JP2000321843A - Color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate extraction of an image forming unit from a device and to correctly set the image forming body to an image exposure optical system again after being loaded. SOLUTION: In this device, each of the image forming units 30 is contained in a shared retaining member 200 and each drum bearing part 30A is kept in a state where they may be moved a little in a vertical or horizontal direction at notched parts 200A by being energized by pressure plates 202 and on the other hand, each optical exposure system 12 is supported by a shared box body 100 and V shaped notched parts 100A are provided in specified positions of each optical axis. Then, the retaining member 200 and the box body 100 are in a separated state when the image forming unit 30 is attached or detached and at the time of image formation, drum bearing parts 30A and the notched parts 100A are abutted and positional relationship between each exposure optical system 12 and each image forming unit 30 are correctly set again.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an electrophotographic color image forming apparatus for forming a color image by superimposing color toner images formed on a plurality of image carriers.

[0002]

2. Description of the Related Art Many color image forming apparatuses for forming a color image by superposing toner images on a plurality of image forming bodies employ an electrophotographic image forming method. FIG. 1 shows a main part of one type of color image forming apparatus.

In FIG. 1, A, B, C, and D are image forming units arranged in parallel in the order of image processing, and are units for forming single-color toner images of yellow, magenta, cyan, and black, respectively. .

Each image forming section includes a drum-shaped image forming body 1, a charging unit 2, an exposing unit 3, a developing unit 4, and a cleaning unit 5 as shown in an image forming unit A for forming a yellow toner image. Each is provided with forming means. With such a configuration, first, a yellow toner image is formed on the image forming body 1 of the image forming unit A. This toner image is transferred to the transfer paper P conveyed by the transfer belt 8 in the direction of the arrow by the operation of the transfer device 7.

During this time, in the image forming section B, a magenta toner image is formed on the image forming body, and the transfer paper P
Is transferred in a manner superimposed on the yellow toner image. Similarly, a cyan toner image is superimposed by the image forming unit C, and a black toner image is superimposed by the image forming unit D, so that a color toner image is formed on the transfer paper P. The transfer paper P on which the color toner image has been formed in this manner is continuously conveyed to the fixing device 6 by the transfer belt 8 to fix the toner image and finish copying.

[0006]

The start of image exposure in each exposure unit is performed in synchronization with the movement of the position of the transfer paper P, that is, the conveyance of the transfer belt 8, so that each exposure unit 3 and the image forming body 1 If there is a variation in the positional relationship, the superposition of the toner images does not match, and as a result, a color shift occurs, and the image quality is greatly reduced. This is the same in the case of a system in which an intermediate transfer belt is used in place of the transfer belt 8 and a toner image is superimposed on the belt and collectively retransferred onto transfer paper.

The present invention has solved and improved the above points,
It is an object of the present invention to provide a color image forming apparatus in which the positional relationship between an exposure unit and an image forming body in each image forming unit is always maintained with extremely high accuracy to obtain a high-quality color image.

[0008]

The object of the present invention is to provide yellow,
A color image forming apparatus having magenta, cyan, and black image forming units, and sequentially superimposing a toner image formed by each of the image forming units on a belt-shaped intermediate transfer body or a transfer material; Is achieved by a color image forming apparatus in which the position of the image forming body is set with reference to the exposure optical systems arranged in parallel (first invention).

[0009] Alternatively, the above object is for yellow, magenta,
Each of the image forming units is mounted in a color image forming apparatus that has cyan and black image forming units and sequentially superimposes the toner images formed by the respective image forming units on a belt-shaped intermediate transfer body or a transfer material. This is achieved by a color image forming apparatus characterized in that the position of the image forming body is movably attached to and detached from the position, and the position of the image forming body is set on the basis of each exposure optical system by the movement of each exposure optical system arranged in parallel. 2).

[0010] Further, the object is to provide yellow, magenta,
Each of the image forming units has cyan and black image forming units, and each of the image forming units is movably attached to and detached from a mounting position, and the position of the image forming body with respect to each exposure optical system arranged in parallel by the movement of each image forming unit. Is established (third invention).

