Patent Publication Number: US-8534810-B2

Title: Liquid discharge head unit and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosures herein relate to a liquid discharge head unit and an image forming apparatus. 
     2. Description of the Related Art 
     An inkjet recording apparatus is generally known as a liquid discharge recording type image forming apparatus, such as a printer, a facsimile machine, or a plotter, or a combination of these functions, that includes a recording head formed, for example, of a liquid discharge head (liquid droplet discharge head) discharging liquid droplets. The liquid discharge recording type image forming apparatus is configured to discharge ink droplets from its recording head onto a transferred sheet, thereby forming an image on the transferred sheet. The formation of the image includes recording, printing, and imaging. There are two types of the liquid discharge image forming apparatus including: 1) a serial type image forming apparatus in which the recording head discharges ink droplets onto the transferred sheet for forming an image while traveling in a main-scanning direction; and 2) a line type image forming apparatus in which the recording head discharges ink droplets onto the transferred sheet for forming an image without traveling in any direction. Note that the transferred sheet is not limited to paper, but may be any media such as an overhead projector (OHP) film insofar as ink droplets or other liquid droplets are adhered to the media. Such media are also referred to as “recording media to be recorded”, “recording media”, “recording paper”, and “recording forms”. 
     Note that in this application, the “liquid discharge recording type image forming apparatus” indicates an image forming apparatus that forms an image onto media such as paper, string, fiber, fabric, leather, metal, plastic, glass, wood, and ceramics by discharging liquid onto such media. Note also that “forming an image” or “image formation” not only indicates providing an image having some kind of meanings onto the media such as characters and symbols, but also indicates an image without having any meanings such as patterns (i.e., by simply discharging ink droplets onto the media). Further, “ink” is not limited to those generally called “ink”, but may include those called “liquid” used as a generic name capable of forming an image, such as recording liquid, fixing liquid, and “liquid”. The ink of this application also includes a specimens for deoxyribonucleic acid (DNA), resist, patterning material, resin, and the like. Moreover, the “image” is not limited to the image applied to a two-dimensional object, but may include the image applied to a three-dimensional object and to the image formed of a molded object. 
     In the liquid discharge recording type image forming apparatus, if air bubbles are mixed into the liquid discharge head (i.e., recording head), the liquid discharge head may discharge liquid droplets in wrong directions or may discharge in an inappropriate fashion. Thus, it may be necessary to efficiently eliminate air bubbles present in ink supply paths. 
     Japanese Patent Application Publication No. 2009-126044 (hereinafter referred to as “Patent Document 1”) discloses an example of an air bubble eliminating technology for eliminating air bubbles from a head tank (synonymously used with a “sub-tank” or “buffer tank” insofar as the ink tank is integrally formed in a head). In the air bubble eliminating technology disclosed in Patent Document 1, a recording head is configured to include a first ink chamber having an inflow port to which ink is flown from an ink supply source; a second ink chamber to which ink is supplied from the first ink chamber; a discharge port configured to discharge ink from the second ink chamber, thereby performing a recording operation; a first discharge port configured to discharge a fluid from the first ink chamber; a second discharge port configured to discharge a fluid from the second ink chamber; and a liquid-air separation unit provided between the first ink chamber and the first discharge port, or between the second ink chamber and the second discharge port, and configured to regulate the discharge of the liquid, where a fluid resistance from the inflow port to the first discharge port is lower than resistance from the inflow port to the second discharge port. 
     Another example of the air bubble eliminating technology for eliminating air bubbles from the head tank is disclosed in Japanese Patent Application Publication No. 2002-086748 (hereinafter referred to as “Patent Document 2”). In the air bubble eliminating technology disclosed in Patent Document 2, a sub-tank is configured to include an ink chamber having an ink introducing part and an ink discharge part, the ink introducing part and the ink discharge part both arranged in an upper part of the ink chamber. The ink chamber includes a supplementary feed valve formed of a compression spring configured to shut off an ink introducing path by closely pressing a ball against a valve seat formed of an elastic member having the ink introducing path provided in the upper part of ink chamber and a sealing part formed of an elastic member having a constantly closed slit in the center of the sealing part provided in the ink discharge part of the ink chamber, where air bubbles mixed inside the ink chamber are appropriately discharged from the slit of the sealing part. 
