JP2014123004A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that employs a direct transfer system using an organic photoreceptor as an image carrier so as to effectively prevent failure in images due to sudden influx of transfer current to the image carrier at a transfer nip part.SOLUTION: An image forming apparatus includes an image carrier having an organic photosensitive layer formed on the surface, a transfer member, a high-voltage power source unit, and a control unit. The high-voltage power source unit can select constant current control or constant voltage control as a control method of a voltage applied to the transfer member. The control unit controls the voltage applied from the high-voltage power source unit to the transfer member in the constant current control during transfer of toner images onto a recording medium at a transfer nip part, and switches the control of the high-voltage power source unit from the constant current control to the constant voltage control after an image area on the recording medium passes through the transfer nip part and before a rear end of the recording medium passes through the transfer nip part.

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile, and more particularly to control of a transfer voltage when transferring toner on an image carrier onto a recording medium.

  Conventionally, in an image forming apparatus using an electrophotographic process, an electrostatic latent image is formed on an image carrier (photosensitive drum) whose surface is uniformly charged, and toner formed according to the electrostatic latent image A method is generally used in which an image is transferred onto a conveyed recording medium and then fixed on the recording medium. Then, the charge remaining on the photosensitive drum after the toner image is transferred is removed by photostatic discharge, and the surface of the photosensitive drum is uniformly charged again to prepare for the formation of the next electrostatic latent image.

  When the toner image on the photosensitive drum is transferred onto the recording medium, a transfer voltage having a polarity opposite to that of the toner is applied to a transfer member such as a transfer roller that forms a transfer nip portion with the photosensitive drum. At this time, it is known to control the transfer voltage with a constant current in order to ensure transfer performance against fluctuations in the resistance value of the recording medium, resistance fluctuations in the transfer roller, and the like due to environmental conditions.

  Here, the transfer current is a parameter that affects the charging stability performance of the photosensitive drum. That is, it is desirable to control so that the amount of charge applied from the transfer roller to the photosensitive drum does not vary between transfer and non-transfer (between sheets). If the amount of charge applied from the transfer member to the photosensitive drum between papers is extremely different from that at the time of transfer, or if the charge having the opposite polarity to that at the time of transfer is applied, the surface of the photosensitive drum Large fluctuations in the potential occur, resulting in a loss of image quality.

  However, when the current is controlled between the sheets with the same current value as that at the time of transfer, a discharge phenomenon occurs at and near the transfer nip. Generally, since the uniformity of the transfer roller surface is not so high, the discharge distribution is not uniform. Therefore, partial non-uniformity of the discharge distribution at the transfer nip portion appears in the image, and therefore it is desirable to control the current value so that the voltage does not cause the discharge phenomenon to be significant. For example, when the constant current value during transfer is controlled at 9 μA, the current value between the sheets is controlled at about 2 to 3 μA, and the voltage value at that time is about 300V.

  FIG. 8 is a graph showing the relationship between the elapsed time and the transfer current when the transfer voltage is controlled at a constant current and the target value of the current is changed at a timing immediately before the trailing edge of the sheet passes through the transfer nip portion. In FIG. 8, the control value (target value) of the transfer current at the time of paper passing is 9 μA, and the target value of the transfer current is changed to 2 μA immediately before passing through the rear end of the sheet (indicated by the vertical broken line in the figure). .

  While the sheet is passing through the transfer nip portion, the load resistance of the transfer nip portion is a total value of the resistances of the transfer member, the photosensitive drum, and the sheet. After the trailing edge of the sheet passes through the transfer nip portion, the load resistance of the transfer nip portion is limited to only the transfer member and the photosensitive drum, and the load resistance rapidly decreases by the amount of resistance of the sheet. As a result, the constant current control temporarily fails until the voltage value decreases due to the feedback of the constant current circuit functioning, and an excessive current flow (peak in FIG. 8) occurs in the transfer nip portion. Occur.

