Patent Publication Number: US-2002001476-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO THE RELATED APPLICATION  
     [0001] “This is a continuation of PCT Application No. PCT/JP99/00555, filed Feb. 9, 1999.” 
    
    
     
       TECHNICAL FIELD  
       [0002] The present invention relates to an image forming apparatus having a fixing unit in which an image formed by at least one print assembly is transferred and fixed to a paper sheet.  
       BACKGROUND ART  
       [0003] An image forming apparatus of an electrostatic recording type comprises a print assembly including a photosensitive member (photosensitive drum), a charger, an optical head for forming an electrostatic latent image on the photosensitive member, a developing unit for converting the electrostatic latent image into a toner image and a transfer element (transfer roller) for transferring the toner image to the paper. The charger, the optical head, the developing unit and the transfer element are arranged around the photosensitive member to form a charged toner image while the photosensitive member makes one rotation. The charged toner image is transferred from the photosensitive member to the paper by the transfer element, and the paper, after the image is fixed in the fixing unit arranged behind the print assembly, is delivered.  
       [0004] An image forming apparatus is available which is capable of multicolor recording and comprises a plurality of print assemblies arranged in series and one fixing unit. In the case where developers of four colors including yellow, magenta, cyan and black are used, for example, the image forming apparatus comprises four print assemblies each containing one developer. The toner image formed by the print assembly of each color is superposed on the paper conveyed on a paper conveyor belt and converted into a color image.  
       [0005] The image forming apparatus further comprises a top cover and an end cover movably mounted on a frame. The top cover is arranged to cover a plurality of print assemblies. By opening the top cover, the component members of the print assemblies arranged in the frame can be repaired or replaced. The end cover, on the other hand, is arranged to cover the fixing unit. By opening the end cover, the fixing unit can be exposed.  
       [0006] Especially in a color image forming apparatus, the fixing unit is required to fix the developers of four colors, and therefore as compared with a monochromatic image forming apparatus, requires strict heating and pressing conditions. Even when the jamming of paper occurs in the fixing unit, the paper jammed in the fixing unit can be removed by opening the end cover. Once the paper has been wound around the fixing rollers, however, it is necessary to remove the fixing unit from the frame of the image forming apparatus and remove the paper from the fixing roller pair by opening the nip of the fixing rollers.  
       [0007] The fixing unit is secured to the frame by screws, and for the fixing unit to be removed from the frame, the screws must be removed. The removing of the screws requires a tool such as a screw driver. It is troublesome for the operator of the image forming apparatus to use a tool. Further, after removing the paper, it is necessary to mount the fixing unit on the frame and secure it on the frame by fastening the screws. In such a case, it is sometimes forgotten to tighten the screws. The job of removing the fixing unit from the frame for removing the paper wound around the fixing roller and subsequently remounting it on the frame can be comparatively troublesome.  
       [0008] On the other hand, the paper is conveyed to the fixing unit by a paper conveyor belt, and a paper guide is arranged at the inlet of the fixing unit. The fixing rollers of the fixing unit are driven by a motor other than the drive motor for the paper conveyor belt, and the paper is conveyed by the fixing rollers of the fixing unit at a rate lower than that of the paper conveyor belt. Thus, the paper is caused to curve upward with respect to a straight line connecting the outlet end of the paper conveyor belt and the nip of the fixing rollers. The input paper guide is protruded upward slightly from the straight line connecting the outlet end of the paper conveyor belt and the nip of the fixing rollers to cause the paper to curve upward. By causing the paper to curve upward, the paper is curved in a cylindrical form so that rigidity is given to the paper and the paper is prevented from wrinkling.  
       [0009] In the case where the paper is curved between the outlet end of the paper conveyor belt and the nip of the fixing rollers, however, the rear end of the paper is released into a free state suddenly after leaving the outlet end of the paper conveyor belt, often with the result that the rear end of the paper jumps up. The rear end of the paper, once it has jumped up, may be fouled by rubbing against the upper paper suction guide and the print assembly. The amount of jumping up of the rear end of the paper increases with the paper thickness. Especially, the rear end of a thick paper sheet jumps up to a large extent.  
       [0010] Further, an oil supply felt is arranged upstream of the nip of the upper one of the fixing rollers, while a cleaning felt is arranged downstream of the nip. The oil supply felt is for coating oil on the fixing roller to prevent the paper from sticking to the fixing roller, while the cleaning felt is for scraping off the toner attached to the fixing rollers. The oil must be supplied uniformly over the width of the fixing rollers by the oil supply felt so that the toner can be scraped off efficiently, by the cleaning felt, over a long time. Most of the toner is attached and scraped off at the forward end of the cleaning felt, after which the cleaning efficiency and the service life of the cleaning felt are reduced.  
       SUMMARY OF THE INVENTION  
       [0011] An object of the present invention is to provide an image forming apparatus in which the fixing unit can be easily mounted and demounted.  
       [0012] Another object of the invention is to provide an image forming apparatus in which the paper is not fouled.  
       [0013] Still another object of the invention is to provide an image forming apparatus in which the cleaning unit can scrape off the toner from the fixing unit efficiently over a long time.  
       [0014] An image forming apparatus according to the invention comprises a frame, at least one print assembly capable of forming an image, a fixing unit including a pair of fixing rollers for holding and conveying a paper sheet, a nip control mechanism for moving the pair of the fixing rollers between a nip position and a non-nip position, and fixing unit securing means movable with the nip control mechanism for securing and releasing the fixing unit to and from the frame, and paper sheet conveyor means for conveying the paper sheet to the fixing unit through the print assembly.  
       [0015] In this configuration, the fixing unit is fixed to the frame by the fixing unit securing means which is movable with the nip control mechanism. The fixing unit, when demounted from the frame or mounted on the frame, therefore, is automatically secured (locked) or released (unlocked) by the operation of the nip control mechanism.  
       [0016] Preferably, at least one print assembly includes a plurality of print assemblies each including a photosensitive member, a charger for charging the photosensitive member, latent image forming means for forming an electrostatic latent image on the photosensitive member, a developing unit for developing the latent electrostatic image, and a transfer element for transferring the developed toner image to the paper sheet, wherein the latent image forming means is mounted on a top cover which is movably mounted on the frame to cover the print assembly.  
