Patent Publication Number: US-6212352-B1

Title: Color image forming apparatus spacially separating toner image heat-fusion from toner image transfer to a recording medium

Description:
FIELD OF THE INVENTION 
     The present invention relates to a color image forming apparatus using an electrophotographic process, such as a copying machine, a facsimile machine, and a printer. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a typical color image forming apparatus using an electrophotographic process, such as a copying machine, a facsimile, and a printer, forms a toner image on the surface of a photosensitive body, then transfers the same onto a recording sheet while feeding the recording sheet to a fuser, and fuses the toner image onto the recording sheet, thereby turning the toner image into a permanent image. 
     Recently, an image forming apparatus of a different type has been put in practical use. To be more specific, this type of color image forming apparatus tranfers a toner image formed on the photosensitive body onto an intermediate transfer belt, then heats the toner image thereon, and fixes the toner image onto a recording sheet upon transfer. Examples of the image forming apparatus of this type are disclosed in Japanese Laid-open Patent Application No. 44220/1996 (Tokukaihei No. 8-44220) (which is referred to as prior art A, hereinafter), and Japanese Laid-open Patent Application No. 114282/1997 (Tokukaihei No. 9-114282) (which is referred to as prior art B, hereinafter), for example. 
     The image forming apparatus of prior art A is mainly arranged to form a color image. As shown in FIG. 10, this image forming apparatus includes a photosensitive body  84 , and toner images T of respective colors developed on the surface of the photosensitive body  84  are transferred and superimposed successively on an intermediate transfer belt  80  by means of a transfer roller  81  which also serves as a driving roller. 
     The intermediate transfer belt  80  is bridged across the transfer roller  81 , a sub-roller  82  also serving as a belt tension roller, and a thermal heater  83 . A pressing roller  85  is provided to oppose the thermal heater  83  through the intermediate transfer belt  80 . The pressing roller  85  either keeps a space from or touches the intermediate transfer belt  80  as the case may be. 
     To be more specific, the pressing roller  85  keeps a space from the intermediate transfer belt  80  while the toner images of respective colors are being transferred thereon, and touches the same when the above toner image transferring step is completed. 
     A recording paper P is transported to a pressing section (fixing nip section) Y of the pressing roller  85  against the thermal heater  83 , and a superimposed multi-color toner image T′ is fixed onto the recording paper P upon transfer by the heat conveyed from the thermal heater  83  and the pressure applied by the pressing roller  85 . 
     Also, as shown in FIG. 11, the image forming apparatus of prior art B includes an intermediate transfer belt  95  bridged across supporting rollers  92  and  93  in such a manner to oppose a photosensitive belt  91 . 
     In this image forming apparatus, a toner image T is formed on the photosensitive belt  91  and subsequently transferred onto the intermediate transfer belt  95 . Then, a transferred toner image T′ on the intermediate transfer belt  95  is heat-fused by a heating roller  96  touching an outer circumferential surface of the intermediate transfer belt  95  and having a halogen lamp serving as a heat source  96   a  inside, after which the toner image T′ is fixed onto a recording material P upon transfer at a pressing section (fixing nip section ) Y of a pressing roller  97  against the supporting roller  93 , both provided at the downstream end of the heating roller  96 . 
     In this image forming apparatus, the toner alone is heat-fused in a satisfactory manner by heating the toner and intermediate transfer belt  95  having a small heat capacity before fixing the toner image onto the recording material P. Thus, the recording material P does not have to be heated, thereby attaining an effect that fixing energy is reduced to one third. 
     However, the image forming apparatus of prior art A has the following problems. 
     1) To attain satisfactory fixing strength, a temperature at the interface between the toner and recording paper P must be raised quite high. Particularly, in case of a color image, a multi-layer toner image in multiple colors must be heated at the fixing nip section for a considerable time to convey the heat to the interface between the toner and recording paper P, thereby imposing a limit on the acceleration of a fixing rate. 
     2) Since a color image is composed of multi-layer toner image portions and single-layer image portions having their respective layer thicknesses, if the entire color image is heated to attain a satisfactory fixing strength at the multi-layer toner image portions, the single-layer toner image portions are heated too much, thereby causing a so-called high-temperature offset. Conversely, if the entire color image is heated to attain a satisfactory fixing strength at the single-layer toner image portions, the multi-layer toner image portions are not heated satisfactorily, thereby causing fixing deficiency. 
     Also, the image forming apparatus of prior art B has the following problems. 
     3) Even if the toner is heat-fused in a satisfactory manner before the toner image is fixed onto the recording material P upon transfer, satisfactory fixing strength may not be attained if the recording material P is too cold. This happens because the toner is cooled and turned into solid as it confers its heat to the recording material P before it impregnates into microscopic spaces among the fibers of which the recording material P is made. To eliminate this inconvenience and constantly attain satisfactory fixing strength, the recording material P must be pre-heated or the toner and intermediate transfer belt  95  must be heated exceedingly to raise temperatures thereof well above a fixing temperature of the toner (for example, up to 170° C. when a fixing temperature of the toner is 102° C.), so that a temperature of the recording material P is also raised sufficiently high at the fixing nip section Y. Thus, in case of a single-layer toner image for a monochrome image, such heating requires more or less the same fixing energy as in the conventional method. 
     4) The toner and intermediate transfer belt  95  heated by the heating roller  96  touch the supporting roller  93  before they are pressed against the recording material P. Thus, if the supporting roller  93  is not heated as high as the toner and intermediate transfer belt  95 , the toner and intermediate transfer belt  95  conveys the heat to the supporting roller  93 , and the toner is cooled and turned into solid before it is fixed onto the recording material P upon transfer. This problem may be solved by heating the supporting roller  93  sufficiently, but a warm-up period is undesirably extended in turn. 
     Further, prior arts A and B share the following problem. 
     5) Since the toner fixing properties depend not only on the temperatures of the thermal heater and heating roller, but also on the temperature of the pressing roller, the fixing properties readily vary right after the warm-up and with operation conditions, such as continuous feeding. The fixing properties also vary with the kinds of the recording paper P (recording material P) (normal papers, envelopes, postcards, OHPs, labels, etc.) or thickness (basis weight) thereof. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a color image forming apparatus which can accelerate a fixing rate while maintaining stable fixing properties for a multi-layer toner image of a color image having different layer thicknesses with any kind of recording papers under any operation condition, and requires a shorter warm-up period while attaining excellent heat efficiency. 
     To fulfill the above and other objects, a color image forming apparatus of the present invention is furnished with: 
     an image carrying body supported in such a manner that a toner image forming surface thereof is allowed to circulate; 
     an image forming section for forming a multi-color toner image on the image carrying body by sequentially superimposing toner images in respective colors; 
     a heating section for heat-fusing the multi-color toner image; and 
     a pressing section for pressing against the image carrying body from the toner image forming surface side, 
     wherein: 
     the multi-color toner image heat-fused is fixed onto a recording material upon transfer at a pressing position of the pressing section against the image carrying body; and 
     the pressing position of the pressing section against the image carrying body is spaced apart from a heating position where the multi-color toner image is heated by the heating section. 
     According to the above arrangement, since a position where the toner image is heated is spaced apart from a position where the toner is fixed onto the recording material upon transfer, the pressing position of the pressing section against the image carrying body can be set to keep a space from the above heating position, for example, at the downstream end of the heating position along a circulation direction of the image carrying body. 
     Consequently, a long portion can be used to heat the toner image along the circulation direction regardless of the pressing position, thereby making it possible to extensively heat the toner image formed on the image carrying body. For this reason, even a multi-layer toner image in multiple colors can be fused uniformly in a shorter time regardless of the layer thickness. 
     Thus, a multi-layer toner image in multiple colors can be fixed more rapidly. Also, even when the layer thickness of the multi-layer toner image varies, uniform and stable fixing properties can be attained. 
     For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view schematically showing an arrangement of a color laser printer in accordance with an example embodiment of the present invention; 
     FIG. 2 is a view explaining Examples of a fixing test where the fixing test is carried out by the color laser printer of FIG. 1; 
     FIG. 3 is a view explaining Comparative Examples by way of comparison with Examples of FIG. 2; 
     FIG. 4 is a graph showing a relation of temperatures versus complex viscosity of the toner used in the fixing test; 
     FIG. 5 is a view explaining an Example of a test for confirming an effect of a pressing roller of the color laser printer of FIG. 1; 
     FIG. 6 is a view explaining a Comparative Example of the test for confirming an effect of the pressing roller of the color laser printer of FIG. 5; 
