Image forming apparatus with adsorption means

An image forming apparatus includes an imager for forming an image at an image forming position on an image supporting member, a movable carrier for carrying the image supporting member on a first side thereof and for conveying the image supporting member to the image forming position in order to perform the image formation, an adsorption device for causing the image supporting member to be ewlectrostatically adsorbed or adhered onto the carrier device. The adsorption device is provided at an upstream side of the image forming position in the direction of movement of the carrier. The adsorption device comprises a corona charger provided at a second side of the carrier opposite to the first side. A contact member for contacting the second side of the carrier is provided adjacent to the corona charger.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an image forming apparatus, such as an 
electrophotographic apparatus, an electrostatic recording apparatus or the 
like, and more particularly, to an image forming apparatus which is 
suitable when a transfer material is adsorbed or adhered onto a carrying 
sheet for electrostatically transferring a toner image on an image 
carrying member to the transfer material held on the carrying sheet. 
2. Description of the Related Art 
In a color electrophotographic copier, a transfer drum is configured by 
winding a carrying member generally in the form of a carrying sheet such 
as a high-resistance film, around a drum-like frame. A transfer material 
is supplied to the transfer drum and wound therearound, and an end of the 
transfer material is mechanically fixed and held by a gripper disposed at 
a part of the circumferential surface of the transfer drum. Alternatively, 
the transfer material is fixed and held on the carrying sheet by 
electrostatically providing an adsorption force between the carrying sheet 
and the transfer material while winding the transfer sheet (as described 
in Japanese Patent Public Disclosure (Kokai) No. 55-32079 (1980)). In this 
way, an image supporting member (the transfer material) is carried by 
carrying means (the transfer drum) to a transfer position. 
In the process of rotating the transfer drum a plurality of times, a toner 
image having plural colors is electrostatically transferred from an image 
carrying member (such as a photosensitive drum) to the surface of the 
transfer material. For this purpose, a corona charging device is disposed 
behind the carrying sheet at the transfer position, or a conductive roller 
is disposed in rotational contact with the transfer material held on the 
carrying sheet, whereby electric charges having a polarity opposite to 
that of the toner image are supplied to the transfer material. 
Subsequently, in order to discharge the transfer material from the 
carrying sheet, the electrostatic adsorption force (Coulomb force) 
produced between the transfer material and the carrying sheet is weakened 
using a corona discharger, and a separation claw is inserted between the 
transfer material and the carrying sheet to separate the transfer 
material. 
In the conventional image forming apparatus as described above, however, a 
gap is occasionally produced locally between the transfer material and the 
carrying sheet when holding the transfer material on the carrying sheet. 
This happens when a transfer material holding surface of the carrying 
sheet is deformed. Such deformations are commonly caused by a cleaning 
member which bears against the carrying sheet when cleaning the carrying 
sheet, electrostatic forces produced by charging of the carrying sheet, 
thermal deformations from a fixing device situated near the moving region 
of the carrying sheet, the reaction of the carrying sheet when the 
transfer material contacts the carrying sheet, and the like. Transfer 
efficiency decreases where there is a gap, and there may arise a region 
where transfer is not achieved at all. Furthermore, when there is a large 
deformed region in the carrying sheet, jamming easily occurs, especially 
if the carrying sheet is convex relative to the transfer material holding 
surface. Finally, gaps decrease the effectiveness of the electrostatic 
adsorption of the transfer material, producing deviation in the position 
of the transfer material on the carrying sheet. These phonomena are even 
more pronounced when the transfer material is held on the carrying sheet 
with an electrostatic adsorption force, i.e. without using a mechanical 
gripper. 
If portions of the transfer material become detached from the carrying 
sheet after image transfer, contamination of the image transferred 
occasionally occurs, caused by the contact of the transfer material with 
members around the transfer drum which disturbs the image on the transfer 
material. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above-described 
problems. 
It is an object of the present invention to provide an image forming 
apparatus for reliably adsorbing an image supporting member onto carrying 
means. 
It is a further object of the present invention to provide an image forming 
apparatus which removes distortions and deformations in such carrying 
means, and in which an image supporting member can be adhered closely to 
the carrying means without producing a gap. 
