Abstract:
A fixing device for fixing a recording sheet, including: a heating roller, a pressure roller, and an exterior heating roller; wherein the hardness of the elastic layer of the heating roller is less than that of the elastic layer of the pressure roller, and wherein the heat conductivity of the elastic layer of the heating roller is equal to or less than 0.2 W/m K, and the heat conductivity of the elastic layer of the pressure roller is equal to or greater than 0.4 W/m K.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a fixing device in which a recording material carrying a toner image is heated and is pressed between a heating roller and a pressure roller so that fixing is achieved, and an image forming apparatus including the same, and in particular, a fixing device wherein an external heating roller is provided which is in contact with the surface of the heating roller, and an image forming apparatus including the same.  
         [0002]     In an image recording apparatus employing an electrophotographic method, such as a copying machine, a facsimile device, and a laser printer, widely used is a fixing device wherein toner images are formed on a photoconductor, and after the toner images are transferred onto a recording sheet, the sheet is conveyed between a heating roller, which houses a heat source in a cored cylinder and generates heat through an elastic layer which is formed on the outer circumferential surface of the cored cylinder, and a pressure roller which comes in contact with the heating roller, and thereby the toner images are permanently fixed onto the recording sheet by heat and pressure.  
         [0003]     Recently, due to the increased speed of image recording apparatus, feeding rate of the recording sheets also has become higher, and further, due to colorization of image and improvement of image quality, a rubber layer, having lower heat conductivity and being normally used for the elastic layer of the heating roller, is required to become thicker, and therefore, it is necessary that the heat source mounted in the cored cylinder must supply an increased amount of heat.  
         [0004]     However, when heat from the heat source in the cored cylinder is increased excessively, the elastic rubber layer is damaged by the heat, and the durability of the elastic rubber layer is deteriorated, whereby a problem results in that the elastic layer is unglued from the cored cylinder.  
         [0005]     In order to overcome these problems, proposed are a fixing device and an image recording apparatus using the fixing device, providing an auxiliary heating roller which comes into pressure-contact with the heating roller, and heats the surface of the heating roller from the outside (Patent Documents 1 and 2), or a fixing device and an image recording apparatus, providing an auxiliary heating roller which heats the surface of the pressure roller from the outside (Patent Document 3). 
        Patent Document 1: Japanese Tokkai 2001-22214     Patent Document 2: Japanese Tokkai 2003-280435     Patent Document 3: Japanese Tokkai 2002-49265        
 
         [0009]     However, in both of these technologies disclosed in Patent Documents 1 and 2, since a pressure roller is a hard roller, though high image quality such as halftone is required, reproductivity of the image is not sufficient. Further, in technology disclosed in Patent Document 3, a heat source is stored in a first fixing roller (heating roller), it takes long times to heat up the surface of the elastic layer to the level of fixing temperature, and if the heating capacity of the heat source is increased too large, the elastic layer made of a rubber is affected by the heat deterioration, which is a problem.  
       SUMMARY OF THE INVENTION  
       [0010]     An objective of the invention is to solve the above problems and to provide a fixing device having such a structure wherein an external heating roller comes into contact with the circumferential surface of the heating roller not having an interior heat source, and further, another objective is, by employing an image forming apparatus using said fixing device, though being a simple structure, to provide a fixing device for producing stable quality without problems such as the deterioration of the durability due to the heat deterioration of the elastic rubber layer of the heating roller, resulting in delamination between the elastic layer and the cored cylinder, and still further to provide an image recording apparatus providing with the fixing device mentioned above.  
         [0011]     The objectives of the present invention are attained by the structures described below.  
         [0012]     Structure 1  
         [0013]     A fixing device for fixing a recording sheet carrying a toner image, including: 
        (1) a heating roller which rotates and comes into contact with a surface carrying the toner image, of the recording sheet, including, 
            a cored cylinder,     an elastic layer which is formed of a rubber on an exterior surface of the cored cylinder, and     a toner releasing layer formed of a fluorine resin on a surface of the elastic layer;    
            (2) a pressure roller which comes into contact with the heating roller to nip the recording sheet carrying the toner image, including, 
            a heat source for heating the pressure roller,     a cored cylinder for including the heat source,     an elastic layer formed of a rubber on an exterior surface of the cored cylinder of the pressure roller, and     a toner releasing layer formed of a fluorine resin on a surface of the elastic layer of the pressure roller; and    
            (3) an exterior heating roller which comes into contact with the heating roller to heat the heating roller, including, 
            a heat source for heating the exterior heating roller,     a cored cylinder for housing the heat source of the exterior heating roller, and     a toner releasing layer formed of a fluorine resin on an exterior surface of the cored cylinder of the exterior heating roller;    
            wherein Asker-C hardness of the elastic layer of the heating roller is less than Asker-C hardness of the elastic layer of the pressure roller, and     wherein the heat conductivity of the elastic layer of the heating roller is equal to or less than 0.2 W/m K, and the heat conductivity of the elastic layer of the pressure roller is equal to or greater than 0.4 W/m K.        
 
