Image forming apparatus having a first mode for forming a multicolor image of restricted length and a second mode for forming a monocolor image of unrestricted length

The invention provides a color image forming apparatus capable of forming a mono-color image and a multi-color image on a rotatable photoreceptor having a given circumferential length. In the apparatus, there are provided a selection switch for selecting either a mono-color mode for forming a mono-color image or a multi-color mode for forming a multi-color image, and a control circuit responsible to a selection signal of the selection switch so that, when the multi-color made is selected, an image size of a multi-color image is restricted within a maximum size corresponding to the circumferential length of the photoreceptor.

BACKGROUND OF THE INVENTION 
The present invention relates to a color image forming apparatus of an 
electrophotographic type and particularly to a compact color image forming 
apparatus wherein a copy mode is changeable between a color mode and a 
monochromatic mode. 
In an electrophotographic image forming apparatus for forming exclusively 
monochromatic images such as black images or the like, the diameter of a 
photoreceptor drum that is an image carrier is required to be small for 
the purpose of making the apparatus small, and the drum with a small 
diameter has been put to practical use recently. 
Even in an image forming apparatus of a transfer drum type in which a 
transfer drum is used as an image forming apparatus capable of forming a 
full color image, there has been known an apparatus employing a 
photoreceptor drum whose diameter is small. 
However, an image forming apparatus of a transfer drum type tends to be 
large in size because of its transfer drum used therein and to be 
complicated in structure and expensive in cost because timing adjustment 
for latent image forming on a photoreceptor drum and for 
image-transferring onto an image-transfer paper from a transfer drum 
(rotational synchronization between a photoreceptor drum and a transfer 
drum) is necessary to assure the registration for an toner image of each 
color. Besides, a diameter of a photoreceptor drum is made small, which 
requires a structure of a so-called rotary developing unit wherein only 
one of plural developing sub-units containing respectively different color 
toners is located at the position successively to face a photoreceptor 
drum for developing, which further causes a complicated structure. 
As a solution of the aforesaid problems on an image forming apparatus of a 
transfer drum type, on the other hand, there is known an image forming 
apparatus wherein no transfer drum is needed and a full color toner image 
is formed on a photoreceptor drum and then is transferred correctively 
onto an image-transfer paper or onto a sheet for OHP use. 
For the purpose of obtaining a compound image such as a full color image or 
the like up to B4 size maximum and of making the diameter of a 
photoreceptor drum small as far as possible, in an image forming apparatus 
of this type, there has been used a method wherein a compound toner image 
including a full color toner image is formed through plural turns of a 
photoreceptor drum (for example, a toner image of each color is formed 
through one turn of a photoreceptor drum). 
When a compound toner image is formed through plural turns of a 
photoreceptor drum as in the case mentioned above, namely when an entire 
one toner image is formed on a photoreceptor drum, the size of a toner 
image formed is restricted by the length in the axial direction and the 
circumferential length of the photoreceptor drum. In this connection, 
Japanese Patent Publication Open to Public Inspection No. 229165/1987 
(hereinafter referred to as Japanese Patent O.P.I. Publication) has a 
description saying that the circumferential length on a photoreceptor drum 
from an image-wise exposure section to a cleaning member is made longer 
than the length of the maximum copy paper size. 
From the aforesaid Japanese Patent O.P.I. Publication No. 229165/1987, it 
is understood that the maximum size of an image transfer paper is limited 
when a diameter of a photoreceptor drum is fixed to a certain value and 
thereby the circumferential length is fixed, while the diameter 
(circumferential length) of the photoreceptor drum is determined by the 
desirable maximum size of an image-transfer paper. This merely shows the 
relation between the circumferential length of a photoreceptor drum and 
the maximum copy size, and it is nothing but providing an image forming 
apparatus wherein the selection of one size determines the other size. 
