Abstract:
A functional liquid tank is adapted to receive functional liquid from a functional liquid supply source and to supply the functional liquid to an inkjet functional liquid droplet discharge head. The functional liquid tank includes a tank body, a liquid column pipe, upper and lower limit liquid level detection units, a float member, and a float-regulating member. The float member is accommodated within the tank body with a small gap formed between the float member and an inside wall of the tank body. The float-regulating member is configured and arranged to regulate a descending end position of the float member when the float member descends as a liquid level within the tank body descends.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Japanese Patent Application No. 2009-068339 filed on Mar. 19, 2009. The entire disclosure of Japanese Patent Application No. 2009-068339 is hereby incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a functional liquid tank for receiving the supply of functional liquid from a functional liquid supply source and supplying the functional liquid to an inkjet functional liquid droplet discharge head. The present invention also relates to a method for replenishing functional liquid in a droplet discharge device and to a droplet discharge device. 
         [0004]    2. Related Art 
         [0005]    In conventional practice, there has been known a method for filling a droplet discharge head with functional liquid. According to this method, the functional liquid is not replenished in a droplet discharge device, but the flow channels in the head of a droplet discharge head are filled with functional liquid by feeding and suctioning (see Japanese Laid-Open Patent Application No. 2004-188410). This functional liquid filling method is implemented primarily when the droplet discharge head is replaced, and the method comprises a pressurizing liquid feed step in which compressed air is supplied to an airtight liquid supply sub-tank, the functional liquid in the liquid supply sub-tank is fed to the droplet discharge head, and the functional liquid fills the flow channels in the head of the droplet discharge head; and a suction step following the pressurizing liquid feed step, in which a suction unit is driven and functional liquid is suctioned from the nozzles of the droplet discharge head. Thus, the droplet discharge head is subjected to a filling action for feeding the functional liquid and a subsequent functional liquid suctioning action, thereby making an initial filling possible and preventing air bubbles from remaining in the flow channels in the head. 
       SUMMARY 
       [0006]    When different functional liquids are introduced into a functional liquid supply device for supplying functional liquid to a functional liquid droplet discharge head, if the conventional method for filling functional liquid described above is emulated, the old functional liquid introduced into the filled flow channel (entire flow channel) of the functional liquid supply device will be entirely removed at this time, after which fresh functional liquid is introduced using the filling action and suction action described above. However, it is known from experience that once all of the functional liquid in all of the flow channels has been removed, the discharge characteristics of the filled functional liquid droplet discharge head deteriorate. According to one method considered for replenishing functional liquid, the functional liquid is gradually replenished in the flow channels by repeatedly suctioning old functional liquid without emptying the sub-tank while fresh functional liquid is simultaneously introduced. 
         [0007]    Since the sub-tank described above is provided with an upper limit detection sensor for detecting the upper limit of the functional liquid level and a lower limit detection sensor for detecting the lower limit of the functional liquid level, another option that can be considered is to use these sensors and replenish functional liquid by alternately repeating a removal step for removing liquid until the surface of old functional liquid retained in the sub-tank is sensed by the lower limit detection sensor, and a liquid feeding step for feeding fresh functional liquid into the sub-tank. However, in this case, a large amount of old functional liquid remains below the lower limit detection sensor, and a large amount of functional liquid is therefore consumed until the sub-tank is completely replenished with fresh functional liquid. 
         [0008]    In such cases, a method has been considered for ending the removal step after timer control is performed and a specified amount of time elapses following the detection of the liquid level by the lower limit detection sensor. However, when there is only a certain small amount of functional liquid remaining below the lower limit detection sensor and the viscosities of the functional liquids differ, the rate at which the liquid level descends varies due to different amounts of pressure lost, and problems occur in that the liquid cannot be sufficiently removed, excessive liquid is removed and the gas in the sub-tank flows out to the functional liquid droplet discharge head, and the like. 
         [0009]    An object of the present invention is to provide a functional liquid tank, a method for replenishing functional liquid in a droplet discharge device, and a droplet discharge device wherein old functional liquid can be replaced with fresh functional liquid appropriately and efficiently. 
         [0010]    A functional liquid tank according to one aspect is adapted to receive functional liquid from a functional liquid supply source and to supply the functional liquid to an inkjet functional liquid droplet discharge head. The functional liquid tank includes a tank body, a liquid column pipe, an upper limit liquid level detection unit, a lower limit liquid level detection unit, a float member and a float-regulating member. The liquid column pipe is coupled to the tank body. The upper limit liquid level detection unit faces the liquid column pipe, and is configured and arranged to detect an upper limit liquid level in the tank body. The lower limit liquid level detection unit faces the liquid column pipe, and is configured and arranged to detect a lower limit liquid level in the tank body. The float member is accommodated within the tank body with a small gap formed between the float member and an inside wall of the tank body. The float-regulating member is configured and arranged to regulate a descending end position of the float member when the float member descends as a liquid level within the tank body descends. 
