A DEVELOPING APPARATUS

Abstract: A developing apparatus is provided wherein a magnet (11) is stationarily disposed within a sleeve (10) for carrying a developer. Adjacent an outer periphery of the magnet, there are only two magnetic poles having different polarities. They are diametrically opposed with respect to a center (O₁₀) of the sleeve (10). One of the magnetic poles forms a magnetic field in the developing zone (7), whereas the other magnetic pole forms a magnetic field in the zone (S) for regulating the thickness of the layer of the developer on the sleeve (10).

Description: The present invention relates to developing apparatus for developing a latent image, for example an electrostatic latent image produced by an electrophotographic process or an electrostatic recording process. A conventional developing device is shown in figure 1 and has a rotatable developer carrying member 2, hereinafter called a "developing sleeve" which is disposed close to a cylindrical electrostatic latent image bearing member 1 and is rotatable in a direction indicated by an arrow B. The image bearing member 1, hereinafter referred to as "photosensitive drum" is rotatable in the direction indicated by arrow A. The developing sleeve 2 is in the form of a cylinder having, for example, an outer diameter of not less than about 16mm and a thickness of 0.75mm. The developing sleeve 2 is disposed in an opening of a developer container 3 which contains a developer 5, the opening being adjacent to the photosensitive drum. In this manner, a developing zone 7 is established at a position where the developing sleeve 2 is close to the photosensitive drum 1. A columnar magnet 4 having a number of magnetic poles is disposed inside the developing sleeve 2 and functions as means for generating a stationery (non-rotatable) magnetic field. The magnet 4 can be permanent magnet or an electromagnet having two pairs of magnetic poles N1 and S1, N2 and S2 which are disposed around the circumference of the magnet equidistantly. The magnet 4 produces magnetic lines of force in the space adjacent to the outer periphery of the developing sleeve 2, by which, for example, a one component developer 5 mainly containing toner particles and usable in a jump developing method is carried on the outer surface of the developing sleeve 2. The outer surface of the developing sleeve 2 carries a layer of the developer 5 whose thickness is regulated by a regulating member 6 located at a position upstream of the developing zone 7 with respect to the rotational direction B of the developing sleeve 2. The regulating member 6 is spaced from the developing sleeve 2 to provide a predetermined gap S therebetween and it is positioned opposite one of the magnetic poles N1 in the example shown. After its thickness has been regulated by the regulating member 6, the layer of developer 5 on the sleeve 2 is conveyed to the developing zone 7 where toner is supplied to the photosensitive drum 1 to convert an electrostatic latent image thereon into a visible form. However, if it is attempted to provide a large magnetic force in order to obtain a good quality of image with a multi-pole magnet, a large diameter magnet has to be used with the result that it is difficult to reduce the size of the apparatus and also its cost is increased. On the other hand, if the magnetic force provided by the magnet is small, it is difficult for the regulating member 6 to produce a developer layer having the appropriate thickness and erection of the developer chains in the developing zone may take place to an insufficient extent. Another known form of developing apparatus is disclosed in US patent 4548489 which has a rotatable developer carrying member of cylindrical construction in which a stationary magnet is located which has two poles, both of which are located between the center and the edge of the cylinder. The device also has a regulating blade which is magnetic and which, with the magnet located in the developer carrying member produces a magnetic field which allows the developer to form a magnetic brush from which toner is transferred to an image bearing member for development. A voltage is applied between the developer carrying member and the image bearing member to bring about transfer of the developer to the image bearing member from the developer carrying member. However, in the arrangement disclosed in this patent no magnetic field is present in the developing zone. A problem with which the present invention is concerned is the provision of developing apparatus including a rotatable developer carrier of relatively small size but which can nevertheless produce developed images of good quality. According to the present invention, there is provided an apparatus for developing an electrostatic latent image, comprising a photoconductive image bearing member, a rotatable cylindrical member disposed adjacent to or in contact with the image bearing member so as to define therebetween a developing zone at which developer on the cylindrical member is transferred to the image bearing member, means for supplying developer to the cylindrical member, means for regulating the thickness of a layer of developer formed on the cylindrical member, and means located in the cylindrical member for producing a stationary magnetic field and having only two magnetic poles which are located substantially on a line passing through the centre of the cylindrical member, with one of the magnetic poles being disposed adjacent to an outer periphery of the magnetic field producing means so that the regulating means is within the magnetic field provided by the pole, characterised in that: the other magnetic pole is also located adjacent to the outer periphery of the magnetic field producing means and diametrically opposite to said one magnetic pole with reference to the centre of the cylindrical member with the developing zone being within the magnetic field produced by the other magnetic pole; and the diameter of the cylindrical member is not less than 6mm and not more than 20mm. The invention will now be further described, by way of example only, with reference to the accompanying drawing in which: Figure 1 is a sectional view of a conventional developing apparatus. Figure 2 is a sectional view of a developing apparatus according to an embodiment of the present invention. Figure 3 is a sectional view of a developing apparatus according to another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 2, there is shown a developing apparatus according to a first embodiment of the present invention, wherein a photosensitive drum 1 has a diameter not more than 120 mm, preferably, not more than 60 mm. The developing apparatus includes a developing sleeve 10 in the form of a cylinder and made of non-magnetic material, and preferably having an outer diameter of not less than 6 mm and not more than 20 mm, and preferably having a thickness of not less than 0.25 mm and not more than 1.5 mm. Further preferably, in order to further assure prevention of scattering or the like of the developer 5, the outer diameter of the developing sleeve 10 is further preferably not less than 8 mm and not more than 16 mm. To the developing sleeve 10, an alternating voltage is applied upon developing operation by a bias voltage source 15. By the application of the bias voltage, an oscillating electric field is formed so that an electric field in the direction from the sleeve 10 to the drum 1 and the electric field in the direction from the photosensitive drum 1 to the sleeve 10 are produced alternately in the developing zone 7. By the oscillating electric field, the developer makes a vibratory motion in the developing zone 7, whereby the developer is deposited on the image area of the latent image. A stationary magnetic field generating means is contained in the developing sleeve 10 and is in the form of a permanent magnet or an electromagnet. The magnetic field generating means 11 is, in this embodiment, a columnar magnet 11 having only one couple of magnetic poles N and S having different magnetic poles disposed adjacent to an outer periphery thereof. The magnetic poles N and S are diametrically opposed, that is, the magnetic poles N and S and the rotational center of the developing sleeve 10 are on a line. The magnet 11 may be produced in conventional manners. For example, it may be formed as an integral magnet having only two magnetic poles N and S. As an another alternative, two magnets may be adhered so that the magnetic poles having different polarities are remote from and opposed to each other. As a further alternative, two cut-away portions are formed in a magnet supporting member, and two magnets are mounted in the cut-away portions so that the magnetic poles having different polarities are disposed remote from and opposed to each other. In any case, the magnetic pole N and the magnetic pole S are disposed diametrically opposed with respect to the center of the sleeve 10 and on the outer surface of the magnet 11 concentrically disposed with the sleeve 10. With this arrangement of the two magnetic poles, the diameter of the sleeve 10, and therefore, the diameter of the magnet 11 can be reduced without loosing the strong magnetic force property of the magnet 11. Therefore, strong magnetic field can be produced on the outer periphery of the sleeve 10, despite the small diameters thereof. The clearance between the magnet 11 and the sleeve 10 is not less than 0.25 mm and not more than 1 mm. One of the magnetic poles of the magnet 11, the magnetic pole N, for example, is disposed at a position opposite to the developing zone 7 and substantially on an extension line XY of a line connecting the rotational center O₁ of the photosensitive drum 1 and the rotational center O₁₀ of the developing sleeve 10. The other magnetic pole, that is, the magnetic pole S is disposed directly opposed to the developing zone 7 and substantially on the line XY. Thus, the magnetic pole S forms, in the developing zone 7, a magnetic field for erecting magnetic brush of the magnetic developer. Opposed to the magnetic pole N, there is disposed a regulating member 12. The regulating member 12 is disposed on or adjacent the extension of a line connecting the rotational center O1 of the photosensitive drum 1 and the rotational center 010 of the