Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF T~E IN~ENTION
The present invention relate~ to an improved transfer
~y~tem for transferring a developed electrostatic image from
a Xerographic plate to a transfer member such as a sheet f
~aper. More particularly, the invention is directed to a
transfer system including an electrically biased transfer roll
and a transfer corona generating device having an improved
turn-on time.
In conventional xerogrzlphy, a xerographic plate
comprising a layer of photosensitive insulating material
af~ixed to a conductive backing i9 used to support electro-
static latent images. In the xerographic process, the photo~
sensitive sur~3ce is electrostatically charged, and the charged
surface is then exposed to a light pattern of the image being
reproduced to thereby discharge the surface in the areas where
light strikes the surface. The undischarsed areas of the surface
thus form an electrostatic charge pattern (an electrostatic
latent imaga) conforming to the original pattarn. The latent
image is then developed by contacting it with a finely divided
electrostatically attractable powder referred to as "toner".
Toner is held on the image areas by the electrostatic charge
on the surace. Where the charge is greater, a greater amount
of toner is depvsited. Thus, a toner image is produced in
conformity with a light image of the copy ~eing reproduced.
Generally, the developed image is then transferred to a suitable
tran~fer member (e.g~, paper), and the image is af~ixed thereto
to form a permanent record of the original documentO
In the practice of xerography, the transfer member
is caused to move in synchronized contact with the p~oto-
sensitive surface during the transfer operation, and an electrical
potential opposite from the polarity of the toner is applied
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to the ~ide of the paper remote ~rom the photosen~itive
surace to electro tatically attract the toner image from the
surface to the paperO
~ modern high ~peed duplicatirly machine sometimes
utilizes an electrically biased transfer roll to effect the
image transfer~ Although a ~iased transfer roll system ~ffects
very good to excellent copy quality, certain copy quality deects
are still produced. These copy quality deficiencies may be
classified into two general catagories, poor transfer of
ima~es (hollow characters and blotchy solids), and the
redistribution of toner in and around images (toner disturbances,
Lichtenburg figures, and blur). These deficiencies are present
to some exten~ in other prior art trans~er systems.
Also high ~peed duplicating machines utilize
numerous corona generating devices for applyin-J various
charges to either a photoreceptor or a transfer member such
as a sheet of paper. In many instances, it is necessary to
turn the device on as instantly as possible. Prior art
arrangements do not permit these devices to be actuated as
rapidly as desired~
SUMMARY OF THE INVENTION
The present invention i~ primarily directed to an
improved transfer system for overcoming the a~ove mentioned
problems. The transfer system includes an electrically biased
transer roll in combination with a transfer corona generating
device, preferably an ultra-high field device. A sheet of
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paper or other transfer member is fed between the transfer
~ roll ~nd the photosensitive suxface whereby transfer is
- ~ partially effacted, and the sheet is electrostat~cally tacked
to the photosensitive surface. Transfer of the developed
electrostatic image to the sheet is completed as the latter
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passe.s under -the transfer corona generat.iny device. In passiny
unde.r the transfer corona yenera-ting device, however, the lead
edge of the sheet preEerably is not charged; only the body and
trail edge of the sheet are charged.
After the transfer operation, the sheet passes
under a detack corona generating device in order to lessen
the electrostatic attraction between the photosensitive surface
and the sheet so that the latter can be rernoved and transported
to a fusiny device. When passing under the detack corona
generating device, the body and trail edge of the sheet prefer-
~ ably receive a charge which is of a smaller magnitude than the
charge deposited on the lead edge of the sheet in order ko
prevent the developed image from being retransferred from the
she~t to the photosen~itive surface.
In order that the transfer corona generating device
can be up to full effectiveness almost instantaneously, a
j dummy or standby electrical load is provided together with a
switching arrangement to switch the power supply from standby
load directly to the corona generating device~
; 20 In accordance with one aspect of -this invention
there is provided an improved transfer system for transferring
a developed electrostatic image from a charged photoreceptor
to a transfer member, the improvement comprising the combination
: ~ of a transfer roll in contact with the photoreceptor, and a
transfer corona generating device located adjacent to both the
~ ~ photoreceptor and the transfer roll, and means for applying
a first charge to the transfer member with the transfer roll,
~. ~ and for subsequently applyiny a second charge to the transfer
~ ~ member with the transfer corona generating device, the first
and second charges each being of the same polarity as the
charge on the photoreceptor.
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~ In accorda.lce with another aspect of this invention
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the.re is providecl an improved process ~o~ transferring a
developed electrostatic image from a charyed photoreceptor to
a transfer men~er, the improvement comprising:
(a) pressing the transEer member into con-tact wlth
the developed image while sim-ultaneously applying a first
charge to the transfer member o~ the same polarity as the
charge on the photoreceptor, the first charge being sufficient
to electrostatically tack the transfer memb~r to the photo-
receptor; and then
(b) applying a second charge to the transfer member
of the same polarity as the first charge.
BRIEF DESCRIPTION ~F THE DRAWINGS
Figure 1 is a schematic sectional view of an electro-
static reproduction machine emhodying the principles of the
present invention.
Figure 2 is a schematic diagram of the circuit for
; controlling both the trans~er and the detack corona generating
devices.
DESCRIPTION OF THE INVENTION
0 E'or a general understanding of an electrostatic
reproduction machine in which the present invention may be
incorporated~ reference is made to Figure 1. As in all
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electrostatic reproduction machines of the type illustrated, a
light image of an original is projected onto the photo~ensitive
surfa~e of a xerographic plate to form an electrostatic latent
image thereon. Thereafter, the latent image i3 developed with
an oppositely charged developing material comprising carrier
beads and toner particles triboeLectrirally adhering thereto to
form a xerographic powder image corre~ponding to the latent
image on the photosensitive surface. The powder image is then
electrostatically transferred to a trans~er member such as a
sheet of paper to which it may be fixed by a fusing device
whe~eby the toner image is caused permanently to adhere to
the transfer member.
In the illustrated machine 10, an original 12 to be
copied is placed upon a transparent support platen 14 fixedly
arranged in an illumination assembly indicated generally by
the reference numeral 16. While upon the platen, the illumination
assembly flashes light rays upon the original, thereby producing
image rays corresponding to the informational areas on the
original. The image rays are projected by means vf an optical
system 18 to an exposure station 20 for exposing the surface
of a moving xeroyraphic plate in the form of a flexible photo-
- conductive belt or photoreceptor 2~ In moving in the direction
indicated by the arrow, prior to reaching the exposure station
20, that portion of the belt being exposed would have been
uniformly charged to approximately ~900 volts by a corona
generating device 24 located at a belt run extending between
the belt supporting rollers 26 and 28. The exposure station
extends between the roller 28 and a third roller 30.
The exposura of the photosensitive surface of the
belt to the light image discharges the surface in the areas struck
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7~by light whereby an electrostatic latent image remains on
the belt in image configuration corresponding to the light
image projected from the original on the support platen.
As the belt continues its movement, the latent image passes
around the roller 30 and through a developing station 32
wh~re a developing apparatus indicated generally by the
reference numeral 34 is positioned. The developing apparatus
34 comprises a plurality of magnetic brushes 36 which carry
developing material to the surface of the upwardly moving
belt 22. As the developing material is applied to the belt~
toner particles in the development material are el0ctrostatically
attxacted to the charged photosensitive surface to form a powder
image (an electrostatic developed image).
The developed electrostatic image is transported
by the belt 22 to a transfer station 38 where a sheet of
paper is moved at a speed in synchronism with the moving belt
in order to effect transfer of the developed image~ Located
at the transfer station 38 i~ a transfer roll 40 which is
arranged on the frame of the machine to contact the back side
of the sheet o paper as the latter is moved or fed between
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the belt and the transfer roll. The roll 40 is electrically
biased with sufficient voltage 90 that the developed image on
the belt may be electrostatically attracted to the adjacent
side of a sheet of paper as the latter is hrought into contact
therewith. The transer is initiated by the transfer roll 40,
but is completed by the ultra-high field corona generating
device 42. The corona generating device 42 applies a higher
charge to the sheet than do~s the transfer roll, the former
producing a field of approximately 40 to 60 volts per micron,
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the latter producing a ield of approximately 27 to 32 volts
per micron. The trans~er roll 40 applies a charge to the
entire sheet as it moves between the roll and the belt 22.
Asjwill be explained below, however, the transfer corona
generating device 42 does not apply charge to the lead edge
of the sheet, but only to the remainder thereof.
A suitable sheet transE)ort mechanism transports
sheets of paper seriatim from a E~aper handling mechanism
indicated generally by the reference numeral 44 to the developed
image on the belt as the same is carxied around the roller 26.
In passing from the paper handling mechanism to the transfer
roll 40, each sheet contacts a plurality of registration
fingers 46, the operation of which will ~e more fully described
below.
As a sheet emerges from the transfer station 38, a
charge is deposited thereon by a detack corona genPrating
device 48 to lessen theelectrostatic attraction between the
belt 22 and the sheet so that the latter can be removed by
a vacuum stripping and transport mechanism 50, the device 48
having a plastic shield. The sheet is thereater retained on
the underside o~ the vacuum stripping tran~port mechanism 50 for
movement into a fuser assembly indicated generally by the reference
numeral 52 wherein the powder image on the sheet is permanently
affixed thereto. After fusing, the finished copy is discharged
at a suitable point ~or collection. The ton~r particles
remaining as residue on the belt 22 are carried by the belt
to a cleaning apparatus 54. The cleaning apparatus 54 comprises
a corona discharge device 56 for neutralizing charyes remaining
on the untransferred toner particles, a rotating brush 58
mounted within a housing 60, and a vacuum outlet 62.
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Referring to Figure 2, the operation o~ the pre~ent
invention will now be de~cribed in more detail. ~he movement
of each sheet fed to the transfer station 38 i5 synchronized
so that the sheet contacts the rotating registration fingers
46. As the registration fingers 46 rotate out o~ the path
of the sheet, a reset pulse is sent to the timing circuit 64
to reset the latter to zero. ~hle timing circuit now begins
to count the clock pulses generated by the detector 660 the
latter counting the teeth on the main drive gear 6~ of the
machine 10. Since both the numbler of teeth on the gear 68
and the rotation speed o~ the gear are known, and since all
the subsystems of the machine 10 are synchronized with the main
drive gear, the location of a sheet at any particular time
after registration can be accurately determined.
Wnlen the sheet is at the proper location with respect
to the transfer roll 40, an output pulse is delivered from the
timing cixcuit to a suitable constant current power supply 70
to actuate th0 latter s~ that the transfer roll 40 applies
a first charge to ~he entire sheet. When the sheet reaches
the proper location with respect to the transfer corona
generating device 42, an output pulse is delivered from the
timing circuit to the reed relay switch 72 to switch a suitable
constant current power supply 74 from the dummy load 76 to the
transfer corona generating device 42.
For this particular application, the resistance o~
dummy Ioad 76 equals the resistance from the corona wire of
t~e corona generating device 42 to the photoreceptor 22. Since
the shield current is returned to the power supply and is not
part of the controlled current, the dummy~load resistance is
Vo/ip, where V~ LS the power supply voltage and ip is the current
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between the corona wire and the photoreceptor. This arrangement
lessens the turn-on time considerably since this time is a
function of the resonse time of the power supply. As stated
above, the transfer corona generating device 42 preferably
apI~lies a second charge to all but the lead edge of the sheet,
the second charge being of a greater magnitude than the first
charge applied by the transfer roll 40. By not applying a
charge to the lead edge of the sheet with the corona generating
device 42, the sheet is~easier to detack. The same output
pulse used to switch the reed relay switch 72 is also delivered
to the pulse delay unit 78 where it is delayed for a pre-
determined time before being delivered to the power supply 80.
In this manner, the transfer roll 40, and the corona generating
devices 42 and 48 are actuated at the proper times so that the
appropriate charges are deposited on the sheets passing there-
under. Preferably, the detack corona generating device is
actuated so as to apply a larger detacking charge to the lead
edge of a sheet than to the remainder of the sheet as
described in U. S. Patent Number 3,970,381, issued July 20th,
1976 and assigned to Xerox Corporation. If desirad, however,
; the same detacking charge may be applied to the entire sheet.
The ultra-high transfer corona generating device
augments the transfer roll, the latter now serving to apply
-! a charge to the entire sheet to initiate transfer as well as
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~ to tack the sheet to the photosensitive surface of the belt.
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Since each sheet is now rolled into contact with and electro-
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statically tacked to the photosensitive surface of the ~elt
by the transfer roll~ the ultra-high field transfer corona
generating device very effectively completes the transfer
operation to produce a very stable unfused image on the sheet,
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~n image much more stable than can be pro~uced with a prior art
transfer system utilizing a transfer roll only.
If enough of a charge i8 placed on a sheet of paper,
thçn an electric fi01d large enough for air breakdown is
generated. Air breakdown takes place (according to Paschen's
Law) in the region with the largest paper to photoreceptor
gap. Because each sheet of paper has been rolled into contact
with and electrostatically tacked to the photosensitive surface
of the belt before the application o~ the ultra-high ield,
~elective air breakdown occurs, i.e, the breakdown occurs
in the background regions adjacent to toner imayes; thuQ,
each sheet is in contact wlth the developed image and this
tends to prevent breakdown in image areas and confines the
breakdown to background areas. This results in a negative
charge on the image sic~e of the sheet of paper which tends to
reduce the electrostatic toner repulsion forces. As a result,
a very stable unfused image is produced on each sheet, and
little or no redistribution of toner in and around images
occurs. In addition, the image produced by the present
transfer system is less sensitive to paper resistivity. Thus,
a wider selection of copy paper is available~for use with the
present transfer system~ In addition to stabilizing the unfused
image on each sheet of paper, the air breakdown that occurs
tends to decrease the net charge on the background toner,
thus resulting in reduced backgxound transfer.
In acldition, in utilizing the present transfer
system as compared to one utiliæing a trans~er roll only, the
requirements relatîng to temperature, humidity, and transfer
roll material can be relaxed somewhat because the present
system i5 not as dependent on the transfer roll.
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Also, tests have indicated thak the present transfer
system reduces photoreceptor filming as compared to a txansfer
system utilizing a transfer roll only; filming life can be
extended by at least a factor of 1:hree. Although the filming
process is a very complicated interaction between various
machine subsystems, the most important interaction iR believed
to be that between ~ine toner parlicles (less than 4 micron~)
and the cleaning brushr The present transer system txansfers
significantly more o~ the fine toner particles, thus, decrea~ing
the number of ~ine toner particles on the cleaning brush;
consequently; the photoreceptor ~ilming rate is substantially .
reduced. AlsoO because increased brush speed or increased
brush interference with the photoreceptor increases the filming .
rate, it may be possible to decrease both the speed and the
interference by utilizing the present invention.
While the invention has been described with reference
to the structure disclosed, it is not confined to the details
set forth, but, is intended to cover such modifications or
changes as may come within the scope of the following ~laim~.
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