Patent Publication Number: US-9409405-B2

Title: Method of controlling liquid ejecting apparatus and liquid ejecting apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of controlling a liquid ejecting apparatus, and a liquid ejecting apparatus. 
     2. Description of the Background Art 
     An inkjet printer which records an image on a recording medium such as printing paper by ejecting minute ink droplets from a plurality of nozzles of a head unit toward the recording medium while moving the recording medium relative to the head unit has been hitherto used (as disclosed, for example, in Japanese Patent Application Laid-Open No. 2013-71410). An inkjet printer for color printing includes a plurality of head units corresponding to inks of respective colors and arranged in a direction in which the recording medium is moved relative to the head units. 
     In the inkjet printer disclosed in Japanese Patent Application Laid-Open No. 2013-71410, each of the head units for the respective colors includes a plurality of heads arranged in a staggered configuration and each having a plurality of nozzles. In a liquid ejecting apparatus, such as the aforementioned inkjet printer, which ejects liquid toward a recording medium, ink to be ejected toward the recording medium is held in a meniscus shape in each of the nozzles provided in the heads. If the ink is not ejected for a long period of time, a volatile ingredient contained in the ink vaporizes at each nozzle, so that the ink is increased in viscosity or is solidified. As a result, there is apprehension about ejection failures of ink from the nozzles and about unevenness in density of an image recorded on the recording medium. 
     If ink containing a precipitable ingredient is not ejected for a long period of time, there is apprehension that the precipitable ingredient is precipitated inside the heads and in ink flow passages, so that the flow passages are narrowed down. 
     To solve the aforementioned problems, there has been hitherto known a maintenance technique (what is called a purge) in which a positive pressure or a negative pressure is applied to the ink inside the heads to force the ink out of the nozzles of the heads, thereby ensuring the stable operation of the heads. 
     Japanese Patent Application Laid-Open No. 2012-30516 discloses a “pressurized purge” technique in which a positive pressure is developed inside a head to force ink in the head out of nozzles (for example, paragraphs 0043 to 0052). 
     Japanese Patent Application Laid-Open No. 2012-30516 also discloses a technique in which a supply valve is interposed in a supply flow passage which connects the head and a sub-tank and in which the supply valve is closed, and is then opened after the pressure inside the sub-tank is increased to a designated pressure, whereby pressure waves provided to the head during the pressurized purge are made sharp. 
     Japanese Patent Application Laid-Open No. 2010-162783 discloses what is called a “negative pressure purge” technique in which caps are brought into contact with nozzle surfaces of heads and suction pumps connected to the caps are driven to decrease the pressure in spaces defined by the nozzle surfaces and the caps, thereby forcing ink out of the nozzles by suction. 
     As disclosed in paragraphs 0050 to 0052 in Japanese Patent Application Laid-Open No. 2010-162783, a positive pressure is developed inside the heads by applying pressure to the interior of the heads during the negative pressure purge. This allows the negative pressure purge, with the positive pressure held in the heads. It is said that this prevents air bubbles from being drawn into the nozzles when the caps are separated after the negative pressure purge. It is also said that the pressure applied to the heads is cut off by closing a supply path shut-off valve interposed in a supply flow passage connecting a sub-tank and the heads after the pressure application to the heads, to return the pressure in the heads to atmospheric pressure, whereby ink is prevented from dripping down after the separation of the caps. 
     In an inkjet printer having a plurality of heads, it is ideal to discharge ink from the heads at the same flow rate during the aforementioned “pressurized purge” because the problems of ejection failures of ink and unevenness in density are solved to the same degree. However, there are differences in the amount of ink discharge during the “pressurized purge” between the heads in actuality because of the differences in flow passage resistance from the sub-tank between the heads. This gives rise to problems of unevenness in density during image recording and excess ink consumption during the pressurized purge. 
     An inkjet printer having a plurality of heads presents another problem. When it is desired to eliminate an ejection failure in a specific head, the apparatus configuration as disclosed, for example, in Japanese Patent Application Laid-Open No. 2010-162783 is required to conduct a purge on all of the heads. To conduct the purge on the heads which require no purge because no ejection failure occurs therein results in the extra ink consumption. 
     When ink menisci are formed in nozzles by decreasing the pressure inside the heads from the sub-tank side after the pressurized purge, there are cases in which air bubbles enter the ink at the nozzles. This might cause an ejection failure in such nozzles. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to reduce the amount of ink consumption during a pressurized purge in an inkjet printer having a plurality of heads. 
     To solve the aforementioned problems, a first aspect of the present invention is intended for a method of controlling a liquid ejecting apparatus for a recovery operation of a head unit, the liquid ejecting apparatus including the head unit having a plurality of heads each including a plurality of nozzles for ejecting a liquid, a liquid reservoir for temporarily storing the liquid for supply to the head unit, a plurality of supply passages for supplying the liquid from the liquid reservoir therethrough to the respective heads, and a plurality of open/close parts provided in the respective supply passages, the open/close parts being switchable between an open position independently ensuring the communication between the liquid reservoir and the heads and a closed position independently closing off the communication between the liquid reservoir and the heads. The method comprises the steps of: a) pressurizing the liquid inside the liquid reservoir to a state of positive pressure higher than atmospheric pressure, while maintaining all of the open/close parts in the closed position, to thereby performing pre-pressurization; b) bringing a first one of the open/close parts into the open position to perform a first purge, the step b) being performed after the step a); and c) making the liquid reservoir open to the atmosphere, the step c) being performed after the step b). 
     In the method according to the first aspect, a first one of the open/close parts is brought into the open position in the step b) of performing the first purge, whereby the liquid is forced out of one of the heads. This allows a selective purge of the head which requires a purge to thereby reduce the amount of consumption of ink during the purge. 
     In the method according to the first aspect, the step c) of making the liquid reservoir open to the atmosphere is performed after the step b) of performing the first purge. Thus, with the communication between the head brought into the open position in the step b) and the liquid reservoir provided through a supply passage, the liquid reservoir is made open to the atmosphere, so that the head is also made open to the atmosphere. This reduces the amount of consumption of ink ejected from the nozzles after the purge. 
     A second aspect of the present invention is intended for a liquid ejecting apparatus for ejecting a liquid onto a recording medium to record an image thereon. The liquid ejecting apparatus comprises: a head unit having a plurality of heads each including a plurality of nozzles for ejecting the liquid; a liquid reservoir for temporarily storing the liquid for supply to the head unit; a plurality of supply passages for supplying the liquid from the liquid reservoir therethrough to the respective heads; a plurality of open/close parts provided in the respective supply passages, the open/close parts being switchable between an open position independently ensuring the communication between the liquid reservoir and the heads and a closed position independently closing off the communication between the liquid reservoir and the heads; a pressurizing part for pressurizing the liquid inside the liquid reservoir; a depressurizing part for depressurizing the liquid inside the liquid reservoir; an open-to-atmosphere part for making the liquid inside the liquid reservoir open to the atmosphere; and a controller for controlling the switching of the open/close parts between the open position and the closed position, the pressurization of the liquid inside the liquid reservoir by means of the pressurizing part, the depressurization of the liquid inside the liquid reservoir by means of the depressurizing part, and the process of making the liquid inside the liquid reservoir open to the atmosphere by means of the open-to-atmosphere part, the controller performing the following operations: a pre-pressurization operation for pressurizing the liquid inside the liquid reservoir by means of the pressurizing part while maintaining all of the open/close parts in the closed position; a first purge operation for bringing a first one of the open/close parts into the open position after the pre-pressurization operation; and an open-to-atmosphere operation for making the liquid reservoir open to the atmosphere by means of the open-to-atmosphere part after the first purge operation. 
     In the liquid ejecting apparatus according to the second aspect, a first one of the open/close parts is brought into the open position in the first purge operation, whereby the liquid is forced out of one of the heads. This allows a selective purge of the head which requires a purge to thereby reduce the amount of consumption of ink during the purge. 
     In the liquid ejecting apparatus according to the second aspect, the open-to-atmosphere operation is performed after the first purge operation. Thus, with the communication between the head brought into the open position in the first purge operation and the liquid reservoir provided through a supply passage, the open-to-atmosphere part makes the liquid reservoir open to the atmosphere, so that the head is also made open to the atmosphere. This reduces the amount of consumption of ink ejected from the nozzles after the purge. 
     In this manner, the inkjet printer having the plurality of heads according to the present invention achieves the reduction in the amount of consumption of ink during the pressurized purge. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view schematically showing the configuration of a liquid ejecting apparatus according to a first preferred embodiment of the present invention; 
         FIG. 2  is a plan view schematically showing the configuration of the liquid ejecting apparatus according to the first preferred embodiment; 
         FIG. 3  is a bottom view schematically showing a head according to the first preferred embodiment; 
         FIG. 4  is a block diagram showing an ink supply system according to the first preferred embodiment; 
         FIG. 5  is a flow diagram showing a recovery step according to the first preferred embodiment; 
         FIG. 6  is a flow diagram showing an initialization step according to the first preferred embodiment; 
         FIG. 7  is a flow diagram showing a pre-pressurization step according to the first preferred embodiment; 
         FIG. 8  is a flow diagram showing a first purge step according to the first preferred embodiment; 
         FIG. 9  is a flow diagram showing a re-pressurization step according to the first preferred embodiment; 
         FIG. 10  is a flow diagram showing a second purge step according to the first preferred embodiment; 
         FIG. 11  is a flow diagram showing an open-to-atmosphere step according to the first preferred embodiment; 
         FIG. 12  is a flow diagram showing a meniscus formation step according to the first preferred embodiment; 
         FIG. 13  is a timing diagram of the recovery step according to the first preferred embodiment; 
         FIG. 14  is a flow diagram showing the recovery step according to a second preferred embodiment of the present invention; 
         FIG. 15  is a flow diagram showing the first purge step according to the second preferred embodiment; 
         FIG. 16  is a flow diagram showing the second purge step according to the second preferred embodiment; 
         FIG. 17  is a timing diagram of the recovery step according to the second preferred embodiment; 
         FIG. 18  is a block diagram showing the ink supply system according to a third preferred embodiment of the present invention; and 
         FIG. 19  is a timing diagram of the recovery step according to the third preferred embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments according to the present invention will now be described with reference to the drawings. A direction in which a recording medium  7  to be described later is transported is referred to hereinafter as a “transport direction”. 
     First Preferred Embodiment 
     A schematic configuration of a liquid ejecting apparatus  1  according to a first preferred embodiment of the present invention will be illustrated with reference to  FIGS. 1 to 4 . A coordinate system in which an XY plane is defined as a horizontal plane and a Z axis is defined to extend in a vertical direction is additionally shown, as appropriate, in  FIGS. 1 to 3  for purposes of clarifying a directional relationship. In the coordinate system shown in  FIGS. 1 to 3 , directions pointed by the arrows shall be positive (+), and directions opposite from those pointed by the arrows shall be negative (−). 
     &lt;1-1. Overall Configuration of Liquid Ejecting Apparatus&gt; 
       FIG. 1  is a side view schematically showing the configuration of the liquid ejecting apparatus  1 .  FIG. 2  is a plan view schematically showing the configuration of the liquid ejecting apparatus  1 .  FIG. 3  is a bottom view schematically showing a head which ejects ink.  FIG. 4  is a block diagram showing an ink supply system. 
     The liquid ejecting apparatus  1  includes a transport mechanism  10  for transporting the recording medium  7  in the transport direction, a plurality of head units  2 , a supply system  3  for supplying ink to the head units  2 , cap members  61  opposed to the respective head units  2  during a recovery operation in which the maintenance of the head units  2  is carried out, and a drainage system  6  for collecting and draining ink ejected from the head units  2  during the recovery operation. The liquid ejecting apparatus  1  further includes a controller  8  for controlling the aforementioned components. 
