Note: Descriptions are shown in the official language in which they were submitted.
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IMAGING MBTHOD AND APPARTUS
Field of the Invention
The present invention relates to image transfer
techniques and apparatus for use in electrophotography.
Background of the Invention
Liquid toner images are developed by varying the
density of pigmented solids in a developer material on a
latent image bearing surface in accordance with an imaged
pattern. The variations in density are produced by the
corresponding pattern of electric fields extending outward
from the latent image bearing surface. The fields are
produced by the different latent image and background
voltages on the latent image bearing surface and a voltage
on a developer plate or roller.
In general, developed liquid toner images comprise
carrier liquid and toner particles and are not homogeneous.
Typically, a liquid toner developer contains about 1.5~ to
2% solids and a developed image contains about 15% solids.
The developed image has a higher density region closer to
the latent image bearing surface and "fluffy", i.e. loosely
bound, region further away from the latent image bearing
surface .
In order to improve transfer of a developed image from
the latent image bearing surface to a substrate, it is most
desirable to ensure that, before transfer, the pigmented
solids adjacent background regions are substantially removed
and that the density of pigmented solids in the developed
image is increased, thereby compacting or rigidizing the
developed image. Compacting or rigidizing of the developed
image increases the image viscosity and enhances the ability
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1 of the image to maintain its integrity under the stresses
2 encountered during image transfer, It is also desirable that
3 excess liquid be removed from the latent image bearing
4 surface before transfer. '
It is known in the prior art, as described in U.S.
6 Patent 3,955,533, to employ a reverse roller spaced about
7 50 microns from the latent image bearing surface to shear
8 off the carrier liquid and pigmented solids in the region
9 beyond the outer edge of the image and thus leave relatively
l0 clean areas above the background.
11 The technique of removing carrier liquid is known
12 generally as metering. An alternative metering technique,
13 described in U.S. patents 3,767,300 and 3,741,643, employs
14 an air knife, but has not been particularly successful due
to sullying of the background as a result of turbulence,
16 Corona discharge has also been used to compress and remove
17 liquid from a developed liguid image.
18 In U.S. patent 3,957,016, the use of a positive biased
19 metering roller is proposed wherein the metering roller is
maintained at a voltage intermediate the image and
21 background voltages to clean the background while somewhat
22 compacting the image.
23 Tn the prior art it is known to effect image transfer
24 from a photoreceptor onto a substrate backed by a charged
roller. Unless the image is rigidi~ed before it reaches the
26 nip of the photoreceptor and the roller, image squash and
27 flow may occur. This is particularly true if the substrate
28 is a non-porous material, such as plastic.
29 In the prior art, liquid toner images are generally
transferred to substrates by electrophoresis, whereby the
31 charged image moves groin the latent image bearing surface to
~2 the substrate through the carrier liquid under the influence
33 of an electric field produced by a high voltage, associated
34 with the substrate, which is of opposite polarity to the
charge on the image particles.
36 The voltage and thus the field strength available for
37 electrophoretic transfer are limited by the danger of
38 electrical breakdown which can occur at both the input and
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1 output edges of the nip, due to the minimum of the Paschen
2 curve being at about 8 micxons. Thus, according to the
3 Paschen curve, the voltage difference at the nip preferably
,4 should not exceed abaut 360 volts, in order, to avoid
electrical breakdown and possible damage to the image and
6 latent image bearing surface.
7 ~lectrophoretic compaction of images prior to transfer
8 thereof is described in U.S. Patent 4,286,039 which shows a
9 metering roller followed by a negatively biased squeegee
roller. The squeegee roller is operative both for compacting
11 the image and for removing excess liquid.
12 U. S. Patents 4,690,539 and 4,708,460 describe
13 apparatus for removing substantially all of the carrier
14 liquid from a liquid image ~n an image transfer member,
prior to transfer to the final substrate.
16 U. S. Patent 4,684,238 describes the use of an
17 electrified roller spaced away~from a liquid image on an
18 intermediate transfer member. The stated object of this
19 mechanism is the compacting of the image and the removal of
liquid therefrom.
21 U. S. Patent 4,796,048 describes a system for
22 transferring a liquid tones image from a photoconductor to
23 an image transfer member. The image transfer member is urged
24 against the photoconductor during transfer to squeegee
Carrier liquid away from the non-image areas. The image
26 areas are kept in a spaced relationship from the
27 intermediate transfer member by spacer particles in the
28 tuner material as described in U: S. Patent Number
29 4,582,774. This toner material is the only toner described
in U. S. Patent 4,796,048 as being a suitable toner.
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1 SUMM~ItY OF T'IiiM 1NVEP1TI~~1
2 The present invention seeks to provide improved
3 apparatus far enhancement of image transfer.
4 In a preferred embodiment of the invention a liquid
- 5 toner image is transferred from an image forming surface to
an intermediate transfer member for subsequent transfer to a
7 final substrate. The liquid toner image includes a liquid
8 portion including carrier liquid and a solids portion
including pigmentcad polymeric toner particles which are
essentially non-.soluble in the carrier liquid at room
11 temperature, and the polymer portion of which forms
12 substantially a single phase with carrier liquid at elevated
13 temperatures. An imaging method is provided which includes
14 the steps of concentrating the liquid toner image to a given
non-volatile solids percentage by compacting the solids
16 portion thereof and removing carrier liquid therefrom;
17 transferring the liquid toner image to an intermediate
18 transfer member; heating the liquid toner image on the
19 intermediate transfer member to a temperature at least as
high as that at which the polymer portion of the toner
21 particles and the carrier liquid form substantially a single
22 phase at the given solids percentage; and transferring the
23 heated liquid toner image to a final substrate.
24 In a preferred embodiment of the invention a liquid
toner image is transferred from an image farming surface to
2~ an intermediate transfer member for subsequent transfer to a
27 final substrate. The liquid toner image includes a liquid
28 portion including carrier liquid and a solids portion
29 including toner particles. An imaging method is provided
which includes the steps of concentrating the liquid toner
31 image by compacting the solids portion thereof and removing
32 carrier liquid therefrom such that the image has a non-
33 volatile solids percentage of between, 20~ and 35%;
34 transferring the liquid toner image to an intermediate
transfer member; and transferring the liquid toner image to
3s a final substrate.
37 In a preferred embodiment of the invention, the step of
38 concentrating includes the simultaneous application of an
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1 electric field to compact the solids portion of the image
2 and of pressure to remove liquid from the image.
3 Zn preferred embodiments of the invention the non-
4 volatile solids percentage can be about 20, 25%,,:30% or 350
5 or greater after the step of concentration.
6 In a preferred embodiment of the invention the single
7 phase is a liquid phase. Alternatively or additionally, in a
8 preferred embodiment of the invention the step of
9 concentrating is operative to increase the solids percentage
to a value at which phase separation cannot occur.
11 There is also provided, in a preferred embodiment of
i2 the invention, imaging apparatus utilizing a liquid
13 developer comprising carrier liquid and pigmented polymeric
14 toner particles which are essentially non-soluble in the
carrier liquid at room temperature, and the polymer portion
16 of which form substantially a single phase with carrier
17 liquid at elevated temperatures, the apparatus including: an
18 image forming surface, apparatus, utilizing the liquid
19 developer, for forming a liquid toner image having a liquid
portion including carrier liquid and a solids portion
21 including toner particles, on the image forming surface,
22 apparatus for concentrating the liquid toner image to a
23 given non-volatile solids percentage by compacting the
24 solids portion of the liquid toner image and removing
carrier liquid therefrom; apparatus for transferring the
26 liquid toner image to an intermediate transfer member after
27 concentration thereof, apparatus for heating the liquid
28 toner image on the intermediate transfer member to a
29 temperature at least as high as that at which the polymer
portion of the toner particles and the carrier liquid form
31 substantially a single phase at the given concentration and
32 apparatus for transferring the liquid toner image, after
33 heating thereof, to a final substrate.
34 There is further provided in a preferred embodiment of
the invention, imaging apparatus utilizing a liquid
36 developer, the apparatus including: an image forming
37 surface, apparatus utilizing the liquid developer, for
38 forming a liquid toner image having a liquid portion
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1 including carrier liquid and a solids portion including
2 toner particles, on the image forming surface, apparatus for
3 concentrating the liquid toner image by compacting the
4 solids portion thereof and removing carrier liquid '
therefrom, including apparatus far increasing the nor.-
6 volatile solids percentage of the liquid toner image to
7 between 20% and 35%, apparatus for transferring the liquid
8 toner image to an intermediate transfer member and apparatus
9 for transferring the liquid toner image from the
intermediate transfer member to a final substrate.
11 In a preferred embodiment of the invention the
12 apparatus for concentrating includes apparatus for the
13 simultaneous application of an electric field to compact the
14 solids portion of the image and of mechanical pressure to
remove liquid from the image. In a preferred embodiment of
16 the invention the apparatus far concentrating includes an
Z7 electrified squeegee roller urged against the image forming
1$ surface.
1~ In a preferred embodiment of the application the single
phase is a liquid phase. Alternatively ar additionally, the
21 apparatus far concentrating is operative to increase the
22 solids percentage to a value at which phase separation
23 cannot occur.
24 In a preferred embodiment o~ the invention the imaging
apparatus also includes optical radiation apparatus far
26 discharging both image and background areas prior to image
27 transfer to the image transfer member. In a preferred
28 embodiment of the invention the optical radiation apparatus
2~ includes at least one light emitting diode. In a preferred
embodiment, the optical radiation apparatus includes at
31 least two radiation sources radiating different color light.
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1 BRIEF OE8CRIPTION OF THE DRATaTIN~S
The present invention will be understood and
3 appreciated more fully from the follawing detailed
description, taken in conjunction with the drawings in
" 5 which:
Fig. 1 is a simplified sectional illustration of
7 electrophotographic apparatus constructed and operative in
8 accordance with a preferred embodiment of the present
9 invention; and
Fig. 2 is part of a partial simplified typical phase
11 diagram for a preferred liquid toner for the present
12 invention.
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Detailed Description of Preferred Embodiments
Reference is now made to Fig. 1 which illustrates
electrophotographic imaging apparatus constructed and operative in
accordance with a preferred embodiment of the present invention.
The invention is described for liquid developer systems with
negatively charged toner particles, and negatively charged
photoconductors, i.e., systems operating in the reversal mode.
For other combinations of toner particle and photoconductor
polarity, the values and polarities of the voltages are changed,
in accordance with the principles of the invention.
The invention can be practiced using a variety of liquid
developer types but is especially useful for liquid developers
comprising carrier liquid and pigmented polymeric toner particles
which are essentially non-soluble in the carrier liquid at room
temperature, and which solvate carrier liquid at elevated
temperatures. This is a characteristic of the liquid developer of
Example 1 of U.S. Patent 4,794,651. Part of a simplified phase
diagram of a typical toner of this type is shown in Fig. 2. This
diagram represents the states of the polymer portion of the toner
particles and the carrier liquid. The pigment in the particles
generally takes little part in the process, and references herein
to ~~single phase~~ and to ~~solvation~~ refer to the state of the
polymer part of the toner particles together with the carrier
liquid.
In a preferred embodiment of the invention a liquid developer
is prepared by mixing 10 parts of Elvax II 5950*
(E. I. du Pont) and 5 parts by weight of Isopar L* (Exxon) at low
speed in a jacketed double planetary mixer connected to an oil
heating unit for one hour, the heating unit being set at 130°C.
A mixture of 2.5 parts by weight of Mogul L* carbon black (Cabot)
and 5 parts by weight of Isopar L* is then added to the mix in the
double planetary mixer and the resultant mixture is further mixed
for one hour at high speed. 20 parts by weight of Isopar L* pre-
heated to 110°C
* Trade marks
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are added to the mixer and mixing is continued at high speed for
one hour. The heating unit is disconnected and mixing is continued
until the temperature of the mixture drops to 40°C
100 g of the resulting material is mixed with 120 g of Isopar
L' and the mixture is milled for 19 hours in an attritor to obtain
a dispersion of particles. The material is dispersed in Isopar L'
to a solids content of 1.5% by weight.
The preferred liquid developer prepared comprises toner
particles which are formed with a plurality of fibrous extensions
or tendrils as described in U.S. Patent 4,794,651. The preferred
liquid developer is characterized in that when the concentration
of toner particles is increased above 20%, the viscosity of the
material increases greatly, apparently in approximately and
exponential manner.
A charge director, prepared in accordance with Example 1 of
US Patent No. 5,047,306 issued September 10, 1991 is added to the
dispersion in an amount equal to about 3% of the weight of the
solids in the developer.
As in conventional electrophotographic systems, the apparatus
of Fig. 1 typically comprises a drum 10 arranged for rotation
about an axle 12 in a direction generally indicated by arrow 14.
Drum 10 is formed with a cylindrical photoconductor surface 16.
A corona discharge device 18 is operative to generally
uniformly charge photoconductor surface 16 with a negative charge.
Continued rotation of drum 10 brings charged photoconductor
surface 16 into image receiving relationship with an exposure unit
including a lens 20, which focuses an image onto charged
photoconductor surface 16, selectively discharging the
photoconductor surface, thus producing an electrostatic latent
image thereon. The latent image comprises image areas at a given
range of potentials and
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1 background areas at a different potential. The image may be
2 laser generated as in printing from ~a computer or it may be
3 the image of an original as in a copier.
4 Continued, rotation of drum 10 brings charged
.5 photoconductor surface 16, bearing the electrostatic latent
6 iaage, into a development unit 22, which is operative to
7 apply liquid developer, comprising a solids portion
8 including pigmented toner particles and a liquid portion
9 including carrier liquid, to develop the electrostatic
latent image. The developed image includes image areas
11 having pigmented toner particles thereon and background
12 areas. Development unit 22 may be a single color developer
13 of any conventional type, or may be a plurality of single
14 color developers for the production of full color images as
is known in the art. Alternatively, full color images may be
16 produced by changing the liquid toner in the development
17 unit when the color to be printed is changed. Alternatively,
18 highlight color development may be employed, as is known in
19 the art.
In accordance with a preferred embodiment of the
21 invention, fol2owing application of toner thereto,
22 photoconductor surface 16 passes a typically charged
23 rotating roller 26, preferably rotating in a direction
24 indicated by an arrow 28. Typically the spatial separation
of the roller 26 from the photoconductor surface 16 is about
26 50 microns. Roller 26 thus acts as a metering roller as is
27 known in the art, reducing the amount of carrier liquid on
28 the background areas and reducing the amount of liquid
29 overlaying the image.
Preferably the potential on roller 26 is intermediate
31 that of the latent image areas and of the background areas
32 on the photoconductor surface. Typical approximate voltages '
33 are: roller 26: -500 V, background area: -1000 ~7 and latent
34 image areas: -150 V.
The liquid toner image which passes roller 26 should be
36 relatively free of pigmented particles except in the region
3'~ of the latent image.
38 Downstream of roller 26 there is preferably provided a
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rigidizing roller 30. Rigidizing roller 30 is preferably
formed of resilient polymeric material, such as polyurethane
which may have only its natural conductivity or which may be
filled with carbon black to increase its conductivity.
According to one embodiment of the invention, roller 30
is urged against photoconductor surface 16 as by a spring
mounting (not shown). The surface of roller 30 typically
moves in the same direction and with the same velocity as
the photoconductor surface to remove liquid from the image.
Preferably, the biased squeegee described in U.S.
Patent 4,286,039 is used as the roller 30. Roller 30 is
biased to a potential of at least several hundred and up to
several thousand Volts with respect to the potential of the
developed image on photoconductor surface 16, so that it
repels the charged pigmented particles and causes them to
more closely approach the image areas of the photoconductor
surface 16, thus compacting and rigidizing the image.
In a preferred embodiment of the invention, rigidizing
roller 30 comprises an aluminum core having a 20 mm
diameter, coated with a 4 mm thick carbon-filled
polyurethane coating having a Shore A hardness of about 30-
35, and a volume resistivity of about 108 ohm-cm. Preferably
roller 30 is urged against photoconductor surface 16 with a
pressure of about 40-70 grams per linear cm of contact,
which extends along the length of the drum. The core of
rigidizing roller 30 is energized to between about -1800 and
-2800 volts, to provide a voltage difference of preferably
between about 1600 and 2700 volts between the core and the
photoconductor surface in the image areas. Voltage
differences of as low as 600 volts are also useful.
After rigidization under these conditions and for the
preferred toner, the solids percentage in the image portion
is believed to be as high as 35% or more, when carrier
liquid absorbed as plasticizer is considered as part of the
solids portion. It is preferable to have an image with at
least 20-30% solids, after rigidizing. When the solids
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percentage is calculated on a non-volatile solids basis, the
solids percentage is preferably above 20% and is usually
less than 30%. Values of 25% have been found to especially
useful. At these concentrations the material has a paste
s like consistency.
Alternatively, the carbon filled polyurethane can be
replaced by unfilled polyurethane with a volume resistivity
of about 3 x 101°, and the voltage is adjusted to give proper
rigidizing.
Downstream of rigidizing roller 30 there is preferably
provided a plurality of light emitting diodes (LEDs) 29 to
discharge the photoconductor surface, and equalize the
potential between image and background areas. For process
color systems, where yellow, magenta and cyan toners are
used, both red and green LEDs are provided to discharge the
areas of the photoconductor behind the developed image as
well as the background areas.
Downstream of LEDs 29 there is provided an intermediate
transfer member 40, which rotates in a direction opposite to
that of photoconductor surface 16, as shown by arrow 41. The
intermediate transfer member is operative for receiving the
toner image from the photoconductor surface and for
subsequently transferring the toner image to a receiving
substrate 42, such as paper.
Various types of intermediate transfer members are
known and are described, for example, in U.S. Patent
4,684,238 and in U.S. Patent No. 5,636,349 issued June 3,
1997and U.S. Patent No. 5,028,964 issued July 2, 1991.
In general, intermediate transfer member 40 is urged
against photoconductor surface 16. One of the effects of the
rigidization described above is to prevent substantial
squash or other distortion of the image caused by the
pressure resulting from the urging. The rigidization effect
is especially pronounced due to the sharp increase of
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1 viscosity with concentration for the preferred toner.
2 Transfer of the image to interanediate transfer mer"ber
3 40 is preferably sided by providing electrical bias to the
.4 intermediate transfer member 40 to attract the charged toner
thereto, although other methods known im the art may be
6 employed. Subsequent transfer of the image to substrate 42
7 is preferably aided by heat and pressure, with pressure
8 applied by a backing roller 43, although other methods known
9 in the art may be employed.
l0 It has been noted that when the negatively biased
11 squeegee roller of U.S. Patent 4,286,039, with high negative
12 voltage, is utilized as the roller 30, the voltage
13 difference between the intermediate transfer member and the
14 photoconductor surface, required to transfer the image to
i5 the intermediate transfer member is sharply reduced. It is
16 believed that this reduction is possibly due to current flow
17 tending to equalize and discharge the potential of image and
18 background areas on the image bearing surface. LEDs 29
19 discharge both image and non-image areas and are operative
20 to further reduce this voltage difference.
21 For the particular illustrative example described
22 herein, the intermediate transfer member voltage is between
23 --300 V and 0 V where no pre~transfer LEDs are used and
24 between X200 V and +500 V where they are used.
25 Following transfer of the toner image to the
26 intermediate transfer member, photoconductor surface 16 is
27 engaged by a cleaning roller 50, which typically rotates in
28 a direction indicated by an arrow 52, such that its surface
29 moves in a.directian opposite to the movement of adjacent
30 photoconductor surface 16 which it operatively engages.
31 Cleaning roller 50 is operative to scrub and clean surface
32 16. A cleaning material, such as toner, may be supplied to
33 the cleaning roller 50, via a conduit 54. A wiper blade 56
34 completes the cleaning of the photoconductor surface. Any
35 residual charge left on photocanductor surface 16 is removed
36 by flooding the photoconductor surface with light from a
37 lamp 58.
38 In a.multi-color system, subsequent to completion of
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the cycle for one color , the cycle is sequentially repeated
for the other colors which are sequentially transferred from
photoconductor surface 16 to intermediate transfer member
40. The single color images may be sequentially transferred
to the paper, in alignment, or may alternatively be overlaid
on the intermediate transfer member and transferred as a
group to the substrate 42.
Details of the construction of the surface layers of
preferred intermediate transfer members are shown in
assignee's U.S. Patent No. 5,089,856 issued February 18,
1992.
Generally, the image is heated on intermediate transfer
member 40 in order to facilitate its transfer to substrate
42. This heating is preferably to a temperature above a
threshold temperature of substantial solvation of the
carrier liquid in the toner particles.
As seen in Fig. 2, when the image is heated, the state
of the image, i.e. of the polymer portion of the toner
particles and the carrier liquid, depends on several
factors, mainly on the temperature of the intermediate
transfer member and on the concentration of toner particles.
Thus, if the percentage of toner particles is "A" and the
intermediate transfer member temperature is "Y" the liquid
image separates into two phases, one phase being
substantially a liquid polymer/ carrier-liquid phase and the
other phase consisting mainly of carrier liquid. On the
other hand, if the percentage of toner particles is "B" at
the same temperature, then substantially only one phase, a
liquid polymer/carrier-liquid phase will be present. It is
believed to be preferable that separate liquid
polymer/carrier-liquid and liquid phases do not form to any
substantial degree, as will be the case for example if the
concentration is "C".
This type of phase separation is believed to be
undesirable on the intermediate transfer member. It is
believed that an absence of substantial phase separation of
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this type in the image on the intermediate transfer member
results in improved image quality, including an improvement
in line uniformity.
It is understood that heating the image on the
intermediate transfer member is not meant to completely dry
the image, although some evaporation of carrier liquid may
result. Rather, the image on the intermediate transfer
member remains a viscous liquid until its transfer to the
final substrate.
The invention has been described by a specific
embodiment utilizing an electrified squeegee roller for
concentrating the liquid toner image on the photoconductor
surface. Alternatively other methods of concentrating the
image, i.e., compacting the solids portion thereof and
removing liquid therefrom, can be utilized provided they
concentrate the image to the extent required. These methods
include the use of separate solids portion compactors and
liquid removal means, such as those described in U.S. Patent
No. 5,028,964 issued July 2, 1991. Alternatively the
apparatus may utilize a solids portion compactor followed by
an intermediate transfer member urged against the
photoconductor to remove liquid from the image. As a further
alternative, the commutated intermediate transfer member
described in U.S. Patent No. 5,028,964 may be used to
provide both solids portion compacting and liquid removal,
just prior to transfer to the intermediate transfer member.
Furthermore the concentrating step may take place on
the intermediate transfer member after transfer of the
liquid toner image thereto and before heating the image.
It will be appreciated by persons skilled in the art
that the present invention is not limited by what has been
particularly shown and described herein above. Rather the
scope of the present invention is defined only by the claims
which follow:
