JP2003057918A - Color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color image forming device that minimizes transfer image disturbance while ensuring transfer capability. SOLUTION: In the color image forming device having photoreceptor drums (image carriers) 1 and transfer rollers (transfer means) 12a to 12d corresponding to the photoreceptor drums 1, a second transfer nip length is greater than a first transfer nip, wherein the first transfer nip length is the length of the nip formed between the first transfer roller 12a and an electrostatic carrying belt (transfer material carrying means) 4 and the second transfer nip length is the length of the nip formed between each of the second transfer rollers 12b to 12d and the electrostatic carrying belt 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus using electrophotography.

[0002]

2. Description of the Related Art A tandem type color image forming apparatus using an electrophotographic technique will be outlined below.

Four image carriers are arranged at equal intervals in the central portion of the image forming apparatus, and all four image carriers are moved in the same direction at the same speed. Then, a charging unit, an exposure device, a developing device, and a cleaning unit are arranged corresponding to each image carrier. Then, each developing device is filled with magenta toner, cyan toner, yellow toner, and black toner, respectively.

The image forming process will be described below.

Generally, an image carrier for holding toner on an electrostatic latent image is composed of a drum or a belt having a photosensitive layer formed by coating a photoconductor on a metal substrate such as aluminum. Here, examples of the photoconductor include OPC (organic photo-semiconductor), A-Si (amorphous silicon), and CdS.
(Cadmium sulfide), Se (selenium) or the like can be used.

Then, the image carrier is driven in a fixed direction in response to a print start signal, and the surface of the image carrier is charged up (charged) to a constant potential by existing charging means such as rollers and corona. The potential of the image carrier at this time is referred to as “VD potential”.

Further, the surface of the image bearing member is irradiated (exposure) with laser light (exposure means) which is ON / OFF controlled based on a signal from the controller while scanning in the longitudinal direction.
The electric potential of the image carrier at this time is called “VL electric potential”. Since the light irradiation position is charged down by an amount corresponding to the exposure amount, an electrostatic latent image is formed on the surface of the image carrier. Then, a developing device attaches toner to the electrostatic latent image on the surface of the image carrier to visualize the electrostatic latent image as a toner image. here,
The developing device includes an elastic roller that comes into contact with the image carrier and rotates in the forward direction with respect to the moving direction of the image carrier at a constant peripheral speed difference, and a thin layer of toner is coated on the elastic roller to keep the toner constant. It is composed of a toner regulating member which gives the electric charge of
Then, by controlling the surface potential of the elastic roller to be an appropriate value between the VD potential and the VL potential, the charged toner is attached only to the VL portion on the image carrier.

Next, a method of transferring the toner image on the image carrier to the transfer material will be described.

The electrostatic transport belts that are in contact with all four image carriers are moved in the forward direction at substantially the same speed with respect to the moving direction of the image carriers. Here, the electrostatic transport belt has, for example, a thickness of 50 to 300 μm and a volume resistivity of 10 ⁹ to 10 ¹⁶ Ω.
-A resin material such as PVdF (polyvinylidene fluoride), polyamide, polyimide, PET (polyethylene terephthalate), polycarbonate, etc. having a thickness of about 0.5 cm and a thickness of 0.5 to
CR of 2 mm, volume resistivity of 10 ⁹ to 10 ¹⁶ Ω · cm
A rubber material such as (chloroprene rubber), EPDM (ethylene-propylene-diene terpolymer), NBR (nitrile butadiene rubber), and urethane rubber is used.
In some cases, carbon, Zn, etc. may be added to these materials.
A conductive filler such as O, SnO ₂ or TiO ₂ may be dispersed to adjust the volume resistivity to about 10 ⁷ to 10 ¹¹ Ω · cm.

Then, four transfer rollers as elastic rollers are arranged on the back surface of the electrostatic transport belt facing the image carrier, corresponding to the respective image carriers. An appropriate positive DC bias is independently applied to each transfer roller, and an electric field is formed in the nip portion between the electrostatic transport belt and the transfer roller to transfer the toner image to the transfer material by electrostatic force.

The fed transfer material is carried to the electrostatic transport belt, and by applying a positive or negative DC bias to the attraction roller disposed in contact with the electrostatic transport belt, the transfer material is transferred to the electrostatic transport belt. It is electrostatically adsorbed. Then, the transfer material is conveyed to the facing portion of each image carrier while being adsorbed, and an electric field is formed between the image carrier and the transfer material by the action of the positive DC bias applied to the transfer member. The images are sequentially transferred to the transfer material. Then, after the transfer of all four colors is completed, the transfer material is separated from the electrostatic transport belt.

On the other hand, with the present technology, it is not possible to transfer 100% of the toner from the image carrier to the transfer material. Therefore, the toner remaining on the image carrier is removed by the existing cleaning means such as a blade or a brush. Mechanically removed from the image carrier.

Further, the transfer material is conveyed to a fixing unit and heated and pressed by a pair of rollers to be fixed with a toner image.

[0014]

In the above system, the transfer of the toner to the transfer material is performed in the nip portion (hereinafter referred to as the transfer nip) formed between the electrostatic transport belt and the transfer roller. Therefore, by increasing the transfer nip, it becomes possible to further improve the transfer ability, that is, to transfer more toner on the image carrier.

On the other hand, the action of the transfer electric field does not stop within the transfer nip region, and the electric field also acts on the air gap region between the transfer roller and the electrostatic transport belt outside the transfer nip region. For example, in a low-humidity environment or the like, abnormal electric discharge may occur in the air gap region due to the action of the electric field.
Then, the toner image on the transfer material may be disturbed by a shock or the like due to the abnormal discharge. Hereinafter, this is referred to as "transfer image disorder".

When the transfer nip is increased, the working area of the transfer electric field is expanded outward. If an abnormal discharge from the transfer roller occurs when the transfer material is completely separated from the image carrier, the toner does not have a mechanical or electrostatic holding effect by the image carrier, resulting in image disturbance. Is likely to occur.

In the latter-stage transfer, it is necessary to transfer the toner so as to be superposed on the toner image transferred in the front stage. Since the charging polarities of the toner are the same, when performing color superposition, it may not be possible to secure sufficient transferability by repelling the toner that has already been transferred to the transfer material at the time of transfer in the subsequent stage. It is desirable that the transcription site at the latter stage has a structure capable of further enhancing the transcription ability. or,
Since the toner image transferred in the front part passes through the transfer part in the latter part, it will be affected by abnormal discharge in a plurality of transfer parts, and in the end, the transfer image is more likely to be disturbed. Become.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a color image forming apparatus capable of minimizing the occurrence of transfer image disturbance while ensuring transfer ability. Especially.

[0019]

In order to achieve the above object, the invention according to claim 1 is a color image forming apparatus having a plurality of image carriers and a plurality of transfer means corresponding thereto, and the plurality of images. A first image bearing member of the bearing members, a second image bearing member disposed downstream of the first image bearing member, the first image bearing member and the second image bearing member. And a transfer material transporting means for transporting the transfer material while moving at a substantially same speed in the forward direction with respect to the moving direction of the first image carrier and the second image carrier, A first transfer unit arranged at a position facing the first image carrier via a transfer material conveying unit, and a second transfer unit arranged at a position facing the second image carrier. The first transfer means and the second transfer means are arranged in contact with the transfer material conveying means, respectively. After the toner image formed on the first image carrier is transferred onto the transfer material by the first transfer unit, the toner image formed on the second image carrier is transferred to the second transfer unit. Is transferred onto the transfer material by means of the first transfer means and the transfer material conveying means, and the nip length formed by the first transfer means and the transfer material conveying means is referred to as a first transfer nip length. When the formed nip length is the second transfer nip length, the second transfer nip length is set to be larger than the first transfer nip length.

According to a second aspect of the present invention, in the first aspect of the present invention, a DC bias is applied to the first transfer means and the second transfer means, whereby the first image carrier and the The toner image on the second image carrier is electrostatically transferred onto a transfer material.

According to a third aspect of the invention, in the first or second aspect of the invention, elastic rubber rollers are used as the first transfer means and the second transfer means.

According to a fourth aspect of the invention, in the invention of the first, second or third aspect, the contact pressure of the first transfer means with respect to the first image carrier is more than the contact pressure of the second transfer means. Two
The contact pressure with respect to the image carrier is set to a large value.

According to a fifth aspect of the invention, in the invention of the third or fourth aspect, the hardness of the elastic rubber roller used for the second transfer means is higher than the hardness of the elastic rubber roller used for the first transfer means. It is characterized by being set small.

According to a sixth aspect of the invention, in the invention of the third, fourth or fifth aspect, the elastic rubber roller used for the second transfer means is larger than the outer diameter of the elastic rubber roller used for the first transfer means. It is characterized in that the outer diameter of is set large.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

<Embodiment 1> FIG. 1 is a sectional view of an essential part of a tandem type color image forming apparatus according to Embodiment 1 of the present invention. FIG. 2 is a transverse sectional view showing a layer structure of a photosensitive drum.
FIG. 3 is an explanatory diagram of a working area of a transfer electric field.

As shown in FIG. 1, four photosensitive drums 1, which are image bearing members having a diameter of 30 mm, are arranged at equal intervals in the central portion of the image forming apparatus, and these four photosensitive drums 1 are all in the same direction (the direction of the arrow in the figure). ) At a peripheral speed of 94 mm / sec.
A charging roller 5, an exposure device 7, a developing device 8 and a cleaning blade 11 are arranged corresponding to each photosensitive drum 1. In the developing device 8, from the right side (transfer upstream side) of FIG. 1, magenta toner, cyan toner,
It is filled with yellow toner and black toner.

The image forming process will be described below.

As shown in FIG. 2, the photosensitive drum 1 is constructed by applying a photosensitive layer on an aluminum cylinder 1a.
The photosensitive layer is usually an insulator, and the substance is not particularly limited as long as it is a photoconductor having a feature of becoming a conductor when irradiated with light having a specific wavelength. Mainly OPC
(Organic optical semiconductor), A-Si (amorphous silicon), CdS (cadmium sulfide), Se (selenium), etc. are often used.

The photosensitive drum 1 is constructed by forming a charge generation layer 1b and a charge transport layer 1c, which is a dielectric layer, on an aluminum cylinder 1a, and a hole-electron pair is formed in the charge generation layer 1b by light irradiation. They are generated, and they are responsible for the flow of electric charge. The charge generation layer 1b has a film thickness of 0.2 μm
The charge transport layer 1c is composed of a polycarbonate in which a hydrazone compound having a film thickness of about 17 μm is dispersed.

In this embodiment, the charging roller 5 is used as a charging member for uniformly charging the surface of the photosensitive drum 1. This charging roller 5 covers, for example, a metal cored bar with a conductive elastic rubber made of EPDM, urethane rubber, CR, NBR or the like having a thickness of about 3 mm, and a thickness of 200 to 600 μm thereon.
A medium resistance layer having a volume resistivity of about 10 ⁶ Ω · cm is provided, and a protective layer having a thickness of about 10 μm is further provided thereon.

The charging roller 5 spring-presses the cored bar portions at both ends thereof to bring the surface layer into contact with the photosensitive drum 1 and rotate the driven roller relative to the photosensitive drum 1. When a bias equal to or higher than the discharge start voltage (about 550 V) is applied to the core of the charging roller 5, discharge is generated near the nip between the charging roller 5 and the photosensitive drum 1. As a result, charges are accumulated on the surface of the charge transport layer 1c of the photosensitive drum 1. At this time, the surface potential of the photosensitive drum 1 is approximately a value obtained by subtracting the discharge start voltage from the applied bias. In this embodiment, the applied bias is -125.
The voltage is set to 0V, and the surface of the photosensitive drum 1 is charged up to -700V. The surface potential of the photosensitive drum 1 after this charging step is called "VD potential".

On the surface of the photosensitive drum 1 held at the VD potential by the discharge of the charging roller 5, the exposure device 7 is used to perform scanning exposure while controlling the ON / OFF of the light source based on the signal from the controller. An electrostatic latent image is formed on. That is, at the light irradiation position on the photosensitive drum 1, hole-electron pairs are formed in the charge generation layer 1b, and the holes recombine with the electrons accumulated on the surface of the photosensitive drum 1 through the charge transport layer 1c. , The charge of that part goes down. That is, the absolute value of the surface potential of the photosensitive drum 1 decreases. In this embodiment, a semiconductor laser is used as the light source. The potential on the photosensitive drum 1 at the light irradiation position is hereinafter referred to as "VL" potential. The VL potential depends on the amount of electric charge generated by light irradiation. That is, it depends on the film thickness of the charge generation layer 1b, the laser exposure amount, and the like. In the present embodiment, these values are controlled so that the VL potential becomes −150V.

After that, the electrostatic latent image on the photosensitive drum 1 is visualized by the developing device 8 arranged at a position facing the photosensitive drum 1. This visualization is performed as follows.

That is, toner is stored in each developing device 8, and these toners are supplied to the developing roller 3 by a stirring member or the like. Here, the developing roller 3 is
The developing roller 3 is disposed in contact with the photosensitive drum 1 and rotates in the forward direction with a peripheral speed difference of about 180% with respect to the rotating direction of the photosensitive drum 1. What covers etc. and coats the dielectric layer on it etc. is used.

A toner regulating blade 23 is brought into contact with the upstream side of the contact portion of the photosensitive drum 1 on the developing roller 3 in the counter direction with a force of about 20 g of linear pressure to coat the developing roller 3 with a thin layer of toner. The toner regulating blade 23 also plays a role of negatively charging a certain amount of toner when the toner passes through. As the toner regulating blade 23, for example, a metal leaf spring having elasticity such as phosphor bronze or SUS, or urethane rubber, silicon rubber supported by a metal leaf spring, or a rubber surface coated with nylon is used. Then, an appropriate bias between VD and VL (hereinafter referred to as "developing bias") is applied to the cored bar portion of the developing roller 3. As a result, due to the action of the electric field formed between the photosensitive drum 1 and the developing roller 3, V1 on the photosensitive drum 1 is
Only the toner on the developing roller 3 corresponding to the portion is transferred onto the photosensitive drum 1 to complete the development.

Next, a method of transferring the toner image on the photosensitive drum 1 to the transfer material will be described.

The electrostatic transport belt 4 is arranged so as to come into contact with all of the four photosensitive drums 1. The electrostatic transport belt 4 is supported by two rollers, a drive roller 14 and a tension roller 13, which maintains an appropriate tension.

By driving the drive roller 14, the electrostatic transport belt 4 moves in the forward direction at substantially the same speed as the photosensitive drum 1. The electrostatic transport belt 4 has a thickness of 50 to 300 μm and a volume resistivity of 1 as an example.
0 ^{9 ~10 16 Ω} · cm approximately PVdF (polyvinylidene fluoride), polyamides, polyimide, PET (polyethylene terephthalate), resin material or the thickness of the polycarbonate 0.5 to 2 mm, a volume resistivity of 10 ⁹ to 10 ¹⁶ Omega・
cm (CR) (chloroprene rubber), EPDM (ethylene-propylene-diene terpolymer), NBR
A rubber material such as (nitrile butadiene rubber) or urethane rubber is used. In some cases, a conductive filler such as carbon, ZnO, SnO ₂ , or TiO ₂ may be dispersed in these materials to adjust the volume resistivity to about 10 ⁷ to 10 ¹¹ Ω · cm.

Further, transfer rollers 12a to 12d are arranged in contact with the photosensitive drums 1 on the rear surface of the electrostatic transport belt 4 so as to correspond to the respective photosensitive drums 1. In this embodiment, the transfer rollers 12a to 12d have an outer diameter of 12 m.
A roller having m and Asker C hardness of 30 ° was used. As the transfer rollers 12a to 12d, for example, a metal cored bar covered with an elastic body such as EPDM, urethane rubber, CR or NBR whose volume resistivity is adjusted to 10 ⁸ Ω · cm can be used. The transfer rollers 12a to 12d are driven and rotated by the movement of the electrostatic transport belt 4.

Further, a suction roller 19 having an outer diameter of 12 mm is disposed in contact with the surface of the electrostatic transport belt 4 facing the drive roller 14. At this time, the drive roller 14 is grounded. The suction roller 19 may have the same configuration as the charging roller 5, for example. That is, the adsorption roller 19 covers a metal cored bar with a conductive elastic rubber made of EPDM, urethane rubber, CR, NBR or the like having a thickness of about 3 mm, and has a thickness of 200 to 600 μm and a volume resistivity of 10 on it.
Providing a medium resistance layer of about ⁶ Ω · cm, and further 10 μ on it
The protective layer of about m is provided. Since the protective layer has a function of preventing toner and the like from adhering to the surface, it is often made of a fluorine resin or the like.

The suction roller 19 has 10 cored bar portions on both ends thereof.
By spring-pressurizing with a linear pressure of about 100 gf / cm, it is pressed against the drive roller 14 via the electrostatic transport belt 4 and is driven to rotate with respect to the movement of the electrostatic transport belt 4.

The transfer material fed from the cassette 20 or a multi-feeder (not shown) is controlled by the registration roller 21 so as to be synchronized with the formation of an electrostatic latent image by laser exposure, and the electrostatic conveyance belt 4 is controlled. And is passed through the nip portion between the suction roller 19 and the electrostatic transport belt 4. In this state, a bias of a discharge start voltage (500 V to 1.0 kV depending on the situation) is applied to the core of the suction roller 19 to generate discharge near the nip between the suction roller 19 and the electrostatic transport belt 4. Charge the transfer material surface. At the same time, a mirror image charge having a polarity opposite to that of the surface of the transfer material is generated on the surface of the electrostatic transfer belt 4, and the transfer material is electrostatically adsorbed to the electrostatic transfer belt 4 by their electrostatic interaction. In this state, the transfer material is stably transferred to each transfer site by the electrostatic transfer belt 4.

At the time of transferring the toner image to the transfer material, an appropriate positive DC bias is independently applied to each of the transfer rollers 12a to 12d to generate a transfer electric field, and the toner image of each color is sequentially transferred to the transfer material. To do so. The DC bias value is 0.3 depending on the usage conditions and the type of transfer material.
It varies in the range of kV to 3.5 kV. After the transfer of all four colors is completed, the transfer material is subjected to curvature separation at the tension roller 13 portion. The method of pressing the transfer rollers 12a to 12d against the electrostatic transport belt 4 will be described later.

On the other hand, with the current technology, it is not possible to transfer 100% of the toner from the photosensitive drum 1 to the transfer material.
If the toner remaining on the photosensitive drum 1 is left as it is, it is transferred to the transfer material at the next rotation, and the image is disturbed.
In order to prevent this, in the present embodiment, the photosensitive drum 1
The cleaning blade 11 is brought into contact with the rotation direction of the counter in the direction of the counter to mechanically remove the transfer residual toner from the photosensitive drum 1, and the removed toner is collected in the waste toner collecting section.

Further, the transfer material is conveyed to a fixing means (not shown),
The toner image is permanently fixed to the transfer material by the fixing means using an existing method.

Now, a method of pressing the transfer rollers 12a to 12d against the electrostatic transport belt 4 will be described.

The transfer rollers 12a to 12d are pressed against the photosensitive drum 1 via the electrostatic conveyance belt 4 by pressing the cored bar portions at both ends using springs 24a to 24d. The linear pressures of the transfer rollers 12a to 12d are as shown in Table 1.

[0049] At this time, when the transfer rollers 12a to 12d which are elastic bodies having an outer diameter of 12 mm and an Asker C hardness of 30 ° in this embodiment are used, the transfer formed by the transfer rollers 12a to 12d and the electrostatic conveyance belt 4 is performed. The nip is as shown in Table 1. This is a result in the state where the bias is not applied to the transfer rollers 12a to 12d.

FIG. 3 shows the area of action of the transfer electric field in this structure. As the transfer nip increases from the first color to the fourth color, the action of the electric field spreads to the area where the transfer material is completely separated from the photosensitive drum 1. That is, the transfer electric field is acting even in a region where the effect of mechanically or electrostatically pressing the toner against the transfer material in the vertical direction cannot be obtained.

The transfer of the toner to the transfer material is performed at the transfer nip. For this reason, if the transfer nip is large, the transfer capability is improved. In the present embodiment, by adopting a configuration in which the transfer nip at the transfer portion in the subsequent stage where repulsion with the toner transferred to the transfer material occurs during color superposition becomes large, good transfer is achieved regardless of the station. Ability is obtained.

