JP2018120219A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of suppressing an image defect caused by an electrification unevenness of a photoreceptor caused by a transfer process.SOLUTION: The image forming apparatus 100 includes a photoreceptor 1, a charging member 2, an application device 30 for applying a charging voltage in which a DC voltage and an AC voltage are superimposed on the charging member 2, an exposure device 3, a developing device 4, a transfer member 5, control means 50 for making the value of the peak-to-peak voltage of the AC voltage of the charging voltage applied to the charging member 2 larger than the value of the peak-to-peak voltage of the AC voltage when the charging portion a is passed through the region of the image region other than the predetermined region X while the predetermined region X passes through the charging portion a, if the region of the photoreceptor 1 including the position on the photoreceptor 1 that has passed through the transfer portion N is defined as a predetermined region X at the time when the rear end in the conveyance direction of the recording material P passes through the transfer portion N.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile apparatus using an electrophotographic system.

  In an electrophotographic image forming apparatus, a dark portion potential (charged potential) VD on a photosensitive member formed by charging processing by a charging unit and a dark portion potential VD portion on a photosensitive member formed by exposure by an exposing unit. An electrostatic latent image is formed with a contrast with the bright portion potential VL. The toner is transferred from the developer carrying member to the bright portion potential VL due to the development contrast which is a potential difference between the bright portion potential VL formed on the photoreceptor and the developing bias applied to the developer carrying member. A toner image is formed on the body. The toner image formed on the photoreceptor is transferred to a transfer medium such as a recording material (paper or the like) by a transfer bias applied to a transfer unit.

  In an electrophotographic image forming apparatus, an image called “memory” in which uneven charging (charging history, potential unevenness) of the photoconductor due to the transfer process of the toner image from the photoconductor to the transfer body appears as density unevenness in the image. Defects may occur. The toner charged with the same polarity as the charged polarity of the photosensitive member is attached to the light portion potential VL portion where the charging polarity of the photosensitive member is negative and the absolute value of the negative dark portion potential VD is reduced to form a toner image. Further, a configuration in which the toner image is directly transferred to the recording material will be further described as an example.

  In such a configuration, for example, when the recording material is peeled off from the photoconductor after transfer, a peeling discharge occurs between the rear end in the conveyance direction of the recording material and the photoconductor, and positive charges are generated on the photoconductor. May be easier to move. When the positive charge that has moved onto the photosensitive member is relatively small, it is removed at the time of charging the photosensitive member by a charging unit located downstream of the transfer unit in the rotational direction of the photosensitive member. However, if the amount of positive charge transferred onto the photoconductor exceeds a certain amount, it cannot be removed by the charged portion, and the absolute value of the dark portion potential VD in the region on the photoconductor where the peeling discharge has occurred is small. turn into. As a result, the development contrast in that region becomes larger than in other regions, and the amount of toner adhering during development increases, resulting in a high image density. As described above, due to partial uneven charging of the photoconductor due to the transfer process, horizontal black streak-like “memory” (high density portion extending along the rotation axis direction of the photoconductor) occurs in the image to be formed thereafter. There are things to do.

  In order to suppress this “memory”, a peeling discharge prevention electrode extending in the rotation axis direction of the photosensitive member is provided near the peeling position between the photosensitive member and the recording material, and a voltage having a polarity opposite to the transfer bias is applied to the peeling discharge electrode. The structure to apply is proposed (patent document 1).

JP-A-6-3970

  However, in the above conventional configuration, it is necessary to add a space for installing the peeling discharge preventing electrode and a drive circuit for generating a voltage having a polarity opposite to that of the transfer bias, leading to an increase in cost. Further, the “memory” due to the uneven charging of the photoconductor due to the transfer process is not only due to the peeling discharge between the rear end in the recording material conveyance direction and the photoconductor. The “memory” may be generated when the leading end in the conveyance direction of the recording material enters the transfer unit, or may be generated due to the influence of a toner image passing through the transfer unit. In the configuration in which the peeling discharge preventing electrode is provided in the vicinity of the peeling position between the photoreceptor and the recording material, it is difficult to suppress “memory” in these cases. Therefore, there is a demand for a method for suppressing the occurrence of “memory” by an approach different from the conventional method.

  An object of the present invention is to provide an image forming apparatus capable of suppressing image defects caused by uneven charging of a photoreceptor due to a transfer process.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention includes a rotatable photosensitive member, a charging member that charges the photosensitive member with a charging unit, an application device that applies a charging voltage in which a DC voltage and an AC voltage are superimposed on the charging member, An exposure device that exposes the charged photoreceptor to form an electrostatic image on an image area on the photoreceptor; a developing device that supplies toner to the electrostatic image on the photoreceptor to form a toner image; When a voltage is applied, a transfer member that transfers the toner image on the photosensitive member to the recording material at the transfer portion, and a rear end in the transport direction of the recording material passes through the transfer portion when passing through the transfer portion. When a region of the photoconductor including a position on the photoconductor is a predetermined region, a peak-to-peak voltage of the alternating voltage of the charging voltage applied to the charging member while the predetermined region passes through the charging unit. Values other than the predetermined area An image forming apparatus characterized by comprising a control unit for controlling to be larger than the value of the peak-to-peak voltage of the AC voltage when the region to be the image area passes through the charging portion.

  According to another aspect of the present invention, a rotatable photosensitive member, a charging member that charges the photosensitive member with a charging unit, and an application device that applies a charging voltage in which a DC voltage and an AC voltage are superimposed on the charging member; An exposure device that exposes the charged photoreceptor to form an electrostatic image on an image area on the photoreceptor, and a developing device that supplies toner to the electrostatic image on the photoreceptor to form a toner image. A transfer member that transfers a toner image on the photosensitive member to a recording material when a voltage is applied, and a tip of the recording material in the conveyance direction passes through the transfer portion when passing through the transfer portion. When a region of the photoconductor including a position on the photoconductor is a predetermined region, a peak-to-peak voltage of the alternating voltage of the charging voltage applied to the charging member while the predetermined region passes through the charging unit. Value in the predetermined area An image forming apparatus comprising: a control unit that performs control so that an outer region that becomes the image region passes a value of a peak-to-peak voltage of the AC voltage when passing through the charging unit. The

  According to another aspect of the present invention, a rotatable photoconductor, a charging member that charges the photoconductor with a charging unit, and an application that applies a charging voltage in which a DC voltage and an AC voltage are superimposed on the charging member. A device, an exposure device that exposes the charged photoreceptor to form an electrostatic image on an image area on the photoreceptor, and a developer that supplies toner to the electrostatic image on the photoreceptor to form a toner image A device, a transfer member that transfers a toner image on the photosensitive member to a transfer target in a transfer portion when a voltage is applied, and the photosensitive member that has reached the transfer portion in a state where a larger amount of toner is carried. When the area of the photoconductor including the body area is a predetermined area, the value of the peak voltage of the alternating voltage of the charging voltage applied to the charging member while the predetermined area passes through the charging unit, Before the predetermined area Image forming apparatus region to be the image area and having a control unit for controlling to be larger than the value of the peak-to-peak voltage of the AC voltage when passing through the charging portion is provided.

  According to the present invention, it is possible to suppress image defects caused by uneven charging of the photoreceptor due to the transfer process.

1 is a schematic sectional view of an image forming apparatus according to Embodiment 1. FIG. FIG. 3 is a functional block diagram illustrating a charging bias control mode according to the first exemplary embodiment. FIG. 3 is a sequence chart diagram of charging bias control according to the first exemplary embodiment. FIG. 3 is a flowchart of charging bias control according to the first exemplary embodiment. It is a sequence chart figure of charge via control of Example 1 and comparative examples 1 and 2. FIG. 6 is a functional block diagram illustrating a charging bias control mode according to a second exemplary embodiment. FIG. 6 is a sequence chart of charging bias control according to the second embodiment. 6 is a schematic cross-sectional view of an image forming apparatus according to Embodiment 3. FIG. FIG. 10 is a flowchart of charging bias control according to the third embodiment. 6 is a schematic cross-sectional view of an image forming apparatus according to Embodiment 4. FIG. FIG. 10 is a schematic view of the vicinity of a charging unit in Example 4. FIG. 6 is a sequence chart diagram of charging bias control according to a fourth embodiment. FIG. 10 is a schematic diagram in the vicinity of a transfer portion in Example 5. FIG. 10 is a flowchart of charging bias control according to a fifth embodiment. 1 is a schematic cross-sectional view of a main part of an intermediate transfer type image forming apparatus.

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

[Example 1]
1. Image forming apparatus <Overall configuration and operation of image forming apparatus>
FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 of the present embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic laser printer.

  A photosensitive drum 1, which is a rotatable drum type (cylindrical) photosensitive member (electrophotographic photosensitive member) as an image carrier, is rotationally driven in the direction of arrow Rd (clockwise) by a driving source (not shown). Is done. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential having a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-shaped charging member as a charging unit. During the charging process, a predetermined charging bias (charging voltage) is applied to the charging roller 2 by a charging power supply (high voltage power supply circuit) 30 as an applying means (applying device). In the present embodiment, an oscillating voltage in which a DC voltage (DC voltage, DC component) and an AC voltage (AC voltage, AC component) are superimposed is applied to the charging roller 2 as a charging bias.