Therefore, even in the case of forming a monochromatic image such as, for 
example, a black toner image produced by a single developing sub-unit or a 
red toner image produced through developing a latent image formed 
collectively through the operation of both developing sub-units containing 
respectively yellow toner and magenta toner, the size of a toner image 
formed on a photoreceptor drum or the maximum size of an image-transfer 
paper has been limited by the predetermined circumferential length of a 
photoreceptor drum, being affected by an image formed by plural turns of a 
photoreceptor drum such as a full color image or the like. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide an image forming apparatus wherein 
the aforesaid limitation or restriction is eliminated when a monochromatic 
image is formed and the size of a toner image to be formed on a 
photoreceptor drum is restricted when a toner image of plural colors 
(color developed image) is formed on the photoreceptor drum through the 
plural turns of the photoreceptor drum. 
Aforesaid object of the invention is attained by a color image forming 
apparatus comprising a selection means which selects a monochromatic mode 
wherein a monochromatic image is formed through the development using a 
single developing sub-unit or using simultaneously plural developing 
sub-units or a color mode wherein a color developed image which is 
composed, in a laminated type, of plural images of different colors formed 
by using plural developing sub-units in succession is formed on an 
image-carrier, and a controlling means that limits, when the aforesaid 
color mode is selected, the size of a color developed image within a size 
corresponding to the circumferential length of the aforesaid image-carrier 
.

DETAILED DESCRIPTION OF THE INVENTION 
Examples of the invention will be explained as follows. The first example 
is a color image forming apparatus wherein a developing unit comprising 
therein developing sub-units containing respectively yellow (Y), magenta 
(M) and cyan (C) color developers and a developing unit comprising a 
developing sub-unit containing black (BK) color developer are 
interchangeable, and a full color mode and a monochromatic mode can be 
switched by changing the aforesaid developing units. FIG. 1 is a schematic 
composition diagram showing an example of a color image forming apparatus 
of the invention. 
In the FIG. 11 is a photoreceptor drum having thereon a photoconductor 
layer and 15 is a charging unit that charges uniformly the surface of 
photoreceptor drum that rotates in the arrowed direction. 
A document (unillustrated) placed on platen glass 2I is read by color image 
reading device 22 and image data of the document obtained through the 
reading by means of the image reading device 22 are supplied to laser 
exposure device 23 after the image-processing. From the laser exposure 
device 23, image light modulated with image data of the document is 
projected on the photoreceptor drum 11 and the image light causes an 
electrostatic latent image to be formed on the surface of photoreceptor 
drum 11 which has been charged by charging unit 12 uniformly. 
The electrostatic latent image thus obtained is converted to a toner image 
on the surface of photoreceptor drum 11 by means of developing sub-units 
31C, 31M and 31Y all provided in the first developing unit 30. Developing 
sub-unit 31C is one containing cyan (C) color developer, developing 
sub-unit 31M is one containing magenta (M) color developer and 31Y is one 
containing yellow (Y) color developer. 
The numeral 13 is a feeding means for an image-transfer material, 14 is a 
transfer unit that transfers a toner image formed on photoreceptor drum 11 
onto an image-transfer material fed into by feeding means 13, 15 is a 
separating unit that separate from photoreceptor drum 11 the 
image-transfer material having thereon the toner image transferred from 
photoreceptor drum 11, 16 is a conveyance means that transports the 
separated image-transfer material to fixing unit 17, 18 is a 
paper-receiving tray that receives the image-transfer material on which a 
toner image has been fixed, 19 is a pre-cleaning neutralizing unit that 
makes it easy to remove residual toners staying on photoreceptor drum from 
which a toner image has been transferred, and 20 is a cleaning unit that 
removes the residual toners from the surface of photoreceptor drum 11. 
In the color image forming apparatus of the first example, there is 
provided image-forming unit 10 comprising at least photoreceptor drum 11 
and cleaning unit 20, and both image-forming unit 10 and the first 
developing unit 30 are capable of being split, being mounted on or 
dismounted from the main body of apparatus. Incidentally, each of 10a and 
10b is a guide member that guides image-forming unit 10 when it is mounted 
or dismounted. 
