Patent Publication Number: US-11648777-B2

Title: Liquid ejection apparatus and method of controlling liquid ejection apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a liquid ejection apparatus including a liquid ejection head that ejects a liquid and to a method of controlling a liquid ejection apparatus. 
     Description of the Related Art 
     In a liquid ejection head that ejects an ink by means of an action of heaters, as ink ejection is repeated, a component in the ink is heated at high temperature. This results in a phenomenon that the component turns into a substance which is difficult to dissolve or disperse, and this gets attached to the surface of an upper protection layer on each heater. This attached substance is what is called “kogation”. In a case where kogation is attached to and builds up on the upper protection layer, the thermal energy applied from the heater cannot be sufficiently transferred to the ink, so that the thermal energy to be applied to the ink decreases. This may lead to a failure to achieve desired ejection. Such a deterioration in ejection performance may be a cause of image unevenness. 
     Japanese Patent Laid-Open No. 2008-105364 discloses that iridium or ruthenium is used as the material of an upper protection layer that chemically and physically protects electrothermal conversion elements, and a surface (heat applying portions) of the upper protection layer is dissolved into a liquid by an electrochemical reaction to remove kogation. 
     During the electrochemical reaction, bubbles are generated at the surface of the upper protection layer. In the case where bubbles are generated as above, the electrochemical reaction stops due to the bubbles, which makes it impossible to perform sufficient kogation removal. To address this, Japanese Patent Laid-Open No. 2008-105364 discloses a method in which, in kogation removal, a recovery process involving covering each ejection head with a cap to suck the liquid (ink) therein is performed to remove the generated bubbles by suction. 
     Here, with a method such as in Japanese Patent Laid-Open No. 2008-105364, there is a possibility that bubbles interfere with the kogation removal and therefore sufficient kogation removal cannot be achieved. In particular, in the case of a long line liquid ejection head, the suction recovery process is usually performed nozzle by nozzle by scanning the cap along the nozzle arrays in the liquid ejection head. Thus, in a case of performing kogation removal simultaneously on a plurality of heat applying portions, there will be nozzles with bubbles generated due to the kogation removal but not sucked. In a case where bubbles generated by kogation removal separate the liquid and the heat applying portions, it interferes with the electrochemical reaction, which may lead to a failure to achieve sufficient kogation removal. 
     Also, in a case of providing a capping unit that covers all nozzles in the liquid ejection head and perform suction with it, there is a problem that the size of the apparatus configuration including pipes and a suction pump increases and the cost increases. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention provides a liquid ejection apparatus and a method of controlling a liquid ejection apparatus capable of preventing bubbles generated by kogation removal from interfering with the kogation removal. 
     To this end, a liquid ejection apparatus of the present invention includes: a liquid ejection head that has an electrothermal conversion element which heats a liquid, a heat applying portion to which heat of the electrothermal conversion element is applied, a pressure chamber in which a bubble is generated in the liquid by heat of the heat applying portion, an electrode provided in the pressure chamber and electrically connectable to the heat applying portion through the liquid, which contains an electrolyte, a supply valve capable of opening and closing a supply path through which to supply the liquid to the pressure chamber, and a collection valve capable of opening and closing a collection path into which to collect the liquid from the pressure chamber, and that ejects the liquid from an ejection port communicated with the pressure chamber by means of an action of the electrothermal conversion element; a voltage application unit capable of applying a voltage to the heat applying portion and the electrode; a supply pump that supplies the liquid to the pressure chamber; and a control unit that, in a cleaning process, performs control which causes the voltage application unit to apply the voltage to the heat applying portion and the electrode, opens the supply valve, closes the collection valve, and drives the supply pump. 
     According to the present invention, it is possible to provide a liquid ejection apparatus and a method of controlling a liquid ejection apparatus capable of preventing bubbles generated by kogation removal from interfering with the kogation removal. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating a schematic configuration of a liquid ejection apparatus; 
         FIG.  2    is a schematic diagram illustrating a liquid circulation path in the printing apparatus; 
         FIG.  3 A  is a perspective view illustrating a liquid ejection head; 
         FIG.  3 B  is a perspective view illustrating the liquid ejection head; 
         FIG.  4    is an exploded perspective view illustrating constituent components or units of the liquid ejection head; 
         FIG.  5 A  is a view illustrating a channel member that distributes an ink to ejection modules; 
         FIG.  5 B  is a view illustrating the channel member that distributes the ink to the ejection modules; 
         FIG.  5 C  is a view illustrating the channel member that distributes the ink to the ejection modules; 
         FIG.  5 D  is a view illustrating the channel member that distributes the ink to the ejection modules; 
         FIG.  5 E  is a view illustrating the channel member that distributes the ink to the ejection modules; 
         FIG.  6    is a transparent view illustrating how ink channels in a printing element substrate and the channel member are connected; 
         FIG.  7    is a view illustrating a cross-section along line VII-VII of  FIG.  6   ; 
         FIG.  8 A  is a view illustrating an ejection module; 
         FIG.  8 B  is a view illustrating the ejection module; 
         FIG.  9 A  is a view illustrating a printing element substrate; 
         FIG.  9 B  is a view illustrating the printing element substrate; 
         FIG.  9 C  is a view illustrating the printing element substrate; 
         FIG.  9 D  is a view illustrating the printing element substrate; 
         FIG.  10    is a perspective view illustrating a cross-section of the printing element substrate; 
         FIG.  11 A  is a cross-sectional view illustrating part of a printing element substrate; 
         FIG.  11 B  is a cross-sectional view illustrating part of the printing element substrate; 
         FIG.  12 A  is a cross-sectional view illustrating a pressure chamber; 
         FIG.  12 B  is a cross-sectional view illustrating the pressure chamber; 
         FIG.  13 A  is a diagram illustrating a circulation path for kogation removal; 
         FIG.  13 B  is a view illustrating an ink flow in a pressure chamber in the kogation removal; 
         FIG.  14 A  is a diagram illustrating another circulation path for kogation removal; 
         FIG.  14 B  is a view illustrating another ink flow in a pressure chamber in the kogation removal; 
         FIG.  15 A  is a view illustrating a pressure chamber in a tilted liquid ejection head; 
         FIG.  15 B  is a view illustrating the pressure chamber in the tilted liquid ejection head; 
         FIG.  15 C  is a view illustrating the pressure chamber in the tilted liquid ejection head; 
         FIG.  16 A  is a diagram illustrating a circulation path for kogation removing cleaning; 
         FIG.  16 B  is a diagram illustrating a circulation path for kogation removing cleaning; 
         FIG.  17 A  is a view illustrating a pressure chamber in a tilted liquid ejection head; 
         FIG.  17 B  is a view illustrating the pressure chamber in the tilted liquid ejection head; 
         FIG.  17 C  is a view illustrating the pressure chamber in the tilted liquid ejection head; and 
         FIG.  18    is a flowchart illustrating a kogation removing cleaning process. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. 
