JP4346994B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, a facsimile, or the like to which electrophotography is applied.

  For example, a developer image (toner image) is developed with a developer on an image carrier having an electrostatic latent image formed on the surface by electrophotography or electrostatic recording, such as a copying machine, laser beam printer, or facsimile. In recent years, an image forming apparatus for forming and transferring an image to a transfer material to form an image has been downsized. For example, in an image forming process of a general electrophotographic image forming apparatus, There is a limit to miniaturizing each process of charging, exposure, development, transfer, fixing, and cleaning. Further, the transfer residual developer (toner) remaining on the surface of the image carrier after transferring the toner image from the image carrier is often collected by the cleaner provided with a cleaner around the image carrier. The waste toner is preferably not discharged from the viewpoint of environmental protection.

  Therefore, this cleaner is removed, and the transfer residual toner on the image carrier after transfer is removed from the image carrier by "development simultaneous cleaning", and the so-called cleaner-less system is configured to collect and reuse it in the developing device. An image forming apparatus that employs a toner recycling process has appeared.

  “Development simultaneous cleaning” means, for example, in an electrophotographic image forming apparatus, the developer (toner) remaining on the image carrier after the transfer is removed during the development process after the next image forming process, that is, the image carrying process. The photosensitive member is uniformly charged, and the charged surface is exposed to form an electrostatic latent image. During development of the electrostatic latent image, the DC voltage applied to the developing device and the surface potential of the photosensitive member This is a method of collecting by the fog removal potential difference Vback which is a potential difference. According to this method, the transfer residual toner is collected by the developing device and reused after the next image forming step, so that waste toner can be eliminated and the running cost of printing can be reduced. Further, the cleanerless has a great space advantage, and the image forming apparatus can be greatly downsized.

  By the way, in the development simultaneous cleaning described above, the transfer residual toner on the image carrier is collected by a charging unit that performs a charging process for uniformly charging the image carrier before forming the electrostatic latent image, and the image is subjected to the charging process. The carrier is charged to a desired potential, and the collected toner is discharged onto the photoreceptor and collected by the developing device.

  Therefore, as the charging means, in the charging device disclosed in Patent Document 1, even when a simple member such as a charging roller or a fur brush is used as the contact charging member in the contact charging, the charging uniformity is excellent and long-term. It has become possible to realize stable direct injection charging over a short period of time, and to realize ozone-less direct injection charging with a simple configuration at a low applied voltage. In this configuration, conductive particles are present on the contact surface between the contact charging member and the surface to be charged, and there is a difference in the relative speed between the surface of the contact charging member and the surface to be charged. , To solve the problems caused by charging failure.

  Further, in Patent Document 2 and Patent Document 3, the conductive particles for promoting charging are included in the developer, that is, supplied from the developing device together with the toner. With such an easy process and a mechanical configuration, supply of conductive particles for promoting charging and a toner recycling process are realized.

  However, in the contact charging method, since the charging member is brought into contact with the member to be charged, the charging member easily picks up the deposit on the member to be charged and becomes dirty.

  Excessive contamination of the contact charging member causes charging unevenness and the like, thereby reducing charging performance. In the object to be charged and the image forming apparatus, the image carrier is charged by contact charging, and an electrostatic latent image corresponding to the target image information is formed on the charging surface of the image carrier, and the electrostatic latent image is In an image forming apparatus in which a developer image (toner image) is visualized by a developing unit and image formation is performed, as the number of times of image formation increases, toner adheres to and accumulates on the contact charging member. Go.

  In particular, when the image forming apparatus described above is a cleanerless system apparatus that does not have a cleaning device that removes the transfer residual toner from the surface of the image carrier after the toner image is transferred to the transfer material, the image carrier is used. This is more remarkable because the transfer residual toner on the body reaches the contact charging member as it is and adheres to and mixes with the charging member.

  In the case of a cleaner-less system, there is not a little, but when the charging means collects the transfer residual toner, the transfer residual toner is temporarily attached to the contact charging member. In some cases, the transfer residual toner may fall off from the contact charging member. In particular, the untransferred toner held on the contact charging member is an insulating particle having an electric charge, and therefore, a potential change occurs on the photosensitive member, and the toner is easily transferred at that portion.

Since the toner adhering to the image carrier before the charging step is located in a portion upstream of the image forming region of the created image in the image carrier rotating direction, it is a non-image region such as during image formation (between sheets). In such a case, since the development process is not performed, the image is not collected by the developing device but remains on the image carrier, which is liable to cause inconveniences such as image contamination.
Japanese Patent Laid-Open No. 10-307454 JP-A-10-307455 JP-A-10-307456

  Accordingly, an object of the present invention is to have a charging unit that charges the image carrier by contact charging. Using the charging unit, the transfer residual developer on the image carrier is recovered and discharged, and the discharged toner is discharged. In an image forming apparatus that executes a cleanerless system in which developer is collected and reused by a developing device, generation of abnormal images due to toner adhesion to charging means is avoided, and good image formation is maintained over a long period of time. With the goal.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention includes an image bearing member rotatable, a charging unit that charges the image bearing member, an exposure means for forming an electrostatic latent image on the image bearing member charged by said charging means, A developing unit that develops the electrostatic latent image formed on the image carrier , and a transfer unit that transfers the toner image developed by the developing unit from the image carrier to a transfer material. The means forms a charging nip portion with the image carrier, and the residual toner not transferred to the transfer material by the transfer means but remaining on the image carrier is upstream of the charging nip portion in the rotation direction of the image carrier. in the by transferring the charging means after being retained, to an image forming apparatus for recovering the residual toner discharged to the image carrier by subsequent to the charging means and the developing means discharging the residual toner on the image bearing member from Stomach,
And an application power source for applying a voltage to the transfer unit so that a current flowing through the transfer nip formed by the transfer unit and the image carrier is a constant current. Before and after the transfer nip, a voltage is applied to the transfer means so that the current flowing through the transfer nip portion becomes a constant current, and before and after the transfer nip portion is not transferred to the transfer material state. In the image forming apparatus , the transfer unit is electrically floated, and then a voltage is applied to the transfer unit so that the current flowing through the transfer nip portion becomes the constant current again .

According to an embodiment of the present invention, the charging unit is a charging roller that rotates in a direction opposite to a rotation direction of the image carrier on a contact surface with the image carrier.

According to another embodiment of the present invention, the developing means develops a toner image on the image carrier and simultaneously collects the collected toner from the image carrier.

According to the present invention , the potential step generated at the leading edge of the paper due to transfer can be kept within a specified range, and as a result, adhesion of transfer residual toner to the charging member can be reduced, and adverse effects in the image due to transfer can be prevented. It becomes possible. That is, when the transfer material enters the transfer nip part, the transfer means is in an electrically floating state. The toner is prevented from being discharged. Therefore, it is possible to prevent the transfer residual toner from dropping from the charging unit onto the image carrier, and it is possible to prevent image defects due to adhesion of the transfer residual toner to the image carrier.

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

Example 1
FIG. 1 shows a schematic structural model diagram of an example of an image recording apparatus according to the present invention. In this embodiment, the image recording apparatus shown in FIG. 1 is a laser printer in which a toner recycling system is implemented using an electrophotographic process, a contact charging method, and a cleanerless.

  First, an overall schematic configuration of the printer shown in FIG. 1 will be described.

  In this embodiment, a rotating drum type negative OPC photosensitive member (negative photosensitive member) 1 (hereinafter referred to as “photosensitive drum 1”) having an outer diameter of 24 mm is used as an image carrier. The photosensitive drum 1 is rotationally driven in a clockwise direction indicated by an arrow at a constant speed of 85 mm / sec (= process speed PS, printing speed).

  As the charging means, a conductive elastic roller 2 (hereinafter referred to as “charging roller 2”) as a contact charging member disposed in contact with the photosensitive drum 1 with a predetermined pressing force is used. A charging nip n is formed between the drum 1 and the drum 1. The charging roller 2 adopts an injection charging method, that is, a direct charging method, and holds (supports) conductive particles having conductivity for the purpose of promoting charging, that is, charging promoting particles m, on the outer peripheral surface thereof. In the charging nip n between the photosensitive drum 1 and the charging roller 2, the charge accelerating particles m are interposed.

  The charging roller 2 is rotationally driven in the opposite direction (counter) to the rotation direction of the photosensitive drum 1 at the charging nip n, and comes into contact with the surface of the photosensitive drum 1 with a speed difference, here, about 80% of the process speed. . Further, at the time of image formation by the printer, a predetermined charging bias is applied to the charging roller 2 from the charging bias application power source S1. As a result, the peripheral surface of the photosensitive drum 1 is uniformly contact-charged to a predetermined polarity and potential by the injection charging method.

  In the charging roller 2 of this embodiment, the switch SW of the charging bias application power source S1 is controlled to be switched to the p1 contact side during image formation by the printer, and the DC voltage of the DC power source SDC is about −500 V on the cored bar 2a of the charging roller 2. Is applied, and the surface of the photosensitive drum 1 is directly charged to a voltage of approximately −500 V, which is substantially equal to the applied DC voltage.

  An exposure apparatus (laser beam scanner) 3 including a laser diode, a polygon mirror, and the like is provided as an exposure means that is a latent image forming means. The laser beam scanner 3 outputs a laser beam whose intensity is modulated in accordance with a time-series electric digital pixel signal of target image information, and scans and exposes the uniformly charged surface of the photosensitive drum 1 with the laser beam. By this scanning exposure L, an electrostatic latent image corresponding to target image information is formed on the surface of the photosensitive drum 1.

  The electrostatic latent image on the surface of the photosensitive drum 1 is reversely developed as a developer image (toner image) at a development site a by a developing device 4 as a developing unit. The developing device 4 contains a developer t, and charging accelerator particles m are added to the developer t.

  The transfer roller 5 has a medium resistance as a contact transfer means. The transfer roller 5 is brought into contact with the photosensitive drum 1 by a predetermined pressure to form a transfer nip (transfer portion) b. A transfer material P as a recording medium is fed to the transfer portion b from a paper feed portion (not shown) at a predetermined timing, and a predetermined transfer bias voltage is applied to the transfer roller 5 from a transfer bias application power source S3. The one-side toner image is sequentially transferred onto the surface of the transfer material P fed to the transfer portion b.

  The transfer material P introduced into the transfer portion b is nipped and conveyed by the transfer nip which is the transfer portion b, and the toner image formed and supported on the surface of the photosensitive drum 1 on the surface side is sequentially pressed with electrostatic force and pressing force. Transferred by pressure.

  The transfer material P that has been fed to the transfer portion b and has received the transfer of the toner image on the photosensitive drum 1 side is separated from the surface of the photosensitive drum 1 and introduced into a fixing device 6 such as a thermal fixing system. After being fixed, it is discharged out of the apparatus as an image formed product (print, copy).

  Since the printer of this embodiment is cleanerless, residual transfer developer (transfer residual toner) remaining on the surface of the photosensitive drum 1 after the transfer of the toner image to the transfer material P is removed by a dedicated cleaner (cleaning device). Instead, as the photosensitive drum 1 rotates, it reaches the development site a via the charging nip n, and is collected and reused in the development unit 4 by simultaneous development cleaning.

  Here, the cleaning of the transfer residual toner on the photosensitive drum 1 by the cleanerless system will be described in detail.

  In the cleanerless system, the residual toner remaining on the surface of the photosensitive drum 1 after being transferred by the transfer roller 5 is conveyed to the charging nip portion n between the photosensitive drum 1 and the charging roller 2 by the movement of the surface of the photosensitive drum 1 and charged. Adheres to and mixes in.

  Here, since the toner that is a conventional developer is an insulator, the adhesion and mixing of the transfer residual toner to the charging roller 2 is a factor that causes a charging failure in charging the photosensitive drum 1.

  However, in this embodiment, since the charge accelerating particles m are present in the charging nip n between the photosensitive drum 1 and the charging roller 2, it is possible to maintain a precise contact property and contact resistance of the charging roller 2 to the photosensitive drum 1. Therefore, regardless of the contamination of the charging roller 2 due to the transfer residual toner, the ozoneless direct charging can be stably maintained for a long time with a low applied voltage, and uniform chargeability can be provided.

  Therefore, the transfer residual toner adhering to and mixed in the charging roller 2 is gradually discharged from the charging roller 2 onto the photosensitive drum 1 in the charging process, and reaches the developing portion a along with the movement of the surface of the photosensitive drum 1. At the same time, development is simultaneously cleaned (collected).

  That is, at the time of development in the image forming process, that is, subsequently, the photosensitive drum 1 is charged and exposed to form a latent image, and at the time of developing the latent image, the fog removal bias of the developing unit 4 is applied to the developing unit 4. It is recovered by the fog removal potential difference Vback, which is the potential difference between the DC voltage and the surface potential of the photoreceptor, and is reused (toner recycled).

  In the case of reversal development as in the printer of the present embodiment, this simultaneous development cleaning is performed from the dark part potential of the photosensitive drum 1 to the electric field for collecting toner to the developing unit 4 and from the developing unit 4 to the bright part potential of the photosensitive drum 1. This is done by the action of an electric field for attaching the toner.

  By the way, in the development simultaneous cleaning described above, the transfer residual toner on the photosensitive drum 1 is collected by the charging roller 2 that performs the charging process, and the photosensitive drum 1 is charged to a desired potential in the charging process, and the collected toner is removed. The toner image is discharged onto the photosensitive drum 1 and collected by the developing device 4, but the toner image is transferred to the transfer material P by leaving the charging roller 2, the developing device 4 and the transfer residual toner for carrying out the cleanerless system in this embodiment. The configuration of the transfer roller 5 in this embodiment will be described. Note that the charging unit, the developing device, and the transfer unit are not limited thereto.

  First, a charging roller 2 as a flexible contact charging member as a charging means in this embodiment is formed by forming a middle resistance layer 2b of rubber or foam on a core metal 2a, and charging uniformity. The injection charging (direct charging) is excellent and stable over a long period of time.

  That is, as a charging means, the photosensitive drum 1 as a member to be charged is disposed in pressure contact with a predetermined pressing force against elasticity, and in this embodiment, a charging nip portion n having a nip width of 2 to 3 mm is formed. Has been.

  The charging roller 2 holds (supports) conductive particles having conductivity on the outer peripheral surface thereof, that is, charging acceleration particles m, and a charging nip n between the photosensitive drum 1 and the charging roller 2. The charge accelerating particles m are interposed.

  That is, as described above, in the cleanerless system, the transfer residual toner that has passed through the transfer portion b adheres to and mixes with the charging roller 2 in the charging roller 2. In this embodiment, since the charge accelerating particles m are present in the charging nip n between the photosensitive drum 1 and the charging roller 2, it is possible to maintain a precise contact property and contact resistance of the charging roller 2 to the photosensitive drum 1. Regardless of contamination of the transfer roller 2 by the transfer residual toner, ozone-less direct charging can be stably maintained for a long time with a low applied voltage, and uniform chargeability can be provided.

  Here, the middle resistance layer 2b is formulated with a resin such as urethane, for example, conductive particles such as carbon black, a sulfurizing agent, a foaming agent, and the like, and is formed in a roller shape on the core metal 2a. After the formation of the middle resistance layer 2b, the surface is polished as necessary to produce a charging roller 2 which is a conductive elastic roller having a diameter of about 18 mm and a longitudinal length of about 220 mm.

  The roller resistance of the charging roller 2 of this example was measured and found to be 100 kΩ. The roller resistance is such that 100V is applied between the core metal 2a and the aluminum drum in a state in which the charging roller 2 is pressure-bonded to an aluminum drum having an outer diameter of 24 mm so that the core metal 2a of the charging roller 2 is loaded with a total pressure of 1 kg. , Measured.

Here, it is important that the charging roller 2 as a contact charging member functions as an electrode. In other words, it is necessary to provide a sufficient contact state with the object to be charged by providing elasticity, and at the same time to have a sufficiently low resistance for charging the object to be moved. On the other hand, it is necessary to prevent voltage leakage when there is a low breakdown voltage defect portion such as a pinhole in the member to be charged. When an electrophotographic photosensitive member is used as the photosensitive drum 1 to be charged, a resistance of 10 ⁴ to 10 ⁷ Ω is desirable in order to obtain sufficient charging properties and leakage resistance.

  Further, it is desirable that the surface of the charging roller 2 has micro unevenness so that the charge accelerating particles m can be held.

  Further, if the hardness of the charging roller 2 is too low, the shape is not stable and the contact property with the member to be charged is deteriorated. If the hardness is too high, the charging nip n cannot be secured between the member and the member to be charged. In addition, since the micro-contact property to the surface of the member to be charged is deteriorated, the ASKER-C hardness is preferably 25 degrees to 50 degrees.

  The material of the charging roller 2 is not limited to an elastic foam, but as an elastic material such as EPDM, urethane, NBR, silicone rubber, IR, etc., carbon black, metal oxide, etc. for resistance adjustment. Examples thereof include a rubber material in which a conductive substance is dispersed, and those obtained by foaming these materials. It is also possible to adjust the resistance using an ion conductive material without dispersing the conductive material.

  Further, in this embodiment, the charging roller 2 is moved at the charging nip n by about 80% of the process speed so that the surface of the charging roller 2 and the surface of the photosensitive drum 1 move at the same speed in opposite directions. Rotated in the direction. That is, the surface of the charging roller 2 as the contact charging member has a speed difference with respect to the surface of the photosensitive drum 1 as the member to be charged.

  Next, the developing device (developing device) 4 that collects the developer discharged by the charging roller 2 in the developing step and reuses it will be described.

  The developing device 4 of this embodiment is a reversal developing device using a one-component magnetic toner (negative toner) as the developer t. To the developer t, charge promoting particles m are added.

  The developing device 4 has a non-magnetic rotating developing sleeve 4a as a developer carrying member enclosing a magnet roll 4b, and the toner t in the developing device 4 is regulated in the process of being conveyed on the rotating developing sleeve 4a. The blade 4c is subjected to layer thickness regulation and charge application.

  The toner t coated on the developing sleeve 4a is transported to a developing site (developing area) a which is a facing portion between the photosensitive drum 1 and the sleeve 4a by the rotation of the sleeve 4a. A developing bias voltage is applied to the sleeve 4a from a developing bias applying power source S2.

In this embodiment, the developing bias voltage is
DC voltage: -350V
AC voltage: peak-to-peak voltage 900V, frequency 1.8kHz,
It was set as the superposition voltage of the rectangular wave comprised by these.

  As a result, the electrostatic latent image on the photosensitive drum 1 side is reversely developed with the toner t.

The transfer roller 5 serving as transfer means will be described. The toner image formed on the photosensitive drum 1 is transferred to the transfer material P in this embodiment by the transfer unit. The transfer roller 5 as the transfer unit includes a core bar 5a made of SUS, aluminum, and the like. On the outer circumference, a volume resistance of about 1 × 10 ⁵ to 1 × 10 ¹² Ω · cm, for example, carbon black dispersed in EPDM, or medium resistance elastic rubber layer 5b such as NBR rubber is coated in a roller shape. It has a configuration.

The roller resistance is approximately 1 × 10 ⁸ Ω in an environment of 23 degrees and 50%, and the transfer roller 5 is pressure-bonded to an aluminum drum having an outer diameter of 24 mm so that the core metal 5a of the transfer roller 5 is loaded with a total pressure of approximately 6N. Then, while rotating at a constant peripheral speed of 85 mm / sec (= process speed PS, printing speed), 2 kV was applied between the metal core 5a and the aluminum drum, and measurement was performed. The outer diameter is about 15 mm, and the hardness is about 35 degrees when an ASKER-C hardness meter is used and 6N is applied. The bearing is pressed and supported by a spring material with a total load of 7 N so that both ends of the core metal can be rotated by the bearing and the transfer roller 5 is always in contact with the photosensitive drum 1. The transfer roller 5 is driven to rotate.

  In the present embodiment, as described above, the charging roller 2 as the contact charging member that realizes the direct injection charging as described in the background art is used as the charging means for collecting the transfer residual toner. As a cleaner-less system in the image forming apparatus of this embodiment, a plurality of image forming modes are provided, and charging is prevented from adhering to and mixed in the charging roller 2 as a contact charging member in the normal image forming process described above. It is equipped with a contact charging member cleaning mode (roller cleaning mode) that efficiently removes transfer residual toner, which is a factor, from the charging roller 2 when the image forming apparatus is not forming an image, for example, between papers. The contamination level of the transfer residual toner on the roller 2 is always kept low so that good chargeability and image recording can be stably maintained over a long period of time.

  That is, the voltage application to the charging roller 2 as the contact charging member is changed in accordance with the image forming mode. As an image forming mode, a DC voltage is applied to uniformly charge the photosensitive drum 1 as an image carrier (1) a mode M1 which is a non-roller cleaning mode, and (2) a DC during non-image formation. And a mode M2 which is a roller cleaning mode in which a voltage in which a voltage and an AC voltage (cleaning bias) are superimposed is applied. Normal image formation is performed in the mode M1, but the mode is switched to the M2 mode at a timing at which image formation is not performed, such as paper interval, post-rotation, and power-on aging. Note that the frequency of the AC voltage in the mode M2 is about 5 to 500 Hz. Hereinafter, (1) mode M1 and (2) mode M2 will be described.

  (1) Mode M1: In this embodiment, at the time of image formation by the printer, the switch SW of the charging bias application power source S1 is controlled to be switched to the p1 contact side by a sequence control circuit (not shown), and the metal core 2a of the charging roller 2 is controlled. When the DC voltage -500 V of the DC power source SDC is applied to the photosensitive drum 1, the surface of the photosensitive drum 1 is directly charged to a voltage of about -500 V, which is substantially equal to the applied DC voltage, and image formation is executed.

(2) Mode M2: In this embodiment, the switch SW of the charging bias application power source S1 is controlled to be switched to the p2 contact side by a sequence control circuit (not shown) at the time of paper non-image formation. By connecting the AC power supply SAC in series with the DC power supply SDC, the DC voltage: −550 V is applied to the core metal 2 a of the charging roller 2.
AC voltage: peak-to-peak voltage of 100 V, frequency, for example, 200 Hz, rectangular wave,
A superimposed voltage of DC voltage and AC voltage is applied.

In the mode M2, the developing sleeve 4a of the developing device 4 has the same recording time as the image recording.
DC voltage: -350V
AC voltage: A superimposed voltage composed of a peak-to-peak voltage of 900 V, a frequency of 1.8 kHz, and a rectangular wave is applied.

  By maintaining these bias relationships, the toner accumulated on the charging roller 2 is developed on the photosensitive drum 1, that is, the toner collected on the charging roller 2 is discharged to the photosensitive drum 1 side. The discharged toner can be collected by the Vback of the developing device 4.

  In other words, the charging property can be maintained by efficiently removing the toner adhering to the charging roller, which becomes a charging inhibiting factor, at the time of non-image formation.

  Here, as a feature of the present invention, in the mode M2, the voltage applied to the charging roller 2, that is, the cleaning bias, is the difference between the potential of the photosensitive drum 1 and the potential of the charging roller 2 on the downstream side of the charging nip n in the movement direction of the photosensitive drum 1. Set so that the difference is large.

  However, in the charging method using direct charging as in this embodiment, a potential approximately equal to the potential applied to the charging roller 2 that is a contact charging member is applied to the surface of the photosensitive drum 1 that is a charged body. And the photosensitive drum 1 are less likely to cause a potential difference.

  Naturally, it is necessary to uniformly charge the surface of the photosensitive drum 1 during image formation. However, in the roller cleaning mode, which is a mode M2 including steps performed during non-image recording, an AC bias (cleaning bias) is applied to reduce the potential difference. It needs to be generated. Accordingly, it is desirable to select a cleaning bias so that a potential difference occurs between the surface potentials of the charging roller 2 and the photosensitive drum 1 inside the charging nip n and on the downstream side in the moving direction of the photosensitive drum 1. Therefore, it is more preferable to set the bias according to the photosensitive drum speed.

  As described above, in this embodiment, the charging roller 2 realizing the direct injection charging as described in the background art is used as the charging unit for collecting the transfer residual toner. When used, as described in the background art, in the contact charging method, the charging roller 2 as a charging member is brought into contact with the member to be charged, here, the photosensitive drum 1, so that the charging roller 2 removes the deposit on the photosensitive drum 1. It is easy to pick up and become dirty, and it is easy to cause inconveniences such as charging unevenness, dropout of transfer residual toner from the charging roller 2 before the charging process is performed on the surface of the photosensitive drum 1, and image staining.

  As such an image defect due to the toner adhering to the charging roller 2 in the cleanerless system, as shown in FIG. 2, the horizontal stripe due to the potential step at the leading edge of the paper, which is the portion that first reaches the transfer portion b of the paper P as the transfer material. Occurrence is increased. Here, a horizontal streak occurs at a position around 145 mm from the leading edge of the paper. FIG. 2 (a) shows the appearance of white streaks in the halftone and solid black images, and FIG. 2 (b) shows the appearance of fog lines in the solid margin images. ing. This corresponds to a position corresponding to one rotation of the charging roller 2 and one rotation of the transfer roller 5. The generation mechanism will be described below. In the present embodiment, since the charging roller 2 is a charging unit that performs negative charging, in this specification, changing the potential of the photosensitive drum 1 in the negative direction is described as “potential increases”. The change in the + direction is described as “the potential decreases”. Therefore, if the charging direction by the charging means is positive, these are opposite.

  With reference to the schematic view of FIG.

  Since it is cleanerless, the transfer residual toner remaining on the drum 1 after transfer reaches the charging roller 2 and is transferred and accumulated on the charging roller 2 rotating in the direction opposite to the rotation direction of the drum 1. Go. For example, when a pattern with a high printing rate, for example, a solid black image or a 1-dot 1-space period image, that is, 1 d / 1 s, is taken, the amount of toner remaining after transfer increases, and the toner that does not completely transfer onto the charging roller 2 Is transferred to the charging roller 2 and accumulated in an area α near the nip exit in the rotation direction of the charging roller 2 in the charging nip n.

  The toner accumulated in the area α is held on the surface of the drum 1 by the force F due to the electric field E generated by the potential relationship between the drum 1 and the charging roller 2, and the accumulated amount is reduced by this potential relationship. It increases or is discharged on the charging roller 2.

  By the way, on the surface of the drum 1, while the image forming process is performed in the mode M1 or the mode M2, when the transfer material P enters the transfer portion b, the surface potential changes in the transfer portion b at that time. Due to the change, there is a portion where the surface potential of the photosensitive drum 1 is momentarily increased at the boundary portion between before and after the transfer material enters. That portion is conveyed toward the charging roller 2 by the rotation of the photosensitive drum 1. A portion where the potential suddenly increases becomes a “potential step”.

  That is, the surface state of the photosensitive drum 1 that moves between the transfer roller 5 and the charging roller 2 is the region A where the transfer material P was present when passing through the transfer portion b, that is, the region where the transfer was performed, and the transfer portion. There is a region B where the transfer material P does not exist when passing through b, that is, a region where transfer is not performed, and the transfer material P tip (paper tip) is the same place when it just enters the transfer part b. In the surface portion of the drum 1 that has passed through the transfer portion b, there is a step difference in surface potential between the portion where the potential rises and the portion immediately before the sheet passing. That is, a “potential step” is generated.

  The “potential step” portion generated in the transfer portion b reaches the charging nip n by the rotation of the photosensitive drum 1 and reaches the area α. The fact that the potential step on the drum 1 arrives in the area α means that “the electric field formed immediately before the potential step portion arrives at the charging roller 2” and “the electric field formed immediately after the potential step portion passes”. This means that there will be a difference. That is, there is a large change in the potential difference in the area α before and after passing through the potential step portion.

  Here, in the case of a potential step where the potential becomes low, such as the paper leading edge where the transfer material P enters the transfer portion, the accumulated residual toner is discharged and discharged to the drum 1 via the charging roller 2. If the discharged toner is in an exposure region, exposure by a laser or the like is hindered and an image to be formed cannot be formed, resulting in a white stripe or the like as shown in FIG. If it is a solid white portion outside the exposure area, the exposure is not hindered, but the portion that cannot be recovered by development reaches the transfer portion b and appears on the image as a fog as shown in FIG. appear. The image defect as described above is one streak of about 145 mm shown in FIGS.

  In order to prevent this streaking, it is necessary to instantaneously increase the potential generated by the potential step on the drum 1, that is, to reduce the potential step to a predetermined size that does not cause streaking.

  At such a potential step at the leading edge of the paper due to the transfer, there is a timing at which the transfer roller 5 and the drum 1 are separated at the moment when the transfer material P enters the nip b between the transfer roller 5 and the drum 1. The impedance between the transfer roller 5 and the photosensitive drum 1 changes greatly, and a point β on the photosensitive drum 1 that is not controlled is formed. That is, the drum 1 potential is discontinuous between the potential of the drum 1 portion B that has passed the transfer portion b before the timing tβ and the potential of the drum 1 portion A that has passed the transfer portion b after the timing tβ. become. That is, at this moment, the point β is not exposed and is not affected by the transfer. Then, the point β becomes the potential Vd by the charging process performed by the image forming process, that is, the potential streak near −500 V reaches the charging roller 2 as it is.

  FIG. 4 shows a profile of the photosensitive drum 1 potential at a portion that has passed the transfer current and the transfer portion b when the above-described potential streak is generated.

  The horizontal axis is a time axis, and the timing is corrected for each item on the basis of one drum surface. In this figure, there is a portion where the transfer current sharply increases at the point β corresponding to the leading edge of the paper. The B side at the time earlier than the point β indicates the drum 1 potential that has passed through the transfer portion b before the transfer material P arrives, and the A side at the time of the rising and gradually decreasing at the moment of passing the point β holds the transfer material P. The potential of the drum 1 that has passed through the transferred portion b is shown. At point β, the drum potential rises to a value close to the charging potential Vd. This is a potential streak.

  As a method of preventing this, that is, as a method of preventing excessive toner adhesion to the photosensitive drum 1 and toner stagnation in the area α, the previous history is also in the state where the charging potential Vd is around −500 V. The transfer material P is sent to the transfer part b. For this purpose, the transfer current flowing in the transfer at the potential B immediately before the transfer material P enters the transfer portion b and the potential A at the point β immediately after the transfer material P may be constant or within a specified range.

  Therefore, the generation state of the leading edge streaks was confirmed while varying the transfer current value at the leading edge of the paper. The results are shown in Table 1. Here, in FIG. 4, the potential A drops and becomes a substantially constant potential (−100 V) in the state where the hand transfer material P is sandwiched between the transfer portions b, but Table 1 shows the potential at the point β. The plus side is in the same direction as the value of the current flowing through the transfer during image formation. The post-transfer potential profiles of current values of 0, 1, 2, 4 μA in Table 1 are also shown in FIG. The surface potential diameter was measured using a MODEL 344 surface potential meter manufactured by TREK.

  From the results, it can be said that when the drum 1 current at the transfer portion b passing portion immediately before and after the sheet passing is 2.0 μA or less, the generation of streaks is almost no problem, and when the current is 1.0 μA or less, the streaks are not generated. Recognize.

  “1.0 μA or less” means that the drum potential at the transfer portion b passage portion immediately after passing the sheet has a potential larger on the − side than 450 V, and the drum potential A immediately after passage of the transfer portion b is the charging potential. Since Vd is −500 V, streaking does not occur when the drum potential difference immediately before and after the sheet passing is smaller than 50 V.

  In the present specification, since the transfer is performed when the transfer material passes through the transfer portion, before and after the sheet passing is synonymous with before and after the transfer action is performed.

  In other words, the influence of the transfer in the transfer step on the drum potential is that there is no streaking problem if the difference between A and B is about 100V, and no streaking occurs if it is about 50V or less.

  In this embodiment, the transfer bias is applied so that the moment when the leading edge of the paper enters is 1.0 μA or less. At that time, the transfer bias at the time of entering the leading edge of the paper was set to 0 V, for example, which was confirmed to be 1.0 μA or less in a preliminary experiment. The profile of the post-transfer drum potential at that time is shown in FIG. At this time, a sudden increase in potential at the point β, which causes streaks, did not occur.

  In this embodiment, the voltage is set to 0 V and 1.0 μA or less. However, due to the influence of the charged charge of the drum, there is a phenomenon that a current flows to some extent without applying a transfer bias, so depending on the resistance of the transfer roller 5. This is not limited to 0V. Here, if the transfer roller has a higher resistance than that of the present embodiment, the transfer bias can be increased so that the current does not exceed 1.0 μA. If the transfer roller has a low resistance, the transfer bias can be increased. Apply with lowering.

  As described above, it is possible to prevent adverse effects in an image by keeping the transfer current within a specified range and keeping the potential step within a specified range.

  In other words, the value of the current flowing through the transfer portion b immediately before the transfer material P reaches the transfer roller 5 is 2 μA or less in the same direction as the current flowing through the transfer roller 5 in the image forming process, preferably 1 μA or less. Then, regarding the surface potential between the transfer portion b and the charging roller 2 on the surface of the photosensitive drum 1, the surface potential of the portion B immediately upstream in the moving direction of the photosensitive drum 1 at the portion where the transfer material tip is sandwiched by the transfer portion b. If the difference with respect to the charging potential Vd is 100 V or less, preferably 50 V or less, it is possible to prevent the residual toner from dropping on the photosensitive drum 1 from the charging roller 2, and an image due to adhesion of the transfer residual toner to the photosensitive drum 1. Defects can be prevented.

  In this embodiment, as described above, the charging roller 2 is directly charged by the charge injection method. However, the charge promoting particles m interposed between the charging roller 2 and the photosensitive drum 1 are developed. It is added to the toner t as an agent. Therefore, the developer t will be described.

  In the image forming apparatus of this embodiment, the one-component magnetic toner t used as a developer is prepared by mixing a binder resin, magnetic particles, and a charge control agent, and kneading, pulverizing, and classifying each step. Further, it is prepared by adding the charge accelerating particles m, a fluidizing agent and the like as external additives. The weight average particle diameter of the toner t was 7 μm.

In this example, tin oxide having a specific resistance of 10 ⁶ Ω · cm and an average particle diameter of 3 μm was used as the charge promoting particles m. Then, 1 part by weight of the charge accelerating particles m was added to 100 parts by weight of the classified toner t, and the particles were uniformly dispersed by a mixer and accommodated in the developing device 4.

  As the material for the charge promoting particles m, various conductive particles such as conductive inorganic particles such as other metal oxides, mixtures with organic substances, or those obtained by subjecting them to surface treatment can be used.

Since the particle resistance transmits and receives charges through the particles, the specific resistance needs to be 10 ⁴ Ω · cm or more and 10 ¹² Ω · cm or less, and preferably 10 ¹⁰ Ω · cm or less. If it is less than 10 ⁴ Ω · cm, it may be difficult to apply a charge to the toner, or the charging uniformity may be impaired. If it is larger than 10 ¹² Ω · cm, there is a problem that charging cannot be performed.

The resistance was measured by the tablet method and normalized. That is, about 0.5 g of a powder sample is placed in a cylinder with a bottom area of 2.26 cm ² and 15 kg of pressure is applied to the upper and lower electrodes. At the same time, a voltage of 100 V is applied and the resistance value is measured. Resistance was calculated.

  The particle size is desirably 0.01 μm or more and 50 μm or less in order to obtain good charging uniformity.

  In the present invention, the particle diameter when the particles constitute an aggregate is defined as the average particle diameter as the aggregate. For the measurement of the particle size, 100 or more samples were extracted from observation with an optical or electron microscope, the volume particle size distribution was calculated with the maximum horizontal chord length, and the 50% average particle size was determined.

  There is no problem that the charge promoting particles m exist not only in the state of primary particles but also in the state of aggregation of secondary particles. In any aggregate state, the form is not important as long as the function as the charge promoting particles can be realized as the aggregate.

  The charge accelerating particles m are desirably white or nearly transparent so that they do not hinder the latent image exposure, particularly when used for charging a photoreceptor, and are therefore preferably non-magnetic. Furthermore, considering that the charge accelerating particles are partially transferred from the photosensitive drum 1 to the transfer material P, it is preferable that the color recording is colorless or white. In order to prevent light scattering by particles during image exposure, the particle size is preferably less than the constituent pixel size. As the lower limit of the particle size, 10 nm is considered to be the limit as a particle that can be stably obtained.

  Contact charging of the photosensitive drum 1 is performed in a state where the charge accelerating particles m are present in the charging nip n between the photosensitive drum 1 and the charging roller 2, but in the charging nip n between the photosensitive drum 1 and the charging roller 2. Due to the presence of the charge accelerating particles m, even if the charging roller has a large frictional resistance due to the lubricant effect of the particles m, it is difficult to make contact with the photosensitive drum 1 with a speed difference as it is. It is possible to easily and effectively bring it into contact with the surface of the photosensitive drum 1 with a speed difference, and the charging roller 2 is placed on the surface of the photosensitive drum 1 through the particles m. It becomes a structure which contacts closely and contacts the photosensitive drum 1 surface more frequently.

  That is, in the present embodiment, as described above, in the charging nip n, the surface of the charging roller 2 has a speed difference with respect to the surface of the photosensitive drum 1 as a member to be charged. By providing a speed difference between the roller 2 and the photosensitive drum 1, the chance of the charge accelerating particles m to contact the photosensitive drum 1 at the nip portion between the charging roller 2 and the photosensitive drum 1 is greatly increased, and high contactability is achieved. The charge accelerating particles m present in the nip portion between the charging roller 2 and the photosensitive drum 1 can rub the surface of the photosensitive drum 1 without any gap so that the charge can be directly injected into the photosensitive drum 1. Direct charging (injection charging) is dominant in the contact charging of the photosensitive drum 1 by 2 due to the interposition of the charge accelerating particles m.

  As a configuration for providing a speed difference, the charging roller 2 is rotationally driven to provide a speed difference from the photosensitive drum 1. Preferably, in order to temporarily collect and level the transfer residual toner on the photosensitive drum 1 carried to the charging nip n on the charging roller 2, the rotation direction is opposite to the moving direction of the surface of the photosensitive drum 1. It is desirable to configure it to rotate. That is, it is possible to preferentially directly charge the toner by temporarily separating the transfer residual toner on the photosensitive drum 1 by reverse rotation.

  Therefore, high charging efficiency that cannot be obtained by conventional roller charging or the like can be obtained, and a charging potential almost equal to the voltage applied to the charging roller 2 can be applied to the photosensitive drum 1. Thus, even when the charging roller 2 is used as the contact charging member, a voltage equivalent to the charging potential required for the photosensitive drum 1 is sufficient as the bias applied to the charging roller 2 and stable and no discharge phenomenon is used. A safe contact charging method or apparatus can be realized.

  In addition, since the charging roller 2 is in contact with the photosensitive drum 1 with a speed difference, the pattern of the residual transfer toner from the transfer portion b to the charging nip portion n is disturbed and broken, resulting in a halftone image. , The previous image pattern portion does not appear as a ghost.

  When the charge promoting particles m drop off from the charging nip n or the charging roller 2 due to durability, the printer is operated, and the charge promoting particles m contained in the developer t of the developing device 4 are developed as described above. It moves to the surface of the photosensitive drum 1 at the part a, and is carried to the charging nip n through the transfer portion b as the surface of the photosensitive drum 1 moves, and is continuously supplied to the charging nip n. Therefore, good chargeability due to the presence of the charge accelerating particles m in the charging nip n is stably maintained over a long period of time.

  That is, the charge accelerating particles dropped from the charging nip n and the charging roller 2 are collected by the developing device 4 at the development site a, mixed into the developer t, and recycled.

  In this way, by carrying the charge accelerating particles m in advance on the surface of the charging roller 2, the direct charging performance can be exhibited without any trouble from the very beginning of the use of the printer.

  If the amount of the charge accelerating particles m intervened in the charging nip n between the photosensitive drum 1 as the image carrier and the charging roller 2 as the contact charging member is too small, the lubricating effect by the particles m cannot be sufficiently obtained, and charging is performed. Since the friction between the roller 2 and the photosensitive drum 1 is large, it is difficult to rotationally drive the charging roller 2 to the photosensitive drum 1 with a speed difference. That is, the driving torque becomes excessive, and if the surface is rotated forcibly, the surfaces of the charging roller 2 and the photosensitive drum 1 are scraped.

  Furthermore, the effect of increasing the contact opportunity due to the charge accelerating particles m may not be obtained, and sufficient charging performance cannot be obtained. On the other hand, when the amount of the inclusion is too large, dropping of the charge accelerating particles m from the charging roller 2 is remarkably increased, which adversely affects image formation.

Thus, experiments have shown that the amount of intervening is desirably 10 ³ pieces / mm ² or more. If it is lower than 10 ³ pieces / mm ² , sufficient lubrication effect and contact opportunity increase effect cannot be obtained, and charging performance is lowered. On the other hand, if it exceeds 5 × 10 ⁵ particles / mm ² , dropping of the particles to the photosensitive member 1 is remarkably increased, resulting in insufficient exposure of the photosensitive drum 1 regardless of the light transmittance of the particles themselves. If it is 5 × 10 ⁵ particles / mm ² or less, the amount of particles falling off can be kept low, and adverse effects can be improved. Since the amount of particles dropped on the photosensitive drum 1 in the intervening amount range was 10 ² to 10 ⁵ particles / mm ² , the existing amount having no problem in image formation is 5 × 10 ⁵ particles. / Mm ² or less is desired. That is, the intervening amount of 10 ^{3 to} 5 × 10 ⁵ pieces / mm ² is preferable.

  A method for measuring the intervening amount and the existing amount on the photosensitive drum 1 will be described. It is desirable that the amount of intervening is directly measured at the charging nip n between the charging roller 2 and the photosensitive drum 1, but most of the particles present on the photosensitive drum 1 before contacting the charging roller 2 move in the opposite direction while contacting. Therefore, in the present invention, the amount of particles on the surface of the charging roller 2 immediately before reaching the charging nip portion n is used as the intervening amount.

  Specifically, the rotation of the photosensitive drum 1 and the charging roller 2 is stopped in a state where no charging bias is applied, and the surfaces of the photosensitive drum 1 and the charging roller 2 are video microscope (OLYMPUS OVM1000N) and digital still recorder (DELTIS manufactured). SR-3100). The charging roller 2 is in contact with the slide glass under the same conditions as the case where the charging roller 2 is in contact with the photosensitive drum 1, and the contact surface of the charging roller 2 is 10 or more with a 1000 × objective lens from the back surface of the slide glass. I took a picture. In order to separate individual particles from the obtained digital image, binarization processing was performed with a certain threshold value, and the number of regions where particles were present was measured using desired image processing software. Further, the abundance on the photosensitive drum 1 was also measured by photographing the same on the photosensitive drum 1 with the same video microscope. The amount of the intervening was adjusted by setting the blending amount of the charge accelerating particles m in the developer t of the developing device 4. Generally, the charge accelerating particles m are blended in an amount of 0.01 to 20 parts by weight with respect to 100 parts by weight of developer (toner).

  As described above, the charge accelerating particles m are supplied to the charging roller 2, so that the toner releasability from the top of the charging roller 2 is improved, that is, the releasability is increased. The discharge of toner to the photosensitive drum 1 side is promoted, the charging roller 2 contaminated with toner is efficiently cleaned, and ozone-less direct charging and a toner recycling system can be executed without any problems with a low applied voltage, thereby achieving high quality. It is possible to maintain image formation over a long period of time and to maintain high-quality image formation over a long period of time even after outputting an image with a high image ratio.

  Further, contact charging of the photosensitive drum 1 is performed in a state where the charge accelerating particles m exist in the charging nip n between the photosensitive drum 1 and the charging roller 2, but in the charging nip n between the photosensitive drum 1 and the charging roller 2. Due to the presence of the charge accelerating particles m, even if the charging roller has a large frictional resistance due to the lubricant effect of the particles m, it is difficult to make contact with the photosensitive drum 1 with a speed difference as it is. It is possible to easily and effectively bring it into contact with the surface of the photosensitive drum 1 with a speed difference, and the charging roller 2 is placed on the surface of the photosensitive drum 1 through the particles m. It becomes a structure which contacts closely and contacts the photosensitive drum 1 surface more frequently.

  In addition, when the toner of the electrostatic latent image on the photosensitive drum 1 side is developed by the developing device 4, an appropriate amount of the charge accelerating particles m added to the developer t of the developing device 4 is transferred to the photosensitive drum 1 side together with the toner at the development site a. To do. Thereafter, the toner image on the photosensitive drum 1 is attracted to the transfer material P side and actively transferred in the transfer portion b due to the influence of the transfer bias, but the charge promoting particles m on the photosensitive drum 1 are conductive. The transfer material P is not positively transferred to the transfer material P side, and remains substantially adhered and held on the photosensitive drum 1.

  Thus, the presence of the charge accelerating particles m substantially adhered and held on the surface of the photosensitive drum 1 also has an effect of improving the transfer efficiency of the toner image from the photosensitive drum 1 side to the transfer material P side.

Example 2
In the present embodiment, the overall configuration of image formation and the execution of the cleaner system are substantially the same as those in the first embodiment. However, in this embodiment, the control when the paper leading edge enters the transfer portion b is further controlled by constant current control. It is characterized by performing by.

  As described in the first embodiment, the current value at that time is preferably set to a current value that has the least influence on the potential of the drum 1. That is, it may be 2.0 μA or less, preferably 1.0 μA or less. As in the present embodiment, a value at which the drum potential is the most continuous is preferable. In this example, constant current control of 0 A was performed.

  A schematic diagram of each profile in which the transfer voltage value, the transfer current value, and the potential change on the drum 1 according to this embodiment are represented by the reference timing on the drum 1 is shown by a curve σ in FIG. It can be seen that the potential step at the leading edge of the paper is small when the current is 0 μA.

  When the leading edge of the paper enters the transfer portion b, after the 0 A constant current control, the voltage is raised to a voltage corresponding to a constant current of 4 μA within the image guarantee area (5 mm in this embodiment) on the paper. In this embodiment, a voltage value corresponding to a 4 μA constant current control value is set in advance as a target value, so that it can be quickly started up and compatible with normal image transferability.

  In the process of setting the transfer bias at the time of continuous paper feeding or other bias applied by the pre-rotation transfer roller 5 resistance detection control to 0 A, the transfer bias is gradually decreased from the previous transfer bias to 0 A. The configuration is such that 0 A constant current control is performed at the time.

  As described above, in order to keep the potential step within a specified range, the transfer to the drum potential by the transfer is controlled by controlling so that the influence of the transfer at the moment when the leading edge of the paper enters the transfer portion is eliminated, that is, by a constant current of 0A. By eliminating the influence, it is possible to eliminate the potential step at the leading edge of the paper and to prevent horizontal streaks.

Example 3
In this embodiment, the overall configuration of the image forming apparatus and the cleanerless system are almost the same as those in the first embodiment. However, in this embodiment, when the front end of the paper enters the transfer portion, the transfer roller 5 is electrically connected. Is to float. If the transfer roller 5 is electrically floated, the transfer roller 5 and the photosensitive drum 1 are equipotential, and no potential step is generated during transfer.

  The profile of the potential change on the drum 1 of this example is shown as a curve δ in the graph of FIG. It can be seen that there is almost no potential step at the leading edge of the paper.

  However, there is a case where an instantaneous potential step to be switched to the float occurs. This is an influence due to the electrostatic capacity of the transfer roller 5, but the streak that is a problem in the present embodiment is a level at which there is no influence. It was.

  When the leading edge of the paper enters the transfer portion b, after the transfer roller 5 is floated, it is pulled up to a voltage corresponding to a constant current of 4 μA within the image guarantee area of 5 mm in this embodiment on the paper. In this embodiment, a voltage value corresponding to a 4 μA constant current control value is set in advance as a target value, so that it can be quickly started up and compatible with normal image transferability.

  The process of floating from the transfer bias during continuous paper feeding or other bias applied by the transfer roller resistance detection control of the previous rotation, etc., is when the transfer bias is gradually lowered from the previous transfer bias to 0 μA. It took the form of making it float.

  As described above, in order to keep the potential step within a certain regulation, control is performed so as not to affect the transfer at the moment when the leading edge of the paper enters the transfer portion b, that is, by making the transfer roller 5 float, By eliminating the influence on the drum potential, it is possible to eliminate the potential step at the front end of the paper and to prevent horizontal streaks.

  As described in the first to third embodiments, in the cleaner-less process, in order to prevent the residual toner accumulated on the charging member from dropping, one reason for dropping the residual toner accumulated on the charging member is as follows. It is necessary to “reduce the potential step on the drum formed after passing through the transfer portion”. As a factor that creates a potential step difference in the image during such image formation, there is a moment when the transfer material enters the transfer portion. When the transfer material enters the transfer section, the potential on the drum changes greatly. In order to prevent the remaining toner accumulated on the charging member from falling off, it is necessary to keep the potential step within the range where it does not fall off. It is.

  Therefore, in the present invention, the potential step is kept within a certain regulation to prevent the adverse effect in the image, or the control that eliminates the influence of the transfer at the moment when the leading edge of the paper enters the transfer portion, that is, 0 μA in the transfer bias. By eliminating the influence of the transfer by controlling the constant current, it is possible to eliminate the potential step at the front end of the paper due to the transfer. In addition, a configuration that eliminates the influence of transfer at the moment when the paper leading edge enters the transfer portion, that is, the transfer roller is floated and is equal to the drum potential, thereby eliminating the potential step at the paper leading edge due to the transfer, and the cleaner. Image defects due to streaks and fog that tend to occur in the less process were avoided.

  The dimensions, materials, shapes, and relative positions of the components of the image forming apparatus described above are not intended to limit the scope of the present invention only to those unless otherwise specified.

  In addition, the charging residual member is not collected by the charging member, and has a storage means for storing the developer remaining on the image carrier after the transfer process by the transfer means, separately from the charging member. The stored developer may be discharged onto the image carrier, and the developing unit may collect the discharged developer.

  The present invention is particularly effective in an image forming apparatus that employs a cleanerless system. However, even if the cleanerless system is not employed, the present invention is effective in preventing contamination on the surface of the charging member.

1 is a schematic configuration diagram illustrating an example of an overall configuration of an image forming apparatus according to the present invention. It is explanatory drawing which shows the white stripe (FIG. 2 (a)) and fogging stripe (FIG.2 (b)) which are examples of an image defect. FIG. 6 is an explanatory diagram illustrating a state in which a transfer residual developer adheres to a charging member. 6 is a graph showing changes in the surface potential of the image carrier near the front end of the paper. 6 is a graph showing changes in the surface potential of the image carrier near the front end of the paper.

Explanation of symbols

1 Photosensitive drum (image carrier)
2 Charging roller (charging member)
4 Developer (Developer)
5 Transfer roller (transfer means)
t Toner (Developer)
m Charge accelerating particles

Claims (3)

A rotatable image carrier, a charging unit for charging the image carrier, an exposure unit for forming an electrostatic latent image on the image carrier charged by the charging unit, and an image bearing member formed on the image carrier. Developing means for developing the electrostatic latent image, and transfer means for transferring the toner image developed by the developing means from the image carrier to a transfer material , wherein the charging means is charged with the image carrier. The charging means is formed after forming a nip portion and retaining the residual toner not transferred to the transfer material by the transfer means but remaining on the image carrier on the upstream side in the rotation direction of the image carrier from the charging nip portion. In the image forming apparatus, after that, the residual toner is discharged from the charging unit to the image carrier, and the residual toner discharged to the image carrier by the developing unit is collected .
And an application power source for applying a voltage to the transfer unit so that a current flowing through the transfer nip formed by the transfer unit and the image carrier is a constant current. Before and after the transfer nip, a voltage is applied to the transfer means so that the current flowing through the transfer nip portion becomes a constant current, and before and after the transfer nip portion is not transferred to the transfer material state. In the image forming apparatus , the transfer unit is electrically floated, and then a voltage is applied to the transfer unit so that the current flowing through the transfer nip portion becomes the constant current again . The image forming apparatus according to claim 1, wherein the charging unit is a charging roller that rotates in a direction opposite to a rotation direction of the image carrier on a contact surface with the image carrier. The image forming apparatus according to claim 1, wherein the developing unit develops a toner image on the image carrier and simultaneously collects the collected toner from the image carrier.

Images