It is preferable that developing sub-units 31C, 31M, and 31Y are of a 
non-contact developing type wherein a developer layer which is thinner 
than the clearance between a developing sleeve surface and photoreceptor 
drum 11 surface, is formed on the developing sleeve, and from the 
developer sleeve, toner flies to the electrostatic image on photoreceptor 
drum 11 to stick thereon, because the development can be made without 
disturbing the toner image formed previously on photoreceptor drum 11, the 
switchover between operation and non-operation for developing can simply 
be made by switching the bias voltage to be applied on the developing 
sleeve, and because developing sub-units do not need to be moved for the 
switchover. 
The main bodies of developing sub-units 31C, 31M and 31Y all of which are 
to be mounted on developing unit 30 in parallel are mostly the same in 
structure, and they are constituted as shown in FIG. 2 in which, however, 
developing sub-unit 31Y only is shown. 
In the developing sub-unit 31Y in FIG. 2, there are provided 
toner-containing device 310, agitating vane wheel 311, agitating vane 
wheel 312, developing sleeve 313, magnet roll 314 and toner replenishing 
roller 315. Cover member 319 is provided over the developing sub-unit 31Y 
to cover the inner toner-containing device 310, agitating vane wheel 311, 
agitating vane wheel 312, developing sleeve 313, magnet roll 314 and 
toner-replenishing roller 315, and thereby to prevent toner spewing. 
Incidentally, the clearance between developing sleeve 313 and 
photoreceptor drum 11 is kept by a roll (unillustrated) provided in 
coaxial with developing sleeve 313 and is kept in contact with 
photoreceptor drum 11. 
On the bottom of toner-containing device 310, there is contained toner T 
which is dredged up toward the upper portion on the left side in FIG. 2 
with dredging plate 310a that rotates clockwise. Toner T dredged up by 
dredging plate 310a is moved to the left side in FIG. 2 by 
toner-replenishing roller 315 and is fed to toner-replenishing vane wheel 
311 which, in cooperation with toner-agitating vane wheel 312, agitates 
magnetic carrier and toner T. Toner-agitating vane wheels 311 and 312 
rotate in opposite direction each other, thereby the conveyance distance 
can be made long and magnetic carrier and toner are agitated and mixed 
fully. Two-component developer D composed of magnetic carrier and toner is 
transported to developing sleeve 313 after being agitated and mixed. 
Inside developing sleeve 313, there is provided magnet roll 314 having 
therein fixed magnetic poles, and both developing sleeve 313 and magnetic 
roll 314 cause a magnetic brush of developer D to be formed. 
On the surface of developing sleeve 313, a thin layer of developer D is 
formed by the magnetic brush mentioned above. Developer thin layer forming 
means 317 such as, for example, a magnetic cylindrical bar arranged to 
oppose to the magnetic pole of magnet roll 314 is brought near to 
developing sleeve 313 so that developer D may be formed to be thinner than 
the clearance between deVeloping sleeve 313 and photoreceptor drum 11. 
From the surface of the thin layer of developer, toner T flies, under the 
influence of AC bias, toward electrostatic latent image on photoreceptor 
drum 11, thus an electrostatic latent image formed on the surface of 
photoreceptor drum 11 is developed to be a toner image. 
During the development, bias voltage composed of both D.C. and A.C. 
components keeps being applied on developing sleeve 313, thus the 
fluctuation of developing conditions between developing sub-units can be 
adjusted by controlling the bias voltage through the method mentioned 
later. 
FIG. 3 shows the sectional view of the first developing unit 30 provided 
therein with developing sub-units 31Y, 31M and 31C and the first 
developing unit 30 is capable of being mounted on or dismounted from the 
unit mounting section 40 provided on the main body. 
The first developing unit 30 is a framework having its opening on the side 
of photoreceptor drum 11, wherein guide member 39 composed of rollers or 
the like is provided, and each of developing sub-unit 31C. 31M and 31Y 
arranged in parallel each other is guided by guide member 39 and pushed 
toward photoreceptor drum 11 by elastic member 32 provided and energized 
on the backside of each developing sub-unit. 
