JP2003140405A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the staining of the backside of recording paper caused by the staining of a transfer roll due to the adhesion of toner by a toner band or the like formed by the lag of timing to apply intermediate bias in an image forming device in which the intermediate bias for electrifying and the intermediate bias for developing are switched to electrifying bias and developing bias and applied during the image non-forming. SOLUTION: This image forming apparatus is provided with a control means for performing control that bias for rising whose output value is smaller than a regular output value for transfer is applied as transfer bias before starting the transfer of a toner image. The control means performs the control that the bias for falling whose output value is smaller than the regular output value for the transfer is applied as the transfer bias after finishing the transfer of the toner image instead of or concurrently with the application of the bias for rising.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus typified by a printer, a copying machine, a facsimile, a multi-function machine or the like for forming an image formed of a toner image, and more particularly, a transfer roll of a transfer device is a toner or a carrier. The present invention relates to an image forming apparatus capable of suppressing back surface stains on a recording medium such as a recording paper, which is caused by stains due to adherence of ink.

[0002]

2. Description of the Related Art Generally, in an image forming apparatus such as a printer using an electrophotographic system, an image composed of a toner image is generally formed as follows.
That is, the image bearing surface of an image bearing member such as a rotating photosensitive member is uniformly charged by a charging device, and then the charged image bearing surface is exposed to light according to image information from a latent image forming device to be statically charged. An electrostatic latent image is formed, and the electrostatic latent image is developed by a one-component or two-component developer supplied from a developing device to be visualized as a toner image, and then the toner image is directly transferred to a recording medium by a transfer device. Or, transfer via an intermediate transfer member, and finally,
By fixing the toner image transferred onto the recording medium, an image composed of the toner image is formed on the recording medium.

Further, in such an image forming apparatus, as a charging device thereof, a charging is made by superposing a direct current and an alternating current on a contact charging member such as a charging roll which is in contact with or close to the image bearing surface of the image bearing member. There is one that uses a charging device of a type that applies a bias to perform charging.

By the way, when such a type of charging device is used, there is an advantage that a uniform and good charging potential can be obtained by applying a charging bias containing an AC component. The image bearing surface (for example, the photosensitive layer) is damaged and abraded due to repeated discharge due to the AC component at the time of charging, and as a result, the life of the image bearing member is shortened.

Therefore, in an image forming apparatus using such a charging device, in order to avoid such a problem, at the time of non-image formation in which an image composed of a toner image is not formed (for example, before the image formation is started, the image is formed). After the formation is completed, during a continuous image formation, etc.), as shown in FIG. 12a, only a predetermined DC voltage that does not include an AC component and is different from the charging bias (Vc) for the contact charging member. There is a configuration in which the intermediate charging bias (Vc-m) is switched and applied. This aims to reduce the time when the AC component is applied as much as possible to reduce the deterioration of the image bearing surface of the image bearing member and extend its life.

However, the intermediate bias for charging (Vc-
In the case where m) is applied during non-image formation, as shown in FIG. 12B, the image-bearing surface area of the image carrier corresponding to the non-image formation is charged to a certain surface potential (Vm). Therefore, it is necessary to prevent such an area of the image bearing surface from being developed by the developing device (when the toner is not supplied and attached) when passing through the developing device. Therefore, unlike the developing bias (Vd) applied between (the developer carrying member of the developing device) and the image carrying member, the developing intermediate bias (Vd-m) in which the voltage value of the DC component is changed is It is necessary to apply in synchronization with the switching timing of the charging intermediate bias (Vc-m).

[0007]

However, in such an image forming apparatus which applies the charging intermediate bias and the developing intermediate bias by switching during non-image formation,
There is a problem that the back surface of the recording medium is stained as described below.

That is, when the application of the charging bias and the intermediate bias for charging is switched, one of the alternating current (AC) and the direct current (DC) or both of the rising and falling timings are output at the normal reference timing (dotted line T). ₀
1, T ₀ 2) or a rise or fall timing at the time of direct current (DC) output at the time of switching the application between the developing bias and the developing intermediate bias is a normal reference timing (dotted line T ₀ 1 , T ₀ 2). When such a switching timing deviation occurs, the toner adheres to the image bearing surface of the image bearing member in a belt shape (in a direction parallel to the rotation axis direction of the image bearing member) corresponding to the deviation amount. "Toner Band"
A unique “carrier band” in which the carrier of the two-component developer adheres in a band shape is formed. As a result, the toner of the toner band or the carrier of the carrier band (strictly, the carrier in which the toner is attached to the surroundings) is transferred to the (primary or secondary) transfer roll of the transfer device, and then the transfer is performed. Thus, the back surface of the recording medium is transferred and adheres to the back surface of the recording medium that passes through the roll thereafter, so that the back surface of the recording medium becomes dirty.

FIG. 13 shows that the AC application switching timing of the charging bias (Vc) is deviated (the start thereof is delayed from the reference timing T ₀ 1 by α1 and the fall thereof is only from the reference timing T ₀ 2 to α2. This shows a pattern in which the toner bands (TB1, TB2) are generated due to the fact that they are accelerated. FIG. 14a shows the state of the surface potential of the image carrier and the developing bias when the AC bias rise timing of the charging bias is deviated by α1 and the toner band TB1 is generated in the above pattern, and FIG. 14b shows the charging bias. 7 shows the state of the surface potential of the image carrier and the developing bias, etc. in the case where the timing of the fall of the alternating current is shifted by α2 and the toner band TB2 is generated. Toner band T at this time
It is presumed that B1 and TB2 are formed as if the belt-shaped electrostatic latent image having a reduced surface potential is developed, and then transferred to the transfer roll side by a normal transfer action.

Further, FIG. 15 shows that the direct current application switching timing of the developing bias (Vd) is deviated (its start is advanced from the reference timing T ₀ 1 by β1 and its fall is started from the reference timing T ₀ 2). Toner band (TB3, TB4) due to β2 delay)
Shows the patterns generated respectively. FIG.
a is a toner band TB3 in which the DC rising timing of the developing bias is deviated by β1 in the above pattern.
16B shows the state of the surface potential of the image bearing member and the developing bias when the toner band T is generated. In FIG. 16B, the DC fall timing of the developing bias is shifted by β2.
It shows the state of the surface potential of the image carrier and the developing bias when B4 occurs. At this time, the toner bands TB3 and TB4 are formed so that the belt-shaped electrostatic latent image having a low surface potential is developed by the application of the intermediate bias for charging, and then the reference timing T
It is presumed that the transfer is performed to the transfer roll side by the normal transfer action by the transfer bias applied earlier and longer than ₀ 1, T ₀ 2.

