JP2009204665A - Developing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing device of hybrid development system that can prevent image memory without increasing drive torque of each roller, and to provide an image forming apparatus provided with the developing device. <P>SOLUTION: The developing device includes: a first magnetic body with a plurality of first magnetic portions, a conveying roller that has a cylindrical conveying sleeve rotated along the periphery of the first magnetic body, and that conveys developer containing non-magnetic toner and magnetic carrier while holding the developer on the outer peripheral face of the conveying sleeve; a second magnetic body with a second magnetic portion; and a cylindrical developing sleeve rotated along the periphery of the magnetic body. The second magnetic portion is disposed between the first and second areas and on the upstream side of the developing roller in its rotating direction, and holds the magnetic substance on the second magnetic portion via the developing sleeve. Thus, the force with which development residual toner is attracted to the developing sleeve is decreased to avoid image memory. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus and a developing device used in the image forming apparatus.

  As a developing method employed in an electrophotographic image forming apparatus, a one-component developing method using only toner as a developer and a two-component developing method using toner and a carrier as developers are known.

  In the one-component development method, generally, the toner is frictionally charged by passing the toner through a regulating portion between the toner carrying member and a regulating plate provided by pressing the toner carrying member, and a toner thin layer having a desired thickness is formed. Since it can be held on the outer peripheral surface of the toner carrier, it is advantageous in terms of simplification of the configuration of the developing device, size reduction, and cost reduction. Further, since a toner layer having an upper limit of a thickness of several tens of μm is formed on the toner carrier, the toner carrier can be set in a state where a minute gap is maintained between the image carrier and As a result, a strong electric field is formed between the toner carrier and the image carrier to obtain a high toner moving speed, and a high-definition high-quality image can be obtained. However, in the one-component development method, the deterioration of the toner is likely to be promoted by the strong stress received in the restricting portion, the toner charge amount is likely to decrease with durability, and the surface of the restricting plate and the toner carrier surface is not easily removed from the toner and other external surfaces. Contamination with the additive lowers the charge imparting property to the toner, resulting in problems such as fogging, making it difficult to extend the life of the developing device.

  On the other hand, the two-component development method is advantageous in terms of toner deterioration because the toner is charged by frictional contact by mixing and stirring with the carrier, so that the stress received by the toner is small. In addition, since the carrier, which is a charge imparting member to the toner, has a surface area larger than that of the toner particles, it is relatively resistant to contamination by toner and other external additives, and is advantageous in extending the life of the developer. It is. However, the magnetic brush formed by the carrier on the developer conveying roller has a length that is 20 to 50 times the thickness of the toner layer on the toner carrier in the one-component developing system, and is not microscopic. It is formed uniformly. As a result, the electric field between the image carrier and the image carrier must be set to be weaker than in the case of the one-component development method in consideration of leakage prevention and the like, and at least a part of the magnetic brush is in contact with the image carrier. Since it is necessary to set a gap between the image carrier and the image carrier, the toner movement speed to the image carrier is slow, and the toner image on the image carrier is scraped off by the magnetic brush. This is inferior to the one-component development method.

  As a developing method incorporating the advantages of both the one-component developing method and the two-component developing method, Patent Document 1 discloses a magnetic pole body after toner charging in the state of a two-component developer as in the two-component developing method. While the developer is held on the outer peripheral surface in a magnetic brush state by a magnetic force with a conveyance roller (supply roller) enclosing the toner, the developer is conveyed to an area facing the development roller by the rotation, and conveyed by the action of an electric field formed in this area Only the toner from the developer on the roller is supplied to the developing roller to form a thin toner layer on the developing roller, and this toner layer is conveyed to an area (developing area) facing the image carrier by the rotation of the developing roller. Thus, a so-called hybrid development system has been proposed in which one-component development for developing an electrostatic latent image on an image carrier is performed. According to this developing system, it is possible to achieve both a long life of the developing device and high image quality.

  However, in the hybrid development method, if the toner remaining on the developing roller without being used for development is not sufficiently separated and collected from the developing roller in the developing device, an image memory (or development history phenomenon) occurs. There was a problem. The problem is that the toner on the developing roller after development is scraped and collected by the magnetic brush on the conveying roller while supplying new toner from the conveying roller to the developing roller in the area where the conveying roller and the developing roller are opposed to each other. The image memory is particularly prominent when the electric field formed in the opposite area is set closer to the toner supply in order to cope with the increase in the image forming speed. appear.

  As a countermeasure to the problem of the image memory, in Patent Document 2 and Patent Document 3, according to the developing device of the hybrid development system including the transport roller (magnetic roller) and the development roller, the region where the transport roller and the development roller face each other. In Japanese Patent Application Laid-Open No. H11-260, it is proposed to magnetize the developing roller with a different magnetic pole that faces one magnetic pole of the conveying roller, thereby enhancing the binding force of the magnetic brush and preventing the occurrence of image memory.

JP 2003-15380 A JP 2006-106027 A JP 2006-106028 A

  However, in the above patent document, the magnetic force is weakened so as to weaken the magnetic force at the circumferential edge of the magnetic pole disposed in the developing roller (the portion corresponding to the developer roller held away from the developer roller surface). Proposals have been made to set this, but the difficulty of setting the magnetization will increase, and the productivity will be affected. Further, when the binding force of the magnetic brush is increased, the torque required for driving the transport roller and the developing roller is also increased, and there is a concern that the driving motor is increased in cost and size.

