Patent Publication Number: US-8543044-B2

Title: Fixing device and image forming apparatus having sheet separation device

Description:
RELATED APPLICATION 
     The present application is based on Patent Application No. 2010-077218 filed at the Japan Patent Office on Mar. 30, 2010 and which is hereby incorporated herein in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a fixing device for fixing a toner image on a sheet of paper by a nip section made up of a fixing member and pressure member, and an image forming apparatus equipped with this fixing device. 
     BACKGROUND 
     In the image forming apparatus using electrophotographic process such as a photocopier, printer, facsimile and multi-functional peripheral provided with the functions thereof; a latent image corresponding to a manuscript is formed on a photoreceptor, and toner is applied to this latent image, which is thereby developed. This developed toner image is transferred onto the sheet of paper. After that, the toner image transferred on the sheet is fixed in position and is then ejected. 
     One of the aforementioned fixing devices for fixing the toner image is a fixing device using a heated roller fixing method, wherein the sheet with the toner image transferred thereon is sandwiched and conveyed by the nip section formed of a fixing roller (fixing member) incorporating a halogen heater and others as a heating means, and a pressure roller (pressure member) for applying pressure to the fixing roller. The sheet is heated and pressed while it is conveyed. This fixing device is used over an extensive range because of simple configuration. 
     Another of the aforementioned fixing devices for fixing the toner image is a fixing device using a belt fixing method, wherein the endless fixing belt (fixing member) is applied to a heating roller incorporating a halogen heater and others, and a fixing roller; and a pressure roller for applying pressure to the fixing roller through a fixing belt is provided. While the sheet with a toner image transferred thereon is sandwiched and conveyed by the nip section formed of a fixing belt and pressure roller, the sheet is heated and pressed during this time. Such a fixing device has a small heat capacity of the fixing belt This reduces the warming up time, and provides an advantage of energy conservation. 
     In this case, the toner of the toner image of the sheet is heated while the sheet is passed through the nip section. Since the toner has an adhesive strength, the sheet passing through the nip section may stick to the surface of the fixing roller and fixing belt, and may wind the roller without being separated, with the result that a paper jam may occur. The separability will be reduced especially when the coated sheet of a smaller basis weight for printing (thin paper) is used. 
     In the meantime, if the fixing roller size is increased in order to ensure the sufficient nip width to cope with an increasing speed of the image forming apparatus, the roller curvature at the outlet of the fixing nip will be reduced, with the result that the separation performance will be reduced. 
     A separation member is used to cope with the reduction in the separation performance. This means uses a blower means and blower duct to blow air to the outlet side of the nip section for the purpose of separating sheets from the fixing member. 
     An example of using such a separation member is found in a fixing device having been disclosed. This means is provided with a gas discharge means made up of a separation guide plate whose one side approaches the surface of the fixing member and which is arranged lying in the circumferentially moving direction of the fixing member, and a plurality of nozzles for discharging pulse-like compressed air from the area sandwiched in-between the fixing member and the surface of the separation guide plate facing the same, toward the gap between the fixing member and one side of the separation guide plate approaching the same, a plurality of the aforementioned nozzles being arranged perpendicular to the circumferentially moving direction of the fixing member (refer to Unexamined Japanese Patent Application Publication No. 2005-202043). 
     Unexamined Japanese Patent Application Publication No. 2005-202043 uses the discharge of compressed air by the gas discharge means to separate the leading edge of the sheet, and uses a separation guide plate to separate the subsequent portion up to the trailing edge. 
     However, the separation failure of the sheet having passed through the nip section and winding of the sheet on the surface of the fixing member such as a fixing roller or fixing belt occur not only to the leading edge of the sheet, but also to the portion subsequent to the leading edge. Further, even when the leading edge of the sheet is pulled out by the sheet ejection roller, uneven separation remains on the wound portion. Thus, when air is to be blown to the outlet side of the nip section to separate the sheet from the fixing roller and others, air is preferably blown on a continuous basis. 
     However, after having separated the sheet from the fixing belt or fixing roller, the continuously blown air is repelled from the nip section to create a turbulent flow such as an eddy. The turbulent flow may cause the sheet to flap and may disturb conveyance of sheets. Particularly the thin sheet of smaller rigidity and smaller basis weight is subjected to such a turbulent flow, with the result that a conveyance trouble such as a jam may be caused. This adverse effect will be increased if the air flow is increased to improve the separation performance. 
     In view of the problems described above, it is an object of the present invention to provide a fixing device capable of minimizing flapping of the sheet caused by the air blown to separate the sheet from a fixing member and ensuring the stable sheet conveyance thereby. 
