Image forming apparatus with an improved cleaning means

An image forming machine such as a copier and a printer for forming a toner image on a recording sheet. The machine includes: a photoreceptor for holding a toner image on its surface; a charger for charging the surface of the photoreceptor; a laser writing unit for exposing the surface of the photoreceptor so that a latent image is formed on the surface; a developer for developing the latent image with toner so that the toner image is formed on the surface of the photoreceptor in which the developer includes a developing sleeve for providing toner on the sleeve so that the toner is transferred from the sleeve to the surface of the photoreceptor; a transferor for transferring the toner image from the surface of the photoreceptor to the recording sheet; a cleaning unit for cleaning a residual toner on the surface after the toner image is transferred by the transferor; and an exposing unit for selectively exposing an area of the residual toner on the surface so that an electrical potential difference is created between the residual area of toner and the photoreceptor.

BACKGROUND OF THE INVENTION 
The present invention relates to a means for preventing toner remaining on 
an image carrier after the image carrier is cleaned by a cleaning device 
of an image forming apparatus such as a copying machine or a printer. 
An electrophotographic color image forming apparatus such as a copy is 
generally comprises a platen on which a document is placed, a scanning 
optical system composed of a document-illuminating lamp reciprocating 
under the platen, a reflection mirror, a fixed projection lens and a 
reflection mirror, a photoreceptor drum which is charged by a charging 
electrode to a certain level of voltage and rotates, a developing unit 
which superposes a plurality of color toners on an electrostatic latent 
image formed on the surface of the photoreceptor drum by the scanning 
optical system for developing a multicolor image, a transfer electrode 
which transfers a toner image onto a recording sheet that is a transfer 
material conveyed toward the lower part of the photoreceptor drum surface, 
a separating electrode and a neutralizing electrode, a fixing unit which 
fixes the toner image transferred onto the recording sheet, an ejecting 
unit for the transfer material, and a cleaning unit that removes toner and 
dust remaining on the photoreceptor surface after transferring. 
Due to the foregoing, necessary color images are recorded in succession on 
transfer materials conveyed one by one intermittently along the 
predetermined conveyance path. 
After that, the photoreceptor drum cleaned by the cleaning unit is charged 
through a predetermined image process to be ready for the following image 
forming cycle. 
A cleaning blade of the cleaning unit in a color image forming apparatus is 
away from a photoreceptor drum while a multicolor image is being formed on 
the circumferential surface of the photoreceptor drum, and the blade is 
brought into contact with the circumferential surface of the photoreceptor 
drum again after the necessary color image is formed and transferred onto 
a transfer material for cleaning residual toner. After the cleaning blade 
has completed a necessary cleaning by scraping off toner, when the 
cleaning blade leaves the photoreceptor drum, a part of toner collected on 
the edge of the cleaning blade remains on the circumferential surface of 
the photoreceptor drum in the form of a streak. 
When residual toner just adhering to the photoreceptor drum arrives at the 
developing unit in the course of a rotation of the photoreceptor drum, and 
passes through a relatively narrow gap of about 500 .mu. between the 
surface of the photoreceptor drum and a developing sleeve of each rotating 
developing unit, the residual toner enters the developing unit and is 
mixed therein. It does not cause any problem in a monochromatic image 
forming apparatus. In a color image forming apparatus, however, yellow 
toner in the developing unit, for example, is mixed with the residual 
toner whose color is different from that of the yellow toner, resulting in 
deterioration of a color tone of the yellow toner and an adverse effect on 
color images to be developed. 
An object of the invention is to solve the problem mentioned above and to 
provide an image forming apparatus wherein, even when toner remains on a 
photoreceptor drum of a color image forming apparatus that forms 
multicolor images after cleaning, the toner does not enter a developing 
unit so that normal color images can be formed. 
SUMMARY OF THE INVENTION 
The object of the invention mentioned above can be attained by an image 
forming apparatus comprising a developing unit in which an electrostatic 
latent image formed on an image carrier by a charging unit and an image 
writing means is given toner to be developed and a cleaning means that 
cleans toner remaining on the image carrier after transferring, wherein a 
residual area of toner remaining on the image carrier after cleaning is 
exposed to light by an exposure means and thereby an electrical potential 
difference is provided between the exposed residual toner area of the 
image carrier and a developer carrier. 
In the apparatus mentioned above, the above-mentioned image writing means 
can be used also for exposure for the area where toner remains after 
cleaning on the image carrier. 
