JP2019207332A - Image forming apparatus - Google Patents

Abstract

To prevent an image defect due to attachment of toner to a charging member, while reducing the amount of discharge at a charging part.SOLUTION: An image forming apparatus can execute a first image forming mode of performing image formation by rotating an image carrier and a second image forming mode of performing image formation by rotating the image carrier at a higher rotation speed than that of the first image forming mode; a control unit controls voltage application means and pre-charging exposure means such that the difference between the absolute value of a voltage applied to the voltage application means and the absolute value of a surface potential of the image carrier exposed by the pre-charging exposure means becomes larger during the execution of the second image forming mode than during the execution of the first image forming mode.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine that forms an image by electrophotography.

  An electrophotographic image forming apparatus such as a copying machine or a laser beam printer irradiates an electrophotographic photosensitive member (photosensitive drum) uniformly charged by a charging unit with light corresponding to image data to form an electrostatic image (latent image). ). Then, developer toner, which is a recording material, is supplied from the developing device to the electrostatic image to be visualized as a toner image. This toner image is transferred from a photosensitive drum to a recording material such as recording paper by a transfer device. A recorded image is formed by fixing the toner image on a recording material with a fixing device.

  Since the charging system has advantages such as low ozone and low power, a contact-type charging device is often used in which a charging member is brought into contact with the photosensitive drum for charging. However, when a contact-type charging device is used, the toner remaining on the photosensitive drum without image formation comes into contact with the charging member, so that part of the toner adheres to the charging member and an image defect due to a charging failure occurs. Sometimes.

  In recent years, in order to reduce the size of an image forming apparatus, a cleaning member that cleans a photosensitive drum and a “cleanerless system” image forming apparatus that does not include a waste toner storage unit have been proposed. The cleanerless system is a system that allows the toner to be reused without requiring a waste toner container by allowing the developing device to collect the toner remaining on the photosensitive drum again. When image formation is performed by a cleaner-less system, toner remaining on the photosensitive drum without image formation is not cleaned, and thus particularly easily adheres to the charging member.

  Japanese Patent Application Laid-Open No. 2004-228688 discloses a configuration that suppresses the toner remaining on the photosensitive drum from adhering to the charging member by discharging the surface of the photosensitive drum after the transfer and before charging again. ing. By neutralizing the surface of the photosensitive drum before charging, the charging contrast, which is the potential difference between the charging voltage applied to the charging member and the surface potential of the photosensitive drum, increases, and the amount of discharge at the charging unit increases. Thus, by adjusting the charge of the toner remaining on the photosensitive drum, toner adhesion to the charging member can be reduced, and the occurrence of image defects can be suppressed.

JP2009-192941A

  However, when the charge contrast is increased to increase the amount of discharge at the charging portion, the image quality may be deteriorated due to wear due to discharge deterioration on the surface of the photosensitive drum or adhesion of discharge products.

  Accordingly, an object of the invention according to the present application is to provide an image forming apparatus capable of suppressing an image defect due to toner adhesion to a charging member while suppressing a discharge amount at a charging portion.

  In order to achieve this object, an image forming apparatus according to the present invention is an image forming apparatus for forming a toner image on a recording material, and is a rotatable image carrier and a contact portion in contact with the image carrier. A charging member that charges the image carrier at the contact portion, and a first exposure unit that exposes the image carrier charged by the charging member to form an electrostatic latent image; A developing member that supplies toner to the electrostatic latent image and develops the toner image on the image carrier, a voltage applying unit that applies a voltage to the charging member, and the image carrier An intermediate transfer member that forms a transfer portion in contact with the body and carries the toner image formed on the image carrier at the transfer portion; and the toner image carried on the intermediate transfer member A transfer member to be transferred to the transfer portion and the transfer portion in the rotation direction of the image carrier A second exposure unit that exposes the image carrier on the downstream side and on the upstream side with respect to the contact portion, and a control unit that controls the voltage application unit and the second exposure unit. In the image forming apparatus, a first image forming mode in which the image carrier is rotated to form an image, and a first image forming mode in which the image carrier is rotated at a rotational speed greater than the first image forming mode. The image forming mode 2 can be executed, and the control unit includes an absolute value of the voltage applied to the voltage applying unit and an absolute value of the surface potential of the image carrier exposed by the second exposure unit. The voltage application unit and the second exposure unit are controlled such that the difference between the voltage application unit and the second exposure unit is greater than that during execution of the first image formation mode. Features.

  As described above, according to the present invention, it is possible to suppress image defects due to toner adhesion to the charging member while suppressing the amount of discharge at the charging unit.

1 is a diagram illustrating an image forming apparatus according to Embodiment 1. FIG. FIG. 3 is a control block diagram according to the first embodiment. FIG. 3 is a diagram illustrating discharge between the charging roller and the photosensitive drum according to the first exemplary embodiment. FIG. 6 is a diagram illustrating the movement of transfer residual toner in the cleaner-less configuration according to the first embodiment. FIG. 6 is a diagram illustrating the movement of retransfer toner in a cleaner-less configuration according to Embodiment 1. 1 is a diagram illustrating a primary transfer configuration of an image forming apparatus according to Embodiment 1. FIG. FIG. 6 is a diagram illustrating a state of charge contrast according to the first embodiment. FIG. 6 is a flowchart illustrating an image forming operation according to the first exemplary embodiment. FIG. 6 is a diagram illustrating a relationship between a discharge start voltage and a surface potential of a photosensitive drum according to Example 2. FIG. 10 is a diagram illustrating an image forming apparatus according to a third embodiment. 6 is a diagram illustrating a primary transfer configuration of an image forming apparatus according to Embodiment 3. FIG. FIG. 10 is a diagram illustrating an image forming apparatus according to a fourth embodiment.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in the following embodiments should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions. Therefore, unless specifically stated, the scope of the present invention is not limited to them.

1. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention particularly relates to an image forming apparatus using a so-called drum cleanerless system that does not have a cleaning unit on an image carrier. FIG. 1 is a schematic diagram illustrating an example of a color image forming apparatus. The configuration and operation of the image forming apparatus according to the present embodiment will be described with reference to FIG. The image forming apparatus is a tandem type printer provided with an image forming station which is an image forming unit a to d. The first image forming station a is yellow (Y), the second image forming station b is magenta (M), the third image forming station c is cyan (C), and the fourth image forming station d is black (Bk). ) For each color. Since the configuration of each image forming station is the same except for the color of the toner to be accommodated, in the description of the image forming station, one image forming station will be representatively described.

  Each image forming station includes a rotatable drum-shaped electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1, a charging roller 2 as a charging unit, an exposure device 3, and a developing device 4. . The photosensitive drum 1 is an image carrier that carries a toner image while being driven to rotate at a peripheral speed (process speed) that is a predetermined rotational speed in the direction of an arrow in FIG. An image forming operation is started, and the photosensitive drum 1 is rotationally driven. The surface of the photosensitive drum 1 is uniformly charged to a predetermined potential with a predetermined polarity by the charging roller 2 during rotational driving, and is subjected to exposure according to an image signal by the exposure device 3. In this embodiment, the charging polarity is negative. Thereby, an electrostatic latent image corresponding to the color component image of the target color image is formed. Next, the electrostatic latent image is developed by the developing device 4 at the development position and visualized as a toner image.

The charging roller 2 as a charging member is in contact with the surface of the photosensitive drum 1 by a predetermined pressure contact force, and is driven to rotate with respect to the photosensitive drum 1 by friction with the surface of the photosensitive drum 1. A contact position between the charging roller 2 and the photosensitive drum 1 is defined as a contact portion. A predetermined DC voltage (bias) is applied to the rotating shaft of the charging roller 2 from a charging voltage power source 20 as charging voltage applying means shown in FIG. 2 according to an image forming operation. In the present embodiment, the charging roller 2 is electrically conductive having a thickness of 1.5 (mm) and a volume resistivity of about 1 × 10 ⁶ (Ωcm) on a metal shaft having a diameter of 5.5 (mm). What provided the elastic layer which consists of an elastic body is used. In accordance with the image forming operation, a surface of the photosensitive drum 1 is discharged by applying a DC voltage of −1000 (V) from the charging voltage power source 20 to the rotating shaft of the charging roller 2 and discharging between the charging roller 2 and the photosensitive drum 1. Is charged. Discharge between the charging roller 2 and the photosensitive drum 1 is generated in the vicinity of the upstream and downstream in the rotation direction of the photosensitive drum 1 at the contact portion. The balance between the discharge amount near the upstream and the discharge amount near the downstream changes as the peripheral speed of the photosensitive drum 1 changes. However, if the charging voltage is constant, the photosensitive drum 1 does not depend on the peripheral speed of the photosensitive drum 1. The surface potential of 1 is almost constant. In this embodiment, the dark portion potential Vd on the surface of the photosensitive drum 1 immediately after passing through the contact portion of the photosensitive drum 1 is set to −500 (V) regardless of the peripheral speed of the photosensitive drum 1. In order to measure the surface potential of the photosensitive drum 1, a surface potential meter Model 344 manufactured by Trek was used.

