Note: Descriptions are shown in the official language in which they were submitted.
'- 2156978
The present invention pertains to
improvements in the field of dynamic printing. More
particularly, the invention relates to an improved
multicolor electrocoagulation printing method and
apparatus.
In US Patent N~ 4,895,629 of January 23,
1990, Applicant has described a high-speed
electrocoagulation printing method and apparatus in
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 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 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 dispersion containing the olefinic
substance and the metal oxide is applied onto the
surface of the positive electrode in a manner so as to
form on the electrode surface micro-droplets of
olefinic substance containing the metal oxide. As
described in the aforementioned patent, this may be
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achieved by means of a device comprising a rotatable
brush provided with a plurality of radially extending
horsehair bristles having extremities contacting the
electrode surface, and a distribution roller arranged
in spaced-apart parallel relation to the brush such as
to contact the bristles thereof at their extremities.
The distribution roller has a plurality of peripheral
longitudinally extending grooves and is partially
immersed in a bath containing the dispersion. As the
distribution roller rotates in the dispersion, the
grooves are filled with the dispersion which is thus
transferred to the bristles to coat the extremities
thereof. Rotation of the brush, on the other hand,
causes the coated bristles to transfer the dispersion
onto the surface of the positive electrode and thereby
form the desired micro-droplets of olefinic substance
containing the metal oxide. Instead of a brush, use can
be made of a roller provided with a plurality of
radially extending strips of chamois leather adapted to
contact the electrode surface, the strips being coated
in the same manner as the bristles. Rotation of such a
roller causes the coated strips to impinge upon the
surface of the positive electrode such as to transfer
thereon the dispersion and thereby form the desired
micro-droplets of olefinic substance containing the
metal oxide.
The electrocoagulation printing ink which is
injected into the gap 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 a pigment is
used, a dispersing agent is added for uniformly
dispersing the pigment into the ink.
21S6978
When a polychromic image is desired, the
negative and positive electrodes, the positive
electrode coating device and the ink injector 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 stations for
being imprinted with the differently colored images in
superimposed relation.
Since each printing unit of the above
multicolor printing apparatus requires a high precision
cylinder which is usually in stainless steel, as a
positive electrode, such an apparatus is not only
cumbersome but also very costly. Moreover, as several
high precision cylinders are required for forming
differently colored images of coagulated colloid, 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.
_ 21S6978
According to one aspect of the invention,
there is provided a multicolor electrocoagulation
printing method comprising the steps of:
a) providing a single positive electrode
formed of an electrolytically inert metal and having a
continuous passivated surface moving at substantially
constant speed along a predetermined path, the
passivated surface defining a positive electrode active
surface;
b) forming on the positive electrode active
surface a plurality of dots of colored, coagulated
colloid by electrocoagulation of an electrolytically
coagulable colloid in the presence of a coloring agent,
the dots of colored, coagulated colloid being
representative of a desired image;
c) bringing a substrate into contact with the
dots of colored, coagulated colloid to cause transfer
of the colored, coagulated colloid from the positive
electrode active surface onto the substrate and thereby
imprint the substrate with the image; and
d) repeating steps (b) and (c) several times
to define a corresponding number of printing stages
arranged at predetermined locations along the aforesaid
path and each using a coloring agent of different
color, and to thereby produce several differently
colored images of coagulated colloid which are
transferred at respective transfer positions onto the
substrate in superimposed relation to provide a
polychromic image.
The present invention also provides, in a
further aspect thereof, an apparatus for carrying out a
method as defined above. The apparatus of the invention
comprlses:
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- a single positive electrode formed of an
electrolytically inert metal and having a continuous
passivated surface defining a positive electrode active
surface;
- means for moving the positive electrode active
surface at a substantially constant speed along a
predetermined path; and
- a plurality of printing units arranged at
predetermined locations along the path, each printing
unit comprising:
- means for forming on the positive electrode
active surface a plurality of dots of colored,
coagulated colloid by electrocoagulation of an
electrolytically coagulable colloid in the presence of
a coloring agent of different color, the dots of
colored, coagulated colloid being representative of a
desired image; and
- means for bringing a substrate into contact
with the dots of colored, coagulated colloid at a
respective transfer station to cause transfer of the
colored, coagulated colloid from the positive electrode
active surface onto the substrate and thereby imprint
the substrate with the image;
whereby to produce several differently colored images
of coagulated colloid which are transferred at the
respective transfer stations onto the substrate in
superimposed relation to provide a polychromic image.
