Abstract:
A transfer material carrying member for carrying a transfer material for receiving an image from an image bearing member, includes a first layer having a thickness Ha; and a second layer adjacent to the first layer, the second layer having a thickness of Hb, wherein the first layer has a dimension which changes by Xa due to a change in an ambient condition, and the second layer has a dimension which changes by Xb due to the change in the ambient condition, and wherein ¦Xa−Xb¦&lt;Ha+Hb.

Description:
FIELD OF THE INVENTION AND RELATE ART  
         [0001]    The present invention relates to an image forming apparatus, for example, a copying machine, a facsimile machine, a printer, or the like, which forms an image with the use of an electrophotographic method or an electrostatic recording method. It also relates to a transfer medium bearing member employed by such an image forming apparatus.  
           [0002]    In an image forming apparatus, for example, an electrophotographic image forming apparatus, the peripheral surface of a cylindrical electrophotographic photoconductive member (photoconductive drum) as an image bearing member is uniformly charged, and an electrostatic latent image is formed on the uniformed charged surface in accordance with image formation data. This electrostatic latent image is visualized with the use of developer; a so-called toner image is formed. Then, the toner image is transferred from the photoconductive drum onto a piece of transfer medium (recording medium), and is fixed to the transfer medium, to obtain a copy or a print.  
           [0003]    Some of the image forming apparatuses are color image forming apparatuses capable of forming a full-color image as well as a monochromatic image. These color image forming apparatuses can be divided into two groups according to the manner in which a full-color image is formed. In one group, a color image forming apparatus comprises a plurality of image forming stations, each of which has its own photoconductive drum, and in each of which a toner image, which is different in color from the toner image formed in the other stations, is formed on the photoconductive drum. A plurality of the thus formed toner images different in color are consecutively transferred in layers onto the same recording medium borne on a transfer medium bearing member, to form a full-color image. In the other group, a color image forming apparatus comprises only a single image forming station with a single photoconductive drum. In a full-color image forming operation, a plurality of tone images different in color are formed in succession on the same photoconductive drum after the preceding toner image is transferred onto the recording medium borne on a transfer medium bearing member. In these groups of image forming apparatuses, recording medium is conveyed by a transfer bearing member, for example, an endless belt suspended around a plurality of rollers, a cylinder formed by stretching a sheet of specific material around a cylindrical skeletal frame, or the like.  
           [0004]    [0004]FIG. 3 shows the general structure of an example of a color image forming apparatus. An image forming apparatus  100  comprises a plurality of image forming stations Py, Pm, Pc, and Pk. In each image forming station, a toner image different in color from the toner image formed in the other stations is formed. The toner image formed in each stations is consecutively transferred onto the same recording medium to form a color image.  
           [0005]    The image forming apparatus comprises a transfer belt  51  as a transfer medium bearing member, which is an endless belt and is suspended around four rollers: a driving roller  52  and three supporting rollers  53   a ,  53   b , and  53   c . Located above the transfer belt  51  in this embodiment are four image forming stations Py, Pm, Pc, and Pk for forming yellow, magenta, cyan, and black images, correspondingly. Since the four image forming stations Py, Fm. Pc, and Pk are the same in structure, the structures of the image forming stations will be described in detail with reference to the image forming station Py for forming a toner image of a first color (yellow). In the drawings, the elements in each image forming station, which are the same in function as those in the other stations, are given the same referential codes, but are differentiated from those in the other stations by addition of subscripts y, m, c, and k, correspondingly to the referential codes Py-Pk for the yellow, magenta, cyan, and black image forming stations.  
           [0006]    Referring to FIG. 4, the image forming station Py for the first color has a cylindrical photoconductive member (photoconductive drum)  1   y  as an image bearing member During an image forming operation, the photoconductive drum  1   y  is rotationally driven in the direction indicated by an arrow mark A by a driving means (unshown), and the peripheral surface of the photoconductive drum  1   y  is uniformly charged by a magnetic brush type charging apparatus as a charging means. Then, the charged photoconductive drum  1   y  is exposed to an image exposure light L representing the yellow component of an original, by an exposing apparatus (LMD based scanning apparatus)  3   y . As a result, an electrostatic latent image in accordance with the inputted image formation data is formed on the peripheral surface of the photoconductive drum  1   y.  Next, the electrostatic latent image on the photoconductive drum  1   y  is developed into a yellow toner image by a developing apparatus  4   y.    
           [0007]    At the same time as the yellow toner image on the photoconductive drum  1   y  reaches a transfer nip between the peripheral surface of the photoconductive drum  1   y  and a transfer belt  51 , a recording medium P. for example, a piece of recording paper, which is fed into the image forming apparatus main assembly from a recording medium cassette  80  as a recording medium storage by a sheet feeding roller  81  or the like, is delivered to the transfer nip by a registration roller  82 . In the transfer nip, electrical charge, which is opposite in polarity to the toner, is applied to the recording medium P, on the reverse side, that is, the side on which the image is not going to be transferred and is in contact with the transfer belt  51 , by a transfer charge blade  54  as a transfer charging device charged with transfer bias. As a result, the toner image on the photoconductive drum  1   y  is transferred onto the transfer medium P, on the top side. A transferring apparatus  5  (belt type transferring apparatus) comprises the transfer belt  51 , rollers  52 ,  53   a ,  53   b , and  53   c , and transfer charge blades  54   y - 54   k.    
           [0008]    After the transfer of the yellow toner image onto the recording medium P, the recording medium P is conveyed to the image forming station Pm for a second color (magenta), as the transfer belt  51  moves in the direction indicated by an arrow mark f.  
           [0009]    The image forming station Pm for the second .color is the same in structure as the image forming station Py for the first color. Thus, the same processes as those carried in the image forming station Py are carried out in the image forming station Pm. That is, a latent image is formed on the photoconductive drum  1   m , and the magenta developing apparatus  4 m develops the latent image into a magenta toner image with the use of magenta toner. Then, the magenta toner image is transferred onto the recording medium P, in a manner to be layered on the yellow toner image, by the function of the transfer charge blade  54   m , in the transfer nip.  
           [0010]    Next, a cyan toner image and a black toner image are formed in the image forming stations Pc for a third color and the image forming station Pk for a fourth color, respectively, and are transferred onto the recording medium P by the transfer charge blades  54   c  and  54   k , in a manner to be layered on the preceding two toner images, in the corresponding image forming stations. Consequently, a color image, or a composite of four layers of toner images different in color, is formed on the recording medium P. At this point, the color image is yet to be fixed.  
           [0011]    After the transfer of the four toner images onto the recording medium P, the recording medium P is conveyed to a fixing apparatus  6  which comprises a fixing roller  6   a  containing a heating means, and a driving roller  6   b . In the fixing apparatus  6 , the toner images on the recording medium P are fixed, as a permanent full-color image, to the surface of the recording medium P by the application of heat and pressure by the fixing roller  6   a  and driving roller  6   b . After the fixation of the toner images, the recording medium P is discharged into an external delivery tray (unshown), or the like, of the image forming apparatus.  
           [0012]    After the recording medium P is separated from the transfer belt  51 , the transfer belt  51  is removed of the electrical charge on the reverse side, by a combination of a grounded electrically conductive fur brush  11  and a grounded transfer belt driving roller  52 . Further, the foreign substances, for example, toner particles (residual toner particles), paper dust, and the like, on the transfer belt  51 , are removed by a transfer belt cleaner  12  comprising a urethane rubber blade and the like, to be prepared for the next image formation cycle.  
           [0013]    On the portion of each of the photoconductive drums  1   y - 1   k , which has just passed the transfer nip, residual toner particles, that is, toner particles which failed to be transferred onto the recording medium P, are present, although only by a small amount. These residual toner particles are scraped away, electrostatically and mechanically, and are temporarily absorbed, by the magnetic brush of each of the magnetic brush type charging apparatuses  2   y - 2   k.    