     FIG. 7 is a view explaining another Comparative Example of the test for confirming an effect of the pressing roller of the color laser printer of FIG. 5; 
     FIG. 8 is a view schematically showing an arrangement of a color laser printer in accordance with another example embodiment of the present invention; 
     FIG. 9 is a view schematically showing an arrangement of a color laser printer in accordance with still another example embodiment of the present invention; 
     FIG. 10 is a view schematically showing an arrangement of a conventional image forming apparatus; and 
     FIG. 11 is another view schematically showing an arrangement of a conventional image forming apparatus. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring to the accompanying drawings, the following description will describe example embodiments of the present invention. 
     Embodiment 1 
     Referring to FIGS. 1 through 7, the following description will describe an example embodiment of the present invention. In the present embodiment, a color laser printer is explained as an example color image forming apparatus of the present invention. 
     To begin with, a color laser printer of the present embodiment will be explained briefly. 
     As shown in FIG. 1, the color laser printer includes four visible image forming units  10 Y,  10 M,  10 C, and  10 B as image forming means, and a transfer/fixing unit  20 . 
     The transfer/fixing unit  20  includes an intermediate transfer belt  21  as an image carrying belt, a heating roller  22  as heating means, a fixing roller  23 , a pressing roller  24 , a tension roller  25 , and a cleaning roller  26 , all of which are allowed to turn or rotate while their axes of rotation being aligned parallel to each other. 
     The intermediate transfer belt  21  is an image carrying body, bridged across the heating roller  22  and tension roller  25 , for carrying toner images formed by each of the four visible image forming units  10 Y,  10 M,  10 C, and  10 B. The fixing roller  23  and pressing roller  24  are provided at the downstream end of the heating roller  22  along the turning direction of the intermediate transfer belt  21  as a pair of rollers opposing each other through the intermediate transfer belt  21 . The pressing roller  24  is pressed against the fixing roller  23  at a predetermined pressure by unillustrated pressing means. 
     The cleaning roller  26  is provided at the downstream end of the fixing roller  23  and pressing roller  24  along the turning direction of the intermediate transfer belt  21  in such a manner to oppose the tension roller  25  through the intermediate transfer belt  21 . The cleaning roller  26  is pressed against the tension roller  25  at a predetermined pressure by an unillustrated pressing device. 
     The aforementioned four visible image forming units  10 Y,  10 M,  10 C and  10 B are provided sequentially at the downstream end of the cleaning roller  26  along the turning direction of the intermediate transfer belt  21  in such a manner to oppose the outer circumferential surface of the intermediate transfer belt  21 . 
     Each visible image forming unit includes a photosensitive drum  11  surrounded by a charging roller  12 , laser beam irradiating means  13 , a developer  14 , a transfer roller  15 , and a cleaner  16 . In FIG. 1, the aforementioned components are labeled with their respective numerical references for the visible image forming unit  10 Y alone and the numerical references are omitted for the rest for the explanation&#39;s convenience. 
     The developer  14  in each visible image forming unit withholds a different color. To be more specific, yellow (Y) toner is withheld in the developer  14  of the visible image forming unit  10 Y, and likewise, magenta (M) toner in the developer  14  of the visible image forming unit  10 M, cyan (C) toner in the developer  14  of the visible image forming unit  10 C, and black (B) toner in the developer  14  of the visible image forming unit  10 B. 
     Each visible image forming unit transfers the toner image sequentially to the intermediate transfer belt  21  in the following manner. 
     That is, the surface of the photosensitive drum  11  is electrically charged uniformly by the charging roller  12 , and exposed by a laser beam emitted from the laser beam irradiating means  13  in accordance with image information, whereby an electrostatic latent image is formed thereon. Later, the electrostatic latent image on the photosensitive drum  11  is developed to a toner image by the developer  14  in each of the visible image forming units  10 Y,  10 M,  10 C, and  10 B. Each visible toner image is transferred onto the intermediate transfer belt  21  in the transfer/fixing unit  20  sequentially by the transfer roller  15 , to which a bias voltage of a polarity opposite to the polarity of the toner is applied. 
     Next, the transfer/fixing unit  20  will be explained in detail. 
     The intermediate transfer belt  21  used in the present embodiment is a 80 mm-long and 40 μm-thick endless belt made of metallic nickel produced by means of electrocasting whose outer circumferential surface is coated with a 150 μm-thick releasing layer made of silicone rubber. Note that the arrangement of the intermediate transfer belt  21  is not limited to the above disclosure, and for example, it may be a 30 μm-thick polyimide belt whose outer circumferential surface is coated with a 150 μm-thick releasing layer made of silicone rubber. 
     The heating roller  22  is composed of an aluminum drum  22   a  having a diameter of 15 mm and a thickness of 0.5 mm and coated with a 20 μm-thick coating layer  22   b  made of fluoride rubber. A halogen lamp (heater lamp)  27  of 800 W is provided inside the heating roller  22  as a heat source for heating the surface thereof. 
     The coating layer  22   b  is provided to secure a driving force for the intermediate transfer belt  21 . For this reason, the coating layer  22   b  is made as thin as possible so as not to deteriorate the heating properties of the heating roller  22  with respect to the intermediate transfer belt  21 . In addition, a metallic filler is attached to the coating layer  22   b  to improve heat conductivity. 
     As shown in FIG. 1, the intermediate transfer belt  21  is wound around the heating roller  22  at a contact angle of about 180 degrees, whereby a heating section X (heating length of 23.5 mm, for example) for heat-fusing the toner image T transferred onto the intermediate transfer belt  21  is formed at the inner circumferential surface of the intermediate transfer belt  21 . 
     A thermistor  28  is provided to touch the heating roller  22  at an outer circumferential portion other than the heating section X. Thus, a surface temperature of the heating roller  22  is detected and the ON/OFF state of the halogen lamp  27  is controlled based on the detected surface temperature, thereby allowing the surface temperature to remain at a predetermined level (herein, 170° C.). Here, the warm-up requires substantially no waiting time, because it only takes 10 or less seconds to raise the surface temperature of the heating roller  22  to 170° C. since the halogen lamp  27  is switched ON. 
     The fixing roller  23  is provided on the side of the inner circumferential surface of the intermediate transfer belt  21 , and composed of an aluminum column  23   a  having a diameter of 20 mm and coated with a 5 mm-thick heat-resistant elastic layer  23   b  made of silicone rubber. 
     The pressing roller  24  is composed of an aluminum drum  24   a  having a diameter of 30 mm and a thickness of 5 mm and coated with a 10 μm-thick releasing layer  24   b  made of fluoride resin (polytetrafluoroethylene or PTFE for short). Note that the drum  24   a  is not necessarily made of aluminum, and high heat conducting materials, such as carbon steel, can be used as the case needed. 
     In addition, the pressing roller  24  is provided on the outer circumferential surface of the intermediate transfer belt  21  at a position opposing the fixing roller  23 . Here, the pressing roller  24  and fixing roller  23  form a pair of rollers serving as pressing means. The pressing roller  24  is pressed against the fixing roller  23  at a predetermined pressure (herein, 180 N) by an unillustrated pressing device through the intermediate transfer belt  21 , whereby a fixing nip section Y is formed. 
     Herein, a width of the fixing nip section Y (fixing nip width) is set to 4.5 mm. Also, since the fixing nip section Y is composed of two rollers: the soft elastic fixing roller  23  and a hard (metal) pressing roller  24 , a cross section of the fixing nip section Y forms a convex shape curving upward. Thus, the recording paper P transported to the fixing nip section Y is readily separated from the intermediate transfer belt  21  even if a separating claw is not provided therein. 
     As has been explained, in the above-arranged color laser printer, the length of the heating section X (heating length) is set to 23.5 mm, and the length of the fixing nip section Y (fixing nip length) formed at the downstream end of the heating section X is set to 4.5 mm, while the heating section X is spaced apart from the fixing nip section Y serving as the fixing section where the toner image is fixed onto the recording paper P upon transfer. The above lengths are located at intervals along the turning direction of the intermediate transfer belt  21 . Therefore, a heating length far longer than the length of the fixing nip section Y can be readily provided. Hence, if a turning rate of the intermediate transfer belt  21  is increased, the toner image on the intermediate transfer belt  21  can be fused in a reliable manner. Thus, compared with conventional image forming apparatus which heat-fuses the toner upon fixing onto the recording paper, a higher processing rate can be attained. 
     Here, let L 1  be a length of the heating section X and L 2  be a length of the fixing nip section Y, then 2L 2 &lt;L 1 &lt;10L 2 , and preferably, 3L 2 &lt;L 1 &lt;8L 2 , and more preferably, 4L 2 &lt;L 1 &lt;6L 2 . 
     Next, heat capacities of the intermediate transfer belt  21  and pressing roller  24  will be explained. 
     A heat capacity Cb of the intermediate transfer belt  21  per unit area is computed as follows: 
     Given that: 
     a specific heat of Ni electrocasting section: 0.439 J/g° C. 
     a specific gravity of Ni electrocasting section: 8.9 g/cm 3    
     a specific heat of silicone rubber: 1.09 J/g° C. 
     a specific gravity of silicone rubber: 1.05 g/cm 3  then, 
     
       
           Cb =(0.439×8.9×0.004)+(1.09×1.05×0.015)=0.0327 J/° C. 
       
