Patent Publication Number: US-11040538-B2

Title: Inkjet printer and maintenance method

Description:
RELATED APPLICATIONS 
     This application claims the benefit of Japanese Application No. 2018-177970, filed on Sep. 21, 2018, the disclosure of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an inkjet printer and a maintenance method of maintaining a head in an inkjet printer. 
     Description of the Background Art 
     An inject printer for commercial printing prints an image on a surface of elongated strip-shaped printing paper by ejecting ink from a plurality of heads while transporting the printing paper along a predetermined transport path. The inject printer of such a type is provided with ink tanks as sources of inks of corresponding colors, C, M, Y, and K. The ink tank and the head are connected through a pipe. Ink stored in the ink tank is supplied through the pipe to the head by the pressure of a pump provided in the pipe. 
     The conventional inkjet printer is disclosed in Japanese Patent Application Laid-Open No. 2016-187884, for example. 
     SUMMARY OF THE INVENTION 
     The head of the inkjet printer ejects ink droplets from a large number of tiny nozzles. In some cases, these nozzles encounter clogging to be caused by drying of ink or adherence of foreign substances. To eliminate or prevent such nozzle clogging, the inkjet printer is subjected to the process of supplying the head with a maintenance solution such as a cleaning solution regularly. 
     In the conventional inkjet printer, the head has been supplied with the maintenance solution by replacing an ink tank as a source of ink by a tank of the maintenance solution. However, this method necessitates replacement of the ink in an ink supply path as a whole by the maintenance solution. In particular, if an intermediary tank is provided on the ink supply path, ink in the intermediary tank is also required to be replaced by the maintenance solution. This results in mass consumptions of the ink and the maintenance solution during supply of the maintenance solution. 
     A mechanism of supplying the maintenance solution to each head may be provided separately from the ink supply path. However, individually providing the mechanism of supplying the maintenance solution to each head unfortunately causes size increase of the printer as a whole. 
     The present invention is intended to provide an inkjet printer and a maintenance method capable of supplying a maintenance solution instead of ink to a plurality of heads while suppressing size increase of the printer, and capable of reducing the consumptions of the ink and the maintenance solution. 
     One aspect of this application is intended for an inkjet printer. 
     The inkjet printer comprises: a first head that ejects first ink toward a printing medium; a second head that ejects second ink of a different color from the first ink toward the printing medium; a first ink supply mechanism that supplies the first ink to the first head; a second ink supply mechanism that supplies the second ink to the second head; and a maintenance solution supply mechanism that supplies a maintenance solution to the first head and the second head. The first ink supply mechanism includes: a first ink pipe through which the first ink is supplied from a first tank to the first head; and a first three-way valve provided on a path along the first ink pipe. The second ink supply mechanism includes: a second ink pipe through which the second ink is supplied from a second tank to the second head; and a second three-way valve provided on a path along the second ink pipe. The maintenance solution supply mechanism includes a maintenance pipe through which the maintenance solution is supplied from a maintenance tank storing the maintenance solution to the first three-way valve and the second three-way valve. 
     Switching the first three-way valve and the second three-way valve allows supply of the maintenance solution instead of ink to the first head and the second head. The maintenance solution is supplied to the first head and the second head from the common maintenance tank. This makes it possible to suppress size increase of the printer. 
     The maintenance pipe is connected to the first ink pipe and the second ink pipe at positions downstream of the first tank and the second tank. This eliminates the need of replacement of the inks in the first tank and the second tank by the maintenance solution during supply of the maintenance solution. This allows reduction in the consumptions of the inks and the maintenance solution. 
     Preferably, the maintenance pipe includes: a common pipe extending from the maintenance tank; a first branch pipe connecting the common pipe and the first three-way valve; and a second branch pipe connecting the common pipe and the second three-way valve. 
     Even if the ink of a small amount flows backward from the first three-way valve into the first branch pipe or from the second three-way valve into the second branch pipe, this ink is unlikely to reach the common pipe. This makes it possible to reduce the occurrence of mixture of ink colors through the common pipe. 
     Preferably, each of the first branch pipe and the second branch pipe has a length of 30 mm or more. 
     The probability of reach of the ink from the first three-way valve or the second three-way valve to the common pipe is reduced further. This makes it possible to reduce the occurrence of mixture of ink colors through the common pipe to a greater extent. 
