JP2006047885A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of suppressing a defective image such as density difference due to toner sticking to a developer carrier in a non-image formation area upon pre-rotation, post-rotation or the like. <P>SOLUTION: The image forming apparatus 10 is provided with the image carrier 1, a charging means 2, an electrostatic image forming means 3 and a developing device 4 for developing an electrostatic image formed on the image carrier 1 by using a two-component developer having nonmagnetic toner and magnetic carrier. Therein, with regard to a potential difference (Vback) between a potential of the image carrier 1 charged by a charging means 2 and a direct current component of a development bias voltage applied to the developing device 4, a potential difference (Vback1) in the non-image formation area is set to be smaller than a potential difference (Vback2) in image formation area. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus, such as a copying machine or a printer, provided with a developing device that develops an electrostatic image formed on an image carrier with a developer, and in particular, a non-magnetic toner and a magnetic carrier. The present invention relates to an image forming apparatus using a two-component developing system using a two-component developer having

  2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, in particular, an image forming apparatus for forming a chromatic color image, a two-component developing system in which a nonmagnetic toner (toner) and a magnetic carrier (carrier) are mixed and used as a developer Is widely used. The two-component development method has advantages such as image quality stability and apparatus durability, as compared with other currently known development methods.

When an electrostatic image (latent image) formed on a photosensitive drum as an image carrier is developed into a toner image in an image forming apparatus using a two-component development method, the following is generally performed as follows. First, the photosensitive drum is uniformly charged to a white background portion potential V _D by charging means. A developing bias voltage is applied to the developing sleeve as the developer carrying member, and the developing sleeve is set to the same potential as the DC component Vdc of the developing bias voltage. At this time, the potential difference between the white background portion potential V _D and the DC component Vdc of the developing bias voltage is set to a desired fog removal potential difference Vback. Further, the image portion (development portion) on the photosensitive drum is exposed to the light portion potential V _L which is exposed and attenuated by the exposure means (electrostatic image forming means). The toner on the developing sleeve moves to the photosensitive drum due to the contrast potential difference Vcont, which is the difference between the DC component Vdc of the developing bias voltage. Thus, the electrostatic image formed on the photosensitive drum is developed as a toner image.

For example, when using negative toner charged to a negative polarity, the DC component of the development bias voltage prevents this negative toner from adhering to a white background portion where the toner should not adhere to the photosensitive drum and causing “fogging”. Vdc is set to a potential on the plus side of the white background portion potential V _D. Hereinafter, the toner adhering to the white background is referred to as “fogging toner”.

  When the fog removal potential difference Vback is small, the toner attracting force to the developing sleeve is weakened, and the fog toner easily adheres to the white background portion of the photosensitive drum. Conversely, when the fog removal potential difference Vback is large, the Coulomb force due to the fog removal potential difference applied to the magnetic carrier charged to the positive polarity by charging the toner to the negative polarity becomes larger than the magnetic carrying force with the developing sleeve, The carrier easily adheres to the white background portion of the photosensitive drum.

  Therefore, the fog removal potential difference Vback is set to an appropriate potential difference depending on the magnetic flux density of the developing pole of the developing sleeve and the characteristics of the toner and the carrier.

  However, it has been found that there are the following problems particularly in an image forming apparatus using the two-component development method.

  The relationship between the potential of the photosensitive drum and the potential of the developing sleeve at the time of the pre-rotation from the start of the image forming operation to the start of image formation in the image forming area or the post-rotation after the image formation in the image forming area is completed is The state of the fog removal potential difference Vback continues for a while. For this reason, the negative toner in the developer is pressed in a direction toward the developing sleeve having a more positive potential. When the two-component development method is used, toner accumulates on the surface of the developing sleeve instead of the rising edge (magnetic brush) of the developer formed on the developing sleeve due to the fog removal potential difference Vback in the non-image forming area. To go. That is, in the non-image forming region, the same state as that in which the developing operation with the toner is performed on the developing sleeve is obtained. As a result, the developing sleeve is in a state where there is much toner adhesion.

  When the toner adhering to the surface of the developing sleeve increases, for example, negative toner charged to a negative polarity is an insulator, so the apparent potential on the surface of the developing sleeve is a potential more negative than the DC component Vdc of the developing bias voltage. Become. Therefore, when an electrostatic image is formed on the photosensitive drum in the image forming area, the apparent development contrast potential difference Vcont becomes large. As a result, the image density becomes high.

  After that, when the toner adhering to the surface of the developing sleeve is consumed in the developing process in the image forming region, the potential of the surface of the developing sleeve becomes the same potential as the DC component Vdc of the developing bias voltage. As a result, the development contrast potential difference Vback becomes normal, and a desired image density comes out.

  Therefore, when halftone and solid images that consume a large amount of toner are developed, only the density of the image corresponding to the portion of the developing sleeve to which the toner has adhered as described above is increased, and image defects such as density steps are caused. May occur.

Note that Patent Document 1 describes the relationship between the charging voltage of the photosensitive drum and the developing bias voltage of the developing unit in an image forming section where the electrostatic latent image is developed and a non-image forming section where the electrostatic latent image is not developed. An image printing apparatus that reduces the potential difference is disclosed. However, Patent Document 1 merely discloses that toner having a polarity opposite to the normal charging polarity is prevented from adhering to the photosensitive drum in the non-image section. That is, Patent Document 1 uses the two-component developer and the above-mentioned problem peculiar to the two-component development method, that is, the ear of the developer formed on the developing sleeve due to the fog removal potential difference Vback in the non-image forming region. There is no description regarding the problem that toner accumulates on the surface of the developing sleeve instead of standing (magnetic brush).
Japanese Patent Laid-Open No. 7-209971

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of suppressing image defects such as density steps due to toner adhesion to a developer carrying member in a non-image forming region during pre-rotation and post-rotation. It is.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image carrier, charging means for charging the image carrier, electrostatic image forming means for forming an electrostatic image on the image carrier charged by the charging means, A developing device that develops the electrostatic image formed on the image carrier using a two-component developer including a non-magnetic toner and a magnetic carrier, and charged by the charging unit. In the image forming apparatus, the potential difference between the potential of the image carrier and the direct current component of the developing bias voltage applied to the developing device is smaller in the non-image forming area than in the image forming area.

