Patent Publication Number: US-6712460-B2

Title: Inkjet printing head

Description:
This application is based on Patent Application No. 2001-246240 filed Aug. 14, 2001 in Japan, the content of which is incorporated hereinto by reference 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sub-tank of an inkjet printing head and, more particularly, to an inkjet printing head having a mechanism for discharging gases that have entered and accumulated in the sub-tank. 
     2. Description of the Related Art 
     In an enclosed type liquid container, gases that have been dissolved in a liquid in the container and gases that have passed through members of the container from outside are accumulated with the passage of time. An increase in the quantity of gases in the liquid container cancels a negative pressure required for holding the liquid in the container. Especially, in the case of an inkjet printing head having such an enclosed type light container, this disables proper ejection of the liquid and results in leakage of the liquid from the nozzle. 
     A countermeasure to this problem is to minimize gases trapped in the liquid container by employing materials having low gas permeability as materials of members used for the container, but it is still insufficient to eliminate the accumulated gases completely. 
     Referring to inkjet cartridges that are one type of liquid containers in the related art, such cartridges are frequently replaced when liquids in the liquid containers have run out, and the above problem has been avoided by replacing the cartridges in most cases. In such cases, however, the cartridges must be thrown away with liquids left in the containers, which undesirably results in the waste of containers and liquids. In the case of inkjet cartridges that are used with liquids repeatedly being charged, the above-described situation remains and there is no solution to this problem at all. 
     Another possible countermeasure to the problem is to discharge the accumulated gases to the outside periodically. However, when this is attempted by taking advantage of a recovery operation in the related art for sucking a liquid from a nozzle, the accumulated gases cannot be discharged efficiently because a liquid in the container is also discharged, and only the gases may finally remain in the container. 
     A possible measure to avoid this is to form a channel for communication with outside separately from an ink channel to a nozzle. A discharge port provided at the bottom of a liquid container sufficiently works as a channel for charging a liquid. However, when the accumulated gases are discharged, the liquid is discharged first, leaving the accumulated gases without discharging the gases as in the case of the recovery operation in the related art. Even if a chimney-like configuration is employed in which the opening of the channel is located at the top of the container, a consuming efficiency of a liquid can be decreased in the case of a container having a movable section constituted by a sheet, for example, because the chimney can impair the movability of the sheet depending on the position where the chimney is formed. 
     SUMMARY OF THE INVENTION 
     The present invention confronts the above problems and provides an inkjet printing head having a liquid container from which the accumulated gases can be easily discharged. 
     According to the present invention this is achieved in an inkjet printing head, characterized in that it has a movable section constituted by a deformable film sheet, a spring for imparting a negative pressure, and a sub-tank having a supply/discharge channel for supplying ink and discharging accumulated gases, the sub-tank being intermittently supplied with ink from a main tank and reserving the ink, and in that the supply/discharge channel is provided in a position where it does not interfere with the movable section and the spring. 
     The sub-tank may have a frame, and the supply/discharge channel may be formed in the frame. 
     An opening of the supply/discharge channel is preferably formed in an upper part of the sub-tank, and the opening may be formed at a ceiling section of the sub-tank. 
     The ceiling section of the sub-tank may be inclined toward the opening of the supply/discharge channel. 