     The liquid ejecting apparatus  1  is capable of performing an image recording step for recording an image on the recording medium  7 , and a recovery step for carrying out the maintenance of the head units  2  in response to operating instructions from the controller  8 . The liquid ejecting apparatus  1  is used as what is called a one-pass type inkjet recording apparatus which records a desired image on the recording medium  7  by ejecting ink droplets from the head units  2  while the recording medium  7  passes under the head units  2  only once. 
     The transport mechanism  10  includes a plurality of rollers  13  arranged in the Y direction and each extending in the X direction. An unwinder  11  which holds the recording medium  7  wound therearound in a roll form is provided on the negative Y (−Y) side of the rollers  13 , and a winder  12  which holds the recording medium  7  wound therearound in a roll form is provided on the positive Y (+Y) side of the rollers  13 . Each of the unwinder  11  and the winder  12  includes a motor (not shown), and is electrically connected to the controller  8 . 
     At least one of the rollers  13  of the transport mechanism  10  is provided with an encoder  14  for detecting the transport speed of the recording medium  7  transported in the Y direction. The encoder  14  is electrically connected to the controller  8 , and outputs the detected transport speed to the controller  8 . The controller  8  controls the rotation of the motor of the winder  12 , based on the output from the encoder  14 , whereby the recording medium  7  is transported at a constant speed in the positive Y direction so as to pass under the head units  2 . 
     During the transport of the recording medium  7 , the motor of the unwinder  11  applies a load (tension) in the negative Y direction to the recording medium  7 , so that the recording medium  7  over the rollers  13  is transported smoothly without becoming wavy. 
     The head units  2  are provided on the positive Z side of the transport mechanism  10  (i.e., over the transport mechanism  10 ) as seen in  FIG. 1 . When the controller  8  provides an image recording operation instruction to be described later to the head units  2 , based on print data inputted from an external input device (not shown) such as a PC, the head units  2  performs the image recording step for ejecting ink toward the recording medium  7 . When the controller  8  provides a recovery operation instruction to be described later to the head units  2 , the head units  2  performs the recovery step for ejecting ink toward the cap members  61  to be described later. 
     In the first preferred embodiment, the liquid ejecting apparatus  1  includes the plurality of head units  2 . As shown in  FIG. 1 , the four head units  2  are arranged in the Y direction, and supply ink of different colors, such as cyan (C), magenta (M), yellow (Y) and black (K) as seen from the negative Y side, to the recording medium  7 . Only the head unit  2  for the K ink on the positive Y side will be described as a representative hereinafter because the four head units  2  are substantially similar in structure, in image recording step and in recovery step to each other except the colors of the ink. 
     The number of head units  2  is four in the first preferred embodiment, but is not limited to this for the practice of the present invention. The liquid ejecting apparatus  1  may include either one head unit  2  or not less than five head units  2 . The colors of ink are not limited to the aforementioned four colors, but may include other colors, e.g. white, orange and green. The arrangement of the colors of ink is not limited to that described above. For example, the head units  2  may be arranged in the following order: Y, M, C and K as seen from the negative Y side. 
     As shown in plan view of  FIG. 2 , each of the head units  2  includes two heads: a first head  21  on the negative X side, and a second head  22  on the positive X side for purposes of convenience. 
     It will be assumed that each of the head units  2  includes two heads for the illustration of the first preferred embodiment. However, the number of heads in each head unit  2  is not limited to two for the practice of the present invention. Each head unit  2  may include not less than three heads. When each head unit  2  includes not less than three heads, the heads may be arranged in a staggered configuration in the Y direction. 
     The head units  2  used in the first preferred embodiment have a width approximately equal to the width of the recording medium  7  as measured in the X direction. The width of the head units  2  is not limited to this for the practice of the present invention. The head units  2  used in the present invention may have a width smaller or greater than the width of the recording medium  7  as measured in the X direction. 
       FIG. 3  is a bottom view schematically showing the configuration of the first head  21 . The first head  21  has a nozzle surface  211  referred to as a bottom surface on the negative Z side and including a plurality of nozzles  212  for ejecting ink. 
     The nozzles  212  are arranged at equal spacings corresponding to a predetermined dot density in the X direction. The predetermined dot density is dependent on a resolution required for an image recorded on the recording medium  7 . Examples of the predetermined dot density used are 360 dpi (dots per inch) and 720 dpi. A dot density of 720 dpi is used in the first preferred embodiment. 
     In the first preferred embodiment, the nozzles provided in first head  21  are aligned in two rows arranged in the Y direction and each including 17 nozzles arranged in the X direction, as shown in  FIG. 3 . The arrangement of the nozzles in first head  21  is not limited to this for the practice of the present invention. More nozzles may be provided in a single head. For example, 200 nozzles arranged at predetermined spacings in the X direction may be provided in a single head. 
     The first head  21  according to the first preferred embodiment is a head of what is called a piezoelectric type. The first head  21  includes a plurality of pressure chambers not shown, and a plurality of piezoelectric elements corresponding to the plurality of pressure chambers. The pressure chambers communicate with the respective nozzles  212 . When an ejection signal that is an electric signal is sent from the controller  8  to each of the piezoelectric elements, each of the piezoelectric elements is deformed to exert pressure on the ink which fills a corresponding one of the pressure chambers. When the pressure in each pressure chamber is increased, the ink is ejected from a corresponding one of the nozzles  212 . 
     The first head  21  according to the present invention is not limited to the head of the piezoelectric type. For example, the first head  21  according to the present invention may be what is called a thermal head which heats the ink in the pressure chambers by means of a heater to generate bubbles, thereby increasing the pressure in the pressure chambers. 
     The second head  22  is similar in internal configuration to the first head  21 , and includes nozzles provided in the same manner as in the first head  21 . For this reason, the bottom surface of the second head  22  is not shown. 
     In the aforementioned manner, the first head  21  and the second head  22  in the first preferred embodiment constitute “heads each including nozzles” in the present invention, and the head units  2  each including the first head  21  and the second head  22  constitute a “head unit” in the present invention. 
     Referring again to  FIG. 1 , the cap members  61  are described next. The cap members  61  include a cap moving mechanism not shown. The cap moving mechanism is electrically connected to the controller  8 . In response to an operating instruction from the controller  8 , the cap moving mechanism moves the cap members  61  between a separated position in which the cap members  61  are separated from the head units  2  and an opposed position in which the cap members  61  are opposed to the head units  2  to cover the nozzle surfaces  211  of the respective head units  2 . 
     During a time period over which the head units  2  record no images on the recording medium  7 , the cap members  61  are in the opposed position to cover the nozzle surfaces  211  having the nozzles  212  of the head units  2 . This suppresses the vaporization of a solvent in the ink from ink meniscus surfaces formed in the nozzles  212  and the resultant solidification or agglomeration of ink near the nozzles  212  during the time period over which no images are recorded on the recording medium  7 . 
     The cap members  61  are connected through piping  62  to the drainage system  6 . When ink is ejected from the nozzles  212  while the cap members  61  in the opposed position cover the nozzles  212 , the ink flows from the cap members  61  through the piping  62  to the drainage system  6 . Thus, the ink is drained from the cap members  61 . 
     The drainage system  6  includes a drain tank (not shown) and a drain pump (not shown). The ink ejected into the cap members  61  is sent by the drain pump through the piping  62  to the drain tank, and is stored in the drain tank. 
     In the case where the head units  2  record an image on the recording medium  7 , the cap members  61  are placed in the separated position by the cap moving mechanism prior to the start of the image recording step. As shown in  FIG. 1 , the separated position is a position where the transport of the recording medium  7  is not prevented, and is on the negative Z side of the head units  2 , with the recording medium  7  therebetween, in the first preferred embodiment. 
     The supply system  3  supplies ink to the head units  2 . The details of the supply system  3  will be described later. 
     The recording medium  7  is an elongated strip-shaped sheet material having a surface which is opposed to the head units  2  and on which an image is to be recorded. A variety of materials may be used for the recording medium  7 . Examples of the material of the recording medium  7  include paper such as plain paper and coated paper, resin films including polyethylene terephthalate (PET) and the like, metal such as an aluminum plate. In the first preferred embodiment, plain paper often used for inkjet liquid ejecting apparatuses is used as the recording medium  7 . Although an elongated strip-shaped sheet material is used as the recording medium  7  in the first preferred embodiment, the shape of the recording medium  7  is not limited to this for the practice of the present invention. Flat sheets may be used as the recording medium  7  in the present invention. 
     The controller  8  controls the operations of the components of the liquid ejecting apparatus  1 . As schematically shown in  FIG. 1 , the controller  8  according to the first preferred embodiment is formed by a computer including an arithmetic processor  81  such as a CPU, a memory  82  such as a RAM, and a storage part  83  such as a hard disk drive. As shown in  FIGS. 1 and 4 , the controller  8  is electrically connected to the transport mechanism  10 , the four head units  2 , the cap moving mechanism of the cap members  61  and components of the supply system  3  to be described later. 
     The controller  8  temporarily reads a computer program  831  and data  832  which are stored in the storage part  83  onto the memory  82 . The arithmetic processor  81  performs arithmetic processing based on the computer program  831  and the data  832 , so that the controller  8  controls the operations of the components of the liquid ejecting apparatus  1 . Thus, the image recording step and the recovery step to be described later in the liquid ejecting apparatus  1  proceed. It should be noted that the controller  8  may be formed by electronic circuitry. 
     For the recording of an image based on print data on the upper surface of the recording medium  7  in the image recording step, the controller  8  controls the ejection of ink from the respective nozzles  212  by using the operating instructions provided to the head units  2 . Thus, the controller  8  controls the ejection position and ejection rate of ink in the respective nozzles  212  in the image recording step. 
     The control of the ejection position of ink in the liquid ejecting apparatus  1  according to the first preferred embodiment is exercised by controlling the ejection timing of the ink from the nozzles  212 . In the image recording step according to the first preferred embodiment, ink is ejected from the nozzles  212  while the controller  8  controls the unwinder  11  and the winder  12  in accordance with an input from the encoder  14  to transport the recording medium  7  at a constant speed in the transport direction. While passing under the head units  2 , the recording medium  7  receives ink ejected from predetermined ones of the nozzles  212  of the head units  2 . Thus, the impact positions of ink on the recording medium  7  in the transport direction are determined by the ejection timing of the ink from the nozzles  212 . 
     In the first preferred embodiment, the ejection rate and ejection timing of ink are controlled by an ejection signal sent from the controller  8  to the piezoelectric elements. The controller  8  generates the ejection signal to be outputted to the piezoelectric elements, based on the print data inputted from an external input mechanism and positional information about the recording medium  7 . 
     &lt;1-2. Supply System&gt; 
     Next, the configuration of the supply system  3  for supplying ink to the head units  2  will be described with reference to  FIG. 4 .  FIG. 4  is a block diagram showing a peripheral configuration of the supply system  3 .  FIG. 4  shows the first head  21 , the second head  22 , the cap member  61  and the controller  8  in addition to the supply system  3 . 
     The supply system  3  includes a main tank  325 , a sub-tank  321 , a pressurizing pump  341  and a depressurizing pump  361 . 
     The main tank  325  is an ink tank or ink pouch for storing ink therein, and is replaceably disposed inside or outside the liquid ejecting apparatus  1 . The main tank  325  is connected through a main pipe  326  to the sub-tank  321 . A main valve  327  and a liquid feed pump  328  are interposed in the main pipe  326 . 
