Patent Publication Number: US-7708373-B2

Title: Ink-jet head and ink-jet printer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2005-282732, filed on Sep. 28, 2005, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink-jet printer and an ink-jet head which jet inks onto a recording medium. 
     2. Description of the Related Art 
     As a color ink-jet printer which jets a plurality of color inks onto a recording medium such as a recording paper to record a color image and/or letter onto the recording medium, there is widely known an ink-jet printer having a construction in which a serial-type ink-jet head which jets a plurality of color inks from nozzles while moving in a direction (scanning direction) orthogonal to a feeding direction of the recording paper or the like. For example, an ink-jet head described in Japanese Patent Application Laid-open No. 2003-220705 has four cavity plates each of which has a cavity for jetting one of four color inks (cyan (C), magenta (M), yellow (Y), and black (K)), and in each of the cavity plates has two nozzle rows formed therein, each rows having a plurality of nozzles arranged in the feeding direction. The four cavity plates are positioned so as to be aligned in the scanning direction. Thus, as shown in  FIG. 19A  for example, when three kinds of nozzle groups  201  which jet three color inks respectively, are positioned in the order of cyan ( 201 C), magenta ( 201 M), and yellow ( 201 Y) from the left, and when an ink-jet head  200  moves to the right, the three color inks (I C , I M , and I Y ) land on the recording paper in the order of Y (yellow), then to M (magenta), and then to C (cyan) (Y-M-C order), thereby forming a dot as shown in  FIG. 18B . 
     SUMMARY OF THE INVENTION 
     In order to increase the recording speed, when the ink-jet head  200  reciprocatingly moves (reciprocates) in the scanning direction (left and right direction), it is possible to jet the inks from nozzles both during when the ink-jet head  200  moves to the left (during the leftward movement) and when the ink-jet head  200  moves to the right (during the rightward movement). In this case, however, as shown in  FIGS. 18A and 18B , during the rightward movement of the ink-jet head  200 , the three color inks land on the recording paper in the order of Y to M, then to C (Y-M-C order). Since the color of the ink droplet which lands last is cyan, the tint or hue of color of the image (recorded image) tends to be biased toward cyan which landed last (to be more bluish). On the other hand, as shown in  FIGS. 19A and 19B , during the leftward movement of the ink-jet head  200 , the inks land in the order of C to M, then to Y (C-Y-M order). In this case, since the color of the ink droplet which lands last is yellow, the tint or hue of color of the image tends to be biased toward yellow (to be more yellowish), Therefore, the color tint or hue thus varies suddenly (abruptly) between an area  202  on which the inks landed during the rightward movement and an area  203  on which the inks landed during the leftward movement, thereby forming a band pattern (color banding) in the scanning direction and thus degrading the printing quality. 
     An object of the present invention is to suppress the degradation of printing quality due to the difference in color tint between an area on which the inks land when an ink-jet head moves toward one side of a scanning direction and an area on which inks land when the ink-jet head moves toward the other side of the scanning direction. 
     According to a first aspect of the present invention, there is provided an ink-jet head which discharges a plurality of different color inks onto a recording medium, including a plurality of nozzle groups which jet the inks onto the recording medium respectively, and each of which is formed by a nozzle row formed by a plurality of nozzles arranged in a predetermined direction; 
     wherein each of the nozzle groups has a high density portion formed therein and positioned at a central portion thereof in the predetermined direction, and two low density portions which are positioned at both sides in the predetermined direction, respectively, of the high density portion and in which the nozzles are arranged in the predetermined direction at a spacing distance greater than a spacing distance at which the nozzles are arranged in the high density portion. 
     In a case that a general ink-jet head is reciprocated when a plurality of different color inks are jetted from a plurality of nozzles, respectively, during a first movement (movement directed away from a standby position of the ink-jet head; outgoing movement) and during a second movement (movement directed toward the standby position of the ink-jet head; returning movement), the order of landing of the inks during the first movement and the order of landing of the inks during the second movement are different. Therefore, an area on which the inks landed during the first movement and another area on which the inks landed during the second movement are different in tint of color (color tint), thereby adversely affecting the printing quality. According to the first aspect of the present invention, each of the nozzle groups has a high density portion and two low density portions which are positioned at both sides, respectively, of the high density portion. Accordingly, for example, it is possible to form two areas in the recording medium by jetting inks from the high density portion both during the first movement and the second movement, and at the same time, it is possible to form an intermediate area of an intermediate color tint between the two areas by jetting the inks from the two low density portions, respectively, to be landed on a portion between the two areas. In this case, since the color tint gradually varies, color banding, which would be otherwise caused due to the difference in color tint between the area on which the inks landed during the first movement and the another area on which the inks landed during the second movement, can be made inconspicuous and thus the printing quality is improved. 
     According to a second embodiment of the present invention, there is provided an ink-jet printer which discharges a plurality of different color inks onto a recording medium to perform printing on the recording medium, including: 
     the ink-jet head of the present invention; and 
     a feeding mechanism which feeds the recording medium in an orthogonal direction orthogonal to the predetermined direction. In this case, printing can be performed at high speed while suppressing the color banding. 
