Transferring device and an image forming apparatus

A transferring device for transferring on a record paper a toner image formed onto an image forming member is disclosed. The transferring device comprises a transfer belt with a dielectric layer formed on a conduction belt member; a driving device for running the transfer belt; a transfer roller for contacting the transfer belt to an image forming member with pressure; a corona charges arranged at a position opposite to the transfer belt for charging the transfer belt with a charge having a polarity opposite to the charging polarity of the toner; a grid electrode arranged between the corona charger and the transfer belt; a plurality of bias sources connected to the grid electrode for biassing the grid electrode at respective potentials; and a change-over circuit for changing over the connection between the grid electrode and the bias source, whereby the grid potentials are selectively changed over in accordance with the characteristics of a record paper to be used.

BACKGROUND OF THE INVENTION 
The present invention relates to a transferring device for transferring on 
a record paper a toner image formed onto an image forming member, such as 
a photosensitive drum and an image forming apparatus utilizing such a 
transferring device. 
In an image forming apparatus, such as an electrophotographic printing 
apparatus or a copying device, an electrostatic latent image is formed 
onto a photosensitive member or body, the electrostatic latent image is 
developed to make a toner image, the toner image is transferred onto a 
recording medium, such as, a record paper and the transferred image is 
subjected to a fixing step to form a hard copy. Various kinds of 
transferring devices for transferring the toner image formed onto the 
photosensitive body are well known, for example, the belt transferring 
device disclosed in Japanese Patent Application Laid-open No. 61-117583. 
In this known belt transferring device, a transfer belt formed by a 
dielectric material is stretched between two drive rollers, the transfer 
belt is charged at a uniform potential by a corona charger, the record 
paper carried in synchronism is electrostaticaly attracted onto the 
transfer belt, and then a toner image transfer process is performed while 
carrying the record paper under the state of holding it to the transfer 
belt. After transferring the toner image onto the record paper, charges on 
the surface of the transfer belt are uniformly removed by a charge remover 
which is composed of a high A.C. supply source and a corona charger and 
then the transfer belt stands by for next transferring step. That is, the 
transfer belt is subjected to a charge removing process before every 
charging to decrease the potential of the transfer belt surface to a given 
surface potential, after which the transfer belt is again subjected to a 
charging process to always make the potential of the transfer belt a given 
certain surface potential. 
In the known belt transferring device, the record paper is carried under 
the state of holding it on the transfer belt and a toner image is 
transferred onto the record paper with the use of charges on the transfer 
belt, so that the carrying and the transferring for the record paper may 
be performed with high reliability. 
In order to record an image having uniform image density on the record 
paper, it is always necessary to hold the charging potential of the 
transfer belt at a certain potential. Therefore, in the conventional belt 
transferring device, an A.C. charge remover is arranged between the 
transferring position and the charging position of the transfer belt, and 
then once the potential of the transfer belt surface is decreased to a 
given potential by the A.C. charge remover, the transfer belt surface is 
again charged to a certain potential. In order to perform A.C. charge 
removing, it is necessary to provide a high A.C. supply source and a 
corona charger, so that manufacturing cost becomes high and the 
construction becomes complicated. 
Also, a copying machine and a printing apparatus of duplex type have been 
put into pratical use which are capable of recording image on both a 
surface and a back surface of the record paper. In this image forming 
device of duplex type, toner image transferred on one surface of the 
record paper is fixed with a heat fixing device such as heat roll fixing 
device, and then the record paper is turned upside down so as to transfer 
a toner image on the back surface of the record paper. When the record 
paper passes through the heat fixing device, however, the percentage of 
moisture content of the record paper becomes decreased substantially, so 
that the electrostatic capacity becomes remarkably small. Therefore, in an 
image forming device using the conventional transferring device, when the 
image recording is performed on the back surface of the record paper, an 
electric field caused between a photosensitive drum and the record paper 
at transferring and for effecting the transfer becomes decreased 
substantially so that the transferring efficiency becomes decreased 
greatly. Thus the image density also becomes decreased, thereby causing a 
decrease of image quality, or a generation of edging phenomenon. 
The record paper has large hygroscopic property so that the percentage of 
moisture content of the record paper is largely changed under the ambient 
environment. 
