Abstract:
An ink jet recording apparatus has a sub tank that supplies ink from an ink cartridge to an ink jet head carried by a reciprocating carriage. The sub tank has an ink container formed of two opposed resin films welded together around their peripheries, and resin sheet members are attached to the ink container over the weld portion for expanding the ink container and restoring it to its original shape after the ink container is deformed by ink flowing in or out of the ink container. The sheet members may be polyimide tape strips or the like that have higher resilience than the material of the ink container and prevent plastic deformation of the ink container due to expansion and compression thereof as ink flows in and out of the ink container.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a recording apparatus, and more particularly, to an ink jet recording apparatus. 
     Conventionally, as described in Japanese Patent Application Laid-open No. 2005-34999, there is disclosed a system which maintains a constant pressure by providing a pressure reducing pump and an atmosphere communication valve to a sub tank, sensing a pressure in the sub tank with a pressure sensor, and driving the pressure reducing pump or driving the atmosphere communication valve based on the pressure value. In addition, an ink jet printer has ink jet heads for individual ink colors, so that sub tanks are provided for individual colors. 
     However, in the conventional technique, it is necessary to provide pressure reducing pumps that are driven independently to the individual sub tanks. Therefore, the number of components increase, and the system is upsized. In addition, the ink jet head, the sub tank, the pressure reducing pump, the pressure sensor, and the like are mounted on a carriage, resulting in an increase of weight of the carriage. Therefore, a drive motor for driving the carriage is required to be upsized. The increase of the number of components and the upsizing of the drive motor cause an increase of cost. Further, because of contact between ink and air, dissolving of air containing oxygen and nitrogen in the ink occurs, which causes a rapid decrease of pressure accompanying air bubbles when an abrupt flow of ink occurs during a printing operation or a cleaning operation. The air bubbles generate pressure relaxation effect of the ink jet heads and cause ink discharge failure. In addition, if the ink temperature is raised by an increase of temperature of the ink jet heads, acceptable gas dissolving amount in the ink is decreased. Therefore, air bubbles are apt to be generated. 
     On the other hand, a sub tank having sealed structure has been devised, in which a tension force of a spring or a weight is utilized so that a negative pressure is generated inside. Further description is given with reference to  FIG. 5 .  FIG. 5  is a schematic diagram of an ink container of a conventional sub tank. Two sheets are welded at the periphery thereof so as to have an inlet  9  and an outlet  10  for ink. In order to generate a negative pressure in the ink container  2 , a biasing portion  37  uses a tension force of a spring, a weight, or the like for biasing. The internal pressure can be changed by expanding or compressing the ink container  2 . In this conventional sub tank, a material and a hardness of the ink container  2  of the sub tank forming the sealed structure largely affect the generated internal pressure. In other words, in order to enhance a gas barrier property, it is necessary to form the ink container  2  of the sub tank by using sheets of structure having both ink resistance and gas barrier property. Therefore, multilayered structure made of plastic thin films or aluminum foil is adopted, and elastic sheets are used. However, as the sheets become harder, a change of pressure increases due to a volume change of the sheets. Thus, it becomes difficult to generate an appropriate back pressure with respect to the ink jet heads. Naturally, the sheet materials for the ink container  2  of the sub tank are restricted, and the softest material among them is selected. However, as the material becomes softer, the ink container  2  is apt to change its volume and has little change of pressure while having poor ability to maintain the shape. If the ink container  2  has once irregular creases  38  and  39 , it cannot be restored so that stability of the pressure and repeatability of deformation cannot be maintained. In addition, there is another problem that a fluctuation of pressure among individual ink containers  2  of the sub tank is large. 
     The reason for those problems is that as the ink container  2  of the sub tank is being filled with ink and is being expanded, the irregular creases  38  and  39  are generated partially at the periphery of the ink container of the sub tank so as to generate a distortion at the periphery, and the distortion causes a plastic deformation. Thus, stability of the operation repeating compression and expansion of the ink container  2  of the sub tank is inhibited when the ink flows in or out of the ink container  2  of the sub tank. Further, there is a fluctuation of deformation of the ink container  2  of the sub tank, which causes a pressure fluctuation among individual ink containers  2  of the sub tank. 