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each of the inventions forms a color image in which toner images superimposed on an intermediate transfer member are collectively transferred onto a transfer material and in which the toner images are superimposed and transferred directly onto a transfer material. Although the present invention is directed to an apparatus, in the present embodiment, only the former color image forming apparatus using an intermediate transfer member will be described with reference to FIGS.

FIG. 2 is a sectional view showing the basic structure of a color image forming apparatus common to the inventions, and FIG.
FIG. 4 is a main part view of a color image forming apparatus according to the invention of FIG.
5 and FIG. 5 show the second invention, and FIGS. 6 and 7 show the third invention.
1 is a configuration diagram of each color image forming apparatus according to the invention of FIG.

In FIG. 2, reference numeral 10 denotes a photosensitive drum as an image carrier for each color, 11 a scorotron charger as charging means for each color, 12 an exposure optical system as image writing means for each color, 13 Denotes a developing device which is a developing means for each color;
Is a cleaning device that is a cleaning unit for each color,
Reference numeral 15 denotes an intermediate transfer belt, which is an intermediate transfer member. The photosensitive drum 10, the scorotron charger 11, the developing device 13, and the cleaning device 14 are respectively yellow (Y), magenta (M), cyan (C), and yellow. Image forming units 30 (Y), 3 for forming each toner image of black (K)
0 (M), 30 (C), and 30 (K) are integrated and arranged in parallel in the drawing of the intermediate transfer belt 15.

A photosensitive drum 10 serving as an image carrier has a conductive layer, an a-Si layer, or an organic photosensitive layer (OPC) on the outer periphery of a cylindrical metal substrate formed of, for example, an aluminum material.
And the like, and is rotated counterclockwise as indicated by the arrow in FIG. 2 with the conductive layer grounded.

Scorotron charger 11 as charging means
Performs a charging action (negative charging in the present embodiment) by a control grid maintained at a predetermined potential and corona discharge of the same polarity as toner by the corona discharge electrode, so that a uniform potential is applied to the photosensitive drum 10. give. As the corona discharge electrode of the scorotron charger 11, a saw-tooth electrode or a needle electrode can also be used.

In the exposure optical system 12, which is an image writing means, the exposure position on the photosensitive drum 10 is located on the downstream side in the rotation direction of the photosensitive drum 10 with respect to the scorotron charger 11 for each color described above. Thus, it is arranged around the photosensitive drum 10. The exposure optical system 12 includes the photosensitive drum 10
For example, a linear exposure element in which a plurality of LEDs (light emitting diodes) are arranged in an array as light emitting elements of image exposure light arranged in the main scanning direction in parallel with the drum axis, and a light focusing property as an imaging element This is an exposure unit composed of an optical transmitter (trade name: Selfoc lens array). Other laser optical systems can be used as the exposure optical system 12. The exposure optical system 12 for each color performs image exposure on the photosensitive layer of the photosensitive drum 10 in accordance with image data of each color read by a separate image reading device and stored in a memory, and is placed on the photosensitive drum 10 for each color. An electrostatic latent image is formed.

The developing device 13 as a developing means keeps a predetermined gap with respect to the peripheral surface of the photosensitive drum 10 and
Rotating in the forward direction with the rotation direction of 0, for example, a thickness of 0.5 to 1
mm, an outer diameter of 15 to 25 mm, and has a cylindrical developing sleeve 131 made of non-magnetic stainless steel or aluminum material. Inside the developing sleeve 131, yellow (Y), magenta (M), cyan (C) ) And black (K) one-component or two-component developers. The developing device 13 is kept in a non-contact state with the photosensitive drum 10 by a contact roller (not shown) with a predetermined gap, for example, 100 to 500 μm, and a developing device in which a DC voltage and an AC voltage are superimposed on the developing sleeve 131. By applying a bias, non-contact reversal development is performed, and a toner image is formed on the photosensitive drum 10.

