Patent Publication Number: US-9841706-B2

Title: Image forming apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus including a container for accommodating powder and a rotatable member mounted to the container. 
     Conventionally, in the image forming apparatus using an electrophotographic type or an electrostatic recording type, a toner image is formed on a drum-shaped or belt-shaped electrophotographic photosensitive member or electrostatic recording dielectric member by an appropriate image forming process. This toner image is directly transferred onto a transfer(-receiving) material (direct transfer type) or is secondary-transferred onto the transfer material after being once primary-transferred onto an intermediary transfer member (intermediary transfer type. In transfer of the toner image from an image bearing member such as a photosensitive member, an electrostatic recording dielectric member or an intermediary transfer member onto a transfer-receiving member, for example, a transfer member provided so as to be contactable to the image bearing member is used. In such an image forming apparatus, a cleaning means for collecting toner from the image bearing member or the transfer member is provided. Toners to be collected include, for example, toner (transfer residual toner) remaining on the members after a transfer step, toner (fog toner) deposited on a non-image region, and toner (patch toner) of various control images. 
     For example, in an image forming apparatus of an intermediary transfer type using an electrophotographic process, patches for positioned alignment between the image and a transfer material and for adjusting a color that are formed on the intermediary transfer member in some cases. The patches on the intermediary transfer member are formed in the non-image region such as a region (sheet interval) between an image and a subsequent image, and therefore are transferred onto a secondary transfer member in some cases. The toner transferred on the secondary transfer member is removed by the cleaning means and is collected in a cleaning container. As the secondary transfer member, a rotatable member such as a roller or an endless belt wound around a plurality of rollers is used. In this image forming apparatus, when a gap is formed between the secondary transfer member and the cleaning container, the toner scatters to an outside through the gap, so that an inside of the image forming apparatus is contaminated with the toner. Therefore, the gap between the secondary transfer member and the cleaning container has been sealed with a seal (sealing) member. In the image forming apparatus, the seal member has been widely used for filling (closing) the gap not only between the secondary transfer member and the cleaning container but also between a container for accommodating powder and a rotatable member. 
     As the seal member, a brush-shaped member formed with furs of synthetic fibers or a sheet-shaped member formed of PET (PET sheet) is used in some cases. In the case where the brush-shaped member or the PET sheet is used, a contact pressure to the rotatable member is liable to become relatively large, so that fusion of the toner to the rotatable member is liable to generate. On the other hand, as the seal member, a sheet-shaped member formed of polyurethane (urethane sheet) is used in some cases. The polyurethane sheet is relatively low in rigidity and the contact pressure to the rotatable member does not readily become excessive, and therefore, it can be said that the urethane sheet is a seal member suitable for a portion where the seal member directly slides with the toner. 
     Japanese Laid-Open Patent Application 2007-140062 discloses that the urethane sheet is contacted to a surface of a photosensitive member in order to suppress scattering of a toner through a gap between the photosensitive member and a developing device. 
     However, in the case where the sheet-shaped seal member (hereinafter simply referred to as a sheet member) such as the urethane sheet described above is used, it turned out that the following problem arose. 
     That is, when the rotatable member is mounted to the container, the sheet member enters a portion below the rotatable member. For that reason, the sheet member is checked by eye observation during an operation such as during assembling or when the rotatable member is replaced by a service person and then is required to be pulled out (turned up) to a normal position by a manual operation in some cases. Thus, the pulling-out of the sheet member complicates the operation and causes damage of the rotatable member and the sheet member. 
     In this regards, it would be considered that a constitution in which the sheet member is made detachably mountable to the container and is mounted to the container after the rotatable member is mounted during replacement or the like of the rotatable member or the sheet member is retracted from the rotatable member is synchronism with mounting and demounting of the rotatable member. However, in this case, complification and upsizing of a device constitution are liable to generate. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising: a movable member unit including a movable member on which a toner image is carried or a movable member configured to feed a recording material on which the toner image is carried, wherein the movable member unit movably supports the movable member; and a container connectable with the movable member unit and including a cleaning member, a sheet member and an urging member, wherein the cleaning member removes a toner on the movable member with movement of the movable member in contact with the movable member in a state in which the movable member unit and the container are connected with each other, wherein the sheet member extends in a widthwise direction perpendicular to a movement direction of the movable member and is supported by the container in one end side thereof with respect to the movement direction of the movable member, and at least during cleaning of the movable member, a part of the sheet member in the other end side where a free end of the sheet member is positioned in a first state in which the part of the sheet member contacts the movable member codirectionally with the movement direction of the movable member, wherein the urging member is adjacent to the sheet member in an upstream side of the sheet member with respect to the movement direction of the movable member and is disposed at a position overlapping with neighborhoods of ends of the sheet members with respect to the widthwise direction, and the part of the sheet member in the other side where the free end of the sheet member is positioned is in a second state in which the part of the sheet member is sandwiched between the movable member and the urging member counterdirectionally with the movement direction of the movable member when the movable member unit and the container are connected with each other, and wherein when a friction coefficient between the sheet member and the movable member is μ1 and a friction coefficient between the sheet member and the urging member is μ2, the following relationship is satisfied: μ1&gt;μ2. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image forming apparatus. 
         FIG. 2  is a schematic sectional view of a secondary transfer device. 
         FIG. 3  is a schematic sectional view of the secondary transfer device for illustrating a state of mounting of a belt unit to a cleaning container. 
       In  FIG. 4 , (a) and (b) are enlarged sectional views for illustrating a behavior of a sheet member during the mounting of the belt unit. 
       In  FIG. 5 , (a) and (b) are enlarged sectional views for illustrating a behavior of the sheet member during automatic pulling-out of the sheet member. 
         FIG. 6  is a schematic view showing a positional relationship among a driving roller of a secondary transfer belt, the sheet member and an urging member. 
         FIG. 7  is a schematic view for illustrating a force exerted during the automatic pulling-out of the sheet member. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An image forming apparatus according to the present invention will be described with reference to the drawings. 
     Embodiment 1 
     1. General Constitution and Operation of Image Forming Apparatus 
       FIG. 1  is a schematic sectional view of an image forming apparatus  100  according to Embodiment 1 of the present invention. 
