Abstract:
A fixing apparatus includes a fixing device having a heating roller which rotates and a press roller which rotates in tight contact with the heating roller, and adopted to heat and press a medium with an unfixed image by passing the medium between the rollers, thereby fixing the unfixed image, a detection device which detects temperatures of the heating and heating rollers, and a control device which variably controls rotating states of the heating and press rollers based on the temperatures detected by the detection device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a fixing apparatus and image forming apparatus in, e.g., a color electronic copying machine, printer, or facsimile machine, for fixing an unfixed toner image on a recording medium by fusing.  
           [0002]    Some color electronic copying machine has a photosensitive belt for carrying a latent image. A plurality of developing units for selectively supplying toners of respective colors (Y, M, C, and K) are disposed to oppose the photosensitive belt. In this color electronic copying machine, toner images of respective colors formed on the photosensitive belt are transferred onto, e.g., an intermediate transfer body, in an overlaying manner to form a color image. The color image is then transferred at once onto a recording sheet. The recording sheet transferred with the color image is fed to a fixing device. The fixing device fixes the color image on the recording sheet.  
           [0003]    The fixing device has a fixing roller serving as a heating roller incorporating a heating source, and a press roller to be urged by the fixing roller. When the recording sheet transferred with the color image passes between the fixing roller and press roller, it is heated and pressed, so the color image is fixed.  
           [0004]    In a fixing device of this type, in order to shorten the warm-up time, the heating source such as a heater lamp is arranged only in the fixing roller with which the toner image comes into contact. The press roller does not have a heating source in it. Hence, the press roller need to efficiently receive heat from the fixing roller with the heating source.  
           [0005]    The recording sheet passes between the fixing roller and press roller. When image fixing operation is repeated, heat is not sufficiently conducted to the press roller. Then, the surface temperature of the press roller decreases gradually, and defective fixing occurs.  
           [0006]    Particularly, in fixing an image such as a color image with a large toner attaching amount, the temperature of the press roller largely influences the fixing properties. If the temperature of the press roller is low, defective fixing tends to occur.  
           [0007]    Assume that after copy operation is started, the image of a recording sheet which is fed first between the fixing roller and press roller is to be fixed. In this case, sometimes the press roller does not reach a temperature sufficiently high for fixing. Then, defective fixing occurs.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention has been made in view of the above situations, and has as its object to provide a fixing apparatus and image forming apparatus which can fix an image after the press roller is heated to a sufficiently high temperature.  
           [0009]    A fixing apparatus according to the present invention comprises a fixing device, having a heating roller which rotates and a press roller which rotates in tight contact with the heating roller, and adopted to heat and press a medium with an unfixed image by passing the medium between the rollers, thereby fixing the unfixed image, a detection device which detects temperatures of the heating and heating rollers, and a control device which variably controls rotating states of the heating and press rollers based on the temperatures detected by the detection device.  
           [0010]    An image forming apparatus according to the present invention comprises a rotatable image carrier which carries an electrostatic latent image, an image forming device which forms the electrostatic latent image on the image carrier, a developing device which supplies a developer to the electrostatic latent image, formed by the image forming device, by rotation of a developing roller, thereby forming a developer image, a transfer device which transfers the developer image developed by the developing device onto a medium, a fixing device, having a heating roller which rotates and a press roller which rotates in tight contact with the heating roller, and adopted to heat and press a medium with an unfixed image by passing the medium between the rollers, thereby fixing the unfixed image, a detection device which detects temperatures of the heating and heating rollers, and a control device which variably controls rotating states of the heating and press rollers based on the temperatures detected by the detection device.  
           [0011]    An image forming apparatus according to the present invention is an image forming apparatus comprising a rotatable image carrier which carries an electrostatic latent image, an image forming device which forms the electrostatic latent image on the image carrier, a plurality of developing devices which selectively supply developers of different colors to the electrostatic latent image, formed by the image forming device, by rotation of a developing roller, thereby forming developer images of respective colors, a transfer device which transfers the developer images of the respective colors developed by the plurality of developing devices onto a medium in an overlaid manner, a fixing device, having a heating roller which rotates and a press roller which rotates in tight contact with the heating roller, and adopted to heat and press a medium with an unfixed image by passing the medium between the rollers, thereby fixing the unfixed image, a detection device which detects temperatures of the heating and heating rollers, and a control device which variably controls rotating states of the heating and press rollers based on the temperatures detected by the detection device.  
