Image forming apparatus having charging means

Abstract: An image forming apparatus includes a movable image bearing member (1); and a charging device extending in contact with the image bearing member in a direction crossing with a movement direction of the image bearing member, the charging device including a conductive wire (10) extending in the crossing direction.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus such as an electrophotographic or electrostatic recording apparatus in the form of a copying machine, a laser beam printer or the like, more particularly to an image forming apparatus having charging means contactable to an image bearing member such as a photosensitive member or a dielectric member to electrically charge (or discharge) it. As for means for charging the photosensitive member or the dielectric member in an image forming apparatus, a corona discharging device is widely used. It is disposed faced to it and comprises a wire electrode and a shield enclosing the wire electrode. Corona discharging device is an effective means to uniformly charge the member to be charged up to a predetermined potential. However, it is disadvantageous in that it requires a high voltage source and a relatively large amount of ozone is produced by corona discharging action. In place of the corona discharging device, a contactable type charging device has been proposed. It comprises a charging member supplied with a voltage and contacted to the member to be charged so as to charge it. It is advantageous in that the required voltage is low and that the amount of produced ozone is very small. It is noted as means in place of the corona discharging apparatus for the purpose of charging the photosensitive member, the dielectric member or another image bearing member or another member to be charged in an image forming apparatus (Japanese Laid-Open Patent Applications Nos. JP-A-57178267, JP-A-56104351, JP-A-5840566 JP-A-58139156 and JP-A-58150975). U.S. Patent No. 4,851,960 which has been assigned to the assignee of this application proposes a contact type charging method and apparatus in which a vibratory voltage (a voltage level periodically changes) having a peak-to-peak voltage which is not less than twice the charge starting voltage relative to the member to be charged, is applied to the charging member to provide uniform charge. As for the charging member, it is conventionally in the form of a roller or a blade (EP-A-0312230). Figure 10 shows an example of a conventional image bearing member using a contact type chargeable member in the form of a roller to charge the image bearing member. The image forming apparatus is an image transfer type electrophotographic apparatus (copying machine, a laser beam printer or the like). It comprises an electrophotographic photosensitive member (rigid member) which will hereinafter be called "photosensitive drum", in the form of a drum. It is driven in the clockwise direction at a predetermined peripheral speed (process speed). A charging roller 2 comprises a conductive core metal 2a made of iron, stainless steel (SUS) or the like and a conductive rubber layer 2b made of EPDM or the like containing carbon and covering the outer periphery thereof. The roller 2 is contacted to the surface of the photosensitive drum 1 by urging the core metal 2a adjacent the opposite longitudinal ends thereof by an urging means not shown such as spring, and it rotates following the rotation of the photosensitive drum 1 in this example. An external voltage source 9 supplies voltage to the charging roller 2. It applies to the charging roller 2 a vibratory voltage provided by a predetermined DC voltage superposed with an AC voltage having a peak-to-peak voltage which is not less than twice the charge starting voltage relative to the photosensitive member, through a contact spring 8 and a core metal 2a. The outer periphery of the photosensitive drum 1 rotated is charged to a predetermined potential. Then, the charged photosensitive member is exposed to image light by charging means 3. The image light is image light of an original or a scanning laser beam imagewisely modulated, so that an electrostatic latent image is sequentially formed on the photosensitive member in accordance with the intended image information. Subsequently, a developing device supplied toner to the surface of the photosensitive drum 1 from a developing sleeve 4 to develop the latent image into a toner image on the photosensitive drum 1. An image transfer roller 5 is press-contacted to the photosensitive drum 1 at a position downstream of the developing sleeve 4 in the rotational direction of the photosensitive drum. A transfer material 7 is fed from an unshown sheet feeder to a nip (transfer position) between the photosensitive drum 1 and the transfer roller 5. It is supplied at such a timing that when the leading edge of the toner image on the photosensitive drum 1 reaches the nip, a leading edge of the transfer material 7 reaches the nip. To the transfer roller 5 a transfer bias is applied from voltage source, so that the toner image is transferred from the photosensitive drum 1 surface to the transfer material 7 gradually in the process of the transfer material 7 passing through the nip. The surface of the photosensitive drum 1 after the toner image transfer is cleaned by a cleaning device 6 so that the residual toner or the like is removed, and therefore, is prepared for the repeated image formation. Figure 11 shows another example of the charging member 20 in the form of a blade rather than the charging roller 2 (Figure 10). It comprises a conductive core 20a and a conductive rubber blade 20b. The charging blade 20 is contacted to the surface of the photosensitive drum 1 co-directionally or counter-directionally. A voltage is applied to the core plate 20a to charge the photosensitive drum 1 surface. The system using the charging roller or the charging blade involves the following problems. (1) Where the surface of the charging member is rubber directly contacted to the photosensitive drum (image bearing member), and when the charging roller or the blade is press-contacted to the photosensitive drum for a long period of time, the plasticizer of the rubber oozes out from the rubber layer and is deposited on the photosensitive drum surface with the result of flowing image. (2) If the size of the transfer material is large as in the case of A3 size, the length of the charging roller becomes large. When the charging roller is urged to the photosensitive drum by pressing means at the opposite ends, the central portion thereof may be out of contact from the surface of the photosensitive drum, with the result of improper charging. (3) With the increase of the process speed, the frequency of the primary voltage source supplied to the charging roller or the blade is required to be increased in order to prevent non-uniform charging. If the frequency is increased, the vibration of the photosensitive drum, charging roller or the blade is produced with the result of larger noise. (4) In an image forming apparatus which is recently required to be reduced in the size, the size of the photosensitive drum and the size of the charging means are decided to be further reduced. SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to provide an image forming apparatus wherein the possible production of flow of image attributable to the foreign matter oozed from the charging means and deposited on the image bearing member, is prevented. It is another object of the present invention to provide an image forming apparatus wherein the improper charging is prevented by improving contact between the image bearing member and the charging means. It is a further object of the present invention to provide an image forming apparatus wherein charging noise is reduced. It is a further object of the present invention to provide an image forming apparatus having a small size and low cost charging means. These objects are achieved by an image forming apparatus according to claim 1 and by a process unit detachably mountable to an image forming apparatus according to claim 18. The objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view of a charging device in an image forming apparatus according to an embodiment of the present invention. Figure 2 is a top plan view of the charging device of Figure 1. Figure 3 is a perspective view of the charging device of Figure 1. Figures 4, 6, 14, 16, 17, 25, 26, 27, 29A and 29B are side views of charging devices according to other embodiments. Figure 5A is a top plan view of the charging device of Figure 4. Figure 5B is a perspective view of the charging device of Figure 4. Figure 7 is a top plan view of the charging device of Figure 6. Figure 8 is a perspective view of the charging device of Figure 6. Figures 9A, 9B, 9C, 9D, 9E, 9F, 9G, 9H, 9I and 9K are sectional views of the charging wire. Figures 10 and 11 are side views of conventional image forming apparatus. Figure 12 is a side view of an image forming apparatus according to another embodiment of the present invention. Figure 13 is a perspective view of a charging device in an image forming apparatus according to a further embodiment of the present invention. Figure 15 is a top plan view of the charging device of Figure 14. Figure 18 is a top plan view of a charging device of Figure 17. Figure 19 is a perspective view of a charging device of Figure 17. Figure 20 is a side view of a surface coating device for the charging wire. Figure 21 is a circuit diagram which is equivalent to the charging device of Figure 17. Figures 22 and 23 illustrates leakage of current at a pin hole of the photosensitive member. Figure 24 is a circuit equivalent to the case wherein the charging wire comprising only the core metal. Figure 28 is a graph of noise relative to the width (diameter) of the conductive member. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be described. Referring to Figure 12, there is shown an image forming apparatus according to an embodiment of the present invention. In this Figure, the same reference numerals as in Figure 10 are assigned to the elements having the corresponding functions, and the detailed description therefor is omitted. The photosensitive drum 1 (image bearing member), charging means 10, 11 and 12 for the photosensitive drum 1, the cleaning device and the developing device are supported on a frame P (hatched portion) of a process unit. The frame of the process unit P is detachably mountable to a main assembly of the image forming apparatus along a guide G in a direction perpendicular to the sheet of the drawing. The process unit P may contain at least one of the photosensitive drum 1 and the charging device 10, 11 or 12. Figure 1 shows an enlarged view of the charging device used in the image forming apparatus of Figure 12. Referring to Figure 1, Figures 2 and 3, the description will be made as to a contactable wire type charging device. Figures 2 and 3 are top plan view and a perspective view of the charging device of Figure 1. A photosensitive drum 1 is a member to be charged and comprises a drum base 1b of aluminum and a photosensitive layer 1a thereon made of organic photoconductive (OPC) in this embodiment. It has an outer diameter of 30 mm in this embodiment. It is rotated in the clockwise direction at a predetermined speed. A charging wire 10 functions as a contact type charging member and may be made of conductive wire of iron, copper, aluminum, stainless steel or the like. Urging members 11 and 11 adjacent the opposite longitudinal ends thereof press-contact the charging wire to the photosensitive drum 1 surface. The charging wire extends parallel with a generating line of the drum. The charging wire 10 may be made of tungsten plated with gold and has an outer diameter of 60 microns in this embodiment. Springs 12 and 12 normally urge the urging members 11 and 12 toward the photosensitive drum 1. The charging wire between the urging members 11 and 11 extends in a direction perpendicular to the movement direction of the photosensitive drum 1, and is press-contacted to the surface of the photosensitive drum 1 covering an effective charging width of the photosensitive drum 1. To the charging wire 10, a voltage is applied from the voltage source 9 through a contact 8, by which a vibratory voltage in the form of a DC biased AC voltage is applied thereto so that a charging electric field is formed between the wire 10 and the drum 1, so that a voltage having a periodically changing voltage level is applied. Preferably, the vibratory voltage has a peak-to-peak voltage which is not less than twice a charge starting voltage between the wire 10 and the photosensitive member 1. Then, between the wire 10 and the drum 1, a vibratory voltage is sufficient to effect transfer and transfer-back of the electric charge. The waveform of the vibratory voltage may be sine rectangular, triangular or pulse-like. The charging is possible using the DC voltage only. With this structure, the photosensitive drum is rotated at a peripheral speed of 40 mm/sec, and the charging wire 10 is supplied with a DC voltage component of -700 V and with an AC voltage component having a peak-to-peak voltage of 1800 Vpp and a frequency of 250 Hz, from the voltage source 9. It has been confirmed that the surface of the photosensitive member 1a is uniformly charged approximately to -700 v, and good image is produced. As described hereinbefore, it is desirable that the peak-to-peak voltage of the AC component or the vibratory voltage is not less than twice the charge starting voltage which is the voltage at which the charging of the member to be charged starts when only a DC voltage is applied to the charging member, since then the member is uniformly charged. In this embodiment, the charge starting voltage with the DC voltage only was -500 V, and therefore, the peak-to-peak voltage providing the uniformity was approximately 1100 Vpp. It has been confirmed that there is no flow of the image attributable to the plasticizer oozed from the conductive rubber as in the case of the conventional charging roller or the charging blade, and no local improper charging by the non-contact between the charging member and the photosensitive member, or significant charging noise. Referring to Figures 6, 7 and 8, a charging device according to another embodiment of the present invention will be described. Figures 7 and 8 are a top plan view and a perspective view of the charging device of Figure 6. In this embodiment, two charging wires are used. The two charging wires 10A and 10B are extended parallel with each other and are press-contacted to the surface of the photosensitive drum 1 by urging members 11 and 11. They are supplied with voltages. Even if foreign matter is deposited on one of the charging wires, or even if one of them is broken, the charging operation can be kept by the remaining charging wire. The number may be three or more. Referring to Figures 4 and 5, a charging device according to a further embodiment of the present invention will be described, in which the non-contact between the charging wire and the photosensitive drum is prevented. Figure 4 is a side view of the charging device, and Figure 5 is a partly broken-away top plan view. In this embodiment, the charging wire 10 is urged to the photosensitive drum surface 1 by an elastic member 14 which is made of moltprane or the like and which is long enough to cover the entire length of the wire 10, by which the urged state is maintained over the entire length of the charging wire. The elastic member 14 has an elongated rigid holder plate 13 at a side opposite from the side of the wire urged to the photosensitive member. The holder plate 13 is made of DERLIN or the like. The holder plate 13 is urged by an unshown pressing member toward the photosensitive drum 1 with the elastic member 14 sandwiched between the holder plate 13 and the surface of the photosensitive drum 1 against the elastic force thereof, by which the entire length of the charging wire 10 is stably and uniformly pressed to the photosensitive drum 1, thus establishing the cross contact therebetween. Therefore, even if the charging wire 10 is supplied from the external voltage source 9 with the vibratory voltage having a large peak-to-peak voltage, the vibration of the charging wire 10 is not significant, as compared to the case of Figures 1 - 3. Even if a DC voltage only is supplied to the charging wire, the charging wire 10 does not dance upon the application thereof. As a result, the charging wire 10 is maintained in contact with the photosensitive drum 1 surface, so that the stabilized charging is possible. When the unevenness of the photosensitive drum surface 1 is large, this is particularly effective. In addition, as contrasted to the conventional charging blade, the silica particles and toner particles passed under the cleaning blade 6 are collected by the moltprane 14, and therefore, the charging wire 10 is not contaminated. As shown in Figure 13, two or more charging wires 10 may be used, similarly to the cases of Figures 6, 7 and 8. Figures 14 and 15 show a further embodiment of the charging device. In this embodiment, the charging wire 10 is made parallel to the generating line of the drum, and the opposite ends thereof are confined by urging members 11a and 11a made of DERLIN or the like. The portion of the charging wire between the confining members 11a and 11a is contacted to the surface of the photosensitive drum 1 by a resilient force of an urging sheet 11b which is made of polyethylene terephthalate (Myler (trade name)) having a thickness of 100 microns, and which is fixed on a cleaning blade holder 6a at one of its long sides. With this structure, the charging operation is stabilized, and the urging member 11b is not directly contacted to the photosensitive drum 1, and therefore, the photosensitive drum is not scraped by the urging member. Other materials of the urging member 11b include stainless steel, aluminum, PET, copper sheet or the like. Figure 16 shows a further embodiment of the charging device. In this embodiment, the cleaning blade 6 is used as the urging member for the charging wire 10. A backside of the cleaning blade 6 is provided with a positioning groove 6b for accommodating the charging wire 10. The groove 6b extends in the longitudinal direction of the blade 6, that is, in the direction of the generating line of the photosensitive drum. The charging wire 10 is in the groove 6b and is bonded by a bonding agent if necessary. The edge of the cleaning blade 6 is press-contacted to the photosensitive drum 1 surface, and the charging wire 10 at the backside of the blade 6 is urged to the surface of the photosensitive drum 1. With this state, the cleaning blade is fixedly mounted on an unshown mounting member. With this structure, no additional member is required to urge the charging wire 10, and the structure around the photosensitive drum is very simple. Figures 17 - 19 show a further embodiment of the charging device. The charging wire 10 functioning as the contactable charging member has a core metal 10c made of tungsten and having a diameter of 60 microns and a coating of N-methoxymethyl nylon (Tresin, available from Teikoku Kagaku Sangyo Kabushiki Kaisha) having a thickness of 10 microns as a resin surface layer 10d. The core metal 10c may be of a conductive wire made of iron, copper, aluminum, stainless steel or the like. In order to enhance the bonding strength between the surface layer 10d and the metal core 10c, one or more primer layer may be used. The volume resistivity of the primer layer or layers are not required to be smaller than that of the surface layer 10d, but the film thickness may be selected within such a range that the contact type charging wire 10 is capable of charging the photosensitive drum 1. The resin coated charging wire 10 is produced in the following manner. First, n-methoxymethyl nylon is solved in a solvent such as a mixture of methanol and toluene to produce n-methoxymethyl nylon resin liquid. The liquid 100d is contained in a container 200 as shown in Figure 20. The core metal 10c of tungsten wire is repeatedly dipped thereinto to form the resin layer 10d on the surface of the tungsten wire. The moving speed and the drying period of the tungsten wire 10c are so determined that the thickness of the resin layer 10d is 1 - 1000 microns. Designated by a reference numeral 100 is a drier. The charging wire 10 thus produced is extended substantially in parallel with the drum generating line in contact with the surface of the photosensitive drum 1 by the urging members 11 and 11 made of DERLIN or the like. The urging member 11 and 11 are urged by the springs 12 to normally urge the wire to the drum. The charging wire 10 between the urging members 11 and 11 is press-contacted to the surface of the photosensitive drum 1. To the core metal 10c of the charging wire 10, a vibratory voltage which is an AC biased DC voltage is supplied from the voltage source through the contact 8. By the application of the vibratory voltage, the voltage level applied to the charging wire 10 periodically changes. More particularly, the peak-to-peak voltage of the vibratory voltage is not less than twice the charge starting voltage relative to the photosensitive member. The vibratory voltage has a sine, rectangular, triangular and pulse waveform. The member to be charged can be charged by the DC voltage only. Since the core metal 10c of the charging wire 10 is made of tungsten, the electric resistance is substantially 0 ohm, and the electric resistance of the surface layer 10d is approximately 30 M-ohm, and the electrostatic capacity thereof is approximately 1700 pF. Therefore, the reactance (capacitance) = 1/2 πfC is approximately 0.4 M-ohm when the core metal 10c is supplied with a bias voltage of 250 Hz. Since the resistance of the core metal 10c of the charging wire 10 is negligibly small as compared with the resistance of the surface layer 10d, the resistance of the charging wire 10 is substantially equal to the resistance of the surface layer 10d. The resistance and the electrostatic capacitance of the charging wire 10 is determined in the following manner. The charging wire is pressed to the drum made of aluminum, and a resistance measuring device and an electrostatic capacitance measuring device are connected between the core metal 10c and the aluminum drum. The obtained values are converted to the values per 1 cm². The electrostatic capacitance per 1 cm² of the photosensitive drum (the member to be charged) is approximately 140 pF. Figure 24 shows an equivalent circuit of the charging device. Since, R >> 1/2 πfC, C >> Cd The AC voltage V AC can be applied to the photosensitive drum substantially without loss, where R is a resistance of the charging wire; f is a frequency of the applied bias voltage; C is an electrostatic capacitance of the charging wire; and Cd is the electrostatic capacitance of the photosensitive drum. In the above inequations, the difference by one order is sufficient. Since the resistance R of the surface layer 10d of the charging wire is sufficiently large, a pin hole of the photosensitive drum if any does not result in leakage of the current between the charging wire and the pin hole. Therefore, the voltage drop of the power supply system can be prevented. The resistance R of the charging wire 10 is 8.51 x 10² ohm ≦ R ≦ 2.20 x 10¹² ohm. The former part of the inequation is the requirement for the prevention of the leakage. and the latter part thereof is the requirement for the prevention of improper charging. The electrostatic capacitance C of the charging wire 10 is 1 pF ≦ C ≦ 100 nF. The electrostatic capacitance Cd of the photosensitive drum 1 (the member to be charged) 0.1 pF ≦ Cd ≦ 10 nF. Using the above-described structure, the photosensitive drum 1 was rotated at a peripheral speed of 40 mm/sec. To the core metal 10c of the charging wire 10, a vibratory voltage having a DC component of -700 V and an AC component having a peak-to-peak voltage of 1800 Vpp and a frequency of 250 Hz, was applied from the power source 9. Then, the surface of the photosensitive member 1A was uniformly charged to approximately -700 V. Good images were produced without problem attributable to the pin hole. The present invention is particularly advantageous when the peak-to-peak voltage of the AC component is not less than twice the charging starting voltage which is the voltage at which the charging of the member to be charged starts upon application only of the DC voltage, since then the charging becomes uniform. In this embodiment, the charge starting voltage was -560 V when the DC voltage only was applied. Here, it is assumed that the resistance of the charging wire 10 with the surface layer 10b relative to the photosensitive drum 1 is R1, the volume resistivity of the core metal 10a is ρ1, the volume resistivity of the surface layer 10b is ρ2, the radius of the core metal 10a is r1, the radius of the contact charging wire 10 is r2, the (effective) length of the charging wire 10 is L, and the nip width is d (= 2 x r2). Since the core metal 10a of the charging wire 10 is made of iron, the volume resistivity P1 is 9.41 x 10⁻⁸ ohm.cm and therefore, it is substantially zero. Therefore, R1 = (ρ2 x ℓnr2/r1)/Ld The volume resistivity ρ2 has been confirmed to be preferably 10⁵ ohm.cm - 10¹² ohm.cm in order to provide good charging operation. As described in the foregoing, according to this embodiment, at least the contact portion of the conductive base of the contactable charging wire with the member to be charged is coated with at least one resistance layer, and the volume resistivity of the outermost layer of the charging member is made larger than the volume resistivity of the base of the charging wire. Therefore, even if the member to be charged has a pin hole, the voltage drop of the power supply system attributable to the current leakage can be prevented, and therefore, the improper charging of the entire range where the charging member is in contact with the member to be charged, can be prevented. The contactable charging wire may be a bare metal wire without coating. However, if, for example, a pin hole is produced in the surface of the photosensitive drum (the member to be charged) in the image forming apparatus for some reason or another (shot by foreign matter or introduction thereof, for example), the current leakage occurs between the charging wire and the pin hole of the photosensitive drum with the result of significant voltage drop of the power supply system, when the pin hole comes to the charging wire. Therefore, if the leakage occurs, the improper charging occurs in the entire longitudinal range contacting the photosensitive drum. In the actual image, it becomes a black stripe in the case of a reverse development, and it becomes a white stripe in the case of a regular development. The black or white stripe appears repeatedly with the period of the rotation of the photosensitive drum, and therefore, the image quality is significantly degraded. Figure 22 shows this situation. This Figure is a sectional view when the pin hole h of the photosensitive drum 1 comes right below the bare charging member 10c without the coating. As will be understood, if the charging wire 10c is of a bear metal such as iron, the electric current I easily flows into the pin hole h along the length of the charging wire 10c, in other words, the large current path is produced toward the pin hole h, and therefore, the voltage easily drops. By coating the charging wire in the manner described above, the volume resistivity of the surface layer of the charging wire is larger than that of the conductive core, and therefore, the current toward the surface defect such as pin hole along the surface layer becomes smaller. Therefore, even if there is the pin hole or other defect, the voltage drop of the power supply system attributable to the leakage can be prevented. Therefore, the improper charging does not occur over the entire contact region between the charging member and the member to be charged. In the case of the image forming apparatus, the above-described degrading of the image quality can be avoided. Without increasing the voltage of the supply system, the sufficient voltage can be supplied, and therefore, the improper charging or non-uniform charging attributable to the insufficient AC voltage, can be prevented. The advantageous effects are particularly significant, when the peak-to-peak voltage of the vibratory voltage is not less than twice the charge starting voltage which is the voltage at which the charging of the member to be charged starts when a DC voltage only is applied. When the charging member is a bare core metal 10c without the surface layer 10d is directly contacted to the photosensitive drum, and a voltage is applied to the charging member, the equivalent circuit is as shown in Figure 24. The resistance r of the core metal 10c is substantially 0 ohm. As described hereinbefore, if the photosensitive drum 1 has a pin hole, the voltage leaks with the result of the non-uniform stripe produced along the length of the photosensitive drum 1. In this embodiment, the charging member has the surface layer of a dielectric material on the conductive wire, and the reactance of the charging member relative to the vibratory voltage (AC voltage) is smaller than the resistance of the charging wire. The charging member has the dielectric layer or the surface layer comprising a resistance layer and a dielectric layer at least at a contact portion with the member to be charged, by which the charging member has the electrostatic capacity component or a resistance and electrostatic capacity components. The resistance is sufficiently large so as to prevent the leakage between the charging member and a pin hole of the member to be charged even if the member to be charged (photosensitive drum or the like) has a pin hole due to shot by or introduction of foreign matter. The electrostatic capacity is made sufficiently larger than that of the member to be charged, by which the reactance of the charging member relative to the AC voltage is made sufficiently smaller than the resistance of the charging wire. As a result, even if the member to be charged has a pin hole, the voltage drop of the power supply system due to the leakage can be prevented, so that the improper charging of the entire range along the length of the charging member where it is contacted to the member to be charged, can be prevented. Therefore, in the case of the image forming apparatus, the black stripe (reverse development) and white stripe (regular development) which is attributable to the leakage and which appears periodically in accordance with the rotation of the photosensitive drum, can be prevented. And therefore, the degradation of the quality of the image thereby can be prevented. In addition, without increasing the AC voltage, the sufficient voltage can be applied to the member to be charged. Therefore, the improper or non-uniform charging due to the insufficient AC voltage can be prevented. This advantageous effects are particularly significant when the peak-to-peak voltage of the vibratory voltage applied to the contact type charging wire is not less than twice the charge starting voltage which is the voltage at which the charging of the member to be charged starts when only the DC voltage is applied. Since the charging wire does not have the charging layer made of rubber or the like as in the case of the charging roller or the charging blade, there is no production of the image flow attributable to the oozed plasticizer of the conductive rubber layer, local improper charging attributable to the rising of the charging member, or the charging noise. In the embodiment shown in Figures 17 - 19, the wire 10 may be press-contacted to the surface of the drum 1 by an elongated elastic member made of moltprane or the like over the entire length of the charging wire 10, as described hereinbefore. Two or more charging wires may be disposed in parallel with each other. In Figure 25, the charging member comprises: conductive wire 10e having a diameter d of 60 microns and made of tungsten; and a resistance member 10f having a volume resistivity of 10¹⁰ ohm.cm made of fluorinated resin containing carbon (EMLARON (trade name), available from Nippon Ajison Kabushiki Kaisha) and having a thickness of 25 microns. In the case of this contactable charging member 10, the charging width L was 200 microns when the diameter d of the conductive wire 10c was 60 microns. With the above-structure, the photosensitive drum 1 was rotated at a peripheral speed of 40 mm/sec, and the charging wire 10 was supplied with a vibratory voltage having a DC voltage component of -700 V and an AC voltage component having a peak-to-peak voltage 1800 Vpp and a frequency of 250 Hz. Then, the surface of the photosensitive member 1a was uniformly charged approximately to -700 V without improper image due to pin holes or without charging noise or very low charging noise. In this embodiment, the provision of the elastic member 10 reduces the production of the charging noise. The flow of the image attributable to the plasticizer oozed out from the conductive rubber layer as in the case of the charging roller or the charging blade, was not observed. In addition, the partial improper charging attributable to the rising of the charging member was not observed. The advantageous effects of this embodiment is particularly significant when the peak-to-peak voltage of the vibratory voltage is not less than twice the charge starting voltage which is the voltage at which the charging of the member to be charged starts when only a DC voltage is applied. In this embodiment, the charge starting voltage when only the DC voltage was applied was -560 V. With decrease of diameter, width or thickness of the wire (conductive member) to which the voltage is applied, that is, with the decrease of the width of the nip between the conductive wire and the member to be charged, the level of the charging noise can be reduced to the practical extent or can be practically eliminated. With the decrease of the width, the charging width becomes small, and as a result, the resistance of the charging portion becomes large. Then, it becomes more difficult for the current to flow, and therefore, the improper charging action easily occurs. In Figure 26, the contactable charging member 10 comprises a conductive wire (conductive member) 10a having a circular cross-section and a coating 10b (resistance material) on the outer periphery of the conductive wire. This is press-contacted to the surface of the photosensitive drum 1 (member to be charged) in the direction substantially parallel to the generating line of the drum by confining members 11 and an urging members 12. A voltage is applied to the conductive wire 10a from the voltage source 9 to effect the charging of the photosensitive drum 1. If the width of the area in which the conductive wire 10a of the charging member 10 is faced to the photosensitive drum 1 (diameter of the wire 10a) is d, and the charging width is L, the charging width L is substantially equal to the contact area between the charging member 10 and the drum 1. The charging width L is smaller than the width d. If the width d is reduced for the purpose of reducing the charging noise, the charging width L decreases with the result of possibility of improper charging. The charging width L can be determined in the following manner. The photosensitive drum 1 is stopped. A bias voltage is applied to the conductive wire 10a from the voltage source 9 for several minutes. Next, the drum 1 is developed with a halftone level and an image is transferred onto the transfer material. Then, on the transfer material, the developed charge memory image is transferred. The width of the charge memory is the charging width L. A charge limitation t means that if the surface of the charging member 10 is away from the surface of the photosensitive drum 1 by a distance not less than t, the charging is not possible. This is determined by the Passhen's Law. Under the normal ambient condition, it is several tens microns. Therefore, in this system, the reduction of the width d in order to reduce the produced charging noise has to be made in consideration of maintenance of sufficient charging width L, and therefore, there is a limit. In this embodiment, the surface of the resistance member contactable to the member to be charged is substantially flat. Therefore, as shown in Figure 25, the charging width L becomes larger than the width d. The contactable charging member 10 of Figure 25 uses a conductive wire having a circular cross-section as the conductive member 10e, and the conductive wire is attached to the bottom surface of an elongated elastic member 10g made of moltprane or the like. The bottom side of the elastic material 10e having having the wire 10e is entirely coated with a high resistance material. The top surface of the elastic material 10g opposite from the coating layer 10f is mounted to an urging plate 10h made of rigid material. Since the coating layer 10f is made of resistance material, and since the surface contactable to the member to be charged 1 is substantially flat, the charging width (contact width) L is larger than the width d with which the conductive wire is faced to the drum. The coating layer 10f side (resistance member) of the contactable charging member 10 is urged to the surface of the photosensitive drum 1 against the elasticity of the elastic member 10g, and a voltage is applied to the wire 10a to charge the photosensitive drum 1 surface. In this case, the charging width L increases relative to the width d, and therefore, the width d may be