Abstract:
The back pressure of an ink tank is controlled. A porous body is arranged inside the ink tank, the lower end of the porous body contacts an ink, and the ink ascends inside the porous body due to a capillary force. The upper end of the porous body is not immersed in the ink, and a maximum ascending force acts upon the ink stored in the ink tank. Therefore, the ink does not leak from a discharge head located lower than the ink tank. Further, because the ink is accumulated under the porous body and the amount of the ink contacting the porous body is small, components of the ink are less susceptible to deterioration.

Description:
[0001]    This application is a continuation of International Application No. PCT/JP2009/067337, filed on Oct. 5, 2009, which claims priority to Japan Patent Application No. 2008-260442, filed on Oct. 7, 2008. The contents of the prior applications are herein incorporated by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a liquid drop discharge device for printing by discharging a discharge liquid (ink). 
         [0003]    In an ink jet discharge device for printing by discharging liquid drops through nozzles, a print head and an ink tank are ordinarily connected by a dedicated use flow path, so that the ink stored inside the ink tank is fed to the print head through this flow path. 
         [0004]    The fed ink is pushed out as ink drops through a nozzle hole with a pressure wave generated by a pressure generating device (for instance, in an on-demand type ink jet head, an actuator (such as, a heater, a piezoelectric element or the like)) placed inside the print head. 
         [0005]    At this time, in order to efficiently discharge the ink drops through the nozzle hole, the meniscus of the ink (the surface state of the ink) at a portion of the nozzle hole needs to be stably held when the print head is in no operation. In order to maintain the meniscus, a force that counters a force by which the ink drop spontaneously falls under gravitation must be given to the ink. 
         [0006]    However, the above-mentioned on-demand type print head is equipped with a liquid drop ejecting mechanism, it is never provided with a mechanism for preventing the ink from leaking through the print head when the print head is in no operation. Therefore, a method in which a pressure (back pressure) is applied in order to prevent the leakage of the ink is used. 
         [0007]    However, the minimum control range of the back pressure required in the ink jet discharge device is around about 10 mm H 2 O or less; and thus, a difference in pressure between an outside atmosphere and the back pressure is extremely small. For this reason, it is difficult in a system using a conventional vacuum pump or the like to control this pressure range with good precision. 
         [0008]    As one of the conventional art techniques for providing the back pressure, there are numerous examples in which a network porous body is provided inside an ink tank (See JP Patent No. 2683187, JP Patent No. 3163864, JP Patent No. 3513979 and JPA 2007-62189). 
         [0009]    According to this technique, a capillary force, which is generated by fine pores of the porous body when the ink is sucked into the porous body, is utilized as the back pressure to hold the ink. It is possible to control the capillary force based on dimensions, the material, the shapes of the fine pores, etc. In this method, however, there is a problem in that a component of the ink is adsorbed on the porous body because the back pressure control accuracy is low and a large amount of the ink is contained in the porous body. 
         [0010]    As another method to provide the back pressure, there is an example in which such a porous body as mentioned above is not used (See, JPA 2006-123562). According to this reference, a movable lid, which is movable up and down, is provided at an interface portion between a gas and the liquid of the stored ink, and the ink is directly held by generating a negative pressure at this lid portion. However, in this method, a separate spring mechanism, which is to push up the movable lid, is necessary so as to hold the negative pressure, which may pose a limitation on the structure of the device. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention has been accomplished to solve the above-mentioned problems, and is aimed at providing a technique which can control the back pressure inside an ink tank with high accuracy. 
         [0012]    In order to solve the above-mentioned problems, the present invention is directed to a discharge device including a discharge head, an ink tank, and a valve device, wherein when the valve device is opened, the discharge head is connected with the ink tank; whereas, when the valve device is closed, the discharge head is shielded from the ink tank and wherein a suction member having a porous body is arranged inside the ink tank, and in a liquid-stored state that the ink is stored inside the ink tank, at least a part of the suction member contacts an inner wall face of the ink tank, and is held stationarily to the ink tank by a static friction force, and the ink ascends up to a midway portion of the porous body due to a capillary force. 
         [0013]    The present invention is directed to the discharge device, wherein the discharge head is provided with a discharge opening at a position lower than the ink tank. When the valve device is opened, the ink inside the ink tank moves to the discharge head through the valve device and a height at which the ink ascends in the porous body becomes smaller as compared to a state in which the valve device is closed, so that a suction force is generated to pull the ink upwardly, and the ink does not leak through the discharge opening. 
