Inkjet head and nozzle plate of inkjet head

An inkjet head includes plural nozzles that eject ink. The nozzles are arranged so that (a) the nozzles are arranged in a first direction on an ink ejection surface to form a plurality of rows parallel to one another; and (b) when the nozzles are projected from a second direction, which is parallel to the ink ejection surface and perpendicular to the first direction, onto a virtual straight line extending in the first direction, projective dots of the nozzles are arranged at equally spaced intervals on the virtual straight line. A spatial frequency, which is determined based on an appearance interval of a most-distant adjacent projective dot pair in the first direction, is lower than a spatial frequency corresponding to a peak value of a visual transfer function.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inkjet head having pressure chambers arrayed in a matrix.

2. Description of the Related Art

JP-A-2003-237078 discloses an inkjet head having a large number of pressure chambers arrayed in a matrix.FIG. 11Ais a schematic view of nozzle arrays when the inkjet head disclosed in JP-A-2003-237078 is used as a line head. In the inkjet head shown inFIG. 11A, sixteen nozzles are present in each belt-like region R delimited by a large number of straight lines extending in a paper conveyance (sub-scanning) direction. As for the sixteen nozzles108, the coordinate in a head longitudinal (main scanning) direction and the coordinate in the paper conveyance (sub-scanning) direction differ from one nozzle to another. When the sixteen nozzles108are projected from the sub-scanning direction onto a virtual straight line extending in the main scanning direction, sixteen projective dots are obtained. The sixteen projective dots are separated at equally spaced intervals corresponding to a printing resolution. Assume that the sixteen nozzles108are numbered (1)-(16) in order from the nozzle whose corresponding projective dot is leftmost. Then, the sixteen nozzles108(1), (9), (5), (13), (2), (10), (6), (14), (3), (11), (7), (15), (4), (12), (8) and (16) are arranged in that order from below. When each belt-like region R is divided equally into four small regions r1, r2, r3and r4by straight lines extending in the sub-scanning direction, four nozzles108are arranged on a straight line in each small region. Each belt-like region R includes one and the same array pattern of sixteen nozzles108.

When ink is ejected at short ejection intervals sequentially from each nozzle108in such an inkjet head, a large number of straight lines extending in the sub-straight line can be printed so as to be separated at equally spaced intervals equal to the intervals of the aforementioned projective dots as shown inFIG. 11B. Because of the narrow intervals between adjacent ones of the straight lines, the range where the large number of straight lines are printed is observed actually as if it were a filled region.

SUMMARY OF THE INVENTION

In the inkjet head disclosed in JP-A-2003-237078, the distance between a nozzle108(1) belonging to one belt-like region R and a nozzle108(16) belonging to another belt-like region R on the left side of the one belt-like region R is very long in the sub-scanning direction as shown inFIG. 11A. Consider that a large number of straight lines are printed as shown inFIG. 11B. When the attachment angle of the ink-jet head is slightly tilted, the interval between the straight line formed by ink ejected from the nozzle108(1) and the straight line formed by ink ejected from the nozzle108(16) with respect to the main scanning direction becomes longer than any other interval between adjacent straight lines as shown inFIG. 1C. As a result, periodic bandings101appear in a print so as to give observers a feeling of wrongness.

To prevent bandings from occurring, the inkjet head has to be attached to a printer body with very high accuracy. However, the attachment of the inkjet head with high accuracy results in complication of its manufacturing process and increase of its cost.

It is therefore an object of the present invention to provide an inkjet head, which can obtain a preferable printing result without demanding high accuracy in attachment of the inkjet head.

An inkjet head according to one embodiment of the invention includes a plurality of nozzles that eject ink. The nozzles are arranged so that (a) the nozzles are arranged in a first direction on an ink ejection surface to form a plurality of rows parallel to one another; and (b) when the nozzles are projected from a second direction, which is parallel to the ink ejection surface and perpendicular to the first direction, onto a virtual straight line extending in the first direction, projective dots of the nozzles are arranged at equally spaced intervals on the virtual straight line. Each of adjacent projective dot pairs includes two projective dots adjacent to each other. A most-distant adjacent projective dot pair represents an adjacent projective dot pair having a longest distance between two rows, which two nozzles corresponding to two projective dots thereof belong to, among the adjacent projective dot pairs. A spatial frequency, which is determined based on an appearance interval of the most-distant adjacent projective dot pair in the first direction, is lower than a spatial frequency corresponding to a peak value of a visual transfer function.

With this configuration, bandings corresponding to the most-distant adjacent projective dot pairs, which occur due to the inclined attachment angle of the inkjet head, can be made inconspicuous when the inkjet head is used as a line head. Accordingly, a preferable printing result can be obtained without demanding high accuracy in attachment of the ink-jet head.

The visual transfer function (VTF) is a function expressing human sensitivity of visual recognition with respect to a spatial frequency. The visual transfer function is an evaluation criteria of objective print quality with reduced personal dispersion. This evaluation criteria is used for evaluation such that human psychological factors sensuously determining whether the print quality is good or bad is added to quantitative factors of printing in a field of a hard copy using an inkjet system. The visual transfer function is obtained on an experimental basis of sampling a large number of human beings. The visual transfer function draws a curve having a peak value in a specific frequency and having a smaller value as the spatial frequency is farther from the specific frequency. For example, a problem of banding is evaluated using a visual transfer function. On the assumption that N designates a spatial frequency corresponding to a peak value of the visual transfer function, the human sensitivity to banding is the highest when the spatial frequency is N. As the spatial frequency is lower than N or higher than N, the sensitivity to banding is lowered.

According to one embodiment of the invention, a nozzle plate of an inkjet head includes a plurality of nozzles that eject ink. The nozzles are arranged so that (a) the nozzles are arranged in a first direction on an ink ejection surface to form a plurality of rows parallel to one another; and (b) when the nozzles are projected from a second direction, which is parallel to the ink ejection surface and perpendicular to the first direction, onto a virtual straight line extending in the first direction, projective dots of the nozzles are arranged at equally spaced intervals on the virtual straight line. Each of adjacent projective dot pairs includes two projective dots adjacent to each other. A most-distant adjacent projective dot pair represents an adjacent projective dot pair having a longest distance between two rows, which two nozzles corresponding to two projective dots thereof belong to, among the adjacent projective dot pairs. A spatial frequency, which is determined based on an appearance interval of the most-distant adjacent projective dot pair in the first direction, is lower than a spatial frequency corresponding to a peak value of a visual transfer function.

With this configuration, bandings corresponding to the most-distant adjacent projective dot pairs, which occur due to the inclined attachment angle of the inkjet head, can be made inconspicuous when the inkjet head having the nozzle plate set forth above is used as a line head. Accordingly, a preferable printing result can be obtained without demanding high accuracy in attachment of the inkjet head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

<Overall Structure of Head>

Description will be made about an inkjet head according to a first embodiment of the invention.FIG. 1is a perspective view of an inkjet head1according to this embodiment.FIG. 2is a sectional view taken on line II-II inFIG. 1. The ink-jet head1has a head body70for ejecting ink onto paper, and a base block71disposed above the head body70. The head body70has a rectangular planar shape extending in a main scanning direction. The base block71is a reservoir unit in which two ink reservoirs3are formed. The ink reservoirs3serve as ink flow paths from which ink is supplied to the head body70.

