U.S. patent number 3,645,770 [Application Number 05/033,117] was granted by the patent office on 1972-02-29 for improved method for developing xerographic images.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Thomas J. Flint.
United States Patent |
3,645,770 |
Flint |
February 29, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
IMPROVED METHOD FOR DEVELOPING XEROGRAPHIC IMAGES
Abstract
An improved method for developing latent electrostatic images by
forming a surface coating of developer material including a
magnetic component and an electroscopic component, transporting the
coating of developer material along a path past latent
electrostatic images to be developed, and uniformly charging the
outer surface of the coating of a polarity opposite that of the
latent electrostatic images. At development an outer layer portion
of the coating is deflected into close proximity with the latent
images to be developed and oscillating toward and away from the
images in the development zone to effect a continuous undulating
flow pattern of the outer layer of the coating.
Inventors: |
Flint; Thomas J. (Herkimer,
NY) |
Assignee: |
Xerox Corporation (Rochester,
NY)
|
Family
ID: |
26709304 |
Appl.
No.: |
05/033,117 |
Filed: |
April 16, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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723041 |
Apr 22, 1968 |
3552355 |
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Current U.S.
Class: |
430/120.1;
430/122.1; 427/145; 427/598; 430/903; 399/267; 118/15; 427/469;
427/560 |
Current CPC
Class: |
G03G
15/09 (20130101); Y10S 430/104 (20130101) |
Current International
Class: |
G03G
15/09 (20060101); G03g 013/22 () |
Field of
Search: |
;96/1 ;117/17.5 ;118/637
;355/3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Van Horn; Charles E.
Assistant Examiner: Cooper, III; John C.
Parent Case Text
This application, is a division of U.S. Pat. No. 3,552,355.
Claims
What is claimed is:
1. An improved method for (xerographically reproducing document
information) developing latent electrostatic images comprising
forming a surface coating of developer material including a
magnetic component and an electroscopic component, transporting the
coating of developer material along a predetermined path past
latent electrostatic images to be developed, applying a uniform
charge to the outer surface of said coating of a polarity opposite
that of the latent electrostatic images physically deflecting by
wave forming means an outer portion of the coating into close
proximity with the latent images to be developed, and
simultaneously oscillating the deflected portion of the coating
toward and away from the images in the development zone to effect a
continuous undulating flow pattern of the outer layer of the
coating.
Description
This invention relates to electrostatographic copying and,
particularly, to an improved method and apparatus for the
deposition of visible powder material on an electrostatic latent
image as in the development of a xerographic image or the like.
In xerography, it is usual to form an electrostatic image on a
sensitized surface. One method of doing this is to charge a
photoconductive insulating surface and then dissipate the charge
selectively by exposure to a pattern of activating radiation as set
forth, for instance, in U.S. Pat. No. 2,297,691 to Chester F.
Carlson. Whether formed by this means or any other, the resulting
electrostatic charge pattern is conventionally developed by the
deposition of an electroscopic material thereon through
electrostatic attraction whereby there is formed a visible image of
electroscopic particles corresponding to the electrostatic
image.
A common process of applying the developer to the electrostatic
image described in U.S. Pat. No. 2,618,552 to E. N. Wise involves
cascading a finely divided colored material called a "toner"
deposited on a slightly more coarsely divided material called a
"carrier" across the electrostatic image areas. The toner and
carrier being rubbed against each other while cascading, impart an
electrostatic charge to each other by triboelectric charging. When
a carrier particle, bearing on its surface oppositely charged
particles of toner, across an area on the image surface having an
electrostatic charge, the charge on the surface exerts greater
attraction for the toner than does the carrier and retains the
toner in the charged areas and separates it from the carrier
particles. The carrier particles, being oppositely charged and
having greater momentum, will not be retained by the charged areas
of the plate. When a toned carrier particle passes over a
noncharged area of the plate, the electrostatic attraction of the
carrier particles for the toner particles is sufficient to retain
the toner on the carrier preventing deposition in such areas as the
carrier particles momentum carries both toner and carrier past.