           [0012]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0013]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
         [0014]    [0014]FIG. 1 is a view showing the internal arrangement of a color electrophotographic copying machine according to the first embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a view showing the arrangement of a fixing apparatus;  
         [0016]    [0016]FIG. 3 is a graph showing the quality of image fixing properties depending on the temperature changes of a fixing roller and press roller;  
         [0017]    [0017]FIG. 4 is a graph showing temperature changes occurring when the fixing and press rollers are rotated at a low speed;  
         [0018]    [0018]FIG. 5 is a graph showing temperature changes occurring when the fixing and press rollers are rotated at a normal speed;  
         [0019]    [0019]FIG. 6 is a graph showing temperature changes occurring when the fixing and press rollers are rotated at a high speed;  
         [0020]    [0020]FIG. 7 is a graph showing temperature changes occurring when the fixing and press rollers are rotated at a high speed with the press roller being at a temperature lower than a reference temperature;  
         [0021]    [0021]FIG. 8 is a graph showing temperature changes occurring when the fixing and press rollers are rotated at a low speed with the fixing roller being at a temperature lower than the reference temperature;  
         [0022]    [0022]FIG. 9 is a graph showing temperature changes occurring when the fixing and press rollers are rotated at a normal speed with the press roller being at a temperature lower than the reference temperature;  
         [0023]    [0023]FIG. 10 is a graph showing temperature changes occurring when the fixing and press roller are rotated at the normal speed with the fixing roller being at a temperature lower than the reference temperature; and  
         [0024]    [0024]FIG. 11 is a graph showing temperature changes occurring when the heating and press rollers are intermittently rotated in the standby mode where image fixing is not performed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    [0025]FIG. 1 is a view showing the arrangement of a color electrophotographing apparatus as an image forming apparatus according to the first embodiment of the present invention.  
         [0026]    This color electrophotographing apparatus has an image forming unit  1 . The image forming unit  1  has a flexible photosensitive belt  2  serving as an image carrier. The photosensitive belt  2  is looped between a plurality of first to third rollers  3   a  to  3   c  with a predetermined tension to travel in the direction of arrows.  
         [0027]    A charging device  4 , laser exposure device  5 , and developing devices  6 Y to  6 K are disposed around the photosensitive belt  2  along its traveling direction. The charging device  4  charges the photosensitive belt  2  at a predetermined potential. The laser exposure device  5  serves as an image forming device for forming an electrostatic latent image on the charged photosensitive belt  2 . The developing devices  6 Y to  6 K visualize the electrostatic latent image formed on the photosensitive belt  2  by supplying yellow (Y), magenta (M), cyan (C), and black (K) toners as developers to it.  
         [0028]    Furthermore, an intermediate transfer drum  7 , discharge lamp  10 , and cleaner device  9  are disposed around the photosensitive belt  2  along its traveling direction. The intermediate transfer drum  7  serves as a rotatable transfer device for temporarily holding the toner image formed on the photosensitive belt  2 . The cleaner device  9  removes the toner left on the photosensitive belt  2 .  
         [0029]    That portion of the photosensitive belt  2  which extends between the first and second rollers  3   a  and  3   b  opposes the developing devices  6 Y to  6 K through a predetermined gap. That portion of the photosensitive belt  2  which extends between the second and third rollers  3   b  and  3   c  is in tight contact with the outer surface of the intermediate transfer drum  7 .  
         [0030]    Either one of the first to third rollers  3   a  to  3   c  is connected to a driving motor (not shown). Upon rotation of the driving motor, the first to third rollers  3   a  to  3   c  are rotatably driven in the direction indicated by arrows at a predetermined speed.  
         [0031]    A sheet cassette  12  for storing sheets P as media with a predetermined size is provided below the image forming unit  1 . The sheet cassette  12  has a feed roller  13  for taking up the sheets P one by one. The sheet P taken up by the feed roller  13  is conveyed upward along a convey path  14  extending in the vertical direction.  
         [0032]    A convey roller pair  17 , an aligning roller pair  18 , a transfer roller  16  serving as a transfer device, a fixing apparatus  20 , and a delivery roller pair  21  are sequentially disposed along the convey path  14  in the convey direction of the sheet P.  
         [0033]    The convey roller pair  17  clamps and conveys the sheet P. The aligning roller  18  temporarily stops the sheet P conveyed to it, corrects tilt of the sheet P with respect to the convey direction, and causes the leading end of the sheet P to coincide with the leading end of the toner image on the intermediate transfer drum  7 . The transfer roller  16  opposes the intermediate transfer drum  7  and transfers the toner image formed on the intermediate transfer drum  7  onto the sheet P. The fixing apparatus  20  fixes the toner image transferred to the sheet P. The delivery roller pair  21  delivers the sheet P. A delivery tray  23  for receiving the sheet to be delivered is provided on the delivery side of the delivery roller pair  21 .  
