Note: Descriptions are shown in the official language in which they were submitted.
4770
BACKGROUND OF ~HE IN~NTION
The lnvention relates to an lmage transfer
unit for use in an electrophotographic copying machine
which incorporates a transfer roller to which a bias volt-
age is applied.
An image transfer unit of the kind described ;
iB confronted with the problem of smearing of the back
side of the transfer sheets that results from the deposi- ~ ;
tion on the surface o~ the transfer roller of a toner, - ;
which i~ floating in ~he internal space of the copying machine
or which iB dri~en from non-image area~ on a photosensitive
member. ~he deposlted toner contaminates th¢ back ~ur$ace
of the transfer sheet which is held between the photosen-
sitive member and the transfer roller during the image
transfer step. Therefore, it 19 nece~sary to pro~ide some
cleanlng unit for removln~ toner which 1B depo~ited on
the tran~fer roller ~n order to pre~ent a ~mearing of the
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back side of the transfer sheet. ; `;
Conventional means for cleaning the trans-
fer roller comprises a blade which i8 brought into abutting
relatlon~hip therewith to ~crape any toner therefrom, or
comprises a rotatable brush which i8 urged against the
transfer roller to wipe off any toner thereon. These ~:
arrangements require some form of toner processing means
which collects the separated toner or which prevents re~
deposition of the separated toner onto the transfer roller. ;~; ~
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SUMM~RY OF THE IN~EN~ION ~ ~
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In accordance with the invention, there
1s provided an image trans~er unit which comprises a~
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electrically conductive image transfer roller di~po~ed
in abuttlng relationship with a photosen~itlve member~
an electrically conductlve, rotatable cleaning means dis-
posed in abuttin~ relation~hip with the $mage transfer
roller, and electrical bias means connected with the clean
ing means and having a switchable polarity. Durlng an
image transfer Btep~ a bias ~oltage i8 applied to the tran~-
fer roller of the opposite polarity ~rom that of the toner
which forms the visual image, causing the visual image to
be trans~erred onto a copy ~heet. ~oner attachlng to the
transfer roller i9 electrostatically transferred onto the
cleaning meanR. During a cleanlng step 3 a bias ~oltage of
the same polarity as the toner i8 applled to the cleaning
means to transfer the toner which attaches to the cleanl~g
means back to the photosensitive member through the trans-
fer roller. Slnce the photo6ensitive member i8 chsrged
to the o~posite polarity from the toner, lt reaaily attracts
the toner. The toner which is transferred back to the
photosensitive member is removed ~y a further cleaning unit which
ls associated wlth the photosensitive member. In thls
manner, the image transfer unlt according to the lnventlon
represents simple mean~ which reliably removes any toner
attaching to the transfer roller.
~here~ore, lt is an obJect of the inven-
tion to provide an lmpro~ed image transfer unit having
cleaning mean~ associated wlth an image transfer roller.
It i8 another ob~ect of the ln~ention to
pro~ide an image trans~er unit of the kind described which
iB eimple ln con~truction and capable of reliably removi~g
a~y toner attaching to t~e transfer roller.
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More particularly, there is provided:
A transfer unit for use in an electrophotographic
copying machine of the type having a photosensitive member on
which a toner image is formed said unit comprising:
electrically conductive transfer roller means for
pressing a transfer sheet against the photosensitive member for ~
transferring a toner image formed on the photosensitive member : ~:
onto said transfer sheet;
electrically conductive, rotatable cleaning means .
disposed in abutting relationship with the transfer roller means,
for cleaning toner therefrom;
electrical bias means for producing a bias voltage
of switcnable polarity; and
switching means, connecting said bias means to sai~ ~:
cleaning means during the transfer of the toner image onto the
transfer sheet, for applying a bias voltage of the opposite
polarity from that of the toner to the transfer roller means `
through the cleaning means, such that the toner attaching to the
transfer roller means is electrostatically transferred to the
: 20 cleaning means to clean the transfer roller means, and after
completion of an image transfer step, for switching the bias
voltage applied to the cleaning means to the same polarity as
that o~ the toner to transfer the toner attaching to the clean-
ing means back to the photosensitive member through the transfer ~ -;
roller means, thus cleaning both the cleaning means and the tran- :
trans~er roller means. .. .
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31L~B4~70
There is also provided: ~
A transfer unit for use in an electrophotographic
copying machine of the type having a photosensitlve member on
which a toner image is formed, said unit comprising:
electrically-conductive transfer roller means for
pressing a transfer sheet against the photosensitive member ~o
transfer a toner image formed on the photosensitive member onto
said transfer sheet;
electrically-conductive rotatable cleaning means
disposed in abutting relationship with the toner transfer rol-
ler means, for cleaning toner therefrom;
first means for connecting a first bias voltage
of switchable polarity to said transfer roller means;
second means for connecting a second bias voltage
of switchable polarity to said cleaning means, said second bias
voltage being greater than said first bias voltage; and
switching means, connectea to said first and second
connecting means, for applying said first and second bias vol-
tages with a polari~y~opposite to that of the toner respectively
to said transfer roller means and said cleaning means such that
the toner attaching to the transfer roller means is electro-
statically transferred to the cleaning means to clean the trans-
fer roller means during the transfer of the toner image onto the
transfer sheet, and after completion of an image transfer step, ~;
for applying said first and second bias voltages with the same
polar.ity as that of the toner respectively to said transfer
roller means and said cleaning means back to the photosensitive
member th~ough the transfer roller means, thus cleaning both the
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cleaning means and the transfer roller me.~ns. :~
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The above and other ob jects of the invention
will become apparent as the description proceeds with
reference to the drawing~.
_RIEF DESCRIITION OF THE_DRAWINGS ~ ,
~ig. 1 1B a schematic view of one embodi-
ment of the invention;
Fig. 2 is a ~ragmentary perspective view
of the embodiment shown in Fig. l;
Fig. 3 is a circuit diagram showing the
electrical equivalent circuit of the embodiment shown in
Figs. 1 and 2;
Fig. 4 is a ~chematic illustration, graph-
ically showing the transfer of toner between the cleaning
roller and the transfer roller;
Fig. 5 graphically shows the proportion
of the transferred.toner as a function of the resistance
of the cleanlng roller; ~ `
~igs. 6 and 7 are schematic views illustrat- ;i :;.
ing other embodiments of the invention; and
Fig. 8 is a circuit diagram showing the
equivalent clrcuit of the embodiment shown in Fig. 7.
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DE~AI~ED DESCRIPTION 0~ PREFERRED EMBODIM~NTS
Referring to Fig. 1, there i~ shown a
photosensitive member 2 in the form of a belt which carriee . : .
a developed toner image 1 thereon. ~he belt 2 i9 adapted ~ .
to be driven by a,grounded roller 3 to run along a path ~ :
indicated by appropriate arrows, which includes an image tranS~e
roller 5. A blae ~oltage o~ the opposite polarlty from
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that of the toner ls applied to the transf~r roller 5, and
as a portion of the belt whlch carrlea the toner image
approaches the transfer roller 5, a transfer ~heet 4 is
fed into superimposed and abutting relationshlp with the
belt, whereby the toner image 1 is transferred onto the
transfer sheet 4.
The lmage transfer roller 5 comprises a :;
metallic roller having lts surface coated with an ~lec-
trically conduct.ive rubber material, and a bias voltage
0 lS applied thereto through a cleaning roller 6 having a similar
surface coating of an electrically conductive rubber mate-
rial which i8 disposed ln abutting relationship with the
surface of the transfer roller 5. The cleanlng roller 6
~ is connected to either of a pair of d-c. high voltage sources 7,
8 of oppoqite polaritie~ through a switch 9. The cleaning
roller 6 has a diameter which i9 reduced as compared with
the diameter of the transfer roller 5 so that contact ~ .
occurs therebetween at varying point~ during the rotation
thereof. :
Fig~ 2 1~ a fragmentary perspective view
showing a specific con~truetion. The trans~er roller 5
is mounted on a metal shaft 5a which 1R mounted ln a pair
of insulating sideplates 12 of a transfer unit by means
of bearings 13. ~he cleaning roller 6 has it~ support :
shaft also rotatably mounted in the sideplates 12 by means ~ ~:
of bearings 14, whlch however are dlsplaceably mounted.
An electrically conductive ~-shaped lever 15 is rockably
mounted on one o~ the sideplates 12 by a ~crew 16, and a
coiled tension spring 18 is disposed between an opening
15a formed in the end of one limb of the le~er 15 and a
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Rcrew 17 fixed to the ~ideplate 12, thus urg1ng the lever
15 to rotate clockwise about the ~cr~w 16. The other
limb of the lever 15 engages a groove 14a formed ln the
bearing 14 and is thus biased by the spring 18 to urge the
cleaning roller into abutting engagement with the transfer
roller 5. One end of a lead wire 19 is secured to the
le~er 15 by a set screw 20, and the other end of the lead
wire 19 is connected with a power supply assembly 21. ~he
~ransfer unit also includes a cover 22 whlch also serves a~
a guide for the transfer sheets when separated from the
transfer roller 5.
Fig. 3 shows the electrical equivalent
c~rcuit of the above embodiment. A character R2 represent~
the resistance of the cleaning roller which i9 8ubgtan-
tially equal to or greater than the resl~tance Rl of the
transfer roller. As a consequence, when a bias voltage
i~ applied from the cleaning roller to the transfer roller
during an image transfer step, and a portion of the developer
toner on the photosensiti~e member which lie8 out~ide the
reg~on of a transfer cheet lnitially attaches to the trans-
fer roller, the tonerwhich attaches to the transfer
roller is then direct}y transferred, for the most part, to
the cleaning roller because of the relative magnitudes of
the resistances of the transfer roller and the cleaning ;
Z5 roller, and accordingly the greater attraction of the higher vol-
tage on the cleaning roller than that on the transfer roller (it
will be appreciated that the voltage on the cleaning roller will `
be still higher by an amount corresponding to the voltage
drop when both are of the same resistance). As a result,
the toner which remains attached to the transfer roller i8 ~;
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~70
considerably reduced, thereby effectively preventing a
smearing of the back 6urface of the transfer sheet by con-
tact with the transfer roller.
After completion of an ~mage transfer
step, the bias voltage is changed by switch 9 to the opposite
polarity from the polarity utilized during the transfer step,
that is to the same polarity as the toner. Thereupon, the toner
attaching to the cleaning roller will be transferred to
the lower ~oltage transfer roller and then to the photosensitive
member which is charged to the same polarity as the toner,
whereby the surface of both the transfer roller and the
cleaning roller will revert to the clean condition which they
assumed before the transfer step took place. The toner
attaching to the photosen~itive member lc carried thereon
to a cleaning station where it i6 removed ~rom the surface
of the photosensitive member.
In the above embodiment, the cleanlng roller
has a resistance which is greater than or substantially
equal to the resistance of the transfer roller. However,
thi8 is not essential~ and the cleaning roller may have a
resi~tance which i9 less than that of the transfer ro~ler.
~he reason ~or this will be understood by reference to
Fig. 4 where it will be noted that there exists a very thin
air layer A ln the region of contaot between the tranRfer `
roller ~ and the cleaning roller 6, and a voltage drop VD
acro~s the resiBtance of the air layer A and the resistance
Of the olean~ng roller 6 produce~ an attraction of the
toner. Such an air layer will be eliminated when both
rollers are urged agal~st each other with a pressure on
the order of several ~ilograms per square centimeter7 but
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remains or is maintained when a normal pressure of one kilo-
gram per square centimeter is employed. As a consequence,
lt can be said that the transfer of the toner from the
transfer roller ~ to the cleaning roller 6 takes place
independently from the resistance of the tran~fer roller 5
ana i9 ~olely determined by the resistance o~ the ~leaning
roller 6. ;~
Referring to Fig. 5, it will be seen that
when the resistance of the cleani~g roller i~ too low a~ ~
will be the oase with a metallic roller, the proportion i
of transferred toner wlll be reduced. It fOllOWB therefore ~;
that a roller having a certain high value of resistance,
such as a roller of an electrically conductive rubber, i8
preferred. However, an excesBively high resistance re~ults
ln charging the roller surface, again reducing the propor- ;
tion of transferred toner.
Fig. 6 shows an arrangement for another
embodiment of the invention. This embodiment differs `
from the ~irst mentioned embodiment ln that the cleaning
roller is replaced by an a~sembly comprising a palr of
electrically conduotive (metallic) roller~ 31 and 32, and
a belt 33 formed of an electrically oonducti~e rubber
which extend~ around these rollers 31, 32. The conductive
roller 32 i8 connected with a pair of d.c. high blas ~olt-
age sources 34, 35 of opposite polarities through a switch
36. The function of these elements is similar to those -
mentio~ed above and therefore will not be repeated.
Fig. 7 shows an arrangement which represents
a further embodiment of the inYention. In this embodiment9
a bias voltage 1~ applied to the transfer roller through
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a cleaning roller and al60 through a separate resistor
R3 in order to prevent any significant change 1n the bia~
voltage applied to the transfer roller or a resulting
degradation in the transfer efficiency i~ the event the
resistance of the roller undergoe3 a conslderable change
as re~ult of the deposition of the toner onto the tran~er
roller and ~he cleaning roller. The electrical equi~alent
circuit i~ shown ~n Fig. 8, where lt will be noted that
the re~i~tance Rl of the transfer roller and the resistance
R2 ~ the cleaning roller will vary as the to~er atta¢hes
to the roller surface, with a corre~pond~ng cha~ge ln the
bia~ ~oltage ~pplied. Since the lnitlal value of the bias
~oltage is cho~en 80 as to provide an optimum transfer
efflclency, a change of the bia~ voltage from such value
implieQ a reduction ln the image transfer e~ficlency. ~he
connectlon of the re~istance R3 between the tran~fer roller
and the blas source will compensate for any change ln the
~alue~ of the resistance Rl and the resistance R2 ~ as
to malntain the bia~ ~oltage in a range which pre~ents a ~
reduction ln the lmage transfer efficiency. `
In the three embodiments described above,
the photosensitive member u~ed is in the rOrm of a belt,
but it should.be und~rstood that a drum-~haped photosen-
sitive member may be used as well. The polarity
of the bias voltage is determined by the characteris-
tic o~ the photoconduative film on the photosensitive
member used. Where a ~elenic photo3e~sltlve member i~
u6ed, lt is rharged to the positlve polarity when the mem-
ber l~ unifor~ly charged, 80 that the bias voltage during
~0 the trans~er atep l~ al~o of the poslt~e polarity and i8
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lOB4770 :,
switched to the negatl~e polarity during the cleaning etep.
Conversely, when the photosensitive member compri~es a
material such as 2inc oxide, it is charged to the negative
polarity inltially, ~o that the bia~ voltage applied during
the transfer ~tep will be of the negative polarity and 1R
switched to the poslti~e polarity duri~g the cleaning step.
In the above description, thP back transfer
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of the toner which attaches to the cleaning means to the
photosensitive member 1s performed ~ubseque~t to the com~
pletion o~ the image transfer, but may be performed after .i .
the termination of a copying cycle or cycles.
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