More particularly, the specific resistance of the record paper becomes 
greatly decreased under high humidity, and thus the transferring 
efficiency becomes decreased, resulting in a decrease of image density. 
The image recording is also performed on record papers having various 
different thicknesses. In this case the distance between the transfer belt 
surface and the photosensitive drum surface is changed in accordance with 
the thickness of the record paper to be used so that field intensity for 
attracting the toner image to the record paper is changed depending on 
whether thick record paper or thin record paper is used, thereby 
decreasing the transfer efficiency in case of recording the image on the 
thick record paper, resulting in a decrease of image density. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the above described 
disadvantages of the conventional transferring device. 
It is another object of the present invention to provide a transferring 
device having simple construction and capable of always holding the 
transfer potential at a certain potential without utilizing high A.C. 
supply source, as well as capable of always obtaining desired transfer 
efficiency even though the properties of record paper to be used, such as 
the percentage of moisture content, the specific resistance and thickness 
or the like of record paper is changed. 
It is a further object of the present invention to provide a recording 
image having high image density and sharpness without causing a decrease 
image quality, or a generation of edging phenomenon. 
According to the present invention, there is provided a transferring device 
comprising a transfer belt with a dielectric layer formed on a conduction 
belt member; a driving means for running the transfer belt; a transfer 
roller for contacting the transfer belt to an image forming member with 
pressure; a corona charger arranged opposite to the transfer belt for 
charging the transfer belt with a charge having a polarity opposite to 
charging polarity of toner; a grid electrode arranged between the corona 
charger and the transfer belt; a plurality of bias sources connected to 
the grid electrode for biasing the grid electrode at respective 
potentials; and a change-over circuit for changing over the connection 
between the grid electrode and the bias source, whereby the grid 
potentials are selectively changed over in accordance with the 
characteristics of a record paper to be used. 
According to the present invention, there is also provided an image forming 
apparatus of a duplex type capable of first performing an image recording 
on one recording surface of the record paper and then an image recording 
on the other recording surface of the record paper, the apparatus 
comprising an image forming member; a transfer belt arranged at a location 
opposite to the image forming member and having a dielectric layer formed 
on a conductive belt member; a driving means for running the transfer belt 
at the same speed as the peripheral speed of the image forming member; a 
transfer roller for contacting the transfer belt to the image forming 
member with pressure; a corona charger arranged opposite to the transfer 
belt for charging the transfer belt with charge having a polarity opposite 
to charging polarity of toner; a grid electrode arranged between the 
corona charger and the transfer belt; first and second bias sources 
connected to the grid electrode for biasing the grid electrode at given 
potentials; and a change-over circuit for changing over the connection 
between the grid electrode and the bias source; whereby the change-over 
circuit connects the first bias source to the grid electrode in case of 
recording an image on the one recording surface of the record paper, and 
connects the second bias source having a biasing voltage larger than that 
of the first bias source to the grid electrode in case of recording an 
image on the other recording surface of the record paper. 
In the belt transferring device for transferring the toner image formed on 
the photosensitive member onto the record paper with the use of charges on 
the transfer belt, the amount of charges on the transfer belt is limited, 
so that the effective transfer field formed between the surface of the 
record paper and the surface of the photosensitive member is largely 
changed by means of electrostatic capacity of the record paper lying 
between the transfer belt and the surface of the photosensitive member. 
That is if the percentage of moisture content of the record paper becomes 
decreased or the thickness of the record paper becomes increased, the 
voltage assigned to the record paper becomes increased and thus the 
effective transferring field becomes decreased. In this case, if the 
surface potential of the transfer belt is always set high, the 
deterioration of the transfer belt is promoted due to electrical stress. 
