JP2017211505A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that, when performing constant voltage control of a primary transfer electric field, can apply an appropriate primary transfer voltage to primary transfer members with a simple configuration.SOLUTION: An image forming apparatus comprises: a plurality of image carriers; an intermediate transfer body; a plurality of primary transfer members; a plurality of voltage application devices; and a control part. The intermediate transfer body has a plurality of colors of toner images formed on the surfaces of the plurality of image carriers sequentially laminated thereon. The primary transfer members are brought into pressure contact with the image carriers with the intermediate transfer body therebetween, and primarily transfer the plurality of colors of toner images developed on the image carriers onto the intermediate transfer body. The plurality of voltage application devices apply a primary transfer voltage having a polarity opposite to that of toner respectively to the plurality of primary transfer members to form primary transfer electric fields between the intermediate transfer body and primary transfer members. The control part performs constant voltage control of the voltage application device, of the plurality of voltage application devices, connected to the primary transfer member located on the most downstream side in the direction of movement of the intermediate transfer body, and performs constant current control of the other voltage application devices.SELECTED DRAWING: Figure 3

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 the tandem type color image forming apparatus, when the toner images of the respective colors formed on the photosensitive drum are superimposed on the intermediate transfer belt, the primary image is first formed on the intermediate transfer belt in the image forming unit on the upstream side in the belt traveling direction. When the transferred toner image passes through the primary transfer electric field in the downstream image forming unit, discharge and charge injection to the toner image occur, and the charge amount of the toner increases. Toner having a high charge amount has a strong electrostatic adhesion to the intermediate transfer belt. In such a case, if a strong secondary transfer electric field is generated in order to ensure the secondary transfer efficiency of the toner image, the non-charged toner that is primarily transferred in the image forming section upstream in the belt traveling direction is discharged. As a result, reverse charging is facilitated, and the secondary transfer efficiency is lowered. In addition, a strong secondary transfer electric field tends to cause a discharge phenomenon between the recording medium and the intermediate transfer belt, and causes a reduction in quality of the secondary transfer image.

  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. There is a problem that (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.

  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 voltage to each primary transfer member with a simple configuration when the primary transfer electric field is controlled at a constant voltage.

  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 plurality of primary transfer members are in pressure contact with the respective image carriers with the intermediate transfer member interposed therebetween, and primarily transfer a plurality of color toner images developed on the respective image carriers onto the intermediate transfer member. The plurality of voltage application devices are connected to each of the plurality of primary transfer members, apply a primary transfer voltage having a polarity opposite to that of the toner to each primary transfer member, and provide a predetermined primary between the intermediate transfer member and each primary transfer member. A transfer electric field is formed. The control unit controls formation of the primary transfer electric field by the plurality of voltage application devices. The control unit performs constant voltage control on 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, and performs constant current control on the other voltage application devices. .

  According to the first configuration of the present invention, only the voltage application device connected to the primary transfer member on the most downstream side in the moving direction of the intermediate transfer member is set to constant voltage control, and the other voltage application devices are set to constant current control. The primary transfer current does not become excessive in the primary transfer portion on the most downstream side where the toner has already been transferred onto the intermediate transfer body in the primary transfer portion on the upstream side, and an appropriate current value can be obtained. Therefore, an increase in the toner charge amount on the intermediate transfer body before the secondary transfer can be suppressed, and high secondary transfer efficiency can be secured. In addition, it is possible to effectively suppress the density unevenness of the image transferred onto the recording medium in the secondary transfer portion.

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. Graph showing transition of toner charge amount in each primary transfer part Graph showing the relationship between toner charge and motor The graph which shows transition of the primary transfer current at the time of applying a fixed voltage to the primary transfer rollers 6a-6d and performing double-sided continuous printing

  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 are used in which a roller body made of EPDM foam rubber having a volume resistance value of 6.8 logΩ is formed on the outer peripheral surface of a metal shaft having a diameter of 8 mm. 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 connected to the charging roller 21 in the chargers 2a to 2d, and the developing bias power source 43 is connected to 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.

  By the way, when the yellow toner image primarily transferred to the intermediate transfer belt 8 in the image forming portion Pa passes through the primary transfer electric field in the image forming portions Pb to Pd on the downstream side in the traveling direction of the intermediate transfer belt 8, the yellow toner image is transferred to the toner image. Discharge and charge injection occur, and the charged state of the toner changes. Further, when the magenta toner image primarily transferred in the image forming portion Pb passes the primary transfer electric field in the downstream image forming portions Pc and Pd, or the cyan toner image primarily transferred in the image forming portion Pc is downstream. A similar phenomenon also occurs when passing through the primary transfer electric field in the image forming unit Pd on the side. Due to this change in the charged state of the toner, a highly charged toner that is difficult to be secondarily transferred or a reverse polarity charged toner is generated, and a factor that causes a defective transfer image when the toner is secondarily transferred onto the paper P by the secondary transfer roller 9 It has become.