On the external portion of the first developing unit 30, there is provided 
engaging member 33 which engages with rail 41 provided on the inner 
surface of unit mounting section 40, thus the first developing unit 30 may 
be inserted along rail 41, from the front side of the apparatus main body. 
Further, on the external portion of the first developing unit 30, there is 
provided mark member 34 such as a protrusion or the like, and under the 
state that the first developing unit 30 is mounted on unit mounting 
section 40, detecting member 42 such as a photocoupler or a microswitch 
detects that the first developing unit 30 is mounted. 
Unit mounting section 40 is provided with sliding portion 43 that is 
slidable on roller 143 provided on the main body and is capable of sliding 
from side to side. On the main body side, in addition to the above, there 
is provided eccentric cam 144 capable of swinging by 90.degree. and it 
engages with cam guide 44 provided on unit mounting section 40. Therefore, 
depending on the position on eccentric cam 144, the location of unit 
mounting section 40 is controlled. In the position shown in FIG. 3 wherein 
unit mounting section 30 is controlled to be at left, developing sub-units 
31C, 31M and 31Y located inside of the first developing unit 30 are 
energized by elastic member 32 to be at left, thus a prescribed clearance 
suitable for the development is formed between photoreceptor 11 and 
developing sleeve 11 of each developing sub-unit. Under such an 
operational posture, stopper plate 145 that is coaxial with eccentric cam 
144 is positioned to be in a chain line and it prevents developing unit 30 
to be mounted or dismounted. 
When eccentric cam 144 is swung by 90.degree., unit mounting section 40 
moves to the right and each of developing sleeves 313 of developing 
sub-units 31C, 31M and 31Y leaves photoreceptor drum 11, thereby creating 
the non-operation attitude. In this non-operation attitude, aforesaid 
stopper plate 145 also leaves to enable the first developing unit 30 to be 
mounted on or dismounted from unit mounting section 40. 
There is provided the second developing unit 50 that is mostly the same as 
the first developing unit 30 in external form and is capable of being 
mounted on unit mounting section 40 in place of the first developing unit 
30. 
Inside the second developing unit 50, there is provided developing sub-unit 
51 BK containing black (BK) developer. Developing sub-unit 51 BK is the 
same as developing sub-unit 31Y explained previously in terms of the 
structure which will be omitted. Since the consumption of black toner is 
large compared with other color toner, there is provided 
toner-replenishing member 521 which enables toner-containing device 510 to 
be replenished with toner. 
FIG. 4 shows the state of operation wherein the second developing unit 50 
is mounted on unit mounting section 40. 
Developing sub-unit 51 BK containing black (BK) developer is guided by 
guide member 59 in the second developing unit 50, and is energized to push 
photoreceptor drum 11 by elastic member 52 provided and energized on the 
back side of the developing sub-unit. At the location corresponding to 
engaging member 33 of the aforesaid first developing unit 30 on the 
outside of the second developing unit 50, there is provided engaging 
section 53 through which the second developing unit can be mounted on or 
dismounted from unit mounting section 40 along rail 41. Therefore, when 
the second developing unit 50 is mounted on unit mounting section 40 and 
eccentric cam 144 is operated to move unit mounting section 40 to the left 
position, the clearance between the developing sleeve of developing 
sub-unit 51BK and photoreceptor drum 11 is kept to be a pre-determined 
developing clearance to create an operation state wherein it is possible 
to develop. The second developing unit 50 also is equipped with mark 
member 54 corresponding to a portion to be detected, and detecting member 
42 detects that the second developing unit 50 is mounted. Namely, 
detecting member 42 detects, by detecting mark member 34 or 54, whether 
the first developing unit 30 is mounted or the second developing unit 50 
is mounted. When the first developing unit 30 is mounted, a full color 
mode or a monochromatic color mode is selected, and when the second 
developing unit 50 is mounted, the mode is switched to the monochromatic 
mode. 