     A known valve may be used as the main valve  327 . The main valve  327  is electrically connected to the controller  8 , and is switched between an open position and a closed position in response to an operating instruction from the controller  8 . When the main valve  327  is in the open position, the main tank  325  and the sub-tank  321  communicate with each other. When the main valve  327  is in the closed position, the communication between the main tank  325  and the sub-tank  321  is closed off. 
     The liquid feed pump  328  is a pump for carrying the ink inside the main tank  325  to the sub-tank  321 . A known pump may be used as the liquid feed pump  328 . The liquid feed pump  328  is electrically connected to the controller  8 . When the main valve  327  is brought into the open position while the liquid feed pump  328  is driven in response to an operating instruction from the controller  8 , the ink stored in the main tank  325  is carried from the main tank  325  through the main pipe  326  to the sub-tank  321 . 
     The sub-tank  321  is a tank for temporarily storing therein the ink for supply to the first head  21  and the second head  22 , and constitutes a “liquid reservoir” according to the present invention. The sub-tank  321  includes a liquid level sensor  322 . 
     The liquid level sensor  322  is a sensor for detecting the liquid level of the ink stored in the sub-tank  321 . A known sensor may be used as the liquid level sensor  322 . The liquid level sensor  322  is electrically connected to the controller  8 , and inputs the result of detection of the liquid at a predetermined level in the sub-tank  321  as a signal to the controller  8 . 
     Upon judging that the liquid level of the ink stored in the sub-tank  321  is lower than a predetermined first level, based on the signal from the liquid level sensor  322 , the controller  8  brings the main valve  327  into the open position and drives the liquid feed pump  328 . Upon judging that the liquid level of the ink stored in the sub-tank  321  is higher than a predetermined second level, based on the signal from the liquid level sensor  322 , the controller  8  stops driving the liquid feed pump  328  and brings the main valve  327  into the closed position. Although the second level is higher than the first level in the first preferred embodiment, the first level and the second level may be at the same position. Thus, the liquid level in the sub-tank  321  is adjusted to within a predetermined fixed range. 
     The sub-tank  321  is connected through a pressure regulating pipe  331  to a three-way valve  332 . A known three-way valve may be used as the three-way valve  332 . The three-way valve  332  is connected to three pipes. Specifically, the three-way valve  332  is connected to a pressurizing pipe  342  in communication with the pressurizing pump  341  and a depressurizing pipe  362  in communication with the depressurizing pump  361  in addition to the pressure regulating pipe  331  in communication with the sub-tank  321 . The three-way valve  332  is electrically connected to the controller  8 , and is selectable between two pipe communicating states (i.e., a pressurizing pipe communicating state and a depressurizing pipe communicating state) in response to an operating instruction from the controller  8 . 
     When the pressurizing pipe communicating state is selected by the controller  8 , the pressure regulating pipe  331  and the pressurizing pipe  342  communicate with each other through the three-way valve  332 , and the communication between the pressure regulating pipe  331  and the depressurizing pipe  362  is closed off. 
     When the depressurizing pipe communicating state is selected by the controller  8 , the pressure regulating pipe  331  and the depressurizing pipe  362  communicate with each other through the three-way valve  332 , and the communication between the pressure regulating pipe  331  and the pressurizing pipe  342  is closed off. 
     The sub-tank  321  is connected through a supply pipe  311  to a first branch pipe  312  and a second branch pipe  314 . The first branch pipe  312  is connected to the first head  21 , and a first head valve  313  is interposed in the first branch pipe  312 . The second branch pipe  314  is connected to the second head  22 , and a second head valve  315  is interposed in the second branch pipe  314 . For distinction from the nozzle surface  211  of the first head  21  in expressive terms, the nozzle surface of the second head  22  is referred to as a “nozzle surface  221 ”. 
     A known valve may be used as the first head valve  313 . The first head valve  313  is electrically connected to the controller  8 . The first head valve  313  is switched between an open position and a closed position in response to an operating instruction from the controller  8 . When the first head valve  313  is in the open position, the first head  21  and the sub-tank  321  communicate with each other. When the first head valve  313  is in the closed position, the communication between the first head  21  and the sub-tank  321  is closed off. 
     A known valve may be used as the second head valve  315 . The second head valve  315  is electrically connected to the controller  8 . The second head valve  315  is switched between an open position and a closed position in response to an operating instruction from the controller  8 . When the second head valve  315  is in the open position, the second head  22  and the sub-tank  321  communicate with each other. When the second head valve  315  is in the closed position, the communication between the second head  22  and the sub-tank  321  is closed off. 
     The first branch pipe  312  and the second branch pipe  314  in the first preferred embodiment constitute “supply passages” for supplying the liquid from the liquid reservoir (i.e., the sub-tank  321 ) to the heads (i.e., the first head  21  and the second head  22 ) in the aforementioned manner according to the present invention. The first head valve  313  and the second head valve  315  constitute “open/close parts” provided in the respective supply passages (i.e., the first branch pipe  312  and the second branch pipe  314 ) according to the present invention. 
     A pressurizing valve  343  is interposed in the pressurizing pipe  342  connecting the pressurizing pump  341  and the three-way valve  332  to each other. Part of the pressurizing pipe  342  which is closer to the three-way valve  332  with respect to the pressurizing valve  343  is branch-connected to an open-to-atmosphere pipe  351  and a pressure sensor  346  respectively. 
     A known valve may be used as the pressurizing valve  343 . The pressurizing valve  343  is electrically connected to the controller  8 . The pressurizing valve  343  is switched between an open position and a closed position in response to an operating instruction from the controller  8 . When the pressurizing valve  343  is in the open position, the pressurizing pump  341  and the three-way valve  332  communicate with each other. When the pressurizing valve  343  is in the closed position, the communication between the pressurizing pump  341  and the three-way valve  332  is closed off. 
     A known pressurizing pump for applying pressure to a gas may be used as the pressurizing pump  341 . The pressurizing pump  341  is electrically connected to the controller  8 . When the pressurizing valve  343  is brought into the open position and the three-way valve  332  is brought into the pressurizing pipe communicating state while the pressurizing pump  341  is driven in response to an operating instruction from the controller  8 , the interior of the sub-tank  321  is pressurized with a pressurized gas through the pressurizing pipe  342  and the pressure regulating pipe  331 . 
     The pressurizing pump  341 , the pressurizing valve  343 , the pressurizing pipe  342 , the three-way valve  332  and the pressure regulating pipe  331  in the first preferred embodiment constitute a “pressurizing part” for pressurizing the liquid reservoir (sub-tank  321 ) in the aforementioned according to the present invention. 
     The open-to-atmosphere pipe  351  has a first end connected to the pressurizing pipe  342 , and a second end open to the atmosphere. An open-to-atmosphere valve  352  is interposed in the open-to-atmosphere pipe  351 . A known valve may be used as the open-to-atmosphere valve  352 . The open-to-atmosphere valve  352  is electrically connected to the controller  8 . The open-to-atmosphere valve  352  is switched between an open position and a closed position in response to an operating instruction from the controller  8 . When the open-to-atmosphere valve  352  is in the open position, the open-to-atmosphere pipe  351  and the pressurizing pipe  342  are open to the atmosphere. At this time, when the three-way valve  332  is brought into the pressurizing pipe communicating state, the gas and ink in the sub-tank  321  are also open to the atmosphere. When the open-to-atmosphere valve  352  is in the closed position, the communication between the open-to-atmosphere pipe  351  and pressurizing pipe  342 , and the second end of the open-to-atmosphere pipe  351  open to the atmosphere is closed off, so that the open-to-atmosphere pipe  351  and the pressurizing pipe  342  are not open to the atmosphere. 
     The open-to-atmosphere pipe  351 , the open-to-atmosphere valve  352 , the pressurizing pipe  342 , the three-way valve  332  and the pressure regulating pipe  331  in the first preferred embodiment constitute an “open-to-atmosphere part” for making the liquid reservoir (sub-tank  321 ) open to the atmosphere in the aforementioned manner according to the present invention. 
     The pressure sensor  346  is a sensor for detecting the pressure of a gas inside the pressurizing pipe  342 . A known sensor may be used as the pressure sensor  346 . The pressure sensor  346  is electrically connected to the controller  8 , and inputs the result of detection as a signal to the controller  8 . 
     A depressurizing valve  363  is interposed in the depressurizing pipe  362  connecting the depressurizing pump  361  and the three-way valve  332  to each other. Part of the depressurizing pipe  362  which is closer to the three-way valve  332  with respect to the depressurizing valve  363  is branch-connected to an open-to-atmosphere pipe  355  and a pressure sensor  366  respectively. 
     A known valve may be used as the depressurizing valve  363 . The depressurizing valve  363  is electrically connected to the controller  8 . The depressurizing valve  363  is switched between an open position and a closed position in response to an operating instruction from the controller  8 . When the depressurizing valve  363  is in the open position, the depressurizing pump  361  and the three-way valve  332  communicate with each other. When the depressurizing valve  363  is in the closed position, the communication between the depressurizing pump  361  and the three-way valve  332  is closed off. 
     A known depressurizing pump for reducing the pressure of a gas may be used as the depressurizing pump  361 . The depressurizing pump  361  is electrically connected to the controller  8 . When the depressurizing valve  363  is brought into the open position and the three-way valve  332  is brought into the depressurizing pipe communicating state while the depressurizing pump  361  is driven in response to an operating instruction from the controller  8 , the interior of the sub-tank  321  is depressurized with a depressurized gas through the depressurizing pipe  362  and the pressure regulating pipe  331 . 
     The depressurizing pump  361 , the depressurizing valve  363 , the depressurizing pipe  362 , the three-way valve  332  and the pressure regulating pipe  331  in the first preferred embodiment constitute a “depressurizing part” for depressurizing the liquid reservoir (sub-tank  321 ) in the aforementioned according to the present invention. 
     The open-to-atmosphere pipe  355  has a first end connected to the depressurizing pipe  362 , and a second end open to the atmosphere. An open-to-atmosphere valve  356  is interposed in the open-to-atmosphere pipe  355 . A known valve may be used as the open-to-atmosphere valve  356 . The open-to-atmosphere valve  356  is electrically connected to the controller  8 . The open-to-atmosphere valve  356  is switched between an open position and a closed position in response to an operating instruction from the controller  8 . When the open-to-atmosphere valve  356  is in the open position, the open-to-atmosphere pipe  355  and the depressurizing pipe  362  are open to the atmosphere. When the open-to-atmosphere valve  356  is in the closed position, the communication between the open-to-atmosphere pipe  355  and depressurizing pipe  362 , and the second end of the open-to-atmosphere pipe  355  open to the atmosphere is closed off, so that the open-to-atmosphere pipe  355  and the depressurizing pipe  362  are not open to the atmosphere. 
     The pressure sensor  366  is a sensor for detecting the pressure of a gas inside the depressurizing pipe  362 . A known sensor may be used as the pressure sensor  366 . The pressure sensor  366  is electrically connected to the controller  8 , and inputs the result of detection as a signal to the controller  8 . 
     In the first preferred embodiment, the sub-tank  321  is disposed above the first head  21  and the second head  22 . For the formation of ink menisci in the nozzles  212  of the first head  21  and the second head  22 , it is hence necessary that the pressure inside the first head  21  and the second head  22  is regulated to a negative pressure lower than atmospheric pressure. 
     In the first preferred embodiment, the first head valve  313  and the second head valve  315  are brought into the open position, and the depressurizing pump  361  is used to depressurize the sub-tank  321  in the aforementioned manner, whereby ink in the first head  21  and the second head  22  is at the negative pressure. This allows the formation of ink menisci in the nozzles  212 . 
