JP6504111B2 - Image forming device - Google Patents

Description

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

  Conventionally, a toner image of different colors is primarily transferred onto an endless intermediate transfer belt by a plurality of image forming units, and then sequentially superimposed, and then secondarily transferred and fixed on a recording medium to form a color image. A color image forming apparatus of the formula is known.

  In the tandem type color image forming apparatus, when the toner images of the respective colors formed on the photosensitive drums are superposed on the intermediate transfer belt, the image forming portion on the upstream side in the belt advancing direction first forms the primary on the intermediate transfer belt. When the transferred toner image passes the primary transfer electric field of the downstream image forming unit, discharge and charge injection to the toner image occur, and the charge amount of the toner becomes high. The highly charged toner has a strong electrostatic adhesion to the intermediate transfer belt. In such a case, if a strong secondary transfer electric field is generated to secure the secondary transfer efficiency of the toner image, the toner of which the amount of charge is not high which is primarily transferred in the image forming portion on the upstream side in the belt traveling direction is discharged. By this, it becomes easy to reversely charge, and the secondary transfer efficiency is lowered. In addition, a strong secondary transfer electric field is likely to cause a discharge phenomenon between the recording medium and the intermediate transfer belt, which also causes deterioration in the quality of the secondary transfer image.

  The control method of the primary transfer electric field described above includes constant current control and constant voltage control. The constant current control can stabilize the primary transfer electric field to the air gap between the intermediate transfer belt and the photosensitive drum with little influence of the impedance change between the primary transfer roller and the photosensitive drum which changes depending on the durability and environment. On the other hand, the transfer electric field is changed by the printing pattern. Specifically, when forming a small print pattern (short solid image) in the longitudinal direction, the resistance of the solid portion increases and the transfer current leaks to the outside of the solid portion, so it is necessary to increase the current There is. As a result, in the case of forming a large print pattern (Long Solid Image) in the longitudinal direction, an excessive transfer current (electric field) is generated, and the toner charge amount on the intermediate transfer belt becomes large. There is a problem that (transfer image quality) deterioration occurs.

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

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

Unexamined-Japanese-Patent No. 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 is a problem that the formation efficiency is reduced. In addition, providing a circuit for detecting the impedance in each image forming unit also leads to an increase in cost.

  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 performing constant voltage control of a primary transfer electric field in view of the above problems.

  In order to achieve the above object, according to a first aspect of the present invention, there is provided 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. It is a forming device. In 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 brought into pressure contact with the respective image bearing members with the intermediate transfer member interposed therebetween, and primarily transfer toner images of a plurality of colors developed on the respective image bearing members onto the intermediate transfer member. A plurality of voltage application devices are connected to each of the plurality of primary transfer members, and a primary transfer voltage having a polarity opposite to that of the toner is applied to each primary transfer member to form a predetermined primary between the intermediate transfer member and each primary transfer member. Form a transfer field. The control unit controls formation of a primary transfer electric field by a plurality of voltage application devices. The control unit performs constant voltage control of the voltage application device connected to the primary transfer member positioned on the most downstream side in the moving direction of the intermediate transfer member among the plurality of voltage application devices, and constant current control of 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 constant voltage control, and the other voltage application devices are constant current control. The primary transfer current does not become excessive at the most downstream primary transfer portion where the toner has already been transferred onto the intermediate transfer member at the upstream primary transfer portion, and an appropriate current value can be obtained. Therefore, it is possible to suppress an increase in the toner charge amount on the intermediate transfer member before the secondary transfer, and to ensure high secondary transfer efficiency. Further, it is possible to effectively suppress density unevenness of the image transferred onto the recording medium in the secondary transfer portion.

FIG. 1 is a schematic cross-sectional view showing an entire configuration of an image forming apparatus 100 according to an embodiment of the present invention. A partial enlarged view around the intermediate transfer belt 8 in FIG. 1 Block diagram showing an example of a control path of the image forming apparatus 100 of the present invention Graph showing transition of toner charge amount at each primary transfer portion Graph showing relationship between toner charge amount and mottle Graph showing transition of primary transfer current when printing is performed on both sides continuously by applying constant voltage to primary transfer rollers 6a to 6d

  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 type 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 disposed in order from the upstream side in the transport direction (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 and magenta are formed by the respective steps of charging, exposure, development and transfer. And black (hereinafter also referred to as Y, C, M, and BK) are sequentially formed.