  Accordingly, the present invention provides a developing device capable of preventing an image memory without increasing the driving torque of each roller in the hybrid developing type developing device, and an image forming apparatus including the developing device. Objective.

  In order to achieve this object, a developing device of the present invention includes a first magnetic pole body having a plurality of first magnetic pole portions and a cylindrical conveying sleeve that is driven to rotate around its outer periphery, and includes a non-magnetic toner and a magnetic sleeve. A transport roller that transports a developer including a carrier while holding the developer on the outer peripheral surface of the transport sleeve, a second magnetic pole body having a second magnetic pole portion, and a cylindrical developing sleeve whose outer periphery is rotationally driven. A developing roller disposed opposite to the conveying roller via the first area and opposed to the image carrier via the second area, and between the conveying roller and the developing roller. Between the developing roller and the image carrier, and a first electric field forming unit that moves the non-magnetic toner in the developer held by the conveying roller to the developing roller. Forming a second electric field and developing A non-magnetic toner held by a roller is moved to an electrostatic latent image on the image carrier and developed to form a toner image, and the second magnetic pole portion includes: The developing sleeve is disposed in a region during which the developing sleeve rotates from the second region to the first region, and holds the magnetic body above the second magnetic pole portion via the developing sleeve. It is what.

  According to the developing device of the present invention, the magnetic particles are held above the second magnetic pole portion, and the toner remaining on the developing roller without being consumed when developing the image carrier is caused to collide with the magnetic particles. By allowing the toner to pass, it is possible to reduce the variation in the thickness of the toner and reduce the adhesion force between the developing sleeve and the toner, thereby supporting the separation and recovery of the toner from the developing sleeve. In this way, the generation of the image memory can be avoided more reliably.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. In the following description, terms indicating a specific direction (for example, “up”, “down”, “left”, “right”, and other terms including them, “clockwise direction”, “counterclockwise” ”) Is used to facilitate understanding of the invention with reference to the drawings, and the present invention should not be construed as being limited by the meaning of these terms. Further, in the image forming apparatus and the developing apparatus described below, the same reference numerals are used for the same or similar components.

[1. Image forming apparatus]
FIG. 1 schematically shows a main part of an electrophotographic image forming apparatus according to an embodiment of the present invention. The image forming apparatus may be any of a copier, a printer, a facsimile machine, and a multi-function machine having a combination of these functions. The image forming apparatus 1 includes a photoconductor 12 that is an image carrier. In this embodiment, the photoconductor 12 is a cylindrical body, but the present invention is not limited to such a form, and an endless belt type photoconductor can be used instead. The photoreceptor 12 is drivingly connected to a motor (not shown), and is rotated in the direction of arrow 14 based on the driving of the motor. Around the photoconductor 12, a charging station 16, an exposure station 18, a developing station 20, a transfer station 22, and a cleaning station 24 are arranged along the rotation direction of the photoconductor 12.

  The charging station 16 includes a charging device 26 that charges the photosensitive layer, which is the outer peripheral surface of the photosensitive member (image carrier) 12, to a predetermined potential. In this embodiment, the charging device 26 is represented as a cylindrical roller, but instead, other types of charging devices (for example, a rotary or fixed brush-type charging device, a wire-discharge-type charging device) are used. Can also be used. In the exposure station 18, the image light 30 emitted from the exposure device 28 disposed in the vicinity of the photosensitive member 12 or away from the photosensitive member 12 travels toward the outer peripheral surface of the charged photosensitive member 12. A passage 32 is provided. On the outer peripheral surface of the photoconductor 12 that has passed through the exposure station 18, an electrostatic latent image is formed that includes a portion where the image light is projected and the potential is attenuated and a portion where the initial charged potential is substantially maintained. In this embodiment, the portion where the potential is attenuated is the electrostatic latent image portion, and the portion where the charged potential is substantially maintained is the electrostatic latent image non-image portion. The developing station 20 includes a developing device 34 that visualizes the electrostatic latent image using a powder developer. Details of the developing device 34 will be described later. The transfer station 22 includes a transfer device 36 that transfers a visible image formed on the outer peripheral surface of the photoreceptor 12 to a sheet 38 such as paper or film. In the embodiment, the transfer device 36 is represented as a cylindrical roller, but other types of transfer devices (for example, wire discharge transfer devices) can also be used. The cleaning station 24 includes a cleaning device 40 that collects untransferred toner remaining on the outer peripheral surface of the photoconductor 12 without being transferred to the sheet 38 at the transfer station 22 from the outer peripheral surface of the photoconductor 12. In the embodiment, the cleaning device 40 is shown as a plate-like blade, but other types of cleaning devices (for example, a rotary type or a fixed type brush type cleaning device) may be used instead.

  When the image forming apparatus 1 having such a configuration forms an image, the photoconductor 12 rotates clockwise based on the driving of a motor (not shown). At this time, the outer peripheral portion of the photoreceptor passing through the charging station 16 is charged to a predetermined potential by the charging device 26. The charged outer periphery of the photoreceptor is exposed to image light 30 at the exposure station 18 to form an electrostatic latent image. The electrostatic latent image is conveyed to the developing station 20 along with the rotation of the photosensitive member 12, where it is visualized as a developer image by the developing device 34. The visualized developer image is conveyed to the transfer station 22 along with the rotation of the photosensitive member 12, and is transferred to the sheet 38 by the transfer device 36 there. The sheet 38 to which the developer image has been transferred is conveyed to a fixing station (not shown), where the developer image is fixed to the sheet 38. The outer peripheral portion of the photosensitive member that has passed through the transfer station 22 is conveyed to the cleaning station 24 where the developer remaining on the outer peripheral surface of the photosensitive member 12 without being transferred to the sheet 38 is recovered.

