JP2004117572A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent toner having a positive normal polarity and toner having a polarity opposite the normal polarity from sticking to a transfer means with the result that the back of a recording medium is soiled or transfer failure and hence image failure are caused, in an image forming apparatus of a contact transfer system. <P>SOLUTION: The image forming apparatus includes: an electrifying means which electrifies a photoreceptor; an exposure means which forms an electrostatic latent image; a developing means which develops an electrostatic latent image; a recording medium supply means which supplies a recording medium; and the transfer means which transfers a toner image to the recording medium in a nip where the transfer means abuts on the photoreceptor. In the initial stage of an image forming operation, during a period (t3) from the moment when a portion of the surface of the photoreceptor in which electrification is started has come to the nip to the moment when a portion of the photoreceptor whose surface potential has become constant reaches the nip, a voltage having the polarity that is the same as that of the toner is applied to the transfer means. During the image forming operation thereafter, a voltage having the polarity which is opposite that of the toner is applied to transfer a toner image to the recording medium. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus in which a transfer bias applied to a transfer unit in a contact transfer type image forming apparatus is controlled to eliminate toner stain on the transfer unit.
[0002]
[Prior art]
In a conventional electrophotographic image forming apparatus, the transfer of a toner image to a recording medium has been performed using a transfer unit such as a corotron which discharges in a non-contact state with a photoconductor. However, in recent years, from the viewpoint of transferability, a contact member such as a belt or a roller is pressed against the photoconductor, and a sheet-shaped recording medium mainly made of paper is passed between the photoconductor and the contact member, so that the transfer roller side The transfer bias of the contact transfer method for transferring a transferable toner image formed on the surface of a photoreceptor to a recording medium by the action of a transfer bias applied to a recording medium has been increasingly used.
[0003]
However, transfer using a contact transfer type transfer unit is effective in terms of transferability, but the transfer unit is stained by toner unnecessary for image formation as long as the transfer unit is in contact with the photoconductor. There was a problem.
[0004]
FIG. 3 is a diagram for explaining an example of a conventional method proposed to prevent the transfer roller from being stained in the contact transfer method as described above. The photosensitive drum drive, the photosensitive member potential, the developing bias, and the transfer bias are illustrated. A timing chart showing the relationship is shown (for example, see Patent Document 1). When the recording paper passes through the transfer position (ie, the nip formed by the photoconductor and the transfer roller) in order to clean the transfer roller in the reversal development method using a laser or an LED, the polarity of the toner is opposite to that of the toner. When a voltage is applied to the transfer roller to transfer the toner to the recording paper, and when the recording paper does not pass through the nip portion, the surface potential of the photoconductor is made lower than when the image is formed. The transfer roller is cleaned by applying a voltage that is polar and higher than the surface potential of the photoreceptor so that the toner adhered to the transfer roller is repelled from the transfer roller when paper is not passed. .
[0005]
[Patent Document 1]
JP-A-01-319078
[Problems to be solved by the invention]
In the above-described conventional image forming apparatus, a voltage having the same polarity as the toner is always applied to the transfer unit (transfer roller) when paper is not passed. There is variation in the charge amount, and the toner includes a toner having a polarity opposite to the normal polarity. For the toner having such a charging behavior opposite to that of the normal toner, if the transfer unit is cleaned as described above, the transfer unit may be strongly attracted to the transfer roller, thereby contaminating the transfer unit. . Although the amount of toner exhibiting the opposite charging behavior to normal is not large in quantity, it accumulates and increases with repetition of image formation, and the accumulated toner stains the back surface of the recording medium or causes transfer. There is a problem that a defect is caused to cause an image defect.
[0007]
The present invention has been made in view of the above-described circumstances, and analyzes a behavior due to charging of a photoreceptor, a behavior due to application of a developing bias voltage, and the like, and controls a voltage to be applied according to each situation. It has been found that the control can be performed without contaminating the transfer roller, and based on this discovery.
