Transfer belt for electrophotographic apparatus and method of manufacturing the same

Disclosed is a transfer belt for an electro-photographic apparatus, comprising a seamless reinforcing layer made of a continuous string, and a semi-conductive rubber layer formed on the reinforcing layer.

BACKGROUND OF THE INVENTION
 The present invention relates to a transfer belt for an electrophotographic
 apparatus and a method of manufacturing the same, particularly, to a
 transfer belt used in place of a transfer roll or a transfer drum in an
 electrophotographic apparatus, particularly, a plain paper copying machine
 (xerography) or a color printer, and to a method of manufacturing the
 same.
 In a transfer system of toner from a photo-conductive drum included in a
 copying machine, a color printer, a facsimile, etc. using an
 electrophotographic system to a paper sheet, it was customary to use a
 transfer roll made of a sponge rubber having a semi-conductivity. In
 preparing the conventional transfer roll, an electrically conductive
 powder is added to an ethylene-propylene rubber to impart a
 semi-conductivity having an electrical resistivity of 10.sup.8 to 10.sup.9
 .OMEGA..multidot.cm to the rubber, followed by forming the rubber into a
 sponge roll having an Asker C type hardness of about 30.degree. (Japan
 Hardcopy '91, p27).
 In a transfer system using the transfer roll, the roll has such a small
 diameter as about 15 mm. Therefore, it is impossible to secure a wide nip
 width with the photoconductive drum, resulting in failure to achieve a
 high copying speed. In a color copying machine or a color printer, the
 copying speed is only about 10 sheets/minute.
 It is conceivable to widen the nip width by increasing the diameter of the
 transfer roll. In this case, it is certainly possible to increase the
 copying speed. However, the apparatus itself is rendered bulky, making it
 impractical to increase the diameter of the transfer roll. Under the
 circumstances, it is proposed to widen the nip width by using a transfer
 belt in place of the transfer roll ("Electrography" by The Society of
 Electrophotography of Japan 30(3), 54(1991)).
 FIG. 8 shows the concept of a copying machine. As shown in the drawing, a
 transfer roll 2, a developing roll 3 and a charging roll 4 for charging a
 photo-sensitive drum 1 are arranged in the vicinity of the photoconductive
 drum 1. Also, a fuser roller 5 is arranged downstream of the
 photoconductive drum 1. In the copying machine of the construction shown
 in FIG. 8, a toner image is formed on the photoconductive drum 1 by the
 developing roll 3 and, then, transferred onto a paper sheet 6 by the
 transfer roll 2. Further, the toner image transferred onto the paper sheet
 6 is heated by the fuser roller 5 so as to be melted and fixed.
 FIG. 9 shows the concept exemplifying a transfer belt system. The members
 of the system common with FIGS. 8 and 9 are denoted by the same reference
 numerals so as to omit the explanation thereof in the following
 description. As shown in FIG. 9, a corona transfer device 7 and a charging
 device 8 are arranged close to and slightly apart from the photoconductive
 drum 1. A transfer belt 10, which is driven by driving rolls 9a, 9b and
 supported by a supporting roll 9c, is stretched so as to be transferred
 between the photo-conductive drum 1 and the corona transfer device 7. As
 apparent from FIG. 9, the nip width between the photoconductive drum 1 and
 the transfer belt 10 can be widened.
 The transfer roll and the transfer belt are required to exhibit a stable
 electrical resistivity, i.e., a semi-conductivity of 10.sup.7 to 10.sup.13
 .OMEGA..multidot.cm. Particularly, these roll and belt are required to be
 low in fluctuation of the electrical resistivity under various
 environments. To be more specific, the transfer roll and the transfer belt
 are required to exhibit less than an exponent of fluctuation in the
 resistivity under various conditions such as a high temperature-high
 relative humidity RH of 30.degree. C. and 80%, a low temperature-low
 relative humidity RH of 23.degree. C. and 5%, or an ordinary
 temperature-ordinary relative humidity RH of 23.degree. C. and 55%. The
 reasons for the requirement of the stable resistivity are that, if the
 resistivity is unduly low, the voltage is lowered, resulting in failure
 for the roll or belt to be charged, and that, if the resistivity is unduly
 high, current is unlikely to flow through the roll or belt, leading to a
 low charging potential.
 What should also be noted is that ozone is generated within the copying
 machine or the printer. Naturally, it is important for the transfer roll
 or the transfer belt to exhibit a high resistance to ozone and a
 reasonable heat resistance and not to contaminate the photoconductive
 drum. In addition to these requirements, the transfer belt is required not
 to run zigzag during operation of the copying machine, to be low in
 elongation relative to a predetermined tension, to exhibit an excellent
 dimensional stability, and not to bear a permanent set over a long period
 of time. In general, the transfer belt is said to receive a tension of 2
 to 3 Kgf/cm and to exhibit an elongation of about 5%. In other words, the
 belt is elongated by about 5% during operation of the copying machine.
 Further, the belt surface is required to readily release the toner and to
 have a small friction coefficient.