[2. Development device]
As shown in FIG. 1, the developing device 34 generally includes a developing roller 48 facing the photosensitive member 12 in the developing area 96, a conveying roller 54 facing the developing roller 48 in the supply / recovery area 88, a non-magnetic toner and a magnetic carrier. A developer agitating chamber 66 for agitating a two-component developer containing the toner and an electric field forming device 110. In the developer used in this embodiment, the toner is negatively charged and the carrier is positively charged by frictional contact, but the toner is positively charged and the carrier is negatively charged. There may be.

  The housing 42 includes an opening 44 that is open toward the photosensitive member 12, and a developing roller 48 that is a toner conveying member is provided in a space 46 formed in the vicinity of the opening 44. The developing roller 48 is disposed in parallel with the photosensitive member 12 and a predetermined developing gap 50 from the outer peripheral surface of the photosensitive member 12.

  The developing roller 48 includes a cylindrical developing sleeve 45 that is supported so as to be rotatable in the direction of an arrow 78, and a cylindrical magnet body (second magnetic pole body) 47 that is fixedly disposed in the developing sleeve so as not to rotate. And have. The conductive magnet body 47 is electrically connected to an electric field forming device 110 described later, whereby a predetermined developing bias is applied to the developing roller 48.

According to the developing device according to the present invention, the magnet 47 has the holding magnetic pole N (second magnetic pole) magnetized on the outer periphery thereof. More specifically, as shown in the enlarged view of FIG. 2, the holding magnetic pole N is disposed while the developing sleeve 45 of the developing roller 48 moves from the developing region 96 to the supply / recovery region 88. That is, when the line connecting the developing roller 48 and the centers O ₁ and O ₂ of the conveying roller 54 is defined as the base line BL, the holding magnetic pole N serves as the center of the developing roller 48 in the clockwise direction in FIG. Magnetized at position θ).

  As will be described later, the developing device according to the present embodiment transports a small amount of magnetic carrier as magnetic particles M by controlling the voltage between the transport roller 54 and the developing roller 48 by the electric field forming device 110 when it is not during image formation. The roller 54 is configured to be supplied to the developing roller 48. That is, the magnetic particles M made of a magnetic carrier are held at a fixed position with respect to the magnet body 47 by the magnetic force of the holding magnetic pole N to form a magnetic brush. That is, when the developing roller 48 rotates, the magnetic particles M slide with respect to the developing sleeve 45 and are in a stationary position with respect to the magnet body 47 (via the developing sleeve 45) above the holding magnetic pole N. Retained.

  Generally, toner remaining on the developing sleeve 45 without being consumed when developing the photosensitive member 12 (hereinafter referred to as “development residual toner”) is used for the latent image portion and the non-image portion (toner of the toner). The film thickness varies in accordance with (consumption). When toner having a variation in film thickness is subjected to development again without being uniformed, a so-called image memory (or development history phenomenon) occurs. On the other hand, according to the developing device according to the embodiment, when the developing residual toner that adheres to and rotates on the developing sleeve 45 collides with the magnetic particles M held above the holding magnetic pole N and passes through the developing sleeve. As well as the adhesion force with 45, the film thickness variation is reduced or eliminated. In this way, by reducing the adhesion of the development residual toner to the development sleeve 45, it becomes easier to separate and collect the development residual toner from the development sleeve 45, and also eliminates variations in film thickness even if there is residual development toner. By doing so, the generation of the image memory can be avoided more reliably.

  Here, the force acting on the magnetic particles M in contact with the developing sleeve 45 will be examined microscopically. As shown in FIG. 2, the holding magnetic pole N acts on the magnetic particle M with a circumferential magnetic field attractive force Fc and a radial magnetic field attractive force Fr. At this time, in order to hold the magnetic particles M above the holding magnetic pole N without rotating together with the developing sleeve 45 even though the developing sleeve 45 rotates in the direction of the arrow 78, The frictional force μ · Fr between the magnetic particles M (μ is the coefficient of friction between the developing sleeve 45 and the magnetic particles M) needs to be smaller than the circumferential magnetic field attractive force Fc by the holding magnetic pole N (that is, Fc > Μ · Fr). As will be described in an experimental example described later, when the friction coefficient μ is 1 or less, it has been confirmed that the above relational expression is satisfied, and the holding magnetic pole N holds the magnetic particles M above the developing sleeve 45. Thus, the generation of the image memory can be avoided.

  In this embodiment, the magnetic carrier contained in the two-component developer is used as the magnetic particle M that reduces the adhesion force of the residual toner and eliminates the variation in the film thickness. For example, it may have an arbitrary form, and a magnetic metal plate (not shown) can be used. In this case as well, the friction coefficient μ between the developing sleeve 45 and the magnetic metal plate is set to 1 or less.

  In addition to or instead of the magnetic particles M held above the holding magnetic pole N, a slit 51 extending in the central axis direction is provided in the magnet body 47 of the developing roller 48 as shown in FIG. In this case, the electrical attraction force by the developing roller 48 with respect to the undeveloped toner may be weakened. The slit 51 has a length in the axial direction of the developing roller 48 (the depth direction in FIG. 2). As a result, the adhesion force of the development residual toner to the development sleeve 45 can be reduced, and the development residual toner can be more reliably separated and collected from the development sleeve 45, thereby avoiding the occurrence of image memory.

