Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
The present invention relates to paper coaters
generally, and in particular to an improved doctor
assembly for a dip roll coating system and a method of
coating paper, in which film split and streaking
patterns in a coating applied onto a paper web are
signiEicantly minimized.
Paper coating processes continue to be performed at
faster web speeds to increase productivity. At the same
time, paper quality continues to rise because of printer
demands, necessitating increasingly higher coat weights
to produce paper of the required quality. The standard
paper coating system for high coat weights and high web
speeds has been the dip roll coater. Essentially, to
apply a high weight of coating onto the surface of a
paper web traveling at a high rate of speed, a dip roll
applicator applies an excess of coating onto the web as
it i~ carried through a nip between the dip roll and a
backing roll. Downstream Prom the dip roll, the coating
is doctored on the web by a blade.
Dip roll coating application has long been plagued
by narrow ~treaky coating lay caused by film splittlng
that occur9 as the dip roll rotates away from the web on
the outgoing side oE the backing roll/dip roll nip. At
high web speeds, the dip roll produces a severe film
split pattern in the excess coating layer applied onto
the web, i.e.,`separations or thin areas occur in the
coating, extending along the direction of web travel.
The narrow coating bands created at the backing roll/dip
roll nip travel on the coated web to the blade, and as
coater speeds increase, especially above 2500 fpm and
faster, the film split pattern becomes unacceptably
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severe. When this nonuniform layer of excess coating
impacts the blade, it exerts varying impulse forces
again3t the blade and is doctored nonuniformly. In
consequence, the film split pattern is not completely
removed by the blade and appears as narrow machine
direction banding in the finished sheet, and the higher
the coat weight, coating material viscosity and web
speed, the more pronounced is the film split pattern.
In addition, when the excess coating impacts the doctor
blade, some spatters back and lands on the web upstream
of the blade. This turbulent spattering becomes more
violent as coater speeds increase, producing blotchy
patterns that are visible in the finished sheet. ~;
Objects of the Inve_tion
An object oE the invention is to provide a dip roll ~ -
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paper coatlng system, and method of operation oP the
system, having an improved doctor assembly that
signiEicantly minimlzes streaky film spllt patterns in
coating applied onto a paper web.
~ nother ob~ect ls to provide ~uch a system and
method, in which the doctor assembly slgnlEicantly
minlmlzes fllm split patterns by rewetting the excess
coating layer on the web just prior to doctoring the
coating.
A further ob~ect is to provide such a system and
method, ln which an eddy current pool of coating is
developed lmmediately upstream of a doctor blade of the `~
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doctor assembly for rewetting the excess coating on the
web to smooth and level the coating lmmediately prior to
` blading the same. `
; j;; Yet another object is to provide such a system and
method, in which the doctor assembly prevents coating
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impacting the blade from spattering back onto the web
surEace upstream of the blade.
Summary of the Invention
In accordance with the present invention, a paper
coating system comprises means for applying an excess
layer of liquid coating material onto a surface of a
paper web traveling at a relatively high rate of speed,
and a downstream doctor assembly for metering and
leveling the excess coating on the web. The doctor
assembly include~ a doctor blade having a tip extended
against and across the web surface, and means for
forming coating removed from the web by the blade into
an eddy current pool of coating extending transversely
across-and in contact with the coating layer on the web ~;~
immediately upstream oE the blade and downstream of and
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spaced from the means Por applying. The eddy current
pool of coating rewets, smooths and levels the excess
coating layer on the web before it reaches the blade, so
the coating layer doctored by the blade on the web is
very uniform, variations in impulse forces of the
coating layer against the blade are minimized, and the
blade doctors the coating on the web very uniformly. In
a contemplated embodiment, the means for applying ~-
comprise3 a dip roll applicator and the web travels at a
speed of at least 2500 fpm.
