JP2013217987A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that employs an image carrier cleaner-less system including an intermediate transfer body having a constant potential on the whole periphery by a current supplied from secondary transfer means, and cleans charging means while reducing attachment of toner to the secondary transfer means in removing toner attached to the charging means.SOLUTION: An image forming apparatus includes execution means for executing a recovery step for controlling a secondary transfer bias power source 6, forming potential gradient between a charging roller 2 and a secondary transfer roller 9 via a photoconductor drum 1 and an intermediate transfer belt 5 during non-image formation period, and recovering a toner attached to the charging roller 2 by an intermediate transfer belt cleaning device 11 via the photoconductor drum 1 and the intermediate transfer belt 5.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer having a function of forming an image on a recording material such as a sheet.

In recent years, image forming apparatuses using an electrophotographic system have been reduced in size and cost, and various proposals have been made.
For example, Patent Document 1 does not provide an image carrier cleaning unit that collects toner (transfer residual toner) remaining on the image carrier after the toner image formed on the image carrier is transferred onto the intermediate transfer member. An image forming apparatus of a system (image carrier cleaner-less system) has been proposed. According to this method, since the transfer residual toner is collected by the developing unit, the image carrier can be cleaned without providing a cleaning unit on the image carrier. This eliminates the need for a dedicated space for holding the collected toner, and makes it possible to reduce the size of the developing device and the image forming apparatus as a whole and to reduce the cost by reducing the number of members.
However, the image carrier cleanerless type image forming apparatus has a problem that the toner adheres to and accumulates on the charging unit while repeating the image forming operation and various image stabilization controls using the toner. This occurs when toner charged to a polarity opposite to the normal charging polarity in the transfer residual toner adheres to the charging unit due to a potential difference between the voltage applied to the charging unit and the surface potential of the image carrier. As a result, there is a concern that the charging ability of the charging means changes and the surface of the image carrier cannot be uniformly charged, resulting in fluctuations in image density.
On the other hand, many methods for cleaning the toner adhering to the charging means have been proposed. For example, in Patent Document 2, a cleaning process of the contact charging unit is provided at a predetermined timing, a potential gradient is formed between the transfer member provided with the cleaning unit and the contact charging unit, and the toner attached to the contact charging unit is removed. Collected on a transfer body provided with a cleaning means due to a potential difference. The collected toner is collected by a cleaning means provided on the transfer body. In this way, by performing cleaning of the charging unit as needed at a predetermined timing, it is possible to suppress fluctuations in charging ability due to toner adhesion of the charging unit.

Further, in the process in which the toner passes through the secondary transfer portion during non-image formation, such as cleaning of the toner adhering to the charging unit and various image stabilization controls using the toner, there are the following problems. That is, when the toner passes through the secondary transfer portion during non-image formation, there is a problem that the toner adheres to the secondary transfer unit due to a potential difference and causes backside staining of the recording material.
On the other hand, various methods have been proposed as methods for suppressing toner adhesion to the secondary transfer means. For example, in Patent Document 3, when there is no recording material in the secondary transfer unit, a voltage having the same polarity as the toner passing through the secondary transfer unit is applied to the secondary transfer unit, and the toner is transferred by electrostatic force to the secondary transfer unit. It is made not to adhere to. In Patent Document 4, when there is no recording material in the secondary transfer portion, the intermediate transfer member and the secondary transfer unit are separated from each other so that the toner is not physically attached to the secondary transfer unit. As described above, it is possible to suppress the occurrence of stains on the back side of the recording material by electrostatically or physically suppressing toner adhesion to the secondary transfer unit.

JP-A 64-20587 JP 2001-074448 A JP 2000-147915 A JP 2000-019863 A

By the way, the applicant of the present invention has provided an image forming apparatus including an intermediate transfer member that has a substantially constant potential around the entire circumference by power feeding from a power feeding unit for the purpose of downsizing and cost reduction of an image forming apparatus using an electrophotographic system. A device is proposed.
In this image forming apparatus, a low-resistance intermediate transfer member is provided, and a current supplied from a secondary transfer unit as a power supply unit flows in the circumferential direction of the intermediate transfer member, so that the entire circumference of the intermediate transfer member is substantially constant. It becomes a potential. In such a configuration, primary transfer is performed at each primary transfer portion due to a potential difference between the intermediate transfer member and the surface of each color image carrier. According to this method, it is not necessary to provide power supply means for each primary transfer unit, and related members / wiring can be omitted. Therefore, power saving and downsizing of the image forming apparatus can be reduced, and cost can be reduced. Connected.
However, in the image carrier cleaner-less type image forming apparatus provided with the intermediate transfer body whose entire circumference is at a substantially constant potential due to the current supplied from the secondary transfer means, the following is concerned. That is, in the process of cleaning the toner attached to the charging unit, when the toner collected on the intermediate transfer member passes through the secondary transfer unit, the conventional method suppresses the toner adhesion to the secondary transfer unit. There is a concern that it is not possible.
This is because the potential of the intermediate transfer member is given by the current supplied from the secondary transfer means, and primary transfer is performed in each primary transfer portion. That is, when a voltage having the same polarity as the toner passing through the secondary transfer portion is applied to the secondary transfer unit, or the intermediate transfer member and the secondary transfer unit are separated, the potential of the intermediate transfer member changes and 1 The next transfer cannot be performed. Therefore, in the conventional method, in the step of cleaning the toner attached to the charging unit, the charging unit cannot be cleaned while reducing the toner adhesion to the secondary transfer unit.

  The present invention has been made in view of the above circumstances, and has the following objects. That is, when cleaning the toner adhering to the charging means in an image carrier cleanerless type image forming apparatus provided with an intermediate transfer body having a constant potential on the entire circumference by the current supplied from the secondary transfer means. This is to clean the charging means while reducing toner adhesion to the next transfer means.

In order to achieve the above object, the present invention provides:
An image carrier on which an electrostatic latent image is formed;
Contact charging means for uniformly charging the image carrier in contact with the image carrier;
A developing member capable of contacting and separating from the image carrier, and developing the electrostatic latent image formed on the image carrier into a toner image while the developing member is in contact with the image carrier; Developing means capable of collecting toner adhering to the body;
An endless rotatable intermediate transfer body onto which a toner image on the image carrier developed by the developing means is transferred at a primary transfer portion;
A transfer member disposed so as to be in contact with the intermediate transfer member and forming a secondary transfer portion with the intermediate transfer member, and is primarily transferred to the intermediate transfer member by applying a voltage. A transfer member for secondary transfer of the toner image to the recording material at the secondary transfer portion;
Voltage applying means for applying a voltage to the transfer member;
A collecting means for collecting toner adhering to the intermediate transfer member on the downstream side in the rotation direction of the intermediate transfer member with respect to the secondary transfer portion of the intermediate transfer member;
An image forming apparatus for forming an image on a recording material,
The voltage applying unit is controlled so that the image carrier and the developing unit are separated from each other between the contact charging unit and the transfer member via the image carrier and the intermediate transfer member in a non-image formation state. An executing means for executing a recovery step for recovering the toner adhering to the contact charging means by the recovery means via the image carrier and the intermediate transfer body by forming a potential gradient between them; It is characterized by.