It is a further object of the present invention to provide an image forming 
apparatus capable of forming superior images on an image supporting member 
carried on carrying means by providing superior adhesive property between 
the image supporting member and the carrying means. 
It is a further object of the present invention to provide an image forming 
apparatus which prevents the occurrence of jamming of an image supporting 
member by removing distortions in such carrying means. 
It is a further object of the present invention to provide an image forming 
apparatus which prevents contamination in an already-transferred image due 
to the contact of an image supporting member with other members around the 
image supporting member by reliably carrying the image supporting member 
on carrying means. 
In one aspect of the invention, there is provided an image forming 
apparatus including image forming means for forming an image on an image 
supporting member at an image forming position, carrying means for 
carrying the image supporting member on a first side thereof and for 
conveying the image supporting member to the image forming position, 
adsorption means for causing the image supporting member to be adsorbed 
onto the carrying means, the adsorption means being provided at an 
upstream position relative to the image forming position, and a contact 
member provided adjacent the adsorption means at the second side of the 
carrying means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be hereinafter 
explained with reference to the drawings. 
In FIG. 1, a photosensitve drum 1 as an image carrying member is rotatably 
supported at the center of the drum, and is rotatably driven in the 
direction of the arrow. Facing the outer circumferential surface of the 
photosensitive drum 1, a charger 2, an optical system 3 and a developer 
supplier 4 are disposed in the direction of rotation of the drum 1. The 
charger 2 provides a uniform charged amount on the photosensitive drum 1. 
The optical system 3 provides an optical image subjected to color 
separation with a predetermined timing or a light beam L corresponding 
thereto on the surface of the photosensitive drum 1 to form an 
electrostatic latent image. A laser-beam exposure apparatus or the like is 
used for the optical system 3. The developer supplier 4 is of movable type 
which faces the surface of the photosensitive drum 1 and moves in the 
tangential direction, and is provided with four developing devices 4M, 4C, 
4Y and 4BK housing four-color developers (toners), that is, magenta, cyan, 
yellow and black developers, respectively. The developer supplier 4 faces 
the developing device selected in accordance with the optical image of the 
color selected by the optical system 3 or the irradiation of the 
corresponding light beam L relative to the photosensitive drum 1, and 
electrostatically provides toner to develop a toner image on the surface 
of the photosensitive drum 1. 
A transfer drum 6 for carrying a transfer material, such as paper or the 
like, as an image supporting member is situated in the direction of 
rotation of the photosensitive drum 1, in contact with or slightly apart 
from the surface of the photosensitive drum 1. On the transfer drum 6, a 
carrying sheet 6a consisting of a dielectric sheet, such as polyvinylidene 
flouride resin or the like, is cylindrically wound between cylindrical end 
frames (see FIG. 6). A corona charger 7 for transfer is disposed at the 
side opposite to the holding surface of the carrying sheet 6a facing the 
photosensitive drum 1. The transfer drum 6 is rotatably driven in the 
direction of the arrow. A corona charger 19 for adsorption is disposed at 
the side opposite to the holding surface of the transfer material by the 
transfer drum 6 at an upstream position in the direction of movement of 
the transfer drum 6 relative to the transfer position, that is, the 
position where the photosensitive drum 1 faces the corona charger 7 for 
transferring a toner image from the photosensitive drum to the transfer 
material. A conductive roller 20 is disposed at the side of the holding 
surface for the transfer material, facing the corona charger 19 for 
adsorption of the transfer material onto the transfer drum 6. Together, 
the corona charger 19 and conductive roller 20 comprise charging means. 
Corona dischargers 10 and 11 for removing charges are disposed facing both 
surfaces of the carrying sheet 6a at a downstream position in the 
direction of movement of the transfer drum 6 relative to the transfer 
position. Pushing rollers 12 and 13, which selectively deform on the 
carrying sheet 6a for separating the transfer material from the carrying 
sheet 6a, are also disposed facing both surfaces of the carrying sheet 6a, 
and a claw 14 for separation is disposed nearby. At a further downstream 
position, there is provided a brush roller 15 for cleaning the holding 
surface of the carrying sheet 6a and, if necessary, a brush-type charge 
remover 16 or corona discharger (not illustrated) for removing adhesion 
force (residual Coulomb force or van der Waals force). 