     The line type image forming apparatus, for example, includes a recording head unit having liquid discharge head integrally having corresponding heads and tanks for supplying liquids to the heads (i.e., head-tanks) arranged in an entire width direction of a recording medium. In the line type image forming apparatus having such a configuration, ink supplied from main-tanks is distributed in the order of the corresponding sub-tanks and distributors (distributing members) to distribute the ink to the corresponding heads. Note that the head tanks include corresponding filtering members configured to filter impurities from the ink. 
     However, nozzles provided in each of the heads are extra finely made (e.g., the nozzle diameter of φ24 μm). Thus, if a dissolved oxygen rate of the ink is high, oxygen is gradually accumulated in the ink, which may eventually cause an ink discharge malfunction. If air bubbles are mixed in the ink within the ink supply path including the head tanks, the ink containing the air bubbles may be distributed without eliminating the air bubbles or the dissolved oxygen rate of the ink may be increased. As a result, the heads may not be able to discharge appropriate amounts of the liquids (ink) or may not be able to discharge the liquids (ink) at all. Accordingly, it may be necessary to eliminate such air bubbles from the ink. 
     As described above, if the air bubbles are mixed into the liquid (ink) inside the head tanks, the recoding heads may not be able to appropriately discharge the ink, resulting in liquid discharge malfunctioning. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of at least one embodiment of the present invention to provide a liquid discharge head unit and an image forming apparatus having the liquid discharge head unit capable of reducing air bubble being mixed from the head tanks to heads, which substantially eliminate one or more problems caused by the limitations and disadvantages of the related art. 
     According to one embodiment, there is provided a liquid discharge head unit that includes a liquid container tank configured to store a liquid; and a head configured to receive the liquid from the liquid container tank to discharge liquid droplets therefrom, the head and the liquid container tank being integrally formed. In the liquid discharge head unit, the head includes a common channel configured to supply the liquid to a plurality of liquid chambers communicating with nozzles configured to discharge the liquid droplets, and the common channel includes a supply port configured to introduce the liquid from outside, and a discharge port configured to discharge the introduced liquid outside. In the liquid discharge head unit, the liquid container tank includes a container configured to contain the liquid to be supplied to the head and including an upstream chamber and a downstream chamber separated via a filtering member, a supply path configured to supply the liquid from the downstream chamber of the container to the supply port of the head, a discharge path configured to discharge the liquid discharged from the discharge port of the head, a first communication path via which the downstream chamber communicates with the discharge path, and a second communication path via which the upstream chamber communicates with the discharge path, where a fluid resistance of the second communication path is higher than a fluid resistance of the first communication path. 
     According to another embodiment, there is provided an image forming apparatus that includes the liquid discharge head unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and further features of embodiments will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic front diagram illustrating an overall configuration of an example of an image forming apparatus according to an embodiment; 
         FIG. 2  is a plan diagram illustrating main components of the image forming apparatus according to an embodiment; 
         FIG. 3  is a diagram illustrating an ink supply system provided in the image forming apparatus according to an embodiment; 
         FIG. 4  is a side diagram illustrating main components of the ink supply system of  FIG. 3 ; 
         FIG. 5  is a schematic sectional diagram illustrating a head unit according to a first embodiment; 
         FIG. 6  is a flowchart illustrating an air bubble eliminating operation; 
         FIG. 7  is a schematic sectional diagram illustrating a head unit according to a second embodiment; 
         FIG. 8  is a schematic sectional diagram illustrating a head unit according to a third embodiment; 
         FIG. 9  is a side diagram illustrating main components of the head unit of  FIG. 8 ; 
         FIG. 10  is a cross sectional diagram of the head unit of  FIG. 8  taken along an A-A line; 
         FIG. 11  is a schematic sectional diagram illustrating a head unit according to a fourth embodiment; 
         FIG. 12  is a side diagram illustrating main components of the head unit of  FIG. 11 ; 
         FIG. 13  is a cross sectional diagram of the head unit of  FIG. 11  taken along a B-B line; 
         FIG. 14  is a schematic sectional diagram illustrating a head unit according to a fifth embodiment; 
         FIG. 15  is a side diagram illustrating main components of the head unit of  FIG. 14 ; 
         FIG. 16  is a cross sectional diagram of the head unit of  FIG. 14  taken along a C-C line; 
         FIG. 17  is a diagram illustrating an ink supply system provided in a head unit according to a sixth embodiment; 
         FIG. 18  is a side diagram illustrating main components of the ink supply system of  FIG. 17 ; and 
         FIG. 19  is a flowchart illustrating an air bubble eliminating operation carried out in the ink supply system of  FIG. 17 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments will be described with reference to the accompanying drawings. First, an example of an image forming apparatus according to an embodiment is described with reference to  FIGS. 1 and 2 . Note that  FIG. 1  is a schematic configuration diagram illustrating an overall configuration of the image forming apparatus according to an embodiment and  FIG. 2  is a schematic plan diagram illustrating the image forming apparatus of  FIG. 1 . 
