Patent Publication Number: US-7717535-B2

Title: Liquid ejection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-019691, filed on Jan. 27, 2006, the entire content of which is incorporated herein by reference. 
   BACKGROUND 
   1. Technical Field 
   The present invention relates to a liquid ejection apparatus such as an inkjet printer. 
   2. Background Art 
   Typically, an inkjet printer is known as a liquid ejection apparatus that ejects liquid from a recording head, or a liquid ejection head, onto a target. The inkjet printer ejects ink onto a recording paper sheet as a target. The printer includes a recording head and a transport mechanism. The recording head is mounted in a carriage that reciprocates in a main scanning direction. The transport mechanism transports the recording paper sheet in a sub scanning direction. Nozzles are formed in the recording head for ejecting the ink, which is supplied from a cartridge (a liquid retaining body), onto the recording paper sheet. 
   A predetermined amount of ink is retained in the cartridge. The retaining amount of the ink gradually decreases as printing on the recording paper sheet is repeated. Japanese Laid-Open Patent Publication No. 2000-158665 describes a printer that indicates the need for replacement by a unused cartridge when the retaining amount of the ink in the cartridge becomes less than or equal to a predetermined threshold value. 
   The printer of Japanese Laid-Open Patent Publication No. 2000-158665 is an off-carriage type in which the cartridge is installed in a component other than a carriage. The ink is retained in an ink pack in the cartridge and flows to a recording head through an ink supply line through hydraulic head of the ink in the ink pack with respect to the recording head. The hydraulic head value P representing the hydraulic head applied to each nozzle of the recording head is obtained by the following equation:
 
 P=p 3−( p 1 +p 2)
 
   In the equation, p 1  represents total resistance acting on the ink that has flown through the ink supply line from an ink pack side end to a recording head side end, which is dynamic pressure of the ink. p 2  represents stiffness resistance of the ink pack. p 3  represents the difference between the hydraulic head at an ink outlet of the ink pack and the hydraulic head in a nozzle of the recording head. 
   When printing by the printer of the aforementioned document is being performed or after such printing is completed, it is determined whether the hydraulic head value P is less than or equal to a minimum value that allows ink ejection from the nozzles of the recording head P (hereinafter, referred to as a “critical hydraulic head”). If it is determined that the pressure hydraulic head value P is less than or equal to the critical hydraulic head, it is indicated that the retaining amount of the ink in the ink pack is less than or equal to the threshold value. The critical hydraulic head is a predetermined fixed value. The threshold value is set in correspondence with the critical hydraulic head. The dynamic pressure p 1  of the ink and the stiffness resistance p 2  of the ink pack vary in correspondence with the temperature in the proximity of the printer (hereinafter, referred to as an “environmental temperature”), the length of the ink supply line, viscosity of the ink, and the remaining amount of the ink in the ink pack. Nonetheless, the dynamic pressure p 1  of the ink and the stiffness resistance p 2  of the ink pack are set to values determined under the most undesirable conditions. 
   The dynamic pressure p 1  of the ink and the stiffness resistance p 2  of the ink pack, with which the hydraulic head value P is calculated, increase as the environmental temperature drops. Therefore, the printer of the aforementioned publication sets the dynamic pressure p 1  of the ink and the stiffness resistance p 2  of the ink pack to values corresponding to a lower temperature (for example, 10° C.) to avoid ink shortage when printing is being carried out. 
   However, if the actual environmental temperature of the printer is higher (for example, 25° C.), the dynamic pressure p 1  of the ink and the stiffness resistance p 2  of the ink pack, which are set to the aforementioned values corresponding to the lower temperature, exceed the values corresponding to the actual environmental temperature. As a result, regardless of that a sufficient amount of ink is retained in the ink pack, it may be determined that the hydraulic head value P is less than or equal to the critical hydraulic head. In other words, the printer may indicate the need for replacement of the cartridge regardless of the sufficient retaining amount of the ink in the ink pack. 
   SUMMARY 
   Accordingly, it is an objective of the present invention to provide a liquid ejection apparatus that optimally determines the amount of liquid retained in a liquid retaining body before urging the replacement of the liquid retaining body due to decrease of the retaining amount of the liquid. 
   To achieve the foregoing objective and in accordance with a first aspect of the present invention, a liquid ejection apparatus including a replaceable liquid retaining body, a liquid ejection head, a retaining amount detecting section, a temperature detecting section, a threshold value setting section, and a determining section is provided. The liquid ejection head ejects a liquid supplied by the liquid retaining body through a nozzle. The retaining amount detecting section detects a retaining amount of the liquid in the liquid retaining body. The temperature detecting section detects an environmental temperature in the proximity of the liquid ejection apparatus. The threshold value setting section sets a threshold value, in accordance with which it is determined whether the liquid retaining body needs be replaced by a unused liquid retaining body, such that the higher the temperature detected by the temperature detecting section, the lower the threshold value becomes. The determining section determines whether the retaining amount detected by the retaining amount detecting section has become less than or equal to the threshold value. 
   In accordance with a second aspect of the present invention, a liquid ejection apparatus including a replaceable liquid retaining body, a liquid ejection head, a retaining amount detecting section, an indication device, a control section, a temperature detecting section, and a threshold value setting section is provided. The liquid ejection head ejects a liquid supplied by the liquid retaining body through a nozzle. The retaining amount detecting section detects a retaining amount of the liquid in the liquid retaining body. The control section compares the retaining amount detected by the retaining amount detecting section with a predetermined threshold value. When the retaining amount becomes less than or equal to the threshold value, the control section activates the indication device to urge replacement of the liquid retaining body. The temperature detecting section detects the temperature in the proximity of the liquid ejection apparatus. The threshold value setting section sets the threshold value such that the higher the temperature detected by the temperature detecting section becomes, the lower the threshold value becomes. 
   Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
       FIG. 1  is a plan view schematically showing an inkjet printer according to a first embodiment of the present invention; 
       FIG. 2  is a cross-sectional view schematically showing a cartridge of the inkjet printer of  FIG. 1 ; 
       FIG. 3  is a diagrammatic view showing a maintenance unit of the inkjet printer of  FIG. 1 ; 
       FIG. 4  is a plan view showing a pressurization unit of the inkjet printer of  FIG. 1 ; 
       FIG. 5  is a block diagram representing the electric configuration of the inkjet printer of  FIG. 1 ; 
       FIG. 6  is a table representing the relationship between the stiffness resistance of an ink pack and the dynamic pressure of ink in an ink supply line and the environmental temperature; 
       FIG. 7  is a table representing the relationship between the environmental temperature and the threshold value; 
       FIG. 8  is a flowchart representing a routine of a replacement indication determining procedure; 
       FIG. 9  is a perspective view schematically showing an inkjet printer according to a second embodiment of the present invention; and 
       FIG. 10  is a map representing the relationship between the environmental temperature and the threshold value according to the second embodiment. 
   

   DESCRIPTION OF THE EXEMPLIFIED EMBODIMENTS 
   A first embodiment of the present invention will now be described with reference to  FIGS. 1 to 8 . In the description herein, a “front-and-rear direction”, a “left-and-right direction”, and an “up-and-down direction” correspond to the arrows in  FIGS. 1 and 3 . 
   As shown in  FIG. 1 , an inkjet printer (hereinafter, referred to as a printer)  10  as a liquid ejection apparatus includes a body frame  11  having a substantially box-like shape. A platen  12  extends in the body frame  11  and along the left-and-right direction. A non-illustrated recording paper sheet is supplied to the platen  12  by a paper feeder mechanism having a non-illustrated paper feeder motor. A bar-like guide member  13  is provided in the body frame  11  and extends parallel with the longitudinal direction (the left-and-right direction) of the platen  12 . 
   The guide member  13  is passed through the carriage  14  and supports the carriage  14  in such a manner as to allow the carriage  14  to reciprocate along the axial direction of the guide member  13 . The carriage  14  is connected to a carriage motor  16  through a timing belt  15  wound around a pair of pulleys  15   a . The carriage  14  is driven by the carriage motor  16  to reciprocate in the axial direction of the guide member  13 . 
   A recording head  17 , or a liquid ejection head, is provided on the surface of the carriage  14  opposed to the platen  12 . Valve units  18  are mounted in the carriage  14  to temporarily retain ink, or liquid, and supply the ink to the recording head  17 . The number of the valve units  18  corresponds to the number of the colors (or the types) of the ink. In the first embodiment, four valve units  18  are employed. Nozzles  19  (see  FIG. 3 ) are defined in the lower surface of the recording head  17 . The nozzles  19  eject ink droplets onto the recording paper sheet (not shown), which is supplied to the platen  12 . A thermistor SE 1  (see  FIG. 3 ) is arranged in the lower surface of the recording head  17 . 
   A cartridge holder  20  is provided on a right end of the body frame  11 . A plurality of cartridges  21  are removably provided in the cartridge holder  20 . In the first embodiment, four cartridge holders  20  are provided in the cartridge holder  20 . Referring to  FIG. 2 , each of the cartridges  21  includes a casing  22  having a rectangular cross section. An ink pack  23 , or a liquid retaining body, is provided in an air chamber  24  defined in each of the casings  22 . The ink packs  23  are filled with ink of different colors in correspondence with the cartridges  21 . 
   An IC chip  25  is arranged on an outer surface of each casing  22 . When the cartridges  21  are mounted in the cartridge holder  20 , the IC chips  25  are electrically connected to a control section  60  (see  FIG. 5 ) of the printer  10 . 
   Each of the ink packs  23  is formed from a flexible film and has a bag-like shape. In  FIG. 2 , the two-dotted chain lines represent the state of the ink pack  23  when the ink has not yet been consumed, or the retaining amount of the ink is a maximum value (an initial value). In the drawing, the solid lines represent the state of the ink pack  23  when the ink has been slightly consumed and reduced. Each of the ink packs  23  is connected to the corresponding one of the valve units  18  in the carriage  14  through a corresponding one of ink supply lines  26 , or liquid supply lines. The ink is ejected from each ink pack  23  to the exterior through an outlet  23   a , which is a joint portion between the ink pack  23  and the corresponding ink supply line  26 . The ink flows from an end  26   a  of the ink supply line  26  corresponding to the ink pack  23  to an end  26   b  of the ink supply line  26  corresponding to the recording head  17  and is temporarily retained in the corresponding valve unit  18 . 
