A charging method and a charging device

Abstract: A charging device for charging a member (1) to be charged includes a charging member (2) for electrically charging a member to be charged, a spacer (2c) for maintaining a clearance between said charging member (2) and the member (1) to be charged, not less than 5 microns and not more than 300 microns, the charging member (2) being supplied with a voltage having a waveform obtained as a sum of an AC voltage and a DC voltage, wherein a peak-to-peak voltage of the vibratory voltage is not less than twice the absolute value of the charge starting voltage relative to the member to be charged.

Description: The present invention relates to a charging device usable with an image forming apparatus such as an electrophotographic machine or an electrostatic recording machine, in which a member to be charged such as a photosensitive member or a dielectric member is uniformly charged or discharged. As for means for uniformly charging the member to be charged such as the image bearing member to a predetermined potential of a predetermined polarity, a corona discharger such as a corotron or scorotron is widely used since the uniformity of the charging is sufficiently provided. However, the corona dischargers involve the drawbacks that it requires an expensive high voltage source, that it requires much space therefor and for the shield for the high voltage source or the like, that the production of ozone is relatively large, which requires means against the production and larger size of the device and a higher cost. Recently, therefore, the consideration is made as to the contact type charging device and apparatus in place of the corona discharger involving the above problems. In the contact type system, a charging member is contacted to the member to be charged such as the image bearing member, while the charging member is supplied with a voltage which may be a DC voltage of 1 - 2 kV or a DC biased AC voltage, by which the member to be charged is charged to a predetermined polarity. It includes a roller charging type (JP-A-56 091 253), blade charging type JP-A-60 147 756, charging and cleaning type (JP-A-56 165 166). The contact type charging has the advantages that it is possible to lower the voltage of the voltage source, that the production of ozone is very slight, if any, the structure is simple with a small size, that the cost is low, and the like. Referring to Figure 6, there is shown an example of an image forming apparatus having such a contact type charging device for uniformly charging the surface of the image forming apparatus. The image forming apparatus comprises an electrophotographic photosensitive member 1, which will hereinafter be called "photosensitive drum", and which is rotated in a direction A indicated by an arrow at a predetermined peripheral speed (process speed). A charging roller 20 is the charging member, and comprises a conductive core (shaft) 20a made of steel or stainless steel or the like, and an outer layer 20b, thereon, made of EPDM or the like containing carbon to provide a predetermined low resistance. The charging roller 20 is supported by bearings at the opposite longitudinal ends of the core metal 20a for free rotation, and parallel to the generating line of the photosensitive drum 1. It is urged to the photosensitive drum 1 with a predetermined pressure so that it rotates following the rotation of the photosensitive drum 1. The apparatus further comprises an external voltage source to the charging roller 20. It supplies a voltage which is a sum of a DC voltage and an AC voltage having a peak-to-peak voltage which is not less than the charge starting voltage between the charging roller 20 and the photosensitive layer. The voltage is supplied to the charging roller 20 through sliding contacts 8 contacted to the ends of the core metal 20a. The surface of the photosensitive drum 1 is sequentially charged by the charging roller 20 which is supplied with such a voltage and which is contacted to the surface to a predetermined potential of a predetermined polarity. The uniformly charged surface of the rotating photosensitive drum 1 thus uniformly charged is exposed to image light 3 bearing the intended image formation (print information) through an unshown exposure means such as analog exposure means fot an original document, a laser scanner, LED array, liquid crystal shutter array, or the like through a slit or by way of scanning means. By doing so, an electrostatic latent image of the intended information is formed sequentially on the surface of the rotating photosensitive drum 1. The thus formed electrostatic latent image is developed by a developing device (developing roller) 4 into a toner image, which is in turn transferred onto a transfer material 7 at an image transfer station between the photosensitive drum 1 and a transfer roller 5 supplied with an image transfer bias voltage. The transfer material 7 is