JPH1026885A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an image in which a fixed density is maintained by separating two-component developer at least on the developer carrier of a developing unit into toner and carrier in the case of successive development of images at low image ratios, thereby preventing toner charge-up and deterioration of carrier. SOLUTION: Every time image formation is carried out the image ratio is counted by a video counter, and when a cumulative count value resulting from past 1000 sheets of original documents becomes smaller than a set value, the developer is separated into the toner and carrier. A separation sleeve 49 is set in a toner recovery chamber 48 having an opening in its upper part, which is set adjacent to the first stirring chamber 41 of the developing unit 4. By applying a bias to the separation sleeve 49, between a developing sleeve 43 and itself, by means of a power source 50, the toner is caused to fly to the sleeve 49 from the developer on the developing sleeve 43 by means of an electric field. The separated toner sticking to the surface of the separation sleeve 49 is scraped with a blade 51 whose edge is in contact with the surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a printer.

[0002]

2. Description of the Related Art FIG. 3 is a sectional view showing a schematic configuration of a conventional color image forming apparatus. The present image forming apparatus has a digital color image reader unit at the top and a digital color image printer unit at the bottom.

In the reader section, the original 30 is placed on an original platen glass 31 and exposed and scanned by an exposure lamp 32 so that a reflected light image from the original 30 is condensed on a full color sensor 34 by a lens 33 and color separation is performed. Obtain an image signal. The color separation image signal is processed by a video processing unit (not shown) via an amplifier (not shown) and sent to a printer unit.

In the printer section, a photosensitive drum 1 serving as an image carrier is rotatably supported in the direction of an arrow. Around the photosensitive drum 1, a pre-exposure lamp 11, a corona charger 2, a laser exposure optical system 3, a potential sensor 12. Four developing units 4Y, 4C, 4M, 4K of different colors, light amount detecting means 1 on the drum
3. The transfer device 5 and the cleaning device 6 are arranged.

In the laser exposure optical system 3, an image signal from the reader unit is converted into an optical signal by a laser output unit (not shown), and the converted laser light (optical image) E is reflected by the polygon mirror 3a. Then, the surface of the photosensitive drum 1 is linearly scanned (raster scanned) through the lens 3b and the mirror 3c.

The printer unit includes a photosensitive drum 1 during image formation.
Is rotated in the direction of arrow R1, and after the charge is eliminated by the pre-exposure lamp 11, the photosensitive drum 1 is uniformly charged by the charger 2,
An optical image E is emitted for each separation color to form an electrostatic latent image on the surface of the photosensitive drum 1.

Next, a predetermined developing device is operated for each latent image of each separation color to develop the image, and an image is formed on the photosensitive drum 1 using a resin-based toner, that is, a toner image. In this example, the developing units 4Y, 4C, 4M, and 4K contain a two-component developer in which a toner and a magnetic carrier are mixed, and yellow, cyan, magenta, and black toners are used as the toners. . These developing devices 4Y to 4K are
The operation of the eccentric cams 24Y, 24C, 24M, and 24K causes the photosensitive drum 1 to be selectively approached in accordance with the latent image.

On the other hand, the recording material cassette 7a, 7b or 7
c, the recording material is conveyed to the image transfer unit facing the photosensitive drum 1 via the conveyance system and the transfer device 5, and the toner image formed on the photosensitive drum 1 is transferred onto the recording material in the image transfer unit. You.

In the present embodiment, the transfer device 5 includes a transfer drum 5a.
And a transfer charger 5b and an adsorption charger 5c around the transfer charger 5b.
And an attraction roller 5g, an inner charger 5d, an outer charger 5e, and the like opposed thereto. The transfer drum 5a is supported so as to be driven to rotate, and a recording material carrying sheet 5f is integrally extended in a cylindrical shape in a peripheral opening area. As the recording material supporting sheet 5f, a dielectric sheet such as a polycarbonate film is used. The recording material is an adsorption charger 5
Due to the action of c and the suction roller 5g, the sheet is electrostatically adsorbed on the surface of the recording material carrying sheet 5f, and is repeatedly rotated through the image transfer section according to the rotation of the transfer drum 5a.

The toner image on the photosensitive drum 1 is moved to an image transfer section as the photosensitive drum 1 rotates, and is transferred to the transfer drum 5.
The toner image is transferred onto the recording material passing through the image transfer portion by the transfer of the transfer member a by the operation of the transfer charger 5b. In this manner, a desired number of color toner images, in this example, four color toner images are superimposedly transferred onto the recording material on the transfer drum 5a, and a full-color image is formed.

In the case of one-sided full-color image formation, when the transfer of the four-color toner image onto the recording material is completed, the separation claw 8 is formed.
a, the recording material is separated from the transfer drum 5a by the action of the separation push-up roller 8b and the separation charger 5h, and the separated recording material is sent to the heat roller fixing device 9 to be fixed, thereby completing the image formation, and the one-sided full color image Is discharged onto the tray 10. After the transfer, the photosensitive drum 1 is cleaned by a cleaning device 6 to remove the residual toner on the surface, and then subjected to the image forming process again.

When a full-color image is to be formed on both sides of the recording material, the conveyance path switching guide 19 is driven immediately after the recording material has left the fixing device 9, and the recording material is passed through the conveyance vertical path 20 to the reverse path. 21a. Thereafter, by the reverse rotation of the reversing roller 21b, the recording material is retreated in a direction opposite to the direction in which the recording material is fed, with the rear end when fed in as the head,
It is stored in the intermediate tray 22. Thereafter, the recording material is sent from the intermediate tray 22 to the transfer drum 5a, and a full-color image is formed on the other surface of the recording material again by the above-described image forming process, thereby obtaining a recording material of a double-sided full-color image.

Incidentally, the powder scattered from the photosensitive drum 1, the developing devices 4Y to 4K, the cleaning device 6, etc. adhere to the recording material carrying sheet 5f of the transfer drum 5a, or the recording material jams (paper jam). At this time, the toner adheres and oil adheres to the recording material adhered at the time of forming a double-sided image, thereby causing contamination. On the other hand, the cleaning of the recording material carrying sheet 5f is performed by the action of the fur brush 14 and the backup brush 15 facing each other across the recording material carrying sheet 5f, and the action of the oil removing roller 16 and the backup brush 17. Such cleaning is performed before or after image formation, and as needed when a jam occurs.

In this embodiment, the gap between the recording material carrying sheet 5f and the photosensitive drum 1 is reduced by operating the eccentric cam 25 at a desired timing and operating the cam follower 5i integrated with the transfer drum 5a. The configuration can be set arbitrarily. For example, during standby or when the power is off, the distance between the transfer drum 5a and the photosensitive drum 1 is increased,
Can be independently rotated.

[0015]

In the above-described image forming apparatus, when an image having a low image ratio (duty) is formed, the amount of toner consumed in the developer carried on the developing sleeves of the developing devices 4Y to 4K. And a large part of the developer on the developing sleeve returns to the developing device while being attached.

If the developing operation is repeated for a long period of time with the developer adhered, the toner of the developer is charged up (excessively charged) or the carrier is deteriorated, and the image density cannot be kept constant.

It is an object of the present invention to separate at least the developer on the developer carrier of the developing device into toner and carrier when the development of an image having a low image ratio continues, thereby increasing the charge of the toner and deterioration of the carrier. An object of the present invention is to provide an image forming apparatus capable of preventing an image and obtaining an image having a constant density by development.

[0018]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image carrier, a latent image forming means for forming an electrostatic latent image corresponding to an image signal on the image carrier, and a two-component developing device for developing the latent image to visualize it as a toner image. And a video counter for counting the image ratio of the image based on the image signal and integrating the counted value, and developing the developing unit when the integrated count value is equal to or less than a predetermined value. An image forming apparatus comprising: a separating unit that separates an agent into a toner and a carrier.

According to one aspect of the present invention, the separating means applies a bias between the rotating conductive member disposed to face the developer carrying member of the developing means and the conductive member. And a bias power supply for generating an electric field for applying the toner from the developer carried on the developer carrier to the conductive member. The conductive member is housed in a toner collection container that communicates with a container of the developing unit at a portion facing the developer carrier,
Further, a member for scraping off the toner attached to the conductive member can be brought into contact with the conductive member.

According to another aspect of the present invention, the separating means blows compressed air toward the surface of the developer carrier of the developing means, and blows off the developer carried on the developer carrier. Spraying means, a toner collection container having a mesh through which the toner can pass, and communicating with a container of the developing means at a position sandwiching the air blowing means and the developer carrier, and suctioning the inside of the toner collection container. A suction means for guiding the blown-down developer into the container and containing the developer, and a stirring means for stirring the developer contained in the toner collection container,
The developer is separated into the toner and the carrier by the stirring of the developer by the stirring means and the suction of the toner by the suction means.

According to still another aspect of the present invention, the separating means blows compressed air toward the surface of the developer carrier of the developing means to blow down the developer carried on the developer carrier. An air blowing means, a toner collection container communicating with the container of the developing means at a position sandwiching the air blowing means and the developer carrier, and having a mesh through which toner can pass, and a suction force in the toner collection container. The developer that has been blown down is guided and accommodated in the container by intermittently suctioning the developer, and the developer contained in the toner collection container is agitated to be separated into toner and carrier. Suction means.

In any case, when the developer is separated by the separating means, the developer in the developing means is newly sucked up and carried on the developer carrying body by rotation of the developer carrying body, and the developer is separated. Can be. In addition, a replenishing tank for replenishing the developer in the developing unit with toner is provided, and the separated toner can be transferred to the toner replenishing tank and used. Further, a plurality of developing means may be provided around the image carrier.

[0023]

Embodiments of the present invention will be described below in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a sectional view showing a developing unit in an embodiment of the image forming apparatus of the present invention. A major feature of the present invention is that a means for separating a developer into a toner and a carrier is provided in a developing device. Since the configuration of the image forming apparatus of the present invention is basically the same as that of the conventional image forming apparatus shown in FIG. 3, it will be described below with reference to FIG.

In the image forming apparatus, the reader 30 places the original 30 on the original platen glass 31 shown in FIG. 3, scans the original 30 with the exposure lamp 32, and collects the reflected light image from the original 30 on the full color sensor 34 by the lens 33. When illuminated, a color separation image signal is obtained. The color separation image signal is
The signal is processed by a video processing unit (not shown) via an amplifier (not shown) and sent to a printer unit.

According to the present invention, a video counter (not shown) is provided in the reader unit, and the image signal processed by the video processing unit is also sent to the video counter. The video counter obtains an image ratio based on an image signal from the video processing unit, and counts (counts) a count value of the image ratio.

In the printer section, the photosensitive drum 1 rotates in the R1 direction, the photosensitive drum 1 is neutralized by the pre-exposure lamp 11, is uniformly charged by the charger 2, and irradiates a light image E for each separation color. Then, an electrostatic latent image for each of the separated colors is formed on the photosensitive drum 1, and the latent images are developed by the corresponding developing units 4Y to 4K.

As shown in FIG. 1, the developing devices 4 (4Y to 4Y)
K) is divided into a first stirring chamber 41 and a second stirring chamber 42 which are partitioned by a partition 47 having two communication ports (not shown).
At the opening of the first stirring chamber 41 facing the photosensitive drum 1, a developing sleeve 43 that rotates in the direction of arrow A is arranged. Inside the developing sleeve 43, a magnet roller 44 is non-rotatably arranged. Stirring screws 45a and 45b are installed in the stirring chambers 41 and 42, and rotate in the directions of arrows B and C, respectively.

A concentration sensor 46 is provided in the upper portion of the second stirring chamber 42, and detects the concentration of the developer (mixing ratio of toner and carrier) in the second stirring chamber 42 by an optical reflection method. The second stirring chamber 4 is moved from a toner supply tank (not shown) so that the developer in the developing device 4 has a predetermined concentration.
2 is supplied with toner.

According to the present invention, the first stirring chamber 41
A toner collection chamber 48 opened at the top is installed adjacent to the toner collection chamber.
A separation sleeve 49 that rotates in the direction of arrow D is provided, and a bias for toner separation between the separation sleeve 49 and the developing sleeve 43 can be applied to the separation sleeve 49 by a bias power supply 50. The toner separating bias is a DC voltage that forms an electric field for causing toner to fly from the developer on the developing sleeve 43 toward the separating sleeve 49, or an AC voltage superimposed thereon. A blade 51 for scraping off the separated toner adhered to the surface of the separation sleeve 49 is brought into edge contact with the surface.

A bias power source 50 for toner separation is used.
In some cases, a bias power supply for the developing bias can be used, and in that case, it is not necessary to provide the bias power supply 50 independently.

Now, a latent image is formed on the photosensitive drum 1,
When the latent image reaches the developing position of the developing device 4 corresponding to the rotation of the photosensitive drum 1 in the R1 direction, the developing sleeve 43 of the developing device 4 is driven to rotate, and is pressed by the pressing cam. And is positioned with respect to the photosensitive drum 1. At the same time, a developing bias in which the AC voltage and the DC voltage are superimposed is applied to the developing sleeve by the developing bias power supply, and development of the latent image is started.

According to the present invention, each time an image is formed, the video counter obtains the image ratio based on the image signal from the video processing unit, and counts (integrates) the value for the past 1000 originals. If the video count value is smaller than a preset value, it is determined that the image frequency with a low image ratio is high. If the amount of consumption of the toner in the developer carried on the developing sleeve is small and the developing operation is repeated for a longer period of time while the carrier and the toner are adhered, the charge-up (excessive charging) and deterioration of the toner occur. In order to prevent this, the following operation for separating the developer into the toner and the carrier is performed.

That is, after the development of the latent image on the photosensitive drum 1 is completed, the application of the developing bias is stopped, and the pressing of the developing sleeve 43 by the pressing cam is released. Developing sleeve 4
9. While the rotation of the stirring screws 45a and 45b is continued, the separation sleeve 49 is newly rotated, and at the same time, a bias is applied between the separation sleeve 49 and the developing sleeve 43 by the bias power supply 50.

The developer in the second stirring chamber 42 is conveyed in one direction in the longitudinal direction while being stirred by the stirring screw 45b, and is transferred into the first stirring chamber 41 through the communication port at one end of the partition. You. The developer in the first stirring chamber 41 is transported in one direction opposite to the above while being stirred by the stirring screw 45a.
It is carried on the developing sleeve 43 by the suction pole of the magnet roller 44 inside, and is conveyed by the rotation of the developing sleeve 43.

When the developer carried on the developing sleeve 43 reaches a position facing the separating sleeve 49, an electric field formed between the developing sleeve 43 and the separating sleeve 49 due to the application of a bias causes the developer in the developing sleeve 43 to be separated. The toner flies in the direction of the separation sleeve 49 and adheres to the surface of the separation sleeve 49. As a result, the developer toner and the carrier are separated on the developing sleeve 43, and the carrier remains on the developing sleeve 43.

The separation toner adhered to the separation sleeve 49 reaches the blade 51 by the rotation of the separation sleeve 49, where the blade 51 causes the separation sleeve 49.
Of the collecting chamber 48 at the lower end of the collecting chamber 48.
8a, and is transferred to the toner replenishing tank by appropriate means. The separated toner sent to the toner replenishing tank is mixed there with toner for replenishment, and the charged charge is reduced.
Therefore, charge-up of the toner is prevented.

The carrier remaining on the developing sleeve 43 is rotated by the rotation of the developing sleeve 43 to cause the stirring screw 45 to rotate.
a, and the magnet roller 4
Due to the repulsion pole of No. 4, it is peeled off from the surface of the developing sleeve 43. Therefore, the developing operation is not repeated in a state where the carrier remains on the developing sleeve 43 for a long time, and the deterioration of the carrier is prevented.

The carrier peeled off from the developing sleeve 43 falls into the first stirring chamber 41 and is stirred by the stirring screw 45.
a, and is transferred into the second stirring chamber 42 from the other end of the partition 47. And the carrier, together with the other carriers in the second stirring chamber 42,
The toner is replenished from the toner replenishing tank by the detection of the density sensor 46, and is mixed by the stirring of the stirring screw 45b, whereby the developer is recovered to a predetermined density. Therefore, an image having a constant density can be obtained by development.

In the above, when the developer is separated by the separation sleeve 49, the rotation of the developing sleeve 43 is continued, so that the developer in the developing container 4 is newly sucked up and carried on the developing sleeve 43, The developer inside was separated. However, the present invention is not limited to this, and the developer can be separated only from the developer originally carried on the developing sleeve 43, and the state in which the developer adheres to the developing sleeve 43 can be used. Thus, the original object of preventing charge-up of the toner and deterioration of the carrier by repeating the developing operation for a long time can be achieved.

Embodiment 2 FIG. 2 is a sectional view showing a developing unit according to another embodiment of the present invention. The major feature of the present embodiment is that compressed air is used as the developer separating means for the developer.

As shown in FIG. 2, an air injection passage 54 having one end as an injection port 54a and the other end as a discharge port 54b is installed above the stirring screw 45b across the inside of the second stirring chamber 42. The discharge port 54 b is desired in the first stirring chamber 41 toward a position near the stirring screw 45 a between the discharge port 54 b and the developing sleeve 43. Injection port 54 of air injection path 54
a is connected to an air compressor (not shown) installed outside the first stirring chamber 46.

A toner collection chamber 48 communicating with the first stirring chamber 41 at the top is installed adjacent to the first stirring chamber 41. In the collection chamber 48, a stirring screw 53 rotating in the direction of arrow E is installed. A mesh 53 through which only the toner can pass is attached below the screw 53. An air fan (not shown) having a blowing direction reversed in the direction of the arrow as suction means is connected to an opening 48a at the lower end of the toner collecting chamber 48.

According to this embodiment, the toner in the developer is separated as follows. In the same manner as in the first embodiment, the image ratio of the past 1000 originals is counted by a video counter, and if it is determined that the image frequency with a low image ratio is high, the development bias of the developing bias The application is stopped, and the developing sleeve 43 is pressed by the pressure cam.
Is released, and the stirring screw 52 of the toner collecting chamber 48 is newly rotated. While the rotation of the developing sleeve 49 and the stirring screws 45a and 45b is maintained, the developer in the second stirring chamber 42 is transferred to the first stirring chamber 41, and the developer in the first stirring chamber 41 is transferred onto the developing sleeve 49. To continue the operation of sucking and conveying.

Next, compressed air is blown into the air injection path 54 from an injection port 54a at one end thereof by an air compressor, and the compressed air is blown through the injection path 54 from the discharge port 54b at the other end to the developing sleeve 43. The developer carried on the developing sleeve 43 is blown off. At the same time, the inside of the toner collecting chamber 48 is sucked by the air fan from the opening 48a at the lower end thereof, and the developer blown down from above the developing sleeve 43 is sent into the toner collecting chamber 48 for storage.

By performing the above operation for a predetermined time, the developer in the first and second stirring chambers 41 and 42 is substantially removed from the toner collecting chamber 4.
When the developing sleeve 43 is accommodated in the developing sleeve 8, the rotation of the developing sleeve 43 and the stirring screws 45a and 45b is stopped.

The developer contained in the toner collection chamber 48 is stirred by the stirring screw 52 and
The toner is sucked out by suction from the opening 48a at the lower end, and the developer is separated into the carrier and the toner. The separated toner is transferred to the toner replenishing tank through the mesh 53 by suction from the opening 48a at the lower end of the collection chamber through a passage (not shown), where it is mixed with the toner for replenishment, and the charged charge is reduced. .

After the above-described separating operation is performed for a predetermined time, when the toner in the developer accommodated in the toner collecting chamber 48 is almost separated, the rotation of the stirring screws 45a and 45b is resumed, and The accumulated carriers are transferred from the collection chamber 48 into the first stirring chamber 41, and then into the first stirring chamber 41.
From the second stirring chamber 42. The transported carrier is supplied with toner from the toner replenishing tank detected by the density sensor 46 together with other remaining carriers in the second stirring chamber 42, and is mixed by stirring by the stirring screw 45 b, thereby obtaining a predetermined density. Of developer.

In the above, the developer can be separated only from the developer originally carried on the developing sleeve 43.

According to the present embodiment, the developing operation is not repeated for a long time while the developer is adhered to the developing sleeve 43, so that the charge-up of the toner and the deterioration of the carrier can be prevented.

Embodiment 3 This embodiment is different from Embodiment 2 shown in FIG. 2 in that the operation of separating the developer housed in the toner collecting chamber 48 is performed by using the stirring screw 52 without stirring. Opening 4
The feature is that the suction was performed only from the position 8.

That is, in this embodiment, the suction force is intermittently made to be large or small by the air fan and the toner collecting chamber 4
8, the toner is sucked out while stirring the developer in the collection chamber 48 by the fluctuating suction force, and the developer is separated into the toner and the carrier. Others were the same as Example 2.

According to this embodiment as well, it is possible to prevent the charge operation of the toner and the deterioration of the carrier from being performed without repeating the developing operation for a long period of time with the developer adhered to the developing sleeve 43.

[0054]

As described above, according to the present invention,
When the development of an image having a low image ratio continues, the image ratio is easily detected by counting the image ratio with a video counter, and at least the two-component developer on the developer carrier of the developing device is transferred to the toner and the carrier. As a result, toner charge-up and carrier deterioration can be prevented and can be used for development, and an image with a constant density can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a developing device in an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a sectional view showing a developing device according to another embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a conventional image forming apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 4 Developing device 41, 42 Stirring chamber 43 Developing sleeve 44 Magnet roller 45a, 45b Stirring screw 46 Density sensor 48 Toner recovery chamber 49 Toner separation sleeve 50 Power supply 51 Blade 53 Mesh 54 Air injection path

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tsuneji Masuda 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc.

Claims (8)

[Claims] 1. An image carrier, a latent image forming means for forming an electrostatic latent image corresponding to an image signal on the image carrier, and a two-component developing means for developing the latent image and visualizing the latent image as a toner image. A video counter that counts the image ratio of the image based on the image signal and integrates the counted value; and, when the integrated count value is equal to or less than a predetermined value, sets the developer of the developing unit to toner. An image forming apparatus comprising: a separating unit that separates a carrier from a carrier. A separating unit configured to apply a bias between the rotating conductive member disposed to face the developer carrier of the developing unit and the developer carrier to the conductive member, 2. The image forming apparatus according to claim 1, further comprising a bias power supply for generating an electric field for causing the toner to fly from the developer carried on the body to the conductive member. 3. A toner collecting container, wherein the conductive member is accommodated in a toner collecting container communicating with a container of the developing means at a portion facing the developer carrying member,
3. The image forming apparatus according to claim 1, wherein a member for scraping off the toner attached to the conductive member is brought into contact with the conductive member. 4. An air blowing means for blowing compressed air toward the surface of the developer carrier of the developing means to blow down the developer carried on the developer carrier, an air blowing means, and a developing means. A toner collection container provided with a mesh through which the toner can pass, which is communicated with the container of the developing means at a position sandwiching the agent carrier, and suctioning the inside of the toner collection container,
The suction device includes suction means for guiding the blown-down developer into the container and accommodating the developer, and stirring means for agitating the developer accommodated in the toner collection container. 2. The image forming apparatus according to claim 1, wherein the developer is separated into the toner and the carrier by suctioning the toner. 5. An air blowing means for blowing compressed air toward the surface of the developer carrying member of the developing means to blow down the developer carried on the developer carrying means, an air blowing means and a developing means. A toner collection container provided with a mesh through which the toner can pass, which is communicated with the container of the developing means at a position sandwiching the agent carrier, and suctioning the inside of the toner collection container with an intermittent difference in suction force The suction means for guiding the developer blown down into the container and accommodating the developer and stirring the developer contained in the toner collecting container to separate the developer into a toner and a carrier. Image forming device. 6. When the developer is separated by the separating means, the developer is separated while the developer in the developing means is newly sucked up and carried on the developer carrying body by rotation of the developer carrying body. An image forming apparatus according to any one of Items 1 to 5. 7. The toner supply device according to claim 1, further comprising a replenishing tank for replenishing the toner in the developing means with toner, and transferring the separated toner to the replenishing tank for use. Image forming device. 8. The image forming apparatus according to claim 1, wherein a plurality of developing means are provided around the image carrier.