     In the ink-jet printer of the present invention, the ink-jet head may be reciprocapable in the orthogonal direction; and the ink-jet head may jet the inks from the plurality of nozzle groups, respectively, both during movement of the ink-jet head toward one side of the orthogonal direction and during movement of the ink-jet head toward the other side of the orthogonal direction. In this case, the color banding can be reduced even when high-speed printing is performed while reciprocating the ink-jet head. In addition, since the inks are overlappingly landed at the two low density portions, even when the paper feed precision is varied or fluctuated, there will be no area at which the ink does not land at all. Accordingly, any white, streak-like areas are not formed on the recording medium on which the recording has been performed. 
     In the ink-jet head and the ink-jet printer of the present invention, two nozzle groups, among the plurality of nozzle groups, may be arranged at positions shifted from each other in the predetermined direction. With respect to the two nozzle groups, low density portions of one of the two nozzle groups and low density portions of the other of the two nozzle groups may be arranged adjacently in the predetermined direction. Alternatively, with respect to the two nozzle groups, low density portions of one of the two nozzle groups and low density portions of the other of the two nozzle groups may partially overlap with each other as viewed in a direction orthogonal to the predetermined direction. According to the construction, a plurality of kinds of intermediate areas are formed, for example, between two areas, onto which the inks jetted from the high density portions during the first movement and the second movement and landed thereon, respectively. Accordingly, the change of color tint is made gradual, thereby making the color banding to be inconspicuous and thus improving the printing quality. 
     In the ink-jet head and the ink-jet printer of the present invention, the spacing distance at which the nozzles are arranged in the low density portions of each of the nozzle groups may be two times the spacing distance at which the nozzles are arranged in the high density portion. In this case, for example, a spacing distance (resolution) between landing positions (dots) in the areas at which the inks jetted from the two low density portions landed during the first movement and the second movement, respectively, can be made same as a spacing distance between landing positions in an area at which the inks jetted from the high density portions landed. 
     In the ink-jet head and the ink-jet printer of the present invention, the spacing distance at which the nozzles are arranged in the low density portions of each of the nozzle groups may be increased toward both ends, in the predetermined direction, of each of the nozzle groups. In this case, since the spacing distance between the nozzles in the low density portions is varied in a stepwise manner, it is possible to form, with two low density portions, a plurality of kinds of intermediate areas between the two areas onto each of which the inks were jetted from the high density portion during one of the first movement and the second movement, without shifting the plurality of nozzle groups in the predetermined direction. Thus, the color banding is made to be inconspicuous. 
     In the ink-jet head and the ink-jet printer of the present invention, each of the nozzle groups may have two nozzle rows in which the nozzles are arranged at the predetermined spacing distance in the predetermined direction; 
     the two nozzle rows may be equal in a length in the predetermined direction; and 
     the two nozzle rows may be arranged at positions shifted from each other in the predetermined direction. In this case, nozzle groups each of which has a high density portion and two density portions are formed by arranging two nozzle rows, having the same length and in which the nozzles are arranged at a same spacing distance, to be arranged at positions shifted from each other in the predetermined direction. Accordingly, the construction of the nozzle groups is simplified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic construction view of an ink-jet printer according to an embodiment of the present invention; 
         FIG. 2  is a plan view of an ink-jet head; 
         FIG. 3  is a plan view of a jetting unit; 
         FIG. 4  is a plan view of a nozzle plate; 
         FIG. 5  is a partially enlarged view of  FIG. 3 ; 
         FIG. 6  is a sectional view taken on line VI-VI of  FIG. 5 ; 
         FIG. 7  is a sectional view taken on line VII-VII of  FIG. 5 ; 
         FIG. 8  is a block diagram showing an electrical construction of the ink-jet printer; 
         FIG. 9A  is a diagram showing a state in which color inks land during a first movement of the ink-jet head, and  FIG. 9B  is a partial sectional view of a recording paper showing the order in which the color inks land during a second movement; 
         FIG. 10A  is a diagram of a state in which the color inks land during the first movement of the ink-jet head, and  FIG. 10  is a partial sectional view of the recording paper showing the order in which the color inks land during the second movement; 
         FIG. 11  is a diagram showing a state in which the color inks land during the first movement of an ink-jet head according to a first modification; 
         FIG. 12  is a diagram showing a state in which the color inks land during the second movement of the ink-jet head according to the first modification; 
         FIG. 13  is a diagram showing a state in which the color inks land during the first movement of an ink-jet head according to a second modification; 
         FIG. 14  is a diagram showing a state in which the color inks land during the second movement of the ink-jet head according to the second modification; 
         FIG. 15  is a diagram showing a state in which the color inks land during the first movement of an ink-jet head according to a third modification; 
         FIG. 16  is a diagram showing a state in which the color inks land during the second movement of the ink-jet head according to the third modification; 
         FIG. 17  is a diagram showing a nozzle group according to a fourth modification; 
         FIG. 18A  is a diagram showing a conventional ink-jet head in a state in which the color inks land during a rightward movement of the conventional ink-jet head, and  FIG. 18B  is a partial sectional view of a recording paper showing the order in which the color inks land during the rightward movement of the conventional ink-jet head; and 
         FIG. 19A  is a diagram showing the conventional ink-jet head in a state in which the color inks land during a leftward movement of the conventional ink-jet head, and  FIG. 19B  is a partial sectional view of the recording paper showing the order in which the color inks land during the leftward movement of the conventional ink-jet head. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be explained. This embodiment is an example in which the present invention is applied to a color ink-jet printer which jets four color inks (cyan, magenta, yellow, and black) from nozzles onto a recording paper (recording medium). 
     First, a general construction of an ink-jet printer  100  will be explained. As shown in  FIG. 1 , the ink-jet printer  100  includes a carriage  4  which is movable in a left and right direction of  FIG. 1 ; a serial type ink-jet head  1  which is provided on the carriage  4  and jets inks onto a recording paper  7 ; feed rollers  5  (feed mechanism) which feed or transport the recording paper  7  forward in  FIG. 1 ; and a control unit  6  (see  FIG. 8 ) which controls overall operations of the ink-jet printer  100 . The ink-jet printer  100  reciprocates the ink-jet head  1  integrally with the cartridge  4  in the left and right direction (scanning direction: first direction), and jets four color inks (cyan (C), magenta (M), yellow (Y), and black (K)) onto the recording paper  7  from nozzles  20  (see  FIGS. 3 to 7 ) which are formed on a lower surface of the ink-jet head  1 , while feeding the recording paper  7  forward by the feed rollers  5 . Accordingly, a desired color image and or letter can be recorded onto the recording paper  7 . In the following explanation, reference numerals or symbols having “K”, “C”, “M”, and “Y” attached thereto indicate correspondence to the black, cyan, magenta, and yellow ink, respectively. 
     Next, the ink-jet head  1  will be explained. As shown in  FIG. 2 , the ink-jet head  1  has four jetting units  8  which are same in structure, and the four jetting units  8  have four kinds of nozzles  20  for jetting the four colors, respectively. These four jetting units  8  are attached to and made integral to a head frame  9 . As shown in  FIG. 2 , the four jetting units  8  ( 8 K,  8 C,  8 M, and  8 Y) are aligned in the scanning direction in the order of black, cyan, magenta, and yellow from the left side. 
     Because all of the four jetting units  8  have the same structure, one of these jetting units will now be explained with reference to  FIGS. 3 to 7 . Each of the jetting units  8  has a channel unit  2  in which an ink channel including nozzles  20  and pressure chambers  14  are formed; and a piezoelectric actuator  3  which is arranged on an upper surface of the channel unit  2  and which applies a jetting pressure to ink inside the pressure chambers  14 . 
     First, the channel unit  2  will be explained. As shown in  FIGS. 6 and 7 , the channel unit  2  has a cavity plate  10 , a base plate  11 , a manifold plate  12 , and a nozzle plate  13 , and these four plates  10  to  13  are joined together in a laminated state (laminated layers). The cavity plate  10 , the base plate  11 , and the manifold plate  12  are stainless steel plates, and the ink channel including a manifold  17 , the pressure chambers  14 , and the like which will be explained later on, can be easily formed by etching in the plates  10  to  12 . The nozzle plate  13  is formed, for example, of a synthetic high-molecular resin material such as polyimide and is adhered to a lower surface of the manifold plate  12 . Alternatively, similarly to the three plates  10  to  12 , the nozzle plate  13  may also be formed a metal material such as stainless steel. 
     As shown in  FIGS. 3 and 5  to  7 , in the cavity plate  10  positioned topmost among the four plates  10  to  13 , a plurality of pressure chambers  14 , arranged in arrays (rows) on a plane, are formed as holes penetrating through the plate  10 , and the pressure chambers  14  are covered by the base plate  11  and a vibrating plate  30 , which will be explained later, from below and above, respectively. The pressure chambers  14  are arranged in four rows aligned in a paper feeding direction (up and down direction in  FIG. 3 ). Each of the pressure chambers  14  is formed to a substantially elliptical shape which is long in the scanning direction (left and right direction in  FIG. 3 ). 
     As shown in  FIGS. 5 to 7 , communicating holes  15  and  16  are formed in the base plate  11  at positions overlapping in a plan view with both ends, respectively, of each of the pressure chambers  14 . Further, in the manifold plate  12 , three manifolds  17  which extend in the paper feeding direction (up and down direction in  FIG. 3 ) are formed so as to overlap in plan view with the pressure chambers  14 , aligned in the paper feeding direction, at portions of the pressure chambers on a side of the communication holes  15 . These three manifolds  17  are communicated with an ink supply port  18  formed in the vibrating plate  30  which will be explained later, and an ink from an ink tank (not shown) is supplied into the manifolds  17  via the ink supply port  18 . Furthermore, a plurality of communicating holes  19  which are communicated with the communicating holes  16 , respectively, are formed in the manifold plate  12  at positions each overlapping in plan view with an end, of one of the pressure chambers  14 , at a side opposite to the one of the manifolds  17 . 
     Moreover, the nozzles  20  are formed in the nozzle plate  13  at positions each overlapping in plan view with one of the communicating holes  19 . As shown in  FIGS. 3 and 4 , each of the nozzles  20  overlaps with one of the pressure chambers  14 , arranged into four rows, at the end thereof on the side opposite to one of the manifolds  17 ; and the nozzles  20  are arranged at an equal spacing distance in the paper feeding direction (up and down direction in  FIG. 2 ; second direction), in the nozzle plate  13  at areas which do not overlap with any one of the three manifolds  17 ; and the nozzles  20  form a nozzle group  21  which includes four nozzle rows  20   a  to  20   d  which are aligned in the scanning direction. 
