Patent Publication Number: US-2023150263-A1

Title: Inkjet recording apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent This application is based on and claims the benefit of priority from Japanese Patent Application No. 2021-184879 filed on Nov. 12, 2021, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to an inkjet recording apparatus, and particularly relates to a cooling mechanism for cooling a recording head that ejects ink. 
     Inkjet recording apparatuses that record an image by ejecting ink from nozzles provided in recording heads are capable of forming a high-definition image and thus have been widely used as recording apparatuses such as facsimile machines, copiers, and printers. 
     In such an inkjet recording apparatus, the temperature of the recording head varies depending on factors such as the operation history and the installation environment of the apparatus, and this may cause uneven image density. A drastic rise in temperature of the recording head may cause an ink discharge error and a discharge failure. Further, in an inkjet recording apparatus capable of recording color images, temperature unevenness among inkjet heads for different colors may cause variations in color tone of images. 
     A cause of such inconvenience is that IC chips (CPU) mounted in recording heads are liable to malfunction under a temperature equal to or higher than a predetermined temperature. Thus, in order to maintain ink ejection capacity, it is necessary to keep the temperature of a recording head, especially the temperature of an IC chip, within a constant range. 
     SUMMARY 
     According to an aspect of the present disclosure, an inkjet recording apparatus includes a conveying portion, a recording portion, a temperature detecting portion, a cooling mechanism, and a control portion. The conveying portion conveys a recording medium. The recording portion includes a line head for each color, the line head including a plurality of recording heads that are arranged along a width direction of the recording medium orthogonal to a conveyance direction of the recording medium, and that execute a printing process by ejecting ink onto the recording medium conveyed by the conveying portion. The temperature detecting portion detects a temperature of the recording heads. The cooling mechanism cools the recording heads based on a result of detection by the temperature detecting portion. The control portion controls the cooling mechanism. The cooling mechanism includes a storing portion, a circulation path, a heat dissipating portion, and a pump, and the cooling mechanism is provided one for each of the line heads. The storing portion stores a cooling liquid. The circulation path has opposite ends thereof connected to the storing portion, is arranged to be branched into a plurality of branch paths so as to pass near the recording heads, and receives heat of the recording heads by means of the cooling liquid circulating therein. The heat dissipating portion is arranged in the circulation path, at a position downstream from the recording heads with respect to a circulation direction of the cooling liquid, and dissipates heat of the cooling liquid. The pump is arranged in the circulation path, at a position upstream from the recording heads with respect to the circulation direction of the cooling liquid, and causes the cooling liquid to circulate in the circulation path. If there exists, among the recording heads, a recording head that is predicted to undergo a temperature rise as compared with others of the recording heads, when driving the cooling mechanism, the control portion increases a flow amount or circulation time of the cooling liquid in whichever of the branch paths serves as a flow path of the cooling liquid flowing toward the recording head that is predicted to undergo a temperature rise, as compared with in others of the branch paths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a side sectional view schematically showing a configuration of a printer according to an embodiment of the present disclosure, 
         FIG.  2    is a side sectional view showing a structure around a first belt conveying portion, a recording portion, and a second belt conveying portion of the printer; 
         FIG.  3    is a plan view of the first belt conveying portion and the recording portion of the printer as viewed from above; 
         FIG.  4    is a block diagram showing an example of a control path of the printer; 
         FIG.  5    is a schematic diagram of a cooling unit used in the printer of the present embodiment; 
         FIG.  6    is a flow chart showing an example of recording-head cooling control executed in the printer before printing is started; and 
         FIG.  7    is a flow chart showing an example of recording-head cooling control executed in the printer during a printing process. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the accompanying drawings.  FIG.  1    is a diagram schematically showing a configuration of a printer  100  employing an inkjet printing method according to an embodiment of the present disclosure.  FIG.  2    is a side sectional view showing a structure around a first belt conveying portion  5 , a recording portion  9 , and a second belt conveying portion  12  of the printer  100 .  FIG.  3    is a plan view of the first belt conveying portion  5  and the recording portion  9  of the printer  100  as viewed from above. 
