Patent Publication Number: US-7905576-B2

Title: Liquid ejection apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid ejection apparatus (mechanism) and an image forming apparatus, and more particularly to a liquid ejection apparatus including an ink supply system and a serial head connected with a sub tank for supplying ink, the sub tank being connected to a main tank for supplying the ink. 
     2. Description of the Related Art 
     In recent years, due to requirements for higher image quality, particular emphasis has been given to research into stabilizing the ejection volume of liquid droplets and improving the accuracy of the depositing positions. Japanese Patent Application Publication No. 2004-42395 discloses technology of stabilizing the flight direction of the ink droplets.  FIGS. 11A to 11E  are diagrams showing the progress of the ejection of ink from a nozzle according to Japanese Patent Application Publication No. 2004-42395. As shown in  FIGS. 11A to 11E , a second bubble generation chamber  26 B which abuts against a nozzle plate  40  (orifice substrate), is provided. Thereby, the ink that is to be ejected through expansion of an ejection bubble  48  is prevented from reversely flowing into the supply flow channel, the ejection speed of an ink droplet  50  is increased, and also the flight direction of the ink droplet  50  is stabilized. 
     However, in the inkjet printer described in Japanese Patent Application Publication No. 2004-42395, since the second bubble generation chamber  26 B abutting against the nozzle plate  40  is provided as shown in  FIGS. 11A to 11E , then a liquid pool  70  occurs at the corner sections of the second bubble generation chamber  26 B when the expansion of the ejection bubble  48  progresses to a state (shown in  FIG. 11B ) immediately before the ejection bubble  48  is incorporated into the atmosphere. Hence, when the ejection bubble  48  is discharged and is incorporated into the atmosphere ( FIGS. 11D and 11E ), then the liquid pool  70  is also discharged in the form of a droplet, resulting in the generation of a satellite droplet  52 . The frequency of occurrence of dot disturbance may be thereby increased, and consequently, it is difficult to achieve the desired dot shape. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of these circumstances, an object thereof being to provide a liquid ejection apparatus which prevents the occurrence of a liquid pool at the corner sections of a bubble generation chamber so that a satellite droplet is not generated when liquid is ejected from a nozzle. 
     In order to attain the aforementioned object, the present invention is directed to a liquid ejection apparatus comprising: a nozzle plate in which a nozzle is formed; a substrate which is opposite to the nozzle plate and is provided with a heat generating element; a bubble generation chamber which has an internal diameter greater than an internal diameter of the nozzle, at a boundary between the bubble generation chamber and the nozzle plate; a liquid storage chamber which stores liquid to be supplied to the bubble generation chamber through a supplying channel; a heat generating element drive device which drives the heat generating element to generate and expand an ejection bubble in the liquid in the bubble generation chamber so that the liquid in the bubble generation chamber is ejected from the nozzle by means of the ejection bubble; and an air bubble incorporation unit which incorporates air bubbles into the liquid stored in the liquid storage chamber. 
     In this aspect of the present invention, the air bubbles are intentionally incorporated into the liquid in the liquid storage chamber by the air bubble incorporation unit, thereby increasing the content ratio of the air bubbles in the liquid. Hence, while the liquid is pushed by the ejection bubble generated by the heat generating element and is ejected from the nozzle in the form of a liquid droplet, the air bubbles incorporated into the liquid coalesce into an air pool at a corner section of the bubble generation chamber at the vicinity of the nozzle plate (at the boundary between the nozzle plate and the bubble generation chamber). Therefore, the liquid pool section does not occur, and accordingly it is possible to prevent the occurrence of the satellite droplets. Consequently, an image can be formed by means of dots having desired shapes. 
     Preferably, the bubble generation chamber includes: a first bubble generation chamber section in which the ejection bubble is generated by the heat generating element; and a second bubble generation chamber section which is interposed between the nozzle plate and the first bubble generation chamber section and regulates a flow direction of the liquid caused by the ejection bubble to be a direction toward the nozzle. 
     In this aspect of the present invention, the direction of the liquid flow due to the expansion of the ejection bubble is restricted to the direction toward the nozzle in the second bubble generation chamber, and hence the ejection efficiency of the liquid is improved and it is possible to achieve greater stability of the ejection volume. 
     Preferably, the air bubble incorporation unit includes a porous member disposed in the liquid storage chamber. 
     In this aspect of the present invention, when the liquid in the liquid storage chamber is consumed due to the liquid supply to the bubble generation chamber and the liquid storage chamber is then replenished with the liquid in accordance with the liquid ejection from the nozzle, then air is absorbed in the air bubble incorporation unit constituted by the porous member disposed in the liquid storage chamber, and the absorbed air is incorporated into the liquid in the liquid storage chamber and the bubble generation chamber, in the form of air bubbles. Consequently, it is possible to incorporate the air bubbles into the liquid in the bubble generation chamber, in a reliable fashion, by means of a simple structure. Therefore, the liquid pool section is reliably prevented from occurring, and hence it is possible to prevent the occurrence of the satellite droplets and an image can be formed by means of dots having desired shapes. 