The head body70includes a flow path unit4in which ink flow paths are formed, and a plurality of actuator units21bonded to the upper surface of the flow path unit4by an epoxy-based thermosetting bonding agent. The flow path unit4and the actuator units21have a configuration in which a plurality of thin sheets are laminated and bonded to one another. In addition, a flexible printed circuit (FPC)50serving as a feeder member is bonded to the upper surface of each actuator unit21by solder, and led to left or right.

FIG. 3is a plan view of the head body70. As shown inFIG. 3, the flow path unit4has a rectangular planar shape extending in one direction (main scanning direction). InFIG. 3, a manifold flow path5provided in the flow path unit4and serving as a common ink chamber is depicted by the broken line. Ink is supplied from the ink reservoirs3of the base block71to the manifold flow path5through a plurality of openings3a. The manifold flow path5branches into a plurality of sub-manifold flow paths5aextending in parallel to the longitudinal direction of the flow path unit4.

Four actuator units21each having a trapezoidal planar shape are bonded to the upper surface of the flow path unit4. The actuator units21are arrayed zigzag in two lines so as to avoid the openings3a. Each actuator unit21is disposed so that its parallel opposite sides (upper and lower sides) extend in the longitudinal direction of the flow path unit4. Oblique sides of adjacent ones of the actuator units21overlap each other partially in the width direction of the flow path unit4.

The lower surface of the flow path unit4opposite to the bonded region of each actuator unit21serves as an ink ejection region where a large number of nozzles8(seeFIG. 6) are arrayed in a matrix. Pressure chamber groups9are formed in the surface of the flow path unit4opposite to the actuator units21. Each pressure chamber group9has rhomboid pressure chambers10(seeFIG. 6) arrayed in a matrix. In other words, each actuator unit21has dimensions ranging over a large number of pressure chambers10.

Returning toFIG. 2, the base block71is made of a metal material such as stainless steel. Each ink reservoir3in the base block71is a substantially rectangular hollow region formed to extend in the longitudinal direction of the base block71. The ink reservoir3communicates with an ink tank (not shown) through an opening (not shown) provided at its one end, so as to be always filled with ink. The ink reservoir3is provided with two pairs of openings3barranged in the extending direction of the ink reservoir3. The openings3bare disposed zigzag so as to be connected to the openings3ain the regions where the actuator units21are not provided.

A lower surface73of the base block71projects downward near the openings3bin comparison with their circumferences. The base block71abuts against the flow path unit4only in near-opening portions73aprovided near the openings3bin the lower surface73. Thus, any region of the lower surface73of the base block71other than the near-opening portions73ais separated from the head body70, and the actuator units21are disposed in these separated regions.

The base block71is fixedly bonded into a recess portion formed in the lower surface of a grip72aof a holder72. The holder72includes the grip72aand a pair of flat plate-like protrusions72bextending from the upper surface of the grip72ain a direction perpendicular to the upper surface so as to put a predetermined interval therebetween. Each FPC50bonded to the corresponding actuator unit21is disposed to follow the surface of the corresponding protrusion72bof the holder72through an elastic member83of sponge or the like. A driver IC80is disposed on the FPC50disposed on the surface of the protrusion72bof the holder72. The FPC50is electrically connected to the driver IC80and the actuator unit21of the head body70by soldering so that a driving signal output from the driver IC80can be transmitted to the actuator unit21.

A substantially rectangular parallelepiped heat sink82is disposed in close contact with the outside surface of the driver IC80so that heat generated in the driver IC80can be dissipated efficiently. A board81is disposed above the driver IC80and the heat sink82and outside the FPC50. Seal members84are put between the upper surface of the heat sink82and the board81and between the lower surface of the heat sink82and the FPC50respectively so as to bond them with each other.

FIG. 4is an enlarged view of the region surrounded with the one-dot chain line inFIG. 3. As shown inFIG. 4, in the flow path unit4opposite to the actuator units21, eight sub-manifold flow paths5aextend in parallel to the longitudinal direction of the flow path unit4. A large number of individual ink flow paths are connected to each sub-manifold flow path5aso as to extend from the outlet thereof to the corresponding nozzle8.FIG. 6is a sectional view showing an individual ink flow path. As is understood fromFIG. 6, each nozzle8communicates with the corresponding sub-manifold5athrough a pressure chamber10(here “pressure chamber10” designates a representative of the pressure chambers10a,10b,10cand10ddepicted inFIG. 4) and an aperture, that is, diaphragm13. In such a manner, in the head body70, an individual ink flow path7is formed for each pressure chamber10so as to extend from the outlet of the sub-manifold5ato the nozzle8through the aperture13and the pressure chamber10.

As is understood fromFIG. 6, the head body70has a laminated structure in which a total of 10 sheet materials of actuator unit21s, a cavity plate22, a base plate23, an aperture plate24, a supply plate25, manifold plates26,27and28, a cover plate29and a nozzle plate30are laminated. Of those sheet materials, the nine plates excluding the plate of the actuator units21constitute the flow path unit4.

In each actuator unit21, four piezoelectric sheets41-44(seeFIG. 8) are laminated, and electrodes are disposed, as will be described in detail later. Of the piezoelectric sheets41-44, only the uppermost layer is set as a layer (hereinafter referred to as “layer having an active portion”) having a portion serving as an active portion when an electric field is applied thereto. The other three layers are set as inactive layers having no active portion. The cavity plate22is a metal plate in which a large number of rhomboid holes for forming spaces of the pressure chambers10are provided within the range where the actuator unit21is pasted. The base plate23is a metal plate in which communication holes23aand23bare provided for each pressure chamber10of the cavity plate22so that the communication hole23amakes communication between the pressure chamber10and the aperture13while the communication hole23bmakes communication between the pressure chamber10and the nozzle8.

The aperture plate24is a metal plate in which for each pressure chamber10of the cavity plate22a communication hole between the pressure chamber10and the corresponding nozzle8is provided in addition to a hole which will serve as the aperture13. The supply plate25is a metal plate in which for each pressure chamber10of the cavity plate22a communication hole between the aperture13and the sub-manifold flow path5aand a communication hole between the pressure chamber10and the corresponding nozzle8are provided. Each of the manifold plates26,27and28is a metal plate in which for each pressure chamber10of the cavity plate22a communication hole between the pressure chamber10and the corresponding nozzle8is provided in addition to a corresponding sub-manifold flow path5a. The cover plate29is a metal plate in which for each pressure chamber10of the cavity plate22a communication hole between the pressure chamber10and the corresponding nozzle8is provided. The nozzle plate30is a metal plate in which a nozzle8is provided for each pressure chamber10of the cavity plate22.

The ten sheets21to30are aligned and laminated to one another so that individual ink flow paths7are formed as shown inFIG. 6. Each individual ink flow path7first leaves upward from the sub-manifold flow path5aand extends horizontally in the aperture13. Then the individual ink flow path7goes upward again and extends horizontally in the pressure chamber10again. After that, the individual ink flow path7turns obliquely downward so as to leave the aperture13for a while, and then turns vertically downward so as to approach the nozzle8.