       [0017] Preferably, the nip control mechanism moves with the end cover when the end cover moves from the open position to the close position.  
       [0018] Preferably, the frame has a positioning pin inserted into a hole of the fixing unit for positioning the fixing unit, and the fixing unit securing means includes a lock member adapted to engage the positioning pin in cooperation with the nip control mechanism.  
       [0019] Preferably, the nip control mechanism includes a manually operable level, the lock member has a cam plate movable with a lever, and the positioning pin has a small-diameter portion adapted to engage the cam plate.  
       [0020] According to another feature of the invention, an image forming apparatus comprises a frame, at least one print assembly capable of forming an image, a pair of fixing rollers for holding and conveying a paper sheet, an inlet-side paper guide arranged to be movable vertically with respect to a paper passage in accordance with a thickness of the paper sheet for guiding the paper sheet between the fixing rollers, and a paper sheet conveyor belt for conveying the paper sheet to the fixing unit through the print assembly and the inlet-side guide.  
       [0021] In this configuration, the inlet-side paper guide is vertically movable, and therefore the paper is prevented from jumping up when the rear end thereof leaves the paper sheet conveyor means.  
       [0022] Preferably, the inlet-side paper guide is composed of an elastic member which is flexible in accordance with a thickness of the paper sheet.  
       [0023] Preferably, the image forming apparatus comprises means for moving the inlet-side paper guide in accordance with the kind of the paper.  
       [0024] According to still another feature of the invention, an image forming apparatus is characterized by comprising a frame, at least one print assembly capable of forming an image, a fixing unit including a pair of fixing rollers for holding and conveying a paper sheet, a paper sheet conveyor belt for conveying the paper through the print assembly and the fixing unit, and means capable of changing a speed of the fixing rollers relative to the speed of the paper conveyor belt in accordance with the type of the paper.  
       [0025] In this configuration, the paper is prevented from curling up when the rear end of the paper leaves the paper conveyor belt by changing the speed of the fixing rollers.  
       [0026] According to a further feature of the invention, an image forming apparatus is characterized by comprising a frame, at least one print assembly capable of forming an image, a fixing unit including a pair of fixing rollers for holding and conveying a paper sheet, and a cleaning member having at least two stripes of cleaning felt arranged so that contact pressures of the two strips of cleaning felt against the fixing roller are changed in the rotational direction of the fixing roller to clean the surface of the fixing roller, and paper conveyor means for conveying the paper sheet through the print assembly and the fixing unit.  
       [0027] In this configuration, the contact pressure of at least two stripes of felt can be changed, and therefore the fouling is not concentrated on one of the cleaning felts, thereby making it possible to scrape off the toner efficiently over a long life. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0028] The present invention will be described in detail with reference to the embodiments shown in the accompanying drawings, in which:  
     [0029]FIG. 1 is a schematic side view showing an image forming apparatus according to an embodiment of the invention;  
     [0030]FIG. 2 is a schematic side view showing a part of the image forming apparatus of FIG. 1, with the end cover thereof in an open position;  
     [0031]FIG. 3 is a plan view of the image forming apparatus of FIG. 1;  
     [0032]FIG. 4 is a plan view showing a part of the image forming apparatus of FIG. 1, with the end cover omitted;  
     [0033]FIG. 5 is a partial sectional view of the apparatus, showing the positioning pin provided on the support member of the frame and the fixing unit mounting member;  
     [0034]FIG. 6 is a side view showing the print assembly of FIG. 1;  
     [0035]FIG. 7 is a schematic side view showing a part of the paper sheet conveyor belt and the fixing unit;  
     [0036]FIG. 8 is a rear view of the fixing unit;  
     [0037]FIG. 9 is a plan view of the fixing unit;  
     [0038]FIG. 10 is a plan view showing a part of the fixing unit, with the cover omitted;  
     [0039]FIG. 11 is a diagram showing the fixing unit of FIG. 10 in the nip position;  
     [0040]FIG. 12 is a diagram showing the detailed configuration in the fixing unit;  
     [0041]FIG. 13 is a diagram showing the fixing unit of FIG. 12 in the nip position;  
     [0042]FIG. 14 is a sectional view of the oil supply unit;  
     [0043]FIG. 15 is a plan view showing the oil supply felt of FIG. 14;  
     [0044]FIG. 16 is a plan view showing the cleaning unit of FIG. 12;  
     [0045]FIG. 17 is a diagram showing an example of the fixing unit having a manually operated paper guide;  
     [0046]FIG. 18 is a diagram showing the fixing unit with the paper guide of FIG. 17 at a lower position;  
     [0047]FIG. 19 is a plan view showing the fixing unit of FIG. 17;  
     [0048]FIG. 20 is a diagram showing the control panel;  
     [0049]FIG. 21 is a block diagram showing an example of speed control of the fixing rollers;  
     [0050]FIG. 22 is a diagram showing an example of selected values used for speed control of the fixing rollers;  
     [0051]FIG. 23 is a flowchart showing an example of the speed control of the fixing rollers; and  
     [0052]FIG. 24 is a flowchart showing an example of the speed control of the fixing rollers. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     [0053] FIGS.  1  to  3  show an image forming apparatus  10  according to an embodiment of the invention. This image forming apparatus  10  is configured as a full-color printer. The image forming apparatus  10  has a frame  12  on which a top cover  14  and an end cover  16  are movably mounted. FIG. 1 shows the top cover  14  in the closed position and the end cover  14  in the open position. FIG. 2 shows the top cover  14  is in the closed position and the end cover  16  in the closed position. By opening the top cover  14  and the end cover  16 , it is possible to obtain access to the members in the image forming apparatus  10 . When the top cover  14  is opened, therefore, component elements  20  of a print assembly can be repaired or replaced, while by opening the end cover  16 , a fixing unit  32  can be repaired and the paper jam can be removed.  
