Patent Publication Number: US-8113636-B2

Title: Inkjet head and inkjet recording apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2007-184480, filed Jul. 13, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet head and an inkjet recording apparatus, which can maintain a constant pressure in the vicinity of nozzles. 
     2. Description of the Related Art 
     For example, PCT National Pub. No. 2002-533247 discloses the following, as an inkjet head with the pressure loss improved. The inkjet head comprises nozzles, an inlet manifold, an outlet manifold, and an array of fluid chambers connected to the manifolds. The inkjet head further comprises a member for generating fluid flows running through the inlet manifold and the chambers in the array into the outlet manifold. 
     In the inkjet head, fluid flows running through the chambers sufficiently prevent foreign matters in the fluid from remaining in the nozzles. In the meantime, demand for such ink-circulating inkjet heads has increased as being highly-reliable inkjet heads, because of high cleanliness of ink channels thereof. 
     However, in ink-circulating inkjet heads, the pressure in the vicinity of nozzles varies by influence of the length of the pipes of the ink channels and the diameter of the ink channels. Further, to prevent variations in ink discharge, it is necessary to keep a constant pressure in the vicinity of the nozzles. To control the pressure in the vicinity of the nozzles to a constant value in the above conventional inkjet head, it is necessary to adjust the channel resistance of the ink channels, and adjust the pressure of the ink tank. According to these methods, high accuracy is required in adjustment of the channel resistance and adjustment of the tank pressure, and there is room for improvement. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an inkjet head which can control the pressure in the vicinity of the nozzles to a constant value with a simple structure. 
     Another object of the present invention is to provide an inkjet recording apparatus which can control the pressure in the vicinity of the nozzles to a constant value with a simple structure. 
     To achieve the above object, an inkjet head according to an aspect of the present invention comprises: a nozzle to discharge liquid droplets; a pressure chamber which is configured to communicate with the nozzle and filled with liquid; a supply section which is configured to communicate with the pressure chamber and supplies the liquid to the pressure chamber; a recovery section which is configured to communicate with the pressure chamber and recovers the liquid from the pressure chamber; a bypass channel which is independent of the pressure chamber and connects the supply section with the recovery section; a pressure-control liquid chamber which is connected at one end to the bypass channel and connected at the other end to the atmosphere; and a porous member which is contained inside the pressure-control liquid chamber. 
     To achieve the above object, an inkjet head according to another aspect of the present invention comprises: a nozzle to discharge liquid droplets; a pressure chamber which is configured to communicate with the nozzle and filled with liquid; a supply section which is configured to communicate with the pressure chamber and supplies the liquid to the pressure chamber; a recovery section which is configured to communicate with the pressure chamber and recovers the liquid from the pressure chamber; a pressure-control liquid chamber which is connected at one end to the supply section and the recovery section and connected at the other end to the atmosphere; a porous member which is contained inside the pressure-control liquid chamber; a first communicating channel which connects the supply section with the pressure-control liquid chamber; and a second communicating channel which connects the recovery section with the pressure-control liquid chamber, wherein channel resistance from the nozzle to the first communicating channel is equal to channel resistance from the nozzle to the second communicating channel. 
     To achieve the above object, an inkjet recording apparatus according to an aspect of the present invention comprises: an inkjet head; a tank to supply liquid to the inkjet head; and a circulation mechanism which circulates the liquid between the inkjet head and the tank, wherein the inkjet head includes: a nozzle to discharge liquid droplets; a pressure chamber which is configured to communicate with the nozzle and filled with the liquid; a supply section which is configured to communicate with the pressure chamber and supplies the liquid to the pressure chamber; a recovery section which is configured to communicate with the pressure chamber and recovers the liquid from the pressure chamber; a bypass channel which is independent of the pressure chamber and connects the supply section with the recovery section; a pressure-control liquid chamber which is connected at one end to the bypass channel and connected at the other end to the atmosphere; and a porous member which is contained inside the pressure-control liquid chamber. 
     To achieve the above object, an inkjet recording apparatus according to another aspect of the present invention comprises: an inkjet head; a tank to supply liquid to the inkjet head; and a circulation mechanism which circulates the liquid between the inkjet head and the tank, wherein the inkjet head includes: a nozzle to discharge liquid droplets; a pressure chamber which is configured to communicate with the nozzle and filled with the liquid; a supply section which is configured to communicate with the pressure chamber and supplies the liquid to the pressure chamber; a recovery section which is configured to communicate with the pressure chamber and recovers the liquid from the pressure chamber; a pressure-control liquid chamber which is connected at one end to the supply section and the recovery section and connected at the other end to the atmosphere; a porous member which is contained inside the pressure-control liquid chamber; a first communicating channel which connects the supply section with the pressure-control liquid chamber; and a second communicating channel which connects the recovery section with the pressure-control liquid chamber, and channel resistance from the nozzle to the first communicating channel is equal to channel resistance from the nozzle to the second communicating channel. 
     According to the present invention, it is possible to provide an inkjet head which can control the pressure in the vicinity of the nozzles to a constant value with a simple structure. 
     Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a schematic diagram illustrating an inkjet recording apparatus according to a first embodiment. 
         FIG. 2  is a perspective view of an inkjet head illustrated in  FIG. 1 . 
         FIG. 3  is a perspective view of a head main body of the inkjet head illustrated in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the head main body illustrated in  FIG. 3 , taken along line F 4 -F 4  of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the head main body illustrated in  FIG. 3 , taken along line F 5 -F 5  of  FIG. 3 . 
         FIG. 6  is a schematic diagram of an inkjet recording apparatus according to a second embodiment. 
         FIG. 7  is a perspective view of a head main body of an inkjet head illustrated in  FIG. 6 . 
         FIG. 8  is a cross-sectional view of the head main body illustrated in  FIG. 7 , taken along line F 8 -F 8  of  FIG. 7 . 
         FIG. 9  is a cross-sectional view of the head main body illustrated in  FIG. 7 , taken along line F 9 -F 9  of  FIG. 7 . 
         FIG. 10  is a perspective view of an inkjet head of an inkjet recording apparatus according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of an inkjet recording apparatus according to the present invention will be described below with reference to drawings. 
     As illustrated in  FIG. 1 , an inkjet recording apparatus  11  comprises an inkjet head  12  which discharges liquid droplets to sheet-like recording media, a tank  13  to supply liquid to the inkjet head  12 , and a circulation mechanism  14  which circulates the liquid between the inkjet head  12  and the tank  13 . The tank  13  stores liquid inside. The liquid is formed of ink (pigment ink) or the like which can form characters and images on sheet-like recording media. The tank  13  has an air release valve  15 , and can set the internal pressure to the atmospheric pressure by opening the air release valve  15 , and change the internal pressure from the atmospheric pressure by closing the air release valve  15 . 
     The circulation mechanism  14  has a circulating channel  16  having an annular shape, a pump  17  which is provided at a given point inside the circulating channel  16  and circulates a liquid in the circulating channel in the direction indicated by an arrow, and a filter member  18  provided at the given point in the circulating channel  16 . Foreign matters mixed in the liquid can be collected by the filter member  18 . 
     As illustrated in  FIG. 2 , the inkjet head  12  has a head main body  21 , a pair of circuit boards  22  attached to the head main body  21 , a supply port  23  to supply the liquid to the head main body  21 , a recovery port  24  which recovers the liquid from the head main body  21 , and an air release port  25  which is connected to the head main body  21 . Each circuit board  22  has head drive ICs  26 . The supply port  23  and the recovery port  24  are connected to the circulating channel  16 . 
       FIG. 3  is a perspective view of an internal portion of the head main body  21 , with part of the head main body  21  cut away. As illustrated in  FIG. 3 , the head main body  21  has a nozzle plate  31 , a plurality of nozzles  32  formed in the nozzle plate  31 , pressure chambers  33  corresponding to the respective nozzles  32 , drive elements  34  which are arranged on both sides of each pressure chamber  33  and hold each pressure chamber  33  therebetween, a supply channel  35  and a recovery channel  36  which communicate with the pressure chambers  33 , a bypass channel  37  which is independent of the pressure chambers  33  and connects the supply channel  35  with the recovery channel  36 , a pressure-control liquid chamber  38  which is connected at one end to the bypass channel  37  and opened at the other end to the atmosphere, and a porous member  39  which is contained inside the pressure-control liquid chamber  38 . 
     The nozzles  32  are formed in a line on the nozzle plate  31 . Each nozzle  32  can discharge liquid droplets. The pressure chambers  33  are formed of a plurality of groove portions formed in a piezoelectric member  43 . The pressure chambers  33  are configured to communicate with the respective nozzles  32 , and to be filled with the liquid. The piezoelectric member  43  is formed by bonding two piezoelectric element plates made of lead zirconium titanate (PZT). The two plates are bonded such that their polarization directions are opposite to each other. The drive elements  34  are formed of columns configured to be adjacent to both sides of each pressure chamber  33 . 
     When the inkjet recording apparatus  11  receives instruction from the user to start printing in the state where the pressure chamber  33  is filled with a liquid, a control section (not shown) of the inkjet recording apparatus  11  outputs a printing signal for the inkjet head  12  to the head drive ICs  26 . The head drive ICs  26  which have received the printing signal apply a drive pulse voltage to the drive elements  34 . Thereby, a pair of drive elements  34  perform share mode deformation and are curved away from each other. Then, the drive elements  34  are returned to the original positions to pressurize the liquid in the corresponding pressure chamber  33  held between the drive elements  34 , and thereby liquid droplets are ejected from the relevant nozzle  32 . 
     As illustrated in  FIG. 5 , the supply channel  35  is connected to the pressure chambers  33  through a shared liquid chamber  44 . The supply channel  35  is also connected to the supply port  23  through a rectifier section  45 . The supply channel  35  can supply the liquid to the pressure chambers  33 . The recovery channel  36  is connected to the recovery port  24 , and can recover the liquid from the pressure chambers  33 . The term “supply section” in the claims indicates a concept including the supply port  23  and the supply channel  35 . The term “recovery section” in the claims indicates a concept including the recovery port  24  and the recovery channel  36 . 
     As illustrated in  FIG. 3 , the bypass channel  37  is provided independent of the pressure chambers  33 , in a position close to the end portion of inkjet head  12 . An opening portion  37 A is formed at a given point inside the bypass channel  37 , and the bypass channel  37  communicates with the pressure-control liquid chamber  38  through the opening portion  37 A. As illustrated in  FIG. 4 , the pressure-control liquid chamber  38  has an air release hole  46 , and thereby the internal portion of the pressure-control liquid chamber  38  is opened to the atmosphere. The air release hole  46  is connected to the air release port  25 . 
     The porous member  39  is a spongy member which can absorb liquid and hold the liquid therein. The porous member  39  is formed of polyurethane foam or the like. The porous member  39  has a plurality of minute holes  40 , which communicate with each other. The porous member  39  exhibits a capillary phenomenon for liquid. The material of the porous member  39  is not limited to the above, but the porous member  39  may be formed of a laminated fiber structure made of a thermoplastic resin. 
     As illustrated in  FIG. 4 , the porous member  39  has a first portion  39 A which is densely formed, and a second portion  39 B which is sparsely formed. The first portion  39 A is disposed in a position adjacent to the bypass channel  37 . The second portion  39 B is disposed in a position adjacent to the air release hole  46 . As illustrated in  FIG. 1 , the first portion  39 A is disposed under the second portion  39 B. The intensity of the capillary action is inversely proportional to the inside diameter of the pipe. Therefore, for example, when the density of the porous member  39  increases and the inside diameter of the pipe is narrowed as in the first portion  39 A, the capillary action of the first portion  39 A is larger than that of the second portion  39 B. 
     To use the inkjet recording apparatus  11  having the above structure, first, the pump  17  is driven in the state where the air release valve  15  of the tank  13  is opened, and thereby the inkjet head  12  is filled with the liquid. Then, when a certain volume of liquid is filled in the porous member  39  in the pressure-control liquid chamber  38 , filling of the liquid is stopped. In this state, the capillary action of the porous member  39  influences the pressure chambers  33 , and the pressure of the pressure chambers  33  is controlled to a weak negative pressure. 
     More specifically, when the capillary action of the porous member  39  influences the liquid, the liquid is drawn up and rises. The liquid stops at a position where the pressure of meniscuses of the nozzles  32  balances the capillary action. In the inkjet head  12 , the liquid is opened to the atmosphere in the nozzles  32  and the porous member  39 . When the liquid rises under the capillary action, meniscuses of the nozzles  32  recede upward. Specifically, the pressure in the vicinity of the nozzles  32  is maintained at a negative pressure, which is lower than the atmospheric pressure. 
     Next, after the air release valve  15  is closed, the pump  17  is driven to circulate the liquid through the circulating channel  16 . Also in this state where the liquid is circulated, the pressure chambers  33  in the vicinity of the nozzles  32  is maintained at a weak negative pressure by the capillary action of the porous member  39 . 
     Specifically, when the pressure in the inkjet head  12  is about to fall, the amount of the liquid maintained inside the porous member  39  is reduced. In this case, the liquid level of the liquid impregnated in the porous member  39  lowers, and the liquid is maintained only by the dense first portion  39 A of the porous member  39 . In this state, the porous member  39  exhibits an intense capillary action, and functions to draw the liquid from the tank  13  and the circulating channel  16  into the porous member  39 . Thereby, an exhaustion of the liquid in the inkjet head  12  is prevented, and the pressure in the inkjet head  12  is maintained at a constant weak negative pressure. 
     In addition, when the pressure in the inkjet head  12  is about to rise, the amount of the liquid impregnated into the porous member  39  increases, and the liquid level of the liquid rises. In this case, the liquid level of the liquid reaches the sparse second portion  39 B. In this state, the porous member  39  exhibits a weak capillary action. As a result, the amount of the liquid drawn from the tank  13  and the circulating channel  16  into the inkjet head  12  is reduced, and the pressure in the inkjet head  12  can be maintained at a constant weak pressure. 
     According to the first embodiment described above, the inkjet recording apparatus  11  has the inkjet head  12 , the tank  13  to supply the liquid to the inkjet head  12 , and the circulation mechanism  14  which circulates the liquid between the inkjet head  12  and the tank  13 . The inkjet head  12  has the nozzles  32  to discharge liquid droplets, the pressure chambers  33  which are configured to communicate with the respective nozzles  32  and filled with the liquid, the supply channel  35  which is configured to communicate with the pressure chambers  33  and supplies the liquid to the pressure chambers  33 , the recovery channel  36  which is configured to communicate with the pressure chambers  33  and recovers the liquid from the pressure chambers  33 , the bypass channel  37  which is independent of the pressure chambers  33  and connects the supply channel  35  with the recovery channel  36 , the pressure-control liquid chamber  38  which is connected at one end to the bypass channel  37  and opened at the other end to the atmosphere, and the porous member  39  which is contained inside the pressure-control liquid chamber  38 . 
     According to the above structure, since the inkjet head  12  has the pressure-control liquid chamber  38  opened to the atmosphere, it is possible to maintain the inside of the inkjet head  12  at almost the atmospheric pressure. In addition, since the porous member  39  is disposed inside the pressure-control liquid chamber  38 , it is possible to cause capillary action to influence the liquid. Thereby, the liquid is drawn upward to cause the meniscuses of the nozzles  32  to recede, and the pressure in the vicinity of the nozzles  32  can be maintained at a weak negative pressure lower than the atmospheric pressure. Therefore, it is possible to maintain a suitable pressure for liquid discharge in the vicinity of the nozzles  32  of the inkjet head  12 , regardless of the length of the pipe of the circulating channel  16 , the diameter of the circulating channel  16 , and the pressure of the tank  13 . 
     In this case, the porous member  39  has the first portion  39 A which is densely formed, and the second portion  39 B which is sparsely formed, and the first portion  39 A is disposed under the second portion  39 B. According to this structure, when the liquid is scarce in the inkjet head  12  and the pressure is about to fall, the liquid level of the liquid in the pressure-control liquid chamber  38  lowers, and the liquid is impregnated and held only in the first portion  39 . Therefore, an intense capillary action influences the liquid, and the liquid is drawn from the tank  13  and the circulating channel  16  into the inkjet head  12 . This prevents scarcity of the liquid in the inkjet head  12 . 
     On the other hand, when the liquid in the inkjet head  12  increases and the pressure is about to rise, the liquid level of the liquid in the pressure-control liquid chamber  38  rises, and the liquid is impregnated and held in both the first portion  39 A and the second portion  39 B of the porous member  39 . In this case, a weak capillary action influences the liquid in the pressure-control liquid chamber  38 , and thus the amount of the liquid drawn from the tank  13  and the circulating channel  16  into the inkjet head  12  is reduced. This prevents supply of a large amount of liquid to the inkjet head  12 , and rise of the pressure in the inkjet head  12 . 
