JPH09277504A - Ink jet printing method and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform ink jet printing of high quality. SOLUTION: In an ink jet printing apparatus scanning a printing head 60 jetting ink from a plurality of nozzles along the surface of a printing medium and feeding the printing medium in the direction crossing the scanning direction of the printing head at a right angle, feed means 20-40 stepwise feeding the printing medium by the distance corresponding to 1/n (n: integer) printing width per one scanning of the printing head 50 and a feed control part RCC are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inkjet printing method and apparatus, and more particularly to an improved inkjet printing method and apparatus for performing high quality image formation.

[0002]

2. Description of the Related Art When performing color drawing with an inkjet printing apparatus, for example, Y (yellow), M (magenta), C
Inks of four primary colors of (cyan) and K (black) are used. Ink heads (hereinafter simply referred to as “heads”) are provided for each of the four primary color inks, and various colors are expressed by combinations of ink dots ejected from the respective heads. The combination of ink dot colors and the pattern formed by them are determined by the drawing data supplied from the control device (computer).

The head scans the surface of the print medium while ejecting ink at a predetermined dot timing. The ejection timing is determined based on the output signal of the encoder that represents the scanning position of the head.

FIG. 14 schematically shows such a situation. As shown in FIG. 14, the ejection timing of each head is determined by an encoder signal, and each ink dot is printed (printed) at the same position p1, p2, p3, ... In the scanning direction.

For example, an encoder signal is output for each head moving distance of 70 μm (moving time of 100 μs), ink is ejected from each head based on the encoder signal, and ink dots are formed at a pitch of 70 μm. This makes 360d
Full-color printing with a resolution of pi (dot per inch) is performed.

In practice, the ink is ejected by the operation of the electronic circuits that drive the heads based on the encoder signals. The head has a plurality of nozzles and prints a band area corresponding to the loci of the plurality of nozzles in one scan. Each time the head is scanned, the print medium or the head is step-fed by the width of the band area (printing width). In this way, printing on the print medium proceeds sequentially.

When it is necessary to increase the print density, the same band region is scanned twice and the same pattern is printed twice. This makes it possible to print with high density. At this time, the print medium or the head is fed by one step for every two scans of the head.

[0008]

Due to an error in the step feed amount of the print medium, an edge overlap or a gap may occur between adjacent band areas. When printing is performed with high density by double-strike, the overlapped part is equivalent to 4-strike and the density is higher, and the gap is whiter than the double-strike part. Becomes more prominent.

Therefore, the boundary portion between the adjacent band areas appears as a streak. This streak is called an inter-line streak or banding. This line-to-line streak appears as an unnecessary striped pattern on the printed pattern and deteriorates the quality of drawing.

The present invention has been made to solve the above problems, and an object thereof is to realize an ink jet printing method and apparatus for performing high quality printing.
Another object of the present invention is to realize an ink jet printing method and apparatus for performing high quality printing.

[0011]

Means for Solving the Problem (1) A first invention for solving the problem is to scan a printer head, which ejects ink from a plurality of nozzles, along a surface of a print medium, and to print the print medium and the print medium. In an inkjet printing method in which at least one of the printer heads is relatively conveyed in a direction orthogonal to the scanning direction of the printer head, the relative conveyance of at least one of the print medium and the printer head The inkjet printing method is characterized by performing stepwise steps by a distance corresponding to 1 / n (n: an integer of 2 or more) of the printing width formed by the trajectory of the plurality of nozzles per scan.

According to the first invention for solving the problem, the print medium is conveyed stepwise by a distance of 1 / n of the printing width per scan of the printer head. By using n times of multiple printing according to the above, it is possible to make the line streaks inconspicuous, and thereby to realize an inkjet printing method for performing high-quality printing.

In the first invention for solving the problem, it is preferable that n is 2 in order to minimize the increase in printing time. Further, in the first invention for solving the problem, it is preferable to perform multi-gradation printing on the print medium in order to obtain a high-quality print.