     As shown in a  FIG.  1   , in the printer  100 , in a lower portion inside a printer main body  1 . a sheet feeding cassette  2   a  is arranged as a sheet containing portion, and a manual sheet feeding tray  2   b  is provided on an outer right side surface of the printer main body  1 . Downstream of the sheet feeding cassette  2   a  in a sheet conveying direction (that is, to the upper right of the sheet feeding cassette  2   a  in  FIG.  1   ), a sheet feeding device  3   a  is arranged. Downstream of the manual sheet feeding tray  2   b  in the sheet conveying direction (that is, to the left of the manual sheet feeding tray  2   b  in  FIG.  1   ), a sheet feeding device  3   b  is arranged. By these sheet feeding devices  3   a  and  3   b , sheets P are separated from each other to be conveyed one by one. 
     Inside the printer  100 , a first sheet conveying path  4   a  is provided. The first sheet conveying path  4   a  is located to the upper right of the sheet feeding cassette  2   a . and to the left of the manual sheet feeding tray  2   b . A sheet P sent out from the sheet feeding cassette  2   a  passes through the first sheet conveying path  4   a  to be conveyed vertically upward along a side surface of the printer main body  1 . A sheet P sent out from the manual sheet feeding tray  2   b  passes through the first sheet conveying path  4   a  to be conveyed substantially horizontally leftward. 
     At a downstream end of the first sheet conveying path  4   a  with respect to the sheet conveying direction, a pair of registration rollers are provided. Further, closely downstream of the pair of registration rollers  13 . the first belt conveying portion (conveying portion)  5  and the recording portion  9  are arranged. The pair of registration rollers  13  on one hand correct skewed feeding of a sheet P, and on the other hand send out a sheet P toward the first belt conveying portion  5  with timing coordinated with an ink ejection operation executed by the recording portion  9 . 
     Further, between the pair of registration rollers  13  and the first belt conveying portion  5 , there is arranged a contact image sensor (CIS)  20  for detecting a position of an edge of a sheet P in a width direction of the sheet P (a direction perpendicular to the sheet conveying direction). 
     The first belt conveying portion  5  is provided with a first conveying belt  8  which is an endless belt wound around a first driving roller  6  and a first driven roller  7 . A sheet P sent out from the pair of registration rollers  13  passes under the recording portion  9  while being sucked and held on a conveying surface  8   a  (an upper surface in  FIG.  1   ) of the first conveying belt  8 . 
     At a position that is inside the first conveying belt  8  and opposite a back side of the conveying surface  8   a  of the first conveying belt  8 , a first sheet sucking portion  30  is provided. The first sheet sucking portion  30 , having a large number of holes  30   a  for air suction formed in an upper face thereof and a fan  30   b  arranged inside thereof, is capable of sucking air downward through the upper face thereof. The first conveying belt  8  is also provided with a large number of vent holes (not shown) for air suction. This configuration allows the first belt conveying portion  5  to convey a sheet P while sucking and holding the sheet P on the conveying surface  8   a  of the first conveying belt  8 . 
     The recording portion  9  performs recording of an image on a sheet P conveyed thereto by being sucked and held on the conveying surface  8   a  of the first conveying belt  8 . As shown in  FIGS.  2  and  3   , the recording portion  9  includes a head housing  10  and four line heads  11 C,  11 M,  11 Y, and  11 K supported in the head housing  10 . These line heads  11 C to  11 K each have a recording area as wide as or wider than a sheet P conveyed, are each supported at such a height that a predetermined gap (for example, 1 mm) is formed with respect to the conveying surface  8   a  of the first conveying belt  8 , and each have three recording heads  17  arranged in a staggered array along a sheet width direction (a BB′ direction in  FIG.  3   ) orthogonal to the sheet conveying direction. The recording heads  17  each have an ink ejection surface, in which a large number of ink ejection nozzles  18  are arrayed. 
     The recording heads  17  constituting the line heads  11 C to  11 K are fed with ink of four colors (cyan, magenta, yellow, and black) stored in ink tanks (not shown) corresponding to the colors of the line heads  11 C to  11 K. 