  The surface of the charged photosensitive drum 1 is exposed by the exposure device 3. The exposure device 3 is an exposure unit that forms an electrostatic latent image on the surface of the uniformly charged photosensitive drum 1. The exposure apparatus 3 includes a laser output unit that outputs laser light modulated in accordance with an input time-series electric digital pixel signal, a rotary polygon mirror (polygon mirror), an fθ lens, a reflecting mirror, and the like. Then, the surface of the photosensitive drum 1 is subjected to main scanning exposure. An electrostatic latent image corresponding to image information is formed by the main scanning exposure and the sub scanning by the rotation of the photosensitive drum 1. In the present embodiment, the exposure amount is adjusted so that the bright portion potential Vl that is the surface potential of the photosensitive drum 1 after being exposed by the exposure device 3 becomes −150 (V).

  The developing unit 4 as a developing device includes a developing roller 41 as a developing member as a toner carrier and a non-magnetic one-component developer (hereinafter referred to as toner), and develops an electrostatic latent image as a toner image. That is, the developing means performs a developing action on the photosensitive drum 1. The developing roller 41 is in contact with the photosensitive drum 1 with a predetermined contact width according to the image forming operation. The developing roller 41 is at a peripheral speed larger than the peripheral speed of the photosensitive drum 1 so that the surface movement direction of the developing roller 41 is in the forward direction with the surface movement direction of the photosensitive drum 1 at the portion facing the photosensitive drum 1. Driven by rotation. The developing unit 4 and the main body of the image forming apparatus 100 are provided with a mechanism for controlling the developing contact / separation state of the developing roller 41 and the photosensitive drum 1 (not shown). It has been. The developing roller 41 and the photosensitive drum 1 are brought into contact with each other according to an image forming operation or the like, and are separated when the operation is stopped. Further, as shown in FIG. 2, a predetermined DC voltage is applied to the core of the developing roller 41 from the developing voltage power supply 40 in accordance with the image forming operation. In this embodiment, the developing voltage is −350 (V ) DC voltage is applied. The toner is a non-magnetic toner having a negative charging property as a normal charging polarity manufactured by a suspension polymerization method, and has a volume average particle diameter of about 6.0 (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. In this embodiment, the toner manufactured by the suspension polymerization method is used. However, 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 this embodiment, the image forming apparatus 100 that reversely develops the electrostatic latent image with toner charged to the same negative polarity as the charging polarity of the photosensitive drum 1 is used. However, the present invention can also be applied to the image forming apparatus 100 in which the charging polarity of the photosensitive drum 1 and the charging polarity of the toner are positive. The present invention can also be applied to an electrophotographic apparatus in which an electrostatic latent image is positively developed with toner charged to a polarity opposite to the charging polarity of the photosensitive drum 1, in which case the charging of the photosensitive drum is performed. Toner charged to a polarity opposite to the polarity becomes normal polarity toner. In addition, although non-magnetic toner is used, a magnetic toner may be used.

The toner image formed on the photosensitive drum 1 is transferred to the intermediate transfer belt 10 by the primary transfer roller 14 which is a primary transfer member. The intermediate transfer belt 10 is disposed so as to contact the photosensitive drums a, b, c, and d, and has a surface resistivity of 1 × 10 ¹¹ (Ω / □) when measured by applying 100 (V). ) Was used. The intermediate transfer belt 10 has a thickness of 100 to 200 (μm), and is formed by endless resin films such as PVdf (polyvinylidene fluoride), nylon, PET (polyethylene terephthalate), and PC (polycarbonate). The intermediate transfer belt 10 is stretched by a plurality of stretching members 11, 12, and 13, and is rotated at substantially the same peripheral speed as that of the photosensitive drum 1 in a direction in which the intermediate transfer belt 10 moves in the circumferential direction at a facing portion in contact with the photosensitive drum 1. Driven. The toner image formed on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 10 in the process of passing through a transfer nip portion that is a contact portion between the photosensitive drum 1 and the intermediate transfer belt 10. In the transfer nip portion, the primary transfer roller 14 is disposed so as to face the photosensitive drum 1 directly via the intermediate transfer belt 10. In this embodiment, as shown in FIGS. 1 and 2, primary transfer is performed by applying 500 (V) as a primary transfer voltage from the primary transfer voltage power supply 140 to the primary transfer roller 14.

  However, part of the toner on the photosensitive drum 1 remains on the photosensitive drum 1 as untransferred toner without being transferred. The transfer residual toner remaining on the photosensitive drum 1 is a normal polarity toner having a small charge amount or a reverse polarity toner having a reverse polarity charge. Further, the toner transferred to the intermediate transfer belt 10 by the primary transfer roller 14 is also charged with a reverse polarity by receiving a discharge when passing through the transfer nip portion of the downstream station in the rotation direction of the intermediate transfer belt 10. It may be a reversal polarity toner. The reversal polarity toner is electrically attached as a retransfer toner to the photosensitive drum 1 in the downstream station. The transfer residual toner and the retransfer toner will be described in detail later.

  When the exposure device 3 is the first exposure unit, the pre-charging exposure unit 5 serving as the second exposure unit exposes and neutralizes the surface of the photosensitive drum 1 after the primary transfer over the entire length thereof, thereby removing the photosensitive drum 1. It plays the role of reducing the absolute value of the surface potential and leveling the potential. Thereby, the potential difference (hereinafter referred to as charging contrast) between the charging voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1 immediately before the photosensitive drum 1 passes through the contact portion can be increased. Details of the charging contrast will be described later. As shown in FIG. 3, the surface of the photosensitive drum 1 is exposed by the pre-charging exposure device 5. At this time, the region of the photosensitive drum 1 exposed by the pre-charging exposure device 5 is a region between the contact portion between the transfer portion and the charging roller 2 in the rotation direction of the photosensitive drum 1. In the present embodiment, as shown in FIG. 1, the pre-charging exposure device 5 is disposed downstream of the transfer portion and upstream of the contact portion in the rotation direction of the photosensitive drum 1. When the surface of the photosensitive drum 1 is exposed by the pre-charging exposure device 5 and the charging contrast is increased, the amount of discharge in the vicinity of the charging unit upstream between the charging roller 2 and the photosensitive drum 1 increases. When the discharge amount increases, the transfer residual toner and the retransfer toner on the photosensitive drum 1 can be charged to normal polarity. When the transfer residual toner or the retransfer toner is charged to the normal polarity, an electrostatic repulsive force acts on the charging roller 2 to which the voltage on the normal polarity side is applied, and the toner is difficult to adhere to the charging roller 2. Thus, the toner contamination of the charging roller 2 is suppressed. Further, the pre-charging exposure device 5 exposes the surface of the photosensitive drum 1 after the primary transfer over the entire longitudinal area, and eliminates the surface potential unevenness of the photosensitive drum 1 after the primary transfer by eliminating the static electricity. This has the effect of suppressing image defects such as image density unevenness due to potential unevenness. The surface of the photosensitive drum 1 that has been neutralized by the pre-charge exposure device 5 is recharged to Vd, which is a dark portion potential, by discharge when entering the contact portion, and thereafter, development, primary transfer, and static elimination steps are repeated. .