Further, in the image forming apparatus mentioned above, it is possible to 
provide a potential difference between an exposed image carrier and a 
developer carrier by exposing, with an exposure means provided between a 
cleaning means and a charging means before charging the image carrier, an 
area on the image carrier where toner remains after cleaning, and by 
charging the image carrier excluding the area where toner remains.

DETAILED DESCRIPTION OF THE INVENTION 
An example of the invention will be explained as follows, referring to FIG. 
1 representing a diagram showing how toner held by a potential difference, 
FIG. 2 representing a side view showing an exposure means forming a 
potential difference, FIG. 3 representing a side view showing another 
example of the invention, FIGS. 4(1)-4(3) show the states of patch 
exposure, charging and image writing in FIG. 3, FIG. 5 representing a 
sectional side view showing the structure of a color forming apparatus, 
and FIGS. 6(1)-6(4) show an image forming system. 
The invention, however, is not limited to the present example. 
An image forming apparatus of the invention will be explained as follows, 
referring to an example of a color printer. 
As shown in FIG. 5, color image forming apparatus 1 comprises laser-writing 
unit B, image forming section C and sheet-feeding section D so that a 
color image can be recorded through the following process. Formation of a 
multicolor image in the present example, to start with, is carried out in 
accordance with an image forming system shown in FIG. 6. Namely, data 
obtained in a color image data input section (FIG. 6 (1)) where an 
original image is scanned by an image sensor are subjected to arithmetic 
processing in a data processing section (FIG. 6 (2)) for preparing image 
data which are stored in an image memory (FIG. 6 (3)) temporarily. Then, 
the image data in the image memory are taken out in the course of 
recording and are inputted, for example, in color image forming apparatus 
1 shown in FIG. 5 that is a recording section (FIG. 6 (4)). Namely, when 
color signals outputted from an image reading unit that is separate from 
the above-mentioned printer are inputted in laser-writing unit B, a laser 
beam generated by a semiconductor laser (not shown) is subjected to rotary 
scanning by means of polygon mirror 52 rotated by driving motor 51, its 
optical path is deflected by mirror 54 through a lens and is projected on 
the circumferential surface of photoreceptor drum 30 that is an image 
carrier and is charged uniformly in advance to a certain predetermined 
electric charge by charging unit 55 that is a charging means, and a bright 
line is formed. 
On the other hand, an index (not shown) provided at a specific position on 
photoreceptor drum 30 with respect to the secondary scanning direction is 
detected by a photosensor (not shown), and then the signals thus detected 
and used as a reference to start modulation of a semiconductor laser made 
by image signals, and thus the primary scanning direction is determined. 
After the start of scanning, a laser beam (not shown) is detected by an 
index sensor (not shown) with respect to the primary scanning direction, 
and a laser beam thus modulated scans the surface of the photoreceptor 
drum 30. Therefore, the primary scanning by means of the laser beam and 
the secondary scanning by means of rotation of the photoreceptor drum 30 
form a latent image corresponding to the first color on the surface of the 
photoreceptor drum 30. This latent image is developed by developing unit 
31Y containing yellow (Y) toner (image-visualizing medium), for example, 
in a plurality of developing units provided in parallel in the present 
example, and thus, a Y toner image is formed on the surface of the 
photoreceptor drum 30. The toner image thus obtained passes, while being 
held on the photoreceptor drum 30, through cleaning unit 59 that is a 
cleaning means being away from the surface of the photoreceptor drum 30 to 
enter the copy cycle for the second color. 
Namely, the photoreceptor drum 30 on which a Y toner image is formed is 
charged again by the above-mentioned charging unit 55, and then signals of 
the second color outputted from the image reading unit are inputted in the 
above-mentioned writing unit B, and thus writing on the surface of the 
photoreceptor drum 30 is conducted in the same way as in the occasion for 
the signals for the first color mentioned above, for forming a latent 
image. This latent image is developed by developing unit 31M containing 
magenta (M) toner as the second color, for example. The M toner image is 
formed on the above-mentioned Y toner image which has already been formed. 
In the same manner as the foregoing, a latent image formed through the 
third image signals is developed by developing unit 31C containing cyan 
(C) toner, and then, after charging again, a latent image formed through 
the fourth image signals is developed by developing unit 31B containing 
black (B) toner, and thus, the C toner image and the B toner image are 
superimposed on the circumferential surface of the photoreceptor drum 30 
to form a color toner image on the surface of the photoreceptor drum 30. 
