Image transferring method using an intermediate transfer body and image forming apparatus for practicing the same

In an image forming apparatus, a potential deposited on the rear of a transfer body is selected to be zero or of the same polarity as the charge of an image carrier at least at a part of a nip formed for image transfer. In this condition, image transfer conditions allowing a minimum of toner scattering to occur at the time of image transfer are set up against, e.g., a change in the resistance of the transfer body ascribable to aging. Therefore, an image with a minimum of toner scattering is achievable.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image transferring method using an 
intermediate transfer body, and an image forming apparatus for practicing 
the same. More particularly, the present invention is concerned with an 
image transferring method of the kind transferring a toner image from a 
photoconductive element or similar image carrier to a sheet or similar 
recording medium by way of an intermediate transfer body, an image 
transferring method of the kind transferring a toner image from a 
photoconductive element or similar image carrier to a sheet or similar 
recording medium by use of a belt capable of conveying the sheet, and a 
copier, printer, facsimile apparatus or similar image forming apparatus 
for practicing either one of the two methods. 
2. Discussion of the Background 
It is a common practice with an electrophotographic image forming 
apparatus, particularly a full-color image forming apparatus, to transfer 
a toner image from a photoconductive element to a sheet by two consecutive 
steps, i.e., a primary transfer step and a secondary transfer step. In the 
primary transfer step, consecutive toner images of different colors each 
are transferred from the photoconductive element to an intermediate 
transfer body implemented as a belt by way of example. In the secondary 
transfer step, the toner images transferred to the transfer body one above 
the other are collectively transferred to a sheet. For the primary 
transfer, an electric field is formed by a bias applied to one or both of 
two rollers over which the transfer body is passed. The two rollers are 
positioned at both sides of the photoconductive element. Alternatively, 
the two rollers may be connected to ground, in which case a bias will be 
applied to a contact member located at the center of a nip between the 
photoconductive element and the transfer body. The intermediate transfer 
body is often formed of a material having a medium volume resistivity 
(10.sup.8 .OMEGA.cm to 10.sup.13 .OMEGA.cm) or a medium surface 
resistivity (10.sup.7 .OMEGA. to 10.sup.12 .OMEGA.). With this kind of 
intermediate transfer body, it is possible to discharge a transfer charge 
applied from a charge applying means at the time of image transfer without 
resorting to a corona discharger or similar discharging means, or to 
reduce a required discharge output even when such discharging means is 
used. 
However, the problem with the image forming apparatus of the type effecting 
the primary and secondary image transfer is that it is apt to blur the 
resulting image due to toner scattered around at the two image transfer 
steps. This kind of toner scattering varies with a transfer voltage and a 
transfer current. 
Generally, the transfer current, transfer voltage and other transfer 
conditions are initially set before the shipment of the apparatus in such 
a manner as to minimize the above toner scattering while implementing the 
maximum toner transfer efficiency. However, the range of transfer 
conditions realizing both a high transfer efficiency and the satisfactory 
reduction of toner scattering is narrow. This, coupled with the fact that 
the optimal transfer conditions depend on the varying environmental 
conditions and the varying characteristics of the photoconductive element 
and intermediate transfer body, make it difficult to noticeably reduce the 
toner scattering. Specifically, when environmental conditions including 
temperature and humidity vary, the amount of charge to deposit on toner 
and the resistance of the transfer body also vary. Therefore, constant 
transfer conditions would lower the transfer efficiency or would bring 
about the toner scattering. Particularly, when the resistance of the 
transfer body decreases, the transfer voltage relatively exceeds its 
optimal value and aggravates the toner scattering due to, e.g., discharge 
occurring at an image transfer position. 
To cope with the varying environmental conditions, it has been customary to 
provide the apparatus with a temperature sensor and a humidity sensor. 
Transfer conditions experimentally determined beforehand are selectively 
set up on the basis of the outputs of the above sensors, thereby 
compensating for a change in environment. On the other hand, a medium 
resistance material consisting of a resin and carbon black or similar 
conductive filler dispersed in the resin tends to lower its resistance 
with the elapse of time. As for an intermediate transfer body formed of 
such a medium resistance material, deterioration ascribable to aging is 
compensated for by the rough experiential estimation of the tendency of 
deterioration and varying the transfer conditions in accordance with the 
estimated tendency. 
Japanese Patent Laid-Open Publication No. 4-45470 discloses an image 
forming apparatus of the type using a conveyor belt for image transfer and 
obviating pretransfer by causing a sheet and a photoconductive element to 
start contacting each other at a position upstream of an image transfer 
region. Japanese Patent Laid-Open Publication No. 4-186387 teaches an 
image forming apparatus of the type including a transfer drum and 
eliminating pretransfer by locating means for shielding an electric field 
at a position upstream of electric field forming means. 
However, the above conventional image forming apparatuses each executes 
correction on the basis of experimental data or experiential data. Such 
apparatuses therefore cannot readily cope with operating conditions 
particular to the individual user or execute adequate correction. 
When the intermediate transfer body or the transfer body for conveying a 
sheet is formed of a medium resistance material, the toner scattering at 
the time of image transfer is particularly noticeable. Specifically, when 
the intermediate transfer body is formed of a medium resistance material, 
the transfer charge applied from the charge applying means is capable of 
migrating even to the portions of the transfer body outside of the nip 
over which the image carrier and transfer body contact each other. As a 
result, a potential gradient and therefore an electric field is formed 
even on the surface of the intermediate transfer body outside of the nip. 
Particularly, an electric field formed at the inlet of the nip acts on the 
toner image carried on the image carrier at a position upstream of the nip 
in the direction of movement of the intermediate transfer body. As a 
result, the toner image is partly transferred from the image carrier to 
the intermediate transfer body before it reaches the nip (pretransfer), 
resulting in the fall of image quality. Further, in some kind of image 
forming apparatus, the undesirable electric field is formed at a position 
downstream of the nip and disturbs the toner image having been desirably 
transferred to the intermediate transfer body. This also brings about the 
toner scattering, irregular image density, local omission and other 
various kinds of defects. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an image 
forming apparatus capable of preserving transfer conditions causative of a 
minimum of toner scattering against, e.g., a change in the resistance of a 
transfer body ascribable to aging, and thereby insuring an image with a 
minimum of toner scattering at all times. 
It is another object of the present invention to provide an image forming 
apparatus capable of reducing toner scattering at the time of image 
transfer from an image carrier to an intermediate transfer body or from an 
image carrier to a sheet carried on a conveyor belt, thereby insuring 
desirable images. 
It is still another object of the present invention to provide an image 
forming apparatus capable of setting up optimal transfer conditions based 
on a potential deposited on the rear of a transfer body or a current to 
flow to the rear of the same. 
It is yet another object of the present invention to provide an image 
transferring method capable of reducing an undesirable electric field 
between an image carrier and an intermediate transfer body, and an image 
forming apparatus for practicing the same. 
In accordance with the present invention, a method of transferring a toner 
image from an image carrier to a transfer body contacting the image 
carrier or to a recording medium supported by the transfer body forms an 
electric field for image transfer by an electrical manipulation at a 
contact position where the image carrier and transfer body contact each 
other. A reducing manipulation is executed for reducing the electric field 
such that at at least a part of the contact position a potential deposited 
on the transfer body is zero or of the same polarity as a charge deposited 
on the image carrier. 
Also, in accordance with the present invention, an image forming apparatus 
includes an image carrier for forming a toner image thereon by being 
charged. A transfer body is held in contact with the image carrier at a 
contact position for transferring the toner image to a recording medium by 
an electric field for image transfer formed at the contact position. A 
reducing electrode causes, at at least a part of the contact position, a 
potential deposited on the transfer member to be zero or of the same 
polarity as a charge deposited on the image carrier. 
Further, in accordance with the present invention, an image forming 
apparatus includes an image carrier for forming a toner image thereon by 
being charged. A transfer body is held in contact with the image carrier 
at a contact position for transferring the toner image to a recording 
medium by an electric field for image transfer formed at the contact 
position. A reducing electrode is connected to ground for reducing the 
transfer electric field. A current Inip to flow from the reducing 
electrode to ground is selected to be smaller than zero inclusive when the 
image carrier is chargeable to the negative polarity or greater than zero 
inclusive when the image carrier is chargeable to the positive polarity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To better understand the present invention, brief reference will be made to 
a conventional image forming apparatus of the type concerned, particularly 
the scattering of toner to occur at the time of image transfer. 
As shown in FIG. 1, the conventional image forming apparatus, generally 10, 
includes an image carrier in the form of a photoconductive drum 12. An 
intermediate image transfer body having a medium resistance is implemented 
as a belt 14 and held in contact with the drum 12. A bias roller 16 for 
image transfer and playing the role of charge applying means is located 
downstream of the nip N between the drum 12 and the belt 14 in the 
direction in which the belt 14 moves. A bias of, e.g., 800 V (absolute 
value) is applied to the bias roller 16. A ground roller 18 is positioned 
upstream of the nip N in the direction of movement of the belt 14. The 
ground roller 18 is connected to ground, but it is a specific form of an 
electrode which is connected to ground or applied with a preselected bias. 
Because the belt 14 has a medium resistance, a potential gradient 24 
(indicated by hatching) occurs on the belt 14 and extends from the 
downstream side toward the upstream side of the nip N in the direction of 
movement of the belt 14. The potential gradient 24 is 300 V (absolute 
value) at the inlet 20 of the nip N and 600 V (absolute value) at the 
outlet 22 of the nip N. As a result, an electric field for image transfer 
is formed at the nip N. In FIG. 1, the gradient 24 is represented by a 
straight line extending from a charge applying position to a discharging 
position. In practice, however, because the gradient contacts the drum 12 
at the nip N, the inclination of the straight line changes at the nip N or 
the straight line is partly replaced with a curve of secondary degree or 
similar nonlinear gradient. 
In another specific arrangement, a corona discharger, a roller, brush or 
blade for image transfer or similar charge applying means is located at 
the nip N. An electrode connected to ground or applied with a bias is 
located upstream of the nip N in the direction of movement of the belt 14. 
With this arrangement, it is also possible to generate, based on the 
medium resistance of the belt 14, the potential gradient 24 on the belt 
14. The gradient 24 extends from a charge applying position in the nip N 
toward the upstream side. 
However, the problem with the image forming device 10 is that the charge 
applied by the bias roller 16 can migrate even to the portions of the belt 
14 outside of the nip N because of the medium resistance of the belt 14. 
As a result, an electric field is formed even in the above portions of the 
belt 14, lowering the quality of the resulting toner image. Particularly, 
the electric field formed at the inlet 20 of the nip N acts on a toner 
image formed on the drum 12 at a position 26 preceding the nip N and 
different from the expected image transfer position. This causes a part of 
the toner to be transferred from the drum 12 to the belt 14 at the 
position 26 and thereby causes the toner to be scattered around. 
Consequently, characters, lines or similar images are blurred or otherwise 
lowered in image quality. FIG. 2A shows a specific image 28 formed on the 
drum 12 while FIG. 2B shows a blurred image 28a transferred from the drum 
12 to the belt 14. 
Preferred embodiments of the present invention will be described with 
reference to the accompanying drawings hereinafter. 
1st Embodiment 
Referring to FIG. 3, an electrophotographic image forming apparatus 
embodying the present invention is shown and generally designated by the 
reference numeral 30. Briefly, the apparatus 30 has a single 
photoconductive element or image carrier and, for example, four developing 
units facing the photoconductive element and each being A assigned to a 
particular color. Toner images of different colors and sequentially formed 
on the photoconductive element are sequentially transferred to an 
intermediate image transfer belt one above the other. The resulting 
composite toner image is collectively transferred to a sheet or similar 
recording medium. As a result, a color image is formed on the sheet. 
As shown in FIG. 3, the photoconductive element is implemented as a drum 
32. The drum 32 is made up of a hollow core formed of aluminum and a 
function separated photoconductive layer formed on the core, although not 
shown specifically. The photoconductive layer is made up of a base layer, 
a charge generating layer, and a charge conveying layer, not shown. The 
photoconductive layer is about 28 .mu.m thick and has a capacity of about 
90 pF/cm.sup.2. During image formation, the drum 32 is rotated by a drive 
source, not shown, in the direction indicated by an arrow in FIG. 3. A 
charger 34 is implemented by a scorotron charger and uniformly charges the 
surface of the drum 32 to about -650 V to -700 V. A laser beam 36 scans 
the charged surface of the drum 32 in accordance with image data, 
electrostatically forming a latent image of -100 V to -500 V. Such a 
procedure is repeated to sequentially form latent images corresponding to 
four different colors, e.g., black (BK), cyan (C), magenta (M), and yellow 
(Y). 
