Patent Publication Number: US-2022234344-A1

Title: Liquid ejection apparatus

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2021-10973 filed on Jan. 27, 2021. The entire content of the priority application is incorporated herein by reference. 
     BACKGROUND ART 
     There is known an ink jet printer having a function of inspecting whether or not ink droplets are normally ejected from a nozzle and determining a method for cleaning the nozzle based on the result of the inspection. 
    
    
     
       DETAILED DESCRIPTION 
       In such an ink jet printer, a viscosity change of ink in accordance with a temperature change may affect performance of ejecting ink droplets from the nozzle. Therefore, if, after the nozzle inspection, a considerable period of time elapses before cleaning, the temperature is likely to change within the period, and the determined cleaning method may not be appropriate. 
       According to the present disclosure, even when there is a temperature change in a period from the nozzle inspection to the nozzle cleaning, the nozzle is appropriately cleaned. 
         FIG. 1  is a schematic configuration of a printer. 
         FIG. 2  shows a detection electrode disposed in the cap and a connection between the detection electrode and the high-voltage power supply circuit and the determination circuit. 
         FIG. 3A  shows a change in the potential of the detection electrode when ink is ejected from the nozzle. 
         FIG. 3B  shows a change in the potential of the detection electrode when ink is not ejected from the nozzle. 
         FIG. 4  is a plan view of the ink jet head. 
         FIG. 5A  is an enlarged view of the VA portion of  FIG. 4 . 
         FIG. 5B  is a cross-sectional view taken along a line VB-VB of  FIG. 5A . 
         FIG. 6  is a block diagram showing the electrical configuration of the printer. 
         FIG. 7  is a flowchart showing the processing by the controller. 
         FIG. 8A  shows a table in which the temperature range is associated with the drive potential. 
         FIG. 8B  shows a table in which the number of failure nozzles is associated with the recovery operation. 
         FIG. 9  is a flowchart showing processing by the controller according to the first modification. 
         FIG. 10  is a flowchart showing the processing by the controller according to the second modification. 
         FIG. 11  is a flowchart showing processing by the controller according to the third modification. 
         FIG. 12  shows a table in which the temperature range is associated with the drive waveform in the fourth modification. 
     
    
    
     Hereinafter, an illustrative embodiment will be described with reference to the accompanying drawings. 
     General Configuration of Printer 
     As illustrated in  FIG. 1 , a printer  1  includes a carriage  2 , a subtank  3 , an inkjet head  4 , a platen  5 , a conveyance roller  6 , a conveyance roller  7 , and a maintenance unit  8 . 
     The carriage  2  is supported by a guide rail  11  and a guide rail  12  each extending in a scanning direction. The carriage  2  is connected to a carriage motor  86  (refer to  FIG. 6 ) via a belt. The carriage  2  is configured to, in response to the carriage motor  86  being driven, move in the scanning direction along the guide rails  11 ,  12 . The scanning direction corresponds to a right-left direction as defined in  FIG. 1 . 
     The subtank  3  is mounted on the carriage  2 . The printer  1  further includes a cartridge holder  13 . The cartridge holder  13  is configured to accommodate four ink cartridges  14  that are attachable thereto and detachable therefrom. In the cartridge holder  13 , the ink cartridges  14  are arranged next to each other in the scanning direction. The ink cartridges  14  store respective colored ink. Ink is an example of liquid. More specifically, the ink cartridges  14  store black ink, yellow ink, cyan ink, and magenta ink, respectively, in this order from the rightmost ink cartridge  14  in the scanning direction. The subtank  3  is connected, by respective corresponding tubes  15 , to the ink cartridges  14  attached to the cartridge holder  13 . Such a configuration may thus enable supply of ink of four colors to the subtank  3  from the respective ink cartridges  14 . 
     The inkjet head  4  is mounted on the carriage  2  and is connected to a lower end portion of the subtank  3 . The inkjet head  4  is supplied with ink of four colors from the subtank  3 . The inkjet head  4  has a nozzle surface  4   a . The nozzle surface  4   a  may be a lower surface of the inkjet head  4 . The nozzle surface  4   a  has four nozzle rows  9  arranged next to each other in the scanning direction. The nozzle rows  9  include nozzles  10 . More specifically, the nozzles  10  are arranged in rows extending in a conveyance direction orthogonal to the scanning direction to form the nozzle rows  9 . The inkjet head  4  is configured to eject ink from the nozzles  10 . In the inkjet head  4 , black ink is ejected from the nozzles  10  belonging to the rightmost nozzle row  9  in the scanning direction. Yellow ink is ejected from the nozzles  10  belonging to the nozzle row  9  to the left of the black nozzle row  9 . Cyan ink is ejected from the nozzles  10  belonging to the nozzle row  9  to the left of the yellow nozzle row  9 . Magenta ink is ejected from the nozzles  10  belonging to the nozzle row  9  to the left of the cyan nozzle row  9 . 
     The platen  5  is disposed below the inkjet head  4  and faces the nozzles  10 . The platen  5  extends in the scanning direction and has a dimension corresponding to the entire width of a sheet P to be conveyed. The platen  5  is configured to support from below a sheet P being conveyed. The sheet P is an example of a recording medium. The conveyance roller  6  is disposed upstream from the inkjet head  4  and the platen  5  in the conveyance direction. The conveyance roller  7  is disposed downstream from the inkjet head  4  and the platen  5  in the conveyance direction. The conveyance rollers  6 ,  7  are connected to a conveyance motor  87  (refer to  FIG. 6 ) via gears. The conveyance rollers  6 ,  7  are configured to, in response to the conveyance motor  87  being driven, rotate to convey a sheet P in the conveyance direction. 