     The image forming apparatus according to an embodiment is a line-type image forming apparatus that includes a main body  1 , a paper feed tray  2  configured to accumulate sheets P and feed the sheet one at each feed, an output tray  3  configured to accumulate the printed sheets P, a transfer unit  4  configured to transfer the sheet P from the paper feed tray  2  to the output tray  3 , an image forming unit  5  having head modules  51  forming recording heads configured to carry out printing by discharging liquid droplets onto the sheet P transferred by the transfer unit  4 , a head cleaner device  6  serving as a maintenance and recovery mechanism configured to maintain the recording heads of the image forming unit at a predetermined timing after printing, a transfer guide unit  7  configured to open or close the head cleaner device, an ink tank unit  8  formed of sub-tanks configured to supply ink to the respective head modules  51  of the image forming unit  5 , and a main-tank unit  9  configured to supply ink to the ink tank unit  8 . 
     The main body  1  is formed of front, rear and side plates and stays, and the sheets P accumulated on the paper feed tray  2  are fed to the transfer unit  4  one sheet at each time separated by a separation roller  21  and a paper feed roller  22 . 
     The transfer unit  4  includes a transfer driving roller  41 A, a transfer driven roller  41 B and an endless transfer belt  43  that is looped over the transfer driving roller  41 A and the transfer driven roller  41 B. The endless transfer belt  43  includes suction pores (not illustrated) in its surface, and a suction fan  44  configured to suction the sheet P is arranged beneath the endless transfer belt  43 . Further, transfer guide rollers  42 A and  42 B supported by (not-illustrated) guides are respectively arranged above the transfer driving roller  41 A and the transfer driven roller  41 B such that the transfer guide rollers  42 A and  42 B are brought into contact with the endless transfer belt  43  by their self weights. 
     The endless transfer belt  43  is configured to circumferentially travel by the rotational movement of the transfer driving roller  41 A, which is driven by a (not-illustrated) motor. The sheet P is suctioned onto the endless transfer belt  43  by the suction fan  44  such that the sheet P is transferred by the rotational traveling of the endless transfer belt  43 . Note that the transfer driven roller  41 B, the transfer guide rollers  42 A and  42 B are rotationally driven by the rotational traveling of the endless transfer belt  43 . Note also that a non-printing liquid discharge cleaner device  45  is arranged beneath the endless transfer belt  43  to remove non-printing liquid discharge attached to the endless transfer belt  43 . 
     The image forming unit  5  formed of the head modules  51  is movably arranged above the transfer unit  4  in a direction indicated by an arrow A, and the head modules  51  are configured to discharge printing liquid to carry out printing on the sheet P. The image forming unit  5  is configured to be located to a position above the head cleaner device  6  when the image forming unit  5  is cleaned by the head cleaner device  6  that carries out a maintenance and recovery operation (i.e., cleaning operation) and be located back to a position illustrated in  FIG. 1  when the image forming unit  5  carries out an image forming operation. 
     The image forming unit  5  includes the head modules (recording head unit)  51 A,  51 B,  51 C and  51 D each having an array of liquid discharge head units (hereinafter simply caller “head units”)  101  each integrally formed of heads and corresponding head tanks. The head modules  51 A,  51 B,  51 C and  51 D of the image forming unit  5  are arranged in a line-base member (not illustrated) along a sheet transfer direction. 
     In the image forming unit  5 , one of the two nozzle arrays in each of the head modules  51 A and  51 B discharges yellow (Y) liquid droplets and the other one of the nozzle arrays discharges magenta (M) liquid droplets, while one of the two nozzle arrays in each of the head modules  51 C and  51 D discharges cyan (C) liquid droplets and the other one of the nozzle arrays discharges black (K) liquid droplets. That is, in the image forming unit  5 , two of the head modules  51  configured to discharge droplets of the same color are arranged in the sheet transfer direction. The two head modules  51  form one nozzle array, a length of which corresponds to the sheet width of the sheet P. 
