Multicolor image forming apparatus with a shutter to prevent mixture of developers in plural developing devices

An image forming apparatus includes a plurality of the developing devices for developing the latent image formed on an image bearing member. The plural developing devices are each movable between its developing position for developing the latent image on the image bearing member and its retracted position away from the image bearing member. The image forming apparatus includes a shutter for selectively opening and closing the developing aperture of the developing device of one of the developing devices. The shutter opens and closes the developing aperture of said one of the developing devices in association with movement of the other developing device between the developing position and the retracted position.

FIELD OF THE INVENTION AND RELATED ART 
The present invention relates to an image forming apparatus such as an 
electric photographic copying apparatus, a laser beam printer and an 
electrostatic printing apparatus, more particularly to an image forming 
apparatus comprising a plurality of developing devices which contain 
different color developers. The present invention is applicable to a 
multi-color image forming apparatus capable of forming images in two or 
more different colors, but it is also applicable to a so-called full-color 
image forming apparatus. The present invention is not limited to the case 
where the developing devices contain different color developers, but it is 
also applicable to the case where the contained developers are different, 
for example, in the polarity of the electric charge of the developers, 
more particularly, the developing devices contain the respective 
developers which are electrically charged to different polarities, wherein 
a positive or negative latent image can be developed by selecting the 
developing devices. 
Recently, the demand for the color copying and color printing has increased 
in the field of general office work as well as in a special field, and 
therefore, a color image forming apparatus is desired which can be 
operated not only by an expert operator but also by an ordinary person. It 
is often required that an original containing two or more color images, or 
full-color a original, is reproduced or printed in a desired color or 
colors. In the full-color image forming apparatus as well as in the 
multi-color image forming apparatus, it is required to meet the demand to 
efficiently produce the images in two or three colors. 
To satisfy those requirements, the image forming apparatus comprises a 
plurality of developing devices, and each of the developing devices 
contains the respective developers which are different in color. For 
example, a two color image forming apparatus comprises one developing 
device for black developer which can provide the most frequently used 
black image and one developing device for red, blue or green images. This 
is a typical arrangement in the case of a two color image forming 
apparatus. 
FIG. 3 illustrates a conventional color electrophotographic copying 
apparatus capable of providing two color images, for example, black and 
red images. 
The color electrophotographic copying apparatus has the general 
construction wherein a photosensitive drum 1, having an 
electrophotographic photosensitive layer, is disposed generally at the 
center of the copying apparatus and is detachably and rotatably supported 
on an unshown shaft having a rotational axis O in a known manner. The 
photosensitive drum 1, in this example, is rotatable in the clockwise 
direction in FIG. 3, that is, the direction of an arrow X. The apparatus 
further comprises around the photosensitive drum 1, a primary charger 2 
substantially right above the photosensitive drum 1, a multi-color 
developing device 3 at the right side of the photosensitive drum 1 in FIG. 
3, an image transfer charger 8 substantially right below the 
photosensitive drum 1 and a cleaning device 9 at the left side of the 
photosensitive drum 1 in FIG. 3. In the upper portion of the 
electrophotographic copying apparatus, there is an optical system (not 
shown), which is effective to project a light image of the original (light 
signals in accordance with an image to be recorded in the case of a laser 
beam printer or the like) onto the photosensitive drum 1 at the image 
exposure station 6 disposed between the primary charger 2 and the 
multi-color developing apparatus 3, so as to form an electrostatic latent 
image on the photosensitive drum 1. The latent image thus formed is 
developed by a developing device, and the developed image is transferred 
onto a sheet by a transfer charger 8. After the image transfer, the image 
is fixed by an unshown image fixing device. 
The multi-color developing device 3 includes two developing devices 4 and 5 
so as to provide a black developed image and a red developed image. Due to 
the above described arrangement of the copying apparatus (FIG. 3), in most 
cases, the developing device 5 disposed downstream with respect to the 
direction of movement of the surface of the photosensitive drum 1 is 
disposed below the upstream developing device 4 with respect to the 
direction of the gravity. In the copying apparatus having the structures 
described above, as shown in FIG. 4, when the upstream developing device 4 
is in operation, the developer (toner) T released from the surface of the 
developing sleeve 4a of the developing device 4 can reach the downstream 
developing device 5 under the influence of gravity and with the aid of the 
flow of the air produced by the rotation of the photosensitive drum 1. As 
a result, the developer is mixed with the different developer on the 
developing sleeve 5a of the downstream developing device 5. If this 
occurs, the image developed by the downstream developing device 5 may have 
a color which is different from the intended color. 
Furthermore, when the rotational direction of the developing sleeve 4a of 
the upstream developing device 4 is such that the direction of the 
peripheral movement is the same as that of the photosensitive drum 1, the 
toner T on the developing sleeve 4a is easily scattered to the downstream 
developing device 5 under the influence of the inertia force provided by 
the rotation of the developing sleeve 4a. 