A potential sensor 38 senses the charge potential of the drum 32 and the 
potential of the exposed portions of the drum 32. A controller, not shown, 
controls the charging condition and exposing condition on the basis of the 
output of the potential sensor 38. Developing units 40BK, 40C, 40M and 40Y 
constitute a developing section, and each stores toner of a particular 
color. The developing units 40BK-40Y each develops the latent image of 
associated color formed on the drum 32 so as to produce a toner image. 
Specifically, the developing units 40BK-40Y each store a dry 
two-ingredient type developer, i.e., toner and carrier mixture and 
deposits toner of negative polarity on the low potential portions of the 
drum 32. These type of developing units are generally referred to as 
reversal type developing units. 
A bias power source for development, not shown, applies a bias voltage of 
about -500 V to -550 V to each of the developing units 40BK-40Y. If 
desired, an AC component may be superposed on the bias. A sensor 42 senses 
the amount of toner deposited on the drum 32. The sensor 42 is implemented 
as a photosensor capable of sensing the amount of toner deposition on the 
basis of the optical reflectance of the drum 32. The controller controls 
process conditions in response to the output of the sensor 42. 
The toner images formed on the drum 32 are sequentially transferred to an 
endless intermediate transfer belt 44. Let the transfer of the toner image 
from the drum 32 to the intermediate transfer belt 44 be referred to as 
belt transfer for simplicity. The belt 44 is passed over a drive roller 
46, a driven roller 48, a roller 50 facing a belt cleaning unit 66, an 
inlet roller 52, and an outlet roller 54. A drive source, not shown, 
causes the belt 44 to rotate via the drive roller 46. A moving mechanism, 
not shown, selectively moves the part of the belt 44 between the inlet 
roller 52 and the outlet roller 54 into or out of contact with the drum 
32. When the belt 44 and drum 32 contact each other, they form a nip N for 
image transfer therebetween. 
In the illustrative embodiment, the part of the belt 44 between the inlet 
roller 52 and the outlet roller 54 is 36 mm long while the belt 44 is 350 
mm in its lengthwise direction. The belt 44 is implemented as a single 
medium resistance layer consisting of a fluorine-contained resin and 
carbon black dispersed in the resin. In the embodiment, the belt 44 is 
about 150 .mu.m thick and has, when it is new, a surface resistivity of 
about 5.times.10.sup.9 .OMEGA./cm.sup.2 and a volume resistivity of about 
1.times.10.sup.11 .OMEGA.cm. The volume resistivity (pv) was measured for 
10 seconds by using a measuring unit Hiresta IP (MCP-HT260) (trade name) 
available from Mitsubishi Petrochemical, a probe HRS Robe (trade name), 
and bias voltages of 100 V (pv) and 500 (ps). If desired, the volume 
resistivity may be measured by a method prescribed by JIS (Japanese 
Industrial Standards) K6911. 
The surface resistivity was measured by use of Hiresta IP (trade name) 
available from Yuka Denshi although us may be made of a method prescribed 
by JIS K6911. 
The belt 44 may be formed of polycarbonate or a similar resin. In the 
illustrative embodiment, the inlet roller 52 is formed of a conductive 
material and connected to ground while the outlet roller 54 is connected 
to a transfer bias power source, not shown, for image transfer. The 
transfer bias power source applies a positive voltage Vt to the outlet 
roller 54. That is, an indirect transfer voltage applying system is used. 
Power source control means, not shown, controls the voltage Vt to be 
applied from the transfer bias power source to the outlet roller 54. 
A precleaning discharger 56 controls the charge of the toner remaining on 
the drum 32 after the belt transfer. A cleaning brush 58 and a cleaning 
blade 60 constituting a drum cleaning device remove such residual toner 
whose charge has been controlled by the precleaning discharger 56. 
Further, a discharge lamp 62 dissipates the charge remaining on the drum 
32. The charger or charging means 34, exposing section or exposing means, 
developing units or developing means 40BK-40Y, belt or transfer body 44, 
and transfer bias power source constitute toner image forming means in 
combination. 
To form a toner image of first color (BK), the drum 32 is uniformly charged 
by the charger 34 and then exposed by the exposing section. The resulting 
BK latent image is developed by the developing unit 40BK and then 
transferred to the belt 44. As a result, a BK toner image is formed on the 
belt 44. The toner left on the drum 32 after the image transfer is removed 
by the precleaning discharger 56, cleaning brush 58, and cleaning blade 
60. Subsequently, the charge left on the drum 32 is dissipated by the 
discharge lamp 62. 
A procedure for forming a toner image of second color (C) is identical with 
the above procedure up to the step of developing a latent image formed on 
the drum 32. The resulting C toner image is transferred from the drum 32 
to the belt 44 over the BK toner image existing on the belt 44. 
Thereafter, the toner and charge remaining on the drum 32 are removed by 
the precleaning charger 56, cleaning brush 58 and cleaning blade 60 and 
the discharge lamp 62, respectively. 
A procedure for forming a toner image of third color (M) is also identical 
with the above procedure up to the step of developing a latent image 
formed on the drum 32. The resulting M toner image is transferred from the 
drum 32 to the belt 44 over the BK and C toner images held in register. 
Thereafter, the toner and charge remaining on the drum 32 are removed in 
the same manner as described above. 
A procedure for forming a toner image of fourth color (Y) is also identical 
with the above procedure up to the step of developing a latent image 
formed on the drum 32. The resulting Y toner image is transferred from the 
drum 32 to the belt 44 over the BK, C and M toner images held in register, 
completing a full-color image. Thereafter, the drum 32 is cleaned by the 
precleaning charger 56, cleaning brush 58, cleaning blade 60, and 
discharge lamp 62. The voltage Vt to be applied from the transfer bias 
power source to the outlet roller 54 may be sequentially increased every 
time a toner image is transferred from the drum 32 to the belt 44. 
A sheet S is fed from a sheet feed section to between the belt 44 and a 
roller 64 such that its leading edge meets the leading edge of the 
full-color image carried on the belt 44. The roller 64 is pressed against 
the drive roller 46 with the intermediary of the belt 44, forming a nip 
between the roller 64 and the belt 44. A bias power source, not shown, 
applies a positive transfer voltage to the roller 54. This transfer 
voltage is applied the sheet S between the roller 64 and the belt 44 from 
the rear of the sheet S. As a result, the full-color image is transferred 
from the belt 44 to the sheet S. Let the image transfer from the belt 44 
to the sheet S be referred to as sheet transfer. In this sense, the roller 
64 will be referred to as a sheet transfer roller 64. The full-color image 
on the sheet S is fixed by a fixing unit, not shown. The belt cleaning 
unit 66 mentioned earlier removes the toner remaining on the belt 44 after 
the sheet transfer. 
With an intermediate transfer body implemented by the belt 44, it is 
possible to reduce the overall size of the apparatus 30 because process 
units around the belt 44 can be laid out with greater freedom. However, 
the advantages of the illustrative embodiment are also achievable with an 
intermediate transfer body in the form of a drum or a roller. 
In the illustrative embodiment, the charging condition, the resistance of 
the belt or intermediate transfer body 44, and the output of the transfer 
bias power source and so forth are selected such that the potential Vnip 
on the rear of the belt (not contacting the drum 32), as measured in at 
least a part of the nip N, is zero or of the same polarity as the charge 
deposited on the drum 32. Why the potential on the rear of the belt 44 is 
measured is as follows. Originally, the potential on the front of the belt 
44 should preferably be described as the charge potential of the belt 44. 
In practice, however, the potential on the front of the belt 44 
(contacting the drum 32) cannot be directly measured at the nip N. 
Hereinafter will be described a relation between the potential on the rear 
of the belt 44 and the potential on the front of the belt 44, as measured 
at the nip N, with reference to FIG. 4. 
As shown in FIG. 4, assuming the resistance of the belt 44 and the transfer 
bias voltage stated earlier, the front of the belt 44 facing the drum 32 
is charged to negative several ten volts in the vicinity of the nip N and 
charged to minus several hundred volts in the vicinity of the transfer 
bias roller 54. This is because the difference in the distance between the 
roller 54 and the front of the belt 44 and the distance between the roller 
54 and the rear of the belt 44 decreases with an increase in distance from 
the roller 54. In the vicinity of the nip N, the front of the belt 44 is 
charged more to the negative side than the rear of the same due to the 
negative charge of the drum 32. Therefore, if the potential on the rear of 
the belt 44 is zero or of negative polarity, a negative potential is 
surely deposited on the front of the belt 44. It follows that the electric 
field causative of the scattering of toner at the inlet of the nip N can 
be reduced. Presumably, under conditions actually implementing image 
transfer, the above relation between the front and the rear of the belt 44 
holds even when the resistance of the belt 44 and the transfer bias are 
varied. There are also shown in FIG. 4 a conductive brush 70 and a 
transfer bias power source 72. 
The range in which the potential Vnip on the rear of the belt 44 is zero or 
of the same polarity as the charge of the drum 32 must be varied in 
accordance with the width .iota. of the nip N and other mechanical 
conditions and the transfer characteristic of toner itself. In this case, 
a prerequisite is that an electric field be reduced at a position 
preceding the nip N in order to obviate a blurred image. Another 
prerequisite is that the effective nip width .iota. be as long as possible 
in order to prevent the transfer ratio from being lowered. Assume that the 
drum 32 and belt 44 start contacting each other at a position O shown in 
FIG. 4 and start leaving each other at a position L also shown in FIG. 4. 
Then, to meet the above prerequisites, the charging condition, the 
resistance of the belt 44, the output of the transfer bias power source 
and so forth should be optimally selected such that the potential Vnip on 
the rear of the belt 44 is zero or of the same polarity as the charge of 
the drum 32 at a position X lying in a range of 0.ltoreq.X.ltoreq.L/2 at 
the nip N. While this range meets the above prerequisites, the 
illustrative embodiment is practical even when the position X does not lie 
in such a range due to, e.g., the charging condition of the drum 32 and 
the developing condition. 
Optimal image transfer conditions will be described hereinafter. In this 
embodiment, the distance between the inlet roller 52 and the position O 
was selected to be 8 mm. The width .iota. of the nip N was selected to be 
20 mm. The distance between the position L and the outlet roller 54 was 
selected to be 8 mm. As shown in FIG. 3, the potential sensor 68 is 
located at the rear of the belt 44 at the nip N. The potential sensor 68 
measures the potential Vnip on the rear of the belt 44 over a range of 
about 4 mm whose center is positioned 7 mm remote from the position O. The 
sensor 68 therefore measures the mean value of the rear potentials Vnip of 
the belt 44 lying in the range of 5 mm&lt;X&lt;9 mm. 
FIG. 5 is a graph showing a relation between the transfer voltage Vt 
applied from the transfer bias power source to the outlet roller 54 and 
the rear potential of the belt 44. Specifically, the rear potential of the 
belt 44 was measured while the charged portion of the drum 32 was passed 
through the transfer position without being exposed (identical with the 
image of a white sheet). The rear potential of the belt 44 varies due to 
irregularity in the resistance of the belt 44. In light of this, the rear 
potential of the belt 44 was measured over one full turn of the belt 44, 
and a mean value of the measured potentials was determined. The 
measurement showed that the rear potential Vnip of the belt 44 is zero or 
of the same polarity as the charge of the drum 32 when the transfer 
voltage Vt is lower than 800 V inclusive. With this embodiment, therefore, 
it is possible to implement transfer conditions causing a minimum of toner 
scattering to occur when the transfer voltage Vt is lower than 800 V 
inclusive, while preventing the transfer efficiency from being lowered. 
FIG. 6 shows how the toner scattering and the transfer efficiency vary with 
respect to the transfer voltage Vt. As shown, the toner scattering was 
ranked by observing a toner image transferred to the belt 44 (about 0.3 mm 
wide line image) in an enlarged scale. Rank 5 and rank 1 are respectively 
representative of the smallest scattering and the greatest scattering, 
respectively. The transfer efficiency was determined in terms of the 
weights of toner measured before and after the transfer of a solid toner 
image by a suction scheme. 
