Charging member with a bridging electrode structure and charging device using same in a detachable process unit in an image forming apparatus

Abstract: A charging member for charging a member (1) to be charged includes a blade member (2), a supporting member (4) which supports the blade member (2), and an electrode layer (3) which is provided on the blade member (2) and is connected to the supporting member (4). The electrode layer (3) is formed after connecting the blade member (2) to the supporting member. A charging device uses the charging member. A process unit including the charging device is detachable relative to an image forming apparatus.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a charging member for charging a member to be charged, an image forming apparatus, such as an electrophotographic apparatus or the like, having the charging member, and a process unit detachable relative to the apparatus. 2. Description of the Related Art A corona discharger, such as a corotron, a scorotron or the like, having a wire electrode and a shield electrode surrounding the wire electrode and having an excellent charging uniformness has been widely used as a means for uniformly charging the surface of an image carrying member, such as a photosensitive member, a dielectric member or the like, serving as a member to be charged in an image forming apparatus, such as an electrophotographic copier, an electrophotographic printer, a recording apparatus or the like. However, the corona discharger has the following problems: an expensive high-voltage power supply is needed. Space is needed for the charger itself, as shield space for the high-voltage power supply, and the like. A large amount of corona products, such as ozone and the like, are produced, and hence additional means and mechanisms are needed in order to deal with the corona products. These factors result in a large and expensive apparatus. In consideration of the above-described problems, the adoption of a contact charging method has recently been studied as an alternative to the corona discharger. In contact charging, by contacting a contact charging member, to which a voltage (for example, a DC voltage of about 1 - 2 kV (kilovolts), or a superposed voltage composed of a DC voltage and an AC voltage) is applied from a power supply, to the surface of an image carrying member, serving as a member to be charged, the surface of the image carrying member is charged at a predetermined potential. Various contact charging methods have been devised, for example, a roller charging method (Japanese Patent Application Public Disclosure (Kokai) No. 56-91253 (1981)), a blade charging method (Japanese Patent Application Public Disclosure (Kokai) Nos. 56-104349 (1981) and 60-147756 (1985)), and a charging-and-cleaning method (Japanese Patent Application Public Disclosure (Kokai) No. 56-165166 (1981)), (U.S. Patent No. 4,387,980 corresponds to Japanese Patent Document Nos. 56-91253 and 56-104349.). Among such contact charging methods, the blade charging method is particularly effective for a small image forming apparatus because it provides for an inexpensive and compact apparatus. One of the problems associated with the contact charging methods including the blade charging method is as follows: if a pinhole portion (a surface defect portion in a member to be charged) is present in an image carrying member, such as a photosensitive member or the like, a spark discharge is apt to occur between a contact charging member, to which a voltage is applied, in contact with the surface of the image carrying member in order to charge the suface of the image carrying member and the pinhole portion in the image carrying member. If such discharge occurs once, a so-called "charge leak" phenomenon will easily subsequently occur on the surface of the image carrying member wherein charged electric charges are not held not only on the pinhole portion but also over the entire surface (the direction of the generatrix of a rotating image carrying member) of the charged region including the pinhole portion in contact with the contact charging member. For purpose of baekground information, FIGS. 6(A) and 6(B) illustrate a model for explaining the charge leak phenomenon. In FIG. 6(A), a photosensitive member 1 serves as an image carrying member (a member to be charged) whose surface moves in the direction of the arrow. Pinhole portions P are present in the photosensitive member 1. A blade member 2 (hereinafter termed a "charging blade") of a contact charging member to which a voltage is applied is in contact with the surface of the photosensitive member 1 in order to charge the surface. FIG. 6(B) is an equivalent circuit of FIG. 6(A). The pinhole portions P in the photosensitive member 1 have lower resistance values than other portions. Hence, when the charging blade 2 contacts the pinhole portions P or the surface of the charging blade 2 comes close to the pinhole portions P, spark discharges S are apt to occur between the charging blade 2 and the pinhole portions P. When the discharges S occur, potentials V A , V B , --- V z at respective portions applied on the surface of the photosensitive member 1 in the direction of the longitudinal direction of the photosensitive member 1 (the direction of the generatrix of the photosensitive member 1) become almost 0 V (volt). As a result, electric charges cannot be held on the surface of the photosensitive member 1 over the entire surface of the contact charging region including the pinhole portions P in contact with the charging blade 2. When the above-described charge leak portions are produced in the charging processing of the surface of the photosensitive member 1, image portions corresponding