JP6565790B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a tandem color image forming apparatus in which a plurality of image forming units including a photoconductor are arranged in parallel along an intermediate transfer member.

  Conventionally, a tandem that forms a color image by primary transfer of toner images of different colors onto an endless intermediate transfer belt by a plurality of image forming units and sequentially superimposing them, followed by secondary transfer and fixing on a recording medium A color image forming apparatus of the type is known.

  In a tandem type color image forming apparatus, a voltage (transfer reverse bias) having the same polarity as the toner may be applied in a primary transfer unit that superimposes toner images of respective colors formed on a photosensitive drum on an intermediate transfer belt. is there. For example, when the toner is periodically supplied from the developing device to the photosensitive drum in order to maintain the cleaning performance of the surface of the photosensitive drum by the cleaning blade, the toner on the photosensitive drum is restricted from moving to the intermediate transfer belt. Therefore, a reverse transfer bias is applied.

  In the above case, if the transfer reverse bias is smaller than an appropriate value, the toner supplied onto the photosensitive drum moves to the intermediate transfer belt, and the amount of toner supplied to the photosensitive drum decreases. As a result, the cleaning performance by the cleaning blade is deteriorated and the toner external additive slips out. On the other hand, when the transfer reverse bias is larger than an appropriate value, the toner on the photosensitive drum is discharged and the toner is reversely charged. Therefore, the supplied toner is easily moved to the intermediate transfer belt, and the amount of toner supply is reduced. In addition, the surface potential of the photosensitive drum increases due to an excessive transfer reverse bias, and the image density decreases when the next toner image is formed.

  The above-described primary transfer electric field control methods include constant current control and constant voltage control. Constant current control is almost unaffected by the impedance change between the primary transfer roller and the photosensitive drum, which varies depending on the durability and environment, and can stabilize the primary transfer electric field against the gap between the intermediate transfer belt and the photosensitive drum. On the other hand, the transfer electric field changes depending on the print pattern. Specifically, when forming a small print pattern (Short Solid Image) in the longitudinal direction, the resistance of the solid part increases and the transfer current leaks outside the solid part, so it is necessary to increase the current. There is. As a result, when a large print pattern (Long Solid Image) is formed in the longitudinal direction, an excessive transfer current (electric field) is generated, and the toner charge amount on the intermediate transfer belt is increased, so that an image is transferred at the secondary transfer portion. (Transfer image quality) Deterioration occurs.

  On the other hand, the constant voltage control has the advantage that the transfer electric field does not change depending on the print pattern, but is affected by the impedance change between the primary transfer roller and the photosensitive drum. For this reason, when performing constant voltage control of the primary transfer electric field, it is necessary to apply a voltage in accordance with the resistance of a member such as a primary transfer roller or an intermediate transfer belt.

  Therefore, in Patent Document 1, a circuit for measuring a current value is provided in a high-voltage power supply that applies a voltage to the transfer roller, and the current value when the voltage is applied is detected to determine the resistance (impedance) of the transfer roller. The correct voltage is determined. Further, since the resistance values of the transfer roller and the intermediate transfer belt are highly temperature dependent, in Patent Document 2, the resistance of the secondary transfer member is detected based on the detection result of the temperature detection means, and the secondary transfer voltage is corrected and controlled. A method is disclosed.

JP-A-5-313522 JP 2004-62086 A

  However, in a tandem type color image forming apparatus having four color image forming units, it takes time to detect the impedance between the primary transfer roller disposed in each image forming unit and the photosensitive drum. There was a problem that the formation efficiency was lowered. In addition, if a circuit for detecting impedance is provided in each image forming unit, there is a problem that the cost is increased.

  SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of applying an appropriate primary transfer reverse voltage to each primary transfer member with a simple configuration when the primary transfer reverse electric field is controlled at a constant voltage. To do.

  In order to achieve the above object, a first configuration of the present invention is an image including a plurality of image carriers, an intermediate transfer member, a plurality of primary transfer members, a plurality of voltage application devices, and a control unit. Forming device. On the intermediate transfer member, toner images of a plurality of colors formed on the surfaces of a plurality of image carriers are sequentially stacked. The primary transfer member is pressed against each image carrier with the intermediate transfer member interposed therebetween, and primarily transfers the toner images of a plurality of colors developed on each image carrier onto the intermediate transfer member. The voltage application device is connected to each of a plurality of primary transfer members, and applies a primary transfer voltage having a polarity opposite to that of the toner or a primary transfer reverse voltage having the same polarity as that of the toner to each of the primary transfer members. A predetermined primary transfer electric field or primary transfer reverse electric field is formed between the members. The control unit controls formation of a primary transfer electric field or a primary transfer reverse electric field by a plurality of voltage application devices. When applying the primary transfer voltage, the control unit performs constant voltage control of at least one of the plurality of voltage application devices, and when applying the primary transfer reverse voltage, performs constant voltage control of all of the plurality of voltage application devices. . The image forming apparatus detects a current flowing between any one of the primary transfer members and an image carrier facing the primary transfer member by a voltage application device that is controlled at a constant voltage when a primary transfer voltage is applied. It has a current detector. The control unit detects an impedance between the primary transfer member and the image carrier using the current value detected by the current detection unit and the voltage value applied to the primary transfer member where the current value is detected. The primary transfer reverse voltage applied to each primary transfer member from all voltage application devices is corrected based on the detected impedance.

  According to the first configuration of the present invention, a current flowing between any one of the primary transfer members and the opposite image carrier is detected by a voltage application device controlled at a constant voltage when the primary transfer voltage is applied. Then, the impedance between the primary transfer member and the image carrier is detected, and the primary transfer reverse voltage applied to each primary transfer member from all voltage application devices is corrected based on the detected impedance. Therefore, when determining the primary transfer reverse voltage to be applied to a plurality of primary transfer members, only the impedance between any one primary transfer member and the image carrier need be detected. Accordingly, the impedance detection time can be shortened, the printing waiting time can be shortened, and the image forming efficiency can be improved. Further, since it is not necessary to provide a current detection unit necessary for impedance detection in the voltage application device, it contributes to simplification of the control path of the image forming apparatus and cost reduction.

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. 1 is a partially enlarged view around the intermediate transfer belt 8 in FIG. 1 is a block diagram illustrating an example of a control path of an image forming apparatus 100 according to the present invention. A graph showing the transfer rate of toner from the photosensitive drums 1a to 1d to the intermediate transfer belt 8 when the primary transfer reverse voltage applied to the primary transfer rollers 6a to 6d is changed. The graph which shows transition of the surface potential of the photoconductive drums 1a-1d when a primary transfer reverse voltage is applied between the papers during continuous printing. A graph showing voltage-current characteristics when impedance detection is performed between the primary transfer roller 6d located on the most downstream side in the moving direction of the intermediate transfer belt 8 and the photosensitive drum 1d.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic view showing a configuration of a tandem color image forming apparatus of the present invention, and FIG. 2 is an enlarged view of the vicinity of an intermediate transfer belt 8 in FIG. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and yellow, cyan, magenta, and the like are respectively performed by charging, exposure, development, and transfer processes. And black (hereinafter also referred to as Y, C, M, and BK) images are sequentially formed.

  These image forming portions Pa to Pd are respectively provided with photosensitive drums 1a to 1d for carrying visible images (toner images) of the respective colors, and further an intermediate transfer belt that rotates clockwise in FIG. 8 is provided adjacent to each of the image forming portions Pa to Pd. In this embodiment, the photosensitive drums 1a to 1d are formed by forming a photosensitive layer made of amorphous silicon (a-Si) on the outer peripheral surface of a drum base tube having a diameter of 40 mm.

  The paper P onto which the toner image is transferred is accommodated in a paper cassette 16 below the image forming apparatus 100 and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b.