   A pressurization unit  27 , or a pressurizing-supplying device, is provided in the vicinity of the cartridge holder  20 . The pressurization unit  27  supplies pressurized air to the air chambers  24  of the cartridges  21  through an air supply line  28 . The pressurization unit  27  includes a pressurization pump  29 , a pressure sensor  30 , and an atmospheric relief valve  31 . The air supply line  28  is divided into four branches by a distributor  32 , which is arranged downstream from the atmospheric relief valve  31 . The four branches are each connected to the corresponding one of the cartridges  21 . 
   An end of each of the branches of the air supply line  28  is provided in the air chamber  24  through the casing  22  of the corresponding cartridge  21 . Therefore, when the pressurization pump  29  of the pressurization unit  27  is actuated, air is supplied under pressure from the pressurization pump  29  to the air chambers  24  of the cartridges  21  through the air supply line  28 . The pressure of the pressurized air acts to squash the ink pack  23  of each air chamber  24 , thus sending the ink from the ink pack  23  to the corresponding valve unit  18  through the ink supply line  26 . 
   A home position of the carriage  14  is defined at a position near the right end of the body frame  11 . A maintenance unit  33  is arranged at the home position. As shown in  FIG. 3 , the maintenance unit  33  has a cap  34 , which forms a sealing body. The cap  34  has a rectangular box-like shape with an upper opening and is connected to a non-illustrated lift mechanism. When the carriage  14  is located at the home position, the lift mechanism raises the cap  34  to a position at which the cap  34  contacts the lower surface of the recording head  17 . This airtightly seals the nozzles  19  of the recording head  17 . An outlet port  35  is defined in the bottom wall of the cap  34 . An outlet tube  36  is connected to the outlet port  35 . The outlet port  35  is connected also to a waste ink tank  39 . A suction pump  37  is provided in an intermediate portion of the outlet tube  36 . 
   The pressurization unit  27  will hereafter be explained. 
   As shown in  FIG. 4 , the pressurization pump  29 , the pressure sensor  30 , and the atmospheric relief valve  31  are secured to a securing plate  40  to form a unit. The pressurization pump  29  is a diaphragm type pump (a volume type pump) and has an elastic member  41 , which is formed of synthetic resin and has a bellows-like lidded cylindrical shape. An air accumulation chamber (not shown) is defined in the elastic member  41  and sealed by a sealing portion  42 . An air outlet tube  43  is connected to the sealing portion  42  to introduce the air out of the pressurization pump  29 . 
   A pressing member  44  is secured to the distal end of the elastic member  41 . The pressing member  44  has a flat plate-like base  45  and a piston  46  formed integrally with the base  45 . A non-illustrated cam groove is defined in the outer circumferential surface of the piston  46 . The pressing member  44  has a first gear  47  through which the piston  46  is passed. The first gear  47  is thus supported rotatably about the piston  46 . 
   A sliding portion  48  is provided between the base  45  and the first gear  47 . The sliding portion  48  is secured to a projection  47   a  formed integrally with the first gear  47  and has a projection (not shown) projecting from the backside of the sliding portion  48 . The projection slides along the cam groove, which is defined in the outer circumferential surface of the piston  46 . When the first gear  47  rotates about the piston  46 , the sliding portion  48  rotates about the piston  46 . In this state, the projection slides along the cam groove. This causes the piston  46  to linearly reciprocate in the axial direction (direction A of  FIG. 4 ) of the piston  46  in accordance with the shape of the cam groove. 
   As a result, the elastic member  41  engaged with the base  45  also extends and contracts as the piston  46  linearly reciprocates. This correspondingly increases and decreases the volume of the air accumulation chamber (not shown), supplying the air into the air outlet tube  43 . In the first embodiment, the piston  46  and the sliding portion  48  form a cam mechanism  49 . 
   A pressurization motor  50 , which is a drive source of the pressurization pump  29 , is secured to the securing plate  40 . The pressurization motor  50  is a motor rotatable in opposing forward and reverse directions. A motor gear  51  is secured to the output shaft of the pressurization motor  50 . A wall  40   a  projects from an end of the securing plate  40  with a support shaft  52  projecting from the wall  40   a . A second gear  53  that is engageable with the motor gear  51  is rotatably supported by the support shaft  52 . 
   The second gear  53  is engaged with the first gear  47 . In the first embodiment, the motor gear  51 , the first gear  47 , and the second gear  53  form a gear mechanism  54 . The rotation of the motor gear  51  of the pressurization motor  50  is transmitted to the first gear  47  through the gear mechanism  54  and converted into linear reciprocation by the cam mechanism  49 . This causes the elastic member  41  to extend and contract. 
   When the piston  46  is moved closer to the elastic member  41  through operation of the gear mechanism  54  and the cam mechanism  49 , the elastic member  41  contracts (air drainage), supplying the air from the air accumulation chamber (not shown) to the air outlet tube  43 . In this state, if the piston  46  is moved separately from the elastic member  41  through operation of the gear mechanism  54  and the cam mechanism  49 , the elastic member  41  extends (air suction), introducing the atmospheric air into the air accumulation chamber (not shown). As the pressurization pump  29  repeatedly performs the air drainage and the air suction in this manner, the air is sent through the air outlet tube  43  and the pressure in the air chamber  24  of each of the cartridges  21  increases in a stepped manner. 
   The air outlet tube  43  is connected to the pressure sensor  30 . The pressure sensor  30  detects the pressure of the air sent from the pressurization pump  29  and outputs an electric signal in correspondence with the detection value of the pressure of the air. The pressure sensor  30  is connected to the atmospheric relief valve  31  through a communication pipe  55 . 
