Patent Publication Number: US-9427979-B2

Title: Inkjet recording apparatus that conveys recording medium while applying negative pressure

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2014-244664, filed on Dec. 3, 2014. The contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to inkjet recording apparatuses. 
     An inkjet apparatus that ejects ink onto a recording medium may address a problem of nozzle clogging in a recording head by adopting a known paper dust removal technique. 
     An inkjet recording apparatus of one known example is provided with a paper dust collector located upstream of a recording head in a conveyance direction of a recording medium. The paper dust collector has a vertical wall and a downstream wall. The vertical wall stands vertically upward. The downstream wall extends from the top end of the vertical wall in a downstream direction in the conveyance direction of the recording medium. 
     The paper dust collector collects paper dust generated during conveyance of the recording medium before the paper dust reaches the recording head. This can reduce subsequent attachment of paper dust to the recording head. 
     SUMMARY 
     An inkjet recording apparatus according to the present disclosure includes a recording head, a conveyance section, and a negative pressure applying section. The recording head ejects ink onto the recording medium. The conveyance section has a conveying surface on which the recording medium is to be placed, and conveys a recording medium while the recording medium is placed on the conveying surface. The conveying surface has a plurality of holes. The negative pressure applying section includes an airflow chamber that has an upper wall having a plurality of holes and in which negative pressure for the recording medium is created. The negative pressure applying section sucks the recording medium by the negative pressure through the holes in the upper wall and the holes in the conveying surface to cause the recording medium to be sucked on the conveying surface. Negative pressure applied through a plurality of first holes among the holes in the upper wall is greater than negative pressure applied through a plurality of second holes among the holes in the upper wall. The first holes are located in a first region of the upper wall. The second holes are located in a second region of the upper wall. The first region is located upstream of a head facing region of the upper wall in a conveyance direction of the recording medium. The head facing region is located opposite to the recording head with the conveying surface therebetween. The second region is located downstream of the first region in the conveyance direction of the recording medium and includes the head facing region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates structure of an inkjet recording apparatus according to an embodiment. 
         FIG. 2  illustrates structure of an image forming section illustrated in  FIG. 1 . 
         FIG. 3  illustrates structure around a plate member illustrated in  FIG. 2 . 
         FIG. 4  is a cross sectional perspective view illustrating structure of a conveyor belt, a guide member, and a negative pressure applying section illustrated in  FIG. 2 . 
         FIG. 5  is a plan view illustrating structure of the guide member illustrated in  FIG. 4 . 
         FIG. 6A  is a plan view illustrating structure of a groove and a through hole formed in the guide member illustrated in  FIG. 5 . 
         FIG. 6B  is a cross sectional view illustrating structure of the groove and the through hole formed in the guide member illustrated in  FIG. 5 . 
         FIG. 7  is a plan view of the guide member illustrated in  FIG. 2 . 
         FIG. 8  illustrates a first configuration example of an airflow chamber illustrated in  FIG. 2 . 
         FIG. 9  illustrates a second configuration example of the airflow chamber illustrated in  FIG. 2 . 
         FIG. 10  is an enlarged view of a first space in the airflow chamber illustrated in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure with reference to the accompanying drawings. In the drawings, the like reference numerals represent similar components and explanation thereof is not repeated. 
     First, an inkjet recording apparatus  1  according to the present embodiment will be described with reference to  FIG. 1 .  FIG. 1  illustrates structure of the inkjet recording apparatus  1  according to the present embodiment. The inkjet recording apparatus  1  includes an apparatus housing  100 , a sheet feed section  2 , an image forming section  3 , a sheet conveyance section  4 , and a sheet ejecting section  5 . The sheet feed section  2  is disposed in a lower part of the apparatus housing  100 . The image forming section  3  is disposed above the sheet feed section  2 . The sheet conveyance section  4  is disposed at a side of the image forming section  3  (right side in  FIG. 1 ). The sheet ejecting section  5  is disposed at the other side of the image forming section  3  (left side in  FIG. 1 ). 
     The sheet feed section  2  includes a sheet feed cassette  21 , a sheet feed roller  22 , and a guide plate  23 . The sheet feed cassette  21  is for storing recording sheets P and is attachable to and detachable from the apparatus housing  100 . The sheet feed roller  22  is located above one end of the sheet feed cassette  21  (right end in  FIG. 1 ). The guide plate  23  extends between the sheet feed roller  22  and the sheet conveyance section  4 . 
     The sheet feed cassette  21  is loaded with a plurality of recording sheets P. In the following description, a recording sheet P is referred to simply as a “sheet” for the sake of convenience. A sheet P is an example of a “recording medium”. The sheet feed roller (pickup roller)  22  feeds sheets P one at a time in the conveyance direction of the sheet P by picking up the uppermost sheet P stored in the sheet feed cassette  21 . The guide plate  23  guides the sheet P picked up by the sheet feed roller  22  to the sheet conveyance section  4 . 
     The sheet conveyance section  4  includes a sheet conveyance path  41 , a pair of first conveyance rollers  42 , a pair of second conveyance rollers  43 , and a pair of registration rollers  44 . The sheet conveyance path  41  substantially defines a C-shape. The pair of first conveyance rollers  42  is located at the entry of the sheet conveyance path  41 . The pair of second conveyance rollers  43  is located at an intermediate location on the sheet conveyance path  41 . The pair of registration rollers  44  is located at the exit of the sheet conveyance path  41 . 
     The pair of first conveyance rollers  42  is a pair of rollers (a pair of feed rollers) that feeds a sheet P in the conveyance direction of the sheet P. The sheet P fed from the sheet feed section  2  is caught between the first conveyance rollers  42  and forwarded to the sheet conveyance path  41 . Also, the pair of second conveyance rollers  43  is a pair of feed rollers. The sheet P forwarded from the pair of first conveyance rollers  42  is caught between the pair of second conveyance rollers  43  and forwarded toward the pair of registration rollers  44 . 