     The image forming apparatus  100  in this embodiment is a tandem laser beam printer which is capable of forming a full-color image using an electrophotographic type and which employs an intermediary transfer type. 
     The image forming apparatus  100  includes, as a plurality of image forming portions (stations), first to fourth image forming portions SY, SM, SC and SK for forming images of yellow (Y), magenta (M), cyan (C) and black (K), respectively. In this embodiment, constitutions and operations of these four image forming portions SY, SM, SC and SK are substantially the same except that the colors of toners used in a developing step described later are different from each other. Accordingly, in the following, in the case where particular distinction is not required, suffixes Y, M, C and K for representing elements for associated colors are omitted, and the elements will be collectively described. 
     The image forming portion S includes a photosensitive drum  1  which is a drum-shaped electrophotographic photosensitive member as a first image bearing member. The photosensitive drum  1  is rotationally driven in an arrow R 1  direction. At a periphery of the photosensitive drum  1  of the image forming portion S, along a rotational direction of the photosensitive drum  1 , the following process devices are provided in the listed order. First, a charger  2  as a charging means is disposed. Next, an exposure device (laser scanner)  3  as an exposure means is disposed. Next, a developing device  4  as a developing means is disposed. Next, primary transfer rollers  5  which are roller-shaped primary transfer members as primary transfer means. Next, a drum cleaning device  6  as a photosensitive member cleaning means is disposed. 
     A surface of the rotating photosensitive drum  1  is electrically charged substantially uniformly to a predetermined polarity (negative in this embodiment) and a predetermined potential by the charger. The charged photosensitive drum  1  is exposed to light depending on image information by the exposure device  3 , so that an electrostatic latent image (electrostatic image) depending on the image information is formed on the photosensitive drum  1 . The electrostatic latent image formed on the photosensitive drum  1  is developed (visualized) with the toner as a developer by the developing device  4 , so that the toner image is formed on the photosensitive drum  1 . The developing device  4  includes a developer container for accommodating the toner and a developing roller as a developer carrying member rotatably provided to the developer container, and supplies the toner, fed to an opposing portion to the photosensitive drum  1  while being carried on a surface of the rotating developing roller, to the photosensitive drum  1  depending on the electrostatic latent image. In this embodiment, a reverse developing method is used. That is, the toner charged to the same polarity as a charge polarity of the photosensitive drum  1  is deposited on an exposed portion of the photosensitive drum  1  where an absolute value of the potential is lowered by exposing to light the surface of the photosensitive drum  1  after the photosensitive drum  1  is uniformly charged. 
     Incidentally, the electrostatic latent image formed by the exposure device  3  is a group of small dotted images, and by changing a density of the dotted images, it is possible to change a density of the toner image to be formed on the photosensitive drum  1 . In this embodiment, each of the color toner images is about 1.5-1.7 in maximum density, and is about 0.4-0.6 mg/cm 2  in toner amount per unit area at the maximum density. 
     As a second image bearing member, an intermediary transfer belt  7  constituted by a rotatable endless belt is provided in contact with the surfaces of the photosensitive drums  1 Y,  1 M,  1 C,  1 K of the image forming portions SY, SM, SC, SK. The intermediary transfer belt  7  is stretched by a plurality of stretching rollers (supporting members) including a driving roller  71 , a tension roller  72 , and a secondary transfer opposite roller  73 . The driving roller  71  transmits a driving force from a driving motor (not shown) as a driving means to the intermediary transfer belt  7  and thus moves (rotates) the intermediary transfer belt  7 . The intermediary transfer belt  7  is rotationally driven by the driving roller  71  in an arrow R 1  direction in  FIG. 1 . In this embodiment, a peripheral speed of the intermediary transfer belt is 250-300 mm/sec. The tension roller  72  controls the tension of the intermediary transfer belt  7  at a constant level. The secondary transfer opposite roller  73  opposes a secondary transfer roller  82  described later via the intermediary transfer belt  7  and a secondary transfer belt  81  described later, so that a secondary transfer portion (secondary transfer nip) N 2  is formed. 
     As the intermediary transfer belt  7 , a belt prepared by incorporating carbon black as an antistatic agent in an appropriate amount into a resin material such as polyimide or polycarbonate, or various rubbers may suitably be used, for example. The intermediary transfer belt  7  may preferably have a volume resistivity of about 1×10 9 -1×10 14  Ω·cm and a thickness of about 0.07-0.1 mm. 
     In the inner peripheral surface (back surface) side of the intermediary transfer belt  7 , the above-described primary transfer rollers  5 Y,  5 M,  5 C,  5 K are disposed corresponding to the photosensitive drums  1 Y,  1 M,  1 C,  1 K, respectively. Each primary transfer roller  5  is urged toward an associated photosensitive drum  1  via the intermediary transfer belt  7 , so that a primary transfer portion (primary transfer nip) N 1  where the intermediary transfer belt  7  and the photosensitive drum  1  contact each other is formed. Further, in the outer peripheral surface (front surface) side of the intermediary transfer belt  7 , at a position opposing a secondary transfer device  10  as a secondary transfer means is provided. As specifically described later, the secondary transfer device  10  includes a secondary transfer belt  81  constituted by an endless belt and includes a secondary transfer roller  82  disposed in an inner peripheral surface side of the secondary transfer belt  81 . The secondary transfer roller  82  is urged toward the secondary transfer opposite roller  73  via the intermediary transfer belt  7  and the secondary transfer belt  81 , so that the secondary transfer portion (secondary transfer nip) N 2  where the intermediary transfer belt  7  and the secondary transfer belt  81  contact each other is formed. Further, in the outer peripheral surface side of the intermediary transfer belt  7 , at a position opposing the driving roller  71 , an intermediary transfer belt cleaner  24  as an intermediary transfer member cleaning means is provided. 
