JP2009151119A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which recovers electrification performance by efficiently removing toner left after transfer which adheres to an electrifying member by easy control. <P>SOLUTION: Frequency of AC voltage to be applied to an charging roller 2 is switched from ordinary 1.5 kHz to 15 kHz, and simultaneously DC voltage to be applied to the charging roller 2 is switched to 0V, and DC voltage applied to a toner electrifying member 7 is switched from ordinary 600V to 900V. When the frequency of the AC voltage is about ordinary 1.5 kHz, the toner left after transfer is hardly discharged to a photoreceptor drum 1 from a surface of the charging roller 2. However, if the frequency of the AC voltage is set to 10 kHz or more, the toner left after transfer is completely discharged from the surface of the charging roller 2. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus for charging an image carrier by applying a DC voltage superimposed on an AC voltage to a charging member brought into contact with the image carrier, and more specifically, to transfer residual toner adhering to the charging member to the image carrier. It is related with the control which makes it discharge.

  2. Description of the Related Art A contact charging type image forming apparatus that charges a surface of an image carrier by applying a DC voltage in which an AC voltage is superimposed on a charging member such as a charging roller brought into contact with the image carrier is widely used.

  In addition, an image forming apparatus using a simultaneous development cleaning system that puts transfer residual toner remaining on the image carrier away from the recording material or intermediate transfer body back to the developing device and reuses it for developing electrostatic images is put into practical use. Has been.

  Patent Document 1 discloses a tandem type intermediate transfer type image forming apparatus in which four image forming units of a simultaneous development cleaning type that performs contact charging using a charging member are arranged along an intermediate transfer belt. Here, a toner charging device is arranged between the toner image transfer portion and the charging member, so that the toner charged to the opposite polarity on the image carrier or the toner with insufficient charge amount has a normal polarity that can be used for development. It is charged.

  Patent Document 2 discloses an image forming apparatus in which a toner uniformizing device is disposed between a toner image transfer portion and a toner charging device. Here, toner particles having different toner charge amount, charge polarity, and adhesion density are dispersed and neutralized to suppress variation in the result of charging the toner by the toner charging device.

  Japanese Patent Application Laid-Open No. 2003-228688 discloses control for discharging toner adhering to a charging roller to an image carrier by reducing an AC voltage applied to the charging roller during non-image formation. Here, when the toner adheres to the charging roller and the charging performance is lowered, the charging performance of the charging member is recovered by returning the toner to the image carrier side by controlling the AC voltage alternately in two steps of high and low.

JP 2001-183905 A JP 2001-215798 A JP 2005-31325 A

  In an image forming apparatus of the simultaneous development cleaning method in which contact charging is performed using a charging member, since there is no cleaning device upstream of the charging member, toner particles that follow the image carrier reach the charging member and part of the toner particles Adhere to. When toner particles accumulate on the surface of the charging member and the charging performance of the charging member is impaired, charging failure and charging unevenness occur.

  In the image forming apparatus disclosed in Patent Document 2, since the toner charging device charges the transfer residual toner to a normal polarity, it is difficult to adhere to a charging member to which a DC voltage having the same polarity as the normal polarity is applied. However, actually, the transfer residual toner is gradually deposited on the charging member through contact with the image carrier, and the charging performance of the charging member is gradually decreased.

  Providing a dedicated cleaning device for the charging member causes problems such as an arrangement space for the cleaning device, disposal of the cleaned transfer residual toner, and an increase in driving resistance of the charging member.

  In the control disclosed in Patent Document 3, toner can be removed from the surface of the charging member only by controlling the AC voltage applied to the charging member without providing such a dedicated cleaning device.

  However, the charged state and the attached state of the toner particles adhering to the charging member are various, and it is difficult to separate the toner particles from the charging member only with a DC electric field of several hundred volts formed between the charging member and the image carrier. There is.

  In particular, for toner particles having a small mass and a small charge amount or having a reversed charge polarity, it is difficult to overcome the intermolecular attractive force with the surface of the charging member by such a DC electric field alone. However, when a high voltage of several thousand volts is applied to obtain a sufficient separation effect, the image carrier is damaged.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can efficiently remove untransferred toner adhering to a charging member by simple control and restore charging performance.

  The image forming apparatus according to the present invention includes an image carrier, a charging member that rotates in contact with the image carrier, and a charging voltage obtained by superimposing an alternating voltage on a direct current voltage on the charging member. A charging power source means for charging, an electrostatic image forming means for forming an electrostatic image on the charged image carrier, and supplying the charged toner to the image carrier to develop the electrostatic image into a toner image And developing means. Then, during non-image formation, a potential difference is formed between the charging member and the image carrier so that the toner adhering to the charging member moves to the image carrier, and the charging power supply unit is controlled. And a control means for applying an AC voltage having a frequency higher than that at the time of image formation to the charging member.

  In the image forming apparatus of the present invention, the excitation acceleration for the charged toner particles is increased by increasing the frequency of the AC voltage. As a result, toner particles that cannot be separated from the surface of the charging member can be separated under the potential difference formed between the charging member and the image carrier unless the frequency of the AC voltage is increased. This is because the increase in excitation acceleration causes the toner particles to vibrate and reduces the intermolecular attractive force between the surface of the charging member and the toner particles. The toner particles released from the intermolecular attractive force restraint with the surface of the charging member move to the image carrier by the intermolecular attractive force with the contacted image carrier and the electric field formed between the image carrier and the image carrier. .

  Therefore, the transfer residual toner adhering to the charging member can be efficiently removed by simple voltage control to restore the charging performance of the charging member.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. As long as a DC voltage superimposed with an AC voltage having a higher frequency than that at the time of image formation is applied to the charging member at the time of non-image formation, the present invention may replace a part or all of the configuration of the embodiment as an alternative configuration. Other embodiments replaced with can also be implemented.

  Accordingly, not only a one-drum type image forming apparatus using one image carrier, but also a tandem type image forming apparatus in which a plurality of image carriers having different development colors are arranged along a recording material conveyance body or an intermediate transfer body. But it can be done.

  The transfer medium to which the toner image is transferred from the image carrier is not limited to a recording material, and can be implemented by a full-color or monochrome image forming apparatus using an intermediate transfer belt.

