JP2003280410A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent an excessive electric charge from being imparted to the rear end of a transfer material at the time of transferring a toner image on a photoreceptive drum to the transfer material by a transfer roller. <P>SOLUTION: In an image forming apparatus possessing at least an image carrier carrying an image to be transferred to the transfer material and a member for transfer to form a transfer nip part through which the transfer material passes by coming into contact with the image carrier and to transfer the image carried by the image carrier to the transfer material by applying transfer bias to the member for transfer, the transfer bias is set as constant voltage control (Vt) until the transfer material passes through the transfer nip part and reaches the rear end and is switched to constant current control (I) at the rear end. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image carrier such as a photoconductor (photoreceptor), a dielectric, a magnetic body, and an intermediate transfer body, and an appropriate image such as an electrophotographic system, an electrostatic recording system, and a magnetic recording system. A transferable image corresponding to the desired image information is formed and carried by the forming means, and the transferable image on the side of the image carrier is transferred to a transfer material such as paper by the transfer means to separate the transfer material from the image carrier. The present invention relates to an image forming apparatus that outputs an image formed product by executing an image forming process including a step of separating a transfer material from the image bearing member by an erasing member for accelerating.

[0002]

2. Description of the Related Art Conventionally, a corona charger or a roller member has been known as a transfer means for transferring a toner image on an image carrier to a transfer material such as paper by applying a high voltage. Actually, it is used in many electrophotographic image forming apparatuses.

Among them, the corona charger has a merit that since the charging is carried out in a non-contact manner, the damage to the image bearing member can be minimized and that the transfer material is not selected when it is used as the transfer member.

However, ozone and NOx are generated by corona discharge, and a means for removing these is required.
In addition, when a corona charger is used as the transfer means, the structure becomes complicated, and there is a problem that a measure is required to prevent the transfer material from sticking into the corona charger.

On the other hand, in order to solve these problems, a roller member has recently been used as a charger. In a charging device using a roller member, a semiconductive rubber material such as EPDM, NBR, urethane rubber, epichlorohydrin, and silicone rubber is generally used as an elastic layer and a conductive base shaft also serving as a power feeding electrode. Is composed of at least one layer.

The elastic layer may be composed of a sponge-like member in order to maintain the uniformity of the contact portion with the image carrier.

Since such a roller charging device can apply a smaller voltage than a charging device using a corona charger, it reduces the generation of ozone and NOx, and when used as a roller member for transfer, it does not transfer. Since the toner image on the image carrier is transferred to the transfer material by contacting the material, the transfer material is sandwiched and conveyed by the photosensitive member as the image carrier and the roller member, and the conveyance of the transfer material is stabilized. Is possible.

When such a roller member is used as the transfer roller, the bias voltage applied to the transfer roller is such that the transfer voltage is applied from the power source when the transfer material does not exist between the image carrier and the transfer roller. In many cases, it is a voltage value determined by the ATVC method or the like, which is calculated by a preset control formula from the generated voltage when a constant current is applied to the roller by constant current control.

By the way, the transfer material having undergone the transfer of the toner image in the transfer step is electrostatically adsorbed to the image carrier, but the transfer material is separated from the image carrier by the discharging means.

The charge removing unit removes the electric charge applied to the transfer material by the transfer roller member and promotes separation of the transfer material from the image carrier.
A structure in which a charge eliminating needle is sandwiched by insulating members, or a brush-shaped one is used.

Further, in the charge eliminating means, the charge eliminating needle is grounded or grounded via a varistor, a resistor or the like to reduce the charges applied to the transfer material and promote the separation.

In particular, when it is difficult to separate using a paper strainer such as A3 and full bleed, a charge biasing member having a reverse polarity to the transfer bias is applied to a charge removing member such as a charge removing needle or a brush to accelerate the separation. Is applied at a constant current or a constant voltage to reduce the charge applied to the transfer material by the transfer bias, and promote the separation of the transfer material from the image carrier.

[0013]

However, the image forming apparatus shown in the conventional example has the following problems.

During the transfer process, in order to transfer the toner image on the photosensitive drum as an image carrier from the transfer roller to the transfer material, a voltage having a polarity opposite to that of the toner image is applied by the above-mentioned ATVC control or the like. Is applied.

At this time, charges are applied to the margin (non-image area) areas of the leading edge and the trailing edge of the transfer material in the same manner as the image area where the toner image exists, so that the non-image area is in a state of excessive charge. Become.