fed from unshown feeding mechanism at a predetermined timing in association with the image on the photosensitive drum 1. The transfer material 7 now having received the toner image is separated from the surface of the photosensitive drum 1, and is introduced into an image fixing apparatus where the toner image is fixed thereon. The surface of the photosensitive drum after the image transfer is cleaned by a cleaner so that the residual toner or the like is removed therefrom, and the photosensitive drum 1 is prepared for the next image forming operation. The following drawbacks of the contact type charging device have been found: 1. Production of tracks of the charging device: The outer layer 20a of the charging roller 20 is made of EPDM, for example, as described above. The EPDM material is contacted to the surface of the photosensitive drum 1 which is the member to be charged. Plasticizer contained in the EPDM may ooze out thereof and may be transferred to the surface of the photosensitive drum 1 during the period in which the photosensitive drum 1 is not rotated, with the result of tracks of the roller on the photosensitive drum 1. This deteriorates the image quality. 2. Production of charging noise: When the AC voltage is applied to the charging roller contacted to the photosensitive drum, the charging roller may vibrate with the result of noise (charging noise). The problems of the tracks of the charging roller and of the charging noise is common to all types of contact chargers. U.S. Patent Specification No. 3,935,517 discloses a charging device in which a rotatable drum is supplied with constant current to charge a dielectric medium which is spaced from the drum. European Patent Specification No. 0,280,542 discloses a drum-type charger which contacts the member to be charged and which utilises a vibratory charging voltage. Accordingly, it is a concern of the present invention to provide a charging device which does not involve the problem of the tracks of the charging device but in which the advantages of the contact type charging device are substantially maintained. Accordingly, from one aspect the present invention provides a charging device as set out in claim 1. From a second aspect the present invention provides a method as set out in claim 9. In order that the present invention may be more readily understood embodiments thereof will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a sectional view of an image forming apparatus according to an embodiment of the present invention. Figure 2 is a front view of a charging device used in the image forming apparatus of Figure 1. Figures 3A, 3B and 3C are sectional views of charging devices according to other embodiments of the present invention having different roller warp preventing means. Figure 4 is a front view according to a further embodiment of the present invention using a charging blade. Figure 5 is a side view thereof. Figure 6 is a sectional view of an image forming apparatus using a contact type charging device. DESCRIPTION OF THE PREFERRED EMBODIMENT Embodiment 1 Referring to Figure 1, there is shown an exemplary image forming apparatus using a proximity (non-contact) type charging device for uniformly charging an image bearing member. Figure 2 is front view of the part adjacent the charging device. The same reference numerals as in Figure 6 are assigned to the the elements having the corresponding functions, and the detailed description thereof have been omitted. The photosensitive drum 1 of this embodiment comprises an aluminum base 1b and a photosensitive layer 1a on the outer surface of the base 1b, which is made of organic photoconductor (OPC). The photosensitive drum 1 has an outer diameter of 30 mm. The charging roller is designated by a reference numeral 2. it comprises a conductive core (shaft) 2a of aluminum, steel or the like, an outer layer 2b of EPDM or the like having a resistance lowered by addition of carbon or the like to a predetermined level, and a spacer ring layers 2c and 2c of nylon, teflon or the like mounted on the outer layer 2b along the circumferential periphery thereof at the longitudinal opposite ends. The charging roller 2 is supported by unshown bearings at the longitudinal opposite ends of the core metal 2a to be substantially parallel with the generating line of the photosensitive drum 1. The charging roller 2 is confined by urging means 10 and 10 such as springs adjacent the opposite ends of the charging roller 2, so that the charging roller 2 is pressed to the photosensitive drum 1 at the spacer ring layers 2c and 2c at a predetermined pressure. Therefore, the portion of the charging roller 2 between the spacer layers 2c and 2c is maintained out of contact from the photosensitive drum 1 with a clearance t, corresponding to the thickness of the spacer ring layer 2c and 2c. The charging roller 2 may be rotated following the rotation of the photosensitive drum 1 or may be positively