Patent Publication Number: US-9411275-B2

Title: Image forming apparatus having partition configured to separate air flow and sheet feeding paths

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus for forming a toner image on a sheet. The image forming apparatus may be a copying machine, a printer, a facsimile machine, or a complex machine having a plurality of functions of such machines, using an electrophotographic process. 
     In a conventional electrophotographic type image forming apparatus, a toner image is formed on a sheet using toner containing a parting material (wax), and the toner image is fixed by heating and pressing by a fixing device. 
     It is known that in the fixing process, the wax contained in the toner is gasified and is condensed immediately thereafter. 
     SUMMARY OF THE INVENTION 
     Much condensed wax (many fine particles (dust) having a particle size of about several nm-several hundreds nm) floats in the neighborhood of the sheet entrance of the fixing device. Most of the wax may scatter widely with air flow with the possible result of an adverse influence on the image. It is desirable to prevent wide scattering of the wax immediately after the condensation with the air flow. 
     On the other hand, with an electromagnetic induction type fixing device disclosed in Japanese Laid-open Patent Application 2010-217580, a heat generating element is provided adjacent a coil holder in order to prevent the wax from fixing and accumulating on a coil holder. More specifically, the coil holder is heated by the heat generating element to liquefy the wax to let the wax drop down from the coil holder. 
     In another example, with the fixing device disclosed in Japanese Laid-open Patent Application 2011-112708, the fine particles deposited on a fixing roller are removed by a cleaning web with the aid of a trapping material, contained in the cleaning web, for trapping the fine particle. 
     However, with the fixing devices disclosed in Japanese Laid-open Patent Application 2010-217580 and Japanese Laid-open Patent Application 2011-112708, it is not possible to suppress wide-range scattering of the dust existing in the neighborhood of the sheet entrance, inside of the machine. 
     Accordingly, it is an object of the present invention to provide an image forming apparatus in which particles having a predetermined particle size are produced from the parting material from scattering over a wide range. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising an image forming device configured to form, in a first position, a toner image on a sheet using toner containing a parting material; a fixing device configured to fix the toner image formed on the sheet by said image forming device in a second position, by heat and pressure; a fan configured to flow air along an air flow path between said image forming device and said fixing device; and a partition configured and positioned to substantially separate between the air flow path and a sheet feeding path from the first position to the second position. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross sectional view of a fixing device according to Embodiment 1 of the present invention. 
         FIG. 1B  is an exploded perspective view of the fixing device. 
         FIG. 2  is an exploded perspective view of a heating unit. 
         FIG. 3  is a schematic front sectional view of an image forming apparatus of Embodiment 1. 
         FIG. 4  is a schematic view illustrating a state in which a right-hand door is closed. 
         FIG. 5  is a schematic view illustrating a state in which the right-hand door is opened. 
       In  FIG. 6 , (a) is an enlarged view of a nip portion in part (a) of  FIG. 1 , and (c) is a schematic illustration of a layer of a pressing roller (b) is a schematic illustration of a layer of a sleeve. 
         FIG. 7  illustrates relationship between a passing region width of a toner image, a maximum feeding width of the sheet, a region width of a sheet-like member. 
         FIGS. 8( a ) and 8( b )  illustrate a dust coalescing and deposition phenomena. 
         FIG. 9  illustrates a dust producing position. 
         FIG. 10  is a graph of dust density in the neighborhood of the sleeve. 
         FIG. 11  is an illustration of air flow in the neighborhood of the sleeve. 
         FIGS. 12( a ) and 12( b )  illustrate a fixing device according to Embodiment 2. 
         FIG. 13  is an illustration of a fixing device according to Embodiment 3. 
         FIG. 14  is an illustration of a fixing device according to Embodiment 4. 
       In  FIG. 15 , (a), (b) and (c) are perspective views of sheet-like members used in the fixing devices of Embodiments 2, 3 and 4. 
         FIG. 16A  is a schematic cross sectional view of a fixing device according to Embodiment 5. 
         FIG. 16B  is an exploded perspective view of the fixing device. 
         FIG. 17  is a schematic view illustrating an air flow adjacent to the pressing roller. 
         FIG. 18  is a schematic view illustrating a relation between a sheet interval (between adjacent sheets in the continuous sheet processing) and a length of a feeding guide measured in a recording material feeding direction. 
         FIG. 19  is a schematic view of a state in which an upstream portion of the feeding guide is raised to an open position. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 1 
     (1) General Arrangement of Image Forming Apparatus: 
       FIG. 3  is a schematic longitudinal front view of an image forming apparatus  1  according to this embodiment. The image forming apparatus  1  is a four full-color laser beam printer (color electrophotographic image forming apparatus) using an electrophotographic process. It forms a image on a recording material (sheet of paper, OHP sheet, label or the like) on the basis of an electrical image signal supplied to a control circuit portion (controlling means, CPU) from an external host apparatus B, such as a personal computer or an image reader. 
     control circuit portion A supplies and receives various electrical information between itself and the external host apparatus B and an operating portion C, and effects overall control of the image forming operation of the image forming apparatus  1  in accordance with a predetermined control program and/or a reference table. Here, in the image forming apparatus  1  of  FIG. 3 , the front side of the sheet of the drawing is a front side of the apparatus and the rear side of the sheet of the drawing is a rear side of the apparatus. The left and right directions are left and right as seen from the front side. The up and down directions are based on the direction of gravity. 
     The image forming apparatus  1  comprises, as image forming devices (image forming stations, first to fourth image forming stations  5  ( 5 Y,  5 M,  5 C,  5 K). The stations  5  are arranged in a substantially central portion in the main assembly  1 A of the image forming apparatus  1 , along a substantially horizontal direction from a left side to the right side. 
     The stations  5  comprise respective electrophotographic processing mechanisms that have similar structures. Each station  5  of this embodiment includes a rotatable drum-type electrophotographic photosensitive member (drum)  6  as an image bearing member on which an image is formed. It also comprises a charging roller (charging means)  7 , a cleaning member (cleaning means)  41  and a developing unit (developing means)  9 , at process means actable on the drum  6 . 
     In a first station  5 Y, a yellow (Y) developer (toner) is accommodated in a toner accommodation chamber of a developing unit  9 . In the second station  5 M, magenta (M) toner is accommodated in a toner accommodation chamber of a developing unit  9 . In the third station  5 C, cyan (C) toner is accommodated in a toner accommodation chamber of a developing unit  9 . In the fourth station  5 K, black (K) toner is accommodated in a toner accommodation chamber of a developing unit  9 . 
     Below each station  5 , there is provided a laser scanner unit  8  functioning as an image forming device (image forming station, exposure means) for the drum  6  of the image forming station  5 . Above each station  5 , there is provided a transfer unit (intermediary transfer belt unit)  10  functioning as an image forming device (image forming station). 
     The transfer unit  10  includes a driving roller  10   a  at the right side ( FIG. 3 ), a tension roller  10   b  at the left side, and an intermediary transfer belt member (belt)  10   c  as the intermediary transfer member stretched around the rollers. Inside the belt  10   c , first to fourth primary transfer rollers  11  opposing to the drums  6  of the stations  5  are provided in parallel with each other. Upper surface portions of the drums  6  of the stations  5  contact a lower surface of the lower traveling portion of the belt  10   c  at a position of each primary transfer roller  11 . The contact portion therebetween establishes a primary transfer portion. 
     Outside a belt bending portion of the driving roller  10   a , there is provided a secondary transfer roller  12  functioning as the image forming device (image forming station). The contact portion between the belt  10   c  and the secondary transfer roller  12  is a secondary transfer portion where the image is transferred onto the sheet P. Outside the belt bending portion of the tension roller  10   b , a transfer belt cleaning device  10   d  is disposed. 
     Below the laser scanner unit  8 , a sheet feeding cassette  2  is provided. A cassette  2  can be inserted into and drawn out of the main assembly  1 A of the apparatus. In the right side in the main assembly  1 A of the apparatus, there is provided an upward sheet feeding path (longitudinal path, substantially vertical recording material feeding path) D for feeding the sheet P fed from the cassette  2 , upwardly. 
     Along the sheet feeding path D, there are provided, in the order from the lower side to the upper side, a roller pair of a feeding roller  2   a  and a retarding roller  2   b , a pair of registration rollers, a secondary transfer roller  12 , a fixing device  103 , a flapper  15   a , a pair of discharging rollers. An upper surface of the main assembly  1 A of the apparatus constitutes a discharging tray (discharged sheet stacking portion)  16 . 
     On a right-hand surface side of the main assembly  1 A of the apparatus, a manual insertion feeding portion (multi-purpose tray)  3  is provided. The manual insertion feeding portion  3  can be folded to the main assembly  1 A of the apparatus as indicated by the chain lines (closed state) when not used. In use, it is opened as indicated by the indicated solid lines. 
     The operations for forming a full-color image will be described. The control circuit portion A causes the execution of the image forming operation of the image forming apparatus  1  in response to a print start signal. More particularly, the drums  6  of the stations  5  are rotated at predetermined speeds in the clockwise direction indicated by the arrow, in timed relation with the image forming operation. The belt  10   c  is also rotated in the counterclockwise direction of an arrow R (codirectionally with the peripheral movement of the drum) at a speed corresponding to the speed of the drum  6 . Also, the laser scanner unit  8  is driven. 
     In synchronism with the driving, the charging roller  7  supplied with a predetermined charging bias voltage in each station  5  electrically charges the surface of the drum  6  uniformly to a predetermined polarity and potential. The laser scanner unit  8  scans in the main scanning direction the surface of the drum  6  with the laser beam modulated in accordance with the image information signal for the corresponding color (Y, M, C, K). By this, an electrostatic latent image is formed on the surface of the drum  6  in accordance with the image information signal of the corresponding color. The electrostatic latent image thus formed is developed into a toner image (developer image) by a developing roller (developing member) of the developing unit  9 . The developing roller is supplied with a predetermined developing bias voltage. 
     By the above-described electrophotographic image forming process operation, a Y chromatic toner image corresponding to the Y color component of the full-color image is formed on the drum  6  of the first station  5 Y, and is primary-transferred onto the belt  10   c . On the drum  6  of the second station  5 M, a M chromatic toner image corresponding to the M color component of the full-color image is formed, and it is primary-transferred superimposedly onto the Y color toner image already transferred on the belt  10   c.    
     On the drum  6  of the third station  5 C, a C chromatic toner image corresponding to the C color component of the full-color image is formed, and it is transferred superimposedly onto the Y color+M color toner images already transferred on the belt  10   c . On the drum  6  of the fourth station  5 K, a K chromatic toner image corresponding to the K color component of the full-color image is formed, and it is transferred superimposedly onto the Y color+M color+C color toner image already transferred on the belt  10   c.    
     To each of the first to fourth primary transfer rollers  11 , a primary transfer bias of a predetermined potential and the polarity opposite to the charge polarity of the toner is applied at predetermined control timing. In this manner, Y color+M color+C color+K color full-color unfixed toner image is synthetically formed on the moving belt  10   c . The unfixed toner image is fed to the secondary transfer portion by the continuing rotation of the belt  10   c . In each station  5 , the surface of the drum  6  after the primary-image transfer onto the belt  10   c  is cleaned by a cleaning member (cleaning blade)  41  so that the primary-untransferred toner is removed, thus preparing for the next image forming step. 
     On the other hand, one sheet P in the cassette  2  is picked up and is fed to the registration roller pair  4  by the feeding roller  2   a  and the retarding roller  2   b  at predetermined control timing. In the case of the manual insertion feeding mode, the sheet P is picked up by the feeding roller  3   a  from the manual insertion tray  3  and is fed to the registration roller pair  4  by the feeding roller pair  3   b.    
     The sheet P is fed to the secondary transfer portion with predetermined control timing by the registration roller pair  4 . In the secondary transfer roller  12 , a secondary transfer bias voltage of a predetermined potential of the polarity opposite to the charge polarity of the toner is applied at the predetermined control timing. By this, the four color superimposed toner image is secondary-transferred all together from the belt  10   c  onto the surface of the sheet P, while the sheet P is being nipped and fed in the secondary transfer portion. In this embodiment, the station  5 , the transfer unit  10 , and the secondary transfer roller  12  constitute the image forming station for forming the toner image on the sheet P by the toner containing a parting material. 
     The sheet P leaving the secondary transfer portion is separated from the belt  10   c  and is fed to the fixing device  103  functioning as a fixing portion, where the toner image is heated and fixed into a fixed image on the sheet P. In this embodiment, the sheet P is fed upwardly in the feeding path leading to the fixing device  103  from the secondary transfer portion, up to the fixing device  103 . 
     The sheet P is passed through the fixing device  103  upwardly and is fed under the flapper  15   a  held at a first attitude a indicated by solid lines and is discharged onto the discharging tray  16  by the discharging rollers  14 . The secondary-untransferred toner remaining on the surface of the belt  10   c  after the secondary-transfer onto the sheet P is removed from the surface of the belt by the transfer belt cleaning device  10   d , and the cleaned belt  10   c  is used repeatedly for the image forming operation. 
     In the both-side-printing mode, the sheet P having the fixed image on one side is not discharged onto the sheet discharge tray  16  after leaving the fixing device  103 , but is refed to a duplex print feeding portion (re-circulation feeding path)  15   b  so that it is subjected to the printing operation on the second side thereof. More particularly, in this case, the P sheet leaving the fixing device  103  passes an upper side of the flapper  15   a  switched to a second attitude b indicated by broken lines, and is fed toward the sheet discharge tray  16  by the reverse feeding portion (switch-back roller pair)  15 . 
     When a downstream end portion of the sheet P with respect to the feeding direction reaches the flapper  15   a , the flapper  15   a  is returned to the first attitude a, and the reverse feeding portion  15  is reversely driven. By this, the sheet P is fed reversely (downwardly) in the duplex print feeding portion  15   b  and is refed to the registration roller pair  4  through the feeding roller (pair  15   c ,  3   b ). Thereafter, similarly to the case of the one-sided image forming mode, the sheet P is fed through the secondary transfer portion, the fixing device  103  and the discharging roller pair  14  and is discharged onto the sheet discharge tray  16 , as a duplex print. 
     Above the manual insertion feeding portion  3 , a right-hand door  130  is provided to permit access to the sheet feeding path (longitudinal path) D and to the fixing device  103  in the jam clearance and maintenance operations. The right-hand door  130  with the manual insertion feeding portion  3  can be opened and closed relative to the main assembly  1 A of the apparatus about a rotational shaft  130   a  of the door. 