Here, since a charge having a polarity opposite to the potential on the drum is applied to the front end and the rear end of the transfer material in the transfer process, especially when the transfer material is thin paper (paper having a weak stiffness) The material sticks to the photosensitive drum, which causes separation failure and trailing edge splash, which hinders stable transfer material transfer, and the transfer material sticks to the suction metal plate that forms the transfer path to the fixing device. There is a case where an electric charge at the trailing end of the transfer material is transferred to the fixing device due to peeling discharge when it is removed and an offset phenomenon (peeling offset) is caused due to the creation of a memory.

Therefore, after the toner image on the photosensitive drum is transferred to the transfer material, it is transferred from the transfer means to the charge removing member in order to reduce the charge given to the transfer material by the transfer means (for example, a transfer roller member). It is preferable to apply a constant current or a constant voltage having a polarity opposite to that of the voltage applied to the material to reduce the separation of the transfer material, the trailing edge splash, the attraction to the sheet metal of the transfer sheet, and the peeling offset phenomenon.

However, when a constant voltage is applied to the charge removing member, the resistance and strain of the transfer material may change depending on the degree of moisture absorption of the transfer material, and the charge removing ability of the transfer material by the charge removing member may be insufficient.

Further, in order to perform uniform charge removal of the transfer material,
A constant current is applied to the charge removing member, but at that time, the transfer bias by the transfer unit leaks to the charge removing member, the transfer bias applied to the transfer material drops, and a transfer defect may occur at the tip of the transfer material. there were.

Further, it is desired to increase the speed of the image forming apparatus and increase the capacity of the charge eliminating member by diversifying the transfer material, which may cause a cost increase of the high voltage power source and the like.

The object of the invention according to the present application is to overcome the above-mentioned problems in the image forming apparatus of the contact electrostatic transfer system, improve the transportability of the transfer material, and cause peeling discharge which may occur when the transfer material is removed from the fixing device. An object of the present invention is to provide an image forming apparatus capable of preventing an offset phenomenon and preventing a transfer failure due to a leakage current from a charge eliminating member.

[0022]

In order to achieve the above object, the present invention determines a transfer bias applied to a transfer member such as a transfer roller at the front or rear end of a transfer material during a transfer process. Current control is performed, and constant voltage application is performed in other cases.

At this time, at the front end and the rear end of the transfer material where the transfer material plunges into or comes out of the transfer nip, there is the transfer material in the transfer nip portion, and the transfer material cannot be sandwiched in the transfer nip portion and the photosensitive drum. There are areas where the transfer member is in direct contact. In this case, since the transfer material, which is a resistor, does not exist, the current escapes to the side of the image carrier such as the photosensitive drum as compared with the case where the transfer material is sandwiched in the entire transfer nip portion as viewed from the transfer member. If the transfer voltage becomes transiently low and the leading and trailing edges of the transfer material do not exist in the entire transfer nip,
Since the electric charge supplied by the transfer member is reduced from the range where the transfer material is sandwiched in the entire transfer nip,
It adheres to an image bearing member such as a photosensitive drum to prevent a separation failure, a trailing edge splash, and a peeling offset phenomenon that occurs when the fixing device is removed.

The bias applied to the discharging means is constant voltage at the tip of the transfer material, and is constant current when the discharging material covers the discharging means.

Therefore, since the bias applied to the charge eliminating means is controlled at a constant voltage with respect to the leading edge of the transfer material, the transfer bias is prevented from dropping and the transfer failure due to the insufficient transfer bias at the leading edge of the transfer material is prevented. To prevent.

When the transfer material, which is a resistor, covers the charge removing means and the transfer bias does not drop, the charge removing means controls the charge removing means with a constant current so that the charge imparted by the transfer member is transferred to the transfer material. It is possible to reduce uniformly regardless of resistance, and it is possible to realize stable paper conveyance regardless of paper type and environment.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION (First Embodiment) FIG. 1 shows a laser beam printer (hereinafter, LBP) embodying the present invention.
Call. 1) shows a schematic configuration of the image forming apparatus of FIG.

In FIG. 1, the image forming apparatus comprises a photosensitive layer 11b on an electrically conductive support 11a as an image bearing member to be charged.
Forming the photosensitive drum 11, a charging roller 12, which is a charging device connected to a charging power source 18 for applying an alternating voltage superposed with a direct current voltage, an exposure light 13, a developing device 14 connected to a developing power source 19, a transfer power source 20. The main components are a transfer roller 15 which is a transfer member, a charge eliminating needle 24, a cleaner 16, paper conveyance guides 21 and 22, and a fixing device 17 which are connected to each other.

In the image forming apparatus constructed as described above, the photosensitive drum 11 rotates in a fixed direction, and the charging roller 12 is pressed against the photosensitive drum 11 and is driven by the photosensitive drum 1.
The surface of No. 1 is uniformly charged and then exposed to the exposure light 13 to form an electrostatic latent image on the photosensitive drum 11.

The electrostatic latent image is visualized as an image of toner, which is a developer powder, by the developing device 14, and the toner image is conveyed by the conveying guide 21 to the photosensitive drum 11 and the transfer roller 15.
The image is transferred onto the transfer material 23 that is conveyed during the transfer. afterwards,
The transfer material 23 is separated from the photosensitive drum 11 by the charge removal needle 24 to which a bias is applied, and moves on the conveyance guide 22.
When the transfer material 23 is introduced into the fixing device 17 and heated under pressure in the fixing device 17, it becomes a permanent image. On the other hand, the waste toner remaining on the photosensitive drum 11 is collected by the cleaner 16.

Here, in the present embodiment, the transfer roller 1
5 determines the transfer bias by the control shown in FIG.

In FIG. 2, first, before a print signal comes and the toner image on the photosensitive drum 11 is transferred onto the transfer material, the transfer roller 15 performs pre-rotation in contact with the photosensitive drum 11.

During the pre-rotation, a constant current value Io is applied to the transfer roller 15, the average of the voltage values at that time is detected, and the value is taken as Vto. Based on the value of Vto, the resistance value of the transfer material is taken into consideration, the transfer bias Vt is determined based on a predetermined control formula, and when printing is started, the transfer bias is applied to the transfer material by this transfer bias Vt. Then, when the rear end of the transfer material reaches the transfer position, a constant current I that is the same as the current flowing when the transfer bias Vt is applied is applied to the rear end of the transfer material, and the applied constant voltage value Vt is turned off. Therefore, the amount of electric charge applied to the transfer material by the transfer roller 15 is reduced.

At this time, the transfer bias Vt applied to the transfer roller 15 when the transfer material 23, which is a resistor, is sandwiched by the transfer nip portion is based on the average value Vto of the voltage values detected by the detection current. Therefore, when a constant current value is applied at the rear end of the transfer material, the voltage is set to the voltage value V
It is also possible to reduce the voltage to the to level, turn off the constant current value I between the sheets, and apply the sheet-to-sheet bias.

When such control is performed, the transfer material 23
At the rear end, as shown in FIG. 3, there is a portion where the transfer material 23, which is a resistor, does not exist in the transfer nip portion. Therefore, the transfer material 23 is sandwiched in the entire transfer nip portion N when viewed from the transfer roller 15. Current (I1) flows to the side of the photosensitive drum 11 compared to the case in which the transfer voltage is transiently lowered, and the amount of charge supplied from the transfer roller 15 to the trailing edge of the transfer material 23 is reduced. The charge amount in the area smaller than the transfer nip width N at the rear end of the transfer material 23, which is the image area, is less than that in the image area.

Generally, in the case of the image forming apparatus using the transfer roller 15, the transfer nip width N is about 1 to 3 mm, and the charge amount in the area of 1 to 3 mm or less at the rear end of the transfer material 23 is reduced. It will be.

Therefore, it is possible to prevent the trailing edge splash due to the sticking to the photosensitive drum and the peeling offset due to the peeling discharge that occurs when the fixing device is removed.

Next, as shown in FIG. 4, the static elimination unit 24 includes the static elimination needle 42 with the insulation member 41 and the insulation member 43.
It has a configuration of sandwiching.

The charge elimination needle 42 is made of a SUS plate having a thickness of 0.1 mm, and its tip is formed in a saw-tooth shape along the width direction of the photosensitive drum as shown in FIG.

