Patent Publication Number: US-7212769-B2

Title: Process cartridge and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority from JP 2004-053106, filed Feb. 27, 2004, the subject matter of which is incorporated herein by reference in its entirety. 
   BACKGROUND 
   Known image forming apparatuses, such as laser printers, generally include an attachable/detachable process cartridge that includes a photosensitive drum for carrying a toner image, which is formed by developing an electrostatic latent image on the photosensitive drum. The toner image carried on the photosensitive drum is generally transferred onto a sheet that is fed between the photosensitive drum and a transfer roller. The transfer roller is disposed so as to confront the photosensitive drum. 
   In some image forming apparatuses, such as the image forming apparatus disclosed in Japanese Laid-Open Patent Publication No. 2000-267547, a transfer roller is provided in a process cartridge. Process cartridges including transfer rollers are generally larger in size. An image forming apparatus that is capable of accommodating such a process cartridge generally needs to allocate a larger space for installing/removing the larger-sized process cartridge into/from the image forming apparatus. Thus, the overall size of such an image forming apparatus must also be larger. 
   To avoid such problems, in some image forming apparatuses, a transfer roller is provided on a frame of the image forming apparatus rather than on a process cartridge. Because the transfer roller is provided on the frame of the image forming apparatus, a smaller process cartridge can be produced. As a result, the amount of space allocated in the image forming apparatus for installing and removing the process cartridge may be reduced and the overall size of the image forming apparatus may also be reduced. 
   When a transfer roller is provided on a frame of an image forming apparatus rather than on a process cartridge, a portion of the photosensitive drum that faces the transfer roller will be exposed when the process cartridge is detached/removed from the image forming apparatus. Such exposure of the photosensitive drum can result in damage to the photosensitive drum. 
   To reduce the possibility of such damage to a photosensitive drum, an openable/closeable shutter for covering the exposed part of the photosensitive drum can be provided on the process cartridge. When the process cartridge is removed from the image forming apparatus, the shutter is closed to cover the part of the photosensitive drum that would otherwise be exposed. When the process cartridge is set in the image forming apparatus, the shutter is opened to expose part of the photosensitive drum. 
   Providing such a shutter, however, increases the number of components needed to manufacture a process cartridge, and causes complex structures to be included on the process cartridge. As a result, the cost of manufacturing the process cartridge, and, in turn, the image forming apparatus capable of accommodating such a process cartridge increases. 
   SUMMARY 
   According to an exemplary aspect of the invention, a smaller sized process cartridge that employs a simple structure for reducing, and preferably preventing, damage to an image carrying member without increasing the manufacturing cost of the process cartridge is provided. In various exemplary embodiments employing one or more aspects of the invention, a smaller sized image forming apparatus that is capable of accommodating the attachable/detachable process cartridge is provided. 
   According to another exemplary aspect of the invention, a process cartridge that includes a frame with a first side wall, a second side wall, a bottom wall and a front wall, an image carrying member, a guide member, a first opening in the bottom wall, and a second opening in the bottom wall is provided. The image carrying member is supported by and extends between the first side wall and the second side wall in a manner that permits the image carrying member to rotate, and the image carrying member projects, at least in part, beyond a bottom surface of the bottom wall. The guide member extends between the first side wall and the second side wall and forms part of the bottom wall. The first opening in the bottom wall extends from the first side wall to the second side wall and is provided between the front wall and the guide member. The second opening in the bottom wall extends from the first side wall to the second side wall and is provided between the guide member and the image carrying member. 
   According to another exemplary aspect of the invention, a process cartridge that includes a guide member and is capable of being removably installed in an image forming apparatus that includes a transfer device, for transferring a developer image from an image carrying member to a transfer medium, is provided. The image carrying member is positioned to oppose the transfer device when the process cartridge is removably installed in the image forming apparatus. The guide member is provided adjacent to and extending along a length of the image carrying member. In various exemplary embodiments, the guide member is positioned so that, when the process cartridge is installed in the image forming apparatus, the guide member guides the transfer medium toward the image carrying member while supporting a side of the transfer medium on which the developer image is not transferred, and the guide member covers at least a part of the image carrying member. 
   According to another exemplary aspect of the invention, an image forming apparatus including a casing with a transfer device capable of transferring a developer image from an image carrying member to a transfer medium, and an attachable/detachable process cartridge is provided. In various exemplary embodiments, the process cartridge includes a frame with a first side wall, a second side wall, a bottom wall and a front wall, an image carrying member, a guide member, a first opening in the bottom wall and a second opening in the bottom wall. The image carrying member is supported by and extends between the first side wall and the second side wall in a manner that permits the image carrying member to rotate, and the image carrying member projects, at least in part, beyond a bottom surface of the bottom wall. The guide member extends between the first side wall and the second side wall and forms part of the bottom wall. The first opening in the bottom wall extends from the first side wall to the second side wall and is provided between the front wall and the guide member. The second opening in the bottom wall extends from the first side wall to the second side wall and is provided between the guide member and the image carrying member. 