     SUMMARY 
     The aforementioned object of the present invention can be achieved by the following Structures:
     1. To achieve at least one of the abovementioned objects, a fixing device conveying a sheet carrying a toner image under heating and pressurization at a nip section and fixing the toner image on the sheet reflecting one aspect of the present invention comprises a fixing member provided with a heating member; a pressure member pressing on the fixing member and forming the nip section which sandwiches the sheet carrying the toner image; a blower duct equipped with an outlet opening toward a vicinity of a sheet outlet of the nip section; a blower member for blowing air in a vicinity of the sheet outlet through the blower duct; and a guide device provided with a guide member for guiding the sheet ejected from the nip section arranged on an upper side of a sheet conveyance path; and, further comprising a biasing member for biasing the sheet toward the guide member, in the vicinity of the nip section.   2. In the abovementioned fixing device of item 1 , wherein the biasing member biases the sheet toward the guide member, using an exhaust gas flow moving along an exhaust gas flow path formed through the guide device between the blower duct and the sheet.   3. In the abovementioned fixing device of item 2 , wherein a sectional area of the exhaust gas flow path is increased in size in a direction in which the exhaust gas flow moves.   4. In the abovementioned fixing device of item 1 , wherein the guide device is configured to comprise a rib.   5. In the abovementioned fixing device of item 1 , wherein the guide device is configured to comprise a net.   6. In the abovementioned fixing device of item 1 , wherein the guide device is configured to comprise a belt.   7. In the abovementioned fixing device of item 1 , wherein at least a portion of the guide device facing the sheet travels at a speed higher than a conveying speed of the sheet.   8. In the abovementioned fixing device of item 1  further comprising; a separation claw arranged on a lower side of the sheet conveyance path to separate the sheet from the pressure member.   9. An image forming apparatus comprises the abovementioned fixing device of item 1.   

     Effects of the Invention 
     The above description shows that the flapping of the sheets can be minimized. Accordingly, the deformation of sheet and occurrence of paper jam can be avoided, and stable conveyance of the sheet is ensured, without a faulty image being formed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram representing an example of the image forming apparatus in the present invention; 
         FIG. 2  is a cross sectional view representing an example of the fixing device in the present invention; 
         FIG. 3  is an enlarged view showing the vicinity of a sheet outlet of a nip section N in  FIG. 2 ; 
         FIG. 4  is a diagram showing a layout example of a blower duct and a fan; 
         FIG. 5  is a diagram showing the configuration of the equipment used in Test 1; 
         FIG. 6  is a diagram showing the data obtained by measuring the vibration in sheet flapping in Test 1; 
         FIG. 7  is a diagram showing the result of the frequency analysis of the data obtained by measuring the vibration in sheet flapping; 
         FIG. 8  is a diagram showing the result of Test 1; 
         FIG. 9  is a front view of an upper guide plate  221  as viewed from the direction of arrow mark X 1  in  FIG. 2 ; 
         FIG. 10  is a diagram showing the air flow and sheet P conveyance path; 
         FIG. 11  is a diagram showing the configuration of the equipment used in Test 2; 
         FIG. 12  is a diagram showing the result of Test 2; 
         FIG. 13  is a diagram showing an Example 2; 
         FIG. 14  is a front view as observed from the direction of arrow mark X 1  in  FIG. 13 ; 
         FIG. 15  is a diagram showing an Example 3; 
         FIG. 16  is a diagram showing the roller of a comb-shaped silicone rubber; 
         FIG. 17  is a diagram showing an Example 4; 
         FIGS. 18   a  and  18   b  are front view as observed from the direction of arrow mark X 1  in  FIG. 17 ; 
         FIG. 19  is an enlarged view showing the layout along the height of the roller  431 ; 
         FIG. 20  is a schematic diagram showing an example of the sheet P flapping; and 
         FIG. 21  is a diagram showing an Example 5. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes the embodiments of the present invention with reference to figures, without the present invention being restricted thereto. 
     In the first place, the following describes an example of the image forming apparatus in the present invention with reference to the configuration diagram of  FIG. 1 : 
     The present image forming apparatus is made up of an image forming apparatus body GH and an image reading device YS. The image forming apparatus body GH is what is commonly called the tandem type color image forming apparatus, and includes a plurality of sets of image forming sections  10 Y,  10 M,  10 C and  10 K, belt-shaped intermediate transfer belt  5 , sheet feed and conveying means and fixing device  8 , and others. 
     An automatic document feeder  201 , and an image reading device YS made up of a document image scanning/exposure device  202  are mounted on the upper part of the image forming apparatus body GH. The document d placed on the document platen of the automatic document feeder  201  is conveyed by the conveying means. The image on side or both sides of the document is scanned and exposed by the optical system of the document image scanning/exposure device  202 , and is read into the line image sensor CCD. 
