JP6157162B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine that forms an image by a two-component development system.

  An electrophotographic image forming apparatus has been recognized for its image quality and stability, and has recently entered the light printing market. However, in order to increase the print volume, the market demands a further increase in image output, and a technique for improving the problems associated with the increase in the speed is required.

  In an electrophotographic image forming apparatus, an electrostatic latent image (electrostatic image) corresponding to image information is formed on a photoreceptor, and the electrostatic latent image is developed (visualized) as a toner image.

  As a development process corresponding to high-speed image output, a two-component developer in which toner (non-magnetic toner particles) and carrier (magnetic carrier particles) are mixed at a predetermined ratio (hereinafter also simply referred to as “developer”). It is the mainstream to use a two-component development method using The reason is the stability of image quality due to the charging stability of the toner.

  In a developing device that employs a two-component development system, a developer carrier is formed on a developer carrier that forms a heading (hereinafter also referred to as a “magnetic ear”) in which the developer is connected in a brush shape. And the photosensitive member are conveyed to a developing area (developing unit) facing each other. Then, in the development area, the magnetic spikes are rubbed against the surface of the photosensitive member, and a developing bias is applied to the developer carrying member to supply the toner in the developer to the image portion of the electrostatic latent image on the photosensitive member. Then, the electrostatic latent image is developed as a toner image.

  Generally, the developer carrying member is a rotatable cylindrical developing sleeve, and a magnet roller having a plurality of magnetic poles is disposed therein as magnetic field generating means. The magnet roller is usually fixedly arranged, and the magnetic sleeve on the surface of the developing sleeve is conveyed by moving the developing sleeve relative to the magnet roller.

  Incidentally, one of the reasons for making it difficult to increase the image output speed for the development process is a decrease in developability. Here, the decrease in developability means that a necessary amount of toner having a certain charge amount cannot be supplied to the image portion of the electrostatic latent image on the photoreceptor. This is considered to be because the time required for the photosensitive member to pass through the developing area decreases as the peripheral speed of the photosensitive member increases, so that a necessary amount of toner cannot be supplied to the developing area within that time.

  Of course, it is possible to improve the developability by increasing the peripheral speed of the developing sleeve and increasing the developer transportability (increasing the transport amount per unit time) as the image output speeds up. However, if the peripheral speed of the developing sleeve is increased too much, deterioration of the developer is promoted, and stable developability cannot be obtained over a long period of time. Further, the increase in centrifugal force may cause the developer to scatter. Therefore, a means for improving developability has been demanded without depending on the peripheral speed of the developing sleeve.

In order to improve developability, it is known that it is effective to lower the electrical resistance of the carrier. Patent Document 1 proposes to use a carrier (10 ³ to 10 ⁷ Ω · cm) having a low electric resistance as a developing device corresponding to a high-speed output (peripheral speed of photoconductor ≧ 500 mm / s). Yes.

  According to the study by the present inventors, when the electric resistance of the carrier is low, the potential of the carrier in the vicinity of the photosensitive member is close to the potential of the developing sleeve, and a large electric field is generated in a narrow gap near the photosensitive member. It is thought that the sex improves.

In this specification, a carrier having a volume resistivity of 10 ⁷ Ω · cm or less (typically 10 ² to 10 ⁷ Ω · cm) is referred to as a “low resistance carrier”, and a volume resistivity is 10 ⁸ Ω · cm or more. Carriers (typically 10 ⁸ to 10 ¹⁰ Ω · cm) are referred to as “high resistance carriers”.

JP-A-10-104868

  However, as a result of studies by the present inventors, when a low-resistance carrier is used, carrier adhesion to the image portion of the electrostatic latent image on the photoreceptor occurs when an image with a high printing rate (image ratio) is continuously output. It turns out that it may be easy to do.

  This is because the electric resistance of the developer is low, so that the charge is easily injected from the developing sleeve to the photoconductor through the developer. As a result, the charge polarity of the carrier is reversed and the electrostatic latent image on the photoconductor is imaged. This is considered to adhere to the part.

  According to the study by the present inventors, in order to improve the developability, it is desired to “lower the electrical resistance of the developer” and “increase the toner amount”. However, when a low-resistance carrier is used, it is difficult to suppress carrier adhesion over a long period of time even if the toner concentration of the developer is increased in order to increase the electric resistance of the developer. Here, the toner concentration of the developer is a weight ratio of the toner to the total weight of the developer including the toner and the carrier (hereinafter also referred to as “T / D ratio”).

  On the other hand, when a high-resistance carrier is used, the carrier adhesion is improved over a long period of time, but the developability may be lowered. When a high-resistance carrier is used, it is difficult to improve developability even if the T / D ratio is decreased in order to reduce the electrical resistance of the developer. On the other hand, it was difficult to improve developability even when the T / D ratio was increased.

  The observation results and reasons for these phenomena will be described in more detail later in comparison with the present invention.

  Thus, according to the study by the present inventors, it is difficult to suppress carrier adhesion over a long period of time when a low-resistance carrier is used. Thus, both improvement in developability and suppression of carrier adhesion are achieved. It is difficult. Therefore, in order to achieve both improvement of developability and suppression of carrier adhesion, it is desired to improve developability when using a high resistance carrier.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of improving developability even when carrier adhesion is suppressed over a long period of time using a high-resistance carrier.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to an image carrier on which an electrostatic image is formed and a developer that is rotatable and includes a toner and a carrier . A first developer carrying member for developing toner by supplying toner to an electrostatic image on the surface ; and a rotation direction of the first developer carrying member fixedly disposed in a hollow portion of the first developer carrying member And a first magnetic field generating means having a plurality of magnetic poles, and a developer that is rotatable and includes toner and a carrier, and conveys the developer, and in the moving direction of the image carrier, than the first developing region. In the upstream second development area, toner is supplied to the electrostatic image on the surface of the image carrier for development, and the developer after passing through the second development area is used as the first developer. A second developer carrying member to be transferred to the carrying member; A second developer carrier disposed at a position facing a predetermined magnetic pole upstream of an odd pole from a magnetic pole corresponding to the first development region in the rotation direction of the first developer carrier among the magnetic poles of A second magnetic field generating means fixedly disposed in a hollow portion of the second developer carrier and having a plurality of magnetic poles in the rotation direction of the second developer carrier, and the first magnetic field generator during image formation. between the developer carrying member and said second developer carrying member, wherein the bets toner of the predetermined said formed by magnetic poles of the first developer carrying member the developer in the bristles on the surface of the a potential difference forming means for forming a potential difference to the second side of the developer carrying member is moved to the first side of the developer carrying member, Ru der image forming apparatus characterized by having a.

  According to the present invention, it is possible to improve developability even when carrier adhesion is suppressed over a long period of time using a high-resistance carrier.

1 is a schematic cross-sectional view illustrating a schematic configuration of a main part of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic cross-sectional view of a developing device according to an embodiment of the present invention. FIG. 6 is a schematic diagram illustrating a behavior of a developer on a developing sleeve in an embodiment of the present invention. 1 is a schematic cross-sectional view of a developing device of Example 1. FIG. 6 is a schematic cross-sectional view of a developing device of Comparative Example 1. FIG. 6 is a schematic cross-sectional view of a developing device of Comparative Example 2. FIG. FIG. 3 is a schematic diagram illustrating a behavior of a developer on a developing sleeve in Embodiment 1. 6 is a schematic diagram illustrating a behavior of a developer on a developing sleeve in Comparative Example 1. FIG. 6 is a schematic diagram illustrating a behavior of a developer on a developing sleeve in Comparative Example 2. FIG. It is typical sectional drawing of the developing device for demonstrating the other form of an electroconductive member. It is typical sectional drawing of the developing device for demonstrating the other form of an electroconductive member. It is typical sectional drawing of the developing device for demonstrating the other form of an electroconductive member. It is a schematic diagram which shows the outline | summary of the measuring apparatus of the volume resistivity of a carrier. 6 is a schematic cross-sectional view of a developing device of Example 2. FIG. 10 is a schematic cross-sectional view of a developing device of Comparative Example 3. FIG. 10 is a schematic cross-sectional view of a developing device of Comparative Example 4. FIG. 10 is a schematic cross-sectional view of a developing device of Comparative Example 5. FIG. FIG. 10 is a schematic diagram illustrating a behavior of a developer on a developing sleeve in Embodiment 2. 12 is a schematic diagram illustrating the behavior of a developer on a developing sleeve in Comparative Example 3. FIG. 10 is a schematic diagram illustrating the behavior of a developer on a developing sleeve in Comparative Example 4. FIG. 10 is a schematic diagram showing the behavior of a developer on a developing sleeve in Comparative Example 5. FIG. 6 is a schematic cross-sectional view of a developing device of Example 3. FIG. 10 is a schematic cross-sectional view of a developing device of Comparative Example 6. FIG. FIG. 10 is a schematic diagram illustrating a behavior of a developer on a developing sleeve in Embodiment 3. 10 is a schematic diagram illustrating the behavior of a developer on a developing sleeve in Comparative Example 6. FIG. It is a schematic diagram which shows the difference in the uniformity of a solid image. FIG. 6 is a schematic diagram illustrating the behavior of the developer on the developing sleeve when the toner concentration of the developer is high. It is a figure which shows the analysis result which simulates the space potential distribution between a photoconductor and a developing sleeve.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

[First Embodiment]
First, a first embodiment of the present invention will be described.

1. Overall Configuration and Operation of Image Forming Apparatus FIG. 1 is a schematic cross-sectional view showing a schematic configuration of a main part of an image forming apparatus 100 according to an embodiment of the present invention.

  The image forming apparatus 100 includes a cylindrical (drum type) electrophotographic photosensitive member (photosensitive member) 1 as an image carrier. The photoreceptor 1 is rotationally driven in the direction of arrow R1 in the figure. Around the photosensitive member 1, the following units are arranged in order along the rotation direction. First, a charging device 2 as a charging unit is arranged. Next, an exposure apparatus 3 as an exposure unit is arranged. Next, a developing device 4 as a developing unit is arranged. Next, an intermediate transfer unit 5 as a transfer device is arranged. Next, a photoreceptor cleaner 6 is disposed as a photoreceptor cleaning means.

  The intermediate transfer unit 5 includes an intermediate transfer belt 51 that is an endless belt as an intermediate transfer member. The intermediate transfer belt 51 is stretched around a drive roller 52, a secondary transfer counter roller 53, and a driven roller 54. The intermediate transfer belt 51 rotates (circulates) in the direction of arrow R <b> 2 in the drawing when a rotational driving force is transmitted to the driving roller 52. On the inner peripheral surface side of the intermediate transfer belt 51, a primary transfer roller 55, which is a roller-shaped primary transfer member serving as a primary transfer unit, is disposed at a position facing the photoreceptor 1. The primary transfer roller 55 is pressed against the photosensitive member 1 via the intermediate transfer belt 51, and a primary transfer portion (primary transfer nip) T1 where the intermediate transfer belt 51 and the photosensitive member 1 are in contact with each other is formed. On the outer peripheral surface side of the intermediate transfer belt 51, a secondary transfer roller 56 that is a roller-like secondary transfer member as a secondary transfer unit is disposed at a position facing the secondary transfer counter roller 53. The secondary transfer roller 56 is pressed against the secondary transfer counter roller 53 via the intermediate transfer belt 51, and a secondary transfer portion (secondary transfer nip) where the intermediate transfer belt 51 and the secondary transfer roller 56 are in contact with each other. T2 is formed.