According to the present invention, therefore, the surface potential of 
the transfer belt is selectively set in accordance with the property of 
the record paper to be used, such as the percentage of moisture content, 
the thickness or the like of the record paper. If high surface potential 
is set for the transfer belt, the effective transferring field formed 
between the surface of the photosensitive member and the surface of the 
record paper becomes also increased in accordance with the increase of 
surface potential so that a substantially large effective transferring 
field may be obtained even though the percentage of moisture content of 
the record paper becomes decreased and the thickness thereof becomes 
increased, thereby providing a sharp image with high image density. If 
common image recording is performed on the record paper, the surface 
potential of the transfer belt is set comparatively low. In this manner, a 
sharp image may always be reproduced and the lifetime of the transfer belt 
may be more prolonged. To this end, according to the present invention, a 
mesh-like grid electrode is provided between the corona charger and the 
transfer belt, a plurality of bias sources are connected to the grid 
electrode and the grid potentials are changed-over in accordance with the 
property of record paper to be used, so that the potential of transfer 
belt may be set at the transfer potential in accordance with the property 
of the record paper to be used by means of change-over of the grid 
potential. Even though the percentage of moisture content and the 
thickness of the record paper are changed, the surface potential of the 
transfer belt may also be changed-over, thereby reproducing sharp image 
with high image density on the record paper. 
When the present invention is applied to the image forming apparatus of 
duplex type for perferming the image recording on both surfaces of the 
record paper, the surface potantial of the transfer belt is automatically 
set higher in case of recording or copying images on the back surface of 
the record paper, so that images having good image density may be 
reproduced on both surfaces of the record paper.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENT 
Now to the drawings, there is shown one embodiment of an image forming 
apparatus according to the present invention. In this embodiment, a 
printing apparatus of duplex type is described. In this embodiment, as an 
image forming member, a photosensitive drum 1 is used which comprises a 
photosensitive layer formed on an outer perpheral surface of a base drum 
composed of aluminum (Al). The outer peripheral surface of the 
photosensitive drum 1 is uniformly charged by a corona charger 2 with 
positive charges over the whole surface thereof, after which auxiliary 
illuminating light from an auxiliary light source 3 and illuminating light 
from a printing head 4 are irradiated on the drum 1 to form a latent 
image. The auxiliary light source 3 irradiates uniformly the whole surface 
of the photosensitive drum 1. The printing head 4 is constituted by an LED 
array formed by arranging for example a plurality of light emitting 
elements in a straight line over the width of the image to be recorded. 
This LED array 4 is irradiated according to the image signal supplied from 
the image forming device and a very small spot of irradiated light focused 
by a SELFOC (trade name) lens is illuminated onto the photosensitive drum 
1. The latent image formed by the illuminated light from the auxiliary 
light source 3 and the LED array 4, is developed by a developing device 5, 
to form a toner image. In this case, the positive charged toner is used to 
effect reverse development. A record paper 7 is supplied from feeding 
cassette 6a or 6b in synchronization with the photosensitive drum 1. The 
record paper 7 is taken out one by one from a cut sheet feeder by means of 
feeding rollers, carried by carrying rollers, and carried to a 
transferring position subject to proper timings for synchronization with 
rotation of the photosensitive drum 1 by a pair of registering rollers 8a 
and 8b. A transfer belt 9 is arranged so as to face the photosensitive 
drum 1. The transfer belt 9 is composed of a conductive plastic sheet 
having an insulating dielectric layer thereon and is stretched between 
four rollers 10a, 10b, 10c and 10d with a capability of running. This 
transfer belt 9 is run at the same speed as the running speed and in the 
same direction as the running direction of the photosensitive drum 1 and 
is charged with a polarity opposite to the charging polarity of toner by 
means of the corona charger 11. The record paper 7 carried therein in 
synchronization with the rotation of the photosensitive drum 1 is 
contacted to the transfer belt 9 through a pair of rollers 10b and 10e, 
electrostatically attracted thereto and carried to a transfer position in 
accordance with the transfer belt. At this transfer position, a transfer 
roller 12 composed of an elastic and conductive rubber roller is arranged 
at the rear side of the transfer belt. The transfer belt 9 and the record 
paper 7 are lightly depressed onto the surface of the photosensitive drum 
1 with the use of elastic force. When the record paper 7 is contacted onto 
the surface of the photosensitive drum, the electric field formed by 
charges on the surface of the transfer belt transfers the toner particles 
present on the surface of the photosensitive drum onto the record paper. 