     The pair of registration rollers  44  performs skew correction on the sheet P having been conveyed by the second conveyance rollers  43 . The pair of registration rollers  44  temporarily holds the sheet P to synchronize the conveyance of the sheet P and image formation, and then feeds the sheet P to the image forming section  3  according to timing of the image formation. 
     The image forming section  3  includes a conveyor belt  32  and recording heads  34 . The conveyor belt  32  conveys the sheet P fed from the pair of registration rollers  44  in a predetermined direction (leftward in  FIG. 1 ). The recording heads  34  form an image on the sheet P being conveyed on the conveyor belt  32 . Detailed structure of the image forming section  3  will be described later with reference to  FIG. 2 . The image forming section  3  additionally includes a conveyance guide  36  located downstream (to the left in  FIG. 1 ) of the recording heads  34  in the conveyance direction of the sheet P. 
     The conveyance guide  36  guides the sheet P discharged from the conveyor belt  32  to the sheet ejecting section  5 . The sheet ejecting section  5  includes a pair of ejection rollers  51  and an exit tray  52 . The exit tray  52  is secured to the apparatus housing  100  so as to protrude outward from an exit port  11  formed in the apparatus housing  100 . 
     The pair of ejection rollers  51  forwards the sheet P toward the exit port  11  after the sheet P passes through the conveyance guide  36 . The exit tray  52  guides the sheet P ejected by the pair of ejection rollers  51 . The sheet P is ejected out of the apparatus housing  100  by the pair of ejection rollers  51  through the exit port  11  formed in a side surface of the apparatus housing  100  (a left side surface in  FIG. 1 ). The sheet P ejected through the exit port  11  is stacked in the exit tray  52 . 
     Next, a description will be given of the image forming section  3  with reference to  FIG. 2 .  FIG. 2  illustrates structure of the image forming section  3  illustrated in  FIG. 1 . 
     As illustrated in  FIG. 2 , the image forming section  3  includes a conveyance section  31 , a negative pressure applying section  33 , the recording heads  34 , and a plate member  35 . The recording heads  34 , which specifically are four types of recording heads  34   a ,  34   b ,  34   c , and  34   d , each include a plurality of nozzles (not illustrated). Ink is ejected through the plurality of nozzles so as to form images such as characters and figures on a sheet P. The recording heads  34   a ,  34   b ,  34   c , and  34   d  are substantially identical in structure and may therefore be generally referred to as recording heads  34  without distinguishing therebetween. 
     The conveyance section  31  conveys a sheet P in a predetermined direction (leftward in  FIG. 2 ) and includes a belt speed detecting roller  311 , a placing roller  312 , a drive roller  313 , a tension roller  314 , a pair of guide rollers  315 , and the conveyor belt  32 . 
     The conveyance section  31  is located opposite to the four types of recording heads  34  ( 34   a ,  34   b ,  34   c , and  34   d ) in the apparatus housing  100 . The conveyor belt  32  is stretched around the belt speed detecting roller  311 , the drive roller  313 , the tension roller  314 , and the pair of guide rollers  315 . The conveyor belt  32  is driven to circulate in the conveyance direction of the sheet P (counterclockwise in  FIG. 2 ) to convey the sheet P. The conveyor belt  32  is an example of an “endless belt”. 
     The tension roller  314  tensions the conveyor belt  32  in order to prevent sagging of the conveyor belt  32 . 
     The belt speed detecting roller  311  is located upstream (to the right in  FIG. 2 ) of the negative pressure applying section  33  in the conveyance direction of the sheet P and rotates by friction with the conveyor belt  32 . The belt speed detecting roller  311  includes a pulse plate (not illustrated) that integrally rotates with the belt speed detecting roller  311 . The circulation speed of the conveyor belt  32  is measured by measuring the rotational speed of the pulse plate. 
     The drive roller  313  is located downstream (to the left in  FIG. 1 ) of the negative pressure applying section  33  in the conveyance direction of the sheet P. The drive roller  313  is preferably located in cooperating relation with the belt speed detecting roller  311  so as to ensure the flatness of the conveyor belt  32  at regions opposite to the recording heads  34 . 
     The drive roller  313  is driven to rotate by a motor (not illustrated) to circulate the conveyor belt  32  counterclockwise in  FIG. 2 . 
     The pair of guide rollers  315  is located below the negative pressure applying section  33  to secure space below the negative pressure applying section  33 . This arrangement can prevent a portion of the conveyor belt  32  below the negative pressure applying section  33  from touching the negative pressure applying section  33 . 
     The four types of recording heads  34  ( 34   a ,  34   b ,  34   c , and  34   d ) are arranged in order from upstream to downstream in the conveyance direction of the sheet P. The recording heads  34   a ,  34   b ,  34   c , and  34   d  each include a plurality of nozzles (not illustrated) arranged in a width direction of the conveyor belt  32  (direction perpendicular to the drawing surface in  FIG. 2 ). The recording heads  34   a ,  34   b ,  34   c , and  34   d  are referred to as a line type recording heads. From this follows that the inkjet recording apparatus  1  is a line head inkjet recording apparatus. 
     The negative pressure applying section  33  applies negative pressure to the sheet P through the conveyor belt  32 , causing the sheet P to be sucked onto the conveyor belt  32 . The negative pressure applying section  33  is located on the rear surface (underside in  FIG. 2 ) of the conveyor belt  32  and opposite to the four types of recording heads  34  with the conveyor belt  32  therebetween. The negative pressure applying section  33  includes an airflow chamber  331  that is open at the top, a guide member  332  that closes the open top of the airflow chamber  331 , a negative pressure creating section  336 , and a gas outlet  337 . 
