Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
The present invention relates generally to the
formation and development of charge patterns and more
particularly to the formation of imagewise non-uniform
charge patterns and the development thereof with finely
divided marking material.
In conventional xerography, a photoconductive surface
is uniformly charged in the dark with a charge of one polarity.
The charged surface is exposed to a pattern of radiation to
which it is sensitive, and the charge is dissipated in the
radiation-struck areas. An imagewise uniform charge pattern
remains in the non-radiation-struck areas.
The imagewise unifarm charge pattern is normally
developed by contaating the surface with a finely divided,
aol~red toner which carries a charge of the opposite polarity.
Because opposite polarities attract, the toner particles adhere
to the photoconductive surface in the area of the uniform
charge pattern.
The toner particles are most usually charged to the
opposite polarity prior to development by rubbing contact
with a carrier material. The carrier material is one which is
removed from the toner material in the triboelectric series. The
carrier material is usually in the form of particles of a larger
size than the toner particles; although the carrier may, in some
cases, be a liquid.
The toner is usually applied to the surface by
cascading or flowing the toner or a toner-carrier combination
(generally referred to as developer) across the surface. Other
well known toner application methods include magnetic brush
development, electrophoretic development and out-of-contact
liquid development, such as that described in U.S. Patent No.
3,084,043 to Gundlach.
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Normal xerographic development has met with great
commercial success. However, there remain areas where improve-
ment is desirable. For example, photoconductive surfaces useful
in most commercial xerographic development should be from
about 10 to about 60 microns thick. Such thicknesses can be
expensive and complicated to manufacture. Any imaging process
which enables the use of thinner photoconductive layers would
be an improvement.
The toner-carrier combination which is well known
in normal xerography is somewhat dependent on the ambient relative
humidity for successful operation. The humidity is preferably
lower. Proper triboelectric charging of the toner is difficult
if the humidity is too high.
Another diffi¢ulty of the toner-carrier combination
i~ that t~e carrier can become co~ted with a thin layer of toner
material after long periods of use. This is generally referred
to as carrier aging. Such coated carrier material cannot be used
efficiently to triboelectrically charge the toner material.
An imaging process which enables the use of a toner
material which does not have to be charged to one polarity or
another before development is also desirable. A toner material
which is readily useful without a carrier material would also
be an improvement.
In normal xerography, the toner particles adhere to
(develop) the photoconductive surface at the point of charge
differential. For example, in normal xerography a plate is
charged to about 1,000 v. and then imagewise exposed. Exposure
reduces the charge in the light struck areas to about 200 v.,
leaving about 800 v. in non-light struck areas. The line between
a 200 v. area and an 800 v. area on a surface attracts toner
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particles thte~ . However, solid area coverage of a
large area of uniform 800 v. charge cannot normally be accomplished
without the aid of such sophisticated and complex mechanisms as
magnetic brush developers or development electrode systems. U. S.
Patent No. 2,777,418 to Gundlach shows a typical development
electrode used to achieve said area coverage of a large uniform
charge pattern using charged toner. A development system which
would make available solid area coverage without such complex
mechanisms and with uncharged toner is desirable.
Even when magnetic brush development and a developer
electrode are used to achieve solid area development, the problem
of "developer starvation" is observed. This undesirable
phenomenon manifests itself as a reduction of density as
large solid areas are developed. The reduction can be quite
dramatia and unattractive,
It is generally understood to occur because of the
limited speed at which the typical toner-carrier type of developer
can provide sufficient toner particles of the proper polarity.
The development of a charge pattern by a toner particle of
one charge leaves a net opposite charge on the carrier. This
results in the carrier attracting the remaining toner with an
increased attraction, making it more difficult for the remaining
toner to leave the carrier and develop subsequent charge patterns.
Normally this undesirable situation can be remedied only by
replenishing the carrier with toner. A marking method which
avoids developer starvation would be useful.
In normal xerography, a developed image must often-
times be 'ransferred to a receiving sheet if it is to be useful.
Such transfer is a critical operation which must be handled
carefully and with great control to achieve complete transfer
while avoiding smearing.
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There are normal xerographic methods which avoid
transfer by coating a photoconductive layer on a conductive
paper and developing the image directly on the coated paper.
However, these methods require conductive papers and expensive
coating treatments during manufacture. They also often lend
themselves to liquid (bath) development which can be relatively
slow and which sometimes can produce damp copies having an
unpleasant odor.
An imaging system which can avoid the difficulties
associated with the normal xerographic transfer step is desir-
able. An imaging system which enables development directly onto
the final copy while avoiding the need for a photoconductive
coating on the final copy and the need for liquid development
also would be useful,
Other image dev~lopment systems have been achieved
which do not overcome these disadvantages. For example, in
V. S. Patent No. 3,318,698, F. A. Schwertz discloses a means
for creating a charge pattern on an insulating surface. The
surface is first frosted in an imagewise pattern and then
uniformly charged. The frosted areas retain less of a charge
than do the unfrosted areas, and a charge differential is
created between the charged and uncharged areas. The charge
differential of Schwertz is developable by well known xerographic
methods. However, because the charge pattern of Schwertz is
all of one polarity, it requires a toner which is triboelectrically
charged to the opposite polarity. The system disclosed by
Schwertz also has the solid area coverage problems discussed
above in connection with charge differential development.
In U. S. Patent No. 3,043,217 to L. E. Walkup, there
is disclosed a development system in which a charge pattern is
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formed on an insulating surface and is developed with a typical
xerographic developer. Although this system has many uses, it
also shares many of the disadvantages of the Schwertz method.
In U. S. Patent No. 3,250,636 to R. A. Wilferth,
a magnetic imaging system is disclosed. In that system, a
non-uniform pattern of magnetic microfields is established in
a magnetizable layer. The uniform pattern is selectively
removed by Curie point erasure, leaving an imagewise pattern
of magnetic microfields. Curie point erasure is a well known
technique and comprises heating a magnetiæed material above
a known critical temperature at whiah its molecules become
disoriented and the material loses its magnetic properties.
Curie poin~ erasure is sometimes accomplished by such techniques
as flash heating a magneti~ed material with a Xenon flash
lamp while protecting the image area~with a mask.
While the technique of Wilferth avoids the solid
area coverage problems of the prior art, it requires a
magnetizable imaging layer and magnetically attractable toner
particles and it is not compatible with well known optical
imaging methods. Also, it is generally limited to forming dark
images because magnetically attractable toners are most
usually of a rust or black color.
Maksymiak, in U. S. Patent No. 3,759,222 discloses
the use of a non-uniform charge pattern on a transfer member
to transport magnetic toner particles to an imaging member
which carries a magnetic image. The transfer member of
Maksymiak comprises a conductive drum coated with a thin
dielectric layer on which is supported a conductive screen.
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A potential difference is established between the screen
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and the drum so that a non-uniform charge pattern exists
over the surface of the drum. The transport member of
Maksymiak does not provide an imagewise non-uniform charge
pattern and does not overcome the difficulties of magnetic
imaging pointed out above.
The existence of field gradients at the edges of
xerographic charge patterns and in periodic xerographic
charge patterns has been disclosed by H. E. J. Neugebauer
(Appl. Opt. 3, 385 (1964) and R. M.Schaffert, Phot. Sci.
Eng. 6, 197 (1962). However, the use of conductive toners
to attempt to develop such field gradients results in dis-
charge and loss of the image. The use af charged insulat-
ing toners or uncharged insulating toners results only in
edge development, as described above.
SummarY of the Invention
It is, therefore, an object of an aspect of the
present invention to overcome the disadvantages of the
prior art.
It is an object of an aspect of the present
lnvention to furnish an apparatus for imagewise marking a
photoconductive imaging surface.
It is an object of an aspect of the present
invention to furnish an apparatus for marking a photoconduc-
tive imaging surface having a thin photoconductive layer.
It is an object of an aspect of the present
invention to disclose an apparatus for creating an image
having a good solid area coverage.
It is an object of an aspect of the present
invention to supply an apparatus for marking an imaging
surface which avoids the phenomenon of "developer starvations".
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It is an object of an aspect of the present
invention to furnish an apparatus for development of a non-
uniform charge pattern with an uncharged marking particle.
An aspect of the invention is as follows:
An apparatus for producing a visible image on a
photoconductive imaging surface which comprises: (a) a
means for forming an imagewise non-uniform charge pattern
having an average strength of at least about 100 volts on a
photoconductive surface having a thickness of at least
about 1.25 microns; and (b) a means for contacting the
surface with finely divided uncharged marking particles
having a dielectric constant of at least about 1.5 more
than the surrounding medium.
The means for ~orming an imagewise non-uniform
charge pattern lncludes, in a preferr~d embodiment, means
for uniformly charging a photoconductive surface, means
for exposing the surface to a regular pattern of dark and
light and means for subsequently exposing the surface to
; an imagewise pattern of light. Other useful means for
forming an imagewise non-uniform charge pattern are
described below.
The means for contacting the imagewise non-
uniform char~e pattern with finely divided uncharged mark-
ing particles can be selected from a variety of useful
means such as fluidized beds, magnetic and electric
brushes and arrangements for flowir.~ ..,arking particles
over the photoconductive surface.
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Brief Descri~tion Of The Drawings
The invention will now be described with reference
to the drawings in which:
Fig. 1 shows a charge pattern of the prior art and
its effect on charged and uncharged marking particles.
Fig. 2 shows a non-uniform charge pattern of the
present invention and its effect on an uncharged marking particle.
Fig. 3 shows a method for making an imagewise
non-uniform charge pattern on an insulating surface.
Figs. 4A-F show methods for making an imagewise
non-uniform charge pattern on a photoconductive surface.
Figs. 5A-C show methods for contacting a surface
containing a non-uniform charge pattern with marking particles.
Figs. 6A-~ show a method for transferring a developed
imag~ to a reaeiving surface.
Figs. 7A and B show methods for fixing the developed
image to a surface.
Fig. 8 shows an imaging apparatus for performing
the method of the present invention.
Detailed Description
Referring more specifically to Fig. l, there is
shown schematically and in cross-section an imaging member
supporting a charge pattern typical of the prior art.
Imaging member 1 comprises photoconductive layer 2
on grounded conductive substrate 3. The charge pattern is
typical of those used in xerography. It includes areas 4 of
relatively high charge and areas 5 of relatively low charge.
Lines 6 depict the shape of the electric field between
the positive charges in areas 4 and 5 of the charge pattern and
the corresponding negative charges a~ the interface between layer
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2 in the central part of areas 4 and 5. However, at the edges
of areas 4 and 5 and at the location where areas 4 and 5 meet,
the shape of the field is defined by loops. The lines 6 defining
the loops indicate by their varied closeness a high concentration
of charge (or a convergence of the field) in areas where the lines
are closest.
Paricles 7 show the behavior of charged toner
particles in the presence of such a field. It is well
known in the prior art that such particles tend to preferentially
develop charge differentials such as the location where areas
4 and 5 meet. That is, particles 7 ~end to preferentially
develop the areas of greater field stren~th.
Particles 7 are negatively charged and is attracted
to the positively charged image, as shown by the arrows. The
~orce of the attraction is accordlng to the well known
formula F = qE, where F = force; q = charge and E = field.
The phenomenon of preferential edge development is explained
by this relation of force, charge and field strength. The-
charge particles 7 experience a stronger force of adhesion to
~urface 2 at locations where the concentration of lines of
force on surface 2 is more dense.
; The effect of the prior art charge patterns of Fig. 1
on an uncharged particle is shown by particles 8. A dipole
is induced in particIes 8 so as to reduce the field inside
the particle. The extent of this polarization depends on the
field strength and the dielectric constant, K, of the particles 8.
In a uniform field the forces exerted by the field on the charges
at each end of particles 8 are equal and opposite, so no net
force results. ~owever, in a convergent field, depicted by
lines 6 where they form loops, a net force results in the
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direction of convergence. For small spherical particles 8
in air the net force is given by the equation:
F = 2~ ~0 a 3 K-l ~ (E2
where:
F = force in Newtons
= permittivity of space (8.85 x 10 12)
a = radius
K = dielectric constant
= field gradient del vector
E = electrostatic field.-
In Fig. 1 uncharged particles 8 are preferentiallydrawn to the areas of charge differential (or field conver~ence).
It is seen in Fig. 1 that neither charged nor uncharged particles
7 or 8 develop the areas of uniform charge between the locations
of charge differential or field convergence.
Referring more specifically to Fig. 2, there is
shown schematically and in cross-section a non-uniform charge
pattern in accordance with the present invention. The non-uniform
charge pattern is created by an alternating positive and negative
charge pattern 9 on surface 10. Lines 11 indicate the direction
of the electric field created by pattern 9.
The effect of such a field on uncharged dielectric
particles 12 can be seen by observing the force vector
~` arrows beside particles 12. The resultant force on particles
12 is down toward surface 9, even though particles 12 are
uncharged. The mechanism of the attraction of particles 12
to the points of convergence of the lines of force on surface 9
is more fully explained in connection with the description of
particle 8 in Fig. 1. Howeverj in contrast with Fig. 1
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: . . . .. . . . ..
where the downward force is observed only at the location of
gross charge differential, the resultant downward force of Fig. 2
is observed at substantially all locations on surface 10 where
pattern 9 exists. Such a phenomenon results in a substantially
uniform coating of surface 10 in the areas of the non-uniform
charge pattern whenever contacted with uncharged particles such
as particles 12.
Suitable surfaces for use in the practice of the
present invention include those which will support an alternating
charge pattern or voltage pattern. Any such suitable surface
may be used. Insulating coatings and photoconductive layers
are typical of suitable surfaces. For example, the insulating
coating may be a thin dielectric layer placed on ordinary paper
or, more preferably, on conductive paper.
It is observed that thé~m~inimum useful voltage
in charge pattern 9 which can attract particles 12 is about
100 v. Thus, the surface should be capable of supporting at least
a 50 v. potential by virtue of its dielectric strength and thick-
ness. A ~50 v. potential which alternates with a -50 v. potential
provides the total 100 v. potential difference normally required
to attract uncharged toner particles. If, for example, the surface
is a vinyl resin, a thickness of at least about 0.5 micron is
necessary to support a potential of 50 v., although thicker
coatings are more typically encountered. No maximum thickness of
the insulating layer has been identified, and practical considera-
tions can be allowed to limit the maximum thickness of the layer.
Typical well known photoconductive materials such as
selenium, selenium alloys, zinc oxide and PVK/TNF, all of which
~re useful in the present invention, will generally support
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about 40 v. per micron in the dark. ~ecause a potential of at
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least abo~t 100 v. is required to attract particles 12, the
minimum useful thickness is about 1.25 microns when the surface
is one of these photoconductors and the non-uniform charge
pattern is an alternating +50 v. / -50 v. potential. ~s
explained further in connection with Fig. 4B, thinner photoconductive
layers enable greater resolution.
To be useful, particles 12 should have a dielectric
constant higher than their surrounding medium. Dielectric constant
is here viewed as a measure of the ability of a material to be
internally polarized responsive to an electric field. The more
readily polarizable, the higher the dielectric constant. For example,
free space as well as air has a dielectric constant of about 1.0
and conductiv~ metals, such as silver, have a dielectric
constant of infinity.
In the present invention, particles 1~ are found to
be useful whenever their dielectric constant is at least about l.S
greater than the surrounding medium. For example, when the
surrounding medium is air, the dielectric constant of particles
12 is normally at least about 2.5. In rare cases a dielectric
constant of less than 2.5 is encountered in useful particles,
such as in toner particles made from air-containing materials.
Typical of such materials are styrofoam microspheres similar
to those used in the manufacture of light weight paper. Higher
dielectric constants are often encountered.
It is important that the particles not discharge
the image. In some cases, such as when layer lO is a
selenium photoconductor, particles 12 should be insulating
at least for the time of the development step. However,
in other cases, such as when layer lO is an insulating
layer, metal particles 12 can be used without fear of discharging
the image.
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Particle 12 can be of any size suitable for imagewise
marking on the surface of layer 10. ~lthough the particle may
be any useful size, its upper size limit is usually determined
the desire for high resolution images. Particles which are
larger than about 30 microns sometimes produce images which have
an undesirable grain or roughness and a lack of fine definition.
Generally speaking, the smaller the particles, the
higher the achievable resolution. ~iowever, there is a lower
practical limit for particle size which is caused by handling
difficulties except in liquid suspension development systems.
It is observed that particles having an average
diameter of less than about 5 micron~ often become airborn to
cause unwanted dust acaumulation. Particles smaller than
about 5 miarons also give rise to transfer difficulties whenever
the developed image is transferred to a receiver sheet and
to cleaning difficulties in preparing the imaging surface for
recycling.
Resolution of the developed image is also determined,
to some extent, by the frequency o the non-uniformity in the
charge pattern. The frequency of the non-uniformity can be
defined as the reciprocal of the period. The period is the
distance between x and y in Fig. 2. The frequency can be varied
to give any suitable resolution in the developed image. Periods
which provide typically useful frequencies are from about 25
microns to about 175 microns with toner particles which range
from about 5 to about 30 microns.
Cosmetically undesirable roughness begins to appear
when toner particles of 10 microns are used to develop non-uniform
charge patterns having a period of 175 microns,or greater. This
relationship of 10 micron particles to a 175 micron period (0.06)
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can conveniently be used as a threshold ratio for cosm~tically
desirable development of charge patterns.
Particles 12 can be dyed and pigmented in a great
variety of colors as long as the colorant does not adversely
effect the dielectric constant of the particles. Any suitable
dye or pigment can be used to color particles 12. The manu-
facture of colored materials, such as thermoplastics, having
dielectric constants of around 3.0 is well known in the art.
Particles of such material are readily available from commercial
sources. Such variously colored materials are useful in the
development of non~uniform imagewise charge patterns in a wide
range of colors.
This ability to develop non-uniform charge patterns in
a wide varlety o colors is an important advantage of the present
invention. ~he great variety of colors and hues which are
possible is discussed by E. J. G. Balley in his paper "The
Coloration of Plastics", Journal a the Society af Colorists and
Dyers, p. 571-578j Dec. 1969.
- Referring more specifically to Fig. 3, there is
shown in cross-section a method for forming a non-unifoxm
charge pattern on an insulating surface.
In Fig. 3, a corona charging device 13 is driven
i by-an alternating positive and negative current supply 14 as
it moves across the surface of layer 15 in the direction shown
by the arrow. DeVice 13 leaves in its path a non-uniform pattern
16 of positive and negative charges. To increase definition,
charging is preferably through slit 20 in shield 21. Lines 17
indicate the shape of the electric field resulting from the
non-uniform pattern.
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Surface 15 is any suitable insulating layer, as
described in connection with Fig. 2. Corona chargin~ ~evice-~,
such as device 13, are well known in the art.
The alternating positive and negative charge applied
to corona device 13 can be any useful charge, as described in
connection with Fig. 2. The resulting surface potential typically
varies between +50 v. and +500 v. A more commonly encountered
charge density on the insulating surface after charging as
shown in Fig. 3 is ~300 v. A non-uniform charge pattern that
varies between +50 v. and -50 v. provides a lOOv. net potential
difference which is adequate to attract uncharged particles.
It will be clear tO those skilled in the art that a
stylus could be substituted for corona device 13. A stylus
whioh makes point contact with the imaging surface would be
preferred. It will also be apparent that the charge applied to
a stylus or to corona device 13 could alternate between
+ or - and O instead of alternating between + and - as does
.
current supply 14. Further, a stylus can be selectively
acti~ated to produce an imagewise non-uniform charge pattern
responsive to such remote control as a computer or an optical
scanner. -
Referring more specifically to Figs. 4A-F, there are
shown in cross-section various methods for establishing an image-
wise non-uniform charge pattern on a photoconductive surface.
In Fig. 4A, there is shown imaging member 27 which
includes photoconductive layer 28 and conductive substrate 29.
Any of the well known photoconductive layers typical of those
common to xerography are useful. A variety of such layers
will be apparent to those familiar with the art.
A uniform pattern of charge 30 is placed on layer
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28 by corona device 31 as it moves across the surface of layer 28
in the direction shown by the arrow. Lines 32 show the direction
of the field created by pattern 30 across layer 28. Conductive
substrate 29 is grounded, and a layer of negative charge 33
arises at the interface between substrate 29 and layer 28
when layer 28 is charged in the dark, as is well known in
xerography.
In Fig. 4B, charged layer 27 of Fig. 4A is exposed
to radiation to which it is sensitive from light source 34.
Exposure is through screen 35 so that radiation from light
source 34 strikes layer 28 in a regular screen pattern.
Uniform charge 30 of Fig. 4A is discharged in the areas where
light passes through screen 35 to strike photoconductive layer
28. Layer 28 is made conductive in the light-struck areas.
The ~oreen exposure leaves non-uniform charge pattern 36 on the
surface of layer 28.
Lines 37 show the shape of the electric field created
by non-uniform charge pattern 36. Lines 37 defining the field
extend from the positive charges on top of layer 28 to the
negative charges at the interface between layer 28 and substrate
29. The shape of the field is seen to be suitable for development
by uncharged marking particles as discussed in detail in
connection with Fig. 2. It is also seen that finer regular
microfields defined by lines 37 are possible in thinner layers
28, giving rise to higher definition.
In Fig. 4C, non-uniform charge pattern 36 of Fig. 4B
is exposed to an imagewise pattern of radiatlon to which layer
28 is sensitive. The radiation is from light source 34, and it
passes through transparency 38 and optical system 39. Trans-
parency 38 blocks some of the radiation from light source 34 and
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allows some light to pass through system 39 to strike layer
28 in an imagewise pattern.
In the light struck area of layer 28, non-uniform
charge pattern 36 is dissipated, because layer 28 becomes
conductive in the light struck area. In the non-light struck
area, the non-uniform charge pattern remains on photoconductive
layer 28. By such imagewise exposure, non-uniform charge
pattern 36 is made to be an imagewise non-uniform charge pattern.
It will be apparent to those skilled in the art that
the imagewise exposure of Fig. 4C could precede the exposure to
the screen pattern of Fig. 4B with substantially the same results.
In a useful variation of this process the screen
exposure of Fig. 4B and the imagewise exposure of Fig. 4c are
combined. The resulting non-uniform charge pattern in the
llght struck areas ls developable as explained in connection
with Fig. 2. The uniform charge pattern in the non-light struck
areas is not developable ~y the uncharged particles such as
particles 12 of Fig. 2. Thus, a dark-for-light reverse image
of the original is created.
An alternative method of forming an imagewise charge
pattern is depicted in cross-section in Figs. 4D-E.
-~ In Fig. 4D, a photoconductive layer 40 is placed on
;~ grounded conductive substrate 41 to form imaging member 42 similar
to member 27 of Fig. 4A. Insulating screen 43 is positioned on
layer 40, and corona charged device 44 is moved across screen
43 in the direction shown by the arrow. A positive charge is
applied to corona charging device 44, and a layer of positive
charge is placed on photoconductive layer 40 in a pattern
matching the openings in screen 43.
Fig. 4E shows the non-uniform charge pattern 4S
established on layer 40 by charging through screen 43. Because
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photoconductive layer 40 is supported on grounded conductive
substrate 41, negative charges are drawn to the interface between
substrate 41 and layer 40. Lines 46 depict the ~lectric field
cxeated on layer 40.
Non-uniform charge pattern 45 on member 42 is made
to be an imagewise non-uniform charge pattern by an imagewise
exposure step such as that shown in Fig. 4C.
Still another useful procedure for placing a non-
uniform charge pattern on a surface is shown in Fig. 4F. Imaging
member 47 includes grounded conductive substrate 48 on which a
substantially regular pattern of photoconductive d~posits 49 have
been placed. A leaky insulator layer 27 ~resistivity from about
1012 to about 1013 ohm cm) surrounds deposits 49.
Corona device 50 moves over member 47 in the dark in
the direction shown by the arrow while a positive charge is applied
to it. A positive charge is left on deposits 49, after the
field across the leaky insulator collapses. The shape of the
fields between the charges on deposits 49 and the corresponding
aharges of opposite polarity in substrate 48 are shown by lines 52.
A non-uniform charge pattern is established on member 47.
The non-uniform charge pattern on member 47 can be
made into a useful imagewise non-uniform charge pattern by the
procedure shown in Fig. 4C for member 27.
Figs. 5A-C show various useful methods for developing
imagewise non-uniform charge patterns.
In Fig. 5A, there is shown schematically and in
perspective view an arrangement for developing an imagewise non-
uniform charge pattern by flowing colored, uncharged particles
of marking material across the pattern. Non-uniform charge
pattern 53 is in the imagewise form of an x on the surface of
mem~er 54. It is to be understood that in Figs. 5A-C the imaging
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surface, such as the surface of member 54, can be either an
i,nsulating layer or a photoconductor as discussed in detail in
connection with Fig. 2. It is also to be understood that imagewise
patterns, such as pattern 53, can be formed by any suitable method,
such as the methods discussed in connection with Figs. 3 and 4.
Particles 55 are poured onto the surface of member 54
from container 56 and allowed to flow across member 54 and the
image it contains. When particles 55 encounter a non-uniform
field, they are drawn to it by the resultant downward vector of
the forces in the field as described in connection with Fig. 2.
In the flowing method of dev~lopment, particles 55
which do not adhere surface 54 to mark image 53 are collected
in sump 57 and may be reused.
Fig. 5B shows in cross-section a method for contacting
an imaging surface using a fluidized~bed-of marking partiales.
Imaging surface 58 is a drum which carries an
imagewise non-uniform charge pattern on its surface. The drum
rotates so that surface 58 passes through a fluidized bed of
marking particles. Any suitable method for creating a fluidized
bed of marking particles may be used~ In Fig. 5B, the method
for creating a fluidized bed is a combination of vibration
and air flow.
Particles 59 are held in container 60. Container
60 is vibrated by vibrating motor 61 which is fixedly attached
to stationary member 62. While particles 59 are vibrated, air
is passed through them from compressed air supply 63. The air
enters the bottom of container 60 through nozzles 64 and is
spread by impedence plate 1-15.
The combination of vibration and air flow create
a fluidized bed of particles 59. Surface 58 passes readily
through the bed and particles 59 uniformly contact surface 58.
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Insulating plug 114 prevents particles 59 from spilling from
container 60 without disturbing the non-uniform imagewise charge
pattern on the surface of drum 58. Particles 59 are attracted
to surface 58 by the imagewise non-uniform charge pattern
thereon. In Fig. 5B, imagewise pattern 65 can be readily
identified where it has been marked by particles 59.
Fig. 5C shows in cross-section a magnetic brush
development system which is also useful for bringing marking
particles into contact with a surface bearing a non-uniform
charge pattern.
A supply of marking particles 66 impregnated'with a
magnetic material is held in container 67. Ro'ller 68 is
_
positioned to contact the surface of the supply of particles
66. Roller 68 is divided in~o isolated segments substantially
as shown in the drawing. ' ' ,'
Alternate segments are of magnetically opposite
polarities.
~ arking particles 66 are picked up from container
67 by roller 68 as it rotates in the direction shown by the
arrow. A "brush~ of particles 66 held on the surface of roller
68 is brushed against surface 69 as roller 68 rotates. Surfàce
69 contains an imagewise non-uniform charge pattern which
attracts particles 66 from brush 70. The strength o'f the
magnetic fields holding brush 70 to roller 68 i5 weaker than
the imagewise non-uniform field on surface 69. Particles 66
attracted to surface 69 mark imagewise non-uniform charge
pattern 71.
The segmented roller 68 could alternatively be charged
to positive and negative potentials instead of having segments
of opposite magnetic polarity. In the case of oppositely
-21-
charged segments, the uncharged particles are held on roller 68
to form brush 70 by the non-uniform fields above the surface of
roller 68. In such an alternative embodiment, the toner particles
are not necessarily magnetic and can be of a greater variety of
colors.
A surface bearing a non-uniform charge pattern can also
be developed with a liquid developer having marking particles
suspended in a liquid carrier. However, suitable combinations
of carrier and toner particles are difficult to select because
the present development system relies on neutral particles
(O zeta potential). Most particles suspended in a liquid have
a finite zeta potential and will react with coulombic forces
in the presence of a field. For this reason, the liquid develop-
ment system is not prèferred.
In some instances, it will be desirable to fix the
particles directly onto the surface which they mark. In other
instances, it will be desirable to transfer the marked image to
a receiver sheet.
TranRfer of marking particles from a surface on which
marking occurs to a receiving surface is illustrated in Figs.
6A-B.
; Fig. 6A shows in cross-section a grounded imaging
member 76 on which marking particles 77 have been deposited in
an imagewise pattern. Member 76 includes photoconductive layer
; 74 and grounded conductive substrate 75. Particles 77 are
held to member 76 by a non-uniform charge pattern and have
no charge of their own. In the transfer method illustrated
here, the marking particles are first given a negative charge
. by corona device 78 as it moves past the particles as shown by
the arrow.
-22-
, , .
;
v
The next step of this transfer method is shown in
Fig. 6B which is also a cross-sectional view. Negatively
charged particles 77 are sandwiched between member 76 and
receiver sheet 79. Although any suitable receiver sheet may
be used, receiver sheets are typically paper.
Corona device 80 moves across the back of receiver
sheet 79 and places a uniform positive charge on it. The
positive charge creates a transfer field of sufficient strength
to tack the negatively charged toner particles 77 to receiver
sheet 79 so that particles 77 remain attached to sheet 79 when
sheet 79 is separated from surface 76 as shown in Fig. 6B.
Whether transferred to a receiver sheet or left on
the original surface, it ls often desirable to fix the marking
particl~s. Tw~ suitable fixing procedures are shown in Figs.
7A and B.
Fig. 7A shows in cross-section a heat fusing fixing
means. Receiver sheet 81 is separated from imaging surface 82,
as shown. Imaging surface 82, in this embodiment, is typically
a drum having either an insulating surface or a photoconductive
surface. Recelver sheet 81 in such an embodiment is typically
a paper web.
Marking particles 83 are transferred to receiving
surface 81 by any suitable process, such as the one typified
by that shown in Figs. 6A and B. Particles 83 are transferred to
receiving surface 81 in an imagewise pattern corresponding to
the imagewise non-uniform charge pattern developed on surface 82
(see Fig. 2).
Marking particles 83 typically are made from colored
thermoplastic material. The material is selected to melt at
high temperatures.
-23-
As the receiving surface moves in the direction shown
by the arrow, it passes under fusing means 84. Fusing means 84
is an electric coil which radiates heat when a current is passed
through it. A reflector directs heat from the coil onto thermo-
plastic marking particles 83 causing them to fuse together and
to combine with the material of receiving sheet 81. The heat
reduceR the viscosity of the toner material so that it can flow
into the receiver sheet as an ink. Such fusing and combining
is generally referred to as fixing.
Heat fusing,systems are well known in the xerographic
arts, and a variety of useful such systems will readily come to
mind; For example, heat fusing with xenon flash lamps or ovens
are useful alternative to the system shown in Fig. 7A.
Another useful fusing system is shown in perspective
vlew in Fig. 7B. Surface 85 i5 similar to surface 54 of Fig.
SA, and developed (marked) non-uniform charge pattern 86 is
similar to pattern 53 of Fig. SA. Developed charge pattern 86
is to be fixed directly onto surface 85 without being first
transferred to a reseiving surface.
Lacquer 87 is sprayed onto pattern 86 to attach it
to surface 85. Lacquer is sprayed onto pattern 86 by aerosol
can 88. Other similar suitable fixing means which are useful
in the present invention will be readily apparent to those
skilled in the art. For example, solvent vapor fixing could be
used. Trichloroethylene and l,l,l-trichloroethane are two well
known vapor solvents for soluble thermoplastic resins. ~he
spray fixing method shown in Fig. 7B is preferred for both
lacquer and solvents because it minimizes the possibility of,
disturbing the marking particles during fixing.
Fig. 8 shows in cross-section an imaging apparatus
-24-
1$1~
which performs the method of the present invention to produce
images on a receiving sheet. The apparatus of Fig. 8 is
illustrative of a variety of similar devices which are useful
to mark imagewise non-uniform charge patterns on receiving
sheets or on original surfaces.
Drum 89 rotates in the direction indicated by the
arrow. Its outer surface is a 6 micron layer of arsenic
doped selenium, a photoconductive material. The photoconductive
layer is coated onto a conductive substrate. The substrate is
grounded as shown.
As drum 89 rotates, it reoelves a uniform negative
charge on its photoconductive outer surface from corona
device 90. The charging is done in the dark, and a uniform charge
pattern is set up on the surface of drum 89 (see Fig. 4A).
At screen exposure statioin `91, a transparent tube 92
with a screen pattern marked on its surface rotates as shown
by the arrow. Fluorescent tube 93 and reflector 94 direct
light through the screen pattern on tube 92 to project a
light and dark screen pattern on the charged surface of drum
89. The rotational speed of tube 92 is synchronous with the
peripheral speed of drum 89 so that the peripheral speed of
drum 89 and the speed of the projected screen pattern on drum
89 are substantially the same. If the screen pattern is a line
pattern parallel with the direction of rotation of drum 89, a
non-moving optical system could be used.
The charge is dissipated in the light struck areas,
leaving a non-uniform charge pattern on the surface of drum 89
~see Fig. 4B).
It is to be understood that the regular non-uniform
charge pattern on the face of drum 89 could also be accomplished
-25-
. - . . .
with a stylus as discussed above.
When the drum moves to imaging station 95, it is
exposed to imagewise pattern of light 96. SIide 97 modifies
light from the light source so that imagewise pattern of light
96 passes through optical system 99 and strikes the non-uniform
charge pattern on the surface of drum 89. The non-uniform
charge pattern is dissipated in the light-struck areas, leaving
an imagewise non-uniform charge pattern (see Fig. 4C).
It will be understood by those familiar with the
xerographic arts that resolution of the imagewise non-uniform
charge pattern on the surface of drum 89 is improved whenever
transparency 97 is moved synchronously with drum 89 during
exposure. Such movement avolds "smearing" of th~ projected
imag~, Another well known technique for minimizing image
"smearing" is to use flash illumination of light source 98
while transparency 97 is held stationary.
It is to be understood that the imagewise non-uniform
charge pattern could be placed on surface 89 entirely by styli
and that, in the case of styli charging, surface 89 could be
an insulating layer.
The imagewise non-uniform charge pattern on the surface
of drum 89 is marked by contacting the surface with marking
particles at development station 100. A supply af marking
particles 101 is held in reservoir 102. The particles are brought
into contact with the surface of drum 89 by brush 103. Brush 103
is similar in construction and operation to the brush described
in detail in Fig. 5C. Marking particles 101 mark the imagewise
non-uniform aharge pattern on the surface of drum 89 as described
in greater detail in connection with Fig. 2.
. Particles 101 which mark t~he non-uniform charge pattern
.
-~6-
~l$~,~,V
are transferred from the surface of drum 89 to receiving sheet 104
substantially by the process shown in Figs. 6A and B. The
particles are first charged to a positive potential by corona
device 105. Receiving sheet 104, which in most cases is a paper
web, is unwound from supply roll 106 and around guide rollers
107 and 108. Positively charged particles 101 are sandwiched
between sheet 104 and drum 89 as the sheet and drum move in
contact between guide rollers 107 and 108.
A negative charge is applied to the back of sheet
104 by corona device 109. The negative charge attracts
positively charged marking particles lal so that the particles
adhere to sheet 104 when th0 sandwich is separated at roller 108.
In the exemplary embodiment of Fig. 8, transferred
partiales 101 are fused to sheet 104 by heat from xenon flash
lamp 110. Heat fusing is explained in greater detail in
connection with Fig. 7A.
After fusiny, sheet 104 is rewound on take-up
roll 111. It is clear that sheet 104 could be cut into
convenient page sizes instead of being rewound. It will be
equally clear to those familiar with the arts of xerography
and paper handling that sheet 104 could be supplied in cut
pages rather than as a web without altering the basic trans-
ferring and fixing steps.
The surface of drum 89 is prepared for subsequent
cycles by cleaning with brush 112. Brush 112 rotates against
the drum surface to remove residual marking particles 101 and
lint from web 104. The surface is then uniformly exposed to
light from fluorescent tube 113 to discharge any remaining
charge from the photoconductive surface.
The invention is described~below by way of example.
-27-
l~ V
Example I
Photoconductive layers of selenium are evaporat~d
onto six aluminum plates (Samples A-G) by well known vacuum
coating techniques. The plates have dimensions of about 9 inches
by 12 inches. The thickness of the photoconductive layer is varied
as shown in Chart I-l below.
Each plate is uniformly charged in the dark to a surface
potential giving an internal field of about 15 v. per micron, and
then exposed to a light through a 50 percent hard dot tint screen
available from Bychrome so that a screen pattern of light and
dark falls on the charged photoconductive layer. Exposure is
by a 40 watt tungsten l~mp.
Subsequently, the selenium layer ls exposed to an
lmage projected onto it by a slide projector. The image is
a dark "X" on a white field.
Blue marking particles of about 10 microns diameter
made from a copolymer of polystyrene and polymethylmethacrylate,
a thermoplastic resin, pigmented with phthalocyanine blue, are
flowed across the surface in the dark by pouring. The particles
have a dielectric constant of about 3Ø The beads are uncharged.
~: :
Chart I-l
Sample Se Thickness (microns) Results of Procedure
A 1 No Image Developed
B 2 Poor Image Developed
C 2.5 Useful Image Developed
D 5 Good Image Developed
E 10 Good Image Developed
' F 25 Good Image Developed
G ! 60 Good Image Developed
-28-
6~
.
Sample E of Chart I-l lS charged to various voltages
as shown in Chart I-2, below. After charging, it is exposed
to a screen pattern and contacted with marking particles as
in Chart I-l except that yel~low marking particles are used.
The yellow particles are formed from a copolymer of polystyrene
and polymethylmethacrylate, and are colored with benzidene
yellow pigment. The particles have a dielectric constant of
about 3Ø
After each development attempt, Sample D is cleaned
by brushing with a nylon bristle brush and exposure to a 30
watt fluorescent tube.
Chart I-2
Sample Charge Results
E 25 v. No Development
E 50 v. Poor Development
E 80 v. Good Development
E 100 v. Good Development
E 200 v. Good Development
It is seen f_om Chart I-l that an inorganic
photoconductor coated on a conductive substrate is useful in
the present invention at coating thicknesses of at least about
2 microns in the presence of a field having a strength of about
15 v./micron. It is seen from Chart I-2 that brushing the
light-discharging are useful in recycling a plate for use in
the method of this invention.
It is seen from Charts I-l and I-2 that uncharged
thermoplastic resins of various colors are useful in marking
the charge pattern on the surface. It is also seen from Charts
I-l and I-2 that flowing the marking particles across the
surface is a useful contacting technique. One useful method
-29-
i6~
for producing an imagewise non-uniform charge pattern is
apparent from Example I.
Example II
Samples H-N are prepared in the same coating thicknesses
as were Samples A-G, respectively, except that PVK/TNF, a well
known organic photoconductive material, is used. Samples H-N
are examined by the same procedure outlined for Samples A-G,
except that the photoconductive surfaces are contacted with
magenta uncharged marking particles made from the same resin
colored with bonadur red pigment.
Substantially the same re~ults are observed, indicating
that organic photoconduative materials are useful in the present
invention to about the same degree as inorganic photoconductive
~aterials. Further indication is provided that the process of
the present invention allows marking in a variety of colors.
Example III
Samples E and L from Exam,ples I and II are charged to
+800 v. and then exposed to a 40 watt incandescent lamp through
a transparent screen which carries a 500 line/inch Ronci ruling.
After charging, the screen is separated from the
photoconductive sur~ace and the surface is exposed to a projected
image as in Examples I and II except that the image is a
silhouette of a gavel. The,gavel silhouette is projected by
a 500 watt bulb through a transparency in a Kodak Carousel~
projector from a distance of 5 feet.
Samples E and L are then contacted with uncharged
black marking particles by the brush method. A brush arrangement
is constructed substantially as shown in Fig. 5C. The segments
of the brush roller are, of magnetically opposite polls. The
reservoir is filled with 19.5 micron averaged diameter marking
traJc n~arks
~30~
i6~
particles available from the 3M Company under the trade-
name A-09 Toner. The particles are thermoplastic resin
impregnated with Fe3O4. In each development step by
the brush method and by subsequent development methods
5 (see Chart III-l), the gavel handle is contacted by --
the developer prior to the gavel head. The results
are noted in Chart III-l.
Samples E and L are then cleaned by brushing
and exposure to light and subjected to the same
procedure again, except that instead of brush develop-
ment, fluidized bed development and flowiny development
are used. A fluidized bed is constructed substantially
as shown in Fig. 5B. The reservoir is caused to
vibra~e at a frequency of about 100 Hz. while air of
about 2 atmospheres pressure is passed through a bed
of A-09 Toner particles.
Flowing development is achieved as in
Examples I and II (Fig. 5A), except A-09 developer
particle~ are used.
Finally, Samples E and L are subjected to a
xerographic process wherein they are uniformly charged
to +800 v. and then exposed directly to the gavel
silhouette. After exposure, the samples are developed
by xerographic systems, cleaned and reused, as with
systems of the present invention. After the first
exposure Xerox 914 developer is cascaded over the
samples. Subsequently, the samples are developed by
the same toner held in a fluidized bed and held by a
magnetic brush. The results of these development
procedures are also noted in Chart III-l.
* trade mark
-31-
.
6~
Chart III-l
les Type of Imaging Type of Development Results
E & LPresent Invention Brush Good Solid
Area Coverage
E ~ LPresent Invention Flowing Good Solid
Area Coverage
E & LPresent InventionFluidized Bed Good Solid
Area Coverage
E & ~Xerography Brush Solid Area
Coverage Except
Developer Starva-
tion Noted *
E ~ LXerography Cascade Edge
Development
E & LXerography Fluidized Bed Edge
Development
* Developer staxvation is a well know~
descriptive phrase in xerography indicating
that good solid area coverage in one area
detracts from solid area co~v,erage in another
area. In Example III, the gavel handle', which ,''
encountered the magnetic brush'first, i5 solidly
developed; but a corresponding light strea~ is
observed in the body of the gavel's silhouette.
' It is seen from Example III that the present invention
.
provides solld area marking of images without developer
starvation in situations where xerographic processes cannot.
Further, it is seen that flowing, fluidized bed-and brush
development are all useful in the present invention. An
alternative method of forming a non-uniform charge pattern
on a photoconductive surface is seen.
Example IV
Samples O-R'are four specimens of Xerox Mobile Printer
paper, which is a conductive paper having a thin insulating
overcoating. Each sample is charged by a moving stylus while
the paper is held against a grounded conductive support. The
stylus is a bar which moves across the insulating coating at the
-
-32-
. . .
6~i~
rate of 1 inch/sec. The bar is alternatively charged to +100
v. at a frequency of 1,000 Hz. and is alternatively turned on
and off each 1/2 inch.
Aft~r charging, the marking material of Example II
is cascaded across the surface. The results are observed and
recorded in Chart IV-l, below.
Chart IV-l
leCoating Thickness Results
0 0.5 micron No Development
P 1.0 micron Useful Development
Q 5.0 microns Good Development
R 15.0 microns Good Development
The image developed on Samples P-R is a repeating
stripe pattern.
It is seen from Example IV that insulating layers ,
are useful imaging surfaces for use in the pr~sent invention and
that they can be coated onto such convenient support surfaces
as ordinary paper. It is also seen that insulating layers as
thin as 1 micron are useful in the method of the present
nvention with potentials of as little as 100 v. It is also
seen from Example IV that styIi can be successfully used to
put down an imagewise non-uniform charge pattern useful in the
present invention.
Example V
An imagewise non-uniform charge pattern of the gavel
silhouette of Example III is formed on Sample E by the process
of Example III. Development of the pattern is attempted using
a variety of 10 micron average particles having various dielectric
constants. The resuLts of the development efforts are noted
in Chart V-l, below.
. ~ . : . ,
.' . ' , . . ' ~ ~
G~ I
Chart V-l ~
I
Particle Material Dielectric Constant Results
Microspheres 1.5 Poor Development
Poly(styrene) 2.45 Good Development
Poly(methyl methacrylate~/ 4.0 Good Development
Poly(vinyl chloride)
Melamine-formaldehyde 7.9 Good Development
resin
It is seen that particles having a dielectric constant
of at least about 2.0 provide useful results.
Example VI
Sample E is charged and exposed to an optical image
as in Example IV. After exposure, lt is contacted with the
marking particles o~ Example I, e~cept that the particles are
of a di~ferent size. After each development attempt, Sample E
is cleaned, re-imaged, and development with another size
particle is tried. The various particle sizes used and the ~- ~
results observed are shown in Chart VI-l.
Chart VI-l
Particle Size Results
2 microns Image developed, but toner .
` dust covered equipment and
caused some undesirable
background on the imaging
sur~ace.
4 microns Image developed, but toner
dust covered equipment and
caused some undesirable
background on the imaging
surface.
5 microns Good Development
10 microns Good Development
20 microns Good Development
30 microns Good development. Image
roughness is visible upon
magni~ication.
40 microns Good development, but
roughness is visible with-
out magnification and is
distracting ~rom the image.
-~4-
. : .
6~
It i8 seen from Chart VI-l that from about 5 to about
30 microns in diam~ter is a preferred size range for use in the
present invention.
Example VII
In an effort to determine a preferred threshold ratio
of marking particle size to frequency of the non-uniform charge
pattern, 10 micron particles of the marking material of Example
III are used to mark the stripe patterns of Example IV (Sample E)
when they are placed on an insulating layer at various frequencies.
In Chart VII-l, the frequencies and the development results
are recorded. Frequency is defined as the reciprocal of the period.
the period is the distance between x and y in Fig. 2.
Chart VII-l
Period of Stylus Results
25 microns ,~ ~-Good ,
50 microns Good
100 microns Good
150 microns Good
175 microns Roughness begins
to show in developed
image.
200 microns Roughness in image is
distraating~
It is seen from Chart VII-l that the preferred
threshold ratio for marking particle size/frequency is about
10 microns/175 microns or 0.06. Ratios below 0.06 are not
preferred, although they might be useful for some purposes.
Example VIII
A magnetic imaging surface formed from CrO2 uniformly
embedded in a plastic sheet is provided. The sheet is uniformly
magnetlzed in a non-uniform magnetic pattern by moving it past a
stationary recording head. The sheet is then exposed to the
-35-
: .
~$~
gavel silhouette as were Samples E and L in Example III.
Cascade development of the sheet is then attempted with 3M
A-O9 Toner.
The sheet is cleaned, remagnetized and development is
attempted with the marking particles of Examples I and II.
When development is attempted with 3M A-O9 black toner,
which is magnetically attractable, excellent solid area coverage
over the entire sheet is observed. No imagewise pattern is seen.
When development is attempted with the marking particles
of Examples I and II, no development occurs.
It is seen from the observed results of Example VIII
that although non-uniform magnetic patterns result in good
solid area coverage with magnetically attraatable toner, magnetic
sUrfaces are not compatible with optical imaging. It is further
seen that magnetic patterns are not markable by the colorful
marking particles of Examples I and II.
The above description, examples and drawings will be
sufficient to enable one skilled in the art to make and use the
present invention and to distinguish it from other inventions and
from what is old. It will be appreciated that other variations
and modifications will occur to those skilled in the art upon
reading the present disclosure. These are intended to be within
the scope of this invention.
* tr~ nqrk
--36--
.