  In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general matter of the image forming apparatus shown by patent documents 1-3, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted. In the description, the reference symbols in parentheses attached to the component names used in the claims are examples for helping understanding of the invention, and are not intended to limit the configuration to the members of the embodiment. Absent.

<First Embodiment>
FIG. 1 is an explanatory diagram of a configuration of the image forming apparatus according to the first embodiment, and FIG. 2 is a schematic diagram of a layer configuration of a photosensitive drum and a charging roller.

  The image forming apparatus 100 is a laser beam printer of a contact charging method, a reverse developing method, a simultaneous development cleaning method, and an electrophotographic method.

(Image carrier)
As shown in FIG. 1, the photosensitive drum 1 is a rotating drum type electrophotographic photosensitive member as an image carrier. The photosensitive drum 1 is formed to have an outer diameter of 60 mm by disposing a negatively charged organic photoconductor (OPC) on the surface, and an arrow R1 at a process speed (peripheral speed) of 100 mm / sec centering on the central support shaft. It is rotationally driven in the direction.

  As shown in FIG. 2, the photosensitive drum 1 includes an undercoat layer 1b for suppressing the interference of light and improving the adhesion of the upper layer on the surface of an aluminum cylinder (conductive drum base) 1a, and a photocharge generation layer 1c. And three layers of the charge transport layer 1d are applied.

(Charging member)
As shown in FIG. 1, the charging roller 2 is a contact charging device (contact charger, roller charger) that uniformly charges the peripheral surface of the photosensitive drum 1.

  The charging roller 2 has a length in the longitudinal direction of 300 mm, and both ends of the cored bar 2a are rotatably held by bearing members (not shown). The charging roller 2 is urged in the direction of the photosensitive drum 1 by the pressing spring 2 e so as to come into pressure contact with the surface of the photosensitive drum 1 with a predetermined pressing force, and rotates following the rotation of the photosensitive drum 1. A pressure contact portion between the photosensitive drum 1 and the charging roller 2 is a charging portion (charging nip portion) a.

  A charging voltage of a predetermined condition is applied to the cored bar 2a of the charging roller 2 from the power source S1. By contacting and rotating the charging roller 2 to which the charging voltage is applied, the peripheral surface of the photosensitive drum 1 is contact-charged to a predetermined polarity and potential.

(Charging power supply means)
The power source S1 applies a charging voltage to the charging roller 2 at the time of image formation, and the charging voltage is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac).

  Specifically, a vibration voltage obtained by superimposing a sine wave having a frequency f = 1.5 kHz and a peak-to-peak voltage of 1500 Vpp on a DC voltage of −600 V is applied to the charging roller 2, and the peripheral surface of the photosensitive drum 1 is −600 V ( The contact charging treatment is uniformly applied to the dark potential Vd).

  As shown in FIG. 2, the charging roller 2 has a three-layer configuration in which a lower layer 2b, an intermediate layer 2c, and a surface layer 2d are sequentially laminated from the bottom around an outer periphery of a central core metal (support member) 2a. The lower layer 2b is a foamed sponge layer for reducing charging noise, and the surface layer 2d is a protective layer for preventing leakage even if the photosensitive drum 1 has a defect such as a pinhole.

More specifically, the specification of the charging roller 2 is as follows.
Core 2a: Stainless steel round bar with a diameter of 6 mm.
Lower layer 2b: Carbon dispersion foamed EPDM, specific gravity 0.5 g / cm ³ , volume resistivity 10 ² to 10 ⁹ Ωcm, layer thickness 3.0 mm, length 320 mm.
Intermediate layer 2c: Carbon-dispersed NBR rubber, volume resistance value 10 ² to 10 ⁵ Ωcm, layer thickness 700 μm.
Surface layer 2d: Tin oxide and carbon are dispersed in a fluorine resin resin resin, volume resistance value 10 ⁷ to 10 ¹⁰ Ωcm, surface roughness (JIS standard 10-point average surface roughness Ra) 1.5 μm, layer thickness 10 μm.

  The cleaning film 2f which is an example of the toner rubbing member is a charging roller cleaning member having flexibility. The cleaning film 2f forms a contact nip with the charging roller 2 on the surface in the vicinity of the free end whose one end is fixed to the support member 2g.

  The support member 2g is arranged in parallel to the longitudinal direction of the charging roller 2, and is driven by a drive motor (not shown) to reciprocate with a constant amplitude in the longitudinal direction. As a result, the toner particles adhering to the surface layer 2d of the charging roller 2 are rubbed against the cleaning film 2f, charged to normal polarity (negative polarity), and transferred to the photosensitive drum 1. Also, contaminants on the surface layer 2d (fine powder toner, external additives, etc.) are removed.

(Electrostatic image forming means)
The exposure device 3 is a laser beam scanner using a semiconductor laser, and forms an electrostatic image on the surface of the charged photosensitive drum 1.

  The exposure device 3 outputs a laser beam modulated in response to an image signal sent from a host process such as an image reading device (not shown), and exposes the uniformly charged surface of the photosensitive drum 1 to an exposure position. Laser scanning exposure L (image exposure) is performed at b. As the potential of the portion irradiated with the laser beam by the laser scanning exposure L decreases, electrostatic images corresponding to the image information subjected to the scanning exposure are sequentially formed on the photosensitive drum 1.

(Development means)
The developing device 4 is a two-component magnetic brush developing type reversal developing device, and supplies toner to the electrostatic image formed on the photosensitive drum 1 to visualize the electrostatic image.

  The developer carrier 4b is formed of a non-magnetic material in a cylindrical shape, and is arranged rotatably in the developing container 4a with a part of the outer peripheral surface exposed to the outside.

  The developer carrying member 4b carries the two-component developer 4e which is stirred and charged by the developer stirring member 4f, and is rotated around the fixed magnetic pole 4c inserted in the developing sleeve 4b while being fixed non-rotatingly. To do.

  The developer coating blade 4d regulates the layer thickness of the two-component developer 4e carried on the developer carrier 4b.

  The toner hopper 4g stores replenishment toner and replenishes the developing container 4a.