FIG. 4 is a graph showing the transition of the toner charge amount in each primary transfer portion, and the toner charge amount on the intermediate transfer belt 8 when printing is performed in a total of 200% Y100% and C100% in the entire longitudinal direction of the paper. It shows the transition. In FIG. 4, toner having an initial charge amount of 30 μC / g is used, and the toner layer thickness per unit area on the photosensitive drums 1a to 1d is set to 0.5 mg / cm ² . In FIG. 4, the yellow primary transfer position (hereinafter referred to as Y position) is a Y color transition, and the cyan primary transfer position (hereinafter referred to as C position) and thereafter are Y + C mixed color charge amount transitions. In the image forming apparatus 100 of the present embodiment, the maximum color mixture density from Y to BK is designed to be 260%.

  Therefore, the setting of the primary transfer current (hereinafter also simply referred to as transfer current) that flows between the primary transfer roller 6a and the photosensitive drum 1a at the Y position is a transfer current that satisfies 100% transfer of the Y color, and the C position. In this case, the transfer current is necessary and sufficient for C100% transfer on Y100%, and the transfer current setting at the magenta primary transfer position (hereinafter referred to as M position) is M60% transfer on 200% Y + C color mixture. The transfer current is necessary and sufficient for M100% transfer on Y + C mixed color 160%.

  In the case of constant current control (data series in ◇ in FIG. 4), the transfer current setting at the final black primary transfer position (hereinafter referred to as the BK position) is set to transfer BK 60% over Y to M color mixture 200%. The necessary and sufficient transfer current is set, and the necessary and sufficient transfer current is set to transfer 100% of BK onto 160% of Y to M mixed colors. In the case of FIG. 4, transfer currents (transfer electric fields) at the Y position, C position, M position, and BK position are −25 μA, −27 μA, −29 μA, and −32 μA, respectively. As a result, the toner charge amounts at the Y position, the C position, the M position, and the BK position are 33 μC / g, 36 μC / g, 42 μC / g, and 49 μC / g, respectively.

  On the other hand, when only the BK position is set to the constant voltage control (data series of □ in FIG. 4), the conditions are the same as above. Transfer currents (transfer electric fields) at the Y position, the C position, and the M position are −25 μA, −27 μA, and −29 μA, respectively, and the transfer voltage at the BK position is −1.8 kV. As a result, the toner charge amounts at the Y position, the C position, the M position, and the BK position are 33 μC / g, 36 μC / g, 42 μC / g, and 43 μC / g, respectively.

  As shown in FIG. 4, when only the BK position is controlled at a constant voltage, the toner charge amount becomes small after a 200% Y + C mixed color toner image has passed the BK position. This is because in the constant current control, upstream color (Y, C, M) toner is transferred at the primary transfer position (Y position, C position, M position) upstream of the intermediate transfer belt 8 in the traveling direction from the BK position. Regardless of whether or not the image is transferred, a transfer electric field of −32 μA is always formed. On the other hand, in the constant voltage control, when a toner image of 200% Y + C color mixture exists on the intermediate transfer belt 8, the transfer voltage is set to a transfer voltage sufficient to transfer 60% at the BK position. The transfer electric field is about −22 μA.

  As described above, when the BK position is under constant current control, when the upstream color toner has already been transferred onto the intermediate transfer belt 8, the transfer electric field becomes excessive, and the intermediate transfer belt is discharged by the discharge at the primary transfer portion. The toner charge amount on 8 is increased.

FIG. 5 is a graph showing the relationship between the toner charge amount and mottle. “Mottle” in the present specification means variation in image density (spotness) defined as an image evaluation standard in ISO (International Organization for Standardization), and specifically 12.7 mm. This is expressed by the standard deviation of the density of each region when an image of × 12.7 mm is divided into 10 × 10 = 100 regions. That is, the density unevenness is smaller as the value of the motor is lower. In FIG. 5, similarly to FIG. 4, toner having an initial charge amount of 30 μC / g is used, and the toner layer thickness per unit area on the photosensitive drums 1a to 1d is set to 0.5 mg / cm ² . When calculating the motor, the secondary transfer current applied to the secondary transfer roller 9 is optimized to a current value at which the motor becomes the lowest according to the toner charge amount on the intermediate transfer belt 8.