     The maintenance unit  8  includes a cap  71 , a suction pump  72 , and a waste liquid tank  73 . The cap  71  is disposed to the right of the platen  5  in the scanning direction. When the carriage  2  is located at a maintenance position, the nozzles  10  face the cap  71 . The maintenance position is further to the right than the platen  5  in the scanning direction. 
     The cap  71  is movable upward and downward selectively by control of a cap up-and-down mechanism  88  (refer to  FIG. 6 ). The cap up-and-down mechanism  88  may have a similar configuration to a known cap up-and-down mechanism. The description of the cap up-and-down mechanism in JP2012-206396A is incorporated herein by reference. In a state where the nozzles  10  and the cap  71  face each other when the carriage  2  is located at the maintenance position, the cap up-and-down mechanism  88  moves the cap  71  upward. As the cap  71  moves upward, an upper end portion of the cap  71  intimately contacts the nozzle surface  4   a  of the inkjet head  4  to cover the nozzles  10 . The cap  71  may not be limited to have such a configuration to intimately contact the nozzle surface  4   a  to cover the nozzles  10 . Alternatively, the cap  71  may intimately contact a frame surrounding the nozzle surface  4   a  of the inkjet head  4  to cover the nozzles  10 . 
     The suction pump  72  may be a peristaltic pump. In this case, the suction pump  72  is connected to both the cap  71  and the waste liquid tank  73 . With this configuration, the maintenance unit  8  may perform a suction purge. In a suction purge, in response to the suction pump  72  being driven in a state where the cap  71  covers the nozzles  10 , ink is sucked from the nozzles  10  of the inkjet head  4  by the suction pump  72 . The suction purge is an example of a second recovery operation. Ink discharged from the inkjet head  4  by the suction purge is stored in the waste liquid tank  73 . 
     In the present embodiment, one of three types of suction purge, i.e., weak purge, medium purge, and strong purge, may be selectively performed. In the medium purge, more ink is discharged than in the weak purge. In the strong purge, more ink is discharged than in the medium purge. By adjusting either or both of the driving time and the driving speed of the suction pump  72 , one of the weak purge, the medium purge and strong purge may be selectively performed. 
     For the sake of convenience, in the illustrative embodiment, the cap  71  covers all the nozzles  10  of the inkjet head  4  and ink is discharged from the inkjet head  4  through each nozzle  10  in a suction purge. Nevertheless, in other embodiments, the cap  71  may include a first capping portion and a second capping portion, each of which may cover corresponding nozzles  10  of the inkjet head  4 . The first capping portion may cover the nozzles  10  belonging to the rightmost nozzle row  9  from which black ink is ejected, and the second capping portion may cover the nozzles  10  belonging to the remaining nozzle rows  9  from which respective color inks (e.g., yellow, cyan, and magenta inks) are ejected. In this case, the cap up-and-down mechanism  88  may include a switching valve. In a suction purge, the cap up-and-down mechanism  88  may move the first capping portion and the second capping portion simultaneously to cover the respective corresponding nozzles  10 . A destination with which the suction pump  72  is communicated may be switched between the first capping portion and the second capping portion by the switching valve. Such a configuration may enable the suction pump  72  to communicate with the first capping portion or the second capping portion of the cap  71  as appropriate, thereby discharging black ink and color inks selectively from the inkjet head  4 . Alternatively, the maintenance unit  8  may include a cap  71  and a cap up-and-down mechanism  88  for each nozzle row  9 . Such a configuration may enable ink to be discharged from the nozzles  10  of the inkjet head  4  on a nozzle row  9  basis. 
     As illustrated in  FIG. 2 , a detection electrode  76  is disposed in the cap  71 . The detection electrode  76  has a flat rectangular shape. The detection electrode  76  is connected to a high-voltage power supply circuit  77  via a resistor  79 . In ejection determination, the high-voltage power supply circuit  77  applies a certain positive potential (e.g., approximately 600 V) to the detection electrode  76 . The inkjet head  4  is maintained at a ground potential. Thus, a certain potential difference is caused between the inkjet head  4  and the detection electrode  76 . A determination circuit  78  is connected to the detection electrode  76 . The determination circuit  78  compares a potential indicated by a signal received from the detection electrode  76  with a threshold potential Vt, and outputs a determination signal responsive to the comparison result. 
     More specifically, in ejection determination, a certain potential difference is caused between the inkjet head  4  and the detection electrode  76 . Thus, when the inkjet head  4  ejects ink from a target nozzle  10 , the ejected ink gets electrically charged. In a state where the carriage  2  is positioned at the maintenance position, the inkjet head  4  is driven in a check mode for ejecting ink from each nozzle  10 . Thus, in a case where ink is normally ejected from a particular target nozzle  10  toward the detection electrode  76 , as shown in  FIG. 3A , as the charged ink approaches the detection electrode  76 , the potential at the detection electrode  76  decreases from the potential Va until the charged ink reaches the detection electrode  76 . When the ink reaches the detection electrode  76 , the potential at the detection electrode  76  reaches the potential Vb that is lower than the threshold potential Vt. After the charged ink reaches the detection electrode  76 , the potential at the detection electrode  76  gradually increases to the potential Va from the potential Vb. That is, the potential at the detection electrode  76  changes in the driving period Td of the inkjet head  4 . In a case where the potential at the detection electrode  76  exceeds the threshold potential Vt in the driving period Td, it is determined that ink has been normally ejected from the target nozzle  10  and thus that the target nozzle  10  is not a failure nozzle. 
