Patent Publication Number: US-11383521-B2

Title: Apparatus configured to discharge liquid

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2019-038924, filed on Mar. 4, 2019, and Japanese Patent Application No. 2020-023672, filed on Feb. 14, 2020. The contents of which are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus configured to discharge liquid. 
     2. Description of the Related Art 
     Conventionally, an inkjet recording apparatus forms an image by discharging ink liquid in the form of droplets from liquid discharge heads onto a conveyed recording sheet. 
     The ink liquid is supplied to each of the liquid discharge heads by generating negative pressure in a sub tank to which the ink liquid is supplied from a main tank. For example, Japanese Unexamined Patent Application Publication No. 2017-209844 discloses a configuration in which a sub tank is arranged for each of two head arrays that are arranged in a zig-zag manner on an array base such that orientations of head surfaces are aligned. In the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2017-209844, if the head arrays are arranged so as to be inclined together with the array base, a hydraulic head difference occurs between the two arrays that are arranged in a zig-zag manner on the array base; therefore each of the sub tanks is arranged at an appropriate height position in a vertical direction. 
     However, when various droplets are to be discharged onto a recording sheet placed on a curved surface, such as a conveying drum, each of liquid discharge heads that are used for different kinds of liquid, such as different ink colors, to be discharged is arranged so as to face the curved surface at a different inclination in accordance with the curved surface. Therefore, the hydraulic head difference varies between heads that are arranged at different inclinations. Consequently, quality of images that are formed in accordance with the positions of the head arrays used to form the images vary, which is a problem. 
     The present invention has been conceived in view of the foregoing situations, and an object of the present invention is to provide an apparatus configured to discharge liquid, where the apparatus is capable of preventing, with a simple configuration, variation in quality of images that are formed in accordance with different inclinations of liquid discharge heads. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an apparatus configured to discharge liquid includes a plurality of liquid discharge modules arranged at different inclinations in the apparatus. Each of the plurality of liquid discharge modules includes a liquid discharge head, a containing member, and a holding member. The liquid discharge head is configured to discharge liquid. The containing member is configured to contain liquid to be supplied to the liquid discharge head. The holding member is configured to hold the containing member. The holding member of each of the plurality of liquid discharge modules includes a first adjuster configured to adjust a position of the containing member relative to the liquid discharge head in a vertical direction in the apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of an entire configuration of an apparatus configured to discharge liquid according to an embodiment; 
         FIG. 2  is an external perspective view illustrating an example of a configuration of a liquid discharge module according to a first embodiment; 
         FIGS. 3A and 3B  are diagrams illustrating an example of a configuration of a single sub tank; 
         FIG. 4  is a diagram for explaining mounting positions of the sub tanks in the liquid discharge modules; 
         FIGS. 5A and 5B  are diagrams illustrating an example of a configuration of a liquid discharge module according to a second embodiment; 
         FIG. 6  is a diagram illustrating a state in which an external bracket (base bracket) is mounted on the liquid discharge module; 
         FIG. 7  is a diagram illustrating a state in which an adjustment bracket is mounted on the base bracket; 
         FIG. 8  is a diagram illustrating an example of a mounting position in a case where the sub tank is mounted at an adjusted height; 
         FIG. 9  is a diagram illustrating another example of the mounting position in a case where the sub tank is mounted at an adjusted height; 
         FIG. 10  is a diagram illustrating an example of a mounting position in a case where the sub tank is mounted with adjustment in a rotation direction; 
         FIG. 11  is a diagram illustrating another example of the mounting position in a case where the sub tank is mounted with adjustment in the rotation direction; 
         FIG. 12  is a diagram illustrating an example of an attachment state in which the sub tank is attached to the liquid discharge module by using the base bracket and the adjustment bracket; 
         FIG. 13  is a diagram illustrating an example of setting of the sub tanks of the liquid discharge modules that are arranged at different angles along with an outer periphery of a conveying drum; 
         FIGS. 14A and 14B  are diagrams illustrating an example of a configuration of a liquid discharge module according to a modification of the second embodiment; 
         FIGS. 15A and 15B  are diagrams illustrating an example of an attachment state in a case where a plurality of sub tanks are attached to a main body of a single liquid discharge module; 
         FIGS. 16A and 16B  are diagrams illustrating an example of an attachment state in a case where a plurality of sub tanks are attached to the main body of the single liquid discharge module; and 
         FIG. 17  is a diagram for explaining mounting positions of sub tanks in liquid discharge modules according to a modification of the first embodiment. 
     
    
    
     The accompanying drawings are intended to depict exemplary embodiments of the present invention and should not be interpreted to limit the scope thereof. Identical or similar reference numerals designate identical or similar components throughout the various drawings. 
     DESCRIPTION OF THE EMBODIMENTS 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. 
     As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In describing preferred embodiments illustrated in the drawings, specific terminology may be employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that have the same function, operate in a similar manner, and achieve a similar result. 
     An embodiment of the present invention will be described in detail below with reference to the drawings. 
     Exemplary embodiments of an apparatus configured to discharge liquid will be described in detail below with reference to the drawings. The embodiments described below are mere examples and not limited thereto. 
     Embodiment 
     In the present application, a “liquid discharge head” is a functional component that discharges and ejects liquid from a nozzle. The liquid to be discharged is not specifically limited as long as the liquid has a viscosity and surface tension that allow the liquid to be discharged from the head; however, it is preferable that the liquid has a viscosity of 30 mPa/s or below when heated and cooled under normal temperature and normal pressure. More specifically, the liquid may be a solution, a suspension, an emulsion, or the like that contains a solvent such as water or an organic solvent, a colorant such as a dye or a pigment, a function providing material such as a polymerizable compound, a resin, or a surfactant, a biomaterial such as DNA, amino acid, protein, or calcium, or an edible material such as a natural pigment, and, the liquid may be used for uses such as ink for inkjet, a surface treatment liquid, a liquid for forming a constituent element of an electron element or a light-emitting element or for forming an electronic circuit resist pattern, and a material liquid for three-dimensional modeling. 
     In the present application, the “apparatus configured to discharge liquid” is an apparatus that includes a liquid discharge head or a liquid discharge unit, and drives the liquid discharge head to discharge liquid. The apparatus configured to discharge liquid includes not only an apparatus that is able to discharge liquid to a target to which liquid can adhere, but also an apparatus that discharges liquid into the air or liquid. 
     The “apparatus configured to discharge liquid” may further include means related to feed, convey, and eject a target to which liquid can adhere, and include a pre-processing apparatus, a post-processing apparatus, and the like. 
     For example, the “apparatus configured to discharge liquid” may be an image forming apparatus that is an apparatus for forming an image by discharging ink onto a sheet, and a stereoscopic modeling device (three-dimensional modeling device) that models a stereoscopic modeled object (three-dimensional modeled object) by discharging modeling liquid onto powder layers in which powders are laminated. 
     Further, the “apparatus configured to discharge liquid” is not limited to an apparatus by which a significant image, such as a character or a graphic, is visualized by discharged ink. For example, an apparatus that forms a pattern or the like that does not have a meaning in itself and an apparatus that models a three-dimensional image may be adopted. 
