Abstract:
A discharge mechanism includes: a rotation shaft; a pair of roll members disposed on the rotation shaft at different positions in an axial direction; and a protrusion that protrudes from a section of the rotation shaft sandwiched between the pair of roll members, wherein a distance from a center of the rotation shaft to a tip of the protrusion is smaller than a radius of each of the pair of roll members, and the protrusion includes a projecting part that projects in a direction of rotation of the rotation shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 U.S.C. 119 from Japanese Patent Application No. 2010-239066, which was filed on Oct. 25, 2010. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a discharge mechanism and an image-forming device. 
         [0004]    2. Related Art 
         [0005]    Various technologies are being developed to improve a process of discharging a recording medium from an image-forming device. 
       SUMMARY 
       [0006]    In one aspect of the present invention, there is provided a discharge mechanism including: a rotation shaft; a pair of roll members disposed on the rotation shaft at different positions in an axial direction; and a protrusion that protrudes from a section of the rotation shaft sandwiched between the pair of roll members, wherein a distance from a center of the rotation shaft to a tip of the protrusion is smaller than a radius of each of the pair of roll members, and the protrusion includes a projecting part that projects in a direction of rotation of the rotation shaft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Exemplary embodiments of the present invention will now be described in detail with reference to the following figures, wherein: 
           [0008]      FIG. 1  is a drawing for explaining an overall configuration of an image-forming device according to an exemplary embodiment of the present invention; 
           [0009]      FIG. 2  is a drawing showing a configuration of a discharge unit and its surroundings; 
           [0010]      FIG. 3  is a drawing showing an envelope; 
           [0011]      FIG. 4  is a perspective view showing a configuration of a discharge unit and its surroundings; 
           [0012]      FIG. 5  is a drawing showing an enlarged view of a section of a discharge rod sandwiched between discharge rollers; 
           [0013]      FIG. 6  is another drawing showing an enlarged view of a section of a discharge rod sandwiched between discharge rollers; 
           [0014]      FIG. 7  is a drawing showing an arrangement of protrusions in an axial direction; 
           [0015]      FIG. 8  is a drawing showing an axial distance between a pair of discharge rollers and an axial distance between each pair of protrusions in relation to a flap of an envelope; 
           [0016]      FIGS. 9A-9C  are drawings for explaining an operation of a protrusion without a hook portion provided thereon; 
           [0017]      FIGS. 10A-10C  are drawings for explaining an operation of a protrusion with a hook portion provided thereon; and 
           [0018]      FIGS. 11A-11F  show modifications of a protrusion having a hook portion. 
       
    
    
     DETAILED DESCRIPTION 
     1. Exemplary Embodiment 
     1-1. Overall Configuration 
       [0019]    In this exemplary embodiment, a recording medium refers to a sheet-like member on which an image is to be formed by image-forming unit  500 . A recording medium typically is a sheet of paper or an envelope made of paper, but it may be a sheet of plastic. 
         [0020]    In the following description and drawings, directions will be indicated using X-axis, Y-axis, and Z-axis, which intersect perpendicularly to one another. The coordinate system represented by X-axis, Y-axis, and Z-axis is a right-handed one. A direction in which an X component (a component represented on X-axis) increases along X-axis will be referred to as X(+) direction, while a direction in which an x component decreases along X-axis will be referred to as X(−) direction. The same applies in the case of each of a Y component and a Z component. 
         [0021]      FIG. 1  is a drawing for explaining an overall configuration of image-forming device  1  according to an exemplary embodiment of the present invention.  FIG. 1  is a schematic diagram of an inside of image-forming device  1  as viewed in Z(−) direction. 
         [0022]    Supply unit  600  includes a container for containing recording media such as a sheet of paper or an envelope. When the container is set in housing  800 , the recording media contained in the container become ready for supply. 
         [0023]    Conveying unit  700  takes out the recording media from supply unit  600  one sheet at a time, and conveys the recording media to image-forming unit  500 . 