     According to information included in image data received from an external computer or the like, the line heads  11 C to  11 K sequentially eject ink of cyan, magenta, yellow, and black from the ink ejection nozzles  18  of the recording heads  17  corresponding to printing positions toward a sheet P conveyed by being sucked and held on the conveying surface  8   a  of the first conveying belt  8 . As a result, on the sheet P, there is formed a full-color image having ink of the four colors, namely, cyan, magenta, yellow, and black, superposed one on top of another. 
     Downstream of the first belt conveying portion  5  with respect to the sheet conveying direction (a left side in  FIG.  1   ), the second belt conveying portion  12  is arranged. A sheet P having had an image recorded thereon at the recording portion  9  is sent to the second belt conveying portion  12 , and the ink which has been ejected onto a surface of the sheet P is dried while the sheet P passes through the second belt conveying portion  12 . 
     The second belt conveying portion  12  includes a second conveying belt  40  which is an endless belt wound around a second driving roller  41  and a second driven roller  42 . The second conveying belt  40  is caused by the second driving roller  41  to rotate counterclockwise in  FIG.  2   . The sheet P having had an image recorded thereon by the recording portion  9  and having been conveyed by the first belt conveying portion  5  in an arrow-X direction is transferred to the second belt conveying belt  40  to be conveyed in an arrow-Z direction in  FIG.  2   . 
     At a position that is inside the second conveying belt  40  and is opposite a back side of the conveying surface  40   a  of the second conveying belt  40 , a second sheet sucking portion  43  is provided. The second sheet sucking portion  43 , having a large number of holes  43   a  for air suction formed in an upper face thereof and a fan  43   b  arranged inside thereof, is capable of sucking air downward through the upper face thereof. The second conveying belt  40  also has a large number of vent holes (not shown) for air suction formed therein. This configuration allows the second belt conveying portion  12  to convey a sheet P while sucking and holding the sheet P on the conveying surface  40   a  of the second conveying belt  40 . 
     At a position that is downstream of the second belt conveying portion  12  with respect to the sheet conveying direction and is close to a left side surface of the printer main body  1 , a decurler portion  14  is provided. A sheet P having had the ink thereon dried at the second belt conveying portion  12  is sent to the decurler portion  14 , where a curl developed in the sheet P is corrected. 
     Downstream of (in  FIG.  1   , above) the decurler portion  14  with respect to the sheet conveying direction, a second sheet conveying path  4   b  is provided. In a case where duplex recording is not to be performed, after passing through the decurler portion  14 , the sheet P is conveyed through the second sheet conveying path  4   b , via a pair of discharge rollers, onto a sheet discharge tray  15  provided on an outer left side surface of the printer  100 . In a case where duplex recording is to be performed, the sheet P on one side of which recording has been completed passes through the second belt conveying portion  12  and the decurler portion  14 , and then passes through the second sheet conveying path  4   b  to be conveyed into a reverse conveying path  16 . In the reverse conveying path  16 , the sheet conveying direction is switched to turn the sheet P upside down, and then the sheet P passes through an upper portion of the printer  100  to be conveyed to the pair of registration rollers  13 . Then, the sheet P is conveyed, with its unrecorded surface up, back to the first belt conveying portion  5 . 
     Under the second belt conveying portion  12 , a maintenance unit  19  is arranged. When executing maintenance of the recording heads  17 , the maintenance unit  19  moves to under the recording portion  9  to wipe off the ink ejected (purged) from the ink ejection nozzles  18  of the recording heads  17 , and collects the wiped-off ink. 
       FIG.  4    is a block diagram showing an example of a control path of the printer  100 . In addition to the above-mentioned configuration, the printer  100  further includes belt driving motors  21  and  22 , a temperature detecting sensor  23 , an operation panel  27 , a storage portion  28 , a communication portion  29 , and a cooling unit  50 . 
     The belt driving motors  21  and  22  respectively cause the first driving roller  6  and the second driving roller  41  to rotate to thereby cause the first conveying belt  8  and the second conveying belt  40  to rotate. 
     The temperature detecting sensor  23  is provided in the recording portion  9  inside the printer  100 , and detects an ambient temperature of the recording heads  17 . Note that the temperature detecting sensor  23  may instead detect a surface temperature or an inner temperature of the recording heads  17 . A result of detection by the temperature detecting sensor  23  is transmitted to a cooling unit control portion  110   b . 
