Multicolor image forming apparatus using chargers with opposite polarity

An image forming apparatus wherein a multicolored image of an original is formed in color on a sheet in one cycle of image forming operation. The apparatus comprises a plurality of developing devices which are filled with respective developers colored differently from each other and a scorotoron charger disposed between the developing devices for recharging on electrostatic latent image on a photosensitive member processed by the upstream developing device with the polarity opposite to the polarity charged beforehand. The developing device located upstream of the scorotron charger functions to develop the imaged portions including primary information which were charged with first potential. The scorotron charger recharges the electrostatic latent image with the opposite polarity, thereby lowering the potential of the other imaged portions including background information which were charged with second potential. The image portions charged with the second potential are developed in low density by the downstream developing device, portions with the second potential finely contrast with each other can be formed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electrophotographic image forming 
apparatus, and more specifically, an image forming apparatus wherein a 
multicolored image of a multicolored original is formed on sheets in one 
cycle of image forming operation based on the electrophotographic 
processing. 
2. Description of Related Art 
Multicolored originals to be subjected to a copying operation are generally 
those which have black letters marked with a color marker and corrections 
in color ink. A variety of methods and apparatuses for copying a bicolored 
image of such an original on a sheet in one cycle of the image forming 
operation (including one cycle of a charging process, an exposing process 
and a transferring process) have been discussed and developed. For 
example, Japanese patent Laid Open Publication No. 55-38561 discloses a 
high-speed multicolor printing method. According to the publication, after 
an electrostatic latent image is formed having two values, a plurality of 
developing devices are impressed with respective developing bias voltages 
different from each other so that each portion of the latent image having 
a different value can be developed. 
In a method for developing an electrostatic latent image having two values 
by applying different developing bias voltages, if the density is set high 
so that the marked portions are developed densely, the marked portions and 
lettered portions will be mixed with each other in the border portions. 
Because of this, the contrast of the letters and the mark will be weaken, 
and there will be a difficulty in reading the lettered portions packed 
with primary information. On the other hand, if the density is set low so 
that the marked portions are developed thinly corrected portions will be 
developed thinly, too, so that there will be a difficulty in reading the 
correction. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to attain a quality 
image where lettered portions and marked portions finely contrast with 
each other and corrected portions are copied clearly with use of an 
electrophotographic multicolored image forming apparatus for forming and 
developing an electrostatic latent image corresponding to a multicolored 
original on a photosensitive member and transferring the image on a sheet. 
To attain the above-mentioned object, an electrophotographic image forming 
apparatus according to the present invention comprises a photosensitive 
member; means for charging said photosensitive member with specified 
polarity; means for forming an electrostatic latent image of a plurality 
of imaged portions which have different potential from each other and 
non-imaged portions with substantially lower potential on said 
photosensitive member; a plurality of developing devices disposed around 
said photosensitive member, which are filled with respective developers 
colored differently from each other; means for impressing said developing 
devices with respective developing bias voltages in accordance with the 
potential of the imaged portions; and means disposed between said 
developing devices for recharging said photosensitive member with the 
polarity opposite to that charged by said charging means. Preferably, said 
recharging means may be a scorotron charger. Further, its charging wire is 
impressed with a voltage with the polarity opposite to that charged by 
said charging means, and its grid is impressed with a voltage with the 
same polarity as that charged by charging means and a value lower than the 
potential of the imaged Portions processed by the upstream developing 
device and higher than the potential of the non-imaged portions. 
A case of copying a bicolored original with use of the apparatus with the 
constitution above is hereinafter described step by step. Suppose the 
bicolored imaged portions to be formed of lettered-in-black portions 
having primary information, marked-in-red portions coloring some parts of 
the lettered-in-black portions and corrected-in-red portions. First, in an 
exposing process, a positive electrostatic latent image is formed having 
two potential values in accordance with the bicolored original image. The 
lettered-in-black portions remain potential which is almost as high as the 
surface potential initially charged, and the marked-in-red portions and 
the corrected-in-red portions have lower potential. A developing bias 
voltage which is effective only to the lettered-in-black portions with 
high potential being impressed on the first developing device, the 
lettered-in-black portions are developed by the first developing device. 
