JP2614315B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention forms a latent image on an image carrier by an electrophotographic system or an electrostatic recording system, and converts the latent image into a visible image by a developing unit. The present invention relates to an image forming apparatus that obtains an image by transferring the visible image to a transfer material by a transfer device, and in particular, has a plurality of developing units, and develops a latent image formed on an image carrier. The present invention can be suitably embodied in a multicolor image forming apparatus that obtains an image by sequentially transferring a toner image onto a transfer material by a transfer device.

(Prior art)

Conventionally, various multicolor image forming apparatuses have been proposed and commercially available. FIG. 11 shows an example of a typical color electrophotographic copying machine.

In FIG. 11, a color electrophotographic copying machine, which will be described in detail later, forms a latent image on an image carrier 3 which is an electrophotographic photosensitive drum in this example, and divides the latent image into a plurality of, for example, yellow and magenta. Developing units 1Y and 1 containing toner having an average particle diameter of 12 μm based on four kinds of resins, cyan, and black.
The rotary developing device 1 provided with M, 1C, and 1Bk forms a color toner image, and these toner images are transferred to a sheet-shaped transfer material such as paper attached to a drum-shaped transfer device 9 disposed close to the surface of the photosensitive drum 3. To obtain a color image by superimposing and transferring the color image.

As schematically shown in FIG. 7, a transfer device 9 used in such a color electrophotographic copying machine includes a pair of annular members arranged in parallel with each other, a cylindrical member 9a in this example,
9b is disposed concentrically, the two are integrally connected by a connecting member 9c, and further, over the entire circumference, a transfer material carrier usually formed of a dielectric material such as polyvinylidene fluoride resin (PVdF) sheet; That is, a transfer drum in which a dielectric sheet 93 is stretched and formed in a cylindrical shape is used.

Further, a gripper 91 is disposed on the outer surface of the connecting member 9c of the transfer drum 9 to grip and transfer the transfer material P supplied in time with the toner image on the photosensitive drum 3, and transfer the transfer material P to the transfer charger. Each time the toner image of each color is transferred to the transfer material at the transfer position where the transfer drum 10 is located, the color image is formed by transferring the toner image of each color to the transfer material, and then the gripper 91 is opened to separate the transfer material from the transfer drum. The separated transfer material is fed to the fixing unit 17 by the conveying unit 16.

Conventionally, in the transfer drum 9, the dielectric sheet 93
In order to increase the transfer nip by increasing the transfer nip and obtain a clear transfer image, the dielectric sheet 93 is pushed up from the inside of the transfer drum 9 toward the photosensitive drum 3 in the vicinity of the transfer charger 10. For this purpose, a back-up member 18 is provided as shown in FIGS. The back-up member 18 is made of an elastic material such as mylar and applies an appropriate pressure to the dielectric sheet 93.

On the other hand, in recent years, in order to meet the demand for higher image quality of images, the latent image is made finer, and in order to improve the reproducibility of the latent image, in order to improve the latent image, the particle size is smaller than that of the toner as the developer, that is, the average particle size is 10 μm or less. Toner particles having a particle size of about 8 μm have been used. In general, as the particle size of the toner particles decreases, the specific surface area increases, so that the charge amount per unit mass, Van der Waalska, and the like increase. As a result, the adhesion between the photosensitive drum and the toner particles becomes stronger, so that the dielectric sheet member is made uniform in order to maintain good transferability, and the role of the back-up member for obtaining the transfer nip is greater than ever. Has become. For this reason,
The pressure control on the dielectric sheet by the back-up member
It has become very important.

[Problems to be solved by the invention]

However, in the above conventional example, along with the durable use,
Permanent distortion occurs in the back-up member, and the pressing force obtained in the initial stage necessary for performing good transfer cannot be maintained, and a defective image having transfer unevenness or missing transfer occurs at about 20,000 A-size copies. Was.

Here, the problem of the occurrence of permanent distortion will be described with reference to FIGS.

8 to 10 are enlarged cross-sectional views of a main part showing a relational arrangement of a portion where the connecting member 9c of the transfer drum 9 exists, the transfer charger 10 and the back-up portion 18. FIG.

As shown in FIG. 8, in the interior of the transfer drum 9, the dielectric sheet 93 of the transfer drum 9 extends from the introduction side of the transfer drum 9 toward the rotation downstream direction in the vicinity of the transfer charger 10. There is provided a back-up member 18 for pressing the pressure.
When the back-up member 18 presses the dielectric sheet 93,
It can be seen that a large force is applied to the back-up member 18 at the fixed end 18A. Next, as shown in FIG. 9, when the connecting member 9c of the transfer drum 9 passes through the transfer position, that is, when it passes on the back-up member 18, the back-up member 18 is moved by the thickness S of the connecting member 9c. Bend over. For this reason, a very large force is applied to the portion 18A and / or the portion 18B that contacts the transfer charger 10. Therefore, the back-up member 18
Extremely large force is applied to this, causing permanent distortion.

Apart from this problem, when a paper jam occurs or the transfer drum 9 is inspected, the transfer drum also needs to rotate in the direction different from the normal image forming sequence, that is, reverse rotation. However, at this time, as shown in FIG. 10, since the end of the connecting member 9c collides with the tip of the back-up member 18, it is difficult to reversely rotate the transfer drum 9 when the transfer drum 9 rotates in the reverse direction. Alternatively, a large impact force was applied to the 18A portion of the back-up member to cause permanent deformation. For this reason, there is a disadvantage that the usability is poor, and as described above, a permanent distortion is caused in the back-up member, and the pressing force is reduced to cause an image defect.

Accordingly, an object of the present invention is to reduce the permanent distortion of the backup member itself in the transfer device, reduce the rate of reduction of the pressing force due to the permanent distortion of the backup member, and form a high-quality image, and furthermore, to use the transfer device easily. It is an object of the present invention to provide an image forming apparatus capable of improving image quality.

[Means for Solving the Problems] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention relates to an image forming apparatus for forming a visible image on an image carrier and transferring the visible image to a transfer material by a transfer device to obtain an image, wherein the transfer device supports the transfer material. A moving sheet-shaped transfer material carrier, and an elastic body that is located on the back surface of the transfer material carrier and biases the transfer material carrier toward the image carrier.
A first back-up member having a free length in contact with the transfer material carrier, and a second back-up member having a free length shorter than the free length of the first back-up member. 1, characterized in that the second back-up member is provided close to the second back-up member.

〔Example〕

Next, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

FIG. 6 shows an embodiment in which the present invention is embodied in a color electrophotographic copying machine. In this embodiment, the photosensitive drum 3 constituting the image carrier has a diameter of 80 mm and rotates in the direction of the arrow. Around the photosensitive drum 3, a primary charger 4, a rotary developing device 1 having developing devices 1Y, 1M, 1C, and 1Bk, a transfer drum 9 having a diameter of 160 mm constituting a transfer device, and a cleaning device 12
Is arranged. The program speed is set to 160mm / sec. Optical information corresponding to an image information signal color-separated by a laser optical system including a polygon mirror unit 8 and a laser reflecting mirror 7 is irradiated onto the photosensitive drum 3 to form a latent image on the photosensitive drum 3.

The latent image on the photosensitive drum 3 is frictionally charged with the carrier particles in the developing device at a developing position facing the photosensitive drum 3 by the rotation of the rotary developing device. To form a visible image (visible toner image).

A cassette transfer material P (hereinafter, referred to as transfer paper) is sent out of the cassette by a paper feed roller, passes through a first pair of registration rollers 19 and a second pair of registration rollers 6, and is sent to a paper feed guide 20. The transfer paper is timed by the first and second registration rollers 19 and 6, and the leading end is held by a gripper 91 on the transfer drum 9.
It is held on an upper transfer material carrier, that is, a dielectric sheet 93 made of polyvinylidene fluoride resin (PVdF) or the like.

The transfer paper held on the dielectric sheet 93 is transferred to the transfer charger 10
As a result, the toner image on the photosensitive drum 3 is transferred.

The transfer process is repeated for each color, and when the transfer is completed, the gripper 91 is opened and guided by the separation claw 15 to the transport means 16 while being discharged by the corona chargers 11, 13, and 14.
The transfer paper guided to the conveying means 16 is fixed by fixing means 17 having a pair of rollers, and is discharged onto a tray.

The toner image remaining on the photosensitive drum 3 after the transfer process was cleaned by the cleaning device 12, and the photosensitive drum 3 was prepared for the next image formation.

Further, as shown in FIG. 1, a back-up member 22 having elasticity extending from the introduction side of the dielectric sheet 93 of the transfer drum 9 to the downstream side in the moving direction within the discharge width of the transfer charger 10. Provided. 21 is the support. The back-up member 22 is made of a synthetic resin film such as polyethylene or polypropylene polyester, and is disposed over the entire length of the transfer portion in the longitudinal direction. The back-up member 22 presses the dielectric sheet 93 by its own elastic force, and the leading end on the dielectric sheet side has a position where the transfer paper has finished contacting the photosensitive drum or a position where the contact starts. Or a position corresponding to a position as close as possible.

If the transfer drum 9 is simply pressed, a back-up member may be provided from the side opposite to the introduction side of the dielectric sheet to the upstream side in the moving direction. The effect of suppressing image scattering caused by the start of transfer before the transfer material comes into contact cannot be obtained. Therefore, as shown in this embodiment, the pressing member is on the introduction side of the dielectric sheet (downstream from the transfer charger).
Therefore, it is preferable to provide it toward the downstream side in the moving direction.

The present inventors have proposed a photosensitive drum 3 and a dielectric sheet 93 shown in FIG.
As shown in an imaginary diagram when the backing member 22 is removed, the back-up member 22 is attached to the first and second two synthetic resin films 221, 222.
Formed. At this time, the first back-up member, 22
1 was made of polyethylene terephthalate resin having a width of 300 mm, a free length of 17 mm and a thickness of 125 μm. The second backup member 222 was formed of polyethylene terephthalate resin having a width of 300 mm, a free length of 7 mm, and a thickness of 100 μm. The tip of the second backup member 222 and the dielectric sheet 93 of the transfer drum 9
The distance from the surface was about 5 mm, and it did not come into contact with the connecting portion 9c having a thickness of 5 mm. Then, as shown in FIG.
The back-up members were provided close to each other.

From the initial state, the present inventors repeatedly performed image formation of A4 size paper output, and measured a change in the pressing force of the backup member with respect to the output number. At the same time, the change in the pressing force of the back-up member having the configuration in the conventional example having the same initial pressure was also measured. These results are shown in FIG.

In FIG. 4, two solid lines indicate a range of pressure that does not cause transfer unevenness or transfer loss. If the pressure is below the lower solid line, a sufficient pressing force cannot be obtained. Insufficient adhesion may occur, causing a decrease in transfer efficiency, unevenness, and the like, resulting in poor transfer.
On the other hand, if it exceeds the upper solid line, an excessive pressure is applied from the back-up member to the transfer drum side, which may cause rotation unevenness of the transfer drum and deteriorate image quality. The pressing force of the back-up member having the configuration shown in the conventional example is out of the pressure range in which good transfer can be performed with about 20,000 copies, while the back-up member of the present invention even after performing 100,000 or more copies. It can be seen that the pressure is in the pressure range where good transfer can be performed. It is believed that the durability of the back-up member was improved in this manner for the following reasons.

This will be described with reference to FIG. FIG. 3 shows a state in which the first and second backup members 221 and 222 according to the present invention are in contact with the connecting portion 9c of the transfer drum 9. By providing the second back-up member 222, the first back-up member 2212 exerts an excessively large force at the fixed end and the shield plate portion of the transfer charger 10 as shown in the above-described conventional example. , And a force is applied over the entire area where the second back-up member 222 is present, so that the generation of permanent distortion can be reduced and the durability can be improved. As a result, a stable image can be formed over a long period of time.

Separately, when the transfer drum is rotated in the reverse direction, the second back-up member 222 does not come into contact with the connecting member 9c as can be clearly understood from FIG. It collides with only one back-up member 221. For this reason, reverse rotation is possible with about half the force as compared with the conventional art, and the usability is improved. And second
Since the back-up member 222 does not receive any force due to the connecting portion 9c, permanent distortion does not occur during reverse rotation. Therefore, it is possible to improve the durability against a decrease in the pressing force, and it is possible to form a stable image for a long period of time.

As a result of further research, the present inventors have found that when the particle size of the toner, which is a developing material formed on the photosensitive drum 3, is set to 10 μm or less and 8 μm on average, the present invention It has been found that the transfer device provides a greater effect. As described above, since the toner having a small particle diameter has an increased adhesive force to the photosensitive drum, the transferability generally decreases. For this reason, the area where transfer unevenness does not occur also becomes narrow. In particular, it is necessary to set the lower limit of the pressing force to a value larger than when the toner having the average particle diameter of 12 μm is used. However, the use of the back-up member according to the present invention makes it possible to continue to form a stable and sufficient image over a long period of time.

In this embodiment, the number of the back-up members is two. However, the number of the back-up members is three or more. When at least one of the free lengths is different from the other, the same effect can be obtained. Further effects can be obtained if the transfer drum is not in contact with the connecting portion.

〔The invention's effect〕

As described above, according to the image forming apparatus of the present invention, the back-up member for pressing the transfer material carrier is constituted by a plurality of elastic members, and the free length of one of the back-up members is the length in contact with the transfer material carrier. Since the free length of the other back-up member is set shorter than that of the one back-up member, the reduction rate of the pressing force due to the permanent distortion of the back-up member is reduced, and the rotation of the transfer drum is required at the time of reverse rotation. It is possible to reduce the permanent strain on the back-up member at the same time as the force is reduced, so that there is an advantage that a high quality image is obtained and the usability is improved.

[Brief description of the drawings]

1, 2, 3, and 5 are cross-sectional views of a main part near a transfer portion of an image forming apparatus embodying the present invention, and FIG. 4 is a diagram illustrating a back-up member according to the present invention and a back-up member according to a conventional example. FIG. 6 is a schematic view of an image forming apparatus embodying the present invention, FIG. 7 is a perspective view of a transfer drum,
FIGS. 8, 9, and 10 are cross-sectional views of main parts near a transfer section of a conventional image forming apparatus, and FIG. 11 is a schematic view of the conventional image forming apparatus. 3 is a photosensitive drum (image carrier), 10 is a transfer charger, 93 is a dielectric sheet (transfer material carrier), and 221 and 222 are backup members.

Claims (4)

(57) [Claims] 1. An image forming apparatus for forming a visible image on an image carrier and transferring the visible image to a transfer material by a transfer device, wherein the transfer device carries the transfer material and moves. A sheet-shaped transfer material carrier, and first and second backup members, which are located on the back side of the transfer material carrier and are made of an elastic body for urging the transfer material carrier toward the image carrier, Wherein the first and second back-up members have a free length in contact with the transfer material carrier, and the second and back-up members have a free length shorter than the free length of the first back-up member. An image forming apparatus, which is provided adjacent to the image forming apparatus. 2. The transfer device has a pair of annular members arranged in parallel with each other and a connecting member for connecting the annular members, and the sheet-like transfer material carrier is formed by the annular member and the connecting member. The image forming apparatus according to claim 1, wherein the image forming apparatus is provided so as to be stretched over the formed annular area. 3. The image forming apparatus according to claim 2, wherein a free length of said second back-up member is set to a length which does not abut on said connecting member. 4. The transfer device is provided on the back side of a transfer material carrier.
A transfer charger provided opposite to the image carrier, wherein the first and second backup members are arranged in a moving direction of the transfer material carrier from a downstream side of the transfer charger to a downstream side of the moving direction. Claims 1 that are provided at an angle
Item 4. The image forming apparatus according to item 2 or 3.