JPH11249396A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve the speed-up of a device without image noises, such as fog and void, and decrease in development efficiency. SOLUTION: The image forming device 1 forms an electrostatic latent image by exposing the surface of an electrified photoreceptor 2, forms a developer image by developing the electrostatic latent image with developer, and transfers the developer image to copying paper S. In the image forming device 1, a developing device 5 having a plurality of developing rollers, 11 and 12, is disposed on the periphery of the photoreceptor, and a development bias which repeats an alternation of the application of an AC bias and quiescence is applied to at least one of the developing rollers, (upstream developing roller 11), while being superposed on a DC bias.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus to which an electrophotographic system is applied, such as a copying machine, a printer, and a facsimile.

[0002]

2. Description of the Related Art In this type of image forming apparatus, the surface of a photoreceptor uniformly charged by a charging device is image-exposed by an exposing device, and the potential of a black nucleus portion (a portion where toner adheres) of a formed image is exposed. Is reduced to form an electrostatic latent image, and the electrostatic latent image is visualized by toner of a developer, and then the developer image is transferred to a copy paper and fixed on the copy paper.

However, in the image forming apparatus, when the potential of the blank portion of the formed image on the photosensitive member is reduced due to the variation of the bias light emission of the laser exposure by the charger and the exposure device, the toner of the developer is also applied to the blank portion. There is a problem in that so-called fogging, that is, the generation of fog, occurs. vice versa,
When the potential of the blank portion of the formed image becomes high, there is a problem that a so-called void occurs in which a carrier for transporting the toner constituting the developer adheres to the blank portion.

In Japanese Patent Publication No. 2-14706, the application of an AC bias and a pause are alternately performed in a developing area where the photosensitive member and a developing device for supplying a developer to the surface of the photosensitive member are opposed to each other. There is provided a developing method in which development is completed in a step of repeatedly applying a developing bias and returning a toner of a developer to a developing device side.

[0005]

In the image forming apparatus to which the above-mentioned developing method is applied, the problem of image noise such as fog and voids can be solved, but the developing efficiency is not always good. That is, the area (margin) in which an image can be formed without generation of the fog or void can be widened, but on the contrary, the potential of the DC component (DC component) of the developing bias and the potential of the highest image density portion are reduced. Is smaller. Therefore, it is possible to secure a certain image density, but if the speed of the apparatus is increased to shorten the time required for image formation, it is difficult to follow the image density. In view of the above, an object of the present invention is to increase the speed of the apparatus without image noise such as fogging and voids and without lowering the development efficiency.

[0006]

In order to solve the above-mentioned problems, an image forming apparatus according to the present invention forms an electrostatic latent image by exposing the surface of a charged photoreceptor, and develops the electrostatic latent image. An image forming apparatus that forms a developer image by visualizing with a developer and transfers the developer image to copy paper, wherein a developing device having a plurality of developing rollers on an outer peripheral portion of the photoconductor is disposed; The developing bias is applied to at least one of the developing rollers by superimposing the developing bias on the DC bias, which alternately alternately applies and suspends the AC bias.

According to the image forming apparatus, a plurality of developing rollers are disposed in the developing device, and a DC bias is applied to at least one of the developing rollers in order to alternately apply an AC bias and alternately suspend. Between the developing roller and the photoconductor,
The step of causing the developer to fly from the developing roller to the photoconductor and the step of returning the developer from the photoconductor to the developing roller are repeated. Therefore, even if a toner or a carrier constituting a developer is supplied to a blank portion of an image to be formed on the photosensitive member in the flying step, the toner or the carrier is returned to the developing roller side in the retraction step. Therefore, image noise such as fog and voids can be prevented. In addition, since a large number of developing rollers are provided, it is not necessary to supply a required amount of developer according to the image density to the photosensitive member in a single (one developing roller) flying step of the developer. As a result, even if the system speed of the image forming apparatus is increased, an image can be formed with a desired density without fail.