         [0034]    Full-color printing operation of the above color electrophotographing apparatus will be described.  
         [0035]    First, the surface of the photosensitive belt  2 , the rear surface of the photosensitive layer of which is grounded to 0 V, is uniformly charged by the charger 4 to −700 V. Then, the laser exposure device  5  is driven in response to a yellow image signal from a control unit (not shown) to form a yellow latent image on the photosensitive belt  2 . The potential of this electrostatic latent image is about −100 V.  
         [0036]    Before forming the yellow latent image, the yellow developing unit  6 Y is moved toward the photosensitive belt  2 . In synchronism with this, a developing roller  31 Y is rotated, and a voltage of −300 V is applied to it. Upon movement of the developing unit  6 Y, when the developing roller  31 Y comes into contact with the photosensitive belt  2 , the latent image is developed, and a yellow toner image is formed on the photosensitive belt  2 .  
         [0037]    The yellow toner image on the photosensitive belt  2  is electrostatically transferred to the intermediate transfer drum  7  to which a voltage of +1 kV is applied, and the toner left on the photosensitive belt  2  is scraped with the blade of the cleaner device  9 . The surface charges left on the photosensitive belt  2  are removed by the discharge lamp  10 .  
         [0038]    The surface of the photosensitive belt  2  is charged again, and a magenta latent image is formed this time, in the same manner as described above. This latent image is developed by the magenta developing unit  6 M to form a magenta toner image, which is overlaid on the yellow toner image on the intermediate transfer drum  7 . The same cycle is repeated for cyan (C) and black (K) to form, on the intermediate transfer drum  7 , a color image in which toner images of four colors are overlaid.  
         [0039]    At this time, the sheet P is supplied from the sheet cassette  12  and fed along the convey path  14 . The sheet P is sandwiched by the convey roller pair  17 , conveyed to the resist roller pair  18 , aligned, and fed to a portion between the intermediate transfer drum  7  and transfer roller  16 . A voltage of +2 kV to 3 kV is applied to the transfer roller  16 . Hence, the toner images of four colors formed on the intermediate transfer drum  7  are transferred to the sheet P at once. After that, the sheet P on which the toner images of four colors are transferred is fed to the heat roll type fixing unit  20 . The toner images are fixed by fusion onto the sheet P, to form a color image on the sheet P.  
         [0040]    The toner left on the intermediate transfer drum  7  is removed by the cleaner device  9  using a brush to which a voltage of +1.5 kV is applied.  
         [0041]    [0041]FIG. 2 is a view showing the arrangement of the fixing apparatus  20 .  
         [0042]    The fixing apparatus  20  has a fixing roller  25  and press roller  26 . The fixing roller  25  serves as a heating roller, and the press roller  26  is brought into tight contact with the lower portion of the fixing roller  25 . The fixing roller  25  has a heater lamp  27  serving as a heat source in it.  
         [0043]    Temperature sensors  28  and  29  are disposed in the vicinities of the fixing roller  25  and press roller  26 . The temperature sensors  28  and  29  serve as a detection device for detecting the surface temperatures of the fixing roller  25  and press roller  26 . The temperature sensors  28  and  29  are connected to a control device  31  through a transmission circuit. The control device  31  is connected to a driving motor  32  through a control circuit. The driving motor  32  rotatably drives the fixing roller  25  and press roller  26 . The control device  31  variably controls the rotational speed of the fixing roller  25  and press roller  26  through detection temperatures transmitted from the temperature sensors  28  and  29 .  
         [0044]    The sheet P passes between the fixing roller  25  and press roller  26  with its color image-side surface and its surface opposite to the color image side being in contact with the fixing roller  25  and press roller  26 , respectively. Hence, the sheet P is heated and pressed, so the color image is fixed to the sheet P.  
         [0045]    [0045]FIG. 3 shows the quality of the image fixing properties depending on the temperature changes of the fixing and press rollers  25  and  26 .  
         [0046]    Even when the fixing roller  25  had reached a predetermined temperature, if the temperature of the press roller  26  was low, defective fixing such as toner separation occurred.  
         [0047]    FIGS.  4  to  6  show results obtained by measuring the temperatures of the fixing roller  25  and press roller  26  while changing their rotational speed when the fixing apparatus  20  is to be warmed up from room temperature. More specifically, temperatures were measured in cases wherein the rotational speed of the rollers  25  and  26  was higher and lower, respectively, than the rotational speed of the rollers  25  and  26  determined as the reference in the image forming apparatus. How the temperatures rose was thus examined.  
         [0048]    [0048]FIG. 4 shows temperature changes occurring when the fixing and press rollers  25  and  26  are rotated at a speed lower than the reference speed. In this case, the temperature of the fixing roller  25  rose faster than when it was rotated at the reference speed, while the temperature of the press roller  26  rose slowly.  
         [0049]    [0049]FIG. 5 shows temperature changes occurring when the fixing and press rollers  25  and  26  are rotated at the reference speed.  
         [0050]    [0050]FIG. 6 shows temperature changes occurring when the fixing and press rollers  25  and  26  are rotated at a speed faster than the reference speed. In this case, the temperature of the press roller  26  rose faster than when it was rotated at the reference speed.  
         [0051]    A description will be made on cases wherein verification was performed in the actual state by utilizing the results shown in FIGS.  4  to  6 .  
         [0052]    In practice, in continuous image fixing, when the press roller  26  or fixing roller  25  had not reached the reference temperature at which fixing was possible, particularly after the image was fixed, control operations as shown in FIGS.  7  to  9  were performed.  
         [0053]    [0053]FIG. 7 shows a state wherein, when the fixing roller  25  has reached the predetermined temperature and the temperature of the press roller  26  is low, the rotational speed of the rollers  25  and  26  is increased before the sheet P reaches the fixing apparatus  20 . According to FIG. 7, the press roller  26  could be heated to the reference temperature or higher within a predetermined period of time with which the sheet P reaches the fixing apparatus  20 . This is probably due to the following reason. As the rollers  25  and  26  were rotated at a high speed, the contact distance between them increased. Accordingly, more heat shifted from the fixing roller  25  with the heater lamp  27  to the press roller  26 .  
         [0054]    “Post-fixing rotation” described in FIG. 7 and FIGS.  8  to  10  to be described later refers to a state wherein the rollers  25  and  26  rotate immediately after the toner image is fixed to the sheet P. “Pre-rotation” refers to a state wherein a process such as development or transfer takes place. “Fixing” refers to a state wherein the toner image is actually being fixed on the sheet P with the fixing apparatus  20 .  
         [0055]    [0055]FIG. 8 shows a case wherein, when the press roller  26  has reached the predetermined temperature and the temperature of the fixing roller  25  is low, the rotational speed of the rollers  25  and  26  is increased. According to FIG. 8, the fixing roller  25  could rise to the predetermined temperature within a predetermined period of time. This is probably due to the following reason. As the rollers  25  and  26  were rotated at a low speed, the contact distance between them within a predetermined period of time was shortened. Accordingly, the press roller  26  was less deprived of heat, and the fixing roller  25  itself was heated well, so the temperature rise rate of the fixing roller  25  increased.  
         [0056]    As described above, the rotational speed of the rollers  25  and  26  is changed until the sheet P reaches the fixing apparatus  20  in accordance with the states of the fixing roller  25  and press roller  26  with respect to the predetermined temperatures. Thus, the rollers  25  and  26  can be efficiently set to the predetermined temperatures at which fixing is possible.  
         [0057]    When the temperatures of both the fixing roller  25  and press roller  26  are lower than the reference temperature, the rollers  25  and  26  are rotated at an ordinary recording rotational speed to increase their temperatures.  
         [0058]    The rotational speed of the rollers  25  and  26  must be appropriately selected in accordance with the materials of the rollers  25  and  26 , the output from the heat source  27 , and the heating method.  
         [0059]    [0059]FIG. 9 shows a case wherein, when the fixing roller  25  has reached the predetermined temperature and the temperature of the press roller  26  is low, the rollers  25  and  26  are rotated at the ordinary speed without increasing their speed as in FIG. 7.  
         [0060]    In this case, the temperature rise of the press roller  26  was slow, and the press roller  26  could not be heated to the reference temperature before the sheet P reached the fixing apparatus  20 . Accordingly, defective fixing sometime occurred.  
         [0061]    [0061]FIG. 10 shows a case wherein, when the press roller  26  reaches the predetermined temperature and the fixing roller  25  has a low temperature, the fixing roller  25  and press roller  26  are rotated at the ordinary speed without decreasing their speed as in FIG. 8. In this case, the temperature of the fixing roller  25  did not rise in time, and defective fixing sometimes occurred.  
         [0062]    The second embodiment of the present invention will be described.  
         [0063]    In the first embodiment described above, the rotational speed of the fixing and press rollers  25  and  26  is controlled during ordinary image forming operation. In the second embodiment, the rotational speed of fixing and press rollers  25  and  26  is controlled after a copy start command is received and until a first sheet P reaches a fixing apparatus  20 .  
         [0064]    For example, assume that the fixing roller  25  maintains a certain predetermined temperature but the press roller  26  cannot rise to a necessary temperature with the ordinary rotational speed. In this case, the rotational speed of the rollers  25  and  26  is increased while steps such as pre-fixing development and transfer are performed, so the press roller  26  reaches the necessary temperature.  
         [0065]    Conventionally, fixing is performed while the fixing roller  25  maintains a certain predetermined temperature. Sometimes, when the image of the sheet P which is sent first to the fixing apparatus  20  is fixed, defective fixing occurs.  
         [0066]    To examine the cause for this, the temperatures of the fixing roller  25  and press roller  26  were measured. The temperature of the press roller  26  while the sheet P passed was low. The reason for this was clarified as follows. Before a copy start command was received, rotation of the fixing and press rollers  25  and  26  was stopped, and the temperature of the press roller  26  decreased. With only “pre-rotation” after the copy start command was received, the temperature of the press roller  26  did not rise to a value sufficiently high for fixing.  
         [0067]    In view of this, while detecting the temperature of the press roller  26 , when the detected temperature was low, the rollers  25  and  26  were rotated at a high speed during “pre-rotation” after the copy start command. Then, the press roller  26  reached the predetermined temperature for image fixing, and fixing was performed well. In this manner, when the temperature of the press roller  26  was detected upon reception of the copy start command and the rotational speed was changed before the image reached the fixing apparatus  20 , the press roller  26  could reach a temperature sufficiently high for fixing.  
         [0068]    The third embodiment of the present invention will be described.  
         [0069]    The third embodiment refers to the standby mode wherein image fixing is not performed. In the standby mode, the temperature of a press roller  26  which is not in contact with an image to be fixed is detected. If the temperature is not a predetermined value, a fixing roller  25  is rotated intermittently to maintain the temperature of the press roller  26  at a certain constant level, as shown in FIG. 11.  
         [0070]    More specifically, in the standby mode, when rotation of the rollers  25  and  26  is stopped, the temperature of the press roller  26  is detected. If the temperature is equal to the predetermined value or less, the rollers  25  and  26  are rotated. The press roller  26  is thus maintained at a temperature to which it can rise within a predetermined fast copy time.  
         [0071]    “Predetermined temperature” refers to a temperature from which, when a copy start command is output, the press roller  26  can reach a temperature at which fixing can be performed with the predetermined fast copy time. When the fixing and press rollers  25  and  26  are rotated, heat of the fixing roller  25  is conducted to the press roller  26 .  
         [0072]    Conventionally, in the standby mode, the temperature of the press roller  26  was not detected, but only the temperature of the fixing roller  25  was detected and maintained at a constant value. After that, when image fixing was performed with a predetermined fast print time interval, defective fixing sometimes occurred in the first print obtained immediately after the standby mode.  
         [0073]    In order to find the cause for this, the temperature of the press roller  26  was measured. During image fixing, the temperature of the press roller  26  was low, which was not sufficiently high for fixing. The reason for this was clarified as follows. In the standby mode, the temperature of the press roller  26  was excessively low. Accordingly, image fixing was subsequently performed before heat was not sufficiently absorbed in print “pre-rotation” by contact with the fixing roller  25 .  
         [0074]    The standby temperature of the press roller  26 , which was necessary to reach the temperature necessary for fixing, within the predetermined fast print time, was obtained from the temperature necessary for fixing. The rollers  25  and  26  were intermittently rotated in the standby mode in order to maintain the standby temperature.  
         [0075]    In this embodiment, this temperature was reached when the rollers were rotated through almost two to three revolutions in the standby mode. After that, this operation was performed when necessary while detecting the temperature of the press roller  26 . In this manner, the image could be fixed well within the predetermined fast print time.  
         [0076]    In the above embodiments, the photosensitive belt  2  and intermediate transfer drum  7  were used as the image carrier and intermediate transfer body, respectively. However, the present invention is not limited to this, and a photosensitive drum and an intermediate transfer belt may be used as the image carrier and intermediate transfer body, respectively. Alternatively, a photosensitive drum and intermediate transfer drum may be used as the image carrier and intermediate transfer body, or a photosensitive belt and intermediate transfer belt may be used as the image carrier and intermediate transfer body. Any combination will do as far as the toner images can be transferred in an overlaid manner and transferred onto a medium at once.  
         [0077]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.