           [0014]    As the amount of the transfer residual toner particles in the magnetic brush of each of the magnetic brush type charging apparatuses  2   y - 2   k  increases, the electrical resistance of the magnetic brush itself increases, and eventually, the magnetic brush fails to sufficiently charge the photoconductive drum. As a result, difference in electrical potential is created between the magnetic brush and the peripheral surface of the photoconductive drum, causing the transfer residual toner particles in the magnetic brush to electrostatically transfer onto the photoconductive drum. After transferring onto the photoconductive drum, the transfer residual toner particles are electrostatically taken into the developing apparatus, to be consumed during the following image formation cycles.  
           [0015]    In the above described image forming apparatus  100 , the toner images formed in the image forming stations Py, Pm, Pc, and Pk must be precisely aligned,.and therefore, the transfer belt  51  as a transfer medium bearing member, which holds and conveys the transfer medium P, must be stable. In the image forming apparatus  100  in this embodiment, the recording medium P is electrostatically held to the transfer belt  51  with the use of electrostatic adhesion rollers  55  and  56 . The electrostatic adhesion roller  56  is grounded. As the recording medium P enters an electrostatic adhesion nip in which the electrostatic adhesion rollers  55  and  56  oppose to each other with the interposition of the transfer belt  51 , a positive bias of 1 kV is applied to the electrostatic adhesion roller  55  to electrostatically adhere the recording medium P to the transfer belt  51 .  
           [0016]    The above described electrostatic adhesion of the recording medium P, and the toner image transfer in each of the image forming stations Py, Pm, Pc, and Pk, are significantly affected by the electrical properties (electrical resistance, dielectric constant, and the like) and mechanical properties (thickness, mechanical strength, surface properties, and the like) of the transfer belt  51 .  
           [0017]    First, regarding the electrical properties of the transfer belt  51 , for example, electrical resistance, if the electrical resistance of the transfer belt  51  is lower than a certain level, the biases applied to the transfer charge blade  54  and electrostatic adhesion roller  55  interfere with each other through the transfer belt  51 , and the electrical charge given to the transfer belt  51  by the transfer charge blade  54  and electrostatic adhesion blade  55  is likely to attenuate. As a result, toner images are disturbed after they are transferred onto the recording medium P, and the electrostatic force for keeping the recording medium P adhered to the transfer belt  51  weakens.  
           [0018]    On the other hand, if the electrical resistance of the transfer belt  51  is a higher than a certain level, the absolute values of the biases applied to the transfer charge blade  54  and electrostatic adhesion roller  55  must be greater, which is likely to trigger abnormal electrical discharge in the transfer nip and electrostatic adhesion nip, and the abnormal electrical discharge results in an image of inferior quality.  
           [0019]    Next, regarding mechanical properties, for example, thickness, if the thickness of the transfer belt  51  is less than a certain level, the transfer belt  51  is insufficient in mechanical strength, being likely to break and/or stretch, and therefore, is not stable, whereas if the thickness the transfer belt  51  is more than a certain level, the absolute values of the biases applied to the transfer charge blade  54  and electrostatic adhesion roller  55  must be greater as they must be if the electrical resistance of the transfer belt  51  is higher than a certain level, rendering the transfer belt  51  unsatisfactory.  
           [0020]    In other words, the transfer belt  51  is sometimes required to satisfy two mutually contradictory requirements, even regarding only one of the aforementioned physical properties. As one of the solutions to this problem, a multilayered transfer belt ( 51 ) disclosed in Japanese Laid-open Patent Application 2-148074 is frequently used. This patent application proposes that various functions of the transfer belt ( 51 ) be divided among the plurality of functional layers. More specifically, in order to prevent the transfer belt from failing to be satisfactorily removed of the electrical charge thereon, while providing the transfer belt with a sufficient amount of mechanical strength, the transfer belt is multilayered; it is provided with a surface layer, the electrical resistance of which has been adjusted to a sufficiently low level, and a base layer which is mechanically strong.  
           [0021]    However, when a transfer belt  51  having a plurality of layers different in function is employed, the transfer belt  51  sometimes warps as shown in FIG. 11, which shows the widthwise cross section of the transfer belt  51  as seen from the direction to which the transfer belt  51  advances. As is evident from the drawing, the belt  51  sometimes warps at both edges.  
           [0022]    The studies made by the inventors of the present invention revealed that this phenomenon, or the warping, was caused by the difference in the coefficient of linear expansion among the plurality of functional layers. More specifically, the warping of the transfer belt  51  occurs when the plurality of layers formed of resinous material are different in the ratio at which their measurements fluctuate due to either or both of the ambient temperature and humidity of the transfer belt  51 .  
           [0023]    If warping such as the above described occur to the belts or sheets, for example, the transfer belt  51  as a transfer medium bearing member, which are involved in the image forming processes within the image forming apparatus  100 , the belts or sheets fail to uniformly contact their counterparts. For example, the transfer belt  51  fails to uniformly contact the photoconductive drum  1  with the interposition of the recording medium P, in the transfer nip, causing the transfer charging means to fail to uniformly charge the transfer belt  51 , and further, a gap is created between the recording medium P and transfer belt  51 , along the both edges of the recording medium P in terms of the widthwise direction of the transfer belt  51  as shown in FIG. 12. As a result, the toner images are improperly transferred, resulting in a full-color image of inferior quality.  
         SUMMARY OF THE INVENTION  
         [0024]    Thus, the primary object of the present invention is to prevent the transfer medium bearing member employed by an image forming apparatus, from suffering from deformation such as warping caused by the changes in the environmental factors such as temperature or humidity.  
           [0025]    Another object of the present invention is to provide an image forming apparatus capable of always producing an excellent image, more specifically, an image which does not suffer from defects which result from unsuccessful image transfer, by preventing the transfer medium bearing member from suffering from deformation such as warping caused by the changes in the environmental factors such as temperature or humidity.  
           [0026]    According to an aspect of the present invention for achieving the above objects, a transfer medium bearing member for holding and conveying a transfer medium onto which an image on an image bearing member is to be, or has been, transferred, comprises a minimum of first and second layers laminated to each other, and the amount Xa of the change in the length of the first layer, the amount Xb of the change in the length of the second layer, the thickness Ha of the first layer, and thickness Hb of the second layer, satisfy the following inequity; 
           ¦ Xa−Xb¦&lt;Ha+Hb.   
           [0027]    According to another aspect of the present invention, in an image forming apparatus comprising: an image forming means for forming an image on an image bearing member; a transfer medium bearing member for holding and conveying a transfer medium; and a transferring means for transferring an image on the image bearing member onto the transfer medium being held and conveyed by the transfer bearing member, the transfer medium bearing member comprises a minimum of first and second layers laminated to each other, and the amount Xa of the change in the length of the first layer, the amount Xb of the change in the length of the second layer, the thickness Ha of the first layer, and thickness Hb of the second layer, satisfy the following inequity: 
           ¦ Xa−Xb¦&lt;Ha+Hb.   
           [0028]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is a rough sectional view of an embodiment of a transfer medium bearing member in accordance with the present invention.  
         [0030]    [0030]FIG. 2 is a rough sectional view of another embodiment of a transfer medium bearing member in accordance with the present invention.  
         [0031]    [0031]FIG. 3 is a sectional view of an embodiment of an image forming apparatus in accordance with the present invention, for showing the general structure thereof.  
         [0032]    [0032]FIG. 4 is an enlarged sectional view of one of the image forming stations in the image forming apparatus in FIG. 3.  
         [0033]    [0033]FIG. 5 is an enlarged sectional view of the charging means and its adjacencies in the image forming station in FIG. 4.  
         [0034]    [0034]FIG. 6 is an enlarged sectional view of the developing apparatus and its adjacencies in the image forming station in FIG. 4.  
         [0035]    [0035]FIG. 7 is a sectional view of another embodiment of an image forming apparatus in accordance with the present invention.  
         [0036]    [0036]FIG. 8 is a perspective view of a transfer drum in accordance with the present invention.  
         [0037]    [0037]FIG. 9 is a perspective view of a transfer drum, a portion of the peripheral surface of which has been slightly dented.  
         [0038]    [0038]FIG. 10 is a rough sectional view of another embodiment of a transfer medium bearing member in accordance with the present invention.  
