Patent Publication Number: US-8529039-B2

Title: Inkjet recording apparatus having thick manifolds for large volume circulation of ink

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet recording apparatus, and more particularly to technology for circulating ink in a line head constituted of a plurality of head modules. 
     2. Description of the Related Art 
     An inkjet recording apparatus has a recording head (inkjet head) in which a plurality of nozzles are arranged on an ejection face, and records an image on a recording medium by ejecting ink droplets from the nozzles while moving the recording head and the recording medium relatively to each other. The ink ejection method of the recording head includes a piezoelectric method, which ejects an ink droplet from a nozzle by applying pressure to the ink inside a pressure chamber using the displacement of a piezoelectric element, and a thermal method, which ejects an ink droplet from a nozzle by means of the pressure generated when bubbles are produced inside a pressure chamber due to thermal energy produced by a heating element, such as a heater, or the like. 
     The inkjet recording apparatuses include a serial type and a line type. The serial type apparatus has a recording head in which a nozzle row is arranged in the conveyance direction of the recording medium, and performs recording by intermittently repeating reciprocal movement of the recording head in the width direction of the recording medium (the main scanning direction; the direction perpendicular to the paper conveyance direction) and conveyance of the recording medium. The line type apparatus has a recording head in which a nozzle row is arranged in the width direction of the recording medium, and performs recording by simply moving the recording medium relatively in the paper conveyance direction (the sub-scanning direction) with respect to the recording head. The line type apparatus has a merit in that the recording speed can be raised over that of the serial type apparatus, and is used widely in various industrial fields. 
     Various technologies have been proposed for the recording heads of the inkjet recording apparatuses; however, in the line type apparatus, it is not practicable to form a single recording head that corresponds to the full width of the recording medium, as a single body, from a silicon wafer, glass, or the like, due to problems relating to the method of manufacture, the production yield, heat generation, cost, and the like. Hence, in the line type apparatus, a long line head having a length corresponding to the full width of the recording medium is formed by aligning a plurality of recording heads (hereinafter referred to as “head modules”) which are shorter than the full width of the recording medium, in the width direction of the recording medium, in such a manner that simultaneous recording over the full width of the recording medium can be performed. 
     In the inkjet recording apparatus, if bubbles are present in ink in flow channels inside the recording heads, then these can give rise to ejection defects, and the like, and hence the ink is circulated between the recording heads and a tank that is open to the air, and the bubbles are thereby collected in the tank and released into the air. 
     For example, Japanese Patent Application Publication No. 2007-069419 discloses an inkjet recording apparatus in which a line head is constituted of a plurality of head modules, and in order to suppress variation in the ink circulation volume in the respective head modules, ink is supplied from the tank to the head modules and the ink is collected (circulated) from the head modules to the tank, through flow channels including a main flow channel, which is arranged commonly in respect of the plurality of head modules, and a plurality of branch flow channels, which branch from the main flow channel and extend to the respective head modules. However, in Japanese Patent Application Publication No. 2007-069419, the object is to prevent stagnation of bubbles in the branching points between the main flow channel and the branch flow channels, but there is no investigation of the issue of using a manifold having a sufficient thickness in order to achieve a large-volume circulation of ink. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of these circumstances, an object thereof being to provide an inkjet recording apparatus which is able to achieve stable ink circulation in a composition which employs sufficiently thick manifolds to achieve a large-volume circulation of ink to a line head constituted of a plurality of head modules while preventing bubbles from arriving at the head modules. 
     In order to attain the aforementioned object, the present invention is directed to an inkjet recording apparatus, comprising: a plurality of recording head modules each having supply ports and discharge ports for liquid; a liquid supply manifold which is a liquid chamber having a liquid inlet port to which a first main flow channel is connected, the liquid supplied from a liquid tank through the first main flow channel being stored in the liquid supply manifold, the liquid supply manifold being connected to the supply ports of the recording head modules through first branch flow channels; a liquid collection manifold which is a liquid chamber having a liquid outlet port to which a second main flow channel is connected, the liquid to be collected to the liquid tank through the second main flow channel being stored in the liquid collection manifold, the liquid collection manifold being connected to the discharge ports of the recording head modules through second branch flow channels; a first bypass flow channel which connects the liquid supply manifold to the liquid collection manifold; and a liquid circulation device which circulates the liquid sequentially to the liquid supply manifold, the recording head modules and the liquid collection manifold, wherein: the liquid supply manifold and the liquid collection manifold each have heights in a vertical direction which enable a gas getting mixed with the liquid to separate from the liquid in the vertical direction; and an end of the first bypass flow channel is connected to an upper side in the vertical direction of an end of the liquid supply manifold on a side opposite to a side where the liquid inlet port is arranged. 
     According to this aspect of the present invention, the liquid supply manifold and the liquid collection manifold each have heights in the vertical direction whereby the gas getting mixed with the liquid can separate from the liquid in the vertical direction, the first bypass flow channel connecting these manifolds together is provided, and one end of the first bypass flow channel is connected to the upper side in the vertical direction (and desirably the upper end face) of the end of the liquid supply manifold on the side opposite to the side where the liquid inlet port is arranged. Hence, bubbles which have entered through the liquid inlet port of the liquid supply manifold collect on the side where the first bypass flow channel is connected and are conveyed by following the flow of the liquid to the liquid collection manifold through the first bypass flow channel, without passing through the recording head modules, and are expelled to the exterior through the liquid outlet port. Furthermore, since the manifolds each have sufficient thicknesses (internal flow channel cross-sectional areas), there is little pressure loss in the manifolds and the pressure difference between the recording head modules can be reduced. 
     Preferably, the other end of the first bypass flow channel is connected to a lower side in the vertical direction of an end of the liquid collection manifold on a side opposite to a side where the liquid outlet port is arranged. 
     According to this aspect of the present invention, the bubbles expelled to the liquid collection manifold do not stagnate in the vicinity of the connection between the first bypass flow channel and the liquid collection manifold, and the bubble expulsion characteristics from the liquid supply manifold to the liquid collection manifold are improved. 
     Preferably, the liquid inlet port is arranged to a lower side in the vertical direction of the liquid supply manifold. 
     According to this aspect of the present invention, since the liquid inlet port is arranged on the lower side in the vertical direction of the liquid supply manifold where no bubbles are present, it is possible to obtain a stable liquid flow volume which is free of the effects of bubbles. 
     Preferably, the liquid outlet port is arranged to an upper side in the vertical direction of the liquid collection manifold. 
     According to this aspect of the present invention, since the liquid outlet port is arranged at a position where the bubbles inside the liquid collection manifold are liable to collect, the bubble expulsion characteristics are improved. 
     Preferably, the inkjet recording apparatus further comprises: an opening and closing valve which is arranged in the first bypass flow channel; and a valve control device which controls opening and closing operations of the opening and closing valve, wherein the valve control device connects the liquid supply manifold to the liquid collection manifold through the first bypass flow channel by opening the opening and closing valve when performing expulsion of bubbles. 