made smaller to sufficiently suppress or substantially eliminate the production of the charging noise, while maintaining the sufficient charging width L. By doing so, the charging operation is stabilized. The cross-section of the conductive member 10e is not limited to circle, but may be rectangular, as shown in Figure 27. In the devices shown in Figures 17 - 19, the following measurements were effected. The voltage applied to the conductive member 10a of the charging wire 10 was a vibratory voltage containing a DC voltage component of -700 V and an AC voltage component having a peak-to-peak voltage of 1800 Vpp and the frequency of 250 Hz. The width d of the conductive member 10a, that is, a diameter thereof was changed. Figure 28 shows the results of measurements of the charging noise. In Figure 28, the abscissa represents the width (diameter) d of the conductive member, and the ordinate represents the magnitude of the produced noise (dB). If the width d exceeds 1000 microns, the noise level exceeds 50 dB which is a practical noise limit. This applied to the AC voltage having the peak-to-peak voltage and the frequency usually applied to the charging member. The reasons are considered as follows. In the case of the contact charging, upon application of the external bias to the contact charging member, the contact charging member starts to vibrate. By the motion of the air sandwiched between the contact charging member and the member to be charged, and the charging noise is produced. When, however, the charging wire 10 is thin as in this embodiment and as contrasted to the conventional contact type charging roller having a large diameter the air sandwiched by the charging member 10 and the member to be charged 1 decreased, and therefore, the noise production is decreased. If, however, the width d is too small, not more than 4 microns, then the area of the charging portion decreases with the result of increased charging resistance and the improper charging. The output images are deteriorated as shown in the Table below. From the foregoing measurements, it has been confirmed that when the member to be charged is charged by a conductive member supplied with a voltage, directly or through a resistance member, the width d of the conductive member faced to the member to be charged can be set so as to satisfy 5 microns < d < 1000 microns, by which the produced charging noise can be within a practically tolerable range. When a coating layer 10b (resistance material) is used on the conductive member 10a, the thickness may have something to do with the width d. However, the thickness is generally small, and therefore, it will substantially suffice that the width d of the conductive member 10a is determined as described above. Rather, if the coating layer 10b is used, the elastic property of the coating layer 10b reduces the charging noise. As shown in Figures 6 - 8, where plural charging wires are used, the charging noise produced by the charging wires can be reduced by confining the width d of each of the conductive members of the charging wires 10a and 10b. However, it would be possible that the total noise would be large, and therefore, it is desirable that the sum of the conductive members of the charging wires is within the above-described range. As shown in Figure 29A or Figure 29B, the contact type charging member 10 may be in the form of an integral molded member comprising one or more conductive members 10a and a resistance member 10b, wherein the conductive member or members embedded in the thickness of the resistance member 10b. This is suitable for mass-production, and therefore, the cost can be decreased. As described in the foregoing, the cross-section of the charging wire 10 is not necessarily circular, but may be triangular, rectangular, oval or the like, as shown in Figures 9A, 9B and 9C. In these cases, the contact area between the photosensitive drum 1 and the charging wire can be larger, and therefore, as compared with the circular charging wire, the stabilized charging becomes possible. As shown in Figures 9D, 9E, 9F and 9G, the outer periphery of the core metal of the charging wire 10 may be coated with dielectric surface layer 10₁ made of resin or the like (the thickness is 10 - 1000 microns, for example). Or, as shown in Figures 9H, 9I, 9J and 9K, the surface layer 10₁ may be formed at least at the contact portion with the member to be charged. Usable-materials for the surface layer 10₁ includes n-methoxymethyl nylon (Toresin (trade name), available from Teikoku Kagaku Sangyo Kabushiki Kaisha, Japan), polyvinyl butyral resin material, polyurethane resin material, ethylenevinyl acetate resin, styrene butadiene resin material, fluorinated resin material in which low resistance powder (carbon or the like) is dispersed (EMLARON (trade name), available from Nihon Agison Kabushiki Kaisha), reproduced cellulose. As described hereinbefore, if the rubber is not used, it is preferable because of no liability of the plasticizer attaching to the photosensitive member. However, in order simply to prevent the leakage, the usable material includes epychlorohydrine, urethane, Cr, NBR rubber or the like. In this case, it is preferable that the resin layer is provided on the side where the charging member is contacted to the photosensitive member. It is also possible to use heat-shrinking tube. The resin layer may comprise two or more layers. Among the above-described embodiments, if the charging member contacting the member to be charged is in the form of a wire (outer diameter of 5 - 1000 microns), the silica particles or the toner particles having passed under the cleaning device 6 are easily passed on the wire of the charging member, as contrasted to the case of the charging blade. Therefore, they are not so easily deposited on the charging wire. In the foregoing, the vibratory voltage applied between the charging member and the photosensitive member contains the voltage provided by periodically rendering a DC voltage on and off into a rectangular wave voltage. The foregoing embodiments apply to the case of decreasing the electric potential of the member to be charged (discharging) as well as increasing the potential of the member to be charged. 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 scope of the following claims.

