Patent Publication Number: US-6705700-B2

Title: Liquid discharge head, and head cartridge and image forming apparatus using such liquid discharge head

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid discharge head having nozzles for discharging liquid, and a head cartridge and an image forming apparatus that use such a liquid discharge head. 
     2. Description of the Related Art 
     Ink discharging methods of the ink jet recording type that have been widely used today may be roughly classified into a method that uses an electro-thermal converter (heater) as a discharge energy generating element that is used for discharging an ink droplet, and a method using a piezoelectric element for the same purpose. In both methods, it is possible to control discharge of an ink droplet using an electrical signal. For example, the principle of the ink droplet discharging method using the electro-thermal converter is that, by giving an electrical signal to an electro-thermal converter, ink near the electro-thermal converter is boiled in an instant and, based on rapid growth of a bubble that is generated due to a phase change of ink thereupon, an ink droplet is discharged at high speed. On the other hand, the principle of the ink droplet discharging method using the piezoelectric element is that, by giving an electrical signal to a piezoelectric element, the piezoelectric element displaces and, based on a pressure generated upon this displacement, an ink droplet is discharged. The former method has merits such that it is not necessary to so much space for the discharge energy generating element, a configuration of an ink jet print head is simple and thus integration of nozzles is easy, and so on. 
     Recently, following the increasing processing speed of personal computers and the spread of the Internet and so on, the demand for high-speed processing of color images have been increasing more and more, i.e. the demand for quick printing of recorded materials with high fineness and high gradation, that is, so-called extremely high-quality recorded materials, has been increasing, so that printers that can produce high-quality images at high speed have been demanded. 
     For obtaining a high-quality image with high fineness and high gradation, such a method is suitable that performs recording by discharging a very small ink droplet from each nozzle. On the other hand, for speed-up, it is necessary to discharge ink droplets repeatedly at a short period from nozzles. Further, it is also required that a carriage mounting thereon a print head moves at high speed synchronously with a response frequency of the head. When performing recording by repeatedly discharging a very small ink droplet from each nozzle as described above, a stripe  101 , for example, is generated in an image of a paint-out printing portion, i.e. a solid printing portion  100 , of a bar graph as shown in FIG.  10 . The stripe  101  just corresponds to a transition portion between nth scan and (n+1)th scan. 
     FIG. 11B is an enlarged view of this transition portion, and FIG. 11A shows the state thereupon wherein ink droplets  102  are discharged from a head  103 . When image data is solid, nozzles SEG 0  to SEG 255  are all driven at a high response frequency. Thus, due to discharge of ink droplets  102  from those nozzles in end regions of the image data, the air with viscosity around the discharged ink droplets moves at a speed substantially equal to that of the ink droplets. Then, the air over the whole discharge port array moves in the same direction as the ink droplets  102  so that a pressure-reduced state is generated in that portion. Accordingly, the air other than the air around the discharged ink droplets moves toward the pressure-reduced portion, so that air flows as shown by arrows in FIG. 11A are generated. As a result, a discharge direction of the ink droplets  102  discharged from the nozzles located in end regions of the nozzle array is dethroughted or mis-aligned toward the center of the nozzle array from expected positions due to the air flows, i.e. inward jetting of the ink droplets occurs. Further, due to air flows generated upon movement of a carriage in a main scanning direction upon recording, air flows toward the center of the nozzle array are generated, so that the discharge direction of the ink droplets  102  discharged from those nozzles located in the end regions of the nozzle array are mis-aligned toward the center of the nozzle array due to the air flows. As a result, there has been a problem that hit positions of the ink droplets on a recording medium are mis-aligned to cause the stripe  101  as shown in FIG. 11B ((dot) mis-alignment). If the discharge amount of ink is increased for preventing the generation of the stripe  101 , waviness is caused on the recording medium due to overflow of ink from the recording medium or absorption of ink into the recording medium, thereby deteriorating a recording image. Particularly, in the image formation with high fineness and high resolution, it is important to reduce graininess and reproduce fine lines, and thus, it is required that a dot size be as small as possible. Accordingly, also from this aspect, it is not desirable. On the other hand, if a period for repetitive discharge of ink droplets is prolonged, the generation of air flows is eased. However, the speed of the printer is lowered, so that it is not possible to satisfy the users&#39; demand for high-speed printing. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a liquid discharge head that is capable of high-speed recording and can reduce generation of a stripe in a recording image, and further provide a head cartridge and an image forming apparatus that use such a liquid discharge head. 
     As a result of detailed review by the inventors, the following has been made clear. 
     (a) Not only the ink droplets discharged from those discharge ports located at end portions of the discharge port array are mis-aligned. For example, even in the state where ink droplets  102  are discharged from discharge ports of part of a head  103  as shown in FIG. 12A, the ink droplets  102  discharged from the discharge ports located at end portions in the discharge port array direction are mis-aligned toward an intermediate portion of the discharge ports. On the other hand, a stripe  101  between solid portions  100  becomes conspicuous in an image as shown in FIG. 12B when it appears at a transition portion between recording scans. Thus, particularly the end portions of the discharge port array causes a problem. 
     (b) As the number of discharging discharge ports upon recording is reduced, a mis-alignment amount of liquid droplets discharged from discharge ports at end portions of an image is reduced. It is estimated that this is because, due to the reduction in number of the discharging discharge ports, a pressure-reduced state at the intermediate portion of the discharging discharge ports is weakened so that air flows that cause mis-alignment become reluctant to occur. A relationship between the number of discharged liquid droplets and the mis-alignment is shown in FIG. 6 as a specific example, which will be described later. No mis-alignment in a direction perpendicular to the main scanning direction due to air flows occurs when only one liquid droplet is discharged. As the number of the discharged liquid droplets increases, the mis-alignment amount is increases. It is also estimated that, also in case of reduction in discharged liquid droplet amount, air flows become reluctant to occur inasmuch as a pressure-reduced state-at the intermediate portion of the discharging discharge ports is weakened. 
     (c) Further, as resolution in a sub-scanning direction for recording is lowered, the mis-alignment amount of liquid droplets discharged from discharge ports at the end portions is reduced. 
     (d) The present inventors have made detailed review based on the foregoing air flow model, and configured such that a volume of each of discharge ports at end portions is greater than a volume of each of discharge ports at an intermediate portion. As a result, the mis-alignment amount of liquid droplets discharged from the discharge ports at the end portions was reduced. As described above, it is estimated that this is because, due to the reduction in volume of the intermediate portion of the discharge port array, a pressure-reduced state caused by flying of liquid droplets at the intermediate portion was weakened. By incorporating it into a liquid discharge head that performs gradation recording by preparing discharge ports whose discharge amounts of liquid droplets are different, the foregoing misalignment amount can be achieved without increasing the size of the head. 
     For accomplishing the foregoing object based on the foregoing results of review, according to the present invention, there is provided a liquid discharge head having a plurality of discharge port arrays each having a plurality of discharge ports and each arranged substantially in parallel to a print medium conveying direction, and a plurality of discharge energy generating portions for discharging liquid from the discharge ports, respectively, the liquid discharge head moved to scan in a direction crossing the conveying direction, the liquid discharge head characterized in that each discharge port array includes first discharge ports each for discharging a liquid droplet of a first volume, and second discharge ports each for discharging a liquid droplet of a second volume being smaller than the first volume, the discharge port arrays that are adjacent to each other have each of the first and second discharge ports as a pair in the scanning direction, and at least one of the discharge port arrays is a first discharge port array including first discharge port groups respectively having the first discharge ports arranged on both end sides of the at least one discharge port array, each of the first discharge port groups including an end discharge port of the first discharge ports that discharges liquid contributing to image formation, the first discharge port array further including at least one second discharge port group having the second discharge ports arranged between the first discharge port groups. 