         [0014]    The present invention is directed to the discharge device, wherein when the amount of the ink inside the ink tank is constant, the suction member is stationary due to the static friction force; whereas, when the amount of the ink inside the ink tank is decreased by a predetermined amount or more, the suction force becomes greater than the static friction force, and the suction member moves downwardly. 
         [0015]    The present invention is directed to the discharge device, wherein the ink tank is provided with an outside connection opening to connect an interior space of the ink tank with an outside atmosphere to which the discharge opening is exposed. 
         [0016]    The back pressure inside the ink tank can be controlled in a minimum control pressure range of 1 mm H 2 O. Because the back pressure is controlled at high accuracy, leakage of the ink through the discharge opening is prevented, and the meniscus is stabilized. Because the meniscus is stabilized, the discharged state of the liquid drops (such as, the discharged amount of the liquid drops through the discharge opening and accuracy in an impinged position) is stabilized. Since the amount of the ink contacting the porous body is small, the components of the ink are difficult to be changed in quality. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1(   a ) is a side view of a printer apparatus, and  FIG. 1(   b ) is a plan view of the printer. 
           [0018]      FIG. 2  is a sectional view illustrating one embodiment of the discharge device according to the present invention. 
           [0019]      FIG. 3  is a sectional view illustrating one embodiment of a discharge head. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIGS. 1(   a ) and ( b ) are a side view and a plane view of the printer. The printer  1  includes a table  7  and a movable arm  8  arranged above the table  7 . 
         [0021]    Rails  37  are extended along sides of the table  7 , and the movable arm  8  is reciprocated in extending directions of the rails  37  by means of a moving unit (not shown). In  FIG. 1(   a ), the rails  37 , the movable arm  8  and a device to ride the movable arm  8  on the rails  37  are omitted. 
         [0022]    The movable arm  8  is equipped with one or plural discharge devices  2 . Each of the discharge devices  2  has at least one discharge head  3 . The discharge head  3  is provided with a discharge opening (not shown), and the discharge openings of the discharge heads  3  are exposed at a bottom face of the movable arm  8 . 
         [0023]    The height from a front surface of the table  7  to the bottom face of the discharge head  3  is set larger than the thickness of a substrate  6  as an object to be treated so that the discharge heads  3  move together with the movable arm  8  above the table  7  without contacting the substrate  6  placed on the table  7 . 
         [0024]    Each discharge device  2  has a discharge unit  20 , and the discharge head  3  is connected to the discharge unit  20 . The number of the discharge heads  3  connected to one discharge unit  20  may be one or more. In this embodiment, the printer  1  has plural (four in this embodiment) discharge units  20 , and plural (two in this embodiment) discharge heads  3  are connected to each discharge unit  20 . In addition, the printer  1  may be equipped with one or more discharge units  20 . 
         [0025]    Each discharge unit  20  has the same structure, and one discharge unit  20  is shown and explained in a schematically sectional view of the discharge device of  FIG. 2 , which replaces explanation of the plural discharge units  20 . The discharge unit  20  has an ink tank  11 . 
         [0026]    An outside connection opening  47  is provided in a ceiling of the ink tank  11 , and an inflow opening  45  and an outflow opening  46  are provided in a bottom face, respectively. One ends of flow paths  9 ,  12 ,  13  (such as, pipes) are connected to the outside connection opening  47 , the inflow opening  45 , and the outflow opening  46 , respectively. 
         [0027]    The other end of the flow path  13 , which is connected to the outside connection opening  47 , is connected to an outside atmosphere (the open air atmosphere in this embodiment) of the discharge device  2 , and as later discussed, the interior space of the ink tank  11  is connected to the outside atmosphere at least when the ink is to be stored in the ink tank  11  and during when the ink is to be discharged through the discharge head  3 . 
         [0028]    A main tank  4  is provided outside the discharge device  2 . 
         [0029]    The other end of the flow path  9 , which is connected to the inflow opening  45 , is connected to the main tank  4 ; the other end of the flow path  12 , which is connected to the outflow opening  46 , is connected to the discharge head  3 ; and cocks (valve devices)  17 ,  18  are provided on ways of the flow paths  9  and  12 . 
         [0030]    The ink is stored in the main tank  4 ; and when the cock  17  between the main tank  4  and the inflow opening  45  is opened, the ink tank  11  is connected to the main tank  4 , and the ink inside the main tank  4  is introduced into the ink tank  11  through the flow path  9 , the cock  17  and the inflow opening  45 . On the other hand, when the cock  17  is closed, the introduction of the ink is stopped. A reference numeral  21  in  FIG. 2  denotes the ink that is introduced and stored inside the ink tank  11 . 