         [0024]    Image-forming unit  500  forms an image on a surface of a recording medium by an electrophotography process using a developer. Specifically, image-forming unit  500  includes a photosensitive member that holds a latent image, an exposure device that exposes the photosensitive member to cause the photosensitive member to hold a latent image, a developer supply device that supplies a developer to the latent image held by the photosensitive member, and a transfer device that transfers a developed image from the photosensitive member to the recording medium. The developer may contain black toner, for example. 
         [0025]    Fixing unit  400  heats the toner that has been caused to adhere to the surface of the recording medium by image-forming unit  500 , so that the toner melts and an image is fixed on the recording medium. 
         [0026]    Discharge unit  100  catches the recording medium, on which an image has been fixed by fixing unit  400 , in cooperation with assist unit  200 , and discharges the recording medium onto stacking unit  300 . 
         [0027]    Stacking unit  300  stacks and holds recording media discharged from discharge unit  100 . 
       1-2. Configuration of Discharge Unit 
       [0028]      FIG. 2  is a drawing showing a configuration of discharge unit  100  and its surroundings. This drawing is a schematic diagram as viewed in Z(−) direction. 
         [0029]    Discharge unit  100  includes discharge rod  101 , discharge rollers  102 , first protrusions  111 , second protrusions  112 , third protrusions  113 , and fourth protrusion  114 . Discharge rod  101  is a rod-shaped member having axis O at its center, and is caused to rotate about axis O by a drive unit (not shown). On a circumference of discharge rod  101  are provided a pair of discharge rollers  102   a  and  102   b , which are spaced apart from each other in an axial direction (in the following description, where it is not necessary to distinguish these rollers, they will be simply referred to as “discharge rollers  102 ”), first protrusions  111 , second protrusions  112 , third protrusions  113 , and fourth protrusion  114 . 
         [0030]    Discharge rollers  102  each are a member whose cross-sectional view taken perpendicularly to discharge rod  101  is a circle with its center coinciding with axis O. Discharge rollers  102 , which are provided on discharge rod  101 , catch a recording medium in cooperation with assist roller  202  of assist unit  200 , which will be described later, and are caused to rotate about axis O of discharge rod  101  in a direction of arrow D 0  to discharge the recording medium onto stacking unit  300 . 
         [0031]    Each of first protrusions  111 , second protrusions  112 , third protrusions  113 , and fourth protrusion  114  (in the following description, summarily referred to as “protrusions”) is provided in a section of discharge rod  101  sandwiched between discharge rollers  102   a  and  102   b . Therefore, these protrusions are caused to rotate about axis O together with rotation of discharge rod  101 . 
         [0032]    A distance from axis O of discharge rod  101  to a tip of each protrusion is smaller than a radius of discharge roller  102  (more precisely, a radius of a circle represented by a cross-section of discharge roller  102  taken along a plane perpendicular to axis O). In other words, rotation of an outer end of each of these protrusions describes a circle whose radius is smaller than the radius of discharge roller  102 . 
         [0033]    Assist unit  200  includes assist rod  201 , assist rollers  202 , and guide roller  203 . Guide roller  203  is a roller rotating around assist rod  201 , and determines a position of assist rod  201  relative to discharge rod  101  when a circumferential surface of guide roller  203  abuts a plate-like member (not shown) provided on housing  800 . 
         [0034]    Assist rollers  202   a  and  202   b  (in the following description, where it is not necessary to distinguish these rollers, they will be simply referred to as “assist rollers  202 ”) are rollers rotating around assist rod  201 , and are provided at positions on assist rod  201  opposed to discharge rollers  102   a  and  102   b . A diameter of each assist roller  202  is larger than that of assist rod  201 . Since the distance between assist rod  201  and discharge rod  101  is determined by guide roller  203 , a clearance between discharge rollers  102  and assist rollers  202  is also adjusted. Assist rollers  202  operate following an operation of discharge rollers  102 , to catch a recording medium in cooperation with opposed discharge rollers  102  and discharge the recording medium onto stacking unit  300 . Path P indicated by a long- and double-short dashed line is a path of a recording medium conveyed by discharge rollers  102  and assist rollers  202 . 