As is apparent fromFIG. 6, the pressure chambers10and the apertures13are provided on different levels in the laminated direction of the respective plates. Consequently, in the flow path unit4opposite to the actuator units21, as shown inFIG. 4, an aperture13communicating with one pressure chamber10can be disposed in a position where it overlaps another pressure chamber10adjacent to the one pressure chamber10in plan view. As a result, the pressure chambers10are brought into close contact with one another and arrayed with high density. Thus, high-resolution image printing can be attained by the inkjet head1occupying a comparatively small area.

Escape grooves14for letting a surplus bonding agent out are provided in the upper and lower surfaces of the base plate23and the manifold plate28, the upper surfaces of the supply plate25and the manifold plates26and27and the lower surface of the cover plate29so as to surround the openings formed in the bonded surfaces of the respective plates. The presence of the escape grooves14can prevent variation in flow path resistance from being caused by projection of the adhesive agent into each individual ink flow path when the respective plates are bonded to one another.

<Details of Flow Path Unit>

Refer toFIG. 4again. A pressure chamber group9having a large number of pressure chambers10is formed within a range where each actuator unit21is attached. The pressure chamber group9has a trapezoidal shape substantially as large as the range where the actuator unit21is attached. Such a pressure chamber group9is formed for each actuator unit21.

As is apparent fromFIG. 4, each pressure chamber10belonging to the pressure chamber group9is configured to communicate with its corresponding nozzle8at one end of its long diagonal, and to communicate with the sub-manifold flow path5athrough the aperture13at the other end of the long diagonal. As will be described later, individual electrodes35(seeFIGS. 7 and 8) are arrayed in a matrix on the actuator unit21so as to be opposed to the pressure chambers10respectively. Each individual electrode35has a rhomboid shape in plan view and is one size smaller than the pressure chamber10. Incidentally, inFIG. 4, the nozzles8, the pressure chambers10, the apertures13, etc. which should be depicted by broken lines are depicted by real lines in order to making the drawing understood easily.

The pressure chambers10are disposed contiguously in a matrix in two directions, that is, an array direction A (first direction) and an array direction B (second direction). The array direction A is the longitudinal direction of the ink-jet head1, that is, the direction in which the flow path unit4extends. The array direction A is parallel to the short diagonal of each pressure chamber10. The array direction B is a direction of one oblique side of each pressure chamber10, which is at an obtuse angle θ with respect to the array direction A. The two acute angle portions of each pressure chamber10are located between two adjacent pressure chambers. Incidentally, the array direction A is parallel to the main scanning direction.

The pressure chambers10disposed contiguously in a matrix in the two directions, that is, the array direction A and the array direction B, are separated at an equal distance corresponding to 37.5 dpi from each other in the array direction A. In each actuator unit21, sixteen pressure chambers10are arranged in the array direction B.

The large number of pressure chambers10disposed in a matrix form a plurality of pressure chamber rows in parallel to the array direction A shown inFIG. 4. The pressure chamber rows are divided into a first pressure chamber row11a, a second pressure chamber row11b, a third pressure chamber row11cand a fourth pressure chamber row11din accordance with their relative positions to the sub-manifold flow path5ain view from a direction (third direction) perpendicular to a plane ofFIG. 4. Four sets of the first to fourth pressure chamber rows11a-11dare disposed periodically in order of11c,11d,11a,11b,11c,11d, . . . ,11bfrom the upper side of the actuator unit21toward the lower side thereof.

In the pressure chambers10aforming the first pressure chamber row11aand the pressure chambers10bforming the second pressure chamber row11b, the nozzles8are unevenly distributed on the lower side of the plane ofFIG. 4with respect to a direction (fourth direction) perpendicular to the array direction A in view from the third direction. The fourth direction is parallel to the sub scanning direction. Specifically, in each pressure chamber10a, the nozzle8is substantially opposite to the lower end acute angle portion of the pressure chamber10ain view from the third direction. In each pressure chamber10b, the nozzle8is opposite to a longitudinally central portion of a pressure chamber10cadjacent to the right lower of the lower end acute angle portion of the pressure chamber10bin view from the third direction. On the other hand, in the pressure chambers10cforming the third pressure chamber row11cand the pressure chambers10dforming the fourth pressure chamber row11d, the nozzles8are unevenly distributed on the upper side of the plane ofFIG. 4with respect to the fourth direction in view form the third direction. Specifically, in each pressure chamber10c, the nozzle8is opposite to a position separated slightly on the right upper from the upper end acute angle portion of the pressure chamber10cin view from the third direction. In each pressure chamber10d, the nozzle8is opposite to a portion near the longitudinally lower end of a pressure chamber10cadjacent to the right upper of the upper end acute angle portion of the pressure chamber10din view from the third direction.

In each of the first and fourth pressure chamber rows11aand11d, at least half the region of each pressure chamber10a,10doverlaps the sub-manifold flow path5ain view from the third direction. In each of the second and third pressure chamber rows11band11c, almost the whole region of each pressure chamber10b,10cdoes not overlap the sub-manifold flow path5ain view from the third direction. Accordingly, in any pressure chamber10belonging to any pressure chamber row, the width of the sub-manifold flow path5acan be expanded as much as possible to supply ink to each pressure chamber10smoothly while the nozzle8communicating with the pressure chamber10is prevented from overlapping the sub-manifold flow path5a.

FIG. 5Ais a schematic diagram showing only the nozzles formed in the nozzle plate30depicted inFIG. 4. As shown inFIG. 5A, a plurality of lines parallel to the array direction A are formed by the nozzles8. Here, a line formed by a plurality of nozzles8communicating with the pressure chambers10awill be referred to as a nozzle array row12a, a line formed by a plurality of nozzles8communicating with the pressure chambers10bwill be referred to as a nozzle array row12b, a line formed by a plurality of nozzles8communicating with the pressure chambers10cwill be referred to as a nozzle array row12c, and a line formed by a plurality of nozzles8communicating with the pressure chambers10dwill be referred to as a nozzle array row12d. A total of sixteen lines of the nozzle array rows12a-12care formed. The head row at the top of the plane ofFIG. 5Ais a nozzle array row12c, which is followed by fourteen rows12d,12a,12c,12b,12d,12a, . . . ,12aarranged periodically in that order toward the bottom of the plane. The tail row, that is, the sixteenth row is a nozzle array row12b.

In the inkjet head1according to this embodiment, think about two belt-like regions R11and R12adjacent to each other, each region R11, R12having a width (678.0 μm) corresponding to 37.5 dpi in the array direction A and extending in the fourth direction. In each belt-like region R11, R12, only one nozzle8is distributed to any row of the sixteen nozzle array rows12a-12dshown inFIG. 5A. That is, when such a belt-like region R11, R12is defined in any position within an ink ejection region corresponding to one actuator unit21, sixteen nozzles8are always distributed in the belt-like region R11, R12. The positions of projective dots P1, P2, . . . , and P16obtained by projecting the sixteen nozzles8from the fourth direction onto a virtual straight line L extending in the array direction A are separated at equally spaced intervals corresponding to 600 dpi, which is a resolution in printing.

