Abstract:
A fixing device for fixing image information on a recording medium passing a nip of the present invention includes a rotatable member for transferring and conveying a non-fixed image, a first heating means for heating an image present on the rotatable member and a rotatable pressing member forming a nip in combination with the rotatable member. A heat pipe extends in parallel to the axis of the rotatable member. A radiating member is positioned at one end of the heat pipe while a heating member is positioned at the other end of the heat pipe.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a fixing device for use in a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a transfixing type of fixing device configured to melt with heat a toner image transferred to a fixing roller at a secondary image transferring position and then transfer and fix the toner image to a sheet or recording medium by tertiary image transfer.  
         [0003]     2. Description of the Background Art  
         [0004]     A transfixing type of fixing device directly heats a toner image transferred to a fixing roller with a radiation source and a reflector substantially surrounding the radiation source. In this type of fixing device, when a toner image is transferred to the fixing roller, toner is directly heated by radiant heat in part of the fixing roller where the toner image is present while the surface of the fixing roller is directly heated in the other part where the toner image is absent. This brings about a problem that the temperature of the fixing roller becomes irregular along the image pattern after the toner image has been fixed on the sheet. As a result, when the next toner image is transferred to the fixing roller at the secondary image transfer position, the irregularity in temperature, corresponding to the previous image pattern, is likely to remain as a temperature history and effect the above toner image. Image defects include, e.g., irregularities in gloss and fixation and granularity.  
         [0005]     A conventional fixing system applicable to an image forming apparatus fixes a toner image on a recording medium by heating and pressing the toner image. It is a common practice with such a fixing system to fix a toner image on a recording medium by melting the toner image with heat radiated from a halogen lamp or causing a heat-resistant film including a thin metallic layer or conductor to contact a recording medium implemented as an endless belt or a hollow cylinder. In any case, however, a fixing device generates a considerable amount of thermal energy which is likely to effect various units around the fixing device.  
         [0006]     On the other hand, a heat pipe is capable of rapidly transporting, despite a small temperature difference, a great amount of thermal energy by use of latent heat derived from evaporation or condensation. A heat pipe is a heat conduction device in which a small amount of working liquid is sealed and having thermal conductivity several hundred times as high as the conductivity of copper, which is originally high.  
         [0007]     Japanese patent laid-open publication Nos. 59-019976, 08-137277 and 2004-086091, for example, each disclose a particular image forming apparatus using a heat pipe as cooling means for protecting image forming sections other than heating means from heat output from the heating means. On the other hand, Japanese patent laid-open publication Nos. 60-154071 and 2000-29341, for example, propose to use a heat pipe as a fixing roller itself in order to reduce a warm-up time in combination with an induction heating effect, thereby realizing an on-demand fixing system that heats a fixing device only during intervals between consecutive printing.  
         [0008]     Further, paying attention to the extremely high thermal conductivity of a heat pipe, Japanese patent laid-open publication No. 2001-201978, for example, proposes to use a heat pipe as a heat diffusing member for preventing, in a continuous print mode, heat from rising in the zones of a fixation nip outside of a sheet passing zone. Likewise, Japanese patent laid-open publication No. 2002-244450 proposes to use a heat pipe as temperature uniforming means for obviating temperature differences between a sheet passing portion and portions outside of the same.  
         [0009]     Moreover, Japanese patent laid-open publication No. 2002-055552 teaches a fixing device of the type using a heat-resistant film whose thermal capacity is small and configured to reduce the irregular temperature distribution of the film in the widthwise direction. For this configuration, a heat pipe is included in a temperature irregularity reducing member positioned outside of a second roller body.  
       SUMMARY OF THE INVENTION  
       [0010]     It is an object of the present invention to provide a fixing device capable of reducing a warm-up time and obviating irregularity in temperature ascribable to the image history of the previous image forming step occurring on a fixing member as well as irregularity in temperature in the lengthwise direction ascribable to the continuous feed of sheets of a relatively small size.  
         [0011]     A fixing device for fixing image information on a recording medium passing a nip of the present invention includes a rotatable member for transferring and conveying a non-fixed image, a first heating means for heating an image present on the rotatable member and a rotatable pressing member forming a nip in combination with the rotatable member. A heat pipe extends in parallel to the axis of the rotatable member. A radiating member is positioned at one end of the heat pipe while a heating member is positioned at the other end of the heat pipe. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:  
         [0013]      FIG. 1  is a view showing the general construction of an image forming apparatus to which the present invention is applied;  
         [0014]      FIG. 2  is a view showing a fixing device embodying the present invention included in the apparatus of  FIG. 1 ;  
         [0015]      FIG. 3  is a view showing the configuration of a heat pipe;  
         [0016]      FIG. 4  is a view showing an alternative embodiment of the present invention; and  
         [0017]      FIG. 5  is a view showing another alternative embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     Referring to  FIG. 1  of the drawings, an image forming apparatus to which the present invention is applied is shown and implemented as a tandem color copier by way of example. As shown, the color copier, generally  1 , includes an image forming section  1 A arranged at the center of the copier body, a sheet feeding section  1 B positioned below the image forming section  1 A and an image scanning section, not shown, positioned above the image forming section  1 A.  
         [0019]     The image forming section  1 A includes an intermediate image transfer belt  2 , which is a specific form of an intermediate image transfer body. An arrangement for forming images of colors complementary to separated colors is positioned on the upper run or image transfer surface of the intermediate image transfer belt (simply belt hereinafter)  2 . More specifically, photoconductive elements or image carriers  3 Y,  3 M,  3 C and  3 B are arranged side by side along the image transfer surface of the belt  2  for carrying toner images of complementary colors, i.e., yellow, magenta, cyan and black, respectively. An optical writing unit or means  5  ( 5 Y,  5 M,  5 C and  5 B) is located above the photoconductive elements  4 Y through  4 B.  
         [0020]     In the illustrative embodiment, the photoconductive elements  3 Y through  3 B are implemented as drums rotatable in the same direction, i.e., counterclockwise in  FIG. 1 . Arranged around the drums  3 Y,  3 M,  3 C and  3 B are chargers  4 Y,  4 M,  4 C and  4 B, developing devices  6 Y,  6 M,  6 C and  6 B and drum cleaners.  8 Y,  8 M,  8 C and  8 B, respectively. Primary image transferring devices  7 Y,  7 M,  7 C and  7 B respectively face the drums  3 Y,  3 M,  3 C and  3 B with the intermediary of the belt  2 . It is to be noted that the suffices Y, M, C and B correspond to the colors of toners stored in the developing devices  6 Y,  6 M,  6 C and  6 Y, respectively. The belt  2  is passed over a drive roller  9  and a driven roller  10  and rotatable clockwise, as indicated by an arrow in  FIG. 1 . A belt cleaner  11  is located to face the driven roller  10  with the intermediary of the belt  2  so as to clean the surface of the belt  2 .  
         [0021]     In operation, the charger  4 Y uniformly charges the surface of the drum  3 Y. The optical writing unit  5  scans the charged surface of the drum  3 Y with a light beam in accordance image data to thereby form a latent image on the drum  3 Y. Subsequently, the developing device  7 Y, applied with a preselected bias for development, develops the latent image with yellow toner for thereby producing a yellow toner image. The yellow toner image thus formed is transferred from the drum  3 Y to the belt  2  by the primary image transferring device  7 Y also applied with a preselected bias. Such a sequence of steps are executed with the other drums  3 M  3 C and  3 Y also except for the color of toner. As a result, toner images of different colors are sequentially transferred to the belt  2  one above the other, completing a full-color toner image or non-fixed image on the belt  2 . The toners left on the drums  3 Y through  3 B after the primary image transfer are removed by the drum cleaners  8 Y through  8 B, respectively. Further, the drums  3 Y through  3 B thus cleaned are discharged by quenching lamps, not shown, and prepared for the next image formation thereby.  
         [0022]     The sheet feeding section  1 B includes a sheet tray  16  loaded with a stack of sheets or recording media P. A pickup roller  17  pays out the top sheet P from the sheet tray  16  toward a registration roller pair  19  while separating it from the underlying sheets P. The registration roller pair  19  once stops the sheet P to correct a skew and again drives the sheet P toward a nip N such that a preselected position of the sheet P in the direction of conveyance meets the leading edge of the full-color toner image transferred to an image transferring and fixing roller  13 .  
         [0023]     More specifically, a fixing device  12  is positioned in the vicinity of the drive roller  9  and includes a press roller or rotatable pressing member  14  in addition to the image transferring and fixing roller or rotatable member  13 . The press roller  14  is pressed against the image transferring and fixing roller  13  by a spring or biasing means, so that the nip N is formed between the two rollers  13  and  14 .  
         [0024]     The full-color toner image, labeled T in  FIG. 1 , completed on the belt  2  is transferred from the belt  2  to the image transferring and fixing roller  13  by an electrostatic force derived from a bias applied to the drive roller  9  by secondary image transferring means not shown. The bias may be a DC bias or an AC-biased DC bias by way of example. The toner image T will sometimes be referred to as toner or toner particles hereinafter, as the case may be.  
         [0025]     As shown in  FIG. 1 , a heat insulation plate  20  is interposed between the belt  2  and the image transferring and fixing roller  13  and plays the role of a heat screening member or heat transfer control member for controlling the heat radiation or heat transfer from the roller  13  to the belt  2 . The heat insulation plate  20 , which may be mounted on either one of the fixing device body and the copier body, is formed with an opening so as to reduce heat radiation to the belt  2  without obstructing the secondary image transfer from the belt  2  to the image transferring and fixing roller  13 .  
         [0026]     The heat transfer control member  20  should preferably be implemented as a glossy plate with a low radiation ratio, more preferably two metallic sheets positioned at opposite sides of a small gap or an insulator. Further, use may be made of a thin plate having a microheat pipe structure used to cool a CPU (Central Processing Unit) mounted on a notebook size personal computer, in which case the heat transfer control member can be held at low temperature for controlling heat transfer at a high level. With the heat insulation plate  20 , it is possible to reduce temperature elevation of the belt or intermediate image transfer body  2  to a certain degree and therefore to control the thermal deterioration of the belt  2 .  
         [0027]     The image transferring and fixing roller  13  and press roller  14  each are implemented by a hollow cylindrical core  53  formed of aluminum or similar metal and coated with a 0.05 mm to 0.5 mm thick elastic layer and a 10 μm to 30 μm thick parting layer formed of PFA (perfluoroalcoxy), PTFE (polytetrafluoroethylene) or similar fluorine-based resin. If desired, the elastic layer and parting layer may be implemented as a tube fitted on the core  53 . Halogen heaters or heating means  15  are disposed in the vicinity of the surface of the image transferring and fixing roller  13  and the inside of the press roller  14  in order to heat toner present on the surfaces of the rollers  13  and  14 . The outside diameter of the image transferring and fixing roller  13  is selected to be great enough to cover the length of a single sheet P or above because the roller  13  has to hold an image extending over the length of the sheet P.  
         [0028]     Further, the surface temperature of the image transferring and fixing roller  13  and that of the press roller or second transferring and fixing roller  14  are sensed by a respective thermistor, not shown, each positioned in a non-image zone. A temperature controller, not shown, controls each of the halogen heaters  15  on or off in accordance with the above surface temperatures.  
         [0029]     If desired, the parting layer of each of the image transferring and fixing roller  13  and press roller  14  may be implemented by conductive fluorine-based resin in which carbon or similar conductive substance is dispersed and connected to the core  53  thereby. Such an alternative structure allows an image transfer bias to be applied between the two parting layers, allowing the image transfer bias to be lowered and reducing toner scattering at the time of image transfer.  
         [0030]     Reference will be made to  FIGS. 2 and 3  for describing a preferred embodiment of the fixing device in accordance with the present invention. As shown in  FIG. 2 , the toner image T carried on the belt  2  is transferred to the image transferring and fixing roller  13  with the drive roller  9  and roller  13  being held in contact with each other or spaced from each other by a small gap. Radiation energy, emitted from the halogen heater  15  adjoining the image transferring and fixing roller  13 , is condensed by a reflector  32  to heat the toner image present on the roller  13 , making the toner image half-melted.  
         [0031]     After the switch-on of the printer body, but before the start of a printing step, the surface temperature of the image transferring and fixing roller  13  is controlled to a preselected temperature by the halogen heaters  15  associated with the roller  13  and press roller  14  as well as by induction heating, which will be described specifically later. The surface temperature of the image transferring and fixing roller  13  is controlled to 100° C. or below, preferably to about 80° C., so as to protect the belt  2  from excessive load.  
         [0032]     At the stat of a printing step, the halogen heater  15  disposed in the press roller  14  and an induction coil, which will be described later, are turned off while power is fed to the halogen heater  15  substantially surrounded by the reflector  32 . Just after the toner image has been moved away from a copying region where the reflector  32  and halogen heater  15  are present, the temperature distribution in the toner layer is higher at the side facing the halogen heater  15 , i.e., the side expected to face the sheet P than at the side contacting the image transferring and fixing roller  13 . More specifically, while part of the temperature of the toner layer contacting the image transferring and fixing roller  13  is about 80° C., as stated earlier, part of the same facing the halogen heater  15  needs a temperature high enough to be sufficiently strongly fixed on the sheet P, which is higher than 80° C.  
         [0033]     We found that the temperature of the interface between a toner image and a sheet or recording medium high enough to implement sufficient fixing strength was about 110° C. to about 120° C. Moreover, it is apparent that because the temperature of toner particles far smaller in thermal capacity than a recording medium is sharply lowered on contacting a recording medium, the fixing step cannot be completed at the outlet of a nip for fixation unless the toner temperature is raised at the inlet of the nip. For example, assuming that the toner is heated to about 150° C. by radiation when moved away from the reflector  32 , then even the portions of the surface of the image transferring and fixing roller  13  where the toner image is absent directly receives radiation energy and is heated thereby.  
         [0034]     The surface of the transferring and fixing roller  13  is colored in black so as to increase the radiation energy absorption ratio at the time of startup. As a result, the interface between the toner particles and the image transferring and fixing roller  13  has heretofore been lower than the surface temperature of the roller  13  directly receiving the radiation energy, i.e., the surface temperature of the roller  13  has become irregular in the pattern of the toner image. The irregularity in temperature has remained on the image transferring and fixing roller  13  even after the toner image has been transferred to a recording medium at the nip.  
         [0035]     In the illustrative embodiment, the image transferring and fixing roller  13  is implemented by a heat pipe  22  in order to reduce the irregularity in temperature stated above as far as possible. More specifically, a working liquid  23  sealed in the heat pipe  22  uniforms the temperature distribution in the lengthwise direction of the image transferring and fixing roller  13  for thereby obviating the irregularity in temperature. It follows that even when another toner image pattern is transferred from the belt  2  to the image transferring and fixing roller  13 , the history of the previous toner image pattern is not left on the roller  13 , so that a high-quality toner image free from irregularity in fixation and irregular gloss and other image defects is achievable.  
         [0036]     As shown in  FIG. 3 , a radiator  29  is held in contact with one end of the heat pipe  22  while fins  29  are positioned around the radiator  27  and formed of aluminum or similar metal with high thermal conductivity. At the time of startup, the heat pipe  22  is rotated while the fins  29  are held stationary, i.e., the heat pipe  22  idles at connecting portions. When a solenoid-operated clutch  28  is energized, the fins  29  start rotating together with the heat pipe  22 .  
         [0037]     Positioned at the other end of the heat pipe  22  is a heating device made up of a heat generating layer  25 , a heat insulating layer  24  and an induction coil  26 . More specifically, the heating device includes a planar heating body adhered to the end of the heat pipe  22  and generating heat when a current is fed thereto or includes a metallic layer of, e.g., Ag formed on the above end and the induction coil  26  for induction heating. At the time of startup, the heating device with such a configuration generates heat in addition to heat generated by the halogen heater  15  and reflector  32  with the result that heat is rapidly transferred by the heat pipe  22  to thereby noticeably reduce the warm-up time. After the surface of the image transferring and fixing roller  13  has been heated to a preselected temperature, the heating device is turned off to operate as a usual heat pipe.  
         [0038]     When image patterns noticeably different in the area of a solid image from each other are continuously passed or when sheets of relatively small size are continuously passed, the fins  29  are rotated together with the heat pipe  22  via the solenoid-operated clutch  28  in the same manner as stated above. In addition, a fan  30 , positioned at one side of the fins  29 , is rotated to produce an air flow in a direction indicated by arrows in  FIG. 3 , allowing the temperature to be rapidly uniformed in the lengthwise direction.  
         [0039]     An alternative embodiment of the fixing device in accordance with the present invention will be described with reference to  FIG. 4 . As shown, this embodiment is identical with the previous embodiment except that the heat pipe  22  is held in contact with the outer periphery of the transferring and fixing roller  13 . The structure of the heat pipe  22  itself is identical with the structure shown in  FIG. 3 . Structural elements identical with the structural elements of the previous embodiment are designated by identical reference numerals, and detailed description thereof will not be made in order to avoid redundancy.  
         [0040]     In the illustrative embodiment, a halogen heater  15  is disposed in the image transferring and fixing roller  13  also. The three halogen heaters  15  in total further reduce the warm-up time in cooperation with induction heating. In a fixing step that begins after the surface of the image transferring and fixing roller  13  has reached the preselected temperature, only the halogen heater  15  substantially surrounded by the reflector  32  continuously heats the toner and image transferring and fixing roller  13 . Although irregularity in temperature may occur after the fixation of the toner image T on the sheet P as in the previous embodiment, the illustrative embodiment causes the heat pipe  22  held in contact with the image transferring and fixing roller  13  to rotate before the roller  13  again contacts the belt  2 . This successfully obviates the above irregularity because of the effect of the working liquid  23 .  
         [0041]     In the illustrative embodiment, the heat pipe  22 , contacting the image transferring and fixing roller  13 , may be implemented as a cleaning roller, in which case the surface of the cleaning roller is formed of a material lower in parting ability than the surface of the roller  13 . With this configuration, it is possible to remove toner particles left on the image transferring and fixing roller  13  after the image transfer for thereby insuring desirable images free from smears.  
         [0042]      FIG. 5  shows another alternative embodiment of the fixing device in accordance with the present invention. This embodiment differs from the previous embodiments in that the image transferring and fixing member is implemented by an image transferring and fixing belt  33  passed over the fixing roller  13  and heat pipe  22 . In this configuration, the toner image T is transferred from the belt  2  to the image transferring and fixing belt  33 . The structure of the heat pipe  22  itself is identical with the structure shown in  FIG. 3 . Structural elements identical with the structural elements of the embodiment described first are designated by identical reference numerals, and detailed description thereof will not be made in order to avoid redundancy.  
         [0043]     In the illustrative embodiment, too, a halogen heater  15  is disposed in the fixing roller  13  also. The three halogen heaters  15  in total further reduce the warm-up time in cooperation with induction heating. In a fixing step that begins after the surface of the image transferring and and fixing belt  13  has reached the preselected temperature, only the halogen heater  15  substantially surrounded by the reflector  32  continuously heats the toner and belt  13 . Although irregularity in temperature may occur on the transferring and fixing belt  33  after the fixation of the toner image T on the sheet P as in the previous embodiments, the illustrative embodiment causes the heat pipe  22  held in contact with the image transferring and fixing belt  13  to rotate before the belt  13  again contacts the belt  2 . This successfully obviates the above irregularity because of the effect of the working liquid  23 . The illustrative embodiment is advantageous over the previous embodiments in that a sufficient period of time is available between the nip and the position where the image transferring and fixing belt  3  again contacts the belt  2  and further uniforms temperature.  
         [0044]     In summary, it will be seen that the present invention provides a fixing device achieving a short warm-up time and obviating irregularity in temperature ascribable to the image history of the previous image forming step left on a fixing member and irregularity in temperature in the lengthwise direction ascribable to the continuous feed of sheets of relatively small size.  
         [0045]     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.