Image forming apparatus

Abstract: The present invention provides an image forming apparatus comprising: an image bearing member (3); an image forming means for forming a toner image on the image bearing member (3), the average volume particle diameter of toner being 3 - 10 µm; a movable transfer sheet bearing means (93) for bearing a transfer sheet; and a transfer member for transferring the toner image formed on the image bearing member onto the transfer sheet carried by the transfer sheet bearing means at a transfer station, the transfer member comprising a fiber brush (23) engageable by the transfer sheet bearing means (93) at a side opposite to the image bearing member (3) at the transfer station.

Description: The present invention generally relates to a black and white, mono-color or full-color image forming apparatus, and more particularly, it relates to a full-color image forming apparatus such as a full-color electrophotographic copying machine, printer and the like wherein an image is obtained by transferring and superimposing visualized images or toner images formed on an image bearing member by an electrophotographic technique or an electrostatic recording technique on a transfer sheet carried by a transfer sheet bearing member by the use of the transfer electric field. In the past, various kinds of full-color image forming apparatuses have been proposed. In such image forming apparatuses, an electrostatic latent image is formed on a photosensitive drum acting as an image bearing member and then is visualized as a toner image with yellow toner (having an average volume particle diameter of 11.0 - 14.0 µm) contained in one of a plurality of developing devices. The yellow toner image is transferred onto a transfer sheet carried by a transfer drum acting as a transfer sheet bearing member at a transfer station. The transfer drum includes a pair of cylinders arranged on both ends, a connecting portion connecting these cylinders to each other, and a dielectric sheet (transfer sheet bearing member) for covering an opening obtained by cutting off the drum portion while remaining the cylinders and the connecting portion. In order to transfer the image onto the transfer sheet at the transfer station, a transferring corona discharger (transfer member) comprising a wire electrode and a shield electrode surrounding the wire electrode is disposed within the transfer drum. Magenta, cyan and black toner images are formed on the image bearing members in the same manner as the yellow toner image, and the same transfer sheet is brought to the transfer station by four times to transfer these four color toner images thereonto. After these four color toner images have been transferred onto the transfer sheet, the sheet is separated from the transfer drum. Thereafter, the transfer sheet is sent to a fixing device, where the four color toner images are melt and combined to form a full-color image fixed to the transfer sheet. As in the above-mentioned conventional technique, although the image obtained by the full-color electrophotographic copying machine which has been practically used provides the improved image quality regarding the color reproductibity and the color tone reproductibity, it is hard to say that the image is closely reproduced from the color original. Further, unlike to the television image, photo-image or color printed matter image, since the reproduced or copied image is not immediately compared with the actual object, the images obtained by the full-color electrophotographic copying machines which have been practically used are not necessarily satisfactory for the persons who are accustomed to see images which are made more beautiful than the actual images. Although the techniques that toner having the smaller particle diameter is used for improving the image quality have been developed, if the toner having the smaller particle diameter is used in the above-mentioned conventional image forming apparatus, since the corona discharger is used to transfer the toner image onto the transfer sheet, there arise the following problems. Generally, as the diameter of the toner particle is decreased, the BET specific surface area measured by the nitrogen gas adsorption technique is increased, so that the friction charging feature of the toner regarding the friction chargeable members such as carriers in developer, a developing sleeve (developer bearing member) and a regulating member for regulating an amount of the developer on the developing sleeve is enhanced, with the result that an amount of the frictionally charged toner is increased, thus worsening the movement of the toner. Further, when the specific surface area is increased, an amount of moisture adsorbed on the surface is increased, so that the change rate of the friction charging feature of the toner due to the environemental change is also increased, thus increasing the amount of the frictionally charged toner, particularly under the low humidity circumstance. By way of an example, conventionally, the toner that the distribution of toner particle sizes corresponds to the average volume particle diameter of 11.0 - 14.0 µm has been used. However, as mentioned above, when the toner having the smaller particle diameter, for example, the toner having the average volume particle diameter of 8.0 - 4.0 µm and the greater friction charging amount (particularly, under the low humidity circumstance) thereby worsening the movement of the toner is used, the toner is strongly adhered to an electrostatic latent image electrostatically (for example, by the Coulomb force or reflection force), with the result that the stronger transfer electric field or transfer current must be supplied to transfer the toner image onto the transfer sheet. That is to say, conventionally, although high voltage of 6 - 8 kV was applied from a power source to the wire electrode of the transferring corona discharger so that a part of the total corona discharging current (about 10 - 30%), i.e., current of a few µA - 20 µA is applied toward the photosensitive drum as the transfer current, if the toner having the smaller particle diameter is used as mentioned above, the further stronger transfer current is required. In view of the construction of the transferring corona discharger, since the transfer current must be applied toward the photosensitive drum via the dielectric shield, the impedance of the corona discharge toward the photosensitive drum becomes greatly high, with the result that the corona discharge is effected actively between the corona discharger and the shield which is earthed and which has the lower discharge impedance. Consequently, almost all of the total corona discharging current will be flown toward the shield. Accordingly, even if the current to be applied is increased to obtain the stronger transfer current, the increment of the transfer current toward the photosensitive drum will be limited to a certain extent. Thus, it is required for not only providing a power source having the greater voltage capacity and current capacity but also using insulators having the greater pressure resistance for the wiring, thereby making the apparatus bulky and expensive. Further, since the transferring corona discharger is used, the corona discharge is generated to produce ozone and nitrogen oxides which affect a bad influence upon the photosensitive drum. Further, in the case where the corona discharge has the positive polarity (i.e., the toner has the negative polarity), if the corona application