Assume that sixteen nozzles8belonging to one belt-like region R11are numbered (1) to (16) respectively in order of increasing distance from the left end of projective dots obtained by projecting the sixteen nozzles8onto the virtual straight line L extending in the array direction A. The sixteen nozzles8(1), (2), (3), (4), . . . , and (16) are arranged in that order from the bottom. That is, as shown inFIG. 5A, the sixteen nozzles8are arranged substantially in a straight line from the left bottom to the right top in the belt-like region R11. In the following description, the array pattern of the nozzles8within the belt-like region R11will be referred to as an array pattern AP11. The array pattern AP11has a feature that the nozzle8located in the left end with respect to the array direction A belongs to the tail row, while the nozzle8located in the right end belongs to the head row.

Assume that sixteen nozzles8belonging to one belt-like region R12are numbered (1) to (16) respectively in order of increasing distance from the left end of projective dots obtained by projecting the sixteen nozzles8onto the virtual straight line L extending in the array direction A. The sixteen nozzles8(9), (8), (10), (7), (11), (6), (12), (5), (13), (4), (14), (3), (15), (2), (16) and (1) are arranged in that order from the bottom. That is, as shown inFIG. 5A, the sixteen nozzles8are arranged substantially in a downward-convex V-shape in the belt-like region R12. In the following description, the array pattern of the nozzles8within the belt-like region R12will be referred to as an array pattern AP12. The array pattern AP12has a feature that the nozzle8located in the left end with respect to the array direction A belongs to the head row, while the nozzle8located in the right end belongs to a row other than the tail row. In addition, the nozzle8in connection with the ninth projective dot from the left end belongs to the tail row, while the nozzle8in connection with the sixteenth projective dot from the left end, that is, the right end projective dot, belongs to the row adjacent to the head row on the tail row side.

The belt-like region R11and the belt-like region R12appear alternately. That is, the array pattern AP11and the array pattern AP12appear alternately with respect to the array direction A. Accordingly, in each nozzle array row12a-12d, the nozzles8having two kinds of predetermined intervals different from each other appear alternately.

As for any pair of projective dots adjacent to each other on the virtual straight line L in connection with nozzles8in the belt-like region R11, the nozzles8corresponding to the two projective dots belong to rows deviating from each other by only one row. On the other hand, as for any pair of projective dots adjacent to each other on the virtual straight line L in connection with nozzles8in the belt-like region R12, the nozzles8corresponding to the two projective dots belong to rows deviating from each other by two rows, except that the nozzles8corresponding to the projective dots P8and P9belong to rows deviating from each other by one row. That is, within the belt-like region R12having a V-shaped nozzle array, the nozzles8in connection with the projective dots on the left side are arranged in the array direction A with being displaced in turn from the left top of the plane (seeFIG. 5) toward the right bottom thereof. On the contrary, the nozzles8in connection with the projective dots on the right side are arranged in the array direction A with being displaced in turn from the left bottom of the plane toward the right top thereof likewise. The right side and left side center the projective dot P9corresponding to the nozzle8in the tail row. In a direction perpendicular to the array direction A, the nozzle8in connection with the projective dot P8is disposed adjacently to the projective dot P9. Further, in the head row direction, the nozzles8in connection with the projective dots on the right side of the projective dot P9and the nozzles8in connection with the projective dots on the left side of the projective dot P9are disposed alternately and in order of increasing distance from the nozzle8corresponding to the projective dot P9. As for all the projective dots on the virtual straight line L, of a plurality of adjacent projective dot pairs each comprised of two projective dots adjacent to each other on the virtual straight line L, an adjacent projective dot pair (most-distant adjacent projective dot pair) comprised of the projective dot P1corresponding to the left end of the belt-like region R1and the projective dot P16corresponding to the right end of the belt-like region R12are associated with two nozzles8belonging to two rows, which are the most distant from each other. The two nozzles8corresponding to the most-distant adjacent projective dot pair belong to rows deviating from each other by fourteen rows. The most-distant adjacent projective dot pair appears periodically in the array direction A. The appearance interval of the most-distant adjacent projective dot pair is a distance corresponding to 18.75 dpi (1356 μm), which is half as long as 37.5 dpi. The distance is expressed to be 0.74/mm (=1/1.356 mm) by spatial frequency.

In addition, as shown inFIG. 4, a large number of circumferential spaces15each having the same shape and same size as each pressure chamber10are arrayed in a straight line all over the long side of the paired parallel sides of the trapezoid of the pressure chamber group9in the head body70. The circumferential spaces15are defined by the actuator unit21and the base plate23closing holes formed in the cavity plate22and each having the same shape and the same size as each pressure chamber10. That is, no ink flow path is connected to any circumferential space15, and no individual electrode35to be opposed is provided in any circumferential space15. That is, there is no case that any circumferential space15is filled with ink.

On the other hand, in the head body70, a large number of circumferential spaces16are arrayed in a straight line all over the short side of the paired parallel sides of the trapezoid of the pressure chamber group9. Further, in the head body70, a large number of circumferential spaces17are arrayed in a straight line all over each oblique side of the trapezoid of the pressure chamber group9. Each of the circumferential spaces16and17penetrates the cavity plate22in a region of an equilateral triangle in plan view. No ink flow path is connected to any circumferential space16,17, and no individual electrode35to be opposed is provided in any circumferential space16,17. That is, in the same manner as the circumferential spaces15, there is no case that any circumferential space16,17is filled with ink.

Next, description will be made about the configuration of each actuator unit21. A large number of individual electrodes35are disposed in a matrix on the actuator unit21so as to have the same pattern as the pressure chambers10. Each individual electrode35is disposed in a position where the individual electrode35overlaps the corresponding pressure chamber10in plan view.

FIG. 7is a plan view of an individual electrode35. As shown inFIG. 7, the individual electrode35is constituted by a primary electrode region35aand a secondary electrode region35b. The primary electrode region35ais disposed in a position where the primary electrode region35aoverlaps the pressure chamber10, so that the primary electrode region35ais received in the pressure chamber10in plan view. The secondary electrode region35bis connected to the primary electrode region35aand disposed out of the pressure chamber10in plan view.

FIG. 8is a sectional view taken on line VII-VII inFIG. 7. As shown inFIG. 8, the actuator unit21includes four piezoelectric sheets41,42,43and44formed to have a thickness of about 15 μm equally. The piezoelectric sheets41-44are formed as continuous stratified flat plates (continuous flat plate layers) to be disposed over a large number of pressure chambers10formed within one ink ejection region in the head body70. When the piezoelectric sheets41-44are disposed as continuous flat plate layers over a plurality of pressure chambers10, the individual electrodes35can be disposed on the piezoelectric sheet41with high density, for example, by use of a screen printing technique. Accordingly, the pressure chambers10to be formed in positions corresponding to the individual electrodes35can be also disposed with high density. Thus, high-resolution images can be printed. The piezoelectric sheets41-44are made of a lead zirconate titanate (PZT) based ceramics material having ferroelectricity.