The two-component developer 4e is a mixture of a nonmagnetic toner and a magnetic carrier. The magnetic carrier has a resistance value of about 10 ¹³ Ω · cm and an average particle size of 40 μm. The non-magnetic toner has an average particle diameter of 7 μm, and is rubbed with the magnetic carrier in the process of being stirred by the developer stirring member 4f to be charged with negative polarity of normal polarity.

  The developer carrying member 4b is disposed in close proximity to the photosensitive drum 1 while maintaining the closest distance (referred to as S-Dgap) to the photosensitive drum 1 at 350 μm. A facing portion between the photosensitive drum 1 and the developer carrier 4a is a developing portion c. The developer carrier 4b is driven to rotate in the direction opposite to the traveling direction of the photosensitive drum 1 in the developing unit c.

  A part of the two-component developer 4e in the developer container 4a is adsorbed and held as a magnetic brush layer on the outer peripheral surface of the developer carrier 4b by the magnetic force of the fixed magnetic pole 4c, and is formed into a predetermined thin layer by the developer coating blade 4d. Layered. The two-component developer 4e is rotated and conveyed to the developing unit c as the developer carrier 4b rotates, and comes into contact with the surface of the photosensitive drum 1 and rubs appropriately.

(Development power supply means)
The power source S2 applies a predetermined development voltage to the developer carrier 4b.

  In the first embodiment, the developing voltage for the developer carrier 4b is an oscillating voltage obtained by superimposing a DC voltage (Vdc) and an AC voltage (Vac).

  More specifically, it is an oscillating voltage in which a rectangular wave of AC voltage: frequency f = 7.5 kHz, peak-to-peak voltage; 1.8 kVpp is superimposed on DC voltage: −450V.

  As a result, the toner in the two-component developer 4e carried on the developer carrying member 4b and conveyed to the developing section c is selectively attached to the electrostatic image on the photosensitive drum 1 by the electric field of the developing voltage, and static The electric image is developed into a toner image. Toner adheres to the exposed bright portion of the photosensitive drum 1 and the electrostatic image is reversely developed.

(Development process)
The two-component developer 4e on the developer carrier 4b that has passed through the developing part c is returned to the developer reservoir in the developer container 4a with the subsequent rotation of the developer carrier 4b.

  The toner density in the two-component developer 4e in the developing container 4a is detected by an optical toner density sensor (not shown), and the toner hopper 4g is driven and controlled according to the detection result. By supplying the toner in the toner hopper 4g to the two-component developer 4e in the developing container 4a, the toner concentration in the two-component developer 4e in the developing container 4a is maintained within a substantially constant range. The toner replenished to the two-component developer 4e is stirred and mixed into the two-component developer 4e by the stirring member 4f.

  Transfer means / fixing means: The transfer roller 5 is brought into pressure contact with the photosensitive drum 1 with a predetermined pressing force, and the pressure nip portion is a transfer portion d. The recording material P is fed at a predetermined control timing from a paper feeding mechanism unit (not shown) to the transfer unit d.

  The recording material P fed to the transfer part d is nipped and conveyed between the rotating photosensitive drum 1 and the transfer roller 5. Meanwhile, a positive transfer voltage (+2 kV in the first embodiment) having a polarity opposite to the normal polarity of the toner is applied to the transfer roller 5 from the power source S3, so that the recording material P conveyed through the transfer portion d is supplied. The toner images carried on the photosensitive drum 1 are sequentially transferred onto the surface.

  The recording material P, onto which the toner image has been transferred through the transfer portion d, is sequentially separated from the photosensitive drum 1 and conveyed to the fixing device 6 (heat roller fixing device). Output as (print, copy).

<Simultaneous development cleaning>
The image forming apparatus 100 employs a cleaner-less system in which a slight amount of residual toner remaining on the surface of the photosensitive drum 1 that has passed through the transfer portion d is reused by the development device 4 performing simultaneous development cleaning. Therefore, a dedicated cleaning device for removing the transfer residual toner is not provided on the downstream side of the transfer portion d.

  The untransferred toner that has passed through the transfer portion d is conveyed to the developing portion c through the charging portion a and the dew-rear portion b as the photosensitive drum 1 continues to rotate, and is simultaneously cleaned by the developing device 4.

  In the first embodiment, the developer carrier 4b of the developing device 4 is rotated in the direction opposite to the traveling direction of the surface of the photosensitive drum 1 in the developing unit c. It is advantageous for the recovery.

  Since the untransferred toner on the photosensitive drum 1 passes through the exposure part b, the exposure process is performed on the untransferred toner. However, since the amount of the untransferred toner is small, no significant influence appears.

  However, in order to effectively perform simultaneous development cleaning by the developing device 4, the transfer residual toner carried to the developing unit c is charged to a normal polarity, and the charge amount is within a range suitable for development by the developing device 4. It needs to be adjusted.

  A toner whose charging polarity is reversed to a positive polarity or a toner whose charging amount is insufficient cannot be recovered from the photosensitive drum 1 to the developing device 4 and is transferred to a recording material, causing a defective image.

  Therefore, a toner uniformizing device (toner uniforming member 8) is provided downstream of the transfer portion d in the rotation direction of the photosensitive drum 1, and the adhesion density and charge amount of the transfer residual toner on the photosensitive drum 1 are uniformized. Yes.

  A positive DC voltage having a polarity opposite to that of the charging roller 2 is applied to the toner uniformizing member 8 from a power source S5. Specifically, a DC voltage of +300 V is variably applied to the toner uniformizing member 8 during image formation.

  Further, a toner charging device (toner charging member 7) is provided downstream of the toner uniformizing member 8 in the rotation direction of the photosensitive drum 1, and the transfer residual toner dispersed and discharged by the toner uniforming member 8 has a normal polarity. Is negatively charged.

  A negative DC voltage having the same polarity as that of the charging roller 2 is applied to the toner charging member 7 from a power source S4. Specifically, a DC voltage of −600 V is variably applied to the toner charging member 7 during image formation.

