Edge dam for paper coating apparatus and method

An edge dam assembly for an applicator for applying a film of coating material on a moving web of paper carried through an application zone, is characterized by a seal element at each side end of the zone. To prevent leakage of coating liquid past the seal element, air is introduced through the element and into the side end of the zone adjacent to the element. The air forms an air barrier pocket at the side end of the zone, which is void of coating liquid, whereby coating liquid is maintained in the zone inwardly of the seal elements.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved edge dam for use with 
applicators of the trailing blade type for applying a coating liquid to a 
moving web of paper. 
Conventional applicators of the trailing blade type include means for 
applying coating liquid to a paper web that is usually supported and 
carried by a backing roll. Such applicators include a chamber having an 
opening extending across and parallel to the web, together with a doctor 
blade at a trailing side of the opening for leveling the coating and a 
front wall or orifice plate extending from a leading side of the opening 
toward the web and defining an orifice for exit of excess coating material 
from an application zone between the wall and doctor blade. Coating liquid 
is supplied to the chamber and thence through the chamber opening into the 
application zone, and to seal end spaces between the front wall and doctor 
blade at side ends of the zone to prevent escape of coating material 
laterally of the web, edge dams are provided thereat. 
For applicators of the foregoing type, the edge dam often comprises a felt, 
plastic or metal material configured to fill a somewhat triangular shaped 
opening at either end of the application zone defined by the doctor blade, 
orifice plate and backing roll. Ideally, the edge dam absolutely prevents 
leakage of any coating liquid to exterior of the applicator. However, that 
can require that the edge dam be in contact with the paper web, and wear 
or abrasion of the edge dam or marking and tearing of the web may occur. 
Consequently, the edge dam is only positioned closely adjacent to, but 
spaced from, the web, and some coating liquid seeps therepast. 
In use of edge dams, an effort is made to minimize leakage of coating 
liquid. However, it is usually very difficult, if not impossible, to mount 
edge dams at the side edges of the application zone in sufficiently close 
proximity to the paper web to allow only an acceptably small amount of 
leakage. In practice, coating liquid often leaks past the edge dam, with 
the result that the web edges become contaminated by stickers of coating 
liquid, which bind together adjacent edges of a wound paper roll and 
prevent the roll from later being properly unwound. 
If leakage of coating liquid past edge dams could be eliminated or at least 
significantly minimized, it would then be possible to coat only the center 
portion of a paper web, leaving narrow uncoated strips along opposite side 
edges of the web. This would advantageously reduce or eliminate coating 
material buildup on backing rolls following the coater as well as edge 
stickers on the coater backing roll and web. Unfortunately, conventional 
edge dams do not ordinarily permit the foregoing to be accomplished. 
OBJECTS OF THE INVENTION 
An object of the present invention is to provide an improved edge dam for a 
paper coater which eliminates or very significantly minimizes leakage of 
coating material therepast. 
Another object is to provide such an edge dam which accommodates 
introduction of air into the side end of an application zone of a coater 
to generate an air barrier pocket, which is void of coating liquid, at the 
side end of the zone adjacent to the edge dam, whereby coating liquid is 
maintained inwardly of the edge dam and leakage is prevented. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an improved edge dam assembly for 
an applicator for applying coating liquid to a moving web of paper carried 
through an application zone, wherein the application is of the type having 
a body defining a chamber therein with an elongate opening to the chamber 
positionable generally adjacent to and transversely across the web, the 
chamber receiving coating liquid and directing the same through the 
opening and into the application zone for being applied onto the web, 
comprises seal means mountable at a side end of the opening and zone and 
means for introducing a gas under pressure through the seal means and into 
the zone to generate at the side end of the zone and adjacent the seal 
means a pressurized gas barrier pocket which is substantially void of 
coating liquid, whereby coating liquid in the zone is maintained inwardly 
of the seal means and prevented from leaking therepast. 
The invention also contemplates an improved method of applying coating 
liquid to a moving web of paper by applying coating liquid to one surface 
of the web as it is carried through an application zone having spaced 
front and rear edges and laterally spaced side ends, which comprises the 
steps of forming and maintaining a reservoir of coating liquid on the web 
in the zone by introducing coating liquid into a chamber having front and 
rear walls the upper ends of which define the front and rear edges of the 
zone and an opening to the zone, and flowing the coating liquid in the 
chamber through the opening and into the zone; doctoring the coating 
liquid on the web at the rear edge of the zone; and maintaining the 
coating liquid in the zone by substantially sealing the front, rear and 
side edges of the zone. In this connection, the improvement comprises the 
steps of sealing each side edge of the zone by substantially closing the 
outermost end of the side edge, and introducing into the side edge of the 
zone adjacent to and inwardly of its outermost end a flow of gas to 
generate at the side edge and adjacent the outermost end a pressurized gas 
barrier pocket which is substantially void of coating liquid, whereby 
coating liquid is prevented from moving past the side edge of the zone. 