(2) A second invention for solving the problem is to scan a printer head for ejecting ink from a plurality of nozzles along the surface of a print medium and at least one of the print medium and the printer head. In an inkjet printing apparatus that relatively conveys a print medium in a direction orthogonal to a scanning direction of the printer head, relative conveyance of at least one of the print medium and the printer head is performed by the plurality of nozzles per scan of the printer head. 1 / n (n: 2) of the printing width formed by the locus of
The inkjet printing apparatus is characterized in that the inkjet printing apparatus is provided with a conveying means for performing stepwise operation by a distance corresponding to the above integer).

According to the second aspect of the invention for solving the problem, the print medium is conveyed stepwise by a distance of 1 / n of the printing width per scan of the printer head. With the multi-printing performed n times, the line streaks can be made inconspicuous, whereby an ink jet printing apparatus for high-quality printing can be realized.

In the second invention for solving the problem, it is preferable that n is 2 in order to minimize the increase in printing time. Further, in the second invention for solving the problem, it is preferable to perform multi-gradation printing on the print medium in order to obtain high quality print.

Further, in the second invention for solving the problem, the printer head has a pair of unit heads for respectively ejecting inks of the same color having different densities to perform multi-gradation printing. It is preferable in terms.

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a block diagram of an inkjet printing apparatus. This device is an example of an embodiment of the present invention. The configuration of the present apparatus shows an example of the embodiment of the apparatus of the present invention. The operation of the apparatus shows an example of an embodiment of the method of the present invention.

In FIG. 1, the print medium 11 unwound from the original roll 10 is conveyed in the direction of the arrow by the rotation of the rollers 20 and 30. Laura 2
0 and 30 have a cylindrical shape. The rollers 20 and 30 have a length exceeding the width of the print medium 11 in the axial direction (direction perpendicular to the paper surface of FIG. 1).

The print medium 11 is an example of an embodiment of the print medium of the present invention, and is, for example, cloth.
The fabric may be a fabric of either natural or synthetic fibers. Of course, the print medium 11 is not limited to cloth, and may be paper, plastic sheet, or the like.

The pinch roller 40 presses the print medium 11 against the roller 20. The pinch roller 40 is also a cylindrical roller having the same length as the roller 20.
The conveyance force of the roller 20 is accurately transmitted to the print medium 11 by being pressed by the pinch roller 40.

The print medium 11 is conveyed at a conveying speed corresponding to the rotation speed of the roller 20. The roller 30 rotates at the same speed as the roller 20 and conveys the print medium 11.
The conveyance of the print medium 11 by the rollers 20 and 30 is controlled by the conveyance control unit RCC. Rollers 20 and 3
A drawing table 50 is provided between 0. The print medium 11 is conveyed on the drawing table 50.

The rollers 20, 30, the pinch roller 40, and the conveyance control unit RCC are an example of an embodiment of the conveyance means in the present invention. The printer head 60 faces the print medium 11 on the drawing table 50 and ejects ink to print on the print medium 11. The printer head 60 is an example of an embodiment having a plurality of printer heads according to the present invention.

The printer head 60 is the head carriage 6
1 The head carriage 61 is configured, for example, in a box shape, and accommodates therein an electronic circuit for a head drive, which will be described in detail later. Printer head 60
Ink ejection from the printer is controlled by the printing control unit PRC.

The head carriage 61 is a scanning control unit SCC.
The surface of the print medium 11 is controlled to be scanned in the direction perpendicular to the paper surface of FIG. When the head carriage 61 does not scan, the print medium 11
Stand by at the standby position (home position) off the surface of.

A linear encoder 70 is provided over the entire scanning range of the head carriage 61 including the home position. An optical sensor 62 is provided on the head carriage 61 corresponding to the linear encoder 70, and a scanning position signal (encoder signal) of the head carriage 61 is generated. The encoder signal is input to the printing control unit PRC.

The computer CPU is a printing control unit PRC,
The scanning control unit SCC and the conveyance control unit RCC are controlled to perform a printing operation. The display unit DIS on the computer CPU
And the operation unit OPC is connected. The operator uses these to operate the computer CPU in an interactive manner to perform a desired printing operation.

FIG. 2 shows an arrangement of unit heads in the printer head 60. FIG. 2 is a view of the printer head 60 as seen from the ink ejection nozzles. In FIG. 2, the x direction is the scanning direction of the printer head 60, and the y direction is the transport direction of the print medium 11.

