COMPACT ELECTROPHOTOGRAPHIC PRINTING APPARATUS HAVING AN IMPROVED DEVELOPEMENT MEANS AND A METHOD FOR OPERATING THE SAME

Abstract: A compact and simple structured electrophotographic printing apparatus having a single magnetic developer unit (13), wherein an accumulation of toner particles is formed on a photosensitive member (11) at a downstream from the magnetic brush developer unit (13). The accumulation of the toner particles is formed by transferring a photosensitive film (11) and a magnetic brush (13) in rubbing contact to the film (11) in mutually opposite directions, or by deforming a magnetic field generated by the magnetic developer unit using a magnetic piece. A recording electrode (15) is disposed on a sleeve (13b) of the magnetic developer (13) facing the photosensitive film (11). Bias voltages having polarities opposite to each other with respect to the photosensitive film (11) are respectively applied to the recording electrode (15) and the sleeve (13b). An optical beam is projected imagewise on the photosensitive film (11) at a region facing the recording electrode (15). Thus, by the aid of toner particles existing between the recording electrodes (15) and the photosensitive film (11) and the accumulation of the toner particles, sensitizing, developing the scavenging are carried out simultaneously, thus a toner image is produced on the photosensitive film (11). For a magnetic developer unit having a rotatable sleeve (13b), a specially designed recording electrode (15) is proposed. The photosensitive member (11) may be in the form of a solid flat plane, a drum, or a flexible belt-like film.

Description: BACKGROUND OF THE INVENTION This invention relates to an electrophotographic printing apparatus and a method for operating the same. More particularly, the present invention is relates to an improved developing means for forming a toner image and a operating method thereof. There have been developed various electrophotographic printers in which a latent electrostatic image is formed by projecting an optical beam imagewise onto a photoconductive layer. By depositing toner particles on the photoconductive layer, the resulting latent electrostatic image is developed into a toner image which is thereafter transferred onto a recording paper and fixed thereon. Principle of a well known prior art method is described referring to Fig.1 to Fig.4. Fig.1 is a block diagram of a prior art electrophotographic printing apparatus according to a process referred to as Carlson method wherein corona dischargers are employed. A photosensitive drum 101 comprising a photo-­ conductive layer 102 such as a selenium layer is rotated in a direction as indicated by an arrow 100, and the surface of the photosensitive drum 101 is uniformly charged (positively in this case) by covering with ions generated by a corona discharging device 103 as shown in Fig.1. Subsequently, the photoconductive layer 102 is imagewise exposed to an optical beam such as a laser beam emitted from an optical image source 104. The resulting electrostatic latent image corresponding to an object pattern to be reproduced is developed by depositing electro­ statically charged toner particles on the photosensitive layer 102, employing a magnetic brush developer 105. Then the toner image is electrostatically attracted and transferred onto a printing paper 110 which is charged in the opposite polarity to that of the toner particles employing another corona discharger 106, and is fixed on the printing paper 110 with an image fixer 107. The charges retained in the photosensitive layer 102 and residual toner particles remaining on the photosensitive layer 102 are neutralized by a corona discharger 108, and the discharged toner particles are wiped away by a fur brush 109. Thus, one cycle of the electrophotographic printing process is over. The corona discharger requires a high voltage such as several thousand volts, and is very sensitive to the atmosphere condition such as a humidity and dusts contained in the air. In addition, ozone gas is generated during the corona discharge, providing operators with a health hazard. Thus, the use of the corona discharging device causes problems such as unstable printing operation, health hazard of operators and cost increase of the device. In order to overcome the disadvantages described above, an electrophotographic device without corona discharging device has been developed recently. For example, an electrophotographic printing apparatus is disclosed in Japanese Laid-Open Provisional Application No.57-119375, issued on July 24, 1982, to Y.Nishigaki. Fig.2 is a block diagram illustrating the configuration of the apparatus. A photosensitive film 115 formed of photosensitive medium is composed of a transparent substrate 111, a transparent electrode 112 formed of ITO (Indium-Tin-Oxide), a photoconductive layer 113, 65 µm thick, formed of cadmium sulfide (CdS), and a white insulator layer 114 formed of titanium oxide (TiO), for example, which are laminated in the recited order one above another. A magnetic brush developer 116 is placed facing the photosensitive film 115. An optical beam is emitted from an optical image source including an optical source 117 such as a laser generating source, a rotating polygon mirror 118, and an fϑ lens 119. The optical beam is imagewise projected onto a portion of the photosensitive film 115 facing the magnetic brush developer 116 from the side of the transparent substrate 111, making the exposed portion of the photoconductive layer 113 conductive. Since a bias voltage is applied between the magnetic brush developer 116 and the transparent electrode 112, the photo-­ carriers generated in the exposed portion of photoconductive layer 113 are attracted by an electrostatic force toward the white insulator 114 and blocked thereby, forming an electro­ static latent image. Consequently, the electrostatic field between the electrostatic latent image and the magnetic brush developer 116 is fairy strong. On the other hand, the electric field across the non-exposed portion of the photosensitive film which still remains non-conductive is rather weak since the photoconductive layer 113 has a large thickness in comparison with that of the white insulator 114. Thus, charged toner particles carried by the magnetic brush developer 116 in contact with the photosensitive film 115 are attracted to the exposed portion of the photosensitive film 115 and are not attracted to the non-exposed portion, forming a visual image on the photo­ sensitive film 115. Thus formed toner image is transferred on a recording sheet. Electric charges of the electrostatic latent image and of the residual toner particles left on the photo­ sensitive film 115 are gradually discharged until the next printing process starts, and collected magnetically by the magnetic brush developer 116. Although it is an advantage of the electrophotographic printing method described above that a corona discharging device employing a high voltage is not necessary, a relatively thick photoconductive layer 113 is required to have a satisfactory contrast, because the toner image formation is done by utilizing the difference between the adhering forces generated by electric fields, namely Coulomb force, in exposed and non-exposed areas as described above. Unfortunately, the fabrication of a thick photoconductive layer having a uniform thickness is rather difficult and the material cost becomes high. Furthermore, reduction of the photo-sensitivity of the photoconductive layer 113 and increase in the bias voltage applied between the transparent electrode 112 and magnetic brush developer 116 becomes inevitable as the thickness of the photoconductive layer 113 increases. In addition, when