Charging device

Abstract: The invention provides an apparatus for charging an imaging surface (10a) of photoreceptor. The apparatus forms a magnetic brush on a cylinder facing (22) with a space the photoreceptor by a magnet (23) disposed in the cylinder. The cylinder (22) and the magnet (23) are relatively rotatable to each other so that the magnetic brush moves around the cylinder (22) and comes in contact with the imaging surface (10a) of the photoreceptor. An electric bias (24) source is provided to apply an electric bias voltage superimposed DC bias voltage and AC bias voltage betweeen the imaging surface (10a) of the photoreceptor and the cylinder (22), whereby the imaging surface (10a) is charged by the magnetic brush under the electric bias voltage. The magnetic brush is also used for cleaning the imaging surface (10a) of photoreceptor.

Description: BACKGROUND OF THE INVENTION The present invention relates to a charging device as defined by the features of the preamble of claim 1 employing a magnetic brush which charges uniformly an image-forming object with electricity in an image forming apparatus such as an electrophotographic copying machine or the like. Heretofore, there has generally been used a corona charging unit for charging an image-forming object such as a photoreceptor drum or the like in an image forming apparatus of an electrophotographic type. In the corona charging unit, high voltage is impressed on a discharge wire around which an intensive electric field is caused for gaseous discharge, and charged ions produced by the gaseous discharge are adsorbed on the image-forming object, thus the image-forming object is charged. Such conventional corona charging unit employed in an image forming apparatus has an advantage that an image-forming object is not damaged when it is charged because it can be charged without touching mechanically the corona charging unit. However, the corona charging unit has a disadvantage that there are a risk of an electric shock and an electric leakage due to high voltage used therein, and ozone produced in gaseous discharge is harmful for the human body and they shorten life of the image-forming object. Further, charging voltage of a corona charging unit is unstable because it is highly affected by temperature and humidity, and the corona charging unit requires several seconds to obtain stable charging voltage after inputting of high voltage, which is a serious problem when an image forming apparatus of an electrophotographic type is used as a communication terminal or an information processor. Many disadvantages of a corona charging unit mentioned above are caused by gaseous discharge accompanying the charging. In this connection, as a charging device capable of charging an image-forming object without requiring gaseous discharge as in the corona charging unit and without giving mechanical damage to the image-forming object, there is disclosed a charging device in Japanese Patent Publication No. JP-A-59133569 (hereinafter referred to as Japanese Patent O.P.I. Publication) wherein a magnetic brush formed by adherence of magnetic particles on a cylinder holding therein a magnet can brush, for charging, the surface of an image-forming object. However, even in the case of the charging device disclosed in the aforementioned Japanese Patent Publication, it has been impossible to charge an image-forming object uniformly with perfect stability. SUMMARY OF THE INVENTION The first object of the invention is to solve the aforementioned problems and to provide a charging device capable of charging uniformly with perfect stability without requiring any occurrence of ozone. The aforementioned first object of the invention can be attained by a charging device as defined in claim 1. This charging device or apparatus comprises a cylinder which is rotatable around magnets having magnetic poles outside and a magnetic brush composed of a magnetic particle layer adhered on the cylindrical surface. The cylinder is moved, for charging an image-forming object, in the direction identical with or opposite to the moving direction of the image-forming object so that the magnetic brush impressed with voltage may brush the image-forming object, wherein the magnetic particles have an electrical resistivity of 10 5 to 10 12 Ω·cm and the voltage to be impressed on the aforementioned magnetic brush is DC voltage containing AC bias component. The second object of the invention is to provide a charging and cleaning apparatus wherein no ozone is produced. This object is attained by an apparatus as defined in claim 9. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing schematic constitution of an electrostatic recording apparatus equipped with a charging device that attains the first object of the invention, Fig. 2 is a sectional view showing an example of a charging device cf the invention, and Fig. 3 is a charging characteristic diagram obtained by changing frequency and voltage of AC voltage component. Fig. 4 is a sectional view showing schematic constitution of an image forming apparatus provided with a charging and cleaning device which attains the second object of the invention, Fig. 5 is a sectional view showing an example of a charging and cleaning device of the invention, Fig. 6 is a graph showing characteristics of a high gamma photoreceptor, and Fig. 7 is a sectional view showing an example of constitution of the high gamma photoreceptor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With regard to the particle size of magnetic particles used for the invention, when an average particle size of magnetic particles is large, a magnetic brush generally tends to be uneven, resulting in uneven charging, even when charging while giving vibration by means of an electric field, because (a) each bristle of the magnetic brush formed on a brush carrier is coarse. For solving this problem, it is enough to reduce an average particle size of carrier particles, and experiments have shown that an effect of reduced average particle size starts appearing when the average particle size is 100 µm or less, and the problem of the aforesaid (a) does not happen substantially when the average particle size is 70 µm or less. However, when particles are too small, the particles tend to adhere to the surface of an image carrier, or to scatter during charging. These phenomena are related to the intensity of a magnetic field acting on particles and the intensity of magnetization of particles caused by the magnetic field, and they generally appear clearly when an average particle size of particles is 30 µm or less. From the foregoing, an average particle size for magnetic particles of 100 µm or less is preferable and that ranging from 70 µm to 30 µm is more preferable. Incidentally, the intensity of magnetization of 20 - 200 Am 2 /kg (20-200 emu/g) is preferably used. For obtaining the aforementioned magnetic particles, the particles of ferromagnetic materials such as iron, chromium, nickel and cobalt, or the compounds thereof or alloy such as, for example, tri-iron tetroxide, γ-ferric oxide, chromium dioxide, manganese oxide, ferrite and manganese-copper alloy, or the surfaces of these magnetic substances are covered by resins such as styrene type resin, vinyl type resin, ethylene type resin, rosin denatured resin, acrylic resin, polyamide resin, epoxy resin and polyester resin, or the particles are made from resins wherein fine particles of magnetic materials are dispersed, and particles thus obtained are subjected to the particle size selection by means of a conventional average particle size selecting means. Incidentally, magnetic particles each being formed to be a sphere provide an advantage that a particle layer formed on a carrier can be uniform and high voltage can be impressed on the carrier. Namely, magnetic particles each being formed to be a sphere provide advantages that (1) magnetization in the direction of a major axis of a magnetic particle is eliminated and thereby a layer can be formed uniformly, preventing occurrence of a partial area with lower electrical resistance and of unevenness in layer thickness, and (2) resistance of magnetic particles is increased and edges seen on a conventional particle are eliminated, thereby, concentration of electric field to the edge does not take place, accordingly, even if high bias voltage is impressed on a carrier for magnetic particles, uniform discharging is carried out on the surface of an image carrier, preventing uneven charging. As a sphere particle providing the aforementioned effect, it is preferable that magnetic particles are formed so that electrical resistivity of carrier particles thereof may be 10 5 Ω·cm or more, and especially not more than 10 12 Ω·cm. This electrical resistivity corresponds to the value obtained by reading a current value after putting particles in a container having a sectional area of 0.5 cm 2 and tapping, applying a load of 1 kg/cm 2 on the stuffed particles, and applying voltage between the load and an electrode on the bottom for generating an electric field of 1000 V/cm. When a voltage is impressed on a particle carrier under the condition of low electrical resistivity, an electric charge transfers to magnetic particle and thereby magnetic particles tend to stick on the surface of an image carrier, or breakdown of bias voltage tends to occur. When the electrical resistivity is high, on the other hand, the electric charge does not enter a magnetic particle and thereby no charging is carried out. In a summary of the foregoing, it is an optimum condition that a magnetic particle is subject to a rounding treatment so as to eliminate protrusion such as a needle-shaped portion or an edge. As a result, the magnetic particle has a ratio of the major axis and the minor axis of 3 or less and an electrical resistivity is not less than 10 5 Ω·cm, preferably not less than 10 5 Ω·cm and not more than 10 12 Ω·cm. These spherical magnetic particles are manufactured through selecting magnetic substances which are spherical or close to a sphere, using fine particles of magnetic substances as far as possible for particles of a magnetic substance fine particle dispersion type, applying a rounding treatment after forming dispersion resin particles, and through causing particles to have dispersion resin particles by means of a method of spray dry. The foregoing represents conditions for magnetic particles. Next, the conditions for a magnetic particle carrier on which a particle layer is formed and an image carrier is charged will be explained. As a magnetic particle carrier, a particle carrier capable of being impressed with bias voltage is used. In particles, there is preferably used a particle carrier having the structure wherein a magnetic body having a plurality of magnetic poles is provided inside the sleeve the rotatable cylinder, on the surface of which the particle layer is formed. In the case of the particle carrier mentioned above, a particle layer formed on the surface of the sleeve moves while undulating like a wave. Therefore, fresh magnetic particles are also supplied in succession, the unevenness in thickness can be leveled fully by the aforementioned wave undulation so that no problem may be caused in practical use even when there caused unevenness in thickness of particle layer on the surface of the sleeve. It is preferable that the speed of carrying magnetic particles caused by the rotation of the sleeve, or further by the rotation of the magnetic body is mostly identical to or higher than the moving speed of the image carrier. Further, the direction for carrying particles caused by the rotation of the sleeve may either be the same or be opposite. When considering the cleaning efficiency, however, the opposite direction is better than the same direction for the cleaning efficiency. However, the invention is not limited to the foregoing. Further, with regard to the thickness of a particle layer formed on the particle carrier, the thickness that can be scraped off sufficiently by a brush thickness regulating plate to be leveled is preferable, and it is desirable that the clearance between a particle carrier and an image carrier is 100 - 5,000 µm. When the clearance between the surface of the particle carrier and that of the image carrier is smaller than 100 µm, it gets difficult to form bristles of a magnetic brush that can electrify uniformly the surface of the image carrier, and it gets impossible to supply sufficient magnetic particles to the charging portion, thus stable charging can not be carried out. When the clearance exceeds 5000 µm, on the other hand, a particle layer is formed coarsely and uneven charging tends to take place, and further, efficiency of transfer of electric charge to the particles is reduced, thus sufficient charging can not be carried out. As stated above, when the clearance between the particle carrier and the image carrier is extremely small or extremely large, it gets impossible to adjust the thickness of the particle layer on the particle carrier for that clearance. However, when the thickness is in the range of 100 - 5000 µm, it is possible to adjust the thickness of the particle layer properly for that clearance. The reason for this is that the occurrence of swept marks caused by brushing of a magnetic brush can be prevented. In the concept for achieving the second object of the invention, residual toners adhering on the image carrier after development are removed by brushing of the magnetic brush, and the magnetic brush device in the present invention is suitable for a regular image forming apparatus wherein positive development is conducted. For example, in an image forming apparatus wherein an image carrier is charged positively, positive development is carried out with toners charged negatively. Therefore, when the magnetic particles which charges toner negatively are used, toners being brushed adhere on magnetic particles of a magnetic brush, thus toners are collected from the image carrier. When toners are mixed in a charging magnetic brush, the resistance of the magnetic brush is increased, resulting in the reduction of charging efficiency. Therefore, the magnetic brush which has finished brushing the image carrier is caused to contact with a collection roller impressed with positive DC voltage higher than charging voltage, thus it is possible to collect the toners on the magnetic brush to the collection roller. The invention includes also a cleaning device which is suitable for a reversal image forming apparatus wherein reversal development is conducted. For example, in an image forming apparatus wherein an image carrier is charged positively, reversal development is conducted with toners charged positively. Therefore, when the magnetic particles which charge the toner positively are used, the toners adhere to a magnetic particle of a magnetic brush while it is brushing, thus toners are collected from the image carrier. In this case, voltage to be impressed on the magnetic brush is the same as toners in terms of polarity. Therefore, even if a part of toners adhere to a photoreceptor as an image carrier, it does not affect image formation wholly because the polarity of toners remains unchanged. However, when toners are mixed in a charging magnetic brush the resistance of the magnetic brush is increased, resulting in the reduction of charging efficiency. Therefore, the magnetic brush which has finished brushing the image carrier is caused to contact with a collection roller impressed with positive or negative DC voltage that is lower than charging voltage, thus it is possible to collect toners on the magnetic brush to the collecting roller. Fig. 1 is a sectional view showing schematic constitution of an electrophotographic recording apparatus equipped with a charging device that achieves the first object of the invention. In the figure, the numeral 10 is a photoreceptor drum that is an image forming object rotating in the arrowed direction, and it is surrounded by charging unit 20, neutralizing unit 12, image-wise exposure L from an exposure unit, developing unit 30, transfer roller 13 and cleaning unit 50, all of which will be explained later. In the basic operation for copy process in the present example, when a command for the start of copying is sent from an unillustrated operation unit to an unillustrated control unit, the photoreceptor drum 10 starts rotating in the arrowed direction due to the control made in the control unit. When the photoreceptor drum 10 rotates, its circumferential surface is charged uniformly by the charging unit 20 which will be explained later, and passes through the neutralizing unit 12. The neutralizing unit 12, due to the control of the aforementioned control unit, neutralizes the charge at a marginal area outside an image area by means of illumination with LED, for example. However, in the reversal development which will be described later, the neutralizing unit 12 is not needed. On the photoreceptor drum 10, image writing is conducted, for example, by a scanning exposure unit that scans an unillustrated document or by an image writing unit by means of laser beam L, thus electrostatic latent images corresponding to the image of document are formed. The developing unit 30 contains therein two-component developer which is stirred by stirring screws 33A and 33B and then adheres on the external surface of developing sleeve 31 that rotates outside magnet roller 32 for forming a magnetic brush of developers. On the developing sleeve 31, predetermined bias voltage is impressed and development is conducted in the developing area that faces the photoreceptor drum 10. In this case, when an ordinary scanning optical system is used for forming an latent image, regular development is carried out, while when image-wise exposure by means of a laser beam is conducted, reversal development is usually conducted. The aforementioned electrostatic latent image on the photoreceptor drum 10 is developed by the developing unit 30 to become a visible toner image. From the sheet feed cassette 40, recording sheet P is fed out one sheet by one sheet by the first sheet feed roller 41. The recording sheet P thus fed out is sent onto the photoreceptor drum 10 by the second sheet feed roller 42 that operates synchronously with the aforementioned toner image on the photoreceptor drum 10. The toner image on the photoreceptor drum 10 is transferred onto the recording sheet P by an action of transfer roller 13, and then the recording sheet is separated from the photoreceptor drum 10. The recording sheet P on which the toner image has been transferred is sent to an unillustrated fixing unit through transport means 80, and then is sandwiched between a heat fixing roller and a pressure roller so that the toner image may be melted and fixed on the recording sheet. After that, the recording sheet is ejected to the outside of the apparatus. The surface of the photoreceptor drum 10 that rotates while holding thereon residual toners which have failed to be transferred onto the recording sheet P is scraped by cleaning unit 50 equipped with blade 51 and is cleaned for the next cycle of copying. Fig. 2 is a sectional view showing an example of charging device 20 of the invention to be used for the electrostatic recording apparatus shown in Fig. 1. In the figure, the numeral 21 represents magnetic particles wherein spherical ferrite particles coated to be conductive are used. As an alternative, it is possible to use conductive magnetic resin particles which are obtained through thermal kneading of magnetic powder and resin and pulverizing thereof. For the better charging, each of particles is adjusted so that its external shape may be spherical having the particle size of 50 µm and its resistivity may be 10 8 Ω·cm. The numeral 22 is a conductive cylinder made of nonmagnetic metal, 23 is a column-shaped magnet bar (roll) arranged inside the conductive cylinder 22. The magnet bar 23 shown in the figure is magnetized to have an S-pole and an N-pole outside, and the conductive cylinder 22 is supported rotatably against the fixed magnet bar 23. Further, the magnet bar 23 which has an equally divided magnetic poles may also rotate. The conductive cylinder 22 is rotated so that its circumferential speed at the position where it faces the photoreceptor drum 10 may be 1.2 - 2.0 times faster than that of the photoreceptor drum 10 and the direction of the movement of the conductive cylinder at the aforementioned position may be the same as that of the photoreceptor drum 10. The photoreceptor 10 is composed of conductive base material 10b and photoreceptor layer 10a which covers the conductive base material 10b, and the conductive material 10b is grounded. The numeral 24 is a power source for bias voltage which applies bias voltage between the aforementioned conductive cylinder 22 and the conductive base material 10b, and the conductive cylinder 22 is grounded through the power source for bias voltage 24 and a protective resister 28. The aforementioned power source for bias voltage 24 is a power source that supplies AC bias voltage in which AC component is superimposed on DC component set to the value of voltage identical to the value of voltage to be charged. The value of the bias voltage depends on the clearance distance between the conductive cylinder 22 and the photoreceptor drum 10 and the surface voltage of the photoreceptor when its clearance, is kept in the range of 0.1 - 5 mm, to be charged preferable charging conditions were successfully obtained by supplying AC bias voltage in which AC component of 200 - 3500 V was superimposed, as Peak - Peak voltage (V p-p ), on DC component of 500 - 1000 V which is mostly the same as voltage to be charged. Incidentally, in the power source for bias voltage 24, DC component is controlled on the constant-voltage control basis and AC component is controlled on the constant-current control basis. The numeral 25 is a casing which forms a storage portion for the aforementioned magnetic particles 21, and the aforementioned conductive cylinder 22 and magnet-bar 23 are arranged in this casing 25. On the outlet of the casing 25, there is provided regulating plate 26 which regulates the thickness of the layer of magnetic particles 21 which adhere to the conductive cylinder 22 and carried, thus the gap between the photorecptor drum 10 and the conductive cylinder 22 is filled with the layer of magnetic particle 21 having the regulated thickness. Next, operations of the aforementioned charging device 20 will be explained as follows. When the conductive cylinder 22 is rotated in the arrowed direction at the circumferential speed that is 1.2 - 2.0 times higher than that of the photoreceptor drum 10 while the photoreceptor drum 10 is rotated in the arrowed direction, magnetic particles 21 adhering to the conductive cylinder 22 and being carried thereby are connected magnetically each other to form a brush at the position on the conductive cylinder 22 facing the photoreceptor drum 10, due to lines of magnetic force of magnet bar 23, thus a so-called magnetic brush is formed. The magnetic brush is conveyed in the rotating direction of the conductive cylinder 22 and touches and brushes the photoreceptor layer 10a of the photoreceptor drum 10. Since the aforementioned bias voltage is applied between the conductive cylinder 22 and the photoreceptor drum 10, electric charges are injected into the photoreceptor layer 10a through conductive magnetic particles 21, thus charging is conducted. In this case, due to the application of AC bias voltage, a vibration contributes to charge injection from the magnetic brush, resulting in extremely stable and uniform charging. The stirring plate 27 is a rotating object having around its shaft plate-shaped members which correct the deviation of magnetic particles 21. Incidentally, Fig. 3 shows the results obtained after changing the frequency and voltage for the components of AC voltage to be impressed on sleeve 2 in the example mentioned above, In Fig. 3, a vertically-hatched zone is an area where dielectric breakdown tends to occur, an obliquely-hatched zone is an area where uneven charging tends to occur, and unhatched zone is a desirable area where stable charging can be conducted. As is clear in the figure, the desirable area slightly changes depending on a change of AC voltage component. A waveform of AC voltage component may also be a square wave or a chopping wave, without being limited to a sine wave. Further, in Fig. 3, a low frequency zone that is shaded with fine dots is an area where uneven charging takes place because of a low frequency. For obtaining non-charging in the present example, it is possible to do so by causing DC component in bias voltage to be zero. Further, when the direction of poles NS of magnet bar 23 is turned to be in parallel with a tangent at a position facing the photoreceptor drum 10, bristles of a magnetic brush are arranged by a horizontal magnetic field to be in parallel with the direction of a tangent at a point facing the photoreceptor drum 10 and thereby the tip of the magnetic brush is separated from the photoreceptor drum 10, thus it is possible to create the state of non-charging. Incidentally, when much toners that remain on the surface of the photoreceptor drum 10 without being cleaned due to the operation of an apparatus for a long time enter a layer of magnetic particle 21, the resistance of the magnetic brush is increased and charging efficiency is deteriorated. Therefore, toners must be prevented from entering a layer of magnetic particles. For this purpose, it is possible to design magnetic particles so that they have frictional electrification with toners and it is possible to provide, in the charging device 20, a collection roller to be impressed with voltage for generating an electric field that attracts toners for causing the entered toners to be adhered by an electric field and collected. When the polarity of DC bias voltage to be impressed on the conductive cylinder 22 is the same as that of charged toners, the toners easily adhere to the photoreceptor, and thereby it is possible to prevent the toners from entering. Especially when the charging polarity of the photoreceptor drum 10 is the same as that of the toners as in the case of an image forming apparatus conducting reversal development, the charging polarity of the photoreceptor 10 is identical to the polarity of the toners in a developing unit, thus no fogging appears on images in the course of developing, resulting in a preferable combination. Or, in order to remove the entered toners, when AC bias voltage wherein AC component is superimposed on DC component whose polarity is opposite to that in the aforementioned charging to the photoreceptor 10 is impressed by power source for bias voltage 24 controlled by a power source control means is applied between the conductive cylinder 22 and the photoreceptor drum 10, unlike the aforementioned charging of the photoreceptor drum 10, the toners and dusts adhering to magnetic particles 21 or entered in casing 25 move toward the photoreceptor drum 10 and adhere thereto. In this case, since bias voltage is AC bias voltage in particular, it is possible to cause the toners and the dusts to move from the magnetic particles 21 to the photoreceptor drum 10 efficiently to adhere thereto, thus it is possible to remove the toners and the dusts entered magnetic particles 21. With regard to timing for removing the adhering substances, it is possible to remove them during non-image-forming period, such as, for example, the period of power-on for an image forming apparatus for warming-up. Or, by releasing adhered substances to non-image-forming portion between each image to be accumulated there during a period of image formation, image quality is not lowered even in the case of continuous image formation. On the occasion of non-image-forming, power source control means controls the aforementioned power source for bias voltage 24 so that it may supply AC bias voltage wherein AC component is superimposed on DC component having the polarity opposite to that in the aforesaid charging. Namely, when AC bias voltage wherein AC component at 200 V - 3500 V is superimposed as (V p-p ) on DC voltage at -100 V - -1000 V is applied, toners adhering to magnetic particles 21 move to the photoreceptor drum 10 and adhere thereto. Furthermore, DC component only may be impressed unlike the case in the present example wherein AC component is superimposed on DC component. However, it is possible to remove toners and dusts adhering to magnetic particles 21 more efficiently if AC component is also superimposed. In any event, charging means 20 can be refreshed to recover its charging efficiency when substances accumulated on magnetic particles 21 are caused to adhere to the photoreceptor drum 10 to be removed collectively by cleaning means 50. Thus, it is possible not only to charge stably at all times but also to keep the surface of the photoreceptor drum 10 clean constantly. Therefore, it is possible to form images stably at all times without deteriorating image quality of toner images to be formed. The present invention can provide a charging device wherein applying voltage can be low because electric charges can be injected directly into a photoreceptor drum, occurrence of ozone can be prevented, and extremely stable and uniform charging can be carried out due to superimposing AC bias voltage. An example achieving the second object of the invention will be explained as follows. Fig. 4 is a sectional view showing the schematic constitution of an image forming apparatus equipped with a charging unit and a cleaning unit both of the present invention. In the figure, the numeral 101 is a drum-shaped photoreceptor that is an image forming object which rotates in the arrowed direction (clockwise), and it is surrounded by charging and cleaning unit 120, developing unit 130, and transfer belt 150. The photoreceptor 101 is a high-γ type photoreceptor composed of photosensitive layer 1A, interlayer 1B and conductive support 1C as shown in Fig. 7. The thickness of the photosensitive layer is 5 - 100 µm and preferably is 10 - 50 µm. In the photoreceptor 101, there is used drum-shaped conductive support 1C made of aluminum having thereon interlayer 1B that is made from ethylene-vinylacetate copolymer and has thickness of 0.1 µm on which photosensitive layer 1A having layer thickness of 35 µm is provided. As the conductive support 1C, there is used a drum made of aluminum, steel or copper, and in addition to that, a belt-shaped one wherein a metallic layer is laminated or evaporated on a paper plastic film, or a metallic belt such as a nickel belt prepared through electroforming method may be used. On the other hand, it is preferable that the interlayer has a hole mobility so that the photoreceptor may withstand high voltage of ±500 - 2000 V, and when charging positively, for example, electron may be prevented from being injected from conductive support 1C and excellent steep light decay characteristic by avalanche phenomenon may be obtained. It is therefore preferable that charge transport substances of a positively charging type described in Japanese Patent Publication No. JP-A-63 44 662 proposed by the applicant of the invention are added to the interlayer 1B in quantity of not more than 10% by weight. As the interlayer 13, it is generally possible to use the following resins, or example, used in a photosensitive layer for electrophotography. (1) Vinyl type polymer such as polyvinyl alcohol (poval) (2) Nitorogen-containing vinyl polymer such as polyvinylamine (3) Polyether type polymer such as polyethyleneoxide (4) Acrylic acid type polymer such as polyacrylic acid and its salt (5) Methacrylic acid polymer such as polymethacrylic acid and its salt (6) Cellulose ether type polymer such as methylcellulose (7) Polyethyleneimine type polymer such as polyethyleneimine (8) Polyamino acid such as polyalanine (9) Starch and its derivative