As shown in FIG. 6, although the scattering tends to increase with an 
increase in transfer voltage Vt, it lies in rank 4 or above when the 
voltage Vt is lower than 800 V inclusive; ranks 4 and 5 are acceptable in 
practical use. Further, although the transfer efficiency decreases with a 
decrease in transfer voltage Vt, a transfer efficiency of 90% or above is 
achieved if the voltage Vt is about 500 V or above. Therefore, when the 
transfer conditions were selected such that the rear potential Vnip of the 
belt 44 was smaller than zero inclusive, the scattering of toner was 
successfully reduced. More preferably, when the transfer conditions were 
so selected as to set up a relation of -100 V.ltoreq.Vnip.ltoreq.0, not 
only the reduction of toner scattering but also a sufficient transfer 
efficiency were achieved. 
While the drum 32 included in the embodiment is chargeable to the negative 
polarity, use may be made of a drum chargeable to the positive polarity. 
If the drum 32 is chargeable to the positive polarity, then the toner will 
be charged to the positive polarity, and a negative transfer bias will be 
applied. In such a case, the transfer conditions will be selected such 
that the potential Vnip on the rear of the belt 44 is greater than zero 
inclusive at the position X lying in the range of 0.ltoreq.X.ltoreq.L/2. 
This also successfully reduces the toner scattering. In the above 
embodiment, the output of the transfer bias power source is controlled by 
the power source control means in order to vary the rear potential Vnip of 
the belt 44. Alternatively, the output of the charger 34 or the resistance 
of the belt 44 may be controlled for the same purpose. 
A first comparative example relating to this embodiment is as follows. The 
comparative example differs from the embodiment in that the inlet roller 
52 and outlet roller 54 are rearranged in order to vary the width .iota. 
of the nip N. In the comparative example, the distance between the inlet 
roller 52 and the contact start position O was selected to be 12 mm, the 
width .iota. was selected to be 10 mm, and the distance between the leave 
start position and the outlet roller 54 was selected to be 14 mm. The 
measuring range of the potential sensor 68 is about 4 mm. The sensor 68 
therefore determines the mean value of the rear potentials Vnip of the 
belt 44 over the range of 1 mm&lt;X.ltoreq.5 mm. 
FIG. 7 shows a relation between the transfer voltage Vt applied to the 
outlet roller 54 and the rear potential Vnip of the belt 44 and particular 
to the first comparative example. As shown, the rear potential Vnip is 
smaller than zero inclusive only when the transfer voltage Vt is zero. 
FIG. 8 shows a relation between the transfer voltage Vt and the toner 
scatter level and transfer efficiency and also particular to the 
comparative example. As shown, a range wherein the toner scatter level is 
4 or above and the transfer efficiency is 90% or above was not achievable 
at all. 
2nd Embodiment 
This embodiment is the same as the first comparative example as to the 
width L of the nip N, but different from the latter as to the resistance 
of the belt 44. In the embodiment to be described, the belt 44 had a 
volume resistivity of about 1.times.10.sup.11 .OMEGA.cm when it was new. 
It is to be noted that an image transfer body applicable to this 
embodiment has its resistance range determined by the output and capacity 
of the transfer power source. For example, even when the resistance of the 
transfer body is as low as 1.times.10.sup.7 .OMEGA.cm, the transfer body 
is usable only if a power source capable of causing an intense current to 
flow is used. While a transfer body generally used allows a current of 
about several ten microamperes to flow, even a transfer body having a low 
resistance can be used if the current is increased to several 
microamperes. 
Further, a transfer bias of several kilovolts is generally applied to a 
transfer body. Even a transfer body having a high resistance is usable if 
it is implemented as a single layer and if a transfer bias of about 10 kV 
is applied. Assume that the transfer body has a double layer structure. 
Then, even when the volume resistivity of the entire transfer body in the 
thicknesswise direction is about 1.times.10.sup.13 .OMEGA.cm, the transfer 
body is usable in the general voltage and current range only if the 
surface layer is about 1.times.10.sup.13 .OMEGA.cm and if the base layer 
is 1.times.10.sup.10 .OMEGA.cm. Therefore, the illustrative embodiment is 
practicable with a volume resistivity range of from 1.times.10.sup.7 
.OMEGA.cm to 1.times.10.sup.13 .OMEGA.cm. The volume resistivity range 
available with this embodiment can even be 1.times.10.sup.8 .OMEGA.cm to 
1.times.10.sup.12 .OMEGA.cm in the case of a single layer or up to 
1.times.10.sup.13 .OMEGA.cm in the case of a double layer from, e.g., the 
power source cost standpoint. 
A relation between the transfer voltage applied to the outlet roller 54 and 
the rear potential Vnip of the belt 44 and particular to the second 
embodiment is also shown in FIG. 7. As shown, in this embodiment, the rear 
potential Vnip was smaller than zero inclusive when the transfer voltage 
Vt was lower than 1,600 V inclusive. FIG. 9 shows how the toner scatter 
level and transfer efficiency vary in accordance with the transfer voltage 
Vt. 
In the second embodiment, as in the first embodiment, although the toner 
scattering tends to increase with an increase in transfer voltage Vt, rank 
4 or above acceptable in practice was achieved when the voltage Vt was 
lower than 1,600 V inclusive. Although the transfer efficiency decreases 
with a decrease in transfer voltage Vt, a transfer voltage of 90% or above 
was attained when the voltage Vt was about 1,200 V or above. Therefore, by 
selecting transfer conditions implementing the rear potential Vnip smaller 
than zero inclusive, it was possible to reduce the toner scattering. 
Preferably, a relation of -60 V.ltoreq.Vnip.ltoreq.0 was set up in order 
to reduce the toner scattering and increase the transfer efficiency. 
As stated above, the first and second embodiments each has various 
unprecedented advantages, as enumerated below. 
(1) The image forming apparatus has the toner image forming means including 
the photoconductive drum or movable image carrier 32. The charger 34 and 
exposing section constitute the image forming means for electrostatically 
forming a latent image on the drum 32. The developing units 40BK-40Y play 
the role of developing means for developing the latent image to produce a 
corresponding toner image. The intermediate transfer belt or endless 
transfer body 44 is passed over a plurality of rollers 46, 48, 50, 52 and 
54. The belt 44 contacts the drum 32 between two, 52 and 54, of the 
rollers 46-54, forming the nip N. The toner image is transferred from the 
drum 32 to the belt 44 at the nip N. The bias power source applies a 
charge opposite in polarity to the toner to at least one of the two 
rollers 52 and 54. In this configuration, in at least a part of the nip N, 
the potential on the rear of the belt 44 is selected to be zero or of the 
same polarity as the charge deposited on the drum 32. Therefore, an 
electric field for image transfer in the above part of the nip N is 
weakened, so that the generation of an electric field in a gap preceding 
the nip N is reduced. This successfully obviates the migration of the 
toner at the position preceding the nip N and thereby allows a minimum of 
toner to be scattered at the time of image transfer. 
(2) Assume the position O where the drum 32 and belt 44 start contacting 
each other, and the position L where they start leaving each other. Then, 
the potential on the rear of the belt 44 is selected to be zero or of the 
same polarity as the charge of the drum 32 at any position X of the nip N 
lying in the range of 0.ltoreq.X.ltoreq.LK/2. This reduces the strength of 
an electric field in the vicinity of the inlet of the nip N and thereby 
obviates the migration of the toner at the position preceding the nip N, 
allowing a minimum of toner to be scattered at the time of image transfer. 
(3) The potential sensor or potential measuring means 68 senses the 
potential Vnip on the rear of the belt 44 at the particular position X 
mentioned above. Therefore, optimal image transfer conditions can be set 
up on the basis of the output of the sensor 68, so that the toner 
scattering at the time of image transfer is reduced. 
3rd Embodiment 
This embodiment is similar to the first embodiment except that it 
additionally includes a control unit for effecting the measurement of the 
rear potential Vnip at the time of power up of the apparatus and every 
time the image forming cycle is repeated a preselected number of times. At 
the time of power up, the charging condition and developing condition are 
optimized, and then the transfer voltage Vt to be applied from the 
transfer bias power source to the belt 44 is optimized. 
Specifically, as shown in FIG. 10, at the time of power up, the charging 
condition and developing condition are optimized as a general process 
control. This optimization is conventional and will not be described 
specifically. Subsequently, the voltage Vt to be applied from the transfer 
bias power source to the belt 44 is optimized. At this instant, the drum 
32 being rotated by the drive source is charged to about -650 V by the 
charger 34 and then passed through the developing units 40BK-40Y without 
being exposed. The developing units 40BK-40Y using the reversal 
development system do not operate in the same manner as when they form the 
image of a white sheet. When the charged portion of the drum 32 arrives at 
the belt transfer position, the potential sensor 68 senses the rear 
potential of the belt 44. Thereafter, the cumulative number of sheets 
output after the last setting of the transfer voltage Vt is reset to zero. 
This is followed by a stand-by state. When a preselected number of sheets 
are output after the power up, the transfer voltage Vt is set. This is 
also followed by a stand-by state. 
To obviate the influence of the irregular resistance distribution of the 
belt 44, it is preferable that the sensor 68 senses the rear potential 
Vnip of the belt 44 derived from a single transfer voltage Vt over one 
full turn of the belt 44, and that the mean value of the measured 
potentials Vnip be used as a value for control. Specifically, as shown in 
FIG. 11, after the start of formation of a white sheet image, the transfer 
voltage Vt is applied. In this condition, the rear potential Vnip is 
measured. If the rear potential Vnip is smaller than zero inclusive, it 
may be possible to increase the voltage Vt. Therefore, the voltage Vt is 
increased by one step .DELTA. V to Vt+.DELTA. V, and again the potential 
Vnip is measured. Such a procedure is repeated until the rear potential 
Vnip exceeds zero. Because the potential Vnip exceeding zero is excessive, 
a voltage Vt' occurred one step before, i.e., Vt'=Vt-.DELTA.V is set as an 
optimal transfer voltage. While the transfer voltage Vt shown in FIG. 5 
has an initial value of 0 V and sequentially increases by a step of 200 V, 
the initial value may be selected to be several hundred volts in order to 
reduce the voltage setting time. Further, the interval between the steps 
of the voltage Vt may be reduced to 50 V in order to effect more precise 
control over the voltage Vt. 
In this embodiment, the transfer voltage Vt is controlled such that the 
maximum voltage in the range implementing the rear potential Vnip smaller 
than zero inclusive at the position X is set as an optimal transfer 
voltage. Specifically, the power source control means controls the output 
of the transfer bias power source such that the transfer voltage Vt 
becomes equal to an optimal transfer voltage. When the belt 44 is new, the 
voltage Vt of 800 V is set as an optimal voltage, as stated earlier. 
Again, the range in which the rear potential Vnip is zero or of the same 
polarity as the charge of the drum 32 may not lie in the range of 
0.ltoreq.X.ltoreq.L/2, depending on the transfer conditions. 
Thereafter, when the usual image forming cycle was repeated with 5,000 
sheets without any optimal transfer voltage setting stated above, the 
toner scatter rank fell from initial 4.0 to 3.5 in a halftone area. The 
volume resistivity of the belt 44 was lowered to about 5.times.10.sup.9 
.OMEGA.cm. When the optimal transfer voltage setting was again effected in 
the above-described manner, a characteristic represented by dots in FIG. 5 
was attained; the optimal transfer voltage was determined to be 600 V and 
set. Under this condition, an even image with a minimum of toner 
scattering from its halftone area over to its solid area was produced. 
Every time the image forming cycle is repeated with the preselected number 
of sheets, the control unit executes the rear potential measurement and 
then sets an optimal transfer voltage based on the result of measurement. 
When the volume resistivity of the belt 44 is lower than about 
1.times.10.sup.8 .OMEGA.cm inclusive, the current output from the transfer 
bias power source and flowing through the belt 44 increases. As a result, 
the condition implementing the rear potential Vnip smaller than zero 
inclusive is not attainable. In such a case, the range realizing the 
scatter rank 4 or above and the transfer efficiency of 90% or above did 
not occur. 
As stated above, the third embodiment has the following advantages. 
(1) Assume the position O where the drum 32 and belt 44 start contacting 
each other, and the position L where they start leaving each other. Then, 
the potential sensor or potential sensing means 68 measures the potential 
of the rear of the belt 44 at any position X of the nip N lying in the 
range of 0.ltoreq.X.ltoreq.LK/2. The control unit causes the sensor 68 to 
sense the potential Vnip at the nip N at the time of image transfer. The 
control unit controls the operation of the toner image forming means such 
that the potential Vnip is zero or of the same polarity as the charge of 
the drum 32. Therefore, by measuring the potential Vnip periodically and 
setting up a condition capable of reducing the toner scattering, it is 
possible to maintain transfer conditions causing a minimum of scattering 
to occur against, e.g., a change in the resistance of the belt 44 
ascribable to aging, and therefore to insure images with a minimum of 
toner scattering. 
(2) The means for controlling the operation of the toner image forming 
means is implemented as the power source control means which controls the 
output of the transfer bias power source. Therefore, the transfer 
conditions causing a minimum of toner scattering to occur can be 
maintained against, e.g., a change in the resistance of the belt 44 
ascribable to aging, insuring images with a minimum of toner scattering. 
4th Embodiment 
In a fourth embodiment, the distance between the inlet roller 52 and the 
contact start position O was selected to be 8 mm, the width l of the nip N 
was selected to be 20 mm, and the distance between the leave start 
position L and the outlet roller 54 was selected to be 8 mm, as in the 
first embodiment. In this embodiment, as shown in FIG. 12, a conductive 
brush 70 is located at the rear of the belt 44 at the nip N. The brush 70 
is held in contact with the rear of the belt 44 over a range whose center 
is 7 mm remote from the contact start position O. 
The brush 70 is 340 mm wide in its lengthwise direction and about 4 mm wide 
in the direction of movement of the belt 44. The brush 70 contacts the 
rear of the belt 44 at the position X lying in the range of 5 mm&lt;X&lt;9 mm. 
The position X lies in the range of O.ltoreq.X.ltoreq.L/2 of the nip N. 
The inlet roller 52 is connected to ground by a conductor. A transfer bias 
power source 72 applies a transfer bias to the outlet roller 54. The brush 
70 is implemented by twenty-four carbon-containing 360 denier acrylic 
filaments. The filaments have a resistance of about 1.times.10.sup.7 
.OMEGA.cm. 
As shown in FIG. 13, at the time of production of the apparatus, an ammeter 
74 is connected between the brush 70 and ground in order to set the 
transfer voltage. The ammeter 74 is connected such that its brush 70 side 
and its ground side are of positive polarity and negative polarity, 
respectively. In this condition, while the power source control means 
varies the transfer voltage being applied from the power source 72 to the 
outlet roller 54, the ammeter 74 measures a current Inip flowing from the 
brush 70 to ground. The optimal transfer voltage is determined on the 
basis of the result of measurement, and the transfer voltage is controlled 
to the optimal voltage. 
FIG. 14 shows a relation between the transfer voltage Vt applied to the 
outlet roller 54 and the current flown from the brush 21 to ground and 
determined by the above measurement. For the measurement, the charged 
portion of the drum 32 was passed through the exposure position without 
being exposed (white sheet image). Because the current to flow from the 
brush 70 to ground varies due to the irregular resistance distribution of 
the belt 44, the current to flow from the brush 70 to ground was measured 
over one full turn of the belt 44, and the mean value of such currents was 
produced. The measurement showed that in the range of Vt.ltoreq.800 V the 
current Inip to flow from the brush 70 to ground is smaller than 800 V 
inclusive (a current flows from ground to the brush 70, or electrons flow 
from the brush 70 to ground). With the illustrative embodiment, transfer 
conditions causing a minimum of toner scattering to occur are achievable 
in the range of Vt.ltoreq.800 V. 
The above current Inip will be described by use of experimental data. A 
first belt was formed of carbon-dispersed ETFE (ethylene 
tetrafluoroethylene) and 150 .mu.m thick. The first belt had a surface 
resistivity of 10.sup.9 .OMEGA. to 10.sup.10 .OMEGA., a volume resistivity 
of 10.sup.10 .OMEGA.cm to 10.sup.11 .OMEGA.cm, and a specific inductive 
capacity of 11.+-.3. A second belt was formed of carbon-dispersed 
polycarbonate and 150 .mu.m thick. The second belt had a surface 
resistivity of 10.sup.8 .OMEGA. to 10.sup.9 .OMEGA. and a volume 
resistivity of 10.sup.8 .OMEGA.cm to 10.sup.9 .OMEGA.cm. 
A current to flow through the brush 70 and a potential to deposit on the 
inlet roller 52 at the time of image transfer were measured and compared 
in order to see the aggravation of toner scattering ascribable to the 
decrease in the resistance of the belt 44. FIG. 15 shows currents flown 
through the brush 70. For the measurement, use was made of a nip ground 
type bias application system, type D. In FIG. 15, the ordinate indicates 
the current flown through the brush 70 (nip brush current) while the 
abscissa indicates the transfer bias voltage. 
As shown in FIG. 16, two different current components presumably flow 
through the brush 70, i.e., a forward current I.sub.1 derived from the 
positive transfer bias applied to the outlet roller 54, and a reverse 
current I.sub.2 flowing toward the negative charge deposited on the 
non-image area of the drum 32. The current Inip and the toner scatter 
level vary, depending on the relation between the currents I.sub.1 and 
I.sub.2. As for the first belt, the current I.sub.2 is greater than the 
current I.sub.1 over the transfer bias range of from 0 V to about +800 V, 
so that the current Inip is of negative polarity. However, the current 
I.sub.1 increases when the transfer voltage exceeds +800 V, resulting in 
the current Inip of positive polarity. It is noteworthy that a transfer 
bias which balances the two currents I.sub.1 and I.sub.2 and thereby 
reduces the nip brush current to zero is coincident with an optimal 
transfer bias determined by the other methods. 
When the current Inip is of negative polarity, the negative charge is 
predominant in the portion of the belt 44 around the brush 70 and reduces 
the electric field around the inlet of the nip N. As a result, the toner 
scattering at the time of image transfer is reduced. Conversely, when the 
current Inip is of positive polarity, the positive charge is predominant 
in the above portion of the belt 44 and increases the electric field 
around the inlet of the nip N, aggravating the toner scattering. 
Under the optimal transfer conditions, a current to flow through the first 
belt is 0 .mu.A while a current to flow through the second belt is as 
great as about 20 .mu.m. This is simply ascribable to the low resistance 
of the second belt which increases the current I.sub.1. Further, when the 
transfer bias is 0 V, the current to flow through the second belt 
increases toward the positive side more than the current to flow through 
the first belt. This indicates that the low resistance belt slightly 
aggravates the toner scatter level, compared to the other belt. 
In this embodiment, the toner scattering and transfer efficiency varied in 
the same manner as in the first embodiment (FIG. 6) with respect to the 
transfer voltage Vt. When the power source control means so set the 
transfer voltage as to satisfy the relation of Inip.ltoreq.0 the scatter 
rank of 4.0 or above was achieved. Preferably, when a transfer voltage 
satisfying a relation of -3 .mu.A.ltoreq.Inip.ltoreq.0 was set, both the 
scatter range of 4.0 or above and the transfer efficiency of 90% or above 
were achieved. 
If desired, the conductive brush or conductive member 70 may be replaced 
with a conductive roller. In any case, it is preferable to use a 
conductive brush or a roller of low hardness capable of reducing the 
pressure to act on the belt 44. Should the mechanical pressure to act on 
the belt 44 at the nip N be excessive, defective image transfer, e.g., 
blank characters would occur. When the drum 32 is chargeable to the 
positive polarity, the current Inip to flow from the brush 70 to ground 
should be greater than zero inclusive. 
A second comparative example was identical with the fourth embodiment 
except for the position of the brush 70. In the comparative example, the 
brush 70 was held in contact with the rear of the belt 44 over a range 
whose center was spaced from the contact start position O of the nip N by 
12 mm. The brush 70 was about 4 mm wide in the direction of movement of 
the belt 44 and held in contact with the rear of the belt 44 at the 
position X lying in the range of 10 mm&lt;X 21 14 mm. Specifically, as shown 
in FIG. 17, the brush 70 contacts the rear of the belt 44 at the position 
X greater than L/2, as distinguished from the brush 70 of the fourth 
embodiment contacting the rear of the belt 44 at the position X lying in 
the range of 0.ltoreq.X.ltoreq.L/2. While the comparative example 
implemented the scatter rank of 4.0 or above when the transfer voltage Vt 
was lower than 1,000 V inclusive, it lowered the transfer efficiency of a 
solid image to about 85% because the substantial nip width subjected to a 
sufficient electric field was reduced. 
The fourth embodiment achieves the following advantages. 
(1) The conductive member 70 is held in contact with the rear of the belt 
or transfer body 44 and connected to ground. The transfer bias power 
source 72 is connected only to the downstream side of the nip N in the 
direction of movement. This weakens the electric field around the inlet of 
the nip N and thereby obviates the migration of toner at a position 
preceding the nip N. Consequently, the toner scattering at the time of 
image transfer is successfully reduced. 
(2) The conductive member 70 is located at the position X lying in the 
range of O.ltoreq.X.ltoreq.L/2 stated earlier. This prevents the transfer 
efficiency from being lowered and thereby reduces the toner scattering. 
(3) The current Inip to flow from the conductive member 70 to ground is 
selected to be smaller than zero inclusive when the drum 32 is chargeable 
to the negative polarity or selected to be greater than zero inclusive 
when the drum 32 is chargeable to the positive polarity. As a result, 
transfer conditions causing a current to flow to the rear of the belt 44 
at the former half of the nip N are set up. This reduces the strength of 
the electric field around the inlet of the nip N and thereby obviates the 
migration of toner at a position preceding the nip N. Consequently, the 
toner scattering at the time of image transfer is successfully reduced. 
(4) The ammeter or current measuring means 74 is provided for measuring the 
current Inip to flow from the conductive member 70 to ground. Therefore, 
optimal transfer conditions can be set on the basis of the result of 
measurement, reducing the toner scattering. 
(5) The conductive member 70 is implemented as a brush having conductive 
filaments implemented by an acrylic resin containing fine carbon 
particles. Generally, acrylic fibers are strong enough to withstand a long 
time of use without being broken or falling off. This reduces the toner 
scattering over a long period of time and obviates defective image 
transfer ascribable to aging. The carbon-containing acrylic resin 
filaments may be replaced with, e.g., stainless steel filaments having a 
diameter of about 5 .mu.m to 8 .mu.m, acrylic resin, nylon, polyester, 
rayon or similar resin filaments plated with metal, filaments consisting 
of a resin and fine particles of carbon, metal or similar conductive 
substance dispersed in the resin, or carbon filaments or similar 
conductive or semiconductive filaments produced by carbonizing, e.g., 
resin filaments. Such conductive filaments and semiconductive filaments 
may be used either individually or in combination. Further, to adjust the 
strength of the brush or the resistance of the tips of its filaments, the 
conductive or semiconductive filaments may be used in combination with, 
e.g., acryl, nylon, polyester or rayon filaments. 
5th Embodiment 
This embodiment is similar to the fourth embodiment except that it 
additionally includes a control unit for effecting the measurement of the 
current Inip to flow from the brush 70 to ground at the time of power up 
of the apparatus and every time the image forming cycle is repeated a 
preselected number of times. At the time of power up, the charging 
condition and developing condition are optimized, and then the transfer 
voltage Vt to be applied from the transfer bias power source to the belt 
44 is optimized. 
Specifically, as shown in FIG. 18, at the time of power up, the charging 
condition and developing condition are optimized as general process 
control. This optimization is conventional and will not be described 
specifically. Subsequently, the voltage Vt to be applied from the transfer 
bias power source to the belt 44 is optimized. At this instant, the drum 
32 being rotated by the drive source is charged to about -650 V by the 
charger 34 and then passed through the developing units 40BK-40Y without 
being exposed. The developing units 40BK-40Y using the reversal 
development system do not operate in the same manner as when they form the 
image of a white sheet. The ammeter 74 measures the current Inip to flow 
from the brush 70 to ground when the charged portion of the drum 32 
arrives at the belt transfer position. 