  Further, the image forming apparatus 100 includes a transfer material supply device (not shown) for supplying the transfer material P to the secondary transfer portion T2, a fixing device 10 as a fixing unit for fixing the toner image to the transfer material P, and the like. Is provided.

  At the time of image formation, the surface of the rotating photoreceptor 1 is uniformly charged by the charging device 2 to a predetermined potential having a predetermined polarity (in this embodiment, negative polarity). The surface of the charged photoreceptor 1 is subjected to scanning exposure by the exposure device 3 according to image information. As a result, an electrostatic latent image (electrostatic image) corresponding to the exposure image pattern is formed on the surface of the photoreceptor 1. The electrostatic latent image formed on the photosensitive member 1 is developed as a toner image by the developing device 4 in a developing region (developing portion) D where a developing sleeve 41 of the developing device 4 and a photosensitive member 1 described later face each other. . The developing device 4 forms magnetic spikes with a developer on the developing sleeve 41 and conveys the magnetic spikes to the development area D. In this embodiment, in the development region D, the magnetic brush is rubbed against the surface of the photoconductor 1 and a developing bias is applied to the developing sleeve 41, so that the toner in the developer is static on the photoconductor 1. The electrostatic latent image is developed by supplying the image portion of the electrostatic latent image. Note that the toner may be transferred from the magnetic brush to the electrostatic latent image by bringing the developer magnetic brush close to the photosensitive member 1 without contacting it. In the present embodiment, the developing device 4 develops the electrostatic latent image on the image carrier by a reversal development method. In other words, the image portion (exposure portion) on the photoreceptor 1 where the absolute value of the potential has been reduced by exposure after being uniformly charged is set to the same polarity as the charge polarity of the photoreceptor 1 (negative polarity in the present embodiment). By attaching the charged toner, the electrostatic latent image is developed as a toner image. The configuration and operation of the developing device 4 will be described in detail later.

  The toner image formed on the photoreceptor 1 is transferred (primary transfer) onto the intermediate transfer belt 51 that moves at a substantially equal speed to the peripheral speed of the photoreceptor 1 by the action of the primary transfer roller 55 in the primary transfer portion T1. Is done. At this time, a primary transfer bias having a predetermined potential is applied to the primary transfer roller 55 from a primary transfer power source (not shown) having a polarity opposite to the toner charging polarity (normal charging polarity) at the time of development. The toner image formed on the intermediate transfer belt 51 is transferred and transferred between the intermediate transfer belt 51 and the secondary transfer roller 56 by the action of the secondary transfer roller 56 in the secondary transfer portion T2. Transferred (secondary transfer) on top. At this time, a secondary transfer bias having a predetermined potential is applied to the secondary transfer roller 56 from a secondary transfer power source (not shown) having a polarity opposite to the charging polarity of the toner at the time of development. The transfer material P is conveyed to the secondary transfer portion T2 by the transfer material supply device in synchronization with the toner image on the intermediate transfer belt 51.

  The transfer material P onto which the toner image has been transferred is separated from the intermediate transfer belt 51 and conveyed to the fixing device 10. The transfer material P is heated and pressed in the process of being sandwiched and conveyed by the heating roller 11 and the pressure roller 12 in the fixing device 10, and the toner image is fixed thereon as a fixed image. The transfer material P on which the toner image is fixed is then discharged outside the apparatus main body of the image forming apparatus 100 as a final image output product.

  Further, the toner (primary transfer residual toner) remaining on the photoreceptor 1 without being transferred to the intermediate transfer belt 51 after the primary transfer process is removed from the photoreceptor 1 by the photoreceptor cleaner 6 and collected. Thereby, the photoreceptor 1 is repeatedly used for image formation. Further, toner (secondary transfer residual toner) remaining on the intermediate transfer belt 51 without being transferred to the transfer material P after the secondary transfer step is removed from the intermediate transfer belt 51 by an intermediate transfer member cleaning unit (not shown). Removed and recovered. Thereby, the intermediate transfer belt 51 is repeatedly used for image formation.

  The image forming apparatus 100 includes a plurality of sets of image forming means including the above-described photoconductor 1, charging device 2, exposure device 3, developing device 4, primary transfer roller 55, photoconductor cleaner 6, and the like. It may be possible to form. For example, yellow (Y), magenta (M), cyan (C), and black (K) image forming units as a plurality of image forming units are arranged along the moving direction of the image transfer surface of the intermediate transfer belt 51. It may be. In this case, for example, when a full color image is formed, the toner images of the respective colors formed on the respective photoreceptors 1 in the respective image forming units are primarily transferred so as to be superimposed on the intermediate transfer belt 51 in the respective primary transfer units T1. The Thereafter, the multiple toner images for color images on the intermediate transfer belt 51 are secondarily transferred collectively onto the transfer material P in the secondary transfer portion T2.

  In the present embodiment, a rotatable cylindrical a-Si photosensitive member 1 in which an a-Si photosensitive layer is formed on a conductive substrate is used as the photosensitive member 1. The photosensitive member 1 is rotationally driven at a predetermined peripheral speed in the direction of arrow R1 in the figure by a driving motor (not shown) as a driving means.

  In the present embodiment, the charging device 2 is of a magnetic brush charging type. A magnetic brush charging type charging device includes a rotatable cylindrical charging sleeve and a magnet roller disposed therein. The charging sleeve is brought into contact with the surface of the photosensitive member 1 while conveying the spikes (charged magnetic brush) of magnetic particles formed on the surface thereof. Then, by applying a predetermined charging bias to the charging sleeve, the surface of the photoreceptor 1 is typically charged by an injection charging method. By using a charging device of a magnetic brush charging system, it is possible to significantly improve image flow and micro charging unevenness due to discharge products.

  The exposure apparatus 3 includes an analog exposure apparatus that projects a document image and a digital exposure apparatus such as a laser scanner or an LED array. In this embodiment, a digital exposure apparatus, particularly a laser scanner is used.

2. Next, the configuration of the developing device 4 of the present embodiment will be described in more detail. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the developing device 4 of the present embodiment.

  The developing device 4 includes a rotatable cylindrical developing sleeve 41 as a developer carrying member that carries and conveys the developer, and a magnetic field generating unit that is fixedly disposed in the inside (hollow portion) of the developing sleeve 41. And a magnet roller 42 composed of a cylindrical permanent magnet. The developing sleeve 41 and the magnet roller 42 constitute a developing member 43 that supplies developer to the developing region D. The developing sleeve 41 is disposed to face the photoconductor 1 so as to leave a predetermined space (gap) between the developing sleeve 41 and the photoconductor 1. In the present embodiment, the developing sleeve 41 is disposed such that the closest distance between the developing sleeve 41 and the photosensitive member 1 is 300 μm.

  Here, the longitudinal direction (rotation axis direction) of the developing sleeve 41 is substantially parallel to the longitudinal direction (rotation axis direction) of the photoreceptor 1. The magnet roller 42 is disposed substantially concentrically with the developing sleeve 41 so as to open a predetermined interval (gap) between the inner surface of the developing sleeve 41 and the longitudinal direction of the developing sleeve 41 ( It extends over almost the entire area in the direction of the rotation axis).

  Further, the developing device 4 includes a developing container 46 that stores a two-component developer including toner and a carrier. The developing container 46 is provided with an opening 46a at a position facing the photosensitive member 1, and the developing sleeve 41 is disposed so that a part of the opening 46a is exposed from the opening 46a. In the present embodiment, the developing sleeve 41 is rotatably supported by the developing container 46, while the magnet roller 42 is fixed to the developing container 46. In addition, a screw 45 as an agitating and conveying member that agitates the developer and conveys the developer toward the developing sleeve 41 is disposed inside the developing container 46.

  Further, a regulating blade 44 as a developer regulating member is disposed at the edge of the developing container 46 upstream of the opening 46a in the rotation direction of the developing sleeve 41 (in the direction of arrow R3 in the figure). The regulating blade 44 is disposed to face the developing sleeve 41 so as to leave a predetermined interval (gap) between the regulating blade 44 and the developing sleeve 41. Further, the regulation blade 44 extends so as to be opposed over substantially the entire region in the longitudinal direction (rotation axis direction) of the developing sleeve 41.

  Further, the conductive member 7 is disposed downstream of the regulating blade 44 and upstream of the developing region D in the rotation direction of the developing sleeve 41 (in the direction of arrow R3 in the figure). The conductive member 7 is disposed to face the developing sleeve 41 so as to leave a predetermined interval (gap) between the conductive member 7 and the developing sleeve 41. In addition, the conductive member 7 extends so as to face almost the entire region in the longitudinal direction (rotational axis direction) of the developing sleeve 41.

  A developing power source (transmitting device) 8 is connected to the developing sleeve 41 as a developing bias applying means (first power source) for applying a developing bias (developing voltage). The developing power source 8 includes a waveform signal oscillator 81 and a high voltage power source 82 that amplifies a signal generated by the waveform signal oscillator 81 and applies a developing bias to the developing sleeve 41.

  More specifically, in this embodiment, an oscillating voltage obtained by superimposing a rectangular wave having a frequency of 6 kHz and a peak-to-peak voltage of 1.5 kV on a DC voltage of −330 V is used as the developing bias. According to the study by the present inventors, the peak-to-peak voltage of the AC component of the developing bias is preferably 0.7 kV to 1.8 kV. If the peak-to-peak voltage of the AC component of the developing bias is greater than 1.8 kV, discharge traces are likely to occur. The discharge trace means that a strong electric field is applied to the development region due to a large peak-to-peak voltage, and a discharge is induced through a low electrical resistance material taken into the developing device 4. This is a scratch on the photoreceptor caused by dielectric breakdown of the portion. The low electrical resistance material is, for example, shaving powder of the developing sleeve 41. On the other hand, when the peak-to-peak voltage of the AC component of the developing bias is smaller than 0.7 kV, the toner is not easily rearranged on the photosensitive member 1, so that the image uniformity is likely to deteriorate rapidly. The frequency of the AC component of the developing bias is preferably 4 kHz to 12 kHz. If the frequency of the AC component of the developing bias is greater than 12 kHz, the toner will not easily follow the change in the polarity of the developing bias, which tends to cause deterioration in character reproducibility and image uniformity. On the other hand, when the frequency of the AC component of the developing bias is less than 4 kHz, the number of toner rearrangements in the developing area is reduced, so that the image uniformity is likely to be rapidly lowered. Note that the waveform of the developing bias is not limited to a rectangular wave. For example, another waveform such as a sine wave may be used.

  In the present embodiment, the developing sleeve 41 is rotationally driven in the direction of arrow R3 in the drawing. That is, in the developing region D, the moving direction of the surface of the developing sleeve 41 and the moving direction of the surface of the photoreceptor 3 are the same direction. However, the present invention is not limited to this, and in the developing region D, the developing sleeve 41 and the photosensitive member 1 are rotationally driven so that the surface of the developing sleeve 41 and the surface of the photosensitive member 1 move in the opposite directions. It is good also as a structure to be.