The above process, referred to as "cascade carrier development,"
has a high-development latitude and is particularly noteworthy in
freedom from background deposition. Further, the process is
dependable, operates with high efficiency under extreme humidity
conditions and is easily converted to give either positive or
reverse reproduction of the original to be copied. The process also
has certain limitations. Thus, cascade carrier development gives
little or no solid area coverage, that is, solid colored areas such
as those presented by block letters develop only around the
periphery leaving a white or undeveloped area in the center. Again,
relying largely on gravity to move the carrier across the
image-bearing surface, the process requires relatively large
carrier particle sizes for best efficiency. As a result, using
cascade development at high speeds places undue frictional stress
upon the photoconductor surface and the developing materials as
well as the equipment necessary to produce cascade movement of
developing material. In other words at high speeds, the use of
two-component developer material requires low impact of developing
materials on photoreceptors and tightly sealed developer housings
in order to prevent scattering and loss of toner particles and the
usual carrier beads. Then, too, there is a tendency for smaller
carrier particles to be retained on the plate thereby interfering
with transfer of the toner image. Closely related to the cascade
carrier development is magnetic brush development as disclosed in
U.S. Pat. No. 2,832,311. In this process a granular carrier is
selected having ferromagnetic properties and selected relative to
the toner in a triboelectric series so as to impart the desired
electrostatic polarity to the toner and carrier as in cascade
carrier development. On inserting a magnet into such a mixture of
toner and magnetic granular material the carrier particles align
themselves along the lines of force of the magnet to assume a
brushlike array. The toner particles are electrostatically coated
on the surface of the granular magnetic carrier particles.
Development proceeds as in regular cascade carrier development on
moving the magnet over the surface bearing the electrostatic image
so that the "bristles" of the magnetic brush contact the
electrostatic image-bearing surface.
Magnetic carrier development gives good coverage of solid areas and
is eminently suitable for machine application by reason of the
greater compactness of the developer system and freedom from
dependence on gravity which limits the placement of a cascade
carrier system around a rotary drum. Against these advantages,
magnetic development is inherently less efficient than cascade
development. In magnetic development only part of the "brush"
contacts the image-bearing surface. In addition, the magnetic field
restricts the motion of the carrier particles interfering with the
individual toner particles smoothly rolling across the image
surface. As one consequence of this, a higher concentration of
toner is generally essential in magnetic carrier development. By
reason of this and the electrical characteristics which result in
solid area coverage, the process gives a high-background deposition
and is generally characterized by poor development latitude.
As a consequence of these development techniques, toner powder
images are formed and the toner consumed must be replenished to the
developer mixture substantially in proportion to the amount
consumed by complicated dispensing devices. Various attempts have
been made to devise a single component development system in which
toner particles are used without carrier beads, but thus far, none
have been entirely successful.
It is therefore an object of the invention to improve the
development of electrostatic latent images.
It is another object of the invention to provide method and
apparatus for the development of electrostatic latent images
utilizing a magnetic toner material.
It is another object of the invention to enable high image quality
at very high development speeds.
It is a further object of the invention to effect optimum solid
image quality with minimum background conditions during
electrostatic development processing.
It is still a further object of the invention to produce solid area
images while at the same time effecting line copy images at very
high speeds using a minimum of developing materials and mechanical
parts and equipment and thus extensively reducing the impact and
frictional wear on the photoreceptor and the developing
materials.
It is still a further object of the invention to obviate the need
for regulating toner concentration in a developer mixture in
proportion to the amount consumed.
These and other objects of the invention are attained by utilizing
a magnetically controlled toner which is applied to an
electrostatic latent image in an undulating pattern at the
development station of an electrostatic reproduction machine. Means
are provided for imparting a uniform charge of proper polarity to
the toner to effect high quality development for both line and
solid images.
A preferred form of the invention is shown in the accompanying
drawings, wherein:
FIG. 1 is a schematic sectional view of a typical xerographic
reproduction machine embodying the principles of the invention;
FIG. 2 is a side-sectional view of the development apparatus
according to the present invention;
FIG. 3 is an end-sectional view of the development apparatus taken
along line 3--3 of FIG. 2;
FIG. 4 is an enlarged view of a circled portion of FIG. 3, and
FIG. 5 is an isometric view partly broken away of the development
apparatus .
For a general understanding of a typical xerographic processing
system in which the invention may be incorporated, reference is
made to FIG. 1 in which various components of a typical system are
schematically illustrated. As in all xerographic systems, a light
image of an original to be reproduced is projected onto the
sensitized surface of a xerographic plate to form an electrostatic
latent image thereon. Thereafter, the latent image is developed
with the same or an oppositely charged developing toner material,
depending upon negative-to-positive or positive-to-positive mode of
reproduction, to form a xerographic powder image corresponding to
the latent image on the plate surface. The powder image is then
electrostatically transferred to a support surface such as a sheet
of paper or the like to which it may be fused by a fusing device
whereby the powder image is caused permanently to adhere to the
support surface.
For purposes of the present disclosure, the xerographic
reproduction machine includes an exposure station at which a light
or radiation pattern of a document 10 to be reproduced is projected
by a lens 11 onto an electrostatographic surface, such as a
xerographic drum 12.