conductive toner particles are employed, a plain paper having relatively low resistivity can not be used as a recording medium because the charges of the deposited toner particles are easily discharged. Thus, a specially treated medium, for example, a paper coated with an insulative layer must be used. In order to overcome the above-described disadvantages, further improved electrophotographic printing apparatus is disclosed in U.S. series No.762,431 by Kimura et al. Fig.3 is a block diagram of an electrophotographic printing apparatus which comprises a electrophotographic printing drum 140, a first magnetic brush developer 125, a second magnetic brush developer 131, an optical image source 128, an optical discharger 137, an image transferring means 135, and an image fixer 138. A photosensitive film 124 is formed on the electrophotographic printing drum 140, being composed of a transparent substrate 121, a transparent electrode 122, a photoconductive layer 123, which are laminated in the recited order one above another. The transparent electrode 122 is grounded. Hereby, the photo­ sensitive film 124 has no insulator layer formed atop thereon for blocking photo-carriers, because the photoconductive layer 123 has electrical trap potentials underneath the top surface thereof which is a feature of the apparatus. As a result, the thickness of the photosensitive film 124 is fairly reduced. Such a photoconductive material is commercialy available. For example, an organic photoconductive material supplied from the Eastman KODAK Co. under model SO-102. The first magnetic brush developer 125 and the second magnetic brush developer 131 are arranged apart from each other by a predetermined distance. Both are conventional ones having a rotating magnet roller and a stationary sleeve of non-magnetic material arranged co-axially. The toner particles employed are magnetic conductive ones or magnetic non-conductive ones which are carried by magnetic particle carriers, and supplied to the magnetic brush developer 125. Thereby, the toner particles are magnetized by rotating magnetic fields generated by the rotating magnet roll, being formed into a lot of toner particle chains extending in the radial direction of the sleeve of the developer 125, thus a so-called magnetic brush is formed. The magnetic brush also rotates but in the opposite direction to that of the magnet roller. Bias voltages having the opposite polarities to each other are applied to the first and the second magnetic brush developers which are supplied from the power sources 126 and 132 respectively. The optical image source 128 includes a self-focusing lens (a product of Nippon Plate Glass LTD, commercially available brand name is SELFOC lens), and LED array, and emits an optical beam. The printing process employing the above-described printing apparatus is described. The printing drum 140 is rotated in an angular direction indicated by an arrow 120 with a constant rotating speed. During one cycle of the rotation, the relevant printing steps are performed sequentially. The printing steps are as follows: Toner particles negatively charged, for example, are attracted to the photoconductive layer 123 by electric field generated by a bias voltage of negative polarity supplied from the power source 126, and a uniformly distributed toner image 127, a solid image, is developed on the surface of the photosensitive film 124. Subsequently, the solid image 127 is moved to a exposing station 130 wherein an optical beam emitted from the optical image source 128 is imagewise irradiated onto the photo­ sensitive film 124 from the rear side thereof, i.e., from the side of the transparent substrate 121. The exposed portions of the photoconductive layer 123 are made conductive by positive photo-carriers generated in the photoconductive layer 123. The positive photo-carriers reach a trapping potential existing close to the top surface of the photoconductive layer 123, trapped by the trap potential and fixed there even after the turning-off of the laser beam, thus forming a electro­ static latent image 129 in the photoconductive layer 123. Subsequently, using the second magnetic brush developer 131, a reversed bias voltage (positive) is applied to the developer 131 to release the toner particles 134 deposited on the non-exposed portion of the photosensitive film 124. The released particles 134 are recovered into the second magnetic brush developer 131. The toner particles on the exposed portion of the photoconductive layer 123 remain adhering to the surface by a electrostatic force generated by the trapped charges, even through a small part of the toner particles may be released. Thus a visual toner image 133 is developed on the photosensitive film 124. The toner image 133 is proceeded to a transferring station where the toner image 133 is transferred to a printing paper 136 by the conventional image transferring means 135, and is fixed onto the recording paper 136 by a conventional image fixer 138. The trapped photo-­ carriers forming the electrostatic latent image 129 is discharged by the optical discharger 137. The remaining toner particles 139 on the photosensitive film 124 are entirely collected by the first magnetic brush developer 125. Thus, the printing drum 140 is recycled to perform a new printing operation. In the above-described photoconductive layer 123, the mobility of the photo-carrier generated therein by an optical exposure is rather slow, requiring a time sufficient to complete the formation of the electrostatic latent image. This is the reason why the exposing station 130 is necessary to secure a time. On the other hand, with respect to a photoconductive layer made of cadomium sulfide (CdS), selenium (SE), and photosensitive organic materials, for example, wherein photo-carriers have a high mobility, there is no need to secure an exposing time separately, and the exposure and the first development can be performed simultaneously which is realized in an electrophototographic printing apparatus shown in Fig.4. The apparatus of Fig.4 differs from the apparatus of Fig.3 only at this point. The apparatus of Fig.4 has a further advantage that the toner particle layer formed at the first magnetic brush developer 125 has a thicker toner image at the exposed portions than the toner particle layer at the non-exposed portions, because electric field at the exposed portions is stronger than that of non-exposed portions. As a result, some image contrast of the toner particle layer appears already at this printing stage. This is apparently advantageous for the electrophoto- graphic printing for obtaing a denser toner image. However, the electrophotographic printing apparatuses of Fig.3 and Fig.4, needs two magnetic brush developers for solid image developing and image developing, providing a complicated structure and high cost to the electrophotographic printing apparatuses. A low voltage electrophotography and its compact apparatus is disclosed in U.S. Patent No.4,545,669, issued on Oct. 8, 1985, to Hays et al.. As illustrated in Fig.3 of the Patent, an electrophotographic apparatus having a simple structure is disclosed. The apparatus includes a single magnetic brush roller and an imaging member, or a belt-like photosensitive film. The toner chains of the magnetic brush roller are moving in the same direction of movement as the belt-like imaging member by the aid of a driving roller system. The imaging member is flexible and deflected such that the magnetic brush roller is intimately in contact with the imaging member, securing a contacting length therebetween sufficient to form a 'sensitizing nip', and a 'development nip' which is immediately adjacent to the sensitizing nip at a downstream thereof. On a stationary shell, or a sleeve of the magnetic brush developer, an electrically insulated strip which serves as an electrode for sensitizing nip is disposed at the upper side of the nip. By this configuration, the magnetic brush developer, in cooperation with bias voltages, play all the roles of an exposing means, a first magnetic brush developer, and a second magnetic brush developer described above: toner particles supplied to the magnetic brush developer is developed uniformly by an electric field generated by a bias voltage Vs applied to the strip, and simultaneously, an electrostatic latent image is formed in the imaging member at the sensitizer nip by an imagewise rear exposure of an optical beam emitted from an electronic imaging source. Thereafter, the toner particles deposited on non-­ exposed portions of the imaging member are released and scavenged by an electric field of the opposite gradient to that of the sensitizing nip, thus a toner image is formed on the surface of the imaging member. The magnetic brush developer surface speed is taken much higher than that of the imaging member speed such as preferably by two to four times. As described before, a time is necessary to create a electrostatic latent image in a photoconductive layer or to develop a toner image. Therefore, a contacting length between the imaging member and the magnetic brush developer is indispensable. According to the apparatus of Hays et al., the length appears to be provided by utilizing the flexibility of the imaging member employed, that is, the contact is realized along a contact arc which is obtained by a small dipping of the magnetic brush developer into a relaxingly tensioned flexible belt-like imaging member. Apparently, when a solid flat imaging member, particularly, a photosensitive roller is employed, it may be impossible to have a contacting length between a magnetic brush developer and the imaging member, sufficient to develop an toner image and scavenge the relevant toner particles. Such limitation of the material for the imaging member is undesirable and disadvantageous. SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus and a method for overcoming the above-described disadvantage. It is a further object of the present invention to provide an improved apparatus and a method for the formation of a toner image on a photosensitive medium. It is another object of the present invention to provide a compact and simple electrophotographic apparatus employing a single sensitizing and developing means which is applicable to a photosensitive medium in the form of a drum, flat plate, or a flexible deflected plate. These and other objects of the present invention are accomplished by an electrophotographic printing apparatus according to the present invention. Fig.5 is a block diagram illustrating the principle of a structure and operation of the apparatus. A photosensitive member 1 is composed of a transparent substrate 1a, a transparent electrode 1b, and a photoconductive layer 1c, which are laminated in the recited order one above another. The transparent electrode 1b is electrically grounded. The photoconductive layer has trap potentials underneath its top surface. Photo-carriers of one polarity (in Fig.5, the polarity is assumed to be positive) are generated by the irradiation of an optical beam and trapped by the trap potentials. A conventional magnetic brush developer 2 having a rotatable magnet roller 2a, a stationary sleeve 2b, and magnetic toner chains 5, is placed in rubbing contact with the surface of the photoconductive layer 1c, and has a strip-like, electrically insulated recording electrode 4 which is formed on the sleeve 2b. The photosensitive member 1 is transferred in a direction indicated by an arrow X and the magnetic toner chains 5, namely a magnetic brush, is transferred in a tangential direction indicated by an arrow Y. That is, the photosensitive member 1 and the magnetic toner brush 5 in contact with the photosensitive member 1, move in the directions opposite to each other. A bias voltage of opposite polarity (negative in Fig.5) to that of photo-­ carriers of the photoconductive layer 1c is supplied to the recording electrode 4 from a voltage source 6, and another bias voltage of the opposite polarity (positive in Fig.5) to the preceding bias voltage is applied to the sleeve 2b from a voltage source 7. Single component developing material of conductive toner particles or two-components developing materials containing non-conductive toner particles and magnetic toner carriers are charged with the polarity (negative in Fig.5) opposite to that of the photo-carriers, and supplied to the surface of the photoconductive layer 1c by the magnetic brush developer 2. An optical image source 8 is disposed such that an optical beam emitted therefrom is incident on the rear side, namely the side of the substrate 1a, of the photosensitive member 1 at a spatial portion A which is exactly facing the recording electrode 4 with a predetermined distance therebetween. The optical beam thus projected to the photosensitive member 1 forms an imagewise electrostatic latent image just below the surface of the photoconductive layer 1c. Consequently, the electrostatic field between the electrostatic latent image and the magnetic brush developer 2 is fairy strong. As the photosensitive member 1 and the magnetic toner chains 5 moves in rubbing contact with each other and in opposite directions to each other, an accumulation of the toner particles is caused mechanically in a spatial portion B which is continuously distributed adjacent to the spatial portion A. At the spatial portion A, between the photoconductive layer 1c and the recording electrode 4, the charged toner particles are attracted by the bias voltage supplied from the voltage source 6 to both of the exposed portion and non-exposed portion of the photosensitive member 1, thus forming a solid image. At the exposed portion, namely, the portion corresponding to the electrostatic latent image, a thicker toner particle layer is formed than that of the non-exposed portion because of the strong electrical field generated by the trapped photo-carriers as described above. Subsequently, the solid image is moved to the spatial portion B, that is, just in rubbing contact with the accumulated toner particles, to which a bias voltage of opposite polarity to that of the preceding spatial portion A is applied. Almost of the toner particles deposited on the exposed portion are still attracted by the trapped photo-carriers in the photoconductive layer 1c, even though a small part of the toner particles are released. On the other hand, the toner particles developed on the non-exposed portion are neutralized by the electric field therein and released magnetically from the surface. Thus a visual toner image 9 is formed on the photoconductive layer 1c which is thereafter transferred on a recording sheet (not shown). Electric charges of the electrostatic latent image and of the residual toner particles left on the photosensitive member 1 are gradually discharged until the next printing process starts, and collected magnetically by the magnetic brush developer 2. It is apparent that the spatial portion A is acting as a sensitizing and a first developing region and the spatial portion B a second developing region or a scavenging region. As the accumulation 5a of the toner particles is the feature of the present invention, a more detailed description will be given referring to Fig.6 of a partial, schematic, cross-sectional views of a magnetic brush developer 2. Hereby, the toner particles are assumed to be charged negatively in advance. In the figure, there are illustrated configurations of toner chains when the magnetic toner chains 5 and the relevant photo- sensitive member 1 are in the same direction of movement (Fig.6a) as in a prior art electrophotographic printing apparatus, and in the opposite directions (Fig.6b) as of the present invention. In Fig.6a, toner chains 50 and 51, for example, contacting with the surface of the photosensitive member 1, develop a solid toner image thereon. Normally, the ends of the toner chains 5 extending radially on a sleeve 2b of the magnetic brush developer 2, form a circular. Since there is no accumulation of the toner particles, the toner chains 52 and 53, for example, depart from the surface of the photoconductive layer 1c rapidly after the contact with the surface. Accordingly, there may not be a time sufficient to release or scavenge the toner particles developed on the non-exposed portions of the photoconductive layer 1c. As a result, a clear visual toner image may not be obtained. Apparently, this is ascribed to an insufficient contacting region formed between the photo­ conductive layer 1c and the toner chains 5. As a counter measure can be done, a sufficient contacting region can be achieved only by slightly dipping the outer circle of the toner chains into the photosensitive member 1 as Hays et al. teach, but this can be possible when the photosensitive member 1 is flexible and deflectable. When the photosensitive member 1 is a drum , a solid flat plate, or a strongly tensioned belt-like film, such a counter measure has no effect as pointed out before. On the contrary, as shown in Fig.6b, when the photosensitive member 1 and the toner chains 5 are moved in opposite directions to each other, a quantity of toner particles comprising toner chains 56 through 59 are, for example, accumulated simply being caused mechanically, that is, by the relatively opposed movements of the toner particles chains 5 and the photosensitive member 1. As a result, the contacting range of the toner chains to the photosensitive member 1 is substantially extended. Of cause, the quantity of the accumulated toner particles depends on a distance between the sleeve 2b and the surface of the photoconductive layer 1c. With an adequate dimension of the distance, the range of the contacting region reaches, for example, up to 10 mm. Since a negative voltage is applied to a recording electrode 4, the toner particle chains 54 and 55 are attracted in a direction Q, to the photoconductive layer 1 where an electrostatic latent image is simultaneously formed, and a solid toner image is formed thereon. Therefore, the region between the recording electrode 4 and the photosensitive member 1 in cooperation with bias voltages, can be regarded as a sensitizing and first developing region, corresponding to the spatial portion A of Fig.5. Adjacent to the above-described region, another region of accumulated toner particles which is outside the recording electrode 4, is subject to the opposite electric field generated by a positive voltage of the sleeve 2b. The negatively charged toner particles deposited on non-exposed portions of the photosensitive member 1, are attracted toward the sleeve 2b in a direction P, released from the photosensitive member 1, and recovered into a hopper (not shown) of the magnetic brush developer 2. The toner particles deposited on the exposed portions of the photosensitive member 1, are also attracted toward the sleeve 2b, but are still much more strongly attracted toward the photosensitive member 1 by photo-carriers trapped in the photoconductive layer 1c. As a result, a small part of the toner particles thereon may be released, but almost part of the toner particles deposited on the exposed portion still remain. Thus, by the aid of accumulation of the toner particles, the function region including a sensitizing and first developing region, and a second developing or scavenging region, is extended to have a length sufficient to form a clear toner image on the photosensitive member 1. As a conclusion, the present invention features the accumulation of toner particles, which is formed mechanically by the mutually opposite movement directions of the relevant photosensitive member and magnetic brush. However, the accumulation can be formed magnetically by deforming the configuration of the associated magnetic field by the aid of magnetic pieces. An embodiment of this type is also disclosed. Furthermore, an electrophotographic printing apparatus according to the present invention, having a magnetic brush with a rotatable sleeve is disclosed which is effectively operative when non-conductive toner particles are employed. These features and advantages will be subsequently apparent as more fully hereinafter described and claimed, reference being had to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 to Fig.4 are block diagrams of prior art electrophotographic printing apparatuses; Fig.5 is a block diagram of an electrophotographic printing apparatus according to the present invention for explaining the principle of the structure and its operation; Fig.6a and Fig.6b are respectively a partial schematic cross-sectional view of a prior art magnetic brush developer and that of the present invention; Fig.7 is a block diagram of a first embodiment of the present invention, illustrating its configuration; Fig.8 is a cross-sectional view of a photosensitive film employed in the electrophotographic printing apparatus of Fig.7; Fig.9 is a perspective view of a strip-like recording electrode 15 of Fig.7, illustrating the structure; Fig.10 is a diagram illustrating a relationship between a bias voltage applied to the recording electrode 15 in volt and optical density of printed toner images; Fig.11 is a diagram illustrating the relationship between the distance between the recording electrode 15 and the top surface of the photosensitive film 11 in mm, and optical density of the printed toner images and that of the associated background noise; Fig.12 is a diagram illustrating a relationship between the distance between the recording electrode 15 and the top surface of the photosensitive film 11 in mm, and optical density of the printed toner images and that of background noise, with respect to an experimental printing; Fig.13 is a block diagram of the second embodiment, illustrating the principle thereof; Fig.14(a) and Fig.14(c) are schematic cross-sectional views of a magnetic brush developer, a photosensitive layer and a magnetic piece, illustrating a configuration of the associated magnetic field; Fig.14(b) and Fig.14(d) are schematic cross-sectional views of a magnetic brush developer, a photosensitive layer and a magnetic piece, illustrating a configuration of the associated magnetic toner particle layers; Fig.15 is a schematic cross-sectional view of an electrophotographic printing apparatus of a third embodiment of the present invention, illustrating its principle of configuration; and Fig.16 is a perspective view of a magnetic brush developer of a third embodiment. PREFERABLE EMBODIMENTS OF THE PRESENT INVENTION Fig.7 is a block diagram of a first embodiment of the present invention. A circulating photosensitive film 11 is composed of a transparent substrate 11a made of polyethylene phthalate, 100 µm thick, a transparent electrode 11b formed of ITO (Indium-Tin-Oxide), an organic photoconductive layer, 10 µm thick, composed of a carrier generating layer (CGL) 11c formed of phthalocyanine and carrier transfer layer (CTL) 11d, formed of oxazole, all of which are laminated in the recited order one above another as shown in a cross-sectional view of Fig.8. Although being not illustrated in the figure, the photosensitive member 11 may be covered by a thin protection film layer for protecting the surface of the photosensitive member 11 from a mnechanical damage. The film layer is, for example, 1 µm thick and made of resistive material having a resisivity from 10 10 to 10¹³ Ω cm such as titanium oxide. Of course, a photosensitive member having an insulator layer atop the member as shown in Fig.2 is also applicable. The above described configurations including the protection layer or insulator layer atop the surface of the member are also applicable to the following embodiments. The transparent electrode 11b is electrically grounded, and the photosensitive film 11 is circulated in a direction indicated by an arrow X by a driving roller 12 driven by a driving source (not shown). An ordinary magnetic brush developer 13 comprises a stationary sleeve 13b, a magnet roller 13a which is rotatable in a rotating direction Z inside the sleeve 13b, and a magnetic brush, or toner chains 18, which are caused to rotate in a direction Y. Accordingly, the photosensitive film 11 and the magnetic brush 18 moves in directions opposite to each other at the contacting point thereto. A strip-like recording electrode 15, 1 to 5 mm in width, is attached on the sleeve 13b in parallel with the axis of the sleeve 13b, being insulated by a polyimide film 14 from the sleeve 13b as shown in a perspective view of Fig.9. Since photo-carriers of the photosensitive film 11 are holes, a negative bias voltage ranging from -100 V to -500 V, preferably, from -150 V to -300 V, is applied to the recording electrode 15 from a first voltage source 16. On the other hand, a positive bias voltage ranging from 0 V to +50 V, preferably, from +10 V to +30 V, is applied to the sleeve 13b from a second voltage source 17. Negatively charged conductive magnetic toner particles 18 having a preferable toner resistivity of 10² to 10 10 Ωcm is supplied to the magnetic brush developer 13. An optical image source 19 comprising a LED array 19a, and self-focusing lens array 19b is disposed such that a rear exposure is possible and the optical axis of the LED array 19a is incident perpendicularly on the longitudinal center line of the recording electrode 15. The electrophotographic printing apparatus of the first embodiment further comprises a conventional transfer means including a transferring roller 22 made of a conductive rubber material, a third voltage source 23, a thermal fixer 25, and an optical discharger 28. The third voltage source 23 supplies a bias voltage ranging +200 V to +600 V to the transferring roller 22 which is pressed mechanically toward the relevant printing paper 21 and the photosensitive film 11 supported by a guide roller 12. The printing paper 21 is transferred in a direction W. The printing process is described in the following. The photosensitive film 11 and the magnetic brush 18 are driven in the directions X and Y respectively, forming an accumulation of negatively charged toner particles as illustrated in Fig.6. The recording electrode 15 and the sleeve 13b are respectively negatively and positively biased. Consequently, on the top surface of the photosensitive film 11, there is formed a function region including a region A for sensitizing and a first developing, and a region B for second developing or scavenging of a toner image. Apparently, the region B is positioned adjacent to the region A at a downstream of the movement of the photosensitive film 11. An optical beam emitted from the optical image source 19, is imagewise projected onto the photosensitive film 11 at the region A from the rear side of the photosensitive film, namely from the side of the transparent substrate 11a. Thereby, an electrostatic latent image in the CTL layer 11d just under its top surface thereof, strongly attracts toner particles on the exposed portions. At the same time, the electric field generated by the negative bias voltage attracts the toner particles onto the exposed and non-exposed portions of the photosensitive film 11, forming a solid toner image. As soon as the solid image is advanced into the region B, there applied a bias voltage supplied from the second voltage source 17 to the sleeve 13b, attracting the toner particles of the solid image thereto, releasing the toner particles developed on the non-exposed portions and a small part of those on the exposed portions. The released toner particles are collected and scavenged by the magnetic brush developer 13. Thus, there is formed a visual, clear toner image 20 which is processed in a conventional manner: the toner image 20 is transferred onto the printing paper 21 by the aid of the transferring roller 22 and fixed on the printing paper 21 by the thermal fixer 25 to be a permanent toner image 26. Residual toner particles 27 left on the photosensitive film 11 after transferring the toner image 20, are neutralized by the optical discharger 28 and magnetically collected by the magnetic brush developer 13. The photo-carriers and residual charges contained in and/or on the photosensitive layer 11 are neutralized by the optical discharger 28. Thus, an electrophotographic printing cycle is over. Fig.10, Fig.11, and Fig.12 are diagrams illustrating empirical results with respect to the electrophotographic printing apparatus of Fig.7. The diagram of Fig.10 illustrates a relationship between the bias voltage applied to the recording electrode 15 in volt and optical density of printed toner images, wherein the bias voltage of the sleeve 13a is +15 V, the resistivity of the relevant toner particles is 10⁶ Ωcm, transferring speed of the photosensitive film 11 is 5 cm/sec, the width of the recording electrode 15 is 3 mm, and the distance between the surface of the photosensitive layer 11 and the recording electrode 15 is 0.35 mm. As can be seen from the diagram, recording voltage in absolute value not lower than 150 V is sufficient to assure a high quality printing image having an OD value higher than 1.0, being accompanied by no background noise. This implies that the accumulation of the toner particles has a length in the transferring direction sufficient to provide an electric field for scavenging and a mechanically rubbing effect to the photo­ sensitive layer 11 in region B in order to collect the toner particles to be scavenged. Fig.11 is a diagram illustrating a relationship between the distance between the recording electrode 15 and the top surface of the photosensitive film 11 in mm, and optical density of the printed toner images and of the associated background noise. As the distance is increased exceeding 0.45 mm, the OD value of the background noise increases very rapidly. This implies that a too large distance fails to have an accumulation of the toner particles, resulting in looseing a capability of collecting the toner particles to be scavenged. Furthermore, an experimental printing is carried out regarding the electrophotographic printing apparatus of the first embodiment, wherein the photosensitive film 11 and the magnetic brush, or toner chains 18 are moved in the same direction at the region A in the same manner as the apparatus of Hays et al.. Thereby, no accumulation of the toner particles occurs at the regions B. Fig.12 is a diagram illustrating a relationship between the distance between the recording electrode 15 and the top surface of the photo­ sensitive film 11 in mm, and optical density of the printed toner images and caused background noise with respect to the above-described experimental printing. For the distance ranging from 0.25 mm to 0.45 mm for which a desirable results are obtained in the preceding experiment as shown in Fig.11, a substantially undesirable results are obtained. That is, the OD values of the printed toner images and that of the associated background noise are almost the same. This means that there are no contrast on the printing paper, and is ascribed to the lack of the accumulation of the toner particles for scavenging the relevant toner particles. The results described above contradicts to the results of Hays et al.. The contradiction is considered to be ascribed to the fact that unlike the electrophotographic printing apparatus of Hays et al., the photosensitive film 11 is tensioned tightly allowing little presence of the contact arc between the photosensitive film 11 and the magnetic brush 18. The photosensitive member 11 of the above-described first embodiment is assumed to be a belt-like flexible photosensitive film as shown in Fig.7. However, the present invention is applicable to a printing apparatus having a photosensitive member in a shape of a solid plane, or a drum. In fact, with respect to a modified electrophotographic printing apparatus of the first embodiment having a photosensitive printing drum of 142 mm in outer diameter and a magnetic brush developer having a sleeve of 30 mm in outer diameter on which toner chains 0.5 mm long, almost the same experimental results are obtained as those illustrated in Fig.10 to 12. Meanwhile, an electrophotographic printing apparatus of the first embodiment is required an accurate setting of the gap distance, such as from 250 µm to 500 µm, between the photosensitive film 11 and the recording electrode 15, requesting the associated operator a delicate adjustment and maintenance skill of the apparatus. This may be a disadvantage. There is disclosed an electrophotographic printing apparatus of a second embodiment of the present invention for overcoming the above-described disadvantage of a first embodiment. Fig.13 is a block diagram of a printing apparatus of the second embodiment, illustrating the principle thereof. In comparison with the apparatus of the first embodiment, the apparatus of the second embodiment additionally includes a magnetic field modulating means 30 which is a rectangular magnetic piece, for example, and is placed at a counter side of the magnetic brush developer 2 with respect to the photosensitive member 1. Consequently, the shape of the magnetic flux lines emanated from the rotating magnet roll 2a is modulated such that these magnetic flux lines are concentrated near around the magnetic piece 30. The resulting magnetic flux and toner particles distribution is illustrated in schematic cross-sectional views of Fig.14(a) to FIg.14(d) wherein like reference numerals denote like parts in accordance with Fig.5. As can be seen from Fig.14(a), magnetic flux lines 31 in a loop shape emanated from the magnet roll 2a, are attracted to the magnetic piece 30, being concentrated to the edges of the magnetic piece 30. As a result, the magnetic flux lines 32 passing through a region corresponding to the region B indicated in Fig.5 are also densely concentrated in the region. Consequently, bundles of magnetic flux lines elongated and densely formed toner chains are formed as illustrated in a schematic cross-sectional view of Fig.14(b). Thus, a sensitizing and first developing region A' and a second developing or scavenging region B' positioning at a downstream of movement of the photosensitive member 1 from the region A', are formed by the aid of magnetic field deformed by the magnetic piece 30. Apparently, these accumulations of the toner particles are formed magnetically, not mechanically. In the second embodiment, therefore, the directions of movement of the photosensitive member 1 and the magnetic brush 5 or the magnetic brush developer 2 are not limited in any manner. In addition, the distance between the recording electrode 4 and the photosensitive member 1 is not critical. In fact, with the distance thicker than 450 µm, there occurs no background noise. The distance of 1.5 mm is confirmed to be useful. Of course, the magnetic piece 30 is not limited to one piece, or a rectangular one. Any magnetic piece effective to form an adequate magnetic field for forming the region A' and region B' is applicable. Furthermore, by placing another magnetic piece 32 as illustrated by dotted lines in Fig.13, another region C' of accumulation of toner particles is formed, as shown in Fig.14(d). The region C' is located upstream of the movement of the photosensitive member 1 from the region A', and is useful for collecting residual toner particles still remaining on the photosensitive member 1 after image transfer from the photosensitive member 1 to a printing paper (not shown). Fig.14(c) illustrates the relevant magnetic field configuration including another bundle of magnetic flux lines 33. The accumulations of toner particles, or the region A', region B', or region C' may be formed closely contacted to each other or separated from each other. The essential condition required for printing is that each region should have a length sufficient to provide a time for implementing the assigned functions such as sensitizing, developing, or scavenging. Fig.15 is a schematic cross-sectional view of an electrophotographic printing apparatus of a third embodiment of the present invention, illustrating the principle of its configuration. With respect to Fig.1, like reference numerals designate like parts. Before proceeding further, a brief description of toner particles will be provided. As described before, there are two types of the toner particles currently employed in the present invention: single-component magnetic conductive toner particles, and two-components magnetic non-conductive toner particles. The single-component magntic conductive toner particles is applicable to the printing apparatuses of the first embodiment and the second embodiment wherein a stationary sleeve is used, because toner chains are easily formed by a fairly low magnetic field intensity generated by a rotating magnet roller of the relevant magnetic brush developer. However, under a rather humid environment, a plain paper can not be used because the moisture in the air reduces electrical resistance of the surface of a printing paper. As a result, the electric charges of the toner image and the printing paper is discharged accompanied by a flowed toner image flow, and a low quality toner image. Accordingly, conductive toner particles are not suitable for obtaining a high quality printing images when a plain paper is used as a recording sheet. On the other hand, the use of non-conductive toner particles has an advantage enabling the use of plain recording papers, because the toner charges are secured by an insulative thin film covering the surface of the toner particles regardless the electrical surface resistance of the printing paper. Instead of this advantage, the non-conductive toner particles must be transferred by the aid of toner carriers with which the non-conductive toner particles are mixed uniformly in order to distribute the toner particles over each carrier particles. The carriers are usually small ball-shaped particles of approximately 10 to 15 µm in diameter. The materials of the carriers are iron, ferrite, etc. for carriers of high magnetism, and magnetic resins for low magnetism. With the low magnetism carriers, toner chains or magnetic brush are easily rotated in accordance with the magnetic field generated by the rotating magnet roller. Consequently, a stationary sleeve is usable, however, there is a disadvantage that the carriers tend to be transferred onto the associated photosensitive member, causing background noise thereon. To the contrary, carriers with high magnetism are not transferred onto the photosensitive member. However, chains of carriers are not rotated by the rotating magnetic field. This is considered that the magnetizing direction of high magnetism particles changes quickly in response to the change of external magnetic field. In addition, the non-conductive toner particles attached to the carriers tend to be shifted toward the sleeve of the magnetic brush developer, forming a layer of the toner particles thereon. Therefore, the sleeve is prepared rotatably in the magnetic brush developer in order to rotate the carrier chains, and scrape the layer of the toner particles occasionally by the aid of the associated cutter blade. This configuration of the magnetic brush developer is well-known. In this case, the recording electrode proposed in the first and second embodiments of the present invention, is not applicable, because the electrode is forced to rotate with the sleeve. In order to overcome the above described problem, a specially designed recording electrode is disclosed in an electrophotographic printing apparatus of a third embodiment. Fig. 15 is a cross-sectional block diagram illustrating the principle of the third embodiment. Except for a rotatable sleeve 42b, recording electrodes 40a, 40b, and the associated feeding means 43, 44, the configuration of Fig.15 is the same as that of Fig.5, wherein like reference numerals denote like parts. A magnetic brush developer 42 includes a rotatable magnet roller 2a and a rotatable sleeve 42b arranged co-axially. On the surface of the sleeve 42b, a plurality of stripe-like recording electrodes 40 are formed mutually in parallel in the longitudinal direction of the sleeve 42b with a predetermined circular pitch. Each electrode is electrically isolated from the sleeve 42b by an insulator film. In line with the recording electrodes 40, a recording contact terminal 43 and a second developing or scavenging terminal 44 are arranged as shown in Fig.15. Both contact terminals are formed stationary. The contact terminal 43 is for feeding a recording bias voltage supplied from a recording bias voltage source 41 to a recording electrode 40 which comes into a region A, a sensitizing and first developing region as described before (the electrode 40 is designated by a reference numeral 40a for clarity). The contact terminal 44 is for feeding a scavenging bias voltage supplied from a scavenging bias voltage source 41' to a recording electrodes 40 other than the electrodes 40a (the electrodes are designated by a reference numeral 40b). The polarity of the recording bias voltage (negative voltage in Fig.15, for example) is opposite to that of photo-carriers of the relevant photoconductive layer lc. The recording bias voltage is applied to the electrode 40a through the contact terminal 43. The scavenging bias voltage having the same polarity as that of the photo- carriers are applied to the electrodes 40b through the contact terminal 44. At a portion of a photosensitive film 1 facing the region A, an electrostatic latent image is formed by an imagewise rear exposure to an optical beam which is emitted from an optical image source 8 including laser optical system, LED array optical system, or liquid crystal shutter optical system, etc. Two-components toner particles including carriers of ferrite particles of 10 µm in average diameter and non-conductive toner particles, are supplied to the magnetic brush developer 2 and transferred into a region A existing between the photosensitive member 1 and the magnetic brush developer 2. When a recording electrode 40 comes into the region A and is in contact with the contact terminal 43, a recording bias voltage ranging from -100 V to -700 V, preferably from -500 V to -600 V, is applied to the region A, depositing toner particles on both exposed and non-exposed portions of the photosensitive layer 1. Thus the first developing is carried out. However, if the recording electrode 40 is not transferred in the region A, the first developing is not implementable. The formation of the electrostatic latent image, therefore, is desirable to be performed synchronizing with the movement of the recording electrodes, namely, with the rotation of the sleeve 2b, in order to achieve a high quality printed toner image. Of course, the electrodes 40 and the contact terminal 43 can be designed such that always one or two recording electrodes 40 are activated. Thereby, the synchronization described above might be unnecessary, however the recording electric field applied to the region A may swing cyclically resulting in rather lower printing quality. Subsequently, the second developing is carried out in the region B under the application of a bias voltage ranging from 0 to +100 V, preferably from +20 to +50 V to the contact terminal 44. The following printing process is the same as described with the first and second embodiments. The recording electrodes and the relevant contact terminals described above are applicable to an electro­ photographic printing apparatus according to the present invention, which employ a rotatable sleeve of the associated magnetic brush developer. Thus two-components toner particles including carriers made of ferromagnetic material become available achieving a high quality printing images on a plain paper. Fig.16 as a perspective view of a magnetic brush developer of a third embodiment. A sleeve 154 and a magnet roller 155, are arranged co-axially, and both are rotatable. Stripe-like recording electrodes are attached to the surface of the sleeve 154, extending in the longitudinal direction of the sleeve 154, and being electrically isolated from the sleeve 154 and from other recording electrodes through insulating films151 of polyimide which are individually inserted between each recording electrode 150 and the sleeve 154. Of course, there may be used an insulating layer covering whole cylindrical surface of the sleeve 154, commonly isolating the recording electrodes 150. A recording contact terminal 152, desirably made of a conductive spring material such as phosphor copper is disposed in contact with one of the recording electrodes 150, which is designated as 150a. A scavenging contact terminal 153 is a ring-like elastic electrode made of phosphor copper. A portion of the ring is cut away, allowing the presence of a portion of the recording contact terminal 152 which contacts directly to the recording electrode 150a. The scavenging contact terminal 153 contacts to the recording electrode 150b through contacting furs 156 which are planted on the inner surface of the terminal ring. The top surfaces of the recording electrodes 150b are bridged to the scavenging terminal 153. These furs 156 are made of conductive rayon fibers, carbon fibers, and metal fibers. Although the recording electrodes150 shown in the figures, are disposed on the sleeve 154 projecting from the surface of the sleeve 154, it is desirable that the recording electrodes 150 are embedded into the sleeve 154 such that the top surfaces of the recording electrodes 150 are just on the cylindrical outer surface of the sleeve 154. This is for the convenience of scraping attached non-conductive toner particles layer which is generated during a long operating time, from the surface of the sleeve 154. In the above description, there have been described electrophotographic printing apparatuses according to the present invention, however, the invention is also applicable to a display device wherein an optical image is formed directly on a semi-transparent screen. The present invention has been described referring to several embodiments, however, the present invention permits various modifications within the scope the subject matter of the present invention. Since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described and accordingly, all suitable modifications and equivalents may restored to, falling within the scope of the invention and the appended claims and their equivalents.

