JP4523354B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a step of charging an image carrier such as an electrophotographic photosensitive member or an electrostatic recording dielectric using a charging roller, and more specifically, an external additive is irregular on the surface of the charging roller. The present invention relates to an image forming apparatus capable of preventing the occurrence of an image defect due to adhesion to an image.

  In recent years, in an image forming apparatus such as a copying machine, a laser beam printer, or a facsimile, a charging member to which a charging bias voltage is applied is brought into contact with an image carrier such as an electrophotographic photosensitive member or an electrostatic recording dielectric to be charged. The contact charging method has been put into practical use. In addition, as a charging member using a contact charging method, a charging roller using conductive or semiconductive rubber is widely employed from the viewpoint of uniformity and stability of charging.

  However, when a toner containing an external additive such as titanium oxide particles or silica particles is used as a developer, the external additive having a low electric resistance is not transferred to the transfer material and is transferred to the surface of the image carrier. Further, the toner may remain, and may pass through a cleaning unit such as a cleaning blade and move to the charging position of the image carrier, and may eventually move to the surface of the charging roller. Since the external additive remaining on the surface of the image carrier depends on the pattern of the formed image, the external additive adheres in a streak pattern along the circumferential direction of the charging roller. Therefore, a difference occurs in the rotation axis direction of the charging roller.

Also, when the external additive adheres in a streak pattern along the circumferential direction of the charging roller, and when the electrical resistance of the charging roller increases significantly due to deterioration of the rubber or changes in the usage environment of the image forming apparatus A phenomenon occurs in which the external additive migrates again to the surface of the image carrier. In such a case, there arises a problem that black streaks appear in the formed image depending on the portion to which the external additive is attached.
On the other hand, in the image forming apparatus described in Patent Document 1, a so-called transfer residual toner that remains on the surface of the photosensitive drum without being transferred onto the transfer target and eventually adheres to the surface of the charging roller. By adjusting the charging bias applied to the charging roller, the toner is discharged onto the photosensitive drum.
JP 2003-307914 A

  However, as described above, the phenomenon that the external additive adheres to the surface of the charging roller is a phenomenon that occurs locally depending on the pattern of the formed image, and does not occur uniformly in the rotation axis direction of the charging roller. . Therefore, even if the charging bias voltage applied to the charging roller is controlled as in the image forming apparatus described in Patent Document 1, the charging bias voltage is applied substantially uniformly in the rotation axis direction of the charging roller. Therefore, it is difficult to completely remove the locally added external additive from the surface of the charging roller.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of image defects due to the toner external additive adhering to the surface of the charging roller.

In order to achieve the above object, an image forming apparatus according to the present invention includes:
In an image forming apparatus comprising a drum-shaped image carrier, a charging roller, a developing device, and a transfer roller,
Toner band forming means for supplying toner containing at least toner particles and an external additive from the developing unit to form a toner band parallel to the rotation axis direction of the image carrier on the surface of the image carrier; ,
Toner particle transfer means for transferring the toner particles of the toner band to the transfer roller surface using the transfer roller;
When a region of the surface of the image bearing member after the toner particles of the toner band are transferred faces the charging roller, a bias voltage is applied to the charging roller to leave the external additive remaining in the region. An external additive attaching means for attaching the surface to the surface of the charging roller;
It is characterized by providing.

  According to the image forming apparatus, at the time of non-image formation, the toner band forming means is used to supply toner from the developing device to the surface of the image carrier, and a toner band parallel to the rotation axis direction of the image carrier is formed. At the same time, the toner particle transfer means is used to transfer the toner particles contained in the toner band to the transfer roller surface, thereby forming an external additive band parallel to the rotation axis direction on the surface of the image carrier. can do.

  Further, since the external additive remaining on the surface of the image carrier is a fine particle and has a strong adhesion to the surface of the image carrier, the image carrier can be passed through a cleaning means such as a cleaning blade. It reaches the charged position of the body. Therefore, when the external additive band moves to the charging position of the image carrier, the external additive band is applied to the charging roller by applying a bias voltage to the charging roller using the external additive attaching means. It can be transferred to the surface. Thus, the external additive adheres to the surface of the charging roller in parallel with the rotation axis direction of the charging roller and uniformly in the rotation axis direction.