     Respective distribution tanks  54  configured to supply ink to the head units  101  are provided above the head modules  51 , where the distribution tanks  54  and the head units  101  are connected via respective tubes  55 . Respective sub-tanks  81  are arranged upstream of the distribution tanks  54 , where the distribution tanks  54  and the sub-tanks  81  are connected via respective supply tunes  82 . Respective main-tanks  91  configured to store ink are arranged upstream of the sub-tanks  81 , where the main-tanks  91  and the sub-tanks  81  are connected via respective supply paths formed of the supply tubes  92 . 
     The transfer guide unit  7  is arranged downstream of the transfer unit  4  and configured to guide the transferred sheet P being discharged from the transfer unit  4  to the output tray  3 . That is, the transferred sheet P is discharged onto the output tray  3  while being guided by the transfer guide unit  7 . The output tray  3  includes a pair of side fences  31  configured to regulate the sheet P alignment in the width direction and an end fence  32  to regulate a front-end alignment of the sheet P. 
     The maintenance and recovery mechanism (i.e., head cleaner device)  6  includes four array cleaner units  61 A to  61 D configured to face the respective head modules  51  of the image forming unit  5 . Each of the cleaner units  61  includes cap members  62  to cap nozzle surfaces of the respective heads  101  and wiping members (wiper members)  64  to wipe the nozzle surfaces of the respective heads  101 . Note that each array of the cap members  61  may be individually moved in an upward direction and a downward direction. Further, respective suction pumps  63 A to  63 D serving as suction unit are arranged beneath the cleaner unit  61  and configured to suction ink from the nozzles of the head units  101  while the nozzle surfaces of the head units  101  are being capped with the cap members  62 . 
     In the image forming apparatus according to an embodiment, when ink is suctioned from the nozzles of the head units  101  of the head modules  51  with the nozzle surfaces being capped by the cleaning units  61 , the entire transfer unit  4  turns, after stopping printing, on the transfer driven roller  41 B as a fulcrum point in a downward direction indicated by a bidirectional arrow B as illustrated in  FIG. 1 . Or, when the respective wiping members  64  wipes ink attached to the nozzle surfaces of the head units  101  of the head modules  51  after printing, the entire transfer unit  4  turns, after stopping printing, on the transfer driven roller  41 B as a fulcrum point in a downward direction indicated by a bidirectional arrow B as illustrated in  FIG. 1 . Accordingly, sufficient space may be provided between the image forming unit  5  and the transfer unit  4  for allowing the movement of the image forming unit  5 . The space formed between the transfer unit  4  and the image forming unit  5  when the cleaning is carried out (the cleaning time) may be larger than the space formed between them when an image forming operation is carried out (the image forming time). At this moment, a transfer guide plate  71  of the transfer guide unit  7  may also turn on a fulcrum  72  in an upward direction indicated by a bidirectional arrow C in  FIG. 1  such that an upper part of the head cleaner device  6  is opened. 
     When the transfer unit  4  and the transfer guide unit  7  are released, the image forming unit  5  is moved in a sheet passing direction (indicated by an arrow A) and then stopped at a position above the head cleaner device  6 . In this state, the cleaner units  61  are raised for carrying out cleaning operations (maintenance and recovery operations) on the respective head modules  51 . 
     Next, an ink supply system including the head modules  51  of the image forming apparatus is described in more detail with reference to  FIGS. 3 and 4 . The sub-tank  81  and the distribution tank  54  of the head module  51  are connected via the supply tube  82  such that appropriate negative pressures for maintaining meniscus of the nozzles are generated due to a hydraulic head differential (in a range of −20 to −70 mmAq) between the sub-tank  81  and the nozzle surfaces of the head unit  101 . As will be described later, the head unit  101  includes a head  201  configured to discharge a liquid and a head tank  202  configured to supply ink to the head  201 . 