     
     On the other hand, a heat capacity Cr of the pressing roller  24  per unit area can be found as follows: 
     Given that: 
     a specific heat of aluminum: 0.9 J/g° C. 
     a specific gravity of aluminum; 2.7 g/cm 3    
     a specific heat of PTFE: 1.05 J/g° C. 
     a specific gravity of PTFE: 2.14 g/cm 3  then, 
     
       
           Cr=[ 0.9×2.7×(1.5 2 −1 2 )π+1.05×2.14×(1.501 2 −1.5 2 )π]/3π)=1.01 J/° C. 
       
     
     In other words, the heat capacity Cr of the pressing roller  24  is set to a value more than 30 times of the heat capacity Cb of the intermediate transfer belt  21 . In short, it is set as CrCb (Cr&gt;30×Cb). Although it will be described below, this arrangement is provided to minimize a rise in temperature of the pressing roller  24  by the heat transmitted from the intermediate transfer belt  21 . 
     Further, the aforementioned heating roller  22 , fixing roller  23 , and pressing roller  24  are aligned with respect to each other in such a manner that the intermediate transfer belt  21  goes into the fixing nip section Y along a tangential line direction. For example, the heating section X is positioned at the inner circumference surface of the intermediate transfer belt  21 . This arrangement makes it possible to prevent a drop in temperatures of the toner image T and intermediate transfer belt  21  heated by the heating roller  22 , which happens when they touch the fixing roller  23  too long at the upstream end of the fixing nip section Y along the turning direction of the intermediate transfer belt  21 . 
     The tension roller  25  is composed of an aluminum column  25   a  having a diameter of 16 mm and coated with a coating layer  25   b  made of silicone rubber. The tension roller  25  exerts predetermined tension so that the intermediate transfer belt  21  is not bent. 
     The cleaning roller  26  is composed of an aluminum column  26   a  having a diameter of 16 mm with a 2 mm-thick felt  26   b  made of Nomex of Dupont being wound around in spiral. 
     Next, the operation of the above-arranged color laser printer will be explained in the following. 
     To begin with, the halogen lamp  27  provided inside the heating roller  22  is switched ON while the intermediate transfer belt  21  is kept stopped, whereby a temperature on the surface of the heating roller  22  is raised to a predetermined level (herein, 170°). The intermediate transfer belt  21  is kept stopped during the warm-up period for the two following reasons: 
     1) to prevent a delay in the rise of the temperature of the surface of the heating roller  22  that otherwise would result from a transfer of heat away from the surface of heating roller  22  when intermediate transfer belt  21  was moving; and 
     2) to prevent an increase in temperature of the pressing roller  24  by the heat transmitted from the heated intermediate transfer belt  21 . 
     Then, when the warm-up of the heating roller  22  is completed, the heating roller  22 , fixing roller  23 , and pressing roller  24  are driven to rotate, so as to turn the intermediate transfer belt  21  and sequentially transfer and superimpose the toner images formed by the visible image forming units  10 Y,  10 M,  10 C, and  10 B in their respective colors on the intermediate transfer belt  21 . 
     A superimposed toner image T formed by the above transferring is heat-fused at the heating section X through the intermediate transfer belt  21  by the heat conveyed from the heating roller  22  whose temperature is maintained at 170° C. 
     Later, the toner image T fused on the intermediate transfer belt  21  is transported to the pressing section (fixing nip section Y) of the pressing roller  24  against the fixing roller  23  as the intermediate transfer belt  21  is turned further. A recording paper P is fed into a space between the intermediate transfer belt  21  and pressing roller  24  at the above transportation timing. 
     Then, the recording paper P transported to the fixing nip section Y is pressed against the pressing roller  24  through the intermediate transfer belt  21 , whereby the fused toner image is fixed onto the recording paper P upon transfer. At the fixing nip section Y, the heat of the intermediate transfer belt  21  and toner image T is conveyed to the fixing roller  23 , pressing roller  24 , and recording paper P, all of which are maintained at a temperature around the ambient temperature. 
     Consequently, the toner transferred and fixed onto the recording paper P is cooled and turned into solid, thereby increasing the cohesion among the toner layers. Thus, the offset (residual) of the toner on the surface of the intermediate transfer belt  21  can be prevented without applying thereon a releasing agent, such as silicone oil. 
     On the other hand, if the offset of the toner occurs on the outer circumferential surface of the intermediate transfer belt  21  by paper jam or the like, the cleaning roller  26  scrapes off the offset toner and paper dust adhering thereon as it touches the same at the downstream end of the fixing nip section Y. 
     Also, as has been explained, the intermediate transfer belt  21  not only has a small heat capacity, but also conveys its heat to the fixing roller  23 , pressing roller  24 , and recording paper P at the fixing nip section Y. Further, the intermediate transfer belt  21  releases its heat to an atmosphere in a long transportation segment at its downstream end, and conveys its heat to the pressing section of the tension roller  25  and cleaning roller  26 . Thus, the intermediate transfer belt  21  is cooled sufficiently by the time it returns to the pressing section against the visible image forming unit  10 Y, and does not give any adverse effect to the photosensitivities of the photosensitive drum  11 . 
     The above-arranged color laser printer is tested in various manners, and the characteristics, functions and effects of the color image forming apparatus of the present invention will be explained with reference to the test results. 
     To begin with, the results of the fixing test conducted by the above-arranged color laser printer will be set forth below. 
     (First Test) 
     Herein, tests (hereinafter, referred to as Examples 1-7) and comparative tests (hereinafter, referred to as Comparative Examples 1-7) are conducted using the above-arranged color laser printer. Here, the transfer/fixing unit  20  of FIGS. 2 and 3 is used commonly in Examples 1-7 and Comparative Examples 1-7. 
     In Examples 1-7, as shown in FIG. 2, the heating roller  22  is activated to raise its temperature from room temperature to a predetermined temperature while the intermediate transfer belt  21  is kept stopped. Subsequently, as soon as the intermediate transfer belt  21  is started to turn, a non-fused toner image T is transferred onto the intermediate transfer belt  21 , whereby the non-fused toner image T is heat-fused at the heating section X by the heating roller  22  maintained at the predetermined temperature. 
     Here, since the heating length of the heating section X is set as long as 23.5 mm, and a heat capacity of the toner image T and intermediate transfer belt  21  is quite small, the toner image T is heated in a satisfactory manner. To be more specific, even in case of a double-layer toner image T, the top layer of the toner image T is exposed to air, but can be heated as high as the surface temperature of the heating roller  22 . Consequently, the toner image T heat-fused at the temperature as high as the surface temperature of the heating roller  22  is fixed onto the recording paper P upon transfer at the fixing nip section Y. 
     In Comparative Examples 1-7, as shown in FIG. 3, a non-fused toner image T is not transferred onto the intermediate transfer belt  21 , but as is in the conventional image forming apparatus, it is transferred onto the recording paper P before being heat-fused, and the toner image T is fixed onto the recording paper P as the recording paper P is transported to the fixing nip section Y along with a intermediate transfer belt  21  heated at the heating section X. 
     In other words, in Comparative Examples 1-7, the non-fused toner image T is not heated as high as the surface temperature of the heating roller  22 . In this case, the non-fused toner image T is merely heated at the fixing nip section Y which is far shorter than the above heating section X. 
     In the fixing tests, toner A (magenta) having a softening point of 102° C. and toner B (magenta) having a softening point of 120° C. are used, whose relations of the temperature versus viscosity (complex viscosity) are respectively shown in FIG.  4 . Also, the fixing rate is set to 85 mm/s. The test results of Examples 1-7 and Comparative Examples 1-7 conducted under the above conditions are set forth in Table 1 below. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 RECORDING 
                   
                   
                   
               
               
                   
                 KIND 
                 NUMBER 
                 PAPER 
                 EXAMPLE/ 
                 FIXABLE 
                 TEMP. AT 
               
               
                 TEST 
                 OF 
                 OF TONER 
                 (BASIS 
                 COMPARATIVE 
                 TEMP. 
                 INTERFACE 
               
               
                 No. 
                 TONER 
                 LAYER(S) 
                 WEIGHT) 
                 EXAMPLE 
                 (° C.) 
                 (° C.) 
               