  The development area D is an area that contributes to the development of the electrostatic latent image in the movement direction of the respective surfaces of the photoreceptor 1 and the development sleeve 41. More specifically, the development area D refers to an area on the photosensitive member 1 and a corresponding developing sleeve 41 where toner transfer may occur when development is performed with the development sleeve 41 and the photosensitive member 1 stopped rotating. Point to the upper area.

  In the example shown in FIG. 2, the magnet roller 42 has magnetic poles S1, N2, S2, N3, and N1 as a plurality of magnetic poles in the circumferential direction. These magnetic poles S1, N2, S2, N3, and N1 are arranged in this order in the direction opposite to the rotation direction of the developing sleeve 41 (the direction opposite to the direction in which the developer passes through the developing region D).

  The magnetic pole name “S” indicates the S pole of the magnet, and “N” indicates the N pole of the magnet. In this specification, for easy understanding, the position on the developing sleeve 41 corresponding to the magnetic pole on the magnet roller 42 may be simply referred to as the position of the magnetic pole on the developing sleeve 41.

  In the example shown in FIG. 2, the magnetic pole S <b> 1 is a magnetic pole (developing pole, developing main pole) corresponding to the developing area D in the circumferential direction of the developing sleeve 41. More specifically, the magnetic pole corresponding to (or existing in) the development area D is a magnetic force generated from a plurality of magnetic poles of the magnet roller 42 that is used for development in the development area D. The magnetic pole held on the developing sleeve 41 is the magnetic pole closest to the photoreceptor 1 in the circumferential direction of the developing sleeve 41. In other words, the magnetic pole corresponding to the development region D is a magnetic pole that forms on the developing sleeve 41 a spike of developer that is closest to or in contact with the photoreceptor 1. In the present embodiment, the spikes of the developer formed on the developing sleeve 41 by the developing pole S 1 come into contact with the photoreceptor 1 in the developing region D. Note that the magnetic pole holds the developer spike on the developing sleeve 41 or is formed on the developing sleeve 41. The magnetic pole is at the end (base end) of the developer spike on the developing sleeve 41 side. It is adjacent.

  A magnetic pole (toner biasing pole) N2 is disposed adjacent to the upstream side of the magnetic pole S1 in the rotation direction of the developing sleeve 41. That is, the toner biasing pole N2 is a magnetic pole that is one pole upstream of the developing pole S1 in the rotation direction of the developing sleeve 41.

  In the example shown in FIG. 2, the conductive member 7 is disposed at a position facing the toner biasing pole N <b> 2 and in contact with the magnetic spike on the developing sleeve 41.

  In this embodiment, when the conductive member 7 is viewed along the longitudinal direction (rotation axis direction) of the developing sleeve 41, at least the surface facing the outer peripheral surface of the developing sleeve 41 (the whole in the present embodiment), The developing sleeve 41 has a substantially concentric circular arc shape. That is, in the present embodiment, at least the surface of the conductive member 7 that faces the outer peripheral surface of the developing sleeve 41 has a curvature that is substantially the same as the curvature of the outer peripheral surface of the developing sleeve. The conductive member 7 is arranged such that a distance (closest distance) L between the surface of the conductive member 7 on the developing sleeve 41 side and the outer peripheral surface of the developing sleeve 41 is 600 μm.

  In this embodiment, the conductive member 7 is disposed at a position facing the toner biasing pole N2 that is one pole upstream from the developing pole S1 corresponding to the developing area D in the rotation direction of the developing sleeve 41. It is not limited. The conductive member 7 may be disposed at a position facing a predetermined magnetic pole upstream of the odd pole from the developing pole S1 corresponding to the developing area D in the rotation direction of the developing sleeve 41. The reason will be described later.

  The conductive member 7 is connected to a toner energizing power source (transmitting device) 9 as toner energizing bias applying means (second power source) for applying a toner energizing bias (toner energizing voltage) described later. ing. The toner energizing power source 9 includes a waveform signal oscillator 91 and a high voltage power source 92 that amplifies the signal generated by the waveform signal oscillator 91 and applies a toner energizing bias to the conductive member 7.

  More specifically, in this embodiment, a DC voltage of −530 V is used as the toner bias bias. However, the present invention is not limited to this. Details of the toner bias will be described later.

  In the example shown in FIG. 2, a magnetic pole (regulatory pole) S2 is arranged adjacent to the upstream side of the toner biasing pole N2 in the rotation direction of the developing sleeve 41. That is, the regulation pole S2 is a magnetic pole that is two poles upstream of the development pole S1 in the rotation direction of the development sleeve 41. In the example shown in FIG. 2, the regulation blade 44 is disposed at a position facing the regulation pole S <b> 2 and in contact with the magnetic spike on the developing sleeve 41. The regulating blade 44 regulates the layer thickness of the developer on the developing sleeve 41 in cooperation with the magnetic force generated by the magnetic pole S2.

  Here, being opposed to the magnetic pole is not only in the case of being positioned on the straight line in the radial direction of the magnet roller 42 passing through the peak position of the magnetic flux density in the normal direction of the magnetic pole, but more than on the same line. This includes the case where the developing sleeve 41 is displaced from the upstream side and the downstream side in the rotational direction. Generally, it is positioned on the radial straight line of the magnet roller 42 that passes through the tip of the magnetic spike (including the one before the complete startup) that has risen on the developing sleeve 41 by the magnetic force of the magnetic pole (including the one before the developing sleeve 41 radially). If you do. Typically, it is located in a range of about ± 30 ° in the circumferential direction of the magnet roller 42 with reference to a radial line of the magnet roller 42 passing through the peak position of the magnetic flux density in the normal direction of the magnetic pole. Is assumed.

  In the example shown in FIG. 2, the magnetic pole N3 is disposed adjacent to the upstream side of the regulation pole S2 in the rotation direction of the developing sleeve 41, and the magnetic pole N1 is disposed adjacent to the upstream side of the magnetic pole N3. At the magnetic pole N3, the developer is pumped up on the developing sleeve 41. The magnetic poles N3 and N1 are adjacent repulsive magnetic poles, and the developer on the developing sleeve 41 is peeled from the developing sleeve 41 between the magnetic poles N3 and N1 and mixed with the developer inside the developing container 46. The

  The arrangement of the magnetic poles of the magnet roller 42 is not limited to the example shown in FIG. If the requirements of the present invention to be described in more detail below are provided, the specific magnetic pole arrangement of the magnet roller 42 can be changed as appropriate. For example, the magnetic pole arrangement as in Example 1 (FIG. 4) described later is used. May be.

3. Next, in order to facilitate understanding of the present invention, before describing the behavior of the developer in the developing device 4 of the present embodiment, the behavior of the developer in the conventional developing device will be described. explain.

  As described above, when the low resistance carrier is used, the carrier for the image portion of the electrostatic latent image on the photosensitive member is continuously output when an image having a high printing rate (image ratio) is used. It has been found that adhesion can easily occur. Here, the carrier adhesion is a phenomenon in which the carrier adheres to the photosensitive member where only toner should adhere from the magnetic spike.

  The reason why the carrier adhesion is likely to occur is considered as follows. Compared to the electrical resistance of the carrier, the electrical resistance of the toner is much higher. Therefore, when a low-resistance carrier is used, the electrical resistance of the developer is low in the first place, and the electrical resistance of the developer is further lowered as the toner is consumed. For this reason, electric charges are easily injected from the developing sleeve to the photoreceptor via the developer. As a result, for example, when using a negatively chargeable toner, the carrier should be charged positively, but the carrier is negatively charged (negative), and the image portion of the electrostatic latent image on the photoconductor Adhere to.

  Accordingly, the present inventors have studied by increasing the toner concentration (T / D ratio) of the developer in order to increase the electric resistance of the developer when a low resistance carrier is used. However, in this case, it has been difficult to suppress carrier adhesion over a long period of time. Since carrier adhesion can occur with only a small current path in the developer, it has been difficult to suppress this when a low-resistance carrier is used.

  On the other hand, when a high-resistance carrier is used, the carrier adhesion is improved over a long period of time, but the developability may be lowered. Therefore, the present inventors have studied by reducing the T / D ratio in order to reduce the electric resistance of the developer when a high-resistance carrier is used. However, in this case, it has been difficult to improve developability. This is considered to be because the electric resistance of the developer is lowered and the electric field is increased by decreasing the toner amount, but the toner amount in the developer is absolutely insufficient. Conversely, in order to increase the amount of toner in the developer, the T / D ratio was increased and examined. However, in this case as well, it was difficult to improve developability. This is considered to be because the electric resistance of the developer is further increased although the toner amount is increased.

  Table 1 summarizes the evaluation results of developability and carrier adhesion as described above, with an acceptable level of “◯” and an unacceptable level of “X”. In Table 1, “low T / D ratio” and “high T / D ratio” indicate a case where the T / D ratio is low and high with respect to “Ref” as a reference. In the column showing the evaluation results, “o” before “/” or “x” is the developability result, and after is the result of carrier adhesion.

  In order to verify in detail the phenomenon related to the above-mentioned evaluation results, the present inventors used a high-speed video camera (FASTCAM SA1 manufactured by Photon) and observed the behavior of the toner in the development region.

  As a result, it has been found that the toner that easily flies from the developing sleeve side to the photosensitive member side by the developing bias in the magnetic spike on the developing sleeve is mainly the toner near the photosensitive member. The reason is considered as follows.

  FIG. 28 shows a calculation result of a space potential distribution in an electrostatic field when a dielectric (carrier) is placed between parallel plates. The calculation was performed by using a Laplace equation and a general difference method as a numerical analysis method (the 59th Japan Imaging Academy Technical Seminar “Easy Electric Field Calculation Using Spreadsheet”).

  As shown in FIG. 28, when a dielectric (carrier) is placed in a space between parallel plates and a voltage is applied between the parallel plates, the dielectric (carrier) having a dielectric constant larger than that of the surrounding space is inside. The interval between equipotential lines becomes sparse. As a result, the interval between equipotential lines formed in the space between the flat plate (photosensitive member) and the tip of the dielectric (carrier) becomes close. That is, it is considered that a stronger electric field acts on the toner in the vicinity of the photoconductor among the toner present in the magnetic spike, and as a result, the toner in the vicinity of the photoconductor is likely to fly.

  Actually, as a result of observing the carrier after development under the conditions where the developability did not reach an allowable level at the low T / D ratio shown in Table 1, the carrier in the vicinity of the developing sleeve was covered with toner. The carrier in the vicinity of the photoreceptor was almost not covered with toner. From this, it is considered that the reason why the developability did not reach an allowable level at a low T / D ratio is that the number of toners in the vicinity of the photoreceptor is absolutely insufficient.

  As described above, when a low-resistance carrier is used, it is difficult to suppress carrier adhesion over a long period of time, and thus it is difficult to achieve both improvement in developability and suppression of carrier adhesion. Therefore, in order to achieve both improvement of developability and suppression of carrier adhesion, it is desired to improve developability when using a high resistance carrier.