In contrast to conventional dynamic and
static printing methods and apparatuses where a central
impression cylinder is used to convey a web to
planetary printing units for impression by respective
plate cylinders, the electrocoagulation printing method
and apparatus of the invention utilize a single
positive electrode on which dots of colored, coagulated
2156978
colloid are formed in sequence and the substrate which
is generally in the form of a web travels independently
of the positive electrode, from one printing unit to
another, so as to contact the colored, coagulated
S colloid in sequence. The invention enables one to
significantly improve the registration of the
differently colored images upon their transfer onto the
web or other substrate, thereby providing a polychromic
image of high definition.
The essence of the invention is of course not
limited to electrocoagulation printing, but also
extends to other dynamic printing techniques, such as
xerography, ionography and magnetography.
According to a broad aspect of the invention,
there is thus provided a multicolor dynamic printing
method comprising the steps of:
a) providing a single support member having a
continuous surface moving at substantially constant
speed along a predetermined path;
b) forming on the surface a colored image
with a printing ink containing a coloring agent;
c) bringing a substrate into contact with the
colored image to cause transfer of the image from the
surface onto the substrate and thereby imprint the
substrate with the image; and
d) repeating steps (b) and (c) several times
to define a corresponding number of printing stages
arranged at predetermined locations along the aforesaid
path and each using a coloring agent of different
color, and to thereby produce several differently
colored images which are transferred at respective
transfer positions onto the substrate in superimposed
relation to provide a polychromic image.
2156978
According to yet another broad aspect of the
invention, there is provided a multicolor dynamic
printing apparatus comprising:
- a single support member having a continuous surface;
- means for moving the surface at a substantially
constant speed along a predetermined path; and
- a plurality of printing units arranged at
predetermined locations along the path, each printing
unit comprising:
- means for forming on the surface a colored
image with a printing ink containing a coloring agent
of different color; and
- means for bringing a substrate into contact
with the colored image at a respective transfer station
to cause transfer of the image from the surface onto
the substrate and thereby imprint the substrate with
the image;
whereby to produce several differently colored images
which are transferred at the respective transfer
stations onto the substrate in superimposed relation to
provide a polychromic image.
Where the desired image is reproduced by
electrocoagulation of a colloid, the positive electrode
used 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 the
aforementioned US Patent No. 4,895,629. In later case,
the printing units are arranged around the positive
cylindrical electrode.
When use is made of a positive electrode of
cylindrical configuration rotating at substantially
constant speed about its central longitudinal axis,
step (b) of the above electrocoagulation printing
method is carried out by:
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i) providing a plurality of negative
electrolytically inert electrodes electrically
insulated from one another and arranged in rectilinear
alignment to define a series of corresponding negative
electrode active surfaces disposed in a plane parallel
to the longitudinal axis of the positive electrode and
spaced from the positive electrode active surface by a
constant predetermined gap, the negative electrodes
being spaced from one another by a distance at least
equal to the electrode gap;
ii) coating the positive electrode active
surface with an olefinic substance and a metal oxide to
form on the surface micro-droplets of olefinic
substance containing the metal oxide;
iii) filling the electrode gap with a
substantially liquid colloidal dispersion containing
the electrolytically coagulable colloid, the coloring
agent, a liquid dispersing medium and a soluble
electrolyte;
iv) electrically energizing selected ones of
the negative electrodes to cause point-by-point
selective coagulation and adherence of the colloid onto
the olefin and metal oxide-coated positive electrode
active surface opposite the electrode active surfaces
of the energized negative electrodes while the positive
electrode is rotating, thereby forming the dots of
colored, coagulated colloid; and
v) removing any remaining non-coagulated
colloid from the positive electrode active surface.
As explained in US Patent No. 4,895,629,
spacing of the negative electrodes from one another by
a distance which is equal to or greater than the
electrode gap prevents the negative electrodes from
undergoing edge corrosion. On the other hand, coating
~- ' 21~6978
of the positive electrode with an olefinic substance
and a metal oxide prior to electrical energization of
the negative electrodes weakens the adherence of 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.
Examples of suitable electrolytically inert
metals from which the positive and negative electrodes
can be made are stainless steel, platinum, chromium,
nickel and aluminum. The positive electrode is
preferably made of stainless steel or aluminum 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.
