Note: Descriptions are shown in the official language in which they were submitted.
ARRANGEMENT FOR EBB COATING
FIELD OF THE INVENTION
The invention relates to an apparatus for one-sided or two-
Dodd coating of a basically vertically traveling web, for
instance a paper web, by providing nonpressurized supply of
coating substance onto the web and evening the coating
substance sticking thereto by pulling the web through a
coating nip formed by a flexible coaxing blade and a back-up
member.
BACKGROUND OF THE INVENTION
In web coating arrangements, in which the web is pulled
vertically through a nip formed by a blade or the like and a
back-up member, the spreading of coating substance onto the
web usually takes part prior the nip by means of a spreading
lo apparatus. In a known arrangement the spreading apparatus
comprises two rolls forming a nip through which the web is
pulled. The coating substance is supplied onto at least one
of these rolls the applicator roll, wherefrom it is
transferred to and spread onto the web. At high web speeds,
one difficulty is due to web slip in the spreading nip
because the applicator roll moves slower than the web. This
causes uneven coating substance transfer to the web. Another
known coaler construction comprises spreading nozzles in
I
lieu of applicator rolls. However, with this arran6eme~t it
is difficult to spread the coating substance uniformly on
the web. A common feature of both these known constructions
it that, in the spreading phase, relatively great amounts of
surplus coating substance are removed from the web and from
the spreading unit. This excess amount has to be pumped back
to the spreading point, and this forms an extra, us-
economical step of the process. Further, another recycling
circuit has to be provided for risque in the coating
substance scraped off by the evening blade or blades. This
complicates the construction end increases the manufacturing
costs.
In coating units provided with a non-pressure coating
applicator, that is, a coating substance applicator under
atmospheric pressure, the substance attached to traveling
web is leveled by a flexible blade. In such coating units,
however, a phenomenon called barring causes problems in the
form of coating layer variations in the longitudinal
direction of the web, especially at high coating speeds
(Giver 500 main Exactly why barring occurs has not been
completely clarified. An improved embodiment of the
invention is based on the assumption that one reason for the
occurrence of barring is due to non-uniform thickness of the
coaling layer on the web before the smoothing nip. This
result in movements in the smoothing blade, and an uneven
coating is produced because of these movements.
~2~7~
US-Patent 3192895 discloses a coaler in which a coating
chamber it provided by a metering bar, which regulates the
flow of coating substance between said bar and the
horizontally moved web. The drawback of this arrangement is
that the gravity acts against coating substance between the
bar and the flexible blade. The bar only forms an outlet
slot adjustment but does not contribute to the level quality
prior the flexible blade.
In an extrusion applicator, in which the coating substance
lo is applied on the web under pressure, the situation is
different. In applicators of this type, a fairly long
smoothing gap is used in connection with the applicator for
equalizing the coating substance pressure. The invention is
not related to such coating methods.
OBJECT AND SUMMARY OF THE INVENTION
The object of the invention is to provide a web coating en-
rangement, in which the spreading of the coating substance,
also at high web speeds, is carried out in a reliable and
economical way and with a homogeneous and even result. An
arrangement according to the invention has low manufacturing
costs and the amount of recycled coating substance is
minimized. This results in an improved coating economy. Also
the coating substance scraped off from the web by the
coating nip blades, is collected for reuse- without any
pumping action.
4 31L:~213~
According to the invention there is provided apparatus
for an one-sided or two-sided coating of a vertically
traveling web comprising a coating applicator, in which
said web is arranged in contact with a coating substance
held at atmospheric pressure, and a flexible blade co-
operating with a back-up member for leveling a coating
substance layer attached at said web, said applicator
comprising a flow regulator which is arranged to form
a clearance between said regulator and said web, said
clearance being mainly of even height or being inclined
in the travel direction of said web, said height being
smaller than a thickness of an eventual coating substance
attached to said web without a co-operation with said
regulator and said web, said applicator further being
provided with means for arranging a gravity controlled
over-flow and a back stream substance flux, said back-
stream flux being governed by the coating substance
supply to said applicator, the viscosity of said coat-
in substance, the speed of said web travel and a slot
of said applicator so that only a small amount of said
coating substance flows downwardly through said slot
during normal operation of said apparatus, said slot
forming a web entrance at the bottom of said applicator
Thus, in the invention, the web is moved past a vertical
coating applicator in which the web is in direct contact
with the coating substance under atmospheric pressure.
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pa
Said applicator comprises a clearance formed between
the web and a flow regulator, which clearance is of even
height or arranged inclined in the web travel direction.
This height is adjusted smaller than a thickness of an
eventual substance layer formed without said regulator.
At the bottom of the applicator there is a slot forming a
web entrance, through which the web comes in contact with
the coating substance. The coating substance supply, the
width of the bottom slot, the viscosity of the substance
and the speed of the web are so adapted, that they effect-
lively govern gravity controlled back stream flux of coating
substance through said slot. Hereby only a small amount
of the coating substance flows downwardly through the slot
during normal operation of the apparatus. The applicator
is further provided with gravity controlled over-flow
means.