     SUMMARY OF THE INVENTION 
     In order to solve the above-mentioned problem, the present invention disposes expansion means at a position of an ink container of a sub tank, which is apt to cause a plastic deformation, to thereby prevent the plastic deformation. A reinforcing material that has higher resilience than the material of the ink container of the sub tank is fixed. Alternatively, an external force is applied to the part that is apt to cause the plastic deformation by the expansion means in the deformation direction, to thereby prevent the plastic deformation of the ink container of the sub tank. As the reinforcing material, it is possible to use a resin sheet, a resin or metal plate, or the like. The reinforcing material may be fixed to the ink container by an adhesive or by welding. By fixing the reinforcing material to the ink container of the sub tank, it is possible to control the deformation due to expansion or compression of the ink container when ink flows in or out. Alternatively, a spring may be used as the external force. 
     According to the present invention, a change of internal pressure due to a plastic deformation of the ink container of the sub tank is prevented so that the internal pressure of the sub tank can be maintained to be constant. Thus, a fluctuation of pressure among individual ink containers is suppressed. Ink discharge from the ink jet head is stabilized and a discharge failure can be prevented, so that deterioration of print image quality can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a perspective view of a sub tank of a recording apparatus according to the present invention, 
         FIG. 2  is a perspective view illustrating structure of a recording apparatus according to an embodiment of the present invention, 
         FIG. 3  is a schematic diagram illustrating a first form of an ink container of the sub tank that is used for the recording apparatus of the present invention, 
         FIG. 4  is a schematic diagram illustrating a second form of the ink container of the sub tank that is used for the recording apparatus of the present invention, 
         FIG. 5  is a schematic diagram of an ink container of a conventional sub tank, and 
         FIG. 6  is a diagram illustrating structure of a sub tank according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are described with reference to the attached drawings. 
     First, a recording apparatus in its entirety is described with reference to  FIG. 2 .  FIG. 2  is a perspective view illustrating structure of the recording apparatus according to an embodiment of the present invention. An ink jet recording apparatus  20  conveys a sheet-like or plate-like recording medium  29  as illustrated in  FIG. 2  by a broken line and records images and characters corresponding to image data on a surface of the recording medium  29  by an ink jet method. The recording medium  29  is a sheet or the like made of paper, fabric, or a synthetic resin such as polyester or PVC. 
     The ink jet recording apparatus  20  includes a conveyor roller  19  that is driven by a motor to rotate, and a pressure roller unit  18  which presses the conveyor roller  19 . The recording medium  29  is sandwiched between the conveyor roller  19  and the pressure roller unit  18  and is conveyed by rotation of the rollers. 
     A platen  25  which supports the recording medium  29  is disposed on the downstream side of the conveyor roller  19  and the pressure roller unit  18  in a conveying direction. This platen  25  is disposed at a position where an ink jet head  14  prints and records images and characters on the recording medium  29 . In addition, beneath the platen  25 , a heater is provided for heating the platen  25 . This heater heats the platen  25  and also the recording medium  29 , so as to facilitate fixation of ink discharged onto the recording medium  29 . 
     The platen  25  is disposed so as to extend in a direction perpendicular to the conveying direction of the recording medium  29 . In addition, the conveyor roller  19  and the pressure roller unit  18  make a pair, and 19 pairs of them are arranged at a predetermined interval, e.g., at an interval of 87.65 mm on the same axis line of the position adjacent to the platen  25 . 
     The ink jet recording apparatus  20  is equipped with suction means for sucking the recording medium  29  to the platen  25 . The suction means includes a plurality of suction holes  27  penetrating through the platen  25 , a plurality of suction fans  28  which suck air, and a suction chamber  26  which communicates the suction holes  27  with the suction fan  28 . The interior of the suction chamber  26  is divided into four cells, and each of the cells is equipped with the suction fan  28 . 