         [0039]    [0039]FIG. 11 is a widthwise sectional view of a transfer belt, which has warped.  
         [0040]    [0040]FIG. 12 is an enlarged widthwise sectional view of one of the two widthwise edges of the transfer belt, which has warped. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    Hereinafter, preferred embodiments of a laminar transfer medium bearing member and an image forming apparatus in accordance with the present invention will be described in detail with reference to the appended drawings.  
         [0042]    Embodiment 1  
         [0043]    First, an embodiment of an image forming apparatus in accordance with the present invention will be described. This embodiment of image forming apparatus is basically the same in structure as a conventional image forming apparatus, except for the structure of the transfer belt as a transfer medium bearing member.  
         [0044]    [0044]FIG. 3 is a sectional view of an example of a color image forming apparatus, for showing the structure thereof. This image forming apparatus comprises a plurality of image forming stations Py, Pm, Pc, and Pk, each of which forms a toner image different in color from the toner image formed in the other stations. The toner images formed in the plurality of image forming stations are consecutively transferred in layers onto the same recording medium to form a multicolor or full-color image.  
         [0045]    This embodiment of the image forming apparatus  100  has an endless transfer belt  51  as a transfer medium bearing member, which is suspended by being wrapped around four rollers, which are a driving roller  52  and three supporting rollers  53   a ,  53   b , and  53   c . In this embodiment, four image forming stations Py, Pm, Pc, and Pk for forming yellow, magenta, cyan, and black images, correspondingly, are located above the transfer belt  51 . Since all the image forming stations are the same in structure, the structures of the image forming stations will be described in detail with reference to the image forming station Py for forming a toner image of a first color (yellow). In the drawings, the elements in each image forming station, which are the same in function as those in the other stations, are given the same referential codes, but are differentiated from those in the other stations by addition of subscripts y, m, c, and k, correspondingly to the referential codes Py-Pk for the yellow, magenta, cyan, and black image forming stations. Incidentally, when differentiation is unnecessary, the subscripts will be omitted.  
         [0046]    Referring to FIG. 4, the image forming station Py for the first color has a cylindrical electrophotographic photoconductive member, that is, a photoconductive drum  1   y , as an image bearing member, which is located in the approximate center of the station. During an image forming operation, the photoconductive drum  1   y  is rotationally driven about a drum supporting central axle in the direction indicated by an arrow mark A at a predetermined peripheral velocity (process speed). As the photoconductive drum  1   y  is rotated, the peripheral surface of the photoconductive drum  1   y  is uniformly charged by a magnetic brush type charging apparatus  2   y  as a contact charging means. In this embodiment, it is negatively charged. Then, the charged photoconductive drum  1   y  is exposed to an exposure light L protected from an exposing apparatus  3   y  (LED based exposing apparatus) while being modulated with image formation signals. As a result, an electrostatic latent image in accordance with the image formation data is formed on the peripheral surface of the photoconductive drum  1   y . Next, the electrostatic latent image on the photoconductive drum  1   y  is developed into a toner image by a developing apparatus  4   y . In this embodiment, the latent image is reversely developed.  
         [0047]    Referring to FIG. 3, meanwhile, a plurality of recording media P, for example, sheets of recording paper, stored in a recording medium cassette  80  as a recording medium storage, are fed one by one into the image forming apparatus main assembly by a sheet feeding roller  81 , and are delivered by a registration roller  82  with a predetermined timing, to the transfer nip in which the peripheral surface of the photoconductive drum  1   y  and the transfer belt  51  of a transferring apparatus  5  oppose each other In the transfer nip, the yellow toner image on the photoconductive drum  1   y  is transferred onto the recording medium P. The transferring apparatus  5  comprises the transfer belt  51  (transfer medium bearing member), a group of rollers  52 ,  53   a ,  53   b , and  53   c , and transfer charge blades  54  of the image forming stations.  
         [0048]    After the transfer of the yellow toner image onto the recording medium P, the recording medium P advances to the image forming station Pm for a second color (magenta) as the transfer belt  51  moves in the direction indicated by an arrow mark f.  
         [0049]    The image forming station Pm for the second color is the same in structure as the image forming station Py for the first color. Thus, the same processes as those carried in the image forming station Py are carried out in the image forming station Pm. That is, a latent image is formed on the photoconductive drum  1   m , and is developed with the use of magenta toner. Then, the magenta toner image is transferred onto the recording medium P, in a manner to be layered on the yellow toner image, by the function of the transfer charge blade  54   m , in the transfer nip.  
         [0050]    Next, a cyan toner image and a black toner image are formed in the image forming stations Pc for a third color and the image forming station Pk for a fourth color, respectively, and are transferred onto the recording medium P by the transfer charge blades  54   c  and  54   k , in a manner to be layered on the preceding two toner images, in the corresponding image forming stations. Consequently, a color image, or a composite of four layers of toner images different in color, is formed on the recording medium P. At this point, the color image is yet to be fixed.  
         [0051]    After the transfer of the four toner images onto the recording medium P, the recording medium P is conveyed to a fixing apparatus  6 . While the recording medium P is passing through the fixing apparatus  6 , the toner particles are melted and fused with the recording medium P by heat and pressure. The fixing apparatus  6  comprises a fixing roller  6   a  containing a heating means, and a driving roller  6   b . After the fixation of the toner images, the recording medium P is discharged into an external delivery tray (unshown), or the like, of the image forming apparatus, to be accumulated therein.  
         [0052]    After the recording medium P is separated from the transfer belt  51 , the transfer belt  51  is removed of the electrical charge on the reverse side, by a combination of a grounded electrically conductive fur brush  11  and a grounded transfer belt driving roller  52 . Further, the foreign substances, for example, toner particles (residual toner particles), paper dust, and the like, on the top surface of the transfer belt  51 , are removed by a transfer belt cleaner  12  comprising a urethane rubber blade and the like, to be prepared for the next image formation cycle.  
         [0053]    On the portion of each of the photoconductive drums  1   y - 1   k , which has just passed the transfer nip, residual toner particles, that is, toner particles which failed to be transferred onto the recording medium P, are present, although only by a small amount. These residual toner particles are scraped away, electrostatically and mechanically, and are temporarily absorbed, by the magnetic brush of each of the magnetic brush type charging apparatuses  2   y - 2   k . As the amount of the transfer residual toner particles in the magnetic brush of each of the magnetic brush type charging apparatuses  2   y - 2   k  increases, the electrical resistance of the magnetic brush itself increases, and eventually, the magnetic brush fails to sufficiently charge the photoconductive drum. As a result, difference in electrical potential is created between the magnetic brush and the peripheral surface of the photoconductive drum, causing the transfer residual toner particles in the magnetic brush to electrostatically transfer onto the photoconductive drum. After transferring onto the photoconductive drum, the transfer residual toner particles are electrostatically taken into the developing apparatus, to be consumed during the following image formation cycles.  
         [0054]    As for the photoconductive member for this embodiment, it is desired to employ an ordinary organic photoconductive member. Preferably, the organic photoconductive member provided with a surface layer formed of a material with an electrical resistance in a range of 10 9 -10 14  Ω•cm, or an amorphous silicon based photoconductive member is employed, so that electrical charge can be directly injected to prevent ozone generation and to reduce electric power consumption, as well as to improve the efficiency with which the photoconductive drum is charged.  