  Next, the image forming units Pa to Pd will be described. There are charging devices 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. An exposure unit 4 that forms an electrostatic latent image by exposing the toner, and developing devices 3a, 3b, 3c, and 3d that develop the electrostatic latent images formed on the photosensitive drums 1a to 1d to form toner images, Cleaning devices 5a, 5b, 5c and 5d for removing the developer (toner) remaining on the photosensitive drums 1a to 1d are provided. Further, a belt cleaning device 19 having a belt cleaning brush 19a facing the driven roller 10 with the intermediate transfer belt 8 interposed therebetween is disposed on the upstream side in the rotation direction of the intermediate transfer belt 8 with respect to the photosensitive drum 1a.

  The intermediate transfer belt 8 is stretched around a driven roller 10, a driving roller 11, and a tension roller 20. The intermediate transfer belt 8 has a three-layer structure including a base layer, an elastic layer, and a coat layer. The base layer is in contact with the primary transfer rollers 6a to 6d, and the coat layer is in contact with the photosensitive drums 1a to 1d.

The base material layer becomes a basic material constituting the intermediate transfer belt 8 and imparts a predetermined rigidity, withstands the processing conditions when laminating the coat layer. Further, when the intermediate transfer belt 8 is manufactured, It is preferable that it is excellent in heat resistance, slipperiness, and other physical properties. As a material of such a base material layer, for example, a resin such as PI (polyimide), PAI (polyacrylimide), PVDF (polyvinylidene fluoride) which is difficult to expand and contract is used. Also, carbon black is dispersed as a conductive agent in the resin, and the volume resistivity when 500 V is applied is adjusted to about 10 ⁹ to 10 ¹² Ω · cm.

  When the toner image on the surface of the intermediate transfer belt 8 is secondarily transferred to the paper P, the elastic layer uniformly applies the pressure contact force of the intermediate transfer belt 8 and the secondary transfer roller 9 to the entire paper P, and the superimposed toner image. This is a layer for preventing the occurrence of a void image. As the material of the elastic layer, an elastic material such as CR (chloroprene rubber) or thermoplastic polyurethane elastomer (TPU) is used. The coat layer protects the elastic layer and imparts releasability to the intermediate transfer belt 8. As the material of the coating layer, a fluororesin such as PVDF (polyvinylidene fluoride) or PTFE (polytetrafluoroethylene) is used.

  In this embodiment, an ion conductive laminated elastic belt having a volume resistance value of 10.5 log Ω · cm in which an elastic layer formed of TPU is laminated on a PVDF base material layer and PTFE is coated as a coating layer is used. The linear speed of the intermediate transfer belt 8 is 350 mm / sec.

  The primary transfer rollers 6 a to 6 d are in contact with the photosensitive drums 1 a to 1 d with a predetermined pressure across the intermediate transfer belt 8, and the toner images formed on the photosensitive drums 1 a to 1 d are transferred onto the intermediate transfer belt 8. Primary transfer. First DC power supplies 44a to 44d are connected to the primary transfer rollers 6a to 6d, respectively, and the first DC power supply 44d detects a primary transfer current flowing between the primary transfer roller 6d and the photosensitive drum 1d. A current detection unit 50 is connected. In the present embodiment, as the primary transfer rollers 6a to 6d, electronically conductive rollers in which a roller body made of EPDM foam rubber having a resistance value of 6.8 logΩ is used on the outer peripheral surface of a metal shaft having a diameter of 8 mm are used. The secondary transfer roller 9 is connected to a second DC power supply 45 for applying a secondary transfer voltage.

  When image data is input from a host device such as a personal computer (hereinafter referred to as a personal computer) connected to the image forming apparatus 100, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging devices 2a to 2d. Then, the exposure unit 4 irradiates the surfaces of the photosensitive drums 1a to 1d with light, and electrostatic latent images corresponding to the image signals are formed on the photosensitive drums 1a to 1d. Next, toner of each color of yellow, magenta, cyan, and black is supplied from the developing devices 3a to 3d onto the photosensitive drums 1a to 1d and adheres to the electrostatic latent image, thereby forming by exposure from the exposure unit 4. A toner image corresponding to the electrostatic latent image thus formed is formed.