driven codirectionally with the photosensitive drum 1, or it may be rotated in the opposite direction, or it may be unrotated. The charging roller is supplied with an oscillating voltage (the voltage level periodically changes with time) having a waveform provided by biasing an AC voltage with a DC voltage, through sliding contacts 8 contacted to the ends of the core metal 2a. The uniform charging is accomplished by such a voltage setting that a charge starting voltage V TH when only a DC voltage is applied to the charging roller and the peak-to-peak voltage Vpp of the AC voltage component of applied voltage satisfy Vpp≥ 2 V TH . The charge starting voltage is determined in the following manner. Only DC component is applied to the charging member contacted to the image bearing member (photosensitive member) having a zero surface potential. The voltage of the DC component is gradually increased. The surface potentials of the photosensitive member are plotted with respect to the DC voltage applied thereto with a predetermined increment of the voltage, for example, 100 volts. The first point of the voltage is the one at which the surface potential of the photosensitive member appears, and about ten surface potential are plotted at each 100 volt increment, for example. Using least square approximatrion, a straight line is drawn from the plots. The DC voltage reading at which the straight line and the line representing the zero surface potential as the charge starting voltage. The waveform may be rectangular, triangular, pulsewise or simple DC form. The charging operation has been performed with the following conditions: the clearance t between the central portion of the charging roller 2 and the photosensitive drum 1 by the spacer ring layer 2c and 2c: 30 microns the peripheral speed of the photosensitive drum 1: 15 π mm/sec the voltages supplied to the charging roller 2 from the voltage source 9: DC of -700 V, and AC of 1500V and 255 Hz. It has been confirmed that the surface of the photosensitive drum 1 is charged uniformly to approx. -700 V. The following table shows the results of the experiments where the clearance t is changed by the spacer ring layer 2c and 2c from 1 micron to 500 microns. t (microns) image quality track of roller charging noise 1 G N N 3 G S S 5 G G G 50 G G G 100 G G G 150 G G G 200 G G G 250 G G G 300 G G G 350 S G G 400 N G G 500 N G G 1) Image quality: If the clearance is not more than 300 microns the charging is good with the result of good image quality. If the clearance is larger than 350 microns, it exceeds the electric insulation breakdown range, with the result of disability of the normal charging action, and therefore, image quality is degraded. In Table 1, "G" means good image quality; "S" means images with slight defect; and "N" means no good image. 2) Track of roller In the range of 1 - 3 microns of the clearance t, the pits and projections of the charging roller surface are partly in contact with the surface of the photosensitive drum. Therefore, the tracks of the charging roller were observed by the plasticizer contained in the outer layer 2 b of the charging roller. If it is larger than 5 microns, the surface of the charging roller is not in contact with the surface of the photosensitive drum, and therefore, the track of the charging roller is not observed. In the Table, "G" means no track of the roller observed; "S" means the tracks appear slightly; and "N" means tracks are observed. 3) Charging noise In the range of 1 - 3 microns of the clearance t, the pits and projections of the charging roller surface are partly in contact with the surface of the photosensitive drum. Therefore, when an external bias is applied to the charging roller, the charging noise is produced. If it is not less than 5 microns, the charging roller is out of contact with the photosensiitive drum, and therefore, the charging noise is not produced. In the Table, "G" means no production of the charging noise; "S" means slight production of the charging noise; and "N" means production of the charging noise. From the experiments and evaluation in the foregoing, it will be understood that the good results can be provided if the clearance between the charging roller and photosensitive drum is 5 - 300 microns. In the structure where the charging roller is urged to the photosensitive drum adjacent the opposite ends of the charging roller 2 by urging means 10 and 10, and is contacted to the photosensitive drum at the spacer ring layer 2c and 2c, if the urging force by the urging means is too strong, the charging roller 2 may be warped between the spacer ring layer 2c and 2c in the upward direction, with the result of the larger clearance between the central portion of the charging roller and the photosensitive drum than the clearance defined by the spacer ring layer 2c and 2c. Therefore, even if the thickness of the spacer ring layer 2c and 2c is set to 300 microns, it is possible