JP2010256528A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can yield an excellent image by preventing a rear end memory image from occurring even if excessive transfer current flows at an edge portion at the rear end of recording material. <P>SOLUTION: In the image forming apparatus, an image is formed by making the recording material pass through a transfer nip part between a surface of a photoreceptor drum and a transfer means abutting thereon, and transferring a toner image on the surface of the photoreceptor drum to the recording material by the transfer means to which transfer bias is applied. The image forming apparatus includes: a current detection means for detecting current flowing from the transfer means to the photoreceptor drum when the transfer bias is applied to the transfer means; and a control means for controlling image forming operation. In the image forming apparatus, if the current detected by the current detection means exceeds a predetermined threshold when the rear end of preceding recording material passes through the transfer nip part during the continuous printing of the recording material, the control means controls feed of following recording material so that an interval, between the rear end of the preceding recording material and the leading end of the following recording material when the following recording material reaches the transfer nip part, is equal to or more one round (65 mm) of the surface of the photoreceptor drum. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine that forms an image by an electrophotographic method, and more particularly to suppression of image deterioration by a rear end memory of the photosensitive drum.

  FIG. 11 is a schematic configuration diagram illustrating an example of a conventional electrophotographic image forming apparatus. The image forming apparatus includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1 that rotates in the direction of an arrow a (clockwise), and a charging roller 2, an exposure device 3, a developing device 4, and a transfer around the drum-shaped electrophotographic photosensitive member. A roller 5 and a cleaning device 6 are provided. On the upstream side in the conveyance direction of the recording material P at the transfer nip N between the photosensitive drum 1 and the transfer roller 5, a paper feed cassette 7, a paper feed roller 8, a pre-feed sensor 9, a registration roller pair 10, a top sensor 11, a transfer A guide 12 is provided. On the downstream side of the transfer nip N in the conveyance direction of the recording material P, a static elimination needle 13, a conveyance guide 14, a fixing device 15, and a discharge roller pair 16 are provided.

  During image formation, a transfer bias Vtr necessary for transferring a toner image is applied to the transfer roller 5 from a transfer bias power source (not shown), and passes through a transfer nip N between the photosensitive drum 1 and the transfer roller 5. Charge is applied to the recording material P. As a result, the toner image on the photosensitive drum 1 is transferred onto the recording material P. After the transfer, the voltage applied to the transfer roller 5 by constant voltage control is switched to the transfer weak bias (transfer bias Vtr is turned off or weak bias Vlow smaller than the transfer bias Vtr). Prevents the occurrence of paper marks. As shown in FIG. 12, the transfer bias is switched such that the image forming area (printable area) S of the recording material P is set to the inner side by bmm (for example, 5 mm) from the front end P1, the rear end P2, and the left and right ends P3. This will be explained with an example. That is, as shown in FIG. 13, switching from the transfer bias Vtr after transfer to the weak bias Vlow is performed at a distance c (c <b) from the rear end P2 of the recording material P so that the toner does not scatter due to insufficient transfer current. This is performed in the non-image area S ′ which is inner by mm (see Patent Document 1). Here, the conveyance direction of the recording material P is an arrow A direction.

  In this conventional example, the photosensitive drum 1 is a negatively charged OPC photosensitive member, and is driven to rotate in the direction of arrow a at a predetermined process speed. The charging roller 2 contacts the surface of the photosensitive drum 1 with a predetermined pressing force, and charges the photosensitive drum 1 to a predetermined polarity and potential by a charging bias applied from a charging bias power source (not shown). The exposure apparatus 3 includes a laser diode (not shown) that emits laser light, a collimator lens, a polygon mirror, an fθ lens, and the like. The exposure device 3 emits a laser beam L that is on / off controlled in accordance with input image information and is uniformly charged by the charging roller 2 on the photosensitive drum 1 in a direction perpendicular to the rotation direction. Scan for exposure. By this exposure, the electric charge in the portion scanned with the laser beam L is removed, and an electrostatic latent image is formed on the surface of the photosensitive drum 1. The developing device 4 includes a rotatable developing sleeve 4a provided with a magnet roller (not shown) fixed therein. Then, a thin layer of developer (toner) is coated on the developing sleeve 4a, and the toner is attached to the electrostatic latent image formed on the surface of the photosensitive drum 1 at the developing position to develop it as a toner image (visualization). ) A developing bias is applied to the developing sleeve 4a from a developing bias power source (not shown). The transfer roller 5 is brought into contact with the surface of the photosensitive drum 1 with a predetermined pressing force to form a transfer nip portion N. The transfer bias applied from a transfer bias power source (not shown) causes the transfer between the photosensitive drum 1 and the transfer roller 5. The toner image on the surface of the photosensitive drum 1 is transferred to the recording material P at the transfer nip portion N between them. The transfer roller 5 is rotated in the direction of arrow b (counterclockwise). The fixing device 15 has a fixing roller 15a and a pressure roller 15b, and heats and presses the recording material P on which the toner image is transferred between the fixing roller 15a and the pressure roller 15b, so that the surface of the recording material P is heated. Heat-fix the toner image.

  Next, an image forming operation of the above-described image forming apparatus will be described. When an image forming operation start signal is issued, the recording material P in the paper feed cassette 7 is fed one by one by the paper feed roller 8 and conveyed to the registration roller pair 10. At this time, the conveyance of the recording material P is detected by the pre-feed sensor 9. On the other hand, the photosensitive drum 1 is rotationally driven in the direction of arrow a by a driving means (not shown), and is charged to a predetermined potential by the charging roller 2. Then, the laser beam L corresponding to the image signal is irradiated on the photosensitive drum 1 from the exposure device 3, and the potential on the photosensitive drum 1 is increased at the portion irradiated with the laser beam L, so that an electrostatic latent image is formed. Is done. Then, toner can be attached to the electrostatic latent image formed on the photosensitive drum 1 by the developing sleeve 4a of the developing device 4 to which a developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1 is applied. Visualize. After the leading edge of the recording material P is detected by the top sensor 11, the recording material P is transferred by the registration roller pair 10 via the transfer guide 12 in synchronism with the rotation of the photosensitive drum 1. Is fed to the transfer nip N between the two. Then, a transfer bias having a polarity opposite to that of the toner (positive polarity) is applied to the transfer roller 5 from a transfer bias power source (not shown), and the toner image is transferred from the photosensitive drum 1 onto the recording material P. The recording material P onto which the toner image has been transferred is neutralized by the neutralizing needle 13 to which a charge having a polarity (negative polarity) opposite to that of the transfer roller 5 is applied, and is separated from the photosensitive drum 1 by the weight of the recording material P. . The recording material P separated from the photosensitive drum 1 is conveyed to the fixing device 15 through the conveyance guide 14, and the transferred toner image is heated on the surface of the recording material P by heating and pressing by the fixing roller 15 a and the pressure roller 15 b of the fixing device 15. The paper is fixed and discharged to the outside by a pair of paper discharge rollers 16.

  On the other hand, the surface of the photosensitive drum 1 after the transfer of the toner image is cleaned by removing residual toner (transfer residual toner) and other adhering substances by the cleaning device 6 and used for the next image formation. By the way, at the time of image formation described above, a transfer bias Vtr necessary for transferring a toner image is applied to the transfer roller 5 from a transfer bias power source (not shown), and a transfer nip portion between the photosensitive drum 1 and the transfer roller 5 is applied. Charge is applied to the recording material P passing through N. As a result, the toner image on the photosensitive drum 1 is transferred onto the recording material P. After the transfer, the voltage applied to the transfer roller 5 by constant voltage control is switched to the transfer weak bias (transfer bias Vtr is turned off or weak bias Vlow smaller than the transfer bias Vtr). Prevents the occurrence of paper marks. As shown in FIG. 12, the transfer bias is switched such that the image forming area (printable area) S of the recording material P is set to the inner side by bmm (for example, 5 mm) from the front end P1, the rear end P2, and the left and right ends P3. This will be explained with an example. That is, as shown in FIG. 13, switching from the transfer bias Vtr after transfer to the weak bias Vlow is performed at a distance c (c <b) from the rear end P2 of the recording material P so that the toner does not scatter due to insufficient transfer current. This is performed in the non-image area S ′ which is inner by mm (see Patent Document 1). Here, the conveyance direction of the recording material P is an arrow A direction.

JP 2001-75378 A

  In the conventional transfer bias switching described above, the timing of switching from the transfer bias to the weak bias at the rear end P2 of the recording material P is such that the rear end P2 of the recording material P passes through the top sensor 11 upstream of the transfer nip N. From the time, the distance between the top sensor 11 and the transfer nip N, and the conveyance speed of the recording material P, the time for the trailing edge P2 of the recording material P to reach the transfer nip N is determined by back calculation. However, the distance between the top sensor 11 and the transfer nip N varies slightly depending on each image forming apparatus to be manufactured. Also, the conveyance speed of the recording material P depends on the outer diameter of the transfer roller 5 and the paper of the recording material P. Slightly changes depending on the seed and printing ratio. For this reason, there is a problem that the timing for switching from the transfer bias to the weak transfer bias is slightly shifted due to these variations. Further, as shown in FIG. 14, the top sensor 11 includes a sensor lever 21 that is movable by the contact of the passing recording material P, and a photo interrupter 22 that detects the movable sensor lever 21 in a non-contact manner. The leading edge and the trailing edge of the recording material P to be detected are detected. However, in the conventional top sensor 11 described above, the movable range of the sensor lever 21 differs depending on where the recording material P passes on the sensor lever 21. For this reason, an error occurs in the time until the photo interrupter 22 detects the sensor lever 21, and this also causes a problem that the transfer bias switching timing is shifted. In addition, since the shift in the transfer bias switching timing increases as the process speed increases, it is difficult to absorb this shift in the non-image area at the rear end of the recording material P due to the recent increase in the speed of the image forming apparatus. It has become. For this reason, as shown in FIG. 15A, when the transfer bias switching timing is shifted to the rear end side of the recording material P, an excessive transfer current flows to the photosensitive drum 1 side at the edge portion of the rear end of the recording material P. The potential fluctuates due to an excess current flowing on the photosensitive drum 1 at the rear end of the recording material P. A phenomenon occurs in which this potential fluctuation appears as a streak on the photosensitive drum 1 during subsequent printing on the recording material P. In the present specification, the streaky potential unevenness on the photosensitive drum is referred to as a rear end memory, and an image generated on a subsequent recording material by the rear end memory is referred to as a rear end memory image.

  The present invention has been made under such circumstances, and even when an excessive transfer current flows at the edge portion of the trailing edge of the recording material, the occurrence of a trailing edge memory image is prevented and a good image is obtained. An object of the present invention is to provide an image forming apparatus that can be obtained.

  In order to solve the above problems, in the present invention, the image forming apparatus is configured as described in the following (1) to (3).

(1) A charging unit that uniformly charges the surface of the photosensitive drum, an exposure unit that exposes the surface of the photosensitive drum charged by the charging unit to form an electrostatic latent image, and the exposure unit. Developing means for developing the electrostatic latent image with toner, and transfer means for transferring the toner image on the surface of the photosensitive drum developed by the developing means to a recording material. The recording material is passed through a transfer nip between the transfer means and the transfer means, and the toner image on the surface of the photosensitive drum is transferred to the recording material by the transfer means to which a transfer bias is applied to form an image. In the image forming apparatus to perform,
A current detection means for detecting a current flowing from the transfer means to the photosensitive drum when the transfer bias is applied to the transfer means; and a control means for controlling an image forming operation.
In the continuous printing of the recording material, when the trailing edge of the preceding recording material passes through the transfer nip portion, the control unit is configured to detect the current when the current detection unit exceeds a predetermined threshold. The interval between the trailing edge of the preceding recording material and the leading edge of the succeeding recording material when the succeeding recording material reaches the transfer nip portion is equal to or more than one round of the surface of the photosensitive drum. An image forming apparatus for controlling feeding of the succeeding recording material.

(2) formed by a charging unit that uniformly charges the surface of the photosensitive drum, an exposure unit that exposes the surface of the photosensitive drum charged by the charging unit to form an electrostatic latent image, and the exposure unit. A developing unit that develops the electrostatic latent image with toner; and a transfer unit that transfers the toner image on the surface of the photosensitive drum developed by the developing unit to a recording material. The surface of the photosensitive drum is in contact with the surface. The recording material is passed through a transfer nip portion with the transfer device, and the toner image on the surface of the photosensitive drum is transferred to the recording material by the transfer device to which a transfer bias is applied, thereby forming an image. In the image forming apparatus,
A current detection means for detecting a current flowing from the transfer means to the photosensitive drum when the transfer bias is applied to the transfer means; and a control means for controlling an image forming operation.
In the continuous printing of the recording material, when the trailing edge of the preceding recording material passes through the transfer nip portion, the control unit is configured to detect the current when the current detection unit exceeds a predetermined threshold. When the photosensitive drum is charged by the charging unit after the detection current detected by the current detection unit exceeds the predetermined threshold, a charging output is generated in a region of the surface of the photosensitive drum where the detection current exceeds the predetermined threshold. An image forming apparatus that performs correction that temporarily increases.

  According to the present invention, there is provided an image forming apparatus capable of obtaining a good image by preventing the occurrence of a rear end memory image even when an excessive transfer current flows in the rear end edge portion of the recording material.

Schematic configuration diagram showing a transfer bias control system in Embodiment 1 Side view showing transfer nip portion between photosensitive drum and transfer roller Front view showing the transfer nip between the photosensitive drum and the transfer roller Diagram showing transfer roller resistance measurement method The figure which shows the transfer bias switching timing in Example 1. FIG. The figure which shows the transfer current at the time of transfer bias switching in Example 1 The figure which shows the timing chart in Example 1. The figure which shows the timing chart in Example 2. The figure which shows the timing chart in Example 3. The figure which shows the timing chart in Example 4. Sectional drawing which shows schematic structure of image forming apparatus The figure explaining the image formation area and margin of a recording material in a conventional example Explanatory drawing of image forming area and transfer bias switching position of recording material in conventional example The figure which shows the structure of the top sensor in a prior art example The figure which shows the relationship between the space between papers and the occurrence of the trailing edge memory

  Hereinafter, the form for implementing this invention is demonstrated in detail by an Example.

  An “image forming apparatus” that is Embodiment 1 will be described. The schematic configuration of this embodiment is the same as that shown in FIG.

  In this embodiment, the photosensitive drum 1 is a negatively charged OPC photosensitive member, and is driven to rotate in the direction of arrow a at a predetermined process speed. The charging roller 2 contacts the surface of the photosensitive drum 1 with a predetermined pressing force, and charges the photosensitive drum 1 to a predetermined polarity and potential by a charging bias applied from a charging bias power source (not shown). The exposure apparatus 3 includes a laser diode (not shown) that emits laser light, a collimator lens, a polygon mirror, an fθ lens, and the like. The exposure device 3 emits a laser beam L that is on / off controlled in accordance with input image information and is uniformly charged by the charging roller 2 on the photosensitive drum 1 in a direction perpendicular to the rotation direction. Scan for exposure. By this exposure, the electric charge in the portion scanned with the laser beam L is removed, and an electrostatic latent image is formed on the surface of the photosensitive drum 1. The developing device 4 includes a rotatable developing sleeve 4a provided with a magnet roller (not shown) fixed therein. Then, a thin layer of developer (toner) is coated on the developing sleeve 4a, and the toner is attached to the electrostatic latent image formed on the surface of the photosensitive drum 1 at the developing position to develop it as a toner image (visualization). ) A developing bias is applied to the developing sleeve 4a from a developing bias power source (not shown). The transfer roller 5 is brought into contact with the surface of the photosensitive drum 1 with a predetermined pressing force to form a transfer nip portion N. The transfer bias applied from a transfer bias power source (not shown) causes the transfer between the photosensitive drum 1 and the transfer roller 5. The toner image on the surface of the photosensitive drum 1 is transferred to the recording material P at the transfer nip portion N between them. The transfer roller 5 is rotated in the direction of arrow b (counterclockwise). The fixing device 15 has a fixing roller 15a and a pressure roller 15b, and heats and presses the recording material P on which the toner image is transferred between the fixing roller 15a and the pressure roller 15b, so that the surface of the recording material P is heated. Heat-fix the toner image.

  Next, an image forming operation of the above-described image forming apparatus will be described. When an image forming operation start signal is issued, the recording material P in the paper feed cassette 7 is fed one by one by the paper feed roller 8 and conveyed to the registration roller pair 10. At this time, the conveyance of the recording material P is detected by the pre-feed sensor 9. On the other hand, the photosensitive drum 1 is rotationally driven in the direction of arrow a by a driving means (not shown), and is charged to a predetermined potential by the charging roller 2. Then, the laser beam L corresponding to the image signal is irradiated on the photosensitive drum 1 from the exposure device 3, and the potential on the photosensitive drum 1 is increased at the portion irradiated with the laser beam L, so that an electrostatic latent image is formed. Is done. Then, toner can be attached to the electrostatic latent image formed on the photosensitive drum 1 by the developing sleeve 4a of the developing device 4 to which a developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1 is applied. Visualize. After the leading edge of the recording material P is detected by the top sensor 11, the recording material P is transferred by the registration roller pair 10 via the transfer guide 12 in synchronism with the rotation of the photosensitive drum 1. Is fed to the transfer nip N between the two. Then, a transfer bias having a polarity opposite to that of the toner (positive polarity) is applied to the transfer roller 5 from a transfer bias power source (not shown), and the toner image is transferred from the photosensitive drum 1 onto the recording material P. The recording material P onto which the toner image has been transferred is neutralized by the neutralizing needle 13 to which a charge having a polarity (negative polarity) opposite to that of the transfer roller 5 is applied, and is separated from the photosensitive drum 1 by the weight of the recording material P. . The recording material P separated from the photosensitive drum 1 is conveyed to the fixing device 15 through the conveyance guide 14, and the transferred toner image is heated on the surface of the recording material P by heating and pressing by the fixing roller 15 a and the pressure roller 15 b of the fixing device 15. The paper is fixed and discharged to the outside by a pair of paper discharge rollers 16.

  On the other hand, the surface of the photosensitive drum 1 after the transfer of the toner image is cleaned by removing residual toner (transfer residual toner) and other adhering substances by the cleaning device 6 and used for the next image formation.

  FIG. 1 is a schematic configuration diagram illustrating a transfer bias control system of the image forming apparatus according to the present exemplary embodiment, FIG. 2 is a side view illustrating the vicinity of a transfer nip portion between a photosensitive drum and a transfer roller, and FIG. 3 is a photosensitive drum. 2 is a front view showing the vicinity of a transfer nip portion between the transfer roller and the transfer roller. Note that the same members as those in the image forming apparatus in FIG. 11 are denoted by the same reference numerals, and redundant descriptions such as image forming operations are omitted.

A Transfer Roller As shown in FIG. 2 and FIG. 3, the transfer roller 5 of this embodiment is a solid (filled meat) using rubber such as EPDM, silicone, NBR, urethane on a core metal 5a such as iron or SUS. Or a rubber roller formed with a foamed sponge-like medium resistance elastic layer 5b. A roller having a hardness of 25 to 70 degrees (when Asker-C / 1 kg load) and a resistance value of 10 ⁶ to 10 ¹⁰ Ω was used. The elastic layer 5b of the transfer roller 5 was obtained by performing secondary vulcanization after primary vulcanization and then polishing the surface so that the outer diameter shape has a desired dimension. In the transfer roller 5 used in this example, an elastic layer (medium resistance elastic layer) 5b made of NBR ion conductive sponge rubber having a diameter of 1 × 10 ⁸ Ω was formed on an iron core 5a having a diameter of 6 mm. Is. A sponge type conductive / elastic roller having a roller hardness of 30 degrees (Asker-C / total load of 1000 g), an outer diameter of 12.5 mm, and a longitudinal dimension of 218 mm. Further, the transfer roller 5 is pressed against the photosensitive drum 1 by the pressing force 5d from the core metal 5a at both ends of the longitudinal axis through the bearing 5c by the pressure spring 5d, thereby forming a transfer nip portion N. In this embodiment, the transfer roller 5 is pressed against the photosensitive drum 1 with a total pressure of 1 kg.

  FIG. 4 is a diagram illustrating a method for measuring the resistance of the transfer roller 5. As shown in FIG. 4, the transfer roller 5 is brought into contact with the aluminum cylinder 40 with a total pressure of 1000 g (500 g on one side) and rotated, and an arbitrary voltage (for example, +2.0 KV) is applied from the DC high voltage power source 41 to the metal core 5a. To do. Then, the maximum value and the minimum value of the voltage value generated at both ends of the resistor 42 at that time are read by the voltmeter 43. An average value of the voltage values flowing in the circuit is obtained from the read voltage value, and the resistance value of the transfer roller 5 is calculated. The measurement environment was a temperature of 20 ° C. and a humidity of 60%.

B Transfer Bias Control A description will be given with reference to FIG. In this image forming apparatus, the transfer bias control is performed by the PTVC (Programmable Transfer Voltage Control) control method described below. In the PTVC control method, a passing signal of the recording material P conveyed to the transfer nip N between the photosensitive drum 1 and the transfer roller 5 is input from the top sensor 11 to the control device (DC controller) 30. Then, the control device 30 outputs a PWM signal having a pulse width corresponding to a desired transfer output voltage to a low-pass filter (Low Pass Filter) 31. The pulse width of the PWM signal is stored in advance in a transfer output table (not shown) in the control device 30.
This PWM signal is converted to DC by the low-pass filter 31, amplified by an amplifier (AMP) 32 to become a transfer output voltage Vt, and input to the transfer high-voltage power supply 33. The transfer high-voltage power supply 33 applies a transfer voltage Vtr to the transfer roller 5 based on the input transfer output voltage Vt. The current value It flowing at this time is detected by the current detection circuit 34, and a signal corresponding to the current value It is input to the control device 30 via the A / D converter 35. When constant voltage control is desired, the PWM signal preset in the control device 30 and a transfer output correspondence table are used for determination, and a PWM signal having a pulse width corresponding to a desired voltage value is output. When constant current control is desired, the pulse width of the PWM signal from the control device 30 is gradually increased, and a signal corresponding to the current value It input to the control device 30 is a desired current value (constant current value). Continue until the value corresponding to. Then, constant current control is performed by following the voltage (pulse width) with a change in current value.

Next, transfer bias control in this embodiment will be described.
When an image formation signal (print signal) is received from a host computer (not shown) and uniform charging of the photosensitive drum 1 is completed, PTVC detection is performed once with the photosensitive drum 1 and the transfer roller 5 in direct contact with each other. . In this PTVC detection, the output voltage from the transfer high-voltage power supply 33 is gradually increased, and the voltage value when the transfer current reaches a preset constant current value is held in the control device 30 as Vto. The transfer voltage Vtr to be applied at the time of transfer is determined based on the detected voltage value Vto and the following transfer control expression stored in the controller 30 in advance.
Vtr = αVto + β (1)
Note that Vto in the equation (1) is a voltage generated when a predetermined detection current is passed through the transfer roller 5 at the time of PTVC detection, and α and β are constants arbitrarily determined by the transfer system.

  After the determination of the transfer voltage Vtr, the printing operation is started when the preparation for image formation is completed, and the recording material P is transferred between the photosensitive drum 1 and the transfer roller 5 in synchronization with the toner image on the photosensitive drum 1. Feed to the nip N. The recording material P and the toner image on the photosensitive drum 1 are synchronized by a timer count after the recording material P passes through the top sensor 11, and at the same time as the leading edge of the recording material P enters the transfer nip portion N, Transfer is performed by applying a constant transfer voltage Vtr. When the recording material P passes the top sensor 11, the timer count is started again, and the time for the trailing end of the recording material P to reach the transfer nip N is calculated backward. Then, a transfer weak bias (between sheets) applied between the first sheet and the second sheet (hereinafter referred to as sheet spacing) of the recording sheet P to which the transfer bias Vtr is conveyed a certain time before the trailing edge of the recording sheet P passes. Bias) Switch to Vlow. For example, if the distance between the top sensor 11 and the transfer nip N is D (mm) and the process speed is S (mm / sec), the rear end of the recording material P passes through the top sensor 11 and then reaches the transfer nip N. The time T to reach is T = D / S (sec). When it is desired to switch the transfer bias to the weak transfer bias at a position c (mm) from the rear end of the recording material P, the rear end of the recording material P passes through the top sensor 11 and then (D−c) / S ( sec) and then the transfer bias is switched to the transfer weak bias.

C Transfer Bias Switching Next, transfer bias switching (switching from transfer bias to weak transfer bias) by transfer bias control in this embodiment will be described. In this embodiment, as shown in FIG. 5, the image forming area (printable area) S of the recording material P is set 5 mm inside from the front end P1 (not shown), the rear end P2, and the left and right ends P3. explain. In this case, as shown in FIG. 5, switching from the transfer bias Vtr after transfer to the weak bias Vlow is performed in the non-image area S ′ at a distance of 2 mm from the rear end P2 of the recording material P. This is determined so that the transfer bias switching timing is about ± 3 mm due to the timing error of the top sensor 11 and the like, and the toner in the image area is not scattered by the influence of the weak bias Vlow. It is the position.
On the other hand, when the switching timing from the transfer bias Vtr to the weak bias Vlow is shifted to the rear end side of the recording material P due to a timing error of the top sensor 11 or the like, a rear end memory image is generated. That is, when an excessive transfer current flows at the trailing edge portion of the recording material P due to the influence of the transfer bias Vtr, the potential on the photosensitive drum 1 fluctuates due to the excess current, and the next recording material P The rear end memory image is generated in the image area. In particular, it occurs remarkably in the recording material P in a state where the resistance value is low and the current tends to flow at the rear end of the recording material P.

D Processing by Detection of Transfer Overcurrent FIG. 6 is a diagram showing a change in transfer current at the switching timing from the transfer bias Vtr to the weak bias Vlow. The transfer current that flows during transfer varies depending on the resistance values of the recording material P and the transfer roller 5. In general, the transfer current that flows due to the transfer bias Vtr at the time of printing is controlled to 8 to 10 μA, and the transfer current that flows due to the weak transfer bias Vlow at the time of paper is controlled to 5 to 7 μA when the sheet is not passing. However, when the transfer bias switching timing is delayed due to the above-described time lag and the transfer bias Vtr is applied to the trailing edge portion of the recording material P, an overcurrent of 12 μA or more may flow on the photosensitive drum 1. is there.

Therefore, in this embodiment, the current value flowing through the photosensitive drum 1 is detected every 1 msec by the current detection circuit 34 between 6 mm from the inner side 5 mm from the rear end of the paper (± 3 mm before and after the transfer bias switching timing, during time T1). . When the control device 30 calculates that the average current value of the detected current value for 10 msec (1 mm) is 12 μA or more (corresponding to a predetermined threshold in the claims), it is assumed that an overcurrent flows on the photosensitive drum 1. to decide. Then, the feed timing of the next recording material P ′ and the image writing in synchronization therewith are changed and controlled, and the output timings of charging, exposure and development are also changed and controlled. As shown in FIG. 15A, the above-mentioned rear end memory image has a memory formed on the photosensitive drum 1 by the current intensively flowing at the rear end of the first recording material P. This is a phenomenon that occurs as a streak image on the second recording material P after one round. However, as shown in FIG. 15B, the memory on the photosensitive drum 1 is attenuated every time charging is repeated, and this memory is generally lost when printing is performed on the second round of the photosensitive drum 1. Therefore, if the gap between the sheets (the distance between the trailing edge of the first recording material P and the leading edge of the second recording material P ′) is equal to or greater than one turn of the photosensitive drum 1, the trailing edge memory image is unlikely to appear on the image. .
Therefore, in this embodiment, when the control device 30 determines that an overcurrent of 12 μA or more is flowing on the drum 1, the space between the sheets up to the next recording material P is one turn of the photosensitive drum 1. Secure at least a minute. As a result, the timing for feeding the next recording material P is delayed from the predetermined timing, and the output timing of charging, exposure, development, and transfer bias is changed in synchronization with the writing of the image.

FIG. 7 shows a timing chart for switching output ON and OFF for paper feeding, top sensor, charging, exposure, development, transfer bias, and overcurrent detection during continuous printing in this embodiment.
In this embodiment, when a plurality of continuous print signals are received from a host computer (not shown), first printing (image formation) is started and paper feeding is started (S1). When the recording material P passes through the top sensor 11 and a top signal (Top signal) is output (turned on) (S2), the control device 30 starts a timer count. Then, the transfer bias Vtr is applied at the leading end of the recording material P in synchronization with the image on the photosensitive drum 1 at the transfer nip N (S3). When the rear end of the recording material P passes through the top sensor 11 and the top signal (Top signal) is turned off (S4), the control device 30 starts the timer count again, and the recording material is recorded at the transfer nip portion N. Predicting the inside of the rear end 5 mm of P, overcurrent detection is started (S5). Immediately thereafter, the transfer voltage Vtr is switched to the transfer weak bias Vto at a timing 2 mm inside the rear end of the recording material P (S6). Then, after the overcurrent detection is activated for a time T1, at the timing when the trailing edge of the recording material P reaches the position 1 mm after the transfer nip portion N, the overcurrent detection is completed and the control device 30 detects the overcurrent. It is determined whether or not it has been detected (S7). As a result, when an overcurrent is not detected, as shown by the dotted line in FIG. 7, the feeding operation of the next recording material P ′ is performed at a timing at which 65 mm space between the recording material P and the recording material P is secured. Start (S8). Thereafter, when the leading edge of the recording material P ′ passes through the top sensor 11 and a top signal (Top signal) is output (ON) (S9), the control device 30 starts a timer count. Then, in synchronization with the image position of the transfer nip portion N, charging, exposure, development, and output of the transfer bias Vtr onto the photosensitive drum 1 are started (S10, 11, 12, 13).

On the other hand, when an overcurrent is detected in S7, as shown by the solid line in FIG. 7, the feeding operation of the next recording material P ′ is started at a timing when 80 mm is maintained between the recording material P and the sheet. (S14). Thereafter, when the leading edge of the recording material P ′ passes through the top sensor 11 and a top signal (Top signal) is output (ON) (S15), the control device 30 starts timer counting. Then, in synchronism with the image position of the transfer nip N, charging, exposure, development, and output of the transfer bias Vtr on the photosensitive drum 1 are started at any time (S16, 17, 18, 19). The image forming apparatus of this embodiment has a process speed of 100 m / sec, A4 / 16.5 ppm (number of A4 prints per minute), a sheet interval of 65 mm during continuous paper feeding, and other various positional relationships. It is as follows.
・ The distance between the recording material leading edge detection position of the top sensor 11 and the transfer nip is 45 mm.
-The distance between the recording material trailing edge detection position of the top sensor 11 and the transfer nip is 25 mm.
・ Distance between paper feed roller and transfer nip is 60mm
・ The drum circumference is 75mm.
・ Distance between charging and transfer nip is 40mm (35mm in the rotation direction)
-Distance between exposure and transfer nip is 35mm (40mm in the rotation direction)
・ Distance between development and transfer nip is 20mm (55mm in the rotation direction)
・ Overcurrent detection operating time T1 = 60 msec (operating distance 6 mm)

  As described above, in this embodiment, when an overcurrent is detected at the rear end of the recording material P, the output start timing of the next recording material P ′ is uniformly 150 msec (15 mm). Equivalent) Delay the paper interval from 65 mm to 80 mm. In this way, by ensuring at least one drum circumference (75 mm) between the paper sheets, the trailing edge memory area generated at the trailing edge of the recording material P passes through a large amount of charging once, and then the next recording material P ′. To reach the image area. Therefore, the rear end memory is almost lost by repeating charging, and the rear end memory image can be prevented from being generated on the image of the recording material P ′.

E Evaluation In this example, a transfer roller having a resistance of 1.0E + 08Ω (when 2 kV is applied at HH) in a high-temperature and high-humidity environment (hereinafter referred to as HH) at 30 ° C. and 85% is used, and a low resistance with a moisture content of 10% or more. This is an example in which 31 sheets of paper, 8% medium resistance paper, and 6% or less high resistance paper are printed continuously. Table 1 shows an evaluation result obtained by confirming the generation level of the rear end memory image. As described above, in the present embodiment, when an overcurrent of 12 μA or more is detected at the rear end of the recording material P, the paper interval is increased from 65 mm to 80 mm, and the image writing timing is changed at a timing synchronized therewith. Comparative Example 1 is an example in which, even when an overcurrent flows, the interval between sheets remains 65 mm and the image writing timing is not changed. In addition, the images printed on the recording material P were evaluated by two types: an isolated dot image in which the rear end memory image is conspicuous and a dark halftone (hereinafter referred to as HT) that is relatively inconspicuous.

  From the above results, in Comparative Example 1, since the paper interval was always shorter than one drum, it was confirmed that the rear end memory image was generated with a considerable frequency including high resistance paper and HT image which are relatively inconspicuous. On the other hand, in the present embodiment, when the overcurrent is detected, the interval between the sheets is extended to one or more rounds of the drum. In some stricter conditions, when low-resistance paper that is prone to overcurrent flow and isolated dots that have a noticeable rear-end memory image are printed, the rear-end is required even if one or more drum cycles are secured. It has been difficult to completely suppress the generation of memory images. However, the improvement effect from the comparative example 1 is clear, and with respect to a memory having a slight generation level such as medium resistance paper, high resistance paper, and HT image, the rear end memory image is hardly generated in this embodiment. confirmed.

  As described above, in this embodiment, when an overcurrent is detected at the rear end of the recording material P, the sheet feeding timing of the next recording material P ′ and the image writing timing are corrected, and the interval between the sheets is equal to or more than one round of the drum. It is set as the structure to provide. Thanks to this, even if potential unevenness (rear edge memory) occurs on the photosensitive drum due to overcurrent, this unevenness is smoothed by giving this area one more charge (charging is performed twice or more). be able to. As a result, it is possible to prevent a rear end memory image generated on the next recording material P ′ and obtain a good image. When the overcurrent greatly exceeds 15 μA or more, the space between the papers may be provided for two or more drums, so that the region subjected to the overcurrent may be charged more once or more. Further, the amount of correction for the feeding timing of the succeeding paper and the image writing timing may be further adjusted according to the current value of the overcurrent detection.

  In the embodiment, it is assumed that the interval between sheets does not change during conveyance. However, when using a plurality of paper feed cassettes, the gap between sheets may change during conveyance, but the point is that when the subsequent recording material reaches the transfer nip portion, the gap between the sheets more than one round of the drum. If it is possible, the rear end memory area is charged twice or more, so that the generation of the rear end memory image can be suppressed.

  An “image forming apparatus” that is Embodiment 2 will be described. In the present embodiment, in the same configuration as in the first embodiment, when an overcurrent is detected at the trailing end of the preceding recording material P, the potential unevenness on the photosensitive drum caused by the overcurrent is smoothed. In this example, the charging output is temporarily corrected to be slightly stronger at the timing of charging the uneven area. Note that functions, operations, and the like of the respective units of the image forming apparatus are the same as those described in the first embodiment, and thus description thereof is omitted.

  FIG. 8 is a timing chart for switching the output ON and OFF of the top sensor, charging, exposure, development, transfer bias, and overcurrent detection in this embodiment. The description of the same operation as that of the first embodiment is omitted. In this embodiment, when the recording material P passes through the top sensor 11 and a top signal (Top signal) is output (ON) (S2), the control device 30 starts a timer count (S3), and the recording material P An overcurrent is detected at the rear end (S7). Thereafter, at the timing when the rear end memory area on the photosensitive drum first reaches the charger, the charging output is temporarily changed to Vh1 (S20). Thereafter, the charging output Vh1 is continuously applied for the time T2 when the overcurrent is detected, and then returned to the original sheet-to-paper charging output Vpr * (S21). Then, when the time for charging the rear end memory area comes again after the time for one rotation of the photosensitive drum has elapsed, the charging output is changed to Vh1 again (S22) (the charging output in the claims is temporarily changed). This is equivalent to performing correction for increasing the number of revolutions of the photosensitive drum). When the time T2 has elapsed again, the charging output is returned to the original charging output Vpr (S23). Various parameters in this example are as follows.

Time T2
10 msec (minimum unit of overcurrent detection) or more and less than 60 msec (overcurrent detection time) Inter-sheet charging output VPr = −1000 V
Corrected charging output Vh1 = −1100V

  As described above, in the present embodiment, when an overcurrent is detected at the rear end of the recording material P, the charging output is temporarily corrected to be slightly stronger by Δ100 V and output to the region on the photosensitive drum 1 that has received the overcurrent. Further, the generation level of the rear end memory image was confirmed by the same method as described above. Table 2 shows the evaluation results.

  From the above results, the improvement effect from Comparative Example 1 is clear. Similar to the first embodiment, the rear end memory image is slightly generated in the isolated dot image of the low resistance paper, but a good image can always be obtained under other conditions. As described above, in this embodiment, when an overcurrent is detected at the rear end of the recording material P, the charging output is temporarily corrected to be slightly stronger at the timing of charging the potential unevenness on the photosensitive drum caused by the overcurrent. ing. As a result, it is possible to further smooth the potential unevenness on the photosensitive drum as compared with the case where charging is performed with a normal charging output, and the generation level of the rear end memory image in a high-temperature and high-humidity environment is suppressed, and a good image is obtained. be able to.

  An “image forming apparatus” that is Embodiment 3 will be described. In this embodiment, when an overcurrent is detected at the rear end of the preceding recording material P, the space between the sheets is provided for one or more rotations of the photosensitive drum (80 mm), and the photosensitive is performed in the two charging until the next recording material P ′. This is an example in which the charging output is strongly output twice in the region of the drum 1 that has received the overcurrent. Note that the image forming apparatus according to the present exemplary embodiment has the same configuration as that of the first exemplary embodiment, and a description thereof will be omitted. Further, description of functions and operations of each unit of the image forming apparatus will be omitted.

In this embodiment, as in the first embodiment, when an overcurrent is first detected at the rear end of the recording material P, the timing of paper feeding and image writing is corrected, and the interval between the sheets is equal to or more than one rotation of the photosensitive drum (80 mm). Secure. Then, in order to smooth the potential unevenness on the photosensitive drum caused by this overcurrent, the charging output is corrected to be stronger in both of the two charging operations up to the next recording material P ′. FIG. 9 is a timing chart for switching the output ON and OFF for the top sensor, charging, exposure, development, transfer bias, and overcurrent detection in this embodiment. The description of the same operation as that of the first embodiment is omitted.
In this embodiment, when the recording material P passes through the top sensor 11 and a top signal (Top signal) is output (ON) (S2), the control device 30 starts the timer count, and the trailing edge of the recording material P is reached. Then, an overcurrent is detected (S7). Then, at the timing when the rear end memory area on the photosensitive drum first reaches the charger, the charging output is temporarily changed to Vh1 (S20). Thereafter, the charging output Vh1 is continuously applied for the time T2 when the overcurrent is detected, and then returned to the original sheet-to-paper charging output Vpr * (S21). When the time for one rotation of the photosensitive drum has elapsed and the timing for charging the rear end memory area comes again, the charging output is changed to Vh1 again (S22). When the time T2 has elapsed again, the charging output is returned to the original charging output Vpr (S23). Various parameters in this example are as follows.

Time T2
10 msec (minimum unit of overcurrent detection) or more and less than 60 msec (overcurrent detection time) Inter-sheet charging output VPr = −1000 V
Corrected charging output Vhigh = -1100V

  As described above, in this embodiment, when an overcurrent is detected at the rear end of the recording material P, a space between the sheets of the photosensitive drum is one round (80 mm) or more. In the two charging operations up to the next recording material P ′, the charging output is output slightly stronger by Δ100 V in both the areas on the photosensitive drum 1 that have received the overcurrent. Therefore, the generation level of the rear end memory image was confirmed by the same method as described above. Table 3 shows the evaluation results.

  From the above results, in this embodiment, even when the isolated dot image is printed with low resistance paper that cannot be suppressed even in the first and second embodiments, the generation of the rear end memory image is suppressed, and a good image is always obtained. be able to. As described above, in this embodiment, when an overcurrent is detected at the rear end of the recording material P, the feeding timing of the next recording material P ′ and the image writing timing are shifted, and the interval between the sheets is equal to or more than one rotation of the photosensitive drum. It is set as the structure to provide. Thanks to this, even if potential unevenness occurs on the photosensitive drum due to an overcurrent, this region can be charged twice or more, so that the potential unevenness can be smoothed. In addition, since the charging output is temporarily corrected to be slightly stronger at the timing of charging the potential unevenness on the photosensitive drum caused by this overcurrent, the potential unevenness is smoothed more than charging with the regular charging output. Can be realized. As a result, it is possible to suppress the generation of the trailing edge memory image and obtain a good image even for low resistance paper in a high temperature and humidity environment.

  An “image forming apparatus” that is Embodiment 4 will be described. The present embodiment is an example in which the charge output correction amount differs between the first charge correction and the second charge correction in the same configuration as the third embodiment. Note that the image forming apparatus according to the present exemplary embodiment has the same configuration as that of the first exemplary embodiment, and a description thereof will be omitted. Further, description of functions and operations of each unit of the image forming apparatus will be omitted.

  As described in the second and third embodiments, in order to prevent the rear end memory image, it is wise to smooth the uneven potential of the photosensitive drum due to the overcurrent generated at the rear end of the recording material P by charging. As in the third embodiment, if one sheet of photosensitive drum is provided between the sheets, the potential unevenness region can be charged once more, and the potential unevenness can be further smoothed by correcting the charging output to be stronger. Can be

  However, if the correction of the charging output is too strong, the photosensitive drum will be uneven in potential due to overcharging, and the phenomenon of white streaks will occur, contrary to the trailing edge memory image that occurs as black streaks. It can happen. In particular, in the second charge correction, it is preferable to reduce the charge correction amount slightly in consideration of the potential unevenness being smoothed in the first charge correction. In addition, the second charging is a timing after the start of image writing on the next recording material P ′, and the influence of overcharging may appear as a white streak image on the next recording material P ′. There is also.

Therefore, in this embodiment, the following operations are executed as shown in the timing chart of FIG. In this embodiment, when the recording material P passes through the top sensor 11 and a top signal (Top signal) is output (ON) (S2), the control device 30 starts the timer count, and the trailing edge of the recording material P is reached. Then, an overcurrent is detected (S7). Then, at the timing when the rear end memory area on the photosensitive drum first reaches the charger, the charging output is temporarily changed to Vh2 (S20). Thereafter, the charging output Vh2 is continuously applied for the time T2 when the overcurrent is detected, and then returned to the original sheet-to-paper charging output Vpr * (S21). Then, when the time for charging the rear end memory area comes again after the time for one rotation of the photosensitive drum has elapsed, the charging output is changed to Vh3 in which the correction amount is smaller than that of the first charging Vh2 (S24). . This processing is equivalent to weakening the degree of correction that temporarily increases the charging output as the circulation proceeds. When the time T2 has elapsed again, the charging output is returned to the original charging output Vpr (S25). Various parameters in this example are as follows.
Time T2 = 10 msec (minimum unit of overcurrent detection) to 60 msec (overcurrent detection time)
Paper-to-paper charging output VPr = -1000V
Corrected charging output Vh2 = −1120V
Corrected charging output Vh1 = −1050V

  As described above, in this embodiment, the correction of the charge output for preventing the occurrence of the rear end memory image prevents the occurrence of white streaks due to overcharging. In the first time when the potential unevenness is large, the charging is slightly strengthened, and in the second time when the potential unevenness is slightly smoothed, the charging is corrected extremely weakly. As a result, even in the same evaluation as in Example 1 shown in Table 4, the generation of the rear end memory image is not seen, and the occurrence of white stripes due to overcharging can be suppressed.

An “image forming apparatus” that is Embodiment 5 will be described. This embodiment is an example in which the correction amount of the charging output is changed according to the current value of the overcurrent detection regarding the correction of the charging output of the second, third, and fourth embodiments. Note that functions, operations, and the like of each unit of the image forming apparatus are the same as those described in the first to fourth embodiments, and thus description thereof is omitted.
As described in the second to fourth embodiments, in order to prevent the rear end memory image, it is wise to smooth the uneven potential of the photosensitive drum due to the overcurrent generated at the rear end of the recording material P by charging. However, since this potential unevenness varies depending on the current value flowing at the rear end of the recording material P, it is preferable to determine the correction value of the charging output based on this current value. For example, if the overcurrent is about 12 μA, the rear end memory image can be prevented by correcting the charging output slightly strongly. However, if the overcurrent exceeds 15 μA, a considerable amount of correction is required. Further, assuming that the overcurrent of 15 μA or more flows, if the correction of the charging output is strengthened, as described above, the white streak phenomenon may occur due to the influence of overcharging. Therefore, in this embodiment, as shown in Table 5, the charging output correction amount is changed according to the current value of the overcurrent detection.

  As described above, in this embodiment, by changing the correction amount of the charging output according to the current value of the overcurrent detection, when the overcurrent of 15 μA or more flows, the correction of the charging output is strong and reversed. In addition, if the overcurrent is less than 12 μA, the charging output correction is set to be weak. Therefore, it is possible to always provide the optimum charging output for smoothing the potential unevenness on the photosensitive drum which is different depending on the amount of the flowing current. As a result, even in the same evaluation as in Example 1 shown in Table 6, the generation of the rear end memory image is not observed, and at the same time, the generation of white streaks due to overcharging can be suppressed, and a good image can always be obtained. .

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging 3 Exposure apparatus 4 Developing apparatus 5 Transfer roller 30 DC controller N Transfer nip part

Claims (6)

A charging unit that uniformly charges the surface of the photosensitive drum; an exposure unit that exposes the surface of the photosensitive drum charged by the charging unit to form an electrostatic latent image; and an electrostatic latent image formed by the exposure unit. A developing means for developing an image with toner; and a transfer means for transferring a toner image on the surface of the photosensitive drum developed by the developing means to a recording material, the transfer means contacting the surface of the photosensitive drum and the transfer means. The image forming apparatus performs image formation by passing the recording material through a transfer nip between the photosensitive drum and the transfer unit to which a transfer bias is applied to transfer a toner image on the surface of the photosensitive drum to the recording material. In
A current detection means for detecting a current flowing from the transfer means to the photosensitive drum when the transfer bias is applied to the transfer means; and a control means for controlling an image forming operation.
The control means performs subsequent processing when the current detected by the current detection means exceeds a predetermined threshold when the trailing edge of the preceding recording material passes through the transfer nip portion during continuous printing of the recording material. When the recording material reaches the transfer nip portion, the interval between the trailing edge of the preceding recording material and the leading edge of the succeeding recording material is equal to or more than one round of the surface of the photosensitive drum. An image forming apparatus that controls feeding of the subsequent recording material.   When the current detected by the current detection means exceeds a predetermined threshold when the trailing edge of the preceding recording material passes through the transfer nip portion during continuous printing of the recording material, the subsequent recording material is The succeeding recording material so that the distance between the trailing edge of the preceding recording material and the leading edge of the succeeding recording material when reaching the transfer nip portion is equal to or more than one round of the surface of the photosensitive drum. The photosensitive drum exceeds the predetermined threshold when the charging unit charges the photosensitive drum after the current detected by the current detection unit exceeds the predetermined threshold. The image forming apparatus according to claim 1, wherein correction for temporarily increasing the charging output is performed in a surface area of the image forming apparatus. A charging unit that uniformly charges the surface of the photosensitive drum; an exposure unit that exposes the surface of the photosensitive drum charged by the charging unit to form an electrostatic latent image; and an electrostatic latent image formed by the exposure unit. A developing means for developing an image with toner; and a transfer means for transferring a toner image on the surface of the photosensitive drum developed by the developing means to a recording material, the transfer means contacting the surface of the photosensitive drum and the transfer means. The image forming apparatus performs image formation by passing the recording material through a transfer nip between the photosensitive drum and the transfer unit to which a transfer bias is applied to transfer a toner image on the surface of the photosensitive drum to the recording material. In
A current detection means for detecting a current flowing from the transfer means to the photosensitive drum when the transfer bias is applied to the transfer means; and a control means for controlling an image forming operation.
In the continuous printing of the recording material, when the trailing edge of the preceding recording material passes through the transfer nip portion, the control unit is configured to detect the current when the current detection unit exceeds a predetermined threshold. When the photosensitive drum is charged by the charging unit after the detection current detected by the current detection unit exceeds the predetermined threshold, a charging output is generated in a region of the surface of the photosensitive drum where the detection current exceeds the predetermined threshold. An image forming apparatus that performs correction to temporarily increase.   4. The control unit according to claim 2, wherein correction for temporarily increasing a charging output for a region of the surface of the photosensitive drum where the detected current exceeds a predetermined threshold is performed over a plurality of turns of the photosensitive drum. The image forming apparatus described in 1.   The control means reduces the degree of correction that temporarily increases the charging output as the rotation proceeds when the correction that temporarily increases the charging output is performed over a plurality of rotations of the photosensitive drum. 5. The image forming apparatus according to 4.   When the control means performs correction for temporarily increasing the charging output, correction for temporarily increasing the charging output based on a detected current value when a detection current detected by the current detection means exceeds a predetermined threshold. The image forming apparatus according to claim 2, wherein the correction amount is changed.

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