               
                   
               
             
            
               
                 1 
                 A 
                 1 
                 80 g 
                 1 
                 170 
                 125 
               
               
                   
                   
                   
                   
                 1 
                 170 
                 110 
               
               
                 2 
                 B 
                 1 
                 80 g 
                 2 
                 180 
                 135 
               
               
                   
                   
                   
                   
                 2 
                 180 
                 120 
               
               
                 3 
                 A 
                 2 
                 80 g 
                 3 
                 170 
                 124 
               
               
                   
                   
                   
                   
                 3 
                 185 
                 111 
               
               
                 4 
                 A 
                 3 
                 80 g 
                 4 
                 167 
                 124 
               
               
                   
                   
                   
                   
                 4 
                 190 
                 111 
               
               
                 5 
                 A 
                 4 
                 80 g 
                 5 
                 165 
                 125 
               
               
                   
                   
                   
                   
                 5 
                 195 
                 111 
               
               
                 6 
                 A 
                 1 
                 52 g 
                 6 
                 175 
                 126 
               
               
                   
                   
                   
                   
                 6 
                 180 
                 110 
               
               
                 7 
                 A 
                 1 
                 128 g  
                 7 
                 170 
                 125 
               
               
                   
                   
                   
                   
                 7 
                 170 
                 109 
               
               
                   
               
            
           
         
       
     