developing sleeve 10. One end of the regulating member 12 is mounted on the container 3, and the other end is spaced from the outer surface of the developing sleeve 10 with a predetermined gap S. That end of the regulating member 12 which is opposed to the sleeve 10 is within the magnetic field formed by the magnetic pole N, and therefore, the regulating member 12 is effective to form a thin layer of the developer in the magnetic field. If the regulating member 12 is made of a magnetic material such as iron, the magnetic field is concentrated on the regulating member 12, so that a magnetic curtain is formed in the gap S by a strong magnetic field, so that a developer layer having a thickness quite smaller than the clearance S can be formed. As an example of the electrostatic latent image to be developed, a latent image constituted by a non-image portion of approximately -200 V and an image portion of approximately -700 V is formed on the photosensitive drum 1. The gap in the developing zone between the surface of the photosensitive drum 1 and the surface of the developing sleeve 10 is 300 µm. The developing bias applied to the developing sleeve 10 is a combined AC voltage having a peak-to-peak voltage of approximately 1.3 KV and having a frequency of 1.5 KHz and a DC voltage of -250 V. A thickness of the developer layer formed on the outer surface of the developing sleeve 10 is approximately 70 - 80 µm. The outer diameter of the developing sleeve is approximately 10 mm, for example, and the magnetic force of the magnet 11 is so selected that it is 700 Gausses at each of the magnetic poles S and N on the outer surface of the developing sleeve 10. The distance from the outer surface of the magnet 11 to the outer surface of the developing sleeve 10 is approximately 1.2 mm. Since the thickness of the developing sleeve 10 is 0.75 mm, the clearance between the outer surface of the magnet 11 and the inner surface of the developing sleeve 10 is approximately 0.45 mm. The magnetic force provided by the two-pole magnet 11 acts strongly on the outer surface of the developing sleeve 10 in the developing zone 7, so that the magnetic brush of the developer 5 strongly erects, and therefore, the developer is easily released from the sleeve 10 under the influence of the oscillating electric field, thus increasing the development efficiency. In addition, since the diameter of the developing sleeve 10 is small, a strong magnetic field for conveying the developer is formed on the surface of the sleeve, so that the developer 5 is sufficiently conveyed into the developing zone 7. For those reasons, a good quality image having a high image density can be provided without foggy background. Furthermore, since the diameter of the developing sleeve 10 is small, the developing zone 7 is limited to the area where the photosensitive drum and the developing sleeve 10 is most close and in the close neighborhood thereof, whereby an image having a good faithfullness in the half-tone image can be produced. In the foregoing embodiment described with Figure 2, a one component developer has been used, but in the following embodiment, two component developer 13 is used which contains magnetic carrier particles and non-magnetic toner particles. Referring to Figure 3, this embodiment is shown. In this embodiment, the two component developer 13 contains magnetic carrier particles made of, for example, ferrite having a particle size of approximately 70 - 50 microns coated with silicone resin, and non-magnetic toner particles made of, for example, toner particles having an average particle size of approximately 10 µm and 0.6 % of colloidal silica, the toner particles being made of 100 parts of styrene-butadiene copolymer resin and 5 parts of copper phthalocyanine pigment. When the sleeve 10 rotates, the carrier particles flow adjacent the surface of the sleeve 10 in the container 3, whereby toner particles are conveyed on the sleeve 10 in the container 3. A regulating member 14 is disposed substantially on a line PQ which is away from the extension XY toward the downstream in the direction of the rotation of the developing sleeve 10 by an angle ϑ (approximately 15 degrees) as viewed from the center O10 of the sleeve 10. The regulating member 14 has one end mounted to the container 3 and the other end disposed within the magnetic field provided by the magnetic pole N and spaced from the surface of the developing sleeve 10 by a predetermined clearance S, whereby the amount of the toner particles 13 conveyed out of the container is regulated properly and prevents the toner scattering and the production of foggy background. To the developing sleeve 10, a developing bias voltage is applied from the voltage source 15. The bias voltage is, for example, a combination of an alternating voltage having a frequency of 1.6 KHz and a peak-to-peak voltage of approximately 1.3 KV and a DC voltage of approximately -250 V. Due to the strong magnetic field provided by the magnetic pole S in the developing zone 7, the magnetic brush of the developer is sufficiently erected, and therefore, the toner particles are easily released from the carrier particles