Next the electrostatic latent image whose some parts have been already 
developed is charged with the polarity opposite to the latent image by the 
scorotron charger. Thereby, the potential of the marked-in-red portions is 
slightly lowered. However the charge is not effective to the edge portions 
of them, and the potential of the edge portions is not lowered. Then, a 
developing bias voltage which is close to the lowered potential of the 
center portions of the marked-in-red portions being impressed on the 
second developing device, the marked-in-red portions and the 
corrected-in-red portions are developed by the second developing device. 
Thereby, the edge portions with high potential, that is, the outlines of 
the marked-in-red portions and the corrected-in-red portions are developed 
thickly, and the marked-in-red portions overlapping the lettered-in-black 
portions are developed thinly. 
The toner image formed in this way is transferred onto a sheet and fixed 
thereon. Thus, in one cycle of image forming operation, a quality image 
whose lettered-in-black portions finely contrast with marked-in-red 
portions and whose corrected-in-red portions are clear is formed on a 
sheet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention is hereinafter described in 
reference to the accompanying drawings. 
FIG. 1 is a schematic view of an electrophotographic copying machine which 
is an embodiment of the present invention. Numeral 1 is a conventional 
photosensitive drum, and it has a photosensitive layer on its 
circumferential surface and can be driven to rotate in the direction of 
the arrow a. Around the photosensitive drum 1, devices and members as 
explained below are disposed. 
An electric charger 2 charges the photosensitive drum 1 with specified 
potential on the surface (first electric charge), and its charging wire is 
connected to a power source 21. 
An image exposure device 3 works to form an electrostatic latent image of 
an original on the surface of the photosensitive drum 1 in the well-known 
slit exposure method, and it comprises an exposure lamp, a mirror, a lens, 
etc. 
Developing devices 5 and 5' are according to the well-known magnetic 
brushing principle, and they comprise developing sleeves 51 and 51' 
respectively. The developing sleeves 51 and 51' incorporate respective 
magnet rollers 52 and 52' whose circumferential surfaces are polarized 
with N and S. The developing sleeves 51 and 51', which also function as 
developing electrodes, are connected to power sources 53 and 53' for 
impressing a developing bias respectively. The developer is a mixture of 
magnetic carriers and insulative toner. The carriers and the toner are 
charged with the opposite polarity to each other by the frictional 
electrification between them, and the insulative toner is charged so as to 
have the polarity opposite to that charged by the electric charger 2. In 
order to perform development in two colors, the first developing device 5 
is filled with developer including black toner, and the second developing 
device 5' is filled with developer including red toner. 
Further, non-contact development is preferable in the second developing 
process, otherwise there is a fear of scratching black toner which was 
stuck on the photosensitive drum 1 in the first developing process. 
Between the two developing devices 51 and 51', a scorotron charger 4 
functioning as means for forming an outline image is disposed. The 
scorotron charger 4 recharges the photosensitive drum 1 on the surface 
(second electric charge). Its charging wire is connected to a power source 
41, and its grid 42 is connected to a power source 43. The power source 41 
impresses the charging wire with a voltage with the polarity opposite to 
that charged by the electric charger 2. The grid 42 is impressed with a 
voltage by the power source 43 so as to have the same polarity as the 
voltage impressed on the electric charger 2. Further, the voltage to be 
impressed on the grid 42 in this moment have to be higher than the surface 
potential of the non-imaged portions of the electrostatic latent image. 