     In a case where ink is not ejected from the particular target nozzle  10  although the inkjet head  4  is driven in the check mode, no ink is between the inkjet head  4  and the detection electrode  76 . Thus, as shown in  FIG. 3B , the potential at the detection electrode  76  is maintained almost constant at the potential Va in the driving period Td of the inkjet head  4 . That is, the potential at the detection electrode  76  does not exceed the threshold potential Vt in the driving period Td of the inkjet head  4 . In this case, it is determined that ink has not been normally ejected from the target nozzle  10  and thus that the target nozzle  10  is a failure nozzle. 
     In the illustrative embodiment, a positive potential is applied to the detection electrode  76  by the high-voltage power supply circuit  77 . Nevertheless, in other embodiments, a negative potential (e.g., approximately −600 V) may be applied to the detection electrode  76  by the high-voltage power supply circuit  77 . In such a case, in a state where the carriage  2  is positioned at the maintenance position, the inkjet head  4  may be driven in the check mode. In response to this, in a case where ink is normally ejected from a particular target nozzle  10  toward the detection electrode  76 , as the charged ink approaches the detection electrode  76 , the potential at the detection electrode  76  may increase from the potential Va by exceeding a threshold potential Vt until the charged ink reaches the detection electrode  76 . When the charged ink reaches the detection electrode  76 , the potential at the detection electrode  76  reaches a particular peak potential. After the charged ink reaches the detection electrode  76 , the potential at the detection electrode  76  may gradually decrease to the potential Va from the particular peak potential. 
     Inkjet Head 
     Next, a configuration of the inkjet head  4  will be described in detail. As illustrated in  FIGS. 4, 5A, and 5B , the inkjet head  4  includes a channel unit  21  and a piezoelectric actuator  22 . 
     The channel unit  21  includes plates  31 ,  32 ,  33 ,  34 , and  35 . The plate  31  may be a bottom plate of the channel unit  21 . The plate  32  is disposed above the plate  31 . The plate  33  is disposed above the plate  32 . The plate  34  is disposed above the plate  33 . The plate  35  is disposed above the plate  34 . The channel unit  21  includes individual channels  41  and four common channels  42 . 
     The individual channels  41  are arranged in rows in the conveyance direction so as to form four individual channel rows  29  for the four nozzle rows  9 . That is, the channel unit  21  includes the individual channel rows  21  arranged next to each other in the scanning direction. 
     Each individual channel  41  includes a nozzle  10 , a pressure chamber  51 , a descender  52 , and a restrictor channel  53 . In each individual channel  41 , a nozzle  10  and a left end portion of a pressure chamber  51  in the scanning direction are connected to each other via a descender  52  such that the nozzle  10  and the pressure chamber  51  are in communication with each other. A restrictor channel  53  is connected to a right end portion of the pressure chamber  51  in the scanning direction such that the restrictor channel  53  and the pressure chamber  51  are in communication with each other. Configurations of the nozzle  10 , the pressure chamber  51 , the descender  52 , and the restrictor channel  53  and positional relationships therebetween are known by a person ordinary skilled in the art. Therefore, a detailed description thereof will be omitted. 
     The four common channels  42  are disposed at respective positions corresponding to the four individual channel rows  29 . Each common channel  42  extends in the conveyance direction and overlaps, in the vertical direction, right end portions of corresponding individual channels  41  belonging to the corresponding individual channel rows  29 . Each common channel  42  is connected to right end portions of corresponding restrictor channels  53  so that the common channel  42  and the corresponding restrictor channels  53  are in communication with each other. The restrictor channels  53  constitute the corresponding individual channels  41 . Each common channel  42  is configured to receive ink supplied thereto via an inlet  42   a  defined at an upstream end portion of the channel unit  21  in the conveyance direction. 
     The piezoelectric actuator  22  includes a diaphragm  61 , a piezoelectric layer  62 , a common electrode  63 , and individual electrodes  64 . The diaphragm  61  may be made of a piezoelectric material containing lead zirconate titanate, a main component of which is a mixed crystal of lead titanate and lead zirconate. The diaphragm  61  is disposed on an upper surface of the plate  35  that may be one of the plates  31  to  35  constituting the channel unit  21 , and covers the pressure chambers  51 . The piezoelectric layer  62  is made of the same piezoelectric material used for the diaphragm  61 . The piezoelectric layer  62  is disposed on an upper surface of the diaphragm  61  and continuously extends over the pressure chambers  51 . In the illustrative embodiment, the diaphragm  61  and the piezoelectric layer  62  are both made of a piezoelectric material. Nevertheless, in other embodiments, the diaphragm  61  may be made of an insulating material, such as a synthetic resin material, other than the piezoelectric material. 