  After the above operations are performed at the first, second, third, and fourth image forming stations a, b, c, and d, respectively, the first color yellow toner image, the second color magenta toner image, and the third color The cyan toner image and the fourth color black toner image are formed on the intermediate transfer belt 10. The toner images formed at each image forming station are sequentially transferred onto the intermediate transfer belt 10 to obtain a composite color image of a plurality of colors corresponding to the target color image. The four color toner images on the intermediate transfer belt 10 are fed by the sheet feeding means 50 in the process of passing through the secondary transfer portion formed by the intermediate transfer belt 10 and the secondary transfer roller 15 as a secondary transfer member. It is collectively transferred onto the surface of the recording material P. The secondary transfer roller 15 contacts the intermediate transfer belt 10 with a pressing force of 50 (N) to form a secondary transfer portion. The secondary transfer roller 15 is driven to rotate with respect to the intermediate transfer belt 10. In this embodiment, when the toner on the intermediate transfer belt 10 is secondarily transferred to the recording material P, the secondary transfer voltage power supply 150 applies 1000 (V) to the secondary transfer roller 15 as shown in FIG. Is applied.

  Thereafter, the recording material P carrying the four-color toner images is introduced into the fixing device 30, where the four-color toners are melted and mixed and fixed to the recording material P by being heated and pressurized. The toner remaining on the intermediate transfer belt 10 after the secondary transfer is cleaned and removed by the cleaning device 17. The cleaning device 17 includes a cleaning blade that contacts the outer peripheral surface of the intermediate transfer belt 10, scrapes off toner remaining on the intermediate transfer belt 10, and collects the toner in the intermediate transfer belt cleaning device 17. The intermediate transfer belt cleaning device 17 is disposed on the downstream side of the intermediate transfer belt 10 in the rotation direction of the intermediate transfer belt 10 with respect to the secondary transfer portion so as to collect toner adhering to the intermediate transfer belt 10. Yes.

  FIG. 2 is a block diagram illustrating a schematic control mode of a main part of the image forming apparatus 100 according to the present exemplary embodiment. The controller 200 exchanges various types of electrical information with the host device, and supervises the image forming operation of the image forming apparatus 100 by the control unit 202 via the interface 201 in accordance with a predetermined control program and reference table. Control. The control unit 202 includes a CPU 151 that is a central element that performs various arithmetic processes, a memory 152 such as a ROM and a RAM that are storage elements, and the like. The RAM stores sensor detection results, counter count results, calculation results, and the like, and the ROM stores control programs, data tables obtained in advance through experiments, and the like. Each control object, sensor, counter, and the like in the image forming apparatus 100 are connected to the control unit 202. The control unit 202 controls a predetermined image forming sequence by controlling transmission / reception of various electrical information signals and driving timing of each unit. For example, the control unit 202 controls the voltage applied by the charging voltage power source 20, the developing voltage power source 40, the exposure device 3, the pre-charging exposure device 5, the primary transfer voltage power source 140, and the secondary transfer voltage power source 150. Yes. The image forming apparatus 100 forms an image on the recording material P based on an electrical image signal input to the controller 200 from the host device. Examples of the host device include an image reader, a personal computer, a facsimile, and a smartphone.

2. Cleanerless System A phenomenon that occurs in the operation of an individual process cartridge when the cleanerless system according to the present embodiment is implemented will be described with reference to FIG.

  As shown in FIG. 4A, after the toner image developed on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 10, a part of the toner that has not been primarily transferred is primarily transferred onto the photosensitive drum 1. It remains as residual toner. When there is a cleaning member, the primary transfer residual toner is collected by the cleaning member. However, in the case of a cleanerless system, there is no cleaning device for collecting the primary transfer residual toner. Therefore, the toner on the photosensitive drum 1 enters the charging roller 2 as it is without being cleaned. The primary transfer residual toner that enters the charging roller 2 is a normal polarity toner or a reverse polarity toner with a small charge amount. As shown in FIG. 4 (b), these primary transfer residual toners are discharged by an electric field due to a charging voltage in the gap portion before the contact portion, and have a negative polarity having the same polarity as that of the photosensitive drum 1. Charged. Since the primary transfer residual toner has a small amount of charge, it is easily affected by discharge, and tends to become negative polarity toner having normal polarity by discharge. Therefore, since the charging voltage is negative and larger than the surface potential of the photosensitive drum 1 at the contact portion, the primary transfer residual toner charged to the negative polarity is attached to the charging roller 2 as shown in FIG. It passes through the charging roller 2 without being attached. A part of the reverse polarity toner that has entered the charging roller 2 without being discharged is electrically attracted to the charging roller 2 and may cause image defects as dirt on the charging roller 2. The primary transfer residual toner that has passed through the contact portion reaches the laser irradiation position as the photosensitive drum 1 rotates. Since the primary transfer residual toner is not so large as to block the laser beam of the exposure device 3, it does not affect the process of forming the electrostatic latent image on the photosensitive drum 1, and the contact portion between the developing roller 41 and the photosensitive drum 1. To. As shown in FIG. 4D, the toner in the non-exposed portion on the photosensitive drum 1 is the potential relationship between the surface potential of the photosensitive drum 1 and the developing voltage (dark portion potential (Vd) of the photosensitive drum 1) = − 500 ( V), development voltage = −350 (V)), and is electrically collected on the developing roller 41 side. In FIG. 4 (e), the toner in the exposed portion on the photosensitive drum 1 has a potential relationship between the surface potential of the photosensitive drum 1 and the developing voltage (light portion potential (Vl) of the photosensitive drum 1 = −150 (V), development). Voltage = −350 (V)), the toner remains on the photosensitive drum 1 without being collected by the developing roller 41. However, toner is electrically supplied from the developing roller 41 to the exposed portion on the photosensitive drum 1. Therefore, the primary transfer residual toner is transferred again together with the toner supplied from the developing roller 41. The development voltage in this embodiment is expressed as a potential difference from the ground potential. Therefore, the development voltage = −350 (V) is interpreted as having a potential difference of −350 (V) with respect to the ground potential (0 (V)) depending on the development voltage applied to the core of the development roller 41. Is done. This also applies to the charging voltage and transfer voltage described later.

  In this way, the primary transfer residual toner that is not transferred to the recording material P and remains on the photosensitive drum 1 is collected on the developing roller 41 in the non-exposed portion, and from the photosensitive drum 1 together with the newly developed toner in the exposed portion. Transcribed. The toner collected on the developing roller 41 is mixed with the toner in the developing unit 4 and used again.

  Next, a phenomenon that occurs when there are a plurality of image forming stations using a cleanerless system will be described with reference to FIG. In the present embodiment, as shown in FIG. 1, four image forming stations are arranged in parallel. For example, image formation is performed at the yellow image forming station a arranged at the most upstream in the rotation direction of the intermediate transfer belt 10. Consider the case. Here, in the description of the phenomenon, a which is an image forming station arranged at the uppermost stream and b which is an image forming station arranged downstream thereof are used. Since the same phenomenon as b occurs in the image forming stations c and d arranged further downstream, the description is omitted.

  The yellow toner on the intermediate transfer belt 10 that has been primarily transferred at the image forming station a disposed at the uppermost stream passes through a transfer nip portion of the image forming station b that is a cartridge disposed downstream. Before passing, as shown in FIG. 5A, the yellow toner on the intermediate transfer belt 10 is partially reversed in polarity due to discharge at the transfer nip formed by the photosensitive drum 1b and the primary transfer roller 14b. To do. This toner behaves as a retransfer toner. The retransfer toner is a positive polarity toner whose polarity is reversed, and is transferred onto the photosensitive drum 1b due to a potential difference between the photosensitive drum 1b and the primary transfer roller 14b. The retransfer toner transferred onto the photosensitive drum 1b enters the charging roller 2b as it is in a cleanerless system without a cleaning member.

  During the image forming operation, the charging voltage applied to the charging roller 2b has a negative polarity and the retransfer toner has a positive polarity. Therefore, as shown in FIG. 5B, the retransfer toner was retransferred onto the photosensitive drum 1b. An electric field acts on the toner in a direction in which the toner is electrically attracted to the charging roller 2b side. However, before entering the charging roller 2b, discharge occurs in the gap between the photosensitive drum 1b and the charging roller 2b, and negative charge is supplied to the toner on the photosensitive drum 1b. As a result, the retransfer toner changes to the same polarity as the charging roller 2b, and therefore passes through the charging roller 2b without being electrically attached. However, a part of the positive transfer retransfer toner may not be made negative by discharge. The retransfer toner that has not been made negative is electrically moved toward the charging roller 2b as shown in FIG. The positive polarity toner adhering to the charging roller 2b is gradually injected by the influence of the charging voltage applied to the charging roller 2b when image formation is continued, and shifts from positive polarity to negative polarity. . When shifting to the negative polarity, since the charging voltage applied to the charging roller 2b is repelled, the retransfer toner is gradually transferred onto the photosensitive drum 1b. As a result, as shown in FIG. 5D, the retransfer toner is collected in the developing unit 4b simultaneously with the image formation. However, if image formation is continued with a large amount of toner adhering to the charging roller 2, retransfer toner gradually accumulates and charging inhibition occurs. As a result, the surface of the photosensitive drum 1 cannot be uniformly charged to a desired potential, and an image defect due to charging failure occurs. By performing the cleaning operation for cleaning the charging roller 2 before the image defect occurs, the image problem can be suppressed. However, when performing image formation continuously, the cleaning operation of the charging roller 2 cannot be performed, which greatly affects the image. This is because if another operation is performed in the case where image formation is performed continuously, downtime will occur accordingly. Therefore, it is desirable to perform image formation in a state where toner is not attached to the charging roller 2 and a cleaning operation is not performed as much as possible.