     According to this aspect of the present invention, when performing the expulsion of bubbles, the opening and closing valve is opened so as to connect the liquid supply manifold and the liquid collection manifold through the first bypass flow channel, and at other times, the opening and closing valve is closed so as to shut off the connection between the liquid supply manifold and the liquid collection manifold through the first bypass flow channel. Hence, it is possible to suppress variation in the liquid circulation volume due to irregular movement of the bubbles and the liquid. 
     Preferably, the inkjet recording apparatus further comprises: a second bypass flow channel which connects the liquid supply manifold to the liquid collection manifold, wherein an end of the second bypass flow channel is connected to the lower side in the vertical direction of the end of the liquid supply manifold on the side opposite to the side where the liquid inlet port is arranged, and the other end of the second bypass flow channel is connected to the lower side in the vertical direction of the end of the liquid collection manifold on the side opposite to the side where the liquid outlet port is arranged. 
     According to this aspect of the present invention, it is possible to reduce the liquid temperature difference between the recording head modules. 
     Due to the liquid inside the liquid supply manifold flowing to the liquid collection manifold through the second bypass flow channel, the flow speed of the liquid in the vicinity of the end of the liquid supply manifold on the side opposite to the side where the liquid inlet port is arranged is kept at or above a prescribed value, temperature variation due to exchange of heat between the liquid and the surrounding air in the vicinity of the end portion is suppressed and difference in the liquid temperature between the recording head modules can be reduced. 
     Furthermore, by connecting together the lower sides in the vertical direction of the manifolds (and desirably, the lower end faces thereof) through the second bypass flow channel, it is possible to prevent bubbles from entering into the second bypass flow channel, and a circulation having a stable flow volume can be achieved. 
     Preferably, a heat insulating member is arranged on an outer circumferential surface of the liquid supply manifold. 
     According to this aspect of the present invention, by reducing the exchange of heat between the liquid supply manifold and the surrounding air, the temperature difference between a recording head module that is connected to a position close to the liquid inlet port in the liquid supply manifold and a recording head module that is connected to a position distant from the liquid inlet port is reduced, and the temperature difference between the recording head modules can be restricted. 
     Preferably, a heat insulating member is arranged on an outer circumferential surface of the liquid collection manifold. 
     According to this aspect of the present invention, by providing a heat insulation member on the outer circumferential surface of the liquid collection manifold, and not just on the liquid supply manifold, it is possible to achieve a more stable state of the liquid circulation, without being affected by the surrounding air. 
     Preferably, the inkjet recording apparatus further comprises: a first pressure determination device which determines an internal pressure of the liquid supply manifold; and a second pressure determination device which determines an internal pressure of the liquid collection manifold, wherein the liquid circulation device serves as a pressure adjustment device which adjusts the internal pressures of the liquid supply manifold and the liquid collection manifold to prescribed pressures in accordance with determination results obtained by the first and second pressure determination devices. 
     According to this aspect of the present invention, since the pressure determination devices are arranged in the manifolds, which are the common flow channels closest to the recording head modules, then it is possible to achieve the ink circulation of large volume, with high accuracy (since the liquid flow channel branches, then it is difficult to measure the pressure in the whole of the line head at a position closer to the recording head modules than the manifolds, because of the effects of the head modules). 
     Preferably, the first pressure determination device is disposed in the end of the liquid supply manifold on the side opposite to the side where the liquid inlet port is arranged; and the second pressure determination device is disposed in the end of the liquid collection manifold on the side opposite to the side where the liquid outlet port is arranged. 
     According to this aspect of the present invention, since the pressure in the portion of each manifold where the flow speed is slowest is measured, then it is possible to obtain measurement values (pressure values) including little effect of dynamic pressure and even more accurate ink circulation can be achieved. 
     According to the present invention, the liquid supply manifold and the liquid collection manifold each have heights in the vertical direction whereby the gas getting mixed with the liquid can separate from the liquid in the vertical direction, the first bypass flow channel connecting these manifolds together is provided, and one end of the first bypass flow channel is connected to the upper side in the vertical direction (and desirably the upper end face) of the end of the liquid supply manifold on the side opposite to the side where the liquid inlet port is arranged. Hence, bubbles which have entered through the liquid inlet port of the liquid supply manifold collect on the side where the first bypass flow channel is connected and are conveyed by following the flow of the liquid to the liquid collection manifold through the first bypass flow channel, without passing through the recording head modules, and are expelled to the exterior through the liquid outlet port. Furthermore, since the manifolds each have sufficient thicknesses (internal flow channel cross-sectional areas), there is little pressure loss in the manifolds and the pressure difference between the recording head modules can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and advantages thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a general schematic drawing of an inkjet recording apparatus according to an embodiment of the present invention; 
         FIG. 2  is a principal plan diagram showing the periphery of a printing unit of the inkjet recording apparatus; 
         FIGS. 3A and 3B  are plan view perspective diagrams showing embodiments of the composition of a printing head; 
         FIG. 4  is a cross-sectional diagram showing the inner composition of an ink chamber unit; 
         FIG. 5  is a principal block diagram showing the control system of the inkjet recording apparatus; 
         FIG. 6  is a schematic drawing showing the composition of an ink supply system according to a first embodiment; 
         FIG. 7  is a schematic drawing showing the composition of an ink supply system according to a second embodiment; 
         FIG. 8  is a schematic drawing showing the composition of an ink supply system according to a third embodiment; 
         FIG. 9  is a schematic drawing showing the composition of an ink supply system according to a fourth embodiment; 
         FIG. 10  is a schematic drawing showing the composition of an ink supply system according to a fifth embodiment; 
         FIG. 11  is a schematic drawing showing the composition of an ink supply system according to a sixth embodiment; and 
         FIG. 12  is a schematic drawing showing the composition of an ink supply system used in evaluation experiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General Configuration of Inkjet Recording Apparatus 
       FIG. 1  is a general configuration diagram of an inkjet recording apparatus according to an embodiment of the present invention. As illustrated in  FIG. 1 , the inkjet recording apparatus  10  includes: a printing unit  12  having a plurality of recording heads (hereafter, also simply called “heads”)  12 K,  12 C,  12 M, and  12 Y provided for the respective ink colors; an ink storing and loading unit  14  for storing inks of K, C, M and Y to be supplied to the printing heads  12 K,  12 C,  12 M, and  12 Y; a paper supply unit  18  for supplying recording paper  16 ; a decurling unit  20  removing curl in the recording paper  16 ; a suction belt conveyance unit  22  disposed facing the nozzle face (ink-droplet ejection face) of the printing unit  12 , for conveying the recording paper  16  while keeping the recording paper  16  flat; a print determination unit  24  for reading the printed result produced by the printing unit  12 ; and a paper output unit  26  for outputting image-printed paper (printed matter) to the exterior. 
     In  FIG. 1 , a magazine for rolled paper (continuous paper) is shown as an example of the paper supply unit  18 ; however, more magazines with paper differences such as paper width and quality may be jointly provided. Moreover, papers may be supplied with cassettes that contain cut papers loaded in layers and that are used jointly or in lieu of the magazine for rolled paper. 