     Next, a second embodiment including an inkjet recording apparatus  51  is explained below with reference to  FIGS. 6 and 9 . Although the inkjet recording apparatus  51  of the second embodiment is different from the first embodiment in the structure of an inkjet head  52 , the other constituent elements thereof are the same as those of the first embodiment. Therefore, constituent elements which are different from the first embodiment are mainly explained, and the same constituent elements as the first embodiment are denoted by the same respective reference numbers and not explained in the second embodiment. 
     The inkjet head  52  has a head main body  21 , a pair of circuit boards  22  attached to the head main body  22 , a supply port  23  to supply liquid to the head main body  21 , a recovery port  24  to recover the liquid from the head main body  21 , and an air release port  25  connected to the head main body  21 . 
     As illustrated in  FIGS. 6 and 7 , the head main body  21  has a nozzle plate  31 , a plurality of nozzles  32  formed in the nozzle plate  31 , pressure chambers  33  corresponding to the respective nozzles  32 , drive elements  34  which are arranged on both sides of each pressure chamber  33  and hold each pressure chamber  33  therebetween, a supply channel  35  and a recovery channel  36  which communicate with the pressure chambers  33 , a pressure-control liquid chamber  38  which is connected at one end to the supply channel  35  and the recovery channel  36  and opened at the other end to the atmosphere, a porous member  39  which is contained inside the pressure-control liquid chamber  38 , a first communicating channel  53  which connects the supply channel  35  with the pressure-control liquid chamber  38 , and a second communicating channel  54  which connects the recovery channel  36  with the pressure-control liquid chamber  38 . 
     The nozzles  32  are formed in a line on the nozzle plate  31 . The supply channel  35  is connected to the supply port  23 , and can supply the liquid to the pressure chambers  33 . The recovery channel  36  is connected to the recovery port  24 , and can recover the liquid from the pressure chambers  33 . The channel diameter and the length of the supply channel  35  are equal to the channel diameter and the length of the recovery channel  36 . Further, the channel diameter and the length of the first communicating channel  53  are equal to the channel diameter and the length of the second communicating channel  54 . Therefore, the channel resistance from the nozzles  32  to the first communicating channel  53  is equal to the channel resistance from the nozzles  32  to the second communicating channel  54 . The term “supply section” in the claims indicates a concept including the supply port  23  and the supply channel  35 . Further, the term “recovery section” in the claims indicates a concept including the recovery port  24  and the recovery channel  36 . 
     The pressure-control liquid chamber  38  has an air release hole  46 . The air release hole  46  is connected to the air release port  25 . The porous member  39  is a spongy member which can absorb liquid and hold the liquid therein, and is formed of polyurethane foam or the like. The porous member  39  has a plurality of minute holes  40 , which communicate with each other. The porous member  39  exhibits a capillary phenomenon for liquid. 
     As illustrated in  FIGS. 7 and 9 , the porous member  39  has a first portion  39 A which is densely formed, and a second portion  39 B which is sparsely formed. The first portion  39 A is disposed in a position adjacent to the first communicating channel  53  and the second communicating channel  54 . The second portion  39 B is disposed in a position adjacent to the air release hole  46 . The first portion  39 A is disposed under the second portion  39 B. 
     To use the inkjet recording apparatus  51  having the above structure, first, the pump  17  is driven in the state where the air release valve  15  of the tank  13  is opened, and thereby the inkjet head  52  is filled with the liquid. Then, when a certain volume of liquid is filled in the porous member  39 , filling of the liquid is stopped. In this state, the capillary action of the porous member  39  influences the pressure chambers  33 , and the pressure of the pressure chambers  33  is controlled to a weak negative pressure which is suitable for liquid discharge. 
     Next, after the air release valve  15  is closed, the pump  17  is driven to circulate the liquid through the circulating channel  16 . Also in this state where the liquid is circulated, the pressure chambers  33  in the vicinity of the nozzles  32  is maintained at a weak negative pressure by the capillary action of the porous member  39 . 
     More specifically, when the pressure in the inkjet head  52  is about to fall, the amount of the liquid maintained inside the porous member  39  is reduced. In this case, the liquid level of the liquid impregnated in the porous member  39  lowers, and the liquid is maintained only by the dense first portion  39 A of the porous member  39 . In this state, the porous member  39  exhibits an intense capillary action, and functions to draw the liquid from the tank  13  and the circulating channel  16  into the porous member  39 . Thereby, the pressure in the inkjet head  52  is increased, and an exhaustion of the liquid in the inkjet head  52  is prevented. 