     The placing roller  312  is a driven roller. The placing roller  312  is located opposite to the guide member  332  with the conveyor belt  32  therebetween. The placing roller  312  guides the sheet P that has been fed from the pair of registration rollers  44  onto the conveyor belt  32  so that the sheet P is sucked onto the conveyor belt  32 . 
     The guide member  332  supports the sheet P through the conveyor belt  32 . The guide member  332  is an example of a “conveyor plate”. The guide member  332  has through holes  335 . The guide member  332  is formed from, for example, a metallic material. Specifically, the guide member  332  may be made from die-cast aluminum or a pressed metal plate. Alternatively, the guide member  332  may be made from resin to provide excellent slidability of the guide member  332  against the conveyor belt  32 . Note that although grooves  334  (see  FIGS. 3 and 4 ) are not illustrated in  FIG. 2 , the through holes  335  pass through the guide member  332  from a bottom surface of a corresponding one of the grooves  334  located in the upper surface of the guide member  332 . 
     For the sake of convenience, the present embodiment describes the guide member  332  as part of the negative pressure applying section  33 . Alternatively, however, the guide member  332  may be described as part of the conveyance section  31  because the guide member  332  supports the conveyor belt  32  as described above. 
     The airflow chamber  331  forms a space (hereinafter referred to as a “negative pressure creating space”)  3311  in which negative pressure for sucking the sheet P onto the conveyor belt  32  is created. The airflow chamber  331  in the present embodiment is a box-shaped member that is a tube having an open top and a closed bottom. The airflow chamber  331  has side walls that are secured at the top to the guide member  332 . The open top of the airflow chamber  331  is covered with the guide member  332 . That is, the guide member  332  in the present embodiment serves as an upper wall of the airflow chamber  331 . 
     The negative pressure creating section  336  creates negative pressure in the airflow chamber  331 , and may for example be a fan or a vacuum pump. The negative pressure creating section  336  is disposed under the airflow chamber  331 , specifically, connected to the bottom surface  3312  of the airflow chamber  331 . The negative pressure creating section  336  creates negative pressure in the airflow chamber  331  by discharging air outward of the airflow chamber  331  through the gas outlet  337 . The negative pressure created in the airflow chamber  331  acts on the sheet P through suction holes  321  (see  FIG. 4 ) in the conveyor belt  32  and the through holes  335  in the guide member  332  to suck the sheet P onto the conveyor belt  32 . As a result, the sheet P is sucked on the conveyor belt  32 . In the above manner, the conveyance section  31  conveys the sheet P while sucking the sheet p onto the conveyor belt  32 . 
     The plate member  35  is located upstream of the recording heads  34  in the conveyance direction of the sheet P (to the right in  FIG. 2 ). In other words, the plate member  35  is located between the recording head  34   a  and the placing roller  312 . The plate member  35  corresponds to part of a “gap forming section”. A gap present between the plate member  35  and the guide member  332  corresponds to a narrow gap  35   a , which will be described later. 
     Next, a description will be given of operation of the inkjet recording apparatus  1  with reference to  FIG. 1 . First, the sheet feed roller  22  picks up a sheet P from the sheet feed cassette  21 . The sheet P is then guided by the guide plate  23  to the pair of first conveyance rollers  42 . 
     The sheet P is fed by the pair of first conveyance rollers  42  into the sheet conveyance path  41  and then conveyed by the pair of second conveyance rollers  43  in the conveyance direction of the sheet P. The sheet P comes to stop upon contact with the pair of registration rollers  44  where skew correction of the sheet P is performed. The sheet P is subsequently fed to the image forming section  3  by the pair of registration rollers  44  in synchronization with timing of image formation. 
     The sheet P is guided to the conveyor belt  32  by the placing roller  312  and sucked onto the conveyor belt  32 . Preferably, the sheet P is guided to the conveyor belt  32  such that the widthwise center of the sheet P coincides with the widthwise center of the conveyor belt  32 . The sheet P covers some of the suction holes  321  (see  FIG. 4 ) in the conveyor belt  32 . The negative pressure applying section  33  sucks air through the guide member  332  and the conveyor belt  32 . That is, the negative pressure applying section  33  creates negative pressure in the airflow chamber  331 . The negative pressure acts on the sheet P and thus the sheet P is sucked onto the conveyor belt  32 . The sheet P is conveyed in the conveyance direction of the sheet P as the conveyor belt  32  circulates. 
     The sheet P is then conveyed on the conveyor belt  32  sequentially to the regions opposite to the four types of recording heads  34   a ,  34   b ,  34   c , and  34   d . While the sheet P is conveyed on the conveyor belt  32 , the four types of recording heads  34   a ,  34   b ,  34   c , and  34   d  eject ink of respective colors toward the sheet P. This forms an image on the sheet P. 
     The sheet P is conveyed from the conveyor belt  32  to the conveyance guide  36 . Once passed through the conveyance guide  36 , the sheet P is fed toward the exit port  11  by the pair of ejection rollers  51  and ejected through the exit port  11  to be guided along the exit tray  52  out of the apparatus housing  100 . 
     Next, a description will be given of structure around the plate member  35  with reference to  FIG. 3 .  FIG. 3  illustrates the structure around the plate member  35  illustrated in  FIG. 2 . 