     In the case of the configuration in which roll paper is used, a cutter  28  is provided as illustrated in  FIG. 1 , and the continuous paper is cut into a desired size by the cutter  28 . The cutter  28  has a stationary blade  28 A, whose length is not less than the width of the conveyor pathway of the recording paper  16 , and a round blade  28 B, which moves along the stationary blade  28 A. The stationary blade  28 A is disposed on the reverse side of the printed surface of the recording paper  16 , and the round blade  28 B is disposed on the printed surface side across the conveyor pathway. When cut papers are used, the cutter  28  is not required. 
     In the case of a configuration in which a plurality of types of recording paper can be used, it is preferable that an information recording medium such as a bar code and a wireless tag containing information about the type of paper is attached to the magazine, and by reading the information contained in the information recording medium with a predetermined reading device, the type of paper to be used is automatically determined, and ink-droplet ejection is controlled so that the ink-droplets are ejected in an appropriate manner in accordance with the type of paper. 
     The recording paper  16  delivered from the paper supply unit  18  retains curl due to having been loaded in the magazine. In order to remove the curl, heat is applied to the recording paper  16  in the decurling unit  20  by a heating drum  30  in the direction opposite from the curl direction in the magazine. The heating temperature at this time is preferably controlled so that the recording paper  16  has a curl in which the surface on which the print is to be made is slightly round outward. 
     The decurled and cut recording paper  16  is delivered to the suction belt conveyance unit  22 . The suction belt conveyance unit  22  has a configuration in which an endless belt  33  is set around rollers  31  and  32  so that the portion of the endless belt  33  facing at least the nozzle face of the printing unit  12  and the sensor face of the print determination unit  24  forms a plane. 
     The belt  33  has a width that is greater than the width of the recording paper  16 , and a plurality of suction apertures (not shown) are formed on the belt surface. A suction chamber  34  is disposed in a position facing the sensor surface of the print determination unit  24  and the nozzle surface of the printing unit  12  on the interior side of the belt  33 , which is set around the rollers  31  and  32 , as illustrated in  FIG. 1 . The suction chamber  34  provides suction with a fan  35  to generate a negative pressure, and the recording paper  16  on the belt  33  is held by suction. 
     The belt  33  is driven in the clockwise direction in  FIG. 1  by the motive force of a motor (not shown) being transmitted to at least one of the rollers  31  and  32 , which the belt  33  is set around, and the recording paper  16  held on the belt  33  is conveyed from left to right in  FIG. 1 . 
     Since ink adheres to the belt  33  when a marginless print job or the like is performed, a belt-cleaning unit  36  is disposed in a predetermined position (a suitable position outside the printing area) on the exterior side of the belt  33 . Although the details of the configuration of the belt-cleaning unit  36  are not shown, examples thereof include a configuration in which the belt  33  is nipped with cleaning rollers such as a brush roller and a water absorbent roller, an air blow configuration in which clean air is blown onto the belt  33 , and a combination of these. In the case of the configuration in which the belt  33  is nipped with the cleaning rollers, it is preferable to make the line velocity of the cleaning rollers different from that of the belt  33  to improve the cleaning effect. 
     A roller nip conveyance mechanism, in place of the suction belt conveyance unit  22 , can be employed. However, there is a drawback in the roller nip conveyance mechanism that the print tends to be smeared when the printing area is conveyed by the roller nip action because the nip roller makes contact with the printed surface of the paper immediately after printing. Therefore, the suction belt conveyance in which nothing comes into contact with the image surface in the printing area is preferable. 
     A heating fan  40  is disposed on the upstream side of the printing unit  12  in the conveyance pathway formed by the suction belt conveyance unit  22 . The heating fan  40  blows heated air onto the recording paper  16  to heat the recording paper  16  immediately before printing so that the ink deposited on the recording paper  16  dries more easily. 
     The printing unit  12  is a so-called “full line head” in which a line head having a length corresponding to the maximum paper width is arranged in a direction (main scanning direction) that is perpendicular to the paper conveyance direction (sub scanning direction). Each of the printing heads  12 K,  12 C,  12 M, and  12 Y constituting the printing unit  12  is constituted by a line head, in which a plurality of ink ejection ports (nozzles) are arranged along a length that exceeds at least one side of the maximum-size recording paper  16  intended for use in the inkjet recording apparatus  10  (see  FIG. 2 ). 
     The printing heads  12 K,  12 C,  12 M, and  12 Y are arranged in the order of black (K), cyan (C), magenta (M), and yellow (Y) from the upstream side, along the feed direction of the recording paper  16  (hereinafter, referred to as the sub-scanning direction). A color image can be formed on the recording paper  16  by ejecting the inks from the printing heads  12 K,  12 C,  12 M, and  12 Y, respectively, onto the recording paper  16  while conveying the recording paper  16 . 
     By adopting the printing unit  12  in which the full line heads covering the full paper width are provided for the respective ink colors in this way, it is possible to record an image on the full surface of the recording paper  16  by performing just one operation of relatively moving the recording paper  16  and the printing unit  12  in the paper conveyance direction (the sub-scanning direction), in other words, by means of a single sub-scanning action. Higher-speed printing is thereby made possible and productivity can be improved in comparison with a shuttle type head configuration in which a head reciprocates in a direction (the main scanning direction) orthogonal to the paper conveyance direction. 
     Although the configuration with the KCMY four standard colors is described in the present embodiment, combinations of the ink colors and the number of colors are not limited to those. Light inks or dark inks can be added as required. For example, a configuration is possible in which heads for ejecting light-colored inks such as light cyan and light magenta are added. 
     As illustrated in  FIG. 1 , the ink storing and loading unit  14  has tanks for storing the inks of K, C, M and Y to be supplied to the heads  12 K,  12 C,  12 M, and  12 Y, and the tanks are connected to the heads  12 K,  12 C,  12 M, and  12 Y by means of channels, which are omitted from figures. The ink storing and loading unit  14  has a warning device (for example, a display device or an alarm sound generator) for warning when the remaining amount of any ink is low, and has a mechanism for preventing loading errors among the colors. 
     The print determination unit  24  has an image sensor (line sensor) for capturing an image of the ink-droplet deposition result of the printing unit  12 , and functions as a device to check for ejection defects such as clogs of the nozzles in the printing unit  12  from the ink-droplet deposition results evaluated by the image sensor. 
     The print determination unit  24  of the present embodiment is configured with at least a line sensor having rows of photoelectric transducing elements with a width that is greater than the ink-droplet ejection width (image recording width) of the heads  12 K,  12 C,  12 M, and  12 Y. This line sensor has a color separation line CCD sensor including a red (R) sensor row composed of photoelectric transducing elements (pixels) arranged in a line provided with an R filter, a green (G) sensor row with a G filter, and a blue (B) sensor row with a B filter. Instead of a line sensor, it is possible to use an area sensor composed of photoelectric transducing elements which are arranged two-dimensionally. 
     The print determination unit  24  reads a test pattern image printed by the heads  12 K,  12 C,  12 M, and  12 Y for the respective colors, and the ejection of each head is determined. The ejection determination includes measurement of the presence of the ejection, measurement of the dot size, and measurement of the dot deposition position. 