[0008]
[Means for Solving the Problems]
The present invention provides a charging unit that charges a photoconductor, an exposure unit that exposes the photoconductor charged by the charging unit to form an electrostatic latent image, and develops the electrostatic latent image to form a toner image. Developing means for forming, recording medium supply means for supplying a recording medium to the photoconductor in synchronization with the rotation of the photoconductor, and a nip portion which is driven by the rotation drive of the photoconductor and abuts on the photoconductor. A transfer unit for transferring a toner image to the recording medium, wherein a voltage having the same polarity as that of the toner when the charged portion of the surface of the photoreceptor comes to the nip portion at the beginning of the image forming operation; Is applied to the transfer means, and when a portion where the surface potential of the photoreceptor becomes constant comes to the nip portion, a voltage having a polarity opposite to that of the toner is reapplied to the transfer means, and the toner image is transferred to the recording medium. This is characterized by transferring Therefore, it is a transfer means (transfer roller, a transfer belt, etc.) in the image forming operation initial adhesion of the toner to that so as to prevent.
[0009]
Further, the present invention is characterized in that a voltage having a polarity opposite to that of the toner is continuously applied to the transfer unit during the image forming operation, so that it is not necessary to switch the voltage applied to the transfer unit during the image forming operation.
[0010]
Further, after the end of the image forming operation, when a portion where the surface potential of the photoconductor starts to decrease reaches the nip portion, a voltage having the same polarity as that of the toner is applied to the transfer unit, and the surface of the photoconductor is charged before charging. The application of a voltage having the same polarity as that of the toner to the transfer unit is terminated when the portion that has returned to the state described above comes to the nip portion. This is to prevent it.
[0011]
Further, a voltage is not applied to the transfer unit during the initial period of the image forming operation, and until the surface of the photosensitive member, which has been charged by the charging unit, reaches the nip portion. In this case, the toner is prevented from adhering to the transfer means.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a schematic diagram for explaining an example of an image forming apparatus to which the present invention is applied. As is well known, a photosensitive drum 1, a charging unit 2 for charging the photosensitive drum 1, and a charged photosensitive drum 1 An exposing unit 3 for exposing the body drum 1 to form an electrostatic latent image, a developing unit 4 for developing the electrostatic latent image to form a toner image, and a nip portion driven by the rotation of the photosensitive drum 1 Transfer means 5 for transferring the toner image onto the recording medium P, a peeling means 6 for peeling the recording medium P from the photosensitive drum 1, a fixing means 7 for fixing the transferred toner image to the recording medium P, And a cleaning unit 8 for cleaning the toner adhered to the photosensitive drum 1.
[0013]
Hereinafter, in order to facilitate understanding of the present invention, the operation of a general image forming process in an image forming apparatus will be described with specific numerical values including potential fluctuations during the operation of each unit. I do. The photoconductor drum 1 includes a conductive substrate made of metal or resin, an undercoat layer formed on the surface thereof, and a photosensitive layer formed thereon. The photosensitive layer includes a relatively thin carrier generation layer (CGL) formed on the undercoat layer, and a carrier transfer layer (CTL) mainly formed of polycarbonate or the like formed on the outermost layer. The photosensitive drum 1 having a diameter of 30 mm is provided rotatably in the direction of the arrow. The surface of the photoreceptor drum 1 is uniformly charged to a predetermined charge amount (−600 V) by a scorotron charger using a corona charger as the charging means 2, and forms a predetermined electrostatic latent image potential by the exposure means 3. This forms an electrostatic latent image. Carriers are generated in the carrier generation layer (CTL) by the exposure, and the electric charges charged on the surface of the photoconductor drum 1 are offset by the carriers moved in the carrier moving layer. That is, the irradiation area is attenuated to -100 V by the irradiation of the optical information, and this portion forms an electrostatic latent image.
[0014]
The electrostatic latent image formed on the photosensitive drum 1 is transported to a developing area (developing section) where the photosensitive drum 1 rotates and comes into contact with the developer adhered on the developer carrier 41 of the developing unit 4. You. The developer carrier 41 that rotates in the direction of the arrow opposite to the rotation direction of the photosensitive drum 1 is pressed against the photosensitive drum 1. Then, the toner in the developer carried on the developer carrying member 41 moves to and adheres to the electrostatic latent image on the photosensitive drum 1, whereby the electrostatic latent image is visualized and developed. A predetermined bias voltage is applied to the developer carrier 41 from a connected power supply (not shown). In this way, the negatively charged toner adheres to the potential attenuated portion by the reversal development method, and a toner image is formed. Note that the rotation direction of the developer carrier 41 is not limited to the direction opposite to the rotation direction of the photosensitive drum 1.