The primary electrode region35aof each individual electrode35formed on the piezoelectric sheet41which is the uppermost layer has a rhomboid planar shape which is substantially similar to the pressure chamber10as shown inFIG. 7. A lower acute angle portion in the rhomboid primary electrode region35ais extended to be connected to the secondary electrode region35bopposite to the outside of the pressure chamber10. A circular land portion36electrically connected to the individual electrode35is provided on the tip of the secondary electrode region35b. As shown inFIG. 8, the land portion36is opposed to a region of the cavity plate22where no pressure chamber10is formed. The land portion36is, for example, made of gold containing glass frit. The land portion36is bonded onto the surface of an extended portion of the secondary electrode portion35bas shown inFIG. 7. Although the FPC50is not shown inFIG. 8, the land portion36is electrically connected to a contact point provided in the FPC50. To establish this connection, it is necessary to press the contact point of the FPC50against the land portion36. Since no pressure chamber10is formed in the region of the cavity plate22opposed to the land portion36, the connection can be achieved surely by sufficient pressure.

A common electrode34having the same contour as the piezoelectric sheet41and having a thickness of about 2 μm is put between the piezoelectric sheet41which is the uppermost layer and the piezoelectric sheet42which is under the piezoelectric sheet41. The individual electrodes35and the common electrode34are made of a metal material such as Ag—Pd based metal material.

The common electrode34is grounded in a not-shown region. Consequently, the common electrode34is kept in constant potential or the ground potential in this embodiment equally over all the regions corresponding to all the pressure chambers10. In addition, the individual electrodes35are connected to a driver IC80through the FPC50including a plurality of lead wires which are independent of one another in accordance with the individual electrodes35. Thus, the potential of each individual electrode35can be controlled correspondingly to each pressure chamber10.

<Method for Driving Actuator Unit>

Next, description will be made about a method for driving each actuator unit21. The piezoelectric sheet41in the actuator unit21has a polarizing direction in the thickness direction thereof. That is, the actuator unit21has a so-called unimorph type configuration in which one piezoelectric sheet41on the upper side (that is, distant from the pressure chambers10) is set as a layer where an active portion exists, while three piezoelectric sheets41-43on the lower side (that is, close to the pressure chambers10) are set as inactive layers. Accordingly, when the individual electrodes35are set at positive or negative predetermined potential, each electric-field-applied portion between electrodes in the piezoelectric sheet41will act as an active portion (pressure generating portion) so as to contract in a direction perpendicular to the polarizing direction due to piezoelectric transversal effect, for example, if an electric field is applied in the same direction as the polarization.

In this embodiment, a portion between each primary electrode region35aand the common electrode34in the piezoelectric sheet41acts as an active portion which will generate a strain due to piezoelectric effect when an electric field is applied thereto. On the other hand, no electric field is applied from the outside to the three piezoelectric sheets42-44under the piezoelectric sheet41. Therefore, the three piezoelectric sheets42-44hardly serve as active portions. As a result, mainly the portion between each primary electrode region35aand the common electrode34in the piezoelectric sheet41contracts in a direction perpendicular to the polarizing direction due to piezoelectric transversal effect.

On the other hand, the piezoelectric sheets42-44are not affected by any electric field, they are not displaced voluntarily. Therefore, between the piezoelectric sheet41on the upper side and the piezoelectric sheets42-44on the lower side, there occurs a difference in strain in a direction perpendicular to the polarizing direction, so that the piezoelectric sheets41-44as a whole want to be deformed to be convex on the inactive side (unimorph deformation). In this event, as shown inFIG. 8, the lower surface of the actuator unit21constituted by the piezoelectric sheets41-44is fixed to the upper surface of the diaphragm (cavity plate)22which defines the pressure chambers. Consequently, the piezoelectric sheets41-44are deformed to be convex on the pressure chamber side. Accordingly, the volume of each pressure chamber10is reduced so that the pressure of ink increases. Thus, the ink is ejected from the corresponding nozzle8. After that, when the individual electrodes35are restored to the same potential as the common electrode34, the piezoelectric sheets41-44are restored to their initial shapes so that the volume of each pressure chamber10is restored to its initial volume. Thus, the pressure chamber10sucks ink from the sub-manifold flow path5a.

According to another driving method, each individual electrode35may be set at potential different from the potential of the common electrode34in advance. In this method, the individual electrode35is once set at the same potential as the common electrode34whenever there is an ejection request. After that, the individual electrode35is set at potential different from the potential of the common electrode34again at predetermined timing. In this case, the piezoelectric sheets41-44are restored to their initial shapes at the same timing when the individual electrode35has the same potential as that of the common electrode34, the volume of the pressure chamber10increases in comparison with its initial volume (in the state where the individual electrode35and the common electrode34are different in potential), so that ink is sucked into the pressure chamber10through the sub-manifold flow path5a. After that, the piezoelectric sheets41-44are deformed to be convex on the pressure chamber10side at the timing when the individual electrode35is set at different potential from that of the common electrode34. Due to reduction in volume of the pressure chamber10, the pressure on ink increases so that the ink is ejected. In the inkjet head1described above, the actuator units21are driven suitably in accordance with the conveyance of a printing medium. Thus, characters, graphics, etc. can be drawn with a resolution of 600 dpi.

<Example of Operation in Printing>

As an example of operation in printing, description will be made about a case where a straight line extending in the array direction A is printed with a resolution of 600 dpi. Here, assume that a printing medium is conveyed from the bottom side to the top side inFIG. 5Awith respect to the head body70. In accordance with the conveyance of the printing medium, the sixteen nozzles8in the belt-like region R11are operated as follows. That is, the nozzle8(1) belonging to the bottom nozzle array row12binFIG. 5Aejects ink first, and the nozzle8belonging to the row just above the bottom nozzle array row12bis next selected to eject ink. In such a manner, the nozzles8(2), (3) and (4) are selected to eject ink in turn. In this event, the nozzle position is displaced in the array direction A by a fixed distance whenever the selected nozzle array row is moved from the lower side to the upper side by one nozzle array row. Accordingly, within a range corresponding to the belt-like region R11, ink dots are formed adjacently to one another at equally spaced intervals of 600 dpi sequentially toward the right in the array direction A.

On the other hand, the sixteen nozzles8in the belt-like region R12are operated in accordance with the conveyance of the printing medium as follows. That is, the nozzle8arrayed in the bottom nozzle array row12binFIG. 5Aejects ink first, and the nozzle8arrayed in the row just above the bottom nozzle array row12bis next selected to eject ink. In such a manner, the nozzles8are selected to eject ink in turn. In this event, the displacement of the nozzle position in the array direction A whenever the selected nozzle array row is moved from the lower side to the upper side by one nozzle array row is not fixed. Accordingly, within a range corresponding to the belt-like region R12, the intervals between ink dots formed sequentially in the array direction A in accordance with the conveyance of the printing medium are not fixed to 600 dpi.

That is, as shown inFIG. 5A, in accordance with the conveyance of the printing medium, ink is ejected first from the nozzle8(9) arrayed in the bottom nozzle array row12binFIG. 5A, so that a dot array is formed on the printing medium. After that, in accordance with the conveyance of the printing medium, the position where a straight line should be formed reaches the position of the nozzle8(8) arrayed in the second nozzle array row12afrom the bottom, and ink is ejected from the nozzle8(8). As a result, a second ink dot is formed at a position displaced from the first formed dot position to the left side in the array direction A by an interval corresponding to 600 dpi.