The gap which is defined between the positive
and negative electrodes can range from about 50 u to
about 100 u, the smaller the electrode gap the sharper
are the dots of coagulated colloid produced. Where the
electrode gap is of the order of 50 u, the negative
electrodes are the preferably spaced from one another
by a distance of about 75 ~.
Examples of suitable olefinic substances
which may be used to coat the surface of the positive
electrode 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. The olefinic
substance is advantageously applied onto the positive
'~ 2156978
electrode active surface in the form of an oily
dispersion containing the metal oxide as dispersed
phase. Examples of suitable metal oxides include
aluminum oxide, ceric oxide, chromium oxide, cupric
oxide, magnesium oxide, manganese oxide, titanium
dioxide and zinc oxide; chromium oxide is the preferred
metal oxide. Depending on the type of metal oxide used,
the amount of metal oxide may range from about 20 to
about 60% by weight, based on the total weight of the
dispersion. Preferably, the olefinic substance and the
metal oxide are present in the dispersion in
substantially equal amounts. A particularly preferred
dispersion contains about 50 wt.% of oleic acid or
linoleic acid and about 50 wt.% of chromium oxide.
The oily dispersion containing the olefinic
substance and the metal oxide is advantageously applied
onto the positive electrode active surface by providing
a distribution roller extending parallel to the
positive cylindrical electrode and having a peripheral
coating comprising an oxide ceramic material, applying
the oily dispersion onto the ceramic coating to form on
a surface thereof a film of the oily dispersion
uniformly covering the surface of the ceramic coating,
the film of oily dispersion breaking down into micro-
droplets containing the olefinic substance in admixturewith the metal oxide and having substantially uniform
size and distribution, and transferring the micro-
droplets from the ceramic coating onto the positive
electrode active surface. As explained in Applicantls
copending Canadian patent application No. 2,113,535
filed January 14, 1994, the use of a distribution
roller having a ceramic coating comprising an oxide
ceramic material enables one to form on a surface of
such a coating a film of the oily dispersion which
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2156978
uniformly covers the surface of the ceramic coating and
thereafter breaks down into micro-droplets containing
the olefinic substance in admixture with the metal
oxide and having substantially uniform size and
distribution. The micro-droplets formed on the surface
of the ceramic coating and transferred onto the
positive electrode active surface generally have a size
ranging from about 1 to about 5u.
A particularly preferred oxide ceramic
material forming the aforesaid ceramic coating
comprises a fused mixture alumina and titania. Such a
mixture may comprise about 60 to about 90 weight % of
alumina and about 10 to about 40 weight ~ of titania.
According to a preferred embodiment of the
invention, the oily dispersion is applied onto the
ceramic coating by disposing an applicator roller
parallel to the distribution roller and in pressure
contact engagement therewith to form a first nip, and
rotating the applicator roller and the distribution
roller in register while feeding the oily dispersion
into the first nip, whereby the oily dispersion upon
passing through the first nip forms a film uniformly
covering the surface of the ceramic coating. The micro-
droplets are advantageously transferred from the
distribution roller to the positive electrode by
disposing a transfer roller parallel to the
distribution roller and in contact engagement therewith
to form a second nip, positioning the transfer roller
in pressure contact engagement with the positive
electrode to form a third nip, and rotating the
transfer roller and the positive electrode in register
for transferring the micro-droplets from the
distribution roller to the transfer roller at the
second nip and thereafter transferring the micro-
'~ 2156978
droplets from the transfer roller to the positiveelectrode at the third nip.
Preferably, the applicator roller and the
transfer roller are each provided with a peripheral
covering of a resilient material which is resistant to
attack by the olefinic substance, such as a synthetic
rubber material. For example, use can be made of a
polyurethane having a Shore A hardness of about 50 to
about 70 in the case of the applicator roller, or a
Shore A hardness of about 60 to about 80 in the case of
the transfer roller.