  Photosensitive drums 1a to 1d for carrying visible images (toner images) of respective colors are disposed in these image forming portions Pa to Pd, respectively, and the intermediate transfer belt is further rotated in the clockwise direction in FIG. 8 are provided adjacent to the respective image forming portions Pa to Pd. In the present embodiment, a photosensitive drum made of amorphous silicon (a-Si) is formed on the outer peripheral surface of a drum tube having a diameter of 40 mm as the photosensitive drums 1a to 1d.

  The sheet P to which the toner image is transferred is accommodated in the sheet cassette 16 at the lower part of the image forming apparatus 100, and is conveyed to the secondary transfer roller 9 through the sheet feeding roller 12a and the registration roller pair 12b.

  Next, the image forming units Pa to Pd will be described. Around and below the photosensitive drums 1a to 1d rotatably disposed, charging devices 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d, and image information on the respective photosensitive drums 1a to 1d And an developing unit 3a, 3b, 3c and 3d for developing the electrostatic latent image formed on the photosensitive drums 1a to 1d to form a toner image. Cleaning devices 5a, 5b, 5c and 5d are provided to remove the developer (toner) remaining on the photosensitive drums 1a to 1d. Further, a belt cleaning device 19 provided with a belt cleaning brush 19a facing the driven roller 10 with the intermediate transfer belt 8 interposed therebetween is disposed on the upstream side of the photosensitive drum 1a in the rotational direction of the intermediate transfer belt 8.

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

The base material layer serves as a basic material constituting the intermediate transfer belt 8 to impart a predetermined rigidity, withstands the processing conditions at the time of laminating the coat layer, and further, processing workability when manufacturing the intermediate transfer belt 8; It is preferable to be excellent in heat resistance, slipperiness and other various physical properties. As a material of such a base material layer, for example, a resin such as PI (polyimide), PAI (polyacrylimide) or PVDF (polyvinylidene fluoride) which is difficult to expand and contract is used. In addition, carbon black as a conductive agent is dispersed in the resin, and the volume resistivity at the time of 500 V application 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 onto the sheet P, the elastic layer causes the pressing force of the intermediate transfer belt 8 and the secondary transfer roller 9 to uniformly act on the entire sheet P. It is a layer to prevent the generation of a hollow image. As a material of the elastic layer, an elastic material such as CR (chloroprene rubber) or a thermoplastic polyurethane elastomer (TPU) is used. The coat layer protects the elastic layer and imparts releasability to the intermediate transfer belt 8. As a material of the coating layer, a fluorine resin 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 layer and PTFE is coated as a coat layer, is used. The linear velocity 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 on the intermediate transfer belt 8. Primary transfer to 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. The current detection unit 50 is connected. In the present embodiment, as the primary transfer rollers 6a to 6d, an electron conductive roller is used in which a roller body made of EPDM foamed rubber having a volume resistance value of 6.8 log Ω is formed on the outer peripheral surface of a metal shaft having a diameter of 8 mm. Further, a second DC power supply 45 for applying a secondary transfer voltage is connected to the secondary transfer roller 9.

  When image data is input from a host device such as a personal computer (hereinafter referred to as a personal computer) connected to image forming apparatus 100, first, charging devices 2a to 2d uniformly charge the surfaces of photosensitive drums 1a to 1d. Then, the surfaces of the photosensitive drums 1a to 1d are irradiated with light by the exposure unit 4, and electrostatic latent images corresponding to the image signals are formed on the respective photosensitive drums 1a to 1d. Next, toners of respective colors of yellow, magenta, cyan, and black are supplied onto the photosensitive drums 1a to 1d from the developing devices 3a to 3d, and are attached to the electrostatic latent image, thereby forming the exposure unit 4 by exposure. 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 the primary transfer rollers 6a to 6d disposed so as to be in pressure contact with the photosensitive drums 1a to 1d with the intermediate transfer belt 8 interposed therebetween. During this time (primary transfer portion), an electric field is applied at a predetermined transfer voltage, and the yellow, magenta, cyan and black toner images on the photosensitive drums 1a to 1d 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 intermediate transfer belt 8 starts to rotate in the counterclockwise direction in FIG. 1 with the rotation of drive roller 11 by a belt drive motor (not shown), sheet P is transferred from intermediate transfer belt 8 from registration roller 12b at a predetermined timing. The toner image is conveyed to the nip portion (secondary transfer nip portion) of the secondary transfer roller 9 and the intermediate transfer belt 8 provided close to each other, and the toner image is secondarily transferred onto the sheet P from the intermediate transfer belt 8 at the nip portion. . The sheet P on which the toner image has been transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressed when passing through the nip portion (fixing nip portion) of the fixing roller pair 13 so that the toner image is fixed 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 as it is (or after being distributed to the reverse conveying path 18 by the branch unit 14 and the image is formed on both sides) by the discharge roller pair 15 to the discharge tray 17.