  In general, the untransferred toner remaining on the photosensitive drum 1 without being transferred to the recording material P at the transfer portion d is mixed with reverse toner charged to the opposite polarity. For this reason, the transfer residual toner is discharged once by being brought into contact with the toner equalizing member 8 and then charged with the normal polarity to the transfer remaining toner by being brought into contact with the toner charging member 7, whereby the transfer in the developing device 4 is performed. The removal and collection of residual toner is ensured. Therefore, the occurrence of a ghost image formed by transferring the transfer residual toner image onto the recording material is strictly prevented.

  The toner uniformizing member 8 and the toner charging member 7 are constituted by brush members having appropriate conductivity, and are arranged with the brush tips contacting the photosensitive drum 1.

  More specifically, the brush member is obtained by adjusting a resistance value by dispersing a resistance adjusting agent such as carbon or metal powder in fibers such as rayon, acrylic, and polyester.

In the brush member, the thickness of one fiber is preferably 30 denier or less and the flocking density is preferably 7750 to 77500 / cm ² (50,000 to 500,000 / inch ² ) or more.

In the first embodiment, the brush fiber thickness is 6 denier, the flocking density is 15500 / cm ² (100,000 / inch ² ), the length from the fixed end to the free end of the fiber is 5 mm, and the resistance value of the brush is 5 × 10 ^4. A brush member of Ω · cm was used.

  The contact width (length in the sub-scanning direction) of the brush member at the contact portion f of the toner uniformizing member 8 and the photosensitive drum 1 and the contact portion e of the toner charging member 7 and the photosensitive drum 1 is 5 mm, respectively. The amount of penetration into each is 1 mm.

  The toner uniformizing member 8 and the toner charging member 7 are driven by a driving mechanism (not shown) to reciprocate in the longitudinal direction of the photosensitive drum 1 with an amplitude of 2.5 mm and a frequency of 2.0 Hz.

  The untransferred toner remaining on the photosensitive drum 1 after escaping to the recording material P at the transfer portion d is conveyed to the contact portion f between the toner uniformizing member 8 and the photosensitive drum 1 and contacts the toner uniformizing member 8. The charge amount is made uniform.

  The transfer residual toner on the photosensitive drum 1 that has been made uniform by the toner uniformizing member 8 is conveyed to the contact portion e between the toner charging member 7 and the photosensitive drum 1, and contacts the toner charging member 7 to have a negative polarity with normal polarity. They are charged together.

  The developing device 4 is a cleaner-less system that cleans the untransferred toner by proceeding simultaneously with the development of the toner image on the photosensitive drum 1.

  Since the charge amount of the transfer residual toner is controlled to an appropriate amount using the toner charging member 7, the charge amount of the transfer residual toner becomes substantially equal to the toner charge amount at the time of development, and the transfer residual toner on the photosensitive drum 1 is developed. It is fully collected in the device 4. The toner charging member 7 controls the charge amount of the transfer residual toner to an appropriate charge amount in which the developing device 4 can develop the electrostatic latent image on the photosensitive drum. Is made efficient.

  However, even if the toner uniformizing member 8 and the toner charging member 7 are arranged, a very small amount of toner reaches the charging roller 2 as the reversal toner and responds to the electric field of the charging roller 2 to the charging roller 2. Adhere to. In some cases, a part of the toner adhering to the charging roller 2 is changed to a reversal toner on the surface of the charging roller 2.

  In any case, the reversal toner adheres to the surface of the charging roller 2, and the reversal toner gradually accumulates as the number of image formations accumulates to contaminate the surface of the charging roller 2. The accumulated reversal toner prevents the charging roller 2 from charging the photosensitive drum 1 to a desired potential, causing the photosensitive drum 1 to be poorly charged and causing a defective image such as fogging.

  The phenomenon that the reversal toner adheres to the charging roller 2 becomes more prominent as the reversal toner generated in image formation increases. As the cumulative number of images formed by the image forming apparatus 100 increases, more specifically, the cumulative usage time of the two-component developer 4e stored in the developing device 4 increases. This is because, when the two-component developer 4e is continuously stirred for a long period of time, the charge imparting ability of the magnetic carrier in the two-component developer 4e to the non-magnetic toner is lowered, and the toner replenished from the hopper 4g is sufficiently supplied. This is because it becomes impossible to charge up to the charge amount.

  Therefore, in the image forming apparatus 100, the reversal toner accumulated on the charging roller 2 is discharged to the photosensitive drum 1 at the time of non-image formation to restore the charging processing capability of the photosensitive drum 1 by the charging roller 2.

<Control means>
FIG. 3 is a timing chart of the sheet interval control when the charging roller cleaning control is not performed, and FIG. 4 is a timing chart of the sheet interval control when the charging roller cleaning control is performed. FIG. 5 is an explanatory diagram of the relationship between the frequency of the AC voltage superimposed on the charging voltage and the charging roller cleaning effect.

  As shown in FIG. 1, the control unit 10 controls the power sources S1, S2, and S4 to execute charging roller cleaning control during continuous image formation (so-called paper interval).

  As shown in FIG. 3 with reference to FIG. 1, when the charging roller cleaning control is not performed, the AC voltage and DC voltage applied to the charging roller 2, and the AC voltage and DC voltage applied to the developer carrier 4b. Is the same between papers during image formation and non-image formation. The direct current voltage applied to the toner uniformizing member 8 and the direct current voltage applied to the toner charging member 7 are not changed between the sheets during image formation and non-image formation.

  As shown in FIG. 4 with reference to FIG. 1, in the charging roller cleaning control, an AC voltage (frequency) and a DC voltage applied to the charging roller 2 and a DC voltage applied to the developer carrier 4b are used for image formation. It is changed between the time and the sheet interval during non-image formation. The DC voltage applied to the toner charging member 7 is also changed between the paper during image formation and during non-image formation.

  The controller 10 increases the frequency of the AC voltage output from the power source S1 from the frequency f = 1.5 kHz during image formation to f = 15 kHz during non-image formation, thereby facilitating the separation of the toner from the charging roller 2. ing.

  As shown in FIG. 5, the charging roller cleaning shown in FIG. 4 was experimented by varying the frequency of the AC voltage applied to the charging roller 2 to determine the amount of discharge residual toner discharged from the surface of the charging roller 2. Was measured.