     As illustrated in  FIG. 3 , the plate member  35  is secured to a head base  37 . The head base  37  is a plate-like member for securing the recording head  34  in place. The head base  37  corresponds to part of the “gap forming section”. A distance H across the narrow gap  35   a  is set so as to allow air flowing into the narrow gap  35   a  from surrounding space to have a higher flow velocity in the narrow gap  35   a  than before flowing into the narrow gap  35   a . The distance H herein is a length of the narrow gap  35   a  in a direction perpendicular to the upper surface of the conveyor belt  32 . In other words, the distance H is a vertical length (distance) of the narrow gap  35   a . Specifically, the narrow gap  35   a  is formed between the lower surface of the plate member  35  and the upper surface of the conveyor belt  32  such that the vertical distance H is equal to or shorter than a threshold distance HS that is set in advance (for example, 3 mm) The plate member  35  is formed from an electrical conductor (metal such as aluminum) that is earthed. The upper surface of a part of the conveyor belt  32  that is in contact with the guide member  332  is an example of a “conveying surface”. According to the present embodiment, the vertical distance H across the narrow gap  35   a  measures, for example, 2 mm. 
     The description given above with reference to  FIG. 3  is directed to a situation in which the sheet P is sufficiently thin relative to the vertical distance H across the narrow gap  35   a . Preferably, the vertical distance H across the narrow gap  35   a  is adjusted according to the thickness of the sheet P. Specifically, for example, it is preferable to lift the plate member  35  up and down according to the thickness of the sheet P to keep the distance between the upper surface of the sheet P and the lower surface of the plate member  35  substantially constant (for example, 2 mm). 
     The head base  37  has holes  371  and  372  for allowing air to flow into the narrow gap  35   a . The hole  371  is located downstream (to the left in  FIG. 3 ) of the plate member  35  in the conveyance direction of the sheet P, and the hole  372  is located upstream (to the right in  FIG. 3 ). The holes  371  and  372  are elongated in the width direction of the sheet P (direction perpendicular to the drawing surface of  FIG. 3 ). 
     The present embodiment is directed to a configuration in which the head base  37  has the holes  371  and  372  elongated in the width direction of the sheet P. Alternatively, however, the head base  37  may have holes having a different shape. The head base  37  may for example have a plurality of substantially cylindrical holes arranged in the width direction of the sheet P. 
     The holes  371  and  372  in the head base  37  allow air to flow into the narrow gap  35   a  and then into the airflow chamber  331  sequentially through the suction holes  321  in the conveyor belt  32  and the through holes  335  in the guide member  332 . In other words, the airflow chamber  331  is under negative pressure created by the negative pressure creating section  336  (for example, at a pressure differing from the atmospheric pressure by about 0.005 atm≈about 500 Pa). Therefore, air present in the narrow gap  35   a  is drawn into the airflow chamber  331  sequentially through the suction holes  321  in the conveyor belt  32  and the through holes  335  in the guide member  332 . In addition, as air is drawn out of the narrow gap  35   a  to the airflow chamber  331 , air is drawn into the narrow gap  35   a  through the holes  371  and  372  in the head base  37 . 
     As described above, air flows along paths indicated by arrows FD 1  and FD 2  in  FIG. 3 . In addition, the vertical distance H across the narrow gap  35   a  is set to be equal to or shorter than the threshold distance HS that is set in advance. Consequently, the flow velocity increases in the narrow gap  35   a . The flow velocity in the narrow gap  35   a  is preferably at least 6.0 m/sec, for example. 
     As described above, air blowing along the path indicated by the arrow FD 1  flows from upstream to downstream in the conveyance direction of the sheet P in the narrow gap  35   a  (to the left in  FIG. 3 ). Consequently, as illustrated in  FIG. 3 , paper dust PD attached to the leading edge (left edge in  FIG. 3 ) of the sheet P can be removed and collected into the airflow chamber  331 . By contrast, air blowing along the path indicated by the arrow FD 2  flows from downstream to upstream in the conveyance direction of the sheet P in the narrow gap  35   a  (to the right in  FIG. 3 ). Consequently, paper dust PD attached to the trailing edge (right edge in  FIG. 3 ) of the sheet P can be removed and collected into the airflow chamber  331 . This can ensure effective removal of paper dust attached to the sheet P. 
     As described above, the plate member  35  is formed from an earthed electrical conductor and thus will not be charged. Therefore, the plate member  35  can be ensured not to attract paper dust even though the paper dust may be charged. 
     As described above, attachment of the plate member  35  can be facilitated by securing the plate member  35  to the head base  37 . In addition, the head base  37  has the holes  371  and  372  allowing air to flow into the narrow gap  35   a  and thus is able to ensure smooth flow of air into the narrow gap  35   a.    
     The present embodiment is directed to a configuration in which the plate member  35  is secured to the head base  37 . Alternatively, however, the plate member  35  may be secured to the apparatus housing  100  illustrated in  FIG. 1 . For example, the apparatus housing  100  may be provided with a securing member extended therefrom to hold the plate member  35  at opposite ends in the width direction of the plate member  35  (direction perpendicular to the drawing surface of  FIG. 3 ). In this configuration, no component member obstructs air flowing into the narrow gap  35   a  from downstream and upstream in the conveyance direction of the sheet P. Therefore, the flow velocity of air in the narrow gap  35   a  can increase to a greater extent. Consequently, paper dust can be removed more effectively. 
     As illustrated in  FIG. 3 , the plate member  35  has tapered portions  351  such that the distance across the narrow gap  35   a  in the direction perpendicular to the upper surface of the conveyor belt  32  is greater toward either edge of the plate member  35  in the conveyance direction of the sheet P (horizontal direction in  FIG. 3 ). Specifically, one of the tapered portions  351  that is on the right in  FIG. 3  is formed such that the distance across the narrow gap  35   a  in the direction perpendicular to the upper surface of the conveyor belt  32  is greater toward the upstream edge of the plate member  35  in the conveyance direction of the sheet P (the horizontal direction in  FIG. 3 ). Similarly, one of the tapered portions  351  that is on the left in  FIG. 3  is formed such that the distance across the narrow gap  35   a  in the direction perpendicular to the upper surface of the conveyor belt  32  is greater toward the downstream edge of the plate member  35  in the conveyance direction of the sheet P (the horizontal direction in  FIG. 3 ). In other words, the tapered portions  351  are formed at an upstream end and a downstream end of the plate member  35  in the conveyance direction of the sheet P such that the plate member  35  is thinner toward either edge of the plate member  35  in the conveyance direction of the sheet P. 