     The air bubble incorporation member may be disposed in any position inside the liquid storage section, and desirably, the air bubble incorporation member is disposed in a position such that the air and liquid can pass through the air bubble incorporation member and the air bubbles are thereby incorporated into the liquid in a reliable fashion even in the case of minute increase or decrease in the amount of liquid in the liquid storage section. For example, in the case of a composition in which the bubble generation chamber and the nozzle are disposed below the liquid storage chamber and the liquid for supply flows in a downward direction, if the air bubble incorporation member is disposed in an upper (uppermost) position in the liquid storage chamber, the circumstances of the air bubble incorporation member are changeable even during little increase or decrease in the amount of liquid. Therefore, it is preferable that the air bubble incorporation member is disposed in an upper (uppermost) position in the liquid storage chamber, since change in the liquid level is liable to cause the air bubbles incorporation even during minute increase or decrease in the amount of liquid. 
     Preferably, the air bubble incorporation unit includes: a filter member; and a pump which supplies the air bubbles to the liquid in the liquid storage chamber through the filter member. 
     In this aspect of the present invention, by supplying air bubbles from the pump through the filter member, air is incorporated into the liquid in the liquid storage chamber and the bubble generation chamber, in the form of air bubbles. Therefore, it is possible to incorporate the air bubbles into the liquid in the bubble generation chamber, in a reliable fashion. Consequently, the liquid pool section is reliably prevented from occurring, and therefore it is possible to prevent the occurrence of the satellite droplets and an image can be formed by means of dots having desired shapes. 
     Preferably, the second bubble generation chamber section has a tapered shape to regulate the flow direction of the liquid caused by the ejection bubble to be the direction toward the nozzle. 
     In this aspect of the present invention, the second bubble generation chamber section has a tapered shape, and therefore, the ejection efficiency of the liquid can be improved more reliably and greater stability of the ejection volume can be achieved. 
     Preferably, a following condition is satisfied: (D 2 −D 1 )/16≦d≦D 1 /8, where d is a diameter of the air bubbles incorporated in the liquid by the air bubble incorporation unit, D 1  is the internal diameter of the nozzle, and D 2  is the internal diameter of the bubble generation chamber at the boundary between the bubble generation chamber and the nozzle plate. 
     In this aspect of the present invention, it is possible to cause the air bubbles incorporated into the liquid to combine together efficiently, thereby generating the air pool at the corner section of the bubble generation chamber in the vicinity of the nozzle plate (at the boundary between the nozzle plate and the bubble generation chamber), and hence the generation of the satellite droplets can be prevented effectively at the same time as improving the ejection characteristics. 
     Since the diameter d of the air bubbles to be incorporated into the liquid by the air bubble incorporation unit has a correlation with the pore diameter of the air bubble incorporation unit, then it is possible to adjust the diameter of the air bubbles to be incorporated into the liquid by appropriately selecting the pore diameter of the air bubble incorporation unit. 
     In order to attain the aforementioned object, the present invention is also directed to an image forming apparatus comprising any one of the liquid ejection apparatuses described above. 
     According to the present invention, the occurrence of the liquid pool at a corner section of the bubble generation chamber is prevented, and hence the occurrence of a satellite droplet can be prevented when the liquid is ejected from the nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and benefits thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a general schematic drawing of a liquid ejection apparatus in which an ink supply system is provided with a liquid ejection mechanism, according to an embodiment of the present invention; 
         FIG. 2  is a compositional diagram of a recording head according to an embodiment of the present invention; 
         FIG. 3  is a diagram showing the diameter of a nozzle and the internal diameter of a second bubble generation chamber; 
         FIGS. 4A to 4C  are cross-sectional diagrams for illustrating states of the ink level while ink is being supplied from a main tank to an ink storage section; 
         FIGS. 5A to 5E  are diagrams showing the progress of the ejection of ink from a nozzle according to an embodiment of the present invention; 
         FIG. 6  is a diagram showing an air bubble incorporation unit according to another embodiment of the present invention; 
         FIG. 7  is a principal plan diagram showing the peripheral area of a print unit of an inkjet recording apparatus according to an embodiment of the present invention; 
         FIG. 8  is an illustrative diagram showing a nozzle face of a head and a sensor face of a print determination unit; 
         FIG. 9  is a general schematic drawing of the inkjet recording apparatus according to an embodiment of the present invention; 
         FIG. 10  is a principal block diagram showing the system composition of the inkjet recording apparatus according to an embodiment of the present invention; and 
         FIGS. 11A to 11E  are diagrams showing the progress of the ejection of ink from a nozzle according to the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described below with reference to the accompanying drawings. In the present specification, the “upward” direction in the liquid ejection apparatus indicates a direction toward an air connection channel, whereas the “downward” direction indicates a direction toward an ejection head (hereinafter referred to also as “head”). 