To obviate the influence of the irregular resistance distribution of the 
belt 44, it is preferable that the ammeter 74 measures the rear current 
Inip derived from a single transfer voltage Vt over one full turn of the 
belt 44, and that the mean value of the measured currents Inip be used as 
a value for control. Specifically, as shown in FIG. 18, after the start of 
formation of a white sheet image, the transfer voltage Vt is applied. In 
this condition, the current Inip is measured. If the rear potential Inip 
is smaller than zero inclusive, it may be possible to increase the voltage 
Vt. Therefore, the voltage Vt is increased by one step .DELTA. V to 
Vt+.DELTA. V, and again the current Inip is measured. Such a procedure is 
repeated until the current Inip exceeds zero. Because the current Inip 
exceeding zero is excessive, a voltage Vt' occurred one step before, i.e., 
Vt'=Vt-.DELTA.V is set as an optimal transfer voltage. While the transfer 
voltage Vt shown in FIG. 14 has an initial value of 0 V and sequentially 
increases by a step of 200 V, the initial value may be selected to be 
several hundred volts in order to reduce the voltage setting time. 
Further, the interval between the steps of the voltage Vt may be reduced 
to 50 V in order to effect more precise control over the voltage Vt. 
The transfer voltage Vt is controlled such that the maximum voltage in the 
range implementing the current Inip to flow from the brush 70 to ground 
and smaller than zero inclusive is set as an optimal transfer voltage. 
Specifically, the power source control means controls the output of the 
transfer bias power source such that the transfer voltage Vt becomes equal 
to an optimal transfer voltage. When the belt 44 is new, the voltage Vt of 
800 V is set as an optimal voltage, as stated earlier. 
Thereafter, when the usual image forming cycle was repeated with 5,000 
sheets without any optimal transfer voltage setting stated above, the 
toner scatter rank fell from initial 4.0 to 3.5 in a halftone area. The 
volume resistivity of the belt 44 was lowered to about 5.times.10.sup.9 
.OMEGA.cm. When the optimal transfer voltage setting was again effected in 
the above-described manner, a characteristic represented by dots in FIG. 
14 was attained; the optimal transfer voltage was determined to be 600 V 
and set on the basis of Inip.ltoreq.0. Under this condition, an even image 
with a minimum of toner scattering from its halftone area over to its 
solid area was produced. Every time the image forming cycle is repeated 
with the preselected number of times, the control unit executes the 
current measurement and then sets an optimal transfer voltage based on the 
result of measurement. 
A third comparative example is identical with the fifth embodiment except 
that the brush 70 was implemented as a SUS brush whose filaments had a 
diameter of about 20 .mu.m. Although the comparative example was as 
desirable as the fifth embodiment as to the initial transfer voltage 
setting, it caused scratches to occur on the rear of the belt 44 when the 
image forming cycle was repeated with several hundreds of sheets. Powder 
ascribable to the scratches deposited on the surfaces of the rollers in 
the form of protuberances. As a result, defective transfer occurred in the 
belt transfer section and sheet transfer section. 
The fifth embodiment has the following advantages. 
(1) The ammeter or current measuring means 74 measures the current Inip to 
flow from the conductive member 70 to ground. The operation of the toner 
image forming means is controlled such that the current Inip is smaller 
than zero inclusive when the drum 32 is chargeable to the negative 
polarity or is greater than zero inclusive when the drum 32 is chargeable 
to the positive polarity. In this condition, the current to flow to the 
rear of the belt or transfer member 44 is measured periodically in order 
to set up transfer conditions capable of reducing the toner scattering. 
This insures transfer conditions causing a minimum of toner scattering to 
occur against, e.g., a change in the resistance of the belt 44 due to 
aging, and thereby frees toner images from noticeable scattering. 
(2) The operation of the toner image forming means is controlled by power 
source control means controlling the output of the transfer bias power 
source 72. This also insures transfer conditions causing a minimum of 
toner scattering to occur against, e.g., a change in the resistance of the 
belt 44 due to aging, and thereby frees toner images from noticeable 
scattering. 
(3) The conductive member 70 is implemented as a brush consisting of an 
acrylic resin and carbon-containing fine conductive filaments dispersed in 
the resin. The member 70 therefore reduces the toner scattering at the 
time of image transfer and obviates defective image transfer ascribable to 
aging. 
In the first to fifth embodiments, the transfer body 44 is implemented as 
an intermediate transfer belt via which a toner image is transferred from 
the drum 32 to a sheet at the nip N. The apparatus is therefore small in 
size and reduces the toner scattering at the time of image transfer from 
the drum 32 to the body 44. 
While the foregoing embodiments have concentrated on an image forming 
apparatus using an intermediate image transfer system, the present 
invention is not limited to such embodiments. 
6th Embodiment 
Referring to FIG. 19, a sixth embodiment of the present invention will be 
described As shown, an image forming apparatus, generally 80, includes a 
conveyor belt or transfer belt 82 for supporting and conveying a sheet. A 
photoconductive element is implemented as a drum 84. The drum 84 is made 
up of a hollow core formed of aluminum and a function separated 
photoconductive layer formed on the core, although not shown specifically. 
The photoconductive layer is made up of a base layer, a charge generating 
layer, and a charge conveying layer, not shown. The photoconductive layer 
is about 28 .mu.m thick and has a capacity of about 90 pF/cm.sup.2. During 
image formation, the drum 32 is rotated by a drive source, not shown. A 
charger 86 is implemented by a scorotron charger and uniformly charges the 
surface of the drum 84 to about -650 V to -700 V. A laser beam 88 scans 
the charged surface of the drum 84 in accordance with image data, 
electrostatically forming a latent image of -100 V to -500 V. 
A developing unit 90 develops the latent image in order to produce a 
corresponding toner image. The developing unit 90 stores a dry 
two-ingredient type developer and deposits negatively charged toner on the 
low potential portions of the drum 84 (reversal development). A bias power 
source for development applies a bias voltage of about -500 V to -550 V 
with or without an AC component superposed thereon to the developing unit 
90. 
The endless belt 82 is passed over a drive roller 92 and a driven roller 94 
and caused to rotate by a drive source, not shown, via the drive roller 
92. A sheet S is fed from a sheet feed section, not shown, to a 
registration roller pair 96. The registration roller pair 96 drives the 
sheet S toward the belt 82 such that the leading edge of the sheet S meets 
the leading edge of the toner image carried on the drum 84. The drum 84 
and belt 82 contact each other and form a nip N therebetween. A bias 
roller 98 is held in contact with a part of the rear of the belt 82 
located downstream of the nip N in the direction of rotation of the belt 
82. A part of the belt 82 between the bias roller 98 and and driven roller 
94 is held in contact with the drum 84. 
The nip N is about 10 mm wide while the belt 82 is 350 mm wide in its 
lengthwise direction. A conductive brush 100 is held in contact with the 
rear of the belt 82 between a position where the drum 84 and belt 82 start 
contacting each other and a position 5 mm remote from that position. The 
brush 100 is implemented by twenty-four 360 denier carbon-containing 
acrylic filaments. The filaments have a resistance of about 
1.times.10.sup.7 .OMEGA.cm. The brush 100 is connected to ground by a 
conductor. 
The belt 82 consists of a rubber layer having a medium resistance and a 
fluorine-based coating layer formed on the rubber layer. The rubber layer 
is formed of a chloroprene rubber and EDPM mixture and carbon black 
dispersed in the mixture. The rubber layer is about 500 .mu.m thick and 
has a volume resistivity of about 1.times.10.sup.10 .OMEGA.cm when the 
belt 82 is new. The coating layer is about 10 .mu.m thick and has a 
surface resistivity of 1.times.10.sup.11 .OMEGA.cm/cm.sup.2 when it is 
new. 
The driven roller 94 and brush 100 are connected to ground. A transfer bias 
power source, not shown, is connected to the bias roller 98 and applies 
the positive transfer voltage Vt to the roller 98. The transfer voltage Vt 
is controlled by power source control means, not shown. The sheet S driven 
by the registration roller pair 96 is conveyed to the nip N by the belt 
82. At the nip N, the toner image is transferred from the drum 84 to the 
sheet S. Because the sheet S is electrostatically retained on the belt 82, 
it can be easily separated from the drum 84 on moving away from the nip N. 
With the belt 82, therefore, it is possible to reduce sheet jams and other 
troubles. 
A cleaning brush 102 and a cleaning blade 104 remove the toner left on the 
drum 84 after the image transfer. Further, a discharge lamp 106 dissipates 
the charge also left on the drum 84. The sheet S with the toner image is 
separated from the belt 82 due to curvature at a position where the drive 
roller 92 is located. Subsequently, the toner image is fixed on the sheet 
S by a fixing unit 108. 
The charger or charging means 86, exposing section or exposing means, 
developing unit or developing means 90, sheet or recording medium S, belt 
82 and bias power source constitute toner image forming means in 
combination. When a bias voltage of 2,600 V was applied from the bias 
power source to the bias roller 98 under usual image forming conditions, 
the output current of the bias power source was about .+-.150 .mu.A. The 
resulting toner scatter rank was 4.5. 
As stated above, the transfer body 82 of this embodiment is implemented as 
a conveyor belt for temporarily supporting the sheet S thereon. The toner 
image formed on the drum or image carrier 84 is transferred from the sheet 
S at the nip N. Then, the conveyor belt conveys the sheet to the next 
step. This reduces sheet jams and reduces the toner scattering at the time 
of image transfer from the drum 84 to the sheet S carried on the belt 82. 
Because the belt 82 has a volume resistivity of 10.sup.7 .OMEGA.cm to 
10.sup.13 .OMEGA.cm, it is possible to control the transfer conditions on 
the basis of the potential on the rear of the belt 82 or the current to 
flow to the rear of the belt 82. 
7th Embodiment 
This embodiment is applied to a color copier. FIG. 20 shows the general 
construction of the color copier while FIG. 21 shows a photoconductive 
element and an intermediate transfer belt included in the embodiment 
together with arrangements around them. As shown, the color copier, 
generally 110, is made up of a color image reading device (color scanner 
hereinafter) 112 and a color image recording device (color printer 
hereinafter) 114. 
In the color scanner 112, a lamp 118 illuminates a document 116 laid on a 
glass platen 125. The resulting imagewise reflection from the document 116 
is focused onto a color image sensor 124 via a mirror group 120 including 
mirrors 120a, 120b and 120c, and a lens 122. The color image sensor 124 
separates the incident color information into, e.g, red (R), green (G) and 
blue (B) components and transforms them to corresponding electric image 
signals. In the illustrative embodiment, the image sensor 124 is made up 
of B, G and R color separating means and a CCD (Charge Coupled Device) or 
similar photoelectric transducer and reads the three colors at the same 
time. The R, G and B image signals output from the image sensor 124 are 
transformed to black (BK), cyan (C), magenta (M) and yellow (Y) color 
image data by an image processing section, not shown, on the basis of 
their intensity levels. Specifically, in response to a scanner start 
signal synchronous to the operation of the color printer 114, the optics 
including the lamp and mirrors scans the document 116 from the right to 
the left, as indicated by an arrow in FIG. 20, outputting image data of 
one color. The optics repeatedly scans the document 116 four times in 
total in order to sequentially output the BK, C, M and Y image data. 
An optical writing unit 126 is included in the color printer 114 and 
transforms the color image data received from the color scanner 112 to an 
optical signal and scans a photoconductive drum or image carrier 128 with 
the optical signal, thereby electrostatically forming a latent image on 
the drum 128. The writing unit 126 includes, e.g., a semiconductor laser 
126a, a laser control section, not shown, a polygonal mirror 126b, a motor 
126c for rotating the mirror 126b, an f/.theta. lens 126d, and a mirror 
126e. 
The drum 128 is rotated counterclockwise, as indicated by an arrow in FIG. 
20. Arranged around the drum 128 are a drum cleaning unit 130 including a 
precleaning discharger, a discharge lamp 132, a charger or main charger 
134, a potential sensor 136, a BK (black) developing unit 138, a C (cyan) 
developing unit 140, an M (magenta) developing unit 142, a Y (yellow) 
developing unit 144, a density pattern sensor 146, and an intermediate 
transfer belt 148. 