In general, when an external additive adheres to the surface of the charging roller, the electrical resistance locally increases at the adhered portion. Therefore, in an ordinary image carrier charging process, the image bearing member is charged at the adhered portion. Current does not flow easily, so that the charging potential on the surface of the image carrier corresponding to the corresponding portion is lowered and black streaks occur.
However, when the external additive uniformly adheres to the surface of the charging roller in the direction of the rotation axis, the entire external additive acts as a uniform resistance film, and the electrical resistance of the entire charging roller increases. However, there is no local high resistance in the direction of the rotation axis. Therefore, if the value of the charging bias voltage is appropriately controlled in the circumferential direction of the charging roller, it is possible to prevent an adverse effect on the formed image due to the adhesion of the external additive.

In addition, by forming a film made of an external additive such as titanium oxide particles on the surface of the charging roller, it is possible to prevent foreign matters other than the toner component from adhering to the surface of the charging roller.
In the image forming apparatus according to the present invention, a bias voltage applied to the charging roller when a region of the surface of the image carrier after the toner particles in the toner band are transferred faces the charging roller. A bias voltage obtained by superimposing an AC voltage on a DC voltage is preferable.

  When a DC bias voltage (DC voltage) is applied to the charging roller, the amount of external additive tends to increase at the central portion in the rotation axis direction of the charging roller due to the influence of discharge. On the other hand, when a bias voltage obtained by superimposing an AC voltage on a DC voltage (DC superimposed AC voltage) is applied to the charging roller, the electric field strength of the bias voltage becomes uniform in the direction of the rotation axis. It is possible to improve the uniformity of the adhesion amount. In addition, the effect of the AC voltage can prevent the resistance value of the charging roller from being affected by the use environment of the image forming apparatus.

  According to the image forming apparatus of the present invention, it is possible to prevent the occurrence of a locally high-resistance portion in the rotation axis direction of the charging roller, and the black streaks that occur as the local electrical resistance of the charging roller increases. It is possible to prevent image defects such as these.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing an example of an image forming apparatus of the present invention, and shows a configuration around a photosensitive drum 11 which is a drum-shaped image carrier. Around the photosensitive drum 11, a charging roller 13, an exposure device 15, a developing device 17, a transfer roller 19, a transfer paper 21, a cleaning roller 23, a cleaning blade 25, and a charge eliminating lamp 27 are disposed.

  In the image forming apparatus shown in FIG. 1, an electrostatic latent image corresponding to a formed image is formed by charging the surface of the photosensitive drum 11 using a charging roller 13 and exposing it using an exposure device 15. . Next, a toner (not shown) is supplied to the electrostatic latent image on the photosensitive drum 11 using the developing unit 17 so that the electrostatic latent image is visualized as a toner image (development). Further, a desired image is formed by transferring the toner image on the photosensitive drum 11 onto the transfer material 21 using the transfer roller 19. After the transfer process, the toner remaining on the photosensitive drum 11 is removed by the cleaning roller 23 and the cleaning blade 25, and the surface of the photosensitive drum 11 is neutralized by irradiating with a neutralizing lamp.

FIG. 2 is an explanatory view schematically showing an operation of attaching an external additive to the surface of the charging roller.
In the image forming apparatus of the present invention, a toner band parallel to the rotation axis direction of the photosensitive drum is formed by the toner band forming unit during non-image formation. That is, as shown in FIG. 2, charging of the charging roller 13 is stopped according to the circumferential length of the photosensitive drum 11 forming the toner band, and the surface potential of the photosensitive drum 11 is made substantially zero. The toner is supplied from the developing unit 17 to a portion of the surface of the photosensitive drum 11 where the surface potential becomes substantially zero, and a toner band 31 parallel to the rotation axis direction of the photosensitive drum 11 is formed on the photosensitive drum 11. Is formed.

  Next, the toner particles in the toner band formed on the surface of the photosensitive drum are transferred to the surface of the transfer roller by the toner particle transfer unit. That is, as shown in FIG. 2, when the toner band 31 formed on the surface of the photosensitive drum 11 moves to a position facing the transfer roller 19, the charging bias voltage of the charging roller 13 with respect to the transfer roller 19. As a result, a bias voltage (normal transfer bias voltage) opposite in polarity is applied, whereby the toner particles 33 in the toner band 31 and the external additive 35 released from the toner particles 33 are separated, and the toner particles 33 are separated. Moves to the transfer roller 19. At this time, the external additive 35 in the toner band 31 remains on the photosensitive drum 11.