  Therefore, in the present embodiment, as shown in FIG. 6, the generation of retransfer toner is suppressed by supplying a current to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10. FIG. 6 is an enlarged view of the image forming station b in FIG. 5 near the transfer nip. As shown in FIG. 6, a discharge is caused in the gap upstream of the transfer nip portion between the photosensitive drum 1 and the intermediate transfer belt 10, and most of the toner formed on the intermediate transfer belt 10 is made negative in advance, so that the transfer nip portion is formed. Let it pass. By passing a current through the circumferential direction of the intermediate transfer belt 10 to the transfer nip portion, the voltage applied from the primary transfer voltage power supply 140 is reflected on the intermediate transfer belt 10, so that the vicinity of the upstream of the transfer nip portion. As a result, a discharge occurs between the photosensitive drum 1 and the intermediate transfer belt 10. When the toner image formed at the upstream station is held on the intermediate transfer belt 10 at the time of discharge, the toner image on the intermediate transfer belt 10 is discharged by discharge before the toner image enters the transfer nip portion. Charged to the normal polarity side, that is, negative polarity. Therefore, the toner image on the intermediate transfer belt 10 can be prevented from being retransferred to the photosensitive drum 1 when passing through the downstream transfer nip portion.

3. As described above, in a cleaner-less system without a cleaning member, residual transfer toner or retransfer toner on the photosensitive drum 1 contacts the charging roller 2 and adheres to the charging roller 2 depending on conditions. To do. The toner adhesion to the charging roller 2 is mainly governed by electrical adhesion. Therefore, the influence of the charge of the toner and the discharge on the charging roller 2 is large. If the charge of the toner adhering to the surface of the photosensitive drum 1 is a negative polarity having a normal polarity, the adhesion to the charging roller 2 can be reduced. Even if the charge of the toner is unstable, the toner on the photosensitive drum 1 is negatively charged by the discharge from the charging roller 2 to which the negative voltage is applied to the surface of the photosensitive drum 1. You can pass through the contact. Therefore, toner adhering to the charging roller 2 can be suppressed by appropriately urging the toner remaining on the photosensitive drum 1 to discharge with the charging roller 2.

  As shown in FIG. 3, the discharge by the charging roller 2 is performed in the gap between the charging roller 2 and the photosensitive drum 1 immediately before the contact portion. Then, a negative potential is formed on the photosensitive drum 1 by discharging. Here, if the toner adheres on the photosensitive drum 1 as shown in FIG. 3, the way of receiving the discharge changes depending on the speed at which the toner passes through the gap. Specifically, when the peripheral speed of the toner on the photosensitive drum 1 passing through the gap is small, it takes a long time for the toner to be discharged, so it is easy to optimize the polarity of the toner. As a result, the toner discharged on the charging roller 2 is easily charged to a negative polarity. On the other hand, when the peripheral speed of the toner on the photosensitive drum 1 passing through the gap is large, the time for the toner to be discharged is short. Therefore, toner that has not been negatively charged due to insufficient discharge may adhere to the charging roller 2 as it is. From the above phenomenon, it can be seen that the state of toner adhesion to the charging roller 2 changes depending on the peripheral speed of the photosensitive drum 1.

  Next, charging contrast that affects toner adhesion to the charging roller 2 will be described. The amount of discharge between the charging roller 2 and the photosensitive drum 1 varies depending on the charging contrast expressed by the potential difference between the charging roller 2 and the photosensitive drum 1. As the charging contrast, which is the difference between the charging voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1, increases, the amount of discharge increases. As described above, considering that the toner is negatively charged by discharge, the polarity of the toner can be optimized when the charging contrast is large. On the other hand, when the charge contrast is small, there are cases where the discharge amount is insufficient and the polarity of the toner cannot be optimized. As a result, the toner cannot be attracted onto the photosensitive drum 1 in some cases, thereby causing the toner to adhere to the charging roller 2.

  However, repeated discharge in a state where the charging contrast is large may lead to discharge deterioration or shaving of the photosensitive drum 1 and may reduce image quality. Therefore, in order to optimize the polarity of the toner, it is necessary to increase the charge contrast, but from the viewpoint of image quality, it is desirable to suppress the discharge amount as much as possible.

  Therefore, in order to suppress toner adhesion to the charging roller 2, it is necessary to control the charging contrast, which is the potential difference between the peripheral speed of the photosensitive drum 1, the charging voltage applied to the charging roller 2, and the surface potential of the photosensitive drum 1. .

  Therefore, in this embodiment, the charging contrast is changed according to the peripheral speed of the photosensitive drum 1. As an example of the mode for changing the peripheral speed of the photosensitive drum 1 at the time of image formation, changing according to the type of the recording material P can be mentioned. The photosensitive drum 1 is rotated by a main motor (not shown). In the present embodiment, the main motor also serves as the rotation of the intermediate transfer belt 10. When the toner image transferred onto the intermediate transfer belt 10 is transferred to the recording material P at the secondary transfer portion, the transferability differs depending on the recording material P. Therefore, it is necessary to change the rotation speed by the main motor according to the recording material P. There is. For example, when the recording material P is thick and difficult to transfer, it is necessary to reduce the peripheral speed of the intermediate transfer belt 10 by a rotation speed switching means (not shown) and change it to a condition that facilitates transfer. is there. At the same time, the peripheral speed of the photosensitive drum 1 is also reduced. Therefore, under the above conditions, the charging contrast is reduced in order to optimize the discharge conditions.

4). Photosensitive Drum Peripheral Speed and Charge Contrast Control The relationship between the peripheral speed of the photosensitive drum 1 and the charge contrast at the time of image formation, which is a feature of the present embodiment, is described in detail in order to suppress the above-described image adverse effects.

  The image forming apparatus 100 according to the present embodiment controls the exposure amount of the pre-charging exposure apparatus 5 so that the charging contrast is reduced as the peripheral speed of the photosensitive drum 1 is decreased. As shown in Table 1, the image forming apparatus 100 according to the present embodiment has five peripheral speeds of the photosensitive drum 1 of 50, 75, 100, 125, and 150 (mm / sec). When using a recording material P that is difficult to transfer or fix toner, such as thick paper or rough paper, a mode with a low peripheral speed is adopted, and a recording material P that is easy to transfer or fix, such as thin paper or smooth paper, is used. When used, image formation is executed in a mode with a high peripheral speed. Then, the surface potential of the photosensitive drum 1 after exposure by the pre-charging exposure device 5 is controlled with respect to the peripheral speed of each photosensitive drum 1, and the charging pre-exposure device 5 has a relationship of charging contrast as shown in Table 1. Control exposure. As described above, the smaller the peripheral speed is, the longer the time for receiving the discharge on the photosensitive drum 1 is, so the charging contrast is controlled to be reduced accordingly. On the other hand, the higher the peripheral speed, the shorter the time for receiving the discharge on the photosensitive drum 1, so the charging contrast is increased accordingly. Specifically, in Example 1, the charging contrast is 750, 800, 850, 900, and 950 with respect to the peripheral speed of the photosensitive drum 1c in five stages of 50, 75, 100, 125, and 150 (mm / sec), respectively. It is controlled to become (V). FIG. 7 is a schematic diagram showing the magnitude of the charge contrast at 50 (mm / sec), which is the mode with the lowest peripheral speed, and 150 (mm / sec), which is the mode with the highest peripheral speed. Regardless of the peripheral speed of the photosensitive drum 1, the charging voltage is set to -1000 (V), and the exposure contrast before charging is changed to change the charging contrast. At 50 (mm / sec), the surface potential of the photosensitive drum 1 after exposure before charging was set to -250 (V). On the other hand, at 150 (mm / sec), the surface potential of the photosensitive drum 1 after exposure before charging was set to −50 (V).