A transfer charger 6 functions to form an electric field on the reverse 
side of a copying sheet 10 traveling in the direction of the arrow b so 
that a toner image formed on the photosensitive drum 1 by the developing 
devices 5 and 5' can be transferred onto the sheet 10. (The mechanism of 
forming a toner image will be described later.) The charging wire is 
impressed with a voltage by a power source 61 so as to have the polarity 
opposite to that of the insulative toner. 
A separation charger 7 functions to form an ac electric field at a place 
where the sheet 10 passes immediately after the image transfer section so 
that the charge on the sheet 10 is erased, whereby the sheet 10 is removed 
from the surface of the photosensitive drum 1. The charging wire is 
impressed with an ac voltage by a power source 71. 
A cleaning device 8 has a blade for removing the residual toner from the 
surface of the photosensitive drum 1. 
An eraser lamp 9 removes the residual charge from the surface of the 
photosensitive drum 1 by an irradiation in order to make the 
photosensitive drum 1 ready for the next copying operation. 
The formation of a bicolored image by the above constituted copying machine 
is hereinafter described step by step referring to definite values of 
potential and so on. Further, the description is hereinafter given in 
connection with a case of using non-magnetic insulative toner. 
(i) First Charging Process 
The photosensitive drum 1 is charged with specified potential on the 
surface by the electric charger 2. In this embodiment, the surface 
potential is +500 V. 
(ii) Exposing Process 
An original is subjected to a slit exposure, and the image is projected 
onto the surface of the photosensitive drum 1 with even potential. In this 
case, the original image is colored with red and black. So, an 
electrostatic latent image is formed on the photosensitive drum 1, as 
shown in FIG. 2a, has lettered-in-black portions A', marked-in-red 
portions A and corrected-in-red portions B. The surface potential (Vo) of 
the lettered-in-black portions A' remain +500 V, and the surface potential 
(Vred) of the marked-in-red portions A and the corrected-in-red portions B 
is lowered to +280 V. The surface potential (Vi) of the other portions, 
that is, non-imaged potions is lowered to +40 V. 
(iii) First Developing Process 
In this process, the electrostatic latent image with two values of surface 
potential, which was formed in said exposing process, is developed with 
black toner by the first developing device 5. In this moment, the 
developing sleeve 51 is impressed with a developing bias voltage (Vb1) of 
+300 V, which is lower than the surface potential (Vo) of the 
lettered-in-black portions A' and higher than the surface potential (Vred) 
of the marked-in-red portions A and the corrected-in-red portions B, by 
the power source 53. Thereby, the black loner Is stuck only on the 
lettered-in-black portions A' with the potential (Vo). 
FIGS. 2b and 3a show potential of the electrostatic latent image and 
adhesion of toner at the time of completing the first developing process. 
Character Tb in FIG. 3a show black toner stuck on the photosensitive drum 
1. 
(iv) Second Charging Process Outline Image Forming Process) 
The photosensitive drum 1 holding the electrostatic latent image whose 
potential distribution is as shown in FIG. 2b is charged with the polarity 
opposite to that of the electrostatic latent image by the scorotron 
charger 4 which is impressed with a voltage of -5 kV by the power source 
41. The grid 42 is impressed with a voltage (Vg) of +150 V which is lower 
than the surface potential (Vred) of the marked-in-red portions A and the 
corrected-in-red portions B and higher than the surface potential (Vi) of 
the non-imaged portions. 
At that time, electric force as shown by the arrows in FIG. 3a is formed 
between the surface of the photosensitive drum 1 and the grid 42. The 
electric lines of force around the edge portions of the electrostatic 
latent image do not stretch toward the grid 42 but curve outward of the 
latent image. Negative ions produced by the scorotron charger 4 travel to 
the photosensitive drum 1 along the electric lines of force, and the 
negative ions which reached the surface of the photosensitive drum 1 lower 
the surface potential to almost the grid voltage (Vg). Thereby, the 
surface potential of the marked-in-red portions A is lowered to almost the 
voltage (Vg) except for the edge portions. In portions on which the black 
toner Tb is stuck, the negative ions give negative charge to the black 
toner Tb, so that the value of the charge on the black toner Tb is raised. 