voltage is too high, the streamer discharge will occur immediately between the discharging wire and the shield having the lower impedance, thus causing the irregularity in the discharge which leads to the distortion of the transferred image. Incidentally, if the transfer sheet is contacted with the dielectric sheet by using a conductive metal sheet or a conductive roller (referred to as "conductive contacting member" hereinafter) to which the voltage is applied, in place of the corona discharger, the above problem can be eliminated. That is to say, in this case, unlike to the corona discharger, almost all of the discharging current does not flow toward the shield, and thus, the transfer of the toner onto the transfer sheet can be achieved with the low potential, low current and high efficiency without producing ozone. For example, JP-A-61 232 478 discloses a transfer by applying voltage to brushes in contact with a belt effected by shutting a switch. However, it does not teach the arrangement of any other regulating member. Moreover, if such conductive contacting member is used, since the transfer sheet is engaged by such member with straight line contact, the sheet is also contacted by the dielectric sheet with straight line or surface contact. In this condition, although it appears macroscopically that the transfer sheet is uniformly contacted with the dielectric sheet, microscopically, there arises the irregular contacting condition between the transfer sheet and the dielectric sheet along the longitudinal direction of the photosensitive drum. Accordingly, if the low voltage is applied to the conductive contacting member, the transfer charging current will be insufficient at weaker contacting areas (between the transfer sheet and the dielectric sheet), with the result that there is the difference in the transferring efficiency along the longitudinal direction of the photosensitive drum. Consequently, in the toner image transferred to the transfer sheet, there arises the irregularity in the toner density along a direction parallel to an advancing direction of the transfer sheet. On the other hand, if the high voltage is applied to the conductive contacting member, the space discharge will be generated at the weaker contacting areas where there are microscopic spaces between the dielectric sheet and the conductive contacting member, thus creating the irregularity in the toner density, discharge unevenness and/or transfer void in the transferred image. Accordingly, the proper voltage applicating condition is limited to a narrower range. Further, even within the proper voltage applicating condition, the difference in the contacting condition between the conductive contacting member and the dielectric sheet leads in the difference in the transferring efficiency. Thus, even if the mono-color image has the slight irregularity in the toner densiy, when such images are superimposed on the single transfer sheet to form the multi-color toner image by using the multi-color image forming apparatus, the irregularity in the toner density will be noticeable. In order to eliminate or reduce the difference in the contacting condition between the conductive contacting member and the dielectric sheet, the contacting pressure between the dielectric sheet and the conductive contacting member may be increased. However, if the contacting pressure is too high, the dynamic friction force between the conductive contacting member and the dielectric sheet will be increased to increase the load acting on the dielectric sheet, with the result that the dielectric sheet is displaced or distorted, thus displacing or distorting the transfer sheet carried by the dielectric sheet. Consequently, when this technique is applied to the multi-color image forming apparatus wherein various color toner images are transferred and superimposed on the single transfer sheet, there arise the discrepancy in colors and/or the irregularity in the color density in the multi-color image. Further, during the transferring operation, since the toner is strongly urged against the photosensitive drum, the transferring efficiency is decreased, and the service lives of the dielectric sheet and of the conductive contacting member are shortened, resulting in the frequent replacement of them, which leads to the cost-up. Further, when the transfer drum as mentioned above is used, if the contacting pressure between the dielectric sheet and the conductive contacting member is high, whenever members thicker than the dielectric sheet (such as the connecting portion for supporting the cylinders on both ends of the drum, and grippers for gripping a leading end of the transfer sheet) pass through the contacting area, the strong impact occurs at the leading and trailing ends of such members, thus scattering the toner on the transfer sheet or causing the discrepancy in colors in the image. In addition, since the conductive contacting member has the sheet-shape or roller-shape, it cannot be re-contacted with the dielectric sheet immediately after it passes through the trailing ends of the connecting portion and the grippers. Particularly, since it is difficult to attain the proper contact between the trailing ends of the connecting portion and the grippers, and the dielectric sheet, the transferring efficiency is considerably worsened at the leading end of the transfer sheet, i.e., at the leading end of the image, thus causing the transfer void at the leading end of the image. An object of the present invention is to provide an image forming apparatus which can form an image with high quality without any transfer drawbacks such as the irregularity in image density, discharge unevenness, irregularity in color, discrepancy in colors, scattering of toner and/or transfer void at the leading end of the image, even when toner having the smaller particle diameter is used. Another object of the present invention is to provide an image forming apparatus which can utilize a current supplied from a power source to a transfer member as a transfer current without wastefulness. A further object of the present invention is to provide an image forming apparatus which can form an image with a low cost without producing ozone by supplying a low voltage and a low current to a transfer member. A still further object of the present invention is to provide an image forming apparatus having the high transferring efficiency and the wide transfer latitude. A further object of the present invention is to provide an image forming apparatus wherein a transfer sheet is closely and uniformly contacted with an image bearing member with a low pressure during a transferring operation. The other objects and characteristics of the present invention will be apparent from the follwoing descriptions. Fig. 1 is an elevational sectional view of a transfer station of an image forming apparatus; Fig. 2 is an elevational sectional view of a transfer station of an image forming apparatus; Fig. 3 is an elevational sectional view of a full-color electrophotographic copying machine embodying the present invention; Fig. 4 is a perspective view of a transfer drum; Fig. 5 is a schematic elevational sectional view of an image forming apparatus; Fig. 6 is a schematic elevational sectional view of an image forming apparatus; Figs. 7 to 11 are partial elevational sectional views of transfer stations of image forming apparatuses according to 1 st to 5th embodiments of the present invention, respectively; Fig. 12 is an enlarged elevational