The intermediate transfer belt 15, which is an intermediate transfer member, has a volume resistivity of 10 ⁸ to 10 ¹⁶ Ω · cm, preferably 10 ⁹ to 10 ⁹ Ω · cm.
An endless belt of 10 ¹² Ω · cm, for example, a conductive material dispersed in engineering plastics such as modified polyimide, thermosetting polyimide, ethylene tetrafluoroethylene copolymer, polyvinylidene fluoride, nylon alloy, and the like. It is a seamless belt having a two-layer structure in which a 0.05 mm semiconductive film substrate is coated with a fluorine coating having a thickness of 5 to 50 μm, preferably as a toner filming preventing layer. As the base of the intermediate transfer belt 15, a semiconductive rubber belt having a thickness of 0.5 to 2.0 mm in which a conductive material is dispersed in silicon rubber or urethane rubber can also be used. The intermediate transfer belt 15 is stretched around a driving roller 15A, a ground roller 15B, and a driven roller 15C, which are roller members, respectively.
It is rotated clockwise as indicated by the arrow in FIG. When driven by a drive motor (not shown), the drive roller 15A rotates,
The intermediate transfer belt 15 is driven and rotated. The rotation of the intermediate transfer belt 15 causes the ground roller 15B and the driven roller 15C to rotate following the rotation.

The transfer unit 16A, which is a transfer unit for each color,
The photosensitive drum 10 for each color with the intermediate transfer belt 15 interposed
The corona discharger is provided so as to face each other, and forms a transfer area for each color between the intermediate transfer belt 15 and the photosensitive drum 10 for each color. By applying a DC voltage of the opposite polarity (positive polarity in the present embodiment) to the transfer device 16A for each color to form a transfer electric field in the transfer area,
The toner image on the photosensitive drum 10 for each color is transferred onto the intermediate transfer belt 15.

The fixing device 17 as a fixing means is constituted by two roller-shaped fixing members, a fixing roller 17a having a heater inside and a pressure roller 17b.
The recording paper P is nipped and conveyed in the nip T between the a and the pressure roller 17b, and heat and pressure are added to fix the toner image on the recording paper P conveyed in the nip T.

Next, the image forming process will be described.

The photosensitive drum 10 of the yellow (Y) image forming unit 30 (Y) disposed at the most upstream position in the rotation direction of the intermediate transfer belt 15 by the start of a photosensitive member drive motor (not shown) at the start of image recording. Are rotated in the counterclockwise direction indicated by the arrow in FIG. 2, and at the same time, the application of the potential to the K photoconductor drum 10 is started by the charging action of the Y scorotron charger 11.

After a potential is applied to the Y photosensitive drum 10, image writing is started by the Y exposure optical system 12 using a first color signal, that is, an electric signal corresponding to the Y image data, and the Y photosensitive drum 10 is exposed. Y of the original image on the surface of the body drum 10
An electrostatic latent image corresponding to the image is formed.

The latent image is reversely developed in a non-contact state by a Y developing unit 13 to form a yellow (Y) toner image in accordance with the rotation of the Y photosensitive drum 10.

The Y toner image formed on the Y photoreceptor drum 10 as an image carrier by the above-described image forming process is subjected to intermediate transfer in a Y transfer area by a Y transfer unit 16A as transfer means. Intermediate transfer belt 15 as a body
Transcribed above.

Next, the intermediate transfer belt 15 is synchronized with the Y toner image, and is given a potential by the charging action of the M scorotron charger 11 by the magenta (M) image forming unit 30 (M). An image is written by the exposure optical system 12 using a second color signal, that is, an electrical signal corresponding to M image data, and is formed on the M photoconductor drum 10 by non-contact reversal development by the M developing unit 13. In the M transfer area, the M toner image is superimposed on the Y toner image by the M transfer unit 16A as the transfer unit.

By the same process, the third toner image formed on the C photosensitive drum 10 by the cyan (C) image forming unit 30 (C) is synchronized with the Y and M superposed toner images. In the transfer area of C, the C toner image corresponding to the C image data by the color signal is superimposed on the Y and M toner images by the C transfer unit 16A as the transfer means. Are formed together, and Y,
Synchronized with the superposed M and C toner images, black (K)
The K toner image formed on the C photosensitive drum 10 by the image forming unit 30 (K) corresponding to the C image data based on the fourth color signal is a transfer unit in the K transfer area. By the transfer device 16A of K, the aforementioned Y,
A K toner image is superimposed on the M and C toner images, and a Y, M, C and K superimposed color toner image is formed on the intermediate transfer belt 15.

The toner remaining on the peripheral surface of the photosensitive drum 10 for each color after the transfer is transferred to a cleaning device 14 as a photosensitive drum cleaning unit, or to the photosensitive drum 1.
Cleaning is performed by a cleaning blade made of a rubber material that has come into contact with the cleaning blade.