     A post-drying unit  42  is disposed following the print determination unit  24 . The post-drying unit  42  is a device to dry the printed image surface, and includes a heating fan, for example. It is preferable to avoid contact with the printed surface until the printed ink dries, and a device that blows heated air onto the printed surface is preferable. 
     A heating/pressing unit  44  is disposed following the post-drying unit  42 . The heating/pressing unit  44  is a device to control the glossiness of the image surface, and the image surface is pressed with a pressure roller  45  having a predetermined uneven surface shape while the image surface is heated, and the uneven shape is transferred to the image surface. 
     The printed matter generated in this manner is outputted from the paper output unit  26 . The target print (i.e., the result of printing the target image) and the test print are preferably outputted separately. In the inkjet recording apparatus  10 , a sorting device (not shown) is provided for switching the outputting pathways in order to sort the printed matter with the target print and the printed matter with the test print, and to send them to paper output units  26 A and  26 B, respectively. When the target print and the test print are simultaneously formed in parallel on the same large sheet of paper, the test print portion is cut and separated by a cutter (second cutter)  48 . The cutter  48  is disposed directly in front of the paper output unit  26 , and is used for cutting the test print portion from the target print portion when a test print has been performed in the blank portion of the target print. The structure of the cutter  48  is the same as the first cutter  28  described above, and has a stationary blade  48 A and a round blade  48 B. 
     Although not illustrated in  FIG. 1 , the paper output unit  26 A for the target prints is provided with a sorter for collecting prints according to print orders. 
     Structure of the Head 
     Next, the structure of heads  12 K,  12 C,  12 M and  12 Y is described. The heads  12 K,  12 C,  12 M and  12 Y of the respective ink colors have the same structure, and a reference numeral  50  is hereinafter designated to any of the heads. 
       FIG. 3A  is a plan view perspective diagram showing an embodiment of the structure of a head  50 , and  FIG. 3B  is a partial enlarged view of same. Furthermore,  FIG. 4  is a cross-sectional diagram showing the inner composition of an ink chamber unit (a cross-sectional diagram along line  4 - 4  in  FIG. 3B ). 
     As shown in  FIG. 3A , the head  50  according to the present embodiment is a full line type head module having a nozzle row of a length corresponding to the full width of the recording paper  16 , and is constituted of a plurality of short head modules  100 A,  100 B, . . . , which are arranged and joined together in a staggered matrix configuration. In each of the short head modules  100 A,  100 B, . . . , a plurality of nozzles  51  are arranged two-dimensionally. The structure of the head modules  100 A,  100 B, . . . , is the same, and a reference numeral  100  is hereinafter designated to any of the head modules, unless specified otherwise. 
     As illustrated in  FIGS. 3A and 3B , each head module  100  has a structure in which a plurality of ink chamber units  53 , each having the nozzle  51  forming an ink droplet ejection hole, a pressure chamber  52  corresponding to the nozzle  51 , and the like, are disposed two-dimensionally in the form of a staggered matrix, and hence the effective nozzle interval (the projected nozzle pitch) as projected in the lengthwise direction of the head (the main scanning direction perpendicular to the paper conveyance direction) is reduced and high nozzle density is achieved. 
     In the present embodiment, the full line type head  50  is constituted of the short head modules  100  ( 100 A,  100 B, . . . ) arranged and joined together in the staggered matrix configuration; however, the composition of the head  50  is not limited to this and although not shown in the drawings, it is also possible, for example, to compose the head by arranging short head modules in a single row. 
     The pressure chambers  52  provided corresponding to the respective nozzles  51  are formed with an approximately square planar shape, as shown in  FIG. 3B . The nozzle  51  and an ink inlet port  54  are arranged in respective corners on a diagonal of the pressure chamber  52 . 
     As shown in  FIG. 4 , each of the pressure chambers  52  is connected to a common liquid chamber  55  through the ink inlet port  54 . Furthermore, each of the pressure chambers  52  is connected to a nozzle flow channel  60 , which is connected to a common circulation flow channel  64  through an independent flow channel  62 . Each head module  100  is provided with a supply port  66  and a discharge port  68 . The supply port  66  is connected to the common liquid chamber  55 , and the discharge port  68  is connected to the common circulation flow channel  64 . In other words, the supply port  66  and the discharge port  68  of the head module  100  are connected through the ink flow channels inside the head module (common liquid chamber  55 , pressure chambers  52 , common circulation flow channel  64 , and the like), and as described below, the ink supplied to the supply port  66  from the exterior of the head module is circulated through the ink flow channels inside the head module and discharged through the discharge port  68  to the exterior of the head module. 
     It is desirable that the individual flow channels  62  are connected to the nozzle flow channels  60  in the vicinity of the nozzles  51  as shown in  FIG. 4 . Since the ink is thereby allowed to circulate in the vicinity of the nozzles  51 , it is then possible to prevent increase in the viscosity of the ink inside the nozzles  51 , and to achieve stable ejection. 
     Piezoelectric elements  58  respectively provided with individual electrodes  57  are bonded to a diaphragm  56  which forms the upper face of the pressure chambers  52  and also serves as a common electrode, and each piezoelectric element  58  is deformed when a drive voltage is supplied to the corresponding individual electrode  57 , thereby causing ink to be ejected from the corresponding nozzle  51 . When ink is ejected, new ink is supplied to the pressure chambers  52  from the common liquid chamber  55  through the ink inlet ports  54 . 
     In the present embodiment, a piezoelectric element  58  is used as an ink ejection force generating device which causes ink to be ejected from a nozzle  51 , but it is also possible to employ a thermal method in which a heater is provided inside the pressure chamber  52  and ink is ejected by using the pressure of the film boiling action caused by the heating action of this heater. 
     As illustrated in  FIG. 3B , the high-density nozzle head according to the present embodiment is achieved by arranging a plurality of ink chamber units  53  having the above-described structure in a lattice fashion based on a fixed arrangement pattern, in a row direction which coincides with the main scanning direction, and a column direction which is inclined at a fixed angle of θ with respect to the main scanning direction, rather than being perpendicular to the main scanning direction. 
     More specifically, by adopting a structure in which a plurality of ink chamber units  53  are arranged at a uniform pitch d in line with a direction forming an angle of θ with respect to the main scanning direction, the pitch P of the nozzles projected so as to align in the main scanning direction is d×cos θ, and hence the nozzles  51  can be regarded to be equivalent to those arranged linearly at a fixed pitch P along the main scanning direction. Such configuration results in a nozzle structure in which the nozzle row projected in the main scanning direction has a high nozzle density of up to 2,400 nozzles per inch. 
     When implementing the present invention, the arrangement structure of the nozzles is not limited to the example shown in the drawings, and it is also possible to apply various other types of nozzle arrangements, such as an arrangement structure having one nozzle row in the sub-scanning direction. 
     Configuration of Control System 
       FIG. 5  is a principal block diagram showing the control system of the inkjet recording apparatus  10 . The inkjet recording apparatus  10  comprises a communication interface  70 , a system controller  72 , a memory  74 , a motor driver  76 , a heater driver  78 , a print controller  80 , an image buffer memory  82 , a head driver  84 , and the like. 