         [0035]    Stacking unit  300  includes a plate member bent at edge  303  to form bottom portion  301  and side portion  302 . The recording media caught and discharged by discharge rollers  102  and assist rollers  202  are stacked on bottom portion  301 . Since bottom portion  301  is inclined with respect to a direction of gravity (Y(−) direction), the recording media stacked on bottom portion  301  tend to slide down in a direction of arrow D 1 . Side portion  302  abuts ends of the recording media to block sliding down of the recording media in the direction of arrow D 1 . 
         [0036]    Now, an explanation will be given of envelope V serving as a recording medium that is caught and discharged by discharge rollers  102  and assist rollers  202  along path P. Envelope V is contained in supply unit  600  in an open state, and after an image including characters representing a name of an addressee, a destination address, and the like, is formed on a front side, for example, envelope V is discharged by discharge unit  100 . 
         [0037]      FIG. 3  is a drawing showing a configuration of envelope V. Envelope V is divided into envelope main body V 1  and closure portion V 2 , which is also referred to as a “flap,” by folding line V 3 . Envelope V is closed when flap V 2  is folded along folding line V 3  and is glued to envelope main body V 1 . The shape of flap V 2  shown in  FIG. 3  is a triangle (isosceles triangle) including folding line V 3  as a base. 
         [0038]    When envelope V is discharged by discharge unit  100 , envelope V is in an open state, and thus, flap V 2  is not folded along folding line V 3  to be in contact with or to be close to envelope main body V 1 . It should be noted, however, that in a case where there is a crease along folding line V 3  so that envelope V tends to be folded in a convex shape in a downward direction (Y(−) direction), envelope V may be held in stacking unit  300  in a state in which it is folded along folding line V 3  as shown in  FIG. 2 . In this state, envelope V is held with envelope main body V 1  extending along bottom portion  301 , while flap V 2  extends along side portion  302 . 
       1-3. Configuration of Protrusions 
     1-3-1. Arrangement of Protrusions in Direction of Rotation 
       [0039]      FIG. 4  is a perspective view showing a configuration of discharge unit  100  and its surroundings. As shown in this drawing, discharge rod  101  extends along Z-axis, and two discharge rollers  102   a  and  102   b  are provided such that they are spaced apart from each other by a predetermined distance in an axial direction of discharge rod  101  (Z-axis direction). Discharge roller  102   a  is positioned on a side in a Z(−) direction with respect to discharge roller  102   b.    
         [0040]      FIG. 5  is a drawing showing an enlarged view of a section of discharge rod  101  sandwiched between discharge rollers  102   a  and  102   b . As shown in this drawing, fourth protrusion  114  is provided at a center of this section in the axial direction (Z-axis direction). Discharge rod  101  is caused to rotate in the direction of arrow D 0 , and with respect to this direction of rotation, at a position spaced in a rearward direction from fourth protrusion  114  by one quarter of a turn (90 degrees) are provided first protrusions  111   a  and  111   b  (in the following description, where it is not necessary to distinguish these protrusions, they will be simply referred to as “first protrusions  111 ”). First protrusion  111   a  is positioned on a side in a Z(−) direction with respect to first protrusion  111   b.    
         [0041]      FIG. 6  is another drawing showing an enlarged view of a section of discharge rod  101  sandwiched between discharge rollers  102   a  and  102   b , where discharge rod  101  has been rotated by a half turn (180 degrees) from the state shown in  FIG. 5 . With respect to the direction of rotation indicated by arrow D 0 , at a position spaced in a rearward direction from first protrusions  111  by one quarter of a turn (90 degrees) are provided second protrusions  112   a  and  112   b  (in the following description, where it is not necessary to distinguish these protrusions, they will be simply referred to as “second protrusions  112 ”). Second protrusion  112   a  is positioned on a side in a Z(−) direction with respect to second protrusion  112   b.    
         [0042]    Also, with respect to the direction of rotation, at a position spaced in a rearward direction from second protrusions  112  by one quarter of a turn (90 degrees) are provided third protrusions  113   a  and  113   b  (in the following description, where it is not necessary to distinguish these protrusions, they will be simply referred to as “third protrusions  113 ”). Third protrusion  113   a  is positioned on side in a Z(−) direction with respect to third protrusion  113   b.    