     The toner image formed on the photosensitive drum  1  as described above is electrostatically transferred (primary-transferred) onto the rotating intermediary transfer belt  7  by the action of the primary transfer roller  5  at the primary transfer portion N 1 . At this time, to the primary transfer roller  5 , a primary transfer bias (primary transfer voltage) of an opposite polarity (positive in this embodiment) to a normal charge polarity of the toner is applied. As a result, a primary transfer current is supplied to the primary transfer portion N 1 . For example, during full-color image formation, the respective color toner images formed on the photosensitive drums  1 Y,  1 M,  1 C,  1 K are successively transferred superposedly onto the intermediary transfer belt  7  at the respective primary transfer portions N 1 . As a result, multiple toner images, for a full-color image, obtained by the superposed four color toner images are formed on the intermediary transfer belt  7 . A deposited matter such as the toners (primary-transfer residual toners) remaining on the photosensitive drums  1  after the predetermined transfer step is removed and collected from the photosensitive drums  1  by the drum cleaners  6 . 
     The toner images formed on the intermediary transfer belt  7  are sent to the secondary transfer portion N 2  by rotation of the intermediary transfer belt  7 . On the other hand, the transfer material (recording material) P, such as paper, accommodated in a transfer material cassette (not shown) is fed one by one by a feeding roller (not shown) and then is fed to the secondary transfer portion N 2  by a registration roller pair  21 . The registration roller pair  21  once stops the fed transfer material P and then supplies the transfer material P to the secondary transfer portion N 2  in synchronism with the feeding of the toner images on the intermediary transfer belt  7  to the secondary transfer portion N 2 . With respect to the feeding direction of the transfer material P, on a side upstream of the secondary transfer portion N 2 , the following guiding members  22   a  and  22   b  as feeding guides for regulating a feeding path of the transfer material P are provided. First, on the front surface side of the intermediary transfer belt  7 , a secondary transfer upstream upper guiding member  22   a  as a feeding guide for regulating behavior such that the transfer material P approaches the surface of the intermediary transfer belt  7  is disposed. Further, a secondary transfer upstream lower guiding member  22   b  for regulating behavior such that the transfer material P is spaced from the surface of the intermediary transfer belt  7  is disposed. The transfer material P passes through between these guiding members  22   a  and  22   b.  That is, by these guiding members  22   a  and  22   b,  a feeding path of the transfer material P from the registration roller pair  21  to the secondary transfer portion N 2  is regulated. 
     Then, at the secondary transfer portion N 2 , the toner images on the intermediary transfer belt  7  are electrostatically transferred (secondary-transferred) onto the transfer material P, sandwiched and fed between the intermediary transfer belt  7  and the secondary transfer belt  81 , by the action of the secondary transfer device  10 . At this time, to the secondary transfer roller  82 , a secondary transfer bias (secondary transfer voltage) of an opposite polarity (positive in this embodiment) to the normal charge polarity of the toner is applied. As a result, a secondary transfer current is supplied to the secondary transfer portion N 2 . A deposited matter such as the toners (secondary-transfer residual toners) remaining on the intermediary transfer belt  7  after the secondary transfer step is removed and collected from the intermediary transfer belt  7  by the intermediary transfer belt cleaner  74 . 
     The transfer material P on which the toner images are transferred is separated from the intermediary transfer belt  7  and then from the secondary transfer belt  81 , and thereafter is fed to a fixing device  24  by a pre-fixing feeding device  23 . Then, after unfixed toner images are fixed on the transfer material P by the fixing device  24 , the transfer material P is discharged (outputted) to an outside of an apparatus main assembly of the image forming apparatus  100 . 
     2. Secondary Transfer Device 
     Next, a basic structure of the secondary transfer device  10  in this embodiment will be specifically described.  FIG. 2  is a schematic sectional view of the secondary transfer device  10 . The secondary transfer device  10  is constituted by including a belt unit  8  and a cleaning unit  9 . 
     First, the belt unit  8  will be described. The belt unit  8  includes the secondary transfer belt  81  constituted by the endless belt as a transfer member. The secondary transfer belt  81  is stretched by a plurality of stretching rollers (supporting members) including the secondary transfer roller  82 , a separation roller  83 , a tension roller  84  and a driving roller  85 . The secondary transfer roller  82  sandwiches the intermediary transfer belt  7  and the secondary transfer belt  81  between itself and the secondary transfer opposite roller  73 , so that the secondary transfer portion N 2  is formed. The separation roller  83  separates the transfer material P, after passing through the secondary transfer portion N 2 , from the secondary transfer belt  81 . The tension roller  84  is urged from the inner peripheral surface side toward the outer peripheral surface side of the secondary transfer belt  81  by a spring (not shown) as an urging means, so that a tension is imparted to the secondary transfer belt  81 . The driving roller  85  transmits a driving force from a driving motor (not shown) as a driving means to the secondary transfer belt  81  and thus moves (rotates) the secondary transfer belt  81 . The secondary transfer belt  81  is rotationally driven in an arrow R 3  direction in  FIG. 1  by the driving roller  85 . Further, the belt unit  8  includes an opposing roller  86  as an opposing member to an upstream fur brush  91  described later. 
     The respective rollers are disposed along a rotational direction of the secondary transfer belt  81  in the order of the secondary transfer roller  82 , the separation roller  83 , the tension roller  84 , the opposing roller  86  and the driving roller  85 . Each of the secondary transfer roller  82 , the separation roller  83 , the tension roller  84  and the opposing roller  86  is rotated with rotation of the secondary transfer belt  81 . 
     In this embodiment, as the secondary transfer belt  81 , a belt prepared by incorporating carbon black as an antistatic agent in an appropriate amount into a resin material, such as polyimide or polycarbonate, or the like belt can be suitably used. The secondary transfer belt  81  is about 1×10 9 -1×10 14  Ω·cm in volume resistivity and about 0.07-0.1 mm in thickness. Further, the secondary transfer belt  81  used in this embodiment is about 100 MPa or more and 10 GPa or less in Young&#39;s modulus as measured by a tensile test method (JIS K 6301), and thus may preferably be sufficiently hard. In this embodiment, the secondary transfer belt  81  is formed of polyimide (PI). 