     Preferably, the first ink supply mechanism further includes a first pump located downstream of the first three-way valve on the path along the first ink pipe. The second ink supply mechanism further includes a second pump located downstream of the second three-way valve on the path along the second ink pipe. The inkjet printer further comprises a controller that controls the first three-way valve, the second three-way valve, the first pump, and the second pump. For switching of the first three-way valve from a normal state of forming connection to the first tank to a maintenance state of forming connection to the maintenance tank, the controller makes the switching while the first pump is being driven and continues driving of the first pump until the first three-way valve is switched again to the normal state. For switching of the second three-way valve from a normal state of forming connection to the second tank to a maintenance state of forming connection to the maintenance tank, the controller makes the switching while the second pump is being driven and continues driving of the second pump until the second three-way valve is switched again to the normal state. 
     The occurrence of backflow of the ink is reduced from the first three-way valve or the second three-way valve to the maintenance pipe. This makes it possible to reduce the occurrence of mixture of ink colors through the maintenance pipe. 
     Preferably, the first ink is black ink, the second ink is ink of a color other than black, and the first three-way valve is connected to a position of the maintenance pipe downstream of the second three-way valve. 
     The first three-way valve corresponding to the black ink is connected to the position of the maintenance pipe downstream of the second three-way valve corresponding to ink of a different color. This makes it possible to reduce the occurrence of mixture of the black ink to exert large influence on a printing result with the ink pipe corresponding to the different color through the maintenance pipe. 
     Preferably, each of the first tank and the second tank is an intermediary tank that temporarily stores ink supplied from a source of the ink. 
     Preferably, the maintenance solution is a cleaning solution used for cleaning the first head and the second head. 
     Another aspect of the invention is intended for a maintenance method employed in an inkjet printer that supplies first ink from a first tank to a first head through a first ink pipe and supplies second ink of a different color from the first ink from a second tank to a second head through a second ink pipe. The method is for maintaining the first head and second head. 
     The maintenance method comprises the steps of: a) supplying a maintenance solution from a maintenance tank to the first head by switching a first three-way valve provided in the first ink pipe; and b) supplying the maintenance solution from the maintenance tank to the second head by switching a second three-way valve provided in the second ink pipe. 
     Preferably, in the step a), while a first pump located downstream of the first three-way valve on a path along the first ink pipe is being driven, the first three-way valve is switched from a normal state of forming connection to the first tank to a maintenance state of forming connection to the maintenance tank, and driving of the first pump is continued until the first three-way valve is switched again to the normal state. In the step b), while a second pump located downstream of the second three-way valve on a path along the second ink pipe is being driven, the second three-way valve is switched from a normal state of forming connection to the second tank to a maintenance state of forming connection to the maintenance tank, and driving of the second pump is continued until the second three-way valve is switched again to the normal state. 
     The occurrence of backflow of the ink is reduced from the first three-way valve or the second three-way valve toward the maintenance tank. This makes it possible to reduce the occurrence of mixture of ink colors through a pipe for supply of the maintenance solution. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the configuration of an inkjet printer; 
         FIG. 2  shows the configurations of four ink supply mechanisms and that of a maintenance solution supply mechanism; 
         FIG. 3  shows connection between a controller and each part in the inkjet printer; 
         FIG. 4  is a flowchart showing a procedure of supplying a maintenance solution to a first head; and 
         FIG. 5  shows the configuration of a maintenance solution supply mechanism according to a modification. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will be described below by referring to the drawings. 
     &lt;1. Configuration of Inkjet Printer&gt; 
       FIG. 1  shows the configuration of an inkjet printer  1  according to a preferred embodiment of the present invention. The inkjet printer  1  is an apparatus that prints a color image on a surface of printing paper  9  that is an elongated strip-shaped printing medium by ejecting ink toward the printing paper  9  from a plurality of heads  60  while transporting the printing paper  9 . As shown in  FIG. 1 , the inkjet printer  1  includes a printer body  10 , four ink supply mechanisms  20 , one maintenance solution supply mechanism  30 , and a controller  40 . 
     &lt;1-1. Printer Body&gt; 
     The printer body  10  includes a transport mechanism  50 , four heads  60 , and four sub-tanks  70 . 
     The transport mechanism  50  is a mechanism that transports the printing paper  9  in a transport direction along the length of the printing paper  9 . The transport mechanism  50  of the preferred embodiment includes an unwinding part  51 , a plurality of transport rollers  52 , and a winding part  53 . The printing paper  9  is unwound from the unwinding part  51 , and is transported along a transport path configured using the transport rollers  52 . Each of the transport rollers  52  rotates about a horizontal axis while contacting the printing paper  9  to guide the printing paper  9  downstream along the transport path. The transported printing paper  9  is collected on the winding part  53 . 
