Patent Publication Number: US-7905584-B2

Title: Image forming apparatus and image forming method

Description:
TECHNICAL FIELD 
     The present specification describes an image forming apparatus and an image forming method, and more particularly an image forming apparatus and an image forming method for forming an image on a recording medium by discharging a liquid. 
     DISCUSSION OF THE BACKGROUND 
     An image forming apparatus, such as a copying machine, a printer, a facsimile machine, a plotter, or a multifunction printer having two or more of copying, printing, scanning, and facsimile functions, forms an image on a recording medium (e.g., a sheet) by a liquid discharging method. For example, a liquid discharging head (e.g., a recording head) discharges a liquid (e.g., an ink drop) onto a conveyed sheet to form an image on the sheet. 
     Image forming apparatuses using the liquid discharging method include a serial type image forming apparatus and a line type image forming apparatus. In the serial type image forming apparatus, a recording head is mounted on a carriage. While the carriage moves in a main scanning direction, the recording head discharges an ink drop onto a sheet intermittently fed in a direction perpendicular to the main scanning direction (i.e., a sub-scanning direction) so as to form an image on the sheet. In the line type image forming apparatus, a plurality of recording heads are attached to a carriage in a manner that nozzles of the recording heads are arranged in a main scanning direction (i.e., a width direction of a sheet) to form a nozzle train. While a sheet is conveyed in a direction perpendicular to the main scanning direction, the recording heads discharge an ink drop onto the sheet to form an image on the sheet. 
     In an exemplary background image forming apparatus, a detector for detecting an inner air pressure in a recording head is provided in the recording head. An adjuster for adjusting the inner air pressure in the recording head is provided in a liquid supplier for supplying ink to the recording head. For example, the adjuster includes a pump adjusting mechanism for adjusting pumping of ink from a tank containing ink to a nozzle of the recording head. Namely, the pump adjusting mechanism adjusts a height for which ink is pumped from a liquid level in the tank up to the nozzle of the recording head. While the recording head discharges an ink drop, the inner air pressure in the recording head is measured. The adjuster is controlled to maintain the inner air pressure in the recording head in a predetermined range. 
     In a liquid discharging head, ink in a nozzle of the liquid discharging head needs to form a proper meniscus so as to discharge an ink drop with a stable discharging property (e.g., ink volume and ink dropping speed) and to prevent dripping of ink from the nozzle. For example, when an image forming apparatus includes a head tank (e.g., a sub tank) for supplying ink to the liquid discharging head, a negative pressure is formed in the head tank. When an image forming apparatus includes a liquid cartridge (e.g., an ink cartridge), a liquid absorber provided in the liquid cartridge forms a negative pressure. 
     An image forming apparatus may be located in an aircraft to provide an in-flight service such as printing and copying. However, background image forming apparatuses may not properly work in the aircraft because the background image forming apparatuses are not designed for usage in the aircraft. For example, air pressure decreases in the aircraft as the aircraft ascends. Accordingly, outer air pressure becomes smaller than inner air pressure in a liquid discharging head. As a result, ink may drip from a nozzle of the liquid discharging head. 
     SUMMARY 
     This patent specification describes a novel image forming apparatus. One example of a novel image forming apparatus includes at least one liquid discharging head and a pressure release member. The at least one liquid discharging head is configured to discharge a liquid to form an image. The pressure release member is configured to adjust inner air pressure in the at least one liquid discharging head to be continuously equivalent to outer air pressure. 
     This patent specification further describes a novel image forming method. One example of a novel image forming method includes discharging a liquid with at least one liquid discharging head to form an image, and adjusting inner air pressure in the at least one liquid discharging head with a pressure release member so that the inner air pressure is continuously equivalent to outer air pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  is a plane view of an image forming device and a sub-scanning direction conveyer of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a perspective view of a head tank of the image forming device shown in  FIG. 2 ; 
         FIG. 4  is an exploded perspective view of the head tank shown in  FIG. 3 ; 
         FIG. 5  is a schematic side view of the head tank shown in  FIG. 3 ; 
         FIG. 6  is a schematic sectional view of the head tank taken on line A′-A′ shown in  FIG. 5 ; 
         FIG. 7  is a block diagram of a controller of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 8  is a top view of an air release driving mechanism and a pressure release mechanism of the image forming device shown in  FIG. 2 ; 
         FIG. 9  is a plane view of the air release driving mechanism shown in  FIG. 8 ; 
         FIG. 10  is a plane view of the air release driving mechanism shown in  FIG. 9  during an air release operation; 
         FIG. 11A  is a sectional view of an air release mechanism of the head tank shown in  FIG. 4 ; 
         FIG. 11B  is a sectional view of the air release mechanism shown in  FIG. 11A  during an air release operation; 
         FIG. 12  is a plane view of the pressure release mechanism shown in  FIG. 8 ; 
         FIG. 13  is a plane view of the pressure release mechanism shown in  FIG. 12  during an air release operation; 
         FIG. 14  is a plane view of a pressure release mechanism according to another exemplary embodiment; 
         FIG. 15  is a plane view of the pressure release mechanism shown in  FIG. 14  during an air release operation; 
         FIG. 16  is a flowchart illustrating an operation of a pressure release mechanism according to yet another exemplary embodiment; 
         FIG. 17  is a flowchart illustrating an operation of a pressure release mechanism according to yet another exemplary embodiment; 
         FIG. 18  is a block diagram of an image forming apparatus according to another exemplary embodiment; 
         FIG. 19  is a flowchart illustrating an operation of the image forming apparatus shown in  FIG. 18 ; 
         FIG. 20  is a block diagram of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 21  is a block diagram of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 22  is a flowchart illustrating an operation of the image forming apparatus shown in  FIG. 21 ; 
         FIG. 23  is a block diagram of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 24  is a block diagram of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 25  is a flowchart illustrating an operation of the image forming apparatus shown in  FIG. 24 ; 
         FIG. 26  is a block diagram of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 27  is a block diagram of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 28  is a flowchart illustrating an operation of the image forming apparatus shown in  FIG. 27 ; 
         FIG. 29  is a sectional view of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 30  is a block diagram of the image forming apparatus shown in  FIG. 29 ; 
         FIG. 31  is a flowchart illustrating an operation of the image forming apparatus shown in  FIG. 30 ; 
         FIG. 32  is a flowchart illustrating an operation of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 33  is a plane view of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 34  is a plane view of the image forming apparatus shown in  FIG. 33  during an air release operation; 
         FIG. 35  is a plane view of an image forming apparatus according to yet another exemplary embodiment; 
         FIG. 36  is a plane view of the image forming apparatus shown in  FIG. 35  during an air release operation; 
         FIG. 37  is a block diagram of the image forming apparatus shown in  FIG. 35 ; and 
         FIG. 38  is a flowchart illustrating an operation of the image forming apparatus shown in  FIG. 37 . 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 1 , an image forming apparatus  1  according to an exemplary embodiment is explained. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  includes an exposure glass cover  10 , a reader  11 , an engine unit  100 , ink cartridges  26 , a cartridge holder  26 A, a paper tray unit  4 , an output conveyer  6 , an output tray  7 , and a controller  300 . The reader  11  includes an exposure glass  12 , optical scanning systems  15  and  18 , a lens  19 , and a scanning element  20 . The optical scanning system  15  includes a light source  13  and a mirror  14 . The optical scanning system  18  includes minors  16  and  17 . The engine unit  100  includes an image forming device  2  and a sub-scanning direction conveyer  3 . The image forming device  2  includes a carriage guide  21 , a carriage  23 , recording heads  24 , and head tanks  25 . The sub-scanning direction conveyer  3  includes a conveying belt  31 , a conveying roller  32 , a tension roller  33 , a charging roller  34 , a guide  35 , pressing rollers  36 , a separating nail  37 , and a conveying roller pair  38 . The paper tray unit  4  includes a paper tray  41 , a feeding roller  42 , a friction pad  43 , a registration roller pair  44 , and a feeding motor  45 . The output conveyer  6  includes conveying roller pairs  61 ,  62 , and  63  and an output roller pair  64 . 
     The image forming apparatus  1  can be included in any of a copying machine, a printer, a facsimile machine, a plotter, and a multifunction printer including copying, printing, scanning, and facsimile functions. In this non-limiting exemplary embodiment, the image forming apparatus  1  functions as a color copying machine for forming a color image on a recording medium. 
     The exposure glass cover  10  is provided on the reader  11  and presses an original placed on the reader  11 . The reader  11  (e.g., a scanner) is disposed in an upper portion of the image forming apparatus  1  and above the output tray  7 , and scans an image on the original to generate image data (e.g., print data). The engine unit  100  has a structure unitizing the image forming device  2  with the sub-scanning direction conveyer  3 , and is attachable to and detachable from the image forming apparatus  1 . The image forming device  2  forms an image on a recording medium according to the image data generated by the reader  11 . The paper tray unit  4  loads a recording medium (e.g., a plurality of sheets P), which is not limited to paper. The paper tray unit  4  is disposed in a bottom portion of the image forming apparatus  1 . The paper tray unit  4  separates an uppermost sheet P from the other sheets P to feed the sheets P one by one towards the sub-scanning direction conveyer  3 . The sub-scanning direction conveyer  3  turns a direction in which a sheet P fed from the paper tray unit  4  is conveyed by about 90 degrees so that the sheet P opposes the image forming device  2 , and conveys the sheet P towards the output conveyer  6 . For example, the image forming device  2  discharges a liquid (e.g., an ink drop) onto a sheet P at an opposing position at which the image forming device  2  opposes the sub-scanning direction conveyer  3 , while the sheet P is intermittently conveyed by the sub-scanning direction conveyer  3 . The output conveyer  6  conveys the sheet P towards the output tray  7 . The output tray  7  is disposed in the upper portion of the image forming apparatus  1 , and receives the sheet P conveyed by the output conveyer  6 . 
     In the reader  11 , an original having an image thereon is placed on the exposure glass  12  facing down. The exposure glass cover  10  is provided on the exposure glass  12  and presses the original towards the exposure glass  12 . The optical scanning systems  15  and  18  move and scan the image on the original. The light source  13  irradiates light onto the original placed on the exposure glass  12 . The mirror  14  deflects the light reflected by the original towards the mirror  16 . The mirror  16  further deflects the light deflected by the mirror  14  towards the mirror  17 . The mirror  17  further deflects the light deflected by the mirror  16  towards the lens  19 . The lens  19  irradiates the light deflected by the mirror  17  towards the scanning element  20 . The scanning element  20  converts the light into an image signal. The image signal is digitized and processed to generate image data. 