         [0043]    Further, with respect to the direction of rotation, at a position spaced in a rearward direction from third protrusions  113  by one quarter of a turn (90 degrees) is provided fourth protrusion  114 . Namely, in a rearward direction with respect to the direction of rotation of discharge rod  101 , first protrusions  111 , second protrusions  112 , third protrusions  113 , and fourth protrusion  114  are arranged in the order stated, spaced apart from one another at an angular interval of one quarter of a turn (90 degrees). In other words, in a section of discharge rod  101  sandwiched between discharge rollers  102   a  and  102   b , respective types of protrusions are provided at four different positions in the direction of rotation of discharge rod  101 . 
         [0044]    At least one of the four types of protrusions is provided with a hook portion. A hook portion is a portion projecting in the direction of rotation from a tip portion of a protrusion (i.e., an outer end portion of a protrusion from discharge rod  101 ). In this exemplary embodiment, first protrusions  111  and third protrusions  113  each are provided with a hook portion, while second protrusions  112  and fourth protrusion  114  are not. Detailed explanation of the hook portion will be given later. 
       1-3-2. Arrangement of Protrusions in Axial Direction 
       [0045]      FIG. 7  is a drawing showing an arrangement of protrusions in an axial direction (Z-axis direction). The length of the section of discharge rod  101  sandwiched between discharge rollers  102   a  and  102   b , i.e., the length from a surface of discharge roller  102   a  on a Z(+) side to a surface of discharge roller  102   b  on a Z(−) side is length L 0 . The length from a surface of first protrusion  111   a  on a Z(+) side to a surface of first protrusion  111   b  on a Z(−) side is length L 1 . The length from a surface of second protrusion  112   a  on a Z(+) side to a surface of second protrusion  112   b  on a Z(−) side is length L 2 . The length from a surface of third protrusion  113   a  on a Z(+) side to a surface of third protrusion  113   b  on a Z(−) side is length L 3 . There is a relationship between L 0 , L 1 , L 2 , and L 3 , that is, L 0 &gt;L 1 &gt;L 2 &gt;L 3 . 
         [0046]      FIG. 8  is a drawing showing an axial distance between a pair of discharge rollers  102  and an axial distance between each pair of protrusions in relation to flap V 2  of envelope V. As discharge rollers  102  rotate, envelope V is discharged in a direction of arrow D 2 , and thus, envelope main body V 1  is discharged first, and flap V 2  is discharged subsequently. Flap V 2  has a shape in which its width (a length in a direction that is parallel with folding line V 3  and is perpendicular to the direction of arrow D 2 ) becomes smaller in a rearward direction with respect to the direction of arrow D 2 . Thus, edge portion E of flap V 2  of envelope V shown in  FIG. 3  is an example of “a trailing end having a shape in which a width becomes smaller in a rearward direction with respect to a direction of discharge.” 
         [0047]    Region V 20  is a portion of flap V 2  where a widthwise dimension is equal to or larger than L 0 . Region V 21  is a portion of flap V 2  where a widthwise dimension is smaller than L 0  and is equal to or larger than L 1 . Region V 22  is a portion of flap V 2  where a widthwise dimension is smaller than L 1  and is equal to or larger than L 2 . Region V 23  is a portion of flap V 2  where a widthwise dimension is smaller than L 2  and is equal to or larger than L 3 . Region V 24  is a portion of flap V 2  where a widthwise dimension is smaller than L 3 . 
         [0048]    Thus, when discharge rollers  102  are in contact with region V 20  of flap V 2 , discharge rollers  102  function to discharge envelope V in the direction of arrow D 2 . However, discharge rollers  102  do not contact regions V 21 -V 24 , which are positioned on a back side of region V 20  with respect to the direction of arrow D 2  (direction of discharge). Therefore, once region V 20  has passed discharge rollers  102 , discharge rollers  102  no longer function to discharge envelope V. At this time, as shown in  FIG. 2 , flap V 2  moves around folding line V 3  as an axis, to incline in a direction of arrow D 3  to a position indicated by broken lines. 