     The four heads  60  are arranged at regular intervals along the transport path of the printing paper  9 . Each of the heads  60  includes a plurality of nozzles aligned parallel to the width direction of the printing paper  9  (a horizontal direction perpendicular to the transport direction). The four heads  60  eject ink droplets of corresponding colors, black (K), cyan (C), magenta (M), and yellow (Y) to become color components of a color image from the nozzles toward a surface of the printing paper  9 . 
     The four heads  60  include a head  60  that ejects color K ink (first ink), a head  60  that ejects color C ink (second ink), a head  60  that ejects color M ink (third ink), and a head  60  that ejects color Y ink (fourth ink), and these heads  60  will be called a “first head  61 ,” a “second head  62 ,” a “third head  63 ,” and a “fourth head  64 ” respectively. In the preferred embodiment, the first head  61 , the second head  62 , the third head  63 , and the fourth head  64  are aligned in this order along the transport path of the printing paper  9 . Meanwhile, the first to fourth heads  61  to  64  may be aligned in different order. 
     The four sub-tanks  70  are containers for storing inks to be ejected. The four sub-tanks  70  store inks of corresponding ones of the colors K, C, M, and Y. Each of the four heads  60  is connected to the sub-tank  70  storing ink of a color to be ejected through a pipe. Ink supplied from the ink supply mechanism  20  described later is stored once in the sub-tank  70  arranged in the printer body  10 . Each of the heads  60  ejects the ink stored in the sub-tank  70  from the nozzles in response to an image to be printed. 
     The four sub-tanks  70  include a sub-tank  70  connected to the first head  61 , a sub-tank  70  connected to the second head  62 , a sub-tank  70  connected to the third head  63 , and a sub-tank  70  connected to the fourth head  64 , and these sub-tanks  70  will be called a “first sub-tank  71 ,” a “second sub-tank  72 ,” a “third sub-tank  73 ,” and a “fourth sub-tank  74 ” respectively. The first sub-tank  71  stores the color K ink to be used by the first head  61 . The second sub-tank  72  stores the color C ink to be used by the second head  62 . The third sub-tank  73  stores the color M ink to be used by the third head  63 . The fourth sub-tank  74  stores the color Y ink to be used by the fourth head  64 . 
     While the inkjet printer  1  operates, the first head  61  ejects droplets of the color K ink to print a single-color image of the color K on a surface of the printing paper  9 . The second head  62  ejects droplets of the color C ink to print a single-color image of the color C on the surface of the printing paper  9 . The third head  63  ejects droplets of the color M ink to print a single-color image of the color M on the surface of the printing paper  9 . Further, the fourth head  64  ejects droplets of the color Y ink to print a single-color image of the color Y on the surface of the printing paper  9 . These four single-color images are superimposed on each other to form a color image on the surface of the printing paper  9 . 
     A drying processor for drying the inks ejected on the surface of the printing paper  9  may further be provided downstream of the four heads  60  along the transport path. The drying processor is to dry the inks by blowing heated gas toward the printing paper  9  and evaporating a solvent in the inks adhering to the printing paper  9 , for example. The drying processor may be configured to dry the inks by light irradiation. Alternatively, the drying processor may be configured to heat and dry the inks adhering to the printing paper  9  by winding the printing paper  9  on a heat roller. 
     &lt;1-2. Ink Supply Mechanism&gt; 
     The four ink supply mechanisms  20  are mechanisms that supply inks of corresponding colors from an ink tank installed outside the printer body  10  to the sub-tanks  70  inside the printer body  10 .  FIG. 2  shows the configurations of the four ink supply mechanisms  20  and that of the maintenance solution supply mechanism  30  described later. 
     The four ink supply mechanisms  20  include an ink supply mechanism  20  that supplies the color K ink to the first head  61 , an ink supply mechanism  20  that supplies the color C ink to the second head  62 , an ink supply mechanism  20  that supplies the color M ink to the third head  63 , and an ink supply mechanism  20  that supplies the color Y ink to the fourth head  64 , and these ink supply mechanisms  20  will be called a “first ink supply mechanism  21 ,” a “second ink supply mechanism  22 ,” a “third ink supply mechanism  23 ,” and a “fourth ink supply mechanism  24 ” respectively. 
     As shown in  FIG. 2 , the first ink supply mechanism  21  includes a first ink tank holder  211 , an upstream first ink pipe  212 , a first intermediary tank  213 , and a downstream first ink pipe  214 . 