On each of sleeves of the developing units 31Y, 31M, 31C and 31B, there is 
impressed an A.C. bias voltage or further a D.C. bias voltage so that the 
photoreceptor drum 30 whose base is grounded may be subjected to reversal 
development (jumping development) in a non-contact manner. Incidentally, 
in a developing unit which is out of developing operation, its developing 
sleeve stops rotating and a bias voltage to be impressed is cut so that 
toner images formed on the photoreceptor drum 30 may not be damaged and 
unnecessary toner may not be supplied to a latent image. 
The color toner image formed on the circumferential surface of the 
photoreceptor drum 30 in the manner mentioned above is transferred, at 
transfer electrode 56 provided as a transfer means, onto a transfer 
material representing a recording medium that is fed out from the 
above-mentioned sheet-feeding section D by a sheet-feeding belt and 
synchronized with the above-mentioned color toner image by a timing 
roller. For this transferring, the transfer electrode 56 impresses high 
voltage from a high voltage power source having a polarity opposite to 
that of the toner. 
Thus, the transfer material onto which the color toner image has been 
transferred is separated from the surface of the photoreceptor drum 30 by 
separating electrode 57, and then is conveyed into fixing unit 70 by a 
conveyance belt, for fixing of the color toner image. After the fixing is 
completed, the transfer material is conveyed to sheet-ejecting unit 80 to 
be ejected onto tray 1A. 
On the other hand, the photoreceptor drum 30 from which the color toner 
image has been transferred to the transfer material and also the transfer 
material has been separated, further rotates clockwise so that residual 
toner on the photoreceptor drum 30 may be removed for cleaning by blade 
59A of cleaning unit 59 that is in pressure contact with the photoreceptor 
drum 30. After cleaning is completed, the blade 59A leaves the 
photoreceptor drum 30 again for the following process of color image 
forming. 
Thus, desired color images are recorded on the transfer material. 
Next, there will be explained a potential difference forming means of the 
invention that forms a potential difference between a photoreceptor drum 
and a developing sleeve so that a toner image can be held on the 
photoreceptor drum. 
The photoreceptor drum 30 which has been cleaned is charged again in a 
predetermined image forming process. An exposure means for an area on the 
circumferential surface of the photoreceptor drum 30 where toner stays 
even after cleaning employs a laser beam of laser writing unit B as shown 
in FIG. 2. Namely, the same exposure means is used both for the area of 
residual toner d and for forming images, and the same laser exposure L is 
used for the area of residual toner d prior to the laser exposure L for 
image forming. Therefore, the details of the exposure means in the present 
example shown in FIG. 2 are the same as those shown in FIG. 5, and 
explanation thereof is omitted accordingly. 
As shown in FIG. 1, V.sub.H represents a charging voltage impressed on 
photoreceptor drum 30, V.sub.DC represents D.C. components of a developing 
bias voltage, V.sub.AC represents A.C. components of the same, and V.sub.L 
represents an exposure voltage. The charging voltage V.sub.H is set to a 
range from -500V to -900V and preferably to -650V. V.sub.DC representing 
D.C. components of a developing bias voltage is set to a range from -400V 
to -800V and preferably to -500V, while A.C. component superimposed on 
D.C. component is set to a range of 2-3 KV and preferably to 2.8 KV. 
Exposure voltage V.sub.L is set to a range from 0V to -300V and preferably 
to -200V. An amount of electric charges of toner to be used is within a 
range from -10 .mu.C/g to -30 .mu.C/g and preferably within a range from 
-15 .mu.C/g to -20 .mu.C/g. 
As shown in FIG. 1, there is formed potential difference H between 
developing bias voltage V.sub.DC as well as V.sub.AC with a base of GND 
(ground) and exposure voltage V.sub.L, due to laser exposure L. Therefore, 
in case of reversal development, it becomes easy by the developing bias 
for toner having an amount of electric charges ranging from -10 .mu.C/g to 
-30 .mu.C/g to fly electrically toward the surface of the photoreceptor 
drum 30, the potential of which is lowered by the exposure. 
Residual toner d stuck to the circumferential surface of the photoreceptor 
drum 30 due to potential difference H thus formed arrives at developing 
unit 31Y shown in FIG. 2 when the photoreceptor drum 30 rotates in the 
arrowed direction. Then the residual toner passes through the developing 
sleeve that is rotating while being away from the circumferential surface 
of the photoreceptor drum 30 by about 500 .mu.. It is possibly happened 
that toner is flown and adhered by the developing bias to the location 
where the residual toner d occurs; however, the toner in not flown back to 
the developing sleeve. Therefore, the residual toner does not enter the 
developing unit 31Y. In the same way, the residual toner d passes through 
developing sleeves of developing units 31M, 31C and 31B located at the 
downstream side of the developing unit 31Y to be adjacent thereto while 
the residual toner is sticking, and then arrives at transfer electrode 56. 
Since the residual toner d is sticking to a non-image area of the 
photoreceptor drum 30, it is not transferred onto the transfer material, 
and the photoreceptor drum 30 is cleaned by cleaning unit 59 again after 
color images on the photoreceptor drum have been transferred. Since the 
residual toner is usually the mixture of Y.M.C.B. toners, especially in 
case that the residual toner enters the developing unit 31Y, it is 
impossible to form a normal Y toner image. 
Therefore, by using the aforementioned method, excellent color images can 
be recorded constantly in a way that no toners enter developing units 31Y, 
31M, 31C and 31B and no color images transferred are affected. 
Next, another example to cause residual toner d to be stuck to the 
photoreceptor drum 30 by forming a potential difference will be explained 
as follows, referring to FIG. 3 and 4. 
As shown in FIG. 3, there is provided patch exposure light source 20 such 
as, for example, LED or a tungsten lamp which exposes an area of residual 
toner d at the location over cleaning unit 59 and before charging unit 55 
at the downstream side in the arrowed direction of rotation of the 
photoreceptor drum 30. The patch exposure light source 20 is also used for 
removal of residual electric charges on the photoreceptor drum 30. 
Exposure voltage V.sub.L of the patch exposure light source 20 to be 
charged is the same exposure voltage V.sub.L as that of laser exposure L 
in the aforementioned example. An area of residual toner d on the 
circumferential surface of the cleaned photoreceptor drum 30 is subjected 
to patch exposure L.sub.1 made by the patch exposure light source 20, 
resulting in exposure voltage V.sub.L identical to that in the 
aforementioned example, for example, a voltage of -200. Thus, potential 
difference H is formed between a developing sleeve whose developing bias 
voltage V.sub.DC and V.sub.AC are set to the values identical to those in 
the above-mentioned example. This patch exposure L.sub.1 is arranged to 
expose at an arbitrary position between the cleaning unit 59 and the 
charging unit 55. 
As stated above, charging voltage V.sub.H to be impressed on the 
photoreceptor drum 30, developing bias voltage D.C. component V.sub.DC and 
A.C. component V.sub.AC, and a value of electric charges amount of toner 
.mu.C/g are all set to be exactly the same as those in the example stated 
previously. 
An upper portion of FIG. 4 (1) shows the state of ON and OFF of patch 
exposure L.sub.1 wherein patch exposure L.sub.1 is ON for exposure for the 
area of residual toner d and OFF for other area. A middle portion of FIG. 
4 (2) shows the state of ON and OFF of charging unit 55 for the 
photoreceptor drum 30 wherein the photoreceptor drum 30 that rotates in 
the arrowed direction after cleaning is subjected to charging in 
accordance with a predetermined image forming process, and when the area 
of residual toner d which has been subjected to patch exposure L1 arrives 
at charging unit 55, the charging unit 55 is turned off and an uncharged 
area is formed. A lower portion of FIG. 4 (3) shows the state of image 
writing made by laser writing unit B for the upper and middle portions. 
Incidentally, residual toner d stays at a non-image area as shown in the 
figure. 
Since no charging is conducted on an area of residual toner d, patch 
exposure L1 is carried out and potential difference H identical to that in 
the example mentioned above is formed and kept, and thus, residual toner d 
passes through developing sleeves of developing units 31Y, 31M, 31C and 
31B in the same manner as in the foregoing while being stuck to the 
photoreceptor drum 30 through adhesive power of a predetermined value. 
Therefore, residual toner d does not enter any of the developing units 
31Y, 31M, 31C and 31B and is removed by cleaning unit 59 after an aimed 
color image has been transferred. 
Turning on and off for the patch exposure light source 20 and the charging 
unit 55 as mentioned above are carried out through the control stored in a 
CPU (not shown) in advance. 
Due to the potential difference formed by an exposure means or a charging 
means between a photoreceptor drum and a developing sleeve in the 
invention, residual toner staying on the photoreceptor drum even after 
cleaning is stuck to the photoreceptor drum through adhesive power, being 
prevented from entering the developing units. Therefore, the color tone of 
color toner in the developing unit is not disturbed and excellent color 
images are recorded on a transfer material constantly.