     The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view showing a general structure of an inkjet printing apparatus having an ink intermittent supply system utilizing a sub-tank according to the present invention; 
     FIG. 2 schematically shows an inkjet printing apparatus utilizing an intermittent supply system that employs a normally connected tube mechanism; 
     FIGS. 3A and 3B shows schematic sectional views of a sub-tank utilizing an ink supply/accumulated gas discharge mechanism as a first embodiment of the present invention, FIG. 3A being a horizontal sectional view of the sub-tank, FIG. 3B being a vertical section view of the sub-tank; 
     FIG. 4 is a schematic enlarged horizontal sectional view of the supply/discharge channel in FIGS. 3A and 3B; 
     FIG. 5 is a schematic enlarged vertical sectional view of the supply/discharge channel in FIGS. 3A and 3B; 
     FIGS. 6A and 6B are schematic sectional views of a sub-tank utilizing an ink supply/accumulated gas discharge channel as a second embodiment of the present invention, FIG. 6A being a vertical sectional view of the sub-tank, FIG. 6B being a sectional view taken along the line A—A in FIG. 6A; 
     FIG. 7 is a schematic sectional view of a sub-tank utilizing an ink supply/accumulated gas discharge channel as a third embodiment of the present invention; 
     FIG. 8 is a schematic sectional view of a sub-tank utilizing an ink supply/accumulated gas discharge channel as a fourth embodiment of the present invention; 
     FIG. 9 is a perspective view of an ink tank (sub-tank) according to the present invention; 
     FIGS. 10A,  10 B and  10 C are illustrations of a step of molding a tank sheet of the ink tank in FIG. 9; 
     FIG. 11A is an illustration of a step of manufacturing a spring unit of the ink tank in FIG. 9, and FIG. 11B is an illustration of a step of manufacturing a spring/sheet unit of the ink tank in FIG. 9; 
     FIGS. 12A and 12B are illustrations of a spring/sheet/frame unit of the ink tank in FIG. 9; 
     FIG. 13 is an illustration of a step of combining the spring/sheet unit and the spring/sheet/frame unit of the ink tank in FIG. 9; 
     FIGS. 14A and 14B are sectional views of major parts at the combining step shown in FIG. 13; 
     FIG. 15 is an illustration of a step of mounting the ink tank in FIG. 9; and 
     FIG. 16 is a sectional view of major parts of the ink tank in FIG. 15 in a mounted state. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to the drawings. 
     (Example of Structure of Inkjet Printing Apparatus) 
     FIG. 1 is a schematic plan view showing a general structure of an inkjet printing apparatus having an intermittent ink supply system utilizing a sub-tank according to the present invention. 
     In the structure in FIG. 1, a printing head unit  1  is replaceably mounted on a carriage  2 . The printing head unit  1  has a plurality of printing heads and an ink tank container or chamber which contains a plurality of ink tanks (also referred to as “second ink tanks” or “sub-tanks” in relation to first ink tanks or main tanks described later) for directly supplying ink to the plurality of printing heads, and there is provided a connector (not shown) for transmitting signals such as a drive signal for driving the head section to cause an ink ejecting operation of a nozzle. The carriage  2  on which the printing head unit  1  is positioned and replaceably mounted is provided with a connector holder (electrical connecting section) for transmitting signals such as the drive signal to the printing head unit  1  through the connector. 
     The carriage  2  is guided and supported by a guide shaft  3  provided on a main body of the apparatus and extending in a main scanning direction such that it can be moved back and forth along the guide shaft. The carriage  2  is driven and controlled with respect to its position and movement by a main scanning motor  4  through transmission mechanisms such as a motor pulley  5 , a driven pulley  6 , and a timing belt  7 . For example, a home position sensor  10  in the form of a transmission type photo-interrupter is provided, and a blocking plate  11  is disposed in a fixed part of the apparatus associated with a home position of the carriage such that it can block an optical axis of the transmission type photo-interrupter. Thus, when the home position sensor  10  passes through the blocking plate  11  as a result of the movement of the carriage  2 , the home position is detected, and the position and movement of the carriage can be controlled using the detected position as a reference. 
     Printing medium  8  that are printing paper or plastic sheets are separately fed one by one from an automatic sheet feeder (hereinafter referred to as an ASF) 14  by rotating a pick-up roller  13  with an ASF motor  15  through a gear. Further, the medium is transported through a position (printing section) in a face-to-face relationship with a surface of the printing head unit  1  where ejection openings are formed as a result of the rotation of a transport roller  9  (sub scanning). The transport roller  9  is driven by transmitting the rotation of a line feed (LF) motor  16  through a gear. 
     At this time, judgment on whether the paper has been fed and decision of a print starting position on the printing medium in a sub scanning direction is performed based on output of a paper end sensor  12  for detecting the presence of a printing medium disposed upstream of a printing position on a printing medium transport path. The paper end sensor  12  is used to detect a rear end of a printing medium  8  and to decide a final printing position on the printing medium in the sub scanning direction based on the detection output. 
     The printing medium  8  is supported by a platen (not shown) at a bottom surface thereof such that a flat surface is formed in a portion thereof to be printed. In doing so, the printing head unit  1  carried by the carriage  2  is held such that the surface thereof where the ejection openings are formed protrudes downward from the carriage in parallel with the printing medium  8 . For example, the printing head unit  1  is an inkjet printing head unit having a structure for ejecting ink utilizing thermal energy and having an electrothermal transducer for generating thermal energy that causes film boiling of ink. That is, the printing head of the printing head unit  1  performs printing by utilizing the pressure of bubbles generated as a result of film boiling of ink caused by the thermal energy applied by the electrothermal transducer to eject ink. Obviously, a different type of unit such as a unit that ejects ink utilizing a piezoelectric device may be used. 