         [0011]    According to this configuration, when, e.g., functional liquid is removed from the functional liquid tank via the functional liquid droplet discharge head while the functional liquid tank or the functional liquid droplet discharge head is being replaced, the functional liquid level and the float in the tank main body both fall. When the float reaches the float-regulating member, the falling position of the float is kept at a midway position even if the liquid level continues to fall, and an amount of functional liquid lower than the lower limit liquid level in the tank body is therefore reliably provided to a certain extent during liquid removal. It is thereby possible to perform one liquid removal without completely emptying the functional liquid tank; i.e., while leaving a small amount of functional liquid in the functional liquid tank, even in cases in which liquid is removed through timer control using the lower limit liquid level detection unit as a trigger. Regulating the position of the float also makes it possible to easily remove functional liquid remaining in the small gaps between the tank body and the float. Therefore, the mixture of old functional liquid remaining in the functional liquid tank and fresh functional liquid newly fed in can be kept to a small amount, and the efficiency of replenishing functional liquid can be improved. 
         [0012]    In this case, it is preferable that the float-regulating member keeps the float member in a position between the lower limit liquid level detection unit and a bottom connecting hole of the liquid column pipe. 
         [0013]    According to this configuration, if the float is kept in a lower position relative to the position of the lower limit liquid level detection unit, it is possible to avoid states in which the gas in the upper space coming in contact with, and being readily absorbed by, the functional liquid when the liquid level has fallen below the position of the lower limit liquid level detection unit. If the liquid level is higher than the position of the bottom connecting hole of the liquid column pipe, the functional liquid in the liquid column pipe can easily be removed during liquid removal. 
         [0014]    In this case, the float-regulating member preferably includes a protrusion formed on a lower portion of the inside wall of the tank body, a protrusion extending upwardly from a bottom surface of the tank body, an immersed member resting on a bottom surface of the tank body, or a hanging protrusion suspended from a bottom surface of the float member. 
         [0015]    According to these configurations, the descending end position of the float can be accurately regulated using a simple structure. 
         [0016]    In a method for replenishing functional liquid in a droplet discharge device according to another aspect, the droplet discharge device includes the functional liquid tank according to the above aspect, a functional liquid supply source configured and arranged to supply the functional liquid to the functional liquid tank, and an inkjet functional liquid droplet discharge head configured and arranged to receive the functional liquid from the functional liquid tank. The method for replenishing functional liquid includes changing the functional liquid supply source from first functional liquid to second functional liquid in a state in which an entire flow channel extending from the functional liquid supply source to the functional liquid droplet discharge head is filled with the functional liquid, and replenishing the first functional liquid with the second functional liquid. The replenishing the first functional liquid with the second functional liquid includes repeating, for a predetermined number of times, suctioning the functional liquid from discharge nozzles of the functional liquid droplet discharge head until a predetermined amount of time elapses after the lower limit liquid level detection unit detects the lower limit liquid level, the predetermined amount of time being monitored by a timer control so that the liquid level in the tank body after the timer control reaches a position immediately above a level of an outlet port of the tank body, and feeding the functional liquid from the functional liquid supply source to the functional liquid tank until the upper limit liquid level detection unit detects the upper limit liquid level. In the suctioning of the functional liquid from the discharge nozzles of the functional liquid droplet discharge head, the float-regulating member regulates the descending end position of the float member so that an amount of the functional liquid remaining within the tank body when the timer control begins ensures that the liquid level in the tank body after the timer control reaches the position immediately above the level of the outlet port of the tank body regardless of the viscosity of the functional liquid. 
         [0017]    A droplet discharge device according to another aspect includes the functional liquid tank according the above aspect, a functional liquid supply source, an inkjet droplet discharge head, a flow channel switching unit, a functional liquid flow channel, a liquid-feeding unit, a suctioning unit, and a control unit. The functional liquid supply source includes two main tanks accommodating first functional liquid and second functional liquid, respectively, and is configured and arranged to supply the functional liquid to the functional liquid tank. The inkjet droplet discharge head is configured and arranged to receive the functional liquid from the functional liquid tank. The flow channel switching unit is configured and arranged to switch flow channels of the two main tanks. The functional liquid flow channel connects the flow channel switching unit and the functional liquid droplet discharge head with the functional liquid tank being located in a path of the functional liquid flow channel. The liquid-feeding unit is configured and arranged to feed the functional liquid of the main tanks to the functional liquid tank via the functional liquid flow channel. The suctioning unit is configured and arranged to suction the functional liquid from discharge nozzles of the functional liquid droplet discharge head. The suctioning unit is detachably connected to a nozzle surface of the functional liquid droplet discharge head. The control unit is configured to control the flow channel switching unit to change the functional liquid supply source from the first functional liquid main tank to the second functional liquid main tank in a state in which the functional liquid flow channel is filled with the functional liquid. The control unit is further configured to control the liquid-feeding unit and the suctioning unit to replenish the first functional liquid with the second functional liquid by repeating, for a predetermined number of times, suctioning by the suctioning unit the functional liquid from the discharge nozzles of the functional liquid droplet discharge head until a predetermined amount of time elapses after the lower limit liquid level detection unit detects the lower limit liquid level, the predetermined amount of time being monitored by a timer control so that the liquid level in the tank body after the timer control reaches a position immediately above a level of an outlet port of the tank body, and feeding by the liquid-feeding unit the functional liquid from the functional liquid supply source to the functional liquid tank until the upper limit liquid level detection unit detects the upper limit liquid level. The float-regulating member is configured and arranged to regulate the descending end position of the float member so that an amount of the functional liquid remaining within the tank body when the timer control begins ensures that the liquid level in the tank body after the timer control reaches the position immediately above the level of the outlet port of the tank body regardless of the viscosity of the functional liquid. 