       FIGS. 3 and 4  are schematic views illustrating the state in which the right-hand door  130  is closed. The image forming apparatus  1  is operable in the state that the right-hand door  130  is closed. In the jam clearance and/or maintenance operations, the right-hand door  130  is opened by rotating about the shaft  130   a  in the clockwise direction in  FIG. 3 .  FIG. 5  is an illustration of the state in which the right-hand door  130  is opened. When the right-hand door  130  is opened, the portion of the sheet feeding path (longitudinal path) D from the registration roller pair  4  to the fixing device  103 , the fixing device  103  and the duplex print feeding portion  15   b  are exposed. By this, the jam clearance and the maintenance operation can be carried out. 
     The secondary transfer roller  12  and the feeding guides  130   b ,  130   c  above and below it are disposed adjacent to the right-hand door  130 . When the right-hand door  130  is closed, the secondary transfer roller  12  is contacted to the outside of the belt bending portion of the driving roller  10   a  of the transfer unit  10  to establish the secondary transfer portion. The feeding guide  130   b  guiding the sheet is disposed opposed to a sheet-like member (flexible sheet the functioning as a partition, which will be described hereinafter. The feeding guides  130   b ,  130   c  and the feeding guide  17  constitute the substantially upward sheet feeding path (longitudinal path) D. 
     When the right-hand door  130  is closed, the feeding guide  130   b  functions to guide, to the fixing device  103 , the side of the sheet P not having the transferred unfixed image and having passed through the secondary transfer portion. In other words, the feeding guide  130   b  is a guide between the transfer unit  10  and the fixing device  103 . It introduces the sheet P from the transfer unit  10  (secondary transfer portion) to the fixing device  103  while guiding the side opposite the toner image carrying side. 
     In order to suppress a temperature rise of the image forming station (intermediary transfer member) by the heat resulting from operation of the fixing device  103  and operations of the electrical parts such as a motor, an air flow path is formed therebetween. More specifically, a fan  150  is disposed as a cooling and/or ventilation means. The fan  150  is provided in a front side of the main assembly  1 A of the apparatus. By this, the temperature of the image forming station can be suppressed below a predetermined temperature. 
     The fan  150  sucks the ambient air which is lower in temperature than the temperature inside of the apparatus and blows it into between the image forming station and the fixing device  103  through the front side of the main assembly  1 A of the apparatus. The air is discharged through a louver (unshown) to the outside of the main assembly  1 A of the apparatus. In this embodiment, the fan  150  is an air flow forming means for providing the air flow  28  ( FIG. 4 ) for ventilation of the opening space adjacent the feeding guide  130   b  of the apparatus. 
     (2) Fixing Device  103 : 
       FIG. 1A  is a schematic cross sectional view of the fixing device  103  in this embodiment, and  FIG. 1B  is an exploded perspective view of the fixing device  103 . In this embodiment, the fixing device  103  is an image heating apparatus of a belt (film) heating type and a pressing member driving type, using a planar (narrow plate-like) heater  101   a  such as a ceramic heater as a heating source. Such a type of heating apparatus is known by Japanese Laid-open Patent Application Hei 4-44075, for example. 
     The fixing device  103  is elongated in a directing direction perpendicular to the feeding direction (sheet feeding direction) X of the sheet in a sheet feeding path plane. The fixing device  103  generally comprises a heating unit  101 , a pressing roller  102  as a back-up member (pressing member), and a casing (fixing casing)  100  accommodating them. The casing encloses the heating unit  101  and the pressing roller  102  so as to permit passage of the sheet therethrough. 
       FIG. 2  is an exploded perspective view of the heating unit  101 . It also shows the pressing roller  102 . The heating unit  101  is an assembly comprising a heater holder  104 , the planar heater  101   a , a pressing stay  104   a , an endless belt-like fixing sleeve  105  as a heating member, and sleeve flanges  106 L,  106 R provided at one end portion side and another end portion side. 
     The holder  104  is a trough like elongated member having a substantially half-arc cross-section and is made of a heat resistive resin material such as a liquid crystal polymer. The heater  101   a  is a plate-like elongated heat generating element having a low thermal capacity, such as a ceramic heater, the temperature of which can rise steeply by electric power supply thereto and is held by the holder  104  along the length of the holder in a central portion with respect to the circumferential direction at an outer side of the holder  104 . The stay  104   a  is an elongated rigid member having a U-cross-section and disposed inside of holder  104  and is made of metal such as steel or the like. The sleeve  105  is loosely fitted around the assembly including the holder  104 , the heater  101   a  and the stay  104   a.    
     The flanges  106 L,  106 R provided at one end portion side and the other end portion side are molded products of heat resistive resin material having symmetrical configurations. The flanges  106 L,  106 R are holding members holding the sleeve  105 . The sleeve  105  is rotatably held between the flanges  106 L,  106 R so that it is limited and kept in shape at the opposite ends. 
     As shown in  FIG. 2 , the flanges  106 L,  106 R each includes a flange portion  106   a , a shelf portion  106   b  and an urged portion  106   c . The flange portion  106   a  limits the movement of the sleeve  105  in a thrust direction of the sleeve  105  by being contacted by the end surface of the sleeve  105 , and has an outer configuration larger than outer configuration of the sleeve  105  by a predetermined amount. The shelf portion  106   b  is provided on an inner surface of the flange portion  106   a  and is arcuate to hold the sleeve and keep the cylindrical shape thereof at the inner surface thereof at the end portion. The urged portion  106   c  is on the outer side of the flange portion  106   a  to receive an urging force T of an urging means (unshown). 
       FIG. 6( b )  is a schematic illustration of the layer structure of the sleeve  105  in this embodiment. The sleeve  105  is a composite layer member including a laminated endless shape (cylindrical) base layer  105   a , a primer layer  105   b , an elastic layer  105   c  and a parting layer  105   d  in the order named from the inside toward the outside. The sleeve  105  is a thin and low thermal capacity member having an overall flexibility, and is substantially cylindrical in a free state. 
     The base layer  105   a  is a base layer of metal such as SUS (stainless steel), and in order to endure thermal stress and mechanical stress, it has a thickness of approx. 30 μm. The primer layer  105   b  on the base layer  105   a  is made of an approx. 5 μm thick electroconductive primer in which a proper amount of electroconductive particles such as carbon are dispersed. 
     The elastic layer  105   c  deforms, when pressing the toner image, to close-contact the parting layer  105   d  to the toner image. The parting layer  105   d  is made of PFA resin material which exhibits an excellent parting property and heat resistivity in order to assure a deposition suppressing property of the toner and the paper dust. The thickness thereof is approx. 20 μm from the standpoint of assuring the heat transfer property. The PFA resin material has an excellent parting property and the heat resistivity, but it is relatively easily damaged, too, and therefore, it is preferable that the sheet-like member  120  having the flexibility is contacted to the fixing sleeve  105  codirectionally with the peripheral moving direction of the fixing sleeve  105 , as will be described hereinafter. 
       FIG. 6( c )  schematically illustrates a layer structure of the pressing roller  102  in this embodiment. The pressing roller  102  is an elastic roller including a metal core  102   a  composed of metal (aluminum and steel), an elastic layer of silicone rubber or the like, and a parting layer  102   c  coating the elastic layer  102   b . The parting layer  102   c  is a tube of fluorine resin material of PFA or the like and is fitted around the elastic layer. The circumferential length of the sleeve  105  and the circumferential length of the pressing roller  102  are substantially the same. 
     The casing  100  comprises an inner frame of an elongated metal plate including a base plate  109 , a stay  108 , one end portion side plate  107 L, and another end portion side plate  107 R. The casing  100  comprises an outer frame member mounted to the outside of the inner frame, the outer frame member of elongated heat resistive resin material including a rear cover  110 , a first upper cover  111 , a front lower cover  112 , a second upper cover  113 , one end portion side cover  117 L, and another end portion side cover  117 R. In  FIG. 1B , parts such as the second upper cover  113  are omitted for better illustration. 
     The pressing roller  102  is rotatably supported between one end portion side plate  107 L and the other end portion side plate  107 R of the inner frame by the bearings (unshown), at the one end portion side and the other end portion side of the metal core  102   a.    
     The heating unit  101  is extended in parallel with the pressing roller  102  between the one end portion side plate  107 L and the other end portion side plate  107 R of the inner frame with the heater ( 101   a ) side opposed to the pressing roller  102 . 
     The flanges  106 L and  106 R at the one end portion side and the other end portion side of the heating unit  101  are slidably engaged with guiding holes formed (unshown) elongated toward the pressing roller  102  in the side plates  107 L and  107 R. The flanges  106 L and  106 R are urged toward the pressing roller  102  by urging means (unshown) at a predetermined urging force T. 
     By the urging force the entirety of the flanges  106 L,  106 R, the stay  104   a  and the holder  104  are moved toward the pressing roller  102 . Therefore, the heater  101   a  is urged toward the pressing roller  102  through the sleeve  105  against an elasticity of the elastic layer  102   b  with a predetermined urging force. By this, a nip (fixing nip)  101   b  having a predetermined width measured in the recording material feeding direction X is formed between the sleeve  105  and the pressing roller  102 .  FIG. 6( a )  is an enlarged view of the nip  101   b  in  FIG. 1A . 
     The fixing operation of the fixing device  103  will be described. The control circuit portion A rotates the pressing roller  102  at a predetermined control timing at a predetermined speed in the clockwise direction indicated by an arrow R 102  in  FIG. 1A . The pressing roller  102  is rotated by a driving force transmitted from the driving source (unshown) to the driving gear G ( FIG. 2 ) integral with the pressing roller  102 . 
     By the pressing roller  102  being rotated, a rotational torque is applied to the sleeve  105  by a frictional force relative to the pressing roller  102  in the nip  101   b . By this, the sleeve  105  is rotated in the counterclockwise direction indicated by an arrow R 105  substantially at a speed corresponding to the speed of the pressing roller  102  around the holder  104  and the stay  104   a  while the inner surface thereof is sliding in close-contact with the heater  101   a.    
     In addition, the control circuit portion A starts the electric power supply to the heater  101   a  from the voltage source portion (unshown). The electric power supply to the heater  101   a  is effected through electric power supply connectors  101   d L,  101   d R ( FIG. 2 ) mounted to one end portion side and the other end portion side of the heater  101   a . By the electric power supply, the temperature of the heater  101   a  rapidly rises all over the effective length thereof. The temperature rise is detected by a thermister TH as temperature detecting means provided on a back side (side opposite the nip  101   b  side) of the heater  101   a.    
     The control circuit portion A controls the electric power supply to the heater  101   a  so that the heater temperature detected by the thermister TH is raised to and maintained at a predetermined set target temperature. In this embodiment, the set target temperature is approx. 170 degree C. 
     In such a state of the fixing device, the sheet P carrying an unfixed toner image is fed from the secondary transfer portion to the fixing device  103 . The sheet P is guided along the feeding guide  130   b  and the guide surface  110   a  of the rear cover  110  and is introduced to the entrance  101   c  of the nip and is nipped and fed by the nip  101   b.    
     The sheet P is heated with the heat of the heater  101   a  through the sleeve  105  while being nipped and fed by the nip  101   b . The unfixed toner image S is melted by the heat of the heater  101   a , and is fixed by the pressure applied in the fixing nip  101   b  into a fixed image (heat-pressure fixing). The sheet P discharged from the nip  101   b  is conveyed to an outside of the fixing device  103  by the fixing and sheet discharging roller pair  118 . 
     The casing  100  of the fixing device  103  is provided with a sealing member  120 . The sealing member  120  has one end, with respect to the recording material feeding direction X, which extends toward the sleeve  105  to close the gap between the casing  100  and the sleeve  105 . The other end of the sealing member  120  extends to oppose to the surface of the feeding guide  130   b  to block the space existing upstream of the nip  101   b  (with respect to the recording material feeding direction X) from the air flow  28  ( FIG. 4 ). 
     More specifically, the sheet-like member  120  having a flexibility as the sealing member is stuck on the sticking surface of the front lower cover  112  of fixing device  103 , and one end portion thereof is in contact with the sleeve  105 . The sheet-like member  120  is made of a fluorinated resin material having both of a heat resistivity, a slidability and an elasticity, and is urged to the sleeve  105  by the elastic force thereof to seal between the front lower cover  112  and the sleeve  105 . 
     The sheet-like member  120  is inclined relative to a perpendicular direction to the surface of the sleeve  105 , and the sleeve  105  side end portion of the sheet-like member  120  is co-directional with the peripheral moving direction of the sleeve  105 . Because of the co-directional arrangement, the load applied to the sleeve  105  is reduced to suppress the damage to the surface thereof. 
     On the other hand, as shown in  FIGS. 3 and 4 , the other end portion side of-the sheet-like member  120  extends to oppose the feeding guide  130   b  and projects to the neighborhood of the belt  10   c  of the transfer unit (image forming station)  10  with a gap therefrom. As described hereinbefore, adjacent to the feeding guide  130   b , the air flow  28  ( FIG. 4 ) is provided to maintain the temperature of the image forming station at or below a predetermined temperature. The other end portion side of-the sheet-like member  120  substantially blocks the air flow  28  so as not to produce air flow at least in the neighborhood of an upstream part of the nip  101   b  (neighborhood of the upstream part with respect to recording material feeding direction). 
     A predetermined gap is provided between the belt  10   c  and the sheet-like member  120  not to positively contact them to each other, by which the load applied to the belt  10   c  is lowered to prevent the damage to the surface of the belt  10   c.    
       FIG. 5  is an illustration of the state in which the right-hand door  130  is opened about the rotational shaft  130   a  for the jam clearance or maintenance operation. When the fixing device  103  is taken out for maintenance operation, the fixing device  103  is pulled out of the main assembly  1 A of the apparatus in the direction of an arrow  27 , and in order to carry out the mounting and demounting operation using a small space, the other end portion side of the sheet-like member  120  desirably has an elasticity (flexibility). 
     In this embodiment, a free end, which is one end portion of the sheet-like member  120 , is contacted to the fixing sleeve  105 , and the other end portion projected from the fixing device  103  is also a free end, while the stick portion  112   a  thereof is stuck on the lower front cover  112 . Therefore, it is flexible in the direction of an arrow  29 . With such a structure, operativity is improved when the sheet is taken out for an upstream side of the fixing nip  101   b  for the purpose of jam clearance. 
     The distance from the feeding guide  130   b  to the sheet-like member  120  is such that the sheet-like member  120  is spaced therefrom by at least 10 mm. This is because if an obstructing material exists opposing the feeding guide  130   b , the unfixed image of the sheet which is being fed to the fixing device  103  may rub the obstructing material due to the possible curling or fluttering, with the result of the occurrence of an image defect. 
     In addition, as shown in  FIG. 7 , a dimension W1 (width with respect to the sheet feeding direction) of the sheet-like member  120  measured in the longitudinal direction of the sleeve  105  will be described. It is larger than a maximum printing area width W2 (entire area of the passing range of the toner image  121  (S)) printed on the sheet in the nip  101   b  at least. 