     Description of Ink Supply System 
       FIG. 1  is a general schematic drawing of a liquid ejection apparatus in which an ink supply system is provided with a liquid ejection mechanism, according to an embodiment of the present invention. 
     As shown in  FIG. 1 , the liquid ejection apparatus includes a head  17  (liquid ejection mechanism), a coupling unit  12 , a main tank  13 , a sub tank  11 , a cap unit  14 , a suction pump  16 , and the like. 
     The sub tank  11  includes: an ink storage section  18  which forms a liquid storage chamber for storing ink; and an air connection channel  19  which serves as a connection path to the atmosphere. A dividing plate  21  is interposed between the ink storage section  18  and the air connection channel  19 . The ink storage section  18  is provided for each color, and a relatively small quantity of ink, corresponding to an amount for printing images on several sheets or several tens of sheets, is stored in the ink storage section  18  provided for each color. 
     A connection port is provided in the dividing plate  21 , and the connection port is filled with an air-liquid separating member  22  through which only air to pass and liquid is inhibited from passing. The air-liquid separating member  22  has a surface which is adjacent to the ink storage section  18  (is located on the side of the ink storage section  18 ) and coated with a hydrophobic material. A plurality of fine pores are formed in the air-liquid separating member  22  so that air can pass through the air-liquid separating member  22 , and these fine pores are formed by laser processing using irradiation of laser light. Furthermore, the air-liquid separating member  22  may be a multifilament fiber body formed by bonding together sheets made of a fibrous resin, a fibrous metal, or the like, to form a laminated body, and then calcining this laminated body. 
     Moreover, in the present embodiment, an air bubble incorporation member  25  is provided inside the ink storage section  18 . The air bubble incorporation member  25  is a porous member that has a function of allowing both air and liquid to pass through the air bubble incorporation member  25 . Therefore, air is allowed to pass through the air bubble incorporation member  25  and to be absorbed in the member  25 , and moreover, the liquid can also be absorbed in the member  25  due to the capillary force. Consequently, the absorbed air is incorporated into the absorbed liquid in the form of air bubbles. The air bubble incorporation member  25  may be disposed in any position inside the ink storage section  18 , and desirably, the air bubble incorporation member  25  is disposed in a position such that the air and ink pass in the air bubble incorporation member  25  and air bubbles can be incorporated into the ink in a reliable fashion even when the amount of the increase or decrease of the liquid level in the ink storage section  18  is minute. In the present embodiment, as shown in  FIG. 1 , the air bubble incorporation member  25  is disposed in a position such that the air bubble incorporation member  25  is in contact with the dividing plate  21  and the air-liquid separating member  22 , which are located above the ink storage section  18 . Moreover, the air bubble incorporation member  25  may be formed by injection molding of resin, metal die casting or machining processing. 
     A suction port  23  for suctioning the air from the exterior is provided with the air connection channel  19 , while on the other hand, a supply port  24  for supplying ink from the exterior is provided with each ink storage section  18 . The ink storage section  18  is preferably filled with an ink holding member (such as a sponge) in order that the negative pressure of the head  17  can be controlled to a very high degree of accuracy. 
     During printing, the head  17 , which is coupled integrally with the sub tank  11 , ejects ink droplets in accordance with image signals while moving reciprocally back and forth over a scanning print region A 1  shown in  FIG. 1 , to form an image on the recording medium (not shown). 
       FIG. 2  is a diagram showing the structure of the head  17 . As shown in  FIG. 2 , the head  17  includes a silicon substrate  34 , a first bubble generation chamber forming layer  36 , a second bubble generation chamber forming layer  38 , a nozzle plate  40 , and the like. A state where the nozzle plate  40  is situated in an upper portion of the head  17  is depicted in  FIG. 2 , whereas in  FIG. 1 , the nozzle plate  40  is depicted as being located in a lower potion of the head  17 . 
     The silicon substrate  34  is provided with a heater  42  which is a heating element and arranged at a position corresponding to a bubble generation chamber  26  (including a first bubble generation chamber  26 A and a second bubble generation chamber  26 B) described below. When a prescribed drive signal is supplied to the heater  42 , an ejection bubble (a bubble that is generated from evaporation of the ink) is generated and is expanded inside the bubble generation chamber  26  because of the heat generated by the heater  42 , and an ink droplet is then ejected from the nozzle  32  because of the pressure created by this ejection bubble. Furthermore, a supply channel  46  and a common flow channel  44  for supplying ink to the bubble generation chamber  26  from the ink storage section  18  of the sub tank  11  are formed in the head  17 . The supply channel  46  is formed in the silicon substrate  34 . 