         [0029]     Asker-C Hardness:  
         [0030]     The roller having the elastic layer was measured by Asker-C type hardness meter which was manufactured by Kobunshi Keiki Co., Ltd. Asker-C type hardness meter is suitable for measuring the hardness of the roller having the elastic layer of the fixing device.  
         [0031]     Structure 2  
         [0032]     The fixing device described in structure 1, wherein the pressure roller is released from the heating roller when the recording material sheet carrying the toner image is not conveyed.  
         [0033]     Structure 3  
         [0034]     A color image forming apparatus which performs a double sided printing, including the fixing device described in structure 1 or 2.  
         [0035]     The effects of the invention will be described below.  
         [0036]     Based on the invention described in structure 1, since there is no need to mount the heat source in the heating roller, the cored cylinder is not heated excessively, and thereby, prevented is deterioration of roller life due to the heat deterioration as well as the unglue which may result between the elastic layer and the cored cylinder. Further, since heating is not performed through the cored cylinder of the heating roller and the elastic layer, a fixing device featuring high heat conductivity and lower electric power consumption can be realized.  
         [0037]     Based on the invention of structure 2, since the pressure roller is released from the heating roller when the recording material is not fed between them, prevented is permanent deformation of the elastic layer of the heating roller and the elastic layer of the pressure roller.  
         [0038]     Based on the invention of structure 3, it is possible to provide the color image forming apparatus which can produce the color image of high quality on the front and back pages of the recording sheet, without any problem caused by deterioration of the elastic layer of the fixing device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]      FIG. 1  is a cross sectional schematic view of an embodiment of a color duplex image forming apparatus employing the fixing device of the present invention.  
         [0040]      FIG. 2  is a cross sectional view showing the structure of the first embodiment of the fixing device of the present invention.  
         [0041]      FIG. 3  is a cross sectional view showing the structure of the second embodiment of the fixing device of the present invention.  
         [0042]      FIG. 4  is a cross sectional view showing an exaggerated condition in which recording material P has been nipped between heating roller  31  and pressure roller  32 , both of which are under pressure contact. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]     The following describes the embodiments of the present invention. The assertive statements regarding the embodiments of the present invention indicate the best mode. They do not restrict the meaning of the terminologies of the present invention or the technological scope.  
         [0044]      FIG. 1  is a cross sectional view of the color duplex image forming apparatus showing one embodiment of an image forming apparatus according to the present invention.  
         [0045]     This color duplex image forming apparatus is called a tandem type color image forming apparatus, and has multiple sets of image forming units  10 Y,  10 M,  10 C and  10 K, an endless transfer belt unit  7 , paper feed means and heat fixing device  24 . A document readout apparatus SC is installed on the top of the main unit A of the image forming apparatus.  
         [0046]     The image forming unit  10 Y forming an yellow image has charging means, arranged around a drum-shaped photoconductor  1 Y as a first image carrier, exposure means  4 Y, developing means  4 Y, a primary transfer roll  5 Y as primary trans means, and cleaning means  6 Y. The image forming unit  10 M for forming a magenta image has a drum-shaped photoconductor  1 M as a first image carrier, charging means  2 M, exposure means  3 M, developing means  4 M, a primary transfer roll  5 M as primary transfer means, and cleaning means  6 M. The image forming apparatus  10 C for forming a cyan image has a drum-shaped photoconductor  1 C as a first image carrier, charging means  2 C, exposure means  3 C, developing means  4 C, primary transfer roll  5 C as primary transfer means, and cleaning means  6 C. The image forming apparatus  10 K for forming a black image has a drum-shaped photoconductor  1 K as a first image carrier, charging means  2 K, exposure means  3 K, developing means  4 K, a primary transfer roll  5 K as primary transfer means and cleaning means  6 K.  
