JP3332672B2 - Image forming device - Google Patents

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 for transferring an image such as a developed image formed on an image carrier to a transfer material held on a transfer material carrying member.

[0002]

2. Description of the Related Art In an electrophotographic color copying machine, for example, a toner image is formed on a rotating cylindrical image carrier such as a photosensitive drum, and the toner image is transferred to a transfer material to form a monochrome image. When an image is obtained, the toner image is fixed, and when a multi-color image or a full-color image is obtained, the toner image formation and transfer process is repeated a predetermined number of times to superimpose and transfer the toner image onto a transfer material. By fixing, a predetermined color image is obtained. Usually, the transfer material is carried on a transfer drum having a configuration in which a transfer material support member, usually a transfer material support sheet such as a high-resistance film, is wound in a cylindrical shape on a drum type frame. That is, the transfer material supplied to the transfer drum is fixed and held on the peripheral surface of the transfer drum by applying an electrostatic attraction force between the transfer material carrying sheet and the transfer material at the transfer material supply position. It is wound around a transfer material carrying sheet (for example,
5-32079). Then, the transfer material is transferred to the transfer position by rotating the transfer drum, and the toner image is electrostatically transferred from the image carrier to the transfer material as described above to obtain a predetermined color image.

The transfer material is fed from a paper feed unit, and is sent to a suction position on a transfer material carrying sheet at a predetermined timing by a pair of conveying rollers. At the suction position, a corona charger is disposed on the back surface of the transfer material carrying sheet, and a conductive roller (charge injection roller) that is in contact with the transfer material to be attracted and held is disposed on the transfer material carrying sheet. The transfer material is electrostatically attracted to the transfer material carrying sheet by applying a charge of the opposite polarity to the toner image to the transfer material by the corona charger and the conductive roller.

Here, the conductive roller is driven by the transfer drum, and is configured to abut on the transfer drum to suck and transfer the transfer material at the same transfer speed as the transfer drum (transfer material carrying sheet). ing. It is also desired that the transfer speed of the transfer material by the transfer roller pair is the same as that of the transfer drum.

[0005]

However, in the above-described conventional configuration, the transfer drum is not fixed due to the dimensional accuracy of the components of the transfer drum, the dimensional accuracy of the pair of transport rollers, the rotational accuracy of the drive motor, or the wear of the transport rollers. It is extremely difficult to completely match the two transport speeds of the pair and the transport roller pair.

For example, when the speed of the transfer roller pair is smaller than the transfer speed of the transfer drum, the transfer roller pair pulls the rear side of the transfer material relative to each other, which may cause the transfer material to shift in position. As shown in FIG. 5A, the transfer material carrying sheet is deformed to cause a transfer failure or a transfer position shift between the first color toner image and the toner image of the next color and thereafter. Further, at the moment when the rear end of the transfer material passes through the pair of transport rollers, the transfer material carrying sheet is restored at a stretch,
The shock causes transfer unevenness and the like.

On the other hand, when the speed of the transfer roller pair is larger than the transfer speed of the transfer drum, the transfer roller pair pushes out the transfer material relative to each other, and as shown in FIG. Or the transfer material floats from the transfer material carrying sheet, and the same problems as described above occur.

[0008] This problem becomes more remarkable when the transfer material is a large size or a thick transfer material. This is a serious drawback in the copier market where high image quality is required, especially in the color copier market where color reproducibility is required to be improved. In addition, misalignment of the transfer material on the transfer material carrying sheet causes poor suction and suction jam, and also causes a jam in a separation process of separating the transfer material from the transfer material carrying sheet in a later process. Cause.

In order to solve the above-mentioned problem, there has already been a proposal to cancel the transfer material conveying means located upstream in the transfer material conveying direction.

However, opening all the transfer material conveying means for all the transfer materials requires a large-scale apparatus.
The durability of the machine parts also goes down. In the case of a thick recording material, the conveyance force may be insufficient with only the transfer attraction force.

Accordingly, it is an object of the present invention to perform an accurate transfer material adsorption step that does not cause a displacement of the transfer material on the transfer material carrier, thereby forming a good image, and improving the durability of mechanical parts. It is another object of the present invention to provide an image forming apparatus that takes account of the above.

