Patent Publication Number: US-6704535-B2

Title: Fiber-reinforced intermediate transfer member for electrophotography, and electrophotographic apparatus including same

Description:
FIELD OF THE INVENTION AND RELATED ARTS 
     The present invention relates to an intermediate transfer member for temporarily holding an image in an image forming process according to electrophotography, and an electrophotographic apparatus including the intermediate transfer member. 
     An electrophotographic apparatus including an intermediate transfer member is very effective for forming a color image by sequentially superposing and transferring a plurality of component color images. For example, it is possible to decrease color deviation in superposing respective color toner images compared with a transfer process described in Japanese Laid-Open Patent Application (JP-A) 63-301960. Moreover, it is possible to transfer an image from the intermediate transfer member onto a recording medium or transfer-receiving material without necessitating holding means, such as glipper means, sucking means or curvature means (as disclosed in FIG. 1 of JP-A 63-301960), so that the recording medium can be selected from a wide variety of materials, including thin paper (40 g/m 2 ) to thick paper (200 g/m 2 ), wide to narrow medium, and long to short medium. Accordingly, transfer can be performed onto an envelope, a post card and even label paper, etc. 
     Because of such advantageous features, color copying machines and color printers using intermediate transfer members have already been available on the market. 
     The intermediate transfer member may assume a shape of a drum or a belt, and a belt-shaped intermediate transfer member is advantageous because it allows a reduction in entire apparatus cost and a broad latitude for disposition designing thereof. 
     However, a conventional intermediate transfer belt comprising resin or rubber has caused the following difficulties when supported under tension about a roller. 
     (1) The permanent elongation of the intermediate transfer belt is gradually increased until the belt slips on the roller so that a deviation (color deviation) is caused between respective color toner images when the toner images are transferred from the photosensitive member onto the intermediate transfer belt (primary transfer), thus failing to provide clear images. 
     (2) When the belt is biased on the roller to its side, the belt edge is abutted against the flange, etc., to be rubbed and damaged in some cases. 
     Against these problems, e.g., JP-A 3-293385 has proposed to reinforce a rubber-made intermediate transfer belt with backing polyamide woven cloth. In this case, however, as there is a large difference in electrical resistivity between the rubber and the woven cloth, the intermediate transfer belt is caused to have a very large thicknesswise resistivity depending on the material and the thickness of the woven cloth, thus failing to effect good electrostatic transfer. Further, the trace of the woven cloth can appear in the product image, preventing high-quality images. 
     On the other hand, JP-A 6-149079 discloses an intermediate transfer belt comprising PVDF or polycarbonate, but the belt lacks elasticity by itself, so that transfer failure such as hollow image dropout (i.e., middle image portion surrounded by a contour of image being not transferred) is liable to occur, and the belt is liable to be torn or cracked due to resin fatigue under repetitive use, thus involving problems regarding durability. 
     In addition to the above, U.S. Pat. No. 5,409,557, JP-A 62-293270, JP-A 3-293385 and JP-A 3-69166 have disclosed reinforcement of intermediate transfer belts with fiber, etc. 
     Accordingly, we have made various intermediate transfer belts of rubbery elastic material reinforced with fiber. As a result, we have discovered the occurrence of a new-type of difficulty (hereinafter called “core trace image”) that the fiber trace appears as a density irregularity in a product image in some cases. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an intermediate transfer member which is free from permanent elongation even in repetitive use, excellent in durability and free from the occurrence of core trace image. 
     Another object of the present invention is to provide an electrophotographic apparatus including such an intermediate transfer member and capable of providing clear images free from color deviation or core trace image. 
     As a result of our study, it has been discovered that the above-mentioned core trace image has occurred in the case where the fibers constituting the core member embedded within the base layer of the intermediate transfer member are not disposed with a proper spacing between adjacent fibers. 
     According to the present invention accomplished based on the above finding and further study, there is provided an intermediate transfer member comprising a base layer, and a core member embedded within the base layer and comprising fibers disposed with a spacing of 50-300 μm between adjacent fibers. 
     According to the present invention, there is further provided an electrophotographic apparatus, comprising: 
     an electrophotographic photosensitive member, 
     charging means for charging the electrophotographic photosensitive member, 
     imagewise exposure means for exposing imagewise the charged electrophotographic photosensitive member to form an electrostatic image, 
     developing means for developing the electrostatic image to form a toner image on the electrophotographic photosensitive member, and 
     an intermediate transfer member for temporarily receiving the toner image by transfer from the electrophotographic photosensitive member, wherein the intermediate transfer member comprising a base layer, and a core member embedded within the base layer and comprising fibers disposed with a spacing of 50-300 μm between adjacent fibers. 
     These and other objects, features and advantages of the present invention will become more apparent upon a 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 
     FIG. 1 is a perspective view for illustrating an embodiment of the intermediate transfer member according to the invention. 
     FIG. 2 is a perspective view for illustrating another embodiment of the intermediate transfer member according to the invention. 
     FIGS. 3 and 5 are partial internal oblique views each illustrating an example of core member embedded within an intermediate transfer member according to the invention. 