The transfer material P, which is separated by the claw 14 and on which a 
toner image is formed, is supplied toward a fixing roller 18 via a 
conveyor 17, and the image developed by the toner on the transfer material 
P is fixed. 
At an immediately upstream position in the direction of movement of the 
transfer drum P from the corona charger 19 as charging means and the 
conductive roller 20, there is provided transfer-material supply means for 
supplying the transfer material P to the holding surface of the carrying 
sheet 6a by guiding it into a guide 22 via resist rollers 21, 21. 
In the present embodiment, at a position where the transfer material P is 
supplied to the carrying sheet 6a, there is provided distortion correction 
means 23 made of an elastic supporting member, such as an elastic sheet 
150 .mu.m thick, is provided contacting the surface opposite to the 
holding surface of the carrying sheet 6a and adjacent to the corona 
charger 19. As shown in FIG. 5, the correction means 23 consists of an 
elastic sheet 50 extending toward the direction of the center shaft of the 
transfer drum 6 and a holding member 51 for holding the elastic sheet 50. 
The elastic sheet 50 is made of a dielectric sheet, such as a polyester 
film or the like. The elastic sheet 50 presses and holds the carrying 
sheet 6a from the inner side for correcting mechanical deformation due to 
the brush roller 15 and the like or deformation due to an electrostatic 
force by the corona charger 19. In this case, the elastic force of the 
correction means 23 may be provided in a direction so as to stretch the 
carrying sheet 6a. This direction is a direction in which the free end of 
the elastic sheet 50 extends in a downstream direction relative to holding 
member 51. However, the elastic sheet 50 may be provided at the direction 
opposite to the above-described direction. Although the elastic sheet 50 
is provided at a more upstream side than the adsorption position in the 
direction of movement of the transfer drum 6 in FIG. 1, it is also 
possible to provide it on the opposite side of the adsorption position. 
Thus, it is possible to reduce distortion in the radial direction of the 
carrying sheet 6a. 
As described above, various cases are possible for the arrangement of the 
elastic sheet 50. However, the most preferred is a case in which the 
elastic sheet 50 is provided close to the corona charger 19 at a more 
upstream side than the adsorption position in the direction of movement of 
the carrying sheet 6a. In this position, the distortion of the carrying 
sheet 6a in the radial direction can substantially be prevented. For 
example, it becomes possible to reduce the shock due to the contact of the 
transfer material P with the carrying sheet 6a that is produced when the 
direction of supplying the transfer material P to the carrying sheet 6a 
differs from the tangential direction at the contact point of the carrying 
sheet 6a with the roller 20 as shown in FIG. 1. Furthermore, by providing 
the free-end position of the elastic sheet 50 within the corona discharge 
region when the elastic sheet 50 is provided in the direction as shown in 
FIG. 1, it is also possible to regulate the discharge region in the 
circumferential direction of the corona charger 19. In this case, the 
boundary between the portion where electric charges for adsorption are 
supplied to the carrying sheet 6a and the portion where the charges are 
not provided becomes sharp, and it is possible to perform an adsorption 
which is better than in a case in which electric charges for adsorption 
are gradually supplied without regulating the corona discharge region. 
In FIG. 1, a charge remover 27 removes static electric charges on the 
surface of the photosensitive drum 1, and a cleaning blade 28 removes 
waste toner. If necessary, a corona discharger 29 may be provided near the 
claw 14 for separation to perform AC corona discharge for the purpose of 
preventing disturbance in image due to discharge at peeling caused when 
the transfer material P is separated from the carrying sheet 6a. 
In such a configuration, when a color image passing through, for example, a 
green filter is first irradiated on the surface of the photosensitive drum 
1 by the optical system 3 in a state in which the surface of the 
photosensitive drum 1 is uniformly charaged by the primary charger 2, a 
latent image consisting mainly of the magenta component among color images 
is formed thereon. In synchronization with the feeding of the latent 
image, the developer supplier 4 moves the developing device 4M housing the 
magenta developer in the tangential direction to the photosensitive drum 1 
to face it relative to the photosensitive drum 1, and electrostatically 
provides the toner to develop a magenta image on the photosensitive drum 
1. 
On the other hand, the transfer material P is guided into the guide 22 with 
the function of the resist rollers 21, 21, and is further supplied to the 
position of the conductive roller 20 along the surface of the carrying 
sheet 6a. The transfer material P is electrostatically adsorbed and held 
onto the carrying sheet 6a having curvature at the adsorption position by 
supplying the carrying sheet 6a with electric charges having a polarity 
identical to that of the charger 7 for transfer by the corona charger 19. 
Electrostatic adsorption is produced by the following process. A voltage 
(for example, positive) having a polarity identical to that of the corona 
charger 7 for transfter is applied to the corona charger 19. Positive 
charges are therefore supplied to the dielectric carrying sheet 6a, and 
current thereby flows through the grounded conductive roller 20 to induce 
negative charges in the transfer material P. Hence, the positive charges 
on the carrying sheet 6a and the negative charges on the transfer material 
P attract each other. Electric charges are thus supplied on the carrying 
sheet 6a and the transfer material P by the corona charger 19 and the 
conductive roller 20. 
The transfer material P electrostatically adsorbed on the carrying sheet 6a 
is sent to the transfer position where the photosensitive drum 1 faces the 
corona discharger 7 for transfer, that is, the image forming position 
where the toner image is formed on the transfer material P by the movement 
of the transfer drum 6. 
In this case, the feed timing of the resist rollers 21, 21 is in 
synchronization with the timing for forming the latent image by the 
optical system 3, and both timings coincide with each other at the 
transfer position. At the transfer position, an electric field for 
transfer is generated by the corona charger 7 for transfer which supplies 
electric charges having a polarity opposite to that of the toner, and the 
toner on the photosensitive drum 1 is held on the transfer material P by 
electric charges supplied to the carrying sheet 6a. 
The electric charges in the residual toner on the photosensitive drum 1 are 
removed by the charge remover 27. The residual toner is then removed by 
the blade 28, and the surface of the photosensitive drum 1 is cleaned. On 
the other hand, the transfer material P adsorbed on the carrying sheet 6a 
is moved in accordance with the rotation of the transfer drum 6 while 
carrying the toner image, and passes through between the corona 
dischargers 10 and 11. At this moment, the corona dischargers 10 and 11 
are not energized, and the pushing rollers 12 and 13 are separated from 
the carrying sheet 6a. The brush roller 15, the corona discharger or 
brush-type charge remover 16 and the conductive roller 20 are also 
separated from the carrying sheet 6a. Accordingly, the transfer material P 
passes through between the coroner charger 19 and the conductive roller 20 
without disturbing the toner image on the transfer material P held by 
Coulomb force, and is sent again to the transfer position. 
As to the energization of the corona charger 19 and the contact of the 
conductive roller 20 with the transfer material P, before the front end of 
the toner image on the transfer material P reaches the above-described 
position of the corona charger 19 and the conductive roller 20, the 
voltage applied to the corona charger 19 has been turned off, and, as 
mentioned above, the conductive roller 20 has been separated from the 
carrying sheet 6a. Hence, when the transfer material P passes through 
between the corona charger 19 and the conductive roller 20, electric 
charges for adsorption are not supplied to the transfer material P. 
Furthermore, before the front end of the toner image reaches the transfer 
position, image formation by the magenta developer has been completed, and 
the optical system 3 has already irradiated a color image passing through 
a red filter on the photosensitive drum 1. The developer supplier 4 shifts 
the developing device 4C against the photosensitive drum 1, and 
electrostatically provides the toner toward the latent imaage to develop a 
cyan image on the photosensitive drum 1. For this purpose, a toner image 
by the cyan developer is transferred overlapped with the preceding toner 
image by the magenta developer at the transfer position. 