In some instances, depending on the type of
olefinic substance used, Applicant has noted that the
film of oily dispersion only partially breaks down on
the surface of the ceramic coating into the desired
micro-droplets. Thus, in order to ensure that the film
of oily dispersion substantially completely breaks on
the ceramic coating into micro-droplets of olefinic
substance containing the metal oxide and having
substantially uniform size and distribution, step
~b)(ii) of the electrocoagulation printing method of
the invention is preferably carried out by providing
first and second distribution rollers extending
parallel to the positive cylindrical electrode and each
having a peripheral coating comprising an oxide ceramic
material, applying the oily dispersion onto the ceramic
coating of the first distribution roller to form on a
surface thereof a film of the oily dispersion uniformly
covering the surface of the ceramic coating, the film
of oily dispersion at least partially breaking down
into micro-droplets containing the olefinic substance
in admixture with the metal oxide and having
substantially uniform size and distribution,
transferring the at least partially broken film from
~ 2156978
the first distribution roller to the second
distribution roller so as to cause the film to
substantially completely break on the ceramic coating
of the second distribution roller into the desired
micro-droplets having substantially uniform size and
distribution, and transferring the micro-droplets from
the ceramic coating of the second distribution roller
onto the positive electrode active surface. Preferably,
the ceramic coatings of the first distribution roller
and the second distribution roller comprise the same
oxide ceramic material.
According to a preferred embodiment, the oily
dispersion is applied onto the ceramic coating of the
first distribution roller by disposing an applicator
roller parallel to the first distribution roller and in
pressure contact engagement therewith to form a first
nip, and rotating the applicator roller and the first
distribution roller in register while feeding the oily
dispersion into the first nip, whereby the oily
dispersion upon passing through the first nip forms a
film uniformly covering the surface of the ceramic
coating.
According to another preferred embodiment,
the at least partially broken film of oily dispersion
is transferred from the first distribution roller to
the second distribution roller and the micro-droplets
are transferred from the second distribution roller to
the positive electrode by disposing a first transfer
roller between the first distribution roller and the
second distribution roller in parallel relation
thereto, positioning the first transfer roller in
pressure contact engagement with the first distribution
roller to form a second nip and in contact engagement
with the second distribution roller to form a third
, ~ ~ 15~q78
nip, rotating the first distribution roller and the
first transfer roller in register for transferring the
at least partially broken film from the first
distribution roller to the first transfer roller at the
second nip, disposing a second transfer roller parallel
to the second distribution roller and in pressure
contact engagement therewith to form a fourth nip,
positioning the second transfer roller in pressure
contact engagement with the positive electrode to form
a fifth nip, and rotating the second distribution
roller, the second transfer roller and the positive
electrode in register for transferring the at least
partially broken film from the first transfer roller to
the second distribution roller at the third nip, then
transferring the micro-droplets from the second
distribution roller to the second transfer roller at
the fourth nip and thereafter transferring the micro-
droplets from the second transfer roller to the
positive electrode at the fifth nip.
Where the positive cylindrical electrode
extends vertically, step (b)(iii) of the above
electrocoagulation printing method is advantageously
carried out by continuously discharging the colloidal
dispersion onto the positive electrode active surface
from a fluid discharge means disposed adjacent the
electrode gap at a predetermined height relative to the
positive electrode and allowing the colloidal
dispersion to flow downwardly along the positive
electrode active surface, the colloidal dispersion
being thus carried by the positive electrode upon
rotation thereof to the electrode gap to fill same.
Preferably, excess colloidal dispersion flowing
downwardly off the positive electrode active surface is
- 14 -
9 7 g
,
collected and the collected colloidal dispersion is
recirculated back to the fluid discharge means.
The colloid generally used is a linear
colloid of high molecular weight, that is, one having a
molecular weight comprised between about 10,000 and
about 1,000,000, 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 anionic copolymer of acrylamide
and acrylic acid having a molecular weight of about
250,000 and sold by Cyanamid Inc. under the trade mark
ACCOSTRENGTH 86. The colloid is preferably used in an
amount of about 6.5 to about 12% by weight, and more
preferably in an amount of about 7% by weight, based on
the total weight of the colloidal dispersion. Water is
preferably used as the medium for dispersing the
colloid to provide the desired colloidal dispersion.
The colloidal dispersion 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. The
electrolyte is preferably used in an amount of about
6.5 to about 9% by weight, based on the total weight of
the dispersion. The coloring agent can be a dye or a
pigment. Examples of suitable dyes which may be used to
color the colloid are the water soluble dyes available
from HOECHST such a 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 V01 Pina for coloring in magenta and
_
6q7~
Anti-Halo Dye Oxonol Yellow N. Pina for coloring in
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 in yellow. A
dispersing agent is added for uniformly dispersing the
pigment into the dispersion. Examples of suitable
dispersing agents include the non-ionic dispersing
agent sold by ICI Canada Inc. under the trade mark
SOLSPERSE 27000. The pigment is preferably used in an
amount of about 6.5 to about 12% by weight, and the
dispersing agent in an amount of about 0.4 to about 6%
by weight, based on the total weight of the dispersion.