   The atmospheric relief valve  31  is arranged between the communication pipe  55  and the air supply line  28  and has a valve opening lever  56 . When the valve opening lever  56  is depressed, the atmospheric relief valve  31  operates to expose the air supply line  28  to the atmospheric air. When the valve opening lever  56  is in a non-depressed state, the air supply line  28  is disconnected from the atmospheric air. The air is thus supplied from the pressurization pump  29  to the air chambers  24  of the cartridges  21  through the air supply line  28 . A non-illustrated valve opening mechanism is provided in the vicinity of the valve opening lever  56 . The valve opening mechanism includes a gear mechanism connected to the pressurization motor  50  and a pressing member capable of pressing the valve opening lever  56 . When the pressurization motor  50  rotates in a reverse direction, the pressing member presses the valve opening lever  56 . 
   A home position detector  57  is arranged in the vicinity of the pressing member  44  of the pressurization pump  29  on the securing plate  40 . The home position detector  57  detects the position of the elastic member  41 . The home position detector  57  is formed by a limit switch or a photo sensor or the like and has a detection lever  58 . When the elastic member  41  extends maximally, or the elastic member  41  is located at its home position, the base  45  of the pressing member  44  depresses the detection lever  58 . In this state, the home position detector  57  outputs a detection signal. 
   The electric configuration of the above-described inkjet printer  10  will be explained with reference to  FIG. 5 . 
   As illustrated in  FIG. 5 , the printer  10  has a control section  60 . The thermistor SE 1 , the pressure sensor  30  and the like are connected to the control section  60  and provide electric signals to the control section  60 . The paper feeder motor (not shown), the carriage motor  16 , the pressurization motor  50 , and the suction pump  37  are connected to the control section  60 . The control section  60  drives the motors  16 ,  50 , the suction pump  37  and the like. A plurality of (in the first embodiment, four) lamps  61 , or indication devices provided in correspondence with the cartridges  21 , are connected to the control section  60 . Each of the lamps  61  is selectively turned on and off in correspondence with the retaining amount (the remaining amount) of the ink in the ink pack  23  of the corresponding one of the cartridges  21 . 
   The control section  60  includes a CPU  62 , with a ROM  63  and a RAM  64  connected to the CPU  62 . The ROM  63  stores various types of control programs and tables in accordance with which the printer  10  is controlled. Specifically, the ROM  63  stores a table representing the relationship between the temperature in the proximity of the printer  10  (hereinafter, referred to as the environmental temperature) and the stiffness resistance of each ink pack  23 , a table representing the relationship between the environmental temperature and the dynamic pressure of the ink in each ink supply line  26 , and a table representing the relationship between the environmental temperature and the threshold value. With reference to the threshold value, a consumed cartridge  21  is replaced with a unused cartridge  21 , or a cartridge  21  an ink pack  23  of which is fully filled with ink. 
   The ROM  63  further stores, in advance, hydraulic head difference, which is the difference between the hydraulic head at the ink outlet  23   a  of each ink pack  23  and the hydraulic head in each nozzle  19  of the recording head  17 . In other words, the ROM  63  functions as a hydraulic head difference memory section that stores the hydraulic head difference between the hydraulic head at the ink outlet  23   a  of each ink pack  23  and the hydraulic head in each nozzle  19  of the recording head  17 . In the first embodiment, the hydraulic head difference is a fixed value regardless of the environmental temperature and the remaining amount of the ink in each ink pack  23 . The hydraulic head difference is set by a test or through simulation. The RAM  64  stores various types of information that are rewritten as needed when the printer  10  is in operation. 
   Next, the tables stored in the ROM  63  will be explained with reference to  FIGS. 6 and 7 . 
   The table of  FIG. 6  represents the relationship between the environmental temperature T and the stiffness resistance PA of each ink pack  23  and the relationship between the environmental temperature T and the total resistance acting on the ink that has flown in each ink supply line  26  from the end  26   a  corresponding to the ink pack  23  to the end  26   b  corresponding to the recording head  17  (which is, hereinafter, referred to as “ink dynamic pressure PB”). 
   Referring to  FIG. 6 , in the ROM  63 , the stiffness resistance PA of each ink pack  23  is set to a first stiffness resistance PA 1  and the ink dynamic pressure PB is set to a first dynamic pressure PB 1  for the environmental temperature T lower than 15° C. (for example, 10° C.). 
   In the ROM  63 , the stiffness resistance PA of each ink pack  23  is set to a second stiffness resistance PA 2  and the ink dynamic pressure PB is set to a second dynamic pressure PB 2  for the environmental temperature T higher than 15° C. but not higher than 20° C. (for example, 18° C.). The second stiffness resistance PA 2  is less than the first stiffness resistance PA 1  and the ink dynamic pressure PB 2  is less than the ink dynamic pressure PB 1 . 
   In the ROM  63 , the stiffness resistance PA of each ink pack  23  is set to a third stiffness resistance PA 3  and the ink dynamic pressure PB is set to a third dynamic pressure PB 3  for the environmental temperature T higher than 20° C. but not higher than 25° C. (for example, 22° C.). The third stiffness resistance PA 3  is less than the second stiffness resistance PA 2  and the ink dynamic pressure PB 3  is less than the ink dynamic pressure PB 2 . 