  The surface of the charged photosensitive drum 1 is scanned and exposed with a laser beam L corresponding to image information by an exposure device (exposure device, laser optical system, laser scanner device) 3 as an exposure unit, and electrostatically is applied on the photosensitive drum 1. A latent image (electrostatic image) is formed. The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) using toner as a developer by a developing device (developing device) 4 as developing means, and a toner image is formed on the photosensitive drum 1. Is done. The developing device 4 includes a developing roller 4a as a developer carrying member that carries and conveys toner. During the developing process, a predetermined developing bias (developing voltage) is applied to the developing roller 4a by a developing power source (high voltage power circuit) (not shown). In the present embodiment, the developing device 4 uses a one-component developer as a developer, and causes the toner to fly from the developing roller 4a disposed in close proximity to the photosensitive drum 1 to the photosensitive drum 1 (one-component jumping development method). ). In this embodiment, the photosensitive drum 1 is exposed to an exposed portion on the photosensitive drum 1 from which at least a part of the electric charge has been removed (the absolute value of the potential is reduced) by being exposed after being uniformly charged. The toner charged to the same polarity as the charged polarity adheres. In this embodiment, the normal charging polarity of the toner, which is the charging polarity of the toner at the time of development, is negative.

  The toner image formed on the photosensitive drum 1 is transferred onto a recording material (transfer material, sheet) P such as paper by the action of a transfer roller 5 which is a roller-like transfer member as transfer means. The transfer roller 5 is urged (pressed) toward the photosensitive drum 1 by a pressure spring (not shown) that is an urging member as an urging means, and is a contact portion between the photosensitive drum 1 and the transfer roller 5. A transfer portion (transfer nip) N is formed. The transfer roller 5 is driven to rotate as the photosensitive drum 1 rotates. In this embodiment, the transfer roller 5 is configured by providing a conductive elastic layer around a metal core (core material) and has an outer diameter of 12.5 mm. The transfer roller 5 is an example of a transfer member disposed so as to directly face the photosensitive drum 1 when the recording material P as a transfer target is not in the transfer portion N. The toner image on the photosensitive drum 1 is transferred by electrostatic force and pressure onto the recording material P that is nipped and conveyed between the transfer roller 5 and the photosensitive drum 1. During the transfer process, a transfer bias (transfer voltage) that is a DC voltage having a polarity opposite to the normal charging polarity of the toner is applied to the transfer roller 5 by a transfer power supply (high voltage power supply circuit) 18. The recording material P is stored in the recording material cassette 7, fed one by one by the feeding roller 8, transported by the transport roller 9, and supplied to the transfer portion N along the pre-transfer guide 17 that is a guide member. Is done.

  The recording material P to which the toner image has been transferred is transported to the fixing device 12 as a fixing unit along the post-transfer guide 11 after the surface charge amount is reduced by the charge eliminating member 23. In this embodiment, the static elimination member 23 is composed of a conductive member that is electrically grounded (connected to the ground), and comes into contact with the surface of the recording material P opposite to the surface on which the toner has been transferred, so that recording is performed. At least part of the charge of the material P is removed. The recording material P conveyed to the fixing device 12 is heated and pressed by the fixing device 12 to fix the toner image, and then conveyed by the conveying roller 14 and the discharge roller 15, and the apparatus main body M of the image forming apparatus 100. The sheet is discharged onto a discharge tray 16 formed on the upper surface in FIG.

  Further, the toner (transfer residual toner) remaining on the surface of the photosensitive drum 1 during the transfer process is removed from the photosensitive drum 1 and collected by the cleaning device 6 as a cleaning unit. The cleaning device 6 scrapes and collects the transfer residual toner from the surface of the rotating photosensitive drum 1 by a cleaning blade 6a as a cleaning member.

  The image forming apparatus 100 according to the present exemplary embodiment can output an image at a throughput (print speed) of 40 sheets per minute during continuous image formation in which images are continuously formed on a plurality of recording materials P and output. An area on the photosensitive drum 1 where a toner image to be transferred onto one recording material P can be formed is referred to as an image area on the photosensitive drum 1. The image area on the photosensitive drum 1 has a predetermined length in the rotational direction and the rotational axis direction of the photosensitive drum 1 according to the size of the recording material P. The length of the image area on the photosensitive drum 1 in the rotational direction and the rotational axis direction of the photosensitive drum 1 may be smaller or larger than the size of the recording material P, or may be substantially equal, but typically small. An area on the photosensitive drum 1 where no toner image is formed according to the size of the recording material P is referred to as a non-image area on the photosensitive drum 1.

  Here, the position where the charging process by the charging roller 2 in the rotation direction of the photosensitive drum 1 is performed is a charging portion (charging position) a. In the present exemplary embodiment, the charging roller 2 is a minute gap between the charging roller 2 and the photosensitive drum 1 formed on the upstream side and the downstream side of the contact portion between the charging roller 2 and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1. The photosensitive drum 1 is charged by electric discharge generated at least one of the gaps. However, for simplicity, it may be assumed that the contact portion between the charging roller 2 and the photosensitive drum 1 is the charging portion a. Further, the position where exposure by the exposure device 3 in the rotation direction of the photosensitive drum 1 is performed is an exposure portion (exposure position) b. Further, a position where toner is supplied from the developing roller 4a to the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 (a portion where the developing roller 4a and the photosensitive drum 1 are opposed in this embodiment) is a developing portion (developing position) c. It is. Further, the transfer portion N is a position where the toner image is transferred from the photosensitive drum 1 to the recording material P in the rotation direction of the photosensitive drum 1 (in this embodiment, the contact portion between the transfer roller 5 and the photosensitive drum 1). Further, a contact portion between the cleaning blade 6 a and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 is a cleaning portion (cleaning position) d.

<Charging roller>
In this embodiment, the charging roller 2 covers the outer periphery of a core metal (core material) made of iron, stainless steel (SUS) or the like with a conductive elastic layer made of hydrin rubber or the like, and further, urethane rubber as a surface layer. It has the structure which provided the protective layer formed by these. The charging roller 2 is disposed in contact with the surface of the photosensitive drum 1 and rotates following the rotation of the photosensitive drum 1.

  FIG. 2 is a schematic functional block diagram showing the control mode of the charging bias in this embodiment. In this embodiment, the operation of each unit of the image forming apparatus 100 is controlled by a control unit (control circuit) 50 as a control unit provided in the apparatus main body M. The control unit 50 controls the operation of each unit according to signals from various sensors provided in the apparatus main body M in accordance with a program stored in the storage device 40 as a storage unit provided in the apparatus main body M. The storage device 40 is configured by a ROM or a RAM. The control unit 50 reads the charge setting value (DC voltage, peak-to-peak voltage of AC voltage, etc.) stored in the storage device 40 from the storage device 40 and outputs a drive signal input to the charging power supply 30.

  In the present embodiment, the charging power source 30 includes a charging AC power source (AC voltage output unit) 31 and a rectifier circuit (DC voltage output unit) 32. The rectifier circuit 32 generates a DC voltage from the AC voltage from the charging AC power supply 31 based on the drive signal from the control unit 50. The charging power supply 30 outputs a charging bias (DC + AC voltage) obtained by superimposing the AC voltage from the charging AC power supply 31 on the DC voltage generated by the rectifier circuit 32 and applies the charging bias to the charging roller 2.

  By using a DC + AC voltage as the charging bias, the surface of the photosensitive drum 1 can be more stably and uniformly charged as compared with the case where the DC voltage is used alone. Specifically, for example, by applying a charging bias in which an AC voltage having a peak-to-peak voltage (also referred to as “Vpp” here) of 1300 V to a DC voltage of −400 V is applied to the charging roller 2, The surface can be charged almost uniformly to -400V. That is, generally, when the potential difference between the charging roller 2 and the photosensitive drum 1 exceeds a certain magnitude, a discharge occurs between the charging roller 2 and the photosensitive drum 1. When a DC + AC voltage is used, the potential difference between the charging roller 2 and the photosensitive drum 1 that is a condition for causing discharge is determined by the amplitude of the voltage applied to the charging roller 2. Discharge occurs when the amplitude of the voltage applied to the charging roller 2 (the amount of fluctuation from the DC voltage value) exceeds the discharge start voltage Vth. That is, when the AC voltage Vpp applied to the charging roller 2 exceeds twice Vth, when the AC voltage applied to the charging roller 2 takes a peak (maximum value or minimum value), the charging roller 2 and the photosensitive drum. Discharge occurs between 1 and 1. The dark portion potential (charging potential) VD of the photosensitive drum 1 is defined by a DC voltage applied to the charging roller 2 (an average value of voltages applied to the charging roller 2).

<Photosensitive drum>
In general, the photosensitive drum 1 is provided with a photosensitive material such as OPC (organic optical semiconductor), amorphous selenium, or amorphous silicon on a drum-shaped (cylinder-shaped) substrate (conductive substrate) formed of aluminum, nickel, or the like. It has a configuration.

  In this embodiment, the photosensitive drum 1 is a negatively chargeable OPC photosensitive member having an outer diameter of 24 mm, and a charge generation layer and a charge transport layer are arranged in this order on the surface of a conductive substrate formed of an aluminum cylinder. And having a photosensitive layer laminated from the conductive substrate side. In this embodiment, the photosensitive drum 1 is rotationally driven at a peripheral speed (process speed) of 220 mm / sec. In this embodiment, the dark portion potential VD on the photosensitive drum 1 formed by being charged by the charging roller 2 is −400V. In this embodiment, the bright portion potential VL on the photosensitive drum 1 formed by exposure by the exposure device 3 is −100V. Then, the toner is transferred from the developing roller 4a to the bright portion potential VL, whereby the electrostatic latent image is developed as a toner image.

  When the charge transport layer provided on the conductive substrate is scraped, the photosensitive drum 1 may have problems such as charging failure. The wear amount of the photosensitive drum 1 due to use tends to increase as the discharge amount at the time of charging increases, and therefore tends to increase as the AC voltage Vpp of the charging bias increases.

<Recording material conveyance path>
In this embodiment, the recording material cassette 7, the feeding roller 8, the transport roller 9, the top sensor 10, the pre-transfer guide 17, the post-transfer guide 11, the discharge sensor 13, the transport roller 14, the discharge roller 15, and the discharge tray 16 are They are arranged in order along the conveyance path of the recording material P.

  The top sensor 10 detects the leading edge and the trailing edge of the recording material P in the conveyance direction, and outputs a detection signal that is input to the control unit 50. Here, the front end and the rear end in the conveyance direction of the recording material P are also simply referred to as the front end and the rear end of the recording material P, respectively. The output of the top sensor 10 is turned on when the recording material P is in a detection unit where the top sensor 10 detects the recording material P, and turned off when the recording material P is not present in the detection unit. The control unit 50 can detect the timing at which the leading edge and the trailing edge of the recording material P pass through the top sensor 10 based on the output of the top sensor 10. The control unit 50 grasps the positions of the leading end and the trailing end of the recording material P from the ON / OFF timing of the output of the top sensor 10 to form an electrostatic latent image, turn on / off the transfer bias, change the output, etc. Is controlled in synchronization with the movement of the recording material P.

  Further, the process speed (the conveyance speed of the recording material P, the peripheral speed of the photosensitive drum 1) is set in advance to a predetermined value. For this reason, the control unit 50 performs photosensitivity passing through the transfer unit N when an arbitrary position in the conveyance direction of the recording material P passes through the transfer unit N by calculation based on the ON / OFF timing of the output of the top sensor 10. The position on the drum 1 can be detected.

  The discharge sensor 13 detects the trailing edge of the recording material P and outputs a detection signal to be input to the control unit 50 in order to confirm whether or not the recording material P is jammed (paper jam).

2. Control of Charging Bias In this embodiment, the control unit 50 obtains a predetermined region on the photosensitive drum 1 that has passed through the transfer unit N at a predetermined timing, in which the above-mentioned “memory” may occur during image formation. . The control unit 50 performs the following control when the image area on the photosensitive drum 1 is passing through the charging unit a when the predetermined area passes through the charging unit a. That is, the value of Vpp of the AC voltage of the charging bias when the predetermined area passes through the charging portion a is the value of the AC voltage of the charging bias when the area that is an image area other than the predetermined area passes through the charging portion a. It is made larger than the value of Vpp.

  In the image forming apparatus 100 of the present embodiment, the above-described “black” streak-like “memory” is likely to occur due to the peeling discharge that occurs when the trailing edge of the recording material P peels from the photosensitive drum 1 after the transfer. Therefore, in this embodiment, the predetermined area is an area including a position on the photosensitive drum 1 that has passed the transfer portion N when the rear end of the recording material P passes the transfer portion N. In this embodiment, the control unit 50 calculates the photosensitive drum that has passed through the transfer unit N when the rear end of the recording material P passes through the transfer unit N based on the ON / OFF timing of the output of the top sensor 10. A predetermined area including a position on 1 is detected.

  In the present embodiment, when the trailing edge of the recording material P passes the transfer portion N, the position on the photosensitive drum 1 that has passed the transfer portion N is more specifically the trailing edge of the recording material P in the transfer portion N. Is the position on the photosensitive drum 1 in contact. Here, for the sake of simplicity, the position on the photosensitive drum 1 that has passed the transfer portion N when the rear end of the recording material P passes the transfer portion N is defined as “on the photosensitive drum 1 corresponding to the rear end of the recording material P”. Also called “position”.

  FIG. 3 shows the switching timing of the charging bias and the transfer bias and the output ON / OFF of the top sensor 10 when printing is continuously performed on two recording materials (here, recording paper) P in this embodiment. It is a sequence chart figure which shows OFF timing. In FIG. 3, when a predetermined area X including a position on the photosensitive drum 1 corresponding to the rear end of the preceding recording material P first passes through the charging portion a, the charging portion a is transferred to the succeeding recording material P. FIG. 6 is a sequence chart when an image area for a toner image is passing. That is, FIG. 3 is a sequence chart diagram in the case where the conveyance interval (that is, the sheet interval) of the recording material P during continuous image formation is less than one turn of the photosensitive drum 1.

  “T” in FIG. 3 indicates the time from when the position on the photosensitive drum 1 corresponding to the rear end of the recording material P has passed through the transfer portion N to reach the charging portion a. In addition, an arrow written so as to bridge between the charts of the top sensor, the transfer bias, and the charging bias in FIG. 3 indicates that the timing is synchronized between both ends of the arrow. An “image forming period” in the figure indicates a period corresponding to an image area on the photosensitive drum 1. Although not shown in FIG. 3, before passing through the transfer portion N, the surface potential of the image area on the photosensitive drum 1 is set to −400V for the dark portion potential and −100V for the bright portion potential VL. ing. Hereinafter, the DC voltage of the charging bias is also simply referred to as “DC voltage”, and the AC voltage of the charging bias is also simply referred to as “AC voltage”.

  First, the charging bias will be described. At substantially the same time as the image area for the toner image transferred to the preceding recording material P reaches the charging portion a, the application of the charging bias is started. At this time, the DC voltage is Vdc = −400V, and the AC voltage Vpp is Vpp−base = 1300V. Here, “Vpp-base” refers to a reference value of Vpp of the AC voltage when an area other than the predetermined area X in the image area passes through the charging portion a. At this time, a charging bias of −400 ± (1300/2) V is applied to the charging roller 2.

  At substantially the same time as the image area for the toner image transferred to the preceding recording material P finishes passing through the charging portion a, the application of the charging bias is stopped. Then, the application of the charging bias is resumed almost simultaneously with the arrival of the image area on the photosensitive drum 1 for the toner image transferred to the subsequent recording material P to the charging portion a. The DC voltage and AC voltage of the charging bias at this time are the same as described above.

  Thereafter, the AC voltage Vpp is changed from Vpp−base = 1300V to Vpp− in accordance with the timing when the predetermined region X including the position on the photosensitive drum 1 corresponding to the rear end of the preceding recording material P passes through the charging portion a. The top is changed to 1600V. Here, “Vpp-top” refers to the maximum value of the AC voltage Vpp (Vpp increased from Vpp-base) when the predetermined area X in the image area passes through the charging portion a. At this time, a charging bias of −400 ± (1600/2) V is applied to the charging roller 2.

  Here, the length in the rotation direction of the photosensitive drum 1 in the predetermined region X including the position on the photosensitive drum 1 corresponding to the rear end of the recording material P can be set as appropriate so that “memory” can be sufficiently suppressed. it can. The predetermined area X is preferably about 1 mm to 5 mm in the rotational direction of the photosensitive drum 1 across the position on the photosensitive drum 1 corresponding to the rear end of the recording material P, and is typically about 1 mm. It is enough. In this embodiment, the period in which the AC voltage Vpp is set to Vpp-top = 1600 V is set to 30 msec, which is sufficiently long with respect to the region in the rotation direction of the photosensitive drum 1. At this time, it is preferable that the position on the photosensitive drum 1 corresponding to the rear end of the recording material P is a substantially center of the predetermined region X in the rotation direction of the photosensitive drum 1, and the region extending before and after that is the predetermined region X. Further, the amount of increase in Vpp-top with respect to Vpp-base of the AC voltage Vpp can be set as appropriate so that “memory” can be sufficiently suppressed, but about 100 V to 500 V is preferable.

  Further, in this embodiment, before and after the period in which the AC voltage Vpp is set to Vpp-top = 1600V, the Vpp is stepped up / down with the duration Δt = 30 msec and the variation ΔVpp = 100 V (the enlarged portion in FIG. 3). reference). That is, in this embodiment, the control unit 50 determines the value of the AC voltage Vpp from the value when the region other than the predetermined region X passes through the charging unit a to the maximum when the predetermined region X passes through the charging unit a. Control is performed so that the value is gradually increased to the value. Further, the control unit 50 gradually changes the value of the AC voltage Vpp from the maximum value when the predetermined region X passes through the charging unit a to the value when the region other than the predetermined region X passes through the charging unit a. The control is performed to make it smaller. In this embodiment, an area on the photosensitive drum 1 corresponding to a period of 150 msec consisting of a period in which the Vpp of the AC voltage is set to Vpp-top = 1600 V and a period in which Vpp before and after it is stepped up / down is a predetermined area. X.

  Next, the transfer bias will be described. As a transfer bias, a DC voltage of +1000 V (paper bias) is applied during a period in which the recording material P passes through the transfer portion N, and a period in which the recording material P does not pass through the transfer portion N (that is, between papers). ) Is applied with a DC voltage (paper bias) of + 500V.

  Next, the output of the top sensor 10 will be described. The output of the top sensor 10 is turned ON when the leading edge of the recording material P is detected, and turned OFF when the trailing edge of the recording material P is detected.

  As described above, in this embodiment, when the predetermined region X including the position on the photosensitive drum 1 corresponding to the rear end of the recording material P first passes through the charging unit a, the image region is passing through the charging unit a. In some cases, control is performed to change the AC voltage Vpp. On the other hand, when the predetermined area X including the position on the photosensitive drum 1 corresponding to the rear end of the preceding recording material P first passes the charging portion a, the non-image area on the photosensitive drum 1 is passing through the charging portion a. In this case, it is not necessary to perform control to change the AC voltage Vpp. That is, when the conveyance interval of the recording material P during continuous image formation (that is, between the sheets) is equal to or longer than one turn of the photosensitive drum 1, it is not necessary to perform control to change the AC voltage Vpp. In this case, the predetermined area X including the position on the photosensitive drum 1 corresponding to the rear end of the recording material P is charged twice or more before becoming the image area for the toner image transferred to the subsequent recording material P. Can pass through part a. For this reason, the charging unevenness of the photosensitive drum 1 due to the transfer process is greatly reduced, and the occurrence of “memory” is suppressed.

  FIG. 4 is a flowchart showing the flow of charging bias control in this embodiment. When starting the printing (S1), the controller 50 reads the charge setting value stored in the storage device 40 and sets the AC voltage Vpp to Vpp-base (S2). The control unit 50 monitors the output of the top sensor 10 when the leading edge of the preceding recording material P reaches the position of the top sensor 10 (S3). When the control unit 50 detects that the output has been switched from OFF to ON, the photosensitive drum 1 corresponding to the rear end of the preceding recording material P is calculated based on the process speed, the size of the recording material P, and the like. A predetermined area X including the upper position is obtained (S4). Specifically, the control unit 50 determines a period A required for the leading end of the preceding recording material P to reach the transfer portion N after the leading end of the preceding recording material P reaches the top sensor 10. The calculation is performed based on the process speed and the size of the recording material P. In addition, the time when the output of the top sensor 10 is turned on from OFF when the leading edge of the preceding recording material P reaches the position of the top sensor 10 is defined as time T1. The controller 50 determines the position on the photosensitive drum 1 corresponding to the rear end of the recording material P in the transfer portion N when the rear end of the preceding recording material P reaches the transfer portion N from time T1 and period A. The specified area X is determined based on the identification. Further, when the predetermined region X first passes through the charging unit a, the control unit 50 determines whether an image region for a toner image transferred to the subsequent recording material P is passing through the charging unit a. Is determined (S5). When the control unit 50 determines that the image area is an image area in step S5, the predetermined area X is an area where a toner image may be formed only after passing through the charging unit a. A control for changing the Vpp of the AC voltage is executed (S6). That is, the control unit 50 reads the charge setting value stored in the storage device 40. Then, the control unit 50 executes a step-up / down sequence of the AC voltage Vpp so that the maximum value becomes Vpp-top when the predetermined region X on the photosensitive drum 1 passes through the charging unit a. Thereafter, the control unit 50 determines whether or not the printing is finished (S7). If the printing continues, the operations in steps S3 to S7 are repeated, and if the printing is finished, the control is finished (S8). .

  Here, in step S4, the predetermined region X is determined based on the time T1 when the leading edge of the preceding recording material P reaches the top sensor 10, but the reference is not limited thereto. The time T2 when the trailing edge of the preceding recording material P reaches the top sensor 10 may be used as a reference. That is, a period B required from the time when the trailing edge of the preceding recording material P reaches the position of the top sensor 10 to the time when the trailing edge of the preceding recording material P reaches the transfer portion N is calculated. Then, the predetermined region X may be determined based on the time T2 and the period B. The calculation of the period A and the period B can be performed if the process speed and the size of the recording material P are specified. For this reason, the timing which calculates the period A or the period B may be before step S3, before step S2, or before step S1.

  Step S5 is an area in which the predetermined area X on the surface of the photosensitive drum 1 reaches the transfer section N as an image area after passing through the charging section a and before reaching the charging section a. It is only necessary to determine whether or not. For this reason, as long as the predetermined region X is determined in step S4, the timing for performing the determination in step S5 may be before the timing at which the predetermined region X first passes through the charging portion a.

3. Effects According to the present embodiment, it is possible to suppress the occurrence of “memory” in the predetermined area X including the position on the photosensitive drum 1 corresponding to the rear end of the recording material P. This is considered to be because by increasing the AC voltage Vpp and increasing the oscillating electric field, the convergence of the potential of the photosensitive drum 1 is enhanced, and uneven charging of the photosensitive drum 1 can be alleviated.

  In this embodiment, the AC voltage Vpp is stepped up / down. The reason is that in this embodiment, the DC voltage and the AC voltage are generated from a common power source. When the AC voltage Vpp is greatly changed, the DC voltage is changed and the dark portion potential VD is also changed because the power supply is shared. For this reason, there is a concern that density unevenness corresponding to the fluctuation of the dark portion potential VD occurs in the image. Therefore, it is preferable to suppress the fluctuation amount ΔVpp of the AC voltage Vpp per step to such an extent that the density unevenness cannot be visually recognized. In this example, if ΔVpp was 100 V or less, density unevenness could not be visually recognized.

4). Evaluation Experiment Next, an evaluation experiment for confirming the effect of this example will be described.

<Experimental conditions>
Environment: 23 ° C / 50% (normal temperature and humidity environment)
Recording material: VTR with LTR size and basis weight of 75 g / m ² (trade name, manufactured by Xerox)
Process speed: 220mm / sec
Recording material conveyance interval: 20 mm
Throughput: 40ppm (40 sheets per minute)
Photosensitive drum outer diameter: 24 mm
Photosensitive drum durability test: 10,000 continuous prints

<Experiment method>
“Memory” caused by peeling discharge between the rear end of the recording material P and the photosensitive drum 1 in the second image when the printing mode is the plain paper mode and two images with a density of 25% are continuously printed. ”Was confirmed. The case where “memory” occurred was evaluated as x (defect), and the case where it did not occur was evaluated as o (good). Further, an endurance test of the photosensitive drum 1 was performed, and printing was performed using the photosensitive drum 1 after the endurance, and it was confirmed whether or not an image defect was generated due to a charging failure due to scraping of the charge transport layer. The case where an image failure due to “charging failure” occurred was evaluated as x (defect), and the case where it did not occur was evaluated as ○ (good).

  The evaluation experiment was conducted for the present example and Comparative Examples 1 and 2 as comparative objects. In Comparative Example 1, the AC voltage Vpp was constant at 1300 V (same as Vpp-base in this example). In Comparative Example 2, the AC voltage Vpp was constant at 1600 V (same as Vpp-top in this example). FIG. 5 is a sequence showing charging bias switching timing when printing is continuously performed on two recording materials (here, recording paper) P for each of the present embodiment, Comparative Example 1 and Comparative Example 2. It is a chart figure.

<Experimental result>
The results of the evaluation experiment are shown in Table 1.

  In this example, “memory” did not occur, and charging failure did not occur even in printing using the photosensitive drum 1 after durability. In this embodiment, by increasing the Vpp of the AC voltage with respect to the predetermined area X on the photosensitive drum 1 corresponding to the rear end of the recording material P, the convergence of the potential in the predetermined area X is enhanced and “memory” is suppressed. I was able to. In this embodiment, the amount of discharge at the time of charging is larger than that in the case where control for increasing Vpp is not performed (Comparative Example 1), but the photosensitive drum 1 is scraped by minimizing the period for increasing Vpp. The photosensitive drum 1 after the endurance did not fail to be charged.

  On the other hand, in Comparative Example 1, although the charging failure of the photosensitive drum 1 after the endurance did not occur, “memory” occurred. In Comparative Example 1, by uniformly reducing Vpp, the discharge amount at the time of charging becomes smaller than in Comparative Example 2, and the photosensitive drum 1 can be prevented from being scraped. Did not occur. However, in Comparative Example 1, the “memory” could not be suppressed because the AC voltage Vpp for the predetermined area X on the photosensitive drum 1 corresponding to the rear end of the recording material P was not increased.

  In Comparative Example 2, no “memory” occurred, but charging failure occurred in the photosensitive drum 1 after the endurance. In Comparative Example 1, by increasing the AC voltage Vpp uniformly, the convergence of the potential was enhanced over the entire image area of the photosensitive drum 1, and “memory” could be suppressed. However, in Comparative Example 2, the amount of discharge at the time of charging was significantly increased compared to Comparative Example 1, and the amount of abrasion of the photosensitive drum 1 was increased, resulting in defective charging of the photosensitive drum 1 after durability.

  As described above, according to this embodiment, it is possible to suppress the occurrence of “memory” while suppressing the charging failure due to the shaving of the photosensitive drum 1. Thus, according to this embodiment, it is possible to suppress image defects caused by uneven charging of the photosensitive drum 1 due to the transfer process.

[Example 2]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the image forming apparatus according to the first exemplary embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted.

  FIG. 6 is a schematic functional block diagram showing a control mode of the charging bias in the present embodiment. In the present embodiment, the charging power source 30 includes a charging AC power source (AC voltage output unit) 31 and a charging DC power source (DC voltage output unit) 33. The charging power source 30 outputs a charging bias (DC + AC voltage) obtained by superimposing the AC voltage from the charging AC power source 31 on the DC voltage from the charging DC power source 33 and applies it to the charging roller 2. Thus, in the present embodiment, the DC voltage and the AC voltage are generated from individual power sources.

  FIG. 7 is a sequence chart showing charging bias switching timing when printing is continuously performed on two recording materials (here, recording paper) P in the present embodiment. In FIG. 7, as in the first embodiment, when the predetermined area X including the position on the photosensitive drum 1 corresponding to the rear end of the preceding recording material P passes through the charging portion a for the first time, FIG. 6 is a sequence chart diagram when an image region for a toner image transferred to a recording material P is passing.

  In the present embodiment, the AC voltage Vpp is increased from Vpp-base to Vpp-top without stepping up / down the AC voltage Vpp as in the first embodiment, and from Vpp-top to Vpp-base. Reduce. In the present embodiment, since the DC voltage and the AC voltage are respectively generated from separate power sources, it is less necessary to suppress the variation in the DC voltage by reducing the variation in the Vpp of the AC voltage as in the first embodiment. It is. In this embodiment, the region on the photosensitive drum 1 corresponding to a period of 150 msec in which the AC voltage Vpp is Vpp-top = 1600 V is the predetermined region X.

  As described above, according to the present embodiment, the same effects as those of the first embodiment can be obtained, and since it is not necessary to step up / down the AC voltage Vpp, the sequence can be simplified as compared with the first embodiment. It becomes.

[Example 3]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the image forming apparatus according to the first exemplary embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted.

  FIG. 8 is a schematic cross-sectional view of the image forming apparatus 100 of the present embodiment. In this embodiment, the image forming apparatus 100 includes a temperature / humidity sensor 19 inside the apparatus main body M as an environment detection unit that detects at least one of temperature and humidity inside or outside the apparatus main body M. In the present embodiment, the temperature / humidity sensor 19 detects the temperature and relative humidity inside the apparatus main body M, and inputs a signal indicating the detection result to the control unit 50. In this embodiment, the control unit 50 determines the usage environment of the image forming apparatus 100 based on the detection result of the temperature / humidity sensor 19, and changes the increase amount of the AC voltage Vpp according to the determination result. In other words, in this embodiment, the control unit 50 determines the predetermined region relative to the value of Vpp of the AC voltage when a region other than the predetermined region X passes through the charging unit a based on the temperature and relative humidity inside the apparatus main body M. The amount of increase in the value of Vpp of the AC voltage when X passes through the charging portion a is changed.

  Table 2 shows an example of the relationship between the determination result of the use environment of the image forming apparatus 100, the dark portion potential VD, the AC voltage Vpp, and the AC voltage Vpp increase amount. Here, the normal temperature and humidity environment is typically 23 ° C./50% environment, the low temperature and low humidity environment is typically 15 ° C./10% environment, and the high temperature and high humidity environment is typically 32.5 ° C. / Refers to an 80% environment.

  The dark portion potential VD of the photosensitive drum 1 is desirably controlled so as to keep the amount of toner transferred onto the photosensitive drum 1 during development constant. In this embodiment, the set value (control value) of the dark portion potential VD is determined to be a predetermined value based on the detection result of the temperature / humidity sensor 19. Since the charge amount of the toner changes depending on the temperature and humidity at which the toner is placed, in order to keep the amount of toner transferred onto the photosensitive drum 1 at the time of development, an appropriate development according to the charge amount of the toner is required. It is desirable to set the contrast. Therefore, the absolute value of the dark portion potential VD is relatively increased (−450 V) in a low temperature and low humidity environment where the toner charge amount is relatively high and a relatively large development contrast is required. On the other hand, in a high temperature and high humidity environment where the toner charge amount is relatively low and a relatively small development contrast is sufficient, the absolute value of the dark portion potential VD is relatively small (−350 V). However, the “memory” described in the first embodiment is more likely to occur in a high-temperature and high-humidity environment where the absolute value of the dark portion potential VD is smaller than in a high-temperature and low-humidity environment where the absolute value of the dark portion potential VD is large. Therefore, it is desired that the increase amount of the AC voltage Vpp is also changed to an appropriate value according to the dark portion potential VD.

  Therefore, in this embodiment, AC voltages Vpp-base and Vpp-top are set as shown in Table 2 in a low temperature and low humidity environment, a normal temperature and normal humidity environment, and a high temperature and high humidity environment, respectively. That is, in this embodiment, the amount of increase in the AC voltage Vpp in the room temperature and humidity environment is larger than that in the low temperature and humidity environment, and the increase in the AC voltage Vpp in the high temperature and humidity environment is higher than that in the room temperature and humidity environment. Increase the amount. If the “memory” does not cause a problem in a low-temperature and low-humidity environment, the control for changing the AC voltage Vpp may not be performed. As for step-up / down of the AC voltage Vpp, for example, the variation ΔVpp can be the same in each environment, and the number of steps and the duration Δt can be changed in each environment. Alternatively, the fluctuation amount ΔVpp can be changed in each environment within a range where the fluctuation of the DC voltage does not become a problem, and the number of steps and the duration Δt can be made the same in each environment. Similarly to the second embodiment, when using the charging power supply 30 configured to individually generate the DC voltage and the AC voltage, it is not necessary to step up / down the AC voltage Vpp in any environment. .

  FIG. 9 is a flowchart showing the flow of charging bias control in the present embodiment. The processing of steps S1 to S8 in the flowchart of FIG. 9 is the same as the processing of steps S1 to S8 in the flowchart of FIG. 4 described in the first embodiment. In the present embodiment, the process of step S10 is added. That is, in this embodiment, the control unit 50 determines the usage environment of the image forming apparatus 100 based on the detection result of the temperature / humidity sensor 19 after the process of step S1 (S10). Then, in step S6, the control unit 50 reads out the charge setting value stored in the storage device 40 and changes the AC voltage Vpp in accordance with the use environment of the image forming apparatus 100 determined in step S10. Run.

  As described above, according to this embodiment, it is possible to more appropriately suppress the occurrence of “memory” according to the use environment of the image forming apparatus 100.

[Example 4]
Next, another embodiment of the present invention will be described. The configuration and operation of the image forming apparatus according to the present exemplary embodiment are substantially the same as those of the image forming apparatus according to the first exemplary embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted.

  FIG. 10 is a schematic cross-sectional view of the image forming apparatus 100 of the present embodiment. In this embodiment, the image forming apparatus 100 includes a transfer belt 20 as a recording material carrier. The transfer belt 20 is stretched around a driving roller 21 and a driven roller 22 as a plurality of stretching rollers (supporting members) and stretched with a predetermined tension. The transfer belt 20 rotates (circulates) in the direction of arrow Rb (counterclockwise) in the figure when the drive roller 21 is driven to rotate.

As the transfer belt 20, PVDF (vinylidene fluoride resin), ETFE (tetrafluoroethylene-ethylene copolymer resin), polyimide, PET having a thickness of 50 to 200 μm and a volume resistivity of about 10 ⁹ to 10 ¹⁶ Ω · cm. A resin film formed of (polyethylene terephthalate), polycarbonate, or the like can be used. Alternatively, as the transfer belt 20, the surface of a base layer made of rubber such as EPDM having a thickness of about 0.5 to 2 mm is covered with a surface layer made of a material in which a fluororesin such as PTFE is dispersed in urethane rubber. Things can be used.

  On the inner peripheral surface (back surface) side of the transfer belt 20, a transfer roller 5 similar to that of the first embodiment is disposed so as to face the photosensitive drum 1 with the transfer belt 20 interposed therebetween. The transfer roller 5 is pressed toward the photosensitive drum 1 via the transfer belt 20, and is a transfer portion (transfer nip) that is a contact portion (contact portion when there is no recording material P) between the photosensitive drum 1 and the transfer belt 20. N is formed. In this embodiment, the recording material P that has passed the pre-transfer guide 17 is carried on the transfer belt 20 and conveyed to the transfer portion N. The toner image formed on the photosensitive drum 1 is transferred onto the recording material P carried on the transfer belt 20 by the action of the transfer roller 5 in the transfer portion N.

  The configuration having the transfer belt 20 as in this embodiment has good separation of the recording material P from the photosensitive drum 1. Further, this configuration has good conveyance stability of the recording material P. Therefore, this configuration has an advantage that image formation can be easily performed at high speed.

  In the image forming apparatus 100 having such a configuration, due to partial charging unevenness on the photosensitive drum 1 due to the transfer process, the image to be formed thereafter has a “white” streak “memory” (in the direction of the rotation axis of the photosensitive drum 1). A portion with a low concentration extending along the side) may occur. FIG. 11 is a schematic cross-sectional view of the vicinity of the transfer portion N when the leading edge of the recording material P enters the transfer portion N. An arrow from the transfer roller 5 to the photosensitive drum 1 in FIG. 11 schematically shows a path through which the transfer current flows. When the leading edge of the recording material P enters the transfer portion N, a gap A where the photosensitive drum 1 and the transfer belt 20 do not contact each other is formed, and an area where the transfer current hardly flows is generated. In the region where the transfer current hardly flows, the positive charge moving on the photosensitive drum 1 is small, and the absolute value of the dark portion potential VD becomes large. As a result, the development contrast in that region becomes smaller than in other regions, the amount of toner adhering during development is reduced, and the image density is reduced. As described above, due to partial charging unevenness of the photosensitive drum 1 due to the transfer process, a “white” stripe-shaped “memory” may occur in an image to be formed thereafter.

  Therefore, in this embodiment, the leading end of the recording material P passes through the transfer portion N in the predetermined region X on the photosensitive drum 1 that has passed through the transfer portion N at a predetermined timing where “memory” may occur. A region including a position on the photosensitive drum 1 that has sometimes passed through the transfer portion N is defined. In the present embodiment, as in the first embodiment, the control unit 50 calculates the transfer unit when the leading edge of the recording material P passes the transfer unit N by calculation based on the ON / OFF timing of the output of the top sensor 10. A predetermined area X including a position on the photosensitive drum 1 that has passed N is detected.

  In the present embodiment, when the leading edge of the recording material P passes through the transfer portion N, the position on the photosensitive drum 1 that has passed through the transfer portion N is more specifically in contact with the leading edge of the recording material P in the transfer portion N. This position is on the photosensitive drum 1. Here, for the sake of simplicity, the position on the photosensitive drum 1 that has passed the transfer portion N when the leading end of the recording material P passes the transfer portion N is “the position on the photosensitive drum 1 corresponding to the leading end of the recording material P”. Also called.

  FIG. 12 shows the timing for switching between the charging bias and the transfer bias and the ON / OFF of the output of the top sensor 10 when printing is continuously performed on two recording materials (here, recording paper) P in this embodiment. It is a sequence chart figure which shows OFF timing. FIG. 12 shows a toner image in which the charging portion a is transferred to the recording material P when the predetermined region X including the position on the photosensitive drum 1 corresponding to the leading end of the recording material P first passes through the charging portion a. It is a sequence chart figure in case the image area for is passing.

  “T” in FIG. 12 indicates the time from when the position on the photosensitive drum 1 corresponding to the leading edge of the recording material P has passed through the transfer portion N to reach the charging portion a. Moreover, the arrow described so as to bridge between the charts of the top sensor, the transfer bias, and the charging bias in FIG. 12 indicates that the timing is synchronized between both ends of the arrow. Although not shown in FIG. 12, the surface potential of the image area on the photosensitive drum 1 is set to −400V for the dark portion potential and −100V for the bright portion potential VL before passing through the transfer portion N. ing.

  At substantially the same time as the image area for the toner image transferred to the preceding recording material P reaches the charging portion a, application of the charging bias to the charging roller 2 is started. At this time, the DC voltage is Vdc = −400V, and the AC voltage Vpp is Vpp−base = 1300V.

  Thereafter, the AC voltage Vpp is changed from Vpp-base = 1300V to Vpp-top in accordance with the timing at which the predetermined area X including the position on the photosensitive drum 1 corresponding to the leading edge of the preceding recording material P passes through the charging portion a. = 1600V. Further, in this embodiment, Vpp is stepped up / down in the same manner as in the first embodiment before and after the period in which the AC voltage Vpp is set to Vpp−top = 1600 V (see the enlarged portion in FIG. 12).

  Here, the length of the predetermined region X including the position on the photosensitive drum 1 corresponding to the leading edge of the recording material P in the rotation direction of the photosensitive drum 1 can be appropriately set so that “memory” can be sufficiently suppressed. . The predetermined area X is preferably about 1 mm to 5 mm in the rotation direction of the photosensitive drum 1 with the position of the photosensitive drum 1 corresponding to the leading end of the recording material P interposed between them, and it is sufficient if the area is typically about 1 mm. is there. In this embodiment, the period in which the AC voltage Vpp is set to Vpp-top = 1600 V is set to 30 msec, which is sufficiently long with respect to the region in the rotation direction of the photosensitive drum 1. At this time, it is preferable that the position on the photosensitive drum 1 corresponding to the leading edge of the recording material P is the approximate center of the predetermined area X in the rotation direction of the photosensitive drum 1, and the area extending before and after the predetermined area X is the predetermined area X.

  At substantially the same time as the image area for the toner image transferred to the preceding recording material P finishes passing through the charging portion a, the application of the charging bias is stopped. The application of the charging bias is resumed almost simultaneously with the arrival of the image area for the toner image transferred to the subsequent recording material P at the charging portion a, and the charging bias is controlled in the same sequence as described above. .

  Normally, when the predetermined area X is an area including the position on the photosensitive drum 1 corresponding to the tip of the recording material P, the charging section a is the recording material when the predetermined area X first passes through the charging section a. An image area for the toner image to be transferred to P is passing. However, as in the case of the first embodiment, when the predetermined region X including the position on the photosensitive drum 1 corresponding to the leading edge of the recording material P first passes through the charging portion a, the charging portion a is not on the photosensitive drum 1. When the image region is passing, control for changing the AC voltage Vpp does not have to be performed. Similarly to the second embodiment, when using the charging power supply 30 configured to individually generate the DC voltage and the AC voltage, it is not necessary to step up / down the AC voltage Vpp. Similarly to the third embodiment, the increase amount of the AC voltage Vpp may be changed according to the use environment of the image forming apparatus 100.

  The transfer bias switching timing and the ON / OFF timing of the top sensor 10 in the sequence chart of FIG. 12 are the same as those in the sequence chart of FIG. 3 described in the first embodiment. Further, the control flow of the charging bias in the present embodiment is the same as that in the flowchart of FIG. 4 described in the first embodiment.

  According to this embodiment, it is possible to suppress the occurrence of “memory” in the predetermined area X including the position on the photosensitive drum 1 corresponding to the leading edge of the recording material P. As in the case of the first embodiment, by increasing the AC voltage Vpp and increasing the oscillating electric field, the convergence of the potential of the photosensitive drum 1 is enhanced, and uneven charging of the photosensitive drum 1 can be alleviated. This is considered to be possible.

  As described above, the control according to the present invention is also effective for suppressing the “memory” in the form of white streaks due to the leading edge of the recording material P passing through the transfer portion N.

  In the configuration having the transfer belt 20 as in the present embodiment, the Vpp of the AC voltage is changed with respect to the predetermined region X including the position on the photosensitive drum 1 corresponding to the rear end of the recording material P as in the first embodiment. Control may be performed. As a result, similar to the first embodiment, it is possible to suppress the occurrence of horizontal black streak-like “memory”. Further, in the configuration in which the photosensitive drum 1 and the transfer roller 5 directly face each other as in the first embodiment, the AC is related to the predetermined region X including the position on the photosensitive drum 1 corresponding to the front end of the recording material P as in the present embodiment. Control for changing the voltage Vpp may be performed. As a result, as in the present embodiment, it is possible to suppress the occurrence of horizontal white stripe-like “memory”.

[Example 5]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus according to the present exemplary embodiment are the same as those of the image forming apparatus according to the first exemplary embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted.

  The “memory” may also be generated due to the influence of the toner image passing through the transfer portion N. FIG. 13 is a schematic cross-sectional view of the vicinity of the transfer portion N when the recording material P to which the toner T adheres passes through the transfer portion N. An arrow from the transfer roller 5 to the photosensitive drum 1 in FIG. 13 schematically shows a path through which the transfer current flows. In the area where a lot of toner adheres on the recording material P, the toner becomes a resistor and the transfer current hardly flows. In the region where the transfer current hardly flows, the positive charge moving on the photosensitive drum 1 is small, and the absolute value of the dark portion potential VD becomes large. As a result, the development contrast in that region becomes smaller than in other regions, the amount of toner adhering during development is reduced, and the image density is reduced. As described above, due to partial charging unevenness of the photosensitive drum 1 caused by the transfer process, a “memory” is generated in which a portion having a low density corresponding to a portion where the toner amount of the toner image previously formed on the image is large is generated. Sometimes.

  Therefore, in this embodiment, the predetermined area X on the photosensitive drum 1 that has passed through the transfer portion N at a predetermined timing in which “memory” may occur is set as the next area. That is, this is a region including a position on the photosensitive drum 1 where the toner image satisfying a predetermined condition based on the image information passes through the transfer portion N when passing through the transfer portion N. In the present embodiment, the control unit 50 has a function as a determination unit that determines whether an image formed in the image area is an image in which “memory” is likely to occur based on image information regarding each image area. If the control unit 50 determines that the “memory” is an image that is likely to occur, the control unit 50 executes control to change the Vpp of the AC voltage according to the determination result.

  In the present embodiment, the control unit 50 controls the recording material P on the recording material P as an index value that correlates with the toner amount of the toner image on the photosensitive drum 1 that reaches the transfer portion N or on the recording material P based on the image information. The printing rate of the toner image is calculated. Then, the control unit 50 determines that “memory” is likely to occur when the calculated printing rate exceeds a predetermined threshold. In other words, in this embodiment, the predetermined area X in which “memory” is likely to occur is a photosensitive drum that has passed through the transfer portion N when a toner image whose index value correlating with the toner amount exceeds a predetermined threshold value passes through the transfer portion N. The area including the position above 1 is used, and in particular, the printing rate is used as the index value.

  The printing rate of the toner image on the recording material P is calculated by counting the number of pixels to which toner is attached in the image data for each page (for each image area), and the total number of pixels to which the toner is attached to the total number of pixels on the page. It can be calculated based on the ratio (area ratio). For example, if the entire surface is a solid image printed at the maximum density, the printing rate is 100%. The threshold of the printing rate for determining whether or not the “memory” is likely to occur can be set by performing a test or the like in advance, and is set to 80% in this embodiment. In particular, in this embodiment, the image area of each page is divided into 20 in the conveyance direction of the recording material P, the above determination is performed for each divided area, and the AC voltage Vpp is increased according to the determination result. The number of divisions of the divided areas can be set as appropriate so that “memory” can be sufficiently suppressed.

  In this embodiment, the position of the toner image on the recording material P on the recording material P is more specifically the position on the photosensitive drum 1 that has passed the transfer portion N when a toner image having a printing rate exceeding the threshold value passes the transfer portion N. This is the position on the photosensitive drum 1 in contact with the divided area where the printing rate exceeds the threshold. Here, for the sake of simplicity, this position is also referred to as “a position on the photosensitive drum 1 corresponding to a divided area where the printing rate exceeds the threshold value”.

  Here, the length in the rotation direction of the photosensitive drum 1 in the predetermined area X including the position on the photosensitive drum 1 corresponding to the divided area where the printing rate exceeds the threshold value is appropriately set so that “memory” can be sufficiently suppressed. be able to. Typically, the predetermined area X is approximately the same as the length of the divided area in the conveyance direction of the recording material P, or 1 mm to each upstream and downstream in the rotation direction of the photosensitive drum 1 across the length range of the divided area. It is sufficient to make the region longer by about 5 mm. In the present embodiment, as in the first embodiment, the AC voltage Vpp is Vpp-base = 1300V and Vpp-top = 1600V. The period during which the AC voltage Vpp is set to Vpp−top = 1600 V is substantially the same as the length of the divided area in the conveyance direction of the recording material P. In the present embodiment, Vpp is stepped up / down in the same manner as in the first embodiment before and after a period in which the AC voltage Vpp is set to Vpp-top = 1600V.

  Also in this embodiment, as in the case of the first embodiment, when the predetermined area X first passes through the charging portion a, when the non-image area on the photosensitive drum 1 is passing through the charging portion a, Control to change the AC voltage Vpp may not be performed. Similarly to the second embodiment, when using the charging power supply 30 configured to individually generate the DC voltage and the AC voltage, it is not necessary to step up / down the AC voltage Vpp. Similarly to the third embodiment, the increase amount of the AC voltage Vpp may be changed according to the use environment of the image forming apparatus 100.

  FIG. 14 is a flowchart showing the flow of charging control in this embodiment. The processes of steps S1 to S3 and S5 to S8 in the flowchart of FIG. 14 are the same as the processes of steps S1 to S3 and S5 to S8 in the flowchart of FIG. 4 described in the first embodiment. In this embodiment, the processes of step S20 and step S21 are added, and the process of step S22 is performed instead of the process of step S4 in the flowchart of FIG. In other words, in the present embodiment, the control unit 50 divides the image area into 20 parts in the conveyance direction of the recording material P with respect to the toner image that next passes through the transfer part N after the process of step S2, and records each divided area. The printing rate of the toner image on the material P or on the surface of the photosensitive drum 1 is calculated (S20). The control unit 50 determines whether or not the printing rate of each divided area exceeds a threshold value (80%) (S21). The divided area where the printing rate exceeds the threshold is an area of the photosensitive drum 1 that has reached the transfer portion N in a state where a larger amount of toner is carried. If there is a divided area where the printing rate exceeds the threshold, the process proceeds to step S3. If there is no divided area where the printing rate exceeds the threshold, the process proceeds to step S7. When the control unit 50 detects that the output of the top sensor 10 is turned ON in step S3, the control unit 50 obtains a predetermined region X including a divided region in which the printing rate exceeds the threshold based on the process speed and the like. (S22). The controller 50 is an area where the predetermined area X on the surface of the photosensitive drum 1 reaches the transfer section N as an image area after passing through the charging section a and before reaching the charging section a. It is determined whether or not there is (S5).

  The index that correlates with the toner amount of the toner image on the recording material P or on the photosensitive drum 1 that reaches the transfer portion N is not limited to the printing rate. In step 21, it is only necessary to determine whether or not the amount of toner carried on the photosensitive drum 1 is larger than a predetermined amount. Therefore, for example, a value obtained by counting the signal for causing the exposure apparatus 3 to emit the laser light L according to the image information for each divided region may be used as an index. This is because the longer the period during which the laser beam L is emitted, the more toner adheres to the photosensitive drum 1, and the value obtained by counting the signal corresponds to the amount of toner that adheres to the photosensitive drum 1. In this case, in step S21, the control unit 50 counts the signal for each divided region, and determines whether or not the counted value exceeds a preset threshold value.

  According to the present embodiment, the occurrence of “memory” in the predetermined area X including the position on the photosensitive drum 1 corresponding to the divided area where the printing rate exceeds the threshold can be suppressed. As in the case of the first embodiment, by increasing the AC voltage Vpp and increasing the oscillating electric field, the convergence of the potential of the photosensitive drum 1 is enhanced, and uneven charging of the photosensitive drum 1 can be alleviated. This is considered to be possible.

  As described above, the control according to the present invention is also effective for suppressing “memory” due to the influence of the toner image passing through the transfer portion N.

  Note that the control of setting the region of the photosensitive drum 1 including the region of the photosensitive drum 1 that has reached the transfer portion N in a state where a larger amount of toner is carried as in the present embodiment to the predetermined region X is performed in the fourth embodiment. Thus, the present invention can also be applied to a configuration having the transfer belt 20. As a result, even in the configuration having the transfer belt 20, the occurrence of “memory” due to the influence of the toner image passing through the transfer portion N can be suppressed as in the present embodiment.

[Other embodiments]
As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example.

  In the above embodiment, the image forming apparatus is a monochrome image forming apparatus having only one image forming unit, but the present invention is also applicable to an image forming apparatus having a plurality of image forming units such as a color image forming apparatus. Can do. For example, in a direct transfer type image forming apparatus having a transfer belt 20 as shown in FIG. 10, a tandem type image in which a plurality of (for example, four) image forming units are arranged side by side along the moving direction of the transfer belt 20. Forming devices are well known. Each image forming unit includes, for example, a photosensitive drum 1, a charging roller 2, an exposure device 3, a developing device 4, a transfer roller 5, and a cleaning device 6 similar to those shown in FIG. In such an image forming apparatus, for example, yellow, magenta, cyan, and black toner images formed on the photosensitive drums 1 of a plurality of (for example, four) image forming units are recorded on the transfer belt 20. The images are sequentially transferred so as to be superimposed on the material P. Also in such an image forming apparatus, each image forming unit can perform the same charging bias control as in the above-described embodiments. When the control of the fifth embodiment is applied, a predetermined area X on the photosensitive drum 1 of one image forming unit has a toner image satisfying a predetermined condition formed in another image forming unit in the one image forming unit. It may be an area including a position on the photosensitive drum 1 that has passed through the transfer portion N when passing through the transfer portion N.

  Embodiment 5 can also be applied to an intermediate transfer type image forming apparatus. FIG. 15 is a schematic cross-sectional view of a main part of an intermediate transfer type image forming apparatus. Elements having the same or corresponding functions or configurations as those of the image forming apparatus of the above-described embodiment are denoted by the same reference numerals. The intermediate transfer type image forming apparatus has an intermediate transfer belt 60 constituted by, for example, an endless belt as an intermediate transfer body, instead of the recording material carrier in the direct transfer type image forming apparatus shown in FIG. The intermediate transfer belt 60 is stretched around a plurality of stretching rollers and stretched with a predetermined tension. On the inner peripheral surface side of the intermediate transfer belt 60, for example, a primary transfer roller 5 as a primary transfer unit is disposed so as to face the photosensitive drum 1 with the intermediate transfer belt 60 interposed therebetween, and the photosensitive drum 1 and the intermediate transfer belt 60 are arranged. A primary transfer portion N1 that is a contact portion with the first is formed. For example, a secondary transfer roller 70 as a secondary transfer unit is disposed on the outer peripheral surface side of the intermediate transfer belt 60 so as to face one of the plurality of stretching rollers, and the intermediate transfer belt 60, the secondary transfer roller 70, The secondary transfer portion N2 that is the contact portion is formed. In the intermediate transfer type image forming apparatus, the toner image formed on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 60 by the action of the primary transfer roller 5 to which a voltage is applied in the primary transfer portion N1. The toner image primarily transferred onto the intermediate transfer belt 60 is secondarily transferred to the recording material P such as paper by the secondary transfer roller 70 to which a voltage is applied in the secondary transfer portion N2. In such an image forming apparatus, “memory” may be generated due to the influence of the toner image passing through the primary transfer portion N1 in the same manner as described in the fifth embodiment. Therefore, in such an image forming apparatus, for example, the same charging bias control as that in the fifth embodiment can be performed.

  In FIG. 15, only one image forming unit is shown for simplicity. However, in the intermediate transfer type image forming apparatus having the intermediate transfer belt 60, as in the case of the direct transfer type image forming apparatus, a plurality of (for example, four) image forming units along the moving direction of the intermediate transfer belt 60. A tandem type image forming apparatus in which are arranged side by side is well known. In such an image forming apparatus, for example, yellow, magenta, cyan, and black toner images formed on the photosensitive drums 1 of a plurality of (for example, four) image forming units are superimposed on the intermediate transfer belt 60. Thus, the images are sequentially transferred. As in the case of the image forming apparatus of the direct transfer method, when the control of the fifth embodiment is applied, a predetermined condition that the predetermined area X on the photosensitive drum 1 of one image forming unit is formed by another image forming unit. It is possible to detect based on a toner image satisfying the above.

