Patent Publication Number: US-9421784-B2

Title: Liquid ejecting apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of, and claims priority under 35 U.S.C. §120 on, U.S. application Ser. No. 12/862,238, filed Aug. 24, 2010, which claims priority under 35 U.S.C. §119 on Japanese Patent Application No. 2009-200904, filed Aug. 31, 2009. The content of each application identified above is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to a liquid ejecting apparatus including liquid ejecting heads ejecting a liquid such as ink. 
     2. Description of Related Art 
     In the past, as this kind of liquid ejecting apparatus, an ink jet printer (hereinafter, simply referred to as a “printer”) disclosed in JP-A-11-342634 was suggested. The printer disclosed in JP-A-11-342634 includes a plurality of head units (printing heads) as liquid ejecting heads ejecting ink as a liquid to a target such as a print sheet and also includes an ink tank and a sub-tank storing the ink to supply the head units. A purge operation of removing bubbles or solid matter in the ink from the head unit is performed by pressurizing the ink tank by the driving of an air pump, supplying the ink from the ink tank to each head unit via a circulation forward passage of the ink, and storing some of the ink, which is not discharged by each head unit, in the sub-tank via a circulation backward passage. After the purge operation, the ink temporarily stored in the sub-tank is returned to the ink tank and is reused. 
     SUMMARY OF INVENTION 
     An advantage of some aspects of the invention is that it provides an improved liquid ejecting apparatus with passages described below. 
     According to an aspect of the invention, there is provided a liquid ejecting apparatus that includes liquid ejecting heads ejecting a liquid. The liquid ejecting apparatus includes a tank that stores the liquid; a first passage through which the liquid flows from the tank to the liquid ejecting heads; and a second passage through which the liquid flows from the liquid ejecting heads to the tank. The first and second passages define a path along which the liquid flows between the tank and the liquid ejecting heads. The liquid ejecting heads includes a first head and a second head. The first passage includes a third passage connecting to the tank, a fourth passage branching from the third passage at a first position to allow the third passage to communicate with the first head via the fourth passage, and a fifth passage branching from the third passage at a second position to allow the third passage to communicate with the second head via the fifth passage. A passage length from the tank to the first position is shorter than a passage length from the tank to the second position. The fourth passage and the fifth passage are of the same length and cross-sectional area. A cross-sectional area of the third passage is larger than a cross-sectional area of the fourth passage. A passage resistance from the tank to the first position and a passage resistance from the tank to the second position are substantially the same. A passage resistance of the fourth passage is much larger than a passage resistance from the tank to the first position. 
     According to the aspect of the invention, a difference in the flow rate of the liquid supplied during a liquid ejecting operation between the liquid ejecting heads can be kept small. Moreover, a difference in the pressure of the liquid between the liquid ejecting heads can be kept small. 
     According to the aspect of the invention, an advantage can be obtained in that the difference in the pressure of the liquid between the liquid ejecting heads can be kept small, while the difference in the flow rate of the liquid supplied during a liquid ejecting operation between the liquid ejecting heads can be kept small. 
     The liquid ejecting apparatus according to the above aspect of the invention may further include a heating unit heating the liquid to supply heated liquid to the liquid ejecting heads. A flow rate of the liquid supplied to the first head via the fourth passage is set to be greater than a maximum flow rate of the liquid ejected by the first head. 
     According to the aspect of the invention, the liquid can be prevented from flowing backward due to an insufficient supply of liquid. Therefore, it is possible to prevent the temperature of the liquid in the liquid ejecting head from becoming unstable as a result of the cooled liquid flowing backward into the liquid ejecting head again. As a consequence, since the temperature of the liquid in each liquid ejecting head is kept at a necessary heating temperature, a stable ejection performance can be ensured for each liquid ejecting head. 
     The liquid ejecting apparatus according to the above aspect of the invention may further include a depressurizing unit depressurizing the tank. The depressurizing unit may depressurize the tank during an ejection operation of the first head. 
     According to the aspect of the invention, stable ejection performance can be ensured, while preventing excessive ejection of the liquid from the first head or the leakage of the liquid. 
     In the liquid ejecting apparatus according to the above aspect of the invention, the depressurizing unit may be controlled so that a pressure of the tank becomes a depressurization value corresponding to a flow rate of the liquid circulated in the sixth passage. 
     According to the aspect of the invention, since the pressure of the tank is controlled to the reduced pressure value in accordance with the flow rate of the liquid circulated in the sixth passage by the depressurizing unit, the liquid pressure of the liquid ejecting head can be maintained at a stable value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a schematic view illustrating a printer according to an embodiment. 
         FIG. 2  is a block diagram illustrating the electric configuration of the printer. 
         FIG. 3  is a schematic side view illustrating an ink supply system including a sub-tank and printing head. 
         FIG. 4  is a schematic side sectional view illustrating a first heating device. 
         FIG. 5  is a schematic top sectional view illustrating a second heating device of which some constituent elements are removed. 
         FIG. 6  is a sectional view illustrating the second heating device taken along the line VI-VI of  FIG. 5 . 
         FIG. 7  is schematic sectional view illustrating the second heating device taken along another direction different from that of  FIG. 6 . 
         FIG. 8  is a partially exploded sectional view schematically illustrating the printing head in which a temperature keeping device is installed. 
         FIG. 9  is a flowchart illustrating the routine of ink supply control. 
         FIG. 10  is a flowchart illustrating the routine of a first cleaning process. 
         FIG. 11  is a flowchart illustrating the routine of a second cleaning process. 
         FIG. 12  is schematic view illustrating a part of a printer according to a modified example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described with reference to  FIGS. 1 to 11 . 
     As shown in  FIG. 1 , an ink jet printer (hereinafter, abbreviated to a “printer  11 ”), which is an example of a liquid ejecting liquid, includes a printing section  12  that performs printing on a target (a film or the like) (not shown) with UV (ultraviolet) ink (ultraviolet curing ink), which is an example of a liquid. The printer  11  according to this embodiment is provided with a radiation unit (not shown) radiating ultraviolet rays to the target subjected to the printing by the printing section  12  and curing the UV ink landed to the target. The UV ink contains a pigment component having low dispersion stability and has a property in which the pigment component readily settles down. 
     The printing section  12  includes a holder unit  14  mounted with an ink cartridge  13  storing the UV ink and a main tank  15  having a substantially cylindrical shape with a bottom portion and disposed below the holder  14  in the direction of gravity. In the holder unit  14 , a hollow ink supply needle  17  detached from and mounted on a extraction unit  16  of the ink cartridge  13  is mounted at the mount position indicated by a two-dot chain line in  FIG. 1 . A first ink supply pipe  18  having an upstream end  18   a  communicating with the inside of the ink supply needle  17  is connected to the holder unit  14 , and a downstream end  18   b  of the first ink supply pipe  18  is disposed in the main tank  15 . The main tank  15  is configured so that the allowable storage amount of UV ink is sufficiently larger than that of the UV ink stored in the ink cartridge  13 . On the side wall of the main tank  15 , a plurality (in this embodiment, two liquid level sensors) of main-side liquid level sensors  19  and  20  are disposed to detect the liquid level of the UV ink remaining in the main tank  15  on the basis of the location of the liquid level A 1  of the UV ink. The main-side liquid level sensors  19  and  20  are disposed at different positions in the direction of gravity. 
     In the printer  12 , a churning device  21  is disposed to churn the UV ink stored in the main tank  15 . The churning device  21  includes a churning motor  22  supplying a driving source, a shaft member  23  rotated by the driving of the churning motor  22 , and a plurality of blade members  24  disposed in the front end (the lower end in  FIG. 1 ) of the shaft member  23 . 
     The printing section  12  includes a sub-tank  25 , for which the allowable storage amount of UV ink is smaller than that of the main tank  15 , a first liquid supply unit  26  which supplies the UV ink from the main tank  15  to the sub-tank  25 . The first liquid supply unit  26  includes: a second ink supply pipe  27  having an upstream end  27   a  disposed inside the main tank  15  and a downstream end  27   b  connected to the sub-tank  25 ; and first pump  29  sucking the UV ink stored in the main tank  15  by the driving of a first driving motor  28  and discharging the UV ink to the sub-tank  25 . A first on-off valve (for example, an electromagnetic valve)  30  operated to permit or regulate flow of the UV ink between the tanks  15  and  25  is disposed in the second ink supply pipe  27  on the side closer to the sub-tank  25  than the first pump  29 . 
     The sub-tank  25  includes a tank main body forming a bottom cylinder and a cover covering the opening of the tank main body. On the side wall of the sub-tank  25 , a sub-tank liquid level sensor  31  is disposed to detect the remaining amount of UV ink temporarily stored in the sub-tank  25 . An ON signal is output from the sub-tank liquid level sensor  31 , when a liquid level A 2  of the UV ink in the sub-tank  25  is located at the same position as or higher than the position at which the sub-tank liquid level sensor  31  is installed. In the sub-tank  25 , a first temperature sensor  32  is installed to detect the temperature of the UV ink stored in the sub-tank  25  and a sub-tank heater  33  is disposed to heat the UV ink. A pressure adjusting device  34  increasing or decreasing the pressure of the sub-tank  25  is connected to the sub-tank  25 . 
     The pressure adjusting device  34  includes: a second pump  36  which is driven to pressure-feed the gas in the sub-tank  25  by a second driving motor  35  so as to pressurize the inside of the sub-tank  25 ; and a second on-off valve (for example, an electromagnetic valve)  37  which is opened when the second pump  36  is driven and which is closed when the second pump  36  is not driven. The pressure adjusting device  34  further includes: a third pump  39  which is driven to discharge the gas from the sub-tank  25  by a third driving motor  38  so as to depressurize the inside of the sub-tank  25 ; and a pressure opening plate  40  which is opened to the air until the pressure is increased to the pressure set in the sub-tank  25 . Moreover, the pressure adjusting device  34  includes a third on-off valve (for example, an electromagnetic valve)  41  which is opened when at least one of the third pump  39  and the pressure opening plate  40  is driven and which is closed when none of the third pump  39  and the pressure opening plate  40  are driven. 
     An ink ejecting unit  42  ejecting the UV ink toward the target is disposed in the printing section  12 . The ink ejecting unit  42  includes a plurality (in this embodiment, four printing heads) of printing heads  43  (a liquid ejecting head (liquid ejecting unit)). Each of the printing heads  43  appropriately ejects the UV ink supplied to the inside of the printing head  43  from nozzles. Each of the printing heads  43  includes a second temperature sensor  44  detecting the temperature of the UV ink supplied to the inside of the printing head  43  and a head heater  45  keeping the temperature of the UV ink therein. 
     The UV ink stored in the sub-tank  25  is supplied to each printing head  43  via a second liquid supply unit  46 . The second liquid supply unit  46  includes a third ink supply pipe  47  (supply passage) having an upstream end  47   a  disposed in the vicinity of the bottom portion of the sub-tank  25 . The third ink supply pipe  47  includes: one common pipe  47   b  on the upstream side; and a plurality (in this embodiment, four connection pipes) of connection pipes  48  (connection passages) branching in parallel from the common pipe  47   b  and disposed on the downstream side so as to be connected to the printing heads  43  respectively and to correspond to the printing heads  43  individually. In the third ink supply pipe  47 , a fourth pump  50  is disposed to suck the UV ink from the sub-tank  25  and send the UV ink to the printing heads  43  by the driving of a fourth driving motor  49 . A fourth on-off valve (for example, an electromagnetic valve)  51  operated to allow and regulate flow of the UV ink from the sub-tank  25  to the printing heads  43  and a damper  52  attenuating pulsation of the UV ink supplied through the fourth pump  50  are disposed in the third ink supply pipe  47  closer to the printing heads  43  than the fourth pump  50 . As the first to fourth pumps, a reciprocating pump such as a diaphragm pump, a tube pump, a piston pump, or a plunger pump may be used, or a rotary pump such as a gear pump, a vane pump, or a screw pump may be used. 
     Each connection pipe  48  is designed to have a passage cross-section area S 2  smaller than a passage cross-section area S 1  of the common pipe  47   b . The UV ink flowing inside each connection pipe  48  is heated by a supply passage heater  54  controlled on the basis of a signal detected by a third temperature sensor  53 . 
     A plurality (in this embodiment, four ink circulation pipes) of ink circulation pipes  55  corresponding to the printing heads  43  individually are disposed between the printing heads  43  and the sub-tank  25 . Each of the upstream ends  55   a  of the ink circulation pipes  55  is connected to each of the printing heads  43 . Each of downstream ends  55   b  of the ink circulation pipes  55  is disposed inside the sub-tank  25 . The ink circulation pipes  55  have a passage cross-section area S 3  smaller than the passage cross-section area S 1  of the common pipe  47   b  and larger than the passage cross-section area S 2  of the connection pipe  48  (where S 1 &gt;S 3 &gt;S 2 ). A fifth on-off valve (for example, an electromagnetic valve)  56  operated to allow or regulate the flow of the UV ink from each printing head  43  to the sub-tank  25  is disposed in each ink circulation pipe  55 . 
     The printing section  12  includes a transport unit (not shown) transporting the target. The printing is performed on the target by ejecting the UV ink by the printing heads  43  to the target transported by the transport unit. The transport unit includes a known transport mechanism such as a roller type transport mechanism, a belt type transport mechanism, or a rotation drum type transport mechanism and a transport motor  57  (see  FIG. 2 ). The transport unit transports the target, when the transport mechanism is driven by the transport motor  57  (see  FIG. 2 ). 
     The printer  11  having the above-described configuration is operated as follows. That is, the ink cartridge  13  is disposed at a standby position at which the ink supply needle  17  is not inserted into the extraction unit  16 . When the liquid level A 1  of the UV ink in the main tank  15  is lowered and the ON state of the first main tank liquid level sensor  19  disposed above becomes the OFF state, a detachable motor is driven on the basis of a control instruction from a control device  60 , which is described below. Then, a press member as a pressurizing device (not shown) disposed above the holder unit  14  moves the ink cartridge  13  disposed at the standby position downward against the urging force of an urging unit. As a result, the ink cartridge  13  is mounted on the holder unit  14  disposed at the mount position into which the ink supply needle  17  is inserted. The UV ink stored in the ink cartridge  13  is taken out to the main tank  15  via the ink supply needle  17  and the first ink supply pipe  18 . At this time, in the main tank  15 , the UV ink is churned by the churning device  21  for a predetermined time. 
