Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates generally to xeroy~aphlc
copierY, and more parkicularly to an improved applic~tor for
depositing triboelectrically-charged toner particles onto an
electrostatically-charged latent image to render visible and
develop this image.
In the xerographic technique, a photoconductive
insulating layer whose sur~ace is uniormly charged electrically
is first exposed to an illuminated pattern of light and shadow
of the intelligence to be recorded. The blanket charge on the
layer is selectively dissipated by the illumunated pattern to
yield a latent electrostatic image. Thereafter, to develop the
image, finely-divided pigmented thermoplastic powder or toner
is deposited on the latent image, the toner particles adhering to
the electrostatically-charged areas in proportion to the
charges thereo~.
In a plain paper xerographic printer such as that
disclosed in the Boulton patent 2,987,037, the photoconductive
insulating layer is supported on a rotating drum and the toner -
image developed on the surface of this layer is transferred from
the drum onto a sheet of ordinary paper. The developed image on
the paper is then fixed thereto by heat which fuses the toner
particles to the paper. It is also known to effect such fusion
by the application of pressure rather than heat.
In a treated-paper xerographic printer such as that
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disclosed in the Shelffo patent 3,520,604, there is no need to
transfer the developed toner image from the photoconductive
insulating layer~ for in this instance use is made o~ paper
coated with photoconductive ~inc oxide particles dispersed in
a film-forming resin binder. The coated surface of the paper is
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subjected ~o a blanke~ electrostatia charye which is then
exposed to the light pattern to be recorded to create a latent
image thereon. This latent ima~e is developed by toner which
is directly fixed onto the treated paper, thereby obviating the
transfer step characteristic of an untreated paper printer.
The present invention is concerned primarily with
apparatus adapted to deposit toner onto an electrostaticall~-
charged latent image formed on a photoconductive layer, the
invention being fully applicable both to treated and plain paper
xerographic printers.
Among the known types of toner development systems are
cascade, brush and magnetic brush development as well as powder
cloud and liquid development. A cascade development system,
such as that disclosed in the Shaffert patent 2,576,047, is
adequate for making ordinary line copies, but it has limited
value where solid area development is required. For solid area
development producing a good copy of an original which includes
photographs as well as printed text, brush applicators have -
greater utility.
29 In a brush development system such as the one des-
cribed in Boulton patent 2,987,037, use is made of a soft ~ur
brush which acts not only to pick up and transfer toner particles
onto a latent electrostatic image, but also to impar~ a tribo-
electxic charge to particles actually brought into contact with
the brush hairs.
Currently, a widely used type of toner applicator in
; commercially-available æerographic printers is the magnetic brush
system, such as that disclosed ~n the Mor~e Fatent 3,67~,532, A
magnetls brush developer requires a developer mix constituted by
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ferxomagnetic iron particles intermingled with toner powder.
This mix is picked up by a magnet having a bristle-like or
brush formation, the iron particles being magnetically attracted
to the bristles and the toner particles adhering electrostati-
cally to the iron particles. The bristles are electrically
conductive and contribute to the transfer of toner to the
electrostatically-charged latent image.
A xerographic printer including a magnetic brush
development system involves relatively costly and complex
apparatus and is subject with prolonged use to contamination
and other problems. Moreover, the iron particles are physically
worked in the course of operation and the properties thereof are
degraded, making it necessary from time to time to shut down the
machine to replace the iron particles in the mix.
In order to overcome the drawbacks of existing
magnetic brush development systems, it is known to provide toner
particles having a ferromagnetic core covered with a thermoplastic
resin sheath, thereby avoiding the need for a mix. But such
two-component particles are expensive and add substan~ially to
the cost of making copies.
SUMMARY OF THE INVENTION
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In view o the foregoing, it is the main ob~ect of
this invention to provide an improved applicator adapted to
impart a triboelectric charge to toner particles and to form a
thin, uniform layer thereof which is applied to a latent electro-
static image, thereby to develop this image.