     The “target to which liquid can adhere (corresponding to a “discharge target”)” is an object to which liquid can adhere at least temporarily, and represents an object to which liquid adheres and sticks, an object to which liquid adheres and penetrates, and the like. Specifically, the target includes all of objects to which liquid adheres, such as a target recording medium including a sheet, a recording paper, a recording sheet, a film, a cloth, and the like, an electronic component including an electronic substrate, a piezoelectric element, and the like, and a medium including a powder layer (powdered layer), an organ model, an examination cell, and the like, unless specifically limited. 
     A material of the “target to which liquid can adhere” may be any material, such as paper, thread, fiber, fabric cloth, leather, metal, plastic, glass, wood, or ceramics, to which liquid can adhere at least temporarily. 
     Furthermore, the “apparatus configured to discharge liquid” is an apparatus in which the liquid discharge head and the target to which liquid can adhere move relative to each other, but is not limited thereto. Specifically, a serial-type apparatus that moves the liquid discharge head, a linear-type apparatus that does not move the liquid discharge head, and the like may be adopted. 
     Moreover, the “apparatus configured to discharge liquid” includes a treatment liquid applying apparatus that discharges treatment liquid onto a sheet to apply the treatment liquid to a surface of the sheet in order to modify the surface of the sheet, a jet granulation apparatus that ejects composition liquid that is obtained by dispersing raw materials in a solution, and forms fine grains of the raw materials through granulation. 
     The “liquid discharge unit” is a unit in which functional components/mechanisms are integrated with the liquid discharge head, and is an assembly of components related to discharging of liquid. The “liquid discharge unit” is constructed by combining at least a containing member, such as a sub tank, and a holding member for holding the containing member with the liquid discharge head. The containing member contains liquid to be supplied to the liquid discharge head. 
     Here, integration includes, for example, a state in which the liquid discharge head and the functional components/mechanisms are fixed together by fastening, bonding, engaging, or the like, and a state in which one of the liquid discharge head and the functional components/mechanisms is held so as to be movable relative to the other one of them. Furthermore, the liquid discharge head and the functional components/mechanisms may be configured so as to be detachably attached to each other. 
     The “apparatus configured to discharge liquid” according to the embodiments will be described below by taking an image forming apparatus as an example. Meanwhile, three-dimensional orthogonal coordinate axes (an X-axis, a Y-axis, and a Z-axis) set in each of the drawings will be appropriately referred to in the explanation. 
     Entire Configuration 
     An image forming apparatus  1  illustrated in  FIG. 1  includes a sheet feed unit  20 , a registration adjusting unit  30 , an image forming unit  10 , a drying unit  40 , and a paper ejection unit  50 . The image forming apparatus  1  includes a conveying means that is arranged over all of the units from the sheet feed unit  20  to the paper ejection unit  50  and that conveys a recording sheet  22  as one example of a discharge target from the sheet feed unit  20  to the paper ejection unit  50 . As illustrated in  FIG. 1 , the image forming apparatus  1  includes various means along a conveying path of the recording sheet  22 . The image forming apparatus  1  performs a series of operation for forming an image on the recording sheet  22  by controlling the conveying means and the various means. Each of the units illustrated in  FIG. 1  will be described in detail below. 
     The sheet feed unit  20  includes a sheet feed tray  26  in which the recording sheets  22  are stacked, and a supply means that supplies the recording sheets  22  one by one to the registration adjusting unit  30 . In  FIG. 1 , an example is illustrated in which an air separation unit  24  that separates the recording sheets  22  from one another by an air method is illustrated. The air separation unit  24  blows air to a bundle of the recording sheets  22 , so that the recording sheets  22  are separated from one another. The recording sheets  22  that are separated from one another are picked up one by one by a pick-up roller of the supply means and supplied to the registration adjusting unit  30 . A method for separating the recording sheets  22  is not limited to the air method, but an arbitrary method may be used. For example, it may be possible to use a method in which the recording sheets  22  that are picked up as a bundle by the pick-up roller are separated from one another by a separation roller. 
     The registration adjusting unit  30  includes a registration roller  32 . The registration roller  32  adjusts registration (adjusts timing) of the recording sheet  22  fed from the sheet feed unit  20 , and feeds the recording sheet  22  to the image forming unit  10 . 
     The image forming unit  10  includes a conveying drum  12  that adsorbs the recording sheet  22  and feeds the recording sheet  22  in a single direction, and a plurality of liquid discharge modules (one example of the “liquid ejection unit”)  14  that discharge droplets (described as ink droplets as one example) onto the recording sheet  22  to form an image. 
     The conveying drum  12  has a cylindrical shape, and a portion represented by a circular shape in  FIG. 1  corresponds to a top surface portion of the cylinder. An outer periphery (corresponding to a “conveying surface”) provided on a side surface portion of the cylinder is formed in a depth direction of a sheet surface along a circumference of the circle of the top surface portion, and makes an entire loop along the circumference of the circle. The conveying drum  12  holds a leading end of the recording sheet  22  fed from the registration adjusting unit  30  by using a sheet clamper  16  and rotates in a counterclockwise direction in the example illustrated in  FIG. 1 . The conveying drum  12  includes a plurality of air suction holes on the outer periphery, and rotates while sucking a back surface side of the recording sheet  22  by using a suction pump. The conveying drum  12  causes the recording sheet  22  to come into close contact with the outer periphery of the conveying drum  12 , and conveys the recording sheet  22  in the counterclockwise direction along the outer periphery. Namely, in this example, a shape of a curved surface of the outer periphery corresponds to a shape of a surface on which the recording sheet  22  is placed. 
     Each of the liquid discharge modules  14  includes a liquid discharge head that discharges liquid (ink liquid in this example), a containing member that contains ink liquid to be supplied to the liquid discharge head, and a holding member that holds the containing member. Each of the liquid discharge heads includes one or a plurality of arrays of nozzle holes (nozzle arrays) that are arranged in a line in the depth direction of the sheet surface in  FIG. 1 . Each of the liquid discharge modules  14  discharges liquid from the nozzle array of each of the liquid discharge heads onto the recording sheet  22  that is conveyed by the conveying drum  12 , so that an image is formed. In the example illustrated in  FIG. 1 , the sheet clampers  16  are arranged at three positions on the outer circumference of the conveying drum  12 . Therefore, it is possible to form images on the three recording sheets  22  by single rotation of the conveying drum  12 . Details of the liquid discharge modules  14  will be described later with reference to the drawings. 
     The drying unit  40  includes a drier unit  42 . The drier unit  42  dries the recording sheet  22  that carries the formed image and that is conveyed from the image forming unit  10 , and prevents the recording sheet  22  from being curled. By causing the recording sheet  22  to pass through the drier unit  42 , moisture of ink evaporates and the recording sheet  22  is dried. 
     In the example illustrated in  FIG. 1 , a mechanism that deals with duplex printing is included. If the duplex printing is to be performed, a conveying direction is changed to a direction toward the image forming unit  10  by causing a sheet reversing unit  60  to reverse the recording sheet  22  that has been dried, and the sheet reversing unit  60  feeds the reversed recording sheet  22  to the image forming unit  10 . The recording sheet  22  fed to the image forming unit  10  is fed to the conveying drum  12  again to form an image on a back side, after timing is adjusted by a registration roller  64  just before the recording sheet  22  is fed to the conveying drum  12 . 