         [0049]    In flap V 2  that has moved to the position indicated by the broken lines in  FIG. 2 , region V 21  shown in  FIG. 8 , which has a widthwise dimension smaller than L 0  and equal to or larger than L 1 , comes into contact with first protrusions  111   a  and  111   b  (see  FIG. 7 ), which are spaced apart from each other by distance L 1 , and thus, region V 21  is conveyed by first protrusions  111   a  and  111   b  in the direction of arrow D 2 . 
         [0050]    Also, region V 22  of flap V 2 , which has a widthwise dimension smaller than L 1  and equal to or larger than L 2 , comes into contact with second protrusions  112   a  and  112   b , which are spaced apart from each other by distance L 2 , and thus, region V 22  is conveyed by second protrusions  112   a  and  112   b  in the direction of arrow D 2 . 
         [0051]    Similarly, region V 23  of flap V 2 , which has a widthwise dimension smaller than L 2  and equal to or larger than L 3 , comes into contact with third protrusions  113   a  and  113   b , which are spaced apart from each other by distance L 3 , and thus, region V 23  is conveyed by third protrusions  113   a  and  113   b  in the direction of arrow D 2 . 
         [0052]    Then, region V 24  of flap V 2  comes into contact with fourth protrusion  114 , and is conveyed in the direction of arrow D 2 . 
       1-3-3. Hook Portion of Protrusion 
       [0053]    Next, explanation will be given of an operation of a hook portion of a protrusion. 
         [0054]      FIGS. 9A-9C  are drawings for explaining an operation of a protrusion without a hook portion provided thereon. The above-described second protrusions  112  and fourth protrusion  114  are protrusions without a hook portion provided thereon. A protrusion without a hook portion includes straight flat plate W extending radially, i.e., perpendicularly to the direction of axis O (Z-axis direction) of discharge rod  101 . Straight flat plate W is provided on a circumferential surface of discharge rod  101 , and is caused to rotate together with rotation of discharge rod  101  in the direction of arrow D 0 . As shown in  FIG. 9A , surface W 0  of straight flat plate W facing in the direction of arrow D 0  comes into contact with trailing end V 0  of envelope V (more specifically, flap V 2  of envelope V), and pushes envelope V along the direction of rotation of discharge rod  101 . 
         [0055]    At this time, depending on an inclination of envelope V relative to surface W 0 , trailing end V 0  of envelope V may be caused to slide, owing to inertia acting on envelope V, in a direction of arrow Db or a direction along surface W 0  and away from discharge rod  101 , as shown in  FIG. 9B . In such a case, if trailing end V 0  moves beyond a length of extension of straight flat plate W, as shown in  FIG. 9C , surface W 0  of straight flat plate W disengages from trailing end V 0 , so that the protrusion does not function to discharge envelope V. 
         [0056]    On the other hand,  FIGS. 10A-10C  are drawings for explaining an operation of a protrusion with a hook portion provided thereon. The above-described first protrusions  111  and third protrusions  113  each are protrusions with a hook portion provided thereon. Each of these protrusions includes straight flat plate W extending radially, i.e., perpendicularly to the direction of axis O (Z-axis direction) of discharge rod  101 , and hook portion Wp. Hook portion Wp is a member projecting from a tip portion of straight flat plate W in the direction of rotation of discharge rod  101  (in a frontward direction with respect to the direction of arrow D 0 ) perpendicularly to straight flat plate W. As shown in  FIG. 10A , when surface W 0 , which faces in the direction of arrow D 0 , of straight flat plate W of a protrusion comes into contact with trailing end V 0  of envelope V to push envelope V in the direction of rotation of discharge rod  101 , trailing end V 0  may be caused to slide in the direction of arrow Db. However, as shown in  FIG. 10B , the sliding trailing end V 0  comes to abut hook portion Wp, and thus, does not move further in the direction away from discharge rod  101 . Then, upon further rotation of discharge rod  101  in the direction of arrow D 0 , straight flat plate W of the protrusion pushes envelope V, thereby to discharge envelope V in a direction of arrow Df, as shown in  FIG. 10C . 
       2. Modifications 
       [0057]    An exemplary embodiment has been described in the foregoing. The exemplary embodiment may be modified as described below. The following modifications may be used in any combination. 