The invention also provides a method of metering
and leveling an excess layer of liquid coating material
on a web of paper traveling at a relatively high rate of
speed, which includes the step of extending a tip of a
doctor blade against and across the web surface to
doctor the coating on and remove coating from the
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surface. Also included is the step of forming the
coating removed by the blade into an eddy current pool
of coating extending transversely across the web in
contact with the excess coating layer on the web
immediately upstream of the blade to rewet, smooth and
level the excess coating layer on the web beEore it
reaches the blade. The coating layer doctored by the
blade on the web is therefore very uniform, so the blade
doctors the coating very uniformly. In a contemplated
embodiment of the method, coating liquid is applied onto
the web by a dip roll applicator and the web i9 moved at
a speed of at least 2500 fpm.
The foregoing and other objects, advantages and
features of the invention will become apparent upon a
consideration of the following detailed description,
when taken in conjunction with the accompanying
drawings.
Brief Deseription of the Drawings
,!~ Fig. 1 il1ustrates a dlp roll paper coating system
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having a doetor assemb1y conEigured aeeording to the one
embodiment oE the invention, and
Fig. 2 is an enlarged view oE the doctor assembly.
Detailed Deseription
The paper eoating system of Fig. 1 i5 for applying
liquid eoating material onto a surface of a web of paper ! ' 'i
20 earried on a baeking roll 22 in a direetion shown by `
an arrow 24. The system ineludes a dip roll applieator
26, mounted for rotation in a pan 28 having an inlet 30
through whieh coating liquid is introduced to maintain a
~ reservoir of eoating in eontaet with the roll and an
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outlet 32 through which excess coating leaves the pan.
The backing and dip rolls extend transversely of the web
in the cross machine direction, and the dip roll is
rotated in a direction indicated by an arrow 34, usually
at a rate such that its surface travels slightly slower
than the web, so that coating picked up on its surface
from the pan is carried and transferred to the web
surface as the web travels through a nip between the dip
and backing rolls. The dip roll transfers an excess of
coating to the web, and downstream from the dip roll a
tip 38 of a doctor blade 40 of a doctor assembly 42
extends against and transversely across the web to meter
and level the excess coating on the web.
Paper coating processes continue to be performed at
faster web speeds to increase productivity, and paper
quality continues to rise because of printer demands,
necessitating higher coat weights to achieve the
required quality. The standard coating system for high
coat weights and high web speeds has been the dip roll
coater. ~owever, at high web speeds the excess coating
layer applied onto the web by the dip roll develops
streaky Eilm split patterns as the dip roll rotates away
from the web on the outgoing ~ide of the dip
roll/baeking roll nip, i.e., separations or thin layers
oeeur in the coating, extending along the direction of
web travel, When the nonuniEorm excess layer of coating
is doctored by a`bl!ade of a conventional doctor I ;
assembly, lt exerts varying impulse forces against the
blade and ls doetored nonuniformly. In consequence, a
film split pattern occurs in the finished ~heet, that
appears as narrow machine direction banding, and the
higher the coating viscosity and coat weight, and the
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faster the speed of travel of the web, the more
¦ pronounced is the pattern. At web speeds on the order
of ~500 fpm and faster, with conventional doctor
assemblies the film split pattern becomes unacceptable.
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A further problem encountered with dip roll coaters
having conventional doctor assemblies, which becomes
more severe with increasing web speeds, is that when the
excess coating on the web impacts the doctor blade, some
spatters back and lands on the web upstream of the
blade. The spattering becomes more violent as coater
speed increases, and result in blotchy patterns that are
visible in the coating on the finished sheet.
In overcoming these disadvantages and to enable a
dip roll to apply coating onto a paper web traveling at
high speed, without occurrence oE streaky film split
patterns in the finished sheet, the invention provides a
novel doctor assembly in which a "shear plate" is
upstream oE a doctor blade and spaced Prom both the web
and blade to deEine a gap wlth the web and an elongate
chamber with the blade. Excess coatlng removed from the
web by the blade Elows into the chamber and is drained
erom a lower end oE and uniformly along the longitudinal
extent of the chamber to control the height of liquid in
an upper end oE the chamber, and the shear plate is
positioned to control the size of the gap. Draining of
coating from the chamber is controlled by a valve that
i3 opened by an amount to maintain an eddy current pool
of coating at.the blade/web nip, but to prevent a
significant reverse flow of coatlng through and
formation of a liquid seal in the gap 80 that coatlng in
the chamber i9 not pressurized. The eddy current pool
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redeposits coating onto the web immediately upstream of
the blade, and the shear plate is simultaneously
adjusted to define a gap that is sufficiently small to
prevent any outflowing coating from spattering onto the
incoming web, yet sufficiently large to prevent forming
a liquid seal in the gap.
Redeposition of coating by the eddy currqnt pool
just prior to the blade rewets the excess coating layer
on the web to smooth and level streaky film split
patterns caused by dip roll application. In addition,
eliminating an excessive reverse flow of coating through
the gap between the shear plate and web controls an
otherwise violent turbulence of coating that can be seen
returning from the blade in the upstream direction in
conventional dip roll coating systems. Such turbulence
becomes more pronounced as coater speeds increase,
causing spattering of coating onto the web upstream of
the blade and blotchy patterns that are visible in the
coating on the finished sheet. The invention therefore
improves averall coating lay not only by rewetting the ;
excess coating layer on the web immediately prior to
doctoring the layer, but also by eliminating turbulent
spattering oE coating back onto the web upstream oE the
blade. In addition, the ~hear plate protects the blade
from continually being impacted by a spray oE coating
emanating from the dip roll/backing roll nip to further
minimize impulse,Eorces of coating against the blade. A
high coat weight may therefore be applied onto a high
speed web with only minimal, if any, film split and/or ~;
blotchy patterns appearing in the coating on the
finished sheet.
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Referring to the drawings, the novel doctor
assembly 42 is downstream of and spaced Erom the dip
i roll 26. It extends transversely across the web 20 in
the cross machine direction, and includes rearward or
downstream frame members 44a-e. The blade 40 i3 clamped
¦ at its lower end to the frame member 44c by a pneumatic
tube 46, and the blade tip 38 is urged against the web
wlth a force controlled by pressure in a pneumatic tube
1 48. The assembly also includes a front wall 50 on which
is carried a vertically adjustable shear plate 52. The
shear plate has a generally planar upper surface 54 that
is elongate in the direction of web travel and spaced
- from the web to define with the web an elongate gap 56
that may progressively increase in size in the upstream
direction, with overall gap size being controlled by
vertical adjustment of the shear plate. A rearward
surface 58 of the shear plate is spaced from the doctor
blade to define therebetween an upper end of an elongate
,
chamber 60 that extends transversely across the web.
From lts upper end, the chamber extends downwardly
between the frame members 44c-e and the front wall 50 to
a lower end closed by a generally ~-shaped wall 62
carried by both the frame member 44e and a base wall 64.
; A draln conduit 66 extends longitudinally across the ~;
chamber lower end, a plurality oE passages extend ~;~
through and are spaced longitudinally along the conduit,
and thè conduit interior connects through a drain line
68 to an inlet to an adjustable drain valve 70. A drain
line 72 connects an outlet from the valve to a coating `
~ return line 74.
; In operation of the doctor assembly 42, as the
excess coating layer applied onto the web 20 by the
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upstream dip roll 26 contacts the doctor blade 48,
excess coating bladed from the web flows into the
chamber 60, substantially fills the chamber and, by
virtue of movement o the web 20 across the top of the
chamber, forms a uniform eddy current pool of very low
pressure, low turbulence coating material extending
transversely across and in contact with the web upstream
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of and at the blade/web nip. This is accomplished by
adjusting the valve 7~ to drain coating from and
uniformly along the longitudinal extent of the chamber
lower end at a rate to control the height and pressure
of coating in the chamber upper end, and by vertically
adjusting the shear plate 52 so its upper surface 54
establishes a gap 56 of desired size.
Specifically, the valve 70 is adjusted to control
accumulation of coating in the chamber in a manner to
maintain the eddy current pool 76 of coating in the
chamber upper end to be of a size sufficiently large to ~`
rewet the coating on the web but sufficiently s~all so
that no or only a limited amount oE coating flows from
the chamber reversely through the gap 56. The valve is
opened by an amount to establish and maintain the eddy
current pool at and ~ust upstream o the blade/web nip,
but to prevent Eormation of a liquid seal in the gap,
since a seal would adversely cause pressurization of
coatlng in the chamber as coating and/or air are carried
into the chamber by the web. It is important that the ;~
eddy current pool be maintained at a very low pressure,
preEerably a pressure approaching zero psi, to avoid
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~ undesirable turbulence of the eddy current pool and
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pressure pulsations on the blade that can cause
nonuniformlties in doctoring~ It is also important that
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coating be drained uniformly along the longitudinal
extent of the chamber lower end to uniformly control the
height of the eddy current pool longitudinally along the
chamber upper end. To aid in adjusting the amount of
opening oE the valve, while visually observing the
upstream end of the gap, the valve may be opened to a
point where coating liquid just flows slowly reversely
out of the gap, which limited flow also keeps the shear
plate upper surface 54 clean. The valve must be opened
enough, however, to prevent a flow oE coating, reversely
through the gap, of a volume sufficient to cause
spattering of coating exiting the gap onto the web
upstream of the gap, and certainly enough to prevent
forming a liquid seal in the gap and pressurization of
the chamber. Coating drained from the chamber through
the valve then flows through the lines 72 and 74.
In addition to adjusting the valve 70, the shear
plate 52 is also vertically adjusted on the front wall
50 to control the gap 56 to be of a size sufficiently
small to prevent spattering oE any outflowing coating
onto the web, yet sufficiently large to prevent forming
a liquid seal in the gap.
The eddy current pool 76 of coating contacts and
rewets the excess coating layer applied onto the web 20
by the dip roll 26, which excess layer has yet to reach
the doctor blade 40. Thus, there is a redeposition oE
coating onto the web to smooth and eliminate, or at
least substantially minimize, nonuniformities such as
streaky film split patterns in the excess coating layer
just upstream of the blade, resulting in a much more
uniform coating layer for being doctored by the blade.
In consequence, the blade doctors the coating very
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uniformly, and a high coat weight may be applied onto
the web without coating nonuniformities appearing on the
finished sheet. Simultaneously, adjustment of the valve
70 to limit any outflow of coating reversely through the
gap 56 between the shear plate 52 and web, together with
adjustment of gap size, prevents pressurization of the
eddy current pool and controls spattering of coating
toward the dip roll, so blotchy patterns are eliminated
or at least minimized on the finished sheet. Should any
spattering occur, the spatter pattern will be
substantially smoothed and leveled, prior to blading, by
the eddy current pool of coating.
The invention therefore provides an improved dip
roll paper coating system and a method of coating paper,
in which overall coating lay i8 significantly improved
by rewetting the excess coating layer on the web
immediately prior to blading the layer, thereby to -~
substantially eliminate film split and blotchy patterns
that normally occur in coating applied onto a high speed
web by conventional dlp roll coating systems. In
addition, the shear plate 52 protect~ the coater blade
40 from continually being impacted by a coating spary
normally emanating from the outgoing side of the dip
roll nip, which minimizes pressure pulsations on the ;~
blade for improved coating uniformity. By virtue of the
ad~ustable valve 70 and the shear plate, the doctor
assembly can be controlled to account for variations in
web speed, excess coating thickness and coating
viscosity, in a manner to provide a very uniform coating
on the finished sheet.
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;~ While one embodiment of the invention has been
described in detail, various modifications and other
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embodiments thereof may be devised by one skilled in the
. art without departing from the spirit and scope of the
. invention, as defined in the appended claims.
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