     The sub-tank  81  is a packing type sub-tank. That is, the sub-tank  81  includes a flexible pack  83  and a closed case  84 . Since the packing type sub-tank will not allow ink to be directly exposed to air (atmosphere), an increase in the viscosity of the ink due to evaporation of water in the ink may be prevented. Further, since the packing type sub-tank may maintain a constant amount of dissolved oxygen in the ink, air bubble accumulation in the ink inside the head unit  101  may be prevented. 
     The sub-tank  81  is connected to a pressure pump  82  (tube pump) configured to pressurize a space between the flexible pack  83  and the closed case  84 . When the maintenance operation before printing is carried out after a predetermined time has elapsed, the pressure pump  85  pressurizes the internal case  84  of the sub-tank  84  to supply ink to the head tank  201  of the head unit  101 , which then causes the nozzles of the head  201  to discharge the ink. Note that the maintenance operation is performed after the image forming unit  5  has been moved to a position above the cleaner device  6 . 
     The main-tank  91  configured to store ink are arranged upstream of the sub-tank  81 , where the main-tank  91  and the sub-tank  81  are connected via the supply path formed of the supply tube  92 . Further, a solenoid valve  93  is provided within the supply path of the supply tube  92 , such that ink is supplied from the main-tank  91  to the sub-tank  81  by opening or closing the solenoid valve  93 . 
     Moreover, an air discharge path  155  and a solenoid valve  156  configured to open or close the air discharge path  155  are provided above the distribution tank  54 . Air is discharged from the distribution tank  54  by opening the solenoid valve  156  when ink is initially filled in the distribution tank  54  or when air is accumulated inside the distribution tank  54 . Note that a slope is formed in a top surface of a common channel inside the distribution tank  54  in order to facilitate the discharge of air from the distribution tank  54 . 
     Further, supply tubes  55  for supplying ink are connected between the head tanks  201  of the respective head units  101  and the respective distribution tanks  54 . Moreover, discharge tubes  56  for discharging ink are connected to the head tanks  201  of the respective head units  101  such that the respective discharge tubes  56  are connected to the common path  57 . Thus, the common path  57  to which the respective discharge tubes  56  are connected is connected to the distribution tank  54  via a circulating path  59  and a circulating pump  58 . 
     Next, details of the head unit  101  according to a first embodiment is described with reference to  FIG. 5 .  FIG. 5  is a schematic sectional diagram illustrating the head unit  101 . The head unit  101  includes a head  201  configured to discharge a liquid and a head tank  202  configured to store ink that is supplied to the head  202 . The head  201  and the head tank  202  serving as a liquid container tank are integrally formed in the head unit  101 . Note that “integrally formed” indicates that the head  201  and the head tank  202  are connected via tubes or pipes; however, it also indicates that the head  201  and the head tank  202  securely fixed by screws and O-rings in the connections of channels between the head  201  and the head tank  202 . 
     The head  201  includes nozzles  211  configured to discharge liquid droplets, liquid chambers  212  communicating with the respective nozzles  211 , a common liquid chamber (a common channel)  213  configured to supply ink to the respective liquid chambers  212 , a common liquid chamber ink supply port (supply opening)  214  configured to supply (introduce) ink to the common liquid chamber (common channel)  213 , and a common liquid chamber ink discharge port (discharge opening)  215  configured to discharge ink from the common liquid chamber (common channel)  213 . 
     The head tank  202  includes a tank case (tank main body)  221  and an ink container  223  configured to contain ink inside the tank case  221 . The ink container  223  includes a filter upstream chamber  223 A and a filter downstream chamber  223 B that are separated by a filtering member  224  configured to filter impurities in the ink. 
     The filter upstream chamber  223 A is communicating with an ink supply port  225  connected to an ink supply tube  55 , whereas the filter downstream chamber  223 B is communicating with an ink supply channel  226  communicating with the common liquid chamber ink supply port  214  of the head  201 . 
     Further, the tank case  221  includes a discharge path  228  configured to discharge ink discharged from the common liquid chamber discharge port  215  communicating with the common liquid chamber discharge port  215  of the head  201 , and the discharge path  228  is communicating with an ink discharge port  229  to which an ink discharge tube  56  is connected. 
     The tank case  221  further includes a first communication path  231  serving as an air bubble eliminating path via which the filter downstream chamber  223 B is communicating with the discharge path  228 . The tank case  223  further includes a second communication path  232  serving as an air bubble eliminating path via which the filter upstream chamber  223 A is communicating with the discharge path  228 . The filtering member  234  is arranged between a first communicating section of the discharge path  228  communicating with the first communication path  231  and a second communicating section of the discharge path  228  communicating with the second communication path. 