         [0055]    Referring to FIG. 5, in this embodiment, the photoconductive drum I is a negatively chargeable organic photoconductive member, and comprises a base member  1 A, which is an aluminum drum with a diameter of 30 mm, and a photoconductive layer  1 B which comprises five sub-layers: first to fifth sub-layers counting from the innermost layer. It is rotationally driven at a predetermined peripheral velocity (process speed), for example, 120 mm/sec. The innermost sub-layer of the photoconductive layer  1 B is an undercoat layer, which is an electrically conductive layer with a thickness of 20 μm and is provided to repair the defects of the base drum  1 A. The second sub-layer is a positive charge transfer prevention layer, which plays a role in preventing the positive charge infected from the base drum  1 A from cancelling the negative charge injected into the peripheral surface of the photoconductive drum  1 . It is a  1  μm thick layer formed of a mixture of Amiran and methoxyl nylon, and its electrical resistance has been adjusted to approximately 10 6  Ω•cm, or a medium resistance. The third sub-layer is a charge generation layer with a thickness of approximately 0.3 μm, and is a resin layer in which disazo pigments has been dispersed. It generate combinations of positive and negative charges. The fourth sub-layer is a charge transfer layer, which is formed of polycarbonate resin in which hydrazone has been dispersed. It is a P-type semiconductor. Therefore, the negative charge given to the peripheral surface of the photoconductive drum  1  is not allowed to go through this layer, and only the positive charge generated in the third layer (charge generation layer) can be transferred to the peripheral surface of the photoconductive drum  1 . The fifth sub-layer, or the outermost layer, is a charge injection layer, which is formed by coating a mixture of dielectric resin as binder, and microscopic particles of SnO 2 , which is electrically conductive particles and has been dispersed in the dielectric binder. More concretely, microscopic particles of SnO 2  doped with antimony, that is, electrically conductive transparent filler, to reduce its electrical resistance (to render it electrically conductive) are dispersed in dielectric resin by 70 wt. %, and the thus formulated mixture is coated on the fourth-sub-layer to a thickness of approximately 3 μm with the use of an appropriate coating method, for example, dipping coating method, spraying coating method, roller coating method, beam coating method, or the like, to form the charge injection layer. The diameter of antimony particle is approximately 0.03 μm.  
         [0056]    The charging means employed in this embodiment is a contact charging means which charges the photoconductive drum  1  by contacting the photoconductive drum  1 . Referring to FIG. 5, it is a magnetic brush type charging apparatus  2  of a rotational sleeve type, which comprises: a stationary magnetic roller  2 A (charge magnetic roller) with a diameter of 16 mm; a nonmagnetic SUS sleeve  2 B (charge sleeve) rotationally fitted around the charge magnetic roller  2 A; and a magnetic brush layer  2 C, that is, a layer of magnetic particles (magnetic carrier) held to the peripheral surface of the charge sleeve  2 B by the magnetic force of the charge magnetic roller  2 A.  
         [0057]    As the magnetic particles for forming the magnetic brush layer  2 C, such magnetic particles that are 10-100 μm in average particle diameter, 20-250 Am 2 /kg in saturation magnetization, and 1×10 2 -1×10 10  Ω•cm in resistivity are preferable. In consideration of the presence of insulative defects, such as a pin hole, in the photoconductive drum  1 , employment of magnetic particles with specific resistivity of no less than 1×10 6  Ω•cm is preferable. In order to improve the charging performance of the charging means, the electrical resistance of the magnetic particles is desired to be as small as possible. In this embodiment, magnetic particles which are 25 μm in average particle diameter, 250 Am 2 /kg in saturation magnetization, and 5×10 6  Ω•cm in resistivity, are employed, and 40 g of such magnetic particles is magnetically adhered to the peripheral surface of the sleeve  2 B to form the magnetic brush layer  2 C. Incidentally, as for the measurement of the resistance value of the magnetic particles, 2 g of magnetic particles was placed in a metallic cell having a bottom area of 228 cm 2 , and the resistance value was measured by applying a voltage of 100 V with the presence of a load of 6.6 kg/cm 2  upon the magnetic particles in the cell.  
         [0058]    As the magnetic particles, resinous magnetic particles or single component magnetic particles, for example, magnetite particles, are employed. As for the composition of the magnetic particles, resinous magnetic particles are formed by dispersing magnetic substance and carbon black in resinous substance to make the resinous substance magnetic and electrically conductive, and to adjust the electrical resistance of the resinous substance, whereas single component magnetic particles are coated with resin for electrical resistance adjustment.  
         [0059]    The magnetic brush type charging apparatus  2  is disposed so that its magnetic brush layer  2 C contacts the peripheral surface of the photoconductive drum  1 . In this embodiment, the width of the contact nip n (charge nip) between the magnetic brush layer  2 C and photoconductive drum  1  is 6 mm. The charge sleeve  2 B is rotational driven at a peripheral velocity of 150 mm/sec, versus the peripheral velocity of, for example, 100 mm/sec for the photoconductive drum  1 , in the direction indicated by an arrow mark B so that the moving direction of the peripheral surface of the charge sleeve  2 B in the contact nip n becomes opposite to the moving direction A of the peripheral surface of the photoconductive drum  1  in the contact nip n. While the charge sleeve  2 B is rotationally driven as described above, a predetermined charge bias voltage is applied to the charge sleeve  2 B from an electrical power source. As a result, the peripheral surface of the photoconductive drum  1  is rubbed by the magnetic brush layer C to which the charge bias is being applied, and the surface of the photoconductive layer  1 B of the photoconductive drum  1  is uniformly charged to a predetermined potential level; in other words, the primary charge is injected into the photoconductive drum  1 . Increasing the peripheral velocity of the charge sleeve  2 B increases the frequency with which the transfer residual toner particles on a given area of the peripheral surface of the photoconductive drum  1  come into contact with the magnetic brush layer  2 C, improving therefore the efficiency with which the transfer residual particles are recovered into the magnetic brush layer  2 C.  
         [0060]    [0060]FIG. 6 shows the general structure of the developing apparatus  4  with which this embodiment of the image forming apparatus  100  is equipped. In this embodiment, the developing apparatus  4  is a contact type developing apparatus which uses two component developer (two component based magnetic brush type developing apparatus). Referring to FIG. 6, it has a development sleeve  41 , which is rotationally driven in the direction of an arrow mark C. Within the hollow of the development sleeve  41 , a magnetic roller  42  (development magnetic roller) is stationarily disposed Within a developer container  46 , in which developer T is stored, a couple of stirring screws  43  and  44  are disposed. Further, the developing apparatus  4  is provided with a regulation blade  45 , which is positioned so that the its edge is placed close to the peripheral surface of the development sleeve  41  to form a thin layer of developer T on the peripheral surface of the development sleeve  41   
         [0061]    The development sleeve  41  is disposed so that the distance between the peripheral surfaces of the development sleeve  41  and photoconductive drum  1  becomes approximately 450 μm at least during development, making it possible for a thin layer  5 A of the developer T formed on the peripheral surface of the development sleeve  41  to contact the peripheral surface of the photoconductive drum  1  for development.  
         [0062]    The developer T used in this embodiment is a mixture of toner t and magnetic carrier c. The toner t is in the form of a microscopic particle with an average particle diameter of 8 μm produced by pulverization, and externally contains titanium particles with an average particle diameter of 20 nm by 1 wt %. The carrier c is magnetic carrier, which is 205 Am 2 /kg in saturation magnetization and 35 μm in average particle diameter. The mixing ratio between the toner t and carrier c in the developer T is 6:94 in weight ratio.  
         [0063]    At this time, the development process in which the electrostatic latent image on the peripheral surface of the photoconductive drum  1  is visualized by the developing apparatus  4  which uses a two component magnetic brush based developing method, and the developer T circulating system, will be described. First, as the development sleeve  41  rotates, the developer T is adhered to the development sleeve  41  at the point correspondent to magnetic pole N 2  of the development magnetic roller  42 , forming a developer T layer. The developer T layer having been adhered to the development sleeve  41  is conveyed to the point correspondent to magnetic pole S 2 , as the development sleeve  41  further rotates. While the developer T layer on the development sleeve  41  is conveyed to the point correspondent to pole S 2 , it is regulated in thickness by the regulation blade  45  positioned perpendicular to the development sleeve  41 . As a result, a thin layer Ta of the developer T is formed on the peripheral surface of the development sleeve  41 . As the thin layer Ta of the developer T borne on the development sleeve  41  is conveyed to the position correspondent to pole N 1 , the thin layer Ta of the developer T is made to crest, and the electrostatic latent image on the photoconductive drum  1  is developed by this crested portion of the thin layer Ta of the developer T. Thereafter, the developer T on the development sleeve  41  is returned into the developer container  46  by the repulsive magnetic field generated by poles N 3  and N 2 .  