     [0054]FIG. 3 is a plan view of the image forming apparatus  10  having the top cover  14  and the end cover  16 . The image forming apparatus  10  includes a control panel CP at the right end of FIG. 3. In the drawing, the right end where the control panel CP is located is the front side of the apparatus and the left end is the rear side of the apparatus. The end cover  16  is located at the end remote from the control panel CP. An unlock button  17  for the top cover  14  is located on the front side of the apparatus. The unlock button  17  is operatively coupled to a lock member  60  of FIG. 1, and the lock is released by depressing the unlock button  17  so that the top cover  14  can be opened. FIG. 1 includes a cover lift mechanism  62  urged by a spring, such that when the lock is released, the cover lift mechanism  62  pushes up the top cover  14  by a predetermined amount and thus the top cover  14  can be manually opened.  
     [0055] In FIGS. 1 and 2, the top cover  14  includes a cover portion  14 A and side plate portions  14 B. The portion of the top cover  14  visible in FIG. 3 is the cover portion  14 A, and the side plate portions  14 B extend rearward and downward from the sides of the cover portion  14 A. The side plate portions  14 B of the top cover  14  are pivotally coupled to pivotal support members  14 X located at the rear portion of the frame  12 . The end cover  16  also has a cover portion  16 A and side plate portions  16 B which are pivotally coupled to the pivotal support members  16 X located under the pivotal support members  14 X of the top cover  14 .  
     [0056]FIG. 4 is a plan view showing a part of the frame  12  and the top cover  14  with the end cover  16  omitted. The frame  12  has vertically extending side plate members  12 A. The side plate portion  14 B of the top cover  14  is pivotally coupled to the pivotal support member  14 X arranged on the side plate member  12 A of the frame  12 . Further, the top cover  12  has a connecting portion  14 D arranged vertically at the rear end portion of the cover portion  14 A and coupling the top cover  12  and the end cover  16  to each other. In FIGS. 1 and 2, the connecting portion  14 D includes an L-shaped coupling slit  14 E, and a pin  16 E extending from the side plate portion  16 B of the end cover  16  is inserted in the coupling slit  14 E. Thus, the end cover  16  is opened and closed with the top cover  12 . Specifically, while the top cover  14  moves to a slightly opened position from the closed position, the end cover  16  remains stationary, and as the top cover  14  opens further, the end cover  16  opens with the top cover  14 . In addition, as shown in FIG. 1, the end cover  16  only can be opened independently of the top cover  14 . This is because the pin  16 E of the side plate portion  16 B of the end cover  16  is urged by a spring (not shown) arranged in the side plate portion  16 B to be protruded or retreated from the surface of the side plate portion  16 B. The pin  16 E, when protruded from the surface of the side plate portion  16 B, engages the coupling slit  14 E. When retreated toward the surface of the side plate portion  16 B against the spring force, on the other hand, the pin  16 E comes off from the coupling slit  14 E, so that the end cover  16  and the top cover  14  are decoupled and the end cover  14  can be moved separately.  
     [0057] In FIGS. 4 and 5, the frame  12  further includes support members  12 B inside the side plate member  12 A, the support member  12 B having a vertically extending pin  18 . The support member  12 B is a portion of the frame  12  that supports the fixing unit  32 . The positioning pin  18  and the support member  12 B will be described in detail later.  
     [0058] In FIG. 1, the image forming apparatus  10  comprises four print assemblies  20 B,  20 C,  20 M and  20 Y. An endless paper sheet conveyor belt  22  is arranged for the four print assemblies  20 B,  20 C,  20 M and  20 Y. The paper sheet conveyor belt  22  is formed of an appropriate transparent synthetic resin material and arranged around four rollers  24   a ,  24   b ,  24   c  and  24   d . The roller  24   a  is a drive roller and also functions as an AC neutralizing roller for removing the charge from the paper sheet conveyor belt  22 . The roller  24   b  is a driven roller and also functions as a charging roller for attaching charge to the paper sheet conveyor belt  22 . The rollers  24   c  and  24   d  are both guide rollers. The roller  24   d  is a tension roller for giving an appropriate tension to the paper sheet conveyor belt  22 .  
     [0059] A hopper  26  is arranged under the paper sheet conveyor belt  22 . A bundle of paper sheets is stacked in the hopper  26 . The paper sheets P are supplied, one by one, by a pick roller  28  from the hopper  26 , and transported to the paper sheet conveyor belt  22  by paper feed rollers  30 . A paper sheet P is delivered to the print assemblies  20 B,  20 C,  20 M and  20 Y by the paper sheet conveyor belt  22  and printed or recorded. The paper sheet P thus recorded is conveyed to the fixing unit  32  and discharged into the stacker formed on the upper surface of the top cover  14  through an appropriate guide roller (not shown) arranged on the end cover  16 .  
     [0060] The paper sheet conveyor belt  22  is charged by the driven roller  24   b  and, therefore, the paper sheet P is electrostatically attracted and held by the paper sheet conveyor belt  22  when introduced to the paper sheet conveyor belt  22  from the driven roller  24   b . Thus, the paper sheet P is held in a fixed relative position with respect to the paper sheet conveyor belt  22 . On the other hand, the drive roller  24   a  functions as a neutralizing roller, so the charge is removed and the paper sheet P can be easily released from the paper sheet conveyor belt  22  when passing the drive roller  24   a . The paper sheet P released from the paper sheet conveyor belt  22  proceeds to the fixing unit  32 .  
     [0061] The four print assemblies  20 Y,  20 M,  20 C and  20 B have identical structures. The print assembly  20 Y includes a developer having a yellow toner component, and the print assembly  20 M includes a developer having a magenta toner component. The print assembly  20 C includes a developer having a cyan toner component, and the print assembly  20 B includes a developer having a black toner component. Thus, these print assemblies  20 Y,  20 M,  20 C and  20 B form a full-color toner image by printing a yellow toner image, a magenta toner image, a cyan toner image and a black toner image, respectively, on the paper P held and moved by the paper sheet conveyor belt  22 .  