     In the liquid discharge head of the invention as configured above, the first discharge port groups having the discharge ports each discharging a liquid droplet of a large volume are arranged on both end sides of the discharge port array so as to include end discharge ports that contributes to image formation, and a volume of each of liquid droplets discharged from the discharge ports forming the second discharge port group arranged between the first discharge port groups is set small. With this configuration, a pressure-reduced state caused by flying of the liquid droplets discharged from the intermediate portion of the discharge port array is weakened, so that the mis-alignment amount of the liquid droplets discharged from the discharge ports located at the end portions of the discharge port array can be reduced. 
     Further, the liquid discharge head of the present invention can achieve reduction of the foregoing mis-alignment of the liquid droplets without increasing the size of the head in the configuration that can perform gradation recording. 
     It may be configured that a discharge port area of each of the first discharge ports forming the first discharge port group is greater than a discharge port area of each of the second discharge ports forming the second discharge port group. 
     It may be configured that each of the discharge energy generating portions has an electro-thermal converter that generates thermal energy for causing film boiling in liquid to discharge the liquid from the discharge ports. 
     According to the present invention, a head cartridge is characterized by comprising the liquid discharge head of the present invention and a liquid tank storing liquid to be supplied to the liquid discharge head. 
     It may be configured that the liquid tank is detachable relative to the liquid discharge head through attaching/detaching means. 
     According to the present invention, an image forming apparatus is characterized by comprising a mounting portion for the liquid discharge head of the present invention, wherein an image is formed on a print medium using liquid discharged from discharge ports of the liquid discharge head. 
     As described above, since the image forming apparatus of the present invention forms an image on a print medium using the liquid discharge head of the present invention, the amount of mis-alignment in which a liquid droplet is not discharged to an expected position on the print medium, which has been the conventional problem, is reduced. Thus, even if solid printing is carried out, a high-quality print image with high fineness and high gradation can be obtained without generating a stripe. 
     It may be configured that the mounting portion has a carriage that is movable for scanning in a direction crossing a print medium conveying direction. 
     Further, it may be configured that the liquid discharge head is detachably mounted on the carriage through attaching/detaching means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a schematic configuration of one preferred embodiment wherein an image forming apparatus according to the present invention is applied to an ink jet printer; 
     FIG. 2 is a perspective view showing an external appearance of one preferred embodiment in a disassembled state wherein a head cartridge according to the present invention is applied to the ink jet printer shown in FIG. 1; 
     FIG. 3 is a perspective view of a print head in the head cartridge shown in FIG. 2; 
     FIG. 4 is a cutaway perspective view showing a schematic configuration of the main part of the print head-shown in FIG. 3; 
     FIG. 5 is a sectional view, taken along line  5 — 5 , of the print head shown in FIG. 4; 
     FIG. 6 is a graph showing a relationship between the number of discharged liquid droplets and mis-alignment amount; 
     FIG. 7 is a plan view showing arrays of discharge ports and electro-thermal converters of a print head according to a first preferred embodiment of the present invention; 
     FIG. 8 is a cutaway plan view showing arrays of discharge ports and electro-thermal converters of a print head according to a second preferred embodiment of the present invention; 
     FIG. 9 is a cutaway plan view showing arrays of discharge ports and electro-thermal converters of a print head according to a third preferred embodiment of the present invention; 
     FIG. 10 is a diagram showing one example for explaining a stripe generated upon recording by a conventional print head; 
     FIG. 11A is a conceptual diagram exemplarily showing one example of an ink discharging state caused by a conventional ink jet printer; 
     FIG. 11B is a conceptual diagram exemplarily showing one example of a solid image formed on a print medium in one pass; 
     FIG. 12A is a conceptual diagram exemplarily showing another example of an ink discharging state caused by a conventional ink jet printer; and 
     FIG. 12B is a conceptual diagram exemplarily showing another example of a solid image formed on a print medium in one pass. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
     Numerical values shown in the following respective embodiments are only an example, and the present invention is not limited thereto. Further, the present invention is not limited to the respective embodiments, but may include combinations thereof, and is further applicable to other techniques to be contained in the concept of the present invention as defined in the appended claims. 
