JPH10207246A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device equipped with an intermediate transfer body by which a high-quality transfer image can be obtained by preventing such a phenomenon that toner is splashed to the circumference of a multi-color image from occurring and preventing the void of a background part from occurring around a line image. SOLUTION: This image forming device is equipped with an image carrier 1 on which an electrostatic latent image corresponding to image information is formed, a developing device 4 visualizing the electrostatic latent image formed on the carrier 1 by toner as a toner image, a primary transfer device 5 primarily transferring the toner image carried on the carrier 1, the intermediate transfer body 7 carrying the primarily transferred toner image and secondary transfer devices 9 and 10 secondary transferring an unifixed toner image on the transfer body 7 on a recording medium P. A belt or a drum whose structure is constituted of two or more layers is used as the transfer body 7. Then, the dielectric constant of a material constituting the surface layer is larger than that of a material constituting the lower layer. Besides, it is preferable that the volume resistivity of the transfer body 7 is >=10<13.5> Ωcm.

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 such as a laser printer, a facsimile, and a composite device thereof. More specifically, the present invention relates to an image forming apparatus in which a toner image formed on an image carrier is once 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. .

[0002]

2. Description of the Related Art An image forming apparatus employing an intermediate transfer member system is disclosed, for example, in Japanese Patent Application Laid-Open No.
No. 192,282. Generally, this type of image forming apparatus will be described with reference to FIG.
Intermediate transfer member output from a position detection device (12) for detecting a position detection mark of the intermediate transfer member (7) by an optical detection method
Based on the position detection signal of (7), the control section of the image forming apparatus controls the exposure timing by the image exposure means (3) according to the image information, thereby correcting the image shift and forming the image. While controlling the operation of the apparatus, an electrostatic latent image is formed on the latent image carrier (1) such as a photosensitive drum at a good timing. The electrostatic latent image is visualized as a toner image by a developing device (4), the formed unfixed toner image is primarily transferred to an intermediate transfer member (7), and the unfixed toner image is further transferred to the intermediate transfer member (7).
Is transferred to a recording medium (P) such as a sheet, and a desired image is formed on the recording medium (P). In particular yellow, magenta, cyan, developing device corresponding to black colors with the (4 ₁ to 4 _4), in a color image forming apparatus for forming a full-color image by superimposing an unfixed toner image of each color, the position detecting device Image exposure means for the latent image carrier (1) based on the position detection signal of the intermediate transfer body (7) detected in (12)
By controlling the timing of the exposure according to (3),
The composite image on the intermediate transfer body (7) after the primary transfer is a recording medium (P)
Is transferred all at once.

The primary transfer from the latent image carrier (1) to the intermediate transfer member (7) or the secondary transfer of the unfixed toner image from the intermediate transfer member (7) to the recording medium (P) is performed by an electrostatic transfer method. It is performed by That is, when a voltage having a polarity opposite to that of the toner is applied to the corona discharger (5) disposed on the rear surface side of the transfer target at each transfer position, the unfixed toner image is transferred from the latent image carrier (1) to the intermediate transfer member. The image is transferred from the intermediate transfer member (7) to the recording medium (P). 2. Description of the Related Art In a conventional color image forming apparatus, a method is known in which an unfixed composite image of each color formed in advance on a latent image carrier is directly multiplex-transferred onto a recording medium conveyed by conveying means such as a conveying belt. ing. In this transfer method, the image quality of a color image formed on a recording medium is affected by many factors such as the thickness and surface characteristics of the recording medium and the transport characteristics of the recording medium with respect to the latent image carrier. However, in the color image forming apparatus using the above-described intermediate transfer body, the composite image that has already been multiplex-transferred onto the intermediate transfer body is transferred to a recording medium. There is an advantage that occurrence of image disturbance and color unevenness at the time can be effectively prevented.

[0004]

However, in such an image forming apparatus, the unfixed toner image is transferred from the latent image carrier to the intermediate transfer body by the electric field generated by the primary transfer device. The following problems occur. If the volume resistivity of the intermediate transfer member is low, the charge given by the primary transfer device such as a corona discharger or a bias roll during the primary transfer from the latent image carrier to the intermediate transfer member is charged by the conductivity of the intermediate transfer member itself. The charge is eliminated. As a result, the electrostatic force that holds the charge of the unfixed toner image transferred from the latent image carrier to the intermediate transfer member does not work, so that the electrostatic repulsion between the toners and the fringe electric field near the image edge are reduced. There is a problem that the toner is scattered around the image due to the force, and an image with large noise is formed. In particular, this phenomenon appears remarkably around an image having a large amount of toner per unit area, such as a multi-color image, and becomes a fatal image quality defect for a color image forming apparatus. As means for preventing such a phenomenon that toner is scattered around an image, Japanese Patent Application Laid-Open No. 3-192282 discloses a method.
It is known to increase the volume resistivity of a material constituting an intermediate transfer member, as disclosed in Japanese Patent Application Laid-Open Publication No. H11-163,036. That is, the charge of the toner primarily transferred from the latent image carrier to the intermediate transfer member is held by the primary transfer device with the electrostatic force of the charge accumulated on the intermediate transfer member, thereby preventing the toner from scattering around the image. Things.

[0005] However, according to the test and research of the present inventors,
It has been found that when the volume resistivity of the intermediate transfer member is increased to a region where the toner scattering phenomenon does not occur, the following problem newly occurs. If there is an image portion having a relatively small toner weight per unit area in the background portion of the line image having a large toner weight per unit area, the height of the line image and the background portion naturally differ. When such an image is secondarily transferred from the intermediate transfer body to the recording medium, when the recording medium comes into close contact with the image on the intermediate transfer body, the image of the background portion around the line is affected by the height of the line image. A gap occurs between the recording medium and the recording medium. For this reason, the interaction between the electric field generated during the secondary transfer and the electric field formed by the charges accumulated in the intermediate transfer member causes a discharge phenomenon in a gap around the line image, and the toner in the background around the line image is recorded. An image quality defect such as a blank area around the line image occurs without being transferred to the medium. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-described problems, and to prevent a phenomenon in which toner scatters around a line image, and further, to prevent a white portion around a line image from occurring. Another object of the present invention is to provide an image forming apparatus provided with an intermediate transfer member capable of obtaining a high-quality transfer image.

[0006]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems, and focused on the dielectric constant of the constituent material of the intermediate transfer member having a multilayer structure. By making the dielectric constant of the surface layer higher than that immediately below, the transfer failure of the toner forming the background around the line image is substantially prevented, and the inventors have found that the image quality defect does not occur. It is what led to.
That is, the image forming apparatus of the present invention includes an image carrier on which an electrostatic latent image corresponding to image information is formed, and a developing device for visualizing the electrostatic latent image formed on the image carrier as a toner image with toner. A primary transfer device for primary transfer of a toner image carried on an image carrier, an intermediate transfer body for carrying the primary-transferred toner image, and a secondary transfer of an unfixed toner image on the intermediate transfer body to a recording medium Wherein the intermediate transfer member has a layer structure of two or more layers, and the dielectric constant of the constituent material of the surface layer is larger than the dielectric constant of the constituent material of the lower layer.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail mainly with reference to the drawings. The image forming apparatus of 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, the present invention is applied to a normal mono-color image forming apparatus, a color image forming apparatus that repeats a primary transfer of a toner image carried on an image carrier such as a photosensitive drum to an intermediate transfer body sequentially. In the image forming apparatuses shown in the drawings, components having the same functions are denoted by the same reference numerals. FIG. 1 is an explanatory diagram showing an outline of an image forming apparatus of an intermediate transfer belt type. In FIG. 1, around an image carrier 1 composed of a photoreceptor drum, along the rotation direction thereof,
A charger 2, an image writing unit 3, a developing device 4, a primary transfer device 5, a cleaning device 6, and the like are arranged. Further, an intermediate transfer belt 7 that 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 8a, 8b, 8c and a backup roll 9. Have been. At a position facing the backup roll 9 and the belt transport roll 8a, a bias roll 10 and a belt cleaner 11 are disposed via the intermediate transfer belt 7, respectively. Bias roll 1
A portion where 0 presses against the backup roll 9 becomes a secondary transfer portion, and a secondary transfer voltage is applied between the two rolls 9 and 10 constituting the secondary transfer device. Further, a position detection device 12 for optically detecting a position detection mark of the intermediate transfer belt 7 is disposed between the belt transport rolls 8b and 8c so as to be separated from the surface of the intermediate transfer belt 7.

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 formed as a toner image visualized by the developing device 4 containing the toner corresponding to the color. This toner image is transferred to the image carrier 1
When the primary transfer device 5 and the primary transfer device 5 pass through the opposed primary transfer portion, the primary transfer device 5 electrostatically primary-transfers the toner onto the intermediate transfer belt 7. Thereafter, similarly, the second color and the third color are placed on the intermediate transfer belt 7 carrying the toner image of the first color.
The primary transfer is performed so that the toner images of the color and the fourth color are sequentially superimposed, and finally a full-color multiplex toner image is obtained. The multiplex toner image is electrostatically and collectively transferred to a recording medium (hereinafter, represented by a sheet P) supplied to the secondary transfer unit from the sheet feed tray 13 at a predetermined timing. 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. The residual toner and residual charge are removed from the image carrier 1 and the intermediate transfer belt 7 after the transfer by the cleaning device 6 and the belt cleaner 11, respectively, to prepare for the next image forming process.

[0009] The developing device 4 includes a plurality of developing units ₄₁ to ₄ containing the toner corresponding to the electrostatic latent image of each color.
That is, each developing device contains toner of each color of yellow (Y), magenta (M), cyan (C), and black (K). 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. A voltage of 1 to 5 kV is applied to the primary transfer device 5,
By the action of the electric field generated between the image carrier 1 and the primary transfer device 5, the toner image carried on the image carrier 1 is primarily transferred to the intermediate transfer belt 7 as described above. The backup roll 9 forms a counter electrode of the bias roll 10. The layer structure of the backup roll 9 may be a single layer or a multilayer. For example, in the case of a single-layer structure, it is constituted by 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. In the case of a two-layer structure, a single-layer roll having a suitably adjusted volume resistivity is used as a core layer, and is formed of a conductive resin layer whose outer peripheral surface is coated with, for example, a fluororesin. Examples of the fluorine-based resin include a tetrafluoroethylene (TFE) -hexafluoropropylene copolymer (FEP) and a perfluoroalkoxy resin (PFA). The volume resistivity of the backup roll 9 is 1
It is preferably 0 ⁸ [Omega] cm or more.

A bias roll 10 for forming a transfer electrode
Is separated from the transfer belt 7 while the toner image carried on the image carrier 1 is primarily transferred onto the intermediate transfer belt 7, and the toner image carried on the transfer belt 7 is secondarily transferred onto the paper P. In this case, the transfer belt 7 is configured to be pressed against the transfer belt 7 and pressed against the backup roll 9. The layer structure of the bias roll 10 is not particularly limited. For example, in the case of a two-layer structure, the bias roll 10 includes a core layer and a coating layer covering the surface thereof. The core layer is made of silicone rubber, urethane rubber, EPDM, or the like or a foam thereof in which conductive fine powder is dispersed. The coating layer is preferably made of the above-mentioned fluororesin in which conductive fine powder is dispersed. The volume resistivity of the bias roll 10 is preferably in the range of 10 ⁴ to 10 ⁶ Ωcm. A core roll of the backup roll 9 or an electrode roll of good electrical conductivity pressed against the roll 9 or a bias roll 10
And a secondary transfer voltage of 1 to 6 kV is applied between the rolls 9 and 10. The above-described formation of the electrostatic latent image is performed by the control unit of the image forming apparatus.
The timing of exposure by the image writing means 3 is controlled based on an output signal from the position detection device 12 for detecting the position detection mark. At the same time, the primary transfer of each color toner image formed on the image carrier 1 and the intermediate transfer belt 7
The timing of the secondary transfer of the multiple toner image carried on the
It is controlled based on the output signal.

According to the present invention, the intermediate transfer belt shown in FIG. 1 can be diverted to an image forming apparatus in which an intermediate transfer drum is used. In the color image forming apparatus shown in FIG. 2 having the intermediate transfer drum, an intermediate transfer drum 15 is provided on the intermediate transfer belt 7, and a drum cleaner 11 'is provided on the belt cleaner 1.
1, a position detecting device 12 'for detecting a position detecting mark of the intermediate transfer drum corresponds to the position detecting device 12, respectively. Further, the belt transport rolls 8a, 8b, 8c are of course unnecessary, and the counter electrode member corresponding to the backup roll 9 is not necessarily required. Since the color image forming apparatus having such a configuration operates almost in the same manner as the image forming apparatus shown in FIG. 1, the description of its operation will be omitted. Figures 1 and 2
When a multicolor image other than full color is formed in the image forming apparatus shown in (1), toner corresponding to the multicolor image is stored in two or three developing units. 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. When housed in
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.

In the present invention, the intermediate transfer member has a layer structure of two or more layers. To prevent the conventional scattering of toner and white spots, the dielectric constant of the constituent material of the surface layer is changed to the dielectric constant of the constituent material of the lower layer. Need to be bigger. When the intermediate transfer drum 15 is used as the intermediate transfer member, an intermediate transfer belt is provided on an outer peripheral surface of a cylindrical base material made of aluminum, stainless steel (SUS), copper, or the like having a thickness of 3 mm or more so that the drum 15 is not deformed. Material similar to 7 is coated. For example, in the case of an intermediate transfer member having a two-layer structure, as shown in FIG. 3A, the intermediate transfer member includes a surface layer a and a lower layer b. The surface layer a has a function of preventing the occurrence of color misregistration due to, for example, a load change during driving of the intermediate transfer body. When the intermediate transfer body is formed of a single layer of the surface layer a, the bias roll is applied to the toner image. Since there is a possibility that a hollow character is generated due to concentration of stress, the lower layer b is preferably made of a material having a high flexibility. Incidentally, the hardness of the backup roll is determined by JI
It is also possible to avoid occurrence of a hollow character by setting S A to 60 ° or less. The surface layer is made of a resin material and, if necessary, a conductive material. Examples of the resin material include polyimide (PI) having a large Young's modulus, a copolymer of polyimide and polyamide, and polyvinylidene fluoride (P).
VdF) and the like are preferably used. Examples of the conductive agent include metals or alloys such as carbon black, graphite, aluminum, nickel, copper, copper alloy, and SUS; tin oxide, zinc oxide, titanium oxide, indium oxide, antimony oxide, potassium titanate, and tin oxide-oxide. One or more fine powders of conductive metal oxides such as indium composite oxide and tin oxide-antimony oxide composite oxide are used. These conductive agents may be coated with insulating fine particles such as barium sulfate. The thickness of the surface layer is 50
It is preferably in the range of from 150 to 150 µm, especially from 70 to 100 µm.

The lower layer is made of polyfluoroethyl vinyl ether (FEV) crosslinked with PTFE and polyisocyanate.
E), ethylene-TFE copolymer (ETFE), FEP
It is preferable to use a flexible fluorine-based polymer material such as polyethylene terephthalate (PET) or the like as a constituent material. The thickness is preferably 10 μm or more so that electric charge does not penetrate through the layer of the intermediate transfer member during the primary transfer. As described above, there is a relationship of the dielectric constant of the surface layer (a)> the dielectric constant of the lower layer (b). Generally, the dielectric constant of the surface layer is in the range of 8 to 60 (frequency 1 kHz), and the dielectric constant of the lower layer is It is desirable that the number be 3 or more. Here, the difference between the dielectric constant of the surface layer and the dielectric constant of the lower layer is preferably at least 2, particularly preferably 4 or more.

In the case of an intermediate transfer member having a three-layer structure, as shown in FIG. 3B, it comprises a surface layer a ', a lower layer (intermediate layer) b', and a base layer c. The surface layer a 'has a lower layer b in the case of a two-layer structure.
It is preferable that the base layer c has a function of preventing the occurrence of color misregistration, similarly to the surface layer a, as in the case of the surface layer a. When a material having high hardness is used for the surface layer a ', a hollow character may be generated. Therefore, a lower layer b' is provided as a countermeasure. The surface layer having the three-layer structure is made of the following resin materials and, if necessary, the above-described conductive agent. As the resin material, polycarbonate (P
C), polyurethane, polyester ether ketone, P
VdF, ETFE, or the like is used. Its thickness is 10 μm
It is preferable to make the above. The lower layer and the base layer are desirably made of the same material as the lower layer and the surface layer of the two-layer structure, respectively. Also in the intermediate transfer member having the three-layer structure, the relationship between the dielectric constant of the surface layer and the dielectric constant of the lower layer is the same as in the case of the two-layer structure. Generally, the dielectric constant of the surface layer is 10 or more, and the dielectric constant of the lower layer is preferably in the range of 3 to 6, and the difference between the two dielectric constants is 2 or more.

To transfer a toner image to an intermediate transfer member by the electrostatic transfer method, the volume resistivity of the intermediate transfer member is important, and the intermediate transfer member of the present invention has a volume resistivity of 10 ^13.5 Ωcm or more. That is, when the volume resistivity is equal to or more than the above value,
Since the intermediate transfer member is sufficiently charged and the charge of the toner image is sufficiently retained on the intermediate transfer member, scattering of the toner image due to electrostatic repulsion between toners and fringe electric field near the image edge is prevented, and There is no disturbing. The operation of the present invention is as follows. The image forming apparatus according to the present invention includes:
In the intermediate transfer member (7, 15) having a layer structure of more than one layer, the dielectric constant of the constituent material of the surface layer is larger than the dielectric constant of the constituent material of the lower layer. Therefore, since the capacitance of the surface of the intermediate transfer member carrying the toner image can be increased, even if the image of the background portion is around the line image on the intermediate transfer member, the line image around the line image is not transferred during the secondary transfer. Since the influence of the fringe electric field is reduced, it is possible to prevent the occurrence of white spots due to poor transfer around the line image. Further, it is possible to prevent the toner scattering phenomenon that occurs remarkably around an image having a large toner weight per unit area. Further, since the inside of the image forming apparatus due to the toner scattering phenomenon can be prevented, the maintenance of the image forming apparatus can be facilitated. In addition, since the amount of untransferred toner decreases, the cleaning member (1
1) The service life is extended and the reliability of the image forming apparatus is improved.

[0016]

EXAMPLES Next, 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. 4 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. The components having the same functions as those of the components of the image forming apparatus shown in FIG. 1 are denoted by the same reference numerals in FIG. In FIG. 4, 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 uniformly charging the surface, a developing device 4 for developing an electrostatic latent image written on the image carrier 1 into a toner image of each color, A corotron (primary transfer device) 5 for transferring a toner image to the intermediate transfer belt 7, and a cleaning device 6 having a static eliminator and a cleaning blade are provided. The developing device 4 has a developing device containing toner of each color of Y, M, C, and K, and develops and visualizes the electrostatic latent image with the toner of each color. The intermediate transfer belt 7 includes the backup roll 9 and the belt transport rolls 8a, 8b, 8
c, and moves in the tangential direction while abutting the surface of the image carrier 1. On the front side of the transfer belt 7 that carries the unfixed toner image, the bias roll 10 and the belt cleaner 11 are disposed to face the backup roll 9 and the belt transport roll 8a, respectively. The position detection device 12 is disposed between the belt transport roll 8c and the backup roll 9, and the paper P carrying the secondary-transferred toner image is placed between the backup roll 9 and the belt transport roll 8a. A peeling claw 29 to be peeled off from the transfer belt 7 is arranged.

An extractable paper feed tray 13 is provided below the main body of the image forming apparatus U, and a pickup roller 30 is disposed above the paper feed tray 13. This pickup roller 3
A pair of feed rolls 31 for preventing double feeding of the paper P, a paper transport roll 32, a guide member 33 for guiding the paper P, and a registration roll 34 are sequentially arranged on the downstream side of 0. On the downstream side of the secondary transfer section, a transport belt 3 for transporting a sheet P carrying a secondary-transferred toner image sequentially
5. A fixing device 14 for fixing an unfixed toner image on the paper P, a pair of discharge rolls 36 for discharging the paper P on which the fixed image is formed outside the apparatus, and a paper discharge on which the discharged paper P is placed. A tray 37 is provided.

(Operation of Image Forming Apparatus) The surface of the image carrier 1 rotating in the direction of arrow A is uniformly charged 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, Y, M, C, and K toner images are sequentially formed. Image carrier 1
After the toner image is transferred onto the intermediate transfer belt 7, the surface of the cleaning device 6 is cleaned by the blade of the cleaning device 6. Here, the optical writing control device 27 first reads out the Y-color digital signal of the first color corresponding to the optical image that has been color-separated by the B-color filter.
8 to write an electrostatic latent image corresponding to the Y color on the surface of the image carrier 1. The timing of the writing of the electrostatic latent image is controlled by an optical writing control device 27 based on a position detection signal output from a position detection device 12 that optically detects a position detection mark of the intermediate transfer belt 7. Will be The electrostatic latent image is developed by the developing device Y in the developing device 4 into a first color toner image visualized in the Y color, and the toner image moves to the primary transfer unit. In the primary transfer section, an electric field having a polarity opposite to the charging polarity is applied to the toner image from the corotron 5 disposed on the back side of the intermediate transfer belt 7 so that the Y color toner image that has reached the primary transfer section is statically transferred. The primary transfer is performed by moving the transfer belt 7 in the direction of arrow B while electrically attracting the transfer belt 7.

The intermediate transfer belt 7 moves at the same cycle as the image carrier 1 while adsorbing and carrying the Y toner image. When the transfer of the first color toner image is completed, the writing of the electrostatic latent image corresponding to the light image color-separated by the G filter by the output from the optical writing control device 27 is started in the same manner as described above. Is done.
When the transfer start position of the transfer belt 7 carrying the Y toner image reaches the primary transfer section, the M toner image of the second color is transferred by the corotron 5. continue,
The electrostatic latent images corresponding to the C and K colors are visualized by the developing units C and K, and the transfer of the C toner image and the K toner image is performed in the same manner as the transfer of the M toner image. In this way, a multiple toner image superimposed on each color is formed on the intermediate transfer belt 7. Until the toner images of each color are primarily transferred onto the transfer belt 7, the bias roll 10, the peeling claw 29, and the belt cleaner 11 disposed on the front surface side of the transfer belt 7 are in the retracted positions separated from the transfer belt 7. 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 30 at a predetermined timing, and fed by a pair of feed rolls 31 and a paper transport roll 32, and a pair of registration rolls 3
The operation is temporarily stopped at step 4. The sheet P is then transferred from the registration roller 34 to the secondary transfer section in synchronization with the transfer of the multiple toner images of each color (Y, M, C, K) transferred onto the intermediate transfer belt 7 to the secondary transfer section. Transported to the department. In the secondary transfer section, the bias roll 10 is in pressure contact with the backup roll 9 via the intermediate transfer belt 7. Then, the conveyed sheet P passes through the secondary transfer section by the press-contact conveyance between the rolls 9 and 10 and the movement of the transfer belt 7. At this time, by applying a transfer voltage having a polarity opposite to the charging polarity of the toner image, the multiple toner image adsorbed and carried on the transfer belt 7 is secondarily transferred to the sheet P from the surface of the transfer belt 7.

Although the transfer of a full-color image has been described above, when a single-color image is formed, the intermediate transfer belt 7 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 29,
The sheet is placed on the conveyance belt 35 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 37 by a pair of discharge rollers 36. When the secondary transfer is completed, the intermediate transfer belt 7 is cleaned by a belt cleaner 11 provided downstream of the secondary transfer unit to prepare for the next transfer.

(Manufacture of Intermediate Transfer Belt) Example 1 18 parts by weight of carbon black per 100 parts by weight of resin component was applied to polyimide varnish (polyimide varnish for heat-resistant film using N-methylpyrrolidone as a solvent; U Varnish-S:
Ube Industries, Ltd.) and mixed well with a mixer. The obtained membrane-forming stock solution is 168 mm in diameter and 500 m in height.
m of stainless steel cylindrical mold, 120 ℃
While drying with hot air for 120 minutes. Then, the cylindrical film removed from the semi-cured state is covered with an iron core, and the temperature is increased from 120 ° C. to 350 ° C. over 30 minutes to evaporate the solvent. The main curing of dehydrating and condensing the acid was performed. The obtained carbon black-dispersed polyimide film having a thickness of 80 μm was cut into a width of 320 mm to form a seamless belt having a volume resistivity of 10 ^10.5 Ωcm. Next, the above-mentioned seamless belt is used as a surface layer (a), and an FEP-based rubber paint (Daily latex GLS-213: Daikin Industries, Ltd.)
Co., Ltd.) by a spray coating method, and then heated at 250 ° C. for 20 minutes to obtain 5
A lower layer (b) having a thickness of 0 μm was formed. Here, the dielectric constant of the carbon black-containing polyimide constituting the surface layer (a) is 3
1.3, the permittivity of the fluorine coat layer constituting the lower layer (b) is 5.6, and the permittivity of the surface layer (a)> lower layer
The relationship of the dielectric constant of (b) holds. The volume resistivity of the intermediate transfer belt was 10 ¹⁴ Ωcm.

Example 2 The seamless belt formed in Example 1 was used as a base layer (c), a fluorine coat layer was used as a lower layer (b '), and a 15 μm thick surface layer made of PVdF was spray-coated thereon.
(a ') was formed. The dielectric constant of the high dielectric material PVdF constituting the surface layer (a ') is 10, and the surface layer (a')
And the dielectric constant of the lower layer (b ') holds. The volume resistivity of the manufactured intermediate transfer belt having a three-layer structure was 10 ¹⁴ Ωcm. In the present invention, the measurement of the dielectric constant is performed according to the ASTM test method, and the LCR made by HP is used.
Aluminum was vapor-deposited on the back surface of the sample using a meter HP4274A, and an AC voltage of 5 V having a frequency of 1 kHz was applied between the sample and a main electrode having a diameter of 18 mm. The measured value after one minute was taken as the dielectric constant of the sample.

(Image Quality Evaluation Test) A high-density line image having an image density of 100% is formed on several intermediate transfer belts on a low-density background portion having an image density of 30%, and the resulting toner is subjected to secondary transfer. The image was scanned for the image density in the direction of the background portion-(missing portion)-high density portion-(missing portion)-background portion, and the density profile was plotted. The image density was determined by measuring the reflectance of a spot light having a width of 35 μm. With respect to the obtained density profile, the distance (width) of the density reduction region of the background portion, that is, the missing portion was measured, and the image quality after the secondary transfer was evaluated. In addition, when the width of the missing portion is less than 0.2 mm, the image quality is judged as “〇”, and when the width is 0.2 mm or more, there is an image quality defect “×”.
It was determined. Table 1 shows the results. Test N
FIG. 5 shows the density profile of the fixed image on the paper secondary-transferred from the intermediate transfer belt of No. 1 when scanning in the above direction.

[0026]

[Table 1]

(Intermediate Transfer Drum) Example 3 As a surface layer (a) containing carbon black-containing polyimide as a constituent, a lower layer containing a fluorine coat layer as a constituent
(b) was coated on a cylindrical aluminum base material having an outer diameter of 111 mm and a thickness of 10 mm to produce an intermediate transfer drum having the same layer structure as in Example 1. When the above-described image quality evaluation test was performed on the intermediate transfer drum, a favorable image having no white spots on the background portion was similarly obtained.

[0028]

According to the present invention, at the time of secondary transfer, it is possible to prevent the occurrence of white spots due to poor transfer around a line image, and to prevent toner from scattering around an image having a large toner weight per unit area. Can be prevented. Further, since the toner scattering phenomenon is prevented and the amount of untransferred toner on the recording medium is reduced, the life of the cleaning member is extended, the contamination inside the image forming apparatus is suppressed, and the maintenance of the image forming apparatus is facilitated. Increases reliability.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an image forming apparatus of the present invention including an intermediate transfer belt.

FIG. 2 is an explanatory diagram of an image forming apparatus of the present invention including an intermediate transfer drum.

FIG. 3 is a sectional view of a layer of an intermediate transfer member according to the present invention.

FIG. 4 is an overall view of an image forming apparatus shown as one embodiment of the present invention.

FIG. 5 is a density profile showing background omission occurring around a high density image when a low density image is formed around a high density image.

[Explanation of symbols]

U: image forming apparatus, P: recording medium, a: surface layer, b: lower layer, c: base layer, 1: image carrier, 4: developing device, 5: primary transfer device, 7: intermediate transfer belt, 9, 10 ... secondary transfer device, 15 ... intermediate transfer drum.

Claims (4)

[Claims] An image carrier on which an electrostatic latent image corresponding to image information is formed; a developing device for visualizing the electrostatic latent image formed on the image carrier as a toner image with toner; A primary transfer device for primary transfer of the toner image carried on the
An intermediate transfer member for carrying the primary-transferred toner image; and a secondary transfer device for secondary-transferring the unfixed toner image on the intermediate transfer member to a recording medium, wherein the intermediate transfer member has two or more layers. An image forming apparatus having a structure, wherein a dielectric constant of a constituent material of a surface layer is higher than a dielectric constant of a constituent material of a lower layer. 2. The intermediate transfer member has a volume resistivity of 10 ^13.5
The image forming apparatus according to claim 1, wherein the value is Ωcm or more. 3. The image forming apparatus according to claim 1, wherein the type of the intermediate transfer member is a belt. 4. The image forming apparatus according to claim 1, wherein the type of the intermediate transfer member is a drum.