Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to an improved
electrostatographic processing system for producing 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 copy having
- little or no adverse quality aspects such as blur, hollow
characters or other causes of image degradation.
In a conventional form 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
lS 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 imag~ 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 oE which may be depleted o
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 transEer 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 electrosta-tic transfer device.
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As the surface o~ 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 ul-tra-high field type.
~ To enhance toner transfer especially in areas of the trans-: ~er 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 ei-ther 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 ~enerating 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
is also capable of effecting the transfer of larger toner
particles including the sizes disclosed herein. The bias
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 transEer 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
and any of the processing steps utilized in electrostato-
graphic processing. In the U. S. ~atent No. 4,027,960,
there is a specific disclosure of a dual transer system
and circuitry therefor. However, the devices in this
system are mainly directed to minimizing various copy
quality deficiencies disclosed in that patent and are
not correlated to the parameters of the components of
developing material and a specifically constructed toner
fixing device.
It is therefore an object of an aspect of the
present invention to produce xerographic images of the
highest quality.
It is an object of an aspect of the invention
to integrate xerographic processing steps and parameters
which has a combined action adapted to effect the repro-
duction of images of very high ~uality.
Various aspects of the invention are as follows:
An electrostatographic processing system having
developer mixture of finely-divided toner particles
electrostatically clinging to the surface of carrier
particles; a corona device for uniformly charging the
photosensitive surface prior to production of an electrostatic
latent image; a development apparatus or developing
the latent image; a transfer station whereat support
material is placed into contact with the photosensitive
surface bearing a developed image for receiving the same;
corona generating means at the transfer station having
an electrode adapted to spray ions upon the adjacent side
of the support material for effecting transfer of toner
particles from the developed image to the support material;
an electrically biased roller at the transfer station
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- arranged ~or contacting the adjacent side of the support
material as the same passes through the transfer station
for effecting transfer of toner particles from the developed
- image to the support material; and an imagng fixing station
whereat support material is conveyed for effecting the
fixing of a transferred image thereon, the improvement
comprising:
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 less than about 20.0 percent less than 5
microns.
An electrosta~ographic processing system having
developer mixture of finely-divided toner particles electro-
statically clinging to the surface of carrier particles;
a corona dev.Lce for uniformly charging the photosensitive
surface prlor to production of an electrostatic latent
image; a development apparatus for developing the latent
image; a transfer station whereat support material is
placed into contact with the photosensi-tive surface ~earing
a developed imageb~r receivingthe same; corona generating
means at the transfer station having an electrode adapted
to spray ions upon the adjacent side of the support
material for effecting transfer of toner particles from
the developed image to the support material; an electrically
hiased roller at the transfer station arranged for contacting
the adjacent side of the support material as the same
passes through the transfer station for effecting transfer
of toner particles from the developed image to the support
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~ ;~
material; and an imaging fixing station whereat support
material is conveyed for effecting the fixing of a trans-
ferred image thereon, the improvement comprising:
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, and a particle
size distribution with a median diameter by vol~me 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 more than
about 13.5 percent less than 5 microns.
The above ob~ects and others are accomplished
and will be more fully understood from the following
description of apparatus shown by way of example in the
accompanying drawings wherein:
Fig. 1 is a schematic view of an electrostatic
type reproduction machine embodying the principles of
the present invention; and
Fig. 2 is a fragmentary, enlarged view of the
transfer station of the machine shown in Fig. 2 and assoc-
- iated circuitry.
The processing system for the present invention
envisions the use of developer material within what is
dis~losed and claimed in the U.S. Patent No. 3,969,251.
Specifically, the present processing system contemplates
- the use of toner having a particle size in the range of
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about 5 microns to about 20 microns, and coated ferrite
carrier materials, having a volume average particle diameter
of about 10~ 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-
sensitive 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
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 1~, 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 photoconductive
belt or photoreceptor 22~ In moving in the direction in-
dicated by the arrow, prior to reaching the e~posure 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 ~o the light image discharges the surface in the
areas struck by light whereby an electrostatic latent image
- remains on the belt in image conflguration 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 3~ preferably comprises a plurality of magnetic
brushes 35 which carry developing material to the surface
o~ 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 br~sh biasing in accordance with the particular
toner/carrier electrostat~c 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
of paper seriatim from a paper supply system indicated
generally by the reference numeral 44 to the developed image
on the belt as the same is carried around the roller 26.
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23
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
for movement into a fuser assembly indicated generally by
the reference numeral 56 wherein the powder image on the
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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 roller type comprising a lower heated
roller 68 having a Quartz lamp 69 supported along the axis
thereof, and an upper pressure roller 70. For further
details of such a fusing apparatus reference is made to
U.S. Patent No. 3,268,351. Each sheet of paper b~aring
lS a toned image to-be-fused is conveyed in an inverted con-
dition by the transport 54, and into the nip of the rollers
68, 70. As each sheet moves through the nip in contact
with the heated roller 68, fixing of the toned images upon
the sheets takes place.
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 pheno~enon
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,
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23
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 distri-
bution over 20 microns diameter is left in the developer
mixture.
For the hot roller fusing apparatus useful 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 mi-
crons. Such a fusing apparatus may have its lower heater
roller 68 coated with TeflonG~ material (trademark of the
E.I. DuPont Corp.) as the outer layer thereby providing
a relatively hard surface for the fusing function.
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 trans~er
function by this roller. The corona emitting transfer
device ~8 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
'nave a negative electrostatic charge and the photoreceptor
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22 would have been charged with a positive charge ~y 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 ~8 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
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
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art of photography, the quantitative measure of processing
is "density". In the case o~ the former however, use of
"reflection density" as the measure is more relevant since
- the object of measurement is an opaque reproduction or copy.
A~ is well known in the reproduction field, reflection den-
sity is defined as
D = log10 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 particu-
larly, the relationship of toner particles to their support-
ing carrier beads has been the area which allows the most
effective means for density variation or control. This
relationship 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
impacted by one or more choices. The effect of an impact
on one a~pect of quality may be more damaging than the gain
on another aspect resulting from a choice of parameters.
In any event, the con-trollable 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.~ 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. ~eyond this point, the adding o~ additional -toner
is not conventional since known Eactors which aclversely af-
fects quality, such as an increase in background, smearing,
etc. will appear.
With use in the present invention, however t it
has been found that the adding of more toner particles to
a mixture of developing materials having the particle size
and distribution discussed above produced a still higher
quality of copy printing not envisioned by conventional use
and experimentation. As stated above, merely adding toner
particles to standard developing mixtures beyond preset amounts
determined by empirical data will increase the number of
background 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.
Additional toner particles are utilized in 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 appro~imately 1.3
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produced copy printing of solid areas with quality far su-
perior to the quality of copy printing exhibiting a density
of 1Ø It has also been found that the transfer efficiency
for solid area coverage increased significantly, approxi-
mately 20~, than what was achievable with the conventional
toner content in conventional developer mixtures.
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
but is intended to cover such modifications or changes as
may come within the scope of the Eollowing claims.
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