     FIG. 4 is a sectional view showing an example of disposition of a core member within an intermediate transfer member according to the invention. 
     FIG. 6 is a partial perspective view showing an embodiment of the intermediate transfer member according to the invention having a coating layer. 
     FIG. 7 is a partial side view for illustrating a relationship between an intermediate transfer member according to the invention and a roller. 
     FIG. 8 is a side view for illustrating an apparatus for measuring an electric resistance of an intermediate transfer member according to the invention. 
     FIGS. 9 and 10 are side views each illustrating an electrophotographic apparatus according to the invention. 
     FIG. 11 is a perspective view of another embodiment of the intermediate transfer member according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinbelow, the intermediate transfer member according to the present invention will be described with respect to some embodiments in the form of a belt but need not be restricted to such a belt form. 
     FIG. 1 is a perspective illustration of an embodiment of the intermediate transfer member according to the present invention. Referring to FIG. 1, the intermediate transfer member comprises a base layer  1  and a core member  2  embedded within the base layer  1 . The base layer  1  may comprise an elastic material, such as a rubber or elastomer, or resin. The core member  2  may comprise fibers in the form of a spiral, rings or woven cloth (or textile) as shown in FIG.  2 . In view of the easiness of production and cost, it is preferred to embed fibers in the form of a spiral or a woven cloth within the base layer  1 . 
     By embedding the core member  2  within the base layer  1 , it becomes possible to provide an intermediate transfer member free from permanent elongation and having excellent durability. 
     In the case of embedding fibers in the form of a spiral within the base layer  1 , adjacent fibers  2  within the base layer are disposed substantially parallel to each other as shown in FIG.  3 . 
     In the intermediate transfer member according to the present invention, the adjacent fibers  2  within the base layer  1  are disposed with a spacing of 50-3000 μm, preferably 100-2000 μm, further preferably 200-1800 μm. In case where the fiber spacing is below 50 μm, because of a substantial difference in resistivity between the fiber and the base layer, the electrical properties of the intermediate transfer member can be remarkably changed, so that good electrostatic transfer becomes difficult. On the other hand, if the fiber spacing is larger than 3000 μm, the resultant intermediate transfer belt is liable to be accompanied with noticeable surface waving, so that the resultant image is liable to be accompanied with an image density irregularity (core trace image) attributable to a surface unevenness on the intermediate transfer member. 
     Also in the case of using a core member  1  in the form of a woven cloth as shown in FIG. 5, the fiber spacing should be 50-3000 μm, preferably 100-2000 μm, further preferably 200-1800 μm, both longitudinally and laterally. 
     Also in the case of using a core member in the form of parallelly arranged fiber rings, the fiber spacing should be 500-3000 μm, preferably 100-2000 μm, further preferably 200-1800 μm. 
     Herein, the fiber spacing between adjacent fibers is based on a value measured as an arithmetic mean of spacings I 1 , I 2 , I 3 , I 4  and I 5  between arbitrarily selected adjacent 6 fibers (i.e., 5 spacings) as shown in FIG.  4 . The 6 adjacent fibers should be selected in a region having the highest probability of carrying a toner image, i.e., in a region of ±5 cm from a center line C (i.e., totally 10 cm with the line C as a center line) shown in FIG.  1 . 
     The core member can be composed of a plurality of superposed layers of woven cloths. In such a case, the woven cloth disposed closest to the outer surface (i.e., the surface for carrying a toner image) should have a fiber spacing in the range of 50-3000 μm. 
     Examples of the elastic material constituting the base layer  1  may include: natural rubber, isoprene rubber, styrene-butadiene rubber, butyl rubber, ethylene-propylene rubber, ethylene-propylene-diene terpolymer (EPDM), chloroprene rubber, chlorosulfonated polyethylene, chlorinated polyethylene, acrylonitrile-butadiene rubber, urethane rubber, syndiotactic 1,2-polybutadiene, epichlorohydrin rubber, acrylic rubber, silicone rubber, fluorine rubber, polysulfide rubber, polynorbonene rubber, hydrogenated nitrile rubber, thermoplastic elastomers (such as those of the polystyrene type, polyolefin type, polyvinyl chloride type, polyurethane type, polyamide type, polyester type, and fluorine-containing resin type); styrene resins (i.e., homopolymers and copolymers of styrene or substituted styrene) inclusive of polystyrene, poly-α-methylstyrene, styrene-butadiene rubber, styrene-vinyl chloride copolymer, styrene-vinyl acetate-copolymer, styrene-maleic acid copolymer, styrene-acrylate copolymers (such as styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-butyl acrylate copolymer, styrene-octyl acrylate copolymer, and styrene-phenyl-acrylate copolymer), styrene-methacrylate copolymers (such as styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, and styrene-phenyl methacrylate copolymer), styrene-methyl α-chloroacrylate copolymer, and styrene-acrylonitrile-acrylate copolymers, methyl methacrylate resin, butyl methacrylate resin, ethyl acrylate resin, butyl acrylate resin, modified acrylic resins (such as silicone-modified acrylic resin, vinyl chloride resin-modified acrylic resin and acrylic urethane resin), vinyl chloride resin, styrene-vinyl acetate copolymer, vinyl chloride-vinyl acetate copolymer, rosin-modified maleic acid resin, phenolic resin, epoxy resin, polyester resin, polyester-polyurethane resin, polyethylene, polypropylene, polybutadiene, polyvinylidene chloride, ionomer resin, polyurethane resin, silicone resin, fluorine-containing resin, ketone resin, ethylene-ethyl acrylate copolymer, xylene resin, polyvinyl butyral resin, polyamide resin, and modified polyphenylene oxide resin. These elastic materials may be used singly or in mixture of two or more species. The above are, however, not exhaustive. 
     The base layer  1  may preferably have a hardness (JIS-A hardness) of 10-95 deg., more preferably 20-80 deg., further preferably 25-70 deg., so as to obviate transfer failure, particularly hollow dropout image (i.e., a phenomenon that a central portion except for a contour is not sufficiently transferred). Accordingly, the base layer  1  may preferably comprise a rubber or elastomer or a soft resin among those enumerated above. 