For example, when the load resistance including the sheet is 500 MΩ, a voltage of (500 × 10 ⁶ Ω) × (2 × 10 ⁻⁶ A) = 1 kV is required to set the transfer current to the target value of 2 μA. There is. When the trailing edge of the sheet passes from this state and the load resistance decreases to 100 MΩ, the transfer current temporarily becomes (1 × 10 ³ V) / (100 × 10 ⁶ Ω) = 1 × 10 ⁻⁵ A = 10 μA. .

  As a result, in the photosensitive layer of the photosensitive drum, the charging performance of the history portion (transfer memory) that has been exposed to the transfer current is reduced, and the surface potential of that portion is reduced, so that the toner adheres excessively and the image is removed. Black streaks appear on the screen. This phenomenon occurs in the case of a monochrome reversal development system in which the photoconductor drum is an organic photoconductor and the charge history received by the photoconductor drum at the transfer nip portion cannot be canceled only by light neutralization. This is particularly noticeable when a transfer roller is used as the transfer member and the transfer voltage is only a DC voltage and no AC voltage is superimposed.

  In view of this, a method for suppressing the generation of a transfer memory due to charge injection into the photosensitive drum when the trailing edge of the recording medium passes through the transfer nip portion has been proposed. Patent Document 1 discloses a recording in which a toner image is transferred. After the image area of the medium passes through the transfer position, the maximum voltage that can be applied is applied to the transfer charging unit, and the reverse voltage (same polarity as the toner) of the transfer voltage is applied to the transfer charging unit. Discloses an image forming apparatus in which after passing through the transfer position, the non-transfer state (current flowing through the transfer charging means is 0 μA) is disclosed.

Japanese Patent Laid-Open No. 2000-242098

  According to the method of Patent Document 1, in a transfer drum type color image forming apparatus in which the charge history on the photosensitive drum is relatively gentle, the positive correction to the photosensitive drum when a sudden resistance change of the transfer system occurs. In addition to preventing charge injection, the transfer current rising speed at the tip of the recording medium is also secured.

  However, in the case of the direct transfer method in which the toner image is directly transferred onto the recording medium using the transfer member, the change in load resistance when the recording medium is present in the transfer nip portion and when the recording medium is not is different from that in the transfer drum method. It gets bigger. For this reason, even if the method disclosed in Patent Document 1 is used, it is not possible to sufficiently prevent a rapid transfer current from flowing into the photosensitive drum in the transfer section, and there is a possibility that an image malfunction due to the transfer memory may occur.

  In view of the above problems, the present invention can effectively prevent an image defect caused by a rapid flow of a transfer current to an image carrier in a transfer nip portion in a direct transfer method using an organic photoreceptor as an image carrier. An object is to provide a forming apparatus.

  In order to achieve the above object, a first configuration of the present invention includes an image carrier, a transfer member, a high-voltage power supply unit, and a control unit, and a transfer nip formed by the image carrier and the transfer member. And an image forming apparatus that transfers a toner image carried on an image carrier onto a recording medium. The image carrier has an organic photosensitive layer formed on its surface and carries a toner image. The transfer member is disposed to face the image carrier. The high-voltage power supply unit has a constant current circuit that applies a voltage to the transfer member by constant current control and a constant voltage circuit that applies a voltage to the transfer member by constant voltage control, and is opposite to the charging polarity of the toner on the transfer member. Apply polarity voltage. While the toner image is transferred to the recording medium at the transfer nip portion, the control portion controls the voltage applied from the high voltage power source portion to the transfer member by constant current control, and the image area of the recording medium passes through the transfer nip portion. Thereafter, the control method of the voltage applied from the high-voltage power supply unit to the transfer member is switched from constant current control to constant voltage control before the trailing edge of the recording medium passes through the transfer nip portion.

  According to the first configuration of the present invention, it is possible to always ensure a certain transfer performance even if the resistance of the recording medium or the transfer member fluctuates due to fluctuations in environmental conditions such as temperature and humidity. In addition, by switching from constant current control to constant voltage control after the image area of the recording medium passes through the transfer nip portion and before the trailing end of the recording medium passes, the trailing end of the recording medium moves the transfer nip portion. Even if a sudden drop in resistance occurs during removal, a significant change in output voltage can be suppressed. Accordingly, it is possible to effectively prevent an excessive current from flowing into the image carrier and a black streak resulting therefrom while maintaining a predetermined transfer performance during transfer. .

1 is a schematic cross-sectional view illustrating an overall configuration of an image forming apparatus 100 according to an embodiment of the present invention. Partial enlarged view of the periphery of the image forming portion P in FIG. 1 is a circuit diagram showing an example of a high-voltage power supply unit 20 used in the image forming apparatus 100 of the present invention. 1 is a block diagram showing an example of a control path used in the image forming apparatus 100 of the present invention. 7 is a flowchart showing a transfer current control procedure in the image forming apparatus 100 of the present invention. Graph showing the relationship between the elapsed time and the transfer current when switching from constant current control to constant voltage control at the timing immediately before the trailing edge of the paper passes the transfer nip. Graph showing the relationship between the elapsed time and transfer voltage when a two-step control value is set as the control voltage during constant voltage control A graph showing the relationship between the elapsed time and the transfer current when the transfer voltage is controlled at a constant current in a conventional image forming apparatus and the target value of the current is changed at a timing immediately before the trailing edge of the sheet passes the transfer nip portion.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 according to an embodiment of the present invention, and FIG. 2 is a partially enlarged view around an image forming portion P in FIG. In the main body of the image forming apparatus (for example, a monochrome multifunction peripheral) 100, an image forming portion P for forming a monochrome image by each process of charging, exposure, development, and transfer is disposed.

  In the image forming unit P, along the rotation direction of the photosensitive drum 1 (counterclockwise direction in FIG. 1), the charging unit 2, the exposure unit 3, the developing device 4, the transfer roller 7, the cleaning device 8, and the static eliminating device. (Not shown) is provided. In the image forming unit P, the image forming process for the photosensitive drum 1 is executed while rotating the photosensitive drum 1 in the counterclockwise direction in FIG.

  The photosensitive drum 1 is formed, for example, by laminating a photosensitive layer on an aluminum drum, and the charging unit 2 charges the surface. Then, an electrostatic latent image in which charging is attenuated is formed on the surface that has received a laser beam from the exposure unit 3 described later. Here, as the photosensitive layer, an organic photosensitive layer (OPC) that generates less high-resolution images with less ozone during charging is used.

  The charging unit 2 uniformly charges the surface of the photosensitive drum 1. For example, a contact-type charging device that applies a voltage in a state where a charging roller (charging member) is in contact with the surface of the photosensitive drum 1 is used as the charging unit 2. Instead of the contact-type charging device, a corona discharge device that discharges by applying a high voltage using a thin wire or the like as an electrode may be used. The exposure unit 3 irradiates the photosensitive drum 1 with a light beam (for example, a laser beam) based on the document image data read by the image reading unit 18 to form an electrostatic latent image on the surface of the photosensitive drum 1.

  The developing device 4 forms toner images by attaching toner to the electrostatic latent image on the photosensitive drum 1. The toner is supplied to the developing device 4 from the toner container 5 via the intermediate hopper 6. Here, a single-component developer (hereinafter also simply referred to as toner) composed only of a magnetic positive polarity toner component is accommodated in the developing device 4.

  The transfer roller 7 is disposed opposite to the photosensitive drum 1, and a transfer nip portion N is formed between the transfer roller 7 and the photosensitive drum 1. The transfer roller 7 is connected to a high voltage power supply unit 20 for applying a transfer voltage, and is transferred to a sheet conveyed through the sheet conveyance path 11 without disturbing the toner image formed on the surface of the photosensitive drum 1. To do. The configuration of the high voltage power supply unit 20 will be described later. Further, on the upstream side and the downstream side of the transfer nip portion N, paper detection sensors 19a and 19b for detecting the passage of the paper are arranged.

  The cleaning device 8 includes a cleaning roller, a cleaning blade, and the like that are in line contact with the longitudinal direction of the photosensitive drum 1 (a direction perpendicular to the paper surface in FIGS. 1 and 2). After the toner image is transferred to the paper, Residual toner remaining on the surface of the photosensitive drum 1 is removed.

  The image reading unit 18 includes a scanner lamp that illuminates the document during copying, a scanning optical system equipped with a mirror that changes the optical path of reflected light from the document, and a condensing lens that focuses the reflected light from the document to form an image. And a CCD sensor or the like (both not shown) that converts the imaged image light into an electrical signal, and reads a document image and converts it into image data. The image data is stored in a temporary storage unit 94 (see FIG. 4) in the control unit 90.

  When performing a copy operation, the image reading unit 18 reads the image data of the document and converts it into an image signal. On the other hand, in the image forming unit P, the surface of the photosensitive drum 1 rotating in the counterclockwise direction in the drawing is uniformly charged by the charging unit 2, and the exposure unit is based on the document image data read by the image reading unit 18. 3 emits a laser beam (light beam) on the photosensitive drum 1 to form an electrostatic latent image based on the image data on the surface of the photosensitive drum 1. Thereafter, the developing device 4 attaches toner to the electrostatic latent image to form a toner image.

  A sheet (recording medium) is conveyed from the sheet storage unit 10 via the sheet conveyance path 11 and the registration roller pair 13 at a predetermined timing toward the image forming unit P on which the toner image is formed as described above. In the image forming unit P, a transfer voltage having a polarity opposite to that of the toner is applied from the high voltage power supply unit 20 to the transfer roller 7, and the toner image on the surface of the photosensitive drum 1 electrically attracted by the transfer roller 7 is transferred onto the sheet. The Thereafter, the toner remaining on the surface of the photosensitive drum 1 is removed by the cleaning device 8 in preparation for the subsequent formation of a new electrostatic latent image. Further, residual charges on the surface of the photosensitive drum 1 are removed by light irradiation from a static eliminator (not shown).

  The sheet on which the toner image has been transferred is separated from the photosensitive drum 1, conveyed to the fixing unit 9, and heated and pressed to fix the toner image on the sheet. The paper that has passed through the fixing unit 9 is sorted in the transport direction by the transport guide member 16 disposed in the branching section of the paper transport path 11, and is sent as it is (or after being sent to the reverse transport path 17 and copied on both sides). The paper is discharged to the paper discharge unit 15 via the discharge roller pair 14.

  FIG. 3 is a circuit diagram showing a detailed configuration of the high-voltage power supply unit 20. The high-voltage power supply unit 20 includes a DC power supply 21 that generates a transfer voltage, a constant current circuit 23 (indicated by a solid line in FIG. 3) and a constant voltage circuit 25 (indicated by a broken line in FIG. 3) connected to the DC power supply 21. Become.

  The constant current circuit 23 includes a control circuit 27a, an operational amplifier 29a, a comparison power supply 30a, and a resistor 31a. In the constant current circuit 23, the voltage value Vs at both ends of the resistor 31a is determined by the current value flowing through the resistor 31a, and the difference between the voltage value Vs and the voltage value Vref of the comparison power supply 30a becomes an input of the control circuit 27a. The output current is controlled.

  The constant current circuit 23 performs control so that Vs changes when the load resistance changes abruptly, resulting in a constant current. In other words, when the load suddenly decreases, a time lag occurs until the circuit feedback is activated, and a current larger than the target value temporarily flows.

  The constant voltage circuit 25 includes a control circuit 27b, an operational amplifier 29b, a comparison power supply 30b, and resistors 31b and 31c. In the constant voltage circuit 25, the output voltage is determined by the ratio (R1 / R2) of the voltage value Vref of the comparison power supply 30b and the resistance values R1 and R2 of the resistors 31b and 31c.

  In the case of the constant voltage circuit 25, even if the load resistance changes abruptly, there is no change in the values of Vref, R1, and R2 that determine the output voltage. Absent.

  Next, the control path of the image forming apparatus 100 of the present invention will be described. FIG. 4 is a block diagram illustrating an example of a control path used in the image forming apparatus 100 of the present invention. It should be noted that since various control of each part of the apparatus is performed when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) that transmit a control signal to each device in the temporary storage unit 94, the counter 95, and the image forming apparatus 100 for storing image data and the like, and receive an input signal from the operation unit 50. The I / F (interface) 96 is provided. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like. The counter 95 adds up the number of printed sheets and counts it.

  In addition, the control unit 90 transmits a control signal from the CPU 91 to the respective units and apparatuses in the image forming apparatus 100 through the I / F 96. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 96. Examples of each part and device controlled by the control unit 90 include the image forming unit P, the image reading unit 18, the high voltage power supply unit 20, the paper detection sensors 19a and 19b, and the operation unit 50.

  The operation unit 50 is provided with a liquid crystal display unit 51, LEDs 52 that indicate various states, and a numeric keypad 53, and the user operates the operation unit 50 to input instructions, whereby various settings of the image forming apparatus 100 are performed. And execute various functions such as image formation. The liquid crystal display unit 51 displays the state of the image forming apparatus 100, displays the image forming status and the number of copies, and can be used as a touch panel to perform various settings such as functions such as double-sided printing and black-and-white reversal, magnification setting, and density setting. It has become. The numeric keypad 53 is used for setting the number of copies to be printed and for inputting the other party's FAX number when the image forming apparatus 100 also has a FAX function.

  FIG. 5 is a flowchart showing a transfer current control procedure in the image forming apparatus 100 of the present invention. The control of the transfer current flowing through the transfer roller 7 will be described along the steps of FIG. 5 with reference to FIGS. 1 to 4 as necessary.

  First, when a print command is transmitted by the operation of the operation panel 50 by the user and printing is started (step S1), a transfer voltage is applied to the transfer roller 7 by constant voltage control using the constant voltage circuit 25 (step S1). S2). Note that the transfer voltage at this time is set to about 300 V at which the discharge phenomenon between the photosensitive drum 1 and the transfer roller 7 is not significant, and the transfer current is controlled to 2 to 3 μA.

  The paper is conveyed from the paper storage unit 10 to the transfer nip N in accordance with the image formation timing at the image forming unit P. Then, the control unit 90 determines whether or not the leading end of the sheet has reached the transfer nip N (step S3), and immediately after the leading end of the sheet has reached the transfer nip N (Y in step S3), the transfer current is controlled. Is switched from constant voltage control to constant current control using the constant current circuit 23 (step S4). The transfer current at this time is controlled to 9 μA, and the average value of the transfer voltage is about 1 kV.

  Note that whether or not the leading edge of the sheet has reached the transfer nip N is determined by the elapsed time after the sheet detection sensor 19a disposed upstream of the transfer nip N in the sheet conveyance direction detects the leading edge of the sheet. Is done. Thereafter, the image area of the paper reaches the transfer nip portion N, and the toner image formed on the photosensitive drum 1 is transferred to the image area (step S5).

  Next, the control unit 90 determines whether or not the image area of the paper has passed the transfer nip N (step S6), and immediately after the image area has passed the transfer nip N (Y in step S6), the transfer is performed. The current control is switched from constant current control to constant voltage control (step S7).

  FIG. 6 shows changes in the transfer current when the constant current control is switched to the constant voltage control at the timing immediately before the trailing edge of the sheet passes the transfer nip portion. In FIG. 6, the control value (target value) of the transfer current at the time of paper passing (at the time of constant current control) is 9 μA, and the control is switched to the constant voltage control immediately before passing through the rear end of the paper (indicated by the vertical broken line in the figure). The target value of the transfer voltage is set to 300 V, and the transfer current is controlled to 2 to 3 μA.

  Even when the control is switched as described above, since the load resistance changes abruptly when the trailing edge of the sheet passes through the transfer nip N, a certain change in the transfer current as shown in FIG. 6 occurs. To do. However, the change value of the current is gentle and does not have a sharp peak as in the graph of FIG.

  Then, it is determined whether or not printing of a predetermined number of sheets has been completed (step S8). If printing has been completed, the process is terminated. On the other hand, if printing continues, the process returns to step S2, and the same procedure is repeated thereafter (steps S2 to S8).

  According to the control shown in FIG. 5, the transfer current to the transfer roller 7 is controlled by constant current control when the toner image is transferred to the paper, so that the paper and the transfer roller 7 can be controlled according to changes in environmental conditions such as temperature and humidity. Even if the resistance fluctuates, a certain transfer performance can be ensured. In addition, by switching from constant current control to constant voltage control after the image area has passed through the transfer nip N and before the trailing edge of the paper has passed, a sudden resistance is generated when the trailing edge of the paper leaves the transfer nip. Even if the decrease occurs, a significant change in the output voltage can be suppressed, and an excessive current does not flow into the photosensitive drum 1.

  Accordingly, it is possible to effectively prevent the occurrence of an excessive current flow to the photosensitive drum 1 and the black streaks caused by the current flow when the rear end of the sheet passes while ensuring a predetermined transfer performance at the time of transfer. . In addition, the control voltage between the sheets (during constant voltage control) is set to a value lower than the average value of the output voltage (transfer voltage) during image transfer (during constant current control). The transfer voltage can be smoothly shifted to the control voltage between sheets by shortening the fall of the transfer voltage when switching to the control.

  In particular, when a transfer voltage having a reverse polarity (negative polarity) to that of toner consisting only of a DC voltage is applied to the photosensitive drum 1 having a single layer of an organic photosensitive layer (OPC) on its surface, charge history tends to remain. However, by performing the control as described above, generation of charge history is suppressed, and image defects can be prevented.

  In the control shown in FIG. 5, the control of the transfer current is switched from the constant voltage control to the constant current control immediately after the leading edge of the sheet reaches the transfer nip N. When the toner enters, the load resistance of the transfer nip portion N varies in the direction of increasing. As a result, even if the control when the leading edge of the sheet enters is constant current control, the current changes in a decreasing direction, so that an excessive current does not flow into the photosensitive drum 1, and the influence on the image is small. . Therefore, there is no problem even if the constant voltage control is switched to the constant current control immediately before the leading edge of the sheet enters the transfer nip portion N.

  By the way, even if the control as shown in FIG. 5 is performed and the occurrence of the charge history due to the excessive current flowing into the photosensitive drum 1 when the trailing edge of the sheet passes through the transfer nip portion N can be prevented, Depending on the response speed of the unit 20, the transfer image quality at the trailing edge of the sheet may be deteriorated.

  In general, when switching the control value of the high-voltage power supply, a time of several tens of ms (usually about 20 ms to 50 ms) is required for the voltage value (current value) to be stabilized at the control value. When this time is converted into distance, when the process speed (linear speed of the photosensitive drum 1) is 147 mm / s, about 3 mm to 7 mm is required, and the charge history at the rear end of the paper is completely erased. Before the trailing edge of the sheet enters the transfer nip portion N, it is necessary to switch from constant current control to constant voltage control. As a result, there is a possibility that the toner image is insufficiently attached (transfer defect) in the image area near the rear end of the paper, and the toner image is disturbed before being conveyed to the fixing unit 9 and image defect may occur. is there.

  On the other hand, if the control value (target value) when switching to constant voltage control is set to a low value (for example, 0 V) in advance, the fall time of the transfer voltage when switching from constant current control to constant voltage control can be shortened. it can. However, when the control voltage between the sheets (during constant voltage control) is too low, the transfer voltage between the sheets remains as a history on the photosensitive drum 1, which hinders uniform charging of the surface of the photosensitive drum 1. is there.