 A transfer belt prepared by extruding a polyurethane rubber in a
 cylindrical form, followed by vulcanizing the extrudate and subsequently
 polishing the surface of the extrudate and, then, coating the surface of
 the extrudate with a fluorine resin is disclosed in "Electrography" by The
 society of Electrophotography of Japan 33(1), 43(1994)".
 However, a reinforcing layer is not incorporated in the polyurethane rubber
 transfer belt noted above, with the result that relaxation of stress takes
 place if tensity of the transfer belt is continued. In this case, a
 permanent set is generated so as to give rise to loosening of the belt. It
 follows that the transfer belt fails to be rotated as desired so as to
 distort the toner image. In other words, the transfer belt fails to
 perform its function.
 An additional defect inherent in the polyurethane rubber transfer belt is
 that the rubber tends to be markedly affected by the environment,
 particularly, by the humidity. In some cases, depolymerization of the
 polyurethane rubber is brought about by the humidity, with the result that
 the hardness of the transfer belt is markedly lowered. In addition, the
 resistivity of the transfer belt under an environment of a low humidity is
 more than 10 times as high as that under an environment of a high
 humidity, which is a decisive defect. Needless to say, the resistivity of
 the polyurethane rubber is markedly lowered with increase in humidity of
 the environment.
 Further, the polyurethane rubber is poor in resistance to ozone and heat
 generated within the copying machine or the printer.
 A method of manufacturing an electrically conductive rubber belt is also
 disclosed in Japanese Patent Disclosure (Kokai) No. 4-99446. It is
 disclosed that a roll coater is arranged on a rotating shaft, and a tank
 housing a solid rubber paste mixed with a conductivity-imparting agent is
 moved from one side so as to coat the shaft with the solid rubber paste
 and to crosslink the rubber. The method disclosed in this prior art is
 directed to formation of a solid rubber single layer and is intended to
 prevent non-uniformity of the electrical resistivity over the entire
 rubber layer which may be derived from the heat history in the forming or
 vulcanizing step and from difference in pressure applied to the rubber
 layer.
 BRIEF SUMMARY OF THE INVENTION
 An object of the present invention is to provide a transfer belt for an
 electrophotographic apparatus, comprising a seamless reinforcing layer
 formed of a continues string and a semi-conductive rubber layer formed on
 the reinforcing layer, and exhibiting a dimensional stability over a long
 period of time, a stability in electrical resistivity under various
 environments, a high resistance to ozone and an excellent heat resistance.
 Another object is to provide a transfer belt for an electrophotographic
 apparatus, comprising a semi-conductive rubber layer and a sponge-like
 compressible layer having a semi-conductivity, which is formed on the
 rubber layer, so as not to do damage to a photoconductive drum and to
 permit widening the nipping width.
 Still another object of the present invention is to provide a method of
 manufacturing a transfer belt for an electrophotographic apparatus, the
 transfer belt exhibiting a dimensional stability over a long period of
 time, a stability in electrical resistivity under various environments, a
 high resistance to ozone and an excellent heat resistance, and the method
 comprising the steps of coating a mandrel with a semi-conductive rubber
 paste, continuously winding a string about the mandrel from one end of the
 mandrel while rotating the mandrel so as to form a seamless reinforcing
 layer, forming a semi-conductive rubber layer on the reinforcing layer,
 and withdrawing the structure consisting of the reinforcing layer and the
 semi-conductive rubber layer from the mandrel.
 According to an aspect of the present invention, there is provided a
 transfer belt for an electro-photographic apparatus, comprising a seamless
 reinforcing layer formed of a continuous string, and a semi-conductive
 rubber layer formed on the reinforcing layer.
 According to another aspect of the present invention, there is provided a
 method of manufacturing a transfer belt for an electrophotographic
 apparatus, comprising the step of coating a mandrel with a semi-conductive
 rubber paste, the step of continuously winding a string about the mandrel
 from one end of the mandrel while rotating the mandrel so as to form a
 seamless reinforcing layer, the step of forming a semi-conductive rubber
 layer on the reinforcing layer, and the step of withdrawing the structure
 consisting of the reinforcing layer and the semi-conductive rubber layer
 from the mandrel.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 Let us describe the present invention more in detail with reference to the
 accompanying drawings.
 The transfer belt of the present invention comprises a seamless reinforcing
 layer. The seamless reinforcing layer is of a single layer structure,
 which is prepared by continuously winding a continuous string 11 about a
 mandrel 12 from one end portion of the mandrel 12, as shown in FIG. 5B.
 Alternatively, the seamless reinforcing layer is of a double layer
 structure, which is prepared by continuously winding continuous string 11
 about the mandrel 12 from one end portion of the mandrel 12, as shown in
 FIG. 5A. The transfer belt comprising the reinforcing layer is
 substantially prevented from being transferred zigzag.