         [0031]    Meanwhile, when the cock  18  between the discharge head  3  and the outflow opening  46  is opened, the ink  21  inside the ink tank  11  is fed into the discharge head  3  through the outflow opening  46 , the cock  18  and the flow path  12 ; whereas, when the cock  18  is closed, the ink tank  11  is shielded from the discharge head  3 , and the feeding of the ink  21  is stopped. 
         [0032]    A suction member  15  is arranged inside the ink tank  11 , and the suction member  15  includes a porous body  10  shaped in a columnar form, and a ring-shaped sealing member  16  covering the lateral face of the porous body  10 , the sealing member  16  being held stationarily to the porous body  10 . 
         [0033]    Both bottom faces of the porous body  10  are exposed, one of the exposed both bottom faces is directed upwardly, and the other is directed downwardly. Because the inflow opening  45  is provided in the bottom face of the ink tank  11 , when the ink  21  is fed from the main tank  4  to the ink tank  11  and stored inside the ink tank  11 , a lower portion of the porous body  10  including the bottom face is in contact with the ink  21 . 
         [0034]    The porous body  10  is made of a spongy resin, a fibrous metal, a sintered body (such as, a ceramics in which pores are formed), an unwoven cloth or the like, for example, and its wettability to the ink  21  is made high (contact angle θ&lt;45°). 
         [0035]    The suction member  15  and the internal space of the ink tank  11  are columnar, and their sectional shapes in a horizontal plane are the same. The suction member  15  is made slightly smaller than the internal space of the ink tank  11 ; and a gap  19  is formed between the lateral face of the suction member  15  and the inner wall face of the ink tank  11 . 
         [0036]    Further, the wettability to the ink  21  of the lateral face of the suction member  15  (the sealing member  16  in this embodiment) and the inner wall face of the ink tank  11  are also made high; and the gap  19  between the lateral face of the suction member  15  and the inner wall face of the ink tank  11  is set so narrow that the ink  21  may ascend due to a capillary force. Therefore, the ink  21  stored in the ink tank  11  can be soaked up into the interior of the porous body  10  and the gap  19  between the suction member  15  and the inner wall face of the ink tank  11  by the capillary force. 
         [0037]    The ascending height of the ink  21  due to the capillary force (the height from the liquid surface H which the ascending portion of the ink  21  due to the capillary force is excluded, a water head difference h) can be calculated by a theoretical formula (1) of the capillary force. 
         [0000]        h= 2 T  cos θ/ρ gr   Theoretical formula (1):
 
         [0038]    In the above theoretical formula (1), h is the water head difference (m), T is the surface tension (N/m) of the ink, θ is the contact angle of the ink to the porous body  10 , ρ is the density of the ink (kg/m 3 ), g is the gravitational acceleration (m/s 2 ), and r is the radius (m) of a capillary tube. 
         [0039]    A reference numeral h 1  in  FIG. 2  denotes a water head difference inside the porous body  10 , and a reference numeral h 2  in the same  FIG. 2  denotes the water head difference of the gap  19 . 
         [0040]    A part of the lateral face of the suction member  15  contacts the inner wall face of the ink tank  11 , so that the porous body  10  is held stationarily to the ink tank  11  by the static friction force. The length of the suction member  15  (the distance between both the bottom faces of the porous body  10 ) is set shorter than the height from the bottom to the ceiling of the tank  11 . Even after the water head differences h 1 , h 2  reach the theoretical values determined by the above-discussed theoretical formula (1), the ink  21  is continuously fed; and when the pressure of the ink  21  exceeds the total of the static friction force and the weight of the suction member  15 , the suction member  15  is pushed up. 
         [0041]    The suction member  15  is surrounded by the inner wall face of the ink tank  11 , the suction member  15  is supported by the inner wall face of the ink tank  11 , and ascends, while the lower end is kept directed downwardly and in contact with the ink  21 . The lower end of the porous body  10  is spaced from the bottom face of the ink tank  11 ; and the ink  21  is stored in a liquid-storing space  41  between the lower end of the porous body  10  and the bottom face of the ink tank  11 . 