     In this embodiment, the secondary transfer roller  82  as a bias application member is constituted by providing, on a core metal (core material), an elastic layer formed with an ion-conductive foamed rubber (NBR rubber). This secondary transfer roller  82  is 24 mm in outer diameter, 6.0-12.0 μm in surface roughness Rz of the surface layer, and 1×10 5 -1×10 7 Ω in electric resistance as measured under application of a voltage of 2 kV in an N/N (23° C./50% RH) environment. Further, to the secondary transfer roller  82 , a secondary transfer bias voltage source (high-voltage source)  87  ( FIG. 1 ) as a secondary transfer bias applying means is connected. The secondary transfer bias voltage source  87  is capable of supplying a variable bias and is constituted so that a desired secondary transfer bias can be applied to the secondary transfer roller  82 . By applying the secondary transfer bias to the secondary transfer roller  82 , not only are the toner images transferred from the intermediary transfer belt  7  onto the transfer material P fed to the secondary transfer portion, but also the transfer material P is attracted to the secondary transfer belt  81  by a supplied electrostatic force. In this embodiment, the secondary transfer bias is applied to the secondary transfer roller  82  so that a current of, e.g., +40 to +60 μA flows. 
     The secondary transfer belt  81  wound around the surface of the secondary transfer roller  82  is moved in the arrow R 3  direction in  FIG. 1 , so that the transfer material P attracted to the surface of the secondary transfer belt  81  at the secondary transfer portion N 2  is fed to a downstream side. Then, at a time when the transfer material P on the secondary transfer belt  81  reaches a position of the separation roller  83  disposed adjacent to and downstream of the secondary transfer roller  82  with respect to the rotational direction of the secondary transfer belt  81 , the transfer material P is separated from the surface of the secondary transfer belt  81  by curvature of the separation roller  83 . Then, the transfer material P separated from the secondary transfer belt  81  is fed to the fixing device  24  as described above. 
     Next, the cleaning unit  9  will be described. Onto the secondary transfer belt  81  which is an object-to-be-cleaned of the cleaning unit  9 , the following toners are transferred. The toners include fog toner in a sheet interval between images during continuous image formation, toner for an adjusting toner image such as a density patch, toner for a toner image which was formed on the intermediary transfer belt  7  during (paper) jamming, and toner of the like. A deposited matter such as the toner causes back surface contamination of the transfer material P, and therefore, it is desired that the deposited matter is removed from the secondary transfer belt  81 . For that reason, in this embodiment, in the secondary transfer device  10 , the cleaning unit  9  using an electrostatic fur brush cleaning type is provided. 
     The cleaning unit  9  includes a cleaning container  90 . The cleaning container  90  is an example of a state accommodating powder and provided with an opening. The cleaning container  90  also has a function as a supporting member (casing, frame) for the above-described belt unit  8 . Although the belt unit  8  will be specifically described later, as shown in  FIG. 3 , the belt unit  8  is detachably mounted to the cleaning container  90 . In a state in which the belt unit  8  is mounted to the cleaning container  90 , the secondary transfer belt  81  exposes to an outside of the cleaning container  90  from a part of a portion wound about the driving roller  85  to a part of a portion wound about the separation roller  83  with respect to movement direction of the surface thereof. In this state, the secondary transfer belt  81  rotates. The secondary transfer belt  81  of the belt unit  8  is an example of a rotatable member which is mounted to the container and which is rotatable while exposing at least a part of the surface thereof to the outside through the opening of the container. 
     Further, the cleaning unit  9  includes an upstream fur brush  91  and a downstream fur brush  94  which are rotatable roller-shaped fur brushes (fur brush rollers) as cleaning members. With respect to the rotational direction, the upstream fur brush  91  is disposed in an upstream side and the downstream fur brush  94  is disposed in a downstream side. Specifically, the upstream fur brush  91  is disposed at a position downstream of the tension roller  84  and upstream of the driving roller  85  with respect to the rotational direction of the secondary transfer belt  81  so as to contact the outer peripheral surface (front surface) of the secondary transfer belt  81 . At a position opposing the upstream fur brush  91  via the secondary transfer belt  81 , the opposing roller  86  as the opposing member is disposed in contact with the inner peripheral surface (back surface). In this embodiment, the opposing roller  86  is constituted by a metal roller and is grounded electrically. The downstream fur brush  94  is disposed so as to contact the outer peripheral surface of the secondary transfer belt  81  wound around the driving roller  85 . In this embodiment, each of the upstream fur brush  91  and the downstream fur brush  94  is constituted by planting electroconductive nylon brushes in a core material. Each of the upstream fur brush  91  and the downstream fur brush  94  is rotationally driven in an arrow direction in  FIG. 1 . 
     Further, an upstream collecting roller  92  rotatable in contact with the upstream fur brush  91  is provided inside the cleaning container  90  of the cleaning unit  9 . The upstream collecting roller  92  not only collects the deposited matter such as the toner collected from the surface of the secondary transfer belt  81  by the upstream fur brush  91  but also functions as a bias applying member (bias roller) for applying a bias (voltage) to the upstream fur brush  91 . 
     Further, a downstream collecting roller  95  rotatable in contact with the downstream fur brush  94  is provided inside the cleaning container  90  of the cleaning unit  9 . The downstream collecting roller  95  not only collects the deposited matter such as the toner collected from the surface of the secondary transfer belt  81  by the downstream fur brush  94  but also functions as a bias applying member for applying a bias (voltage) to the downstream fur brush  94 . To each of the upstream collecting roller  92  and the downstream collecting roller  95 , a collecting bias voltage source (not shown) is connected. Further, to the upstream collecting roller  92 , a bias (voltage) of an opposite polarity (positive in this embodiment) to a normal charge polarity of the toner is applied, and to the downstream collecting roller  95 , a bias of the same polarity (negative in this embodiment) as the normal charge polarity of the toner is applied. Each of the upstream collecting roller  92  and the downstream collecting roller  95  is rotationally driven in an arrow direction in  FIG. 1 . 