After transferring of the toner particles the record paper 7 is carried by 
electrostatic attraction onto the surface of the transfer belt and 
separated at the position of the roller 10c and then entered into a heat 
roll fixing device 13, in which the toner particles are fixed onto the 
record paper. After the fixing of the toner particles, the record paper 7 
is delivered to a delivery stacker 15 face down by carrier rollers 13a, 
13b and delivery rollers 14a, 14b. If images are to be recorded on the 
back surface of the record paper, the record paper 7 is carried upwards by 
a paper passing route changing mechanism (not shown) after passing through 
the fixing device 13 and delivered to an intermediate tray 16 through a 
carrying route positioned the upper side of the apparatus by means of few 
pairs of carrying rollers. In this case the copied surface of the record 
paper on which images are recorded is now located on the upper side. The 
record paper 7 is then carried through a passing route 17 by means of a 
switching back mechanism (not shown) in accordance with a back surface 
copying instruction from the printer, and once again carried to the 
transfer position under the state of holding it to the transfer belt after 
taking account of the synchronous timing by the registering rollers 8a, 
8b. After transfer on the back surface, the record paper 7 is delivered to 
a delivering cassette 15 through the fixing device 13. After the transfer 
process, the charges on the photosensitive drum 1 are removed uniformly by 
a charge removing device 18 such as a lamp so that the surface potential 
of the photosensitive drum is decreased to the residual potential. The 
residual toner particles on the photosensitive drum 1, are then removed by 
means of a brush cleaning device 19, and then the photosensitive drum 1, 
is again charged by the corona charger 2 over the whole surface thereof so 
as to form next latent image. 
FIG. 2 shows a detailed construction of the transferring device according 
to the present invention. The transfer belt 9 comprises a conduction belt 
member formed by inpregnating a conductive material such as carbon with 
plastic material. The conductive belt member has an outer peripheral 
surface which is coated by a dielectric layer to form an endless belt. The 
transfer belt 9 has the dielectric layer which is positioned to face the 
photosensitive drum 1. The transfer belt 9 is charged over its surface 
with the surface potential of, for example, 1100 V by means of the 
charging device 11 which comprises a corona charger 21 having a corona 
wire connected to a shield casing 20 and a high D.C. supply source (not 
shown) and a mesh-shaped grid electrode 23 attached to the shield casing 
20 through an insulative supporting member 22. The grid electrode 23 is 
connected to a Zener diode 24 serving as a bias source. When corona ions 
having a polarity opposite to the charging polarity of toner are released 
from the corona charger 20, a part of said ions arrive at the surface of 
the transfer belt 9 after passing through openings of the mesh-shaped grid 
electrode 23, and another part of the corona ions are captured by the grid 
electrode 23 and thus the charges thereof flow to the ground through the 
Zener diode 24. As a result of this, the grid electrode 23 is in the self 
biased state and thus the grid electrode 23 is biased at breakdown voltage 
of the Zener diode 24 due to constant-voltage thereof. If the grid 
electrode 23 is arranged near the surface of the transfer belt 9, the 
transfer belt 9 is charged at the voltage equal to the grid voltage, since 
the Zener diode 24 serves as a bias source of the grid electrode. Even 
though, for example, the transfer belt 9 passes by the front surface of 
the charging device 11, or after passing through the transferring position 
the surface potential of the transfer belt is charged or the surface 
potential is changed partly, the surface of the transfer belt is charged 
at the set potential only after passing through the charging device. 