sectional view of a portion of Fig. 11; and Fig. 13 is a schematic elevational sectional view of an image forming apparatus according to a 6th embodiment of the present invention. The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. Fig. 3 is an elevational sectional view of a full-color electrophotographic copying machine embodying an image forming apparatus according to the present invention. The electrophotographic copying machine comprises a photosensitive drum (image bearing member) 3 rotatably mounted and rotated in a direction shown by the arrow, and an image forming means disposed around the drum. Although the image forming means may be of any type, in the illustrated embodiment, it includes a primary charger 4 for uniformly charging the photosensitive drum 3, an exposure means 8 such as a laser beam exposure device for illuminating color decomposed (color separated) light images or other light images equivalent to them onto the photosensitive drum 3 in response to image information from an original and the like to form an electrostatic latent image on the drum 3, and a rotary developing device 1 for visualizing the electrostatic latent image on the photosensitive drum 3. The rotary developing device 1 comprises four developer containers 1Y, 1M, 1C, 1BK containing yellow color developer, magenta color developer, cyan color developer and black color developer, respectively, and a substantially cylindrical housing rotatably mounted and supporting these four developer containers 1Y, 1M, 1C, 1BK. The rotary developing device 1 is so designed that a desired developer container is brought into a position confronting to an outer peripheral surface of the photosensitive drum 3 to develop the electrostatic latent image on the drum with the developer contained in that container so that four color images can be obtained. The visualized image or toner image on the photosensitive drum is transferred onto a transfer sheet P fed and carried by a transfer device 9. In the illustrated embodiment, the transfer device 9 is constituted by a transfer drum rotatably mounted, and, as best seen in Fig. 4, the transfer drum 9 comprises a frame comprised of a pair of cylinders 9a, 9b disposed on both ends of the drum and a connecting portion 9c connecting the cylinders 9a, 9b to each other, and a transfer sheet bearing member 93 covering an opening obtained by cutting off the drum portion while remaining the cylinders 9a, 9b and the connecting portion 9c. The transfer sheet bearing member 93 is constituted by a film-shaped dielectric sheet having a thickness of 150 µm and made of, for example, polyvinylidene fluoride resin and the like. Further, a gripper 7 for gripping a leading end of the transfer sheet P supplied from a sheet supply means is formed on the connecting portion 9c. Further, within the transfer drum 9, there are disposed a transfer member 23 at a position where this drum is contacted with the photosensitive drum 3, and an inner charge removing discharger 13 at a downstream side of the transfer member in a rotational direction of the transfer drum. And, outer charge removing dischargers, 11, 14 are also arranged outside of the transfer drum in correspondence to the inner discharger 13. The connecting portion extends to a direction perpendicular to a shifting direction of the transfer drum and can be contacted with the transfer member 23 as the transfer drum is rotated. Now, a method for forming a full-color image by using the full-color electrophotographic copying machine as mentioned above will be briefly described. After the photosensitive drum 3 is uniformly charged by the primary charger 4, the electrostatic latent image is formed on the photosensitive drum 3 by illuminating the light image E corresponding to the image information onto the drum by means of the exposure means 8. The electrostatic latent image is visualized on the photosensitive drum 3 with resin-based toner having the average volume particle diameter of 6.0 µm by means of the rotary developing device 1. On the other hand, the transfer sheet P is fed to the transfer drum 9 through a sheet supply guide 18 by means of a pair of regist rollers 6 in synchronous with the image. The leading end of the transfer sheet P is gripped by the gripper 7, and then the sheet is conveyed by the transfer drum 9 in the direction shown by the arrow. Then, by supplying the charge having the polarity opposite to that of the toner to the transfer sheet from the back of the transfer sheet bearing member or dielectric sheet 93 by means of the transfer member 23, the toner image on the photosensitive drum 3 is transferred onto the transfer sheet P. After a predetermined number of transferring processes have been completed, the charge is removed from the transfer sheet by the dischargers 11, 13, 14 and the transfer sheet P is separated from the transfer drum 9 by means of a separating pawl 15. Then, the separated transfer sheet is conveyed by a convey belt 16 toward a fixing device 17, where the various color toner images transferred and superimposed on the transfer sheet are fused and combined to form a full-color image on the transfer sheet. Thereafter, the transfer sheet is ejected out of the machine. On the other hand, after the transferring operation, the residual toner remaining on the photosensitive drum 3 is removed by a cleaning device 12 for the preparation for the next image forming operation. Further, the surface of the dielectric sheet of the transfer drum 9 is cleaned by a cleaning device 5 comprising a fur brush and an auxiliary cleaning means 8 for the preparation for the next image forming operation. The transfer member will be further described in detail with reference to Fig. 1 showing an elevational view of a portion near the transfer station of the copying machine of Fig. 3. As seen in Fig. 1, a conductive brush (conductive contacting member) 23 which is an embodiment of the transfer member is disposed within the transfer drum 9 and is urged against the back of the dielectric sheet 93 at a position where the photosensitive drum 3 is contacted with the transfer drum. The conductive brush 23 is connected to a bias voltage source 30 which applies the transfer electric field to the brush. In this embodiment, while the conductive brush was constituted by conductive fibers obtained by dyeing acrylic fine fibers with copper sulfide and by making the fibers conductive, any other conductive or semiconductive fibers, such as stainless steel fibers each having a diameter of 8 - 15 µm, electroplated resin fibers such as acrylic resin, nylon, polyester rayon or the like, fibers obtained by kneading or mixing resin with conductive powder such as carbon powder, metal powder and the like, or carbon fibers obtained by carbonizing resin fibers may be used. The volume resistivity of the conductive brush 23 can be 10 10 Ωcm or less and preferably be 10 8 Ωcm or less. The conductive brush 23 having the elasticity is arranged so that it extends from an upstream side to a downstream side of a moving direction of the dielectric sheet 93 and is urged against the dielectric sheet 93 at or near a position where the transfer sheet carried by the dielectric sheet 93 starts to be separated from the photosensitive drum, or, at or near a position where the transfer sheet starts to be contacted with the photosensitive drum. Unlike to the above mentioned conventional contacting condition between the dielectric sheet and the conductive contacting member, wherein the sheet-shaped or roller-shaped conductive contacting member is contacted with the dielectric sheet with line or surface contact, according to the present invention, since the contacting condition between the dielectric sheet 93 and the conductive brush 23 is accomplished by contacting each of the elastic fine fibers with the dielectric sheet 93 individually, the brush can follow the surface of the dielectric sheet regardless of the difference in the contacting pressure along the longitudinal direction of the contacting area, the difference in roughness of the surface of the dielectric sheet 93 and/or the step due to the existance of the connecting portion 9c, so that the individual fiber can be contacted with the dielectric sheet uniformly and softly. Further, in consideration of the diameter, material and density of the fibers, the more uniform and soft contacting condition can be attained without increasing the contacting pressure. During the transferring operation, the voltage is applied from the bias voltage source 30 to the conductive brush 23 to transfer the toner image on the photosensitive drum 3 onto the transfer sheet carried by the dielectric sheet 93. When the transfer electric field generating means as mentioned above was incorporated into the full-color electrophotographic copying machine of Fig. 3, the toner image was obtained with inversion phenomenon by forming the electrostatic latent image on the photosensitive drum 3 negatively charged and by using the toner having the average volume particle diameter of 6.0 µm. In the present invention, the available average volume particle diameter of the toner is 10.1 - 3.0 µm and is preferably 8 - 5 µm. When the average volume particle diameter of the toner exceeds 10.0 µm, the toner can be used with the conventional transfer member, as well as that of the present invention. In addition, if the average particle diameter of the toner is smaller than 3.0 µm, pulverized particles are increased in the manufacture of the toner, with the result that the fog phenomenon and/or deterioration of the image are caused, and much time and energy are required for powdering the toner, thus leading to the cost-up. The average volume particle diameter of the toner is measured as follows. The Corta counter TA - II type (manufactured by Corta Inc. in U.S.A.) was used as a measuring means, to which an interface (manufactured by Hitachi Co. in Japan) for outputting average number distribution and average volume distribution and a CX - 1 personal computer (manufactured by Canon Co. in Japan) were connected. Electrolytic solution was prepared by diluting the first class sodium chloride with water to obtain aqueous solution including 1% NaCl. The surface-active agent, preferably alkyl benzene sulfonate, of 0.1 - 5 ml was added to the electrolytic solution of 100 - 150 ml as dispersant, and an sample of 0.5 - 50 mg was further added to the solution. The electrolytic solution suspending the sample therein was subjected to the dispersion treatment by the supersonic dispersing device for 1 - 3 minutes. Then, the particle size distribution of particles of 1 - 40 µm was measured by the Corta counter TA - II by using the apertures of 100 µm and 8 µm, thus seeking the average volume distribution and the average number distribution. The average volume particle diameter was determined by the average volume distribution and the average number distribution so obtained. The toner used in this embodiment was constituted by resin, dye and a small amount of additive for improving the charge controlling ability and the lubricativity and was negatively charged by frictionally contacting with the carrier particles in the developer container. In this case, the frictional charging amount of the toner was measured by means of the Faraday gauge by directly sucking the toner image formed on the photosensitive drum 3 into the gauge. As a result, the frictional charging amount of the toner was about -10 to -45 µc/g. Incidentially, in the conventional technique, the frictional charging amount of the toner was about -10 to -25 µc/g. The toner image formed as mentioned above was then transferred onto the transfer sheet by the transferring means according to the illustrated embodiment. Then, the transfer sheet was separated from the dielectric sheet 93 and was subjected to the fixing treatment by the fixing device 17 to obtain the mono-color image. Further, after a plurality of toner images were sequentially transferred onto the transfer sheet by repeating the above-mentioned processes, the transfer sheet was separated from the dielectric sheet 93 and was subjected to the fixing treatment by the fixing device 17 to obtain the full-color image. Both the mono-color and full-color images were excellent, without the irregularity in image density, discharge unevenness or void, irregularity in color, discrepancy in colors, scattering of toner and transfer void at the leading end of the image, which was caused in the above-mentioned conventional technique. The same test was performed under the low humidity condition of 10% RH in 15°C which is severe for the discharging phenomenon. Also in this case, the excellent images could be obtained. In this case, the frictional charging amount of the toner was about -45 µc/g. In the illustrated embodiment, the diameter of the cylindrical dielectric sheet 93 was 160 mm, the moving speed of the dielctric sheet was 160 mm/sec, the diameter of the photosensitive drum 3 was 80 mm, and the moving speed of the photosensitive drum was 160 mm/sec. The transfer bias voltage applied to the conductive brush 23 might be +0.7 ∼ 3.5KV, and the current value might be a few µA - 35 µA. This leads to the remarkable reduction in the capacity of the power source and the remarkable cost-down, in comparison with the transferring operation using the corona discharger (having the total discharge current value of about +500 µA). Fig. 2 shows, in place of the fixed condcutive brush 23 in the embodiment of Fig. 1, a roller-shaped conductive brush, i.e., a conductive fur brush roller 25 is used. The other elements are the same as those in the embodiment of Fig. 1, and, thus, the explanation thereof will be omitted. The conductive fur brush roller 25 comprises a metal core 26 having a diameter of 8 mm, and a cloth web which is closely adhered around the metal core and on which a large number of conductive fibers 27 are closely mounted. Each fiber 27 has a length of 5 mm to define the outer diameter of the roller to 18 mm. While the conductive fibers 27 were obtained by dyeing acrylic fine fibers with copper sulfide and by making the fibers conductive, any other conductive or semi-conductive fibers, such as stainless steel fibers each having a diameter of 8 - 15 µm, electroplated resin fibers such as acrylic resin, nylon, polyester rayon or the like, fibers obtained by kneading or mixing resin with conductive powder such as carbon powder, metal powder and the like, or carbon fibers obtained by carbonizing resin fibers may be used. The volume resistivity of the fur brush roller can be 10 10 Ωcm or less and preferably be 10 8 Ωcm or less. The conductive fur brush roller 25 having the elasticity is arranged so that it is urged against the dielectric sheet 93 at or near a position between the position where the transfer sheet carried by the dielectric sheet 93 starts to be separated from the photosensitive drum and the position where the transfer sheet starts to be contacted with the photosensitive drum. Further, the conductive fur brush roller 25 may be driven at the same speed as that of the moving dielectric sheet 93, or may be rotatingly driven at a speed different from that of the dielectric sheet, if necessary. Also in this embodiment, as a result that the same test as that in the embodiment of Fig. 1 was performed by using the conductive fur brush roller 25 as the transfer member, the same technical effect or advantage could be obtained. That is to say, also in this embodiment, the excellent images without the irregularity in image density, discharge unevenness or void, irregularity in color, discrepancy in colors, scattering of toner, transfer void at the leading end of the image and distortion of the image could be obtained under various circumstances. Fig. 5 shows in contrast to the embodiments of Fig. 1, 2 wherein a full-color electrophotographic copying machine having the rotary developing device and the transfer drum was used as the transferring means, a full-color electrophotographic apparatus having a transfer belt. Referring to Fig. 5, the full-color electrophotographic apparatus comprises first, second, third and fourth image forming portions Pa, Pb, Pc, Pd arranged sequentially within a frame of the apparatus. The image forming portions Pa, Pb, Pc, Pd include exclusive electrophotographic photosensitive drums 3a, 3b, 3c, 3d, respectively. Around the photosensitive drums 3a, 3b, 3c, 3d, primary chargers 4a, 4b, 4c, 4d, exposure means 8a, 8b, 8c, 8d, developing devices 1a, 1b, 1c, 1d each including toner having the same average volume particle diameter as the aforementioned one, and cleaning devices 12a, 12b, 12c, 12d are disposed, respectively. Further, a transfer belt 93 is mounted around rollers 50, 51, 52 and extends therebetween below the photosensitive drums 3a, 3b, 3c, 3d. The belt is rotated in a direction shown by the arrow and constitutes a transfer station. With this arrangement, first of all, a latent image corresponding to an yellow color component of an original image is formed on the photosensitive drum 3a of the first image forming portion Pa by means of the primary charger 4a and the exposure means 8a. Then, this latent image is developed with yellow toner in the developing device 1a to form an yellow toner image. The yellow toner image is then transferred onto a transfer sheet P carried by the transfer belt 93 at the transfer station. Although the transfer belt 93 can be constituted by various kinds of dielectric sheets or composite sheets, in this embodiment of Fig. 5, it is constituted by a sheet made of polyvinylidene fluoride (PVdF) and having a thickness of 150 µm. On the other hand, while the yellow toner image is being transferred onto the transfer sheet P, a latent image corresponding to a magenta color component of the original image is formed at the second image forming portion Pb. This latent image is then developed with magenta toner in the developing device 1b to form a magenta toner image. When the transfer sheet P on which the yellow toner image has been transferred at the first image forming portion 1a is brought into a position of the transfer station relating to the second image forming portion 1b, the magenta toner image is transferred and superimposed on the transfer sheet P. In this way, a cyan toner image and a black toner image are sequentially formed at the third and fourth image forming portions Pc, Pd in the same manner as the above, and these cyan and black toner images are transferred and superimposed on the same transfer sheet P at predetermined positions. When all of the toner images are transferred onto the single transfer sheet, the superimposed toner images on the transfer sheet are fixed to the sheet P by a fixing device 17 to obtain a full-color image. On the other hand, after each toner image has been transferred from the photosensitive drum to the transfer sheet, the residual toner remaining on each of the photosensitive drums 3a, 3b, 3c, 3d is removed by the corresponding cleaning devices 12a, 12b, 12c, 12d for the preparation for the next latent image formation. In the full-color electrophotographic apparatus having the above construction, the transfer sheet P is moved from right to left (Fig. 5) by the transfer belt 93 together with the latter to pass through the image forming portions Pa, Pb, Pc, Pd sequentially. According to the embodiment of Fig. 5, in place of the conventional corona discharger, conductive brushes 23a, 23b, 23c, 23d each having the same construction as that of the first embodiment are used in connection with the 1st to 4th image forming portions. That is to say, in Fig. 5, conductive brushes 23a, 23b, 23c, 23d are disposed within the transfer belt 93 in such a manner that these brushes are urged against the back surface of the transfer belt 93 at positions where the photosensitive drums 3a, 3b, 3c, 3d are contacted with the transfer belt, respectively. Further, the conductive brushes 23a, 23b, 23c, 23d are connected to corresponding bias voltage sources 30a, 30b, 30c, 30d for applying transfer electric fields to the conductive brushes 23a, 23b, 23c, 23d, respectively. In this embodiment, as mentioned above, the conductive brushes 23a, 23b, 23c, 23d are constituted in the same manner as that shown in the embodiment of Fig. 1, and each of the brushes is arranged so that it extends from an upstream side to a downstream side of a moving direction of the transfer belt 93 and is urged against the transfer belt 93 at or near a position where the transfer sheet carried by the transfer belt 93 starts to be separated from the corresponding photosensitive drum, or, at or near a position where the transfer sheet starts to be contacted with the corresponding photosensitive drum. Also in this embodiment, as a result that the same test as that in the embodiment of Fig. 1 was performed by using the conductive brushes 23 (23a, 23b, 23c, 23d) as the transfer electric field generating means, the same technical effect or advantage could be obtained. That is to say, also in this embodiment, the excellent images without the irregularity in image density, discharge unevenness or void, irregularity in color, discrepancy in colors, scattering of toner, transfer void at the leading end of the image and distortion of the image could be obtained under various circumstances. Further, since the frictional load acting on the transfer belt can be maintained to the smaller extent, the displacement, distortion and/or hunting of the transfer belt can be prevented, thus avoiding the distortion of the image due to the hunting of the belt. Fig. 6 shows, in place of the fixed conductive brushes 23a, 23b, 23c, 23d in the embodiment of Fig. 5, roller-shaped conductive brushes, i.e., conductive fur brush rollers 25 (25a, 25b, 25c, 25d) are used. Fig. 6 shows only characteristic portions of the fourth embodiment. The elements same as those shown in Fig. 5 are designated by the same reference numerals, and the detailed explanation thereof will be omitted. In this embodiment, the conductive fur brush rollers 25a, 25b, 25c, 25d are constituted in the same manner as the conductive fur brush roller 25 in the embodiment of Fig. 2, and are arranged so that each of the conductive fur brush rollers is urged against the transfer belt 93 at or near a position between the position where the transfer sheet carried by the transfer belt 93 starts to be separated from the corresponding photosensitive drum and the position where the transfer sheet starts to be contacted with the corresponding photosensitive drum. Further, the conductive fur brush rollers may be driven at the same speeds as that of the moving dielectric sheet 93, or may be rotatingly driven at speeds different from that of the dielectric sheet, if necessary. Also in this embodiment, as a result that the same test as that in the embodiment of Fig. 5 was performed by using the conductive fur brush rollers 25 as the transfer members, the same technical effect or advantage could be obtained. That is to say, also in this embodiment, the excellent images without the irregularity in image density, discharge unevenness or void, irregularity in color, discrepancy in colors, scattering of toner, transfer void at the leading end of the image and distortion of the image could be obtained under various circumstances. Further, since the frictional load acting on the transfer belt can be maintained to the smaller extent, the displacement, distortion and/or hunting of the transfer belt can be prevented, thus avoiding the distortion of the image due to the hunting of the belt. Fig. 7 shows a first embodiment of the transfer member of Fig. 1. The same elements as those shown in Fig. 1 are designated by the same reference numerals, and the detailed explanation thereof will be omitted. For example, in Fig. 1, since the transfer electric field is formed between the photosensitive drum 3 and the conductive brush 23 even at an area (shown by the letter A) upstream of the position where the transfer sheet carried by the dielectric sheet 93 starts to be contacted with the photosensitive drum 3, the discharge phenomenon occurs in a small space near the area A. Thus, it is feared that the toner image on the photosensitive drum 3 is scattered before it is transferred onto the transfer sheet, thus causing the transfer unevenness and/or transfer void. However, in this embodiment, as shown in Fig. 7, an electric field regulating member 22 is attached to the conductive brush 21 between the dielectric sheet 93 and the conductive brush (transfer member) 21 at an upstream side of a position B where the transfer sheet carried by the dielectric sheet 93 starts to be contacted with the photosensitive drum 3. The electric field regulating member 22 serves to weaken or shield the transfer electric field at the upstream side of the transfer position. In the illustrated embodiment, the electric field regulating member 22 has a thickness of 50 µm or 75 µm and is made of polyethylene terephthalate; however, it may be constituted by a conventional insulative resin sheet. With this arrangement, since the voltage is uniformly applied along a longitudinal direction of the conductive brush 21, the electric field regulating member 22 is not charged unevenly, unlike to the corona discharge. The conductive brush 21 was constituted by a sheet made of stainless steel (SUS) and having a thickness of 50 µm, and a number of conductive and elastic fibers upstandingly adhered to the sheet by a conductive adhesive. However, any other conductive or semi-conductive fibers, such as stainless steel fibers each having a diameter of 8 - 15 µm, electroplated resin fibers, fibers obtained by kneading or mixing resin such as acrylic resin, nylon, polyester rayon or the like with conductive powder such as carbon powder, metal powder and the like, or conductive fibers obtained by dyeing fibers with copper sulfide and by making the fibers conductive. Further, a conductive brush including such fibers may be used. The volume resistivity of the conductive brush 21 can be 10 10 Ωcm or less and preferably be 10 8 Ωcm or less. In this embodiment, since the area upstream of the conductive brush 21 is regulated by the electric field regulating member 22, the scattering of the toner, transfer unevenness and/or transfer void which are caused due to the above-mentioned discharge phenomenon can be prevented, thus obtaining the excellent image without distortion. Further, in the illustrated embodiment, as a result of various tests performed by varying the volume resistivity of the dielectric sheet 93, it was found that, when the dielectric sheet having the volume resistivity of about 10 8 - 10 16 Ωcm (preferably, 10 9 - 10 12 Ωcm) was used, the discharge phenomenon caused by the separation of the transfer sheet P from the dielectric sheet 93, and the discrepancy in colors and/or the irregularity in color due to the insufficient adhesion between the dielectric sheet and the transfer sheet were not generated, and the transfer efficiency was satisfactory. Further, when the dielectric sheet 93 having the volume resistivity smaller than 10 12 Ωcm was used, since the potential generated after the charging due to the transferring operation was quickly reduced, the transferring efficiency in the transferring of 2nd - 4th toner images or in the continuous transferring was not reduced, or reduced to the negligible extent. Therefore, it was not required for providing a corona discharger for removing the charge, thus making the apparatus more small-sized and cheaper and reducing the generation of ozone. Further, when the dielectric sheet 93 having the relatively great volume resistivity, for example, of about 10 16 Ωcm was used, by using the dielectric sheet having dielectric constant of 4 or more, the sufficient transfer efficiency could be obtained by bias voltage of 1.0 - 3.0 kV which is considerably smaller than the bias voltage of 6 - 8 kV required for the transferring due to the corona current. Fig. 8 shows a second embodiment of the present invention. In this embodiment, in place of the conductive brush according to the embodiment of Fig. 7, a conductive fur brush roller 24 is used. The conductive fur brush roller 24 comprises a metal core having a diameter of 8 mm, and a cloth web which is closely adhered around the metal core with a conductive adhesive and on which a large number of conductive fibers are closely mounted. Each fiber has a length of 5 mm to define the outer diameter of the roller to 18 mm. As the conductive fibers, for example, conductive or semi-conductive fibers, such as stainless steel fibers each having a diameter of 8 - 15 µm, electroplated resin fibers, fibers obtained by kneading or mixing resin such as acrylic resin, nylon, polyester rayon or the like with conductive powder such as carbon powder, metal powder and the like, or conductive fibers obtained by dyeing fibers with copper sulfide and by making the fibers conductive. The volume resistivity of the conductive fur brush roller 24 can be 10 10 Ωcm or less and preferably be 10 8 Ωcm or less. Also in this embodiment, since the area upstream of the conductive fur brush roller 24 is regulated by the electric field regulating member 22, the same technical effect or advantage as that of the embodiment of Fig. 7, whereby the scattering of the toner, transfer unevenness and/or transfer void which are caused due to the above-mentioned discharge phenomenon can be prevented, thus obtaining the excellent image without distortion. Fig. 9 shows a third embodiment of the present invention. In this embodiment, in place of the electric field regulating member 22 according to the embodiment of Fig. 7, an electric field regulating member 22 having a two-layer construction comprised of an insulator layer 22a and a conductor layer 22b. The conductor layer 22b is insulated from the conductive brush 21 and is earthed. According to this embodiment, the electric field generated between the photosensitive drum 3 and the conductive brush 21 at an area upstream of the position where the transfer sheet carried by the dielectric sheet 93 starts to be conducted with the photosensitive drum 3 is guarded and blocked by the earthed conductor layer 22b. The electric field regulating member 22 according to this embodiment is formed by depositing a metal chrome (Cr) as the conductor layer 22b on one surface of polyethylene terephthalate (PET) sheet (insulator layer 22a) having a thickness of 50 µm. The insulator layer 22a may be made of conventional insulative resins other than the PET resin, and the conductor layer 22b laminated onto the insulator layer may be formed from other deposited metals, or may be formed from conductive or semi-conductive material obtained by dispersing conductive material such as carbon, metal powder and the like in resin or rubber to control the resistive value. Also in this embodiment, since the area upstream of the conductive brush 21 is regulated by the electric field regulating member 22, the same technical effect or advantage as that of the embodiment of Fig. 7, whereby the scattering of the toner, transfer unevenness and/or transfer void which are caused due to the above-mentioned discharge phenomenon can be prevented, thus obtaining the excellent image without distortion. In this embodiment, while the electric field regulating member 22 had the two-layer construction comprising the insulator layer 22a and the conductor layer 22b laminated thereon, since the conductor layer 22b is earthed, even when an additional insulator layer for improving the insulation effect is laminated on the conductor layer 22b, the same advantage can be obtained. In the fourth embodiment shown in Fig. 10, in place of the electric field regulating member 22 according to the embodiment of Fig. 8, the electric field regulating member 22 having the two-layer construction comprising the insulator layer 22a and the conductor layer 22b shown in Fig. 9 is used. Also in this case, the same technical effect or advantage as that of Fig. 9 can be obtained. The transfer electric field generating means shown in Figs. 7 to 10 can be applied to the full-color image forming apparatus of Fig. 5 wherein the plurality of photosensitive drums are abutted against the single transfer belt to perform the multiple transferring operations, and the same technical advantage can be expected. Next, a fifth embodiment of the transfer electric field generating means which can be applied to the transfer station of the image forming apparatus of Fig. 3 is shown in Fig. 11. Incidentally, the same elements as those in the aforementioned embodiments are designated by the same reference numerals, and the detailed explanation thereof will be omitted. According to this embodiment, as shown in Fig. 11, a transfer electric field generating means 33 comprising a conductive brush 23, an elastic dielectric sheet 31 and an electric field regulating member 22 is supported by a metal rigid body (in the illustrated embodiment, a support member 35 made of SUS) within a transfer drum 9 at a position where the transfer drum 9 is opposed to the photosensitive drum 3. The elastic dielectric sheet 31 is mounted on the support member 35, the conductive bursh 23 is mounted on the elastic sheet 31 and urged againt the back surface of the dielectric sheet 93, and the electric field regulating member 22 is supported by the support member 35 via a space 34. A free end of the conductive brush 23 is protruded toward the dielectric sheet 93 between the elastic sheet 31 and the electric field regulating member 22. The transfer electric field generating means 33 is inclined from a position at an upstream side of the rotating direction of the transfer drum 9 toward a position where the dielectric sheet 93 is abutted against the photosensitive drum 3, so that, when the support member 35 is turned, the free end of the conductive brush 23 is contacted with the back surface of the dielectric sheet 93. The elastic dielectric sheet 31 and the electric field regulating member 22 are formed from polyethylene terephthalate (PET) sheets each having thickness of 125 µm, 50 µm, respectively. However, material and thickness of these members are not limited to the above. With this arrangement, in the transfer electric field generating means 33, the elastic dielectric sheet 31 urges the conductive brush 23 toward the dielectric sheet 93 during the transferring operation, so that the conductive brush 23 and the dielectric sheet 93, the dielectric sheet 83 and the transfer sheet P carried by the dielectric sheet and positioned at the transfer position, and the transfer sheet P and the photosensitive drum 3 are closely contacted to each other, respectively, without any clearance or space therebetween. The electric field regulating member 22 blocks a part of the transfer electric field generated at the conductive brush 23 by applying the voltage thereto, at an area upstream of the transfer position, thereby limiting an area influenced upon the transfer electric field to an area d E other than the area upstream of the transfer position. The conductive brush 23 is arranged so that it extends from an upstream side to a downstream side of the moving direction of the dielectric sheet 93 and is urged against the dielectric sheet 93 at or near a position where the transfer sheet P carried by the dielectric sheet 93 finishes to be contacted with the photosensitive drum 3, or at or near a position where the transfer sheet starts to be contacted with the photosensitive drum. More specifically, the transfer electric field falls within a so-called nip d N where the transfer sheet P is being contacted with the photosensitive drum 3. Particularly, if the transfer electric field acts on the transfer sheet P before it is contacted with the photosensitive drum 3, the distortion of the image will occur by the scattering of the toner due to a so-called pre-transfer. However, in the transfer electric field generating means 33 according to this embodiment, since the transfer electric field area is regulated by the electric field regulating member 22 to prevent the transfer electric field from acting on the transfer sheet P before the latter is contacted with the photosensitive drum 3, it is possible to prevent the distortion of the image. Further, as shown in Fig. 12, since the conductive brush 23 is abutted against the dielectric sheet 93 in an inclined fashion, it is not feared that the fine fibers in the brush 23 come down, thus maintaining the proper contact between the brush and the dielectric sheet 93. The image formed on the transfer sheet is excellent, without the irregularity in image density, discharge void, irregularity in color, discrepancy in colors, scattering of toner and transfer void at the leading end of the image. Fig. 13 shows the sixth embodiment of an image forming apparatus according to the present invention. In this embodiment, a transfer sheet bearing member is constituted by an endless transfer belt 29 mounted around rollers 50, 51, 52 and extending therebetween. The transfer belt is moved in synchronous with the photosensitive drum 3. The transfer belt 29 is abutted against or disposed in the vicinity of the photosensitive drum 3 to define an image transfer station therebetween. At the image transfer station, a transfer electric field generating means 33 as same as that of Fig. 12 is disposed inside (back side) of the endless transfer belt 29. Similar to the embodiment of Fig. 12, the transfer electric field generating means 33 comprises a conductive brush 23, an elastic dielectric sheet 31 supporting the brush, and an electric field regulating member 22 disposed between the conductive brush 23 and the transfer belt 29, and is supported by a support member 35. The image forming operation effected by this image forming apparatus is fundamentally the same as that of the aforementioned apparatus, except that the transfer belt 29 is used in place of the transfer drum 9 including the dielectric sheet 93. That is to say, the surface of the photosensitive drum 3 is uniformly charged by the primary charger 4, and an electrostatic latent image is formed on the photosensitive drum by illuminating the light image E corresponding to the image information by means of the exposure means. Then, the electrostatic latent image is developed with the aforementioned toner in the developing device 1 to form the toner image on the photosensitive drum 3. By the rotation of the photosensitive drum 3, when the toner image is brought into the transfer station, the electric field is applied to the image transfer station and the toner image on the photosensitive drum 3 is transferred onto the transfer sheet P carried by the transfer belt 29. After the toner image is transferred to the transfer sheet, the latter is conveyed away from the image transfer station by a conveying force of the transfer belt 29 to reach the downstream end of the transfer belt, where the transfer sheet is separated from the transfer belt 29. Then, the transfer sheet is fed to the fixing device 17, where the toner image is fixed to the transfer sheet. Then the transfer sheet is ejected out of the image forming apparatus. It is feared that, when the transfer sheet P is separated from the transfer belt 29, the toner is scattered onto the image due to the peeling discharge. To avoid this, a charge removing charger 36 is disposed in the vicinity of a position where the transfer sheet is separated from the transfer belt 29, so that the charges can be removed from the transfer sheet and the transfer belt. After the transferring operation, the residual toner remaining on the photosensitive drum 3 is removed by the cleaning device 12 for the preparation for the next image formation. Also in this embodiment, similar to the aforementioned embodiments, since the toner image on the photosensitive drum 3 is transferred onto the transfer sheet P while generating the transfer electric field at the image transfer station by applying the voltage to the conductive brush 23 urged against the back surface of the dielectric sheet 93 via the elastic sheet 31 and blocking the transfer electric field at the area upstream of the image transfer station by the electric field regulating member 22 of the transfer electric field generating means 33, the excellent image without the irregularity in image density, discharge void, irregularity in color, discrepancy in colors, scattering of toner and transfer void at the leading end of the image can be obtained. Further, the ozone is also prevented. Incidentally, it should be noted that the transfer electric field generating means 33 of Fig. 12 can be used in place of the transfer members 23a - 23d of Fig. 5. As mentioned above, while the present invention was explained in connection with various embodiments thereof, the present invention is not limited to such embodiments, and, therefore, various alterations and modifications can be effected without the departure from the scope of the appended claims. The present invention according to claim 1 provides an image forming apparatus comprising: an image bearing member; an image forming means for forming a toner image on the image bearing member, the average volume particle diameter of toner being 3 - 10 µm; a movable transfer sheet bearing means for bearing a transfer sheet; and a transfer member for transferring the toner image formed on the image bearing member onto the transfer sheet carried by the transfer sheet bearing means at a transfer station, the transfer member comprising a fiber brush engageable by the transfer sheet bearing means at a side opposite to the image bearing member at the transfer station.

Claims: An image forming apparatus comprising: an image bearing member (3); an image forming means (4, 8, 1) for forming a toner image on said image bearing member (3); a movable transfer sheet bearing means (9, 29) for bearing a transfer sheet (P); an electrically conductive transfer member (21, 23, 24, 25) for transferring a toner image formed on said image bearing member (3) onto a transfer sheet (P) carried by said transfer sheet bearing means through a transfer electric field generated at a transfer station, said transfer member (23, 24, 25) being contactable with said transfer sheet bearing means (9, 29) at a side opposite to said image bearing member (3) at said transfer station, characterized by a regulating member (22) for regulating a transfer electric field between said image bearing member (3) and said transfer member (21, 23 to 25), said regulating member having a dielectric member and being disposed upstream of said transfer member in a moving direction of said transfer sheet bearing means (9, 29) and adjacent to said transfer member. An image forming apparatus according to claim 1, further including an electric power source (30) for supplying an electric power to said transfer member (21, 23, 24, 25) to generate a transfer electric field between said image bearing member (3) and said transfer member (23, 24, 25). An image forming apparatus according to claim 1 or 2, wherein the volume resistivity of said transfer member (21, 23, 24, 25) is 10 10 Ω cm or less. An image forming apparatus according to any of claim 1 or 2, wherein the volume resistivity of said transfer member (21, 23, 24, 25) is 10 8 Ω cm or less. An image forming apparatus according to any of claims 1 to 4, wherein said transfer sheet bearing means (9, 29) includes a dielectric sheet (93) for bearing the transfer sheet. An image forming apparatus according to any claims 1 to 5, wherein said regulating member (22) is insulative. An image forming apparatus according to any of claims 1 to 6, wherein, in use, a plurality of the toner images are transferred and superimposed on the transfer sheet (P) carried by said transfer sheet bearing means (9, 29). An image forming apparatus according to claim 7, wherein, in use, after the transferring and superimposing, a full-color image is formed on the transfer sheet (P). An image forming apparatus according to any of claims 1 to 8, wherein said transfer member comprises a fiber brush. An image forming apparatus according to any of claims 1 to 9, further including an elastic sheet (31) for supporting said transfer member and for urging said transfer member (21, 23, 24, 25) against said transfer sheet bearing means (9, 29). An image forming apparatus according to claim 10, wherein said elastic sheet is dielectric. An image forming apparatus according to any of claims 1 to 11, wherein said regulating means (22) has an electrically conductive layer electrically earthed to a surface opposite to said transfer member (21, 23 to 25). An image forming apparatus according to any of claims 1 to 12, wherein an average particle diameter of the toner of the toner image to be formed on said image bearing member (3) is about 3 to 10 µm.

Inventors: MIURA YASUSHI, TAKEUCHI TATSUO, TSUNEMI TAKEO, WAKI KENICHIRO, WATANAVE AKIRA, MIURA, YASUSHI, TAKEUCHI, TATSUO, TSUNEMI, TAKEO, WAKI, KENICHIRO, WATANABE, AKIRA, WATANAVE, AKIRA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G03G15
IPC codes: G03G15