  According to another aspect of the present invention, an image carrier, a charging unit that charges the image carrier, and an electrostatic image forming unit that forms an electrostatic image on the image carrier charged by the charging unit. An electrostatic image formed on the image carrier is provided with a developer carrier that carries a two-component developer comprising a non-magnetic toner and a magnetic carrier and conveys the developer to a position facing the image carrier. In an image forming apparatus having a developing device that develops using a component developer and a driving unit that rotationally drives the developer carrier, the potential of the image carrier charged by the charging unit and the developer carrier An image forming apparatus characterized in that a potential difference with a DC component of a developing bias voltage applied to a body is smaller than a potential difference when the developer carrying member is driven, when the developer carrying member is not driven. Is provided.

  According to the present invention, it is possible to suppress image defects such as a density difference due to toner adhesion to the developer carrying member in a non-image forming area such as at the time of pre-rotation and after rotation.

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

Example 1
[Overall Configuration and Operation of Image Forming Apparatus]
First, the overall configuration and operation of an embodiment of an image forming apparatus to which the present invention can be applied will be described. FIG. 1 shows a cross section of an image forming apparatus 10 of this embodiment. The image forming apparatus 10 of this embodiment is a tandem type intermediate transfer type color image forming apparatus. The image forming apparatus 10 is a recording material according to an image information signal from a document reading device 20 provided in the image forming apparatus main body A (or an external host device such as a personal computer that is communicably connected to the image forming apparatus main body A). A full-color image can be formed on P (recording paper, plastic film, cloth, etc.).

  In the image forming apparatus of this embodiment, as shown in FIG. 1, four image forming stations Pa, Pb, Pc, and Pd are juxtaposed in series in the image feed direction as image forming means. Each of the image forming stations Pa to Pd includes cylindrical electrophotographic photosensitive members (photosensitive drums) 1a, 1b, 1c, and 1d as image carriers, charging devices 2a, 2b, 2c, and 2d as charging means, and exposure means. Exposure devices 3a, 3b, 3c and 3d (electrostatic image forming means), developing devices 4a, 4b, 4c and 4d as developing means, cleaning devices 5a, 5b, 5c and 5d as cleaning means, and primary Primary transfer devices 7a, 7b, 7c and 7d are provided as transfer means. Further, intermediate transfer as an intermediate transfer member so as to pass between the photosensitive drums 1a, 1b, 1c, and 1d of the image forming stations Pa, Pb, Pc, and Pd and the primary transfer devices 7a, 7b, 7c, and 7d. The belt 12 is arranged so as to be able to go around in the direction of the arrow in the figure.

  Each of the developing devices 4a, 4b, 4c, and 4d contains a developer for each developing color of yellow, magenta, cyan, and black. As the developer, a two-component developer including a nonmagnetic toner (toner) and a magnetic carrier (carrier) is used. The toner and the carrier are mixed at a predetermined mixing ratio, and a predetermined amount is filled in each of the developing devices 4a, 4b, 4c, and 4d.

  For example, when a four-color full-color image of yellow, magenta, cyan, and black is formed, the surfaces of the photosensitive drums 1a, 1b, 1c, and 1d that rotate in the direction of the arrow in the drawing are charged at a predetermined timing. It is charged by the action of the charging bias voltage applied to 2b, 2c, 2d. The charged photosensitive drums 1a, 1b, 1c, and 1d are, for example, exposure devices (laser scanner devices in this embodiment) 3a, 3b, 3c, and the like in accordance with color-separated image information signals from the document reading device 20. Scan exposure is performed by 3d. As a result, electrostatic images are formed on the respective photosensitive drums 1a, 1b, 1c, and 1d according to the color-separated image information signals. The electrostatic images formed on the photosensitive drums 1a, 1b, 1c, and 1d are developed as toner images by the developing devices 4a, 4b, 4c, and 4d, respectively. In this way, toner images of respective colors are sequentially formed on the photosensitive drums 1a, 1b, 1c, and 1d.

  The toner images formed on the photosensitive drums 1a, 1b, 1c, and 1d are 1 at the primary transfer positions n1a, n1b, n1c, and n1d where the photosensitive drums 1a, 1b, 1c, and 1d face the intermediate transfer belt 12, respectively. Primary transfer is performed on the intermediate transfer belt 12 by the action of the primary transfer bias voltage applied to the next transfer devices 7a, 7b, 7c, and 7d. As a result, a multiple toner image in which the toners of the respective colors are superimposed on the intermediate transfer belt 12 is formed.

  Further, the transfer material P accommodated in the transfer material cassette 13 is conveyed to the secondary transfer position n2 where the secondary transfer device 11 as the secondary transfer means and the intermediate transfer belt 12 face each other. The toner image carried on the intermediate transfer belt 12 is secondarily transferred to the transfer material P by the action of the secondary transfer bias voltage applied to the secondary transfer device 11 at the secondary transfer position n2.

  Thereafter, the recording material P is separated from the intermediate transfer belt 12 and conveyed to the fixing unit 9. In the fixing unit 9, the toner image is fixed on the recording material P by heating and pressing. Thereafter, the recording material P on which the toner image is fixed is discharged out of the apparatus as a recorded image.

  Further, an intermediate transfer body cleaning device 14 as an intermediate transfer body cleaning unit is provided downstream of the secondary transfer position n2 to the transfer material P in the traveling direction of the intermediate transfer belt 12. The intermediate transfer body cleaning device 14 includes an intermediate transfer body cleaning blade that cleans fog toner, transfer residual toner, and the like attached to the surface of the intermediate transfer belt 12. The intermediate transfer belt 12 is cleaned by always contacting the intermediate transfer member cleaning blade. On the other hand, the toner remaining on the photosensitive drums 1a, 1b, 1c, and 1d is cleaned by cleaning devices 5a to 5d including fur brushes, blade means, and the like.

  A single color or a multicolor image can be formed by selectively using a single or a plurality of desired image forming stations.

[Developer]
Next, the developing device 4 will be described with reference to FIG. In the image forming apparatus 10 of the present embodiment, the configuration of each of the image forming stations Pa, Pb, Pc, and Pd is substantially the same except that the development colors are different. The subscripts “a”, “b”, “c”, and “d” given to the reference numerals in the drawing are omitted so as to indicate that the elements belong to any one of the image forming stations.

  As shown in FIG. 2, the developing device 4 includes a developing container 101 that stores a two-component developer (developer) including a non-magnetic toner (toner) and a magnetic carrier (carrier). An opening 100 is provided on the side of the developing container 101 facing the photosensitive drum 1. A developing sleeve 102 made of a non-magnetic material as a developer carrying member is disposed so as to be partially exposed from the opening 100. Inside the developing sleeve 102, a magnet roller 103 having a plurality of magnetic poles is fixedly disposed along the developer conveying direction by the developing sleeve 101. Further, the developing container 101 is provided with stirring screws 106 and 107 as developer transport members that transport the developer while stirring. Further, in order to form a thin layer of developer on the surface of the developing sleeve 102, a regulating blade 105 as a developer regulating member is disposed to face the developing sleeve 102.

  A developing bias power source (high voltage power source) 110 is connected to the developing sleeve 102 as voltage applying means for applying a developing bias voltage to the developing sleeve 102. The developing bias power supply 110 can supply a DC voltage (DC voltage) or a voltage obtained by superimposing a DC voltage and an AC voltage (AC voltage). Hereinafter, the DC voltage component of the developing bias voltage applied from the developing bias power source 110 to the developing sleeve 102 is also referred to as “developing DC bias”, and the AC voltage component is also referred to as “developing AC bias”.

  In this embodiment, a voltage obtained by superimposing a DC voltage and an AC voltage is used as the developing bias voltage. However, the present invention is not limited to this, and only the DC voltage can be used as the developing bias voltage. As the AC voltage of the developing bias voltage, a rectangular wave is used in this embodiment, but an arbitrary waveform can be used.

  The developing process by the two-component magnetic brush method and the developer circulation system in this embodiment will be described.

  First, as the developing sleeve 102 rotates, the developer is pumped up to the developing sleeve 102 by the pumping magnetic pole N2. The developer pumped up by the developing sleeve 102 is regulated by a regulating blade 105 disposed opposite to the developing sleeve 102 at a predetermined interval (SB gap) in the process of being conveyed to the magnetic poles S2 and N1. Thus, a thin layer of the two-component developer is formed on the developing sleeve 102. Here, when the developer carried as a thin layer on the developing sleeve 102 is conveyed to the developing main pole S1, a magnetic brush (protruding spike) is formed by the magnetic force. The magnetic brush formed in the shape of a spike approaches or comes into contact with the photosensitive drum 1. Then, the toner in the magnetic brush moves to the photosensitive drum 1 according to the electrostatic image formed on the photosensitive drum 1 by the action of the developing bias voltage applied to the developing sleeve 102. As a result, the electrostatic image on the photosensitive drum 1 is developed as a toner image. The developer on the developing sleeve 102 that has developed the electrostatic image is then collected in the developing container 101 by the repulsive magnetic field of the magnetic poles N3 and N2.

In this example, as a magnetic carrier, the saturation magnetization for an applied magnetic field of 240 kA / m is 24 Am ² / kg, the specific resistance at an electric field strength of 3000 V / cm is 1 × 10 ⁷ to ⁸ Ω · cm, and the weight average particle size is 50 μm Ferrite magnetic carrier was used. Further, as the non-magnetic toner, a negatively chargeable polyester resin toner having a weight average particle diameter of 7.2 μm, in which hydrophobic colloidal silica is externally added to colored resin particles, was used.

  As the magnetic carrier, a resin magnetic carrier produced by a polymerization method using a binder resin, a magnetic metal oxide, and a nonmagnetic metal oxide as starting materials may be used. The method for producing these magnetic carriers is not particularly limited. A styrene acrylic resin toner may be used as the nonmagnetic toner.

  Further, in this embodiment, a developer obtained by mixing the above-described magnetic carrier and non-magnetic toner in a weight ratio of 93: 7 was used. This value should be appropriately adjusted depending on the charge amount of the toner, the carrier particle size and specific gravity, the configuration of the image quality forming apparatus, etc., and does not necessarily follow this value.

[Operation timing]
Next, with reference to FIGS. 3 to 5, the operation timing and action in the image forming apparatus 10 of the present embodiment will be described in detail.

  FIG. 3 shows the timing of driving the photosensitive drum 1 (rotational driving), applying the charging bias voltage by the charging device 2, driving the developing sleeve 102 (rotating driving), and applying the developing DC bias and developing AC bias to the developing sleeve 102. FIG. FIG. 4 is a diagram showing the positional relationship of each device arranged around the photosensitive drum 1 while paying attention to one of the image forming stations Pa to Pd.