Claims: An image forming apparatus, comprising a moveable image bearing member (1) , characterized by a coated or uncoated conductive charging wire (10) for charging said image bearing member (1) , said charging wire (10) extending in a direction perpendicular to the movement direction of said image bearing member (1) and being press-contacted to the surface of said image bearing member (1) such that an effective charging width of said image bearing member (1) is covered. An image forming apparatus according to claim 1, further characterized by means (9) for applying a voltage between said charging wire (10) and said image bearing member (1) to electrically charge said image bearing member (1) . An image forming apparatus according to claim 2, characterized in that said voltage has a periodically changing voltage level. An image forming apparatus according to claim 3, characterized in that said voltage has a peak-to-peak voltage which is not less than twice a charge starting voltage of said charging wire (10) relative to said image bearing member (1) . An image forming apparatus according to claim 1, characterized in that said charging wire (10) is coated by a dielectric layer (10d) contactable to said image bearing member (1) . An image forming apparatus according to claim 5, characterized in that said dielectric layer (10d) is of a resin material. An image forming apparatus according to claim 6, characterized in that said resin material is a non-rubber resin material. An image forming apparatus according to claim 3 or 4, characterized in that said charging wire (10) has a dielectric layer (10d) contactable to said image bearing member (1) , wherein a reactance of said charging wire (10) relative to a voltage of said charging wire (10) is smaller than a resistance of said charging wire (10) . An image forming apparatus according to claim 8, characterized in that an electrostatic capacity of said charging wire (10) is larger than that of said image bearing member (1) . An image forming apparatus according to claim 1, further characterized by supporting means (11, 12) for supporting said charging wire (10) . An image forming apparatus according to claim 10, characterized in that said supporting means (11, 12) extends in said direction perpendicular to said moving direction of said image bearing member (1) to urge said charging wire (10) to said image bearing member (1) over a length in said perpendicular direction. An image forming apparatus according to claim 1, characterized in that said charging wire (10) is faced to said image bearing member (1) within a width d, where 5µm < d < 1000µm. An image forming apparatus according to claim 1 or 12, characterized in that a charging width measured in said movement direction of said image bearing member (1) is larger than a width of said charging wire (10) measured in the same direction. An image forming apparatus according to claim 1, characterized in that said charging wire (10) is operated when a latent image is formed on said image bearing member (1) . An image forming apparatus according to claim 6 or 7, characterized in that said dielectric layer (10d) is made of n-methoxymethyl nylon, polyvinyl butyral resin material, polyurethane resin material, ethylene vinyl acetate resin material, styrene butadiene resin material, fluorinated resin material in which low resistance powder is dispersed, or regenerated cellulose. An image forming apparatus according to claim 1, characterized in that said image bearing member (1) is an organic photoconductor material. An image forming apparatus according to claim 1, further characterized by a process unit (P) detachably mountable to said image forming apparatus and containing said image bearing member (1) and said charging wire (10) . A process unit detachably mountable to an image forming apparatus, characterized by a) a moveable image bearing member (1) ; and b) a coated or uncoated conductive charging wire (10) for charging said image bearing member (1) , said charging wire (10) extending in a direction perpendicular to the movement direction of said image bearing member (1) and being press-contacted to the surface of said image bearing member (1) such that an effective charging width of said image bearing member (1) is covered. A process unit according to claim 18, characterized in that said charging wire (10) is coated by a dielectric layer (10d) contactable to said image bearing member (1) . A process unit according to claim 19, characterized in that said dielectric layer (10d) is of a resin material. A process unit according to claim 20, characterized in that said resin material is a non-rubber resin material. A process unit according to claim 18, characterized in that an electrostatic capacity of said charging wire (10) is larger than that of said image bearing member (1) . A process unit according to claim 18, further characterized by supporting means (11, 12) for supporting said charging wire (10) . A process unit according to claim 23, characterized in that said supporting means (11, 12) extends in said direction perpendicular to said moving direction of said image bearing member (1) to urge said charging wire (10) to said image bearing member (1) over a length in said perpendicular direction. A process unit according to claim 18, characterized in that said charging wire (10) is faced to said image bearing member (1) within a width d, where 5µm < d < 1000µm. A process unit according to claim 18 or 25, characterized in that a charging width measured in said movement direction of said image bearing member (1) is larger than a width of said charging wire (10) measured in the same direction. A process unit according to claim 18, characterized in that said charging wire (10) is operated when a latent image is formed on said image bearing member (1) . A process unit according to claim 20 or 21, characterized in that said dielectric layer (10d) is made of n-methoxymethyl nylon, polyvinyl butyral resin material, polyurethane resin material, ethylene vinyl acetate resin material, styrene butadiene resin material, fluorinated resin material in which low resistance powder is dispersed, or regenerated cellulose. A process unit according to claim 18, characterized in that said image bearing member (1) is an organic photoconductor material.

Inventors: KISU HIROKI C O CANON KABUSHIK, YAMADA HIROMICHI C O CANON KAB, KISU, HIROKI, C/O CANON KABUSHIKI KAISHA, YAMADA, HIROMICHI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: G03G15
IPC codes: G03G15