The static elimination needle 42 is a transfer paper serving as a transfer material used for image formation on the second surface during double-sided printing (especially, paper that is curled in the curvature direction of the photosensitive drum 1 because it is dry).
In order to promote the separation of 23 from the photosensitive drum 1, a voltage having a polarity opposite to that of the transfer voltage is applied, a constant voltage is applied to the leading edge of the paper, and when the leading edge of the paper covers the static elimination member (static elimination unit 24), It is a constant current.

That is, the static elimination needle 4 is attached to the tip of the transfer material.
By controlling the bias applied to 2 with a constant voltage, the drop of the transfer bias is prevented, and the transfer failure due to the insufficient transfer bias at the leading end of the transfer material is prevented. That is,
When the control is performed with a constant current, the current flows into the static elimination needle and the transfer voltage becomes low, but when the control is performed with a constant voltage, the transfer voltage is constant even if the current flows into the static elimination needle. , The current simply increases, and there is no problem in transferability.

When the transfer material, which is a resistor, covers the charge removing member and the transfer bias does not drop, the charge removing member is controlled by a constant current so that the charge imparted by the transfer roller that is the transfer means is controlled. Can be reduced uniformly regardless of paper resistance, and stable paper transport can be realized regardless of paper type and environment. That is, when the constant current control of the charge removing member is performed, the current flowing between the transfer member and the charge removing member can be kept substantially constant without depending on the resistance unevenness of the transfer material such as paper and the potential of the photoconductor. Specifically, the amount of current on the transfer material where the charge is high increases, and the current on the transfer material where the charge is low decreases. Therefore, it is possible to reduce unevenness in the charge amount on the transfer material, and it is possible to make the charge amount on the transfer material uniform.

The process speed of the laser beam printer shown in the first embodiment is set to about 110 mm / sec, and the transfer bias is controlled by the detection current Io applied to the transfer roller 15 during the pre-rotation as shown in FIG. To 7 μA
Then, a constant current control was applied to a region corresponding to 5 mm of the rear end of the transfer material, and a constant voltage based on the voltage detected by the detection current Io was applied to the rest of the region.

The transfer roller 15 has a hardness of 30 ° (Asker C 500 gf) and has a transfer nip width N of 2 mm. Therefore, in the area of about 2 mm at the rear end of the transfer material 23, the transfer roller 15 is formed. The amount of charge provided by 15 will decrease.

As a comparative example, Table 1 shows the case where only a constant voltage is applied at the time of printing by using the conventional ATVC control and the constant current value applied at the rear end of the transfer material 23.
-1 is 6 μA, Example 1-2 is 7 μA, and Example 3-
In the case of No. 3, it was set to 8 μA, and image formation was performed for each. At that time, the quality of the transferred state, the state of the trailing edge splashing, and the presence or absence of peeling offset were examined. The paper used as the transfer material in this experiment has an A3 size and a basis weight of about 65 g / m @ 2.

[0046]

[Table 1]

It can be seen from Table 1 that the constant current value applied to the trailing edge of the paper must be at least 7 μA. However, when the constant current value is increased, the trailing edge is splashed and the peeling offset is effective, but the current flowing to the photosensitive drum side is increased and a memory is generated on the photosensitive drum, which causes an image defect. Therefore, attention must be paid to the magnitude of the applied current.

As shown in Table 1, by using the image forming apparatus according to the present embodiment, the amount of electric charge applied to the rear end of the transfer material 23 by the transfer roller 15 is the transfer material 23 when viewed from the transfer roller 15. In comparison with a case where the transfer nip N is sandwiched across the transfer nip N, a current flows to the photosensitive drum 11 side, so that the transfer voltage is transiently lowered, and the transfer material 23 is narrower than the transfer nip width N at the rear end thereof. The amount of charge in the area is
A smaller amount of electric charge is applied to the other transfer nip N than in the area where the transfer material 23 is completely sandwiched. Therefore, the trailing edge caused by sticking to the photosensitive drum,
Peeling offset can be prevented.

(Second Embodiment) In the second embodiment, the same laser beam printer as in the first embodiment is used, and the process speed is about 150 mm / sec.

The transfer bias control of this embodiment is shown in FIG.
As shown in, the detection current applied to the transfer roller 15 at the time of the front rotation is set to 9 μA, the area corresponding to 5 mm of the front end and the rear end of the paper is subjected to the constant current control, and the rest is based on the voltage detected by the detection current Io. A constant voltage Vt was applied.