The developing units 138, 140, 142 and 144 respectively include developing 
sleeves 138a, 140a, 142a and 144a, paddles 138b, 140b, 142b and 144b, and 
toner content sensors 138c, 140c, 142c and 144c. The developing sleeves 
138a-144a each are rotatable with a developer deposited thereon contacting 
the surface of the drum 128 so as to develop the latent image. The paddles 
138b-144b each are rotatable in order to scoop up the associated developer 
while agitating it. The toner content sensors 138c-144c each are 
responsive to the toner content of the associated developer. In a stand-by 
state, the developers in all the developing units 138-144 are held in 
their inoperative positions. 
The intermediate transfer belt 148 is passed over a drive roller 150, a 
belt transfer bias roller 152, a ground roller 154, and a plurality of 
driven rollers. A motor, not shown, causes the belt 148 to rotate via the 
drive roller 150, as will be described specifically later. A belt cleaning 
unit 156 and a sheet transfer unit 158 are arranged around the belt 148. 
The belt cleaning unit 156 includes a brush roller 156a, a rubber blade 
156b, and a mechanism 156c for moving the unit 156 into and out of contact 
with the belt 148. The sheet transfer unit 158 includes a sheet transfer 
bias roller 158a, a roller cleaning blade 158b, and a mechanism 158c for 
moving the unit 158 into an out of contact with the belt 148. 
The printer 114 additionally includes a pick-up roller 160 for feeding the 
sheet S to between the sheet transfer unit 158 and the belt 148, a 
registration roller pair 162, sheet cassettes 164, 166, 168 and 170 each 
storing sheets of particular size, and a manual feed tray 172 assigned to 
OHP (OverHead Projector) sheets and relatively thick sheets. There are 
also shown in FIG. 20 a sheet conveying unit 174, a fixing unit 176, and a 
copy tray 178. 
The operation of the color copier 110 will be described on the assumption 
that it sequentially forms a BK image, C image, M image and Y image in 
this order, although such an order is only illustrative. On the start of 
operation, the color scanner 112 starts reading the BK image data at a 
preselected time. The formation of a latent image using a laser beam 
starts on the basis of the BK image data. Let the latent image based on 
the BK image data be referred to as a BK latent image. This is also true 
with the other colors C, M and Y. Before the leading edge of the BK latent 
image arrives at a developing position assigned to the BK developing unit 
138 (BK developing position hereinafter), the developing sleeve 138a is 
caused to start rotating in order to develop the leading edge to the 
trailing edge of the BK latent image. As a result, BK toner deposited on 
the sleeve 138a develops the BK latent image and thereby produces a 
corresponding BK toner image. As soon as the trailing edge of the BK 
latent image moves away from the BK developing position, the developer on 
the sleeve 138a is brought to its inoperative position. This is completed 
at least before the leading edge of a C latent image based on the C image 
data arrives at the BK developing position. To render the developer 
inoperative, the sleeve 138a is rotated in the reverse direction. 
The BK toner image is transferred from the drum 128 to the front of the 
belt 148 being rotated at the same speed as the drum 128. For such belt 
transfer, a preselected bias voltage is applied to the belt transfer bias 
roller 152 while the drum 128 and belt 148 are held in contact with each 
other. 
In parallel with the belt transfer, a procedure for forming a C toner image 
is executed with the drum 128. Specifically, the color scanner 112 starts 
reading the C image data at a preselected time. The formation of a latent 
image using a laser beam starts on the basis of the C image data. After 
the trailing edge of the BK latent image has moved a way from a developing 
position assigned to the C developing unit 140 (C developing position 
hereinafter), but before the leading edge of the C latent image arrives at 
the C developing position, the developing sleeve 140a is caused to start 
rotating in order to develop the leading edge to the trailing edge of the 
C latent image. As a result, C toner deposited on the sleeve 140a develops 
the C latent image and thereby produces a corresponding C toner image. As 
soon as the trailing edge of the C latent image moves away from the C 
developing position, the developer on the sleeve 140a is brought to its 
inoperative position. This is also completed at least before the leading 
edge of an M latent image based on the M image data arrives at the C 
developing position. The C toner image is transferred from the drum 128 to 
the belt 148 over and in accurate register with the BK toner image 
existing on the belt 148. 
An M toner image and a Y toner image are formed in the same manner as the 
BK and C toner images. As a result, the BK, C, M and Y toner images are 
sequentially transferred from the drum 128 to the belt 148, completing a 
four-color composite image. 
After the first or BK toner image has been fully transferred to the belt 
148, the belt 148 is driven by any one of a constant speed forward system, 
a skip forward system and a back-and-forth (or quick return) system or by 
any efficient combination thereof matching with a copy size from the copy 
speed standpoint. The constant speed forward system causes the belt 148 to 
rotate at a low speed in a preselected direction during image transfer. 
The skip forward system releases the belt 148 from the drum 128, causes 
the belt 148 to skip forward until the image forming position of the belt 
148 returns to the toner image position of the drum 128, again brings the 
belt 148 into contact with the drum 128, and repeats such a procedure 
thereafter. The back-and-forth system releases the belt 148 from the drum 
for 128, stops the forward movement of the belt 148, causes the F belt 148 
to move in the reverse direction until the image forming position of the 
belt 148 returns to the toner image position of the drum 128, again causes 
the belt 148 to move forward, and repeats such a procedure. 
During the belt transfer of the second, third and fourth colors, the belt 
cleaning unit 156 is spaced from the surface of the belt 148 by the 
mechanism 156c. The sheet transfer bias roller 158a is usually spaced from 
the belt 148. The mechanism 156c brings the roller 158a into contact with 
the belt 148 at the time when the four-color composite image is to be 
collectively transferred from the belt 148 to the sheet S. In this 
condition, a preselected bias voltage is applied to the roller 158a. As a 
result, the composite toner image is transferred from the belt 148 to the 
sheet S. The sheet S is fed from any one of the sheet cassettes 166-170 
designated via an operation panel, not shown, and then driven by the 
registration roller pair 162 when the leading edge of the composite image 
carried on the belt 148 is to arrive at the sheet transfer position. 
The sheet S carrying the composite toner image is conveyed to the fixing 
unit 176 by the conveying unit 174. In the fixing unit 176, a heat roller 
176a and a press roller 176b cooperate to fix the toner image on the sheet 
S. The sheet S coming out of the fixing unit 176 is driven out to the tray 
178 as a full-color copy. 
The drum cleaning unit 130 (precleaning discharger, brush roller and rubber 
blade) removes the toner left on the drum 128 after the belt transfer, and 
the discharge lamp 132 dissipates the charge also left on the drum 128. 
After the sheet transfer, the mechanism 156c brings the belt cleaning unit 
156 into contact with the belt 148 so as to clean the surface of the belt 
148. 
In a repeat copy mode, the operation of the color scanner 112 and the image 
formation on the drum 128 advance to the second BK (first color) step 
after the first Y (fourth color) step at a preselected timing. After the 
transfer of the composite toner image from the belt 148 to the sheet S, 
the second BK toner image is transferred from the drum 128 to the area of 
the belt 148 having been cleaned by the cleaning unit 156. 
While the foregoing description has concentrated on a tetracolor copy mode, 
a tricolor or a bicolor copy mode can also be effected if the above 
procedure is repeated a number of times corresponding to the desired 
number of colors and the desired number of copies. In a monocolor copy 
mode, only the developing unit assigned to the desired color is maintained 
operative until the desired number of copies have been produced. In this 
case, the belt 148 is driven forward at a constant speed in contact with 
the drum 128, and the belt cleaner 156 is held in contact with the belt 
148. 
Arrangements characterizing this embodiment will be described hereinafter. 
As shown in FIG. 22, the belt transfer bias roller 152 is located 
downstream of the nip (primary transfer nip) between the drum 128 and the 
belt 148. A bias is applied to the bias roller 152. In this sense, the 
bias roller 152 plays the role of charge applying means. The ground roller 
154 connected to ground is located upstream of the nip N. The bias roller 
152 and ground roller 154 support the belt 148 and press it against the 
drum 128. A brush or nip contact member 180 is held in contact with the 
rear of the belt 148 at the center of the nip N, preventing the toner on 
the drum 128 from being pretransferred just before it arrives at the nip 
N. The brush 180 is implemented by, e.g., conductive filaments and 
connected to ground. 
In the illustrative embodiment, the transfer charge applied from the bias 
roller 152 to the belt 148 is discharged by the brush 180. As a result, 
the transfer charge applied to the belt 148 does not migrate or scarcely 
migrates from the position where the brush 180 contacts the belt 148 to 
the upstream side in the direction of movement of the belt 148. It follows 
that no charge or substantially no charge exists on the belt 148 at the 
inlet of the nip where the drum 128 and belt 148 do not contact each 
other. Therefore, no potential gradient or substantially no potential 
gradient is produced at the inlet of the nip N, so that an electric field 
adversely effecting the image is absent. As shown in FIG. 23, a potential 
gradient 182 (indicated by hatching) on the belt 148 extends only to the 
brush 180. This is contrastive to the potential gradient 24 shown in FIG. 
1. In the above condition, the potential on the belt 148 upstream of the 
position where the brush 180 contacts the belt 148 is substantially zero 
or zero or of the same polarity as the charge potential of the drum 128. 
How the brush 180 discharges the belt 148 has already been described in 
detail. 
As stated above, in an image forming apparatus of the type transferring a 
toner image from a photoconductive element to a sheet by way of an 
intermediate transfer body passed over rollers, the seventh embodiment 
obviates a problem ascribable to a transfer bias applied to downstream one 
of two rollers located at both sides of a nip between the photoconductive 
element and the intermediate transfer body. Such a transfer bias has 
heretofore generated an excessive electric field slope and therefore an 
electric field extending to the upstream roller, bringing about the 
pretransfer of toner. For example, the seventh embodiment successfully 
reduces the scattering of toner shown in FIG. 2B to a noticeable degree, 
as determined by experiments. The words "to a noticeable degree" mean that 
in practice the drum 128 and toner bear a negative electric field and 
cause so me transfer or pretransfer to occur. However, this kind of 
pretransfer does not critically disturb images. 
Assume the same process conditions as described in relation to the 
conventional configuration shown in FIG. 1 and including the electrical 
characteristic and other properties and material of the intermediate 
transfer body, the moving speed of the intermediate transfer body, the 
properties and material of the toner, etc. Then, the seventh embodiment 
may lower the transfer efficiency, compared to the conventional 
configuration. When a voltage at the outlet of the nip shown in FIG. 23 
should be 60 V as in the configuration of FIG. 1 in order to attain the 
same transfer efficiency, it suffices to apply a transfer bias (e.g. 1 kV) 
higher than the conventional bias (e.g. 800 V). Alternatively or in 
addition, the area of the nip N between the drum 128 and the belt 148 may 
be increased. For example, the part of the nip upstream (or downstream) of 
the brush 180 may be extended in addition to the application of the higher 
transfer bias. Of course, the various process conditions including the 
electrical characteristic and moving speed of the belt 148 may be suitably 
selected instead of varying the conventional transfer bias and area of the 
nip. 
Assume that the brush 180 included in the arrangement of FIG. 23 exerts an 
excessive pressure on the drum 128. Then, the contact pressure acting 
between the drum 128 and the belt 148 at the nip N increases to such a 
degree that thin lines, for example, are locally omitted in a vermicular 
condition. FIG. 24A shows a specific image 184 formed on the drum 128 
while FIG. 24B shows an image 184a, corresponding to the image 184, but 
transferred to the drum 148 in a vermicular condition. In light of this, 
when the pressure of the brush 180 is excessive, it may be controlled to 
an adequate value. Alternatively, the brush 180 may be inclined such that 
the contact angle between the brush 180 and the belt 148, i.e., the angle 
between a line normal to the line of the belt 148 tangential to the drum 
128 at the nip N and the brush 180 (see FIG. 25) ranges from 20 degrees to 
60 degrees, thereby reducing the above pressure. 
8th Embodiment 
A color copier to be described includes a color scanner similar to that of 
the color copier shown in FIG. 20, and operates basically in the same 
manner as the copier of FIG. 20. The copier of this embodiment differs 
from the copier of FIG. 20 mainly in the configuration and operation of 
the color printer. 