  In FIG. 5, the horizontal axis indicates the frequency of the AC voltage applied to the charging roller 2. The vertical axis represents the residual rate of the transfer residual toner remaining on the surface of the charging roller 2 after the AC roller voltage of each frequency is applied and the charging roller cleaning control is performed for a predetermined time. A lower remaining rate indicates that a larger amount of untransferred toner is discharged from the surface of the charging roller 2. If no untransferred toner is discharged, the remaining rate becomes 100%, and all the remaining transfer toners are discharged. When spilled out, the remaining rate becomes 0%.

The method for measuring the residual rate of the transfer residual toner was as follows.
(1) The transfer residual toner adhering to the charging roller 2 before carrying out the charging roller cleaning control was magnified 10 times with a digital microscope (VK8500 manufactured by Keyence Corporation) to capture an image.
(2) The captured enlarged image was subjected to a binarization process using image processing software (Mitani Shoji Co., Ltd. WinRoof), and the adhesion area of the transfer residual toner was calculated.
(3) Next, the transfer residual toner attached on the charging roller after the charging roller cleaning control is calculated in the same manner as described above, and the transfer residual toner after the control is performed on the transfer residual toner area before the control is performed. The area ratio was calculated.

  As shown in FIG. 5, when the frequency of the alternating voltage is around 1.5 kHz, which is normal, almost no transfer residual toner is discharged from the surface of the charging roller 2. However, if the frequency of the AC voltage is 10 kHz or more, the transfer residual toner is sufficiently discharged from the surface of the charging roller 2.

  Therefore, in the first embodiment, the frequency of the AC voltage is switched to 15 kHz so that the untransferred toner can be peeled off more stably from the surface of the charging roller 2.

  Here, if the frequency applied to the charging roller 2 is sufficiently high even during image formation, there is no need to bother switching between sheets. However, since the charging roller 2 is in contact with the photosensitive drum 1, the vibration of the charging roller 2 propagates to the photosensitive drum 1 and adversely affects the toner image developed by the developing device 4. There is a possibility that toner scattering increases. For this reason, in the first embodiment, during image formation, the conventional frequency is set to 1.5 kHz, and only the interval between sheets during non-image formation is switched to 15 kHz.

<Separation of toner particles by AC voltage>
6 is an explanatory diagram of the binding force acting on the toner attached to the charging roller, FIG. 7 is an explanatory diagram of the separating force acting on the toner attached to the charging roller, and FIG. 8 is the action of the AC voltage on the toner attached to the charging roller. It is explanatory drawing of.

  As the adhesion force of particles with charge such as toner, electrostatic adhesion force represented by mirror force and non-electrostatic force represented by van der Waals force and liquid bridge force It is roughly divided into two types: general adhesive force. The surface of the charging roller 2, the surface of the toner, and the surface of the photosensitive drum 1 are sufficiently hydrophobized, and the liquid cross-linking force is almost negligible. Van der Waals forces are dominant.

  As shown in FIG. 6A, the transfer residual toner that is nearly spherical has only a point contact with the surface of the charging roller 2 at several points, so that the true contact area with the surface of the charging roller 2 is very large. Small, van der Waals forces are usually negligible.

  As shown in FIG. 6B, however, the untransferred toner adhering to the surface of the charging roller 2 passes through the contact portion (charging portion a) with the photosensitive drum 1 after being attached many times and is flattened. Has been. For this reason, the untransferred toner adhering to the surface of the charging roller 2 greatly increases the real contact area with the surface of the charging roller 2 and increases to a level at which Van der Waals force cannot be ignored.

  Accordingly, even if a potential difference is provided between the charging roller 2 and the photosensitive drum 1 and a Coulomb force is applied to the transfer residual toner, it is difficult to separate the transfer residual toner from the surface of the charging roller 2.

  As shown in FIG. 7A, when the real contact area between the transfer residual toner and the surface of the charging roller 2 is very small, non-electrostatic adhesion force, specifically, Van der Waals force (Fv) ) Can be ignored. Therefore, by adjusting the potential difference between the surface of the charging roller 2 and the surface of the photosensitive drum 1 with respect to the reflection force (Fq) of the transfer residual toner, the Coulomb force (Fe) acting on the transfer residual toner is sufficiently increased. Then, it is possible to peel off the transfer residual toner from the surface of the charging roller 2.

  As shown in FIG. 7B, when the real contact area between the transfer residual toner and the surface of the charging roller 2 is very large, the van der Waals force (Fv) is significantly increased. For this reason, it is necessary to sufficiently increase the Coulomb force (Fe) with respect to the sum of the Van der Waals force (Fv) and the reflection force (Fq) of the transfer residual toner. It is necessary to increase the potential difference on the surface of the drum 1.

  However, if the potential difference between the surface of the charging roller 2 and the surface of the photosensitive drum 1 is increased, excessive discharge may occur near the contact portion (charging portion a) between the charging roller 2 and the photosensitive drum 1 to damage the photosensitive drum 1. There is. For this reason, a sufficient potential difference cannot be provided.

  Therefore, as shown in FIG. 4, the frequency of the alternating voltage (charging AC) superimposed on the charging voltage is increased from 1.5 kHz during image formation to 15 kHz during charging roller cleaning control to improve the separation. . The reason why the charging roller cleaning effect is enhanced by increasing the frequency of the AC voltage will be described below.

  As shown in FIG. 8A, when the charging roller 2 is vibrated by an AC voltage, the transfer residual toner attached to the surface of the charging roller 2 has an adhesion force and a peeling force.

  The adhesion force of the transfer residual toner to the surface of the charging roller 2 includes a reflection force (Fq) that is an electrostatic adhesion force, and a van der Waals force (fv) that is a non-electrostatic adhesion force. Is working.

  In the formula (2), A represents a Hammer constant determined by the substance, Z represents a separation distance between the transfer residual toner and the surface of the charging roller 2, and d represents a particle size of the transfer residual toner.

  Here, the reflection force (Fq) is a force acting on one transfer residual toner, but the Van der Waals force (fv) is exerted between each point on the transfer residual toner surface and the surface of the charging roller 2. It is power. Therefore, the van der Waals force (Fv) acting on one transfer residual toner can be expressed as the sum of van der Waals forces (fv) at each point.

