Liquid ejection head

A liquid ejection head includes an array of first nozzle holes, an array of second nozzle holes, first pressure chambers to which an ejection pressure is applied for liquid ejection from the first nozzle holes, second pressure chambers to which an ejection pressure is applied for liquid ejection from the second nozzle holes, and a manifold. The manifold includes a first channel communicating with the first pressure chambers, a first opening located at the first channel and through which liquid enters from an exterior, a second channel communicating with the second pressure chambers, a second opening located at the second channel and through which liquid exits to the exterior, a first communication passage communicating one end of the first channel with one end of the second channel, and a second communication passage communicating the other end of the first channel with the other end of the second channel.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2019-069604 filed on Apr. 1, 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 nozzles for liquid ejection, pressure generating chambers communicating with the nozzles, a manifold communicating with the pressure generating chambers, an inflow passage through which liquid is supplied from a tank at an exterior of the liquid ejection head to the manifold, and an outflow passage through which liquid exits from the manifold toward the tank. The inflow passage and the outflow passage are respectively located on one side and the other side of the manifold which extends in a first direction.

SUMMARY

In the known liquid ejection head, when liquid flows from the inflow passage, via the manifold, to the outflow passage, components, such as pigments, contained in the liquid may settle, causing differences in concentration of the liquid in the first direction.

Aspects of the disclosure provide a liquid ejection head configured to reduce differences in liquid concentration.

According to one or more aspects of the disclosure, a liquid ejection head includes a first nozzle hole array, a second nozzle hole array, a plurality of first pressure chambers, a plurality of second pressure chambers, and a manifold. The first nozzle hole array includes a plurality of first nozzle holes arranged in a first direction. The second nozzle hole array is spaced from the first nozzle hole array in a second direction crossing the first direction, and includes a plurality of second nozzle holes arranged in the first direction. An ejection pressure is applied to the plurality of first pressure chambers for liquid ejection from the first nozzle holes. An ejection pressure is applied to the plurality of second pressure chambers for liquid ejection from the second nozzle holes. The manifold communicates with the plurality of first pressure chambers and the plurality of second pressure chambers. The manifold includes a first channel communicating with the plurality of first pressure chambers, a first opening which is located at the first channel and through which liquid enters from an exterior, a second channel communicating with the plurality of second pressure chambers, a second opening which is located at the second channel and through which liquid exits to the exterior, a first communication passage communicating one end of the first channel with one end of the second channel, and a second communication passage communicating the other end of the first channel with the other end of the second channel.

DETAILED DESCRIPTION

Illustrative embodiments of the disclosure will be described with reference to the drawings.

First Illustrative Embodiment

A liquid ejection apparatus10including a liquid ejection head20(hereinafter referred to as a “head”) according to a first illustrative embodiment is configured to eject liquid. Hereinafter, the liquid ejection apparatus10will be described by way of example as applied to, but not limited to, an inkjet printer.

As shown inFIG. 1, the liquid ejection apparatus10employs a line head type and includes a platen11, a transport unit, a head unit16, tanks12, and a controller13. The liquid ejection apparatus10may employ a serial head type or other types than the line head type.

The platen11is a flat plate member to receive thereon a sheet14and adjust a distance between the sheet14and the head unit16. Herein, one side of the platen11toward the head unit16is referred to as an upper side, and the other side of the platen11away from the head unit16is referred to as a lower side. However, the liquid ejection apparatus10may be positioned in other orientations.

The transport unit may include two transport rollers15and a transport motor (not shown). The two transport rollers15are disposed parallel to each other while interposing the platen11therebetween in a transport direction, and are connected to the transport motor. When the transport motor is driven, the transport rollers15rotate to transport the sheet14on the platen11in the transport direction.

The head unit16has a length greater than or equal to the length of the sheet14in a direction (an orthogonal direction) orthogonal to the transport direction of the sheet14. The head unit16includes a plurality of heads20.

Each head20includes a stack structure including a channel unit and a volume changer. The channel unit includes liquid channels formed therein and a plurality of nozzle holes21aopen on a lower surface (an ejection surface31a). The volume changer is driven to change the volume of a liquid channel. In this case, a meniscus in a nozzle hole21avibrates and liquid is ejected from the nozzle hole21a. The head20will be described in detail later.

Separate tanks12are provided for different kinds of inks. For example, each of four tanks12stores therein a corresponding one of black, yellow, cyan, and magenta inks. Inks of the tanks12are supplied to corresponding nozzle holes21a.

The controller13includes a processor such as a central processing unit (CPU), memories such as a random access memory (RAM) and a read only memory (ROM), and a driver integrated circuits (ICs) such as an application specific integrated circuit (ASIC). In the controller13, 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 controller13executes ejection from the head unit16and transport of sheets14. The head unit16is controlled to eject ink from the nozzle holes21a. A sheet14is transported in the transport direction. Printing progresses by execution of ink ejection and sheet transport.

As described above, each head20includes the channel unit and the volume changer. As shown inFIGS. 2 and 3, the channel unit is formed by a stack of a plurality of plates, and the volume changer includes piezoelectric elements50and a vibration plate38.

The plurality of plates include a nozzle plate31, a first channel plate32, a second channel plate33, and an accommodating plate34. These plates are stacked in this order in a stacking direction.

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 nozzles40aand40b, a plurality of individual channels, and a manifold60. The manifold60includes a first channel61and a second channel62. Each element will be described in detail later.

The nozzle plate31includes first nozzles40aand second nozzles40bformed therethrough in the stacking direction. The ejection surface31aof the nozzle plate31has openings (first nozzle holes41a) of the first nozzles40aand openings (second nozzle holes41b) of the second nozzles40brespectively arranged in a first direction as a first nozzle array and a second nozzle array.

The first direction is orthogonal to the stacking direction and may be parallel or inclined relative to the orthogonal direction (a direction orthogonal to the transport direction of the sheet14). A second direction is a direction orthogonal to the stacking direction and crossing (e.g., orthogonal to) the first direction, and may be parallel or inclined relative to the transport direction.

The first nozzle array and the second nozzle array are arranged parallel to each other, at an interval from each other in the second direction. The first nozzle holes41aand the second nozzle holes41bmay be arranged in a staggered manner in the first direction or may be arranged side by side in the second direction.

The individual channels include first individual channels each connected to a corresponding first nozzle40aand to the first channel61, and second individual channels each connected to a corresponding second nozzle40band to the second channel62. The first channel61and the second channel62sandwich the first individual channels and the second individual channels, which are respectively arranged to be next to the first nozzles40aand the second nozzles40b.

Each first individual channel includes a throttle channel43, a first pressure chamber44a, and a descender45which are arranged in this order. Each second individual channel includes a throttle channel43, a second pressure chamber44b, and a descender45which are arranged in this order. The first pressure chambers44aand the second pressure chambers44bare spaced from each other in the second direction and arranged in the first direction in an arrangement region C.

The first channel plate32includes the first channel61, the second channel62, the throttle channels43and the descenders45. Each descender45penetrates the first channel plate32in the stacking direction and is connected, at its upper end, to a corresponding pressure chamber44aor44band, at its lower end, to a corresponding nozzle40aor40b.

The first channel61and the second channel62are recessed from a lower surface of the first channel plate32and extend in the first direction. The first channel61has an L-shape in cross-section orthogonal to its extending direction and includes, at a lower portion of the L shape, a protrusion61aprotruding toward the first nozzles40a. The second channel62has an L-shape in cross-section orthogonal to its extending direction and includes, at a lower portion of the L shape, a protrusion62aprotruding toward the second nozzles40b.

Lower openings of the first channel61and the second channel62are covered by a damper film37. The damper film37is a flexible film and deforms to reduce a pressure change in liquid in the first channel61and the second channel62.

Each throttle channel43extends upward from a corresponding protrusion61aor62ato penetrate the first channel plate32in the stacking direction and is connected, at its upper end, to a corresponding pressure chamber44aor44b.

The second channel plate33includes the pressure chambers44aand44b. Each pressure chamber44aor44bis recessed from a lower surface of the second channel plate33, extends in the second direction, and is connected, at its one end, to a corresponding throttle channel43and, at its other end, to a corresponding descender45.

The second channel plate33includes a vibration plate38over the pressure chambers44aand44b. The vibration plate38may be separate from the second channel plate33. In this case, the pressure chambers44aand44bmay penetrate the second channel plate33in the stacking direction, and the vibration plate38may be stacked on the second channel plate33to cover upper openings of the pressure chambers44aand44b.

The accommodating plate34defines a first accommodating space46for accommodating therein piezoelectric elements50. The first accommodating space46is located over the pressure chambers44aand44bto be recessed from a lower surface of the accommodating plate34and extends in the first direction. An electronic circuit with a driver IC55is located on the accommodating plate34.

Each piezoelectric element50includes a common electrode51, a piezoelectric layer52, and an individual electrode53which are arranged in this order on the vibration plate38. Each individual electrode53is provided over a corresponding pressure chamber44aor44b. The common electrode51entirely covers the vibration plate38. In this case, a piezoelectric element50is formed by an active portion of a piezoelectric layer52, which is sandwiched by an individual electrode53and the common electrode51.

Each individual electrode53is electrically connected to the driver IC55. The driver IC55receives control signals from the controller13(FIG. 1) and generates drive signals (voltage signals) selectively to the individual electrodes53. In contrast, the common electrode51is constantly maintained at a ground potential.

In response to a drive signal, an active portion of each selected piezoelectric layer52expands and contracts in a surface direction, together with the two electrodes51and53. Accordingly, the vibration plate38corporates to deform to increase and decrease the volume of a corresponding pressure chamber44aor44b. This applies a pressure to the corresponding pressure chamber44aor44bwhich in turn ejects liquid from a nozzle40aor40b.

The manifold60is a common channel through which liquid is supplied to the individual channels and is connected to the individual channels. The manifold60includes the first channel61, the second channel62, a first opening63, a second opening64, a first communication passage65, and a second communication passage66.

The first channel61is connected to the first individual channels, while the second channel62is connected to the second individual channels. The first channel61and the second channel62extend longer than the arrangement region C in the first direction to sandwich therebetween in the second direction the first individual channels and the second individual channels. The first channel61and the second channel62have the same dimension in the first direction and the same cross-sectional area orthogonal to the first direction.

The first opening63is located at a center of the first channel61in the first direction. The first opening63is connected to an inner space of a first port71via a first hole located at an upper portion of the first channel plate32.

The first port71is, for example, cylindrical and is attached to an upper surface of the first channel plate32to surround the first hole and protrude upward from the upper surface. Thus, the first channel61communicates with the inner space of the first port71via the first opening63. The first port71may be integral with the first channel plate32.

The second opening64is located at a center of the second channel62in the first direction. The second opening64is connected to an inner space of a second port72via a second hole located at an upper portion of the first channel plate32.

The second port72is, for example, cylindrical and is attached to an upper surface of the first channel plate32to surround the second hole and protrude upward from the upper surface. Thus, the second channel62communicates with the inner space of the second port72via the second opening64. The second port72may be integral with the second channel plate32.

The first opening63and the second opening64are located on a straight line extending in the second direction and have the same size and shape. The first opening63is located on an opposite side of the first channel61from the first pressure chambers44ain the second direction. The second opening64is located on an opposite side of the second channel62from the second pressure chambers44bin the second direction.

The first communication passage65is connected to one end of the first channel61and to one end of the second channel62. The second communication passage66is connected to the other end of the first channel61and to the other end of the second channel62. Thus, the first channel61and the second channel62communicate with each other via the first communication passage65and the second communication passage66.

The first communication passage65and the second communication passage66are defined by a forming member35which is made of, for example, resin and fixed to the first channel plate32. The forming member35may be integral with the first channel plate32.

The cross-sectional area defined by the first communication passage65to be orthogonal to its axis is less than that of each of the first channel61and the second channel62. The cross-sectional area defined by the second communication passage66to be orthogonal to its axis is less than that of each of the first channel61and the second channel62. The cross-sectional area of the first communication passage65is equal to that of the second communication passage66.

The first communication passage65is curved such that the inclination of its portion farther from the first channel61changes from the first direction further toward the second direction. Likewise, The second communication passage66is curved such that the inclination of its portion farther from the second channel62changes from the first direction further toward the second direction. Thus, the first communication passage65and the second communication passage66are curved to protrude away from each other and are symmetrical with respect to a line while sandwiching the first channel61and the second channel62.

Thus, the manifold60, which includes the first channel61, the second channel62, the first communication passage65, and the second communication passage66, surrounds the first pressure chambers44aand the second pressure chambers44bin a direction orthogonal to the stacking direction, that is, in a direction including the first direction and the second direction. When viewed in the stacking direction, the first opening63and the second opening64are symmetrical with respect to a center point60aof the manifold60.

<Liquid Flow in Manifold>

As shown inFIG. 2, the first port71is connected to a subtank80via a supply conduit at which a pressure pump83is located. The second port72is connected to the subtank80via a return conduit at which a negative pressure pump84is located. The subtank80, which may be disposed above the head20, is connected to a tank12(FIG. 1).

When the pumps83and84are driven, liquid from the subtank80passes through the supply conduit81to flow, via the inner space of the first port71and the first opening, into a center of the first channel61. In this case, liquid flows, from above, into the first channel61and flows down the first channel61to collide with its bottom. This disperses components, such as pigments, contained in the liquid, thereby reducing settling of the components.

The liquid flow branches at the center in the first direction toward opposite ends of the first channel61along the first direction. Part of the liquid flows into the first individual channels, passes the throttle channels43, the first pressure chambers44a, and the descenders45in this order, reaches the first nozzles40a, and is ejected from the first nozzles41a.

The remaining liquid flows from one end of the first channel61into the first communication passage65and from the other end of the first channel61into the second communication passage66. The liquid flows along the curved communication passages65and66. The direction of liquid flow gradually changes toward one side in the first direction, toward the second direction, and toward the other side in the first direction. Thus, liquid flows smoothly in the communication passages65and66to discharge air bubbles.

The liquid flows from the communication passages65and66into the second channel62, through opposite ends toward a center of the second channel62in the first direction. Part of the liquid flows into the second individual channels, passes the throttle channels43, the second pressure chambers44b, and the descenders45in this order, reaches the second nozzles40b, and is ejected from the second nozzle holes41b.

As the remaining liquid, the liquid from the first communication passage65and the liquid from the second communication passage66meet at the center in the second channel62. This disperses the components contained in the liquid, thereby reducing settling of the components.

Then, the liquid is discharged from the center of the second channel62, via the second opening64and the inner space of the second port72, and returns, via the return conduit, to the subtank80. Thus, the liquid not having flown into the individual channels circulates between the subtank80and the manifold60.

Printing is performed by liquid ejection and sheet transport. As shown inFIG. 4, by ejection of liquid from the nozzles40aand40b, liquid dots (first dots t1) ejected from the first nozzle holes41aare formed in the first direction, and liquid dots (second dots t2) ejected from the second nozzle holes41bare formed in the first direction.

In the head20, the manifold60includes the first channel61, the first opening63, the second channel62, the second opening64, the first communication passage65, and the second communication passage66. The first channel61communicates with the first pressure chambers44aand includes the first opening63through which liquid enter from an exterior. The second channel62communicates with the second pressure chambers44band includes the second opening64through which liquid exits to the exterior. The first communication passage65communicates one end of the first channel61with one end of the second channel62. The second communication passage66communicates the other end of the first channel61with the other end of the second channel62.

As liquid flows in the manifold60, components contained in the liquid settle down, causing a decrease in concentration of the liquid. Nevertheless, the first opening63is located at the first channel61and the second opening64is located at the second channel62. Thus, a distance between any portion of the first channel61and the first opening63and a distance between any portion of the second channel62and the second opening64are relatively small, thereby reducing differences in liquid concentration.

This may reduce differences in liquid concentration in the first direction of partial images i formed by liquid ejected through the nozzles41aand41bfrom the channels61and62, respectively. Consequently, the degradation of image quality due to differences in liquid concentration may be reduced.

Because liquid flows from the first channel61to the second channel62, the liquid in the first channel61is higher in concentration than the liquid in the second channel62. Herein, the first channel61and the second channel62are arranged in the second direction.

First dots t1formed by the liquid ejected from the nozzle holes41acommunicating with the first channel61are thicker than second dots t2formed by the liquid ejected from the nozzle holes41bcommunicating with the second channel62. Among the first dots t1arranged in the first direction, a first dot t1closer to a center in the first direction is thicker. Among the second dots t2arranged in the first direction, a second dot t2closer to a center in the first direction is thinner.

A thicker first dot t1compensates for a thinner second dot t2. Consequently, the degradation of image quality due to differences in liquid concentration may be reduced.

Unlike the above-described embodiment, providing the openings63and64at the communication passages65and66, respectively, increases the sizes of the communication passages65and66depending on the sizes of the openings63and64, respectively. However, in the head20, the openings63and64are not provided at the communication passages65and66, respectively. Thus, the communication passages65and66are reduced in size, regardless of the size and position of the openings63and64. The manifold60, which is single and common to the first nozzle array and the second nozzle array, makes the head20compact.

In the head20, the first communication passage65and the second communication passage66are curved to protrude away from each other. This structure allows liquid to flow along the curved communication passages65and66without stagnation, thereby discharging air bubbles.

In the head20, the cross-sectional area defined by the first communication passage65to be orthogonal to its extending direction is less than that defined by the first channel61to be orthogonal to its extending direction. Likewise, the cross-sectional area defined by the second communication passage66to be orthogonal to its extending direction is less than that defined by the second channel62to be orthogonal to its extending direction. This structure makes the flow velocity of liquid in the communication passages65and66higher than that in the channels61and62, respectively, thereby discharging air bubbles from the communication passages65and66, via the channel62and the second opening64, to the exterior.

In the head20, the cross-sectional area of the first communication passage65is equal to that of the second communication passage66, and the cross-sectional area of the first channel61is equal to that of the second channel62. This structure makes the flow path resistance of the first communication passage65equal to that of the second communication passage66, and the flow path resistance of the first communication passage61equal to that of the second channel62. This makes similar changes in liquid concentration in the first channel61in response to the distance from the first opening63to those in the second channel62in response to the distance from the second opening64.

In the head20, the first opening63is located at a center of the first channel61in an extending direction of the first channel61. Likewise, the second opening64is located at a center of the second channel62in an extending direction of the second channel62. This structure may prevent an increase in length in the first direction of the first channel61from the first opening63and an increase in length in the first direction of the second channel62from the second opening64, thereby reducing differences in liquid concentration in the channels61and62in the first direction.

In the head20, a center63aof the first opening63is located opposite to the first pressure chambers44arelative to a center of the first channel61in a direction orthogonal to the extending direction of the first channel61. A center64aof the second opening64is located opposite to the second pressure chambers44brelative to a center of the second channel62in a direction orthogonal to the extending direction of the second channel62.

Unlike this embodiment, providing the openings63and64at sides of the channels61and62closer to the pressure chambers44aand44b, respectively, causes stagnation of the liquid at sides farther from the pressure chambers44aand44b. In contrast, providing the openings63and64at sides of the channels61and62farther from the pressure chambers44aand44b, respectively, reduces stagnation of the liquid there. In response to liquid ejection from the nozzles40aand40b, liquid flows into the pressure chambers44aand44b, respectively. Thus, the liquid flows at sides closer to the pressure chambers44aand44b, thereby reducing stagnation of the liquid there.

First Modification

In a head20according to a first modification of the first illustrative embodiment, as shown inFIG. 5, each connection portion of a first channel161connected to a corresponding communication passage65or66may have a cross-sectional shape asymptotically similar to that of the corresponding communication passage65or66. Each connection portion of a second channel162connected to a corresponding communication passage65or66may have a cross-sectional shape asymptotically similar to that of the communication passage65or66. The elements other than the above-described elements are similar, in structure, function, and effect, to those of the first illustrative embodiment and will not be described repeatedly.

For example, each of the channels161and162includes a pair of tapered portions168sandwiching a straight portion167therebetween in the first direction. Each straight portion167is located in a corresponding arrangement region C and has a uniform cross-section in the first direction.

One and the other tapered portions168are located at one and the other ends of a corresponding arrangement region C. Each tapered portion168is tapered from one end or the other end of the corresponding arrangement region C and has a small diameter end and a large diameter end which is larger in dimension than the small diameter end. Each large diameter end is connected to an end of a corresponding straight portion167and has the same size and shape as the corresponding straight portion167. Each small diameter end is connected to an end of a corresponding communication passage65or66and has the same size and shape as the corresponding communication passage65or66.

Each tapered portion168is continuously tapered from a corresponding straight portion167toward a corresponding communication passage65or66without a step or a corner between the tapered portion168and the corresponding straight portion167and between the tapered portion168and the corresponding communication passage65or66. This allows a smooth liquid flow along the tapered portion168between the corresponding straight portion167and communication passage65or66, thereby efficiently dispersing the liquid components and discharging air bubbles.

Second Modification

In a head20according to a second modification of the first illustrative embodiment, as shown inFIG. 6, a first opening163and a second opening164may be located symmetrically with respect to the center point60aof the manifold60which surrounds the first pressure chambers44aand the second pressure chambers44b. The elements other than the above-described elements are similar, in structure, function, and effect, to those of the first illustrative embodiment and will not be described repeatedly.

In contrast, as shown inFIG. 2, the first opening63and the second opening64are located to interpose the center point60atherebetween in the second direction. The first opening163and the second opening164are not so limited as long as the first and second openings163and164are symmetrical with respect to the center point60a. For example, as shown inFIG. 6, the first opening163is closer to the second communication passage66in the first direction than the second opening164. In this case, a center163aof the first opening163, the center point60a, and a center164aof the second opening164are arranged in this order on the same straight line.

This structure may prevent an increase in length in the first direction from the first opening163to opposite ends of the first channel61and from the second opening164to opposite ends of the second channel62, thereby reducing differences in liquid concentration in the first direction. In addition, the first opening163and the second opening164are offset from each other in the first direction and are not aligned in the second direction. This facilitate piping between the first port71on the first opening163and the second port72on the second opening164.

Third Modification

In a head20according to a third modification of the first illustrative embodiment, as shown inFIG. 7, a first port171whose inner space communicates with the first opening63may increase in diameter toward the first channel61. A second port172whose inner space communicates with the second opening64may decrease in diameter in a direction away from the second channel62. The elements other than the above-described elements are similar, in structure, function, and effect, to those of the first illustrative embodiment and will not be described repeatedly.

The first port171has a first upper end opening171aand a first lower end opening171bwhich is smaller in dimension than the first upper end opening171a. The first port171has a shape of a corn without its tip, such as a circular truncated cone, and has a straight inclined surface such that a cross-sectional area of the first port171increases from the first upper end opening171atoward the first lower end opening171b.

The first lower end opening171bis connected, via the first hole, to the first opening63of the first channel61, and the inner space of the first port171communicates, via the first opening63, with the first channel61. Liquid flows from the first port171into the first channel61in a vertical direction and in a direction orthogonal to the vertical direction. This evenly disperses the liquid in a wide range of the first channel61.

The second port172has a second upper end opening172aand a second lower end opening172bwhich is smaller in dimension than the first upper end opening172a. The second port172has a shape of a corn without its tip, such as a circular truncated cone, and has a straight inclined surface such that a cross-sectional area of the second port172decreases from the second lower end opening172btoward the second upper end opening172a.

The second lower end opening172bis connected, via the second hole, to the second opening64of the second channel62, and the inner space of the second port172communicates, via the second opening64, with the second channel62. When liquid flows from the second channel62into the second port172, the flow velocity increases as the second port172decreases in diameter. This efficiently discharges air bubbles from the second channel62.

Second Illustrative Embodiment

In a head20according to a second illustrative embodiment, as shown inFIG. 8, a first unit21and a second unit22may be arranged in a second direction. The elements other than the above-described elements are similar, in structure, function, and effect, to those of the first illustrative embodiment and will not be described repeatedly.

Specifically, the first unit21includes first nozzle holes41af, second nozzle holes41bf, first pressure chambers44af, second pressure chambers44bf, and a manifold60f. The manifold60fincludes a first channel61f, a first opening63f, a second channel62f, a second opening64f, a first communication passage65f, and a second communication passage66f.

The second unit22includes first nozzle holes41as, second nozzle holes41bs, first pressure chambers44as, second pressure chambers44bs, and a manifold60s. The manifold60sincludes a first channel61s, a first opening63s, a second channel62s, a second opening64s, a first communication passage65s, and a second communication passage66s.

The first unit21and the second unit22are arranged next to each other in the second direction. The first unit21and the second unit22are arranged such that in the second direction, the second channel62fand the first channel61s, which are next to each other, are located between the first channel61fand the second channel62s.

