JPH10228188A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device having an intermediate transfer body which can hold a toner image, without causing the scatter and discharge skip of a primarily transferred toner image and be naturally discharged from a certain color transfer to the next color transfer, at the time of forming a full color image. SOLUTION: The image forming device is provided with an image carrier 1 on which the toner image corresponding to image information is formed, the intermediate transfer body 8 to which the toner image formed on the image carrier 1 is primarily transferred and a bias roll 11 for secondarily transferring an unfixed toner image primarily transferred to the intermediate transfer body 8 to a recording medium P. The surface resistivity and volume resistivity of the intermediate transfer body 8 are set within the range of 10<10> -10<14> Ω/squ. and <=10<13> Ωcm respectively. When the surface resistivity and the volume resistivity are expressed by 10<x> Ω/squ. and 10<y> Ωcm, between the surface resistivity and the volume resistivity, the relation of x>=y is established.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrophotographic copying machine,
The present invention relates to an image forming apparatus using an electrophotographic method, such as a laser printer, a facsimile, and a composite device thereof. More specifically, an intermediate transfer method in which a toner image formed on an image carrier is primarily transferred to an intermediate transfer body, and then this is secondarily transferred to a recording medium such as paper to obtain a reproduced image. Related to an image forming apparatus.

[0002]

2. Related Background Art Conventionally, a toner image formed on an image carrier such as a photosensitive drum is primarily transferred to an intermediate transfer member, and the toner image is secondarily transferred from the intermediate transfer member to a recording medium such as paper. As a forming apparatus, an apparatus disclosed in JP-A-62-206567 and the like is known. For example, a conventional belt transfer type image forming apparatus will be described below with reference to FIG. Image carrier according to image information
The electrostatic latent image formed in (1) is developed with toner in the developing device (4) and is visualized as a toner image. This toner image is electrostatically transferred to the intermediate transfer belt (8) in the primary transfer section while being carried on the image carrier (1). When transferring a multicolor image, the primary transfer is repeated for each color toner stored in the developing device (4). After completion of the primary transfer, when the unfixed toner image of a desired hue carried on the intermediate transfer belt (8) moves to the secondary transfer unit, the paper P is transferred to the secondary transfer unit in synchronization with the transfer. Conveyed. When the paper P passes through the secondary transfer section while receiving the pressing force between the backup roll (10) and the bias roll (11),
By applying a transfer voltage between (10, 11), the toner image carried on the transfer belt (8) is secondarily transferred to the paper P. The surface of the image bearing member (1) on which the primary transfer process of the toner image has been completed is cleaned by the cleaning device (6) and the static eliminator
The residual toner and electric charge are removed by (7), and the surface of the intermediate transfer belt (8) where the secondary transfer process is completed is removed by the belt cleaner (12) and the static eliminator to remove the residual toner and electric charge. Prepare for the process.

In the above-described image forming apparatus, there are the following devices that define the resistance value of the intermediate transfer member. For example, the volume resistivity is 10 ⁹ to 10 ¹² Ωc
m, the surface resistivity is 10 ⁷ to 10 ¹⁵ Ω / □, and the volume resistivity is 10 ⁸ to 10 ⁹ Ωcm.
No. 3871, the surface resistivity of a two-layer belt is 10
^{0 ~10 13 Ω} / □ in and volume resistivity 10 ⁰ ~10 ¹³ Ωc
m Hei 5-213504 discloses a surface resistivity of 10 ⁰
Japanese Unexamined Patent Publication (Kokai) No. 6-149081 having a density of from 10 to 10 ¹³ Ω / □.

[Problems to be solved by the invention]

If the surface resistivity of the intermediate transfer member is lower than a predetermined range, the electric charge in the vicinity of the nip portion between the image carrier and the intermediate transfer member is extended along the surface of the intermediate transfer member to a region wider than the nip portion. And the gap transfer occurs at the pre-nip of the primary transfer portion, causing image dispersion. Furthermore,
The charge holding the toner image once transferred onto the intermediate transfer member is attenuated, and the toner on the toner image scatters (scatters).
Resulting in. On the other hand, if the height is higher than the predetermined range, an excessive discharge phenomenon occurs at the post nip of the primary transfer portion where the intermediate transfer member is separated from the image bearing member, and the portion becomes white, resulting in discharge omission. On the other hand, if the volume resistivity of the intermediate transfer member is too high, static elimination cannot be performed spontaneously by the next image forming cycle while being charged, and thus a static elimination mechanism is required. In addition, along the intermediate transfer member, charging, exposure, development,
In a tandem type image forming apparatus in which a plurality of image forming units for transfer are provided in a plurality of rows, the toner image on the intermediate transfer member is not disturbed (unsteady) from transfer of one color to transfer of the next color. The intermediate transfer member itself, which holds the image, must be uncharged, that is, must be naturally neutralized. For this purpose, the balance between the surface resistivity and the volume resistivity of the intermediate transfer member must be considered.

[0005] However, in the conventional image forming apparatus, although the respective appropriate regions of the surface resistivity and the volume resistivity of the intermediate transfer member are known, the two types of resistivity related to the primary transfer have an appropriate relationship with each other. Is not known, and when at least one of the two resistivity values deviates from an appropriate region, a transfer failure such as transfer unevenness or retransfer may occur. Therefore, the present invention has been made in view of the above-described state of the art, and its purpose is to hold a toner image without causing scattering or discharge omission of a toner image to be primarily transferred, An object of the present invention is to provide an image forming apparatus provided with an intermediate transfer member capable of being naturally neutralized from transfer of one color to transfer of the next color when forming a full-color image.

[0006]

Means for Solving the Problems The inventors of the present invention have made intensive studies to eliminate the above-mentioned transfer failure caused by the resistance value of the intermediate transfer member. In the process of studying the relationship between the primary and secondary transfer efficiencies, the area of surface resistivity where scattering of toner images and discharge omission do not occur, and the area of volume resistivity where static elimination is performed by the next image forming cycle The present inventors have found that there is a predetermined correlation, and have accomplished the present invention. That is, the image forming apparatus of the present invention includes an image carrier on which a toner image corresponding to image information is formed, an intermediate transfer body on which a toner image formed on the image carrier is primarily transferred, and a primary image on the intermediate transfer body. A bias roll for secondarily transferring the transferred unfixed toner image to a recording medium, wherein the surface resistivity of the intermediate transfer member is in the range of 10 ¹⁰ to 10 ¹⁴ Ω / □, and the volume resistivity is 10 10
¹³ [Omega] cm or less, and when the surface resistivity was expressed 10 ^x Ω / □ and a volume resistivity at 10 ^y [Omega] cm, characterized in that it is a x ≧ y.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail mainly with reference to the drawings. The present invention is not particularly limited as long as it is an intermediate transfer type image forming apparatus using an intermediate transfer belt or a drum. For example, a normal mono-color image forming apparatus having only a single color toner in a developing device, or a color image forming apparatus in which a toner image carried on an image carrier such as a photosensitive drum is repeatedly primary-transferred to an intermediate transfer body sequentially The present invention is applied to a tandem-type color image forming apparatus and the like in which a plurality of image carriers having developing units for each color are arranged in series. In the image forming apparatuses shown in the following drawings, components having the same functions are denoted by the same reference numerals.

FIG. 1 is an explanatory view showing an outline of an image forming apparatus of an intermediate transfer belt type. In FIG. 1, a charging unit 2, an image writing unit 3, a developing unit 4, a primary transfer unit 5, a cleaning unit 6, and a cleaning unit 6 are sequentially arranged around an image carrier 1 formed of a photosensitive drum along the rotation direction. An electric device 7 and the like are arranged. Further, an intermediate transfer belt 8 which moves between the image carrier 1 and the primary transfer device 5 in the direction of the arrow while abutting on the surface of the image carrier 1 is stretched around belt transport rolls 9a, 9b, 9c and a backup roll 10. Have been. At a position facing the backup roll 10 and the belt transport roll 9a, a bias roll 11 and a belt cleaner 12 are disposed via the intermediate transfer belt 8, respectively. A portion where the primary transfer device 5 presses the image carrier 1 via the intermediate transfer belt 8 serves as a primary transfer portion, and a primary transfer voltage is applied between the image carrier 1 and the primary transfer device 5. A portion where the bias roll 11 presses the backup roll 10 serves as a secondary transfer portion, and a secondary transfer voltage is applied between the two rolls 10 and 11.

In the color image forming apparatus shown in FIG.
After the surface of the image carrier 1 rotating in the direction of the arrow is uniformly charged by the charger 2, the image is written by the image writing means 3 such as a laser beam.
An electrostatic latent image of the first color is formed. The electrostatic latent image is visualized by the developing device 4 containing the toner corresponding to the color, and a toner image is formed. When the toner image passes through the primary transfer portion, the toner image is electrostatically primarily transferred onto the intermediate transfer belt 8 by the primary transfer device 5. Similarly, the second color, the third color, and the fourth color toner images are primarily transferred onto the intermediate transfer belt 8 carrying the first color toner image so as to be sequentially superimposed, and finally full color. A multiple toner image is obtained. Here, the developing device 4, a plurality of developing units 4 ₁ to accommodating the toner corresponding to the electrostatic latent image of each color
It has 4 ₄ . That is, each developing unit has a black
(K), yellow (Y), magenta (M), and cyan (C) toners are stored. The recording medium (hereinafter, represented by paper P) supplied from the paper feed tray 13 at a predetermined timing when the multiplex toner image passes through the secondary transfer unit.
Are collectively transferred electrostatically. The sheet P to which the toner image has been transferred is conveyed to the fixing device 14 and subjected to a fixing process, and then discharged outside the apparatus. After the primary transfer, the residual toner and the electric charge are removed from the image carrier 1 by the cleaning device 6 and the static eliminator 7, and the intermediate transfer belt 8 after the secondary transfer is the belt cleaner 1.
2 removes the residual toner and prepares for the next image forming process.

FIG. 2 is a schematic structural view of the tandem type color image forming apparatus. In this image forming apparatus, a developing device containing toner of each color is arranged on a plurality of image carriers 1 arranged in series, and a toner image of each color is sequentially transferred to an intermediate transfer belt 8.
The primary transfer is different from the color image forming apparatus shown in FIG. That is, the surface of the image bearing member ₁ 1 to 1 ₄ charger 2
After being uniformly charged by ₂₁ to _24, the image writing section 3 ₁ for irradiating by converting an original image into electrical signals for each color laser light, etc.
The ~ 3 _4, an electrostatic latent image of each color is formed. Each electrostatic latent image developing device containing the toner corresponding to the color 4 ₁
The ~ 4 ₄ is visible, toner images of respective colors are formed. The primary transfer unit 5 ₁ to 5 ₄ are image bearing member 1 ₁
~ 1 ₄ opposite disposed, the respective color toner images are sequentially primarily transferred electrostatically onto the intermediate transfer belt 8. This primary transfer is performed when the intermediate transfer belt 8 carrying the first color toner image passes through the second and subsequent primary transfer portions.
Subsequent secondary transfer is the same as in the color image forming apparatus shown in FIG. In the image forming apparatus shown in FIG. 2, by arranged two image bearing member, to two disposed an appropriate developing units to each image carrier from among the developing device ₄₁ to _4, 2
It can also be used as a continuous tandem type full color image forming apparatus.

According to the present invention, the intermediate transfer belt 8 shown in FIGS. 1 and 2 can be replaced with a known intermediate transfer drum, and the operation is almost the same as that of the above-described image forming apparatus. In a color image forming apparatus having an intermediate transfer drum, the backup roll 10 is not always necessary. In the case of forming a multicolor image other than full color in the image forming apparatus shown in FIGS. 1 and 2, two or three developing units contain toner corresponding to the multicolor image. Further, an electrostatic latent image is formed on the image carrier 1 by the image writing means 3 which has been subjected to image processing so as to form a monochromatic electrostatic latent image, and only the toner corresponding to the color is developed by the developing device 4. In this case, the image forming apparatus shown in FIGS. 1 and 2 can be applied to a monocolor image forming apparatus. Further, the photosensitive drum (1) can be replaced with a known belt photosensitive member.

As the primary transfer device 5, a corona transfer device such as a corotron, a transfer roll, a transfer blade or the like is used. The layer structure of the primary transfer device 5 may be a single layer or a multilayer. For example, in the case of a single-layer structure,
Foamed or non-foamed silicone rubber, urethane rubber, E
It is composed of a roll in which an appropriate amount of conductive fine powder such as carbon black is mixed with PDM or the like. Its resistance value is 10 ⁵ -1
It is preferably in the range of 0 ¹⁰ Ω. A voltage of 1 to 5.5 kV is applied to the primary transfer device 5, and the primary transfer device 5 carries the voltage on the image carrier 1 as described above by the action of the electric field generated between the image carrier 1 and the primary transfer device 5. The transferred toner image is primarily transferred to the intermediate transfer belt 8. The backup roll 1
0 forms a counter electrode of the bias roll 11. The layer structure of the backup roll 10 may be either a single layer or a multilayer. For example, in the case of a single-layer structure, it is constituted of a roll in which an appropriate amount of conductive fine powder such as carbon black is mixed with silicone rubber, urethane rubber, EPDM, or the like. 2
In the case of a layered structure, it is constituted by a roll in which the outer peripheral surface of an elastic layer made of the above rubber material is covered with a high resistance layer. The surface resistivity of the backup roll 10 is 10 ⁷ to 1
It is preferably in the range of 0 ¹¹ Ω / □.

Bias roll 11 for forming transfer electrode
Is separated from the transfer belt 8 while the toner image carried on the image carrier 1 is primarily transferred onto the intermediate transfer belt 8, and the toner image carried on the transfer belt 8 is secondarily transferred onto the paper P. In this case, the transfer belt 8 is configured to be pressed against the transfer belt 8 and pressed against the backup roll 10. The layer structure of the bias roll 11 is not particularly limited. For example, in the case of a three-layer structure, the bias roll 11 includes a core layer, an intermediate layer, and a coating layer covering the surface thereof. The core layer is made of silicone rubber, urethane rubber, E
The foam is a foam such as PDM, and the intermediate layer is composed of these non-foams. The coating layer is preferably made of a fluororesin in which conductive fine powder is dispersed. Examples of the fluorine-based resin include a tetrafluoroethylene (TFE) -hexafluoropropylene copolymer (FEP) and a perfluoroalkoxy resin (PFA). It is preferable that the volume resistivity of the bias roll 11 is 10 ⁷ Ωcm or less. Further, a two-layer structure excluding the intermediate layer may be employed. The backup roll 10 and the bias roll 11
The secondary transfer voltage of 1 to 6 kV is usually applied between the shafts. Instead of applying a voltage to the shaft of the backup roll 10, a voltage may be applied between the electrode member having good electrical conductivity pressed against the roll 10 and the bias roll 11. As the electrode member, a metal roll, a conductive rubber roll, a conductive brush, a metal plate, a conductive resin plate, or the like is used.

In the present invention, the belt-like or drum-like intermediate transfer member may have a single-layer structure or a multilayer structure.
The multi-layer structure has problems such as delamination between layers and abrasion of the surface layer with time, and a single-layer structure is preferably used in view of productivity and cost. As a single-layer material,
A high-strength film made of a conductive agent dispersed in polyimide, a copolymer of polyimide and polyamide, polyethersulfone, polycarbonate resin, etc. is preferred to prevent the occurrence of color misregistration due to load fluctuation when driving the intermediate transfer body. Used for Examples of the conductive agent include metals or alloys such as carbon black, graphite, aluminum, nickel, copper, and copper alloys, tin oxide, zinc oxide, titanium oxide, potassium titanate, tin oxide-indium oxide composite oxide, and tin oxide-. One or more fine powders such as a conductive metal oxide such as an antimony oxide composite oxide are used. Further, an insulating fine particle such as barium sulfate may be coated with these conductive agents. Further, the film (substrate) can be covered with a surface layer made of a highly flexible material in order to prevent the generation of hollow characters due to the concentration of stress caused by the pressing force of the bias roll.
Such materials include TFE polymer, ethylene-TF
Fluoropolymer materials such as E copolymer, FEP, and polyfluoroethyl vinyl ether crosslinked with polyisocyanate, and polyesters are preferably used. The thickness of the surface layer is preferably in the range of 10 to 50 μm.

Further, an intermediate layer made of an elastic material may be interposed between the base layer and the surface layer so that the intermediate layer has a function of preventing the generation of hollow characters. As the elastic material, any rubber material such as fluorine-based rubber and silicone rubber can be used. The thickness of the intermediate layer is 20-65
It is preferably in the range of μm. In the case of a three-layer structure, the surface layer has a thickness of 1 μm so as to prevent the intermediate transfer member from sticking to the surface of the intermediate transfer member due to the thickness.
It is preferable to use the above-mentioned fluorine-based polymer material having a size of μm or less. When an intermediate transfer drum is used as the intermediate transfer member, the outer peripheral surface of a cylindrical substrate made of aluminum, stainless steel (SUS), copper, or the like having a thickness of 3 mm or more is coated with the same material as described above so that the drum is not deformed. Is done. The thickness of the intermediate transfer member is 50 to 200 μm, particularly 70 to 100 μm.
It is preferably in the range of μm.

In the present invention, the intermediate transfer member has a surface resistivity in the range of 10 ¹⁰ to 10 ¹⁴ Ω / □ and a volume resistivity of 1
0 ¹³ Ωcm or less, and the resistivity (absolute value) of the former is equal to or larger than that of the latter. The reason will be described with reference to FIG. FIG. 3 shows that the peripheral speeds of the image carrier and the intermediate transfer member are set to 30, 50, 200, and 500 mm / sec.
Here, the allowable ranges of the surface resistivity and the volume resistivity of the intermediate transfer member are shown. As shown in FIG. 3, the lower limit of the surface resistivity is:
It is determined by the spread of the charges near the primary transfer portion and the start of image variation due to early decay. This is because if the surface resistivity is lower than a predetermined value at each peripheral speed, image deviation occurs, and the higher the peripheral speed, the lower the allowable value of the surface resistivity. In addition, the upper limit of the surface resistivity is determined by the start of white spots due to electric discharge generated when the intermediate transfer member is separated from the image carrier at the post nip of the primary transfer portion. This is because discharge loss occurs at 10 ¹⁴ Ω / □ or more irrespective of the peripheral speed. On the other hand, the upper limit of the volume resistivity is determined by whether or not the charge is naturally removed by the time the next color is transferred. This is because when the volume resistivity becomes higher than a predetermined value at each peripheral speed, the charge remains and the intermediate transfer member is charged until the next transfer, and the higher the peripheral speed, the lower the allowable value of the volume resistivity becomes. Become.
If the volume resistivity is lower than a predetermined value, the insulation resistance is reduced and the intermediate transfer member is destroyed due to leakage. Therefore, the volume resistivity is preferably 10 ⁵ Ωcm or more. For the above reasons, in order to avoid transfer failure in the primary transfer section, as shown in FIG. 3, the surface resistivity of the intermediate transfer member is in the range of 10 ¹⁰ to 10 ¹⁴ Ω / □, and the volume resistivity is It can be seen that the surface resistivity is equal to or less than 10 ¹³ Ωcm and the surface resistivity is equal to or larger than the volume resistivity. Then, if the resistance value of the intermediate transfer member has such a relationship,
The process speed in the image forming apparatus of the present invention can be increased to 500 mm / sec.

Next, the charging state in the surface direction and the volume direction of the intermediate transfer member at the primary transfer section in the tandem type color image forming apparatus will be described with reference to FIG. When the first color toner image is primarily transferred, it is charged both in the surface direction and in the volume direction of the intermediate transfer body. Before the transfer of the second color, the toner image of the first color must be held while maintaining the charged potential of the intermediate transfer body at a predetermined value or more in the surface direction so as to prevent image fluctuation. On the other hand, in the volume direction, the charged potential must be attenuated to 0 V before receiving the transfer of the second color so that the charge is not accumulated. The same applies to the transfer of the third and fourth colors. Since the charging potential of the intermediate transfer member is proportional to its resistance value, when the space between the primary transfer portions is short as in a tandem color image forming apparatus, the surface resistivity is equal to the volume resistivity as described above. Or larger. That is, in order to perform primary transfer without causing image variation, the surface resistivity is set to 10 ^× Ω.
/ □ and when represents a volume resistivity at 10 ^y [Omega] cm, x
≧ y.

The operation of the present invention is as follows. According to the image forming apparatus of the present invention, the surface resistivity of the intermediate transfer member is set to 10 ¹⁰ to 1.
By setting it to 0 ¹⁴ Ω / □, transfer failure does not occur in the primary transfer portion. That is, since the surface resistivity is 10 ¹⁰ Ω / □ or more, the electric charge does not spread to a region wider than the nip portion along the surface of the intermediate transfer member near the nip portion of the primary transfer portion. Does not occur. Furthermore, the electric charge that carries the transferred toner image on the intermediate transfer member is retained, and the toner on the toner image does not scatter. In addition, the surface resistivity is 10 ¹⁴ Ω
Since it is / □ or less, an excessive discharge phenomenon does not occur in the post nip of the primary transfer portion. On the other hand, by setting the volume resistivity of the intermediate transfer member to 10 ¹³ Ωcm or less, the intermediate transfer member is naturally discharged by the next primary transfer, and the charge removing mechanism becomes unnecessary. Therefore, even if the charge elimination mechanism is not provided, the charge is not accumulated in the next and subsequent cycles, so that the toner image is not scattered due to the charge-up. Further, by making the absolute value of the surface resistivity equal to or greater than the absolute value of the volume resistivity, the toner image can be formed even when the interval between the primary transfer portions is short, particularly as in a tandem image forming apparatus. The toner image can be held on the surface of the intermediate transfer member without disturbing the image, and the charge is naturally removed in the volume direction of the intermediate transfer member.

[0019]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited to the following Examples. (Image Forming Apparatus) FIG. 5 is an overall view of a digital color copying machine provided with an intermediate transfer belt as an image forming apparatus of the present invention. In FIG. 5, light emitted from a document illumination lamp 22 moving along the lower surface of a document (not shown) placed on a platen 21 and reflected by the document is reflected by a moving mirror unit 23, a lens 24, and a fixed mirror. The image is converged on the CCD of the image reading unit via the line 25. The CCD converts the original image into an electric signal for each color by using a large number of photoelectric conversion elements and three color filters of red (R), green (G), and blue (B). The electric signal is input to the image processing circuit 26, and the image processing circuit 26 has an image memory that converts a document image reading signal input for each color into a digital signal and stores it. The optical writing control device 27 reads out the image data of the image processing circuit 26 at a predetermined timing and outputs it to the light beam writing device 28. Light beam writing device 28
Writes an electrostatic latent image corresponding to each color on the image carrier 1 composed of a photosensitive drum rotating in the direction of the arrow. The image writing means 3 is composed of these numbers 21 to 28.

Around the image carrier 1, a charger 2 for charging the surface to a predetermined dark potential, a developing device 4 for developing the electrostatic latent image written on the image carrier 1 into toner images of each color,
A primary transfer roll 5 for transferring a toner image of each color to the intermediate transfer belt 8, a cleaning device 6 having a cleaning blade, and a static eliminator 7 are arranged. The developing device 4 has a developing device containing toner of each color of K, Y, M, and C, and develops and visualizes the electrostatic latent image with the toner of each color. The intermediate transfer belt 8 is stretched around the belt transport rolls 9a, 9b (tension rolls) and 9c and the backup roll 10, and moves in a tangential direction while abutting on the surface of the image carrier 1. On the front side of the transfer belt 8 carrying the unfixed toner image, the bias roll 11 and the belt cleaner 1 are opposed to the backup roll 10 and the belt transport roll 9a, respectively.
2 are arranged. An electrode member 29 connected to a power supply that applies a secondary transfer voltage having the same polarity as the toner is pressed against the backup roll 10. In addition, between the backup roll 10 and the belt transport roll 9a, a peeling claw 30 for peeling the paper P carrying the secondary-transferred toner image from the transfer belt 8 is arranged.

An extractable paper feed tray 13 is provided below the main body of the image forming apparatus U, and a pickup roller 31 is arranged above the paper feed tray 13. This pickup roller 3
A pair of feed rolls 32 for preventing double feeding of the paper P, a paper transporting roll 33, a guide member 34 for guiding the paper P, and a registration roll 35 are sequentially arranged downstream of 1. On the downstream side of the secondary transfer section, a transport belt 36 for transporting a sheet P carrying a secondary-transferred toner image sequentially, a fixing device 14 for fixing an unfixed toner image on the sheet P, a fixed image A pair of discharge rolls 37 for discharging the paper P on which the paper P is formed outside the apparatus, and a paper discharge tray 38 for mounting the discharged paper P are arranged.

(Operation of Image Forming Apparatus) The surface of the image carrier 1 rotating in the direction of arrow A is charged to a predetermined dark potential by the charger 2, and an electrostatic latent image is written by the light beam writing device 28. . The electrostatic latent image on the image carrier 1 is developed by the developing device 4 into an unfixed toner image. In the formation of the toner image, a toner image of a first color is formed first, and thereafter, every time the image carrier 1 rotates for a predetermined time, a toner image of a second color to a fourth color is formed. In this embodiment, K, Y,
M and C color toner images are sequentially formed. After the toner image is transferred to the intermediate transfer belt 8, the surface of the image carrier 1 is cleaned by the cleaning device 6, and the charge is removed by the charge remover 7. Here, the optical writing control device 27 first reads out the digital signal image-processed to the first K color and outputs it to the light beam writing device 28. This writing device 28 writes an electrostatic latent image corresponding to K color on the surface of the image carrier 1. The electrostatic latent image corresponding to the color K is developed into a visualized toner image of the color K by the developing device K in the developing device 4 and moves to the primary transfer section. In the primary transfer section, an electric field having a polarity opposite to the charging polarity is applied to the toner image from the primary transfer roll 5 disposed on the back side of the intermediate transfer belt 8 so that the K color toner image reaches the primary transfer section. Is primarily transferred by moving the transfer belt 8 in the direction of arrow B while electrostatically attracting the transfer belt 8 to the transfer belt 8.

The intermediate transfer belt 8 moves at the same period as the image carrier 1 while adsorbing and carrying the K toner image. When the transfer of the K toner image of the first color is completed, the output of the optical writing control device 27 causes the blue (B) filter to be used by the output of the optical writing controller 27 until the transfer start position of the K toner image on the transfer belt 8 reaches the primary transfer portion. Writing of the electrostatic latent image corresponding to the color-separated light image is started. When the transfer start position of the transfer belt 8 carrying the K toner image reaches the primary transfer portion, the transfer of the second color Y toner image is performed by the primary transfer roll 5. Subsequently, the electrostatic latent images corresponding to the light images color-separated by the green (G) and red (R) filters are developed.
The transfer of the M toner image and the C toner image is performed in the same manner as the transfer of the Y toner image. In this way, a multiple toner image superimposed on each color is formed on the intermediate transfer belt 8. Until the toner images of each color are primarily transferred onto the transfer belt 8, the bias roll 11, the peeling claw 29, and the belt cleaner 12 disposed on the surface side of the transfer belt 8 are in the retracted positions separated from the transfer belt 8. Is held.

On the other hand, the paper P stored in the paper feed tray 13
Are taken out one by one by a pickup roller 31 at a predetermined timing, fed by a pair of feed rolls 32 and a paper transport roll 33, and temporarily stopped by a pair of registration rolls 35. The sheet P is then transferred from the registration roll 35 to the secondary transfer section in synchronization with the transfer of the multiple toner image of each color (K, Y, M, C) transferred onto the intermediate transfer belt 8 to the secondary transfer section. It is transported to the transfer section. In the secondary transfer section, the bias roll 11 is in pressure contact with the backup roll 10 via the intermediate transfer belt 8.
Then, the conveyed sheet P passes through the secondary transfer section by the press-contact conveyance between the rolls 10 and 11 and the movement of the transfer belt 8. At this time, a transfer voltage having the same polarity as the charging polarity of the toner image is applied to the electrode member 29, so that the multiple toner image adsorbed and carried on the transfer belt 8 is secondarily transferred from the surface of the transfer belt 8 to the sheet P. .

Although the transfer of a full-color image has been described above, when a single-color image is formed, the intermediate transfer belt 8 is used.
When, for example, a K-color toner image primary-transferred above moves to the secondary transfer section, the toner image is immediately transferred to the paper P. In the case of forming an image of a plurality of colors, the desired hue is selected, and when the multicolor toner image superimposed on those colors moves to the secondary transfer section, the toner image may be transferred to the paper P. . As described above, the paper P on which the toner image has been transferred to the desired hue is peeled by the operation of the peeling claw 30,
The sheet is placed on the conveyance belt 36 and is conveyed to the fixing device 14. After fixing the unfixed toner image to the permanent image in the fixing device 14, the sheet P is discharged to a discharge tray 38 by a pair of discharge rollers 37. When the secondary transfer is completed, the intermediate transfer belt 8 is cleaned by a belt cleaner 12 provided downstream of the secondary transfer unit, and prepares for the next transfer.

(Primary Transfer Section and Secondary Transfer Section) The specific structures of the primary transfer section and the secondary transfer section in the image forming apparatus shown in FIG. 5 are as follows. As the image carrier (1), the outer diameter is 84 mm, the surface of which is coated with polycarbonate.
OPC photosensitive drum was used. As the primary transfer roll (5), a carbon black-dispersed urethane foam roll having an outer diameter of 18.7 mm was used. This primary transfer roll
(5), the volume resistivity at the applied voltage 1kV in the test chamber environment is 10 ^7.5 [Omega] cm. As the intermediate transfer belt (8), a carbon black-dispersed polyimide resin having a thickness of 80 μm was used. This transfer belt (8) has a surface resistivity of 10 ¹² Ω / □ and a volume resistivity of 10 ¹⁰ Ωcm.
The method of measuring the resistance value of the intermediate transfer belt (8) is as follows. Micro ammeter (R8340)
A: A main electrode having an outer diameter d of 1.6 cm and a guard electrode having an inner diameter D of 3.0 cm (HR probe: manufactured by Mitsubishi Yuka Co., Ltd.) were used as probes. The measurement table (manufactured by Mitsubishi Yuka Co., Ltd.) used an insulating surface when measuring the surface resistivity, and used a conductive surface when measuring the volume resistivity. Then, the resistance value R (Ω) was measured at an applied voltage of 100 V, a load of 2 kg, and a measurement time of 30 seconds. Assuming that the thickness of the belt is t (cm), the surface resistivity ρs
(Ω / □) and volume resistivity ρv (Ωcm) are represented by the following equations. ρs = 2πR / ln (D / d) ρv = πd ² R / 4t

As the backup roll (10), a rubber roll having an outer diameter of 28 mm and having a two-layer structure in which a SUS shaft having an outer diameter of 14 mm was covered with an elastic layer and a high resistance layer was used. The elastic layer is made of EPDM molded to a thickness of 5 mm, and the high-resistance layer is 2 m thick in which carbon black is dispersed.
m of EPDM. The surface resistivity of the backup roll (10) was 10 ⁹ Ω / □. A SUS roll having a diameter of 14 mm was used as the electrode member (29). The electrode member (29) was pressed against a backup roll (10) 180 ° away from the secondary transfer portion. Bias roll (1
As 1), a three-layer roll having the same outer diameter as the backup roll (10) was used. The roll (11) is formed by sequentially covering a grounded SUS shaft having an outer diameter of 14 mm with a urethane foam rubber having a thickness of 5 mm, a urethane rubber having a thickness of 2 mm, and a fluorine resin layer having a thickness of 30 μm.
Carbon black is dispersed in each layer, and the volume resistivity of the bias roll (11) at an applied voltage of 1 kV in a test room environment is 10 ⁶ Ωcm.

(Primary and Secondary Transfer Efficiency) In the image forming apparatus of the embodiment, the peripheral speeds of the image carrier and the intermediate transfer belt were changed to 50, 200 and 500 mm / sec, respectively, while changing the applied voltage. The transfer efficiency was measured. 6 and 7 show the relationship between the applied voltage and the primary transfer efficiency and the secondary transfer efficiency at that time. As shown in FIGS. 6 and 7, the higher the peripheral speed, the higher the applied voltage at which both the primary and secondary transfer efficiencies peak, and if the optimal applied voltage is set according to the peripheral speed, the transfer efficiency can be reduced. There is no difference. That is, when the surface resistivity and the volume resistivity of the intermediate transfer member satisfy the above conditions, the peripheral speeds of the image carrier and the intermediate transfer member in the image forming apparatus of the present invention are temporarily set to 500.
mm / sec.

Next, in the image forming apparatus of the embodiment in which a voltage of -2 kV was applied to the electrode member, the surface resistivity ρs of the intermediate transfer belt was set to 10 ¹⁰ Ω / □ and 10 ¹² Ω / □, While changing the resistivity ρv, a primary transfer current at which good transferability was obtained was determined. Ρv (Ωc
FIG. 8 shows the relationship between m) and the primary transfer current (μA). As shown in FIG. 8, when the primary transfer current is smaller than a predetermined value, the transfer electric field is weakened, and transfer unevenness (transfer failure) occurs. On the other hand, if the value is larger than the predetermined value, an excessive current flows, so that the toner is charged to the opposite polarity, so that the toner transferred to the intermediate transfer belt is transferred to the image carrier again. As a result, the image density decreases. When ρv is changed while ρs is kept constant, as ρv increases, the resistance value also increases, so that a large amount of current needs to flow, and the primary transfer current required to avoid the occurrence of transfer failure increases. .
On the other hand, if the applied voltage is increased and the current flows too much, the above-described retransfer occurs. To avoid this, the primary transfer current must be reduced as ρv increases. Comparing the magnitudes of ρs, when ρs is low at 10 ¹⁰ Ω / □, ρv at which good transferability is obtained is low, and the required primary transfer current is small, but retransfer occurs. The upper limit of the current is also reduced. As can be seen from FIG. 8 showing the relationship between the resistance value of the intermediate transfer member and the primary transfer current at which good transferability can be obtained, ρ
It is shown that s is equal to ρv or larger in absolute value.

[0030]

According to the image forming apparatus of the present invention, a discharge occurs in which image dispersion occurs in the pre-nip of the primary transfer portion, toner scatters on the upper portion of the toner image, or toner comes off white in the post nip of the primary transfer portion. No dropout occurs. In addition, the intermediate transfer body is naturally neutralized by the next primary transfer, and a neutralization mechanism is not required. In particular, even if the distance between the primary transfer portions is short, as in a tandem type color image forming apparatus, the toner image can be held on the intermediate transfer member without disturbing the toner image. Is neutralized by the next primary transfer.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus of the present invention.

FIG. 2 is a schematic configuration diagram of another image forming apparatus of the present invention.

FIG. 3 is a graph showing an allowable range of a surface resistivity and a volume resistivity of an intermediate transfer member.

FIG. 4 is a graph illustrating a charging state in a surface direction and a volume direction of an intermediate transfer body in a primary transfer unit of a tandem type color image forming apparatus.

FIG. 5 is an overall view of an image forming apparatus shown as one embodiment of the present invention.

FIG. 6 is a graph showing a relationship between a primary applied voltage and a primary transfer efficiency when peripheral speeds of an image carrier and an intermediate transfer member are changed.

FIG. 7 is a graph showing the relationship between the secondary applied voltage and the secondary transfer efficiency when the peripheral speeds of the image carrier and the intermediate transfer member are changed.

FIG. 8 is a graph showing a relationship between a resistance value of an intermediate transfer member and a primary transfer current at which good transferability is obtained.

[Explanation of symbols]

U: image forming apparatus, P: recording medium (paper), 1: image carrier, 4: developing device, 5: primary transfer device, 8: intermediate transfer belt, 10: backup roll, 11: bias roll.

Claims (2)

[Claims] An image bearing member on which a toner image corresponding to image information is formed, an intermediate transfer member on which a toner image formed on the image bearing member is primarily transferred, and an unfixed image primarily transferred on the intermediate transfer member. A bias roll for secondary transfer of the toner image to a recording medium, wherein the intermediate transfer body has a surface resistivity of 10 ¹⁰ to 10 ¹⁴
Ω / □, volume resistivity is 10 ¹³ Ωcm or less, and x ≧ y when the surface resistivity is represented by 10 ^x Ω / □ and the volume resistivity is represented by 10 ^y Ωcm. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the intermediate transfer body has a single-layer structure.