     Further, inside the cleaning container  90 , the cleaning unit  9  includes an upstream cleaning blade  93  as a removing member provided in contact with the upstream collecting roller  92 . The upstream cleaning blade  93  scrapes the deposited matter such as the toner off the upstream collecting roller  92 , and the deposited matter is collected in a toner collecting unit  10  described later. Further, inside the cleaning container  90 , the cleaning unit  9  includes a downstream cleaning blade  96  as the removing member provided in contact with the downstream collecting roller  95 . The downstream collecting blade  96  scrapes the deposited matter such as the toner off the downstream collecting roller  95 . Further, inside the cleaning container  90 , the cleaning unit  9  includes a collecting screw  97  as a feeding member. 
     The toner transferred on the secondary transfer belt  81  is transferred from the secondary transfer belt  81  onto the upstream fur brush  91  to which the positive bias (voltage) is applied by the upstream collecting roller  92 . Then, the toner transferred on the upstream fur brush  91  is transferred onto the upstream collecting roller  92  and then is scraped off the upstream collecting roller  92  by the upstream cleaning blade  93 . To the upstream collecting roller  92 , the positive bias is applied, so that the toner charged to the normal charge polarity of the toner is collected from the secondary transfer belt  81  by the upstream fur brush  91 . Most of the negative toner is transferred onto the upstream collecting roller  92  and is scraped off the roller  92  by the upstream cleaning blade  93 . However, there is a toner which is transferred from the secondary transfer belt  81  onto the upstream fur brush  91  but which passes through a contact portion between the upstream fur brush  91  and the upstream collecting roller  92  without being transferred from the upstream fur brush  91  onto the upstream collecting roller  92 . This toner can be returned from the upstream fur brush  91  to the secondary transfer belt  81  when the toner contacts the secondary transfer belt  81  again. In many cases, this toner is the positive toner, and therefore, is transferred onto the downstream fur brush  94  supplied with the negative bias, by the downstream collecting roller  95 . The toner is transferred from the downstream fur brush  94  onto the downstream collecting roller  95 , and then is scraped off the roller  95  by the downstream cleaning blade  96 . 
     The toners collected by the upstream cleaning blade  93  and the downstream cleaning blade  96  as described above drop onto the collecting screw  97  immediately below the blades. Then, the toners are carried to a collecting toner path (not shown) by the collecting screw  97  and are finally fed into a residual (waste) toner container (not shown). 
     3. Seal Member 
     A seal (sealing) member for sealing a gap between the cleaning container  90  and the secondary transfer belt  81  in the secondary transfer device  10  will be described. Here, a direction substantially perpendicular to a surface movement direction of the secondary transfer belt  81  (i.e., substantially parallel to rotational axes of the stretching rollers  82 ,  83 ,  84  and  85 ) is also referred to as a “thrust direction”. Further, with respect to the thrust direction, an end portion side corresponding to a front side of the drawing sheet of  FIG. 2  is a “front side”, and an end portion side corresponding to a rear side of the drawing sheet of  FIG. 2  is a “rear side”. 
     Referring to  FIG. 2 , inside the cleaning container  90 , the toners scraped off by the fur brushes  91  and  94  and the cleaning blades  93  and  96  are filled. For that reason, the toners are likely to leak out to the outside of the cleaning container  90  through the gap between the belt unit  8  and the cleaning container  90 . In the neighborhood of the fur brushes  91  and  94 , the fur brushes  91  and  94  scrapes the toners off the surface of the secondary transfer belt  81  and scatter the toners with rotation thereof. Particularly, in the neighborhood of the downstream fur brush  94 , there is an edge portion (edge portion of the cleaning container  90  in an upstream side with respect to the surface movement direction of the secondary transfer belt  81  exposed from the cleaning container  90 )  90   b  which forms an opening  90   a  ( FIG. 3 ) of the cleaning container  90  and which extends in the thrust direction. For that reason, the toner is liable to blow out through a gap between the edge portion  90   b  and the secondary transfer belt  81  while moving along air flow by rotation of the secondary transfer belt  81 . 
     For that reason, the gap between the edge portion  90   b  of the cleaning container  90  and the secondary transfer belt  81  is sealed with the seal member. In this embodiment, as the seal member, a sheet member  11  which is a sheet-shaped seal member relatively low in rigidity is used. Particularly, in this embodiment, as the sheet member  11 , an urethane sheet which is a sheet formed of polyurethane is used. 
     The sheet member  11  is a flexible sheet which is disposed along the thrust direction, which has a predetermined length with respect to each of a longitudinal direction and a widthwise direction substantially perpendicular to the longitudinal direction and which has a substantially rectangular shape in plan view. Further, the sheet member  11  is fixed to the edge portion  90   b  of the cleaning container  90  along the thrust direction in a predetermined range in one end portion (fixed end) side with respect to the widthwise direction, and contacts the surface of the secondary transfer belt  81  wound about the driving roller  85  in a predetermined range in the other end portion (free end) side. In this embodiment, the sheet member  11  contacts the secondary transfer belt  81  in a substantially entire region of the secondary transfer belt  81  with respect to the thrust direction. Further, in this embodiment, the sheet member  11  is disposed in contact with the surface of the secondary transfer belt  81  from above and deforms so as to follow the surface of the secondary transfer belt  81  by its own weight, and thus surface-contacts the surface of the secondary transfer belt  81  ((b) of  FIG. 5 ). As a result, the sheet member  11  performs a function of suppressing scattering of the toner through the gap between the edge portion  90   b  of the cleaning container  90  and the secondary transfer belt  81 . That is, the sheet member  11  contacts the surface of the secondary transfer belt  81  along the thrust direction in a (codirectional) state in which the free end thereof faces toward a downstream side with respect to the surface movement direction of the secondary transfer belt  81  (i.e., in a normal state described later), and thus seals between the secondary transfer belt  81  and the cleaning container  90 . 
     Here, as the seal member contacting the secondary transfer belt  81  in the substantially entire region with respect to the thrust direction, when a brush-shaped member or a foam material which is urged against the surface of the secondary transfer belt  81  is used, these members collect the scattered toner. For this reason, by friction with the surface of the secondary transfer belt  81 , fusion of the toner generates, so that an image defect is caused and a load torque becomes large. Further, when the sheet member such as the PET sheet relatively high in rigidity is used as the seal member, there is a possibility that the secondary transfer belt  81  is damaged and a sealing performance changes by abrasion due to use of the sheet member. For that reason, as in this embodiment, it is preferable that the sheet member which is relatively low in rigidity and which is capable of relatively lowering the contact pressure is used. 