     As shown in  FIG. 3 , each of the four nozzle rows  20   a  to  20   d  has the same number of nozzles  20 . In the nozzle rows  20   a  to  20   d , spacing distance (pitch P) at which the nozzles  20  are arranged in a direction in which the nozzles are arranged (nozzle arrangement direction) is equal, and all of the nozzle rows  20   a  to  20   d  are equal in length in the nozzle arrangement direction (that is, in the number of nozzles  20  are equal among the nozzle rows  20   a  to  20   d ). Further, as shown in  FIG. 3 , the nozzle row  20   c  (third row from the left) is shifted with respect to the nozzle row  20   a  (first row from the left) just by a distance of ½ of the pitch P (0.5 P) toward the downstream of the paper feeding direction (downward in  FIG. 3 ). Furthermore, the nozzle row  20   b  (second row from the left) and the nozzle row  20   d  (fourth row from the left) are shifted just by a distance of ( 9/4) times the pitch P (2.25 P) toward the downstream of the paper feeding direction, with respect to the nozzle row  20   a  and the nozzle row  20   c , respectively. Accordingly, as shown in  FIG. 4 , the nozzle group  21 , which includes the four nozzle rows  20   a  to  20   d , has a high density portion  22 , in which the nozzles are arranged in the paper feeding direction at a spacing distance of 0.25 P, and two low density portions  23  and  24  in which the nozzles are arranged at a spacing distance greater than that in the high density portion  22 . The high density portion  22  is arranged at a central portion, of the nozzle plate  13 , in the paper feeding direction. The low density portions  23  and  24  are arranged at both sides (upper and lower sides in  FIG. 4 ) in the paper feeding direction of the high density portion  22 , and the spacing distance between the nozzles (nozzle spacing distance) thereof in the paper feeding direction is 0.5 P (two times that of the high density portion  22 ). The reason as to why the high density portion  22  and the low density portions  23  and  24  which differ in nozzle spacing distances are provided will be explained in detail later. 
     As shown in  FIG. 6 , each of the manifolds  17  is communicated with one of the pressure chambers  14  via one of the communicating holes  15 , and each of the pressure chambers  14  are communicated with one of the nozzles  20  via the communicating holes  16  and  19 . In this manner, a plurality of individual ink channels  25  each from one of the manifolds  17  to one of the nozzles  20  via one of pressure chambers  14  are thus formed in the channel unit  2 . 
     Next, the piezoelectric actuator  3  will be explained. As shown in  FIGS. 3 and 5  to  7 , the piezoelectric actuator  3  has the vibrating plate  30  arranged on an upper surface of the channel unit  2 ; a piezoelectric layer  31  formed continuously on an upper surface of the vibrating plate  30  to cover the pressure chambers  14 ; and a plurality of individual electrodes  32  formed on an upper surface of the piezoelectric layer  31  corresponding to the pressure chambers  14 , respectively. 
     The vibrating plate  30  is a metal plate of substantially rectangular shape and has an electrically conductive property. The vibrating plate  30  is formed, for example, of an iron-based alloy such as stainless steel, a copper-based alloy, a nickel-based alloy, a titanium-based alloy, or the like. The vibrating plate  30  is arranged on the upper surface of the cavity plate  10  so as to cover the pressure chambers  14  and is joined to the cavity plate  10 . The vibrating plate  30  is constantly held at a ground potential and is positioned opposite to or facing the individual electrodes  32 . Accordingly, the vibration plate  30  serves also as a common electrode for making an electric field act in the piezoelectric layer  31  between the individual electrodes  32  and the vibrating plate  30 , in a thickness direction of the piezoelectric layer  31 . 
     On the upper surface of the vibration plate  30 , the piezoelectric layer  31 , mainly composed of a lead zirconate titanate (PZT) which is a ferroelectric solid solution of lead zirconate and lead titanate. The piezoelectric layer  31  is formed continuously so as to cover the pressure chambers  14 . The piezoelectric layer  31  can be formed, for example, by an aerosol deposition (AD method) in which ultra-fine particulate material is collided onto an objective surface at high velocity so as to make the particulate material to deposit on the objective surface. Other than the AD method, the piezoelectric layer  31  can be also formed by using a method such as a sol-gel method, a sputtering method, a hydrothermal synthesis method, a chemical vapor deposition (CVD method), or the like. Still alternatively, the piezoelectric layer  31  can be formed by cutting a piezoelectric sheet, obtained by calcinating a green sheet of PZT, and then by bonding the piezoelectric sheet to the vibration plate  30 . 
     On the upper surface of the piezoelectric layer  31 , the individual electrodes  32  are formed to correspond to the pressure chambers  14 , respectively. Each of the individual electrodes  32  is substantially elliptic in a plan view, is smaller to some extent than one of the pressure chambers  14  in a plan view, and is formed at a position overlapping in a plan view with a central portion of one of the pressure chambers  14  to which the individual electrode  32  corresponds. Further, the individual electrodes  32  are formed of an electrically conductive material such as gold, copper, silver, palladium, platinum, titanium, or the like. Furthermore, a plurality of contact points  35  are drawn each from left end of one of the individual electrodes  32  (one end of one of the individual electrodes  32  on the side of the manifold  17 ), toward the left side in  FIG. 3 . These contact points  35  are connected to contact points, respectively, of a flexible wiring member (not shown) such as a flexible printed circuit (FPC) or the like. The contact points  35  are electrically connected via this wiring member to a driver IC  27  (see  FIG. 8 ) which applies a drive voltage selectively to the individual electrodes  32 . The individual electrodes  32  and the contact points  35  can be formed by a method such as screen printing, the sputtering method, a vapor deposition, or the like. 
