Patent Publication Number: US-2015070415-A1

Title: Liquid ejecting apparatus and liquid circulation method of liquid ejecting apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-187565, filed Sep. 10, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a liquid circulation-type liquid ejecting apparatus and a liquid circulation method of the liquid ejecting apparatus. 
     BACKGROUND 
     There is a liquid circulation-type liquid ejecting apparatus that removes bubbles included in liquid by circulating the liquid between a liquid tank and a liquid ejecting head. 
     However, in the liquid circulation-type liquid ejecting apparatus, it is concerned that the pressure in the liquid ejecting head is drastically decreased by absorption pressure by a circulation pump when the circulation of the liquid is started in order to remove the bubbles. If the pressure in the liquid ejecting head is greatly decreased, when the circulation of the liquid is started, it is concerned that an ejection head absorbs the air from a nozzle. 
     JP-A-2005-125670 is an example of the related art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram schematically illustrating an ink circulation mechanism of an inkjet printer according to a first embodiment. 
         FIG. 2  is a view schematically illustrating a structure in a circumference of a nozzle of an inkjet head according to the first embodiment. 
         FIG. 3  is a block diagram schematically illustrating a control system that mainly controls the ink circulation mechanism of the inkjet printer according to the first embodiment. 
         FIG. 4  is a diagram illustrating an example of pressure fluctuations in an ink chamber when circulation of ink is started by the ink circulation mechanism according to the first embodiment. 
         FIG. 5  is a flow chart illustrating control of the circulation of the ink according to the first embodiment. 
         FIG. 6  is a timing chart illustrating the control of the circulation of the ink according to the first embodiment. 
         FIG. 7  is a flow chart illustrating control of circulation of ink according to a second embodiment. 
         FIG. 8  is a timing chart illustrating the control of the circulation of the ink according to the second embodiment. 
         FIG. 9  is a timing chart illustrating the control of circulation of ink according to a comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     An object of an exemplary embodiment is to provide a liquid ejecting apparatus and a liquid circulation method of the liquid ejecting apparatus in which liquid ejecting performance is favorably maintained by preventing absorption of bubbles from a nozzle when circulation of liquid is started in order to remove bubbles or foreign substances mixed into the liquid. 
     In order to achieve the object, a liquid ejecting apparatus according to the embodiment includes a liquid ejecting head that ejects liquid from an ejection portion; a tank that stores the liquid; a circulation transportation portion that includes a first transportation portion for transporting liquid from the tank to the liquid ejecting head, and a second transportation portion for transporting the liquid from the liquid ejecting head to the tank; an actuator that circulates the liquid of the circulation transportation portion; and a control portion that controls the actuator so as to maintain pressure in the liquid ejecting head to be negative pressure with respect to atmospheric pressure when circulation of the liquid in the circulation transportation portion is started and to start the circulation of the liquid after the pressure in the liquid ejecting head is increased. 
     First Embodiment 
     An inkjet printer that forms an image on a recording medium by ejecting ink is described with reference to  FIGS. 1 to 6 , as an example of a liquid ejecting apparatus according to a first embodiment. The ink circulation mechanism  10  of the inkjet printer  50  illustrated in  FIG. 1  includes an inkjet head  12  which is a liquid ejecting head, and an ink tank  13  which is a tank that stores ink  15  which is liquid. The ink circulation mechanism  10  includes an ink supply path  14  which is a first transportation portion, an ink circulation channel  16  which is a second transportation portion, a circulation pump  17  which is a transportation pump, a filter  18 , a pressure sensor  20 , a pressure adjustment pump  21 , an air communication passage  22 , and an electromagnetic valve  23 . The circulation pump  17  and the pressure adjustment pump  21  configure an actuator. The ink circulation mechanism  10  is tightly sealed from the air. 
     The inkjet head  12  includes an inflow port  30  connected to the ink supply path  14 , and an outflow port  31  connected to the ink circulation channel  16 . The inkjet head  12  includes a head body  32 , a nozzle plate  33  on which a nozzle  33   a  which is an ejection portion is formed, and an actuator  34 , as illustrated in  FIG. 2 . (The plurality of nozzles  33   a  are formed on the nozzle plate  33 , but one nozzle  33   a  is illustrated in  FIG. 2 . The nozzle plate  33  includes the actuator  34  for each nozzle  33   a. ) 
     The inkjet head  12  includes an ink chamber  36  formed in the head body  32 . The ink chamber  36  is conductively connected to the inflow port  30  and the outflow port  31 . The inkjet head  12  includes the actuator  34  in a protruding portion  32   a  formed on the head body  32 . The actuator  34  is configured with a piezoelectric element such as a piezo (PZT) element. In the ink chamber  36 , a gap between the actuator  34  and the nozzle plate  33  is narrow in an arrangement area  36   a  of the actuator  34 . The arrangement area  36   a  of the actuator  34  in the ink chamber  36  has a smaller cross section of an ink channel than that of the other areas. 
     The inkjet head  12  forms a piezoelectric element by applying a voltage to the piezoelectric element of the actuator  34 , generates the pressure fluctuation in the ink  15  in the ink chamber  36 , and ejects an ink drop from the nozzle  33   a.  The actuator  34  of the inkjet head  12  is not limited to the piezoelectric element, and may have a configuration in which an ink drop is ejected from the nozzle  33   a  by using thermal energy such as a heater. 
     The ink tank  13  includes an ink layer  13   a  formed of the ink  15  and an air space  13   b  inside the ink tank  13 . An upstream end  14 a of the ink supply path  14  and a downstream end  16   a  of the ink circulation channel  16  exist in the ink layer  13   a.  For example, bubbles mixed in the ink  15  from the nozzle  33   a  in the inkjet head  12  are collected into the ink tank  13  through the ink circulation channel  16 , and are absorbed into the air space  13   b  in the ink tank  13 . For example, a flexible resin tube or a metal pipe formed of stainless steel or the like is used as the ink supply path  14  and the ink circulation channel  16 . 
     The circulation pump  17  exists in the ink circulation channel  16 . The circulation pump  17  circulates the ink  15  in a circulation direction indicated by an arrow s in the circulation channel  10   a  including the inkjet head  12 , the ink tank  13 , the ink supply path  14 , and the ink circulation channel  16 . For example, a tube pump, a diaphragm pump, or a piston pump is used as the circulation pump  17 . 
     The filter  18  exists on the downstream of the circulation pump  17  of the ink circulation channel  16  in the circulation direction and removes foreign substances mixed in the ink  15 . For example, a mesh filter formed of polypropylene, nylon, polyphenylene sulfide, or stainless steel or the like is used as the filter  18 . 
     The pressure sensor  20  detects pressure of the air space  13   b  of the ink tank  13 . The pressure sensor  20  uses, for example, a semiconductor piezoresistance pressure sensor. The semiconductor piezoresistance pressure sensor includes a diaphragm that receives pressure from the outside, and a semiconductor strain gauge formed on a surface of the diaphragm, and detects the pressure by converting a change of an electric resistance caused by a piezoresistance effect generated by the strain gauge according to the deformation of the diaphragm caused by the pressure from the outside into an electric signal. 
     The pressure adjustment pump  21  sends the air into the ink tank  13 , and increases the pressure in the circulation channel  10   a.  The pressure adjustment pump  21  uses, for example, a tube pump or a bellows pump. 
     One end of the air communication passage  22  exists in the air space  13   b  of the ink tank  13 , and the other end exists in the outside of the ink tank  13 , that is, in the air. The electromagnetic valve  23  opens and closes the air communication passage  22 . 
     With reference to the block diagram illustrated in  FIG. 3 , a control system  60  that mainly controls the ink circulation mechanism  10  of the inkjet printer  50  is described. The control system  60  includes, for example, a system controller  100  that controls the entire body of the inkjet printer  50 , a memory  101 , an interface (I/F)  102 , and a control panel  103 . The control system  60  includes a head driving circuit  104  that drives the inkjet head  12 , a circulation pump driving circuit  105  that drives the circulation pump  17 , an A/D converter  106  that converts the detection result of the pressure sensor  20 , a pressure adjustment pump driving circuit  107  that controls a pressure adjustment pump  21 , and a valve driving circuit  108  that controls the electromagnetic valve  23 . 
     The system controller  100  includes a Central Processing Unit (CPU)  100   a,  a Read Only Memory (ROM)  100   b,  and a Random Access Memory (RAM)  100   c.  The system controller  100  executes various processes of an inkjet printer  50  including a process relating to the circulation of the ink  15  in a circulation channel  10   a  of an ink circulation mechanism  10 . The various processes are realized by executing a computer program stored in the ROM  100   b  or the like by the CPU  100   a.    
     The memory  101  stores image data to be a print target. The image data may be data received from the external device connected to the inkjet printer  50 , and may be data read by a scanner included in the inkjet printer  50 , or the like. The interface (I/F)  102  is, for example, an I/O port, and relays data transmission and reception performed between the control panel  103  and the system controller  100 . The control panel  103  includes various operation buttons, touch panels, and the like. 
     The head driving circuit  104  drives the inkjet head  12  corresponding to the command of the system controller  100 . For example, if the command from the system controller  100  is to control the printing of an image, the head driving circuit  104  selectively drives the actuator  34  of the inkjet head  12  according to the image data stored in the memory  101 , and forms an image on a recording medium. 
     The circulation pump driving circuit  105  drives the circulation pump  17  according to a command from the system controller  100 . The A/D converter  106  performs A/D conversion on an analog signal output by the pressure sensor  20 , and outputs a digital signal generated by the conversion to the system controller  100 . The pressure adjustment pump driving circuit  107  drives the pressure adjustment pump  21  corresponding to the command from the system controller  100 . The valve driving circuit  108  opens and closes the electromagnetic valve  23  according to the control of the system controller  100 . 
     The inkjet printer  50  circulates the ink  15  of the circulation channel  10   a  in the ink circulation mechanism  10 , for example, when power supply is input or when the inkjet printer returns from a power saving mode. The inkjet printer  50  circulates the ink  15  of the circulation channel  10   a  and removes bubbles or foreign substances mixed in the ink  15 . 
     According to the first embodiment, the pressure in the ink chamber  36  of the inkjet head  12  is increased when the driving of the circulation pump  17  in the ink circulation mechanism  10  is started. Even if the pressure in the ink chamber  36  is increased, and the absorption pressure when the driving of the circulation pump  17  is started is too strong, the bubbles are prevented from being absorbed from the nozzle  33   a.    
     A principle of the first embodiment is described with reference to  FIG. 4 . The pressure in the ink chamber  36  when the inkjet head  12  is stopped maintains negative pressure in a degree in which the ink  15  is not leaked from the nozzle  33   a,  and the bubbles are not absorbed from the nozzle  33   a.  The negative pressure is a pressure when the atmospheric pressure is caused to be zero. The inkjet head  12  maintains the ink chamber  36  to be in the negative pressure, and the leakage of the ink  15  from the ink chamber  36  is prevented by the meniscus generated in the nozzle  33   a.  When the circulation of the ink  15  in the circulation channel  10   a  is started, the pressure in the ink chamber  36  of the inkjet head  12  is decreased compared to when the circulation is stopped. For example, if the pressure in the ink chamber  36  is equal to or lower than a bubble absorption pressure D (kPa) when the circulation is started, the bubbles are absorbed from the nozzle  33   a.    
     When the circulation is started, if the pressure in the ink chamber  36  is decreased by the absorption pressure of the circulation pump  17 , for example, as indicated by a solid line α, the pressure in the ink chamber  36  does not reach the bubble absorption pressure D (kPa), the bubbles from the nozzle  33   a  are not attracted. On the other hand, for example, if the pressure in the ink chamber  36  is greatly decreased by the absorption pressure of the circulation pump  17  as indicated by a dotted line β, the pressure in the ink chamber  36  is greatly decreased, the pressure is decreased to be equal to or lower than the bubble absorption pressure D (kPa), and the bubbles are absorbed from the nozzle  33   a.    
     According to the first embodiment, when the driving of the circulation pump  17  is started, the pressure in the ink chamber  36  is increased from a stopped-time pressure B (kPa) to a safety margin pressure A (kPa). If the pressure in the ink chamber  36  is increased to the safety margin pressure A (kPa), even if the absorption pressure when the driving of the circulation pump  17  is started is great, and the pressure in the ink chamber  36  is decreased as indicated by a solid line γ. Even if the absorption pressure when the driving of the circulation pump  17  is started is great, the pressure in the ink chamber  36  is prevented from being decreased to be equal to or lower than the bubble absorption pressure D (kPa). As long as the ink chamber  36  is maintained to be in the negative pressure, the safety margin pressure A (kPa) is optional according to characteristics of the inkjet head  12 . 
     Control of the circulation of the ink  15  by the ink circulation mechanism  10  is described with respect to  FIGS. 5 and 6 . For example, if an instruction of circulating the ink  15  is input from the control panel  103  at a time t 1  in  FIG. 6 , the ink circulation mechanism  10  starts circulation of the ink  15 . The circulation of the ink  15  may be started by switching on the power supply of the inkjet printer  50 . The system controller  100  determines whether a pressure value P in the ink tank  13  which is detected by the pressure sensor  20  reaches a threshold value P 1  (ACT  197 ). If the pressure value P in the ink tank  13  is the threshold value P 1 , the pressure in the ink chamber  36  of the inkjet head  12  becomes the safety margin pressure A (kPa). 
     After the circulation of the ink  15  is started, if the pressure value P in the ink tank  13  reaches the threshold value P 1  (Yes in ACT  197 ), the system controller  100  proceeds to ACT  202  and drives the circulation pump  17  at a rated flow rate. 
     If the pressure value P does not reach the threshold value P 1  (No in ACT  197 ), the system controller  100  increases the pressure in the ink tank  13  (ACT  198 ). The system controller  100  increases the pressure in the ink tank  13  by controlling the pressure adjustment pump driving circuit  107  so that the pressure adjustment pump  21  is driven to increase the pressure (ACT  198 ). Otherwise, the system controller  100  may increase the pressure in the ink tank  13  by controlling the valve driving circuit  108  so as to drive the electromagnetic valve  23 . The pressure in the ink tank  13  is increased through the air communication passage  22  by driving the electromagnetic valve  23 . 
     If the pressure value P in the ink tank  13  reaches the threshold value P 1  at a time t 2  (Yes in ACT  200 ), the system controller  100  stops the pressure adjustment pump  21  (ACT  201 ), and starts to drive the circulation pump  17  (ACT  202 ). The system controller  100  controls the circulation pump driving circuit  105  so as to drive the circulation pump  17  at the rated flow rate. The circulation pump  17  is started to be driven, and starts to circulate the ink  15  in the circulation channel  10   a  at the rated flow rate in the direction of the arrow s. 
     If the pressure value P in the ink tank  13  reaches the threshold value P 1 , the pressure in the ink chamber  36  becomes the safety margin pressure A (kPa). If the pressure value P in the ink tank  13  reaches the threshold value P 1 , even if the absorption pressure of the circulation pump  17  is strong when the ink  15  starts to circulate at the rated flow rate, the pressure in the ink chamber  36  is not decreased to be equal to or lower than the bubble absorption pressure D (kPa). When the ink  15  starts to circulate, the bubbles are not absorbed from the nozzle  33   a  to the ink chamber  36  of the inkjet head  12 . 
     Thereafter, while causing the pressure value P in the ink tank  13  to satisfy P 3 ≦P≦P 2 , the system controller  100  continues the circulation of the ink  15  (ACTS  203  and  204 ). In the inkjet head  12 , if the pressure value P in the ink tank  13  is in the scope of P 3 ≦P≦P 2 , the ejection amount and the ejection speed of the ink drop from the nozzle  33   a  are stabilized and a prominent ink ejection property can be obtained. 
     A pressure value P 2  in the ink tank  13  is a maximum value of the pressure scope in which the inkjet head  12  can exhibit the prominent ink ejection performance when the ink  15  is circulated. A pressure value P 3  in the ink tank  13  is a minimum value of the pressure scope in which the inkjet head  12  can exhibit the prominent ink ejection performance when the ink  15  is circulated. The pressure values P 2  and P 3  in the ink tank  13  are optional according to characteristics of the inkjet head  12 , and are not limited to a specific value. 
     If the pressure value P in the ink tank  13  is not in the scope of P 3 ≦P≦P 2  (No in ACT  203 ), the system controller  100  adjusts the pressure in the ink tank  13  (ACT  204 ). The system controller  100  adjusts the pressure in the ink tank  13  by controlling the pressure adjustment pump driving circuit  107  so that the pressure adjustment pump  21  is driven to increase or decrease the pressure (ACT  204 ). Otherwise, the system controller  100  may adjust the pressure in the ink tank  13  by controlling the valve driving circuit  108  so as to drive the electromagnetic valve  23 . 
     For example, if the pressure value P is less than the pressure value P 3  at a time t 3 , the pressure adjustment pump driving circuit  107  increases the pressure in the ink tank  13  by driving the pressure adjustment pump  21  to increase the pressure (ACT  204 ). For example, if the pressure value P is equal to or greater than the pressure value P 2  at a time t 4 , the pressure adjustment pump driving circuit  107  decreases the pressure in the ink tank  13  by driving the pressure adjustment pump  21  to decrease the pressure (ACT  204 ). 
     Since the pressure in the ink tank  13  is maintained in the scope of P 3 ≦P≦P 2 , while the circulation of the ink  15  continues (ACTS  203 ,  204 , and  206 ), the foreign substance in the ink  15  of the circulation channel  10   a  is removed from the filter  18 . The bubbles mixed in the ink  15  are collected into the ink tank  13  and absorbed into the air space  13   b  in the ink tank  13 . 
     For example, if a user instructs to stop circulating the ink  15  from the control panel  103  at a time t 5 , the system controller  100  determines to stop the circulation of the ink  15  (Yes in ACT  206 ). By the determination of stopping the circulation (Yes in ACT  206 ), the system controller  100  stops the circulation pump  17  by controlling the circulation pump driving circuit  105  (ACT  207 ). The system controller  100  may determine to stop the circulation of the ink  15  at the time t 5  not by the instruction of the user, but by a passage of a certain time from the start of the circulation of the ink  15 . The system controller  100  ends the control of the circulation of the ink  15  of the ink circulation mechanism  10  by stopping the circulation pump  17  in ACT  207 . 
     According to the first embodiment, bubbles, foreign substances, or the like included in the ink  15  are removed by circulating the ink  15  in the circulation channel  10   a  by the ink circulation mechanism  10 . It is possible to enhance a quality of the image printed by the inkjet printer  50  by favorably maintaining the ink ejection performance of the inkjet head  12 . 
     According to the first embodiment, when the circulation pump  17  of the ink circulation mechanism  10  is started be driven, the pressure in the ink chamber  36  of the inkjet head  12  is increased to the safety margin pressure A (kPa) in advance by increasing the pressure value P in the ink tank  13  to the threshold value P 1 . Though the pressure of the ink chamber  36  is decreased by the absorption pressure due to the start of the driving of the circulation pump  17 , it is possible to maintain the pressure in the scope in which the bubbles from the nozzle  33   a  are not attracted without causing the pressure to be equal to or lower than the bubble absorption pressure D (kPa). In the inkjet head  12 , it is possible to prevent the bubbles from being absorbed from the nozzle  33   a  regardless of the absorption pressure when the circulation of the ink  15  is started. The ink circulation mechanism  10  favorably maintains the ink ejection performance of the inkjet head  12  by preventing the bubbles from being mixed again into the ink  15  when the circulation of the ink  15  is started so that the quality of the printed image is enhanced. 
     Second Embodiment 
     The inkjet printer according to the second embodiment is described with reference to  FIGS. 7 and 8 . The second embodiment can be obtained by controlling the pressure value and the circulation flow rate in the ink tank when the circulation of the ink is started further to the first embodiment. According to the second embodiment, the same configurations with the configurations described in the first embodiment are denoted by the same reference numerals and the detailed descriptions are not provided. 
     According to the second embodiment, for example, if the instruction of the circulation of the ink  15  is input from the control panel  103 , the ink circulation mechanism  10  starts the circulation of the ink  15 . 
     The system controller  100  drives the circulation pump  17  according to the flow chart illustrated in  FIG. 7  (ACT  221 ). The system controller  100  drives the circulation pump  17  so that the flow rate of the ink  15  that circulates the circulation channel  10   a  of the ink circulation mechanism  10  becomes N (ml/min) by controlling the circulation pump driving circuit  105  (ACT  221 ). 
     If a rated flow rate of the ink  15  that circulates the circulation channel  10   a  of the ink circulation mechanism  10  is set to be F (ml/min), the flow rate N (ml/min) of the ink  15  is, for example, a value obtained by dividing a rated flow rate F by the number of divisions X (N=F/X). The flow rate N (ml/min) of the ink  15  is a flow rate having a size that does not cause the pressure in the ink chamber  36  to be equal to or lower than the bubble absorption pressure D (kPa) when the circulation of the ink  15  is started regardless of the pressure value of the stopped-time pressure B (kPa) of the inkjet head  12  illustrated in  FIG. 4 . The flow rate N (ml/min) of the ink  15  is optional according to characteristics of the inkjet head  12 . 
     After circulating the ink  15  in a circulation channel  10   a  by driving the circulation pump  17 , the system controller  100  determines whether the pressure value P in the ink tank  13  reaches a minimum pressure value P 5  (ACT  222 ). 
     The minimum pressure value P 5  is a pressure value that causes the pressure value in the ink chamber  36  of the inkjet head  12  to have a margin to reach the bubble absorption pressure D (kPa) of  FIG. 4 . Even if the pressure value P in the ink tank  13  is slightly decreased from the minimum pressure value P 5 , the inkjet head  12  does not reach the bubble absorption pressure D (kPa) for a while, and does not absorb the bubbles from the nozzle  33   a.  The minimum pressure value P 5  of the ink tank  13  is optional according to characteristics of the inkjet head  12 , and is not limited to a specific value. 
     If the pressure value P in the ink tank  13  does not decrease to the minimum pressure value P 5  (No in ACT  222 ), the system controller  100  proceeds to ACT  227  in order to standby for a time T. The circulation pump  17  and the inkjet head  12  of the ink circulation mechanism  10  are directly connected to each other though the ink circulation channel  16 . Therefore, when the circulation of the ink  15  starts in the circulation channel  10   a,  or the flow rate of the ink  15  of the circulation channel  10   a  increases, the pressure in the ink tank  13  decreases. However, if a certain period of time passes, since the ink  15  is supplied to the inkjet head  12  through the ink supply path  14 , the pressure value in the ink tank  13  gradually returns. As the certain period of time passes, the pressure value in the ink tank  13  converges on a certain pressure value. After standing by for the time T for which the pressure value in the ink tank  13  converges on a certain pressure value in ACT  227 , the system controller  100  proceeds to ACT  224 . The time T until the pressure value in the ink tank  13  converges on a certain pressure value is optional according to characteristics of the ink circulation mechanism  10 , and is not limited to a specific value. 
     If the pressure value P in the ink tank  13  is equal to or lower than the minimum pressure value P 5  (Yes in ACT  222 ), the system controller  100  increases the pressure in the ink tank  13  (ACT  223 ). The system controller  100  increases the pressure of the ink tank  13 , for example, by the difference between the pressure value P detected by the pressure sensor  20  and the minimum pressure value P 5  by controlling the pressure adjustment pump driving circuit  107  so that the pressure adjustment pump  21  is driven to increase the pressure (ACT  223 ). Subsequently, the system controller  100  proceeds to ACT  224 . 
     In ACT  224 , if the pressure value P in the ink tank  13  reaches a return pressure value P 4  (Yes in ACT  224 ), the system controller  100  proceeds to ACT  225 . If the pressure value P in the ink tank  13  does not reach the return pressure value P 4  (No in ACT  224 ), the system controller  100  proceeds to ACT  223 , and increases the pressure in the ink tank  13 . The return pressure value P 4  is a pressure value to be a criterion for a timing of increasing the flow rate of the ink  15  that circulates in the circulation channel  10   a.    
     The return pressure value P 4  is a high pressure value to a degree in which the pressure in the ink chamber  36  does not decrease to the bubble absorption pressure D (kPa) even if the flow rate of the ink  15  that circulates in the circulation channel  10   a  increases later. 
     If the pressure value P in the ink tank  13  is the return pressure value P 4 , the pressure in the ink chamber  36  of the inkjet head  12  is maintained to be the negative pressure with respect to the atmospheric pressure, and the nozzle  33   a  do not absorb the bubbles. The return pressure value P 4  in the ink tank  13  is optional according to characteristics of the inkjet head  12 , and is not limited to a specific value. 
     In ACT  225 , the system controller  100  increases the flow rate of the ink  15  that circulates the circulation channel  10   a  of the ink circulation mechanism  10  by N (ml/min). In the same manner as in ACT  221 , the system controller  100  drives the circulation pump  17  by controlling the circulation pump driving circuit  105  (ACT  225 ). 
     After the flow rate of the ink  15  is increased by N (ml/min) in ACT  225 , the system controller  100  determines whether the flow rate of the ink  15  that circulates the circulation channel  10   a  reaches the rated flow rate F (ml/min) (ACT  226 ). 
     The flow rate of the ink  15  is determined by an output value of the pressure sensor  20  or a driving command sent by the system controller  100  to the circulation pump driving circuit  105 . If the flow rate of the ink  15  that circulates the circulation channel  10   a  does not reach the rated flow rate F (ml/min) (No in ACT  226 ), the system controller  100  proceeds to ACT  222 . The system controller  100  gradually increases the flow rate of the ink  15  by executing ACTS  222  to  227  until the flow rate of the ink  15  that circulates the circulation channel  10   a  reaches the rated flow rate F (ml/min). 
     If the flow rate of the ink  15  that circulates the circulation channel  10   a  reaches the rated flow rate F (ml/min) (Yes in ACT  226 ), the system controller  100  executes ACTS  203 ,  204 ,  206 , and  207  in the same manner as in the first embodiment. The system controller  100  ends the circulation of the ink  15  of the ink circulation mechanism  10  by stopping the circulation pump  17  in ACT  207 . 
     According to the second embodiment, the pressure fluctuation in the ink chamber  36  of the inkjet head  12  when the driving of the circulation pump  17  is started is illustrated in  FIG. 8 . For example, the stopped-time pressure B (kPa) of the ink chamber  36  of the inkjet head  12  is set to be −0.3 (kPa). The pressure value of the ink chamber  36  when the ink tank  13  is in the return pressure value P 4  is set to be −1.2 (kPa), and the pressure of the ink chamber  36  when the ink tank  13  is in the minimum pressure value P 5  is set to be −3.0 (kPa). The bubble absorption pressure D (kPa) of the ink chamber  36  of the inkjet head  12  is set to be −5.0 (kPa). 
     According to ACT  221 , the circulation of the ink  15  is started at the flow rate N (ml/min) in the circulation channel  10   a  at a time t 10 . The pressure of the ink chamber  36  by the absorption pressure of the circulation pump  17  gradually returns after being decreased from −0.3 (kPa) to, for example, −2.5 (kPa). According to ACT  224 , if the pressure value in the ink tank  13  is detected to reach the return pressure value P 4  (time t 11 ), the pressure of the ink chamber  36  returns up to −1.2 (kPa). 
     According to ACT  225 , the flow rate of the ink  15  in the circulation channel  10   a  is increased by N (ml/min) at the time t 11 . If the pressure value in the ink tank  13  decreases, for example, to the minimum pressure value P 5  by increasing the flow rate of the ink  15  (time t 12 ), the pressure in the ink tank  13  is increased according to ACT  223 . The ink chamber  36  is prevented from reaching −5.0 (kPa) which is the bubble absorption pressure D (kPa) by increasing the pressure in the ink tank  13  in advance. 
     After the pressure in the ink tank  13  is increased, according to ACT  224 , it is detected whether the pressure value in the ink tank  13  reaches the return pressure value P 4  (time t 13 ), if the pressure of the ink chamber  36  returns up to −1.2 (kPa), the flow rate of the ink  15  of the circulation channel  10   a  is increased by N (ml/min) 
     In the same manner, if the pressure value of the ink chamber  36  is equal to or lower than the minimum pressure value P 5 , while the pressure in the ink tank  13  is increased, the increase of the flow rate of the ink  15  is repeated X times. After the flow rate of the ink  15  that circulates the circulation channel  10   a  reaches the rated flow rate F (ml/min) by repeating the increase of the flow rate of the ink  15  X times (Yes in ACT  226 ), the ink  15  is circulated at the rated flow rate F (ml/min). 
     The foreign substances mixed in the ink  15  of the circulation channel  10   a  are removed from the filter  18 , while the ink  15  is circulated at the rated flow rate F (ml/min) (ACT  203 ,  204 , and  206 ). The bubbles mixed into the ink  15  is collected in the ink tank  13 , and absorbed into the air space  13   b  in the ink tank  13 . 
     According to the second embodiment, the pressure fluctuation of the ink chamber  36  when the circulation of the ink at the rated flow rate F is started from an ink stopped state is illustrated in  FIG. 9  as a comparative example. If the circulation of the ink  15  starts at the rated flow rate F (ml/min) in the circulation channel  10   a  at a time t 20 , after the pressure of the ink chamber  36  by the absorption pressure of the circulation pump  17  drastically decreases from −0.3 (kPa), for example, to −5.5 (kPa) (time t 21 ), the pressure of the ink chamber  36  gradually returns. However, since the pressure of the ink chamber  36  is equal to or lower than −5.0 (kPa) of the bubble absorption pressure D (kPa) at the time t 21 , the bubbles may be mixed from the nozzle  33   a.    
     According to the comparative example, the pressure of the ink chamber  36  is drastically decreased in order to suddenly circulate the ink at the rated flow rate F (ml/min) from the ink stopped state. Meanwhile, according to the second embodiment, the decrease of the pressure of the ink chamber  36  is small since the circulation starts at a low ink flow rate and the ink flow rate is gradually increased to the rated flow rate F (ml/min). 
     According to the second embodiment, in the same manner as in the first embodiment, it is possible to favorably maintain the ink ejection performance of the inkjet head  12  and enhance the quality of the printed image by removing bubbles or foreign substances included in the ink  15 . 
     According to the second embodiment, the circulation of the ink  15  in the circulation channel  10   a  at a low ink flow rate is started when the driving of the circulation pump  17  of the ink circulation mechanism  10  is started. After the start of the circulation, while the pressure of the ink chamber  36  of the inkjet head  12  is adjusted, the flow rate of the ink  15  is gradually increased until the flow rate reaches the rated flow rate F (ml/min). 
     The absorption pressure due to the start of driving the circulation pump  17  of the ink circulation mechanism  10  is alleviated, and the ink chamber  36  drastically decreases the pressure, so that the pressure reaches a value which is equal to or lower than the bubble absorption pressure D (kPa). Regardless of the absorption pressure when the circulation of the ink  15  is started, it is possible to prevent the nozzle  33   a  of the inkjet head  12  from absorbing the bubbles. Further, when the circulation of the ink  15  is started, it is possible to maintain the pressure in the ink chamber  36  of the inkjet head  12  in a certain appropriate scope. 
     The liquid ejecting apparatus according to the embodiment described above is not limited to the inkjet printer, and may be used in a facsimile apparatus, a copying machine, or the like that form images in an inkjet method. Further, the liquid ejecting apparatus can eject liquid in addition to the ink. The liquid ejecting apparatus that ejects liquid other than the ink may be an apparatus that ejects liquid including, for example, conductive particles for forming a wiring pattern of the printed wiring board. 
     According to at least one embodiment described above, it is possible to favorably maintain the ink ejection performance of the inkjet head and enhance the quality of the printed image by circulating the ink by the ink circulation mechanism and removing bubbles, foreign substances, or the like in the ink. The pressure of the ink chamber  36  is decreased by the absorption pressure by the start of driving of the circulation pump  17 , but the pressure in the ink chamber when the circulation of the ink is started is appropriately maintained and the absorption of the bubbles from the nozzle is prevented so that it is possible to provide the printed image in a good quality by the prominent ink ejection performance. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.