         [0042]    After a predetermined amount of the ink  21  is stored in the liquid-storing space  41  and before the upper end of the porous body  10  reaches and contacts the ceiling of the ink tank  11 , the cock  17  is switched from the opened state to the closed state, the main tank  4  is shielded from the ink tank  11 , and the feeding of the ink from the main tank  4  to the ink tank  11  is stopped. The ascending of the suction member  15  is stopped, and it is held stationarily to the ink tank  11  by the static friction force. 
         [0043]    As discussed above, the entire lower end of the porous body  10  is in close contact with the ink  21  and furthermore, the gap  19  between the suction member and the inner wall face of the ink tank  11  is set so narrow that the capillary force may act, so that the ascending force is applied to the entire portion of the liquid surface H of the ink  21  in the liquid-storing space  41  by the capillary force. 
         [0044]    The length of the porous body  10  is set such that the height from the liquid surface H to the upper end of the porous body  10  is greater than the theoretical values of the water head differences h 1 , h 2 , so that the ink  21  does not reach the upper end of the porous body  10 , which keeps it in a dried state. Therefore, the maximum capillary force always acts for the ink  21  in the liquid storing space  41 . 
         [0045]    When the ink  21  is to be discharged, the cock  17  is closed to shield the ink tank  11  from the main tank  4 , and the cock  18  is opened to connect the ink tank  11  with the discharge heads  3 .  FIG. 3  is an enlarged sectional view of a part of the discharge head  3 , and the discharge head  3  includes an ink chamber  31  and a discharge opening  36  connected to the ink chamber  31 , and the flow path  12 , which connects the discharge head  3  to the ink tank  11 , is connected to the ink chamber  31  in the discharge head  3 . The ink, which is fed from the ink tank  11  to the discharge head  3 , is fed to the ink chamber  31 , and is exposed to the outside atmosphere of the discharge head  3  through the discharge opening  36 . 
         [0046]    The discharge opening  36  is positioned lower than the ink tank  11 , and connected to the same atmosphere (the open air atmosphere) as that to which the outside connection opening  47  is connected. The liquid surface H inside the ink tank  11  is located higher than the surface of the ink  21  exposed inside the discharge opening  36 , and no bubble is contained in the ink  21 . Therefore, the ink  21  inside the ink chamber  31 , the liquid-storing space  41  and the porous body  10  is pulled downwardly by the weight of the ink  21  under the liquid surface, so that the water head differences h 1 , h 2  decrease beyond the theoretical values. 
         [0047]    The wettability to the ink  21  of the porous body  10 , the sealing member  16  and the inner wall face of the ink tank  11  are set high such that when the ink tank  11  is connected with the discharge head  3 , and the wettability to the ink  21  is set high so as to prevent the water head difference h 1 , h 2  being zero. When the descending force and the ascending force for the ink  21  are in balance, the decrease in the water head differences h 1 , h 2  stops. Therefore, pull-up forces, which are differences between the theoretical value and actual values of the water head differences h 1 , h 2 , are applied to the ink  21 . 
         [0048]    Meanwhile, if there is a fear that the ink  21  leaks out from the discharge opening  36  during when the water head differences h 1 , h 2  decrease, the head connecting state is turned on after the discharge device  2  is placed at an evacuation site outside a substrate  6 . After the decrease in the water head differences h 1 , h 2  stops and the ink  21  from the discharge opening  36  does not leak, the discharge device  2  is located above the substrate  6 . 
         [0049]    An actuator  35  (such as, a heater, a piezoelectric element or the like) is provided in the ink chamber  31 . When the actuator  35  is heated or deformed by applying electric current therethrough in order to apply a pressing force to the ink  21  inside the ink chamber  31 , the ink  21  is discharged through the discharge opening  36 , and lands on the substrate  6 . 
         [0050]    Since the outflow opening  46  is positioned lower than the lower end of the porous body  10  (the bottom face of the ink tank  11  in this embodiment), when the liquid amount in the ink chamber  31  descends as the ink  21  is discharged through the discharge opening  36 , the ink  21  is refilled into the ink chamber  31  from the ink tank  11 . 
         [0051]    At this time, because the ink is not refilled into the ink tank  11  from the main tank  4 , the liquid surface H of the ink  21  in the liquid-storing space  41  descends, and a simultaneous descending force acts upon the ink  21  sucked in the porous body  10  by the capillary force. 
         [0052]    When the liquid surface H descends, the capillary force serving to hold the ink  21  becomes larger, and when the capillary force exceeds the static friction force between the suction member  15  and the ink tank  11 , the suction member  15  moves downwardly. Therefore, even when the ink is not refilled, the suction member  15  moves downwardly, and the state in which the lower end of the suction member  15  is in contact with the liquid surface H of the ink  21  is maintained. 