     In the image forming device  2 , the carriage guide  21  (e.g., a guide rod) supports the carriage  23  together with a guide stay (not shown) in a state that the carriage  23  is movable in a main scanning direction. The carriage  23  carries the recording heads  24 . The recording heads  24  serve as liquid discharging heads for discharging an ink drop onto a sheet P sent from the paper tray unit  4  according to the image data generated by the wader  11 . The head tanks  25  are mounted on the carriage  23  and contain inks to be discharged from the recording heads  24 , respectively. 
     The ink cartridges  26  contain black, cyan, magenta, and yellow inks, respectively, and are attachable to and detachable from the cartridge holder  26 A disposed on a front of the image forming apparatus  1 . The black, cyan, magenta, and yellow inks contained in the ink cartridges  26  are supplied to the head tanks  25  via tubes (not shown), respectively. 
     In the paper tray unit  4 , the paper tray  41  loads a plurality of sheets P. The paper tray  41  is attachable to and detachable from the image forming apparatus  1 . The feeding roller  42  and the friction pad  43  feed the sheets P from the paper tray  41  one by one towards the registration roller pair  44 . The registration roller pair  44  feeds the sheet P fed by the feeding roller  42  and the friction pad  43  towards the sub-scanning direction conveyer  3 . The feeding motor  45  includes a HB (hybrid) type stepping motor and serves as a driver for rotatably driving the feeding roller  42  and the registration roller pair  44  via a feeding clutch (not shown). 
     In the sub-scanning direction conveyer  3 , the conveying belt  31  is formed in an endless belt-like shape and is looped over the conveying roller  32  and the tension roller  33 . The conveying roller  32  serves as a driving roller for rotatably driving the conveying belt  31 . The tension roller  33  serves as a driven roller for being rotatably driven by the conveying roller  32  via the conveying belt  31 , and applies tension to the conveying belt  31 . The charging roller  34  applies an alternating voltage (e.g., an alternating current bias voltage) to the conveying belt  31  to charge a surface of the conveying belt  31 . The guide  35  opposes the image forming device  2  and guides the rotating conveying belt  31 . The pressing rollers  36  oppose the conveying roller  32  via the conveying belt  31  and press the sheet P conveyed on the conveying belt  31  towards the conveying belt  31 . The separating nail  37  separates the sheet P bearing an image formed by the image forming device  2  from the conveying belt  31 . The conveying roller pair  38  feeds the sheet P separated from the conveying belt  31  towards the output conveyer  6 . 
     In the output conveyer  6 , the conveying roller pairs  61 ,  62 , and  63  feed the sheet P bearing the image and sent from the engine unit  100  towards the output roller pair  64 . The output roller pair  64  feeds the sheet P bearing the image onto the output tray  7 . 
     The controller  300  controls operations of the image forming apparatus  1 . 
       FIG. 2  is a plane view of the image forming device  2  and the sub-scanning direction conveyer  3 . As illustrated in  FIG. 2 , the image forming device  2  further includes a timing belt  29 , a driving pulley  28 A, a driven pulley  28 B, a main scanning motor  27 , a back stay  101 B, a front side plate  101 F, a rear side plate  101 R, a linear scale  128 , a photo sensor  129 , a maintenance-recovery mechanism  121 , and an idle discharge receiver  126 . The sub-scanning direction conveyer  3  further includes a sub-scanning motor  131 , a timing belt  132 , and a timing roller  133 . The recording heads  24  include four recording heads  24 K,  24 C,  24 M, and  24 Y. The head tanks  25  include head tanks  25 K,  25 C,  25 M, and  25 Y. The maintenance-recovery mechanism  121  includes caps  122 A,  122 B,  122 C, and  122 D, a wiper blade  124 , and an idle discharge receiver  125 . The idle discharge receiver  126  includes openings  127 A,  127 B,  127 C, and  127 D. 
     The timing belt  29  is looped over the driving pulley  28 A and the driven pulley  28 B. The main scanning motor  27  rotates the driving pulley  28 A. The rotating driving pulley  28 A rotates the timing belt  29 . The rotating timing belt  29  rotates the driven pulley  28 B. The carriage  23  is attached to the timing belt  29 . Thus, the main scanning motor  27  moves the carriage  23  via the driving pulley  28 A, the driven pulley  28 B, and the timing belt  29  in a main scanning direction (i.e., directions A). Namely, the rotating timing belt  29  moves the carriage  23  movably supported by the carriage guide  21  and a guide stay (not shown) provided on the back stay  101 B in the main scanning direction. The carriage guide  21  is bridged between the front side plate  101 F and the rear side plate  101 R. 
     The recording heads  24  are mounted on the carriage  23  and discharge an ink drop in a shuttle method. For example, while a sheet P is conveyed on the conveying belt  31  in a sub-scanning direction (i.e., a direction B), the recording heads  24  mounted on the carriage  23  and moving in the directions A discharge an ink drop onto the sheet P to form an image on the sheet P. The recording heads  24 K,  24 C,  24 M, and  24 Y discharge black, cyan, magenta, and yellow inks, respectively. The head tanks  25 K,  25 C,  25 M, and  25 Y mounted on the carriage  23  supply the black, cyan, magenta, and yellow inks to the recording heads  24 K,  24 C,  24 M, and  24 Y, respectively. 
     Multiple types of recording heads including piezo, thermal, and electrostatic types may be used as the recording heads  24 . The piezo type recording head uses a piezoelectric element as a pressure generator (e.g., an actuator) for applying pressure on ink in an ink flow route (e.g., a pressure generating room) to deform a vibration board forming walls of the ink flow route, so that a changed volume of the ink flow route discharges an ink drop. The thermal type recording head uses a heat generating resistance body to generate a bubble by boiling ink in an ink flow route, so that pressure of the bubble discharges an ink drop. The electrostatic type recording head uses a vibration board forming walls of an ink flow route and an electrode, which oppose each other, so that the vibration board deformed by an electrostatic force generated between the vibration board and the electrode changes a volume of the ink flow route and discharges an ink drop. The recording heads  24  may include one or more liquid discharging heads including a train of one or more nozzles for discharging liquid in one or more colors. According to this non-limiting exemplary embodiment, each of the recording heads  24 K,  24 C,  24 M, and  24 Y includes a nozzle for discharging a liquid drop in a single color. 
     The linear scale  128  (e.g., an encoder sheet) includes a slit and is stretched between the front side plate  101 F and the rear side plate  101 R along the main scanning direction in which the carriage  23  moves. The photo sensor  129  (e.g., a transmission photo sensor or an encoder sensor) is provided on the carriage  23  and detects the slit of the linear scale  128 . The linear scale  128  and the photo sensor  129  form a linear encoder for detecting movement of the carriage  23 . 
     The maintenance-recovery mechanism  121  is disposed in a non-printing area near one end of the carriage guide  21  in the main scanning direction in which the carriage  23  moves. The maintenance-recovery mechanism  121  maintains and recovers conditions of the nozzles of the recording heads  24 . The caps  122 A,  122 B,  122 C, and  122 D cap the nozzles of the recording heads  24 K,  24 C,  24 M, and  24 Y, respectively. The cap  122 A moisturizes the nozzle and sucks ink from the nozzle. The caps  122 B,  122 C, and  122 D moisturize the nozzles, respectively. The wiper blade  124  wipes the nozzles of the recording heads  24 . The idle discharge receiver  125  receives an ink drop discharged from the recording heads  24  during idle discharge and not used for printing. 
     The idle discharge receiver  126  is disposed in another non-printing area near the other end of the carriage guide  21  in the main scanning direction in which the carriage  23  moves. The openings  127 A,  127 B,  127 C, and  127 D receive ink drops discharged from the recording heads  24  during idle discharge and not used for printing. 
     The sub-scanning motor  131  rotates the timing belt  132 . The rotating timing belt  132  rotates the timing roller  133 . The rotating timing roller  133  rotates the conveying roller  32 . The rotating conveying roller  32  rotates the conveying belt  31  in a sheet conveyance direction (i.e., the subscanning direction or the direction B). 
     Referring to  FIGS. 3 to 6 , the following describes one example of the head tank  25 .  FIG. 3  is a perspective view of the head tank  25 .  FIG. 4  is an exploded perspective view of the head tank  25 .  FIG. 5  is a schematic side view of the head tank  25 .  FIG. 6  is a schematic sectional view of the head tank  25  taken on line A′-A′ shown in  FIG. 5 . 
     As illustrated in  FIG. 4 , the head tank  25  (e.g., the head tank  25 K,  25 C,  25 M, or  25 Y depicted in  FIG. 2 ) includes an ink container  200 , a case  201 , a film member  202 , an elastic member  203 , a bulge  202 A, a reinforcing member  204 , a full detecting lever  206 , supports  207 , an ink tube  216 , connectors  212  and  213 , a filter  215 , an air flow path  221 , an air release opening  231 , a storage  226 , an air release mechanism  232 , and detecting electrodes  241  and  242 . The air release mechanism  232  includes a holder  233 , a valve seat  234 , a ball  235 , a spring  236 , and a pressing member  237 . 
     As illustrated in  FIG. 5 , the head tank  25  further includes an ink input path  211  and an ink output path  214 . The air flow path  221  includes an entrance path  222  and a flow path  223 . 
     As illustrated in  FIG. 4 , the ink container  200  contains ink. The case  201  forms the ink container  200 . The film member  202  has flexibility and is attached (e.g., adhered or welded) to the case  201  to seal an opening of the ink container  200 . The elastic member  203  (e.g., a spring) is provided between the case  201  and the film member  202  to apply a force to the film member  202  in a direction separating the film member  202  away from the case  201 . The bulge  202 A is formed on the film member  202  to correspond to the elastic member  203 . The reinforcing member  204  is attached to an outer surface of the bulge  202 A. 
     The full detecting lever  206  detects a full condition in which the ink container  200  is full of ink. The supports  207  are provided on one side of the case  201  and swingably support the full detecting lever  206 . 