With the formation of the surface image of K on the photosensitive drum 10 of K, a recording paper P as a transfer material is fed from a paper feed cassette 18 as a transfer material storage means to a feed roller 18A.
, And is conveyed to a timing roller 19 as a transfer material feeding unit. The driving of the timing roller 19 synchronizes with the color toner image carried on the intermediate transfer belt 15 to transfer the transfer area of the transfer unit 16B. Is sent to

The color toner image on the intermediate transfer belt 15 is transferred onto the recording paper P by applying a voltage of the opposite polarity (positive polarity in this embodiment) to the toner to the transfer unit 16B, and the recording paper P holds the color toner image. The paper P is removed from the intermediate transfer belt 15 by the static elimination action of the paper separating AC static eliminator 16C.
Is transported to the fixing device 17 after being separated from the peripheral surface of the device, and the toner is welded and fixed, and is discharged to the upper surface of the device via the paper discharge roller 20. On the other hand, the intermediate transfer belt 15 from which the recording paper P has been separated is cleaned by removing the residual toner in the cleaning device 21, and the process proceeds to a new image forming process.

(Embodiment 1) An example of the configuration of a color image forming apparatus according to the first invention will be described with reference to FIG.

Each of the exposure optical systems 12 is supported and integrated on the inner surface of a lid-shaped casing 100 having an exposed surface side opened.

The housing 100 has a V-shaped notch 100A at the end face of the side wall provided opposite to the drum bearing 30A of the photosensitive drum 10 protruding from the front and rear surfaces of each image forming unit 30. Further, each of the exposure optical systems 12 is formed in a concave portion 100B formed on the basis of the cutout portion 100A.
Are sandwiched and supported by a pair of support members 12A.
Therefore, the notch 100A is provided in each of the exposure optical systems 1
2, since the position and distance on the optical axis are set with high precision and then integrated, they are used as positioning portions of each image forming unit 30 as described later.

On the other hand, each of the image forming units 30 is detachably supported inside a frame-shaped holding member 200 having both open sides.

That is, the holding member 200 is provided with a pair of columns 203 for slidably engaging the pressing plate 202 at the bent portions 201 on the front and rear side edges, respectively. Is constantly biased toward the housing 100 side.

The holding member 200 further has cutouts 200A on its front and rear sides for receiving the drum bearings 30A of each image forming unit 30, and the drum bearings 30A are mounted on the pressing plate 202. It is engaged so that it can move slightly in the horizontal direction when it is placed.

The drum bearing portion 30A has a flat surface facing the pressing plate 202, that is, a lower surface of the bearing portion 30A in FIG. 3, and has an appropriate urging force received from the pressing plate 202 to form an image forming unit. 30 is held at a position slightly separated from the intermediate transfer belt 15.

The casing 100 and the holding member 200 are brought into a combined state as shown in FIG.
Each notch 100A of the housing 100 and the drum bearing 30A of each image forming unit 30 supported by the holding member 200 come into contact with each other, and as a result, the drum bearing 30A, that is, the center of rotation of the photosensitive drum 10 becomes the notch 100A. , That is, the position on the optical axis of the exposure optical system 12, and at the same time, the peripheral surface of the photosensitive drum 10 is set to the image forming position of the exposure optical system 12 fixed to the housing 100.

The notch 100A and the drum bearing 30
The contact state of A is detected by the photo sensor S installed in the housing 100 as shown in FIG. The photosensor S includes a light emitting unit and a light receiving unit, and the contact state is confirmed by detecting whether the amount of reflected light from the peripheral surface of the drum bearing unit 30A reaches the amount of reflected light at the time of contact. Otherwise, measures such as displaying a warning indicating that the image formation is impossible on the operation panel or disabling the operation of the image forming process are taken.

(Embodiment 2) An example of the configuration of a color image forming apparatus according to the second invention will be described with reference to FIGS.

The apparatus shown in FIG.
In this example, the apparatus main body is divided into an upper main body I and a lower main body II, and the upper main body I is rotated clockwise about the support shaft H so that the lower main body I is rotated. This is a so-called clamshell type color image format device that can be opened and closed with respect to II.