Next, in accordance with the conveyance of the printing medium, the position where a straight line should be formed reaches the position of the nozzle8(10) arrayed in the third nozzle array row12dfrom the bottom, and ink is ejected from the nozzle8(10). As a result, a third ink dot is formed at a position displaced from the first formed dot position to the right side in the array direction A by an interval corresponding to 600 dpi. Further, in accordance with the conveyance of the printing medium, the position where a straight line should be formed reaches the position of the nozzle8(7) arrayed in the fourth nozzle array row12bfrom the bottom, and ink is ejected from the nozzle8(7). As a result, a fourth ink dot is formed at a position displaced from the first formed dot position to the left side in the array direction A by a distance twice as long as an interval corresponding to 600 dpi. Further, in accordance with the conveyance of the printing medium, the position where a straight line should be formed reaches the position of the nozzle8(11) arrayed in the fifth nozzle array row12cfrom the bottom, and ink is ejected from the nozzle8(11). As a result, a fifth ink dot is formed at a position displaced from the first formed dot position to the right side in the array direction A by a distance twice as long as an interval corresponding to 600 dpi.

In such a manner, the nozzles8are selected in turn from one located at the bottom inFIG. 5Ato one located at the top inFIG. 5A, so that ink dots are formed. In this event, on the assumption that N designates the number suffixed to each nozzle8shown inFIG. 5A, the nozzle8(N) forms an ink dot at a position displaced from the first formed dot position in the array direction A by a distance corresponding to (scale n(=N−9))×(interval corresponding to 600 dpi). A positive sign of the scalendesignates displacement to the right side in the array direction A, and a negative sign of the scalendesignates displacement to the left side in the array direction A. When the selection of the sixteen nozzles8is terminated finally, seven dots are formed on the right side in the array direction A with respect to the ink dot formed by the nozzle8(9) in the bottom nozzle array row12binFIG. 5Aso as to be separated at intervals corresponding to 600 dpi. On the other hand, eight dots are formed on the left side in the nozzle array row12blikewise. When a nozzle8in the belt-like region R11belongs to the same row as a nozzle8in the belt-like region R12, the nozzles8eject ink concurrently. As a result, a straight line extending in the array direction A with a resolution of 600 dpi as a whole can be drawn.

Incidentally, each of the neighborhoods of the opposite end portions (oblique sides of the actuator unit21) in the array direction A of each ink ejection region has a correlation with the neighborhood of an opposed one of the opposite end portions in the array direction A of an ink ejection region corresponding to another actuator unit21opposed in the width direction of the head body70. Thus, printing with a resolution of 600 dpi can be performed continuously in the array direction A using the two actuator units21.

As another example of operation in printing, description will be made about the case where a large number of straight lines extending in the sub-scanning direction (fourth direction) are printed adjacently to one another at equally spaced intervals of 600 dpi. In this case, any nozzle8belonging to any belt-like region R11, R12ejects ink sequentially at short ejection intervals.FIG. 5Bshows an example of printing when the inkjet head1is attached with high accuracy so that the inkjet head1hardly tilts. Such a range where a large number of straight lines have been printed with a resolution of 600 dpi is observed as if it were a filled region. Here, such a range is illustrated as a set of a large number of lines for the sake of explanation. As is also understood fromFIG. 5B, no banding appears in the print surface in this case.

FIG. 5Cshows an example of printing when the attachment angle of the inkjet head1is slightly inclined so that the sub-scanning direction and the array direction A do not cross at right angles. In this case, as is also understood fromFIG. 5C, bandings91appear in the print surface. The bandings91appear at positions corresponding to the most-distant adjacent projective dot pairs. Accordingly, the appearance interval of the bandings91is a distance corresponding to 18.75 dpi, which is equal to the interval of the most distant projective dot pairs in the array direction A. The bandings91appear thus the positions corresponding to the most-distant adjacent projective dot pairs for the following reason. When the attachment angle of the inkjet head1is inclined, the distance between adjacent two of printed straight lines increases as rows, which two nozzles corresponding to two projective dots adjacent to each other belong to, are more distant from each other.

FIG. 9shows a graph drawing a visual transfer function which is a function expressing the relationship between a spatial frequency depending on the appearance interval of bandings and the human sensitivity of visual recognition to the spatial frequency. The visual transfer function (VTF) curve depicted inFIG. 9is obtained from the expression

VTF=5.05×exp⁡(-0.138×x×f×π/180)×{1-exp⁡(-0.1×x×f×π/180)}
wherexdesignates the observation distance andfdesignates the spatial frequency. InFIG. 9, the visual transfer function is calculated with assuming that x=30 cm.

In the visual transfer function shown inFIG. 9, the sensitivity reaches a peak value when the spatial frequency is about 1/mm. That is, banding is the most conspicuous when the spatial frequency thereof is about 1/mm. As the spatial frequency is lower or higher than 1/mm, the sensitivity of visual recognition becomes lower, and the banding becomes more inconspicuous.

In this embodiment, the spatial frequency of the most-distant adjacent projective dot pairs and the spatial frequency of the bandings91corresponding thereto are about 0.74/mm (=1/1.356 mm). At this time, the value of sensitivity of the visual transfer function is about 0.9 on the assumption that the value is 1 when the spatial frequency is 1/mm. Thus, the bandings formed on a printing medium can be made more inconspicuous than those in the spatial frequency 1/mm. As a result, a preferred printing result in which visual deterioration in image quality is suppressed can be obtained without attaching the inkjet head1with high accuracy. In addition, the cost required for attaching the inkjet head1can be reduced so that a printer can be manufactured at a low cost.

Particularly, in this embodiment, two nozzles8corresponding to two projective dots forming each most distant adjacent projective pair belong to two lines which are outermost rows (head row and tail row) of sixteen lines. Therefore, bandings are apt to occur even when the head tilts slightly. It is, however, possible to make the bandings inconspicuous even in such a case.

The appearance interval of the most-distant adjacent projective dot pairs in the array direction A is a distance twice as long as the width (37.5 dpi) of each belt-like region R11, R12. Accordingly, the spatial frequency of the bandings91caused by the inclined attachment angle of the inkjet head1can be lowered on a large scale. As a result, the bandings can be made more inconspicuous.

Further, a large number of nozzles8are arrayed in each nozzle array row12a-12dso that two kinds of predetermined intervals different from each other appear alternately. Accordingly, each array of nozzles8has regularity so that it becomes easy to manufacture the inkjet head land particularly to manufacture the nozzle plate30in which the nozzles8are formed.

In view of making the bandings inconspicuous, it is preferable that the spatial frequency of the bandings91is made smaller than about 0.74/mm. For example, it is preferable that the spatial frequency is not higher than about 0.65/mm (spatial frequency corresponding to 80% of the sensitivity peak value), and it is more preferable that the spatial frequency is not higher than about 0.5/mm (spatial frequency corresponding to 70% of the sensitivity peak value). To make the spatial frequency of the bandings91lower, the appearance interval of the most-distant adjacent projective dot pairs may be increased.