     The ink jet recording apparatus  20  is equipped with a carriage  15  housing the ink jet head  14 , which discharges ink droplets to the recording medium  29  for recording, and a sub tank  1  that supplies the ink to the ink jet head  14 , a conveyor belt  24  which is coupled to the carriage  15 , a drive pulley  21  and a driven pulley  22  for moving the conveyor belt  24 , a drive motor  23  which drives the drive pulley  21  to rotate, and a guide rail  17  which guides scanning of the carriage  15  in the direction perpendicular to the conveying direction of the recording medium  29 . The carriage  15  is adapted to move in a sliding manner in a direction crossing the conveying direction of the recording medium  29 , e.g., the direction perpendicular to the same, that is, in a direction as illustrated in the diagram by the bidirectional arrow AB. 
     In addition, the ink to be supplied to the ink jet head  14  is stored in an ink cartridge  16 . The ink cartridge  16  is a cartridge ink container that is exchangeable. The ink cartridge  16  is exchanged when ink runs out. 
     The sub tank  1  is disposed in an ink flow path for supplying the ink from the ink cartridge  16  to the ink jet head  14 . The ink cartridge  16  supplies the ink to the sub tank  1 , and the sub tank  1  supplies the ink to the ink jet head  14 . The sub tank  1  is described later. 
     The ink jet head  14 , the sub tank  1 , the ink cartridge  16 , and the ink flow path are provided for each color of ink. Structural elements including the conveyor roller  19 , the pressure roller unit  18 , the drive pulley  21 , the driven pulley  22 , the drive motor  23 , the guide rail  17 , and the like are attached to a pedestal  13 . 
     Next, the sub tank  1  is described.  FIG. 1  is a perspective view of the sub tank of the recording apparatus according to the present invention. The sub tank  1  generally includes an ink container  2  which stores the ink temporarily, and pressure adjustment means which changes an internal pressure of the ink container  2 . 
     The ink container  2  has an inlet  9  that is connected to the ink cartridge  16  and is supplied with the ink, and an outlet  10  that is connected to the ink jet head  14  and delivers the ink. 
     The ink container  2  is formed by welding two rectangular film-like sheets to each other at the periphery thereof. Three peripheral sides of the ink container  2  are welded, and the remaining side is provided with the inlet  9  and the outlet  10  that are welded at a weld portion  11  for sealing. The ink container  2  is constituted by using at least one type of films made of resins such as polyethylene, polypropylene, polyethylene terephthalate, nylon, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, and an ethylene-vinyl alcohol copolymer for securing strength, ink resistance, and gas barrier property. The ink container  2  may be constituted by using a plurality of laminated films. Further, if it is necessary to secure higher gas barrier property, metal foil or a metallized film made of aluminum or other metal as an intermediate layer. The periphery of the ink container  2  is provided with expansion means  3 . The expansion means  3  is described later. 
     A first pressure adjustment plate  5  is fixed to one of the flat surfaces of the ink container  2 , and a second pressure adjustment plate  6  is fixed to the other opposed flat surface. Four shafts are fixed to a base plate  4 , and the second pressure adjustment plate  6  is fixed to the ends of the shafts with screws. An attachment portion thereof is substantially the end portion of a bent portion  7  that is formed by bending each end portion of the second pressure adjustment plate  6  by 90 degrees and further bending the end portion thereof by 90 degrees. Expansion of the ink container  2  is defined by the length of the shafts. In addition, the first pressure adjustment plate  5  and the second pressure adjustment plate  6  can be fixed to the ink container  2  with an adhesive, double coated tape, or the like. 
