Image bearing belt and image forming apparatus using same

The present invention provides an image bearing belt wherein a toner image formed on an electrophotographic photosensitive member is temporarily transferred to the image bearing belt and the toner image transferred to the image bearing member is used in a system for transferring the toner image onto a transfer material. It comprises a rubber layer having a thickness of 0.5 mm or more, and a high resistive layer having at a transfer position where the toner image is transferred in an image forming apparatus, a thickness of 100 .mu.m or less and having an average net resistance value greater than that of the rubber layer at that transfer position by ten times or more, thereby forming a good multi toner image.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus such as a 
copying machine having an intermediate transfer belt or a printer having 
an intermediate transfer belt, in which an image is formed by 
electrostatically transferring an image formed on an image bearing belt 
onto a transfer material. 
2. Related Background Art 
In conventional color image forming apparatuses, various systems such as 
electrophotographic systems, heat-transfer systems, ink jet systems or the 
like have been utilized. Among them, image forming apparatuses having the 
electrophotographic system are superior to other image forming apparatuses 
from a view point of high speed operation, high image quality and 
silentness and have recently been used widely. 
In such electrophotographic image forming apparatuses, there have been used 
various methods such as a multi-developing method in which, after color 
images are superimposed on a surface of a photosensitive member, the 
images are collectively transferred for image formation, a multi-transfer 
method in which a development/transfer cycle is repeated, or an 
intermediate transfer method in which, after various color developed 
images were once transferred onto an intermediate transfer member 
successively, the images are collectively transferred onto a transfer 
material. Among them, the intermediate transfer method has been noticed in 
the points that there is no color mixing between developing devices and 
that it can be applied to various media. 
The intermediate transfer member may be of roller type or of belt type. An 
intermediate transfer belt is superior to an intermediate transfer roller 
in the points that it has greater flexibility than the intermediate 
transfer roller and that separation ability between the transfer material 
and the belt (after second transferring) is excellent due to the fact that 
a curvature of the belt can be increased at a second transfer position 
where the developed images are collectively transferred onto the transfer 
material. 
In general, the intermediate transfer belt is formed from a resin film made 
of PVdF, nylon, PET or polycarbonate and having a thickness of 100 to 200 
.mu.m and volume resistivity of about 10.sup.11 to 10.sup.16 .OMEGA.m. By 
using such a thin resin film, since great electrostatic capacity of the 
order of several hundreds to several thousands of pF can be obtained at a 
transfer nip, stable transfer current can be achieved. 
However, when the intermediate transfer belt having the thickness of 200 
.mu.m or less is repeatedly flexed by support rollers during rotation, 
wrinkles are formed on the surface of the belt, thereby causing the image 
uneven. Further, since the belt may be torn through the wrinkles, a 
service life of the belt is decreased. In addition, since the resin film 
cannot be extended, if instantaneous great tension is applied to the belt, 
the belt cannot absorb such a great force, with the result that the belt 
will be torn. The image forming apparatus is frequently stopped 
instantaneously due to sheet jam treatment, or inadvertent door open 
caused by an operator's erroneous operation. In such a case, the 
intermediate transfer belt may be torn. 
Further, if the thickness of the resin film is increased to improve the 
service life of the belt, the belt cannot follow the driving roller and/or 
a driven roller to make the rotation of the belt unstable, with the result 
that misalignment of registration occurs, thereby worsening the image 
quality of the color image. In addition, since a friction force is small, 
slip is easily generated, thereby making the drive unstable. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a new intermediate 
transfer member which can eliminate the drawbacks of the conventional 
intermediate transfer members made of resin film. 
Another object of the present invention is to provide an image forming 
apparatus using such a new intermediate transfer member. 
The other object of the present invention is to provide an intermediate 
transfer member onto which toner images can effectively be transferred in 
a superimposed fashion and wherein the toner images can effectively be 
transferred onto a transfer material, and an image forming apparatus which 
can output a color toner image with high quality. 
To achieve the above object, according to the present invention, there is 
provided an image bearing belt wherein toner images formed on an 
electrophotographic photosensitive member are temporarily transferred to 
the image bearing belt and the toner images transferred to the image 
bearing member are used in a system in which the toner images are 
transferred onto a transfer material. It comprises a rubber layer having a 
thickness of 0.5 mm or more and a high resistive layer having a thickness 
of 100 .mu.m or less and an average net resistance value at a transfer 
position (where the toner images are transferred in an image forming 
apparatus) greater than that of the rubber layer at that transfer position 
by ten times or more. 