     Reference numeral  50  represents a recovery system mechanism that has a cap member used for an operation of recovering suction of ink from the printing head unit  1  and for protecting the surface of the printing head where the ejection openings are formed. The cap member can be set in positions where it is joined to and detached from the surface where the ejection openings are formed by a motor that is not shown. Operations such as the suction recovery operation of the printing head are performed by generating a negative pressure in the cap member by a suction pump which is not shown in the joined state. The surface of the printing head where the ejection openings are formed can be protected by keeping the cap member in the joined state when the printing apparatus is not used. 
     Reference numeral  51  represents a valve unit provided on the printing head unit side for coupling the printing head unit  1  to a first ink tank (hereinafter referred to as a main tank). Reference numeral  54  represents a valve unit provided at the ink supply source side to be paired with the valve unit  51 . Reference numeral  52  represents a valve unit provided on the printing head unit side for coupling the printing head unit  1  to an air pump unit. Reference numeral  53  represents a valve unit provided on an air pump unit side to be paired with the valve unit  52 . 
     The valve units  51  through  54  are in contact and coupled with the respective valve units to allow ink and air to flow between the valve units when the carriage  2  is located at the home position outside a printing area in the main scanning direction or at a position in the vicinity of the same. The valve units are decoupled from each other when the carriage  2  moves away the position toward the printing area, and the valve units  51  and  54  automatically enter a closed state as a result of the decoupling. On the contrary, the valve unit  52  is always in an open state. 
     Reference numeral  55  represents a tube member that is coupled with a main tank  57  to supply ink to the valve unit  54 . Reference numeral  56  represents a tube member for an air pressure or pneumatic circuit, the tube member  56  being coupled with a pump unit  58  for pressurization and depressurization. Reference numeral  62  represents a suction and exhaust port of the pump unit  58 . 
     It is not essential to configure each of the tube members as an integral unit, and it may be configured by combining a plurality of tube elements. A plurality of first and second ink tanks, tubes and valve units communicating therebetween are provided, corresponding to the number of printing heads. 
     (Another Example of Structure of Inkjet Printing Apparatus) 
     The intermittent supply system in FIG. 1 has a structure in which the valve units are coupled only when the second ink tank is charged with ink and in which the ink supply system between the first and second ink tanks is spatially disconnected during a printing operation. An intermittent supply system may be employed in which the ink channel or a fluid path is blocked with a valve instead of such disconnection to achieve fluid isolation between the first and second ink tanks. 
     FIG. 2 schematically shows an inkjet printing apparatus in which an intermittent supply system utilizing a normally connected tube mechanism is used. For simplicity, FIG. 2 does not show parts which can be configured similarly to those in FIG.  1  and which are not related to the description of the supply system of the present example. 
     In FIG. 2, reference numeral  70  represents a flexible tube for an air pressure circuit that is connected to a second ink tank of a printing head unit at one end thereof and connected to a pump unit  58  for pressurization and depressurization through an electromagnetic valve unit  72  and a tube member  56  for the air pressure circuit at another end thereof. Reference numeral  71  represents a flexible tube for supplying ink that is connected to the second ink tank of the printing head unit at one end thereof and connected to first ink tank  57  through the electromagnetic valve unit  72  and a tube member  55  for supplying ink at another end thereof. 
     That is, an intermittent supply system may be configured even using such a normally connected tube mechanism by interposing units for opening to form and closing to block a channel such as the electromagnetic valve unit  72  and by controlling the opening and closing of the same appropriately during an operation of charging the second ink tank with ink and a printing operation. 
     (Sub-tank and Manufacturing Method Thereof) 
     Here, a sub-tank (a second ink tank) to which an accumulated gas discharge mechanism according to the present invention is applied and a manufacturing method thereof will be described with reference to FIGS. 9-16. 
     FIG. 9 is a perspective view of an ink tank (a sub-tank)  127  manufactured through steps as described below, the tank having an enclosed structure in which top and bottom spring/sheet units  114  are mounted to openings at the top and bottom of a square frame  115 . As will be described later, the spring/sheet unit  114  is constituted by a spring unit  112  including a spring  107  and a pressure plate  109  and a flexible tank sheet (flexible member)  106 . The frame  115  is formed with a first ink supply port  128  for supplying ink from the ink tank  127  to a printing head and a second ink supply port  129  for introducing ink from a main tank to the ink tank  127 . 