  Next, details of the operation of the present embodiment will be described using the flowchart of FIG.

  First, the recording material P designated by the user before image formation is fed to the paper feed cassette 7 (S1). For the recording material P designated by the user in S1, conditions necessary for image formation are transmitted to the control unit 202 based on the information of the recording material P stored in advance by the memory 152 (S2). Specifically, the peripheral speed of the photosensitive drum 1, the charging voltage necessary for forming the charging contrast, and the development voltage necessary for forming the image are set. Thereafter, image formation is performed under conditions suitable for the recording material P transmitted in S2 (S3). Thereafter, the image formation on the recording material P is completed, and the sheet is discharged to the outside of the image forming apparatus 100 by paper conveyance (S4).

  Next, the effect of the present embodiment will be described. In order to confirm the effect of this example, the following test was performed, and the toner contamination was confirmed and the influence of discharge was compared.

  First, in order to confirm toner contamination, a test was conducted by the following method. At the image forming station b, a solid black image with a vertical band is printed at the center in the direction orthogonal to the conveying direction of the recording material P, and after continuous printing of 30 sheets, a half-tone image is continuously printed at the image forming station c. Print. As a result, the longitudinal belt image printed at the image station b is transferred to the intermediate transfer belt 10 and then re-transferred to the photosensitive drum 1c of the image forming station c disposed downstream in the rotation direction of the intermediate transfer belt 10 for charging. It adheres to the roller 2c. Due to this toner adhesion, a toner adhesion portion and a non-toner adhesion portion are formed on the charging roller 2, and a difference in surface potential on the photosensitive drum 1 due to discharge occurs between them. This difference in surface potential has an effect on image formation, resulting in density unevenness due to density differences occurring on the entire halftone image. By utilizing the fact that the density unevenness level becomes worse as the toner adhesion amount is larger, the toner stain suppression effect of the charging roller 2 is confirmed by comparing the density unevenness levels of the entire halftone image.

  Next, in order to confirm the influence of the discharge, a test was performed by the following method. In the image forming station b, an operation of printing an image with a printing rate of 4 (%) with 100 sheets of continuous printing is repeated 200 times. After the repeated operation, a full-tone image is printed on the entire surface to check the influence of the image. When the influence of the discharge is strong, image adverse effects such as image flow due to discharge products generated by discharge on the surface of the photosensitive drum 1 and streaks on the halftone image due to scraping of the surface of the photosensitive drum 1 occur. To do. By using this phenomenon, the influence of discharge can be compared.

  As the conditions of Example 1, the test was performed for each of the cases where the peripheral speed of the photosensitive drum 1 during image formation was 50, 75, 100, 125, and 150 (mm / sec). Further, the charging contrast was set to 750, 800, 850, 900, 950 (V) by changing the exposure amount of the pre-charging exposure device 5 for each peripheral speed of the photosensitive drum 1. The test results of Example 1 are shown in Table 2. As a comparison with Example 1, as shown in Tables 3 and 4, the charge contrast was not changed according to the peripheral speed of the photosensitive drum 1, and a test under the same conditions was performed separately. Table 3 shows a comparatively small charge contrast as Comparative Example 1 (50 (mm / sec) condition of Example 1), and Table 4 shows a comparatively large charge contrast as Comparative Example 2 (150 of Example 1). (Condition of (mm / sec)).

  In the setting of the peripheral speed and the charge contrast of the photosensitive drum 1 of Example 1 shown in Table 2, the density unevenness of the entire halftone due to the toner contamination of the charging roller 2 and the influence of the discharge are affected at any peripheral speed of the photosensitive drum 1. It can be suppressed. In the first exemplary embodiment, the discharge amount for the toner on the photosensitive drum 1 is appropriately controlled according to the peripheral speed of the photosensitive drum 1, and by adjusting the discharge amount so as to suppress toner adhesion without excessive discharge, This is because the discharge deterioration of the photosensitive drum 1 can be suppressed.

  On the other hand, when the charging contrast is set to 750 (V) regardless of the peripheral speed of the photosensitive drum 1 under the conditions of Comparative Example 1 shown in Table 3, the influence of the discharge does not appear, but the photosensitive drum 1 Density unevenness occurs in the medium to high speed region. Under the conditions of Comparative Example 1, when the peripheral speed of the photosensitive drum 1 is high, the toner on the photosensitive drum 1 cannot be sufficiently charged, and the toner partially adheres to the charging roller 2. This is because when the peripheral speed of the photosensitive drum 1 increases, the time that can be discharged by passing the toner through the charging roller 2 is short, so that the discharge to the toner becomes insufficient and the toner adheres to the charging roller 2. This suggests that it cannot be suppressed.

  Therefore, as in Comparative Example 2 shown in Table 4, it is considered to uniformly increase the charging contrast regardless of the peripheral speed of the photosensitive drum 1. When the charging contrast is set to 950 (V) regardless of the peripheral speed of the photosensitive drum 1, density unevenness does not occur at any peripheral speed of the photosensitive drum 1. This is because the toner is sufficiently discharged due to the large charge contrast. However, as compared with the first embodiment, the charging contrast in the middle to low speed region of the photosensitive drum 1 is increased, which causes image defects due to the influence of discharge, particularly in the low speed region of the photosensitive drum 1. Yes. In the low speed region of the photosensitive drum 1 of Comparative Example 2, since the discharge amount is larger than the discharge amount necessary for image formation, the photosensitive drum 1 is excessively discharged as compared with Example 1. This excessive discharge is considered to have an adverse effect on the photosensitive drum 1. From the above results, it can be seen that the larger the charging contrast, the greater the influence of the discharge, and in particular, the influence appears when the peripheral speed of the photosensitive drum 1 is small. This is because, as described above, when the peripheral speed of the photosensitive drum 1 is low, the length of the discharge from the photosensitive drum 1 and the charging roller 2 is long and the deterioration tends to occur.

  Therefore, as in the present embodiment, setting the charging contrast according to the peripheral speed of the photosensitive drum 1 suppresses discharge deterioration of the photosensitive drum 1 while suppressing image defects due to toner contamination of the charging roller 2. It turns out that it is effective to do. The smaller the peripheral speed of the photosensitive drum 1, the more the charging roller 2 can be prevented from becoming dirty even if the charging contrast is reduced. The lower the peripheral speed, the longer the upstream discharge time around the unit area of the photosensitive drum 1. Because. That is, the amount of upstream discharge received per unit area of the photosensitive drum 1 increases, and toner contamination of the charging roller 2 is suppressed.

  Therefore, in the present embodiment in which the first image forming mode in which image formation is performed by rotating the photosensitive drum 1 and the second image forming mode in which the rotation speed is higher than that in the first image forming mode can be executed. Control like this. The difference between the absolute value of the voltage applied to the charging voltage power supply 20 and the absolute value of the surface potential of the surface of the photosensitive drum 1 exposed by the pre-charging exposure device 5 is greater in the second image forming mode than in the first image forming mode. The charging voltage power source 20 and the pre-charging exposure apparatus 5 are controlled so that the direction becomes larger. The difference between the absolute value of the voltage applied to the charging voltage power source 20 and the absolute value of the surface potential of the surface of the photosensitive drum 1 exposed by the pre-charging exposure device 5 indicates charging contrast. When the pre-charge exposure device 5 is controlled, the exposure amount for exposing the surface of the photosensitive drum 1 by the pre-charge exposure device 5 is controlled to be larger in the second image formation mode than in the first image formation mode. To do. At this time, if there is a difference in charge contrast, the amount of discharge will be different, so that the exposure by the pre-charge exposure device 5 is controlled not to be performed in the first image forming mode but only in the second image forming mode. May be. This is because when the pre-charge exposure is not performed, the absolute value of the surface potential of the photosensitive drum 1 is increased as compared with the case where the pre-charge exposure is performed, thereby reducing the charge contrast and the discharge amount. Therefore, the discharge amount can be set larger in the second image forming mode in which the pre-charging exposure is performed than in the first image forming mode in which the pre-charging exposure is not performed.