The upper main body I accommodates the housing 100 for supporting the respective exposure optical systems 12, and when the lower main body II is closed, the notches 100A are connected to the lower main body as shown by dashed lines. II, which engages with the drum bearing portion 30A of each image forming unit 30 to regulate each position of each image forming unit 30 in the horizontal direction and the vertical direction so that the peripheral surface of each photosensitive drum 10 is transferred to the intermediate transfer belt. 15 is pressed against the peripheral surface of the housing 15 with an appropriate pressing force, and
The imaging position of each exposure optical system 12 supported at 0 is also set on each drum surface, so that image formation is possible.

When the upper main body I is opened from the lower main body II, the casing 100 is retracted upward as shown by a solid line to release the position regulation with respect to the drum bearing 30A. Each of the image forming units 30 is safely detached from the upper part of the holding member 200 individually by being separated by the intermediate transfer belt 15.

The apparatus shown in FIG. 5 is an example in which the image forming units 30 are arranged side by side in the vertical direction. The apparatus main body is divided into a right main body IA and a left main body IIA, and the right main body IA has a support shaft. This is a so-called vertical clamshell type color image forming apparatus which can be opened and closed with respect to the left main body IIA by rotating clockwise about H.

In this apparatus as well, each image forming unit 30 accommodated in the left main body IIA is set to a state where an image can be formed by closing the right main body IA accommodating the exposure optical system 12, and the image can be formed by opening the same. Although the state is canceled, each of the image forming units 30 is not
Is prevented by the inclined surface of the holding member 200, and accidental detachment from the holding member 200 is prevented, so that attachment and detachment can be performed safely as in the previous example.

The upper main body I and the right main body I
A is opened more widely than the illustrated angle, and is locked at each position by a lock member or the like at each of the closed and open positions.

(Embodiment 3) A configuration example of a color image forming apparatus according to the third invention will be described with reference to FIGS.

The apparatus shown in FIG.
Are arranged in parallel in the vertical direction, and the apparatus main body is divided into a right main body IB and a left main body IIB, and the right main body IB
Is a so-called vertical clamshell type color image forming apparatus which can be opened and closed with respect to the left main body IIB by rotating clockwise about the support shaft H.

The right main body IB houses the above-mentioned holding member 200 for supporting each image forming unit 30 and the intermediate transfer belt 15. When the left main body IB is closed with respect to the left main body IIB, each image forming unit is indicated by a dashed line. The housing 100 in which the 30 drum bearings 30A are housed in the left main body IIB
And the horizontal and vertical positions of each image forming unit 30 are regulated so that the peripheral surface of each photosensitive drum 10 is pressed against the peripheral surface of the intermediate transfer belt 15 with an appropriate pressing force. At the same time as the contact, the surface of the photosensitive drum is also set at the image forming position of each exposure optical system 12 so that image formation is possible.

The holding member 200 and the intermediate transfer belt 15 are sandwiched and supported between a pair of substrates 300 before and after being integrated via a connecting member 300A. The substrate 300 is a driving roller 15A of the intermediate transfer belt 15.
Is supported on the left main body IIB side, and a pair of guide pins 300P are slidably engaged with the guide grooves G of the right main body IB. The intermediate transfer belt 15 is rotated following the support shaft h as a fulcrum.

The apparatus shown in FIG. 7 is an example in which the image forming units 30 are arranged side by side in the horizontal direction. The apparatus main body is divided into an upper main body IC and a lower main body IIC. This is a clamshell type color image forming apparatus which can be opened and closed with respect to a lower main body IIC by rotating about an axis H as a fulcrum.

Also in this apparatus, each exposure optical system 12 accommodated in the lower main body IIC is set to a state in which an image can be formed by closing the upper main body IC accommodating the image forming unit 30, and is released by opening the same. However, each of the image forming units 30 is prevented by the crank provided in the notch portion 200A to prevent the image forming unit 30 from being accidentally dropped from the holding member 200, and can be safely attached and detached as in the previous example.

The right main body IB and the upper main body IC are each opened more widely than the illustrated angle, and are locked by a lock member or the like at the respective closed and open positions.

[0054]

According to the present invention, the image exposure optical system and the image forming unit can be freely separated and united irrespective of the posture, and the image forming unit can be easily attached and detached (claims 1 and 2). The relative position between the image exposure optical system and each image forming unit is set with high accuracy (claims 3 to 5), and the state is confirmed from outside (claim 6).