[0015]
After the development, the toner attached to the photosensitive drum 1 is transported to a predetermined transfer area (nip). A recording medium P such as paper is supplied to the transfer area by a recording medium supply unit such as a paper supply unit, and comes into contact with the toner image on the photosensitive drum 1. The transfer unit 5 includes a contact transfer roller provided with a high-voltage power supply. In order to transfer the toner image to the recording medium P, a positive voltage opposite to the polarity of the toner is applied. As a result, the toner moves to the recording medium P, and the toner image is transferred. However, the form of the transfer means 5 is not limited to a roller, but may be a form such as a belt. Here, the peripheral speed of the transfer roller is set to be 1.03 times the peripheral speed of the photosensitive drum 1 in accordance with the drive of the photosensitive drum 1.
[0016]
This transfer roller has a diameter of 14.3 mm and a roller elastic body length of 301 mm, and is pressed against the photosensitive drum 1 so as to form a nip portion of about 2.5 mm. By using a roller having a medium resistivity of about 10 ⁷ to 10 ¹¹ Ω · cm as the resistivity, the toner on the photosensitive drum 1 can be transferred to the recording medium P satisfactorily. The elastic body of the roller uses a solid rubber such as EPDM (foam rubber) in which conductive carbon is dispersed, NBR (nitrile rubber) or urethane rubber.
[0017]
Next, after the recording medium P is separated from the photosensitive drum 1 by the peeling unit 6, the toner on the recording medium P is fixed by the fixing unit 7 by, for example, heat melting or pressure. Thereafter, the recording medium 1 is discharged outside the apparatus. Further, the surface of the photosensitive drum 1 after the transfer is cleaned by the cleaning unit 8.
[0018]
In the image forming apparatus described above, various settings were made in the following manner in consideration of photoreceptor photoresponsiveness and developability.
366 mm / s linear velocity of developer carrier
Linear speed of photoreceptor drum 122 mm / s
Printing speed (A4): 27 CPM
Resolution: 600 dpi
Laser power: 0.40 mW
Charging potential: -600 V
Development bias: -450 V
Toner adhesion prevention voltage -1000V
Transfer bias: +1000 V
[0019]
FIG. 2 is a timing chart showing the relationship between the photosensitive drum drive, the photosensitive member surface potential, the developing bias, and the transfer bias during the image forming operation in the image forming apparatus shown in FIG. 1. First, the photosensitive drum 1 is driven. As a trigger, the photosensitive drum 1 is charged by the charging means 2. However, even if a voltage is applied to the band electrode of the charging means 2, some time is required for the photosensitive drum 1 to reach the predetermined voltage. This is the time called the rising period, and generally requires about 300 ms. The developer carrier 41 and the like of the developing unit 4 also start rotating at the same time as the photosensitive drum 1 is driven, and the toner has a charge amount for each particle due to friction with the carrier. The developing bias (DVB) = 0 is maintained during a period (t1) until the portion where the application of the voltage of the photosensitive drum 1 starts to reach the developing section by the rotation driving of the photosensitive drum 1. That is, the developing bias (DVB) = 0 is maintained for the portion of the photosensitive drum 1 where the surface potential (Vo) = 0. When Vo = 0 and DVB = 0, the amount of toner adhering to the photosensitive drum 1 in the developing unit is small. This is because there is no toner inquiry between the photosensitive drum 1 and the developing means 4 due to the electric field. Therefore, during this period, since the developing unit is not controlled by the electric field and is in an unstable state, toner of any polarity adheres to the surface of the photosensitive drum 1.
[0020]
Therefore, during the period (t2) until the surface of the photosensitive drum 1 whose charging is started by the charging unit 2 reaches the nip portion, no transfer bias is applied to the transfer unit 5. This is because if a voltage is applied to the transfer means 5, the toner attached to the surface of the photosensitive drum 1 is a toner of any polarity, so that no matter what charge is applied, the toner having the opposite polarity to the polarity is transferred. This is because the toner firmly adhered to the transfer means 5 remains on the transfer means 5 as it is, which undesirably causes the back surface of the recording medium P to be stained and causes transfer failure. Accordingly, by applying no transfer bias to the transfer unit 5, the amount of toner adhering to the transfer unit 5 from the surface of the photosensitive drum 1 is reduced to a very small amount of physical adhesion, and the adhesion is very weak. As a result, the back surface is cleaned with dirt or the like that is invisible to the user, so that the occurrence of image defects can be ignored.