  In this embodiment, the potential difference between the potential of the photosensitive drum 1 charged by the charging device 2 and the DC component of the developing bias voltage applied to the developing bias 102 of the developing device 4 is set to a non-image forming area rather than the potential difference in the image forming area. Reduce the potential difference at.

  In this specification, the image forming area means a portion (timing) on the photosensitive drum 1 corresponding to an area where a toner image finally transferred onto the recording material P can be formed. In the case of printing on the material P, a portion where characters are not printed is also included in the image forming area. The image portion in the image forming area means a portion on the photosensitive drum 1 corresponding to the area of the toner image that is finally transferred onto the recording material P. For example, printing is performed on the recording material P. In that case that character. The non-image forming area means a part other than the image forming area. Typically, the non-image forming area is typically a pre-rotation operation (before the image forming operation is started and the image formation in the image forming area is started). In the multi-rotation operation), the image area includes a portion (timing) corresponding to the post-rotation operation that is the preparatory operation after the image formation is completed. Furthermore, application to a portion (timing) corresponding to the space between the recording materials P and the recording material P in a series of image forming operations for a plurality of recording materials P may be contemplated.

  3 and 4, the image forming operation is started and the photosensitive drum 1 is started to be driven. At the same time, the application of the charging bias voltage to the charging device 2 is started. Is charged. When the charged position of the photosensitive drum 1 reaches the position facing the developing sleeve 102 after time T1 due to the rotation of the photosensitive drum 1, the first developing DC bias (non-image area development) to the developing sleeve 102 by the high voltage power source 110 is reached. Application of DC bias) Vdc1 is started, and driving of the developing sleeve 102 is started.

  Thereafter, during the time T2, multiple pre-rotations are performed to stabilize the potential of the photosensitive drum 1. When the potential of the photosensitive drum 1 is stabilized, an electrostatic image is formed on the image forming area on the photosensitive drum 1 by the exposure device 3. Almost simultaneously with the movement of the image forming area (electrostatic latent image) to the position facing the developing sleeve 102, the application of the developing AC bias Vac is started, whereby the second developing DC bias (image area) is applied to the developing sleeve 102. A development bias voltage in which a development DC bias) Vdc2 and a development AC bias Vac are superimposed is applied. Thus, the electrostatic image in the image forming area is developed.

  In addition, the application of the development AC bias Vac is stopped almost at the same time as the image forming region finishes passing through the portion facing the development sleeve 102. Further, the first developing DC bias Vdc1 is applied again until the charging end position on the photosensitive drum 1 comes to a position facing the developing sleeve 102.

  In this embodiment, it takes 100 msec for the development DC bias to stabilize, and 100 msec for the development AC bias to stabilize. Therefore, in this embodiment, the development DC bias is changed from the first development DC bias to the second development DC bias 200 msec before switching from the non-image forming area to the image forming area, and the second development DC is performed 100 msec after the image forming area. The bias Vdc2 is changed to the first development DC bias Vdc1. However, the present invention is not limited to such an accurate timing, and a difference occurs depending on the main body of the image forming apparatus that implements the present invention. As will be described in more detail below, the fog removal potential difference Vback may be made substantially smaller in the non-image forming area than in the image forming area.

FIG. 5 shows a potential relationship between the photosensitive drum 1 and the developing sleeve 102 during the image forming operation. Here, the symbols in FIG. 5 mean the following.
V _D : Potential of the photosensitive drum 1 uniformly charged by the charging device 2 (white background portion potential)
V _L : latent image potential after exposure by the exposure apparatus 3 (bright part potential)
Vdc1: first developing DC bias (the potential of the developing sleeve 102 when the first developing DC bias is applied)
Vdc2: second developing DC bias (the potential of the developing sleeve 102 when the second developing DC bias is applied)

As shown in FIG. 5, the surface of the photosensitive drum 1 is uniformly charged to V _D = −700 V by the charging device 2. The image portion of the image forming area is exposed by the exposure device and becomes V _L = −200V.

The developing DC bias when developing in the image forming area, that is, the second developing DC bias is −550V. The development contrast potential difference Vcont, which is the difference between the potential (bright part potential) V _L (= −200 V) of the exposure part by the laser of the exposure apparatus 3 and the second development DC bias Vdc2 (= −550 V), is 350V. With this development contrast potential difference Vcont, toner is transferred from the development sleeve 102 onto the photosensitive drum 1. Further, the fog removal potential difference (second fog removal potential difference) Vback2 which is the difference between the white background portion potential V _D (= −700 V) in the image forming area and the second development DC bias Vdc2 (= −550 V) is 150 V. . By this fog removal potential difference Vback2, the fog toner is pulled back to the developing sleeve 4 side, and the fog toner does not adhere to the photosensitive drum 1.

  On the other hand, the development DC bias in the non-image forming area, that is, the first development DC bias Vdc1 is −650V. Accordingly, in the non-image area, the fog removal potential difference (first fog removal potential difference) Vback1 which is the difference between the white background portion potential Vd (= −700 V) and the first development DC bias Vdc1 (= −650 V) is 50 V. .

  The principle of the present invention will be further described. As described above, in the two-component development method using a developer including a non-magnetic toner and a magnetic carrier, it is formed by a potential difference between the potential of the photosensitive drum 1 (white background portion potential) and the development DC bias in the non-image forming area. When the fog removal potential difference Vback is large, the negative toner in the negatively charged developer moves toward the developing sleeve 102 having a low negative potential, that is, a positive potential in the developing nip facing the photosensitive drum 1. Pressed in the direction. When the two-component development method is used, the toner is not formed on the surface of the developing sleeve 102 (magnetic brush) but on the surface of the developing sleeve 102 by the fog removal potential difference Vback in the non-image forming area. Accumulate. In other words, in the non-image forming region, the same state as that in which the developing operation with the toner is performed on the developing sleeve 102 is obtained. As a result, the toner adheres to the developing sleeve 102.