Thus, the potential distribution of the photosensitive drum 1 at the time 
of completing this process is shown by FIG. 2c. The non-imaged portions 
remain having the low potential (Vi), about +40 V. The outlines of the 
marked-in-red portions A, which are formed inside the edges with almost 
the same width as letters in the corrected-in-red portions B, and the 
corrected-in-red portions B have high potential (Vred), about +280 V. The 
surface potential of the center portions of the marked-in-red portions A 
including the portions on which the black toner is stuck has been lowered 
to almost the grid voltage (Vg), +150 V. 
Further, the value of the charge on the latent image with the opposite 
polarity provided in this process may be higher or lower. 
(v) Second Developing process 
In this process, the electrostatic latent image processed in said second 
charging process is developed with red toner by the second developing 
device 5'. Regarding a developing bias voltage (Vb2) to be impressed on 
the developing sleeve 51' by the power source 53', the following two cases 
should be considered separately. 
(1) Vg.ltoreq.Vb2&lt;&lt;Vred 
The grid voltage (Vg) is +150 V, and the potential (Vred) of the outlines 
of the marked-in-red portions A and the corrected-in-red portions B is 
+280 V. The impressed developing bias voltage (Vb2) is +170 V. In this 
case, the outlines of the marked-in-red portions A are comparatively 
strong, so that the lettered-in-black portions A' are enclosed with the 
red outlines (Refer to FIG. 3b). On the other hand, the red toner Tr is 
not stuck on the corrected-in red portions B so much. Accordingly, in this 
case, it is preferred to use bright red toner. 
(2) Vi&lt;Vb2.ltoreq.Vg 
Concretely, the impressed developing bias voltage (Vb2) is +80 V. In this 
case, all the marked-in-red portions A including the portions C in FIG. 3b 
are developed in red, but all the portions other than the outlines are 
stuck with less toner because of the lower potential. Accordingly, the 
marked-in-red portions A are on the whole thin, so that the marked-in-red 
portions A and the lettered-in-black portions A' are never mixed with each 
other. Therefore, the black letters can be read without interruption of 
red mark. On the other hand, the corrected-in-red portions B are developed 
clearly. 
Thus, in the exposing process, an electrostatic latent image is formed, two 
values of potential being distributed. Then, through the subsequent 
processes, the electrostatic latent image is colored with black and red so 
that the lettered-in-black portions including primary information and the 
marked-in-red portions emphasizing some parts of the lettered-in-black 
portions can keep a fine contrast with each other and that the 
corrected-in-red portions can be strong enough. The transfer charger 6 
discharges electricity with negative polarity, and thereby the toner is 
transferred onto the sheet 10. Thereafter, the image is fixed on the sheet 
by a fixing device not shown in the drawings. 
Although the present invention has been described in connection with the 
preferred embodiment thereof, it is to be noted that various changes and 
modifications are apparent to those who are skilled in the art. Such 
changes and modifications are to be understood as included within the 
scope of the present invention as defined by the appended claims, unless 
they depart therefrom. 
Especially, in the embodiment above, the present invention has been 
described in connection with a bicolor image forming apparatus wherein an 
electrostatic latent image is formed after an original colored with black 
and red so as to have two values of potential. However, even if an 
original has two colors other than black and red, the present invention 
can be applied to form an electrostatic latent image with two values of 
surface potential in accordance with the colors. Also, if a color filter 
is used in consideration of the spectral sensitivity characteristic of the 
photosensitive layer, the present invention can be applied to make a copy 
of an original with any two colors. 
Further, each of the potential (Vo), (Vred), (Vg), (Vb1) and (Vb2) is just 
an example, and each potential can be changed depending on conditions, 
especially, type of photosensitive layer and toner.