  As shown in Expression (2), the Van der Waals force (Fv) greatly depends on the distance (separation distance Z) between the surface of the charging roller 2 and each point on the surface of the transfer residual toner. Therefore, the van der Waals force (Fv) increases as the actual contact area between the surface of the charging roller 2 and the transfer residual toner increases.

  On the other hand, the following forces act as the peeling force of the transfer residual toner from the surface of the charging roller 2. That is, the Coulomb force (Fe) due to the electric field (E) formed by the potential difference between the surface of the charging roller 2 and the surface of the photosensitive drum 1 and the vibration driving force (Fω) that works by applying vibration due to the AC voltage to the charging roller 2. ) And act. The vibration driving force (Fω) is a force acting on one transfer residual toner.

  In Expression (3), q represents the charge amount of one transfer residual toner. In Equation (4), m is the mass of one transfer residual toner, a is the vibration amplitude acting on the transfer residual toner, and ω is the vibration frequency acting on the transfer residual toner.

  When the Van der Waals force (Fv) in Formula (5) is large, the transfer residual toner is removed from the surface of the charging roller 2 unless the potential difference (electric field) between the surface of the charging roller 2 and the surface of the photosensitive drum 1 is sufficiently increased. Can not be peeled from. It is necessary to increase the potential difference so that a Coulomb force (Fe) larger than the sum of the reflection force (Fq) and van der Waals force (Fv) acts on the transfer residual toner.

  However, if the potential difference is increased, the discharge between the surface of the charging roller 2 and the surface of the photosensitive drum 1 cannot be controlled, and the surface of the photosensitive drum 1 or the charging roller 2 may be locally damaged. .

  Here, the function of the vibration driving force (Fω) is important. It can be seen from Equation (4) that the vibration driving force (Fω) increases by increasing the frequency of the AC voltage applied to the charging roller 2. When the vibration driving force (Fω) acts on the transfer residual toner on the surface of the charging roller 2, the transfer residual toner vibrates on the surface of the charging roller 2.

  As shown in FIG. 7B, at this time, the distance (separation distance Z) between the surface of the charging roller 2 and each surface of the transfer residual toner also increases, and therefore the van der Waals force (Fv). Becomes extremely small and almost negligible.

  As shown in the equation (4), the vibration driving force (Fω) greatly depends on the applied vibration frequency ω, and generally has a frequency higher than the high frequency side (specifically, 10 kHz or higher) of the audio frequency range (more than 20 kHz is generally higher). Known as working with ultrasound).

  As described above, by increasing the frequency of the AC voltage applied to the charging roller 2 and applying the vibration driving force (Fω) to the transfer residual toner, the van der Waals force (Fv) can be reduced to an almost negligible level. . As a result, the potential difference between the surface of the charging roller 2 and the surface of the photosensitive drum 1 is adjusted so that the Coulomb force (Fe) acting on the untransferred toner becomes sufficiently larger than the reflection force (Fq) of the untransferred toner. By doing so, the transfer residual toner can be peeled off from the surface of the charging roller 2 and discharged onto the photosensitive drum 1.

  As shown in FIG. 5, since the vibration driving force (Fω) is not sufficient until the frequency is around 1 kHz, almost no transfer residual toner is discharged from the surface of the charging roller 2.

  However, the discharge amount of the transfer residual toner varies greatly between 1 kHz and 10 kHz, and the transfer residual toner is sufficiently discharged from the surface of the charging roller 2 at 10 kHz or more.

  Therefore, in order to sufficiently obtain the vibration driving force (Fω) for peeling off the transfer residual toner from the surface of the charging roller 2, it is necessary to set the frequency of the AC voltage to 10 kHz or more. In the first embodiment, 15 kHz is selected.

<Voltage control other than AC voltage>
FIG. 9 is an explanatory diagram of the effect of switching the DC voltage applied to the charging roller, and FIG. 10 is an explanatory diagram of the DC voltage applied to the toner charging member.

  Next, switching of the DC voltage applied to the charging roller 2, the DC voltage applied to the toner charging member 7, and the DC voltage applied to the developer carrier 4b will be described.

  As shown in FIG. 4 with reference to FIG. 1, in the charging roller cleaning control, the DC voltage applied to the charging roller 2 is changed from −600 V to 0 V (off) in synchronization with the timing of switching the frequency of the AC voltage (between sheets). ). In addition, the DC voltage applied to the toner charge amount member 7 is changed from −600 V to −900 V and the DC voltage applied to the developer carrying member 4 b is changed from −450 V to 0 V with a time difference according to the rotation of the photosensitive drum 1. Switch.

  The reason why the DC voltage applied to the charging roller 2 is switched is that the transfer residual toner having a charge of mainly opposite polarity attached to the surface of the charging roller 2 is peeled off from the surface of the charging roller 2 to the photosensitive drum 1 in a coulomb direction. This is because force (Fe) is applied.

  As shown in FIG. 9A, the surface potential of the charging roller 2 to which a DC voltage of −600 V is applied during image formation is −600 V. On the other hand, the surface potential of the photosensitive drum 1 converges at about −580 V because a slight loss of follow-up occurs when charging is performed by the charging roller 2 having a surface potential of −600 V. As a result, there is a slight potential difference of about 20 V at the contact portion (charging portion a) between the charging roller 2 and the photosensitive drum 1, and the positive charge contained in the transfer residual toner on the surface of the photosensitive drum 1 is transferred. The toner particles are transferred to the surface of the charging roller 2.

  As shown in FIG. 9B, during the charging roller cleaning control, the DC voltage applied to the charging roller 2 is set to 0V, and the DC voltage applied to the toner charging member 7 is set to −900V. As a result, a potential difference of about 600 V is generated between the charging roller 2 and the photosensitive drum 1 (charging portion a: FIG. 1) in the opposite direction to that during image formation.

  Accordingly, contrary to the time of image formation, a relatively large Coulomb force (Fe) acts on the transfer residual toner attached to the surface of the charging roller 2, and the transfer residual toner attached to the surface of the charging roller 2 becomes photosensitive drum. It is exhaled to the surface of 1.