under the influence of the oscillating electric field, the toner particles are easily released from the carrier particles. For those reasons, a good quality image with a high density can be produced without foggy background. The angle ϑ formed between the line PQ connecting that end of the regulating member 14 which is opposed to the sleeve 10 and the center O10 of the sleeve and the downstream line XY with respect to the rotational direction of the sleeve 10, is preferably not less than 5 degrees and not more than 20 degrees. If the angle ϑ is close to 5 degrees, the confining force to the magnetic carrier particles provided by the magnetic field becomes strong at the position of the regulating member 14, thus increasing the ratio of the toner to the carrier particles in the developer carried to the developing zone 7, whereas if the angle ϑ is close to 20 degrees, the ratio of the toner in the developer conveyed on the sleeve 10 to the developing zone 7 decreases. The regulating member 14 may be made of magnetic material or non-magnetic material, or may be made of magnetic material and non-magnetic material overlaid one another. The present invention is applicable to the apparatus wherein the sleeve 10 is supplied with a DC voltage from a voltage source 15, wherein a DC electric field is formed in the developing zone 7. In this case, it is preferable that the clearance S is larger so that the magnetic brush of the developer erected from the surface of the sleeve in the developing zone 7 is contacted to the surface of the drum 1. As for the two component developer, a mixture of magnetic carrier particles and weakly magnetic toner particles can be used. Also, the present invention is applicable to a contact type developing apparatus wherein the developing sleeve surface is contacted to the photo-sensitive drum in the developing zone, as disclosed, for example, in U.S. Patent No. 4,444,864. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

Claims: Apparatus for developing an electrostatic latent image, comprising a photoconductive image bearing member (1), a rotatable cylindrical member (10) disposed adjacent to or in contact with the image bearing member (1) so as to define therebetween a developing zone (7) at which developer on the cylindrical member (10) is transferred to the image bearing member (1), means (3) for supplying developer (5) to the cylindrical member (10), means (12) for regulating the thickness (S) of a layer of developer formed on the cylindrical member (10), and means (11) located in the cylindrical member (10) for producing a stationary magnetic field and having only two magnetic poles which are located substantially on a line passing through the center (O₁₀) of the cylindrical member (10), with one of the magnetic poles being disposed adjacent to an outer periphery of the magnetic field producing means (11) so that the regulating means (11) is within the magnetic field provided by the pole, characterised in that: the other magnetic pole is also located adjacent to the outer periphery of the magnetic field producing means (11) and diametrically opposite to said one magnetic pole with reference to the centre (O₁₀) of the cylindrical member (10) with the developing zone (7) being within the magnetic field produced by the other magnetic pole; and the diameter of the cylindrical member (10) is not less than 6mm and not more than 20mm. Apparatus according to claim 1, wherein the cylindrical member is in the form of a drum (1) and the two magnetic poles are substantially on a line connecting the centers (0₁,0₁₀) of the drum (1) and the cylindrical member (10). Apparatus according to claim 2, wherein the regulating means (12) is adjacent to the line (0₁, 0₁₀) connecting the centres of the drum (1) to the cylindrical member (10); and the developer supplying means (3) contains a one-component developer. Apparatus according to claim 2, wherein the regulating means (14) makes an angle (O) of 5-20 o with the line (0₁- 0₁₀) connecting the centers of the the drum (1) to the cylindrical member (10); and the developer supplying means (3) contains a two component developer of magnetic carrier particles and non-magnetic toner particles. Apparatus according to claim 4, wherein the regulating means (14) makes an angle (O) of about 15 o . Apparatus according to any preceding claim wherein the regulating member (12) is of a magnetic material. Apparatus according to any preceding claim, wherein the cylindrical member (10) has a thickness of not less 0.25mm and not more than 1.5mm. Apparatus according to claim 7, wherein the cylindrical member (10) and the magnetic field generating means (11) are separated by a gap, said gap being not less than 0.25mm and not more than 1mm. Apparatus according to any preceding claim, further comprising means (15) for applying a bias voltage between the image bearing member (1) and the cylindrical member (10) so as to produce an oscillating electric field between the image bearing member (1) and the cylindrical member (10).

Inventors: KUBO, TAKAHIRO, TAJIMA, HATSUO
Applicants: CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G03G15
IPC codes: G03G15