     As described above, the plate member  35  is provided with the tapered portions  351  such that the distance across the narrow gap  35   a  in the direction perpendicular to the upper surface of the conveyor belt  32  is greater toward either edge of the plate member  35  in the conveyance direction of the sheet P (the horizontal direction in  FIG. 3 ). This configuration enables reduction in pressure loss of air flowing along the plate member  35 . Therefore, the flow velocity of air in the narrow gap  35   a  can increase to remove paper dust even more effectively. 
     Next, a description will be given of structure of the conveyor belt  32 , the guide member  332 , and the negative pressure applying section  33 , with reference to  FIG. 4 .  FIG. 4  is a cross sectional perspective view illustrating the structure of the conveyor belt  32 , the guide member  332 , and the negative pressure applying section  33  illustrated in  FIG. 2 . 
     As illustrated in  FIG. 4 , the conveyor belt  32 , the guide member  332 , the airflow chamber  331 , and the negative pressure creating section  336  are located in order from top to bottom. The conveyor belt  32  has a plurality of suction holes  321  perforated therethrough. 
     The following describes the suction holes  321  in the conveyor belt  32 . As illustrated in  FIG. 4 , the suction holes  321  are formed in the conveyor belt  32  at substantially equal intervals. The suction holes  321  each have a diameter of, for example, 2 mm. The spacing between adjacent suction holes  321  is, for example, 8 mm. 
     The guide member  332  has a plurality of grooves  334  in the upper surface (surface facing toward the conveyor belt  32 ). The grooves  334  have a shape of an oval elongated in the conveyance direction of the sheet P. 
     With reference to  FIG. 5 , the following describes the grooves  334  and the through holes  335  formed in the guide member  332 .  FIG. 5  is a plan view illustrating structure of the guide member  332  illustrated in  FIG. 4 . As illustrated in  FIG. 5 , the guide member  332  has the grooves  334  each having a shape of an oval elongated in the conveyance direction of the sheet P (horizontal direction in  FIG. 5 ). The grooves  334  are arranged in a plurality of rows that are next to one another in the width direction of the guide member  332  (vertical direction in  FIG. 5 ). Each groove  334  has a through hole  335  that penetrates the guide member  332  in the thickness direction thereof substantially at the center of the groove  334  in the conveyance direction of the sheet P (the horizontal direction in  FIG. 5 ). Each through hole  335  is substantially circular in cross section. 
       FIG. 5  indicates, in dashed lines, a projected position of the plate member  35  on the guide member  332 . The projected image of the plate member  35  on the guide member  332  overlaps with two columns of through holes  335 , one at an upstream side in the conveyance direction of the sheet P (left in  FIG. 5 ) and the other at a downstream side (right in  FIG. 5 ). The grooves  334  containing the through holes  335  that are in the upstream column in the conveyance direction of the sheet P (to the left in  FIG. 5 ) each extend further upstream beyond the upstream edge (left edge in  FIG. 5 ) of the projected image of the plate member  35 . Similarly, the grooves  334  containing the through holes  335  that are in the downstream column in the conveyance direction of the sheet P (to the right in  FIG. 5 ) each extend further downstream beyond the downstream edge (right edge in  FIG. 5 ) of the projected image of the plate member  35 . 
     Next, a description will be given of the grooves  334  and the through holes  335  of the guide member  332  with reference to  FIGS. 6A and 6B .  FIG. 6A  is a plan view illustrating the structure of the groove  334  and the through hole  335  formed in the guide member in  FIG. 5 .  FIG. 6B  is a cross sectional view illustrating the structure of the groove  334  and the through hole  335  formed in the guide member in  FIG. 5 . 
     As illustrated in  FIG. 6A , the groove  334  has the through hole  335  that penetrates the guide member  332  in the thickness direction thereof substantially at the center of the groove  334  in the conveyance direction of the sheet P (horizontal direction in  FIG. 6A ). As illustrated in the  6 B, the groove  334  is continuous with the through hole  335 , and therefore negative pressure created in the airflow chamber  331  affects an inner region of the groove  334  through the through hole  335 . The through hole  335  has a tapered portion  335   a  formed at an upper mouth and a tapered portion  335   b  formed at a lower mouth. 
     As described above, the grooves  334  are located in a region opposite to the plate member  35 . Therefore, negative pressure created in the airflow chamber  331  affects the inner regions of the grooves  334  through the through holes  335 . This can further facilitate flow of air along the paths indicated by the arrows FD 1  and FD 2  indicated in  FIG. 3 . Consequently, more effective removal of paper dust is enabled. 
     As described above, the tapered portion  335   a  at the upper mouth and the tapered portion  335   b  at the lower mouth of each through hole  335  are effective to reduce pressure loss of air flowing through the through hole  335 . This can further facilitate flow of air along the paths indicated by the arrows FD 1  and FD 2  in  FIG. 3 . Consequently, more effective removal of paper dust is enabled. 
     The present embodiment is directed to a configuration in which each through hole  335  has both the tapered portions  335   a  and  335   b  respectively at the upper mouth and the lower mouth. Alternatively, however, each through hole  335  may have one tapered portion at either the upper or lower mouth. 