In FIG. 2, each of the yellow head Y, the magenta head M, the cyan head C and the black head K is a rectangular unit head having the same size, and they are arranged in a diagonal direction.

The nozzle surface of the unit head is, for example, as shown in FIG. In FIG. 3, the nozzle NZL is 32
They are formed in two rows each. The pitch of the nozzles in each row is, for example, 140 μm, and the two rows are formed with a half pitch offset. As a result, 70μ as a whole
64 nozzles of m pitch are formed. That is, 3
A head is constructed that provides a print resolution of 60 dpi. Here, the 64 nozzles are an example of the embodiment of the plurality of nozzles in the present invention.

FIG. 4 shows a block diagram of the ink ejection control system. In FIG. 4, the computer CPU writes the drawing data in the parallel / serial converters KPS, MPS, CPS and YPS.

These parallel / serial converters are
S is a parallel / serial converter for black drawing data, MPS is for magenta drawing data, CPS is for cyan drawing data, and YPS is for yellow drawing data.

The drawing data is drawing data representing a dot pattern of each unit color. The dot pattern drawing data of each unit color is written in the corresponding parallel / serial converter by the address control circuit ADC controlled by the computer CPU.

The drawing data is composed of, for example, 64-bit parallel drawing data for each color. The 64 bits correspond to 64 nozzles of the unit head, and specify the ON / OFF of ink ejection for each nozzle.

The ejection / transfer control circuit CNT is an optical sensor 62.
Encoder signal given from the computer CPU
Parallel-serial converters KPS to YPS based on the control signal from the clock source and the clock signal from the clock source OSC.
Control.

Parallel / serial converters KPS to YPS
Under the control of the ejection / transfer control circuit CNT, the drawing data is converted into parallel / serial data and transferred to the corresponding heads KHD, MHD, CHD and YHD.

Injection / transfer control circuit CNT, clock source O
The SC, the address control circuit ADC, and the parallel / serial converters KPS to YPS are included in the printing control unit PRC. The head KHD is a black (K) head, the head MHD is a magenta (M) head, the head CHD is a cyan (C) head, and the head YHD is a yellow (Y) head. These heads correspond to the black head K to the yellow head Y shown in FIG. 2, respectively.

Each of the heads KHD to YHD has a built-in serial / parallel converter (not shown), and converts the transferred serial drawing data into parallel drawing data, which is used as an ON / OFF control signal for 64 nozzles.

Parallel / serial converters KPS to YPS
In addition, the injection timing signals supplied from the injection / transfer control circuit CNT are supplied to the drivers KDR, MDR,
Fill in CDR and YDR.

Drivers KDR, MDR, CDR and Y
The DR and the heads KHD, MHD, CHD and YHD are mounted on the head carriage 61. These are parallel
The serial converters KPS to YPS are connected by a flexible cable CBL.

The drivers KDR to YDR generate drive signals based on the respective ejection timing signals and drive the heads KHD, MHD, CHD and YHD, respectively. As a result, the heads KHD to YHD eject ink in a dot pattern according to the drawing data.

Next, the operation of the ink ejection control system thus constructed will be described. An operation explanatory view is shown in FIGS. As shown in FIG. 5C, the computer C
The drawing data for the second ejection is written by the PU into the parallel / serial converters KPS to YPS. In addition, 1
The drawing data for the second ejection is the head K at the previous timing.
It has already been transferred to HD to YHD. However, the first time, the second time, ... Represent a relative order.

Next, based on the encoder signal indicating the first injection position as shown in FIG. 5A, the injection / transfer control circuit CNT outputs an injection timing signal as shown in FIG. 5B. This is transmitted to the drivers KDR to YDR through the parallel / serial converters KPS to YPS, and the first ejection of each of the heads KHD to YHD is performed based on the drawing data already supplied.

As a result, ink is ejected to the dot position p1 on the print medium 11 as shown in FIG.
In FIG. 6, one dot is represented by each head for simplification of description. Each dot belongs to a different band area (row) for each head.

Based on the first injection timing signal,
As shown in FIG. 5D, the parallel / serial converter K
PS to YPS start serial transfer of the drawing data.
As a result, the parallel / serial converter KPS
The second drawing data written in YPS is transferred to the serial / parallel converters in the corresponding heads KHD to YHD.