     In Table 1, “fixable temperature” means a temperature of the heating roller  22  at which satisfactory fixing strength is obtained as the result of the bending test of the toner image fixed onto the recording paper P while the temperature of the heating roller  22  is varied by 5° C. each time. 
     Also, “temperature at interface” means a logically computed value of a temperature at the interface between the toner and paper at the fixing nip section Y when the heating roller  22  is heated to the fixable temperature, which is computed by a linear heat transmission simulation by means of the calculus of finite difference. The logical value is used because an actual temperature at the interface between the toner and paper is very difficult to measure. 
     In the following, a computing method of the temperature at the interface between the toner and paper by means of the logical computation will be explained. 
     A linear equation of non-steady heat conduction is expressed as:                  ∂   T       ∂   t       =       λ     ρ                 c       ·           ∂   2        T       ∂     x   2         .               (   1   )                         
     where T, t, x, λ, ρ, and c represent temperature, time, distance, heat conductivity, specific gravity, and specific heat, respectively. 
     By introducing a difference equation by means of spacial discrete from Equation (1) above, we get:                T        (     x   ,     t   +   τ       )       =       T        (     x   ,   t     )       +           λ                 τ       ρ                   ch   2              [       T        (       x   -   h     ,   t     )       -     (     T        (     x   ,   t     )       )     +     (       T        (       x   +   h     ,   t     )       -     T        (     x   ,   t     )         )       ]       .               (   2   )                         
     where h represents a distance from one lattice to another and τ represents a very short period of time. Here, if temperatures T at three adjacent lattices (x−h), (x), and (x+h) keeping a very small distance (h) from each other at a given time (t) are known, then temperatures T (x, t+τ) at a very short period of time τ later, that is, at (t+τ), can be computed from Equation (2) above. 
     Equation (2) above is a difference equation within the same material, but a difference equation on the interface between different materials (a) and (b) can be computed in the same manner as Equation (2) above, which is expressed as:                T        (     x   ,     t   +   τ       )       =       T        (     x   ,   t     )       +         2      τ           ρ   a          c   a          h   a       +       ρ   b          c   b          h   b           [                    λ   a       h   a                       (       T        (       x   -   h     ,   t     )       -     (     T        (     x   ,   t     )       )     +         λ   b       h   b                       (       T        (       x   +   h     ,   t     )       -     T        (     x   ,   t     )         )         ]                   (   3   )                         
     The other analysis conditions are set forth as follow: 
     {circle around (1)} heat transfer in the axial and circumferential directions is not concerned, and heat transfer in the thickness (linear) direction alone is concerned; 
     {circle around (2)} irradiation and heat transmission by natural convection current are concerned as a heat loss from the surface of the fixing roller; 
     {circle around (3)} physical properties depend on a temperature of air alone; 
     {circle around (4)} the toner layer is uniform (printing at 100%) and the layer thickness does not vary before and after the fixing; 
     {circle around (5)} the variance of the physical properties with a change in phase of the toner is not concerned; 
     {circle around (6)} contact heat resistance is not concerned; 
     {circle around (7)} a temperature at the interface at the instance when two subjects (for example, toner and fixing roller) touch with each other at an inlet of the fixing nip section is defined as an average of the temperatures of the two subjects before they touch with each other which are weighted with the heat capacity; and 
     {circle around (8)} evaporation heat of the moisture contained in the recording paper is not concerned. 
     Thus, if temperatures of the intermediate transfer belt  21  before entering into the fixing nip section, fixing roller  23 , pressing roller  24 , toner, recording paper P, and atmosphere are measured as the initial conditions at the time t=0, then a change in temperature at each component after the lapse of an arbitrary time τ can be computed by the above computation method. In the present fixing tests, since the fixing rate is set to 85 mm/s and the fixing nip length is set to 4.5 mm, a temperature at the interface between the toner and paper over a time τ=4.5/85=53 ms is computed in accordance with the above equation. 
     Note that the above computing method is an example for finding a temperature at the interface between the toner and paper, and it should be appreciated that the temperature at the interface between the toner and paper can be found by any other applicable method. For example, the above method adopts the explicit method of the difference equation, but the implicit method of the difference equation can be adopted as well. Alternatively, the linear equation may be expanded to a quadratic equation to further improve the computation accuracy. Further, besides the heat transmission simulation, a temperature at the interface between the toner and paper may be inferred empirically based on the results of various kinds of tests. 
     Next, the test results will be detailed in the following. 
     As has been explained, in Examples, the non-fused toner image T is heat-fused on the intermediate transfer belt  21 , and fixed onto the recording paper P at the fixing nip section Y upon transfer. The result of Test No. 1 reveals that, in case of a single-layer toner image, a temperature of the heating roller  22  (≈temperature of toner image T) must be set to a temperature (170° C.) far higher than the toner softening point (102° C.) to secure satisfactory fixing strength. 
     Also, the result of the Comparative Example, where the non-fused toner image T is transferred onto the recording paper P and fixed thereon upon heating together with the recording paper P at the fixing nip section Y, reveals that the fixable temperature must be set to 170° C. as well. Thus, in case of a single-layer toner image, the fixable temperatures are the same in the Example and Comparative Example. 
     Thus, in case of a monochrome laser printer handling a single-layer toner image, the fixing energy is about the same whether the toner image is heat-fused and fixed onto the recording paper upon transfer or the toner image is fixed onto the recording paper upon heating with the recording paper. 
     The reason why is as follows. Even when the toner image T is heat-fused and fixed onto the recording paper upon transfer as it is in the Example, if the recording paper P is too cold, no matter how satisfactorily the toner is heat-fused, the toner is cooled and turned into solid as its heat is conveyed to the recording paper P before it impregnates into the microscopic spaces among the fibers of which the recording paper P is made. To solve this problem, the toner and intermediate transfer belt  21  must be heated exceedingly to raise the temperature of the recording paper P satisfactorily as well. Further, although it will be described below, if a temperature at the interface between the toner and paper is low, the same fixing strength can be attained more readily by fixing the toner image after it is transferred onto the recording paper. 
     However, in case of a double-layer toner image, the result of the Test No. 3 reveals the following. That is, in the case of the Example, the fixable temperature of the heating roller  22  is the 170° C., which is the same as the fixable temperature in the case of the single-layer toner image (Test No. 1). By contrast, in the case of the Comparative Example, a temperature of the heating roller  22  must be raised to 185° C., which is 15° C. higher than the temperature in the case of the single-layer toner image. Thus, in the case of the double-layer toner image, the fixing energy can be saved in the Example compared with the Comparative Example. 
     The reason why is as follows. In the Example, since the heating section X of the heating roller  22  is sufficiently long (23.5 mm), the toner image can be heated uniformly regardless of the layer thickness. Whereas in the Comparative Example, since a temperature at the interface between the toner and paper must be raised sufficiently high at the fixing nip section Y (fixing nip length: 4.5 mm), which is far shorter than the heating section X, the heat is not conveyed sufficiently to the interface between the toner and paper if the toner layer is too thick. 
     In the case of triple-layer and quadruple-layer toner images thicker than the double-layer toner images, the results of Test Nos. 4 and 5 reveal that, in the Examples, the fixable temperatures of the heating roller  22  are 167° C. and 165° C., respectively, which are about the same as the one in the case of the single-layer toner image (Test No. 1). 
     On the other hand, the results of the Test Nos. 4 and 5 reveal that, in the Comparative Examples, the fixable temperatures of the heating roller  22  are 190° C. and 195° C., respectively, which are higher by 23° C. and 30° C. than those in the corresponding Examples. In other words, in Comparative Examples, the fixing energy must be increased compared to the fixing energy required in the Examples. 
     As can be understood from the foregoing, the color image forming apparatus of the present embodiment, which heat-fuses the toner image T on the intermediate transfer belt  21  and fixes the same onto the recording paper P upon transfer, can offer a merit that the fixing energy can be saved when used as an apparatus for forming a color image by transferring a superimposed multi-layer toner image onto the recording sheet. 
     In the case of a color image composed of the toner image T having different layer thicknesses, if the toner image T is fixed onto the recording paper upon heat-fusing at the fixing nip section by the conventional image forming apparatus, the following problem occurs. That is, the conventional image forming apparatus can not attain stable fixing properties, because the single-layer toner image portion is heated too much and causes a high-temperature offset, while the double-, triple-, or quadruple-layer toner image is not heated sufficiently, and a low-temperature offset or fixing deficiency occurs. 
     On the other hand, in the case of the color image forming apparatus which heat-fuses the toner image T on the intermediate transfer belt  21  and fixes the same onto the recording paper P upon transfer like in the Examples, as the test results reveal, the fixing conditions do not have to be changed with the toner layer thickness (the fixable temperature is maintained at 170° C.). Thus, the color image forming apparatus arranged in the same manner as the Examples can offer a merit that constant, stable fixing properties can be attained without causing fixing deficiency and high-temperature offset. 
     The results of Test Nos. 1-5 reveal that, in the Examples, sufficient fixing strength can be attained regardless of the kinds of the recording papers and toner layer thickness when a temperature at the interface between the toner and paper is 125° C. or above with the toner A, and 135° C. or above with the toner B. 
     As shown in FIG. 4, when a temperature of the toner A is 125° C. and a temperature of the toner B is 135° C., both the toner A and toner B have toner viscosity (complex viscosity) of 10000 (1.E+04) poise. Thus, it can be understood that if a temperature at the interface between the toner and paper is raised to a level such that keeps the toner viscosity at 10000 poise or below, sufficient fixing strength can be attained in a reliable manner. 
     On the other hand, the condition for attaining sufficient fixing strength in the Comparative Examples is: a temperature at the interface between the toner and paper is 110° C. or above with the toner A, and 120° C. or above with the toner B. Thus, the toner image T can be fixed onto the recording paper P at a temperature 15° C. lower than those in the Examples. The reason why the substantially same fixing strength is obtained at a lower temperature in the Comparative Examples is assumed that, in case that the non-fused toner image is transferred onto the recording paper and fixed thereon upon heating like in the Comparative Examples (conventional method), static electricity acts on the toner by the electric charges generated on the back side of the recording paper upon transfer, and the toner readily impregnates into the microscopic spaces among the fibers of which the recording paper is made. 
     Besides the above-described method (method of computing a temperature at the interface between the toner and paper), the necessary fixing conditions can be found more easily in the following manner. 
     Since the test results reveal that the fixable temperature is 170° C. for the toner A having a softening point of 102° C., and 180° C. for the toner B having a softening point of 120° C., sufficient fixing properties can be secured if the heating roller  22  is controlled to stay at a temperature higher than the softening point by 60° C. or more. 
     The toner is heated for a sufficiently long period by the heating roller  22  at the heating section X. Thus, a temperature of the toner is raised substantially as high as the temperature of the heating roller  22 , and the heat is hardly lost while the toner is transported from the heating section X to the fixing nip section Y. Thus, a temperature of the toner right before it goes into the fixing nip section Y to be fixed onto the recording paper P upon transfer is substantially the same as the temperature of the heating roller  22 . 
     Thus, let T 3  be the softening point temperature of the toner and T 4  be the temperature of the toner right before it is fixed onto the recording paper P upon transfer, then sufficient fixing properties can be secured if a temperature of the heating roller  22  is set to satisfy: T 4 −T 3 ≧60(° C.). 
     Next, the results of the Test Nos. 1, 6 and 7 reveal that, in the Examples, the fixable temperature remains in a range between 170-175° C. and does not substantially vary regardless of the kinds (basis weights) of the recording papers P. 
     On the other hand, in the case of the typical conventional fixing device adopting a heating roller method used in the Comparative Examples, the fixing temperature must be varied with the basis weights of the recording papers. To be more specific, the fixing temperature is raised when the basis weight is increased to prevent the deterioration of the fixing properties with a rise in temperature. Conversely, the fixing temperature is lowered when the basis weight is reduced to prevent too much improvement on the fixing proprieties, so that the toner is not fixed exceedingly. The reason why is as follows. In the conventional fixing device adopting the heating roller method, the heating of the toner depends on not only temperature of the fixing roller, but also the temperature of the pressing roller. Thus, under these conditions, when the basis weight (thickness of the recording paper) varies, so does a quantity of heat conveyed from the pressing roller to the toner through the recording paper. 
     However, the color image forming apparatus of the Examples has the following characteristics. 
     {circle around (1)} Since the heating roller  22  serving as the heat source and the pressing roller  24  are spaced apart, the heat is not directly conveyed to the pressing roller  24 . 
     {circle around (2)} Since the intermediate transfer belt  21  is kept stopped while the heating roller  22  is warming up, the heat is not conveyed to the pressing roller  24  during the warm-up period, either. 
     {circle around (3)} Since the high heat-insulating recording paper P is interposed in a space between the intermediate transfer belt  21  and pressing roller  24  when the toner image T is fixed onto the recording paper P upon transfer, the heat is hardly conveyed to the pressing roller  24 . 
     {circle around (4)} Since a heat capacity of the pressing roller  24  is far larger than a very small heat capacity of the intermediate transfer belt  21 , a temperature of the pressing roller  24  is not raised much by the heat conveyed from the intermediate transfer belt  21 , even when intermediate transfer belt  21  touches the pressing roller  24  directly after the recording paper P is released and before the next recording paper P is transported. 
     {circle around (5)} Since the pressing roller  24  is made of a high heat conducting material (aluminum), heat conveyed to the surface of the pressing roller  24  is quickly conveyed and distributed to the interior thereof, so that a temperature is uniform throughout the pressing roller  24 , and therefore, a temperature of the pressing roller  24  does not rise much. 
     For the reasons specified above, in the Examples, a temperature of the pressing roller  24  is maintained at a temperature (20-40° C.) substantially the same as the ambient temperature, and hardly affects the fixing of the toner at the fixing nip section Y. For this reason, the fixing properties hardly vary even if the basis weight of the recording paper P changes. In other words, as the result of the Test No. 6 reveals, if the recording paper P has a small basis weight, the fixable temperature tends to rise because the heat of the intermediate transfer belt  21  is readily released to the pressing roller  24  through the recording paper P, but the variance is as small as 5° C. and negligible. 
     The same can be said when the kinds of the recording papers P (normal papers, envelopes, postcards, labels, etc.) are changed. 
     Thus, the color image forming apparatus of the Examples can readily attain constant, stable fixing properties regardless of the kinds or thickness (basis weights) of the recording paper P. 
     Next, to prove the effects of the pressing roller  24  in the above-arranged color laser printer, the following test is conducted. 
     (Second Test) 
     An arrangement of an apparatus used in the present test is depicted in FIGS. 5 through 7. 
     The test (hereinafter, referred to as Example 8) using the above-arranged color laser printer is conducted to evaluate the quality of the fixed image formed by the transfer/fixing unit  20  of FIG.  5 . 
     To be more specific, in Example 8, a non-fused toner image T is transferred on the intermediate transfer belt  21 , and heat-fused at the heating section X by the heating roller  22  controlled to stay at a predetermined temperature. Then, a recording paper P is fed to the fixing nip section Y, and the heat-fused toner image T is fixed onto the recording paper P upon transfer. The above series of image forming process is repeated continuously for 100 papers. A temperature of the pressing roller  24  is monitored by a temperature sensor  31  every time a certain number of papers have been fed, and the quality of the fixed image is evaluated as well. 
     In Comparative Example 8, the quality of the fixed image is evaluated in the same manner as above except that the pressing roller  24  of FIG. 5 is replaced with a pressing roller  30  of FIG.  6 . 
     To be more specific, the pressing roller  30  of Comparative Example 8 is composed of an aluminum column  30   a  having a diameter of 15 mm coated with a 7.5 mm-thick elastic layer  30   b  made of foam silicone rubber, which is covered with a tube of a 50 μm-thick releasing layer  30   c  made of fluoride resin (tetrafluoroethylene-parfluoroalkylvinyl ether copolymer or PFA for short). Here, a heat capacity Cr′ of the pressing roller  30  per unit area in Comparative Example 8 is computed in the following manner by taking only the heat capacity of the PFA tube  30   c  serving as the releasing layer into consideration, because the elastic foam silicone rubber layer  30   b  has excellent heat insulation: 
     Given that: 
     specific heat of PFA: 1.05 J/9° C. 
     specific gravity of PFA 2.14 g/cm 3 , then, 
     
       
         Cr′=[1.05×2.14×(1.505 2 −1.5 2 )π]/(3π)=0.011 J°/C. 
       