     Next, the operation of the piezoelectric actuator  300  upon jetting the ink will be explained. When a drive voltage is applied from the driver IC  27  selectively to the plurality of individual electrodes  32 , a potential difference is generated between a certain individual electrode  32  among the individual electrodes  32 , which is disposed on the piezoelectric layer  31  and to which the drive voltage is applied, and the vibration plate  30  as the common electrode which is disposed under the piezoelectric layer  31  and maintained at ground potential, thereby generating an electric field in a thickness direction of the piezoelectric layer  31  in a portion of the piezoelectric layer  31  sandwiched between the individual electrode  32  and the vibration plate  30 . At this time, when a direction in which the piezoelectric layer  31  is polarized and the direction of the electric field are same, the portion of the piezoelectric layer  31 , which is positioned directly below the individual electrode  32  applied with the drive voltage, expands in the thickness direction in which the piezoelectric layer  31  is polarized and contracts in a horizontal direction (direction parallel to the plane of the piezoelectric layer  31  and orthogonal to the polarization direction). Then, accompanying with the contracting deformation of the piezoelectric layer  31 , the vibration plate  30  is deformed to project toward a pressure chamber  14 , among the pressure chambers  14 , corresponding to the individual electrode  32 . Accordingly, the volume of the pressure chamber  14  is decreased to apply pressure to the ink in the pressure chamber  14 , thereby jetting a droplet of the ink from a nozzle  20  communicating with the pressure chamber  14 . 
     Next, an explanation will be given mainly about an electrical construction of the control unit  6 , with reference to a block diagram in  FIG. 8 . The control unit  6  is constructed of a Central Processing Unit (CPU); a Read Only Memory (ROM) which stores a various kinds of programs and data, and the like for controlling entire operations of the ink-jet printer  100 ; and a Random Access Memory (RAM) which temporarily stores data and the like which are processed in the CPU; and the like. 
     Data concerning a character and/or an image to be recorded are inputted into the control unit  6  from an input device  50  such as PC. When data are inputted from the input device  50 , the control unit  6  outputs a drive signal, based on the data, to the driver IC  27  of the ink-jet head  1 , to a carriage drive motor  40  which drives the carriage  4  in the scanning direction, to a feed motor  41  which rotatingly drives the feed rollers  5 , and the like. That is, the control unit  6  reciprocates the ink-jet head  1  in the scanning direction by the carriage drive motor  40  while feeding the recording paper  7  forward by rotating the feed rollers  5  by the feed motor  41 , and makes the plurality of color inks be jetted onto the recording paper  7  by the ink-jet head  1 . Further, the control unit  6  controls the driver IC  27 , the motors  40  and  41 , and the like so that upon moving the ink-jet head  1  to reciprocate in the scanning direction, the inks are jetted from the nozzle groups  21  of the four jetting units  8 , respectively, both during the rightward movement (first moving direction) in  FIG. 1  and during the leftward movement (second moving direction) in  FIG. 1 . In this case, the recording speed is made faster in comparison to a case where the inks are jetted during only one of the movement in the first moving direction and the movement in the second moving direction. 
     In a case that the four color inks are jetted onto the recording paper  7  both during the movement of the ink-jet head in the first moving direction (outgoing movement; first movement) and during the movement of the ink-jet head in the second moving direction (returning movement; second movement), the order in which the color inks land differs during the first movement and during the second movement as shown in  FIGS. 18 and 19  described above. Therefore, a difference in color tint occurs between an area onto which the inks landed during the first movement and an another area onto which the inks landed during the second movement, and thus color banding clearly appears and the printing quality degrades. 
     As shown in  FIG. 4 , however, in the ink-jet head  1  of the present embodiment, the nozzle group  21  of each jetting unit  8  has a high density portion  22 , positioned at the central portion in the paper feeding direction, and two low density portions  23  and  24  which are positioned at both sides, respectively, in the paper feeding direction of the high density portion  22  and in which the nozzles are arranged at a spacing distance greater than that in the high density portion  22 , thereby making it possible to suppress the above-described color banding. The operation of the nozzle groups  21  will be explained in detail with reference to  FIGS. 9A ,  9 B and  10 A,  10 B. 
     First, as shown in  FIG. 9A , when the inks are jetted while the ink-jet head  1  is moved toward the right (in the first moving direction), then, as shown in  FIG. 9B , the three color inks (I C , I M , and I Y ) land on the recording paper  7  in the order that the three kinds of nozzle groups  21 C,  21 M, and  21 Y are positioned from the right side, which is the downstream side of the movement direction of the ink-jet head  1 , namely in the order of Y (yellow) to M (magenta), then to C (cyan) (Y-M-C order), during the first movement. Therefore, the color tint of the color image recorded during the first movement tends to be a color tint which is biased toward cyan which is the ink landed last (more bluish tint). In this process, in an area  60  of the recording paper  7 , onto which the inks jetted from the high density portions  22  land, dots are formed at spacing distance of 0.25 P in the paper feeding direction (up and down direction of  FIG. 9A ). On the other hand, in areas  61  and  62  of the recording paper  7 , onto which the inks from the two low density portions  23  and  24  land, respectively, dots are formed at spacing distance of 0.5 P in the paper feeding direction. 