       FIG.  1    is a view illustrating a schematic configuration of a liquid ejection apparatus (hereinafter, also referred to as “printing apparatus”)  1000  to which the present embodiment is applicable. The printing apparatus  1000  in the present embodiment is an apparatus that circulates each of liquids such as inks between a buffer tank  1003  (see  FIG.  2   ) and a liquid ejection head  3  and ejects the ink in the form of ink droplets out of the circulated inks. Also, each liquid ejection head  3  is a so-called long line head having a length corresponding to the width of a print medium, and performs printing by ejecting the liquid (liquid droplets) from a plurality of ejection ports arrayed in the width direction of the print medium. Note that the present invention is also applicable to so-called serial liquid ejection apparatuses that perform printing with a scan over a print medium. 
     The printing apparatus  1000  includes a conveyance drum  1  that rotationally conveys a transfer body, and the line liquid ejection heads  3  disposed substantially perpendicularly to the surface of the conveyance drum  1 . The liquid ejection heads  3  perform one-pass continuous printing on the transfer body attached to the surface of the conveyance drum  1  while rotationally conveying it, and, from the printed transfer body, the printed object is transferred onto a print medium  2  attached to a transfer drum  4 . The print medium  2  is not limited to a cut sheet and may be a continuous roll sheet. 
     The printing apparatus  1000  includes five single-color liquid ejection heads ( 3   a ,  3   b ,  3   c ,  3   d , and  3   e ) for five types of inks that are CMYKT (cyan, magenta, yellow, black, and transparent function inks). 
       FIG.  2    is a schematic diagram illustrating an ink circulation path in the printing apparatus  1000  in the present embodiment. Note that while  FIG.  2    illustrates only the flow path for the ink of one color among the CMYKT inks for a simple explanation, the main body of the printing apparatus is actually provided with circulation paths for the five colors. The circulation path in the printing apparatus  1000  is provided with a buffer tank  1003 , supply units  220 , an ejection unit  300 , and negative pressure control units  230 , and the ink is circulated through the circulation path by driving first circulation pumps  1001  and  1002  and a second circulation pump  1004 . The liquid ejection head  3  includes the ejection unit  300 , the negative pressure control units  230 , the supply units  220 , connection parts  111 , valves  241   a  and  241   b  serving as supply valves, and valves  242   a  and  242   b  serving as collection valves. The valves  241   a  and  241   b  are configured to be capable of opening and closing supply channels extending to the ejection unit  300 . The valves  242   a  and  242   b  are configured to be capable of opening and closing collection channels extending from the ejection unit  300 . 
     Also, the buffer tank  1003 , serving as a sub tank, has an atmosphere communication port (not illustrated) through which the inside and outside of the tank communicate with each other, and is capable of discharging air bubbles in the ink to the outside. The buffer tank  1003  is also connected to a replenishment pump  1005 . In a case where the ink is consumed at the liquid ejection head  3  as a result of ejecting the ink from the ejection ports in the liquid ejection head for printing, suction recovery, or the like, which involves ejection of the ink, the replenishment pump  1005  transfers the consumed amount of the ink from a main tank  1006  to the buffer tank  1003 . The two first circulation pumps  1001  and  1002  draw the ink through one of the connection parts  111  of the liquid ejection head  3  and transfer them to the buffer tank  1003 . Positive displacement pumps having an ability to transfer a fixed amount of liquid are preferably used as the first circulation pumps. While specific examples include tube pumps, gear pumps, diaphragm pumps, syringe pumps, and so on, it is also possible to employ, for example, general pumps configured to ensure a fixed flow rate with a fixed flow rate valve or a relief valve disposed at the exit of the pump. 
     While the liquid ejection head  3  is driven, the valves  241   a  and  241   b  and the valves  242   a  and  242   b  are open. As the first circulation pump (higher-pressure side)  1001  and the first circulation pump (lower-pressure side)  1002  are driven, fixed amounts of the ink flow through a common supply channel  211  and a common collection channel  212  and thus circulate. The flow rates of the ink are preferably set at or above such flow rates that the temperature difference among the printing element substrates  10  in the liquid ejection head  3  does not affect the print image quality. Incidentally, if the flow rates are set high, the difference in negative pressure among the printing element substrates  10  will be excessively large due to pressure drops in the channels in the ejection unit  300 , which will cause density unevenness in the image. It is therefore preferred to set the flow rates with the differences in temperature and negative pressure among the printing element substrates  10  taken into account. 
     The negative pressure control units  230  are provided between the paths of the second circulation pump  1004  and the ejection unit  300 . The negative pressure control units  230  have a function of operating so as to maintain the pressures on the downstream sides of the negative pressure control units  230  (i.e., the ejection unit  300  side) at preset constant pressures even in a case where the flow rates of the ink in the circulation system vary due to a difference in printing ratio. Any mechanisms may be used as two pressure adjustment mechanisms forming the negative pressure control units  230  as long as they are capable of controlling the pressures on the downstream sides of the negative pressure control units  230  within certain ranges of variation centered at desired preset pressures. 
     In one example, mechanisms similar to so-called pressure-reducing regulators can be employed. In the case of using pressure-reducing regulators, it is preferred to pressurize the upstream sides of the negative pressure control units  230  with the second circulation pump  1004  via the supply units  220 , as illustrated in  FIG.  2   . This makes it possible to reduce the impact of the hydraulic head pressure of the buffer tank  1003  on the liquid ejection head  3  and thus enhance the degree of freedom in arrangement of the buffer tank  1003  in the printing apparatus  1000 . 
     The second circulation pump  1004  only needs to be one that exerts a certain pump head pressure or higher within the range of ink circulation flow rates to be used during the driving of the liquid ejection head  3 , and a turbo-pump, a positive displacement pump, or the like can be used. Specifically, a diaphragm pump or the like can be employed. Also, instead of the second circulation pump  1004 , a hydraulic head tank can be employed which is disposed with a certain hydraulic head difference relative to the negative pressure control units  230 , for example. 
     The negative pressure control units  230  include two pressure adjustment mechanisms whose control pressures are set to be different from each other. 
     The ejection unit  300  is provided with individual supply channels  213   a  and individual collection channels  213   b  communicating with the common supply channel  211 , the common collection channel  212 , and the printing element substrates  10 . The communication of the individual channels  213  with the common supply channel  211  and the common collection channel  212  generates flows such that part of the ink flows from the common supply channel  211  to the common collection channel  212  through channels inside the printing element substrates  10  (see the arrows in  FIG.  2   ). This is because the pressure adjustment mechanisms are connected to the common supply channel  211  and the common collection channel  212 , and a differential pressure is generated between the two common channels. 
       FIGS.  3 A and  3 B  are perspective views illustrating a liquid ejection head  3  in the present embodiment. The liquid ejection head  3  is a line liquid ejection head in which  16  printing element substrates  10  each capable of ejecting the ink of 1 color are arranged in a straight line (in-line arrangement). The liquid ejection heads  3  that eject the inks of the five colors are configured similarly to each other. 
     As illustrated in  FIGS.  3 A and  3 B , the liquid ejection head  3  includes the printing element substrates  10 , flexible wiring substrates  40 , and an electric wiring substrate  90  provided with signal input terminals  91  and power supply terminals  92 . The signal input terminals  91  and the power supply terminals  92  are electrically connected to a control unit in the printing apparatus  1000 , and respectively supply ejection drive signals and electrical power necessary for ejection to the printing element substrates  10 . The number of signal input terminals  91  and power supply terminals  92  can be made smaller than the number of printing element substrates  10  by gathering wirings in the electric circuit in the electric wiring substrate  90 . This can reduce the number of electrically connected components that need to be detached in a case of mounting the liquid ejection head  3  to the printing apparatus  1000  or in a case of replacing the liquid ejection head  3 . 
     The connection parts  111 , which are provided at both end portions of the liquid ejection head  3 , are connected to an ink supply system of the printing apparatus  1000 . The ink is supplied from the supply system of the printing apparatus  1000  to the liquid ejection head  3  through one of the connection parts  111  and, after passing through the liquid ejection head  3 , the ink is collected into the supply system of the printing apparatus  1000  through the other of the connection parts  111 . Thus, the liquid ejection head  3  is configured such that the ink can be circulated through the paths in the printing apparatus  1000  and the paths in the liquid ejection head  3 . 
       FIG.  4    is an exploded perspective view illustrating constituent components or units of the liquid ejection head  3 . The liquid ejection head  3  includes the ejection unit  300 , the supply units  220 , the electric wiring substrate  90 , and ejection unit support parts  81 . 
     In the liquid ejection head  3 , a second channel member  60  included in the ejection unit  300  ensures the stiffness of the liquid ejection head  3 . The ejection unit support parts  81  are provided at both end portions of the second channel member  60 . The ejection unit  300  is mechanically coupled to a carriage of the printing apparatus  1000  via the ejection unit support parts  81 , so that the liquid ejection head  3  is positioned by the carriage. The supply units  220 , which include the negative pressure control units  230 , and the electric wiring substrate  90  are coupled to the ejection unit support parts  81 . Filters  100  are incorporated in each of the two liquid supply units  220 . 
     The two negative pressure control units  230  are set to control the pressures in the ink paths in the liquid ejection head  3  respectively with different, higher and lower negative pressures. In a case where the negative pressure control units  230  on the higher-pressure side and the lower-pressure side are installed respectively at both end portions of the liquid ejection head  3 , ink flows are generated in the common supply channel  211  (see  FIG.  2   ) and the common collection channel  212  (see  FIG.  2   ) extending in the longitudinal direction of the liquid ejection head  3 . As a result, the ink between the common supply channel  211  and the common collection channel  212  promotes heat exchange, thereby reducing the difference between the temperatures in the two common channels. Accordingly, the plurality of printing element substrates  10  provided along the common channels are less prone to have a temperature difference among them, and thus print unevenness due to a temperature difference is less prone to occur. 
     The ejection unit  300  includes a plurality of ejection modules  200  and a channel member  210 , and a cover member  130  is attached to the surface of the ejection unit  300  on the print medium side. Here, the cover member  130  is a frame-shaped member provided with a long opening  131 , and the printing element substrates  10  and sealing members  110  included in the ejection modules  200  are exposed from the opening  131 . A frame portion around the opening  131  serves as a contact surface to be in contact with a cap that covers the liquid ejection head  3  during a standby period for printing to suppress drying of the ink in ejection ports  13 . Since the contact between the frame portion around the opening  131  and the cap forms a closed space within the cap, the contact surface of the frame portion around the opening  131  and the ejection port surface of the ejection unit  300  are preferably formed flat. 
     Next, the channel member  210  of the ejection unit  300  will be described. The channel member  210  is first channel members  50  and the second channel member  60  laminated together, and distributes the ink supplied from the supply units  220  to the ejection modules  200 . The channel member  210  also functions as a channel member through which the ink to be circulated is returned from the ejection modules  200  to the supply units  220 . The second channel member  60  of the channel member  210  is a channel member in which the common supply channel  211  (see  FIG.  2   ) and the common collection channel  212  (see  FIG.  2   ) are formed, and has a function of mainly ensuring the stiffness of the liquid ejection head  3 . For this reason, the material of the second channel member  60  is preferably one having sufficient corrosion resistance against the ink and high mechanical strength. Specifically, SUS, Ti, alumina, or the like is preferred. 
       FIGS.  5 A to  5 E  are views illustrating the channel members that distribute the ink supplied from the supply units  220  to the ejection modules  200 .  FIG.  5 A  is a view illustrating the surfaces of the first channel members  50  to which to mount the ejection modules  200 , and  FIG.  5 B  is a view illustrating the opposite surfaces of the first channel members  50 , which are to be in contact with the second channel member  60 . A first channel member  50  is provided for each ejection module  200 , and the plurality of first channel members  50  are arrayed. By employing a structure divided for each individual ejection module  200  and arraying a plurality of modules as described above, it is possible to adjust to the long liquid ejection head. 
     Communication ports  51  in the first channel members  50  fluidly communicate with the ejection modules  200  (see  FIG.  4   ), and individual communication ports  53  in the first channel member  50  fluidly communicate with communication ports  61  in the second channel member  60 .  FIG.  5 C  is a view illustrating the surface of the second channel member  60  to be in contact with the first channel members  50 .  FIG.  5 D  is a cross-sectional view of a middle portion of the second channel member  60  in its thickness direction.  FIG.  5 E  is a view illustrating the surface of the second channel member  60  to be in contact with the supply units  220 . Of two common channel grooves  71  in the second channel member  60 , one is the common supply channel  211 , and the other is the common collection channel  212 , through each of which the ink flows in the longitudinal direction of the liquid ejection head  3  from one end side to the other end side. 
       FIG.  6    is a transparent view illustrating how the ink channels in a printing element substrate  10  and the channel member  210  are connected. The paired common supply channel  211  and common collection channel  212  extending in the longitudinal direction of the liquid ejection head  3  are provided in the channel member  210 . The communication ports  61  in the second channel member  60  are positioned with and connected to the individual communication ports  53  in the first channel members  50 . As a result, supply paths are formed through which some communication ports  72  (see  FIG.  5 E ) in the second channel member  60  communicate with some communication ports  51  in the first channel members  50  through the common supply channel  211 . Similarly, collection paths are formed through which some communication ports  72  (see  FIG.  5 E ) in the second channel member  60  communicate with some communication ports  51  in the first channel member  50  through the common collection channel  212 . 
       FIG.  7    is a view illustrating a cross-section along line VII-VII of  FIG.  6   . The common supply channel  211  is connected to the ejection modules  200  through some communication ports  61 , individual communication ports  53 , and communication ports  51 . In other words, the individual supply channels  213   a  (see  FIG.  2   ) are formed so as to include some communication ports  61 , individual communication ports  53 , and communication ports  51 . Although not illustrated in  FIG.  7   , in another cross-section, the individual collection channels  213   b  are connected to the ejection modules  200  through similar paths. 
     In each printing element substrate  10 , channels communicating with its ejection ports  13  are formed, and these enable part or all of the ink supplied not to be ejected from the ejection ports  13  that are not involved in an ejection operation but to pass these ejection ports  13  and circulate. While an ejection operation is performed, the circulating ink is ejected from the ejection ports  13  in the form of ink droplets. Also, the common supply channel  211  and the common collection channel  212  are connected to the negative pressure control unit  230  (higher-pressure side) and the negative pressure control unit  230  (lower-pressure side), respectively, through the supply units  220 . The differential pressure between the negative pressure control unit  230  (higher-pressure side) and the negative pressure control unit  230  (lower-pressure side) generates an ink flow in which the ink flows from the common supply channel  211 , passes the ejection ports  13  in the printing element substrates  10 , and then flows to the common collection channel  212 . 
       FIGS.  8 A and  8 B  are views illustrating an ejection module  200 .  FIG.  8 A  is a perspective view of the ejection module  200 , and  FIG.  8 B  is an exploded view thereof. The ejection module  200  includes a printing element substrate  10 , a support member  30 , and flexible wiring substrates  40 . 
     An example of a method of manufacturing the ejection module  200  will be described. Firstly, the printing element substrate  10  and the flexible wiring substrates  40  are bonded onto the support member  30  provided with communication ports  31 . Then, terminals  16  on the printing element substrate  10  and terminals  41  on the flexible wiring substrates  40  are electrically connected to each other by wire bonding, and thereafter the wire-bonded portions (electrically connected portions) are covered with sealing members  110  to be sealed. Terminals  42  on the flexible wiring substrates  40  opposite to the printing element substrate  10  are electrically connected to connection terminals (see  FIG.  3 A ) on the electric wiring substrate  90 . The support member  30  is a support that supports the printing element substrate  10  and also is a channel member through which to bring the printing element substrate  10  and the channel member  210  into fluid communication with each other. Thus, a support member that has high flatness and can be joined sufficiently reliably to the printing element substrate  10  is preferred. Its material is preferably alumina or a resin material, for example. 
     Note that a plurality of terminals  16  are disposed respectively at both edge portions of the printing element substrate  10  along the direction of a plurality of ejection port arrays (the longer edge portions of the printing element substrate  10 ). Moreover, for the one printing element substrate  10 , there are disposed two flexible wiring substrates  40  to be electrically connected to the terminals  16 . Such a configuration can shorten the maximum distance from each terminal  16  to the corresponding printing element and accordingly reduce the voltage drop and signal transfer delay occurring in a wiring portion in the printing element substrate  10 . 
       FIGS.  9 A to  9 D  are views illustrating a printing element substrate  10 .  FIG.  9 A  is a schematic view illustrating the surface of the printing element substrate  10  where its ejection ports  13  are disposed, and  FIG.  9 B  is a schematic view illustrating the back surface in  FIG.  9 A .  FIG.  9 C  is a schematic view illustrating the back surface of the printing element substrate  10  in a state where a lid member  20  provided on the back surface of the printing element substrate  10  in  FIG.  9 B  is removed.  FIG.  9 D  is an enlarged view of a portion A in  FIG.  9 A .  FIG.  10    is a perspective view illustrating a cross-section of the printing element substrate  10 . 
     The printing element substrate  10  includes a base plate  11  formed by laminating a plurality of layers on a silicon base, an ejection port forming member  12  made of a photosensitive resin, and the lid member  20  joined to the back surface of the base plate  11 . A plurality of ejection port arrays  14  are formed in the ejection port forming member  12  of the printing element substrate  10 . Note that the direction of extension of the ejection port arrays  14  each being a plurality of arrayed ejection ports  13  will be hereinafter referred to as “ejection port array direction”. Printing elements  15  are formed in the base plate  11  while grooves extending in the ejection port array direction and forming the supply paths  18  and the collection paths  19  are formed on the back side. The printing elements  15  are elements that generate energy to be utilized for liquid ejection. 
     As illustrated in  FIGS.  9 C and  10   , the printing element substrate  10  is provided with the supply paths  18  and the collection paths  19  extending in the ejection port array direction, and along each ejection port array  14  there is a supply path  18  provided on one side thereof and a collection path  19  provided on the other side thereof. Also, the supply paths  18  and the collection paths  19  are provided alternately in a direction crossing the ejection port array direction. Moreover, as illustrated in  FIG.  9 D , along the ejection port array direction, a plurality of supply ports  17   a  connected to the supply paths  18  are arrayed to form supply port arrays, and a plurality of collection ports  17   b  form collection port arrays such that the liquid flows out into the collection paths  19 . 
     The lid member  20  of a sheet shape is laminated on the surface of the base plate  11  opposite the surface where the ejection port forming member  12  is provided. The lid member  20  is provided with a plurality of openings  21  communicating with the supply paths  18  and the collection paths  19 . Each opening  21  in the lid member  20  communicates with a communication port  51  (see  FIG.  7   ) in a first channel member  50  through a communication port  31  (see  FIG.  8 B ) in the support member  30 . The lid member  20  functions as a lid that forms part of the walls of the supply paths  18  and the collection paths  19  formed in the base plate  11  of the printing element substrate  10 . 
     The lid member  20  is preferably a member having sufficient corrosion resistance against the link. Moreover, the opening shapes and positions of the openings  21  are required to be highly accurate. It is therefore preferred to use a photosensitive resin material or a silicon plate as the material of the lid member  20  and to provide the openings  21  by a photolithography process. As described above, the lid member converts the pitch between the channels by means of the openings  21  and, considering the pressure drop, is desirably thin and desirably formed of a film-shaped member. 
     As illustrated in  FIG.  10   , at each of positions corresponding to the ejection ports  13 , a printing element  15  is disposed which is a heat generating resistive element that generates a bubble in the ink by means of thermal energy. Pressure chambers  23  each including a printing element  15  therein are defined by partitions  22  (see  FIG.  9 D ). The printing elements  15  are electrically connected to the terminals  16  (see  FIG.  9 A ) by electric wirings provided in the printing element substrate  10 . Each printing element  15  generates heat based on a pulse signal inputted from a control circuit in the printing apparatus  1000  via the electric wiring substrate  90  (see  FIG.  4   ) and the corresponding flexible wiring substrate  40  (see  FIG.  8 A ) to thereby boil the ink. The force of a bubble generated by the boiling of the ink ejects an ink droplet from the ejection port  13 . Note that, as will be described later, the printing elements  15  are covered with the plurality of layers provided in the base plate  11 , but the printing elements  15  in  FIGS.  9 D and  10    are schematically illustrated in the surface of the base plate  11 . 
     The ink flow inside the printing element substrate  10  will be described. The supply paths  18  and the collection paths  19  formed by the base plate  11  and the lid member  20  are connected respectively to the common supply channel  211  and the common collection channel  212  in the channel member  210 , and a differential pressure is present between the ink flowing through the supply paths  18  and the ink flowing through the collection paths  19 . During ink ejection from a plurality of ejection port  13  in the liquid ejection head  3 , at each ejection port not performing an ejection operation, this differential pressure causes the ink to flow from the supply path  18  to the collection path  19  through the supply port  17   a , the pressure chamber  23 , and then the collection port  17   b  (arrows C in  FIG.  10   ). 
     With this ink flow, the ink present in each ejection port  13  and pressure chamber  23  not involved in printing and thickened due to evaporation through the ejection port  13 , as well as bubbles, foreign matter, and the like, can be collected into the collection path  19 . The ink flow also makes it possible to suppress thickening of the ink in the ejection port  13  and the pressure chamber  23 . The ink collected in the collection path  19  passes through the corresponding openings  21  in the lid member  20  and the corresponding communication ports  31  (see  FIG.  7   ) in the support member  30 , and is collected through the corresponding communication ports  51 , the individual collection channel  213   b , and the common collection channel  212  in the channel member  210  in this order to thereby circulate through the circulation path in the printing apparatus  1000 . 
     Note that, as illustrated in  FIG.  2   , the ink having flowed in from one end of the common supply channel  211  in the ejection unit  300  is not entirely supplied to the pressure chambers  23  through the individual supply channels  213   a . Specifically, there is a portion of the ink that does not flow into the individual supply channels  213   a  but flow into one of the supply units  220  from the other end of the common supply channel  211 . By including these paths that allow the ink to flow without passing through the printing element substrates  10 , it is possible to suppress backflow of the circulating ink flow even in the case of including printing element substrates  10  with narrow channels in which the flow resistance is large as in the present embodiment. Since the thickening of the ink in the pressure chambers  23  and in the vicinity of the ejection ports  13  can be suppressed as above, it is possible to suppress the occurrence of misdirected ejection and ejection failure in the liquid ejection heads  3 . 
       FIGS.  11 A and  11 B  are cross-sectional views illustrating part of a printing element substrate  10 .  FIG.  11 A  is a diagram of the surface of the printing element substrate  10  provided with heat applying portions  124   a , illustrating a region around some heat applying portions  124   a  in a schematic enlarged view.  FIG.  11 B  is a schematic cross-sectional view along line XIB-XIB of  FIG.  11 A . Incidentally, a second adhesion layer  122  illustrated in  FIG.  11 B  is omitted in  FIG.  11 A . Note that, in order to generate a bubble in the ink, each heat applying portion  124   a  contacts the ink and applies heat to the ink. 
     The base plate  11  included in the printing element substrate  10  is formed by laminating a plurality of layers. In the present embodiment, a heat accumulation layer formed of a thermally oxidized film, a SiO film, a SiN film, or the like is disposed on a silicon base. Also, heat generating resistive elements  126  serving as the printing elements  15  are disposed on the heat accumulation layer. Electrode wiring layers made of a metallic material such as Al, Al—Si, or Al—Cu and serving as wirings are connected to the heat generating resistive elements  126  via plugs  128  made of tungsten or the like. The plugs  128  are disposed so as to be paired with the respective heat generating resistive elements  126 , and a portion of each heat generating resistive element  126  where a current flows through the plug  128  functions as a heat generating portion for ink ejection. On the heat generating resistive elements  126 , an insulating protection layer  127  is disposed so as to cover the heat generating resistive elements  126 . The insulating protection layer  127  is formed of, for example, a SiO film, a SiN film, or the like. 
     A first protection layer  125  and a second protection layer  124  are disposed on the insulating protection layer  127 . These protection layers serve to protect the surfaces of the heat generating resistive elements  126  from chemical and physical impacts resulting from the heat generation of the heat generating resistive elements  126 . For example, the first protection layer  125  is made of tantalum (Ta), and the second protection layer  124  is made of iridium (Ir). Also, the protection layers made of these materials have electrical conductivity. 
     Moreover, a first adhesion layer  123  and the second adhesion layer  122  are disposed on the second protection layer  124 . The first adhesion layer  123  serves to improve adhesion between the second protection layer  124  and another layer. The first adhesion layer  123  is made of, for example, tantalum (Ta). The second adhesion layer  122  serves to protect other layers from the ink and to improve adhesion to the ejection port forming member  12 . The second adhesion layer  122  is made of, for example, SiC or SiCN. 
     The ejection port forming member  12 , in which the ejection ports  13  are formed, is joined to the surface of the base plate  11  on the second adhesion layer  122  side, and forms channels including the pressure chambers  23  by being joined to the base plate  11 . The ejection port forming member  12  has the partitions  22  each provided between adjacent heat applying portions  124   a , and the pressure chambers  23  are defined by these partitions  22 . 
     In ejection of the ink, on each of the heat applying portions  124   a  of the second protection layer  124 , which cover the heat generating resistive elements  126  and contact the ink, the temperature of the ink rises instantaneously, so that a bubble is generated in the ink and the bubble then disappears, thereby causing cavitation. For this reason, the second protection layer  124  including the heat applying portions  124   a  is made of iridium (Ir), which has high corrosion resistance and also high cavitation resistance. The second protection layer  124  is desirably made of a material containing iridium (Ir), ruthenium (Ru), or platinum (Pt). 
     In the present embodiment, the base plate  11  is provided with electrodes  129  in the same layer as the second protection layer  124  as counterparts of the heat applying portions  124   a . Also, the electrodes  129  as counterparts of the heat applying portions  124   a  are connected to a voltage application unit capable of applying voltage. The ink contains an electrolyte, and the heat applying portions  124   a  and the electrodes  129  as their counterparts are configured to be connectable to each other through the ink. As a voltage is applied to the heat applying portions  124   a  and the electrodes  129 , an electrochemical reaction occurs through the ink, which dissolves a voltage-applied dissolvable layer on the one of the heat applying portions  124   a  and the electrodes  129  that electrochemically serves as an anode. That is, as the voltage is applied such that the heat applying portion  124   a  is the anode side, the voltage-applied dissolvable layer on the heat applying portion  124   a  is dissolved by an electrochemical reaction. This enables removable of kogation attached to the surface. 
     The electrodes  129  are disposed downstream of the heat applying portions  124   a  in the direction of flow of the ink from the supply ports  17   a  toward the collection ports  17   b . Further, in a case where the supply ports  17   a  are disposed on one side of an array of a plurality of heat applying portions  124   a  and the collection ports  17   b  are disposed on the other side thereof, as illustrated in  FIG.  9 D , the electrodes  129  are disposed on the collection ports  17   b  side of the array of heat applying portions  124   a . Incidentally, to reduce the load of the manufacturing process, the electrode layer forming the electrodes  129  is preferably made of the same material as the second protection layer  124  (iridium) as well. Also, since iridium does not form an oxidized film, using it as the material of the second protection layer  124  including the heat applying portions  124   a  can suppress blocking of the dissolution into the ink by an oxidized film. 
     With no ink being present between the heat applying portions  124   a  and the electrodes  129 , the heat applying portions  124   a  and the electrodes  129  are not electrically connected to each other. However, with an ink containing an electrolyte being filled between the heat applying portions  124   a  and the electrodes  129 , currents flow between the heat applying portions  124   a  and the electrodes  129  through the ink, so that an electrochemical reaction occurs at the interfaces between the heat applying portions  124   a  and electrodes  129  and the ink. By the electrochemical reaction, the surfaces of the heat applying portions  124   a  are dissolved into the ink. Accordingly, kogation attached to the surfaces of the heat applying portions  124   a  can be removed. The anode electrode side undergoes dissolution of its metal. Hence, in the case of removing kogation on the heat applying portions  124   a  (hereinafter referred to as kogation removing cleaning), a voltage is applied to the heat applying portions  124   a  and the electrodes  129  such that the heat applying portions  124   a  are the anode side and the electrodes  129  are the cathode side. 
       FIGS.  12 A and  12 B  are cross-sectional views illustrating a pressure chamber  23 .  FIG.  12 A  illustrates the pressure chamber  23  in an ink circulation period, i.e., an ejection period.  FIG.  12 B  illustrates the pressure chamber  23  during an electrochemical reaction. In the pressure chamber  23  in the ink circulation period, the ink flows into the pressure chamber  23  from the supply port  17   a  and is discharged from the collection port  17   b . In the electrochemical reaction in kogation removing cleaning, an anode reaction (oxidation reaction) occurs on the heat applying portion  124   a , and a cathode reaction (reduction reaction) occurs on the electrode  129 . As a result, bubbles are generated from the heat applying portion  124   a  and the electrode  129  as illustrated in  FIG.  12 B . 
     In the printing apparatus  1000  in the present embodiment, in kogation removing cleaning, the kogation on the heat applying portions  124   a  is removed and, at the same time, the inks are caused to flow with the pressure in the pressure chambers  23  raised. In this way, the bubbles generated are efficiently discharged out of the ejection ports  13  along with the inks. 
       FIGS.  13 A and  13 B  are a diagram and a view respectively illustrating a circulation path for kogation removing cleaning among the circulation paths employed by the printing apparatus  1000  in the present embodiment, and an ink flow in a pressure chamber  23  in the kogation removing cleaning. In the circulation path in  FIG.  13 A , the valves  242   a ,  242   b ,  241   a , and  241   b  provided to the channels connected to the common supply channel  211  and the common collection channel  212  are such that the valves  242   a  and  242   b  are closed, and the valves  241   a  and  241   b  are open. In this state, the ink is supplied from liquid feed pumps P 2  and P 3 , which serve as supply pumps, to the head side. As a result, unlike the liquid flows generated during an ejection operation, during which the liquid flows in from the supply ports and flows out from the collection ports, the liquid flows in from both the supply ports and the collection ports. Further, the flow rate of the liquid to be fed from the liquid feed pump P 3  per unit time is set larger than the flow rate of the liquid to be fed from the liquid feed pump P 2  per unit time, so that the amount of inflow from the supply ports  17   a  per unit time is larger than the amount of inflow from the collection ports  17   b  per unit time. 
       FIG.  13 B  illustrates the ink flow in a pressure chamber  23  in the circulation path of  FIG.  13 A . The ink flows in from the supply port  17   a  and the collection port  17   b , so that the inside of the pressure chamber  23  becomes pressurized. As a result, bubbles are discharged from the ejection port  13  with the ink flow. Inside the pressure chamber  23 , bubbles are generated at the heat applying portion  124   a  and the electrode  129 , as illustrated in  FIG.  12 B . Thus, the amount of inflow per unit time from the collection port  17   b , which is closer to the electrode  129 , is made larger. In this way, the bubbles generated at the heat applying portion  124   a  and the electrode  129  can be efficiently discharged from the ejection port  13 . 
       FIGS.  14 A and  14 B  are a diagram and a view respectively illustrating another circulation path for kogation removing cleaning among the circulation paths employed by the printing apparatus  1000  in the present embodiment, and an ink flow in a pressure chamber  23  in the kogation removing cleaning. In the circulation path in  FIG.  14 A , the valves  242   a ,  242   b ,  241   a , and  241   b  provided to the channels connected to the common supply channel  211  and the common collection channel  212  are such that the valves  242   a ,  242   b , and  241   a  are closed, and the valve  241   b  is open. In this state, the ink is supplied from the liquid feed pump P 3  to the head side, so that the ink flows in from the collection ports  17   b  but no ink flows in from the supply ports  17   a .  FIG.  14 B  illustrates the ink flow in a pressure chamber  23  in the circulation path of  FIG.  14 A . The ink flows in from the collection port  17   b , so that the inside of the pressure chamber  23  becomes pressurized. As a result, bubbles are discharged from the ejection port  13  with the ink flow. Depending on the size and amount of bubbles generated at the heat applying portion  124   a  and the electrode  129 , the bubbles can be efficiently discharged from the ejection port  13  by causing the ink to flow in from the collection port  17   b  and causing no ink to flow in from the supply port  17   a.    
     In the printing apparatus  1000  in the present embodiment, a plurality of liquid ejection heads  3  are provided along the surface of the conveyance drum  1 , as illustrated in  FIG.  1   . Thus, the liquid ejection heads  3  are set such that some of them are set to be tilted with respect to the vertical direction (the direction of arrow Z). Specifically, in  FIG.  1   , the liquid ejection heads  3   a ,  3   b ,  3   d , and  3   e  are set to be tilted. Moreover, it can be seen that the liquid ejection heads  3   a  and  3   b  and the liquid ejection heads  3   d  and  3   e  differ in tilting direction. 
     Buoyancy acts on bubbles generated in the pressure chambers  23 . Accordingly, in the case where the liquid ejection head  3  is tilted, bubbles generated in its pressure chambers  23  are affected by the tilt so as to move to a higher position in the pressure chambers  23 . 
       FIGS.  15 A to  15 C  are views illustrating a pressure chamber  23  in a tilted liquid ejection head  3 . Suppose that the liquid ejection head  3  is tilted in such a direction that the electrodes  129  are positioned higher than the heat applying portions  124   a  and, in this state, bubbles are generated at the heat applying portions  124   a  and the electrodes  129  in the pressure chambers  23 . In this case, the bubbles gather on the electrode  129  side, as illustrated in  FIG.  15 A . In such a state where the bubbles gather on the electrode  129  side, the ink is caused to flow in from the supply ports  17   a  and the collection ports  17   b , and the flow rate of the liquid to be fed from the liquid feed pump P 3  per unit time is set larger than the flow rate of the liquid to be fed from the liquid feed pump P 2  per unit time. In this way, the generated bubbles can be efficiently discharged from the ejection ports  13  (see  FIG.  15 B ). Alternatively, depending on the size and amount of bubbles, the ink may be fed from the liquid feed pump P 3  to the head side to thereby cause the ink to flow in from the collection ports  17   b  and cause no ink to flow in from the supply ports  17   a . In this way too, the bubbles can be efficiently discharged from the ejection ports  13  (see  FIG.  15 C ). 
       FIGS.  16 A and  16 B  are diagrams illustrating circulation paths for kogation removing cleaning different from the circulation paths in  FIGS.  13 A and  14 A . In the circulation path in  FIG.  16 A , the valves  242   a ,  242   b ,  241   a , and  241   b  provided to the channels connected to the common supply channel  211  and the common collection channel  212  are such that the valves  242   a  and  242   b  are closed, and the valves  241   a  and  241   b  are open. In this state, the ink is supplied from the liquid feed pumps P 2  and P 3  to the head side, and the flow rate of the liquid to be fed from the liquid feed pump P 2  per unit time is set larger than the flow rate of the liquid to be fed from the liquid feed pump P 3  per unit time. On the other hand, in the circulation path in  FIG.  16 B , the valves  242   a ,  242   b , and  241   b  are closed, and the valve  241   a  is open. In this state, the ink is fed from the liquid feed pump P 2  to the head side. 
       FIGS.  17 A to  17 C  are views illustrating a pressure chamber  23  in a liquid ejection head  3  tilted in the direction opposite to that in  FIGS.  15 A to  15 C , and corresponds to the circulation paths illustrated in  FIGS.  16 A and  16 B . Suppose that the liquid ejection head  3  is tilted in such a direction that the electrodes  129  are positioned lower than the heat applying portions  124   a  and, in this state, bubbles are generated at the heat applying portions  124   a  and the electrodes  129  in the pressure chambers  23 . In this case, the bubbles gather on the side of the pressure chambers  23  opposite to the electrodes  129 , as illustrated in  FIG.  17 A . In such a case, the ink is circulated through the circulation path illustrated in  FIG.  16 A  to thereby cause the ink to flow in from the supply ports  17   a  and the collection ports  17   b  as illustrated in  FIG.  17 B . Moreover, the flow rate of the liquid to be fed from the liquid feed pump P 2  per unit time is set larger than the flow rate of the liquid to be fed from the liquid feed pump P 3  per unit time. In this way, the generated bubbles can be efficiently discharged from the ejection ports  13 . Alternatively, the ink is fed from the liquid feed pump P 2  to the head side to thereby cause the ink to flow in from the supply ports  17   a  and cause no ink to flow in from the collection ports  17   b  (see  FIG.  16 B ). With such a circulation path too, the bubbles can be efficiently discharged from the ejection ports  13  (see  FIG.  17 C ), depending on the size and amount of the bubbles. 
       FIG.  18    is a flowchart illustrating a kogation removing cleaning process in the present embodiment. The kogation removing cleaning process in the present embodiment will be described below using this flowchart. A CPU in the printing apparatus  1000  performs the series of processes illustrated in  FIG.  18    by loading program code stored in a program memory to a data memory and executing it. Alternatively, the functions of some or all of the steps in  FIG.  18    may be implemented with hardware such as an ASIC or an electronic circuit. Meanwhile, the symbol “S” in the description of each process means a step in the flowchart. 
     Upon start of the kogation removing cleaning process, in S 1 , the CPU stops the circulation of the ink circulating through a circulation path. Then, in S 2 , the CPU closes the valves  242   a  and  242   b  and drives the liquid feed pumps P 2  and P 3  (or one of them) to thereby pressurize the inside of the pressure chambers  23 . Thereafter, in S 3 , the CPU applies a voltage to the heat applying portions  124   a  and the electrodes  129  to thereby apply potentials thereto. As a result, kogation removal is performed on the heat applying portions, and the removed kogation is discharged from the ejection ports  13  along with the bubbles generated at the heat applying portions  124   a  and the electrodes  129 . Then, the CPU stops the voltage application in S 4 , and stops the driven liquid feed pumps (P 2 , P 3 ) in S 5 . 
     Thereafter, in S 6 , the CPU performs suction wiping that sucks and wipes the surface of the liquid ejection head  3  where the its ejection ports  13  are provided. Then, in S 7 , the CPU starts the stopped ink circulation, which is the end of the process. 
     As described above, in each long liquid ejection head with a liquid circulated therethough, the pressure chambers  23  are pressurized and a voltage is applied to the heat applying portions  124   a  and the electrodes  129  to perform kogation removing cleaning. In this way, it is possible to provide a liquid ejection apparatus and a method of controlling a liquid ejection apparatus capable of suppressing an increase in apparatus size and cost and performing kogation removal without a decrease in output. 
     Another Embodiment 
     Another embodiment of the present invention will be described below. Note that the basic configuration in this embodiment is similar to that in the above embodiment, and the characteristic configuration will therefore be described below. 
     In the description of the above embodiment, potentials are applied such that the heat applying portions  124   a  are the anode side and the electrodes  129  are the cathode side. However, in a case where the electrochemical reaction is continued in a state where the polarities of the heat applying portions  124   a  and the electrodes  129  are kept constant, such as the heat applying portions  124   a  being the anode side and the electrodes  129  being the cathode side, the electrolyte component in the ink gets attached to the heat applying portions  124   a  and the electrodes  129 . Consequently, the electrolyte component may cover the surfaces of the heat applying portions  124   a  and the electrodes  129 . In the case where the electrolyte component covers the surfaces of the heat applying portions  124   a , it blocks the dissolution of the heat applying portions  124   a  in the electrochemical reaction, which may lead to a failure to remove the kogation attached to the surfaces. 
     To solve this, in the present embodiment, the polarity of the voltage to be applied is regularly inverted so that the heat applying portions  124   a  and the electrodes  129  will alternately be the anode side and the cathode side. Such voltage application with the polarity switched back and forth enables removal of the electrolyte component in the ink attached to the surfaces of the heat applying portions  124   a  and the electrodes  129 . It is therefore possible to remove the kogation on the heat applying portions  124   a  while also suppressing attachment of the electrolyte component to the surfaces of the heat applying portions  124   a.    
     Note that while the heat applying portions  124   a  are the cathode side, the heat applying portions  124   a  do not dissolve and therefore kogation removal is not performed. While the heat applying portions  124   a  are the anode side, kogation removal is performed by means of an electrochemical reaction. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2020-154781 filed Sep. 15, 2020, which is hereby incorporated by reference wherein in its entirety.