the vibration plate  55  together with the other plates. 
     In order to securely bond the plates, an excessive amount of the adhesive agent is applied to the facing surfaces. Thus, an excessive adhesive agent flows from a bonding zone between the upper surface of the nozzle plate  40  and the lower surface of the first channel plate  41 . The lower surface of first channel plate  41  includes the return throttle channels  36  with a small cross-sectional area. If a large amount of excessive adhesive agent flows into the return throttle channels  36 , the return throttle channels  36  may be clogged. 
     However, to cope with this, the lower surface of the first channel plate  41  includes the return-side relief groove  81 , the second relief groove  90 , and the dummy return channels  73 . An excessive adhesive agent flows into these grooves and channels to be trapped there. An excessive adhesive agent flowing into the return-side relief groove  81  and the dummy return channels  73  passes the return-side relief groove  81  and exits, via the communication passage  82 , to the exterior of the stack structure  25 . This may reduce filling of the return relief groove  81  and the dummy return channels  73  with an excessive adhesive agent, and reduce clogging of the narrow return throttle channels  36  with the excessive adhesive agent flowing there, instead of flowing into the groove  81  and the channels  73 . 
     In this case, the return-side relief groove  81 , which is branched and formed into a meshed pattern, provides a plurality of paths through which an excessive adhesive agent flows from the dummy return channels  73 , via the return-side relief groove  81 , to the communication passage  82 . Even when the return-side relief groove  81  is partially clogged with an excessive adhesive agent, an excessive adhesive agent flowing into the dummy return channels  72  is discharged, via unclogged paths of the return-side relief groove, to the communication passage  82 . This may reliably reduce filling of the return throttle channels  36  with an excessing adhesive agent. 
     Once the facing surfaces are bonded in the stack structure  25 , the upper-end opening of the communication passage  82  is covered with the lid  83 . Thus, the communication passage  82  and the dummy channels  70  are shut from the exterior. 
     &lt;Effects&gt; 
     In the head  20 , the first relief groove  80  is connected to the dummy channels  70 . This allows an excessive adhesive agent overflowing from the bonding zone between the facing surfaces to flow into the dummy channels  70  and to the first relief groove  80 . This may reduce the amount of excessive adhesive agent flowing into the individual channels  30  and reduce clogging of the narrow return channels  36  of the individual channels  30  with the excessive adhesive agent. 
     In the stack structure  25  of the head  20 , the dummy channels  70  include, at a layer provided with the return throttle channels  36 , the dummy return channels  73  which respectively communicate with the dummy chambers  71  and have a smaller cross-sectional area than the dummy chambers  71 . The first relief groove  80  is connected to the dummy return channels  73 . 
     For example, the first channel plate  41  includes, at its lower surface, the return throttle channels  36  and the dummy return channels  73  to which the return-side relief groove  81  is connected. Thus, any excessive adhesive agent flowing into the dummy return channels  73  flows from the dummy return channels  73  to the return-side relief groove  81 . This may reduce clogging of the narrow dummy return channels  73  with the excessive adhesive agent. Without such a clog in the dummy return channels  73 , the excessive adhesive agent is prevented from flowing into the return throttle channels  36 , instead of flowing into the dummy channels  70 . This may reduce clogging of the return throttle channels  36  with the excessive adhesive agent. 
     In the head  20 , the stack structure  25  includes the grooved plate formed with the first relief groove  80  and the dummy return channels  73 . The first relief groove  80  and the dummy return channels  73  are open on one and same surface of the two facing surfaces. 
     Thus, the grooved plate (the first channel plate  41 ) may be machined, from its lower surface, to form therein the return-side relief groove  81  of the first relief groove  80  and the dummy return channels  73 . The return-side relief groove  81  and the dummy return channels  73  are formed in the same surface. This may facilitate forming the return-side relief groove  81  and the dummy return channels  73  while adjusting the positional relation therebetween. 
     In the head  20 , the dummy return channels  73  are arranged in the array direction. The first relief groove  80  includes the first groove portions  81   a  each connected to one end of a corresponding dummy return channel  73 , and the second groove portions  81   b  each connected to corresponding at least two first groove portions  81   a . Each second groove portion  81   b  extends in the array direction in a curved manner to surround the nearest one of the ends of the dummy return channels  73 . 
     This allows each second groove portion  81   b  to uniformly trap therein an excessive adhesive agent around the one end of a corresponding dummy return channel  73 . This may reduce the amount of excessive adhesive agent flowing into the dummy return channels  73 , reduce clogging of the dummy return channels  73  with the excessive adhesive agent, and thus clogging of the return throttle channels  36  with the excessive adhesive agent. 