to the charge leak portions in an output image appear as white stripes in normal development and black stripes in reversal development, causing deterioration in image quality. The pinholes P are apt to be produced, for example, during the production of an image carrying member (a member to be charged), such as a photosensitive member or the like, due to scratching, or due to dielectric breakdown. It is rather difficult to completely eliminate pinholes. In order to prevent the above-described charge leaks, it is necessary to increase the electric resistance of the charging blade material. Since the charging blade 2 is pressed with a proper pressure utilizing rubber elasticity, the distance (the free length of the blade) between the distal end of a rigid blade supporting member made, for example, of sheet metal and a portion of the charging blade 2 in contact with the member to be charged must be considerably larger than the thickness of the charging blade 2. Accordingly, when a voltage is applied from the blade supporting member to the charging blade 2, the voltage drop in the charging blade 2 becomes large, causing a decrease in the potential of the portion of the charging blade 2 in contact with the member to be charged. Hence, it is necessary to attach a back electrode to the charging blade 2, but there has been no excellent means for producing a charging blade having a back electrode. Moreover, document EP-A-0 308 185 see preamble of claims 1, 7, 14 and 20 discloses a charging device wherein a charging member is constituted of a resistive blade member being in contact with a member to be charged and a conductive member arranged between the blade member and a supporting member. SUMMARY OF THE INVENTION The present invention has been made in consideration of the above-described problems. According to various aspects, the present invention pertains to a charging member as defined in claim 1, a process unit as defined in claim 7, an electrophotographic image forming apparatus as defined in claim 14, and a method for making a charging member as defined in claim 20. These and other objects and features of the present invention will become more apparent from the following detailed description taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a charging blade portion of a contact charging means; FIG. 2 is a schematic diagram of an image forming apparatus incorporating a contact charging means using a charging blade; FIG. 3(A) is a diagram illustrating how charging blades are formed; FIG. 3(B) is an enlarged view of a cut distal-end portion of a charging blade; FIG. 3(C) is a diagram showing a state wherein a coated electrode-layer material has moved on a side end of the cut distal-end portion of the blade; FIG. 4(A) is a diagram illustrating another example of the configuration of a charging blade; FIG. 4(B) is a diagram showing a state wherein an electrode-layer material has moved on a side-end portion of the blade; FIGS. 5(A) and 5(B) illustrate still another example of the configuration of a charging blade; FIG. 6(A) is a model diagram for explaining a charge leak phenomenon; and FIG. 6(B) is an equivalent circuit of FIG. 6(A). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be explained with reference to the drawings. FIG. 2 is a schematic diagram of the configuration of a principal part of an image forming apparatus which incorporates a contact charging means using a contact charging member according to the present invention as the charging processing means for an image carrying member. In FIG. 2, a rotating-drum-type electrophotographic photosensitive member (termed hereinafter a "photosensitive drum") serves as an image carrying member. The photosensitive drum 1 is composed of an organic photoconductive layer la which is a surface layer, and a grounded conductive substrate 1b made, for example, of aluminum for supporting the organic photoconductive layer 1a. The photosensitive drum 1 is rotatably driven in the clockwise direction as shown by arrow A at a predetermined circumferential speed (process speed). The photosensitive drum 1 is uniformly charged at a predetermined polarity and a predetermined potential during its rotation by a charging blade 2 serving as a contact charging member of a contact charging device (to be described later). Subsequently, the charged surface of the photosensitive drum 1 is subjected to exposure L (for example, exposure by an analog optical system for imaging and exposing the image of an original, scanning exposure by a digital optical system including a laser-beam scanner, an LED array or the like) in accordance with object image information at an exposing portion. Thus, an electrostatic latent image corresponding to the object image information is formed. The formed latent image is then subjected to normal or reversal development using toner by a developing unit 7. On the other hand, a transfer material Pa is fed from a paper feed mechanism (not shown), and is supplied to a space (transfer portion) between the photosensitive drum 1 and a transfer roller 8 (for example, a corona charger may also be used), serving as transfer means, with a predetermined timing by registration rollers 10. The developed image formed on the photosensitive drum 1 is sequentially transferred to the fed transfer material Pa. The transfer material Pa passing through the transfer portion is separated from the surface of the photosensitive drum 1, and is guided into a fixing unit (not shown) by feed means 11. The image on the transfer material Pa is fixed in the fixing unit. Unnecessary particles remaining on the surface of the photosensitive drum 1 after image transfer are removed by a cleaning unit 9, and the photosensitive drum 1 is