         [0047]     The endless transfer belt unit  70  has an endless transfer belt  70  as a second semiconducting endless belt-shaped image carrier wound and rotatably supported by multiple rolls  71 ,  72 ,  73 ,  74  and  76 , and supported.  
         [0048]     Images formed by the image forming units  10 Y,  10 M,  10 C and  10 K are sequentially transferred onto the endless transfer belt  70  rotated by the primary transfer rolls  5 Y,  5 M,  5 C and  5 K, whereby a synthesized color image is formed. Sheet P as a transfer material (transfer material is hereafter called sheet P) that is a recording medium stored inside a paper feed cassette  20  is fed by paper feed means  21 , and is sent to the secondary transfer means  5 A via multiple intermediate rolls  22 A,  22 B,  22 C and  22 D and a resist roll  23 , whereby the color image is transferred on sheet P in one operation. The sheet P where the color image has been transferred is subjected to fixing process by a heat fixing device  24 , and is gripped by an ejection roll  25 . Then it is placed on the ejection tray  26  outside the apparatus.  
         [0049]     Cleaning means  6 A removes residual toner from the endless transfer belt  70  having curve-separated sheet P after color image has been transferred onto secondary transfer means  5 A.  
         [0050]     In addition, when the images are printed on the both sides of sheet P, sheet P which is carrying a transferred image on one of its sides is processed in fixing device  30 , and after sheet P is ejected by paired fix ejecting rollers  30 A, revering lever  27  is moved counterclockwise, and sheet P is guided toward paired turnaround rollers  28 A,  28 B, and  28 C. When the back end of sheet P, guided by turnaround rollers  28 A,  28 B, and  28 C, passes between turnaround rollers  28 A and  28 B, and approaches reversing paired rollers  28 C, paired turnaround rollers  28 C are reversed, and sheet P is conveyed toward turnaround roller  29  via paired turnaround rollers  28 E and  28 F. sheet P, reversed by turnaround roller  29 , passes between paired intermediate rollers  22 D and paired registration rollers  23 , and is conveyed toward secondary transfer means  5 A, whereby an image is transferred onto the other side of sheet P.  
         [0051]     Sheet P, carrying the transferred image, is processed in fixing device  30 , and arrives at transfer changeover lever  27  via fix ejecting rollers  30 A. Reversing lever  27  then rotates clockwise, before sheet P arrives. Whereby sheet P is nipped by paired ejecting rollers  25 , and ejected onto tray  26 , located outside the apparatus.  
         [0052]     During the image formation process, the primary transfer roller  5 K is always kept in pressure contact with photoconductor  1 K. Other primary transfer rollers  5 Y,  5 M and  5 C are kept in pressure contact with respective corresponding photoconductors  1 Y,  1 M and  1 C only during color image formation.  
         [0053]     Secondary transfer means  5 A is in pressure contact with endless transfer belt  70  only when sheet P passes and secondary transfer is performed.  
         [0054]      FIG. 2  is a cross sectional view showing the structure of the first embodiment of the fixing device of the present invention.  
         [0055]     In  FIG. 2 , fixing device  30  includes heating roller  31  excluding the heat source, pressure roller  32  being in pressure-contact with heating roller  31 , and external heating roller  33  being also in pressure contact with heating roller  31 .  
         [0056]     Heating roller  31  is composed of cored cylinder  311  formed of a metal or a hard resin, elastic layer  312  formed of a rubber on the surface of cored cylinder  311 , and fluorine resin layer  313  formed on the surface of elastic layer  312 . Pressure roller  32  is composed of cored cylinder  321  formed of a metal or a hard resin, elastic layer  322  formed of a rubber on the surface of cored cylinder  321 , and fluorine resin layer  323  (which is a toner releasing layer) formed on the surface of elastic layer  322 .  
         [0057]     External heating roller  33  is composed of roller member (which is a cored cylinder)  331  formed of aluminum or aluminum alloy, and heat source  332  which is a halogen lamp mounted within roller member  331 . The surface of roller member  331  is covered with fluorine resin layer  333  (which is a toner releasing layer).  
         [0058]     Before recording sheet P, on which toner images have been transferred, approaches fixing device  30 , the heat supplied from previously energized and heated by heat source  332  reaches fluorine resin layer  313  of heating roller  31  through roller member  331  which is rotating counter-clockwise, and heats elastic layer  312  of heating roller  31  through fluorine resin layer  313 . Since pressure roller  32  is pressed to heating roller  31 , the heat supplied from heat source  332  is conducted from fluorine resin layer  313  of heating roller  31  to elastic layer  322  through fluorine resin layer  323  of pressure roller  32 , and much more heat is supplied to elastic layer  322  of pressure roller  32 , because elastic layer  322  of pressure roller  32  is formed of a material having higher thermal conductivity than elastic layer  312  of heating roller  31 . The heat conductivity of elastic layers  312  and  322  are set to adequate values by changing the amount of additives of an absorbent such as carbon black.  
         [0059]     In  FIG. 2 , when the image forming apparatus is under normal operating conditions, pressure roller  32  is brought into pressure-contact with heating roller  31  by a spring member, which is not illustrated. When the image forming apparatus is under a power-off condition or a stand-by condition for a long time, pressure roller  32  is released from heating roller  31  by any of the well-known means using a motor or a cam mechanism, which is not illustrated. Permanent deformation of heating roller  31  and pressure roller  32  is thereby prevented by the above releasing process.  
         [0060]      FIG. 3  is a schematic view showing the structure of the second embodiment of the fixing device of the present invention.  
         [0061]     A position adjusting means of fixing device  30  shown in  FIG. 3  is a method wherein pressure roller  32  is in pressure-contact at a the predetermined position. Both ends of rotating shaft  32 A of pressure roller  32  is supported by pressing lever  32 C one end of which is supported by pivoted supporting member  32 B. The other end of pressing lever  32 C is controlled by cam  32 D to a predetermined pressing position. Cam  32 D is driven by a driving source which is not illustrated. When the position of cam  32 D is changed, the pressing point onto pressing lever  32 C is consequently changed, and thereby, when recording sheet P is nipped, the center distance between heating roller  31  and pressure roller  32  is changed, and the nipping depth at fixing area N can be adjusted to a predetermined depth. Cam  32 D is always in pressure-contact with pressing lever  32 C by a spring.  
         [0062]      FIG. 4  is a schematic view showing the condition in which recording sheet P has been nipped between heating roller  31  and pressure roller  32 , both of which are in pressure-contact with each other. Since the hardness of elastic layer  312  of heating roller  31  is set to be less than the hardness of elastic layer  322  of pressure roller  32 , elastic layer  312  of heating roller  31  is deformed as shown in  FIG. 4 , and thereby the top surface of recording sheet P is forced from the surface of heating roller  31  due to the hardness of sheet P, whereby sheet P easily separates from heating roller  31 . The hardness of elastic layers  312  and  322  is set to adequate values by changing the amount of additives of filling agent such as an inorganic filler or a vulcanizing agent.  
         [0063]     The inventor of the present invention performed the experiments described below, being practical examples 1 and 2, and comparative examples 1-4, using the fixing device shown in  FIGS. 2 and 3 , and the color duplex image forming apparatus shown in  FIG. 1 .  
         [0064]     (Common Elements)  
         [heading-0065]     1. Heating Roller  
         [none]    
       