  When the toner adhering to the surface of the developing sleeve 102 increases, the negative toner charged to the negative polarity is an insulator, so the apparent potential on the surface of the developing sleeve 102 becomes a potential on the negative side further than the developing DC bias Vdc. . For this reason, when an electrostatic image is formed on the photosensitive drum 1 in the image forming area, the apparent development contrast potential difference Vcont increases. As a result, the image density becomes high.

  In the present embodiment, the fog removal potential difference Vback is reduced in the non-image forming area with respect to this problem. Thereby, as described above, it is possible to suppress the toner from being pressed against the developing sleeve 102 by the fog removal potential difference Vback in the non-image forming area, and the developing sleeve 102 being developed in the non-image area. Then, the toner adhering to the developing sleeve 102 can be reduced, and the change in the apparent potential on the surface of the developing sleeve 102 can be prevented.

  Table 1 shows the relationship between the fog removal potential difference Vback in the non-image forming area, the amount of toner adhering to the developing sleeve 102, and the surface potential of the developing sleeve 102 with the toner adhering thereto.

  The amount of toner adhered on the developing sleeve was measured as follows. The developer (two-component developer) on the developing sleeve 102 is removed after the front multi-rotation and the post-rotation operations which are non-image forming areas are performed in each Vback shown in Table 1. Thereafter, the toner adhering to the developing sleeve 102 was collected with a tape, and the density was measured using an X-Rite 500 color reflection densitometer manufactured by X-Rite.

  The surface potential of the developing sleeve 102 was measured as follows. The developer (two-component developer) on the developing sleeve 102 is removed. Thereafter, the developing sleeve 102 is grounded with the toner adhering to the developing sleeve 102, and a Trek 344 surface potential meter probe made by Trek is installed so that the clearance from the developing sleeve 102 is 1 mm. The surface potential was measured.

  Table 1 shows the results for the black developing device 4d, and the higher the reflection density, the greater the amount of toner adhering to the surface of the developing sleeve 102. A similar experiment was performed for the developing devices 4 for other colors, but the toner adhesion tendency to the surface of the developing sleeve 102 and the change tendency of the surface potential of the developing sleeve 102 showed substantially the same results. .

  As shown in Table 1, when the fog removal potential difference Vback during non-image formation is small, the amount of toner adhering to the developing sleeve 102 is small, and the surface potential on the developing sleeve 102 is almost 0V. However, as the fog removal potential difference Vback is increased, the amount of toner adhering to the developing sleeve 102 increases and the surface potential on the developing sleeve 102 becomes a negative potential.

  This is because the potential of the developing sleeve 102, which is a conductor grounded to the ground, is 0V, but the negatively charged negative toner is an insulator, so that it appears that the negative toner is adhered on the developing sleeve 102. This is because the potential of 102 becomes negative.

  As described above, when the potential of the developing sleeve 102 is negative due to the toner adhering to the surface, when the electrostatic latent image is formed on the photosensitive drum in the image forming region, the potential of the developing sleeve 102 is developed. Since the potential is on the minus side of the development DC bias applied to the sleeve 102, the apparent development potential Vcont is increased. For this reason, the image density is increased. When the toner adhering to the developing sleeve 102 is consumed in the developing process in the image forming area, the potential of the developing sleeve 102 becomes the same as the developing DC bias. As a result, the development potential becomes normal and a desired image density is obtained.

  According to the study by the present inventors, if there is a difference of 10 V or more between the state where the toner is attached to the developing sleeve 102 and the state where the toner is not attached, a halftone image or a solid image is detected. When an image is formed, an image defect such as a density step may occur.

  As shown in Table 1, in the configuration of this embodiment, in order to reduce the toner adhesion amount at the time of non-image formation and to prevent the occurrence of image defects such as the above-described density step, the fog removal potential difference Vback at the time of non-image formation is set to It is desirable to make it 80V or less. Conversely, when the fog removal potential difference Vback is set to 20 V or less, the electric field regulating force due to the electric field generated by the fog removal potential difference Vback is almost eliminated, so that the fog toner easily adheres to the photosensitive drum 1. When the fog toner easily adheres to the photosensitive drum 1 as described above, the amount of consumed toner and the amount of waste toner increase, which is not preferable.

  Therefore, in the configuration of the present embodiment, the fog removal potential difference (first fog removal potential difference) Vback1 in the non-image formation region is preferably smaller than the fog removal potential difference (second fog removal potential difference) Vback2 in the image formation region. In addition, the fog removal potential difference (first fog removal potential difference) Vback1 in the non-image area is set to 20 (V) <Vback ≦ 80 (V).

  Here, FIG. 6 is a schematic control block diagram of the image forming apparatus 10 of the present embodiment. The image forming apparatus 10 includes a control unit 150 that controls the overall operation of the apparatus. The control unit 150 includes a CPU 151 that is a central element (control means) of control, and the CPU 151 includes a ROM 152 that stores programs executed by the CPU 151 and various data, a RAM 153 that is used as a working memory, and the like. It is connected. The CPU 151 causes the image forming apparatus 10 to perform a sequence operation according to data, programs, and the like stored in the ROM 152. In this embodiment, the CPU 151 transmits a control signal to the developing bias power source 110, the developer carrier driving means (driving motor) 120 for rotating the developing sleeve 102, the charging bias power source 130, etc., and according to the chart of FIG. An image forming operation is performed. The developer carrier driving means 120 may drive the developing sleeve 102 independently. For example, the developer driving means 140 and the driving source may be shared.