  Unlike the above embodiment, when it is assumed that the polarity of the main magnetic pole of the transport roller closest to the holding magnetic pole N is different, it becomes more difficult to hold the magnetic particles M above the holding magnetic pole N. Therefore, it is preferable that the polarity of the main magnetic pole of the transport roller closest to the holding magnetic pole N is the same as that of the holding magnetic pole N as in the embodiment.

  The developing roller 48 can be formed of a cylindrical member made of a metal such as aluminum that has been subjected to a surface treatment. Examples of the surface treatment include polyester resin, polycarbonate resin, acrylic resin, polyethylene resin, polypropylene resin, urethane resin, polyamide resin, polyimide resin, polysulfone resin, polyether ketone resin, vinyl chloride resin, vinyl acetate resin, and silicone resin. A resin coating such as a fluororesin, or a rubber coating such as silicone rubber, urethane rubber, nitrile rubber, natural rubber, or isoprene rubber is used, but is not limited thereto. In addition, a conductive agent may be added to the inside or the surface of the coating. As the conductive agent, an electronic conductive agent or an ionic conductive agent can be used. Examples of the conductive agent include, but are not limited to, carbon black particles such as ketjen black, acetylene black, and furnace black, metal powder, and metal oxide fine particles. Examples of the ionic conductive agent include cationic compounds such as quaternary ammonium salts, amphoteric compounds, and other ionic polymer materials, but are not limited thereto.

  Returning to FIG. 1, another space 52 is formed behind the developing roller 48. In the space 52, a transport roller 54 as a developer transport member is disposed in parallel with the developing roller 48 and through an outer peripheral surface of the developing roller 48 and a predetermined supply / recovery gap 56. The conveyance roller 54 includes a magnet body 58 (first magnetic pole body) fixed in a non-rotatable manner and a conveyance sleeve supported around the magnet body 58 so as to be rotationally driven in the direction of the arrow 80 that is the same direction as the developing roller 48. 60. Above the conveying sleeve 60, a regulating plate 62 fixed to the housing 42 and extending in parallel with the central axis of the conveying sleeve 60 is disposed so as to oppose a predetermined regulating gap 64.

  The magnet body 58 has a plurality of magnetic poles (first magnetic pole portions) that face the inner surface of the transport roller 54 and extend in the central axis direction of the transport roller 54. In the embodiment, the plurality of magnetic poles are opposed to the magnetic pole S1 near the upper inner peripheral surface portion of the transport roller 54 near the regulating plate 62 and the left inner peripheral surface portion of the transport roller 54 near the supply and recovery gap 56. Main magnetic pole N1, magnetic pole S2 facing the lower inner peripheral surface portion of the transport roller 54, and two adjacent magnetic poles N2, N3 of the same polarity facing the right inner peripheral surface portion of the transport roller 54.

  A developer stirring chamber 66 is formed behind the transport roller 54. The stirring chamber 66 includes a front chamber 68 formed in the vicinity of the transport roller 54 and a rear chamber 70 separated from the transport roller 54. A front screw 72 that is a pre-stirring and conveying member that conveys the developer while stirring the developer from the front surface to the back surface of the drawing is rotatably disposed in the front chamber 68, and the rear chamber 70 is rotated from the back surface to the front surface of the drawing. A rear screw 74 that is a rear stirring member transporting member that transports the developer while stirring is disposed rotatably. As shown in the figure, the front chamber 68 and the rear chamber 70 may be separated by a partition wall 76 provided therebetween. In this case, the partition portions near both ends of the front chamber 68 and the rear chamber 70 are removed to form a communication passage, and the developer that has reached the downstream end of the front chamber 68 passes through the communication passage. The developer that has been fed to 70 and reaches the downstream end of the rear chamber 70 is fed to the front chamber 68 via a communication passage.

  In order to efficiently move the toner from the conveying sleeve 60 of the conveying roller 54 to the developing sleeve 45 of the developing roller 48 in the supply region 90, the developing roller 48 and the conveying roller 54 are electrically connected to the electric field forming device 110. .

  FIG. 3 shows a specific example of the electric field forming device 110. The electric field forming device 110 includes a first power source 112 and a second power source 114. Here, the first and second power sources 112 and 114 constitute a first electric field forming unit, and the first power source 112 constitutes a second electric field forming unit.

The first power source 112 includes a DC power source 118 connected between the developing roller 48 and the ground 116. The DC power supply 118 applies a DC voltage V _DC1 (for example, −300 volts) having the same polarity as the charging polarity of the toner 6 to the developing roller 48. The second power source 114 has a DC power source 120 and an AC power source 122 connected in series between the transport roller 54 and the ground 116. As a result, the conveying roller 54 has a predetermined direct current component V _DC2 (for example, −400 volts) from the direct current power supply 120 and a predetermined alternating current component (the amplitude _VP-P is, for example, 1,500 volts) from the alternating current power supply 122. A superposed vibration voltage is applied.

By applying the voltage V _DC1 to the developing roller 48 as described above, an electric field (second electric field) is formed between the photoconductor 12 and the developing roller 48. By the action of this electric field, the negative polarity toner on the developing roller 48 is electrostatically charged based on the potential difference between the developing roller 48 (V _DC1 : −300 volts) and the electrostatic latent image portion (V _L : −80 volts). It adheres to the latent image portion. At this time, the negative polarity toner adheres to the electrostatic latent image non-image portion due to a potential difference between the developing roller 48 (V _DC1 : −300 volts) and the electrostatic latent image non-image portion (V _H : −600 volts). There is nothing.