     Referring back to  FIG. 4 , a description will be given of the relative positions of the suction holes  321  in the conveyor belt  32  and the grooves  334  in the guide member  332 . The conveyor belt  32  has the suction holes  321  arranged in a plurality of rows in the conveyance direction of the sheet P. The rows of suction holes  321  are next to one another in the width direction of the conveyor belt  32  (direction perpendicular to the conveyance direction of the sheet P) such that the suction holes  321  in adjacent rows are staggered. As illustrated in  FIG. 4 , the respective rows of the suction holes  321  in the conveyor belt  32  are located opposite to the rows of the grooves  334  in the guide member  332 . 
     Each groove  334  is arranged so as to be opposite to at least two of the suction holes  321  at all times. The suction holes  321  that are opposite to the grooves  334  change one-by-one as the conveyor belt  32  circulates. 
     The airflow chamber  331 , which is under negative pressure created by the negative pressure creating section  336 , is in communication with the suction holes  321  in the conveyor belt  32  through the through holes  335  and the grooves  334  of the guide member  332 . 
     Therefore, negative pressure is applied to the suction holes  321  of the conveyor belt  32  and thus the conveyor belt  32  can convey a sheet P with the sheet P sucked onto the conveyor belt  32 . 
       FIG. 7  is a plan view of the guide member  332  (upper wall of the airflow chamber  331 ) in  FIG. 2 . 
     Rectangular regions  75  ( 75   a ,  75   b ,  75   c , and  75   d ) in  FIG. 7  are regions of the guide member  332  that are located opposite to the respective recording heads  34  (hereinafter referred to as head facing regions). The conveyor belt  32  is located between the recording heads  34  and the guide member  332 . More precisely, the head facing regions  75  are regions of the guide member  332  that face the respective recording heads  34  with the conveyor belt  32  therebetween. The head facing region  75   a  faces the recording head  34   a . The head facing region  75   b  faces the recording head  34   b . The head facing region  75   c  faces the recording head  34   c . The head facing region  75   d  faces the recording head  34   d.    
     Note that the image forming section  3  includes a single recording head  34  for each of the four types but may include a plurality of recording heads  34  of each of the four types. In a configuration with a plurality of recording heads  34  of each type, the recording heads  34  of each type are staggered in the width direction of the guide member  332  (direction perpendicular to the conveyance direction of the sheet P). 
     Referring to  FIG. 7 , a rectangular region  71  is a given region of the guide member  332  located upstream of the head facing regions  75  in the conveyance direction of the sheet P (to the right in  FIG. 7 ). Hereinafter, the given region is referred to as a “first region”. The first region  71  in the present embodiment corresponds to a region where the plate member  35  is located, that is, a region opposite to the plate member  35  with the conveyor belt  32  therebetween. In other words, the narrow gap  35   a  is located above the first region  71 . 
     Referring further to  FIG. 7 , a rectangular region  72  is located downstream of the first region  71  in the conveyance direction of the sheet P (to the left in  FIG. 7 ) and includes the head facing regions  75 . Hereinafter, the rectangular region  72  is referred to as a “second region”. Ink ejection toward the sheet P (image formation) is performed above the second region  72 . Hereafter, a space  341  (see  FIGS. 8 and 9 ) above the second region  72  in which image formation is performed is referred to as an “image formation space”. 
     The negative pressure applying section  33  in the present embodiment applies greater negative pressure through first through holes  335   c  (first holes) than that through second through holes  335   d  (second holes). Here, the first through holes  335   c  are located in the first region  71 . The second through holes  335   d  are located in the second region  72 . The first and second through holes  335   c  and  335   d  are included among the through holes  335 . 
     In the above configuration, the amount of air sucked through each first through hole  335   c  is greater than that of air sucked through each second through hole  335   d . The amount of air sucked through a through hole  335  herein means an amount of air sucked through the through hole  335  per unit time. As a result, the flow velocity of air (air flowing toward the airflow chamber  331 ) to be sucked through the first region  71  (in the narrow gap  35   a ) increases. The inkjet recording apparatus  1  with the above configuration accordingly can efficiently collect paper dust upstream of the image formation space  341  in the conveyance direction of the sheet P. Thus, in the inkjet recording apparatus  1 , the amount of paper dust conveyed to the image formation space  341  can be reduced. This can result in effective prevention of attachment of paper dust to the nozzles. 
     In the above configuration, the flow velocity of air to be sucked through the second region  72  (in the image formation space  341 ) decreases. Thus, in the inkjet recording apparatus  1 , paper dust can be prevented from stirring up in the image formation space  341  and accordingly be further prevented from being attached to the nozzles. 
     Any of various schemes may be adopted as a scheme for setting the negative pressure applied through the first through holes  335   c  to be greater than that applied through the second through holes  335   d . For example, any of the following schemes can be adopted.
     Scheme 1: The area of an opening of each first through hole  335   c  is set greater than that of an opening of each second through hole  335   d.      Scheme 2: The depth of each first through hole  335   c  is set shallower than that of each second through hole  335   d.      Scheme 3: The negative pressure creating section  336  is disposed under the bottom surface  3312  of the airflow chamber  331  below the first region  71 .   Scheme 4: The depth of a region of the airflow chamber  331  below the first region  71  (depth of the negative pressure generation space  3311 ) is set greater than that of a region of the airflow chamber  331  below the second region  72 . Further, the negative pressure creating section  336  is connected to a portion of the bottom surface  3312  of the airflow chamber  331  that corresponds to the region having the greater depth.   Scheme 5: The airflow chamber  331  (the negative pressure generation space  3311 ) is partitioned into a first space  331   a  located in correspondence with the first region  71  and a second space  331   b  located in correspondence with the second region  72 . Negative pressure in the first space  331   a  is set greater than that in the second space  331   b.      