  The primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d are arranged by primary transfer rollers 6a to 6d arranged so as to be in pressure contact with the photosensitive drums 1a to 1d with the intermediate transfer belt 8 interposed therebetween. An electric field is applied at a predetermined transfer voltage during the interval (primary transfer portion), and the toner images of yellow, magenta, cyan and black on the photosensitive drums 1 a to 1 d are primarily transferred onto the intermediate transfer belt 8. The toner remaining on the surfaces of the photosensitive drums 1a to 1d after the primary transfer is removed by the cleaning devices 5a to 5d.

  When the intermediate transfer belt 8 starts to rotate counterclockwise in FIG. 1 as the drive roller 11 is rotated by a belt drive motor (not shown), the sheet P is transferred from the registration roller 12b to the intermediate transfer belt 8 at a predetermined timing. The toner image is conveyed to a nip portion (secondary transfer nip portion) between the secondary transfer roller 9 and the intermediate transfer belt 8 provided close to each other, and the toner image is secondarily transferred from the intermediate transfer belt 8 onto the paper P at the nip portion. . The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined color image is formed. It is formed. The sheet P on which the color image is formed is discharged to the discharge tray 17 by the discharge roller pair 15 as it is (or after being distributed to the reverse conveyance path 18 by the branching unit 14 and the image is formed on both sides).

  Next, the control path of the image forming apparatus 100 of the present invention will be described. FIG. 3 is a block diagram showing an example of a control path used in the image forming apparatus 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 control signals to and receive input signals from the operation unit 60, such as a temporary storage unit 94 that stores image data and the like, a counter 95, and each device in the image forming apparatus 100. 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. As each part and device controlled by the control unit 90, for example, the image forming units Pa to Pd, the exposure unit 4, the primary transfer rollers 6a to 6d, the intermediate transfer belt 8, the secondary transfer roller 9, the image input unit 40, the bias Examples include the control circuit 41, the operation unit 60, and the like.

  The image input unit 40 is a receiving unit that receives image data transmitted from the personal computer or the like to the image forming apparatus 100. The image signal input from the image input unit 40 is converted into a digital signal and then sent to the temporary storage unit 94.

  The bias control circuit 41 is connected to the charging bias power source 42, the developing bias power source 43, the first DC power sources 44 a to 44 b, the second DC power source 45, and the belt cleaning bias power source 46, and these signals are output by an output signal from the control unit 90. Each power source is operated, and according to a control signal from the bias control circuit 41, each of these power sources is charged by the charging bias power source 42 to the charging roller 21 in the chargers 2a to 2d and the developing bias power source 43 by the developing device 3a. -3d, magnetic roller 27 and developing roller 29, first DC power supplies 44a-44d to primary transfer rollers 6a-6d, second DC power supply 45 to secondary transfer roller 9, and belt cleaning bias power supply 46 to belt cleaning. A predetermined voltage is applied to each brush 19a.

  The in-machine temperature sensor 51 detects the temperature inside the image forming apparatus 100, particularly the temperature around the primary transfer rollers 6a to 6d, and is arranged in the vicinity of the image forming portions Pa to Pd.

  The operation unit 60 is provided with a liquid crystal display unit 61 and LEDs 62 indicating various states. The user operates the stop / clear button of the operation unit 60 to stop image formation, and operates the reset button to operate the image. Various settings of the forming apparatus 100 are set to a default state. The liquid crystal display unit 61 displays the state of the image forming apparatus 100, and displays the image forming status and the number of copies to be printed. Various settings of the image forming apparatus 100 are performed from a printer driver of a personal computer.

