Charging device and method

Abstract: A charging device for charging a member (1) to be charged includes a charging member to which a voltage is applicable to charge the member to be charged, the charging member having a magnetic brush (2) of magnetic particles (23) contactable to the member (1) to be charged, and supporting member for supporting the magnetic particles; wherein the the magnetic particles (23) have a resistance value of 1x10 4 -1x10 7 Ohm when a DC voltage of 1-1000V is applied thereto.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to a charging device having a charging member contactable to a member to be charged such as photosensitive member or dielectric member. The charging device is preferably usable with an image forming apparatus such as copying machine or printer, and with a process cartridge detachable from the apparatus. As a charging device for an electrophotographic apparatus, a corona charging type has been mainly used which comprises a wire and shield. Recently, however, a contact charging type becomes increasingly used from the standpoint of environment problem, since the ozone product due to it is much less. As one of charging members used for the contact charging type, a magnetic brush is known. With the magnetic brush charging type, the contact chance between a member to be charged and the charging member can be increased, and therefore, it is suitable to an injection charging type by which the current flows through the contact portion between the photosensitive member as the member to be charged and the charging member to inject the charge into the photosensitive member. If, however, the use is made with magnetite as the magnetic particles, the voltage dependence property of the resistance value gives rise to the following problems. Even if the resistance value of the magnetic brush of the magnetic particle of the magnetite is not less than 1x10 4 Ohm with which a pin hole leakage does not occur when 100V DC voltage is applied, the resistance of the magnetic brush is lower with the application voltage at the time of charging (for example - 700V) to such an extent that the leakage occurs at the pin hole of the photosensitive member, with the result of a lateral line in the form of the charging nip extending in the longitudinal direction, on the image as a lateral line. On the other hand, even if the resistance of the magnetic particle is not more than 1x10 7 Ohm with which the charging defect does not occur upon 100V application, the resistance value of the magnetic brush changes during the actual charging operation with the result of the charging defect. SUMMARY OF THE INVENTION Accordingly, it is a principal concern of the present invention to provide a charging device and an image forming apparatus wherein the charge uniformity is improved, and the leakage through the pin hole of the surface of the member to be charged is prevented. European Patent Specification No. EP-A-0598483 discloses an image forming apparatus which uses a magnetic brush to charge an image carrier. The disclosure of this prior document does not provide a solution to the problems already discussed. In accordance with a first aspect of the present invention there is provided a charging device as set out in claim 1. In accordance with a second aspect of the invention there is provided a charging method as set out in claim 8. These and other features and advantages of the present invention will become more apparent upon a consideration of the following invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of an image forming apparatus according to embodiment 1. Figure 2 shows an enlargement a longitudinal section of a photosensitive member and a principle of charge injection according to embodiment 1. Figure 3 is a graph of an application voltage vs. a resistance value of magnetic particles. Figure 4 shows method of measuring a resistance of magnetic particles. Figure 5 is a graph of a rotational frequency of magnetic brush vs. charging fog. Figure 6 is a schematic view of an image forming apparatus according to embodiment 2. Figure 7 is a graph of a charging time vs. photosensitive member potential. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the accompanying drawings, the embodiments of the present invention will be described. < Embodiment 1 > Figure 1 shows a laser beam printer of electrophotographic type as an an example of an image forming apparatus having a charging device according to an embodiment of the present invention. The structure and operation will be described briefly. The image forming apparatus comprises a drum type electrophotographic photosensitive member (photosensitive member) 1 as the image bearing member. The photosensitive member 1 has a diameter of 30mm and is an OPC photosensitive member, which is rotated in the arrow R1 direction at a process speed (peripheral speed) of 100mm/sec. To the photosensitive member 1, an electroconductive magnetic brush as a