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.

DETAILED DESCRIPTION 
Referring to the drawings, there is indicated generally at 20 an applicator 
of a type with which an edge dam or edge seal assembly, indicated 
generally at 22 and configured in accordance with the teachings of the 
invention, is particularly adapted for use. The applicator is of the 
trailing blade type and applies a pigment bearing liquid coating to a 
moving web of paper 24 carried on a resilient backing roll 26 in a 
direction indicated by an arrow 28. The applicator includes a housing 30 
having a coating liquid chamber defined between a front or leading wall 32 
and a rear or trailing wall 34 of the housing. The walls extend generally 
transversely across the backing roll and taper toward an open upper end or 
elongate application zone 36 of the chamber. Coating liquid introduced 
into the chamber under pressure flows through a metering slot 38 between 
the front and rear walls, as shown by an arrow, into the application zone 
and against the web of paper. Although not shown, it is understood that 
opposite sides of the housing, and therefore of the metering slot, are 
closed by side walls extending between the front and rear walls. 
During operation, the coating applicator 20 is positioned closely adjacent 
the backing roll 26 with the application zone 36 facing the surface of the 
paper web 24. A flexible doctor blade 40 at the trailing end of the zone 
is clamped against the housing rear wall 34 and urged against the web by 
any suitable means (not shown), such for example as by a pair of 
transversely extending air tubes as taught in U.S. Pat. No. 4,250,211. The 
doctor blade serves several functions, one of which is to level and meter 
the coating applied to the web and another of which is to form a seal at 
the trailing end of the zone to prevent escape therepast of excessive 
amounts of coating liquid. An orifice plate 42 is mounted and vertically 
adjustable on the upper end of the housing front wall 32, such that an 
upper free edge 44 of the plate extends toward but is closely spaced from 
the web to define therewith a narrow orifice 46 extending transversely of 
the web at the leading end of the zone, through which excess coating 
material escapes from the zone. A copious excess of coating material is 
supplied to the chamber and thence through the metering slot 38 into the 
application zone, such that material escaping through the orifice 46 forms 
a liquid seal in the orifice and provides, along with the edge dams 22, 
the orifice plate and the doctor blade, a substantially closed application 
zone through which the paper web passes for being coated by coating liquid 
under pressure. 
To the extent described, the applicator is of the type disclosed in 
aforementioned U.S. Pat. No. 4,250,211, issued to Damrau et al. and 
assigned to the assignee of the present invention, the teachings of which 
are specifically incorporated herein by reference. 
To seal the spaces at the side ends of the application zone 36 between the 
orifice plate 42, backing roll 26 and doctor blade 40, an edge dam 
assembly 22 is positioned at each side end of the zone. As will become 
apparent, in improving upon conventional edge dams the edge dam assembly 
of the invention seals coating liquid in the application zone partially by 
means of providing a pressurized air barrier within the zone immediately 
inwardly of the edge dam, which maintains coating liquid out of contact 
with the edge dam and substantially prevents escape of coating liquid from 
the zone. 
With reference to the drawings, each edge dam assembly comprises a main 
body 48, which may be of nylon, metal or any other suitable material. At 
one end of the main body is carried a somewhat triangular shaped seal 
element 50 which is configured to generally fill the opening at an end of 
the application zone 36 between the orifice plate 44, doctor blade 40 and 
backing roll 26, with the element being in sealed engagement with the 
orifice plate and doctor blade but spaced from the backing roll. A passage 
is formed longitudinally through the main body, and is provided with an 
air inlet fitting 52 at one end and a somewhat elongate air outlet nozzle 
54 at an opposite end at the seal element. The passage and air nozzle 
internal diameters may be on the order of about 3/16 inch, and the air 
inlet fitting connects with compressed air from a supply thereof (not 
shown) through an air line 56, whereby compressed air flows through the 
passage to the air outlet nozzle for exit from an outer end 58 of the 
nozzle. A slide plate 60 extending longitudinally along the main body is 
slidably connected with a lower surface of the body by means of a dovetail 
slot, and is longitudinally adjustable with respect to the main body by 
means of a rod 62 having a ring 64 at its outer end, thereby to extend an 
end of the plate a selected distance beyond the end of the main body. To 
secure the slide plate in position once adjusted, a set screw 66 carried 
in a flange 68 of the main body may be rotated into engagement with the 
rod. 