     The control device  60  of the printer  11  measures an amount of ink consumed by the printing heads  43 . Therefore, when it is determined on the basis of the measurement result that a predetermined amount of UV ink in the sub-tank  25  is consumed from the state of the liquid level A 2  where the sub-tank liquid level sensor  31  is turned on, the first pump  29  is driven to supply the UV ink from the main tank  15  to the sub-tank  25 . When the liquid level A 2  of the UV ink in the sub-tank  25  is raised and the OFF state of the sub-tank liquid level sensor  31  becomes the ON state, the control device  60  stops driving the first pump  29  and stops supplying the UV ink from the main tank  15  to the sub-tank  25 . 
     By driving the fourth pump  50  while depressurizing the sub-tank  25  by the pressure adjusting device  34  upon the printing, the UV ink is supplied from the sub-tank  25  to the printing heads  43  via the third ink supply pipe  47 , the UV ink flows from the printing heads  43  to the ink circulation pipes  55 , and then the UV ink flows back to the sub-tank  25 . The ink is supplied to the printing heads  43  by circulating the ink through the third ink supply pipe  47  and the ink circulation pipes  55  between the sub-tank  25  and the printing heads  43 . The ink stored in the sub-tank  25  is gradually decreased by the amount of ink consumed by ejecting the ink from the nozzles of the printing heads  43 . 
     In the printer  11 , temperature control is executed in such a manner that the UV ink in the sub-tank  25  and the third ink supply pipe  47  is heated by the sub-tank heater  33  and the supply passage heater  54 , respectively and the temperature of the UV ink supplied in the heated state is kept in the printing heads  43  by the head heaters  45 . In the printer  11 , a first cleaning operation is performed to remove the bubbles of the ink in the printing heads  43 . Moreover, a second cleaning operation is performed to prevent and solve the clogging of the nozzle of the printing heads  43 . 
     The printing section  12  is configured to eject the UV ink of plural colors to the target. The printing section  12  includes the holder unit  14 , the tanks  15  and  25 , and printing units including ink ejecting units  42  for respective colors. In this embodiment, however, only a printing unit for a mono-color (for example, white) will be described. The description of the printing units for other colors is omitted for easy understanding of the specification. In the following description, the UV ink is also just referred to as the ink. 
     Next, the electric configuration of the printing section  12  according to this embodiment will be described with reference to  FIG. 2 . As shown in  FIG. 2 , the printer  11  includes the control device  60  controlling an ink supply system and a printing system as a whole. In an input/output interface of the control device  60 , the first main tank liquid level sensor  19 , the second main tank liquid level sensor  20 , the sub-tank liquid level sensor  31 , and a pressure sensor  58  detecting pneumatic pressure of the sub-tank  25  are electrically connected as sensors of the ink supply system. In the input/output interface of the control device  60 , the first temperature sensor  32 , the four second temperature sensors  44 , and the third temperature sensor  53  are electrically connected as heating control sensors. 
     In the input/output interface of the control device  60 , the four printing heads  43  and the transport motor  57  are electrically connected as control targets of the printing system. In the input/output interface of the control device  60 , the first driving motor  28 , the second driving motor  35 , third driving motor  38 , and the fourth driving motor  49  for driving the pumps; the first on-off valve  30 , the fourth on-off valve  51 , and the four fifth on-off valves  56  for opening and closing the passage; and the second on-off valve  37 , the third on-off valve  41 , and the pressure opening plate  40  forming the pressure adjusting device  34  are electrically connected as control targets of the ink supply system. 
     In the input/output interface of the control device  60 , the sub-tank heater  33  for heating the ink, the supply passage heater  54  for heating the ink, and the four head heaters  45  for keeping the temperature of the ink are electrically connected. 
     The control device  60  further includes a computer  61  (microcomputer) executing a variety of control on the basis of the detection results input from the sensors  19 ,  20 ,  31 ,  32 ,  44 ,  53 , and  58 , a head drive controller  62  controlling the driving of the printing heads  43 , a motor drive controller  63  controlling the driving of the motors  57 ,  22 ,  28 ,  35 ,  38 , and  57 , a plate drive controller  64  controlling the on-off valves  30 ,  37 ,  41 ,  51 , and  56  and the pressure opening plate  40 , and a heater drive controller  65  controlling the heating of the heaters  33 ,  45 , and  54 . 
     The control device  60  controls a print operation, a transport operation, a pump operation, a plate drive operation, a heating operation, and the like, when the computer  61  gives control detail (instruction value) instruction to the drive controllers  62  to  65 . Here, the computer  61  includes a CPU  67 , a ROM  68 , and a RAM  69 . The ROM  68  stores program data used for the CPU  67  to perform a variety of control and various data including setting values used to perform the variety of control. The RAM  69  temporarily stores the calculation results and the like of the CPU  67 . Some areas of the RAM  69  are used as a buffer developing print data input from a host apparatus (not shown), for example. The drive controllers  62  to  65  are structured by ASIC (Application Specific Integrated Circuit) and various drive circuits, and the like. A plurality of the CPUs  67  may be installed to control a printing system (transport system and ejection system), the ink supply system, and a heating system, individually. 
     For example, the computer  61  performs duty control to control the amount of ink ejected from the nozzles of the printing head  43  by instructing a duty value D corresponding to the amount of ink ejected by the head drive controller  62 . At this time, the duty value D instructed by the computer  61  is varied in the range from 0% to 100%. The amount of ink ejected (which is equal to the amount of ink ejected per one ejection) is increased nearly in proportion to an increase in the duty value (%). When ink droplets are ejected from all of the nozzles in each ejection period by instructing the duty value of 100% (FULL duty) to all of the printing heads  43 , the amount of ink (ink ejection rate Qh) ejected from the nozzles of the printing heads  43  per unit time becomes the maximum. 
     In the printer  11  according to this embodiment, the first cleaning operation is performed to remove the bubbles in the ink of the printing heads  43  using the ink circulation flow and the second cleaning operation (nozzle cleaning operation) is performed to prevent and solve the clogging of the nozzles by forcibly discharging the ink from the nozzles  84  (see  FIG. 8 ) of the printing heads  43 . 
     For example, when the same ink (for example, the same color ink) of the ink cartridge  13  is consumed and thus the ink cartridge  13  is replaced, bubbles in ink may be mixed therein via the ink supply needle  17  upon mounting the ink cartridge  13  on the holder unit  14 . Alternatively, when the ink cartridge  13  is replaced by a new ink cartridge of another ink (for example, different color ink), all of the ink in the tanks and the passages is replaced with the different color ink, and an initial filling operation is performed to fill the passages with the different color ink. Alternatively, a gas may penetrate in portions, in which the resin tube is used in the ink supply pipes  18 ,  27 , and  47  and the ink circulation pipes  55 , and the air dissolved in the ink of the passages may become the bubbles when the printer  11  is not used for a long time. When the ink cartridge is replaced, the initial filling is performed, or the printer is not used for a long time, bubbles may gather at the corners of an area on the upstream side of a filter  83  (see  FIG. 8 ) in the printing head  43  or bubbles are captured in the filter  83 . For this reason, the first cleaning operation is performed mainly to remove the bubbles in the ink of the printing head  43  as a whole using the ink circulation flow. That is, the computer  61  shown in  FIG. 2  performs the first cleaning operation, when measurement time T of an internal timer measuring elapsed time from the detection time of replacement of the ink cartridge, the detection time of the initial filling, or the end time of the previous second cleaning operation reaches first cleaning time T 1 . 
     The second cleaning operation is performed to prevent and solve the clogging of the nozzles of the printing head  43 , when a cleaning instruction is received by the operation of a user or the cleaning operation is scheduled to be performed. That is, the computer  61  shown in  FIG. 2  performs the second cleaning operation, when the cleaning instruction is received by the operation of a user or the measurement time T of the internal timer measuring the elapsed time from the end time of the previous second cleaning operation reaches the second cleaning time T 2 . 
     The second cleaning operation is performed by driving the second pump  36  (pressurizing pump) and pressurizing an air chamber  25   a  in the sub-tank  25  to pressurize the ink of the sub-tank  25 , supplying the ink in the pressurized state from the sub-tank  25  to the printing heads  43  via the ink circulation pipes  55 , and forcibly discharging the ink from the nozzles of the printing heads  43 . Therefore, the second cleaning operation is performed in two steps: a step (pressurizing step) of closing the passages of the ink circulation pipes  55  and accumulating the ink pressure on the upstream side including the sub-tank  25 ; and a step (a valve opening step) of opening the passages of the ink circulation pipes  55  at a time at which the ink pressure is accumulated up to a target value and flowing the ink to the downstream side at once to forcibly discharge the ink from the nozzles  84 . 
     That is, in the pressure accumulation step of the second cleaning operation, the second pump  36  (pressurizing pump) is driven to pressurize the ink of the sub-tank  25  in the state where the on-off valves  30 ,  41 ,  51 , and  56  are closed and the on-off valve  37  is opened and the driving of the pumps  29 ,  39 , and  50  is also stopped. 
     In this embodiment, there is used a selection cleaning operation of selecting M (where 1≦M&lt;N) printing heads  43  to be subjected to the second cleaning operation among all (N) of the printing heads  43  and cleaning only the selected M printing heads  43 . In the printer  11 , a nozzle inspection device (not shown) capable of inspecting the clogging of the nozzles is installed in each printing head  43 . Only the printing heads  43  determined as requiring cleaning on the basis of the inspection result of the nozzle inspection device become cleaning targets. 
     For example, the strengths of the plural-phase steps are prepared in the second cleaning operation. When the second cleaning operation is repeatedly instructed by the operation of a user, a strong cleaning operation is selected due to an increase in the number of operations and a strong cleaning operation is selected on the basis of a long elapsed time from the time at which the previous cleaning operation is performed. In the pressure accumulation step, the control device  60  starting the driving of the second pump  36  determines that the pressure accumulation step ends, when the pressure sensor  58  detects the pressure (pneumatic pressure) of the air chamber  25   a  and the pressure reaches a target pressure corresponding to the selected cleaning strength. After the pressure accumulation step ends, the second cleaning operation is performed only on the M printing heads  43  by opening only the on-off valves  56  of the ink circulation pipes  55  connected to the M printing heads  43  determined as requiring cleaning on the basis of the detection results of the nozzles among the N on-off valves  56 . 
     In this embodiment, the passage resistances of the third ink supply pipe  47  and the ink circulation pipes  55  are set as follows. That is, a passage resistance R (≅R 2 &gt;R 1 ) of the third ink supply pipe  47  (supply passage) and a passage resistance R 3  of the ink circulation pipes  55  are set to satisfy a relation of R&lt;R 3 . Therefore, the amounts of ink supplied to the printing heads  43  can be made equal to each other. Moreover, the ink pressures of the printing heads  43  can be kept low, while a difference between the ink pressures of the printing heads  43  is kept small. As a consequence, the ink from the nozzles of each printing head  43  can be prevented from leaking during the printing. Moreover, an appropriate amount of ink droplets can also be ejected since the ink pressure in each printing head  43  falls within an allowable range. 
     A passage resistance R 1  of the common pipe  47   b  of the third ink supply pipe  47 , a passage resistance R 2  of the connection pipe  48 , and the passage resistance R 3  of the ink circulation pipe  55  are set to satisfy a relation of R 1 &lt;R 3 &lt;R 2 . Therefore, the amounts of ink supplied to the printing heads  43  can be made equal to each other. Moreover, the ink pressures of the printing heads  43  can be kept low, while the difference between the ink pressures of the printing heads  43  is kept small. 
     Here, among the passage resistances R 1 , R 2 , and R 3 , the passage resistance R 1  of the common pipe  47   b  is set to be the smallest and the passage resistance R 2  of the connection pipe  48  is set to be largest. Then, the ink pressures can be made nearly equal to each other in the entrances of the connection pipes  48  on the common pipe  47   b . By setting the passage resistance R 2  of each connection pipe  48  to be very large, the amounts of ink supplied to the printing heads  43  can be made nearly equal to each other. The ink pressure has a tendency to be increased in the printing head  43 , as the passage resistance R 3  of the ink circulation pipe  55  is large. However, since the passage resistance R 3  of the ink circulation pipe  55  is set to be small, the ink pressure of the printing head  43  can be kept low. At this time, since ink supply rates Qin to the printing heads  43  are nearly equal to each other and ink ejection rates Qh from the printing heads  43  are different from each other, ink circulation rates Qout (=Qin−Qh) become different from each other between the printing heads  43 . However, since the passage resistance R 3  of the ink circulation pipes  55  is set to be small, a pressure loss P 3 loss (=Qout·R 3 ) of the ink circulation pipe  55 , which is calculated by a product of the ink circulation rate Qout and the passage resistance R 3 , has a small value. Therefore, since the pressure losses P 3 loss of the printing heads  43  are considered to be nearly equal to each other, the ink pressures of the printing heads  43  can be made nearly equal to each other between the printing heads  43 . 
     The reason for allowing the passage resistance R 1  of the common pipe  47   b  and the passage resistance R 3  of the ink circulation pipe  55  to satisfy the relation of R 1 &lt;R 3  is that the passage resistance R 3  of the ink circulation pipe  55  is made as small as possible. Above all, at least upon the printing, the ink circulation rate Qout is smaller than the ink supply rate Qin since the ink is ejected by the printing heads  43 . Therefore, the diameter of the ink circulation pipe  55  is designed to be small by the amount of ink ejected by the printing heads  43 , and thus the ink circulation pipes  55  are designed to be miniaturized. Moreover, it is necessary to allow the variation in the ink pressure of the printing head  43  within ±50 Pa. Therefore, the passage resistance R 3  of the ink circulation pipe  55  is determined so that the variation in the ink pressure of the printing head  43  falls within ±50 Pa in a range of the variation in the ink circulation rate Qout between the maximum printing time and non-printing time. 
     Since it is necessary to allow the variation in the ink pressure in a given printing mode to fall within ±50 Pa, the passage resistance R 2  of the connection pipe  48  is set to be five or more times the passage resistance R 3  of the ink circulation pipe  55  (where R 2 ≧5·R 3 ). Therefore, even when the printing is performed in the given printing mode, the variation in the ink pressure of the printing head  43  falls within ±50 Pa. Accordingly, the amount of ink ejected from the nozzles of the printing head  43  can be stabilized. 