     [0062]FIG. 6 shows a print assembly  20 Y. The other print assemblies  20 M,  20 C and  20 B also have a similar structure, and therefore only the print assembly  20 Y will be explained in detail. The print assembly  20 Y includes a photosensitive member (photosensitive drum)  36 , which is rotationally driven in the direction of the arrow in FIG. 6. A precharger  38 , an optical head (LED beam scanner)  40 , a developing unit  42 , a transfer element (transfer roller)  44  and a toner cleaner  46  are arranged, in that order, around the photosensitive member  36 .  
     [0063] The precharger  38  arranged above the photosensitive member  36  is configured as a corona charger or a scorotoron charger, for example. The surface of the photosensitive member  36  is sequentially and uniformly charged by the precharger  38 . The optical head  40  is arranged behind the precharger  38  to write an electrostatic latent image to the charge area of the photosensitive member  36  with a LED beam. Specifically, the LED beam is switched on and off based on the binary image obtained from the computer or the word processor thereby to write the electrostatic latent image as a dot image.  
     [0064] The electrostatic latent image written to the photosensitive member  36  is electrostatically developed as a charged toner image by a toner of a predetermined color of the developing unit  42 . Then, the charged toner image is electrostatically transferred to the paper sheet P by the transfer element  44  located under the photosensitive member  36 . The transfer element  44  is formed as a conductive transfer roller of a porous material (sponge). The transfer element  44  is pressed against the photosensitive member  36  through the paper sheet conveyor belt  22 , and the charge of the polarity opposite to the charged toner image is applied to the paper sheet P conveyed by the paper sheet conveyor belt  22 , so that the charged toner image on the photosensitive member  36  is electrostatically transferred to the paper sheet P from the photosensitive member  36 .  
     [0065] The paper sheet P to which the charged toner image has been transferred is released by the paper sheet conveyor belt  22  and proceeds to the fixing unit  32 . The toner that is not transferred to the paper sheet P is attached to the surface of the photosensitive member  36  when the transfer to the paper P is finished. The residual toner is removed by the toner cleaner  46 , returned to the developing unit  42  by a toner recirculator, including a screw conveyor  48  or the like, and reused as a developing toner.  
     [0066] The developing unit  42  includes a developer container  50  for holding a one-component developer composed of a toner (pulverized particles of colored resin). A magnet roller, i.e. a developing roller  52  is arranged in the opening of the developer container  50  with a part of the surface thereof exposed. The developing roller  52  includes a shaft core portion fixedly supported by the two side wall portions of the developer container  50  and a sleeve of a nonmagnetic material such as aluminum arranged rotatably around the core portion. When the developing unit  42  is in operation, the sleeve of the developing roller  52  is rotated in the direction indicated by arrow in the drawing.  
     [0067] A detachable toner hopper  53  of the developer container  50  is arranged above the developer container  50 . The toner hopper  53  contains the toner, and whenever the toner in the developer container  50  is reduced in amount, the toner in the toner hopper  53  is supplied to the developer container  50 . In the case where the toner in the toner hopper  53  is depleted as the result of being supplied to the developer container  50 , the fact is indicated as an error. In refilling the toner, the old toner hopper  53  depleted of the toner is replaced by a toner hopper  53  filled with new toner. The interior of the toner hopper  53  is segmented into two chambers by a wall  53 - 1 . One of the chambers is filled with new toner and the other chamber is empty before the toner hopper  53  is mounted. When the toner hopper  53  is mounted on the apparatus, the waste toner recovered by the toner cleaner  46  is recovered and stored in the empty chamber through a screw conveyor  48 . By replacing the toner hopper  53 , therefore, the new toner can be refilled while at the same time disposing of the waste toner. The screw conveyor  48  includes a screw arranged in a recovery pipe, whereby the toner is conveyed.  
     [0068] When the developing unit  42  is mounted in the apparatus, the exposed surface of the developing roller  52 , i.e. the sleeve is placed in opposed relation with the surface of the carrier such as the photosensitive member carrying the electrostatic latent image. The bottom wall portion of the developer container  50  makes up a developer pool, in which a reset roller  54  is arranged. The reset roller  54  is rotatably supported by the two side wall portions of the developer container  50  and, when the developing unit  42  is in operation, is rotationally driven in the direction of the arrow. The reset roller  54  recovers the developer that remains on the developing roller  52  without being supplied to the photosensitive member  36 .  
     [0069] With the rotation of the developing roller  52 , the developer is conveyed to the developing area opposed to the photosensitive member  36 . In order to maintain a predetermined amount of the developer conveyed to the developing area by the developing roller  52 , a developer control blade  56  is mounted on the lower side of the developing roller  52  in an opposed relation to the photosensitive member  36 .  
     [0070] The following process is carried out in the developing unit  42 . In the case where the toner component of the developer is charged negatively, for example, an area uniformly charged negatively by the precharger  38  is formed on the rotational surface of the photosensitive member  36 . Once the charged area of the photosensitive member  36  is irradiated with the LED beam emitted from the optical head  40 , the negative charge is released from the irradiated portion and thereby a potential difference occurs. In other words, an electrostatic latent image is written in the charged area of the photosensitive member  36  as a potential difference. Assuming that the potential of the charged area of the photosensitive member  36  is −600 volts, for example, the potential of the electrostatic latent image  52  is lowered to about −15 volts. On the other hand, the developing roller  52  is impressed with a negative developing bias voltage of, say, −400 volts, so that an electric field is formed between the developing roller  52  and the photosensitive member  36 . The toner component charged negatively flies toward the photosensitive member  36  due to the electric field between the developing roller  52  and the photosensitive member  36 . The toner component thus is attached and is used for development on the photosensitive member  36 .  
     [0071] As the paper sheet P is introduced from the driven roller  24   b  of the belt conveyor means  10  to the printing unit and passes through the print assemblies  20 Y,  20 M,  20 C and  20 B sequentially, as shown in FIG. 1, therefore, the paper sheet P is formed with toner images of four colors in superposed relation thereby to form a full-color image. Then, the paper sheet P is fed from the drive roller  24   a  of the belt conveyor means toward the fixing unit  32  of a heat-roller type where the full-color image is thermally fixed on the paper sheet P.  