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 colloidal
dispersion, and the collected colloidal dispersion in
admixture with the collected non-coagulated colloid is
recirculated back to the aforesaid fluid discharge
means.
Further features and advantages of the
invention will become more readily apparent from the
following description of preferred embodiments 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
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~ lJ~q78
according to a preferred embodiment of-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;
Figure 3 is a view similar to Fig. 2, but
showing a different embodiment;
Figure 4 is a fragmentary perspective view of
the apparatus illustrated in Fig. 1, showing one of the
printing heads used for electrocoagulation of the
colloid; and
Figure 5 which is on the same sheet of
drawings as Fig. 2 is a fragmentary longitudinal view
of the printing head illustrated in Fig. 4.
Referring first to Fig. 1, there is
illustrated a multicoior electrocoagulation printing
apparatus comprising a central positive electrode 20 in
the form of a revolving cylinder and four identical
printing unlts 22 arranged around the cylindrical
electrode 20. In the embodiment shown, the first
printing unit 22A at the left of the figure is adapted
to print in yellow color, the second printing unit 22B
in magenta color, the third printing unit 22C in cyan
color and the fourth printing unit 22D in black color.
The cylindrical electrode 20 extends vertically and has
a shaft 24 which is driven by a motor (not shown) for
rotating the electrode about a vertical axis coincident
with the shaft 24. A substrate in the form of a
continuous web 26 is fed to the printing units for
being imprinted with differently colored images which
are transferred at respective transfer stations onto
the web in superimposed relation to provide a
polychromic image, the web 26 being guided to the
respective transfer stations by guide rollers 28.
q 7 ~
As best shown in Fig. 2, the printing units
22 each comprise a cleaning device 30 for cleaning the
surface 32 of the positive electrode 20, a positive
electrode coating device 34 for coating the surface 32
with an olefinic substance and a metal oxide, a
polishing brush 36 for polishing the olefin and metal
oxide-coated surface 32, a device 38 for discharging a
colloid onto the surface 32, a printing head 40
provided with negative electrodes 42 for
electrocoagulating the colloid to form on the positive
electrode surface 32 dots of colored, coagulated
colloid representative of a desired image and a soft
rubber squeegee 44 for removing any remaining non-
coagulated colloid from the surface 32. Each printing
unit 22 further includes a pressure roller 46 for
bringing the web 26 into contact with the dots of
colored, coagulated colloid to cause transfer of the
colored, coagulated colloid onto the web 26 and thereby
imprint the web with the image. As shown in Fig. 1, the
provision of two pairs of diametrically opposed
pressure rollers 46 arranged about the cylindrical
electrode 20 prevents the electrode 20 from flexing
since the forces exerted by the rollers 46 of each pair
cancel each other out.
The positive electrode cleaning devices 30
each comprise a rotating brush 48 and two high pressure
water injectors 50 arranged in a housing 52. Each brush
48 is provided with a plurality of radially extending
bristles 54 made of horsehair and having extremities
contacting the surface 32. Any coagulated colloid
remaining on the surface 32 after transfer of the dots
of colored, coagulated colloid at the transfer station
of a preceding printing unit is thus removed by the
- 18 -
A
q 7~
brush 48 and washed away by the powerful jets of water
produced by the injectors 50.
The positive electrode coating devices 34
each comprise a vertically extending distribution
roller 56, an applicator roller 58 extending parallel
to the distribution roller 56 and in pressure contact
engagement therewith to form a nip 60, and a transfer
roller 62 extending parallel to the roller 56 and in
contact engagement therewith to form a nip 64. The
transfer roller 62 is in pressure contact engagement
with the positive electrode 20 to form a nip 66 and
permit the roller 62 to be driven by the positive
electrode 20 upon rotation thereof. Each coating device
34 further includes a feeding device 68 for supplying
to the applicator roller 58 the olefinic substance in
the form of an oily dispersion containing the metal
oxide as dispersed phase.