         [0018]    According to these configurations, when the functional liquid tank is repeatedly subjected to the removal step (removal process) and liquid feeding step (liquid feeding process), the old (first) functional liquid filling all of the flow channels is gradually removed from the functional liquid droplet discharge head, and fresh (second) functional liquid is fed from the functional liquid supply source to the functional liquid tank while in contact with the interface with the old functional liquid. In the functional liquid tank, the interface is dissolved, the fresh functional liquid and old functional liquid mix together, and fresh functional liquid is gradually replenished through repetitive iterations of the partial replenishing step (partial replenishing process). When the partial replenishing step (partial replenishing process) is repeated a predetermined number of times, old functional liquid not only in the functional liquid tank but in the flow channels in the head of the droplet discharge head will ultimately be replenished with fresh functional liquid until a predetermined replenishment ratio (corresponding to complete replenishment) is reached. Thus, old functional liquid can be replaced with fresh functional liquid in a short amount of time, because old functional liquid is removed while fresh functional liquid is being introduced at the same time. Furthermore, fresh functional liquid can be replenished while it is in a state of contact with, or mixed with, old functional liquid; and, when introduced, the fresh functional liquid can be prevented as much as possible from being exposed to air. Therefore, it is possible to effectively prevent fresh functional liquid from being altered by air and air bubbles remaining in the functional liquid flow channels. Since as much functional liquid as possible can be removed in the removal step (removal process), a large amount of functional liquid is suctioned in a single removal step, and functional liquid can be efficiently replenished without any effect on the discharge characteristics of the functional liquid droplet discharge head. 
         [0019]    In this case, it is preferable for the first and second functional liquids to be drawing inks having different compositions, or that the first and second functional liquids be a drawing ink and a detergent. 
         [0020]    According to this configuration, it is not only when the fresh and old drawing inks are replenished, but also when a washing step is implemented during the replenishing of new and old drawing inks that fresh drawing ink can be introduced without being exposed to air and without the functional liquid tank having to be emptied. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Referring now to the attached drawings which form a part of this original disclosure: 
           [0022]      FIG. 1  is a perspective view of a droplet discharge device; 
           [0023]      FIG. 2  is a schematic plan view of a head unit; 
           [0024]      FIG. 3  is an external perspective view of a functional liquid droplet discharge head; 
           [0025]      FIG. 4  is a piping system diagram of a functional liquid supply device; 
           [0026]      FIG. 5  is a side view of a suction unit; 
           [0027]      FIG. 6  is a cross-sectional view schematically depicting the surroundings of a sub-tank according to the first embodiment; 
           [0028]      FIG. 7  is a flowchart of the method for replenishing functional liquid; 
           [0029]      FIG. 8  is a cross-sectional view schematically depicting the surroundings of a sub-tank according to the second embodiment; 
           [0030]      FIG. 9  is a cross-sectional view schematically depicting the surroundings of a sub-tank according to the third embodiment; and 
           [0031]      FIG. 10  is a cross-sectional view schematically depicting the surroundings of a sub-tank according to the fourth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0032]    The following is a description, made with reference to the accompanying drawings, of a droplet discharge device which applies the method for replenishing functional liquid according to an embodiment of the present invention. This droplet discharge device is incorporated in a flat panel display manufacturing line. The device uses a functional liquid droplet discharge head that brings in special ink or a functional liquid that is, e.g., a luminescent resin liquid, and the device forms the color filter of a liquid crystal display device, the light-emitting elements that serve as pixels of an organic EL device, or the like. 
         [0033]    A droplet discharge device  1  is composed of an X-axis table  2  provided on an X-axis support base  11  supported on a stone surface plate, wherein the X-axis table extends in an X-axis direction which is a main scanning direction and moves a workpiece in the X-axis direction; a Y-axis table  3  provided on a pair of Y-axis support bases  13  spanning across the X-axis table  2  via a plurality of support braces  12 , wherein the Y-axis table extends in a Y-axis direction which is a sub-scanning direction; and thirteen carriage units  4  on which a plurality of functional liquid droplet discharge heads  14  (not shown in  FIG. 1 ) are mounted; wherein the thirteen carriage units  4  are suspended from the Y-axis table  3 , as shown in  FIG. 1 . Furthermore, the droplet discharge device  1  comprises a chamber  5  for housing these devices in an atmosphere whose temperature and humidity are managed, and a functional liquid supply unit  6  having three functional liquid supply devices  15  (see  FIG. 4 ) which pass through the chamber  5  and supply functional liquid from the exterior of the chamber  5  to the functional liquid droplet discharge heads  14  therein. The functional liquid droplet discharge heads  14  are dischargeably driven in synchronization with the driving of the X-axis table  2  and the Y-axis table  3 , whereby functional liquid droplets in three colors (red, green, and blue) supplied form the functional liquid supply unit  6  are discharged and a predetermined drawn pattern is drawn on the workpiece. 
