Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF T~:E INVENTION
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In the art of xerography as disclosed in U.S. Patent
~,297,691 to Carlson, a xerographic plate comprising a layer of
photoconducting and insulting material on a conducting backing
is given a u~iform electric charge over its entire surface and
is then exposed to the subject matter to be reproduced usually
by conventional projection techniques. This exposure results
in discharge of the photoconductive plate whereby an electrostatic
~j latent image is formed. Development of the latent charge pattern
is effected with an electrostatically charged, ~inely divided
I material such as an electroscopic powder, that is brought into
i surface contact with the photoconductive layer and is held thereon
electrostatically in a pattern corresponding to the electrostatic
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latent image. Thereafter, the developed image may be fixed by
any suitable means to the surface on which it has been developed
or may be transferred to a secondary support surface to which
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; it may be fixed or utilized by means known in the art.
In any method employed for forming electrostatic images,
they are usually made visible by~a development step. Various
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developing systems are well known and include cascade, brush
development, magnetic brush, powder cloud and liquid developments,
to cite a fewO In connection with these various developing
~ systems, it is known that a conductive control electrode as, for
;~ example-, disclosed in U.S. Patents 2,808,023, 2j777,418, 2,573,881
and others, is highly effective in influencing electrostatic
gradients to develop images having varying charge gradients and
~ having relatively ~;arge solid image areas.
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At the same time, when developing images generally devoid of
solid areas and consisting primarily of lined-copy lmages,
; superior results are generally obtainable without the electrode
in place.
Another important development technique is disclosed
in U.S. Patent 2,895,847 issued to Mayo. This particular develop~
ment process employs a support member such as a web, sheet or
other member termed a "donor" which carries a releasable layer
of electroscopic marking particles to be brought into close
contact wlth an image bearing plate for deposit in conformity
with the electrostatic image to be developed. In donor or
transfer development of this type, the electrical properties of
the donor are a factor for development in response to the area
characteristics of the latent charge image. Specifically,
electrically insulating donors respond best with line copy, while
electrlcally conductive donors respond best with solid areas in
a manner compa~able to the control electrode. Ac~ordingly, prior
attempts to provide development flexibility on a p~actical basis
for development of any kind o~ image, such as solid area versus
line copy, have met with difficulty. This has resulted in
limitations on the usual copying system and has necessitated
selectivity with regard to particular materials to be reproduced.
As mentioned above, transfer development broadly in-
volves bringing a layer of toner to an image photoconductor where
- toner particles will be transferred from the layer to the imaged
areas. In one transfer development technique, the layer of
toner particles is applied to a donor member which is capable
of retaining the particles on its surface and then the donor
member i5 brought into close proximity to the surface of the
photoconductor. In the closely spaced position, partlcles of
toner in the toner layer on the donor member, are attracted to
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the photoconductor by the electLostatic charge on the photo-
conductor so that development takes place. In this technique
the toner particles must traverse an air gap to reach the imaged
regions of the photoconductor. In two other transfer techiques
the toner-laden donor actually contacts the imaged photoreceptor
and no air gap is involved. In one such technique, the toner-
laden donor is rolled in non-slip relationship into and out of
contact with the electrostatic latent image to develop the image
in a single rapid step. In another such technique, the toner-
laden donor is skidded across the xerographic surface. Skiddingthe toner by as much as the width of the thinnest line will
double the amount of toner available or development of a line
which is perpendicular to the skid direction and the amount of
` skidding can be increased to achieve greater density or greater
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area coverage.
It is to be noted, therefore, that the term "trans~er
development" is generic to development techniques where (1) the
toner layer is out of contact with the imaged photoconductor and
the toner particles must traverse an air gap to e~fect development,
(2) the toner layer is brought into rolling contact with the
imaged photoconductor to effect development, and (3) the toner
layer is brought into contact with the imaged photoconductor and
skidded acroæs the imaged surface to effect development. Transfer
development has also come to be known as "touchdown development".
In connection with transfer type development, it is
known that by applying a controlled bias to a donor member
characterized by appropriake electrical resistance while in
contact with a plate being developed, that the donor functions
to effect results-similar to a control electrode described above~
That is, by applying a bias potential to the rear surface of the
donor member when presenting developer into contact with an
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electrostatic latent image, it becomes much more effective than
an insulating or highly resistive unbiased donor for developing
images haviny relatively large solid areas, as well as the various
gradations of charge commonly associated with continuous tone
images. At the same time, when developing images generally de-
void of so]id areas and gradations in tone and consisting
primarily of line copy images, substantially greater image
~- exposure latltude can still be obtained by developing with the
donor in its inherently more resistive state without the benefit
! 10 of the corona bias applied thereto.
number of transfer type development systems were
advanced in which background development was minimizedO In U.S.
Patent 31232,190 to Wilmott, a transfex type development system
is disclosed in which the charged toner particles are typically
stored on a donor member and development is accomplished by
transferring the toner from the donor to the image regions on
~` the photoconductive surface across a finite air gap caused by
the spacial dieposition of~said donor and image surface.
Activation of the-toner particles, i.e., removal from the donor
surface, and attraction onto the image regions (development)
was primarily due to the influence of the electrostatic force
field associated with the charged photoconductive plate surface.
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- For this reason, the spacial positioning of the two coacting
members (donors and photoconducting surface) in relation to each
-other was critical. Should the members be in too close proximity
excessive background development occurs, while too great a
distance results in inadequate development.
- In the application of an electrical field to a transfer
; development system, a problem of background development arose.
This was due to the fact that, while applying a bias across the
; development zone enhanced the deposition of the electroscopic
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particles onto the charge image pattern, the charged toner was
also motiva~ed onto the uncharged or background areas of the
pattern, thereby resulting in a background development.
In U.SO Patent 2,289,~00 to Moncrieff-Yeates, there is
disclosed an out of contact transfer development system in which
a continuous and uniform force field is established within the
transfer zone and assists the electrostatic force field associated
with the charged imaging element during activation and develop-
ment. The application of this type of electrical force field
cannot, h~wever, simply permit the toner particles to be trans-
ported over a wider gap. Because the force field is continuous
and uniform, no additional control is afforded over the develop-
:~ ment process. Therefore, the electrostatic force field associated
with the latent image still remains the predominant mechanism
by which the toner particles are both activated and attracted
to the imaged area of the photoconductive surface.
In copending Canadian application Serial Number 191,594
:~ there is described a donor development system in which a high
fsequency bias is applied between a spacially disposed image
~bearing su~face and a donor. The bias is created by applying
: the voltag~ from an alternating.~current power supply between theplate and donor at frequencies of from about 10 to 3,Q00 kilo-
; ~ cycles/sec. while the gap between the donor and image retaining
member can be up to about 7 mils (1 mil equals 1/1000 of an inch).
While such a system results in good quality line
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copy images, it has been found that superior quality in both
line and continuous tone images can be attained utilizing a
square pulse signal having proper frequencies and duty cycle
voltage amplitudes in a transfer development system.
As can be ascertained from the above, the art of
xerographic development, and in particular transfer development,
would be significantly advanced if a pulsed bias could be used
to improve both line and continuous tone quality in transfer
development.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with this invention there is provided
an apparatus for developing a latent electrostatic image
recorded on an image retaining member comprising: (a~ a donor
member for supporting a uniform layer of electroscopic devel-
oping material adjacent to the image retaining member, said
donor member and image retaining member ~eing spatially
disposed as to create a space gap between both members;
~) means to introduce a pulse bias across said gap, said pulse
being comprised o~ an activation potential segment in which
,~: 2a electroscopic particles are released from the donor member and
a development.potential segment o different polarity in which
. : the electroscopic particles in non-image areas are attracted
towards the donor thereby preventing particle deposition in the
non-image areas.
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- . This invention will become more apparent upon
consideration of the following detailed disclosure, along with
specific embodiments of the invention, especially when taken
in conjunction with the accompanying drawings herein.
30 Figure 1 is a cross-sectional view of a continuous
; automatic xerographic copying machine utilizing the developing
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technique o this inventionO
Figure 2 is a graphic illustration of the charactex-
: istics of the contrslled pulsation techni~ue utilized in the
. instant invention~
Figure 3 is a cross-sectional view of the developm~nt
system of the present invention illustrating the particular
mechanism thereof~
DETAILED DESCRIPTIO~ OF THE DRAWINGS
. . Referring now specifically to Figure 1, there is
illustrated a continuous xerographic machine adapted to form
an electrostatic reproduction of a copy onto a paper sheet,
web or the likeO The apparatus includes the xerographic plate 10
in th~ form of a cylindri~al drum which comprises the photocon-
ductive insulating peripheral surface on a conductive s~bstratus
-~ ~ aboveO The drum is mounted on an alxle 15 ~or rotation, and
; driven by a motor 1~ thxough belt 17 connected to pulley 18 .
: secured to the shaft or axle 15.
Positioned adjacent the path of motion of the surface
of tha drum 10 is a charging element 20 comprising, for èxample.
20 a positive polarity corona discharye electrode consisting of
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a fine wire suitably connected to a high-voltage source 22 or
potentially high enough to cause a corona discharge from the
electrode onto the surface of the drum 10. Subsequent to the
charging station 20 in the direction of rotation of the drum, is
an exposure station 23 generally comprising suitable means for
imposing a radiation pattern reflected or projected from an
original copy 24 or to the surface of the xerographic drum. To
effect exposure, the exposure station is shown to include a pro-
jection lens 25 or other exposure mechanism as is conventional
in the art, preferably operating with slit projection methods to
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focus the movlng lmage at the exposure slit 26.
Subsequent to the exposure station is a developing
station, generally designated 30, as will be further described
below for rendering the latent image visible. Beyond the develop-
ing station~is a transfer station 31 adapted to transfer a
deyeloped image from the surface of the drum to a transfer web
32 that is advanced from supply roll 33 into contact with the
surface o~ the xerographic drum at a point beneath a transfer
electrode 34. After transfer, the web desirably continues through
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a fusing or fixing device 35 onto a take-up roll 36 being driven
through a slip clutch arrangement 37 from motor 16. Desirably~
electrode 34 has a corona discharge operably connected to a
high-voltage source 40 whereby a powder image developed on the
surface of the drum is transferred to the web surface. Fusing
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device 35 primarily fixes the transferred powder image onto the
web to yield a xerographic print. After transfer, the xerographic
drum 10 continues to rotate past a cleaning station 41 in which
; residual powder on the drum's surface is removed. This may in-
clude, for example, a rotating brush 42 driven by a motor 43
through a belt 44 whereby the brush bristles bear against the
surface of the drum to remove residual developer therefrom.
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Optionally, further charging means, illumination means, or the
like, may effect electrical or controlled operations.
Operative at the developing station 30 is a donor member
50 in the form of a cylindrical roll, as will be further des-
cribed, which revolves about a center axis 51. Rotation of the
donor is effected by means of an axle 51 being driven by a motor
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S5 operating through a belt 56, preferably to drive the cylinder
in the same direction as the surface rotation of the drum. The
speeds of the donor member and drum may be substantially the same
i 10 or the donor member can travel` at speeds as high as 5 to 10 times
as fast as the peripheral speed of the drum to effect a greater
development in imaged areas. Also affixed to donor member 50 is
a pulse generator source 61 for applying the pulsed bias potentials
`~ of the ins~ant invention.
Between the donor member 50 and the drum 10, there is
maintained a spacial gap 70 of from about 2 to 20 mils (1 mil
equals 1/1000 of an inch). The actual development step within
` the purview of the instant invention is achieved maintaining a
gap of between 2 to 7 mils between the rotating donor and photo-
I ZO receptor utilizing a pulsed electrical field to establish the
i ~ proper field relationships whereby optimum line and solid develop-
ment is~effected with a minimum of background deposition. Any
type of pulse generating source, including combinations of D.C.
s~urces, which will effect the requisite pulsing (to be discussed
hereinafter) will be s~itable within the purview of the present
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invention.
; Adjacent one portion of the path of motion of the
developer donor member 50 is a powder loading station which may,
for example, comprise a developer hopper 57 containing a quantity
of developer product 58 which may be a form of a toner or
electroscopic powder. The hopper opens against the donor member
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whereby the cylinder passes in contact with the developer supply
and is contacted uniformly with the toner powder as the donor
passes through the developer. Other loading mechanisms may, of
course, be employed including a dusting brush or the like, as is
known in the art.
While the donor member of Figure 1 has been described
in the terms of a cylindrical element, it is to be understood
that said donor may be in the form of web, belt, or roll, or any
other structure capable of operating within the purview of the
instant invention. A preferred donor element of the present in-
vention is a microfield donor consisting of a milled aluminum
cylinder over which a thin layer of insulating enamel is placed,
on which enamel layer there is a thinner layer of copper etched
in the orm af a grid pattern. The enamel layer would have a
thlckness of about 2 x 10 3 inches, while the copper grid layer
would be in the order of 5 x 10 4 inches in thickness. The
typical grid pattern on a donor member of thls type general'ly
has from about 120 to 150 lines per inch with the ratio of
nsulator-to-grid surface areas being about 1.25 to 1Ø
In order that a donor member function in accordance
with the instant invention, it must first be characterized by
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sufflcient strength and durability to be employed for continuous
recycling, and in addltion should preferably comprise an electri-
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cal insulator or at least possess sufficient high electrical
resistance of approximately 1012 ohrn-cm or greater. This is
not to be considered an absolute limitation, since the resistivity
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~ requirement will become less than about 1011 ohm-cm and below
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with reduced time period of exposure between the particular in~
crernental area of the donor and the xerographic plate. Hence,
the use of donor material of too low a resistivity permits ex-
cessive penetration of charge from the corona discharge source
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into the donor within the time-of contact. As a result, as the
low resistivity donor advances from charged to uncharged areas
of the electrostatic latent image, the charges induced into the
bulk of the donor causes excessive deposition of toner in these
; uncharged or background areas. At the same time, however, for
development speeds giving shorter contact times, materials of
lower resistivity may be used. Materials found suitable for this
purpose include Teflon*, polyethylene terephthalate (Mylar*), and
i polyethylene.
In carrying out a preferred method of development with-
in the purview of the present invention, a microfield donor of
the type described above is used as member 50 of Figure 1.
Generally, the four basic steps in carrying out a development
process are loading the donor with toner, corona charging the
toner (see corona charging element 71 of Figure 1), passing the
`~ tonex to the electrostatic latent image on the photoconductive
surface, and cleaning residual toner from the donor member so as
~ to allow repetition of the process. In the actual practice of
; ~ development of most machines, there are additional steps such as
20~ agglomerate toner removal and corona discharging of the donor
~ member, which steps are auxiliary to the development process.
;~ ~ In loading a~microfield donor of the type described
above, a bias is applied to the grid which establishes strong
electrical fringe fields between the copper grid and the grounded
aluminum ~ubstrate. As the donor is rotated through a bed of
vibrating toner, these fields collect toner on the donor in both
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grid and the enamel insulator areas. In the next process step
this layer of toner is then charged negatively using a negative
corona (see 71 of Figure 1). As the toner passes peripherally
adjacent the spacially disposed photoconductive layer havlng
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the elec~rostatic image dispos~d thereon, a square pulse of
certain potentials (see 61 of Figure 1) is applied by the pulse
generator at the donor to effect development. The overall effect
of the pulsed bias is an oscilLating negative and positive
potential between the xerographic plate and the donor and the
xerographic plate and facilitates continuous tone development.
Referring now to Figure 2, the pulse cycle contemplated
in the instant invention is demonstrated. Basically, the single
pulse cycle is considered in two components, namely, a negative
part described as activation and defined by an activation
potential Va which operates for a time Ta, and a posi~ive part
described as development transfer, defined by a potential Vd which
operated for a time Td. The number of times per second a pulse
cycle is repeated is defined as the repetition rate R, where
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R = k _. Where the activation and development times are
T + T
a d
given in microseconds (1 sec. = 1,000,000 microseconds), and k
is a proportionality constant, 1000, the repetition rate is given
i~; in kilo-Hertz (KHz). A zero volt reference is used for all
-; voltage levels. In reality, the pulse is not perfect in shape;
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'- neglected. In utilizing the microfield donor elements described
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above, the pulse is usually applied to both the grid and aluminum
substrate.
- As can be seen in Figure 2 any definition of parameters
of a square pulse have to account for an activation potential Va,
an activation time Ta, a development potential Vd, and a
repetition (or frequency) rate. These parameters may be varied
to accommodate donor-photoreceptor spacings of from 2 to 20 mils
(1 mil = 1/1000 of an inch). Activ~tion times Ta betweQn 10 and
200 microseconds and development times Td between 100 and 500
microseconds (repetition rates between about 1 1/2 and 10 kilo-
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Hertz) give improved results. Best results are obtained with
spacïngs between 2 and 7 mils, activation times between 30 and
70 microseconds, and development times between 100 and 180 micro-
seconds (repetition rates between about 4 and 8 kilo-Hertz).
Typical times are 50 microsecond activation time and 150 micro-
second development time, resulting in a repetition rate of 5 kilo-
Hertz.
The activation potential at spacings of from 2 to 7
mils is about -lS0 volts or greater (i.e. -150 volts, 200 volts,
etc.). The development potential at these spaces is about +400
~ volts or greater (~450 volts). Ranges of the activation potential
;~ ~ (Va)l are from about -150 to -450 volts. The development potential
varies from abou* +400 to +800 volts. Any combination of Va and
Vd can be used, the preference being that the peak amplitude of
~,~ the pulse bias, i.e., the difference between Va and Vd, not exceed
8QO volts.
While not to be construed as limiting, a general des-
oription o possible mechanism occurring at the development
; lnterface, i.e., the space gap between the donor and photoconduc-
tive surface, is shown in Figure 3. As shown, the blas level
during the activation portion of the pulse is such that the
negative toner particles experience a field force in thé direction
of-the photoreceptor 10 comprised of a substrate 11 and photo~
conductiue.layer 12~ This force is in addition to the force pro-
duced by the potential on the photoreceptor and, fox this reason,
the image areas produce a higher activation force than the non-
image or background areas. The duration of the activating field
is important in that a fraction of this time is spent breaking
the toner-donor bond, while the remainder is used to drive the
toner toward the imaged element. Therefore, the actual position
of the toner particle in the gap is dependent upon the forces
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applied, as well as the time duration of the activating force.
A similar analysis can be applied to what happens during the
actual development part of the cycle. The bias levels which are
established during the development part of the pulse are such
that a negative toner particle in the gap experiences a field
force away from the photoreceptor. By means of this mechanism
toner not caught up in the field caused by the imaged areas is
drawn onto the donor away from the non-image or background areas.
The experimental work carried out in devaloping the
instant invention utilized simple bench-type apparatus. A Xerox
`~ 813 size cylindrical donor containing a grid of 120 lines per
~` inch was loaded by rotating through a vibrating tray of toner
-~ and then charged negatively. The actual transfer development
;` step was completed by rolling the donor over a halogen doped
selenium plate. The donor-to-photoreceptive spacing was main-
`'~ tained by plastic shim stock placed on the edges of the plate.
Nominal spacings of from 2 to 7 mi:Ls were used on most tests~
~ Since the primary objective of the experimentation was to in-
; ~ vestigate development variables, the charging and loading functions
-` 20 were kept reasonably constant. Typical toner layers were 2 to
i~ 2 1j2 mils thick and were checked optically. The charge on
the toner layer was monitored by reading the potential above
the toner layer after charging. Then the image ~uality measure-
ments were made on semimicro densitometer systems and pulse
variabIes were set and monitored on an oscilloscope at all phases
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of experlmentation.
Since many-chages could be made, the above invention
and many apparently widely different embodiments of this in-
vention could be made without departing from the scope thereof,
it is intent that all matter contained in the drawings and speci-
fications should be interpreted as illustrative and not, in any
sense, limiting.
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