repeatedly used for forming images. The image forming apparatus of the present embodiment is constituted as a process unit 6 wherein the four process devices, that is, the photosensitive drum 1, the charging blade 2, the developing unit 7 and the cleaning unit 9, are incorporated as a unit with a predetermined mutual positional relationship. The process unit 6 can be mounted by inserting it into the main body of the image forming apparatus along supporting rails 12, 12' in the direction perpendicular to the plane of FIG. 2. The process unit 6 is also detachable from the main body of the image forming apparatus. The process unit 6 may comprise the photosensitive drum 1 and the charging blade 2. By sufficiently inserting the process unit 6 within the main body of the image forming apparatus, the main body of the apparatus and the process unit 6 are mechanically and electrically coupled with each other, and the image forming apparatus assumes an operable state. FIG. 1 is a model diagram of the contact charging device portion of the image forming apparatus shown in FIG. 2. The substrate of the charging blade 2 is, for example, an elastic rubber blade 2a which is 1 - 2 mm thick and which is made of hydrin, EPDM (ethylene/propylene/diene terpolymer), urethane or the like whose volume resistivity is controlled to about 10 7 - 10 9 Ω·cm. The base portion of the charging blade 2 is mounted on a conductive rigid supporting member 4, made of a steel plate or the like, as a unit using an adhesive or the like. Alternatively, the charging blade 2 and the supporting member 4 are molded and held as a unit by injecting the blade material into a metal mold. By setting the free length ℓ (the distance between the distal end of the blade supporting member and the portion of the charging blade 2 in contact with the photosensitive drum 1) of the charging blade 2 to about 5 - 15 mm, the contact angle  (the angle made by the distal end of the charging blade 2 and the downstream tangent line from the contact point of the charging blade 2 with the photosensitive drum 1 in the direction of the movement of the surface of the photosensitive drum 1 at the contact point) relative to the photosensitive drum 1 to about 8° - 25°, and the contact pressure to about 0,4-4 kg/m (4 - 40 gr/cm), the distal end of the charging blade 2 contacts the photosensitive drum 1 in the counter direction (the contact angle is an acute angle) relative to the rotation of the photosensitive drum 1. The contact of the charging blade 2 with the photosensitive drum 1 may also be in the forward direction (the contact angle is an obtuse angle) relative to the rotation of the photosensitive drum 1. On a surface (i.e., the back of the charging blade 2) opposite to a surface in contact with the photosensitive drum 1 is formed a back electrode 3 by printing with a conductive paint. The back electrode 3 bridges the conductive rigid supporting member 4 and the supporting charging blade 2 such that the two are electrically connected. As described above, the contact charging member includes the charging blade 2 having the rubber blade 2a and the electrode layer 3, and the supporting member 4. A power supply 5 for applying a voltage to the charging blade 2 applies to the conductive rigid supporting member 4 of the charging blade 2, for example, a DC voltage corresponding to a potential necessary for the photosensitive drum 1, or a bias voltage obtained by superposing an alternating voltage having a peak-to-peak voltage at least twice the discharge starting voltage (V TH ) determined from the charging blade 2 and the photosensitive drum 1 with the DC voltage in order to obtain uniform charging. The above-described superposed bias voltage is a voltage whose value periodically changes. It may, for example, be a sinusoidal-wave AC voltage, or a rectangular-wave AC voltage which is formed by periodically switching on and off a DC power supply. As described above, by applying a bias voltage to the conductive rigid supporting member 4, a voltage is applied to the charging blade 2 via the supporting member 4 and the back electrode 3 electrically connected thereto. As a result, an electric field is produced at the contact portion between the charging blade 2 and the photosensitive drum 1, and the surface of the photosensitive drum 1 is thereby uniformly charged at a predetermined polarity and a predetermined potential. In FIG. 3(A), the rubber blade 2a, serving as the substrate of the charging blade 2, is sized to provide two sheets of charging blades having a predetermined size. If the rubber blade 2a is cut along its longitudinal central axis C - C, two charging blade substrates having the predetermined size are obtained. Conductive rigid supporting members 4, 4' are connected to the left and right side portions of the rubber blade 2a having the size for two sheets as one body symmetrically relative to the axis C - C using an adhesive or the like. Subsequently, the back electrode 3 having a volume resistivity of 10 2 - 10 3 Ω · cm and having a cross-like patterned region, as shown by hatching, is formed as a layer symmetrically relative to the longitudinal central axis C - C by printing with a conductive paint made, for example, of polyurethane and the like on the back of the rubber blade 2a. Any material having a volume resistivity of 10 5 Ω · cm or less may be used for the back electrode 3. In this case, the supporting members 4, 4' and the back electrode 3 are electrically connected by forming part of the back electrode 3 so as to extend on respective surfaces of the previously connected supporting members 4, 4'. After the above-described printing process, two charging blades 2 are obtained by cutting the rubber blade 2a along the longitudinal central axis C - C. As described above, by forming the back electrode 3 as a layer over the supporting members 4, 4' from the back of the rubber blade 2a after connecting the rubber blade 2a to the supporting members 4, 4', the supporting members 4, 4' and the rubber blade 2a can be electrically connected at the same time as the back electrode 3 is provided. Hence, the production process of the charging member is simplified. If the charging member is formed by connecting the back electrode 3 and the supporting member 4 so as to superpose with each other after forming the back electrode on the rubber blade 2a, and the photosensitive drum 1 is rotatably moved while contacting the charging blade 2 to the photosensitive drum 1, the connected portion may easily peel according to a state wherein the back electrode 3 is formed, or the positional accuracy of the charging blade 2 or the back electrode 3 relative to the photosensitive drum 1 may decrease. Moreover, if an adhesive is used for connecting the back electrode 3 and the supporting member 4 by superposing them, the adhesive must be conductive. Hence, the usable range of adhesives is limited. Accordingly, by forming the back electrode 3 over the rubber blade 2a and the supporting members 4, 4' after connecting the rubber blade 2a to the supporting members 4, 4', as described above, the connecting force between the rubber blade 2a and the supporting member 4 can be stabilized for a long period, and the positional accuracy of the charging blade 2a relative to the photosensitive drum 1 increases. As a result, it is possible to perform stable charging. In addition, the usable range of adhesives is increased. By cutting the rubber blade 2a after forming the back electrode 3 as described above, an excellent finish accuracy for the cut surface C 1 of the rubber blade 2a can be obtained, and the charging blade 2 can be provided without having an electrode-layer material deposited on the portion of the charging blade 2 in contact with the member 1 to be charged. The back electrode 3 need not be formed on the entire surface of the back of the rubber blade 2a, but it is sufficient if there are a back portion of the rubber blade 2a corresponding to the distal-end portion of the rubber blade 2a in contact with the member 1 to be charged, and a connecting portion for electrically connecting that portion to the supporting member 4, serving as the voltage supply side, as the T-like pattern (the pattern after cutting along the axis C - C) in the present embodiment. If a back electrode 3 1 is formed as a layer by coating an electrode-layer material on the back of the rubber blade 2a after the connection/cutting, as shown in FIG. 3(B), electric charge leaks may occur in some cases, for example, due to the movement of the coated electrode-layer material in the neighborhood of the contact portion, as shown by reference numeral 32 in FIG. 3(C). In the case of FIG. 3(B), even if the back electrode 3 is formed on the back of the rubber blade 2a with highly accurate printing, a case may arise wherein the electrode-layer material moves on one or both of the right and left end portions, as shown by reference numeral 3 3 in FIG. 4(B). The presence of such moved electrode-layer material 3 3 may cause other types of electric charge leaks. In order to prevent such a problem, it is effective to previously form the pattern of the back electrode 3 so that the width T 2 of the charging blade 2 is larger than the width T 1 of the electrode layer in the direction of the generatrix (the longitudinal direction) of the photosensitive drum 1, as shown in FIG. 4(A). Alternatively, as shown in FIG. 5(A), the width of the rubber blade 2a may be increased by α and α at its right and left end sides, respectively, the supporting members 4, 4' may be attached to the rubber blade 2a, and the back electrode 3 may then be formed. Subsequently, the rubber blade 2a may be cut along its central axis C - C, and the extra widths α and α at the right and left end sides may be removed by cutting along lines U 1 - U 1 and U 2 - U 2 . Thus, an excellent finish accuracy for the right and left end sides of the rubber blade 2a can be obtained, as shown in FIG. 5(B), and it is possible to eliminate the trouble of electric charge leaks due to the movement of the electrode-layer material onto the right and left end sides. The pattern of the back electrode 3 in the embodiment shown in FIGS. 5(A) and 5(B) has a ]-like shape composed of a portion along the distal-end side of the rubber blade 2a and portions along the right and left end sides of the rubber blade 2a on the back of the rubber blade 2a. The back electrode 3 may be coated on the entire surface of the back of the rubber blade 2a. For example, the back electrode 3 may be first formed on the entire surface of the back of the rubber blade 2a using spray coating followed by the above-described cutting process. In the case of providing two sheets, the rubber blade 2a may be cut along its center line C - C. Although an explanation has been provided by referring to a rubber blade 2a, the charging blade 2 may also be composed of a sheet material or a film material. The back electrode 3 may be formed and configured in the same manner as explained above. As explained above, the present invention has the following effects: By forming an electrode layer on a charging blade, serving as a contact charging member, after connecting the blade to its supporting member as one body, it is possible to stabilize the connecting force between the two members for a long period, to accurately contact the blade to a member to be charged and thereby perform stable charging, and to simplify the production process of the charging member.