     3. Mounting Operation 
     A mounting operation of the belt unit  8  to the cleaning container  90  will be described.  FIG. 3  is a schematic sectional view of the secondary transfer device  10  for illustrating a state during mounting of the belt unit  8  to the cleaning container  90 . In  FIG. 4 , (a) and (b) are enlarged sectional views for illustrating a behavior of the sheet member  11  during the mounting of the belt unit  8 . 
     In this embodiment, the belt unit  8  is disposed above the cleaning unit  9  with respect to the vertical direction. Then, the mounting of the belt unit  8  to the cleaning container  90  is made so that the belt unit  8  is dropped into the cleaning container  90  from above. Further, the sheet member  11  is disposed so that the free end thereof is positioned within a moving range of the belt unit  8  when the belt unit  8  is assembled with the cleaning container  90  ( FIG. 3 , (a) of  FIG. 4 ). That is, the free end of the sheet member  11  is positioned within a moving locus of the secondary transfer belt  81  when the secondary transfer belt  81  is mounted to the cleaning container  90 . In other words, when the secondary transfer belt  81  is mounted to the cleaning container  90 , the free end of the sheet member  11  is positioned inside a region formed by a contour of the secondary transfer belt  81  as seen in the movement direction of the secondary transfer belt  81 . For that reason, the sheet member  11  is trampled by the secondary transfer belt  81  when the secondary transfer unit  8  is mounted to the cleaning container  90 , and thus is in a state in which the sheet member  11  is caught inside the cleaning container  90  ((b) of  FIG. 4 ). In this case, the free end of the sheet member  11  is in a (counterdirectional) state in which the free end faces an upstream side with respect to the surface movement direction of the secondary transfer belt  81 . 
     Incidentally, in  FIG. 3  and (a) and (b) of  FIG. 4 , the sheet member  11  is illustrated on the assumption that the sheet member  11  extends along a rectilinear line in the widthwise direction in a state before the mounting of the belt unit  8 . However, even in the state before the mounting of the belt unit  8 , the sheet member  11  bends downwardly toward the inside of the cleaning container  90  depending on rigidity thereof in some cases. 
     Here, in the state in which the sheet member  11  is caught as shown in (b) of  FIG. 4 , an attitude of the free end of the sheet member  11  is not stabilized, and therefore, a stable sealing effect over the substantially entire region of the sheet member  11  with respect to the thrust direction cannot be expected. For that reason, it would be considered that an operator manually pulled out (turns up) the sheet member  11  to a normal position. However, in order to pull out the sheet member  11  manually, in general, the operation is performed in a narrow place and therefore a tool having a sharp tip or the like is required to be used in some cases. For that reason, the secondary transfer belt  81  and the sheet member  11  are liable to be damaged, so that the image defect and worsening of a degree of the toner scattering can be caused. 
     Further, it would be considered that the sheet member  11  is made detachable from the cleaning container  90  and is mounted to the cleaning container  90  after the belt unit  8  is mounted to the cleaning container  90 . Alternatively, it would be considered that a mechanism for retracting the sheet member  11  from the belt unit  8  in synchronism with mounting and demounting of the belt unit  8  relative to the cleaning container  90  is provided. However, in these methods, the operation is complicated, and an increase in number of parts and a complicated mechanism are required. 
     Further, it would be also considered that the sheet member  11  is mounted in the belt unit  8  side, but an operation when part exchange of the belt unit  8  itself is performed becomes complicated. 
     For these reasons or the like, in this embodiment, the sheet member  11  is mounted in the cleaning container  90  side and is held is a state in which the sheet member  11  enters the moving rang of the belt unit  8 . 
     Incidentally, the mounting of the belt unit  8  to the cleaning container  90  is performed not only during assembling of the image forming apparatus  100  but also when the belt unit  8  is mounted to or demounted from the cleaning container  90  in order to replace the belt unit  8  (particularly the secondary transfer belt  81 ). 
     4. Automatic Pulling-Out of Sheet Member 
     Next, automatic pulling-out of the sheet member  11  in this embodiment will be described. In  FIG. 5 , (a) and (b) are enlarged views for illustrating a behavior of the sheet member  11  during the automatic pulling-out of the sheet member  11 . Incidentally, herein, a state of the sheet member  11  as shown in (b) of  FIG. 4  described above is also referred to as a “caught state”, and a state of the sheet member  11  as shown in (b) of  FIG. 5  also referred to as a “normal state”. 
     In this embodiment, the urging member  12  which is urged against the surface of the secondary transfer belt  81  in a normal operation state and which does not contact the sheet member  11  is provided in the cleaning container  90 . The normal operation state is a state of the belt unit  8  after the sheet member  11  is placed in the normal state by an operation performed first after the mounting of the belt unit  8  described later. The urging member  12  is disposed so as to sandwich and urge, between itself and the secondary transfer belt  81 , the sheet member  11  placed in the caught state by the belt unit  8 , when the belt unit  8  is mounted to the cleaning container  90 . Typically, as this time, the sheet member  11  is sandwiched only by the urging member  12  between the secondary transfer belt  81  and the urging member  12 . Then, during the operation performed first after the mounting of the belt unit  8 , the sheet member  11  in the caught state is automatically pulled out (turned up, raised) by rotation of the secondary transfer belt  81 , and thus is placed in the normal state. For this reason, during the mounting of the belt unit  8  to the cleaning container  90 , even when the sheet member  11  is in the caught state, the operator is not required to perform a particular operation. Then, during subsequent first actuation of the image forming apparatus  100  (i.e., the belt unit  8 ), the sheet member  11  is automatically put in the normal state in which the sheet member  11  is disposed at the normal position. This will be specifically described. 