     At this time, if the sub-tank  321  is excessively depressurized, there is apprehension that air bubbles are drawn into the nozzles  212 . To prevent this, the controller  8  controls the driving of the depressurizing pump  361  and the open position of the depressurizing valve  363  or the open-to-atmosphere valve  356 , based on the value of pressure detected by the pressure sensor  366 , to thereby regulate the negative pressure inside the first head  21  and the second head  22 . 
     The supply system  3  and the peripheral configuration thereof are described above. The liquid ejecting apparatus  1  according to the first preferred embodiment further includes a timer not shown. The timer measures the amount of time elapsed since an operating instruction was provided for the control of each component. For example, the timer measures the amount of time elapsed since the transition was made from the closed position to the open position in each of the valves such as the pressurizing valve  343 . The timer is electrically connected to the controller  8 , and the results of measurement are written one by one in the memory  82  of the controller  8 . 
     &lt;1-3. Recovery Step&gt; 
     Next, the recovery step of the first head  21  and the second head  22  in the liquid ejecting apparatus  1  according to the first preferred embodiment will be described.  FIG. 5  is a flow diagram of the recovery step according to the first preferred embodiment.  FIGS. 6 to 12  are flow diagrams of steps included in the recovery step according to the first preferred embodiment.  FIG. 13  is a timing diagram of the recovery step according to the first preferred embodiment. 
     Immediately before the execution of the recovery step, the liquid ejecting apparatus  1  is in a state of what is called a “waiting time period” during which no images are recorded on the recording medium  7  (i.e., the image recording step is not being executed). 
     During the waiting time period, the liquid ejecting apparatus  1  brings the depressurizing valve  363 , the first head valve  313  and the second head valve  315  into the open position, and brings the three-way valve  332  into the depressurizing pipe communicating state while driving the depressurizing pump  361 . This forms ink menisci in the nozzles  212  to maintain the ink not dripping down from the nozzles  212 . At this time, the open-to-atmosphere valve  356  is in the closed position. 
     In the liquid ejecting apparatus  1  according to the first preferred embodiment, if ink is not ejected from the nozzles  212  of the first head  21  and the second head  22  for a long period of time (i.e., the aforementioned “waiting time period” continues for a long period of time), there is apprehension that a volatile ingredient contained in the ink vaporizes, so that the ink is increased in viscosity or is solidified. This might cause ejection failures of the ink from the nozzles  212  and unevenness in density of an image recorded on the recording medium  7 . 
     If the ink used in the liquid ejecting apparatus  1  contains a precipitable ingredient and is not ejected for a long period of time, there is apprehension that the precipitable ingredient is precipitated in places where the ink is stored and places serving as flow passages of ink, such as the first head  21 , the second head  22 , the first branch pipe  312 , the second branch pipe  314 , the supply pipe  311  and the interior of the sub-tank  321 . This might result in the problems that there arises unevenness in density of an image recorded on the recording medium  7  and that the flow passages of the ink are narrowed down. 
     To solve the problems as described above, the liquid ejecting apparatus  1  according to the first preferred embodiment performs the recovery step to be described below in response to a recovery operation instruction from the controller  8 . Specifically, the liquid ejecting apparatus  1  cleans the interiors of the heads by performing a “pressurized purge” in which pressure is applied to the ink in the first head  21  and the second head  22  in sequential order to force the ink out of the nozzles  212  of the first head  21  and the second head  22  toward the cap members  61 . 
     The recovery step according to the first preferred embodiment will be described with reference to the flow diagram of  FIG. 5  as appropriate. First, upon selecting the computer program  831  relating to the recovery operation stored in the storage part  83 , the controller  8  instructs the components of the liquid ejecting apparatus  1  to perform a recovery process. Thus, the liquid ejecting apparatus  1  performs operations to be described below. 
     The selection of the computer program  831  in the controller  8  may be made by inputting from a manipulation part not shown in the liquid ejecting apparatus  1  or upon being triggered by detecting that ink has not been ejected from the nozzles  212  for a fixed period of time by means of the timer not shown in the liquid ejecting apparatus  1 . Alternatively, an appropriate computer program  831  may be selected from among a plurality of computer programs  831  relating to the recovery process in accordance with conditions of the recovery operation. For example, computer programs  831  different from each other in the amount of ink forced out in first and second purge steps to be described later may be prepared, so that a computer program  831  which provides an appropriate amount of ink forced out is selected in accordance with the length of the “waiting time period” immediately before the execution of the recovery step. 
     In the recovery operation instruction from the controller  8 , the controller  8  initially executes an initialization operation instruction upon the liquid ejecting apparatus  1 . Thus, the liquid ejecting apparatus  1  performs an initialization step (Step S 1 ) for setting each of the components thereof at its initial position for the recovery process. 
       FIG. 6  is a flow diagram showing the details of the initialization step. Upon starting to perform the initialization step, the controller  8  initially provides an operating instruction to the cap moving mechanism not shown, thereby placing the cap members  61  in the opposed position which is opposed to the nozzle surfaces  211  and  221  of the first and second heads  21  and  22  (Step S 11 ). When the cap members  61  are already in the opposed position, the opposed position of the cap members  61  is maintained. 
     Next, the controller  8  checks to see that the liquid level of the ink stored in the sub-tank  321  is within a predetermined fixed range, based on a signal from the liquid level sensor  322  (Step S 12 ). Upon judging that the liquid level of the ink is not within the fixed range (specifically, that the liquid level of the ink is lower than the predetermined first level), the controller  8  brings the main valve  327  into the open position and drives the liquid feed pump  328  to carry the ink from the main tank  325  to the sub-tank  321 . Then, upon judging that the liquid level of the ink becomes higher than the predetermined second level, based on the signal from the liquid level sensor  322 , the controller  8  stops driving the liquid feed pump  328  and brings the main valve  327  into the closed position to stop carrying the ink. 
     As described above, the step of checking the amount of ink stored in the sub-tank  321  is performed before a purge step to be described below. This prevents changes in purge conditions resulting from the charging of ink into the sub-tank  321  in the course of forcing ink from the heads in the purge step. 
     After checking the charging of ink into the sub-tank  321  in Step S 12 , the controller  8  next sets the opening and closing of the valves at respective locations of the supply system  3  to their initial positions for the recovery process (Step S 13 ). In Step S 13 , the controller  8  provides an operating instruction to bring the first head valve  313 , the second head valve  315  and the depressurizing valve  363  into the open position. Specifically, when these valves are already in the open position, the open position thereof is maintained. The valves in the closed position, if any, are changed to the open position. The three-way valve  332  is maintained in the depressurizing pipe communicating state. The pressurizing valve  343 , the open-to-atmosphere valves  352  and  356 , and the main valve  327  are brought into the closed position. 
     Next, the controller  8  checks to see that the pressurizing valve  343  is in the closed position, and drives the pressurizing pump  341  (Step S 14 ). Thus, the gas in part of the pressurizing pipe  342  extending from the pressurizing pump  341  to the pressurizing valve  343  is pressurized to a predetermined pressure. 
     Next, the controller  8  brings the first head valve  313  and the second head valve  315  to the closed position (Step S 15 ). This completes the initialization step. 
     The aforementioned steps (Steps S 11  to S 15 ) included in the initialization step need not be performed in the aforementioned order. The order of the aforementioned steps (Steps S 11  to S 15 ) may be changed, as appropriate, or be performed at the same time without inconsistencies. 
     After the initialization step is completed, the liquid ejecting apparatus  1  next performs a pre-pressurization step (Step S 2 ) for pressurizing the ink stored in the sub-tank  321  for pre-pressurization while the first head valve  313  and the second head valve  315  remain in the closed position. 
       FIG. 7  is a flow diagram showing the details of the pre-pressurization step. Upon starting to perform the pre-pressurization step, the controller  8  initially provides an operating instruction to bring the pressurizing valve  343  into the open position (Step S 21 ). Next, the controller  8  switches the three-way valve  332  to the pressurizing pipe communicating state (Step S 22 ). This causes the gas inside the pressurizing pipe  342  pressurized by driving the pressurizing pump  341  to pressurize the ink stored in the sub-tank  321 , so that the ink in the sub-tank  321 , the supply pipe  311 , part of the first branch pipe  312  closer to the supply pipe  311  with respect to the first head valve  313  and part of the second branch pipe  314  closer to the supply pipe  311  with respect to the second head valve  315  are brought into a state of positive pressure higher than atmospheric pressure. 
     Next, the controller  8  performs a first pressure monitoring step (Step S 23 ) for judging whether the pressure inside the pressurizing pipe  342  in communication with the sub-tank  321  is less than a predetermined pressure (referred to hereinafter as a “first pressure”) or not, based on a signal from the pressure sensor  346 . Upon judging that the pressure inside the pressurizing pipe  342  is not less than the first pressure, the controller  8  then completes the pre-pressurization step, and performs a first purge step (Step S 3 ) for performing a first purge. 
       FIG. 8  is a flow diagram showing the details of the first purge step. Upon starting the first purge step, the controller  8  initially provides an operating instruction to bring the first head valve  313  into the open position (Step S 31 ). This causes the supply pipe  311  and the sub-tank  321  which are brought into the state of positive pressure by the pre-pressurization step (Step S 2 ) to communicate with the first head  21 . Thus, the ink inside the first head  21  is brought into the state of positive pressure, so that the ink is forced out of the nozzles  212  of the first head  21 . That is, the opening of the first head valve  313  starts the pressurized purge in the first head  21 . 
     Forcing the ink out of the nozzles  212  of the first head  21  in the aforementioned manner eliminates the problems of the increased viscosity and solidification of ink in the nozzles  212  of the first head  21 . Also, when the ink contains a precipitable ingredient, this eliminates the problem of the precipitation of the ink inside the first head  21 . 
     After the first head valve  313  is brought into the open position, the timer not shown measures the amount of time elapsed since the first head valve  313  was brought into the open position. The value measured by the timer is inputted as a signal to the controller  8 . The controller  8  judges whether a predetermined time period has elapsed since the first head valve  313  was brought into the open position or not (Step S 32 ). When the controller  8  judges in Step S 32  that the predetermined time period has elapsed (i.e., the answer to Step S 32  is YES), the pressurizing valve  343  is brought into the closed position (Step S 33 ), and the first head valve  313  is brought into the closed position (Step S 34 ). This completes the first purge step (Step S 3 ). 
     In the flow diagram of  FIG. 8 , the first head valve  313  is closed (Step S 34 ) after the pressurizing valve  343  is closed (Step S 33 ). However, the order in which these valves are closed is not limited to this for the practice of the present invention. The first head valve  313  may be brought into the closed position before the pressurizing valve  343 . Alternatively, the pressurizing valve  343  and the first head valve  313  may be brought into the closed position at the same time. 
     When the controller  8  brings the first head valve  313  into the closed position in Step S 34 , the ink inside the first head  21  and the ink inside the supply pipe  311  are isolated from each other, so that the ink inside the sub-tank  321  and the ink inside the supply pipe  311  stop pressurizing the ink inside the first head  21 . After the pressurization stops, the ink inside the first head  21  continues to be ejected from the nozzles  212  of the first head  21 . Thus, the pressure of the ink inside the first head  21  decreases gradually from the state of positive pressure toward atmospheric pressure. 