     The first bubble generation chamber forming layer  36  made from a photosensitive resin, or the like, is stacked on the silicon substrate  34 , and the first bubble generation chamber  26 A is formed in the first bubble generation chamber forming layer  36 . The second bubble generation chamber forming layer  38  is stacked on the first bubble generation chamber forming layer  36 , and the second bubble generation chamber  26 B connected to the first bubble generation chamber  26 A is formed in the second bubble generation chamber forming layer  38 . The bubble generation chamber  26 , which is filled with ink that is to be ejected from the nozzle  32  (before the ejection), is constituted by the first bubble generation chamber  26 A and the second bubble generation chamber  26 B. The second bubble generation chamber forming layer  38  forms a side wall  38 A of the second bubble generation chamber  26 B. The side wall  38 A is inclined at an angle of approximately 10 to 40 degrees with respect to the central axis of the second bubble generation chamber  26 B, and thus forms a circular conical tapered shape in which the internal diameter decreases in terms of a direction toward the nozzle  32 . Therefore, in the second bubble generation chamber  26 B, the liquid flow direction in which the ink is caused to flow due to the expansion of the ejection bubble  48  is regulated to be a direction toward the nozzle  32 , and therefore the ejection efficiency of the ink can be improved and the ejection volume can be stabilized. 
     The nozzle plate  40  is stacked on the second bubble generation chamber forming layer  38 , and the nozzles  32  forming ink ejection ports are formed in this nozzle plate  40 .  FIG. 3  is a diagram showing the relationship between the diameter of a nozzle and the internal diameter of a second bubble generation chamber at the vicinity of the nozzle plate  40 . As shown in  FIG. 3 , the nozzle  32  has a diameter D 1  smaller than the diameter D 2  of the second bubble generation chamber  26 B formed in the second bubble generation chamber forming layer  38 , at the boundary between the second bubble generation chamber forming layer  38  and the nozzle plate  40 . 
     Moreover, as shown in  FIG. 1 , the coupling unit  12 , which forms a supply connection device, is provided with joints  27  and  28 . The joint  27  is connected to a suction pump  16 , which forms a suction device. On the other hand, the joint  28  is connected to the main tank  13 , which is a base tank for storing the ink. A valve (not shown) is provided for each of the joints  27  and  28 . A suction tube (not illustrated) is provided in the cap unit  14 . By applying a negative pressure with a suction pump  16  through the suction tube in the cap unit  14 , the cap unit  14  suctions and expels the ink from the ejection port of the head  17 . Moreover, a replenishment port (not illustrated) is provided between the ink storage section  18  and the head  17 . 
     As shown in  FIG. 1 , a valve  31  for controlling the suctioning from the joint  27 , and a valve  33  for controlling the suctioning from the cap unit  14 , are connected to the suction pump  16 . 
     The ink supply system having the above-described composition functions as follows. 
     Firstly, when the remaining amount of ink in the ink storage section  18  in the sub tank  11  becomes low, then the head  17  moves from the scanning print region A 1  to a maintenance region A 2 , and the sub tank  11  is coupled with the coupling unit  12 . In this case, the suction port  23  is coupled to the joint  27  that is connected to the suction pump  16  for suctioning air, while on the other hand, the supply port  24  is coupled to the joint  28  that is connected to the main tank  13 . In this situation, air passes through and is absorbed in the air bubble incorporation member  25 . 
     Next, when the valve  31  (shown in  FIG. 1 ) is opened and air suctioning is implemented through the joint  27  and the suction port  23  by means of the suction pump  16 , then the pressure in the air connection channel  19  is reduced.  FIGS. 4A to 4C  are cross-sectional diagrams showing a change in the ink level when the ink is supplied from the main tank  13  to the ink storage section  18 . Since air can pass through the air-liquid separating member  22 , then an empty region  18 A above the liquid surface of the ink (indicated by diagonal hatching) stored in the ink storage section  18  shown in  FIG. 4A  assumes a reduced pressure state. The ink is therefore supplied from the main tank  13  to the ink storage section  18 , through the joint  28  and the supply port  24 . As a result of this, the ink level in the ink storage section  18  rises as shown in  FIG. 4B , and eventually, as shown in  FIG. 4C , the ink level rises to a level such that the air bubble incorporation member  25  is immersed in the ink and the ink liquid surface makes contact with the air-liquid separating member  22 . The ink storage section  18  is filled with the ink in this way. In this case, the ink permeates the air bubble incorporation member  25  due to the capillary force, and the air that has been absorbed in the air bubble incorporation member  25  is incorporated into the ink in the form of air bubbles. 
     When the ink surface in the ink storage section  18  makes contact with the air-liquid separating member  22 , then the rise of the ink level is halted since the air-liquid separating member  22  has a function for inhibiting the passage of liquid. The suction force of the suction pump  16  is set to be lower than the liquid passage inhibiting force (liquid repelling force) of the air-liquid separating member  22 . 
     Moreover, a configuration is adopted in which the ink storage sections  18  are arranged in alignment independently for the color inks, and the air connection channel  19  is provided for all the color inks as a single common chamber. In this case, when the suction operation is carried out by the suction pump  16 , the ink supply operations from the main tanks  13  are started simultaneously for all the color inks stored in the ink storage sections  18 . It is often the case that the remaining ink amount varies between the ink storage sections  18  storing the color inks; however, since the air-liquid separating member  22  has a function for inhibiting the passage of the liquid, as stated previously, then the ink supply terminates sequentially as the ink storage sections of the respective inks become full and the inks comes into contact with the air-liquid separating member  22 . 