     Scheme 4 will be described specifically with reference to  FIG. 8 . Then, Scheme 5 will be described specifically with reference to  FIGS. 9 and 10 . 
       FIG. 8  illustrates a first configuration example of the airflow chamber  331  illustrated in  FIG. 2 . Although the grooves  334  are not illustrated in  FIG. 8 , the respective through holes  335  are located in the bottom surfaces of the respective grooves  334  formed in the upper surface of the guide member  332 . 
     The airflow chamber  331  in the first configuration example has a first portion A 1  having a greater depth and a second portion A 2  having a shallower depth. A distance L 1  represents the depth of the airflow chamber  331  in the first portion A 1 , that is, the distance between the bottom surface  3312  and the guide member  332  serving as the upper wall in the first portion A 1 . A distance L 2  represents the depth of the airflow chamber  331  in the second portion A 2 , that is, the distance between the bottom surface  3312  and the guide member  332  serving as the upper wall in the second portion A 2 . The distance L 1  is greater than the distance L 2 . 
     The bottom surface  3312  in the first portion A 1  includes a third region  73 . The third region  73  herein is a region of the bottom surface  3312  of the airflow chamber  331  that is located in correspondence with the first region  71 . In other words, the third region  73  is located opposite to the plate member  35  with the conveyor belt  32  and the guide member  332  therebetween and below the narrow gap  35   a . By contrast, the bottom surface  3312  in the second portion A 2  includes a fourth region  74 . The fourth region  74  herein is a region of the bottom surface  3312  of the airflow chamber  331  that is located in correspondence with the second region  72 , that is, a region below the image formation space  341 . 
     The negative pressure creating section  336  is connected to the bottom surface  3312  of the first portion A 1 , for example, in the third region  73 . Another negative pressure creating section  336  connected to the bottom surface  3312  may be provided in the second portion A 2 , in addition to one connected to the bottom surface  3312  in the first portion A 1 . 
     In the configuration illustrated in  FIG. 8 , the negative pressure applied through the first through holes  335   c  can be set greater than that applied through the second through holes  335   d . Thus, the flow velocity of the air flowing above the first region  71  (in the narrow gap  35   a ) increases. The inkjet recording apparatus  1  in the above configuration can effectively collect paper dust upstream of the image formation space  341  in the conveyance direction of the sheet P (to the right in  FIG. 8 ). Thus, in the inkjet recording apparatus  1 , the amount of paper dust conveyed to the image formation space  341  can be reduced, thereby enabling effective prevention of attachment of paper dust to the nozzles. In the above configuration, the flow velocity of air flowing above the second region  72  (in the image formation space  341 ) decreases. Thus, in the inkjet recording apparatus  1 , paper dust can be prevented from stirring up in the image formation space  341  and accordingly prevented from being attached to the nozzles. 
       FIG. 9  illustrates a second configuration example of the airflow chamber  331  illustrated in  FIG. 2 . Although  FIG. 9  does not illustrate the grooves  334 , the respective through holes  335  are located in the bottom surfaces of the respective grooves  334  formed in the upper surface of the guide member  332 . 
     In the second configuration example, the airflow chamber  331  (the negative pressure generation space  3311 ) is partitioned into the first space  331   a  located in correspondence with the first region  71  and the second space  331   b  located in correspondence with the second region  72 . A portion A 3  of the guide member  332  (the upper wall) that forms the first space  331   a  includes the first region  71 . In the above configuration, negative pressure created in the first space  331   a  causes air to be sucked through the first through holes  335   c  in the first region  71 . A portion A 4  of the guide member  332  (the upper wall) that forms the second space  331   b  includes the second region  72 . In the above configuration, negative pressure created in the second space  331   b  causes air to be sucked through the second through holes  335   d  in the second region  72 . 
     The negative pressure applying section  33  in the second configuration example includes two negative pressure creating sections  336  (first and second negative pressure creating sections  336   a  and  336   b ) and two gas outlets  337  (first and second gas outlets  337   a  and  337   b ). The first negative pressure creating section  336   a  is connected to the bottom surface  3312  of a portion A 5  of the airflow chamber  331  that forms the first space  331   a . The second negative pressure creating section  336   b  is connected to the bottom surface  3312  of a portion A 6  of the airflow chamber  331  that forms the second space  331   b.    
     The first negative pressure creating section  336   a  discharges air outward of the first space  331   a  through the first gas outlet  337   a  to create negative pressure in the first space  331   a . The second negative pressure creating section  336   b  discharges air outward of the second space  331   b  through the second gas outlet  337   b  to create negative pressure in the second space  331   b.    
     Here, the operating rates of the first and second negative pressure creating sections  336   a  and  336   b  (the rotational speed of each fan in a configuration in which the negative pressure creating sections  336  are fans) are set so that the amount of air discharged by the first negative pressure creating section  336   a  per unit time is greater than that of air discharged by the second negative pressure creating section  336   b  per unit time. In the above configuration, the negative pressure in the first space  331   a  is greater than that in the second space  331   b.    
     In the configuration illustrated in  FIG. 9 , the negative pressure applied through the first through holes  335   c  can be set greater than that applied through the second through holes  335   d . Thus, the flow velocity of air flowing above the first region  71  (in the narrow gap  35   a ) increases. Therefore, paper dust can be effectively corrected upstream of the image formation space  341  in the conveyance direction of the sheet P (to the right in  FIG. 9 ), that is, in the narrow gap  35   a  in the inkjet recording apparatus  1 . Thus, in the inkjet recording apparatus  1 , the amount of paper dust conveyed to the image formation space  341  can be reduced, thereby enabling effective prevention of attachment of paper dust to the nozzles. In the above configuration, the flow velocity of air flowing above the second region  72  (in the image formation space  341 ) decreases. Thus, in the inkjet recording apparatus  1 , paper dust can be prevented from stirring up in the image formation space  341  and accordingly prevented from being attached to the nozzles. 