     Referring to  FIGS. 2 to 5 , in this embodiment, the urging member  12  is provided in the neighborhood of the sheet member  11 . Specifically, the urging member  12  is mounted to the cleaning container  90  at a position upstream of the sheet member  11  in the normal state with respect to the surface movement direction of the secondary transfer belt  81  and corresponding to each of end portions of the driving roller  85  with respect to the thrust direction. The urging member  12  is fixed to a surface of the cleaning container  90  opposing the secondary transfer belt  81  wound about the driving roller  85  adjacent to the edge portion  90   b  of the cleaning container  90  and extending toward the inside of the cleaning container  90 . In this embodiment, as the urging member  12 , a pile formed with PTFE fibers is used. 
     The urging member  12  is urged against the surface of the secondary transfer belt  81  wound about the driving roller  85  when the sheet member  11  is in the normal state. In this embodiment, the urging member  12  is compressed together with the secondary transfer belt  81  between the driving roller  85  and the cleaning container  90  by mounting the belt unit  8  to the cleaning container  90 . As a result, the urging member  12  is urged against the surface of the secondary transfer belt  81 . The urging member  12  also has a function as an end portion seal (sealing) member for suppressing the toner scattering with respect to the thrust direction in each of the front side and the rear side of the driving roller  85  in the neighborhood of the edge portion  90   b  when the sheet member  11  is in the normal state. 
       FIG. 6  is a schematic view showing a positional relationship among the driving roller  85 , the sheet member  11  and the urging member  12 . With respect to the thrust direction, a maximum of a length of the transfer material P fed to the secondary transfer portion N 2  is a sheet passing width S. Further, with respect to the thrust direction, a maximum of a length of the image formed on the intermediary transfer belt  8  is an image region T. Further, with respect to the thrust direction, a distance between the front-side urging member  12  and the rear-side urging member  12  is a region U between the urging members  12 . At this time, in this embodiment, the relationship: T&lt;S&lt;U (provided that a smaller region is included in a larger region) is satisfied. Particularly, the relationship T&lt;U (provided that the smaller region is included in the larger region) is satisfied, so that the fusion of the toner can be suppressed. That is, when the urging member  12  is disposed within the image region T, the urging member  12  collects the toner, so that the fusion of the toner is liable to generate by friction of the urging member  12  with the secondary transfer belt  81  but can be suppressed by disposing the urging member  12  in a region out of the image region T. 
     As described above, when the belt unit  8  is mounted to the cleaning container  90 , the sheet member  11  is caught. At this time, in this embodiment, the cleaning container  90  is provided with the urging member  12 , and therefore, as shown in (b) of  FIG. 12 , the sheet member  11  is sandwiched between the secondary transfer belt  81  and the urging member  12  at each of end portions thereof with respect to the thrust direction. In this embodiment, only the urging member  12  sandwiches the sheet member  11  between itself and the secondary transfer belt  81 . 
     Thus, the urging member  12  urged against (press-contacted to) the surface of the secondary transfer belt  81  is provided upstream of a contact portion between the sheet member  11  in the normal state and the secondary transfer belt  81  with respect to the surface movement direction of the secondary transfer belt  81 . Further, the urging member  12  is disposed at a position where the sheet member  11  can be sandwiched between the urging member  12  and the secondary transfer belt  81  in the case where the free end of the sheet member  11  is deformed so as to face the upstream side of the secondary transfer belt  81  with respect to the surface movement direction of the secondary transfer belt  81 . That is, the urging member  12  is disposed at a position where the urging member  12  can sandwich the sheet member  11  in a state in which the free end of the sheet member  11  is deformed so as to face the upstream side of the secondary transfer belt  81  with respect to the surface movement direction, between itself and the secondary transfer belt  81 . 
       FIG. 7  is a schematic view showing forces exerted on the sheet member  11  in the caught state or the like member. A friction coefficient between the sheet member  11  and the secondary transfer belt  81  is μ1, and a friction coefficient between the sheet member  11  and the urging member  12  is μ2. Specifically, μ1 is a friction coefficient between a surface (a surface of the sheet member  11  in the normal state in an opposite side from the secondary transfer belt  81 ) of the sheet member  11  in the caught state in a side facing the secondary transfer belt  81  (rotatable member) and the surface of the secondary transfer belt  81 . Further, μ2 is a friction coefficient between a surface (a surface of the sheet member in the normal state in a side facing the secondary transfer belt  81 ) of the sheet member  11  in the caught state in a side facing the urging member  12  and the surface of the urging member  12 . At this time, the relationship: μ1&gt;μ2 is satisfied. 
     Further, when the sheet member  11  is in the caught state, the urging member  12  urges the sheet member  11  toward the driving roller  85  via the secondary transfer belt  81 . For that reason, stress N exerted on a boundary between the sheet member  11  and the secondary transfer belt  81  and stress N exerted on a boundary between the sheet member  11  and the urging member  12  are equal to each other. 
     Here, a force μ1×N for pulling out (turning up) the sheet member  11  by the secondary transfer belt  81  on the driving roller  85  is F 1 , and a force μm2×N for holding the sheet member  11  by the urging member is F 2 . Further, when the sheet member  11  changes from the state of (b) of  FIG. 4  to the state of (a) of  FIG. 5 , the sheet member  11  to be pulled out is gradually moved to the normal position (normal state) while being gradually bent. Of reaction forces generated during the bending due to the resilience of the sheet member  11 , i.e., of bending elastic forces of the sheet member  11 , a component force with respect to the rotational direction of the driving roller  85  is F 3 . In this case, when a relationship between the force F 1  for pulling out the sheet member  11  and a resultant of the force F 2  for holding the sheet member  11  and the component force F 3  of the bending elastic forces due to the resilience of the sheet member  11  is F 1 &gt;(F 2 +F 3 ), the sheet member  11  is pulled out. If the sheet member such as PET sheet relatively high in rigidity is used as the sheet member and F 1 &lt;(F 2 +F 3 ) is satisfied, the sheet member cannot be pulled out. 