       FIG. 3  is a schematic view illustrating the ink supply system including the sub-tank and the printing head. As shown in  FIG. 3 , the sub-tank  25  is disposed above the printing head  43  in the direction of gravity. In this embodiment, the printing head  43  has no pressure adjusting valve. Therefore, the ink pressure of the nozzles  84  of the printing head  43  is adjusted using a liquid head difference H which is a distance between the height of the liquid level A 2  in the sub-tank  25  and the surface height Anozl of an ink meniscus in the nozzles of the printing head  43 . 
     Here, the ink pressure of the nozzles  84  of the printing head  43  is influenced not only by the liquid head difference H between the liquid level A 2  in the sub-tank  25  and the surface height Anozl of the ink meniscus in the nozzles but also by the passage resistance of the ink flowing in the passages including the third ink supply pipe  47  and the ink circulation pipe  55  and the ink pressure of the sub-tank  25 . In this embodiment, therefore, the ink pressure of the ink meniscus in the nozzles  84  of the printing head  43  is adjusted so as to have an appropriate value by controlling the air chamber  25   a  in the sub-tank  25  by the negative pressure to set the pressure of the sub-tank  25  to be negative by the pressure adjusting device  34 . 
     In the printing head  43 , the pressure chamber (not shown) communicating with the nozzle  84  (see  FIG. 8 ) is provided in each nozzle. Therefore, when a pressure generating element disposed in each nozzle on the opposite side of the nozzle in the pressure chamber is driven, the pressure chamber is expanded and contracted. The ink sucked to the pressure chamber upon expanding the pressure chamber is ejected from the nozzle  84  upon contracting the pressure chamber. At this time, the surface height Anozl of the ink meniscus in the nozzle  84  is determined depending on the ink pressure (that is, the ink pressure in the range of the nozzles) of the pressure chamber. In order to keep the ink ejection performance stable, the surface height Anozl of the ink meniscus has to be maintained at an appropriate position in the nozzle  84 . For example, when the surface of the ink meniscus in the nozzle  84  is located inside the nozzle due to the fact that the ink pressure of the pressure chamber is too low, the amount of ink ejected may be insufficient or ejection mistakes may easily occur. Alternatively, when the surface of the ink meniscus in the nozzle protrudes in a circular-surface form from the nozzle opening due to the fact that the ink pressure of the pressure chamber is too high, the amount of ink ejected is excessive or leakage of ink from the nozzle may occur. In this embodiment, therefore, ink supply control is performed so that the ink pressure of the ink meniscus is kept at an appropriate value. 
     Hereinafter, the ink supply control will be described with reference to  FIG. 3 . Here, the liquid head difference H (liquid surface height difference) between the surface height Anozl of the ink meniscus in the nozzle and the passage resistance R 1  of the common pipe  47   b , the passage resistance R 2  of the connection pipe  48 , the passage resistance R 3  of the ink circulation pipe  55 , and the liquid level A 2  of the ink in the sub-tank  25  is set to a negative pressure value Pdec (depressurization value) of the sub-tank  25 . 
     In this example, the amount of ink supplied from the fourth pump  50  (supply pump) is constant at 20 N (cc/minute). Here, P(H) denotes the pressure generated by the liquid head difference H, P 1 loss denotes a pressure loss caused by the passage resistance R 1  of the common pipe  47   b , P 2 loss denotes a pressure loss caused by the passage resistance R 2  of the connection pipe  48 , and P 3 loss denotes a pressure loss caused by the passage resistance R 3  of the ink circulation pipe  55 . As for the pressure loss P 3 loss, the ink circulation rate Qout of the ink circulation pipe  55  varies in accordance with the duty value D and the ink circulation rate Qout can be represented by a function Qout(D) of the duty value D. The pressure loss P 3 loss can also be represented by a function P 3 loss(D) (=R 1 ·Qout(D)) of the duty value D. 
     An ink pressure Ph of the ink meniscus in the nozzle of the printing head  43  can be expressed as Ph=P(H)+Pdec−P 1 loss−P 2 loss+P 3 loss(D). The pressure is expressed as a positive pressure (&gt;0) and a negative pressure (&lt;0) on the assumption that 1 atmosphere pressure is “0”. Since Pdec is a negative pressure, a relation of Pdec&lt;0 is satisfied. 
     In this embodiment, in order to maintain the ink pressure Ph at a target value appropriate for the ink ejection, the third pump  39  (depressurizing pump) and the pressure opening plate  40  are controlled so that the pressure of the air chamber  25   a  becomes a target negative pressure value Pdectrg in response to the pressure loss P 3 loss(D) (=Qout(D)·R 3 ) varying in accordance with the ink circulation rate Qout(D). The target negative pressure value Pdectrg is expressed as Expression of Pdectrg=Po·Ph(H)−P 3 loss(D). Here, Po is an integer expressed as Po=Phtrg+P 1 loss+P 2 loss on the assumption that the target value of Ph is Phtrg. 
     The ink ejection rate Qh of the printing head  43  is varied depending on a printing mode, even when the duty value D is the same. Therefore, the printing mode is taken into consideration, when the target negative pressure value Pdectrg is requested. Examples of the printing mode include a high-speed printing mode), where print speed is preferred over a print quality, and a low-speed printing mode (high-quality printing mode, where print quality is preferred over printing speed. In the high-speed printing mode, the ink ejection rate Qh (cc/minute) is larger than that of the low-speed printing mode due to the fact that the printing speed is high, even when the same image is printed. Therefore, each function P 3 loss(D) is prepared for both the high-speed printing mode and the low-speed printing mode in the ROM  68 . In addition, the target negative pressure value Pdectrg is calculated using the above expression to which the function P 3 loss(D) is applied in accordance with the printing mode read from the ROM  68 . 
     In this embodiment, the computer  61  of the control device  60  calculates the target negative pressure value Pdectrg using the expression of Pdectrg=Po−Ph(H)−P 3 loss(D) on the basis of the liquid head difference H determined from the printing mode and the liquid surface height Hsub of the liquid level A 2  of the ink in the sub-tank  25  and the duty value D for the control of the printing head. Then, the computer  61  controls the third driving motor  38  for the third pump  39  (depressurizing pump) and the pressure opening plate  40  so that a real negative pressure value Pdet detected by the pressure sensor  58  is identical to the target negative pressure value Pdectrg. 
     On the assumption that h is a distance from the liquid surface height Hsub and the inner bottom surface of the sub-tank  25  to the height (≅the surface height Anozl of the ink meniscus) of the nozzle opening, the liquid head difference H is calculated by the expression of H=Hsub+h. Here, the liquid surface height Hsub is calculated by the expression Hsub=Hsubo+ΔA, using the given liquid surface height Hsubo, obtained when the liquid level is detected by the sub-tank liquid level sensor  31 , as a reference and a liquid level variation ΔA of the sub-tank  25  after the detection. The liquid level variation ΔA is calculated by dividing the amount of ink supplemented from the first pump  29  to the sub-tank  25  after the detection of the sub-tank liquid level sensor  31  and the amount of ink varied in the sub-tank  25  obtained by the measurement result of the amount of ink ejected and consumed by the printing head  43  or the calculation result by the cross-section area parallel to the liquid level of the sub-tank  25 . Of course, a liquid level sensor detecting the amount of liquid of the sub-tank  25  is provided to calculate the liquid surface height Hsub on the basis of a detection value of the liquid level sensor. 
     For example, when a print amount is small and the duty value D is relatively small, the ink circulation rate Qout becomes large. When the print amount is large and the duty value D is relatively large, the ink circulation rate Qout becomes small. When the ink circulation rate Qout is large, the pressure loss P 3 loss determined by the product of the passage resistance R 3  and the ink circulation rate Qout is large and an increase in the ink pressure of the ink meniscus is relatively large. For this reason, the target negative pressure value Pdectrg is set to be large on the depressurization side. Alternatively, when the ink circulation rate Qout is small, the pressure loss P 3 loss determined by the product of the passage resistance R 3  and the ink circulation rate Qout is small and an increase in the ink pressure of the ink meniscus is relatively small. For this reason, the target negative pressure value Pdectrg is set to be small on the depressurization side. 
     Next, a heating system will be described in which the ink is heated during the supply of the ink from the sub-tank  25  disposed in the printer  11  to the printing head  43  and the temperature of the heated ink supplied to the printing head  43  is kept in the printing head  43 . 
     As shown in  FIG. 1 , the heating system includes a first heating device  71  (first heating unit) preliminarily heating the ink of the sub-tank  25  supplied from the main tank  15  to the sub-tank  25  via the second ink supply pipe  27  so as to have a target temperature and a second heating device  72  (second heating unit) heating the ink, which is supplied to the third ink supply pipe  47  in the state where there is a slight gap in the temperature of the ink heated in the sub-tank  25 , at the positions of the connection pipes  48  so as to have the target temperature while eliminating the temperature gap. The heating system further includes temperature keeping devices  73  (third heating unit) installed in each printing head  43  to keep the temperature of the heated ink supplied to each printing head  43  via the third ink supply pipe  47  at the target temperature. 
     The first heating device  71  includes a sub-tank heater  33  (tank heater) disposed inside the sub-tank  25  and a first temperature sensor  32  detecting the temperature of the ink in the sub-tank  25 . The control device  60  performs heating control of the sub-tank heater  33  so that the temperature (the temperature of the ink at the position of the first temperature sensor  32 ) detected by the first temperature sensor  32  becomes a first target temperature (target value), which is the target temperature of the ink in the sub-tank  25 . 
     The second heating device  72  includes a supply heater  54  heating the heated ink supplied from the sub-tank  25  at the position of the connection pipes  48  of the third ink supply pipe  47 , a heat transfer member  74  (heating block) heating the connection pipes  48  by transferring the heat of the supply heater  54 , and a third temperature sensor  53  detecting the temperature of the heat transfer member  74 . The control device  60  performs heating control of the supply heater  54  so that the temperature (surface temperature of the heat transfer member  74 ) detected by the third temperature sensor  53  becomes a second target temperature (target value). 
     The temperature keeping device  73  includes a head heater  45  keeping the temperature of the heated ink of the printing head  43  and a second temperature sensor  44  detecting the temperature of the head heater  45 . The control device  60  performs the heating control of the head heater  45  so that the temperature (surface temperature of the head heater  45 ) detected by the second temperature sensor  44  becomes a third target temperature (target value) to maintain the temperature of the ink of the printing head  43 . 
     Next, the configurations of the first heating device  71 , the second heating device  72 , and the temperature keeping device  73  will be described in detail.  FIG. 4  is a sectional view illustrating the sub-tank  25  including the first heating device  71 .  FIG. 5  is a schematic sectional view illustrating the second heating device  72 .  FIG. 6  is a partially sectional view schematically illustrating the second heating device  72  taken along the line VI-VI of  FIG. 5 .  FIG. 7  is a schematic sectional view illustrating the cross-section of the second heating device  72  in a direction perpendicular to  FIG. 6 .  FIG. 8  is a partially exploded sectional view schematically illustrating the printing head  43 . 
     First, the configuration and function of the first heating device  71  will be described. As shown in  FIG. 4 , the sub-tank  25  includes a cylindrical tank main body  25   b  having a bottom and a cover  25   c  blocking the opening of the tank main body  25   b . The sub-tank  25  is made of a material with a relatively low heat conductivity, a high heat resistant property, and a corrosion resistant property into which the ink rarely intrudes. An example of the material is glass. For example, when a heater is disposed on the outer wall surface of a metal container made of a stainless steel or the like, the heat is transferred from the inner circumferential surface of the container toward the ink. Therefore, it takes a relatively long time to heat the ink up to the first target temperature (for example, 40° C.). In this embodiment, however, since the sub-tank heater  33  is dipped into the ink in the sub-tank  25 , the ink near the sub-tank heater  33  located in a slightly lower portion of the middle is first heated. Therefore, it takes a relatively short time until the average temperature of all of the ink reaches the target temperature. Since the sub-tank  25  is made of an inorganic material (such as glass) of which the heat conductivity is smaller than that of metal, it is difficult for the heat of the heated ink to dissipate from the wall of the sub-tank  25  to the outside. Therefore, the time required to heat the ink to the first target temperature is short. 
     Here, the ink is intermittently supplied to the sub-tank  25 . Of the ink heated to the first target temperature, the ink with the normal temperature flows in. In this embodiment, as shown in  FIG. 4 , the first temperature sensor  32  is disposed at the position separated by a predetermined distance from an ink inflow port  25   d  (liquid inflow portion) from the main tank  15  in the ink of the sub-tank  25 . Here, the first temperature sensor  32  is disposed under the arrangement condition that the first temperature sensor  32  is disposed at the position opposite to the ink inflow port  25   d  relative to an imaginary surface perpendicular to an imaginary line binding the center of the ink inflow port  25   d  and the sub-tank heater  33  and passing through the center of the sub-tank heater  33 . When the temperature sensor  32  is located near the ink inflow port  25   d , the temperature of the ink cooled immediately when the ink starts flowing in is detected by the sub-tank heater  33  and thus the sub-tank heater  33  heats the ink rapidly. At this time, during the flowing of the ink, the flow of the ink has a mixing effect on the ink of the sub-tank  25 , whereby the temperature of the ink is increased while the ink is mixed. Therefore, the temperature of the entire ink easily increases. However, since the mixing operation resulting from the flow of the ink disappears after the end of the ink inflow, the temperature of the ink near the ink inflow port locally increases. When the temperature of the ink locally reaches the target heating temperature, the sub-tank heater  33  stops heating the ink at this time in spite of the fact that the temperature of the ink in other positions is low. For this reason, a temperature distribution of the ink of the sub-tank  25  occurs. Moreover, the center of the sub-tank heater  33  upon determining the arrangement condition of the first temperature sensor  32  refers to a circular center in the circular heater as in this embodiment. 
     In this embodiment, as shown in  FIG. 4 , the sub-tank heater  33  is separated by the predetermined distance from the ink inflow port  25   d  in the sub-tank  25 . Therefore, the temperature of the ink near the ink inflow port  25   d  locally reaches the target temperature, as described above, but it is easy to avoid the occurrence of the temperature distribution in which the temperature of the ink farthest from the ink inflow port  25   d  on the opposite side is considerably lower than the target temperature. 