[0021]
When the portion of the surface of the photosensitive drum 1 where the application of the voltage has started reaches the developing unit, the application of the same voltage as that used for image formation to the developing unit 4 is started. However, even if a voltage is applied to the band electrode of the charging means 2, a rising period is required for the photosensitive drum 1 to reach a predetermined voltage. It takes some time for 4 to reach the predetermined voltage. A portion where the surface potential becomes a predetermined constant potential (hereinafter referred to as a rising portion) from a portion where the voltage on the surface of the photosensitive drum 1 is started to be applied (hereinafter referred to as a rising portion) is a rising period (a time period during which the portion passes through the nip portion by the rotation drive). The length of t3) differs depending on the specifications of the band electrode or the developing means. During the rising period (t3), the surface potential of the photosensitive drum 1 and the bias voltage of the developing unit 4 are unstable, and are not preferable for use in image formation. Therefore, image formation is performed after the surface potential of the photosensitive drum 1 and the bias potential of the developing unit 4 become constant.
[0022]
In the rising period (t3), if the potential difference between the surface potential (Vo) of the photosensitive drum 1 and the developing bias potential (DVB) can be controlled within the range of (Vo-DVB) =-100 to -200 V, the developing means 4 Adhesion of toner to the surface of the photosensitive drum 1 can be prevented. However, it is difficult to control within the range of (Vo-DVB) =-100 to -200 V, and if this is achieved, the cost increases.
[0023]
The value of (Vo-DVB) in the rising period (t3) is in the range of (Vo-DVB) ≒ 0 to -400 V depending on the characteristics of the photosensitive drum 1, the characteristics of the band electrode of the charging unit 2, and the characteristics of the developing unit 4. May have various values. Therefore, depending on the voltage state, any of the normal polarity toner, the opposite polarity toner, and the uncharged toner may possibly adhere to the surface of the photosensitive drum 1 from the developing unit 4. The distribution of the charge amount of the toner is overwhelmingly large for the normal polarity toner compared to the other toners. Therefore, it is considered that the toner that may adhere to the transfer unit 5 during the rising period (t3) has the largest amount of toner of normal polarity.
[0024]
Therefore, in the present invention, in the rising period (t3), a voltage having the same polarity as that of the normal polarity toner is applied to the transfer unit 5 in order to prevent the toner of the normal polarity from adhering to the transfer unit 5. By applying the toner adhesion preventing voltage to the transfer unit 5 during the rising period (t3), the normal polarity toner most likely to be attached to the surface of the photosensitive drum 1 does not adhere to the transfer unit 5, so that the transfer unit 5 5 can be prevented.
[0025]
After the rising period (t3), when a portion where the surface potential of the photosensitive drum 1 and the potential of the developing unit 4 reach a predetermined voltage reaches the nip, the transfer unit 5 transfers the photosensitive drum to the photosensitive drum by image formation. A predetermined voltage having a polarity opposite to that of the toner is applied to transfer the toner image adhered to the surface of the recording medium 1 onto the recording medium P, and an image forming operation is performed.
[0026]
That is, before reaching the nip portion, light information is irradiated by the exposure means 3 onto the photosensitive drum 1 whose surface potential has become constant at −600 V by the charging means 2, and the potential of the irradiated part is −100 V. descend. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 1. When the surface of the photosensitive drum 1 on which the electrostatic latent image is formed reaches the developing unit 4 to which a voltage of -450 V is applied, the above-mentioned potential attenuated portion is visualized by the normal polarity toner by the reversal developing method, and the toner image is formed. It becomes. On the other hand, a recording medium P such as a recording sheet supplied from a conveyance path by a recording medium forming unit such as a sheet feeding unit includes a photosensitive drum 1 and a transfer unit 5 pressed against the photosensitive drum 1 in timing with a toner image. A nip is formed. When the recording medium P passes, a transfer bias of +1000 V is applied to the transfer unit 5 by a power supply (not shown), so that the toner image is transferred to the recording medium P. The recording medium P to which the toner image has been transferred is discharged through a peeling / fixing process by the peeling means 6 and the fixing means 7.