  Further, an oscillating electric field (first electric field) as illustrated in FIG. 4 is formed in the supply / recovery region 88 between the conveying roller 54 and the developing roller 48. As a result, the toner supply action of moving the negative toner from the conveying roller 54 to the developing roller 48 is performed in a time zone region where the electric potential of the conveying roller 54 is negative with respect to the electric potential of the developing roller 48. The toner collects the negative polarity toner from the developing roller 48 side to the conveying roller 54 in a time zone region on the plus side with respect to the potential of the developing roller 48. Further, in the time zone region for the toner collecting action, an electrostatic force in the direction of moving to the developing roller 48 side (carrier supply direction) acts on the positive polarity carrier on the conveying roller 54, but to the developing roller 48. The carrier movement is suppressed.

In this embodiment, the electric field formed between the transport roller 54 and the developing roller 48 is an oscillating electric field, and the first power source 112 averages the nonmagnetic toner from the transport roller 54 to the developing roller 48. Although it is configured to generate an oscillating electric field in the moving direction, the present invention is not limited to this. As long as toner can be supplied from the conveying sleeve 60 to the developing roller 48 in the supply region 90, the electric field is a DC electric field. Also good. That is, the first power source 112 may be configured to generate a DC electric field in a direction in which the nonmagnetic toner is moved from the transport roller 54 to the developing roller 48. In order to obtain higher toner charging performance, the strength of the toner separation electric field is preferably 2.5 × 10 ⁶ V / m or more.

Next, the operation of the developing device 34 configured as described above will be described.
During image formation, the developing sleeve 45 and the conveying sleeve 60 rotate in the directions of arrows 78 and 80, respectively, based on driving of a motor (not shown). The front screw 72 rotates in the direction of arrow 82 and the rear screw 74 rotates in the direction of arrow 84. As a result, the developer stored in the developer stirring chamber 66 is stirred while being circulated and conveyed between the front chamber 68 and the rear chamber 70. As a result, the toner and the carrier contained in the developer come into frictional contact with each other and are charged with opposite polarities. In the embodiment, it is assumed that the carrier is positively charged and the toner is negatively charged. Carrier particles are considerably larger than toner particles. Therefore, the negatively charged toner adheres around the positively charged carrier mainly based on the electrical attraction force of both.

  The charged developer is supplied to the transport roller 54 in the process of being transported through the front chamber 68 by the front screw 72. The developer supplied from the front screw 72 to the conveyance roller 54 is held on the outer peripheral surface of the conveyance sleeve 60 by the magnetic force of the magnetic pole N3 in the vicinity of the magnetic pole N3. The developer held on the transport sleeve 60 constitutes a magnetic brush along the magnetic lines formed by the magnet body 58, and is transported counterclockwise based on the rotation of the transport sleeve 60. The amount of developer that is held on the magnetic pole S <b> 1 in the opposed region (regulation region) 86 of the restriction plate 62 is restricted by the restriction plate 62 to a predetermined amount. The developer that has passed through the regulation gap 64 moves together with the conveying sleeve 60 while being held by the magnetic force of the magnetic pole S1, and is conveyed to a region (supply / recovery region) 88 where the developing roller 48 and the conveying roller 54 face each other.

  The supply / recovery region 88 includes an upstream region (supply region) 90 and a downstream region (recovery region) 92 with respect to the rotation direction of the transport sleeve 60, and adheres to the carrier by an electric field formed in the supply region 90. The developing toner is electrically supplied to the developing roller 48, and the developing residual toner is scraped off by the magnetic brush formed along the magnetic field lines of the magnetic pole N1 by the electric field formed in the collecting region 92, and is transferred to the conveying sleeve 60. To be recovered. The consumed toner is supplied again in the developer storage chamber 66. Thus, even when toner is consumed during development, the toner ratio of the developer on the conveyance roller 54 is maintained substantially constant.

The toner held on the developing roller 48 in the supply area 90 is conveyed counterclockwise with the rotation of the developing roller 48, and is an area (developing area) 96 where the photoconductor 12 and the developing roller 48 face each other. It adheres to the electrostatic latent image portion formed on the outer peripheral surface. In the image forming apparatus according to the embodiment, a predetermined negative potential V _H is applied to the outer peripheral surface of the photoreceptor 12 by the charging device 26, and the electrostatic latent image image portion on which the image light 30 is projected by the exposure device 28 is predetermined. attenuated until the potential V _L, an electrostatic latent image non-image portion of the image light 30 is not projected by the exposing device 28 maintains a substantially charge potential V _H. Accordingly, in the developing region 96, the negatively charged toner 6 adheres to the electrostatic latent image portion due to the action of the electric field formed between the photosensitive member 12 and the developing roller 48, and the electrostatic latent image portion. The latent image is visualized as a developer image.

  In this embodiment, during non-image formation, that is, when the electrostatic latent image on the photoreceptor 12 is not visualized as a developer image, the electric field forming device 110 is controlled to develop a small amount of magnetic carrier instead of toner. The electric field in the supply / recovery region 88 is controlled so as to be electrically supplied to the roller 48. The magnetic carrier thus supplied to the developing roller 48 is held above the holding magnetic pole N of the magnet body 47 as magnetic particles M via the photosensitive member 12.