The transfer roller 15 used in the second embodiment has a hardness of 30 ° (Asker C500 gf) and a transfer nip width N of 2 mm. Therefore, the transfer material 23 has a front / rear end of about 2 mm. In the area of
Will reduce the amount of charge given by.

[0052]

[Table 2]

In Table 2, as a comparative example, the case where a constant voltage is applied at the time of printing using the conventional ATVC control, and the constant current value applied at the rear end of the paper is 7 μm in Example 2-1.
A, 8 μA in Example 2-2, 90 μ in Example 2-3
A was set as A and image formation was performed for each. Then, the images and the conveyance and separation performances at that time were examined. The separation performance was examined in a high temperature / high humidity environment (HH), and evaluated on the second side during double-sided printing, which is disadvantageous in separating the photosensitive drum. The paper used in this experiment is of A3 size and has a basis weight of about 65 g / m @ 2.

From Table 2, the constant current value applied to the trailing edge of the paper is
It can be seen that a minimum of 8 μA is required. However, when the constant current value is increased, although the trailing edge and the peeling offset are effective, the current flowing to the photosensitive drum 11 side is increased and a memory is generated on the photosensitive drum 11, causing an image defect. Therefore, attention must be paid to the constant current applied.

As shown in Table 2, by using the image forming apparatus of the present embodiment, the amount of electric charge applied from the transfer roller 15 to the front / rear end of the transfer material 23 when viewed from the transfer roller 23 is the transfer material 23. Since a current flows to the photosensitive drum 11 side as compared with the case where the sheet is sandwiched across the transfer nip N,
The transfer voltage transiently decreases, and at the front / rear end of the transfer material 23, the charge amount in the area narrower than the transfer nip width N and the area in which the transfer material 23 is completely sandwiched in the other transfer nips N. Less charge will be imparted. Therefore, it is possible to prevent the HH second surface separation failure, the trailing edge splash, and the peeling offset due to the sticking to the photosensitive drum.

(Third Embodiment) In the present embodiment,
The same laser beam printer as in the first embodiment is used, and the process speed is 110 mm / sec.

In such a laser beam printer, the transfer bias control is changed according to the size of the transfer material 23 in the present embodiment.

Specifically, in the case of a paper having a width larger than the width of the letter, the detection current applied to the transfer roller 15 at the time of pre-rotation is set to 7 μA, and the rear end of the transfer material 23 corresponding to 5 mm is subjected to constant current control. Otherwise, the constant voltage Vt calculated by the average voltage Vto detected by the detection current Io is applied, and in the paper size width less than the letter size horizontal,
The conventional ATVC control of applying the constant voltage Vt calculated by the average voltage Vto with the detection current Io applied to the previous rotation is adopted.

This effect is as described in Table 1 in the first embodiment, but when a constant current is applied, as shown in FIG. In addition, the photosensitive drum 1 in which the transfer material 23 that is a resistor does not exist
A large amount of current may flow into the 1st side, which may cause a memory in the photosensitive drum 11 to cause an image defect. However, in the present embodiment, transfer bias control is switched depending on the paper size (in the case of FIG. 7, By the conventional control method), the image defect can be prevented.

When the transfer material 23 has a smaller size, it is possible to separate the transfer material 23 using the strain of the transfer material 23 without switching the transfer bias.

Further, the control in the case where the transfer material 23 has a large size may be performed by switching the transfer bias between the front end and the rear end of the transfer material 23, as shown in the second embodiment.

(Fourth Embodiment) In the present embodiment,
The same laser beam printer as in the first embodiment was used. The transfer bias was controlled in the same manner as in the second embodiment by applying a constant current to the front and rear ends of the transfer material 23 corresponding to 5 mm, and in other regions with a constant voltage.

At this time, the bias applied to the discharging needle 42 is a constant voltage from the print start time (T0) to the time when the tip of the transfer material 23 covers the discharging needle 42 (T1) as shown in FIG. After that, a constant current is applied until the time (T2) during which the transfer material 23 passes through the charge removal unit 24.

In this way, the tip of the transfer material 23 has the static elimination needle 4
Until it reaches 2, the drop of the transfer bias is prevented by controlling the bias applied to the static elimination needle 42 with a constant voltage, and the transfer failure due to the insufficient transfer bias at the tip of the transfer material 23 is prevented.