  As described above, in order to improve developability, both “reducing the electrical resistance of the developer” and “increasing the toner amount” are desired, but simply controlling the T / D ratio. It is difficult for both of these to work as conflicting effects and to be compatible.

4). Next, the behavior of the developer in the developing device 4 of the present embodiment will be described.

  FIG. 3 schematically shows the behavior of the developer from the position of the regulating electrode S2 on the developing sleeve 41 to the position of the developing electrode S1 in the example shown in FIG. 2, developed in the moving direction of the surface of the developing sleeve 41. .

  The developer (two-component developer) including the toner t and the carrier c is disposed at a position facing the developing sleeve 41 and the magnetic pole S2 as the developing sleeve 41 rotates in the direction of arrow R3 in the drawing. It is conveyed to the gap with the regulating blade 44. The developer on the developing sleeve 41 is subjected to layer thickness regulation in the gap between the developing sleeve 41 and the regulating blade 44 as the developing sleeve 41 further rotates, and a desired amount of developer is applied to the developing sleeve 41. (P101).

  Thereafter, the developer on the developing sleeve 41 subjected to the regulation of the layer thickness is further moved between the conductive sleeve 7 and the developing sleeve 41 disposed at a position facing the magnetic pole N2 as the developing sleeve 41 further rotates. It is conveyed (P102). At this time, in this embodiment, the magnetic spike M rising from the surface of the developing sleeve 41 by the magnetic force of the magnetic pole N <b> 2 comes into contact with the conductive member 7.

  Here, in this embodiment, at least during image formation, the toner t in the magnetic brush M on the developing sleeve 41 in contact with the conductive member 7 is moved from the conductive member 7 side to the developing sleeve 41 side. A potential difference is formed between the developing sleeve 41 and the conductive member 7. That is, the toner biasing bias is applied from the toner biasing power source 9 to the conductive member 7 so that such a potential difference is formed between the developing power source 8 and the developing sleeve 41 to which the developing bias is applied. The Therefore, an electric field E1 is formed between the conductive member 7 and the developing sleeve 41 so that the toner t moves from the conductive member 7 side to the developing sleeve 41 side. As a result, under the action of the electric field E1, the toner t in the vicinity of the conductive member 7 moves to the vicinity of the developing sleeve 41 (P103).

  Thereafter, the magnetic brush M on the developing sleeve 41 is conveyed from between the conductive member 7 and the developing sleeve 41 by further rotating the developing sleeve 41 (P104). Then, as the position approaches the position of the magnetic pole S1, the upper and lower sides in the figure are reversed (half-rotated) by one pole between the magnetic pole N2 and the magnetic pole S1 (P105). This is because the magnetic spike M falls along the magnetic field line of the magnetic pole N2 as the developing sleeve 41 moves, and then rises again along the magnetic field line of the magnetic pole S1. At this time, the magnetic head M has a side closer to the magnetic pole as a base end portion, and the tip end portion on the opposite side falls or rises, so that the magnetic sleeve M performs a rotational movement on the developing sleeve 41 in the same direction as the developing sleeve 41. In the figure, the top and bottom are reversed. Here, the reversal of the magnetic spike M does not mean that a series of the magnetic spike M is strictly rotated. When moving from the position of the previous magnetic pole to the position of the subsequent magnetic pole, the developer (carrier c) that was on the distal end side from the developing sleeve 41 at the position of the previous magnetic pole is moved to the proximal end side. It is only necessary that the rate of movement is relatively greater than the reverse.

  At this time, the toner t in the magnetic spike M moved to the vicinity of the developing sleeve 41 by the action of the electric field E1 between the conductive member 7 and the developing sleeve 41 is restrained on the carrier c or between the magnetic spikes M. Thus, the moved state is held substantially. Therefore, the toner t held in the vicinity of the developing sleeve 41 in the magnetic brush M is arranged in the vicinity of the photoconductor 1 this time by the magnetic brush M being inverted.

  Thereafter, the magnetic brush M on the developing sleeve 41 is disposed between the photosensitive member 1 and the developing sleeve 41 in the developing region D by further rotating the developing sleeve 41. At this time, in this embodiment, the magnetic spike M rising from the surface of the developing sleeve 41 by the magnetic force of the magnetic pole S 1 comes into contact with the photosensitive member 1. In the developing region D, the toner t in the magnetic spike M on the developing sleeve 41 is exposed from the developing sleeve 41 side under the action of an electric field (developing electric field) E2 between the photosensitive member 1 and the developing sleeve 41. It moves to the body 1 side (P106).

  As described above, in this embodiment, in the development region D, the toner amount near the developing sleeve 41 is decreased and the toner amount near the photoconductor 1 is increased without changing the absolute toner amount. Therefore, developability can be improved. That is, as described above, a stronger electric field acts on the toner in the vicinity of the photoconductor 1 among the toners present in the magnetic ears. It is considered that a larger amount of toner can be moved onto the photoreceptor 1 by increasing the amount of toner at a site where such a strong electric field acts. Further, when the toner amount in the vicinity of the developing sleeve 41 is decreased, the electric resistance of the developer is drastically decreased because the normal toner has a resistance higher than that of the magnetic carrier by 5 digits or more. When the electric resistance of the developer decreases, the electric potential in the vicinity of the photosensitive member 1 approaches the electric potential of the developing sleeve 41, so that the electric field strength is further increased. As a result, it is considered that more toner can be moved onto the photoreceptor 1. By synergistically acting, it is considered that more toner can be moved onto the photoreceptor 1 in the development region D without changing the absolute toner amount.

  Here, in order to move the toner from the conductive member 7 side to the developing sleeve 41 side, the conductive member 7 has the same polarity as the toner charging polarity at the time of development (negative polarity in this embodiment) and an absolute value. Applies a toner bias having a potential larger than the potential of the DC component of the developing bias. That is, the potential of the DC component of the toner biasing bias is set higher than the potential of the DC component of the developing bias toward the charging polarity of the toner during development. In the present embodiment, the toner bias is a DC voltage, but may be an oscillating voltage in which a direct current component and an alternating current component are superimposed (see the second embodiment).

  In the present embodiment, the developing power supply 8 and the toner energizing power supply 9 cause the toner on the developer carrying body formed by the toner energizing electrode to flow from the conductive member side to the developer carrying body side. A potential difference forming means for forming a potential difference to be moved is configured.

Further, according to the study by the present inventors, the maximum electric field strength E formed between the conductive member 7 and the developing sleeve 41 in order to move the toner from the conductive member 7 side to the developing sleeve 41 side is it is preferably ^{1.0 × 10 4 ~2.0 × 10 4} V / cm. Here, the maximum electric field strength E is the electric field when the electric field for moving the toner formed between the conductive member 7 and the developing sleeve 41 from the conductive member 7 side to the developing sleeve 41 side is maximum. It is strength. Further, the closest distance between the conductive member 7 and the developing sleeve 41 is L. In this embodiment, the toner charging polarity during development is negative, the toner biasing bias is a DC voltage having the same polarity as the toner charging polarity during development, and the development bias is the toner charging during development. This is an oscillating voltage obtained by superimposing a DC voltage and an AC voltage having the same polarity as the polarity. Therefore, in the present embodiment, the maximum electric field strength E is obtained by the following equation.
Electric field strength E =
((Peak value of positive polarity of developing bias) − (DC voltage applied to conductive member)) / distance L

When the electric field intensity E is larger than 2.0 × 10 ⁴ V / cm, the toner tends to be excessively unevenly distributed in the vicinity of the developing sleeve 41 and directly adheres to the toner that is not restrained by the carrier in the magnetic ear or the developing sleeve 41. Toner increases. Since such toner has no adhesion or is very weak, it tends to cause fog (a phenomenon in which unintended toner adheres to a non-image portion on the photoreceptor) in the development region D. On the other hand, when the electric field strength E is smaller than 1.0 × 10 ⁴ , the toner cannot fly from the carrier, and it is difficult for the toner to be unevenly distributed in the vicinity of the developing sleeve 41, so that the developability is difficult to improve.

5. Examples etc. <Developing Devices of Examples and Comparative Examples>
Using the developing device 4A of Example 1 (FIG. 4) according to the present invention and the developing devices 4B and 4C of Comparative Example 1 (FIG. 5) and Comparative Example 2 (FIG. 6) as comparative examples, Carrier adhesion was evaluated. 4, 5, and 6 are schematic cross-sectional views illustrating the configurations of the developing devices 4 </ b> A, 4 </ b> B, and 4 </ b> C of Example 1, Comparative Example 1, and Comparative Example 2, respectively. In each of the developing devices 4A, 4B, and 4C, the same or corresponding elements as those of the developing device 4 described with reference to FIG.

  The developing device 4A of Example 1 (FIG. 4) differs from the developing device 4 shown in FIG. 2 in the specific magnetic pole arrangement of the magnet roller 42, but the other configuration is substantially the same as the developing device 4 shown in FIG. The same. In the developing device 4A of Example 1, the magnet roller 42 has magnetic poles S1, N2, S2, N3, S3, S4, and N1 as a plurality of magnetic poles in the circumferential direction. These magnetic poles S 1, N 2, S 2, N 3, S 3, S 4, N 1 are arranged in this order in the direction opposite to the rotation direction of the developing sleeve 41. In the developing device 4A according to the first exemplary embodiment, the developer sleeve 41 is opposed to the toner biasing pole N2 that is one pole upstream from the developing pole S1 in the rotation direction of the developing sleeve 41, and is in contact with the magnetic spike on the developing sleeve 41. A conductive member 7 is disposed. The conductive member 7 is fixed to the support 71. In addition, a regulating blade 44 is disposed at a position facing the regulating pole N3 that is three poles upstream of the developing pole S1 in the rotation direction of the developing sleeve 41 and in contact with the magnetic spike on the developing sleeve 41.

  The developing device 4B of Comparative Example 1 (FIG. 5) has substantially the same configuration as the developing device 4A of Example 1 except that the arrangement of the conductive members 7 is changed as follows. . In the developing device 4B of the comparative example 1, in the rotation direction of the developing sleeve 41, the conductive surface is located at a position facing the toner biasing pole S2 that is two poles upstream from the developing pole S1 and in contact with the magnetic spike on the developing sleeve 41. A member 7 is arranged. The conductive member 7 is fixed to the support 71.

  The developing device 4C of Comparative Example 2 (FIG. 6) has substantially the same configuration as the developing device 4A of Example 1 except that the conductive member 7 and the support 41 are not provided.

<Evaluation experiment>
First, the developing devices 4A, 4B, and 4C of Example 1, Comparative Example 1, and Comparative Example 2 described above are mounted on the image forming apparatus 100 of the present embodiment, and developability when a solid image is formed. An evaluation experiment on carrier adhesion was performed.

The two-component developer was used and the particle size r _c is 35μm carriers, those having a particle size r _t is formed by mixing the toner of negatively chargeable 5.5 [mu] m (negative charging property), the. The toner density (T / D ratio) x of the developer was 8%. The carrier true density ρ _c is 3.5 g / cm ³ , the carrier volume resistivity is 1 × 10 ¹⁰ Ω · cm, the toner true density ρ _t is 1.0 g / cm ³ , and the toner volume resistivity is 1 ×. 10 ¹⁸ Ω · cm. The method for measuring each parameter will be described later.