         [0034]    The droplet discharge device  1  also comprises a maintenance device  8  composed of a flushing unit  16 , a wiping unit  17 , a discharge performance inspection unit  18 , and a suction unit  19  described hereinafter. These units are provided for the maintenance of the functional liquid droplet discharge heads  14 , allowing for the function preservation and function restoration of the functional liquid droplet discharge heads  14 . 
         [0035]    The carriage units  4  each comprise a head unit  20  composed of twelve functional liquid droplet discharge heads  14  and a carriage plate  21  for supporting the twelve functional liquid droplet discharge heads  14  divided into two groups of six, as shown in  FIG. 2 . The carriage units  4  are suspended from bridge plates  22  extending across the tops of the pair of Y-axis support bases  13 . The carriage units  4  are designed so that natural water heads are used from sub-tanks (functional liquid tanks)  48  provided on the bridge plates  22 , and functional liquid is supplied to the functional liquid droplet discharge heads  14  via pressure regulation valves  23  (see  FIG. 4 ). The number of carriage units  4  and the number of functional liquid droplet discharge heads  14  mounted on each carriage unit  4  are arbitrary. 
         [0036]    Each of the functional liquid droplet discharge heads  14  is a so-called double inkjet head, comprising a functional liquid feeder  31  having two connecting pins  34 , two head substrates  32  joined to the side of the functional liquid feeder  31 , and a head main body  33  joined to the bottom of the head substrates  32  and filled with functional liquid therein, as shown in  FIG. 3 . The connecting pins  34  are connected to the functional liquid supply unit  6 , and functional liquid is supplied to the functional liquid feeder  31 . The head main body  33  is configured from a cavity (piezoelectric element)  35 , and a nozzle plate  36  having a nozzle surface  37  in which numerous discharge nozzles  38  are formed. When the functional liquid droplet discharge head  14  is dischargeably driven, functional liquid droplets are discharged from the discharge nozzles  38  by the pumping action of the cavity  35 . 
         [0037]    The functional liquid supply unit  6  comprises three functional liquid supply devices  15  corresponding to the three colors red, green, and blue, as shown in  FIG. 4 . The functional liquid supply unit  6  comprises nitrogen gas supply facilities  41  for supplying compressed nitrogen gas for controlling main tanks (functional liquid supply sources)  50 , sub-tanks  48 , and the like; compressed air supply facilities  42  for supplying compressed air for controlling various opening and closing valves; gas ventilation facilities  43  for ventilating gas from these components; and an air bubble expulsion unit  45 . The three functional liquid supply devices  15  are connected to functional liquid droplet discharge heads  14  corresponding to the three colors red, green, and blue, whereby functional liquid of the corresponding color is supplied to the functional liquid droplet discharge heads  14  of each color. 
         [0038]    Each of the functional liquid supply devices  15  comprises a tank unit  47  having two main tanks  50 ,  50  constituting a functional liquid supply source, thirteen sub-tanks  48  provided in correspondence with the carriage units  4 , and functional liquid flow channels  49  for connecting the tank unit  47  and the functional liquid droplet discharge heads  14 . The functional liquid in the main tanks  50  is pressurized by the compressed nitrogen gas from the nitrogen gas supply facilities  41  connected thereto, and is selectively supplied to the thirteen sub-tanks  48  via the functional liquid flow channels  49 . At this time, the various opening and closing valves are controlled to open and close by the compressed air from the compressed air supply facilities  42 . At the same time, the sub-tanks  48  are opened to the atmosphere via the gas ventilation facilities  43 , and the necessary amount of functional liquid is received. The functional liquid of the sub-tanks  48  is supplied to the functional liquid droplet discharge heads  14  via the functional liquid flow channels  49  by the driving of the connected functional liquid droplet discharge heads  14  while a predetermined water head pressure is maintained. Instead of feeding the liquid by gas pressure as described above, the liquid may be fed by pumping. 
         [0039]    The tank unit  47  comprises a pair of main tanks  50 ,  50  as functional liquid supply sources, a pair of weight measurement devices  51 ,  51  for respectively measuring the weight of the pair of main tanks  50 ,  50 , and a switching mechanism (flow channel switching unit)  52  for switching the communication to the functional liquid flow channels  49  from one main tank  50  to the other main tank  50 . Connected to each of the main tanks  50  is a nitrogen gas supply facility  41  and a gas ventilation facility  43  for performing positive pressure control for pressure-feeding the functional liquid and negative pressure control (equivalent to opening to the atmosphere) for returning the functional liquid. 