       FIG. 10  is an enlarged view of the first space  331   a  in the airflow chamber  331  illustrated in  FIG. 9 . 
     As illustrated in  FIG. 10 , the airflow chamber  331  is partitioned into the first and second spaces  331   a  and  331   b  by a partition plate  339  such that third through holes  335   e  are located in a region of the first space  331   a . That is, the third through holes  335   e  are located in the portion A 3  of the guide member  332  (the upper wall) that forms the first space  331   a . The third through holes  335   e  herein are each located in the bottom surface of a corresponding one of grooves  334   e  that extend from the first region  71  further downstream in the conveyance direction of the sheet P (to the left in  FIG. 10 ) beyond the first region  71 . 
     As described above, negative pressure applied from the airflow chamber  331  through the through holes  335  affects the inner regions of the grooves  334  where the through holes  335  are located. Accordingly, even in a configuration in which the third through holes  335   e  are located outside the first region  71 , negative pressure applied through the third through holes  335   e  affects also a space above the first region  71  as long as at least a part of the respective grooves  334   e  that have the third through holes  335   e  are located in the first region  71 . Partition of the airflow chamber  331  as above can allow comparatively high negative pressure in the first space  331   a , that is, negative pressure applied through the through holes  335  located in the first space  331   a  to affect the space above the first region  71 . 
     The collection member  338  for collecting foreign matter such as paper dust is disposed at a downstream end of the first gas outlet  337   a  in a direction of airflow. The collection member  338  may be a filter, for example. The collection member  338  collects paper dust mixed with air that is to be discharged outward of the first space  331   a . In the above configuration, a situation in which paper dust sucked in the first space  331   a  is discharged through the first gas outlet  337   a  and scattered in the inkjet recording apparatus  1  can be prevented. Note that another collection member  338  may be disposed at the downstream end of the second gas outlet  337   b  in a direction of airflow, in addition to the collection member  338  at the first gas outlet  337   a.    
     An embodiment of the present disclosure has been described so far with reference to the accompanying drawings. Note that the present disclosure is not limited to the above embodiment, and a wide range of alterations can be made to the embodiment so long as such alterations do not deviate from the intended scope of the present disclosure (e.g., (1) to (5) below). The drawings are schematic illustrations that emphasize elements of configuration in order to facilitate understanding thereof. Therefore, properties of each of the elements, such as thickness, length, and number thereof, may differ from actual properties of the element. The properties of each of the elements, such as shape and dimension thereof described above are mere examples and not limited specifically. A wide range of variations of the properties can be made to the embodiment so long as such variations do not deviate from the intended scope of the present disclosure. 
     (1) The plate member  35  is disposed at a location upstream of the recording heads  34  in the conveyance direction of the sheet P and corresponding to the first region  71  in the present embodiment. However, the plate member  35  may be dispensed with. Even in a configuration without the plate member  35 , it is only required that the negative pressure applying section  33  applies greater negative pressure through the first through holes  335   c  than that through the second through holes  335   d.    
     (2) The negative pressure applying section  33  includes, but is not limited to, the two negative pressure creating sections  336  for the airflow chamber  331  in the second configuration example. Alternatively, a single negative pressure creating section may be provided. In a configuration with a single negative pressure creating section  336 , the negative pressure creating section  336  is disposed, for example, under the bottom surface  3312  of a portion  5 A of the airflow chamber  331  that forms the first space  331   a . A gap is formed between the partition plate  339  and the upper wall, a side wall, or the bottom wall of the airflow chamber  331  so as to allow air to move between the first and second spaces  331   a  and  331   b . In the above configuration, negative pressure in the first space  331   a  can be greater than that in the second space  331   b.    
     (3) The present embodiment describes a configuration in which the conveyor belt  32  conveys the sheet P in the image forming section  3 . Alternatively, however, the image forming section  3  may employ a different method for conveying a sheet P. For example, a plurality of conveyance rollers may be used to convey the sheet P. In this variation, negative pressure is preferably applied through a gap between adjacent conveyance rollers. 
     (4) The above embodiment describes a configuration in which the narrow gap  35   a  is formed by the plate member  35 . This, however, should not be construed as limiting. The narrow gap  35   a  may be formed in another way. For example, the head base  37 , which is located upstream of the recording head  34  in the conveyance direction of the sheet P, may be provided with part extending toward the conveyor belt  32  so as to form the narrow gap  35   a . This variation can simplify the structure. 
     Alternatively, instead of the plate member  35 , a belt stretched around two rollers may be employed to form the narrow gap  35   a . Specifically, this variation employs a drive roller, a driven roller, and an endless belt in such position that the endless belt stretched around the drive roller and the driven roller is substantially parallel to the upper surface of the conveyor belt  32 . The narrow gap  35   a  is formed between the lower surface of the endless belt and the upper surface of the conveyor belt  32 . In this variation, once a region of the endless belt located on a lower side is contaminated with paper dust, the endless belt can be circulated to position a region not yet contaminated with paper dust on the lower side. This can effectively reduce the required frequency of paper dust removal from the endless belt by, for example, a service person. 
     (5) The above embodiments describe a configuration in which the guide member  332  and the airflow chamber  331  are separate components. The guide member  332  may be integral with the airflow chamber  331 . This variation enables prevention of unintentional release of negative pressure from the airflow chamber  331  (air flowing into the airflow chamber  331  through a gap between the guide member  332  and the airflow chamber  331 ).