     FIGS. 10A to  14 B illustrate a method of manufacturing such an ink tank  127 . 
     First, FIGS. 10A,  10 B, and  10 C are illustrations of steps of forming the flexible tank sheet  106  with a convex shape. 
     A sheet material  101  for forming the tank sheet  106  is formed from a raw material into a sheet having a large size, and the sheet material  101  is an important factor of the performance of the ink tank. The sheet material  101  has low permeability against gases and ink components, flexibility, and durability against repeated deformation. Such preferable materials include PP, PE, PVDC, EVOH, nylon, and composite materials with deposited aluminum, silica or the like. It is also possible to use such materials by laminating them. In particular, excellent ink tank performance can be achieved by laminating PP or PE that has high chemical resistance and PVDC that exhibits high performance in blocking gases and vapors. The thickness of such a sheet material  101  is preferably in the range from about 10 μm to 100 μm taking softness and durability into consideration. 
     As shown in FIG. 10A, such a sheet material  101  is formed into a convex shape using a forming die  102  having a convex portion  103 , a vacuum hole  104 , and a temperature adjusting mechanism (not shown). The sheet material  101  is absorbed by the vacuum hole  104  and formed into a convex shape that is compliant with the convex portion  103  by heat from the forming die  102 . After being formed into the convex shape as shown in FIG. 10B, the sheet material  101  is cut into a tank sheet  106  having a predetermined size as shown in FIG.  10 C. The size is only required to be suitable for manufacturing apparatus at subsequent steps and may be set in accordance with the volume of the ink tank  127  for containing ink. 
     FIG. 11A is an illustration of a step of manufacturing the spring unit  112  used for generating a negative pressure in the ink tank  127 . A spring  107  that is formed in a semicircular configuration in advance is mounted on a spring receiving jig  108 , and a pressure plate  109  is attached to the same from above through spot welding using a welding electrode  111 . A thermal adhesive  110  is applied to the pressure plate  109 . A spring unit  112  is constituted of the spring  107  and the pressure plate  109 . 
     FIG. 11B is an illustration of a step of mounting a spring unit  112  to the tank sheet  106 . The spring unit  112  is positioned on an inner surface of the tank sheet  106  placed on a receiving jig (not shown). The thermal adhesive  110  is heated using a heat head  113  to bond the spring unit  112  and the tank sheet  106  to form a spring/sheet unit  114 . 
     FIG. 12A is an illustration of a step of welding the spring/sheet unit  114  to the frame  115 . The frame  115  is secured to a frame receiving jig  116 . After the flame  115  is positioned and placed on the jig  116 , a sheet absorbing jig  117  surrounding the frame  115  absorbs the spring/sheet unit  114  to a vacuum hole  117 A to hold the unit  114  and the frame  115  without relative misalignment. Thereafter, a heat head  118  is used to thermally weld annular joint surfaces of a top side circumferential edge of the frame  115  in FIG. 12A and a circumferential edge of the tank sheet  106  of the spring/sheet unit  114 . Since the sheet absorbing jig  117  sets the top circumferential edge of the frame  115  in FIG.  12 A and the circumferential edge of the tank sheet  106  of the spring/sheet unit  114  in a uniform face-to-face relationship, the bonding surfaces are quite uniformly thermally welded and sealed. Therefore, the sheet absorbing jig  117  is important for thermal welding in order to provide uniform sealing. 
     FIG. 12B is an illustration of a step of cutting off a part of the tank sheet  106  protruding from the frame  115  with a cutter (not shown). A spring/sheet/frame unit  119  is completed by cutting off the part of the tank sheet  106  protruding from the frame  115 . 
     FIG. 13, FIG. 14A, and FIG. 14B are illustrations of steps of thermally welding another spring/sheet unit  114  fabricated through the above-described steps to such a spring/sheet/frame unit  119 . 