     The first communication path  231  and the second communication path  232  are formed such that the fluid resistance of the first communication path  231  and that of the second communication path  232  are each higher than the fluid resistance of the common liquid chamber  213  of the head  201 , and the fluid resistance of the first communication path  231  and that of the second communication path  232  are each higher than the fluid resistance of the ink supply channel  226 . Further, the second communication path  232  is formed such that the fluid resistance of the second communication path is higher than the fluid resistance of the first communication path  231 . 
     For example, the first communication path  231  communicating with the filter downstream chamber  223 B has a size of φ1.5*a length 20 mm and the second communication path  232  communicating with the filter upstream chamber  223 A has a size of φ1.5*a length 20 mm, compared to the ink supply path  226  having a size of φ2.5*a length 2 mm. In general, the fluid resistance R is expressed by R=128 μL/(πd 4 )*1.045 [Pa·s/m 3 ] (μ: viscosity, L: length of round tube, d: radius). In this case, the fluid resistance of the second communication path  232  is approximately five times the fluid resistance of the first communication path  231 . 
     Next, an air bubble eliminating operation carried out on the head unit  101  having the above configuration is described with reference to a flowchart illustrated in  FIG. 6 . First, a circulation pump  58  is activated such that a flow is circulated in the order of the distribution tank  54 , the head tank  202 , a common path  57  and the distribution tank  54  (see also  FIG. 4 ). While the flow is circulated in an above-described fashion, air bubbles are gradually accumulated in an upper part of the distribution tank  54  such that the accumulated air bubbles form a large air ball. Then, after the circulation pump  58  is operated for a predetermined time (e.g., 10 to 30 sec.), the circulation pump  58  is deactivated. Thereafter, the sub-tank  81  is pressurized such that ink is supplied from the sub-tank  81  to the distribution tank  54  while air is discharged by opening (releasing) a valve  56  above the distribution tank  54 , simultaneously. Further, a liquid surface detecting sensor (not-illustrated) is provided at the upper part of the distribution tank  54 . When the liquid surface detecting sensor detects that a surface of the liquid in the distribution tank  54  reaches a predetermined level (predetermined height), the pressurizing operation of the sub-tank  81  is simultaneously stopped and the valve  56  above the distribution tank  54  is closed. The air bubbles mixed within the ink supply path and the head tank  202  are thus eliminated by the air bubble eliminating operation described above. 
     Next, elimination of air bubbles in the ink inside the head unit  101  is described. For example, an air bubble  501  is mixed into the filter upstream chamber  223 A of the head tank  202 , an air bubble  502  is mixed into the filter downstream chamber  223 B of the head tank  202 , and an air bubble  503  is mixed into the common liquid chamber  213  of the head  201 . In this case, the air bubble  503  mixed in the common liquid chamber  213  may be eliminated easily by carrying out the above-described circulation operation. The air bubble  502  mixed in the filter downstream chamber  223 B located downstream of the filtering member  224  may be eliminated via the first communication path  231  and the discharge path  228 . Further, the air bubble  501  mixed in the filter upstream chamber  223 A located upstream of the filtering member  224  may be eliminated via the second communication path  232  and the discharge path  228 . 
     In this case, since the fluid resistance of the second communication path  232  is higher than the fluid resistance of the first communication path  231 , the amount of ink passing through the second communication path  232  is more restricted than the amount of ink passing through the first communication path  231 . 
     As a result, the flow amount Qc flowing in the first communication path  231 , the flow amount Qa flowing in the second communication path  232  and the flow amount Qb flowing downstream of the filtering member  234  inside the discharge path  228  become approximately the same. 
     That is, if the fluid resistance of the first communication path and the fluid resistance of the second communication path  232  are the same, the ink flowing inside the first communication path  231  receives both the fluid resistance of the first communication path  231  and the fluid resistance of the filtering member  234 . As a result, the flow amount Qc flowing in the first communication path  231  may be increased (i.e., Qc&gt;Qa, Qb), which may lengthen the time for eliminating the air bubbles. 