  The development residual toner generated at the time of image formation collides with the magnetic carrier held on the development sleeve 45 to reduce the adhesive force and film thickness variation with the development sleeve 45, and the toner in the recovery region 92 is reduced. The collection efficiency can be improved and the occurrence of image memory can be avoided.

  Further, as described above, the magnet body 47 of the developing roller 48 has the slit 51 extending in the central axis direction with the developing sleeve 45 so as to weaken the electrical attraction force by the developing roller 48 with respect to the development residual toner. It is configured. As a result, the toner collection efficiency in the collection area 92 can be further improved, and the occurrence of image memory can be avoided more reliably.

  When the toner is consumed from the developer in this manner, it is preferable that an amount of toner corresponding to the consumed amount is supplied to the developer. For this purpose, the developing device 34 includes means for measuring the mixing ratio of the toner and the carrier accommodated in the housing 42. In addition, a toner replenishment section 98 is provided above the rear chamber 70. The toner supply unit 98 includes a container 100 for storing toner. An opening 102 is formed at the bottom of the container 100, and a supply roller 104 is disposed in the opening 102. The replenishing roller 104 is drivingly connected to a motor (not shown), and the motor is driven based on the output of the means for measuring the mixing ratio of toner and carrier so that the toner drops and replenishes the rear chamber 70.

[3. Specific materials of developer]
Specific materials of the toner, carrier, charged particles, and other particles included in the developer will be described.

〔toner〕
As the toner, a known toner that has been conventionally used in an image forming apparatus can be used. The toner particle size is, for example, about 3 to 15 μm. A toner containing a colorant in a binder resin, a toner containing a charge control agent or a release agent, and a toner holding an additive on the surface can also be used.

  The toner can be produced by a known method such as a pulverization method, an emulsion polymerization method, or a suspension polymerization method.

  The binder resin used for the toner is not limited. For example, styrene resin (monopolymer or copolymer containing styrene or styrene-substituted product), polyester resin, epoxy resin, vinyl chloride resin, phenol resin. , Polyethylene resin, polypropylene resin, polyurethane resin, silicone resin, or any mixture of these resins. The binder resin preferably has a softening temperature in the range of about 80 to 160 ° C and a glass transition point in the range of about 50 to 75 ° C.

  The colorant used in the toner is a known material such as carbon black, aniline black, activated carbon, magnetite, benzine yellow, permanent yellow, naphthol yellow, phthalocyanine blue, first sky blue, ultramarine blue, rose bengal, lei. Key red or the like can be used. In general, the addition amount of the colorant is preferably 2 to 20 parts by weight with respect to 100 parts by weight of the binder resin.

  As the charge control agent, materials conventionally known as charge control agents can be used. Specifically, for the positively charged toner, for example, nigrosine dyes, quaternary ammonium salt compounds, triphenylmethane compounds, imidazole compounds, and polyamine resins can be used as charge control agents. For the negatively charged toner, metal-containing azo dyes such as Cr, Co, Al, and Fe, salicylic acid metal compounds, alkyl salicylic acid metal compounds, and curixarene compounds can be used as charge control agents. The charge control agent is preferably used at a ratio of 0.1 to 10 parts by weight with respect to 100 parts by weight of the binder resin.

  As the release agent, a known release agent conventionally used as a release agent can be used. As the material for the release agent, for example, polyethylene, polypropylene, carnauba wax, sazol wax, or a mixture of them as appropriate is used. The release agent is preferably used at a ratio of 0.1 to 10 parts by weight with respect to 100 parts by weight of the binder resin.

  As other additives, a fluidizing agent that promotes fluidization of the developer may be added. As the fluidizing agent, for example, inorganic fine particles such as silica, titanium oxide, and aluminum oxide, and resin fine particles such as acrylic resin, styrene resin, silicone resin, and fluorine resin can be used. In particular, it is preferable to use a material hydrophobized with a silane coupling agent, a titanium coupling agent, silicone oil, or the like. The fluidizing agent is preferably added at a ratio of 0.1 to 5 parts by weight with respect to 100 parts by weight of the toner. The number average primary particle size of these additives is preferably 9 to 100 nm.

[Carrier]
As the carrier, a known carrier that has been generally used can be used. Either a binder type carrier or a coat type carrier may be used. The carrier particle size is not limited, but is preferably about 15 to 100 μm.

  The binder type carrier is obtained by dispersing magnetic fine particles in a binder resin, and those having fine particles or a coating layer charged positively or negatively on the surface can be used. The charging characteristics such as the polarity of the binder type carrier can be controlled by the material of the binder resin, the chargeable fine particles, and the type of the surface coating layer.

  Examples of the binder resin used for the binder-type carrier include thermoplastic resins such as vinyl resins, polyester resins, nylon resins, polyolefin resins, and the like typified by polystyrene resins, and curable resins such as phenol resins. .

  Magnetic fine particles of the binder type carrier include spinel ferrite such as magnetite and gamma iron oxide, and magnets such as spinel ferrite and barium ferrite containing one or more metals other than iron (Mn, Ni, Mg, Cu, etc.). Plumbite type ferrite, iron or alloy particles having an oxide layer on the surface can be used. The shape of the carrier may be granular, spherical, or needle-shaped. In particular, when high magnetization is required, it is preferable to use iron-based ferromagnetic fine particles. In consideration of chemical stability, it is preferable to use ferromagnetic fine particles of magnetoplumbite type ferrite such as spinel ferrite and barium ferrite containing magnetite and gamma iron oxide. A magnetic resin carrier having a desired magnetization can be obtained by appropriately selecting the type and content of the ferromagnetic fine particles. The magnetic fine particles are suitably added in an amount of 50 to 90% by weight in the magnetic resin carrier.

