U.S. patent number 3,900,001 [Application Number 05/345,424] was granted by the patent office on 1975-08-19 for developing apparatus.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Lawrence J. Fraser, Delmer G. Parker.
United States Patent |
3,900,001 |
Fraser , et al. |
August 19, 1975 |
Developing apparatus
Abstract
An electrostatographic developing apparatus for applying
developer material to a developer receiving surface in conformity
with an electrostatic charge pattern wherein the developer is
transported from the developer supply to a development zone while
in a magnetic brush configuration and thereafter, transported
through the development zone in magnetically unconstrained blanket
contact with the developer receiving surface.
Inventors: |
Fraser; Lawrence J. (Rochester,
NY), Parker; Delmer G. (Rochester, NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
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Family
ID: |
26844209 |
Appl.
No.: |
05/345,424 |
Filed: |
March 27, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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146704 |
May 25, 1971 |
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Current U.S.
Class: |
399/269;
399/277 |
Current CPC
Class: |
G03G
15/09 (20130101) |
Current International
Class: |
G03G
15/09 (20060101); G03g 013/00 () |
Field of
Search: |
;118/623,637,636
;117/17.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rimrodt; Louis K.
Attorney, Agent or Firm: Ralabate; James J. Kolasch; Donald
C. Chapman; Ernest F.
Parent Case Text
This is a division of application Ser. No. 146,704, filed May 25,
1971, now abandoned.
Claims
What is claimed is:
1. Electrostatographic developing apparatus for applying developer
material to a developer receiving surface in conformity with an
electrostatic charge pattern, said apparatus comprising stationary
multipole magnet means to form dry magnetically attractable
developer material in a magnetic brush configuration in a region
extending from a developer supply to a development zone formed
adjacent the developer receiving surface; means to transport the
dry magnetically attractable developer material in the magnetic
brush configuration from the developer supply to the development
zone, said development zone formed between the developer receiving
surface and the transport means; means to transport said developer
through said development zone in magnetically unconstrained blanket
contact with said charge pattern; and means to discharge unused
developer from said development zone.
2. Electrostatographic developing apparatus for applying developer
material to a developer receiving surface in conformity with an
electrostatic charge pattern, said apparatus comprising an endless
movable transport surface in communication with a developer supply
and a development zone, stationary multipole magnet means
positioned interior of said endless transport surface whose lines
of force are sufficient to maintain developer present on the
movable transport surface in a brush configuration on that portion
of said transport surface between said developer supply and said
development zone, but are insufficient to maintain the developer in
a brush configuration in the development zone whereby said
developer is transported throgh said development zone in
magnetically unconstrained blanket contact with said developer
receiving surface during development of said charge pattern, and
means to discharge unused developer from said development zone.
3. Apparatus of claim 2 further comprising a second endless movable
transport surface in communication with said developer supply and
said development zone and positioned interior of said endless
transport surface stationary multipole magnetic field producing
means whose lines of force are sufficient to maintain developer on
the movable transport surface in a brush configuration on that
portion of said transport surface between said developer supply and
said development zone, but are insufficient to maintain the
developer in a brush configuration in the development zone during
development of the charge pattern, said second endless movable
transport surface having means for rotating the surface in a
direction opposite that of the first endless movable transport
surface whereby simultaneous development occurs with motion of the
developer with and against the motion of the developer receiving
surface.
4. Apparatus of claim 2 wherein said endless movable transport
surface comprises a nonmagnetic conductive rigid cylindrical
sleeve.
5. Apparatus according to claim 4 wherein said stationary multipole
magnet means comprises a continuous annular matrix of magnetizable
material, only the sector between the developer supply and
development zone having been permanently magnetized.
6. Apparatus according to claim 1 including means to transport the
developer receiving surface through the development zone in the
same direction as the direction of the developer.
7. Apparatus according to claim 1 including means to transport the
developer receiving surface through the development zone in the
direction opposite the direction of the developer.
8. Apparatus for developing an electrostatic latent image present
on an imaging surface comprising means defining a development zone
between developer transporting means and said imaging surface,
stationary multipole magnet means to form magnetically attractable
dry developer material in a magnetic brush configuration in a
region extending from a developer supply to said developer zone,
means to transport said developer material in the magnetic brush
configuration from the developer supply to the development zone,
means to place said developer in magnetically unconstrained dense
closely packed blanket moving contact with the imaging surface in
said development zone at a rate to provide development of the
electrostatic latent image on the imaging surface, and means to
discharge unused developer from the development zone.
9. An electrostatographic developing apparatus for applying
developer material to a developer receiving surface in conformity
with an electrostatic charge pattern, said apparatus comprising
stationary multipole magnet means to form dry, magnetically
attractable developer in a magnetic brush configuration from a
developer supply to a development zone formed between the developer
receiving surface and a stationary conductive surface, means to
transport said developer material in the magnetic brush
configuration, from the developer supply to the development zone,
means to catapult the developer from the transport means into the
development zone in magnetically unconstrained contact with said
developer receiving surface and means to discharge unused developer
from said development zone.
10. An electrostatographic developing apparatus for applying
developer material to a developer receiving surface in conformity
with an electrostatic charge pattern, said apparatus comprising
stationary magnet means to transport dry, magnetically attractable
developer in a magnetic brush configuration from a developer supply
to a development zone formed between the developer receiving
surface and a stationary conductive surface, means to catapult the
developer from the transport means into the development zone in
magnetically unconstrained contact with said developer receiving
surface; means to apply a potential of the same polarity and about
the same magnitude as present on said developer receiving surface
on said stationary conductive surface; and means to discharge
unused developer from said development zone.
11. Electrostatographic developing apparatus for applying developer
material to a developer receiving surface in conformity with an
electrostatic charge pattern, said apparatus comprising a plurality
of endless movable transport surfaces in communication with a
developer supply and a development zone; stationary multipole
magnet means positioned interior of said endless transport surfaces
whose lines of force are sufficient to maintain developer present
on the movable transport surface in a brush configuration on that
portion of said transport surface between said developer supply and
said development zone, but are insufficient to maintain the
developer in a brush configuration in the development zone whereby
dry magnetically attractable developer material is transported in a
magnetic brush configuration from said developer supply to said
development zone formed between the developer receiving surface and
the transport means, and through said development zone in
magnetically unconstrained blanket contact with said developer
receiving surface during development of said charge pattern; and
means to discharge unused developer from said development zone.
Description
BACKGROUND OF THE INVENTION
This invention relates to imaging systems and more particularly, to
an improved electrostatographic development apparatus.
The formation and development of images on the surface of
photoconductive materials by electrostatic means is well known. The
basic electrostatographic process, as taught by C. F. Carlson in
U.S. Pat. No. 2,297,691 involves placing a uniform electrostatic
charge on a photoconductive insulating layer, exposing the layer to
a light and shadow pattern to dissipate the charge on the areas of
the layer exposed to the light and developing the resulting
electrostatic latent image by depositing on the image a finely
divided electroscopic material referred to in the art as "toner."
The toner will normally be attracted to those areas of the layer
which retain a charge, thereby forming a toner image corresponding
to the electrostatic latent image. This powder image may then be
transferred to a support surface such as paper and may be
subsequently permanently affixed to the support surface as by heat.
Instead of latent image formation by uniformly charging the
photoconductive layer and then exposing the layer to a light and
shadow pattern, one may form the latent image directly by charging
the layer in image configuration. The powder image may be fixed to
the photoconductive layer if elimination of the powder image
transfer step is desired. Other suitable fixing means such as
solvent or overcoating treatment may be substituted for the
foregoing heat fixing step.
Several techniques are known for applying electroscopic marking
particles to the electrostatic latent image to be developed.
Included within this group are the "cascade" development technique
disclosed by E. N. Wise in U.S. Pat. No. 2,618,552, the "powder
cloud" technique disclosed by C. F. Carlson in U.S. Pat. No.
2,221,776 and the "magnetic brush" process disclosed, for example,
in U.S. Pat. No. 2,874,063. The magnetic brush development
technique has proven to be particularly satisfactory in the
development of large solid areas. In this technique, a fine marking
powder called toner is mixed with a relatively coarse magnetically
attractable carrier material and, through the phenomenon of
triboelectrification, the toner particles become triboelectrically
charged and cling to the surface of the carrier material. In
magnetic brush development, the brush may consist of a magnet with
the magnetically attachable carrier such as iron filings or other
ferromagnetic powder attached to it by magnetic attraction in a
chainlike arrangement simulating the fibers of a brush. When the
toner material is applied to the brush, the toner particles cling
to the ferromagnetic fibers by triboelectric attraction. Image
development is accomplished by brushing the imaging surface with
the brush to enable the toner particles to be electrostatically
attracted by a charge of opposite polarity on the imaging surface
to develop the electrostatic latent image.
In its most suitable and practical configuration, the magnetic
brush development system comprises a rotatable cylindrical shell or
sleeve surrounding a stationary bar magnet to provide the necessary
magnetic field. Upon rotation, the developer is magnetically
attracted to the rotatable shell from the developer reservoir and
formed into a brush and upon being rotated is delivered to the
development zone while in a brush configuration. Typically, the
magnetic field is produced from a single large heavy expensive
permanent bar magnet of high energy product material which must be
placed off axis inside the cylindrical sleeve thereby requiring
awkward mountings and minimizing freedom of structural design. In
addition, when only a single magnetic field generating means is
used, the single magnet must be strong enough that the holding
force at the most distant point on the cylindrical roller be
adequate. When the magnetic field is produced by a single magnet,
the magnetic attractive force cannot be adjusted at different
points on the roller circumference to perform the various functions
of pickup, holding, erection of a brush and release of the carrier
in an optimum manner. Furthermore, the field with the bar magnet
configuration extends far beyond the roller surface where it is
needed to perform the attractive function and may interact with the
field of adjacent rollers or interfere with other operations.
Additionally, stray fields can permeate the flowing developer where
they tend to increase friction and interfere with crossmixing and
may also cause eddy current losses in nearby moving parts.
In these magnetic brush development systems, it is also generally
desirable to regulate or control the thickness of the developer
layer carried on the roller by moving the roller past a metering
blade, which in addition to requiring fairly sensitive adjustment
may abrade the moving developer and increase the number of
collisions between adjacent carrier particles contributing to
increased impaction of toner material on the carrier beads with a
consequent reduction in developer life. Toner impaction in the
carrier is generally to be avoided since the impaction of any
material on the surface of the carrier will alter the triboelectric
relationship between the carrier and the toner particles and may
contribute to the subsequent flaking off of impacted material from
the carrier leading to uneven development on the imaging surface.
Further areas of toner impaction on carrier beads may be present in
magnetic brush development systems where the flow of developer is
interrupted and started by mechanical scrapers or gates or by
rotating the bar magnet inside the roller. The gates contribute to
carrier wear and impaction of toner material into the carrier
particles. In addition, in the actual development zone where the
magnetic brush is brushed across the imaging surface, the developer
must be passed through a development zone defined by a narrow
restrictive opening between the roller and the imaging surface so
that the toner particles may be electrostatically attracted from
the carrier particles to the imaging surface. To provide sufficient
toner particles to the imaging surface, it is generally necessary
to compress the developer bristles thereby making toner adhering to
carrier particles remote from the edges of the bristles available
for development. To accomplish this compression in the development
zone, it is generally necessary to force the developer through the
development zone with a consequent increase in the energy or power
required during development.
An additional problem encountered with magnetic brush development
is in the scratchy prints which are frequently obtained as a result
of the magnetically attractable carrier particles being pulled or
pushed through developed solid areas as magnetically linked chain
segments. Furthermore, with a magnetic field generating structure
comprising a single dipole bar magnet, after the formation of the
developer in a brush configuration on an applicator surface, little
mixing of the carrier and toner is accomplished on the applicator
surface prior to reaching the development zone. As a result,
charging of the toner material may be inadequate in places and the
toner may not be evenly distributed throughout the developer both
resulting in lower quality development.
Various attempts have been made to solve or minimize the
difficulties described above. In each of these proposals, however,
while certain problems may be alleviated to one extent or another,
deficiencies still exist in the remaining areas of difficulty.
There is a continuing need for improvements minimizing or
eliminating all of the foregoing mentioned and other
difficulties.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to provide a
development apparatus which overcomes the above noted
deficiencies.
It is another object of this invention to provide a development
technique and apparatus requiring reduced power to accomplish
development.
It is another object of this invention to provide a development
technique and apparatus capable of reduced carrier impaction.
It is another object of this invention to provide a development
technique and apparatus capable of providing increased developer
life.
It is another object of this invention to provide a development
technique and apparatus of increased flexibility and simplicity of
design.
It is another object of this invention to provide a development
technique and apparatus which limits the presence of magnetic
fields to only those areas where required.
It is another object of this invention to provide a development
technique and apparatus capable of producing improved solid area
development.
It is another object of this invention to provide a development
technique and apparatus with automatic self metering of the
developer to the developer applicator.
It is another object of this invention to provide a development
technique and apparatus capable of producing prints with more
uniform solid areas.
The above objects and others are accomplished, generally speaking,
by providing an electrostatographic developing technique and
apparatus for applying developer material to a developer receiving
surface in conformity with an electrostatic charge pattern wherein
the developer is transported from a developer supply to a
development zone while present in a brush configuration and the
developer is transported through the development zone in a
magnetically unconstrained blanket contact with the developer
receiving surface. More specifically, the development zone is
substantially free of the influence of the magnetic field
generating means used to maintain the developer in a brush
configuration during the transport of developer to the development
zone. Since in the development zone the developer is not
magnetically constrained and there are substantially no magnetic
cohesive forces between adjoining magnetically attractable beads
the developer will fall off the developer donor member by its own
weight when placed upside down. Typically, the developer is present
on a developer donor member in the development zone in a brushless,
dense closely packed blanket contact with the developer receiving
surface.
While the magnetic field present in the development zone is
insufficient to maintain the developer in the form of a magnetic
brush, the magnetic field generating means are sufficient to
maintain the developer during transport from the developer supply
to the development zone in the form of a magnetic brush. The use of
small multipole magnetic field generating structures such as
commercially available flexible magnets and ceramic magnets enables
the design and control of the magnetic field in such a manner that
the effectively short range magnetic fields can be tailored to
perform certain functions while at the same time not influencing
other nearby functions. This degree of control over the individual
toner and carrier particles in the developer enables the regulation
of the contacts or collisions between particles and with external
surfaces and enables the transport of developer material only to
desired areas.
Further, the removal of the magnetic field from the development
zone reduces the forces between the carrier beads permitting them
to roll more freely in contact with the developer receiving member
thereby reducing friction between the developer receiving member
and the developer donor member. Surprisingly with this degree of
control, the power required in the development zone and toner
impaction on the carrier are significantly reduced. In addition,
copies of improved print quality may be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features and advantages of the present invention may be
further illustrated by reference to the accompanying drawings in
which:
FIG. 1 is a schematic illustration of development apparatus
according to the present invention.
FIG. 2 is a schematic illustration of an alternative embodiment
according to the present invention.
FIG. 3 is a schematic illustration of an alternative embodiment
according to the invention.
FIG. 4 is a graphical comparison of the power requirements for
development with three different apparatus.
In FIG. 1, an electrostatographic imaging surface 11 such as a
photoconductive insulating layer in the configuration of a drum,
only a portion of which is illustrated, and which may have been
previously uniformly charged and exposed to a light and shadow
pattern in conventional manner is passed through a development zone
defined by the space between the drum surface and the developer
applicator or donor member 15. Developer applicator 15 comprises a
stationary shaft 13 of high magnetic permeability which may
function as a magnetic shunt or keeper and affixed to the
stationary shaft about at least a portion thereof a plurality of
stationary magnetic members 14 which may be individual small bar
magnets or small strips or sectors of flexible rubber magnetic
material or a sector of a ceramic magnet which has been magnetized
in a manner to provide a plurality of magnetic pole members.
Adjacent magnetic pole members are of opposite polarity on the
outer surface to provide the desired magnetic field generating
structure. The individual magnetic members 14 may be positioned
adjacent to one another or if desired, separated by a small
distance. During operation, the magnetic field produced by the
plurality of stationary magnetic members 14 should be sufficient to
magnetically support the developer in the form of a brush while
being transported from the developer supply or reservoir along the
surface of rotatably mounted conductive cylindrical sleeve member
12 as the sleeve member is rotated within the reservoir 10 to the
development zone defined by the gap between imaging surface 11 and
the surface of sleeve 12. The lines of force of the magnetic field
producing means are insufficient to maintain the brush
configuration of the developer in the development zone and outside
this zone there is substantially no contact between the developer
and the developer receiving member. If desired, uniformity of
composition of the developer may be maintained in the developer
reservoir 10 by means of rotating vaned mixing members 16.
FIG. 2 illustrates an alternative embodiment according to the
present invention wherein the developer donor member comprises an
endless movable transport surface such as belt 21 supported and
driven around cylindrical driving rollers 20 and 29. Within the
driving roller which advances the belt into the development zone is
placed an annular multipole magnetic field generating structure
concentric with the axis of the roller and along that portion of
the roller which transports developer to the development zone.
Typically, this magnetic field generating structure comprises a
plurality of magnetic members 23 fixedly positioned interior of
roller 20 about shaft 28 with, if desired, a magnetic shunt or
keeper 22 positioned in between the shaft and the magnetic members.
In this embodiment developer will be magnetically attracted to the
nonmagnetic conductive belt 21 and transported into the development
zone defined by the space between the belt 21 and an image bearing
surface, for example, an insulating material 25 driven through the
development zone by means of positioning and driving rollers 24.
With this structure and due to the comparatively small size of the
magnetic field the developer may be transported while in the
bristle configuration of a magnetic brush from the developer supply
to the development zone and thereafter transported through the
development zone in a magnetically unconstrained configuration.
Following development, the developer may be discharged from the
development zone and fall by gravity into developer reservoir 26.
If desired vaned mixing members 27 may be employed to move the
developer from the discharge portion to the pickup area of the
developer reservoir. Alternatively, following discharge from the
development zone the developer may be magnetically attracted to
donor belt and conveyed from the discharge portion through the
developer supply and into the development zone while magnetically
constrained in a brush configuration.
In FIG. 3 is depicted an alternative embodiment where the developer
may be transported from a developer supply while being magnetically
constrained in a brush configuration and thereafter pitched or
catapulted into a magnetic field free region development zone and
subsequently discharged from the development zone and magnetically
recaptured and returned to the developer reservoir. In this
embodiment, the development zone is generally defined by that gap
between the imaging surface 30 and a conductive member 32 serving
as a development electrode in well known manner. Developer in the
developer reservoir 31 is picked up from the reservoir onto
rotatably mounted cylindrical roller 33 containing a concentric
multipole magnetic field generating means 34 housed within in a
manner described with respect to either FIGS. 1 or 2 and the
developer is transported around roller 33 while in a brush
configuration and pitched or catapulted into the above described
development zone. The peripheral speed of the developer donor
member is such that at the top of the roller when the influence of
the magnetic field is terminated the developer is thrown from the
surface of the roller 33 and into the development zone. Depending
on the specific configuration of the developer donor member and the
structure of the magnetic field generating means the speed with
which the roller 33 must be moved to transport developer from the
developer supply to the development zone and catapult or pitch the
developer therein may be readily determined by the artisan. Upon
discharge of the developer from the development zone, the developer
may be magnetically recaptured in the form of a magnetic brush on
cylindrically mounted rotatable roller 35 having concentrically
mounted within a stationary multipole magnetic field means 36
similar to that described in FIGS. 1 and 2. If desired, the
developer may then be transported back to the developer supply.
It should be understood that the above described three figures of
the drawings are intended to be exemplary only of the development
technique and apparatus of the present invention. The basic
principals of this invention may be easily modified to provide the
requisite control of developer in any particular development
scheme. For example, it may be desirable in certain instances to
employ a plurality of developer donor rollers similar to roller 15
in FIG. 1 in the development zone, particularly when high speed
development is desired. Alternatively, developer may be conveyed to
the transporting and donor roller 15 of FIG. 1, for example, by
means of a second roller containing multipole magnetic field
generating means. In addition, with the use of adjacent roller
developer donor members rotating in opposite directions, both
rollers containing annular multipole magnetic field generating
means, simultaneous development with motion of the developer with
and against the motion of the imaging surface may also be
obtained.
Any suitable multipole magnetic field producing means of any
suitable structure may be employed in the practice of this
invention. Typical materials may be readily selected by the artisan
from the commercially available ceramic magnets and flexible
magnets. In general, the ceramic magnets are prepared by extruding
a composition comprising ferrite powder and a ceramic binder and
the resulting shape is then sintered at high temperatures, cooled
and magnetized. If required, the ceramic magnets may be finished by
grinding but it is quite difficult to drill or machine them. The
flexible magnets in general, consist of finely divided ferrite
particles such as barrium ferrite uniformly dispersed in a
generally nonmagnetic rubber or plastic binder. These materials are
generally prepared by extruding a large magnetic matrix containing
the ferrite particles into the desired shape and permitting the
resinous plastic or elastomeric material to harden or cure to a
solid state. The binder material may contain any additional agent
such as vulcanizing agents, plasticizers and the like to aid in the
compounding of the matrix. In addition to barrium ferrite, lead and
strontium ferrite are also particularly effective particles which
form good permanent magnets.
The multipole magnetic field producing means may be of any suitable
shape or configuration. They may, for example, be in the form of a
plurality of strips of the flexible rubber magnets with each strip
representing a single dipole magnet or alternatively may be in the
form of a flexible sheet which is appropriately shaped and which
may contain a plurality of individual dipole magnetic field
producing members. In addition, the use of shaped ceramic magnets
of any desired configuration may be employed. Particularly
satisfactory results may be had with extruded annular sections made
of ferrite loaded rubber which may be adequately magnetized along a
portion of its perimeter.
The multipole magnetic field producing materials may be magnetized
in any suitable manner. Typically, they are magnetized with a
standard impulse discharge type or DC type magnetizer, as is well
known in the art. Since the magnetic field producing means of this
invention may be magnetized prior to assembly of the magnets in
their appropriate place and since several poles may be placed on
one face or surface of these magnets, the flexibility and design in
fabricating is vastly improved. The multipole magnetic field
producing means may produce a magnetic field of any desired
strength. Typically, the peak magnetic field at the surface of the
developer donor member in the transporting zone is of the order of
from about 200 to about 1000 oersted. Generally, the lower value is
characterized by the magnetic field which is capable of holding the
developer on the surface of the donor member in a brush
configuration while the upper value is a practical value
established by the geometry and material characteristics. The short
range nature of the fields produced from these multipole magnetic
field producing members assures that the fields are confined to the
immediate region near the magnetic members and accordingly, the
fields can be eliminated at any given point if desired.
The developer donor member may be of any suitable size, structure
or configuration, including that of a cylindrical roller or belt
and may generally be described as an endless movable transport
surface. With the appropriate use and placement of a short range
magnetic field producing means control of the magnetic field in the
substantially field free areas is readily facilitated. Typically,
cylindrical roller donor members having a diameter of from about 1
inch to several inches may be employed. Positioned interior of this
roller may be a plurality of a single annular magnet or rectangular
bar magnets or rubber magnets having widths of the order of about
one quarter to about 1 inch and thicknesses between about
one-sixteenth to about one-half inch.
The donor member which may, for example, be the rotatable
cylindrical sleeve of FIG. 1 may be of any suitable conductive
material, rough or smooth surfaced. Since the multipole magnetic
field producing means employed herein are relatively low energy
product materials with short range field configurations it is
generally preferred to keep this donor member quite close to the
magnetic material. To insure that the magnetic field does not
interfere with other magnetic fields in any particular applicator
configuration, and to provide the best use of the magnetic
material, it may be desirable to employ a magnetic shunt or keeper
in the manner depicted in FIG. 1. Any suitable high magnetic
permeability material of suitable configuration may be employed for
this purpose.
In operation, the flow of developer from the developer reservoir to
the development zone may be readily interrupted by the rotation of
the otherwise stationary multipole magnet and particularly the
annular multipole magnet about an axis. Since the fields are short
ranged and well defined, the magnet need not be moved very far to
cut the developer flow. When the developer flow is to be
interrupted, the magnet may be rotated in the direction of the
developer flow so that all the developer then present on the roller
may be transported through the development zone and no additional
developer will be transported thereby leaving the roller clean. A
simple cam mechanism on the shaft of the magnetic support member
may be provided for this rotation.
With the relatively low energy short range field producing magnets,
the developer will generally be present on the donor member in a
brush thickness of the order of the pole spacing between the poles
of the magnetic field producing structure. Typically, this is of
the order of from about one eighth to about three-eights of an
inch. The developer is held on the surface of the donor member in
the form of a brush by the high gradient short range magnetic field
and as the donor member moves past the several poles of the
magnetic field producing structure, the individual developer
particles tumble under the effects of the changing direction of the
magnetic field while moving as a mass in the direction of movement
of the moving donor surface. It is currently believed that this
tumbling contributes to increased uniformity of developer and
thereby more uniform development. The magnetic field in the
transport zone is generally sufficient to form and maintain this
brush against the influence of gravity, inertia and centrifugal
force while the developer is transported from a suitable supply to
the development zone. The donor member transports the developer
through the development zone in a magnetically unconstrained
configuration so that the developer has freedom of movement within
the development zone.
With the use of a roller donor member and an arcuate imaging
surface as depicted in FIG. 1, development will take place in the
center portion of the nip formed between the roller applicator and
the imaging surface and in this zone substantially no magnetic
field is present. In the interior of the donor member, the magnetic
field producing structure is so positioned that the developer may
be transported in the brush configuration from the developer supply
to a position on the donor member where the mere physical movement
of the donor member enables the developer to be further transported
to and through the development zone. In the case of a cylindrical
donor member, for example, this may be readily accomplished by
positioning the multipole magnetic field producing structure
interior of that arcuate portion of the cylindrical roller required
in transporting the developer from the developer supply to the
development zone. Within the development zone, any magnetic field
present is insufficient to restrict the movement of the developer
material.
While the development zone may be described as being substantially
free of magnetic field this does not mean that the field is
strictly zero oersteds. Clearly the effects of the earth's magnetic
field which is about 0.5 oersted are present. It is intended that
the magnetic field present in the developer zone be significantly
reduced such that it does not affect the structure of the developer
during development. Typically, the magnetic field in the
development zone will be reduced to less than about 10% of the
field applied for forming the developer brush. This generally will
provide a magnetic field within the development zone of about 50
oersteds or less.
Any suitable spacing between the donor member and the imaging
surface within a development zone may be employed. Typically, the
spacing is generally dependent upon the quantity of developer
desired to be present during development. Typically, development
zone spacings of from about 0.020 to about 0.120 inches have proven
to be adequate. With such spacing, the movable donor member with
the developer material physically adhering to the surface moves
through the development zone and forces the developer to pass
through the development zone creating the desired intimate flowing
contact between the developer and the imaging surface to provide
uniform development.
Generally, both the donor member and the surface upon which an
electrostatic charge pattern is to be developed are both moved
through the development zone and may be moved either in the same
direction or in opposite directions. While it is currently believed
that there is wide latitude in the choice of speed and direction of
movement of the developer donor member and the speed of
development, it is generally preferred to move the developer donor
member at a rate faster than the surface upon which to develop the
electrostatic charge pattern in order to insure an adequate supply
of developer for development. Typically, a ratio of about 3 to 1 of
the peripheral speed of the developer donor to the speed of the
imaging surface in the development zone has been found to be
effective. Generally, the individual speed of the donor member is
such that the magnetic brush formed by the multipole magnetic field
producing structure is undisturbed merely by the effects of
centrifugal force. If desired, the size of the development zone may
be increased with the use of a belt type donor member which may be
adjacent a greater portion of the imaging surface, and thereby
insure more uniform development. As will be more fully apparent
hereinafter, development in an essentially field free region
reduces the power required to force the developer present on the
donor member through the development zone. It is currently believed
that in the absence of a strong magnetic field, there are no
significant magnetic linkages between adjoining magnetically
attractable particles and as such, there is freedom of motion of
the developer particles within the development zone and reduced
friction between the imaging surface and the donor member. An
additional advantage of this development technique is that the
developer donor member may function as a development electrode in
conventional manner to provide improved solid area coverage and if
desired, reversal development. Typically, reversal development may
be obtained by applying to the electrode a potential of the same
polarity and about the same magnitude as that in the charged areas
of the imaging surface.
The development technique and apparatus according to the present
invention may be employed to develop an electrostatic charge
pattern present on any suitable electrostatographic imaging
surface. Basically, any surface upon which an electrostatic charge
pattern may be formed and maintained for a short period of time may
be employed. Typical electrostatographic imaging surfaces include
dielectrics such as plastic coated papers and photoconductors.
Typical photoconductors include photoconductive materials on an
electrically conductive support member such as brass, aluminum,
nickel, steel or the like. The support member may be of any
convenient thickness, rigid or flexible, and may be in any desired
form such as sheet, web, plate, cylinder, drum or the like and may
also comprise other materials such as metalized paper and plastic
coated sheets.
Development may be accomplished with any suitable developer
materials. Typically, as is well known in the art, development may
be accomplished with electroscopic marking particles referred to in
the art as toner and grossly larger magnetically attractable
carrier beads which are selected in accordance with their
triboelectric properties so that when they are brought into contact
each material becomes electrically charged to a polarity opposite
to that of the other. Thus, one material may be charged positively
if the other material is below it in a triboelectric series and
negatively if the other material is above it in a triboelectric
series. To provide a magnetically attractable developer some
portion of the developer is made of magnetically attractable
material. Typically, the granular carrier material may, therefore,
comprise iron filings or other ferromagnetic powder such that when
under the influence of a magnetic field, a chainlike arrangement of
magnetic particles simulating the fibers of a brush is formed. The
toner material clings to the ferromagnetic fibers by triboelectric
attraction. Typically, the grossly larger carrier particles which
may have average particle diameters of from about 50 microns to
about 600 microns may be coated with a material to provide the
desired triboelectric relationship between the carrier and the
toner material. The carrier material is generally of sufficient
mass to avoid adherence to the electrostatic image during
development. Any suitable dyed or pigmented electroscopic marking
material may be employed. Typical well known materials include gum
copal, gum sandarac, rosin, phenolformaldehyde resins,
methylacrylic resins, polystyrene resins, polypropylene resins,
epoxy resins, polyethylene resins and mixtures thereof. These
toners generally have average particles diameters of from about 1
and about 30 microns.
As discussed above, when development is accomplished in a region
essentially free of the constraining effects of the magnetic field,
the power necessary to accomplish development is surprisingly found
to be markedly reduced as will be apparent from the following
nonlimiting comparative examples which describe and compare
preferred methods and apparatus of the present invention. Example I
is a practice according to the present invention while Examples II
and III are presented for comparative purposes only. Unless
otherwise specified, all amounts, proportions and percentages are
by weight.
EXAMPLE I
An electrophotographic plate comprising a surface layer of
selenium, 60 microns thick, on a conductive substrate is positively
charged to 900 volts and exposed to a light and shadow pattern in
conventional manner. Development of the electrostatic latent image
present on the selenium plate is obtained with a developer
applicator similar to that depicted in FIG. 1 by placing a
plurality of about one quarter inch square bar magnets cut from a
piece of Plastiform (available from Leyman Corporation, Cincinnati,
Ohio) around a steel keeper such that the pole closest to the
vertical from the advancing side of the roller applicator to the
development zone is approximately 30.degree. to the left of the
vertical and that the last bar magnet placed adjacent to the
discharge portion of the development zone is approximately
60.degree. to the right of the vertical. The rotatable roller is a
11/2 inch diameter stainless steel roller and development is
obtained with a developer comprising 250 micron steel carrier beads
and Xerox 914 toner, the toner being present in the concentration
of about 11/2%, by weight, of the developer. The plate is moved
through the development zone at a spacing of about 0.05 inches in a
direction both with and against the direction of the developer
donor roller. The power dissipated in the development zone is
determined by measuring the torque on the applicator roller
necessary to maintain a constant roller surface speed of about 20
inches per second. A reference zero power level is that required to
rotate the roller at a surface speed of 20 inches per second while
the roller is carrying developer but is not in contact with the
plate. Developer is loaded onto the rotating roller at a rate of
about 21 grams per inch second. With the field free zone created by
terminating the magnets 30.degree. from the vertical in the
development zone, the peak field nearest the development zone
occurs about 45.degree. from the vertical where at the roller
surface it is about 450 gauss without developer on the roller and
475 gauss with developer on the roller. The field at the vertical
position is about 10 gauss without developer and 30 gauss when the
roller is carrying developer. Curve A of FIG. 4 graphically
represents the power requirements at various development speeds
with the photoreceptor moving both with and against the direction
of rotation of the roller.
EXAMPLE II
The procedure of Example I is repeated except that the multipole
magnetic field generating means is rotated about the axis of the
roller such that the bar magnets extend around the roller through
the entire development zone and the field free region is present
where developer is thrown from the roller. Curve B of FIG. 4
graphically illustrates the power requirements in this development
technique with the magnetic field present in the development zone
with various speeds of development with the photoreceptor moving
with and against the direction of rotation of the roller.
Comparison of the print quality of Examples I and II reveals that
both techniques provided prints of equal quality even though the
energy dissipated in the development zone differed by an average
factor of about 3 or more.
EXAMPLE III
For further comparison the annular magnetic field producing means
of Examples I and II is replaced by a dipole magnet which produces
a peak magnetic field at the roller surface of about 450 gauss.
Contrasted to the rate of passing developer through the development
zone in Examples I and II of only 21 grams per inch second, the
developer flow rate is about 45 grams per inch second in this
example due to the longer range nature of the magnetic field.
Development is obtained at various development speeds with the
photoreceptor being moved with and against the direction of
rotation of the applicator. Comparison of the print quality of
prints produced in this mode indicates that they are comparable in
quality to those produced in Examples I and II. Curve C of FIG. 4
graphically illustrates the power requirements in the development
zone and clearly demonstrates the extraordinary reduction in power
which may be achieved in practicing the present invention. It is
currently believed that this reduction in power is achieved since
the magnetic brush does not have to be counteracted or collapsed
while forcing the developer through the development zone. Further,
extended life study tests of the developer in development
configurations where development is achieved in a magnetic field
region and in a nonmagnetic field region indicate that the
developer employed in a nonmagnetic field region is believed to
enjoy a greater life span due to reduced toner impaction of the
carrier. In addition, since the short range fields of the annular
multipole magnetic field producing means of this invention during
the transport portion and recapture portion, if used, produce
magnetic brushes of very small dimensions, there is no need for the
use of leveling blades and other devices to regulate the thickness
of developer on the applicator surface. Accordingly, impaction
between the carrier particles is reduced and developer life
extended. In addition, reduced streaking in the development of
solid areas when developing in an essentially field free region has
been observed. The increase in power as the imaging surface and
applicator surface approach synchronous speed as shown in FIG. 4 is
believed to be due to frictional losses since they approach a
maximum because static friction losses which are higher than
sliding friction losses are possible between the developer and both
the applicator surface and imaging surface.
The development technique and apparatus described herein provide a
relatively small compact development system of great versatility
and flexibility in operation. Thus, the development system may be
tailored to produce strong magnetic field influences in areas where
desired and weaker or essentially no magnetic field influences in
other areas. The use of magnetic fields to hold the developer in
the form of a brush from the developer supply to the development
zone and the ability to exclude the magnetic field from areas not
desired permit development in a zone where the developer is
magnetically unconstrained resulting in reduced power requirements
for development. In addition, the increased mixing of developer
mateial during the transport from the developer supply into the
development zone is believed to contribute to a more uniform
development in the development zone. Since the mass of material
that must be moved during each development cycle may be markedly
reduced, the collisions between neighboring carrier particles and
toner particles are markedly reduced and the impaction of toner on
the carrier particles is also reduced resulting in increased
developer life.
Although specific techniques, apparatus and materials are set forth
in the foregoing disclosure and comparative examples using the
development apparatus and technique of this invention, there are
other materials and techniques and modifications of the present
invention which will occur to those skilled in the art upon a
reading of the present disclosure which techniques, modifications
and materials are intended to be included within the scope of this
invention.
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