The web is in direct contact with the coating substance
in the applicator. The moving web then carries away
coating substance, which contributes to a firm adhere
once of coating substance all over the web surface.
The excess coating substance scraped off by the coating
blade is, even at high web speeds collected for imp
mediate reuse without any pumping means. Since only
a small amount of coating substance passes through
the bottom slot downwards, a column of coating sub-
stance is formed in the coating chamber This
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provides a large and uniform contact between the web and the
coating substance. The construction is economical as only a
smell amount of coating substance flows out of the contact
zone. The viscosity of the substance being optimized to the
web speed, no substance flows out of the zone. Hereby no
pumping means are necessary for substance recycling.
The coating substance level can easily be adjusted by means
of a preferably adjustable overflow edge or duct as known
per so. The applicator may have vertical adjustable side
walls, the upper edge of which form the overflow edge. It
should be noted that the coating applicator may have a
length in the traverse direction of the web of 6...8 m in a
full scale coaler. This means that also rather large end
openings will nevertheless allow the applicator to remain
substantially filled with coating substance.
The object of the invention is to reduce the risk of
barring. This is possible by said flow regulator arranged
crosswise to said web and at a distance therefrom The
regulator forms between itself and said web a clearance
extending in the travel direction of the web. In a preferred
embodiment of the invention, this clearance converges in
said travel direction. This arrangement considerably
decreases the thickness variations of the coating substance
layer adhering to the web before the smoothing blade, as
well as other disturbances, and this substantially improves
the quality and uniformity of the coating. The regulator can
be positioned at the coating substance level in the
applicator. This is, however not always absolutely
necessary. The rear edge of the flow apply actor should be
sharp to eliminate disturbing effects on the coating.
301ution resembling the improved embodiment of the invention
Lo shown in several prior publications, like in Patent
Specifications US 3,179,536, item 28, TJS 4,063,531, item 67
and item aye, FIX 55375 and SE 402 222. These known.
arrangements differs from said improved embodiment in that
lo the flow regulating duct is not converging (US 3,179,536,
FIX 55375, SE 402 222 and US 4,063,531, item aye) and in that
there is no contact between the element in question and the
coating layer on the web (US 4,063,531, item 67).
The height of the flow regulating clearance or duct is
dependent on the dry solids content and the viscosity of the
substance. If the coating substance contain much liquid and
has a low viscosity, the clearance height should be reduced.
In practice, the average height of the flow regulating duct
should be 0.5...5 mm.
I The length of the flow regulating duct in the web travel
direction can be selected relatively freely. It has been
found that the length is not of great importance. The length
should be at least the same as the lowest duct height. A
recommendable duct length is 1...200 mm, preferably
10... 100 mm.
The flow regulating duct is converting in the longitudinal
direction of the web. A suitable angle on convergence is
2...20, preferably 4...10. Hydrodynamics forces having 2
favorable effect on the uniformity of the coating layer
occur due to the convergence of the duct. A converging duct
should be of a suitable length, preferably at least 4 mm.
To ensure that the flow regulator operates correctly, it is
important that the coating substance level if. the coating
applicator remains basically constant. The surface-level
regulation must be very accurate, if the distance between
the web and the slow regulator is small. A sufficiently
accurate surface level regulation it usually obtained by
providing the coating applicator with an overflow system
that automatically keeps the surface level at a desired
height. On the other hand the economy of the over-flow
system is back-up by the gravity operation.
A suitable back-up member for the coating nip in one-sided
web coating can be formed as a plate extending vertically
upward slightly above the coating nip and downwards to or
past the bottom level of the collection tank or trough. The
width of the plate should correspond to or exceed the web
width. By this means the web is simply and firmly supported
at its one side and the nip back-up member as well as the
slot limiting member are formed by one single member. The
adjustment of the clearance or the slot relative to the
back-up member is easy to carry out for these stationary
8 I
positioned elements. Another back-up member is a rotating
roll.
In an arrangement according to the invention, the members
located at opposite sides of the web may be fully or partly
identical. For two-sided coating corresponding members can
be arranged at both sides of the web, preferably at the same
vertical level. These applicators may be supplied with the
tame kind or with different kinds of coating substance. In
the latter case, the slot widths and liquid level positions
at both side may he considerably different.
In order to obtain an uniform coating result, the
arrangement can be provided with a load member applying an
adjustable pressure on the flexible coating blade, as known
per so.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described more in detail with
reference to the attached drawing, in which
- Figure 1 is a schematic sectioned view of a first
embodiment of the invention, in which a one-sided web
I coating i 9 shown,
- Figure 2 is a sectional side view of another embodiment
in an apparatus according to the invention.