  An image processing unit (video controller) 160 is connected to the control unit 150. The image processing unit 160 receives an image signal from the document reading device 20 (or an external host device such as a personal computer that is communicably connected to the apparatus main body A), and converts this signal into a signal related to image formation. The data is converted and transmitted to the CPU 151 of the control unit 150. The CPU 151 controls the operation of each part of the image forming apparatus 10 in accordance with such an image forming signal.

  In this embodiment, in particular, the CPU 151 transmits a voltage application signal for starting and stopping the application of the development DC bias and the development AC bias to the development bias power supply 110 at a predetermined timing. Further, the CPU 151 transmits a voltage switching signal for switching the development DC bias between the first development DC bias Vdc1 and the second development DC bias Vdc2 to the development bias power supply 110 at a predetermined timing. On the other hand, the developing bias power supply 110 can apply a developing DC bias or a voltage in which a developing DC bias and a developing AC bias are superimposed on the developing sleeve 102 of the developing device 4. The developing bias power supply 110 includes a voltage switch (voltage switching unit) 111 that switches the developing DC bias applied to the developing sleeve 102 between the first developing DC bias and the second developing DC bias. Thus, the developing bias power supply 111 can selectively apply the first developing DC bias or the second developing DC bias to the developing sleeve 102 in accordance with the voltage switching signal from the CPU 151.

  As described above, according to the present embodiment, the fog removal potential difference Vback in the non-image formation area is made smaller than the fog removal potential difference Vback in the image formation area, and the amount of toner attached to the developing sleeve 102 in the non-image formation area is reduced. Decrease. As a result, an apparent potential change on the surface of the developing sleeve 102 can be prevented, and image defects such as density steps in a halftone image or a solid image can be prevented. Therefore, high image quality can be achieved.

  In this embodiment, the developing DC bias applied to the developing sleeve 102 is changed. However, it may be changed so that the fog removal potential difference Vback is smaller in the non-image forming area than in the image forming area. Therefore, the developing DC bias may be fixed and the charging potential of the photosensitive drum 1 by the charging device 2 may be changed between the non-image forming area and the image forming area. Alternatively, both the charging potential of the photosensitive drum 1 and the development DC bias may be changed between the non-image forming area and the image forming area. In this case, similarly to the developing bias power supply 110, the charging bias power supply 130 is provided with a voltage switch (voltage switching means), and the charging bias power supply 130 is changed in accordance with a charging bias switching signal from the CPU 151 to remove a desired fog. The photosensitive drum 1 may be charged so as to obtain the potential difference Vback. As a result, the same effect as described above can be obtained.

Example 2
Next, another embodiment of the present invention will be described with reference to FIG. Since the basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the first exemplary embodiment, elements having substantially the same or corresponding functions and configurations are denoted by the same reference numerals and are described in detail. Is omitted.

  In Example 1, the magnitude of the fog removal potential difference Vback was changed between the non-image forming area and the image forming area. In contrast, in this embodiment, the fog removal potential difference Vback is changed between when the developing sleeve 102 is not driven and when it is driven.

  FIG. 7 illustrates driving (rotational driving) of the photosensitive drum 1, application of a charging bias voltage by the charging device 2, driving of the developing sleeve 102 (rotational driving), and developing DC bias and developing AC bias for the developing sleeve 102 in this embodiment. It is the chart which showed the timing of application of.

  At the same time when the image forming operation is started and driving of the photosensitive drum 1 is started, application of a charging bias voltage to the charging device 2 is started, and the photosensitive drum 1 is uniformly charged. When the charged position of the photosensitive drum 1 reaches the position facing the developing sleeve 102 after time T1 due to the rotation of the photosensitive drum 1, the first developing DC bias (non-image area development) to the developing sleeve 102 by the high voltage power source 110 is reached. Application of DC bias) Vdc1 is started. Then, multiple forward rotations are performed to stabilize the potential of the photosensitive drum 1.

  When the potential of the photosensitive drum 1 is stabilized, an electrostatic latent image is formed in the image forming area on the photosensitive drum 1 by the exposure device 3. Almost simultaneously with the movement of the image forming area (electrostatic latent image) to the position facing the developing sleeve 102, the driving of the developing sleeve 102 is started. At the same time, the application of the development AC bias is started, whereby a development bias voltage on which the second development DC bias Vdc2 and the development AC bias Vac are superimposed is applied to the development sleeve 102. Thus, the electrostatic image in the image forming area is developed.

  Further, almost simultaneously with the end of the image forming region passing through the portion facing the developing sleeve 102, the driving of the developing sleeve 102 and the application of the developing AC bias Vac are stopped. Further, the first developing DC bias Vdc1 is applied again until the charging end position comes to a position facing the developing sleeve 102.

  In this embodiment, it takes 100 msec for the development DC bias to stabilize, 30 msec for the development sleeve 102 to stabilize, and 100 msec for the development AC bias to stabilize. Therefore, in this embodiment, the development DC bias is changed from the first development bias Vdc1 to the second development bias Vdc2 100 msec before the drive of the development sleeve 102 is started. However, the present invention is not limited to such an accurate timing, and a difference occurs depending on the main body of the image forming apparatus that implements the present invention.

  As described above, in this embodiment, the potential difference between the potential of the photosensitive drum 1 charged by the charging device 2 and the DC component of the developing bias voltage applied to the developing sleeve 102 is determined during the operation of the driving means of the developing sleeve 102 (that is, The potential difference when the driving means is not operating (that is, when the developing sleeve 102 is not driven) is made smaller than the potential difference when the developing sleeve 102 is driven.

  Incidentally, in the developing device 4 as shown in FIG. 2, the developer circulating in the developing device 4 is stressed in the vicinity of passing through the regulating blade 105 due to the rotation of the developing sleeve 102. As a result, the developer deteriorates, such as a reduction in toner charge. Examples of the deterioration of the developer include peeling of a coating material applied to the surface of the carrier in order to stabilize the charge imparting property to the toner, embedding external additives such as silica and titanium in the toner, and the like. .