[0012]

In order to achieve the above object, a first invention according to the present application is to adsorb and hold a transfer material on a transfer material carrier using an adsorbing means and move the transfer material to a transfer position. In an image forming apparatus configured to transfer a developed image transferred and developed on an image carrier to the transfer material, the image forming apparatus is held upstream in a transfer material transport direction from a transfer material suction position of the transfer material carrier. Providing a conveying means having a means for releasing the pressure,
The nipping pressure of the conveying member is released according to the type of the transfer material to be conveyed.

A second invention according to the present invention is characterized in that, in the first invention, the holding pressure releasing means of the conveying member operates only when the transfer material to be conveyed has a basis weight of about 120 g / m ² or more. I do.

According to a third aspect of the present invention, in the second aspect, the clamping pressure of the conveying member is released before the next transfer material is conveyed after releasing the clamping pressure of the conveying member and returning the clamping pressure. In order to return to the initial state, the conveying member is rotated.

[0015]

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIGS. 1 and 2 are a schematic sectional view and a schematic sectional view, respectively, showing an image forming apparatus according to a first embodiment of the present invention. The present invention is applied to an electrophotographic color copying machine. Show the case. This color copying machine has an image carrier, that is, a photosensitive drum 1 which is rotatably supported at its center and rotates in the direction of the arrow, and an image forming means is arranged on the outer peripheral portion thereof. This image forming means may be any means. In this example, a primary charger 2 for uniformly charging the photosensitive drum 1 and a light image color-separated at a predetermined timing or a light beam L corresponding thereto are charged to the photosensitive drum. An optical system 3 including, for example, a laser beam scanning device, which forms an electrostatic latent image on the photosensitive drum 1 by forming an image on the surface of the photosensitive drum 1;
And a developing device 4 that makes the upper electrostatic latent image a visible image are sequentially provided in the rotation direction of the photosensitive drum 1.

In the present embodiment, the developing device 4 is a horizontally movable developing device which moves in a tangential direction in opposition to the outer peripheral surface of the photosensitive drum 1, and includes, for example, a magenta developer, a cyan developer, and a yellow developer. 4 of color developer and black color developer
Four developing units 4M, 4C, each storing a color developer separately;
4Y, 4BK are provided. The horizontal moving developing device 4 uses a moving mechanism (not shown) to move a desired developing device to the outer peripheral surface of the photosensitive drum 1 in response to irradiation of a light image of a selected color by the optical system 3 or irradiation of a light beam L corresponding thereto. And a developer, that is, toner, is electrostatically scattered to develop an electrostatic latent image on the photosensitive drum 1 to form a toner image of a selected color. ing.

The toner image formed on the photosensitive drum 1 is
The image is transferred onto a transfer material carrier or a transfer material P carried and conveyed by the transfer device 6. In the present embodiment, the transfer device 6
The transfer drum 6 is rotatably supported on the transfer drum 6, and the transfer drum 6 is positioned in the rotation direction of the photosensitive drum 1.
The photosensitive drum 1 is opposed to or slightly away from the surface of the photosensitive drum 1. As schematically shown in FIG. 3, the transfer drum 6 has a pair of ring-shaped members 6b, 6b spaced apart and concentrically arranged, and the two are integrally connected by a connecting member (not shown) to form a sheet. The transfer material supporting sheet 6a, which is a dielectric material, is wound cylindrically over the entire circumference between the two ring-shaped members to cover the opening.
Further, as shown in FIG. 1, at the transfer position facing the photosensitive drum 1, on the side opposite to the carrying surface of the transfer material carrying sheet 6a,
That is, a transfer corona charger 7 is provided inside the transfer drum 6.

The transfer drum 6 is driven to rotate in the direction of the arrow shown in the figure, and on the upstream side of the transfer position where the corona charger 7 is disposed, on the side opposite to the transfer material carrying surface of the transfer material carrying sheet 6a.
That is, the suction corona charger 19 is provided inside the transfer drum 6.
However, the suction corona charger 1 faces the transfer material carrying surface of the transfer material carrying sheet 6a (with the transfer material carrying sheet interposed therebetween).
Conductor rollers (charge injection rollers) 20 as charge injection means are disposed respectively (opposed to 9).