     (First Embodiment) 
     One preferred embodiment wherein an image forming apparatus according to the present invention is applied to an ink jet printer will be described in detail referring to FIGS. 1 to  7 . 
     An external appearance of a mechanical portion of the ink jet printer in this embodiment is shown in FIG. 1, an external appearance of a head cartridge used in this ink jet printer is shown in FIG. 2 in a disassembled state, and an external appearance of a print head thereof is shown in FIG.  3 . Specifically, a chassis  10  of the ink jet printer in this embodiment is having a plurality of plate-shaped metal members having a prescribed rigidity, and forms a framework of the ink jet printer. On the chassis  10  are mounted a medium feed portion  11  for automatically feeding a print medium in the form of a sheet toward the inside of the ink jet printer, a medium conveying portion  13  for conveying a print medium fed one by one from the medium feed portion  11  to a given print-position and further conveying the print medium to a medium discharge portion  12 , a print portion for carrying out a prescribed printing operation relative to the print medium conveyed to the print position, and a head recovery portion  14  for carrying out a recovery process relative to the print portion. 
     The print portion comprises a carriage  16  that is supported movably in a main scanning direction along a carriage shaft  15 , and a head cartridge  18  detachably mounted onto the carriage  16  through a head set lever  17 . 
     The carriage  16  to be mounted with the head cartridge  18  is provided with a carriage cover  20  for positioning a print head  19  of the head cartridge  18  in a prescribed mounting position on the carriage  16 , and the foregoing head set lever  17  that engages with a tank holder  21  of the print head  19  and pushes it so as to place the print head  19  in the prescribed mounting position. The head set lever  17  is pivotally mounted on a head set lever shaft (not shown) at an upper portion of the carriage  16 , and provided, at an engaging portion with the print head  19 , with a head set plate (not shown) urged by a spring. By means of a spring force of the head set plate, the head set lever  17  pushes the print head  19  thereby to mount it onto the carriage  16 . 
     One end of a contact flexible print cable (hereinafter referred to as “contact FPC”)  22  (not shown) is connected to another engaging portion, relative to the print head  19 , of the carriage  16 . A contact portion (not shown) formed at such one end of the contact FPC  22  and a contact portion  23 , as external signal input terminals, provided in the print head  19  are brought into contact with each other so as to be electrically connected therebetween, so that exchanges of various information for printing, power feeding to the print head  19 , and so on can be performed. 
     Between the contact portion of the contact FPC  22  and the carriage  16  is provided an elastic member such as rubber. By means of an elastic force of this elastic member and a pushing force of the head set plate, the contact portion of the contact FPC  22  and the contact portion  23  of the print head  19  can be securely contacted therebetween. Another end of the contact FPC  22  is connected to a carriage substrate (not shown) mounted at the back of the carriage  16 . 
     The head cartridge  18  in this embodiment comprises ink tanks  24  storing ink, and the foregoing print head  19  for discharging ink, supplied from the ink tanks  24 , through discharge ports  25  (see FIG. 4) of the print head  19  according to print information. The print head  19  in this embodiment employs the so-called cartridge type wherein the print head  19  is detachably mounted onto the carriage  16 . 
     In this embodiment, for enabling high-quality photographic color printing, the independent six ink tanks  24  can be used for the colors of, for example, black, light cyan, light magenta, cyan, magenta, and yellow, respectively. Each ink tank  24  is provided with an elastically deformable removing lever  26  that is retainable relative to the head cartridge  18 . By operating this removing lever  26 , each ink tank  24  is detachable relative to the print head  19  as shown in FIG.  3 . 