Thus, the optical system 3 irradiates optical images, which are obtained by 
performing color separation relative to an identical image a plurality of 
times while sequentially providing green, red and blue filters, on the 
photosensitive drum 1 to form latent images. The developer supplier 4 
supplies the photosensitive drum 1 with corresponding developers, that is, 
magenta, cyan and yellow developers to perform color development as a 
whole. The sequence of providing the filters and supplying the developers 
can of course be appropriately selected according to requirements. 
After the final toner image, which is an image by the yellow developer in 
the present embodiment, has been transferred onto the transfer material P, 
the corona dischargers 10 and 11 are energized when the transfer material 
P passes through between them to remove the electric charges, and the 
pushing rollers 12 and 13 are pressed against the carrying sheet 6a to 
increase curvature at the pressed portion and to aid peeling of the 
transfer material P from the carrying sheet 6a. The claw 14 for separation 
contacts or comes close to the carrying sheet 6a to separate the front end 
of the transfer material P from the carrying sheet 6a and to supply the 
transfer material P to the fixing rollers 18 via the conveyor 17. The 
fixing rollers 18 then fix the toner image on the transfer material P. In 
separating the transfer material P, disturbance of the image due to 
discharge at peeling may be prevented by the corona discharger 29. The 
surface of the carrying sheet 6a after the separation of the transfer 
material P is cleaned by the brush 15. At this time, if the residual toner 
still continues to keep the electrostatic adhesion force, cleaning cannot 
be performed satisfactorily. The removal of the residual toner by the 
corona discharger or brush-type charge remover 16 provides an effective 
cleaning. 
The reproduction of a color image can thus be performed. When the filters 
are not used in the optical system 3 and the developing device 4BK for the 
black developer is faced against the photosensitive drum 1 in the 
developer supplier 4, a normal black-and-white reproduction can be 
performed. In this case, since only one transfer is performed, there is 
provided the function of each component which corresponds to the case of 
the final development in color reproduction. 
If necessary, in the final stage of color development, the exposure of 
white light and a black image by the black developer may be superposed. 
Particularly in the present embodiment, before the transfer material P is 
electrostatically adsorbed and held on the carrying sheet 6a, uneven 
deformation of the carrying sheet 6a is corrected by the distortion 
correction means 23 to prevent an adsorption state causing a local gap. 
Hence, there is no danger of causing jamming or producing a region where 
transfer is not performed. Although the correction means 23 uses an 
elastic sheet 50 in the present embodiment, it may of course be replaced 
by other configurations. 
In an embodiment shown in FIG. 2, the correction means 23 comprises rollers 
24 and 25 which are disposed facing and adjacent to the corona charger 19 
at upstream and downstream sides relative to the corona charger 19 in the 
direction of movement of the transfer drum 6, respectively. The conductive 
roller 20 also has the function of the correction means 23. The rollers 24 
and 25 aid in correcting uneven deformation of the carrying sheet 6a 
before the transfer material P is received and adsorbed onto the carrying 
sheet 6a. The conductive roller 20 has a crowned shape with a difference 
"a" in diameter as shown in FIG. 3, and adsorbs the transfer material P 
guided between the carrying sheet 6a and the conductive roller 20 first 
from its center. Hence, distortion is pushed out toward the circumference, 
and electrostatic adsorption between the carrying sheet 6a and the 
transfer material P is reliably performed. According to experiments, 
failure in adsorption can be nearly prevented even when the difference "a" 
in diameter is within 50 .mu.m -1mm. 
Although the conductive roller 20 is grounded in the present embodiment, it 
may be connected to voltage application means such as a biasing power 
supply to constitute electric-charge supply means together with the corona 
charger 19. 
In an embodiment shown in FIG. 4, the correction means 23 comprises a pair 
of rollers 26 provided at both sides of the carrying sheet 6a adjacent to 
and facing each other and located immediately upstream of the 
electrostatic adsorption means consisting of the corona charger 19 and the 
conductive roller 20. Rollers 26 are retracted from carrying sheet 6A when 
transfer material P, bearing an unfixed toner image, is recirculated past 
the rollers. The pair of rollers 26 has the function of correcting 
mechanical distortion or distortion caused by the electrostatic force of 
the carrying sheet 6a, as well as closely contacting the transfer material 
P coming from transfer-material supply means with the corrected carrying 
sheet 6a and guiding the transfer material P between the corona charger 19 
and the conductive roller 20. At least one of the pair of rollers 26 may 
of course be formed in the shape of a crown as described above with 
reference to FIG. 3. 