The generally triangular shape of the seal element 50 enables the edge dam 
assembly 22 to be mounted in the space between the doctor blade 40, 
orifice plate 42 and backing roll 26 at a side end of the application zone 
36. For the purpose, and as shown in FIG. 1, the downwardly depending 
flange 68 of the main body 48 has a passage 70 which receives a rod 72 
extending outwardly of the applicator transversely of the application 
zone. The flange is secured to the rod by means of a set screw 74, which 
enables the edge dam assembly to be conveniently removed from the 
applicator by being slid transversely therefrom for cleaning, repair or 
replacement. With the edge dam assembly mounted between the doctor blade 
and orifice plate, the upper end of the triangular shaped seal element 
extends toward and adjacent to but spaced from the backing roll. At the 
same time, the sides of the seal element are positioned against and seal 
with the doctor blade and orifice plate. 
FIGS. 1 and 3 illustrate the mounting arrangement of the edge dam assembly 
22 on the applicator. As is apparent, the configuration and size of the 
seal element 50 is quite important, and the element must substantially 
fill the area between the doctor blade, orifice plate and backing roll. If 
the element is too small, coating liquid will seep therepast. If the 
element is too large, the doctor blade will be held from its doctoring 
position against the paper web and/or the element will engage the backing 
roll or web. 
In operation, air at a pressure greater than the pressure of coating liquid 
in the application zone 36 is introduced through the air inlet fitting 52 
and exits from the outlet end 58 of the nozzle 54 into the zone toward a 
side end thereof adjacent to the seal element 50. Normally, 1-10 psig air 
pressure is sufficient, although the pressure may vary depending upon the 
pressure of coating liquid in the zone. The air forces coating liquid away 
from the side end of the zone and forms a pressurized air barrier pocket 
in the zone at the side edge thereof adjacent to the seal element, which 
pocket is void of coating liquid. Consequently, coating liquid is 
maintained inwardly of the seal element for application on the paper web, 
but cannot escape laterally past the element and out of the application 
zone. As shown in the drawings since the seal element does not rely on a 
closely spaced relationship with the web or backing roll to form a seal, 
the upper edge of the element may be spaced somewhat further from the 
backing roll and paper web and would be the case with conventional edge 
dams, thereby to facilitate a flow of air out of the application zone so 
that the air does not move into the coating liquid therein and cause 
"skips" in the film of coating applied on the web. Together with the flow 
rate of gas into the zone, the seal means is configured, or spaced an 
amount from the web, to accommodate escape of gas from the gas barrier 
pocket to external of the applicator at a rate which maintains a 
substantially constant volume gas barrier pocket. 
As described, coating liquid is introduced into the application zone 36 
through the metering slot 38 which extends along and substantially 
coextensively with the zone, and upon exiting the slot flows upwardly into 
the zone. Absent the slide plate 60, coating liquid introduced into the 
side ends of the zone would tend to flow into the air barrier pocket 
generated inwardly of the seal element 50, thereby destroying the 
integrity of the pocket and causing coating liquid to be applied onto the 
edge of the web, which is desirably left uncoated, and possibly to leak 
past the seal element. However, the slide plate 60 is longitudinally 
adjustable along the main body 48 to extend a selected distance beyond the 
outlet end 58 of the air nozzle 54 and over the portion of the metering 
slot therebeneath, and blocks or deflects coating liquid which would 
otherwise be directed into the air pocket. At the same time, the degree of 
extension of the slide plate beyond the air nozzle, taken together with 
the pressure of air flowing through the nozzle and the longitudinal 
positioning of the edge dam assembly on the rod 72, controls the 
longitudinal extent of the air pocket into the application zone, and 
thereby the width of the uncoated strip on the side end of the web. Should 
some coating liquid nevertheless escape past the seal element 50, a 
surface 76 on the upper surface of the main body is tapered downwardly 
toward the forward end of the applicator and is wider at the air outlet 
end of the assembly than at the air inlet end to present a further barrier 
to coating liquid escaping the applicator, inasmuch as the liquid would be 
required to run uphill to escape. 
As compared with conventional edge dam assemblies which must be positioned 
in very close proximity with the web to control seepage of coating liquid 
from the application zone, and yet nevertheless allow an unacceptable 
amount of coating liquid to escape, the edge dam assembly of the invention 
provides secure seals at the side ends of the zone to significantly 
minimize, if not eliminate, seepage of coating liquid therefrom. By means 
of adjusting the pressure of air introduced into the side end of the 
application zone through the air nozzle 54 and the amount of extension of 
the slide plate 60 beyond the nozzle, the edge dam assembly may be 
controlled so that substantially no coating liquid seeps therepast, 
despite the fact that the assembly may be spaced a reasonable distance 
from the web. Consequently, the edge dam assembly permits a dry or 
uncoated narrow strip to remain on each side edge of the paper web and 
eliminates coating liquid buildup on rolls following the applicator and 
edge stickers on the wound paper roll. 
While one embodiment of the invention has been described in detail, various 
modifications and other 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.