     The first ink tank holder  211  is a base for supporting a first ink tank  81  as a source of the color K ink. The first ink tank  81  is placed at the upper surface of the first ink tank holder  211 . The first ink tank  81  is a container having a cylindrical lateral surface, for example. The first ink tank  81  stores the color K ink in an unused condition therein. Initially, the ink in the first ink tank  81  is stored to an amount set at  200  liters, for example. The first ink tank  81  may be supported directly on a predetermined support surface of the ink supply mechanism  21  without intervention of the first ink tank holder  211 . 
     The upstream first ink pipe  212  is a pipe for supplying the ink from the first ink tank  81  to the first intermediary tank  213 . An upstream end of the upstream first ink pipe  212  is connected to the first ink tank  81 . A downstream end of the upstream first ink pipe  212  is connected to the first intermediary tank  213 . As shown in  FIG. 2 , an upstream first pump  215  is interposed in a path along the upstream first ink pipe  212 . The upstream first pump  215  is a liquid feeder that forms a downstream flow of the ink in the upstream first ink pipe  212 . When the upstream first pump  215  is operated, the color K ink is supplied from the first ink tank  81  to the first intermediary tank  213  through the upstream first ink pipe  212 . 
     The first intermediary tank  213  is a tank storing the color K ink between the upstream first ink pipe  212  and the downstream first ink pipe  214 . The capacity of the first intermediary tank  213  is smaller than that of the first ink tank  81 . The capacity of the first intermediary tank  213  is 20 liters, for example. The first intermediary tank  213  functions as a buffer that stores the color K ink temporarily to prevent shortage of the ink occurring at a location downstream of the first intermediary tank  213 . 
     As shown in  FIG. 2 , a first lower limit sensor  216  and a first upper limit sensor  217  are provided near the first intermediary tank  213 . The first lower limit sensor  216  is a sensor that determines the presence or absence of the ink at a predetermined lower limit position H 1  at the first intermediary tank  213 . The first upper limit sensor  217  is a sensor that determines the presence or absence of the ink at a predetermined upper limit position H 2  at the first intermediary tank  213  higher than the lower limit position H 1 . The first lower limit sensor  216  and the first upper limit sensor  217  may be capacitive sensors, for example. 
     Results of the determinations by the first lower limit sensor  216  and the first upper limit sensor  217  are output to the controller  40  described later. The controller  40  controls the operation of the foregoing upstream first pump  215  on the basis of the results of the determinations by the first lower limit sensor  216  and the first upper limit sensor  217 . By doing so, the liquid level position of the ink in the first intermediary tank  213  is maintained between the lower limit position H 1  and the upper limit position H 2 . 
     The downstream first ink pipe  214  is a pipe for supplying the ink from the intermediary tank  213  to the first sub-tank  71  in the printer body  10 . An upstream end of the downstream first ink pipe  214  is connected to the first intermediary tank  213 . A downstream end of the downstream first ink pipe  214  is connected to the first sub-tank  71 . 
     A first three-way valve  218  and a downstream first pump  219  are interposed in a path along the downstream first ink pipe  214 . The downstream first pump  219  is located downstream of the first three-way valve  218 . The first three-way valve  218  allows switching in pipe connection between a normal state and a maintenance state. In the normal state, a part of the downstream first ink pipe  214  downstream of the first three-way valve  218  is connected to a part of the downstream first ink pipe  214  upstream of the first three-way valve  218 . In the maintenance state, the part of the downstream first ink pipe  214  downstream of the first three-way valve  218  is connected to a maintenance pipe  32  described later. The upstream first ink pipe  212  and the downstream first ink pipe  214  may be provided with an air trap or a vacuum pump interposed at a required position. 
     The downstream first pump  219  is a liquid feeder that forms a downstream flow of the ink in the downstream first ink pipe  214 . When the downstream first pump  219  is driven while the first three-way valve  218  is in the foregoing normal state, the ink stored in the first intermediary tank  213  is supplied to the first sub-tank  71  through the downstream first ink pipe  214 . When the first three-way valve  218  is switched to the foregoing maintenance state while the downstream first pump  219  is being driven, a maintenance solution stored in a maintenance tank  85  described later is supplied to the first sub-tank  71  through the maintenance pipe  32  and the downstream first ink pipe  214 . The first sub-tank  71  is provided with a level sensor not shown in the drawings. The level sensor is a sensor that determines that the liquid level of a liquid stored in the first sub-tank  71  has reduced or increased to a reference level. 