     An operation panel  27  is an operation portion for accepting inputs of various settings. For example, by operating the operation panel  27 , a user can input a size of a sheet P to be set in the sheet feeding cassette  2   a  or in the manual sheet feeding tray  2   b , that is, the size of the sheet P to be conveyed by the first conveying belt  8 . By operating the operation panel  27 . the user can also input the number of sheets P to be printed and give an instruction to start a print job. The operation panel  27  further functions as a notification device that provides notification regarding the operation status of the printer  100 . 
     The storage portion  28  is a memory that stores an operation program for a control device  110  and various kinds of information, and is configured by including a read only memory (ROM), a random-access memory (RAM), a nonvolatile memory, etc. Information set via the operation panel  27  is stored in the storage portion  28 . 
     The communication portion  29  is a communication interface for transmitting and receiving information to and from an external device (for example, a personal computer (PC)). For example, when the user operates a PC to transmit a printing command together with image data to the printer  100 , the image data and the printing command is input to the printer  100  via the communication portion  29 . In the printer  100 , a main control portion  110   a  controls the recording heads  17  based on the image data mentioned above and causes them to eject ink, whereby an image can be recorded on a sheet P. 
     The cooling unit  50  cools the recording heads  17  arranged in each of the line heads  11 C to  11 K of the recording portion  9 . The configuration of the cooling unit  50  will be described later in detail. 
     The printer  100  of the present embodiment includes the control device  110 . The control device  110  is configured by including, for example, a central processing unit (CPU) and a memory. Specifically, the control device  110  includes the main control portion  110   a , the cooling unit control portion  110   b , a sheet feed control portion  110   c , and a maintenance control portion  110   d . 
     The main control portion  110   a  controls operations of various portions of the printer  100 . For example, operations such as driving of various rollers provided inside the printer  100 , ejection of ink from the recording heads  17  during image recording, and the like are controlled by the main control portion  110   a . 
     The cooling unit control portion  110   b , based on the ambient temperature of the recording heads  17  detected by the temperature detecting sensor  23 , determines whether or not the recording heads  17  need to be cooled by the cooling unit  50 ; and if the recording heads  17  need to be cooled, the cooling unit control portion  110   b  transmits a control signal to the cooling unit  50  to drive the cooling unit  50 . 
     The sheet feed control portion  110   c  is a recording medium feeding control portion that controls the pair of registration rollers  13  functioning as a recording medium feeding portion. For example, the sheet feed control portion  110   c  controls the pair of registration rollers  13  based on timing when the CIS  20  detects the rear edge of a sheet P, to thereby control the timing of conveying the subsequent sheet P. 
     The maintenance control portion  110   d  performs control for causing the recording heads  17  to execute the above-described purging of forcibly pushing the ink out of each of the ink ejection nozzles  18 . When causing the recording heads  17  to execute the purging, the maintenance control portion  110   d  also controls driving of the above-described maintenance unit  19  (for example, movement and retreat of the maintenance unit  19  downward of the recording portion  9 ). 
     The control device  110  may further include a calculation portion that performs necessary calculations and a timer that counts time. Or, the functions of the calculation portion and the timer may also be executed by the main control portion  110   a . 
       FIG.  5    is a schematic diagram of the cooling unit  50  used in the printer  100  of the present embodiment.  FIG.  5    shows the cooling unit  50  that cools the recording heads  17  arranged in the line head  11 C; the other line heads  11 Y to  11 K are also each provided with the cooling unit  50  for cooling the recording heads  17  arranged therein, and all the cooling units  50  have completely the same configuration. For convenience of description, a distinction is made among the three recording heads  17  arranged in the line head  11 C, such that the one arranged on the left side (an arrow-B direction side) looking in the sheet conveying direction (the arrow-X direction) will be referred to as the recording head  17   a . the one arranged at the center will be referred to as the recording head  17   b , and the one arranged on the right side (an arrow-B′ direction side) will be referred to as the recording head  17   c . 