  As a countermeasure against this problem, as in the present embodiment, the driving time of the developing sleeve 102 is set to be substantially the same as that of the image forming area, that is, the developing sleeve 102 is driven as much as the image forming area. It is effective to minimize the rotation time. In addition, such a configuration has a problem that the toner is fused to the developing sleeve 102 when the developing sleeve 102 rotates for a long time and the surface roughness of the developing sleeve 102 is reduced, so that the developer transportability is reduced. This is also effective for the life of mechanical parts such as the drive motor and clutch of the developing sleeve 102.

  However, if the driving of the developing sleeve 102 is stopped during the front multi-rotation or the post-rotation, the toner adheres to the developing sleeve 102 described in the first embodiment, and the photosensitive drum is exposed to the photosensitive drum on the peripheral surface of the developing sleeve 102. Concentrate locally only on the part facing 1. For this reason, the amount of toner attached to that portion also increases. As a result, there may occur an image defect in which a stripe-like high density portion along the longitudinal direction of the developing sleeve 102 appears in a halftone image or the like in the subsequent image forming area.

  Therefore, as shown in FIG. 7, the developing DC bias when the driving of the developing sleeve 102 is stopped is set to a voltage that reduces the fog removal potential difference Vback. As a result, similarly to the first embodiment, the amount of toner adhered to the developing sleeve 102 when the developing sleeve 102 is not driven can be reduced. In addition, since the driving of the developing sleeve 102 is stopped, the toner is not supplied to the photosensitive drum 1 due to the low fog removal potential difference Vback. Therefore, there is no problem that the fog toner adheres to the photosensitive drum 1.

  When the developing sleeve 102 is not driven, it corresponds to a non-image forming area, and when the developing sleeve 102 is driven, it corresponds to an image forming area. Therefore, in other words, also in this embodiment, the fog removal potential difference Vback is changed between the non-image forming area and the image forming area.

  FIG. 8 is a schematic control block diagram of the image forming apparatus 10 in this embodiment. The control mode of the present embodiment is basically the same as that shown in FIG. In this embodiment, the CPU 151 transmits a control signal to the developing bias power supply 110, the developer carrier driving means (driving motor) 120 for rotating the developing sleeve 102, the charging bias power supply 130, etc. An image forming operation is performed according to the chart. In particular, in this embodiment, the developer carrying member driving unit 120 includes a drive switching unit (driving switching unit) 121 that can drive and stop the developing sleeve 102. This drive switcher can start and stop the rotational drive of the developing sleeve 102 in accordance with a control signal from the CPU 151. If the developer carrier driving means 120 is independent, the drive of the drive motor itself may be started and stopped, and the developer carrier driving means 120 and, for example, the driving source of the photosensitive member driving means 140 are shared. In this case, the transmission / cutoff of the drive may be switched by turning on / off the clutch.

  As described above, the fog removal potential difference Vback when the developing sleeve 102 is not driven is made smaller than the fog removing potential difference Vback when the developing sleeve 102 is driven, and the amount of toner adhered to the developing sleeve 102 when the developing sleeve 102 is not driven. By reducing this, it is possible to prevent an image defect in which a high density streak occurs in a halftone image. Further, by reducing the fog removal potential difference Vback in the non-image forming area, it is possible to suppress the fog toner from adhering to the photosensitive drum 1. In addition, by stopping the driving of the developing sleeve 102 in the non-image forming area, the driving time of the developing sleeve 102 can be minimized, and the deterioration of the developer and the toner fusion on the surface of the developing sleeve 102 can be suppressed.

Example 3
Next, still another embodiment of the present invention will be described with reference to FIG. Since the basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the first exemplary embodiment, elements having substantially the same or corresponding functions and configurations as those of the first exemplary embodiment are denoted by the same reference numerals. A detailed description will be omitted.

  In the developing device 4 according to the first exemplary embodiment, the regulating blade 105 is disposed so as to face the magnetic pole S2 immediately downstream of the developer lifting pole N2 in the developing sleeve traveling direction. On the other hand, in this embodiment, the regulating blade 105 is disposed so as to face the developer lifting pole N2.

  The developing process by the two-component magnetic brush method and the developer circulation system in this embodiment will be described.

  Inside the developing sleeve 102, similarly to the first embodiment, a magnet roll as a magnetic field generating means having a plurality of magnetic poles is fixedly disposed along the developer conveying direction by the developing sleeve 102.

  First, as the developing sleeve 102 rotates, the developer is pumped up to the developing sleeve 102 by the pumping magnetic pole N2. The developer pumped up by the pumping magnetic pole N2 is regulated by a regulating blade 105 that is substantially opposed to the magnetic pole N2 and arranged with a predetermined interval (SB gap) with respect to the developing sleeve 102. Thus, a thin layer of the two-component developer is formed on the developing sleeve 102. Here, when the developer carried as a thin layer on the developing sleeve 102 is transported to the developing main pole S1 via the transport poles S2 and N1, a magnetic brush (spreading of the developer) is caused by magnetic force. It is formed. The magnetic brush formed in the shape of a spike approaches or comes into contact with the photosensitive drum 1. Then, the toner in the magnetic brush moves to the photosensitive drum 1 according to the electrostatic image formed on the photosensitive drum 1 by the action of the developing bias voltage applied to the developing sleeve 102. As a result, the electrostatic image on the photosensitive drum 1 is developed as a toner image. The developer on the developing sleeve 102 that has developed the electrostatic image is then collected in the developing container 101 by the repulsive magnetic field of the magnetic poles N3 and N2.