     Where necessary, an additional element such as a solenoid valve, a filter, or a degasifier may be provided in each of the paths along the upstream first ink pipe  212  and the downstream first ink pipe  214 . 
     While the second ink supply mechanism  22 , the third ink supply mechanism  23 , and the fourth ink supply mechanism  24  are to supply inks of different colors from the first ink supply mechanism  21  and are connected to different destinations from the first ink supply mechanism  21 , the configurations themselves of the second to fourth ink supply mechanisms  22  to  24  are similar to those of the first ink supply mechanism  21 . More specifically, the second ink supply mechanism  22  includes a second ink tank holder  221  that supports a second ink tank  82 , an upstream second ink pipe  222 , a second intermediary tank  223 , a downstream second ink pipe  224 , an upstream second pump  225 , a second lower limit sensor  226 , a second upper limit sensor  227 , a second three-way valve  228 , and a downstream second pump  229 . The third ink supply mechanism  23  includes a third ink tank holder  231  that supports a third ink tank  83 , an upstream third ink pipe  232 , a third intermediary tank  233 , a downstream third ink pipe  234 , an upstream third pump  235 , a third lower limit sensor  236 , a third upper limit sensor  237 , a third three-way valve  238 , and a downstream third pump  239 . The fourth ink supply mechanism  24  includes a fourth ink tank holder  241  that supports a fourth ink tank  84 , an upstream fourth ink pipe  242 , a fourth intermediary tank  243 , a downstream fourth ink pipe  244 , an upstream fourth pump  245 , a fourth lower limit sensor  246 , a fourth upper limit sensor  247 , a fourth three-way valve  248 , and a downstream fourth pump  249 . The function of each of these parts will not be described repeatedly as it is similar to the function of a corresponding part of the first ink supply mechanism  21 . 
     &lt;1-3. Maintenance Solution Supply Mechanism&gt; 
     The maintenance solution supply mechanism  30  is a mechanism for supplying the maintenance solution to the four heads  60 . In some cases, the head  60  encounters clogging of a nozzle to be caused by drying of the ink or adherence of foreign substances. The maintenance solution supply mechanism  30  supplies the maintenance solution that is a cleaning solution for cleaning the nozzle for eliminating such nozzle clogging, for example. The maintenance solution supplied from the maintenance solution supply mechanism  30  may be a preservative solution for preventing drying while the head  60  is on standby. 
     As shown in  FIG. 2 , the maintenance solution supply mechanism  30  includes a maintenance tank holder  31  and the maintenance pipe  32 . 
     The maintenance tank holder  31  is a base for supporting the maintenance tank  85  as a source of the maintenance solution. The maintenance tank  85  is placed at the upper surface of the maintenance tank holder  31 . Any shape is applicable to the maintenance tank  85 . The maintenance tank  85  stores the maintenance solution therein. Initially, the maintenance solution in the maintenance tank  85  is stored to an amount set at  20  liters, for example. 
     The maintenance pipe  32  is a pipe for supplying the maintenance solution from the maintenance tank  85  to the foregoing first three-way valve  218 , second three-way valve  228 , third three-way valve  238 , and the fourth three-way valve  248 . As shown in  FIG. 2 , the maintenance pipe  32  includes a common pipe  320 , a first branch pipe  321 , a second branch pipe  322 , a third branch pipe  323 , and a fourth branch pipe  324 . An upstream end of the common pipe  320  is connected to the maintenance tank  85 . 
     The first branch pipe  321  extends from a downstream end of the common pipe  320  toward the first three-way valve  218 . A downstream end of the first branch pipe  321  is connected to one port belonging to three ports of the first three-way valve  218  and not connected to the downstream first ink pipe  214 . The second branch pipe  322  extends from a branch position B 2  upstream of the downstream end of the common pipe  320  toward the second three-way valve  228 . A downstream end of the second branch pipe  322  is connected to one port belonging to three ports of the second three-way valve  228  and not connected to the downstream second ink pipe  224 . 
     The third branch pipe  323  extends from a branch position B 3  upstream of the foregoing branch position B 2  of the common pipe  320  toward the third three-way valve  238 . A downstream end of the third branch pipe  323  is connected to one port belonging to three ports of the third three-way valve  238  and not connected to the downstream third ink pipe  234 . The fourth branch pipe  324  extends from a branch position B 4  upstream of the foregoing branch position B 3  of the common pipe  320  toward the fourth three-way valve  248 . A downstream end of the fourth branch pipe  324  is connected to one port belonging to three ports of the fourth three-way valve  248  and not connected to the downstream fourth ink pipe  244 . 