such as starchacetate and amine starch (10) Polymer soluble in a mixed solvent of water and alcohol, such soluble nylon as polyamide The photosensitive layer 1A is basically formed by coating on the interlayer a coating solution prepared by mixing and dispersing photoconductive phthalocyanine fine pigment particle having a particle size of 0.1 - 1 µm and antioxidant in a binder resin solvent without adding any charge transport substances together, drying it and, when necessary, heat-treating it. When using photoconductive materials and charge transport substances in combination, photoconductive pigment as photoconductive material and a small amount of charge transport substance that is one fifth of the photoconductive pigment by weight, preferably one thousandth to one tenth of the photoconductive pigment by weight, and antioxidant are dispersed in binder resin to make the photosensitive layer 1A. When a part of toner images untransferred in the reversal development is remaining on the photoreceptor 101 even after cleaning and charging both related to the invention, a photoreceptor having its spectral sensitivity at the long wavelength side and infrared-rays-permeable toners are necessary so that a beam from a scanning optical system may occur photoconductive action on the photoreceptor not to be shielded by color toner images. Light-decay characteristic of a high-γ type photoreceptor in the present example will be explained as follows. Fig. 6 is a graph showing characteristic of a high-γ type photoreceptor. In the figure, V 1 is charged voltage (V), V 0 is initial voltage (V) just before exposure, L 1 is an energy amount (µJ/cm 2 ) of illuminating light of a laser beam that is needed for the initial voltage V 0 to decay to 4/5 of the initial voltage, and L 2 is an energy amount (µJ/cm 2 ) of illuminating light of a laser beam that is needed for the initial voltage V 0 to decay to 1/5 of the initial voltage. The preferable range of L 2 /L 1 is as follows. 1.0 ≦ L 2 /L 1 ≦ 1.5 The present example has the following conditions. V 1 = 1000 V, V 0 = 950 V, L 2 /L 1 = 1.2 The photoreceptor surface voltage in exposed area is 10 V. There is selected a photoconductive semiconductor that satisfys the relation of (E 1/2 )/(E 9/10 ) ≥ 2 preferably of (E 1/2 )/(E 9/10 ) ≥ 5 wherein, E 1/2 is photosensitivity at the position corresponding to the middle period of exposure where the initial voltage V 0 is decayed down to a half thereof in a light decay characteristic curve, and E 9/10 is photosensitivity at the position corresponding to the initial period of exposure where the initial voltage V o is decayed down to 9/10 thereof. In this case, the photosensitivity is defined in terms of the absolute value of voltage decaying to extremely small quantities of light. In the light decay curve of the present photoreceptor, the absolute value of a differential coefficient for voltage characteristics having the photosensitivity shown in Fig. 6 is small in the case of small quantities of light exposure, and it is decayed sharply as quantities of light exposure increase. To be concrete, a light decay curve shows, in the initial period of exposure, a flat curve for a certain period L 1 representing poor sensitivity characteristic as shown in Fig. 6, but in the middle period of exposure ranging from L 1 to L 2 , the light decay curve changes suddenly to ultra-high γ that falls linearly showing ultra-high sensitivity. It is considered that the photoreceptor actually shows high γ characteristic by an avalanche phenomenon under high potential of +500 - +2000 V. Namely, it is construed that carriers generated on the surface of a photoconductive pigment in the initial period of exposure are trapped effectively in the boundary layer between the photoconductive pigment and a covering resin, thus, light decay is surely inhibited, resulting in an extremely sudden avalanche phenomenon in the middle period of exposure. The photoreceptor of this kind has a special feature that the uneven charging and insufficient cleaning for the photoreceptor is inconspicuous on the image quality because recording is carried out on a binary basis. In the basic operation of copy process in the present example, when a copy start command is sent from an unillustrated operation unit to an unillustrated control unit, the photoreceptor 101 starts rotating in the arrowed direction, being controlled by the control unit. When the photoreceptor 101 rotates, the circumferential surface thereof is cleaned and charged uniformly by charging and cleaning unit 120. On the photoreceptor 101, there is conducted image writing by means of laser beam L, for example, from an unillustrated image writing device, thus, electrostatic latent image corresponding to the image is formed on the photoreceptor 101. DC bias voltage or DC plus AC bias voltage is impressed on the developing sleeve 131 of developing unit 130 and then non-contact development with two-component developer is conducted thereon to form an toner image. In this case, either contact development by means of two-component developer and contact or non-contact development by means of mono-component developer may be used. Toner images thus formed on the photoreceptor 101 are transfered on an image receiving sheet which is sent one by one by the first sheet feed roller from an unillustrated sheet feed cassette and successively sent by the second sheet feed roller 142 synchronizing with the aforementioned toner images, to be moved in the arrowed direction. Namely, the toner image mentioned above is transferred on an image receiving sheet sent onto a transfer belt 150 which is started running before the transfer to make the image receiving sheet contact with the photoreceptor. The aforesaid transfer belt 150 is spread between roller 159 and roller 160 and is rotated by the roller 160 to synchronize with the circumferential speed of the photoreceptor 101, and it is separated from or contacted to the photoreceptor 101 depending respectively on the upward movement or downward movement of bias roller 158. For the aforementioned transfer belt 150, a conductive cloth-padded rubber belt is used as a basic support, and a high resistivity layer or an insulator layer made of an elastic material having the thickness of 0.5 mm is provided on the external surface of the cloth-padded rubber belt. The aforementioned transfer is conducted by the bias roller 