     [0072] In each of the print assemblies  20 Y,  20 M,  20 C and  20 B, the photosensitive member  36 , the charger  38 , the developing unit  42  and the toner cleaner  46  are formed in a single development unit, and each development unit is replaceably mounted on the frame  12 . The optical head  40  is mounted on the top cover  14 . The paper conveyor belt  22  and the rollers  24   a  to  24   d  are integrated as a belt unit, and the transfer element  44  is mounted on the belt unit.  
     [0073]FIG. 7 is a schematic side view showing a part of the paper sheet conveyor belt  22  and the fixing unit  32 . The fixing unit  32  includes a case  64 , an upper fixing roller  66 U and a lower fixing roller  66 L. The upper fixing roller  66 U and the lower fixing roller  66 L are arranged in parallel contact with each other to form a nip N between the upper fixing roller  66 U and the lower fixing roller  66 L. The upper fixing roller  66 U and the lower fixing roller  66 L each have a built-in heat source such as a halogen lamp and are adapted to be heated to 170 to 190° C., for example. Thermistors  67 U and  67 L (FIG. 12) are arranged to detect the temperature of the roller surface. A high pressure of, say, 33 atmosphers is applied between the upper fixing roller  66 U and the lower fixing roller  66 L. The paper sheet is fixed at a high temperature and under a high pressure between the fixing rollers  66 U and  66 L.  
     [0074] A drive motor (not shown) is provided for driving the fixing rollers  66 U and  66 L of the fixing unit  32 , separately from a drive motor (not shown) for driving the paper sheet conveyor belt  22 , so that the sheet conveying speed of the fixing rollers  66 U and  66 L is lower than that of the paper sheet conveyor belt  22 . As a result, the forward end of the paper sheet is nipped by the fixing rollers  66 U and  66 L, while the rear end of the paper sheet is attracted to the paper sheet conveyor belt  22  and thus the paper sheet is curved with respect to a straight line connecting the outlet end of the paper sheet conveyor belt  22  and the nip N of the fixing rollers  66 U and  66 L by an amount corresponding to the overfeed.  
     [0075] The fixing unit  32  includes an inlet-side paper guide  68  and an outlet-side paper guide  70 . An upper guide  72  is arranged above the outlet end portion of the paper sheet conveyor belt  22 . The inlet-side paper guide  68  (the forward end portion thereof) is located slightly above the straight line (indicated by dashed line) connecting the outlet end portion of the paper sheet conveyor belt  22  and the nip N of the fixing rollers and causes the paper sheet to be curved upward. The curved paper sheet P is shown in FIG. 7. By causing the paper sheet to be curved upward in the cylindrical shape, the rigidity of the paper sheet is increased and the paper sheet does not become fouled in the fixing unit  32 .  
     [0076] As described above, a phenomenon, that the rear end portion of the paper sheet jumps up, is observed when the rear end portion of the paper sheet is released from the outlet end portion of the paper sheet conveyor belt  22  with the forward end portion of the paper sheet nipped between the fixing rollers  66 U and  66 L. The rear end portion of the paper is fouled when it jumps up. In view of this, according to this embodiment, the inlet-side paper guide  68  is formed of a metal plate having elasticity and adapted to move vertically with respect to the paper passage in accordance with the thickness and type of the paper sheet P. When the paper sheet tends to jump up, the inlet-side paper guide  68  is pushed down under the pressure of the paper sheet. In view of the fact that the portion which should make up the supporting point of the paper is lowered when the paper tends to jump up, the force to cause the paper to jump is suppressed thereby to prevent the paper sheet jumping up.  
     [0077] The degree to which the rear end portion of the paper jumps up is small for a thin paper sheet and large for a thick paper sheet. The elastic inlet-side paper guide  68  is elastically deformed to a large degree for a thick paper sheet and to a small degree for a thin paper sheet, with the result that the force to cause the paper to jump is properly reduced. The forward end portion of the inlet-side paper guide  68  is protruded upward by about 2 mm from the straight line connecting the outlet end portion of the paper conveyor belt  22  and the nip of the fixing rollers  66 U and  66 L. Therefore, the inlet-side paper guide  68  is properly elastically deformed by about 2 mm for the commercially available thick paper (such as a postcard).  
     [0078] This embodiment employs the inlet-side paper guide  68  elastically deformed under the paper pressure. A similar effect can be attained, however, by moving the inlet-side paper guide  68  using drive means in accordance with the type and thickness of the paper. In FIG. 12, for example, an electromagnetic plunger  74  is arranged to move the inlet-side paper guide  68  downward when the paper tries to jump up. A motor or other drive means can be used in place of the electromagnetic plunger  74 . The inlet-side paper guide  68 , which can be regulated automatically as described above, may also be configured for manual operation (FIG. 17).  
     [0079]FIG. 8 is a rear view of the fixing unit  32 , and FIG. 9 is a plan view of the fixing unit  32 . The fixing unit  32  has a pair of nip control levers  76  in proximity to the end portions thereof. The nip control levers  76  are operatively coupled to the lower fixing roller  66 L, so that the nip N between the fixing rollers  66 U and  66 L can be closed when the nip control levers  76  are moved in the direction of arrows A in FIG. 9, while the nip N can be opened between the fixing rollers  66 U and  66 L when the nip control levers  76  are moved in a direction opposite to the arrows A.  
     [0080] Further, the fixing unit  32  has fixing unit mounting members  78  in proximity to the nip control levers  76 . The fixing unit  32  is mounted on the frame  12  at the mounting members  78 . The mounting members  78  have holes  80  for inserting the positioning pins  18  shown in FIGS. 4 and 5. As shown in FIG. 9, the mounting members  78  of the fixing unit  32  are arranged at the four corners of the fixing unit  32 , but the holes  80  for inserting the positioning pins  18  are arranged at three corners, and the remaining one corner has no hole. However, a hole  80  for inserting the positioning pin  18  can be formed at the remaining one corner. Alternatively, such holes can be formed at the two side corners.  