     As illustrated in  FIG. 5 , the ink input path  211  is provided in the case  201  and guides ink to the ink container  200 . The ink tube  216  is connected to the ink cartridge  26  (depicted in  FIG. 1 ). The connector  212  connects the ink tube  216  with the ink input path  211 , and is attachable to and detachable from the ink input path  211 . 
     The connector  213  is attached to a bottom of the case  201  and supplies ink from the ink container  200  to the recording head  24  (depicted in  FIG. 3 ). The ink output path  214  is provided in the connector  213  and guides ink from the ink container  200  to the recording head  24 . The filter  215  is provided between the ink container  200  and the connector  213 . 
     The air flow path  221  is provided in an upper portion of the case  201  and conveys air from the ink container  200 . The entrance path  222  includes an opening connected to the ink container  200 . The flow path  223  is provided downstream from the entrance path  222  in an air conveyance direction. A downstream portion of the flow path  223  is connected to the air release opening  231  provided in the case  201 . The storage  226  is disposed at a position lower than the air release opening  231 . 
     As illustrated in  FIG. 4 , the air release mechanism  232  is provided in the air release opening  231 . In the air release mechanism  232 , the holder  233  holds the valve seat  234 , the ball  235 , and the spring  236 . The spring  236  applies a force to the ball  235 , serving as a valve disc, in a direction in which the ball  235  contacts the valve seat  234 . The pressing member  237  applies a pressure to the ball  235 . 
     The two detecting electrodes  241  and  242  are attached to the upper portion of the case  201  and detect whether the ink container  200  is empty or almost empty. 
     When the pressing member  237  presses the ball  235 , the air release opening  231  is opened so that air in the ink container  200  is released through the air flow path  221  and the air release opening  231 . Thus, ink is pressingly conveyed from the ink cartridge  26  (depicted in  FIG. 1 ) to the head tank  25 . As a result, the head tank  25  is replenished with ink. Accordingly, air in the head tank  25  is output through the air flow path  221  and the air release mechanism  232 . 
     When the air release mechanism  232  closes the air release opening  231 , air in the ink container  200  is not released. Simultaneously, the cap  122 A (depicted in  FIG. 2 ) caps the recording head  24  (depicted in  FIG. 2 ). Thus, ink in the head tank  25  is sucked via the nozzle of the recording head  24 . Since the elastic member  203  applies a force to the film member  202 , a negative pressure generates in the head tank  25 . 
     The negative pressure generated in the head tank  25  is applied to the recording head  24 . A meniscus is formed at a reference position in the nozzle of the recording head  24 , resulting in a stable ink discharge. 
     Referring to  FIG. 7 , the following describes the controller  300 . As illustrated in  FIG. 7 , the image forming apparatus  1  further includes a communication circuit  303 , a main scanning motor driving circuit  311 , a sub-scanning motor driving circuit  312 , a carriage position detecting circuit  313 , a distance detecting circuit  314 , an AC (alternating current) bias supplier  315 , a feeding motor driving circuit  316 , a maintenance-recovery mechanism driving circuit  317 , a scanner controller  318 , a control panel  319 , an air release mechanism driving circuit  323 , a pressure release mechanism driving circuit  324 , and a head driving circuit  321 . The controller  300  includes a main controller  301  and a print controller  302 . 
     The main controller  301  includes a microcomputer for controlling overall operations of the image forming apparatus  1 . The microcomputer includes a CPU (central processing unit), a ROM (read-only memory), a RAM (random-access memory), a VRAM (video random-access memory), and an I/O (input output). The print controller  302  includes a microcomputer for controlling print operations of the image forming apparatus  1 . 
     The communication circuit  303  sends information about print processing to the main controller  301 . The main scanning motor driving circuit  311  drives the main scanning motor  27  (depicted in  FIG. 2 ). The sub-scanning motor driving circuit  312  drives the sub-scanning motor  131  (depicted in  FIG. 2 ). The main controller  301  controls driving of the main scanning motor  27  and the sub-scanning motor  131  via the main scanning motor driving circuit  311  and the sub-scanning motor driving circuit  312 , respectively, so as to form an image on a sheet P based on the information about print processing sent from the communication circuit  303 . The main controller  301  sends print data to the print controller  302 . 
     The carriage position detecting circuit  313  detects a position of the carriage  23  (depicted in  FIG. 2 ) and sends a detection signal corresponding to the detected position to the main controller  301 . The main controller  301  controls movement (e.g., a position to which the carriage  23  moves and a moving speed of the carriage  23 ) of the carriage  23  based on the detection signal. For example, the carriage position detecting circuit  313  detects the position of the carriage  23  by calculating the number of slits provided on the linear scale  128  (depicted in  FIG. 2 ) disposed along the main scanning direction in which the carriage  23  moves. The photo sensor  129  (depicted in  FIG. 2 ) mounted on the carriage  23  reads the slits. The main scanning motor driving circuit  311  rotatably drives the main scanning motor  27  in accordance with an output value sent from the main controller  301  and corresponding to a moving distance of the carriage  23  so as to move the carriage  23  to a predetermined position at a predetermined speed. The output value includes a PWM (pulse width modulation) output value when PWM control is performed. 
     The distance detecting circuit  314  detects a moving distance of the conveying belt  31  (depicted in  FIG. 2 ) and sends a detection signal corresponding to the detected distance to the main controller  301 . The main controller  301  controls movement (e.g., a moving distance and a moving speed) of the conveying belt  31  based on the detection signal. For example, the distance detecting circuit  314  detects the moving distance of the conveying belt  31  by calculating the number of slits provided on an encoder wheel (not shown) attached to a shaft of the conveying roller  32  (depicted in  FIG. 2 ). A photo sensor (not shown), such as an encoder sensor, reads the slits. The sub-scanning motor driving circuit  312  rotatably drives the sub-scanning motor  131  in accordance with the moving distance sent from the main controller  301  so as to rotate the conveying roller  32 . The rotating conveying roller  32  moves the conveying belt  31  to a predetermined position at a predetermined speed. 
     The main controller  301  controls charging of the conveying belt  31  via the AC bias supplier  315  for applying an alternating current bias to the charging roller  34  (depicted in  FIG. 1 ). The main controller  301  controls driving of the feeding motor  45  (depicted in  FIG. 1 ) via the feeding motor driving circuit  316  for rotatably driving the feeding motor  45 . The main controller  301  controls driving of a motor (not shown) for driving the maintenance-recovery mechanism  121  (depicted in  FIG. 2 ) via the maintenance-recovery mechanism driving circuit  317  for rotatably driving the motor. For example, the motor lifts and lowers the caps  122 A,  122 B,  122 C, and  122 D and the wiper blade  124  (depicted in  FIG. 2 ), and drives a sucking pump (not shown). 
     The main controller  301  controls driving of the reader  11  (depicted in  FIG. 1 ) via the scanner controller  318  for driving the reader  11 . The main controller  301  sends information to be displayed on the control panel  319  to the control panel  319  and receives information specified by a user on the control panel  319  from the control panel  319 . 
     The main controller  301  controls driving of a driver (e.g., a solenoid and a motor) of an air release driving mechanism (described below) via the air release mechanism driving circuit  323 . The air release mechanism driving circuit  323  drives the air release driving mechanism for opening and closing the air release mechanism  232  of each of the head tanks  25  (depicted in  FIG. 4 ). The main controller  301  controls driving of a driver (e.g., a solenoid and a motor) of a pressure release mechanism (described below) via the pressure release mechanism driving circuit  324 . The pressure release mechanism driving circuit  324  drives the pressure release mechanism for opening and closing the air release mechanism  232  of each of the four head tanks  25 . 
     The print controller  302  generates data for driving a pressure generator (not shown) for causing the recording head  24  to discharge an ink drop based on a signal sent from the main controller  301 , the position of the carriage  23  detected by the carriage position detecting circuit  313 , and the moving distance of the conveying belt  31  detected by the distance detecting circuit  314 . The print controller  302  transfers the image data as serial data to the head driving circuit  321 . For example, the print controller  302  sends a transfer clock for transferring the image data and confirming transfer of the image data, a latch signal, and an ink control signal (e.g., a mask signal) to the head driving circuit  321 . The print controller  302  includes a driving wave generator (not shown) and a driving wave selector (not shown). The driving wave generator includes a D/A (digital-analog) converter (not shown) for performing digital-analog conversion on pattern data of a driving signal stored in the ROM, a voltage amplifier (not shown), and a current amplifier (not shown). The driving wave selector selects a driving wave sent to a head driver (not shown). Namely, the print controller  302  generates a driving wave formed of one or more driving pulses (e.g., driving signals) and sends the driving wave to the head driving circuit  321 . 
     The head driving circuit  321  drives the recording head  24  by selectively applying a driving signal to a driving element (e.g., the piezoelectric element described above). The driving signal forms a driving wave provided by the print controller  302  based on image data serially input and corresponding to one line image formed by ink drops discharged by the recording head  24 . The driving element generates energy for causing the recording head  24  to discharge an ink drop. A driving pulse forming a driving wave may be selected to cause the recording head  24  to selectively discharge ink drops (e.g., dots) of different sizes. For example, the recording head  24  may discharge a large ink drop (e.g., a large dot), a medium ink drop (e.g., a medium dot), or a small ink drop (e.g., a small dot). 
     An amount of rotation of the conveying roller  32  for driving the conveying belt  31  is detected so as to control driving of the sub-scanning motor  131  based on the detected amount. The AC bias supplier  315  applies an alternating voltage (i.e., a square-wave, high voltage having positive and negative polarities) to the charging roller  34 . Electric charges having positive and negative polarities are alternately applied to the conveying belt  31  to form strips in the sheet conveyance direction of the conveying belt  31 . The conveying belt  31  is charged in a predetermined charging width to form an uneven electric field. 
     As illustrated in  FIG. 1 , a sheet P is fed from the paper tray unit  4  towards a nip formed between the conveying roller  32  and the pressing roller  36  via the conveying belt  31 . The pressing roller  36  and the conveying roller  32  feed the sheet P onto the conveying belt  31  where electric charges having positive and negative polarities form an uneven electric field. The sheet P is instantly polarized in accordance with directions of the electric field. The rotating conveying belt  31  electrostatically attracts and conveys the sheet P. 