When image reading device 22 reads a document placed on platen glass 21, 
the color separation through filter 9 is conducted. Filter 9 is composed, 
as shown in FIG. 5, of filter 9R, filter 9G, filter 9B and filter 9ND, and 
when reading a document, either one of filter 9ND transmits light before 
reading. The movement of filter 9 from side to side in FIG. 5 (in the 
direction perpendicular to the figure plane in FIG. 1) by means of 
filter-selecting device 22 which will be explained later determines the 
selection of filter among filter 9R, filter 9G, filter 9B and filter 9ND. 
Incidentally, filter 9R, filter 9G and filter 9B transmit respectively red 
light, green light and blue light, and filter 9ND is a neutral filter 
prepared to match the human visual sensation and it transmits all light of 
three colors. 
After the first developing unit 30 is mounted on unit mounting section 40, 
it is detected by detecting member 45 and is inputted into CPU 60 through 
the control circuit shown in FIG. 6. A user can select a monochromatic 
color or a full color, and when the latter is selected, CPU 60 sends to 
the image reading unit 22 a command as follows, because the first 
developing unit 30 is provided therein with developing sub-units 31C, 31M 
and 31Y containing respectively cyan (C) toner, magenta (M) toner and 
yellow (Y) toner. Namely, the command is given so that filter 9R may be 
selected for the development by means of developing sub-unit 31C 
containing cyan (C) toner, filter 9G may be selected for the development 
by means of developing sub-unit 31M containing magenta (M) toner and 
filter 9B may be selected for the development by means of developing 
sub-unit 31Y containing yellow (Y) toner, thus the filter is selected and 
set through filter selection circuit 22a. After a copy button is pressed 
and copy start command 67 is performed. CPU 61 gives a command to process 
control 68 to conduct image forming that is based on a color image forming 
time chart shown in FIG. 7. Namely, each time the photoreceptor drum 11 
makes one turn of its total three turns, each of toner images of yellow 
(Y). magenta (M) and cyan (C) is formed on a latent image formed by means 
of the image light emitted from laser exposure device 4 and projected onto 
photoreceptor drum 11. 
A color toner image thus formed is transferred by the transfer unit 14 onto 
recording paper (not illustrated) which is fed from the paper feeding unit 
13. Transferred toner image on the recording paper is heated and fixed by 
fixing unit 17, then is ejected out of the apparatus after being fixed. On 
the other hand, the photoreceptor drum 11 from which the color toner image 
has been transferred is cleaned by the cleaning unit 20. 
Further, after the second developing unit 50 is mounted on unit mounting 
section 40, it is detected by detecting member 42 and is inputted into CPU 
61 through developing sub-unit discriminating circuit 60. Since the second 
developing unit 50 is equipped with developing sub-unit 51BK containing 
black toner, CPU gives a command to filter selecting circuit 22a so that 
either filter 9G that is a green filter or filter 9ND may be selected. 
After a copy button is pressed and copy start command 67 is performed. CPU 
61 commands image forming process control 68 to conduct an image forming 
based on an image forming time chart in monochromatic mode shown in FIG. 
8. The image forming time chart in this case is the same as that in a 
general monochromatic electrophotographic copying machine. Namely, 
photoreceptor drum 11, after being charged uniformly by charging device 
12, is exposed to light from laser exposure device 23, resulting in the 
formation of an electrostatic latent image. The electrostatic latent image 
is developed by developing unit 51BK and the monochromatic toner image 
thus formed is transferred by means of transfer unit 14 onto a recording 
paper (unillustrated) fed by paper feeding device 13. The transferred 
toner image on the recording paper is heated and fixed by fixing unit 17 
and then the recording paper is ejected out of the apparatus. On the other 
hand, photoreceptor drum 11 from which the monochromatic toner image has 
been transferred is cleaned by cleaning unit 20. 
In the invention, a developed color image is restricted so that it may be 
within the length of a circumferential surface of photoreceptor drum 11 in 
case of a color mode wherein toner images are superposed, which is 
different from a monochromatic color mode that is an image forming mode in 
which toner images are not superposed. Further, a corresponding recording 
paper is also restricted preferably. 