     When the first three-way valve  218  is switched for connection to the maintenance pipe  32  while the downstream first pump  219  is being driven, the maintenance solution is supplied from the maintenance tank  85  to the first sub-tank  71  through the common pipe  320 , the first branch pipe  321 , and the downstream first ink pipe  214 . When the second three-way valve  228  is switched for connection to the maintenance pipe  32  while the downstream second pump  229  is being driven, the maintenance solution is supplied from the maintenance tank  85  to the second sub-tank  72  through the common pipe  320 , the second branch pipe  322 , and the downstream second ink pipe  224 . 
     When the third three-way valve  238  is switched for connection to the maintenance pipe  32  while the downstream third pump  239  is being driven, the maintenance solution is supplied from the maintenance tank  85  to the third sub-tank  73  through the common pipe  320 , the third branch pipe  323 , and the downstream third ink pipe  234 . When the fourth three-way valve  248  is switched for connection to the maintenance pipe  32  while the downstream fourth pump  249  is being driven, the maintenance solution is supplied from the maintenance tank  85  to the fourth sub-tank  74  through the common pipe  320 , the fourth branch pipe  324 , and the downstream fourth ink pipe  244 . 
     &lt;1-4. Controller&gt; 
     The controller  40  is means for controlling the operation of each part in the inkjet printer  1 .  FIG. 3  shows connection between the controller  40  and each part in the inkjet printer  1 . The controller  40  of the preferred embodiment is configured using a computer. As shown in  FIG. 3 , the controller  40  includes a processor  41  such as a CPU, a memory  42  such as a RAM, and a storage  43  such as a hard disk drive. The storage  43  contains a computer program P for implementation of the operation of ejecting ink at the printer body  10 , the operation of supplying ink from the ink supply mechanism  20 , and the operation of supplying the maintenance solution from the maintenance solution supply mechanism  30 . 
     As shown in  FIG. 3 , the controller  40  is electrically connected to each of the foregoing transport mechanism  50 , first head  61 , second head  62 , third head  63 , fourth head  64 , upstream first pump  215 , first lower limit sensor  216 , first upper limit sensor  217 , first three-way valve  218 , downstream first pump  219 , upstream second pump  225 , second lower limit sensor  226 , second upper limit sensor  227 , second three-way valve  228 , downstream second pump  229 , upstream third pump  235 , third lower limit sensor  236 , third upper limit sensor  237 , third three-way valve  238 , downstream third pump  239 , upstream fourth pump  245 , fourth lower limit sensor  246 , fourth upper limit sensor  247 , fourth three-way valve  248 , and downstream fourth pump  249 . 
     The controller  40  reads the computer program P from the storage  43  temporarily onto the memory  42  and the processor  41  performs arithmetic processing on the basis of the read computer program P, thereby controlling the operation of each of the foregoing parts. This makes progress of the operations including ejection of ink at the printer body  10 , supply of ink from the ink supply mechanism  20 , and supply of the maintenance solution from the maintenance solution supply mechanism  30 . 
     &lt;2. Operation of Supplying Maintenance Solution&gt; 
     The following describes maintenance process of supplying the maintenance solution to the head  60  in the foregoing inkjet printer  1 . In the following description, a procedure taken for supplying the maintenance solution to the first head  61  will be described as an example by following the flowchart in  FIG. 4 . 
     First, while the first three-way valve  218  is in the state of forming connection to the first intermediary tank  213  (normal state), the controller  40  starts driving of the downstream first pump  219  (step S 1 ). In this state, the color K ink is supplied from the first intermediary tank  213  to the first sub-tank  71  through the downstream first ink pipe  214 . 
     While the downstream first pump  219  is kept driven, the controller  40  thereafter switches the first three-way valve  218  from the foregoing normal state to the state of forming connection to the maintenance tank  85  (maintenance state) (step S 2 ). By doing so, the maintenance solution is supplied from the maintenance tank  85  to the first sub-tank  71  through the common pipe  320 , the first branch pipe  321 , and the downstream first ink pipe  214 . 