     While the conveying belt  31  intermittently conveys the sheet P, the recording heads  24  discharge ink drops onto the sheet P to form an image on the sheet P. When the separating nail  37  touches a foremost head of the sheet P, the separating nail  37  separates the sheet P from the conveying belt  31  and the conveying roller pair  38  feeds the sheet P towards the output conveyer  6 . 
     As illustrated in  FIG. 2 , when the image forming device  2  is in a standby mode and thereby does not perform a print operation, the carriage  23  is moved to the maintenance-recovery mechanism  121  and the caps  122 A,  122 B,  122 C, and  122 D cap the nozzles of the recording heads  24  to enable the nozzles to retain moisture, respectively. Thus, inks in the nozzles are not dried, preventing a faulty discharge of inks. When the cap  122 A for moisturizing the nozzles and sucking ink from the nozzles caps the recording heads  24 , viscous ink and bubbles are sucked from the nozzles in a recovery operation. The wiper blade  124  wipes the nozzles of the recording heads  24  to remove inks adhered to the nozzles during the recovery operation. The recording heads  24  discharge ink not used for the print operation towards the idle discharge receiver  125  before and during the print operation. Thus, the recording heads  24  may maintain a stable discharging performance. 
     Referring to  FIGS. 8 to 13 , the following describes an air release driving mechanism and a pressure release mechanism according to an exemplary embodiment. 
     As illustrated in  FIG. 8 , the image forming device  2  further includes an air release driving mechanism  401  and a pressure release mechanism  402 . The ink tube  216  includes ink tubes  216 K,  216 C,  216 M, and  216 Y. 
     The air release driving mechanism  401  drives the air release mechanism  232  (depicted in  FIG. 4 ) provided in each of the head tanks  25 K,  25 C,  25 M, and  25 Y. The pressure release mechanism  402 , serving as a pressure release member, drives the air release mechanisms  232  provided in the head tanks  25 K,  25 C,  25 M, and  25 Y, collectively or simultaneously. The air release driving mechanism  401  and the pressure release mechanism  402  are disposed at positions different from each other in the main scanning direction in which the carriage  23  moves. The air release driving mechanism  401  is disposed near the maintenance-recovery mechanism  121  (depicted in  FIG. 2 ). The ink tubes  216 K,  216 C,  216 M, and  216 Y are connected to the ink cartridges  26  (depicted in  FIG. 1 ) and convey black, cyan, magenta, and yellow inks to the ink containers  200  (depicted in  FIG. 4 ) included in the head tanks  25 K,  25 C,  25 M, and  25 Y, respectively. 
       FIG. 9  is a plane view of the air release driving mechanism  401 . As illustrated in  FIG. 9 , the air release driving mechanism  401  includes a solenoid  411 , a support shaft  414 , an air release lever  412 , and a spring  413 . The solenoid  411  includes a plunger  411 A. 
     The solenoid  411  serves as a driver. The support shaft  414  rotatably supports the air release lever  412 . One end of the air release lever  412  is rotatably connected with the plunger  411 A. The spring  413  maintains the air release lever  412  at a position that does not contact the pressing member  237  of the air release mechanism  232 . Namely, the spring  413  closes the air release mechanism  232 . 
     As illustrated in  FIG. 8 , to replenish the head tank  25 Y with a yellow ink when air is released, for example, the carriage  23  moves to a position illustrated in a broken line. Accordingly, the head tank  25 Y moves to a position corresponding to the air release lever  412  (depicted in  FIG. 9 ) of the air release driving mechanism  401 . As illustrated in  FIG. 10 , the solenoid  411  is driven to pull the plunger  411 A in a direction C. Accordingly, the air release lever  412  rotates in a rotating direction D. The rotating air release lever  412  presses the pressing member  237  of the air release mechanism  232 . 
     As a result, the ball  235 , which closes the air release mechanism  232  as illustrated in  FIG. 11A , is pressed by the pressing member  237  towards the spring  236  as illustrated in  FIG. 11B . Thus, the air release mechanism  232  is released (e.g., opened) and the inside of the head tank  25 Y (depicted in  FIG. 8 ) becomes open to air. Accordingly, a yellow ink is supplied from the ink cartridge  26  (depicted in  FIG. 1 ). 
     To replenish another head tank (e.g., the head tank  25 K,  25 C, or  25 M depicted in  FIG. 8 ) with a corresponding (e.g., black, cyan, or magenta ink), such head tank  25 K,  25 C, or  25 M moves to a position corresponding to the air release lever  412  (depicted in  FIG. 9 ). In the head tank  25 K,  25 C, or  25 M, the air release mechanism  232  is released as illustrated in  FIG. 11B  and the inside of the head tank  25 K,  25 C, or  25 M becomes open to air. Accordingly, a black, cyan, or magenta ink is supplied from the ink cartridge  26 . 
       FIG. 12  is a plane view of the pressure release mechanism  402 . As illustrated in  FIG. 12 , the pressure release mechanism  402  includes a solenoid  421 , a support shaft  424 , an air release lever  422 , and a spring  423 . The solenoid  421  includes a plunger  421 A. The air release mechanism  232  includes air release mechanisms  232 K,  232 C,  232 M, and  232 Y. 
     The solenoid  421  serves as a driver. The support shaft  424  rotatably supports the air release lever  422 . One end of the air release lever  422  is rotatably connected with the plunger  421 A. The spring  423  maintains the air release lever  422  not to contact the pressing members  237  of the air release mechanisms  232 K,  232 C,  232 M, and  232 Y, respectively. Namely, the spring  423  closes the air release mechanisms  232 K,  232 C,  232 M, and  232 Y. 
     The air release mechanisms  232 K,  232 C,  232 M, and  232 Y are provided in the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively. The air release lever  422  has a length causing the air release mechanisms  232 K,  232 C,  232 M, and  232 Y included in the head tanks  25 K,  25 C,  25 M, and  25 Y, respectively, mounted on the carriage  23  (depicted in  FIG. 8 ) to oppose the air release lever  422 . 
     To cause inner air pressures in the head tanks  25 K,  25 C,  25 M, and  25 Y (i.e., inner air pressures in the recording heads  24 K,  24 C,  24 M, and  24 Y), respectively, to be continuously equivalent to an outer air pressure, the carriage  23  moves to a position illustrated in a solid line in  FIG. 8 , so that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y included in the head tanks  25 K,  25 C,  25 M, and  25 Y, respectively, oppose the air release lever  422 , as illustrated in  FIG. 13 . 
     As illustrated in  FIG. 13 , when the solenoid  421  is driven to pull the plunger  421 A in a direction E, the air release lever  422  rotates in a rotating direction F. The rotating air release lever  422  presses the pressing members  237  of the air release mechanisms  232 K,  232 C,  232 M, and  232 Y, respectively. Thus, almost simultaneously, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are opened. 
     Accordingly, inner air pressures in the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively, become equivalent to an outer air pressure in an environment where the image forming apparatus  1  (depicted in  FIG. 1 ) is located. When the image forming apparatus  1  is located in an aircraft, for example, the pressure release mechanism  402  maintains the head tanks  25 K,  25 C,  25 M, and  25 Y to continuously release air when the outer air pressure decreases as the aircraft ascends. The inner air pressures in the head tanks  25 K,  25 C,  25 M, and  25 Y, respectively, do not become relatively greater than the outer air pressure, preventing an ink drop from dripping from the nozzles of the recording heads  24  (depicted in  FIG. 1 ). 
     As described above, according to this non-limiting exemplary embodiment, the image forming apparatus  1  includes a plurality of liquid discharging heads (e.g., the recording heads  24 K,  24 C,  24 M, and  24 Y depicted in  FIG. 8 ) for discharging liquids in colors different from each other and a pressure release member (e.g., the pressure release mechanism  402  depicted in  FIG. 8 ) for causing inner air pressures in the plurality of the liquid discharging heads, respectively, to be continuously equivalent to an outer air pressure in an environment where the image forming apparatus  1  is located. Even when the image forming apparatus  1  is located in an environment having a decreased air pressure (e.g., an aircraft or a highland), the pressure release member causes the inner air pressures in the liquid discharging heads, respectively, to be continuously equivalent to the outer air pressure in the environment. Namely, the inner air pressures in the liquid discharging heads, respectively, do not become relatively greater than the outer air pressure in the environment. As a result, an ink drop does not drip from the nozzles of the liquid discharging heads. 
     Referring to  FIGS. 14 and 15 , the following describes a pressure release mechanism  402 A according to another exemplary embodiment. As illustrated in  FIG. 14 , the pressure release mechanism  402 A includes a motor  431 , a pinion  435 , a rack  436 , a moving body  433 , an air release lever  432 , and a support shaft  434 . The motor  431  includes a rotating shaft  431 A. 
     The pressure release mechanism  402 A serves as a pressure release member. The motor  431  serves as a driver for generating a driving force. The pinion  435  is attached to the rotating shaft  431 A of the motor  431 . The rack  436  is attached to the moving body  433 . Namely, the pinion  435  and the rack  436  convert a rotating direction of the driving force generated by the motor  431  into a straight direction for moving the moving body  433 . The moving body  433  moves to swing the air release lever  432 . The support shaft  434  rotatably supports the air release lever  432 . 
     One end of the air release lever  432  rotatably supports the moving body  433 . The other end of the air release lever  432  has a length such that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y included in the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively, oppose such other end of the air release lever  432 . 
     To cause inner air pressures in the head tanks  25 K,  25 C,  25 M, and  25 Y (i.e., inner air pressures in the recording heads  24 K,  24 C,  24 M, and  24 Y depicted in  FIG. 8 ), respectively, to be continuously equivalent to an outer air pressure, the carriage  23  moves to a position illustrated in the solid line in  FIG. 8 , so that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y included in the head tanks  25 K,  25 C,  25 M, and  25 Y, respectively, oppose the air release lever  432 , as illustrated in  FIG. 15 . 
     As illustrated in  FIG. 15 , when the motor  431  is driven in a direction G to move the moving body  433  via the pinion  435  and the rack  436 , the air release lever  432  rotates in a rotating direction H. The rotating air release lever  432  presses the pressing members  237  of the air release mechanisms  232 K,  232 C,  232 M, and  232 Y, respectively. Thus, almost simultaneously, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are opened. 