An applicator in accordance with the invention is
useful in conjunction with a plain paper xerographic printer
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wherein toner particles ar~ applied to a phokoconductive insul-
ating layer formed on a transfer drum, or with a treated paper
printer in which the particles are applied to the treated surface
of the paper. The applicator is suitable for both line and solid
area development.
- More particularly, it is an object o~ this invention
to provide an applicator in which a thin layer of triboelectrically-
charged toner particles is formed uniformly on the surface of a
rotating carrier roll which engages the surface of an advancing
web of treated paper having a latent image thereon, the relative -
motion of the roll and the web being such as to create shear
forces regulating and enhancing the deposition of toner particles
on the latent image.
` Among the significant advantages of a toner applicator
in accordance with the invention for the developer stage of a :
xerographic printer are that the applicator is usable with low~
cost, single component toner having no inherent magnetic proper-
ties, and that nothing need be added to the toner at any time.
~ Moreover, the applicator does not entail the use of a magnetic
`~ 20 or electrostatic carrier.
Yet another object of the invention is to provide an
applicator of simple and inexpensive design which functions
efficiently and reliably, and which requires little maintenance
even after prolonged operation.
Briefly stated, these objects are attained in an
applicator in which a supply of toner powder is contained
withln a trough defined by a first plate having a flexible
flap extending therefrom to engage the surace of the roll at
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one point thereon, thereby formin~ one inclined wall of the
trough, and a ~econd plate having a resilient blade of
dielectric material extending therefrom and biased against
the surface of the roll at another point thereon to form another
inclined wall, the junction of the blade edge and the xoll
surface constituting a tight nip. As the roll turns, toner
particles at the bottom o~ the trough are advanced toward the
blade and forced through the nip, the blade flexing to admit
a thin layer of the particles which adhere to the roll surface.
In the course of such passage, the particles passing
through the tight nip are frictionally rubbed by the edge of
the blade and thereby caused to assume an electrostatic
char~e whose polarity depends on the triboelectric nature of
the toner.
The paper or other medium having an electrostatically-
charged latent image thereon is transported so as to engage
the toner layer carried by the roll in a path wherein the
electrostatically-charged tone particles are attracted to
and transferred to the medium and deposited on the charged
latent image to develop the image. The applicator roll is
preferably rotated at a rate producing a surface speed relative
to the speed of the advancing medium so as to create shear
~orces therebetween acting to regulate the deposition of the
charged toner particles.
O~TLINE OF THE DRAWINGS
For a better understanding of the invention as well
as other objects and further features thereof, reference is
~ade to the following det~iled description to be re~d in
conjunction with the ~ccompanyiny drawings~ wherein;
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Fig. 1 sch~ical~ shows in perspective an
applicator in accordance with the invention;
Fig. 2 is an end view of the applicator,
Fig. 3 shows a detail of the applicator, and
Fig. 4 shows the end of an applicator roll with a
metering roller.
. DESCRIPTION OF INVENTION
Referring now to the drawing, there is shown an
applicator in accordance with the invention adapted to operate
in the development stage of a treated paper xerographic
printer in which copies of documents are made directly on a
web 10 of treated paper The photoelectrostatic technique
; for making copies of a graphic original on a treated or
coated paper is more specifically described in an article
entitled "Electrofax Direct Electrophotographic Printing on
Paper" by C.J. Young et a . in the RCA Review, Vol. 15, No. 4,
pp 469-484, ~ecember 1~54,
In the basic "Electrofax" process described in this
article, a blanket electrostatic charge is imposed on the
coated side o~ web 10 in a dark chamber. The charged surface
is thereafter exposed to a pattern of light and shadow of
the original to be recorded in order to form a latent electro-
static image thereof on the photoconductive surface of this
paper.
In the applicator, the latent image is formed on the
upper ~acq lOA of paper web 10, the web entering the
applicator along an input guide 11 and being drawn therein by a
pair of cooperating rollers 12 and 130 The advancing web is
thenconveyed across a double-humped pressure shoe PS which is
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adapted to press ~he portion of the web bridged across the
humps against the underside of a rotating applicator roll 14,
From the shoe, web 10 travels across an output paper
guide 15 from which it passes into the fixing stage of the
xerographic printer, The nature of the fixing stage forms no
part of the present inventlon, However, it is to be noted that
; the applicator makes use of toner particles which lend
themselves to fusion by pressure, making it possible to provide
a simple, compact and efficient low-cost xerographic printer
requiring no warm-up period as in the case of printers which
; make use of a thermal fixing stage.
: Roll 14 is supported on a shaft 16 operatively
coupled by suitable gears or belts to a variable-speed motor
17, whereby the rotary speed of the roll relative to the
. speed of the advancing web may be adjusted to bring about
optimum transfer conditions,
Associated with roll 14 is an inclined plate 18
extending the full length of the roll and having a flexible
sealing flap 19 extending downwardly from the under side of
the plate. Flap 19 engages the upper surface of roll 14 at a :
position disposed to the right of the vertical plane Y passing
-. through the axis X of the roll~ Also associated with the roll
. is an oppositely-inclined p.late 20 which extends the full
; lenyth of the roll, plate 20 having a blade 21 extend.ing
downwardly from the lower side thereof,
Blade 21 functions to impart an electrostatic charge
to the toner particles and must therefore be fabriaated of
high strength, flexible material having good dieleatroic
properties. Suitable for this purpose are Mylar~ polyethylene,
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polycarbonate and other plastic dielectric materials having
acceptable mechanical and dielectric properties,
As shown in Fig, 2, the plate arrangement is such as
. to bias blade 21 so that its edge presses against the upper
surface of the roll to form a tight nip at the junction of the
ed~e and roll surface at a point displaced to the left of the
vertical plane Y Roll 14 turns in the counter-clockwise
direction, and because blade 21 is biased, the blade is
somewhat deflected, with its edge pressed tangentially against
the moving surface of the roll
The inclined plates in combination with the rolls :~.
and end plates (not shown) define a trough for retaining a
. substantial supply 22 of toner particles therein These
particles are formed of pigmented, low-cost thermoplastic
:' powder such as polyamide resin or polystyrene The particle
sizes may be in a range of about 15 to 35 microns, but
. preferably they lle in a narrow range of about 20 to 30
microns.
The toner particles in the supply thereof which are
at the bottom of the trough and lie against the surface of the
rotating roll are advanced thereby toward blade 21 and forced
through the tight nip. Because of the pressure exerted by .
the blade edge against the roll surface, only a thin layer of
particles is admitted through the nip
As the particles are forced through this tight nip,
; they rub ~rictionally against the dielectric edge of the blade
. and theraby acquire a triboelectric charge whose polarity
~epends on the nature of the particles, The polarity of the
charge to be imparted to the toner depends on the requirements
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of the xerographic printer and on whether one wishes to
develop a negative or positive image,
The choice of dielectric materials for the charge-
inducing blade and for the toner which rubs against the edge
of the blade is governed by the principles of frictional
electricity or tribo-electrification, which relate to the
phenomenon occurring when two dissimilar substances are rubbed
together, In general, it can be said that the potential
difference between two contacting dielectrics is proportional
to the difference between their dielectric constants, the one
having the greater constant being rendered positive, `
Materials are often listed in a triboelectric series wherein
any material on the list, when rubbed against another material
occupying a lower position on the list, becomes positive,
The reason why it is important that the toner
~ particles lie within a narrow size range is to insure
.: frictional engagement between virtually all of the particles
in the layer and the blade edge, Should ~he particles lie
; within a large range, then only the larger sizes thereof would
rub against the edge and assume a charge, whereas the finer
. particles would be neglected and lack proper attractive ~
: properties, But by providing particles all within a narrow
range, substantially all of the particles are
triboelectrically--charged,
In order to provide an electric field between roll :
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14 and the latent electrostatic image formed on the upper face
lOA on web 10~ roll 14 is preferably formed of an electrically-
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conductive elastomeric material suitable as a carrier for the :`
toner such as rubber or Neoprene~ having graphite or other
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conductive material dispersed therein,
~oll 1~ is mounted on electrically-insulated
bearings so that a bias potential may be applied between the
roll and output paper guide 15, The applied bias voltage lies
in a range of from about 10 to 250 volts d.c,, with a sign
appropriate to the type of toner used and the type of print
desired, Alternatively, a suitable Zener diode can be
connected between the roll and the guide plate to limlt the
induced bias to an appropriate level, ~ -
The function of bias in the present system is the
: same as that in two-component magnetic brush developing;
namely, the reduction or elimination of tone deposition in
the non ~ ge areas of the photoconductive surface, In positive
printing wherein toner is deposited on the charged areas of a
` photoconductive surface, the bias used would have the same
sign as the photoconductor charge and would act to hold ~oner .
to the roll, Thus if the charge on the image areas of the
photoconductive surface were 300 volts negative and the charge
on the non-image areas were 50 volts, then a bias oE 100 volts
negative on the roll would act to prevent toner deposition in
.' the non-image areas,
In reversal printing, wherein toner is deposited
. on the discharged areas of the photoconductive surface, the
bias pr~vents background toning by maintaining a voltage on :
; the toner which acts to reduce or eliminate deposition in the
areas of like sign on the photoconductive surface,
. Thus coated on the surface of roll 14 as it departs
from the edge of blade 21 is a thin uniform layer of charged
toner particles, This toner layer carried by the roll engages
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the latent electrostatic image on the upper surface of web 10
and is selectlvel.y transferred thereto, the par~icles being
attracted only to charged areas on the web, The remaining
particles in the layer are not wasted but return through the
sealing flap 19 to the trough region for recycling,
In practice, it has been found desirable to run roll
14 at a high speed, producing relative motion between the
; surface of the roll and the paper web engaged thereby, The
resultant shear or sliding forces therebetween act to
regulate and enhance the deposition of the charged toner
~ particles on the latent image and improves the developed image,
Blades which perform effectively are fabricated from
any one of the following dielectric materials: Teflon, Mylar,
H-Film (DuPont Polyimide)~ polycarbonate and cellulose acetate
in thickness of about 5 mils to 1/16 of an inch, The blade
edge must be straight and smooth to permit uniform contact
with the toner along the entire length of the nip, Nicks or
scratches in the blade edge as minute as 0O002 inches can
~ cause print defectsO In practice, the edge can be bevelled in :
20 either direction and possibly.rounded,
The contact angle assumed by the blade relative to
the roll at the nip is not critical, Any one of the various
possible configurations represented by blades 21A, 21B, 21C,
21D~ 21E and 21F in Fig, 3 will function acceptably, However~ ~ .
; the critical aspect of the system lies in the nip pressure,
; Insufficient pressure will permit toner to pass through the
I nip without proper rubbing contact with the blade, With
:i inadequate rubbing contact~ the layer of toner on roll 14 will
not be uniformly charged; and will give rise to weak and/ar -
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high background prints. On the other hand, e~cessive nip
pressure will red-tce the thickness o~ the toner layer, thereby
producing weak, low-density prints In practice, therefore,
the system is provided with blade adjusting means (not shown),
making it possible to set the nip pressure to an optimum
value
While there has been shown and described a preferred
embodiment of a toner applicator for an electrostatic copier
in accordance with the invention, it will be appreciated that
many changes and modifications may be made therein without,
however, departing from the essential spirit thereof.
Instead of using a first sealing means consisting of
` a plate having a deflectable blade extending therefrom and
formed of dielectric material the said sealing means may
consist of a metering roller 23, Figure 4, which is substantially
parallel to the rotating applicator roll and the surface of
which contacts the surface of the said rotating roll in such
a way that a tight nip is formed therewith which only admits
f a thin layer of the particles drawn from the bottom of the
trough, The metering roller may be driven to rotation
It is also possible instead of a plate of dielectric
material to use a plate of non-dielectric material, for instance
stainless steel, which may be high-polishedp having a
thickness of .002-,003 inches. The blade must be supported 50
that it is electrically insulated from ground~
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