         [0053]    When the liquid surface H descends, the difference between the surfaces of the discharge liquid  21  inside the discharge opening  36  becomes smaller, so that the force by which the ink  21  is pulled downwardly decreases. On the other hand, the water head differences h 1 , h 2  increase, and the raising force based on the capillary force decreases, so that the descending force of the ink  21  is in balance with the ascending force. Thus, the height of the surface (meniscus) of the ink  21  inside the discharge opening  36  does not change. 
         [0054]    When the suction member  15  descends and before the lower end of the porous body  10  reaches the outflow opening  46  or the inflow opening  45 , the cock  18  is turned to a close state to shield the discharge head  3  from the ink tank  11 , and the cock  17  is turned to an open state to refill the ink into the ink tank  11  from the main tank  4 . Since the inflow opening  45  is located under the porous body  10 , at least the upper end of the porous body  10  is kept dry without being wetted with the ink  21 . 
         [0055]    The inflow opening  45  may be located higher than the lower end of the porous body  10 , so long as it is located lower than the upper end of the porous body  10 . However, when the lateral face of the porous body  10  is covered with the sealing member  16 , since the gap between the sealing member  16  and the inner wall face of the ink tank  11  is narrow, the ink  21  is not normally introduced if the inflow opening  45  is opposed to the sealing member  16 . For this reason, it is desirable that the inflow opening  45  is located under the lower end of the porous body  10 . 
         [0056]    As discussed above, although the case where the lateral face of the porous body  10  is covered with the sealing member  16  has been explained, the present invention is not limited thereto. The lateral face of the porous body  10  may be exposed. However, when the suction member  15  moves up and down, it is feared that the porous body  10  is damaged by the friction, if the lateral face of the porous body  10  is exposed. Thus, it is desirable to protect the lateral face of the porous body  10  with the sealing member  16 . 
         [0057]    The porous body  10  to be used in the present invention has a spongy structure having a network structure in which fine pores communicate in random directions. Although its material is not particularly limited, it essentially has a resistance that it is not dissolved in the ink  21  or it is not chemically changed by contacting the ink  21 . For example, an organic resin material (such as, a polyolefin resin or the like) can be recited. 
         [0058]    One example of the requirements for the porous body  10  being described, when the bore (radius) of the ink tank  11  is 10 mm and the storing volume for the ink is 15 ml, the average bore (radius) of the pores is 63 μm, the average porosity is 44.8%, and the height of the member is 10 mm. 
         [0059]    It is to be noted that based on the above one example as a standard, when the sectional area of the ink tank  11  is changed to n times, the average porosity has only to be made 1/n time. Meanwhile, if the storing volume for the ink is similarly changed to n times, the average porosity has only to be made n times. It is to be noted that the requirements for the porous body  10  can be calculated based on the above-discussed theoretical formula (1) of the capillary force. 
         [0060]    The material of the porous body  10  is not limited to a resin, and a metal such as SUS (stainless steel) or the like can be used, so long as it has resistance against the ink  21 . In this case, although a contact angle of the ink  21  to the porous body  10  may fall outside the above requirements, similar effects as the above discussed requirements can be exhibited by surface-treating of the porous body  10 . 
         [0061]    The ink  21  to be used in the present invention is not particularly limited, and besides a colored ink  21  which is added a coloring agent such as a pigment, a dye or the like, inks in which a material for an oriented film, a resin material, a material for a color filter, spacer particles or the like is dispersed or dissolved can be used. 
         [0062]    The sealing member  16  is not particularly limited, so long as it is not dissolved in the ink  21  and it does not chemically change when coming in contact with the ink  21 . In particular, a solvent-resistant resin (such as, a silicone resin, a fluorine resin, a polyolefin resin or the like) can be used. 
         [0063]    As discussed above, the case where the suction member  15  is held stationarily to the ink tank  11  by the static friction force has been explained; however, the invention is not limited thereto. It may be that the entire lateral face of the suction member  15  is closely contacted with the lateral face of the ink tank  11  or the suction member  15  is mechanically fixed to the ink tank  11  by means of a fixing member, an adhesive or the like. In this case, since the suction member  15  does not move up or down even when the liquid surface H goes up and down, an appropriate amount of the ink  21  is successively refilled from the main tank  4 .