     Specifically, when the ink with the normal temperature flowing from the inflow port (an upstream end  27   a ) of the sub-tank  25  flows in by a predetermined distance from the ink inflow port  25   d , the first temperature sensor  32  detects the temperature of the ink with the normal temperature and the sub-tank heater  33  heats the ink. The ink flowing mainly above the sub-tank heater  33  readily joins the flow of the ink circulating from the ink circulation pipes  55  and thus readily flows below the sub-tank heater  33 . The ink moving and flowing down slightly from the ink inflow port  25   d  is heated while the ink flows near the sub-tank heater  33 . Since the ink is mixed by the flow of the ink and is heated while the ink flows from the main tank  15 , the temperature distribution of the ink of the sub-tank  25  rarely occurs. Moreover, since the ink is mixed and heated by the flow of the ink from the ink circulation pipes  55  during the circulation of the ink, the temperature distribution of the ink of the sub-tank  25  rarely occurs. 
     When the ink is locally heated near the sub-tank heater  33 , the heat may have an adverse influence on the ink. For this reason, the first temperature sensor  32  is disposed at the position at which the adverse influence of the heat does not occur. While the ink is heated, a temperature distribution may occur in that the temperature of the ink near the sub-tank heater  33  is high and the temperature of the ink distant from the sub-tank heater  33  is low. For example, when the first temperature sensor  32  is too far away from the sub-tank heater  33 , the temperature of the ink near the sub-tank heater  33  is considerably increased, the temperature of the ink near the position of the first temperature sensor  32  reaches the target heating temperature, and thus the sub-tank heater  33  stops the heating at this time. In this case, since the temperature of the ink near the sub-tank heater  33  is considerably increased and thus the heat may have an adverse influence on the ink. Alternatively, when the first temperature sensor  32  is too close to the sub-tank heater  33 , the sub-tank heater  33  stops the heating at a time, at which the temperature of the ink near the sub-tank heater  33  reaches the target heating temperature, in spite of the fact that the temperature of the ink in the circumference separated from the sub-tank heater  33  is considerably lower than the target heating temperature. For this reason, in order to avoid the occurrence of the temperature distribution at both ends, the first temperature sensor  32  is disposed at the position spaced from the sub-tank heater  33  by an appropriate distance. The position at which the first temperature sensor  32  is disposed is set in the range of the half of the depth from the liquid level A 2  to the sub-tank heater  33  in a depth direction at the center of the intermediate position between the sub-tank heater  33  and a liquid level (for example, the liquid level A 2  in  FIG. 4 ) when the inflow of the ink from the main tank  15  stops. In particular, in this example, the first temperature sensor  32  is disposed at a position slightly closer to the sub-tank heater  33  than the intermediate position between the liquid level A 2  and the sub-tank heater  33  in the range in the depth direction. 
     A pipe portion  47   c  (pipe passage) having a predetermined length and forming a part of the third ink supply pipe  47  on the upstream end thereof is inserted along the bottom surface of the sub-tank  25  so as to extend at the position slightly above the bottom surface of the sub-tank  25 . An inflow port  47   d  of the pipe portion  47   c  is opened at a position which is opposite to the ink inflow port  25   d  and to which the ink flowing from the ink inflow port  25   d  crosses across the inside of the sub-tank  25 . Here, the insertion position of the pipe portion  47   c  is determined under the condition that the pipe portion  47   c  is inserted only by a predetermined length crossing the half or more of the sub-tank  25  until the inflow port  47   d  reaches the position opposite to the ink inflow port  25   d  relative to the imaginary surface perpendicular to an imaginary line binding the center of the ink inflow port  25   d  and the sub-tank heater  33  and passing through the center of the sub-tank heater  33 . Therefore, the ink heated by the sub-tank heater  33  while the ink flows from the main tank  15  to the sub-tank  25  and crosses the inside of the sub-tank  25 , or the ink flowing near the inflow port  47   d  while being mixed and heated to the average temperature flows from the inflow port  47   d  of the pipe portion  47   c , as indicated by an arrow of  FIG. 4 . Therefore, the ink with the normal temperature just flowing in the sub-tank  25  is stopped from flowing from the inflow port  47   d  of the pipe portion  47   c  to the third ink supply pipe  47 . 
     The pipe portion  47   c  extends along the bottom surface of the sub-tank  25 . Therefore, even when the ink flows below the sub-tank heather  33  during the flow of the ink in the pipe portion  47   c , the ink is also heated. The sub-tank heater  33  is disposed at the position separated from the pipe portion  47   c  by an appropriate distance so that the ink passing the pipe portion  47   c  is appropriately heated. Even when the ink which is not sufficiently heated flows from the inflow port  47   d  of the pipe portion  47   c  at the time at which the ink intermittently flows from the main tank  15 , the ink is heated while flowing in the pipe portion  47   c  and passing below the sub-tank heater  33 . Therefore, the ink heated to the first target temperature mainly flows from the sub-tank  25  to the third ink supply pipe  47 . When the ink does not flow from the main tank  15 , the sub-tank heater  33  just heats the ink heated to the first target temperature to the degree of keeping the temperature of the ink. Therefore, even when the ink flowing in the pipe portion  47   c  extending along the bottom surface of the sub-tank  25  flows below the sub-tank heater  33 , the ink is rarely heated by the sub-tank heater  33 . Accordingly, the ink excessively heated in the sub-tank  25  does not flow to the third ink supply pipe  47 . The pipe portion  47   c  extending near the bottom surface of the sub-tank  25  is disposed at the depth slightly below the liquid level A 2  and at the position spaced opposite to the ink inflow port  25   d , to which the ink with the normal temperature flows, with reference to the sub-tank heater  33  in the depth direction. Therefore, the ink passing through the pipe portion  47   c  can be easily prevented from being cooled by the ink with the normal temperature just flowing from the ink inflow port  25   d.    
     The heated ink flows from the ink circulation pipes  55  by circulating the ink supplied from the sub-tank  25  to the printing heads  43  and via the third ink supply pipe  47  again from the printing heads  43  to the sub-tank  25  via the third ink supply pipe  47 . At this time, when the ink with the normal temperature flows from the ink inflow port  25   d  to the sub-tank  25 , the ink with the normal temperature is mixed with the heated ink flowing from the ink circulation pipes  55 , thereby preventing the temperature of the ink of the sub-tank  25  from being rapidly dropped. In addition, even when the temperature distribution of the ink of the sub-tank  25  occurs during the heating after the end of the inflow of the ink with the normal temperature or the temperature is not yet sufficiently stabilized, the temperature of the ink of the sub-tank  25  is averaged and the ink temperature and the target temperature are converged by the inflow of the heated ink, which is slightly cooled from the target temperature, flowing from the ink circulation pipes  55  and the mixing operation caused by the ink flow upon the inflow of the ink. Therefore, a gap between the temperatures of the ink heated at the appropriate temperature in the sub-tank  25  can be reduced. Accordingly, the ink stabilized in the temperature since the gap between the temperatures is reduced can be supplied to the third ink supply pipe  47 . 
     For example, when the ink is not circulated during the printing and only the ink necessary for the printing heads  43  is supplied, the ink is readily cooled at the position where the second heating device  72  is not disposed in the third ink supply pipe  47 . For this reason, the temperature distribution may occur in that the temperatures are different due to a positional difference in the third ink supply pipe  47  in the longitudinal direction. Since it is difficult to solve the temperature distribution just occurring in the third ink supply pipe  47 , the temperature distribution has an influence on the ink ejection performance of the printing heads  43 . In this embodiment, however, since the ink is circulated during the printing from the printing preparation period and the standby period after the end of the printing, the temperature distribution due to the positional difference in the third ink supply pipe  47  in the longitudinal direction does not occur. 
     As described above, the sub-tank heater  33  is disposed at the position slightly above the pipe portion  47   c  extending along the bottom surface of the sub-tank  25 . The sub-tank heater  33  is disposed nearly at the center of the tubular sub-tank  25  in the horizontal direction in the state where the sub-tank heater  33  is located slightly closer to the bottom surface than the depth position of the half of the depth from the liquid level A 2  to the bottom surface when the inflow of the ink from the main tank  15  is stopped. The first temperature sensor  32  is disposed closer to the opposite side (closer to the left end in  FIG. 4 ) of the end of the ink inflow port  25   d  with reference to the center of the circular shape of the sub-tank heater  33  and is disposed at the range in which the intermediate position of the half of the depth from the liquid level A 2  to the sub-tank heater  33  is located at the center. In particular, the first temperature sensor  32  is located at the position closer to the sub-tank heater  33  than the intermediate position of the range. 
     In this way, the ink of the sub-tank  25  is heated up to the first target temperature by the sub-tank heater  33 . However, it is difficult to eliminate the temperature distribution of the ink in the sub-tank  25 . Moreover, when the ink with the normal temperature flows intermittently from the main tank  15 , the temperature distribution has a tendency to occur easily. For this reason, the ink flowing from the sub-tank  25  to the third ink supply pipe  47  via the pipe portion  47   c  is heated mainly to the first target temperature, but there is a slight difference in the temperature. 
     Next, the configuration of the second heating device  72  will be described with reference to  FIG. 1  and  FIGS. 5 to 7 . As shown in  FIG. 1  and  FIGS. 5 to 7 , the second heating device  72  includes a heat transfer member  74  installed inside the connection pipes  48 , a supply passage heater  54  installed in the heat transfer member  74 , and a third temperature sensor  53  installed in the heat transfer member  74  and detecting the temperature of the heat transfer member  74 . The heat transfer member  74  is configured to transfer the heat of the supply passage heater  54  and heat the connection pipes  48 . 
     As shown in  FIGS. 6 and 7 , the heat transfer member  74  includes a heat transfer plate  76  with the same square plate shape and nearly the same size as those of a heat transfer block  75  with a square plate shape. A plurality of guide grooves  75   a  is formed on the surface facing the heat transfer plate  76  of the heat transfer block  75 . The plurality (N) of connection pipes  48  are interposed between the heat transfer block  75  and the heat transfer plate  76  in the state where the connection pipes  48  are received in the guide grooves  75   a , respectively. As shown in  FIGS. 6 and 7 , the supply passage heater  54  is attached to the surface of the heat transfer block  75  of the heat transfer member  74 . The third temperature sensor  53  is attached to the surface of the heat transfer block  75  of the heat transfer member  74  at the position slightly spaced from the supply passage heater  54 . Of course, the third temperature sensor  53  may be attached to the surface of the heat transfer plate  76  opposite to the arrangement position of the supply passage heater  54  of the heat transfer member  74 . 
     In this embodiment, the heat transfer block  75  and the heat transfer plate  76  forming the heat transfer member  74  are made of aluminum-based metal (for example, aluminum or aluminum alloy) with high heat conductivity. The connection pipe  48  is made of an iron-based metal (for example, stainless steel) with high ink corrosion resistant property. In addition, the heat transfer member  74  is joined to the connection pipe  48  received in a guide passage  74   a  by soldering. Of course, when the material of the heat transfer member  74  has low heat conductivity and an ink corrosion resistant characteristic, the guide passage  74   a  of the heat transfer member  74  may be configured as a passage with a circular cross-section, for example, and this passage may be used as the connection pipe. 
     As shown in  FIG. 5 , the N (for example, four) connection pipes  48  extend to be adjacent to each other and nearly parallel to each other at a nearly uniform interval. The connection pipes  48  are installed along a meandering predetermined path. The N connection pipes  48  with a small pipe diameter extend to be long and thin in the meandering path. When the connection pipes  48  extend to be long and thin in the meandering path, the broad contact areas of the connection pipes  48  and the heat transfer member  74  can be ensured. Moreover, the broad contact areas of the connection pipes  48  installed in the heat transfer member  74  and the ink flowing in the connection pipes can be ensured. For this reason, the heat of the supply passage heater  54  can effectively be transferred to the ink flowing in the connection pipes  48  via the heat transfer member  74 . 
     As shown in  FIG. 5 , the connection pipes  48  are installed to be adjacent to each other distantly and parallel to each other at the nearly uniform interval along the meandering path. Therefore, it is possible to realize the pipe structure in which the difference in the temperature hardly occurs between the connection pipes  48 . For example, when the N connection pipes are respectively installed in N independent pipe areas along the meandering path, the temperature in the pipe area of the connection pipe  48  connected to the printing head  43 , in which the flow rate of the ink ejected is large, is relatively lower than that in the other pipe areas. Therefore, the temperature in the pipe area of the connection pipe  48  connected to the printing head  43  consuming a small amount of ink becomes relatively higher than that in the other areas. In this case, a problem may arise in that the temperature of the ink in the connection pipes  48  is irregular between the connection pipes  48  and the temperature of the ink in the printing heads  43  is irregular between the printing heads  43 . 
     In this embodiment, the N connection pipes  48  may be installed to be adjacent to each other at nearly uniform intervals along the meandering path in the same area of the heat transfer member  74 . Therefore, even when the temperature near the connection pipe  48  corresponding to the printing head  43  ejecting a large amount of ink is lowered, the other connection pipes  48  also pass through the area where the temperature is lowered. Accordingly, irregularities are not easily generated in the temperature of the ink in the connection pipes  48  between the connection pipes  48 . 
     The connection pipes  48  extend to be long and thin, and thus the passage resistance R 2  is increased. Even when the pulsation of the fourth pump  50  is transferred up to the entrance of each connection pipe  48  via the common pipe  47   b  without attenuation, the pulsation is attenuated and disappears due to the large dynamic pressure generated when the ink passes through each connection pipe  48 . Accordingly, weak pulsation is not prevented from being transferred to the inside of the printing head  43 . 
     The N connection pipes  48  installed along the meandering path shown in  FIG. 5  have nearly the same length. Therefore, the ink flows in the thick common pipe  47   b , where the loss of pressure is small. The nearly equal ink pressure at the entrance of each connection pipe  48  undergoes nearly the same loss of pressure when the ink passes through the connection pipes  48  with nearly the same passage length. Therefore, the ink pressures of each printing heads  43  are nearly the same between the other printing heads  43 . 
     Next, the structure of the temperature keeping device  73  will be described with reference to  FIG. 8 . As shown in  FIG. 8 , the printing head  43  includes a head main body  80  and a head section  81  fixed to the lower portion of the head main body  80 . An ink chamber  82  is formed inside the head main body  80 . The connection pipe  48  and the ink circulation pipe  55  are connected to each other at the position at which the connection pipe  48  and the ink circulation pipe  55  face each other via the ink chamber  82  in the upper portion of the head main body  80 . In the ink chamber  82 , a filter  83  is disposed at the midway point between the upper portion communicating with the connection pipe  48  and the head section  81 . The filter  83  removes bubbles or foreign particles in the ink flowing in the head section  81  in the ink flowing from the connection pipe  48  to the ink chamber  82 . 