   In the ROM  63 , the stiffness resistance PA of each ink pack  23  is set to a fourth stiffness resistance PA 4  and the ink dynamic pressure PB is set to a fourth dynamic pressure PB 4  for the environmental temperature T higher than 25° C. but not higher than 30° C. (for example, 30° C.). The fourth stiffness resistance PA 4  is less than the third stiffness resistance PA 3  and the ink dynamic pressure PB 4  is less than the ink dynamic pressure PB 3 . 
   Further, in the ROM  63 , the stiffness resistance PA of each ink pack  23  is set to a fifth stiffness resistance PA 5  and the ink dynamic pressure PB is set to a fifth dynamic pressure PB 5  for the environmental temperature T higher than 30° C. (for example, 35° C.). 
   Specifically, the stiffness resistance PA of each ink pack  23  and the ink dynamic pressure PB become less as the environmental temperature T becomes higher. To facilitate understanding, in the first embodiment, it is defined that the stiffness resistances PA of the ink packs  23  of all of the cartridges  21  are equal at a common environmental temperature T. Similarly, it is defined that the ink dynamic pressures PB of all of the cartridges  21  are equal at a common environmental temperature T. 
   The table of  FIG. 7  represents the relationship between the environmental temperature T and a threshold value KI with respect to the retaining amount of the ink of each ink pack  23 . Referring to  FIG. 7 , in the ROM  63 , the threshold value KI is set to a first threshold value KI 1  for the environmental temperature T not higher than 15° C. For example, the first threshold value KI 1  of 8 g is set for the environmental temperature T of 15° C. 
   In the ROM  63 , the threshold value KI is set to a second threshold value KI 2  for the environmental temperature T higher than 15° C. but not higher than 20° C. That is, the second threshold value KI 2  is less than the first threshold value KI 1 . For example, the second threshold value KI 2  of 5 g is set for the environmental temperature T of 16° C. 
   In the ROM  63 , the threshold value KI is set to a third threshold value KI 3  for the environmental temperature T higher than 20° C. but not higher than 25° C. That is, the third threshold value KI 3  is less than the second threshold value KI 2 . For example, the third threshold value KI 3  of 4 g is set for the environmental temperature T of 24° C. 
   In the ROM  63 , the threshold value KI is set to a fourth threshold value KI 4  for the environmental temperature T higher than 25° C. but not higher than 30° C. That is, the fourth threshold value KI 4  is less than the third threshold value KI 3 . For example, the fourth threshold value KI 4  of 3.7 g is set for the environmental temperature T of 26° C. 
   Further, the threshold value KI is set to a fifth threshold value KI 5  for the environmental temperature T higher than 30° C. That is, the fifth threshold value KI 5  is less than the fourth threshold value KI 4 . For example, the fifth threshold value KI 5  of 3.5 g is set for the environmental temperature T of 31° C. 
   Specifically, the ROM  63  functions as a memory section that stores the multiple threshold values KI 1  to KI 5  in correspondence with variation of the environmental temperature T. To facilitate understanding, in the first embodiment, it is defined that, regardless of the type of the ink, the threshold values KI corresponding to the ink packs  23  of all of the cartridges  21  are equal at a common environmental temperature T. 
   Next, a replacement indication determining routine, which is one of control routines executed by the control section  60 , will be explained with reference to  FIG. 8 . In accordance with the replacement indication determining routine, it is determined whether indication of the need for replacement of a cartridge  21  should be carried out. 
   The control section  60  executes the replacement indication determining routine by predetermined cycles of, specifically, one second, for example. In the replacement indication determining routine, the control section  60  receives a signal generated by the thermistor SE 1  and calculates the environmental temperature T based on the signal (in step S 10 ). In other words, in the first embodiment, the control section  60  and the thermistor SE 1  function as a temperature detecting section that detects the environmental temperature T. Specifically, the thermistor SE 1  functions as a temperature signal generating section and the control section  60  functions as a temperature calculating section. 
   Subsequently, the control section  60  sets the threshold value KI in correspondence with the environmental temperature T, which is detected in step S 10  (in step S 11 ). Specifically, the control section  60  reads out the threshold value KI corresponding to the environmental temperature T detected in step S 10  from the threshold values KI 1  to KI 5  stored in the ROM  63 . The control section  60  then sets the obtained threshold value KI in a predetermined area in the RAM  64 . For example, if the environmental temperature T detected in step S 10  is 25° C., the control section  60  reads out the third threshold value KI 3  and sets the threshold value KI 3  in a predetermined area in the RAM  64 . In other words, in the first embodiment, the control section  60  functions as a threshold value setting section that sets the threshold value KI to a lower value as the environmental temperature T detected in step S 10  becomes higher. 
   The control section  60  then reads out the hydraulic head difference PC corresponding to each of the cartridges  21  and sets the obtained hydraulic head differences PC in a predetermined area in the RAM  64  (in step S 12 ). Further, in correspondence with the environmental temperature T detected in step S 10 , the control section  60  sets the ink dynamic pressure B (in step S 13 ). Specifically, the control section  60  reads out the ink dynamic pressure PB corresponding to the environmental temperature T detected in step S 10  from the ink dynamic pressures PB 1  to PB 5  and sets the obtained dynamic pressure PB in a predetermined area of the RAM  64 . For example, if the environmental temperature T from step S 10  is 27° C., the control section  60  reads out the fourth dynamic pressure PB 4  and sets the fourth dynamic pressure PB 4 , which has been read out, in a predetermined area of the RAM  64 . In other words, in the first embodiment, the control section  60  functions as a dynamic pressure detecting section that detects the ink dynamic pressure PB in correspondence with the environmental temperature T, which has been detected in step S 10 . 