         [0064]    To the development sleeve  41 , a combination of DC voltage and AC voltage is applied from an electric power source (unshown). In this embodiment, a combination of a DC voltage of −500 V, and an AC voltage having a frequency of 2,000 Hz and a peak-to-peak voltage of 1,500 Vpp, is applied.  
         [0065]    Generally, in a two component developing method, application of AC voltage improves development efficiency, producing therefore an image of higher quality However, it is likely to trigger fog generation. Thus, normally, in order to prevent the fog generation, a certain amount of difference in potential level is provided between the DC voltage applied to the developing apparatus, and the surface potential of the photoconductive drum  1 .  
         [0066]    Next, the transferring apparatus  5  with which this embodiment of an image forming apparatus is equipped will be described in more detail. Referring to FIG. 3, the transferring apparatus in this embodiment is a belt type transferring apparatus, which comprises the transfer belt  51  as a transfer medium bearing member, which is an endless belt and is suspended around the driving roller  52  and three supporting rollers  53   a ,  53   b , and  53   c , which are follower rollers. The transfer belt  51  is rotationally driven in the direction of the arrow mark f at approximately the same speed as the rotational speed (peripheral velocity) of the photoconductive drum  1 . More specifically, the transfer belt  51  is driven so that the moving speed of the peripheral surface of the photoconductive drum  1  and the moving speed of the transfer belt  51  in the direction of the arrow mark f become approximately the same in the transfer nip between the photoconductive drum  1  and transfer belt  51 .  
         [0067]    When forming an image using this embodiment of image forming apparatus  100 , the toner images formed in the image forming stations Py, Pm, Pc, and Pk, one for one, must be precisely in alignment with each other on the recording medium P, as the recording medium P advances into the image forming stations Py, Pm, Pc, and Pk In order to precisely align the toner images, the recording medium P must be precisely held to the transfer belt  51  and be stably conveyed. Thus, the recording medium P is electrostatically adhered to the transfer belt  51  with the use of electrostatic adhesion rollers  55  and  56 . The adhesion roller  56  is grounded. As the recording medium P enters between the adhesion rollers  55  and  56 , a positive bias of 1 kV is applied to the adhesion roller  55  to electrostatically adhere the recording medium P to the transfer belt  51 .  
         [0068]    The bottom side, in the drawing, of the photoconductive drum  1  of each of the image forming stations Py, Pm, Pc, and Pk is kept in contact with the top surface, in the drawing, of the top side of the loop of the transfer belt  51 . The recording medium P is placed on the top surface of the top side of the loop of the transfer belt  51 , and is conveyed through the transfer nip of each of the image forming stations Py, Pm, Pc, and Pk. In each transfer nip, a predetermined transfer bias is applied to the transfer blade  54  from an electrical transfer bias application power source (unshown). As a result, the recording medium P is changed to the polarity opposite to that of the toner t from its reverse side Consequently, the toner image on the photoconductive drum  1  is transferred onto the top surface of the recording medium P.  
         [0069]    The transfer belt  51  as a transfer medium bearing member employed in this embodiment is an endless belt formed of laminar material having two layers of thermosetting polyimide resin as shown in FIG. 1.  
         [0070]    The width of the transfer belt  51  is 330 mm, which is wide enough for an A 3  printing paper, and the circumference of the transfer belt  51  is approximately 1,037 mm.  
         [0071]    The first layer  51   a  (surface layer) of the transfer belt  51 , which has the surface (transfer medium bearing surface) which contacts the photoconductive drum  1  is 35 μm in thickness, and is formed of thermosetting polyimide resin (PI) in which carbon black (CB) as electrically conductive filler (electrical resistance adjustment agent) has been dispersed to give the transfer belt  51  a surface resistivity (ρs) of 10 13 -10 14  Ω/□. The surface layer  51   a  of the transfer belt  51  in this embodiment contains carbon black as electrical resistance adjustment agent) by 10 wt. %.  
         [0072]    On the other hand, the second layer  51   b  (back layer) of the transfer belt  51 , which has the surface with which the transfer blade  51  contacts, is 40 μm in thickness, and is formed of pure thermosetting polyimide resin, that is, such thermosetting resin that does not contain electrical resistance adjustment agent. Thus, the second layer  51   b  is an dielectric layer.  
         [0073]    The surface layer (first layer)  51   a  and the back layer (second layer)  51   b  are laminated to each other while polyimide resin is in its precursor state (polyamide resin) to form the laminar transfer belt  51  comprising the integrally laminated surface layer  51   a  and back layer  51   b . The precursor of the polyimide resin, or polyamide resin, turns into polyimide resins while the transfer belt  51  is molded.  
         [0074]    Giving the transfer belt  51  a laminar structure as described above, that is, forming the transfer belt  51  by laminating the surface layer  51   a  adjusted in electrical resistance with the use of electrically conductive filler, and the back layer  52   a  with no adjustment in electrical resistance, to divide the functions of the transfer belt  51  between two layers, makes it possible to provide the transfer belt  51  with appropriate electrical properties as well as mechanical strength for withstanding the repetitions of image forming operations. With the provision of the above described structural arrangement, it is possible to provide a mechanically strong transfer belt which does not suffer from the above described problems, such as the interference between the biases applied to the transfer blade  54  and electrostatic adhesion roller  55 , the disturbance of the toner images, and the generation of insufficient amount of recording medium P adhering force, which occur when the electrical resistance of the transfer belt  51  is lower than a certain level, and also, the abnormal electrical discharge in the transfer nips and/or electrostatic adhesion nip, which occurs when the electrical resistance of the transfer belt  51  is higher than a certain level.  
         [0075]    The employment of polyimide resin, which is superior in mechanical strength, as the material for the laminar material for the transfer belt  51 , drastically reduces the number of times by which the transfer belt  51  needs to be replaced due to the breaking, bending, or the like, of the transfer belt  51 , compared to the employment of the thermoplastic resin such as PvdF (polyfluorovinylidene resin) or PC (polycarbonate resin), which has been widely used.  
         [0076]    However, it has been known that thermosetting polyimide resin, which is a crystalline resin, has a tendency to relatively easily absorb moisture, and is large in the coefficient of linear expansion resulting from the moisture absorption. The transfer belt  51  in this embodiment employs a laminar structure. Further, it employs thermosetting polyimide resin as the material therefor, and carbon black as electrical resistance adjustment agent has been dispersed in the surface layer  51   a . Therefore, there is a subtle different in coefficient of linear expansion, in other words, rate of shrinkage, between the surface layer  51   a  and back layer  51   b.    
         [0077]    Generally, if an object has a laminar structure having two layers different in rate of shrinkage, this object warps toward the layer with the smaller rate of shrinkage, due to the changes in ambience, for example, changes in ambient temperature and/or humidity.  
         [0078]    In the case of the endless transfer belt  51  in this embodiment, which is suspended around the plurality of rollers, even if the above described warping occurs, it matters very little as long as the warping concerns the circumferential direction of the transfer belt  51 , because the transfer belt  51  is suspended around the driving roller  52  and three follower rollers  53   a ,  53   b , and  53   c  in a manner to give the transfer belt  51  a constant tension (approximately 3 kgf≈29N) in the circumferential direction of the belt (conveyance direction).  
         [0079]    However, if the transfer belt  51  warps in terms of the width direction by a large amount, the recording P, transfer belt  51 , and photoconductive drum  1  fail to uniformly contact among themselves in terms of the width direction of the transfer belt  51  as described above As a result, it becomes impossible for the transfer charging means such as the transfer charge blade  54  to uniformly charge the transfer belt  51  or the recording medium P. Further, there occur air gaps G (FIG. 12) between the transfer belt  51 , in particular, its edge portions, and the photoconductive drum  1 , and between the transfer belt  51  and the recording medium P, which result in an image of inferior quality (transfer error).  