Second Embodiment

Next, description will be made about a second embodiment of the invention. The configuration of an inkjet head according to this embodiment is similar to that in the first embodiment and the same as the configuration shown inFIGS. 1-8, except the arrays of nozzles. The following description will be made focusing on difference between the both, and redundant description will be omitted to the utmost.

FIG. 10Ais a schematic view showing arrays of nozzles8formed in a nozzle plate30, correspondingly toFIG. 5Aof the first embodiment. A large number of nozzles8are arrayed on sixteen nozzle array rows12a-12dparallel to the array direction A in the same manner as in the first embodiment.

Think about three belt-like regions R21, R22and R23adjacent to one another, each region R21, R22, R23having a width (678.0 μm) corresponding to 37.5 dpi in the array direction A and extending in a direction (fourth direction) perpendicular to the array direction A. In each belt-like region R21, R22, R23, only one nozzle is disposed in each of sixteen nozzle array rows12a-12dshown inFIG. 10A. That is, when such a belt-like region R21, R22, R23is delimited in any position within an ink ejection region corresponding to one actuator unit21, sixteen nozzles8are always disposed in each of the belt-like region R21, R22, R23. The positions of projective dots P1, P2, . . . , and P16obtained by projecting the sixteen nozzles8from the fourth direction onto a virtual straight line L extending in the array direction A are separated at equally spaced intervals corresponding to 600 dpi, which is a resolution in printing.

Assume that sixteen nozzles8belonging to one belt-like region R21are numbered (1) to (16) respectively in order of increasing distance from the left end of projective dots obtained by projecting the sixteen nozzles8onto the virtual straight line L extending in the array direction A. The sixteen nozzles (16), (15), (14), (13), . . . , and (1) are arranged in that order from the bottom. That is, as shown inFIG. 10A, the nozzles8are arranged substantially in a straight line from the left top to the right bottom in the belt-like region R21. In the following description, the array pattern of the nozzles8within the belt-like region R21will be referred to as an array pattern AP21.

Assume that sixteen nozzles8belonging to one belt-like region R22are numbered (1) to (16) respectively in order of increasing distance from the left end of projective dots obtained by projecting the sixteen nozzles8onto the virtual straight line L extending in the array direction A. The sixteen nozzles8(16), (15), (14), (13), (12), (11), (10), (9), (1), (2), (3), (4), (5), (6), (7) and (8) are arranged in that order from the bottom. That is, as shown inFIG. 10A, the eight nozzles8(1) to (8) in the left upper portion of the belt-like region R22are arranged substantially in a straight line from the left bottom to the right top, while the eight nozzles8(9) to (16) in the right lower portion of the belt-like region R22are arranged substantially in a straight line from the right bottom to the left top. The relative positions of the eight nozzles8(9) to (16) in the right lower portion of the belt-like region R22are the same as the relative positions of the eight nozzles8(9) to (16) in the right lower portion of the belt-like region R21respectively. On the other hand, the array of sixteen nozzles8belonging to one belt-line region R23is similar to that in the belt-like region R22. In the following description, the array pattern of the thirty-two nozzles8distributed in the belt-like regions R22and R23will be referred to as an array pattern AP22.

The belt-like regions R21, R22and R23are formed repeatedly and regularly in order of R21, R22, R23, R21, R22, R23. . . That is, the array pattern AP21and the array pattern AP22appear alternately in the array direction A. Accordingly, nozzles8appear at equally spaced intervals on each of lower eight nozzle array rows of the sixteen nozzle array rows, while nozzles8appear at two kinds of predetermined intervals different from each other on each of upper eight nozzle array rows of the sixteen nozzle array rows.

As for any pair of projective dots adjacent to each other on the virtual straight line L in connection with nozzles8in the belt-like region R21, the nozzles8corresponding to the two projective dots belong to rows deviating from each other by only one row. On the other hand, as for any pair of projective dots adjacent to each other on the virtual straight line L in connection with nozzles8in the belt-like region R22or R23, the nozzles8corresponding to the two projective dots belong to rows deviating from each other by one line, except that the nozzles8corresponding to the projective dots P8and P9belong to rows deviating from each other by eight rows. In addition, as for an adjacent projective dot pair of the projective dot P16corresponding to the right end of the belt-like region R21and the projective dot P1corresponding to the left end of the belt-like region R22and an adjacent projective dot pair of the projective dot P16corresponding to the right end of the belt-like region R22and the projective dot P1corresponding to the left end of the belt-like region R23, two corresponding nozzles8belong to rows deviating from each other by eight rows. As for all the projective dots on the virtual straight line L, of a plurality of adjacent projective dot pairs each comprised of two projective dots adjacent to each other on the virtual straight line L, an adjacent projective dot pair (most-distant adjacent projective dot pair) comprised of the projective dot P1corresponding to the left end of the belt-like region R21and the projective dot P16corresponding to the right end of the belt-like region R23are associated with two nozzles8belonging to two rows, which are the most distant from each other. The two nozzles8corresponding to the most-distant adjacent projective dot pair belong to rows deviating from each other by fourteen rows. Such most-distant adjacent projective dot pairs appear periodically in the array direction A. The appearance interval of the most-distant adjacent projective dot pairs is a distance corresponding to 12.5 dpi (=2034 μm), which is one third of 37.5 dpi. This distance is expressed to be 0.49/mm (=1/2.034 mm) by spatial frequency.

As an example of operation in printing, description will be made about a case where a straight line extending in the array direction A is printed with a resolution of 600 dpi. In accordance with the conveyance of the printing medium, the sixteen nozzles8in the belt-like region21are operated as follows. That is, the nozzle8(16) belonging to the bottom nozzle array row12binFIG. 10Aejects ink first, and the nozzle8belonging to the row just above the bottom nozzle array row12bis next selected to eject ink. In such a manner, the nozzles8(15), (14) and (13) are selected to eject ink in turn. In this event, the nozzle position is displaced in the array direction A by a fixed distance whenever the selected nozzle array row is moved from the lower side to the upper side by one nozzle array row. Accordingly, within a range corresponding to the belt-like region R21, ink dots are formed adjacently to one another at equally spaced intervals of 600 dpi sequentially toward the right in the array direction A.

On the other hand, the sixteen nozzles8in each belt-like region22,23are operated in accordance with the conveyance of the printing medium as follows. That is, the nozzle8(16) arrayed in the bottom nozzle array row12binFIG. 10Aejects ink first, and the nozzle8arrayed in the row just above the bottom nozzle array row12bis next selected to eject ink. In such a manner, the nozzles8are selected to eject ink in turn. In this event, before reaching the nozzle8(9), the nozzle position is displaced to the left side in the array direction A by an interval corresponding to 600 dpi whenever the selected nozzle array row is moved from the lower side to the upper side by one nozzle array row. However, in a range from the nozzle8(9) to the nozzle8(1), the nozzle position is displaced to the left side in the array direction A by a distance corresponding to 8× (interval corresponding to 600 dpi). After that, the nozzle position is displaced to the right side in the array direction A by an interval corresponding to 600 dpi whenever the selected nozzle array row is moved from the lower side to the upper side by one nozzle array row. When nozzles8in the belt-like regions R21, R22and R23belong to one and the same row, the nozzles8eject ink concurrently. As a result, a straight line extending in the array direction A with a resolution of 600 dpi as a whole can be drawn.