  Further, the image forming station a arranged at the most upstream in the rotation direction of the intermediate transfer belt 10 may be controlled as follows. The difference between the absolute value of the voltage applied to the charging voltage power supply 20 and the absolute value of the surface potential of the surface of the photosensitive drum 1a exposed by the pre-charging exposure device 5a of the image forming station a arranged at the most upstream is expressed as follows. The same applies to the first image forming mode and the second image forming mode. Since the image forming station a arranged at the uppermost stream has no image forming station arranged upstream thereof, toners of other colors do not pass through the photosensitive drum 1a. That is, no retransfer toner is generated. Therefore, the charging roller 2a is less contaminated than the other image forming stations b, c, d. For this reason, it is not necessary to perform control for suppressing the contamination of the charging roller 2 performed in the other image forming stations b, c, and d. Therefore, the control of the image forming station a may perform image formation without controlling the charge contrast in accordance with the peripheral speed of the photosensitive drum 1, or the change in the charge contrast due to the change in the peripheral speed may be changed to another image forming station b. , C, d may be smaller.

  As described above, the charging contrast is changed in accordance with the peripheral speed of the photosensitive drum 1, and the charging contrast is increased as the peripheral speed increases, so that the toner contamination of the charging roller 2 is suppressed and the photosensitive drum is deteriorated due to discharge. It is possible to suppress the deterioration of image quality associated with.

  In this embodiment, the charging contrast is controlled to a predetermined setting value by changing the exposure amount on the photosensitive drum 1 after the primary transfer by the pre-charging exposure device 5, but the voltage applied to the charging roller 2 is changed. By changing the charging contrast, the charging contrast may be controlled to a predetermined design value. Specifically, the voltage applied to the charging roller 2 by the charging voltage power supply 20 is controlled so that the absolute value is larger when the second image forming mode is executed than when the first image forming mode is executed. However, when the voltage applied to the charging roller 2 is changed, the surface potential of the photosensitive drum 1 after charging also changes, so the potential relationship between the photosensitive drum 1 and the developing roller 41 also changes, and the toner development characteristics change. End up. Therefore, when the charging contrast is controlled by changing the voltage applied to the charging roller 2, the surface potential of the photosensitive drum 1 when reaching the developing roller 41 is constant regardless of the voltage applied to the charging roller 2. It is preferable to control as described above. For example, the background control which makes the surface potential of the photosensitive drum 1 constant by performing non-image part exposure of the non-image forming region by the exposure device 3 can be mentioned. A non-image portion exposure amount and an image portion exposure amount, which are exposure amounts of an image forming region and a non-image forming region by the exposure device 3, are controlled according to the surface potential of the photosensitive drum 1 after charging. Accordingly, the surface potential of the photosensitive drum 1 when reaching the developing roller 41 can be made constant regardless of the magnitude of the voltage applied to the charging roller 2.

  Further, the development voltage may be adjusted together with the background exposure. In that case, the image portion exposure amount to the image forming area of the exposure apparatus 3 may be adjusted according to the change in the developing voltage.

  Further, the transfer voltage may be changed as means for changing the charging contrast. By changing the transfer voltage, the potential of the surface of the photosensitive drum 1 that has been discharged by the transfer voltage can be changed.

  Further, although the present embodiment employs the image forming apparatus 100 having a plurality of image forming stations, it may be applied to an image forming apparatus having a single image forming station.

  In this embodiment, the configuration of an intermediate transfer member using a belt is adopted. However, the recording material P is conveyed between the photosensitive drum 1 and the primary transfer roller 14, and the toner image is transferred onto the recording material P at the transfer nip portion. Alternatively, a direct transfer method for transferring to may be employed.

In Example 1, when the peripheral speed of the photosensitive drum 1 is V,
50 (mm / sec) ≦ V ≦ 150 (mm / sec) (1)
In this case, the charging contrast was controlled to suppress the adverse effects of the image and the discharge due to the toner adhering to the charging roller 2. In the second embodiment, a method for appropriately charging the toner and discharging the photosensitive drum 1 when the peripheral speed of the photosensitive drum 1 is set to be smaller than the range of the expression (1) will be described. Specifically, a case of a peripheral speed smaller than 50 (mm / sec) will be described.

First, in the range of the peripheral speed of the photosensitive drum 1 of Example 1, when the charging contrast is Δ, the charging contrast range is
750 (V) ≦ Δ ≦ 900 (V) (2)
It was. Here, the conditions under which discharge occurs between the charging roller 2 and the photosensitive drum 1 will be described. FIG. 9 is a graph showing the results of measuring the relationship between the charging voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1 in an environment of a temperature of 23 ° C. and a humidity of 50%. When the applied charging voltage is small, the surface potential on the photosensitive drum 1 does not increase at all, but the surface potential starts increasing from a certain voltage value. This is the discharge start voltage Vth. In the present embodiment, −550 (V) is the discharge start voltage Vth. The discharge start voltage Vth is determined from the gap between the charging roller 2 and the photosensitive drum 1, the layer thickness of the photosensitive drum 1, and the layer relative dielectric constant of the photosensitive drum 1. When a charging voltage equal to or higher than the discharge start voltage Vth is applied to the charging roller 2, a charge is placed on the photosensitive drum 1 due to a discharge phenomenon based on Paschen's law. Therefore, assuming that the charging voltage is Vpri, the range in which discharge occurs between the charging roller 2 and the photosensitive drum 1 is expressed by the following equation (3).
| Vpri |> | Vth | ... Formula (3)

As shown in FIG. 9, when the absolute value of the charging voltage Vpri is gradually increased from −400 (V) to the charging roller 2, the increase amount of the charging voltage Vpri is a straight line with a slope of 1 starting from the discharge start voltage Vth. Accordingly, the dark portion potential Vd increases. Therefore, in the range of the charging voltage Vpri of the formula (3), the dark portion potential Vd formed on the photosensitive drum 1 is
| Vd | = | Vpri | − | Vth | (4)
It is represented by Since the dark portion potential Vd is 0 (V) when the discharge start voltage Vth is applied, discharge does not occur in the first place in a range where the charging contrast Δ is less than 550 (V). Therefore, the range of the charging contrast Δ necessary for image formation required for the conditions of Example 2 is expressed by the following equation.
550 (V) ≦ Δ ≦ 750 (V) (5)

  When setting the peripheral speed of the photosensitive drum 1 at 50 (mm / sec) or less, the range of the equation (5) may be selected as appropriate. Preferably, when the range of the formula (5) is set so that the peripheral speed of the photosensitive drum 1 changes linearly from 50 (mm / sec) to 0 (mm / sec), appropriate toner charging and the photosensitive drum are performed. Discharge suppression and image adverse effects can be suppressed.

  In the second embodiment, the range in which the peripheral speed of the photosensitive drum 1 is low has been described. In the third embodiment, a configuration suitable for a range in which the peripheral speed of the photosensitive drum 1 is particularly high will be described.

  As in the third embodiment, when the peripheral speed is further increased as compared with the range of the peripheral speed of the photosensitive drum 1 of the formula (1), which is the condition of the first embodiment, the charging contrast in the range of the formula (2) is further increased. Otherwise, it will be difficult to charge the toner. However, increasing the charging contrast causes deterioration of the photosensitive drum 1 due to discharge as described above. Therefore, in the third embodiment, a configuration is proposed in which the amount of toner retransferred to an image forming station arranged downstream of the rotation of the intermediate transfer belt 10 is reduced without changing the charging contrast. This configuration is effective regardless of the range of the peripheral speed of the photosensitive drum 1, and by reducing the retransfer toner, the amount of discharge necessary for charging the toner can be reduced. In particular, this is effective in a range where the peripheral speed of the photosensitive drum 1 where the charging contrast cannot be increased is large.