     The cooling unit  50  includes a circulation tube (a circulation path)  51 , a radiator (a heat dissipating portion)  60 , a pump  70 , a reservoir tank (a storing portion)  80 , and an electromagnetic valve  81 . The circulation tube  51  has opposite end portions thereof connected to the reservoir tank  80 , and is arranged so as to pass near the recording heads  17   a  to  17   c , and a cooling liquid (here, water) stored in the reservoir tank  80  is caused to circulate in the circulation tube  51  to thereby receive heat of the recording heads  17   a  to  17   c . 
     The circulation tube  51 , at a branching portion  55  located downstream of the electromagnetic valve  81  with respect to a circulation direction of the cooling liquid, branches into three tubes, namely, a first tube  51   a , a second tube  51   b , and a third tube  51   c , which are rejoined into one path at a joining portion  57 . The first, second, and third tubes  51   a ,  51   b , and  51   c  are arranged so as to pass near the recording heads  17   a ,  17   b , and  17   c , respectively. Of the circulation tube  51 , flow paths (forward paths) from the reservoir tank  80  to the recording heads  17   a  to  17   c  are indicated by solid lines, and flow paths (return paths) from the recording heads  17   a  to  17   c  to the reservoir tank  80  are indicated by dotted lines. 
     The first, second, and third tubes  51   a ,  51   b , and  51   c  are in contact with the recording heads  17   a ,  17   b , and  17   c , respectively, via heat receiving portions  53 . The heat receiving portions  53  are formed of a thermally conductive material such as ceramic, for example, and have a high thermal conductivity. The heat receiving portions  53  are arranged adjacent to IC chips  54  mounted on the recording heads  17   a  to  17   c . Note that, instead of providing the heat receiving portions  53 , the first to third tubes  51   a  to  51   c  themselves may be formed as silicone or rubber tubes containing a thermally conductive material such as ceramic to be arranged in contact with the recording heads  17   a  to  17   c . 
     The radiator  60  is arranged downstream from the joining portion  57  of the circulation tube  51  with respect to the circulation direction of the cooling liquid, and dissipates heat of the cooling liquid in the circulation tube  51 . The radiator  60  is provided with a cooling fan (not shown) for air-cooling of the cooling liquid flowing through the radiator  60 . 
     The pump  70  is arranged upstream from the branching portion  55  (the electromagnetic valve  81 ) of the circulation tube  51  with respect to the circulation direction of the cooling liquid, and circulates the cooling liquid in the circulation tube  51 . The reservoir tank  80  temporarily stores the cooling liquid. 
     The electromagnetic valve  81  is arranged in the branching portion  55 , at which the circulation tube  51  is branched into the first to third tubes  51   a  to  51   c . The electromagnetic valve  81  is capable of individually opening and closing flow paths toward the first to third tubes  51   a  to  51   c . 
     The cooling liquid sent out from the reservoir tank  80  by the pump  70  flows toward at least one of the first to third tubes  51   a  to  51   c  via the electromagnetic valve  81  receives heat of the recording heads  17   a  to  17   c  while passing through the heat receiving portions  53 , and is then cooled at the radiator  60 . After being cooled at the radiator  60 , the cooling liquid returns into the reservoir tank  80 . 
     If, at start of printing, the temperature of the recording heads  17   a  to  17   c  is above an upper limit value (for example, 75° C.) of a printable temperature, it is necessary to drive the cooling unit  50  to cool the recording heads  17   a  to  17   c  down to the upper limit value of the printable temperature or below. 
     However, if, in the next printing process (print job), printing is performed with respect to a sheet P of which a width-direction size is a predetermined size (for example, A4 lateral size) or smaller, ink is not ejected from the recording heads  17   a  and  17   c  located on opposite sides in the width direction, but ink is ejected only from the recording head  17   b  located at the center in the width direction. Accordingly, even when the temperature of the recording heads  17   a  and  17   c  is above the upper limit value of the printable temperature, the printing process can be executed as long as the temperature of the recording head  17   b  is equal to or lower than the upper limit value of the printable temperature. 
     Thus, in the present embodiment, by providing the electromagnetic valve  81  at the branching portion  55  of the circulation tube  51  and by switching the flow path of the cooling liquid by means of the electromagnetic valve  81 , cooling is performed focusing on whichever of the recording heads  17   a  to  17   c  is to be used in the next the printing process. Specifically, in starting printing when the temperature of the recording heads  17   a  to  17   c  is above a range of the printable temperature and the next printing is to be performed with respect to a sheet P of which the width-direction size is a predetermined size or less and for printing on which only the recording head  17   b  at the center in the width direction is to be used, the flow path of the cooling liquid is switched by means of the electromagnetic valve  81  to the second tube  51   b  alone to concentrate the flow of the cooling liquid to the recording head  17   b . 