Further, according to the present invention, there is provided an image 
forming apparatus wherein toner images formed on an electrophotographic 
photosensitive member are firstly transferred to an image bearing belt 
temporarily and then the toner images transferred to the image bearing 
member are transferred onto a transfer material (second transferring). It 
comprises an electrophotographic photosensitive member movable along an 
endless path, a toner image forming means for forming a toner image on the 
photosensitive member, a belt-shaped image bearing member onto which the 
toner images formed on the photosensitive member are transferred at a 
first transfer position and including a rubber layer having a thickness of 
0.5 mm or more, and a high resistive layer having a thickness of 100 .mu.m 
or less and an average net resistance value at the first transfer position 
(where the toner images are transferred in an image forming apparatus) 
greater than that of the rubber layer at the first transfer position by 
ten times or more and a transfer means for transferring the toner images 
formed on the belt-shaped image bearing member onto the transfer material 
at a second transfer position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
(First Embodiment) 
FIG. 1 shows a color image forming apparatus using an intermediate transfer 
belt according to the present invention. 
Around a photosensitive drum (image bearing member) 1, there are disposed 
various color developing devices adjacent to each other. These developing 
devices include a black developing device 5, a magenta developing device 
6, a cyan developing device 7 and an yellow developing device 8. A desired 
developing device to be used for development is selected by a means (not 
shown) to be contacted with the photosensitive drum. The photosensitive 
drum 1 is rotated in an anti-clockwise direction. During this rotation, 
the photosensitive drum is uniformly charged by a first charger 2, and 
then, latent images are formed on the photosensitive drum with scanning 
light 4 from a laser exposure optical system 3. 
Then, the latent images are developed by the developing devices 5, 6, 7 and 
8, and toner images formed on the photosensitive drum 1 are successively 
transferred onto an intermediate transfer belt (image bearing belt) 91 at 
a first transfer position by means of a first transfer roller 10. The 
above-mentioned process is effected successively with respect to the 
developing devices 5 to 8. When four color toner images are transferred to 
the intermediate transfer belt 91 (rotated in a clockwise direction) in a 
superimposed fashion, a transfer material 18 is urged against the transfer 
belt by a second transfer roller 111, with the result that the toner 
images are collectively transferred onto the transfer material 18 (second 
transferring). 
The first and second transferring processes will be fully described. 
First of all, if the photosensitive drum is constituted by an OHP 
photosensitive body for effecting the charging with negative polarity, in 
the illustrated embodiment in which inverse development is effected, when 
bright portions generated by the exposure of the laser light 4 are 
developed by the developing devices 5 to 8, toner having negative polarity 
is used. Thus, in order to transfer the toner images formed on the 
photosensitive drum onto the intermediate transfer belt, transfer bias 
having positive polarity is applied to the first transfer roller 10. As 
the first transfer roller 10, a low resistive roller having volume 
resistivity of 10.sup.5 .OMEGA..multidot.cm or less is used. 
Then, in a second transfer position, an opposed roller 121 is opposed to 
the second transfer roller 111 and is used as a counter electrode which 
acts as a support and an electrode and which is earthed or to which 
appropriate bias is applied. In this case, the second transfer roller 111 
to which bias having positive polarity is applied from a bias power source 
21 is urged against the counter roller with the inter position of the 
transfer material 18. 
After the above-mentioned processes were finished, the toner remaining on 
the intermediate transfer belt 91 after the second transferring is removed 
by a cleaner 13, and, thereafter, electricity is removed from the 
intermediate transfer belt 91 by means of an electricity removal charger 
(AC corona charger) 14. In this case, an electrode 16 may be disposed at a 
back side of the intermediate transfer belt 91 in order to improve 
electricity removing efficiency. 
Incidentally, after the first transferring process, the toner remaining on 
the photosensitive drum 1 is removed by a cleaner 19 and electricity is 
removed from the drum by electricity removal exposure 17, thereby 
preparing for next image formation. In FIG. 1, the reference numeral 16 
denotes a tension roller also acting as the electrode; and 15 denotes a 
drive roller for the intermediate transfer belt. 