  As shown in FIG. 10 with reference to FIG. 1, the toner charging member 7 contacts and charges the surface of the photosensitive drum 1 that rubs when the toner particles adhering to the surface of the photosensitive drum 1 are charged to normal polarity. . The surface potential of the photosensitive drum 1 charged by the toner charging member 7 varies depending on the DC voltage applied to the toner charging member 7 and the use environment (temperature and humidity) in which the image forming apparatus 100 is used. This is mainly because the resistance of the brush member forming the toner charging member 7 varies depending on the use environment.

  The control unit 10 records the relationship shown in FIG. 10 and applies an optimum DC voltage to the toner charging member 7 according to the use environment. When the DC voltage applied to the toner charging member 7 is set to -900 V during the charging roller cleaning control, the surface of the photosensitive drum 1 is charged to about -600 V in an environment of 23 degrees C and 50% relative humidity. As a result, a −600 V Coulomb force (Fe) is applied to the positively charged toner particles adhering to the surface of the charging roller 2 to strongly separate it from the surface of the charging roller 2.

  The control unit 10 switches the DC voltage applied to the developer carrier 4b from −450V to 0V while the region of the photosensitive drum 1 subjected to the charging roller cleaning control passes. This is to prevent the toner charged to the normal polarity (negative) and carried on the developer carrier 4b from being transferred to the surface of the photosensitive drum 1 charged to approximately 0 V by the charging roller 2.

<Effect of charging roller cleaning control>
FIG. 11 is an explanatory diagram of the effect of the charging roller cleaning control.

  As shown in FIG. 1, in the charging roller cleaning control, the vibration driving force (Fω) is applied to the untransferred toner by setting the frequency of the AC voltage applied to the charging roller 2 to 15 kHz at the sheet interval timing. This reduces the van der Waals force (Fv) to a negligible level.

  In addition, the DC voltage applied to the charging roller 2 and the toner charging member 7 is switched at the timing between sheets, and the potential difference at the contact portion (charging portion a) between the charging roller 2 and the photosensitive drum 1 is reversed and sufficient from the time of image formation. Make it bigger. As a result, a Coulomb force (Fe) larger than the mirror force (Fq) acting between the transfer residual toner and the charging roller 2 is applied to the transfer residual toner, so that the surface of the charging roller 2 is transferred to the surface of the photosensitive drum 1. The transfer residual toner is discharged.

  That is, by performing charging roller cleaning control, the transfer residual toner does not adhere to or contaminate the surface of the charging roller 2 even in the image forming apparatus 100 adopting the cleaner-less method.

  Experiments were performed on continuous image formation by the image forming apparatus 100 when the charging roller cleaning control is not performed as shown in FIG. 3 and when the charging roller cleaning control is performed every sheet interval as shown in FIG.

  As shown in FIG. 11, when charging roller cleaning control is not performed, image defects start to be observed on 30000 sheets due to charging failure, but this can be prevented by performing charging roller cleaning control.

  Accordingly, it is possible to prevent image defects such as charging failure, fogging, and ghosting, and to form a stable image without image defects over a long period of time.

  In the first embodiment, the charging roller cleaning control is performed at the timing between successive sheets of image formation. However, the charging roller cleaning control with the same voltage control may be performed at other timings. It can be performed at a timing such as a post-processing operation after completion of image formation or a sheet interval after a predetermined number of image formations.

  Further, the image forming conditions may be recorded by a recording device provided in the image forming apparatus 100, and the charging roller cleaning control may be performed at a timing when a predetermined threshold is reached.

Second Embodiment
In the first embodiment, the frequency of the AC voltage applied to the charging roller 2 is changed, but in the second embodiment, in addition to the frequency of the AC voltage, the peak-to-peak voltage (Vpp) of the AC voltage is increased. The charging roller cleaning control is executed.

  The reason for this is that, as shown in the equation (4), the vibration driving force (Fω) is also affected by the vibration amplitude a, so that the vibration driving force (Fω) increases as the peak-to-peak voltage (Vpp) increases. Thus, the efficiency of separating the toner particles from the charging roller 2 is increased.

<Third Embodiment>
In the first embodiment, the removal of toner particles having positive charges attached to the charging roller 2 has been described. In the third embodiment, the removal of toner particles having negative charges attached to the charging roller 2 is removed. Do. This is because the toner particles cannot be removed from the charging roller 2 by the control shown in FIG. 3 if the charge is small even if it has a negative charge.

  At this time, the frequency of the AC voltage is switched from 1.5 kHz at the time of image formation to 15 kHz, and the DC voltage applied to the toner charging member 7 is variably applied from −600 V to −300 V at the time of image formation. As a result, a large Coulomb force (Fe) from the charging roller 2 toward the photosensitive drum 1 is applied to toner particles having negative charges attached to the surface of the charging roller 2.

  In the image forming apparatus 100, since the transfer residual toner is collected by the developing device and reused for developing the electrostatic latent image in the subsequent steps, the waste toner is eliminated, and the trouble is reduced during maintenance. Can do. Further, the cleaner-less configuration is advantageous for downsizing the image forming apparatus.

<Unnecessary toner>
The charging device is not a non-contact charging method using a corona discharge phenomenon such as a scorotron charger, but a conductive charging member (mainly a charging roller using a conductive roller) is used as an image on a photoconductor or the like. It is in direct contact with the carrier. For this reason, a high-capacity high-voltage power supply is not required, which is advantageous in terms of cost and apparatus miniaturization, and generation of ozone due to discharge is suppressed to a very small amount as compared with the corona charging method.

  However, in the cleanerless system, the toner in which the charging polarity present in the transfer residual toner is reversed to the polarity opposite to the normal polarity is likely to adhere to and accumulate on the charging member.

  This is because if the charge polarity of the transfer residual toner on the photosensitive member that passes through the charging member and reaches the developing device is normal, and the charge amount is not the toner charge amount that can be recovered from the photosensitive member by the developing device, This is because the transfer residual toner on the body cannot be removed / collected by the developing device.

  The reason why the toner whose charging polarity is reversed to the reverse polarity from the normal polarity is generated in the transfer residual toner is that there are two toner particles whose charging polarity is originally reversed to the opposite polarity although the amount is small. It may be mixed in the component developer. Further, even when the toner has a regular charge polarity, the charge polarity may be reversed due to the influence of a transfer bias, peeling discharge, or the like.