     The print head  19  comprises a later-described print element substrate  27 , the foregoing tank holder  21 , and so on. FIG. 4 shows a cutaway configuration of the print element substrate  27  of the print head  19  in this embodiment, and FIG. 5 shows a  5 — 5  sectional configuration thereof. The print element substrate  27  in this embodiment is in the form of a silicon substrate having a thickness of 0.5 mm to 1 mm, on which discharge energy generating portions, common ink chambers  31 , ink passages  33 , discharge ports  25 , and so on are formed using a film formation technique. Specifically, the print element substrate  27  is formed with ink supply ports  28  each in the form of an elongate hole penetrating the print element substrate  27 , and stacked with a discharge port plate  5  formed with the discharge ports  25 , through a coating resin layer  36 . On both sides of the ink supply port  28 , a plurality of electro-thermal converters  29  are formed in two lines each extending along a print medium conveying direction, i.e. along a longitudinal direction of the ink supply port  28 . The electro-thermal converters  29  are arranged with a predetermined pitch in each line, and offset by a half pitch between the respective two lines. The electro-thermal converters  29  in each line form a discharge energy generating portion. A distance between the centers of the two lines is 233 μm. In this embodiment, the number of the electro-thermal converters  29  in each is  128 . Apart from the electro-thermal converters  29 , the print element substrate  27  is formed with electrode terminals  30  for electrical connection between the electro-thermal converters  29  and the side of a printer body, electrical wiring (not shown) made of aluminum or the like, and so on, using a film formation technique. 
     An electrical wiring substrate connected to the electrode terminals  30  formed on the print element substrate  27  is for applying electrical signals for discharging ink, to the print element substrate  27 . The electrical wiring substrate has electrical wiring corresponding to the print element substrate  27 , and the foregoing contact portion  23  located at an end portion of such electrical wiring for receiving electrical signals from the printer body. The contact portion  23  is fixed on the back side of the tank holder  21 . A driving signal is given to the electro-thermal converter  29  from a driving IC (not shown) through the electrical wiring substrate, and simultaneously, driving power is fed to that electro-thermal converter  29 . 
     The tank holder  21  detachably holding the ink tanks  24  is formed with ink flow passages extending from the ink tanks  24  to the corresponding ink supply ports  28  of the print element substrate  27 . 
     On the print element substrate  27 , an upper plate member  32  is formed that has the terminals  25  confronting the electro-thermal converters  29 , respectively, through each of the common ink chambers  31  communicating with the corresponding ink supply ports  28 . Specifically, the ink passages  33  each establishing communication between the corresponding discharge port  25  and the common ink chamber  31  are formed between the upper plate member  32  and the print element substrate  27 , and partition walls  34  are formed between the adjacent ink passages  33 . The common ink chambers  31 , the ink passages  33 , the partition walls  34 , and so on are formed along with the upper plate member  32 , like the discharge ports  25 , using a photolithography technique. 
     Liquid supplied from the ink supply port  28  into each ink passage  33  boils following heat generation of the electro-thermal converter  29  exposed to the corresponding ink passage  33  when a driving signal is given to such an electro-thermal converter  29 , and is discharged from the corresponding discharge port  25  due to a pressure of a bubble generated thereupon. In this event, a bubble generated in the liquid chamber  31  is, following growth thereof, brought into the state communicating with the air. 
     In the print head of this embodiment, the discharge ports  25  are arranged in two lines at a pitch of 600 dpi, with the 128 discharge ports  25  in each line. Volumes of discharged ink droplets are 5.0 pl and 2.5 pl, i.e. two kinds, and an ink density is 1.05. Since resolution of the ink jet printer in the carriage scanning direction is 600 dpi and a driving frequency is 15.0 kHz, the moving speed of the carriage is about 635 mm/s and one discharge port carries out discharge per about 66.7 μs at the minimum time interval. Further, a distance from the discharge port surface to the surface of the print medium is 1.5 mm. The ink jet printer carries out unidirectional recording. 