     Thereby, even with the configuration where the single pump  70  is used to cool the three recording heads  17   a  to  17   c  arranged in each of the line heads  11 C to  11 K, it is possible to individually cool the three recording heads  17   a  to  17   c , and thus to effectively cool only a desired one or more of the recording heads  17   a  to  17   c . Also, since only whichever of the recording heads  17   a  to  17   c  is to be used in the next printing process is cooled before printing is started, it is possible to cool only a minimum necessary number of the recording heads  17   a  to  17   c  in a short period of time. As a result, the printing waiting time is shortened, and further the printing efficiency (productivity) is improved. 
       FIG.  6    is a flow chart showing an example of cooling control executed in the printer  100  with respect to the recording heads  17   a  to  17   c  before printing is started. Referring to  FIGS.  1  to  5    as necessary and following the steps shown in  FIG.  6   , a description will be given of a procedure of cooling the recording heads  17   a  to  17   c  executed before printing is started. 
     The main control portion  110   a  determines whether or not a printing request has been received via the communication portion  29  from a host device such as a personal computer (step S 1 ). If no printing request has been received (No in step S 1 ), a printing standby state is maintained. If a printing request has been received (Yes in step S 1 ), the cooling unit control portion  110   b  determines whether or not a temperature T of the recording heads  17   a  to  17   c  detected by the temperature detecting sensor  23  is above an upper limit value T1 of the printable temperature (step S 2 ). 
     If T &gt; T1 holds (Yes in step S 2 ), the main control portion  110   a  acquires the width-direction size of the sheet P from print information included in the printing request (step S 3 ). Then, it is determined whether or not the width-direction size of the sheet P is a predetermined size (here, the A4 lateral size) or less (step S 4 ). If the width-direction size of the sheet P is the A4 lateral size or less (Yes in step S 4 ), only the recording head  17   b  is to be used in the next printing process, and thus the recording heads  17   a  and  17   c  do not need to be cooled, and it is only the recording head  17   b  that needs to be cooled to T1 or below. Thus, from the cooling unit control portion  110   b , a control signal is transmitted to the electromagnetic valve  81  to open only the second tube  51   b  (step S 5 ), and the flow path of the cooling liquid is switched to the second tube  51   b  alone. 
     If the width-direction size of the sheet P is larger than the A4 lateral size (No in step S 4 ), all of the recording heads  17   a  to  17   c  are to be used in the next printing process, and thus a control signal is transmitted to the electromagnetic valve  81  to open the first to third tubes  51   a  to  51   c  (step S 6 ), and the flow path of the cooling liquid is switched to the first to third tubes  51   a  to  51   c . Then, the pump  70  is driven (step S 7 ). 
     Next, the cooling unit control portion  110   b  determines whether or not the temperature T of the recording heads  17   a  to  17   c  detected by the temperature detecting sensor  23  is equal to or lower than the printable temperature T1 (step S 8 ). If T &gt; T1 holds (No in step S 8 ), it is determined that the cooling of the recording heads  17   a  to  17   c  by the cooling unit  50  is still necessary, and the pump  70  continues to be driven. 
     If T ≤ T1 holds (Yes in step S 8 ), the pump  70  is stopped (step S 9 ), and printing is started (step S 10 ). 
     According to the example of the cooling control shown in  FIG.  6   , in starting printing with respect to a sheet P of which the width-direction size is small and for printing on which only the recording head  17   b  at the width-direction center is used, the flow path of the cooling liquid is switched to the second tube  51   b  by means of the electromagnetic valve  81  to concentrate the cooling liquid to the recording head  17   b . In this manner, even with the configuration where the recording heads  17   a  to  17   c  and the pump  70  are not provided in a one-to-one correspondence, it is possible to suppress rise in temperature of the recording head  17   b  in a short period of time. As a result, the printing waiting time is shortened, and the printing efficiency (productivity) is improved. 