The first opening63fand the first opening63sare connected to a supply opening of a subtank80for the same liquid. The second opening64fand the second opening64sare connected to a discharge opening of the subtank80for the same liquid. The openings63f,63s,64f, and64smay be connected to the common subtank80or to separate subtanks80for the same kind of liquid. The same kind of liquid is supplied to the first openings63fand63s, passes through the manifolds60fand60s, and is discharged from the second openings64fand64s, respectively.

A first nozzle hole array of the first nozzle holes41af, a second nozzle hole array of the second nozzle1holes41bf, a first nozzle hole array of the first nozzle holes41as, and a second nozzle hole array of the second nozzle holes41bsare arranged in this order in the second direction and are parallel to each other.

In the first unit21, the first nozzle holes41afand the second nozzle holes41bfare arranged alternately with each other in the first direction. In the second unit22, the first nozzle holes41asand the second nozzle holes41bsare arranged alternately with each other in the first direction.

In each nozzle array, an nth (n is a natural number) first nozzle hole41af, an nth second nozzle hole41bs, an nth second nozzle hole41bf, and an nth first nozzle hole41asare arranged in this order, at equal intervals d, in the first direction. The order of the nozzle holes arranged in the first direction is not so limited as long as the first nozzle holes41afand the second nozzle holes41bfof the first unit21, and the first nozzle holes41asand the second nozzle holes41bsof the second unit22are arranged at equal intervals in the first direction.

In other words, an interval between a first nozzle hole41afand a second nozzle hole41bswhich are adjacent to each other in the first direction is equal to the interval d. An interval between a second nozzle hole41bsand a second nozzle hole41bfwhich are adjacent to each other in the first direction is equal to the interval d. An interval between a second nozzle hole41bfand a first nozzle hole41aswhich are adjacent to each other in the first direction is equal to the interval d.

Further, an nth first nozzle hole41asand an (n+1)th first nozzle hole41afare adjacent to each other, at the interval d, in the first direction. The first nozzle hole41af, second nozzle hole41bs, second nozzle hole41bf, and first nozzle hole41asare arranged at the predetermined intervals d in the first direction and are spaced from each other in the second direction.

Thus, dots formed by liquid ejected from these nozzle holes41aand41bare arranged in the first direction at equal intervals in the first direction to form an image. This may reduce unevenness in color of an image.

The use of the two units21and22makes smaller the interval d between two adjacent ones of the nozzle holes41af,41bf,41as, and41bs. This increases the density of dots in the image.

Further, in the first unit21, the first nozzle holes41afand the second nozzle holes41bfare arranged alternately with each other at equal intervals in the first direction. In the second unit21, the first nozzle holes41asand the second nozzle holes41bsare arranged alternately with each other at equal intervals in the first direction. This allows to selectively use one or a combination of the units21and22, thereby reducing the cost of a product.

The structure of at least one of the first, second, and third modifications may be applied to the head20according to the second illustrative embodiment.

Third Illustrative Embodiment

In a head20according to a third illustrative embodiment, as shown inFIGS. 9 and 10, a manifold60may include, at its upper surface, protruding members protruding downward. The elements other than the above-described elements are similar, in structure, function, and effect, to those of the first illustrative embodiment and will not be described repeatedly.

Specifically, as the protruding members, first protruding members91and second protruding members92are located in a first channel61and a second channel62, respectively. The first protruding members91and the second protruding members92may have the same dimensions and shape.

Each first protruding member91extends in a first direction between a first opening63and a first communication passage65or between the first opening63and a second communication passage66. Each second protruding member92extends in the first direction between a second opening64and the first communication passage65or between the second opening64and the second communication passage66.

The first protruding members91are aligned with a center63aof the first opening63in the first direction, and the second protruding members92are aligned with a center64aof the second opening64. The first protruding members91are located at a center of the first channel61in a second direction, and the second protruding members92are located at a center of the second channel62in the second direction.

Each first protruding member91includes a plurality of first protrusions93arranged at intervals in the first direction. Each second protruding member92includes a plurality of second protrusions94arranged at intervals in the first direction. Each of the protrusions93and94extends longer in the first direction than in the second direction.

Each of the protruding members91and92protrudes downward from an upper surface of a corresponding channel61or62. Liquid flows down along the protruding members91and92in the channels61and62, respectively. This reduces settling down of the liquid components and differences in liquid concentration.