  In Example 3, as shown in FIG. 10, the primary transfer roller 14 is not disposed in the vicinity of the transfer nip portion with the photosensitive drum 1, but is disposed at a position where the photosensitive drum 1 is not directly pressed. At the time of primary transfer, a DC voltage of 1000 (V) is applied from the primary transfer voltage power supply 140 to the primary transfer roller 14, and the current required for primary transfer to the transfer nip portion via the circumferential direction of the intermediate transfer belt 10. Supply. With such a configuration, a voltage corresponding to the applied voltage applied from the primary transfer voltage power supply 140 is also applied to the intermediate transfer belt 10 in the vicinity of the gap formed between the primary transfer roller 14 and the photosensitive drum 1. The Therefore, as shown in FIG. 11, in Example 3, discharge (hereinafter referred to as upstream discharge in the transfer nip portion) is actively generated between the photosensitive drum 1 and the intermediate transfer belt 10 in the vicinity of the upstream portion in the transfer nip portion. To do. When a toner image exists at a position where an upstream discharge occurs in the transfer nip portion, a negative charge is supplied from the photosensitive drum 1 to the toner image on the intermediate transfer belt 10 by this discharge. The toner image on 10 is charged to the normal polarity side. Then, by charging to the normal polarity side, the reverse polarity toner amount of the toner image on the intermediate transfer belt 10 is reduced. As a result, the toner image on the intermediate transfer belt 10 is prevented from being retransferred from the toner image on the intermediate transfer belt 10 to the photosensitive drum 1 when passing through the transfer nip portion of the image forming station on the downstream side of the rotation. You will be able to

  Since other configurations are the same as those of the first embodiment, the description thereof is omitted.

  In the configuration of Example 3, effects similar to those of Example 1 were confirmed. The difference between the third embodiment and the first embodiment is only the difference in the peripheral configuration of the intermediate transfer belt 10 in the above-described image forming apparatus 100 and the set charge contrast value. As shown in Table 5, in Example 3, the charging contrast is set to be 100 (V) smaller than that in Example 1 with respect to each peripheral speed of the photosensitive drum 1.

  From the results of confirming the effects shown in Table 5, in Example 3, as in Example 1, image quality associated with deterioration of the photosensitive drum by the pre-charge exposure device 5 is suppressed while suppressing occurrence of image defects due to toner contamination of the charging roller 2. It turns out that the deterioration can be suppressed. Further, in Example 3, it can be seen that density unevenness can be suppressed even if the charging contrast is made smaller than that in Example 1. In particular, the condition of the charge contrast when the peripheral speed of the photosensitive drum 1 is 100 (mm / sec) is the same as that of the comparative example 1. In the comparative example 1, the adverse effect occurred in the condition. 3 shows that the problem can be solved even under the conditions. In Example 1, when the charging contrast was 900 (V), no image problem occurred until the peripheral speed of the photosensitive drum 1 was 125 (mm / sec). In Example 3, however, 175 (mm / sec) ) Did not occur until. This is because in Example 3, toner retransfer from the intermediate transfer belt 10 to the photosensitive drum 1 can be further suppressed. Specifically, the primary transfer roller 14 is not disposed in the vicinity of the transfer nip portion with the photosensitive drum 1, but is disposed at a position where the photosensitive drum 1 is not directly pressed, so that upstream discharge of the transfer nip portion is likely to occur. Therefore, the amount of retransfer toner is reduced. This is because the upstream discharge time around the unit area of the intermediate transfer belt 10 is extended, and thus the amount of upstream discharge received by the unit area of the intermediate transfer belt 10 is increased. As a result, in the third embodiment, the toner on the intermediate transfer belt 10 is charged to the normal polarity side as compared with the first embodiment, and retransfer of the toner is suppressed. From the above, it can be said that Example 3 is a particularly suitable configuration when the peripheral speed of the photosensitive drum 1 is higher than 150 (mm / sec).

  As described above, by supplying the current to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10, the toner of the charging roller 2 can be obtained even if the charging contrast is smaller than that in the first embodiment. It is possible to suppress the occurrence of image defects due to dirt.

  In this embodiment, the primary transfer roller 14 disposed at a position where the photosensitive drum 1 is not directly pressed is used. However, another contact member that contacts the intermediate transfer belt 10 is transferred to the transfer nip portion via the intermediate transfer belt 10. And a structure for supplying current.

  In the third embodiment, the current is supplied from the primary transfer roller 14 to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10. However, the discharge between the intermediate transfer belt 10 and the photosensitive drum 1 is discharged. It only has to be able to extend the time to do it. This is because the discharge time generated between the intermediate transfer belt 10 and the photosensitive drum 1 contributes to the retransfer toner amount, and it is sufficient that the discharge time can be extended even if the configuration is not as in the third embodiment. .

In Example 4, the intermediate transfer belt 10 having a lower resistance than that of Example 1 in which the surface resistivity of the outer peripheral surface when measured by applying a voltage of 100 (V) is 2 × 10 ¹⁰ (Ω / □) was used. . Here, by using the intermediate transfer belt 10 having a lower surface resistance than that of the first embodiment, the current is not supplied to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10 as in the third embodiment. This is because the upstream discharge of the transfer nip portion is generated. Further, since the resistance is low, it is possible to increase the amount of current flowing, that is, the amount of charge with respect to the toner. The surface resistivity is not limited to the surface resistivity of the outer peripheral surface, and the same effect can be obtained even if an intermediate transfer belt 10 having a surface resistivity of 2 × 10 ¹⁰ (Ω / □) is used. It is done.

  Specifically, as shown in FIG. 12, by using the intermediate transfer belt 10 in which current easily flows in the circumferential direction, a positive voltage applied from the secondary transfer voltage power supply 150 to the secondary transfer roller 15 causes the intermediate transfer belt 10 to flow. And is reflected in the vicinity of each primary transfer upstream. At this time, a transfer current flows as shown by the black line in FIG. The image forming station a closest to the secondary transfer roller 15 has the most transfer current. On the other hand, the farthest image forming station d has a configuration in which the transfer current hardly flows. For example, a voltage may be applied to the tension member 11 in a superimposed manner so that the transfer current does not vary for each image station. From the above, as in the third embodiment, discharge is generated upstream of the transfer nip portion, and the effect of suppressing retransfer can be obtained.

  Since other configurations are the same as those of the first embodiment, description thereof is omitted.

  In the configuration of Example 4, the same effects as those of Example 1 and Example 3 were confirmed. The difference between the fourth embodiment, the first embodiment, and the third embodiment is only the difference in the peripheral configuration of the intermediate transfer belt 10 in the above-described image forming apparatus 100 and the set charge contrast value. As shown in Table 6, the charging contrast is set to 150 (V) smaller in the fourth embodiment than the first embodiment with respect to the respective peripheral speeds of the photosensitive drum 1.

  As described in the third embodiment, in the configuration in which the upstream discharge of the transfer nip portion is generated, the retransfer of the toner from the intermediate transfer belt 10 to the photosensitive drum 1 is suppressed by the upstream discharge of the transfer nip portion. As a result, in the fourth embodiment, the toner on the intermediate transfer belt 10 is charged to the normal polarity side as compared with the first embodiment, and retransfer of the toner is suppressed. Further, as compared with Example 3, the amount of discharge upstream of the transfer nip portion was increased, so that normal charging of the reverse polarity toner could be promoted. As a result, the amount of toner retransferred is reduced, and the overall charge contrast can be reduced. In particular, the conditions of the charge contrast when the peripheral speed of the photosensitive drum 1 is 125 (mm / sec) are the same as those in the comparative example 1. In the comparative example 1, the adverse effect occurred in the conditions. 4 shows that the problem can be solved even under the conditions. In Example 1, when the charge contrast was 950 (V), the peripheral speed of the photosensitive drum 1 was suitable for 150 (mm / sec), but in Example 4, it was 225 (mm / sec). Further, the present invention can be applied even when the peripheral speed of the photosensitive drum 1 is high. From the above, it can be said that the configuration is more suitable when the peripheral speed is higher than 150 (mm / sec).

  As described above, by using the intermediate transfer belt 10 in which a current easily flows in a circumferential direction to some extent, an image defect due to toner contamination of the charging roller 2 occurs even if the charging contrast is smaller than that in the first embodiment. Can be suppressed.