     The paper ejection unit  50  includes a discharge tray  52  on which the recording sheet  22  ejected from the drying unit  40  is stacked. The discharge tray  52  includes a pair of side fences that control a width direction of the recording sheet  22  and an end fence that controls a leading end of the recording sheet  22 . 
     Liquid Discharge Module of First Embodiment 
     A configuration of the liquid discharge module  14  according to the first embodiment will be described in detail below. The liquid discharge module  14  according to the first embodiment includes the liquid discharge head, the containing member that contains liquid to be supplied to the liquid discharge head, and a holding member  143  (see  FIG. 2 ). As the containing member, a sub tank  142  (see  FIG. 2 ), in which when ink liquid gets low, ink liquid is fed with pressure from the main tank to replenish the ink liquid, will be described as an example. 
       FIG. 2  is an external perspective view illustrating an example of the liquid discharge module according to the first embodiment. The liquid discharge module  14  illustrated in  FIG. 2  includes a liquid discharge head  141 , the sub tank  142 , and the holding member  143 . The holding member  143  is a member that fixes the sub tank  142  at a predetermined position relative to the liquid discharge head  141 . While not illustrated in  FIG. 2 , at the time of use, an ink tube for transmitting the ink liquid from the main tank to the sub tank  142 , an ink tube for transmitting the ink liquid from the sub tank  142  to the liquid discharge head  141 , and the like are connected. 
     In the liquid discharge module  14  illustrated in  FIG. 2 , a discharge surface of the liquid discharge head  141  is located at a lower end surface (in the negative Z direction), i.e., at bottom. Although the discharge surface is not illustrated because it is located at bottom in  FIG. 2 , nozzle holes are arranged in a line in the positive Y direction on the discharge surface. Further, a plurality of (for example, two) nozzle arrays are arranged parallel to each other on the discharge surface. The liquid discharge module  14  includes, as one example of the liquid discharge head  141 , a component configured to supply first ink liquid from a first supply port to a first nozzle array and supply second ink liquid from a second supply port to a second nozzle array. The first ink liquid and the second ink liquid may be the same ink liquid or may be different kinds of ink liquid. 
     A first sub tank  142   a  and a second sub tank  142   b  supply ink liquid into the liquid discharge head  141  from corresponding supply ports (a first supply port and a second supply port). The ink liquid is discharged in the form of droplets from different nozzle arrays (a first nozzle array and a second nozzle array) through flow paths, liquid chambers, or the like that are separated from each other inside the liquid discharge head  141 . 
     The first sub tank  142   a  and the second sub tank  142   b  illustrated in  FIG. 2  are mounted by the holding member  143  such that positions (for example, heights or the like) and postures (orientations) relative to the liquid discharge head  141  become the same. For example, when viewed in an XZ plane illustrated in  FIG. 2 , center positions overlap with each other and the postures are the same. In the example illustrated in  FIG. 2 , the first sub tank  142   a  and the second sub tank  142   b  are mounted such that holding positions are located at the same height from the liquid discharge head  141  and respective flexible films face a direction along which the nozzles are arranged on the discharge surface. 
     Sub Tank 
     A configuration of the sub tank will be described below. The first sub tank  142   a  and the second sub tank  142   b  have the same configuration. In the following, the configuration of a single sub tank will be described. 
       FIGS. 3A and 3B  are diagrams illustrating an example of the single sub tank.  FIG. 3A  is a side view of the sub tank, and  FIG. 3B  is a partial cross-sectional view for explaining an internal configuration of the sub tank. In  FIGS. 3A and 3B , some of components are omitted for simplicity of explanation. The omitted components will be appropriately explained with reference to  FIG. 2 . 
     In the sub tank  142  illustrated in  FIG. 3A  and  FIG. 3B , a structure of a container for containing ink liquid is formed at a side of a main body  80 , and an opening is sealed by the flexible film (for example, a flexibly film member  82 ) by bonding or welding, so that an ink container  81  is constructed inside the sub tank  142 . The ink container  81  illustrated in  FIG. 3B  has an approximately circular shape similar to an outer shape of the film member  82  when viewed from a front side illustrated in  FIG. 3A . 
     An elastic member (for example, a spring or the like)  83  that biases the film member  82  outward is arranged between the main body  80  and the film member  82 , i.e., inside the ink container  81 . Here, the film member  82 , the elastic member  83 , and an air open valve mechanism  132  to be described later correspond to a “negative pressure generating means”. The principle of generation of negative pressure by the “negative pressure generating means” will be described later. 
     An ink supply port  84  is arranged in a lower part of the main body  80 . A removable connection part is connected to the ink supply port  84 , so that ink liquid that has been transmitted by pressure from the main tank to the connection part is transmitted and supplied to the ink container  81 . A liquid transmission pump (liquid transmitting unit) that transmits the ink liquid by pressure from the main tank to the sub tank  142  is constructed between the main tank and the sub tank  142 . The ink liquid is fed to the ink supply port  84  from the main tank by the liquid transmission pump. 
     An ink outlet  85  is arranged in an upper part of the main body  80 . The ink outlet  85  discharges the ink liquid to be supplied to the liquid discharge head  141  from the sub tank  142  with the aid of generated negative pressure. The ink outlet  85  and the supply port of the liquid discharge head  141  are connected by an ink tube or the like, so that the ink liquid discharged from the sub tank  142  is supplied to the liquid discharge head  141 . 
     The air open valve mechanism  132  (see  FIG. 2 ) as an air open means for switching between a sealed state and an air open state inside the sub tank  142  is arranged in an upper part of the main body  80 . An air opening hole that communicates with the ink container  81  via an air flow path is opened and closed by the air open valve mechanism  132 , so that the sealed state and the air open state inside the sub tank  142  are switched. By controlling open and close of the air open valve mechanism  132 , the air opening hole is opened and closed. 
     Furthermore, a storage unit that stores therein ink liquid is arranged inside the main body  80 . If an apparatus main body is inclined or oscillated, it is more likely that ink enters the air flow path. Therefore, the storage unit is provided so that ink that has entered from the air flow path can be stored in the storage unit. With this configuration, it is possible to prevent ink from entering the air opening hole and the air open valve mechanism  132 , and prevent the entered ink from being solidified and causing operation failure. 
     Moreover, two detection electrodes  91  and  92  (see  FIG. 2 ) for detecting that an amount of ink in the sub tank  142  reaches a predetermined amount or below (this state will be referred to as a “no-ink state”) is attached in the upper part of the main body  80 . It is possible to determine the no-ink state by detecting a state in which both of the detection electrodes  91  and  92  are immersed in ink and a state in which at least one of the detection electrodes  91  and  92  is not immersed in ink on the basis of a change of a conduction state between the detection electrodes  91  and  92 . 
     Ink Liquid Replenishing Process 
     An ink liquid replenishing process in the sub tank  142  will be described. For example, an ink supply process may be started when it is detected that the amount of ink in the sub tank  142  is equal to or smaller than a lower limit threshold, and the ink supply process may be stopped when it is detected that the amount of ink in the sub tank  142  is equal to or larger than an upper limit threshold. 
     First, the air open valve mechanism  132  of the sub tank  142  is opened to achieve the air open state inside the sub tank  142 . Then, ink is transmitted and supplied from the main tank to the sub tank  142  by the liquid transmission pump. In this case, air inside the sub tank  142  is discharged to the outside through the air opening hole. A biasing force of the elastic member  83  is applied to the film member  82 , so that negative pressure is generated inside the sub tank  142 . 
     In this manner, it is possible to generate negative pressure inside the sub tank  142  by the film member  82  and the elastic member  83 , so that the negative pressure generation mechanism can be simplified. 
     Sub Tank Mounting Position 
       FIG. 4  is a diagram for explaining mounting positions of the sub tanks  142  in the liquid discharge modules  14 .  FIG. 4  is a schematic diagram of the conveying drum  12  and a module structure of each of the liquid discharge modules  14  viewed from the same direction as in the view illustrated in  FIG. 1 , for easy understanding of the mounting positions of the sub tanks  142 . In  FIG. 4 , side surfaces (flexible film sides) of the sub tanks  142  are attached so as to face a front side of the sheet surface. Therefore, the sub tanks  142  are represented by circles because the ink containers  81  have approximately circular shapes. The elastic members  83  that generate negative pressure are arranged in central parts of the sub tanks  142 . Here, the central parts of the circles where the negative pressure is generated in the sub tanks  142  will be referred to as “negative pressure generation positions”. On a discharge surface  1400  of each of the liquid discharge heads  141 , the nozzle array is formed in the same direction as the side surface of the sub tank  142 . 
     As illustrated in  FIG. 4 , each of the liquid discharge modules  14  is arranged on the image forming unit  10  in the apparatus at a different inclination along the conveying surface such that the discharge surface  1400  of the liquid discharge head  141  faces the outer periphery of the conveying drum  12 . The liquid discharge head  141  is attached with predetermined arrangement on each of the liquid discharge modules  14 . As one example, as illustrated in  FIG. 4 , each of the liquid discharge modules  14  is mounted at a different angle such that a perpendicular line of the discharge surface  1400  crosses a rotation axis of the conveying drum  12  at a right angle when the liquid discharge head  141  is attached. The holding member  143  of each of the liquid discharge modules  14  holds the containing member  142  such that a distance between the discharge surface  1400  of the liquid discharge head  141  and the containing member  142  in a direction of a perpendicular line of the discharge surface  1400  (indicated by a chain line in the figure) is larger as an angle between the perpendicular line and a horizontal plane  600  on which the image forming apparatus  1  is arranged is smaller. Meanwhile, it is preferable that the perpendicular line that extends from the discharge surface  1400  and crosses the rotation axis of the conveying drum  12  at a right angle is located on the nozzle array if the nozzle array is a single line in a region where the nozzles are formed (nozzle formation region) on the discharge surface  1400 , for example. Furthermore, if a plurality of nozzle arrays are arranged parallel to one another, it is preferable to arrange the perpendicular line at a position (on a center line) at an equal distance from each of nozzle arrays located at both ends among the nozzle arrays (that is, two nozzle arrays located on the outermost side), for example. 
     Moreover,  FIG. 4  illustrates an example in which the mounting positions of the sub tanks  142  are adjusted when the liquid discharge modules  14  are arranged at different inclinations. The mounting position of the sub tank  142  in each of the liquid discharge modules  14  is adjusted such that the positions of the sub tanks  142  relative to the respective liquid discharge heads  141  in a vertical direction become approximately the same or equivalent among the liquid discharge modules  14 . Here, the vertical direction is a direction perpendicular to the horizontal plane  600  on which the image forming apparatus  1  is arranged. Specifically, in each of the liquid discharge modules  14 , the position of the sub tank  142  in the vertical direction is adjusted such that a hydraulic head difference that is a difference between the position of the nozzle array of each of the liquid discharge heads  141  and the negative pressure generation position of the sub tank  142  in a height direction (vertical direction) become approximately the same or equivalent among the liquid discharge modules  14 . 
     For example, as illustrated in  FIG. 4 , it is assumed that nozzle arrays  17 A and  17 B are arranged on the discharge surfaces  1400  of the liquid discharge heads  141  of two liquid discharge modules  14 A and  14 B. In this case, in each of the liquid discharge modules  14 A and  14 B, a distance at which the sub tank  142  is mounted along the perpendicular line of the discharge surface  1400  is changed on the basis of a position of each of the nozzle arrays  17 A and  17 B. In other words, by individually adjusting the distance at which the sub tank  142  is mounted in the direction of the perpendicular line of each of the discharge surfaces  1400 , the position of the sub tank  142  in the vertical direction is adjusted such that hydraulic head differences in the respective liquid discharge modules  14  become approximately the same or equivalent between the liquid discharge modules  14 . 
     In this manner, the positions of the sub tanks  142  in the vertical direction are adjusted such that the hydraulic head differences become equivalent among the liquid discharge modules  14 . In each of the liquid discharge modules  14 , if the sub tank  142  is mounted at the same distance in the direction of the perpendicular line of the discharge surface of the liquid discharge head  141 , the hydraulic head differences vary among the liquid discharge modules  14  because the liquid discharge modules  14  are arranged at different inclinations. However, if the sub tanks  142  are adjusted with respect to the discharge surfaces in the direction of the perpendicular line, it is possible to set a height between each of the nozzle arrays and each of the negative pressure generation positions in the height direction to be approximately the same or a constant value L, so that it is possible to prevent variation in the hydraulic head difference among the liquid discharge modules  14 . 
     The holding member  143  (see  FIG. 2 ) is a member that holds the sub tank  142  and fixes the sub tank  142  at a predetermined position, and a member that adjusts the position at which the sub tank  142  is held (holding position) with respect to the discharge surface in the direction of the perpendicular line. The sub tank  142  is detachably attached to the holding member  143 . The position for holding the sub tank  142  is adjusted by using, as the holding member  143 , a component that is extendable and retractable in the height direction. With this configuration, it becomes possible to adjust the distance of the sub tank  142  relative to the discharge surface  1400  in the direction of the perpendicular line, so that it becomes possible to adjust the sub tank  142  in the height direction (vertical direction) in the image forming unit  10 . 
     Meanwhile, the liquid discharge module  14  including the holding member  143  (see  FIG. 2 ) may be provided by being attached to the image forming unit  10  in advance, or a user or a maintenance operator may later select and attach the holding member  143  to a mounting position in the image forming unit  10 . 
     Further, an extendable/retractable direction indicates a direction in which the liquid discharge module  14  is separated away from (or comes close to) the outer periphery of the conveying drum  12  when the discharge surface  1400  of the liquid discharge head  141  is arranged so as to face the outer periphery of the conveying drum  12 . For example, mounting portions are arranged as a first adjustment portion such that the sub tank  142  can be mounted by being moved to several positions in the vertical direction with respect to the discharge surface. In this case, the sub tank  142  can be moved to and mounted at a plurality of positions by the single holding member  143 . 
     Furthermore, it may be possible to arrange a means that rotates the sub tank  142  in a circumferential direction about the negative pressure generation position. For example, structures (for example, structures for performing fastening with screws) that determine positions while changing orientation in the circumferential direction are arranged at a plurality of positions. In this case, it becomes possible to always locate the ink outlet  85  in an upper part regardless of the inclination of the liquid discharge module  14 . With this configuration, it becomes easy to discharge air bubbles in an ink supply path extending to the sub tank  142 , without leaving air bubbles inside the sub tank  142 . 
     As described above, according to the first embodiment, it is possible to adjust the positions of the containing members in the vertical direction in the plurality of liquid discharge modules, and it is possible to prevent variation of discharging among the heads with a simple structure. For example, if a position at which a droplet is discharged onto a recording sheet is located on a curved surface, such as a conveying drum, the liquid discharge module is arranged so as to be inclined and face the curved surface for, for example, each of colors of ink liquid in accordance with the shape of the curved surface. Even in this case, it is possible to adjust the heights of the sub tanks by using the holding members such that the hydraulic head differences among head arrays at different inclinations become equivalent. Therefore, it is possible to prevent variation in quality of images that are formed in accordance with the positions of the head arrays used to form the images. 
     Liquid Discharge Module of Second Embodiment 
     As a liquid discharge module according to a second embodiment, a configuration of a liquid discharge module that is an inkjet head assembly will be described below. The liquid discharge module according to the second embodiment includes a liquid discharge head, a containing member that contains ink liquid to be supplied to the liquid discharge head, and a base bracket and an adjustment bracket that are one example of the holding member  143 . The liquid discharge module as the head assembly is able to mount a large number of liquid discharge heads and supply liquid from a single sub tank to, for example, liquid discharge heads arranged in the same line in accordance with a combination of the liquid discharge heads, which is different from the liquid discharge module described in the first embodiment. The liquid discharge module as the head assembly is arranged on the image forming unit  10  at a different inclination such that a discharge surface of each of the liquid discharge heads faces the outer periphery of the conveying drum  12 , similarly to the liquid discharge module illustrated in the first embodiment. 
       FIGS. 5A and 5B  are diagrams illustrating an example of a configuration of the liquid discharge module as the head assembly, as the liquid discharge module according to the second embodiment.  FIG. 5A  illustrates a configuration of an attachment state (attachment state on the front surface side) in which the liquid discharge module is attached to the image forming apparatus  1  illustrated in  FIG. 1  (assumed as a front surface).  FIG. 5B  illustrates a configuration of a side surface of the liquid discharge module and a main tank that supplies ink liquid to a sub tank of the liquid discharge module. Meanwhile,  FIGS. 5A and 5B  illustrate a configuration of the single liquid discharge module. An entire configuration will be schematically described below, and details of each of units of the liquid discharge module will be described later with reference to detailed figures. 
     A liquid discharge module  100  as the head assembly is able to mount and use a large amount of liquid discharge heads on a head attachment plate (head frame)  101  illustrated in a lower part of a main body. An ink manifold  150  is constructed in an upper part of the main body, and it is possible to distribute ink liquid from an ink common path  151  of the ink manifold  150  to each of the liquid discharge heads  141  through a branch path  152  that extends to each of the liquid discharge heads  141  that are attached at respective attachment positions  103 . Here, the ink manifold  150  corresponds to a “liquid supply path”. 
     An external bracket (base bracket)  200  and an adjustment bracket  250  for attaching the sub tank  142  to the main body are mounted on a plate on the front surface side of the liquid discharge module  100 , and the sub tank  142  is attached by the brackets. Here, the base bracket  200  corresponds to a “first holding member”, and the adjustment bracket  250  corresponds to a “second holding member”. The holding member  143  including the bracket (base bracket)  200  and the adjustment bracket  250  holds the liquid discharge head  141  and the sub tank  142 . 
     The ink liquid is transmitted to each of the units through pipes as illustrated in  FIG. 5A  and  FIG. 5B . The ink liquid is transmitted from a main tank  300  to an ink filter  61  through a supply tube (ink tube or the like)  71 , and the ink liquid that has passed through the ink filter  61  is transmitted to the ink supply port  84  located in the lower part of the sub tank  142  through a supply pipe  72 , so that the ink container  81  is replenished with the ink liquid. The ink liquid in the sub tank  142  is transmitted from the ink outlet  85  located in the upper part of the sub tank  142  to the ink common path  151  of the ink manifold  150  through a pipe  73 , and supplied from the branch path  152  to each of the liquid discharge heads  141 . 
       FIG. 6  and  FIG. 7  are external perspective views illustrating steps of attachment to a main body of the liquid discharge module  100 . First, common components of the liquid discharge module  100  illustrated in  FIG. 6  and  FIG. 7  will be described with reference to  FIG. 6 . 
     The liquid discharge module  100  illustrated in  FIG. 6  is able to mount the  24  liquid discharge heads  141 . The head attachment plate  101  illustrated in a lower part of the main body of the liquid discharge module  100  is formed so as to be able to mount six liquid discharge heads in the Y direction illustrated in  FIG. 6  and mount a total of four arrays such that two arrays are arranged on each side across a structure  102  that extends in the Y direction. The liquid discharge heads are attached such that the discharge surfaces face the negative Z direction (downward in this arrangement), and the discharge surfaces are exposed in the negative Z direction from an opening  104  of the head attachment plate  101 .  FIG. 6  illustrates a state in which, as one example, the two liquid discharge heads  141  are attached. Among first to sixth attachment positions in each of the two arrays on one side, the liquid discharge head  141  is attached to the first attachment position  103  in each array. The attachment states at the other attachment positions  103  are not illustrated, but the liquid discharge head  141  is attached and used at each of the other attachment positions  103 . 
     The ink manifold  150  is constructed in the upper part of the main body. The ink manifold  150  distributes the ink liquid to each of the liquid discharge heads  141  from the ink common path  151  of the ink liquid through the branch path  152  extending to each of the liquid discharge heads  141  that are attached to the respective attachment positions  103 . 
       FIG. 6  further illustrates a state in which the external bracket (base bracket)  200  that can adjust the position and the posture of the sub tank  142  (see  FIGS. 5A and 5B ) is mounted on the liquid discharge module  100 . Further,  FIG. 7  illustrates a state in which the adjustment bracket  250  that adjusts the position and the posture of the sub tank  142  is further mounted on the base bracket  200  illustrated in  FIG. 6 . When the sub tank  142  is to be attached, the position and the posture of the sub tank  142  are adjusted by using the adjustment bracket  250 . Here, as one example, a use example will be described in which adjustment in the vertical direction (the Z direction in  FIG. 7 ) and rotation adjustment in a positive direction or a negative direction about the negative pressure generation position serving as a central axis are performed. Meanwhile, the vertical direction described in this example is a direction in which “upward” indicates a direction away from the outer periphery of the conveying drum  12  and “downward” indicates a direction approaching the outer periphery of the conveying drum  12  when the liquid discharge module  100  is arranged such that the discharge surface  1400  of the liquid discharge head  141  faces the outer periphery of the conveying drum  12 . Furthermore, the central axis described above is an axis that is perpendicular to a conveying direction of a discharge target and a liquid discharge direction when the liquid discharge module  100  is arranged such that the discharge surface  1400  of the liquid discharge head  141  faces the outer periphery of the conveying drum  12 . 
     How to Use Brackets 
     Configurations of the base bracket  200  and the adjustment bracket  250  will be first described, and thereafter, how to use the brackets will be described using an example. As illustrated in  FIG. 6 , the base bracket  200  is a plate member that extends in the upward direction (Z direction), i.e., the direction away from the outer periphery of the conveying drum  12 , and includes a plurality of screw holes  201  in accordance with a plurality of mounting positions in the vertical direction such that the mounting position of the adjustment bracket  250  illustrated in  FIG. 7  can be adjusted in the vertical direction (Z direction). By mounting the adjustment bracket  250  by using the different screw holes  201  that are arranged in the direction away from the outer periphery, it is possible to adjust the holding position of the sub tank  142  in the direction away from the outer periphery of the conveying drum  12 . Furthermore, the adjustment bracket  250  illustrated in  FIG. 7  includes a plurality of holes  251  along a rotation direction (circumferential direction) of the sub tank  142  such that the sub tank  142  can be held by being rotated in a positive circumferential direction or a negative circumferential direction about the negative pressure generation position serving as the central axis. 
     Here, the plurality of screw holes  201  of the base bracket  200  are one example of a “first adjuster”, a “first mounting part”, and the like. Further, the plurality of holes  251  of the adjustment bracket  250  are one example of a “second adjuster”, a “second mounting part”, and the like. Meanwhile, the “first adjuster”, the “first mounting part”, the “second adjuster”, and the “second mounting part” are not limited thereto, and other modes may be adopted. 
     When the sub tank  142  is to be attached to the liquid discharge module  100 , the base bracket  200  is mounted on the liquid discharge module  100  with screws or the like, and the adjustment bracket  250  is mounted at an adjusted height on the base bracket  200  with screws. The sub tank  142  is held by being fastened to the adjustment bracket  250  with screws via the holes  251 . In this manner, the sub tank  142  can be detachably attached by detaching and attaching screws from and to the base bracket  200 . Meanwhile, the base bracket  200  may be mounted on the liquid discharge module  100  in advance, or may be integrally configured as a part of the main body of the liquid discharge module  100 . 
       FIG. 8  and  FIG. 9  are diagrams illustrating an example of the mounting position in a case where the sub tank  142  is mounted at an adjusted height.  FIG. 8  illustrates, at (a), a corresponding relationship between the mounting positions of the base bracket  200  and the adjustment bracket  250  when the sub tank  142  is mounted at a low position.  FIG. 8  illustrates, at (b), a result in a case where mounting is performed based on the positional relationship as illustrated at (a) in  FIG. 8 .  FIG. 9  illustrates, at(a), a correspondence relationship between the mounting positions of the base bracket  200  and the adjustment bracket  250  in a case where the sub tank  142  is mounted at a high position.  FIG. 9  illustrates, at (b), a result in a case where mounting is performed based on the positional relationship as illustrated at (a) in  FIG. 9 . 
     As illustrated at (a) in  FIG. 8  (or (a) in  FIG. 9 ), the base bracket  200  includes the screw holes  201  in accordance with a plurality of positions for three-level height adjustment so that the height can be adjusted at three levels. As illustrated in  FIG. 8  (or  FIG. 9 ), the holes  202  that are arranged at six positions in the adjustment bracket  250  are aligned with the six screw holes  201  at a height at which the base bracket  200  is to be mounted, and the adjustment bracket  250  is fastened to the base bracket  200  with screws at this position. The sub tank  142  is fastened at the position of the predetermined holes  251  of the adjustment bracket  250  with screws before the adjustment bracket  250  is fastened to the base bracket  200  with screws. 
       FIG. 10  and  FIG. 11  are diagrams illustrating an example of the mounting positions in a case where the sub tank  142  is mounted while being adjusted in the rotation direction.  FIG. 10  illustrates, at (a), a correspondence relationship of the mounting positions in a case where the sub tank  142  is mounted on the base bracket  200  at a small rotation angle.  FIG. 10  illustrates, at (b), a result in a case where mounting is performed based on the positional relationship as illustrated at (a) in  FIG. 10 .  FIG. 11  illustrates, at (a), a correspondence relationship of the mounting positions in a case where the sub tank  142  is mounted on the base bracket  200  at a large rotation angle.  FIG. 11  illustrates, at (b), a result in a case where mounting is performed based on the positional relationship as illustrated at (a) in  FIG. 11 . Meanwhile,  FIG. 10  and  FIG. 11  illustrate the cases in which the adjustment bracket  250  is inclined with respect to the sub tank  142 . A direction in which the adjustment bracket  250  illustrated at (b) in  FIG. 10  and (b) in  FIG. 11  is adjusted in the vertical direction of the base bracket  200  is represented by the Z direction at (b) in  FIG. 10  and (b) in  FIG. 11 . 
     As illustrated at (a) in  FIG. 10  (or (a) in  FIG. 11 ), the plurality of holes  251  are arranged on the adjustment bracket  250  along the rotation direction (circumferential direction) such that the sub tank  142  can be held by being rotated in the positive circumferential direction or the negative circumferential direction about the negative pressure generation position serving as the central axis. In the sub tank  142 , four screw holes  255  are arranged such that the sub tank  142  can be fastened through the four holes  251  of the adjustment bracket  250  with screws. As illustrated in at (a) in  FIG. 10  (or (a) in  FIG. 11 ), the four screw holes  255  arranged in the sub tank  142  are aligned with the holes  251  at four positions corresponding to a mounting rotation angle among the plurality of holes  251  of the adjustment bracket  250 , and the sub tank  142  is fastened to the adjustment bracket  250  at these positions with screws. With this fastening, the adjustment bracket  250  holding the sub tank  142  is mounted on the base bracket  200  at a certain height adjusted in the Z direction. 
       FIG. 12  is a diagram illustrating an example of an attachment state in which the sub tank  142  is attached to the liquid discharge module  100  by using the base bracket  200  and the adjustment bracket  250 . As one example, three attachment examples are illustrated at (a), (b), and (c) in  FIG. 12 . At (a), (b), and (c) in  FIG. 12 , dashed lines and arrows are illustrated to clarify a relative arrangement relationship. 
       FIG. 12  illustrates, at (a), an attachment example in which the sub tank  142  is attached at a low position in the liquid discharge module  100 .  FIG. 12  illustrates, at (b), an attachment example in which the sub tank  142  is attached to a high position in the liquid discharge module  100 . The sub tank  142  is attached to the high position indicated in the arrow direction at (b) in  FIG. 12 , relative to the low position illustrated at (a) in  FIG. 12 .  FIG. 12  illustrates, at (c), an attachment example in which the sub tank  142  is attached at a high position in the liquid discharge module  100  by being rotated by a predetermined rotation angle. As indicated by the arrow at (c) in  FIG. 12 , the sub tank  142  is attached by being rotated by a predetermined rotation angle from a horizontal line. 