Moreover, the surface potential of the transfer belt may be controlled 
adequately by controlling the bias voltage of the grid electrode. The 
transfer belt which is charged at a given surface potential, for example 
1100 V is run to the position of the pair of rollers 10b, 10e and then the 
record paper 7 is electrostatically attracted by these rollers. That is, 
the record paper 7 is entered between the roller 10b and the back-up 
roller 10e and, the depressed force effected between the rollers 10b and 
10e makes the record paper and the transfer belt closely contacted, 
thereby utilizing the electrostatic attraction advantageously. In this 
case when the back-up roller 10e is in the electrically floating state the 
induction of charges on the record paper becomes insufficient. In the 
present embodiment, therefore, the back-up roller 10e is composed of 
conductive elastic rubber and is connected to a grounded point through a 
resistor R. In this manner, when the back-up roller 10e is constituted by 
a conductive elastic rubber roller and is connected to the grounded point, 
charges may simultaneously be injected into the record paper from the 
back-up roller, at the same time the record paper is contacted closely to 
the transfer belt, so that the record paper may be electrostatically 
attracted to the transfer belt effectively. The record paper 7 
electrostatically attracted to the transfer belt 9 is carried under the 
state of holding it to the transfer belt, and the toner image on the 
photosensitive drum is transferred on the record paper at the position 
opposite to the photosensitive drum 1 by means of an electric field due to 
the charges on the transfer belt, and then the record paper 7 is separated 
from the transfer belt at the position of the roller 10e. After separation 
of the record paper, the transfer belt 9 is run at the cleaning position 
through the roller 10d. In this embodiment, toner and dust adhered to the 
surface of the transfer belt, are removed by a brush roller 25. This brush 
roller 25 comprises conductive elastic fibers which are composed of rayon 
or regenerated cellulosic fibers and nylon fibers impregated with 
conductive materials. These conductive elastic fibers are studded onto the 
outer periphery surface of the brush roller 25. This brush roller 25 is so 
arranged as to contact with the transfer belt 9, and is rotatably and 
journally supported by an insulative bearing which is connected to a drive 
motor (not shown), so as to rotate in a direction opposite to the running 
direction of the transfer belt. Moreover, the brush roller 25 is connected 
through a flange to a bias source V.sub.b of about -300 V having a 
polarity opposite to the charging polarity of toner. A metal recovery or 
collection roller 26 is arranged so as to contact the brush roller 25 at a 
position opposite thereto. This recovery roller 26 is rotatably and 
journally supported through the insulative bearing and connected to a 
negative bias source V.sub.c. This bias source V.sub.c has the same 
polarity as that of a bias source V.sub.b for the brush roller and is set 
to a voltage larger than that of the bias voltage V.sub.b , that is, about 
-600 V. The recovery or collection roller 26 rotates in the same direction 
as that of the brush roller 25 and has a pheripheral speed v.sub.c which 
is set larger than the peripheral speed v.sub.b of the brush roller 25, 
that is v.sub.c &gt;v.sub.b. The collection roller 26 is provided with a 
scraper 27 made of urethane rubber or thin metal arranged so as to touch 
roller 26, thereby scraping off the toners or the like adhered onto the 
outer peripheral surface of the collection roller 26. Toners and dust 
adhered onto the surface of the transfer belt are mechanically swept away 
therefrom by the brush roller 25 to clean the surface of the transfer 
belt. In this case the brush roller 25 is biased at about -300 V, so that 
the toners or dust thus scraped off are attracted to the brush roller 25. 
Toners or dust thus attracted thereto are carried to the position opposite 
to the collection roller 26 and are attracted electrostatically to the 
collection roller 26 by the bias voltage applied thereto, so that these 
toners and dust are scraped off by the scraper 27 and deposited on the 
bottom surface of a housing 28. 
FIG. 3 is a circuit diagram showing the construction of one embodiment of a 
biasing circuit for the grid electrode. As described above, in the image 
forming apparatus of duplex type, after the record paper is passed through 
the heat roll fixing device, the image is recorded on the back surface of 
the record paper, so that the percentage of moisture content of the record 
paper is substantially decreased for recording the image on the back 
surface of the record paper. The electrostatic capacity thereof becomes 
decreased, resulting in an insufficient field intensity for attracting the 
toner particles to back surface of the record paper. According to the 
present invention, the surface potential of the transfer belt may be 
changed from the case of recording the image on the front surface (first 
recording surface) of the record paper to the case of recording the image 
on the back surface (second recording surface) of the record paper. 