     Thus, the flow amount of the ink flowing in the second communication path  232  is restricted by increasing the fluid resistance of the second communication path  232  such that the fluid resistance of the second communication path  232  is higher than the fluid resistance of the first communication path  231 , which may equate the flow amounts Qc, Qa and Qb. In this fashion, the air bubble  502  mixed in the filter downstream chamber  223 B may be efficiently eliminated, and the time required for the air bubble elimination may be reduced. 
     Next, details of a head unit  101  according to a second embodiment is described with reference to  FIG. 7 .  FIG. 2  is a schematic sectional diagram illustrating the head unit  101 . In the head unit  101  according to the second embodiment, a non-return valve (impurity prevention unit)  235  configured to allow the ink to flow in a discharge direction and prevent the ink from flowing in a backward direction is provided in place of the filtering member  234  provided in the discharge path  228  in the first embodiment. 
     With this configuration, the ink that flows from the filter upstream chamber  223  through the second communication path  232  to the discharge path  228  in a normal discharge operation but has not passed through the filtering member  224  is prevented from being mixed inside the head  201  side via the discharge path  228 . 
     Next, details of a head unit  101  according to a third embodiment is described with reference to  FIGS. 8 to 10 . Note that  FIG. 8  is a schematic sectional diagram of the head unit  101 ,  FIG. 9  is a side diagram of the head unit  101  of  FIG. 8  and  FIG. 10  is a sectional front diagram cut across an A-A line of the head unit  101  of  FIG. 8 . In the head unit  101  according to the third embodiment, the filtering member  224  is arranged along an ink supply direction (i.e., a vertical direction), and the filter upstream chamber  223 A and the filter downstream chamber  223 B in the ink container  223  are arranged in a horizontal direction, where an entrance port of the first communication path  231  is provided in a top surface of the filter downstream chamber  223 B and an entrance port of the second communication path  232  is provided in a top surface of the filter upstream chamber  223 A. Other components of the head unit  101  are the same as those illustrated in the first embodiment. 
     With this configuration, an effective area (i.e., an area capable of removing impurities) of the filtering member  224  may be increased while reducing the width of the head tank  202 . Accordingly, the head unit  101  (liquid discharge head unit) may be reduced in size without reducing the effective area of the filtering member  224 . 
     Next, details of a head unit  101  according to a fourth embodiment is described with reference to  FIGS. 11 to 13 . Note that  FIG. 11  is a schematic sectional diagram of the head unit  101 ,  FIG. 12  is a side diagram of the head unit  101  of  FIG. 11  and  FIG. 13  is a sectional front diagram cut across a B-B line of the head unit  101  of  FIG. 11 . As illustrated in  FIG. 11 , in the head unit  101  according to the third embodiment, the second communication path  232  has a portion  232   a  having a cross-sectional area smaller (narrower) than the cress-sectional area of the rest of the second communication path  232  in a direction perpendicular to a liquid flow direction, which may partially increase the fluid resistance of the second communication path  232 . For example, the portion  232   a  of the second communication path  232  has a size of φ0.8*5 mm and the rest of the second communication path  232  has a size of φ1.5*15 mm. 
     Next, details of a head unit  101  according to a fifth embodiment is described with reference to  FIGS. 14 to 16 . Note that  FIG. 14  is a schematic sectional diagram of the head unit  101 ,  FIG. 15  is a side diagram of the head unit  101  of  FIG. 14  and  FIG. 16  is a sectional front diagram cut across a C-C line of the head unit  101  of  FIG. 14 . As illustrated in  FIG. 14 , in the head unit  101  according to the third embodiment, the second communication path  232  has two or more bending portions along a liquid flow direction, which may increase the fluid resistance of the second communication path  232  due to the increased channel length of the second communication path  232 . For example, the second communication has a size of φ1.5*30 mm. 
     Next, a head unit  101  according to a sixth embodiment is described with reference to with reference to  FIGS. 17 and 18 . Note that  FIG. 17  is a diagram illustrating an ink supply system of the head unit  101  according to the sixth embodiment, and  FIG. 18  is a diagram illustrating main components of the ink supply system of  FIG. 17 . In the head unit  101  according to the sixth embodiment, an air-bubble elimination path  157  is provided with the common path  57  such that the common path  57  is communicating with a liquid waste tank (not-illustrated) via the air-bubble elimination path  157 . Further, the air-bubble elimination path  157  is provided with an air-bubble elimination pump  158 . 