     The common electrode  63  is disposed between the diaphragm  61  and the piezoelectric layer  62  and extends therebetween in an entire range within which the common electrode  63  and the diaphragm  61  extend. The common electrode  63  is connected to a power supply via a wiring and is maintained at a ground potential. The individual electrodes  64  are disposed on an upper surface of the piezoelectric layer  62 . The individual electrodes  64  are in a one-to-one relationship with the pressure chambers  51 . Each individual electrode  64  overlaps a central portion of a corresponding pressure chamber  51  in the vertical direction. Each individual electrode  64  is connected to a driver IC  89  (refer to  FIG. 6 ) via a wiring. The ground potential or a drive potential (e.g., approximately 20 V to 30 V) is selectively applied to each individual electrode  64  from the driver IC  89 . A portion of the piezoelectric layer  62  sandwiched between a particular portion of the common electrode  63  and an individual electrode  64  is polarized in a thickness direction of the piezoelectric layer  62 . Such polarized portions are provided corresponding to the arrangement of the common electrode  63  and the individual electrodes  64 . 
     The piezoelectric actuator  22  has particular portions that serve as drive elements  22   a  for applying pressure to ink in the respective pressure chambers  51 . Each drive element  22   a  includes a portion of the diaphragm  61 , a portion of the piezoelectric layer  62 , a portion of the common electrode  63 , and an individual electrode  64 . The portions of the diaphragm  61 , the piezoelectric layer  62 , and the common electrode  63 , and the individual electrode  64  overlap a pressure chamber  51  in the vertical direction. In response to the driver IC  89  switching the potential at a target individual electrode  64  between the ground potential and the drive potential, the potential difference occurring between the target individual electrode  64  and the common electrode  63  is changed. This change causes deformation in the piezoelectric layer  62  and the diaphragm  61 . Thus, the pressure chamber  51  changes in its shape and thus the pressure applied to ink in the pressure chamber  51  changes, whereby ink is ejected from the nozzle  10  being in communication with the pressure chamber  51 . 
     Electrical Configuration of Printer 
     Hereinafter, a description will be provided on an electrical configuration of the printer  1 . As illustrated  FIG. 6 , the printer  1  includes a controller  80 . The controller  80  includes a CPU  81 , a ROM  82 , a RAM  83 , a flash memory  84 , and an ASIC  85 . The controller  80  is configured to control operations of the carriage motor  86 , the inkjet head  4 , the conveyance motor  87 , the cap up-and-down mechanism  88 , the suction pump  72 , the high-voltage power supply circuit  77 , and the driver IC  89 . In the illustrative embodiment, the controller  80  is configured to control the driver IC  89  to control driving of the inkjet head  4 . The controller  80  is configured to receive a determination signal from the determination circuit  78 . 
     The printer  1  further includes a display  69 , an operation interface  70 , a temperature sensor  68 , and a clock unit  67 . The operation interface  70  is an example of an inputting device. The display  69  may be a liquid crystal display disposed at a housing of the printer  1 . The controller  80  is configured to control the display  69  to display information necessary for operating the printer  1 . The operation interface  70  includes buttons disposed at the housing of the printer  1  and a touch screen of the display  69 . In response to a user operating the operation interface  70 , a particular signal is input to the controller  80  from the operation interface  70 . 
     The temperature sensor  68  is configured to detect a temperature of the environment where the printer  1  is placed and to output a temperature signal indicating the temperature. The controller  80  is configured to receive the temperature signal from the temperature sensor  68 . The clock unit  67  is configured to count the time and output a time signal indicating the current time. The control unit  80  is configured to receive the time signal from the clock unit  67 . 
     In the controller  80 , only one of the CPU  81  or the ASIC  85  may perform all processing or a combination of the CPU  81  and the ASIC  85  may perform all processing. Alternatively, the controller  80  may include a single CPU  81  that may perform all processing or include a plurality of CPUs  81  that may share all processing. Alternatively, the controller  80  may include a single ASIC  85  that may perform all processing or include a plurality of ASICs  85  that may share all processing. 
     As shown in  FIG. 8A , the flash memory  84  stores a table in which a temperature range of the temperature T indicated by the temperature signal received from the temperature sensor  68  is associated with a drive potential to be applied to the individual electrode  64 . As shown in  FIG. 8A , the drive voltage is V4 when T3≤T. The drive voltage is V3 when T2≤T&lt;T3. The drive voltage is V2 when T1≤T&lt;T2. The drive voltage is V1 when T&lt;T1. T1, T2, and T3 satisfy the inequality of T1&lt;T2&lt;T3. V1, V2, V3, and V4 satisfy the inequality of V4&lt;V3 &lt;V2 &lt;V1. 
     As shown in  FIG. 8B , the flash memory  84  stores a table in which the number N of failure nozzles is associated with a recovery operation. As shown in  FIG. 8B , the recovery operation is the strong purge when N3≤N. The recovery operation is the medium purge when N2≤N&lt;N3. The recovery operation is the weak purge when N1≤N&lt;N2. The recovery operation is a flushing when N&lt;N1. N1, N2, and N3 satisfy the inequality of N1&lt;N2&lt;N3. The flushing is an operation in which the driver IC  89  is controlled to drive the individual electrodes  64  corresponding to the nozzles  10  in order to discharge ink from the nozzles  10 . The amount of ink discharged by the flushing is less than the amount of ink discharged by the weak purge. 
     Control of Printer by Controller 
     The control of the printer  1  by the controller  80  will be described. The controller  80  controls the operation of the printer  1  by performing processing along the flowchart in  FIG. 7 . 
     The controller  80  waits while the controller  80  does not receive a recording command (S 101  : NO) instructing recording on the recording sheet P and the time indicated by the time signal received from the clock unit  67  is not a predetermined time that is set in advance (S 102 : NO). 