  As described above, in order to maintain the cleaning performance of the photosensitive drums 1a to 1d by the cleaning blade 31, the toner is periodically supplied from the developing devices 3a to 3d onto the photosensitive drums 1a to 1d. At this time, in order to restrict the movement of the toner from the photosensitive drums 1a to 1d to the intermediate transfer belt 8, a primary transfer reverse voltage having the same polarity as the toner (here, positive polarity) is applied to the primary transfer rollers 6a to 6d. The Further, the primary transfer reverse voltage is applied to the primary transfer rollers 6a to 6d so that the residual toner on the photosensitive drums 1a to 1d does not move to the intermediate transfer belt 8 side between the sheets during continuous printing.

  FIG. 4 is a graph showing the transfer rate of toner from the photosensitive drums 1a to 1d to the intermediate transfer belt 8 when the primary transfer reverse voltage applied to the primary transfer rollers 6a to 6d is changed. The primary transfer reverse voltage is constant voltage control so as not to depend on the print patterns formed on the photosensitive drums 1a to 1d, and the transfer performance depends on the value of the current flowing in the primary transfer portion. The current value (primary transfer reverse current) that flows when the primary transfer reverse voltage is applied is taken instead of the voltage value.

  As shown in FIG. 4, the transfer rate without applying the primary transfer reverse voltage is 10%, but when the primary transfer reverse voltage is applied such that a current of 1 μA flows, the transfer rate is reduced to 2%. . The transfer rate was maintained at 2% until the current value reached 4 μA, and thereafter the transfer rate gradually increased as the current value increased. That is, it is optimal that the current value when the primary transfer reverse voltage is applied is slightly flowing. If the current value is too large, the toner on the photosensitive drums 1a to 1d is discharged to reversely charge the toner. It can be seen that the transfer rate increases.

  FIG. 5 is a graph showing changes in the surface potential of the photosensitive drums 1a to 1d when a primary transfer reverse voltage is applied between sheets during continuous printing. In FIG. 5, when a halftone image having an image density of 25% is printed at a resolution of 300 dpi and an appropriate primary transfer reverse voltage with a current value of 4 μA is applied as shown in FIG. 4 (solid line in FIG. 5), The transition of the surface potentials of the photosensitive drums 1a to 1d was compared with the case where an excessive primary transfer reverse voltage with a current value of 10 μA was applied (broken line in FIG. 5).

  As shown in FIG. 5, when an appropriate primary transfer reverse voltage was applied, the surface potential (post-exposure potential) was stable throughout the entire image, and the image density was uniform. On the other hand, when an excessive primary transfer reverse voltage is applied, the surface potential of the leading edge of the image (one revolution of the photosensitive drum) is higher than that of the other portions, and the density of the leading edge of the image is lighter. It was.

  Here, in order to apply an appropriate primary transfer reverse voltage to the primary transfer rollers 6a to 6d, the impedance between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d is detected, and the detected impedance is set. It is necessary to determine the primary transfer reverse voltage to be applied based on this. The constant voltage control has a drawback in that it takes time to detect the impedance.

  In the image forming apparatus 100 of the present invention, when the primary transfer voltage is applied to the four primary transfer rollers 6 b to 6 d using the first DC power supplies 44 a to 44 d, the most downstream side with respect to the traveling direction of the intermediate transfer belt 8. Only the first DC power supply 44d for applying the primary transfer electric field to the primary transfer roller 6d is set to constant voltage control, and the other first DC power supplies 44a to 44c are set to constant current control. Then, a current flowing between the primary transfer roller 6d and the photosensitive drum 1a when a voltage is applied to the primary transfer roller 6d is detected by the current detection unit 50, and the primary transfer is performed based on the detected current value and the applied voltage value. The impedance between the roller 6d and the photosensitive drum 1a is calculated. The controller 90 corrects the primary transfer voltage applied to the primary transfer roller 6d based on the calculated impedance.

  Further, when the primary transfer reverse voltage is applied to the primary transfer rollers 6a to 6d, all of the first DC power supplies 44a to 44d are set to constant voltage control. The control unit 90 corrects the primary transfer reverse voltage applied from the first DC power supplies 44a to 44c to the primary transfer rollers 6a to 6c based on the impedance between the primary transfer roller 6d and the photosensitive drum 1a.