     It is possible to add an electroconductivity-imparting additive in order to adjust the resistivity of the base layer  1 . Examples of the conductivity-imparting agent may include: carbon black, powder of metal such as aluminum or nickel, metal oxide such as tin oxide or titanium oxide, and electroconductive polymers, such as quaternary ammonium salt-containing polymethyl methacrylate, polyvinylaniline, polyvinylpyrrole, polydiacetylene, polyethyleneimine, boron-containing polymers, and polypyrrole. These may be used singly or in combination of two or more species. These conductivity-imparting additives are not exhaustive. In the case of using a conductivity-imparting agent, it is preferred to add 5-40 wt. parts thereof per 100 wt. parts of the elastic material (rubber, elastomer or resin) of the base layer  1 . 
     The base layer  1  may preferably have a thickness of 0.3-2 mm. Too large a thickness of the base layer  1  may result in difficulty in smooth drive. Too small a thickness may fail to provide a sufficient mechanical strength. 
     Examples of the fiber constituting the core member  2  may include: natural fibers of cotton, silk, hemp, and wool; reproduced fibers, such as chitin fiber, alginic acid, and reproduced cellulose fiber; semisynthetic fiber such as acetate fiber; synthetic fiber, such as polyester fiber, nylon fiber, acrylic fiber, polyolefin fiber, polyvinyl alcohol fiber, polyvinyl chloride fiber, polyvinylidene chloride fiber, polyurethane fiber, polyalkyl paraoxybenzoate fiber, polyacetal fiber, aramide fiber, polyfluoroethylene fiber, and phenolic resin fiber; inorganic fiber, such as carbon fiber, glass fiber and boron fiber; and metal fibers, such as iron fiber, and copper fiber. These fibers may be used singly or in combination of two or more species. The fiber may preferably have a thickness in diameter of 2-50 μm, more preferably 20-200 μm, further preferably 50-180 μm. 
     Too thin fiber thickness results in a low-mechanical strength of the intermediate transfer member, thus being liable to leave a problem regarding reliable durability. To thick fiber thickness is liable to result in noticeable core trace image. 
     The fiber thickness referred to herein is based on a value measured in the following manner. 
     &lt;Fiber thickness measurement&gt; 
     (1) An intermediate transfer member (belt) is cut in a thicknesswise direction, and the resultant section is enlarged to an appropriate magnification by appropriate means such as a microscope (FIG.  4 ). 
     (2) An appropriate fiber is selected, and a measured diameter (in case of fiber having a circular section) or a diameter of a circle providing sectional area equal to that of the fiber (in case of fiber having a non-circular section). In case of a yarn or thread composed of a plurality of filaments, the outer contour of the yarn or thread is used to determine the thickness. 
     The fiber used in the present invention may be in the form of a mono-filament or a thread or yarn composed of a twist or twined plurality of fibers. The manner of twisting may be any, including single twist, bias twist or double(-layered) twist. The direction of twist, i.e., left or right, is not questioned. It is also possible to use a thread or yarn of plural fiber species in mixture. The fiber can be used after an appropriate electroconductivity-imparting treatment, as desired. 
     In the present invention, the fibers are disposed to provide a spacing not exceeding 3000 μm between adjacent fibers. In addition, it is preferred to set a thickness B between the upper edge of the fiber and the upper surface of the intermediate transfer member relative to the diameter A of the fiber as shown in FIG. 4 so as to satisfy B/A≧1, whereby the hardness irregularity and surface waving on the outer surface of the intermediate transfer member is further less noticeable. 
     Too large a B value is not desirable because it increases the rigidity of the intermediate transfer belt. Therefore, the value B may preferably be 2-1500 μm. 
     The values A, B and B/A are determined as arithmetic means of respective values measured with respect to arbitrarily selected five adjacent fibers. 
     The outer surface of the intermediate transfer member may preferably be provided with an increased releasability, e.g., by a surface treatment with bleaching powder or by providing a surface-coating layer  101  on the intermediate transfer member  1  as desired as shown in FIG.  6 . 
     The coating layer  101  may be composed as a similar material as that of the base layer  1  as described above but may preferably exhibit a contact angle with water of at least 80 deg. For this purpose, the coating layer  101  may preferably contain a releasability-enhancing additive, such as silicone resin fine powder. The coating layer  101  may preferably have a thickness sufficiently small so as not to impair the resilience of the base layer, more specifically 1-500 μm, further preferably 5-200 μm. The coating layer  101  can also contain an electroconductivity-imparting additive similarly as in the base layer, e.g., in a proportion of 5-40 wt. parts per 100 wt. parts of the elastic material constituting the coating layer  101 . 
     The core member  2  is preferably disposed at a depth within the base layer  1  so as to provide a spacing H from the inner surface (opposite to the outer surface for carrying a toner image) of the intermediate transfer member of at least 0.1 mm as shown in FIG. 6. A spacing H of below 0.1 mm is liable to cause severe fatigue of the outer surface layer, leading to cause cracks in the coating layer in some cases where such a coating layer is provided. 
     The reason for the severe fatigue of the outer surface layer of the intermediate transfer member in the case of the spacing H being smaller than 0.1 mm may be considered as follows. 
     As shown in FIG. 7, an appropriate portion of length L is taken along an intermediate transfer member  20 . When the portion arrives at the position of a roller  65 , an outer surface layer is elongated to a length L+β and an inner surface layer is shrunk to L−α (α, β: positive values), as the core member  1  is not substantially elongated or shrunk because of a larger tensile modulus than the base layer  1 . 
     Thus, the outer surface layer of the intermediate transfer member  20  repetitively causes elongation-shrinkage at each passing of the roller  65 , leading to severe fatigue of the outer surface layer of the intermediate transfer member. 
     Accordingly, in order to minimize the fatigue of the outer surface layer of the intermediate transfer member, it is preferred to minimize β, and this may be accomplished by increasing the spacing H between the core member  2  and the inner surface of the intermediate transfer member so as to have the core member  2  approach the outer surface. 
     The spacing H referred to herein is based on an arithmetic mean of measured values with respect to appropriately selected 5 adjacent fibers. 
     The intermediate transfer member according to the present invention may preferably be in the form of an endless belt or a tube which may preferably be seamless. 
     The intermediate transfer member according to the present invention may preferably have an electrical resistance across the thickness of 1×10 4  ohm to 1×10 11  ohm. Too high a resistance of the intermediate transfer member is liable to cause a lowering in transfer bias voltage within the intermediate transfer member, so that a first color toner image transferred already onto the intermediate transfer member is liable to return to the photosensitive member at the time of transfer of second or subsequent color toner image from the photosensitive member onto the intermediate transfer member, thus failing to provide a desired color image. On the other hand, too low a resistance of the intermediate transfer member results in a remarkable difference in resistance between a portion already carrying a transferred toner image and a portion not carrying a toner image, so that the effective transfer of second or subsequent color toner image can be obstructed to also fail in providing an objective color image. 
     The electrical resistance of the intermediate transfer member referred to herein is based on values measured in the following manner. 
     (1) An intermediate transfer member is wound under tension about a drive roller  200  and a driven metal roller  201 , and the intermediate transfer member is sandwiched between two metal rollers  202  and  203  connected to a DC power supply  204 , a resistor  205  having an appropriate resistance value R 205  and a potentiometer  206 , as shown in FIG.  8 . 
     (2) By operating the drive roller  200 , the intermediate transfer member is moved at a surface velocity of 120 mm/sec. 
     (3) A voltage of 1 kV is applied from DC power supply  204  to read a potential difference Vr between both terminals of the resistor  205  by the potential meter  206 . The measurement is made in an environment of a temperature of 23±5° C., and a moisture of 50±10% RH. 
     (4) A current I is calculated from the measured potential difference Vr as I=Vr/R 205 . 
     (5) The resistance of the intermediate transfer member is calculated as applied voltage (1 kV)/current I. 
     The intermediate transfer member according to the present invention need not be produced through a particularly limited process but may be produced in the following manner. First of all, an elastic material is wound about a metal mold. Then, a core member is wound about the elastic material layer, and the core member is covered with a tubular-shaped elastic material. An adhesive may preferably be applied onto the core member in advance. Finally, the superposed elastic material layers are subjected to vulcanization, and the outer surface of the vulcanized product is abraded, whereby an intermediate transfer member holding a core member within a base layer is formed. An optional coating layer may be further provided, e.g., by spray coating, dip coating or electrostatic coating. 
     An electrophotographic apparatus will now be described with reference to FIG.  9 . 
     The apparatus includes a rotating drum-type electrophotographic photosensitive member (hereinafter called “photosensitive drum”)  1  repetitively used as a first image-bearing member, which is driven in rotation in a counterclockwise direction indicated by an arrow at a prescribed peripheral speed (process speed). The photosensitive drum  1  may preferably be one having an outermost layer containing fine powder of polytetrafluoroethylene resin (PTFE), so as to provide a high transfer efficiency, which may be attributable to an improved toner releasability caused by a lowering in surface energy of the outermost layer by inclusion of the PTFE fine powder. 
     During the rotation, the photosensitive drum  1  is uniformly charged to a prescribed polarity and potential by a primary charger  2  and then exposed to imagewise light  3  supplied from an imagewise exposure means (not shown, e.g., an optical system including means for color separation of a color original image, a scanning exposure system including a laser scanner for emitting laser beam modulated corresponding to time-serial electrical digital pixel signals of image data) to form an electrostatic latent image corresponding to a first color component image (e.g., a yellow color component image) of an objective color image. 
     Then, the electrostatic latent image is developed with a yellow toner Y (first color toner) by a first developing device (yellow developing device  41 ). At this time, second to fourth developing devices (magenta developing device  42 , cyan developing device  43  and black developing device  44 ) are placed in an operation-off state and do not act on the photosensitive drum  1 , so that the yellow (first color) toner image thus formed on the photosensitive drum  1  is not affected by the second to fourth developing devices. 
     An intermediate transfer member  20  is supported about rollers  64 ,  65  and  66  and rotated in a clockwise direction at a peripheral speed equal to that of the photosensitive drum  1 . 
     As the yellow toner image formed and carried on the photosensitive drum  1  passes through a nip position between the photosensitive drum  1  and the intermediate transfer member  20 , the yellow toner image is transferred onto an outer surface of the intermediate transfer member  20  under the action of an electric field caused by a primary transfer bias voltage applied from a primary transfer roller  62  to the intermediate transfer member  20  (primary transfer). 
     The surface of the photosensitive drum  1  after the transfer of the yellow (first color) toner image onto the intermediate transfer member  20  is cleaned by a cleaning device  13 . 
     Thereafter, a magenta (second color) toner image, a cyan (third color) toner image and a black (fourth color) toner image are similarly formed on the photosensitive drum  1  and successively transferred in superposition onto the intermediate transfer member  20  to form a synthetic color toner image corresponding to an objective color image. 
     The transfer bias voltage for sequential transfer in superposition of the first to fourth color toner images from the photosensitive drum  1  onto the intermediate transfer member  20  is supplied in a polarity (+) opposite to that of the toner from a bias voltage supply  29 . The voltage may preferably be in the range of, e.g., +100 volts to +2 kvolts. 
     For secondary transfer of the synthetic color toner image formed on the intermediate transfer member  20  onto a transfer-receiving material P (second image-bearing member), such as recording paper, a secondary transfer roller  63  is supported on a shaft in parallel to the roller (secondary transfer opposing roller)  64  and so as to be contactable onto a lower (but outer) surface of the intermediate transfer member  20 . During the primary transfer steps for transferring the first to fourth color images from the photosensitive drum  1  onto the intermediate transfer member  20 , the secondary transfer roller  63  can be separated from the intermediate transfer member  20 . 
     For the secondary transfer, the secondary transfer roller  63  is abutted against the intermediate transfer member  20 , a transfer-receiving material P is supplied via paper supply rollers  11  and a guide  10  to a nip position between the intermediate transfer member  20  and the secondary transfer roller  63  at a prescribed time and, in synchronism therewith, a secondary transfer bias voltage is applied to the secondary transfer roller  63  from a power supply  28 . Under the action of the secondary transfer bias voltage, the synthetic color toner image on the intermediate transfer member  20  is transferred onto the transfer-receiving material (second image-bearing member) P (secondary transfer). The transfer-receiving material carrying the toner image is introduced into a fixing device to effect heat fixation of the toner image. 
     After completion of image transfer onto the transfer-receiving material P, a charging member  7  for cleaning connected to a bias voltage supply  26  is abutted to the intermediate transfer member  20  to apply a bias voltage of a polarity opposite to that of the photosensitive drum  1 , whereby a transfer residual toner (a portion of toner remaining on the intermediate transfer member  20  without being transferred onto the transfer-receiving material P) is imparted with a charge of the opposite polarity. Then, the charged transfer residual toner is electrostatically transferred back to the photosensitive drum  1  at a nip position or a proximity thereto, whereby the intermediate transfer member  20  is cleaned. 
     Generally, the cleaning of the intermediate transfer member  20  may be performed by any cleaning means, such as blade cleaning, fur brush cleaning, electrostatic cleaning or a combination of these. However, in order to provide a small-sized an inexpensive apparatus, it is preferred to adopt the cleaning scheme as described with reference to FIG. 9, wherein the intermediate transfer member is cleaned by electrostatically transferring the transfer residual toner back to the photosensitive drum  1 . 
     The charging member  7  for cleaning shown in FIG. 9 can assume a various form, such as a metal roller, an electroconductive elastic roller, an electroconductive fur brush, or an electroconductive blade. 
     In the image forming apparatus shown in FIG. 9, it is possible to adopt a cleaning scheme wherein a transfer residual toner on the intermediate transfer member  20  generated during a previous image forming step is transferred back to the photosensitive drum  1  simultaneously with the primary transfer of a toner image from the photosensitive drum  1  onto the intermediate transfer member  20  in a subsequent image forming step, which may be called a concurrent primary transfer-cleaning scheme”). The concurrent primary transfer-cleaning scheme is advantageous in that it is free from a lowering in throughput because of no additional cleaning step. 
     It is also possible to provide a transfer residual toner-recovering member  8  connected to a bias voltage supply as shown in FIG.  10 . 
     The residual toner-recovering member  8  can also assume a various form, such as a metal roller, an electroconductive elastic roller, an electroconductive fur brush, or an electroconductive blade. 
     The residual toner-recovering member  8  may be supplied with a voltage of a polarity opposite to that applied to the charging member  7  for cleaning, thereby electrostatically cleaning the transfer residual toner. 
     Further, in the apparatus shown in FIG. 10, it is possible to apply a bias voltage of a polarity opposite to that of the photosensitive drum  1  to the residual toner-recovering member  8 , thereby charging the transfer residual toner in a residual toner recovery vessel  9 , so that the charged residual toner can be recovered by the cleaning device  13  for the photosensitive drum. This scheme is advantageous in that the residual toner-receiving vessel  9  can be reduced in size. 
     In the above, the intermediate transfer member according to the present invention has been described principally with reference to belt-shaped embodiments, but can also be a drum-shaped one as shown in FIG.  11 . The intermediate transfer member shown in FIG. 11 comprises a cylindrical metal support  100  and a base layer  1  formed thereon including a core member (not specifically shown) embedded within the base layer  1 . The base layer  1  can be covered with an optional coating layer  101  as described. 
     Hereinbelow, the present invention will be described more specifically based on Examples, wherein part(s) means part(s) by weight. 
     EXAMPLE 1 
     A rubber compound having a composition as shown below was wound about a cylindrical metal mold uniformly in a thickness of 0.4 mm. Then, a polyester twisted yarn (diameter=100 μm) coated with an adhesive was spirally wound about the rubber compound layer at a spacing of 0.05 mm (=50 μm) between adjacent yarns, and then further covered with a rubber compound of the composition shown below extruded in advance into a tube, followed by vulcanization and grinding to form a 0.8 mm-thick rubber belt of 250 mm in width and 435 mm in outer peripheral length reinforced with the polyester yarn as the core member. 
     
       
         
           
               
             
               
                   
               
               
                 (Rubber compound) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 SBR rubber 
                 30 
                 part(s) 
               
               
                   
                 EPDM rubber 
                 70 
                 part(s) 
               
               
                   
                 Precipitated sulfur (vulcanizer) 
                 1.5 
                 part(s) 
               
               
                   
                 Zinc white (vulcanization aid) 
                 2 
                 part(s) 
               
               
                   
                 MBT (mercaptobenzothiazole) 
                 1.5 
                 part(s) 
               
               
                   
                 (vulcanization promoter) 
               
               
                   
                 TMTM (tetramethylthiuram monosulfide) 
                 1.2 
                 part(s) 
               
               
                   
                 (vulcanization promoter) 
               
               
                   
                 Carbon black (conductivity-imparting agent) 
                 25 
                 part(s) 
               
               
                   
                 Stearic acid (dispersion aid) 
                 1 
                 part(s) 
               
               
                   
                 Naphthenic process oil (plasticizer) 
                 40 
                 part(s) 
               
               
                   
                   
               
            
           
         
       
     
     The resultant belt was surface-treated with an aqueous solution containing bleaching powder to improve the surface releasability, thereby obtaining an intermediate transfer belt. 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and subjected to evaluation of color deviation, core trace image and transfer performance. 
     The cleaning of the intermediate transfer belt was performed by the concurrent primary transfer-cleaning scheme using an elastic roller having a resistance of 1×10 8  ohm as the cleaning member. The evaluation was performed by continuous printing of a full-color image on 1×10 4  sheets while supplying a current of +40 μA to the cleaning member  7  from the bias voltage supply  26 . The other image forming conditions were as follows. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Surface potential at non-image part: 
                 −550 
                 volts 
               
               
                   
                 Surface potential at image part: 
                 −150 
                 volts 
               
            
           
           
               
               
               
            
               
                   
                 Color developers (for all four colors): 
                 non-magnetic 
               
               
                   
                   
                 monocomponent 
               
               
                   
                   
                 toner 
               
            
           
           
               
               
               
               
            
               
                   
                 Primary transfer voltage: 
                 +500 
                 volts 
               
               
                   
                 Secondary transfer voltage: 
                 +1500 
                 volts 
               
               
                   
                 Process speed: 
                 120 
                 mm/sec 
               
               
                   
                 Developing bias voltage: 
                 Vdc = −400 
                 volts 
               
               
                   
                   
                 Vac = 1600 
                 Vpp 
               
               
                   
                   
                 (f = 1800 
                 Hz) 
               
               
                   
                   
               
            
           
         
       
     
     As a result of evaluation, good electrostatic transfer was possible from the initial stage, and the resultant images were free from core trace image. It was possible to obtain good images free from color deviation attributable to permanent elongation of the belt. The results are inclusively shown in Table 2 appearing hereinafter together with those of other Examples. 
     EXAMPLES 2-6 
     Intermediate transfer belts were prepared in the same manner as in Example 1 except that the spacing between adjacent polyester yarns was changed as shown the following Table 1. The resultant intermediate transfer belts were evaluated in the same manner as in Example 1. The results are inclusively shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                   
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
                 Ex. 6 
               
               
                   
                   
               
               
                   
                 Spacing (mm) 
                 0.1 
                 0.5 
                 1.0 
                 2.0 
                 3.0 
               
               
                   
                   
               
            
           
         
       
     
     EXAMPLE 7 
     A solution of a rubber compound having a composition as shown below was applied onto a cylindrical metal mold to form a 0.07 μm-thick unvulcanized rubber layer. Then, a cotton yarn (diameter: 100 μm) coated with an adhesive was spirally wound about the rubber compound layer at a spacing of 0.70 mm (700 μm) between adjacent layers, and then further covered with a rubber compound of the composition shown below extruded in advance into a tube, followed by vulcanization and polishing to form a totally 1.2 mm-thick rubber belt of 250 mm in width and 435 mm in outer peripheral length reinforced with the cotton yarn as the core member. 
     
       
         
           
               
             
               
                   
               
               
                 (Rubber compound) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 NBR rubber 
                 60 
                 part(s) 
               
               
                   
                 EPDM rubber 
                 40 
                 part(s) 
               
               
                   
                 Precipitated sulfur (vulcanizer) 
                 1.5 
                 part(s) 
               
               
                   
                 Zinc white (vulcanization aid) 
                 2 
                 part(s) 
               
               
                   
                 MBT (vulcanization promoter) 
                 1.5 
                 part(s) 
               
               
                   
                 TMTM (vulcanization promoter) 
                 1.5 
                 part(s) 
               
               
                   
                 Carbon black (conductivity-imparting agent) 
                 25 
                 part(s) 
               
               
                   
                 Stearic acid (dispersion aid) 
                 1.2 
                 part(s) 
               
               
                   
                 Naphthenic process oil (plasticizer) 
                 20 
                 part(s) 
               
               
                   
                   
               
            
           
         
       
     
     Then, the rubber belt was further coated with a surface layer paint of the following composition. 
     
       
         
           
               
             
               
                   
               
               
                 (Surface layer paint) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Polyurethane prepolymer 
                 100 parts 
               
               
                   
                 Isocyanate (hardener) 
                  4 parts 
               
               
                   
                 PTFE resin powder 
                  70 parts 
               
               
                   
                 (average primary particle size of 0.3 μm) 
               
               
                   
                 Methyl ethyl ketone 
                 400 parts 
               
               
                   
                 N-methylpyrrolidone 
                  50 parts 
               
               
                   
                   
               
            
           
         
       
     
     The above paint was sprayed onto the belt, dried to finger-felt dryness and then heated at 120° C. for two hours for complete drying and crosslinking of the paint to form an intermediate transfer belt having a 30 μm-thick tough surface coating layer (outermost layer). 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and subjected to evaluation of color deviation, core trace image and transfer performance in the same manner as in Example 1 except that an elastic layer having a resistance of 1×10 7  ohm was used as the cleaning member  7  and the continuous printing for evaluation was performed on 1.5×10 4  sheets so as to observe the fatigue of the surface coating layer. 
     As a result, good electrostatic transfer was effected from the initial stage and without any problem until 1×10 4  sheets. However, after 1.5×10 4  sheets of continuous image formation, no color deviation was observed. However, linear transfer failure portions were observed at some parts in a solid black image region due to some crack (observed later) in the surface coating layer. Such transfer failure was not noticeable in image regions other than those of solid black image. Thus, the transfer failure was at a practically acceptable level as a whole. The results are shown in Table 2 together with those of other Examples. 
     EXAMPLE 8 
     An intermediate transfer belt was prepared in the same manner as in Example 7 except that first an unvulcanized rubber layer was formed in a thickness of 0.1 mm on a cylindrical metal mold. 
     The resultant intermediate transfer belt was evaluated in the same manner as in Example 7. As a result, good electrostatic transfer was performed to provide good images free from core trace image. Even after continuous printing of full color images on 1.5×10 4  sheets, good images could be obtained without causing color deviation or occurrence of cracks in the coating layer. The results are also shown in Table 2. 
     EXAMPLE 9 
     A rubber compound having the same composition as in Example 7 was extruded into a 0.3 μm-thick tube, which was disposed to cover a cylindrical metal mold. Then, a cotton yarn (diameter: 100 μm) coated with an adhesive was spirally wound about the rubber compound layer at a spacing of 0.70 mm (700 μm) between adjacent layers, and then further covered with a rubber compound of the same composition extruded in advance into a tube, followed by vulcanization and grinding to form a totally 1.2 mm-thick rubber belt reinforced with the cotton yarn as the core member. 
     Then, a surface layer pain of the same composition as that used in Example 7 was sprayed onto the rubber belt, dried to finger-felt dryness and then heated at 120° C. for 2 hours for complete drying and crosslinking of the paint to obtain an intermediate transfer belt having a 30 μm-thick coating layer. 
     The resultant intermediate transfer belt was evaluated in the same manner as in Example 7. As a result, good electrostatic transfer was performed to provide good images free from core trace image. Even after continuous printing of full color images on 1.5×10 4  sheets, good images could be obtained without causing color deviation or occurrence of cracks in the coating layer. The results are also shown in Table 2. 
     EXAMPLE 10 
     An intermediate transfer belt was prepared in the same manner as in Example except that a rubber compound tube of 1.0 mm in thickness was first disposed to cover a metal mold, and the resultant intermediate transfer belt was evaluated in the same manner as in Example 7. 
     As a result, good electrostatic transfer was performed to provide good images free from core trace image. Even after continuous printing of full color images on 1.5×10 4  sheets, good images could be obtained without causing color deviation or occurrence of cracks in the coating layer. The results are also shown in Table 2. 
     EXAMPLE 11 
     A cylindrical metal mold was covered with a preliminarily extruded 0.6 mm-thick tube of a rubber compound having the same composition as in Example 7. Then, a polyester yarn (diameter=200 μm) was spirally wound about the rubber compound layer at a spacing of 1.0 mm between adjacent yarns, and then further covered with a preliminarily extruded tube of the same rubber compound, followed by vulcanization and polishing to form a 0.9 mm-thick reinforced rubber belt. 
     Then, the same surface layer paint as used in Example 7 was sprayed onto the rubber belt, dried to finger-felt dryness and then heated to 120° C. for 2 hours for complete drying and curing of the paint, to obtain an intermediate transfer belt having a 40 μm-thick coating layer. 
     The transfer belt exhibited a thickness B above the core member of 140 μm and thus a ratio B/A (core member thickness =200 μm) of 0.7. 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, good electrostatic transfer was performed to provide good image free from color deviation from the initial stage, but slight core trace image within a practically acceptable extent was observed due to a B/A ratio below 1. 
     The results are also shown in Table 2. 
     EXAMPLE 12 
     A cylindrical metal mold was covered with a preliminarily extruded 0.6 mm-thick tube of a rubber compound having the same composition as in Example 7. Then, a polyester twisted yarn (diameter =140 μm) was spirally wound about the rubber compound layer at a spacing of 1.0 mm between adjacent yarns, and then further covered with a preliminarily extruded tube of the same rubber compound, followed by vulcanization and grinding to form a 0.8 mm-thick reinforced rubber belt. 
     Then, a 40 μm-thick coating layer was formed in the same manner as in Example 1 to obtain an intermediate transfer belt. 
     The transfer belt exhibited a thickness B above the core member of 140 μm and thus a ratio B/A (core member thickness=100 μm) of 1.0. 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, good electrostatic transfer was performed to provide good image free from color deviation from the initial stage, and also free from core trace image. 
     EXAMPLE 13 
     A cylindrical metal mold was covered with a preliminarily extruded 0.6 mm-thick tube of a rubber compound having the same composition as in Example 7. Then, a polyester twisted yarn (diameter=100 μm) was spirally wound about the rubber compound layer at a spacing of 1.0 mm between adjacent yarns, and then further covered with a preliminarily extruded tube of the same rubber compound, followed by vulcanization and polishing to form a 1.0 mm-thick reinforced rubber belt. 
     Then, a 40 μm-thick coating layer was formed in the same manner as in Example 1 to obtain an intermediate transfer belt. 
     The transfer belt exhibited a thickness B above the core member of 340 μm and thus a ratio B/A (core member thickness=100 μm) of 3.4. 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, good electrostatic transfer was performed to provide good image free from color deviation from the initial stage, and also free from core trace image. 
     EXAMPLE 14 
     A cylindrical metal mold was covered with a preliminarily extruded 0.4 mm-thick tube of a rubber compound having the same composition as in Example 7. Then, a polyester yarn (diameter=100 μm) was spirally wound about the rubber compound layer at a spacing of 1.0 mm between adjacent yarns, and then further covered with a preliminarily extruded tube of the same rubber compound, followed by vulcanization and grinding to form a 1.2 mm-thick reinforced rubber belt. 
     Then, a 40 μm-thick coating layer was formed in the same manner as in Example 1 to obtain an intermediate transfer belt. 
     The transfer belt exhibited a thickness B above the core member of 740 μm and thus a ratio B/A (core member thickness=100 μm) of 7.4. 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, good electrostatic transfer was performed to provide good image free from color deviation from the initial stage, and also free from core trace image. 
     EXAMPLE 15 
     A cylindrical metal mold was covered with a preliminarily extruded 0.4 mm-thick tube of a rubber compound having the same composition as in Example 7. Then, a polyester twisted yarn (diameter=100 μm) was spirally wound about the rubber compound layer at a spacing of 1.0 mm between adjacent yarns, and then further covered with a preliminarily extruded tube of the same rubber compound, followed by vulcanization and polishing to form a 2.0 mm-thick reinforced rubber belt. 
     Then, a 40 μm-thick coating layer was formed in the same manner as in Example 1 to obtain an intermediate transfer belt. 
     The transfer belt exhibited a thickness B above the core member of 1540 μm and thus a ratio B/A (core member thickness=100 μm) of 15.4. 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, good electrostatic transfer was performed to provide good image free from color deviation from the initial stage, and also free from core trace image. 
     Comparative Example 1 
     An intermediate transfer belt was prepared in the same manner as in Example 1 except that the polyester yarn was spirally wound at a spacing of 0.045 mm (45 μm). 
     The resultant intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, the resultant images were free color deviation, but good electrostatic transferred could not be effected because the core fiber layer functioned like an insulating layer because of a small spacing of 0.045 mm between adjacent yarns. 
     The results are also shown in Table 1. 
     Comparative Example 2 
     An intermediate transfer belt was prepared in the same manner as in Example 1 except that the polyester yarn was spirally wound at a spacing of 3.5 mm (3500 μm). 
     The resultant intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, the resultant images were free color deviation, but accompanied with core trace image because of a layer spacing of 3.5 mm between adjacent yarns. 
     Comparative Example 3 
     A cylindrical metal mold was covered with a preliminarily extruded tube of a rubber compound having the same composition as in Example 1, followed by vulcanization and grinding to form 1.0 mm-thick rubber belt. 
     Then, the rubber belt was surface-treated with an aqueous solution containing bleaching powder to improve the surface releasability to obtain an intermediate transfer belt. 
     The intermediate transfer belt was incorporated in a full-color electrophotographic apparatus as shown in FIG.  9  and evaluated in the same manner as in Example 1. 
     As a result, the resultant images were free from core trace images (as a matter of course because the transfer belt contained no core member) but were accompanied with noticeable color deviation, thus failing to provide practically acceptable full color image. 
     The results of the above Examples and Comparative Examples are inclusively shown in the following Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Color 
                 Core trace 
                 Transfer 
                 Cracks after 
               