     The signal formed by photoelectric conversion by means of a line image sensor CCD is subjected to analog processing, analog-to-digital conversion, shading correction, image compression and other processing in the image processing section, and is then sent to the exposure means  3 Y,  3 M,  3 C and  3 K. 
     The image forming section  10 Y for forming a yellow (Y) image includes a charging means  2 Y, exposure means  3 Y, development means  4 Y and cleaning means  7 Y arranged around the photoreceptor drum  1 Y. The image forming section  10 M for forming a magenta (M) image has a charging means  2 M, exposure means  3 M, development means  4 M and cleaning means  7 M arranged around the photoreceptor drum  1 M. The image forming section  10 C for forming a cyan (C) image has a charging means  2 C, exposure means  3 C, development means  4 C and cleaning means  7 C arranged around the photoreceptor drum  1 C. The image forming section  10 K for forming the black (K) image has a charging means  2 K, exposure means  3 K, development means  4 K and cleaning means  7 K arranged around the photoreceptor drum  1 K. The latent image forming means is formed by the charging means  2 Y, exposure means  3 Y, charging means  2 M, exposure device  3 M, charging means  2 C, exposure means  3 C, charging means  2 K and exposure means  3 K. 
     The development units  4 Y,  4 M,  4 C and  4 K includes a two-component developer made of small-diameter toner particles and carriers of yellow (Y), magenta (M), cyan (C) and black (K) colors. The toner is formed of a pigment or dye as a coloring reagent, and a wax that assists separation of the toner from a fixing member subsequent to fixing, as well as the binder resin combining these two substances. 
     The intermediate transfer belt  5  is applied around a plurality of rollers, and is rotatably supported. 
     The fixing device  8  fixes the toner image in position in the process wherein the toner image of the sheet P is heated and pressed by a nip section formed between the fixing belt  81  as a heated fixing member and a pressure roller  83  as a pressure member. 
     Thus, the multi-colored image formed by the image forming sections  10 Y,  10 M,  10 C and  10 K is sequentially transferred onto the rotating intermediate transfer belt  5  by the transfer units  6 Y,  6 M,  6 C and  6 K (primary transfer), and a toner image made by superimposition of color images is produced. The sheet P incorporated in a sheet feed cassette  20  is fed by the sheet feed means  21  is conveyed to the transfer means  6 A through the sheet feed rollers  22 A,  22 B,  22 C and  22 D, and the registration roller  23  and others, and a color image is transferred onto the sheet P (secondary transfer). The sheet P with the color image having been transferred thereto is heated and pressed by the fixing device  8 , whereby the color toner image of the sheet P is fixed in position. After that, the sheet is sandwiched by the sheet ejection rollers  24  and is placed on the sheet ejection tray  25  outside the apparatus. 
     In the meantime, the color image is transfer onto the sheet P by the transfer means  6 A. After that, residual toner is removed by a cleaning means  7 A from the intermediate transfer belt  5  by which the sheet P has been curvature-separated. 
     The aforementioned description has referred to an image forming apparatus for forming a color image. However, it can be an image forming apparatus for forming a monochromatic image. Further, use of the intermediate transfer belt is not essential. 
     Further, the fixing device  8  can be designed in a heating roller fixing type configuration wherein the fixing member uses a roller equipped with a heating member. 
     Referring to drawings, the following describes the fixing device  8  of the present invention: 
       FIG. 2  is a cross sectional view representing an example of the fixing device (belt fixing device) in the present invention, and shows the configuration of the Example 1 to be described later. The following description uses an example wherein image forming operation is applied to one hundred A4-sized sheets P per minute. 
     The fixing belt  81  (fixing member) is formed in an endless shape. For example, PI (polyimide) having a thickness of 70 μm is used as a substrate, and the outer peripheral surface of the substrate as an elastic layer is coated with a heat resistant silicone rubber (hardness JIS-A30°) having a thickness of 200 μm Further, the heat-resistant resin having a thickness of 30 μm is coated with PFA (tetrafluoro alkoxy). The external diameter is 168 mm, for example. 
     The heating roller  82  incorporates a halogen heater  82 A as a heating member for heating the fixing belt  81 . For example, the outer peripheral surface of the aluminum-made cylindrical cored bar  82 B having a wall thickness of 4 mm is covered with the resin layer  82 C coated with 30 μm thick PTFE. The external diameter is 90 mm, for example. It should be noted that, to cope with different paper widths, the halogen heaters  82 A include, for example, two 1200-watt heaters, two 750-watt heaters and one 500-watt heaters. These heaters are arranged to provide distribution of heat generations different in the direction of shaft in order to meet different widths of sheets. 
     The fixing roller  83  is coated with a resin layer  83 C wherein the solid cored bar  83 A formed of such a metal as iron is covered with a 20 mm-thick heat resistant silicone rubber (hardness JIS-A5°) as an elastic layer  83 B and is further coated with the 30 μm-thick PTFE heat-resistant resin of low friction. The outer diameter is 90 mm, for example. 