  FIG. 6 is a graph showing voltage-current characteristics when impedance detection is performed between the primary transfer roller 6d located on the most downstream side in the moving direction of the intermediate transfer belt 8 and the photosensitive drum 1d. When the primary transfer reverse voltage at which the current value is 3 μA is set as the target value, the target value is −614 V from the approximate expression (y = 39.9x−734.0) obtained from the voltage-current characteristics shown in FIG. Become.

  Further, it is set to be larger than the resistance values of the primary transfer rollers 6a to 6d. As described above, the resistance of the intermediate transfer belt 8 changes depending on the temperature because it is ionic conductive, but the resistance change due to temperature is small because the primary transfer rollers 6a to 6d are electronically conductive. For this reason, the impedance in the primary transfer portion depends on the resistance value of the intermediate transfer belt 8. Therefore, the target value (−614V) of the primary transfer reverse voltage obtained using the impedance between the primary transfer roller 6d and the photosensitive drum 1d is an appropriate value for the primary transfer rollers 6a to 6c.

  In the image forming apparatus 100 of the present invention, when determining the primary transfer reverse voltage to be applied to the primary transfer rollers 6a to 6d, it is only necessary to detect the impedance of the primary transfer roller 6d. Accordingly, the impedance detection time can be shortened, the printing waiting time can be shortened, and the image forming efficiency can be improved. In addition, since it is not necessary to provide the current detection unit 50 necessary for impedance detection in the first DC power supplies 44a to 44c, it contributes to simplification of the control path and cost reduction of the image forming apparatus 100.

  Further, the resistance of the intermediate transfer belt 8 changes depending on the temperature, and the resistance of the primary transfer roller 6d changes depending on the degree of deterioration of the roller although the change due to temperature is small. Therefore, when the temperature detected by the in-machine temperature sensor 51 has changed by a predetermined temperature from the time of the previous primary transfer voltage correction, or the number of accumulated prints from the time of the previous primary transfer voltage correction counted by the counter 95 is predetermined. When the number of sheets has been reached (when the cumulative driving time of the image forming apparatus 100 has reached a predetermined time), the impedance between the primary transfer roller 6d and the photosensitive drum 1d is detected, and the primary transfer voltage and the primary transfer reverse voltage are detected. It is preferable to perform correction.

  Accordingly, the primary transfer reverse voltage does not become excessive in each primary transfer portion, and an appropriate current value can be obtained. Accordingly, it is possible to maintain the cleaning performance by the cleaning blade 31 by suppressing the movement of the toner supplied onto the photosensitive drums 1 a to 1 d to the intermediate transfer belt 8. In addition, a decrease in image density due to a rise in the post-exposure potential at the leading edge of the image can be effectively suppressed.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above embodiment, only the first DC power supply 44d that applies the primary transfer electric field to the primary transfer roller 6d on the most downstream side with respect to the traveling direction of the intermediate transfer belt 8 is controlled by constant voltage, but the first DC power supply 44d Instead, any one of the first DC power supplies 44a to 44c may be set to constant voltage control, and two or more of the first DC power supplies 44a to 44c may be set to constant voltage control.

  By using the first DC power supply 44d as a constant voltage control as in the above embodiment, the primary transfer current does not become excessive at the primary transfer portion of black to which the upstream color (yellow, cyan, magenta) toner is transferred, An appropriate primary transfer current value can be obtained. As a result, an increase in the toner charge amount on the intermediate transfer belt 8 before the secondary transfer can be suppressed, high secondary transfer efficiency can be ensured, and the density unevenness of the image transferred onto the paper P in the secondary transfer portion can be effectively obtained. Can be suppressed. Accordingly, it is preferable that the first DC power supply 44d that applies the primary transfer electric field to the primary transfer roller 6d on the most downstream side with respect to the traveling direction of the intermediate transfer belt 8 is controlled at a constant voltage.