On the other hand, FIG. 17 shows that the AC application switching timing of the charging bias (Vc) is deviated (its start is advanced from the reference timing T ₀ 1 by α3 and its fall is started from the reference timing T ₀ 2. Carrier band (CB1, CB) due to α4 delay
2) shows the generated patterns. FIG. 18a shows the state of the surface potential of the image carrier and the developing bias and the like when the rising timing of the alternating current of the charging bias is deviated by α3 and the carrier band CB1 is generated in the above pattern, and FIG. It shows the state of the surface potential of the image carrier and the developing bias when the AC falling timing is deviated by α4 and the carrier band CB2 is generated. The carrier bands CB1 and CB2 at this time are formed by the carrier adhering to the surface potential portion that is charged more than the normal charging potential by the AC component of the charging bias applied with a timing shift, and then transferred. It is presumed that it physically transfers and adheres to the transfer roll to which the bias is not applied.

Further, FIG. 19 shows that the application switching timing of the DC voltage of the developing bias (Vd) is deviated (the start thereof is delayed from the reference timing T ₀ 1 by β3, and the fall thereof is the reference timing T ₀ 2). From the carrier band (CB3, CB)
4) shows the generated patterns. FIG. 20a shows the state of the surface potential of the image carrier and the developing bias when the rising timing of the DC voltage of the developing bias is deviated by β3 and the carrier band CB3 is generated in the above pattern, and FIG. 20b shows the developing bias. 7 shows the state of the surface potential of the image carrier when the DC voltage falling timing is shifted by β4 and the carrier band CB4 is generated. At this time, the carrier bands CB3 and CB4 are formed by attaching the carrier together with the toner as if they were developed by the developing bias applied with a timing shift, and then by the action of the transfer bias, the transfer roll or the intermediate roll. It is presumed that the amount of electrostatic adhesion between the transfer body and the image carrier increases, and the transfer body physically adheres to the image carrier.

By the way, as a countermeasure against the back surface stain of the recording medium due to the toner contamination of the transfer roll in the image forming apparatus, a means for changing the polarity of the transfer bias applied to the transfer roll between the image area and the non-image area (Japanese Patent Application Laid-Open No. HEI-KOKAI). 7-33
No. 4015, JP-A-2000-147915, etc.) or a means for preventing the toner from adhering to the transfer roll by exhausting or sucking air from a position near the inside or outside of the transfer roll (Japanese Patent Laid-Open No. H06-6). -95519
No. gazette) is proposed. However, in any of these proposals, when the charging intermediate bias and the developing intermediate bias as described above are switched and applied at the time of non-image formation, the contamination of the transfer roll caused by the toner band or the carry band is caused. There is no particular description about means for avoiding the back surface stain of the recording medium due to the stain on the transfer roll.

The present invention has been made in view of the above circumstances, and its main object is to switch the charging intermediate bias and the developing intermediate bias to the charging bias and the developing bias during non-image formation. It is an object of the present invention to provide an image forming apparatus capable of suppressing backside contamination of a recording medium that occurs due to contamination of a transfer roll of a transfer device due to adhesion of toner or a carrier, as an image forming apparatus that applies a voltage.

[0015]

SUMMARY OF THE INVENTION An image forming apparatus of the present invention is a charging device for charging a rotating image bearing member and an image bearing surface of the image bearing member by applying a charging bias formed by superimposing direct current on alternating current. An apparatus, a latent image forming apparatus that forms an electrostatic latent image according to image information on an image bearing surface of the image bearing member charged by the charging apparatus, and an electrostatic latent image formed by the latent image forming apparatus. A developing device that develops the latent image with toner while applying a developing bias, and a transfer device that transfers the toner image developed by the developing device to a recording medium or an intermediate transfer member by a transfer roll to which a transfer bias is applied. During non-image formation in which an image composed of the toner image is not formed, a charging intermediate bias of only direct current different from the charging bias is switched and applied to the charging device, and the developing bias is applied to the developing device. Is a control for applying a rising bias having an output value smaller than the normal transfer output value as the transfer bias before starting the transfer of the toner image in an image forming apparatus that switches and applies different developing intermediate biases, And a control means for performing one or both of control for applying a falling bias having an output value smaller than the normal transfer output value as the transfer bias after the transfer of the toner image is completed. It is what

Here, before the transfer of the toner image is started,
It is a time before the transfer timing corresponding to the reference start timing of the latent image formation by the latent image forming device, and after the transfer of the toner image is the recording medium size of the latent image formation by the latent image forming device. Is a time later than the transfer timing corresponding to the reference end timing determined according to the above. The developing bias is usually a voltage formed by superimposing an alternating current on a direct current, but may be a voltage of only a direct current voltage. The transfer device is either one or both of a primary transfer device when an intermediate transfer member is not used and a secondary transfer device when an intermediate transfer member is used. The transfer bias is preferably a bias applied by constant current control, but is not particularly limited to this and may be a bias applied by constant voltage control.

According to the image forming apparatus of the invention as described above,
As described above, the timing of the rise and fall of the alternating current when switching the application between the charging bias and the charging bias deviates from the normal reference timing, or the application between the developing bias and the intermediate bias for development. Before the toner band or carrier band reaches the transfer roll, even if a specific toner band or carrier band may be generated due to the deviation of the DC start-up or turn-off timing from the normal reference timing during switching. Before or after the transfer of the toner image, which includes the time when the toner image has finished passing through, or both, by the control operation of the control means, the bias for raising the output value smaller than the normal transfer output value. Is applied or the output value is smaller than the normal transfer output value. For lowering the bias is applied. As a result, the electrostatic action of the toner band or the carrier band due to the transfer bias (a transfer roll (or an intermediate transfer member) is pressed against the image bearing member or the like due to the electrostatic action and the contact range is increased to cause a physical effect). (Including various adhesion effects), the transfer and adhesion to the transfer roll side is reduced. As a result, backside contamination of the recording medium due to contamination of the transfer roll is suppressed.

In addition, the control device in the image forming apparatus of the present invention can more reliably realize the reduction of transfer and adhesion of the toner band and the carrier band to the transfer roll before the start of the transfer of the toner image. It is preferable that the initial bias value applied at the beginning of the raising bias is controlled to be 60% or less (more preferably 30% or less) of the normal transfer output value.