Additionally, in the copying apparatus described above, it is not necessary 
that the developing devices 4 and 5 contain different color developers. 
Generally, however, they contain different color developers, for example, 
black and red developers, and the developing device containing the 
developer having the color selected by an operator on an operation panel 
is shifted toward a predetermined developing position adjacent to the 
photosensitive drum 1, whereby the latent image can be developed by the 
selected color developer. Therefore, when one of the developers is shifted 
to the predetermined developing position, vibration or shock can occur 
with the result of the developer falling or scattering from the developing 
device to enter the other developing device. This, of course, leads to 
mixture of color. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the present invention to provide 
an image forming apparatus wherein an image without color mixture, that 
is, an image in the intended color can be produced. 
It is another object of the present invention to provide an image forming 
apparatus comprising a plurality of developing devices, wherein the 
developer scattering from one of the developing devices is prevented from 
entering the other developing device, by a simple structure. 
These and other objects, features and advantages of the present invention 
will become more apparent upon a consideration of the following 
description of the preferred embodiments of the present invention taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An image forming apparatus according to an embodiment of the present 
invention will be described in conjunction with the drawings. 
FIG. 3 illustrates a part of a two color electrophotographic copying 
apparatus to which the present invention is applicable. 
Referring to FIGS. 1 and 2, there is shown a multi-color developing device 
3 of the color image forming apparatus. As shown in FIG. 3, the 
photosensitive drum 1 is disposed substantially at the center of the 
copying apparatus and is rotatable about a rotational axis O in the 
rotational direction indicated by an arrow X. Around the photosensitive 
drum 1, there are disposed various elements for effecting the 
electrophotographic process. 
In this embodiment, a black color developing device 5 is disposed at a 
lower position and is detachably mounted to a black color developing 
device supporting member 17 movably mounted in the frame 22 of the 
apparatus. The developing device 5 contains the black developer (not 
shown) and is provided with a developing roller (or sleeve) 5a rotatably 
supported with a part thereof exposed through a developing aperture to 
supply the developer to the photosensitive drum 1. The developing roller 
5a, as is well known, includes a sleeve (not shown) of non-magnetic 
material containing therein a magnet. 
A pressing spring 19 is fixed by screws 18 to a rear wall of the black 
developing device supporting member 17. A pressing, cam 21 is rotatable 
about a shaft 20 and engages spring 19. 
By rotation of the pressing cam 21, the black developing device 5 is 
movable between a developing position for developing the latent image on 
the photosensitive drum 1 and a retracted position sufficiently away from 
the photosensitive drum 1. In this embodiment, the black developing device 
5 is made non-operative when it is situated at its retracted position. 
A red color developing device 4 disposed above the black developing device 
5 has similar structures. More particularly, the red developing device 4 
is detachably mounted to a red developing device supporting member 11 
which is movably mounted to the frame 22 of the apparatus. The red 
developing device 4, as is well known, contains a red developer (not 
shown) and is provided with a developing roller (sleeve) 4a with a part 
thereof exposed through a developing aperture to supply developer to the 
photosensitive drum 1. The developing roller 4a includes a sleeve (not 
shown) of non-magnetic material and a magnet contained therein. 
A pressing spring 13 is fixed by screws 12 to a rear wall of the supporting 
member 11. A pressing cam 15 is rotatable about a shaft 14a and engages 
spring 13. 
By the rotation of the pressing cam 15, the red developing device 4 is 
movable between a developing position for developing the latent image on 
the photosensitive drum 1 and a retracted position which is sufficiently 
away from the photosensitive drum 1. In this embodiment, the red 
developing device 4 is made in-operative when situated in its retracted 
position. 
According to this embodiment of the present invention, a shutter is 
swingably supported adjacent an end of the red developing device 
supporting member 11 so as to prevent the unintended developer from 
entering the developing device 5. The shutter 16 has one end in the 
neighborhood of which it is rotatably supported to the supporting member 
11, and a free end 16a in the neighborhood of which it is effective to 
cover the portion of the developing roller 5a of the black developing 
device 5 which is exposed to the photosensitive drum 1 through the 
developing aperture. 
FIG. 1 shows the state wherein the image is developed by red developer, 
more particularly, the red developing device 4 is disposed close to the 
photosensitive drum 1 (developing position), while the black developing 
device 5 is away from the photosensitive drum 1 retracted position). This 
position is achieved by a control signal provided from the operation 
panel, which signal presses the red developing device 4 by the pressing 
cam 15 through the pressing spring 13. 
In this state, the developing device 4a rotates in the direction of 4X to 
supply red developer 4b to the photosensitive drum 1 so as to develop the 
latent image on the photosensitive drum 1 in red. 
At this time, the black developing device 5 is released from the pressing 
cam 21, and therefore, it is urged by an unshown urging means such as a 
spring (not shown) mounted to a frame 22, so that it is moved in the 
direction 17y and is maintained at its retracted position sufficiently 
away from the photosensitive drum 1. 
