Image transferring method and image forming apparatus for transferring toner image from image carrier to recording medium either via or carried on intermediate image transfer belt

Information

  • Patent Grant
  • 6212351
  • Patent Number
    6,212,351
  • Date Filed
    Tuesday, November 23, 1999
    25 years ago
  • Date Issued
    Tuesday, April 3, 2001
    23 years ago
Abstract
An image forming apparatus and method of using the apparatus including: an image carrier; and an intermediate image transfer unit. The intermediate transfer unit includes: an intermediate image transfer belt which is movable while contacting a surface of the image carrier over a preselected distance; a discharging member for discharging a charge deposited on the intermediate image transfer belt at a nip between the intermediate image transfer belt and the image carrier; a charge depositing member for depositing a transfer charge on the intermediate image transfer belt at a position downstream of the nip in a direction of movement of the intermediate image transfer belt, whereby a toner image formed on the image carrier is transferred to the intermediate image transfer belt itself or a recording medium by an electric field formed at the nip; and the discharging member discharging the charge deposited on the intermediate image transfer belt at the nip so that the discharging member is in contact with a surface of the intermediate image transfer belt opposite to a surface contacting the image carrier with a pressure between 0.05 N/cm2 and 2 N/cm2.
Description




BACKGROUND OF THE INVENTION




The present invention relates to an image forming method of the type transferring a toner image from an image carrier to a recording medium via an intermediate image transfer belt, or intermediate image transfer body, or transferring it from the image carrier to a recording medium carried on a transfer belt, or medium carrier, and a copier, printer facsimile apparatus or similar image forming apparatus for practicing the same.




An image forming apparatus of the type transferring a toner image from a photoconductive element or image carrier to an intermediate image transfer belt (primary transfer) is well known in the art. For the primary transfer, use may be made of an indirect bias application system, which applies a bias for image transfer indirectly to the belt. In the indirect bias application system, a bias roller for belt transfer is positioned downstream of a nip between the photoconductive element and the belt while a ground roller is positioned upstream of the nip. The above bias is applied to the bias roller in order to transfer a toner image from the photoconductive element to the belt.




The problem with the above image forming apparatus is that toner is scattered at the time of primary transfer of a toner image from the photoconductive element to the belt. Specifically, at the time of primary transfer, a toner image formed on the photoconductive element is not transferred to a preselected position on the belt, but is scattered around the preselected position and blurred. Particularly, such scattering of toner causes thin lines to lose sharpness.




One cause of the scattering of toner is so-called pretransfer, i.e., the transfer of toner from the photoconductive element to the belt occurring at a position upstream of the nip between the element and the belt in the direction of movement of the element, as well known in the art. Another cause is so-called retransfer, i.e., the transfer of toner from the belt back to the photoconductive element occurring at a position downstream of the above nip. More specifically, as for pretransfer, when the bias is applied to the bias roller, a potential slope occurs between the bias roller and the ground roller and forms an electric field even at the side upstream of the nip, causing the toner to move toward the belt away from the photoconductive element. As for retransfer, the toner image successfully transferred from the photoconductive element to the belt is disturbed by an electric field for image transfer formed at the side downstream of the nip.




Presumably, the above pretransfer and retransfer also occur when a toner image is directly transferred from the photoconductive element to an image transfer belt used to convey a recording medium.




It is a common practice with an image forming apparatus using the intermediate image transfer belt or the transfer belt to cause the belt to contact an object facing it by use of a pressing member. The pressing member presses the surface of the belt opposite to the surface expected to contact the object. However, with this kind of arrangement, it is likely that when the belt is left unused over a long time, both the belt and the object contacting each other over a long time are damaged, and the belt curls complementarily to the contour of the pressing member. The cur led portion of the belt would vary the mechanical contact condition and therefore the image transfer condition on entering the nip and would thereby bring about a defective image ascribable to, e.g., irregular image transfer.




The above problem arises not only in an image forming apparatus including the image transfer belt or the transfer belt, or image transfer body, to which a toner image is transferred from the image carrier, but also in an image forming apparatus including a belt, a pressing member for pressing the belt, and an object which the surface of the belt opposite to the surface pressed by the pressing member contacts.




Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-249842, 8-166731, 8-2409591, and 10-161440.




SUMMARY OF THE INVENTION




It is therefore an object of the present invention to provide an image transferring method capable of obviating pretransfer and retransfer apt to occur during belt transfer, and an image forming apparatus for practicing the same.




It is another object of the present invention to provide an image forming apparatus capable of preventing a belt from curling.




It is another object of the present invention to provide an image forming apparatus capable of preventing a belt from curling and freeing the belt and an object which the belt is expected to contact from damage ascribable to a long time of contact.




In accordance with the present invention, in an image transferring method for discharging, at a nip between an image carrier and an intermediate image transfer belt moving while contacting the surface of the image carrier over a preselected distance, a charge deposited on the belt, depositing a transfer charge on the belt at a position downstream of the nip in the direction of movement of the belt, and transferring a toner image formed on the image carrier to the belt by an electric field formed at the nip, a discharging member for discharging the belt discharges, at the nip, the belt in contact with the surface of the belt opposite to the surface contacting the image carrier with a pressure between 0.05 N/cm


2


and 2 N/cm


2


.




Also, in accordance with the present invention, an image forming apparatus includes an image carrier, and an intermediate image transfer unit. The intermediate image transfer unit includes an intermediate image transfer belt movable while contacting the surface of the image carrier over a preselected distance, a discharging member for discharging a charge deposited on the belt at a nip between the belt and the image carrier, and a charge depositing member for depositing a transfer charge on the belt at a position downstream of the nip in the direction of movement of the belt. A toner image formed on the image carrier is transferred to the belt by an electric field formed at the nip. At the nip, the discharging member discharges the belt in contact with the surface of the intermediate image transfer belt opposite to the surface contacting the image carrier with a pressure between 0.05 N/cm


2


and 2 N/cm


2


.











BRIEF DESCRIPTION OF THE DRAWINGS




The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:





FIG. 1

is a section showing an image forming apparatus embodying the present invention;





FIG. 2

is a view showing a photoconductive element included in the illustrative embodiment together with various units arranged around the element;





FIG. 3

is a view showing an alternative embodiment of the present invention;





FIGS. 4A and 4B

are fragmentary views each showing a specific configuration of moving means included in the embodiment of

FIG. 3

;





FIG. 5

is a table listing biases to be selectively applied to a secondary transfer bias roller included in the embodiment of FIG.


3


;





FIG. 6

is a view showing another alternative embodiment of the present invention;





FIG. 7

is a table listing biases selectively applied to a secondary transfer bias roller included in the embodiment of

FIG. 6

; and





FIG. 8

is a view showing a further alternative embodiment of the present invention.











DESCRIPTION OF THE PREFERRED EMBODIMENTS




Referring to

FIGS. 1 and 2

of the drawings, a preferred embodiment of the present invention is shown which is implemented as a full-color electrophotographic copier by way of example. As shown, the copier is generally mage up of a scanner section or color image reading device


1


and a printer section or color image recording device


2


.




The scanner section


1


includes a lamp


4


for illuminating a document


3


laid on a glass platen. The resulting reflection from the document is incident to a color image sensor


7


via mirrors


5




a


,


5




b


and


5




c


and a lens


6


. The color image sensor


7


separates color image information incident thereto to, e.g., a blue (B), a green (G) and a red (R) component and transforms the B, G and R components to a B, a G and an R electric signal, respectively. To read the three colors at the same time, the image sensor


7


includes color separating means and CCDs (Charge Coupled Devices) or similar photoelectric transducers. An image processing section, not shown, executes color conversion with the B, G and R image signals on the basis of intensity level to thereby output black (Bk), cyan (C), magenta (M) and yellow (Y) color image data. More specifically, in response to a start signal associated with the operation of the printer section


2


, the above scanning optics scans the document in a direction indicated by an arrow A in

FIG. 1

so as to output the color image data. In the illustrative embodiment, every time the optics sans the document, image data of one color is output. Therefore, to output the Bk, C, M and Y color image data, the optics scans the same document four consecutive times.




The printer section


2


includes an optical writing unit or exposing means


8


and a photoconductive drum


10


which is a specific form of an image carrier. The optical writing unit


8


converts the color image data output from the scanner section


1


to optical signals so as to sequentially form electrostatic latent images on the drum


10


. The optical writing unit


8


may be implemented by a semiconductor laser


8




a


, a controller for controllably driving the laser


8




a


, a polygonal mirror


8




b


, a motor


8




c


for driving the mirror


8




b


, an f/θ lens


8




d


, and a mirror


8




e


. The drum


10


is rotated in a direction indicated by an arrow B in

FIG. 1

, i.e., counterclockwise.




Arranged around the drum


10


are a drum cleaning unit or drum cleaning means


11


, a discharge lamp or discharging means


12


, a charger or charging means


13


, a potential sensor or potential sensing means


14


, a Bk developing unit


15


, a C developing unit


16


, an M developing unit


17


, a Y developing unit


18


, a density pattern sensor or density sensing means


19


, and an intermediate image transfer unit


20


. The cleaning unit


11


includes a blade


11




a


, a brush roller or applying means


11




b


for applying a lubricant to the drum


10


, and a precleaning discharger


11




d


. The Bk, C, M and Y developing units


15


-


18


constitute developing means.




The blade


11




a


is constantly held in contact with the drum


10


for removing toner left on the drum


10


after primary transfer. The brush roller


11




b


also constantly held in contact with the drum


10


applies a lubricant to the surface of the drum


10


in order to enhance the cleaning ability of the cleaning unit


11


. Specifically, when a drive mechanism, not shown, connected to the shaft of the brush roller


11




b


causes the roller


11




b


to rotate, the roller


11




b


shaves off solid zinc stearate


11




c


and applies the resulting fine powder of zinc stearate to the drum


10


.




The developing units


15


-


18


respectively include paddles


15




a


-


18




a


, toner content sensors


15




b


-


18




b


, and developing sleeves


15




c


-


18




c


. The paddles


15




a


-


18




a


each play the role of agitating means for agitating a developer while scooping it up. The toner content sensors


15




b


-


18




b


each are toner content sensing means responsive to the toner content of a developer. The developing sleeves


15




c


-


18




c


each are a developer carrier for causing the ear of a developer formed thereon to contact the surface of the drum


10


. While the copier is in a stand-by state, the developing units


15


-


18


each maintain the ear of the developer deposited on the respective developing sleeve in an inoperative position.




The intermediate image transfer unit


20


includes an intermediate image transfer belt or intermediate image transfer body


21


passed over a primary transfer bias roller or charge depositing means


22


, a ground roller or primary pretransfer discharging means


23


, a drive roller or drive means


24


, and a driven roller


25


. The primary transfer bias roller


22


is connected to a primary transfer power supply


28


. A motor, not shown, is drivably connected to the intermediate image transfer belt


21


.




The above belt


21


has a laminate structure made up of a surface layer, an intermediate layer, and a base layer although not shown specifically. The belt


21


is positioned such that the surface layer contacts the drum


10


while the base layer is remotest from the drum


10


. An adhesive layer, not shown, intervenes between the intermediate layer and the base layer. The belt


21


has a volume resistivity of 10


11


Ωcm to 10


14


Ωcm, preferably 10


12


Ωcm to 10


13


Ωcm or more preferably 10


13


Ωcm, as measured by a method prescribed by JIS (Japanese Industrial Standards) K6911.




In the illustrative embodiment, the surface layer and intermediate layer of the belt


21


each have a high resistance while the base layer has a medium volume resistivity of 10


8


Ωcm to 10


11


Ωcm. This configuration is, of course, only illustrative.




A belt cleaning unit


29


adjoins the belt


21


and includes, like the drum cleaning unit


11


, a blade


29




a


and a brush roller


29




b


for applying a lubricant implemented by solid zinc stearate to the belt


21


. The blade


29




a


contacts the belt


21


in an orientation counter to the direction in which the belt


21


moves, as illustrated. The brush roller


29




b


faces the surface of the belt


21


at a position upstream of the position where the blade


29




a


contacts the belt


21


in the direction of movement of the belt


21


. A gear, not shown, is mounted on the shaft of the brush roller


29




b


and rotated to, in turn, rotate the roller


29




b


. As a result, the brush roller


29




b


shaves off the solid zinc stearate and applies the resulting fine powder to the belt


21


. Moving means, not shown, selectively brings the blade


29




a


and brush roller


29




b


into or out of contact with the belt


21


.




An image transfer unit or image transferring means


30


also adjoins the belt


21


and includes a secondary transfer bias roller


31


facing the drive roller


24


, a cleaning blade


32


, and a moving mechanism


33


for selectively moving the unit


30


into or out of contact with the belt


21


.




The printer section


2


further includes a pick-up roller


41


for feeding, via a registration roller pair


42


, a paper or similar recording medium


100


toward a secondary image transfer region between the secondary transfer bias roller


31


and the portion of the belt


21


contacting the drive roller


24


. Paper cassettes


43




a


,


43




b


and


43




c


each are loaded with papers


100


of particular size. A manual feed tray


40


is available for feeding OHP (OverHead Projector) sheets, thick sheets and other special sheets by hand. The printer section


2


additionally includes a conveyor unit


44


, a fixing unit or fixing means


45


including a heat roller


45




a


and a press roller


45




b


, and a copy tray


46


.




The operation of the illustrative embodiment will be described on the assumption that it sequentially forms a Bk, a C, an M and a Y toner image in this order by way of example. On the start of the copying operation, the scanner section


1


reads a document laid on the glass platen. The optical writing unit


8


scans the surface of the drum


10


with a laser beam based on the resulting Bk image data, thereby forming a Bk latent image on the drum


10


. The Bk developing unit


15


develops the Bk latent image with Bk toner to thereby form a Bk toner image. To insure the development of the Bk latent image, the developing sleeve


15




a


of the Bk developing unit


15


is caused to start rotating before the leading edge of the Bk latent image arrives at a developing position assigned to the developing unit


15


. That is, the developer deposited on the developing sleeve


15


is held in an operative position before the leading edge of the Bk latent image arrives at the above developing position. As soon as the trailing edge of the Bk latent image moves away from the developing position, the developer on the sleeve


15




a


is immediately brought to the inoperative position, rendering the developing unit


15


inoperative. This is completed at least before the leading edge of a C latent image to be developed next arrives at the developing position of the Bk developing unit


15


. To render the developer on the sleeve


15




a


inoperative, the sleeve


15




a


may be rotated in the direction opposite to the direction for development.




The Bk toner image formed on the drum


10


is transferred from the drum


10


to the surface of the belt


21


moving at the same speed as the drum


10


(primary transfer).




In parallel with the primary transfer of the Bk toner image, the scanner section


1


again reads the same document at a preselected timing in order to produce C image data. The optical writing unit


8


scans the drum


10


in accordance with the C image data to thereby form a C latent image on the drum


10


. The C developing unit


16


develops the C latent image so as to form a C toner image. The developing sleeve


16




a


of the C developing unit


16


is caused to start rotating after the trailing edge of the Bk latent image has moved away from a developing position assigned to the developing unit


16


, but before the leading edge of the C latent image arrives at the developing position. As soon as the trailing edge of the C latent image moves away from the developing position, the developer on the sleeve


16




a


is immediately brought to the inoperative position, rendering the developing unit


16


inoperative. This is completed at least before the leading edge of an M latent image to be developed next arrives at the developing position of the C developing unit


16


. The C toner image is transferred from the drum


10


to the belt


21


over the Bk toner image existing on the belt


21


(primary transfer).




The above procedure is repeated with an M latent image and a Y latent image also. As a result, the Bk and C toner images and an M and a Y toner image are sequentially transferred from the drum


10


to the belt


21


one above the other in this order, forming a full-color toner image on the belt


21


.




During the interval between the primary transfer of one toner image and that of the next toner image, e.g., the primary transfer of the first or Bk toner image and that of the second or C toner image, the belt


21


is driven by any one of conventional systems including a constant speed forward system, a skip forward system, and a reciprocation or quick return system. If desired, to increase the copy speed, any one of the above drive systems may be selected in accordance with the copy size, or a plurality of them may be efficiently combined.




Briefly, the constant forward system is such that the belt


21


is driven forward at a low speed during primary transfer. The skip forward system is such that after the forward movement effected for the primary transfer in the same manner as in the constant forward system, the belt


21


is released from the drum


10


and then caused to skip forward to a primary transfer start position at a high speed. The reciprocation or quick return system is such that after the belt


21


has been released from the drum


10


in the same manner as in the skip forward system, it is returned in the reverse direction to a primary transfer start position at a high speed.




The belt


21


carrying the full-color image thereon conveys the image to the secondary image transfer region in order to transfer it to the paper


100


(secondary transfer). Usually, the moving mechanism


33


presses the secondary transfer bias roller


31


against the belt


21


at a timing for transferring the toner image to the paper


100


. Subsequently, a preselected bias for secondary transfer is applied to the bias roller


31


in order to form an electric field in the secondary image transfer region. As a result, the toner image is transferred from the belt


21


to the paper


100


. Specifically, the paper


100


is fed from one of the paper cassettes


43




a


-


43




c


designated by the operator via an operation panel, not shown, to the secondary image transfer region via the registration roller pair


42


. The registration roller pair


42


drives the paper


100


toward the secondary image transfer region such that the leading edge of the paper


100


meets the leading edge of the toner image formed on the belt


21


.




The conveyor unit


44


conveys the paper


100


carrying the full-color toner image thereon to the fixing unit


45


. The fixing unit


45


fixes the toner image on the paper


100


with the heat roller


45




a


and press roller


45




b


. The paper or copy


100


is then driven out to the copy tray


46


.




After the primary transfer, the drum cleaning blade


11




a


removes the toner left on the drum


10


, and then the brush roller


11




b


applies zinc stearate to the cleaned surface of the drum


10


.




In a repeat copy mode, the scanner section


1


having output the Y or fourth color image data for the first copy starts the Bk or first color step for the second copy at a preselected timing. The printer section


2


forms a Bk latent image for the second copy on the drum


10


. After the secondary transfer of the first full-color toner image from the belt


21


to the first paper


100


, a Bk toner image for the second copy is transferred from the drum


10


to the portion of the belt


21


having been cleaned by the cleaning blade


29




a.






In a three-color or a two-color copy mode, the illustrative embodiment operates in the same manner as in the above full-color or four-color mode except for the colors of toner. In a one-color copy mode, the developer stored in designated one of the developing units


15


-


18


is constantly held operative until a desired number of copies have been produced. In this case, the belt cleaning blade


29




a


and image transfer unit


30


are held in contact with the belt


21


while the belt


21


is held in contact with the drum


10


. In this condition, the belt


21


is driven forward at a preselected speed.




Part of the above construction and operation unique to the illustrative embodiment will be described more specifically hereinafter. As shown in

FIG. 2

, the primary transfer bias roller


22


is positioned downstream of a nip between the drum


10


and the belt


21


, i.e., a primary image transfer region. The power supply


28


applies a preselected bias for primary transfer to the bias roller


22


. The ground roller or discharging means


23


connected to ground is pressed against the inner surface of the belt


21


by a preselected pressure, so that the belt


21


is pressed against the drum


10


. The ground roller


23


therefore forms the start point of the nip between the drum


10


and the belt


21


.




It is noteworthy that the primary transfer bias roller


22


and ground roller


23


supporting the belt


21


replace a separate charge depositing member and a separate discharging member otherwise located at the above nip, thereby saving cost and space.




Further, in the illustrative embodiment, by simply connecting the ground roller


23


to ground, it is possible to discharge the charge deposited on the belt


21


by the primary transfer bias roller


22


. Consequently, the charge deposited on the belt


21


substantially does not migrate or migrates little to the side upstream of the start point of the nip between the belt


21


and the drum


10


. That is, the charge does not exist or exists little on the belt


21


upstream of the above nip. It follows that an electric field effecting the toner image transferred to the belt


21


does not exit at the side upstream of the nip. This, coupled with the fact that the belt


21


and drum


10


pressed against each other by the ground roller


23


press the toner entered the nip, causes the toner to cohere on the belt


21


.




As stated above, despite the bias applied to the bias roller


22


located downstream of the nip in the direction of movement of the belt


21


, no electric fields causative of pretransfer are formed at the upstream side. In addition, because the toner coheres at the nip, the toner image is disturbed little and prevented from being retransferred to the drum


10


even when subjected to an electric field at the downstream side. The ground roller


23


should preferably be pressed against the belt


21


by a pressure of 0.05 N/cm


2


or above. Should the pressure be excessively low, the effect achievable with the cohesion of the toner would be lost.




On the other hand, should the pressure pressing the ground roller


23


against the belt


21


be excessively high, both the adhesion of the toner to the drum


10


and the adhesion of the same to the belt


21


would increase. If the adhesion of the toner to the drum


10


increases, it is likely that the toner remains on the drum


10


and results in a vermicular image. In light of this, the above pressure should preferably be 2 N/cm


2


or below.




To increase the adhesion of the toner to the belt


21


, the drum


10


and belt


21


each may be formed of a particular material, or the amount of zinc stearate to be applied to the drum


10


and belt


21


may be adjusted. This, however, cannot fully obviate vermicular images because the adhesion is sometimes partly inverted.




A separate discharging member may be located at the above nip and implemented by any one of a brush, a blade and a roller. In such a case, a roller is preferable in consideration of damage to the belt


21


and the movement of the discharging member caused by the movement of the belt


21


. Further, because the separate discharging member would reduce the substantial image transfer region upstream of the discharge position, compared to the ground roller


23


forming the start point of the nip. The separate discharge member should therefore be positioned as close to the start point of the nip as possible. This is successful to form a relatively broad substantial image transfer region and therefore to increase the image transfer efficiency.




As stated above, the illustrative embodiment obviates pretransfer and retransfer of a toner image and thereby insures attractive images free from toner scattering.




Reference will be made to FlG.


3


for describing an alternative embodiment of the present invention also implemented as a full-color electrophotographic copier. This embodiment also includes the scanner section, not shown, and basically operates in the same manner as the previous embodiment. This embodiment differs from the previous embodiment mainly in the construction and operation of the printer section. As shown, the printer section includes the drum


10


. Arranged around the drum


10


are the optical writing unit, not shown, a drum cleaning unit or drum cleaning means


111


, the charger


13


, a revolver type developing unit (revolver hereinafter)


110


, and an intermediate image transfer unit or intermediate image transferring means


120


. The drum cleaning unit


111


includes a cleaning blade


111




a


and a brush roller


111




b


for applying a lubricant or solid zinc stearate


111




c


to the drum


10


. The printer section additionally includes an image transfer unit or image transferring means


130


and a fixing unit or fixing means


145


including a heat roller


145




a


and a press roller


145




b


as well as the paper feed section and controller described in relation to the previous embodiment.




The drum cleaning blade


111




a


is constantly held in contact with the drum


10


for cleaning the surface of the drum


10


after the primary transfer. The brush roller


111




b


is also held in contact with the drum


10


for applying the lubricant


111




c


to the drum


10


in order to enhance the cleaning ability. Specifically, when the brush roller


111




b


is caused to rotate by a drive mechanism, not shown, connected to the shaft of the roller


111




b


, the roller


111




b


shaves off the lubricant


111




c


and applies the resulting fine lubricant powder to the surface of the drum


10


.




The revolver


110


includes a Bk developing section


115


, a C developing section


116


, an M developing section


117


, and a Y developing section


118


. The revolver


110


is rotatable to bring any one of the developing sections


115


-


118


to a developing position where the developing unit faces the drum


10


.




The intermediate image transfer unit


120


includes an intermediate image transfer belt or intermediate image transfer body


121


passed over a primary transfer bias roller


122


, a ground roller or primary transfer predischarging means


123


, a drive roller or belt driving means


124


, a tension roller


125


, a secondary transfer counter roller


126


, and a cleaning counter roller


127


. A primary transfer power source


128


is connected to the primary transfer bias roller


122


. All the rollers over which the belt


121


is passed are electrically conductive, and all the rollers other than the bias roller


122


are connected to ground. The power source


128


applies a preselected bias subjected to constant current or constant voltage control to the bias roller


122


. The belt


121


is identical with the belt


21


of the previous embodiment except that it has a volume resistivity of 10


12


Ωcm to 10


14


Ωcm, preferably 10


13


Ωcm. The surface layer of the belt


121


has a surface resistance of 10


7


Ω/cm


2


to 10


14


Ω/cm


2


.




A belt cleaning blade


129




a


and a brush roller


129




b


for applying a lubricant or zinc stearate


129




c


to the belt


121


adjoin the belt


121


. A moving mechanism, not shown, selectively moves the blade


129




a


and brush roller


129




b


into or out of contact with the belt


121


. Another moving mechanism, not shown, moves the image transfer unit


130


into and out of contact with the belt


121


.




The image transfer unit


130


includes a belt or recording medium carrier


134


for effecting secondary transfer. A belt cleaning blade


132


cleans the surface of the belt


134


. A secondary transfer bias roller


131


faces the secondary transfer counter roller


126


included in the intermediate image transfer unit


120


. A secondary transfer power source


139


is connected to the bias roller


131


. The belt


134


is passed over a first support roller


135




a


located at a paper inlet end, a second support roller


135




b


adjoining the fixing unit


145


, and a third support roller


135




c


facing the belt cleaning blade


132


. The image transfer unit


130


additionally includes a paper discharger


136


and a belt discharger


137


. The belt


134


is formed of PVDF (polyvinyl idene fluoride) and has a volume resistivity as high as 10


13


Ωcm or above. If desired, the belt


134


may be replaced with a drum or any other suitable member.




The operation of the illustrative embodiment will be described on the assumption that a Bk, a C, an M and a Y toner image are sequentially formed in this order. Before the start of an image forming cycle, the drum


10


is rotated counterclockwise, i.e., in a direction indicated by an arrow C in

FIG. 3

, and the charger


13


starts corona discharge. For example, the charger


13


uniformly charges the drum


10


to a preselected negative potential. The belt


121


of the intermediate image transfer unit


120


is driven at the same speed as the drum


10


in a direction indicated by an arrow D in

FIG. 3

, i.e., clockwise.




The scanner section outputs color image data at a preselected timing as in the previous embodiment. The optical writing unit scans the charged surface of the drum


10


with a laser beam in accordance with Bk image data by, e.g., raster exposure. As a result, a Bk latent image is electrostatically formed on the drum


10


. The Bk developing section


115


of the revolver


110


develops the Bk latent image with toner charged to negative polarity (reversal development), thereby forming a Bk toner image.




The Bk toner image is transferred from the drum


10


to the belt


121


by an electric field formed in the primary image transfer region. The electric field is formed by a charge deposited on the belt


121


by the primary transfer bias roller


122


. For example, the power source


128


for primary transfer applies a bias of 1.5 kV to the bias roller


122


for the Bk or first color toner image, a bias of 1.6 kV to 1.8 kV for the C or second color toner image, a bias of 1.8 kV to 2.0 kV for the M or third color toner image, and a bias of 2.0 kV to 2 kV for the Y or fourth color toner image. The drum cleaning blade


111




a


removes the toner left on the drum


10


after the primary transfer, and then the brush roller


111




b


applies the lubricant


111




c


to the drum


10


.




The portion of the belt


121


carrying the Bk toner image is again returned to the primary transfer region as in the previous embodiment. At this time, the belt cleaning blade


129




a


and brush roller


129




b


are released form the belt


121


so as not to disturb the toner image. Also, the first support roller


125




a


and secondary transfer bias roller


131


are so moved as to release the bias roller


131


from the belt


121


. At this instant, the application of the bias from the power source


139


to the bias roller


131


is interrupted. This condition is maintained until the secondary transfer of a full-color toner image from the belt


121


to the paper


100


.




After the primary transfer of the Bk toner image to the belt


121


, the scanner section again reads the same document to output C image data. The optical writing unit forms a C latent image with a laser beam in accordance with C image data as in the previous embodiment. The C developing section


116


of the revolver


110


develops the C latent image to thereby produce a C toner image on the drum


10


.




In the illustrative embodiment, after the trailing edge of the Bk latent image has moved away from the developing position, the revolver


110


is immediately rotated. This rotation of the revolver


110


is completed before the leading edge of the C latent image arrives at the developing position where the C developing section


116


is positioned. In this condition, the developing section


116


develops the C latent image with C toner.




The above procedure is repeated with an M latent image and a Y latent image also. As a result, the Bk and C toner images and an M and a Y toner image are sequentially transferred from the drum


10


to the belt


121


one above the other, completing a full-color toner image.




The belt


121


carrying the full-color toner image conveys the toner image to the secondary image transfer region. At this instant, the secondary transfer bias roller


131


is brought into contact with the belt


121


. Subsequently, a preselected bias for secondary transfer is applied to the bias roller


131


so as to form an electric field in the secondary transfer region. As a result, the full-color toner image is transferred from the belt


121


to the paper


100


. Again, the paper


100


is fed such that the leading edge of the paper


100


meets the leading edge of the toner image at the secondary image transfer region.




The belt


134


of the image transfer unit


130


conveys the paper


100


carrying the full-color toner image to a position where the paper discharger


136


is located. The paper discharger


136


discharges the paper


100


and thereby peels off the paper


100


from the belt


134


. The paper


100


peeled off is conveyed toward the fixing unit


145


. In the fixing unit


145


, the heat roller


145




a


and press roller


145




b


fix the toner image on the paper


100


with heat and pressure. Subsequently, the paper or copy


100


is driven out to a copy tray not shown.




After the secondary transfer, the belt cleaning unit


129




a


is brought into contact with the belt


121


in order to remove the toner left on the belt


121


, and then the brush roller


129




b


applies the fine powder of the lubricant


129




c


to the belt


121


.




After the separation of the paper


100


from the belt


134


, the belt discharger


137


discharges the belt


134


, and then the belt cleaning blade


132


cleans the surface of the belt


134


.




Part of the construction and operation unique to the illustrative embodiment is as follows. As shown in

FIG. 3

, the primary transfer bias roller or charge depositing means


122


is positioned downstream of the nip between the drum


10


and the belt


121


in the direction of movement of the belt


121


, as in the previous embodiment. Also, the ground roller or discharging means


123


is positioned upstream of the above nip and presses the belt


121


against the drum


10


with a pressure between 0.05 N/cm


2


and 2 N/cm


2


. Therefore, the illustrative embodiment is also successful to obviate pretransfer and retransfer and therefore to insure attractive images free from toner scattering.




As shown in

FIG. 3

, the belt


121


is constantly pressed against the drum


10


by the ground roller


123


. This may bring about a problem that when the belt


121


is not driven over a long time, the drum


10


and belt


121


are apt to suffer from damage, and the belt


121


is apt to curl along the circumference of the ground roller


123


. The cur led portion of the belt


121


would vary the mechanical contact condition and therefore image transfer condition on entering the nip, resulting in a defective image ascribable to, e.g., irregular image transfer.




In light of the above, the illustrative embodiment additionally includes moving means for selectively moving the ground roller


123


into or out of contact with the belt


121


. The moving means may be implemented by, e.g., a cam device or a solenoid mechanism. Specifically, as shown in

FIG. 4A

, on the stop of rotation of the belt


121


, the moving means moves the ground roller


123


away from the belt


121


in response to a signal received from control means not shown. As a result, the belt


121


is released from the drum


10


and from the ground roller


123


. Alternatively, as shown in

FIG. 4B

, the ground roller


123


may be moved at least to a position where it does not press the belt


121


, but contacts the belt


123


. With this configuration, it is possible to prevent the belt


121


from being constantly pressed against the drum


10


and therefore to minimize damage to the belt


121


and drum


10


. Moreover, the belt


121


is prevented from curling along the circumference of the ground roller


123


even when held inoperative over a long time, thereby solving the above defective image problem.




While a conventional support roller for supporting the belt


121


has a diameter great enough to obviate the curling of the belt


121


, the illustrative embodiment including the above moving means is practicable with a roller having a relatively small diameter. In the illustrative embodiment, use is made of a roller having a diameter of 30 mm. Because a mechanism for mounting and dismounting the intermediate image transfer unit


120


is usual ly arranged between the opposite runs of the belt


121


together with other mechanisms, the roller diameter should preferably be as small as possible.




A series of experiments were conducted with the illustrative embodiment under the following conditions. The intermediate transfer belt


121


was 0.15 mm thick and 268 mm wide and had an inner peripheral length of 565 mm. The belt


121


was driven at a speed of 200 mm/sec. Further, the belt


121


had an about 1 μm thick surface layer formed of an insulating material and an about 75 μm thick intermediate layer formed of PVDF. The intermediate layer had a volume resistivity of 9×10


12


Ωcm when a voltage of 100 V was applied for 10 seconds or a volume resistivity of 6×10


12


Ωcm when a voltage of 500 V was applied for 10 seconds, as measured at a temperature of 25° C. and a humidity of 45% by a resistance measuring device Hirester IP available from Yuka Denshi. In addition, the belt


121


had an about 75 μm thick base layer formed of PVDF and titanium oxide. The base layer had a volume resistivity of 7×10


7


Ωcm when a voltage of 100 V was applied for 10 seconds, as measured in the above environment by the same measuring device.




The surface layer of the belt


121


had a surface resistance of 10


13


Ω/cm


2


as measured by the above measuring device. To measure the surface resistance, use may be made of a measuring method prescribed by JIS (Japanese Industrial Standards) K6911 in place of the above measuring device.




The primary transfer bias roller


122


was implmented by a metal roller plated with nickel while the ground roller


123


was implemented by a metal roller. The other rollers were formed of metal or conductive resin. The bias roller


122


was applied with a DC voltage of 1.5 kV for the Bk or first color toner image, a DC voltage of 1.7 kV for the C or second color toner image, a DC voltage of 1.9 kV for the M or third color toner image, and a DC voltage of 2.1 kV for the Y or fourth color toner image. The primary image transfer region had a nip width of 10 mm.




In the image transfer unit


130


, the secondary transfer bias roller


131


had a surface layer formed of conductive sponge or conductive rubber and a core layer formed of metal or conductive resin. A particular transfer bias subjected to constant current control was applied to the bias roller


131


for each of different kinds of papers, as shown in FIG.


5


. The secondary image transfer belt


134


was formed of PVDF and had a volume resistivity of 10


13


Ωcm and a thickness of 100 μm.




The paper discharger


136


and belt discharger


137


each were applied only with an AC voltage or an AC+DC voltage from a power supply not shown. The cleaning blade


132


contacted the portion of the secondary transfer belt


134


contacting the third support roller


135




c


in a counter orientation.




In

FIG. 3

, the primary transfer bias roller


122


was located downstream of the nip between the drum


10


and the intermediate transfer belt


121


in the direction of movement of the belt


121


. The ground roller


123


connected to ground was pressed against the belt


121


by a pressure between 0.05 N/cm


2


and 2 N/cm


2


, so that the belt


121


was pressed against the drum


10


. Under the above conditions, the illustrative embodiment successfully obviated pretransfer at the downstream side and retransfer at the upstream side and thereby produced desirable images.




Another alternative embodiment of the illustrative embodiment is shown in FIG.


6


and also implemented as a full-color electrophotographic copier. This embodiment is directed mainly toward a low cost construction. Because this embodiment is similar to the embodiment of

FIG. 3

except for the following, identical structural elements are designated by identical reference numerals.




As shown in

FIG. 6

, this embodiment includes an intermediate image transfer unit


220


including an intermediate image transfer belt


221


. The belt


221


has an overall volume resistivity of 10


10


Ωcm to 10


12


Ωcm. Specifically, the belt


221


includes an intermediate layer having a medium volume resistivity of 10


8


Ωcm to 10


11


Ωcm, and a surface layer having a surface resistance of 10


7


Ω/cm


2


to 10


14


Ω/cm


2


. With the belt


221


having a medium resistance, it is possible to free the surface of the belt


221


from irregular charging after the primary transfer.




A drive roller


224


included in the intermediate image transfer unit


220


is located downstream of the secondary image transfer region, but upstream of the primary image transfer region, in the direction of movement of the belt


221


. A belt cleaning blade


229




a


faces the drive roller


224


. In this sense, the drive roller


224


plays the role of the cleaning counter roller


127


of the previous embodiment at the same time. The reference numerals


229




b


and


229




c


designate a brush roller and a lubricant, respectively.




A secondary bias roller


231


and a power supply


802


constitute image transferring means and replace the image transfer unit of the embodiment shown in FIG.


3


. The bias roller


231


faces the secondary transfer counter roller


126


of the intermediate image transfer unit


220


. This configuration reduces the number of parts necessary for the secondary transfer and thereby reduces the cost, compared to the embodiment shown in FIG.


3


.




In the illustrative embodiment, the secondary transfer bias roller


231


and belt


221


directly nip the paper


100


fed to the secondary image transfer position and drive it toward the heat roller


145




a


and press roller


145




b.






Part of the above construction and operation particular to this embodiment will be described hereinafter. As shown in

FIG. 6

, a ground roller


223


is so positioned as to contact the belt


221


although the former does not press the latter. This prevents the belt


221


from wrapping around the ground roller


223


and therefore prevents it from curling along the circumference of the ground roller


223


even when left inoperative over a long time. This embodiment not only achieves the same advantages as the embodiment of

FIGS. 1 and 2

, but also obviates defective images ascribable to the variation of image transfer condition.




A series of experiments were conducted with the above embodiment under the following conditions. The structural members except for ones to be described hereinafter are identical with the structural members of the embodiment of FIG.


3


. The belt


221


had an intermediate layer formed of PVDF and titanium oxide and had a volume resistivity of 5×10


2


Ωcm when applied with a voltage of 100 V for 10 seconds or a volume resistivity of 2×10


11


Ωcm when applied with a voltage of 500 V for 10 seconds, as measured at a temperature of 25° C. and a humidity of 45% by Hirester mentioned earlier. The surface layer and base layer of the belt


221


were identical with the surface layer and base layer of the belt


121


of the previous embodiment. The belt


221


was moved at a speed of 156 mm/sec.




The bias roller


122


was applied with a DC voltage of 1.7 kV for the Bk or first color toner image, a DC voltage of 1.8 kV for the C or second color toner image, a DC voltage of 1.9 kV for the M or third color toner image, and a DC voltage of 2.0 kV for the Y or fourth color toner image. The bias roller


231


for secondary transfer was formed of conductive rubber. As shown in

FIG. 7

, a particular bias subjected to constant current control was applied to the bias roller


231


for each of different kinds of papers.




As shown in

FIG. 6

, a primary transfer bias roller


222


was located downstream of the nip between the drum


10


and the belt


121


in the direction of movement of the belt


121


. The ground roller


223


was located upstream of the above nip to press the belt


221


toward the drum


10


with a pressure between 0.05 N/cm


2


and 2 N/cm


2


. Under these conditions, the illustrative embodiment successfully obviated pretransfer at the downstream side and retransfer at the upstream side.




A further alternative embodiment of the present invention will be described hereinafter which is applicable to an image forming apparatus of the type including a belt for conveying a paper, OHP sheet or similar recording medium. As shown in

FIG. 8

, the illustrative embodiment is applied to the drum or image carrier


10


in place of the intermediate image transfer body shown and described. In

FIG. 8

, the reference numeral


311




a


designates a cleaning blade while the reference numerals


335




a


and


225




b


designate support rollers. In the illustrative embodiment, a toner image is formed on the drum


10


by a conventional electrophotographic process. The toner image is transferred to the paper


100


at the nip between the drum


10


and a belt


334


included in an image transfer unit


330


.




Specifically, in the image transfer unit


330


, a transfer bias roller or charge depositing means


331


is located downstream of the above nip in the direction of movement of the belt


334


. A power supply, not shown, applies a preselected bias for image transfer to the bias roller


331


. As a result, an electric field is formed at the nip between the drum


10


and the belt


334


, so that a toner image is transferred from the drum


10


to the paper


100


being conveyed by the belt


334


. The belt


334


has a medium volume resistance of 10


8


Ωcm to 10


11


Ωcm.




Part of the above construction unique to the illustrative embodiment is as follows. As shown in

FIG. 8

, the bias roller


331


is located downstream of the nip, as stated above. A ground roller or discharging means


333


is connected to ground and located upstream of the above nip in such a manner as to press the belt


334


toward the drum


10


with a pressure between 0.05 N/cm


2


and 2 N/cm


2


. In this condition, the ground roller


333


pressed against the belt


334


causes the belt


334


to contact the drum


10


and thereby forms the start point of the nip.




In this embodiment, the ground roller


333


discharges the charge deposited on the belt


334


by the bias roller


331


. Therefore, the charge deposited on the belt


334


substantial ly does not migrate or migrates little to the side upstream of the start point of the nip. That is, the charge does not exist or exists little on the belt


334


upstream of the above nip. It follows that an electric field effecting the toner image transferred to the belt


334


does not exit at the side upstream of the nip. This, coupled with the fact that the belt


334


and drum


10


pressed against each other by the ground roller


333


press the toner entered the nip, causes the toner transferred to the paper


100


to cohere.




As stated above, even when the bias is applied to the bias roller


331


located downstream of the nip in the direct ion of movement of the belt


334


, pretransfer does not occur because no electric fields are formed at the upstream side. In addition, the toner image is disturbed little by the downstream electric field because the toner coheres at the nip, obviating retransfer.




All the embodiments shown and described insure attractive images free from toner scattering by obviating pretransfer and retransfer. The characterizing parts of the illustrative embodiments may be replaced with each other.




While each illustrative embodiment has been shown and described as including a ground or discharging means connected to ground, a bias opposite in polarity to the transfer charge may alternatively be applied to the ground roller so long as it does not effect the transfer charge required at the nip.




The bias roller or charge depositing means of any one of the illustrative embodiments may be replaced with any other suitable charge depositing means.




The embodiments described with reference to

FIGS. 1-6

each use a secondary transfer bias roller as secondary transfer charge depositing means. The secondary transfer bias roller may, of course, be replaced with a blade, brush or similar secondary transfer charge depositing means. The embodiments described with reference to

FIGS. 3 and 6

each are operable even in a copy mode other than the full-color copy mode like the embodiment of FIG.


1


.




In all the illustrative embodiments, the photoconductive drum


10


may be replaced with any other suitable image carrier, e.g., a photoconductive belt passed over two or more rollers.




In the embodiments of

FIGS. 1-6

, the intermediate transfer belt may have any suitable electrical characteristic including a surface resistance, structure and thickness matching with image forming conditions.




In the embodiments shown and described, the drum or image carrier


10


is charged to negative polarity while the developing means effects reversal development by using a two-ingredient type developer, i.e., a toner and carrier mixture. If desired, the drum


10


may be charged to positive polarity, and the developing means may use a single ingredient type developer, i.e., toner or may effect positive development.




In summary, the present invention achieves the following various unprecedented advantages.




(1) A charge deposited on an intermediate image transfer belt is discharged by a discharging member at a nip between an image carrier and the belt. This prevents the influence of an electric field for image transfer from extending to the side upstream of the nip in the direction of movement of the belt and thereby obviates pretransfer, i.e., the transfer of toner from the image carrier to the belt at the upstream side. The discharging member contacts the belt with a pressure between 0.05 N/cm


2


and 2 N/cm


2


, so that the belt and image carrier contact with each other with a pressure high enough to cause the toner to cohere at the nip. As a result, a toner image once transferred from the image carrier to the belt is disturbed little by the above electric field at the side downstream of the nip. This successful ly obviates pretransfer and retransfer causative of toner scattering. Should the above pressure be excessively high, the toner would cohere to an excessive degree and would remain on the image carrier at the time of image transfer, resulting in a vermicular image. The pressure of 2 N/cm


2


or below solves such a problem. This advantage is also achievable when the intermediate transfer belt is replaced with a transfer belt or recording medium carrier.




(2) By simply connecting the discharging member to ground, it is possible to reduce a charge deposited on the belt.




(3) The discharging member discharges the belt in the vicinity of the start point of the above nip. Therefore, an image transfer region upstream of the discharging position and contributing to image transfer is broadened, compared to a case wherein the discharging member is located downstream of the start point of the nip. It follows that higher image transfer efficiency is achievable.




(4) Because the belt is not wrapped around the discharging member, the belt is prevented from curling along the circumference of the discharging member even when left unused over a long time. A curled belt would vary the image transfer condition and would thereby bring about a defective image ascribable to, e.g., irregular image transfer.




(5) Moving means is capable of moving the discharging member to a position where the discharging member does not press the belt, but contacts the belt, or a position where it is spaced from the belt. This also achieves the above advantage (4), and in addition reduces damage to the belt and image carrier otherwise pressed against each other. This is also true when the discharging member is replaced with a pressing member.




(6) A roller member, as distinguished from a brush or a blade, reduces damage to the belt even when it exerts a high pressure against the belt. In addition, the roller member does not follow the rotation of the belt when the belt is driven.




(7) Support rollers supporting the belt play the role of a discharging member and a charge depositing member at the same time. This makes it needless to arrange a separate discharging member and a separate charge depositing member and thereby simplifies the construction.




(8) The belt is not wrapped around a pressing member. This is also successful to achieve the above advantage (4).




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.



Claims
  • 1. An image transferring method comprising the steps of:discharging a charge deposited on an intermediate image transfer belt at a nip located between an image carrier and said intermediate image transfer belt; moving said intermediate image transfer belt so as to contact a surface of said image carrier over a preselected distance; depositing a transfer charge on said intermediate image transfer belt at a position downstream of said nip in a direction of movement of said intermediate image transfer belt; transferring a toner image formed on said image carrier to said intermediate image transfer belt by an electric field formed at said nip; and providing a discharging member, for discharging said charge deposited on said intermediate image transfer belt at said nip so that said discharging member is in contact with a surface of said intermediate image transfer belt opposite to a surface contacting said image carrier with a pressure between 0.05 N/cm2 and 2 N/cm2.
  • 2. An image transferring method comprising the steps of:discharging a charge deposited on an image transfer belt, said charge being deposited on said image transfer belt at a nip between an image carrier and said image transfer belt, wherein said image transfer belt is moving while contacting a surface of said image carrier over a preselected distance with an intermediary of a recording medium; depositing a transfer charge on said image transfer belt at a position downstream of said nip in a direction of movement of said image transfer belt; and transferring a toner image formed on said image carrier to said recording medium by an electric field formed at said nip; and using a discharging member to discharge said charge onto said image transfer belt at said nip, said discharging member being in contact with a surface of said image transfer belt opposite to a surface contacting said image carrier with a pressure between 0.05 N/cm2 and 2 N/cm2.
  • 3. An image forming apparatus comprising:an image carrier; and an intermediate image transfer unit, said intermediate image transfer unit comprising: an intermediate image transfer belt, said intermediate image transfer belt being movable while contacting a surface of said image carrier over a preselected distance; a discharging member for discharging a charge deposited on said intermediate image transfer belt at a nip between said intermediate image transfer belt and said image carrier; and a charge depositing member for depositing a transfer charge on said intermediate image transfer belt at a position downstream of said nip in a direction of movement of said intermediate image transfer belt, whereby a toner image formed on said image carrier is transferred to said intermediate image transfer belt by an electric field formed at said nip; and said discharging member discharging said charge deposited on said intermediate image transfer belt at said nip so that said discharging member is in contact with a surface of said intermediate image transfer belt opposite to a surface contacting said image carrier with a pressure between 0.05 N/cm2 and 2 N/cm2.
  • 4. An apparatus as claimed in claim 3, wherein said discharging member is connected to ground.
  • 5. An apparatus as claimed in claim 4, wherein said discharging member adjoins a start point of said nip.
  • 6. An apparatus as claimed in claim 4, wherein a start point of said nip coincides with a position where said discharging member and said intermediate image transfer belt contact each other.
  • 7. An apparatus as claimed in claim 4, wherein said discharging member is located to contact said intermediate image transfer belt without being pressed against said intermediate image transfer belt.
  • 8. An apparatus as claimed in claim 4, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said intermediate image transfer belt into contact with said image carrier and a position where said discharging member contacts said intermediate image transfer belt without being pressed against said intermediate image transfer belt or a position where said discharging member is spaced from said intermediate image transfer belt.
  • 9. An apparatus as claimed in claim 4, wherein said discharging member comprises a roller.
  • 10. An apparatus as claimed in claim 4, wherein said charge depositing member comprises a support roller supporting said intermediate image transfer belt.
  • 11. An apparatus as claimed in claim 3, wherein said discharging member adjoins a start point of said nip.
  • 12. An apparatus as claimed in claim 11, wherein said discharging member is located to contact said intermediate image transfer belt without being pressed said intermediate image transfer belt.
  • 13. An apparatus as claimed in claim 11, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said intermediate image transfer belt into contact with said image carrier and a position where said discharging member contacts said intermediate image transfer belt without being pressed against said intermediate image transfer belt or a position where said discharging member is spaced from said intermediate image transfer belt.
  • 14. An apparatus as claimed in claim 11, wherein said discharging member comprises a roller.
  • 15. An apparatus as claimed in claim 11, wherein said charge depositing member comprises a support roller supporting said intermediate image transfer belt.
  • 16. An apparatus as claimed in claim 3, wherein a start point of said nip coincides with a position where said discharging member and said intermediate image transfer belt contact each other.
  • 17. An apparatus as claimed in claim 16, wherein said discharging member is located to contact said intermediate image transfer belt without being pressed said intermediate image transfer belt.
  • 18. An apparatus as claimed in claim 16, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said intermediate image transfer belt into contact with said image carrier and a position where said discharging member contacts said intermediate image transfer belt without being pressed against said intermediate image transfer belt or a position where said discharging member is spaced from said intermediate image transfer belt.
  • 19. An apparatus as claimed in claim 16, wherein said discharging member comprises a roller.
  • 20. An apparatus as claimed in claim 16, wherein said charge depositing member comprises a support roller supporting said intermediate image transfer belt.
  • 21. An apparatus as claimed in claim 3, wherein said discharging member is located to contact said intermediate image transfer belt without being pressed said intermediate image transfer belt.
  • 22. An apparatus as claimed in claim 21, wherein said discharging member comprises a roller.
  • 23. An apparatus as claimed in claim 21, wherein said charge depositing member comprises a support roller supporting said intermediate image transfer belt.
  • 24. An apparatus as claimed in claim 3, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said intermediate image transfer belt into contact with said image carrier and a position where said discharging member contacts said intermediate image transfer belt without being pressed against said intermediate image transfer belt or a position where said discharging member is spaced from said intermediate image transfer belt.
  • 25. An apparatus as claimed in claim 24, wherein said discharging member comprises a roller.
  • 26. An apparatus as claimed in claim 24, wherein said charge depositing member comprises a support roller supporting said intermediate image transfer belt.
  • 27. An apparatus as claimed in claim 3, wherein said discharging member comprises a roller.
  • 28. An apparatus as claimed in claim 3, wherein said charge depositing member comprises a support roller supporting said intermediate image transfer belt.
  • 29. An image forming apparatus comprising:an image carrier; and an image transfer unit, said image transfer unit comprising: an image transfer belt, said image transfer belt being movable while contacting a surface of said image carrier over a preselected distance with an intermediary of a recording medium; a discharging member for discharging a charge deposited on said image transfer belt at a nip between said image transfer belt and said image carrier; a charge depositing member for depositing a transfer charge on said image transfer belt at a position downstream of said nip in a direction of movement of said image transfer belt, whereby a toner image formed on said image carrier is transferred to the recording medium by an electric field formed at said nip; and said discharging member discharging said charge deposited on said image transfer belt at said nip so that said discharging member is in contact with said image transfer belt at a surface opposite to a surface contacting said image carrier with a pressure between 0.05 N/cm2 and 2 N/cm2.
  • 30. An apparatus as claimed in claim 29, wherein said discharging member is connected to ground.
  • 31. An apparatus as claimed in claim 30, wherein said discharging member adjoins a start point of said nip.
  • 32. An apparatus as claimed in claim 30, wherein a start point of said nip coincides with a position where said discharging member and said image transfer belt contact each other.
  • 33. An apparatus as claimed in claim 30, wherein said discharging member is located to contact said image transfer belt without being pressed against said image transfer belt.
  • 34. An apparatus as claimed in claim 30, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said image transfer belt into contact with said image carrier and a position where said discharging member contacts said image transfer belt without being pressed against said image transfer belt or a position where said discharging member is spaced from said image transfer belt.
  • 35. An apparatus as claimed in claim 30, wherein said discharging member comprises a roller.
  • 36. An apparatus as claimed in claim 30, wherein said charge depositing member comprises a support roller supporting said image transfer belt.
  • 37. An apparatus as claimed in claim 29, wherein said discharging member adjoins a start point of said nip.
  • 38. An apparatus as claimed in claim 37, wherein said discharging member is located to contact said image transfer belt without being pressed against said image transfer belt.
  • 39. An apparatus as claimed in claim 37, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said image transfer belt into contact with said image carrier and a position where said discharging member contacts said image transfer belt without being pressed against said image transfer belt or a position where said discharging member is spaced from said image transfer belt.
  • 40. An apparatus as claimed in claim 37, wherein said discharging member comprises a roller.
  • 41. An apparatus as claimed in claim 37, wherein said charge depositing member comprises a support roller supporting said image transfer belt.
  • 42. An apparatus as claimed in claim 37, wherein a start point of said nip coincides with a position where said discharging member and said image transfer belt contact each other.
  • 43. An apparatus as claimed in claim 42, wherein said discharging member is located to contact said image transfer belt without being pressed against said image transfer belt.
  • 44. An apparatus as claimed in claim 42, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said image transfer belt into contact with said image carrier and a position where said discharging member contacts said image transfer belt without being pressed against said image transfer belt or a position where said discharging member is spaced from said image transfer belt.
  • 45. An apparatus as claimed in claim 42, wherein said discharging member comprises a roller.
  • 46. An apparatus as claimed in claim 42, wherein said charge depositing member comprises a support roller supporting said image transfer belt.
  • 47. An apparatus as claimed in claim 29, wherein said discharging member is located to contact said image transfer belt without being pressed against said image transfer belt.
  • 48. An apparatus as claimed in claim 47, wherein said discharging member comprises a roller.
  • 49. An apparatus as claimed in claim 47, wherein said charge depositing member comprises a support roller supporting said image transfer belt.
  • 50. An apparatus as claimed in claim 47, wherein said intermediate image transfer unit further comprises moving means for moving said discharging member between a position where said discharging member presses said image transfer belt into contact with said image carrier and a position where said discharging member contacts said image transfer belt without being pressed against said image transfer belt or a position where said discharging member is spaced from said image transfer belt.
  • 51. An apparatus as claimed in claim 47, wherein said discharging member comprises a roller.
  • 52. An apparatus as claimed in claim 29, wherein said discharging member comprises a roller.
  • 53. An apparatus as claimed in claim 29, wherein said charge depositing member comprises a support roller supporting said image transfer belt.
Priority Claims (1)
Number Date Country Kind
10-333099 Nov 1998 JP
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Number Date Country
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10-161440 Jun 1998 JP