contact charging member is contacted. The electroconductive magnetic brush 2 has a fixed magnet roller 22 within a rotatable non-magnetic charging sleeve 21, and the magnetic particles 23 are carried on the non-magnetic charging sleeve 21 by the magnetic force of the magnet 22. To the charging member 2, a DC charging bias of -700V is applied from a charging bias application voltage source S1, so that the photosensitive member 1 surface is substantially uniformly charged to approximately - 700V. The thus charged surface 1a of the photosensitive member 1 is exposed to and scanned by a laser beam having an intensity modulated in accordance with time series electric digital pixel signal corresponding to the intended image information, so that an electrostatic latent image thereof is formed. The laser beam is projected from an unshown laser beam scanner including a laser diode, polygonal mirror or the like. The electrostatic latent image is developed into a toner image by using magnetic one component insulative toner. The developing device 3 has a non-magnetic developing sleeve 3a having a diameter of 16mm containing therein a magnet 3b. Negative toner toner is applied on the developing sleeve 3a. It is rotated at the same peripheral speed as the photosensitive member 1 while a fixed gap of 300 µm is maintained therebetween. To the developing sleeve 3a, a developing bias voltage is applied from a developing bias voltage source S2. The voltage is a DC voltage of - 500V biased with a rectangular AC voltage having a frequency of 1800Hz and a peak-to-peak voltage 1600V to effect a jumping development between the developing sleeve 3a and the photosensitive member 1. On the other hand, a transfer material P as a recording material is fed from an unshown sheet feeding portion into a press-contact nip portion (transfer portion) T at a predetermined timing. The press-contact nip portion (transfer portion) T is formed between the photosensitive member 1 and a transfer roller 4 having an intermediate resistance of 10 6 -10 9 Ohm (contact transfer means) press-contacted to the photosensitive member 1 at a predetermined pressure. To the transfer roller 4, a predetermined transfer bias voltage is applied from the transfer bias application voltage source S3. The transfer roller 4 of this embodiment has a roller resistance value of 5x10 8 Ohm, and a DC voltage of +2000V is applied thereto. A transfer material P introduced into the transfer portion T, is advanced by the nip, and the toner image is transferred onto the transfer material P by the pressure and the electrostatic force. The transfer material P now having the transferred toner image thereon is separated from the surface of the drum, and is introduced to a heat fixing type fixing device 5, where the toner image is fixed on the transfer material P. The transfer material P is finally discharged to the outside as a print or copy. On the other hand, the photosensitive member 1, after the toner image is transferred therefrom, is cleaned by a cleaning device 6 so that residual toner or deposited contamination are removed therefrom to be prepared for the next image forming operation. The image forming apparatus of this embodiment is a cartridge type. That is, a process cartridge 20 integrally comprising four process means, namely, the photosensitive member 1, the contact charging member 2, the developing device 3 and the cleaning device 6, is detachably mountable to a main assembly of the image forming apparatus. However, the present invention is applicable to an image forming apparatus of non-cartridge-type. Referring to Figure 2, the description will be made as to the photosensitive member 1. It is of negative charging property, and comprises-an electroconductive base 14 of aluminum having a diameter of 30mm, first - fifth function layers from the bottom. The first layer on the base is an electroconductive primer layer functioning to smooth defects of the aluminum drum base and to prevent moire attributable to the reflection of the laser exposure beam. The second layer is a positive charge injection layer, which functions to prevent the positive charge injected from the aluminum drum base from neutralizing the negative charge applied on the photosensitive member surface. The second layer is an intermediate resistance layer having a thickness of approximately 1 µm. The resistance thereof is adjusted by AMILAN (tradename of polyamide resin material, available from Toray Kabushiki Kaisha, Japan) resin material and methoxymethyl nylon. The third layer is a charge generating layer of disazo pigment dispersed in a resin material and having a thickness of approximately 0,3 µm. It produces a pair of positive and negative charge when it is subjected to laser exposure. The fourth layer is a charge transfer layer of hydrazone dispersed in polycarbonate resin material, and is a P-type semiconductor. Therefore, the negative charge on the photosensitive member surface cannot move through the layer, and can transfer only the positive charge produced in the charge generating layer to the photosensitive member surface. The fifth layer is a charge injection layer as a surface charge injection layer, and is an applied layer of SnO 2 ultra-fine particles dispersed in the light curing acrylic resin material. More particularly, the SnO 2 particles having a particle size of approx. 