Further, the transfer nip is made small in the transfer portion in the former stage so that the abnormal discharge does not reach the area where the transfer material is completely separated from the photosensitive drum 1. As a result, it is possible to suppress the occurrence of the disorder of the transferred image due to the impact due to the abnormal discharge at the transfer portion in the former stage. That is, the transfer image disorder occurs substantially only in the transfer part in the subsequent stage, and the level of the transfer image disorder in the toner transferred in the preceding stage part is as low as the level of the toner transferred in the subsequent stage part. It becomes possible to suppress.

<Second Embodiment> Next, a second embodiment of the present invention will be described with reference to FIGS.

FIG. 4 is a sectional view of an essential part of the tandem type color image forming apparatus according to the present embodiment, and FIG. 5 is an explanatory view of an action area of a transfer electric field. In these drawings, FIGS. The same elements as those shown are denoted by the same reference numerals, and description thereof will be omitted below.

In this embodiment, the Asker C hardness is shown in Table 2.
The transfer rollers 12a to 12d shown in FIG. Other than the Asker C hardness, the same ones as described in the first embodiment are adopted. The outer diameter is 12 mm, and the linear pressure of each transfer roller 12a to 12d on the photosensitive drum 1 is 1
It was set to 5 gf / cm.

[0056] Table 2 also shows the transfer nip formed by the transfer roller 12 and the electrostatic conveyance belt 4 in this case. This is a result in the state where no bias is applied to the transfer roller 12 as in the first embodiment. The smaller the hardness, the easier the elastic body is to collapse.

With respect to the transfer nip itself, the same result as that of the first embodiment is obtained, and as shown in FIG.
A similar electric field effect is formed. That is, also in the present embodiment, the same effect as in the first embodiment can be obtained,
It is possible to suppress the occurrence of disorder of the transferred image due to the impact due to the abnormal discharge to the minimum while ensuring the transfer ability of the color overlapping.

Incidentally, each transfer roller 1 shown in the first embodiment.
A configuration may be adopted in which the transfer nip amount is adjusted for each transfer site by combining with linear increase of the linear pressure of the photosensitive drum 1 of 2a to 12d.

<Third Embodiment> Next, a third embodiment of the present invention will be described with reference to FIGS.

FIG. 6 is a cross-sectional view of the main part of the tandem type color image forming apparatus according to the present embodiment, and FIG. 7 is an explanatory view of the action area of the transfer electric field. In these figures, FIGS. The same elements as those shown are denoted by the same reference numerals, and description thereof will be omitted below.

In this embodiment, the transfer rollers 12a to 12d whose outer diameters are shown in Table 3 are adopted. Except for the outer diameter, the same ones as described in the first embodiment are adopted. Asker C hardness is 30 °. Also, each transfer roller 12a-
The line pressure for the photosensitive drum 1 of 12d is all 15 gf / c
m.

[0062] Table 3 also shows the transfer nip formed by the transfer roller 12 and the electrostatic transport belt 4 in this case. This is a result in the state where no bias is applied to the transfer roller 12 as in the first embodiment. The larger the roller diameter, the larger the contact area with the electrostatic transport belt 4. Therefore, also in the present embodiment, the same result as in the first and second embodiments can be obtained for the transfer nip itself.

Therefore, as shown in FIG. 7, the same electric field action as in the first and second embodiments is formed. That is, also in the present embodiment, the same effects as those of the first and second embodiments can be obtained, and the occurrence of the transfer image disturbance due to the impact due to the abnormal discharge can be suppressed to the minimum while ensuring the transfer ability of the color superposition. It will be possible.

Incidentally, each transfer roller 1 shown in the first embodiment.
A configuration in which the transfer nip amount is adjusted for each transfer portion by combining the linear pressure increase of 2a to 12d with respect to the photosensitive drum 1 and the sequential increase of the roller hardness of each transfer roller 12a to 12d described in the second embodiment. May be adopted.

[0065]

As is apparent from the above description, according to the present invention, in a color image forming apparatus having a plurality of image carriers and a plurality of transfer means corresponding thereto, among the plurality of image carriers, Of the first image carrier, a second image carrier arranged downstream of the first image carrier, and both the first image carrier and the second image carrier. A transfer material conveying unit that is in contact with the first image carrier and the second image carrier and that conveys the transfer material while moving in the forward direction at substantially the same speed, and the transfer medium conveyor unit. A first transfer unit arranged at a position facing the first image carrier via a first transfer unit and a second transfer unit arranged at a position facing the second image carrier. The first transfer means and the second transfer means are arranged in contact with the transfer material conveying means, respectively, and The toner image formed on the transfer material is transferred onto the transfer material by the first transfer means, and then the toner image formed on the second image carrier is transferred onto the transfer material by the second transfer means. , A first transfer nip length is a nip length formed by the first transfer means and the transfer material conveying means, and a nip length formed by the second transfer means and the transfer material conveying means is a first transfer nip length. When the transfer nip length is 2, since the second transfer nip length is set to be larger than the first transfer nip length, it is possible to minimize the occurrence of transfer image disturbance while ensuring the transfer performance. The effect is obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of a color image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing the layer structure of the photosensitive drum of the color image forming apparatus according to the first embodiment of the invention.

FIG. 3 is an explanatory diagram of a working area of a transfer electric field in the color image forming apparatus according to the first embodiment of the present invention.

FIG. 4 is a sectional view of a main part of a color image forming apparatus according to a second embodiment of the present invention.

FIG. 5 is an explanatory diagram of a working region of a transfer electric field in the color image forming apparatus according to the second embodiment of the present invention.

FIG. 6 is a sectional view of a main part of a color image forming apparatus according to a third embodiment of the present invention.

FIG. 7 is an explanatory diagram of a working area of a transfer electric field in the color image forming apparatus according to the third embodiment of the present invention.

[Explanation of symbols] 1 Photosensitive drum (image carrier) 3 developing roller 4 Electrostatic conveyor belt (transfer material conveyor) 5 charging roller 7 Exposure equipment 8 developing device 11 cleaning blade 12a to 12d Transfer roller (transfer means) 24a-24d spring

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H030 AB02 AD05 BB23 BB44 BB46                       BB54 BB63                 2H200 FA04 GA12 GA16 GA17 GA18                       GA23 GA34 GA44 GA47 GA49                       GA56 GB12 HA03 HB12 HB43                       HB45 HB46 HB47 HB48 JA02                       JA08 JA28 JB06 JB25 JB42                       JB45 JB46 JB47 JB48 KA03                       KA12 MA03 MA04 MB04 MB06                       MC01 MC02 NA02 NA09

Claims (6)

[Claims] 1. A color image forming apparatus having a plurality of image carriers and a plurality of transfer means corresponding thereto, comprising: a first image carrier of the plurality of image carriers;
A second image carrier disposed downstream of the image carrier of
Abutting on both the first image carrier and the second image carrier,
In addition, a transfer material conveying unit that conveys the transfer material while moving at a substantially same speed in the forward direction with respect to the moving direction of the first image carrier and the second image carrier, and via the transfer medium carrier unit. A first transfer means arranged at a position facing the first image carrier and a second transfer means arranged at a position facing the second image carrier. A transfer unit and a second transfer unit are respectively arranged in contact with the transfer material conveying unit, and the toner image formed on the first image carrier is transferred onto the transfer material by the first transfer unit. After that, the toner image formed on the second image carrier is transferred onto the transfer material by the second transfer means, and a nip formed by the first transfer means and the transfer material conveying means. The length is the first transfer nip length, and is formed by the second transfer unit and the transfer material conveying unit. A color image forming apparatus, wherein the second transfer nip length is set to be larger than the first transfer nip length when the nip length is set to a second transfer nip length. 2. A toner image on the first image carrier and the toner image on the second image carrier are transferred onto a transfer material by applying a DC bias to the first transfer device and the second transfer device. The color image forming apparatus according to claim 1, wherein the color image forming apparatus performs electrostatic transfer. 3. The color image forming apparatus according to claim 1, wherein elastic rubber rollers are used as the first transfer unit and the second transfer unit. 4. The contact pressure of the second transfer means with respect to the second image carrier is set to be larger than the contact pressure of the first transfer means with respect to the first image carrier. The color image forming apparatus according to claim 1, 2, or 3. 5. The color according to claim 3, wherein the hardness of the elastic rubber roller used for the second transfer means is set smaller than the hardness of the elastic rubber roller used for the first transfer means. Image forming apparatus. 6. The outer diameter of an elastic rubber roller used for the second transfer means is set to be larger than the outer diameter of the elastic rubber roller used for the first transfer means. 5. The color image forming apparatus according to item 5.