[0027]
This is a period (t4) from the end of the transfer to one recording medium P to the arrival of the other one recording medium P at the nip, and the surface potential of the photosensitive drum 1, the developing bias, and the transfer bias During the image forming operation including a period during which the state of the blank portion during image formation is maintained, a pressure having a polarity opposite to that of the normal polarity toner is applied to the transfer unit 5.
[0028]
The fact that toner adheres to a portion of the recording medium P that hits a white background is generally considered to be fogging, which is an undesirable state. In order to prevent this state, a developing bias and a transfer bias are set in a portion corresponding to a blank portion during image formation so that toner adheres little to the photosensitive drum 1 and the recording medium P is not stained. I have. In other words, during the image forming operation, even if the recording medium P is not present, if the balance of the photoconductor surface potential, the developing bias, and the transfer bias is maintained under these conditions, the transfer means is provided with the normal polarity toner and the reverse polarity toner. It becomes possible not to attach any of them.
[0029]
Therefore, during the period (t4) during which the recording medium does not pass through the nip portion during the image forming operation, both the photoconductor potential and the developing bias are in the same state as the blank portion during printing, that is, (Vo) so that the toner does not adhere to the photoconductor. By setting −DVB) = − 100 to −200 V, it is possible to prevent toner from adhering to the photosensitive drum 1. Since the toner does not adhere to the photosensitive drum 1, the transfer unit 5 is maintained in a stain-free state even when the transfer bias is applied under the same conditions as the image formation.
[0030]
After the transfer to the final recording paper is completed, that is, after the image formation is completed, the voltage application to the band electrodes is stopped to lower the surface potential of the photosensitive drum 1. However, even if the application of voltage to the band electrode is stopped, some time is required for the photosensitive drum 1 to reach 0V. This is the time called the falling period, and generally requires a time of about 300 ms, which is the same as the rising period. During the period (t1 ') until the portion of the surface of the photosensitive drum 1 where the potential starts to decrease reaches the developing unit by the rotation of the photosensitive drum 1, the developing bias is maintained under the same conditions as in image formation. That is, similarly to the period (t4) during which the recording medium does not pass through the nip portion during the image forming operation, the potential of the photosensitive drum 1 and the developing bias are both set so that the toner does not adhere to the photosensitive drum 1 during printing. In the same state as that of the sheet, that is, (Vo-DVB) =-100 to -200 V, the toner is prevented from adhering to the photoreceptor, and the same potential difference (Vo-DVB) as that of the blank sheet during printing = -100. Until the portion at -200 V reaches the nip portion (t2 '), a transfer bias under the same conditions as in image formation is applied to keep the transfer means 5 clean.
[0031]
When the portion of the surface of the photosensitive drum 1 where the potential has started to decrease reaches the developing unit by the rotation of the photosensitive drum 1, the application of the voltage to the developing unit 4 is stopped, and the voltage of the developing unit 4 decreases. However, even if the application of voltage to the band electrode is stopped, some time is required for the photosensitive drum 1 to reach 0 V, and similarly, a predetermined time is required for the fall of the developing means 4. A portion from the start of the potential reduction of the surface of the photosensitive drum 1 until the surface potential returns to the state before charging (hereinafter referred to as a falling portion) is the same as the rising portion. If the potential difference (Vo-DVB) between (Vo) and the developing bias (DVB) can be controlled within the range of -100 to -200 V by any method, the adhesion of toner from the developing means 4 to the surface of the photosensitive drum 1 is prevented. can do. As a control method, there is a method of controlling a grid voltage or a stepwise reduction of a surface potential using laser exposure. However, even if any of these control methods is used, the same as in the rising period (t3). There may be a falling period (t5) in which the potential difference between the surface potential of the photosensitive drum 1 and the developing bias is unstable.
[0032]
The value of (Vo-DVB) in the fall period (t5) varies in the range of (Vo-DVB) ≒ 0 to -400 V depending on the characteristics of the photosensitive drum 1, the characteristics of the band electrodes, and the characteristics of the developing means 4. Can be a value. Therefore, during the period (t5) during which the falling portion passes through the nip portion, a voltage having the same polarity as that of the toner is applied to the transfer unit 5 as in the rising period (t3). This makes it possible to prevent the transfer unit 5 from being contaminated by the toner, as in the case of the rising period (t3).