     According to the review by the present inventors, when the number of discharged liquid droplets that are simultaneously discharged from the discharge ports, i.e. when the number of the discharge ports that operate simultaneously, is eight or less on one side in a line, the mis-alignment amount of the discharged liquid droplets at the end portion is several μm or less as shown in FIG. 6, which raises no problem in view of the image quality. 
     Therefore, in this embodiment, as shown in FIG. 7, a discharge port array  60  has first discharge port groups  50  each having, among the 128 discharge ports, the 8 discharge ports  25  located at one of two end portions (thus 16 discharge ports at both end portions) and including the end discharge port  25 ′, and a second discharge port group  51  having the remaining 112 discharge ports located at an intermediate portion between the respective first discharge port groups  50 . In this embodiment, ink droplets each having a volume of 5.0 pl are discharged from the first discharge port groups  50 , and ink droplets each having a volume of 2.5 pl are discharged from the second discharge port group  51 . 
     On the other hand, in a discharge port array  61 , as opposed to the discharge port array  60 , the second discharge port groups  51  are arranged on both end sides of the discharge port array, and the first discharge port group  50  is arranged at an intermediate portion between the second discharge port groups  51 . A diameter of each discharge port  25  of the first discharge port group  50  that discharges 5 pl is set to Ø16.0 μm each of the electro-thermal converters is set to 26×26 μm, a diameter of each discharge port  25  of the second discharge port group  51  that discharges 2.5 pl is set to Ø11.0 μm, and each of the electro-thermal converters is set to 22×22 μm. A distance between the discharge port array  60  and the discharge port array  61  is set to 215 μm. 
     In this embodiment, the formed discharge ports  25  are all used for image formation. On the other hand, if dummy discharge ports are formed that are not used for image formation, the discharge port  25  located at an end among the discharge ports  25  excluding the dummy discharge ports is set as the end discharge port  25 ′. 
     In the ink jet printer of the present invention, those discharge ports with the volume of 5.0 pl are used for one-pass recording. In the print head of this embodiment, the first discharge port groups  50  corresponding to the 5.0 pl discharge ports are arranged being separated between the discharge port array  60  and the discharge port array  61 , i.e. offset in the main scanning direction. Further, the second discharge port group  51  is arranged at the intermediate portion in the discharge port array  60  and the volume of each ink droplet discharged therefrom is reduced to 2.5 pl. As a result, the mis-alignment amount with respect to the 16 discharge ports  25  at the end portions upon one-pass recording was reduced to 5 μm while it was 20 cm in case of the compared conventional head, so that the stripe at the transition portion of the carriage scan did not appear. 
     The foregoing one-pass recording is one example wherein ink droplets were discharged from both the first discharge port groups  50  (5.0 pl discharge ports) and the second discharge port groups  51  (2.5 pl discharge ports). On the other hand, when one-pass recording was carried out by discharging ink droplets only from the first discharge port groups  50  (5.0 pl discharge ports) for achieving high-speed recording, the ink droplets were discharged only from the first discharge port groups  50  located at both end portions with respect to the discharge port array  60 . Accordingly, inasmuch as ink droplets were not charged from the second discharge port group  51  located at the intermediate portion, a pressure-reduced state, which is caused by flying of liquid droplets at the intermediate portion, was not generated. Therefore, the mis-alignment becomes reluctant to occur so that the mis-alignment amount with respect to the 16 discharge ports at the end portions was reduced to 4 μm, and thus, the stripe did not appear at the transition portion of the carriage scan. 
     On the other hand, in the ink jet printer of the present invention, both 5.0 pl discharge ports and 2.5 pl discharge ports are used in case of multi-pass recording. When review was performed with four-pass recording being one of multi-pass recording, since the recording duty per pass was lowered, the end mis-alignment amount was reduced to about 6 μm. When recording was performed using 5.0 pl and 2.5 pl discharge ports, since it is more noticeable when the mis-alignment occurs with 5.0 pl discharge ports, there was no generation of stripes. 
     In this embodiment, each of the first discharge port groups  50  located at both end portions of the discharge port array  60  has the 8 discharge ports  25  and thus the 16 discharge ports  25  in total. However, the present invention is not limited those numbers. 