  Next, control paths of the image forming apparatus 100 of the present invention will be described. FIG. 3 is a block diagram showing an example of control paths used in the image forming apparatus of the present invention. In addition, since various controls of each part of the apparatus are performed when using the image forming apparatus 100, the control path of the entire image forming apparatus 100 is complicated. Therefore, the parts of the control path that are necessary for the implementation of the present invention will be mainly described here.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 as a read only storage unit, a random access memory (RAM) 93 as a read / write storage unit, and a temporary A temporary storage unit 94 for storing image data etc., a counter 95, a plurality of (two in this case) transmitting control signals to each device in the image forming apparatus 100 and receiving input signals from the operation unit 60 At least an I / F (interface) 96. In addition, the control unit 90 can be disposed at any place inside the apparatus body.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and other data that can not be changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during control of the image forming apparatus 100, data temporarily required to control the image forming apparatus 100, and the like. The counter 95 counts and integrates the number of printed sheets.

  Further, the control unit 90 transmits a control signal from the CPU 91 to each part and apparatus in the image forming apparatus 100 through the I / F 96. Further, each portion, a signal indicating the state from the device or an input signal is transmitted to the CPU 91 through the I / F 96. 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, and the bias The control circuit 41, the operation unit 60 and the like can be mentioned.

  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 to a digital signal and then sent to the temporary storage unit 94.

  The bias control circuit 41 is connected to the charging bias power supply 42, the developing bias power supply 43, the first DC power supplies 44a to 44b, the second DC power supply 45, and the belt cleaning bias power supply 46, and outputs control signals from the control unit 90. The respective power supplies are operated by the control signal from the bias control circuit 41, and the charging bias power supply 42 is connected to the charging roller 21 in the chargers 2a to 2d, and the developing bias power supply 43 is connected to the developing device 3a. In the magnetic roller 27 and the developing roller 29 in ~ 3d, the first DC power supplies 44a to 44d to the primary transfer rollers 6a to 6d, the second DC power supply 45 to the secondary transfer roller 9, and the belt cleaning bias power supply 46 belt cleaning Predetermined voltages are applied to the brushes 19a, respectively.

  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 disposed 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. The various settings of the forming apparatus 100 are set to the default state. The liquid crystal display unit 61 is configured to indicate the state of the image forming apparatus 100, and to display the image formation state and the number of print copies. 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 the primary transfer electric field in the image forming portions Pb to Pd on the downstream side in the advancing direction of the intermediate transfer belt 8 Discharge and charge injection occur to change the charged state of the toner. 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, Pd, or the cyan toner image primarily transferred in the image forming portion Pc is downstream The same phenomenon occurs when passing the primary transfer electric field in the image forming portion Pd on the side. Due to the change in the charged state of the toner, highly-charged toner which is difficult to be secondarily transferred and reversely-charged toner are generated, and a factor causing an image of defective transfer when secondarily transferred onto the sheet P by the second transfer roller 9 It has become.

FIG. 4 is a graph showing the transition of the toner charge amount at each primary transfer portion, and the toner charge amount on the intermediate transfer belt 8 when a total of 200% printing of Y 100% and C 100% is performed all over the longitudinal direction of the sheet. It shows the transition. In FIG. 4, a toner with 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 0.5 mg / cm ² . In FIG. 4, the primary transfer position of yellow (hereinafter referred to as Y position) is the transition of the charging amount of Y monochrome, and the primary transfer position of cyan (hereinafter referred to as C position) is the charge amount transition of Y + C mixed color. Further, 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 referred to simply as transfer current) flowing between the primary transfer roller 6a and the photosensitive drum 1a at the Y position is a transfer current satisfying 100% Y color transfer. In this case, the transfer current necessary for C100% transfer on Y100% is sufficient, and the transfer current setting at the primary transfer position of magenta (hereinafter referred to as M position) is M60% transfer on 200% Y + C mixed color The transfer current is sufficient for the M + 100% transfer on the Y + C mixed color 160%.

  In the case of constant current control (data series of ◇ in FIG. 4), the transfer current setting at the final primary transfer position of black (hereinafter referred to as BK position) is to transfer 60% BK onto 200% of Y to M mixed colors. The transfer current setting is necessary and sufficient, and the transfer current setting is necessary and sufficient for BK 100% transfer onto the Y-M mixed color 160%. In the case of FIG. 4, the transfer current (transfer electric field) at the Y position, C position, M position and BK position is -25 μA, -27 μA, -29 μA and -32 μA, respectively. As a result, the toner charge amounts at the Y position, C position, M position and 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 constant voltage control (data series of □ in FIG. 4), the conditions are the same as above. The transfer current (transfer electric field) at the Y position, C position and M position is -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, C position, M position and BK position are 33 μC / g, 36 μC / g, 42 μC / g and 43 μC / g, respectively.

  As shown in FIG. 4, in the case where constant voltage control is performed only at the BK position, the charge amount of the toner is small after the toner image of the Y + C mixed color 200% passes the BK position. The reason is that in 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 BK position in the direction of movement of the intermediate transfer belt 8 A transfer field of -32 .mu.A is always formed, whether it is transferred or not transferred. On the other hand, in constant voltage control, when a toner image of Y + C mixed color 200% exists on the intermediate transfer belt 8, the transfer voltage setting is sufficient to transfer 60% at the BK position. The transfer electric field) is about -22 μA.

  From the above, when the BK position is constant current control, when the upstream color toner is already transferred onto the intermediate transfer belt 8, the transfer electric field becomes excessive, and the intermediate transfer belt is discharged by the primary transfer portion. The toner charge amount above 8 is increased.

FIG. 5 is a graph showing the relationship between the toner charge amount and the mottle. The term "mottle" as referred to in the present specification means the variation (mottledness) of the image density defined as an image evaluation standard in ISO (International Organization for Standardization), and specifically 12.7 mm. The standard deviation of the density of each area when the 12.7 mm image is divided into 10 × 10 = 100 areas is shown. That is, the lower the value of the mottle, the smaller the uneven density. 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 0.5 mg / cm ² . When calculating the mottle, the secondary transfer current applied to the secondary transfer roller 9 is optimized to a current value at which the mottle 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%). When the toner charge amount is 42 μC / g and the mottle is 5.5 to 6 at any of the blue (□ data series) and red (Δ data series), the toner charge amount is 46 to 49 μC / g. In g, the mottle is 7, and the value of mottle increases as the toner charge amount increases. That is, it can be seen that the greater the toner charge amount, the greater the uneven density.

  In the image forming apparatus 100 of the present invention, among the first DC power sources 44a to 44d for applying the primary transfer electric field to the four primary transfer rollers 6a to 6d, the primary side on the most downstream side with respect to the traveling direction of the intermediate transfer belt 8. The first DC power source 44d for applying a primary transfer electric field to the transfer roller 6d is under constant voltage control. As a result, 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, it is possible to suppress an increase in the toner charge amount on the intermediate transfer belt 8 before the secondary transfer, and to ensure a high secondary transfer efficiency. Further, it is possible to effectively suppress density unevenness of the image transferred onto the sheet P in the secondary transfer portion.