When the transfer of the second drawing data is completed, the computer CPU writes the third drawing data to the parallel / serial converters KPS to YPS.

Then, based on the encoder signal indicating the second ejection position, the ejection based on the second drawing data is performed by the heads KHD to YHD as shown in FIG.
Done in Hereinafter, the same operation is repeated.

As the print medium 11 is conveyed, the ink dots of a plurality of unit colors corresponding to the drawing data are overlaid on the dot positions p1, p2, p3, ... And a color image is formed.

At this time, in this apparatus, the following printing is performed in order to eliminate the line spacing. Printing is performed under the control of the computer CPU. The printing operation will be described below.

FIG. 7 shows a specific example of the nozzle arrangement on the ink ejection surface of the unit printer head HD. The 64 nozzles NZL are arranged in a staggered arrangement in four rows. Such a unit printer head HD is scanned in the direction of arrow x orthogonal to the nozzle row.

The print medium 11 is conveyed (sent) in the direction of arrow y which is orthogonal to the scanning direction. The print medium 11 is conveyed relative to the unit printer head HD, and the unit printer head HD may be sent in the reverse direction instead of sending the print medium 11.

The print medium 1 is attached to the 64 nozzles NZL.
No. 1 from the downstream side to the upstream side in the transport direction of 1
1 to # 64 are attached. Nozzle # 1 to nozzle # 6
The range up to 4 is the width of one line (printing width).

A flow chart of the printing operation is shown in FIG. First,
On the stage ST1, drawing is performed by the first scanning. At this time, the upper half (32 nozzles) of the image in one line is the nozzle # 33 in the lower half of the unit printer head HD.
~ Imaged by # 64. Here, the traveling direction (feeding direction) of the print medium 11 is upward.

FIG. 9 shows a schematic diagram of the drawing performed at this time. In FIG. 9, white circles represent printed dots. Each dot draws a pattern corresponding to on / off of ink ejection. However, FIG. 9 shows an example of solid printing for convenience. As shown in FIG. 9, the upper half image of one line is printed on the print medium 11.

After the first scan is completed, the print medium 11 is then fed by half the width of one line on the stage ST2. Next, on the stage ST3, all the images for one line (for 64 nozzles) are drawn using all the nozzles of the unit printer head HD. That is, the upper half of the image for one line is formed by the nozzles # 1 to # 32, and the lower half is formed by the nozzles # 33 to # 64. FIG. 10 shows a schematic diagram of the drawing at this time.

When the print medium 11 is fed by 1/2 line, the upper half image formed in the first scan is the nozzle #
It is in the position of 1 to # 32. Therefore, nozzle # 1
The ink ejected from # 32 to # 32 is superposed on the ink, so that the density of the upper half image is increased. This state is shown by a black circle in FIG. On the other hand, the lower half image is printed at the normal density because it is the first drawing. This state is indicated by a white circle.

After the scanning of this time is completed, it is then determined in stage ST4 whether or not the current drawing is of the final line. If it is not the last line, the process returns to stage ST2 and the print medium 11 is fed by 1/2 line. As a result, the lower half image on the first line drawn on the print medium 11 comes to the positions of the nozzles # 1 to # 32.

Next, in stage ST3, image formation is performed using all 64 nozzles. At this time, as the drawing data, the lower half image of the first row and the upper half image of the second row are used. The images in the lower half of the first line are formed by the nozzles # 1 to # 32. The images in the upper half of the second line are formed by the nozzles # 33 to # 64. As a result, the images representing the lower half of the first row are overlaid and the density is increased.

Hereinafter, the same operation is repeated until the drawing of the final line is performed. As a result, drawing is performed while overlapping by ½ line for each scan. Printing with increased density can be performed by overprinting at overlapping portions.

When the final row is determined in stage ST4, the operation proceeds to stage ST5. Stage S
At T5, 1/2 line feed of the print medium 11 is performed. As a result, the image of the lower half of the last line printed on the print medium 11 comes to the positions of the nozzles # 1 to # 32.