  According to the present invention, the following can be performed in an image carrier cleanerless type image forming apparatus provided with an intermediate transfer body in which the entire circumference has a constant potential by the current supplied from the secondary transfer means. That is, when cleaning the toner adhering to the charging unit, it is possible to clean the charging unit while reducing toner adhesion to the secondary transfer unit.

Sectional drawing which shows schematic structure of the image forming apparatus of Example 1. FIG. Schematic explaining the case of measuring the resistance value of the intermediate transfer belt of Example 1 The figure which shows the timing chart of the cleaning operation | movement of the charging roller in Example 1. FIG. The figure which shows the relationship of each electric potential in the cleaning process in Example 1. The figure explaining the current path at the time of voltage application in the cleaning process of the charging roller The figure which shows the relationship between an applied voltage and a toner adhesion rate in the cleaning process of a charging roller

DETAILED DESCRIPTION Exemplary embodiments for carrying out the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the scope to the following embodiments.
The present invention relates to an image forming apparatus that forms an image on a recording material (recording medium) using an electrophotographic system. Examples of the image forming apparatus include a copying machine, a printer (laser beam printer, LED printer, etc.), a facsimile machine, a word processor, and a multifunction machine (multifunction printer) thereof. The present invention particularly relates to an image forming apparatus using a so-called image carrier cleaner-less system which does not have an image carrier cleaning means.

Example 1
First, an overall configuration and operation of an electrophotographic image forming apparatus will be described as an embodiment of an image forming apparatus to which the present invention can be applied.
FIG. 1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus 100 according to the first embodiment. The image forming apparatus 100 is a full-color laser beam printer that employs an inline method or an intermediate transfer method, and can form a full-color image on a recording material (for example, recording paper, plastic sheet, cloth, etc.) in accordance with image information. The image information is input to the image forming apparatus main body from an image reading apparatus connected to the image forming apparatus main body or a host device such as a personal computer connected to the image forming apparatus main body so as to be communicable.

In FIG. 1, an image forming apparatus 100 includes first and first images for forming images of yellow (Y), magenta (M), cyan (C), and black (K) as a plurality of image forming units, respectively. 2, second and third image forming units SY, SM, SC, and SK. In the present embodiment, the first to fourth image forming units SY, SM, SC, and SK are arranged in a line at a certain interval in a direction intersecting the vertical direction.
In this embodiment, the configurations and operations of the first to fourth image forming units are substantially the same except that the colors of the images to be formed are different. Therefore, in the following description, if no particular distinction is required, the subscripts Y, M, C, and K given to the reference numerals to indicate that they are elements provided for any color are omitted, and the summary I will explain it.

In this embodiment, the image forming apparatus 100 includes four drum-type electrophotographic photosensitive members (hereinafter, photosensitive drums) 1 arranged in parallel in a direction intersecting the vertical direction as a plurality of image carriers.
The photosensitive drum 1 has a photoreceptor layer in which an undercoat layer, which is a functional film, a carrier generation layer, and a carrier transfer layer are sequentially coated on an aluminum drum base (not shown). Then, by a driving force of a drive motor (not shown), it is rotationally driven in a direction indicated by an arrow a (clockwise) at a predetermined peripheral speed according to an image forming operation or the like. In this embodiment, the photosensitive drum 1 is a negatively charged organic photosensitive drum having a diameter of 24 mm, and is driven to rotate at a peripheral speed of 240 mm / sec.
Around the photosensitive drum 1, a charging roller 2, an exposure device 3, and a developing device 4 are arranged.

The charging roller 2 is a contact charging unit that uniformly (uniformly) charges the surface of the photosensitive drum 1 to a potential having a predetermined polarity. The charging roller 2 is in contact with the surface of the photosensitive drum 1 with a predetermined pressing force, and is driven to rotate with respect to the photosensitive drum 1 by friction with the surface of the photosensitive drum 1. A predetermined DC voltage is applied to the rotating shaft of the charging roller 2 from a charging bias power source (not shown) according to the image forming operation. In this embodiment, the charging roller 2 includes a base layer made of a conductive elastic body having a thickness of 1.5 mm and a volume resistivity of about 1 × 10 ⁴ Ωcm on a metal shaft having a diameter of 5.5 mm, and a thickness. A surface layer having a surface resistivity of about 0.05 μm and a volume resistivity of about 1 × 10 ¹⁰ Ωcm is used. A DC voltage of −1100 V is applied to the rotating shaft of the charging roller 2 in accordance with the image forming operation. At this time, when the surface potential of the photosensitive drum 1 was measured with a surface potential meter Model 344 manufactured by Trek, it was about -500V.

  The exposure apparatus 3 includes a laser driver, a laser diode, a polygon mirror, an optical lens system, and the like, and is uniformly charged by irradiating laser light based on image information input from a host computer (not shown). It is exposure means for forming an electrostatic latent image on the surface of the photosensitive drum 1. In this embodiment, the exposure amount is adjusted so that the potential of the surface of the photosensitive drum 1 after being exposed with the maximum light amount of the exposure apparatus 3 becomes −100V.

The developing device 4 includes a developing roller 12 as a developing member (toner carrier) and a non-magnetic one-component toner (hereinafter, toner) as a developer, and develops an electrostatic latent image as a toner image (photosensitive drum). 1 is a developing means that performs a developing action. The developing roller 12 is in contact with the photosensitive drum 1 with a predetermined contact width according to the image forming operation. Then, the developing roller 12 has a peripheral speed faster than the peripheral speed of the photosensitive drum 1, and the surface movement direction of the developing roller 12 is opposite to the surface moving direction and the forward direction of the photosensitive drum 1 at a portion (contact portion) facing the photosensitive drum 1. It is driven to rotate.
Here, the developing roller 12 is provided so as to be able to contact and separate from the photosensitive drum 1. That is, the developing device 4 and the main body of the image forming apparatus 100 are provided with a mechanism (not shown) that controls the contact / separation (development separation) state between the developing roller 12 and the photosensitive drum 1 and develops according to the image forming operation or the like. The roller 12 and the photosensitive drum 1 are brought into contact with each other and are separated when the operation is stopped.

A predetermined DC voltage is applied to the core of the developing roller 12 from a developing bias power source (not shown) according to the image forming operation. In this embodiment, the developing roller 12 is rotationally driven at a peripheral speed of 280 mm / sec according to the image forming operation, and a DC voltage of −300 V is applied to the core metal of the developing roller 12. A uniform toner layer having a toner charge amount per unit mass of about −40 μC / g and a toner amount per unit area of about 0.4 mg / cm ² is formed on the developing roller 12.
Here, the toner charge amount per unit mass and the toner amount per unit area are obtained by sucking and collecting the toner on the developing roller 12 with a suction type Faraday gauge having a filter inside, and measuring the charge measured with the Faraday gauge. And the toner collection area and the filter mass increase. That is, the toner charge amount per unit mass is obtained by dividing the charge measured by the Faraday gauge by the filter mass increase, and the toner amount per unit area is obtained by dividing the filter mass increase by the collection area. It was.

The toner in this embodiment is a non-magnetic toner having a negative charging property produced by a suspension polymerization method, and has a volume average particle diameter of about 6.5 μm. Further, in order to modify the surface property, silicon oxide particles having a volume average particle diameter of about 20 nm are uniformly adhered to the toner surface by about 1.5% of the toner weight. Here, the volume average particle diameter of the toner is a volume average particle diameter measured by a laser diffraction particle size distribution analyzer LS-230 manufactured by Beckman Coulter, Inc. However, in this example, the toner manufactured by the suspension polymerization method was used, but the present invention is not limited to this. For example, the toner manufactured by using another polymerization method such as a pulverization method or an emulsion polymerization method. It may be. In the present embodiment, the developing devices 4Y, 4M, 4C, and 4K contain yellow toner, magenta toner, cyan toner, and black toner, respectively.
That is, in the present embodiment, the developing device 4 is a portion where the charge is attenuated by the exposure of the surface of the photosensitive drum 1 (exposure portion) of the toner charged to the same polarity (negative polarity in this embodiment) as the charging polarity of the photosensitive drum 1. The so-called reversal development is performed, which is adhered to the surface. As a result, the electrostatic latent image on the photosensitive drum 1 (on the image carrier) is developed, and a toner image is formed.

  In this embodiment, the photosensitive drum 1, the charging roller 2 as the process means acting on the photosensitive drum 1, and the developing device 4 are integrated into a cartridge to form a process cartridge 7. The process cartridge 7 can be attached to and detached from the image forming apparatus 100 via mounting means (not shown) such as a mounting guide and a positioning member provided in the main body of the image forming apparatus 100. In this embodiment, the process cartridges 7 for each color all have the same shape, and each of the process cartridges 7 for each color has yellow (Y), magenta (M), cyan (C), and black. Each color toner of (K) is stored. However, in this embodiment, the process cartridge will be described, but the present invention is not limited to this. For example, the developing device 4 may be fixed to the main body of the image forming apparatus 100 so that only the toner is supplied to the developing device 4. It is good also as a structure which can be attached or detached.

  On the other hand, in order to transfer the toner image on the photosensitive drum 1 onto the recording material P facing the four photosensitive drums 1, an intermediate transfer belt 5 as an intermediate transfer member is disposed. In the rotation direction of the photosensitive drum 1, the charging position by the charging roller 2, the exposure position by the exposure device 3, the development position by the developing device 4, and the transfer position of the toner image to the intermediate transfer belt 5 are provided in this order.

The intermediate transfer belt 5 is formed of an endless rotatable belt, and is stretched between a driving roller 51, a secondary transfer counter roller 52, and a tension roller 53 (hereinafter, the three are collectively referred to as a stretching roller). In some cases). Here, the driving roller 51, the secondary transfer counter roller 52, and the tension roller 53 correspond to a stretching member. The secondary transfer counter roller 52 corresponds to a counter member.
Then, all the photosensitive drums 1 are brought into contact with each other on the outer peripheral surface, and are driven to rotate (rotate) in a direction indicated by an arrow b (counterclockwise) in a direction indicated by an arrow b by driving a driving roller 51 to which a driving motor (not shown) is connected. To do. In this embodiment, the intermediate transfer belt 5 circulates (rotates) at the same 240 mm / sec as the peripheral speed of the photosensitive drum 1.
On the inner peripheral surface side of the intermediate transfer belt 5, four primary transfer rollers 8 as primary transfer means are arranged in parallel so as to face each photosensitive drum 1. The primary transfer roller 8 presses the intermediate transfer belt 5 toward the photosensitive drum 1 to form a nip (primary transfer nip) at the primary transfer portion N1 where the intermediate transfer belt 5 and the photosensitive drum 1 are in contact with each other. Further, the secondary transfer roller 9 as a transfer member constituting the secondary transfer unit is in contact with the intermediate transfer belt 5 at a position facing the secondary transfer counter roller 52 on the outer peripheral surface side of the intermediate transfer belt 5. Has been placed.
The secondary transfer roller 9 is in pressure contact with the secondary transfer counter roller 52 via the intermediate transfer belt 5, and a nip (secondary transfer nip) is formed in the secondary transfer portion N <b> 2 where the intermediate transfer belt 5 and the secondary transfer roller 9 are in contact with each other. ). A predetermined DC voltage is applied to the secondary transfer roller 9 from the secondary transfer bias power source 6. Further, an intermediate transfer belt cleaning device 11 as a collecting unit (intermediate transfer member cleaning unit) is provided at a position facing the tension roller 53 on the outer peripheral surface side of the intermediate transfer belt 5. Here, the secondary transfer bias power source 6 applies a voltage to the secondary transfer roller 9 and also applies a voltage to the secondary transfer roller 9 to apply the intermediate transfer belt 5 in contact with the secondary transfer roller 9. This corresponds to voltage applying means for applying a voltage. The secondary transfer roller 9 and the secondary transfer bias power source 6 constitute secondary transfer means.

As the intermediate transfer belt 5, the following can be used. It is made of urethane resin, fluorine resin, nylon resin, polyimide resin, etc., made of elastic material such as silicone rubber, urethane rubber, hydrin rubber, etc., or carbon or conductive powder is dispersed in these. The resistance is adjusted.
In this embodiment, the intermediate transfer belt 5 has a thickness of 0.5 to 3.0 μm on a base material (outer peripheral surface side) in which carbon black is dispersed in a polyphenylene sulfide (PPS) resin having a thickness of 100 μm. A belt provided with a surface layer of high-resistance acrylic resin is used. Further, the resistance value of the intermediate transfer belt 5 of this embodiment was 110 kΩ.
Since the intermediate transfer belt 5 in this embodiment has a low resistance, the measurement is performed using a resistivity meter Hiresta UP manufactured by Mitsubishi Chemical Analytech Co., Ltd. and a Hiresta UP dedicated probe URS probe as a measurement electrode when measured at an applied voltage of 100V. It was out of range.
Therefore, the resistance value of the intermediate transfer belt 5 was measured using an image forming apparatus 100 in this embodiment as shown in FIG. 2A is a schematic diagram for explaining a case where the resistance value of the intermediate transfer belt 5 is measured at the position of the photosensitive drum 1Y, and FIG. 2B is a schematic diagram at the position of the photosensitive drum 1K. 6 is a schematic diagram for explaining a case where a resistance value of a transfer belt 5 is measured. FIG.

In FIG. 2A, as a resistance measuring electrode of the intermediate transfer belt 5, a secondary transfer roller 9M, which is a metal roller having the same shape as the secondary transfer roller 9, and an aluminum drum 1YM excluding the photosensitive layer are respectively provided. They are arranged in place of the next transfer roller 9 and the photosensitive drum 1Y. The secondary transfer roller 9M is connected to the secondary transfer bias power source 6, and the aluminum drum 1YM is grounded via an ammeter. The current flowing when 100 V was applied from the secondary transfer bias power source 6 to the secondary transfer roller 9M was detected, and the resistance value calculated by the following equation was used as the circumferential resistance value of the intermediate transfer belt 5.
Resistance value = applied voltage / detected current value In this embodiment, the distance from the secondary transfer portion N2 to the primary transfer portion N1 of the aluminum drum 1YM is 420 mm on the upper path of the intermediate transfer belt 5 and on the lower path. It was 370 mm. The circumferential resistance value of the intermediate transfer belt 5 measured by this method is 110 kΩ. Also, as shown in FIG. 2B, the circumferential resistance value of the intermediate transfer belt 5 was almost the same when the aluminum drum was changed in position and measured at the position of the aluminum drum 1KM. At this time, the distance from the secondary transfer portion N2 to the primary transfer portion N1 of the aluminum drum 1KM was 710 mm in the upper path of the intermediate transfer belt 5 and 80 mm in the lower path.
As described above, the intermediate transfer belt 5 in this embodiment is applied with a voltage from the secondary transfer bias power source 6 (by a current supplied from the secondary transfer bias power source 6), and thus has a constant potential over the entire circumference. It is comprised so that it may become.

The driving roller 51 has a conductive rubber layer having a high friction and a volume resistivity of 10 ⁵ Ωcm or less on the surface layer in order to drive the intermediate transfer belt 5.
In order to form a secondary transfer nip, the secondary transfer counter roller 52 similarly has a conductive rubber layer having a volume resistivity of 10 ⁵ Ωcm or less on the surface layer. Further, the secondary transfer counter roller 52 rotates following the intermediate transfer belt 5.
The tension roller 53 is made of a metal roller, applies a tension of a total pressure of about 60 N to the intermediate transfer belt 5, and rotates following the intermediate transfer belt 5.

The drive roller 51, the secondary transfer counter roller 52, and the tension roller 53 are grounded via a resistance element having a predetermined resistance value. In this embodiment, all are grounded through a resistance element having a resistance value of 1 GΩ. However, in this embodiment, the resistance value of the resistance element connected to each roller is 1 GΩ, but the present invention is not limited to this. In this embodiment, each roller is grounded via a resistance element having the same resistance value. However, the present invention is not limited to this, and the resistance value of the resistance element connected to each roller may be different. . In this embodiment, the resistance element is connected to each roller. However, the present invention is not limited to this. For example, the roller may be grounded via a Zener diode having a predetermined breakdown voltage.

The primary transfer roller 8 is configured by covering the cored bar surface with an elastic layer. The primary transfer roller 8 is in contact with the photosensitive drum 1 via the intermediate transfer belt 5 at the primary transfer portion N1, and is driven to rotate as the intermediate transfer belt 5 circulates. In this embodiment, the primary transfer roller 8 is an elastic roller having a volume resistivity of 10 ⁵ to 10 ⁹ Ωcm and a rubber hardness of 30 ° in Asker C hardness. In this embodiment, the primary transfer roller 8 is in contact with the photosensitive drum 1 through the intermediate transfer belt 5 with a total pressure of about 9.8 N to form a primary transfer nip. In this embodiment, the primary transfer roller 8 is in an electrically floating state.

The secondary transfer roller 9 is configured by covering the core metal surface with an elastic layer. The secondary transfer roller 9 is in contact with the secondary transfer counter roller 52 via the intermediate transfer belt 5 in the secondary transfer portion N2, and is driven to rotate as the intermediate transfer belt 5 circulates. In this embodiment, the secondary transfer roller 9 is configured by covering the core metal surface with an elastic layer such as an EPDM foam layer, the volume resistivity is 10 ⁷ to 10 ⁹ Ωcm, and the rubber hardness is Asker C hardness of 30. ° Elastic roller is used. In this embodiment, the secondary transfer roller 9 is in contact with the secondary transfer counter roller 52 via the intermediate transfer belt 5 with a total pressure of about 39.2 N to form a secondary transfer nip. In this embodiment, a DC voltage of −3000 to 7000 V can be applied from the secondary transfer bias power source 6 to the secondary transfer roller 9, and 2000 V is applied to the secondary transfer roller 9 according to the image forming operation. A voltage is applied.

  At this time, a current flows from the secondary transfer roller 9 through the secondary transfer nip to the ground through the intermediate transfer belt 5, the stretching roller, and a resistance element having a resistance value of 1 GΩ, and at the primary transfer portion N1, the intermediate transfer belt. A discharge corresponding to the potential difference occurs between the photosensitive drum 5 and the photosensitive drum 1. Thereby, the potential of the intermediate transfer belt 5 is given. When the surface potential of the intermediate transfer belt 5 is measured at each position of the photosensitive drums 1Y, 1M, 1C, and 1K in each image forming unit by using a surface potential meter Model 344 manufactured by Trek, any position of the photosensitive drum 1 is measured. The surface potential of the intermediate transfer belt 5 was about 200V.

At the primary transfer portion N1, an electric field (potential gradient) is formed by the difference between the surface potential of the photosensitive drum 1 (-500 V for the non-exposed portion and -100 V for the exposed portion) and the surface potential (200 V) of the intermediate transfer belt 5. The Here, the direction of the electric field is a direction in which the negatively charged toner moves from the photosensitive drum 1 to the intermediate transfer belt 5 regardless of the non-exposed portion and the exposed portion on the surface of the photosensitive drum 1. By this electric field, the toner image formed on the photosensitive drum 1 is transferred onto the intermediate transfer belt 5.
In the secondary transfer portion N2, negatively charged toner is transferred from the intermediate transfer belt 5 to the secondary transfer roller due to the difference between the surface potential (200V) of the intermediate transfer belt 5 and the voltage (2000V) applied to the secondary transfer roller 9. An electric field in the direction moving to 9 is formed. The toner image on the intermediate transfer belt 5 is transferred onto the recording material P by this electric field.

The intermediate transfer belt cleaning device 11 has a cleaning blade that contacts the outer peripheral surface of the intermediate transfer belt 5, scrapes off the toner on the intermediate transfer belt 5, and collects it in the intermediate transfer belt cleaning device 11. The intermediate transfer belt cleaning device 11 removes toner adhering to the intermediate transfer belt 5 (on the intermediate transfer member) on the downstream side of the intermediate transfer belt 5 in the rotation direction of the intermediate transfer belt 5 with respect to the secondary transfer portion N2. Arranged to collect.
Further, a fixing device 10 as a fixing unit having a fixing roller and a pressure roller is provided on the downstream side in the transport direction of the recording material P in the secondary transfer portion N2.

  At the time of image formation, first, the photosensitive drum 1 is rotationally driven in accordance with an image forming operation start signal and is uniformly charged by the charging roller 2. Next, the uniformly charged surface of the photosensitive drum 1 is scanned and exposed by a laser beam corresponding to the image information output from the exposure device 3, and an electrostatic latent image according to the image information is formed on the photosensitive drum 1. It is formed. Then, the electrostatic latent image formed on the photosensitive drum 1 is developed as a toner image by the developing device 4.

Next, the toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto the intermediate transfer belt 5 by the action of an electric field at the primary transfer portion N1. Here, for example, when forming a full-color image, the process is sequentially performed in the first to fourth image forming units SY, SM, SC, and SK, and the toner images of the respective colors are sequentially superimposed on the intermediate transfer belt 5 to be primary. Transferred to form a full color toner image. Thereafter, the recording material P is conveyed to the secondary transfer portion N2 in synchronization with the movement of the intermediate transfer belt 5.
By applying a voltage from the secondary transfer bias power source 6 to the secondary transfer roller 9, the toner image on the intermediate transfer belt 5 is transferred onto the recording material P by the action of an electric field at the secondary transfer portion N2 ( Secondary transfer). The recording material P to which the toner image has been transferred is conveyed to the fixing device 10. Then, in the nip (fixing nip) formed at the contact portion between the fixing roller and the pressure roller, heat and pressure are applied to the recording material P, and the toner image is fixed on the recording material P. Thereafter, the recording material P is discharged outside the image forming apparatus 100.

  On the other hand, the toner (secondary transfer residual toner) remaining on the intermediate transfer belt 5 without being secondarily transferred onto the recording material P at the secondary transfer portion N2 is subjected to intermediate transfer belt cleaning as the intermediate transfer belt 5 is circulated. It is conveyed to the part facing the device 11. The secondary transfer residual toner is removed and collected from the intermediate transfer belt 5 by the intermediate transfer belt cleaning device 11.

Further, the toner (primary transfer residual toner) remaining on the photosensitive drum 1 without being primary transferred onto the intermediate transfer belt 5 in the primary transfer portion N1 comes into contact with the charging roller 2 as the photosensitive drum 1 rotates. And is conveyed to a portion facing the developing device 4. At this time, the surface of the photosensitive drum 1 is again charged and exposed to form an electrostatic latent image according to the image information. Further, the primary transfer residual toner conveyed to the portion facing the developing device 4 is almost negatively charged. The primary transfer residual toner is an electric field formed by the difference between the surface potential of the photosensitive drum 1 (-500 V for the non-exposed portion and -100 V for the exposed portion) and the voltage (-300 V) applied to the developing roller 12. A part of the toner is collected by the developing device 4.
That is, in the non-exposed portion, the direction of the electric field is the direction in which the negatively charged toner is moved from the photosensitive drum 1 to the developing roller 12, so that the primary transfer residual toner on the photosensitive drum 1 moves to the developing roller 12. Then, it is collected in the developing device 4.
On the other hand, in the exposure unit, the direction of the electric field is a direction in which the negatively charged toner is moved from the developing roller 12 onto the photosensitive drum 1, so the negatively charged toner on the developing roller 12 moves onto the photosensitive drum 1 to be photosensitive. The electrostatic latent image on the drum 1 is developed. At this time, the primary transfer residual toner on the photosensitive drum 1 is also used for developing the electrostatic latent image. As described above, the developing device 4 is configured to develop the electrostatic latent image formed on the photosensitive drum 1 into a toner image and to collect the toner adhering to the photosensitive drum 1.
Note that the image forming apparatus 100 can also form a single or multi-color image using only one or some (not all) desired image forming units.

Next, toner adhesion and accumulation on the surface of the charging roller 2 in this embodiment will be described.
Due to the effect of transfer at the primary transfer portion N1, the primary transfer residual toner contains toner charged to a polarity opposite to the normal charge polarity (positive polarity in this embodiment) at a certain ratio. Therefore, an electric field formed by the difference between the voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1 when the primary transfer residual toner passes through the contact portion with the charging roller 2 as the photosensitive drum 1 rotates. Thus, the positively charged toner in the primary transfer residual toner moves onto the charging roller 2. The toner adhering to the surface of the charging roller 2 occurs each time the primary transfer residual toner passes through the contact portion with the charging roller 2 as the photosensitive drum 1 rotates. Toner accumulates on the surface.

  When toner adheres to the surface of the charging roller 2, the charging performance of the charging roller 2 changes, so that the surface potential of the photosensitive drum 1 changes, leading to fluctuations in image density. Further, as the amount of toner attached to the surface of the charging roller 2 increases, the change in charging performance increases and the change in the surface potential of the photosensitive drum 1 also increases. Therefore, although the influence of one image forming operation is small, if a certain amount or more of toner accumulates on the surface of the charging roller 2, the surface of the photosensitive drum 1 cannot be uniformly charged, and the image density fluctuation cannot be ignored.

Next, cleaning of the charging roller 2 in this embodiment will be described.
In the cleaning operation of the charging roller 2 in this embodiment, an electric field in the opposite direction to that during the image forming operation is formed between the charging roller 2 and the secondary transfer roller 9 via the photosensitive drum 1 and the intermediate transfer belt 5. Is done. Here, the resistance elements connected to the drive roller 51, the secondary transfer counter roller 52, and the tension roller 53 have a role of forming this electric field.
The toner on the charging roller 2 is positively charged and electrostatically adheres to the surface of the charging roller 2. Therefore, by setting the electric field between the charging roller 2 and the photosensitive drum 1 in the direction opposite to that during the image forming operation, it is possible to remove the toner attached to the surface of the charging roller 2. At this time, the voltage applied to the charging roller 2 is opposite to the normal charging polarity of the toner with respect to the surface potential of the photosensitive drum 1, assuming that the discharge start voltage between the charging roller 2 and the photosensitive drum 1 is VaV. It is preferable that it is 100V or more and VaV or less. This is because when the voltage is less than 100 V, an electric field that is sufficiently strong to move the positively charged toner from the charging roller 2 to the photosensitive drum 1 is not formed. On the other hand, when VaV is exceeded, a discharge occurs between the charging roller 2 and the photosensitive drum 1, and a part of the positively charged toner on the charging roller 2 changes to the negatively charged toner and can move to the photosensitive drum 1. This is because it disappears.

  In order to set the electric field between the charging roller 2 and the photosensitive drum 1 in the opposite direction to that during the image forming operation, the cleaning operation (cleaning process, recovery process) of the charging roller 2 performs the image forming operation on the recording material. This is done during non-image formation. In order to prevent image density fluctuation, the charging roller 2 is cleaned before a certain amount of toner accumulates on the surface of the charging roller 2 and the surface of the photosensitive drum 1 cannot be uniformly charged. In this embodiment, the cleaning operation of the charging roller 2 is performed in the post-rotation process, which is the end process of the image forming operation. Further, during continuous printing, the charging roller 2 is cleaned once every 20 sheets of continuous printing. The cleaning operation of the charging roller 2 is executed by a control unit (execution unit) provided in the image forming apparatus.

FIG. 3 is a timing chart of the cleaning operation of the charging roller 2 in this embodiment.
In the drawing, the developing roller 12 is first separated from the surface of the photosensitive drum 1 in accordance with the end of the image forming operation. The voltage applied to the secondary transfer roller 9 is switched from 2000 V to -1200 V at time t0 immediately after the trailing edge of the final toner image on the intermediate transfer belt 5 passes through the secondary transfer portion N2.
Here, the potential of the intermediate transfer belt 5 is given by the current supplied from the secondary transfer roller 9 through the secondary transfer nip. Accordingly, by switching the voltage applied to the secondary transfer roller 9, the surface potential of the intermediate transfer belt 5 changes from 200V to about -960V.

Next, the voltage applied to the charging roller 2 is switched to 0V. As a result, the electric field (potential gradient) between the charging roller 2 and the photosensitive drum 1 is in the opposite direction to that during the image forming operation, and the positively charged toner attached to the surface of the charging roller 2 moves onto the photosensitive drum 1. The positively charged toner that has moved onto the photosensitive drum 1 is conveyed to the primary transfer portion N1 as the photosensitive drum 1 rotates.
In the primary transfer portion N1, the positively charged toner moves from the photosensitive drum 1 to the intermediate transfer belt 5 due to the difference between the surface potential (−500V) of the photosensitive drum 1 and the surface potential (−960V) of the intermediate transfer belt 5. An electric field is formed in the direction of the direction. Due to this electric field, the positively charged toner on the photosensitive drum 1 moves onto the intermediate transfer belt 5.
The surface potential of the intermediate transfer belt 5 is preferably a value that is on the same polarity side as the normal charging polarity of the toner with respect to the surface potential of the photosensitive drum 1 and has good primary transferability.

Here, in general, the case where 90% or more of the toner on the photosensitive drum 1 moves on the intermediate transfer belt 5 in the primary transfer portion N1 is said to have good primary transferability. In this embodiment, the surface potential of the intermediate transfer belt 5 with good primary transferability in the cleaning process of the charging roller 2 was 100 V or more and 1000 V or less with respect to the surface potential of the photosensitive drum 1. Then, the positively charged toner that has moved onto the intermediate transfer belt 5 passes through the secondary transfer portion N2 as the intermediate transfer belt 5 circulates, and is conveyed to a portion facing the intermediate transfer belt cleaning device 11 for intermediate transfer belt cleaning. Collected by the device 11.
Through the above steps, the toner adhering to the surface of the charging roller 2 can be removed.

The time required to set the electric field between the charging roller 2 and the photosensitive drum 1 in the direction opposite to that during the image forming operation requires at least the following time. That is, until all of the positively charged toner moved from the charging roller 2Y of the first image forming unit SY located farthest from the secondary transfer unit N2 to the intermediate transfer belt 5 passes through the secondary transfer unit N2. Is the time.
This is because if the time is shorter than the above time, the direction of the electric field in the primary transfer portion N1 and the secondary transfer portion N2 of each image forming portion changes. In this case, the positively charged toner that has moved onto the intermediate transfer belt 5 is transferred onto the photosensitive drum 1 or 2 by an electric field at the primary transfer portion N1 or the secondary transfer portion N2 on the downstream side in the circulation movement direction of the intermediate transfer belt 5. It moves onto the next transfer roller 9. In this embodiment, the cleaning process time of the charging roller 2 (interval from time t0 to time t1) is 7 sec.
In this embodiment, the dark decay rate of the surface potential of the photosensitive drum 1 is about 1.3 V / sec, and the influence can be almost ignored in the cleaning process of the charging roller 2.

Next, the voltage applied to the secondary transfer roller 9 in the cleaning process of the charging roller 2 of this embodiment will be described.
FIG. 4 is a graph showing the relationship between the voltage applied to the secondary transfer roller 9 in the cleaning process of the charging roller 2 in this embodiment, the potential of the intermediate transfer belt 5 and the surface potential of the photosensitive drum 1. FIG. 5 is a schematic diagram for explaining a current path when a predetermined voltage is applied to the secondary transfer roller 9 in the cleaning process of the charging roller 2. FIG. 5A is a diagram for explaining the case where the voltage applied to the secondary transfer roller 9 is −1300 V or less. FIG. 5B is a diagram for explaining a case where the voltage applied to the secondary transfer roller 9 exceeds −1300V.
Here, the surface potential of the intermediate transfer belt 5 and the surface potential of the photosensitive drum 1 are measured at the position of the third image forming unit SC by using a surface potential meter Model 344 manufactured by Trek. Is a measured value. Further, in this embodiment, the surface potentials of the intermediate transfer belt 5 and the photosensitive drum 1 are obtained by using an average value obtained by performing measurement for 1 second after a predetermined voltage is applied to the secondary transfer roller 9. Yes.

In FIG. 4, when the voltage applied to the secondary transfer roller 9 is −1300 V or less, when a predetermined voltage is applied to the secondary transfer roller 9, the intermediate transfer belt 5 passes through the secondary transfer nip. The current flows to the ground through the tension roller and the resistance element having a resistance value of 1 GΩ (see FIG. 5A). The shared voltage is determined according to the electric resistance, contact resistance, etc. of each member, and the potential of the intermediate transfer belt 5 is given. At this time, a potential difference corresponding to the contact resistance is generated between the secondary transfer roller 9 and the intermediate transfer belt 5. Due to this potential difference, an electric field in the direction in which the positively charged toner moves from the intermediate transfer belt 5 to the secondary transfer roller 9 is formed in the secondary transfer portion N2.

On the other hand, when the voltage applied to the secondary transfer roller 9 exceeds −1300 V, the absolute value of the potential difference between the photosensitive drum 1 and the intermediate transfer belt 5 in the primary transfer portion N1 is The discharge start voltage VbV between the belts 5 is exceeded, and discharge corresponding to the potential difference occurs. As a result, the surface of the photosensitive drum 1 is charged, and the surface potential of the photosensitive drum 1 changes from −500V. In this case, when a predetermined voltage is applied to the secondary transfer roller 9, a current flows to the ground through the secondary transfer nip through the intermediate transfer belt 5, the stretching roller, and a resistance element having a resistance value of 1 GΩ, A discharge current flows between the intermediate transfer belt 5 and the photosensitive drum 1 in the primary transfer portion N1. (See FIG. 5 (b))
This discharge current causes a voltage drop in the intermediate transfer belt 5 and the potential of the intermediate transfer belt 5 is lowered. Therefore, the potential difference corresponding to the contact resistance between the secondary transfer roller 9 and the intermediate transfer belt 5 is rapidly increased as compared with the case where no discharge occurs. That is, the electric field formed in the secondary transfer portion N2 becomes strong, and the amount of positively charged toner that moves from the intermediate transfer belt 5 to the secondary transfer roller 9 increases.

Therefore, in the present invention, in the cleaning process of the charging roller 2, the voltage applied to the secondary transfer roller 9 is applied between the photosensitive drum 1 and the intermediate transfer belt 5 by applying a voltage to the secondary transfer roller 9. The absolute value of the formed potential difference is set to a value that is equal to or less than VbV.
Thereby, the voltage drop of the intermediate transfer belt 5 due to the discharge current is eliminated, and the potential difference between the secondary transfer roller 9 and the intermediate transfer belt 5 is reduced. Therefore, the amount of positively charged toner that moves from the intermediate transfer belt 5 to the secondary transfer roller 9 in the secondary transfer portion N2 is reduced. In this embodiment, the voltage applied to the secondary transfer roller 9 is −1200V.
Here, in this embodiment, the photosensitive drum 1 or toner having a normal charging polarity of negative polarity is used, but the present invention is not limited to this, and the photosensitive drum 1 or toner having a normal charging polarity of positive polarity is used. May be. In that case, the polarity of the voltage applied to each member including the charging roller 2 and the developing device 4 is changed as necessary.

(Comparative Example 1)
Below, the comparative example 1 is demonstrated.
This comparative example basically conforms to the first embodiment, but differs in the following points. In this comparative example, the voltage applied to the secondary transfer roller 9 in the cleaning process of the charging roller 2 is set such that the absolute value of the potential difference between the photosensitive drum 1 and the intermediate transfer belt 5 exceeds VbV. In this comparative example, the voltage applied to the secondary transfer roller 9 was set to −1600V. At this time, the surface potential of the intermediate transfer belt 5 was about −1100V.

Next, evaluation of Example 1 and Comparative Example 1 will be described.
Regarding the configurations of Example 1 and Comparative Example 1, the cleaning effect of the charging roller 2 and the toner adhesion to the secondary transfer roller 9 in the cleaning process of the charging roller 2 were evaluated. Here, all evaluations were performed in an environment of 23 ° C. and 50% RH. Hereinafter, each evaluation will be described individually.

(A) Evaluation of Charging Roller Cleaning Effect In this evaluation, 20 A4 size full-scale black images having an image ratio of 100% are printed continuously. Thereafter, the cleaning operation of the charging roller 2 was performed, and the change in the toner amount on the charging roller 2 before and after the cleaning operation of the charging roller 2 was evaluated.
Specifically, after the toner on the charging roller 2 was collected with a polyester tape, the tape density was measured with a spectral densitometer X-Rite 504 manufactured by Gretag Macbeth Co., and the average value was taken as Dt. Then, the difference (Dt−Dr) between the five-point average value Dr and Dt of the reflection density of the polyester tape itself was obtained and used as the toner density on the charging roller 2. Further, this was obtained before and after the cleaning operation of the charging roller 2, and the toner concentrations on the charging roller 2 before and after the cleaning operation of the charging roller 2 were set to Dp and Da, respectively. The toner removal rate on the charging roller 2 was calculated from the following formula.
Toner removal rate (%) = (1−Da / Dp) × 100
The evaluation criteria for the cleaning effect of the charging roller 2 are as follows.
○: Toner removal rate on charging roller 2 is 90% or more ×: Toner removal rate on charging roller 2 is less than 90%

(B) Evaluation of Toner Adhesion to Secondary Transfer Roller In this evaluation, first, 20 A4 size full-scale black images having an image ratio of 100% are continuously printed. Thereafter, the charging roller 2 is cleaned, and the change in the toner amount on the intermediate transfer belt 5 around the secondary transfer portion N2 when the positively charged toner on the intermediate transfer belt 5 passes through the secondary transfer portion N2 is evaluated. did.
Specifically, the toner concentrations on the intermediate transfer belt 5 before and after the secondary transfer portion N2 are obtained by the same method as in (A), and are set as Dp2 and Da2, respectively. The toner adhesion rate to the secondary transfer roller 9 was calculated from the following equation.
Toner adhesion rate (%) = (1−Da2 / Dp2) × 100

  Table 1 shows the cleaning effect evaluation results of the charging roller (A) of Example 1 and Comparative Example 1. FIG. 6 is a graph showing the relationship between the voltage applied to the secondary transfer roller 9 and the toner adhesion rate to the secondary transfer roller 9 in the cleaning process of the charging roller 2.

Hereinafter, the superiority of the present invention will be described by comparing Example 1 and Comparative Example 1.
First, as shown in Table 1, the cleaning effect of the charging roller 2 was good in both Example 1 and Comparative Example 1. This is because the potential difference between the charging roller 2 and the photosensitive drum 1 is sufficiently large in both Example 1 and Comparative Example 1, and the positively charged toner accumulated on the charging roller 2 is moved from the charging roller 2 onto the photosensitive drum 1. This is because a sufficient electric field is formed.
Further, when the surface of the photosensitive drum 1 after the cleaning operation of the charging roller 2 was visually observed, in Example 1 and Comparative Example 1, there was almost no toner on the photosensitive drum 1. From these facts, it can be seen that in both cases, the positively charged toner accumulated on the charging roller 2 has moved almost entirely from the charging roller 2 to the intermediate transfer belt 5 via the photosensitive drum 1.

Next, as shown in FIG. 4, in the region where the voltage applied to the secondary transfer roller 9 including Comparative Example 1 exceeds −1300 V, the toner adhesion rate to the secondary transfer roller 9 in the cleaning process of the charging roller 2 is high. It is increasing rapidly. This is because when the voltage applied to the secondary transfer roller 9 exceeds −1300 V, the potential of the intermediate transfer belt 5 is lowered as follows. That is, when the voltage applied to the secondary transfer roller 9 exceeds -1300V, the absolute value of the potential difference between the photosensitive drum 1 and the intermediate transfer belt 5 exceeds VbV in the primary transfer portion N1, and the discharge corresponding to the potential difference occurs. Occurs, the voltage of the intermediate transfer belt 5 drops, and the potential of the intermediate transfer belt 5 decreases.
As a result, the potential difference between the secondary transfer roller 9 and the intermediate transfer belt 5 in the secondary transfer portion N2 increases, and the amount of positively charged toner that moves from the intermediate transfer belt 5 to the secondary transfer roller 9 is large. Become. In other words, the backside of the recording material is likely to occur.

  On the other hand, when the applied voltage to the secondary transfer roller 9 including Example 1 is −1300 V or less, the toner adhesion rate to the secondary transfer roller 9 in the cleaning process of the charging roller 2 is 50% or less. This is because when the applied voltage to the secondary transfer roller 9 is −1300 V or less, the absolute value of the potential difference between the photosensitive drum 1 and the intermediate transfer belt 5 at the primary transfer portion N1 is VbV or less. This is because no discharge occurs between the transfer belt 5 and the transfer belt 5. As a result, the potential difference between the secondary transfer roller 9 and the intermediate transfer belt 5 in the secondary transfer portion N2 becomes small, and the amount of positively charged toner that moves from the intermediate transfer belt 5 to the secondary transfer roller 9 is small. Become.

As described above, in this embodiment, the voltage applied to the secondary transfer roller 9 in the cleaning process of the charging roller 2 is set to the following value. This is because when the voltage is applied to the secondary transfer roller 9, the absolute value of the potential difference formed between the photosensitive drum 1 and the intermediate transfer belt 5 in the primary transfer portion N1 is VbV or less (less than the discharge start voltage). ).
Thus, the voltage drop of the intermediate transfer belt 5 due to the discharge generated between the photosensitive drum 1 and the intermediate transfer belt 5 is eliminated, and the potential difference between the secondary transfer roller 9 and the intermediate transfer belt 5 in the secondary transfer portion N2 is reduced. Thus, toner adhesion to the secondary transfer roller 9 can be reduced. Accordingly, in the cleaning process of the charging roller 2, it is possible to clean the charging roller 2 while reducing toner adhesion to the secondary transfer roller 9.

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum, 2 ... Charging roller, 4 ... Developing device, 5 ... Intermediate transfer belt, 6 ... Secondary transfer bias power supply, 9 ... Secondary transfer roller, 11 ... Intermediate transfer belt cleaning device, 12 ... Developing roller

Claims (5)

An image carrier on which an electrostatic latent image is formed;
Contact charging means for uniformly charging the image carrier in contact with the image carrier;
A developing member capable of contacting and separating from the image carrier, and developing the electrostatic latent image formed on the image carrier into a toner image while the developing member is in contact with the image carrier; Developing means capable of collecting toner adhering to the body;
An endless rotatable intermediate transfer body onto which a toner image on the image carrier developed by the developing means is transferred at a primary transfer portion;
A transfer member disposed so as to be in contact with the intermediate transfer member and forming a secondary transfer portion with the intermediate transfer member, and is primarily transferred to the intermediate transfer member by applying a voltage. A transfer member for secondary transfer of the toner image to the recording material at the secondary transfer portion;
Voltage applying means for applying a voltage to the transfer member;
A collecting means for collecting toner adhering to the intermediate transfer member on the downstream side in the rotation direction of the intermediate transfer member with respect to the secondary transfer portion of the intermediate transfer member;
An image forming apparatus for forming an image on a recording material,
The voltage applying unit is controlled so that the image carrier and the developing unit are separated from each other between the contact charging unit and the transfer member via the image carrier and the intermediate transfer member in a non-image formation state. An executing means for executing a recovery step for recovering the toner adhering to the contact charging means by the recovery means via the image carrier and the intermediate transfer body by forming a potential gradient between them; An image forming apparatus. When the execution means forms the potential gradient in the recovery step,
When a voltage is applied to the transfer member by the voltage application unit, an absolute value of a potential difference formed between the image carrier and the intermediate transfer member in the primary transfer portion is
In order to be equal to or lower than the discharge start voltage between the image carrier and the intermediate transfer member,
The image forming apparatus according to claim 1, wherein a voltage is applied to the transfer member by the voltage applying unit.   3. The image formation according to claim 1, wherein the toner image is transferred from the image carrier to the intermediate transfer member by applying a voltage to the transfer member by the voltage applying unit during image formation. apparatus.   The counter member is provided at a position facing the transfer member via the intermediate transfer member, and a resistance element for forming the potential gradient is connected to the counter member. The image forming apparatus according to claim 3.   5. The intermediate transfer member is an intermediate transfer belt, and includes a tension member that stretches the intermediate transfer belt, and the resistance element is connected to the tension member. Image forming apparatus.

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