     Furthermore, by executing feed-forward control (predictive control) based on the sheet-P-size information acquirable in advance before printing is started, instead of by acquiring real-time temperatures of the recording heads  17   a  to  17   c  to switch the flow path of the cooling liquid, it is possible to quickly cool only whichever of the recording heads  17   a  to  17   c  needs to be cooled. 
     Note that, in the control example shown in  FIG.  6   , the pump  70  is stopped when, before printing is started, the recording heads  17   a  to  17   c  are cooled down to the upper limit value T1 of the printable temperature or below, but the pump  70  may continue to be driven even after printing is started, to thereby allow the cooling unit  50  to continue to cool the recording heads  17   a  to  17   c  during the printing process as well. 
     Specifically, in a case where the width-direction size of a sheet P is equal to or less than the A4 lateral size, only the second tube  51   b  is used as the flow path of the cooling liquid, and the pump  70  continues to be driven. In a case where the width-direction size of a sheet P is larger than the A4 lateral size, the first to third tubes  51   a  to  51   c  are used as the flow path of the cooling liquid, and the pump  70  continues to be driven. In this manner, it is possible to maintain the temperature of the recording heads  17   a  to  17   c  constant during a printing process and to suppress rise in temperature of the recording heads  17   a  to  17   c  when printing is finished, and thus, in starting subsequent printing, printing can be started without waiting for the temperature of the recording heads  17   a  to  17   c  to fall, and thus the printing waiting time is reduced and the printing efficiency (productivity) is further improved. 
     Next, a description will be given of another example of cooling control using the cooling unit  50  The temperatures of the recording heads  17   a  to  17   c  do not necessarily rise in a constant manner, and the temperatures of the recording heads  17   a  to  17   c  may rise in manners different from each other. For example, in performing printing on a sheet P having a large width-direction size, ink is ejected from all the recording heads  17   a  to  17   c , and the extent of temperature rise depends on the number of times ink has been ejected. More specifically, whichever of the recording heads  17   a  to  17   c  has ejected ink a larger number of times than the other recording heads undergoes a more remarkable temperature rise than the other recording heads. 
     Thus, a coverage rate is calculated from the image data before printing is started, and a flow amount or circulation time of the cooling liquid during the printing process is adjusted corresponding to the calculated coverage rate. More specifically, printing is performed while driving the cooling unit  50  so as to increase the flow amount or the circulation time of the cooling liquid in whichever of the first to third tubes  51   a  to  51   c  serves as the flow path of the cooling liquid flowing toward whichever of the recording heads  17   a  to  17   c  corresponds to a relatively high coverage-rate region in comparison with whichever of the first to third tubes  51   a  to  51   c  serves as the flow path of the cooling liquid flowing toward whichever of the recording heads  17   a  to  17   c  corresponds to a relatively low coverage-rate region. 
       FIG.  7    is a flow chart showing an example of the cooling control executed in the printer  100  with respect to the recording heads  17   a  to  17   c  during a printing process. Referring to  FIGS.  1  to  5    as necessary, according to the steps shown in  FIG.  7   , a description will be given of a procedure of cooling the recording heads  17   a  to  17   c  during a printing process. Here, although not shown in  FIG.  7   , assume that, at a time point when a printing request is received, the temperature of the recording heads  17   a  to  17   c  is equal to or lower than the upper limit value of the printable temperature. The electromagnetic valve  81  is capable of not only opening/closing the first to third tubes  51   a  to  51   c , but also adjusting the flow amount of the cooling liquid that circulates in the first to third tubes  51   a  to  51   c . 
     The main control portion  110   a  determines whether or not a printing request has been received via the communication portion  29  from a host device such as a personal computer (step S 1 ). If no printing request has been received (No in step S 1 ), a printing standby state is maintained. If a printing request has been received (Yes in step S 1 ), a coverage rate is calculated from the image data included in the printing request (step S 2 ). 