     (Second Embodiment) 
     In the foregoing first embodiment, the number of the discharge ports forming each discharge port array is set to 128. On the other hand, in this embodiment, description will be given about a print head wherein each discharge port array has 256 discharge ports and those discharge ports that discharge ink droplets of a large volume are arranged at an intermediate portion. Those elements having the same functions as those in the first embodiment are assigned the same reference symbols, thereby to omit duplicate explanation thereof. 
     FIG. 8 is a cutaway plan view showing discharge port arrays of the print head in this embodiment. 
     A discharge port array  65  has first discharge port groups  50  arranged at both end portions and each having 8 discharge ports  25 , and second discharge port groups  51  arranged adjacent to the first discharge port groups  50 , respectively, and further has a first discharge port group  50  arranged between the second discharge port groups  51  and having 64 discharge ports  25 . 
     On the other hand, as opposed to the discharge port array  65 , a discharge port array  66  has second discharge port groups  51  arranged at both end portions and each having 8 discharge ports  25 , and first discharge port groups  50  arranged adjacent to the second discharge port groups  51 , respectively, and further has a second discharge port group  51  arranged between the first discharge port groups  50  and having 64 discharge ports  25 . 
     With the foregoing arrays of the discharge ports  25 , recording was carried out under the same condition as the first embodiment. As a result, the mis-alignment amount of the ink droplets discharged from the 16 discharge ports  25  at both end portions was reduced to 5 μm while it was 20 μm in case of the compared conventional head, so that the stripe at the transition portion of the carriage scan did not appear. 
     (Third Embodiment) 
     In the print head described in the foregoing first embodiment, the first and second discharge port arrays  60  and  61  are provided only as one pair. On the other hand, in a print head of this embodiment, the first and second discharge port arrays are provided as a plurality of pairs, so that bidirectional recording with reciprocating scanning can be carried out. Also in this embodiment, those elements having the same functions as those in the first embodiment are assigned the same reference symbols, thereby to omit duplicate explanation thereof. 
     FIG. 9 is a cutaway plan view showing discharge port arrays of the print head in this embodiment. 
     In the print head of this embodiment, 10 discharge port arrays  71  to  75  and  81  to  85  are arranged at a prescribed pitch. Each of the discharge port arrays  73 ,  75 ,  83  and  85  has the first discharge port arrays  50  arranged at both end portions and the second discharge port group  51  arranged therebetween, which is the same as the discharge port array  60  in the first embodiment. On the other hand, each of the discharge port arrays  72 ,  74 ,  82  and  84  has the second discharge port arrays  51  arranged at both end portions, and the first discharge port array  50  is arranged therebetween, which is the same as the discharge port array  61  in the first embodiment. Further, each of the discharge port arrays  71  and  81  is having only the first discharge port group  50 . 
     The discharge port arrays  75 ,  73 ,  71 ,  83  and  85  form a first discharge port array group  90  wherein the discharge ports thereof are arranged such that the ith discharge ports correspond to each other as shown by a broken line in FIG.  9 . On the other hand, the discharge port arrays  74 ,  72 ,  81 ,  82  and  84  form a second discharge port array group  91  wherein the discharge ports thereof are arranged such that the jth discharge ports correspond to each other as shown by a broken line in FIG.  9 . 
     Further, among the 10 discharge port arrays, the outermost discharge port arrays  74 ,  75 ,  84  and  85  discharge cyan (C), the discharge port arrays  72 ,  73 ,  82  and  83  discharge magenta (M), and the innermost discharge port array  71  and  81  adjacent to each other discharge yellow (Y). 
     In this embodiment,  128  discharge ports are arranged in each array at a pitch of 1200 dpi, and ink droplets are discharged toward a print medium from the discharge ports. A volume of each discharged ink droplet is 5.0 pl and an ink density is 1.05, so that the volume is large enough to fill dots relative to resolution of 1200 dpi in a sub-scanning direction (discharge port array direction) so as not to generate a white stripe upon recording. In the ink jet printer, resolution in the carriage scanning direction is 600 dpi, recording resolution is 1200 dpi and a driving frequency is 15. Accordingly, the moving speed of the carriage is about 635 mm/s and one discharge port carries out discharge per about 80 μs at the minimum time interval. Further, a distance from the discharge port surface to the surface of the print medium is 1.5 mm. The ink jet printer carries out bidirectional recording. 
     Based on the detailed review by the present inventor, comparison was made between a head wherein the discharge port groups arranged at both end portions in the discharge port array direction in the discharge port array are offset in an advancing direction (forward) of the main scan relative to the discharge port group arranged at the intermediate portion and a head wherein, as opposite thereto, the discharge port arrays at both end portions are offset in a direction opposite (backward) to the advancing direction of the main scan relative to the discharge port array at the intermediate portion. As a result, it has been made clear that the mis-alignment amount of liquid droplets discharged from the discharge ports at the end portions was 6 μm in the former, which was smaller than 9 μm of the latter. Therefore, in the bidirectional head of this embodiment, in a first discharge port region  92  having the discharge port arrays  72  to  75 , 5.0 pl discharge ports are provided in the array on the left in the figure for each color, while 2.5 pl discharge ports are provided on the right in the figure, on the other hand, in a second discharge port region  93  having the discharge port arrays  82  to  85 , 5.0 pl discharge ports are provided in the array on the right in the figure for each color, while 2.5 pl discharge ports are provided on the left in the figure. For example, with respect to the discharge port arrays  74 ,  75 ,  84  and  85  that discharge cyan ink, assuming that ink droplets are discharged using 5.0 pl discharge ports, by discharging the ink droplets using, in combination, the first discharge port groups  50  located at both end portions of the discharge port array  75  and the first discharge port group  50  located at the intermediate portion of the discharge port array  74  upon forward direction recording, the end mis-alignment amount of the first discharge port arrays  50  located at both end portions of the discharge port array  75  can be reduced. On the other hand, upon reverse direction recording, by discharging the ink droplets using, in combination, the first discharge port groups  50  located at both end portions of the discharge port array  85  and the first discharge port group  50  located at the intermediate portion of the discharge port array  84 , the end mis-alignment amount of the first discharge port arrays  50  located at both end portions of the discharge port array  85  can be reduced. 
     Specifically, by configuring discharge port arrays arranged at both ends in a scanning direction as discharge port arrays each having on both end sides the first discharge port groups  50  that discharge liquid of a large volume, the discharge port array having on both end sides the first discharge port groups  50  that discharge liquid of a large volume can be always located in a forward position with respect to the scanning direction, in each of bidirectional directions upon reciprocating scanning. When the foregoing discharge is carried out, upon forward direction recording, the first discharge port groups  50  on both end sides of the discharge port array  75  first discharge liquid droplets, then the first discharge port group  50  at the intermediate portion of the discharge port array  74  discharge liquid droplets. Specifically, in the discharge port array  75 , since liquid droplets are not discharged from the intermediate portion, but discharged only from both end sides, the liquid droplets discharged from both end sides of the discharge port array  75  are free from an influence of pressure reduction at the intermediate portion of the discharge port array  75  itself. In addition, since the discharge from the intermediate portion of the discharge port array  74  is carried out after the discharge from both end sides of the discharge port array  75  has been performed. Accordingly, the liquid droplets discharged from both end sides of the discharge port array  75  are liable to escape from an influence of the liquid droplets discharged from the intermediate portion of the discharge port array  74 . Therefore, the mis-alignment amount of the liquid droplets at the end portions of the discharge port array can be reduced. 
     In this manner, by equalizing per scan the mis-alignment amounts of two spaced-apart discharge port arrays (e.g. discharge port arrays  75  and  85 ) for the same color, which are scanned through one scan, generation of the stripe can be suppressed.