          external diameter: 65 mm,  
          surface hardness: Asker—C78°,  
          material of cored cylinder: iron,  
          thickness of cored cylinder wall: 3.0 mm,  
          thermal conductivity of cored cylinder: 45 W/mK,  
          material of elastic layer (rubber layer): silicone rubber,  
          rubber hardness of elastic layer: JIS—25°,  
          material of toner releasing layer: PFA,  
          thickness of toner releasing layer: 30 μm,  
          thermal detector of the central image area: non-contact type,  
          thermal detector of the end image area: contact type, 
 
 2. Pressure Roller, 
 
          external diameter: 55 mm,  
          surface hardness: Asker—C83° (excluding comparative example 1),  
          material of cored cylinder: aluminum,  
          thickness of cored cylinder wall: 7.5 mm,  
          thermal conductivity of cored cylinder: 220 W/mK,  
          rubber hardness of elastic layer: JIS—45°,  
          material of toner releasing layer: PFA,  
          thickness of toner releasing layer: 30 μm,  
          heat source: halogen lamp,  
          thermal detector of the central image area: non-contact type,  
          thermal detector of the end image area: contact type, 
 
 3. External Heat Roller 
 
          external diameter: 50 mm,  
          material of cored cylinder: aluminum,  
          thickness of cored cylinder wall: 1.0 mm,  
          elastic layer: not used,  
          material of toner releasing layer: 30 μm,  
          thickness of toner releasing layer: 30 μm,  
          heat source: halogen lamp,  
          contacting pressure: 15 N/cm,  
       
     
         [0098]     The experimental results are shown in Table 1.  
                                                                                                                                                                                                                                                                                                                                                                 TABLE 1                                   Actual           Comparative examples   examples            Items   1   2   3   4   1   2                    Heating   Elastic   Thickness   1.5 mm   1.25 mm       1.5 mm            Roller   layer   Thermal   0.2 W/mK   0.3 W/mK   0.2 W/mK               conductivity            Pressure   Elastic   Hardness   Non   83° Asker-C            roller   layer   Material   elastic   Si   Si   Si Rubber                   layer   rubber   rubber   including TiO 2                         including                       TiO 2                      Thickness       1.25 mm       1.0 mm                    Thermal        0.4 W/mK   0.2 W/mK   0.3 W/mK   0.4 W/mK               conductivity            Press   Pressing force   800 N   700 N            Contact   Automatic   Releasing mechanism is not   Exists           release, while   employed.           sheeet is not            conveyed            Result   Image   Uneven   Highly   No problem           quality   luster   visible                Durability   After   No   Bad   Broken   Broken   No           test   100,000   Problem   Separation   rubber of   rubber of   problems               prints       of sheet.   heating   heating                       Wound sheet   roll at   roll at                       on heat   20,000   50,000                       roller   prints   prints                Fixed results   No problems   Under   No                   fixing   problems                Badly   Crossed   Barely exist, but no problems   No           printed   uneven       Problems           image   luster                  
 
         [0099]     Based on the experimental results by the inventor of the present invention, using the above examples, the inventor obtained the following results.  
         [0100]     In the case of comparative example 1, since the elastic layer is not formed on the pressure roller, an uneven luster is produced on the printed image.  
         [0101]     In the case of comparative example 2, the thickness of the elastic layer of the heating roller is equal to the thickness of the elastic layer of the pressure roller, and thereby the deformed amount of the heating roller is comparatively small while the sheet is nipped, that is, poor separation of the sheet from the heating roller occurs.  
         [0102]     In the case of comparative example 3, the heat conductivity of the elastic layer of the pressure roller is low, and in order to obtain high fixing performance, the heat amount radiated from the heat source in the pressure roller must be high, which raises the likelihood of heat deterioration of the pressure roller.  
         [0103]     In the case of comparative example 4, the heat conductivity of the elastic layer of the heating roller is rather high, and the heat conductivity of the elastic layer of the pressure roller is rather low, which causes that the heat is easily passed through the interior of the heating roller, resulting in the deterioration of fixing performance. In order to maintain high fixing performance, the heat amount radiated from the heat source in the pressure roller must be high, which raises the likelihood of heat deterioration of the pressure roller.