         [0080]    Thus, the inventors of the present invention seriously studies the transfer belt  51  formed of two layers of thermosetting polyimide resin, while paying special attention to the rates of shrinkage of the two layers, and the changes in the measurements of each layer of the transfer belt  51  caused by the changes in ambience (temperature and humidity). In other words, “difference in the measurement change between the two layers”, which could be calculated from the shrinkages and lengths of the two layers, and are affected by the ambient factors such as temperature and humidity, were studied. As a result, it was discovered that when the two layers satisfied certain requirements, the above described problem, or the warping, did not occur.  
         [0081]    More specifically, the sizes of the surface and back layers  51   a  and  51   b  of the transfer belt  51  composed of polyimide resin were measured when the ambient temperature and humidity were 15° C. and 10% RH, respectively, that is, when the ambient temperature and humidity are the lowest and the volume of polyimide resin used in this embodiment was smallest, within the normal environment in which the image forming apparatus  100  in this embodiment was used, and also the sizes were measured when the ambient temperature and humidity were 30° C. and 80% RH), respectively, that is, when the ambient temperature and humidity were the highest and the polyimide resin had swollen to its largest volume, within the normal environment in which the image forming apparatus  100  was used. Then, the difference in the size change between the two layers, the warping of the transfer belt  51 , and the image defects caused by the warping, were studied.  
         [0082]    Next, the method for measuring the changes in the size of each layer will be described.  
         [0083]    First, test pieces were made of each of the resinous materials for the surface layer  51   a  and  51   b . All test pieces were the same in thickness. Then, the dimensions of the test pieces were measured when the temperature and humidity are highest and lowest within the normal environment (15° C./10% RH-30° C./80% RH) in which an image forming apparatus were used. In other words, they were measured in an environment in which the temperature and humidity were 15° C. and 10% RH, and an environment in which the temperature and humidity were 30° C. and 80% RH. Then, the difference in measurements of corresponding test pieces between the two environments, that is, the expansion, or shrinking, of the test pieces, were obtained.  
         [0084]    More concretely, in order to test a laminated transfer belt such as the transfer belt  51  in this embodiment, composed of the surface layer  51   a  which was 1,037 mm in circumference, 330 mm in width, and 35 μm in thickness, and the back layer  51   b  which was 1,037 mm in circumference, 330 mm in width, and 40 μm in thickness, a nonlaminative test piece (i) for the surface layer  51   a  and a nonlaminative test piece (ii) for the back layer  51   b , were made of resinous materials, which were 330 mm and 330 mm in length, 50 mm and 50 mm in width, and 35 μm and 40 μm in thickness, respectively.  
         [0085]    These resinous materials expanded due to the presence of moisture as temperature and humidity increased. In order to compare the surface layer  51   a  and back layer  51   b , in terms of the absolute value in the widthwise expansion of the transfer belt  51  which caused the widthwise warping of the transfer belt  51 , the lengths L (a/low) and L (b/low) of the test pieces for the surface layer (first layer)  51   a  and back layer (second layer)  51   b  in the aforementioned low temperature/low humidity environment, respectively, and the lengths L (a/high) and L (b/high) of the test pieces for the surface and back layers  51   a  and  51   b  in the aforementioned high temperature/high humidity environment, respectively, were measured.  
         [0086]    The elongations (measurement change) of the surface and back layers  51  a and  51   b  were: 
         elongation (Xa) of surface layer =L (a/high)−L (a/low) 
         elongation (Xb) of back layer =L (b/high)−L (b/low). 
         [0087]    Thus, the difference in measurement change between the surface and back layers  51   a  and  51   b  was defined as: 
         difference=|elongation (Xa) of surface layer −elongation (Xb) of back layer|. 
         [0088]    For example, the elongation Xa of the test piece for the surface layer  51   a  of the transfer belt  51 , which was formed of thermosetting polyimide resin in which carbon black had been dispersed by 10 wt. %, and the length of which was 330 mm in length, 50 mm in width, and 35 μm in thickness in the environment in which temperature and humidity were 23° C. and 60% RH, was +180 μm. In other words, the length of the surface layer  51   a  in this embodiment in the high temperature/high humidity environment was 180 μm greater than that in the low temperature/low humidity environment.  
         [0089]    On the other hand, the elongation Xb of the test piece for the surface layer  51   b  of the transfer belt  51 , which was formed of polyimide resin. and the length of which was 330 mm in length, 50 mm in width, and 40 μm in thickness in the environment in which temperature and humidity were 23° C. and 60% RH, was +240 μm. In other words, the length of the surface layer  51   a  in this embodiment in the high temperature/high humidity environment was 240 μm greater than that in the low temperature/low humidity environment.  
         [0090]    Incidentally, it had been known that dispersing filler such as carbon black in a certain resinous substance in the same manner as carbon black is dispersed in the resinous material for the surface layer  51   a  of the transfer belt  51  in this embodiment reduces the shrinkage of the resinous substance in proportion to the amount of the filler.  
         [0091]    Thus, a plurality of test piece for the surface layer  51   a , which were the same in length, that is, 330 mm, but were different in thickness and the amount of the carbon black dispersed in polyimide resin, as shown in Table 1, were made of thermosetting polyimide resin in which carbon black were dispersed, in addition to a test piece for the back layer  51   b , which was 330 mm in length and 35 μm in thickness, but was made of pure polyimide. Then, the elongations Xa for the test pieces containing carbon black, and the elongation Xb for the test piece containing no carbon black, were measured. As is evident from Table 1, the elongation Xb, that is, the elongation for the test piece for the back layer  51   b , was 240 μm.  
         [0092]    Further, in addition to the above described test pieces, a plurality of actual laminar transfer belts  51  were made. They had the surface and back layers  51   a  and  51   b , the specifications of which were as shown in Table 1. These transfer belts were set up in the image forming apparatus  100  in accordance with the present invention, and the images produced by the image forming apparatus  100  in the low temperature/low humidity environment (15° C./10% RH) in which the transfer belts shrank to the smallest length, and in the high temperature/high humidity environment (30° C./80% RH) in which the transfer belts swelled to the largest length, were evaluated. When there were a large amount of difference in the measurement change between the surface and back layers  51   a  and  51   b  of the transfer belt  51 , and therefore, the transfer belt  51  warped as shown in FIG. 12, the recording medium P, transfer belt  51 , and photoconductive drum  1  failed to remain in contact with each other, along the edges of the transfer belt  51 . As a result, transfer errors occurred, resulting in images of inferior quality, which were low in density across the areas correspondent to the edges of the transfer belt  51 . Thus, the images were evaluated with respect to the occurrences of the transfer errors. The results are given in Table 1.  
                                   TABLE 1                           Surface layer   Surface layer   Difference in   Total thickness           Surface layer   thickness (μm)   elongation (μm)   dimensional change (μm)   (μm)   Image                   Pl   35   240    0   75   G       Pl + Carbon (10 wt. %)   35   180   60   75   G       Pl + Carbon (20 wt. %)   35   150   90   75   NG       Pl + Carbon (30 wt. %)   35   120   120    75   NG       Pl + Carbon (10 wt. %)   45   180   60   85   G       Pl + Carbon (20 wt. %)   45   150   90   85   F       Pl + Carbon (30 wt. %)   45   120   120    85   NG       Pl + Carbon (10 wt. %)   55   180   60   95   G       Pl + Carbon (20 wt. %)   55   150   90   95   F       Pl + Carbon (30 wt. %)   55   120   120    95   NG                                          
 
         [0093]    It is evident from the results given in Table I that unless the difference in the absolute value of elongation (Xa and Xb) between the surface and back layers  51   a  and  51   b  of the transfer belt  51  exceed the value of the overall thickness  11   t  (thickness Ha of surface layer  51   a +thickness Hb of back layer  51   b ) of the transfer belt  51 , the formation of a low quality image can be almost completely avoided In other words, satisfying the following inequity (1): 
         difference in elongation (|elongation of surface layer (Xa)−elongation of back layer (Xb)|&lt;overall thickness (Ht=Ha+Hb))  (1) 
         [0094]    prevents the warping of the transfer belt  51 , and therefore, prevents the formation of an image of low quality which results from transfer errors or the like.  