     Referring again to  FIG. 5 , the liquid ejecting apparatus  1  performs a re-pressurization step (Step S 4 ) after the first purge step is completed. The re-pressurization step in Step S 4  is substantially the same as the pre-pressurization step in Step S 2 . The re-pressurization step pressurizes the ink inside the sub-tank  321  again to a predetermined pressure, with the communication between the first and second heads  21  and  22  and the sub-tank  321  closed off. 
       FIG. 9  is a flow diagram showing the details of the re-pressurization step. Upon starting the re-pressurization step, the controller  8  initially provides an operating instruction to bring the pressurizing valve  343  into the open position and to maintain the pressurizing pipe communicating state of the three-way valve  332  (Step S 41 ). Next, the controller  8  performs a first pressure monitoring step (Step S 42 ) for judging whether the pressure inside the pressurizing pipe  342  in communication with the sub-tank  321  is less than a predetermined pressure (which is the “first pressure” identical with that in the pre-pressurization step in the first preferred embodiment) or not, based on a signal from the pressure sensor  346 . Upon judging that the pressure inside the pressurizing pipe  342  is not less than the first pressure, the controller  8  then completes the re-pressurization step, and performs the next step, i.e. a second purge step (Step S 5 ). 
     In the first preferred embodiment, the ink inside the sub-tank  321  is pressurized to the first pressure both in the pre-pressurization step (Step S 2 ) and in the re-pressurization step (Step S 4 ). However, the pressurization of the ink inside the sub-tank  321  is not limited to this for the practice of the present invention. The ink inside the sub-tank  321  may be pressurized to pressures different between the pre-pressurization step and the re-pressurization step. In the case where the use of the same pressure causes a difference between the amount of ink ejected in the first purge step and the amount of ink ejected in the second purge step to be described below because of a difference in flow passage resistance from the sub-tank  321  between the first head  21  and the second head  22  and the like, the sub-tank  321  may be pressurized to the first pressure in the pre-pressurization step whereas the sub-tank  321  is pressurized to a second pressure different from the first pressure in the re-pressurization step for the purpose of making the amounts of ejected ink equal to each other. 
     After the re-pressurization step (Step S 4 ) is completed, the second purge step (Step S 5 ) is performed which brings the second head valve  315  into the open position to force ink out of the nozzles  212  of the second head  22 , thereby performing a second purge. 
       FIG. 10  is a flow diagram showing the details of the second purge step. Upon starting the second purge step, the controller  8  initially provides an operating instruction to bring the second head valve  315  into the open position (Step S 51 ). This causes the supply pipe  311  and the sub-tank  321  which are brought into the state of positive pressure by the re-pressurization step (Step S 4 ) to communicate with the second head  22 . Thus, the ink inside the second head  22  is brought into the state of positive pressure, so that the ink is forced out of the nozzles  212  of the second head  22 . That is, the opening of the second head valve  315  starts the pressurized purge in the second head  22 . 
     Forcing the ink out of the nozzles  212  of the second head  22  in the aforementioned manner eliminates the problems of the increased viscosity and solidification of ink in the nozzles  212  of the second head  22 . Also, when the ink contains a precipitable ingredient, this eliminates the problem of the precipitation of the ink inside the second head  22 . 
     After the second head valve  315  is brought into the open position, the timer not shown measures the amount of time elapsed since the second head valve  315  was brought into the open position. The value measured by the timer is inputted as a signal to the controller  8 . The controller  8  judges whether a predetermined time period has elapsed since the second head valve  315  was brought into the open position or not (Step S 52 ). When the controller  8  judges in Step S 52  that the predetermined time period has elapsed (i.e., the answer to Step S 52  is YES), the pressurizing valve  343  is brought into the closed position (Step S 53 ), and the second head valve  315  is brought into the closed position (Step S 54 ). This completes the second purge step (Step S 5 ). 
     In the flow diagram of  FIG. 10 , the second head valve  315  is closed (Step S 54 ) after the pressurizing valve  343  is closed (Step S 53 ). However, the order in which these valves are closed is not limited to this for the practice of the present invention. The second head valve  315  may be brought into the closed position before the pressurizing valve  343 . Alternatively, the pressurizing valve  343  and the second head valve  315  may be brought into the closed position at the same time. 
     When the controller  8  brings the second head valve  315  into the closed position in Step S 54 , the ink in the second head  22  and the ink in the supply pipe  311  are isolated from each other, so that the ink inside the sub-tank  321  and the supply pipe  311  stops pressurizing the ink inside the second head  22 . After the pressurization stops, the ink inside the second head  22  continues to be ejected from the nozzles  212  of the second head  22 . Thus, the pressure of the ink inside the second head  22  decreases gradually from the state of positive pressure toward atmospheric pressure. 
     Referring again to  FIG. 5 , after the second purge step (Step S 5 ) is completed whereby the pressurized purges of the first head  21  and the second head  22  in each head unit  2  are completed, an open-to-atmosphere step (Step S 6 ) is next performed which reduces the pressure of the ink inside the first head  21  and the second head  22 . 
       FIG. 11  is a flow diagram showing the details of the open-to-atmosphere step. Upon starting the open-to-atmosphere step, the controller  8  initially provides an operating instruction to bring the first head valve  313  and the second head valve  315  into the open position (Step S 61 ). This causes the supply pipe  311  and the sub-tank  321  to communicate with the first head  21  and the second head  22 . 
     Next, the controller  8  provides an operating instruction to bring the open-to-atmosphere valve  352  into the open position (Step S 62 ). This causes the ink inside the first and second heads  21  and  22  caused to communicate with the sub-tank  321  in Step S 61  to be open to the atmosphere through the pressure regulating pipe  331 , the pressurizing pipe  342  and the open-to-atmosphere pipe  351 . Thus, the pressure of the ink inside the first head  21  and the second head  22  which is brought into the state of positive pressure in the first purge step and the second purge step decreases gradually toward atmospheric pressure. 
     Next, the controller  8  judges whether the pressure inside the pressurizing pipe  342  in communication with the sub-tank  321  is greater than a predetermined pressure (which is atmospheric pressure in the first preferred embodiment) or not, based on a signal from the pressure sensor  346  (Step S 63 ). Upon judging that the pressure inside the pressurizing pipe  342  is not greater than atmospheric pressure, the controller  8  then brings the open-to-atmosphere valve  352  into the closed position (Step S 64 ) to complete the open-to-atmosphere step. 
     In judging whether the pressure inside the pressurizing pipe  342  is greater than a predetermined pressure or not in Step S 63 , the predetermined pressure as used in the first preferred embodiment is atmospheric pressure. However, the predetermined pressure is not limited to atmospheric pressure for the practice of the present invention. A pressure higher than atmospheric pressure and lower than the first pressure in the pre-pressurization step and the re-pressurization step may be used as the aforementioned predetermined pressure. 
     Referring again to  FIG. 5 , after the open-to-atmosphere step (Step S 6 ) is completed whereby the pressure of the ink inside the first head  21  and the second head  22  of each head unit  2  is decreased to atmospheric pressure, a meniscus formation step (Step S 7 ) is next performed which forms ink menisci in the nozzles  212  of the first head  21  and the second head  22 . 
       FIG. 12  is a flow diagram showing the details of the meniscus formation step. Upon starting the meniscus formation step, the controller  8  initially checks whether the pressure of the gas in the depressurizing pipe  362  is maintained at a pressure (referred to hereinafter as a “meniscus pressure”) which allows the formation of ink menisci in the nozzles  212  or not, based on a signal from the pressure sensor  366  (Step S 71 ). The controller  8  controls the driving of the depressurizing pump  361  and the opening/closing of the depressurizing valve  363  or the open-to-atmosphere valve  356  to regulate the pressure of the gas in the depressurizing pipe  362  at the meniscus pressure. 
     Upon checking that the pressure in the depressurizing pipe  362  is maintained at the meniscus pressure in Step S 71 , the controller  8  provides an operating instruction to bring the three-way valve  332  into the depressurizing pipe communicating state (Step S 72 ). This depressurizes the gas inside the sub-tank  321  through the pressure regulating pipe  331  and the depressurizing pipe  362  to accordingly depressurize the ink inside the sub-tank  321 . Thus, after the open-to-atmosphere step in Step S 61 , the ink inside the first and second heads  21  and  22  in communication with the sub-tank  321  is also depressurized, so that ink menisci are formed in the respective nozzles  212 . 
     The recovery step in the first preferred embodiment is thus completed. The pressures inside the first head  21  and the second head  22  in the recovery step according to the first preferred embodiment will be described with reference to the timing diagram of  FIG. 13 . 
     The open/closed positions of the valves are shown in upper part of the timing diagram of  FIG. 13 , and the pressures inside the heads are schematically shown in the lower part thereof. The reference characters S 1  to S 7  at the top correspond to Steps S 1  to S 7  in the flow diagram of the recovery step in  FIG. 5 , and the reference characters t 0  to t 13  at the bottom denote points of time. The intervals between the points of time t 0  to t 13  are shown as exaggerated, as appropriate, for purposes of clarifying the steps, and do not necessarily coincide with the actual intervals between the points of time in the recovery step. 
     In the upper part of  FIG. 13 , the open position of each valve is denoted as “OPEN” and the closed position thereof is denoted as “CLOSED”. The pressurizing pipe communicating state of the three-way valve  332  is denoted as “PRESSURIZING” and the depressurizing pipe communicating state thereof is denoted as “DEPRESSURIZING”. The lower part of  FIG. 13  shows the pressures inside the first head  21  and the second head  22 . A positive pressure is shown over atmospheric pressure, and a negative pressure is shown under atmospheric pressure. The inside pressures at the respective points of time will be described later. 
     The recovery step in the first preferred embodiment will be described with reference to  FIGS. 5 and 13 , as appropriate. Upon selecting the computer program  831  relating to the recovery process stored in the storage part  83 , the controller  8  instructs the components of the liquid ejecting apparatus  1  to perform the recovery process. At the time t 0 , the initialization step (Step S 1 ) starts. 
     At the time t 1 , Step S 15  in the initialization step is performed, so that the first head valve  313  and the second head valve  315  are brought into the closed position. At this time, the pressures inside the first head  21  and the second head  22  are held at a state of negative pressure lower than atmospheric pressure for the formation of ink menisci in the nozzles  212 , and are a “meniscus pressure P 13 ” and a “meniscus pressure P 23 ”, respectively. 
     At the time t 2 , Step S 21  and Step S 22  in the pre-pressurization step are performed substantially simultaneously, so that the pressurizing valve  343  is brought into the open position and the three-way valve  332  is switched from the depressurizing pipe communicating state to the pressurizing pipe communicating state. Thus, the ink inside the sub-tank  321  is pressurized, but the first head  21  and the second head  22  are not pressurized to maintain the meniscus pressures to some extent because the first head valve  313  and the second head valve  315  are in the closed position. After the time t 2 , the controller  8  monitors whether the sub-tank  321  is pressurized to not less than the first pressure or not, based on the signal from the pressure sensor  346 . 
     The time t 3  is a point of time at which the controller  8  judges that the sub-tank  321  is pressurized to not less than the first pressure. At this point of time, Step S 31  in the first purge step is performed, so that the first head valve  313  is brought into the open position. This starts the pressurization of the ink inside the first head  21 , so that the ink starts being forced out of the nozzles  212 . The pressure inside the first head  21  is increased to a pressure P 11 . The pressure P 11  is a “purge pressure” required to force the ink inside the first head  21  out of the nozzles  212  in the first purge step (Step S 3 ), and is a pressure applied to the first head  21  when the sub-tank  321  is pressurized to the first pressure. 
     In Step S 32  in the first purge step, a lapse of a predetermined time period since the time t 3  is monitored. The predetermined time period is selected, as appropriate, depending on purge conditions including the amount of ejected ink required for the first head  21  and the like. The time t 4  is a point of time at which the controller  8  judges that the predetermined time period has elapsed since the time t 3 . At this point of time, the pressurizing valve  343  is brought into the closed position (Step S 33 ). Then, the first head valve  313  is brought into the closed position at the time t 5  (Step S 34 ). 
     The time t 4  and the time t 5  are different points of time in the first preferred embodiment, but are not limited to this for the practice of the present invention. The pressurizing valve  343  and the first head valve  313  may be brought into the closed position substantially simultaneously at the time t 4 . 
     At the time t 6 , the opening of the pressurizing valve  343  in the re-pressurization step (Step S 4 ) is performed. After the time t 6 , the controller  8  monitors whether the sub-tank  321  is pressurized to not less than the first pressure or not. 
     The time t 7  is a point of time at which the controller  8  judges that the sub-tank  321  is pressurized to not less than the first pressure after the time t 6 . At this point of time, Step S 51  in the second purge step is performed, so that the second head valve  315  is brought into the open position. This starts the pressurization of the ink inside the second head  22 , so that the ink starts being forced out of the nozzles  212 . The pressure inside the second head  22  is increased to a pressure P 21 . The pressure P 21  is a “purge pressure” required to force the ink inside the second head  22  out of the nozzles  212  in the second purge step (Step S 5 ), and is a pressure applied to the second head  22  when the sub-tank  321  is pressurized to the first pressure. 
     In Step S 52  in the second purge step, a lapse of a predetermined time period since the time t 7  is monitored. The predetermined time period is selected, as appropriate, depending on purge conditions including the amount of ejected ink required for the second head  22  and the like. The time t 8  is a point of time at which the controller  8  judges that the predetermined time period has elapsed since the time t 7 . At this point of time, the pressurizing valve  343  is brought into the closed position (Step S 53 ). Then, the second head valve  315  is brought into the closed position at the time t 9  (Step S 54 ). 
     The time t 8  and the time t 9  are different points of time in the first preferred embodiment, but are not limited to this for the practice of the present invention. The pressurizing valve  343  and the second head valve  315  may be brought into the closed position substantially simultaneously at the time t 8 . 
     At the time t 10 , Step S 61  in the open-to-atmosphere step is performed, so that the first head valve  313  and the second head valve  315  are brought into the open position. 
     The pressures inside the first head  21  and the second head  22  which are caused to reach the pressure P 11  and the pressure P 21 , respectively, by the first purge step and the second purge step decrease gradually after the first purge step and the second purge step as the ink is forced out of the nozzles  212 . Thus, after the completion of the second purge step (the time t 9 ), there are cases in which the pressures inside the first head  21  and the second head  22  do not completely return to atmospheric pressure but in a state of positive pressure as shown in  FIG. 13 . 
     Also, there are cases in which at the time t 10  the pressure of the ink inside the sub-tank  321  is higher than the pressures inside the first head  21  and the second head  22  which decrease as the ink is forced out of the nozzles  212 , and the pressures inside the first head  21  and the second head  22  become higher as shown in  FIG. 13  after the first head valve  313  and the second head valve  315  are brought into the open position at the time t 10 . 
     Subsequently, at the time t 11 , Step S 62  in the open-to-atmosphere step is performed, so that the open-to-atmosphere valve  352  is brought into the open position. Thus, the pressure of the gas by which a positive pressure has been applied to the interior of the sub-tank  321  decreases toward atmospheric pressure, and the pressures of the ink in the sub-tank  321 , the first head  21  and the second head  22  decrease toward atmospheric pressure. The rate at which these pressures decrease is higher than the rate at which the pressures decrease as the ink is forced out of the nozzles  212 . 
     This results from the facts that the flow passage resistance of the ink between the first and second heads  21  and  22  and the sub-tank  321  is sufficiently lower than the flow passage resistance of the ink in the nozzles  212  and that the flow passage resistance of the gas pressurizing the sub-tank  321  between the sub-tank  321  and the pressurizing and open-to-atmosphere pipes  342  and  355  is sufficiently lower than the flow passage resistance of the ink. 
     After the time t 11 , the controller  8  monitors whether the pressure inside the sub-tank  321  is decreased to not greater than the second pressure or not, based on the signal from the pressure sensor  346  (Step S 63 ). The time t 12  is a point of time at which the controller  8  judges that the pressure inside the sub-tank  321  is decreased to not greater than a predetermined value after the time t 11 . At this point of time, Step S 64  in the open-to-atmosphere step is performed, so that the open-to-atmosphere valve  352  is brought into the closed position. The predetermined value is atmospheric pressure in the first preferred embodiment. 
     At the time t 12 , the pressures inside the first head  21  and the second head  22  are decreased to a pressure P 12  and a pressure P 22 , respectively. The pressure P 12  and the pressure P 22  are lower than the pressure P 11  and the pressure P 21 , respectively, and are not less than atmospheric pressure. In the first preferred embodiment both the pressure P 12  and the pressure P 22  are atmospheric pressure. 
     Next, at the time t 13 , Step S 72  in the meniscus formation step is performed, so that the three-way valve  332  is switched from the pressurizing pipe communicating state to the depressurizing pipe communicating state. Thus, the pressures inside the first head  21  and the second head  22  are decreased to the meniscus pressures (i.e., the pressure P 13  and the pressure P 23 ) at which ink menisci are formed in the nozzles  212 . 
     The time t 12  and the time t 13  are different points of time in the first preferred embodiment, but are not limited to this for the practice of the present invention. The three-way valve  332  may be switched from the pressurizing pipe communicating state to the depressurizing pipe communicating state substantially simultaneously with the process of bringing the open-to-atmosphere valve  352  into the closed position at the time t 12 . 
     The drawing of air bubbles into the nozzles during the formation of ink menisci will be described. If the pressures inside the first head  21  and the second head  22  change by large amounts before and after the execution of Step S 72  in the meniscus formation step, the force drawing the ink into the nozzles acts strongly, so that air bubbles are more prone to be drawn into the nozzles  212 . 
     The pressures inside the first head  21  and the second head  22  in the case where the opening of the open-to-atmosphere valve  352  in Step S 62  in the open-to-atmosphere step is not performed are indicated by broken lines in  FIG. 13 . In the case where Step S 62  is not performed, the pressures inside the first head  21  and the second head  22  at the time t 13  are pressures P 14  and P 24 , respectively. The pressures P 14  and P 24  are lower than the pressures P 11  and P 21  and higher than the pressures P 12  and P 22 , respectively. When Step S 72  is performed in such a state to switch the three-way valve  332  to the depressurizing pipe communicating state, the pressure inside the first head  21  decreases from the pressure P 14  to the pressure P 13 , so that the amount of change in the pressure inside the first head  21  is expressed as (P 14 −P 13 ). Also, the pressure inside the second head  22  decreases from the pressure P 24  to the pressure P 23 , so that the amount of change in the pressure inside the second head  22  is expressed as (P 24 −P 23 ). 
     On the other hand, the amounts of change in the pressures inside the first head  21  and second head  22  before and after the execution of Step S 72  are expressed as (P 12 −P 13 ) and (P 22 −P 23 ), respectively, in the case where the first head valve  313  and the second head valve  315  are brought into the open position in the open-to-atmosphere step and the opening of the open-to-atmosphere valve  352  is performed in Step S 62 . The amounts of change in the pressures are smaller by the amounts corresponding to the decrease in the pressures inside the first head  21  and the second head  22  in Step S 62 . Thus, the force drawing the ink into the nozzles  212  in this case is weaker than that in the case where the open-to-atmosphere step is not performed, so that air bubbles are less prone to be drawn into the nozzles  212 . 
     In the first preferred embodiment, the first purge step (Step S 3 ) is performed after the pre-pressurization step (Step S 2 ) is performed, and thereafter the re-pressurization step (Step S 4 ) is performed, following which the second purge step (Step S 5 ) is performed. In the first purge step, a first open/close part is brought into the open position to start forcing the ink from the first head  21 . This varies the pressure of the ink inside the sub-tank  321  from an initial state. In the first preferred embodiment, the subsequent execution of the re-pressurization step (Step S 4 ) allows the pressure of the ink inside the sub-tank  321  prior to the execution of the second purge step to return to the state prior to the first purge step or to be brought into a state preferable for the second purge step. As a result, this provides equal purge conditions in the second purge step and in the aforementioned first purge step (Step S 3 ). 
     Further, the pressurized purge of the first head  21  and the pressurized purge of the second head  22  are executed separately in the first preferred embodiment. As compared with the simultaneous execution of the pressurized purges of the first head  21  and the second head  22 , the separate execution thereof suppresses a difference between the rate at which the ink supplied from the sub-tank  321  flows to the first head  21  and the rate at which the ink supplied from the sub-tank  321  flows to the second head  22  to provide more equal purge conditions. This reduces a difference between the amount of ink forced from the first head  21  during the execution of the first purge step and the amount of ink forced from the second head  22  during the execution of the second purge step to suppress unevenness in density during image recording and excess ink consumption in the first and second purge steps. 
     2. Second Preferred Embodiment 
     Next, a second preferred embodiment according to the present invention will be described. As described above, the purge conditions for the first head  21  and the second head  22  are adjusted in the first preferred embodiment by providing the pre-pressurization step prior to the execution of the first purge step and providing the re-pressurization step prior to the execution of the second purge step. These steps are not limited to this for the practice of the present invention. For example, the re-pressurization step (Step S 4 ) prior to the execution of the second purge step may be dispensed with in the case where there is a small variation in the pressure of ink inside the sub-tank  321  after the first purge step. 
     The second preferred embodiment is a preferred embodiment in which the recovery step in the first preferred embodiment is more simplified. The liquid ejecting apparatus  1  according to the second preferred embodiment is similar to the liquid ejecting apparatus  1  according to the first preferred embodiment shown in  FIGS. 1 to 4 , and will not be described. The second preferred embodiment differs from the first preferred embodiment in that the step corresponding to the re-pressurization step (Step S 4 ) of the first preferred embodiment is not performed in the recovery step and in that the first purge step and the second purge step are more simplified. Other steps similar to those of the first preferred embodiment will not be described, as appropriate. 
       FIG. 14  is a flow diagram showing the recovery step according to the second preferred embodiment. When the controller  8  provides the recovery operation instruction to the liquid ejecting apparatus  1 , the liquid ejecting apparatus  1  according to the second preferred embodiment performs the recovery step shown in  FIG. 14 .  FIG. 15  is a flow diagram showing the first purge step according to the second preferred embodiment.  FIG. 16  is a flow diagram showing the second purge step according to the second preferred embodiment. 
     Upon starting the recovery step in the second preferred embodiment, the controller  8  provides an operating instruction, so that the liquid ejecting apparatus  1  performs the initialization step (Step S 10 ) and the pre-pressurization step (Step S 20 ). In the initialization step (Step S 10 ), the liquid ejecting apparatus  1  performs steps similar to those of the initialization step (Step S 1 ) of the first preferred embodiment. In the pre-pressurization step (Step S 20 ), the liquid ejecting apparatus  1  performs steps similar to those of the pre-pressurization step (Step S 2 ) of the first preferred embodiment. After the pre-pressurization step (Step S 20 ) is completed, the controller  8  next provides an operating instruction to start the first purge step (Step S 30 ). 
       FIG. 15  is a flow diagram showing the details of the first purge step. Upon starting the first purge step, the controller  8  initially provides an operating instruction to bring the first head valve  313  into the open position (Step S 301 ). This causes the supply pipe  311  and the sub-tank  321  which are brought into the state of positive pressure by the pre-pressurization step (Step S 20 ) to communicate with the first head  21 . Thus, the ink inside the first head  21  is brought into the state of positive pressure, so that the ink is forced out of the nozzles  212  of the first head  21 . That is, the opening of the first head valve  313  starts the pressurized purge in the first head  21 . 
     Forcing the ink out of the nozzles  212  of the first head  21  in the aforementioned manner eliminates the problems of the increased viscosity and solidification of ink in the nozzles  212  of the first head  21 . Also, when the ink contains a precipitable ingredient, this eliminates the problem of the precipitation of the ink inside the first head  21 . 
     After the first head valve  313  is brought into the open position, the timer not shown measures the amount of time elapsed since the first head valve  313  was brought into the open position. The value measured by the timer is inputted as a signal to the controller  8 . The controller  8  judges whether a predetermined time period has elapsed since the first head valve  313  was brought into the open position or not (Step S 302 ). When the controller  8  judges in Step S 302  that the predetermined time period has elapsed (i.e., the answer to Step S 302  is YES), the first head valve  313  is brought into the closed position (Step S 303 ). This completes the first purge step (Step S 30 ). The second preferred embodiment differs from the first preferred embodiment in that the pressurizing valve  343  is maintained in the open position after the completion of the first purge step. 
     When the first head valve  313  is closed by the controller  8  in Step S 303 , the ink inside the first head  21  and the ink inside the supply pipe  311  are isolated from each other, so that the ink inside the sub-tank  321  and inside the supply pipe  311  stops pressurizing the ink inside the first head  21 . After the pressurization stops, the ink inside the first head  21  continues to be ejected from the nozzles  212  of the first head  21 . Thus, the pressure of the ink inside the first head  21  decreases gradually from the state of positive pressure toward atmospheric pressure. 
     Referring again to  FIG. 14 , after the first purge step is completed, the liquid ejecting apparatus  1  performs the second purge step (Step S 50 ) which brings the second head valve  315  into the open position to force ink out of the nozzles  212  of the second head  22 . 
       FIG. 16  is a flow diagram showing the details of the second purge step. Upon starting the second purge step, the controller  8  initially provides an operating instruction to bring the second head valve  315  into the open position (Step S 501 ) after a predetermined time period has elapsed since the operating instruction was provided. 
     The supply pipe  311  and the sub-tank  321  are also maintained in the state of positive pressure because the pressurizing valve  343  is maintained in the open position after the completion of the first purge step as described above. When the second head valve  315  is brought into the open position to cause the sub-tank  321  and the second head  22  to communicate with each other in such a state, the ink inside the second head  22  is brought into the state of positive pressure, so that the ink is forced out of the nozzles  212  of the second head  22 . That is, the opening of the second head valve  315  starts the pressurized purge in the second head  22 . 
     By bringing the second head valve  315  into the open position in Step S 501  after the predetermined time period has elapsed since the operating instruction was provided, the processes of forcing the ink out of the nozzles  212  of the first head  21  and the second head  22  are performed independently in terms of time with reliability. Further, this allows the pressure of the ink inside the sub-tank  321  prior to the execution of the second purge step to return to the state prior to the first purge step again during the predetermined time period. 
     Forcing the ink out of the nozzles  212  of the second head  22  in the aforementioned manner eliminates the problems of the increased viscosity and solidification of ink in the nozzles  212  of the second head  22 . Also, when the ink contains a precipitable ingredient, this eliminates the problem of the precipitation of the ink inside the second head  22 . 
     After the second head valve  315  is brought into the open position, the timer not shown measures the amount of time elapsed since the second head valve  315  was brought into the open position. The value measured by the timer is inputted as a signal to the controller  8 . The controller  8  judges whether a predetermined time period has elapsed since the second head valve  315  was brought into the open position or not (Step S 502 ). When the controller  8  judges in Step S 502  that the predetermined time period has elapsed (i.e., the answer to Step S 502  is YES), the pressurizing valve  343  is brought into the closed position (Step S 503 ). This completes the second purge step (Step S 50 ). 
     In the second purge step according to the second preferred embodiment, the second head valve  315  is not brought into the closed position after Step S 501  is performed to force the ink from the second head  22 . When the pressurizing valve  343  is closed by the controller  8  in Step S 503 , the communication the pressurizing pump  341  and the sub-tank  321  is closed off, so that the pressurization of the ink in the sub-tank  321  stops. After the pressurization stops, the ink inside the second head  22  continues to be ejected from the nozzles  212  of the second head  22 . Thus, the pressure of the ink inside the second head  22  decreases gradually from the state of positive pressure toward atmospheric pressure. 
     Referring again to  FIG. 14 , after the second purge step (Step S 50 ) is completed whereby the pressurized purges of the first head  21  and the second head  22  in each head unit  2  are completed, an open-to-atmosphere step (Step S 60 ) is next performed which reduces the pressure of the ink inside the first head  21  and the second head  22 . In the open-to-atmosphere step (Step S 60 ), the liquid ejecting apparatus  1  performs steps substantially similar to those of the open-to-atmosphere step (Step S 6 ) of the first preferred embodiment. 
     In Step S 61  in the open-to-atmosphere step of the first preferred embodiment, both the first head valve  313  and the second head valve  315  are switched from the closed position to the open position. However, the second head valve  315  is already in the open position when the open-to-atmosphere step of the second preferred embodiment is performed. Thus, in the step corresponding to Step S 61  in the second preferred embodiment, the first head valve  313  is switched from the closed position to the open position, and the second head valve  315  is maintained in the open position. The open-to-atmosphere step (Step S 60 ) of the second preferred embodiment differs in such a point from the open-to-atmosphere step (Step S 6 ) of the first preferred embodiment. The remaining steps in the open-to-atmosphere step (Step S 60 ) are performed similarly as in the first preferred embodiment. 
     After the open-to-atmosphere step (Step S 60 ) is performed, the meniscus formation step (Step S 70 ) is performed. In the meniscus formation step (Step S 70 ), the liquid ejecting apparatus  1  performs steps similar to those of the meniscus formation step (Step S 7 ) of the first preferred embodiment. 
     The recovery step of the liquid ejecting apparatus  1  in the second preferred embodiment is thus completed. The pressures inside the first head  21  and the second head  22  in the recovery step according to the second preferred embodiment will be described with reference to the flow diagram of  FIG. 14  and the timing diagram of  FIG. 17 , as appropriate. The open/closed positions of the valves and the pressures inside the heads are schematically denoted in  FIG. 17  in the same manner as in the timing diagram of  FIG. 13 . 
     Upon selecting the computer program  831  relating to the recovery process stored in the storage part  83 , the controller  8  instructs the components of the liquid ejecting apparatus  1  to perform the recovery process. At the time t 0 , the initialization step (Step S 10 ) starts. 
     The steps from the time t 0  to the time t 3  are similar to those of the first preferred embodiment, and will not be described. The time t 3  is a point of time at which the controller  8  judges that the sub-tank  321  is pressurized to not less than the first pressure. At this point of time, Step S 301  in the first purge step is performed, so that the first head valve  313  is brought into the open position. This starts the pressurization of the ink inside the first head  21 , so that the ink starts being forced out of the nozzles  212 . The pressure inside the first head  21  is increased to the pressure P 11 . The pressure P 11  is a “purge pressure” required to force the ink inside the first head  21  out of the nozzles  212  in the first purge step (Step S 30 ), and is a pressure applied to the first head  21  when the sub-tank  321  is pressurized to the first pressure. 
     In Step S 302  in the first purge step, a lapse of a predetermined time period since the time t 3  is monitored. The predetermined time period is selected, as appropriate, depending on purge conditions including the amount of ejected ink required for the first head  21  and the like. The time t 5  is a point of time at which the controller  8  judges that the predetermined time period has elapsed since the time t 3 . At this point of time, the first head valve  313  is brought into the closed position (Step S 303 ). 
     The time t 7  is a point of time after a predetermined time period has elapsed since the time t 5 . During this predetermined time period, the pressure of the ink inside the sub-tank  321  which is decreased from the pre-pressurization step when the first purge step is performed is increased up to a predetermined pressure (the “first pressure” in this preferred embodiment) by the pressurization of the pressurizing pump  341 . At the time t 7 , Step S 501  in the second purge step is performed, so that the second head valve  315  is brought into the open position. This starts the pressurization of the ink inside the second head  22 , so that the ink starts being forced out of the nozzles  212 . The pressure inside the second head  22  is increased to the pressure P 21 . The pressure P 21  is a “purge pressure” required to force the ink inside the second head  22  out of the nozzles  212  in the second purge step (Step S 50 ), and is a pressure applied to the second head  22  when the sub-tank  321  is pressurized to the first pressure. 
     In Step S 502  in the second purge step, a lapse of a predetermined time period since the time t 7  is monitored. The predetermined time period is selected, as appropriate, depending on purge conditions including the amount of ejected ink required for the second head  22  and the like. The time t 8  is a point of time at which the controller  8  judges that the predetermined time period has elapsed since the time t 7 . At this point of time, the pressurizing valve  343  is brought into the closed position (Step S 503 ). At the time t 8 , the first head valve  313  is in the closed position, and the second head valve  315  is in the open position. 
     At the time t 10 , Step S 61  in the open-to-atmosphere step is performed, so that the first head valve  313  is switched from the closed position to the open position, whereas the second head valve  315  is maintained in the open position. The pressures inside the first head  21  and the second head  22  which are caused to reach the pressure P 11  and the pressure P 21 , respectively, by the first purge step and the second purge step decrease gradually after the first purge step and the second purge step as the ink is forced out of the nozzles  212 . Thus, after the completion of the second purge step (the time t 8 ), there are cases in which the pressures inside the first head  21  and the second head  22  do not completely return to atmospheric pressure but in a state of positive pressure as shown in  FIG. 17 . 
     Subsequently, the steps from the time t 11  to the time t 13  are performed in the same manner as in  FIG. 12  of the first preferred embodiment. Specifically, at the time t 11 , the open-to-atmosphere valve  352  is brought into the open position, so that the pressures of the ink inside the first head  21  and the second head  22  decrease toward atmospheric pressure. Then, at the time t 12 , the open-to-atmosphere valve  352  is brought into the closed position. Next, at the time t 13 , the three-way valve  332  is switched from the pressurizing pipe communicating state to the depressurizing pipe communicating state. Thus, the pressures inside the first head  21  and the second head  22  are decreased to the meniscus pressures (i.e., the pressure P 13  and the pressure P 23 ) at which ink menisci are formed in the nozzles  212 . 
     In the liquid ejecting apparatus  1  according to the second preferred embodiment as described above, the execution of the open-to-atmosphere step (Step S 60 ) makes the force drawing the ink into the nozzles  212  weaker than that in the case where the open-to-atmosphere step is not performed, so that air bubbles are less prone to be drawn into the nozzles  212 . 
     In the second preferred embodiment, the first purge step (Step S 30 ) is performed after the pre-pressurization step (Step S 20 ) is performed, and thereafter the second head valve  315  is brought into the open position (Step S 501 ) after a lapse of the predetermined time period in the second purge step (Step S 50 ). In the first purge step, the first open/close part is brought into the open position to start forcing the ink from the first head  21 . This varies the pressure of the ink inside the sub-tank  321  from an initial state. In the second preferred embodiment, the provision of the predetermined delay time period after the operating instruction from the controller  8  in Step S 501  allows the pressure of the ink inside the sub-tank  321  prior to the execution of the second purge step to return again to the state prior to the first purge step. As a result, this provides equal purge conditions in the second purge step and in the aforementioned first purge step (Step S 30 ). 