Specifically, the surface potential is made larger for the case of 
recording the image on the back surface of the record paper in comparison 
to the case of recording the image on the surface thereof. That is, the 
surface potential of the transfer belt may be set at -1100 V in the case 
of recording the image on the first recording side of the record paper and 
set at -1400 V in the case of recording the image on the second recording 
side of the record paper. To this end, as shown in FIG. 3, first Zener 
diode Z.sub.1 having breakdown voltage of -1200 V and second Zener diode 
Z.sub.2 having breakdown voltage of -300 V are connected in series to the 
grid electrode 23, and the cathode electrode of the second Zener diode 
Z.sub.2 is grounded. The junction point of the two Zener diodes Z.sub.1 
and Z.sub.2 is connected to a source electrode of a field effect 
transistor Q.sub.1, and a drain electrode thereof is grounded. A gate 
electrode of the transistor Q.sub.1 is connected to one end of a parallel 
combination of a resistor R and a photo diode of a light coupling element 
P.sub.t. The other end of the parallel combination is connected to a 
source electrode of the transistor Q.sub.1. The anode electrode of a light 
emitting diode of the light coupling element P.sub.t is connected to a 
reference potential V.sub.cc through the resistor R.sub.1, and the cathode 
electrode of the light emitting diode is connected to a drive element 
Q.sub.2 for the light emitting diode. In the case of recording the image 
on the first recording side of the record paper, the light coupling 
element P.sub.t is driven to render the transistor Q.sub.1 conductive. 
When the transistor Q.sub.1 is in the conducting state, the current flows 
in the grid electrode 23 through the first Zener diode Z.sub.1 and the 
field effect transistor Q.sub.1, so that the grid electrode 23 is biased 
at the breakdown voltage of the first Zener diode Z.sub.1, that is, at 
-1200 V, and thus is always set at the surface potential of -1100 V. In 
the case of recording the image on the second recording side of the record 
paper, the light coupling element is held in an OFF state, so that the 
field effect transistor Q.sub.1 becomes non-conductive and thus the 
current flows in the grid electrode 23 flow in the ground through two 
Zener diodes Z.sub.1 and Z.sub.2. Therefore, the grid electrode 23 is 
biased at -1500 V and thus the surface of the transfer belt is charged at 
-1400 V. In this manner, if the image is recorded on the second recording 
surface side of the record paper, when the surface potential of the 
transfer belt is set higher in the negative direction than the case of 
recording the image on the first recording surface side thereof the field 
strength for attracting toner particles on the photosensitive drum 1 to 
the record paper becomes larger in accordance with the increase of the 
surface potential even though the electrostatic capacity of the record 
paper becomes small, resulting in an increase of transfer efficiency. 
FIG. 4 is a circuit diagram showing a modification of the bias circuit. In 
this embodiment, a series combination of three Zener diodes Z.sub.1 
.about.Z.sub.3 is connected to the grid electrode 23 to form four kinds of 
bias sources. First change-over circuit 30 is connected in parallel to the 
Zener diode Z.sub.2 and second change-over circuit 31 is connected in 
parallel to the Zener diode Z.sub.3. The detaching of the Zener diode 
Z.sub.2 from and to the series combination is made by the first 
change-over circuit 30 and the detaching of the Zener diode Z.sub.3 from 
and to the series combination is made by the second change-over circuit 
31. These change-over circuit 30 and 31 are the same as that shown in FIG. 
3, so that the explanation thereof is omitted. In this manner, provided 
that, for example, the breakdown voltages of the Zener diodes Z.sub.1 
.about.Z.sub.3 are -1000 V, -200 V and -400 V, respectively, four kinds of 
bias sources of -1000 V, -1200 V, -1400 V, and -1600 V respectively, may 
be formed, four kinds of grid voltages may be changed-over, in accordance 
with the properties of the record paper, for example, the thickness and 
the specific resistance thereof. That is, when record papers having thin, 
intermediate, thick and high density of large electrostatic capacity are 
utilized, the transferring is performed at low grid voltage in case of 
rcording the image on the thin record paper with respect to the record 
paper having intermediate thickness, and the transferring is performed at 
higher grid voltage in the negative direction in the case of thick and 
high density record papers. As a result, the surface potential of the 
transfer belt may be set at the most suitable value in accordance with the 
thickness and the electrostatic capacity of the record paper, so that the 
highest voltage is not always applied to the transfer belt, and thus the 
lifetime thereof may be prolonged. The percentage of moisture content of 
the record paper is changed largely under condition of environment, so 
that use is made of a humidity sensor, and thus the grid voltage may be 
changed in step-wise manner according to the output of the sensor. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiment is therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.