   Then, the control section  60  sets the stiffness resistance PA of each ink pack  23  in correspondence with the environmental temperature T from step S 10  (in step S 14 ). That is, the control section  60  reads out the stiffness resistance PA corresponding to the environmental temperature T detected in step S 10  from the stiffness resistances PA 1  to PA 5  stored in the ROM  63 . The control section  60  then sets the obtained stiffness resistance PA in a predetermined area of the RAM  64 . For example, if the environmental temperature T detected in step S 10  is 18° C., the control section  60  reads out the second stiffness resistance PA 2  and sets the second stiffness resistance PA 2  in a predetermined area of the RAM  64 . In the first embodiment, the control section  60  functions as a stiffness resistance setting section that sets the stiffness resistance PA of each ink pack  23 . 
   Next, in correspondence with a signal from the pressure sensor  30 , the control section  60  detects the pressure PD applied to the ink in the ink pack  23  of each cartridge  21  by the pressurization unit  27  (in step S 15 ). In the first embodiment, the control section  60  and the pressure sensor  30  function as a pressure detecting section that detects the pressure PD applied to the ink in each ink pack  23  by the pressurization unit  27 . 
   From the results obtained from processing in step S 12  to step  515 , the control section  60  calculates the hydraulic head value P (in step S 16 ). The hydraulic head value P is obtained by the following equation:
 
 P =( PC+PD )−( PA+PB )  (1)
 
   Specifically, the control section  60  calculates a first sum (PC+PD) by adding the hydraulic head difference PC detected in step S 12  to the pressure PD detected in step S 15 . The control section  60  also calculates a second sum (PA+PB) by adding the ink dynamic pressure PB detected in step S 13  to the stiffness resistance PA of each ink pack  23 . Further, by subtracting the second sum (PA+PB) from the first sum (PC+PD), the control section  60  obtains the hydraulic head value P. 
   Subsequently, based on the hydraulic head value P obtained in step S 16 , the control section  60  estimates the retaining amount, which is the remaining amount I, of the ink in the ink pack  23  of each cartridge  21  (in step S 17 ). That is, in the first embodiment, the control section  60 , the thermistor SE 1 , and the pressure sensor  30  function as a retaining amount detecting section that detects the retaining amount I of the ink in each ink pack  23 . 
   The control section  60  then determines whether the retaining amount I of the ink in each ink pack  23 , which has been detected in step S 17 , is less than or equal to the threshold value KI that has been set in step S 11  (in step S 18 ). In the first embodiment, the control section  60  thus functions also as a determining section. If it is determined that the retaining amount I of the ink in each ink pack  23  is higher than the threshold value KI (I&gt;KI) in step S 18 , the control section  60  determines that the amount of the ink retained in each ink pack  23  is sufficient for the current environmental temperature T. The control section  60  thus suspends the replacement indication determining routine. 
   Contrastingly, if it is determined that the retaining amount I of the ink in each ink pack  23  is less than or equal to the threshold value KI (I&lt;KI) in step S 18 , the control section  60  determines that the amount of the ink in each ink pack  23  is insufficient under the current environmental temperature T. In other words, the control section  60  determines that, in printing on the recording paper sheet, ink shortage may occur. In this case, the control section  60  illuminates the lamp  61  corresponding to the cartridge  21  in which the ink shortage may occur, thus indicating the need for replacement of the cartridge  21  to a non-used cartridge  21  (in step S 19 ). 
   The control section  60  then ends the replacement indication determining routine. After the replacement of the cartridge  21  by the unused cartridge  21  is completed, the control section  60  turns off the lamp  61  that has been illuminated in step S 19 . 
   Operation of the printer  10  will hereafter be described. The following description explains, particularly, the operation of the printer  10  when the retaining amount I of the ink in the ink pack  23  of the cartridge  21  becomes less than or equal to the threshold value KI. In the first embodiment, the environmental temperature T of the printer  10  is 25° C. 
   As ink droplets are ejected from the nozzles  19  of the recording head  17  onto the recording paper sheet, which has been supplied by the paper feeder mechanism, the retaining amount I of the ink in the ink pack  23  of each cartridge  21  gradually decreases. The environmental temperature T (25° C.) of the printer  10  is detected in correspondence with a signal generated by the thermistor SE 1 , which is provided in the recording head  17  at predetermined cycles. The threshold value KI corresponding to the environmental temperature T is read out from the corresponding one of the tables stored in the ROM  63 . In the first embodiment, since the environmental temperature T is 25° C., the threshold value KI is set to the third threshold value KI 3 . 
   Subsequently, the hydraulic head difference PC in each of the cartridges  21  is read out from the ROM  63 . Further, the stiffness resistance PA of each ink pack  23  and the ink dynamic pressure PB corresponding to the detected environmental temperature T are read out from the corresponding one of the tables stored in the ROM  63 . In the first embodiment, since the environmental temperature T is 25° C., the stiffness resistance PA of each ink pack  23  and the ink dynamic pressure PB are set to the third stiffness resistance PA 3  and the third dynamic pressure PB 3 , respectively. 
