Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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L~ Lc~ngement
The invention relates to an arrangement for two-sided
coating of a moving web, for instance a paper web, which
arrangement comprises transfer rolls for ~he transfer of
coating substance to the web, and, between the transfer
rolls, a coating nip, in which the coating of the web is
carried out, and, at a considerable distance from the
coatiny nip, spreader means for spreading and evening
the coating substance on the transfer rolls.
In a known arrangement, the moving web passes from
above downwards through a nip between two transfer rolls,
whereby a considerable amount of coating substance accumu-
lates to form a pond above the nip. At high web speeds,
there will be unstability in the pond generating harmful
vibrations in the web. A further drawback is an unavoid-
able splashing of coating substance, which causes extramaintenance work and thus lowers the profitability. An-
other known construction is presented in U.S. Patent No.
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4,198,446 issued on April 15, 1980 to William J. Goetz.
In one embodiment oF this known arrangement~ the web moves
through the coating nip vertically upwards. For two-sided
web coating six rolls are req~ired, which leads to high
manufacturing costs. A third known construction, the so
called gate-roll coating arrangement, also comprises six
rolls with the web moving downwards through the coating
nip. In this construction, the whole roll set has to be
renewed even if only one roll has worn out, which raises
the operational costs of the gate-roll arrangement.
The object of the invention is to provide a reliable
and inexpensive arrangement for ~wo~sided web cooating,
which can also be used at high web speeds.
According to the invention there is provided an
arrangement for two-sided coating of a moving web, which
comprises: first and second transfer rolls for the
transfer of a coating substance to said web, said first
and second transfer rolls defining a single coating nip
therebetween, in which coating nip the coating of said web
is carried out, and, at a considerable distance from said
coating nip, first and second spreader means disposed with
a first spreader nip between said first transfer roll and
said first spreader means and a second spreader nip between
said second transfer roll and said second spreader means,
said first and second spreader means spreading and evening
a coating substance on the first and second transfer
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rolls, respectively; rneans for moving said web in a
substantially vertical direction through said coating
nip from below the nip upwards; coating substance supply
means at said first and said second spreader nips for
supplying a coating substance to form a coating substance
pond at said first and said second spreader nips; and
said arrangement having a maximum of four rolls.
The spreader means can, for instance, be a spreader
nip or slot. From the spreader means the coating sub-
stance is brought by the transfer rolls to the coating
nip, where a stable, inverted pond of coating substance is
formed below the nip. The forces acting on the inverted
pond, the gravity and the counteracting, vertical force
components generated by the moving web and the transfer
rolls, are in balance.
The invention can be used for cardboard coating as
well, whereby the web speed is usually lower than in
paper coating. Also in this case, the supply of coating
substance is so dimensionedl that the coating substance
amount in the inverted pond at the coating nip is so
small, that the pond remains stable. Hence, there will be
no noticeable flow~down or dripping of coating substance
and no extra collector for surplus coating substance is
needed.
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Even if a roll is used a~ spreader memberJ the arrangement
remains con~iderably less complicated than a gate-roll arran-
gement, but i3, nevertheles~, at least a3 e~ficient. By
making the distance between the tran~fer roll and the spreader
roll adjustable, the width of the spreader slot can e sily he
kept at an optimum. It also makes it pos~ible to chose between
a ~preader slot and a spreader nip. By making the line load o~
the ~preader nip adjustable in the longitudinal direction of
the transfer rolls, a uniform ~preading o~ coating ~ubstance
on the transfer roll can easily be obtained.
The coating substance amount on the trans~er roll can be ef-
fectively controlled by running the tran~fer roll and the
spreader roll at dif~erent circumferential speeds. Naturally,
the rolls can al~o have the ~ame speed The circumferential
speed of the spreader roll can be kept lower than the circum-
ferential ~peed of the transfer roll. This generate~ a wiping
e~ect acting on the transfer rolls, which recollect~ the
wiped-o~f coating substance.
By providing the spreader roll mantle with recesses, such a~
grooves or depressions, the coating substance can be trans~er-
red to certain ~ections of the web via the transfer roll. The
recesse~ may extend mainly continuously all over the tranY~er
roll circumference, for instance so, that the distance between
two successive recesse~ is not longer than the circumferential
length of the recesses. By suitable arrangement of the
recesses coating substance is tranqferred to the web in the
form of unbroken or periodically repe~ted stripes.
By running the web through one of the ~preader means, the
space required below the arrangement is minimized. The web is
also more firmly held by two nips and one transfer roll,
whereby web vibrations can effectively be eliminated or
damped.
An uncomplicated and unexpensive modification is obtained by
having at least one spreader means in the form of a flexible
blade. If the line pressure in the spreader nip is made
locally adjustable, a uniform spreading of coating substance
is easily obtained.
The functionally corresponding members at opposite sides of
the coating nip may structurally be mainly equal. Then, both
~ides of the web can be coated with the same kind of coating
substance. Adju~tment of the spreader nip width or its nip
pressure makes it possible to use different types of coating
substances at opposite sides of the web.
~ hen coating opposite web sides with a different type of
coating substance the use of different spreader means at both
sides is natural. Siæe substance can, for instance, be trans-
ferred to one web side by means of a flexible blade acting on
the one transfer roll. If one web side is to be coated spot-
wise or is to be provided with pigmented marks or the like,
the coating qubstance is spread on the transfer roll by means
of a recessed spreader roll.
A favourable coating substance flow in a spreader nip
including a blade is obtained by keeping the angle between the
spreader blade and the trans~er roll small, preferably less
than 20. Due to the wedge geometry the hydrodynamic pressure
acts effectively in a blade nip. Thus, relatively iarge
amount~ of coating substance can be supplied to the spreader
nip. I~ the angle between the flexible spreader blade and the
transfer roll is adjustable, it is possible to compensate for
the variation in hydrodynamic pressure resulting from diffe
rent viscosities of the coating substance.
In the following, the invention will be described more in
detail with reference to the attached drawing, in which
- Figure 1 shows a ~ide view of a first embodiment of the
invention,
- Figure 2 shows a side view of a second embodiment of the
invention,
- Figure 3 show~ a side view of a third embodiment of the
invention.
In the drawing, reference numeral 1 indicates a moving web,
which moves through a coating nip 3 between two transfer rolls
2. A spreader nip 5 iq formed between transfer roll 2 and a
spreader member, a flexible blade 4 or a spreader roll 4a.
Coating sub3tance 8 is supplied to spreader nip 5 through
supply ducts 6. The running direction of web 1 is indicated by
an arrow. At the inlet side of coating nip 39 a small inverted
pond 7 of coating substance is formed. Pond 7 can extend
laterally around the edges of web 1, thus forming an undivided
pond. It is al~o possible to have two separate ponds 7 at
opposite sides of web 1. Coating substance 8 is supplied to a
space above spreader nip 5. Figure 2 ~hows an embodiment, in
which web 1 first runs through spreader nip 5 between one
transfer roll 2 and a spreader roll 4a. Thereafter, web 1
runs through the coating nip 3. This embodiment requires very
little space below the arrangement, since the web is fed
basically horizontally or from above. Figure 3 shows an arran-
gement of the gate-roll type, in which, according to the
invention, only four rolls are used.
The web coating is carried out as follows. Coating substance
is supplied via supply duct 6 to a space above ~preader nip 5,
which space is located between transfer roll 2 and spreader
member 4 or 4a. Spreader nip 5 distributes coating substance
onto transfer roll 2, the rotation of which brings the coating
substance into coating nip 3, where it is transferred to
running web 1. The moving of coating substance from spreader
nip 5 to coating nip 3 can be carried out differently at both
sides of the arrangementO If the web runs through one spreader
nip 5, as Qhown in Figure 2, the web itself acts a~ transfer
member and simultaneously prevents the coating substance from
dropping downwards. The coating substance supply is so dimen-
sioned, that only a small pond 7 is formed in coating nip 3.
The invention is not limited to the embodiments disclosed,
but several modifications thereof are feasible within the
scope of the claims. The supply of coating substance to the
spreader means, for instance, can be arranged by means of slot
- nozzles, laterally extending mainly over the whole transfer
roll length. A flexible spreader blade can be attached to such
a ~upply nozzle.