         [0040]    The switching mechanism  52  comprises a pair of tank flow channels  53 ,  53  connected to the pair of main tanks  50 ,  50 , a tank flow channel coupler  54  connected at the upstream side to the pair of tank flow channels  53 ,  53  and connected at the downstream side to the functional liquid flow channels  49 , and tank opening/closing valves  55  provided in the paths of the tank flow channels  53 ,  53 . Closing one tank opening/closing valve  55  and opening the other tank opening/closing valve  55  causes the connection to the functional liquid flow channels  49  to switch alternately between the pair of main tanks  50 ,  50 . 
         [0041]    Each of the functional liquid flow channels  49  is configured from a main functional liquid flow channels  56  connected at the upstream end to one tank unit  47 ; thirteen branching flow channels  57  for dividing the main functional liquid flow channel  56  into thirteen branches, the branching flow channels being connected to the downstream end of the main functional liquid flow channel  56 ; and individual functional liquid flow channels  58  for connecting the thirteen branching flow channels  57  and the functional liquid droplet discharge heads  14 . Provided in the path of the main functional liquid flow channel  56  is the air bubble expulsion unit  45  for expelling microbubbles in the functional liquid over time via a gas-permeable membrane. 
         [0042]    Each of the individual functional liquid flow channels  58  is configured from a tank-side flow channel  61  connected at the upstream end to one branching flow channel  57 , a head-side flow channel  62  connected at the upstream side to one sub-tank  48 , four branching flow channels  63  connected at the upstream sides to the head-side flow channel  62 , and a plurality of individual flow channels  64  connected at the upstream sides to the four branching flow channels  63 . The functional liquid is thereby branched four ways from each sub-tank  48  and is connected to the respective functional liquid droplet discharge heads  14 . In other words, functional liquid is supplied to the 13×4 functional liquid droplet discharge heads  14  by the thirteen branches of each of the functional liquid flow channels  49  and the four branches of each of the individual functional liquid flow channels  58 . 
         [0043]    The suction unit  19  comprises thirteen cap units  71  in which twelve head caps  74  corresponding to the twelve functional liquid droplet discharge heads  14  are disposed on a cap plate  75 , thirteen raising/lowering mechanisms  72  for raising and lowering the cap units  71  via the support plate  76 , thirteen suction flow channel systems joined to the cap units  71  and having functional liquid suction flow channels, and suction mechanisms (none of which are shown) joined to the suction flow channel systems and each having two disposed liquid tanks corresponding to two pressure levels, as shown in  FIG. 5 . Though not shown in the drawings, the suction unit  19  also comprises compressed a air supply facility for supplying compressed air for control to the pressure control mechanisms and other components, an air ventilation facility for venting air from the components, and a functional liquid disposal facility for disposing of retained functional liquid. 
         [0044]    The cap units  71  are configured from the head caps  74  corresponding to a total of twelve functional liquid droplet discharge heads  14 , two for each color, and the cap plate  75  on which the cap units are mounted. Each of the raising/lowering mechanisms  72  has a raising/lowering cylinder  77  for directly raising and lowering the head caps  74  via the support plate  76 , a pair of linear guides  78  for guiding the raising and lowering by the raising/lowering cylinders  77 , and a base part  79  for supporting these components. 
         [0045]    The suction action of the suction unit  19  is performed by raising the cap units  71  using the raising/lowering mechanisms  72 , bringing in the head caps  74  corresponding to the functional liquid droplet discharge heads  14 , and then suctioning the functional liquid from the discharge nozzles  38  of the functional liquid droplet discharge heads  14  by driving an ejector (not shown). 
         [0046]    A sub-tank (functional liquid tank)  48  according to the present embodiment is described in detail with reference to  FIG. 6 . The sub-tank  48  comprises a sub-tank body (tank body)  81  for retaining functional liquid, a lid float (float member)  82  that floats on the sub-tank body  81  in the manner of a lid, a transparent liquid column pipe  83  appended to the sub-tank body  81 , a liquid level detection mechanism  84  that faces the liquid column pipe  83  and detects the level of retained functional liquid, and an immersed member (float-regulating member)  85 , which is provided at the bottom of the sub-tank body  81  and which regulates the descending end position of the lid float  82  at the upper side of a bottom connecting hole  86  of the liquid column pipe  83 . At the bottom of the sub-tank  48  are provided an inlet port  88  connected to a tank-side flow channel  61  via a sub-tank opening/closing valve  87 , and an outlet port  89  connected to a head-side flow channel  62 . Thus, the configuration is designed so that the functional liquid flows in from below the sub-tank body  81  and also flows out from below. A nitrogen gas supply facility  41  and a gas ventilation facility  43  are also connected to the top of the sub-tank body  81  (see  FIG. 4 ), and the interior of the sub-tank body  81  is configured so as to be capable of opening to the atmosphere when liquid is fed from the main tank 50, and so that the pressure increase to the main tank  50  can be controlled. 