Next, the intermediate transfer belt 91 according to the illustrated 
embodiment will be fully explained. 
In the illustrated embodiment, in consideration of strength and driving 
stability, the intermediate transfer belt 91 is formed from a rubber base 
material 912 having a thickness of 0.8 mm, in place of the conventional 
resin film. 
By the way, since it is difficult to control resistance value of a rubber 
belt having a thickness of 100 .mu.m or more during the manufacture of the 
belt, it is not preferable that such a rubber belt is used as an 
intermediate transfer member in which a high quality image is tried to be 
formed by superimposing the toner images. If the belt having uneven 
resistance is used as the intermediate transfer belt, when the transfer 
bias is applied, current (referred to as "transfer current" hereinafter) 
flowing through the intermediate transfer belt is not stabilized, thereby 
making the image uneven. 
To avoid this, although constant-current control of the second transfer 
power source 21 can be performed, but, in this case, since the same 
current cannot be used for various transfer materials having different 
thicknesses, features and/or widths, it is practically impossible to adopt 
the constant-current control. Further, even when the first and second 
transfer rollers 10, 111 and the opposed roller 121 (at the second 
transfer position) are formed from rubber, foam urethane or the like, it 
is difficult to make resistance values of these rollers uniform, with the 
result that the transfer current becomes unstable by the fluctuation of 
the resistance values, thereby worsening the image quality of the 
transferred image. 
FIG. 2 is a model view of the second transfer position according to the 
illustrated embodiment. 
The intermediate transfer belt 91 is constituted by the rubber base 
material 912 having a thickness of 0.8 mm and made of millable urethane, 
and a surface layer 911 coated on the base material and having a thickness 
of 20 .mu.m and obtained by dispersing iron oxide filler into soluble 
fluoro-material. The coating is effected by a spraying technique, and, 
after coating, the surface layer is polished by a wrapping film. 
The second transfer roller 111 is constituted by a metal core having an 
outer diameter of 6 mm and an outer layer made of foam urethane and coated 
on the metal core and having a thickness of 5 mm, and the opposed roller 
121 is constituted by a metal core having an outer diameter of 20 mm and 
an outer layer made of foam urethane and coated on the metal core and 
having a thickness of 5 mm. The resistance value of the foam urethane used 
for the outer layers is adjusted to have a desired value by dispersing 
resistance adjusting agent such as carbon into the foam urethane. 
The surface layer 911 of the intermediate transfer belt 91 according to the 
illustrated embodiment is formed from material having volume resistivity 
of 2.5.times.10.sup.9 .OMEGA..multidot.cm, and an average net resistance 
value R1 at the second transfer position is 5.0.times.10.sup.7 .OMEGA.. 
Further, the rubber base material 912 of the belt 91 is formed from 
material having volume resistivity of 7.0.times.10.sup.6 
.OMEGA..multidot.cm, and an average net resistance value R2 at the second 
transfer position is 5.0.times.10.sup.5 .OMEGA.. The second transfer 
roller 111 is formed from material having volume resistivity of 
2.0.times.10.sup.5 .OMEGA..multidot.cm, and an average net resistance 
value A at the second transfer position is 8.9.times.10.sup.4 .OMEGA.. 
Further, the opposed roller 121 is formed from material having volume 
resistivity of 5.0.times.10.sup.5 .OMEGA..multidot.cm, and an average net 
resistance value C at the second transfer position is 2.3.times.10.sup.5 
.OMEGA.. 
The net resistance value means a net resistance value value of each member 
at a nip generated at the second transfer position. These net resistance 
values were measured by a method shown in FIG. 11, which will be described 
hereinbelow. 
First of all, only the rubber base material 912a is mounted on a drive 
roller 40 and a driven roller 41 (which are electrically floating) in a 
belt fashion as shown in FIG. 11, and the rubber base material 912a is 
rotated at a speed of 100 mm/sec substantially the same as a rotational 
speed of the intermediate transfer belt 91 in the apparatus of FIG. 1. The 
rubber base material 912a is pinched between a metal roller 43 having a 
diameter of 46.7 mm and earthed via an ampere meter 44 and a metal roller 
42 having a diameter of 14 mm and to which voltage of 1 kv is applied, and 
the net resistance value of the rubber base material 912a is obtained by 
reading a value of the ampere meter 44. 