The distribution roller 56 has a solid core
70 of metal provided with a peripheral coating 72 of
oxide ceramic material. A pair of stub shafts 74 (only
one shown) integral with the core 70 extends outwardly
from the extremities of the roller 56. The applicator
roller 58 and transfer roller 62 also have a solid core
76 of metal, but are provided with a peripheral
covering 78 of polyurethane. The rollers 56 and 58 are
rotated in register by means of a motor (not shown)
driving the shaft 74 of the distribution roller 56. The
drive from the motor rotates the distribution roller 56
in a counterclockwise manner, which in turn transmits a
clockwise rotation to the applicator roller 58.
The feeding device 68 is adapted to discharge
the oily dispersion onto the applicator roller 58 at an
upper portion thereof. The dispersion then flows
downwardly under gravity along the roller 58 and is
-- 19 --
r,A r
~ 15l q 1~
carried to the nip 60 by the roller 58 during rotation
thereof. The dispersion upon passing through the nip 60
forms a film uniformly covering the surface of the
ceramic coating 70 of the distribution roller 56, the
film breaking down into micro-droplets containing the
olefinic substance in admixture with the metal oxide
and having substantially uniform size and distribution.
The micro-droplets formed on the roller 56 are carried
by the latter to the nip 64 where they are transferred
onto the transfer roller 62. The micro-droplets are
then carried by the roller 62 to the nip 66 where they
are transferred onto the positive electrode 20.
The positive electrode coating device 34'
illustrated in Fig. 3 is similar to the device 34 shown
in Fig. 2, except there are two distribution rollers 56
and 56' with an additional transfer roller 62' arranged
therebetween. Such an arrangement ensures that the film
of oily dispersion formed on the distribution roller 56
substantially completely breaks down into the desired
micro-droplets prior to transfer onto the positive
electrode 20, should the film only partially break down
on the surface of the ceramic coating 72 of the
distribution roller 56. As shown, the transfer roller
62' extends parallel to the distribution rollers 56 and
56' and in pressure contact engagement with the roller
56 to form a nip 80 and permit the roller 62' to be
driven by the distribution roller 56 upon rotation
thereof, the transfer roller 62' being in contact
engagement with the distribution roller 56' to form a
nip 64'. The distribution roller 56, applicator roller
58 and transfer roller 62' thus rotate in register. The
second distribution roller 56', on the other hand, is
in pressure contact engagement with the transfer roller
62 to form a nip 82 and permit the roller 56' to be
- 20 -
156q7~
driven by the transfer roller 62 upon rotation thereof.
The distribution roller 56', transfer roller 62 and
positive electrode 20 thus rotate in register. Any
partially broken film of oily dispersion formed on the
surface of the ceramic coating 72 of the distribution
roller 56 is transferred from the roller 56 to the
transfer roller 62' at the nip 80 and thereafter
transferred from the roller 62' to the distribution
roller 56' at the nip 64', the film substantially
completely breaking down on the surface of the ceramic
coating 72 of the roller 56' into the desired micro-
droplets having substantially uniform size and
distribution. The micro-droplets of olefinic substance
containing the metal oxide are then transferred from
the roller 56' to the transfer roller 62 at the nip 82
and thereafter transferred from the roller 62 to the
positive electrode 20 at the nip 66.
The polishing brushes 36 used for polishing
the olefin and metal oxide-coated surface 32 of the
positive electrode 20 are similar to the brushes 48,
each brush 36 being provided with a plurality of
radially extending bristles 54 made of horsehair and
having extremities contacting the surface 32. The
friction caused by the bristles 54 contacting the
surface 32 upon rotation of the brush 36 has been found
to increase the adherence of the micro-droplets onto
the positive electrode surface 32.
As shown in Fig. 4, each printing head 40
comprises a cylindrical body 84 mounted between a pair
of upper and lower arms 86,86' which are pivotally
connected to a column 88 with bushings 90, for pivotal
movement of the printing head 40 between an operative
position (shown in Figs. 1, 2 and 3) whereat the
negative electrodes 42 are spaced from the positive
- 21 -
A
q 7~
-
electrode 20 by a constant predetermined gap 92 and a
cleaning position (shown in Fig. 4) whereat the
negative electrodes 42 are exposed to permit cleaning
thereof. The column 88 is mounted on a horizontal beam
94 provided with a metal reinforcing member 96, the
beam 94 being supported at a predetermined height by a
plurality of vertical beams 98 (only one shown). The
column 88 is fixed at its upper end to an attachment
arm 100 which is connected to the shaft 24 of the
electrode 20. A pair of collars 102,102l fixed to the
column 88 support the upper and lower arms 86 and 86',
respectively. The printing head 40 includes a pair of
stub shafts 104,104l extending through the arms 86 and
86l, respectively, bushings 106 being provided to
enable the body 84 to be rotated about a vertical axis
coincident with the shafts 104,104l and thereby
permitting a greater access to the negative electrodes
42 for cleaning same. A releasable locking mechanism
(not shown) is provided to secure the body 84 in the
desired position.