     As shown in FIG. 13, the spring/sheet/frame unit  119  is mounted on a receiving jig (not shown), and the periphery of the spring/sheet/frame unit  119  is surrounded by an absorbing jig  120  whose position is defined relative to the receiving jig. The receiving jig is in surface contact with an outer planar section  106 A of the tank sheet  106  of the spring/sheet/frame unit  119  to hold the planar section  106 A as shown in FIGS. 14A and 14B. The other spring/sheet unit  114  is absorbed and held by a holding jig  121  at an outer planar section  106 A of the tank  106  thereof. The holding jig  121  then is lowered to fit ends  107 A and  107 B of the spring  107  of the spring/sheet unit  114  and ends  107 A and  107 B of the spring  107  of the spring/sheet/frame unit  119  substantially simultaneously. The ends  107 A of the springs  107  have a convex shape, and the other ends  107 B have a concave shape, which causes them to fit each other respectively an a self-alignment basis. A single spring member is formed with combining those springs  107  as a pair of spring member forming bodies. 
     The holding jig  121  is further lowered to compress the pair of springs  107  as shown in FIG.  14 A. In doing so, the holding jig  121  widely presses the top planar section  106 A of the spring/sheet unit  114  in FIG. 13, i.e., a top flat region of the tank sheet  106  that is formed in a convex configuration. As a result, the position of the planar section  106 A of the tank sheet  106  is regulated, and the spring/sheet unit  114  approaches the unit  119  and the jig  120  located below the same while being kept in parallel with them. Therefore, as shown in FIG. 14B, the circumferential edge of the tank sheet  106  of the spring sheet unit  114  is absorbed and held at the vacuum hole  120 A in contact with a surface of the absorbing jig  120 , and it is also put in a uniform face-to-face relationship with the welding surface (the top joint surface in the same figure) of the frame  115 . In this state, annular joint surfaces of the top circumferential edge of the frame  115  of the spring/sheet/frame unit  119  and the tank sheet  106  of the spring/sheet unit  114  are thermally welded to each other with a heat head  122 . 
     By compressing the pair of springs  107  while thus maintaining parallelism between the planar section  106 A of the tank sheet  106  of the upper unit  114  and the planar section  106 A of the tank sheet  106  of the lower unit  119 , ink tanks  127  having high parallelism between the planar sections  106 A of the pair of tank sheets  106  thereof can be produced on a mass production basis with stability. Since the pair of springs  107  are symmetrically and uniformly compressed and deformed in FIGS. 14A and 14B, there will be no force that can incline the spring/sheet unit  114 , which makes it possible to produce ink tanks  127  having high parallelism between the planar sections  106 A of the pair of tank sheets  106  thereof with higher stability. Further, since the pair of springs  107  are symmetrically and uniformly compressed and deformed in FIGS. 14A and 14B, the interval between the planar sections  106 A of the pair of tank sheets  106  in a face-to-face relationship changes with higher parallelism maintained, which consequently makes it possible to supply ink with stability. Further, the ink tank  127  has high sealing property, pressure resistance, and durability because no force acts to incline the planar section  106 A of the flexible tank sheet  106 . 
     Thereafter, the part of the tank sheet  106  protruding from the frame  115  is cut off to complete the ink tank  127  as shown in FIG.  9 . The interior of the ink tank  127  has an enclosed structure that is in communication with the outside only through the first ink supply port  128  and the second ink supply port  129 . 
     FIG. 15 is an illustration of a step of mounting the ink tank (the sub-tank)  127  to a printing head. 
     A head chip  133  serving as a printing head is mounted in an ink tank containing chamber  130 , and a plurality of ink tanks  127  are mounted in the ink tank containing chamber  130 . The ink tanks  127  are mounted to an ink tank mounting section  131  using welding or bonding. The ink tanks  127  of the present embodiment are mounted with the ink supply ports  128  and  129  located on the bottom thereof. Thereafter, a lid  132  is mounted to an opening of the ink tank containing chamber  130  using welding or bonding to form a semi-enclosed space in the ink tank containing chamber  130 . A printing head having ink tanks is thus configured. The head chip  133  may serve as an inkjet printing head. The inkjet printing head may have a configuration in which an electrothermal transducer is provided to eject ink droplets from an ink ejection port, for example. Specifically, a configuration may be employed in which film boiling of ink is caused by heat generated by the electrothermal transducer and in which ink droplets are ejected from the ink ejection port utilizing the foaming energy. An inkjet cartridge can be configured by combining such an inkjet printing head and ink tanks. 
     FIG. 16 is a sectional view of the ink tank containing chamber  130  in FIG. 15 having ink tanks therein. 
