Note: Descriptions are shown in the official language in which they were submitted.
WO 95/05900 PCT/US94/07358
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1~TON-RECIRCULATING DIE SUPPLIED DOCTORED ROLL COATER
WITB SOLVENT ADDITION
TECHNICAL FIELD
The present invention relates to roll
coaters. More particularly, the present invention
relates to roll coaters without fluid recycling.
BACKGROUND pF THE INVENTION
The production of composite materials by the
coating of layers of fluid substances onto solid
substrates and solidifying the layers by drying or
curing is well known. Composite layered materials
formed by such coating processes are especially useful
as information recording media.
An important requirement for layers of
information recording media is uniformity. Non-
uniformities are defects which can lead to improper
information recording or retrieval. When information
recording layers are formed by coating fluids, a
common source of coating non-uniformity is the.
presence of oversized particulates in the coating
fluid. These oversized particulates result in defects
in the recording layer, and can be the result of
contamination from outside the system, but are more
commonly dried dispersion clumps formed within the
coating fluid during coating. A common source of
clumps is premature solidification of portions of the
coating fluid in localized areas of the coating
apparatus before the coating step.
Coating fluids used to produce magnetic
recording layers, called magnetic recording fluids,
typically include fine particles of magnetic
materials, called magnetic pigments, dispersed in a
liquid binder of polymeric materials, solvents,
reactants, and catalysts. The liquid binder is
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formulated to solidify into a matrix which binds the
pigment into a durable layer suitable for magnetic
recording. Combinations of catalysts and other
ingredients, called activators, initiate and sustain
crosslinking or polymerization reactions during
solidification. The properties of the resulting
magnetic recording layer may be enhanced by additives
such as lubricants, plasticizers, and antistatic
agents.
The solidification of magnetic coating
fluids into magnetic recording layers typically occurs
first by evaporation of the solvent, then by chemical
curing reactions such as crosslinking or
polymerization. Solvent removal is initiated by
coating the fluid as a thin film, since this greatly
increases the surface area available for solvent
evaporation. Curing is typically completed by the
application of heat over time, which accelerates
solvent evaporation and increases the rates of
crosslinking and polymerization. Other forms of
energy, such as ultraviolet light or electron beams,
can promote crosslinking or polymerization.
While open reservoir coating apparatus are
especially prone to localized premature drying of the
coating fluid, these apparatus have many advantages
which either outweigh the drying problem or provide
great incentive to find solutions to it. Two typical
types of reservoirs are the pan and the trough. In
the pan type reservoir, shown in Figure 1, a receiving
roller, such as a gravure cylinder 10, is immersed in
a coating fluid 12 in a reservoir 14. The gravure
cylinder 10 rotates and carries a layer of coating
fluid to a doctor blade 16, which contacts the gravure
cylinder 10 and wipes off the excess fluid, leaving
the remaining fluid on the gravure cylinder 10 to be
carried to a substrate 18 on which it is coated. The
substrate 18 is held close to the gravure cylinder 10
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by a backup roller 20.
The amount of fluid 12 carried by the
gravure cylinder 10 to the substrate 18 can be
governed by providing small fluid-holding pits or
grooves, called cells 32, in the outer surface of the
gravure cylinder 10. By applying an abundance of
coating fluid 12 to the gravure cylinder 10 and then
wiping off the excess fluid with the doctor blade 16,
the fullness of the cells 32 is controlled. Each cell
32 acts as a measuring cup so that the rate of coating
fluid 12 application is closely controlled in both the
downweb and crossweb directions.
Alternatively, smooth rollers, without
cells, can be used and the coating thickness is
controlled with a roller or a doctor blade. The
roller or doctor blade is spaced a small distance from
the surface of the roller to provide an accurately
controlled gap for a layer of coating fluid to be
carried by the surface of the coating roller.
Transfer of the coating fluid to the substrate is
similar to that found in celled gravure coating.
Typical examples of controlling the application of
coating fluid in this manner can be found in U.S.
Patent Nos. 4,864,930; 4,581,994; and 4,534,290.
Alternatively, in offset roll coating, the coating
fluid first is transferred to intermediate rollers and
then is transferred to the substrate. Pan type
reservoirs expose a large area of fluid to the air
where solvent removal occurs, causing premature fluid
drying. Pan systems also typically have stagnant
areas near the pan walls where fluid residence time is
long, resulting in further loss of solvent.