  When the solid image was formed, the photosensitive member 1 was rotated at 1000 mm / s in the direction of the arrow R1 in the figure. The solid image is subjected to charging and exposure steps, and the surface potential (dark potential) of the unexposed portion of the photoreceptor 1 in the development region D is −480 V, and the surface potential (bright potential) of the exposed portion of the photoreceptor 1 is −. It was 130V.

  On the other hand, the developing sleeve 41 was rotated at 1500 mm / s in the arrow R3 direction in the figure. Further, during the formation of the solid image, a developing bias in which a rectangular wave having a frequency of 6 kHz and a peak-to-peak voltage of 1.5 kV was applied to the developing sleeve 41 from the developing power source 8 on a DC voltage of −330V. Further, during the formation of the solid image, a DC voltage of −530 V was applied to the conductive member 7 from the toner energizing power source 9.

  Table 2 shows evaluation results of developability and carrier adhesion of the developing devices 4A and 4B of Example 1 and Comparative Example 1 with respect to the developing device 4C of Comparative Example 2.

  In the developing device 4A of Example 1, the developability was improved as compared with the developing device 4C of Comparative Example 2. On the other hand, in the developing device 4B of Comparative Example 1, the developability was lower than that of the developing device 4C of Comparative Example 2. Further, it was confirmed that both the developing device 4A of Example 1 and the developing device 4B of Comparative Example 1 were at an acceptable level for carrier adhesion. Each evaluation method will be described later.

  The reason why the above results are obtained can be considered as follows. FIG. 7 schematically illustrates the behavior of the developer from the position of the regulating electrode N3 on the developing sleeve 41 to the position of the developing electrode S1 in the developing device 4A of Example 1 in the moving direction of the surface of the developing sleeve 41. Shown in Similarly, FIG. 8 shows the behavior of the developer in the developing device 4B of Comparative Example 1, and FIG. 9 shows the behavior of the developer in the developing device 4C of Comparative Example 2.

  As shown in FIG. 7, in the developing device 4 </ b> A according to the first embodiment, the magnetic spike M on the developing sleeve 41 faces the toner biasing pole N <b> 2 that is one pole upstream from the developing pole S <b> 1 in the rotation direction of the developing sleeve 41. The conductive member 7 is disposed at a position in contact with the conductive member 7. Further, at least during image formation, the toner energizing power source 9 is arranged so that the toner t in the magnetic brush M on the developing sleeve 41 in contact with the conductive member 7 is moved from the conductive member 7 side to the developing sleeve 41 side. A toner biasing bias is applied to the conductive member 7. Thereafter, as the magnetic spike M on the developing sleeve 41 approaches the position of the developing pole S1, the top and bottom in the figure are reversed. Therefore, in the development region D, the toner amount near the developing sleeve 41 decreases and the toner amount near the photoconductor 1 increases. As a result, the electrical resistance of the developer in the vicinity of the developing sleeve 41 decreases and the amount of toner in the vicinity of the photoreceptor 1 increases as compared with the case of the developing device 4C (FIG. 9) of Comparative Example 2. It is thought that developability is improved. On the other hand, since the electric resistance of the carrier is high, carrier adhesion due to charge injection can be suppressed.

  As shown in FIG. 8, in the developing device 4 </ b> B of Comparative Example 1, the magnetic brush M on the developing sleeve 41 faces the toner biasing pole N <b> 2 that is two poles upstream from the developing pole S <b> 1 in the rotation direction of the developing sleeve 41. The conductive member 7 is disposed at a position in contact with the conductive member 7. Further, at least during image formation, the toner energizing power source 9 is arranged so that the toner t in the magnetic brush M on the developing sleeve 41 in contact with the conductive member 7 is moved from the conductive member 7 side to the developing sleeve 41 side. A toner biasing bias is applied to the conductive member 7. However, in Comparative Example 1, the magnetic spike M on the developing sleeve 41 is turned upside down in the figure by one pole between the magnetic pole S2 and the magnetic pole N2 as it approaches the position of the magnetic pole S1. In the figure, the upper and lower sides are reversed by one pole between N2 and the magnetic pole S1, so that one rotation is made in total. Therefore, in the development area D, the toner amount near the developing sleeve 41 increases and the toner amount near the photoreceptor 1 decreases. As a result, the electrical resistance of the developer in the vicinity of the developing sleeve 41 increases and the amount of toner in the vicinity of the photoconductor 1 decreases compared to the case of the developing device 4C (FIG. 9) of Comparative Example 2. It is considered that developability is lowered.

  As shown in FIG. 9, in the developing device 4 </ b> C of Comparative Example 2, the conductive member 7 is not provided, and the toner bias for moving the toner in the magnetic spike M on the developing sleeve 41 is not applied. . Therefore, the toner t in the magnetic brush M conveyed to the development area D is not significantly unevenly distributed on the photoconductor 1 side.

  Next, the same evaluation experiment as described above was performed using the developing device 4A of Example 1 and using carriers having different volume resistivity. The results are shown in Table 3.

It has been found that carriers having a volume resistivity of 10 ⁷ Ω · cm or less are difficult to suppress carrier adhesion due to charge injection, and it is difficult to achieve both improvement in developability. That is, it is preferable to use a high-resistance carrier having a volume resistivity of 10 ⁸ Ω · cm or more, typically 10 ⁸ to 10 ¹⁰ Ω · cm.

  As described above, according to the first embodiment, at the position of the toner biasing pole that is an odd number upstream from the developing pole, the conductive material is transferred so that the toner moves from the conductive member 7 side to the developing sleeve 41 side during image formation. A voltage is applied to the sex member 7. As a result, the toner t in the magnetic brush M at the position of the toner biasing pole is unevenly distributed in the vicinity of the developing sleeve 41. Thereafter, the magnetic brush M is conveyed while rotating between the position of the toner biasing electrode and the position of the developing pole, so that the toner in the magnetic brush M at the developing pole is unevenly distributed in the vicinity of the photoreceptor 1. As a result, in the development region D, it is possible to decrease the toner amount near the developing sleeve 41 and increase the toner amount near the photoreceptor 1 without changing the absolute toner amount. Therefore, developability can be improved. Therefore, carrier adhesion can be suppressed over a long period of time using a high-resistance carrier, and developability can be improved.

  Here, at least in the rotation direction of the developing sleeve, the magnetic poles from the developing pole to the toner energizing pole upstream are alternately made different from each other. In addition, if the toner biasing electrode is disposed upstream of the developing pole from the developing pole in the rotation direction of the developing sleeve, the magnetic brush repeatedly reverses as the developing sleeve rotates, and as a result, the photosensitive member is positioned at the position of the developing pole. The amount of toner in the vicinity of can be increased. However, in order to convey the magnetic spike to the position of the developing pole while keeping the toner moved in the vicinity of the developing sleeve at the position of the toner biasing pole as much as possible, the odd number is preferably 5 or less, More preferably, it is 3 or 1, and most preferably 1.

  In the present embodiment, the conductive member has a shape having substantially the same curvature as that of the outer peripheral surface of the developing sleeve, but the present invention is not limited to this. Typically, as in the present embodiment, at least a surface of the conductive member that faces the outer peripheral surface of the developing sleeve is curved in a direction along the outer peripheral surface of the developing sleeve. Further, for example, a columnar or cylindrical conductive member 7 as shown in FIG. 10 may be used. In this case, the conductive member 7 usually has an outer diameter smaller than the outer diameter of the developing sleeve 41, and its axial direction is typically substantially parallel along the rotational axis direction of the developing sleeve 41. Placed in. Moreover, the cylindrical electroconductive member 7 by which the magnet roller 72 as a magnetic field generation means is fixedly arrange | positioned inside as shown in FIG. 11 may be sufficient. In this case, among the magnetic poles of the magnet roller 72 inside the conductive member 7, the magnetic pole facing the magnet roller 42 inside the developing sleeve 41 is different from the opposing magnetic pole inside the developing sleeve 42. Further, as shown in FIG. 12, the developer regulating member (regulating blade) 44 may have a function of the conductive member 7.

  In the present embodiment, the conductive member 7 is disposed at a position where the magnetic spike M on the developing sleeve 41 contacts, but the present invention is not limited to this. As long as an electric field for moving the toner t in the magnetic ear M to the developing sleeve 41 side can be applied, the conductive member 7 may not be in contact with the magnetic ear M and is close to the magnetic ear M. It may be arranged.

<Measurement method for each parameter>
・ Volume resistivity of carrier:
FIG. 13 is a schematic diagram showing an outline of an apparatus used for measuring the volume resistivity of a carrier. A developing sleeve 203 of a developing device 202 containing a carrier c alone (that is, not including toner) is opposed to an aluminum cylinder (hereinafter referred to as “Al drum”) 201. At this time, the closest distance between the Al drum 201 and the developing sleeve 203 is kept equal to that of the actual machine (that is, the above-described embodiment and comparative example). Then, while rotating the Al drum 201 and the developing sleeve 203 at a predetermined peripheral speed, an AC voltage (Sin wave) is generated between the Al drum 201 and the developing sleeve 203 by a high voltage power source (HVA4321 manufactured by NF). Is applied. At this time, the impedance can be measured by sweeping the frequency of the Sin wave and measuring the response current to the applied voltage with the ammeter 205. The peak voltage of the Sin wave was set so that the electric field strength at the closest position between the Al drum 201 and the developing sleeve 202 was 2 × 10 ⁴ V / cm. Here, the measurement was performed by a dielectric measurement system 501 (126096W manufactured by Solartron).

  The analysis method is as follows. Fitting with an equivalent circuit (RC parallel circuit) using analysis software (Solartron ZView) from Cole-Cole plots plotting each measured value (real part and imaginary part) when the frequency is swept from 1 Hz to 10 kHz . Thereby, the resistance component and the capacitance component of the magnetic carrier can be obtained. From the resistance value and the carrier volume (contact area × nearest distance), the volume resistivity (Ω · cm) of the carrier was determined.

-Volume resistivity of toner: Diameter φ = 6 mm, measurement electrode plate area = 0.283 cm ² , pressure = 1500 g weight, pressure = 96.1 kPa, powder layer thickness at measurement = 0.5 to 1.0 mm As a result, the following measurements were performed. That is, a current value was measured with a minute current meter (YHP4140pA · METER / DC · VOLTAGE · SOUCE) with a DC voltage of 400V, and a volume resistance value was calculated from the measured current value.

-Developer toner density (T / D ratio) x:
About 0.3 g of the developer carried on the developing sleeve was taken, mixed with a mixed solution of water and a surfactant (for example, palm flea detergent), the melted toner and the carrier were separated, and each weight was measured. .

・ True density ρ:
It measured and calculated | required with the dry-type automatic densimeter (Shimadzu Corporation Accupic 1330).

The true density ρt of the toner used in the examples and comparative examples was 1.0 g / cm ³ , and the true density ρ _{c of the} carrier was 3.5 g / cm ³ .