The xerographic drum 12 is detachably secured to a shaft 13 mounted
in suitable bearings in the frame of the machine and is driven in a
counterclockwise direction by a motor at a constant rate that is
proportional to the scan rate for the document being reproduced
whereby the peripheral rate of the drum surface is identical to the
rate of movement of the projected light image of the document. The
drum surface comprises a layer of photoconductive material on a
conductive backing that is sensitized prior to exposure by means of
a corona generating device 14.
The exposure of the drum to the document light image discharges the
photoconductive layer in the areas struck by light, whereby there
remains on the drum an electrostatic latent image in configuration
corresponding to the light image projected from the document. As
the drum surface continues, the electrostatic latent image passes
through a developing station in which there is positioned a
developer apparatus 16 in accordance with the present invention as
will be described hereinafter.
Positioned next and adjacent to the developing station is the image
transfer station which includes a pair of rollers 18 for holding a
support material in the form of paper web P against the surface of
the drum to receive the developed xerographic powder image
therefrom. The web P is moved in synchronism with the rotation of
the drum by means of a takeup roll 20 which drives the support
material P from a supply roll 22. A suitable drive mechanism (not
shown) is connected to the drum 12 for imparting rotation thereto
at a continuous speed. This drive mechanism may be connected to the
takeup roll 20 for imparting rotation thereto thereby producing
movement of the web material P in the same peripheral direction and
at the same speed as the peripheral surface of the drum. In order
to insure identical movement of the two coating surfaces, a
suitable programming device may be utilized to effect continuous
synchronous movement of these surfaces.
The transfer of the xerographic powder image from the drum surface
to the transfer material is effected by means of a corona transfer
device 23 that is located at place of contact between the transfer
material and the rotating drum. The corona transfer device 23 is
substantially similar to the corona discharge device 14 in that it
includes an array of one or more corona discharge electrodes that
are energized from a suitable high-potential source and extend
transversely across the drum surface and are substantially enclosed
within a shielding member.
In operation, the electrostatic field created by the corona
discharge device 23 of appropriate polarity is effective to attract
the toner particles comprising the xerographic powder image from
the drum surface and cause them to adhere electrostatically to the
surface of the transfer material.
Immediately subsequent to the image transfer station, the transfer
material is carried to a fixing device in the form of a fuser
assembly 25 whereby the developed and transferred xerographic
powder image on the sheet material P is permanently fixed thereto.
After fusing, the finished copy is preferably discharged from the
apparatus as a suitable point for collection externally of the
apparatus.
The next and final station in the device is a drum cleaning station
having positioned therein a corona precleaning device 26 similar to
the corona charging device 14 of appropriate polarity, negative for
positive-to-positive mode of reproduction and positive for
negative-to-positive mode of reproduction, to impose an
electrostatic charge on the drum and residual powder adherent
thereto to aid in effecting removal of the powder and a drum
cleaning device under suction in the form of a rotary brush 27
adapted to remove any powder remaining on the xerographic drum.
In general the electrostatic charging of the xerographic drum in
preparation for the exposure step and the electrostatic charging of
the support surface to effect toned image transfer are accomplished
by means of corona generating devices whereby electrostatic charge
on the order of from 700 to 1,000 volts is measured on the
respective surface in each instance. Although any one of a number
of types of corona generating devices may be used, a corona
charging device of the type disclosed in Vyverberg U.S. Pat. No.
2,836,725 is used for both the corona charging device 14 and the
corona transfer device 23, each of which is secured to suitable
frame elements of the apparatus and connected to a suitable
electrical circuit.
Referring now to FIGS. 2-5, there is shown in greater detail the
development apparatus 16 according to the present invention.
Development apparatus 16 comprises a frame 50 on which there is a
trough 51 for containing a supply of magnetic toner material 53.
Magnetic toner material is made up of two components, one of which
is magnetic particle and the other which is an electroscopic
marking resin powder. Any suitable electroscopic marking resin
powder can be used such as those described in U.S. Pat. No.
2,618,551 to Walkup, U.S. Pat. No. 2,618,552 to Wise and U.S. Pat.
No. 2,638,416 to Walkup and Wise.
The magnetic component should be a material which will respond to a
low- or high-frequency magnetic field so that it will readily
transfer the electroscopic binder and preferably can be heated,
thereby causing the electroscopic component of the developer to
melt or flow and become attached to the transferred material.