  Silicone resin, acrylic resin, epoxy resin, fluorine resin, etc. are used as the surface coating material for the binder type carrier. The charge imparting ability of the carrier can be improved by coating and curing these resins on the carrier surface to form a coat layer.

  For example, the charging fine particles or the conductive fine particles are fixed to the surface of the binder-type carrier by, for example, mixing the magnetic resin carrier and the fine particles uniformly, adhering these fine particles to the surface of the magnetic resin carrier, and then mechanically / thermally. This is done by driving fine particles into the magnetic resin carrier by applying a strong impact force. In this case, the fine particles are not completely embedded in the magnetic resin carrier, but are fixed so that a part thereof protrudes from the surface of the magnetic resin carrier. Organic and inorganic insulating materials are used for the chargeable fine particles. Specifically, organic insulating materials include polystyrene, styrene-based copolymers, acrylic resins, various acrylic copolymers, nylon, polyethylene, polypropylene, fluororesin, and cross-linked products thereof such as organic insulating fine particles. is there. The charge imparting ability and the charge polarity can be adjusted to the material of the chargeable fine particles, the polymerization catalyst, the surface treatment and the like. As the inorganic insulating material, negatively charged inorganic fine particles such as silica and titanium dioxide, and positively charged inorganic fine particles such as strontium titanate and alumina are used.

  The coat type carrier is a carrier in which carrier core particles made of a magnetic material are coated with a resin, and like the binder type carrier, chargeable fine particles that are charged positively or negatively can be fixed to the surface of the carrier. The charging characteristics such as the polarity of the coated carrier can be adjusted by selecting the type of the surface coating layer and the electrifying fine particles. As the coating resin, the same resin as the binder resin of the binder type carrier can be used.

  The mixing ratio of the toner and the carrier may be adjusted so as to obtain a desired toner charge amount, and the toner ratio is preferably 3 to 50% by weight, preferably 6 to 30% by weight based on the total amount of the toner and the carrier. .

[4. Experiment)
Next, between the developing sleeve 45 and the magnetic particles M, which is necessary because the magnetic particles M are held above the holding magnetic pole N without rotating together with the developing sleeve 45 using the developing device of this embodiment. The friction coefficient will be described.

As the magnetic particle M, a carrier for bizhub C350 manufactured by Konica Minolta Technologies Inc. having an average particle diameter of about 33 μm was used, which is a coated carrier in which a carrier core particle made of a magnetic material is coated with an acrylic resin.
The developing roller 48 includes a developing sleeve 45 having an outer diameter of 18 mm supported around a cylindrical magnet body 47 so as to be rotationally driven. The magnet body 47 is centered on the developing roller 48 with respect to the base line BL. And having a holding magnetic pole N magnetized on a radius of about 70 degrees with the counterclockwise direction as a positive direction.

At this time, the magnetic flux density (Br, Bc) in the radial direction and the circumferential direction of the developing roller 48 that the magnetic particles M shown in FIG. 2 receive from the holding magnetic pole N are shown in FIG. 4 with respect to the angle (θ) formed with the base line BL. It was confirmed that the transition was as shown in the graph. At this time, at the position where the holding magnetic pole N is formed (θ = about 70 degrees), the radial magnetic flux density (Br) has a peak value, and the circumferential magnetic flux density (Bc) becomes zero. It is distributed symmetrically as the center. The radial magnetic flux density (Br) is plotted with the direction toward the center as the positive direction, and the circumferential magnetic flux density (Bc) is plotted with the direction coinciding with the rotation direction of the developing roller 48 as the positive direction. .
Note that the magnetic attractive force (Fr, Fc) that the magnetic particles M receive from the holding magnetic pole N is proportional to the magnetic flux density and therefore has the same distribution as in FIG.

  Further, the magnetic particles M receive the same vertical drag as the magnetic attractive force (Fr) in the radial direction from the developing sleeve 45, and the developing sleeve 45 receives a frictional force (μ · Fr) obtained by multiplying the vertical drag by a friction coefficient (μ). Receive from. The magnetic particles M are to be held above the holding magnetic pole N and collide with the toner in order to reduce the adhesion force and film thickness variation of the toner with the developing sleeve 45. It is necessary to select the friction coefficient so that the attractive force (Fc) is larger than the frictional force (μ · Fr). Otherwise, that is, when the magnetic attractive force (Fc) in the circumferential direction received by the magnetic particles M is smaller than the frictional force (μ · Fr), the magnetic particles M adhere to the developing sleeve 45 and rotate together therewith, Cannot be achieved.

Here, a function F (θ) of an angle (θ) formed with the base line BL is defined as follows.
F (θ) = μ · Fr−Fc
At this time, if there exists an angle (θ) on the developing sleeve 45 such that the magnetic attractive force (Fc) in the circumferential direction is larger than the frictional force (μ · Fr) (that is, F (θ) <0), The magnetic particles M can be expected to have a desired effect without rotating together with the developing sleeve 45.