Carrier particle size r _c :
The volume average particle size was measured and determined by a particle size distribution measuring device (Microtrack MT3300EX manufactured by Nikkiso Co., Ltd.).

The particle size r _c of the carrier used in the above Examples and Comparative Example was 35 [mu] m.

Toner particle size r _t :
The weight average particle size was measured and determined by a precision particle size distribution measuring device (Multisizer 3 manufactured by Beckman Coulter, Inc.).

The toner particle size r _t used in the above Examples and Comparative Example was 5.5 [mu] m.

<Evaluation method>
・ Developability evaluation:
A potential difference ΔV before and after development was measured with a surface potential meter (MODEL 344 manufactured by TREK). In this embodiment and the comparative example, this surface potential meter can measure the surface potential of the photosensitive member 1 and the toner image on the photosensitive member 1 on the downstream side of the developing region D in the moving direction of the surface of the photosensitive member 1. It was attached to the lower part of the device 4. Then, when ΔV measured by the developing device 4C of Comparative Example 2 was increased, “◯: acceptable level” was evaluated, and when equal or decreased, “X: unacceptable level” was evaluated.

・ Carrier adhesion evaluation:
When the solid image was continuously output, the toner image on the photoconductor was taped (that is, the tape was peeled off after sticking an adhesive tape on the photoconductor on which the solid image was formed). Thereafter, an area (taping area) of about 1 cm ^{2 on the} surface where the toner image of the tape is adhered was observed with a microscope (an optical microscope manufactured by Olympus Corporation), and the number of adhered carriers was counted. And carrier adhesion evaluation was performed according to the following evaluation criteria.
2 / cm ² or less: ○ (acceptable level)
2 / over 1 cm ² : × (Unacceptable level)

[Second Embodiment]
Next, a second embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Therefore, the same reference numerals are given to elements that are the same as or equivalent to those of the first embodiment, and detailed description thereof is omitted.

  In the present embodiment, the developing device has two developing sleeves as a plurality of developer carrying members, and transfers the developer from the upstream developing sleeve to the downstream developing sleeve in the movement direction of the surface of the photosensitive member. It is configured.

  The upstream developing sleeve moves the toner in the magnetic brush on the downstream developing sleeve so that the toner in the magnetic brush in the downstream developing region is unevenly distributed on the photoreceptor side. Have Thereby, in particular, the developability by the downstream developing sleeve is improved. Since the downstream developing sleeve finally develops the electrostatic latent image on the photoreceptor, it is important to improve the developability by the downstream developing sleeve in order to improve the image. .

  The configuration and operation of the developing device of this embodiment will be described in more detail in Example 2 below.

<Developing devices of Examples and Comparative Examples>
The developing device 4D of Example 2 (FIG. 14) according to the present invention and the developing devices 4E and 4F of Comparative Example 3 (FIG. 15), Comparative Example 4 (FIG. 16) and Comparative Example 5 (FIG. 17) as comparative examples. 4G was used to evaluate developability and carrier adhesion. 14, FIG. 15, FIG. 16, and FIG. 17 are schematic cross-sectional views showing the configurations of the developing devices 4D, 4E, 4F, and 4G of Example 2, Comparative Example 3, Comparative Example 4, and Comparative Example 5, respectively. .

  The developing device 4D of Embodiment 2 (FIG. 14) includes an upstream developing sleeve (second developer bearing member) 41a and a downstream developing sleeve (first developer bearing member) 41b as a plurality of developer bearing members. 2 developing sleeves. The upstream developing sleeve 41 a is rotatably arranged so that a part thereof is exposed from the opening 46 a of the developing container 46 on the upstream side in the moving direction of the surface of the photoreceptor 1. Further, the downstream side developing sleeve 41 b is rotatably arranged so that a part thereof is exposed from the opening 46 a of the developing container 46 on the downstream side in the moving direction of the surface of the photoreceptor 1. The longitudinal direction (rotation axis direction) of each of the upstream development sleeve 41a and the downstream development sleeve 41b is substantially parallel to the longitudinal direction (rotation axis direction) of the photoreceptor 1. In the present embodiment, the moving direction of the surface of the photoreceptor 1 is substantially the same as the developer conveyance direction by the upstream and downstream developing sleeves 41 a and 41 b at the portion facing the photoreceptor 1.

  The upstream developing sleeve 41a and the downstream developing sleeve 41b are rotationally driven in the directions indicated by arrows R3a and R3b in the same direction. In the upstream development area Da and the downstream development area Db, the movement direction of the surfaces of the upstream development sleeve 41a and the downstream development sleeve 41b is the same as the movement direction of the surface of the photoreceptor 3. In addition, the moving direction of the surfaces of the upstream developing sleeve 41a and the downstream developing sleeve 41b is opposite in the facing portion between the upstream developing sleeve 41a and the downstream developing sleeve 41b.

  An upstream developing power source (transmitting device) 9 as a developing bias applying means (second power source) 9 is connected to the upstream developing sleeve 41a. The upstream developing power source 9 includes a waveform signal oscillator 91 and a high voltage power source 92 that amplifies the signal generated by the waveform signal oscillator 91 and applies a bias to the upstream developing sleeve 41a. As will be described later, the upstream developing power source 9 also functions as a toner bias applying unit. The downstream developing sleeve 41b is connected to a downstream developing power source 8 as a developing bias applying means (first power source). The downstream developing power supply 8 includes a waveform signal oscillator 81 and a high voltage power supply 82 that amplifies the signal generated by the waveform signal oscillator 81 and applies a bias to the developing sleeve 41.

  An upstream magnet roller (second magnetic field generating means) 42a is fixedly arranged inside the upstream developing sleeve 41a, and a downstream magnet roller (first magnetic field generating means) is arranged inside the downstream developing sleeve 41b. 42b is fixedly arranged. The upstream magnet roller 42a has magnetic poles S11, N12, S12, N13, S13, S14, and N11 as a plurality of magnetic poles in the circumferential direction. These magnetic poles S11, N12, S12, N13, S13, S14, and N11 are arranged in this order in the direction opposite to the rotation direction of the upstream developing sleeve 41a. The downstream magnet roller 42b has magnetic poles S21, N22, N23, S22, and N21 as a plurality of magnetic poles in the circumferential direction. These magnetic poles S21, N22, N23, S22, N21 are arranged in this order in the direction opposite to the rotation direction of the downstream developing sleeve 41b.

  The upstream developing sleeve 41a and the upstream magnet roller 42a constitute an upstream developing member 43a, and the downstream developing sleeve 41b and the downstream magnet roller 42b constitute a downstream developing member 43b.

  In the developing device 4D of Example 2, the upstream developing sleeve 41b is disposed at the following position. That is, in the rotational direction of the downstream developing sleeve 41b, it faces the toner biasing pole N22 that is one pole upstream from the magnetic pole (downstream developing pole) S21 corresponding to the downstream developing area (first developing area) Db, and This is the position in contact with the magnetic spike on the downstream developing sleeve 41b.

  The position of the magnetic pole N14 on the upstream side developing sleeve 41a is opposed to the position of the magnetic pole N22 on the downstream side developing sleeve 41b. The magnetic pole S14 on the upstream developing sleeve 41a is different from the magnetic pole N22 on the downstream developing sleeve 41b. The developer is transferred between the position of the magnetic pole (transfer pole) S14 on the second developer carrier and the position of the magnetic pole (toner biasing pole) N22 on the first developer carrier. .

  In the developing device 4D according to the second embodiment, the regulation blade 44 is disposed at the following position. That is, it is a position that faces the regulation pole N13 that is upstream of the three poles of the upstream development pole S11 in the rotation direction of the upstream development sleeve 41a, and that the magnetic brush on the upstream development sleeve 41a contacts.

  At least during image formation, the following potential difference is formed between the downstream developing sleeve 41b and the upstream developing sleeve 41a. That is, the potential difference is such that the toner t in the magnetic brush M on the downstream developing sleeve 41b in contact with the upstream developing sleeve 41a is moved from the upstream developing sleeve 41a side to the downstream developing sleeve 41b side. More specifically, a toner biasing bias is applied from the upstream developing power source 9 that also functions as a toner biasing unit to the upstream developing sleeve 41a7 so that such a potential difference is formed with the downstream developing sleeve 41b. A developing bias that also functions as a is applied. At this time, a developing bias is applied from the downstream developing power source 8 to the downstream developing sleeve 41b.

  In this embodiment (example and comparative example), the outer diameter of the photoreceptor 1 and the outer diameters of the upstream and downstream developing sleeves 41a and 41b are the same as those in the first embodiment. In this embodiment (Example and Comparative Example), the closest distance between the upstream and downstream developing sleeves 41a and 41b and the photosensitive member 1 is 300 μm. In this embodiment (Example and Comparative Example), the closest distance between the upstream developing sleeve 41a and the downstream developing sleeve 41b is 600 μm.

  Further, in the present embodiment, the downstream developing power supply 8, the upstream developing power supply 9, and the like cause the toner on the first developer carrying member to rise from the second developer carrying member side to the first developer carrier. Potential difference forming means for forming a potential difference to be moved to the developer carrying member side is configured.

  The developing device 4E of Comparative Example 3 (FIG. 15) has substantially the same configuration as the developing device 4D of Example 2 except that the arrangement of the magnetic poles of the downstream magnet roller 42b is changed as follows. Has been. In the developing device 4E of Comparative Example 3, the downstream magnet roller 42b has magnetic poles N21, S22, N22, N23, S23, N24, and S21 as a plurality of magnetic poles in the circumferential direction. These magnetic poles N21, S22, N22, N23, S23, N24, and S21 are arranged in this order in the direction opposite to the rotation direction of the downstream developing sleeve 41b.

  In the developing device 4E of Comparative Example 3, the upstream developing sleeve 41a is disposed at the following position. That is, it is a position that faces the toner biasing pole N22 that is two poles upstream from the downstream developing pole N21 in the rotation direction of the downstream developing sleeve 41b, and that is in contact with the magnetic brush on the downstream developing sleeve 41b. The position of the magnetic pole N14 on the upstream side developing sleeve 41a is opposed to the position of the magnetic pole N22 on the downstream side developing sleeve 41b. The developer is delivered between the position of the delivery pole S14 on the upstream side development sleeve 41a and the position of the toner biasing pole N22 on the downstream side development sleeve 41b. In Comparative Example 3, the arrangement of the magnetic poles of the upstream magnet roller 42a is the same as in Example 2. In Comparative Example 3, as in Example 2, a potential difference is formed between the downstream developing sleeve 41b and the upstream developing sleeve 41a at least during image formation.

  The developing device 4F of Comparative Example 4 (FIG. 16) and the developing device 4G of Comparative Example 5 (FIG. 17) are implemented except that no potential difference is formed between the developing sleeves 41b and 41a on the downstream side and the upstream side. The developing device 4D of Example 2 and the developing device 4E of Comparative Example 3 have substantially the same configuration. That is, in the developing device 4F of the comparative example 4 and the developing device 4G of the comparative example 5, a common developing bias is applied from the common developing power supply 8 to the upstream and downstream developing sleeves 41a and 41b.