  Therefore, a two-stage control value (target value) is set as a control voltage between the sheets (during constant voltage control), and the control value immediately after switching from constant current control to constant voltage control is set to Vm1, constant current control. Control is performed so that 0 ≦ | Vm1 | ≦ | Vm2 | is satisfied when the control value after a predetermined time has elapsed after switching to constant voltage control from Vm2. Specifically, the control value Vm1 immediately after switching from the constant current control to the constant voltage control is set to 0V, and the control value Vm2 after 10 ms from the switching time is controlled to be 300V.

  FIG. 7 shows the relationship between the transfer voltage and the elapsed time in that case. According to the control of FIG. 7, the control value Vm1 (0 V) is 10 ms before the passage of the trailing edge of the sheet (indicated by the vertical broken line in the figure) (1.5 mm before the trailing edge of the sheet when the process speed is 147 mm / s). Switching to constant voltage control, the control value is changed to Vm2 (300 V) when the trailing edge of the sheet passes.

  Compared with FIG. 6, the transfer voltage fall time can be shortened, so that the occurrence of charge history due to excessive current flow into the photosensitive drum 1 when passing the trailing edge of the sheet is suppressed, Deterioration of transfer image quality in an image area near the trailing edge of the paper can also be reduced.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the constant current circuit 23 and the constant voltage circuit 25 shown in FIG. 3 are merely examples, and other circuit arrangements capable of constant current control and constant voltage control may be used.

  In the above embodiment, the image forming apparatus 100 according to the present invention has been described by taking the monochrome multifunction peripheral as shown in FIG. 1 as an example. However, the present invention is not limited to this. The present invention can also be applied to image forming apparatuses such as printers and color printers.

  The present invention can be used in a direct transfer type image forming apparatus using an organic photoreceptor. The use of the present invention ensures a constant transfer performance regardless of fluctuations in the resistance of the recording medium and transfer member due to fluctuations in environmental conditions such as temperature and humidity, and ensures that the trailing edge of the recording medium passes through the transfer nip. The image forming apparatus can effectively prevent the occurrence of image defects due to excessive current flow into the image carrier.

1 Photosensitive drum (image carrier)
2 Charging unit 3 Exposure unit 4 Developing device 7 Transfer roller (transfer member)
DESCRIPTION OF SYMBOLS 8 Cleaning apparatus 9 Fixing part 20 High voltage power supply part 21 DC power supply 23 Constant current circuit 25 Constant voltage circuit 90 Control part 100 Image forming apparatus P Image forming part

Claims (4)

An organic photosensitive layer is formed on the surface, and an image carrier that carries a toner image;
A transfer member disposed opposite to the image carrier;
A high-voltage power supply for applying a voltage having a polarity opposite to the charging polarity of the toner to the transfer member;
A control unit that controls application of a voltage from the high-voltage power supply unit to the transfer member;
An image forming apparatus for transferring a toner image carried on the image carrier onto a recording medium at a transfer nip formed by the image carrier and the transfer member.
The high-voltage power supply unit has a constant current circuit that applies a voltage to the transfer member by constant current control, and a constant voltage circuit that applies a voltage to the transfer member by constant voltage control,
The controller controls the voltage applied from the high-voltage power source to the transfer member by the constant current control while transferring the toner image to the recording medium at the transfer nip, and the image area of the recording medium is After passing through the transfer nip portion, before the rear end of the recording medium passes through the transfer nip portion, the control method of the voltage applied from the high-voltage power supply portion to the transfer member is changed from the constant current control to the constant voltage control. An image forming apparatus characterized by switching.   The image forming apparatus according to claim 1, wherein the control voltage during the constant voltage control is set to a value lower than an average value of output voltages during the constant current control. A two-stage control value is set as the control voltage during the constant voltage control, and the control value immediately after switching from the constant current control to the constant voltage control is switched to Vm1, and the constant current control is switched to the constant voltage control. 3. The image forming apparatus according to claim 2, wherein the following expression (1) is satisfied when a control value after a predetermined time has elapsed is Vm <b> 2:
0 ≦ | Vm1 | ≦ | Vm2 | (1)   The image forming apparatus according to claim 1, wherein the high-voltage power supply unit applies only a DC voltage to the transfer member.

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