The negative electrodes 42 of each printing
head 40 are electrically insulated from one another and
arranged in rectilinear alignment along the length of
the body 84 to define a series of corresponding
negative electrode active surfaces 108, as best shown
in Fig. 5. In the operative position, the printing
head 40 is positioned relative to the positive
electrode 20 such that the surfaces 108 of the negative
electrodes 42 are disposed in a plane parallel to the
central longitudinal axis of the electrode 20 and are
spaced from the positive electrode surface 32 by the
gap 92. The electrodes 42 are also spaced from one
another by a distance at least equal to the electrode
~ ~A~
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gap 92 to prevent edge corrosion of the negative
electrodes.
The device 38 which is used to fill the
electrode gap 92 with a colloidal dispersion containing
an electrolytically coagulable colloid, a dispersing
medium, a soluble electrolyte and a coloring agent
comprises an elongated hollow body 110 defining a
container for receiving the colloidal dispersion and a
fluid discharge nozzle 112 at the lower end of the body
110 for continuously discharging the dispersion onto
the positive electrode surface 32. The body 110 is
fixed to the upper arm 86 such that when the printing
head 40 is in the working position, the nozzle 112 is
disposed adjacent the electrode gap 92 at a
predetermined height relative to the positive electrode
20. As the colloidal dispersion is being discharged
from the nozzle 112 onto the positive electrode surface
32, it flows downwardly along the surface 32 and is
carried by the positive electrode 30 upon rotation
thereof to the electrode gap 92 to fill same. Excess
colloidal dispersion flowing downwardly off the surface
32 is collected in a trough 114 which is connected by
conduit 116 to a reservoir 118. A recirculation pump
120 is connected to the reservoir 118 for recirculating
the collected dispersion back to the device 38 through
conduit 122. The trough 114 has an arcuate outer wall
124 adapted to be contacted by a stop member 126 fixed
to the lower arm 86' when the printing head is moved to
the operative position, for providing the desired
electrode gap 92. A similar stop member 126 is fixed to
the upper arm 86 for contact engagement with an
abutment member ~not shown) disposed above the
electrode 20.
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.
Electrical energizing of selected ones of the
negative electrodes 42 causes point-by-point selective
coagulation and adherence of the colloid onto the
olefin and metal oxide-coated surface 32 of the
positive electrode 20 opposite the electrode active
surfaces 108 of the energized negative electrodes 42
while the electrode 20 is rotating, thereby forming a
series of corresponding dots of colored, coagulated
colloid representative of a desired image. After
electrocoagulation of the colloid, any remaining non-
coagulated colloid is removed from the positive
electrode surface 32 by the squeegee 44 so as to fully
uncover the dots of colored, coagulated colloid adhered
on the surface 32. Any non-coagulated colloid removed
by the squeegee 44 is collected in the trough 114,
mixed with excess colloidal dispersion in the reservoir
118 and the collected non-coagulated colloid in
admixture with the excess colloidal dispersion is
recirculated back to the device 38 by the pump 120, for
discharge onto the positive electrode surface 32.
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 42. Synchronisation
of the data furnished to the printing heads 40 is
ensured by proper electronic circuitry (not shown).
The pressure rollers 46 which serve to bring
the web 26 into contact with the dots of colored,
coagulated colloid at the respective transfer stations
are each in pressure contact engagement with the
positive electrode 20 to form a nip 128 through which
the web 26 is passed and permit the rollers 46 to be
driven by the positive electrode 20 upon rotation
thereof. As the web 26 is contacted with the dots of
- 24 -
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~ 1 56q 7 ~
colored, coagulated colloid, the colored, coagulated
colloid is transferred onto the web 26 to thereby
imprint same with the image. The differently colored
images produced by the printing units 22A, 22B, 22C and
22D are thus transferred onto the web 26 in
superimposed relation to provide a polychromic image.
Since a single positive electrode 20 is used and the
web 26 contacts only the positive electrode surface 32
upon passing through the respective nip 128 of each
transfer station, a polychromic image of high
definition is obtained.
A~