<Evaluation experiment>
The developing bias used in each of the developing devices 4D, 4E, 4F, and 4G of Example 1 and Comparative Examples 3 to 5 is as follows.

Example 2 (developing device 4D):
Upstream development power supply 9
DC: -380V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)
Downstream development power supply 8
DC: -230V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)
Note that the waveform of the output of the upstream developing power supply 9 and the waveform of the output of the downstream developing power supply 8 are synchronized, and the potential difference between the two outputs is kept substantially constant.

Comparative example 3 (developing device 4E):
Upstream development power supply 9
DC: -380V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)
Downstream development power supply 8
DC: -230V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)
Note that the waveform of the output of the upstream developing power supply 9 and the waveform of the output of the downstream developing power supply 8 are synchronized, and the potential difference between the two outputs is kept substantially constant.

Comparative example 4 (developing device 4F):
Development power supply 8
DC: -330V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)

Comparative example 5 (developing device 4G):
Development power supply 8
DC: -330V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)

  Other conditions are the same as in the evaluation experiment described in the first embodiment.

  Table 4 shows the evaluation results of the developability and carrier adhesion of the developing device 4D of Example 2 to the developing device 4F of Comparative Example 4 and the developing device 4E of Comparative Example 3 to the developing device 4G of Comparative Example 5.

  In the developing device 4D of Example 2, the developability was improved as compared with the developing device 4F of Comparative Example 4. On the other hand, in the developing device 4E of Comparative Example 3, the developability was lower than that of the developing device 4G of Comparative Example 5. Further, it was confirmed that both the developing device 4D of Example 2 and the developing device 4E of Comparative Example 3 were at acceptable levels for carrier adhesion.

  The reason why the above results are obtained can be considered as follows. FIG. 18 schematically illustrates the behavior of the developer in the developing device 4D according to the second embodiment, developed in the moving direction of the surfaces of the upstream and downstream developing sleeves 41a and 41b. Similarly, FIG. 19 shows the behavior of the developer in the developing device 4E of Comparative Example 3, FIG. 20 shows the behavior of the developer in the developing device 4F of Comparative Example 4, and FIG. 21 shows the development in the developing device 4G of Comparative Example 5. The behavior of the agent is shown.

  As shown in FIG. 18, in the developing device 4D of the second embodiment, the sheet is conveyed to the position of the magnetic pole (upstream developing pole) S11 corresponding to the upstream developing area (second developing area) Da on the upstream developing sleeve 41a. The magnetic head M thus brought into contact with the photoreceptor 1 in the upstream development area Da. In the upstream development area Da, the toner t in the magnetic brush M on the upstream development sleeve 41a is subjected to the action of the electric field E3 between the photosensitive member 1 and the upstream development sleeve 41a, and the upstream development sleeve. It moves from the 41a side to the photoreceptor 1 side. In the upstream developing area Da, the toner t in the magnetic spike M on the upstream developing sleeve 41a is consumed according to the electrostatic latent image on the photoreceptor 1. Thereafter, the magnetic spike M on the upstream developing sleeve 41a is inverted one by one between the magnetic pole S11 and the magnetic pole N11 on the upstream developing sleeve 41a, and further the magnetic pole on the upstream developing sleeve 41a. It reverses by one pole between N11 and magnetic pole S14, and makes one rotation in total. The magnetic spike M conveyed to the position of the magnetic pole S14 on the upstream developing sleeve 41a is under the electric field E1 between the upstream developing sleeve 41a and the downstream developing sleeve 41b, and the magnetic pole N22 on the downstream developing sleeve 41b. Passed to position. At this time, the toner t in the magnetic brush M on the downstream developing sleeve 41b that has contacted the upstream developing sleeve 41a is subjected to the action of the electric field E1, and from the upstream developing sleeve 41a side to the downstream developing sleeve 41b side. Moving. Thereafter, the magnetic spike M on the downstream developing sleeve 41b is inverted up and down in the figure by one pole between the magnetic pole N22 and the magnetic pole 21 on the downstream developing sleeve 41b. The magnetic spike M conveyed to the position of the downstream development pole S21 on the downstream development sleeve 41b contacts the photoreceptor 1 in the downstream development region Db. In the downstream development area Db, the toner t in the magnetic brush M on the downstream development sleeve 41b is subjected to the action of the electric field E2 between the photosensitive member 1 and the downstream development sleeve 41b, and the downstream development sleeve. It moves from the 41b side to the photoreceptor 1 side.

  Thus, in the developing device 4D of Example 2, the toner biasing pole N22 that is one pole upstream from the downstream developing pole S21 is opposed to the magnetic brush M on the downstream developing sleeve 41b. An upstream developing sleeve 41a is disposed. At least during image formation, a developing bias is applied from the upstream developing power source 9 to the upstream developing sleeve 41a so that the toner t is moved from the upstream developing sleeve 41a side to the downstream developing sleeve 41b side. Thereafter, as the magnetic spike M on the downstream developing sleeve 41b approaches the position of the downstream developing pole S21 on the downstream developing sleeve 41b, the top and bottom in the drawing are reversed. Therefore, in the downstream development area Db, the toner amount near the downstream development sleeve 41b decreases and the toner amount near the photoreceptor 1 increases. As a result, compared with the case of the developing device 4F (FIG. 20) of the comparative example 4, the electrical resistance of the developer in the vicinity of the downstream side developing sleeve 41b decreases and the toner amount in the vicinity of the photoreceptor 1 increases. Therefore, it is considered that developability is improved. On the other hand, since the electric resistance of the carrier is high, carrier adhesion due to charge injection can be suppressed.

  As shown in FIG. 19, in the developing device 4E of Comparative Example 3, the toner biasing pole N22 that is two poles upstream from the downstream developing pole N21 is opposed to and in contact with the magnetic spike M on the downstream developing sleeve 41b. The upstream developing sleeve 41a is disposed at the position. At least during image formation, a developing bias is applied from the upstream developing power source 9 to the upstream developing sleeve 41a so that the toner t is moved from the upstream developing sleeve 41a side to the downstream developing sleeve 41b side. However, after that, in Comparative Example 3, as the magnetic spike M approaches the position of the downstream development pole N21, the top and bottom in the figure are inverted by one pole between the magnetic pole N22 and the magnetic pole S22, and further, the magnetic pole S22 In the figure, the upper and lower sides are reversed by one pole between the magnetic pole N21 and a total of one rotation. Therefore, in the downstream development area Db, the toner amount near the downstream development sleeve 41b increases and the toner amount near the photoreceptor 1 decreases. As a result, as compared with the case of the developing device 4G (FIG. 21) of Comparative Example 5, the electrical resistance of the developer in the vicinity of the downstream side developing sleeve 41b increases and the amount of toner in the vicinity of the photoreceptor 1 decreases. Therefore, it is considered that developability is lowered.

  As shown in FIGS. 20 and 21, in each of the developing devices 4F and 4G of Comparative Examples 4 and 5, no potential difference is formed between the downstream developing sleeve 41b and the upstream developing sleeve 41a. Therefore, after the toner t is consumed from the magnetic brush M on the upstream developing sleeve 41a in the upstream developing area Da and until the downstream developing area Db, the toner t in the magnetic brush M is substantially at one end. It is not moved from the part side to the other end part side. Therefore, the toner t in the magnetic spike M conveyed to the downstream development area Db is significantly unevenly distributed on the photoreceptor 1 side as compared with the state where the toner t is consumed from the magnetic spike M in the upstream development area Da. It will never be done.

  Next, using the developing device 4D of Example 2, the same evaluation experiment as described above was performed using carriers having different volume resistivity. The results are shown in Table 5.

It has been found that carriers having a volume resistivity of 10 ⁷ Ω · cm or less are difficult to suppress carrier adhesion due to charge injection, and it is difficult to achieve both improvement in developability.

  As described above, according to the second embodiment, at the position of the magnetic pole upstream of the magnetic pole corresponding to the downstream development area Db, the toner is transferred from the upstream development sleeve 41a side to the downstream development sleeve 41b side during image formation. A voltage is applied to the upstream developing sleeve 41a so as to move to. That is, the toner is moved from the second developing sleeve side to the first developing sleeve side. Thereby, the toner in the magnetic brush M at the position of the toner biasing pole is unevenly distributed in the vicinity of the downstream developing sleeve 41b. Thereafter, the magnetic brush M is conveyed while rotating between the toner energizing pole and the downstream developing pole, whereby the toner in the magnetic brush M at the downstream developing pole is unevenly distributed in the vicinity of the photoreceptor 1. As a result, in the downstream development area Db, it is possible to reduce the toner amount in the vicinity of the downstream development sleeve 41b and increase the toner amount in the vicinity of the photoreceptor 1 without changing the absolute toner amount. become. Therefore, developability can be improved. Therefore, carrier adhesion can be suppressed over a long period of time using a high-resistance carrier, and developability can be improved.

  Here, a conductive member may be disposed on the upstream developing sleeve in the same manner as in the first embodiment.

[Third Embodiment]
Next, a third embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of the present embodiment are the same as those of the first and second embodiments. Therefore, the same or equivalent elements as those in the first and second embodiments are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the present embodiment, as in the second embodiment, in the configuration in which the developing device includes two developing sleeves as a plurality of developer carriers, the arrangement of the magnetic poles of the magnet roller incorporated in the upstream developing sleeve is changed. Thus, the preferred conditions for improving the developability were investigated in more detail.

  The configuration and operation of the developing device of this embodiment will be described in more detail in Example 3 below.

<Developing devices of Examples and Comparative Examples>
Using the developing device 4H of Example 3 (FIG. 22) according to the present invention and the developing device 4I of Comparative Example 6 (FIG. 23) as a comparative example, developability and carrier adhesion were evaluated. 22 and 23 are schematic cross-sectional views showing the configurations of the developing devices 4H and 4I of Example 3 and Comparative Example 6, respectively.

  The developing device 4H of Example 3 (FIG. 22) is the developing device of Example 2 described in the second embodiment, except that the arrangement of the magnetic poles of the upstream magnet roller 42a is changed as follows. The configuration is substantially the same as 4D (FIG. 14). In the developing device 4H of Example 3, the upstream magnet roller 42a includes magnetic poles N11, S12, N12, S13, and S11 as a plurality of magnetic poles in the circumferential direction. These magnetic poles N11, S12, N12, S13, and S11 are arranged in this order in the direction opposite to the rotation direction of the upstream developing sleeve 41a. In the third embodiment, the upstream development pole N11 is arranged one pole upstream from the delivery pole S11 that delivers the developer to the downstream development sleeve 41b in the rotation direction of the upstream development sleeve 41a. In the developing device 4D (FIG. 14) of the second embodiment, the upstream developing pole S11 is disposed two poles upstream from the delivery pole S14 in the rotation direction of the upstream developing sleeve 41a. In the third embodiment, as in the second embodiment, a potential difference is formed between the downstream developing sleeve 41b and the upstream developing sleeve 41a at least during image formation.

  The developing device 4I of Comparative Example 6 (FIG. 23) has substantially the same configuration as the developing device 4H of Example 3 except that no potential difference is formed between the downstream developing sleeve 41b and the upstream developing sleeve 41a. It is said that. That is, in the developing device 4I of Comparative Example 6, a common developing bias is applied from the common developing power supply 8 to the upstream and downstream developing sleeves 41a and 41b.

