Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
In the art of xerography~as disclosed in U.S. Patent
2,297,691 to Carlson, a xerographic plate comprising a layer
of photoconducting and insulating material on a conducting
backing is given a uniform electric charge over its entire
surface and is then exposed to the subject matter to be repro-
duced usually by conventional projection techniques. This
exposure results in discharge of the photoconductive plate
whereby an electrostatic latent image is formed. Development
of the latent charge pattern is effected with an electrostati-
cally charged, finely divided material such as an electroscopic
powder, that is brought into surface contact with the photo-
conductive layer and is held thereon electrostatically in a
pattern corresponding to the electrostatic latent image. There-
after, the developed image may be fixed by any suitable means
to the surface on which it has been developed or may be trans-
ferred to a secondary support surface to which 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 developing systems are well known and include cascade,
brush development, magnetic brush, powder cloud and liquid
developments, to cite a few. One other important development
technique is disclosed in U.S. Patent 2,895,847 issued to
Mayo. This particular development 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 with an image bear-
ing plate for deposit in conformity with the electrostatic
image to be developed. Development processes of this type are
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termed transfer development.
One form of transfer development broadly involves
bringing a layer of toner to an imaged 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 mernber which
is capable of retaining the particles on its surface and then
the donor member is brought into close proximity to the surface
of the photoconductor. In the closely spaced position, particles
of toner in the toner layer on the donor member are attracted
to the photoconductor by the electrostatic charge on the photo-
conductor so that development takes place. In this technique
the toner particles rnust traverse an air gap to reach the imaged
regions of the photoconductor. The present invention relates
to this type of transfer development, i.e., the toner layer
is out of contact with the imaged photoconductor and the toner
particles must traverse an air gap to effect development.
In U.S. Patent 3,232,190 to Wilmott, a space gap
transfer 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 spatial disposition of
said donor and image surface. Activation of the toner
particles, i.e., removal from the donor surface, and attrac-
tion onto the image regions (development) was primarily due
to the influence of the electrostatic force field associated
with the charged photoconductive plate surface. E~or this
reason, the spatial positioning of the two coacting members
(donors and photoconducting surface) in relation to each other
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was critical. Should the members be in too close proximity,
excessive background development occurs, ~hile too great a
distance results in inade~uate development.
In an attempt to alleviate the criticality of the
spacing between the donor and the photoconductor, a bias poten-
tial was introduced to aid the motivation of the toner to the
charged image areas. Therefore, in U.S. Patent 2,289,400 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 electro-
static force field associated with the charged imaging element -
during activation and development. The application of this
type of electrical force field cannot, however, simply permit
the toner particles to be transported over a wider gap. Because
the force field is continuous and uniform, no additional control
is a~forded over the development 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 application Ser. No.192,003 filed February 7, 1974
there is described a transfer development system which utilizes
a spaced donor-receptor system in combination with a pulsed bias
of different polarities to effect development of imaged areas
while preventing deposition on background areas. m e donor
and photoreceptor preferably operate at spacings between 2 and
7 mils while the frequencies of the pulse are from 4 to 8 kilo
hertz, the negative polarity operating between 30 and 70
microseconds.
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In xerographic development, generally two types of
quali y reproduction are desirable. There is line copy quality
in ~hich reproduction of the letteriny is more important than
~aithulness of reproduction of the original. This occurs in
the case of a withered document with a grey background. The
second type of development is pictorial, i.e., reproduction of
a grey or colour gradient. In this instance faithfulness of
reproduction is esseniial, a perfect example being photographs.
While the aforementioned development system (Serial No. 192,003)
is satisfactory for line copy, it is not effectively adaptable
for pictorial reproduction. The instant invention presents a
transfer development system which enables control of the tonal
response of the development system 50 as to be capable of both
line copy and pictorial reproduction.
BRIEF DESCRIPTION OF THE INVENTION
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In accordance with one aspect of 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
developing material adjacent to the image retaining member, the
donor member and the image retaining member being spatially dis-
posed as,to create a space gap between both members; (b) means
for charging the developing material on the donor member to a
polarity opposite to that of the image retaining member; (c)
means to move the donor member and the charged toner thereon to
the zone where the gap exists; and (d) means to selectively
introduce high and low frequency pulse biases across the gap,
the pulses being comprised of an activation potential segment
in which electroscopic particles are released from the donor
member and a development potential segment of different polarity
in which the electroscopic particles in non-image areas are
attracted towards the donor thereby preventing particle deposi-
tion in the non-image areas.
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In accordance with another aspect of this invention
there is provided a method of development of a charge pattern on
a photoconductive surface of an apparatus including a donor
member for supporting a uniform layer of electroscopic develop-
ing material adjacent to the image retaining member, the donor
member and the image retaining member being spatially disposed
as to create a space gap between both members, and means to
selectively introduce high and low frequency pulse biases across
the gap, the method comprising the steps of: (a) loading the
donor with toner; (b) charging the developing material to a
` polarity opposite to that of the photoconductive surface; and
(c) selectively applying a high or low frequency pulse bias
across the gap, the respective pulse being comprised of an
activation potential segment in which electroscopic particles
are released from the donor member and a development potential
se~ment of different polarity in which the electroscopic
particles in non-image areas are attracted towards the above
thereby preventing particle deposition in the non-image areas.
BRIEF DESCRIPTION OF THE DR~WINGS
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.
Figure 1 is a cross-sectional view of a continuous
automatic xerographic copying machine utilizing the developing
technique of this invention.
Figure 2 is a graphic illustration of the character-
istics of the controlled pulsation technique utilized in the
instant invention.
Figure 3 is a cross-sectional view of the development
system of the present invention illustratiny the particular
mechanism thereof.
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DETAILED DESCRIPTION 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 like. The apparatus includes the xerographic plate
10 in the form of a cylindrical drum which comprises the photo-
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conductive insulating peripheral surface on a conductive sub-
stratus above. The drum is mounted on an axle 15 for rotation,
and driven by a motor 16 through belt 17 connected to pulley
18 secured to the shaft or axle 15.
Positioned adjacent to the path of motion of the
surface of the drum 10 is a charging element 20 comprising, for
example, a positive polarity corona discharge electrode con-
sisting of 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 projection lens 25 or other exposure mechanism as
is conventional in the art, preferably operating with slit
projection methods to focus the moving image 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
developing station is a transfer station 31 adapted to transfer
a developed image from the surface of the drum to a transfer
web 32 that is advanced from supply roll 33 into contact with
the surface of the xerographic drum at a point beneath a trans-
fer electrode 34. After transfer, the web desirably continues
through 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
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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 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 include, 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. 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
described, which revolves about a center axis 51. Rotation
of the donor is effected by means of an axle 51 being driven
by a motor 55 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 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 instant invention.
Between the donor member 50 and the drum 10, there
is maintained a spatial gap 70 of from about 5 to 20 mils (1 mil
eguals 1/1000 of an inch). Preferred spacings, within the
purview of the instant invention, are from about 5 to 10 mils
between the rotating donor and photoreceptor utilizing a
pulsed electrical field to establish the proper field relation-
ll35~S90ships. Any type of pulse generating source, including com-
binations of D.C. sources, which will effect the requisite
pulsing (to be discussed hereinafter) will be suitable within
the purview of the present invention.
Adjacent to 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 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 invention 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 form of a grid pattern. The enamel layer
would have a thickness of about 2 x 10 3 inches, while the
copper grid layer would be in the order of S x 10-~ inches
in thickness. The typical grid pattern on the donor member of
this type generally has from about 120 to 150 lines per inch
with the ratio of insulator-to-grid surface areas about 1.25
to 1Ø
In order that a donor member function in accordance
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with the instant invention, it must first be characterized by
sufficient strength and durability to be employed for continuous
recycling, and in addition should preferably comprise an elec-
trical insulator or at least possess sufficient high electrical
resistance of approximately 1012 ohm-cm or greater. This is
not to be considered an absolute limitation, since the resisti-
vity requirement will become less than about 1011 ohm-cm and
below with reduced time period of exposure between the particular
incremental area of the donor and the xerographic plate. Hence,
the use of donor material of too low a resistivity permits
excessive penetration of charge from the corona discharge
source 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
æ suitable for this purpose include Teflon, polyethylene tere-
phthalate (Mylar), and polyethylene.
In carrying out a pre~erred method o~ development
within the purview of the present invention, a microfield
donor of the type descrihed 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 toner 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
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additional steps such as 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 substrate. As the donor is rotated through
a bed of vibrating toner, these fields collect toner on the
donor in both grid and the enamel insulator areas. In the
next process step this layer of toner is then charged nega-
tively using a negative corona (see 71 of Figure 1). As the
toner passes peripherally adjacent to the spatially disposed
photoconductive layer having the electrostatic image disposed
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
whereby toner is intermittently driven (activated) into the
space gap, thereby being made readily available to the charge
image, and attracted away from background areas.
Referring now to Figure 2, a pulse cycle contemplated
within the purview of 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 positive part described as development transfer, defined by
a potential Vd which operated for a time Td. The negative
segment of the pulse is termed an activation potential because
the toner has been charged negatively, as described above, and
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therefore releases upon the negative potential on the donor.
The number of times per second a pulse cycle is repeated is
defined as the repetition rate R or frequency, where R = k
Ta+Td
Where the activation and development times are given in micro-
seconds (1 sec. = 1,000,000 microseconds), and k is a pro-
por$ionality constant, 1000, the repetition rate is given in
B ~ hertz (KHz). A zero volt reference is used for all voltage
levels. In reality, the pulse is not perfect in shape; however,
rise times are small enough so that they can be neglected. In
utilizing the microfield donor elements described above, the
pulse is usually applied to both the grid and aluminum substrate.
As can be appreciated from Figure 2, four independent
parameters, negative amplitude, negative pulse duration, positive
amplitude, and frequency, offer an infinite combination of
development conditions. However, the present invention relates
to the advantage of being capable of utilizing both high and
low frequencies to control tonal response, the remaining para-
meters being relatively fixed or defined. Additionally,
spacings of from about S to 20 mils can be set at both instant
high and low pulse frequencies. It has been found that high
k;lo
pulse frequencies on the order of about 18-22 ~i~ hertz results
in extended tonal range of development, i.e., development of
the gradient scales of gray are possible. On the other hand,
pulsing at low frequencies of from about 2 to 5 ~ hertz
yeilds strong black-white separation, i.e., excellent line
copy reproduction. The use of a transfer development system
having this dual capability would enhance any xerographic
reproduction device.
As mentioned above, definition of parameters of a
square pulse have to account for an activation potential Va,
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an activation time Ta~ a development potential Vd, and a
repetition (or frequency) rate. While all these parameters
may be varied to accommodate donor-photoreceptor spacings of
from 5 to 20 mils (l mil = l/lO00 of an inch), generally
activation times Ta between 5 and 100 microseconds at frequency
rates of from 2 to 5 and 18-22 ~i~e hertz give optimum results
in the subject invention. Best results are obtained with
spacings between 5 and 10 mils, activation times between 5 and
50 microseconds at the above cited frequencies. Typical times
are from 20 to 50 microseconds activation time at the lower
frequencies and from 5 to 25 microseconds at the higher fre-
quencies.
The activation potential at spacings of from 5 to
20 mils is about -150 volts or greater (i.e., -150 volts, -200
volts, etc.). The development potential at these spacings is
about +400 volts or greater (+450 volts, etc.). Activation
potentials (Va) can be from about -150 to -1400 volts while
development potential varies from about +400 volts to +1000
volts. The greater values of Va and Vd indicated are used
at the larger values of the spacing between the donor and
the photoreceptor. ~he peak value of the activation potential
~a is limited in part by the onset of an electrical breakdown
phenomenon in the air gap 71 between the donor and the photo-
receptor. The peak value of development potential Vd must be
chosen such that the thickness of the electroscopic powder
deposit on the developed image is sufficient for the ultimate
use of the imaging process; i.e., the final copy must be
adequate.
While not to be construed as limiting, a general
description of possible mechanism occurring at the development
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interface, i.e., the space gap between the donor and photo-
conductive surface, is shown in Figure 3. As shown, the bias
level during the activation portion of the pulse is such that
the negative toner particles experience a field force in the
direction of the photoreceptor 10 comprised of a substrate 11
and photoconductive layer 12. This force is in addition to the
force produced by the potential on the photoreceptor and, for
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 image element. Therefore,
the actual position of the toner particles in the gap is
dependent upon the forces 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 bac~ground areas.
To recapitulate, the present invention relates to a
transfer development system which is capable of controlling
tonal response utilizing high and low frequency pulse biasing.
It has been found that the low frequency pulse engenders a
strong contrast or unfaithful response in low density areas of
an image, thereby being excellent for line copy. The high
freqllency pulse results in faithful reproduction of low
density areas (grays) and, therefore, is excellent for pictorial
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quality. Because the high and low frequencies are used select-
ively, the use of a switch or control connected to the pulse
generating device would be appropriate for use in a copy
machine.
The experimental work carried out in developing 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 tyary of
toner and then charged negatively. The actual transfer develop-
ment step was completed by rolling the donor over a halogen
doped selenium plate. The donor-to-photoreceptive spacing was
maintained by plactis shim stock placed on the edges of the
plate. Nominal spacings of from 5 to 20 mils were used on most
tests. Since the primary objective of the experimentation was
to investigate development variables, the charging and loading
functions were kept reasonably constant. Typical toner layers
were 2 to 2-1/1 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 quality
measurements were make on semimicro densitometer systems and
pulse variables were set and monitored on an oscilloscope at all
phases of experimentation.
Since many changes could be made, the above invention
and many appaxently widely different embodiments of this
invention could be made without departing from the scope thereof,
it is intent that all matter contained in the drawings and
specifications should be interpreted as illustrative and not,
in any sense, limiting.