Next, in stage ST6, the images of the lower half of the last row are formed by the nozzles # 1 to # 32. As a result, as shown by the black circles in FIG. 11, the image in the lower half of the last row is hit twice. The dots indicated by double circles are images in the upper half of the final row, the density of which has already been increased by double-strike. In this way, the image of all lines is drawn by double-strike, and a print with high density is obtained.

If it is not necessary to increase the density, the second ink ejection may be stopped. If used together, the print density can be changed in two steps. That is, printing with two gradations can be performed.

Even if an overlapping edge or a gap occurs due to an error in the feeding of the print medium at the boundary portion of each strip area having a width of ½ line by double-printing by overlapping by ½ line, it is caused. The change in the density at the portion is only for one time of the ink ejection. Therefore, a large density change equivalent to double hitting as in the conventional case does not occur, and the line spacing becomes inconspicuous and virtually disappears. That is, it is possible to perform double print with improved quality.

It is preferable that the density (gradation) increase by the double-printing is performed by the minimum number (twice) of the multiple-printing, and the increase of the printing time is minimum. If higher density (multi-tone) printing is required, multiple printing such as three-time printing or four-time printing may be performed. In that case,
The feed amount of the print medium feed is set to 1/3 line feed, 1/4 line feed, etc., and the overlapping portion is printed as 2/3 line and 3/4 line etc., respectively. However, it is needless to say that, for the first row and the last row, in order to perform multiple hits on that portion, repeated hits are controlled in a manner similar to the above double hits.

According to such multiple printing, it is possible to increase the density (gradation) and obtain a higher quality print, and even if there is overlap at the boundary, the relative density change is further increased. Since it becomes smaller, it is possible to more thoroughly reduce the line spacing.

There is also a method of improving the gradation of the print (obtaining a high-quality print) only by the double-strike instead of the multiple-strike of three or more. This will be described below.

FIG. 12 shows an arrangement of unit heads in the printer head 60 used for multi-gradation printing. FIG. 12 is a view of the printer head 60 as seen from the ink ejection nozzle. In FIG. 12, the printer head 6 is in the x direction.
The scanning direction of 0 and the y direction are the conveyance directions of the fabric 11.

In FIG. 12, yellow heads Y1 and Y
2, magenta heads M1 and M2, cyan heads C1 and C
2 and the black heads K1 and K2 are rectangular unit heads having the same size, and they are arranged in a diagonal direction.

The yellow heads Y1 and Y2 eject ink of the same color (yellow) but different in density. For example, a high density ink is supplied to the yellow head Y1, and a low density ink is supplied to the yellow head. Magenta head M
1, M2, cyan heads C1, C2 and black head K
The same applies to 1 and K2. An ink ejection control system similar to that shown in FIG. 4 is provided for each of these unit heads.

Among these unit heads, the yellow heads Y1 and Y2, the magenta heads M1 and M2, the cyan heads C1 and C2, and the black heads K1 and K2 are an example of a pair of unit heads according to the present invention.

Using such a printer head 60,
/ 2 Strike twice. At this time, a combination of unit heads used for double-strike can be formed as shown in FIG. 13 in the case of the yellow heads Y1 and Y2.

In FIG. 13, the combination (1) is a case where the yellow head Y1 is used for the first ink ejection and the second ink ejection. This makes it possible to print with the highest density.

The combination (2) is a case where the yellow head Y1 is used for the first ink ejection and the yellow head Y2 is used for the second ink ejection. This makes it possible to perform printing with a lower density than in the case of (1).

The combination (3) is a case where the yellow head Y2 is used for the first ink ejection and the second ink ejection. As a result, it is possible to print with a lower density than in the case of (2).

The combination (4) is a case where the yellow head Y1 is used for the first ink ejection and the second ink ejection is not performed. This makes it possible to perform printing with a lower density than in the case of (3).

The combination (5) is a case where the yellow head Y2 is used for the first ink ejection and the second ink ejection is not performed. This makes it possible to perform printing with a lower density than in the case of (4).

The combination (6) is the yellow heads Y1 and Y2.
Then, it is a case where ink ejection is not performed. As a result, printing with a yellow density of 0 can be performed. Similar combinations are magenta heads M1 and M2, cyan head C
1, C2 and black heads K1, K2 are also performed. Therefore, it is possible to perform printing with 6 gradations for each unit color and obtain a high-quality print. Even in this case, the effect that the line spacing becomes inconspicuous is maintained as in the case of one head for each color.

