Electrophotographic image forming apparatus having multiple printing stations

In the image forming apparatus of the present invention, printing stations are provided in positions opposed to an endless paper conveying belt, and an electric charger is disposed close to a conveyance roller and on a downstream side of the conveyance roller with respect to a conveyance direction, which conveyance roller is disposed in a conveyance start position of the paper conveying belt, to maintain the surface potential of each recording paper in good condition, whereby it is made possible to prevent the deterioration of the image quality, diminish the power consumption and attain a reduction in size of the apparatus.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
The present invention relates to an electrophotographic type image forming 
apparatus to make printing for a printing paper being conveyed in an 
electrostatically adsorbed state on a paper conveying belt. 
An example of a conventional image forming apparatus will first be 
described with reference to FIG. 3. In a conventional image forming 
apparatus 1, an endless paper conveying belt 2 formed of a dielectric is 
stretched horizontally within the apparatus, and four printing stations 
3.sub.1 to 3.sub.4 of the same structure are disposed side by side along 
the upper surface of the belt 2. 
The structure of the printing stations 3.sub.1 3.sub.4 will now be 
explained. On the upper surface of the paper conveying belt 2 there are 
disposed photosensitive drums 4 side by side, and above and before each of 
the photosensitive drums 4 there are disposed side by side a photoscanner 
5 and a developing unit 6, respectively. On the reverse side of the paper 
conveying belt 2 there are disposed transfer rollers 8 as transfer units 
in positions opposed to the photosensitive drums 4, with a power source 7 
being connected to the transfer rollers 8. Further, a toner cleaner 9 and 
a charger 10 are disposed along the peripheral surface of each 
photosensitive drum 4 in the area from the transfer portion up to the 
scanning portion. At an end portion on the conveyance start side of the 
paper conveying belt 2 there are disposed a conveyance roller 11 and a 
charger 12 above and below, respectively, in an opposed relation to each 
other. On the other hand, at the end portion on the conveyance end side of 
the belt 2 there is disposed a fixing unit 13 in an opposed relation to 
the belt end portion. The developing unit 6 contains toners (not shown) 
of, for example, yellow, magenta, cyan and black in the printing stations 
3.sub.1 to 3.sub.4, respectively. 
In the above construction, first a recording paper 14 fed into the 
apparatus is pressed by the conveyance roller 11 against the upper surface 
of the paper conveying belt 2 which has been electrified by a discharging 
action of the charger 12, and thereby attracted to the belt upper surface 
electrostatically. With movement of the paper conveying belt 2, the 
recording paper 14 thus attracted to the belt is conveyed to each of the 
printing stations 3.sub.1 to 3.sub.4. In each of the printing stations 
3.sub.1 to 3.sub.4, laser light is emitted from the photo scanner 5 to the 
photosensitive drum 4 which has been electrified by a discharging action 
of the charger 10, to scan an electrostatic latent image, to which latent 
image is applied a toner when the photosensitive drum 4 passes the 
developing unit 6 to form a monochromatic image. Then, by a discharging 
action of the transfer roller 8 the monochromatic image is transferred to 
the recording paper 14 being carried on the paper conveying belt 2. In 
this way, monochromatic images are lap-printed successively onto the 
recording paper 14, which paper is then pressed and heated by the fixing 
unit 13 and discharged out of the apparatus. Printing is now over. The 
photosensitive drum 4 after image transfer then passes the toner cleaner 9 
and the charger 10, whereby the residual toner is removed and the drum is 
re-electrified, ready for the next printing. 
In the image forming apparatus 1, as set forth above, the recording paper 
14 is conveyed while being attracted electrostatically to the paper 
conveying belt 2. To ensure this attraction, the paper 14 is pushed 
against the paper conveying belt 2 by means of the conveyance roller 11. 
At this time, however, the electric charge on the belt 2 flows out from 
the roller 11, with the result that the surface potential of the paper 14 
lowers. Therefore, in the conventional image forming apparatus 1 it is 
necessary to use a discharger for image transfer in addition to the 
charger 12 for the paper conveying belt 2, and the four transfer rollers 8 
are provided. Consequently, the power consumption of the image forming 
apparatus 1 is large, so the power source 7 is large in size, and an 
insulation distance is required between adjacent transfer rollers 8. 
Further, it is difficult to reduce the size of the apparatus because of a 
large number of components. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is the first object of the present invention to provide an image forming 
apparatus capable of effecting good transfer even in the absence of 
transfer rollers in printing stations. 
It is the second object of the present invention to diminish the power 
consumption. 
It is the third object of the present invention to provide a small-sized 
image forming apparatus. 
It is the fourth object of the present invention to attain a strong 
adhesion of a recording paper to a paper conveying belt without lowering 
the surface potential of the paper. 
Other objects of the present invention will become apparent from the 
following description. 
According to the present invention, in an electrophotographic type image 
forming apparatus having at least one printing station in a position 
opposed to an endless paper conveying belt stretched in the interior of 
the apparatus, a charger is disposed close to a conveyance roller and on a 
downstream side of the conveyance roller with respect to a conveyance 
direction which conveyance roller is disposed in a conveyance start 
position of the paper conveying belt opposedly to the belt. By this 
arrangement, a recording paper positioned on the paper conveying belt by 
the conveyance roller is electrified by the charger and reaches the 
printing station. Since the surface potential of the recording paper does 
not lower, the transfer of image can be performed by utilizing the surface 
potential and it is possible to omit a discharger in the printing station.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The first embodiment of the present invention will now be described with 
reference to FIG. 1. In a color printer which is an image forming 
apparatus 15 according to this embodiment, a discharger 16, which also 
serves as an electric charger, is disposed behind a conveyance roller 11, 
and in a position opposed to the discharger there is disposed a counter 
electrode 17 through a paper conveying belt 2. And in a first printing 
station 18.sub.1 there is omitted a transfer roller 8. Other 
constructional points are the same as in the foregoing conventional image 
forming apparatus 1. Since the paper conveying belt 2 has insulation 
properties, the conveying roller 11 is insulated from the frame ground 
(earth). 
In the image forming apparatus 15 of the above construction, a recording 
paper 14 fed into the apparatus is held under pressure between the 
conveying roller 11 and the paper conveying belt 2 and thereby positioned. 
In this state the paper 14 is electrified by a discharging action of the 
discharger 16 and thereby attracted to the belt 2. This electrostatic 
attraction is attained by an electric charge counter to that on the 
surface of the recording paper 14 being induced on the counter electrode 
17 side opposed to the discharger 16 through the belt 2. Then, in the same 
manner as in the foregoing conventional image forming apparatus 1 a 
monochromatic image is lap-printed in each Of printing stations 18.sub.1 
to 18.sub.4 to form a color image. In this image forming apparatus 15, the 
first printing station 18.sub.1 is not provided with a transfer roller for 
the following reason. After the recording paper 14 is electrified directly 
by the discharger 16, the surface thereof does not come into abutment with 
any member and is therefore held at a high potential, and this surface 
potential permits the transfer of a toner image onto the paper. Besides, 
since the paper 14 is attracted to the paper conveying belt 2 by virtue of 
a strong attraction, its fore end is prevented from floating from the belt 
2, so there is no fear of image disturbance or jamming. 
All the transfer rollers 8 can be omitted by improving the output of the 
discharger 16 and the insulation of the printing stations 18.sub.1 to 
18.sub.4. 
Further, even when the working atmosphere is in a high humidity condition, 
since the conveyance roller 11 which is in contact with the recording 
paper 14 is insulated from the frame ground, even in the event of slight 
electroconductivity developed on the surface of the paper 14, it is 
impossible that the electric charge imposed thereon by the discharger 16 
will flow out to the frame ground through the conveyance roller 11. That 
is, the surface of the paper 14 after being electrified is maintained at a 
high potential. 
The second embodiment of the present invention will now be described with 
reference to FIG. 2. In a color printer which is an image forming 
apparatus 19 according to this embodiment, there is used a paper conveying 
belt 22 having a dielectric layer 20 as a surface layer and a conductive 
layer 21 as a reverse-side layer, and none of printing stations 23.sub.1 
to 23.sub.4 used in this embodiment are provided with the transfer 
rollers 8, and the counter electrode 17 is omitted. Other constructional 
points are the same as in the image forming apparatus of the previous 
embodiment. 
In such construction, the image forming apparatus 19 functions in the same 
manner as the image forming apparatus 15. In this image forming apparatus 
19, a potential sufficient to carry out the image transfer four times is 
provided to the recording paper 14 by the discharger 16. Further, since 
the surface potential of the paper 14 is maintained in good condition by 
the conductive layer 21 positioned on the reverse side of the paper 
conveying belt 22, all the transfer rollers 8 are omitted. 
Although in the above embodiments the image forming apparatus 15 and 19 are 
used as color printers provided with plural printing stations 18.sub.1 to 
18.sub.4 and 23.sub.1 to 23.sub.4, respectively, the present invention is 
not limited thereto, but is applicable also to a single color printer 
having only one printing station. 
Also in this case, since the dielectric layer 20 of the paper conveying 
belt 22 exhibits a high insulation property, the conveyance roller 11 is 
insulated from the frame ground and the surface potential of the paper 14 
once electrified is maintained in good condition.