     The first pressure adjustment plate  5  is biased in a direction separating it from the second pressure adjustment plate  6  by a force of springs  8  disposed respectively around the four shafts. In other words, the first pressure adjustment plate  5  and the second pressure adjustment plate  6  are biased in the direction separating them from each other, so that the ink container  2  is forced to expand. The force causes a decrease of the internal pressure so as to balance at a constant position. The sub tank  1  has such pressure adjustment means. Here, a head value of a nozzle is denoted by P, and a head value of the sub tank  1  is denoted by h. Then, it is desired that P-h be within the range from −3 to zero (kPa), so as to prevent leakage of the ink from the nozzle of the ink jet head  14  and a malfunction in the discharge. In addition, the base plate  4  is fixed to the carriage  15 . The first pressure adjustment plate  5  and the second pressure adjustment plate  6  cannot be fixed directly to the carriage  15 . Therefore, the attachment does not affect the ink container  2 . 
     Further, if the sub tank  1  and the ink cartridge  16  are communicated with each other, the sub tank  1  may be affected by the head value of the ink cartridge  16  or, if a supply pump is used, the pressure of the supply pump. Therefore, a valve (not shown) is provided in the path, which disconnects between the inlet  9  of the sub tank  1  and the ink cartridge  16  or the supply pump at least during printing operation. 
     In addition, the ink jet head  14  for printing is disposed downstream of the outlet  10 , so that a meniscus at the nozzle tip of the ink jet head  14  and the internal pressure of the sub tank  1  are balanced. Because the ink is supplied from the ink cartridge  16  to the sub tank  1 , in order to prevent the ink jet head  14  from being affected by the head value of the ink cartridge  16  or the pressure of the supply pump during a supplying period, a valve (not shown) for disconnecting the flow path is disposed in the path between the outlet  10  and the ink jet head  14 . During printing operation or when ink is supplied to the sub tank  1 , the valve (not shown) is controlled. 
     Next, the expansion means of the ink container  2  is described.  FIG. 3  is a schematic diagram illustrating a first form of the ink container of the sub tank that is used for the recording apparatus of the present invention. 
     The ink container  2  is constituted of two resin films that are welded and sealed at the periphery of the ink container  2  outside a weld portion boundary  30 . The weld portion boundary  30  is indicated by a solid line on the side to which the inlet  9  and the outlet  10  are provided, and by broken lines on the other three sides. Polyimide tape strips  32 ,  33 , and  34  having a width of 8 mm as a reinforcing material, i.e., the expansion means are applied to the three sides of the ink container  2  across the weld portion boundary  30  for reinforcing the periphery portion. In addition, a polyimide tape strip  31  having a width of 8 mm as the reinforcing material, i.e., the expansion means is applied to one surface of the side to which the inlet  9  and the outlet  10  are provided. In this way, resilience that restores an original shape of the ink container  2  after being deformed by the ink flowing in or out of the ink container is increased by applying the polyimide tape strip  31 . In other words, the original shape of the ink container  2  can be restored easily, so that a plastic deformation can be prevented. The plastic deformation is apt to occur in the periphery of the weld portion or the edge portion of the container. Therefore, the expansion means can prevent the plastic deformation. 
     The place where the plastic deformation is apt to occur is different depending on a size, a material, and a method of fixing of the ink container  2 . Accordingly, the position and method of attaching the expansion means may be different. In the case of  FIG. 1 , the sheet-like expansion means is attached up to the end portion of the periphery. In the example of  FIG. 3 , the polyimide tape strips  31  to  34  are applied so as to cover the weld portion boundary  30 . Further, the polyimide tape strip  31  is applied to only one surface of the ink container  2 , and the polyimide tape strips  31  to  34  are applied to both surfaces. It is because that the side to which the inlet  9  and the outlet  10  for the ink are connected has a smaller plastic deformation than the other sides. 
     The polyimide tape strips may be applied across the weld portion boundary  30  corresponding to the polyimide tape strip  31 . Thus, the reinforcing material is provided on both sides of the weld portion boundary  30  of all four sides, so that the effect can be further increased. 
     As another form, the reinforcing material is not limited to polyimide, and a sheet-like material made of a resin such as rubber, nylon, or the like, which has higher resilience than the material of the ink container  2 , may be used. In addition, as another reinforcing material, a thin plate made of a resin or a metal may be used. The reinforcing material may be fixed to the ink container with an adhesive or by welding. 