Also in this case, the density and the gradation can be further increased by performing the multi-printing three times or more, and the reduction of the line spacing can be further thoroughly reduced.

[0079]

As described in detail above, according to the first invention for solving the problems, the print medium is stepwise conveyed by a distance of 1 / n of the printing width per scanning of the printer head. By doing so, it is possible to make the line streaks inconspicuous by the multiple printing of n times by the 1 / n shift, whereby it is possible to realize the ink jet printing method for performing high-quality printing.

According to the second invention for solving the problem, the print medium is conveyed stepwise by a distance of 1 / n of the printing width per scanning of the printer head. The line spacing can be made inconspicuous by n multiple printings by n shifting, thereby
It is possible to realize an inkjet printing apparatus that performs high-quality printing.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an apparatus according to an example of an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a printer head in an apparatus according to an example of an embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a printer head in an apparatus according to an example of an embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of an ink ejection control system in an apparatus according to an example of an embodiment of the present invention.

FIG. 5 is an operation explanatory diagram of the ink ejection control system in the apparatus according to the example of the embodiment of the present invention.

FIG. 6 is an operation explanatory diagram of the ink ejection control system in the apparatus according to the example of the embodiment of the present invention.

FIG. 7 is a diagram showing a configuration of a printer head in an apparatus according to an example of an embodiment of the present invention.

FIG. 8 is a flowchart of an operation of the apparatus according to the embodiment of the present invention;

FIG. 9 is an explanatory diagram of an operation of the apparatus according to the embodiment of the present invention;

FIG. 10 is an explanatory diagram of an operation of the apparatus according to an example of the embodiment of the present invention;

FIG. 11 is an explanatory diagram illustrating an operation of the apparatus according to an embodiment of the present invention;

FIG. 12 is a diagram showing a configuration of a printer head in an apparatus according to an example of an embodiment of the present invention.

FIG. 13 is a diagram showing a combination of unit heads in the apparatus according to the example of the embodiment of the present invention.

FIG. 14 is a diagram illustrating the operation of a conventional inkjet printer.

[Explanation of symbols]

10 original winding 11 print medium 20,30 roller 40 pinch roller 50 drawing table 60 printer head 61 head carriage 62 optical sensor 70 linear encoder PRC printing control unit SCC scanning control unit RCC transport control unit CPU computer DIS display unit OPC operation unit CNT Injection / transfer control circuit OSC clock source ADC address control circuit KPS, MPS, CPS, YPS parallel / serial conversion circuit KDR, MDR, CDR, YDR driver KHD, MHD, CHD, YHD head

Claims (7)

[Claims] 1. A printer head, which ejects ink from a plurality of nozzles, scans along the surface of a print medium, and at least one of the print medium and the printer head is relatively positioned in a direction orthogonal to the scanning direction of the printer head. In the inkjet printing method of conveying the print medium, the relative conveyance of at least one of the print medium and the printer head is 1 / the printing width formed by the plurality of nozzle tracks per scan of the printer head.
An inkjet printing method, which is performed stepwise by a distance corresponding to n (n: an integer of 2 or more). 2. The inkjet printing method according to claim 1, wherein the n is 2. 3. The inkjet printing method according to claim 1, wherein multi-level printing is performed on the print medium. 4. A printer head, which ejects ink from a plurality of nozzles, scans along the surface of a print medium, and at least one of the print medium and the printer head is positioned relative to a direction orthogonal to the scanning direction of the printer head. In an inkjet printing apparatus that conveys the print medium, the relative conveyance of at least one of the print medium and the printer head is 1 / one of the printing width formed by the trajectory of the plurality of nozzles per scan of the printer head.
An inkjet printing apparatus comprising: a conveying unit that performs a stepwise step by a distance corresponding to n (n: an integer of 2 or more). 5. The ink jet printing apparatus according to claim 4, wherein the n is 2. 6. The inkjet printing apparatus according to claim 4, wherein the printing medium is configured to perform multi-gradation printing. 7. The inkjet printing apparatus according to claim 4, wherein the printer head has a pair of unit heads that respectively eject inks of the same color having different densities.