The paper, which is a resistor, is the static elimination unit 24.
In a state in which the transfer bias does not drop, the charge removal needle 42 is controlled by a constant current, so that the charge applied by the transfer roller 15 can be uniformly reduced regardless of the resistance of the transfer material 23. Therefore, stable transfer of the transfer material 23 can be realized regardless of the transfer material type and environment.

Further, the control of the static elimination unit 24 is changed for the first surface and the second surface of the transfer material, or at the front and rear ends of the transfer material 23 or for promoting the separation from the photosensitive drum 11 and preventing the rear end splashing. In addition, a bias stronger than the bias applied near the center of the transfer material 23 may be applied.

[0067]

As described above, according to the present invention,
Since excessive charges are not supplied to the rear end or both ends of the transfer material by a transfer member such as a transfer roller, the transfer material sticks to a photosensitive drum as an image carrier, resulting in defective separation, It is possible to prevent the conventional problems such as a cause of splashing on the rear end, and further, a peeling offset occurring due to peeling discharge when leaving the fixing device.

Further, when the tip of the transfer material passes through the charge removing means, the charge removing bias is controlled by a constant current, so that the charge imparted to the transfer material by the transfer member such as the transfer roller is applied to the resistance of the transfer material such as paper. It is possible to uniformly reduce the amount of paper regardless of the type of paper and the environment.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an LBP according to a first embodiment of this invention.

FIG. 2 is a waveform diagram of transfer bias control according to the first embodiment.

FIG. 3 is a diagram illustrating an effect of the first embodiment.

FIG. 4 is a schematic diagram of a static elimination unit used in the first embodiment.

5 is a front view of the static elimination needle of FIG. 4. FIG.

FIG. 6 is a waveform diagram of transfer bias control according to the second embodiment.

FIG. 7 is a diagram illustrating a third embodiment.

FIG. 8 is a waveform diagram of transfer bias control according to the fourth embodiment.

[Explanation of symbols]

11 Photosensitive drum, photoconductor 15 Transfer roller N transfer nip 23 Transfer material 24 Static elimination unit

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H200 FA06 GA23 HA02 HB06 HB12                       HB22 HB48 JA02 JA28 JA29                       JA30 KA02 KA07 KA28 KA29                       NA02 NA15 NA16 PA05 PA06                       PA10 PA23 PA29 PA30 PB12                       PB25

Claims (7)

[Claims] 1. A transfer member comprising at least an image carrier that carries an image to be transferred to a transfer material, and a transfer member that forms a transfer nip portion in contact with the image carrier and through which the transfer material passes. An image forming apparatus for transferring an image carried on the image carrier to the transfer material by applying a transfer bias to the transfer member, wherein the transfer material is transferred while the transfer material is passing through the transfer nip portion. An image forming apparatus characterized in that the transfer bias applied to a member is switched between constant voltage control and constant current control. 2. The transfer bias applied to the transfer member is controlled by a constant current with respect to the trailing end of the transfer material passing therethrough, and is otherwise controlled by a constant voltage.
The image forming apparatus described. 3. The transfer bias applied to the transfer member is controlled by a constant current with respect to the leading edge of the transfer material passing therethrough, and is otherwise controlled by a constant voltage.
The image forming apparatus described. 4. The image forming apparatus according to claim 1, wherein the transfer bias applied to the transfer member is controlled with a constant current with respect to the front and rear ends of the transfer material, and is controlled with a constant voltage during the transfer. apparatus. 5. The transfer bias applied to the transfer member is applied while the transfer material is passing through the transfer nip portion.
A first transfer bias application mode for switching between constant voltage control and constant current control and a second transfer bias application mode for only constant voltage control may be switched according to the size of the transfer material passing through the transfer nip portion. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. 6. An image carrier that carries an image to be transferred to a transfer material, a transfer member that forms a transfer nip portion that contacts the image carrier and through which the transfer material passes, and a separation bias is applied. Accordingly, the image carried by the image carrier is at least provided with a charge removing unit that accelerates separation of the transfer material that has undergone the transfer process from the image carrier, and a transfer bias is applied to the transfer member. In the image forming apparatus for transferring the toner to the transfer material, the bias applied to the charge removing unit is controlled by a constant voltage at the front end of the transfer material and by a constant current near the rear end of the transfer material. . 7. The image forming apparatus according to claim 6, wherein the transfer bias is switched between constant voltage control and constant current control while the transfer material is passing through the transfer nip portion.