  In Example 3, as an example of the state of the image forming apparatus, the amount of increase in Vpp of the AC voltage is changed according to the use environment of the image forming apparatus. In particular, in Example 3, the amount of increase in Vpp of the AC voltage was changed according to the temperature and humidity detection results by the temperature and humidity sensor. However, the appropriate value of the increase amount of Vpp of the AC voltage may have a sufficient correlation with at least one of temperature and humidity. Therefore, the amount of increase in the AC voltage Vpp can be changed based on at least one of the environmental temperature and humidity. Typically, the amount of increase in the case of the second temperature higher than the first temperature is made larger than the amount of increase in the case where the temperature is the first temperature. In addition, the amount of increase in the case of the second humidity higher than the first humidity is made larger than the amount of increase in the case where the humidity is the first humidity.

  Further, the state of the image forming apparatus is not limited to the use environment of the image forming apparatus, and may be the life state (usage amount state) of the components of the image forming apparatus. For example, the amount of increase in AC voltage Vpp can be changed in accordance with the usage state of the photosensitive drum. In this case, the control unit 50 uses the number of rotations of the photosensitive drum 1, the rotation time, the charging time (time for performing the charging process), the number of images formed, etc. as information correlated with the usage amount from the start of use of the photosensitive drum 1. Can be sequentially accumulated and stored in the storage device 40 (FIG. 2 and the like). Then, the control unit 50 determines the current use of the photosensitive drum 1 based on the information indicating the relationship between the increase amount of the Vpp of the AC voltage and the usage amount of the photosensitive drum 1 that is obtained in advance and stored in the storage device 40. The amount of increase in the AC voltage Vpp can be determined according to the amount. For example, in a configuration in which the absolute value of the dark portion potential decreases as the usage amount of the photosensitive drum 1 increases, the usage amount of the photosensitive drum 1 is larger than the first usage amount than the increase amount when the usage amount is the first usage amount. The increase amount in the case of a large second usage amount is increased.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging roller 5 Transfer roller 10 Top sensor 19 Temperature / humidity sensor 20 Transfer belt 30 Charging power supply 50 Control part

Claims (26)

A rotatable photoreceptor,
A charging member for charging the photoreceptor with a charging unit;
An application device for applying a charging voltage in which a DC voltage and an AC voltage are superimposed on the charging member;
An exposure device that exposes the charged photoreceptor to form an electrostatic image in an image area on the photoreceptor;
A developing device for supplying toner to the electrostatic image on the photoreceptor to form a toner image;
A transfer member for transferring a toner image on the photosensitive member to a recording material in a transfer portion by applying a voltage;
When the rear end of the recording material in the conveyance direction passes through the transfer unit and the region of the photoconductor including the position on the photoconductor that has passed through the transfer unit is a predetermined region, the predetermined region is the charging unit. The value of the peak voltage of the AC voltage of the charging voltage applied to the charging member while passing through the charging member is set to the value of the AC voltage when the region that becomes the image region other than the predetermined region passes through the charging unit. A control unit that performs control to be larger than the value of the peak-to-peak voltage;
An image forming apparatus comprising:   The image forming apparatus according to claim 1, wherein the transfer member is disposed so as to directly face the photoconductor when the recording material is not in the transfer portion. A recording material carrier that carries and transports the recording material;
The image forming apparatus according to claim 1, wherein the transfer member is disposed so as to face the photoconductor with the recording material carrier interposed therebetween.   The control unit sets the peak-to-peak voltage value of the AC voltage from a value when a region other than the predetermined region passes through the charging unit to a maximum value when the predetermined region passes through the charging unit. And control so that the predetermined area is gradually reduced from the maximum value when the predetermined area passes through the charging section to the value when the area other than the predetermined area passes through the charging section. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   The controller controls the peak-to-peak voltage of the AC voltage when the predetermined region passes through the charging unit with respect to the value of the peak-to-peak voltage of the AC voltage when a region other than the predetermined region passes through the charging unit. The image forming apparatus according to claim 1, wherein an increase amount of the value can be changed.   The image forming apparatus according to claim 5, wherein the control unit changes the increase amount based on at least one of environmental temperature and humidity.   The image forming apparatus according to claim 5, wherein the control unit changes the increase amount based on information correlated with the usage amount of the photoconductor.   The image forming apparatus according to claim 1, wherein the predetermined region is a region that first reaches the charging unit after passing through the transfer unit. A rotatable photoreceptor,
A charging member for charging the photoreceptor with a charging unit;
An application device for applying a charging voltage in which a DC voltage and an AC voltage are superimposed on the charging member;
An exposure device that exposes the charged photoreceptor to form an electrostatic image in an image area on the photoreceptor;
A developing device for supplying toner to the electrostatic image on the photoreceptor to form a toner image;
A transfer member for transferring a toner image on the photosensitive member to a recording material in a transfer portion by applying a voltage;
When the front end of the recording material in the conveyance direction passes through the transfer unit, and the region of the photoconductor including the position on the photoconductor that has passed through the transfer unit is defined as a predetermined region, the predetermined region serves as the charging unit. The AC voltage peak-to-peak value of the charging voltage applied to the charging member during the passage is the peak of the alternating voltage when the region that is the image region other than the predetermined region passes through the charging unit. A control unit that performs control to be larger than the value of the inter-voltage,
An image forming apparatus comprising:   The image forming apparatus according to claim 9, wherein the transfer member is disposed so as to directly face the photoconductor when the recording material is not in the transfer portion. A recording material carrier that carries and transports the recording material;
The image forming apparatus according to claim 9, wherein the transfer member is disposed so as to face the photoconductor with the recording material carrier interposed therebetween.   The control unit sets the peak-to-peak voltage value of the AC voltage from a value when a region other than the predetermined region passes through the charging unit to a maximum value when the predetermined region passes through the charging unit. And control so that the predetermined area is gradually reduced from the maximum value when the predetermined area passes through the charging section to the value when the area other than the predetermined area passes through the charging section. The image forming apparatus according to claim 9, wherein the image forming apparatus is an image forming apparatus.   The controller controls the peak-to-peak voltage of the AC voltage when the predetermined region passes through the charging unit with respect to the value of the peak-to-peak voltage of the AC voltage when a region other than the predetermined region passes through the charging unit. The image forming apparatus according to claim 9, wherein an increase amount of the value can be changed.   The image forming apparatus according to claim 13, wherein the control unit changes the increase amount based on at least one of environmental temperature and humidity.   The image forming apparatus according to claim 13, wherein the control unit changes the increase amount based on information correlated with the usage amount of the photoconductor.   The image forming apparatus according to claim 9, wherein the predetermined region is a region that first reaches the charging unit after passing through the transfer unit. A rotatable photoreceptor,
A charging member for charging the photoreceptor with a charging unit;
An application device for applying a charging voltage in which a DC voltage and an AC voltage are superimposed on the charging member;
An exposure device that exposes the charged photoreceptor to form an electrostatic image in an image area on the photoreceptor;
A developing device for supplying toner to the electrostatic image on the photoreceptor to form a toner image;
A transfer member that transfers a toner image on the photosensitive member to a transfer target in a transfer portion by applying a voltage;
When the area of the photoconductor including the area of the photoconductor that has reached the transfer unit in a state where a toner larger than a predetermined amount is carried is defined as the predetermined area, the charging is performed while the predetermined area passes through the charging unit. The value of the peak voltage of the AC voltage of the charging voltage applied to the member is set to be greater than the value of the peak voltage of the AC voltage when the region that is the image region other than the predetermined region passes through the charging unit. A control unit that performs control to increase, and
An image forming apparatus comprising:   18. The image forming apparatus according to claim 17, wherein the area of the photoconductor that has reached the transfer portion in a state where a toner larger than the predetermined amount is carried is an area having a printing rate larger than a predetermined threshold. apparatus.   The image forming apparatus according to claim 17, wherein the transfer member is disposed so as to directly face the photoconductor when a recording material as the transfer target is not in the transfer unit. . A recording material carrier that carries and conveys the recording material as the transfer object;
19. The image forming apparatus according to claim 17, wherein the transfer member is disposed so as to face the photoconductor with the recording material carrier interposed therebetween. An intermediate transfer member serving as the transfer target, transported to transfer the toner image transferred from the photosensitive member to a recording material;
The image forming apparatus according to claim 17, wherein the transfer member is disposed so as to face the photoconductor across the intermediate transfer body.   The control unit sets the peak-to-peak voltage value of the AC voltage from a value when a region other than the predetermined region passes through the charging unit to a maximum value when the predetermined region passes through the charging unit. And control so that the predetermined area is gradually reduced from the maximum value when the predetermined area passes through the charging section to the value when the area other than the predetermined area passes through the charging section. The image forming apparatus according to claim 17, wherein the image forming apparatus is an image forming apparatus.   The controller controls the peak-to-peak voltage of the AC voltage when the predetermined region passes through the charging unit with respect to the value of the peak-to-peak voltage of the AC voltage when a region other than the predetermined region passes through the charging unit. 23. The image forming apparatus according to claim 17, wherein an increase amount of the value can be changed.   24. The image forming apparatus according to claim 23, wherein the control unit changes the increase amount based on at least one of environmental temperature and humidity.   24. The image forming apparatus according to claim 23, wherein the control unit changes the increase amount based on information correlated with the usage amount of the photoconductor.   26. The image forming apparatus according to claim 17, wherein the predetermined region is a region that first reaches the charging unit after passing through the transfer unit.

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