       FIG. 13  is a diagram illustrating an example of setting of the sub tanks  142  of the liquid discharge modules  100  that are arranged at different angles along with the outer periphery of the conveying drum  12  (see  FIG. 1 ). Each of the liquid discharge modules  100  is set in the image forming unit  10  at a different angle along with the outer periphery of the conveying drum  12 . In other words, the set liquid discharge modules  100  are inclined differently. Therefore, if the positional relationship between the liquid discharge head  141  and the sub tank  142  in each of the liquid discharge modules  100  is the same, the hydraulic head difference varies. While the three attachment examples are illustrated at (a), (b), and (c) in  FIG. 12 , the sub tank  142  is set by appropriately adjusting the height and the rotation angle. 
     Specifically, as illustrated in  FIG. 13 , the sub tanks  142  are assembled at adjusted heights by using the base bracket  200  and the adjustment bracket  250  in accordance with the set positions of the liquid discharge modules  100 , i.e., in accordance with the inclinations of the liquid discharge modules  100 . Further, the sub tanks  142  are assembled at adjusted rotation angles such that the ink outlets  85  are always located in the upper parts as illustrated in  FIG. 13  in accordance with the inclinations of the liquid discharge modules  100 . In the setting of the sub tanks  142  illustrated in  FIG. 13 , the sub tank  142  is attached to a higher position in the liquid discharge module  100  located at a larger inclination so that the hydraulic head difference can always be set to the constant value L (see  FIG. 4 ). Further, the sub tank  142  is attached at a larger rotation angle in the liquid discharge module  100  located at a larger inclination so that the ink outlet  85  can always be located in the upper part. 
     As described above, even in the second embodiment, it is possible to adjust the positions of the containing members in the vertical direction in the plurality of liquid discharge modules with a simple configuration, so that it is possible to prevent variation of discharging among the heads by a simple method. 
     Furthermore, like the base bracket  200 , it is sufficient to adjust the holding position of the containing member in the direction away from the conveying surface by using the holding member that extends in the direction away from the conveying surface along which the discharge target for discharging liquid is conveyed; therefore, it is possible to easily perform adjustment operation. 
     Moreover, it is possible to simplify a structure for holding the containing member by providing the second adjuster that adjusts the position of the containing member in the rotation direction in the adjustment bracket  250 . 
     Furthermore, like the combination of the base bracket  200  and the adjustment bracket  250 , by allowing different mechanisms to adjust the position of the containing member in the vertical direction and the holding position in the rotation direction, it is possible to simplify the adjustment operation and simplify the structure. 
     Moreover, by arranging the plurality of screw holes  201  that are arranged in the direction away from the conveying surface as in the base bracket  200 , it is possible to perform operation of adjusting the position of the containing member in the vertical direction in a visually easy manner. 
     Furthermore, by arranging the plurality of holes  251  that are arranged in the circumferential direction as in the adjustment bracket  250 , it is possible to perform operation of adjusting the position of the containing member in rotation direction in a visually easy manner. 
     Moreover, by allowing the containing member to be detachably attached, it is possible to easily perform operation of exchanging the containing member. 
     Modification of Second Embodiment 
     As a modification of the second embodiment, a configuration of a liquid discharge module that discharges different kinds of ink liquid (for example, ink liquid of different colors) in a main body of a single liquid discharge module will be described. In the following, only components different from the configuration of the liquid discharge module of the second embodiment will be described. 
       FIGS. 14A and 14B  are diagrams illustrating an example of the configuration of the liquid discharge module according to the modification of the second embodiment.  FIG. 14A  and  FIG. 14B  illustrate liquid discharge modules  500  according to the modification, both of which have the same configuration. Here, for easy understanding of an outer shape and an internal configuration of the liquid discharge module  500  according to the modification, the liquid discharge heads  141  attached at different positions in a depth direction are viewed from a front side, i.e., viewed from the same front side as the attachment state of the liquid discharge module  500  in the image forming apparatus  1  (assumed as the front side) as illustrated in  FIG. 1 . A difference between  FIG. 14A  and  FIG. 14B  is the number of attached sub tanks. The sub tank  142  is schematically illustrated to clarify a connection relationship. 
     The liquid discharge module  500  of the modification as illustrated in  FIG. 14A  (the same applies to  FIG. 14B ) is configured to allow attachment to be performed in such a manner that the discharge surfaces  1400  are oriented differently between a set of two arrays (even) on one side and a set of two arrays (odd) on the other side across the structure  102  on the head attachment plate  101 , as compared to the liquid discharge module  100  described in the second embodiment. Specifically, left and right sides of the head attachment plate  101  are inclined at different angles such that the discharge surfaces  1400  of the liquid discharge heads  141  face the outer periphery of the conveying drum  12  across the structure  102 . As illustrated in  FIG. 14A  (the same applies to  FIG. 14B ), each of the liquid discharge heads  141  attached to the liquid discharge module  500  is oriented in a direction in which the discharge surface  1400  faces the outer periphery of the conveying drum  12  at each of the positions on the left and right sides across the structure  102 . 
       FIG. 14A  illustrates a connection example in which the sub tank  142  is attached to each array. In an upper part of a main body of the liquid discharge module  500 , the ink manifold  150  (see  FIG. 6 ) is constructed for each of the arrays of the liquid discharge heads  141 , and the sub tank  142  for supplying ink liquid is individually attached to each of the ink common paths  151  (see  FIG. 6 ). In other words, in the connection example in  FIG. 14A , the four sub tanks  142  are attached. For example, four kinds of ink liquid (CMYK or the like) are assigned to and used in the respective arrays. 
       FIG. 14B  illustrates a connection example in which the sub tank  142  is attached for each set of two arrays on each side. In the upper part of the main body of the liquid discharge module  500 , the ink manifold  150  (see  FIG. 6 ) is constructed for each of the arrays of the liquid discharge heads  141 , the ink common paths  151  (see  FIG. 6 ) are combined for each set of two arrays on each side, and the single sub tank  142  is attached to each set of two arrays. In other words, in the connection example in  FIG. 14B , the two sub tanks  142  are attached. For example, two kinds of ink liquid (two colors or the like) are assigned to and used in the respective sets of two arrays. 
       FIG. 15A  to  FIG. 16B  are diagrams illustrating an example of an attachment state in which the plurality of sub tanks  142  are attached to the main body of the single liquid discharge module  500 . To attach the plurality of sub tanks  142  to the main body of the single liquid discharge module  500 , it is sufficient to increase the adjustment brackets  250  in accordance with the number of the sub tanks  142  to be attached. Here, as one example, an example will be described in which the two sub tanks  142  are attached. 
       FIGS. 15A and 15B  are diagrams illustrating an attachment state of the two sub tanks  142  at a position at which the main body of the liquid discharge module  500  is set in an erected manner, i.e., set without being inclined, in the image forming unit  10 .  FIG. 15A  illustrates a perspective view of the attachment state of the two sub tanks  142 , and  FIG. 15B  illustrates the attachment state of the two sub tanks  142  viewed from above (plan view). 