     Accordingly, inner air pressures in the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively, become equivalent to an outer air pressure in an environment where the image forming apparatus  1  (depicted in  FIG. 1 ) is located. Thus, the pressure release mechanism  402 A may provide effects common to the pressure release mechanism  402  (depicted in  FIG. 13 ). 
     In the pressure release mechanism  402  illustrated in  FIG. 13 , the solenoid  421  does not move when the image forming apparatus  1  is powered off. For example, when the image forming apparatus  1  is powered off after air is released, the air release lever  422  returns to a default position illustrated in  FIG. 12 . Thus, air may not be continuously released. In the pressure release mechanism  402 A illustrated in  FIG. 15 , however, the motor  431  rotates forward and backward to move the pinion  435  and the rack  436 . Therefore, even when the image forming apparatus  1  is powered off after air is released, the air release lever  432  remains at an air release position at which the air release lever  432  presses the pressing members  237  of the air release mechanisms  232 K,  232 C,  232 M, and  232 Y, respectively. Thus, air may be continuously released. 
     In the pressure release mechanism  402  or  402 A, a single, large air release lever (e.g., the air release lever  422  or  432 ) causes the air release mechanisms  232 K,  232 C,  232 M, and  232 Y to release air. However, an air release lever may be provided for each of the air release mechanisms  232 K,  232 C,  232 M, and  232 Y. Namely, one or more air release levers may cause the air release mechanisms  232 K,  232 C,  232 M, and  232 Y to release air, so that inner air pressures in the head tanks  25 K,  25 C,  25 M, and  25 Y, respectively, are continuously equivalent to an outer air pressure. 
     Referring to  FIG. 16 , the following describes an example of operation of the pressure release mechanism  402  (depicted in  FIG. 13 ) according to yet another exemplary embodiment  FIG. 16  is a flowchart of the example of operation of the pressure release mechanism  402 . According to this non-limiting exemplary embodiment, the pressure release mechanism  402  is provided near the maintenance-recovery mechanism  121  (depicted in  FIG. 2 ). The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     In step S 101 , the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ) perform an image forming operation. For example, the recording heads  24 K,  24 C,  24 M, and  24 Y discharge an ink drop to form an image. In step S 102 , the controller  300  (depicted in  FIG. 7 ) determines whether or not the image forming operation is finished. If the image forming operation is finished and thereby the image forming apparatus  1  (depicted in  FIG. 1 ) is in a standby mode (i.e., YES is selected in step S 102 ), the carriage  23  (depicted in  FIG. 2 ) moves to a capping position at which the caps  122 A,  122 B,  122 C, and  122 D (depicted in  FIG. 2 ) cap the recording heads  24 K,  24 C,  24 M, and  24 Y, respectively, in step S 103 . In step S 104 , the caps  122 A,  122 B,  122 C, and  122 D cap the recording heads  24 K,  24 C,  24 M, and  24 Y, respectively. In step S 105 , the pressure release mechanism  402  (depicted in  FIG. 13 ) is activated to cause the air release mechanisms  232 K,  232 C,  232 M, and  232 Y to release air. Namely, the pressure release mechanism  402  continuously releases pressure. 
     When a next image forming operation starts, the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 2 ) perform a negative pressure forming operation. In the negative pressure forming operation, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are closed, and the recording heads  24 K,  24 C,  24 M, and  24 Y discharge a predetermined amount of ink. Namely, an amount of ink contained in the head tanks  25 K,  25 C,  25 M, and  25 Y decreases while the head tanks  25 K,  25 C,  25 M, and  25 Y are closed. Thus, the inside of the head tanks  25 K,  25 C,  25 M, and  25 Y has a negative pressure. 
     As described above, in a standby mode after the image forming operation is finished, the pressure release mechanism  402  is activated to adjust inner air pressures in the plurality of the recording heads  24 K,  24 C,  24 M, and  24 Y, respectively, to be continuously equivalent to an outer air pressure, preventing an ink drop from dripping from the recording heads  24 K,  24 C,  24 M, and  24 Y. 
     Referring to  FIG. 17 , the following describes another example of operation of the pressure release mechanism  402  (depicted in  FIG. 13 ) according to yet another exemplary embodiment  FIG. 17  is a flowchart of the example of operation of the pressure release mechanism  402 . According to this non-limiting exemplary embodiment, a user operates the control panel  319  (depicted in  FIG. 7 ) to activate the pressure release mechanism  402 . The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     In step S 201 , the controller  300  (depicted in  FIG. 7 ) determines whether or not a user specifies a continuous pressure release by using the control panel  319 . If the user specifies the continuous pressure release (i.e., if YES is selected in step S 201 ), the carriage  23  (depicted in  FIG. 2 ) moves to a capping position at which the caps  122 A,  122 B,  122 C, and  122 D (depicted in  FIG. 2 ) cap the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ), respectively, in step S 202 . In step S 203 , the pressure release mechanism  402  is activated (e.g., a driver drives the pressure release mechanism  402 ) to cause the air release mechanisms  232 K,  232 C,  232 M, and  232 Y (depicted in  FIG. 13 ) to release air. Namely, the pressure release mechanism  402  continuously releases pressure. In step S 204 , the controller  300  sets a print reject mode in which no print job (e.g., image forming job) is accepted. 
     If the user cancels the continuous pressure release by using the control panel  319  (i.e., if NO is selected in step S 201 ), the controller  300  determines whether or not the pressure release mechanism  402  is deactivated (e.g., whether or not the continuous pressure release is canceled) in step S 205 . If the pressure release mechanism  402  is deactivated (i.e., if YES is selected in step S 205 ), the air release mechanisms  232 K,  232 C,  232 M, and  232 Y do not continuously release air in step S 206 . In step S 207 , a negative pressure forming operation is performed for the recording heads  24 K,  24 C,  24 M, and  24 Y. In step S 208 , the controller  300  cancels the print reject mode so that an image forming operation is performed. 
     As described above, a user inputs a command for activating the pressure release mechanism  402  by using the control panel  319 . Namely, the user may optionally select a mode for preventing an ink drop from dripping from the recording heads  24 K,  24 C,  24 M, and  24 Y. Thus, the image forming apparatus  1  (depicted in  FIG. 1 ) may prevent dripping of an ink drop caused by change in outer air pressure. 
     Referring to  FIGS. 18 and 19 , the following describes yet another example of operation of the pressure release mechanism  402  (depicted in  FIG. 13 ) according to yet another exemplary embodiment.  FIG. 18  is a block diagram of an image forming apparatus  1 A according to yet another exemplary embodiment.  FIG. 19  is a flowchart of the example of operation of the pressure release mechanism  402 . The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 18 , the image forming apparatus  1 A includes a barometer  451 , a drip preventing controller  452 , and a print controller  453 . The other elements of the image forming apparatus  1 A are common to the image forming apparatus  1  depicted in  FIG. 1 . The barometer  451  serves as an outer air pressure detector for measuring (e.g., detecting) an outer air pressure. The drip preventing controller  452  serves as a pressure release member activator for activating the pressure release member. For example, the drip preventing controller  452  controls an operation of the pressure release mechanism  402  based on a measurement result (e.g., a detection result) provided by the barometer  451 , and sends a command for entering a standby mode to the print controller  453 . The main controller  301  (depicted in  FIG. 7 ) includes the drip preventing controller  452  and the print controller  453 . The barometer  451  may be disposed near the recording head  24 K,  24 C,  24 M, or  24 Y. 
     As illustrated in  FIG. 19 , in step S 301 , the barometer  451  (depicted in  FIG. 18 ) measures an outer air pressure. In step S 302 , the drip preventing controller  452  (depicted in  FIG. 18 ) determines whether or not a measured pressure (e.g., a measured value) is equal to or smaller than a reference pressure (e.g., a reference value). If the measured value is not equal to or is not smaller than the reference value (i.e., if NO is selected in step S 302 ), the drip preventing controller  452  sets a standby mode in which printing is available to the print controller  453  (depicted in  FIG. 18 ) in step S 303 . 
     If the measured value is equal to or smaller than the reference value (i.e., if YES is selected in step S 302 ), the carriage  23  (depicted in  FIG. 2 ) moves to a position corresponding to the air release lever  422  (depicted in  FIG. 13 ) in step S 304 . In step S 305 , the air release lever  422  presses the pressing members  237  (depicted in  FIG. 13 ) so that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y (depicted in  FIG. 13 ) release air from the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively. Namely, the pressure release mechanism  402  (depicted in  FIG. 18 ) is in a continuous pressure release mode. 
     In step S 306 , the barometer  451  measures an outer air pressure again. In step S 307 , the drip preventing controller  452  determines whether or not a measured pressure (e.g., a measured value) is equal to or smaller than a reference pressure (e.g., a reference value). If the measured value is still equal to or smaller than the reference value (i.e., if YES is selected in step S 307 ), the drip preventing controller  452  sets a standby mode in which printing is not available to the print controller  453  in step S 308 . 
     If the measured value is not equal to or is not smaller than the reference value (i.e., if NO is selected in step S 307 ), the pressure release lever  422  moves back to the initial position in step S 309 . Accordingly, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are closed to cancel the continuous pressure release mode so as to stop air release. In step S 310 , a negative pressure forming operation is performed for the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ). The drip preventing controller  452  sets the standby mode in which printing is available to the print controller  453  in step S 303 . 
     As described above, the image forming apparatus  1 A (depicted in  FIG. 18 ) includes the drip preventing controller  452  serving as a pressure release member activator for activating the pressure release mechanism  402  based on the measured outer air pressure. Thus, the image forming apparatus  1 A may automatically enter a mode for preventing an ink drop from dripping from the recording heads  24 K,  24 C,  24 M, and  24 Y. Even when the image forming apparatus  1 A is located in an aircraft or at a high elevation on land, the pressure release mechanism  402  can prevent dripping of an ink drop caused by change in outer air pressure. 
     The barometer  451  for measuring an outer air pressure is provided inside the image forming apparatus  1 A and sends a measurement result (e.g., the measured outer air pressure) to the drip preventing controller  452 . The barometer  451  may measure an outer air pressure near the recording heads  24 K,  24 C,  24 M, and  24 Y. Thus, the drip preventing controller  452  may perform a control operation with an increased precision. 
     Referring to  FIG. 20 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment  FIG. 20  is a block diagram of an image forming apparatus  1 B according to yet another exemplary embodiment. The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 20 , the image forming apparatus  1 B does not include the barometer  451 . The other elements of the image forming apparatus  1 B are common to the image forming apparatus  1 A depicted in  FIG. 18 . The barometer  451 , serving as an outer air pressure detector for measuring an outer air pressure, is provided outside the image forming apparatus  1 B. The barometer  451  sends a measurement result (e.g., a measured outer air pressure) to the drip preventing controller  452  provided inside the image forming apparatus  1 B by wire or wireless. The drip preventing controller  452 , the pressure release mechanism  402 , and the print controller  453  perform operations common to the operations illustrated in  FIG. 19 . 
     The barometer  451  provided outside the image forming apparatus  1 B measures an outer air pressure and sends a measurement result (e.g., the measured outer air pressure) to the image forming apparatus  1 B. Even when the image forming apparatus  1 B is located in an aircraft or otherwise at a high elevation on land, the barometer  451  may be connected to the image forming apparatus  1 B as needed, resulting in decreased manufacturing costs of the image forming apparatus  1 B. 
     Referring to  FIGS. 21 and 22 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment.  FIG. 21  is a block diagram of an image forming apparatus  1 C according to yet another exemplary embodiment.  FIG. 22  is a flowchart of the example of operation of the pressure release mechanism  402 . The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 21 , the image forming apparatus  1 C includes an altimeter  461  and a drip preventing controller  462  instead of the barometer  451  and the drip preventing controller  452  (depicted in  FIG. 18 ), respectively. The other elements of the image forming apparatus  1 C are common to the image forming apparatus  1 A depicted in  FIG. 18 . The altimeter  461  serves as an altitude detector for measuring (e.g., detecting) an altitude at which the image forming apparatus  1 C is located. The drip preventing controller  462  serves as a pressure release member activator for activating the pressure release member. For example, the drip preventing controller  462  controls an operation of the pressure release mechanism  402  based on a measurement result (e.g., a detection result) provided by the altimeter  461  and sends a command for entering a standby mode to the print controller  453 . The main controller  301  (depicted in  FIG. 7 ) includes the drip preventing controller  462  and the print controller  453 . 
     As illustrated in  FIG. 22 , in step S 401 , the altimeter  461  (depicted in  FIG. 21 ) measures an altitude. In step S 402 , the drip preventing controller  462  (depicted in  FIG. 21 ) determines whether or not a measured altitude (e.g., a measured value) is equal to or greater than a reference pressure (e.g., a reference value). If the measured value is not equal to or is not greater than the reference value (i.e., if NO is selected in step S 402 ), the drip preventing controller  462  sets a standby mode in which printing is available to the print controller  453  (depicted in  FIG. 21 ) in step S 403 , because dripping of an ink drop due to a decreased outer air pressure may not occur when the altitude is lower than the reference value. 
     If the measured value is equal to or greater than the reference value (i.e., if YES is selected in step S 402 ), the carriage  23  (depicted in  FIG. 2 ) moves to a position corresponding to the air release lever  422  (depicted in  FIG. 13 ) in step S 404 , because an outer air pressure decreases when the altitude is not smaller than the reference value. In step S 405 , the air release lever  422  presses the pressing members  237  (depicted in  FIG. 13 ) so that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y (depicted in  FIG. 13 ) release air from the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively. Namely, the pressure release mechanism  402  (depicted in  FIG. 21 ) is in a continuous pressure release mode. 
     In step S 406 , the altimeter  461  measures an altitude again. In step S 407 , the drip preventing controller  462  determines whether or not a measured altitude (e.g., a measured value) is equal to or greater than a reference pressure (e.g., a reference value). If the measured value (i.e., the altitude) is still equal to or greater than the reference value (i.e., if YES is selected in step S 407 ), the drip preventing controller  462  sets a standby mode in which printing is not available to the print controller  453  in step S 408 . 
     If the measured value (i.e., the altitude) is not equal to or is not greater than the reference value (i.e., if NO is selected in step S 407 ), an outer air pressure increases. Thus, the pressure release lever  422  moves back to the initial position in step S 409 . Accordingly, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are closed to cancel the continuous pressure release mode so as to stop air release. In step S 410 , a negative pressure forming operation is performed for the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ). The drip preventing controller  462  sets the standby mode in which printing is available to the print controller  453  in step S 403 . 
     As described above, the image forming apparatus  1 C (depicted in  FIG. 21 ) includes the drip preventing controller  462  serving as a pressure release member activator for activating the pressure release mechanism  402  based on the measured altitude. Thus, the image forming apparatus  1 C may automatically enter a mode for preventing an ink drop from dripping from the recording heads  24 K,  24 C,  24 M, and  24 Y. Even when the image forming apparatus  1 C is located in an aircraft or at an elevation on land, the pressure release mechanism  402  may prevent dripping of an ink drop caused by change in altitude relating to change in outer air pressure. 
     The altimeter  461  for measuring an altitude is provided inside the image forming apparatus  1 C and sends a measurement result (e.g., a measured altitude) to the drip preventing controller  462 . Thus, the drip preventing controller  462  may perform a control operation with an increased precision. 
     Referring to  FIG. 23 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment.  FIG. 23  is a block diagram of an image forming apparatus  1 D according to yet another exemplary embodiment. The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 23 , the image forming apparatus  1 D does not include the altimeter  461 . The other elements of the image forming apparatus  1 D are common to the image forming apparatus  1 C depicted in  FIG. 21 . The altimeter  461 , serving as an altitude detector for measuring (e.g., detecting) an altitude at which the image forming apparatus  1 D is located, is provided outside the image forming apparatus  1 D. The altimeter  461  sends a measurement result (e.g., a measured altitude) to the drip preventing controller  462  provided inside the image forming apparatus  1 D by wire or wireless. The drip preventing controller  462 , the pressure release mechanism  402 , and the print controller  453  perform operations common to the operations illustrated in  FIG. 22 . 
     The altimeter  461  provided outside the image forming apparatus  1 D measures an altitude and sends a measurement result (e.g., the measured altitude) to the image forming apparatus  1 D. Even when the image forming apparatus  1 D is located in an aircraft or at an elevation on land, the altimeter  461  may be connected to the image forming apparatus  1 D as needed, resulting in decreased manufacturing costs of the image forming apparatus  1 D. 
     Referring to  FIGS. 24 and 25 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment  FIG. 24  is a block diagram of an image forming apparatus  1 E according to yet another exemplary embodiment  FIG. 25  is a flowchart of the example of operation of the pressure release mechanism  402 . The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 24 , the image forming apparatus  1 E includes a GPS (global positioning system) measuring device  471  and a drip preventing controller  472  instead of the barometer  451  and the drip preventing controller  452  (depicted in  FIG. 18 ), respectively. The other elements of the image forming apparatus  1 E are common to the image forming apparatus  1 A depicted in  FIG. 18 . The GPS measuring device  471  serves as a position detector for measuring (e.g., detecting) a position at which the image forming apparatus  1 E is located. The GPS measuring device  471  is provided inside the image forming apparatus  1 E. The drip preventing controller  472  serves as a pressure release member activator for activating the pressure release member. For example, the drip preventing controller  472  controls an operation of the pressure release mechanism  402  based on a measurement result (e.g., a detection result) provided by the GPS measuring device  471  and sends a command for entering a standby mode to the print controller  453 . The main controller  301  (depicted in  FIG. 7 ) includes the drip preventing controller  472  and the print controller  453 . 
     The GPS measuring device  471  measures a position of the image forming apparatus  1 E and sends a measurement result (e.g., the measured position) to the drip preventing controller  472 . The drip preventing controller  472  estimates an altitude of the position of the image forming apparatus  1 E based on topographic map information and the measured position of the image forming apparatus  1 E stored in an internal memory (not shown). The drip preventing controller  472  estimates an outer air pressure based on the estimated altitude of the position of the image forming apparatus  1 E. The drip preventing controller  472  controls an operation of the pressure release mechanism  402  based on the estimated outer air pressure. The topographic map information may not be stored in the internal memory, but may be obtained by communicating with an external device (not shown) as needed. 
     As illustrated in  FIG. 25 , in step S 501 , the GPS measuring device  471  (depicted in  FIG. 24 ) measures a position of the image forming apparatus  1 E, and estimates an altitude of the position of the image forming apparatus  1 E based on the measured position and the topographic map information so as to estimate an outer air pressure. In step S 502 , the drip preventing controller  472  (depicted in  FIG. 24 ) determines whether or not the estimated outer air pressure (e.g., an estimated value) is equal to or smaller than a reference pressure (e.g., a reference value). If the estimated value is not equal to or is not smaller than the reference value (i.e., if NO is selected in step S 502 ), the drip preventing controller  472  sets a standby mode in which printing is available to the print controller  453  (depicted in  FIG. 24 ) in step S 503 . 
     If the estimated value (e.g., the outer air pressure) is equal to or smaller than the reference value (i.e., if YES is selected in step S 502 ), the carriage  23  (depicted in  FIG. 2 ) moves to a position corresponding to the air release lever  422  (depicted in  FIG. 13 ) in step S 504 . In step S 505 , the air release lever  422  presses the pressing members  237  (depicted in  FIG. 13 ) so that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y (depicted in  FIG. 13 ) release air from the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively. Namely, the pressure release mechanism  402  (depicted in  FIG. 24 ) is in a continuous pressure release mode. 
     In step S 506 , the GPS measuring device  471  measures a position of the image forming apparatus  1 E again to estimate an outer air pressure. In step S 507 , the drip preventing controller  472  determines whether or not the estimated outer air pressure (e.g., an estimated value) is equal to or smaller than a reference pressure (e.g., a reference value). If the estimated value (i.e., the outer air pressure) is still equal to or smaller than the reference value (i.e., if YES is selected in step S 507 ), the drip preventing controller  472  sets a standby mode in which printing is not available to the print controller  453  in step S 508 . 
     If the estimated value (i.e., the outer air pressure) is not equal to or is not smaller than the reference value (i.e., if NO is selected in step S 507 ), for example, if the image forming apparatus  1 E is moved to a location where an outer air pressure is greater than the reference pressure, the pressure release lever  422  moves back to the initial position in step S 509 . Accordingly, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are closed to cancel the continuous pressure release mode so as to stop air release. In step S 510 , a negative pressure forming operation is performed for the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ). The drip preventing controller  472  sets the standby mode in which printing is available to the print controller  453  in step S 503 . 
     As described above, the image forming apparatus  1 E (depicted in  FIG. 24 ) includes the drip preventing controller  472 , serving as a pressure release member activator for activating the pressure release mechanism  402  based on the measured position of the image forming apparatus  1 E. Thus, the image forming apparatus  1 E may automatically enter a mode for preventing an ink drop from dripping from the recording heads  24 K,  24 C,  24 M, and  24 Y. Even when the image forming apparatus  1 E is located on land at an elevation, the pressure release mechanism  402  can prevent dripping of an ink drop caused by change in altitude relating to change in outer air pressure. 
     Referring to  FIG. 26 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment.  FIG. 26  is a block diagram of an image forming apparatus  1 F according to yet another exemplary embodiment. The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 26 , the image forming apparatus  1 F does not include the GPS measuring device  471 . The other elements of the image forming apparatus  1 F are common to the image forming apparatus  1 E depicted in  FIG. 24 . The GPS measuring device  471 , serving as a position detector for measuring (e.g., detecting) a position at which the image forming apparatus  1 F is located, is provided outside the image forming apparatus  1 F. The GPS measuring device  471  sends a measurement result (e.g., a measured position) to the drip preventing controller  472  provided inside the image forming apparatus  1 F by wire or wireless. The drip preventing controller  472 , the pressure release mechanism  402 , and the print controller  453  perform operations common to the operations illustrated in  FIG. 25 . 
     The GPS measuring device  471  provided outside the image forming apparatus  1 F measurement a position of the image forming apparatus  1 F and sends a measurement result (e.g., the measured position) to the image forming apparatus  1 F. Even when the image forming apparatus  1 F is located on land at an elevation, the GPS measuring device  471  may be connected to the image forming apparatus  1 F as needed, resulting in decreased manufacturing costs of the image forming apparatus  1 F. 
     Referring to  FIGS. 27 and 28 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment.  FIG. 27  is a block diagram of an image forming apparatus  1 G according to yet another exemplary embodiment.  FIG. 28  is a flowchart of the example of operation of the pressure release mechanism  402 . The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 27 , the image forming apparatus  1 G includes a drip preventing controller  482  instead of the drip preventing controller  452  (depicted in  FIG. 18 ). The other elements of the image forming apparatus  1 G are common to the image forming apparatus  1 A depicted in  FIG. 18 . A seat belt lock sensor  481  for detecting a lock of a seat belt in an aircraft is connected to the image forming apparatus  1 G. The drip preventing controller  482  serves as a pressure release member activator for activating the pressure release member. For example, the drip preventing controller  482  controls an operation of the pressure release mechanism  402  based on a detection result provided by the seat belt lock sensor  481  and sends a command for entering a standby mode to the print controller  453 . The main controller  301  (depicted in  FIG. 7 ) includes the drip preventing controller  482  and the print controller  453 . 
     The seat belt lock sensor  481  detects whether or not an operator of the image forming apparatus  1 G fastens a seat belt or whether or not an instruction for prompting the operator to lock the seat belt is sent. When the seat belt is locked or is to be locked, the seat belt lock sensor  481  sends a lock detection signal to the drip preventing controller  482  by wireless. 
     As illustrated in  FIG. 28 , in step S 601 , the seat belt lock sensor  481  (depicted in  FIG. 27 ) detects a lock of a seat belt. In step S 602 , the drip preventing controller  482  (depicted in  FIG. 27 ) determines whether or not the seat belt is locked based on a detection signal sent from the seat belt lock sensor  481 . If the seat belt is not locked (i.e., if NO is selected in step S 602 ), the drip preventing controller  482  sets a standby mode in which printing is available to the print controller  453  (depicted in  FIG. 27 ) in step S 603 . 
     If the seat belt is locked (i.e., if YES is selected in step S 602 ), the carriage  23  (depicted in  FIG. 2 ) moves to a position corresponding to the air release lever  422  (depicted in  FIG. 13 ) in step S 604 . In step S 605 , the air release lever  422  presses the pressing members  237  (depicted in  FIG. 13 ) so that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y (depicted in  FIG. 13 ) release air from the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 8 ), respectively. Namely, the pressure release mechanism  402  (depicted in  FIG. 27 ) is in a continuous pressure release mode. 
     In step S 606 , the seat belt lock sensor  481  detects the lock of the seat belt again. In step S 607 , the drip preventing controller  482  determines whether or not the seat belt is locked based on a detection signal sent from the seat belt lock sensor  481 . If the seat belt is still locked (i.e., if YES is selected in step S 607 ), the drip preventing controller  482  sets a standby mode in which printing is not available to the print controller  453  in step S 608 . 
     If the seat belt is not locked (i.e., if NO is selected in step S 607 ), the pressure release lever  422  moves back to the initial position in step S 609 . Accordingly, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are closed to cancel the continuous pressure release mode so as to stop air release. In step S 610 , a negative pressure forming operation is performed for the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ). The drip preventing controller  482  sets the standby mode in which printing is available to the print controller  453  in step S 603 . 
     As described above, the image forming apparatus  1 G (depicted in  FIG. 27 ) includes the drip preventing controller  482 , serving as a pressure release member activator for activating the pressure release mechanism  402  based on the detection signal relating to the lock of the seat belt in the aircraft. Thus, the image forming apparatus  1 G can automatically enter a mode for preventing an ink drop from dripping from the recording heads  24 K,  24 C,  24 M, and  24 Y. Even when the image forming apparatus  1 G is located in an aircraft, the pressure release mechanism  402  can prevent dripping of an ink drop caused by change in outer air pressure. 
     Referring to  FIGS. 29 to 31 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment.  FIG. 29  is a schematic view of an image forming apparatus  1 H according to yet another exemplary embodiment.  FIG. 30  is a block diagram of the image forming apparatus  1 H.  FIG. 31  is a flowchart of the example of operation of the pressure release mechanism  402 . The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As illustrated in  FIG. 29 , the image forming apparatus  1 H includes drip sensors  501  (e.g., drip sensors  501 K,  501 C,  501 M, and  501 Y). Each of the drip sensors  501 K,  501 C,  501 M, and  501 Y includes two detecting electrodes  511  and  512 . The drip sensors  501 K,  501 C,  501 M, and  501 Y serve as a drip detector for detecting dripping of an ink drop from the recording heads  24 K,  24 C,  24 M, and  24 Y, respectively. The detecting electrodes  511  and  512  are arranged in a manner that a small space is provided between the detecting electrodes  511  and  512 . When a liquid (e.g., an ink drop) is adhered to the detecting electrodes  511  and  512 , an electric current flows between the detecting electrodes  511  and  512 . Thus, the drip sensors  501 K,  501 C,  501 M, and  501 Y detect dripping of an ink drop. 
     As illustrated in  FIG. 30 , the image forming apparatus  1 H further includes a drip preventing controller  502  instead of the drip preventing controller  452  (depicted in  FIG. 18 ). The drip sensor  501  replaces the barometer  451  (depicted in  FIG. 18 ). The other elements of the image forming apparatus  1 H are common to the image forming apparatus  1 A depicted in  FIG. 18 . The drip preventing controller  502  serves as a pressure release member activator for activating the pressure release member. For example, the drip preventing controller  502  controls an operation of the pressure release mechanism  402  based on a detection result provided by the drip sensor  501  and sends a command for entering a standby mode to the print controller  453 . The main controller  301  (depicted in  FIG. 7 ) includes the drip preventing controller  502  and the print controller  453 . 
     As illustrated in  FIG. 31 , in step S 701 , the drip sensor  501  (depicted in  FIG. 30 ) detects dripping of an ink drop from the recording head  24 K,  24 C,  24 M, or  24 Y (depicted in  FIG. 29 ). In step S 702 , the drip preventing controller  502  (depicted in  FIG. 30 ) determines whether or not an ink drop drips from the recording head  24 K,  24 C,  24 M, or  24 Y based on a detection result sent from the drip sensor  501 . If the ink drop is not dripped (i.e., if NO is selected in step S 702 ), the drip preventing controller  502  sets a standby mode in which printing is available to the print controller  453  (depicted in  FIG. 30 ) in step S 703 . 
     If the ink drop is dripped (i.e., if YES is selected in step S 702 ), the carriage  23  (depicted in  FIG. 2 ) moves to a position corresponding to the air release lever  422  (depicted in  FIG. 13 ) in step S 704 . In step S 705 , the air release lever  422  presses the pressing members  237  (depicted in  FIG. 13 ) so that the air release mechanisms  232 K,  232 C,  232 M, and  232 Y (depicted in  FIG. 13 ) release air from the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 29 ), respectively. Namely, the pressure release mechanism  402  (depicted in  FIG. 30 ) is in a continuous pressure release mode. 
     In step S 706 , the drip preventing controller  502  receives information relating to a command for canceling the continuous pressure release mode from the control panel  319  (depicted in  FIG. 7 ), for example. In step S 707 , the drip preventing controller  502  determines whether or not the drip preventing controller  502  receives the command for canceling the continuous pressure release mode (e.g., a command for canceling a drip preventing mode). In the image forming apparatus  1 H, once an ink drop is adhered to the detecting electrodes  511  and  512 , the ink drop sticks to the detecting electrodes  511  and  512  unless a user, an operator, a service engineer, or the like of the image forming apparatus  1 H removes the ink drop from the detecting electrodes  511  and  512 . Therefore, the drip preventing controller  502  determines whether or not the drip preventing controller  502  receives the command for canceling the continuous pressure release mode from the control panel  319  or the like after the user, the operator, the service engineer, or the like removes the ink drop. 
     If the drip preventing controller  502  does not receive the command for canceling the continuous pressure release mode from the control panel  319  or the like (i.e., if NO is selected in step S 707 ), the drip preventing controller  502  sets a standby mode in which printing is not available to the print controller  453  in step S 708 . 
     If the drip preventing controller  502  receives the command for canceling the continuous pressure release mode from the control panel  319  or the like (i.e., if YES is selected in step S 707 ), the pressure release lever  422  moves back to the initial position in step S 709 . Accordingly, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y are closed to cancel the continuous pressure release mode so as to stop air release. In step S 710 , a negative pressure forming operation is performed for the recording heads  24 K,  24 C,  24 M, and  24 Y. The drip preventing controller  502  sets the standby mode in which printing is available to the print controller  453  in step S 703 . 
     As described above, the image forming apparatus  1 H (depicted in  FIG. 30 ) includes the drip preventing controller  502  serving as a pressure release member activator for activating the pressure release mechanism  402  based on a detection result relating to whether or not an ink drop drips from the recording head  24 K,  24 C,  24 M, or  24 Y. Thus, the image forming apparatus  1 H can quickly cope with dripping of an ink drop caused by change in outer air pressure, for example. 
     Referring to  FIG. 32 , the following describes yet another example of operation of the pressure release mechanism  402  according to yet another exemplary embodiment  FIG. 32  is a flowchart of the example of operation of the pressure release mechanism  402  (depicted in  FIG. 13 ). The pressure release mechanism  402 A (depicted in  FIG. 15 ) may be used instead of the pressure release mechanism  402 . 
     As described above, when the pressure release mechanism  402  is in a continuous pressure release mode (i.e., when the air release mechanisms  232 K,  232 C,  232 M, and  232 Y depicted in  FIG. 13  release air from the head tanks  25 K,  25 C,  25 M, and  25 Y depicted in  FIG. 2 , respectively), the image forming apparatus (i.e., the image forming apparatus  1 ,  1 A,  1 B,  1 C,  1 D,  1 E,  1 F,  1 G, or  1 H) is in a standby mode in which printing is not available. If the image forming apparatus has a facsimile function, the image forming apparatus may not print a document (e.g., print data) even when the image forming apparatus receives the document. 
     As illustrated in  FIG. 32 , in step S 801 , the image forming apparatus receives a document (e.g., print data). In step S 802 , the controller  300  (depicted in  FIG. 7 ) determines whether or not the image forming apparatus is in the standby mode in which printing is not available. If the image forming apparatus is not in the standby mode in which printing is not available (i.e., if NO is selected in step S 802 ), the image forming apparatus prints the received document in step S 803 . If the image forming apparatus is in the standby mode in which printing is not available (i.e., if YES is selected in step S 802 ), the image forming apparatus stores the received document in a memory (not shown) in step S 804 . If the image forming apparatus is not in the standby mode in which printing is not available when the controller  300  determines whether or not the image forming apparatus is in the standby mode again in step S 802 , the image forming apparatus prints the received document stored in the memory in step S 803 . 
     As described above, even when the pressure release mechanism  402  is activated, the image forming apparatus temporarily prohibits printing when the image forming apparatus receives a print request (e.g., a document). Thus, the image forming apparatus may print the received document without losing it. 
     Referring to  FIGS. 33 and 34 , the following describes an image forming apparatus  1 I according to yet another exemplary embodiment  FIGS. 33 and 34  illustrate a plane view of the image forming apparatus  1 I. 
     As illustrated in  FIG. 33 , the image forming apparatus  1 I includes a pressure release mechanism  602  and a body cover  1 Z. The pressure release mechanism  602  includes an air release lever  622 , a support shaft  624 , and a control lever  625 . The body cover  1 Z includes a hole  626 . The other elements of the image forming apparatus  1 I are common to the image forming apparatus  1  (depicted in  FIG. 1 ). 
     A user manually operates the pressure release mechanism  602  serving as a pressure release member. The air release lever  622  presses the pressing members  237  of the air release mechanisms  232 K,  232 C,  232 M, and  232 Y, respectively. The support shaft  624  rotatably supports the air release lever  622 . The control lever  625  serves as a control member controlled from an outside of the image forming apparatus  1 I. The control lever  625  is integrally provided with the air release lever  622 . A part of the control lever  625  protrudes from the hole  626  formed in the body cover  1 Z towards the outside of the image forming apparatus  1 I. Alternatively, the control lever  625  may not protrude from the body cover  1 Z. For example, when the body cover  1 Z is openable and closable or when the body cover  1 Z includes a cover which is openable and closable, the control lever  625  may be provided in a space in which the user may manually operate the control lever  625  when the body cover  1 Z or the cover included in the body cover  1 Z is opened. 
     When the image forming apparatus  1 I is located in an aircraft or at an elevation otherwise, a decreased outer air pressure may cause dripping of an ink drop from the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ). However, when the user manually swings the control lever  625  in a rotating direction I as illustrated in  FIG. 34 , the air release lever  622  swings in the rotating direction I. Thus, the air release lever  622  presses the pressing members  237 . Almost simultaneously, the air release mechanisms  232 K,  232 C,  232 M, and  232 Y release air from the head tanks  25 K,  25 C,  25 M, and  25 Y (depicted in  FIG. 2 ), respectively. 
     Accordingly, inner air pressures in the head tanks  25 K,  25 C,  25 M, and  25 Y, respectively, become equivalent to an outer air pressure in an environment in which the image forming apparatus  1 I is located. 
     As described above, the image forming apparatus  1 I includes the pressure release mechanism  602  manually operated by the user. Thus, the user may easily switch the pressure release mechanism  602  into a drip preventing mode to prevent dripping of an ink drop caused by change in outer air pressure. Even when the image forming apparatus  1 I is powered off, the pressure release mechanism  602  can prevent dripping of an ink drop. For example, even when the image forming apparatus  1 I is carried by a transportation vehicle (e.g., an aircraft or the like) as a cargo without being operated, the user may swing the air release lever  622  to prevent dripping of an ink drop before the image forming apparatus  1 I is loaded on the transportation vehicle. 
     Referring to  FIGS. 35 to 38 , the following describes an image forming apparatus  1 J according to yet another exemplary embodiment  FIGS. 35 and 36  illustrate a plane view of the image forming apparatus  1 J.  FIG. 37  is a block diagram of the image forming apparatus  1 J.  FIG. 38  is a flowchart of an example of operation of the pressure release mechanism  602 . 
     As illustrated in  FIG. 35 , the image forming apparatus  1 J includes a lever sensor  631 . The other elements of the image forming apparatus  1 J are common to the image forming apparatus  1 I (depicted in  FIG. 33 ). 
     The lever sensor  631  serves as a lever position detector for detecting a position (e.g., a condition) of the control lever  625 . When the control lever  625  is at a standby position at which the air release lever  622  does not press the pressing members  237  as illustrated in  FIG. 35  (i.e., when the pressure release mechanism  602  is not in a continuous pressure release mode), the lever sensor  631  is turned off. When the control lever  625  is at an air release position at which the air release lever  622  presses the pressing members  237  as illustrated in  FIG. 36  (i.e., when the pressure release mechanism  602  is in the continuous pressure release mode), the lever sensor  631  is turned on. According to this non-limiting exemplary embodiment, the lever sensor  631  includes a push switch. However, the lever sensor  631  may include a photo sensor. 
     As illustrated in  FIG. 37 , the image forming apparatus  1 J further includes a print controller  633 . The print controller  633  controls an image forming operation based on a detection result provided by the lever sensor  631 . The main controller  301  (depicted in  FIG. 7 ) includes the print controller  633 . 
     As illustrated in  FIG. 38 , the lever sensor  631  (depicted in  FIG. 35 ) detects a position of the controller lever  625  (depicted in  FIG. 35 ) in step S 901 . In step S 902 , the controller  300  (depicted in  FIG. 7 ) determines whether or not the control lever  625  is at the air release position based on a detection result provided by the lever sensor  631 . If the control lever  625  is not at the air release position (i.e., if NO is selected in step S 902 ), a negative pressure forming operation is performed for the recording heads  24 K,  24 C,  24 M, and  24 Y (depicted in  FIG. 2 ) in step S 903 . In step S 904 , the controller  300  sets a standby mode in which printing is available to the print controller  633  (depicted in  FIG. 37 ). If the control lever  625  is at the air release position (i.e., if YES is selected in step S 902 ), the controller  300  sets a standby mode in which printing is not available to the print controller  633  in step S 905 , until the control lever  625  moves away from the air release position. 
     As described above, the lever sensor  631  detects whether or not the control lever  625  is at the air release position. The image forming apparatus  1 J performs an image forming operation based on a detection result provided by the lever sensor  631 . Therefore, the image forming apparatus  1 J may provide effects common to the image forming apparatus  1 I (depicted in  FIG. 33 ). The lever sensor  631  detects whether or not the control lever  625  is at the air release position, preventing a timeout error caused by an improper negative pressure formed during a negative pressure forming operation performed while air is released. 
     According to the above-described exemplary embodiments, an image forming apparatus (i.e., the image forming apparatus  1 ,  1 A,  1 B,  1 C,  1 D,  1 E,  1 F,  1 G,  1 H,  1 I or  1 J depicted in  FIGS. 2 ,  18 ,  20 ,  21 ,  23 ,  24 ,  26 ,  27 ,  29 ,  33 , or  35 , respectively) includes a plurality of liquid discharging heads (i.e., the recording heads  24 K,  24 C,  24 M, and  24 Y depicted in  FIG. 2 ) for discharging liquids different from each other and a pressure release member (i.e., the pressure release mechanism  402 ,  402 A, or  602  depicted in  FIG. 13 ,  15 , or  33 , respectively) for causing inner air pressures in the plurality of the liquid discharging heads, respectively, to be continuously equivalent to an outer air pressure. For example, when the outer air pressure decreases, the pressure release member adjusts the inner air pressures in the liquid discharging heads, respectively, to be continuously equivalent to the outer air pressure. Thus, even when the inner air pressures in the liquid discharging heads, respectively, relatively increase, the air release member may prevent liquid from dripping from nozzles of the liquid discharging heads. 
     According to the above-described exemplary embodiments, a recording medium, on which the image forming apparatus forms an image, includes paper, strings, fiber, cloth, leather, metal, plastic, glass, wood, ceramics, and/or the like. An image formed by the image forming apparatus includes a character, a letter, graphics, a pattern, and/or the like. A liquid, with which the image forming apparatus forms an image, is not limited to ink but includes any fluid and any substance which becomes fluid when discharged from the liquid discharging head. The liquid discharging head may discharge a liquid not forming an image as well as a liquid forming an image. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     This patent specification is based on Japanese patent application No. 2006-191641 filed on Jul. 12, 2006 in the Japan Patent Office, the entire contents of which are hereby incorporated herein by reference.