     
     Since the heat capacity Cb of the intermediate transfer belt  21  is 0.0327 J/° C., Cr′&lt;Cb in Comparative Example 8. In other words, since the heat capacity Cr′ of the pressing roller  30  is smaller than the heat capacity Cb of the intermediate transfer belt  21 , it is impossible to prevent a rise in temperature of the pressing roller  30  by the heat conveyed from the intermediate transfer belt  21 . 
     Also, Comparative Example 9 is conducted in the manner shown in FIG. 7 by omitting the heating roller  22  and using the pressing roller  30  of FIG.  6 . 
     In other words, Comparative Example 9 is conducted in the same manner as Example 8 and Comparative Example 8 using the above arrangement. 
     The toner A (magenta) having a softening point of 102° C. used in First Test is also used herein. The other test conditions are as follows: 
     recording paper P: 80 g 
     fixing rate: 85 mm/s. 
     The test results of Example 8 and Comparative Examples 8 and 9 are set forth in Table 2 below. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 TEMP. OF 
                   
                   
                   
               
               
                   
                 HEATING 
               
               
                 PRESSING 
                 ROLLER 
                   
                 NUMBERS OF PRINTED PAPERS 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 ROLLER 
                 (° C.) 
                   
                 1 
                 25 
                 50 
                 75 
                 100 
               
               
                   
               
               
                 EXAMPLE 8 
                 170 
                 TEMP. OF 
                 25 
                 29 
                  32 
                  34 
                  35 
               
               
                   
                   
                 PRESSING 
               
               
                   
                   
                 ROLLER (° C.) 
               
               
                   
                   
                 FIXED IMAGE 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 COMPARATIVE 
                 170 
                 TEMP. OF 
                 25 
                 56 
                  75 
                  82 
                  86 
               
               
                 EXAMPLE 8 
                   
                 PRESSING 
               
               
                   
                   
                 ROLLER (° C.) 
               
               
                   
                   
                 FIXED IMAGE 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 X(H) 
               
               
                   
                   
                 TEMP. OF 
                 25 
                 51 
                  70 
                  78 
                  83 
               
               
                   
                 160 
                 PRESSING 
               
               
                   
                   
                 ROLLER (° C.) 
               
               
                   
                   
                 FIXED IMAGE 
                 X(L) 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 COMPARATIVE 
                 170 
                 TEMP. OF 
                 25 
                 80 
                 123 
                 132 
                 138 
               
               
                 EXAMPLE 9 
                   
                 PRESSING 
               
               
                   
                   
                 ROLLER (° C.) 
               
               
                   
                   
                 FIXED IMAGE 
                 ◯ 
                 ◯ 
                 X(H) 
                 X(H) 
                 X(H) 
               
               
                   
                 160 
                 TEMP. OF 
                 25 
                 75 
                 118 
                 128 
                 135 
               
               
                   
                   
                 PRESSING 
               
               
                   
                   
                 ROLLER (° C.) 
               
               
                   
                   
                 FIXED IMAGE 
                 X(L) 
                 ◯ 
                 ◯ 
                 X(H) 
                 X(H) 
               
               
                   
               
               
                 ◯: Satisfactory  
               
               
                 X(H): High-temperature offset  
               
               
                 X(L): Low-temperature offset (fixing deficiency)  
               
            
           
         
       