In the control device configured as described above,
When the gradually increasing rising bias is applied as the rising bias and gradually increases by gradually or continuously, the increasing width of the gradually rising bias is set within 30% of the normal transfer output value. It is preferable to configure so as to perform the control. As a result, when the increase width is set to a large value exceeding 30% of the normal transfer output value, the image is generated by applying a rising bias (a bias having a polarity opposite to the charging bias) that greatly increases and changes. A transfer history difference of reverse polarity occurs on the carrying surface of the carrier, and density unevenness of the image easily occurs due to the history difference, but the occurrence of such image density unevenness is prevented.

Further, when the controller for the image forming apparatus of the present invention applies the gradually decreasing falling bias, which gradually changes and gradually changes as the falling bias, the gradually decreasing falling bias is applied. It is preferable that the control is performed so that the width of decrease of 30% is within 30% of the regular transfer output value. As a result, when the decrease width is set to a large value exceeding 30% of the normal transfer output value, as in the case of applying the gradual increase type rising bias, the falling bias (the falling bias which greatly changes) is changed. The application of a bias having a polarity opposite to that of the charging bias causes a history difference in transfer having a reverse polarity on the carrying surface of the image carrier, and the density difference in the image is likely to occur due to the history difference. Will be prevented.

In the control device configured as described above,
From the viewpoint of more reliably realizing the reduction of transfer and adhesion of the toner band and the carrier band to the transfer roll after the transfer of the toner image, the final output value of the falling bias to be finally applied is a normal transfer. Output value of 60
The output value may be controlled to be not more than% (more preferably not more than 30%). The final output value finally applied is the output value of the bias applied at the final stage in the case of a gradual-decrease type falling bias. If necessary, a bias having an output value of 60% or less of the normal transfer output value may be applied as the falling bias.

Further, the control device in the image forming apparatus as described above, the application start timing of the rising bias is switched to the application timing from the charging intermediate bias to the charging bias or from the developing intermediate bias to the development. It is preferable that the control is performed in synchronism with the timing of switching to the bias application. In this case, the carrier band generated before each of the above timings can be surely prevented from being transferred and attached to the transfer roll by electrostatic action. On the other hand, transfer adhesion of the toner band and the carrier band to the transfer roll, which occurs after the above timings and before the start of the transfer of the toner image, can be reliably prevented.

Further, the control device in the image forming apparatus as described above is arranged such that the application timing of the application of the falling bias is switched to the application of the charging bias to the intermediate bias for charging, or the developing bias to the intermediate for development. It is preferable that the control is performed in synchronism with the timing of switching to the bias application. In this case, the carrier band generated after each of the above timings can surely prevent transfer and adhesion due to electrostatic action on the transfer roll. On the other hand, transfer adhesion of the toner band and the carrier band to the transfer roll after the start of the transfer of the toner image and before the above timings can be reliably prevented.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] FIGS. 1 and 2 show a color image forming apparatus according to Embodiment 1 of the present invention, and FIG. 1 is a schematic view showing the entire structure thereof. FIG. 2 is a sectional view of the main part.

This color image forming apparatus has four image forming units 10Y, 10M and 10C for exclusively forming toner images of four colors of yellow (Y), magenta (M), cyan (C) and black (K). Equipped with 10K,
The four color toner images formed in each of the image forming units 10 (Y, M, C, K) are primarily transferred so as to be superposed on the surface of the intermediate transfer belt 20, and then the recording medium is transferred from the intermediate transfer belt 20. It is a color image forming apparatus capable of forming a so-called full-color image by secondary transfer to the recording paper P side.

Each of the image forming units 10 forms each of the toner images of the four colors by an electrophotographic process, and is located at a position below the intermediate transfer belt 20 in the substantially central portion of the apparatus main body 1. It is arranged in a horizontal state with a gap. Further, in each of the image forming units 10, basically, a photosensitive drum 11 in which an organic photosensitive layer or the like is formed on an outer peripheral surface of a drum base body which is rotationally driven at a predetermined speed in an arrow direction, and the photosensitive drum 11 are provided. Contact type charging device 12 that uniformly charges the surface of the photosensitive drum 11, a latent image forming device 13 that forms an electrostatic latent image by exposing the surface of the photosensitive drum 11 after charging to light according to image information, and A developing device 14 that develops the electrostatic latent image with a developer of a predetermined color, a roll-type primary transfer device 15 that electrostatically transfers the toner image formed by the development to the intermediate transfer belt 20, and a photosensitive drum. 11
Similarly, a blade type drum cleaning device 16 for cleaning the surface of the device is provided.

Among them, the charging device 12 is, as shown in FIG. 2, a charging roll 12a arranged so as to be in contact with the photosensitive drum 11 and rotatable, and a charging polarity of the toner with respect to the charging roll 12a. The charging power supply device 12b is configured to apply a charging bias (Vc) composed of a voltage obtained by superimposing an alternating current on a polar direct current.

The latent image forming device 13 is housed in a housing hermetically sealed box 17 disposed below the four image forming units 10, and is separated for each color from four semiconductor lasers (not shown). After the laser beams LB-Y, LB-M, LB-C, and LB-K emitted according to the image signal are applied to the rotary polygon mirror 18 via a predetermined optical system (lens or the like) to perform deflection scanning, Scanning exposure is performed from diagonally below on each of the photosensitive drums 11 that face each other through a plurality of reflecting mirrors (not shown). Reference numeral 17a in the drawing denotes a light emitting surface portion provided with a transparent glass surface for transmitting and outputting the laser light LB from the inside of the housing hermetically sealed box 17 toward the photosensitive drum 11 of each image forming unit 10.

Further, each developing device 14 uses a two-component developer composed of a non-magnetic toner and a magnetic carrier as the developer G, and as shown in FIG. The developer G is accommodated in the circulation conveyance path portion 41 and is agitated and conveyed by two developer agitating and conveying members (eg, augers) 42 and 43 which rotate in a parallel state, and finally, the photosensitive drum of the housing 40 thereof. 1
The toner is supplied to the developing roller 45 which is partially exposed at the opening 44 facing the first roller 1 and is rotationally driven. The developing roll 45 is provided with a developing power supply device 46 for applying a developing bias composed of a voltage obtained by superimposing an alternating current on a direct current.