The shutter 16 rotatably supported on the red developing device supporting 
member 11 adjacent its one end is moved to the photosensitive drum 1 along 
a cam surface 5c formed in a frame of the black developing device 5, while 
the red developing device 4 moved from the retracted position toward the 
photosensitive drum 1. During this movement, the free end 16a comes to 
cover the exposed portion of the developing roller 5a of the black 
developing device with the aid of gravity (an arrow 16y). As a result, the 
shutter 16 is interposed between the photosensitive drum 1 and the 
developing roller 5a of the black developing device 5. By this, the red 
developer scattering and falling from the red developing device 4 is 
prevented from attaching to the developing roller 5a of the developing 
device 5, so that the mixture of the developers is prevented. 
When, on the other hand, the black image is to be provided, the shaft 14 of 
the pressing cam 15 rotates in the direction of an arrow 14X from the 
state shown in FIG. 1 by a signal given from the operation panel so as to 
release the pressure to the red developing device 4. Therefore, the red 
developing device 4 moves in the direction of an arrow 11X (FIG. 1) by an 
unshown urging means such as a spring mounted to the frame 22 of the 
apparatus, with the result that it is moved to a retracted position which 
is sufficiently away from the photosensitive drum 1 as shown in FIG. 2. 
Simultaneously, the shutter 16 swings in the direction of an arrow 16X 
(FIG. 1) by the cam surface 5c formed on a part of the black developing 
device 5. 
The black developing device 5 starts moving in the direction of an arrow 
17X, that is, toward the photosensitive drum 1 by the rotation of the 
pressing cam 21 about the shaft 20 in the direction of an arrow 20X. 
Together with the movement, the shutter 16 is moved in the direction of an 
arrow 16X by the cam surface 5c formed on the frame of the black 
developing device 5. The developing device 5 is moved to the developing 
position. It is not always necessary that the cam surface 5c is integral 
with the developing device 5, but may be provided on the frame of the 
copying apparatus. The shutter 16 may be made swingable in the directions 
16X and 16Y by means other than the cam surface 5c. 
FIG. 2 illustrates the situation wherein the black image is to be provided. 
The black developing device 5 is pressed toward the photosensitive drum 1 
through the pressing spring 19 by the pressing cam 21 fixed to the shaft 
20, thus taking the developing position wherein it is capable of 
developing the latent image on the photosensitive drum 1. The developing 
roller 5a rotates in the direction shown by an arrow 5X so as to supply 
the black developer 5b to the photosensitive drum 1, thus developing the 
latent image on the photosensitive drum 1 in black. 
At this time, the shutter 16 is between the red developing device 4 and the 
black developing device 5, and does not cover the developing roller 5a of 
the black developing device 5, so that the shutter 16 does not obstruct 
the developing operation of the black developing device. 
In the foregoing embodiment, the shutter 16 is supported on the developing 
device supporting member of that one of the developing devices which is 
located upstream with respect to the rotation of the image bearing member. 
However, this is not limiting, but the shutter may be supported on the 
developing device to be covered or supported on the frame of the 
apparatus. What is required is that a developing aperture of the 
developing device is opened or closed in association with movement of the 
other developer. 
In the embodiment, the developing device is urged by urging means such as a 
spring in the direction away from the photosensitive drum, and the 
developing device is moved to the developing position by the rotation of 
the pressing cam against the urging force of the spring, while the 
developing device is moved back to the retracted position by releasing the 
developing device from the pressure of the pressing cam. However, another 
arrangement is possible, for example, a controllable solenoid or plunger 
may be used to shift the developing devices. 
In the two color image forming apparatus shown in FIG. 3 to which the 
present invention is applicable, a latent image to be reproduced as a red 
image is formed and then developed, and the developed image is transferred 
onto a transfer sheet; and then a latent image to be developed in black is 
formed and developed, and then the developed black image is transferred 
onto the same sheet (having the red image). That is, the image transfer 
operation is effected twice. However, the latent images corresponding to 
the red image and the black image, respectively is formed in a single 
image forming cycle, and then they are developed by the respective color 
developers, and subsequently the developed images are transferred onto the 
transfer sheet simultaneously. 
The present invention is not limited to the copying apparatus having only 
two applicable devices, but is to the image forming apparatus having three 
or more developing devices. 
As described in the foregoing, according to the present invention, a 
developing aperture of one of the developing devices is closed or opened 
in association with movement of the other developing device, by which the 
possibility of the mixture of the developers by the scattering or falling 
thereof, can be prevented by a simple structure, thus providing stabilized 
and high quality images. 
While the invention has been described with reference to the structures 
disclosed herein, it is not confined to the details set forth and this 
application is intended to cover such modifications or changes as may come 
within the purposes of the improvements or the scope of the following 
claims.