     
     Table 2 above reveals that in the color image forming apparatus of Example 8 shown in FIG. 5, the heating roller  22  serving as the heat source is spaced apart from the pressing roller  24 , and the heat capacity of the pressing roller  24  is far larger than the heat capacity of the intermediate transfer belt  21 . In this case, stable fixing properties are obtained because the temperature of the pressing roller  24  does not vary much and remains around room temperature (25-35°), and if the images are printed out successively on the 100 recording papers, the image quality can be maintained satisfactorily without causing the high-temperature offset or fixing deficiency. 
     However, as in Comparative Example 8, even when the heat source is spaced apart from the pressing roller  30 , if the pressing roller  30  has a small heat capacity, a temperature of the pressing roller  30  rises as the images are printed out successively on the 100 recording papers, and the high-temperature offset occurs at the end of the print-out job (while a temperature of the heating roller  22  is kept at 170° C.). On the other hand, if a temperature of the heating roller  22  is dropped (160° C.) to prevent the high-temperature offset, the low-temperature offset occurs on the first recording paper. Thus, the fixing properties are not stabilized. 
     Further, when the fixing roller  23  and pressing roller  24  are provided closely as in Comparative Example 9, a larger quantity of heat is conveyed to the pressing roller  24 . For this reason, a temperature of the pressing roller  24  varies much in Comparative Example 9 compared with the result in Comparative Example 8, thereby making it more dificult to stabilize the fixing properties. 
     Thus, as can be understood from the foregoing test results, to stabilize the fixing properties, the transfer/fixing unit  20  in the color image forming apparatus is arranged in the same manner as Examples 1-8. To be more specific: 
     {circle around (1)} the heating means (heating roller  22 , herein) is spaced apart from the pressing roller  24 ; 
     {circle around (2)} the pressing roller  24  is composed of a high heat conducting member having a larger heat capacity than the intermediate transfer belt  21  (more preferably, a heat capacity per unit area is 30 times or more of the heat capacity of the intermediate transfer belt  21  as in the Example). 
     Embodiment 2 
     Referring to FIG. 8, the following description will describe another example embodiment of the present invention. Since a color image forming apparatus of the present embodiment is identical with its counterpart in Embodiment 1 above except for the heating means, fixing roller  23  and pressing roller  24 , like numerals are labeled to like components and the explanation of these components is not repeated for the explanation&#39;s convenience. 
     As shown in FIG. 8, the color image forming apparatus of the present embodiment includes four visible image forming units  10 Y,  10 M,  10 C, and  10 B as image forming means, and a transfer/fixing unit  40 . 
     The transfer/fixing unit  40  is composed of an intermediate transfer belt  21 , a supporting roller  42 , a fixing roller  43 , a pressing roller  44 , a tension roller  25 , a cleaning roller  26 , and a thermal heater  41 . 
     The intermediate transfer belt  21  is bridged across the supporting roller  42 , fixing roller  43 , and tension roller  25 . The thermal heater  41  is provided fixedly as heating means at the downstream end of the supporting roller  42  along the turning direction of the intermediate transfer belt  21  in such a manner to touch the inner circumferential surface of the intermediate transfer belt  21 . In short, the thermal heater  41  is positioned at the upstream end of the pressing roller  44 . 
     The thermal heater  41  is composed of an alumina ceramic substrate  41   a  having thereon printed a planar Mo-based heating resistor  41   b  (planar heating body) serving as a heating source, a ceramic heater having thereon printed a layer of glass coat  41   c,  a temperature sensor  41   d  provided beneath the ceramic heater, and a heater holder  41   e  supporting the ceramic heater while insulating the heat therefrom. 
     The thermal heater  41  rises up as soon as the current starts to pass through the heating resistor  41   b,  and controls the current passing based on a signal from the temperature sensor  41   d,  so that it stays at a predetermined temperature (170° C., herein). Consequently, a toner image T is transferred on the surface of the intermediate transfer belt  21 , and transported further to the pressing section (heating section X) with the thermal heater  41  to be heat-fused. 
     The fixing roller  43  is composed of an aluminum column  43   a  having a diameter of 20 mm and coated with a 5 mm-thick heat-resistant elastic layer  43   b  made of foam silicone rubber. Since foam silicone rubber has small heat conductivity and excellent heat insulation as well as satisfactory elasticity, sufficient fixing nip length Y can be secured at a low pressure. Also, since the foam silicone rubber has small heat conductivity, a quantity of the heat released from the intermediate transfer belt  21  to the fixing roller  43  can be minimized. Hence, the intermediate transfer belt  21  is allowed to touch the fixing roller  43  at the upstream end of the fixing nip section Y. This means that the intermediate transfer belt  21  can be bridged across the rollers in a more flexible manner, thereby realizing more flexible design for the color image forming apparatus. 
     The pressing roller  44  is composed of an aluminum drum  44   a  having a diameter of 30 mm and a thickness of 3 mm and coated with a 10 μm-thick releasing layer  44   b  made of fluoride resin (polytetrafluoroethylene or PTFE for short). Moreover, the interior of the drum  44   a  is of a heat-pipe structure. In other words, although it is not illustrated in the drawing, a portion touching the intermediate transfer belt  21  in the longitudinal direction of the pressing roller  44  is a heat absorbing section, and a corresponding outside portion is a heat releasing section. 
     A coagulating liquid (not shown) for delivering heat from the heat absorbing section to the heat releasing section is sealed in the drum  44   a,  and a wick  44   c,  which returns the coagulating liquid from the heat releasing section to the heat absorbing section by means of capillary, is provided on the inner circumferential surface of the drum  44   a.    
     As has been explained, by adopting the heat-pipe structure to the pressing roller  44 , the heat conveyed to the pressing roller  44  from the intermediate transfer belt  21  is immediately released from the releasing section, so that the pressing roller  44  is constantly kept substantially as high as the ambient temperature. Consequently, stable fixing properties can be attained regardless of the operation conditions and the kinds or thickness of the recording paper P. 
     In Embodiments 1 and 2 above, the intermediate transfer belt  21  is used as an example image carrying body. However, the image carrying body of the present invention is not limited to a belt, and for example, a drum can be used as the image carrying body as well, and example of which will be explained in Embodiment 3 below. 
     Embodiment 3 
     Referring to FIG. 9, the following description will describe still another example embodiment of the present invention. In the present embodiment, a color laser printer is used as an example image forming apparatus of the present invention. 
     As shown in FIG. 9, unlike the counterparts in Embodiments 1 and 2, the color laser printer of the present embodiment includes an intermediate transfer drum (image carrying drum)  61  as the image carrying body instead of the intermediate transfer belt  21 . 
     Around the intermediate transfer drum  61 , a photosensitive drum  51 , a corona charger  55 , a cleaning roller  66 , and a pressing roller  64  are provided sequentially. 
     Around the photosensitive drum  51 , a charging roller  52 , a laser irradiating device  53 , a developing device  54 , and a cleaner  56  are provided sequentially. 
     The charging roller  52  is provided as charging means for electrically charging the surface of the photosensitive drum  51  uniformly, and the laser irradiating device  53  is provided as laser beam irradiating means for forming an electrostatic latent image on the surface of the photosensitive drum  51  which has been electrically charged uniformly. Assume that the electrostatic latent image is formed for each color. 
     The developing device  54  is provided as visible image forming means for developing the electrostatic latent image formed on the photosensitive drum  51  into a visible image. To form a color image, the developing device  54  includes four developers: a developer  54 Y for forming a yellow toner image; a developer  54 M for forming a magenta toner image; a developer  54 C for forming a cyan toner image; and a developer  54 B for forming a black toner image. 
     To be more specific, the electrostatic latent images formed on the photosensitive drum  51  for their respective colors are developed into visible toner images in their respective colors by the developers  54 Y,  54 M,  54 C, and  54 B, and transported onto the intermediate transfer drum  61  to be superimposed sequentially one on another, whereby a multi-color toner image for a color image is formed on the intermediate transfer drum  61 . 
     The toner and paper dust remaining on the photosensitive drum  51  after the transfer are collected by the cleaner  56  provided at the downstream end of a transfer position Z. 
     The intermediate transfer drum  61  is composed of a stainless drum  61   a  coated with a 150 μm-thick releasing layer  61   b  made of silicone rubber. The toner image is transferred onto the intermediate transfer drum  61  by electrically charging the surface of the releasing layer  61   b  by means of the corona charger  55  to make a potential difference from the photosensitive drum  51 . 
     As previously mentioned, the pressing roller  64  for pressing against the intermediate transfer drum  61  to fix the heat-fused toner image T onto the recording paper P upon transfer, and the cleaning roller  66  for cleaning the residual toner on the intermediate transfer drum  61  are provided on the outer circumferential surface of the intermediate transfer drum  61 . The pressing roller  64  and cleaning roller  66  are of the same arrangements as the aforementioned pressing roller  24  and cleaning roller  26 , respectively. 
     To be more specific, the pressing roller  64  is composed of an aluminum column  64   a  having a diameter of 30 mm and a thickness of 5 mm and coated with a 10 μm-thick releasing layer  64   b  made of fluoride resin (polytetrafluoroethylene or PTFE for short). The cleaning roller  66  is composed of an aluminum column  66   a  having a diameter of 16 mm with a 2 mm-thick felt  66   b  made of Nomex of Dupont being wound around in spiral. 
     The pressing roller  64  is provided to either touch or keep a space with respect to the intermediate transfer drum  61  as the case may be. To be more specific, the pressing roller  64  moves to press against the intermediate transfer drum  61  when fixing the toner image T onto the recording paper P upon transfer, and moves to keep a space from the intermediate transfer drum  61  otherwise. 
     Likewise, the cleaning roller  66  is provided to either touch or keep a space with resect to the intermediate transfer drum  61  as the case may be. To be more specific, the cleaning roller  66  moves to keep a space from the intermediate transfer drum  61  until the toner image T has been transferred onto the recording sheet P, and moves to press against the intermediate transfer drum  61  after the transfer is completed. 
     A coil  62  for heating the intermediate transfer drum  61  by means of electromagnetic induction is provided inside the intermediate transfer drum  61  at the upstream end of its rotating direction. Also, a power source  67  for supplying power to the coil  62  is provided inside the intermediate transfer drum  61 . 
     The power source  67  is controlled by unillustrated control means in such a manner not to pass the current to the coil  62  while the toner images in their respective colors are being transferred from the photosensitive drum  51  to the intermediate transfer drum  61 , and start to pass the current to the coil  62  when the above transferring step is completed. 
     Thus, the coil  62  is provided at the upstream end of the pressing section of the intermediate transfer drum  61  and pressing roller  64 , that is, the fixing nip section Y (fixing nip length: 4.5 mm), and the current is allowed to pass when the power source  67  is turned ON, whereby the heating section X (heating length: 23.5 mm) is formed on the stainless intermediate transfer drum  61  by means of the electromagnetic induction heating. 
     The coil  62  is fixed to a predetermined position in the interior of the stainless (metallic) drum  61   a,  which is determined by taking the heating conditions, such as heating efficiency, into consideration. 
     Next, the operation of the above-arranged color laser printer will be explained with reference to FIG.  9 . 
     To begin with, the photosensitive drum  51  is electrically charged uniformly on the surface by the charging roller  52  as it rotates in a direction indicated by an arrow, and an electrostatic latent image is formed thereon by a laser beam irradiated from the laser irradiating device  53 , which is developed into a visible image by the developing device  54 . 