In the invention, it is also possible to use a color image reading unit in 
which a copy image sensor is employed. Further, developing unit in an 
electrophotographic image forming apparatus can be composed of 4 
developing sub-units containing respectively yellow (Y), magenta (M), cyan 
(C) and black (BK) toners. In this case, while the photoreceptor drum 
makes 4 turns, a yellow (Y) toner image, magenta (M) toner image, cyan (C) 
toner image and black (BK) toner image are formed on the photoreceptor 
drum, and a full color image is obtained by transferring those images on a 
recording paper. Or, it is possible to perform the selection wherein a 
monochromatic image is obtained by driving selectively one or plural 
developing sub units out of the aforesaid 4 developing sub-units and by 
conducting the development of the type of superposing toner images for one 
electrostatic image. Even in this case, it is possible, depending on the 
selection of either a color mode or a monochromatic mode, to restrict 
similarly to the above maximum size of a developed color image so that it 
may be within the length of a circumferential surface of the photoreceptor 
drum. 
In the explanation of the invention, image data are based on the input from 
an image reading device, but they may also be based on the input from a 
frame memory wherein image data prepared by CAD or transmitted image data 
are stored. In this case, when forming a developed color image on any one 
of a host computer, a frame memory, or a control data related to a color 
image forming apparatus or image data, it is preferable to provide a 
control means or control data which restricts the size of the aforesaid 
developed color image within a size equivalent to the circumferential 
length of an image-carrier. As for photoreceptor drum 11 in the present 
example, its dimensions are determined so that its axial length may exceed 
slightly a longitudinal length of a recording paper (width 210 mm x length 
297 mm) in an A size and its circumferential length may exceed slightly a 
lateral dimension, resulting in a diameter of photoreceptor drum 11 that 
is about 80 mm. Therefore, in the color image forming apparatus in the 
example, it is possible to form on the photoreceptor drum a color image 
whose dimension is A4 size (fed laterally)+.alpha. (251 mm). As a result, 
it is possible to form an image as a maximum size in a color mode by the 
use of a recording paper of an A4 size. On the monocolor mode, on the 
contrary, it is possible to form an image with an A3 size recording paper 
as a maximum copy size. In FIG. 9, the relation between photoreceptor drum 
11 whose circumferential surface is developed and the size of a copy paper 
allowing the maximum copy size is illustrated based on the example. 
Recording paper size designating member 65 is set either manually or 
automatically through the automatic reading of a document size. Since the 
set mode of monocolor or color (in the first example, input from 
developing sub-unit discriminating member 60) is inputted in CPU 61, when 
the recording paper size designated per each mode stated above exceeds the 
maximum allowable size, display device 66 indicates that it is impossible 
to copy, thus prohibiting image forming. In a color image forming 
apparatus having a function of reduction, it is naturally possible to form 
an image by providing a program for setting at the allowable maximum size 
of a recording paper and reducing the document image so that it may match 
the recording paper size which has been set. 
In the first example stated above, a color mode and a monochromatic mode 
can be switched by changing the developing unit, an image forming 
apparatus is made small by using photoreceptor drum 11 which is of a 
demand type and is small in its diameter, sequence and process conditions 
for image forming are set depending on either a color mode or a 
monochromatic mode, a large-sized color developed image exceeding the 
circumferential length of a photoreceptor drum is prohibited to be formed 
and a large-sized recording paper is prohibited to be used for copies 
(prints) in a color mode, and a recording paper with the maximum size of 
A4 can be used in a color mode and the one with the maximum size of A3 can 
be used in a monochromatic mode. 
Therefore, the concrete explanation of the prohibition of a large-sized 
copy in the present example may be as follows. Namely, when an image is 
formed based on a color mode with a document in an A3 size placed on 
platen glass 21, either one of the following items (1)-(3) is carried out. 
(1) A large-sized copy is prohibited. In this case, an indication showing 
that it is impossible to make a large-sized copy appears concurrently in 
general. 