As shown in FIG. 26, the color printer in accordance with this embodiment, 
generally 190, includes a photoconductive drum 192. Arranged around the 
drum 192 are a main charger or charging means 194, a drum cleaning unit 
196 including a cleaning blade and a fur brush, an optical writing unit or 
exposing means, not shown, a rotary developing unit (revolver hereinafter) 
or developing means 198, and so forth. The printer 190 additionally 
includes an intermediate transfer unit 200, a fixing unit implemented by a 
roller pair 204, a sheet feed section, not shown, and a controller, not 
shown. 
Assume that in a full-color copy mode, the copier 190 causes its color 
scanner to sequentially read BK, C, M and Y in this order. Then, at the 
beginning of the image forming cycle, a motor, not shown, drives the drum 
192 counterclockwise, as indicated by an arrow in FIG. 26. The main 
charger 194 starts uniformly charging the drum 192 to, e.g., the negative 
polarity by corona discharge. An intermediate transfer belt 206 included 
in the intermediate transfer unit 200 is caused to rotate at the same 
speed as the drum 192 in the direction indicated by an arrow. 
The belt 206 is passed over a primary transfer bias roller 208 playing the 
role of primary charge applying means, a drive roller 210, a tension 
roller 212, a secondary transfer counter roller 214, a belt cleaning 
counter roller 216, and a discharge roller or pre-primary transfer 
discharging means 218. The rollers 208-218 each are formed of a conductive 
material and are connected to ground, except for the primary transfer bias 
roller 208. A primary transfer power source 220 is controlled on a 
constant current or a constant voltage basis and applies a preselected 
transfer bias to the bias roller 208. 
The color scanner starts reading BK color image data at a preselected 
timing. The optical writing unit scans the charged surface of the drum 192 
with a laser beam in accordance with the BK image data by, e.g., raster 
scanning. As a result, a BK latent image represented by the BK image data 
is formed on the drum 192. A BK developing section 198Bk included in the 
revolver 198 develops the BK latent image by reversal development, using 
toner of negative polarity stored therein. As a result, a BK toner image 
corresponding to the BK latent image is formed on the drum 192. 
At a primary transfer position where the drum 192 and belt 206 contact each 
other, the BK toner image is transferred from the drum 192 to the belt 206 
by a transfer electric field. This electric field is formed by the charge 
applied from the primary transfer bias roller 208 to the belt 206. After 
the image transfer, the cleaning unit 196 removes the toner remaining on 
the part of the drum 192 moved away from the primary transfer position. 
The belt 206 in rotation again conveys the BK toner image to the primary 
transfer position. During this conveyance, the toner image BK must be 
protected from disturbance. For this purpose, a pretransfer charger or 
pretransfer charging means (PTC hereinafter) 224, the sheet transfer unit 
202, a belt cleaning charger 226, a belt cleaning blade 228 and a 
lubricant brush 230 arranged around the belt 206 are held in their 
inoperative conditions. That is, the PTC 224 and belt discharger 226 are 
prevented from discharging. The sheet transfer unit 202 includes three 
support rollers 232, 234 and 236 and a secondary transfer bias roller or 
secondary transfer charge applying means 238. The secondary transfer belt 
240 is located at the upstream end of the unit 202 in the direction in 
which a secondary transfer belt or conveyor belt 240 faces the counter 
roller 214. During the conveyance of the BK toner image, the support 
roller 232 and a secondary transfer bias roller or secondary transfer 
charge applying means 238 are spaced from the belt 206 by a mechanism, not 
shown, so that the secondary transfer belt or recording medium conveyor 
240 is spaced from the belt 206. A secondary transfer power source 242 
does not apply any voltage to the secondary transfer bias roller 238. 
Further, the belt cleaning blade 228 and lubricant brush 230 are spaced 
from the belt 206 by a mechanism, not shown. These conditions are also set 
up when toner images are sequentially transferred to the belt 206 one 
above the other. 
The BK image forming step executed with the drum 192 is followed by a C 
image forming step. In the C image forming step, the color scanner starts 
reading C image data at a preselected timing. A C latent image is formed 
on the drum 192 in accordance with the C image data. As soon as the 
trailing edge of the BK latent image moves away from a developing position 
assigned to the revolver 198, the revolver 198 starts rotating. Before the 
leading edge of the C latent image arrives at the developing position, the 
rotation of the revolver 198 is stopped in order to locate a C developing 
section 198C thereof at the developing position. The C latent image is 
developed by C toner stored in the C developing section 198C. Such a 
procedure is repeated with M image data and Y image data so as to 
sequentially form M and Y toner images. Consequently, the Bk, C, M and Y 
toner images are sequentially transferred to the belt 206 one above the 
other, completing a composite color image (four colors at most) on the 
belt 206. 
The belt 206 conveys the composite color image formed thereon to the 
secondary transfer position while having the image uniformly charged by 
the PTC 224. A sheet is fed the secondary transfer position where the belt 
206 and sheet transfer unit 202 face each other, such that the leading 
edge of the sheet meets the leading edge of the image carried on the belt 
206. At this instant, the sheet transfer unit 202 is rendered operative. A 
transfer bias is applied to the secondary transfer bias roller 238 of the 
sheet transfer unit 202 in order to form a transfer electric field. As a 
result, the composite image on the belt 206 is bodily transferred to the 
sheet. A sheet transfer discharger 246 is activated when the sheet 
carrying the toner image and being conveyed by the belt 240 faces the 
discharger 246, so that the sheet is separated from the belt 240. The 
sheet separated from the belt 240 is conveyed toward the fixing roller 
pair 204. The roller pair 204 fixes the toner image on the sheet by 
heating and pressing the sheet. Finally, the sheet is driven out of the 
copier onto a copy tray by an outlet roller pair, not shown. 
After the secondary transfer, the belt discharger 226 discharges the 
surface of the belt 206. Also, the belt cleaning blade 228 is pressed 
against the belt 206 by the previously mentioned mechanism in order to 
remove the toner left on the belt 206. Further, to enhance the cleaning of 
the belt 206 and the transfer of the toner image to the sheet, a 
mechanism, not shown, presses the lubricant brush 230 against the belt 206 
so as to apply a lubricant 247 to the belt 206. The lubricant 247 is 
implemented as a plate-like piece of fine particles of zinc stearate. 
Likewise, after the separation of the sheet, the belt discharger 248 
dissipates charge remaining on the secondary transfer belt 240 while the 
cleaning blade 250 cleans the surface of the belt 240. 
While the foregoing description has concentrated on a tetracolor copy mode, 
a tricolor or a bicolor copy mode can also be effected if the above 
procedure is repeated a number of times corresponding to the desired 
number of colors and the desired number of copies. In a monocolor copy 
mode, only the developing section of the revolver 198 assigned to the 
desired color is maintained operative until the desired number of copies 
have been produced; the belt cleaning blade 228 as well as other members 
are held in their operative conditions. 
As shown in FIG. 27, in this embodiment, the belt 206 is provided with a 
laminate structure consisting of a surface layer 206a, an intermediate 
layer 206b, and a base layer 206c. The surface layer 206a and base layer 
206c respectively constitute an outermost layer contacting the drum 192 
and an innermost layer. An adhesive layer 206d intervenes between the 
intermediate layer 206b and the base layer 206c bonding them together. 
At the primary transfer position, the belt 206 is passed over the primary 
transfer bias roller 208 and belt discharge roller 218 and pressed against 
the drum 192 thereby. In this condition, the drum 192 and belt 206 form a 
nip N having a preselected width therebetween. A belt discharge brush or 
primary transfer discharging means 252 is connected to ground and held in 
contact with the rear of the belt 206 at the nip N. The belt discharge 
brush 252 prevents an undesirable electric field from being formed at the 
inlet of the primary transfer position where the belt 206 approaches the 
drum 192. As shown in FIG. 28, the primary transfer position has a nip 
width Wn while the brush 252 contacts the belt 206 at a position spaced 
from the downstream end of the nip N in the direction of movement of the 
belt 206 by a distance L. The nip width Wn and distance L are so selected 
as to set up preselected transfer conditions. 
A specific example of the eighth embodiment is as follows. The intermediate 
transfer belt 206 was 0.15 mm thick, 368 mm wide and 565 mm long in terms 
of its inner peripheral length. The belt 206 was caused to move at a speed 
of 200 mm/sec. The surface layer 206a of the belt 206 was implemented as 
an about 1 .mu.m thick insulating layer. The intermediate layer 206b was 
constituted by an about 75 .mu.m thick insulating layer formed of PVDF 
(polyvinylidene fluoride) and having a volume resistivity of about 
10.sup.13 .OMEGA.cm. The base layer 206c was constituted by an about 75 
.mu.m medium resistance layer formed of PVDF and titanium oxide and having 
a volume resistivity of 10.sup.8 .OMEGA.cm to 10.sup.11 .OMEGA.cm. The 
belt 206 with such a laminate structure was found to have an overall 
volume resistivity ranging from 10.sup.7 .OMEGA.cm to 10.sup.12 .OMEGA.cm. 
The volume resistivities were measured by a method prescribed by JIS K6911 
and by applying a voltage of 100 V for 10 seconds. The surface layer 206a 
had a surface resistivity of 107 .OMEGA. to 1012 .OMEGA. when measured by 
Hiresta IP mentioned earlier. For the measurement of the surface 
resistivity, use may be made of a surface resistivity measuring method 
prescribed by JIS K6911. 
The primary transfer bias roller 208 was implemented by a metal roller 
plated with nickel. The belt discharge roller 218 was also implemented by 
a metal roller. For the other rollers, use was made of metal rollers or 
conductive resin rollers. DC transfer biases of adequate sizes are applied 
to the bias roller 208. Specifically, 1.0 kV, 1.3 kV to 1.4 kV, 1.6 kV to 
1.8 kV, and 1.9 kV to 2.2 kV were sequentially applied to the bias roller 
208 for the first, second, third and fourth colors, respectively. 
The nip width Wn of the primary transfer position was selected to be 10 mm 
while the distance L was selected to be 7 mm (see FIG. 28). The belt 
discharge brush 252 had conductive filaments formed of a carbon-containing 
resin. 
For the PTC 226, use was made of a charger with a grid. The power source 
254 applied a DC bias voltage of the same polarity as the charge of the 
toner image carried on the belt 206 to the PTC 224. More specifically, a 
DC voltage controlled to a constant current of -500 .mu.A was applied to a 
main wire 225a included in the PTC 224 while a DC voltage ranging from 0 
kV to -3 kV was applied to a grid electrode 224b. 
The secondary transfer bias roller 238 had a surface layer formed of 
conductive sponge or conductive rubber and a core layer formed of metal or 
conductive resin. A transfer bias controlled to a constant current of 10 
.mu.A to 20 .mu.A was applied to the roller 238. The secondary transfer 
belt 240 was 100 .mu.m thick and formed of PVDF and had a volume 
resistivity of 10.sup.10 .OMEGA.cm to 10.sup.13 .OMEGA.cm. 
The sheet transfer discharger 246 was implemented by a discharger to which 
an AC voltage or an AC+DC voltage was applied from a power source, not 
shown. The cleaning blade 250 was held in contact with the portion of the 
secondary transfer belt 240 contacting the support rollers 236. 
9th Embodiment 
Referring to FIG. 29, a ninth embodiment of the present invention will be 
described which is similar to the seventh embodiment except for the 
addition of cost saving features. In FIG. 29, the same structural elements 
as the elements shown in FIG. 26 are designated by the same reference 
numerals, and a detailed description thereof will not be made in order to 
avoid redundancy. 
In a color copier 260 shown in FIG. 29, the intermediate layer 206b of the 
intermediate transfer belt 206 is formed of a material having a medium 
resistance. In addition, the entire belt 206 is configured to have a 
medium resistance. The belt 206 having a medium resistance allows a 
minimum of irregular charge distribution to occur on the belt 206 after 
the primary transfer. For this reason, the copier 260 does not include the 
PTC 224. The drive roller 210 for driving the belt 206 is located at a 
position where the belt 206 moves from the secondary transfer position 
toward the primary transfer position, playing the role of a belt cleaning 
counter roller at the same time. Mainly for a cost reducing purpose, the 
secondary transfer belt 240 shown in FIG. 26 is replaced with an 
arrangement in which the secondary transfer bias roller 238 and the 
portion of the belt 206 contacting the secondary transfer counter roller 
214 directly nip a sheet therebetween. In addition, the sheet discharger 
246, belt discharger 248 and cleaning blade 250 are absent. 