     Next, at the transfer position Z, the toner image formed on the photosensitive drum  51  is transferred onto the intermediate transfer drum  61  which is electrically charged uniformly to a polarity opposite to the polarity of the toner by the corona charger  55 . 
     The aforementioned transferring step is carried out repetitively for each color. The pressing roller  64  and cleaning roller  66  keep a space from the intermediate transfer drum  61  and no current is allowed to pass through the coil  62  until the transferring step is completed. 
     When all the toner images are transferred onto the intermediate transfer drum  61  and a multi-color toner image T is formed, that is, when the transferring step is completed, a high frequency current is passed through the coil  62  from the power source  67 , whereupon the stainless (metallic) drum  61   a  is heated by means of induction heating at a portion corresponding to the heating section X. At this point, the pressing roller  64  moves to press against the intermediate transfer drum  61 . 
     The toner image T is transported to the heating section X from the transfer position Z as the intermediate transfer drum  61  rotates, and heat-fused therein. Then, the toner image T heat-fused on the intermediate transfer drum  61  is fixed onto the recording paper P at the fixing nip section Y upon transfer. 
     After the toner image T is transferred onto the recording paper P from the intermediate transfer drum  61 , the cleaning roller  66  is pressed against the intermediate transfer drum  61  to remove the residual toner and paper dust adhering thereon. 
     As has been explained, in the case that the intermediate transfer drum  61  is used as the image carrying body, the heating section X is separated from the fixing nip section Y, which is a fixing section with the recording paper P, like in Embodiments 1 and 2 above where the intermediate transfer belt  21  is used as the image carrying body. Consequently, a far longer heating length than the fixing nip section Y can be readily realized, and the processing rate can be increased compared with the conventional color image forming apparatus which fixes the toner image onto the recording paper P upon heat-fusing. 
     In the case that the intermediate transfer drum  61  is used as the image carrying body like in the present embodiment, the fixing roller  23 , which is indispensable for the intermediate transfer belt  21 , can be omitted. Consequently, it has become possible to prevent a drop in temperature of the toner caused when the heat of the toner image T fused at the heating section X is conveyed to the fixing roller  23  at the upstream end of the fixing nip section Y, thereby allowing more flexible arrangement for the image forming apparatus. 
     A color image forming apparatus of the present invention may be arranged to comprise: 
     an image carrying body supported in such a manner that a toner image forming surface thereof is allowed to circulate; 
     image forming means for forming a multi-color toner image on the image carrying body by sequentially superimposing toner images in respective colors; 
     heating means for heat-fusing the multi-color image on the image carrying body; and 
     pressing means for pressing the image carrying body from the toner image forming surface side after the multi-color toner image is transferred onto the image carrying body, wherein: 
     the multi-color toner image heat-fused is fixed upon transfer onto a recording material transported to a pressing position of the pressing means against the image carrying body; and 
     the pressing position of the pressing means against the image carrying body is provided at a downstream end of a heating position where the multi-toner image is heated by the heating means along a circulation direction of the image carrying body. 
     The color image forming apparatus may be further arranged in such a manner that, let T 1  be a temperature at an interface between toner and the recording material when the toner image is fixed onto the recording material upon transfer, and T 2  be a temperature of the toner when viscosity of the toner is set to a value such that can realize satisfactory fixing strength, then the toner images and the image carrying body are pre-heated by the heating means to satisfy: T 1 ≧T 2 . 
     Accordingly, the toner image and image carrying body are heated by the heating means in such a manner that the temperature T 1  at the interface between the toner and recording material when the toner image is fixed onto the recording material upon transfer becomes as high as or above the temperature T 2 . Thus, the temperature at the interface between the toner and recording material can be raised to a temperature at which satisfactory fixing strength can be secured. Hence, the fixing properties are not affected adversely by a temperature of the recording material transported to the pressing section between the image carrying body and pressing means. Consequently, it has become possible to prevent fixing deficiency that occurred when the recording material transported to the pressing position of the pressing means against the image carrying body is too cold (absorbs too much heat). 
     The color image forming apparatus may be further arranged in such a manner that the temperature at the interface between the toner and recording material is found from logical computation using temperatures of the toner right before being fixed upon transfer onto the recording material, image carrying body, pressing means, and recording material. 
     Accordingly, although an actual temperature at the interface between the toner and recording material can not be measured easily, the temperature at the interface between the toner and recording material can be found from the logical computation using the temperatures actually measured. 
     The color image forming apparatus may be further arranged in such a manner that, let T 3  be a softening point temperature of the toner and T 4  be a temperature of the toner right before the toner image is fixed onto the recording material upon transfer, then, before fixing the toner image onto the recording material upon transfer, the toner image and image carrying body are heated by the heating means to satisfy: T 4 −T 3 ≧60(° C.). 
     Accordingly, the toner image and image carrying body are heated in such a manner that there is a difference of 60° C. or more between the temperature T 4  of the toner right before the toner image is fixed onto the recording material upon transfer and the temperature T 3  of the toner softening point, and therefore, a temperature at the interface between the toner and recording material is raised sufficiently high to secure the satisfactory fixing strength. Thus, the fixing properties are not affected adversely by a temperature of the recording material transported to the pressing section of the pressing means against the image carrying body. Consequently, it has become possible to prevent fixing deficiency that occurred when the recording material transported to the pressing position of the pressing means against the image carrying body is too cold. 
     The color image forming apparatus may be further arranged in such a manner that the image carrying body is an endless image carrying belt bridged across at least two rollers, and that the heating means is provided in such a manner to oppose an inner circumferential surface of the image carrying belt. 
     Accordingly, since the image carrying body is a belt and the heating means is provided on the inner side of the image carrying belt, heat is conveyed from the inner side to the outer side (toner image forming surface) of the image carrying belt. Moreover, if the heating means is formed long along the circulation direction (moving direction) of the image carrying belt to conform to the shape of the belt, a length of the heating section for heat-fusing the toner can be readily extended. Also, if the heating means also serves as one of the rollers forming belt bridging means, the arrangement can be simplified. 
     Consequently, the heating section can be designed more flexibly in such a manner to heat a multi-color toner image uniformly regardless of the layer thickness, thereby allowing more flexible design for the apparatus. 
     The color image forming apparatus may be further arranged in such a manner that the heating means is a heating roller, which is one of the rollers across which the image carrying belt is bridged, and positioned at an upstream end of the pressing position of the pressing means against the image carrying belt. 
     Consequently, since the image carrying belt is bridged across the heating roller, the surface of the image carrying belt at a portion that touches the heating roller serves as the heating section. In other words, since a portion of the image carrying belt wound around the heating roller serves as the heating section, a length of the heating section can be readily adjusted by changing a diameter of the heating roller or a contact angle of the image carrying belt with respect to the heating roller. Further, the apparatus can be designed in a more flexible manner. 
     It is preferable that a heater lamp is provided inside the heating roller. 
     Since the heater lamp is provided inside the heating roller, the heating action by the heating roller with respect to the heating section can be readily controlled by merely controlling the ON/OFF action of the heater lamp. 
     The color image forming apparatus may be further arranged in such a manner that the heating means is provided somewhere between the pressing position and one of the two rollers across which the image carrying belt is bridged, so that the inner circumferential surface of the image carrying belt slides along the heating means. 
     Accordingly, since the heating means is provided somewhere between the pressing position and one of the two rollers across which the image carrying belt is bridged to let the inner circumferential surface of the image carrying belt slide along the heating means, the heating means can be provided more closely to the pressing position compared with a case where the heating roller, across which the image carrying belt is bridged, is used as the heating means. 
     Consequently, a distance between the heating section to the pressing position can be shortened, and a drop in temperature of the heat-fused toner can be minimized, thereby eliminating fixing deficiency caused by a drop in temperature of the toner image. Thus, the above arrangement can realize more stable fixing properties compared with a case where the heating roller, across which the image carrying belt is bridged, is used as the heating means. 
     It is preferable that the heating means is a thermal heater having a planar heating body which can heat up quickly. 
     When the heating means is a thermal heater having a planar heating body which can heat up quickly, the heating section can be heated up promptly. Also, since the thermal heater is planar, it can be provided closely to the fixing nip section, thereby making it possible to minimize a drop in temperature of the toner while it is transported from the heating section to the fixing nip section. Consequently, a temperature of the fused toner at the fixing nip section can be controlled readily and the fused toner can be fixed onto the recording material in a stable manner. 
     The image carrying body may be an image carrying drum, and the heating means may be provided inside the image carrying drum. 
     When the image carrying body is an image carrying drum and the heating means is provided inside the image carrying drum, the pressing means forms a pressing section (fixing nip section) by pressing against a toner image forming surface of the image carrying drum. In other words, in the case that a roller is used as the pressing means when the image carrying body is a drum, one roller is sufficient. 
     By contrast, the image carrying belt forms the pressing section (fixing nip section) by sandwiching the image carrying belt from both the inner and outer circumferential surfaces sides. In other words, in the case that a roller is used as the pressing means when the image carrying body is a belt, at least two rollers are necessary at the inner and outer circumferential surfaces sides of the image carrying belt, respectively. 
     Thus, when the image carrying belt is used as the image carrying body, heat may be conveyed from the toner to the roller forming the fixing nip section, thereby possibly causing a drop in temperature of the toner and hence fixing deficiency. 
     By contrast, when the image carrying drum is used as the image carrying body, since only one roller is used, a quantity of heat conveyed from the toner to the roller is quite small. Consequently, the apparatus can be designed without worrying about how much heat will be conveyed from the toner to the pressing means, and the apparatus can be designed in a more flexible manner. 
     The color image forming apparatus may be further arranged in such a manner that the image carrying drum has a metallic drum and the heating means is a coil which heats the metallic drum by means of induction heating. 
     When the heating means is the coil that heats the metallic drum inside the image carrying drum by means of induction heating, the coil can heat the metallic drum without touching the same. Consequently, the coil can be provided fixedly to a position inside the image carrying drum which is allowed to rotate. 
     In addition, since the coil heats up and starts to heat the metallic drum to a predetermined temperature by means of induction heating as soon as the current is passed, the warm-up period can be cut shorter. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.