     [0081] The mounting members  78  of the fixing unit  32  are supported by the support members  12 B of the frame  12  shown in FIGS. 4 and 5. Thus, by placing the fixing unit  32  on the support members  12 B of the frame  12  while inserting the positioning pins  18  into the holes  80 , the fixing unit  32  can be mounted snugly on the frame  12 . The positioning pins  18  and the support members  12 B are shown also in FIG. 7. The fixing unit  32  is fixed on the frame  12 , not by screws as in the prior art, but by use of the nip control levers  76  and the positioning pins  18 , as described below.  
     [0082]FIG. 10 is a plan view showing a part of the fixing unit  32  with the cover thereof removed, and FIG. 11 is a diagram showing the fixing unit  32  of the FIG. 10 in the nipped state. As shown in FIG. 5, the mounting members  78  of the fixing unit  32  include members  78 A,  78 B and  78 C. The member  78 A is formed as a grooved stud or a headed pin. The members  78 B and  78 C are plate members. The members  78 A and  78 C are secured on the fixing unit  32 , and the central member  78 B is sandwiched between the members  78 A and  78 C and formed as a cam plate movable relative to the members  78 A and  78 C.  
     [0083] As shown in FIGS. 10 and 11, the central plate member  78 B has a cam portion  78   h , a lock hole  78   i  and guide slots  78   j  and  78   k . The guide slots  78   j  and  78   k  extend in the direction perpendicular to the direction of arrows A in which the nip control levers  76  are moved. The pins  781  and  78 A formed on the members  78 C are inserted into the guide holes  78   j ,  78   k  and  78   i  As a result, the central plate member  78 B is guided movably in the direction perpendicular to the direction of arrows A in which the nip control levers  76  are moved.  
     [0084] The cam portion  78   h  extends in the direction substantially parallel to the arrows A of the nip control levers  76  and has an inclined surface portion. When the nip control levers  76  engage the cam portion  78   h  and move in the direction of arrow A, the central plate member  78 B is moved in the direction of arrow B perpendicular to the direction of arrow A.  
     [0085] A lock hole  78   i  is a long slot extending in the same direction as the guide holes  78   j  and  78   k , and has an enlarged portion  78   m  at an end thereof. The enlarged portion  78   m  of the lock hole  78   i  is in registration with the hole  80  for inserting the positioning pin  18  in the state shown in FIG. 10. The enlarged portion  78   m  of the guide holes  78   j  and  78   k  is shown in FIG. 5 and has the same diameter as the hole  80  of the member  78 C.  
     [0086] As shown in FIG. 5, the positioning pin  18  has a small-diameter portion  18   a  in the longitudinal middle portion thereof. The small-diameter portion  18   a  of the positioning pin  18  is located at the same height as the central plate member  78 B. The diameter of the positioning pin  18  is substantially the same as that of the holes  80  of the plate members  78 A and  78 C, so that the fixing unit  32  can be set in position by the positioning pin  18 . The diameter of the small-diameter portion  18   a  of the positioning pin  18  is substantially the same as the width of the lock slot  78   i.    
     [0087] Thus, when the nip control levers  76  are in the nip open position as shown in FIG. 10, the positioning pins  18  can be inserted into the aligned holes  80  of the plate members  78 C and the enlarged portion  78   m  of the lock hole  78   i  of the plate member  78 B. When the nip control levers  76  are moved in the direction of arrow A from the position shown in FIG. 10, the central plate member  78 B is moved in the direction of arrow B into the state of FIG. 11. In FIGS. 10 and 11, the movement of the central plate member  78 B causes the small-diameter portion  18   a  of the positioning pin  18  to move along the lock hole  78   i  so that the upper and lower annular walls of the small-diameter portion  18   a  of the positioning pin  18  come to engage the side wall of the lock hole  78   i . In this way, the fixing unit  32  is locked or secured to the frame  12  while the mounting portion  78  is kept within the positioning pin  18 .  
     [0088] In removing the fixing unit  32  from the frame  12 , the nip control levers  76  are moved in the direction opposite to the arrows A from the position of FIG. 11. Then, the central plate member  78 B moves in such a manner that each of the holes  80  of the plate member  78 C and the enlarged portion  78   m  of the lock hole  78   i  of the plate member  78 B are aligned. Then, in removing the fixing unit  32  from the frame  12 , the positioning pin  18  comes off from the aligned holes.  
     [0089]FIGS. 12 and 13 show various members in the fixing unit  32 . The upper fixing roller  66 U is substantially supported at a fixed position, while the lower fixing roller  66 L is movable vertically with respect to the upper fixing roller  66 U. For this reason, the lower fixing roller  66 L is supported by a support member  82  which is pivotally coupled to the frame  12  at the pivotal support member  82 X. The pivotal support member  82  is urged by a spring  84  so that the lower fixing roller  66 L is in contact with the upper fixing roller  66 U. The nip control levers  76  are pivotally coupled to the frame  12  at the pivotal support member  76 X and pivotally coupled to the support member  82  at the pivotal support member  76 Y.  
     [0090] Thus, when the nip control levers  76  are moved in the direction of arrow Al in FIG. 12, the lower fixing roller  66 L moves down away from the upper fixing roller  66 U against the force of spring  84  and thus is brought into the nip open position shown in FIG. 13. Under this condition, the force of the spring  84  acts in the nip open direction. When the nip control levers  76  are moved in the opposite direction from the position of FIG. 13, on the other hand, the lower fixing roller  66 L moves up into the nip position shown in FIG. 12 at which it comes into contact with the upper fixing roller  66 U. Under this condition, the force of the spring  84  is exerted in the direction to close the nip.  
     [0091] Further, as shown in FIGS. 1, 12 and  13 , the end cover  16  has a lever drive piece  16 F. The lever drive piece  16 F is arranged in such a manner that when the end cover  16  is moved from the open position to the close position, the end cover  16  comes into contact with the nip control levers  76  remaining in the open position and drives the nip control levers  76  into the nip position.  