     When the recording command is received (S 101 : YES), the controller  80  sets a drive potential to be applied to the individual electrode  64  (S 103 ). In S 103 , the controller  80  sets the drive potential to one of V1, V2, V3, and V4 according to the table shown in  FIG. 8A  based on the temperature T indicated by the temperature signal received from the temperature sensor  68 . 
     Then, the controller  80  executes a recording process (S 104 ). In S 104 , the controller  80  repeats a recording operation in which the inkjet head  4  ejects ink from the nozzles  10  toward the recording sheet P and a conveying operation in which the recording sheet P is conveyed by a predetermined amount to the conveyance rollers  6 ,  7 . In the recording operation, the controller  80  controls the carriage motor  86  to move the carriage  2  in the scanning direction, and the driver IC 89  to switch the potential to be applied to the individual electrode  64  between the ground potential and the drive potential that is set in S 103 . In the conveying operation, the controller  80  controls the conveyance motor  87  to rotate the conveyance rollers  6 ,  7 . 
     When the current time indicated by the time signal received from the clock unit  67  is a predetermined time set in advance (S 102 : YES), the controller  80  executes check mode driving (S 105 ) for checking whether or not a failure has occurred in each of the nozzles  10 . In the check mode driving, the controller  80  drives the inkjet head  4  in a state where the carriage  2  is in the maintenance position to eject ink sequentially from each of the nozzles  10  toward the detection electrode  76 . The controller  80  receives a determination signal from the determination circuit  78  for each of the nozzles  10 . 
     If all of the received determination signals indicate that no failure has occurred (S 106 : NO), the process proceeds to S 101 . If the received determination signal indicates that a failure has occurred (S 106 : YES), the controller  80  stores first failure-nozzle data and first temperature data in the flash memory  84  (S 107 ). The first failure-nozzle data identifies the nozzle  10 , i.e., a failure nozzle, in which a failure has occurred, and includes the number N of the failure nozzles, based on the determination signals received during the check mode driving. The first temperature data is based on the temperature signal received from the temperature sensor  68  during the check mode driving, and indicates a first temperature. The first temperature data may be about the temperature itself indicated by the temperature signal. The first temperature data may alternatively indicate that the temperature indicated by the temperature signal is included in one of the temperature ranges in  FIG. 8A . 
     Then, the controller  80  waits until receiving the recording command (S 108 : NO). When the controller  80  receives the recording command (S 108 : YES), the controller  80  determines whether or not the temperature at a timing of receiving the recording command is in the temperature range at a timing of the check mode driving (S 109 ). In S 109 , the controller  80  determines which temperature range defined in the table in  FIG. 8A  includes the temperature T, i.e., second temperature, indicated by the temperature signal received from the temperature sensor  68 . Then, in S 109 , the controller  80  determines whether the first temperature and the temperature T satisfies a predetermined condition. That is, the controller  80  determines whether or not the determined temperature range matches the temperature range indicated by the first temperature data stored in the flash memory  84 . 
     If the controller  80  determines in S 109  that the determined temperature range matches the temperature range indicated by the first temperature data (S 109 : YES), that is, the controller  80  determines that the first temperature and the temperature T satisfies a predetermined condition, the controller  80  reads out the first failure-nozzle data stored in the flash memory  84  in S 107 , refers to the table in  FIG. 8B  based on the number N of failure nozzles included in the first failure-nozzle data, and sets a recovery operation (S 110 ). 
     In this embodiment, the maintenance unit  8  for performing suction purge and the inkjet head  4  for performing flushing correspond to the “recovery device”. 
     If the controller  80  determines in S 109  that the determined temperature range does not match the temperature range indicated by the first temperature data (S 109 : NO), that is, the controller  80  determines whether the first temperature and the temperature T does not satisfy a predetermined condition, the controller  80  executes the check mode driving process as done in S 105  (S 111 ). In S 111 , the controller  80  executes the check mode driving for checking whether or not a failure has occurred in each nozzle  10 . In the check mode driving, the controller  80  drives the inkjet head  4  in a state where the carriage  2  is in the maintenance position to eject ink sequentially from each of the nozzles  10  toward the detection electrode  76 . The controller  80  receives a determination signal from the determination circuit  78  for each of the nozzles  10 . 
     The controller  80  obtains second failure-nozzle data on the basis of the determination signal. The second failure-nozzle data identifies the nozzle  10 , i.e., a failure nozzle, in which the failure has occurred, and includes the number N of the failure nozzles, on the basis of the determination signal received during the check mode driving. 
     The controller  80  sets a recovery operation on the basis of the number N of failure nozzles included in the second failure-nozzle data and the table in  FIG. 8B  (S 112 ). 
     The controller  80  executes the recovery operation set in S 110  or S 112  (S 113 ). After the recovery operation is executed, the drive potential applied to the drive electrode  64  is set in S 103 , and the recording process is executed in S 104 . 
     Effects 
     If a large temperature change occurs after driving in the check mode until when the recovery operation is performed, the condition of the failure nozzle may change. In the present embodiment, first failure-nozzle data and first temperature data obtained during the check mode driving are stored in the flash memory  84 . When the temperature T during the recovery operation is in the temperature range during the check mode driving, the recovery operation is set based on the first failure-nozzle data and the recovery operation is executed. In other words, it is not necessary to execute the check mode driving immediately before the recovery operation so the recovery operation is quickly completed. 
     When the temperature at the recovery operation is not in the temperature range at the check mode driving, the check mode driving is performed again to obtain the second failure-nozzle data. Then, the recovery operation is set on the basis of the obtained second failure-nozzle data, and the recovery operation is performed. If, after the check mode driving, a large temperature change occurs before the recovery operation, the recovery operation suitable for the nozzle condition is performed on the basis of the second failure-nozzle data obtained by the check mode driving immediately before the recovery operation. 
     The temperature range and the drive potential are associated with each other in accordance with the relationship between the temperature of the ink and the viscosity of the ink, in the inkjet head  4 . In the present embodiment, prior to executing the recovery operation, it is determined whether or not the current temperature is in the temperature range at the check mode driving. When the temperature changes largely since the check mode driving, the viscosity of the ink may also change largely such that the drive potential needs to be changed. Because either the recovery operation based on the first failure-nozzle data or the recovery operation based on the second failure-nozzle data is performed according to the temperature change, the nozzle condition can be appropriately recovered. 
     In the present embodiment, the check mode driving is executed when the time indicated by the clock signal from the clock unit  67  is a predetermined time set in advance. For example, it may be preferable to execute the check mode driving while setting the predetermined time in a period when it is unlikely that a user uses the printer  1 , to avoid disturbing the recording process in the printer  1 . 
     As a result of the check mode driving, it may be allowed to immediately execute the recovery operation when a failure has occurred in the nozzle  10 . If, after the check mode driving, a period until receiving recording command is long, the viscosity of the ink in the inkjet head  4  may increase or decrease during the period. In this case, it is necessary to execute the recovery operation again when receiving the recording command, thereby increasing the consumption of the ink. 
     In the present embodiment, after the check mode driving, the recovery operation is performed using the reception of the recording command as a trigger. Therefore, since the recovery operation is performed immediately before the recording process, only an amount of ink necessary for recovering the nozzle condition is consumed regardless of the length of the period from the check mode driving to the reception of the recording command. 
     In the present embodiment, one of flushing, weak purge, medium purge, and strong purge is selectively performed according to the number of failure nozzles, so that the nozzle condition is appropriately recovered. 
     Modifications 
     The present invention may not be limited to the above-described embodiments, and various modifications may be made within the scope of the claims. 
     The controller  80  of the first modification performs processing to control the printer  1  according to a flowchart of  FIG. 9 . In the flowchart of  FIG. 9 , S 109  in the flowchart of  FIG. 7  is replaced by S 201 . In the flowchart of  FIG. 9 , the first temperature data stored in the flash memory  84  in S 107  is a numerical value of the temperature indicated by the temperature signal received from the temperature sensor  68 . 
     When the controller  80  receives the recording command (S 108 : YES), the controller  80  determines whether the temperature difference ΔT between the temperature T indicated by the temperature signal received from the temperature sensor  68  and the temperature indicated by the first temperature data stored in the flash memory  84  in S 107  is equal to or less than the threshold value ΔTh (S 201 ). That is, the controller  80  determines whether the first temperature and the temperature T satisfies a predetermined condition. 
     If the temperature difference ΔT is less than or equal to the threshold value ΔTh (S 201 : YES), the process proceeds to S 110 . If the temperature difference ΔT is greater than the threshold value ΔTh (S 201 : NO), the process proceeds to S 111 . 
     If a large temperature change occurs after driving in the check mode until when the recovery operation is performed, the condition of the failure nozzle may change. In the first modification, first failure-nozzle data and first temperature data obtained during the check mode driving are stored in the flash memory  84 . The first temperature data is a numerical value of the temperature indicated by the temperature signal received from the temperature sensor  68 . When the temperature difference ΔT between the temperature T at the recovery operation and the temperature at the check mode driving stored in the flash memory  84  is equal to or less than a threshold value, the recovery operation is set based on the first failure-nozzle data and the recovery operation is executed. In other words, it is not necessary to execute the check mode driving immediately before the recovery operation so the recovery operation is quickly completed. 
     In the second modification, the controller  80  controls the printer  1  by performing processing along the flowchart of  FIGS. 10 . S 301 , S 302  and S 303  in the flowchart of  FIG. 10  are replacements for S 111  and S 112  in the flowchart of  FIG. 7 . 
     In the second modification, if the controller  80  determines in S 109  that the temperature T indicated by the temperature signal received from the temperature sensor  68  is not in the temperature range indicated by the first temperature data that is stored in the flash memory  84  in S 107  (S 109 : NO), the controller  80  determines whether or not the temperature T is higher than the temperature at the check mode driving (S 301 ). If the temperature T is higher than the temperature at the check mode driving (S 301 : YES), a recovery operation in which the discharge amount of ink is less than the discharge amount of ink in the recovery operation based on the first failure-nozzle data is set (S 302 ), and the process proceeds to S 113 . 
     If the temperature T is lower than the temperature at the check mode driving (S 301 : NO), a recovery operation in which the discharge amount of ink is greater than the discharge amount of ink in the recovery operation based on the first failure-nozzle data is set (S 302 ), and the process proceeds to S 113 . 
     In general, when the temperature increases, the viscosity of the ink decreases. In the second modification, when the temperature rises significantly after the check mode driving until the recovery operation is performed, the recovery operation with less ink discharge is performed than when the temperature does not change. 
     On the other hand, when the temperature decreases, the viscosity of the ink increases. In the second modification, when the temperature decreases significantly until the recovery operation is performed after the check mode driving, the recovery operation with more ink discharge is performed than when the temperature does not change. Therefore, even when the temperature changes largely after the check mode driving, the recovery operation suitable for the nozzle condition is performed. 