     The head  20  includes an array of dummy channels  73  arranged in the array direction, and an array of return throttle channels  36  arranged in the array direction. The array of return throttle channels  36  is located laterally to the array of dummy channels  73 , in a direction orthogonal to the array direction. The head  20  further includes the edge portion  26  located opposite to the array of return throttle channels  36  relative to the array of the dummy return channels  73  in the direction orthogonal to the array direction. 
     Specifically, the first channel plate  41  includes, at its lower surface, the edge portion  26 , the array of dummy return channels  73 , the array of the return throttle channels  36 , in this order from the end  41   a . Because the edge portion  26  is located near the end  41   a , a relatively greater amount of adhesive agent is applied to the edge portion  26  than to a zone where the dummy return channels  73  and the return throttle channels  36  are formed. This may reliably prevent leakage of liquid from the individual channels  30 , through the end  41   a  of the first channel plate  41 , to the exterior. 
     Even when a relatively greater amount of adhesive agent is applied to the edge portion  26 , an excessive adhesive agent flows from the edge portion  26  into the dummy return channels  73  before flowing into the return throttle channels  36 . This may reduce entry of the excessive adhesive agent into the return throttle channels  36  and reduce clogging of the channels  36  with the excessive adhesive agent. 
     The head  20  includes the communication passage  82  through which the first relief groove  80  communicates with the exterior of the stack structure  25 . Any excessive adhesive agent entering the dummy channels  70  and the return-side relief groove  81  flows to the exterior via the communication passage  82 . This may reduce filling of the dummy channels  70  and the return-side relief groove  81  with an excessive adhesive agent, and reduce the amount of excessive adhesive agent flowing into return throttle channels  36 . 
     The head  20  includes the lid  83  for shutting the communication passage  82  from the exterior. Any bonding failure between plates of the stack structure  25  may cause liquid to leak from the individual channels  30  to the dummy channels  70 . Even in this case, the lid  83 , which shuts the communication passage  82  from the exterior, may prevent discharge of the liquid from the dummy channels  70  via the communication passage  82 . 
     The head  20  includes the second relief groove  90  for trapping therein an excessive adhesive agent between plates. The second relief groove  90  is not connected to the dummy channels  70 . The second relief groove  90  and the communication passage  82  are separate from each other. 
     Because the communication passage  82  is not connected to the second relief groove  90 , no excessive adhesive agent flows from the second relief groove  90  into the communication passage  82 . Thus, the communication passage  82  is used exclusively as a path for an excessive adhesive agent from the return-side relief groove  81 . The excessive adhesive agent in the dummy channels  70  is reliably discharged from the communication passage  82  via the return-side relief groove  81 . This may prevent leakage of the excessive adhesive agent from the dummy channels  70  to neighboring individual channels  30 . 
     In the head  20 , the dummy chambers  71  are filled with no liquid. Thus, discharge of liquid is prevented from the dummy chambers  71 , via the return-side relief groove  81  and the communication passage  82 , to the exterior. 
     In the head  20 , the stack structure  25  includes the grooved plate including the first relief groove  80 . The first relief groove  80  is recessed from either one of the two facing surfaces of the grooved plate and does not penetrate through the two facing surfaces. For example, the grooved plate (the first channel plate  41 ) is continuous, at its an upper portion of the return-side relief groove  81  of the first relief groove  80 , in a direction orthogonal to a direction in which the grooves  80  and  81  are recessed. This may reduce a decrease in strength of the first channel plate  41  due to the return-side relief groove  81 . 
     In the head  20 , the stack structure  25  includes the ejection surface  40   a  where the nozzles  21  are open. The dummy channels  70  are not open on the ejection surface  40   a . If the dummy channels  70  are open on the ejection surface  40   a , wiping off the liquid on the ejection surface  40   a  may cause the liquid to enter the dummy channels  70  via the openings in the ejection surface  40   a . In this case, a sheet placed facing the ejection surface  40   a  may be smeared with the liquid having entered and remaining in the dummy channels  70 . However, the dummy channels  70  are not open on the ejection surface  40   a , not causing such a problem. 
     Second Illustrative Embodiment 
     As shown in  FIG. 4 , a head  20  according to a second illustrative embodiment defers from the head  20  according to the first illustrative embodiment in that each dummy channel  70  includes a dummy supply channel  75  and that a first relief groove  80  includes a supply-side relief groove  84 . The elements other than the above-described elements are similar to those of the first illustrative embodiment and will not be described repeatedly. 