[0033]
When the portion where both the surface potential of the photosensitive drum 1 and the potential of the developing means 4 reach 0 V reaches the nip portion, the application of the voltage to the transfer means 5 is stopped, all the driving is stopped, and the printing operation is completed. I do.
[0034]
(Example)
The image forming apparatus shown in FIG. 1 described using the timing sequence of FIG. 2 is mounted on a printer DM-2705 manufactured by Sharp, and an aging test of 50,000 sheets is performed intermittently on three sheets. As a result of performing the test under three environments of high humidity, it was possible to obtain good printing without back stain on the recording paper even after aging.
[0035]
(Comparative Example 1)
When the image forming apparatus described in the conventional timing sequence shown in FIG. 3 was mounted on a printer DM-2705 manufactured by Sharp, and aging of 50,000 sheets was performed, dirt accumulation on the transfer roller caused back surface dirt on the recording paper. did. In particular, stains under high temperature and high humidity were remarkable.
[0036]
Although the present invention has been described above, the charging voltage, the developing bias, and the transfer bias are controlled under conditions matching the respective machines, and are not limited to the numerical values described above.
[0037]
【The invention's effect】
According to the present invention, in a contact transfer type image forming apparatus, at the beginning of an image forming operation, when the charged portion of the surface of the photoconductor comes to the nip portion, a voltage having the same polarity as the toner is applied to the transfer member, When a portion where the surface potential of the photoconductor reaches a predetermined constant potential comes to the nip portion, a voltage having a polarity opposite to that of the toner is reapplied to the transfer means, and the toner image is transferred to the recording medium. Neither the toner nor the toner having the polarity opposite to the normal polarity adheres to the transfer unit and stains the back surface of the recording medium, or adheres to the transfer unit to cause transfer failure and cause image failure. Can be
[Brief description of the drawings]
FIG. 1 is a schematic diagram for explaining an example of an image forming apparatus to which the present invention has been applied.
FIG. 2 is a timing chart illustrating a relationship among a photosensitive drum drive, a photosensitive member surface potential, a developing bias, and a transfer bias during an image forming operation in the image forming apparatus illustrated in FIG.
FIG. 3 is a timing chart showing a relationship between a photoconductor potential, a developing bias, and a transfer bias in a conventional image forming apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Photoreceptor drum, 2 ... Charging means, 3 ... Exposure means, 4 ... Development means, 5 ... Transfer means, 6 ... Peeling means, 7 ... Fixing means, 8 ... Cleaning means, 41 ... Developer carrier, P ... recoding media.

Claims (4)

Charging means for charging a photoconductor, exposure means for exposing the photoconductor charged by the charging means to form an electrostatic latent image, and developing means for developing the electrostatic latent image to form a toner image A recording medium supply unit that supplies a recording medium to the photosensitive member in synchronization with the rotation of the photosensitive member; and a nip portion that is driven by the rotation of the photosensitive member and abuts on the photosensitive member. An image forming apparatus comprising: a transfer unit for transferring to a recording medium, an image forming operation, and when the charged portion of the surface of the photoconductor reaches the nip, a voltage having the same polarity as the toner is applied to the transfer unit. When a portion where the surface potential of the photoconductor becomes constant comes to the nip portion, a voltage having a polarity opposite to that of the toner is reapplied to the transfer unit to transfer the toner image to the recording medium. An image forming apparatus comprising: 2. The image forming apparatus according to claim 1, wherein a voltage having a polarity opposite to that of the toner is continuously applied to the transfer unit during the image forming operation. 3. The image forming apparatus according to claim 1, wherein after the image forming operation is completed, when a portion where the surface potential of the photoconductor starts to decrease reaches the nip portion, the transfer unit includes the same toner as the toner. 4. Applying a voltage having a polarity same as that of the toner to the transfer unit when a portion of the surface of the photoreceptor that has returned to a state before charging comes to the nip portion. Image forming apparatus. 4. The image forming apparatus according to claim 1, wherein the transfer unit is in an initial stage of an image forming operation and before the surface of the photosensitive member, which has been charged by the charging unit, reaches the nip. Wherein no voltage is applied to the image forming apparatus.

Images