In the image forming apparatus, a developing bias, which alternately applies the AC bias and pauses, is alternately applied to a DC bias to a developing roller on the upstream side in the moving direction of the photosensitive member among the plurality of developing rollers. Preferably, the AC bias is applied to the downstream developing roller while being superimposed on the DC bias. With this configuration, on the surface of the photoreceptor, the downstream developing roller can replenish the portion where the developer is insufficiently supplied by the upstream developing roller, and a predetermined image density can be obtained. It is possible to reliably increase the speed of the apparatus without lowering.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a schematic configuration of an image forming apparatus 1 according to a first embodiment of the present invention. The image forming apparatus 1 generally includes a photoconductor 2, a charger 3, an exposure device 4, a developing device 5, a photoconductor cleaner 6, an eraser 7, and a transport device 8.
Consists of

The photosensitive member 2 has a cylindrical shape and can be driven to rotate in the direction A in FIG. Photoconductor 2 of the present embodiment
Is a photoconductor made of an OPC (organic photoconductor), which has a charging property and a photosensitivity to a negative polarity.

The charger 3 charges the surface of the photosensitive member 2 to a uniform negative polarity.

The exposure device 4 is of a laser exposure type,
The photoconductor 2 is disposed downstream of the charger 3 with respect to the rotation direction A of the photoconductor 2, and forms an electrostatic latent image of a predetermined image on the surface of the photoconductor 2 that is uniformly charged.

As shown in FIG. 2, the developing device 5 is disposed on the outer peripheral portion of the photosensitive member 2 on the downstream side of the laser exposure portion, and generally includes a casing 10, a pair of developing rollers 11, 12, Supply roller 13, doctor blade 1
4, scraper 15, bucket roller 16, screw roller 17, toner hopper 18, toner supply roller 1
Consists of nine.

The casing 10 accommodates a developer therein, and has a supply port 1 at a position facing the photosensitive member 2.
0a is provided. Here, a two-component developer composed of a toner and a carrier is used as the developer.
The toner is obtained by mixing carbon black with a polyester resin, styrene acrylic resin, or the like, and has an average particle size of about 6 to 8 μm. The carrier is a mixture of resin and magnetic powder, and has an average particle size of about 50 μm.
belongs to.

The pair of developing rollers 11 and 12 are for supplying the toner of the developer to the surface of the photoconductor 2, and are provided at a supply port 10 a of the casing 10 with an outer peripheral portion of the photoconductor 2. The photoconductor 2 is disposed so as to be located upstream and downstream with respect to the rotation direction A of the photoconductor 2 at a predetermined interval t2 with an interval t1. These developing rollers 11 and 12 are made of a fixed magnet provided with a large number of magnetic poles inside, and a sleeve is rotatably externally provided. The developing rollers 11 and 12 are set so as to rotate in the same direction as each other at a portion facing the photoconductor 2. I have. In the present embodiment, the interval t1 is 0.3 to 0.9 mm, the interval t2 is 1 to 3 mm, and the rotation speed is 2 to 2.2 with respect to the peripheral speed of the photoconductor 2.
It is set to rotate at twice the peripheral speed.

A bias power supply 20 superimposes a DC (DC) bias on the sleeves of the developing rollers 11 and 12 so as to superimpose a DC (direct current) bias, which alternately switches between applying an AC bias and stopping the operation. FIG. 4 shows a waveform in which the developing bias is superimposed on the DC bias and applied. By doing so, the developing roller 11 and the photosensitive member 2
In the meantime, the step of causing the toner of the developer to fly to the photoconductor and the step of returning the toner from the photoconductor 2 to the developing roller 11 are repeated. In the present embodiment, the PP value (peak-to-peak) of the waveform is 1 kV, the frequency f1 is 1 kHz, and the frequency f2 is 9 kHz. These numerical values have been found by the present inventors through intensive research. In this way, the margin M between fogging and voids described later is the widest, and the development efficiency is improved.

The supply roller 13 supplies the developer to the upstream developing roller 11, and is disposed opposite the upstream developing roller 11 with a predetermined gap therebetween. The supply roller 13 is, similarly to the developing rollers 11 and 12, provided with a sleeve rotatably mounted on a fixed magnet provided with a large number of magnetic poles inside, and at a portion opposed to the upstream developing roller 11 in a direction opposite to each other. It is set to rotate. In the present embodiment, the upstream developing roller 1
The gap between the developing rollers 11 and 12 is about 0.4 to 1 times the peripheral speed of the developing rollers 11 and 12.

The doctor blade 14 regulates the supply of the developer to the upstream developing roller 11 by the supply roller 13 to a fixed amount, and its distal end is positioned above the supply roller 13 with a predetermined gap. The casing 10 is disposed in such a manner that In the present embodiment, the gap between the leading end and the supply roller 13 is 0.5 to 3 mm.
It has been.

The scraper 15 is for removing the developer remaining on the outer peripheral portion of the downstream developing roller 12.
The rear side of the downstream-side developing roller 12 is opposed to the rear-side developing roller 12 with a predetermined gap. In the present embodiment, the gap is about 0.5 mm.
05 to 0.5 mm.

The bucket roller 16 is provided on the casing 1.
The developer in the nozzle 0 is supplied to the supply port 10a side while being stirred, and the developer is supplied to the upstream developing roller 11, and a number of concave portions 16a are formed in an outer peripheral portion thereof.

The screw roller 17, like the bucket roller 16, supplies the developer to the bucket roller 16 while stirring.

The toner hopper 18 is provided in the casing 1
0, the developer is supplied to the bucket roller 1
6 is provided on the upper wall of the casing 10 so as to open to the upper part of the casing 6.

The developer replenishing roller 19 is composed of a malt-brain roller made of an ether urethane foam material.
When the developer in the casing 10 is reduced to a certain amount or less by an ATDC sensor signal or an AIDC sensor signal (not shown), the casing 10 rotates and supplies the developer in the toner hopper 18 into the casing 10.

The photoreceptor cleaner 6 removes developer toner remaining on the surface of the photoreceptor 2.
On the other hand, it is disposed downstream of a nip portion between the photosensitive member 2 and the transport device 8 in the rotation direction A.

The eraser 7 removes static electricity charged on the surface of the photoconductor 2, and is disposed downstream of the photoconductor cleaner 6 in the photoconductor 2.

The transport device 8 transports the copy sheet S, and includes a drive roller 22, a driven roller 23, a transfer transport belt 24 stretched over these rollers 22, 23, a transfer charger 25, and a mylar. 26 and a belt cleaner 27. The transfer charger 25 transfers the copy paper S
The toner is adsorbed on the copy paper S by applying a positive voltage having a polarity opposite to that of the toner of the developer.
The photosensitive member 2 is disposed so as to face the transfer position at the lower end. The mylar 26 pushes up the transfer conveyance belt 24 toward the photoconductor 2 in order to improve the adhesion between the copy paper S and the photoconductor 2, and is disposed above the transfer charger 25. The belt cleaner 27 is for cleaning the outer peripheral surface of the transfer conveyance belt 24, and is disposed on the side of the driven roller 23 so as to be in contact with the transfer conveyance belt 24.

The developer image on the surface of the photoconductor 2 at the transfer position of the photoconductor 2 is aligned with the copy sheet S conveyed by the transfer conveyance belt 24 on the upstream side of the conveyance device 8. For this purpose, a timing roller (not shown) is provided. Further, a pair of upper and lower fixing rollers 29 and 30 having heat generating means therein are disposed downstream of the conveying device 8, and these fixing rollers 29 and 30 and the conveying device 8 are provided.
A guide plate 31 is erected between them.

Next, the copy paper S by the image forming apparatus 1
The following describes the image formation on the substrate. First, the photoreceptor 2 is rotated in the direction A in FIG. 1 and the surface of the photoreceptor 2 is uniformly charged by the charger 3 and then is exposed to laser light by the exposing device 4 to form a predetermined image. An image is formed on the surface of the photoconductor 2 as an electrostatic latent image.

Then, the photosensitive member 2 is
The toner of the developer is separated and supplied to the surface of the photoconductor 2, and the latent image on the surface of the photoconductor 2 is visualized to reversely develop the developer image. The operation of supplying toner to the photoconductor 2 by the developing device 5 will be described later in detail.

The transfer charger 25 applies a voltage having a polarity opposite to that of the toner to the copy paper S conveyed via the transfer conveyance belt 24 of the conveyance device 8, and the transfer conveyance belt 24 uses the mylar 26. The copy paper S is brought into close contact with the photoreceptor 2 by pushing up 24. As a result, the toner electrostatically adhering to the surface of the photoconductor 2 is attracted to the copy paper S, and the developer image is formed on the copy paper S according to the rotation of the photoconductor 2 and the movement of the copy paper S by the transport device 8. Transcribed.

Next, the copy paper S on which the image has been transferred is
It is conveyed in a state of being electrostatically attracted to the transfer conveyance belt 24, is separated at the upper portion of the drive roller 22 by the curvature of the drive roller 22, passes over the guide plate 31,
The sheet passes between a pair of upper and lower fixing rollers 29 and 30 and is thermally fixed, and is discharged to the outside of the image forming apparatus 1 such as a discharge cassette (not shown).

On the other hand, the photoreceptor 2 having completed the transfer operation is cleaned and removed of the toner remaining on the surface by the photoreceptor cleaner 6, and is then discharged by the eraser 7 to prepare for the next image forming process to be performed subsequently.

The portion of the transfer / transport belt 24 where the copy paper S has been adsorbed after the separation of the copy paper S is:
From the drive roller 22 side to the driven roller 23 side, the belt is cleaned by a belt cleaner 27 disposed on the side of the driven roller 23, and the apparatus prepares for the next conveyance of the copy sheet S.

Next, supply of toner to the photosensitive member 2 by the developing device 5 will be described. First, the toner and the carrier constituting the developer are circulated and agitated by the screw roller 17 and the bucket roller 16 to charge the toner to a negative polarity and the carrier to a positive polarity.

Then, a part of the developer, specifically, a part of the developer charged to a predetermined potential, is agitated by the bucket roller 16 and supplied to the supply roller 13 side. On the other hand, the remaining developer not supplied to the supply roller 13 is supplied to the right side of the bucket roller 16 in FIG. 2, and is stirred and charged by the bucket roller 16 as described above.

Next, the developer supplied to the supply roller 13 is regulated to a predetermined amount by the doctor blade 14, and then supplied to the upstream development roller 11.

Here, the upstream developing roller 11 has
As described above, since the bias power supply 20 superimposes the DC bias on the developing bias that alternately controls the application of the AC bias and the pause, the bias between the developing roller 11 and the photoconductor 2 is applied as described above. The process of flying the toner of the developer onto the photoconductor and the process of returning the toner from the photoconductor 2 to the developing roller 11 are repeated. Therefore,
Part of the toner constituting the developer supplied to the upstream developing roller 11 is transferred to the surface of the photoconductor 2 in the flying step, and the developer image is reversely developed on the surface of the photoconductor 2. Further, the toner and the carrier that have adhered to the blank portion of the image formed in this transfer step are pulled back from the photoconductor 2 to the upstream developing roller 11 in the pull-back step. Therefore, the problem of fog and voids can be reliably prevented.

On the other hand, the developer (toner and carrier) remaining on the upstream-side developing roller 11 and the toner and carrier returned to the upstream-side developing roller 11 in the retraction step are separated by the downstream-side developing roller 12. Supplied to

The developer supplied to the downstream-side developing roller 12 is supplied with DC as in the case of the upstream-side developing roller 11.
Due to the developing bias superimposed on the bias, a part of the toner separates and flies again to the surface of the photosensitive member 2 in the flying process, and replenishes the high density portion of the image that is not sufficiently supplied by the upstream developing roller 11. I do.

On the other hand, the developer (the rest of the toner and the carrier) which has not been supplied to the photoreceptor 2 even at the downstream developing roller 12 reaches the position where the scraper 15 is provided, and the downstream developing roller 12 And is returned to the bucket roller 16 side.

If it is detected by a sensor (not shown) that the amount of developer contained in the casing 10 of the developing device 5 has decreased due to the supply of toner to the photosensitive member 2, the developer supply roller 19, the developer in the toner hopper 18 is removed from the casing 1.
Replenish within 0.

FIG. 3 shows the surface potential of the photosensitive member 2 when only a DC bias is applied to the developing roller 11.

In the figure, V2 is the potential of the blank portion in the standard state (non-exposure potential or the potential when the exposure device 4 emits bias light). V4 is the potential of the DC component of the developing bias. V5 is the potential of the highest image density portion (exposure device 4
Is the latent image potential at which the electrostatic latent image is written. V1
Is a value when the potential of V1 is increased due to a variation in bias light emission of laser exposure by the photoconductor 2, the charger 3, and the exposure device 4. V3 is a value opposite to V1 when the potential of V1 is lowered due to a variation in bias light emission of laser exposure by the photoconductor 2, the charger 3 and the exposure device 4.

Here, it can be said that "no fog occurs" not at the time of the potential V2 of the blank portion in the standard state, but at
In the case of V3, it is necessary to prevent fogging from occurring in the blank portion. Further, in order to be able to say that "no void is generated", it is necessary to prevent voids from being generated at the potential V1 of the blank portion in the standard state, not at the potential V2. These confirmations are made by performing a test print of the halftone dot image and determining whether or not white dots have occurred in the halftone dot image.

When fog does not occur on the blank portion at V3, the potential at which fog occurs can be obtained by lowering the grid voltage of the charger 3. Also,
When no void is generated at V1, the potential at which a void is generated can be obtained by increasing the grid voltage of the charger 3. The difference between the potential at which voids are generated and the potential at which fogging is determined is defined as a margin M (V) between fog and voids. It can be said that the larger the margin M between fog and voids, the less image noise such as fog and voids is generated, and the smaller the margin M between fog and voids, the more easily image noise such as fog and voids is generated.

In order to say that "development efficiency is good", the target image density is secured even if the potential difference ΔV between V4 and V5 is small, that is, the high density portion of the formed image is set to a predetermined density. Need to be formed.

In the photosensitive member 2 used in the present embodiment, the photosensitive layer is abraded by the cleaning means such as the photosensitive member cleaner 6 as the photosensitive member 2 is used. Begins to occur. The surface potential at which black spots occur during the life of the photosensitive member 2, that is, the limit of V1 is -800V. V5 depends on the environment, but the maximum is -2
00V. The difference between V2 and V4 is -150V, V1 and V
The difference between 2 and V2 and V3 is -50V. Also,
The difference between V1 and V4 is 200V. Accordingly, at this time, the surface potential of the photoconductor 2 is -800 V for V1 and -75 V for V2.
0V, V3 is -700V, V4 is -600V, V5 is-
200V.

As a result, the margin M between the fog and the void needs to be 100 V or more because the difference between V1 and V3 is 100V. The development efficiency is V4 and V5.
Since the maximum potential difference ΔV is only -400V,
It is necessary to secure a target image density at 0 V or less. Further, it has been confirmed by experiments that the difference between V3 and V4 is 75 V.

In the image forming apparatus 1 of the first embodiment,
While maintaining the above conditions, the developing bias is applied to the developing rollers 11 and 12 by repeating the application of the AC bias and the pause.
Since the supply is performed while being superimposed on the bias, it is possible to reliably prevent the occurrence of image noise such as fog and voids as described above. Further, since the developing device 5 is provided with the two developing rollers 11 and 12, there is no need to supply the necessary amount of toner to the photoreceptor 2 with one toner supply, and the toner is divided into two. Since it suffices to supply, even if the difference between the potential of the DC component of the developing bias and the potential of the highest image density portion is small, an image having a desired density can be formed.

FIG. 5 shows an image forming apparatus 1 to which the developing device 5 of the second embodiment is applied. In the developing device 5 of this image forming apparatus, in the first embodiment, the bias power supply 20 applies a developing bias, which repeats the application of an AC bias and a pause, to both the developing rollers 11 and 12 in a manner superimposed on the DC bias. On the other hand, the bias power supply 20 applies a developing bias that repeats the application and suspension of the AC bias only to the upstream developing roller 11 superimposed on the DC bias, and applies the AC bias to the downstream developing roller 12 by the bias power supply 35. The third embodiment differs from the first embodiment only in that the voltage is applied so as to be superimposed on a DC bias.

The waveform of the developing bias applied to the upstream developing roller 11 by the bias power source 20 is the same as that of the first embodiment shown in FIG. 3, and the waveform of the developing bias applied to the downstream developing roller by the bias power source 35 is shown. Is P
-A sine wave with a P value of 1 kV and a frequency of 6 kHz.

[0052]

[Experimental Examples] The present inventors have proposed an image forming apparatus having the structure described in Japanese Patent Publication No. 2-1706 and the system speed (= image forming speed) of the image forming apparatuses according to the first and second embodiments. In order to investigate the limits of the above, margins M and potential differences ΔV required at various system speeds were obtained, and comparisons were made by applying numerical values corresponding thereto.

Here, the margin M was obtained by adjusting the grid voltage of the charger 3 in each system as described above. The potential difference ΔV is a value that is satisfactory for forming a high density portion of a formed image, and V5 is −200 V at the maximum. Therefore, by fixing V5 to −200 V and adjusting the voltage of V4, I asked.

[0054]

[Table 1]

Table 1 shows the case where the system speed of each image forming apparatus is set to 20 cm / sec. In this case, in the conventional image forming apparatus, the required value of the margin M is 140
(V), the required value of the potential difference ΔV was 460 (V).
In order to secure these necessary values, V4 is -660V because V5 is -200V. As described above, V3 needs to have a difference of 75V from V4, and therefore becomes -735V. As a result, V2 is -805 V, V1 is -875
V, which is the limit of the surface potential of the photoconductor 2 -800 V
Beyond. Therefore, when trying to suppress V1 to -800V, V2 becomes -730V, V3 becomes -660, V4 becomes -585V, and V5 becomes -125V. Thus, V
5 exceeds -200 V, the actual value of the potential difference ΔV is 385 V, and the required value ΔV 460 V cannot be satisfied, so that a desired image density cannot be secured. Therefore, it can be said that the image forming apparatus having the conventional configuration cannot be used at the system speed of 20 cm / sec.

In contrast to the conventional image forming apparatus, the image forming apparatus 1 according to the first embodiment of the present invention has a required margin M
Was 200 V and the potential difference ΔV was 310 V. To secure these required values, V5 is -200V,
V4 is -510V, V3 is -585V, V2 is -685
V and V1 become -785V, and the limit voltage (-
800V), a margin M, a potential difference ΔV, and the like. Therefore, the image forming apparatus 1 of the first embodiment can form an image with a desired image density without generating image noise such as fog and voids even when used at a system speed of 20 cm / sec. I can say.

In the image forming apparatus 1 of the second embodiment, the required margin M is 200 V and the potential difference ΔV is 250
V. In order to secure these necessary values, V5 is-
Since it is 200 V, V4 is -450 V and V3 is -52
5V, V2 is -625V, and V1 is -725V, which satisfies each of the above conditions similarly to the image forming apparatus 1 of the first embodiment. Therefore, the system speed is 20 cm / s.
ec.

[0058]

[Table 2]

Table 2 shows that the system speed of the image forming apparatus 1 of the first embodiment and the second embodiment is 40 cm / sec.
The case where it is set to c is shown. In this case, in the image forming apparatus 1 of the first embodiment, the required margin M is 170 V, and the potential difference △
V is 350 V, and in the image forming apparatus 1 of the second embodiment, the necessary margin M is 170 V and the potential difference ΔV is 280
V. To secure these required values, V1 to V
5 are the values shown in Table 2, and it can be said that the image forming apparatuses 1 of the respective embodiments can satisfy the above conditions, and can be used at a system speed of 40 cm / sec.

[0060]

[Table 3]

Table 3 shows the case where the system speed of both image forming apparatuses 1 of each embodiment is 60 cm / sec. In this case, in the image forming apparatus 1 of the first embodiment, the required margin M is 140 V and the potential difference ΔV is 380 V, and in the image forming apparatus 1 of the second embodiment, the required margin M is 140 V and the potential difference ΔV Was 310V. In order to secure these necessary values, V1 to V5 become the values shown in Table 3, respectively, and the image forming apparatus 1 of each embodiment can satisfy the above conditions, and therefore, the system speed 60c
It can be said that it can be used at m / sec.

[0062]

[Table 4]

Table 4 shows the case where the system speed of the image forming apparatus 1 of each embodiment is 80 cm / sec. In this case, in the image forming apparatus 1 of the first embodiment, the necessary margin M is 120 V, the potential difference ΔV is 420 V,
In the image forming apparatus 1 of the embodiment, the necessary margin M is 1
20V and the potential difference ΔV was 340V. In order to secure these necessary values, V1 to V5 become the values shown in Table 4, respectively. In the image forming apparatus 1 of the first embodiment, the potential difference △
If V is to be secured, V1 exceeds -800 V. If the margin M is to be secured, the actual value of the potential difference ΔV is 405 V, and the required value condition cannot be satisfied. On the other hand, in the image forming apparatus 1 of the second embodiment, each of the above conditions can be satisfied. Therefore, the image forming apparatus 1 of the first embodiment cannot be used at a system speed of 80 cm / sec, but the image forming apparatus 1 of the second embodiment is
It can be said that it can be used even at a system speed of 80 cm / sec.