     In the third modification, a recovery operation corresponding to the failure-nozzle data stored most recently in the flash memory  84  is performed. 
     The controller  80  of the third modification performs processing to control the printer  1  in accordance with the flowchart of  FIG. 11 . The controller  80  performs processing of S 101 , S 102 , S 103 , S 104 , S 105  and S 106  in the same manner as in the present embodiment. When the controller  80  determines in S 106  that a failure nozzle exists (S 106 : YES), the controller  80  stores the failure-nozzle data and the first temperature data in the flash memory  84  (S 401 ). The failure-nozzle data identifies the nozzle  10 , i.e., the failure nozzle, in which a failure has occurred, and includes the number N of failure nozzles, based on the determination signal received during the check mode driving. 
     Then, in S 402 , the controller  80  determines whether or not the temperature T indicated by the temperature signals received from the temperature sensors  68  is in the temperature range indicated by the first temperature data stored in the flash memory  84 . More specifically, the controller  80  determines which temperature range defined in the table shown in  FIG. 8A  includes the temperature T indicated by the temperature signals received from the temperature sensors  68 . Next, the controller  80  determines whether or not the determined temperature range matches the temperature range indicated by the first temperature data that is stored in the flash memory  84  in S 107 . If it matches (S 402 : YES), the controller  80  determines in S 403  whether or not a recording command has been received. While the recording command has not been received (S 403 : NO), the processing proceeds to S 402 . 
     When the temperature T indicated by the temperature signal received from the temperature sensor  68  is not in the temperature range indicated by the first temperature data stored in the flash memory  84  (S 402 : NO), the controller  80  executes the check mode driving (S 404 ). In S 404 , the controller  80  drives the inkjet head  4  in a state where the carriage  2  is in the maintenance position to eject ink sequentially from each of the nozzles  10  toward the detection electrode  76 . The controller  80  receives a determination signal from the determination circuit  78  for each of the nozzles  10 . 
     Subsequently, the failure-nozzle data and the first temperature data stored in the flash memory  84  are updated (S 405 ), and the processing proceeds to S 402 . The failure-nozzle data identifies the nozzle  10 , i.e., the failure nozzle, in which a failure has occurred, and includes the number N of failure nozzles, based on the determination signal received during the check mode driving. The first temperature data is based on the temperature signal received from the temperature sensor  68  during the check mode driving. The first temperature data may be about the temperature itself indicated by the temperature signal. The first temperature data may alternatively indicate that the temperature indicated by the temperature signal is included in one of the temperature ranges in  FIG. 8A . 
     When the controller  80  receives a recording command (S 403 : YES), the controller  80  sets a recovery operation on the basis of the number N of failure nozzles included in the failure-nozzle data and the table in  FIG. 8B  (S 406 ). The controller  80  then executes the recovery operation set in S 406  (S 407 ). After the recovery operation, in S 103  the controller  80  sets a drive potential to be applied to the drive electrode  64 , and executes a recording process in S 104 . 
     In the third modification, after the failure-nozzle data is stored in the flash memory  84  on the basis of the determination signal output from the determination circuit  78  during the check mode driving, the check mode driving is executed every time the temperature T is not in the temperature range indicated by the first temperature data stored in the flash memory  84  before the recovery operation is executed, and the failure-nozzle data stored in the flash memory  84  is updated on the basis of the determination signal output from the determination circuit  78 . Thus, the controller  80  executes the recovery operation set on the basis of the failure-nozzle data stored most recently in the flash memory  84 . 
     In S 402 , as in S 201  described above, the controller  80  may determine whether or not the temperature difference ΔT between the temperature T indicated by the temperature signal received from the temperature sensor  68  and the temperature indicated by the first temperature data that is stored in the flash memory  84  in S 107  is equal to or less than the threshold value ΔTh. In this case, whenever the temperature difference ΔT exceeds the threshold value ΔTh, the controller  80  executes the check mode driving in S 405  to update the failure-nozzle data and the first temperature data stored in the flash memory  84 . 
     According to the third modification, no matter how the temperature T changes after the first check mode driving, the recovery operation suitable for the nozzle condition is performed. 
     In the fourth modification, a table shown in  FIG. 12  is stored in the flash memory  84  in which a temperature range of the temperature T indicated by the temperature signal received from the temperature sensor  68  is associated with a drive waveform of the drive signal to be transmitted from the driver IC  89  to the individual electrode  64 . The temperature range and the drive waveform are associated with each other in accordance with the relationship between the temperature of the ink and the viscosity of the ink, in the inkjet head  4 . The potential of the individual electrode  64  is switched between the ground potential and the drive potential according to the drive waveform. 
     As shown in the table in  FIG. 12 , the drive waveform is W4 when T3≤T. The drive waveform is W3 when T2≤T&lt;T3. The drive waveform is W2 when T1≤T&lt;T2. The drive waveform is W1 when T&lt;T1. T1, T2, and T3 satisfy the inequality of T1&lt;T2&lt;T3. The drive waveforms W1, W2, W3 and W4 in  FIG. 12  are pulse waveforms. The drive waveform W1 has its unique pulse width, number of pulses, and pulse interval, one or more of which is different from those of other waveforms. Similarly, each of drive waveforms W2, W3 and W4 has its unique pulse width, number of pulses, and pulse interval. 