that the clearance between the central portion of the charging roller and the photosensitive drum is larger than the tolerable 300 microns due the warping of the charging roller. Figures 3A, 3B and 3C show the measures against this. In Figures 3A, a confining roller 30 is disposed in the middle of the charging roller 2 to prevent the warping. The roller 30 is supported on the shaft 31, and the roller 30 is urged by a spring 32 to the charging roller 2. Even if the charging roller 2 tends to warp upwardly due to too strong forces by the urging means 10 and 10 with the result of the tendency of warping of the charging roller in the middle region. However, the tendency is suppressed so that the clearance t is between the charging roller 2 and the photosensitive drum 1 is maintained at the level corresponding to the thickness of the spacer ring layer 2c and 2c. Figure 3B shows an example in which the core metal 2a of the charging roller 2 has a diameter which is larger at the central portion than at the longitudinal end portions to suppress the warping of the charging roller 2 at the middle portion. In the example of Figure 3C, the diameter of the core metal 2a is uniformly increased to such an extent that the warping of the charging roller does not occur. The material cost increases corresponding to the increase of the diameter. However, the since the core metal is a straight roller so that it is easy to polish. Totally, this example is low in cost. Embodiment 2 Referring to Figures 4 and 5, there is shown an embodiment wherein the charging member is in the form of a blade. A charging blade 11 is made of conductive material. The longitudinal end portions of the charging blade is mounted on spacer members 12 and 12 by screws 13 and 13. The spacer members 12 and 12 are supported on a stationary member not shown. The clearance t between the charging blade 11 and the photosensitive drum 1 is defined by the spacer members 12 and 12. The voltage is supplied to the charging blade 11 from the voltage source directly through a lead wires 9a. In this example, as will be understood, the charging member 11 is not movable, so that the electric noise due to the electric contacts can be suppressed. In addition, the required space is smaller than in the charging roller, because the necessity for the confining of the charging member by the urging means as in the case of the charging roller, can be eliminated. Therefore, the problem of the warping is avoided. As described in the foregoing, according to the present invention, the proximity type charging device of the present invention substantially maintains the advantages of the contact type charging roller, and additionally advantageous in that tracks of the charging device and the charging noise as produced in the contact type charging device can be avoided. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

Claims: A charging device for electrographic reproduction apparatus comprising: a photosensitive member (1) to be electrically charged; a charging member (2, 11) for electrically charging said member to be charged; voltage application means (9) for applying to said charging member a vibratory voltage the peak-to-peak voltage of which is not less than twice the charge starting voltage of the member to be charged; and characterised by means (2, 12) for maintaining a clearance of not less than 5 microns and not more than 250 microns between said charging member and the surface of said photosensitive member (1). A device according to claim 1, wherein said vibratory voltage is in the form of a DC biased AC voltage. A device according to any one of the preceding claims, wherein said charging member (11) is stationary in use. A device according to claim 3, wherein said charging member is in the form of a blade (11). A device according to either of claims 1 or 2, wherein said charging member (2) is a rotatable roller. A device according to claim 5, wherein said charging member (2) is rotated by movement of the photosensitive member (1). A device according to either claim 5 or claim 6, wherein the roller (2) has one or more spacer rings (2c) to maintain the clearance between the roller (2) and said surface of the photosensitive member (1) to be charged. A device according to any one of the preceding claims, wherein the photosensitive member (1) to be charged is a photosensitive drum. A method of charging a photosensitive member (1) comprising applying a vibratory voltage to a charging member (2, 11), the peak-to-peak voltage of the vibratory voltage being not less than twice the absolute value of the charge starting voltage of the member to be charged, and characterised in that the charging member (2, 11) is held spaced from the surface of the photosensitive member (1) with a clearance of not less than 5 microns and not more than 250 microns.

Inventors: KISU HIROKI, KISU, HIROKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G03G15
IPC codes: G03G15