     Next, as shown in  FIGS. 10A and 10B , the recording paper  7  is then fed in the paper feeding direction (downward in  FIG. 10A ) by the feed roller  5  (see  FIG. 1 ) as shown in  FIG. 10A . In  FIG. 10A , a relative position of the ink-jet head  1  with respect to the recording paper  7  prior to the feeding is indicated by broken lines, and a relative position of the ink-jet head  1  with respect to the recording paper  7  after the feeding is indicated by solid lines. At this time, the recording paper  7  is fed just by a predetermined distance L so that the area  61 , onto which the inks from the low density portion  23  at the upstream in the paper feeding direction (upper side in  FIG. 10A ) landed during the first movement, comes to a position directly below the low density portion  24  at the downstream in the paper feeding direction (lower side in  FIG. 10B ). 
     Then, as shown in  FIG. 10A , when the inks are jetted while the ink-jet head  1  is moved to the left (in the second moving direction), the three color inks (I C , I M , and I Y ) land on the recording paper  7  in the order in which the three kinds of nozzle groups  21 C,  21 M, and  21 Y are positioned from the left side, which is the downstream in the movement direction (outgoing movement) of the ink-jet head  1 , namely in the order of C (cyan) to M (magenta), then to Y (yellow) (C-M-Y order) during the second movement as shown in  FIG. 10B . Therefore, the color tint of the color image which is recorded during the second movement tends to have a color tint which is biased toward yellow which is the ink landed last (yellowish tint). At this time, in an area  63  of the recording paper  7  which is adjacent to the area  61  at the upstream in the paper feeding direction, the inks jetted from the high density portions  22  land so as to form dots at spacing distance of 0.25 P in the paper feeding direction (up and down direction in  FIG. 9A ). 
     On the other hand, the inks, jetted from the low density portion  24  at the downstream in the paper feeding direction, land on portions between the dots formed in the area  61  by the inks jetted from the low density portion  23  at the upstream in the paper feeding direction during the first movement. That is, in the area  61 , the dots formed by the inks jetted in the order of Y to M then to C (Y-M-C order) from the low density portion  23  for each of the three colors during the first movement, and the dots formed by the inks jetted in the order of C to M then to Y (C-M-Y order) from the low density portion  24  for each of the three colors during the second movement, are mixed. These two kinds of dots are of the same dot spacing distance (resolution) as those in the areas  60  and  63 , and are aligned in the paper feeding direction at an spacing distance of 0.25 P. Namely, an intermediate area  61  is thus located (formed) between the area  60  onto which the inks jetted from the high density portions  22  for each of the three colors landed in the Y-M-C order during the first movement, and the area  63  onto which the inks jetted from the high density portions  22  for each of the three colors landed in the C-M-Y order during the second movement. The dot spacing distance in the intermediate area  61  is equal to those in the two areas  60  and  63 , and the color tint of the dots in the intermediate area  61  is a color tint which is intermediate between the color tints of the dots in the areas  60  and  63 . Accordingly, the color tint between the two areas  60  and  63  which differ in the order of landing of the color inks gradually changes because the intermediate area  61  is interposed therebetween, thereby making the color banding to be inconspicuous and thus improving the printing quality. 
     As shown in  FIGS. 3 and 4 , each of the nozzle groups  21  includes four nozzle rows  20   a  to  20   d  of equal length (number of nozzles  20 ) and equal spacing distance between the nozzles  20 . Among the nozzle rows, two adjacent nozzle rows (nozzle rows  20   a  and  20   b , and nozzle rows  20   c  and  20   d ) are arranged at positions shifted from each other in the paper feeding direction, thereby forming the high density portion  22  and the two low density portions  23  and  24 . Thus, although each of the nozzle groups  21  has the high density portion  22  and the low density potions  23  and  24 , the high density portion  22  and the low density potions  23  and  24  are considerably simple in construction and are easily formed. 
     Next, an explanation will be given about modifications in each of which various changes are made to the embodiment. Parts or components of the modification, which are same in construction as those in the embodiment, will be assigned with same reference numerals and any explanation therefor will be omitted as appropriate. 
     First Modification 
     Among the three nozzle groups  21  ( 21 C,  21 M, and  21 Y) which jet three color inks respectively, at least one nozzle group  21  may be arranged at a position shifted in the paper feeding direction with respect to the other nozzle groups  21 . As shown in  FIG. 11 , for example, a nozzle group  21 Y, which is positioned at the right end and jets an yellow ink, is arranged at a position shifted toward the upstream in the paper feeding direction (upper side in  FIG. 11 ) with respect to a cyan nozzle group  21 C and a magenta nozzle group  21 M arranged to the left of the nozzle group  21 Y. Low density portions  23 Y and  24 Y of the yellow nozzle group  21 Y are disposed at positions which are adjacent, in the paper feeding direction (up and down direction in  FIG. 11 ), with respect to low density portions  23 C and  24 C of the cyan nozzle group  21 C and with respect to low density portions  23 M and  24 M of the magenta nozzle group  21 M. 