Further, the primary transfer device 15 has a transfer roll 15a arranged so as to be in contact with the photosensitive drum 11 so as to be rotatable with the intermediate transfer belt 20 interposed therebetween, and a charging polarity of toner with respect to the transfer roll 15a. The primary transfer power supply device 15b applies a transfer bias (It) composed of a DC voltage of opposite polarity. Primary transfer power supply 1
5b is configured to apply a transfer bias (It) having a predetermined output value by a constant current control method.

In each such image forming unit 10, the surface of the photosensitive drum 11 rotating in the direction of the arrow is uniformly charged by the charging device 12, and then the charged surface is colored by the latent image forming device 13. An electrostatic latent image is written by scanning and exposing the laser beam LB corresponding to the decomposed image signal through the light emitting surface portion 17a, and thereafter, the latent image is supplied from the developing device 14 through the developing roller 45. The toner image of a predetermined color is developed by the toner T of the two-component developer G, and finally, the toner image of the intermediate transfer belt 20 passing between the photosensitive drum 11 and the transfer roll 15a of the primary transfer device. The primary transfer is performed on the surface in sequence. Photosensitive drum 1 after this primary transfer
The surface of No. 1 is cleaned by removing residual toner and the like by the blade of the drum cleaning device 16.

Next, the intermediate transfer belt 20 has a two-layer structure in which a surface layer such as silicone rubber is laminated on a belt base material such as a polyimide film having a volume resistivity adjusted by containing a conductive material or the like. An endless belt, which is stretched around a driving roll 21, a backup roll 22 of a secondary transfer unit, a plurality of driven rolls 23, 24, ... And between the driven rolls 23, 24, each of the image forming units. The photosensitive drums 11 are arranged so as to be in contact with the respective photosensitive drums 11, and are rotated in the direction of arrow B by the rotational power of the drive roll 21.

Around the intermediate transfer belt 20, there is provided a secondary transfer roll 25 which presses the intermediate transfer belt 20 against the backup roll 22 to form a pressure contact area of the secondary transfer portion. Further, a secondary transfer bias composed of a DC voltage having the same polarity as the charging polarity of the toner image on the intermediate transfer belt 20 is supplied to the conductive shaft of the backup roll 22 from a secondary transfer power supply device (not shown). The power supply member 27 for the above is disposed so as to be in contact. further,
A blade-type belt cleaning device 26 that cleans the surface of the intermediate transfer belt 20 is disposed at a position facing the drive roll 21 of the intermediate transfer belt 20. A mark detection sensor (not shown) for detecting the home position mark attached to the inner peripheral surface of the belt is provided.

On such an intermediate transfer belt 20 (multiplex)
The transferred toner image is conveyed to the secondary transfer portion facing the secondary transfer roll 25 arranged facing the backup roll 22 as the belt 20 rotates. Then, in the secondary transfer portion, the toner image on the intermediate transfer belt 20 is transferred from the paper storage tray 30 provided below the latent image forming device 13 in accordance with the image formation of the toner image and the secondary transfer timing. It is electrostatically secondarily transferred to the recording paper P of a predetermined size which is sent out by the paper feeding mechanism 31 and is conveyed to the secondary transfer portion through the paper conveying path in which the registration rolls 32 and the like are arranged. The surface of the intermediate transfer belt 20 after the secondary transfer is cleaned by the belt cleaning device 2
Residual toner and the like are removed by the blade 6 and cleaned. The dashed-dotted line in FIG. 1 indicates the conveyance path of the recording paper P.

The recording paper P on which the toner image is transferred from the intermediate transfer belt 20 is naturally peeled off from the intermediate transfer belt 20 when passing through the secondary transfer portion, and then the fixing device 3 is used.
The heating roll 36 and the pressure member (pressure pad, pressure roll, etc.) 37 in FIG. 5 are sent so as to pass through the pressure contact area, and the heat fixing process is performed. The sheet is ejected to the sheet ejection accommodating section 1a). Through the above process, a desired full-color image or the like is formed on the recording paper P.

In this color image forming apparatus, as shown in FIG. 2, the charging power supply unit is controlled by the system control unit 50 including a microcomputer or the like for overall management and control of the operation of each part in the apparatus. 12b, the developing power supply device 46, and the primary transfer power supply device 15b, the following special control is performed.

First, regarding the charging power supply device 12b,
As shown in FIG. 3a, during non-image formation (image formation start
Images in continuous image formation before and after image formation is completed
AC component, etc. (when passing through the inter-image part)
An intermediate charging bias that consists only of a specified DC voltage that does not include
As (Vc-m) is disconnected from the above-mentioned charging bias (Vc).
It is controlled so that it is applied to the charging roll 12a in exchange.
It At this time, as the charging bias, for example, −750 to −
Peak voltage 1.7 to 2.0k for 800V DC
V _pp, Applying a voltage that superimposes alternating current with a frequency of 614 Hz
However, as the intermediate bias for charging, for example, -900 to -1
A DC voltage of 000V is applied.

Further, in the developing power supply device 46, as shown in FIG. 3a, in response to switching and application of the charging intermediate bias during non-image formation, the voltage of the DC component is also applied during non-image formation. The developing intermediate bias (Vd-m) whose value is changed is the above-mentioned developing bias (Vd).
It is controlled so as to be switched to and applied to the developing roll 45. The switching application of the developing intermediate bias is set to be performed in synchronization with the switching application timing of the charging intermediate bias described above. At this time, as the developing bias, for example, a voltage in which a peak-to-peak voltage of 1.2 kV and an alternating current having a frequency of 6 Hz are superimposed on a direct current of -600 V is applied, and as an intermediate bias for development, the same alternating current is applied to a direct current of -240 V in the developing bias. The voltage superimposed on is applied.

Further, in the primary transfer power supply device 15b, as shown in FIG. 3a, an output value smaller than the regular transfer output value (TVh) is set as the primary transfer bias (It) before the transfer of the toner image is started. Is controlled so as to apply a rising bias (It-sm). In this case, the transfer start of the toner image is at least before the transfer timing synchronized with the reference start timing (T _S ) of the latent image formation by the latent image forming device 13 (may be the same).
It is time to. Reference start timing of latent image formation (T
_S ) is determined, for example, by receiving an image signal emitted from a predetermined control device.

In this example, the rising bias (It-
sm) is set to apply a gradually increasing start-up bias that gradually increases so as to change stepwise as shown in FIG. Specifically, a rising bias (It-sm1, It-sm2) that gradually increases in two steps is applied. Further, the first-stage rising bias (It-sm1) applied first is controlled to have an output value (TVm1) that is 60% or less of the regular transfer output value (TVh). At this time, the application time of the first-stage rising bias and the application time of the second-stage rising voltage are equal.

Moreover, this startup bias (It-
sm), that is, the first-stage rising bias (It-
sm1) is a reference timing (T ₀ that switches from application of the intermediate charging bias (Vc-m) to application of the charging bias (Vc) at the time of transition from non-image formation to image formation.
It is controlled so as to be applied in synchronization with 1). In addition, the startup bias (It
-Sm) is the increase width ΔX [= (rise voltage of the second stage: It-sm2)-(rise voltage of the first stage: It-
sm1)] is controlled to be within 30% of the regular transfer output value (TVh).

In this color image forming apparatus, when an image forming command is issued, the control device 50 causes the image forming operation to be started before or after the completion of the image forming operation and when the image forming apparatus is passing through the inter-image portion. Charging power supply device 15
b to the charging roll 15a, a charging intermediate bias (Vc-
m) is applied (so that it can be switched with the charging bias).

As a result, the image bearing surface (organic photosensitive layer) of the photosensitive drum 11 is charged to, for example, -720 V by the charging bias (Vc) applied during image formation.
At the time of non-image formation, a charging intermediate bias (Vc-m) having no AC component is applied to be charged to, for example, -360V (see FIG. 12b). As a result, the deterioration of the image bearing surface of the photosensitive drum 11 due to the discharge of the AC component during charging is reduced at least by the amount of the intermediate charging bias applied, and the life thereof is extended.

During this non-image formation, the controller 5
When 0, the reference timing T _0, which is the application switching timing of the developing intermediate bias (Vd-m) from the developing power supply device 46 to the developing roll 45, is set.
1, is applied in synchronization with T ₀ 2 (so as to be switched to the developing bias Vd). As a result, a developing electric field is formed between the photosensitive drum 11 and the developing roll 45 at a level at which the toner on the developing roll does not move to the photosensitive drum side due to the application of the developing intermediate bias. It is avoided that the developing device 14 develops the area portion of the photosensitive drum 11 which is charged at the time of application (see FIG. 12b).

Next, when the image formation is started, the control device 50 switches the charging bias from the charging power source device 12b to the charging roll 12a and applies it, and at the same time, from the developing power source device 46 to the developing roll 45. The developing bias is switched and applied. Moreover, in synchronization with this switching timing, the primary transfer power supply device 15
b from the primary transfer roll 15a to the primary transfer bias (I
A rising bias (It-sm) of t) is applied. Strictly speaking, the first-stage rising bias (It-sm1) starts to be applied in synchronization with the reference timing T ₀ 1, and the second-stage rising bias (It-sm2) starts to be applied after a predetermined time elapses. It is continuously applied. At this time, a bias (TVh) having a constant current controlled output value of 14 μA is applied as the regular transfer bias, and a constant current control of 7 μA is performed as the first-stage rising bias (It-sm1). Output value bias (TVm
1) is applied, and the second-stage rising bias (It-
As sm2), for example, a bias (TVm2) having an output value controlled to a constant current of 10.5 μA is applied.

The application of the rising bias (It-sm) is performed before the toner image formed on the photosensitive drum 11 through the above-described image forming process by the image forming command is transferred to the intermediate transfer belt 20 ( In this example, the process is finished up to the stage of slightly before the reference start timing T _S of latent image formation). After that, the regular primary transfer bias (It) is continuously applied. By applying the primary transfer bias (It), the toner image on the photosensitive drum 11 is primarily transferred to the intermediate transfer belt 20. The toner image on the intermediate transfer belt 20 is secondarily transferred to the recording paper P at the secondary transfer portion as described above (FIG. 3).

In this color image forming apparatus, since the rising bias (It-sm) of the primary transfer bias (It) is applied as described above, the charging bias and the charging intermediate bias are temporarily provided. The timing of the rise of the AC voltage at the time of switching the application is deviated from the regular reference timing T ₀ 1, or the timing of the rise of the DC voltage at the time of switching the application between the developing bias and the developing intermediate bias is the regular reference timing. T ₀
Due to the deviation from 1, the toner bands (TB1, TB3) and the carrier bands (CB1, CB) as described above are
Even when 3) is formed on the photosensitive drum 11,
As shown in FIG. 4, the toner bands (TB1, TB
3) and carrier bands (CB1, CB3) are less likely to transfer and adhere to the intermediate transfer belt 20.

For example, the toner band TB1 generated due to the timing shift (α1) as shown in FIG.
Since the timing of reaching and passing through the primary transfer portion overlaps with the timing of applying the start-up bias (It-sm) having an output value smaller than the output value of the regular transfer bias, the primary transfer roll 15a and A transfer electric field that is weaker than the normal transfer electric field formed between the photosensitive drums 11 is formed, so that the transfer electric field is less likely to be transferred to the intermediate transfer belt 20 side. Further, the toner band TB3 generated due to the timing shift (β1) as shown in FIG. 15 is applied with a start-up bias (It-sm) when it reaches and passes through the primary transfer portion. Since it is before, the transfer electric field is not formed and is not affected by the transfer electric field, so that the transfer electric field is not transferred to the intermediate transfer belt 20 side.

On the other hand, the carrier band CB1 generated due to the timing shift (α3) as shown in FIG.
Since the timing of reaching and passing through the primary transfer portion is before the start-up bias (It-sm) is applied, the transfer electric field is not formed and is not affected by the effect. As the primary transfer roll 15a is strongly pressed against the photosensitive drum 11 side, the intermediate transfer belt 2
The toner on the photosensitive drum 11 becomes a state in which the toner image 0 and the photosensitive drum 11 come into close contact with the photosensitive drum 11 a little wider.
Since the action of physically adhering to the intermediate transfer belt 20 does not increase, it is not transferred to the intermediate transfer belt 20 side. Further, the carrier band CB3 generated due to the timing shift (β3) as shown in FIG. 19 has a rising bias (It-sm) when the carrier band CB3 reaches and passes through the primary transfer portion.
Is applied, the transfer electric field is not formed and is not transferred to the intermediate transfer belt 20 side by the action (electrostatic repulsive force) of the electric field.

Therefore, in this image forming apparatus, the toner and the carrier forming the toner band and the carrier band hardly adhere to the intermediate transfer belt 20, so that the intermediate transfer belt 20 is used to transfer the toner and carrier to the secondary transfer section. There is almost no transfer and adhesion to the secondary transfer roll 25. As a result, the secondary transfer roll 25 becomes dirty due to the adhesion of the toner and the carrier, and the toner and the carrier are transferred and adhered to the back surface of the recording paper P that subsequently passes through the secondary transfer roll 25, so that the recording paper. The occurrence of the problem that the back surface of P becomes dirty is suppressed.

The rising bias (It-s
With regard to m), the increase width ΔX is suppressed to within 30% (about 25% in this example) of the regular transfer output value (TVh). Therefore, the voltage of the polarity opposite to the charging bias is applied to the photosensitive drum 11. Does not cause unevenness in image density due to the transfer history that is caused by the application of a large variation, and there is a possibility that image quality may be deteriorated (unevenness in density) due to the influence of the application of the bias for start-up. Absent.

[Second Embodiment] FIG. 5 shows a second embodiment of the present invention. In the image forming apparatus according to this embodiment, the controller 50 (FIG. 2) sets the regular transfer output value as the primary transfer bias (It) after the transfer of the toner image is finished, in place of the start-up bias described above. The color image forming apparatus has the same configuration as the color image forming apparatus according to Embodiment 1 except that a falling bias (TV-em) having a voltage value smaller than (TVh) is applied after the transfer of the toner image is completed. Is. The end of the transfer of the toner image is a time after the transfer timing that is synchronized with the end timing (T _E ) determined according to the size of the recording paper P for forming the latent image by the latent image forming device 13. The end timing (T _E ) of the latent image formation at this time is determined, for example, by receiving an image signal emitted from a predetermined control device.

In this example, the control device 50 gradually decreases the transfer power source 15b to the primary transfer roll 15a as a falling bias (It-em) so that it gradually changes as shown in FIG. It is set to apply a rising bias. Specifically, the falling biases (It-em1, It-em) are gradually reduced in two steps.
2) is applied. In addition, the falling bias (It-em) that changes stepwise and gradually decreases has a decreasing width ΔY.
[= (Falling voltage of the first step: It-em2)-(falling voltage of the second step: It-em1)] is controlled to fall within 30% of the regular transfer output value (TVh). It The finally applied second-stage falling bias (I
t-em2) is the regular transfer output value (TVh) of 60.
The voltage value (TVm1) is controlled to be less than or equal to%.
At this time, the application time of the falling bias of the first step and the application time of the falling bias of the second step are the same.

Moreover, this falling bias (It-
em), the second-stage start-up bias (It-e
The application of m2) is synchronized with the reference timing (T ₀ 2) at which the application of the charging bias (Vc) is switched to the application of the intermediate bias for charging (Vc-m) during the transition from the image formation to the non-image formation. Controlled to finish.

Then, in this color image forming apparatus, the image forming operation and the like are performed in the same manner as in the case of the image forming apparatus according to the first embodiment. The operation contents different from those in the first embodiment will be described below.

That is, non-image formation is completed after image formation is completed.
When the time comes, the control device 50 causes
As described, the charging power supply device 15b is connected to the charging roll 1
When an intermediate charging bias (Vc-m) is applied to 5a,
Both for developing from the developing power supply device 46 to the developing roll 45
An intermediate bias (Vd-m) is applied.
To form a latent image corresponding to the transfer end timing of the toner image
End timing (T _E), Power supply for primary transfer
The primary transfer roll 15a is transferred from the device 15b to the primary transfer roll 15a.
Bias (It-em) for falling of As (It) is output.
Forced and applied.

Strictly speaking, the end timing (T
_E ) in synchronization with the first-stage falling bias (It-e
m1) begins to be applied, and after the required time has elapsed, the second-stage falling bias (It-em2) is continuously applied.
At this time, for example, the first-stage fall bias is 1
A bias (TVm3) having a constant current controlled output value of 0.5 μA is applied, and the second-stage falling bias (It-em2) is, for example, a bias having a constant current controlled output value of 7. (TVm4) is applied.
Also, application of the second-stage start-up bias (It-em2) in synchronization with ends with reference timing of switching the application of the charging for the intermediate bias _{(Vc-m) (T 0} 2).
This completes the image forming operation.

In this color image forming apparatus, since the bias for lowering the primary transfer bias (It) (It-em) is applied as described above, it is assumed that the charging bias and the intermediate bias for charging are temporarily set. or shift the timing of the deactivation of the AC voltage is supplied, the switching from the reference timing T ₀ 2 regular, or developing bias and reference timing timing of deactivation is a regular DC voltage is supplied, switching between the developing intermediate bias T ₀
Due to the deviation from 2, the toner bands (TB1, TB3) and the carrier bands (CB1, CB) as described above are
Even when 3) is formed on the photosensitive drum 11,
As shown in FIG. 6, the toner band (TB2, TB
4) and carrier bands (CB2, CB4) are less likely to transfer and adhere to the intermediate transfer belt 20.

For example, the toner band TB2 generated due to the timing shift (α2) as shown in FIG.
Since the timing of reaching and passing through the primary transfer portion overlaps with the timing of applying the falling bias (It-em) having an output value smaller than the output value of the regular transfer bias, the primary transfer roll 15a and A weaker transfer electric field than the normal transfer electric field formed between the photosensitive drums 11 is formed, and as a result, the transfer electric field is less likely to be transferred to the intermediate transfer belt 20 side. The toner band TB4 generated due to the timing shift (β2) as shown in FIG. 15 reaches the primary transfer portion and passes therethrough after the application of the falling bias (It-em) is completed. Therefore, the transfer electric field is not formed and is not affected, so that it is not transferred to the intermediate transfer belt 20 side.

On the other hand, the carrier band CB2 generated due to the timing shift (α4) as shown in FIG.
Since the timing of reaching and passing through the primary transfer portion is before the application of the rising bias (It-sm), the transfer electric field is formed and the action (electrostatic repulsive force) by the electric field is generated. When it is received, it is not transferred to the intermediate transfer belt 20 side. Further, the carrier band C generated due to the timing shift (β4) as shown in FIG.
In B4, since the timing of reaching and passing through the primary transfer portion is after the application of the rising bias (It-sm), the transfer electric field is not formed and is not affected by the action. Due to this action, the primary transfer roll 15a is strongly pressed against the photosensitive drum 11 side, and the intermediate transfer belt 20 comes into a state in which the intermediate transfer belt 20 and the photosensitive drum 11 are in a slightly wider contact state, and the toner on the photosensitive drum 11 is intermediately transferred. Since the action of physically adhering to the belt 20 does not greatly act, it is not transferred to the intermediate transfer belt 20 side.