On the other hand, on the downstream side of the transfer position, two corona dischargers 10 and 11 for static elimination are disposed with the transfer material carrying sheet 6a interposed therebetween, and downstream of the transfer material carrying sheet 6a. The held transfer material P is transferred to the supporting sheet 6a.
The pressing rollers 12, 13 for separating the transfer material from the sheet are arranged with the transfer material holding sheet 6a interposed therebetween, and a separation blade 14 is arranged near the holding surface in the vicinity thereof. Further downstream, a brush roller 15 for cleaning the carrying surface of the transfer material carrying sheet 6a and, if necessary, a corona discharger or a corona discharger for removing the adhesive force (residual Coulomb force and van der Waals force). A brush type static eliminator 16 is provided.

Further, a fixing device 18 comprising a pair of fixing rollers is disposed adjacent to the transfer drum 6, and a fixing blade 14 is provided.
Then, the toner image on the transfer material P separated from the transfer material carrying sheet 6a and conveyed by the conveyor 17 is fixed.

As shown in FIG. 1, around the photosensitive drum 1, a static eliminator 2 for removing electrostatic charges on the surface of the photosensitive drum 1 is provided.
7. A cleaning blade 28 for removing residual toner is provided. If necessary, the separation blade 14
A corona discharger 29 may be installed in the vicinity of, for the purpose of preventing the image from being disturbed by the peeling discharge generated when the transfer material P is separated from the transfer material carrying sheet 6a, and performing an AC corona discharge.

Next, the operation of the image forming apparatus of the present embodiment having such a configuration will be described.

When forming a monochromatic or multicolor (including full-color) color image, first, the surface of the photosensitive drum 1 is uniformly charged by the primary charger 2, and then the optical system 3 For example, when a color image passing through a green filter is irradiated, an electrostatic latent image mainly composed of a magenta component is formed in the color image. In synchronization with the feeding of the electrostatic latent image, the developing device 4 makes a horizontal movement in a tangential direction with respect to the photosensitive drum 1, and arranges a developing device 4M containing magenta developer at a position facing the photosensitive drum 1. Then, a magenta toner image is formed on the photosensitive drum 1.

On the other hand, the transfer material P stored in the cassettes 30 and 31 is sent out only from the one cassette selected by one of the pickup rollers 32 and 33, or the transfer material P is placed on the manual feed tray 34. Is fed by the pair of manual feed rollers 35 and 36, and further fed inside by the action of the pair of transport rollers 37, 38 or 39, 40. Next, the sheet is pulled out by the pulling-out rollers 100 and 101 and fed to the registration roller pair. Then, after the skew of the transfer material P is removed by the stopped registration roller pairs 41 and 42, the transfer material P is transferred to the registration roller pairs 41 and 4.
2 and the upper guide 21 and the lower guide 2 are synchronized.
2 and is guided by these vertical guides and conveyed along the surface (carrying surface) of the transfer material carrying sheet 6a to a transfer material supply position (suction position) where the conductive roller 20 is disposed. .

As shown in FIG. 2, the lower guide 22 is swingably supported about a fulcrum A, and is normally urged clockwise in FIG. The conductive roller 20 is rotatably attached to the distal end of the lower guide 22. Therefore, the conductive roller 20 is driven by a driving means (not shown) to rotate in the direction indicated by an arrow in the figure. By lower guide 2
2 swings around the fulcrum A, the transfer material carrying sheet 6a
Perform a movement that separates from and touches. The eccentric cam 221 is driven in synchronization with the supply timing of the transfer material P.
When the transfer material P is supplied to the supply position, the conductive roller 2
0 is positioned in a state of being in contact with the transfer material carrying sheet 6a as shown in FIG.

After the transfer material P is supplied to the transfer material supply position where the conductive roller 20 is located, the corona charger 19 electrostatically attracts and holds the transfer material P on the transfer material carrying sheet 6a to expose the transfer material P. It is sent to the transfer position where the transfer charger 7 is located, facing the drum 1. Of course, the registration roller 4
The timing of the transfer of the transfer material 1 and the timing of the formation of the latent image by the optical system 3 are synchronized and coincide with each other at the transfer position.

At this transfer position, the corona charger 7 for transfer is operated to generate a transfer electric field, and imparts a charge of a required polarity to the transfer material carrying sheet 6a to transfer the toner image on the photosensitive drum 1 to the transfer material. Transfer onto P.