   These and other optional features and possible advantages of various aspects of this invention are described in, or are apparent from, the following detailed description of exemplary embodiments of systems and methods which implement this invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of the invention will be described in detail with reference to the following figures, wherein: 
       FIG. 1  is a cross sectional view of a portion of an exemplary laser printer employing one or more aspects of the invention in a state where a front cover is closed; 
       FIG. 2  is a cross sectional view of a portion of the laser printer shown in  FIG. 1  in a state where the front cover is open; 
       FIG. 3  is a perspective view of the laser printer shown in  FIG. 1  in a state where an exemplary process cartridge is removed from the printer; and 
       FIG. 4  is a bottom view of the process cartridge shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Throughout the following description, numerous specific concepts and structures are set forth in order to provide a thorough understanding of the invention. The invention can be practiced without utilizing all of these specific concepts and structures. In other instances, well known elements have not been shown or described in detail, so that emphasis can be focused on the invention. 
   In various exemplary embodiments, a process cartridge may be removably installed in image forming apparatus that includes a transfer device for transferring developer images onto transfer media. The process cartridge may include an image carrying member that is capable of carrying a developer image. The image carrying member is disposed to face the transfer device when the process cartridge is removably positioned in the image forming apparatus. A guide member may be disposed on the process cartridge upstream (in a feeding direction of a transfer medium) of a position where the transfer device and the image carrying member face each other and the length of the guide member may extend along a direction of extension of the length of the image carrying member. The guide member may guide the transfer medium toward the image carrying member while supporting a side of the transfer medium opposite from a side on which the developer image is transferred. The guide member may be further positioned to cover a part of the image carrying member such that the covered part of the image carrying member does not face. 
   In various exemplary embodiments, the guide member that guides the transfer medium may be provided on the process cartridge and may cover a part of the image carrying member. Therefore, damage to the image carrying member, which can occur, for example, while the process cartridge is being attached/detached from the image forming apparatus, may be reduced, and preferably prevented, by the guide member. Such a guide member is generally less costly and has a less complicated structure for reducing, and preferably preventing, damage to the image carrying member, than the shutter structure discussed above. The transfer device may be provided in the image forming apparatus, which accommodates such a process cartridge and such an arrangement allows for the manufacture of smaller sized process cartridges. By allowing the manufacture of a smaller sized process cartridge, a smaller sized image forming apparatus, which can attachably/detachably accommodate the process cartridge may also be manufactured. 
   In some exemplary embodiments, at least a part of a guide member of a process cartridge is positioned on an image-carrying-member side of a plane that is tangential to a point on an image carrying member where the image carrying member contacts a transfer roller in an image forming apparatus when the process cartridge is removably installed in the image forming apparatus. 
   By employing such a structure, the guide member may be provided on the process cartridge rather than in the image forming apparatus and there are many advantages to providing the guide member on the process cartridge rather than in the image forming apparatus. For example, the process cartridge can be installed in or removed from the image forming apparatus without needing to avoid the guide member situated in the image forming apparatus. Thus, because it is not necessary to allocate space within the image forming apparatus to help avoid contact between the process cartridge and the guide member of the image forming apparatus during attachment/detachment of the process cartridge when the guide member is provided on the process cartridge, the amount of space in the image forming apparatus allocated for attachment/detachment of the process cartridge may be reduced. 
   Providing the guide member on the process cartridge is also advantageous because when a guide member is provided in an image forming apparatus, and at least a part of a guide member is positioned on an image-carrying-member side of a plane that is tangential to a point on an image carrying member where the image carrying member contacts a transfer roller in the image forming apparatus, when the process cartridge is installed/removed in/from the image forming apparatus, the process cartridge does not need to be guided into/out of the image forming apparatus in a manner that will avoid, as much as possible, the guide member of the image forming apparatus in order to reduce, and preferably prevent, damage to the image carrying member. That is, in embodiments where the guide member is provided on the process cartridge, the guide member does not contact the image carrying member during installation/removal of the process cartridge, and thus damage to the image carrying member by the guide member can be reduced, and preferably avoided. Further, in some embodiments, as the guide member may be positioned on an image carrying member side of a plane that is tangential to a point on an image carrying member where the image carrying member contacts a transfer roller in an image forming apparatus when the process cartridge is installed in the image forming apparatus, a transfer medium may be smoothly guided toward the contact point. 
   In various exemplary embodiments, when a process cartridge is removably installed in an image forming apparatus, a guide member provided on the process cartridge may guide a transfer medium such that a leading edge of the transfer medium is guided so as to contact an image carrying member of the process cartridge upstream (in a direction that a transfer medium is guided) of a contact position between and the image carrying member and a transfer device in the image forming apparatus. 
   By employing such a structure, the leading end of the transfer medium (already in contact with a surface of the image carrying member) may be smoothly guided to the contact position between the image carrying member and the transfer device as the image carrying member rotates. In addition, spatter of developer due to electrical discharge can be reduced, and preferably avoided, thereby promoting favorable developer image transfer. 
   In various exemplary embodiments, a process cartridge may include frame side plates disposed, so as to face each other, at end portions of an image carrying member provided in the process cartridge. The frame side plates may support ends of the image carrying member. A guide member may be formed in a plate-like shape so as to extend between the frame side plates in a longitudinal direction along the length of the image carrying member. By employing such a structure, the guide member can be formed simply, while extending along the image carrying member in a longitudinal direction and covering at least part of the image carrying member. 
   In various exemplary embodiments, a guide member of a process cartridge may be provided with one or more ribs that help provide rigidity to the guide member. By employing such a structure, the enhanced rigidity of the guide member at least reduces, and preferably prevents, deformation which can cause the guide member to contract the image carrying member. Accordingly, damage to the image carrying member caused by such contact may be reduced, and preferably prevented. 
   In various exemplary embodiments, a guide member of a process cartridge may be provided with a flexible film member that extends from the guide member and toward an image carrying member of the process cartridge. The flexible member may be provided at a downstream (in a direction that a transfer medium is guided) end of the guide member. By employing such a structure, the film member may shorten the distance between the guide member and the image carrying member, so that feeding accuracy of the transfer medium may be improved. Even if the film member, which is flexible, contacts the image carrying member, the possibility of damage to the image carrying member is low. Accordingly, the film member may be positioned near the image carrying member without causing damage to the image carrying member. 
   In various exemplary embodiments, a process cartridge may include a developing cartridge that supplies a developer to an image carrying member provided on the process cartridge. The developing cartridge may be removably installed in the process cartridge. By employing such a structure, when the developing cartridge is removed from the process cartridge, exposure of the image carrying member may be reduced because at least part of the image carrying member is covered by the guide member. Thus, damage to the image carrying member may be reduced, and preferably prevented. 
   In various exemplary embodiments, a developing cartridge may have a guide surface that guides a transfer medium from a side of the transfer medium on which a developer image is transferred. By employing such a structure, the transfer medium can be fed with high accuracy. 
   In various exemplary embodiments, an image forming apparatus may include a transfer device that transfers a developer image onto a transfer medium, and a process cartridge. By providing the transfer device on the image forming apparatus, the image forming apparatus may include a smaller-sized process cartridge. Therefore, the image forming apparatus may also, in turn, be reduced in size. 
   In various exemplary embodiments, an image forming apparatus may include a path for removably installing a process cartridge in the image forming apparatus. The path may be provided substantially parallel to a feeding direction of a transfer medium in the image forming apparatus. By employing such a structure, when the process cartridge is removably installed in the image forming apparatus, the process cartridge may be installed and removed in a direction substantially parallel to the feeding direction of the transfer medium. Thus, the space allocated for removably installing the process cartridge in the image forming apparatus may be reduced. Therefore, the image forming apparatus may also, in turn, be reduced in size. 
   In various exemplary embodiments, an image forming apparatus may further include a casing that accommodates a transfer device and a process cartridge, an opening that communicates with the path and is formed in the casing on a front side of the image forming apparatus, and a cover that opens or closes the opening. By employing such a structure, the process cartridge may be removably installed in the image forming apparatus from a front side thereof. Ease of installing or removing the process cartridge may be improved by providing front access. 
   As shown in  FIG. 1 , an exemplary laser printer  1  is provided with a main casing  2  in which a feeder section  4  for feeding sheets  3  and an image forming section  5  for forming images on the fed sheets  3  are disposed. 
   The main casing  2  may have, for example, a substantially box-like shape, as best seen in  FIG. 3 . An opening  44  for removably installing a process cartridge  19  (described below) in the laser printer  1  is formed on a front sidewall of the main casing  2 . A front cover  45  for opening and closing the opening  44  is provided on the front sidewall of the main casing  2 . A lower end of the front cover  45  is pivotally attached to the casing  2  by a hinge  46 . When the front cover  45  is closed by pivoting the front cover  45  about the hinge  46 , the opening  44  is closed. As the front cover  45  is opened by pivoting the front cover  45  about the hinge  46 , the opening  44  is opened, so as to make it possible to install or remove of the process cartridge  19  into or from the main casing  2 . 
   An installation/removal path  47  for removably setting the process cartridge  19  in the main casing  2  is provided inside the main casing  2 . The installation/removal path  47  is substantially parallel to a feeding direction of the sheet  3  (i.e., sheet feeding direction) and communicates with the opening  44 . 
   With the process cartridge  19  set in the main casing  2 , a side where the front cover  45  is disposed is defined as a front side of the laser printer  1  or the process cartridge  19 . The opposite side to the front side (a side where a fixing unit  13  is disposed) is defined as a rear side of the laser printer  1  or the process cartridge  19 . 
   The feeder section  4  includes a sheet supply tray  6  removably installed in a bottom of the main casing  2 , a pick-up roller  7  and a separation pad  8  disposed at an upper portion of the front side of the sheet supply tray  6 , a sheet powder removing roller  9  disposed downstream of the separation pad  8  in the sheet feeding direction, and register rollers  10  disposed downstream of the sheet powder removing roller  9  in the sheet feeding direction. 
   The sheet supply tray  6  is capable of mounting thereon a stack of the sheets  3 . The pick-up roller  7  and the separation pad  8  are provided so as to face each other. The separation pad  8  is pressed against the pick-up roller  7  by a spring (not shown) disposed on an underside of the pad  8 . The sheet powder removing roller  9  is disposed downstream of the separation pad  8  in the sheet feeding direction, so as to face the pick-up roller  7 . 
   The register rollers  10  include a pair of rollers. One of the register rollers  10  disposed on the upper side (upper-side register roller) is provided in the process cartridge  19 . The other one of the register rollers  10  disposed on the lower side (lower side register roller) is provided in the main casing  2 . With the process cartridge  19  set in the main casing  2 , the lower-side register roller  10  provided in the main casing  2  faces and contacts the upper-side register roller  10 , which is provided in the process cartridge  19 , vertically from below. As the lower-side register roller  10  is driven during the sheet feeding, the upper-side register roller  10  follows the rotation of the lower-side register roller  10 . 
   An uppermost sheet  3  on the sheet supply tray  6  is pressed by the pick-up roller  7 . By the rotation of the pick-up roller  7 , the uppermost sheet  3  is nipped between the pick-up roller  7  and the separation pad  8  and separated from the remaining sheets  3 . Sheet powders or fibers on the sheet  3  are removed by the sheet powder removing roller  9 . Then, the sheet  3  is fed to the register rollers  10 , where the sheet  3  is registered and/or skew of the sheet  3  is corrected. Thereafter, the sheet  3  is fed to a transfer position in the image forming section  5 . The transfer position is a position between a photosensitive drum  21  and a transfer roller  34  where a toner image on the photosensitive drum  21  is transferred onto the sheet  3 . 
   The image forming section  5  includes a scanner unit  11 , a process unit  12 , and a fixing unit  13 . 
   The scanner unit  11  is provided in an upper portion of the main casing  2 . The scanner unit  11  includes a laser emitting portion (not shown), a polygon mirror  14  that is driven so as to spin, a lens  15 , and reflecting mirrors  16 ,  17 ,  18 . A laser beam emitted from the laser emitting portion based on image data passes through or reflects off the polygon mirror  14 , the lens  15 , the reflecting mirrors  16 ,  17 ,  18  in this order, to irradiate with the laser beam a surface of the photosensitive drum  21  of the process cartridge  19  at a high speed. 
   The process unit  12  includes the process cartridge  19 , removably installed in the main casing  2 , and the transfer roller  34 , as a transfer device, provided in the main casing  2 . The process cartridge  19  is installed in the main casing  2  below the scanner unit  11 . The process cartridge  19  includes a frame  50  and a developer cartridge  20 , the photosensitive drum  21 , as an image carrying member, a scorotron charger  22 , and a cleaning brush  23 . The developer cartridge  20 , the photosensitive drum  21 , the scorotron charger  22  and the cleaning brush  23  are disposed in the frame  50 . The developer cartridge  20  is attachably/detachably disposed in the frame  50 . 
   As shown in  FIGS. 2 and 3 , the frame  50  includes a pair of side walls  51  disposed in confrontation with each other at a predetermined distance, a front wall  52  connected to front ends of the side walls  51 , a front bottom wall  53  connected to lower ends of the side walls  51  at a front portion thereof, a rear bottom wall  54  connected to lower ends of the side walls  51  at a rear portion thereof, a rear wall  55  connected to rear ends of the side walls  51 , and a top wall  56  connected to upper ends of the side walls  51  at a rear portion thereof. The walls  51 – 56  are integrally formed. The frame  50  is formed substantially in a box shape with a front side of the frame  50  open upward and sideward. The developer cartridge  20  is removably installed in the upward-open portion of the frame  50 . 
   The front wall  52  and the front bottom wall  53  together have a substantially arc-shaped cross section. The front bottom wall  53  and the rear bottom wall  54  face each other in the frontward and rearward direction and have an open area  59  between them. A part of the photosensitive drum  21  is exposed from the open area  59 . 
   As shown in  FIG. 1 , the developer cartridge  20  includes a toner chamber  24 , a supply roller  25 , a developer roller  26 , and a layer thickness regulating blade  27 . 
   The toner chamber  24  is formed as an interior space in a front portion of the developer cartridge  20  that is divided by a partition wall  28 . Formed below the partition wall  28  is a port  29  that allows the communication between the front portion and rear portion of the developer cartridge  20 . 
   The toner chamber  24  contains as a developing agent, for example, a positively chargeable non-magnetic single component toner. The toner may be, for example, polymerized toner that is obtained by copolymerizing polymerizable monomers using a known polymerization method, such as, for example, a suspension polymerization method. The polymerizable monomers may be styrene-based monomers, such as styrene, and acrylic-based monomers, such as acrylic acid, alkyl (C1–C4) acrylate, and/or alkyl (C1–C4) methacrylate. Polymerized toner particles are generally spherical in shape, having excellent fluidity, permitting high-quality image formation. 
   Toner may be mixed with a coloring material, such as carbon black, and wax, as well as an external additive, such as silica, to improve the fluidity of the toner. Average toner particle sizes may be about 6 μm to about 10 μm. 
   An agitator  30  is disposed in the toner chamber  24 . The toner contained in the toner chamber  24  is agitated by the agitator  30  and supplied to the supply roller  25  through the port  29 . 
   The supply roller  25  is rotatably supported in the developer cartridge  20  behind the port  29 . The supply roller  25  may include a metal roller shaft covered by a roller portion formed of conductive foam. The supply roller  25  is driven by a drive force input from a motor (not shown), so as to rotate in a counterclockwise direction, as indicated by the arrow in  FIG. 1 . 
   The developer roller  26  is rotatably supported in the developer cartridge  20 , such that the supply roller  25  and the developer roller  26  contact each other so as to apply pressure to each other. The developer roller  26  includes a metal roller shaft covered by a roller portion formed of, for example, a conductive elastic material. In some embodiments, the roller portion of the developer roller  26  may be formed of conductive urethane rubber or silicone rubber including fine carbon particles. A surface of the roller portion of the developer roller  26  may be coated with urethane rubber or silicone rubber including fluorine. A development bias is applied to the developer roller  26  during development. The developer roller  26  is driven by the drive force input from the motor (not shown), so as to rotate in the same direction (counterclockwise) as the supply roller  25 , as indicated by the arrow in  FIG. 1 . 
   The layer thickness regulating blade  27  includes a blade body  31  formed, for example, of a flexible metal plate, and a pressing portion  32  formed, for example, of insulating silicone rubber having a substantially semicircular shape in cross section. The layer thickness regulating blade  27  is supported by the developer cartridge  20  near the developer roller  26 . The pressing portion  32  presses the surface of the developer roller  26  with the elasticity/flexibility of the blade body  31 . 
   The toner supplied through the port  29  is conveyed to the developer roller  26  by the rotation of the supply roller  25 . When the toner is supplied from the supply roller  25  to the developer roller  26 , the toner is positively charged by the friction formed between them. As the developer roller  26  rotates, the toner supplied by the supply roller  25  to the developer roller  26  moves between the developer roller  26  and the pressing portion  32  of the layer thickness regulating blade  27 , where it is formed into a thin toner layer, with a substantially uniform thickness, on the developer roller  26 . 
   The photosensitive drum  21  is disposed behind the developer cartridge  20  to face the developer roller  26 . The photosensitive drum  21  is of a substantially cylindrical shape and is electrically grounded. The photosensitive drum  21  is supported by the side walls  51  of the frame  50  so as to rotate on a drum shaft  33 , which is disposed along an axis of the drum  21 , in a clockwise direction, as indicated by the arrow in  FIG. 1 . An outermost surface of the photosensitive drum  21  is formed of a positively chargeable photosensitive layer. With the photosensitive drum  21  being supported by the side walls  51 , a lower portion of the photosensitive drum  21  is exposed from the open area  59 , which is defined along an axial direction of the photosensitive drum  21  between the front bottom wall  53  and the rear bottom wall  54 . 
   As shown in  FIG. 3 , the drum shaft  33  protrudes outwardly from the side walls  51 . 
   As shown in  FIGS. 1 and 2 , the scorotron charger  22  is supported by the top wall  56  above the photosensitive drum  21  with a predetermined distance therebetween, to prevent the scorotron charger  22  from contacting the photosensitive drum  21 . The scorotron charger  22  may be, for example, a positively charging scorotron charger that generates corona discharge from a tungsten wire. The scorotron charger  22  uniformly and positively charges the surface of the photosensitive drum  21 . 
   The cleaning brush  23  is supported by the rear wall  55  behind the photosensitive drum  21 . The cleaning brush  23  is disposed so as to confront the photosensitive drum  21  and such that it contacts the surface of the drum  21 . 
   The transfer roller  34  is provided in the main casing  2 . With the process cartridge  19  is set in the main casing  2 , the transfer roller  34  vertically faces and contacts the photosensitive drum  21 . The transfer roller  34  is disposed so as to nip a sheet  3  between the transfer roller  34  and the photosensitive drum  21 . Because the transfer roller  34  is provided in the main casing  2 , a smaller sized process cartridge  19  can be manufactured. The transfer roller  34  includes, for example, a metal roller shaft covered by a roller portion formed of conductive rubber. 
   A transfer bias is applied to the transfer roller  34  during transfer of the toner onto the sheet  3 . The transfer roller  34  is driven by the drive force input from the motor (not shown), so as to rotate in the counterclockwise direction, as shown by the arrow in  FIG. 1 . 
   While the photosensitive drum  21  rotates, the surface of the photosensitive drum  21  is uniformly and positively charged by the scorotron charger  22 . Thereafter, the surface of the photosensitive drum  21  is selectively exposed to the laser beam that is emitted from the scanner unit  11  and which scans across the surface of the drum  21  at a high speed. An electrostatic latent image corresponding to an image to be formed on the sheet  3 , is thereby formed on the surface of the photosensitive drum  21 . 
   Thereafter, as the positively charged toner, which is being carried on the developer roller  26 , is brought into contact with the photosensitive drum  21  based on the rotation of the developer roller  26 , the toner is supplied to the electrostatic latent image on the surface of the photosensitive drum  21 . As a result, the parts of the photosensitive drum  21  that were selectively exposed to the laser beam and thus have a lower potential level than the remaining parts (i.e., non-exposed parts) of the photosensitive drum  21  surface that are uniformly positively charged. Thus, a reverse image is developed on the surface of the photosensitive drum when the electrostatic latent image on the photosensitive drum  21  is made visible by the attraction between the positively charged toner and the lower potential regions (i.e., regions exposed by the laser beam) of the photosensitive drum. 
   When the sheet  3 , which is fed by the register rollers  10 , passes through the transfer position between the photosensitive drum  21  and the transfer roller  34 , the toner image carried on the surface of the photosensitive drum  21  is transferred on the sheet  3  by the application of the transfer bias to the transfer roller  34 . 
   Thereafter, the sheet  3  having the toner image transferred thereon is fed to the fixing unit  13 . 
   The toner which remains on the photosensitive drum  21  after the toner image is transferred, is collected by the developer roller  26 . Sheet powders or fibers attached to the photosensitive drum  21  after the toner image transfer are collected by the cleaning brush  23 . 
   The fixing unit  13  is disposed behind the process cartridge  19 . The fixing unit  13  includes a unit frame  35 , a heat roller  36  and a pressure roller  37 , and feed rollers  38 . The heat roller  36  and the pressure roller  37  are disposed in the unit frame  35 . 
   The heat roller  36  includes, for example, a metal tube accommodating therein a halogen lamp for heat application. The heat roller  36  is driven by the drive force input from the motor (not shown), so as to rotate in the clockwise direction, as indicated by the arrow in  FIG. 1 . 
   The pressure roller  37  is disposed below the heat roller  36 , so as to press the heat roller  36 . The pressure roller  37  includes, for example, a metal roller shaft covered by a roller portion formed of rubber material. The pressure roller  37  is rotated by the rotation of the heat roller  36 . 
   The feed rollers  38  include a pair of rollers. The feed rollers  38  are disposed downstream of the heat roller  36  and the pressure roller  37  in the sheet feeding direction, behind the rollers  36 ,  37 . 
   In the fixing unit  13 , the toner transferred at the transfer position onto the sheet  3  is thermally fixed to the sheet  3  while the sheet  3  passes through between the heat roller  36  and the pressure roller  37 . The sheet  3  having the toner fixed thereon is guided by the feed rollers  38  to a discharge path  39  that is disposed behind the feed rollers  38  so as to extend upwardly toward an upper face of the main casing  2 . Then, the sheet  3  that is conveyed to the discharge path  39  is discharged by the discharge rollers  40 , which are disposed at an upper side of the discharge path  39 , onto a discharge tray  41  formed on the upper face of the main casing  2 . 
   The laser printer  1  is provided with a front chute  42  and a rear chute  43 , as guide members, so as to interpose the transfer roller  34  between them. The front chute  42  guides the sheet  3  toward the transfer roller  34  while supporting an underside of the sheet  3  on which the toner image is not transferred. The rear chute  43 , which is disposed behind the front chute  42 , guides the sheet  3  toward the fixing unit  13  while supporting the underside of the sheet  3  that passed the transfer roller  34 . 
   The rear chute  43  is disposed in the main casing  2 . The rear chute  43  is integrally formed with a substantially U-shaped cover portion that covers a lower portion of the transfer roller  34  and a guide portion that extends rearward from a rear end of the cover portion. 
   As shown in  FIG. 4 , the front chute  42  is provided between the front bottom wall  53  of the frame  50  and the photosensitive drum  21 , at a predetermined distance from the photosensitive drum  21 . The front chute  42  has, for example, a substantially rectangular shape that extends between the side walls  51  of the frame  50  along the axial direction of the photosensitive drum  21 . The front chute  42  is disposed parallel to the axis of the photosensitive drum  21  at a position upstream of a nip portion between the photosensitive drum  21  and the transfer roller  34  in the sheet feeding direction, so as to face the surface of the photosensitive drum  21  and to cover a part of a lower front portion of the photosensitive drum  21  along the axial direction of the drum  21 . With the process cartridge  19  removed from the main casing  2 , the front chute  42  covers a part of the surface of the photosensitive drum  21 . Thus, damage to the photosensitive drum  21  can be reduced, and preferably prevented, without employing a shutter, which can lead increased manufacturing costs. 
   With the process cartridge  19  removed from the main casing  2 , as the developer cartridge  20  is removed from the frame  50 , as shown by broken lines in  FIG. 3 , the amount of exposure of the photosensitive drum  21  is increased. However, the front chute  42  covers the lower front part of the photosensitive drum  21 , so that the exposure of the photosensitive drum  21  can be reduced when the developer cartridge  20  is removed from the frame  50 . Consequently, damage to the photosensitive drum  21  is reduced, and preferably prevented. 
   As shown in  FIG. 2 , the front chute  42  is provided with a flexible film member  48  formed of resin film on a rear end of the front chute  42  (downstream end of the chute  42  in the sheet feeding direction). The film member  48  has a substantially rectangular shape. The front end of the film member  48  is supported by the front chute  42  across the width thereof along the axial direction of the photosensitive drum  21 , and extends toward the photosensitive drum  21 . The rear end of the film member  48  is positioned on the photosensitive drum  21  side of a plane that is tangential to a point on the photosensitive drum  21  where the photosensitive drum  21  contacts the transfer roller  34 , when the process cartridge  19  is positioned in the laser printer  1 . Therefore, the distance between the front chute  42  and the photosensitive drum  21  is shortened by the extending film member  48 . Further, the film member  48  guides a leading end of the sheet  3  to a position upstream of the nip portion between the photosensitive drum  21  and the transfer roller  34 . That is, the front chute  42  that may be provided with the film member  48  is disposed so as to guide the leading end of the sheet  3  into contact with the photosensitive drum  21  at a position upstream of the nip portion where the photosensitive drum  21  and the transfer roller  34  are in contact. 
   By rotation of the photosensitive drum  21 , the leading end of the sheet  3 , which is in contact with the surface of the photosensitive drum  21 , is smoothly guided to the nip portion between the photosensitive drum  21  and the transfer roller  34 , resulting in an improvement in sheet feeding accuracy. Also, as the leading end of the sheet  3  contacts the surface of the photosensitive drum  21  at an upstream side of the nip portion between the photosensitive drum  21  and the transfer roller  34 , electrical discharge between the sheet  3  and the photosensitive drum  21  can be prevented. Thus, spatter of the toner due to electrical discharge can be reduced, and preferably avoided, leading to favorable toner image transfer. 
   Even if the flexible film member  48  flexes and contacts the photosensitive drum  21 , the film member  48  is not likely to damage the photosensitive drum  21 , due to its flexibility. Therefore, the film member  48  can be disposed near the photosensitive drum  21 , while not causing damage to the photosensitive drum  21 . 
   The front chute  42  is provided in the process cartridge  19 . Therefore, even when the rear end of the film member  48  is positioned on the photosensitive drum  21  side of a plane that is tangential to a point on the photosensitive drum  21  where the photosensitive drum  21  contacts the transfer roller  34 , when the process cartridge  19  is positioned in the laser printer  1 , the process cartridge  19  does not have to bypass the front chute  42  when set in or removed from the main casing  2 , in contrast with an apparatus in which the front chute  42  is provided in the main casing  2 , rather than on the process cartridge  19 , to be configured with respect to the photosensitive drum  21 , as discussed above. Thus, the process cartridge  19  can be moved in a substantially straight manner along the installation/removal path  47 . 
   More specifically, in the case where the front chute  42  is provided in the main casing  2 , when the process cartridge  19  is set in the casing  2  to make the photosensitive drum  21  face the transfer roller  34 , the process cartridge  19  has to bypass the film member  48  of the front chute  42 , so as to make the photosensitive drum  21  go over the film member  48  when brought into confrontation with the transfer roller  34 . Also, in the case where the front chute  42  is provided in the main casing  2 , the process cartridge  19  has to bypass the film member  48  of the front chute  42 , from the position where the photosensitive drum  21  faces the transfer roller  34 , when the process cartridge is being removed from the main casing  2 . 
   In contrast to an image forming apparatus in which a front chute  42  is provided on the main body casing, in an exemplary embodiment of an image forming apparatus in which the front chute  42  is provided in the process cartridge  19 , as shown, for example, in  FIG. 1 , while attaching/detaching the process cartridge  19  to/from the image forming apparatus, the process cartridge  19  does not have to bypass the film member  48 . That is, in an exemplary embodiment of an image forming apparatus in which the front chute  42  is provided on the process cartridge  19 , the process cartridge  19  may be moved relative to the main casing  2  in a substantially straight direction along the installation/removal path  47 , which is substantially parallel to the sheet feeding direction. Thus, the height of the installation/removal path  47  necessary to removably install the process cartridge  19  in the main casing  2  can be about the same height as the process cartridge  19 . Thus, a smaller sized installation/removal path  47  can be provided. 
   Further, in an embodiment where the front chute  42  is provided on the process cartridge  19 , when the process cartridge  19  is removably installed in the main casing  2 , the photosensitive drum  21  does not contact the front chute  42 , so damage to the photosensitive drum  21  by the front chute  42  can be prevented. Consequently, the laser printer  1  can be reduced in size based on the reduction in size of the process cartridge  19 , while preventing the photosensitive drum  21  from being damaged by the front chute  42 . 
   As shown in  FIG. 4 , in the exemplary embodiment, a plurality of ribs  49  are aligned on a reverse or undersurface of the front chute  42  along the axial direction of the photosensitive drum  21 . The ribs  49  help provide rigidity to the front chute  42 . Each rib  49  may be formed in a substantially triangular shape, such that the base of the triangular rib  49  is disposed on the downstream side and the vertex is disposed on the upstream side in the sheet feeding direction, as shown in  FIG. 1 . By employing the ribs  49 , the rigidity of the front chute  42  is increased and deformation of the front chute  42 , which can result in contact between the front chute  42  and the photosensitive drum, can be avoided. Thus, the photosensitive drum  21  is further prevented from being damaged by the front chute  42 . 
   In various exemplary embodiments, an underside of the sheet  3  is supported by the front chute  42 . The upper side of the sheet  3 , where the toner image is transferred, is guided by a rear end portion  60  that is provided on the lower side of the developer cartridge  20 . More specifically, the rear end portion  60  on the lower side of the developer cartridge  20  is formed substantially parallel to the front chute  42  and such that there is some distance between them in the vertical direction. The rear end portion  60  is formed as a guide surface that guides the sheet  3  from the upper side thereof. Thus, the sheet  3  can be fed properly to the nip portion between the photosensitive drum  21  and the transfer roller  34 . 
   In the laser printer  1 , the opening  44  for removably installing the process cartridge  19  in the laser printer  1  is formed on a front sidewall of the main casing  2 . Therefore, the process cartridge  19  can be removably installed in the main casing  2  from the front side of the laser printer  1 . Thus, ease of installation or removal of the process cartridge  19  can be improved by the front access. 
   While this invention has been described in conjunction with the exemplary embodiments outlined above, various alternatives, modifications, variations, improvements and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention. Therefore, the invention is intended to embrace all known or later developed alternatives, modifications, variations, improvements and/or substantial equivalents.