     Ink can be reserved in the ink tanks  127 , and the ink is supplied from the first ink supply ports  128  of the ink tanks  127  to a supply channel  136  through a filter  137  and is then further supplied to the head chip  133 . A heater board  134  is bonded to the head chip  133  of the present embodiment to form an inkjet printing head. The heater board  134  is formed with ink paths and orifices and is provided with electrothermal transducers (heaters) to be able to eject ink supplied from the ink tanks  127 . The ink tanks  127  can be charged with ink through the second supply ports  129 . Specifically, a joint seal  138  for preventing ink leakage and allowing ink charging is secured to the second ink supply port  129  with a joint seal plate  139  such that it seals an opening  141  at the bottom of the ink tank containing chamber  130 . The joint seal  138  is constituted by a flexible rubber member and provided with a slit into which a supply pipe in the form of a needle can be inserted. When ink is supplied to the ink tank  127 , the needle-like supply pipe is inserted into the slit of the joint seal  138 , and ink is supplied to the ink tank  127  through the supply pipe. When ink is not supplied to the ink tank  127 , since the slit is closed because of the elasticity of the joint seal  138 , ink will not leak out. Reference numeral  140  represents a communication channel that is in communication with the second supply port  129 , and the communication channel may be formed in advance such that it extends through the frame  115 . 
     The ink tank containing chamber  130  having the generally enclosed structure formed by the lid  132  is in communication with the outside only through a small hole  142 . The interior of the ink tank containing chamber  130  can be isolated from the atmosphere by closing the small hole  142 . The pressure in the ink tank containing chamber  130  can be reduced to increase a negative pressure in the ink tanks  127  by exhausting air from the ink tank containing chamber  130  through the small hole  142 . 
     Ink can be automatically suctioned and supplied into the ink tanks  127  through the second ink supply ports  129  by repeating depressurization and pressurization of the interior of the ink tank containing chamber  130 . At this time, since the springs  107  are elastically deformed with high responsiveness to changes in the pressure in the ink tank containing chamber  130 , the ink tanks can be preferably used as compact ink tanks that are frequently replenished with ink. 
     Instead of a pair of springs  107 , a single spring may be provided which has a configuration that is similar to the combination of the two springs. In this case, the single spring may be mounted to one of a pair of tank sheets  106 ; the tank sheet  106  may then be coupled with a frame  115 ; and the other tank sheet  106  may be coupled with the frame  115  while compressing the single spring. In doing so, the single spring may be simply sandwiched between the pair of tank sheets  106  instead of mounting it to the other one of the pair of tank sheets  106 . 
     At least either of the pair of tank sheets  106  may be constituted by a flexible member. 
     (First Embodiment) 
     A configuration of a sub-tank according to the present invention will now be described with reference to FIGS. 3 to  5 . 
     An accumulated gas discharge mechanism according to the present invention is used in a sub-tank manufactured according to the above-described method of manufacture. The accumulated gas discharge mechanism according to the present invention also serves as an ink supply mechanism. 
     FIGS. 3A and 3B are schematic sectional views of a sub-tank (which corresponds to the ink tank  127  in the above description of the manufacturing method) that a first embodiment of an ink supply/accumulated gas discharge mechanism according to the present invention is utilized. FIG. 3A is a horizontal sectional view of the sub-tank, and FIG. 3B is a vertical sectional view of the sub-tank. FIG. 4 is a schematic enlarged horizontal sectional view of the supply/discharge channel in FIGS. 3A and 3B, and FIG. 5 is a schematic enlarged vertical sectional view of the supply/discharge channel in FIGS. 3A and 3B. 
     Referring to those figures, a sub-tank  200  is constituted by deformable film sheets  201 , a frame  202 , a pair of pressure plates  203  and  204 , and a pair of plate springs  210  for generating a negative pressure in the sub-tank  200 , as described above. FIG. 3A shows a state in which ink in the sub-tank  200  has been used to constrict the sub-tank  200 . The dotted line in FIG. 3A indicates a fully loaded state of the sub-tank  200 . A plurality of the sub-tanks  200  are provided in an ink tank containing chamber (which corresponds to the ink tank containing chamber  130  in the above description of the manufacturing method) of an inkjet printing head which is not shown. Each sub-tank movable section constituted by film sheets  201 , a pair of pressure plates  203 ,  204  and a pair of plate springs  210  as described above is provided substantially in parallel with a bottom wall of the ink tank containing chamber (i.e., such that the frame  202  is perpendicular to the bottom wall of the ink tank containing chamber). 