Magnetic materials suitable for the purposes of the present
invention are magnetic iron and its alloys, such as nickel-iron
alloys, nickel-cobalt-iron alloys, and magnetic oxides, such as
hematite (Fe.sub.2 O.sub.3 ) and magnetite (Fe.sub.3 O.sub.4 ) and
ferromagnetic ferrites. Cobalt and its alloys are also useful, such
as, for example, aluminum-nickel-cobalt, copper-nickel-cobalt, and
cobalt-platinum-manganese alloys. Moreover, other alloys, such as
certain magnetic alloys of aluminum, silver, copper, magnesium and
manganese can likewise be used with satisfactory results. These
materials can be added singly or in mixtures to the electroscopic
powder component. A preferred magnetic material comprises iron
oxide particles under the trademark of I.R.N. No. 100 manufactured
by C. K. Williams Division of Charles Phizer Co.
The magnetic component should be finely divided as this enables it
to be readily mixed or coated with the electroscopic binder
component and greatly increases its pigment value. Also the
magnetic component should be substantially coated or firmly
attached to a relatively larger amount, areawise, of the
electroscopic component in order that the powder will readily be
influenced by and develop electrostatic images since the magnetic
component itself may not be susceptible to electrostatic charges
and not, by itself, developed. Particles sizes of 1 to 20 microns
have been found satisfactory for producing good, clear dense
pictures.
There should be sufficient resin present in the composition so that
the resin containing the magnetic component will respond to the
electrical charges on the plate and thereby develop a picture even
if the magnetic component not be electroscopic. Also, there should
be sufficient resin present to hold the magnetic portion when the
powder is transferred and fixed. The magnetic material should be
present in an amount sufficient to respond to the electromagnetic
field and to carry the resin through such a field, as well as to
have a mass or volume to provide, under the influence of a
high-frequency electromagnetic field, sufficient heat to fuse or
flow the resin attached to it. It has been found that the ratio of
binder or resin to the magnetic component can vary from 19 to 2 to
3 . For the best results, there should be at least 20 percent by
weight of the magnetic particles, but not over 70 percent by
weight, as the higher amounts may contain too little binder to
satisfactorily secure the magnetic portion of the transferred
media.
Magnetic toner material 53 can be readily prepared by first finely
dividing or crushing the resin material, after which it is mixed
with the magnetic material. Thorough mixing is necessary in order
to insure that the magnetic particles have been entirely encased
with the binder. The mixed resin and magnetic powders are melted
and stirred to thoroughly disperse the magnetic powder in the
resin. The mass is then permitted to cool, and preferably is mixed
on a rubber mill where the heated rollers assure sufficient
plasticity to blend the components thoroughly, after which it is
broken into small chunks and again subdivided. It is then
micronized and sieved to size Obviously, other methods can readily
be devised by those skilled in the art for the production of
extremely fine pigmented resin powders of this type where the
pigment particles are magnetic in character.
Journaled for rotation, as by ball bearings 55, mounted on frame
50, is a transport roll 57, which serves to move the toner material
from trough 51 into the image development zone. Roll 57 comprises
alternately spaced magnetic field producing members or ring magnets
59 which are annular in shape and which are alternately spaced by
magnetic insulating members 61 for a purpose to be described.
Magnets 59 and magnetic insulating members 61 are held tightly
together by a pair of end plates 63,64 which are received in frame
50. A sleeve 65 is wrapped about the outer periphery of the
transport roll. Sleeve 65, end plates 63,64 and magnetic insulating
members 61 are made from any suitable nonmagnetic material. Typical
materials comprise glass, or any of the nonmagnetic metals, such
as, brass, aluminum or copper and mixtures thereof.
It is to be understood that the ring magnets 59 conveniently
comprise permanent magnets which exhibit polarities indicated by
letters N and S in FIG. 2 showing north and south poles,
respectively. Thus, magnetic fields are produced which result in
lines of flux passing through sleeve 65 and forming flux
concentrations such that brushlike tufts of magnetic toner material
are formed in projecting relationship to the peripheral surface in
a somewhat undulating pattern due to the flux patterns being
formed. It is desirable to provide independent magnets which are
spaced in the arrangement shown since the flux produced from
magnetic pole to magnetic pole is relatively constant across the
face of the transport roll, thereby overcoming any disadvantages of
long pole pieces where flux distribution may be difficult to
control. To rotate transport roll 57 there is fixed to end plate 64
one end of a shaft 67 connected at the other end thereof to a
driving pulley 69 which can be driven from any suitable power
source.