     [0092] It is possible to remove the fixing unit  32  from the frame  12  for carrying out work, by opening the end cover  16  and operating the nip control levers  76  to open the nip of the fixing rollers  66 U and  66 L. Further, after that, the image can be formed by mounting the fixing unit  32  on the frame  12 , operating the nip control levers  76  to close the nip of the fixing rollers  66 U,  66 L and closing the end cover  16 . Even if, after mounting the fixing unit  32  on the frame  12 , the end cover  16  is closed while forgetting the operation of the nip control levers  76  to close the nip of the fixing rollers  66 U and  66 L, the lever drive piece  16 F of the end cover  16  comes into contact with the nip control levers  76  and drives the nip control lever  76  into the nip position. In FIG. 12, the end cover is shown in the open position and the end cover  16 ′ is shown in the process of being closed. In FIG. 13, the end cover  16  is shown in the closed position.  
     [0093] Further, in FIGS. 12 and 13, an oil supply unit  86 , an oil blade  88  and a cleaning unit  90  are arranged with respect to the upper fixing roller  66 U located on the side of the image-forming surface of the paper sheet P. The oil supply unit  86  and the oil blade  88  are arranged upstream of the nip, and the cleaning unit  90  is arranged downstream of the nip as viewed in the direction of rotation of the fixing roller  66 U. The oil supply unit  86  is provided to prevent the paper sheet P from being stuck to the fixing roller  66 U by coating oil on the upper fixing roller  66 U. The oil blade  88  scrapes off the extra oil coated by the oil supply unit  86  into a uniform state. The cleaning unit  90  is for cleaning off the residual toner or the like attached to the upper fixing roller  66 U.  
     [0094]FIGS. 14 and 15 show the oil supply unit  86  in detail. The oil supply unit  86  includes an oil tank  86   a  arranged above the fixing unit  32 , an oil tube  86   b , a reservoir felt  86   d  arranged in the oil reservoir container  86   c  and an oil supply felt  86   e  arranged in contact with the reservoir felt  86   d.    
     [0095] The oil tube  86   b  extends downward of the oil tank  86   a  into contact with the reservoir felt  86   d . Thus, the oil always stays in the reservoir felt  86   d . The oil supply felt  86   e  includes a vertical portion in contact with and extending upward from the reservoir felt  86   d , a first horizontal portion bent substantially at right angles from the vertical portion and a second horizontal portion bent in the shape of a hair pin from the first horizontal portion. The portion  86   f  bent in the shape of the hair pin is in contact with the upper fixing roller  66 U.  
     [0096] A support plate  86   g  is inserted between the first horizontal portion and the second horizontal portion of the oil supply felt  86   e , and the securing plate  86   h  is arranged on the second horizontal portion. The support plate  86   g  is mounted by a screw  86   i  on a bracket  32   a  which in turn is mounted on the fixing unit  32 . The hole of the support plate  86   g  and the screw  86   i  have a play therebetween, and therefore the support plate  86   g  is movably mounted on the bracket  32   a  so that the oil supply felt  86   e  is movably supported, within an allowable range, on the fixing unit  32 . The securing plate  86   h  integrates the support plate  86   g  and the first horizontal portion and the second horizontal portion of the oil supply felt  86   e  with a screw  86   j.    
     [0097] Springs  87  are arranged in such a position as to urge the securing plate  86   h  downward. Therefore, the oil supply felt  86   e  of the oil supply unit  86  is urged to the upper fixing roller  66 U by the elasticity of the springs  87 . In the embodiment, the two springs  87  are arranged at positions in the vicinity of the ends of the oil supply felt  86   e . In this way, the oil supply felt  86   e  is urged to the upper fixing roller  66 U by the elasticity of the spring  87 , and therefore the contact pressure of the oil supply felt  86   e  against the upper fixing roller  66 U is not varied from one place to another so that the oil is attached uniformly to the upper fixing roller  66 U.  
     [0098] As shown in FIGS. 12 and 16, the cleaning unit  90  includes a felt support member  90   a  and two felts  90   b ,  90   c . The felt support member  90   a  is properly mounted on the fixing unit  32 . The two felts  90   b ,  90   c  are mounted on the felt support member  90   a  in such a manner as to contact the upper fixing roller  66 U. According to this embodiment, the pressure under which the felt  90   b  is in contact with the upper fixing roller  66 U is different from the pressure under which the felt  90   c  is in contact with the upper fixing roller  66 U. The pressure under which the felts  90   b ,  90   c  are contact with the upper fixing roller  66 U, for example, are indicated by the deformation thickness t 1 , t 2  of the felts  90   b ,  90   c , respectively, in contact with the upper fixing roller  66 U, and the difference in deformation thickness between the two felts  90   b  and  90   c  (t 1 -t 2 ) is about 1 mm.  
     [0099] The pressure under which the felt  90   c  first comes into contact with the upper fixing roller  66 U is lower than the pressure under which the felt  90   b  later comes into contact with the upper fixing roller  66 U. In this way, the two felts  90   b  and  90   c  can substantially equally clean the residual toner or the like attached on the upper fixing roller  66 U, thereby preventing one of the felts from being fouled earlier than the other. If the two felts  90   b  and  90   c  are mounted under the same contact pressure, the felt  90   c  coming first into contact with the upper fixing roller  66 U cleans an excessively large proportion of the residual toner and the like and is fouled earlier, resulting in a shorter service life of the fixing rollers as a whole.  
     [0100] FIGS.  17  to  19  are diagrams showing an example in which the inlet-side paper guide  68  can be manually operated. In the case, a manual lever  92  is mounted on the fixing unit  32  for manually operating the inlet-side paper guide  68 . As shown in FIG. 17, the forward end portion of the lever  92  is in substantial contact with the inlet-side paper guide  68 , and the forward end portion of the inlet-side paper guide  68  is located slightly above the nip between the two fixing rollers. As shown in FIG. 18, when using thick paper liable to be curled up, the forward end portion of the inlet-side paper guide  68  is slightly lowered by operating the lever  92 . As a result, the paper is prevented from curling up.  
     [0101] The inlet-side paper guide  68 , if comparatively hard, can be rendered to move in its entirety. As shown in FIG. 19, the inlet-side paper guide  68  is mounted on the fixing unit  32  by a screw  94 A with a play at almost the central portion thereof, and by a spring plate  94 B and a screw  94 C at an outer portion thereof. Thus, the inlet-side paper guide  68 , when depressed by the forward end portion of the lever  92 , can be displaced by the elastic deformation of the spring plate  94 B and the play around the screw  94 A.  