Such measurements of the net resistance value are effected at ten points 
along a shifting direction of the rubber base material 912a and the 
measured values are averaged to determine the average resistance value of 
the rubber base material 912. 
Then, the measurement of the net resistance value of the belt 91 having the 
rubber base material 912a and the surface layer 911 is similarly performed 
to determine the average resistance value of the belt. The average 
resistance value of the surface layer 911 is obtained by subtracting the 
average resistance value of the belt 91 from the average resistance value 
of the rubber base material 912. Further, the metal rollers 42, 43 are 
replaced by the second transfer roller 111 and the opposed roller 121, 
respectively, and, then, by effecting the similar measurements, the 
average resistance values of the second transfer roller 111 and the 
opposed roller 121 are determined. 
In the illustrated embodiment, the rubber belt is used as the intermediate 
transfer belt 91. An advantage of the rubber belt is that, since the 
rubber belt has elasticity, any wrinkles are not created on the belt 
during the rotation of the belt. Further, when the thickness of the belt 
is 0.5 mm or more, if instantaneous great tension is applied to the belt, 
the tension is absorbed by the elasticity of the rubber, thereby 
preventing the tearing of the belt. 
Further, when the thickness of the belt is 3 mm or less, the belt can 
follow the drive roller 15 so that the rotation of the belt is stabilized, 
thereby preventing the deterioration of the image quality due to 
out-of-synchronization for causing the erroneous reproduction of the 
superimposed images. In addition, it was found that, by providing 
fluoro-material having good mold releasing ability on the surface of the 
intermediate transfer belt 91, the cleaning ability for removing the toner 
remaining on the belt after the second transferring process can be 
improved. 
Further, although the net resistance of the rubber base layer 912 is 
changed from 1.5.times.10.sup.5 .OMEGA. to 2.7.times.10.sup.6 .OMEGA., the 
net resistance of the second transfer roller 111 is changed from 
7.5.times.10.sup.4 .OMEGA. to 8.8.times.10.sup.5 and the net resistance of 
the opposed roller 121 is changed from 6.5.times.10.sup.4 .OMEGA. to 
8.9.times.10.sup.5 .OMEGA., in the illustrated embodiment, the apparatus 
is not influenced by the dispersion of such resistance values and the 
stable transfer current can be obtained to effect the uniform 
transferring, thereby obtaining the good image. 
Next, a developing mechanism will be explained. 
FIG. 3 shows an example of an equivalent circuit for the second transfer 
position. In FIG. 3, a symbol A denotes the average net resistance of the 
second transfer roller 111; C denotes the average net resistance of the 
opposed roller 121; R1 denotes the average net resistance of the surface 
layer 911; R2 denotes the average net resistance of the rubber base layer 
912; and B denotes a value of (R1+R2). A symbol Vt denotes the transfer 
bias. 
A total resistance value of this circuit is (A+B+C), and the transfer 
current It flowing through the circuit is as follows: 
EQU It=Vt/(A+B+C) 
since C is sufficiently small in comparison with B, the following relation 
can be obtained: 
EQU B&gt;&gt;A+C 
Thus, 
EQU (A+B+C).perspectiveto.B 
Accordingly, the transfer current can be represented as follows: 
EQU It.perspectiveto.Vt/B 
That is to say, when the net resistance values of the second transfer 
roller 111 and of the opposed roller 121 are smaller than the net 
resistance value of the intermediate transfer belt 91, the transfer 
current It is determined by the net resistance value of the intermediate 
transfer belt 91. 
FIG. 4 shows an equivalent circuit for the second transfer position, 
obtained in consideration of the above relations. In FIG. 4, a symbol R1 
denotes the average net resistance of the surface layer 911; and R2 
denotes the average net resistance of the rubber base layer 912. 
Although It.perspectiveto.(R1+R2)/Vt, since R2 is sufficiently smaller than 
R1 (i.e., R1&gt;&gt;R2), the following relation can be established: 
EQU R1+R2.perspectiveto.R1 
Thus, the transfer current can be represented as follows: 
EQU It.perspectiveto.R1/Vt 
Accordingly, the transfer current It is determined by the average net 
resistance of the surface layer 911. 