  In addition, when forming an image with a high printing rate such as a photographic image, the absolute amount of transfer residual toner increases, and the toner equalizing member and the toner charging member are damped, and the resistance value of the contact portion with the image carrier increases. As a result, the function of the toner uniformizing device and the toner charging device is lowered. As a result, there is a problem that the transfer residual toner is unpatterned, the charging process to the transfer residual toner is insufficient, the reversal toner increases, and the reverse transfer residual toner adheres to or contaminates the surface of the charging member. To do.

  If the transfer residual toner adheres to the surface of the charging member, the charging unit cannot charge the surface potential of the photosensitive member to a desired value, which may cause a vicious circle inducing a new fog in the developing device.

  Further, this problem tends to become worse as the amount of use of the image forming apparatus, specifically, the stirring time of the two-component developer in the developing apparatus is accumulated. This is because the charge imparting ability of the magnetic carrier constituting the two-component developer to the toner decreases with the amount of use, and the newly replenished toner is not sufficiently stirred and mixed in the developing device. This is because the development is performed without reaching the polarity and the charge amount.

  As described above, the image forming apparatus 100 includes the photosensitive member, the charging device, the exposure device, the developing device, the transfer device, and the charging auxiliary member (toner charging member). The transfer residual toner remaining on the body surface is collected by a developing device. In such a cleaner-less system, for example, the frequency of the AC voltage applied to the charging device is increased at an arbitrary timing after image formation, such as between sheets. At the same time, the DC voltage applied to the charging member and the charging auxiliary member is variably controlled so that a force (Coulomb force) to peel off from the charging member acts mainly on the transfer residual toner having the opposite polarity mainly attached to the charging member.

  Accordingly, the transfer residual toner having the reverse polarity attached to the surface of the charging member is periodically discharged to the surface of the photosensitive member, thereby preventing the toner having the reverse polarity from adhering and accumulating on the charging member.

  Stable image formation over a long period of time by preventing the occurrence of poor charging, fogging, ghosting, etc. caused by the opposite polarity toner contained in the transfer residual toner adhering to or contaminating the surface of the charging member It becomes possible.

It is explanatory drawing of a structure of the image forming apparatus of 1st Embodiment. It is a schematic diagram of a layer structure of a photosensitive drum and a charging roller. 6 is a timing chart of sheet interval control when charging roller cleaning control is not performed. 6 is a timing chart of sheet interval control when charging roller cleaning control is performed. It is explanatory drawing of the relationship between the frequency of the alternating voltage superimposed on the charging voltage, and the charging roller cleaning effect. FIG. 6 is an explanatory diagram of a binding force that acts on toner attached to a charging roller. FIG. 6 is an explanatory diagram of a separation force that acts on toner attached to a charging roller. It is explanatory drawing of the effect | action of the alternating voltage with respect to the toner adhering to the charging roller. It is explanatory drawing of the effect which switches the DC voltage applied to a charging roller. FIG. 6 is an explanatory diagram of a DC voltage applied to a toner charging member. It is explanatory drawing of the effect of charging roller cleaning control.

Explanation of symbols

1 Image carrier (photosensitive drum)
2 Charging member (charging roller)
3 Electrostatic image forming means (exposure device)
4 Development means (developing device)
4b developer carrier 4f stirring means (developer stirring member)
5 Transfer roller 6 Fixing device 7 Toner charging device (toner charging member)
8 Toner Uniform Device (Toner Uniform Member)
10 Control means (control unit)
100 Image forming apparatus S1 Charging power supply means (power supply)
S2 Development power supply means (power supply)

Claims (7)

An image carrier;
A charging member that rotates in contact with the image carrier;
Charging power source means for charging the image carrier by applying a charging voltage in which an AC voltage is superimposed on a DC voltage to the charging member;
Electrostatic image forming means for forming an electrostatic image on the charged image carrier;
An image forming apparatus comprising: a developing unit that supplies the charged toner to the image carrier and develops the electrostatic image into a toner image;
At the time of non-image formation, a potential difference is formed between the charging member and the image carrier so that the toner attached to the charging member moves to the image carrier, and the charging power source means is controlled, An image forming apparatus comprising control means for applying an AC voltage having a frequency higher than that during image formation to the charging member.   2. The image forming apparatus according to claim 1, wherein the control unit superimposes a DC voltage lower than that at the time of image formation on the AC voltage of the high frequency and applies the superimposed voltage to the charging member. The developing unit includes a stirring unit that stirs and charges the two-component developer, a developer carrier that rotates while supporting the charged two-component developer, and a direct current in which an AC voltage is superimposed on the developer carrier. Development power supply means for applying a voltage, and reversal development of the electrostatic image, and at the same time removing the transfer residual toner of the image carrier,
The control means controls the developing power supply means so that the area of the image carrier that is in contact with the charging member to which the high-frequency AC voltage is applied passes through the developer carrier, while forming an image. The image forming apparatus according to claim 2, wherein a lower DC voltage is applied to the developer carrier. A toner charging device that applies a voltage to a toner charging member disposed between a transfer unit that transfers the toner image to a transfer medium and the charging member to charge the toner on the surface of the image carrier to a normal polarity; ,
The control means applies a higher DC voltage to the toner charging member than during image formation while the area of the image carrier that is in contact with the charging member to which the high frequency AC voltage is applied passes. The image forming apparatus according to claim 3, wherein:   The image forming apparatus according to claim 1, wherein a frequency of the high-frequency AC voltage is 10 kHz or more. A toner rubbing member that rubs the toner adhering to the charging member and charges the toner to a normal polarity;
While applying a high frequency AC voltage to the charging member, a DC electric field is applied between the charging member and the image carrier so that the toner charged to the normal polarity is urged toward the image carrier. The image forming apparatus according to claim 1, wherein the image forming apparatus is formed in between.   The image forming apparatus according to claim 1, wherein a peak-to-peak voltage of the high-frequency AC voltage is greater than a peak-to-peak voltage of the AC voltage applied during image formation.

Images