     [0102]FIG. 20 is a diagram showing an example of the control panel CP (FIG. 3) arranged on the frame  12  of the image forming apparatus  10 . The control panel CP has the function of controlling the speed of the fixing rollers  66 U and  66 L in accordance with the type of the paper or the processing mode. As described above, the speed at which the paper is conveyed by the fixing rollers  66 U and  66 L is lower than the speed at which the paper is conveyed by the paper conveyor belt  22 , so that the paper P is conveyed while being curved between the paper sheet conveyor belt  22  and the fixing rollers  66 U and  66 L as shown in FIG. 7. When the paper sheet leaves the outlet end portion of the paper sheet conveyor belt  22 , the rear end portion of the curved paper sheet jumps up and the paper sheet is fouled. According to this embodiment, therefore, the inlet-side paper guide  68  is adapted to move down.  
     [0103] If the paper feed rate of the fixing rollers  66 U and  66 L is the same as that of the paper sheet conveyor belt  22 , the paper would not be curved and the problem of jumping up would be obviated. Nevertheless, the drive motor of the fixing rollers  66 U and  66 L is separate from that of the paper sheet conveyor belt  22 , and the paper feed operation of the fixing rollers  66 U and  66 L changes in accordance with the change of the roller diameter due to the heat of the fixing rollers and the type of the paper sheet. Therefore, the paper feed rate of the fixing rollers  66 U and  66 L cannot be the same as that of the paper conveyor belt  22 . Hence, the speed at which the paper is fed by the fixing rollers  66 U and  66 L is set at an optimum value lower than the speed at which the paper is conveyed by the paper conveyor belt  22 . The speed at which the paper sheet is fed by the fixing rollers  66 U and  66 L can be regulated at this optimum setting.  
     [0104]FIG. 21 is a block diagram showing an example of speed control of the fixing rollers. In FIG. 21, the speed of the drive motor of the fixing rollers  66 U,  66 L can be set appropriately in accordance with the type of paper and the printing mode.  
     [0105] In FIG. 20, for example, the control panel CP includes an on-line button, a set-up button, a continue button and a reset button. For setting the speed of the fixing rollers  66 U,  66 L, the special mode is used. To attain the special mode, power is switched on while depressing two of these buttons at the same time. In the shown example, the buttons shown in the lower part of FIG. 20 are displayed.  
     [0106] Upon depression of the menu button, the color mode is displayed, while upon depression of the select button, a selected value is displayed as shown in FIG. 22. The selected value 5 corresponds to the initial value determined based on the optimum setting described above. For the selected value of more than 5, the speed increases by 0.25% of the initial value. For the selected value of smaller than 5, on the other hand, the speed decreases by 0.25% of the initial value.  
     [0107] The color mode indicates that the paper of standard thickness is printed in color. A value suitable for this mode is selected and set by the set button. Then, upon depression of the menu button, the thick paper mode is displayed. The thick paper mode indicates that a paper (120 kg, for example) considerably thicker than a paper of standard thickness is printed in color, in which case a suitable value is selected and set by the set button. Further, upon depression of the menu button, the OHP mode is displayed. The OHP mode indicates the OHP is printed in color, so that a value suitable for this mode is selected and set by the set button. Upon another depression of the menu button, the monochromatic mode is displayed. A value suitable for this mode is selected and set by the set button. In this way, the speed of the drive motor of the fixing rollers  66 U,  66 L is set.  
     [0108]FIG. 23 is a flowchart showing an example of setting the speed of the drive motor of the fixing rollers  66 U,  66 L automatically using a displacement sensor. As shown in FIG. 7, the displacement sensor  98  is arranged above the passage of the paper, for example, for detecting the amount of displacement of the paper P by measuring the distance to the paper P without contact.  
     [0109] In step S 1  of FIG. 23, the speed value V of the drive motor of the fixing rollers  66 U,  66 L is input. This speed value V is the aforementioned optimum setting for starting the operation in the initial stage of operation of the apparatus. This value is subsequently corrected based on the optimum setting and constitutes the value stored on the immediately preceding occasion. In step S 2 , the displacement amount F of the paper P is detected by a non-contact sensor  98 . In step S 3 , it is determined whether the detected displacement amount F of the paper P is larger than a predetermined value V 1  or not. If the result is YES, the process proceeds to step S 4  for calculating the speed change rate V 1 . The speed change rate V 1 , like the selected value of FIG. 22, for example, is stored in a computer as a table of functions of the displacement amount. In step S 5 , the speed value V is determined as the product of the speed value V and the speed change rate V 1 . In step S 6 , the speed value V thus corrected is input to the motor drive circuit for changing the rotational speed of the fixing rollers  66 U,  66 L. In the case where the result of step S 3  is NO, the speed value V not corrected is input to the motor drive circuit for driving the fixing rollers  66 U,  66 L at the same rotational speed as before.  
     [0110]FIG. 24 is a flowchart showing an example of automatically setting the speed of the drive motor for the fixing rollers  66 U,  66 L using a temperature sensor. The temperature sensor is a thermistor  67 U, for example, shown in FIG. 12.  
     [0111] In step S 11  of FIG. 24, the speed value V of the drive motor of the fixing rollers  66 U,  66 L is input. In step S 12 , the temperature T of the fixing roller  66 U is detected. In step S 13 , the speed change rate V 1  is calculated. The speed change rate V 1  is stored in a computer as the product of the functions of temperature. In step S 14 , the speed value V is determined as the product of the speed value V and the speed change rate V 1 . In step S 15 , the speed value V thus corrected is input to the motor drive circuit for changing the rotational speed of the fixing rollers  66 U,  66 L.  
     [0112] It will thus be understood from the foregoing description that according to this invention, the provision of the fixing unit securing means operatively interlocked with the nip control mechanism simplifies the operation of mounting and demounting the fixing unit. Also, the fouling of the paper is suppressed by a movable paper guide or by correcting the speed of the fixing rollers. Further, the cleaning member for scraping off the toner from the fixing unit is efficient and has a long service life.