     As shown in  FIG. 11 , by the jetting of the inks during the rightward movement (first moving direction), five areas  70  to  74  which differ in color tint are formed on the recording paper  7 . Namely, at a central portion in the paper feeding direction, an area  70  of a color tint which is biased toward cyan (bluish tint) is formed by the inks landed from the high density portions  22  ( 22 C,  22 M, and  22 Y) of the three colors in the Y-M-C order. Also, at an upstream in the paper feeding direction of the area  70 , an area  71  is formed, in which dots formed by the inks landed from the cyan and magenta low density portions  23 C and  23 M, and from the yellow high density portion  22 Y in the Y-M-C order, and dots of the yellow ink jetted from the yellow high density portion  22 Y are mixed. At a downstream in the paper feeding direction of the area  70 , an area  72  is formed in which dots formed by the inks landed from the cyan and magenta high density portions  22 C and  22 M and from the yellow low density portion  24 Y in the Y-M-C order, and dots formed by the inks landed from the cyan and magenta high density portions  22 C and  22 M in an order of M to C (M-C order) are mixed. Furthermore, at an upstream end in the paper feeding direction, an area  73  is formed of dots of the yellow ink-jetted from the yellow low density portion  23 Y; and at a downstream end in the paper feeding direction, an area  74  is formed from dots formed of the inks landed from the cyan and magenta low density portions  24 C and  24 M in the M-C order. 
     As shown in  FIG. 12 , after the recording paper has been fed in the paper feeding direction just by the predetermined distance L so that the areas  71  and  73 , onto which the inks from the low density portions  23  at the upstream in the paper feeding direction of the nozzle groups  21  landed, are positioned directly below the low density portions  24  at the downstream in the paper feeding direction, the ink-jet head  1  jets ink again while moving to the left (in the second moving direction). In this process, an area  75 , the color tint of which is biased toward yellow (yellowish tint), is formed by the inks landing from the high density portions  22 C,  22 M, and  22 Y of the three colors in the C-M-Y order. The area  75  is formed adjacent to the area  73  which has been formed at the downstream end in the paper feeding direction during the first movement. Namely, two intermediate areas  71  and  73  are thus positioned between the two areas  70  and  75  onto which the inks from the high density portions  22 C,  22 M, and  22 Y landed during the first movement and the second movement. 
     The intermediate area  71  is positioned at the downstream of the intermediate area  73  in the paper feeding direction. During the second movement, the inks from the two low density portions  24 C and  24 M for cyan and magenta land in the C-M order onto the dots, among the dots formed during the first movement (see  FIG. 11 ), formed only from the yellow ink. Namely, in this area, the dots formed by the inks landed in the Y-M-C order during the first movement, and the dots formed by the inks landed in the Y-C-M order in which the yellow ink landed during the first movement and the cyan and magenta inks landed during the second movement, are mixed. In the intermediate area  73  positioned at the upstream of the intermediate area  71  in the paper feeding direction, the inks from the two high density portions  22 C and  22 M for cyan and magenta land in the C-M order during the second movement onto the dots formed from just the yellow ink during the first movement. Further, during the second movement, the inks land from the two high density portions  22 C and  22 M for cyan and magenta and the yellow low density portion  24 Y in the C-M-Y order. Accordingly, in this area, the dots landed in the Y-C-M order and the dots landed in the C-M-Y order are mixed. 
     Thus, the area  70  in which dots of cyan-biased color tint formed by the inks landing in the Y-M-C order during the first movement; the area  74  in which dots of yellow-biased color tint are formed by the inks landing in the C-M-Y order during the second movement; the intermediate area  71  in which the dots landed in Y-M-C order and having cyan-biased color tint and the dots landed in the Y-C-M order and having magenta-biased color tint are mixed; and the intermediate area  73  in which the dots landed in the Y-C-M order and having magenta-biased color tint and the dots landed in the C-M-Y order and having a yellow-biased color tint are mixed, are formed on the recording paper  7 . Because the two kinds of intermediate areas  71  and  73  are present between the area  70  and the area  75 , the change of color between the area  70  and the area  75  is made gradual, the color banding is thus made inconspicuous, and the printing quality is improved. 
     Second Modification 
     The nozzle groups  21 C,  21 M, and the  21 Y of the three colors which respectively jet the three color inks may be arranged at positions shifted from one another in the paper feeding direction. For example, as shown in  FIG. 13 , an yellow nozzle group  21 Y at the right end is shifted toward the upstream in the paper feeding direction (upper side in  FIG. 13 ) with respect to a magenta nozzle group  21 M at the center, and further a cyan nozzle group  21 C at the left end is shifted toward the upstream in the paper feeding direction with respect to the yellow nozzle group  21 Y. In the paper feeding direction, the low density portions  23  of the nozzle groups  21  of the three colors are positioned adjacent each other in the order of cyan ( 23 C,  24 C), yellow ( 23 Y,  24 Y), and magenta ( 23 M,  24 M) from the upstream side. 