     The pressure roller  84  (pressure member) incorporates a halogen heater  84 A to reduce the temperature rise time immediately after turning on power for the image output device. The outer peripheral surface of the 4 mm-thick cylindrical cored bar  84 B formed of aluminum or the like is covered with a 1 mm-thick heat resistant silicone rubber (hardness JIS-A30°) as an elastic layer  84 C, and is further coated with the resin layer  84 D of 30 μm thick PFA tube. The outer diameter is 90 mm. The halogen heater  84 A is a 700 watt heater, for example. 
     A pressure means (not illustrated) allows the pressure roller  84  to press the fixing roller  83  through the fixing belt  81 . 
     In the aforementioned configuration, when the pressure roller  84  is rotated in the counterclockwise direction by the drive means (not illustrated), the fixing belt  81  and heating roller  82  rotates in the clockwise direction, and the fixing roller  83  also rotates in the clockwise direction. The fixing roller  83  can be driven. The fixing belt  81  is heated by the halogen heater  82 A through the abutting heating roller  82 , and the pressure roller  84  is also heated by the halogen heater  84 A. Since the pressure roller  84  is pressed in the direction of the fixing roller  83  by the pressure means, the sheet P having been fed is heated and pressed in the nip section N formed between the fixing belt  81  wound on the fixing roller  83  and the pressure roller  84 , whereby the toner image is fixed on the sheet P. 
     The following describes the examples of fixing conditions: 
     Fixing load: 2500 N 
     Fixing belt tension: 250 N 
     Fixing belt control temperature: 160 through 200 degrees Celsius 
     Pressure roller control temperature: 80 through 120 degrees Celsius 
     Sheet conveying speed: 500 mm/s 
     Any heating member can be used to heat the fixing belt  81 . For example, it is possible to employ an inductive heating element using an exciting coil. Further, the position for installing the heating member is not restricted to a space within the heating roller  82 . 
     It is also possible to provide a tension roller for providing the fixing belt  81  with tension, and an offset correction roller for controlling the meandering of the belt. 
     If the sheet P having been fixed in position by the aforementioned fixing device  8  is ejected from the nip section N, and sticks to the fixing belt  81  and winds the same thereafter, a paper jam may occur. This requires a positive means to be taken to separate the sheet P from the fixing belt  81 . 
     The fixing device  8  of the embodiment of the present invention uses a pneumatically driven separation member wherein air is blown to the outlet side of the nip section N, and the sheet is separated from the fixing belt  81 . 
     The following describes the pneumatically driven separation member. 
       FIG. 3  is an enlarged view showing the vicinity of a sheet outlet of a nip section N in  FIG. 2 . 
     The blower duct  121  has an outlet  121   a  opening toward the vicinity of the sheet outlet side of the nip section N, and opening  121   c  connected to the outlet  123   a  of the fan  123  as a blower menber. 
     The blower duct  121  is arranged in such a way that the outlet  121   a  will be located at a distance of 25 mm from the nip section N outlet, and the extension of the blower duct  121  will be directed toward the outer peripheral surface of the fixing belt  81  located 10 mm from the nip section N outlet in the rotating direction, from the tangential direction. 
     The outlet  121   a  of the blower duct  121  and the opening  121   c  have a length of about 100 mm. The opening  121   c  and outlet  123   a  of the fan  123  are directly coupled. 
     The fan  123  discharges air from the outlet  121   a  toward the vicinity of the sheet outlet of the nip section N through the blower duct  121  on a continuous basis. The air is blown to the sheet P to separate the sheet P from the fixing belt  81 . The arrow mark b of  FIG. 3  indicates the flow of air to separate the sheet P. The arrow mark e indicates the flow of air after having separate the sheet P. Namely, it shows an exhaust gas flow. 
     A sirocco fan, for example, is preferably used as the fan  123  because of greater static pressure, without the fan  123   b  being restricted thereto. 
     The fan  123  of this embodiment is a sirocco fan having an outer diameter of 97 mm and a width of 33 mm, with a power of 39 W and a static pressure of 1280 Pa. 
     In this embodiment, one blower duct  121  is connected to one fan  123  to form one set. Five sets thereof are arranged across the width of the sheet P (in the longitudinal direction of the nip section N). Namely, five outlets  121   a  of the blower ducts  121  are arranged continuously in a straight line. 
       FIG. 4  is a top view showing a layout example of a blower duct  121  and a fan  123 , as observed from the arrow mark Y 1  of  FIG. 2 . 
     The outlet  121   a  has an opening which measures approximately 60 mm across the width of the sheet P and 1.6 mm along the thickness of the sheet (in the vertical direction of  FIG. 2 ). Accordingly, five blower ducts  121  have an opening with a width of 300 mm and a height of 1.6 mm. The total volume of air discharged from the outlet  121   a  amounts to 0.016 m 3 /s. 
     In the image forming operation, the sheet P is fed out from the registration roller  23  ( FIG. 1 ). At the same time, power is applied to the fan  123  so that air is fed to assist separation of the sheet from the blower duct  121 . In the vicinity of the N outlet, an air flow with a velocity of 30 m per second is formed from the image surface side to the non-image surface side. 
     The sheet P having been separated from the fixing belt  81  is conveyed while being held at the side of the pressure roller  84  by the air flow, and is separated from the pressure roller  84  by the separation claw  86  having a tip end width of 12 m arranged in contact with the pressure roller  84  under a load of about 10 mN. The separation claw  86  is formed of a heat resistant resin with its surface coated with fluorine resin. The abutting load of the separation claw  86  depends on the relationship between the width and material of the claw, and the material of the surface of the pressure roller  84 . The abutting load is set without allowing the pressure roller  84  to be damaged. 
     The separation claw having been used in the conventional fixing device based on electrophotographic process can be employed as the separation claw  86 . Further, approximately 3 through 8 separation claws  86  are installed at prescribed space intervals, with consideration given to the rigidity of the transfer material, and configuration of the fixing device. 
     The sheet ejection guide plate  85  is composed of a great number of ribs or smaller rollers arranged in parallel to the sheet conveying direction below the sheet conveyance path. This arrangement prevents the ejected sheet P from sticking to the sheet ejection guide plate  85 , and avoids paper ejection failure. A pair of sheet ejection rollers  88  is provided on the downstream side in the conveying direction of the sheet ejection guide plate  85 . 
     Since the pressure roller  84  is kept at such a temperature that the toner is not melted (90 through 110 degrees Celsius in this Example), the image is immune to any damage that may be caused by the separation claw  86 , even if an image is located on the side of the pressure roller  84 . To keep the pressure roller  84  at a low temperature, a distance between transfer and fixing positions in excess of the maximum length of the sheet is provided, and the sheet-to-sheet distance is reduced in the range from transfer and fixing positions, thereby reducing the amount of heat transfer from the fixing belt  81  to the pressure roller  84  or cooling the inner periphery and outer periphery of the roller by a fan. 
     Incidentally, if air is blown by the fan  123  on a continuous basis, the blown air separates the sheet P from the fixing belt  81 . After that, the air is repelled by the nip section N, whereby an eddy and a turbulent flow are created. This will cause the sheet P to flap and will disturb the conveyance of the sheet P.  FIG. 20  is a schematic diagram showing an example of flapping. The flapping will cause a conveyance trouble such as a paper jam. 
     In the present invention, a guide device for guiding the sheet P and a biasing member for biasing the sheet P toward the guide device are provided. When the sheet P is biased toward the guide device by the biasing member, the flapping of the sheet P is reduced. 
     EXAMPLE 1 
       FIG. 2  shows Example 1, as described above. 
     The present inventors conducted a test (Test 1) to verify the flapping. The following describes the result of this test: 
     &lt;&lt;Test 1&gt;&gt; 
       FIGS. 5 through 8  shows the Test 1 conducted to verify the relationship between the free length of the sheet P (L 1  of  FIG. 5 ) and flapping. The free length of the sheet P in the sense in which it is used here refers to the distance from the tip end of the nipped sheet P in the sheet conveying direction to the outlet of the nip section N. 
       FIG. 5  is a diagram showing the configuration of the equipment used in Test 1. The nip section N and blower duct  121  used in this test has the same configuration as that of Example 1. The wind velocity in the vicinity of the nip section N outlet is approximately 30 m per second. 
     When the fixing belt  81  was stopped, the sheet P was sandwiched and air was blown from the blower duct  121  so as to change the free length L 1 , whereby the occurrence of flapping was checked. 
     In the first place, a laser displacement meter (LB-01 by Keyence Corp.) LS was placed at a prescribed position on the downstream side of the nip section N in the sheet conveying direction, and the vibration of flapping of the sheet P was measured at a prescribed position. The amplitude of the sheet P was frequency-analyzed at the measured output (voltage (V)), using the FFT analyzer (CF6400 by Ono Sokki Co., Ltd.) (not illustrated) connected to the laser displacement meter. 
       FIG. 6  and  FIG. 7  show how to analyze the flapping. Since the amplitude is formed of a superimposition of various forms of vibration modes, the data having been obtained ( FIG. 6 ) is subjected to Fourier transformation, whereby amplitude Af in each frequency was calculated ( FIG. 7 ). The calculated amplitudes at various frequencies are added in the range from 0 through 100 Hz, to get the amplitude overall value ΣA (0 through 100 Hz). This value is used to evaluate the flapping. 
       FIG. 8  is a diagram showing the result of Test 1 (the relationship between the free length of sheet P and flapping). As the free length L 1  increases, the amplitude overall value increases. In the configuration of the present Example, if the free length has exceeded 150 mm, i.e., if the free length increases over the broken line of  FIG. 8 , the flapping increases when the lower surface of the blower duct  121  is brought in contact. 
     The above study has demonstrated that controlling the flapping of the sheet P before the free length reaches 150 mm is an effective solution. 
     Based on the result of Test 1, an upper guide plate  221  as a guide device on the upper side of the sheet P along the conveyance path (on image surface side) is provided on the lower surface of the blower duct  121  ( FIG. 2 ) in Example 1. 
       FIG. 9  is a front view of an upper guide plate  221  as viewed from the direction of arrow mark X 1  in  FIG. 2 . The upper guide plate  221  has a plurality of ribs  222  as guide members on the side in contact with the sheet P. 
     As shown in  FIG. 2 , the ribs  222  are arranged in such a way as to expand at an angle of θ with reference to the upper guide plate  221  from the position of 50 mm, i.e., from the outlet  121   a  toward the downstream side of sheet conveyance, i.e., in the flow direction of the exhaust gas. The ribs  222  are configured to form a triangle with a length of 100 mm or more to ensure that the tip end of the sheet ejection guide  86  contacts the upper surface of the rib  222 . Namely, ribs  222  are configured in such a way as to expand at an angle of θ from the lower surface of the blower duct  221 . Thus, the upper guide plate  221  is configured to have grooves  230  which are each enclosed by two ribs  222 , and which communicate with each other in the sheet P conveying direction. The sectional area of the groove  230  increases in the direction of exhaust gas flow. 
     Based on the specifications and configuration of the particular fixing device for each model, the angle θ is determined in the Test (Test 2 to be described later). As shown in  FIG. 9 , ten ribs  222  are uniformly arranged at intervals of 30 mm in the direction across the width of sheet P from the sheet conveyance center. Each rib has a width of 2 mm, and the tip end thereof is provided with roundness R 1  for the purpose of increasing the contact area with the sheet. The sheet P at the portion in contact with the rib  222  deflects along the tip end. Thus, the contact area with the sheet can be practically increased by the tip ends provided with roundness R 1 . This leads to a reduced pressure for unit area, and hence prevents an image fem being deteriorated by abrasion. Further, the surface is provided with fluorine coating to minimize the frictional drag with respect to the sheet P and to prevent toner or wax from being attached. 
     The air blown from the outlet  121   a  toward the outlet of the nip section N is repelled by the nip section N outlet and is dispersed to become an exhaust gas flow. The exhaust gas flowing in the vicinity of the lower surface of the blower duct  121  flows away from the outlet of the nip section N, namely, toward the downstream side of sheet conveyance. Thus, the groove  230  constitutes a part of the exhaust gas flow path for the exhaust gas that flows in the vicinity of the lower surface of the blower duct  121 . 
     In the present invention, the sheet P ejected from the nip section N is biased toward the rib  222  by the biasing member located in the vicinity of the nip section N, and is conveyed under the guidance of the rib  222 . 
     In the present invention, the biasing member is made of a plurality of exhaust gas flow paths which are formed of the e aforementioned groove  230  and the sheet P abutting on the rib  222 , wherein the exhaust air is discharged to the vicinity of the sheet outlet and is ejected in the sheet conveying direction. The biasing member is intended to bias the sheet P toward the rib  222  by member of the exhaust gas flowing along the exhaust gas flow path. 
     The following describes the biasing by the exhaust gas flow: 
       FIG. 10  is a diagram showing the air flow and sheet P conveyance path. 
     The sheet P separated from the pressure miler  84  by the separation claw  86  is separated from the blower duct  121  in the vicinity of the outlet of the nip section N by a turbulent flow or eddy caused by blowing of air. 
     As sheet ejection proceeds, the sheet P is separated from the fixing belt  8 , and starts to be raised from the tip end toward the lower surface of the blower duct  121  under the influence of the flow of air repelled by the nip section N, namely, the exhaust gas flow. When the sheet P has reached the rib  222  of the upper guide plate  221  provided on the lower surface of the blower duct  121 , the exhaust gas flows through an exhaust gas flow path provided by the groove  230  which is formed through the rib  222  between the sheet P and blower duct  121  and whose sectional area expands as one goes to the further downstream side. This causes a negative pressure to occur on the side of the exhaust gas flow path, with the result that the sheet P is biased toward the rib  222 , namely, guide plate  221 . 
     According to the aforementioned Test 1, if the length of the sheet P ejected from the nip section N (free length) is below a prescribed value, the sheet P is pressed against the sheet ejection guide  85  by air flow. Flapping occurs only when the length has reached a prescribed value. When the toner is cooled before flapping occurs, an image can be prevented from being deteriorated by the abrasion with the guide of the rib  222 . This free length was about 150 mm in the present Example. 
     The following describes the test (Test 2) for determining the angle θ of the rib  222 : 
     &lt;&lt;Test 2&gt;&gt; 
       FIG. 11  is a diagram showing the configuration of the equipment used in Test 2 conducted to verify the effect of forced absorption of the sheet P toward the rib  222  by the negative pressure of exhaust gas flow. Conditions are the same as those in Test 1, except that the lower surface of the blower duct  121  is provided with the upper guide plate  221 . The procedure for measuring the flapping is also the same as that of Test 1. 
     The upper guide plate  221  is arranged at an angle of θ from the position 50 mm away from the outlet  121   a  on the downstream side in the sheet conveying direction, a triangle is formed wherein the length of the rib  222  is 100 mm. 
     To measure the flapping, the angle θ of the rib  222  was adjusted, and the sheet P was nipped and conveyed by the fixing belt  81  pressure roller  84 . Thus, the flapping with respect to each angle θ was measured. 
       FIG. 12  shows the result of the Test 2. When the angle does not exceed 3 degrees, there is a greater impact of the turbulent flow or eddy caused by blowing from the outlet of the nip section N, despite the increased negative pressure. This increases the amount of flapping. If the angle exceeds approximately 3 degrees (toward the angle greater than that indicated by the broken line in  FIG. 12 ), the impact of the turbulent flow or eddy is reduced, with the negative pressure kept intact, to the point wherein the image is not damaged by the flapping. However, if the angle is excessive, the sheet P will be forcibly pressed against the rib  222  because of a high rigidity of the sheet P when thick paper is passed through the machine. This may lead to a heavy abrasion of the sheet P, and hence damage of the sheet P. 
     Based on the result of Test 2, the angle of the rib is set at 7 degrees in Example 1. 
     Thus, before the length of the sheet P ejected from the nip section N reaches 150 mm, the sheet P is biased toward the rib  222 , and is conveyed under the guidance of the rib  222 . 
     This arrangement ensures the flapping of the sheet P to be reduced. Thus, sheet deformation or paper jam can be prevented, and stable sheet conveyance is provided. Further, image deterioration can be avoided. 
     EXAMPLE 2 
       FIG. 13  is a diagram showing an Example 2.  FIG. 14  is a front view as observed from the direction of arrow mark X 1  in  FIG. 13 . 
     Example 2 shows a modified guide device. A wire net  323  is provided on the plane composed of the tip end of the rib  222  of Example 1, whereby an upper guide plate  321  is formed. Otherwise, the specifications are the same as those of Example 1. 
     The wire net  323  is a member that causes the sheet P to be biased or that guides the sheet P. If the wire net  323  has an excessively small or large mesh, the exhaust gas flow is disturbed. Thus, selection is made not to disturb the exhaust gas flow. 
     In the Example 2, the wire net  323  uses a stainless steel wire net (a wire diameter of 01 mm with 65 meshes per inch). To minimize the frictional drag with respect to the sheet P and to prevent the toner or wax from adhering, the surface is provided with fluorine coating. Further, in Example 2, the wire net  323  is supported by ribs  222  bearing the numbers  1 ,  4 ,  7 ,  10  in  FIG. 14 . 
     In the Example 2, the sheet P is supported by the wire net  323 , whereby the number of the contact points us practically increased, and the force for each contact point can be reduced. This arrangement prevents the image from being deteriorated by abrasion. 
     EXAMPLE 3 
       FIG. 15  is a diagram showing an Example 3. 
     In the Example 3, the endless guide belt  421  which is applied to a plurality of rollers and is rotated by the drive member (not illustrated) as a guide member for biasing the sheet P is arranged on the lower surface of the blower duct  121 . Otherwise, the specifications are the same as those of the Example 1. 
     The guide belt  421  is formed by connecting the ends of the stainless steel wire net (a wire diameter of 0.1 mm with 65 meshes per inch). This Example uses a stainless steel wire net, without the present invention being restricted thereto. For example, a rubber such as an ethylene propylene (EPDM) rubber can be used. Further, the surface is provided with fluorine coating to minimize the frictional drag with respect to the sheet P and to prevent toner or wax from being attached. The belt tension is set at 30 N. 
     In the Example 3, if the guide belt  421  is kept in contact with one and the same position of the sheet P, an image failure such as uneven gloss will result due to the temperature difference between the contact position and non-contact position. Thus, the speed of the guide belt  421  is kept higher than that of the P, without being equal to that of the sheet P. In this Example, the guide belt  421  is operated at 540 mm per second. 