By the way, the net resistance of the surface layer 911 is adjusted to a 
desired value by dispersing the filler into the fluoro-material as the 
base material. In this method, by using filler having good dispersing 
ability and by agitating the filler sufficiently, the evenness of the 
resistance of the surface layer becomes greatly superior to that of the 
rubber. 
Further, in the illustrated embodiment, since the thickness of the surface 
layer is thin (100 .mu.m or less), even when the rubber is used as the 
base material for the surface layer 911, it is possible to maintain the 
evenness of the resistance. Accordingly, the average net resistance value 
of the surface layer 911 at the second transfer position is selected in 
such a manner that it becomes greater than the average net resistance 
value of the rubber base layer 912 at the second transfer position by ten 
times or more so that the transfer current It is governed by the average 
net resistance value of the surface layer 911. 
Similarly, the average net resistance value of the surface layer 911 at the 
second transfer position is selected in such a manner that it becomes 
greater than the average net resistance value of the opposed roller 121 at 
the second transfer position by ten times or more so that the average net 
resistance value of the surface layer 911 at the second transfer position 
becomes greater than the average net resistance value of the second 
transfer roller 111 at the second transfer position by ten times or more. 
In this way, since the transfer current It is determined by the surface 
layer 911 having the uniform resistance, the transfer current It is 
stabilized, with the result that the transferring is also stabilized 
without causing any toner scattering, thereby obtaining the uniform image. 
Now, in consideration of productivity of material, capacity of a power 
source of the apparatus and the like, it is preferable that the average 
net resistance value of the surface layer 911 is smaller than those of the 
rubber base layer 912, second transfer roller 111 and opposed roller 121 
by 1/1000 time or less and practically has 10.sup.7 to 10.sup.9 .OMEGA.. 
Further, in consideration of a service life and bending endurance of the 
belt, it was found that the thickness of the surface layer 911 is 
preferably 5 .mu.m or more and 100 .mu.m or less. 
Further, in consideration of evenness of the image, prevention of slip of 
the transfer material at the second transfer position and the like, it was 
found that average roughness of a central surface of the surface layer 911 
(JIS B 0601) is 0.1 to 1.5 .mu.m. The filler for the surface layer 911 is 
not limited to the iron oxide material as described in the illustrated 
embodiment, but may be titanium oxide material, fluoro-material, carbon 
black, graphite, nylon or the like. The base material into which the 
filler is dispersed may be urethane and the like, as well as the 
above-mentioned fluoro-material. 
(Second Embodiment) 
FIG. 5 shows an image forming apparatus according to a second embodiment of 
the present invention, and FIG. 6 is a model view showing a second 
transfer position of the apparatus of the second embodiment. In the 
following explanation, the same or similar constructural elements as those 
of the first embodiment are designated by the same reference numerals and 
explanation thereof will be omitted. 
An intermediate transfer belt 92 according to the second embodiment has a 
rubber base layer 922 made of urethane having a thickness of 0.7 mm, 
volume resistivity of 2.0.times.10.sup.7 .OMEGA..multidot.cm and an 
average net resistance value of 1.2.times.10.sup.6 .OMEGA. at a second 
transfer position, and a surface layer 921 obtained by dispersing carbon 
into thermo-plastic fluoro-material. Further, the surface layer 921 is 
made of material having volume resistivity of about 1.0.times.10.sup.9 
.OMEGA..multidot.cm and has an average resistance value of 
5.3.times.10.sup.7 .OMEGA.. A thickness of the surface layer 921 is 50 
.mu.m so that, as explained in the first embodiment, dispersion of 
resistance of the surface layer 921 is small. 
Next, manufacturing processes for the belt 92 will now be explained. 
As shown in FIGS. 7A to 7C, rubber base material 922a is entered into a 
centrifugal forming device 32, so that the rubber base material is formed 
to have a thickness of 0.7 mm (step 1). Then, while remaining the rubber 
base material 922a in the centrifugal forming device 32, material for the 
surface layer 921 is entered into the centrifugal forming device and is 
treated, thereby forming the surface layer 921 on the rubber base layer 
922 (step 2). Lastly, the belt 92 is removed from the centrifugal forming 
device 32 and is turned up (step 3). 