Referring to Figure 2, trough type systems
minimize the area of fluid that contacts the air and
reduces fluid residence time. A suitable fluid level
is maintained in the trough 22 by supplying excess
coating fluid 12 to the reservoir 15 and providing an
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opening 28 in the reservoir wall at the height of the
desired fluid level. Coating fluid fills the trough
22 to this level and overflow through a tube 30,
returning to the reservoir 15.
In this configuration, the trough 22 is
supplied with coating fluid 12 by a pump 24, which
feeds the fluid from a reservoir 15 through a filter
26. The coating fluid 12 supplies a gravure cylinder
10, while the surface of the gravure cylinder 10 moves
downwardly past the doctor blade 16. The coating
fluid is then transferred to the substrate 18 by
contact between the substrate 18 and the coating fluid
12 on the gravure cylinder 10, which is maintained by
a backup roller 20. The excess fluid returns to the
reservoir 15. Many trough systems use a top seal 34,
shown in Figure 2, which contacts the roller so the
air volume above the fluid becomes saturated with
solvent. These systems work only if the fluid
remaining on the gravure roller does not contaminate
the top seal. Many fluids dry on the seal causing
flaws. If the system is run without the top seal or
with the seal close to the supply roller, the air
above the pool does not become saturated with solvent
and the fluid in the reservoir dries.
An area in which this is particularly likely
to occur is the region above the wetting line between
the wall of the reservoir and the free surface of the
coating fluid. Since the level of coating fluid in
the reservoir is constantly changing, thin films of
fluid can form on the reservoir walls as the fluid
drains, due to dynamic wetting effects. This film can
sometimes dry before the fluid level rises again, due
to the increased rate of solvent evaporation brought
about by the high surface area to volume ratios found
in thin liquid films. When the fluid level rises and
falls again, another layer of coating fluid is
deposited over the first, adding to the thickness of
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the solid layer formed. The solidified areas can
eventually break off the walls, mix with the coating
fluid, and find their way onto the substrate, where
they show up as flaws.
Trough type systems also typically use an
overflow system which requires costly and complicated
filtering of a catalyzed fluid. Overflow systems that
use the top seals increase the pressure in the system
and increase the likelihood of end seal leaks. A
further problem caused by overflow and recirculation
systems arises from the need to crosslink the polymers
in the coating fluid. Since the effects of catalysts
and other crosslinking agents are often cumulative
over time, any eddies or areas of stagnation increase
the average residence time in the fluid, thereby
increasing the likelihood of premature chemical
solidification. Furthermore, since clumps formed from
catalyzed coating fluids often involve crosslinking or
polymerization reactions, they are not likely to be
redissolved by the coating fluid solvents. An
additional problem which can occur is filter clogging
since recirculating, clump-laden coating fluid
typically passes through a filter, requiring more
frequent cleaning or replacement of the filters.
Eliminating overflow and recirculation reduces
premature solidification by reducing fluid surface
area and eliminates the need for recirculation and
filtration of catalyzed fluids.
Coating apparatus which use fluid reservoirs
and receiving rollers but which supply coating fluid
to the reservoir without overflow and recirculation
are known. A typical method for accomplishing this is
to provide a fluid level sensor in the reservoir which
feeds a signal back to the fluid supply source to
control the rate of supply to the reservoir. U.S.
Patent No. 3,730,089 discloses a mechanical device
which senses the size of the rotating vortex of ink in
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a printing press reservoir, thereby providing an
indication of the ink level. The vortex sensor is
mechanically coupled to an ink supply to provide
additional ink as needed. However, this type of level
sensor introduces additional surfaces on which dynamic
wetting effects can occur, thereby increasing the rate
of clump formation. A different ink level sensor is
disclosed in U.S. Patent No. 4,284,005 which measures
the ink level in the reservoir by a capacitive device
which senses the vertical location of the surface of
the ink in the reservoir and sends an appropriate
signal to the ink supply source. These capacitive
devices are affected by nearby metal and are
unreliable as it is hard to keep clean electrical
connections.
Another approach to liquid depth measurement
is to measure the hydrostatic pressure at the bottom
of the liquid layer, such as with a bubble tube.