[0064]

[Table 5]

Table 5 shows the case where the system speed of the image forming apparatus 1 of each embodiment is 100 cm / sec. In this case, in the image forming apparatus 1 of the first embodiment, the required margin M is 110 V and the potential difference ΔV is 460 V, and in the image forming apparatus 1 of the second embodiment, the required margin M is 110 V and the potential difference ΔV Was 370V. In order to secure these necessary values, V1 to V5 become the values shown in Table 5, respectively, and similarly to the case of the system speed of 80 cm / sec, the image forming apparatus 1 of the first embodiment has a system speed of 100 cm / sec. Although the image forming apparatus 1 according to the second embodiment cannot be used in the second mode, the system speed is 100 cm / sec.
It can be said that c can also be used.

As described above, the conventional image forming apparatus cannot form an image with a desired image density unless the system speed is set lower than 20 cm / sec. On the other hand, the image forming apparatus 1 having the configuration according to the first embodiment of the present invention can increase the system speed to 60 cm / sec.
Further, the image forming apparatus 1 having the configuration of the second embodiment can increase the system speed to 100 cm / sec.

Note that the image forming apparatus of the present invention is not limited to the above configuration. For example, in each of the above embodiments, two developing rollers are provided, but three or more developing rollers may be provided. With such a configuration, the speed can be further increased.

[0068]

As is apparent from the above description, in the image forming apparatus of the present invention, a developing device having a large number of developing rollers is provided, and an AC bias is applied to at least one of the developing rollers. The development bias, which alternately repeats and pauses, is applied so as to be superimposed on the DC bias, so that image noise such as fogging and voids can be prevented from occurring, and one separation flight from the photosensitive member Therefore, since it is not necessary to supply a target amount of toner, the system speed of the apparatus can be increased without lowering the development efficiency.

[Brief description of the drawings]

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

FIG. 2 is a schematic configuration diagram illustrating a developing device provided in the image forming apparatus according to the first embodiment.

FIG. 3 is a graph showing a surface potential of a photoconductor when only a DC bias is applied to a developing roller.

FIG. 4 is a waveform diagram in a state where a developing bias applied to a developing roller is superimposed on a DC bias.

FIG. 5 is a schematic configuration diagram illustrating a developing device provided in an image forming apparatus according to a second embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Photoconductor, 3 ... Charger, 4 ... Exposure apparatus, 5 ... Developing apparatus, 6 ... Photoconductor cleaner, 7 ... Eraser, 8 ... Conveying apparatus, 10 ... Casing, 11, 12 ... Development Roller, 13: Supply roller, 14: Doctor blade, 15: Scraper, 16: Bucket roller, 17 ...
Screw roller, 18: toner hopper, 19: toner supply roller, 20: bias power supply, 22: drive roller, 23: driven roller, 24: transfer / transport belt, 25 ...
Transfer charger, 26 ... Mylar, 27 ... Belt cleaner, 2
9, 30: fixing roller, 31: guide plate, S: copy paper.

Claims (2)

[Claims] An electrostatic latent image is formed by exposing the surface of a charged photoreceptor, and the electrostatic latent image is visualized with a developer to form a developer image, and the developer image is copied. In an image forming apparatus for transferring to paper, a developing device having a large number of developing rollers is provided on an outer peripheral portion of the photoreceptor. Wherein the developing bias is alternately repeated and applied in a manner superimposed on the DC bias. 2. A developing bias superimposed on a DC bias, which alternately applies and pauses the application of the AC bias, to a developing roller on the upstream side in the moving direction of the photoconductor, and is applied to the developing roller on the upstream side in the moving direction of the photoconductor. 2. The image forming apparatus according to claim 1, wherein an AC bias is applied to the developing roller in a manner superimposed on a DC bias.