Note: Descriptions are shown in the official language in which they were submitted.
CA 02355458 2001-08-17
ELECTROCOAGULATION PRINTING METHOD AND
APPARATUS PROVIDING COLOR JUXTAPOSITION
The present invention pertains to improvements in the field of
electrocoagulation printing. More particularly, the invention relates to an
electrocoagulation printing method and apparatus providing color
juxtaposition.
In US Patent No. 4,895,629 of January 23, 1990, Applicant has
described a high-speed elf;ctrocoagulation printing method and apparatus in
1 o which use is made of a positive electrode in the form of a revolving
cylinder
having a passivated surface onto which dots of colored, coagulated colloid
representative of an image are produced. These dots of colored, coagulated
colloid are thereafter contacted with a substrate such as paper to cause
transfer
of the colored, coagulated colloid onto the substrate and thereby imprint the
i5 substrate with the image. As explained in this patent, the positive
electrode is
coated with a dispersion containing an olefinic substance and a metal oxide
prior to electrical energization of the negative electrodes in order to weaken
the
adherence of the dots of coagulated colloid to the positive electrode and also
to
prevent an uncontrolled corrosion of the positive electrode. In addition, gas
z o generated as a result of electrolysis upon energizing the negative
electrodes is
consumed by reaction with the olefinic substance so that there is no gas
accumulation between the negative and positive electrodes.
The electroco;agulation printing ink which is injected into the gap
25 defined between the positive and negative electrodes consists essentially
of a
liquid colloidal dispersion containing an electrolytically coagulable colloid,
a
dispersing medium, a soluble electrolyte and a coloring agent. Where the
coloring agent used is a piigment, a dispersing agent is added for uniformly
dispersing the pigment into the ink. Atter coagulation of the colloid, any
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CA 02355458 2001-08-17
remaining non-coagulated colloid is removed from the surface of the positive
electrode, for example, by scraping the surface with a soft rubber squeegee,
so
as to fully uncover the colored, coagulated colloid which is thereafter
transferred onto the substrate. The surface of the positive electrode is
thereafter
cleaned by means of a plurality of rotating brushes and a cleaning liquid to
remove any residual coagulated colloid adhered to the surface of the positive
electrode.
When a polychromic image is desired, the negative and positive
io electrodes, the positive electrode coating device, ink injector, rubber
squeegee
and positive electrode clearing device are arranged to define a printing unit
and
several printing units each using a coloring agent of different color are
disposed
in tandem relation to produce several differently colored images of coagulated
colloid which are transferred at respective transfer stations onto the
substrate in
superimposed relation to provide the desired polychromic image. Alternatively,
the printing units can be arranged around a single roller adapted to bring the
substrate into contact with the dots of colored, coagulated colloid produced
by
each printing unit, and the substrate which is in the form of a continuous web
is
partially wrapped around the roller and passed through the respective transfer
a o stations for being impriinted with the differently colored images in
superimposed relation.
A polychromic image can also be formed by providing a single
positive electrode in the form of a revolving cylinder, arranging the negative
z5 electrodes, the positive electrode coating device, ink injector, rubber
squeegee
and positive electrode cleaning device to define a printing unit and disposing
several printing units each using a coloring agent of different color around
the
positive cylindrical electrode to produce several differently colored images
of
coagulated colloid which are transferred at respective transfer stations from
the
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CA 02355458 2001-08-17
positive electrode surface onto the substrate in superimposed relation to
provide
the desired polychromic image. The substrate which is in the form of a
continuous web is partially wrapped around the positive electrode and passed
through the respective transfer stations for being imprinted with the
differently
colored images in superinnposed relation. This arrangement is described in
Applicant's US Patent No. .'>,538,601 of July 23, 1996.
Since each printing unit of the above multicolor printing
apparatus requires a positive electrode coating device and cleaning device,
such
to an apparatus is not only cumbersome but also very costly. Moreover, since
the
differently colored images of coagulated colloid are transferred at respective
transfer stations onto the substrate in superimposed relation, and there are
thus
several transfer stations, it 'is difficult to provide a polychromic image in
which
the differently colored images are perfectly superimposed.
It is therefore an object of the present invention to overcome the
above drawbacks and to provide an improved multicolor electrocoagulation
printing method and apparatus of reduced cost and cumbersomeness, capable of
providing a polychromic image of high definition.
According to one aspect of the invention, there is provided a
multicolor electrocoagulation printing method comprising the steps of:
a) providing a positive electrolytically inert electrode having a
continuous passivated surface moving at substantially constant speed along a
predetermined path, the passivated surface defining a positive electrode
active
surface;
b) coating the positive electrode active surface with an olefinic
substance to form on the surface micro-droplets of olefinic substance;
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CA 02355458 2001-08-17
c) forming on the olefin-coated positive electrode active surface a
plurality of colored pixels representative of a desired polychromic image,
each
pixel comprising juxtaposed dots of differently colored, coagulated colloid;
and
d) bringing a substrate into contact with the colored pixels to cause
transfer of the colored pixels from the positive electrode active surface onto
the
substrate and thereby imprint the substrate with the polychromic image.
Step (c) of the method according to the invention is carried out
by:
to i) providing a series of negative electrolytically inert electrodes each
having a cylindrical configuration with a predetermined cross-sectional
dimension and an end surface covered with a passive oxide film, the negative
electrodes being electrically insulated from one another and arranged in
rectilinear alignment so that the end surfaces thereof define a plurality of
i5 corresponding negative elesctrode active surfaces disposed in a plane
spaced
from the positive electrode active surface by a constant predetermined gap,
the
negative electrodes being spaced from one another by a distance smaller than
the electrode gap;
ii) filling the electrode gap with an eletrocoagulation printing ink
z o comprising a liguid col loidal dispersion containing an electrolytically
coagulated colloid, a dispersing medium, a soluble electrolyte and a coloring
agent;
iii) applying to the negative electrodes a pulsed bias voltage ranging
from -1.5 to -40 volts andl having a pulse duration of 15 nanoseconds to 6
z 5 microseconds, the bias voltage applied being inversely and non-linearly
proportional to the pulse duration;
iv) applying to selected ones of the negative electrodes a trigger voltage
sufficient to energize same and cause point-by-point selective coagulation and
adherence of the colloid onl;o the olefin-coated positive electrode active
surface
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CA 02355458 2001-08-17
opposite the electrode active surfaces of the energized electrodes while the
positive electrode active surface is moving, thereby forming dots of colored,
coagulated colloid;
v) removing any remaining non-coagulated colloid from the positive
electrode active surface; and
vi) repeating steps (i) through (v) several times to define a
corresponding number of sprinting stages arranged at predetermined locations
along the aforesaid path and each using a coloring agent of different color to
produce dots of differently colored, coagulated colloid, the distance between
1 o the negative electrodes of each printing stage being at least three times
the
cross-sectional dimension of each negative electrode to permit juxtaposition
of
the dots of differently colored, coagulated colloid, thereby forming the
colored
pixels.
The present invention also provides, in another aspect thereof, an
apparatus for carrying out a method as defined above. The apparatus of the
invention comprises:
- a positive electr~olytically inert electrode having a continuous
passivated surface defining a positive electrode active surface;
2 0 - means far moving the positive electrode active surface at a
substantially constant speedl along a predetermined path;
- means for coating the positive electrode active surface with an olefinic
substance to form on the surface micro-droplets of olefinic substance;
- a plurality of printing units arranged at predetermined locations along
the path, each printing unit comprising:
- a series of negative electrolytically inert electrodes each having a
cylindrical
configuration with a predetermined cross-sectional dimension and an end
surface covered with a passive oxide film, the negative electrodes being
electrically insulated from one another and arranged in rectilinear alignment
so
CA 02355458 2001-08-17
that the end surfaces thereof define a plurality of corresponding negative
electrode active surfaces disposed in a plane spaced from the positive
electrode
active surface by a constant predetermined gap, the negative electrodes being
spaced from one another by a distance smaller than the electrode gap;
- means for filling the electrode gap with an eletrocoagulation printing ink
comprising a liquid colloidal dispersion containing an electrolytically
coagulated colloid, a dispersing medium, a soluble electrolyte and a coloring
agent;
- means for applying to the negative electrodes a pulsed bias voltage ranging
to from -1.5 to -40 volts and having a pulse duration of 15 nanoseconds to 6
microseconds, the bias voltage applied being inversely and non-linearly
proportional to the pulse duration;
- means for applying to selected ones of the negative electrodes a trigger
voltage sufficient to energize same and cause point-by-point selective
coagulation and adherence of the colloid onto the olefin-coated positive
electrode active surface opposite the electrode active surfaces of the
energized
electrodes while the positive electrode active surface is moving, thereby
forming dots of colored, coagulated colloid; and
- means for removing any remaining non-coagulated colloid from the positive
z o electrode active surface;
wherein the printing units each use a coloring agent of different color so as
to
form a plurality of dots of differently colored, coagulated colloid on the
olefin-
coated positive electrode active surface, the distance between the negative
electrodes of each printing; unit being at least three times the cross-
sectional
z5 dimension of each negative electrode to permit juxtaposition of the dots of
differently colored, coagul<~ted colloid, whereby to form a plurality of
colored
pixels representative of a desired polychromic image and each comprising
juxtaposed dots of differently colored, coagulated colloid; and
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CA 02355458 2001-08-17
- means for bringing a substrate into contact with the colored pixels to
cause transfer of the colored pixels from the positive electrode active
surface
onto said substrate and thereby imprint the substrate with the polychromic
image.
Applicant has found quite unexpectedly that by arranging the
negative electrodes of each printing stage or unit so that the distance
between
the negative electrodes is at least three times the cross-sectional dimension
of
each negative electrode, colored pixels representative of a desired
polychromic
to image and each comprisin;~ juxtaposed dots of differently colored,
coagulated
colloid can be formed on the positive electrode active surface. These colored
pixels can thereafter be transferred from the positive electrode active
surface
onto the substrate at a single transfer station so as to imprint the substrate
with
the polychromic image. Moreover, a single positive electrode coating device is
i5 required for coating the positive electrode active surface with the
olefinic
substance.
The positive electrode which is used for electrocoagulation
printing must be made of an electrolytically inert metal capable of releasing
z o trivalent ions so that upon electrical energization of the negative
electrodes,
dissolution of the passive oxide film on such an electrode generates trivalent
ions which then initiate coagulation of the colloid. Examples of suitable
electrolytically inert metals include stainless steel, aluminium and tin.
25 As explained in Applicant's US Patent No. 5,750,593 of
March 12, 1998, a breakdown of passive oxide films occurs in the presence of
electrolyte anions, such as Cl-, Br- and I-, there being a gradual oxygen
displacement from the passive film by the halide anions and a displacement of
adsorbed oxygen from the metal surface by the halide anions. The velocity of
CA 02355458 2001-08-17
passive film breakdown, once started, increases explosively in the presence of
an applied electric field. 'There is thus formation of a soluble metal halide
at the
metal surface. In other words, a local dissolution of the passive oxide film
occurs at the breakdown sites, which releases metal ions into the electrolyte
solution. Where a positive electrode made of stainless steel or aluminium is
utilized in Applicant's electrocoagulation printing method, dissolution of the
passive oxide film on such an electrode generates Fe3+ or A13+ ions. These
trivalent ions then initiate coagulation of the colloid.
to The positive electrode can be in the form of a moving endless belt
as described in Applicant's US Patent No.4,661,222, or in the form of a
revolving cylinder as described in Applicant's US Patent Nos. 4,895,629 and
5,538,601. In the latter case, the printing stages or units are arranged
around the
positive cylindrical electrode. Preferably, the positive electrode active
surface
i5 and the ink are maintained at a temperature of about 35-60°C,
preferably 40°C,
to increase the viscosity of the coagulated colloid in step (c) so that the
dots of
colored, coagulated colloid remain coherent during their transfer in step (d),
thereby enhancing transfer of the colored, coagulated colloid onto the
substrate.
For example, the positive electrode active surface can be heated at the
desired
z o temperature and the ink applied on the heated electrode surface to cause a
transfer of heat therefrom to the ink.
Coating of the positive electrode with an olefinic substance prior
to electrical energization of the negative electrodes weakens the adherence of
25 the dots of coagulated colloid to the positive electrode and also prevents
an
uncontrolled corrosion of the positive electrode. In addition, gas generated
as a
result of electrolysis upon energizing the negative electrodes is consumed by
reaction with the olefinic substance so that there is no gas accumulation
between the negative and positive electrodes. Applicant has found that it is
no
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CA 02355458 2001-08-17
longer necessary to admix a metal oxide with the olefin substance; it is
believed
that the passive oxide film on currently available electrodes contains
sufficient
metal oxide to act as catalyst for the desired reaction.
Examples of suitable olefinic substances which may be used to
coat the surface of the po;>itive electrode in step (b) include unsaturated
fatty
acids such as arachidonic acid, linoleic acid, linolenic acid, oleic acid and
palmitoleic acid and unsaturated vegetable oils such as corn oil, linseed oil,
olive oil, peanut oil, soybean oil and sunflower oil. Oleic acid is
particularly
1 o preferred. The micro-droplets formed on the surface of the positive
electrode
active surface generally have a size ranging from about 1 to about 5 ~u.
The olefin-coated positive active surface is preferably polished to
increase the adherence of t:he micro-droplets onto the positive electrode
active
surface, prior to step (c). For example, use can be made of a rotating brush
provided with a plurality of radially extending bristles made of horsehair and
having extremities contacting the surface of the positive electrode. The
friction
caused by the bristles contacting the surface upon rotation of the brush has
been
found to increase the adlherence of the micro-droplets onto the positive
a o electrode active surface.
Where the positive cylindrical electrode extends vertically, step
(c)(ii) of the above elec~trocoagulation printing method is advantageously
carried out by continuously discharging the ink onto the positive electrode
z5 active surface from a fluid discharge means disposed adjacent the electrode
gap
at a predetermined height relative to the positive electrode and allowing the
ink
to flow downwardly along the positive electrode active surface, the ink being
thus carried by the positive electrode upon rotation thereof to the electrode
gap
to fill same. Preferably, excess ink flowing downwardly off the positive
- 9 -
CA 02355458 2001-08-17
electrode active surface is collected and the collected ink is recirculated
back to
the fluid discharge means.
The colloid generally used is a linear colloid of high molecular
s weight, that is, one having; a weight average molecular weight between about
10,000 and about 1,000,(100, preferably between 100,000 and 600,000.
Examples of suitable colloids include natural polymers such as albumin,
gelatin, casein and agar, and synthetic polymers such as polyacrylic acid,
polyacrylamide and polyvinyl alcohol. A particularly preferred colloid is an
io anionic copolymer of acr',~lamide and acrylic acid having a weight average
molecular weight of about 250,000 and sold by Cyanamid Inc. under the trade-
mark ACCOSTRENGTH 85. Water is preferably used as the medium for
dispersing the colloid to provide the desired colloidal dispersion.
15 The ink also contains a soluble electrolyte and a coloring agent.
Preferred electrolytes include alkali metal halides and alkaline earth metal
halides, such as lithium chloride, sodium chloride, potassium chloride and
calcium chloride. Potassium chloride is particularly preferred. The coloring
agent can be a dye or a pigment. Examples of suitable dyes which may be used
a o to color the colloid are the water soluble dyes available from Hoechst
such as
Duasyn Acid Black for coloring in black and Duasyn Acid Blue for coloring in
cyan, or those available from Riedel-Dehaen such as Anti-Halo Dye Blue T.
Pina for coloring in cyan, Anti-Halo Dye AC Magenta Extra VO1 Pina for
coloring in magenta and Anti-Halo Dye Oxonol Yellow N. Pina for coloring in
25 yellow. When using a pigment as a coloring agent, use can be made of the
pigments which are available from Cabot Corp. such as Carbon Black
Monarch~ 120 for coloring in black, or those available from Hoechst such as
Hostaperm Blue B2G or B3G for coloring in cyan, Permanent Rubine F6B or
L6B for coloring in magenta and Permanent Yellow DGR or DHG for coloring
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in yellow. A dispersing agent is added for uniformly dispersing the pigment
into the ink. Examples of suitable dispersing agents include the anionic
dispersing agent sold by Boehme Filatex Canada Inc. under the trademark
CLOSPERSE 25000.
In step (c) (iii), a pulsed bias voltage ranging from -1.5 to -40
volts and having a pulse duration of 15 nanoseconds to 6 microseconds is
applied to the negative electrodes. As explained in Applicant's Canadian
application No.2,334,265, filed February 6, 2001, this prevents undesirable
to formation of a gelatinous deposit on the surfaces of the negative
electrodes, and
of a low-density blur on the electrocoagulation printed image, while enabling
the negative electrodes to be positioned close to one another with a spacing
therebetween smaller than the electrode gap, without undergoing edge
corrosion. If the pulsed bias voltage is less than -1.5 volts at a pulse
duration of
6 microseconds, the passive oxide film of each negative electrode upon being
energized dissolves into the ink, resulting in a release of metal ions and
edge
corrosion of the negative electrodes. On the other hand, if the pulsed bias
voltage is greater than -40~ volts at a pulse duration of 15 nanoseconds, such
a
voltage is sufficient to cause formation of the gelatinous deposit and low-
a o density blur. If the pulse duration is shorter than 15 nanoseconds, the
negative
electrodes undergo edge corrosion and, if it is longer than 6 microseconds,
there is formation of the gelatinous deposit and of the low-density blur. The
pulse duration must therefore be insufficient for the bias voltage to cause
formation of the gelatinous deposit and the low-density blur, yet sufficient
for
2 5 the bias voltage to protect the negative electrodes against edge
corrosion. Thus,
by operating with a pulsed bias voltage ranging from -1.5 to -40 volts and
having a pulse duration of 1 S nanoseconds to 6 microseconds, preferably about
-2 volts at a pulse duration of 4 microseconds, and by positioning the
negative
electrodes sufficiently close to one another with a spacing therebetween
smaller
- 11 -
CA 02355458 2001-08-17
than the electrode gap, an image resolution as high as 400 lines per inch, or
more, can be obtained without adverse effect. A trigger voltage is then
applied
in step (c) (iv) to selected ones of the negative electrodes to energize same
and
cause point-by-point selectiive coagulation and adherence of the colloid onto
the
olefin-coated positive electrode surface opposite the surfaces of the
energized
electrodes.
Preferably, the negative electrodes each have a cylindrical
configuration with a circular cross-section and a diameter ranging from about
l0 10 pm to about 50 pm. Electrodes having a diameter of about 15 ~m are
preferred. The gap whiclh is defined between the positive and negative
electrodes can range from about 35 ~m to about 100 p,m, the smaller the
electrode gap the sharper are the dots of coagulated colloid produced. Where
the electrode gap is of the order of 50 pm, the negative electrodes preferably
have a diameter of about 15 pm and are spaced from one another by a distance
of about 48 pm. Examples of suitable electrolytically inert metals from which
the negative electrodes can be made include chromium, nickel, stainless steel
and titanium; stainless steel is particularly preferred.
a o After coagulation of the colloid, any remaining non-coagulated
colloid is removed from the positive electrode active surface, for example, by
scraping the surface with a soft rubber squeegee, so as to fully uncover the
colored, coagulated colloid. Preferably, the non-coagulated colloid thus
removed is collected and mixed with the collected ink, and the collected non-
z5 coagulated colloid in admixture with the collected ink is recirculated back
to
the aforesaid fluid discharge means.
- 12 -
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The optical density of the dots of colored, coagulated colloid may
be varied by varying the voltage and/or pulse duration of the pulse-modulated
signals applied to the negative electrodes.
After step (d), the positive electrode active surface is generally
cleaned to remove therefrom any remaining coagulated colloid. According to a
preferred embodiment, the' positive electrode is rotatable in a predetermined
direction and any remaining coagulated colloid is removed from the positive
electrode active surface by providing an elongated rotatable brush extending
to parallel to the longitudinal axis of the positive electrode, the brush
being
provided with a plurality of radially extending bristles made of horsehair and
having extremities contacting the positive electrode active surface, rotating
the
brush in a direction opposite to the direction of rotation of the positive
electrode so as to cause the bristles to frictionally engage the positive
electrode
i5 active surface, and directing jets of cleaning liquid under pressure
against the
positive electrode active surface, from either side of the brush. In such an
embodiment, the positive electrode active surface and the ink are preferably
maintained at a temperature of about 35-60°C by heating the cleaning
liquid to
thereby heat the positive electrode active surface upon contacting same and
2 o applying the ink on the heated electrode surface to cause a transfer of
heat
therefrom to the ink.
Further features and advantages of the invention will become
more readily apparent from the following description of a preferred
a s embodiment as illustrated by way of examples in the accompanying drawings,
in which:
Figure 1 is a schematic top plan view of a multicolor
electrocoagulation printing apparatus according to a preferred embodiment of
- 13 -
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the invention, comprising four printing units each using a coloring agent of
different color;
Figure 2 is a fragmentary sectional view thereof, showing one of
the printing units together with a positive electrode coating device and
cleaning
device;
Figure 3 is a fragmentary longitudinal view of the printing head
of one of the printing unit;
to
Figure 4 is a fragmentary sectional view of the printing head
illustrated in Fig. 3, showing one of the negative electrodes;
Figure 5 is a schematic diagram showing how the negative
i5 electrodes of each printing head one energized in response to an input
signal of
information; and
Figure 6 is an enlarged top plan view of a colored pixel formed
by the multicolor electrocoagulation printing apparatus illustrated in Fig. 1.
Refernng first to Fig. 1, there is illustrated a multicolor
electrocoagulation printing apparatus comprising a central positive electrode
10
in the form of a revolving cylinder and having a passivated surface 12, around
which are arranged a positive electrode cleaning device 14 for cleaning the
surface 12, a positive electrode coating device 16 for coating the surface 12
with an olefinic substance, a polishing brush 18 for polishing the olefin-
coated
surface 12 and four identical printing units 20 adapted to form on the olefin-
coated surface 12 a plurality of colored pixels representative of a desired
polychromic image, each pixel comprising juxtaposed dots of differently
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colored, coagulated colloid. The first printing unit 20A is adapted to produce
dots of coagulated colloid lhaving a black color, the second printing unit 20B
to
produce dots of coagulated colloid having a yellow color, the third printing
20C
to produce dots of coagulated colloid having a magenta color and the fourth
s printing unit to produce dots of coagulated colloid having a cyan color. The
cylindrical electrode 10 extends vertically and has a shaft 22 which is driven
by
a motor (not shown) for rotating the electrode about a vertical axis
coincident
with the shaft 22. The apparatus further includes a pressure roller 24 for
bringing a substrate in the form of a continuous web 26 into contact with the
1 o colored pixels to cause transfer of the colored pixels from the positive
electrode
surface 12 onto the web 26 and thereby imprint the web with the polychromic
image.
As best shown in Fig. 2, the positive electrode cleaning device 14
15 comprises a rotating bru;>h 28 and two high pressure water injectors 30
arranged in a housing 32. The brush 28 is provided with a plurality of
radially
extending bristles 34 made' of horsehair and having extremities contacting the
surface 12 of the positive electrode 10. Any coagulated colloid remaining on
the surface 12 after transfer of the colored pixels onto the web 26 is thus
2o removed by the brush 28 and washed away by the powerful jets of water
produced by the injectors 30.
The positive electrode coating device 16 comprises a vertically
extending distribution roller 36, an applicator roller 38 extending parallel
to the
25 distribution roller 36 and in pressure contact engagement therewith to form
a
nip 40, and a transfer roller 42 extending parallel to the roller 36 and in
contact
engagement therewith to form a nip 44. 'The transfer roller 42 is in pressure
contact engagement with the positive electrode 10 to form a nip 46 and permit
the roller 42 to be driven by the positive electrode 10 upon rotation thereof.
The
- 15 -
CA 02355458 2001-08-17
coating device 16 further includes a feeding device 48 for supplying to the
applicator roller 38 the oledinic substance in the form of an oily liquid.
The distribution roller 36 has a solid core 50 of metal provided
with a peripheral coating 52 of oxide ceramic material. A pair of stub shafts
54
(only one shown) integral with the core 50 extends outwardly from the
extremities of the roller 36. The applicator roller 38 and transfer roller 42
also
have a solid core 56 of metal, but are provided with a peripheral covering 58
of
polyurethane. The rollers 36 and 38 are rotated in register by means of a
motor
to (not shown driving the shaift 54 of the distribution roller 36. The drive
from the
motor rotates the distribution roller 36 in a counterclockwise manner, which
in
turn transmits a clockwise rotation to the applicator roller 38.
The feeding device 48 is adapted to discharge the oily liquid onto
i5 the applicator roller 38 at an upper portion thereof. The liquid then flows
downwardly under gravity along the roller 38 and is carried to the nip 40 by
the
roller 38 during rotation thereof. The liquid upon passing through the nip 40
forms a film uniformly covering the surface of the ceramic coating 52 of the
distribution roller 36, the film breaking down into micro-droplets containing
2 o the olefinic substance and having substantially uniform size and
distribution.
The micro-droplets formed on the roller 36 are carried by the latter to the
nip 44
where they are transferred onto the transfer roller 42. The micro-droplets are
then carried by the roller 42 to the nip 46 where they are transferred onto
the
positive electrode 10.
The polishing brush 18 used for polishing the olefin-coated
surface 12 of the positive electrode 10 is similar to the brush 28, the brush
18
being provided with a plurality of radially extending bristles 34 made of
horsehair and having extremities contacting the surface 12. The friction
caused
- 16 -
CA 02355458 2001-08-17
by the bristles 34 contacting the surface 12 upon rotation of the brush 18 has
been found to increase the: adherence of the micro-droplets onto the positive
electrode surface 12.
s The printing units 20 each comprise a device 60 for discharging
an electrocoagulation printing ink onto the olefin-coated surface 12, a
printing
head 62 provided with negative electrodes 64 for electrocoagulating the
colloid
present in the ink to form on the olefin-coated surface 12 dots of colored,
coagulated colloid and a soft rubber squeegee 66 for removing any remaining
1 o non-coagulated colloid from the surface 12. The electrocoagulation
printing ink
consists of a colloidal dllspersion containing an electrolytically coagulable
colloid, a dispersing medium, a soluble electrolyte and a coloring agent. As
shown in Fig. 3, each printing head 62 comprises a cylindrical electrode
earner
68 with the negative electrodes 64 being electrically insulated from one
another
i5 and arranged in rectilinear alignment along the length of the electrode
earner
68 to define a plurality of corresponding negative electrode active surfaces
70.
Each printing head 62 is positioned relative to the positive electrode 10 such
that the surfaces 70 of the :negative electrodes 64 are disposed in a plane
which
is spaced from the positive electrode surface 12 by a constant predetermined
2 o gap 72. The electrodes 64 are also spaced from one another by a distance
smaller than the electrode ~;ap 72 to increase image resolution. The device 60
is
positioned adjacent the electrode gap 72 to fill same with the
electrocoagulation
printing ink.
25 As shown in Fig. 4, the negative electrodes 64 each have a
cylindrical body 74 made of an electrolytically inert metal and covered with a
passive oxide film 76. They end surface of the electrode body 74 covered with
such a film defines the aforementioned negative electrode active surface 70.
- m -
CA 02355458 2001-08-17
Figure 5 is a schematic diagram illustrating how the negative
electrodes 64 of each printing head 62 are energized in response to an input
signal of information 78 to form dots of colored, coagulated colloid. A pulsed
bias circuit 80 is provided for applying to the negative electrodes 64 a
pulsed
bias voltage ranging from -1.5 to -40 volts and having a pulse duration of 15
nanoseconds to 6 microseconds. The pulsed bias voltage applied by the circuit
80 to the negative electrodf;s 64 is inversely and non-linearly proportional
to the
pulse duration. A driver circuit 82 is also used for addressing selected ones
of
the electrodes 64 so as to apply a trigger voltage to the selected electrodes
and
to energize same. Such an electrical energizing causes point-by-point
selective
coagulation and adherence of the colloid onto the olefin-coated surface 12 of
the positive electrode 10 opposite the electrode active surfaces 70 of the
energized electrodes 64 while the electrode 10 is rotating, thereby forming on
the surface 12 a series of corresponding dots of colored, coagulated colloid.
The driver circuit 82 associated with each printing unit 20 is connected to a
central processing unit (not shown).
The first printing unit 20A utilizes an ink containing a black
coloring agent to produce dots of coagulated colloid having a black color. The
a o second printing unit 20B utilizes an ink containing a yellow coloring
agent to
produce dots of coagulated colloid having a yellow color. The third printing
unit 20C utilizes an ink containing a magenta coloring agent to produce dots
of
coagulate colloid having a magenta color. The fourth printing unit 20D
utilizes
an ink containing a cyan coloring agent to produce dots of coagulated colloid
having a cyan color. The distance between the negative electrodes 64 of each
printing unit 20 is at least three times the diameter of each electrode 64 to
permit juxtaposition of the dots of differently colored, coagulated colloid
during their formation on the surface 12 of the positive electrode 10. Since
electrocoagulation of the colloid present in the ink between the positive
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CA 02355458 2001-08-17
electrode surface 12 and the negative electrode active surface 70 of an
energized negative electrode 64 follows the lowest electrolytic resistive path
between the surfaces 12 and 70, coagulation of the colloid will occur on a
free
area of the surface 12 next to a previously formed dot of differently colored,
coagulated colloid even if the free area of the positive electrode surface 12
is
not perfectly opposite the negative electrode active surface 70 of the
energized
electrode 64. As a result., a plurality of colored pixels representative of a
desired polychromic image are formed on the positive electrode surface 12,
each pixel comprising juxtaposed dots of differently colored, coagulated
1 o colloid.
Figure 6 is an enlarged top plan view of a typical pixel 84 as
formed on the surface 12 of the positive electrode 10. The pixel 84 comprises
a
dot 86A of coagulated colloid having a black color produced by the printing
unit 20A, a dot 86B of coagulated colloid having a yellow color produced by
the printing unit 20B, a dot 86C of coagulated colloid having a magenta color
produced by the printing unit 20C, and a dot 86D of coagulated colloid having
a cyan color produced by the printing of unit 20D. The dots 86A, 86B, 86C and
86D are juxtaposed to one' another and typically each have a diameter D1 of
z o about 30 ~.m when the negative electrodes 64 of the printing units 20 each
have
a diameter of about 15 Vim. The colored pixel 84 has a dimension D2 of about
60 Vim.
The colored pixels 84 are transferred by means of the pressure
z 5 roller 24 from the positive .electrode surface 12 onto the web 26 so as to
imprint
the web with the polychromic image. A polychromic image having a high
definition as well as a resolution as high as 400 lines per inch, or more, can
thus
be obtained.
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