Claims: 1. An electrophotographic printing apparatus comprising: a photosensitive member (1, 11) including a transparent or semi-transparent electrode (1b, 11b) and a photoconductive layer (1c, 11c) formed thereon; a developing means (2, 13) comprising a sleeve (2b, 13b) and a magnetic brush (5, 18) made of developing material, said magnetic brush being in rubbing contact with the top surface of said photosensitive member (1, 11); a recording electrode (4, 15) being disposed on said sleeve (2b, 13b) and electrically insulated from said sleeve (2b, 13b), forming a first region (A, A') between the top surface of said photosensitive member (1, 11); an accumulation (5a) of developing material formed in a second region (B, B') located at a downstream position from said magnetic developing means (2, 13) with respect to the movement of said photosensitive member (1, 11); a first voltage source (6, 16) for supplying a bias voltage of a first polarity with respect to said transparent electrode (1b, 11b) to a recording electrode (4, 15); a second voltage source (7, 17) for supplying a bias voltage of said second polarity with respect to said transparent electrode (1b, 11b) to said sleeve (2b, 13b), said second polarity being opposite to said first polarity; and an optical imaging source (8, 19) for emitting an optical beam, said optical beam imagewise exposing the surface of said photosenstive member (1, 11) which is facing said recording electrode (4, 15) from the side of said transparent electrode (1b, 11b). 2. An electrophotographic printing apparatus recited in claim 1, wherein said magnetic brush (2, 13) made of said developing material is transferred in a direction which is opposite to the movement of said photosensitive member (1, 11), in said first region (A, A'). 3. An electrophotographic printing apparatus recited in claim 1 or 2, wherein said accumulation (5a) of developing material is formed by a movement of said photosensitive member (1, 11) moving in a first direction and a movement of said magnetic brush (2, 13) in a second direction which is opposite to said first direction under a condition that the distance between said recording electrode (4, 15) and the top surface of said photosensitive member (1, 11) is smaller than a predetermined value. 4. An electrophotographic printing apparatus recited in claim 3, wherein said developing materials are selected from single-component toner particles comprising magnetic conductive toner particles and two-components particles including non-conductive toner particles and magnetic toner carriers, and said sleeve (2b, 13b) is stationary and made of non-magnetic material. 5. An electrophotographic printing apparatus recited in claim 1, wherein said electrophotographic printing apparatus further comprising: a magnetic piece (30) for modulating the magnetic field generated by a magnet roller (2a) which is rotatably provided in said sleeve (2b) co-axially, said magnetic piece (30) being disposed at the opposite side to said recording electrode (4) with respect to said photosensitive member (1), facing said magnetic developing means (2), and is arranged in the manner to allow the passage of said optical beam. 6. An electrophotographic printing apparatus recited in claim 5, wherein said accumulation of developing material is formed by said modulated magnetic field, being densely concentrated over said second region (B') of said photoconductive layer (1). 7. An electrophotographic printing apparatus recited in claim 5 or 6, wherein an additive accumulation of developing material is further formed in contact with the top surface of said photosensitive layer (1), said additive accumulation being located at a position upstream from said first region (A') of movement of said photosensitive member (1) and in the proximity of said first region (A'). 8. An electrophotographic printing apparatus recited in claim 7, wherein by said additive accumulation of said developing material, residual developing material remaining on the top surface of said photosensitive member (1) are eliminated to clean said top surface. 9. An electrophotographic printing apparatus recited in claim 1, wherein said first reion (A') and said second region (B') are connected to each other. 10. An electrophotographic printing apparatus recited in claim 1, wherein said developing material are two-components toner particles including non-conductive magnetic toner particles and ferromagnetic toner carriers and said sleeve (2b) of said magnetic developing means (2) is rotatable and made of non-magnetic material. 11. An electrophotographic printing apparatus recited in claim 10, wherein said electrophotographic printing apparatus further comprising: a plurality of stripe-like recording electrodes (40, 40a, 40b) which are disposed on the surface of said rotating sleeve (42b) in parallel with the axis of said rotating sleeve (42b), and individually insulated electrically from said sleeve (42b) and from each other; a first stationary terminal (43) electrically contactable to one of said recording electrodes (40, 40a, 40b) locating in said first region (A'); and a second stationary terminal (44) having a ring-like shape surrounding and being capable of contacting electrically to said recording electrodes (40, 40a, 40b) except for the recording electrode in contact with said first stationary terminal. 12. An electrophotographic printing apparatus recited in claim 11, wherein said second terminal (44) contacts to said recording terminals (40, 40a, 40b) through conductive fur materials planted on the inner surface of said second stationary terminal (44). 13. A method of an electrophotography employing an electrophotographic apparatus having a single magnetic developing means (2, 13, 42), thereby an accumulation (5a) of developing material are formed at a downstream position from said magnetic developing means (2, 13, 42) with respect to the movement of an associated photosensitive member (1, 11) by simultaneously transferring said photosensitive member (1) and a magnetic brush (5, 18) of said magnetic developing means (2, 13, 42) in the mutually opposite directions and in rubbing contact therebetween, applying a first bias voltage to a recording electrode (4, 15) arranged on a portion of a sleeve (2b, 13b, 42b) of said developing means (2, 13, 42) facing said photosensitive member (1, 11), applying a second bias voltage having an opposite polarity to that of said first bias voltage to said sleeve (2b, 13b, 42b), imagewise projecting an optical beam onto a portion of said photosensitive member (1, 11) from the opposite side of said magnetic developing means (2, 13, 42), thus forming an electrostatic latent image in said photosensitive member (1, 11), forming a solid image of said developing material by the aid of said developing material in contact with said photosensitive member (1, 11) scavenging said developing material on non-exposed portions of said photosensitive member (1, 11) into said developing means (2, 13, 42) by the aid of said accumulation (5a) of said developing material, and finally obtaining a developing particle image on said photosensitive member (1, 11).

Inventors: KIMURA, MASATOSHI, NAKAJIMA, JUNZO, SASAKI, SACHIO, WANOU, MASAHIRO
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: G03G15
IPC codes: G03G15