(a) In case of a manual mode, no copying is carried out for the large (A3) 
recording paper selected by an operator. 
(b) In case of an automatic mode, no copying is carried out for the 
large-sized recording paper selected through a document size automatic 
detection (and magnification designation). 
(2) In place of copying for a large-sized copy, either one of the following 
items is carried out. 
(a) When a large size copying is selected through a automatic document size 
detection (and magnification designation (in this case, magnification of 
1:1)), the large size copying is not carried out but a mode of continuous 
copying from an image-wise split document wherein a part of a document 
image is formed on a recording paper and the other part of the document 
image is formed on the next recording paper is automatically selected for 
copying. 
(b) When a large size copying is selected through a document size automatic 
detection, the large size copying is not carried out but a mode of 
automatic reduction is automatically selected for copying. 
(c) When a large size copying is selected through a automatic document size 
detection (and magnification designation (in this case, magnification of 
1:1)), the large size copying is not carried out but the large size 
monochromatic copying in a monochromatic mode is carried out by the use of 
developers in the first process cartridge. 
(3) Large size copying is prohibited and (a) displaying of an instruction 
for continuous copying from an image-wise split document, (b) displaying 
of an instruction for automatic reduction and (c) displaying of an 
instruction for switching to monochromatic copying, are carried out. 
(a) Following a display of an instruction for continuous copying from an 
image-wise split document, an operator designates a mode of continuous 
copying from an image-wise split document, for a copying operation. It is 
naturally possible not to select a copying operation. 
(b) Following a display of an instruction for an automatic reduction, an 
operator designates a mode of an automatic reduction, for a copying 
operation. It is naturally possible not to select a copying operation. 
(c) Following a display of an instruction for switching to a monochromatic 
copying, an operator switches to a monochromatic copying for a copying 
operation, thereby a monochromatic copying for a large size can be carried 
out. It is naturally possible not to select a co operation. 
In the example explained above, a between a monochromatic developing unit 
and a color unit is carried out, a switchover between a color and a 
monochromatic mode is carried out in a mod a color developing unit is 
used, and further, the formation of a color developed image in a large 
size exceeding the circumferential length of a photoreceptor is prohibited 
and also copying on a recording paper is preferably prohibited in a color 
mode. In place of the developing unit, it is possible, by uniting a 
developing unit, a photoreceptor drum and further a cleaning unit into one 
provide a process cartridge which can further be the process cartridge 
having 3 developing sub-units employing respectively yellow (Y), magenta 
(M) and, cyan (C) developers and the second process cartridge having a 
sub-unit employing a black (BK) developer, both cartridges being equipped 
with portions to be detected. Depending on whether the first process 
cartridge is or the second one is mounted on a main body of a color 
forming apparatus, a CPU can discriminate a color mode or a monochromatic 
mode, and it is further to possible discriminate a color mode or a 
monochromatic mode in the first process cartridge and also to limit 
automatically the maximum size of a recording paper based on diameter of a 
photoreceptor drum. 
In the same way as the above, it is possible to limit the size of a color 
developed image within a size equivalent to the circumferential length of 
a photoreceptor drum and to restrict preferably the maximum size of a 
recording paper depending on a monochromatic mode or a color mode even in 
the case where a process cartridge has 4 developing sub-units for yellow 
(Y), magenta (M), cyan (C) and black (BK). 
Furthermore, it is naturally possible, in a push-button mode, to perform 
the control in the invention by providing, on a color image forming 
apparatus, a selection button for selecting either a color mode or a 
monochromatic mode. 
The present invention has made it possible to obtain a small-sized color 
image forming apparatus wherein there is provided a photoreceptor drum 
whose diameter is small. The invention has further made it possible to 
provide both a monochromatic mode of black or a single color and a color 
mode, thus achieving the copying on a recording paper in a large size same 
as that in a large size in an ordinary copying machine, on a monochromatic 
mode, and the copying with the color copying from a large-sized document 
through a reduction or splitting by means of continuous copying from an 
image-wise split document in a color mode.