According to the present embodiment, as shown in FIG. 2, at the timing when the rear end of the transfer material P passes through the conductive roller 20, the eccentric cam 221 is driven to rotate to the bottom dead center by the driving means. You. As a result, the lower guide 22 is urged downward against the elastic force of the coil spring 222, and rotates counterclockwise about the fulcrum A. As a result, the conductive roller 20 is moved to the transfer material carrying sheet 6a. Move to a position that is largely retracted from.

Next, the opening operation and timing of the conveying member for nipping and conveying the transfer material of the image forming apparatus having the above-described configuration will be described. For simplicity of description, only the paper feeding from the upper cassette 30 will be described, but the same applies to the paper feeding from the lower cassette 31.

FIG. 4A shows a state immediately before the feeding operation of the transfer material P. The pickup roller 32 is located at a position retracted from the transfer material P in the cassette 30, and presses against the transfer material P at the sheet feeding timing to feed only one uppermost transfer material P. The transfer material is fed by the pickup roller 32 until the leading end of the transfer material P enters the nip between the pair of transport rollers 37 and 38, and thereafter retreats again (see FIG. 4B). Pickup roller 32 and transport roller pair 37,3
The rotation driving of 8 is almost simultaneous start, and the pickup roller 32 may be rotating at the retracted position. Further, in the present embodiment, the lifting control of the cylindrical pickup roller is performed. However, the rotation control of the semi-cylindrical pickup roller may be performed, or another means capable of performing the same control may be used.

The transfer roller pair 37, 38 and the pull-out roller pair 100, 101 transfer the transfer material P until the leading edge of the transfer material reaches the nip portion of the stopped registration roller pair 41, 42, and further transfer. When the material P is conveyed until a loop is formed to remove the skew, the rotation of the conveying roller pairs 37 and 38 is stopped (see FIG. 4C). Then, at the transfer position, the pair of registration rollers 41 and 42 operate at timing when the transfer material P should coincide with the toner image formed on the photosensitive drum 1. At this time, the eccentric cam 221 makes a half turn, whereby the conductive roller 2
0 comes into contact with the transfer drum 6 and starts to rotate at the same linear speed as the transfer drum 6, and at the same time, the suction corona charger 19 inside the transfer drum 6 also starts operating. The leading end of the transfer material P is introduced into the upper and lower guides 21 and 22 and is guided to the suction position by the rotation of the pair of registration rollers 41 and 42. By the way, when the leading edge of the transfer material P slightly passes through the nip portion between the pair of registration rollers 41 and 42, even if the transfer material has the smallest size in the transfer material transport direction,
The stopped transport roller pair 37, 38 sandwiches the vicinity of the rear end of the transfer material.

In the case of the retard system, since the transfer roller pair has a separating ability, it continues to rotate until the trailing end of the transfer material comes off. In the case of the Duplo system, when the transfer material is nipped between the pull-out roller pairs 100 and 101, It will be in a stopped state.

The transfer of the transfer material P is performed by a pair of registration rollers 41,
42 dominates, the transfer drum 6 (the transfer material carrying sheet 6
Since it is extremely difficult to completely match the transfer speed of the transfer material of (a), when a predetermined amount of suction is performed before the leading edge of the transfer material reaches the transfer position, the registration roller pair 41,
Reference numeral 42 indicates an open posture. That is, the conveying member for nipping and conveying the transfer material is opened by means (not shown) for releasing the nipping pressure.

Although releasing the transport rollers 37 and 38 and the pickup roller is effective against transfer deviation, the transport force is insufficient at the time of thick paper, so it is effective to attach a pull-out roller to assist the transport force. It is. By the way, the speed accuracy of the motor and the processing accuracy of the rollers have been improved, so that the speed difference between the transfer drum and the registration roller can be kept within 1% even if the variation is considered. Therefore, even if the speed of the transfer drum is 150 mm / s and the registration roller is 151 mm / s, the A3 (420 mm × 297
mm), a loop caused by a speed difference between the transfer drum and the registration roller is within 3 mm even when the paper is conveyed. In such a case, if the guide shape is such that the loop can be absorbed even if a loop is formed, opening is not required for a transfer material having a basis weight of less than about 120 g / m ² . In addition, basis weight 120g /
Even if the transfer material is m ² or more, if the pull-out roller is kept rotating with the draw roller being at least 100 mm from the pull-out roller to the suction position and the registration roller being opened at least 2 mm, the pressing force due to the loop of the recording material between the transfer drum and the pull-out roller Does not affect the transfer, there is no need to open the pull-out roller and the transport roller. Further, when the registration roller is opened by 2 mm or more, the engagement of the upper and lower gears of the registration roller is completely disengaged. However, by applying a post-rotation that rotates the gear by 1/4 turn or more after the release state is completed, the gears are completely engaged, and the pressure of the registration roller is stabilized.