     In this case, as shown in  FIGS. 13 and 14 , during the first movement of the ink-jet head  1 , the inks from the three high density portions  22 C,  22 M, and  22 Y land in the order of Y to M then to C (Y-M-C order) to form an area  80  of a cyan-biased color tint; and during the second movement of the ink-jet head  1 , the inks from the three high density portions  22 C,  22 M, and  22 Y land in the order of C to M then to Y (C-M-Y order) to form an area  84  of a yellow-biased color tint. Further, three areas  81 ,  82 , and  83  are formed between the areas  80  and  84 . Among the three areas  81 ,  82 , and  83 , the area  81  positioned at the most downstream side has dots landed in the Y-M-C order and having a cyan-biased color tint and dots landed in the Y-C-M order and having a magenta-biased color tint mixed therein. The central area  82  has dots landing in Y-C-M order and having a magenta-biased color tint and dots landing in C-M-Y order and having yellow-biased color tint mixed therein. The area  83  at the most upstream side is formed of dots landed in the C-M-Y order and has of the same color tint as that of the area  84 . In this modification also, the two kinds of intermediate areas  81  and  82  are present between the area  80  of cyan-biased color tint in which the inks landed in the Y-M-C order and the areas  83  and  84  of yellow-biased color tint in which the inks landed in the C-M-Y order, thereby making any color banding to be inconspicuous and thus improving the printing quality. 
     Third Modification 
     Among two nozzle groups  21  arranged at position which are mutually shifted, low density portions  23  and  24  of one of the two nozzle groups  21  and low density portions  23  and  24  of the other of the two nozzle groups  21  may be positioned so as to be partially overlapped as viewed from the scanning direction. For example, in  FIG. 15 , an yellow nozzle group  21 Y positioned at the right end is shifted toward the upstream in the paper feeding direction with respect to a cyan nozzle group  21 C and a magenta nozzle group  21 M which are positioned to the left of the yellow nozzle group  21 Y; and yellow low density portions  23 Y and  24 Y partially overlap with cyan low density portions  23 C and  24 C and magenta low density portions  23 M and  24 M just by a pitch P as viewed in the scanning direction (left and right direction). In other words, the shift amount of the yellow nozzle group  21 Y in the paper feeding direction is the pitch P. 
     As shown in  FIGS. 15 and 16 , during the first movement of the ink-jet head  1 , the inks from the three high density portions  22 C,  22 M, and  22 Y land in an order of Y to M then to C (Y-M-C order) to form an area  90  of a cyan-biased color tint; and during the second movement of the ink-jet head  1 , the inks from the three high density portions  22 C,  22 M, and  22 Y land in an order of C to M then to Y (C-M-Y order) to form an area  94  of a yellow-biased color tint. Further, three intermediate areas  91 ,  92 , and  93  are formed between the areas  90  and  94 . Among the three areas  91 ,  92 , and  93 , the area  91  at the most downstream side has dots landed in the Y-M-C order and having a cyan-biased color tint and dots landed in an order of Y to C then to M (Y-C-M order) and having a magenta-biased color tint mixed therein. The central area  92  at the center has dots landed in the Y-M-C order and having a cyan-biased color tint and dots formed in an order of C to M then to Y (C-M-Y order) and having a yellow-biased color tint mixed therein. Furthermore, the area  93  at the most upstream side has dots landed in the Y-C-M order and having a magenta-biased color tint and dots landed in C-M-Y order and having a yellow-biased color tin mixed therein. In this modification, since the three intermediate areas  91  to  93  are present between the area  90  of cyan-biased color tint in which the inks landed in the Y-M-C order, and the area  94  of yellow-biased color tint in which the inks landed in the C-M-Y order, color banding is made inconspicuous and the printing quality is thus improved. 
     Fourth Embodiment 
     It is not necessarily indispensable that the nozzle spacing distance in the low density portions are fixed or constant. Alternatively, as shown in  FIG. 17 , the nozzle spacing distance in two low density portions  23 A and  24 A of a nozzle group  21 A may be increased or widened toward both ends in the paper feeding direction, respectively. In this case, since the nozzle spacing distance in each of the low density portions  23 A and  24 A are changed in a stepwise manner, not less than two kinds of intermediate areas can be formed, by the two low density portions  23 A and  24 A, between an area in which dots are formed by the high density portion  22  during the first movement and an area in which dots are formed by the high density portion during the second movement, without arranging the nozzle groups  21 C,  21 M, and  21 Y of the three colors at positions shifted from one another in the paper feeding direction. The difference in color tint between the areas formed by the high density portions  22  can thus be made inconspicuous. It should be noted that the nozzle spacing distance in each of the two low density portions  23 A and  24 A is appropriately set so that the spacing distance between dots formed by the jetting of ink once at a time onto the same area from the two low density portions  23 A and  24 A is same as the spacing distance between dots formed by one time of jetting from the high density portion  22 . Further, with respect to a plurality of nozzle rows forming each of the nozzle groups, it is possible to vary or change the nozzle spacing distance in the paper feeding direction for low density portions for each of the nozzle groups by differing the number of nozzles  20  for each of the nozzle rows forming one of the nozzle groups. 
     In the above-described embodiment and modifications thereof, it is not necessarily indispensable that a nozzle group is formed of four nozzle rows. Alternatively, the nozzle groups may be formed, for example, of one nozzle row or any arbitrary number of nozzle rows. Alternatively, the number of nozzle groups which jet inks of different colors is not limited to four and may be changed as appropriate according to the number of inks used. Still alternatively, when the inks are to be discharged only when the ink-jet head moves toward one side of the scanning direction, for example, the inks may be jetted only from the high density portions of the nozzle groups without jetting the inks from the low density portions of the nozzle groups.