158 when transfer voltage whose polarity is opposite to that of toners is impressed on the bias roller 158. Incidentally, toners sticking to the transfer belt 150 are removed and cleaned by cleaning unit 153. The image receiving sheet onto which toner images have been transferred in the aforesaid manner, is separated from the circumferential surface of the photoreceptor 101 and then is ejected by a sheet delivery roller to the outside of the apparatus after being transported to an unillustrated fixing unit wherein toners on the image receiving sheet are melted and fixed on its surface. On the other hand, the photoreceptor 101 after the image receiving sheet has been separated is neutralized by neutralizing lamp 151 and then is cleaned by the charging and cleaning unit 120 which removes residual toners staying on the photoreceptor 101, to be on standby for the following print cycle. Fig. 5 is a sectional view showing an example of charging and cleaning unit 120 of the invention used for the electrostatic recording apparatus in Fig. 1. In the figure, the numeral 121 represents magnetic particles, and in an embodiment, conductively coated spherical ferrite particles were used. For the excellent charging, the particles are prepared to be a spheric external form, a particle size of 50 µm and specific resistivity of 10 8 Ω·cm. As an alternative, conductive magnetic resin particles obtained by pulverizing magnetic particles and resins mixture as the principal ingredients after thermal kneading thereof may also be used. The numeral 122 is a conductive cylinder made of nonmagnetic metal and the numeral 123 is a bar-shaped magnet bar (roll) arranged inside the conductive cylinder 122. The magnet 123 is magnetized to have therein an S-pole and an N-pole, and the conductive cylinder 122 is supported rotatably against the fixed magnet 123. The magnet 123 may rotate as a homopolar arranged pole. Magnetic force of the magnet 123 is not less than 0.06 T (600 gauss), and the aforementioned magnetic particles 121 are magnetized to 50 Am 2 /kg (emu/g). Further, conductive cylinder 122 is rotated in the direction opposite to the moving direction of the photoreceptor 101 at the point where the conductive cylinder faces to the photoreceptor 101 at the circumferential speed that is 1.2 - 2.0 times higher than that of the photoreceptor 101. The conductive support 101C of the photoreceptor 101 is grounded. The numeral 124 is a power source for bias voltage that applies bias voltage between the aforementioned conductive cylinder 122 and conductive support 101C, and the conductive cylinder 122 is grounded through this power source for bias voltage 24. The aforementioned power source for bias voltage 124 is a power source that supplies AC bias voltage wherein AC component is superimposed on DC component established at the value identical to the voltage to be impressed, and it applies voltage through protective resistor 124a. The conditions for impression of voltage depend upon the distance between the conductive cylinder 122 and the photoreceptor 101 and charging voltage on the photoreceptor 101. It was possible to obtain preferable conditions for charging by supplying AC bias voltage wherein AC component of 200 - 3500 V is superimposed as Peak-Peak voltage on DC component of 500 - 1000 V that is mostly the same as charging voltage to be applied, under the condition that the clearance is kept in the range of 0.1 - 5 mm. For avoiding uneven charging, the frequency of 300 Hz - 10 kHz is preferable. Incidentally, in the power source for bias voltage 124, constant-voltage control is applied to DC component and constant-current control is applied to AC component. The numeral 125 is a casing that forms a storage area for the aforementioned magnetic particles 121, and the aforementioned conductive cylinder 122 and magnet 123 are located in the casing 125. At the outlet of the casing 125, there is provided regulating plate 126 which regulates the thickness of a layer of magnetic particles 121 which adheres to the conductive cylinder 122 and is conveyed thereby so that the thickness of the layer may match the established clearance for development, thus the clearance between the photoreceptor 101 and the conductive cylinder 122 is filled with the layer of magnetic particles 121 regulated in terms of its thickness. The numeral 127 is a toner-collecting roller that is impressed with bias voltage whose polarity is opposite to that for charged toner T, the numeral 128 is a stirring plate which rotates a plate-shaped member around a shaft that corrects ill-balanced magnetic particles layer 121 on the cylinder 122, the numeral 129 is a toner-collecting blade that scrapes off collected toner T from the collecting roller 127, the numeral 191 is a toner-collecting screw that conveys collected toner T to a collecting box or to developing unit 130. Incidentally, in the example mentioned above, the results obtained by changing frequency and voltage of AC voltage component which is impressed on the conductive cylinder 122 are the same as those shown in Fig. 3. Next, how the aforementioned charging and cleaning unit 120 operates will be explained as follows. When the conductive cylinder 122 is rotated in the direction opposite to that of the photoreceptor in the arrowed direction at a peripheral speed that is 1.2 - 2.0 times higher than that of the photoreceptor 101 while rotating the photoreceptor 101 in the arrowed direction, the magnetic particles 121 are magnetically connected each other by lines of magnetic force of the magnet 123 to form a brush, becoming a so-called magnetic brush at the position on the conductive cylinder 122 where the conductive cylinder faces the photoreceptor 101. The magnetic brush is conveyed in the rotating direction of the conductive cylinder 122 and brushes photosensitive layer 101A of the photoreceptor 101 to catch toner T remaining untransferred on the photosensitive layer 101A. Since the aforementioned AC bias voltage is applied between the conductive cylinder 122 and the photoreceptor 101, electric charges are injected into the photosensitive layer 101A through conductive magnetic particles 121, thus, charging is carried out. Since AC bias voltage is specifically employed as bias voltage in this case, it is possible to conduct the charging which is extremely stable. In this occasion, toner T remaining untransferred on the photoreceptor 101 