9 I
DESCRIPTION OF THE PREFERRED Embodiment
erring descriptively to the drawing, reference numeral 1
indicates a moving web, which is pulled through a coating
nip formed between a flexible blade 2 and a back-up member
5 I (Figures 1 and 2). The running direction of the web it
indicated by an arrow. A narrow slot 5 is formed below said
nip. Slot 5 is formed between a slot limiting member
associated with the backup member 19 and the edge 11 of an
adjustable slot width regulator 10. The slot width is
lo infinitely variable up to a maximum of preferably 5 cm.
Back-up member 19 and the limiting member associated
therewith may be separate or combined to a single member 7
which may be a roll, a flexible or a rigid plate or a fixed
web guide member. A coating chamber is formed above slot 5.
This chamber is limited by the web 1, the flexible blade 2,
a back wall 8 and side walls (not shown). The side walls may
pass beyond the side edges of the web as obvious to a man
skilled in art. In this case, it might be suitable to use,
at that portion of the web, a properly arranged backup
member (not shown). It is in most cases more convenient to
slightly reduce the distance between the side walls so that
their edges seal against the web. Thereby, a narrow section
at each web edge is left uncoated. The upper portion 8b of
the coating chamber is during operation of the apparatus
more or less filled with coating substance due to the action
of blade 2, which scrapes off surplus coating substance at
lets it flow back to the lower portion of the coating
1 0 '.
chamber.
A purity plate 9 carries slot width regulator lo, which is
movably suspended so as to allow adjustment of the width of
910t 5. Width regulator 10 may be provided with threaded
members I so arranged, that turning of the members causes
adjustment of the width of slot 5. The slot width regulator
could alto have the form of a rotatable eccentrically
~ournalled bar extending crosswise to the web.
Coating substance 16 is supplied to the chamber through a
lo supply duct 13. A collecting tank 12, open against web l, is
arranged below slot 5. The small amounts of coating
substance flown through slot 5 and collected in tank 12 are
recycled to the chamber through an outlet duct 14, which via
pumping means (not shown) it connected to supply duct 13.
Chamber is provided with means for regulating the level of
coating substance contained therein. For example, coating
substance may flow over the upper edge of the side walls
into tank 12. The edges are preferably vertically
adjustable.
The embodiment of Figure 1 includes a roll 19, which at the
tame time forms the back-up member for flexible blade 2 and
the limiting member for slot 5. Roll 19 is rotated in the
running direction of web 1. There is an overflow wall 23 at
the back of the coating applicator, keeping the level 24 of
I the coating substance 16 at a constant height. At this
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height, the coating applicator has a flow regulator 26,
forming a slightly converging duct 18 at the web. Flow
regulator 26 reduces the amount 27 of coating substance
following web 1 upwards into the upper portion 8b of the
coating chamber. Layer 27 is substantially uniform in
icons, which has a favorable influence on the function
of coating blade 2 and thus helps to avoid barring. The
distance between flow regulator 26 and the web and
preferably also the angular position of the flow regulator
relative to the web are adjustable.
Coating substance is pumped to applicator 8 through pipes
13, and coating substance flown over the overflow wall 23 is
led to a recycling system through a pipe 33. The converging
duct 18 between flow regulator 26 and the web is more
clearly shown in Figure 2, which shows another back-up
member 19. The angle of convergence d is about 10. The
inlet-side duct height is a and the outlet-side duct height
is b. The dimension a is in most cases 1.5...7 mm,
preferably 2...4 my The duct length c should be greater
than b and at least 4 mm, preferably 10... 100 mm.
Due to the convergence of duct 18, there will be a
hydrodynamics force P in the duct. Expressed as force per
width-unit in the Crosby direction of the flow regulator
this force can be calculated from the known formula:
,
p = gnu r in _ 2m-2 1
Tandy my , where
12 ~22
n - the viscosity of the coating substance
- the speed of 'he web 1
d _ the angle of convergence of the dllct
m - a/b (ratio of the duct height at the inlet end and at
the outlet end)
The hydraulic force P has an evening effect on the coating
sub.c~ance layer 27, which improves the effect of the flow
regulator 26.
The embodiments shown in Figure 1 and 2 are based on one-
lo sided web coating. Basically symmetric set-ups relative to
web 1 can be applied in two-sided web coating. Such
embodiments are shown, for instance in British Patent
Application 2085327. It is, however, not necessary to use a
symmetrical coating arrangement. If, for instance, different
coating substances with different viscosity are supplied to
coating chambers at opposite sides of the web, the slot
regulating edges 11 may be at different distances from web
1.
Collecting tank 12 is suspended at coating chamber wall 8.
Tank 12 can also be formed in one piece with wall 8 by pro-
voiding the latter with a long, downwards extending portion
bent at its lower edge towards web 1 to form a trough. The
trough or tank 12 may extend outside the side edges of web
1 .
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The back-up member of the coating nip and the limiting
member of slot 5 may be combined into a single, mainly rigid
web guiding plate.
The invention is not limited to the embodiments shown, but
several modifications thereof are feasible within the scope
ox the claims