     Reference numeral  205  represents a supply/discharge channel that serves as an ink supply mechanism for supplying ink to the sub-tank and also as an accumulated gas discharge mechanism for discharging gases that have entered and accumulated in the sub-tank. As shown in FIGS. 3B and 5, the supply/discharge channel  205  is formed such that it extends in the vertical direction in a vertical frame  221  forming a part of the frame  202  and constituting a side section of the sub-tank. An opening  205 A of the channel  205  is provided in an upper part of the sub-tank  200 . By forming the opening  205 A of the supply/discharge channel  205  in an upper part of the sub-tank  200  in such a manner, gases accumulated (collected) in the upper part of the sub-tank can be efficiently discharged. 
     The supply/discharge channel  205  is preferably provided in a position where it does not interfere with the movable section that forms a part of the sub-tank, i.e., a dead space in the sub-tank. When provided in such a position, the supply/discharge channel  205  shall neither hinder the movement of the movable section nor reduce the consuming efficiency of ink in the sub-tank. Further, the ink capacity of the sub-tank  200  can be maximized by providing the supply/discharge channel  205  in the frame  202  as in the present embodiment. The supply/discharge channel  205  may be formed as a vertical pipe provided in a dead space in the sub-tank  200  apart from the vertical frame  221 . 
     Reference numeral  206  represents a groove formed along the supply/discharge channel  205  and an appropriate number of the grooves are provided as required. In the present embodiment, four grooves  206  are provided as shown in FIG.  4 . Reference numeral  207  represents an ink supply/accumulated gas discharge needle that is inserted in the supply/discharge channel  205  and the needle  207  has an opening  212  at the tip thereof. Reference numeral  209  represents a base that is integrally mounted to a bottom frame  223  for positioning and fixing the sub-tank  200  on the bottom wall of the ink-tank containing chamber of the inkjet printing head. Reference numeral  208  represents an ink supply port provided at the bottom frame  223  and the base  209  constituting the bottom of the sub-tank  200  for supplying ink in the sub-tank to the printing head (not shown). 
     A description will now be made with reference to FIGS. 3A,  3 B, and  5  on ink supply and accumulated gas discharge operations in the sub-tank  200  having the above configuration. 
     When the ink supply/accumulated gas discharge needle  207  is inserted in the supply/discharge channel  205  (or when a valve unit  51  of a printing head unit side and a valve unit  54  of an ink supplying side are connected as shown in FIG. 1 to connect the sub-tank  200  and the main tank  57 ), ink deposits or thickened ink  213  that has been accumulated in the supply/discharge channel  205  are discharged into the sub-tank  200  through the opening  205 A of the supply/discharge channel  205 , as shown in FIGS. 3A,  3 B, and  5 . This indicates that the operation of inserting the needle  207  has an effect of cleaning the supply/discharge channel  205 . The deposits  213  discharged into the sub-tank  200  are discharged from the tank through a printing head unit  1  during a recovery operation of the printing head unit  1 . 
     A valve unit  52  of the printing head side and a valve unit  53  of an air pump side are connected at the same time to couple the air pump with the ink tank containing chamber. 
     When air is sucked from the ink tank containing chamber with an air pump  58  in this state to generate a negative pressure in the containing chamber, the sub-tank  200  expands. Then, the negative pressure in the sub-tank  200  increases accordingly to allow ink to be supplied from the main tank  57  to the sub-tank  200  consequently. Conversely, when air is supplied into the ink tank containing chamber with the air pump  58  to pressurize the interior of the containing chamber, the sub-tank  200  constricts. Then, gases accumulated in the sub-tank  200  can be discharged to the main tank  57  along with the ink in the sub-tank  200 . The gases discharged in the main tank  57  can be released to the atmosphere since the main tank  57  is exposed to the atmosphere, and the returned (discharged) ink can be used again. 
     The inner diameter of the ink supply/accumulated gas discharge needle  207  is preferably made as great as possible to reduce pressure loss in the channel when ink is supplied or discharged. This can make the clearance between the supply/discharge channel  205  and the outer diameter of the needle  207  small to increase pressure loss at the opening  212  of the needle  207 . In order to avoid this, in the present embodiment, the supply/discharge channel  205  has a configuration including the grooves  206  as shown in FIG.  4 . This provides a clearance between the supply/discharge channel  205  and the opening  212  of the needle not to hinder ink from flowing. 