The protruding members91and92respectively extend in the channels61and62in their extending directions (the first direction). Liquid flows along the protruding members91and92in an extending direction of the manifold60. The liquid flow moves air bubbles, thereby reducing stagnation of air bubbles.

Further, the protruding members91and92respectively include the protrusions93and94arranged at intervals. Air bubbles pass through and exit from the intervals between adjacent protrusions93and the intervals between adjacent protrusions94.

In the above-described structure, the first channel61includes the first protruding members91, and the second channel62includes the second protruding members92. Alternatively, the first channel61may include the first protruding members91, and the second channel62may include no second protruding member92. Alternatively, the first channel61may include no first protruding member91, and the second channel61may include the second protruding members.

Fourth Modification

In a head20according to a fourth modification of the third illustrative embodiment, as shown inFIG. 11, first protruding members191may each extend from the first opening61toward the first communication passage65or the second communication passage66so as to be closer to a center of the first channel61in a direction orthogonal to an extending direction of the first channel61. The elements other than the above-described elements are similar, in structure, function, and effect, to those of the third illustrative embodiment and will not be described repeatedly.

Specifically, the first protruding members191include two pairs of first protruding members191eand191f. One pair of first protruding members191eand191fis located between the first opening63and the first communication passage65in the first direction. The other pair of first protruding members191eand191fis located between the first opening63and the second communication passage66in the first direction.

When viewed from above, each pair of first protruding members191eand191fis arranged in a V shape in a direction (including the first direction and the second direction) orthogonal to a vertical direction. The first protruding members191eand191fare spaced from each other in the second direction such that a distance therebetween increases from the first opening63toward the communication passage65or66.

The first protruding member191eincludes a plurality of first protrusions193earranged at intervals on a straight line inclined in a direction between the first direction and the second direction. The first protruding member191fincludes a plurality of first protrusions193farranged at intervals on a straight line inclined in a direction between the first direction and the second direction. The first protrusions193eand the first protrusions193fhave the same size and shape and are arranged side by side in the second direction.

The first protruding members191each extend from the first opening63toward the first communication passage65or the second communication passage66so as to be closer to a center of the first channel61in the second direction. Fluid flows at a higher velocity at a portion closer to the center. Thus, air bubbles are collected, along the first protruding members191, toward the center with a high velocity and are efficiently discharged from the first channel61.

Fifth Modification

In a head20according to a fifth modification of the third illustrative embodiment, as shown inFIG. 11, second protruding members192may each extend from the second opening64toward the first communication passage65or the second communication passage66so as to be farther from a center of the second channel62in a direction orthogonal to an extending direction of the second channel62. The elements other than the above-described elements are similar, in structure, function, and effect, to those of the third illustrative embodiment and will not be described repeatedly.

Specifically, the second protruding members192include two pairs of second protruding members192eand192f. One pair of second protruding members192eand192fis located between the second opening64and the first communication passage65in the first direction. The other pair of second protruding members192eand192fis located between the second opening64and the second communication passage66in the first direction.

When viewed from above, each pair of second protruding members192eand192fis arranged in a V shape in a direction orthogonal to a vertical direction. The second protruding members192eand192fare spaced from each other in the second direction such that a distance therebetween decreases from the second opening64toward the communication passage65or66.

The second protruding member192eincludes a plurality of second protrusions194earranged at intervals on a straight line inclined in a direction between the first direction and the second direction. The second protruding member192fincludes a plurality of second protrusions194farranged at intervals on a straight line inclined in a direction between the first direction and the second direction. The second protrusions194eand the second protrusions194fhave the same size and shape and are arranged side by side in the second direction.

The second protruding members192eand192feach extend from the communication passage65or66toward the second opening64so as to be closer to a center of the second channel62in the second direction. Fluid flows at a higher velocity at a portion closer to the center. Thus, air bubbles entrained from the communication passages65and66into the second channel62flow, along the second protruding members192, toward the second opening64and are collected toward the center with a high velocity. The air bubbles are efficiently discharged, via the second opening64, from the second channel62.

The structure of at least one of the first, second, and third modifications may be applied to the head20according to the third illustrative embodiment. The structure of the fourth modification may be applied to the head20according to the fifth modification.

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.