As described above, the registration roller pair 41,
Even if there is a difference in the transfer speed of the transfer material between the transfer roller 42 and the transfer drum 6, the transfer material is transferred to the pair of registration rollers 41, 4
As a result, the photosensitive drum 1 can be passed through the transfer position at the same speed as the photosensitive drum 1, without being pushed out or pulled relatively to the transfer material carrying sheet 6a.

According to the experiment of the present inventor, a PVDF film having a thickness of 150 μm was used as a transfer material carrying sheet,
This was wound into a cylindrical shape to form a transfer drum having a diameter of 160 mm, and a total current of 150 μA was applied to a corona charger for adsorption.
When a voltage of 6 kV is applied and a grounded metal roller is used as the conductive roller, the rollers other than the conductive roller are opened when the transfer material carrying sheet is attracted by about 50 mm from the leading edge of the transfer material. It was confirmed that the transfer material was conveyed in close contact with the transfer material-carrying sheet without causing suction failure, displacement of the suction position, and the like when the posture was set.

(Second Embodiment) Next, another embodiment to which the present invention is applied will be described with reference to FIG.

This embodiment is also an electrophotographic color copying machine, but has four image forming units 50M, 50C, 50
The configuration is different from the above-described embodiment in that Y and 50BK are linearly arranged. One image forming unit is a photosensitive drum 5
1, an erase lamp 52, a charger 53, a developing unit 54, and a cleaner 55 disposed therearound;
The first image forming unit 50M is magenta, the second image forming unit 50C is cyan, the third image forming unit 50Y is yellow, and the fourth image forming unit 50B.
K forms black color separation toner images, sequentially transfers and superimposes these color separation toner images onto the transfer material P, and collectively fixes them by the fixing device 18 to obtain a full color image.

The transfer material P stored in the cassette 30 is fed out by a pickup roller 32, and further, by the action of a pair of registration rollers 41 and 42, is transferred onto the surface of an endless transfer material carrying member, in this embodiment, a conveyor belt 60. The roller 19 is fed to the suction position where the charger 19 is disposed on the back surface of the conductive roller 20. In the present embodiment, the conductive roller 20 may be at a position where it is always in contact with the conveyor belt 60. The transport belt 60 is stretched around a driving pulley 61, a driven pulley 62, and a tensioner 63,
The transfer material P is held and conveyed to four image forming units, where the toner image is transferred by an internal transfer charger 7 and sent to a fixing device 18.

In the image forming apparatus of the present embodiment having the above-described structure, the pickup roller 32 is retracted from the transfer material at the top of the transfer material in the cassette, or the pressure is released, and the transfer material is attracted to the transport belt 60 by a predetermined amount. When the registration rollers 41 and 42 are released, the registration rollers 41 and 42 are opened, the holding pressure is released, or the rollers are freely rotated.
Is rotated, the transfer material P is introduced to the transfer position of the first image forming unit 50M by the rolling contact of the conductive roller 20, the suction force of the suction corona charger 19, and the conveyance force of the pull-out roller. . It has been confirmed by an experiment that the timing of opening the registration roller is 10 mm to 60 mm after passing through the first image forming unit, and the timing is not absorbed, and no positional deviation occurs.

As a result, the registration roller pair 41, 42
The transfer material P is transferred to the pair of registration rollers 41 and 4 even if a transfer speed difference of the transfer material P occurs between the transfer material P and the transfer belt 60.
2, it is not pushed out or pulled relatively, and is held in close contact with the transport belt 60.

As described above, the roller for nipping and transporting the transfer material P is opened or the nipping pressure is released until the leading end of the transfer material P reaches the transfer position of the most upstream image forming section in the transfer material movement direction. By making it freely rotate,
The transfer belt 60 can be formed without causing a suction displacement, a suction failure, or the like.
The transfer material P could be conveyed in close contact with the sheet.