0.03 µm doped with antimony to lower the resistance thereof are dispersed in the resin material in the amount of 70wt%. The painting liquid thus provided is applied as the charge injection layer into the thickness of approximately 2 µm by dipping. By doing so, the volume resistivity of the photosensitive member surface is lowered to volume resistivity 1x10 12 Ohm.cm from 1x10 15 Ohm.cm in the case of of the charge transfer layer alone. It is preferable that the volume resistivity of the charge injection layer is 1x10 9 -1x10 15 Ohm.cm. The volume resistivity is measured using a sheet-like sample with the voltage of 100V, and it is measured using HIGH RESISTANCE METER 4329A available from Hewlett Packard Japan to which RESISTIVITY CELL 16008A is connected. Referring to Figure 2, the charging device will be described. Designated by 2 in the Figure is an electroconductive magnetic brush as the contact charging member contacted to the photosensitive member 1, and it comprises a non-magnetic electroconductive charging sleeve 21 having an outer diameter of 16mm, a magnet roller 22 therein and magnetic particles 23 on the charging sleeve 21. The magnet roller 22 is fixed, and the charging sleeve 21 is rotatable. The magnetic flux density provided by the magnet is 800x10 -4 T (tesla) at the charging sleeve 21 surface. The magnetic particles 23 are applied on the charging sleeve 21 into a thickness of 1mm and a width of 220mm to form a charging nip with the photosensitive member 1 in a width of approx. 5mm. To the sleeve 21, a DC charging bias of - 700V is applied from charging bias application voltage source S1 so that the surface 1a of the photosensitive member 1 is substantially uniformly charged to - 700V. Figure 5 shows a relation between the rotational frequency of the charging sleeve 21 and image fog due to charging which is indicative of charging power in a reverse development. The fog increases with increase of charging defect when the charge is not sufficiently injected into the photosensitive member, and decreases when the charge is uniformly injected. The positive value of the rotational frequency on the abscissa means the rotation codirectional with the photosensitive member 1 (peripheral movement at the contact portion), the negative value means counterdirectional. As will be understood, the amount of the fog can be decreased by the rotational direction. In the case of the counterdirection, good charging property can be provided since the magnetic particles 23 having departed the charging nip are discharged from the charge-up state during one rotation around the charging sleeve 21, and then the discharged magnetic particles 23 are contacted to the photosensitive member 1. However, in the case of codirection, after the magnetic particles 23 are contacted to the surface of the photosensitive member 1, they sequentially overtake the surface, and therefore, the very charged-up magnetic particles 23 are contacted to the photosensitive member 1 adjacent the exit of the charging nip. For this reason, the charging property is no so good as in the counterdirection. In order to provide the charging property sufficient to prevent fogging not less than 294rpm(peripheral speed 200mm /sec) of the rotational frequency is preferable in the codirectional case, but in the case of opposite direction, it will suffice if the charging sleeve 21 is rotated at a low speed. At the peculiar point of rotational frequency of 0rpm in the graph, the brush is at rest, and the charging property is deteriorated by charge-up. Thus, when the rotational speed of the sleeve 21 is the same, the charging property of less fog can be provided in the counterdirectional case as compared with the codirectional case relative to the movement of the surface of the photosensitive member. The description will be made as to the charging principle when the contact charging member 2 charges the photosensitive member 1. The injection charging type is such that the charge injection is effected into the photosensitive member surface having an intermediate surface resistance by the contact charging member 2 having an intermediate resistance. The injection charging type of this embodiment is not such that the charge is injected into the trap potential of the material of the photosensitive member surface, but the electroconductive particles of the charge injection layer are charged. More particularly, as shown in Figure 2, a fine capacitor constituted by the charge transfer layer 11 as a dielectric member and the aluminum base 14 and the electroconductive particles 12 in the charge injection layer 13 as electrode plates, is charged by the contact charging member 2. The electroconductive particles 12 are substantially electrically independent from each other, so that a kind of fine float electrode is formed. Macroscopically, the photosensitive member surface seems to be charged or discharged uniformly, but actually, a great number of fine charged SnO 2 covers the photosensitive member surface. Therefore, when the image exposure is effected, the electrostatic latent image can be retained since the SnO 2 particles are electrically independent. As for examples of the magnetic particle constituting the magnetic brush 23, the following is considered: Kneaded mixture of resin material and the magnetic powder members such as magnetite is formed into particles, or the one further mixed with electroconductive carbon or the like for the purpose of resistance value control ; Sintered magnetite or ferrite, or the one deoxidized or oxidized is provided for the purpose of control of the resistance value. The above magnetic particles coated with resistance-adjusted coating material (for example, carbon dispersed in the phenolic resin), or plated with metal to adjust the resistance value to a proper level. As for the resistance value of the magnetic particle 23, if it is too high, the charge is not uniformly injected into the photosensitive member 1, with the result of fog image attributable to the fine charging defect. If it is too low, on the contrary, when the photosensitive member surface has a pin hole, the current is concentrated to the pin hole with the result of the voltage drop so that the photosensitive member surface cannot be charged. If this occurs, charging defect in the form of charging nip-like appear on the image. Usually, the resistance value of magnetic particles 23 is measured with one or two application voltages (1-100V), but the resistance value of the magnetic particles 23 changes depending on the applied voltage as shown in graph of Figure 3. The pin hole leakage is determined by the resistance value upon height to the charging member. More particularly, when the pin hole of the photosensitive member comes to the nip portion, the difference between the ground of the photosensitive member base layer and the voltage applied to the magnetic particles, is applied across the magnetic particles at the pin hole portion. Therefore, it is preferable that an excessive current does not flow at this time. In order to accomplish this, the resistance value of the magnetic particle at the maximum application voltage Vmax(V) applied to the charging member is desirably not less than 1x10 4 Ohm. If the resistance value of the magnetic particles is smaller than 1x10 4 Ohm, leakage occurs by the Vmax(V). On the other hand, the charging defect is determined by the resistance value upon low voltage application. In the injection charging type, as shown in Figure 7, with elapse of contact time from the contact start between the photosensitive member and the charging member, the photosensitive member potential (Vd) approaches to the application voltage (Vdc) to the charging member. More particularly, if the photosensitive member potential at the start is OV, then Vc=0, and Vdc=-700V at the time of t=0, and therefore, the voltage (Vdc - Vd) actually applied to the magnetic particles is - 700V. At this time, the charging property is determined by the resistance of the magnetic particle upon 700V application. At a later timing (t=t1), Vd=-500V, and Vdc=-700V, so that the actual voltage across the magnetic particles is - 200V. At this time, the resistance of the magnetic particles upon - 200V application, determines the charging property. Thus, the voltage across the magnetic particles decreases with the approaching to the photosensitive member potential (Vd) to the charging member application voltage (Vdc). The current resistance of the magnetic particles is decisive to the charging property. If the resistance of the magnetic particles upon application of 1V is higher than 1x10 7 Ohm, it is not possible to transfer the charge from the magnetic particles to the photosensitive member within a predetermined period. Therefore, the charging defect occurs. In view of this, the resistance of magnetic particles is preferably not more than 1x10 7 Ohm. The resistance value at the low voltage side is one of the important points in this injection charging type. In the conventional contact charging member, discharge is produced in a small gap, thus charging the photosensitive member, and therefore, the potential difference between the photosensitive member potential and the charging member is required to be higher than a discharge threshold, and therefore, the resistance value at such a low voltage is not a problem. More specific examples will be described. Image formation operations were carried out using the image forming apparatus described above, as to magnetic particles A-D having different resistances. Figure 3 gives the resistance values of magnetic particles A-D at voltages. The results are shown in Table 1. As regards charging property, "G" means that the photosensitive member potential is approx. - 700V after the surface once passed through the charging nip. Sample Resistance (1V) Ohm Resistance (700v) Ohm Vd(Vd) (V) Leakage A 2x10 5 1x10 3 or lower - 700 NG B 8x10 5 3x10 5 - 700 Good C 5x10 7 3x10 6 - 650 Good D 5x10 8 3x10 3 - 630 NG With sample A, the resistance was low upon 700V application, and therefore, the leakage occurred at the pin hole. With sample B, good charging property was exhibited without leakage at the pin hole with the charged level of 700V. With sample C, the resistance upon 1V application was so high that the charging up to 700V was not possible. With sample D, the resistance upon 1V application was so high that the charging to 700V was not possible, and the resistance upon 700V application was so low that the leakage occurred at the pin hole. In this embodiment, it is preferable that the potential of the photosensitive member surface is substantially equal to the application voltage to the charging member after passing through the nip. The potential of the photosensitive member charged by the charging member is preferablely not less than 94% of application voltage. When the application voltage is 700V, the target surface potential is preferably not less than 658V. Sample A is magnetite ; sample B is copper zinc ferrite ; sample C is oxidized copper zinc ferrite ; and sample D is oxidized magnetite of sample A. The ferrite (MO-Fe 2 O 3 ) and magnetite (FeO-Fe 2 O 3 ) have similarities in structure with each other. However, most of ferrite materials have high resistances, whereas in the case of magnetite, the transfer of an electron is quite free between Fe 2+ and Fe 3+ , and therefore, the resistance property shown by A in Figure 3 is exhibited. Also, in the case of ferrite, if the metal ion other than Fe 3+ is smaller than the ionization potential (30.651eV) of Fe 2+ (for example , Al=28.447 and Sc=24.76eV), the transfer of an electron with Fe 3+ is permitted, and therefore, it is predicted that the resistance property shown by A in Figure 3 is exhibited. For this reason, if the third ionization potential of metal other than iron in ferrite is higher than the third ionization potential of iron, such a resistance property that the resistance value is 1x10 4 -1x10 7 Ohms at the application voltage 1-1000V as indicated by B in Figure 3, is exhibited. This is effective for improvement of the charging property and drum pin hole leakage prevention. The resistance value of magnetic particles 23 is measured in the following manner. As shown in Figure 4, 2g of the magnetic particles 23 is placed in a metal cell 7 (bottom area 227mm 2 ) to which voltage is applicable, and thereafter, they are pressed at 6.6kg/cm 2 , and the DC voltage is applied from voltage source S4. Designated by 9 is electrode. The magnetic brush 2 using magnetic particles 23 of copper zinc ferrite having the resistance property B in Figure 3, was formed, and the image evaluation was made using the image forming apparatus. It has been confirmed that the leakage does not occur even if the photosensitive member 1 has a pin hole, and good images have been produced without charging defect. The material of the magnetic particle 23 is not limited to copper zinc ferrite, but resin material carrier is usable if the resistance value is 1x10 4 -1x10 7 Ohms at application voltage 1-1000V. Then, good images can be provided. In the case of ferrite, the material is not limited to copper zinc ferrite. As described above, the third ionization potential of the bivalent metal ion is higher than the third ionization potential of iron ion, since then the resistance value is 1x10 4 -1x10 7 Ohm at application voltage 1-1000V. More particularly, nickel, manganese, magnesium or the like are usable other than copper and zinc. From the standpoint of stability in manufacturing and from the cost, copper zinc ferrite is desirable. The resistance value of 1x10 4 -1x10 7 Ohms upon application voltage 1-1000V may be provided by treating the surface of the magnetic particle 23 to reduce the resistance. < Embodiment 2 > In this embodiment, the untransferred toner after image formation is temporarily collected by charging portion, and is removed by the developing zone, so that a cleaning device only for effecting the cleaning operation is not used. This embodiment is applicable to such an image forming apparatus. The image forming apparatus used in this embodiment is shown in Figure 6. This is the same as embodiment 1, except that the charging member is supplied with an AC biased DC voltage, and that the cleaning device is not used. The AC is applied at the charging portion in order to collect the untransferred toner into the magnetic brush charger and to make uniform the charge polarity of the toner to a regular polarity (the charge polarity is not uniform due to friction among toner particles or friction with the photosensitive member). By doing so, the residual toner is discharged from the magnetic brush to facilitate the collection into the developing zone. In this embodiment, the application voltage applied to the charging member was - 700V, and the AC component had Vpp (peak-to-peak voltage) of 800V and frequency of 1kHz, AC with duty ratio of 50% in the form of rectangular wave. The pin hole leakage in the case of the voltage in the form of AC biased DC, is determined by the maximum application voltage to the charging member. In this embodiment, the resistance of magnetic particlesupon - 1100V application ((- 700) +(- 400)) is to be noted. On the other hand, the charging property is determined by the voltage difference between the DC voltage of the application voltage and the average potential of the photosensitive member surface immediately after one passage through the charging nip once. In this embodiment, the charging substantially to the DC potential is carried out, and therefore, the resistance value of the magnetic particles upon 1V application is to be noted. The magnetic particles used in this embodiment have the resistance of 3x10 5 Ohms upon 1100V application, and 8x10 5 Ohms upon 1V application, as in B in embodiment 1. Therefore, the current does not leak even if the photosensitive member has a pin hole, and the average of the potential of the photosensitive member surface immediately after one passage is 8x10 5 Ohms. Satisfactory charging property can be provided. Thus, also when the AC is superimposed on the DC, the leakage at the pin hole does not occur and the charging property is satisfactory if the resistance value of the magnetic particle is 1x10 4 -1x10 7 Ohm between the application voltage 1V and maximum value. Accordingly, satisfactory images can be provide in the image forming apparatus without the cleaner device.

Claims: A charging device for charging a member to be charged (1), comprising: a charging member to which a voltage is supplied in operation to charge said member to be charged, said charging member having a supporting member (21) for supporting magnetic particles (23) in the form of a magnetic brush (2) of magnetic particles so that the particles are contactable to said member to be charged, and characterised in that the resistance of said magnetic particles is within the range of 1x10 4 -1x10 7 Ohms at any DC voltage applied thereto in the range of 1-1000 volts, wherein the resistance of said magnetic particles is determined for 2g of said magnetic particles placed in a metal cell (7) having a bottom area of 277 mm 2 and compressed at 6.6 kg/cm 2 . A charging device according to claim 1 wherein said member to be charged is an image bearing member. A device according to claim 2, wherein in operation said magnetic brush is moved relative to said member to be charged (1) and the direction of movement of said magnetic particles is opposite from that of said image bearing member at the position at which they contact each other. A device according to either of claims 2 or 3, wherein said image bearing member has a charge injection layer (13) into which charge is injected by contact with said magnetic particles. A device according to claim 4, wherein said charge injection (13) layer has a volume resistivity of 1x10 9 -1x10 15 Ohm.cm. A device according to any one of claims 2 to 5, wherein said magnetic particles comprise a ferrite material containing iron and metal other than iron, and in which the third ionization potential of metal other than iron is higher than the third ionization potential of iron. A process cartridge (20) removably mountable on an electrophotographic reproduction apparatus, the process cartridge including a device as claimed in any of claims 2 to 6. A method of charging the image bearing member (1) of an electrophotographic device comprising the step of applying a DC voltage in the range of 1-V max volts to a charging member, said charging member comprising a supporting member for supporting magnetic particles in the form of a magnetic brush of magnetic particles which contacts the image bearing member, and wherein the resistance of said magnetic particles is within a range of 1x10 4 -1x10 7 Ohms at any DC voltage applied thereto in the range of 1-1000 volts, wherein the resistance of said magnetic particles is determined for 2g of said magnetic particles placed in a metal cell having a bottom area of 277 mm 2 and compressed at 6.6 kg/cm 2 , and where V max volts is a maximum value applied to said charging member when the member to be charged is actually charged.

Inventors: FURUYA TADASHI, ISHIYAMA HARUMI, YANO HIDEYUKI, FURUYA, TADASHI, ISHIYAMA, HARUMI, YANO, HIDEYUKI, FURUYA, TADASHI, CANON K.K., ISHIYAMA, HARUMI, CANON K.K., YANO, HIDEYUKI, CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1995
Main classification: G03G15
IPC codes: G03G9, G03G15