By the function of such correction means 23, the gap between the transfer 
material P and the carrying sheet 6a can actually be maintained to be 50 
.mu.m or less. Hence, failure in adsorption virtually never occurs, and it 
is also possible to prevent jamming. 
Although an explanation has been provided of a configuration in which the 
transfer material P is adsorbed on the carrying sheet 6a in all the 
embodiments described above, the correction means of the present invention 
may of course be applied to a method in which the transfer material P is 
mechanically fixed and held on the carrying sheet 6a by a mechanical 
gripper. Furthermore, in order to adsorb the transfer material P onto the 
transfer drum 6, an opening (or a plurality of pin-hole openings) may be 
provided in a portion where the transfer material P is carried on the 
transfer drum 6, and the transfer material P may be adsorbed by sucking 
air through the opening(s) from within the transfer drum 6. 
As the transfer drum 6, a drum comprising an endless sheet on which the 
transfer material P can be adsorbed at an arbitrary position may be used. 
However, as shown in FIG. 6, it is preferred to use a drum for transfer 
which includes cylindrical rings 6b and 6c at both ends thereof in its 
longer direction and a connecting member 6d for connecting the rings, and 
on which the dielectric carrying sheet 6a is wound and carried in a 
portion 6e cut away on the surface of the frame of the drum, because the 
strength of the transfer drum 6 increases and the carrying sheet 6a is 
hardly deformed. In the case of using the transfer drum 6 as shown in FIG. 
6, the use of the corona charger 19 for adsorption as described above is 
preferred to the use of a conductive roller, to which a biasing voltage is 
applied, in place of the corona charger 19 for adsorption. The reason is 
as follows. If a conductive roller is used for the transfer drum 6 as 
shown in FIG. 6, the transfer drum 6 produces a deviation in rotation when 
the conductive roller runs over the connecting member 6d by the rotation 
of the transfer drum 6, and a deviation in image is thereby produced. 
Accordingly, when the conductive roller is used within the transfer drum, 
it is necessary to provide a mechanism for separating the conductive 
roller from the connecting member 6d when it passes over the connecting 
member 6d. If the corona charger 19 for adsorption is used, however, such 
a separation mechanism becomes entirely unnecessary, and hence the 
figuration becomes simpler. 
Similarly, as the correction means for contacting the inner side of the 
transfer drum, the use of an elastic sheet, as shown in FIG. 1, is 
preferred to the use of rollers, as shown in FIGS. 2 and 4, because the 
possibilities of transfer sheet deviations are reduced. 
Although the above-described embodiments use a method in which the toner 
image formed on the photosensitive drum is transferred on the transfer 
material as the image supporting member, the present invention may also be 
applied to a method which does not require transfer, such as the ink-jet 
method depicted in FIG. 7, in which a recording ink jet head faces an 
endless belt as carrying means and recording is performed by the ink jet 
head on an image supporting member, such as paper or the like, 
electrostatically adsorbed on the belt. Reference numerals in FIG. 7 which 
are the same as the reference numerals in FIGS. 1, 2 and 4 also correspond 
to the same components as in FIGS. 1, 2 and 4. 
As described above in detail, the image forming apparatus according to the 
present invention includes members for correcting distortion in carrying 
means and closely contacting an image supporting member with the carrying 
means. Hence, it is possible reliably to adsorb the image supporting 
member, to prevent jamming of the image supporting member, to achieve a 
high quality image without blank areas, and improve reproducibility of 
color in the case of color reproduction. 
Furthermore, according to the present invention, by securely holding the 
entire surface of an image supporting member on carrying means and 
preventing the detachment of the image supporting member from the carrying 
means, it is possible to prevent contamination of an already-transferred 
image which might occur by contact with provided around the carrying 
means.