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

  As shown in FIG. 6, during continuous printing, the primary transfer current value gradually increases with the passage of time. 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 with the temperature rise in the image forming apparatus 100, and the primary transfer roller 6a It can be seen that the impedance between 〜6d and the photosensitive drums 1a to 1d is changing. 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 and correct the voltage value. The constant voltage control is disadvantageous 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 for applying the primary transfer electric field to the primary transfer roller 6d is constant voltage control, and the other first DC power supplies 44a to 44c are constant current control. It is sufficient to detect only the impedance between the primary transfer roller 6d and the photosensitive drum 1d. Therefore, the detection time of the impedance can be shortened, and the printing waiting time can be shortened to improve the image forming efficiency. Further, since it is not necessary to provide the first direct current power supplies 44a to 44c with the current detection unit 50 necessary for detecting the impedance, it contributes to simplification of the control path of the image forming apparatus 100 and cost reduction.

  The detection of the impedance between the primary transfer roller 6d and the photosensitive drum 1d is carried out by using the current detection unit 50 for the 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 control unit 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 ion conductive, the resistance changes with temperature. Further, since the primary transfer rollers 6a to 6d have electronic conductivity, the resistance change due to temperature is small, but the resistance changes depending on the degree of deterioration of the rollers. Therefore, the impedance between the primary transfer roller 6d and the photosensitive drum 1d also changes due to the environmental temperature or the deterioration with time of the roller.

  Therefore, when the temperature detected by the in-machine temperature sensor 51 has changed by a predetermined temperature since the previous correction of the primary transfer voltage, or the cumulative print count from the time of correction of the previous primary transfer voltage counted by the counter 95 is predetermined. When the number of sheets is reached (when the cumulative movement time of the intermediate transfer belt 8 reaches a predetermined time), the impedance between the primary transfer roller 6d and the photosensitive drum 1d is detected to correct the primary transfer voltage. preferable.

  Others The present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the present invention. For example, in the above embodiment, only the first DC power source 44d for applying the primary transfer electric field to the primary transfer roller 6d for transferring the black image is constant voltage control, but the intermediate transfer belt 8 is located at the most downstream side in the moving direction. The image forming portion Pd is not limited to the black image forming portion, and may be another color (yellow, cyan or magenta).

  Further, the set current value of the primary transfer current described above is only an example, and may be set appropriately according to the specifications of the image forming apparatus, the use environment, and the like.

  The present invention is applicable to a tandem-type color image forming apparatus using an intermediate transfer method. By the use of the present invention, the primary transfer current required when transferring a toner image to the intermediate transfer member can be stably secured, and the primary transfer current on the intermediate transfer member on the upstream side in the moving direction of the intermediate transfer member can be secured. The image forming apparatus can suppress excessive charging of the toner when the transferred toner layer passes through the primary transfer electric field on the downstream side.

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 body)
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 internal temperature sensor 90 control unit 100 image forming apparatus Pa to Pd image forming unit

Claims (3)

With multiple image carriers,
An intermediate transfer member on which toner images of a plurality of colors formed on the surfaces of the plurality of image carriers are sequentially laminated;
A plurality of primary transfer members, which are in pressure contact with the respective image bearing members with the intermediate transfer member interposed therebetween, and primarily transfer onto the intermediate transfer member the toner images of a plurality of colors developed on the respective image bearing members;
Each of the plurality of primary transfer members is connected, and a primary transfer voltage having a polarity opposite to that of the toner is applied to each of the primary transfer members to thereby apply a predetermined primary transfer electric field between the intermediate transfer member and each of the primary transfer members. A plurality of voltage application devices to be formed;
A control unit configured to control formation of the primary transfer electric field by the plurality of voltage application devices;
In an image forming apparatus provided with
The control unit performs constant voltage control on the voltage application device connected to the primary transfer member positioned on the most downstream side in the moving direction of the intermediate transfer member among the plurality of voltage application devices, and applies the other voltage application In addition to constant current control of the device ,
A current detection unit for detecting a current flowing between the primary transfer member positioned 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 at which the current value is detected, between the primary transfer member and the image carrier. An image forming apparatus that detects an impedance between them and corrects the primary transfer voltage applied from the voltage application device to the primary transfer member based on the detected impedance . A temperature detection unit for detecting the temperature inside the image forming apparatus;
The control unit executes correction of the primary transfer voltage when the temperature detected by the temperature detection unit changes by a predetermined temperature or more from the previous correction of the primary transfer voltage . Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the control unit executes the correction of the primary transfer voltage when the accumulated driving time of the intermediate transfer member from the previous correction of the primary transfer voltage becomes equal to or longer than a predetermined time. Image forming device.

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