  FIG. 5 shows the change in mottle when the toner charge amount is changed for green (Y + C mixed color 200%), blue (C + M mixed color 200%), and red (Y + M mixed color 200%). ), Blue (square data series), and red (triangle data series), the toner charge amount is 46 to 49 μC / g, whereas the toner charge amount is 42 μC / g and the motor is 5.5 to 6. In g, the mottle is 7, and the value of the mottle increases as the toner charge amount increases. That is, it can be seen that the density unevenness increases as the toner charge amount increases.

  In the image forming apparatus 100 of the present invention, among the first DC power supplies 44 a to 44 d that apply primary transfer electric fields to the four primary transfer rollers 6 a to 6 d, the primary side on the most downstream side with respect to the traveling direction of the intermediate transfer belt 8. The first DC power supply 44d that applies a primary transfer electric field to the transfer roller 6d is set to constant voltage control. Thus, the primary transfer current does not become excessive at the BK position where the upstream color (Y, C, M) toner is transferred, and an appropriate current value can be obtained. Therefore, an increase in the toner charge amount on the intermediate transfer belt 8 before the secondary transfer can be suppressed, and high secondary transfer efficiency can be secured. Further, it is possible to effectively suppress the density unevenness of the image transferred onto the paper P in the secondary transfer portion.

  FIG. 6 is a graph showing the transition of the primary transfer current when continuous printing is performed by applying a constant voltage (−1.6 kV) to the primary transfer rollers 6 a to 6 d (constant voltage control).

  As shown in FIG. 6, the primary transfer current value gradually increases with time during continuous printing. Here, since the voltage applied to the primary transfer rollers 6a to 6d is constant, the resistance of the primary transfer rollers 6a to 6d and the intermediate transfer belt 8 changes as the temperature in the image forming apparatus 100 rises, and the primary transfer roller 6a. It can be seen that the impedance between .about.6d and the photosensitive drums 1a to 1d changes. In the case of constant voltage control, in order to make the transfer electric field (transfer current) constant, it is necessary to detect the impedance between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d to correct the voltage value. 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, only the first DC power supply 44d that applies 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. Only the impedance between the primary transfer roller 6d and the photosensitive drum 1d may 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. 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.

  The impedance between the primary transfer roller 6d and the photosensitive drum 1d is detected by the current detection unit 50 by using a current detection unit 50 to generate a current flowing between the primary transfer roller 6d and the photosensitive drum 1d when a voltage is applied to the primary transfer roller 6d. The impedance is calculated from the detected current value and the applied voltage value. The controller 90 corrects the primary transfer voltage applied to the primary transfer roller 6d based on the calculated impedance.

  As described above, since the intermediate transfer belt 8 used in the present embodiment is ionic conductive, the resistance varies depending on the temperature. Further, since the primary transfer rollers 6a to 6d are electronically conductive, the resistance change due to temperature is small, but the resistance changes depending on the degree of deterioration of the roller. For this reason, the impedance between the primary transfer roller 6d and the photosensitive drum 1d also changes depending on the environmental temperature and the deterioration of the roller over time.

  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 moving time of the intermediate transfer belt 8 has reached a predetermined time), the impedance between the primary transfer roller 6d and the photosensitive drum 1d can be detected to correct the primary transfer voltage. preferable.

  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-described embodiment, only the first DC power supply 44d that applies the primary transfer electric field to the primary transfer roller 6d that transfers the black image is set to constant voltage control, but is positioned on the most downstream side in the moving direction of the intermediate transfer belt 8. The image forming unit Pd is not limited to the black image forming unit, and may be another color (yellow, cyan, or magenta).

  The set current value of the primary transfer current 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 (4)

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;
A primary transfer electric field is connected between each of the plurality of primary transfer members, and a primary transfer electric field having a polarity opposite to that of toner is applied to each primary transfer member so that a predetermined primary transfer electric field is generated between the intermediate transfer member and each primary transfer member. A plurality of voltage applying devices to be formed;
A control unit for controlling the formation of the primary transfer electric field by the plurality of voltage applying devices;
In an image forming apparatus comprising:
The control unit performs constant voltage control on 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, and applies the other voltage application. An image forming apparatus characterized by performing constant current control of the apparatus. A current detector for detecting a current flowing between the primary transfer member located on the most downstream side in the moving direction of the intermediate transfer member and the image carrier;
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. The image forming apparatus according to claim 1, wherein an impedance between the first transfer voltage and the primary transfer voltage applied to the primary transfer member from the voltage application device is corrected based on the detected impedance. 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 voltage, when the temperature detected by the said temperature detection part changes more than predetermined temperature from correction | amendment of the said primary transfer voltage last time. Image forming apparatus.   The said control part performs correction | amendment of the said primary transfer voltage, when the cumulative drive time of the said intermediate transfer body after predetermined | prescribed correction | amendment of the said primary transfer voltage becomes more than predetermined time. Image forming apparatus.

Images