         [0095]    In the case of the transfer belt  51  in this embodiment, elongations (Xa) and (Xb) of the surface layer (first layer)  51   a  and back layer (second layer)  51   b  were 180 μm and 240 μm, and therefore, the difference (absolute value) in elongation between the two layers was 60 μm. Thus,  
         [0096]    difference in elongation (|180 μm-240 μm &lt;overall thickness (75 μm)).  
         [0097]    In other words, the difference in the elongation between the two layers  51   a  and  51   b  was smaller than the overall thickness 76 μm of the transfer belt  51 , satisfying the above described requirement, and therefore, being capable of preventing the problems which results from the warping.  
         [0098]    As for the requirement regarding the range of the ambience change, that is, the temperature and humidity ranges, it has only to assured that the temperature and humidity are kept within ranges of 15-30° C. and 10-80% RH, respectively, in consideration of the actual environment in which an image forming apparatus is used.  
         [0099]    Incidentally, this embodiment of the image forming apparatus  100  was described as a color image forming apparatus comprising the plurality of image forming stations Py-Pk. However, the application of the present invention is not limited to such an image forming apparatus. That is, obviously, the present invention is also applicable to a monochromatic image forming apparatus such as the one shown in FIG. 4, which comprises only a single image forming station, and forms an image on the a recording medium P being held to, and conveyed by, a transfer belt  51  as a transfer medium bearing member.  
         [0100]    As described above, the present invention can prevent the transfer belt  51  from warping in terms of the width direction. The prevention of the warping of the transfer belt  51  prevents such problems that the transfer belt  51  and/or recording medium P are nonuniformly charged by the transfer charge blade  54  because of the warping of the transfer belt  51 , and/or that air gaps are created between the photoconductive drum  1  and recording medium P, along the widthwise edges of the transfer belt  51 . The prevention of these problems prevents the formation of a defective image which results from the transfer error caused by these problems. In other words, the present invention can prevent the formation of a defective image which results from the warping of the transfer belt  51 .  
         [0101]    Embodiment 2  
         [0102]    Next, another embodiment of the present invention will be described. FIG. 7 shows the general structure of another embodiment of an image forming apparatus in accordance with the present invention.  
         [0103]    The present invention is also applicable to an image forming apparatus such as the image forming apparatus  200  shown in FIG. 7, which is equipped with only one image bearing member on which a plurality of toner images different in color are consecutively formed to be consecutively transferred onto a recording medium P electrostatically adhered to the transfer medium bearing member. The application of the present invention to such an image forming apparatus produces the same beneficial effects as those produced by the first embodiment.  
         [0104]    Referring to FIG. 7, the image forming apparatus  200  in accordance with the present invention has only a single image bearing member, which is an electrophotographic photoconductive member in the form of a rotational cylinder, that is, a photoconductive drum  1 . It also has a primary charging device  2 ′ as a charging means, an exposing apparatus  3 , a developing apparatus group  4 , and a cleaner  9 , which are disposed around the photoconductive drum  1 . The developing apparatus group  4  in this embodiment comprises magenta, cyan, yellow, and black color developing apparatuses  4   m ,  4   c ,  4   y , and  4   k  for forming magenta, cyan, yellow, and black toner images, correspondingly.  
         [0105]    Located diagonally below the photoconductive drum  1  in the drawing is a transferring apparatus  7 A (drum type transferring apparatus) as a transfer medium bearing member, which comprises a sheet  71  (transfer sheet) stretched around a cylindrical skeletal frame.  
         [0106]    Within the hollow of this transfer drum  7 A, an adhesion charge blade  75 , and a transfer charge blade  74  as a transfer charging device, are disposed. On the outward side of the transfer drum  7 A, an adhesion blade  76  is disposed in a manner to oppose the adhesion charge blade  75  across the transfer sheet  71 . The adhesion blade  76  is grounded, and is enabled to be placed in contact with, or separated from, the transfer drum  7 A.  
         [0107]    As an image forming operation begins, the peripheral surface of the photoconductive drum  1  is uniformly charged by the primary charging device  2 ′and is exposed to a laser beam L projected from the exposing apparatus  3 , a laser based exposing apparatus, while being modulated with a first color (yellow) component of a target image. As a result, an electrostatic latent image correspondent to the yellow color component of the target image is formed. This electrostatic latent image is visualized into a yellow toner image by the yellow developing apparatus  4   y.    
         [0108]    Meanwhile, a recording medium P such as a piece of recording paper is fed into the image forming apparatus main assembly from a recording medium cassette  80  as a recording medium storage located in the bottom portion of the apparatus main assembly by a pair of sheet feeder rollers  81  and the like, and is delivered to the transfer drum  7 A by a registration roller  81  in synchronism with the formation of the yellow toner image on the photoconductive drum  1 . The recording medium P is electrostatically adhered to the recording medium bearing portion, that is, the transfer sheet  71 , of the transfer drum  7 A, by the function of the adhesion charge blade  75  to which voltage is being applied, and the function of the adhesion roller  76  which has been temporarily placed in contact with the transfer drum  7 A to adhere the recording medium P to the transfer drum  7 A. After the adhesion of the recording medium P to the transfer drum  7 A, the adhesion roller  76  is separated from the transfer drum  7 A.  
         [0109]    The recording medium P borne an the transfer drum  7 A is conveyed to a transfer nip, or the interface between the photoconductive drum  1  and transfer drum  7 A, by the rotation of the transfer drum  7 A in the direction of an arrow mark B in FIG. 7. In the transfer nip, the yellow toner image on the photoconductive drum  1  is electrostatically transferred onto the recording medium P by the function of the transfer charge blade  74  to which voltage is being applied.  
         [0110]    Processes similar to the above described processes carried out for the yellow color component of the target image are consecutively carried out for the cyan, magenta, and black color components so that the consecutively formed toner images are transferred one after another onto the recording medium P borne on the transfer drum  7 A which is rotating in the direction of the arrow mark B. Consequently, a full-color image composed of four unfixed color toner images, is formed on the recording medium P.  
         [0111]    Thereafter, the recording medium P is separated from the transfer drum  7 A, and is conveyed to a fixing apparatus  6 , which comprises a fixing roller  6   a  equipped with a heating means, and a driving roller  6   b . As the recording medium P is conveyed through the fixing apparatus  6  by the combination of the fixing roller  6   a  and driving roller  6   b , being pinched between the two rollers, the unfixed toner images on the recording medium P are fixed to the recording medium P by heat and pressure; in other words, they are turned into a permanent full-color image. After the fixation of the toner images, the recording medium P is discharged from the apparatus main assembly.  
         [0112]    The transfer residual toner particles, that is, the toner particles remaining on the peripheral surface of the photoconductive drum  1  after the transfer of the toner images, are removed by the cleaner  9  equipped with cleaning means such as a fur brush or an elastic blade. The foreign substances such as toner particles adhering to the transfer sheet  71  of the transfer drum  7 A are removed by the transfer drum cleaner  11  equipped with cleaning means such as a fur brush or an elastic blade.  
         [0113]    Next, referring to FIGS. 8 and 9, the transfer drum  7 A will be further described.  
         [0114]    Referring to FIG. 8, the transfer drum  7 A comprises two circular sub-frames  72 , or base rings  72 , a straight sub-frame  73 , or a base rod  73 , and the transfer sheet  71 . The two base rings  72  are connected by the base rod  73 , forming the cylindrical skeletal frame of the transfer drum  7 A. The transfer sheet  71  is stretched between the two base rings  72  in a manner to wrap the cylindrical skeletal frame in the circumferential direction of the base rings  72 , and pasted to the frame.  
         [0115]    As the material for the transfer sheet  71  employed by the transfer drum  7 A in this embodiment, the same material as that employed in the first embodiment, that is, two layer laminate of thermosetting polyimide resin, is used. After the pasting of the transfer sheet  71  to the frame, the transfer sheet  71  is 330 mm in terms of the width direction of the transfer drum  7 A, and 565 mm (transfer drum  1 A diameter 180 mmπ) in terms of the circumferential direction (transfer medium conveyance direction) of the transfer drum  7 A, in the normal environment in which the apparatus is used.  
         [0116]    Also in this embodiment, the first layer (surface layer)  71   a , the surface of which the transfer charge blade  74  contacts, is formed of thermosetting polyimide, and the surface electrical resistance of which has been adjusted to 10 13 -10 14  Ω•cm by dispersing carbon black as electrically conductive filler in the resin. Its thickness is 35 μm. The surface layer  51   a  of the transfer sheet  71  in this embodiment contains carbon black, that is, electrical resistance adjustment agent, by 10 wt. %.  
         [0117]    On the other hand, the second layer (back layer)  71   b , the surface of which the adhesion charge blade  75  and transfer charge blade  74  contact, is formed of pure thermosetting polyimide resin, in other words, polyimide resin which does not contain electrical resistance adjustment agent and therefore, is dielectric. Its thickness is 40 μm. The two layers of polyimide resin are laminated to each other while polyimide resin is in the precursor state (polyamide resin) to form the laminar transfer sheet  71 , as done when the transfer belt  51  in the first embodiment is formed. The polyamide resin, or the precursor of the polyimide resin, turns into polyimide resin while the two layers of precursor are molded into the laminar transfer sheet  71 .  
         [0118]    The transfer drum  7 A in this embodiment comprises a cylindrical skeletal frame, and a rectangular transfer sheet  71  slightly loosely wrapped around this cylindrical skeletal frame. The cylindrical skeletal frame comprises two sub-frames  72  in the form of a ring, and a straight sub-frame  73  which connects the two rings  72 . The four edges of the rectangular transfer sheet  71 , that is, the portions of the transfer sheet  71 , which correspond in position to the two sub-frames  72  in the form of a ring, and the straight sub-frame  73 , are adhered to the corresponding portions of the cylindrical skeletal frame, with the use of double-side adhesive tape or the like.  
         [0119]    Therefore, the transfer sheet  71  in this embodiment is different from the transfer belt  51  in the first embodiment in that the four edges of the transfer sheet  71  are fixed. In the case of a transfer sheet such as the transfer sheet  71 , if warping occurs to the transfer sheet  71  itself, the transfer sheet  71 , which normally remain cylindrical by being wrapped around the cylindrical skeletal frame, deforms and loses its cylindrical configuration. More concretely, deformations such as a dent D occur to the transfer sheet  71 .  
         [0120]    The occurrence of such deformations creates problems similar to those which result from the warping of the transfer belt  51  in the first embodiment. In other words, the deformation of the transfer sheet  71  prevents the transfer sheet  71 , recording medium P, and photoconductive drum  1  from contact each other uniformly across their surfaces, causing therefore transfer errors, which results in the formation of an image of inferior quality. Further, the deformation of the transfer sheet  71  may cause the recording medium P to be improperly adhered to the transfer sheet  71 . In other words, the deformation of the transfer sheet may have worse effects than the warping of the transfer belt  51 .  
         [0121]    However, the transfer sheet  71  in this embodiment is given a laminar structure, being composed of a surface layer  71   a  formed of thermosetting polyimide resin in which carbon black has been dispersed by 10 wt. %, and a back layer  71   b  formed of polyimide resin, and satisfies the following inequity (1) which was presented before, within the normal environment,in which the apparatus is operated, that is, within a temperature/humidity range of 15° C./10% RH-30° C./80% RH: 
         difference in elongation (elongation of surface layer (Xa)−elongation of back layer (Xb)|&lt; overall thickness (Ht=Ha+Hb))  (1) 
         [0122]    More specifically, when the ambient temperature and humidity was 23° C. and 60% RH , the surface and bottom layer  71   a  and  71   b  are 330 mm and 330 mm in length, and 35 μm and 45 μm, respectively, as they were measured with the use of the method described regarding the first embodiment. The length changes (elongations) Xa and Xb of the two layers  71   a  and  71   b  between when the ambient temperature and humidity were 15° C. and 10% RH, that is, when two layers  71   a  and  71   b  were shortest within the above described normal operational environment, and when the ambient temperature and humidity were 30° C. and 80% RH, that is, when the two layers  71   a  and  71   b  were longest, were 180 μm and 240 μm, respectively, satisfying the above inequity (1).  
         [0123]    The employment of a laminar transfer sheet such as the transfer sheet  71  formed of two layers of thermosetting polyimide can prevent the transfer errors which result as the transfer sheet  71 , recording medium P, and photoconductive drum  1  fail to contact each other uniformly across their surfaces, and also prevent such anomalies as the improper adhesion of the recording medium P to the transfer sheet  71  that affects the formation and conveyance of an image. Therefore, the employment of a laminar transfer sheet such as the transfer sheet  71  makes it possible to form an excellent image.  
         [0124]    As is evident from the above description of the second embodiment, the present invention is also applicable, with excellent results, to an image forming apparatus, the transfer medium bearing member of which is in the form of a sheet pasted to the cylindrical skeletal frame of the transfer drum.  
         [0125]    Also as is evident from the above descriptions, thermosetting polyimide resin, which is a crystalline resin, is superior to thermoplastic resin, in mechanical strength; in other words, the former is more difficult to break than the latter. Therefore, it is preferable as the resinous material for the transfer belt  51  or transfer sheet  71 . Since crystalline resin frequently used as the material for the transfer belt  51  or transfer sheet  71  has a relatively large coefficient of linear expansion, the beneficial effects of the present invention are greater. Principally, however, the application of the present Invention is not limited to an image forming apparatus, the transfer medium bearing member of which is in the form of a belt or sheet and is formed of thermosetting crystalline resin. Obviously, the application of the present invention is not limited to the preceding embodiments of an image forming apparatus, the transfer medium baring member of which was formed of polyimide resin. In other words, the present invention is also compatible with laminar material composed of plastic such as polycarbonate resin, polyethylene-terephthalate resin, polyfluorovinylidene resin, polyethylene-naphthalate resin, polyether-ether-ketone resin, polyether-sulfone resin, polyurethane, or the like, and a laminar transfer belt or transfer sheet, as a transfer medium bearing member, formed of such laminar material, in addition to the above described materials and transfer medium bearing members.  
         [0126]    As for the overall thickness of the transfer belt  1 , it is not limited to 75 μm. It may be in a range of 25-2,000 μm, preferably in a range of 50-150 μm.  
         [0127]    In the above description of the embodiments of the present invention, the transfer belt  51  and transfer sheet  71  were described as a laminar member having two layers: first and second layers. The present invention, however, does not need to be limited to the configuration of these transferring members. In other words, the present invention is also compatible with a laminar transfer medium bearing member having three or more layers. When a laminar transfer medium bearing member has thee or more layers, assuring that adjacent two layers satisfy inequity (1) presented above suffices. In such a case, the overall thickness Ht in inequity (1) is the sum of the thicknesses of the adjacent two layers Referring to FIG. 10, when a laminar transfer medium bearing member has, for example, three layers, that is, first, second, and third layers  51   a ,  51   b , and  51   c , with thicknesses of Ha, Hb, and Hc, correspondingly, the elongations xa, Xb, and Xc of the layers  51   a ,  51   b , and  51   c , correspondingly, caused by the changes in the ambience, sum Ht1 of the thicknesses of the first and second layers  51   a  and  51   b , and sum Ht2 of the second and third layers  51   b  and  51   c , must satisfy the following inequities; 
         different in measurement change (|xa−Xb|&lt;thickness (Ht1=Ha+Hb)  (2) 
         different in measurement change (|Kb−Xc|&lt;thickness (Ht2=Hb+Hc)  (3) 
         [0128]    By configuring the laminar member in manner to satisfy both inequities (2) and (3), the deformation, such as warping, of the laminar member employed by an image forming apparatus, which is caused by the ambient changes, can be prevented, and therefore, an excellent image, that is, an image which does not suffer from defects which result from transfer errors, can be always formed.  
         [0129]    As described above, the present invention makes it possible to provide a transfer medium bearing member which does not suffer from such deformation as warping that is caused by the changes in environmental factors such as temperature and humidity. Further, an image forming apparatus employing a transfer medium bearing member in accordance with the present invention can always form an excellent image, that is, an image which does not suffer from defects which results from transfer errors or the like.  
         [0130]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.