  The set value of the primary transfer reverse voltage described above is merely an example, and may be set as appropriate according to the specifications of the image forming apparatus, the usage environment, and the like.

  The present invention is applicable to a tandem color image forming apparatus using an intermediate transfer system. By utilizing the present invention, it is possible to stably secure a primary transfer current required for transferring a toner image to an intermediate transfer member, and to provide a primary transfer on the intermediate transfer member on the upstream side in the moving direction of the intermediate transfer member. The image forming apparatus can suppress excessive charging of the toner when the transferred toner layer passes through the downstream primary transfer electric field.

1a to 1d Photosensitive drum (image carrier)
2a to 2d Charging device 3a to 3d Developing device 4 Exposure unit 5a to 5d Cleaning device 6a to 6d Primary transfer roller (primary transfer member)
8 Intermediate transfer belt (intermediate transfer member)
9 Secondary transfer roller 19 Belt cleaning device 44a to 44d First DC power supply (voltage application device)
45 Second DC power supply 50 Current detection unit 51 In-machine temperature sensor 90 Control unit 100 Image forming apparatus Pa to Pd Image forming unit

Claims (6)

A plurality of image carriers;
An intermediate transfer body in which a plurality of color toner images formed on the surface of the plurality of image carriers are sequentially laminated;
A plurality of primary transfer members that are pressed against the respective image carriers with the intermediate transfer member interposed therebetween, and that primarily transfer a plurality of color toner images developed on the respective image carriers onto the intermediate transfer member;
The intermediate transfer member and each primary transfer member connected to each of the plurality of primary transfer members and applying a primary transfer voltage having a polarity opposite to that of toner or a primary transfer reverse voltage having the same polarity as that of toner to each of the primary transfer members. A plurality of voltage application devices that form a predetermined primary transfer electric field or primary transfer reverse electric field between
A control unit that controls formation of the primary transfer electric field or the primary transfer reverse electric field by the plurality of voltage applying devices;
In an image forming apparatus comprising:
When applying the primary transfer voltage, the control unit performs constant voltage control of at least one of the plurality of voltage application devices, and when applying the primary transfer reverse voltage, all of the plurality of voltage application devices. The constant voltage control,
A current for detecting a current flowing between any one of the primary transfer members and the image carrier facing the primary transfer member by the voltage application device controlled at a constant voltage when the primary transfer voltage is applied. Having a detector,
The control unit uses the current value detected by the current detection unit and the voltage value applied to the primary transfer member where the current value is detected to determine whether the primary transfer member and the image carrier are used. And detecting the primary transfer reverse voltage applied to each primary transfer member from all the voltage application devices based on the detected impedance, and the toner from the image carrier to the intermediate transfer member. The image forming apparatus is corrected to a voltage value at which a current flows so that the transfer rate of the toner is maintained at 2% .   The controller, when applying the primary transfer voltage, includes only the voltage application device connected to the primary transfer member located on the most downstream side in the moving direction of the intermediate transfer body among the plurality of voltage application devices. The image forming apparatus according to claim 1, wherein constant voltage control is performed. It has a temperature detection unit that detects the temperature inside the image forming apparatus,
The said control part performs correction | amendment of the said primary transfer reverse voltage, when the temperature detected by the said temperature detection part changes more than predetermined temperature from the correction | amendment of the said primary transfer reverse voltage last time. The image forming apparatus according to claim 2.   The control unit executes the correction of the primary transfer reverse voltage when the cumulative driving time of the intermediate transfer body after a previous correction of the primary transfer reverse voltage becomes a predetermined time or more. Alternatively, the image forming apparatus according to claim 2.   The image forming apparatus according to claim 1, wherein a resistance value of the intermediate transfer member is larger than a resistance value of the primary transfer member.   The image forming apparatus according to claim 5, wherein the intermediate transfer member is ion conductive, and the primary transfer member is electronic conductive.

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