Bubble tubes measure hydrostatic pressure in a liquid
using a small tube which is connected to a pressurized
supply of air or gas, called the test gas, placed in
the liquid where the hydrostatic pressure is to be
measured. The flow rate of the test gas is adjusted
until a stream of bubbles form at the end of the tube.
Measuring the gas pressure yields the hydrostatic
liquid pressure, which provides an indication of the .
liquid level. Various types of bubble tubes and ,
bubble tube improvements are disclosed in U.S. Patent
Nos. 2,668,438; 2,755,669; and 4,719,799. Bubble
tubes and the gas supply systems needed to operate
them are commercially available. A problem which
sometimes arises when bubble tubes are used with
solutions of solids in a volatile solvent is that
premature solvent removal and solidification of the
solution can occur on the surface of the tube. As
bubbles exit the tube, the tube inside diameter
becomes coated unless the test gas is solvent
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saturated. This can lead to clump formation and tube
clogging.
One requirement for trough-supplied rotating
rollers are reliable seals between the rotating
surface of the receiving roller and the stationary
reservoir. Many known end seal configurations are
suitable for printing ink or other similar fluids.
However, none can adequately withstand the high level
of abrasiveness and exposure to solvents encountered
in magnetic recording coating fluids and similar -
fluids. One method of sealing the ends of trough
reservoirs, is disclosed in U.S. Patent No. 4,945,832,
includes sealing against the curved peripheral areas
near each end of the receiving roller. The seal is
made of closed cell silicone foam and provides
sufficient flexibility to maintain contact with both
the roller and the trough, while permitting the
distance between the doctor blade and the roller to be
adjusted. An additional seal contacts the end of the
roller. However, when used with magnetic pigments,
these seals are subject to severe wear and leakage.
Additionally, wear products fall into the coating
solution.
Coating apparatus which do not use
reservoirs also are known. U.S. Patent No. 4,332,840
discloses a variety of methods for dispensing coating
fluid through a slot, called an extrusion bar, which
extends across either the moving substrate or a
receiving roller. The coating apparatus also includes
additional rollers, doctor blades, or other metering
devices. These devices for liquid delivery to
rotating rollers involve some form of recirculation
due to excess supply of the coating fluid.
During the coating process, coating must be
stopped for periods of time ranging from minutes to
hours. A typical cause for such stoppage might be web
breaks, or malfunction of some part of the system. On
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such occasions, it is preferred to maintain the coating
system in a standby mode, or idling state, to resume coating
quickly when desired. It is also preferred to maintain the
coating system in an idling state during the start of a
coating run, since many adjustments and other tasks must be
performed as part of setting up the coating process.
However, if the coating apparatus is left idling for more
than a few minutes, volatile solvents can evaporate from the
reservoir, leading to excessive increases in viscosity and
premature solidification of the coating fluid. This results
in agglomerate formation and increased occurrence of coating
defects when coating is resumed.
There is a need to provide a coating apparatus
reservoir and a system for supplying coating fluid to the
reservoir which reduces flaws in the coated product by
reducing conditions leading to premature solidification due
to solvent removal. There is a need for providing a system
for sealing the ends of the reservoir to the roller which is
less subject to wear and leakage than known devices. There
also is a need to match the usage rate to the supply rate to
the reservoir by eliminating the need for recirculation and
to prevent defects from arising due to idle periods.
SUMMARY OF THE INVENTION
An aspect of the present invention provides an
apparatus for coating a fluid onto a substrate.
In one major embodiment of this aspect, the
apparatus comprises:
a coating die having a coating slot, a supply
manifold from which the fluid enters the coating slot,
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feed openings into the supply manifold, and an external wall
along which the fluid flows after exiting through the slot;
a rotatable receiving roller which is located
adjacent the coating die and which applies the fluid onto
the substrate;
a reservoir located between the coating die and
the receiving roller for receiving the fluid flowing along
the coating die, which fluid is applied onto the receiving
roller;
a doctor blade for regulating the amount of the
fluid applied to the receiving roller from the reservoir;
and
means for compensating for solvent evaporation
during idling of the apparatus by removing the fluid from
the reservoir and replacing it with a suitable solvent.
In this embodiment, the apparatus preferably
further comprises means for loading the doctor blade against
the receiving roller.
In another major embodiment, the apparatus
comprises:
an apparatus for coating an amount of a fluid onto
a substrate, comprising:
a coating die having a die slot, a supply chamber
from which the fluid enters the die slot, and an external
wall along which the fluid flows after exiting the die slot;
a rotatable receiving roller disposed adjacent the
coating die and further disposed to apply the fluid to the
substrate; and
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a doctor blade disposed proximate the receiving
roller to regulate the amount of the fluid applied to the
receiving roller;
wherein the coating die, the receiving roller, and
the doctor blade form a reservoir that receives the fluid
flowing along the external wall.
In this embodiment, preferably the doctor blade is
disposed at the bottom of the reservoir. Also preferably,
the receiving roller comprises a gravure cylinder. Also
preferably, the reservoir further comprises first and second
ends and the apparatus further comprises first and second
end seals that seal the first and second ends of the
reservoir, respectively. Further preferably, the apparatus
further comprises means for loading the doctor blade against
the receiving roller.
An aspect of the present invention provides a
compensating apparatus which compensates for solvent
evaporation during idling of such a coating apparatus. The
compensating apparatus comprises:
a pump connected near a bottom of the reservoir to
enable substantially all of the fluid to be pumped from the
reservoir, and
means for adding a solvent to the reservoir,
wherein the adding means comprises means for controlling a
solvent level in the reservoir to add a sufficient solvent
to the reservoir to bring the solvent to a level higher than
that used for the fluid to cover an exit of the die slot.
Coating fluid is supplied from a die to a
reservoir to a rotatable receiving roller, preferably
comprising a rotating gravure cylinder. The rate of
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supplying fluid to the reservoir is at least equal to the
rate of consumption of the coating fluid by coating onto the
substrate. The hydrodynamic design of the coating fluid
flow path reduces premature solidification and agglomerate
formation in the reservoir. The rotating interface between
the ends of the reservoir and the
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gravure cylinder is simply and reliably sealed.
The apparatus includes a coating die having
a coating slot and an external wall along which the
fluid flows after exiting through the slot. The
external wall includes a spillway adjacent the coating
slot opening and the coating die also includes a
supply chamber which feeds fluid into the coating
slot. A rotatable coating cylinder is located
adjacent the coating die to coat the fluid onto the
substrate. A reservoir is located between the coating
die and the coating cylinder. The reservoir receives
the fluid flowing along the coating die before the
fluid is applied onto the coating cylinder. The
reservoir includes first and second ends, a front
surface formed by the external wall of the coating
die, a rear surface formed by the surface of the
coating cylinder, and a bottom surface formed by the
doctor blade. The first and second ends are sealed
against the rear surface. The doctor blade regulates
the amount of liquid applied to the gravure cylinder
from the reservoir. The apparatus also includes a
system for measuring and controlling the depth of the
fluid in the reservoir at a predetermined level.
The apparatus also includes a system which
loads the doctor blade against the receiving roller.
The loading system can repeatedly apply a precise
force to the doctor blade to locate precisely the
doctor blade relative to the receiving roller by
moving the doctor blade in only a horizontal,
translational direction without requiring any angled
or side-to-side adjustments.
End seals seal the ends of the die and the
ends of the reservoir. The end seals can be loaded to
maintain the end seals in position adjacent the ends
of the die to compensate for relaxation of the seals
over time. Also, the end seals can each include a pad
portion which seals against the doctor blade, and a
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wear plate portion which seals against the receiving
roller. The wear plate portion prevents the pad
portion from contacting and wearing against the
receiving roller.
In addition, the coating fluid in the
reservoir can be replaced quickly and conveniently
with a suitable solvent, when coating is not being
performed. This reduces the occurrence of coating
defects arising from excessive evaporation of solvent
and premature solidification of coating fluid.
The invention also encompasses a method for
coating a fluid onto a substrate including extruding
the fluid through a coating die slot, causing the
fluid to flow from the die into a reservoir, applying
the fluid onto a coating cylinder, and applying the
fluid from the coating cylinder onto the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic view of a known pan
roll coater.
Figure 2 is a schematic view of a known
trough roll coater.
Figure 3 is a schematic view of the coating
system of the present invention.
Figure 4 is an exploded view of the end
seals of the present invention.
Figure 5 is an assembled view of the end
seals of Figure 4.
Figure 6 is an exploded view of the end
seals according to another embodiment of the
invention.
Figure 7 is an exploded view of the end
seals according to another embodiment of the .
invention.
Figure 8 is a schematic view of the loading
system of the present invention.
Figures 9A, 9B, 9C, and 9D are perspective
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views of dies according to alternative embodiments of
the present invention.
Figure 10 is a schematic view of the coating
system according to another embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMf30DIMENTS
Figures 3 and 8 show the coating system of
the present invention. Coating fluid 12 is supplied
to a die 42 at a rate substantially equal to the rate
at which it is applied to a moving substrate such as a
web 44. The web 44 can be a flexible film suitable
for magnetic recording tape or diskettes. The
direction in which the web is transported is the
downweb direction and the direction in the plane of
the web perpendicular to the downweb direction is the
crossweb direction.
Coating fluid 12 is extruded from and flows
along one side of the die 42 to a reservoir 46. A
reservoir cover 53 mounted on the die 42 limits fluid
drying. A receiving roller, shown as a gravure
cylinder 48, receives the coating fluid 12 from the
bottom of the reservoir 46. A doctor blade 50 forms
the bottom of the reservoir 46, and wipes off excess
fluid from the surface of the gravure cylinder 48.
The coating fluid 12 is coated from the gravure
cylinder 48 onto the web 44 which passes around a
backup roller 52.
This system is a rotating roller type coater
in which the coating fluid is supplied to the rotating
roller from a die, rather than from an open pan or
trough. The system cleanly and uniformly applies and
doctors fluid onto a roller to coat a web without
recycling excess fluid. This system can handle a wide
range of fluids including inks, adhesives, abrasives,
and non-self-cleaning fluids such as magnetic
dispersions for magnetic media products.
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By combining the feeding and metering steps,
the system reduces the number of critical hardware
components and process variables required for roll
coating. This improves the coating uniformity,
reduces waste, and makes it easier to find the source
of any coating problems. The system reduces the
amount of time and waste produced during equipment '
start-up. Also, cleaning the critical coating head
components can be completed with little effort. Also,
this system enables the reduction or elimination of
filtering after adding catalysts. This extends the
filter life and reduce filtration related costs.
The die 42 is easy to fabricate from readily
available materials such as aircraft grade aluminum.
The die halves 60, 62 can be held together without
stress to avoid warpage by bolting them together at a
mating interface 64. The die 42 ends can be enclosed
and sealed as explained below. Removable bolts
facilitate cleaning. The die 42 includes a die bore
or supply chamber 54 which holds coating fluid 12
provided through a supply tube 56. An infinite
manifold die can be used in combination with gravity
leveling of the reservoir 46 to improve uniformity for
high viscosity fluids without the need to recirculate
fluid through a chamber.
The coating fluid is fed from the supply
chamber 54 through the die slot 58, where it exits and
flows in a continuous film over a spillway 66, and
then along a side wall 68 of the die 42 to the
reservoir 46. The spillway 66 and the side wall 68
need not be separate parts divided by a sharp corner,
as shown in Figure 3. The spillway and the side wall
can be merged into a single straight or curved '
surface. The continuity of the film on the side wall
68 as it merges with the fluid 12 in the reservoir 46
eliminates dynamic wetting effects which might
otherwise occur between the fluid in the reservoir and
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the side wall.
The crossweb length of the die 42 should be
slightly greater than the coated width of the web 44.
If this length is much greater, stagnant areas of
coating fluid could collect in the ends of the
reservoir 46, causing coating flaws.
The die slot 58 is the full length of the
die to improve slot dimensional uniformity and flow
uniformity. For use with magnetic coating fluids, the
thickness t of the die slot 58 is approximately 0.051
cm (0.020 in). To compensate for crossweb differences
in coating fluid requirements if the fluid tends to be
differentially distributed in the pool, the die 42 can
be modified. 'The die slot length and thickness can be
varied, the die bore size can be varied, and the die
feed locations can be selected to yield a level
reservoir and a more uniform coating caliper, as shown
in Figures 9A, 9B, 9C, and 9D, respectively.
The die slot 58 exit is above the highest
level of the reservoir 46 and is vertical to prevent
stagnation and drying of the reservoir surface and
slot wetting. A vertical die orientation also
decreases the system weight, allows air to purge from
the die and fluid, and allows the flow to be viewed
and the slot cleaned. Alternatively, the side wall 68
can slope downwardly at another angle, and the die
slot 58 can be horizontal, or at any intermediate
angle, as long as the exit of the die slot 58 is above
the fluid level in the reservoir 46, and an open solid
surface supports a continuous film of fluid flowing
from the die slot 58 to the reservoir 46. This
requirement is for fluids that are not self-cleaning
and have drying problems.
The top of the reservoir 46 is confined by
gravity and the reservoir shape is designed so that
the reservoir surface is in constant motion to avoid
drying effects. A rectangular reservoir is easy to
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build and maintain and provides a low fluid velocity
area for level sensing. A wider reservoir also works
but the reservoir level should be higher to avoid
complex pool flows. Larger reservoir widths make ,
level control easier because height changes less with
flow rate changes. Lower viscosity fluids work well
over a wider range of reservoir heights due to
increased gravity leveling flow.
The reservoir fluid level should be kept
relatively constant to prevent running with a low
reservoir, starving the doctor blade, and forming
coating voids. The flow rate of coating fluid to the
die 42 and the reservoir 46 level are controlled by
sensing the level of fluid in the reservoir 46
adjacent the gravure cylinder 48 using a static
pressure sensor.
The sensing tip of the sensor is positioned
in a low velocity area, such as just above the doctor
blade and flush with the die surface. The sensor is
an input to a closed loop control metering system to
prevent dynamic pressure effects from causing errors
in the reservoir depth reading. A signal representing
the fluid level is fed back to a pump controller,
which provides feedback to a pump to adjust the rate
of pumping to change the level of fluid in the
reservoir to bring it toward a predetermined level.
Suitable pump controllers are commercially available.
The fluid level sensor can be a modified bubble tube
type sensor. By bubbling the test gas through a layer
of solvent before injecting it into the bubble tube,
solvent vapor can be introduced into the gas. This
greatly reduces the drying of coating fluid in the
region near the bubbles, thereby reducing clump
formation and tube clogging. Other sensors also can
be used.
Other hydrodynamic features of the present
invention are the width w and fluid depth h of the
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reservoir 46. During coating, since the surface of
the gravure cylinder 48 moves downwardly past the
reservoir 46, fluid near the gravure cylinder 48 in
the reservoir 46 tends to be carried downwardly due to
viscous shear effects. As the fluid reaches the
doctor blade 50, the portion of the downwardly moving
fluid which is not able to continue with the gravure
cylinder 48 must turn and flow along the bottom of the
reservoir 46 and away from the gravure cylinder 48,
thereby creating eddy flow. This flow prevents
localized drying and scumming at the edges of the
reservoir and prevents clumping.
It is possible for two or more eddies to
occur in the reservoir 46, if the fluid depth h is
very large in comparison with width w. For a given
set of fluid rheological properties and surface
speeds, the size and shape of the reservoir can be
selected to achieve a minimum of multiple eddying and
unstable flow within the reservoir. For example, for
many magnetic fluids, a width to height ratio of from
0.6 - 0.8 is desirable.
The ends of the reservoir 46 provide a
liquid-containing interface with the coating cylinder
48. One end sealing method, as shown in Figures 4 and
5, includes a circular or partially circular puck 70
mounted at each end of the gravure cylinder 48. The
pucks 70 can be ultra high molecular weight
polyethylene, acetal, or nylon and act as stationary
cylinder extensions which make the dynamic seal. The
pucks 70 are mounted to a frame, which also holds the
gravure cylinder 48, via a slide arrangement which
allows the pucks to be aligned with the gravure
cylinder 48. Alignment can be adjusted independently
of die location. Arms, mounted to the slides, contain
springs or air cylinders which load the pucks 70
against the end of the gravure cylinder 48.
The ends of the die supply chamber 54 and
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reservoir 46 can be sealed with foam seals 75 held by
end plates 76. The foam seals 75 can be a closed cell
polyethylene foam which conforms well to the doctor
blade 50, and is chemically inert to commonly used
coating fluids. The doctor blade 50 can be longer
than the grawre cylinder 48 and penetrate into the
foam seals 75 either by placing it in pre-cut foam or
by cutting the foam with the doctor blade 50. After
the die 42, doctor blade 50, foam seal 75, and end
plates 76 are assembled together, the assembly is
loaded against the grawre cylinder 48 and the pucks
70. The doctor blade 50, the grawre cylinder 48, and
the pucks 70 form the lower seal. As the die assembly
is loaded against the pucks, the foam seal is pinched
between the doctor blade 50 and the pucks 70 forming a
static seal. Alternatively, the end of the die supply
chamber 54 can be sealed with an intermediate plate
and a gasket or sealant to make this seal independent
of the pool seal.
Another sealing method works best when die
mounted. One configuration of this sealing method is
shown in Figure 6. The die supply chamber 54 is
sealed with an end plate 80 and a foam seal 82 or a
sealant. Shoulder bolts mount and locate a keyhole
mounting block 84, the end plate 80, and the foam seal
82 to the die 42. The end plate 80 thickness permits
its outer surface to be flush and coplanar with the
grawre cylinder end. Alternatively, the end plate- -
to-end plate length is up to 0.127 cm (0.050 in) and
preferably 0.025 cm (0.010 in) shorter than the
grawre cylinder length. The doctor blade 50 is
clamped between the die body and a die clamp 100.
Both the doctor blade 50 and the die clamp 100 are
longer than the grawre cylinder 48, preferably by at
least 5 cm (2 in).
A thin [0.127 - 0.025 cm (.010 - .050 in)]
ultra-high molecular weight polyethylene wear plate 86
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is slid on the end of the mounting block 84 as shown.
The wear plate 86 is configured to seal between the
end of the gravure cylinder 48 and the outer surface
of the end plate 80. The wear plate 86 barely clears
the top and front of the deflected doctor blade 50. A
foam pad 88 is inserted in a plastic pad holder 90 and
this assembly is slid on the mounting block 84. The
wear plate 86 protects the foam pad 88 from the
gravure cylinder 48. The foam pad 88 is received in a
slat 92 in the pad holder 90, and the wear plate 86
and the pad holder 90 have openings 94, 96 which
dovetail onto a complementarily-shaped portion 98 of
the mounting block 84. Other shapes and fastening
devices also can be used. The pad holder 90 has a pin
102 on which the tail of the foam pad 88 is slid on to
pull the foam pad around the deflected doctor blade
50.
An air cylinder 104 with a seal spool 106
loads the foam pad 88 and the wear plate 86 against
the end of the gravure cylinder 48 and the end plate
80 of the die assembly. The spool 106 is inserted
through the mounting block 84 then is moved down and
locked to the die assembly. Then the air cylinder 104
can be loaded to push the pad holder 90, foam pad 88,
and wear plate 86 against the gravure cylinder 48 and
the die assembly. The foam pad 88 softly holds the
wear plate 86 against the end of the gravure cylinder
48 forming the majority of the dynamic seal. The foam
pad 88 also fills the gap between the doctor blade 50,
the wear plate 86, and the gravure cylinder 46 to
complete the seal. A sealant can be used in addition
to the foam pad 88 to improve the seal at this triple
contact point. This configuration has a very small
effect on the measured doctor blade-force read-out.
A similar seal configuration shown in Figure
7 uses a die clamp 100' that is about as long as the
gravure cylinder 48 and a doctor blade 50 that is
2167923
WO 95/05900 PCT/US94/07358
-18-
about 0.32 cm (0.125 in.) longer than the gravure
cylinder 48. The foam pad 108 and pad holder 110 are
modified so the foam seals against the end of the
doctor blade 50 instead of on the top of it. This
configuration is very reliable even without using
additional sealant. It also has a negligibly small _
effect on the doctor blade-force read-out.
Separating the reservoir seal from the bore
seal enables the seals to be changed without changing
the die assembly. Also, the foam seals are shielded
and prevented from contacting the gravure cylinder 48.
For example, in the embodiment of Figures 6 and 7, the
foam pad 88 is shielded from and prevented from
contacting the gravure cylinder 48 by the plastic wear
plate 86.
If the fluid is abrasive, a gravure cylinder
end scraper could be used to improve seal life. The
scraper can be air loaded.
The doctor blade 50 is pressed against the
gravure cylinder 48 by a loading system including a
holding device 112. As shown in Figure 8, the holding
device 112 uses a force transducer 114 and a screw
slide 116 to press the doctor blade 50 against the
gravure cylinder 48 with a predetermined force. The
force is sensed by the transducer 114 and a motor 118
rotates the screw 120 of the screw slide 116 to move
the slide 122 of the screw slide 116 and thereby to
move the die 42 to the desired position. The holding
device 112 can include a ball slide 124 on which the
die assembly is mounted. Two ball slides 124, one
located near each end of the die 42, are typically
used to provide equal force at both ends of the doctor
blade 50, while the force transducer 114 can be .
located generally centrally of the die 42.
Alternatively, other types of slides can be used.
This loading system is repeatable, accurate,
and allows for automatic loading based only on
WO 95/05900 216 7 9 2 3 I'CT~S94/07358
-19-
doctoring force using a feedback system 126. Loading
can be preset and applied precisely. The system
allows rapid replacement of a used die and repeatable
die location, and the dies and loading system are
inexpensive and easy to assemble and maintain. Using
a horizontal slide loading system obtains quantifiable
process set-up and enables gravity forces to be
ignored, unlike pivoting systems. If a force sensor
is located between the loading device and the table
then the doctoring force can be directly read without
converting air pressure to force or calculating the
mechanical advantage of linkage devices. Also, air
cylinder drag has no effect on the doctoring force
reading as it does with known pivoting air cylinder
devices.
During the coating process, when coating
must be stopped, it is preferred to maintain the
coating system in an idling state to be able to resume
coating quickly. It is also preferred to maintain the
coating system in an idling state during the start of
a coating run, since many adjustments and other tasks
must be performed as part of setting up the coating
process. However, it is not desirable to completely
remove the coating fluid from the coating system, as
might be done during cleanup of the apparatus after
completion of coating. On the other hand, if the
coating apparatus is left idling for more than a few
minutes, volatile solvents can evaporate from the
reservoir, leading to excessive increases in viscosity
and premature solidification of the coating fluid,
resulting in increased coating defects. Because even
a very small amount of thickening or agglomeration of
the coating fluid can produce defects which last for a
long time after beginning coating, it is highly
desirable to avoid this occurrence. Additionally, it
has been found, contrary to conventional belief, that
merely adding solvent to the reservoir to make up for
WO 95/05900 ~ PCTIUS94107358
-2 0-
that lost by evaporation is insufficient to maintain
the desired viscosity of the coating fluid.
Defects due to solvent evaporation during
the idling state can be greatly reduced by removing
the coating fluid 12 from the reservoir 46 and
replacing it with a suitable solvent. It is preferred
that the replacement solvent be compatible with the
solvent used in the coating fluid, and that it be the
same solvent as that used in the coating fluid.
Figure 10 shows one embodiment of the system
for replacing the coating fluid with solvent. A pump
130 is connected near the bottom of the reservoir 46
through a tube 132 to enable substantially all of the
fluid 12 to be pumped from the reservoir 46. The
coating fluid 12 is pumped through a tube 134 to a wet
scrap container 136. Replacement solvent is added by
any known solvent addition system, such as one
including a solvent supply 138, a control valve 140,
and a delivery tube 142. The solvent level can be
controlled by the level controller (not shown) used in
controlling the coating fluid level in the reservoir
46. Sufficient solvent can be added to the reservoir
46 to bring the solvent to a level higher than that
used for the coating fluid 12 so the solvent can cover
the exit of the die slot 58.
When the coating system is to be placed in
the idling state, web transport is stopped, and the
gravure cylinder 48 rotation can be either slowed or
stopped. When the rotational speed of the gravure
cylinder 48 decreases to a predetermined level, the
supply of coating fluid 12 from the coating fluid
supply 144 to the reservoir 46 is stopped, the pump
130 is activated, and coating fluid 12 is pumped out
of the reservoir 46. Immediately thereafter, solvent
is added from the solvent supply 138, through the
valve 140 and the solvent addition tube 142, to the
reservoir 46 until the level predetermined by the
WO 95/05900 216 7 9 2 3 PCT~S94/07358
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level controller is reached. The coating system can
idle for several hours without forming agglomerates or
other contaminants in the reservoir 46.
Upon resumption of coating, the solvent is
pumped from the reservoir 46, the supply of coating
fluid 12 from the coating fluid supply 144 is resumed,
and the reservoir 46 is filled to the level
predetermined by the level controller. Control of the
relevant valves, pumps, solvent addition apparatus,
and other devices can be accomplished by a known,
commercially available, programmable controller 146.
Because the sequence of functions involved in the
coating fluid and solvent interchange are controlled
by the controller 146, the process occurs
substantially automatically.