  The configuration of the developing device 4 used in this embodiment is different from that of the first embodiment in that the developer is regulated by the pumping magnetic pole N2. Here, the excessive amount of the developer regulated by the regulation blade 105 remains on the development sleeve 102 as an agent pool upstream of the regulation blade 105 in the rotation direction of the development sleeve 102. However, with the configuration as in the present embodiment, the amount of developer in the agent reservoir is smaller than that of the developing device 4 of the first embodiment. As a result, the stress that the developer receives in the vicinity of the regulating blade 105 due to the rotation of the developing sleeve 102 is small as compared with the developing device 4 of the first embodiment because the amount of the agent pool is small. As a result, it is possible to reduce the deterioration of the developer such as peeling off of the coat for adjusting the resistance of the surface of the carrier and embedding of an external additive such as silica or titanium in the toner.

  However, on the other hand, since the stress from the agent reservoir is small in the vicinity of the facing portion between the developing sleeve 102 and the regulating blade 105, the toner adhering to the surface of the developing sleeve 102 due to the fog removal potential difference Vback in the non-image forming area is It is hard to be peeled off from the surface. Therefore, image defects such as density steps in the above-described halftone image and solid image tend to occur remarkably.

  In the developing device 4 configured as in the present embodiment, the amount of toner adhered to the developing sleeve 102 is effectively reduced in the non-image forming region by controlling the fog removal potential difference Vback performed in the first and second embodiments. Can be made. Further, it is possible to prevent an apparent potential change on the surface of the developing sleeve 102 and to prevent image defects such as density steps in a halftone image or a solid image. Moreover, the deterioration of the developer due to stress can be reduced.

  As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the aspect of the said Example.

1 is a cross-sectional view of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a cross-sectional view illustrating a developing device provided in the image forming apparatus of FIG. 1. FIG. 6 is a chart showing the operation timing of the image forming apparatus in one embodiment of the present invention. It is a schematic diagram which shows the positional relationship of each apparatus arrange | positioned around the photosensitive drum. FIG. 6 is an explanatory diagram illustrating a potential relationship between a photosensitive drum and a developing sleeve during an image forming operation. It is a schematic functional block diagram which shows the one control aspect of the fog removal potential difference according to this invention. FIG. 6 is a chart showing the operation timing of an image forming apparatus in another embodiment of the present invention. It is a general | schematic functional block diagram which shows the other control aspect of the fog removal potential difference according to this invention. FIG. 5 is a cross-sectional view of another example of a developing device that can be used in the image forming apparatus of FIG. 1.

Explanation of symbols

1a to 1d Photosensitive drum (image carrier)
2a to 2d charging device 4a to 4d developing device 102 developing sleeve 105 regulating blade 110 high voltage power source (developing bias power source)

Claims (10)

An image carrier, a charging unit for charging the image carrier, an electrostatic image forming unit for forming an electrostatic image on the image carrier charged by the charging unit, and an image carrier formed on the image carrier. An image forming apparatus comprising: a developing device that develops an electrostatic image using a two-component developer including a non-magnetic toner and a magnetic carrier;
The potential difference between the potential of the image carrier charged by the charging unit and the DC component of the developing bias voltage applied to the developing device is smaller than the potential difference in the image forming area. An image forming apparatus.   The image forming apparatus according to claim 1, wherein the potential difference is reduced by changing a DC component of the developing bias voltage.   2. The image forming apparatus according to claim 1, wherein the potential difference is reduced by changing a potential of the image carrier charged by the charging unit.   The developing device further includes a developer carrier that carries a two-component developer and conveys the developer to a position facing the image carrier, and the developing bias is applied to the developer carrier. The image forming apparatus according to claim 1.   Further, the image forming apparatus further includes a driving unit that rotationally drives the developer carrying member, and the potential difference is greater than the potential difference in driving the developer carrying member in the image forming region. 5. The image forming apparatus according to claim 4, wherein the potential difference when the carrier is not driven is reduced.   The developing device further includes a developer regulating member that is disposed opposite to the developer carrying body and regulates the developer carried on the developer carrying body, and inside the developer carrying body, A magnetic field generating means having a plurality of magnetic poles is fixedly disposed along a developer carrying direction by the developer carrier, and the developer regulating member is the developer among the plurality of magnetic poles of the magnetic field generating means. 6. An image forming apparatus according to claim 4 or 5, wherein said image forming apparatus is disposed opposite to a pumping electrode for pumping the two-component developer on the carrier. An image carrier, a charging unit that charges the image carrier, an electrostatic image forming unit that forms an electrostatic image on the image carrier charged by the charging unit, a nonmagnetic toner, and a magnetic carrier Development in which an electrostatic image formed on the image carrier is developed using the two-component developer, the developer carrier being carried to carry the two-component developer and transporting to a position facing the image carrier. In an image forming apparatus comprising: an apparatus; and a driving unit that rotationally drives the developer carrier.
The potential difference between the potential of the image carrier charged by the charging means and the direct current component of the developing bias voltage applied to the developer carrier is greater than the potential difference during driving of the developer carrier. An image forming apparatus characterized by reducing a potential difference when a body is not driven.   The image forming apparatus according to claim 7, wherein the potential difference is reduced by changing a DC component of the developing bias voltage.   9. The image forming apparatus according to claim 7, wherein the potential difference is reduced by changing a potential of the image carrier charged by the charging unit.   The developing device further includes a developer regulating member that is disposed opposite to the developer carrying body and regulates the developer carried on the developer carrying body, and inside the developer carrying body, A magnetic field generating means having a plurality of magnetic poles is fixedly disposed along a developer carrying direction by the developer carrier, and the developer regulating member is the developer among the plurality of magnetic poles of the magnetic field generating means. The image forming apparatus according to any one of claims 7 to 9, wherein the image forming apparatus is disposed opposite to a pumping electrode for pumping the two-component developer on the carrier.

Images