JP2014081631A - Image forming apparatus - Google Patents

Abstract

An object of the present invention is to provide an image forming apparatus in which the time required for a transfer bias to become a target value from an OFF state can be made substantially the same in each transfer unit.
SOLUTION: A plurality of photosensitive drums 51, a plurality of transfer rollers 74 provided corresponding to each of the photosensitive drums 51 for transferring a developer image on the photosensitive drum 51 onto a sheet P, and the plurality of transfer rollers. And an image forming apparatus (color printer 1) including a control device 100 that controls a transfer bias applied to 74. The transfer bias is applied so that at least two transfer rollers 74 have different target values, and the controller 100 sets the same value when the transfer bias is changed from the OFF state to the ON state for each transfer roller 74. After the first predetermined bias is applied, the transfer bias corresponding to the target value is applied.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus including a plurality of photosensitive drums and a plurality of transfer units.

  In general, in an electrophotographic color image forming apparatus, a plurality of photosensitive drums and a plurality of transfer portions that transfer a developer image on the photosensitive drum onto a transfer medium are provided corresponding to different colors (patents). Reference 1).

  In this color image forming apparatus, the transfer bias applied to each transfer unit is set to an optimum target value (different target value) for each color. This is because it is necessary to set comprehensively considering the ease of charging of the developer of each color, the coloring power after printing, and the like.

JP 2011-113007 A

  When a transfer bias input command is issued when the transfer bias is OFF, a predetermined delay time is required until the transfer bias that is the target value rises. This delay time corresponds to the time from when the input command is issued until the output voltage of the operational amplifier used for controlling the transfer bias output reaches a predetermined threshold value. For example, as shown in FIG. 2A, when a delay time T4 until the output voltage of the operational amplifier reaches a predetermined threshold after the input command of the target value I11 of the transfer bias (see the solid line) is issued. The transfer bias starts to rise (see broken line).

  The output voltage of the operational amplifier is determined based on the difference between the voltage corresponding to the target value of the transfer bias and the input voltage. When this difference value is large, the delay time until the output voltage reaches the predetermined threshold is shortened, and when the difference value is small, the delay time until the output voltage reaches the predetermined threshold becomes long. For example, as shown in FIG. 2B, when there is an input command of I12 smaller than the transfer bias target value I11 (see solid line), the delay time until the operational amplifier output voltage reaches a predetermined threshold value. When the delay time T5 longer than T4 has elapsed, the transfer bias starts to rise (see the broken line). As described above, when the transfer bias target value is different in each transfer portion, the transfer bias delay time in each transfer portion changes.

  Note that this delay time does not occur once the transfer bias rises. For example, as shown in FIG. 2C, when the rising transfer bias is changed from I12 to I11, when an input command is input to I11 (see the solid line), the transfer bias is quickly changed from I12 to I11. (Refer to the broken line).

  On the other hand, when performing control to turn on the transfer bias for each transfer unit at the timing when the leading end of the sheet (between the leading end and the printable area) reaches the transfer position of each transfer unit, in order to simplify the control, It is desired that the time until the transfer bias reaches the target value from the OFF state is substantially the same for each transfer unit.

  However, as described above, when the target value of each transfer portion is different, the delay time of each transfer bias from the OFF state to the target value changes, so the transfer bias changes from the OFF state to the target value. It becomes difficult to make the time required for the transfer substantially the same at each transfer portion.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus in which the time required for the transfer bias to reach a target value from the OFF state can be made substantially the same in each transfer unit.

  In order to achieve the above object, an image forming apparatus of the present invention is provided corresponding to each of a plurality of photosensitive drums and the photosensitive drums, and a plurality of transfer units for transferring a developer image on the photosensitive drum onto a transfer medium. And a control unit that controls a transfer bias applied to the plurality of transfer units. The transfer bias is applied to at least two transfer portions so as to have different target values, and the control portion sets the same value when the transfer bias is changed from the OFF state to the ON state for each transfer portion. (1) After applying a predetermined bias, control is performed so that a transfer bias corresponding to the target value is applied.

  According to such a configuration, when the transfer bias is changed from the OFF state to the ON state, the first predetermined bias having the same value is applied to each transfer portion, so that the delay time of each transfer bias can be made the same. . That is, it is possible to make the transfer bias rise time constant in each transfer portion.

  In the above-described configuration, the magnitude of the first predetermined bias can be configured to be equal to or greater than the absolute value of all target values.

  According to such a configuration, since the magnitude of the first predetermined bias is greater than or equal to the absolute value of all target values, the transfer bias can be quickly raised.

  In the above-described configuration, the magnitude of the first predetermined bias can be set to be equal to or less than the absolute value of all target values.

  If the first predetermined bias is too high, the potential of the photosensitive drum facing the transfer portion is likely to decrease and affect printing. With such a configuration, the magnitude of the first predetermined bias is the absolute value of all target values. Since it is less than the value, the potential of the photosensitive drum is unlikely to decrease, and the influence on printing can be reduced.

  In the configuration in which the magnitude of the first predetermined bias is equal to or greater than the absolute values of all target values, the controller is smaller than the absolute values of all target values after applying the first predetermined bias, and each A configuration may be adopted in which a second predetermined bias having the same polarity as the target value is applied, and then a transfer bias corresponding to the target value is applied.

  According to such a configuration, the second predetermined bias having the same polarity as each target value that is smaller than the absolute value of all target values after application of the first predetermined bias that is equal to or greater than the absolute value of all target values. Since it is applied, the influence when a transfer bias larger than the absolute value of the target value is output can be reduced.

  In the configuration described above, the control unit determines a bias correction amount according to the amount of current flowing between the photosensitive drum and the transfer unit immediately before switching from the OFF state to the ON state, and sets a transfer bias to be applied to each transfer unit. A configuration may be adopted in which control is performed to obtain a value obtained by adding a bias correction amount corresponding to each transfer portion to the first predetermined bias.

  When the current flows from the photosensitive drum to the transfer portion, when the current rises from the state where there is no inflow current to the first predetermined bias, when the current flows from the state where there is the inflow current to the first predetermined bias, each amount varies depending on the amount of inflow. Since the delay time of the transfer bias varies, the rise time of the transfer bias at each transfer portion varies.

  However, according to such a configuration, the bias correction amount is determined in accordance with the inflow amount of current for each transfer unit, and the transfer bias applied to each transfer unit is set to the first predetermined bias corresponding to each transfer unit. Since the control is performed so that the bias correction amount is added, it is possible to reduce the difference in the amount of rise of the transfer bias at each transfer portion. That is, since the delay time of each transfer bias can be made substantially the same, the rise time of the transfer bias at each transfer portion can be made substantially the same.

  According to the present invention, the time until the transfer bias reaches the target value from the OFF state can be made substantially the same at each transfer portion.

1 is a cross-sectional view illustrating a color printer according to an embodiment of the present invention. FIG. 5A is a diagram illustrating a state in which a transfer bias delay time is generated, and FIG. 6C is a diagram illustrating a state in which the value of the transfer bias in a rising state is changed. It is a figure which shows control of the transfer bias in each transfer roller. 3 is a flowchart showing control of a transfer bias. It is a figure which shows control of the transfer bias which concerns on a 1st modification. FIG. 10 is a diagram illustrating transfer bias control in which a first predetermined bias is set to a value higher than that in the embodiment of FIG. 3 in a second modification. It is a flowchart which shows control of the transfer bias which concerns on a 2nd modification. FIG. 10A is a diagram showing control of a transfer bias when the inflow current is 0 according to a third modification, and FIG. 10B is a diagram showing control of the transfer bias when there is an inflow current. It is a flowchart which shows control of the transfer bias which concerns on a 3rd modification.

  Next, an embodiment of the present invention will be described in detail with reference to the drawings as appropriate. In the following description, the direction will be described with reference to the user when using the color printer 1. That is, the left side in FIG. 1 is “front”, the right side is “rear”, the back side is “left”, and the front side is “right”. Also, the vertical direction in FIG.

<Overall configuration of color printer>
As shown in FIG. 1, the color printer 1 includes a paper feeding unit 20 that supplies a sheet P as an example of a transfer medium in an apparatus main body 2 and an image forming unit 30 that forms an image on the fed paper P. A paper discharge unit 90 that discharges the paper P on which an image is formed, and a control device 100 as an example of a control unit.

  An opening 2 </ b> A is formed in the upper part of the apparatus main body 2. The opening 2A is opened and closed by an upper cover 3 that is rotatably supported by the apparatus main body 2. The upper surface of the upper cover 3 is a paper discharge tray 4 that accumulates the paper P discharged from the apparatus main body 2, and a plurality of LED attachment members 5 that hold the LED units 40 are provided on the lower surface.

  The paper feeding unit 20 is provided in the lower part in the apparatus main body 2, and a paper feeding tray 21 that is detachably attached to the apparatus main body 2, and a paper supply mechanism that conveys paper P from the paper feeding tray 21 to the image forming unit 30. 22 is mainly provided. The paper supply mechanism 22 is provided on the front side of the paper supply tray 21 and mainly includes a paper supply roller 23, a separation roller 24, a separation pad 25, a paper dust removing roller 26, a pinch roller 27, and a registration roller 29.

  The registration roller 29 is disposed on the upstream side in the conveyance direction of the photosensitive drum 51 to be described later in the conveyance path 28, and regulates the leading end of the sheet P toward the photosensitive drum 51 by being controlled to rotate / stop.

  The paper P in the paper feed tray 21 is separated one by one by the paper supply mechanism 22 and sent upward, and after paper dust is removed in the process of passing between the paper dust removing roller 26 and the pinch roller 27, The direction is changed backward through the conveyance path 28, passes through the registration roller 29, and is supplied to the image forming unit 30.

  The image forming unit 30 mainly includes four LED units 40, four process cartridges 50, a transfer unit 70, and a fixing unit 80.

  The LED unit 40 is swingably coupled to the LED mounting member 5 and is appropriately positioned and supported by a positioning member (not shown) provided in the apparatus main body 2.

  The process cartridge 50 is arranged side by side in the front-rear direction between the upper cover 3 and the paper feeding unit 20, and each of the photosensitive drum 51, the charger 52, the developing roller 53, and the developer for forming an electrostatic latent image. As an example, a toner storage chamber 54 that stores toner is provided.

  In the process cartridge 50, those indicated by reference numerals 50K, 50Y, 50M, and 50C containing toners for black, yellow, magenta, and cyan are arranged in this order from the upstream side in the conveyance direction of the paper P. ing. In this specification, when the transfer roller 74 corresponding to the color of the toner is specified, the symbols K, Y, M, and C are attached to each of black, yellow, magenta, and cyan. And

  The transfer unit 70 is provided between the paper feeding unit 20 and each process cartridge 50, and mainly includes a driving roller 71, a driven roller 72, a conveying belt 73, and a transfer roller 74 as an example of a transfer unit.

  The driving roller 71 and the driven roller 72 are arranged in parallel with a space in the front-rear direction, and a conveyance belt 73 formed of an endless belt is stretched between them. The outer surface of the conveyor belt 73 is in contact with the plurality of photosensitive drums 51.

  In addition, four transfer rollers 74 that sandwich the conveyance belt 73 with the photosensitive drum 51 are provided inside the conveyance belt 73 in correspondence with each of the photosensitive drums 51. A transfer bias is applied to the transfer roller 74 by constant current control during transfer.

  The fixing unit 80 is disposed on the rear side of each process cartridge 50 and the transfer unit 70, and includes a heating roller 81 and a pressure roller 82 that is disposed to face the heating roller 81 and presses the heating roller 81.

  In the image forming unit 30 configured as described above, the surface of each photosensitive drum 51 is uniformly charged by the charger 52 and then exposed by each LED unit 40. As a result, the potential of the exposed portion is lowered, and an electrostatic latent image based on the image data is formed on each photosensitive drum 51. Thereafter, toner is supplied from the developing roller 53 corresponding to the electrostatic latent image, so that the toner image is carried on each photosensitive drum 51.

  When the paper P supplied on the conveyor belt 73 passes between each photosensitive drum 51 and each transfer roller 74 provided inside the conveyor belt 73, the sheet P is formed on each photosensitive drum 51 by applying a transfer bias. The toner image is transferred onto the paper P. Then, as the paper P passes between the heating roller 81 and the pressure roller 82, the toner image transferred onto the paper P is thermally fixed.

  The paper discharge unit 90 mainly includes a paper discharge side transport path 91 formed so as to extend upward from the exit of the fixing unit 80 and to be reversed forward, and a plurality of pairs of transport rollers 92 that transport the paper P. ing. The paper P on which the toner image has been transferred and heat-fixed is transported along a paper discharge side transport path 91 by a transport roller 92, discharged outside the apparatus main body 2, and stored in the paper discharge tray 4.

<Control device>
Next, the control device 100 will be described in detail.
The control device 100 includes, for example, a CPU, a ROM, a RAM, and the like, and is configured to control transfer bias applied to the plurality of transfer rollers 74 according to a program prepared in advance. Further, the control device 100 receives a signal from a known paper passing sensor 110 provided upstream of the black transfer roller 74K in the paper transport direction, and the leading edge of the paper P passes through the paper passing sensor 110. Can be detected.

  The transfer bias is a current or voltage applied to the transfer roller 74 in order to transfer the toner image on the photosensitive drum 51 onto the paper P conveyed on the conveyance belt 73. This transfer bias generates an electrical force for moving the toner from the photosensitive drum 51 to the transfer roller 74. In this embodiment, the transfer bias is controlled at a constant current so that the current flowing through the photosensitive drum 51 and the transfer roller 74 is constant. The transfer bias control method is not limited to constant current control, and may be performed by constant voltage control.

  Different transfer bias (current value) target values are set for the four transfer rollers 74, and the transfer bias corresponding to each target value is controlled to be applied to each transfer roller 74. In this embodiment, the target values are set such that the transfer roller 74K is I1, the transfer roller 74Y is I2, the transfer roller 74M is I3, and the transfer roller 74C is I4. These values are set as appropriate through simulations and experiments. be able to.

  Further, as shown in FIG. 3, when the transfer bias is changed from the OFF state to the ON state, the control device 100 controls the transfer roller 74 to apply a first predetermined bias I5 having the same value. . In FIG. 3, the transfer bias commanded from the control device 100 is indicated by a solid line, and the transfer bias actually flowing through each transfer roller 74 is indicated by a broken line. The magnitude relationship between I1 to I4 is I1> Illustrated as I2> I3> I4.

  The first predetermined bias I5 is a current whose magnitude is equal to or greater than the absolute value of all target values I1 to I4 set for each transfer roller 74, and is applied to each transfer roller 74 at the application time T1. . The application time T1 is, for example, the time from when the leading edge of the paper P passes through the transfer position to the time immediately before the printable area of the paper P (the part located slightly on the leading edge side from the printable area) reaches the transfer position. It can be set as appropriate by simulation or experiment.

  By raising the transfer bias of each transfer roller 74 with the first predetermined bias I5, the delay time of each transfer bias that actually flows can be made the same for each transfer roller 74. That is, the rising time of the transfer bias at each transfer roller 74 can be set to a constant application time T1. Further, by setting the magnitude of the first predetermined bias I5 to be equal to or greater than the absolute value of all target values, the transfer bias can be quickly raised.

  After applying the first predetermined bias I5 at the application time T1, the control device 100 applies a transfer bias corresponding to the target values I1 to I4 to each transfer roller 74. For example, in the case of the transfer roller 74K, the control device 100 changes the transfer bias command value from I5 to I1.

Further, the transfer bias is changed from the OFF state to the ON state when a predetermined time has elapsed after detecting that the leading edge of the paper P has passed through the paper passing sensor 110.
The predetermined time is set for each transfer roller 74 in accordance with the timing at which the leading edge of the paper P (between the leading edge and the printable area) reaches the transfer position of each transfer portion. In this embodiment, the transfer roller 74K, the transfer roller 74Y, the transfer roller 74M, and the transfer roller 74C are turned on in this order. The roller 74M is set to have T02 longer than T01, and the transfer roller 74C has T03 longer than T02. This value can be appropriately set by simulation or experiment.

The control device 100 configured as described above performs control as shown in the flowchart of FIG. In the following description, only the transfer roller 74K will be described as a representative, and the other control is the same, and the description is omitted.
When starting the print control, the control device 100 first determines whether the leading edge of the paper P has passed through the paper passing sensor 110 (S1). When the leading edge of the paper P passes the paper passing sensor 110 (S1; Yes), the control device 100 determines whether a predetermined time T00 has passed (S2).

  If the predetermined time T00 has elapsed (S2; Yes), the control device 100 changes the transfer bias from the OFF state to the ON state for the transfer roller 74K, and applies the first predetermined bias I5 to the transfer roller 74K (S3). ), It is determined whether or not the application time T1 has elapsed (S4).

  When the application time T1 has elapsed (S4; Yes), the control device 100 applies a transfer bias corresponding to the target value I1 to the transfer roller 74K (S5), and determines whether or not the printing time T2 has elapsed (S5). S6). The printing time T2 is, for example, the time required for the printable area of the paper P to pass through the transfer position, and can be set as appropriate by simulation or experiment. When the printing time T2 has elapsed (S6; Yes), the control device 100 turns off the transfer bias applied to the transfer roller 74K (S7).

  As mentioned above, although embodiment of this invention was described, this invention is not limited to the said embodiment. About a concrete structure, it can change suitably in the range which does not deviate from the meaning of this invention.

<First Modification>
In the embodiment, the magnitude of the first predetermined bias I5 is set to be equal to or larger than the absolute values of all the target values I1 to I4. However, as shown in FIG. It is good also as I10 below the absolute value of I4. In FIG. 5, I4, which is the minimum value of I1 to I4, is shown as a representative. Further, the application time T3 is appropriately set by simulation, experiment, etc., and the control method is the same as in the above embodiment, and T1 in step S4 in FIG. 4 may be replaced with T3.

  For example, if the first predetermined bias is too high, the potential of the photosensitive drum 51 facing the transfer roller 74 is lowered, and printing is likely to be affected. However, in this embodiment, since the magnitude of the first predetermined bias I10 is equal to or less than the absolute value of all target values, the photosensitive drums facing the respective transfer rollers 74 are compared with the configuration in which the first predetermined bias is too high. It is possible to reduce the influence on printing due to the decrease in the potential of 51.

<Second Modification>
In the above-described embodiment, control is performed such that the first predetermined bias I5 that is larger than the absolute value of all the target values I1 to I4 is changed to each target value after the first predetermined bias I5 is started, as shown in FIG. Alternatively, the second predetermined bias I6 may be applied after the first predetermined bias I9 is applied, and then the transfer bias corresponding to the target value may be applied. In FIG. 6, the transfer bias commanded from the control device 100 is indicated by a solid line, and the transfer bias actually flowing through each transfer roller 74 is indicated by a broken line.

  The first predetermined bias I9 is a value larger than I5 in the above-described embodiment, and is applied to each transfer roller 74 at the first application time T11. The first application time T11 is shorter than T1, and is set to a time during which the first predetermined bias I9 is not completely applied.

  The second predetermined bias I6 is a current that is smaller than the absolute value of all target values set for each transfer roller 74 and has the same polarity as each target value, and each transfer at the second application time T12. Applied to the roller 74. The second application time T12 is set to a time longer than T11. T11 and T12 can be set as appropriate by simulation, experiment, or the like.

  For example, when the time to reach the target value is shortened, the first predetermined bias may be set to I9 that is higher than I5 in order to shorten the transfer bias delay time (see FIG. 6). In that case, if a transfer bias greater than the absolute value of the target value is output, the print quality may be affected.

  However, in the present embodiment, the second predetermined bias I6 that is smaller than the absolute values of all target values and has the same polarity as each target value is applied before the first predetermined bias I9 is completely applied. It is possible to reduce the influence when a transfer bias larger than the value is output. Further, the time until the transfer bias reaches the target value can be shortened. In addition, it is possible to reduce the influence when the transfer bias is output while shortening the delay time compared to the case where I6 smaller than the absolute value of all target values is applied as the first predetermined bias.

  Such a control apparatus 100 performs control like the flowchart shown in FIG. In the following description, only the transfer roller 74K will be described as a representative, and the other control is the same, and the description is omitted.

  The process up to step S2 is the same as that in FIG. 4, and when the predetermined time T00 has elapsed (S2; Yes), the control device 100 changes the transfer bias from the OFF state to the ON state with respect to the transfer roller 74K. The first predetermined bias I9 is applied to 74K (S31), and it is determined whether or not the first application time T11 has passed (S32).

  When the first application time T11 has elapsed (S32; Yes), the control device 100 applies the second predetermined bias I6 (S33), and determines whether or not the second application time T12 has elapsed (S34). When the second application time T12 has elapsed (S34; Yes), the control device 100 applies a transfer bias corresponding to the target value I1 to the transfer roller 74K (S5), and performs the same control as in FIG.

<Third Modification>
In the embodiment, the control device 100 controls the transfer bias in the OFF state as 0. However, as shown in FIG. 8, the control device 100 performs control assuming the amount of current flowing from the photosensitive drum 51 to the transfer roller 74. It is good also as a structure.

  The control device 100 in this configuration determines the bias correction amount I7 according to the amount of current flowing between the photosensitive drum 51 and the transfer roller 74 immediately before switching from the OFF state to the ON state, and applies it to each transfer roller 74. The transfer bias to be controlled is controlled to a value obtained by adding a bias correction amount I7 corresponding to each transfer roller 74 to the first predetermined bias I5. In FIG. 8, the transfer bias commanded from the control device 100 is indicated by a solid line, and the transfer bias actually flowing through each transfer roller 74 is indicated by a broken line.

  The inflow current is a current that flows from the photosensitive drum 51 to the transfer roller 74 when the transfer bias is turned off. When the inflow current is 0, as shown in FIG. 8A, the bias correction amount I7 is determined to be 0, and the transfer bias is applied with the first predetermined bias I5. When there is an inflow current, as shown in FIG. 8B, the bias correction amount I7 is determined as the value of the inflow current amount, and is a value obtained by adding the bias correction amount I7 to the first predetermined bias I5. A transfer bias is applied.

  Here, when the transfer bias is applied at I8, the transfer bias is raised by the difference between I8 and I7. That is, the transfer bias can be raised with substantially the same delay time as when the transfer bias is raised from the state where there is no inflow current. Thereby, the rising time of the transfer bias in each transfer roller 74 can be set to substantially the same T1.

  Such a control apparatus 100 performs control like the flowchart shown in FIG. In the following description, only the transfer roller 74K will be described as a representative, and the other control is the same, and the description is omitted.

  The process up to step S2 is the same as in FIG. 4, and when a predetermined time has elapsed (S2; Yes), the control device 100 detects the inflow current of the corresponding transfer roller 74K (S301). Then, the bias correction amount I7 is determined according to the inflow current (S302). In this description, it is assumed that the inflow current is equal to the bias correction amount I7.

  When the bias correction amount I7 is determined, the control device 100 applies a transfer bias having a value obtained by adding the bias correction amount I7 to the first predetermined bias I5 to the transfer roller 74K (S303). Take control.

<Other variations>
In the above-described embodiment, the photosensitive drum 51 is exemplified as the photosensitive member. However, the present invention is not limited to this, and may be, for example, a belt-shaped photosensitive member.

  In the above-described embodiment, the transfer roller 74 is exemplified as the transfer portion. However, the present invention is not limited to this, and the transfer portion may be any transfer bias such as a conductive brush or a conductive leaf spring. .

  In the embodiment, the color printer 1 is exemplified as the image forming apparatus. However, a color complex machine or a color copier may be used.

  In the embodiment, the paper P is exemplified as the transfer medium, but an intermediate transfer belt may be used.

  In the embodiment, the predetermined time is the time from when it is detected that the leading edge of the paper P has passed the paper passing sensor 110. However, the present invention is not limited to this, and for example, the registration roller 29 feeds paper. It may be the time from the start.

  In the embodiment, the transfer bias is a positive value, but may be a negative value.

1 Color Printer 51 Photosensitive Drum 74 Transfer Roller 74C Transfer Roller 74K Transfer Roller 74M Transfer Roller 74Y Transfer Roller 100 Controller I5 First Predetermined Bias I6 Second Predetermined Bias I7 Bias Correction Amount I9 First Predetermined Bias P Paper

Claims (5)

A plurality of photosensitive drums;
A plurality of transfer portions provided corresponding to each of the photosensitive drums and transferring a developer image on the photosensitive drum onto a transfer medium;
A control unit for controlling a transfer bias applied to the plurality of transfer units,
The transfer bias is applied so that at least two transfer portions have different target values, respectively.
When the transfer bias is changed from the OFF state to the ON state, the control unit applies a transfer bias corresponding to the target value after applying a first predetermined bias having the same value. An image forming apparatus that controls the image forming apparatus.   The image forming apparatus according to claim 1, wherein the magnitude of the first predetermined bias is equal to or greater than an absolute value of all the target values.   The image forming apparatus according to claim 1, wherein the magnitude of the first predetermined bias is equal to or less than an absolute value of all the target values.   After applying the first predetermined bias, the control unit applies a second predetermined bias that is smaller than the absolute values of all the target values and has the same polarity as each of the target values, and then the target value The image forming apparatus according to claim 2, wherein the image forming apparatus is controlled to apply a transfer bias corresponding to.   The control unit determines a bias correction amount according to an amount of current flowing between the photosensitive drum and the transfer unit immediately before changing from the OFF state to the ON state, and the transfer bias applied to each transfer unit 5. The image formation according to claim 1, wherein a value obtained by adding a bias correction amount corresponding to each transfer portion to the first predetermined bias is controlled. apparatus.

Images