       2-1. Image-Forming Unit 
       [0058]    In the above-described exemplary embodiment, image-forming unit  500  forms an image on a surface of a recording medium by an electrophotography process using a developer. However, formation of an image on a recording medium may be carried out by another process. For example, an image may be formed by ink jet technique. 
       2-2. Protrusions 
       [0059]    (1) In the above-described exemplary embodiment, the protrusions extending from discharge rod  101  include, in a section of discharge rod  101  sandwiched between discharge rollers  102   a  and  102   b , four types of protrusion, i.e., first protrusions  111 , second protrusions  112 , third protrusions  113 , and fourth protrusion  114 , respectively corresponding to four different positions in the direction of rotation of discharge rod  101 . However, the protrusions may include fewer than four types of protrusion or more than four types of protrusion. 
         [0060]    (2) In the above-described exemplary embodiment, of the four types of protrusions, first protrusions  111  and third protrusions are provided with a hook portion. However, it is possible that at least one type of protrusion is provided with a hook portion. 
         [0061]    (3) It is possible that, of the multiple types of protrusion, only two types of protrusion positioned symmetrically about the axis of discharge rod  101  are provided with a hook portion. In this way, in a case where discharge rod  101  is molded integrally with the protrusions by injection of resin into a mold, removal of discharge rod  101  from the mold can be made easier, as compared to a case where three or more types of protrusion are provided with a hook portion. It is to be noted that discharge rod  101  does not have to be molded integrally with the protrusions, and the protrusions may be attached on a circumferential surface of molded discharge rod  101  by means of an adhesive, for example. 
         [0062]    (4) The positions of the protrusions in the axial direction (Z-axis direction) may be the same. Namely, the distance in the axial direction between each pair of protrusions only need be smaller than the distance between the pair of discharge rollers. 
         [0063]    (5) It is to be noted that, in the above-described exemplary embodiment, the protrusions are divided into groups based on the direction in which each protrusion extends away from the axis, and each group is composed of a pair of protrusions that are spaced apart from each other in the axial direction, except for the group composed of fourth protrusion  114 . Of these groups, those composed of a pair of protrusions are arranged such that the distances between the pairs of protrusions provided on discharge rod  101  become progressively smaller in the rearward direction with respect to the direction of rotation of discharge rod  101  (L 1 →L 2 →L 3 ). 
         [0064]    As discharge rod  101  rotates, a trailing end of a recording medium comes into contact with first protrusions  111  that are spaced apart from each other by distance L 1 , and is pushed by first protrusions  111  toward stacking unit  300 . Since the trailing end of the recording medium has a width that becomes smaller in the rearward direction relative to the direction of discharge, after being pushed toward stacking unit  300 , the recording medium will have a width smaller than L 1  at a portion that is closest to discharge rod  101 . At this time, since the protrusions are arranged in the order described in the foregoing, second protrusions  112  spaced apart from each other by distance L 2 , which is smaller than L 1 , come into contact with the trailing end of the recording medium subsequently to first protrusions  111 . Thus, even when the width of the trailing end of the recording medium is smaller than L 1 , second protrusions  112  push the trailing end of the recording medium in the direction of discharge. 
         [0065]    Similarly, subsequently to second protrusions  112 , third protrusions  113  spaced apart from each other by distance L 3 , which is smaller than L 2 , come into contact with the trailing end of the recording medium, and further, subsequently to third protrusions  113 , fourth protrusion  114 , which is provided alone in the axial direction, comes into contact with the trailing end of the recording medium. Thus, the distances between the pairs of protrusions for pushing a trailing end of a recording medium are adapted to become smaller as discharge rod  101  rotates, and therefore, the protrusions sequentially push a trailing end of a recording medium even in a case where the width of the trailing end becomes smaller as the discharge of the recording medium progresses. 
         [0066]    (6) The protrusions do not have to include a group composed of a pair of protrusions. Namely, it is possible that multiple protrusions are provided in a section of discharge rod  101  sandwiched between discharge rollers  102   a  and  102   b  such that the protrusions protrude respectively from at least two different positions in the axial direction. Since the discharge mechanism of an exemplary embodiment of the present invention has a configuration in which the protrusions protruding from at least two different positions in the axial direction are adapted to push a trailing end of a recording medium, it is possible to suppress rotational movement of a recording medium around a contact point between the recording medium and one of the protrusions. 
         [0067]    (7) In the above-described exemplary embodiment, a hook portion is a member projecting from a tip portion of a protrusion in the direction of rotation of discharge rod  101 . However, a hook portion may project from a portion of a protrusion other than a tip portion. Also, an angle between a direction of extension of a hook portion and a direction of extension of a protrusion is not limited to a right angle, and may be an obtuse angle or an acute angle. Moreover, a direction of extension of a protrusion does not have to pass through axis O of discharge rod  101 , and may be curved. 
         [0068]      FIGS. 11A-11F  show modifications of a protrusion having a hook portion. In the above-described exemplary embodiment, a protrusion having a hook portion has the shape shown in  FIG. 11A . Namely, a protrusion in the above-described exemplary embodiment has a shape in which hook portion Wp projects in the direction of rotation of discharge rod  101  (in a frontward direction with respect to the direction of arrow D 0 ) from a tip portion of straight flat plate W extending in a direction passing through axis O (not shown in this drawing) of discharge rod  101 . However, as shown in  FIG. 11B , a protrusion may have hook portion Wp projecting in the direction of rotation of discharge rod  101  from an intermediate position in the direction of extension of straight flat plate W (i.e., from a position that is neither a tip nor a base). 
         [0069]    Also, angle θ between hook portion Wp and straight flat plate W (an angle between a surface of hook portion Wp facing axis O of discharge rod  101  and surface W 0  of straight flat plate W facing in the direction of rotation of discharge rod  101 ) preferably is an acute angle as shown in  FIG. 11D , but may be an obtuse angle as shown in  FIG. 11C , if a frictional force acting between straight flat plate W and a recording medium is sufficiently large. Namely, it is only necessary that a protrusion has a configuration that in which hook portion Wp holds a trailing end of a recording medium that is pushed in the direction of discharge by surface W 0  facing in the direction of rotation of discharge rod  101 , thereby to prevent the trailing end from moving beyond an extension of straight flat plate W. 
         [0070]    Further, as shown in  FIG. 11E , a line drawn in the direction of extension of straight flat plate W does not have to pass through axis O (not shown in this drawing) of discharge rod  101 . Furthermore, as shown in  FIG. 11F , a protrusion may include curved flat plate Wc instead of straight flat plate W. In this case, curved flat plate Wc has concave surface W 0  facing in the direction of rotation of discharge rod  101 , and this surface W 0  and hook portion Wp provided on the tip portion of curved flat plate We serve to push a trailing end of a recording medium in the direction of rotation, while holding the trailing end. 
       2-3. Discharge Rod 
       [0071]    In the above-described exemplary embodiment, discharge rollers  102  and the protrusions are provided on common discharge rod  101 . However, it is only necessary that discharge rollers  102  and the protrusions are adapted to be able to rotate about axis O, which is an axis extending in Z-axis direction. Therefore, discharge rollers  102  and the protrusions may be provided on different rods. For example, in a case where discharge rollers  102  are provided on one rod and the protrusions are provided on another, discharge unit  100  may include a transmission mechanism that engages both of gears provided on outer circumferences of these rods, so that discharge rollers  102  and the protrusions are caused to rotate about common axis O. In this case, discharge unit  100  including the transmission mechanism may be configured such that the rotation speed of discharge rollers  102  is different from that of the protrusions. 
       2-4. Others 
       [0072]    The shape of flap V 2  shown in  FIG. 3  is a triangle (isosceles triangle) including folding line V 3  as a base. However, the shape of flap V 2  may be a trapezoid including folding line V 3  as a longer one of the parallel sides. For example, flap V 2  may have a shape that does not include region V 24  shown in  FIG. 8 . Thus, the above-described envelope V is an example of a recording medium with a trailing end having a shape in which a width becomes smaller in a rearward direction with respect to a direction of discharge. 
         [0073]    The foregoing description of the embodiments of the present invention is provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.