Claims: A charging member for charging a member (1) to be charged, comprising a blade member (2), and a supporting member (4) for supporting said blade member, characterized by an electrode layer (3) provided on the surface of said blade member opposite to the surface contacting the member to be charged after connecting the blade member to the supporting member, said electrode layer at least partially extending on said supporting member to establish an electrical connection between said blade member and said supporting member. A charging member according to claim 1, characterized in that the width of said blade member (2) is larger than the width of said electrode layer (3) in the longitudinal direction of the member to be charged. A charging member according to claim 1, characterized in that said blade member (2) is formed from an elastic material. A charging member according to claim 1, characterized in that said supporting member (4) is made from a conductive material. A charging member according to claim 4, characterized in that said supporting member (4) is rigid. A charging member according to claim 4, characterized in that said member comprises means (5) for applying voltage to said supporting member (4) in order to perform the charging. A process unit detachable relative to an electrophotographic image forming apparatus, comprising an image carrying member (1), and a charging means (2, 4) for charging said image carrying member, said charging means comprising a blade member (2) for contacting said image carrying member, and a supporting member (4) for supporting said blade member, characterized by an electrode layer (3) provided on the surface of said blade member opposite to the surface contacting said image carrying member after connecting the blade member to the supporting member, said electrode layer at least partially extending on said supporting member to establish an electrical connection between said blade member and said supporting member. A process unit according to claim 7, characterized by further comprising developing means (7) for developing a charge latent image generated on said image carrying member (1). A process unit according to claim 7, characterized in that the image carrying member (1) is rotatable and the width of said blade member (2) is larger than the width of said electrode layer (3) in the longitudinal direction of the rotatable image carrying member (1). A process unit according to claim 7 characterized in that said blade member (2) is formed from an elastic material. A process unit according to claim 7, characterized in that said supporting member (4) is formed from a conductive material. A process unit according to claim 7, characterized in that said supporting member (4) is rigid. A process unit according to claim 7, characterized by further comprising means (5) for applying a voltage to said supporting member (4) in order to perform the charging. An electrophotographic image forming apparatus, comprising an image carrying member (1), an image forming means (L) for forming an image on said image carrying member, and a charging means (2, 4) for charging said image carrying member, said charging means comprising a blade member (2a) for contacting said image carrying member, and a supporting member (4) for supporting said blade member, characterized by an electrode layer (3) provided on the surface of said blade member opposite to the surface contacting said image carrying member after connecting the blade member to the supporting member, said electrode layer at least partially extending on said supporting member (4) to establish an electrical connection between said blade member and said supporting member. An image forming apparatus according to claim 14, characterized in that the image carrying member is rotatable and the width of said blade member (2) is larger than the width of said electrode layer (3) in the longitudinal direction of the rotatable image carrying member. An image forming apparatus according to claim 14, characterized in that said blade member (2) is formed from an elastic material. An image forming apparatus according to claim 14, characterized in that said supporting member (4) is formed from a conductive material. An image forming apparatus according to claim 14, characterized in that said supporting member (4) is rigid. An image forming apparatus according to claim 17, characterized by further comprising means (5) for applying a voltage to said supporting member (4) in order to perform the charging. A method for making a charging member for a member to be charged, comprising the steps of providing a blade member (2a), providing a supporting member (4), and connecting said blade member to said supporting member, and characterized by the step of forming an electrode layer (3) on the surface of said blade member opposite to the surface contacting the member to be charged after connecting the blade member to the supporting member, said electrode layer at least partially extending on said supporting member (4) to establish an electrical connection between said blade member and said supporting member. A method according to claim 20, characterized by further comprising the step of sizing the blade member (2) such that the width of said blade member (2) is larger than the width of said electrode layer (3) in the longitudinal direction of a member to be charged by the charging member. A method according to claim 20, characterized by further comprising the step of fabricating said blade member (2) from an elastic material. A method according to claim 20, characterized by further comprising the step of using a rigid supporting member (4).

Inventors: ADACHI HIROYUKI, HOSHIKA NORIHISA, ADACHI, HIROYUKI, HOSHIKA, NORIHISA, ADACHI, HIROYUKI, C/O CANON KABUSHIKI KAISHA, HOSHIKA, NORIHISA, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G03G15
IPC codes: G03G15