   Next, the pressure PD is detected in correspondence with a signal of the pressure sensor  30 . The control section  60  then calculates the hydraulic head values P using the aforementioned equation (1). The control section  60  thus estimates the retaining amount I of the ink in the ink pack  23  of each cartridge  21  using the corresponding one of the hydraulic head values P. 
   Then, it is determined whether the retaining amount I of the ink has become less than or equal to the threshold value KI for each of the cartridges  21 . If it is determined that the retaining amount I of the ink has become less than or equal to the threshold value KI (I≦KI) in any one of the cartridges  21 , the corresponding one of the lamps  61  is turned on. This urges replacement of the cartridge  21 , which has been determined to have an insufficient retaining amount I of ink in the ink pack  23  under the detected environmental temperature T, by a unused cartridge  21 . After the completion of such replacement is detected, the lamp  61  is turned off. 
   The first embodiment has the following advantages. 
   (1) The threshold value KI, in accordance of which it is determined whether any one of the cartridges  21  accommodating the ink packs  23  needs be replaced, is set in correspondence with the environmental temperature T detected based on the signal of the thermistor SE 1 . If the environmental temperature T of the printer  10  is relatively high, the hydraulic head value P of each nozzle  19  of the recording head  17  is relatively low. The threshold value KI is thus set to a relatively small value. This reduces the amount of the ink remaining in the ink pack  23  of the cartridge  21  to be replaced, when replacement of the cartridge  21  is urged due to decrease of the retaining amount I of the ink. 
   (2) The threshold value KI, with reference to which it is determined whether to replace the cartridges  21 , is set by reading out the threshold value KI corresponding to the environmental temperature T detected in correspondence with the signal of the thermistor SE 1  from the ROM  63 . This makes it unnecessary to perform a calculation procedure using an equation. The load of the control section  60  is thus decreased. 
   (3) Since the thermistor SE 1  is provided in the recording head  17 , which ejects ink, the temperature at a position closer to the ink ejected from the nozzles  19  is detected. The threshold value KI is thus set to a value approximate to the lower limit of the hydraulic head (hereinafter, referred to also as “critical hydraulic head”) that allows ejection of the ink from the nozzles  19  of the recording head  17 . 
   (4) When the retaining amount I of the ink in each ink pack  23 , which corresponds to the hydraulic head value P, becomes less than or equal to the threshold value KI, indication by the lamps  61  are performed. The threshold value KI is set to the value corresponding to the critical hydraulic head. This effectively reduces the amount (the retaining amount I) of the ink remaining in the ink packs  23  of the cartridges  21  when the cartridges  21  are replaced. 
   (5) The pressure PD applied to the ink in each ink pack  23  by the pressurization unit  27  is detected in correspondence with a signal of the pressure sensor  30  of the pressurization unit  27 . The first sum is calculated by adding the hydraulic head difference PC read out from the ROM  63  and the pressure PD together. The second sum is obtained by adding the stiffness resistance PA of the ink pack  23  set by the control section  60  and the dynamic pressure PB of the ink in the ink supply line  26  together. 
   In other words, the second sum is set to a value corresponding to the environmental temperature T, which is detected in correspondence with the signal of the thermistor SE 1 . The hydraulic head value P is obtained by subtracting the second sum from the first sum. Therefore, regardless of the presence of the pressurization unit  27 , the amount of the ink remaining in the ink pack  23  of the cartridge  21  that is to be replaced is effectively reduced in correspondence with the environmental temperature T, which is detected based on the signal of the thermistor SE 1 . 
   Next, a second embodiment of the present invention will be explained with reference to  FIG. 9 . A liquid ejection apparatus  10 A of the second embodiment is different from the liquid ejection apparatus  10  of the first embodiment in that cartridges  21  are mounted in a carriage  14 . Therefore, the following explanation will refer only to the differences between the liquid ejection apparatus  10 A of the second embodiment and the liquid ejection apparatus  10  of the first embodiment. 
   As shown in  FIG. 9 , a printer  10 A, or the liquid ejection apparatus, includes a body frame  11  having a substantially box-like shape. A platen  12  is provided in the body frame  11 . A paper feeder mechanism having a paper feeder motor  70  sends a recording paper sheet  71 , or a target, to the platen  12 . A bar-like guide member  13 , which extends parallel with the longitudinal direction (the left-and-right direction) of the platen  12 , is also arranged in the body frame  11 . 
   The guide member  13  is passed through the carriage  14  and supports the carriage  14  in such a manner as to allow the carriage  14  to reciprocate in the axial direction of the guide member  13 . A recording head  17  is provided on the lower surface of the carriage  14 . A plurality of nozzles  19  are formed in the lower surface of the recording head  17 . A plurality of (in the second embodiment, four) cartridges  21  are removably mounted in the carriage  14  at a position above the recording head  17 . Each of the cartridges  21  retains ink in such a manner as to allow supply of the ink to the recording head  17 . 