<Evaluation experiment>
The developing bias used in each of the developing devices 4H and 4I of Example 3 and Comparative Example 6 is as follows.

Example 3 (developing device 4H):
Upstream development power supply 9
DC: -380V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)
Downstream development power supply 8
DC: -230V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)
Note that the waveform of the output of the upstream developing power supply 9 and the waveform of the output of the downstream developing power supply 8 are synchronized, and the potential difference between the two outputs is kept substantially constant.

Comparative example 6 (developing device 4I)
Development power supply 8
DC: -330V
AC: Rectangular wave (frequency 6 kHz, peak-to-peak voltage 1.5 kV)

  Other conditions are the same as in the evaluation experiment described in the first embodiment.

  Table 6 shows the evaluation results of developability and carrier adhesion of the developing device 4H of Example 3 to the developing device 4I of Comparative Example 6.

  In the developing device 4H of Example 3, the developability was improved as compared with the developing device 4I of Comparative Example 6, but the effect was limited. In other words, the developing device 4D of Example 2 was more effective in improving developability than the developing device 4H of Example 3.

  The reason why the above results are obtained can be considered as follows. FIG. 24 shows the behavior of the developer in the developing device 4H according to the third embodiment, developed in the moving direction of the surfaces of the upstream and downstream developing sleeves 41a and 41b. Similarly, FIG. 25 shows the behavior of the developer in the developing device 4I of Comparative Example 6.

  As shown in FIG. 24, in the developing device 4H according to the third embodiment, the toner t in the magnetic brush M is positioned at the upstream developing pole N11 that is one pole upstream from the transfer pole S11 in the rotation direction of the upstream developing sleeve 41a. Is consumed. In the upstream development area Da, the toner t in the magnetic brush M on the upstream development sleeve 41a is moved from the upstream development sleeve 41a side to the photoreceptor 1 side. Thereafter, the magnetic head M is reversed by one pole between the upstream developing pole N11 and the transfer pole S11. Therefore, the magnetic spike M conveyed to the position of the delivery pole S11 has a relatively large amount of toner near the upstream developing sleeve 41a and a small amount of toner on the downstream developing sleeve 41b side. Accordingly, a sufficient amount of toner t is moved from the upstream developing sleeve 41a side to the downstream developing sleeve 41b side by the action of the electric field E1 between the position of the delivery pole S11 and the position of the toner biasing pole N22. May not be possible. For this reason, it is considered that the improvement effect of the developer may be limited as compared with the case of the developing device 4I of Comparative Example 6 (FIG. 25).

  On the other hand, in the developing device 4D (FIG. 18) of the second embodiment, after the toner t in the magnetic brush M is moved from the upstream developing sleeve 41a side to the photoreceptor 1 side in the upstream developing area Da, The magnetic spike M makes one rotation as a whole for the two poles between the upstream development pole S11 and the transfer pole S14. Therefore, the magnetic brush M conveyed to the position of the delivery pole S14 has a relatively small amount of toner near the upstream developing sleeve 41a and a large amount of toner on the downstream developing sleeve 41b side. Therefore, a sufficient amount of toner t is easily moved from the upstream developing sleeve 41a side to the downstream developing sleeve 41b side by the action of the electric field E1 between the position of the delivery pole S14 and the position of the toner biasing pole N22. It is considered a thing.

  As described above, it is preferable that the upstream development pole is located upstream of the even pole from the delivery pole that delivers the developer from the upstream development sleeve 41a to the downstream development sleeve 41b. Thereby, developability can be improved more effectively.

  Here, at least in the rotation direction of the upstream developing sleeve, the magnetic poles from the transfer pole to the upstream developing pole are alternately made different from each other. Further, the upstream developing pole may be disposed upstream of the even pole from the transfer pole in the rotation direction of the upstream developing sleeve. As a result, the magnetic brush repeats reversal with the rotation of the upstream developing sleeve, and as a result, the amount of toner near the downstream developing sleeve can be increased at the position of the transfer pole. However, in order to transfer the magnetic brush to the position of the transfer pole while keeping the toner moved in the vicinity of the upstream development sleeve at the position of the upstream development pole as much as possible, the even number is preferably 6 Hereinafter, it is more preferably 4 or 2, and most preferably 2.

[Fourth Embodiment]
Next, a fourth embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Therefore, the same reference numerals are given to elements that are the same as or equivalent to those of the first embodiment, and detailed description thereof is omitted.

  In the present embodiment, in the same developing device as in the first embodiment, the toner concentration (T / D ratio) x of the developer is changed, and the preferable conditions for improving the developability were investigated in more detail.

<Evaluation experiment>
Using a developing device having substantially the same configuration as the developing device 4A of Example 1 described in the first embodiment, a solid image was output by changing the toner density x of the developer to be used. As a result, it was found that the uniformity of the solid image is lowered when the toner density x of the developer is high.

  FIG. 26 schematically shows a normal solid image (FIG. 26A) and a defective image with reduced uniformity (FIG. 26B). While the normal image has high uniformity, the leading edge of the defective image is dark and the uniformity is low. Since the length y of the leading edge of the image coincides with the circumferential length of the developing sleeve 41, the following reason can be considered.

  FIG. 27 shows the behavior of the developer from the position of the regulating pole S2 on the developing sleeve 41 to the position of the developing pole S1 when the toner density x of the developer is high. When the toner concentration x of the developer is high, the toner t sufficiently covers the surface of the carrier c. Therefore, when the toner t is moved to the developing sleeve 41 side at the toner biasing pole N2, the toner t that cannot directly cover the carrier c and adheres directly to the developing sleeve 41 increases. The toner t has a weak adhesion and is more easily moved than the toner t covering the carrier c. For this reason, it is considered that the image leading edge of the developing sleeve 41 is darkened. That is, in order not to reduce the uniformity of the solid image, it is considered that there is a preferable upper limit on the coating amount of the toner t on the surface of the carrier c. Therefore, a coverage S (%) represented by the following formula 1 was introduced as the ratio of the toner t covering the carrier c.

In the above, the [rho _c true density of carrier ^{(g / cm 3), r} c is the carrier particle diameter ([mu] m), true density of [rho _t toner ^{(g / cm 3), r} t is the toner particle diameter ( μm) and x are the toner concentration (T / D ratio) (%) of the developer. However, for each unit, substitution is made so that the dimensions of the above formula 1 are matched.

Formula 1 is derived as follows. Of the surface area of the carrier (pi] r _c ^2), when the toner number of covering N and (pieces), the coverage and is located at a ratio of the sum (Nπr _t ^2/4) of the toner sectional area relative to the carrier surface area S = Nr _t ² / 4r _c ² × 100 (%). On the other hand, the T / D ratio x is the ratio of the toner mass to the developer mass, and x = (ρ _t V _t × 100) / (ρ _t V _t + ρ _c V _c ). Here, V _t is the total volume of the toner, the V _c is the total volume of carrier. Since the number of toners is N times that of the carrier, V _t / V _c = N (r _t / r _c ) ³ , so N = xρ _c r _c ³ / ρ _t r _t ³ (100 -X) is derived, and the coverage S is expressed by Equation 1. Therefore, the coverage S (%) calculated by the above equation 1 can represent the ratio of the carrier c in the magnetic ear M covered with the toner t.

^{_{ρ c = 3.5g / cm 3,}} r c = 35μm, using a two-component developer ^{_{ρ t = 1.0g / cm 3,}} r t = 5.5μm, the result of evaluating the uniformity of a solid image It shows in Table 7. A case where an image with good uniformity of a solid image as shown in FIG. 26A is formed is “◯ (good)”, and an image with reduced uniformity of a solid image as shown in FIG. The case where it was formed was designated as “× (defect)”.

^{_{ρ c = 3.5g / cm 3,}} r c = 23μm, using ^{_{ρ t = 1.0g / cm 3,}} rt = 5.5μm of the two-component developer, the table the results of evaluation of the uniformity of the solid image It is shown in FIG. The evaluation method is the same as in Table 7.

  From the results shown in Tables 7 and 8, when the coverage S (%) represented by the above formula 1 is larger than 50%, the toner t directly adhered on the developing sleeve 41 increases, and therefore the uniformity of the solid image It has been found that may decrease.

  As described above, in the configuration in which the toner is moved by the conductive member 7, it is preferable that the coverage S (%) represented by the above formula 1 is 50 (%) or less. As a result, it is possible to suppress a decrease in solid image uniformity due to toner being unevenly distributed in the vicinity of the developing sleeve 41 and directly adhering to the developing sleeve 41 at the position of the toner biasing electrode. The coverage S (%) is usually 5 (%) or more.

DESCRIPTION OF SYMBOLS 1 Photoconductor 4 Developing apparatus 7 Conductive member 8 Development power supply (downstream side development power supply, 1st power supply)
9 Toner energizing power source (upstream developing power source, second power source)
41 Development sleeve (developer carrier)
41a Upstream developing sleeve (second developer carrier)
41b Downstream developing sleeve (first developer carrier)
42 Magnet roller 42a Upstream magnet roller (second magnetic field generating means)
42b Downstream magnet roller (first magnetic field generating means)
44 Regulating blade D Development area Da Upstream development area (second development area)
Db downstream development area (first development area)

Claims (2)

An image carrier on which an electrostatic image is formed;
A first developer carrier that carries and conveys a developer that is rotatable and includes toner and a carrier, and that supplies toner to an electrostatic image on the surface of the image carrier in a first development region. Body,
A first magnetic field generating means fixedly disposed in a hollow portion of the first developer carrier and having a plurality of magnetic poles in the rotation direction of the first developer carrier;
Rotatable and carrying and conveying a developer having a toner and a carrier, the image carrier said image bearing member than said first developing region in the direction of movement in the second developing region in the upstream of the A second developer carrying member that supplies toner to the electrostatic image on the surface and develops it, and also transfers the developer after passing through the second development region to the first developer carrying member. The position of the plurality of magnetic poles of the first magnetic field generating means facing a predetermined magnetic pole upstream of the odd poles from the magnetic pole corresponding to the first development region in the rotation direction of the first developer carrier. A second developer carrier disposed in
A second magnetic field generating means fixedly disposed in the hollow portion of the second developer carrier and having a plurality of magnetic poles in the rotation direction of the second developer carrier;
During image formation, the between the first developer carrying member and said second developer carrying member, in the ears on the surface of the predetermined said formed by magnetic poles of the first developer carrying member a potential difference forming means for forming a potential difference to move the bets toner of the developer from said second developer carrying member side to the first side of the developer carrying member,
An image forming apparatus comprising: The potential difference forming means applies a DC voltage applied to the second developer carrying member to the second developer carrying member when a DC voltage applied to the second developer carrying member has the same polarity as the normal charging polarity of the toner. To each of the first developer carrier and the second developer carrier so that the absolute value of the direct current voltage is larger than the absolute value of the DC voltage applied to the first developer carrier. The image forming apparatus according to claim 1, wherein a DC voltage is applied .