  6 to 8 were obtained as graphs plotting the function F (θ) when the friction coefficient (μ) was 0.05, 0.7, and 1.0, respectively. As is apparent from these graphs, when the friction coefficient (μ) is 0.05 and 0.7, the angle at which the circumferential magnetic attractive force (Fc) is larger than the frictional force (μ · Fr). Although (θ) exists in the developing sleeve 45, when the friction coefficient (μ) is 1.0, there is almost no angle (θ) that satisfies such a condition. Therefore, in order to reduce the variation in the adhesion force and film thickness of the toner to the developing sleeve 45 by holding the magnetic particles M above the holding magnetic pole N and colliding with the toner, the developing sleeve 45 and the magnetic particles are reduced. The coefficient of friction between M is preferably 1.0 or less.

When the magnetic particles M are rotated together with the developing sleeve 45 before being captured by the holding magnetic pole N, the centrifugal force (mv ² / r, m: the mass of the magnetic particles M, v: the developing sleeve 45 associated with the rotation). Of the developing sleeve 45). However, even when the peripheral speed of the developing sleeve 45 is high (for example, 900 m / s), the centrifugal force is about 10 ⁻⁸ N, which is sufficiently small compared to the magnetic attractive force (Fr) in the radial direction, and is ignored. be able to.

  In this way, by holding the magnetic particles M above the holding magnetic pole N and causing the undeveloped toner to collide with and pass through the magnetic particles M, the variation in the undeveloped toner thickness can be reduced, and the developing sleeve and the toner By reducing the adhesion force between them and supporting the separation / collection of the undeveloped toner from the developing sleeve, the generation of the image memory can be surely avoided.

1 is a schematic configuration diagram of an image forming apparatus and a developing device. FIG. 2 is an enlarged view of a conveying roller and a developing roller shown in FIG. It is a figure which shows the specific example of an electric field formation apparatus. It is a figure which shows the electric potential relationship of a conveyance roller and a developing roller. It is a graph which shows the magnetic flux density on a development sleeve. It is a graph of the function F ((theta)) when a friction coefficient is 0.05. It is a graph of the function F ((theta)) when a friction coefficient is 0.7. It is a graph of the function F ((theta)) when a friction coefficient is 1.0.

Explanation of symbols

1: image forming apparatus, 2: developer, 4: carrier, 6: toner, 8: charged particles, 10: spent, 12: photoconductor, 16: charging station, 18: exposure station, 20: developing station, 22: Transfer station 24: Cleaning station 26: Charging device 28: Exposure device 30: Image light 32: Passage 34: Development device 36: Transfer device 38: Sheet 40: Cleaning device 42: Housing 44: opening, 45: sleeve, 46: second space, 47: magnet body, 48: developing roller, 49: developing roller center, 50: developing gap, 51: slit, 52: second space, 54: Conveying roller, 55: Conveying roller center, 56: Supply / recovery gap, 58: Magnet body, 59: Straight line connecting each center, 60: Sleeve, 63: Restricting plate, 64 Regulation gap, 66: Developer stirring chamber, 68: Front chamber, 70: Rear chamber, 72: Front screw, 74: Rear screw, 76: Partition, 86: Restriction area, 88: Supply recovery area, 90: Supply area, 92: Collection area, 94: Release area, 96: Development area, 98: Toner replenishment section, 100: Container, 102: Opening section, 104: Replenishment roller, 110: Electric field forming device, 112: First power supply, 114: Second power supply, 116: ground, 118: DC power supply, 120: DC power supply, 122: AC power supply, N1: main magnetic pole (first magnetic pole part), N: holding magnetic pole (second magnetic pole part), M: magnetic particles .

Claims (9)

A first magnetic pole body having a plurality of first magnetic pole portions and a cylindrical conveying sleeve whose outer periphery is driven to rotate, and a developer containing non-magnetic toner and a magnetic carrier on the outer circumferential surface of the conveying sleeve A transport roller for transporting while holding
A second magnetic pole body having a second magnetic pole portion and a cylindrical developing sleeve whose outer periphery is driven to rotate are opposed to the conveying roller through the first area and through the second area. A developing roller disposed opposite the image carrier;
A first electric field forming unit that forms a first electric field between the conveying roller and the developing roller, and moves the non-magnetic toner in the developer held by the conveying roller to the developing roller;
A second electric field is formed between the developing roller and the image carrier, and the nonmagnetic toner held by the developing roller is moved to the electrostatic latent image on the image carrier and developed. A second electric field forming unit for forming a toner image,
The second magnetic pole portion is disposed in a region during which the developing sleeve rotates from the second region to the first region, and the magnetic material of the second magnetic pole portion is interposed through the developing sleeve. A developing device characterized by being held above.   The friction coefficient of the developing sleeve with respect to the magnetic material is 1 or less so that the magnetic material is held above the second magnetic pole portion while sliding on the developing sleeve. The developing device described. The magnetic body consists of a magnetic carrier,
The first electric field forming unit moves the magnetic carrier in the developer held by the transport roller to the developing roller when the second electric field forming unit does not form a toner image. The developing device according to claim 1 or 2.   The developing device according to claim 1, wherein the magnetic body is made of a magnetic metal plate.   The developing device according to claim 1, wherein the second magnetic pole body has a slit having a length in an axial direction of the developing roller.   The developing device according to claim 1, wherein the first and second magnetic pole portions disposed adjacent to the first region have different polarities.   The said 1st electric field formation part forms the direct current | flow electric field of the direction which moves a nonmagnetic toner to the said developing roller from the said conveyance roller, It is characterized by the above-mentioned. The developing device described.   The first electric field forming unit forms an oscillating electric field in a direction in which, on average, non-magnetic toner is moved from the transport roller to the developing roller. The developing device according to any one of the above.   An image forming apparatus comprising the developing device according to claim 1.

Images