     Specifically, the dummy supply channel  75  communicates with a corresponding dummy chamber  71  via a second dummy hole  76 . The second dummy hole  76  is located in a ninth channel plate  49  including the second holes  33 , and penetrates in the stacking direction an upper portion of the dummy supply channel  75  in the ninth channel plate  49 . The second dummy hole  76  is connected, at its upper end, to a second-side end of a corresponding dummy chamber  71  and extends downward from the dummy chamber  71  in the stacking direction. The cross-sectional area defined by the second dummy hole  76  to be orthogonal to the stacking direction is less than that defined by the dummy chamber  71  to be orthogonal to the lateral direction, and is equal to that defined by the second hole  33  to be orthogonal to the stacking direction. 
     The dummy supply channel  75  is connected, at its first-side end, to a lower end of the second dummy hole  76 , and extends toward a second side in the lateral direction. The dummy supply channel  75  is formed by a groove recessed from a lower surface of the ninth channel plate  49  including the supply throttle channels  32 . The cross-sectional area defined by the dummy supply channel  75  to be orthogonal to the lateral direction is less than that defined by the second dummy hole  76  to be orthogonal to the stacking direction, and is equal to that defined by the supply throttle channel  32  to be orthogonal to the lateral direction. 
     The supply-side relief groove  84 , as the first relief groove  80 , traps therein an excessive adhesive agent between an upper surface of an eighth channel plate  48  and a lower surface of the ninth channel plate  49 . The supply-side relief groove  84  is located at an edge portion  26  between an end of the ninth channel plate  49  and the dummy supply channels  75 , and is formed by a groove recessed from a lower surface toward an upper surface of the ninth channel plate  49 . 
     The supply-side relief groove  84  and the dummy supply channels  75  are open on a lower surface of the ninth channel plate  49  and do not penetrate through the upper and lower surfaces of the ninth channel plate  49 . The supply-side relief groove  84  may be formed in the upper surface of the eighth channel plate  48  facing the lower surface of the ninth channel plate  49 . 
     The supply-side relief groove  84  is connected, at its first-side ends, to corresponding second-side ends of the dummy supply channels  75  and extends from that connected portions toward a second side. Similarly to a return-side relief groove  81 , the supply-side relief groove  84  may be curved in a direction orthogonal to the stacking direction, branched, and formed into a meshed pattern. The cross-sectional area defined by the supply-side relief groove  84  to be orthogonal to its extending direction is less than or equal to the cross-sectional area defined by each dummy supply channel  75  to be orthogonal to the lateral direction. 
     In the head  20  according to the second illustrative embodiment, each dummy channel  70  includes a dummy return channel  73  and the dummy supply channel  75 . In the stack structure  25 , each dummy return channel  73  is located at a layer provided with return throttle channels  36 , communicates with a corresponding dummy chamber  71 , and has a less cross-sectional area than the corresponding dummy chamber  71 . In the stack structure  25 , each dummy supply channel  75  is located at a layer provided with supply throttle channels  32 , is connected to a corresponding dummy chamber  71 , and has a less cross-sectional area than the corresponding dummy chamber  71 . The first relief groove  80  includes the return-side relief groove  81  connected to the dummy return channels  73 , and the supply-side relief groove  84  connected to the dummy supply channels  75 . 
     An excessive adhesive agent entering the dummy return channels  73  flows to the return-side relief groove  81 , and an excessive adhesive agent entering the dummy supply channels  75  flows to the supply-side relief groove  84 . This may reduce filling of the dummy return channels  73  and the dummy supply channels  75  with the excessive adhesive agent. This may reduce filling of the return-side relief groove  81  and the dummy return channels  73  with an excessive adhesive agent, and reduce clogging of the narrow return throttle channels  36  and supply throttle channels  32  with the excessive adhesive agent flowing there, instead of flowing into the grooves  81  and  84 . 
     A second relief groove (not shown) may be provided in the lower surface of the ninth channel plate  49  or the upper surface of the eighth channel plate  48  so as not to be connected to the dummy channels  70  and so as to trap therein an excessive adhesive agent. 
     &lt;First Modification&gt; 
     A head  20  according to a first modification of the second illustrative embodiment, as shown in  FIG. 4 , may include a common communication passage  82  through which the return-side relief groove  81  and the supply-side relief groove  84  communicate with an exterior of the stack structure  25 . In this case, the communication passage  82 , which penetrates the first channel plate  41  through the 10th channel plate  50  in the stacking direction, is connected, at the first channel plate  41 , to a second-side end of the return-side relief groove and connected, at the ninth channel plate  49 , to a second-side end of the supply-side relief groove  84 . 