     Next, the controller  40  drives the downstream first pump  219  on the basis of a signal from the level sensor (not shown in the drawings) provided at the first sub-tank  71 . More specifically, if the level sensor determines that a liquid level in the first sub-tank  71  has reduced to the reference level, the controller  40  drives the downstream first pump  219  until the liquid level recovers to the reference level. In response to this, the downstream first pump  219  supplies the maintenance solution to the first sub-tank  71 . When the liquid level has recovered to the reference level, driving of the downstream first pump  219  is stopped. In this way, the controller  40  drives the downstream first pump  219  intermittently (step S 3 ). In parallel with the intermittent driving of the downstream first pump  219 , a cap mechanism not shown in the drawings sucks a nozzle surface of the first head  61 . By doing so, the maintenance solution in the first head  61  is pushed out toward the cap mechanism. Such intermittent driving of the downstream first pump  219  continues for a predetermined length of time to replace the ink in the first sub-tank  71  and the first head  61  by the maintenance solution. In this way, each nozzle of the first head  61  is treated by means of cleaning, for example. 
     After passage of the predetermined length of time, the controller  40  switches the first three-way valve  218  from the foregoing maintenance state to the normal state while keeping the downstream first pump  219  driven (step S 4 ). After the first three-way valve  218  is switched to the normal state, the controller  40  stops driving of the downstream first pump  219  (step S 5 ). By doing so, supply of the maintenance solution from the maintenance solution supply mechanism  30  is stopped. 
     The maintenance solution may be supplied to the second head  62  by operating the second three-way valve  228  and the downstream second pump  229  by following a procedure similar to the foregoing procedure. The maintenance solution may be supplied to the third head  63  by operating the third three-way valve  238  and the downstream third pump  239  by following a procedure similar to the foregoing procedure. The maintenance solution may be supplied to the fourth head  64  by operating the fourth three-way valve  248  and the downstream fourth pump  249  by following a procedure similar to the foregoing procedure. 
     Supplies of the maintenance solution to the first to fourth heads  61  to  64  may proceed individually or simultaneously. 
     As described above, the inkjet printer  1  achieves supply of the maintenance solution instead of ink to each of the four heads  61  to  64  by means of switching of the three-way valve. The maintenance solution is supplied to each of the heads  61  to  64  from the common maintenance tank  85 . Specifically, while the inkjet printer  1  includes the four ink supply mechanisms  20 , it includes one maintenance solution supply mechanism  30 . This makes it possible to suppress size increase of the inkjet printer  1 , compared to a case where the maintenance solution supply mechanism  30  is provided individually for each head  60 . 
     In the inkjet printer  1 , the maintenance pipe  32  is connected to the ink pipes  214 ,  224 ,  234 , and  244  at positions downstream of the intermediary tanks  213 ,  223 ,  233 , and  243  respectively of the ink supply mechanisms  20 . This eliminates the need of replacement of the interiors of the intermediary tanks  213 ,  223 ,  233 , and  243  by the maintenance solution during supply of the maintenance solution. This allows reduction in the consumptions of the inks and the maintenance solution. 
     In particularly, the maintenance pipe  32  of the preferred embodiment includes the common pipe  320 , and the four branch pipes  321 ,  322 ,  323 , and  324  extending from the common pipe  320  toward the three-way valves  218 ,  228 ,  238 , and  248  respectively of the ink supply mechanisms  20 . Thus, even if the ink of a small amount flows backward from any of the three-way valves  218 ,  228 ,  238 , and  248  into a corresponding one of the branch pipes  321 ,  322 ,  323 , and  324 , this ink is unlikely to reach the common pipe  320 . This makes it possible to reduce the occurrence of mixture of ink colors through the common pipe  320 . 
     To reduce the probability of reach of the inks from the three-way valves  218 ,  228 ,  238 , and  248  to the common pipe  320  through the branch pipes  321 ,  322 ,  323 , and  324  respectively, each of the branch pipes  321 ,  322 ,  323 , and  324  preferably has a certain length. More specifically, each of the branch pipes  321 ,  322 ,  323 , and  324  preferably has a length of 30 mm or more. More preferably, each of the branch pipes  321 ,  322 ,  323 , and  324  has a length of 50 mm or more. Meanwhile, if each of the branch pipes  321 ,  322 ,  323 , and  324  is too long, a resistance is increased in a flow path along each of the branch pipes  321 ,  322 ,  323 , and  324 , Thus, the length of each of the branch pipes  321 ,  322 ,  323 , and  324  is preferably set so as not to exceed 100 mm, for example. 