     The ink passing through the filter  83  from the ink chamber  82  flows to the head section  81  and is ejected as ink droplets from a plurality of nozzles  84  opened to a nozzle formation surface  81   a , which is a lower surface of the head section  81 . The same number of pressure chambers (not shown), which respectively communicate with the nozzles  84 , as the number of nozzles are disposed in the head section  81 . The ink droplets are ejected from the nozzle  84  by vibrating one wall portion of the pressure chamber by a pressure generating element disposed in each nozzle  84  and applying ejection pressure to the ink in the pressure chamber. Examples of the pressure generating element include a piezoelectric element, an electrostatic element, and a heater used in a thermal type ink jet printer. 
     As shown in  FIG. 8 , the temperature keeping device  73  keeping the temperature of the heated ink flowing to the ink chamber  82  is disposed on the outer wall surface of the printing head  43 . In the printing head  43 , a head cover  85  (heating member) made of metal is attached to the head section side from the circumference of the nozzle formation surface  81   a  of the head section  81 . The head heater  45  is disposed to come into contact with the head cover  85 . The second temperature sensor  44  disposed in the head heater  45  directly detects the surface temperature of the head heater  45 . 
     With such a configuration, the control range of the temperature can be reduced when the heat of the head heater  45  is controlled so that the temperature detected by the second temperature sensor  44  approaches the third target temperature (target temperature for maintaining temperature). For example, when the ink temperature or the temperature of the head cover  85  and a heat transfer plate  86  is directly detected, the head heater  45  has already been cooled or heated considerably at the time of detecting the cooled ink or the heated ink. For this reason, deviation of the temperature of the head heater  45  becomes relatively large. However, the surface temperature of the head heater  45  is directly detected and the temperature of the head heater  45  can be kept at a nearly constant temperature (the third target temperature). Therefore, since the temperature of the printing heads  43  can be kept at the third target temperature, the advantage of keeping the temperature of the heated ink in the printing head  43  can be obtained. Accordingly, it is possible to avoid a case where the ink is excessively heated or cooled by overshoot, which is a problem when the control range of the temperature is large. In addition, since the control range of the temperature of the head heater  45  is small, the ink in the printing head  43  is kept at the third target temperature. 
     The heat transfer plate  86  configured to cover the side surfaces of the head heater  45  and the head cover  85  in the state where the heat transfer plate  86  comes into contact with the surface of the head heater  45  and the surface of the head cover  85  are disposed on the outer wall of the printing head  43 . The heat of the head heater  45  can be transferred not only to the side surface of the printing head  43  but also the side surface of the printing head  43  via the heat transfer plate  86 . Therefore, the heat of the head heater  45  can further be transferred to the side of the head section  81  and the circumference of the nozzle formation surface  81   a  via the heat transfer plate  86  and the head cover  85 . The heat of the head heater  45  can be directly transferred to the head cover  85  via the contact portion of the end surfaces or can be directly transferred to the side surface of the head cover  85  via the heat transfer plate  86 . Therefore, the heat can effectively transferred to the side surface of the head section  81  and the circumference of the nozzle formation surface  81   a . The temperature keeping device  73  can be effectively keep the ink in the pressure chambers or the nozzles  84  of the head section  81 . As a consequence, the ink droplets ejected from the nozzles  84  can be ejected satisfactorily. 
     In this embodiment, the head main body  80  is formed of a resin base and the portion including the nozzle formation surface  81   a  of the head section  81  is formed of a material with higher heat conductivity than that of the resin base of the head main body  80 . In this embodiment, the portion including the nozzle formation surface  81   a  of the head section  81  is formed of, for example, silicon. The heat conductivity of silicon is higher than that of resin or ceramics, even though the heat conductivity of the silicon is lower than that of metal. In this way, the temperature of the ink in the head section  81  can be kept by transferring the heat of the head heater  45  to the circumference and the side wall of the nozzle formation surface  81   a  of the head section  81  via the heat transfer plate  86  and the head cover  85  and heating the entire head section  81  at nearly the same temperature as the temperature of the head heater  45 . 
     In this case, it is difficult to transfer the heat to the ink in the ink chamber  82 , the passage on the downstream side of the ink chamber  82 , the pressure chamber, or the nozzle  84 , even though the head main body  80  is heated from the outside. In this embodiment, however, the head cover  85 , to which the heat is transferred from the head heater  45  via the heat transfer plate  86 , heats the side of the head section  81  and the circumference of the nozzle formation surface  81   a . Therefore, in the printing head  43  including the head main body  80  made of resin, in which it is difficult to heat the ink of the ink chamber  82 , the ink has a tendency to be gradually cooled while the ink is sent from the ink chamber  82  to the downstream side. However, by heating the head section  81 , the ink in the nozzle  84  or the pressure chamber located on the downstream side of the ink passage is heated. Therefore, since the temperature of the ink is appropriately kept at the third target temperature before the ink is ejected from the printing head  43 , the satisfactory ejection performance of the printing head  43  is ensured. 
     The first heating device  71 , the second heating device  72 , and the temperature keeping device  73  forming the heating system realize a first heating function, a second heating function, and a temperature keeping function by the arrangement structure of the heater, the temperature sensor, and the heat transfer unit (the heat transfer member or the heat transfer plate). Moreover, the first heating function, the second heating function, and the temperature keeping function can be realized by feedback control of the heaters  33 ,  45 , and  54  by the control device  60 . 
     In this embodiment, the computer  61  of the control device  60  performs PID control on the heaters  33 ,  45 , and  54  so that the temperature detected by the temperature sensors  32 ,  44 , and  53  approaches the target temperature. The sub-tank heater  33  controls the temperature rapidly in accordance with a variation in the temperature by performing the PID control in which the control range of the temperature is large, by putting emphasis on P. The supply passage heater  54  performs the PID control so that the control range of the temperature is small in spite of putting the emphasis on P and so that the temperature is controlled rapidly in accordance with the variation in the temperature even though the supply passage heater  54  may not perform the control as the sub-tank heater  33  does. Moreover, the head heater  45  performs the PID control so that the control range of the temperature is the smallest in comparison to the other control even when the difference between the detected temperature and the target temperature is the same as that of the other control and so that the real temperature smoothly follows the third target temperature even when a variation in the temperature deviated from the target temperature occurs in the head heater  45 . 
     Next, ink supply control and cleaning control performed by the computer  61  of the control device  60  will be described. 
     The computer  61  executes the supply control routine for each predetermined time (for example, a predetermined time in the range of 1 to 100 milliseconds). When the power of the printer  11  is turned off, the on-off valves  30 ,  37 ,  41 ,  51 , and  56  are in a closed state. When the printer  11  is turned on, the computer  61  opens the on-off valves  30 ,  41 ,  51 , and  56  and simultaneously drives the third pump  39  and the fourth pump  50 . As a consequence, the inside of the air chamber  25   a  enters a negative pressure state by performing an operation of discharging the air from the air chamber  25   a  by the third pump  39 . The ink pressure of the sub-tank  25  is depressurized since the negative pressure is applied at the liquid level A 2  of the ink in the sub-tank  25 . In this state, by the ejection drive of the fourth pump  50 , the ink is supplied from the sub-tank  25  to the printing heads  43  via the third ink supply pipe  47 . At this time, by ejection drive of the fourth pump  50 , an ink ejection rate Qpump (=the ink supply rate Qin) of 20 N (cc/minute), for example, is supplied via the third ink supply pipe  47 . 
     The lengths (passage lengths) by which the ink flows to the entrances (branch places) of the N connection pipes  48  via the common pipe  47   b  in the third ink supply pipe  47  are different. However, since the passage resistance R 1  of the common pipe  47   b  is small and the loss of the pressure of the ink hardly occurs, the ink supply pressures are nearly the same as each other between the connection pipes  48  at the time at which the ink reaches the entrances of the N connection pipes  48 . The passage resistance R 2  of the ink flowing in the connection pipes  48  having the small pipe diameter and extending to be long and thin along the meandering path (zigzag path) is considerably increased. Therefore, the amounts of ink supplied to the printing heads  43  become equal between the printing heads  43 . The pulsation of the fourth pump  50  is transferred to the entrances of the connection pipes  48  to the small degree that the pulsation is not attenuated by the damper  52 . However, the delivered weak pulsation becomes almost disappears due to the dynamic pressure of the ink flowing in the connection pipes  48  with the large passage resistance R 2 . Accordingly, the pulsation rarely affects the inside of the printing head  43 . 
     Here, in the printing head  43 , the ink is consumed by the amount of ink ejected from the nozzles  84 . At this time, the ink ejection rate Qh of ink corresponding to the duty value D at that time is consumed from the amount 20 N (cc/minute) of ink supplied to the printing heads  43 . In this embodiment, when the printing is performed at the maximum (FULL) duty, the amount of ink consumed per printing head is 10 (cc/minute). The ink supply rate (Qin) (=20 N (cc/minute)) of ink larger than a maximum ink ejection rate Qhmax (=10 N (cc/minute) when all of the N printing heads  43  perform the printing at the maximum (FULL) duty is supplied by the fourth pump  50  (supply pump). Therefore, during either printing or interruption of the printing, the ink circulation rate Qout (=Qin−Qh) which is a rate obtained by subtracting the ink ejection rate Qh from the ink supply rate Qin is circulated from the printing heads  43  to the sub-tank  25  via the ink circulation pipes  55 . Therefore, even when the printing is performed at the maximum duty, the ink is circulated only via the ink circulation pipes  55 . Therefore, the ink flowing from the printing heads  43  to the ink circulation pipes  55  is not returned from the ink circulation pipes  55  to the printing heads  43 . Therefore, it is possible to prevent the deterioration in the ejection characteristics of the printing heads  43  since the UV ink cooled during the flow to the ink circulation pipes  55  is returned to the printing heads  43  again and thus the temperature of the ink in the printing heads  43  falls. 
     The ROM  68  stores a program for a print processing routine shown in the flowchart of  FIG. 9  and used to execute the ink supply control at the time of the printing. When the printing starts, the computer  61  (specifically, the internal CPU  67 ) executes the print processing routine shown in  FIG. 9  to control the supply of the ink at the time of the printing. Hereinafter, the ink supply control executed by the computer  61  at the time of the printing will be described with reference to  FIG. 9 . In the standby state before the printing performed by the printer  11 , the ink is circulated between the sub-tank  25  and the printing heads  43 . However, when a predetermined period expires in the standby state, the circulation of the ink is stopped. Here, when a printing work is received, it is assumed that the circulation of the ink stops. In this case, the on-off valves  51  and  56  of the third ink supply pipe  47  and the ink circulation pipe  55  and the on-off valves  37  and  41  of the pressure adjusting device  34  are in the closed state. The pressure adjusting device  34  is driven to adjust the temperature of the air chamber  25   a  to the target pressure in accordance with a variation in the volume of the air chamber  25   a  corresponding to a variation in the volume of the ink of the sub-tank  25 . 
     First, in step S 10 , the on-off valves are opened to supply the ink to the printing heads  43 . That is, the on-off valve  51  of the third ink supply pipe  47 , the on-off valves  56  of the ink circulation pipes  55 , and the on-off valve  41  of the pressure adjusting device  34  are opened. 
     In step S 20 , heating/temperature keeping control of the ink in the ink supply path and the printing heads is performed. The computer  61  starts the pressurizing/temperature keeping control of the ink when the printer  11  is turned on. In this step, a part of the heating/temperature keeping control performed during the printing is described. That is, the computer  61  controls the temperature of the sub-tank heater  33  on the basis of the detection result of the first temperature sensor  32 . The computer  61  controls the temperature of the supply passage heater  54  on the basis of the detection result of the third temperature sensor  53 . Moreover, the computer  61  controls the temperature of the head heater  45  on the basis of the detection result of the second temperature sensor  44 . 
     In step S 30 , the fourth pump  50  for the ink supply is driven. At this time, the driving of the fourth pump  50  is controlled to satisfy the condition that the ink supply rate Qin (Qin&gt;Qhmax) is larger than the maximum ink ejection rate Qhmax. 
     In step S 40 , the pressure of the air chamber  25   a  of the sub-tank  25  is controlled to become the negative pressure value Pdec based on the duty value D and the liquid head difference H for the printing mode and printing head control. That is, the target negative pressure value Pdectrg is calculated with the expression of Pdectrg=Po−Ph(H)−P 3 loss(D) by selecting P 3 loss(D) corresponding to the printing mode at that time and using the duty value D and the liquid head difference H. The computer  61  controls the third pump  39  (depressurizing pump) and the pressure opening plate  40  so that a real negative pressure value Pdecreal detected by the pressure sensor  58  is equal to the target negative pressure value Pdectrg. As a consequence, the air chamber  25   a  is controlled so that its pressure becomes the target negative pressure value Pdectrg. Specifically, when the absolute value of the real negative pressure value Pdecreal is smaller than the absolute value of the target negative pressure value Pdectrg, the computer  61  depressurizes the air chamber  25   a  until the real negative pressure value Pdectreal is equal to the target negative pressure value Pdectrg, by driving the third driving motor  38  to depressurize the third pump  39 . On the other hand, when the ink is decreased in the sub-tank  25  and the volume of the air chamber  25   a  is increased, the pressure of the air chamber  25   a  is decreased and thus the absolute value of the real negative pressure value Pdecreal becomes larger than the absolute value of the target negative pressure value Pdectrg. In this way, when the absolute value the absolute value of the real negative pressure value Pdectreal is larger than the absolute value of the target negative pressure value Pdectrg, the computer  61  inputs a small amount of air into the air chamber  25   a  until the real negative pressure value Pdectreal is equal to the target negative pressure value Pdectrg, by opening the pressure opening plate  40  and opening the air chamber  25   a  to the atmosphere. 
     Subsequently, in step S 50 , it is determined whether the printing ends. When the printing does not end (that is, the printing is being performed), the process returns to step S 20 . Then, steps S 20  to S 40  are repeated until it is determined that the printing ends in step S 50 . When the printing ends, in step S 60 , the driving of the fourth pump  50  is stopped to stop the supply of the ink and the on-off valves  51  and  56  are closed to block the passages of the third ink supply pipe  47  and the ink circulation pipes  55  after the driving of the fourth pump  50  is stopped. 