     Further, the pressurized purge of the first head  21  and the pressurized purge of the second head  22  are executed separately in the second preferred embodiment. As compared with the simultaneous execution of the pressurized purges of the first head  21  and the second head  22 , the separate execution thereof suppresses a difference between the rate at which the ink supplied from the sub-tank  321  flows to the first head  21  and the rate at which the ink supplied from the sub-tank  321  flows to the second head  22  to provide more equal purge conditions. This reduces a difference between the amount of ink forced from the first head  21  during the execution of the first purge step and the amount of ink forced from the second head  22  during the execution of the second purge step to suppress unevenness in density during image recording and excess ink consumption in the first and second purge steps. 
     Furthermore, the number of times of switching between the open and closed positions of the valves in the second preferred embodiment is less than that in the first preferred embodiment. This shortens the time required for the switching of the valves to allow the recovery process to be performed using more simplified steps. 
     3. Third Preferred Embodiment 
     Next, a third preferred embodiment according to the present invention will be described. The configuration of the ink supply system  3  is not limited to that shown in  FIG. 4  in the first and second preferred embodiments for the practice of the present invention, but may be a configuration as shown in  FIG. 18  for use in the third preferred embodiment. 
     The third preferred embodiment is a preferred embodiment in which the supply system  3  of the first preferred embodiment is partially changed. Specifically, the number of heads is increased by one, so that each of the head units  2  includes three heads. A third head  23  newly added is a head which is not subjected to a purge in the recovery step. The external appearance of the liquid ejecting apparatus  1  according to the third preferred embodiment is substantially similar to that of the liquid ejecting apparatus  1  according to the first preferred embodiment shown in  FIGS. 1 to 3 , and will not be described. Also, the recovery step according to the third preferred embodiment is substantially similar to the recovery step according to the second preferred embodiment. Steps similar to those in the recovery step of the second preferred embodiment will not be described, as appropriate. 
     &lt;3-1. Supply System&gt; 
       FIG. 18  is a block diagram schematically showing the supply system  3  according to the third preferred embodiment. In  FIG. 18 , parts similar to those of the first preferred embodiment are designated by like reference numerals and characters, and will not be described, as appropriate. Differences from the first preferred embodiment will be described. 
     The supply system  3  of the third preferred embodiment differs from that of the first preferred embodiment in that the supply pipe  311  for causing the sub-tank  321  to communicate with the first branch pipe  312  and the second branch pipe  314  is not provided but the first branch pipe  312  and the second branch pipe  314  communicate directly with the sub-tank  321 . 
     Each of the head units  2  in the third preferred embodiment further includes the third head  23  having a nozzle surface  231  including nozzles  212 . The interior of the third head  23  is similar in configuration to that of the first head  21 . 
     The supply system  3  further includes a third branch pipe  316  for providing direct communication between the third head  23  and the sub-tank  321 , and a third head valve  317  interposed in the third branch pipe  316  and switchable between an open position which ensures the communication between the sub-tank  321  and the third head  23  and a closed position which closes off the communication between the sub-tank  321  and the third head  23 . A known valve may be used as the third head valve  317 . The third head valve  317  is electrically connected to the controller  8 . The third head valve  317  is switched between the open position and the closed position in response to an operating instruction from the controller  8 . 
     The supply system  3  of the third preferred embodiment further differs from that of the first preferred embodiment in that the three-way valve  332  for providing communication between the sub-tank  321 , the pressurizing pipe  342  and the depressurizing pipe  362 , and the pressure regulating pipe  331  are not provided, but the pressurizing pipe  342  and the depressurizing pipe  362  communicate directly with the sub-tank  321 . 
     Further, the pressure sensor  346  and the pressure sensor  366  are not provided in the supply system  3  of the third preferred embodiment. Thus, the controller  8  monitors the driving situations (e.g., a driving voltage and the amount of time elapsed since the driving started) of the pressurizing pump  341  and the depressurizing pump  361 , the opening/closing situations (e.g., the amount of time elapsed since the switching was done between the open and closed positions) of the valves and the like to thereby estimate the pressure of the ink inside the sub-tank  321 . 
     &lt;3-2. Recovery Step&gt; 
     Next, the recovery step according to the third preferred embodiment will be described with reference to  FIG. 19 .  FIG. 19  is a timing diagram of the recovery step according to the third preferred embodiment. The open/closed positions of the valves and the pressures inside the heads are schematically denoted in  FIG. 19  in the same manner as in the timing diagram of  FIG. 13 . The flow diagram of the recovery step according to the third preferred embodiment is substantially similar to that of the recovery step according to the second preferred embodiment shown in  FIG. 14  except the control of the third head valve  317 . Steps similar to those in the recovery step of the second preferred embodiment will not be described, as appropriate. 
     Upon selecting the computer program  831  relating to the recovery process stored in the storage part  83 , the controller  8  initially instructs the components of the liquid ejecting apparatus  1  to perform the recovery process. At the time t 0 , the initialization step (Step S 10 ) starts. The initialization step maintains the first head valve  313 , the second head valve  315 , the third head valve  317  and the depressurizing valve  363  in the open position, and maintains the pressurizing valve  343  in the closed position. Also, the driving of the pressurizing pump  341  starts. 
     Next, at the time t 1 , the first head valve  313 , the second head valve  315  and the third head valve  317  are brought into the closed position. Further, the depressurizing valve  363  is brought into the closed position, so that the depressurization of the ink inside the sub-tank  321  by the depressurizing pump  361  is stopped. At this time, the pressures inside the first head  21 , the second head  22  and the third head  23  are held at a state of negative pressure lower than atmospheric pressure for the formation of ink menisci in the nozzles  212 , and are the “meniscus pressure P 13 ”, the “meniscus pressure P 23 ” and a “meniscus pressure P 33 ”, respectively. 
     At the time t 2 , the pressurizing valve  343  is brought into the open position. Thus, the ink inside the sub-tank  321  is pressurized, but the first head  21 , the second head  22  and the third head  23  are not pressurized to maintain the meniscus pressures to some extent because the first head valve  313 , the second head valve  315  and the third head valve  317  are in the closed position. After the time t 2 , the controller  8  monitors whether the sub-tank  321  is pressurized to not less than the first pressure or not, based on the amount of time elapsed since the pressurizing valve  343  was brought into the open position. 
     The time t 3  is a point of time at which the controller  8  judges that the sub-tank  321  is pressurized to not less than the first pressure. At this point of time, Step S 301  in the first purge step is performed, so that the first head valve  313  is brought into the open position. This starts the pressurization of the ink inside the first head  21 , so that the ink starts being forced out of the nozzles  212 . The pressure inside the first head  21  is increased to the pressure P 11 . The pressure P 11  is a “purge pressure” required to force the ink inside the first head  21  out of the nozzles  212  in the first purge step (Step S 30 ), and is a pressure applied to the first head  21  when the sub-tank  321  is pressurized to the first pressure. Subsequently, the steps from the time t 3  to the time t 11  are performed in the same manner as in  FIG. 16  of the second preferred embodiment. The third head valve  317  is maintained in the closed position in the steps from the time t 3  to the time t 11 . 
     At the time t 11 , the open-to-atmosphere valve  352  is brought into the open position, so that the pressures of the ink inside the first head  21  and the second head  22  decrease toward atmospheric pressure. The controller  8  monitors the amount of time elapsed since the open-to-atmosphere valve  352  was brought into the open position to thereby judge whether the pressure of the ink inside the sub-tank  321  reaches not greater than a predetermined value or not. The predetermined value is atmospheric pressure in the third preferred embodiment. 
     When the controller  8  judges that the pressure of the ink inside the sub-tank  321  reaches not greater than atmospheric pressure because of a lapse of a predetermined time period since the time t 11 , the open-to-atmosphere valve  352  is brought into the closed position at the time t 12 . Next, the third head valve  317  and the depressurizing valve  363  are brought into the open position at the time t 13 . Thus, the pressures inside the first head  21 , the second head  22  and the third head  23  are decreased to the meniscus pressures (i.e., the pressure P 13 , the pressure P 23  and the pressure P 33 ) at which ink menisci are formed in the nozzles  212 . 
     In the liquid ejecting apparatus  1  according to the third preferred embodiment as described above, the execution of the open-to-atmosphere step (Step S 60 ) makes the force drawing the ink into the nozzles  212  weaker than that in the case where the open-to-atmosphere step is not performed, so that air bubbles are less prone to be drawn into the nozzles  212 . 
     In the third preferred embodiment, the first purge step (Step S 30 ) is performed after the pre-pressurization step (Step S 20 ) is performed, and thereafter the second head valve  315  is brought into the open position (Step S 501 ) after a lapse of the predetermined time period in the second purge step (Step S 50 ). In the first purge step, the first open/close part is brought into the open position to start forcing the ink from the first head  21 . This varies the pressure of the ink inside the sub-tank  321  from an initial state. In the third preferred embodiment, the provision of the predetermined delay time period after the operating instruction from the controller  8  in Step S 501  allows the pressure of the ink inside the sub-tank  321  prior to the execution of the second purge step to return again to the state prior to the first purge step. As a result, this provides equal purge conditions in the second purge step and in the aforementioned first purge step (Step S 30 ). 
     Further, the pressurized purge of the first head  21  and the pressurized purge of the second head  22  are executed separately in the third preferred embodiment. As compared with the simultaneous execution of the pressurized purges of the first head  21  and the second head  22 , the separate execution thereof suppresses a difference between the rate at which the ink supplied from the sub-tank  321  flows to the first head  21  and the rate at which the ink supplied from the sub-tank  321  flows to the second head  22  to provide more equal purge conditions. This reduces a difference between the amount of ink forced from the first head  21  during the execution of the first purge step and the amount of ink forced from the second head  22  during the execution of the second purge step to suppress unevenness in density during image recording and excess ink consumption in the first and second purge steps. 
     Further, the third preferred embodiment further includes the third head  23 , unlike the first and second preferred embodiments. The third head  23  is shorter in the amount of time elapsed since the previous image recording than the first head  21  and the second head  22 , and does not require the recovery operation using the pressurized purge. The third preferred embodiment sequentially performs the pressurized purges upon the first head  21  and the second head  22  among the three heads, and does not perform the pressurized purge on the third head  23 . This allows the selective purge of the heads which require the pressurized purge. The execution of no purge on the head which does not require the purge reduces the amount of ink consumption in the recovery operation. 
     The three-way valve  332 , the pressure sensor  346  and the pressure sensor  366  are not provided in the third preferred embodiment, unlike the first and second preferred embodiments. Thus, the liquid ejecting apparatus  1  according to the third preferred embodiment includes a smaller number of components. This allows the recovery process to be performed using a more simplified configuration. 
     4. Modifications 
     While the main preferred embodiments according to the present invention have been described hereinabove, the present invention is not limited to the aforementioned preferred embodiments. 
     In the third preferred embodiment are configured so that only the first and second heads are subjected to the purge, but the purge is not performed on the third head. These heads are not limited to this for the practice of the present invention. For example, the first, second and third heads may be configured so that only the first head is subjected to the purge, but the purge is not performed on the second and third heads. 
     The aforementioned configuration allows the purge to be selectively performed on a head in which, for example, an ejection failure occurs, whereas the purge is not performed on a head which does not require the purge. This reduces the amount of ink forced out of the nozzles in the recovery step. 
     The configuration of the details of the liquid ejecting apparatus may differ from that shown in the figures of the present invention. The components described in the aforementioned preferred embodiments and in the modifications may be combined together, as appropriate, without inconsistencies. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.