     The single communication passage  82  is commonly used for the return-side relief groove  81  and the supply-side relief groove  84 , thereby reducing the number of communication passages  82  and downsizing the head  20 . 
     Alternatively, separate communication passages  82  may be provided for the return-side relief groove  81  and the supply-side relief groove  84 . Further, the communication passage  82  may be provided separately from the second relief groove (not shown) provided in the lower surface of the ninth channel plate  49  or the upper surface of the eighth channel plate  48 . 
     Third Illustrative Embodiment 
     As shown in  FIG. 5 , a head  20  according to a third illustrative embodiment defers from the head  20  according to the first illustrative embodiment in that a first relief groove includes a chamber-side relief groove  85  connected to each dummy chamber  71 . The elements other than the above-described elements are similar to those of the first illustrative embodiment and will not be described repeatedly. 
     The chamber-side relief groove  85 , as the first relief groove  80 , traps therein an excessive adhesive agent between an upper surface of a ninth channel plate  49  and a lower surface of a 10th channel plate  50 . The chamber-side relief groove  85  is located at an edge portion  26  between an end of the 10th channel plate  50  and an array of dummy chambers  71 , and is formed by a groove recessed from a lower surface toward an upper surface of the 10th channel plate  50 . The chamber-side relief groove  85  and the dummy chambers  71  are open on the lower surface of the 10th channel plate  50 . The chamber-side relief groove  85  may be formed in the upper surface of the ninth channel plate  49  facing the lower surface of the 10th channel plate  50 . 
     The chamber-side relief groove  85  is connected, at its first-side ends, to corresponding second-side ends of the dummy chambers  71  and extends from that connected portions toward a second side. Similarly to the return-side relief groove  81 , the chamber-side relief groove  85  may be curved in a direction orthogonal to the stacking direction, branched, and formed into a meshed pattern in the lower surface of the 10th channel plate  50 . The cross-sectional area defined by the chamber-side relief groove  85  to be orthogonal to its extending direction is less than the cross-sectional area defined by each dummy chamber  71  to be orthogonal to the lateral direction. 
     A communication passage  82  penetrates the first channel plate  41  through the 10th channel plate  50  in the stacking direction. The communication passage  82  is connected, at the first channel plate  41 , to a second-side end of the return-side relief groove  81  and connected, at the 10th channel plate  50 , to a second-side end of the chamber-side relief groove  84 . Alternatively, separate communication passages  82  may be provided for the return-side relief groove  81  and the chamber-side relief groove  85 . 
     Thus, any excessive adhesive agent flowing into the dummy chambers  71  flows from the dummy chambers  71  to the chamber-side relief groove  85 . This may reduce filling of the dummy chambers  71  with an excessive adhesive agent and reduce the amount of excessive adhesive agent flowing into the individual channels  30 . 
     A second relief groove (not shown) may be provided in the upper surface of the ninth channel plate  49  or the lower surface of the 10th channel plate  50  so as not to be connected to the dummy channels  70  and so as to trap therein an excessive adhesive agent. The second relief groove may be provided separately from the communication passage  82 . 
     &lt;Other Modifications&gt; 
     In each of the above-described illustrative embodiments and modification, the return-side relief groove  81  is formed to be recessed from the lower surface of the first channel plate  41 . However, the return-side relief groove  81  may be formed to penetrate trough the lower and upper surfaces of the first channel plate  41 . Alternatively, the return-side relief groove  81  may be formed to be recessed from the upper surface of the nozzle plate  40  facing the lower surface of the first channel plate  41 . In this case also, the return-side relief groove  81  traps therein an excessive adhesive agent between the first channel plate  41  and the nozzle plate  40 . 
     Any elements may be combined across the above-described illustrative embodiments and the modification unless they are incompatible with each other. For example, the head  20  in the third illustrative embodiment may include a dummy supply channels  75  and a supply-side relief groove  84 , as in the second illustrative embodiment. The head  20  in the third illustrative embodiment may include the communication passage  82  common to a return-side relief groove  81  and a supply-side relief groove  84 , as in the first modification of the second illustrative embodiment. 
     While the disclosure has been described with reference to the specific embodiments thereof, these are merely examples, and various changes, arrangements and modifications may be applied therein without departing from the spirit and scope of the disclosure.