     An air bubble eliminating operation for eliminating air bubbles from an ink supply path and the head tank  202  of this ink supply system is described with reference to  FIG. 19 . 
     First, the air-bubble elimination pump  158  is activated such that a flow is circulated in the order of the distribution tank  54 , the head tank  202 , the air-bubble elimination path  157  and the liquid waste tank. Such an air bubble eliminating operation is conducted for a predetermined time (e.g., in a range of 10 to 30 sec). 
     Next, the air-bubble elimination pump  158  is deactivated, and the sub-tank  81  is pressurized such that ink is supplied from the sub-tank  81  to the distribution tank  54  while air is discharged by opening (releasing) a valve  156  above the distribution tank  54 , simultaneously (i.e., the air bubbles mixed when replacing an ink pack is discharged from the valve above the distribution tank  54 ). 
     A liquid surface detecting sensor (not-illustrated) is provided at the upper part of the distribution tank  54 . When the liquid surface detecting sensor is activated, the pressurizing operation of the sub-tank  81  is stopped and the valve  156  above the distribution tank  54  is closed. 
     The air bubbles mixed within the ink supply path and the head tank  202  are thus eliminated by the air bubble eliminating operation described above. 
     As described above, since the head unit  101  is configured to eliminate air bubbles to the liquid waste tank during the air bubble eliminating operation, the air bubbles may be efficiently eliminated, which may result in liquid waste reduction. 
     Note that examples given above are based on the line type image forming apparatus to which the head unit  101  according to the above-described embodiments are applied; however, the head unit  101  according to the above-described embodiments may also be applied to the serial type image forming apparatus. 
     The liquid discharge head unit according to the embodiments includes the first communication path via which the downstream chamber communicates with the discharge path of the head tank  202 ; the second communication path via which the upstream chamber communicates with the discharge path of the head tank  202 , where the fluid resistance of the second communication path is higher than the fluid resistance of the first communication path. With this configuration, air bubbles mixed into the liquid inside the head tank may be eliminated and air bubbles entered from the head tank may be suppressed. 
     Further, since the image forming apparatus according to the embodiments includes the liquid discharge head unit according to the embodiments, the image forming apparatus may be capable of stably discharging ink for forming images. 
     According to an embodiment, there is provided a liquid discharge head unit that includes a liquid container tank configured to store a liquid; and a head configured to receive the liquid from the liquid container tank to discharge liquid droplets, the head and the liquid container tank being integrally formed. In the liquid discharge head unit, the head includes a common channel configured to supply the liquid to a plurality of liquid chambers communicating with nozzles configured to discharge the liquid droplets, and the common channel includes a supply port configured to introduce the liquid from outside, and a discharge port configured to discharge the introduced liquid outside. Further, in the liquid discharge head unit, the liquid container tank includes a container configured to contain the liquid to be supplied to the head and including an upstream chamber and a downstream chamber separated via a filtering member, a supply path configured to supply the liquid from the downstream chamber of the container to the supply port of the head, a discharge path configured to discharge the liquid discharged from the discharge port of the head, a first communication path via which the downstream chamber communicates with the discharge path, and a second communication path via which the upstream chamber communicates with the discharge path, where a fluid resistance of the second communication path is higher than a fluid resistance of the first communication path. 
     According to an embodiment, there is provided the liquid discharge head unit in which one of a non-return valve and the filtering member is provided between a first communicating section of the discharge path communicating with the first communication path and a second communicating section of the discharge path communicating with the second communication path. 
     According to an embodiment, there is provided the liquid discharge head unit in which the second communication path partially includes a small cross sectional configuration perpendicular to a fluid flow direction of the liquid. 
     According to an embodiment, there is provided the liquid discharge head unit in which the second communication path has a longer channel than that of the first communication path. 
     According to an embodiment, there is provided the liquid discharge head unit in which the filtering member is arranged along a direction toward which the head discharges the liquid droplets. 
     According to an embodiment, there is provided an image forming apparatus that includes the above liquid discharge head unit. 
     Embodiments of the present invention have been described heretofore for the purpose of illustration. The present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention. The present invention should not be interpreted as being limited to an embodiments that are described in the specification and illustrated in the drawings. 
     The present application is based on Japanese Priority Application No. 2010-203360 filed on Sep. 10, 2010, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.