         [0047]    The liquid level detection mechanism  84  comprises an upper limit detection sensor (upper limit liquid level detection unit)  91  that faces the liquid column pipe  83  and detects the upper limit of the functional liquid level, a liquid level detection sensor  92  that is provided in a vertically intermediate position and that detects the functional liquid level during refilling, and a lower limit detection sensor (lower limit liquid level detection unit)  93  for detecting the lower limit of the functional liquid level. The upper limit detection sensor  91  is provided in order to prevent overflowing in the sub-tank  48 , and when the upper limit detection sensor  91  has detected an upper limit liquid level, the liquid feed from the main tank  50  is stopped. The lower limit detection sensor  93  is provided in order to prevent the sub-tank  48  from becoming empty, and when the lower limit detection sensor  93  has detected a lower limit liquid level, the droplet discharge device  1  is stopped when the drawing currently being performed on the workpiece ends. The lower limit detection sensor  93  is also used when functional liquid is removed in the method for replenishing functional liquid described hereinafter. When liquid is removed, timer control is performed using the liquid level detection by the lower limit detection sensor  93  as a trigger. 
         [0048]    The liquid level detection sensor  92  detects a liquid level which accounts for the ideal water head value of the functional liquid droplet discharge heads  14 , and when the functional liquid level is detected by the liquid level detection sensor  92 , a determination is made through coordination with the control device as to whether the liquid level has risen or fallen. Specifically, the liquid level is determined to have fallen when the functional liquid has been reduced by the discharge action from a state in which the liquid level is higher than the liquid level detection sensor  92 , and the liquid level being detected by the liquid level detection sensor  92 . The liquid level is determined to have risen when the functional liquid is increased by the refilling action from a state in which the liquid level is lower than the liquid level detection sensor  92 , the liquid level is detected by the liquid level detection sensor  92 , and a specified time has elapsed thereafter. The functional liquid level in the sub-tank  48  is controlled at a vertically intermediate position by this type of liquid level detection sensor  92 . 
         [0049]    The following is a description of the action of refilling functional liquid from the main tank  50  used as a functional liquid supply source to the sub-tank  48 . When the functional liquid in the sub-tank  48  decreases by a specified amount and the liquid level detection mechanism  84  described above determines that the liquid level has decreased, the sub-tank opening/closing valve  87  is opened and functional liquid is refilled from the main tank  50  to the sub-tank  48 . Since the pressure in the main tank  50  has been increased, opening the sub-tank opening/closing valve  87  and opening the sub-tank  48  interior to the atmosphere causes the functional liquid in the main tank  50  to be automatically fed to the sub-tank  48 . When a specified amount of functional liquid has accumulated in the sub-tank  48  and the liquid level detection mechanism  84  has determined that the liquid in the sub-tank  48  is full, the sub-tank opening/closing valve  87  is closed and the refilling action is ended. 
         [0050]    The lid float  82  is formed from a chemically resistant metal material into a thin rectangular solid shape having a hollow interior. More specifically, the lid float comprises stainless steel sheets machined into a box shape, and is formed into a shape substantially similar to the top surface of the sub-tank body  81  when viewed from above so that there is a small gap in between the two. The immersed member  85  is positioned directly below the lid float  82  and is immersed so as to rest on the bottom surface of the sub-tank body  81 . 
         [0051]    The immersed member  85  functions only in the case of a temporary extreme reduction in the amount of functional liquid remaining in the sub-tank  48  in the method for replenishing functional liquid described hereinafter. The immersed member is configured from a plate-shaped immersed plate  101  and an immersed protrusion  102  standing upright in the middle of the immersed plate  101 . The immersed member  85  is formed so that when it is immersed, the top end thereof is in a higher position than the bottom connecting hole  86  of the liquid column pipe  83 . More specifically, the immersed member is arranged so as to be capable of regulating the descending end position of the lid float  82  so that the bottom surface of the lid float  82  is in a position lower than the lower limit detection sensor  93  and higher than the bottom connecting hole  86 . The immersed member  85  is preferably formed so as to have a height reaching from the bottom surface of the sub-tank body  81  to the top end of the bottom connecting hole  86  of the liquid column pipe  83 . It shall be apparent that the immersed member  85  is configured from a chemically resistant (resistant to the functional liquid) metal having a sufficiently greater relative density than the functional liquid, and the shape thereof is arbitrary. For example, the immersed member may be configured in the form of a mesh in order to have a smaller volume (e.g., a perforated metal), and the mesh may be bent or curved so as to have a predetermined height and may be immersed in the functional liquid. 
         [0052]    The sub-tank body  81 , which is underneath the bottom surface of the lid float  82  whose position is regulated by the immersed member  85 , is designed to have a capacity that will not affect the timer control when the functional liquid is being removed. The details are described hereinafter, but in the method for replenishing functional liquid of the present embodiment, liquid is removed so that the liquid level falls to the vicinity of the outlet port  89 , and timer control is performed with precision so that the amount of a single liquid removal increases and gas does not flow in toward the functional liquid droplet discharge heads  14 . Liquids of different viscosities are used for the replenished functional liquid; i.e., the functional liquid used, and even with functional liquids of different viscosities, the object of the timer control as well as the amount of functional liquid are defined to a certain extent so that the timer can be stably controlled. 