As another example of operation in printing, description will be made about the case where a large number of straight lines extending in the sub-scanning direction (fourth direction) are printed adjacently to each other at equally spaced intervals of 600 dpi. In this case, any nozzle8belonging to each belt-like region R21, R22, R23ejects ink sequentially at short ejection intervals.FIG. 10Bshows an example of printing when the inkjet head1is attached with high accuracy so that the inkjet head1hardly tilts. Such a range where a large number of straight lines have been printed with a resolution of 600 dpi is observed as if it were a filled region. Here, such a range is illustrated as a set of a large number of lines for the sake of explanation. As is also understood fromFIG. 10B, no banding appears in the print surface in this case.

FIG. 1Cshows an example of printing when the attachment angle of the inkjet head1is slightly inclined so that the sub-scanning direction and the array direction A do not cross at right angles. In this case, as is also understood fromFIG. 10C, bandings92appear in the print surface. The bandings92appear in positions corresponding to the most-distant adjacent projective dot pairs. Accordingly, the appearance interval of the bandings92is a distance corresponding to 12.5 dpi, which is equal to the interval of the most distant projective dot pairs. In this embodiment, therefore, the spatial frequency of the most distant adjacent projective dots and the spatial frequency of the bandings92corresponding thereto are about 0.49/mm. In this event, with reference toFIG. 9, the value of sensitivity of the visual transfer function is about 0.65 on the assumption that the value is 1 when the spatial frequency is 1/mm. Thus, the bandings formed on a printing medium can be made much more inconspicuous than those in the spatial frequency 1/mm. As a result, a preferable printing result in which visual deterioration in image quality is suppressed can be obtained without attaching the inkjet head1with high accuracy.

Particularly, in this embodiment, two nozzles8corresponding to two projective dots forming each most distant adjacent projective pair belong to two rows, which are outermost rows (head row and tail row) of sixteen rows. Bandings are apt to occur even when the head tilts slightly. It is, however, possible to make the bandings inconspicuous even in such a case.

The appearance interval of the most-distant adjacent projective dot pairs in the array direction A is a distance three times as long as the width (37.5 dpi) of each belt-like region R21, R22, R23. Accordingly, the spatial frequency of the bandings92caused by the inclined attachment angle of the inkjet head1can be lowered on a large scale. As a result, the bandings can be made more inconspicuous.

Further, the nozzle array rows12a-12dinclude rows in which a large number of nozzles8are arrayed so that two kinds of predetermined intervals different from each other appear alternately, and rows in which a plurality of nozzles8are arrayed at equally spaced intervals. Each array of nozzles8has regularity thus so that it becomes easy to manufacture the inkjet head1and particularly to manufacture the nozzle plate30in which the nozzles8are formed.

Description has been made above about the preferred embodiments of the invention. However, the invention is not limited to the aforementioned embodiments. Various changes on design can be made on the invention within the scope stated in claims. For example, the array patterns of nozzles are not limited to those in the aforementioned first and second embodiments. Any change can be made only if the spatial frequency depending on the appearance period of bandings corresponding to the appearance interval of the most-distant adjacent projective dot pairs is lower than a value corresponding to a peak value of the visual transfer function. Also, the visual transfer function may be calculated with assuming that the observation distance x is equal to or less than 30 cm. Dotted lines shown inFIG. 12shows a visual transfer function with assuming that the observation distance x=20 cm. In this case, the visual transfer function takes a peak value at a spatial frequency about 1.5/mm. On the other hand, the visual transfer function takes about 0.8 at a spatial frequency 0.74/mm (embodiment 1) and about 0.3 at a spatial frequency 0.49/mm (embodiment 2). Thus, when the observation distance x is 20 cm, the embodiments of the invention can also make the bandings formed on a printing medium more inconspicuous than those in the spatial frequency 1.5/mm. _Further, the shapes of flow paths, the shapes of pressure chambers, etc. may be changed suitably.

Also, in the above-described embodiments, a spatial frequency [1/mm] is used as criteria. The spatial frequency can be transformed into a viewing angle ω as follows.

TABLE 1Spatial frequency f [1/mm]Viewing angle ω [1/degree]1.005.2360.76 (embodiment 1)3.9790.49 (embodiment 2)2.566(x = 30 cm)
It is apparent from Table 1 that if the viewing angle ω is equal to less than 4.0 (1/degree), the same effect can be achieved as with a case where the spatial frequency is equal to or less than 0.76 (1/mm).

The aforementioned first and second embodiments have been described about the case where the appearance interval of the most-distant adjacent projective dot pairs in the array direction A is an integral multiple of the width in the array direction A of each belt-like region in which one nozzle is disposed in each of sixteen nozzle array rows. The invention is not limited to the case. Accordingly, the appearance interval of the most-distant adjacent projective dot pairs in the array direction A does not have to be an integral multiple of the width of the belt-like region. When the appearance interval is set as an integral multiple, it is not limited to two or three times. It may be set as four or more times.

The aforementioned first and second embodiments have been described about the case where the nozzle array on each nozzle array row has regularity. However, the nozzle array does not have to have regularity. The nozzle array rows may be arrayed at equally spaced intervals.

Also, in the first embodiment, the two belt-like regions R11and R12, which are different in the array pattern of the nozzles8, appear alternately. However, from the view point of making the banding occurring at a boundary between different belt-like regions further inconspicuous, a combination (array pattern group) of a single array pattern AP11and plural array patterns AP12may be repeated in the array direction A. This modification is similar to the second embodiment in that plural array patterns are repeated. In addition to this similarity, this modification has a feature that the nozzle8(16) located at one end of the array pattern AP12in the array direction A and the nozzle8(1) located at the other end of the array pattern AP12in the array direction A belong to rows adjacent to each other, respectively. Thus, there is no fear that banding may occur at a boundary between the array patterns AP12and AP12.

Furthermore, one of the nozzles8(1), (16) located at both ends of the belt-like region R12in the array direction A belongs to the head row. Also, the nozzles8(10), (11), (16) belonging to (2n−1)th rows (n is a natural number) counted from the head row (that is, 2n-th rows counted from the tail row) are arranged on the right side of the nozzle (9) belonging to the tail row. On the other hand, the nozzles8(1), (2), (8) belonging to 2n-th rows counted from the head row (that is, (2n−1)th rows counted from the tail row) are arranged on the left side of the nozzle8(9) belonging to the tail row. With this configuration, in a range where the array patterns AP12are repeated, any of the nozzles8corresponding to two projective dots, which are adjacent to each other on the virtual straight line L, belong rows adjacent to each other or rows spaced at a single row therebetween. Accordingly, there is no fear that banding occurs even at a position other than the boundary between the array patterns AP12and AP12.

Also, since the array pattern group has plural (three or more) array patterns, the banding occurring at a boundary between different array patterns can be made more inconspicuous.