     The passing range of the toner image  121  is a maximum width of the toner image  121 , that is, more particularly, the a width of the image  121  having a largest printable width. Therefore, W1&gt;W3&gt;W2 is satisfied. 
     In this embodiment, W1&gt;W3 is satisfied, too, where W3 is a width of the maximum sheet (maximum feeding width of the sheet) and is slightly larger than the maximum printing width W2. That is, the width W1 of the sheet-like member  120  is larger than the maximum feeding width W3 of the sheet P. With such a structure, the dust produced in the passing range of the toner image in the nip  101   b  can be blocked assuredly from the air flow  28  ( FIG. 4 ). 
     (3) Parting Wax Contained in Toner Particle: 
     Parting wax (parting material) contained in toner particle S will be described. In the image forming apparatus  1  using the toner S as in the printer, the toner S may be deposited onto the sleeve  105  (toner offset). The offset toner may cause various problems such as an image defect and/or variation in the temperature of the nip  101   b.    
     Under the circumstances, in the image forming apparatus  1  of this embodiment, the parting wax as a parting material is contained in the toner particles S so that the parting wax seeps from the toner particles S in the heating and fixing operation. The parting wax melted by the heating functions to prevent offset by intervening between the sleeve  105  and the toner image on the sheet P. 
     The melting point Tm of the parting wax is approx. 75 degree C. The melting point Tm is selected so that the parting wax in the toner S instantaneously melts to seep into the interface between the toner image and the sleeve  105  when the nip  101   b  is kept at the set target temperature 170 degree C., When the parting wax melts, a part of the parting wax such as low molecular weight component in the parting wax gasifies. The parting wax comprises long chain components, but the lengths thereof are not uniform, and have a predetermined distribution. More particularly, the parting wax comprises a low molecular component having short chains and a low boiling point, and a high molecular component having long chains and a high boiling point, in which the low molecular component gasifies. 
     The gasified wax component is cooled in the air to condense into dust particles having sizes of approx. several tens-several hundreds nm. The wax component dust is sticky and may stick on inside parts of the image forming apparatus  1 , which may cause problems. For example, if the dust is deposited and accumulated on the fixing and sheet discharging rollers  118  or the discharging rollers, the contamination may be transferred onto the sheet P, thus causing a deterioration in the image quality. For another example, in the case that the image forming apparatus  1  is provided with a discharging filter, the dust may be deposited on the discharging filter to clog up. 
     Under the circumstances, in the fixing device  103  of this embodiment, the sheet-like member  120  is provided between the lower front cover  112 , which is a part of the casing  100 , and the sleeve  105 , which is the heating member, to seal therebetween, thus suppressing scattering of the dust in the main assembly  1 A of the apparatus. 
     For better understanding of the function of the sheet-like member  120 , the general property of the dust, and the consideration of the inventors will be described. 
     It is known as general properties of the dust that they coalesce into large particles and that they are deposited on a solid matter in the flow of the dust.  FIGS. 8( a ) and 8( b )  illustrate these properties As shown in  FIG. 8( a ) , a high boiling point substance  20  having the boiling point 150-200 degree C. is placed on a heating source  20   a , and is heated to approx. 200 degree C., by which the high boiling point substance volatiles into volatilized matter  21   a . When the volatile matter  21   a  contacts the normal temperature air, the temperature of the volatile matter  21   a  immediately decreases to below the boiling point temperature to condense in the air, by which it becomes fine dust  21   b  of approx. several nm-several tens nm. This phenomenon is the same as the water vapor condensing into fine droplets when the temperature thereof becomes lower than the due-point temperature. 
     The fine dust  21   b  is moving in the air by the Brownian movement, and they collapse each other to coalesce into larger dust particles  21   c , as is known. The growth ends when the dust size exceeds a certain size because when the dust particles become large, the Brownian movement becomes less active. 
     Consider the case shown in  FIG. 8( b )  in which the air α containing fine dust  21   b  and larger dust particles  21   c  flows against the wall  23  by the air flow  22 . At this time, the large dust particles  21   c  are more easily deposited on the wall  23  than the file dust particles  21   b . Dust particles  21   c  have large inertia, and therefore, impinge on the wall  23  strongly. This occurs also when the air flow speed is as low as is not more than 0.2 m/s, which is outside the measurement limit of an ordinary anemometer. 
     As will be understood from the foregoing, the nature of the dust is to coalesce into large particles, and the nature of large dust particles is that they are susceptible to easy deposition on the parts. The tendency to coalesce depends on the temperature and the density of the dust particles. For example, the coalescence increases when the temperature of the sticky component becomes high, and it becomes soft, and the collision probability of the dust particles increases under a high density. 
     Considering the suppressing measurement of the dust scattering in the image forming apparatus  1  in the light of the nature of such coalescence and dust particles, it is preferable to confine the air containing the dust particles in the region in the neighborhood of the sleeve  105 . The neighborhood of the sleeve  105  is close to the position of the production of the dust particles, and therefore, the dust density is high, and in addition, the ambient temperature is high due to the heat of the surface of the sleeve  105 , and for these reasons, the situation is proper for coalescence of the dust particles. 
     Referring to  FIG. 9  and  FIG. 10 , the position of the production of the dust particles will be described.  FIG. 9  shows the fixing device  103  in which the sheet-like member  120  has been removed. The sheet P carrying the toner image is fed and fed by the nip  101   b . Therefore, the dust particles are produced. In such a state, the dust density has been measured at a point A adjacent the inlet  101   c  of the nip  101   b  and at a point B adjacent the outlet. 
     For the measurement, a high speed response particle sizer FMPS available from TSI Corporation, USA was used. The prediction before the actual measurement had been that the density would be relatively higher at the outlet side where the toner image has been sufficiently by the nip  101   b , but the result was the opposite. The result of measurement was that the position of the production of the dust particles is at the inlet  101   c  of the nip. It is considered that this is because a low molecular weight component of the parting wax is volatilized instantaneously when the high temperature sleeve  105  contacts the toner image, and after having passed through the nip  101   b , the volatilization has been finished. 
     Referring to  FIG. 11  showing a result of simulation, diffusion of the dust particles produced at the inlet  101   c  of the nip, inside of the machine will be described.  FIG. 11  shows the flow of the air from the neighborhood of the nip inlet  101   c  along a path  24 . An arrow F in  FIG. 11  is the direction of gravity. 
     The simulation of the heating and the air flow has been made under the conditions of 170 degree C. at the surface temperature of the sleeve  105 , the rotation in the counterclockwise at a speed V, and the sheet P speed of V upwardly in  FIG. 11 . In the simulation, an ascending air flow due to natural convection around of sleeve  105 , and a film surface air flow  25  caused by the movement of the surface of the sleeve  105  are taken into account. The path  24  has been determined by producing a phantom particle having a zero weight at the nip inlet  101   c  on the simulation program. The method is well used to investigate an air flow path in an air flow simulation. 
     The phantom particle of the zero weight does not have an inertia, and cannot replicate the diffusion by the Brownian movement of actual particles, but quite replicates the discharging flow path of the dust particles. 
     According to the path  24  shown in  FIG. 11 , the dust particles produced at the nip inlet  101   c  move in the clockwise direction along the surface of the sleeve  105  and rise rises through the gap adjacent the roller pair  118  along the sheet P. Between the sleeve  105  and the path  24 , a gap t exists. The gap t is provided by the sleeve surface air flow  25  entering between the path  24  and the sleeve  105 . 
     As described in the foregoing, it is considered that the positions of the production of the dust particles and the coalescence and deposition of the dust particles are at the nip inlet  101   c , and the produced dust particles move along the surface of the sleeve  105 . The sheet-like member  120  shown in  FIGS. 1A and 1B  is provided on the basis of this consideration and has a function of stagnating the dust particles in the region  26  by shutting the flow of the rising dust particles along the surface of the sleeve  105 . It has an additional function, that is, of not diffusing the dust particles stagnated in the range  26  in the image forming apparatus against the air flow  28  of  FIG. 4 . 
     In a bar graph of  FIG. 10 , the right-hand end data indicates the dust density at the point B ( FIG. 9 ) in the case that the sheet-like member  120  is provided. As compared with the case not having the sheet-like member  120 , the dust density at the point B was reduced to approx. ⅕. As a result, the diffusion of the dust particles in the image forming apparatus can be suppressed to reduce the image contamination and/or the filter packing. 
     Dust particles are prevented from moving between the casing  100  and the sleeve  105  by the sheet-like member  120 , and stagnate in the region  26  shown in  FIG. 1A . The temperature and the density of the stagnated dust particles there are so high that the coalescence of them is rapid. The dust particles upsized by the coalescence move toward the sleeve  105  by the rising air flow caused by the natural convection and by the movement of the sheet P. The deposited dust particles melt by the heat of the sleeve  105  and are deposited on the sheet P, but since the dust particles are so fine, the influence on the image is practically negligible. 
     That is, the portion of the sheet-like member  120  between the sleeve  105  and the casing  100  confines in the neighborhood of the nip, the dust particles produced adjacent to the nip  101   b . The enclosed dust particles coalesce and upsize and are deposited on the rotating sleeve  105 . The dust particles deposited on the sleeve  105  are transferred onto the sheet, but do not influence the image because their size is sufficiently small. 
     In addition, by extending the other end portion side of the sheet-like member  120  to the neighborhood of the transfer unit  10 , the peripheral portion of the nip is blocked (partitioned) from the ventilation air flow  28 . Therefore, the wide range diffusion of the dust particles in the image forming apparatus can be suppressed. 
     Embodiment 2 
     Referring to  FIG. 12 , a fixing device  103  according to Embodiment 2 will be described. The apparatus is different from the fixing device  103  of Embodiment 1 in the following respects. One end portion side and another end portion side with respect to a widthwise direction (sheet widthwise direction) of the sheet-like members  120  as a sealing member are provided with respective wall surface portions  120   a  and  120   b  bent toward a feeding guide  130   b  opposing the sheet-like member  120 . The sheet-like member  120  extends over a range width W1 wider than the maximum feeding width W3 of the sheet P. Wall surface portions  120   a  and  120   b  bent toward the feeding guide  130   b  are provided at least at one end portion side with respect to the widthwise direction of the sheet-like member  120 . 
       FIG. 15( a )  is a perspective view of the sheet-like member  120  provided with the wall surface portions  120   a  and  120   b . In the description of this embodiment, the same reference numerals as in Embodiment 1 are assigned to the elements having the corresponding functions in this embodiment, and the detailed description thereof is omitted for simplicity. 
       FIG. 12( a )  is a perspective view of major parts of the apparatus of Embodiment 2,  FIG. 12( b )  is a schematic view of the major parts as seen from above the main assembly of the apparatus. On a stick surface  112   a  of the lower front cover  112 , the sheet-like member  120  is stuck, and the sheet-like member  120  covers the region W1, which is larger than the maximum feeding width W3. The opposite end portions of the sheet-like member  120  with respect to the widthwise direction are provided with the wall surface portions  120   a  that are integrally bent. 
     A fan  150  as the air flow forming means is disposed closer to the inlet  101   c  of the fixing nip than in Embodiment 1. With this structure, air flow  28  includes the flow in the path along the surface of the belt  10   c  from a sheet feeding path to a secondary transfer portion and a fixing device  103 , and the flow in the path from a front side of the image forming apparatus  1  directly to and through a louver  151  provided in a rear surface across the sheet feeding path. 
     In this embodiment, the inlet  101   c  of the nip can be blocked from the air flow  28  entering from a lateral side of the sheet feeding path by the wall surface portions  120   a  and  120   b  of the sheet-like member  120 . More particularly, the dust particles produced in the nip inlet  101   c  are effectively stagnated in a space portion surrounded by the sleeve  105 , the pressing roller  102 , the sheet-like member  120 , the wall surface portions  120   a  and  120   b , and the feeding guide  130   b . As a result, the diffusion of the dust particles in the image forming apparatus can be suppressed to reduce the image contamination and/or the filter packing. 
     Embodiment 3 
     Referring to  FIG. 13 , a fixing device  103  according to Embodiment 3 will be described. The fixing device of this embodiment is different from the fixing device  103  of Embodiment 2 in that only at one end portion side of the sheet-like member  120  with respect to the widthwise direction (sheet widthwise direction) is a sealing member, with a wall surface portion  120   a  bent toward the feeding guide  130   b  opposing the sheet-like member  120 .  FIG. 15( b )  is a perspective view of the sheet-like member  120  is provided with the wall surface portion  120   a . In the description of this embodiment, the same reference numerals as in Embodiment 1 are assigned to the elements having the corresponding functions in this embodiment, and the detailed description thereof is omitted for simplicity. 
       FIG. 13  is a schematic view of major parts of the apparatus according to Embodiment 3 as seen from above a main assembly of the apparatus. The sheet-like member  120  extends over a range width wider than the maximum feeding width W3 of the sheet P. The one end portion of the sheet-like member  120  with respect to the widthwise direction is provided with a wall surface portion  120   a  that is integrally bent. On the other hand, the other end portion is not provided with a wall surface portion. 
     In this embodiment, the inlet  101   c  of the fixing nip can be blocked from a high speed air flow  28  entering from the front side of the image forming apparatus  1 , by the wall surface portion  120   a  of the sheet-like member  120 . 
     According to this embodiment, too, the diffusion of the dust particles in the image forming apparatus can be suppressed to reduce the image contamination and/or the filter packing. In the case of this embodiment, the other end portion side of the sheet-like member  120  where the air flow speed is almost zero is opened, so that the moisture of the sheet feeding path can be properly discharged. As a result, the improper feeding and/or the image defect attributable to the dew condensation can be avoided. 
     Embodiment 4 
     Referring to  FIG. 14 , a fixing device  103  according to Embodiment 4 will be described. In this embodiment, one end portion side and another end portion side with respect to a widthwise direction (sheet widthwise direction) of the sheet-like members  120  as a sealing member are provided with respective wall surface portions  120   a  and  120   b  bent toward a feeding guides  130   b  and  110   a  opposing the sheet-like member  120 . The heights of the wall surface portions  120   a  and  120   b  (lengths in the direction toward the feeding guides  130   b  and  110   a ) are made different from each other.  FIG. 15( c )  is a perspective view of the sheet-like member  120  provided with such wall surface portions  120   a  and  120   b.    
     In the description of this embodiment, the same reference numerals as in the foregoing Embodiments are assigned to the elements having the corresponding functions in this embodiment, and the detailed description thereof is omitted for simplicity. 
       FIG. 14  is a schematic view of major parts of the apparatus according to Embodiment 4 as seen from above a main assembly of the apparatus. The sheet-like member  120  extends over a range width wider than the maximum feeding width W3 of the sheet P. The one end portion of the sheet-like member  120  with respect to the widthwise direction is provided with wall surface portion  120   a  (having a height L1) that are integrally bent. On the other hand, the other end portion side is provided with an integrally bent wall surface  120   b  having a height L2. Here, L1&lt;L2. 
     In addition, as is different from the foregoing embodiments, a fan  150 , which is the air flow forming means, is provided at a rear surface of the image forming apparatus  1 , and a louver  151  is provided at a front side of the image forming apparatus  1 , wherein the air is sucked from the rear surface. In this embodiment, the height L2 of the wall surface portion  120   b  at the other end portion side of the sheet-like member  120  is higher than the height L1 of the wall surface portion  120   a  at the one end portion side. Therefore, the inlet  101   c  of the fixing nip can be blocked from the rear surface of the image forming apparatus  1  where the air flow speed is particularly high. 
     More particularly, the wall surface portions  120   a  and  120   b  are provided at the one end portion side and the other end portion side with respect to the widthwise direction of the member  120 , and the height provided by the bending toward the feeding guide  130   b  of the wall surface portion  120   b  at the main entering side of the air flow  28  is higher than that of the other wall surface portion  120   a.    
     According to this embodiment, too, the diffusion of the dust particles in the image forming apparatus can be suppressed to reduce the image contamination and/or the filter packing. The front side of the image forming apparatus  1  where the air flow speed is very low is opened, so that the moisture of the sheet feeding path can be properly discharged. As a result, the improper feeding and/or the image defect attributable to the dew condensation can be avoided. 
     In this embodiment, it will suffice if the sheet-like member  120  contacts the sleeve  105  by which movement of the dust particles is prevented, and the continuous sheet-like member  120 , which is continuous without a gap, extends to the upstream side of the nip to block from the air flow in the neighborhood of the nip inlet. As long as these functions are provided, the sheet-like member  120  is not limited to those explained in Embodiments 1-4. 
     Embodiment 5 
       FIG. 16A  is a schematic cross sectional view of the fixing device  103  in this embodiment, and  FIG. 16B  is an exploded perspective view of the fixing device  103 . In the description of this embodiment, the same reference numerals as in Embodiments 1-4 are assigned to the elements having the corresponding functions in this embodiment, and the detailed description thereof is omitted for simplicity. 
     In the fixing device  103  of this embodiment, a casing  100  is provided with a first neighborhood  110   b , which extends to a neighborhood of the pressing roller  102  and which has a length larger than a maximum printing area width W2 ( FIG. 7 ) of the sheet P introduced to a nip  101   b . In this embodiment, the first neighborhood  110   b  extends in the widthwise direction of the rear cover  110  inside the rear cover  110  mounted on the outside of an inner frame of the casing  100 . 
     In the fixing device  103  of this embodiment, there is provided a feeding guide  140 , which is disposed upstream of the nip  101   b  with respect to the recording material feeding direction X and which guides a side of the sheet P opposite from toner image carrying side toward the nip  101   b . The feeding guide  140  is provided with a second neighborhood (guide frame)  140   a  which extends to a neighborhood of the pressing roller  102  to guide the sheet P to the nip  101   b . The portion of the feeding guide  140  upstream of the second neighborhood  140   a  with respect to the recording material feeding direction X is called an upstream portion  140   b  of the feeding guide. 
     In the fixing device  103  of this embodiment, a sheet-like member  120  as with Embodiments 1-4 is provided, although not shown in the drawing. 
     As described hereinbefore, the dust particles are produced by the toner S on the sheet, being heated by the nip  101   b  of the fixing device  103 , and therefore, they are mainly produced at a point A in  FIG. 16A , which is upstream of the nip  101   b  with respect to the recording material feeding direction X. The dust particles produced at point A diffuse inside of the machine with the air flow therearound. 
     As described hereinbefore, it is preferable to confine the air containing the dust particles in the neighborhood of the point A (producing position) by which they are stagnated there in a high density to promote the coalescence of the dust particles. The neighborhood of the sleeve  105  and the pressing roller  102  is close to the position of the production of the dust particles, and therefore, the dust density is high, and the ambient temperature is also high due to the heat of the surface of the sleeve  105 , and for this reason, the neighborhood is suitable for the coalescence of the dust particles. 
     In view of these, in this embodiment, the dust particles are confined in the neighborhood of the producing point A to promote the coalescence of the dust particles, thus preventing scattering of the dust particles in the inside of the machine, and this is accomplished by the provision of the feeding guide  140  upstream of the rear cover  110  and the nip  101   b  with respect to the recording material feeding direction X. The structures of the fixing device  103  according to this embodiment will be described. 
     The rear cover  110  is provided with a dust blocking rib (first neighborhood)  110   b  close to the pressing roller  102 . The width of the blocking rib  110   b  in the longitudinal direction of the pressing roller  102  is wide enough to cover the entire area W2 ( FIG. 7 ), in the nip  101   b , of the passing range of the toner image printed on the sheet P. Between the blocking rib  110   b  and the pressing roller  102 , a gap is provided to prevent wearing of them. 
       FIGS. 17( a ) and 17( b )  are illustrations of the air flow between the pressing roller  102  and the rear cover  110 . 
       FIG. 17( a )  shows the case in which the rear cover  110  is not provided with the dust blocking rib  110   b  close to the pressing roller  102 . In this case, the pressing roller  102 , which is a rotatable member, produces a rotation air flow  200  along the rotational moving direction at the surface when the pressing roller  102  rotates. In addition, between the rear cover  110  and the pressing roller  102 , a rise air flow  201  is produced by the air heated by the heat generated by the heating unit  101 . In a nip opposing side of the pressing roller  102  where the directions of the rotation air flow and the rise air flow are opposite to each other, the air flow  200  and the air flow  201  collapse each other, so that a substantially no-airflow space  202  is produced. 
     Because the dust particles are so light, they move with the air flows  200  and  201 . The dust particles carried by the rotation air flow  200  are returned to the nip  101   b , and the dust particles carried by the rise air flow leak to the outside of the fixing device through a gap between fixing and the sheet discharging rollers  118  and/or through a gap of the casing  100 . In addition, the dust particles in the space  202  diffuse in the space  202  by the Brownian movement, and finally are carried by either of the air flows  200  and  201 . As described above, the rise air flow is a particular factor diffusing the dust particles to the outside of the fixing device. 
     In view of this, as shown in  FIG. 17( b ) , it is desirable that the dust blocking rib  110   b  extends at least to the space  202 , and if possible to the region where the rotation air flow is produced toward the pressing roller  102 . By this, the rise air flow  201  is blocked so that the dust particles carried by the rise air flow  201  can be closely closed confined in the fixing device. The distance between the blocking rib  110   b  and the pressing roller  102  can be determined easily through heat air flow simulation or the like. 
     The dust particles produced at the point A diffuse from the neighborhood of the feeding guide  140  as well as through the above-described path. As shown in  FIG. 16A , the dust particles produced at the nip  101   b  are blocked by the sheet P, and do not transfer onto the non-printing side during the sheet fixing operation, but after the fixing operation, the dust particles move from the space between the succeeding sheets to the non-printing side. Thus, moved dust particles move to the duplex print feeding portion  15   b  ( FIG. 3 ) and further move through gaps of the duplex print feeding portion  15   b  to the upper portion reverse feeding portion  15 , and thus into the machine. The thus scattered dust particles gradually accumulate to the extent of transferring onto the sheet P sooner or later, with the result of producing an adverse effect on the image on the sheet. 
       FIG. 18  is a schematic view of the fixing device  103  when a prior sheet P1 and the next sheet P2, with the sheet interval L2 therebetween, are at the point A during continuous sheet processing. In order to suppress the above-described dust scattering, in this embodiment, as shown in  FIG. 18 , the length L1 of the feeding guide  140  of the fixing device  103  extends beyond the sheet interval L2 in the recording material feeding direction X. In order to block the dust particles, the feeding guide  140  does not have an opening in the longitudinal direction within the width of the passing range of the toner image in the nip  101   b , in the range of the length not less than sheet interval L2 at least in the region of the length L1. 
     That is, the feeding guide  140  is longer in the recording material feeding direction X than the sheet interval between the prior sheet P1 and the next sheet P2 during the continuous feeding of the sheets P. In addition, no opening is provided at least within the maximum printing area width W2 ( FIG. 7 ) of the sheet P for introducing into the nip  101   b  at a position of the length not more than the sheet interval L2. 
     For this reason, the second neighborhood (guide frame)  140   a  of the feeding guide  140  and the upstream portion  140   b  of the feeding guide do not have an opening in the longitudinal direction at least within the width of the passing range of the toner image in the nip  101   b.    
     The feeding guide  140  of the fixing device  103  of this embodiment comprises the second neighborhood (guide frame)  140   a  and the upstream portion  140   b  of the feeding guide, which are connected with each other by a hinge shaft  140   c . The second neighborhood is mounted to and supported by the casing  100 . The feeding guide upstream portion  140   b  is rotatable about the hinge shaft  140   c  relative to the second neighborhood  140   a.    
     The upstream portion  140   b  of the feeding guide in a free state rotates in the direction of sagging from the second neighborhood  140   a  about the hinge shaft  140   c  by the weight thereof. The upstream portion  140   b  of the feeding guide is held in the closed attitude and state shown in  FIG. 16A  or  FIG. 18 , in which the top end portion  140   b   1  rotates to abut to and connect with the bottom end portion  140   a   1  of the second neighborhood  140   a.    
     In the closed state of the upstream portion  140   b  of the feeding guide, the feeding guide surfaces of the second neighborhood  140   a  and the feeding guide upstream portion  140   b  provide a continuous feeding guide in the up and down direction. Thus, the feeding guide  140  guides, toward the nip  101   b , the sheet P fed upwardly from the secondary transfer portion, at side of the sheet P opposite from the toner image carrying side. 
     Here, the bottom end portion  140   a   1  of the second neighborhood  140   a  is provided with a sealing member  130   d . The sealing member  130   d  seals a gap in the longitudinal direction between the top end portion  140   b   1  of the feeding guide upstream portion  140   b  and the bottom end portion  140   a   1  of the second neighborhood  140   a  when the feeding guide upstream portion  140   b  is in the closing attitude relative to the second neighborhood  140   a.    
     By this, the feeding guide  140  as a whole has no opening existing in the longitudinal direction at least within the width of the passing range of the toner image in the nip  101   b . The sealing member  130   d  may be provided at the top end portion  140   b   1  of the second neighborhood  140   a , or at each of the bottom end portion  140   a   1  of the second neighborhood  140   a  and the bottom end portion  140   b   1  of the second neighborhood  140   b.    
     By the feeding guide  140  free of opening, the dust particles passed through the gap between adjacent sheets do not directly move to the duplex print feeding portion  15   b  ( FIG. 3 ). 
     According to the fixing device  103  of this embodiment, by the first neighborhood  110   b  extending to the neighborhood of the pressing roller  102 , the dust particles produced at the nip  101   b  and flowing between the pressing roller  102  and casing  100  are confined in the casing. The confined dust particles coalesce with each other to become large particles, which are deposited on the casing  100  and the rotating pressing roller  102 . The dust particles deposited on the pressing roller  102  are transferred to the sheet P, but they are so small that the image is not influenced thereby. 
     In addition, the feeding guide  140  adjacent the pressing roller  102  extends beyond the sheet interval L2, by which the phenomenon that the dust particles produced at the nip  101   b  diffuse through the using sheet interval L2 to contaminate the duplex print feeding portion  15   b  and/or the downstream feeding guide portion of the fixing device can be suppressed. 
     Upon performing a jammed sheet clearance operation, the right-hand door  130  ( FIG. 3 ) is opened to open the sheet feeding path D ( FIG. 5 ). Then, as shown in  FIG. 19 , the feeding guide upstream portion  140   b  of the feeding guide  140  is rotated about the hinge shaft  140   c  toward the right-hand door  130  side up to a substantially horizontal position (open attitude). By this, the portion of the nip inlet  101   c  of the fixing device  103  is opened, and the visibility of the sheet inside the fixing device  103  is as good as with the conventional structure. 
     The rotatability of the upstream portion  140   b  of the feeding guide provides the following advantageous effects. The deterioration of the sheet visibility, by extending the feeding guide  140  toward image forming station (transfer unit  10 ), when the sheet feeding path D is opened by opening the right-hand door  130  upon jam clearance can be suppressed. In other words, the deterioration of the operationality of the apparatus upon the performance of the jam clearance operation, by extending the pressing roller  102  side feeding guide by the rotatable feeding guide upstream portion  140   b , can be suppressed. 
     In addition, with the structure of this embodiment, the high density dust particles stagnate around the feeding guide  140 , and therefore, the feeding guide  140  tends to be contaminated. Therefore, the feeding guide upstream portion  140   b  of the feeding guide  140  is detachable from the hinge shaft  140   c  to facilitate the exchanging operation. 
     When the feeding guide upstream portion  140   b  is contaminated, the feeding guide upstream portion  140   b  is removed from the second neighborhood  140   a  and is cleaned or is replaced with a fresh feeding guide upstream portion  140   b , thus suppressing the adverse effect of the accumulated dust particles on the image quality. Thus, by the detachably mountable structure of the feeding guiding member, the replacement of the guide contaminated by the wax is easy. 
     The feeding guide  140  may have such a structure that the entirety thereof or at least a part of the upstream (of the second neighborhood  140   a ) side feeding guiding portion  140   b  with respect to the recording material feeding direction X is rotatable relative to the casing  100 . In addition, the feeding guide  140  may have such a structure that the entirety thereof or at least a part of the upstream (of the second neighborhood  140   a ) side feeding guiding portion  140   b  with respect to the recording material feeding direction X is mountable and dismountable relative to the casing  100 . 
     &lt;Others&gt; 
     1) in Embodiments 1-5, the image forming apparatus  1  has been described as being a full-color laser beam printer comprising a plurality of drums  6 , but the image forming apparatus may be a monochromatic copying machine or printer comprising one drum  6 . Therefore, the image forming apparatus is not limited to the full-color laser beam printer. 
     2) the sleeve  105  which is a rotatable member in the fixing device  103  may be a flexible and circulatable endless belt stretched around a plurality of stretching members. The sleeve  105  which is a rotatable member may be a rotatable rigid roller member (heat roller). The sleeve  105  which is a rotatable member may be a non-endless web-like member traveling from an unwinding portion in a winding-up portion. 
     3) the heating means for the fixing device is not limited to the planar heater  101   a  of the foregoing embodiments. It may be an inside heating type or outside heating type heater such as an electromagnetic induction heater, a halogen heater, an infrared radiation lamp. 
     4) in the fixing devices of Embodiments 1-5, the pressing roller  102  which is a rotatable member may be an endless belt member. The pressing roller  102  may be a non-rotatable member. For example, it may be a non-rotatable member or the like a pressing pad having a low surface friction coefficient. 
     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 priority from Japanese Patent Application No. 281250/2012 filed Dec. 25, 2012 which is hereby incorporated by reference.