               
                   
                 deviation 
                 image 
                 failure 
                 1.5 × 10 4  sheets 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Ex. 1 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 2 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 3 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 4 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 5 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 6 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 7 
                 A 
                 A 
                 A 
                 B3 
               
               
                 Ex. 8 
                 A 
                 A 
                 A 
                 A 
               
               
                 Ex. 9 
                 A 
                 A 
                 A 
                 A 
               
               
                 Ex. 10 
                 A 
                 A 
                 A 
                 A 
               
               
                 Ex. 11 
                 A 
                 B2 
                 A 
                 — 
               
               
                 Ex. 12 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 13 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 14 
                 A 
                 A 
                 A 
                 — 
               
               
                 Ex. 15 
                 A 
                 A 
                 A 
                 — 
               
               
                 Comp. Ex. 1 
                 A 
                 A 
                 C3 
                 — 
               
               
                 Comp. Ex. 2 
                 A 
                 C2 
                 A 
                 — 
               
               
                 Comp. Ex. 3 
                 C1 
                 A 
                 A 
                 — 
               
               
                   
               
               
                 The evaluation in the above table was performed according to the following standards:  
               
               
                 A: Not occurred.  
               
               
                 B (B2, B3): Occurred but at a practically acceptable level.  
               
               
                 B2: Core trace image was not noticeable at a glance but noticeable by careful observation.  
               
               
                 B3: Some linear transfer portions attributable to cracks in the surface coating layer were observed in a solid black image region at a practically acceptable level.  
               
               
                 C (C1, C2, C3): Occurred at a practically unacceptable level.  
               
               
                 C1: Deviation of respective colors occurred at a level of 300 μm or larger.  
               
               
                 C2: Core trace image was clearly observed.  
               
               
                 C3: Objective colors could not be obtained due to poor transfer efficiency of the respective colors.