       FIG. 4  is a schematic diagram illustrating a second form of the ink container of the sub tank that is used for the recording apparatus of the present invention. An end of a spring  35  is attached to each of four corners of the ink container  2 , and the base plate  4  is provided with fixing means  36 , to which the other ends of the springs  35  are respectively fixed, so that the ink container  2  is pulled to extend in four directions. In this case, because forces are always applied to the ink container  2  so as to extend in the four directions, the ink container  2  causes little distortion or bending so that a plastic deformation can be prevented. 
       FIG. 6  is a diagram illustrating structure of the sub tank according to another embodiment of the present invention. The sub tank includes an outer case. An upper case  40  and a lower case  41  house the ink container  42 . A protrusion  56  of the upper case  40  is inserted into an engaging portion  57  of the lower case  41 , so that the upper case  40  and the lower case  41  are coupled to each other. Further the upper case  40  and the lower case  41  are fixed to each other with screws put into thread grooves  47  of the lower case  41 . 
     The ink container  42  is a flexible container for housing ink. A plate  43  is fixed to one side face of the ink container  42 . The ink container  42  is expanded or compressed in accordance with a volume of the ink in the ink container  42 , so that the plate  43  moves in accordance with the expansion or compression of the ink container  42 . A protruding portion  44  is provided to an end portion of the plate  43 . The ink container  42  is provided with an inlet  51  for filling the ink in and an outlet  52  for delivering the ink. Correspondingly, the lowercase  41  is provided with an inlet opening  53  for the inlet  51  and an outlet opening  54  for the outlet  52 . The inlet  51  and the outlet  52  are connected to the ink path in the recording apparatus. When the ink is filled through the inlet  51 , the ink container  42  is expanded. When the ink is delivered through the outlet  52 , the ink container  42  is compressed. 
     Further, the lower case  41  is provided with a shaft fixing hole  48 . A detection plate  45  is provided with a rotation shaft  49  at one end and a long hole  46  at the other end. The rotation shaft  49  is inserted into the shaft fixing hole  48  so that the detection plate  45  can rotate about the rotation shaft  49 . The protruding portion  44  of the plate  43  is inserted into the long hole  46 . In this way, as the plate  43  moves, the detection plate  45  rotates about the rotation shaft  49 . The lower case  41  is provided with a window  50 . If the ink container  42  is filled enough with the ink, the detection plate  45  does not block the window  50 . If the ink container  42  runs out of the ink, the detection plate  45  blocks the window  50 . In this way, because the detection plate  45  works in accordance with the volume of the ink in the ink container  42 , it is possible to detect the volume of the ink in the ink container  42  from whether the window  50  is blocked or not. Using an optical sensor including a light emission portion and a light receiving portion for detecting a state of the window  50 , the volume of the ink in the ink container  42  can be detected. 
     The ink container  42  is used as the sub tank of the recording apparatus, and the ink is filled in or delivered from the ink container  42 . Therefore, the ink container  42  is expanded and compressed frequently. In addition, if the ink container  42  has an abnormal deformation such as a plastic deformation, the volume of the ink cannot be detected correctly. Therefore, a reinforcing material  55  is provided on both sides of the weld portion boundary at the periphery of the ink container  42 . The plastic deformation of the ink container  42  can be prevented. In addition, it is preferred that the reinforcing material  55  be disposed not to prevent the movement of the plate  43 . For instance, at the overlapping portion of the plate  43  or the protruding portion  44  with the reinforcing material  55 , the reinforcing material  55  having a thickness that does not prevent the movement is applied. In addition, the arrangement is performed so that the plate  43  or the protruding portion  44  does not contact with the reinforcing material  55 . In this way, the reinforcing material  55  does not prevent the movement of the detection plate  45 . 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used for a recording apparatus such as an ink jet printer having a sub tank.