Therefore, the degree of freedom in the layout of the image exposure optical system and the image forming unit in the color image forming apparatus is increased, and as a result, the clamshell type apparatus for opening and closing the image exposure optical system side is provided. A clamshell type device for opening and closing the image forming unit side
And 12) are also realized, and each of the above devices can be configured as a vertical device (claims 9 and 13) or a horizontal device (claims 10 and 14).

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a main part of a color image forming apparatus.

FIG. 2 is a cross-sectional configuration diagram of a color image forming apparatus according to the present invention.

FIG. 3 is an explanatory view showing a support structure of an image exposure optical system and an image forming unit.

FIG. 4 is a configuration diagram of a color image forming apparatus (horizontal type) that opens and closes an image exposure optical system side.

FIG. 5 is a configuration diagram of a color image forming apparatus (vertical type) that opens and closes an image exposure optical system side.

FIG. 6 is a configuration diagram of a color image forming apparatus (vertical type) that opens and closes an image forming unit side.

FIG. 7 is a configuration diagram of a color image forming apparatus (horizontal type) that opens and closes an image forming unit side.

[Explanation of symbols]

 REFERENCE SIGNS LIST 10 photoconductor drum 11 scorotron charger 12 exposure optical system 13 developing device 14 cleaning device 15 intermediate transfer belt 17 fixing device 18 paper feed cassette 30 image forming unit 30A drum bearing 100 housing 100A, 200A notch 100B recess 200 holding member 201 Bending part 202 Pressing plate I Upper body II Lower body IA, IB Right body IIA, IIB Left body S Photosensor H, h Support shaft

Claims (14)

[Claims] 1. A color image forming apparatus having image forming units for yellow, magenta, cyan, and black, and superposing a toner image formed by each of the image forming units on a belt-shaped intermediate transfer body or a transfer material sequentially. 3. The color image forming apparatus according to claim 1, wherein the position of the image forming body is set based on each of the exposure optical systems arranged in parallel in each of the image forming units. 2. The color image forming apparatus according to claim 1, wherein each of the image forming units is detachable. 3. Each of the exposure optical systems is integrated,
3. The color image forming apparatus according to claim 1, wherein a positioning section of the image forming unit is provided in each of the exposure optical systems. 4. The color image forming apparatus according to claim 1, wherein said positioning portions are integrated. 5. The color image forming apparatus according to claim 3, wherein each of the exposure optical systems and each of the positioning portions are integrated. 6. The color image forming apparatus according to claim 1, wherein the color image forming apparatus includes a detecting unit configured to detect a position setting of an image forming body of each image forming unit. apparatus. 7. A color image forming apparatus having image forming units for yellow, magenta, cyan and black, and superimposing toner images formed by the respective image forming units on a belt-like intermediate transfer body or a transfer material sequentially. In the color, each of the image forming units is movably attached and detached at a mounting position, and the position of an image forming body is set based on each of the exposure optical systems by movement of each of the exposure optical systems arranged in parallel. Image forming device. 8. The color image forming apparatus according to claim 7, wherein said exposure optical systems and said image forming body positioning portions are integrated with each other. 9. The color image forming apparatus according to claim 7, wherein the image forming units are arranged in parallel in a vertical direction. 10. The image forming unit according to claim 7, wherein the image forming units are arranged in parallel in a horizontal direction.
3. The color image forming apparatus according to 1. 11. A color image forming apparatus having image forming units for yellow, magenta, cyan, and black, and superimposing toner images formed by the image forming units on a belt-shaped intermediate transfer body or a transfer material sequentially. In the color, each of the image forming units is movably attached and detached at a mounting position, and the position of the image forming body is set based on each of the exposure optical systems arranged in parallel by moving each of the image forming units. Image forming device. 12. The color image forming apparatus according to claim 11, wherein the respective exposure optical systems and the positioning portions of the respective image forming bodies are integrated. 13. The color image forming apparatus according to claim 11, wherein the image forming units are arranged in parallel in a vertical direction. 14. The color image forming apparatus according to claim 11, wherein the image forming units are arranged in parallel in a horizontal direction.