     Furthermore, as printing starts and the ink in the ink storage section  18  is consumed, the liquid level of the ink descends in the air bubble incorporation member  25  from a level such that the liquid contacts with the air-liquid separating member  22  and the liquid level further descends in the ink storage section  18 . Therefore, the empty region  18 A is formed again, air passes through the air bubble incorporation member  25 , and consequently the air is absorbed in the air bubble incorporation member  25  again. 
     In the present embodiment, as shown in  FIG. 1  in particular, the air bubble incorporation member  25  is disposed in a position such that the air bubble incorporation member  25  makes contact with the dividing plate  21  and the air-liquid separating member  22 . In the present embodiment, the head  17 , which is the ink ejection unit, is disposed below the ink storage section  18 , and when the ink is ejected and consumed, the liquid surface of the ink inside the ink storage section  18  gradually moves downwards. Since the air bubble incorporation member  25  is disposed in a position such that the air bubble incorporation member  25  is in contact with the dividing plate  21  and the air-liquid separating member  22 , then even when the liquid surface of the ink moves slightly downward, the air bubble incorporation member  25  is exposed to the empty region  18 A and is able to absorb air therein. Thereupon, when the ink is subsequently supplied to the ink storage section  18  and the ink tank becomes full, then the ink passes through the air bubble incorporation member  25 , and the air that has been absorbed in the air bubble incorporation member  25  is incorporated into the ink in the form of air bubbles. As described above, by disposing the air bubble incorporation member  25  in a position in contact with the dividing plate  21  and the air-liquid separating member  22 , then even when ink is ejected and supplied in very small amounts, it is still possible to incorporate air bubbles into the ink in a reliable fashion. 
     By means of the operations described above, air and ink alternately permeate the air bubble incorporation member  25 , and the air that has been absorbed in the air bubble incorporation member  25  is thereby incorporated into the ink in the form of air bubbles. 
     Next, the function and beneficial effects according to the present embodiment are described in more detail with reference to  FIGS. 5A to 5E , focusing attention on the head  17 . By providing the air bubble incorporation member  25  in the ink storage section  18  as described above, air bubbles are incorporated into the ink and the content ratio of the air bubbles contained in the ink is raised internationally. The ink storage section  18  is connected to the bubble generation chamber  26  (constituted by the first bubble generation chamber  26 A and the second bubble generation chamber  26 B) via the supply channel  46  (shown in  FIG. 2 ), and the bubble generation chamber  26  is filled with the ink in which the content ratio of the air bubbles is intentionally raised. 
       FIGS. 5A to 5E  are diagrams showing the progress of the ejection of ink from the nozzle, according to an embodiment of the present invention. When a prescribed drive signal is applied to the heater  42  provided with the silicon substrate  34  so as to generate heat, then as shown in  FIG. 5A , an ejection bubble  48  is generated and formed due to the boiling of the ink in the vicinity of the heater  42  inside the first bubble generation chamber  26 A. The fine air bubbles contained in the ink are not depicted in  FIGS. 5A to 5E . Thereupon, as the heater  42  continues to heat, the ejection bubble  48  is expanded to a larger size as shown in  FIG. 5B . In this situation, the ink is pushed by the ejection bubble  48 , resulting in the formation of an ink droplet  50  to be ejected from the nozzle  32 , whereas the air bubbles contained in the ink coalesce and collect at the corner sections of the second bubble generation chamber  26 B shown in  FIG. 5B , resulting in the formation of an air pool  49 . In this way, by causing the air pool  49  to be formed at the corners of the second bubble generation chamber  26 B, it is possible to prevent the occurrence of the liquid pool  70 , which is liable to occur in the related art as shown in  FIGS. 11A to 11E . 
     Thereupon, when heat is further generated from the heater  42 , as shown in  FIG. 5C , the ink droplet  50  is ejected from the nozzle  32  whereas the air pool  49  combines with the ejection bubble  48  and connects with the atmosphere. 
     Subsequently, as shown in  FIG. 5D , the ink is supplied again to the bubble generation chamber  26 . Then, the bubble generation chamber  26  is filled with the ink as shown in  FIG. 5E . In this case, as shown in  FIGS. 5D and 5E , when the ink droplet  50  is ejected from the nozzle  32 , there is no occurrence of the satellite droplet  52  described above with reference to  FIGS. 11A to 11E . Consequently, in the present embodiment, it is possible to obtain the desired dot shape. 
     As described above, it is possible to reliably incorporate the air bubbles into the ink inside the bubble generation chamber  26  by means of a simple structure, and the formation of the liquid pool  70  can be reliably prevented from occurring. Consequently, it is possible to prevent the occurrence of the satellite droplet  52 , and an image can be formed by means of dots having desired shapes. 
     As described above with reference to  FIG. 3 , the following relationship is satisfied: D 2 &gt;D 1 , where D 1  represents the diameter of the nozzle  32  and D 2  represents the diameter of the wall  38 A constituted by the second bubble generation chamber forming layer  38  at a vicinity of the nozzle plate  40  (between the nozzle plate  40  and the wall  38 A). In addition to this relationship, the following relationship is preferably satisfied:
 
{( D 2− D 1)/16}≦ d≦ ( D 1/8)  (1)
 
where d represents a diameter of the air bubbles incorporated into the bubble generation chamber  26 .
 
     In the inequality equation (1), the upper limit value (D 1 /8) is the limit value for ensuring the ink ejection characteristics from the nozzle  32 . If the air bubbles having a larger diameter than this limit value are incorporated into the ink, then there is a possibility that an ejection failure may occur and that the variation in the ejection volume of the ink may increase and the image quality may thus be deteriorated. 
     Moreover, the lower limit value {(D 2 −D 1 )/16} is the lower limit value for achieving effective coalescence of the incorporated air bubbles. If the air bubbles having a smaller diameter than this lower limit value are incorporated into the ink, then it becomes difficult to cause the air bubbles to combine together effectively, the air pool  49  shown in  FIG. 5B  is not formed, and there is a possibility that the liquid pool may occur. 
     When air and ink alternately permeate the air bubble incorporation member  25  constituted by a porous member and the air bubbles are thereby incorporated into the ink, then the following relationship is satisfied between the pore diameter in the porous member and the diameter of the air bubbles to be incorporated:
 
g2≦d≦g1  (2)
 
where g 1  represents the maximum pore diameter of the air bubble incorporation member  25 , and g 2  represents the minimum pore diameter of the air bubble incorporation member  25 . By taking into consideration this relationship, the air bubble incorporation member  25  having the desired pore diameter can be selected so as to adjust the diameter of the air bubbles to be incorporated into the ink.
 
     Another possible embodiment for incorporating the air bubbles into the ink is described below.  FIG. 6  is a diagram showing another embodiment for incorporating the air bubbles into the ink. As shown in  FIG. 6 , the ink storage section  18  is supplied with the air bubbles by a pump  62  through a filter member  60 , and the ink storage section  18  supplies the ink to the plurality of bubble generation chambers  26 . In the present embodiment, air is supplied to the ink storage section  18  through the filter member  60 , in the form of fine bubbles, and the air bubbles are thereby incorporated into the ink. In this way, the liquid pool  70  is reliably prevented from occurring, and it is possible to prevent the occurrence of the satellite droplets  52 , and hence an image can be formed by means of dots having desired shapes. In the present embodiment also, the inequality equation (1) is satisfied between the diameter D 1  of the nozzle  32 , the internal diameter D 2  of the second bubble generation chamber at the joining part between the nozzle plate  40  and the wall  38 A formed by the second bubble generation chamber forming layer  38  (the internal diameter D 2  of the wall  38 A formed by the second bubble generation chamber forming layer  38  at the vicinity of the nozzle plate  40 ), and the diameter d of the incorporated air bubbles. 
     Description of Print Unit 
     Next, the print unit including the aforementioned ink supply system is described below.  FIG. 7  is a principal plan diagram showing the periphery of a print unit  29  of an inkjet recording apparatus  10 . The print unit  29  is provided with a carriage  162  which is movable reciprocally along two guide rails  160  extending in the breadthways direction of the recording paper  137  (the main scanning direction). The sub tank  11  including the ink storage sections  18 K,  18 C,  18 M and  18 Y corresponding to the four color inks of black (K), cyan (C), magenta (M) and yellow (Y), and a print determination unit (scanner unit)  143  are detachably mounted on the carriage  162 . The sub tank  11  and the print determination unit  143  can move in the main scanning direction in unison with the carriage  162 . 
     The recording paper conveyance amount measurement sensor (conveyance amount sensor)  165  is a device which measures the conveyance amount in the sub-scanning direction of the recording paper  137 , and the conveyance amount sensor  165  includes photoelectric sensors arranged following a substantially parallel direction with respect to the sub-scanning direction. The amount of conveyance of the recording paper  137  is measured on the basis of sensor signals obtained from this conveyance amount sensor  165 . 
       FIG. 8  is an illustrative diagram showing the nozzle surface of the head  17  and the sensor surface of the print determination unit  143 . As shown in  FIG. 8 , a plurality of the nozzles  32  are arranged in a staggered matrix fashion in the head  17 , and the nozzle density (nozzle pitch h) in the sub-scanning direction is 1200 nozzles per inch. 
     As shown in  FIG. 8 , a plurality of sensors  164  are arranged in a line configuration (a one-dimensional configuration), on the sensor surface of the print determination unit  143 . The sensor density (sensor pitch) in the sub-scanning direction is 1200 sensors per inch, which is the same as the nozzle density of the head  17 , and the print determination section  143  has a reading resolution of 1200 dpi. 
     The sensor width (readable width) of the print determination unit  143  is composed to be broader than the nozzle arrangement width (printable width) of the head  17 . Accordingly, even when relative displacement (position error) occurs between the head  17  and the print determination unit  143  mounted on the carriage  162  (shown in  FIG. 7 ), the print determination unit  143  is able to read the test pattern formed by the head  17 , reliably. 
     General Composition of Inkjet Recording Apparatus 
       FIG. 9  is a general schematic drawing of an inkjet recording apparatus  10  including the print unit described above. This inkjet recording apparatus  10  includes: the print unit  29 , which includes the sub tank  11  having ink storage sections  18 K,  18 C,  18 M and  18 Y corresponding to the four ink colors; the main tank  13 , which stores the inks to be supplied to the respective ink storage sections  18 K,  18 C,  18 M and  18 Y; the coupling unit  12 , which couples with the print unit  29  when the inks are supplied to the respective ink storage sections  18 K,  18 C,  18 M and  18 Y; the suction pump  16 , which is connected to the coupling unit  12 ; a paper supply unit  138  for supplying a recording paper  137 ; a decurling processing unit  139  for removing curl from the recording paper  137 ; a suction belt conveyance unit  141 , which is disposed opposing the nozzle surface (ink ejection surface) of the print unit  29  and conveys the recording paper  137  while holding the recording paper  137  flat; the print determination unit  143 , which reads in the print results of the print unit  29 ; and a paper output unit  146 , which outputs printed recording paper (printed matter) to the exterior. 
     In the case of a configuration in which roll paper is used, a cutter  147  is provided as shown in  FIG. 9 , and the roll paper is cut to a desired size by the cutter  147 . The cutter  147  has a stationary blade  147 A, whose length is not less than the width of the conveyor pathway of the recording paper  137 , and a round blade  147 B, which moves along the stationary blade  147 A. The stationary blade  147 A is disposed on the reverse side of the printed surface of the recording paper  137 , and the round blade  147 B is disposed on the printed surface side across the conveyance path from the stationary blade  147 A. 
     The recording paper  137  delivered from the paper supply unit  138  retains curl due to having been loaded in the magazine. In order to remove the curl, heat is applied to the recording paper  137  in the decurling unit  139  by a heating drum  130  in the direction opposite to the curl direction in the magazine. 
     After decurling, the cut recording paper  137  is delivered to the suction belt conveyance unit  141 . The suction belt conveyance unit  141  has a configuration in which an endless belt  133  is set around rollers  131  and  132  so that the portion of the endless belt  133  facing at least the nozzle face of the print unit  29  forms a plane. 
     The belt  133  has a width that is greater than the width of the recording paper  137 , and a plurality of suction holes (not shown) are formed on the belt surface. A suction chamber  134  is disposed in a position facing the nozzle surface of the print unit  29  on the interior side of the belt  133 , which is set around the rollers  131  and  132 , as shown in  FIG. 9 ; and this suction chamber  134  provides suction with a fan  135  to generate a negative pressure, thereby holding the recording paper  137  on the belt  133  by suction. In the region of the print unit  29 , the head  17  integrated with the sub tank  11  scans (moves above) the recording paper  137  reciprocally back and forth in the front/rear direction of  FIG. 9 . 
     The belt  133  is driven in the clockwise direction in  FIG. 9  by the motive force of a motor (not illustrated) being transmitted to at least one of the rollers  131  and  132 , which the belt  133  is set around, and the recording paper  137  held on the belt  133  is conveyed in the sub-scanning direction (the paper conveyance direction) in  FIG. 9 . 
     Since the ink adheres to the belt  133  when a marginless print job or the like is performed, a belt-cleaning unit  136  is disposed in a predetermined position (a suitable position outside the printing area) on the exterior side of the belt  133 . A heating fan  140  is disposed on the upstream side of the print unit  29  in the conveyance pathway formed by the suction belt conveyance unit  141 . The heating fan  140  blows heated air onto the recording paper  137  to heat the recording paper  137  immediately before printing so that the ink deposited on the recording paper  137  dries more easily. 
     The main tank  13  includes tanks which store the color inks to be supplied to the heads  17  corresponding to the respective ink storage sections  18 K,  18 C,  18 M and  18 Y of the print unit  29  (shown in  FIG. 7 ). Moreover, the main tank  13  also includes a notifying device (such as a display device, an alarm generating device, or the like) that generates a notification when the remaining amount of ink has become low, and a mechanism for preventing incorrect loading of ink of the wrong color. 
     The coupling unit  12  shown in  FIG. 1  and the print unit  29  are arranged in the main scanning direction of the print unit  29 . When the remaining amount of ink in the ink storage section  18  inside the sub tank  11  becomes low, then the head  17  moves from the scanning print region A 1  to the maintenance region A 2 , and the sub tank  11  is coupled with the coupling unit  12 . The inks are then supplied to the respective ink storage units  18 K,  18 C,  18 M and  18 Y, from the main tank  13 , through the coupling unit  12 . 