     As illustrated in  FIG. 15A , the first sub tank  142  is attached to the main body of the liquid discharge module  500  by mounting the adjustment bracket  250  ( 250 - 1 ) on the base bracket  200  as has been described above. The second sub tank  142  is attached to the main body of the liquid discharge module  500  by further mounting the adjustment bracket  250  ( 250 - 2 ) on an outer side of the first sub tank  142  that has been mounted on the base bracket  200 . Here, an example is described in which the adjustment bracket  250 - 2  that is slightly larger than the first adjustment bracket  250 - 1  is used. The second adjustment bracket  250 - 2  has the same function as the first adjustment bracket  250 - 1 . In other words, the second adjustment bracket  250 - 2  can be mounted on the base bracket  200  by being moved in the vertical direction, and can be mounted on the base bracket  200  such that the sub tank  142  is inclined in the circumferential direction. 
     In the example illustrated in  FIG. 15A  and  FIG. 15B , when the main body of the liquid discharge module  500  is erected, the hydraulic head difference can be set to the same between the set of two arrays on one side and the set of two arrays on the other side across the structure  102  by arranging the sub tanks  142  in the same orientation in the Z direction in both of the set of two arrays on one side and the set of two arrays on the other side across the structure  102  (see  FIG. 14B ). 
       FIGS. 16A and 16B  are diagrams illustrating an attachment state of the two sub tanks  142  at a position at which the main body of the liquid discharge module  500  is set in an inclined manner in the image forming unit  10 .  FIG. 16A  illustrates a perspective view of the attachment state of the two sub tanks  142 .  FIG. 16B  illustrates the hydraulic head difference at the position at which the main body of the liquid discharge module  500  is set in the inclined manner. 
     When the main body of the liquid discharge module  500  is set in an inclined manner, the hydraulic head difference varies between the set of two arrays on one side and the set of two arrays on the other side across the structure  102 ; therefore, it is necessary to displace mounting positions of the sub tanks  142  such that the hydraulic head difference in each of the sets of two arrays becomes equivalent at the value L that has been set in the other liquid discharge module  500 . Therefore, as illustrated in  FIG. 16A , the mounting positions on the base bracket  200  using the adjustment bracket  250 - 1  and the adjustment bracket  250 - 2  are displaced between the sub tank  142  that supplies ink liquid to the set of two arrays on one side and the mounting position of the sub tank  142  that supplies ink liquid to the set of two arrays on the other side. Furthermore, if the main body of the liquid discharge module  500  is set in an inclined manner, the orientation of the ink outlet  85  of the sub tank  142  is changed in accordance with the inclination. Therefore, each of the sub tanks  142  is mounted by being rotated by the same rotation amount in the circumferential direction about the negative pressure generation position serving as a center. With this configuration, the ink outlet  85  is located so as to always face upward. 
     In this manner, as illustrated in  FIG. 16B , the hydraulic head difference is set to the constant value L in each of the set of two arrays in one side and the set of two arrays in the other side at a position at which the main body of the liquid discharge module  500  is set in an inclined manner. Meanwhile,  FIG. 16B  illustrates that the hydraulic head differences are set to the constant value L at a boundary line between the sets of two arrays on each side. 
     Here, as one example, the example has been described in which the two sub tanks  142  are attached such that one of the sub tanks  142  is shared with the two arrays on one side and the other one of the sub tanks  142  is shared with the two arrays on the other side across the structure  102  (see  FIG. 14B ). Meanwhile, if the sub tank  142  is attached for each of the arrays, i.e., if the four sub tanks are attached as illustrated in  FIG. 14A , the sub tanks  142  are attached so as to be displaced from one another such that the hydraulic head difference in the nozzle formation region of the discharge surface  1400  of each of the arrays can be set to the constant value L. Even when the four sub tanks  142  are attached, the four sub tanks  142  are sequentially stacked on the outer side by using the adjustment brackets  250  having slightly larger sizes, in the same manner as described above. As another example, it may be possible to attach the plurality of sub tanks  142  on a plate of the single adjustment bracket  250 . 
     Meanwhile, it may be possible to use, for example, liquid of four colors such as black K, cyan C, magenta M, and yellow Y, and other special kinds of liquid as the ink liquid to be discharged from each of the liquid heads. The types of the liquid are not limited to this example. 
     Modification of First Embodiment 
       FIG. 17  is a diagram for explaining mounting positions of the sub tanks  142  in liquid discharge modules  14  according to a modification of the first embodiment. As illustrated in  FIG. 17 , each of the liquid discharge modules  14  is arranged at a different angle such that a perpendicular line extending from the discharge surface  1400  crosses the rotation axis of the conveying drum  12  at a right angle when the liquid discharge head  141  is attached. In other words, the liquid discharge modules  14  are arranged inside the image forming unit  10  such that angles between respective longitudinal sides of the liquid discharge modules  14  and the horizontal plane  600  become different from one another. 
     The holding member  143  of each of the liquid discharge modules  14   a ,  14   b , and  14   c  holds the containing member  142  such that distances D 1 , D 2 , and D 3  between the discharge surfaces  1400  of the liquid discharge heads  141  and the containing members in directions of perpendicular lines (chain lines in the figure) of the discharge surfaces  1400  are larger as angles  81 ,  82 , and  83  between the perpendicular lines and the horizontal plane  600  on which the image forming apparatus  1  is set are smaller. In the present modification, while the positions of the sub tanks  142  in the vertical direction relative to the respective liquid discharge heads  141  in the liquid discharge modules  14   a ,  14   b , and  14   c  are different, it is possible to prevent variation in the hydraulic head difference among the liquid discharge modules  14 , as compared to a case in which the distances D 1 , D 2 , and D 3  are set to be constant regardless of the angles θ 1 , θ 2 , and θ 3 . In other words, by adjusting the sub tanks  142  relative to the discharge surfaces  1400  in the direction of the perpendicular line, it is possible to prevent variation in the height between each of the nozzle arrays and each of the negative pressure generation positions in the vertical direction among the plurality of liquid discharge modules  14 . With this configuration, it is possible to prevent variation in the hydraulic head difference among the liquid discharge modules  14 . 
     In the present modification, the above-described relationship between the angle and the distance is satisfied for all of the five liquid discharge modules, but embodiments are not limited thereto. For example, in  FIG. 17 , the two liquid discharge modules, such as the single liquid discharge module in the center and the adjacent liquid discharge module, may be adopted as targets, and the holding members  143  may be configured to hold the containing members  142  such that the distances D 1  and D 2  between the discharge surfaces  1400  of the respective liquid discharge heads  141  and the containing members in the directions of the perpendicular lines are larger as the angles θ 1  and θ 2  with respect to the horizontal plane  600  are smaller. 
     While the example has been described in the above-described embodiments in which the plurality of liquid discharge modules  14  are arranged so as to face the conveying drum  12 , embodiments are not limited thereto. In a configuration in which a conveying guide plate in a curved shape is arranged instead of the drum and a sheet is conveyed onto the conveying guide plate by a mechanism, such as a conveying roller, it may be possible to arrange a plurality of liquid discharge modules such that the liquid discharge modules face the conveying guide plate. 
     According to an embodiment of the present invention, it is possible to prevent variation in image quality among heads with a simple configuration. 
     The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, at least one element of different illustrative and exemplary embodiments herein may be combined with each other or substituted for each other within the scope of this disclosure and appended claims. Further, features of components of the embodiments, such as the number, the position, and the shape are not limited the embodiments and thus may be preferably set. It is therefore to be understood that within the scope of the appended claims, the disclosure of the present invention may be practiced otherwise than as specifically described herein.