         [0053]    Since the suction unit  19  described above is configured so as to suction functional liquid with a specified pressure, in cases in which the suction unit has a capacity for a small amount of functional liquid when functional liquid is being removed, the rate at which the liquid level falls in the sub-tank  48  increases, and timer control accounting for a functional liquid of a different viscosity becomes unstable. Therefore, the descending end position of the lid float  82  is regulated and the object of timer control as well as the functional liquid amount are defined to a certain extent, so that functional liquid of a different viscosity can be appropriately removed under timer control for a specified amount of time. 
         [0054]    The following is a description, made with reference to  FIG. 7 , of a method for replenishing functional liquid in the functional liquid supply devices  15  (the functional liquid supply unit  6 ). This method for replenishing functional liquid involves removing (incompletely) functional liquid (old functional liquid) filled in the functional liquid supply devices  15  (in all the flow channels) and introducing functional liquid (fresh functional liquid) having a different constitution, wherein the process of removing and filling is repeated. It shall be apparent that the functional liquid replenishing sequence starts from a state in which old functional liquid remains in one main tank  50  and the other main tank  50  filled with fresh functional liquid has been set. After the functional liquid is suctioned from the discharge nozzles  38  of the functional liquid droplet discharge heads  14  by the suction unit  19  and the lower limit detection sensor  93  has detected the liquid level, a partial replenishing step is repeated a predetermined number of times, the partial replenishing step comprising a removal process for removing a predetermined amount of functional liquid from the sub-tanks  48  by timer control for a predetermined amount of time, and a liquid feeding step for pressure-feeding functional liquid from the main tanks  50  to the sub-tanks  48 . 
         [0055]    First, the flow channels are switched by the switching mechanism  52  from the main tank  50  containing old functional liquid currently being used to the main tank  50  filled with fresh functional liquid. Next, the suction unit  19  is driven and the removal process (S 1  to S 5 ) in which liquid is removed from the sub-tanks  48  is started. Reverse-feed pipes connected to the disposal tanks are provided in the functional liquid flow channels  49  extending from the main tanks  50  to the sub-tanks  48 , whereby fresh functional liquid may be fed from the main tanks  50  so as to push out the old functional liquid in the flow channels, and the old functional liquid may be fed to the disposal tanks through the reverse-feed pipes. 
         [0056]    Liquid starts to flow out of the sub-tanks  48 , and when the falling liquid level reaches the position of the lower limit detection sensors  93  and the lower limit detection sensors  93  detect the level of the functional liquid (“Yes” in S 2 ), the liquid removal is switched to timer control (S 3 ). Under timer control, the timer duration is set so that as much functional liquid as possible is removed from the sub-tanks  48  as calculated in advance from the remaining amount of functional liquid and the viscosity of the functional liquid. The liquid column pipes  83  are thereby depleted of functional liquid, and substantially no functional liquid remains in the sub-tanks  48 . More specifically, a sufficient amount of liquid is removed so that the outlet ports  89  are not exposed above the liquid surfaces. 
         [0057]    The sub-tank opening/closing valves  87  are then opened and liquid feeding process for feeding liquid to the sub-tanks  48  (steps S 6  to S 9 ) is started. Specifically, the sub-tank opening/closing valves  87  are left open and the first sub-tank  48  is filled with functional liquid until the upper limit detection sensor  91  of the first sub-tank  48  detects a full liquid level (“Yes” in S 7 ). In this case, the replenishing rate is markedly improved because old functional liquid remains in portions (small gaps) where it does not readily mix with fresh functional liquid, such as the space between the sub-tank  48  and the lid float  82  and the interior of the liquid column pipe  83 . A high replenishing rate is maintained particularly in cases in which the relative density of the fresh functional liquid is greater than that of the old functional liquid. The first partial replenishing process (S 1  to S 9 ) of the first sub-tank  48  thereby concludes. The partial replenishing process is then performed on the second through thirteenth sub-tanks  48 , thereby concluding the first series of partial replenishing process. After the first series of partial replenishing process has ended, a second series of partial replenishing process is performed on the first through thirteenth sub-tanks  48 . The partial replenishing process is then repeated multiple times (a predetermined number of times), thereby concluding functional liquid replenishing in all of the flow channels extending from the main tanks  50  to the functional liquid droplet discharge heads  14 . The number of repetitions is a number that enables the functional liquid suctioned from the functional liquid droplet discharge heads  14  to have a replenishing rate (99.0%) nearly equivalent to complete replenishing, and the number of repetitions is preferably a number determined experimentally in advance so as to achieve the aforementioned replenishing rate. 
