U.S. patent number 3,656,948 [Application Number 04/878,495] was granted by the patent office on 1972-04-18 for selective removal of liquid developer in a cyclical electrophotographic process.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Joseph Mammino.
United States Patent |
3,656,948 |
Mammino |
April 18, 1972 |
SELECTIVE REMOVAL OF LIQUID DEVELOPER IN A CYCLICAL
ELECTROPHOTOGRAPHIC PROCESS
Abstract
An electrostatographic imaging surface is cleaned of liquid
developer by selectively applying to the charged or image areas of
the imaging surface a finely divided, dry, absorbent powder. The
powder may be selectively applied by charging the powder to a
polarity opposite that which the imaging surface bears and
presenting a charged powder loaded applicator adjacent the imaging
surface such that the charged powder is transferred to the imaging
surface with an electrostatic assist from the residual charge on
the imaging surface.
Inventors: |
Mammino; Joseph (Penfield,
NY) |
Assignee: |
Xerox Corporation (Rochester,
NY)
|
Family
ID: |
25372145 |
Appl.
No.: |
04/878,495 |
Filed: |
November 20, 1969 |
Current U.S.
Class: |
430/117.3;
399/249; 134/7; 134/9 |
Current CPC
Class: |
G03G
5/005 (20130101); G03G 21/0088 (20130101) |
Current International
Class: |
G03G
21/00 (20060101); G03G 5/00 (20060101); G03g
013/22 () |
Field of
Search: |
;96/1.4,1 ;134/7.9
;252/410 ;15/3 ;355/15 ;117/17.5,37 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lesmes; George F.
Assistant Examiner: Cooper, III; John C.
Claims
What is claimed is:
1. The method of cyclically developing electrostatic latent images
on a reusable electrostatographic imaging surface comprising the
steps of forming an electrostatic latent image on the imaging
surface, developing the image with a liquid developer, transferring
the developer from the imaging surface to the receiving surface in
image configuration, selectively applying dry absorbent cleaning
powder to the image area of said imaging surface to absorb
substantially all of the liquid developer on the imaging surface
and removing the absorbent powders containing absorbed liquid to
prepare the imaging surface for the next imaging cycle and
repeating the recited steps of forming, developing, transferring,
applying and removing at least one additional time.
2. The method of claim 1 wherein said dry powder has a charge
opposite in polarity to the charge in the image areas of the
imaging surface.
3. The method of claim 1 wherein said dry absorbent powder is
triboelectrically charged by contact with a granular carrier
material.
4. The method of claim 1 wherein the imaging member is a reusable
photoconductor.
5. The method of claim 1 wherein the photoconductor is selected
from the group consisting of selenium and selenium alloys.
6. The method of claim 1 wherein said selective application of
absorbent powder is electrostatically assisted.
7. The method of claim 1 wherein the imaging member is treated with
an amount of absorbent powder at least sufficient to absorb
substantially all the liquid on the imaging member.
8. The method of claim 1 wherein the absorbent powder is
substantially insoluble in the liquid developer.
9. The method of claim 1 wherein the powder has a surface area of
from about 100 to about 800 m.sup.2 /gm.
10. The method of claim 1 wherein the powder has a particle size of
from about 1 to about 20 microns.
11. The method of claim 1 wherein said powder is removed from said
imaging member by contacting the imaging member with an absorbent
fibrous material.
12. The method of claim 1 wherein said absorbent powder is a silica
gel.
13. The method of claim 1 wherein said absorbent powder is a finely
divided carbonaceous material.
14. The method of claim 1 wherein said carbonaceous material is
carbon black.
Description
BACKGROUND OF THE INVENTION
This invention relates to imaging systems, and more particularly,
to improved cleaning systems and techniques.
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 image 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. The transferred
image may subsequently be permanently affixed to a 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 image, 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.
Similar methods are known for applying the electroscopic 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.
Development of an electrostatic latent image may also be achieved
with liquid rather than dry developer materials. In conventional
liquid development, more commonly referred to as electrophoretic
development, an insulating liquid vehicle having finely divided
solid material dispersed therein contacts the imaging surface in
both charged and uncharged areas. Under the influence of the
electric field associated with the charged image pattern, the
suspended particles migrate toward the charged portions of the
imaging surface separating out of the insulating liquid. This
electrophoretic migration of charged particles results in the
deposition of the charged particles on the imaging surface in image
configuration.
A further technique for developing electrostatic latent images is
the liquid development process disclosed by R. W. Gundlach in U.S.
Pat. No. 3,084,043 hereinafter referred to as polar liquid
development. In this method, as electrostatic latent image is
developed or made visible by presenting to the imaging surface a
liquid developer on the surface of a developer dispensing member
having a plurality of raised portions or "lands" defining a
substantially regular patterned surface and a plurality of portions
depressed below the raised portions or "valleys". The depressed
portions of the developer dispensing member contains a layer of
conductive liquid developer which is maintained out of contact with
the electrostatographic imaging surface. Development is achieved by
moving the developer dispensing member loaded with liquid developer
in the depressed portions into developing configuration with the
imaging surface. The liquid developer is believed to be attracted
from the depressed portions of the applicator surface in the
charged field or image areas only. The developer liquid may be
pigmented or dyed. The development system disclosed in U.S. Pat.
No. 3,084,043 differs from electrophoretic development systems
where substantial contact between the liquid developer and both the
charged and uncharged area of an electrostatic latent image bearing
surface occurs. Unlike electrophoretic development systems,
substantial contact between the polar liquid and the areas of the
electrostatic latent image bearing surface not to be developed is
prevented in the polar liquid development technique. Reduced
contact between a liquid developer and the non-image areas of the
surface to be developed is desirable because the formation of
background deposits is thereby inhibited. Another characteristic
which distinguishes the polar liquid development technique from
electrophoretic development is the fact that the liquid phase of a
polar developer actually takes part in the development of a
surface. The liquid phase in electrophoretic developers functions
only as a carrier medium for developer particles. In general the
developer technique disclosed in U.S. Pat. No. 3,084,043 may
provide development with liquid developers having a conductivity of
from about 10.sup.-.sup.4 (ohm-cm).sup.-.sup.1 to about
10.sup.-.sup.14 (ohm-cm).sup.-.sup.1.
An additional liquid development technique is that referred to as
"wetting development" or selective wetting as described in U.S.
Pat. No. 3,285,741. In this technique, an aqueous developer
uniformly contacts the entire imaging surface and due to the
selected wetting and electrical properties of the developer
substantially only the charged areas of the imaging surface are
wetted by the developer. The developer should be relatively
conductive having a resistivity generally from about 10.sup.6 to
10.sup.10 ohm-cm and having wetting properties such that the
wetting angle measured when placed on the imaging surface is
smaller than 90.degree. at the charged area and greater than
90.degree. at the uncharged areas.
While capable of producing satisfactory images, these liquid
development systems in general, suffer deficiencies in certain
areas and are in need of further development and improvement.
Particularly troublesome difficulties are encountered in liquid
development systems employing reusable or cycling
electrostatographic imaging surfaces. In these systems, for
example, a photoconductor such as a selenium or selenium alloy drum
as the photoconductor surface is charged, exposed to a light and
shadow image and developed by bringing the image bearing surface
into developing configuration with an applicator containing
developing quantities of liquid developer thereon. The liquid
developer is transferred according to the appropriate technique
from the developer applicator onto the image bearing surface in
image configuration. Thereafter, the developer pattern on the
electrostatographic imaging surface is transferred to copy paper
and the liquid developer may be absorbed by the paper to form a
permanent print. During the transfer operation not all the liquid
developer is transferred to the copy paper and a considerable
quantity remains on the photoconductor surface. In order to recycle
the imaging surface, this residual developer must be either removed
or its effects immobilized; otherwise it will tend to be present as
background in subsequent cycles. If the liquid developer is
relatively conductive having, for instance, a resistivity less than
about 10.sup.10 ohm centimeters, any residue remaining on the
imaging surface may dissipate any charge subsequently put on it.
Furthermore, lateral conductivity of the liquid developer on the
imaging surface may become excessive and the resolution of the
resulting image will be poor. On repeated cycling, there is also a
progressive accumulation of liquid developer on the imaging surface
since in each cycle, not all the developer is transferred to the
copy paper. This progressive accumulation of developer residue
results in an overall loss of density, deterioration of fine detail
and contributes to increased background deposits on the final copy
particularly since accurate imaging on the imaging surface may by
inhibited.
Until recently, the procedures suggested to be employed to remove
the developer liquid from the imaging surface have been so severe
and complete that there has been a progressive degradation of the
imaging surface particularly a photoconductor surface which
degradation lessens its useful life span. The severity of the
cleaning step is dictated by the fact that on cleaning a film from
a surface, the film is progressively split so that on each separate
cleaning only about one-half the film is removed. The cleaning
solvents that have generally been suggested as adequate cleaning
aids frequently chemically attack the imaging surface and are
hazardous due to their volatility and toxicity. In some instances,
with complete removal of the ink film the electrical properties of
a photoconductor are virtually destroyed by the cleaning operation
after only a few cycles. In other instances, the solvent employed
may also act as a solvent for the resin binder in a binder plate or
may induce crystallization of a thin layer of a selenium
photoconductor. These problems have, however, to a very large
degree, been solved, or at the very least, minimized by the
development of the cycling cleaning system described in copending
United States application for Letters Patent Ser. No. 873,103 by
Joseph Mammino entitled "Cleaning System", and filed Oct. 31, 1969.
Therein described is a cleaning system wherein dry, highly
absorbent powders are placed on the imaging surface to absorb any
residual developer of any particular imaging cycle. After absorbing
the developer the fine powders are removed from the imaging
surface.
When the technique described in the above U.S. Application for
Letters Patent is applied to an automatic machine concept,
difficulties are encountered in handling the dry powder. Since in a
machine each cycle must be completed within a very short period of
time between about 1 to 3 seconds, the powder must be quickly
applied and removed from the imaging surface. The rapidity of the
technique necessary to provide this result produces a considerable
amount of dust during the cleaning. The dust is not only present in
the cleaning zone which to some extent minimizes the cleaning
efficiency with respect to quantity of cleaning powder employed,
but also may be found in other parts of the machine where it may
lead to contamination of material supply such as developer liquid
and paper or may lead to abrasion or fouling of machine components.
Furthermore, the quantity of powder employed in this technique is
relatively large in that the powder is applied indiscriminately to
the entire imaging surface and removed therefrom. While the
cleaning powders may be recycled to some extent, means must be
provided to measure the relative remaining absorbent capacity of
the powder, and when the capacity is reduced below a certain level,
the powder must be removed. To minimize or control these problems,
the cleaning mechanism may be extremely cumbersome and complicated.
It is therefore clear that there is a continuing need for an
improved cleaning technique in a cycling electrostatographic
imaging process.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a developing
system which overcomes the above noted deficiencies.
It is another object of this invention to provide a novel cleaning
system.
It is another object of this invention to provide a cleaning system
which makes more efficient use of cleaning materials.
It is another object of this invention to provide a simple cleaning
system capable of cyclical use.
It is another object of this invention to provide a cleaning means
for cleaning residual liquid developer from a reusable
electrostatographic imaging surface.
It is another object of this invention to provide a simple
mechanical means for cyclical cleaning a reusable imaging
surface.
It is another object of this invention to provide a liquid
development system superior to known systems.
It is another object of this invention to provide a cleaning system
superior to known systems.
The above objects and others are accomplished, generally speaking,
by providing a cycling electrostatographic imaging system having a
cleaning system which enables complete cleaning of residual liquid
developer without degradation of the imaging surface. Finely
divided dry absorbent cleaning powders are selectively applied to
the charged or developed areas of an electrostatographic imaging
surface to remove residual liquid developer from the imaging
surface in any imaging cycle. More specifically, in a liquid
development system employing a cycling or reusable
electrostatographic imaging surface the residual developer
remaining on the imaging surface in any one cycle is cleaned from
the surface by directly placing the highly absorbent dry powders in
the areas on the surface to be cleaned. By so applying the cleaning
powders to the imaging surface, the residual developer is
effectively absorbed by the cleaning powder and with a minimum of
mixing removes all residual developer on the imaging surface
including that which may be present in the background areas and
permits the cleaning powders to be readily removed from the imaging
surface.
Any suitable method may be employed to selectively deposit the
absorbent cleaning powders on the imaging surface. A particularly
preferred technique for ease of operation and simplicity of design
involves placing a charge on the powder so that it may be
electrostatically pulled from a powder applicator to the image or
charged area of the imaging surface. To accomplish this result, the
cleaning powders must be charged to a polarity opposite that which
the electrostatographic imaging surface bears in the image
portions.
Any suitable method may be employed to provide cleaning powders
oppositely charged in polarity to the polarity of the image areas
of the imaging surface. A particularly preferred technique of great
simplicity in design and materials along with excellent functional
results is to triboelectrically charge the cleaning powder. The
phenomena of triboelectrification is present when two dissimilar
materials are brought into contact and rubbed against one another
with each material becoming electrically charged to a polarity
opposite to that of the other. The polarity obtained from the
triboelectric charging may be determined by contacting the two
materials, separating them and detecting the sign of the charge on
each with an electrometer. By doing this with a variety of
different materials, one may arrange the materials in a descending
order from positive to negative such that any one material is
negative with respect to any other material above it in the series.
In such a series, materials above a given one listed will donate
electrons to the material and materials below the given one will
accept electrons from the material. Therefore, by appropriately
selecting materials, the polarity of charge on the cleaning powders
may be regulated.
The finely divided dry, absorbent powders may be triboelectrically
charged in any suitable manner. They may, for example, be brought
into contact with a fur cloth or other solid material. A
particularly preferred manner of charging the cleaning powder is by
contacting and transporting them by means of a granular carrier. In
a typical embodiment, a mass of bead-like solid material and the
dry absorbent powders are agitated to provide the desired charge on
the cleaning powders. The cleaning powders are then transported to
the imaging surface. They may, for example, be pushed, rolled, or
cascaded across the imaging surface. A particularly preferred
technique providing optimum cleaning while conserving materials,
machine space and components is to transfer the charged cleaning
powder to an applicator surface which delivers the cleaning powders
to the imaging surface.
The invention may be further illustrated by reference to the
accompanying drawings in which:
FIG. 1 is a schematic view of an embodiment of an
electrostatographic imaging system employing the cleaning technique
of this invention.
FIG. 2 is a schematic view of a preferred manner of charging,
transporting and applying the charged cleaning powder to the
imaging surface.
FIG. 3 is an alternative preferred manner of transporting the
charged cleaning powder from a powder reservoir and charging
station to the imaging surface.
In the electrostatographic imaging system depicted in FIG. 1, an
electrostatic latent image is placed on the imaging surface here
illustrated as a rotating cylindrical drum photoconductor 10, such
as a selenium drum by uniformly placing a positive charge on the
drum by charging means 12, exposing the charged imaging surface to
a light and shadow image through exposure means 11, and developing
the electrostatic latent image at developing station 33. Developing
station 33 comprises a rotating pattern applicator roll 13 which is
loaded with liquid developer 17 by means of feed roller 16 and
doctored by doctor blade 14 held in place by positioning member 15
to provide liquid developer in the depressed portions of the
applicator surface while the raised portions are substantially free
of developer. The liquid developer 17 may be replenished through
the developer reservoir 18 by any suitable means such as receptacle
19 with a gravity feed to the developer bath 18. The developer on
the imaging surface in image configuration is transferred to a
receiver sheet such as ordinary paper 20, which is moved through
the transfer zone in contact with the drum at the same rate and in
the same direction as the periphery of the drum. The paper to which
the developed image is transferred is held in transfer position by
idlers 21. Transfer may be electrostatically assisted by means of
corotron 31. The residual liquid developer present on the
electrostatographic imaging member is then cleaned from the imaging
member at cleaning station 32. At cleaning station 32, a porous,
resilient roll 25, containing absorbent charged cleaning powder
obtained from bath 24, containing a mixture of cleaning powder 23,
and carrier beads 22, delivers the charged cleaning powder to the
imaging surface. The residual developer on the drum 10, may contact
the applicator roll containing the charged powder, picking the
charged powder out of the applicator in response to the residual
charge pattern left on the imaging surface. The residual liquid
developer and the charged cleaning powder on the imaging surface
next come in contact with a web-type material 27, which is slowly
advanced from a supply reel 30, through idlers 28, into a rubbing
contact with the imaging surface and finally onto takeup reel 26.
This cleaning web is preferably moving slowly in the direction
countercurrent to the direction of the advancing imaging surface so
that the cleanest portion of the web contacts the cleanest portion
of the imaging surface. In the initial stages of contact between
the cleaning web and the imaging surface, the cleaning powder and
residual liquid developer are intimately mixed together and the
cleaning powder substantially and completely absorbs all the liquid
forming a liquid loaded powdery residue. The liquid developer
loaded powdery residue is removed from the imaging surface and
transferred to the cleaning web. This transfer may be assisted
electrostatically by means of a corotron 29, or any other suitable
means such as a biased roll. As a result of the countercurrent
motion of the imaging surface and the cleaning web, the powdery
residue is removed from the imaging surface by the cleanest portion
of the cleaning web. The application of charged dry absorbent
powder to an electrostatographic imaging surface and the removal
from the surface of the powder with the absorbed liquid developer
provides a substantially complete cleaning of the imaging surface
such that upon passing the cleanest portion of the web cleaner the
imaging surface is prepared for the next imaging cycle.
FIG. 2 is a more descriptive view of the particular cleaning powder
applicator station of FIG. 1. In FIG. 2 the powder applicator
device is illustrated as a rotating cylindrical roll which may be
independently driven or driven by contact with the moving imaging
surface. While this applicator surface may be moved in a direction
concurrent or countercurrent with the imaging surface, it is
preferred to move it in a direction concurrent with the imaging
surface to thereby minimize any possible abrading effect between
the powder and the imaging surface and also to minimize any
contamination of either the applicator or the powder supply by
having the liquid developer pushed into it. In other words, by
moving the applicator surface and the imaging surface in
countercurrent directions, a nip of liquid developer would form at
the line of contact between the two which would be moving in a
direction toward the reservoir of cleaning powders. In FIG. 2 the
powder applicator here depicted as a cylindrical roll 35, rotates
in powder reservoir 36 containing a mixture of cleaning powders 38,
and carrier beads 37. An additional supply of cleaning powder is
contained within powder supply chamber 39. A flicker bar 40 is
positioned on the reservoir housing so as to serve to contain the
carrier beads or composite particles in the powder reservoir.
FIG. 3 is an enlarged view of an alternative cleaning powder supply
means in which the powder applicator surface is depicted as a
porous resilient belt 45, driven around support rolls 46, in powder
reservoir 50, containing finely divided absorbent powder 49, and
carrier beads 48.
Any suitable absorbent powder may be employed in the practice of
this invention. The cleaning aids may generally be described as
small sized, highly absorbent, dry powders that function as tiny
sponges. To minimize abrasion the dry powders preferably are softer
than the surface which they are to clean. For a maximum cleaning
effect, the powders are also preferably insoluble in the liquid
developer, are adhesive after they have absorbed the developer
liquid thereby enabling their ready removal from the surface and
further do no introduce anything to contaminate or degrade the
imaging surface. The absorbent powders typically have a surface
area of from about 30 to about 950 m.sup.2 /gm. Preferably, the
powders have a surface area of from about 100 to about 800 m.sup.2
/gm. The cleaning powders of this invention may be of any suitable
size which provides the necessary surface area and absorptive
ability. Typically, the particles range in size from about 0.1 to
about 20 microns. As the size of the particles increases, they
become more abrasive. On the other hand, if their size decreases,
greater caution must be exercised in the handling of the powders.
For these reasons average particle sizes of from about 1 to about
10 microns are preferred with average sizes of from about 3 microns
to about 8 microns providing optimum balance in handling and
performance. The particular cleaning powder should in general be
selected based on the absorptive capacity of the powder for the
particular developer. If the developers are oil base material, the
cleaning powder preferably has a high oil absorption, if the
developer is a polar liquid, the cleaning powder preferably has
high absorptive powers for the polar developer and may have a low
oil absorption. Typically, the preferred absorption capacity is
from about 40 to about 500 milligrams of developer per 100
milligrams of powder.
Typical absorptive microporous materials useful in the practice of
this invention include the finely divided forms of carbon such as
furnace black, channel black, lamp black, bone black and charcoal;
clays such as kaolin and china clay; diatomaceous earth, pumice,
fly ash, infusorial earth; pigments such as titanium dioxide, zinc
oxide and silica. Typical organic pigments having adequate
absorptive capacity include quinacridones, phthalocyanines, and
benzidine yellow.
Especially satisfactory cleaning is obtained with silica gels,
kaolin clays, carbon black, titanium dioxide. Particularly
preferred materials in handling and cleaning ability are the silica
gels having an average particle size of from about 3 microns to
about 11 microns, a surface area of from about 200 m.sup.2 /gm to
about 350 m.sup.2 /gm and a liquid absorption of from about 100 to
about 315 mg liquid/100 mg powder.
The absorbing cleaning powder may be applied to the surface to be
cleaned in any suitable amount. Typically, the absorbing power of
the powder is at least sufficient to absorb all the residual liquid
on the imaging surface. Preferably the amount of cleaning powder
applied provides an absorbing power greater than that necessary to
absorb all the residual liquid. Generally the residual developer is
present on the imaging surface in an amount up to a maximum of
about 2.3 milligrams per square centimeter. For such amounts of
liquid from about 1 percent to about 10 percent by weight of the
developer of the cleaning powder are generally added.
Any suitable applicator means may be employed to transfer the dry
cleaning powder from a powder supply or reservoir to the imaging
surface. Typical materials would include rollers, webs and brushes.
The applicator preferably is porous, soft and resilient to thereby
hold a sufficient amount of dry powder to absorb all the residual
developer and to present the dry powder to the imaging surface
while minimizing any wear or abrasion due to contact between the
two surfaces. A particularly satisfactory group of materials useful
as the powder applicator may be described as porous, cellular
foams. Typical cellular foams that may be employed as a powder
applicator include sponge rubber, foamed polyurethane,
polyethylenes, vinyls, polystyrene, cellulose acetate, phenolics,
ureas, silicones, epoxies.
The granular carrier material may be of any shape although it is
preferred to have them round or nearly round to facilitate their
movements in mixing and to provide less friction. Any suitable
round granular material may be employed. Typical granular materials
include sodium chloride, aluminum potassium chloride, Rochelle
salt, granular zircon, granular silicone, methyl methacrylate,
phenolics, epoxies, ureas, melamines and other resins; glass,
silicone dioxide, flint shot, iron, steel, ferrite, nickel,
carborundum and suitable mixtures thereof. In those instances where
the particular granular material does not impart the necessary
triboelectric properties, the granular material may be covered with
or encased in a suitable covering which imparts the necessary
triboelectric properties so that it will properly charge the dry
cleaning powder when mixed therewith.
To facilitate separation of the dry powder from the granular
material, it is preferred that the carrier particles be
substantially larger than the cleaning particles. Typically, the
granular carrier particles have a particle diameter of between 50
microns to about 600 microns. The granular material should be
sufficiently large not to be trapped in the cellular structure of
the preferred powder applicator layer but should not be so large
that it unduly limits the volume of cleaning powder with which it
may be satisfactorily mixed. For these reasons, the particle
diameter of the granular material is preferably between about 150
microns and about 450 microns. It is particularly preferred that
the granular carrier material be provided with a lubricating
coating to prevent the sticking of one particle to another since
the entire mass of granular material and dry powders when being
agitated should flow like a fluid.
To provide uniform charge on the dry cleaning powders, it is
preferred that the granular carrier material and the powder be
continuously and uniformly mixed during operation. The granular
carrier material and the dry powders may be mixed together in any
proportion which will provide sufficient charge on a sufficient
amount of the dry powders to be transferred to an applicator
surface and on subsequent transfer to the imaging surface remove
all liquid developer. Typically, the mixture of granular carrier
and dry powder comprises from about 10 to about 50 parts by volume
of granular carrier to about 1 part by volume of dry powder. In
obtaining uniform charging on the dry powder, there preferably is
from about 20 to about 35 parts by volume granular carrier to 1
part dry powder.
The applicator surface should be sufficiently porous to hold an
adequate quantity of the dry powder to substantially completely
absorb all the liquid developer remaining on the imaging surface
when pulled from the applicator to the imaging surface. To assist
in transport of the dry powder from the granular carrier to the
applicator, it is preferred to provide an applicator which is
capable of being charged to a polarity opposite that of the dry
powder. A particularly preferred applicator is a cylindrical roll
between about 1 inch to 1 1/2 inch in diameter comprised of a steel
core with an adhesive backed polyurethane sponge cemented around
it.
Since the cleaning powders and techniques of this invention provide
a substantially complete cleaning of residual developer from the
imaging surface on every cycle, no problem of charging and imaging
through a residual liquid film exists and developers of both low
and high conductivity may be employed. Any suitable developer may
be used. Typically, the developers for which the dry absorbent
powders of this invention are effective have a conductivity of from
about 10.sup.-.sup.4 (ohm-cm).sup.-.sup.1 to about 10.sup.-.sup.14
(ohm-cm).sup.-.sup.1. Typical vehicles within this group providing
these properties include glycerol, polypropylene glycol, 2,5
hexanediol, mineral oil, the vegetable oils including castor oil,
peanut oil, coconut oil, sunflower seed oil, corn oil, rapeseed
oil, sesame oil. Also included are mineral spirits, fluorinated
hydrocarbon oil such as duPont's Freon solvents and Krytox oils;
silicone oils, fatty acid esters, kerosene, decane, toluene and
oleic acid. In addition, as is well known in the art, the
developers may contain one or more secondary vehicles, dispersants,
pigments or dyes, viscosity controlling agents or additives which
contribute to fixing the pigment on the copy paper.
Any suitable electrostatographic imaging surface may be cleaned
through the technique of this invention. Basically any surface upon
which an electrostatic charge pattern may be cyclically formed or
developed may be employed. Typical electrostatographic imaging
surfaces include dielectrics such as plastic coated papers, xero
printing master, photoconductors and overcoated photoconductors.
Typical photoconductors that may be employed include selenium and
selenium alloys, cadmium sulfide, cadmium sulfoselenide,
phthalocyanine binder coatings, polyvinyl carbazole sensitized with
2,4,7 trinitrofluorenone. Typical overcoated photoconductors
include those described in U.S. Pat. Nos. 3,234,019 and 3,251,686.
A specific example of an overcoated photoconductor is a selenium
layer on a conductive aluminum substrate overcoated with a thin
film of polyethylene terephthalate. The electrostatographic imaging
surface may be employed in any suitable structure including plates,
belts, or drums may be employed in the form of a binder layer. For
more effective cleaning, it is preferred to provide a surface to be
cleaned which has a very smooth surface and which is non-absorbent
for the liquid developer since generally the more smooth and
uniform the surface, the better will be the cleaning.
After the dry, absorbent powders are applied to the imaging
surface, it is generally preferred to distribute the cleaning
powder over the imaging surface so that the entire surface is
contacted with the dry absorbent powders to thereby insure maximum
absorption of all residual liquid developer in the imaging surface
including the residual developer in the image or developed areas
and those that may be present in the background or undeveloped
areas. When the dry absorbent powders employed in this invention
have absorbed all the residual liquid developer, they are in the
form of a solid powdery residue containing the absorbed developer
within the powder and they must be removed from the imaging surface
prior to the next imaging cycle. Any suitable cleaning system may
be employed. Typical cleaning systems include: wiper blades,
brushes, vacuum suction and cleaning webs. Particularly
satisfactory distributing of cleaning powder and cleaning is
obtained with the use of a fibrous cleaning web moving
countercurrent to the direction of movement of the imaging surface
such that the cleanest portion of the imaging surface is contacted
and finally cleaned by the cleanest portion of the cleaning
surface. The cleaning web may be made of any suitable material.
Typical fibrous cleaning webs include those made from cheesecloth,
flannel, rayon, cotton, and combinations of rayon and cotton,
Dacron, polyester and/or polypropylene fibers. Particularly
satisfactory cleaning is obtained with those fibrous webs which are
substantially homogeneous and thick and have a high absorbent
capacity. As indicated in FIG. 1 of the drawing, transfer of the
dry powder containing absorbed liquid developer to the absorbent
web may be facilitated by electrostatic transfer from the imaging
surface to the web. This may be accomplished by the use of a
corotron or a biased member disposed on the side of the cleaning
web opposite the imaging surface.
In operation, an electrostatic latent image is placed on an
electrostatographic imaging surface in a conventional manner. The
electrostatic latent image is thereafter developed with a liquid
developer according to any of the techniques previously discussed.
Development preferably is obtained with the use of a patterned
surface applicator roller wherein the liquid developer is present
in the depressed portions of the applicator, while the raised
portions are substantially free of developer and the developer is
pulled from the developer applicator to the imaging surface in
image configuration. After transfer of the developer from the
imaging surface to receiver sheet in image configuration the
residual developer remaining on the imaging surface is removed from
the imaging surface according to the technique of this invention.
According to this technique, and particularly according to the
preferred embodiments illustrated in FIGS. 1 and 2 of the drawing,
a mixture of granular carrier material and dry absorbent powder is
provided and agitated to apply a charge to the cleaning powder.
Thereafter, the powder is separated from the carrier and
transferred to a porous resilient applicator which in turn is
positioned close to or in contact with the imaging surface where
the powder is transferred to the imaging surface in image
configuration. Preferably the powder is separated from the granular
carrier material by applying a charge of polarity opposite that as
the powder to the applicator. In a particular example employing a
selenium photoconductor as the electrostatographic imaging surface,
the selenium would be positively charged and the image areas would
remain positively charged following exposure. To be
electrostatically attracted to the image areas of the selenium
imaging surface, the dry absorbent powder should be negatively
charged. Accordingly, the powder applicator is positively charged
to pull the dry powder free from the granular carrier and the
granular carrier also has acquired a positive charge due to the
triboelectric contact between the granular carrier and dry powder.
The polarity of charge placed on the granular carrier by the
triboelectric contact and the polarity of any charge placed on the
powder applicator must, therefore, be the same as the polarity of
charge on the imaging surface. However, to provide the necessary
transfer of dry powder from contact with the granular carrier to
the imaging surface, the magnitude of potential on the powder
applicator preferably is greater than the potential on the granular
carrier and the residual potential on the imaging surface is
greater than that on the powder applicator. However, while
electrostatic transfer may be a principal means of transfer of dry
powder from carrier to powder applicator and from a powder
applicator to the imaging surface it is not the sole means of
transfer. The powder may be transferred to the porous applicator
from the carrier by physical scraping during contact. A
considerable force acting towards transfer to the imaging surface
is the adhesive nature of the liquid residue on the imaging
surface. It is the combination of electrostatic transfer and the
adhesive nature of this liquid which provide the necessary transfer
of the dry cleaning particles to the imaging surface. While the
powder applicator and the imaging surface may come in contact, it
is preferred that they do not come in contact so that any residual
developer on the imaging surface does not offset onto the powder
applicator. Residual developer remaining on the imaging surface
builds up to a thickness of approximately 3 microns. Therefore, in
order to minimize any possible offset to the powder applicator, it
is preferred that a layer of dry absorbent powder be provided on
the powder applicator in excess of 3 microns and preferably from
about 5 to about 10 microns in thickness. Such a relatively thick
layer of powder also facilitates adhesive transfer to the imaging
surface. Once the dry absorbent powder has been placed on the
imaging surface to provide the maximum cleaning efficiency, it is
preferred to uniformly distribute the dry powder over the imaging
surface to absorb all the liquid developer remaining on that
surface. This may be accomplished through any suitable means such
as the cleaning web depicted in FIG. 1. With a cleaning web moving
in a direction countercurrent to the direction in which the imaging
surface is moved the dirtiest portion of the cleaning web would
provide this smearing or distributing function in which all of the
liquid developer would be absorbed by the absorbent powders.
Further upstream in the arch formed by the contact between the
cleaning web and the imaging surface, the cleaning web will remove
all the absorbent powders which, while they have absorbed the
liquid developer, remain powdery and substantially dry. The rate at
which a web of cleaning material is consumed is a function of the
rate of plate movement and the relative rate of the web required to
yield satisfactory cleaning and has been found to vary to some
degree dependent upon the particular cleaning material employed.
Typically, the cleaning web is found to have a speed on the order
of one-fortieth to one four-hundredth of the plate speed.
While the cleaning web and imaging surface may be moved in the same
direction, minimum contact length has been found to occur when the
web and plate are moved in substantially opposite directions.
The absorbent powder applicator may be independently driven or
driven by the imaging surface. Preferably, it will be driven in the
same direction as the imaging surface since if driven in a
direction opposite to that of the imaging surface, the liquid
developer would tend to fill the nip between the two surfaces and
serve to contaminate the roll at this nip. When the absorbent
powder applicator is frictionally driven by the imaging surface, it
is in contact with the imaging surface under the minimal driving
pressure necessary to provide adequate motion. If excessive
pressure is employed, the absorbent powder may be mechanically
pressed onto the entire imaging surface from the powder applicator
thus using more powder than necessary and requiring additional
cleaning effort later to remove this dry powder. The absorbent
powder applicator may be driven at the same speed or a speed
slightly slower or faster than the imaging surface provided the
abrading effect is minimized and there is no transfer of liquid
developer to contaminate the powder applicator.
The agitation of the granular carrier material and the absorbent
powders should be sufficient to provide uniform mixing and charging
of the absorbent powders and preferably takes place in an area
adjacent to and in contact with the powder applicator. Providing
this type of mixing in contact with a portion of the powder
applicator minimizes any possible clogging of the applicator roller
by some wetted powders which may be present on the powder
applicator. By providing a sufficient mixing, the wetted powders
should be removed from the applicator surface and uniformly
distributed throughout the body of the cleaning material. This
agitation may be provided simply by the stirring action of a powder
applicator roller such as that depicted in FIG. 2. Any suitable
means, however, may be employed such as a vibrating or stirring
means. Any powder accumulating on the powder applicator that is
undesirable may be removed from the applicator surface by
positioning a flicking bar or pinch member in contact with the
applicator.
DESCRIPTION OF PREFERRED EMBODIMENT
The following preferred example further defines and describes
preferred materials, methods, and techniques of the present
invention. In the example, all parts and percentages are by weight
unless otherwise specified.
EXAMPLE I
An imaging system similar in configuration to that depicted in FIG.
1 is assembled. A photoconductor in the form of a drum comprising a
surface layer of selenium about 50 microns thick on a conductive
plate is positively charged to about 450 volts and exposed to a
light and shadow image in conventional manner. The electrostatic
latent image thus formed is developed by moving a patterned surface
applicator roll having developing quantities of a developer in the
depressed portions thereof past the image bearing surface so that
liquid developer is pulled out of the depressed portions to the
image bearing surface in image configuration. The speed of
development is about 10 inches per second. The developer employed
is of the following composition by weight:
Drakeol 9 30 parts by weight Microlith CT 18 parts by weight Methyl
violet tannate 3 parts by weight Ganex V216 15 parts by weight
Paraflint RG wax 0.5 parts by weight
Drakeol 9 is a mineral oil manufactured by Pennsylvania Refining
Company having a kinematic viscosity of about 15.7 - 18.1
centistokes at 25.degree.C. and a specific gravity of about 0.85.
Microlith CT is a resinated predispersed carbon black pigment
composed of about 40 percent carbon black pigment and 60 percent
ester gum resin, manufactured by CIBA. Paraflint RG is a hard
synthetic wax available in flake form from Moore & Munger.
Ganex V216 is an alkylated polyvinyl pyrrolidone compound
manufactured by GAF Corporation which serves as additional pigment
dispersant and may also be regarded as a secondary vehicle.
The developer on the photoconductor is transferred to bond paper in
image configuration. A reservoir containing about 5 cubic
centimeters of Syloid 308, a silica gel available from Davison
Chemical Division, W. R. Grace & Company, and about 175 cubic
centimeters of about 450 micron diameter steel spheres. A sponge
roll of about 11/4 inches in diameter composed of a polyurethane
sponge containing about 80 pores perlinear inch on the surface,
available from Scott Paper Company under the name of Scott
Industrial Foam, and about one-eighth inch in thickness wrapped
around a 1 inch aluminum core is rotated in the reservoir so that
the lower portion is below the surface of the mixture of steel
spheres and silica gel. The polyurethane powder applicator roll is
driven by slight contact with the selenium drum at a speed of about
10 inches per second. A rayon web Miracloth, code 9243 manufactured
by Chickapee Mills is advanced in a direction opposite to the
rotating selenium drum along an arc of the drum surface of about 5
inches at a speed of about one one-hundredth that of the drum
speed. The first print obtained with this apparatus is free of
background and has a resolution of about 10 line pairs per
millimeter. After repeated cycling of 1000 prints, no significant
change in print quality is observed. Very little dusting and no
fouling of the mechanical movements by any of the cleaning powder
are observed.
The technique provided by the instant invention provides a
substantially complete cleaning of all residual liquid developer
from an imaging surface without any significant abrasion of the
imaging surface and a very fast and efficient manner and employs
very few mechanical moving parts. It further provides a cleaning
system capable of cyclical use which minimizes dusting and
conserves expendable material by applying only sufficient cleaning
powder to the areas to be cleaned. In addition, the final material
being handled after the cleaning has been accomplished is a
substantially powdery residue.
Although specific materials and operational techniques are set
forth in the above exemplary embodiments using the cleaning
technique of this invention, these are merely intended as
illustrations of the present invention. There are other materials
and techniques than those listed above, which may be substituted
with similar results.
Other modifications of the present invention will occur to those
skilled in the art upon a reading of the present disclosure which
modifications are intended to be included within the scope of this
invention.
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