     Next, the cleaning operation will be described. The printer  11  has a cleaning function to prevent and solve the ejection failure of the printing heads  43 . As described above, the printer  11  according to this embodiment can perform the first cleaning operation to remove bubbles in the ink in the ink chamber  82  of the printing head  43  and the second cleaning operation to prevent and solve the clogging of the nozzles of the printing head  43 . The first cleaning operation is performed when bubbles are mixed or are possibly mixed, for example, when the ink cartridge is replaced, the initial filling is performed, or the printer is not used for a long time. 
     The printer  11  include a nozzle inspecting unit (not shown) detecting whether the nozzle is clogged in each printing head  43 . The control device  60  permits the nozzle inspecting unit to inspect the nozzle of the printing head  43 , when a cleaning instruction is received by the operation of a user and when it is determined that a time elapsed from the end of the previous cleaning operation reaches a predetermined time on the basis of a measurement time of a cleaning timer (not shown). When there is the printing head  43  determined to have the clogged nozzle from the result obtained by inspecting the nozzles by the nozzle inspecting device, the second cleaning operation is selectively performed on the printing head  43  which is not necessary for the first cleaning operation. The ROM  68  in  FIG. 2  stores the program of the processing routine of the first cleaning operation shown in  FIG. 10  and the program of the processing routine of the second cleaning operation shown in  FIG. 11 . 
     First, the first cleaning operation will be described. The computer  61  performs the routine of the first cleaning operation shown in  FIG. 10  at the time of performing the first cleaning operation either when the ink cartridge is replaced, the initial filling is performed, or the printer is not used for a long time. 
     First, in step S 110 , the first on-off valve  30  and the second on-off valve  37  are closed and the third on-off valve  41  and the fourth on-off valve  51  are opened. As a consequence, when the first on-off valve  30  is closed, the communication state between the sub-tank  25  and the main tank  15  are blocked. Simultaneously, when the fourth on-off valve  51  is opened, the sub-tank  25  and the printing heads  43  enter the communication state. Moreover, in the pressure adjusting device  34 , a state is entered in which the second pump  36  does not communicate with the sub-tank  25  and the third pump  39  communicates with the sub-tank  25 . 
     Subsequently, in step S 120 , M fifth on-off valves  56  corresponding to M printing heads  43 , which are targets of the first cleaning operation at this time, are opened and the remaining (N-M) fifth on-off valves  56  are closed among N (in this embodiment, four) fifth on-off valves  56 . Here, the first cleaning operation is performed a plural number of times sequentially in order on the M printing heads  43 . In this step, the M printing heads  43  (hereinafter, referred to as “first cleaning target heads”) are selected as the first cleaning targets and the M fifth on-off valves  56  corresponding to the selected M printing heads  43  are opened. 
     Specifically, M in the first cleaning operation is the maximum number of the cleaning targets per the cleaning operation. K (where M≦K≦N) liquid ejecting heads among N cleaning targets are subjected to the cleaning operation at least |[−K/M]| (where   is Gauss&#39;s notation and | | is an absolute value) times to perform the cleaning operation on all of the K liquid ejecting heads. For example, when the cleaning operation for one liquid ejecting head is performed on K liquid ejecting heads (where M=1), the cleaning operation for one liquid ejecting head is performed K (=|[−K]|) times. Alternatively, when the cleaning operation for two liquid ejecting heads is performed on seven liquid ejecting heads (where M=2 and K=7), the cleaning operation for two liquid ejecting heads is performed three times and the cleaning operation for one liquid ejecting head is performed once, that is, the cleaning operation is performed a total of four (=|[−7/2]|) times. 
     In step S 130 , the fourth pump  50  (supply pump) is driven. That is, the computer  61  drives the fourth driving motor  49  to drive the fourth pump  50 . As a consequence, the ink supplied from the sub-tank  25  to the printing heads  43  via the third ink supply pipe  47  is circulated to the sub-tank  25  via the M ink circulation pipes  55  again. 
     Next, in step S 140 , the third pump  39  (depressurizing pump) is driven. That is, the computer  61  drives the third driving motor  38  to drive the third pump  39 . When the third pump  39  is driven, the sub-tank  25  is depressurized. That is, by discharging the air from the air chamber  25   a  by the third pump  39 , the air chamber  25   a  is depressurized, the negative pressure of the air chamber  25   a  reaches the liquid level A 2 , and thus the ink of the sub-tank  25  is depressurized. 
     Subsequently, in step S 150 , it is determined whether the depressurization of the sub-tank  25  is completed. That is, the computer  61  determines whether an air pressure (the pressure of the sub-tank) Psub of the sub-tank  25  detected by the pressure sensor  58  reaches a target negative pressure value PD (where Psub≦PD). When the relation of Psub≦PD is not satisfied, the driving of the third pump  39  in step S 140  continues. Alternatively, when the relation of Psub≦PD is satisfied, the process proceeds to step S 160 . 
     In step S 160 , it is determined whether a first cleaning time expires. The computer  61  permits a timer (not shown) to measure the elapsed time from the start time of the first cleaning operation when the fourth pump  50  is driven and the ink circulation starts. When a measured time T of the timer reaches a first cleaning time T 1  (hereinafter, also referred to as a “first CL time T 1 ”), which is a time at which the first cleaning operation is performed (T≧T 1 ), the computer  61  determines that the first CL time T 1  expires. When the first CL time T 1  does not expire (when a relation of T≧T 1  is not satisfied), the first cleaning operation continues without change. Alternatively, when the first CL time T 1  expires (when the relation of T≧T 1  is satisfied), the process proceeds to step S 170 . 
     In step S 170 , it is determined whether the first cleaning target head (first CL target head) remains. That is, when the first cleaning operation is not completed on all of the N printing heads  43  and the printing head  43  to be subjected to the first cleaning operation remains, it is determined that the first cleaning target head remains. When the first cleaning target head remains, the process returns to step S 120  and the processes from steps S 120  to S 160  are performed on the remaining first cleaning target head to perform the first cleaning operation. Then, the first cleaning operation is performed on all of the N printing heads  43 . When it is determined that the first cleaning target head does not remain in step S 170 , the process proceeds to step S 180 . 
     In step S 180 , the driving of the fourth pump  50  is stopped and the ink circulation is stopped. The fourth on-off valve  51  and the fifth on-off valve  56  are closed and the third ink supply pipe  47  and the ink circulation pipes  55  are closed. Moreover, by controlling the pressure opening plate  40  and permitting the small amount of air to flow into the sub-tank  25  from the outside, the depressurized state of the sub-tank  25  is returned to the normal pressure in the standby state of the printing. The reduced pressure of the sub-tank  25  in steps S 140  and S 150  is set to the variable target negative pressure value PD in accordance with the ink supply rate Qin per one printing head so that the leakage of the ink does not occur in the nozzles or the very small leakage of the ink occurs, even when the ink supply rate Qin (=the ink circulation rate Qout) per one printing head is N/M times the value (in this embodiment 20 N (cc/minute)) at the time of the printing. 
     During the first cleaning operation, the amount of ink sent by the fourth pump  50  is also 20 N (cc/minute) at the time of the printing. The amount of ink sent is a substantial capability upper limit of the fourth pump  50 . In this embodiment, the amount of ink flowing may not be larger than the amount of ink sent. In this embodiment, the number M of first cleaning target heads is “1”. The first cleaning operation is performed one by one sequentially on the printing heads  43 . M (for example, one) ink circulation pipes  55  corresponding to the first cleaning target heads are opened among the five ink circulation pipes  55  and the remaining (N−M) (for example, three) ink circulation pipes  55  are blocked. Therefore, in this embodiment of M=1, since the three ink circulation pipes  55  are blocked, the ink flows back by 20 N (cc/minute) via the one ink circulation pipe  55  corresponding to the printing head  43  which is the cleaning operation target. 
     All of the 20 N (cc/minute) ink flowing from the sub-tank  25  to the common pipe  47   b  due to the fourth pump  50  is circulated in the path passing through the one printing head  43  which is the first cleaning target. When all of the 20 N (cc/minute) ink corresponding to the N printing heads at the time of the printing flows to the one printing head  43 , the flow speed of the ink flowing in the printing head  43  becomes faster. 
     In this embodiment, as shown in  FIG. 8 , the amount of ink flowing from the connection pipe  48  to the ink chamber  82  of the printing head  43  is N/M times (for example, four times) the amount of ink than the flow rate at the time of the printing. Therefore, the ink flowing from the connection pipe  48  to the ink circulation pipe  55  via the ink chamber  82  flows faster by N/M times the flow speed at the time of the printing. Accordingly, the bubbles gathering at the upper corners of the ink chamber  82  or the bubbles captured by the filter  83  are pushed out by the fast flow speed of the ink and thus are removed from the ink chamber  82 . 
     Here, the ink may leak from the nozzles since the flow speed of the ink is N/M times in each printing head  43  and thus the ink pressure of the printing head  43  is increased. In this embodiment, however, since the sub-tank  25  is depressurized by driving the third pump  39 , the ink pressure of the printing head  43  is also depressurized. Therefore, the increased ink pressure of the ink chamber  82  caused due to an increase in the amount of ink flowing in each printing head is nearly offset by the reduced ink pressure caused due to the depressurization of the sub-tank  25 . As a consequence, the leakage of the ink from the nozzles does not occur. Even though the leakage of the ink occurs, the amount of leaking ink can be reduced to be small. 
     For example, the amount of ink flowing in each printing head is increased and thus the ink in the printing head is pressurized, the bubbles are compressed by the pressurizing force and thus it is difficult to detach the bubbles from the filter. In this embodiment, however, the ink of the ink chamber  82  is depressurized to offset the increased pressure corresponding to the increased amount of ink flowing. Therefore, since the bubbles in the ink of the ink chamber  82  are expanded compared to a case of no depressurization, the bubbles captured by the filter  83  are easily separated from the filter  83 . In this way, by performing the depressurization of the ink for the second cleaning operation in which the amount of ink flowing in each printing head is increased, the leakage of the ink from the nozzle can be prevented or the leakage of the ink from the nozzle can be made small. Moreover, an advantage can be obtained in that the bubbles can be effectively removed. Moreover, a cap may be provided in advance which comes into contact with the nozzle formation surface of the printing head  43 . Then, even when the ink leaks from the nozzle, the leaking ink can be received in the cap upon performing the first cleaning operation. 
     Here, the target negative pressure value PD of the depressurizing control of the first cleaning operation will be described. Since the passage resistance R of the third ink supply pipe  47  is larger than the passage resistance R 3  of the ink circulation pipe  55  (where R&gt;R 3 ), ink pressures Pin are nearly the same as each other at the entrances of the connection pipes  48  at the time of the cleaning operation, as in the printing. A value lowered by the passage resistance R 2  of the connection pipe  48  from the ink pressure Pin becomes ink pressure Phead of the printing head  43 . 
     The ink pressure Phead of the printing head  43 , which corresponds to the closed on-off valve  56  and is a non-cleaning target, is increased, as the ink gradually flows to the printing head  43  via the connection pipe  48 . The flow of the ink passing the connection pipe  48  is stopped at the time at which the increased ink pressure Phead becomes equal to the ink pressure Pin at the entrance. Therefore, the ink pressure Phead of the printing head  43  converges to the same value as that of the ink pressure Pin at the entrance after a while after the cleaning starts. Here, the ink pressure Pin at the entrance can be expressed as the expression of Pin=Psub−P 1 loss=Psub−R 1 ·Qpump by use of the sub-tank pressure Psub, the ink ejection rate Qpump (=Qintotal) of the fourth pump  50  (supply pump), and the passage resistance R 1 . 
     On the other hand, an ink pressure Phcl of the meniscus in the nozzle  84  of the printing head  43 , which corresponds to the opened on-off valve  56  and is the cleaning target, can be expressed as the expression of Phcl=Psub−(N/M)·(P 1 loss+P 2 loss·P 3 loss)+Ph(H), since the ink supply rate Qin is (N/M)·Qintotal/N and the passage resistance of the connection pipe  48  is R 2  when the ink flows to the printing head  43  via the connection pipe  48 . 
     The ink pressure Phncl of the meniscus in the nozzle  84  of the printing head  43  which is the non-cleaning target is expressed as the expression of Phncl=Psub−P 1 loss+Ph(H). 
     The ink pressure Ph at the time of the first cleaning operation can be adjusted by varying the sub-tank pressure Psub, when the total number N of printing heads  43 , the number M of printing heads  43  subjected to the cleaning operation, and the liquid head difference H are determined from the above two expressions. Therefore, in this example, the ink pressures Phcl and Phncl are set to values of a degree that the ink does not leak from the nozzle  84 , and the negative pressure value Pdec of the sub-tank pressure Psub is adjusted. On the assumption that the ink pressure at which the ink does not leak is Phtrg 2  and the target negative pressure values of the sub-tank pressure Psub for the cleaning target and the non-cleaning target are PDcl and PDncl, respectively, to satisfy the relation of Ph=Phtrg 2  and PDcl and PDncl can be expressed as PDcl=Phtrg 2 +(N/M)·(P 1 loss+P 2 loss−P 3 loss)−Ph(H) and PDncl=Phtrg 2 +P 1 loss−Ph(H). The smaller one of PDcl and PDncl determined by the above two expressions is used as the target negative pressure value PD. In this embodiment, when the sub-tank pressure Psub is set to the negative pressure value PD at the time of the first cleaning operation, the leakage of the ink from the nozzle  84  can be prevented. 
     Next, the second cleaning operation will be described. When the cleaning timer measures the predetermined time from the end time of the previous cleaning operation or the cleaning instruction is given by the operation of a user, the computer  61  permits the nozzle inspecting device to inspect the nozzles of each printing head  43 . When it is determined that the printing head  43  of which the nozzle is clogged is present from the nozzle inspection result, this printing head  43  is subjected to the second cleaning operation. When the second cleaning operation is performed, the computer  61  executes the processing routine of the second cleaning operation shown in  FIG. 11 . Hereinafter, the description will be made on the assumption that K printing heads  43  (hereinafter, referred to as second cleaning target heads) to be subjected to the second cleaning operation are present among the N printing heads  43 . 
     In step S 210 , the first on-off valve  30 , the third on-off valve  41 , and the fifth on-off valve  56  are closed and the second on-off valve  37  and the fourth on-off valve  51  are opened. As a consequence, the communication state between the sub-tank  25  and the main tank  15  is blocked and all of the N ink circulation pipes  55  are blocked. Moreover, in the pressure adjusting device  34 , a state is entered in which the second pump  36  communicates with the sub-tank  25  and the third pump  39  does not communicate with the sub-tank  25 . 
     In step S 220 , the second pump  36  (pressurizing pump) is driven. That is, the computer  61  drives the second driving motor  35  to drive the second pump  36 . When the second pump  36  is driven, the sub-tank  25  is pressurized. That is, by sending the air from the outside by the second pump  36 , the air chamber  25   a  is pressurized, the pressurizing force of the air chamber  25   a  reaches the liquid level A 2 , and thus the ink of the sub-tank  25  is pressurized. 
     Subsequently, in step S 230 , it is determined whether the pressurization of the sub-tank  25  is completed. That is, the computer  61  determines whether an air pressure Psub of the sub-tank  25  detected by the pressure sensor  58  reaches a target increased pressure value PA (where Psub≧PA). When the relation of Psub≧PA is not satisfied, the driving of the second pump  36  in step S 220  continues. Alternatively, when the relation of Psub≧PA is satisfied, the process proceeds to step S 240 . 
     In step S 240 , K fifth on-off valves  56  are opened which correspond to the K printing heads  43  of the second cleaning target among the N (in this example, four) fifth on-off valves  56 . As a consequence, when the K fifth on-off valves  56  are opened in the state where the pressure of the sub-tank  25  is sufficiently increased, the pressurized ink is supplied from the sub-tank  25  to the K printing heads  43  via the K ink circulation pipes  55 . At this time, since the third ink supply pipe  47  is closed, the pressurized ink is supplied at one time to the ink chamber  82  of the printing head  43  and the ink is strongly discharged from the nozzles of the printing head  43 . 
     In step S 250 , it is determined whether a second cleaning time expires. The computer  61  permits a timer (not shown) to measure the elapsed time from the start time of the second cleaning operation after the K fifth on-off valves  56  are opened. When a measured time T of the timer reaches a second cleaning time T 2  (hereinafter, referred to as a “second CL time T 2 ”), which is a time at which the second cleaning operation is performed (T≧T 2 ), the computer  61  determines that the second CL time T 2  expires. When the second CL time T 2  does not expire (when a relation of T≧T 2  is not satisfied), the second cleaning operation continues without change. Alternatively, the second CL time T 2  expires (when the relation of T≧T 2  is satisfied), the process proceeds to step S 260 . 
     Subsequently, in step S 260 , the second cleaning operation is stopped by closing the K fifth on-off valves  56  to close the ink circulation pipes  55 . Moreover, the pressure of the sub-tank  25  is returned to the normal pressure in the standby state of the printing by switching the on-off valves  37  and  41  of the pressure adjusting device  34 , driving the third pump  39 , and depressurizing the sub-tank  25 . 
     In the second cleaning operation, unnecessary ink consumption can be reduced, since the air pressure Psub of the sub-tank  25  is increased up to the target increased pressure value PA and then the fifth on-off valves  56  are opened. For example, when the fifth on-off valves  56  are initially opened and the second pump  36  is driven to perform the pressurization, the ink may leak little by little from the nozzle of the printing head  43  while the sub-tank  25  is pressurized up to the target increased pressure value PA. The leaking ink is not strong, does not help to solve the clogging of the nozzle, and thus the ink is consumed unnecessarily. In the second cleaning operation according to this embodiment, however, the sub-tank  25  is sufficiently pressurized and then the fifth on-off valves  56  are opened. Therefore, since the ink discharged from the nozzles are initially strong, thereby helping to solve the clogging of the nozzle, the ink can be prevented from being consumed unnecessarily. 
     As another nozzle cleaning method, there may be considered a method of driving the fourth pump  50  in the state where all of the fifth on-off valves  56  are closed, and supplying the ink from the sub-tank  25  to the printing heads  43  via the third ink supply pipes  47  to forcibly discharge the ink from the nozzles of the printing heads  43 . In this case, however, since the loss of the pressure is large when the ink passes through the connection pipes  48  with the large passage resistance, the sub-tank  25  has to be pressurized by the second pump  36  and a high ink pressurizing force has to be generated on the upstream side by an ejection force of the fourth pump  50 . However, the ink discharged from the nozzles of the printing heads  43  is not strong. In the second cleaning operation according to this embodiment, however, the pressurized ink is supplied to the printing heads  43  via the ink circulation pipes  55  with the small passage resistance. Therefore, the loss of the pressure is small when the pressurized ink passes through the ink circulation pipes  55 . Moreover, the ink can be strongly discharged from the nozzles of the printing heads  43 . 
     In this embodiment, the following advantages can be obtained. 
     (1) The passage resistance R (≅R 2 &gt;R 1 ) of the third ink supply pipe  47  (supply passage) and the passage resistance R 3  of the ink circulation pipe  55  (circulation passage) are set to satisfy the relation of R&lt;R 3 . Therefore, the amounts of ink supplied to the printing heads  43  can be made nearly the same as each other. Moreover, the low ink pressure of the printing heads  43  can be maintained, while the difference in the ink pressure between the printing heads  43  is suppressed to be small. Accordingly, during the printing, an appropriate amount of ink droplets can be ejected within an allowable range of the ink pressure of each printing head  43 , while the leakage of the ink from the nozzles of each printing head  43  can be prevented. 
     (2) The passage resistance R 1  of the common pipe  47   b  of the third ink supply pipe  47 , the passage resistance R 2  of the connection pipe  48 , and the passage resistance R 3  of the ink circulation pipe  55  are set to satisfy the relation of R 1 &lt;R 3 &lt;R 2 . Therefore, the amounts of ink supplied to the printing heads  43  can be made nearly the same as each other. Moreover, the low ink pressure of the printing heads  43  can be maintained, while the difference in the ink pressure between the printing heads  43  is suppressed to be small. The ink circulation rate Qout is smaller than the ink supply rate Qin at least at the time of the printing and thus the ink circulation pipe  55  is configured to have a small diameter by this small amount, the size of the ink circulation pipe  55  can be reduced. 
     (3) In order to allow the variation in the ink pressure of the printing head  43  to be set within ±50 Pa, it is desirable that the passage resistance R 2  of the connection pipe  48  is five or more times the passage resistance R 3  of the ink circulation pipe  55 . Therefore, when the relation of R 2 ≧5·R 3  is satisfied, the variation in the ink pressure of the printing head  43  can be set within ±50 Pa in any printing mode. Accordingly, the amount of ink ejected from the nozzles of the printing head  43  can be stabilized. 
     (4) The ink supply rate Qin of ink larger than the maximum ink ejection rate Qhmax of the printing head  43  is supplied to the printing head  43  (Qin&gt;Qhmax) when the printing is performed with the maximum duty value Dfull (maximum ejection rate). Therefore, even when the printing is performed with the maximum duty value Dfull, the cooled ink flowing from the printing head  43  to the ink circulation pipe  55  can be prevented from flowing backward to the printing head  43 . As a consequence, since the temperature of the ink of the printing head  43  can be stabilized so as to have an appropriate value, the low viscosity of the ink appropriate for the ejection can be maintained in the printing head  43 . Accordingly, since a difference in the ejection performance of the ink between the printing heads  43  can be suppressed, the high print quality can be realized. 
     (5) In order to make the passage resistance R 2  of the connection pipe  48  large, the connection pipe  48  is formed to be long and thin. Therefore, by disposing the second heating device  72  in the connection pipe  48 , the ink flowing in the third ink supply pipe  47  can be effectively heated. 
     (6) By driving the fourth pump in the state where at least one of the on-off valves is closed in the first cleaning operation, the large amount of ink flows to the printing heads  43  of the cleaning targets by circulating the ink along the circulation passages passing from the sub-tank  25  to the printing heads  43  and the sub-tank  25  is depressurized. Accordingly, the bubbles in the ink of the printing heads  43  can be effectively removed. 
     (7) By allowing the third pump  39  to depressurize the sub-tank  25 , the bubbles can be more effectively removed while the bubbles in the ink of the printing heads  43  are suppressed from becoming small. Moreover, the amount of ink discharged from the nozzles  84  of the printing heads  43  can be suppressed to be small. 
     (8) In the second cleaning operation, the fifth on-off valves  56  are opened after the second pump  36  is driven in the state where the fifth on-off valves  56  are closed and the ink of the sub-tank  25  is pressurized (accumulated pressure) up to a predetermined pressure. Therefore, a nozzle cleaning operation can be performed while the ink is suppressed from being discharged unnecessarily during the pressurization. At this time, the fourth on-off valve  51  of the third ink supply pipe  47  with the large passage resistance R is closed and the pressurized ink is sent from the sub-tank  25  to the printing heads  43  via the ink circulation pipes  55  with the small passage resistance R 3 . With such a configuration, the loss of the pressure is small when the pressurized ink is supplied from the sub-tank  25  to the printing heads  43 . Therefore, a strong nozzle cleaning operation can be performed. Moreover, since the heated ink rarely flows to the third ink supply pipe  47  at the time of the second cleaning operation, the heated ink of the third ink supply pipe  47  is not unnecessarily discharged in the nozzle cleaning operation. Accordingly, at the time of the printing after the cleaning operation ends, the heated ink with the low viscosity in the third ink supply pipe  47  is used, and thus satisfactory printing can be performed. 
     (9) Since the sub-tank heater  33  is dipped into the ink of the sub-tank  25 , the average temperature increase speed (heating speed) of the entirety of the ink of the sub-tank  25  can be increased. 
     (10) Since the sub-tank  25  is made of an inorganic material with the heat conductivity lower than metal, the heat of the ink of the sub-tank  25  is hardly dissipated via the wall of the sub-tank  25 . Accordingly, the heating speed of the ink of the sub-tank  25  can be improved accordingly. 
     (11) The pipe portion  47   c  forming a part of the third ink supply pipe  47  on the upstream side in the sub-tank  25  is inserted so as to cross along the bottom surface of the sub-tank  25 , and the inflow port  47   d  of the pipe portion  47   c  is located on the opposite side of the ink inflow port  25   d  from the main tank  15 . Therefore, the ink which is not sufficiently heated immediately after the ink flows from the ink inflow port  25   d  can be prevented from being sent to the third ink supply pipe  47 . 
     (12) Since the first temperature sensor  32  is dipped into the ink of the sub-tank  25 , it is possible to increase a response speed in which the ink is heated after the real temperature of the ink of the sub-tank  25  is dropped. For example, by allowing the first temperature sensor  32  to rapidly detect the temperature of the ink flowing from the main tank  15  with the normal temperature, the sub-tank heater  33  can heat the ink rapidly. Therefore, even when the ink with the normal temperature is flowing, the ink heated at the first target temperature can be mainly supplied to the third ink supply pipe  47 . 
     (13) Since the first temperature sensor  32  is separated from the sub-tank heater  33  by the appropriate predetermined distance, it is possible to prevent the characteristic variation caused due to excessive heating of the ink, which is a problem occurring when the first temperature sensor  32  is too close to the sub-tank heater  33  or it is possible to prevent deterioration in the response and the reduction in the average temperature increase speed of the entirety of the ink of the sub-tank  25 , which are problems occurring when the first temperature sensor  32  is too far away from the sub-tank heater  33 . In particular, the first temperature sensor  32  is disposed in the range of the opposite side of the ink inflow port  25   d  with reference to the center of the sub-tank heater  33  and is disposed within the range (in particular, the position closer to the sub-tank heater  33  than the center position of the range) of the half of the depth between the center position of the half of the depth from the liquid level A 2  to the sub-tank heater  33  at the time of stopping the ink supply from the main tank  15 . Therefore, it is possible to increase the response speed until the start of the heating when the ink with the normal temperature flows into the sub-tank  25  and the average temperature increase speed (the increase speed of the average temperature obtained by averaging the temperature distribution of the ink of the sub-tank  25 ) of the entirety of the ink after the start of the heating. 
     (14) The connection pipes  48  are heated by the heat transfer member  74  of which the temperature is nearly the same as the temperature of the supply passage heater  54  by transferring the heat of the supply passage heater  54  in the state where the connection pipes  48  pass through the heat transfer member  74  (heating block). Therefore, the heated ink in the connection pipes  48  can be heated without a difference in the temperature by transferring the heat from the heat transfer member  74  maintained nearly at the target temperature. 
     (15) By disposing the third temperature sensor  53  in the heat transfer member  74 , the supply passage heater  54  is controlled on the detection result of the surface temperature of the heat transfer member  74 . Therefore, since the heat transfer member  74  can be maintained nearly at the target temperature, the heated ink in the connection pipes  48  can be heated without a difference in the temperature by transferring the heat from the heat transfer member  74  maintained nearly at the target temperature. 
     (16) In the temperature keeping device  73 , the head cover  85  (heating member) transferring and heating the heat of the head heater  45  is disposed on the head side wall from the circumference of the nozzle formation surface  81   a . Therefore, the heat of the head heater  45  is transferred to the circumference of the nozzle formation surface  81   a  via the head cover  85 , and thus the temperature of the printing head  43  can be kept at the target temperature from the nozzle  84  which is the downstream end of the passage. Accordingly, since the liquid in the nozzles  84  or right near the upstream side of the nozzles  84  can be maintained at the appropriate heating temperature, the ink with the low viscosity can be ejected from the nozzles  84  and thus the satisfactory ejection can be realized. 
     (17) The head heater  45  is controlled on the basis of the detection result of the surface temperature of the head heater  45  by disposing the second temperature sensor  44  in the head heater  45 . With such a configuration, since the head heater  45  can be maintained at the target temperature, the heat of the head heater  45  maintained at the target temperature can be transferred to the circumference of the nozzle formation surface  81   a  via the head cover  85 . Therefore, even when the head main body  80  is made of resin, the head section  81  can be maintained at the target temperature. As a consequence, since the liquid in the nozzles  84  or right near the upstream side of the nozzles  84  can be maintained at the appropriate heating temperature, the satisfactory ejection of the ink droplets can be realized. 
     (18) Since the heat of the head heater  45  is transferred to the head cover  85  via the heat transfer plate  86 , the heat can effectively be transferred to the head cover  85 . 
     The above-described embodiment may be modified in the following other forms. 
     The second cleaning operation is not limited to the method of driving the third pump  39  (pressurizing pump). For example, the second cleaning operation may be performed by driving the fourth pump  50  (supply pump). That is, the N fifth on-off valves  56  disposed in the ink circulation pipes  55  are closed to drive the fourth pump  50 . The ink is sent to the printing heads  43  via the third ink supply pipe  47  by the driving of the fourth pump  50  in the state where the flow of the ink is blocked by the fifth on-off valves  56  closing the ink circulation pipes  55  on the downstream side of the printing heads  43 . Therefore, the ink pressure of the printing heads  43  is increased at one time and thus the ink is strongly discharged from the nozzles. 
     The configuration and the method of performing the second cleaning operation (nozzle cleaning operation) to solve the clogging of the nozzles can be used in a configuration and a method shown in  FIG. 12 . For example, by driving the fourth pump  50  in the state where all of the fifth on-off valves  56  are closed, it is possible to use the configuration and the method of discharging the ink from the nozzles of the printing heads  43 . In this case, as shown in  FIG. 12 , N sixth on-off valves  90  are disposed in the connection pipes  48  branching from the third ink supply pipe  47  in parallel. By driving the fourth pump  50  (supply pump) in the state where all of the sixth on-off valves  90  are closed, the pressure of the ink on the upstream side of the sixth on-off valves  90  is made to accumulate. M sixth on-off valves  90  corresponding to the printing heads  43  of the cleaning targets are selectively opened at the time at which the ink pressure is sufficiently increased (at the time at which the accumulation of the pressure ends) to realize the nozzle cleaning operation. In this way, the nozzle cleaning operation may be realized not only by the fifth on-off valves  56  disposed in the ink circulation pipes  55  but also by the fourth pump  50  sending the ink from the sub-tank  25  to the printing heads  43  via the third ink supply pipe  47  and the sixth on-off valves  90  disposed on the connection pipes  48 . In this case, the heated ink stored in the third ink supply pipe  47  is supplied to the printing heads  43  and the ink is discharged to perform the cleaning operation. However, since the printing heads  43  are filled with the heated ink after the end of the cleaning operation, the heated ink is satisfactorily ejected in the next printing. As in the above-described embodiment, when the pressurized ink flows backward in the direction opposite to the supply direction via the ink circulation pipes  55 , the ink cooled in the ink circulation pipes  55  flows into the printing heads  43 . Thereafter, the printing may not start for a while until the ink in the printing heads  43  is heated. In the second cleaning operation, however, the ink flows in the supply direction and the printing heads  43  are filled with the heated ink after the end of the nozzle cleaning operation. Therefore, the printing can start after a relatively short time until the temperature is stabilized. 
     By selectively opening and closing the N sixth on-off valves  90  in  FIG. 12 , the first cleaning operation may be performed. That is, the ink is circulated along the circulation path passing through the M printing heads  43  of the cleaning targets, by driving the fourth pump  50  (supply pump) after the M sixth on-off valves  90  selected as the cleaning targets among the N sixth on-off valves  90  are opened. Of course, when the fifth on-off valves  56  are also disposed in the ink circulation pipes  55 , as in  FIG. 12 , at least the M fifth on-off valves  56  corresponding to the printing heads  43  of the cleaning targets are opened. When the first cleaning operation is performed, the ink pressure Phncl of the printing head  43  of the non-cleaning target blocked by the closed sixth on-off valve  90  is not taken into consideration. Therefore, Phcl may be set as the negative pressure value PD of the sub-tank pressure Psub. The position of the fourth pump  50  serving as the supply pump may be moved to the side of the ink circulation pipes  55  in the state where the liquid can be sent to the circulation direction, and the second cleaning operation may be performed by allowing the ink to flow along the path passing through the third ink supply pipe  47  (supply passage). In this case, the second cleaning operation is performed by driving the third pump  39  (pressurizing unit) in the state where the N sixth on-off valves  90  are closed, pressurizing the sub-tank  25  to form the pressure accumulation state, completing the accumulation of the pressure, opening the M sixth on-off valves  90 , and then sending the ink to the M printing heads  43  via the third ink supply pipe  47  (supply passage). When the first and second cleaning operations are performed by selectively opening and closing all of the sixth on-off valves  90 , the fifth on-off valves  56  of the ink circulation pipes  55  may be eliminated. 
     In the embodiment, the sub-tank  25  serving as a tank may be configured as a plurality of units to correspond to the printing heads  43 , respectively. In this case, the downstream end of the ink circulation pipe  55  may be inserted into or connected to each sub-tank  25 . 
     In the embodiment, one of the main tank and the sub-tank may be provided. By providing only one tank, the configuration may be formed between the one tank and the printing heads  43  to supply and circulate the ink. The ink cartridge may be used as a tank. In this case, when the ink cartridge is mounted on the holder unit, the ink cartridge may be connected to the upstream end of the supply passage and the downstream end of the circulation passage and may be also connected to one end of the passage connected to the second pump  36 , the third pump  39 , and the pressure opening plate  40 . In the ink cartridge, the ink may be stored in a case or an ink pack may be received in the case. 
     In the embodiment, by disposing one variable throttle plate in each ink circulation pipe  55  to adjust a throttle amount of the variable throttle plate, the passage resistance R 3  of the ink circulation pipes  55  may be adjusted together or separately. For example, by controlling the adjustment of the throttle amount of the variable throttle plate in accordance with the duty value D, the ink pressure of the printing heads  43  may be adjusted to an appropriate value. 
     In the embodiment, the negative pressure value upon depressurizing the sub-tank  25  may be obtained in the following method. The negative pressure value is obtained by analyzing print data (liquid ejection processing data), calculating the number of print dots per unit time, estimating the ink ejection rate (cc/minute) from the value corresponding to the number of print dots calculated, and obtaining a negative pressure value corresponding to the estimated ink ejection rate with reference to table data. For example, the maximum ink ejection rate Qhm (cc/minute) during the course (that is, during the printing period) of completing the printing may be calculated on the basis of the print data, a given value Qo may be added to the maximum ink ejection rate Qhm, and the ink supply rate Qin (=Qhm+Qo) may be calculated. For example, the given value Qo is set to a value of the necessary ink circulation rate Qout or the ink circulation rate Qout+a margin rate. In this case, the ink larger than the given value Qo flows in the circulation passage from the start of the printing to the end of the printing. 
     By analyzing the print data (liquid ejection processing data) and sequentially calculating and estimating the amounts of ink ejected after a predetermined time expires in the range from 10 milliseconds to 10 seconds from the present time during the printing on the basis of the analysis result, the depressurization of the sub-tank  25  may be controlled in real time to obtain the negative pressure value corresponding to the amount of ink ejected at that time. Here, the predetermined time corresponds to a response time expressed as the sum of an amount of time required until the pressure of the sub-tank  25  actually becomes the negative pressure value after the pressure control starts to set the pressure of the sub-tank  25  to the negative pressure value (target negative pressure value) and an amount of time required until the liquid pressure of the meniscus of the ink in the nozzles becomes a desired pressure after the pressure of the sub-tank  25  becomes the target negative pressure value. 
     In the embodiment, the ink supply rate Qin may be variable. For example, when Qhmax is variable depending on the printing mode (ejection mode), Qin may be variable in the range in which the relation of Qin&gt;Qhmax is satisfied. When the print data (liquid ejection processing data) can be analyzed and the ink ejection rate Qh can be estimated, Qin may be variable so that the relation of Qin&gt;Qhmax is satisfied in accordance with the estimated ink ejection rate Qh. Alternatively, the ink may be supplied by the ink supply rate Qin satisfying a relation of Qin=Qh+Qoutcnst (where, Qoutcnst is a given value) so that the ink circulation rate Qout is as constant as possible. With such a configuration, even when the ink ejection rates Qh are different from each other between the printing heads  43 , the ink circulation rate Qout can be normally constant (=Qoutcnst). Therefore, the difference in the ink pressure between the printing heads  43  can be almost solved. 
     In the embodiment, the relation of the passage resistances may satisfy a relation of R 3 &lt;R 1 &lt;R 2 . In this case, since the passage resistance R of the third ink supply pipe  47  is determined as the passage resistance R 2  of the connection pipe  48 , the relation of R&gt;R 3  is constantly satisfied. In this way, by allowing the passage resistance R 3  of the ink circulation pipe  55  to be the smallest among the passage resistances, the variation in the ink pressure of the printing heads  43  can further be reduced and thus the difference in the ink pressure between the printing heads  43  can further be reduced. As a consequence, the difference in the size (or weight) of the ink droplets between the printing heads  43  can be made small. 
     In the embodiment, one third ink supply pipe  47  may be disposed in each printing head  43 . With such a configuration, when the passage resistance R of the third ink supply pipe  47  and the passage resistance R 3  of the ink circulation pipe  55  satisfy the relation of R&gt;R 3 , the same advantage can be obtained. 
     The pipe portion  47   c  (pipe passage) may be inserted so as to extend nearly in parallel to the bottom surface of the sub-tank  25 . For example, the pipe passage may be inserted so as to extend nearly in parallel to the bottom surface (or the liquid level) of the sub-tank  25  above the sub-tank heater  33 . Alternatively, the pipe passage may be inserted so as to extend in a direction intersecting the direction nearly parallel to the bottom surface (or the liquid level) of the sub-tank  25 . 
     The shape of the heating block is not limited to the plate shape, but may be a rectangular shape, a cubic shape, a cylindrically columnar shape, a pyramidal shape, or a plate-shaped block with a convex portion, which extends along a portion (pipe passage of the connection pipe) in which the connection pipe passes through the inside thereof, on at least one of the front and rear surfaces. The heating block sufficient when the connection pipes are covered by the heating block is not limited to the configuration in which the connection pipe is disposed between two members (the block and the plate). For example, the connection pipe may penetrate through a through-hole formed in the heating block. 
     The tank may be disposed below the liquid ejecting heads or at the same height of the liquid ejecting heads in the direction of gravity. In this case, in order to ensure the ink pressure necessary in the liquid ejection head, the tank need not be depressurized but may be pressurized by a pressurizing unit during the printing (liquid ejecting operation). 
     The heating unit may be disposed in one of the tank and the supply passage. Alternatively, the heating unit (temperature keeping unit) may not be disposed in the liquid ejecting head. In this case, it is desirable that the chamber or the passage in the liquid ejecting head is covered with a material with a high temperature keeping property to improve the temperature keeping property of the liquid ejecting head. 
     The ink jet printer to which the invention is applied may be any printer such as a line printer, a serial printer, or a page printer. 
     During the standby state before the printing starts, the fourth pump  50  is operated to supply the amount of liquid smaller than the liquid supply rate during the printing. In this case, by applying a negative pressure to the sub-tank by the pressure adjusting device  34 , the liquid is supplied to the degree that the liquid does not leak from the head. Alternatively, even in the state where the pressure adjusting device  34  is not driven, the liquid may be supplied to the degree that the liquid does not leak from the head. 
     In the above-described embodiment, the circulation passage may include one circulation backward passage and a plurality of discharge passages, as in JP-A-11-342634. 
     In the above-described embodiment, the liquid may be supplied from the main tank (ink tank) to the liquid ejecting heads via the supply passage, as in JP-A-11-342634. 
     In the above-described embodiment, a blocking unit is not limited to the on-off valve such as the fourth on-off valve  51 . For example, the fourth pump  50  may serve as the blocking unit. For example, when the fourth pump  50  can block the flow of the liquid like a gear pump, the fourth pump  50  may be used as the blocking unit. In this case, the fourth on-off valve  51  may be eliminated. 
     The unit supplying/stopping the supply of the ink, which is an example of a liquid, may be an on-off valve disposed in the supply passage, in a case where the ink is supplied using the liquid head difference. That is, when the on-off valve is opened, the liquid is supplied from the tank to the liquid ejecting heads using the liquid head difference. When the on-off valve is closed, the supply of the liquid from the tank to the liquid ejecting heads is stopped. 
     The printing head  43  may be a piezoelectric type printing head, an electrostatic type printing head, or a thermal type printing head. 
     The negative pressure value of the sub-tank  25  is variable in accordance with the duty value D, but the negative pressure value may be constant. 
     The ink which is an example of the liquid is not limited to the UV ink. For example, the ink may be thermal cured ink, water-based or oil-based pigment ink, or dye ink. 
     The target is not limited to the resin film, but may be a sheet, a cloth, or a metal film. 
     In the above-described embodiment, the liquid ejecting apparatus is realized as the ink jet printer  11 , but the invention is not limited thereto. The invention is applicable to a liquid ejecting apparatus ejecting or jetting other liquids (including a liquid-formed substance in which particles of a function material are dispersed or mixed in a liquid and a fluid-formed substance such as gel) other than ink. Examples of the liquid ejecting apparatus include: a liquid-formed substance ejecting apparatus ejecting a liquid-formed substance in which a material such as an electrode material or a coloring material (pixel material) used to manufacture a liquid crystal display device, an EL (electroluminescence) display device, and a plane emission display is dispersed or solved; a liquid ejecting apparatus ejecting a bio organic material used to manufacture a bio chip; and a liquid ejecting apparatus ejecting a liquid as a sample used by a precise pipette. In addition, there may be used a liquid ejecting apparatus ejecting a lubricant to a precision instrument such as a clock or a camera by a pin point; a liquid ejecting apparatus ejecting a transparent resin liquid such as ultraviolet cured resin on a substrate to form a minute hemispheric lens (optical lens) used in an optical communication element or the like; a liquid ejecting apparatus ejecting an etchant such as acid or alkali to etch a substrate or the like; and a fluid-formed substance ejecting apparatus ejecting a fluid-formed substance such as gel (for example, physical gel). In addition, the invention is applicable to one thereof. 
     The technical spirit grasped from the above-described embodiment and the modified examples will be described below. 
     (A) There is provided the liquid ejecting apparatus further including a liquid supplying unit supplying the liquid from the tank to the liquid ejecting heads via the supply passage. The passage resistances are set when the liquid supplying unit supplies the liquid supply flow rate of the liquid during the liquid ejection operation of the liquid ejection heads. 
     (B) There is provided the liquid ejecting apparatus further including a depressurizing unit depressurizing the tank. The depressurizing unit depressurizes the tank to the negative pressure upon performing the cleaning operation. 
     (C) There is provided the liquid ejecting apparatus further including: a plurality of on-off valves disposed in the circulation passage for the liquid ejecting heads, respectively; and a supply pump disposed in the supply passage and supplying the liquid from the tank to the liquid ejecting heads. A cleaning operation is performed by driving the supply pump in a state where all of the plurality of on-off valves are closed, sending the liquid to the plurality of liquid ejecting heads, and discharging the liquid from the nozzles of the plurality of liquid ejecting heads.