         [0058]    The following is a description, made with reference to  FIG. 8 , of a sub-tank  48  according to the second embodiment of the present invention. The description primarily focuses on different components in order to avoid superfluous description. In this sub-tank  48 , an extended protrusion  103 , made of stainless steel or a plastic material, is provided on the bottom inside surface of the sub-tank body  81  as a float-regulating member. The extended protrusion  103  is configured from four individual protrusions  104  provided at the same height on the four inside walls of the sub-tank body  81 , and is disposed so as to be positioned above the bottom connecting hole  86  of the liquid column pipe  83 . More specifically, the extended protrusion  103  is disposed so as to be capable of regulating the descending end position of the bottom surface of the lid float  82  at a position lower than the lower limit detection sensor  93  and higher than the bottom connecting hole  86 . Furthermore, the extended protrusion  103  is preferably disposed at the same height as the top end of the bottom connecting hole  86  of the liquid column pipe  83 . In this case as well, the capacity of the sub-tank body  81 , which is underneath the bottom surface of the lid float  82  whose position is regulated by the extended protrusion  103 , is designed so as not to affect the timer control when the functional liquid is being removed. 
         [0059]    The following is a description, made with reference to  FIG. 9 , of a sub-tank  48  according to a third embodiment of the present invention. In this sub-tank  48 , a contact protrusion  105 , made of stainless steel or a plastic material, stands upright in the bottom surface of the sub-tank body  81  as a float-regulating member. The contact protrusion  105  is configured from a plurality of individual contact protrusions  106 , and is formed so that the top ends thereof are positioned above the bottom connecting hole  86  of the liquid column pipe  83 . More specifically, the contact protrusion  105  is formed so as to be capable of regulating the descending end position of the bottom surface of the lid float  82  at a position lower than the lower limit detection sensor  93  and higher than the bottom connecting hole  86 . Furthermore, the contact protrusion  105  is preferably formed so as to reach from the sub-tank body  81  to the same height as the top end of the bottom connecting hole  86  of the liquid column pipe  83 . In this case as well, the capacity of the sub-tank body  81 , which is underneath the bottom surface of the lid float  82 , whose position is regulated by the contact protrusion  105 , is designed so as not to affect the timer control when the functional liquid is being removed. 
         [0060]    The following is a description, made with reference to  FIG. 10 , of a sub-tank  48  according to a fourth embodiment of the present invention. In this sub-tank  48 , a hanging protrusion (float-regulating member)  107 , made of stainless steel or a plastic material, is suspended as a float-regulating member from the bottom surface of the lid float  82 . The hanging protrusion  107  is configured from an attachment plate  108  attached to the bottom surface of the lid float  82 , and a regulating protrusion  109  formed protruding in the middle of the attachment plate  108 . The regulating protrusion  109  is formed in a rod shape or plate shape, and is provided so as to protrude downward in the middle of the attachment plate  108 . The hanging protrusion  107  is also formed so as to have a height reaching from the bottom surface of the sub-tank body  81  to a point above the bottom connecting hole  86  of the liquid column pipe  83 . More specifically, the hanging protrusion  107  is formed so as to be capable of regulating the descending end position of the bottom surface of the lid float  82  at a position lower than the lower limit detection sensor  93  and higher than the bottom connecting hole  86 . Furthermore, the hanging protrusion  107  is preferably formed so as to reach from the sub-tank body  81  to the same height as the top end of the bottom connecting hole  86  of the liquid column pipe  83 . In this case as well, the capacity of the sub-tank body  81 , which is underneath the bottom surface of the lid float  82 , whose position is regulated by the hanging protrusion  107 , is designed so as not to affect the timer control when the functional liquid is being removed. 
         [0061]    According to the configuration described above, during functional liquid removal, the descending of the lid float  82  is regulated at a position lower than the lower limit detection sensor  93  and higher than the bottom connecting hole  86 , and the amount of functional liquid below the lid float  82  is reliably provided to a certain extent. Therefore, even in cases in which liquid is removed by timer control, a small amount of functional liquid remains in the sub-tank  48 , and it is possible to remove this liquid in one removal. The functional liquid remaining in the liquid column pipe  83  and in the small gap between the sub-tank body  81  and the lid float  82  can readily be removed by regulating the position of the lid float  82 . Therefore, old functional liquid can be efficiently and in a short amount of time replenished with fresh functional liquid in the functional liquid supply devices  15 . 
         [0062]    In the present embodiment, functional liquid is replenished using one sub-tank  48  as a replenishing unit, but functional liquid may also be replenished using two sub-tanks  48  as replenishing units. The form of the float-regulating member is not limited to those of the embodiments described above; the float-regulating member need only be capable of regulating the position of the lid float  82  to a predetermined position. For example, the float-regulating member may be in the form of a cord by which the lid float  82  is suspended from the top surface of the sub-tank  48 . 
         [0063]    The fresh functional liquid may be a detergent for washing all flow channels extending from the main tanks  50  to the functional liquid droplet discharge heads  14 . In this case, if the detergent is a solvent of the functional liquid, the number of repetitions described above can be markedly reduced. 
       GENERAL INTERPRETATION OF TERMS 
       [0064]    In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Finally, terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
         [0065]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.