An opposed roller 122 is constituted by a shaft made of SUS and having a 
diameter of 30 mm. A second transfer roller 112 is constituted by a metal 
core having a diameter of 6 mm and a foam urethane layer (having volume 
resistivity of 1.4.times.10.sup.5 .OMEGA..multidot.cm) coated on the metal 
core. An average net resistance value of the second transfer roller at a 
second transfer position is 5.0.times.10.sup.4 .OMEGA.. 
Also in this second embodiment, since a thickness of the intermediate 
transfer belt 92 is included within a range from 0.5 mm to 3.0 mm and the 
average net resistance value of the surface layer 921 is selected to be 
greater than the average net resistance values of the rubber base layer 
922 and of the second transfer roller 112 by ten times or more, the 
service life of the belt is increased and the good image transferring 
could be achieved without influence of the unevenness of resistance. 
Further, in a method used in the second embodiment, since air acts on the 
surface of the belt during the centrifugal formation, roughness of the 
surface is greatly reduced, thereby further improving the evenness of the 
image. 
(Third Embodiment) 
FIG. 8 shows an image forming apparatus according to a third embodiment of 
the present invention, and FIG. 9 is a model view showing a second 
transfer position of the apparatus of the third embodiment. In the 
following explanation, the same or similar constructural elements as those 
of the first embodiment are designated by the same reference numerals and 
explanation thereof will be omitted. 
An intermediate transfer belt 93 according to the third embodiment has a 
rubber base layer 932 made of NBR rubber having a thickness of 0.8 mm, 
volume resistivity of 3.5.times.10.sup.7 .OMEGA..multidot.cm and an 
average net resistance value of 2.2.times.10.sup.6 .OMEGA. at a second 
transfer position, and a surface layer 931 formed from a heat-shrinkable 
tube having a thickness of 30 .mu.m, volume resistivity of 
3.5.times.10.sup.10 .OMEGA..multidot.cm and an average net resistance 
value of 1.0.times.10.sup.8 .OMEGA.. 
Next, manufacturing processes for the belt 93 will now be explained. 
As shown in FIGS. 10A to 10D, rubber base material 932a is wound around a 
cylindrical mold 33. An outer diameter of the mold 33 is equal to an inner 
diameter of the rubber base material 932a (step 1). Then, a 
heat-shrinkable tube is wound around on the mold 33 with the interposition 
of the rubber base material 932a (step 2). Then, hot air is blown onto the 
mold 33 to shrink or contract the rube, thereby forming the surface layer 
931 on the rubber base material 932 (step 3). Lastly, the belt is removed 
from the mold 33 (step 4). 
An opposed roller 123 is constituted by a shaft made of SUS and having a 
diameter of 30 mm. A second transfer roller 113 is constituted by a metal 
core having a diameter of 6 mm and a foam urethane layer (having volume 
resistivity of 1.4.times.10.sup.5 .OMEGA..multidot.cm) coated on the metal 
core. An average net resistance value of the second transfer roller at a 
second transfer position is 5.0.times.10.sup.4 .OMEGA.. 
Also in this third embodiment, since a thickness of the intermediate 
transfer belt 93 is included within a range from 0.5 mm to 3.0 mm and the 
average net resistance value of the surface layer 931 is selected to be 
greater than the average net resistance values of the rubber base layer 
932 and of the second transfer roller 113 by ten times or more, the 
service life of the belt is increased and the good image transferring 
could be achieved without influence of the unevenness of resistance. 
Further, the belt 93 manufactured by this method is characterized that an 
anti-wear feature of the surface layer of the belt is superior to that of 
the surface layer of the belt manufactured in accordance with the first 
embodiment (coated by the spraying technique). 
As mentioned above, according to the present invention, since the image 
bearing belt includes the rubber layer having the thickness of 0.5 mm or 
more, the service life of the belt can be improved. Further, the image 
bearing belt includes the high resistive layer having the average net 
resistance value (at the transfer position) greater than that of the 
rubber layer by ten times or more and the thickness of 100 .mu.m or less, 
even if there is substantial dispersion of net resistance value in the 
rubber layer, the good transferring can be achieved.