     In addition, when the pressure in the inkjet head  52  is about to rise, the amount of the liquid impregnated into the porous member  39  increases, and the liquid level of the liquid rises. In this case, the liquid level of the liquid reaches the sparse second portion  39 B. In this state, the porous member  39  exhibits a weak capillary action. As a result, the amount of the liquid drawn from the tank  13  into the inkjet head  52  is reduced, and the pressure in the inkjet head  52  can be maintained at a constant weak pressure. 
     In the state where the liquid is circulated, the pressure on the upstream side as viewed from the nozzles  32  is higher, and the pressure on the downstream side as viewed from the nozzles  32  is lower. However, since the channel resistance from the nozzles  32  to the first communicating channel  53  is equal to the channel resistance from the nozzles  32  to the second communicating channel  54 , the pressure in the vicinity of the nozzles  32  can be set to the same state as that before the liquid is circulated. 
     According to the second embodiment, the inkjet recording apparatus  51  has the inkjet head  52 , the tank  13  to supply the liquid to the inkjet head  52 , and the circulation mechanism  14  which circulates the liquid between the inkjet head  52  and the tank  13 . The inkjet head  52  has the nozzles  32  to discharge liquid droplets, the pressure chambers  33  which are configured to communicate with the respective nozzles  32  and filled with the liquid, the supply channel  35  which is configured to communicate with the pressure chambers  33  and supplies the liquid to the pressure chambers  33 , the recovery channel  36  which is configured to communicate with the pressure chambers  33  and recovers the liquid from the pressure chambers  33 , the pressure-control liquid chamber  38  which is connected at one end to the supply channel  35  and the recovery channel  36  and opened at the other end to the atmosphere, the porous member  39  which is contained inside the pressure-control liquid chamber  38 , the first communicating channel  53  which connects the supply channel  35  with the pressure-control liquid chamber  38 , and the second communicating channel  54  which connects the recovery channel  36  with the pressure-control liquid chamber  38 . The channel resistance from the nozzles  32  to the first communicating channel  53  is equal to the channel resistance from the nozzles  32  to the second communicating channel  54 . 
     According to the above structure, since there is provided the pressure-control liquid chamber  38  opened to the atmosphere, it is possible to maintain the inside of the inkjet head  52  at almost the atmospheric pressure. In addition, since the porous member  39  is disposed inside the pressure-control liquid chamber  38 , it is possible to cause the capillary action to influence the liquid. Thereby, the liquid is drawn upward to cause the meniscuses of the nozzles  32  to recede, and the pressure in the vicinity of the nozzles  32  can be maintained at a weak negative pressure lower than the atmospheric pressure. Therefore, it is possible to maintain a suitable pressure for liquid discharge in the vicinity of the nozzles  32  of the inkjet head  52 , regardless of the length of the pipe of the circulating channel  16 , the diameter of the circulating channel  16 , and the pressure of the tank  13 . 
     Further, generally, in the case of providing a structure like the first communicating channel  53  and the second communicating channel  54 , there occurs a phenomenon in the porous member  39  that the pressure on the upstream side is high and the pressure on the downstream side is low, when the liquid is circulated. However, according to the second embodiment, the channel resistance from the nozzles  32  to the first communicating channel  53  is equal to the channel resistance from the nozzles  32  to the second communicating channel  54 . Therefore, even in the above structure provided with the first communicating channel  53  and the second communicating channel  54 , it is possible to maintain a negative pressure similar to that in the state where the liquid is not circulated. 
     In this case, the porous member  39  has the first portion  39 A which is densely formed, and the second portion  39 B which is sparsely formed, and the first portion  39 A is disposed under the second portion  39 B. According to this structure, when the pressure is about to fall, the liquid is impregnated and held only in the first portion  39 A. Therefore, an intense capillary action influences the liquid, and the liquid is drawn from the tank  13  and the circulating channel  16  into the inkjet head  52 . This prevents scarcity of the liquid in the inkjet head  52 . 