     In the preferred embodiment, for switching of the three-way valves  218 ,  228 ,  238 , and  248  from the normal state to the maintenance state such as that in step S 2  described above, this switching is made while corresponding ones of the pumps  219 ,  229 ,  239 , and  249  are being driven. The pumps  219 ,  229 ,  239 , and  249  are kept driven until the three-way valves  218 ,  228 ,  238 , and  248  are switched again to the normal state like in steps S 3  to S 5 . By doing so, downstream suction force is applied at the time of switching of the three-way valves  218 ,  228 ,  238 , and  248  to reduce the occurrences of backflows of the inks themselves into the branch pipes  321 ,  322 ,  323 , and  324 . This makes it possible to reduce the occurrence of mixture of ink colors through the maintenance pipe  32  to a greater extent. 
     In the preferred embodiment, the fourth three-way valve  248  corresponding to the color Y is connected to the most upstream position of the maintenance pipe  32 . The third three-way valve  238  corresponding to the color M is connected to the position of the maintenance pipe  32  downstream of the fourth three-way valve  248 . The second three-way valve  228  corresponding to the color C is connected to the position of the maintenance pipe  32  downstream of the third three-way valve  238 . Further, the first three-way valve  218  corresponding to the color K ink is connected to the position of the maintenance pipe  32  downstream of the second three-way valve  228 . 
     The color K ink becomes easily noticeable when mixed with ink of a different color so exerts large influence on a printing result. The foregoing connection of the first three-way valve  218  corresponding to the color K to the most downstream position of the maintenance pipe  32  particularly functions to prevent mixture of the color K ink with different ink through the maintenance pipe  32 . Colors are ranked as follows in terms of the magnitude of influence caused by mixture with ink of a different color: color K &gt;color C &gt;color M &gt;color Y. In consideration of this, in the preferred embodiment, the three-way valves  218 ,  228 ,  238 , and  248  for the corresponding colors are connected to the maintenance pipe  32  in the foregoing order. This makes it possible to reduce the occurrence of mixture of ink colors. Further, even on the occurrence of mixture of ink colors, this still makes it possible to reduce influence on a printing result. 
     The foregoing order in which the three-way valves  218 ,  228 ,  238 , and  248  for the corresponding colors are connected to the maintenance pipe  32  is not always required to match the order in which the heads  60  are aligned at the printer body  10 . 
     &lt;3. Modifications&gt; 
     While the one preferred embodiment of the present invention has been described above, the present invention is not limited to the foregoing preferred embodiment. 
       FIG. 5  shows the configuration of a maintenance solution supply mechanism  30  according to a modification. In the illustration of  FIG. 5 , the maintenance pipe  32  includes a first maintenance pipe  32 A, a second maintenance pipe  32 B, a third maintenance pipe  32 C, and a fourth maintenance pipe  32 D. The first maintenance pipe  32 A is one pipe connecting the maintenance tank  85  and the first three-way valve  218 . The second maintenance pipe  32 B is one pipe connecting the maintenance tank  85  and the second three-way valve  228 . The third maintenance pipe  32 C is one pipe connecting the maintenance tank  85  and the third three-way valve  238 . The fourth maintenance pipe  32 D is one pipe connecting the maintenance tank  85  and the fourth three-way valve  248 . 
     As described above, in the illustration of  FIG. 5 , the four maintenance pipes  32  are arranged in parallel between the maintenance tank  85  and the three-way valves  218 ,  228 ,  238 , and  248 . Specifically, the maintenance pipe  32  in  FIG. 5  does not include a common pipe itself. This further reduces the risk of mixture of ink colors through the maintenance pipe  32 . 
     In the foregoing preferred embodiment, the color K ink, the color C ink, the color M ink, and the color Y ink are called the “first ink,” the “second ink,” the “third ink,” and the “fourth ink” respectively. However, the “first ink” of the present invention may be ink of a color other than the color K. Further, the “second ink” of the present invention may be ink of a color other than the color C. 
     In the foregoing preferred embodiment, the four heads  60  are provided in the printer body  10 . Meanwhile, the number of the heads  60  in the printer body  10  may be one, two or three, or five or more. For example, a head for ejection of ink of a spot color may be provided in addition to heads corresponding to the colors K, C, M, and Y. The ink supply mechanism  20  may be provided for each of ink colors to be used. 
     The ink used in the present invention may either be water-based ink or oil-based ink. The ink used in the present invention may be ink to be dried by evaporation of a solvent, or may be photo-curing ink to be cured by irradiation with light such as ultraviolet light. 
     The foregoing inkjet printer  1  is to print an image on the printing paper  9  as a printing medium. However, the inkjet printer of the present invention may be configured to print an image on a sheet-like printing medium other than general paper (for example, a resin film, metal foil, or glass). 
     The components described in the foregoing preferred embodiment and in the modifications may be combined together, as appropriate, without inconsistencies. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.