     The communication interface  70  is an interface unit for receiving image data sent from a host computer  86 . A serial interface such as USB (Universal Serial Bus), IEEE1394, Ethernet (registered trademark), wireless network, or a parallel interface such as a Centronics interface may be used as the communication interface  70 . A buffer memory (not shown) may be mounted in this portion in order to increase the communication speed. 
     The image data sent from the host computer  86  is received by the inkjet recording apparatus  10  through the communication interface  70 , and is temporarily stored in the memory  74 . The memory  74  is a storage device for temporarily storing images inputted through the communication interface  70 , and data is written and read to and from the memory  74  through the system controller  72 . The memory  74  is not limited to a memory composed of semiconductor elements, and a hard disk drive or another magnetic medium may be used. 
     The system controller  72  is a control unit which controls the respective sections, such as the communication interface  70 , the memory  74 , the motor driver  76 , the heater driver  78 , and the like. The system controller  72  is made up of a central processing unit (CPU) and peripheral circuits thereof, and as well as controlling communications with the host computer  86  and controlling reading from and writing to the memory  74 , and the like, and it generates control signals for controlling the motors  88  of the conveyance system and the heaters  89 . 
     Programs executed by the CPU of the system controller  72  and the various types of data which are required for control procedures are stored in the memory  74 . The memory  74  is used as a temporary storage region for the image data, and it is also used as a program development region and a calculation work region for the CPU. 
     Various control programs are stored in the program storage unit  90 , and the control programs are read out and executed in accordance with commands from the system controller  72 . The program storage unit  90  may use a semiconductor memory, such as a ROM, EEPROM, or a magnetic disk, or the like. An external interface may be provided, and a memory card or PC card may also be used. Naturally, a plurality of these recording media may also be provided. The program storage unit  90  may also be combined with a storage device for storing operational parameters, and the like (not illustrated). 
     The motor driver (drive circuit)  76  drives the motor  88  in accordance with commands from the system controller  72 . The heater driver  78  drives the heater  89  of the post-drying unit  42  and the like in accordance with commands from the system controller  72 . 
     The pump driver  92  is a driver which drives a pump  94  in accordance with the instructions from the system controller  72 . The pump  94  shown in  FIG. 5  includes pumps  124  and  126  of an ink supply system. 
     The print controller  80  has a signal processing function for performing various tasks, compensations, and other types of processing for generating print control signals from the image data stored in the memory  74  in accordance with commands from the system controller  72  so as to supply the generated print control signals (dot data) to the head driver  84 . Necessary signal processing is carried out in the print controller  80 , and the ejection amount and the ejection timing of the ink from the respective recording heads  50  are controlled via the head driver  84 , on the basis of the print data. By this means, desired dot size and dot positions can be achieved. 
     The print controller  80  is provided with the image buffer memory  82 ; and image data, parameters, and other data are temporarily stored in the image buffer memory  82  when image data is processed in the print controller  80 . The aspect illustrated in  FIG. 6  is one in which the image buffer memory  82  accompanies the print controller  80 ; however, the memory  74  may also serve as the image buffer memory  82 . Also possible is an aspect in which the print controller  80  and the system controller  72  are integrated to form a single processor. 
     The head driver  84  generates drive signals for driving the piezoelectric elements  58  (see  FIG. 4 ) of the recording heads  50  of the respective colors, on the basis of dot data supplied from the print controller  80 , and supplies the generated drive signals to the piezoelectric elements  58 . A feedback control system for maintaining constant drive conditions in the recording heads  50  may be included in the head driver  84 . 
     The print determination unit  24  is a block that includes the line sensor as described above with reference to  FIG. 1 , reads the image printed on the recording paper  16 , determines the print conditions (presence of the ejection, variation in the dot formation, and the like) by performing prescribed signal processing, and the like, and provides the determination results of the print conditions to the print controller  80 . 
     According to requirements, the print controller  80  makes various corrections with respect to the recording head  50  on the basis of information obtained from the print determination unit  24 . 
     Composition of Ink Supply System 
     Next, the composition of the ink supply system of the inkjet recording apparatus  10  according to embodiments of the present invention is described. 
     First Embodiment 
       FIG. 6  is a schematic drawing showing the composition of the ink supply system according to a first embodiment of the present invention. In  FIG. 6 , in order to simplify the description, the ink supply system relating to one color only is depicted, and an inkjet recording apparatus of a plurality of colors is provided with a plurality of similar ink supply systems. 
     As shown in  FIG. 6 , the ink supply system of the inkjet recording apparatus  10  according to the present embodiment includes an ink supply manifold  102 , which is a liquid chamber in which the ink supplied from the ink tank (not shown) to the respective head modules  100  is temporarily stored, and an ink collection manifold  104 , which is a liquid chamber in which the ink collected from the head modules  100  to the ink tank is temporarily stored. 
     Each of the ink supply manifold  102  and the ink collection manifold  104  has a long thin shape having the lengthwise direction thereof along the direction in which the head modules  100  are arranged, and is formed to have a sufficient thickness (internal flow channel cross-sectional area) for a gas and the ink to separate in the vertical direction when the gas gets mixed with the ink. 
     The ink tank is a base tank (ink supply source) which stores the ink to be supplied to the head modules  100 , and corresponds to the tank disposed in the ink storage and loading unit  14  shown in  FIG. 1 . The ink tank is constituted of an open-air tank, which is connected to the ink supply manifold  102  through a first main flow channel  106  and is connected to the ink collection manifold  104  through a second main flow channel  108 . The first main flow channel  106  is provided with a first liquid pump  124 , and the second main flow channel  108  is provided with a second liquid pump  126 . 
     An ink inlet port  110  is arranged on a first end of the ink supply manifold  102  in the lengthwise direction thereof (the right-hand end in  FIG. 6 ). The ink inlet port  110  is connected with an end of the first main flow channel  106  (the end on the opposite side to the ink tank). A plurality of first branch flow channels  112  branch off from directly below the ink supply manifold  102 , and ends of the first branch flow channels  112  are connected respectively to the supply ports  66  of the head modules  100 . 
     An ink outlet port  114  is arranged on a first end of the ink collection manifold  104  in the lengthwise direction thereof (the right-hand end in  FIG. 6 ). The ink outlet port  114  is connected with an end of the second main flow channel  108  (the end on the opposite side to the ink tank). A plurality of second branch flow channels  116  branch off from directly below the ink collection manifold  104 , and ends of the second branch flow channels  116  are connected respectively to the discharge ports  68  of the head modules  100 . 
     According to this composition, when the first liquid pump  124  and the second liquid pump  126  are driven, the ink is supplied to the ink supply manifold  102  from the ink tank through the first main flow channel  106 . The supplied ink is distributed to the head modules  100  from the ink supply manifold  102  through the first branch flow channels  112  and the supply ports  66  of the head modules  100 . The ink circulated inside the head modules  100  is collected to the ink collection manifold  104  through the discharge ports  68  of the head modules  100  and the second branch flow channels  116 . The collected ink is returned to the ink tank from the ink collection manifold  104  through the second main flow channel  108 . 