Therefore, also in this image forming apparatus, the toner and the carrier forming the toner band and the carrier band hardly adhere to the intermediate transfer belt 20, so that the secondary transfer portion is passed through the intermediate transfer belt 20. Therefore, it is hardly transferred and attached to the secondary transfer roll 25. As a result, the secondary transfer roll 25 becomes dirty due to the adhesion of the toner and the carrier, and the toner and the carrier are transferred and adhered to the back surface of the recording paper P that subsequently passes through the secondary transfer roll 25, so that the recording paper. The occurrence of the problem that the back surface of P becomes dirty is suppressed.

Further, the above-mentioned falling bias (It-e
Regarding m), the decrease width ΔY is suppressed to within 30% (about 25% in this example) of the regular transfer output value (TVh). Therefore, in the case of applying the rising voltage in the first embodiment, Similarly, the unevenness of the image density due to the transfer history does not occur, and the deterioration of the image quality (uneven density) due to the influence of the application of the falling bias is not caused.

[Third Embodiment] FIG. 7 shows a third embodiment. In the image forming apparatus according to this embodiment, the control device 50 (FIG. 2) uses the primary transfer bias (It) as the primary bias (It-sm) in the first embodiment and the secondary bias in the second embodiment. The color image forming apparatus has the same configuration as the color image forming apparatus according to Embodiments 1 and 2 in which the bias (It-em) is applied together.

In this case, it is assumed that the charging bias and the charging are
Of the AC voltage when switching the application between the intermediate biases for
The standard timing for starting and stopping
Gu T ₀1, T₀Deviation from 2 or development bias
DC when switching the application between the developing and intermediate bias
The timing of voltage rise and fall is a standard reference
Imming T₀1, T₀Due to the deviation from 2,
Toner band (TB1 to TB4) and carrier
Bands (CB1 to CB4) are formed on the photosensitive drum 11.
The toner band or carrier band
Any of the dots will be transferred and attached to the intermediate transfer belt 20.
Can be hardened. As a result, such a toner van
For recording due to the formation of magnetic field or carrier band
It is possible to more reliably prevent the back surface of the paper P from being soiled.
It

[Other Embodiments] In the first and third embodiments, the case has been described in which the gradually increasing rising bias is applied as the rising bias (It-sm), which gradually changes and gradually increases. Alternatively, as illustrated in FIG. 8, a gradually increasing rising bias (It-sm5, It-sm6) that continuously changes and gradually increases may be applied. The gradually increasing start-up bias (It-sm5) shown in FIG. 8a is of a type that gradually increases at a plurality of different increasing rates (inclination of the solid line rising upward in the figure) and gradually increases continuously in a curvilinear manner as a whole. Further, the gradually increasing start-up bias (It-sm6) shown in FIG. 7B is of a type that gradually increases at a constant rate of increase and linearly and continuously increases as a whole. It is preferable to set the output value at the time of initial application of any of the gradual increase type biases to 60% or less of the regular transfer output value (TVh).

Further, in the first and third embodiments, the rising bias (It-sm) is set as the rising bias (It-sm) which is started in only one stage as illustrated in FIG. 9A.
It may be configured to apply m7). The one-stage rising bias (It-sm7) is set to an output value that is at least 60% or less of the regular transfer output value (TVh).
In addition, as the bias (It-sm) for the start-up,
As illustrated in FIG. 7B, the rising bias (It-sm) is applied even before the reference timing T ₀ 1, which is the application switching timing of the charging intermediate bias.
8) may be applied. In this case, the rising bias (It-sm8) is not limited to the type that changes stepwise and gradually increases as shown in the figure, but the type that continuously changes and gradually increases as illustrated in FIG. May be

On the other hand, in the second and third embodiments, the case of applying the gradually decreasing rising bias which gradually changes and gradually decreases as the falling bias (It-em) has been described. As described above, the bias gradually decreases (It-em).
5, It-em6) may be applied.
The gradually decreasing start-up bias (It-em) shown in FIG.
5) is of a type in which it gradually decreases at a plurality of different reduction rates (the slope of the downward-sloping solid line in the figure) and gradually decreases continuously in a curvilinear manner as a whole. The start-up voltage (It-em6) is of a type that gradually decreases at a constant decrease rate and gradually decreases linearly as a whole. It is preferable to set the finally applied output value of any gradual rising bias to 60% or less of the regular transfer output value (TVh).

Further, in the second and third embodiments, as the falling bias (It-em), the falling bias (It-em) that is lowered in only one stage is used as illustrated in FIG. 11a.
It may be configured to apply em7). The one-step fall bias (It-em7) is set to an output value that is at least 60% or less of the regular transfer output value (TVh).
Also, as the startup bias (It-em),
As illustrated in FIG. 7B, the falling bias (It-em) is also applied at a timing after the reference timing T ₀ 2 which is the application switching timing of the charging intermediate bias.
8) may be applied. In this case, the falling bias (It-em8) is not limited to the type that changes stepwise and gradually decreases as shown in the figure, and the type that continuously changes and gradually decreases as illustrated in FIG. May be

Further, in the first to third embodiments, when the primary transfer bias is applied, a special start-up bias (It-
sm) and at least one of the falling bias (It-em) are applied to the primary transfer roll 15a,
Intermediate transfer belt 2 for the toner band or carrier band
The case where the transfer and adhesion to 0 is prevented and the back surface stain of the recording paper P generated on the secondary transfer roll 25 is suppressed has been described. However, the generated back surface stain of the recording paper P is as described above. Applying a rising bias and a falling bias (instead of applying the primary transfer roll 15a or together with the application to the primary transfer roll 15a) to the secondary transfer roll 25 as when applying the primary transfer bias. It may be configured so as to suppress it.

Further, in each of the above-mentioned embodiments, the case where it is applied to the color image forming apparatus using the intermediate transfer belt 20 is illustrated, but in the present invention, the intermediate transfer system is not used, and the toner image is transferred from the photosensitive drum. Needless to say, it may be applied to an image forming apparatus of the type that directly transfers the recording paper P.

[0071]

As described above, according to the present invention,
Since either one or both of the special rising bias and the falling bias are applied when the transfer voltage is applied, the application timing of the charging intermediate bias and the developing intermediate bias that are switched and applied during non-image formation It is possible to prevent the transfer roll from being contaminated due to the toner band and the carrier band formed by the shift. As a result, it is possible to suppress the back surface stain of the recording medium that may be generated by coming into contact with and passing through the transfer roll.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a color image forming apparatus according to a first embodiment.