     As an example, in this embodiment, as the sheet member  11 , a 100 μm-thick urethane sheet (“ESMER URS”, manufactured by Nihon Matai Co., Ltd.) is used. In place of this urethane sheet, in the case where a 50 μm-thick PET sheet is used, a thickness thereof is thin, but is higher in rigidity than the urethane sheet, so that the above-described force F 3  becomes large and therefore the automatic pulling-out as described above cannot be realized. Further, the higher rigidity of the sheet member  11  is liable to impair a mounting and demounting operation of the belt unit  8 , and is also liable to increase in risk of buckling, peeling and the like. 
     As an example, the friction coefficients μ1 and μ2 in this embodiment are as follows. The friction coefficient μ1 was calculated by pressing a roller about which a test piece formed with an urethane sheet substantially identical to the sheet member  11  in this embodiment was wound, against a test piece formed of μ1 identical to the μ1 of the secondary transfer belt  81  in this embodiment by a weight of 300 g and then by obtaining a value thereof from a force with respect to a tangential direction when the roller was rotated. Similarly, the friction coefficient μ2 was calculated by pressing a roller about which the urethane sheet substantially identical to the sheet member  11  in this embodiment was wound, against a test piece formed of PTFE identical to the PTFE of the urging member  12  in this embodiment by a weight of 300 g and then by obtaining a value thereof from a force with respect to a tangential direction when the roller was rotated. When the friction coefficients were measured in an environment of 20° C. in ambient temperature and 30% in relative humidity, a static friction coefficient μ1 was 1.59, and a static friction coefficient was 0.66. Thus, in this embodiment, the above-described relationship: μ1&gt;μ2 is satisfied. 
     Incidentally, values of the above-described friction coefficients μ1 and μ2 correspond to those when all of the sheet member  11 , the secondary transfer belt  81  and the urging member  12  are new (fresh). When the sheet member  11  is used in the image forming apparatus  100 , the toner is deposited on the surface thereof in the normal state in a side facing the secondary transfer belt  81 . On the other hand, the toner is not significantly deposited on the surface thereof in the normal state in an opposite side from the secondary transfer belt  81 . Further, when the sheet member  11  is used in the image forming apparatus  100 , the toner is also deposited on the surface of the urging member  12  contacting the secondary transfer belt  81  in some cases. Here, in general, a frequency of replacement of the secondary transfer belt  81  is higher than a frequency of replacement of the sheet member  11  and the urging member  12 . Therefore, compared with the cleaning container  90  (i.e., the sheet member  11  and the urging member  12 ) used in the image forming apparatus  100 , the case where the belt unit  8  (i.e., the secondary transfer belt  81 ) is replaced will be considered. In this case, when the sheet member  11  is in the caught state, the surface of the sheet member  11  where the toner is deposited and the surface of the urging member  12  where the toner is deposited contact each other. Then, the surface of the new (fresh) secondary transfer belt  81  and the surface of the sheet member  11  where the toner is minimally deposited contact each other. When powder such as the toner exists between the two members, the friction coefficient between the two members tends to lower. However, only between the sheet member  11  in the caught state and the urging member  12 , the toner is significantly deposited, and therefore, even after the sheet member  11  and the urging member  12  are used in the image forming apparatus  100 , the above-described relationship: μ1&gt;μ2 is maintained. 
     Incidentally, during the replacement or the like of the belt unit  8 , the sheet member  11  is returned from the caught state to the normal state in a relatively short time. However, for example, it would be also considered that the sheet member  11  is maintained in the caught state for a relatively long time such as in a period from assembling of the image forming apparatus  100  until the image forming apparatus  100  is first actuated. When the sheet member  11  is maintained in a bent state for a relatively long time, the sheet member  11  gets into a habit of being bent in some cases. However, depending on a material and a dimension (such as a length with respect to the widthwise direction) of the sheet member  11 , even when the sheet member  11  gets into the habit, the sheet member  11  is deformed by its self-weight, so that the sheet member  11  can contact the surface of the secondary transfer belt  81  so as to sufficiently follow the surface of the secondary transfer belt  81  and thus can achieve a sufficient sealing effect. Accordingly, a time in which the sheet member  11  can be maintained in the caught state may appropriately set depending on the material, the dimension or the like of the sheet member  11 . 
     As described above, according to this embodiment, the sheet member  11  sandwiched between the secondary transfer belt  81  and the urging member  12  by being trampled by the secondary transfer belt  81  is automatically pulled out by the rotation of the urging member  12  during an operation of the belt unit  8  performed first subsequently. As a result, it is possible to dispose the sheet member  11  at the normal position without requiring a complicated operation and a complicated mechanism. 
     Other Embodiments 
     The present invention was described based on the specific embodiment, but is not limited to the above-described embodiment. 
     For example, in the above-described embodiment, the rotatable member was the secondary transfer belt as the transfer member for transferring the toner image from the intermediary transfer member onto the transfer material, and the container was the cleaning container for accommodating the toner collected from the secondary transfer belt, but the present invention is not limited thereto. For example, the rotatable member may also be the secondary transfer roller as the transfer member for transferring the toner image from the intermediary transfer member onto the transfer material, or the photosensitive member or the developer carrying member or the like. Further, the container may also be the cleaning container for accommodating the toner collected from the secondary transfer roller or the photosensitive member or the developer container for accommodating the toner to be supplied to the developer carrying member. That is, the present invention can be applied when a sheet member similar to the sheet member in the above-described embodiment can be used as the seal member for sealing between the container accommodating the powder and the rotatable member mounted to the container, and an effect similar to the effect of the above-described embodiment can be obtained. The accommodation of the powder in the container is not limited to storage of the powder, such as the toner before use or the residual (waste) toner after the use, in the container, but may also include the case where the powder such as the toner which simply scatters is confined in the container so as to be prevented from leaking out of the container. 
     In the above-described embodiment, the urging member was urged toward the rotatable member by being compressed between the rotatable member and the container, but the present invention is not limited thereto. The urging member may also be urged toward the rotatable member by a separate member, from the container, such as a spring as an urging means. 
     Further, the mounting of the rotatable member to the container is not limited to support of the rotatable member by the container, but may also include the case where the rotatable member is supported by a separate supporting member from the container and is disposed at a predetermined position relative to the container. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-151221 filed on Jul. 30, 2015, which is hereby incorporated by reference herein in its entirety.