  In this embodiment, the toner image is transferred onto the intermediate transfer belt 10 using the intermediate transfer belt 10 and then transferred onto the recording material P. However, the recording material P is conveyed onto the conveyance belt, A method of directly transferring the toner image onto the recording material P may be employed.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 3 Exposure apparatus 4 Developing part 5 Exposure apparatus before charging 10 Intermediate transfer belt 14 Primary transfer roller

Claims (19)

An image forming apparatus for forming a toner image on a recording material,
An image carrier that is rotatable, a charging member that contacts the image carrier to form a contact portion, and charges the image carrier at the contact portion; and the image that is charged by the charging member An image having first exposure means for exposing the carrier to form an electrostatic latent image, and a developing member for supplying toner to the electrostatic latent image and developing the toner image on the image carrier. Forming part;
Voltage applying means for applying a voltage to the charging member;
An intermediate transfer member that forms a transfer portion in contact with the image carrier and carries the toner image formed on the image carrier in the transfer portion;
A transfer member for transferring the toner image carried on the intermediate transfer member to a recording material;
A second exposure unit that exposes the image carrier downstream of the transfer unit and upstream of the contact unit in the rotation direction of the image carrier;
A control unit that controls the voltage application unit and the second exposure unit;
In an image forming apparatus having
A first image forming mode in which image formation is performed by rotating the image carrier, and a second image forming mode in which image formation is performed by rotating the image carrier at a rotational speed greater than the first image formation mode. Is possible and
The control unit determines that the difference between the absolute value of the voltage applied to the voltage application unit and the absolute value of the surface potential of the image carrier exposed by the second exposure unit is the first image formation. An image forming apparatus, wherein the voltage application unit and the second exposure unit are controlled so that the second image forming mode is larger than the mode executing.   The control unit controls the exposure amount for exposing the image carrier by the second exposure unit to be larger when the second image forming mode is executed than when the first image forming mode is executed. The image forming apparatus according to claim 1.   2. The control unit according to claim 1, wherein the control unit performs control so that the exposure by the second exposure unit is not executed when the first image forming mode is executed, but is executed when the second image forming mode is executed. Or the image forming apparatus according to 2; The first exposure means exposes the image carrier charged by the charging member to form the surface potential that does not form the toner image, and exposes the toner image. Performing image portion exposure with an exposure amount larger than the exposure amount of the non-image portion exposure to form a surface potential;
The control unit is configured to control the exposure amount of the non-image part exposure so that the surface potential of the image carrier that does not form the toner image is constant during image formation. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled.   The control unit includes an absolute value of the voltage applied to the voltage application unit of the image forming unit arranged at the most upstream position in the rotation direction of the intermediate transfer member, and the image forming unit arranged at the most upstream position. Control is performed so that the difference between the absolute value of the surface potential of the image carrier exposed by the second exposure means is the same in the first image forming mode and the second image forming mode. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. An image forming apparatus for forming a toner image on a recording material,
An image carrier that is rotatable, a charging member that contacts the image carrier to form a contact portion, and charges the image carrier at the contact portion; and the image that is charged by the charging member An image forming unit comprising: an exposure unit that exposes the carrier to form an electrostatic latent image; and a developing member that supplies toner to the electrostatic latent image and develops the toner image on the image carrier. ,
Voltage applying means for applying a voltage to the charging member;
An intermediate transfer member that forms a transfer portion in contact with the image carrier and carries the toner image formed on the image carrier in the transfer portion;
A transfer member for transferring the toner image carried on the intermediate transfer member to a recording material;
A control unit for controlling the voltage applying means;
In an image forming apparatus having
A first image forming mode in which image formation is performed by rotating the image carrier, and a second image forming mode in which image formation is performed by rotating the image carrier at a rotational speed greater than the first image formation mode. Is possible and
The control unit has a difference between the absolute value of the voltage applied to the voltage application unit and the absolute value of the surface potential of the image carrier that enters the contact part after passing through the transfer unit, An image forming apparatus, wherein the voltage application unit is controlled so that the time when the second image forming mode is executed is larger than the time when the first image forming mode is executed.   Transferring the toner image to the intermediate transfer member by supplying a current to the image carrier through the intermediate transfer member from a contact member contacting the outer peripheral surface or the inner peripheral surface of the intermediate transfer member; The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. The surface resistivity of the outer peripheral surface or the inner peripheral surface of the intermediate transfer member when 100 (V) is applied to the intermediate transfer member is 2 × 10 ¹⁰ (Ω / □) or less. The image forming apparatus described in 1. The exposure means includes a non-image area exposure for exposing the image carrier charged by the charging member to form the surface potential that does not form the toner image, and the surface potential for forming the toner image. Performing image portion exposure with an exposure amount larger than the exposure amount of the non-image portion exposure to form,
The control unit controls the exposure unit to control the exposure amount of the non-image portion exposure so that the surface potential of the image carrier that does not form the toner image is constant during image formation. The image forming apparatus according to claim 6, wherein the image forming apparatus is an image forming apparatus.   The control unit includes an absolute value of the voltage applied to the voltage application unit of the image forming unit arranged at the most upstream in the rotation direction of the intermediate transfer member, and the contact after passing through the transfer unit. Control for executing the same difference between the absolute value of the surface potential of the image carrier that enters the portion in the first image forming mode and the second image forming mode. The image forming apparatus according to claim 6. An image forming apparatus for forming a toner image on a recording material,
An image carrier that is rotatable;
A charging member that contacts the image carrier to form a contact portion, and charges the image carrier at the contact portion;
A first exposure means for exposing the image carrier charged by the charging member to form an electrostatic latent image;
A developing member for supplying toner to the electrostatic latent image to develop the toner image on the image carrier;
Voltage applying means for applying a voltage to the charging member;
A transfer member that transfers the toner image developed on the image carrier to a recording material at a transfer portion;
Second exposure means for exposing the image carrier downstream of the transfer unit and upstream of the contact unit in the rotation direction of the image carrier;
A control unit that controls the voltage application unit and the second exposure unit;
In an image forming apparatus having
A first image forming mode in which image formation is performed by rotating the image carrier, and a second image forming mode in which image formation is performed by rotating the image carrier at a rotational speed greater than the first image formation mode. Is possible and
The control unit determines that the difference between the absolute value of the voltage applied to the voltage application unit and the absolute value of the surface potential of the image carrier exposed by the second exposure unit is the first image formation. An image forming apparatus, wherein the voltage application unit and the second exposure unit are controlled so that the second image forming mode execution time is larger than the mode execution time.   The control unit controls the exposure amount for exposing the image carrier by the second exposure unit to be larger when the second image forming mode is executed than when the first image forming mode is executed. The image forming apparatus according to claim 11.   12. The control unit according to claim 11, wherein control is performed so that the exposure by the second exposure unit is not executed when the first image forming mode is executed, but is executed when the second image forming mode is executed. Or the image forming apparatus of 12. The first exposure means exposes the image carrier charged by the charging member to form the surface potential that does not form the toner image, and exposes the toner image. Performing image portion exposure with an exposure amount larger than the exposure amount of the non-image portion exposure to form a surface potential;
The control unit is configured to control the exposure amount of the non-image part exposure so that the surface potential of the image carrier that does not form the toner image is constant during image formation. The image forming apparatus according to claim 11, wherein the image forming apparatus is controlled. An image forming apparatus for forming a toner image on a recording material,
An image carrier that is rotatable;
A charging member that contacts the image carrier to form a contact portion, and charges the image carrier at the contact portion;
Exposure means for exposing the image carrier charged by the charging member;
A developing member for supplying toner to the image carrier exposed by the exposure means and developing the toner image on the image carrier;
Voltage applying means for applying a voltage to the charging member;
A transfer member that transfers the toner image developed on the image carrier to a recording material at a transfer portion;
A control unit for controlling the voltage applying means;
In an image forming apparatus having
A first image forming mode in which image formation is performed by rotating the image carrier, and a second image forming mode in which image formation is performed by rotating the image carrier at a rotational speed greater than the first image formation mode. Is possible and
The control unit has a difference between the absolute value of the voltage applied to the voltage application unit and the absolute value of the surface potential of the image carrier that enters the contact part after passing through the transfer unit, An image forming apparatus, wherein the voltage application unit is controlled so that the time when the second image forming mode is executed is larger than the time when the first image forming mode is executed. The exposure means includes a non-image area exposure for exposing the image carrier charged by the charging member to form the surface potential that does not form the toner image, and the surface potential for forming the toner image. Performing image portion exposure with an exposure amount larger than the exposure amount of the non-image portion exposure to form,
The control unit controls the exposure unit to control the exposure amount of the non-image portion exposure so that the surface potential of the image carrier that does not form the toner image is constant during image formation. The image forming apparatus according to claim 15.   The control unit controls the voltage applied to the charging member by the voltage applying unit so that the voltage is larger in absolute value when the second image forming mode is executed than when the first image forming mode is executed. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   18. When the image forming mode is executed, residual toner remaining on the image carrier that is not used for image formation is collected by the developing member. The image forming apparatus described. The image forming apparatus according to claim 1, wherein the toner is a one-component developer.

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