FIG. 2 is a principal part explanatory view showing a configuration of a control system of a charging power source, a developing power source, and a primary transfer power source.

FIG. 3A is a timing chart showing the application timing and state of each bias, and FIG. 3B is an explanatory diagram showing the configuration of a rising bias at the time of primary transfer.

FIG. 4 is an explanatory diagram showing a state on an intermediate transfer belt and a recording sheet when the electric bias application in FIG. 3 is performed.

FIG. 5 shows a main part of the configuration of the second embodiment,
(A) is a timing chart showing the application timing and state of each bias, and (b) is an explanatory view showing the configuration of the falling bias at the time of primary transfer.

FIG. 6 is an explanatory diagram showing a state on an intermediate transfer belt or recording paper when the bias application of FIG. 5 is performed.

FIG. 7 shows a main part of the configuration of the third embodiment,
(A) is a timing chart showing the application timing and state of each bias, and (b) is an explanatory diagram showing a configuration of a rising bias and a falling bias at the time of primary transfer.

FIG. 8 is an explanatory diagram showing another configuration example of the gradually increasing start-up bias.

FIG. 9 is an explanatory diagram showing another configuration example of the rising bias.

FIG. 10 is an explanatory diagram showing another configuration example of the gradual-decrease-type falling bias.

FIG. 11 is an explanatory diagram showing another configuration example of the falling bias.

FIG. 12A is a timing chart showing the timing and state of applying each bias when the charging intermediate bias and the developing intermediate bias are switched and applied during non-image formation, and FIG. 12B.
6A and 6B are explanatory views showing the states of the surface potentials of the photoconductor and the developing voltages during non-image formation and during image formation.

FIG. 13 is an explanatory diagram showing a toner band generation process that occurs at each timing shift in switching application of the charging intermediate bias.

FIG. 14 is an explanatory diagram showing a surface potential state of a photoconductor and a developing bias application state in the case where the timing shifts of FIG. 13 occur.

FIG. 15 is an explanatory diagram showing a toner band generation process that occurs due to a shift in each application timing in switching application of the developing intermediate bias.

16 is an explanatory diagram showing the surface potential state of the photoconductor and the application state of the developing bias when the timing shifts of FIG. 15 occur.

FIG. 17 is an explanatory diagram showing a generation process of a carrier band generated at each timing shift in switching application of the charging intermediate bias.

FIG. 18 is an explanatory diagram showing the surface potential state of the photoconductor and the application state of the developing bias when the timing shifts in FIG. 17 occur.

FIG. 19 is an explanatory diagram showing a generation process of a carrier band generated at each application timing deviation in switching application of the developing intermediate bias.

20 is an explanatory diagram showing the surface potential state of the photoconductor and the application state of the developing bias when the timing shifts of FIG. 19 occur.

[Explanation of symbols]

11 ... Photosensitive drum (image carrier), 12a ... Charging roll (part of charging device), 12b ... Charging power supply device (part of charging device), 14 ... Developing device, 15a ... Primary transfer roll (transfer device) 15b ... Primary transfer power supply device (part of transfer device), 20 ... Intermediate transfer belt (intermediate transfer member), 25 ... Secondary transfer roll (part of transfer device), 46
... power supply device for development, 50 ... control device (control means), P ...
Recording paper (recording medium), X ... increase width, Y ... decrease width.

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H027 DA38 EA01 EA03 EA05 EC20                       EF09 EF13                 2H073 AA01 AA10 BA02 BA04 BA09                       BA21                 2H200 FA08 FA18 HA02 HA29 JA02                       JA28 JA29 JC02 NA02 NA06                       PA03 PA10

Claims (7)

[Claims] 1. A rotating image bearing member, a charging device for charging the image bearing surface of the image bearing member by applying a charging bias formed by superimposing a direct current and an alternating current, and the charging device charged by the charging device. A latent image forming device that forms an electrostatic latent image according to image information on the image bearing surface of an image bearing member, and an electrostatic latent image formed by this latent image forming device is developed with toner while applying a developing bias. And a transfer device for transferring the toner image developed by the developing device to a recording medium or an intermediate transfer member by a transfer roll to which a transfer bias is applied. At the time of formation, a charging intermediate bias of only direct current different from the charging bias is switched and applied to the charging device, and a developing intermediate bias different from the developing bias is switched to the developing device. In the image forming apparatus for applying the voltage, a control for applying a rising bias having an output value smaller than the normal transfer output value as the transfer bias before the transfer of the toner image is performed, and the transfer is performed after the transfer of the toner image is completed. An image forming apparatus comprising: a control unit that performs one or both of a control for applying a falling bias having an output value smaller than the regular transfer output value as a transfer bias. 2. The image forming apparatus according to claim 1, wherein the control unit sets an initial output value of the rising bias applied first to an output value of 60% or less of a normal transfer output value. An image forming apparatus characterized by performing control. 3. The image forming apparatus according to claim 2, wherein the control unit applies a gradually increasing start-up bias that gradually and gradually changes and gradually increases as the start-up bias. An image forming apparatus, characterized in that control is performed such that an increasing width of a gradually increasing start-up bias is within 30% of a regular transfer output value. 4. The image forming apparatus according to claim 1, wherein when the control unit applies a gradual-decrease-type falling bias that gradually decreases by gradually or continuously changing as the falling bias, An image forming apparatus, characterized in that control is performed such that a decreasing width of a gradual decrease type falling bias is within 30% of a regular transfer output value. 5. The image forming apparatus according to claim 4, wherein the control unit sets the final output value finally applied as the falling bias to an output value of 60% or less of a normal transfer output value. An image forming apparatus, characterized in that the image forming apparatus is controlled. 6. The image forming apparatus according to claim 1, wherein the control unit switches the application start timing of the rising voltage from the intermediate charging bias to the charging bias. An image forming apparatus, wherein control is performed in synchronism with timing or timing of switching from application of the developing intermediate bias to development bias. 7. The image forming apparatus according to claim 1, 4 or 5, wherein the control unit applies the end timing of the falling bias from the charging bias to the intermediate charging bias. The image forming apparatus is characterized in that control is performed in synchronism with the timing of switching to or the timing of switching from the developing bias to the application of the developing intermediate bias.