     On the other hand, when the liquid in the inkjet head  52  increases and the pressure is about to rise, the liquid is impregnated and held in both the first portion  39 A and the second portion  39 B of the porous member  39 . In this case, a weak capillary action influences the liquid, and thus the amount of the liquid drawn from the tank  13  and the circulating channel  16  into the inkjet head  52  is reduced. This prevents supply of a large amount of liquid to the inkjet head  52 , and rise of the pressure in the inkjet head  52 . 
     Next, a third embodiment including an inkjet recording apparatus  61  is explained below with reference to  FIG. 10 . Although the inkjet recording apparatus  61  of the third embodiment is different from the first embodiment in the drive elements  34  and the number of nozzle lines of an inkjet head  62 , the other constituent elements thereof are the same as those of the first embodiment. Therefore, constituent elements which are different from the first embodiment are mainly explained, and the same constituent elements as the first embodiment are denoted by the same respective reference numbers and not explained in the third embodiment. 
     The inkjet head  62  has a head main body  21 , a pair of circuit boards  22  attached to the head main body  22 , a supply port  23  to supply liquid to the head main body  21 , a recovery port  24  to recover the liquid from the head main body  21 , and an air release port  25  connected to the head main body  21 . 
     As illustrated in  FIG. 10 , the head main body  21  has a nozzle plate  31 , a plurality of nozzles  32  formed in two lines on the nozzle plate  31 , pressure chambers  33  formed in two lines in correspondence with the nozzles  32  formed in two lines, drive elements  34  which are arranged on both sides of each pressure chamber  33  and hold each pressure chamber  33  therebetween, supply channels  35  and recovery channels  36  which communicate with the pressure chambers  33 , a bypass channel  37  which is independent of the pressure chambers  33  and connects the supply channels  35  with the recovery channels  36 , a pressure-control liquid chamber  38  which is connected at one end to the bypass channel  37  and opened at the other end to the atmosphere, and a porous member  39  which is contained inside the pressure-control liquid chamber  38 . 
     The nozzles  32  are formed in two lines on the nozzle plate  31 . The pressure chambers  33  are formed of a plurality of groove portions which are formed in piezoelectric members  43  arranged in two lines. 
     The supply channels  35  are connected to the supply port  23 , and can supply the liquid to the pressure chambers  33 . The recovery channels  36  are connected to the recovery port  24 , and can recover the liquid from the pressure chambers  33 . 
     The bypass channel  37  is formed in a position close to the end portion of the inkjet head  62 . An opening portion  37 A to communicate with the pressure-control liquid chamber  38  is formed at the given point in the bypass channel  37 . The pressure-control liquid chamber  38  has an air release hole  46 . 
     The porous member  39  is a spongy member which can absorb liquid and hold the liquid therein. The porous member  39  is formed of polyurethane foam or the like. The porous member  39  has a plurality of minute holes  40 , which communicate with each other. The porous member  39  exhibits a capillary phenomenon for liquid. The structure of the porous member  39  is the same as that in the first embodiment. Specifically, the porous member  39  has a first portion  39 A which is densely formed, and a second portion  39 B which is sparsely formed. The first portion  39 A is disposed in a position adjacent to the bypass channel  37 . The second portion  39 B is disposed in a position adjacent to the air release hole  46 . The first portion  39 A is disposed under the second portion  39 B. 
     In the inkjet recording apparatus  61  having the above structure, when the pressure in the inkjet head  62  is about to fall, the porous member  39  exhibits an intense capillary action in the same manner as the first embodiment, and functions to draw the liquid from the tank  13  and the circulating channel  16  into the porous member  39 . Thereby, the pressure in the inkjet head  62  is increased, and exhaustion of the liquid in the inkjet head  62  is prevented. Further, the pressure in the inkjet head  62  is maintained at a constant weak negative pressure. 
     On the other hand, when the pressure in the inkjet head  62  is about to rise, the porous member  39  exhibits a weak capillary action in the same manner as the first embodiment. As a result, the amount of the liquid drawn from the tank  13  into the inkjet head  62  is reduced, and the pressure in the inkjet head  62  can be maintained at a constant weak negative pressure. 
     According to the third embodiment, even in the structure in which the nozzles  32  and the drive elements  34  are arranged in two lines, the pressure in the vicinity of the nozzles  32  can be maintained at a constant negative pressure, by the pressure-control liquid chamber  38  opened to the atmosphere and the porous member  39 . 
     The present invention is not limited to the above embodiments, but can be carried out with various modifications as a matter of course, within a range not departing from the gist of the invention. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.