     In the roller  422  to which the guide belt  421  is applied, air flow may be disturbed if the all-surface roller is used. Thus, a comb-shaped silicone rubber roller is used, as shown in  FIG. 16 . This arrangement provides a space for the exhaust gas to flow, and allows a sufficient negative pressure to be produced. This ensures the sheet P to be conveyed while being biased toward the guide belt  421 . 
     For the sheet P wherein the basis weight is up to 80 g/m 2  or thereabout, the configurations of the aforementioned Examples 1 and 2 meet requirements without any problem. However, under the conditions wherein the basis weight of the sheet P is further reduced, the sheet P may be pressed against the guide members (rib  222  and wire net  323 ), and an image failure may occur to the image surface. To cope with such a situation, the Example 3 provides more effective solution. 
     EXAMPLE 4 
       FIG. 17  is a diagram showing an Example 4.  FIG. 18  is a front view as observed from the direction of arrow mark X 1  in  FIG. 17 .  FIG. 18   a  indicates the cross section BB. 
     In the Example 4 as in the Example 1, ribs  222  are provided, and a rotatably supported roller  431  is provided on the ribs  222  80 mm away from the outlet  121   a.    
     As shown in  FIG. 18   a , one roller  431  is installed at the center, and then others are placed at intervals. A total of five rollers are installed.  FIG. 18   b  shows an example of the configuration of the roller  431 . A surface layer characterized by excellent release characteristics is formed on the surface of the roller  431 . In the present Example, the substrate uses the SUS 303, and the surface layer is provided with bead coating (MRG 1090 by Mizoguchi Kogyo Co., Ltd.). 
       FIG. 19  is an enlarged view showing the layout along the height of the roller  431 . The roller  431  is installed so that the center thereof will be flush with the plane formed by the tip end of the rib  222 . Namely, the roller is installed in such a way that the dimension d 1  of  FIG. 19  will be half the diameter of the roller  431 . In the present Example, d 1 =3.6 mm. 
     The roller  431  is supported by inserting the shaft  431   a  of the roller  431  into the roller supporting section  222   a  provided on the rib  222 . It should be noted that the method of supporting the roller  431  and dimension d 1  are not restricted to the aforementioned examples. 
     This roller  431  ensures smooth conveyance of the recording sheet and prevents the image from being deteriorated by abrasion of the guide. Further, similarly to the case of Example 3, greater efficiency is provided when the basis weight of the sheet P is further reduced from 80 g/m 2 . 
     EXAMPLE 5 
       FIG. 21  is a diagram showing an Example 5. 
     In the present Example, the separation member includes a first separation member made up of a fan  123  and blower duct  121  and a second separation member made up of compressor (not illustrated), a solenoid valve (not illustrated), a blower tube (not illustrated) and nozzle  521 . The first separation member has the same configuration as that of the Example 1. Namely, the Example 5 consists of the configuration of Example 1, plus the second separation member. 
     The nozzle  521  of the second separation member is installed between the blower duct  121  and fixing belt  81 , and blows air to the vicinity where the sheet separation is installed. 
     The following describes the details of the compressor, solenoid valve and nozzle: 
     Compressor: 0.75 kW, 0.8 MPa, 0.00125 m 3 /s (reciprocation type/oil free), accumulator tank capacity; 0.05 m 3    
     Direct acting type solenoid valve: capacity 0.001 m 3 /s (100 kPa), response speed: up to 20 ms; quantity: 2 
     Separation assisting nozzle: nozzle diameter 0.8 mm, quantity 130 (2.5 mm pitch) 
     Nozzle blow outlet position: 25 mm from the outlet of the nip section N 
     The configurations of the upper guide plate  221  as a guide device and the separation claw  86  are the same as those of Example 1. 
     In the image forming operation, the sheet P is fed out of the registration miler  23  ( FIG. 1 ). At the same time, power is supplied to the fan  123 , and air is blown from the blower duct  121  to assist separation of the sheet P. To put it another way, air starts to come out of the first separation member. 
     Synchronously with the leading edge of the sheet P being ejected from the nip section N, the operation of the second separation member starts. Air is blown from the nozzle  521  to separate the leading edge of the sheet P from the fixing belt  81 . In the present Example, the second separation member of higher air pressure is used to separate the leading edge of the sheet P characterized by greater power of adsorption to the fixing belt  81 . In the meantime, after the leading edge of the sheet P has been separated, the first separation member of smaller air pressure is utilized. This technique ensures positive separation of the sheet P and allows the fixing performance and sheet conveying performance to be maintained without being deteriorated. 
     In the present Example, flapping occurs when the second separation member is stopped and air is blown only by the first separation member. This provides the same effect as that of Example 1, using the same configuration as that of Example 1.