   The printer  10 A of the second embodiment supplies the ink from each cartridge  21  directly to the nozzles  19  of the recording head  17 , unlike the printer  10  of the first embodiment that supplies the ink to the nozzles  19  of the recording head  17  through the ink supply lines  26 . Therefore, the hydraulic head value P acting on each of the nozzles  19  of the recording head  17  is calculated using the following equation (2):
 
 P=PC−PA   (2)
 
   In the equation, P represents a value of the hydraulic head applied to each nozzle of the recording head, while PA represents the stiffness resistance of each cartridge. PC represents the hydraulic head difference, or the difference between the hydraulic head at the outlet of each cartridge and the hydraulic head in each nozzle of the recording head. 
   In the second embodiment, a control section  60  detects the environmental temperature T in correspondence with a signal generated by a thermistor SE 1  and reads out a threshold value KI corresponding to the environmental temperature T from a ROM  63 . The obtained threshold value KI is set in a predetermined area of a RAM  64 . In the second embodiment, the control section  60  reads out the hydraulic head differences PC of the cartridges  21  from the ROM  63 . The control section  60  of this embodiment also reads out the stiffness resistance PA of each cartridge  21  corresponding to the detected environmental temperature T from the ROM  63 . The obtained stiffness resistance PA is set in a predetermined area of the RAM  64 . 
   The control section  60  of the second embodiment calculates the hydraulic head values P using the aforementioned equation (2) and estimates the retaining amount I of the ink in each cartridge  21  from the calculation results. The control section  60  of this embodiment then determines whether the retaining amount I of the ink in each of the cartridges  21  is less than or equal to the threshold value KI for all of the cartridges  21 . If it is determined that the retaining amount I of the ink in any one of ink packs  23  is less than or equal to the threshold value KI (I&lt;KI), the corresponding one of the lamps  61  is illuminated. 
   The second embodiment has the following advantage in addition to the advantages (1) to (5) of the first embodiment. 
   (6) The hydraulic head value P of each nozzle  19  of the recording head  17  is calculated by subtracting the stiffness resistance PA of each cartridge  21  set by the control section  60  from the hydraulic head difference PC, which has been set in the ROM  63  in advance. The stiffness resistance PA corresponds to the environmental temperature T detected in correspondence with the signal of the thermistor SE 1 . This effectively reduces the hydraulic head value P of each nozzle  19  of the recording head  17  in correspondence with the environmental temperature T, which has been detected based on the signal of the thermistor SE 1 . 
   The illustrated embodiments may be modified as follows. 
   The printer  10  of the first embodiment may supply the ink from the ink packs  23  to the recording head  17  via the ink supply lines  26  solely through the hydraulic head difference PC, or the difference between the hydraulic head at the outlet  23   a  of the ink pack  23  of each cartridge  21  and the hydraulic head in each nozzle  19  of the recording head  17 , without employing the pressurization unit  27 . In this case, the hydraulic head value P of each nozzle  19  of the recording head  17  is calculated using the following equation (3):
 
 P=PC− ( PA+PB )  (3)
 
   P represents the hydraulic head value of each nozzle of the recording head. PA represents the stiffness resistance of each ink pack. PB represents the dynamic pressure of the ink flowing in each ink supply line. PC represents the hydraulic head difference between the hydraulic head at the outlet of each ink pack and the hydraulic head in each nozzle of the recording head. 
   In each of the illustrated embodiments, the thermistor SE 1  may be located at any suitable position in the printer  10 ,  10 A. For example, the thermistor SE 1  may be arranged in the vicinity of the maintenance unit  33 . 
   In the illustrated embodiments, the map of  FIG. 10  representing the relationship between the environmental temperature T and the threshold value KI may be stored in the ROM  63 . In this case, the threshold value KI is set with reference to the map. Alternatively, an equation representing the relationship between the environmental temperature T and the threshold value KI may be stored in the ROM  63 . In this case, the threshold value KI is set using the equation. 
   Similarly, a map representing the relationship between the environmental temperature T and the stiffness resistance PA of each ink pack  23  (or each cartridge  21 ) may be stored in the ROM  63 . In this case, the stiffness resistance PA of the ink pack  23  (or the cartridge  21 ) is set with reference to the map. Alternatively, an equation representing the relationship between the environmental temperature T and the stiffness resistance PA of the ink pack  23  (or the cartridge  21 ) may be stored in the ROM  63 . In this case, the stiffness resistance PA of the ink pack  23  (or the cartridge  21 ) is set using the equation. 
   Further, a map representing the relationship between the environmental temperature T and the dynamic pressure PB of the ink flowing in each ink supply line  26  may be stored in the ROM  63 . In this case, the ink dynamic pressure PB is set with reference to the map. Alternatively, an equation representing the relationship between the environmental temperature T and the ink dynamic pressure PB may be stored in the ROM  63 . In this case, the ink dynamic pressure PB is set using the equation. 
   In each of the illustrated embodiments, the indication device is embodied by each of the lamps  61 . However, the present invention is not restricted to this. The indication device may be embodied by, for example, a speaker, which gives an audio indication of the need for replacement of a cartridge  21 . Alternatively, the indication device may be embodied by, for example, a liquid crystal display. In this case, a display screen of the liquid crystal display displays the need for replacement of a cartridge  21 . 
   In each of the illustrated embodiments, the liquid ejection apparatus is embodied by the printer  10 ,  10 A. However, the present invention is not restricted to this but may be embodied by, for example, a liquid ejection apparatus used in the manufacture of color filters of liquid crystal displays or in the formation of pixels of organic EL displays.