In each of the above embodiments, the present invention is applied to an electrophotographic color copying machine. However, the present invention is not limited to the above-described embodiments, but may be applied to various other types of copying such as electrophotographic and electrostatic recording. The present invention can be equally applied to image forming apparatuses such as printers and printers. Needless to say, various modifications and changes can be made to members, elements, and the like constituting the image forming apparatus as needed.

This will be described in more detail. Reference numeral 103 denotes a transfer point at which the first developed image is transferred to the recording material. The distance from the pulling roller pair to the pulling roller pair is 100 mm or more, and the speed difference between the transfer belt 60 and the registration roller occurs, the loop of the recording material increases, and the distance is not affected by the pressing force of the recording material. The pull-out rollers 100 and 101 have a sufficient conveying force and are not affected by the conveying rollers 41 and 42. Further, since the guide 102 is a swing type, as the loop between the transfer belt 60 and the registration roller increases, the guide 102
Rises and the recording material does not receive the guide resistance. According to an experiment performed by the present inventors, when a PVDF film having a thickness of 150 μm was used as a transfer material carrying sheet, and a total current of 150 μA and a voltage of −6 kV were applied to a corona charger for adsorption, the transfer material carrying sheet was used. It was confirmed that if the registration roller was opened at the time when the transfer material was sucked by about 80 mm from the leading edge of the transfer material, the transfer material was conveyed in close contact with the transfer material carrying sheet without causing poor suction and displacement of the suction position. .

[0047]

As described above, according to the present invention,
When the transfer material is attracted to the transfer material carrier by a predetermined amount from the leading edge thereof, the holding member releases the holding pressure of the transfer member for holding and transferring the transfer material upstream of the conductive roller, and releases the transfer material. Since it is held in close contact with the body by the attraction force, it is possible to completely prevent the displacement of the suction position and the suction failure, the deformation and transfer failure of the transfer material carrier, and the transfer position deviation due to the difference in the transport speed. In addition, since the holding pressure of the conveying member is released only with the transfer material having the predetermined basis weight or more, the durability of the mechanical parts required for releasing the holding pressure can be increased. Further, if the pulling roller and the transfer position described in the embodiment are maintained in the above-described relationship, it is not necessary to release the pressure of the pulling roller and the paper feeding roller.

[Brief description of the drawings]

FIG. 1 is an overall schematic sectional view showing an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a schematic sectional view of a main part of the image forming apparatus according to the embodiment of the present invention.

FIG. 3 is a perspective view illustrating an example of a transfer device used in the image forming apparatus of FIG.

FIG. 4 is a schematic cross-sectional view for explaining a transfer mode of a transfer material.

FIG. 5 is a schematic sectional view for explaining a defect of a conventional image forming apparatus.

FIG. 6 is a schematic sectional view showing a second embodiment according to the present invention.

[Explanation of symbols]

1, 51: photosensitive drum 6: transfer drum (transfer device) 6a: transfer material carrying sheet 19: adsorption corona charger 20: conductive roller (charge injection roller) 32, 33
Pickup rollers 37, 38 ... Conveyance rollers 41, 42 ...
Registration roller 60: conveyor belt P: transfer material

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G03G 15/16 G03G 15/00 510

Claims (3)

(57) [Claims] 1. A transfer material is attracted and held on a transfer material carrier using a suction means, and the transfer material is transferred to a transfer position to transfer a developed image developed on the image carrier to the transfer material. In the image forming apparatus configured as described above, a transfer unit having a unit for releasing the holding pressure is provided upstream of the transfer material suction position of the transfer material carrier with respect to the transfer material transfer direction, and according to the type of the transfer material to be transferred. An image forming apparatus, wherein the holding pressure of the conveying member is released. 2. The image forming apparatus according to claim 1, wherein the nipping pressure releasing means of the transport member operates only when the transfer material to be transported has a basis weight of about 120 g / m ² or more. 3. The conveying member is rotated to release the nipping pressure of the transport member, return the nipping pressure, and return the nipping pressure of the transport member to an initial state before transporting the next transfer material. The image forming apparatus according to claim 2, wherein: