Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to an improved
electrostatographic processing system for producing high
density/high quality electrostatically developed images
by the transfer of these images to support material, such
as sheets of plain paper with a maximum of toner transfer,
and fixing the transferred images by means which produces
line copy having little or no adverse quality aspects such
as blur, hollow characters or other causes of image degradation.
In a conventional fcrm of electrostatographic
processing, such as xerography, a xerographic plate com-
prising a layer of photosensitive insulating material af-
fixed to a conductive backing is used to support electro-
static latent images. In the xerographic process, the
photosensitive surface is electrostatically charged, and
the charged surface is then exposed to a light pattern of
the image being reproduced to thereby discharge the sur-
face in the areas where light strikes the surface. The
undischarged areas of the surface thus form an electrostatic
- charge pattern (an electrostatic latent image) conforming
to the original pattern. The latent image is then developed
by contacting it with developing material having a finely
divided electrostatically attractable powder referred to
as "toner". Toner is held on the image areas by the electro-
static charge on the surface. Where the charge is greater,
a greater amount of toner is deposited. Thus, a toner image
is produced in conformity with a light image of the copy
being reproduced. Generally, the developed image is then
transferred to a suitable support material (e.g. paper),
and the image is affixed thereto to form a permanent record
of the original document.
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The developing material normally comprises rela-
tively large carrier beads, which may be insulatively coated
metal, and the relatively smaller dry ink particle toner.
Due to the triboelectric relationships between the two,
the smaller toner particles attach themselves to the carrier
in great numbers. As the developing material is brought
into contact with the photoreceptor surface, electrostatic
charges on the photoreceptor tends to separate the toner
particles from the carrier and deposit the same onto the
photoreceptor in accordance with the latent image charge
pattern. The carrier, some of which may be depleted of
toner or partially depleted, is returned to a developer
sump for replenishing with toner particles.
In the practice of xerography, the support ma-
terial is caused to move in synchronized contact with the
photosensitive surface during the transfer operation, and
an electrical potential opposite from the polarity of the
toner is applied to the side of the paper remote from the
photosensitive surface to electrostatically attract the
toner image from the surface to the paper.
Some modern high speed duplicating machines util-
ize a single transfer device such as an electrically biased
transfer roll to effect the image transfer. Although a
biased transfer roll system performing as the sole transfer
device effects good to excellent copy quality, certain copy
quality deficiencies may be present. The most notable of
these is the difficulty of transferring very small sizes
of toner particles, say on the order of 3 to 10 micron
diameter sizes. This inability is apparent because of the
geometry of a roller type electrostatic transfer device.
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As the surface of the roller approaches the nip at which
transfer of toner particles occur, there is a tendency for
voltage breakdown if the transfer potential is too high.
Lowering of the transfer voltage to avoid voltage break-
down, will limit the size of toner particles which can be
made to adhere to support material.
The toner particles contemplated for the purpose
of this invention must be related to a specific arrangement
of image transfer devices utilized during the xerographic
processing step of transferring. The present invention
contemplates a transfer system which includes a transfer
corona generating device of the ultra-high field type.
To enhance toner transfer especially in areas of the trans-
fer material wherein the corona generating device is not
applied, or is unable to effect efficient transfer, there
is combined therewith an electrically biased transfer roller.
The electrical biases on each of these transfer devices
and the form in which they take are effectively related
to the sizes of toner particles on either side of the size
distribution range. The transfer device in the form of
a corona generating device and the electrical potential
impressed thereon is suitable to effect transfer of the
smallest toner particles say down to approximately 3 microns
in diameter. A corona generating device of the type contem-
plated has one or more electrode wires which when energized
with the suitable potential sprays ions on the back side
of the sheet of paper during the transfer step. It is to
be understood that a corona generating device of this type
i6 also capable of effecting the transfer of larger toner
particles including the sizes disclosed herein. The bias
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transfer roller and the electrical potential impressed there-
on acting in unison with the corona generating device en-
hances transfer efficiency and is able to effect transfer
of more of the larger toner particles say from 10 microns
to somewhat larger sizes. In addition, the biased roller
will effect toner transfer in situations wherein the corona
generating device is unable to effect transfer or complete
transfer such for example at the leading edge of each copy
sheet.
As will be described hereinafter, the average
size of the diameters of the toner particles is shifted
downwardly by the classification of toner particles wherein
a large percentage of the larger toner particles are re-
moved. The resultant mixture provides a larger percentage
of smaller particles and a smaller average diameter size.
In effect, there is a finer toner distribution and the
transfer of this distribution with high transfer efficiency
is accomplished by virtue of the present invention by the
above-disclosed arrangement of transfer devices.
From the foregoing it will be appreciated that
the present invention contemplates the integrated action
of certain parameters of toner particles with the utiliza-
tion of two forms of transfer devices having various elec-
trical biases and potentials utilized to effect the transfer
of both small and larger diameter toner particles thereby
providing an efficient processing system for improving the
quality of copy.
A prior issued patent disclosing the use of both
a biased transfer roller and a corona generating device
do not correlate their use with the size of toner particles
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and any of the processing steps utilized in electrostato-
graphic processing. In the u. S. Patent No. 4,027,960,
there is a specific disclosure of a dual transfer system
and circuitry therefore. However, the devices in this
5 system are mainly directed to minimizing various copy
quality deficiencies disclo~ed in that patent and are not
correlated to the parameters of the components of develop~
ing material and a specifically constructed toner fixing
device. ~`
It is therefore an object of an aspect of the -~
present in~ention to produce xerographic images of the
hlghest quality.
It is an object of an aspect of the invention to
integrate xerographic processing steps and parameters
15 which has a combined action adapted to effect the repro-
duction of images of very high quality.
An aspect of the invention is as follows:
An electrostatographic processing system -
having developer mixture of finely-divided toner particles
20 electrostatically clinging to the surface of carrier par-
ticles; a corona device for uniformly charging the photo-
sensitive surface prior to production of an electrostatic
latent image; a development apparatus for developing the
latent image; a transfer station whereat support material
25 is placed into contact with the photosensitive surface
bearing a developed image for receiving the same; and an
imaging fixing station whereat support material is conveyed
for effecting the fixing o a transferred image thereon,
the improvement comprising:
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the toner particles having a particle diameter
in the range of between about 5 microns and about 20 microns,
and a particle size distribution with a median diameter
by volume of about 12.0 microns, that the coarse content
by volume not more than a range of 0-8.0 percent greater
than about 20 microns and that the fine content by number
not more than about 20.0 percent less than 5 microns, and
wherein the ratio of toner content to carrier is such that
there is a solid area reflection optical density o~ fixed ;
images on the support material of approximately 1.3,
corona generating means at the transfer station
having an electrode adapted to spray ions upon the ad~acent
side of the support material for effecting transfer of
toner particles from the developed image to the support
material-
The above objects and others are accomplishedand will be more ully understood from the following des-
cription of apparatus shown by way of example in the ac-
companying drawings wherein:
20 ~ Fig. 1 is a schematic view of an electrostatic
type reproduction machine embodying the prin~iples of the
present invention;
Fig. 2 is a fragmentary, enlarged view of the
transfer station of the machine shown in Fig. 1 and assoc-
iated circuitry, and
Fig. 3 is a cross section of the toner fixingapparatus utilized in the present invention.
The processing system for the present invention
envisions the use of developer material within what is
dlsclosed and claimed in the U. S. Patent No. 3,969,251.
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Specifically, the present processing system contemplates
the use of toner having a particle size in the range of
about 5 microns to about 20 microns, and coated ferrite
carrier materials, having a volume average particle diameter
of about 100 microns. It has been found that the developer
materials of this classification when used in the present
invention provide very improved results over conventional
developer materials. Better than expected results were
obtained when the toner materials had a particle size dis-
tribution with a median diameter by volume of about 12.0
microns against the larger median in conventional use, that
the coarse content by volume not more than a range of 0-
8.0 percent greater than about 20 microns, and that the
fine content by number not more than about 20.0 percent
less than 5 microns. Still better results were achieved
with the coarse content at a percent range of 1.0 to 5.0
and a fine content with a percent range of 0.5 to 13.5.
For a general understanding of an electrostatic
reproduction machine in which the present invention may
be incorporated, reference is made to Fig. 1. As in all
electrostatic reproduction machines of the type illustrated,
a light image of an original is projected onto the photo-
~ensitive surface of a xerographic plate to form an electro-
static latent image thereon. Thereafter, the latent image
is developed with an oppositely charged developing material
comprising carrier beads and toner particles triboelectri-
cally adhering thereto to form a xerographic powder image
corresponding to the latent image on the photsensitive
surface. The powder image is then electrostatically trans-
ferred to a transfer member such as a sheet of paper to
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which it may be fixed by a fusing device whereby the toner
image is caused permanently to adhere to the transfer member.
In the illustrated machine 10, an original 12
to be copied is placed upon a transparent support platen
14 fixedly arranged in an illumination assembly indicated
generally by the reference numeral 16. While upon the
platen, the illumination assembly flashes light rays upon
the original, thereby producing image rays corresponding
to the informational areas on the original. The image rays
are projected by means of an optical system 18 to an ex-
posure station 20 for exposing the surface of a moving
xerographic plate in the form of a flexible phatoconductive
belt or photoreceptor 22. In moving in the direction in-
dicated by the arrow, prior to reaching the exposure station
20, that portion of the belt being exposed would have been
uniformly charged to approximately +800 to +950 volts by
a corona generating device 24 located at a belt run extending
between the belt supporting rollers 26 and 28. The exposure
station extends between the roller 28 and a third roller
30.
The exposure of the photosensitive surface of
the belt to the light image discharges the surface in the
areas struck by light whereby an electrostatic latent image
remains on the belt in image configuration corresponding
to the light image projected from the original on the sup-
port platen. As the belt continues its movement, the latent
image passes around the roller 30 and through a developing
station 32 where a developing apparatus indicated generally
by the reference numeral 34 is positioned. The developing
apparatus 34 preferably comprises a plurality of magnetic
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brushes 35 which carry developing material to the surface
of the upwardly moving belt 22. As the developing material
is applied to the belt, toner particles in the development
material are electrostatically attracted to the charged
photosensitive surface to form a powder image (an electro-
static developed image). The brushes 35 for the apparatus
34 are electrically connected to a d.c. power supply 36
by way of a bus bar 37 to be electrically biased in accor-
dance with the electrical field needed between the brushes
and the photoreceptor 22. A variable resistance device
38 is connected to the circuit to permit variation in the
magnetic brush biasing in accordance with the particular
toner/carrier electrostatic characteristics and the electro-
static charge of the latent image being developed. The
apparatus 34 is electrically insulated from the remaining
structure of the machine so that the electrically conductive
carrier particles do not short out, or cause electrical
shorts relative to the machine.
The developed electrostatic image is transported
by the belt 22 to a transfer station 39 where a sheet of
paper is moved at a speed in synchronism with the moving
belt in order to effect transfer of the developed image.
Located adjacent the transfer station 39 is an electrically
biased transfer roller 40 which is rotatably arranged on
the frame of the machine to receive individual sheets from
a sheet conveyor 41 of a transport mechanism generally
indicated by the reference numeral 42 and to guide each
sheet to the transfer station 39.
The sheet transport mechanism 42 transports sheets
34 of paper seriatim from a paper supply system indicated
_g_
,, , . . ., ., . ., .. .; ,,. , . . .... ... . -
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generally by the reference numeral 44 to the developed image
on the belt as the same is carried around the roller 26.
At the transport station 39, the main transfer
device 48 in the form of a corona generating device having
one or more corona emitting wires is positioned to spray
ions on the back side of a sheet of paper as the latter
is moved or fed between the photoreceptor belt 22 and the
device 48. The transfer roller 40 is supplied with electri-
cal potential from a suitable d.c. source having a polarity
opposite of the toner particles being transferred. Trans-
fer of toner particles by the roller 40 is effected as the
roller comes in contact with the side of a sheet of paper
opposite that to which the developed image is to be trans-
ferred. The corona emitting wire for the device 48 is
electrically biased by means of a supply circuit with suf-
ficient voltage to effect ion spray upon the adjacent side
of each sheet of paper passing therebeneath so that the
developed image on the belt may be electrostatically at-
tracted to its adjacent side of the sheet of paper as the
latter is brought into contact therewith.
As a sheet emerges from the transfer station 39,
a charge is deposited on the leading edge thereof by a de-
tack corona generating device 52 to lessen the electrostatic
attraction between the belt 22 and the sheet so that the
latter can be removed by a vacuum stripping and transport
mechanism 54. With only the leading edge being so charged
for stripping purposes, there is less charge being applied
to the sheet by this detack device so toner disturbance
is eliminated. The sheet is thereafter retained on the
underside of the vacuum stripping transport mechanism 54
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for movement into a fuser assembly indicated generally by
the reference numeral 56 wherein the powder image on the
sheet is permanently affixed thereon. After fusing, the
finished copy is discharged at a suitable point for col-
lection. The toner particles remaining as residue on the
belt 22 are carried by the belt to a rotating brush cleaning
apparatus 58. The cleaning apparatus 58 cooperates with
a corona discharge device 60 for neutralizing charges re-
maining on the untransferred toner particles before being
removed by the rotating brush 58.
The fusing apparatus 56 envisioned in this in-
vention is of the hot, soft roller type comprising a lower
heated roller 68 having a Quartz lamp 69 supported along
the axis thereof, and an upper pressure roller 70. The
lamp 69 serves as a source of thermal energy for the fusing
apparatus. As shown in Fig. 3, the heated roller 68 in-
cludes a metallic core 90, which surrounds the lamp 69.
The outer surface of the core 90 is coated with an "abhesive"
or offsetting material 91, preferably a fluoroelastomer
based on the copolymer of vinylidene fluoride and hexafluoro-
propylene. A preferred example of this coating material
is Viton~material (trademark of the E.I. DuPont Corp) and
at a thickness in the range of 7-lO mils. For additional
information regarding the use of Viton~ as a fuser roller
material, reference is made of the description in the pub-
lication "Product Licensing Index, Research Disclosures",
July 1972, pages 72, 73. A suitable offset preventing oil,
such as silicone oil, may be applied to the fuser roller
surface during fusing operation.
The pressure roller 70 includes a metallic core
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92 having a thick organic rubber outer layer 93. The layer
93 may be of the material known as EPDM under the tradename
EPCAR 346 of the B.F. Goodrich Corp. and having an outer
thin sleeve 94 of PFA Teflon~ material, a trademark of the
E.I. DuPont Corp. For additional information of these materials
for a pressure roller, reference is made to U.S. Patent
No. 4,083,092 which discloses a pressure roller envisioned
for use in the present invention. When in operative contact,
the rollers 68, 70 are held against one another under pressure.
The materials 91 and 93 are of such a hardness
as to produce a relatively large deformed nip area 95 having
a width approximately 1/5 the diameter of the rollers.
As shown in Fig. 3, the pressure is such as to deform the
rubber in the pressure roller. The coating 91 is sufficiently
thick and yet conformable as to conform to the pile heights
of toner images-to-be-fused whether the piles comprise large
toner particles or small toner particles as envisioned in
the present invention. In conventional fuser roller coatings
made for example from Teflo ~, the hardness of the coating
prevents adequate conformability to the various heights
of toner piles in images and also to the variations of toner
sizes. When images-to-be-fused comprise variations of pile
heights, generally only the higher piles come in contact
with the fuser roller surface leaving much of the lower
heights out of contact with the fuser roller. This results
in high graininess of fused images and, for solid area cover-
age, glossiness with or without the accompaniment of images
which can be smeared by touching.
The combination of the deformability of the pressure
roller 70 along with the conformability of the fuser roller
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68 with the developing mixture to be discussed below has
produced very high quality output copies not available with
the separate uses of these features.
It has been found that relatively small toner
particles, or fine toner in a developer mixture tend to
effect high quality, line copy~ However, this phenomenon
is accompanied by the production of what is known as "hollow
characters", that is, some toner particles may occasionally
be depleted from the central regions of various letters.
It is very important then to determine diameter size of
such toner particles and to ocntrol the distribution of
these small size particles.
It has also been found that the use of too large
a diameter for too many toner particles, in other words,
coarse content, will be incapable of producing quality reso-
lution as images will appear blurred. Resolution may be
enhanced by controlling the course content so that a very
narrow percent range by volume for particle size distribu-
` tion over 20 microns diameter is left in the developer mixture.
For the hot roller fusing apparatus described
for the present invention, it has been found that higher
quality for line copy is available when the toner particles
have a size in the range of about 5 microns to about 20
microns with the median size by volume of about 12.0 microns,
that the coarse content by volume not more than about 5.0
percent greater than about 20 microns, and that the fine
content by number not less than about 13.5 percent less
than 5 microns. Such a fusing apparatus may have its lower
heater roller 68 coated with Teflon~ material (trademark
of the E.I. DuPont Corp.) as the outer layer thereby pro-
viding a relatively hard surface for the fusing function.
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In Fig. 2, the image transfer arrangement and
circuitry is illustrated along with the detack device 52.
As shown, the biased roller 40 is electrically connected
to the high voltage, positive d.c. power supply 80 which
is adapted to apply 1-5 kv potential to the roller for
providing approximately +25~A of current for the transfer
function by this roller. The corona emitting transfer de-
vice 48 has its coronode wire 81 connected to the high
voltage, positive d.c. source 82 to having impressed there-
on a potential of 4-8 kv at +90~A. The foregoing polarities
are utilized since it is assumed that the toner particles
have a negative electrostatic charge and the photoreceptor
22 would have been charged with a positive charge by the
corona emitting device 24. It will be understood that these
polarities may be reversed and coordinated in the usual
manner in the event the toner particles have a positive
charge and the latent electrostatic image is of negative
polarity.
The device 48 includes a shield 83 made of a
suitable non-conducting material and a plurality of thin
guide elements 84 which are mounted across the open end
of the device 48. The shield surrounds the coronode wire
81 almost completely except for one side which faces the
photoreceptor belt 22. The elements 84 are made from suit-
able conductive material and serve to prevent the leading
edge of each sheet of paper from entering the cavity of
the shield after being transported through the nip between
the roller 40 and the roller 26 with belt 22 and brought
under the device 48.
The shield 83 is electrically connected by way
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of resistor/capacitor device 85 to the return side of the
power supply 82. Similarly, the elements 84 are electrically
connected to the supply 82 by way of a resistor 86, and is
impressed with a potential approximately +1.0 k.v.d.c. The
elements 84, by being electrically biased, eliminates the
charges being built up on the elements during machine use.
Build up of such charges tend to create unevenness of the
transfer charge placed upon the photoreceptor 22 by the
transfer corona generating device 48. By virtue of the above-
described circuit for the elements 84, the latter become
self-biasing during operation.
In the art of electrostatic printing, as in the
art of photography, the quantitative measure of processing
is "density". In the case of the former however, use of
"reflection density" as the measure is more relevant since
the object of measurement is an opaque reproduction or copy.
As is well known in the reproduction field, reflection den-
sity is defined as
D = logl0 Il/I2 wherein
Il is the measuring incident beam directed upon the test
surface of an opaque copy while I2 is the reflected beam
from the test surface. For a xerographic copy, that is,
a fixed toner image upon support material such as paper,
reflection density is a measure of the quantity of the fixed
tone on a surface viewed by reflected light.
In conventional xerographic machines, density is
usually controlled by the variance of various parameters
associated with the development materials and the electrical
biases utilized in xerographic processing. More particular-
ly, the relationship of toner particles to their supporting
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carrier beads has been the area which allows the most ef-
fective means for density variation or control. This re-
lationship may directly involve the toner particle sizes
and distribution in a developer mixture.
The triboelectric relationships between toner par-
ticles and carrier beads is also a factor in conventional
xerography. Of the many possible variations which may be
chosen and employed, care must be taken to avoid or minimize
any deterioration of other quality factors which may be im-
pacted by one or more choices. The effect of an impact on
one aspect of quality may be more damaging than the gain
on another aspect resulting from a choice of parameters.
In any event, the controllable parameters of development
materials in conventional xerography and the amount of toner
present in a mixture have been such that a resulting density
of approximately l.0 has become the norm for high quality
copying.
Generally, the amount of toner in a developing
mixture is predetermined in accordance with the capability
of the reproduction machine in producing copies of acceptable
quality. Beyond this point, the adding of additional toner
i8 not conventional since known factors which adversely af-
fects quality, such as an increase in background, smearing,
etc. will appear.
In the present invention, however, it has been
found that the adding of more toner particles to a mixture
of developing materials having the particle size and distri-
bution discussed above produced a still higher quality of
copy printing not envisioned by conventional use and experi-
mentation. As stated above, merely adding toner particles
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to standard developing mixtures beyond preset amounts deter-
mined by empirical data will increase the number of back-
ground particles, a prospect which seriously lessons quality.
An excess of toner particles also increases the presence
of toner particles in the adjacent atmosphere which, in turn,
may eventually result in quality degradation.
It is contemplated in the present invention to
utilize additional toner particles to the mixture described
above in amounts which will result in a solid area density
of approximately 1.3 for copy printing. It has been found
by extensive testing, that this additional toner to the
mixture did not impact other aspects of quality, and that
the achievement of a density of approximately 1.3 produced
copy printing of solid areas with quality far superior to
the quality of copy printing exhibiting a density of lØ
It has also been found that the transfer efficiency for solid
area coverage increased significantly, approximately 20%,
than what was achievable with bias roll transfer only and
with the conventional toner content in conventional developer
mixtures.
From the foregoing it will be apparent that there
is disclosed a toner particle size distribution in conjunc-
tion with a transfer system having increased transfer ef-
ficiency of 20% that provides greatly improved line copy
characteristics (minimizes blur and hollow character) and,
in conjunction with higher output density (1.3), yields
exceptional solid area quality.
While the invention has been described with ref-
erence to the structure, and materials disclosed, it is not
to be considered to be confined to the details set forth
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but is intended to cover such modifications or changes as
may come within the scope of the following claims.
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