  Next, the external additive band remaining on the surface of the photosensitive drum is transferred to the surface of the charging roller by the external additive attaching means. That is, as shown in FIG. 2, the band of the external additive 35 separated from the toner particles 33 in the toner band 31 is fine as described above and has a strong adhesion to the photosensitive drum 11. For this reason, it passes through the cleaning means such as the cleaning roller 23 and the cleaning blade 25 and moves to the charging position of the photosensitive drum 11. Therefore, when the external additive 35 band moves to the charging position of the photosensitive drum 11, the external additive 35 band is transferred to the surface of the charging roller 13 by applying a bias voltage to the charging roller 13. Can do.

  On the other hand, the toner particles 33 transferred from the toner band 31 formed on the surface of the photosensitive drum 11 to the transfer roller 19 cause the transfer material 21 for image formation (see FIG. 1) to be contaminated from the back surface. Therefore, after the operation of separating the toner particles 33 and the external additive 35 from the toner band 31 is finished, the transfer roller 19 is applied with a bias voltage having a polarity opposite to that of the normal transfer operation, and thus the transfer roller. 19 is pulled back to the photosensitive drum 11.

This operation is executed after the band of the external additive 35 has moved from the contact position between the photosensitive drum 11 and the transfer roller 19. This is because when the toner particles 33 reach the position of the cleaning roller 23 and the cleaning blade 25 together with the band of the external additive 35, the external additive 35 cannot be transferred onto the charging roller 13 as a uniform band.
Therefore, a reverse bias voltage for transferring the toner particles 33 from the toner band 31 to the transfer roller 19 and a bias voltage for transferring the toner particles transferred on the transfer roller 19 onto the photosensitive drum 11 again. The time T ₁ until the polarity is switched is such that T ₁ > L / S (L: the length of the toner band 31 along the circumferential direction of the photosensitive drum 11, S: the circumferential speed of the photosensitive drum 11). Is set.

In the present invention, the external additive of the toner is not particularly limited, and examples thereof include titanium oxide particles and silica particles. Of these, titanium oxide particles are preferable. For example, the external additive of the toner has a volume average resistance (volume resistivity) of about 10 ¹⁵ Ω · cm so that the volume average resistance (volume resistivity) of titanium oxide is about 10 ⁴ to 10 ⁸ Ω · cm. Therefore, it can be said that the toner particles are not transferred to the transfer roller by the action of the transfer bias voltage.

In the present invention, the image bearing member is not particularly limited, but a photosensitive drum having a photosensitive layer made of amorphous silicon is preferable.
The amorphous silicon (a-Si) forming the photosensitive layer is not particularly limited, and various amorphous silicons such as a-Si, a-SiC, a-SiO, and a-SiON can be mentioned. Among these, a-SiC is particularly suitable as a photosensitive layer material in the present invention because it has a high resistance and provides excellent charging ability, abrasion resistance, and environmental resistance.

About a-SiC, what has the ratio of Si and C (carbon) is a specific value is preferable. In particular, a-Si _1-x C _x (the value of x is 0.3 or more and less than 1) is preferable, and a-Si _1-x C _x (the value of x is 0.5 or more, 0.95 The following is more preferable. Such a-SiC has an extremely high electric resistance of 10 < ¹² > to 10 < ¹³ > [Omega] cm, and the flow of the electrostatic latent image is suppressed on the surface of the photosensitive drum. Also excellent in moisture resistance.

  The present invention is not limited to the above description, and various design changes can be made within the scope of the matters described in the claims.

1 is a perspective view showing an embodiment of an image forming apparatus of the present invention. It is explanatory drawing which shows typically operation which attaches an external additive to the surface of a charging roller.

Explanation of symbols

11 Photosensitive drum 13 Charging roller 17 Developer 19 Transfer roller 31 Toner belt 33 Toner particle 35 External additive

Claims (2)

In an image forming apparatus comprising a drum-shaped image carrier, a charging roller, a developing device, and a transfer roller,
Toner band forming means for supplying toner containing at least toner particles and an external additive from the developing unit to form a toner band parallel to the rotation axis direction of the image carrier on the surface of the image carrier; ,
Toner particle transfer means for transferring the toner particles of the toner band to the transfer roller surface using the transfer roller;
When a region of the surface of the image bearing member after the toner particles of the toner band are transferred faces the charging roller, a bias voltage is applied to the charging roller to leave the external additive remaining in the region. An external additive attaching means for attaching the surface to the surface of the charging roller;
An image forming apparatus comprising:   A bias voltage applied to the charging roller when a region of the surface of the image bearing member after the toner particles of the toner band transferred to the charging roller faces the charging roller is obtained by superimposing an AC voltage on a DC voltage. The image forming apparatus according to claim 1, wherein the image forming apparatus is a bias voltage.

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