As transport roll 57 is moved through the supply of the magnetic
toner material, magnetic field producing members 59 on the roll
surface which is trimmed to a uniform thickness by a doctor blade
71. Typically the thickness of the developer coating after trimming
ranges from about 0.050 inches to about 0.100 inches. After being
trimmed to a uniform thickness on the transport roll, the developer
coating is moved past a corona charging device 73 similar to the
corona charging devices previously described at which time a
uniform charge of a polarity opposite to that of the electrostatic
latent image is applied to the developer coating. Charging
potentials ranging from about 4,500 to about 7,500 volts are
suitable for the development of latent electrostatic images. An
insulating block 74 serves to insulate charging device 73 from
housing 50. The charging causes the surface of the developer
coating to expand or spread slightly from its position prior to
charging. In order to pack down the coating, a baffle element 75 is
positioned adjacent to corona charging device 73 so that there is a
smoothened uniform layer of developer coating presented to the
latent image to be developed.
At the topmost position in the path of transport roll 57, there is
positioned one or more wave forming elements 80 around which
development of the latent image takes place as will become more
apparent. Wave forming elements 80 desirably have an arcuate shape
and are positioned sufficiently close to the transport roll surface
such that a charged layer of the developer coating is deflected
upwardly into close proximity with the latent image to be developed
due to the rotational movement of the roll.
At the same time wave forming elements 80 are oscillated in a
direction transverse to the rotational movement of the transport
roll 57 causing an undulation of the developer coating in the
vicinity of development. As a result, the latent image is
completely submerged in a flowing developer material in an
undulating pattern resulting in optimum development of the latent
image.
Wave forming elements 80 are desirably made out of a conductive
material so as to serve as an electrode to strengthen the
electrostatic fields emanating from the latent image. Hence the
solid area development portion of the image is greatly enhanced.
Typically each of the elements 80 may comprise ribbon shaped steel
which is approximately 0.250 inches wide and about 0.030 to about
0.075 inches thick. The wave-forming elements 80 are shaped tubular
at their ends where they are received in openings 81 formed in
housing 50. To adjust the tension of the wave-forming elements,
tensioning screws 83 are threadingly received at one end of these
elements.
In order to oscillate the wave forming elements 80 magnetic fields
are utilized from a rotatable linear magnet disposed on the
interior of transport roll 57 on a concentric axis therewith.
Rotatable magnet 85 is supported by ball bearings 87 and driven by
any suitable drive as by shaft 89 driven by a pulley 91. It will be
appreciated that when magnet 85 is in the vertical position the
magnetic fields directed toward field elements 80 are greatest and
thus the field elements are oscillated or vibrated due to pulsing
magnetic forces acting upon them.
It has been found that a speed ratio ranging from about 75 to about
125 times of linear magnet 85 to the transport roll 57 results in
developed images of very high quality. Both the ring magnets and
linear magnet 85 may be made from any suitable material, such as,
alnico.
It is preferred that the surface of trough 51 and the outer surface
of transport roll 57 and field elements be coated with a suitable
electrically insulating material, as, for example, ethyl cellulose
so that the charged toner particles do not stick to these surfaces
or become grounded.
In operation, the transport roll rotates in the same direction as
the travel of the xerographic photoreceptor but at 1.5 to about 5
times the photoreceptor speed so that a renewed portion of the roll
continually contacts the latent image. The transport roll
continually picks up magnetic toner material from the trough which
is at a level slightly less than the outside periphery of the roll.
Any suitable means may be used for periodically replenishing the
trough with a new supply of magnetic toner material as it is
consumed. Since all of the magnetic toner material is utilized in
the development of the image, there is no problem of insuring
proper metering and proportionality between carrier and toner
particles as in the prior art development devices. As the transport
roll moves past the magnetic toner material, the magnetic forces
emanating from the ring magnets draw a sufficient amount of the
material to form a developer coating on the periphery which is then
reduced to a uniform thickness by knife blade element 71. The
topmost portion of the coating is charged by charging device 73 and
smoothened by baffle elements 75 for development in the vicinity of
the wave-forming elements 80. Due to the pulsing action of the wave
forming elements, an undulating pattern of developer is flowed
across the latent image resulting in high-quality development.
Since wave forming elements 80 are conductive, image fields are
strengthened and solid area development is effected as well as the
line copy. Also due to the magnetic attraction of the layer of
developer material to the transport roll 57, the background
deposited on the photoreceptor is minimized. If desired, an
electrical bias may be applied to the transport roll to suppress
low electrostatic fields in the background areas.
While the present invention as to its objects and advantages has
been described herein as carried out in a specific embodiment, it
is not desired to be limited thereby; but it is intended to cover
the invention broadly within the spirit and scope of the appended
claims.
* * * * *