 The string is wound about a mandrel by using an apparatus comprising a
 rotary shaft having the mandrel mounted thereto and a transfer mechanism
 for moving a truncated cone-shaped bobbin wound with a large number of
 turns of a string. In each of FIGS. 5A and 5B, a clearance is provided
 between adjacent turns of the string 11 to set forth clearly the
 construction of the reinforcing layer formed of the string. However, it is
 practically desirable to eliminate the clearance substantially completely.
 The reinforcing layer is intended to prevent the transfer belt from
 bearing an elongation set.
 The distance between adjacent turns of the string constituting the
 reinforcing layer is determined by controlling the feeding rate of the
 string. To be more specific, the distance noted above can be decreased to
 increase the string density about the mandrel by decreasing the string
 feeding rate and can be increased by increasing the string feeding rate.
 It is desirable to control the string feeding rate to permit the adjacent
 turns of the string to contact lightly with each other. If a large
 clearance is provided between adjacent turns of the string, each turn of
 the string is made visible, which is not desirable.
 For preparing the string constituting the reinforcing layer, it is possible
 to use various materials including natural fibers such as cotton, hemp,
 silk and rayon; synthetic fibers such as polyester, nylon, polyamide,
 polyimide and aramid; monofilament yarn or multifilament yarn of metallic
 inorganic fibers such as steel carbon and ceramic materials; two ply yarn
 and fibers spun from a mixture thereof. The thickness of the string, which
 is dependent on the thickness of the transfer belt, should desirably be
 0.1 to 0.5 mm because the thickness of the transfer belt is generally
 about 0.5 to 1 mm.
 In order to impart a semi-conductivity to the reinforcing layer formed of a
 string, the reinforcing layer is coated with a rubber paste or an adhesive
 having a semi-conductivity so as to permit the reinforcing layer to be
 impregnated with the particular paste or adhesive. Alternatively, the
 string used for forming the reinforcing layer is impregnated in advance
 with a rubber paste or adhesive having a semi-conductivity. Since a
 semi-conductivity is imparted to the reinforcing layer, a uniform
 conductivity can be guaranteed.
 In order to prevent a permanent set of the transfer belt, it is conceivable
 to line the inner surface of the belt with a film, a woven fabric or an
 unwoven fabric, followed by forming a rubber layer on the lining. It
 should be noted, however, that it is unavoidable to join the end portions
 of the reinforcing layer consisting of the lining and the rubber layer
 with, for example, an adhesive or a string so as to make the reinforcing
 layer endless. As a result, the transfer belt is caused to include a
 stepped portion formed in the step of joining the end portions of the
 reinforcing layer, making it impossible to transfer toner images
 accurately on a paper sheet when the transfer belt is used in, for
 example, a copying machine. Also, the film used as a lining material tends
 to be split and is low in elasticity, with the result that the transfer
 belt lined with the film lacks a restoring force and, thus, tends to run
 zigzag.
 As another method of preventing a permanent set, it is conceivable to
 employ a circular knitting method using a circular knitting material of an
 inlay knitting structure for forming a reinforcing layer, followed by
 forming a rubber layer on the upper surface of the reinforcing layer. This
 method is certainly effective in that use of the circular knitting
 material permits preparing a seamless reinforcing layer. However, it is
 difficult to form a rubber layer on the surface of the reinforcing layer
 and to control strictly the dimension of the transfer belt, particularly,
 the inner diameter of the belt. It is also necessary to prepare an
 apparatus for thickening or thinning the inner diameter of the belt,
 leading to a serious economic burden.
 In the present invention, the reinforcing layer is coated with a
 semi-conductive rubber layer as shown in, for example, FIG. 6. In this
 method, a reinforcing layer (not shown) formed on the surface of the
 mandrel 12, which is kept rotated, is coated with a rubber paste 13
 prepared by dissolving rubber in a solvent by using a doctor blade 14,
 followed by heating and vulcanizing the resultant rubber layer within a
 heating can or oven to coat the reinforcing layer with the rubber layer.
 In the case of using a sponge rubber, the rubber is foamed in the
 vulcanizing step.
 Alternatively, it is possible to laminate a rubber sheet on a reinforcing
 layer formed on the mandrel. It is also possible to prepare a cylindrical
 hose by using an extruder, followed by covering the reinforcing layer
 formed on the mandrel with the resultant hose and subsequently heating and
 vulcanizing the hose. The semi-conductive rubber paste used in the present
 invention includes a rubber paste mixed with a semi-conductivity imparting
 agent, a rubber paste containing a large number of micro-balloons having a
 semi-conductivity, and a rubber paste containing a foaming agent
 exhibiting a semi-conductivity.
 The mandrel used in the present invention includes a mandrel 16 having a
 smooth surface as shown in, for example, FIG. 4A and a mandrel 18 having a
 plurality of axial grooves 17 formed along the circumferential surface as
 shown in, for example, FIG. 4B. In the case of using the mandrel 18 shown
 in FIG. 4B, grooves are formed on the inner surface of the transfer belt
 because axial grooves 17 are formed on the circumferential surface of the
 mandrel 18. It follows that, if teeth are formed on the outer
 circumferential surface of one of the driving rolls 9a and 9b included in
 the transfer belt system shown in FIG. 9 so as to permit these teeth to be
 engaged with the grooves formed on the inner surface of the transfer belt,
 the transfer belt is prevented from running zigzag without fail.
 The semi-conductive rubber layer used in the present invention includes a
 semi-conductive elastomer (solid rubber) layer and a semi-conductive
 sponge-like compressible layer. The material of the semi-conductive
 elastomer layer includes, for example, chloroprene rubber, epichlorohydrin
 rubber, ethylene-propylene rubber, silicone rubber, fluororubber, nitrile
 rubber, acrylic rubber, urethane rubber, styrene-butylene rubber and butyl
 rubber. On the other hand, the material of the compressible layer includes
 a large number of micro-balloons made of a thermoplastic resin. In this
 case, the wall of the micro-balloon is formed of a copolymer of vinylidene
 chloride, acrylonitrile, acrylic acid ester or a methacrylic acid ester.
 Alternatively, an organic or inorganic blowing agent can be added to the
 semiconductive elastomer. In this case, the semiconductive elastomer is
 heated to cause the foaming agent to generate a gas, thereby to prepare a
 foamed compressible layer. It is desirable to use a sponge-like
 compressible layer because no damage is done to the photoconductive drum
 and it is possible to secure a large nip width.
 In the present invention, it is desirable to form an electrical resistance
 control layer on the semi-conductive rubber layer, as required. The
 electrical resistance control layer can be formed by coating the
 semi-conductive rubber layer with a solution prepared by dissolving, for
 example, N-methoxy-methylated nylon in a mixed solvent consisting of
 toluene and methanol. The electrical resistance control layer thus formed
 has a smooth surface and, at the same time, prevents the photoconductive
 drum from being contaminated so as to stabilize the electrical resistance.
 FIGS. 1, 2 and 3 are cross sectional views each showing a transfer belt for
 an electrophotographic apparatus. The transfer belt shown in FIG. 1
 comprises a seamless reinforcing layer 21 formed of a continuous single
 string, a semi-conductive rubber layer 22 formed on the reinforcing layer
 21, and an electrical resistance control layer 23 formed on the
 semi-conductive rubber layer 22. In the transfer belt shown in FIG. 2, a
 semi-conductive sponge layer 24 is interposed between the semi-conductive
 rubber layer 22 and the resistance control layer 23 shown in FIG. 1.
 Further, the transfer belt shown in FIG. 3 comprises a reinforcing layer
 21 having a large number of axial grooves 25 formed on the inner
 circumferential surface, a semi-conductive rubber layer 22 formed on the
 reinforcing layer 21, and a semi-conductive sponge layer 24 formed on the
 semi-conductive rubber layer 22.
 The present invention is also directed to a method of manufacturing a
 transfer belt for an electro-photographic apparatus. The method of the
 present invention includes types (1) to (5) given below depending on the
 type of the semi-conductive rubber layer included in the transfer belt:
 (1) A method of manufacturing a transfer belt for an electrophotographic
 apparatus, comprising the step of coating a mandrel with a semi-conductive
 rubber paste, the step of forming a seamless reinforcing layer by
 continuously winding a string about the mandrel from one end of the
 mandrel while rotating the mandrel, the step of coating the reinforcing
 layer with a rubber paste having a semi-conductivity while rotating the
 mandrel, followed by vulcanizing and surface-polishing the rubber layer so
 as to form a semi-conductive rubber layer, the step of forming an
 electrical resistance control layer on the semi-conductive rubber layer;
 and the step of withdrawing the structure consisting of the reinforcing
 layer, the semi-conductive rubber layer and the resistance control layer
 from the mandrel.
 (2) A method of manufacturing a transfer belt for an electrophotographic
 apparatus, comprising the step of coating a mandrel with a semi-conductive
 rubber paste, the step of forming a seamless reinforcing layer by
 continuously winding a string about the mandrel from one end of the
 mandrel while rotating the mandrel, the step of coating the reinforcing
 layer with a rubber paste having a semi-conductivity and containing a
 large number of micro-balloons while rotating the mandrel, followed by
 vulcanizing and surface-polishing the rubber layer so as to form a
 semi-conductive rubber layer, the step of forming an electrical resistance
 control layer on the semi-conductive rubber layer; and the step of
 withdrawing the structure consisting of the reinforcing layer, the
 semi-conductive rubber layer and the resistance control layer from the
 mandrel.
 (3) A method of manufacturing a transfer belt for an electrophotographic
 apparatus, comprising the step of coating a mandrel with a semi-conductive
 rubber paste, the step of forming a seamless reinforcing layer by
 continuously winding a string about the mandrel from one end of the
 mandrel while rotating the mandrel, the step of coating the reinforcing
 layer with a rubber paste having a semi-conductivity and containing a
 foaming agent while rotating the mandrel, followed by vulcanizing and
 foaming the rubber layer so as to form a semi-conductive rubber layer, the
 step of forming an electrical resistance control layer on the
 semi-conductive rubber layer; and the step of withdrawing the structure
 consisting of the reinforcing layer, the semi-conductive rubber layer and
 the resistance control layer from the mandrel.
 (4) A method of manufacturing a transfer belt for an electrophotographic
 apparatus, comprising the step of coating a mandrel with a semi-conductive
 rubber paste, the step of forming a seamless reinforcing layer by
 continuously winding a string about the mandrel from one end of the
 mandrel while rotating the mandrel, the step of forming an non-vulcanized
 rubber sheet having a semi-conductivity on the reinforcing layer, followed
 by vulcanizing and surface-polishing the rubber layer so as to form a
 semi-conductive rubber layer, the step of forming an electrical resistance
 control layer on the semi-conductive rubber layer; and the step of
 withdrawing the structure consisting of the reinforcing layer, the
 semi-conductive rubber layer and the resistance control layer from the
 mandrel.
 (5) A method of manufacturing a transfer belt for an electrophotographic
 apparatus, comprising the step of coating a mandrel with a semi-conductive
 rubber paste, the step of forming a seamless reinforcing layer by
 continuously winding a string about the mandrel from one end of the
 mandrel while rotating the mandrel, the step of extruding a non-vulcanized
 rubber sleeve having a semi-conductivity onto the reinforcing layer,
 followed by vulcanizing and surface-polishing the rubber sleeve so as to
 form a semi-conductive rubber sleeve layer, the step of forming a
 resistance control layer on the semi-conductive rubber sleeve layer; and
 the step of withdrawing the structure consisting of the reinforcing layer,
 the semi-conductive rubber sleeve layer and the resistance control layer
 from the mandrel.
 In the method of the present invention, the mandrel is coated first with a
 semi-conductive rubber paste. However, it is also possible to use a
 mandrel coated in advance with a semi-conductive rubber paste. In other
 words, it is possible to wind a string about a mandrel coated in advance
 with a semi-conductive rubber paste so as to prepare a seamless
 reinforcing layer. It is also possible to use a mandrel provided with a
 large number of axial grooves arranged in the circumferential direction.
 In this case, it is possible to rotate the mandrel with the axial grooves
 coated with a semi-conductive rubber paste.
 The transfer belt of the present invention comprises a reinforcing layer
 formed of a single continuous string and serving to prevent elongation of
 the transfer belt. Therefore, the permanent set of the transfer belt is
 extremely small if the transfer belt is used under a tension falling
 within a breaking extension. On the other hand, the conventional transfer
 belt is formed of rubber alone and, thus, does not include a reinforcing
 layer. It follows that, if the conventional transfer belt is allowed to
 stand under a certain tension over a long period of time, the belt is
 caused to bear an elongation set. Further, the transfer belt of the
 present invention also comprises a rubber layer, with the result that the
 transfer belt is prevented from running zigzag by the elastic restoring
 force of the transfer belt.
 Let us describe some Examples of the present invention.
 EXAMPLE 1
 Let us describe this Example with reference to FIG. 7 and Table 1.
 Specifically, Table 1 shows the composition of a semi-conductive rubber
 containing a conductivity-imparting agent including a conductive carbon
 black (Ketchen black EC), conductive zinc white, and a potassium titanate
 whisker.
 In the first step, a rubber paste was prepared by sufficiently mixing the
 rubber composition shown in Table 1 with milling rolls, followed by
 dissolving the resultant mixture in toluene. Then, the surface of a
 mandrel 31 shown in FIG. 7, which had a diameter of 150 mm and a width of
 300 mm, was polished and plated with chromium, followed by mounting the
 mandrel 31 to a rotatable shaft. Under this condition, the surface of the
 mandrel 31 was uniformly coated with a silicone oil acting as a release
 agent. Further, the rubber paste noted above was sprayed with a spray gun
 against the surface of the mandrel 31 while rotating the mandrel 31 to
 form a uniform rubber paste layer having a thickness of 0.1 mm after
 drying.
 In the next step, a bobbin 33 wound with a cotton string 32 having a
 thickness of 0.3 mm was rotatably mounted to a moving apparatus arranged
 in front of a carriage, and one end of the string 32 was fixed to one end
 of the mandrel 31. Under this condition, the mandrel 31 was rotated and,
 at the same time, the moving apparatus having the bobbin 33 mounted
 thereon was driven so as to wind the string 32 continuously about the
 mandrel 31 and, thus, to form a reinforcing layer 34. Then, the rubber
 paste prepared in advance was sprayed against the mandrel 31 with a spray
 gun to permit the rubber paste to permeate sufficiently into the
 reinforcing layer 34. Further, a solid semi-conductive rubber layer of the
 composition shown in Table 1, which was prepared separately, was laminated
 on the reinforcing layer, followed by putting the resultant structure in a
 vulcanizer having the temperature controlled at 150.degree. C. for
 vulcanizing the solid rubber layer. The vulcanization was completed in 3
 hours.
 After the vulcanizing treatment, the solid rubber layer was cooled. Then,
 the surface of the rubber layer was polished with a polishing machine so
 as to reduce the outer diameter of the rubber layer to 152 mm, followed by
 applying a sand paper of 400 meshes to the surface of the rubber layer so
 as to set the surface roughness Rz at 2 .mu.m. Then, the surface of the
 rubber layer was coated by a spray gun with a solution prepared by
 dissolving N-methoxy methylated nylon-6 in a mixed solvent consisting of
 toluene and methanol. The solution layer was dried to form a resistance
 control layer. Finally, the structure consisting of the reinforcing layer,
 the rubber layer and the resistance control layer was withdrawn from the
 mandrel 31 so as to manufacture a transfer belt having a semi-conductivity
 for an electrophotographic apparatus, said transfer belt having an inner
 diameter of 150 mm and including a seamless reinforcing layer 34 formed of
 the continuous cotton string 32. The transfer belt thus manufactured was
 found to have a Shore A hardness of 55.degree..
 TABLE 1
 Parts by
 Composition weight
 Ethylene-propylene rubber (trade name of Mitsui EPT 4025 100
 manufactured by Mitsui Chemical Co., Ltd.)
 Zinc oxide 5
 Powdery sulfur 1.5
 Vulcanization promoter (trade name of Nocceller TS 1
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Vulcanization promoter (trade name of Nocceller DM 1
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Stearic acid 1
 Carbon black (trade name of Ketchen black EC manufactured 10
 by Mitsubishi Chemical Co., Ltd.)
 Electrically conductive zinc oxide 20
 Potassium titanate whisker (trade name of Dentol WK .multidot. 200B 20
 manufactured by Otsuka Kagaku Co, Ltd.)
 Calcium carbonate 30
 Softening agent (trade name of Process Oil NS .multidot. 24 30
 manufactured by Nippon Oil Co., Ltd.)
 Sum 219.5
 The transfer belt for an electrophotographic apparatus thus manufactured
 comprises a seamless reinforcing layer formed of a continuous string, a
 semi-conductive rubber layer formed on the reinforcing layer, and a
 resistance control layer formed on the semi-conductive rubber layer. The
 transfer belt was cut in a width of 1 cm. One end of the cut piece of the
 belt was fixed, with a weight of 3 Kgf hung from the other end. Elongation
 of the cut piece of the belt was found to be 2.1% when measured 24 hours
 later and to be 2.5% when measured 500 hours later. Further, a voltage of
 2 KV was applied across the cut piece of the belt under various
 environments so as to measure the electrical resistance of the cut piece.
 The electrical resistance was found to be 1.1.times.10.sup.10 .OMEGA.
 under room temperature and a low humidity (23.degree. C., RH 5%),
 5.5.times.10.sup.9 .OMEGA. under room temperature and an ordinary humidity
 (23.degree. C., RH 55%), and 3.times.10.sup.9 .OMEGA. under room
 temperature and a high humidity (23.degree. C., RH 80%). In conclusion,
 the transfer belt of the present invention was found to be excellent in
 dimensional stability and stability of electrical resistance.
 EXAMPLE 2
 The surface of a mandrel having a diameter of 150 mm and a width of 300 mm
 was polished and plated with chromium. Then, the mandrel was mounted to a
 carriage, followed by uniformly coating the surface of the mandrel with a
 release agent of a silicone oil. On the other hand, a rubber paste was
 prepared by sufficiently mixing the semi-conductive rubber composition
 shown in Table 1 with milling rolls, followed by dissolving the resultant
 mixture in toluene. The solution was uniformly sprayed with a spray gun
 against the surface of the mandrel to form a rubber paste layer having a
 thickness of 0.1 mm after drying.
 In the next step, a bobbin wound with a polyester string having a thickness
 of 0.15 mm was rotatably mounted to a moving apparatus arranged in front
 of the carriage. One end of the polyester string was fixed to one end of
 the mandrel. Under this condition, a moving apparatus having the bobbin
 mounted thereon was driven while rotating the mandrel so as to
 continuously wind the polyester string about the bobbin. When the string
 wound about the bobbin reached the other end of the mandrel, a clutch of
 the moving apparatus was switched so as to wind the string about the
 mandrel from the other end toward said one end so as to form a reinforcing
 layer made of the polyester string, said reinforcing layer being of a
 double layer structure.
 Further, a rubber composition shown in Table 2 was sufficiently mixed with
 milling rolls, followed by dissolving the resultant mixture in toluene so
 as to prepare a rubber paste. As shown in Table 2, the rubber composition
 contained micro-balloons (Expancel 091DE) having a diameter of 40 to 60
 .mu.m. Upon vulcanization, the rubber composition is converted into a
 semi-conductive sponge rubber.
 Then, the reinforcing layer was coated with the rubber paste noted above to
 form a rubber paste layer having a thickness of 1 mm. In this coating
 step, a doctor blade 14 was mounted to the mandrel 12 and the rubber paste
 13 was supplied to the doctor blade 14, as shown in FIG. 6. Under this
 condition, the mandrel 12 was rotated so as to achieve the desired
 coating. After toluene was sufficiently evaporated from the rubber paste,
 the rubber composition was vulcanized for 5 hours within an oven having
 the temperature controlled at 150.degree. C. As a result, the formed
 sponge rubber layer was somewhat shrunk to have an outer diameter of 152.5
 mm.
 In the next step, the surface of the sponge rubber layer was polished on a
 polishing disc to reduce the outer diameter to 152.0 mm. Then, the sponge
 rubber layer was coated with GLP-102NR, which is the trade name of a
 primer manufactured by Daikin Co. Ltd., to form a primer layer, followed
 by spraying GL-213D, which is the trade name of a fluororubber paint
 manufactured by Daikin Co. Ltd., with a spray gun against the primer layer
 so as to form a fluororubber layer having a thickness of 50 .mu.m as a
 resistance control layer on the primer layer. The fluororubber layer
 (resistance control layer) was subjected to a heat treatment at
 150.degree. C. for 30 minutes. Finally, the structure consisting of the
 reinforcing layer of the double layer structure, the semi-conductive
 rubber layer, the semi-conductive sponge layer and the resistance control
 layer was withdrawn from the mandrel 12 so as to manufacture a transfer
 belt having an inner diameter of 150 mm, a thickness of 1 mm and a width
 of 300 mm. The transfer belt thus manufactured was found to have a JIS E
 hardness of 40.degree..
 TABLE 2
 Parts by
 Composition weight
 Ethylene-propylene rubber (trade name of Mitsui EPT 4025 50
 manufactured by Mitsui Chemical Co., Ltd.)
 Ethylene-propylene rubber (trade name of Mitsui EPT 4045 50
 manufactured by Mitsui Chemical Co., Ltd.)
 Powdery sulfur 1.5
 Vulcanization promoter (trade name of Nocceller DM 1
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Vulcanization promoter (trade name of Nocceller TS 1
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Zinc oxide 5
 Stearic acid 1
 Carbon black (trade name of Ketchen black EC manufactured 10
 by Mitsubishi Chemical Co., Ltd.)
 Electrically conductive zinc oxide 20
 Potassium titanate whisker (trade name of Dentol WK .multidot. 200B 20
 manufactured by Otsuka Kagaku Co., Ltd.)
 Calcium carbonate 30
 Softening agent (trade name of Process Oil NS .multidot. 24 30
 manufactured by Nippon Oil Co., Ltd.)
 Micro-balloon (trade name: Expancel 091DE manufactured 10
 by Novel Industry, Inc.)
 Sum 229.5
 The transfer belt for an electrophotographic apparatus thus manufactured
 comprises a continuous seamless reinforcing layer of a double layer
 structure made of a polyester string, a semi-conductive rubber layer
 formed on the reinforcing layer, a semi-conductive sponge layer formed on
 the semi-conductive rubber layer, and a resistance control layer formed on
 the semi-conductive sponge layer.
 The transfer belt was cut in a width of 1 cm. One end of the cut piece of
 the belt was fixed, with a weight of 3 Kgf hung from the other end.
 Elongation of the cut piece of the belt was found to be 3.1% when measured
 24 hours later and to be 3.4% when measured 500 hours later. Further, a
 voltage of 2 KV was applied across the cut piece of the belt under various
 environments so as to measure the electrical resistance of the cut piece.
 The resistance was found to be 1.2.times.10.sup.10 .OMEGA. under room
 temperature and a low humidity (23.degree. C., RH 5%), 6.times.10.sup.9
 .OMEGA. under room temperature and an ordinary humidity (23.degree. C., RH
 55%), and 5.1.times.10.sup.9 .OMEGA. under room temperature and a high
 humidity (23.degree. C., RH 80%).
 EXAMPLE 3
 Prepared was a mandrel having a diameter of 150 mm and a width of 600 mm.
 The mandrel was provided with a large number of axial grooves each having
 a width of 1 mm and a depth of 1 mm. These axial grooves were arranged
 equidistantly in the circumferential surface of the mandrel. The mandrel
 was uniformly coated with a release agent of a silicone oil. On the other
 hand, a rubber paste was prepared by sufficiently mixing the
 semi-conductive rubber composition shown in Table 3 with milling rolls,
 followed by dissolving the resultant mixture in toluene. Then, the rubber
 paste was sprayed with a spray gun against the mandrel while rotating the
 mandrel on a carriage so as to fill the axial grooves of the mandrel with
 the rubber paste.
 On the other hand, a bobbin wound with Cornex string (trade name of aramid
 fiber string manufactured by Teijin Ltd.) having a thickness of 0.15 mm
 and subjected in advance to an adhesive treatment was rotatably mounted to
 a moving apparatus arranged in front of the carriage. One end of the
 aramid fiber string was fixed to one end of the mandrel. Under this
 condition, the moving apparatus having the bobbin mounted thereon was
 driven while rotating the mandrel so as to wind continuously the aramid
 fiber string about the mandrel and, thus, to form a reinforcing layer.
 In the next step, a rubber paste of the composition shown in Table 3 was
 sprayed with a spray gun against the reinforcing layer to permit the
 rubber paste to permeate into the string forming the reinforcing layer.
 Then, a rubber sheet having a thickness of 0.3 mm was prepared by
 sufficiently mixing a rubber composition shown in Table 4 with milling
 rolls, followed by sheeting the mixture with a calender machine. The
 composition shown in Table 4 contains a foaming agent and, thus, generates
 mainly a nitrogen gas when heated so as to be converted into a
 semi-conductive sponge rubber. The rubber sheet thus prepared was wound
 twice about the mandrel, followed by tightening the wound rubber sheet
 with a polyethylene sheet.
 Further, the mandrel was put in an oven having the temperature controlled
 at 150.degree. C. so as to vulcanize the rubber sheet for 5 hours. During
 the vulcanizing step, the foaming agent contained in the rubber
 composition was decomposed so as to generate a gas, with the result that
 the rubber sheet was converted into a semi-conductive sponge layer. When
 cooled, the surface of the sponge layer was polished with a polishing
 machine so as to reduce the outer diameter of the sponge layer to 152 mm.
 The surface layer was found to consist of a semi-conductive sponge rubber
 containing cells each having a diameter of 100 to 150 .mu.m. Finally, the
 structure consisting of the reinforcing layer, the rubber sheet, and the
 semi-conductive sponge rubber layer was withdrawn from the mandrel so as
 to obtain a transfer belt for an electrophotographic apparatus having an
 inner diameter of 150 mm, a thickness of 1 mm, and a large number of axial
 grooves formed on the inner circumferential surface, each of said axial
 grooves having a depth of 1 mm and a width of 1 mm. The transfer belt thus
 obtained was found to have a JIS E hardness of 35.degree..
 TABLE 3
 Parts by
 Composition weight
 Epichlorohydrin rubber (trade name: Epichlomer CG-102, 100
 manufactured by Daiso Inc.)
 Powdery sulfur 1
 Magnesium oxide 5
 Vulcanization promoter (trade name of Nocceller 22 1.5
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Vulcanization promoter (trade name of Nocceller TRA 1
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Lubricant (trade name of DR1000 manufactured by Daiso Inc.) 0.5
 LiCF.sub.3 SO.sub.3 (ionic conductive agent) 0.5
 Electrically conductive zinc oxide (trade name of 23-K 10
 manufactured by Hakusui Kagaku Inc.)
 Carbon black (trade name of Ketchen black EC manufactured by 10
 Mitsubishi Chemical Co., Ltd.)
 Potassium titanate whisker (trade name of H. A. FOB 10
 manufactured by Otsuka Kagaku Co., Ltd.)
 Titanium oxide (trade name of FT-2000 manufactured by 10
 Ishihara Sangyo Kaisha Ltd.)
 Calcium carbonate 20
 TABLE 4
 Parts by
 Composition weight
 Epichlorohydrin rubber (trade name: Epichlomer CG-102, 70
 manufactured by Daiso Inc.)
 Nitrile rubber (trade name of Nipole DN201 manufactured by 30
 Nippon Zeon Inc.)
 Powdery sulfur 1
 Zinc oxide 5
 Vulcanization promoter (trade name of Nocceller 22 1
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Vulcanization promoter (trade name of Nocceller TRA 1
 manufactured by Ouchi Shinko Kagaku Co., Ltd.)
 Aging inhibitor (trade name of Nocrac NBC manufactured by 1
 Ouchi Shinko Kagaku Co., Ltd.)
 LiCF.sub.3 SO.sub.3 (ionic conductive agent) 0.5
 Electrically conductive zinc white (trade name of 23-K 10
 manufactured by Hakusui Kagaku Inc.)
 HAF carbon black 10
 Titanium oxide (trade name of FT-2000 manufactured by 10
 Ishihara Sangyo Kaisha Ltd.)
 Calcium carbonate 20
 Foaming agent (trade name: neocellbon N#5000 manufactured 8
 by Eiwa Kasei Co., Ltd.)
 The transfer belt for an electrophotographic apparatus thus prepared
 comprises a seamless reinforcing layer of a single layer structure made of
 a continuous aramid fiber string, a rubber sheet formed on the reinforcing
 layer, and a semi-conductive sponge rubber layer formed on the rubber
 sheet.
 The transfer belt was cut in a width of 1 cm. One end of the cut piece of
 the belt was fixed, with a weight of 3 Kgf hung from the other end.
 Elongation of the cut piece of the belt was found to be 1.2% when measured
 24 hours later and to be 1.8% when measured 500 hours later. Further, a
 voltage of 2 KV was applied across the cut piece of the belt under various
 environments so as to measure the electrical resistance of the cut piece.
 The resistance was found to be 1.1.times.10.sup.8 .OMEGA. under room
 temperature and a low humidity (23.degree. C., RH 5%), 9.5.times.10.sup.7
 .OMEGA. under room temperature and an ordinary humidity (23.degree. C., RH
 55%), and 5.times.10.sup.7 .OMEGA. under room temperature and a high
 humidity (23.degree. C., RH 80%).
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.