     Next, the main control portion  110   a , based on the coverage rate of the sheet P in the width direction, sets flow amounts of the cooling liquid to circulate in the first to third tubes  51   a  to  51   c  (step S 3 ). For example, if the coverage rate of the center portion of the sheet P in the width direction is twice as high as the coverage rate at the opposite end portions of the sheet P in the width direction, a control signal is transmitted to the electromagnetic valve  8  to increase the flow amount of the cooling liquid to circulate in the second tube  51   b  twice as large as in the first tube  51   a  and the third tube  51   c . Then, printing is started (step S 4 ), and at the same time, the pump  70  is driven (step S 5 ). 
     Thereafter, it is determined whether or not the printing has been finished (step S 6 ), and if the printing is still being performed (No in step S 6 ), the pump  70  continues to be driven. If the printing has been finished (Yes in step S 6 ), the pump  70  is stopped (step S 7 ), and the processing is finished. 
     According to the example of the cooling control shown in  FIG.  7   , in a case of performing printing using a sheet P having a large width-direction size, a coverage rate is calculated from image data Then, based on the coverage rate in the width direction of the sheet P, flow amounts of the cooling liquid to circulate in the first to third tubes  51   a  to  51   c  are set. In this manner, it is possible to adjust the flow amount of the cooling liquid corresponding to a predicted rise in temperature of the recording heads  17   a  to  17   c , and to maintain the temperature of the recording heads  17   a  to  17   c  constant during printing even with the configuration where the recording heads  17   a  to  17   c  and the pump  70  are not provided in one-to-one correspondence. 
     By adjusting the flow amount of the cooling liquid by performing the feed-forward control (predictive control) based on the coverage rate calculable in advance before printing is started instead of acquiring the real-time temperatures of the recording heads  17   a  and  17   b , it is possible to perform cooling focusing on whichever of the recording heads  17   a  to  17   c  is expected to undergo a temperature rise. Moreover, when starting subsequent printing, printing can be performed without waiting for the temperature of the recording heads  17   a  to  17   c  to fall, and thus the printing waiting time is reduced and the printing efficiency (productivity) is improved. 
     Note that, in the control example shown in  FIG.  7   , the flow amounts of the cooling liquid to circulate in the first to third tubes  51   a  to  51   c  are adjusted based on the coverage rate in the width direction, but the circulation time of the cooling liquid to circulate in each of the first to third tubes  51   a  to  51   c  may be adjusted instead. Specifically, if the coverage rate at the center portion of the sheet P in the width direction is twice as high as the coverage rate at the opposite end portions of the sheet P in the width direction, a control signal is transmitted to the electromagnetic valve  81  to prolong open time of the second tube  51   b  twice as long as open time of the first and third tubes  51   a  and  51   c , to thereby increase the circulation time of the cooling liquid to circulate in the second tube  51   b  twice as long as the circulation time of the cooling liquid to circulate in the first and third tubes  51   a  and  51   c . 
     In this manner, as in the control example shown in  FIG.  7   , it is possible to adjust the cooling time for each of the recording heads  17   a  to  17   c  corresponding to their respective predicted temperature rises, and thus to maintain the temperature of the recording heads  17   a  to  17   c  constant during printing. It is also possible to reduce a rise in temperature of the recording heads  17   a  to  17   c  when printing is finished, and thus, when starting subsequent printing, printing can be performed without waiting for the temperature of the recording heads  17   a  to  17   c  to fall, and thus the printing waiting time is reduced and the printing efficiency (productivity) is improved. Moreover, since the electromagnetic valve  81  performs only the operations of opening and closing the first to third tubes  51   a  to  51   c , there is no need of providing a flow amount adjusting mechanism, and thus the configuration of the electromagnetic valve  81  is simplified. 
     It is to be understood that the present disclosure may be practiced in any other manner than specifically described above as embodiments, and various modifications are possible within the scope of the invention. For example, in the above embodiment, the single electromagnetic valve  81  is arranged at the branching portion  55  of the first to third tubes  51   a  to  51   c , but instead, at positions downstream of the branching portion  55 , individual electromagnetic valves  81  may be arranged one for each of the first to third tubes  51   a  to  51   c . 
     The present disclosure is usable in inkjet recording apparatuses that eject ink from recording heads. Use of the present disclosure makes it possible to provide an inkjet recording apparatus in which the installation space for a cooling mechanism for cooling recording heads can be reduced, and that is capable of effectively cooling only a desired recording head.