     In order to achieve this ink circulation, the system controller  72  shown in  FIG. 5  controls the driving of the first liquid pump  124  and the second liquid pump  126  through the drive circuit (pump driver)  92  to adjust the pressures in the ink supply manifold  102  and the ink collection manifold  104  at prescribed pressures. 
     More specifically, a prescribed pressure differential is set between the ink supply manifold  102  and the ink collection manifold  104  in such a manner that the pressure in the ink supply manifold  102  is relatively higher than the pressure in the ink collection manifold  104 , and the driving of the liquid pumps  124  and  126  is controlled in such a manner that a prescribed back pressure (negative pressure) is applied to the ink inside the head modules  100 . 
     To give a more detailed description, the system controller  72  controls the driving of the liquid pumps  124  and  126  so as to satisfy:
 
 P   in   +ΔP   h1   &gt;P   nz1   &gt;P   out   +ΔP   h2   (1)
 
where P in  is the pressure in the ink supply manifold  102 , P out  is the pressure in the ink collection manifold  104 , P nz1  is the pressure (back pressure) in the head modules  100 , ΔP h1  is the pressure differential due to the height differential between the ink supply manifold  102  and the nozzle surfaces of the nozzle modules  100 , and ΔP h2  is the pressure differential due to the height differential between the ink collection manifold  104  and the nozzle surfaces of the head modules  100 .
 
     By controlling the driving of the liquid pumps  124  and  126  in this way, it is possible to achieve a circulation of the ink inside the head modules  100  (and in particular in the vicinity of the nozzles) at all times, irrespective of whether or not the head modules  100  are performing ejection operation. Thus, it is possible to prevent ejection defects caused by increased viscosity of the ink, or the like, and good print quality can be maintained over a long period of time. 
     In the ink supply system of the inkjet recording apparatus  10  having the composition described above, in order to improve the expulsion of bubbles which have entered in the ink supply manifold  102 , a bubble expulsion bypass flow channel  118  is arranged between the ink supply manifold  102  and the ink collection manifold  104 . One end of the bubble expulsion bypass flow channel  118  is connected to a connection port (bubble expulsion port)  120  of the ink supply manifold  102 , and the other end of the bubble expulsion bypass flow channel  118  is connected to a connection portion (bubble introduction port)  122  of the ink collection manifold  104 . 
     The connection port  120  of the ink supply manifold  102  is desirably arranged at the upper side in the vertical direction, and desirably on the upper end face, on a second end of the ink supply manifold  102  in the lengthwise direction thereof opposite to the first end where the ink inlet port  110  is arranged. The bubbles which have entered into the ink supply manifold  102  from the ink inlet port  110  are liable to follow the flow of the ink, in a separated state from the ink, and accumulate at the upper side in the vertical direction on the second end of the ink supply manifold  102 , which is opposite to the side where the ink inlet port  110  is arranged. Hence, the connection port  120  is arranged in the ink supply manifold  102  at the upper side in the vertical direction on the second end in the lengthwise direction thereof, and bubbles which have entered in the ink supply manifold  102  do not stagnate and can be conveyed easily and reliably through the bubble expulsion bypass flow channel  118  to the ink collection manifold  104 . 
     The connection port  122  of the ink collection manifold  104  is desirably arranged at the lower side in the vertical direction, and desirably on the lower end face, on a second end of the ink collection manifold  104  in the lengthwise direction thereof opposite to the first end where the ink outlet port  114  is arranged. If the connection port  122  is arranged at the upper side in the vertical direction of the ink collection manifold  104 , there is a concern that conveyance of bubbles from the ink supply manifold  102  to the ink collection manifold  104  will become difficult due to the effects of bubbles having accumulated inside the ink collection manifold  104 . Hence, the connection port  122  is arranged in the ink collection manifold  104  at the lower side in the vertical direction on the second end in the lengthwise direction thereof, and it is possible to convey the bubbles easily and reliably from the ink supply manifold  102  to the ink collection manifold  104 , without being affected by bubbles which are present inside the ink collection manifold  104 . The bubbles conveyed to the ink collection manifold  104  follow the flow of the ink and collect in the first end in the lengthwise direction on the opposite side thereof (the right-hand side in  FIG. 6 ), and are expelled from the ink outlet port  114  arranged at this position through the second main flow channel  108  to the ink tank and released into the atmosphere. 
     The ink outlet port  114  of the ink collection manifold  104  is desirably arranged at the upper side in the vertical direction, and desirably in the vicinity of the uppermost portion, on the first end in the lengthwise direction of the ink collection manifold  104  (the right-hand side in  FIG. 6 ). The bubbles collected in the ink collection manifold  104  are liable to accumulate on the upper side in the vertical direction thereof. If the ink outlet port  114  is arranged on the lower side in the vertical direction of the ink collection manifold  104 , the bubbles cannot be released into the atmosphere from the ink tank through the second main flow channel  108 . Hence, the ink outlet port  114  is arranged on the upper side in the vertical direction of the ink collection manifold  104 , and it is possible to release the bubbles inside the ink collection manifold  104  through the second main flow channel  108  and the ink tank into the atmosphere. 
     The ink inlet port  110  of the ink supply manifold  102  is desirably arranged at the lower side in the vertical direction, and desirably in the vicinity of the lowermost portion, on the first end in the lengthwise direction of the ink supply manifold  102  (the right-hand side in  FIG. 6 ). If the ink inlet port  110  is arranged on the upper side in the vertical direction of the ink supply manifold  102 , variation occurs in the flow channel resistance due to the effects of bubbles having entered in the ink supply manifold  102 , and it may become impossible to achieve a stable ink flow volume. Hence, the ink inlet port  110  is arranged in the ink supply manifold  102  at the lower side in the vertical direction on the first end in the lengthwise direction thereof, and it is possible to obtain a stable ink flow volume without being affected by the bubbles. 
     In the present embodiment, each of the ink supply manifold  102  and the ink collection manifold  104  is formed to have the same thickness in the lengthwise direction of the manifold; however, the present invention is not limited to this, and it is also possible to form the manifolds in such a manner that, for example, the thickness changes gradually from one end toward the other end in the lengthwise direction as in the sixth embodiment described below (see  FIG. 11 ), or in such a manner that the thickness of the central portion is different from that of either end in the lengthwise direction. Here, it is necessary to take account of the bubble expulsion characteristics when deciding the positions at which the ink inlet port  110 , the ink outlet port  114  and the connection ports  120  and  122  are to be arranged. 
     In the present embodiment, the ink supply manifold  102  and the ink collection manifold  104  have a length equal to or greater than the line head  50  (shown in  FIG. 3A , not shown in  FIG. 6 ) constituted of the head modules  100 , and are arranged substantially parallel with the direction in which the head modules  100  are arranged (the main scanning direction). Thus, the flow channel lengths of the branch flow channels from the ink supply manifold  102  and the ink collection manifold  104  to the respective head modules  100  are uniform between the head modules, the pressure losses in the head modules  100  are made uniform, and the ink can be stably circulated to the head modules  100 . 
     In the present embodiment, as shown in  FIG. 6 , the head modules  100 , the ink collection manifold  104  and the ink supply manifold  102  are arranged in order from the lower side to the upper side in the vertical direction; however, the order in which these elements are arranged is not limited in particular, provided that a prescribed pressure differential is set between the ink supply manifold  102  and the ink collection manifold  104 , in such a manner that circulation of the ink can be achieved. 
     In the present embodiment, each of the ink supply manifold  102  and the ink collection manifold  104  has a sufficient thickness (internal flow channel cross-sectional area) for gas and the ink to separate in the vertical direction even if the gas has gotten mixed with the ink, and hence there is little pressure loss inside the manifolds  102  and  104 , and it is possible to reduce the pressure differential between the head modules  100 . Moreover, even if a gas enters into the ink supply manifold  102  and the ink collection manifold  104 , since this gas stagnates on the upper sides in the vertical direction, then it never reaches the head modules  100  through the branch flow channels  112  and  116 , which are arranged on the lower sides in the vertical direction. 
     Second Embodiment 
       FIG. 7  is a schematic drawing showing the composition of the ink supply system according to a second embodiment of the present invention. In  FIG. 7 , elements which are the same as or similar to those in  FIG. 6  are denoted with the same reference numerals and description thereof is omitted here. 
     In the second embodiment, the bubble expulsion bypass flow channel  118  is provided with a valve (opening/closing valve)  130  as shown in  FIG. 7 . The opening and closing operation of the valve  130  is controlled by the system controller  72  shown in  FIG. 5 . 
     When performing the expulsion of bubbles, the system controller  72  implements control to open the valve  130 , set the ink supply manifold  102  and the ink collection manifold  104  to a connected state through the bubble expulsion bypass flow channel  118 , and move the bubbles in the ink supply manifold  102  to the ink collection manifold  104  through the bubble expulsion bypass flow channel  118 . On the other hand, at other times (when not performing the expulsion of bubbles), the system controller  72  implements control to close the valve  130 , and set the ink supply manifold  102  and the ink collection manifold  104  to a non-connected state through the bubble expulsion bypass flow channel  118 . 
     According to the second embodiment, it is possible to suppress variation in the ink circulation flow volume caused by irregular movement of bubbles. 
     Third Embodiment 
       FIG. 8  is a schematic drawing showing the composition of the ink supply system according to a third embodiment of the present invention. In  FIG. 8 , elements which are the same as or similar to those in  FIG. 6  or  7  are denoted with the same reference numerals and description thereof is omitted here. 
     If the ink supply manifold  102  and the ink collection manifold  104  are composed to have thick dimensions as in the respective embodiments described above, the ink flow rate in the manifolds becomes slow, the ink temperature varies due to exchange of heat with the surrounding air, and there is a concern that a difference will occur in the ink temperature between the head modules  100 . 
     Hence, in the third embodiment, a circulation bypass flow channel  132 , which is separate from the bubble expulsion bypass flow channel  118 , is arranged between the ink supply manifold  102  and the ink collection manifold  104  as shown in  FIG. 8 . Thus, it is possible to circulate the ink directly from the ink supply manifold  102  to the ink collection manifold  104 , without passing through the head modules  100 . 
     A connection port (ink discharge port)  134  to which an end of the circulation bypass flow channel  132  is connected is desirably arranged in the ink supply manifold  102  at the lower side in the vertical direction (desirably, on the lower end face) on the second end in the lengthwise direction thereof (the end on the side opposite to the ink inlet port  110 ; the left-hand end in  FIG. 8 ). 
     A connection port (ink inlet port)  136  to which the other end of the circulation bypass flow channel  132  is connected is desirably arranged in the ink collection manifold  104  at the lower side in the vertical direction (desirably, on the lower end face) on the second end in the lengthwise direction thereof (the end on the side opposite to the ink outlet port  114 ; the left-hand end in  FIG. 8 ). 
     According to the third embodiment, by carrying out ink circulation though the circulation bypass flow channel  132  during the printing operation, it is possible to reduce the difference in the ink temperature between the head modules  100 . 
     Moreover, since the circulation bypass flow channel  132  is connected to the lower sides in the vertical direction (and desirably on the lower end faces) of the ink supply manifold  102  and the ink collection manifold  104 , entering of bubbles into the circulation bypass flow channel  132  is prevented and circulation of a stable flow volume can be achieved. 
     Fourth Embodiment 
       FIG. 9  is a schematic drawing showing the composition of the ink supply system according to a fourth embodiment of the present invention. In  FIG. 9 , elements which are the same as or similar to those in  FIGS. 6 to 8  are denoted with the same reference numerals and description thereof is omitted here. 
     In the fourth embodiment, heat insulating members  140  and  142  are arranged on the outer circumferential surfaces of the ink supply manifold  102  and the ink collection manifold  104  as shown in  FIG. 9 . 
     According to the fourth embodiment, it is possible to reduce the exchange of heat between the ink supply manifold  102  and the surrounding air by means of the heat insulating member  140  arranged on the outer circumferential surface of the ink supply manifold  102 , and temperature difference between the head modules  100  can be further reduced. 
     Moreover, it is also possible to achieve ink circulation in a more stable state, without being affected by the surrounding air, by arranging the heat insulating member  142  also on the outer circumferential surface of the ink collection manifold  104  in addition to the ink supply manifold  102 . 
     Fifth Embodiment 
       FIG. 10  is a schematic drawing showing the composition of the ink supply system according to a fifth embodiment of the present invention. In  FIG. 10 , elements which are the same as or similar to those in  FIGS. 6 to 9  are denoted with the same reference numerals and description thereof is omitted here. 
     In the fifth embodiment, a first pressure sensor  144  is disposed in the ink supply manifold  102  on the second end in the lengthwise direction thereof (the end on the opposite side from the ink inlet port  110 ), and a second pressure sensor  146  is disposed in the ink collection manifold  104  on the second end in the lengthwise direction thereof (the end on the opposite side from the ink outlet port  114 ) as shown in  FIG. 10 . 
     The pressure sensors  144  and  146  are pressure determination devices which respectively measure the internal pressures of the corresponding manifolds  102  and  104 , and the measurement values (pressure values) determined by the pressure sensors  144  and  146  are reported to the system controller  72  shown in  FIG. 5 . 
     The system controller  72  controls the driving of the first liquid pump  124  and the second liquid pump  126  in such a manner that the internal pressures of the manifolds  102  and  104  assume target pressures, on the basis of the measurement values reported from the pressure sensors  144  and  146 . The control method performed by the system controller  72  is similar to the first embodiment and description thereof is omitted here. 
     According to the fifth embodiment, it is possible to obtain measurement values which are little affected by dynamic pressure, by measuring the pressures in the portions of the slowest flow rate in the ink supply manifold  102  and the ink collection manifold  104  (the end portions of the manifolds most distant from the ink inlet port  110  and the ink outlet port  114 , respectively). Hence, it is possible to control the internal pressures of the ink supply manifold  102  and the ink collection manifold  104  with greater accuracy, and further stabilization of the circulation of ink can be achieved. 
     Sixth Embodiment 
       FIG. 11  is a schematic drawing showing the composition of the ink supply system according to a sixth embodiment of the present invention. In  FIG. 11 , elements which are the same as or similar to those in  FIGS. 6 to 10  are denoted with the same reference numerals and description thereof is omitted here. 
     In the sixth embodiment, the ceiling faces of the ink supply manifold  102  and the ink collection manifold  104  (the inner wall faces on the upper sides in  FIG. 11 ) are inclined. 
     The ceiling face  102   a  of the ink supply manifold  102  is inclined in such a manner that the ceiling face  102   a  at the second end (the end on the side of the connection port  120 ) in the lengthwise direction of the ink supply manifold  102  is higher in the vertical direction than the ceiling face  102   a  at the first end of the ink supply manifold  102  (the end on the side of the ink inlet port  110 ). Hence, the bubbles which have entered into the ink supply manifold  102  are liable to collect in the periphery of the connection port  120  by following the inclination of the ceiling face  102   a , in such a manner that the bubbles can be conveyed readily through the bubble expulsion bypass flow channel  118  to the ink collection manifold  104 . 
     The ceiling face  104   a  of the ink collection manifold  104  is inclined in such a manner that the ceiling face  104   a  at the first end (the end on the side of the ink outlet port  114 ) in the lengthwise direction of the ink supply manifold  104  is higher in the vertical direction than the ceiling face  104   a  at the second end of the ink collection manifold  104  (the end on the side of the connection port  122 ). Hence, the bubbles collected in the ink collection manifold  104  are liable to collect in the periphery of the ink outlet port  114  by following the inclination of the ceiling face  104   a , in such a manner that the bubbles can be conveyed readily through the second main flow channel  108  to the ink tank and released into the atmosphere in the ink tank. 
     According to the sixth embodiment, it is possible to improve the bubble expulsion characteristics inside the manifolds. 
     In the sixth embodiment, the composition is described in which the ceiling face  102   a  of the ink supply manifold  102  and the ceiling face  104   a  of the ink collection manifold  104  are constituted of inclined faces (i.e., the ceiling faces are oblique to the bottom faces); however, the present invention is not limited to this, and it is also possible to incline the whole of each manifold in a composition where the ceiling face is parallel to the bottom face, as in the ink supply manifold  102  and the ink collection manifold  104  of the first to fifth embodiments. In this case also, it is possible to improve the bubble expulsion characteristics inside the manifolds. 
     EXAMPLES 
     Specific examples of the respective units of the ink supply system according to the first embodiment (shown in  FIG. 6 ) are described below. 
     It is possible to use an ink supply manifold  102  and an ink collection manifold  104  which have mutually similar shapes. The manifold flow channel length L is 750 mm, the cross-sectional shape of the manifold flow channel is circular, and the diameter of the manifold flow channel is 14 mm (if this diameter is not sufficient, then vertical separation of the air and the liquid does not occur). Polypropylene can be used as the material of the manifolds. 
     The pressure differential set between the ink supply manifold  102  and the ink collection manifold  104  is 4000 Pa. 
     The ink circulation flow volume is 9 ml/sec flow speed at the ink inlet port  110  and 7 ml/sec flow speed at the ink outlet port  114 . 
     The ink used has a viscosity of 6 mPa·s, a surface tension of 36 mN/m, and temperature of 25° C. 
     The number of head modules  100  connected to the ink supply manifold  102  and the ink collection manifold  104  is seventeen (17), and the arrangement spacing M of the head modules  100  is 43 mm. 
     The bubble expulsion bypass flow channel  118  has an internal diameter of 4 mm and a length of 300 mm. The circulation bypass flow channel  132  used in the third to the sixth embodiments (shown in  FIGS. 8 to 11 ) has an internal diameter of 2.5 mm and a length of 150 mm. 
     Each of the first main flow channel  106  and the second main flow channel  108  has an internal diameter of 6 mm. 
     Each of the first branch flow channels  112  and the second branch flow channels  116  has an internal diameter of 4 mm. 
     The bypass flow channels  118  and  132 , and the branch flow channels  112  and  116  used have the diameters which do not produce vertical separation of the air and the liquid. 
     There follows a description of evaluation results to confirm the state of vertical separation of gas and liquid in cylindrical internal flow channels, and the state of entering of bubbles into the branch flow channels, when using the ink under the aforementioned conditions. 
       FIG. 12  is a diagram showing the composition of the evaluation experiments. In  FIG. 12 , the cylindrical internal flow channel was formed inside a pipe  900  made of polypropylene. The ink was sent from an ink tank  906  by a tube pump  908  to the pipe  900 , in which bubbles  902  were mixed with the ink. The ink then flowed through branch flow channel pipes  904  and was received by an ink receptacle  910 . 
     In the evaluation experiments, the following two items were evaluated using the experimental composition shown in  FIG. 12 . 
     &lt;State of Air/Liquid Vertical Separation&gt; 
     Bubbles were intentionally mixed with the ink filling the pipe  900 , and the separated state was visually observed. 
     &lt;Entering of Bubble into Branch Flow Channel&gt; 
     Manifolds for evaluation were formed, in which seventeen (17) branch flow channel pipes  904  having an internal diameter of 4 mm were welded at a pitch of 43 mm to pipes  900  having respective internal diameters of 2.5 mm, 4.0 mm, 6.0 mm, 8.0 mm, 10.0 mm and 14.0 mm, each pipe  900  was filled with a mixture of bubbles and ink, and in this state, the tube pump  908  was driven to cause the ink to flow from the ink tank  906  into the pipe  900  at a flow rate of 9 ml/sec, and the state of entering of bubbles into the branch flow channel pipes  904  was visually observed. 
     Table 1 shows the evaluation results of the experiments. 
                             TABLE 1                          Internal diameter of flow channel (mm)                                             2.5   4.0   6.0   8.0   10.0   14.0                                                     State of air/liquid   Poor   Poor   Fair   Fair   Good   Good       vertical separation   (no   (no   (partial   (partial   (full   (full           separation)   separation)   separation)   separation)   separation)   separation)       Entering of   No   Poor   Poor   Poor   Good   Good       bubble into   assessment   (bubbles   (bubbles   (bubbles   (no   (no       branch flow       entered)   entered)   entered)   bubbles   bubbles       channel                   entered)   entered)                    
As shown in Table 1, if the internal diameter of the flow channel of the pipe  900  was 10 mm or greater, then the air and the liquid were completely separated in the vertical direction inside the pipe  900 , and desirable results could be achieved in that no bubbles entered into the branch flow channel pipes  904 .
 
     The diameter of the manifolds  102  and  104  described above is 14 mm, and it can be seen from these evaluation experiments that this is a desirable dimension. 
     The inkjet recording apparatus according to the present invention has been described in detail above, but the present invention is not limited to the aforementioned examples, and it is of course possible for improvements or modifications of various kinds to be implemented, within a range which does not deviate from the essence of the present invention. 
     It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.