     The amount of ink ejected from the nozzle  10  when the drive waveform W1 is applied to the individual electrode  64  is greater than the amount of ink ejected from the nozzle  10  when the drive waveform W2 is applied to the individual electrode  64 . The amount of ink ejected from the nozzle  10  when the drive waveform W2 is applied to the individual electrode  64  is greater than the amount of ink ejected from the nozzle  10  when the drive waveform W3 is applied to the individual electrode  64 . The amount of ink ejected from the nozzle  10  when the drive waveform W3 is applied to the individual electrode  64  is greater than the amount of ink ejected from the nozzle  10  when the drive waveform W4 is applied to the individual electrode  64 . 
     In the fourth modification, the determination in S 109  is performed using the temperature range shown in the table of  FIG. 12  to determine the drive waveform to be applied to the individual electrode  64 . Since the individual electrode  64  is driven according to the drive waveform corresponding to the temperature range, the nozzle condition can be appropriately recovered rather than controlling the drive of the individual electrode  64  according to only the drive potential. 
     Other Modifications 
     When the printer  1  supports recording at low image quality and recording at high image quality, and when it is determined that the first received recording instruction is for recording at high image quality after the check mode driving in S 105 , S 109  may be executed. When a recording instruction for recording at low image quality is received before receiving a recording instruction for recording at high image quality, recording processing may be executed without obtaining a temperature signal from the temperature sensor  68 . 
     When the controller  80  receives a signal other than the recording command, S 109  may be executed. The signal other than the recording command may be, for example, an ON signal indicating that the power of the printer  1  is turned ON after driving in the check mode. 
     After receiving a time signal indicating that the time is a predetermined time and executing the check mode driving, the recovery operation may be executed after performing an operation which requires a certain amount of time other than the recovery operation. In this case, when a large temperature change occurs during the execution of such operation, the controller  80  may execute another recovery operation other than the recovery operation based on the first failure-nozzle data stored in the flash memory  84 . 
     The execution timing of the check mode driving may be when a predetermined time has elapsed from the latest recording or when a predetermined time has elapsed from the latest check mode driving. 
     As a type of the recovery operation, only a plurality of types of the suction purge in which ink discharge amounts are different may be used, and the flushing may be excluded. As a type of the recovery operation, only a plurality of types of the flushing in which ink discharge amounts are different may be used, and the suction purge may be excluded. 
     Instead of the suction purge, a pressure purge may be performed by providing a pressure pump in the middle portion of the tube  15  connecting the sub-tank  3  and the ink cartridge  14 , and driving the pressure pump while the nozzles  10  are covered with the cap  71 , whereby the ink in the inkjet head  4  is pressurized and discharged from the nozzles  10 . 
     Both suction by the suction pump  72  and pressurization by the pressurization pump may be performed. 
     The check mode driving may be executed for some nozzles  10  of the inkjet head  4 , for example, every other nozzle  10  in each nozzle row  9 . Then it may be estimated whether or not the other nozzles  10  are failure nozzles on the basis of a determination signal output from the determination circuit  78  during the check mode driving. 
     In order to determine whether or not the nozzle  10  is a failure nozzle, a detection electrode extending in the vertical direction may be used. By detecting a change in the potential of the detection electrode when the ink discharged from the nozzle  10  passes through a region in the vicinity of the detection electrode, a failure nozzle may be determined. 
     Alternatively, by detecting an output change from a light-receiving portion due to the ink ejected from the nozzle  10  crossing the light emitted by a light source of the optical sensor, a failure nozzle may be determined. 
     Alternatively, as described in Japanese Patent No. 4929699, a voltage detection circuit for detecting a change in voltage when ink is ejected from the nozzle  10  may be connected to the plate  31  on which the nozzle  10  of the inkjet head  4  is formed, Thus, a failure nozzle may be determined based on a signal output from the voltage detection circuit to the controller  80 . 
     Alternatively, as described in Japanese Patent No. 6231759, the inkjet head  4  may include a temperature sensor element. After a first voltage is applied to drive a heater to eject ink, a second voltage is applied to drive the heater so that ink is not ejected. Then, a failure nozzle may be determined based on a change in temperature detected by the temperature sensor element during a period until a predetermined time elapses after the second voltage is applied. 
     A signal output unit that outputs signals corresponding to the state of the abnormal nozzle may be provided so information on the state of the failure nozzle may be acquired on the basis of the signals from the signal output unit. The state of the failure nozzle includes an abnormality in the ink ejection direction, ink spray, air bubbles in ink, clogging of paper powder, and the like. A recovery operation may be performed based on information on the state of the failure nozzle. A recovery operation may be performed based on information including both the number of failure nozzles and the state of the failure nozzle. 
     The present disclosure may be applied to a printer having a so-called line head extending in the scanning direction. 
     The present disclosure may be applied to a printer that records images on a recording medium such as a T-shirt, a sheet for outdoor advertisement, a jacket for a portable terminal such as a smartphone, a corrugated board, or a resin member. 
     The present disclosure may be applied to an apparatus for discharging liquid resin or metal. 
     While the invention has been described in conjunction with various example structures outlined above and illustrated in the figures, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that may be presently unforeseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the example embodiments of the disclosure, as set forth above, are intended to be illustrative of the invention, and not limiting the invention. Various changes may be made without departing from the spirit and scope of the disclosure. Therefore, the disclosure is intended to embrace all known or later developed alternatives, modifications, variations, improvements, and/or substantial equivalents.