A specific example of the ninth embodiment is as follows. The example is 
similar to the example of the eighth embodiment except for the following. 
The entire belt 206 and the intermediate layer 206b of the belt 206 each 
had a volume resistivity of 10.sup.8 .OMEGA.cm to 10.sup.11 .OMEGA.cm. The 
intermediate layer 206b, like the base layer 206c, was formed of PVDF and 
titanium oxide. The distance L (see FIG. 28) was selected to be 6 mm to 7 
mm. The belt 206 was caused to move at a speed of 156 mm/sec. DC transfer 
biases of adequate sizes are applied to the primary transfer bias roller 
208. Specifically, 1.2 kV, 1.3 kV, 1.4 kV and 1.6 kV were sequentially 
applied to the bias roller 208 for the first, second, third and fourth 
colors, respectively. The secondary transfer bias roller 238 was formed of 
conductive rubber. 
10th Embodiment 
FIG. 30 shows a tenth embodiment of the present invention which is applied 
to an image forming apparatus of the type including a belt or similar 
support member for supporting a sheet, OHP sheet or similar recording 
medium. As shown, an image forming apparatus, generally 270, includes a 
transfer belt 272 playing the role of a recording medium support member. A 
toner image is formed on a photoconductive drum or image carrier 274 by 
the conventional electrophotographic process. The drum 274 and belt 272 
contact each other, forming a nip N therebetween. A transfer bias roller 
276 is located downstream of the nip N in the direction of movement of the 
belt 272. The toner image formed on the drum 274 is transferred to a sheet 
S by a transfer charge applied via the bias roller 276. The belt 272 is 
provided with a medium resistance (10.sup.8 .OMEGA.cm to 10.sup.13 
.OMEGA.cm or 10.sup.7 .OMEGA. to 10.sup.12 !) for the same purpose as 
described in relation to the previous embodiments. 
A potential gradient is formed on the belt 272 due to the transfer charge 
applied via the bias roller 278. The potential gradient forms an electric 
field at the inlet of the nip N. As a result, it is likely that the toner 
image carried on the drum 274 is partly transferred to the sheet S before 
it reaches the nip N due to the above electric field (pretransfer). Such 
an occurrence would lower the quality of the resulting image. In light of 
this, a discharge brush 280 or similar discharging means is disposed in 
the nip N. The discharge brush 280 prevents a potential causative of 
pretransfer at the inlet of the nip N from being generated. 
In the seventh to tenth embodiments, the discharging means is implemented 
as a discharge brush. If desired, the discharge brush may be replaced with 
a blade, roller or similar discharge member. 
The position where the discharging means is located is not limited to one 
included in the tenth embodiment. The crux is that the discharging 
position be located upstream of the bias roller or charge applying means 
152, 208 or 276 in the direction of movement of the intermediate transfer 
belt, but within the nip N. FIG. 31 shows, taking the arrangement of FIG. 
26 or 27 as an example, positions A-E in the nip N where the discharge 
brush 252 may be located and the gradients of the potential V of the belt 
206 particular to the positions A-E. The nip N starts at the position A. 
The position B is intermediate between the position A and the center C of 
the nip N. The position D is intermediate between the position C and the 
position E where the nip N ends. The potential on the belt 206 upstream of 
the position where the discharge brush 252 and belt 206 contact in the 
direction of movement of the belt 206 is of the same polarity as the 
charge deposited on the drum 192. Specifically, the charge potential of 
the belt 206, as measured at the above contact position, is 0 V or is 
around 0 V under some process conditions. The charge potential of the belt 
206 sequentially approaches the charge potential of the drum 192 toward 
the upstream side by being influenced by the drum 192. The charge 
potential of the belt 206 again varies to around 0 V or to 0V. 
As FIG. 31 indicates, the discharging member is capable of obstructing the 
formation of an electric field at the inlet of the nip N in any one of the 
positions A-E. If desired, a plurality of discharging means may be located 
side by side, and each may be provided with a particular configuration. 
Another discharging means may be located upstream of the nip N in the 
direction of movement of the belt 206 in addition to the discharging means 
disposed in the nip N. For example, discharging means independent of the 
discharging means present in the nip N may be located upstream or 
downstream of the nip N with respect to the above direction. 
While the discharge brush in the tenth embodiment is connected to ground, a 
bias opposite in polarity to the transfer charge may be applied to the 
discharge brush so long as it does not influence the transfer charge 
necessary for image transfer at the nip N. 
The photoconductive drum shown in any one of the seventh to tenth 
embodiments may be replaced with any other suitable kind of image carrier, 
e.g., an endless photoconductive belt passed over two rollers. 
The intermediate transfer belt shown in any one of the seventh to ninth 
embodiments may be replaced with any other suitable form of intermediate 
transfer body. The intermediate transfer belt may be provided with any 
suitable thickness and structure (single layer, double layer or the like) 
and formed of any suitable material in conformity to desired image forming 
conditions. 
In the seventh to tenth embodiments, the bias roller is only a specific 
form of transfer charge applying means. The transfer charge applying means 
may apply the transfer charge at a position lying in the nip if the 
position is downstream of the position where the discharge brush or 
similar transfer discharging means is located. 
In any one of the seventh to tenth embodiments, the ground roller playing 
the role of pretransfer discharging means may be replaced with a blade, 
brush or the like. The secondary transfer bias roller included in the 
seventh to ninth embodiments may be replaced with a blade, brush or any 
other suitable secondary transfer charge applying means. 
In the eighth embodiment, the support member for supporting a recording 
medium is implemented as a belt. If desired, a drum or similar support 
member may be substituted for the belt. 
The seventh to ninth embodiments have concentrated on the case wherein the 
photoconductive drum is chargeable to the negative polarity, and the 
developing unit performs reversal development by using a two ingredient 
type developer. The embodiments are also practicable with a 
photoconductive drum chargeable to the positive polarity and/or the 
regular developing system using a single ingredient type developer. 
The voltage and current of the primary transfer applied to the primary 
transfer charge applying means in any one of the embodiments is only 
illustrative and may be replaced with any other voltage and current 
matching desired image forming conditions. 
In summary, it will be seen that the present invention achieves various 
unprecedented advantages, as enumerated below. 
(1) A potential deposited on the rear of a transfer body is zero or of the 
same polarity as the charge of an image carrier at least in a part of a 
nip formed for image transfer. Therefore, an electric field for image 
transfer is weakened at least at a part of the nip. This successfully 
prevents toner from migrating at a position preceding the nip and thereby 
reduces the toner scattering at the time of image transfer. 
(2) Assume that the image carrier and transfer body start contacting each 
other at a position O at the nip, and that they start leaving each other 
at a position L. Then, the potential on the rear of the transfer body is 
zero or of the same polarity as the charge of the image carrier at a 
position X lying in the range of O.ltoreq.X.ltoreq.L/2 at the nip. 
Therefore, the effective nip width can be made as great as possible so as 
to prevent a transfer efficiency from lowering. At the same time, the 
electric field for image transfer in the vicinity o the inlet of the nip 
is weakened. This also prevents toner from migrating at the position 
preceding the inlet of the nip and thereby reduces the scattering of 
toner. 
(3) Potential measuring means is provided for measuring the potential Vnip 
deposited on the rear of the transfer body. Transfer conditions can 
therefore be optimally set up on the basis of the result of measurement, 
reducing the toner scattering at the time of image transfer. 
(4) Control means for causing the potential measuring means to operate at 
the time of image transfer at the nip is also provided. The control means 
controls the operation of toner image forming means such that the 
potential Vnip is zero or of the same polarity as the charge of the image 
carrier. This insures transfer conditions causing a minimum of toner 
scattering to occur at all times against the varying resistance of the 
transfer body due to aging. As a result, an image with a minimum of toner 
scattering is attainable at all times. 
(5) The measurement occurs at the time of image transfer at the position of 
the nip lying in the range of O.ltoreq.X.ltoreq.L/2. This also allows the 
effective nip width to be as great as possible and thereby prevents the 
transfer efficiency from lowering. 
(6) Power source control means controls the output of a transfer bias power 
source and plays the role of means for controlling the operation of the 
toner image forming means. This also insures transfer conditions causing a 
minimum of toner scattering to occur at all times against the varying 
resistance of the transfer body due to aging. As a result, an image with a 
minimum of toner scattering is attainable at all times. 
(7) A conductive member is held in contact with the rear of the transfer 
body and connected to ground. The transfer bias power source is connected 
only to the downstream side in the direction of movement at the nip. As a 
result, the electric field around the inlet of the nip is weakened. This 
prevents the toner from migrating at the position preceding the nip and 
thereby reduces the toner scattering at the time of image transfer. 
(8) A current Inip to flow from the conductive member to ground is selected 
to be smaller than zero inclusive when the image carrier is chargeable to 
the negative polarity or to be greater than zero inclusive when it is 
chargeable to the positive polarity. As a result, transfer conditions are 
set up such that a current flows to the rear of the transfer body at the 
former half of the nip. This weakens the electric field around the inlet 
of the nip and thereby obviates the migration of the toner at the position 
preceding the nip. 
(9) Current measuring means for measuring the current Inip is provided. 
Optimal transfer conditions can therefore be set up on the basis of the 
result of measurement, reducing the toner scattering at the time of image 
transfer. 
(10) The features stated in the above items (8) and (9) are combined in 
order to insure transfer conditions causative of a minimum of toner 
scattering against the varying resistance of the transfer body ascribable 
to aging. 
(11) The conductive member is implemented as a brush having conductive 
filaments consisting of an acrylic resin and fine carbon particles 
dispersed therein. The conductive member is therefore capable of reducing 
toner scattering over a long period of time and obviating defective image 
transfer ascribable to aging. 
(12) The transfer body is implemented as an intermediate transfer belt for 
temporarily supporting a toner image transferred from the image carrier at 
the nip and then transferring it to a sheet or similar recording medium. 
The apparatus is therefore miniature and reduces toner scattering at the 
time of image transfer from the image carrier to the belt. 
(13) The transfer body is implemented as a conveyor belt for temporarily 
supporting the sheet and conveying, after image transfer from the image 
carrier to the sheet, the sheet to the next step. The transfer body 
therefore reduces sheet jams while reducing toner scattering at the time 
of image transfer from the image carrier to the sheet. 
(14) Because the transfer body has a volume resistivity of 10.sup.7 
.OMEGA.cm to 10.sup.13 .OMEGA.cm, transfer conditions can be controlled on 
the basis of the potential on the rear of the transfer body or the current 
to flow to the rear of the transfer body. 
(15) Assume a position where the image carrier contacts the intermediate 
transfer body or the recording medium support member. Then, the influence 
of an electrical manipulation for forming a transfer electric field in a 
gap extremely close to the position where the above two members contact 
can be desirably reduced, compared to a case wherein the above 
manipulation is effected at a position where the two members are spaced 
from each other. This prevents image quality from being lowered due to, 
e.g., pretransfer. 
(16) A contact pressure acting between the image carrier and the 
intermediate transfer body is prevented from increasing to a critical 
degree, compared to a case wherein an electrode member contacts a portion 
of the transfer body contacting the image carrier. This prevents image 
quality from being lowered by such a contact pressure. Even when the 
electrode member is implemented as a rotary body, the oscillation of the 
electrode member is scarcely transferred to the image carrier; otherwise, 
the oscillation would adversely affect the step of forming a toner image 
on the image carrier. 
(17) The electrode member has an elastically deformable contact portion and 
successfully absorbs, e.g., a change in the contact condition between the 
intermediate transfer body and the electrode member which would affect the 
contact pressure between them. Therefore, the electrode member can be 
positioned relative to the transfer body in such a manner as to set up a 
desired contact pressure by a simple positioning mechanism, compared to a 
case wherein the electrode member has a rigid contact portion. 
(18) The influence of the charge which the electrode member failed to 
dissipate on the above gap is reduced, compared to a case wherein the 
charge is dissipated at the position where the image carrier and 
intermediate transfer body contact each other. This reduces the fall of 
image quality ascribable to pretransfer more desirably than when the 
charge is dissipated at the position where the image carrier and transfer 
body start contacting each other. 
(19) The charge deposited on the intermediate transfer body can be more 
fully dissipated than when it is dissipated only at the position where the 
image carrier and transfer body contact each other. This is also 
successful to achieve the above advantage (18). 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure without departing 
from the scope thereof.