     As shown  FIG. 9 , a post-drying unit  142  is disposed following the print unit  29 . The post-drying unit  142  is a device to dry the printed image surface, and includes a heating fan, for example. 
     A heating/pressurizing unit  144  is disposed following the post-drying unit  142 . The heating/pressurizing unit  144  is a device to control the glossiness of the image surface, and the image surface is pressed with a pressure roller  145  having a predetermined uneven surface shape while the image surface is heated, and the uneven shape is transferred to the image surface. 
     The printed matter generated in this manner is outputted from the paper output unit  146 . The target print (i.e., the result of printing the target image) and the test print are preferably outputted separately. In the inkjet recording apparatus  10 , a sorting device (not shown) is provided for switching the outputting pathways in order to sort the printed matter with the target print and the printed matter with the test print, and to send them to paper output units  146 A and  146 B, respectively. When the target print and the test print are simultaneously formed in parallel on the same large sheet of paper, the test print portion is cut and separated by a cutter (second cutter)  148 . The cutter  148  is disposed immediately before the paper output unit  146 , and is used for cutting the test print portion from the target print portion when a test print has been performed in the blank portion of the target print. The structure of the cutter  148  is the same as the first cutter  147  described above, and has a stationary blade  148 A and a round blade  148 B. 
     Description of Control System 
       FIG. 10  is a principal block diagram showing the system configuration of the inkjet recording apparatus  10 . The inkjet recording apparatus  10  includes a communication interface  170 , a system controller  172 , an image memory  174 , a motor driver  176 , a heater driver  178 , a print controller  180 , an image buffer memory  182 , a head driver  184 , a pump driver  190 , and the like. 
     The communication interface  170  is an interface unit for receiving image data sent from a host computer  186 . The image data sent from the host computer  186  is received by the inkjet recording apparatus  10  through the communication interface  170 , and is temporarily stored in the image memory  174 . The image memory  174  is a storage device for temporarily storing an image input via the communication interface  170 , and data is written to and read from the image memory  174  via the system controller  172 . 
     The system controller  172  is a control unit for controlling the various sections, such as the communication interface  170 , the image memory  174 , the motor driver  176 , the heater driver  178 , and the like. The system controller  172  is constituted by a central processing unit (CPU) and peripheral circuits thereof, and the like, and in addition to controlling communications with the host computer  186  and controlling reading and writing from and to the image memory  174 , and the like, it also generates control signals for controlling the heater  42  and the motor  188  in the conveyance system. 
     The motor driver (drive circuit)  176  drives the motor  188  in accordance with commands from the system controller  172 . The heater driver  178  drives the heaters  42  (shown in  FIG. 3 ), a heater in the post-drying unit  142 , and the like, in accordance with commands from the system controller  172 . The pump driver  190  is a driver which drives the pump  16  in accordance with instructions from the system controller  172 . 
     The print controller  180  has a signal processing function for performing various tasks, compensations, and other types of processing for generating print control signals from the image data stored in the image memory  174  in accordance with commands from the system controller  172  so as to supply generated print control signals (dot data) to the head driver  184 . Prescribed signal processing is carried out in the print controller  180 , and the ejection amount and the ejection timing of the ink droplets from the respective print heads  17  are controlled by the head driver  184 , on the basis of the print data. By this control of the head driver  184 , desired dot size and dot positions can be achieved. 
     The print controller  180  is accompanied with the image buffer memory  182 , and image data, parameters, and other data are temporarily stored in the image buffer memory  182  while the image data is processed in the print controller  180 . 
     The heater driver  178  generates drive signals for driving the heaters  42  (shown in  FIG. 3 ) in the heads  17  corresponding to the respective colors on the basis of the print data supplied from the print controller  180 , and the drive signals thus generated are transmitted to the heaters  42 . 
     As stated previously, the print determination unit  143  reads in a test pattern recorded by the heads  17 , and it performs prescribed signal processing, and the like, in order to determine the ink ejection status of the heads  17  (the presence or absence of ejection, the dot sizes, the dot depositing positions, and the like) (in other words, in order to determine variations between the nozzles  32 ). The print determination unit  143  supplies the determination results to the print controller  180 . According to requirements, the print controller  180  makes various corrections with respect to the heads  17  on the basis of information obtained from the print determination unit  143 . 
     The conveyance amount sensor  165  measures the amount of conveyance of the recording paper  137  in the sub-scanning direction, and the sensor signals (conveyance amount information) obtained from the conveyance amount sensor  165  are supplied to the print controller  180 . 
     The image forming apparatus according to an embodiment of the present invention is described in detail above, but the present invention is not limited to the aforementioned embodiments, and it is of course possible for improvements or modifications of various kinds to be implemented, within a range which does not deviate from the essence of the present invention. 
     It should be understood that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.