is adhered electrostatically on the aforementioned magnetic brush which brushes the photosensitive layer and is conveyed to arrive at the collecting roller 127 and then the toner T is transfered by the higher bias voltage applied on the collecting roller 127 to the collecting roller 127. The toner T remaining untransferred and moved to the collecting roller 127 is scraped by collecting blade 129 and drops on the bottom of the casing 125 where the toner T is conveyed by collecting screw 191 to an unillustrated collecting box, or conveyed to the developing unit 130 as recycled toner. When magnetic particles 121 located at the tip of the magnetic brush arrive at the position of stirring plate 128, they are scraped off by the stirring plate 128 and stirred, thus magnetic particles 121 of the magnetic brush are replaced constantly. Further, toner T mixed with magnetic particles 121 is collected immediately as stated above. Therefore, it is possible to avoid that the resistivity of magnetic particles 121 is increased by the mixed toner T and thereby the charging efficiency is lowered, and to achieve constantly the stable and uniform charging, together with the aforementioned AC bias voltage. Furthermore, when the photoreceptor 101 having a specific character of high y is used as shown in the example represented by Fig. 4 and further toner T having a property of being transparent to the infrared ray of a wavelength of not less than 750 nm disclosed in Japanese Patent Publication No. JP-A-2 271 371 is used, no problem happens even if a certain amount of toner T may remain, provided that the toners do not flock at one location. Therefore, the magnetic brush in the present example showed an excellent cleaning effect, and even when toner T is not collected perfectly, the magnetic brush may be an excellent cleaning means because it has a leveling effect of dispersing uniformly toner T having a property of being transparent to the infrared ray on the photoreceptor 101 in the case of forming reversal images using a reversal development method. When obtaining non-charging in the present example, it is possible to do so by causing DC components of bias voltage to be zero. Further, when the direction of poles N - S of magnet 123 is changed to be in parallel with a tangent at the point where the magnet faces the photoreceptor 101, bristles of the magnetic brush are caused by a horizontal magnetic field to be in parallel with a tangent at the point where the magnet faces the photoreceptor 101, thus a tip of the magnetic brush can be separated from the photoreceptor to create the state of non-charging and non-cleaning. Assuming a case where two-component development and positive charging of the photoreceptor are carried out, for example; (a) The toner have to be charged negatively for positive development, that is, a developer whose carrier charges toner negatively, through frictional electrification, is used and (b) For reversal development, the toner have to be charged positively, and a developer whose carrier charges positively the toner T is used. The same magnetic particles as developer carrier can be used and the toner collection also can be carried out effectively to make this process a very preferable charging and cleaning means. Due to the aforementioned constitution of the invention, it is possible to lower the charging voltage and thereby to prevent occurrence of ozone because electric charges are injected directly into a photoreceptor, and it is possible to prevent deterioration of the photoreceptor and to conduct extremely and uniform charging because of AC bias. The remarkable effect of the foregoing was observed when a photoreceptor of high-γ type explained in the example was used. Therefore, it has become possible, in the invention, to provide a charging and cleaning unit wherein a charging unit and a cleaning unit are combined for the use in common to allow an image forming apparatus to be small in size.

Claims: An apparatus for charging an image surface of an image carrying member (10;101) which rotates in a given direction, comprising: a rotatable cylinder (22;122) facing said image carrying member; a magnet (23;123) disposed in said rotatable cylinder, wherein a magnetic brush composed of magnetic particles (21;121) is formed on said rotatable cylinder by the magnetic field of said magnet, and said magnetic brush comes in contact with said image carrying member; and a power source (24;124) for supplying a bias voltage between said image carrying member and said rotatable cylinder, wherein said image surface of said image carrying member is charged by said magnetic brush under said bias voltage, characterized in that said magnetic particles have an electrical resistivity of 10 5 to 10 12 Ω·cm, and said bias voltage is a superposition of an AC and a DC voltage. The apparatus of claim 1, wherein a peak to peak value of said AC voltage is from 200 to 3500 V. The apparatus of claim 1 or 2, wherein a clearance between said rotatable cylinder and said image surface of said image carrying member is 0,1 to 5 mm. The apparatus of anyone of claims 1 to 3, wherein said magnetic particles have a spherical shape. The apparatus of anyone of claims 1 to 4, wherein said magnetic particles have an average particle size not larger than 100 µm. The apparatus of anyone of claims 1 to 5, wherein said magnetic particles have an average particle size of 30 to 70 µm. The apparatus of any one of claims 1 to 6, wherein said magnetic particles have a magnetization of 20 to 200 Am 2 /kg (20 to 200 emu/g). The apparatus of any one of claims 1 to 7, wherein a magnetic field provided on a surface of said rotatable cylinder is substantially parallel to a tangent at a point where the magnet (23;123) pocc said image carrying member when no charging is conducted. The apparatus of claim 1, the apparatus further comprising a means for forming a toner image on said charged image surface of said image carrying member, a means (150) for transferring said toner image from said image surface to a recording medium, wherein said magnetic brush cleans the remaining toner on said image surface after said toner image has been transferred to said recording medium.

Inventors: HANEDA SATOSHI, HANEDA, SATOSHI, HANEDA, SATOSHI, KONICA CORPORATION
Applicants: KONISHIROKU PHOTO IND, KONICA CORPORATION
Application year: 1991
Main classification: G03G15
IPC codes: G03G15, G03G21