     Although four grooves  206  are provided in the present embodiment, this is not limiting the present invention. Referring to the configuration of the supply/discharge channel  205 , the same effect can be achieved by employing a configuration having an elliptic horizontal section or spiral grooves instead of the linear grooves  206  in the present embodiment. 
     (Other Embodiments) 
     FIGS. 6A and 6B show a second embodiment of the present invention. The present embodiment is different from the first embodiment in the position of an opening in an upper part of a supply/discharge channel  205 . Specifically, in the present embodiment, a supply/discharge channel  205  penetrates through a vertical frame  221  forming a part of a frame  202  and connects to an opening  205 A formed at a ceiling section of a sub-tank  200  through a horizontal channel  215  formed in a top horizontal frame  222 . Since gases  216  that have entered the sub-tank  200  are accumulated in an upper part of the sub-tank (in the direction opposite to the direction of gravity) as described above, the gases accumulated in the upper part can be discharged with efficiency higher than that of the first embodiment regardless of the level of ink  217  in the sub-tank  200  by discharging the accumulated gases  216  through the opening  205 A of the supply/discharge channel  205 . 
     In the present embodiment, the horizontal channel  215  is formed by removing a top surface of the top horizontal frame  222  for reasons associated with the die structure and processing of the top horizontal frame  222 . Therefore, in order to configure the horizontal channel  215 , the opening on the top surface must be sealed by thermally welding a seal material  214  for example, as shown in FIG. 6A. A film sheet  201  that forms a part of the sub-tank  200  may be extended and used as the seal material as shown in FIG.  6 B. 
     As a modification of the present embodiment, instead of providing the supply/discharge channel  205  extending upward from a lower part of the tank as in the present embodiment and forming another opening  205 B of the supply/discharge channel (an opening at the side thereof connected to the main tank that supplies ink) in a lower part of the tank, the horizontal channel  215  formed in the top horizontal frame  222  may be extended in the horizontal direction to form the opening  205 B in a side section of the sub-tank, thereby allowing the needle  207  to be inserted in the horizontal direction. Alternatively, the supply/discharge channel  205  may be horizontally formed along and under a bottom surface of the top horizontal frame  222  (the ceiling section of the sub-tank), and the second opening  205 B may be formed in a side section of the sub-tank similarly. 
     FIGS. 7 and 8 show third and fourth embodiments of the invention in which accumulated gases  216  can be discharged with higher efficiency. 
     Referring to FIG. 7, a top surface (ceiling section) of a sub-tank  200  that is an ink container is formed with a slope  218 . Specifically, a bottom surface of a top horizontal frame  222  forming a part of a frame  202  of the sub-tank  200  is formed as an inclined surface ascending toward an opening of a supply/discharge channel  205 . Since accumulated gases  216  are collected in the vicinity of an opening  205 A of the supply/discharge channel  205  in such a configuration, the accumulated gases  216  can be easily and efficiently discharged. In the embodiment shown in FIG. 7, the volumetric capacity of the sub-tank is slightly reduced by the slope  218  provided at the ceiling section of the sub-tank. As a measure to solve this problem without losing the same effect, the sub-tank may be installed in an inclined attitude in an ink tank containing chamber as shown in FIG.  8 . Alternatively, an ink tank containing chamber containing a plurality of the sub-tanks may be installed in an inclined attitude in a printing head. 
     As described above, an inkjet printing head according to the present invention comprises a sub-tank which has a movable section constituted by deformable film sheets, a spring for generating a negative pressure, a supply/discharge channel for supplying ink and discharging accumulated gases, and which is intermittently supplied with ink from a main tank and reserves the ink. The printing head according to the present invention also comprises a configuration in which the supply/discharge channel of the sub-tank is provided in a position where it does not interfere with the movable section and the spring. Therefore, this allows the sub-tank to be constricted until the ink therein is substantially used up, which makes it possible to improve ink consuming efficiency and to reduce the frequency of ink supply from the main tank. 
     Further, since the supply/discharge channel is formed in a frame of the sub-tank, the sub-tank can be provided with a great liquid (ink) containing capacity and manufactured easily. 
     Since an opening of the supply/discharge channel is provided in an upper part of the sub-tank or at a ceiling section of the same that is the top section thereof, gases that have entered and accumulated in the sub-tank can be easily and efficiently discharged without hindering ink supply in spite of the simple structure. In addition, accumulated gases can be more easily and efficiently discharged by tilting a bottom surface of the ceiling section. 
     The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, in the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.