Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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132
MACLEOD
The groove between the wires determines the precise amount of coating material
that will pass through. The initial shape of the coating is a series of stripes, spaced apart
according to the spacing of the wire windings. Almost immediately, normal surface tension
pulls these stripes together, forming
a
flat surface, ready for drying in air or under heat.
Wet film coating thicknesses, accurate to 0.1 mil, can easily be achieved using metering
rods. Mathematically, the average thickness of the area between the wires, and therefore
the coating thickness, is
0.173
times the wire diameter. From a practical standpoint, most
users use a
1
:
10
ratio to select the best theoretical rod size for
a
given wet coating thickness.
A rod wound with an
0.015
in. diameter wire, for example, will ideally produce a wet
film that is
0.0015
in. (1.5 mils) thick.
In
production coating, other physical factors can
influence the actual coating thickness. The most important of these is the phenomenon of
shearing action
of
the liquid. In effect, not all the coating material passes through the
gaps; some liquid adheres to the surface of the wire. The impact of this is usually small,
but it can be significant when small (0.025-.005 in.) wires are used, or when viscosities
are high. In high volume coating operations, the speed of the web, web tension, penetration
into the base material. and other factors will also affect the coating thickness. The sum
of
all these variations seldom exceed
a
20%
difference from the theoretical coating weight;
however, the differences can be large enough to suggest a trial-and-error procedure for
selecting the ideal wire size for each coating application. Because of the relatively low
cost of rods, and the ease
of
changing from one wire size to another, the selection process
is both economical and practical. Most coatings laboratories have a set
of
“lab rods,”
which are shorter versions of production coating rods in a variety
of
wire sizes. To test
a coating for coverage, adhesion. opacity, or other characteristics. drawdown samples are
made and examined.
A
piece of the substrate material is attached to the flat surface, and
some coating liquid is puddled near the top of the substrate sample. A lab rod with a
preselected wire size is chosen and pulled manually through the liquid, meeting a precise
thickness of coating on the substrate.
The quality of test samples made by hand is largely dependent on the skill of the
technician. Variations in rod pressure, angle of stroke, and speed can cause differences
in the thickness and consistency of drawdown samples.
4.0
FILM
THICKNESS
Wire-wound rod applies a finite thickness of liquid to
a
web of base material. In production
coating operations, it is difficult
to
measure the actual thickness of a wet coating before
it starts to dry through evaporation, although there are gages made for this purpose. In
addition, many substrate materials are absorbent,
so
that there may be little or no actual
increase in thickness after coating. If the coating is applied
to
a
hard, nonabsorbent material
and is allowed
to
dry, its dry thickness can be measured accurately with micrometers or
other dry thickness gages. Table
1
compares the calculated wet film thickness to the wire
diameter of metering rods. The
1O:l
ratio between wire diameter and wet film thickness
allows an easy selection of the required rod.
5.0
THE
ROD
COATING STATION
In production coating. the web must pass through a wetting station and then
to
the metering
rod, where a measured thickness
of
coating is allowed to pass between the wires, and the
excess liquid is returned to the reservoir. The coating liquid may be applied at the wetting
WIRE-WOUND ROD COATING
133
Table
1
Wire Size Selection Chart
Wire Wet film Wet film Wire Wet film Wet film
Wire diameter thickness thickness Wire diameter thickness thickness
size (inches) (mils) (microns) size (inches) (mils) (microns)
#o
#2 112
#3
#4
#S
#6
#7
#8
#9
#l
0
#l
1
#l
2
#l
3
#l4
#l5
#l6
#l
7
#l 8
#l9
#20
#22
#24
#26
#28
#3
0
#32
#34
#3 6
None
0.0025
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.010
0.01
1
0.012
0.013
0.014
0.0
1 5
0.016
0.0
I7
0.018
0.0
19
0.020
0.022
0.024
0.026
0.028
0.030
0.032
0.034
0.036
0.00
0.25
0.3
0.4
0.5
0.6
0.7
0.8
0.9
I
.o
1.1
1.2
1.3
1.4
1
.S
1.6
1.7
1
.S
1.9
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
0.0
6.4
7.6
10.2
12.7
15.2
17.8
20.3
22.9
25.4
27.9
30.5
33.0
35.6
38.1
40.6
43.2
45.7
48.3
50.8
55.9
61.0
66.0
71.1
76.2
81.3
86.4
91.4
#3
8
#40
#42
#44
#46
#48
#SO
#52
#S4
#S6
#S8
#60
#62
#64
#66
#68
#70
#72
#74
#76
#78
#80
#82
M4
#86
#88
#90
1
0.038
0.040
0.042
0.044
0.046
0.048
0.050
0.052
0.054
0.058
0.058
0.060
0.062
0.064
0.066
0.068
0.070
0.072
0.074
0.076
0.078
0.080
0.082
0.084
0.086
0.088
0.090
3.8
4.0
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
8.2
8.4
8.6
8.8
9.0
96.5
101.6
106.7
111.8
116.8
121.9
127.0
132.1
137.2
142.2
147.3
152.4
157.5
162.6
167.6
172.7
177.8
182.9
188.0
193.0
198.1
203.2
206.3
213.4
218.4
223.5
228.6
L'
For
coating
up
to
0.190
In.
thlck. use Super-coat
rods
station by several different methods. The web can be immersed directly into
a
tank; or
an applicator can be rotated (Fig.
4)
in the reservoir to transfer the liquid to the web at
the top of its rotation (Fig.
5).
It is important to apply an excess of coating liquid at this
station, to let the metering rod to do its job.
(When an applicator roller is used in
a
rod coating system, the speed
of
the applicator
is not a critical factor.) In addition, the machine operator can adjust the applicator roller
speed within
a
side range, even while the machine is running. The web passes over the
metering rod, which may be stationary or may be rotated slowly. The rotation may be
either in the same direction
as
the web, or in the opposite direction. The choice of stationary
rod, movement with different coaters and with different products. Establishing the ideal
speed of rotation will
also
be different from job to job, and converters experiment to find
the best procedure for each run. The most common procedure however, is to rotate the
rod slowly in the opposite direction
to
the movement of the web. The rotation flushes the
134
MACLEOD
Figure
4
Web
immersion
method.
coating material between the wires, keeping the wire surfaces wet, and preventing setting
up and hardening of some liquids. The rotation also distributes any abrasive wear evenly
on the wires, and prevents flat spots from forming. The purpose of the metering rod is to
remove excess coating liquid, allowing a measured amount
to
pass between the wire
windings. The web should pass above the rod,
to
allow the excess liquid to fall back into
the tank. The web, however, need not be perfectly horizontal, as long as the surplus coating
can return to the tank through gravity. Metering rods for production coating can be made
in a wide variety of sizes. The most common core rod diameters are quite small
(3/16
and 1/4 in.), although sizes up to
1
in. diameter are also used. The main advantages of
small-diameter rods is their low cost and ease
of
storing and handling.
These thin rods must be supported in the coating machine, because they are not
rigid and will deflect with pressure from the web. There are several types of rod holder
in common use; the simplest is a square of rectangular steel bar. with a
“V”
groove
machined into it. This rod holder is mounted between the side frames of the coating
machine, and the metering rods
are
placed in the grooves. The
“V”
groove should be
Figure
5
Applicator
roller
method.
WIRE-WOUND ROD
COATING
135
Figure
6
Rod
holder.
ground and polished to minimize wear on the rod and should be mounted accurately, at
a
right angle to the direction of web travel and parallel to the idler rollers of the machine
(Fig.
6).
The design
of
a rod coating station should ensure that the web makes intimate contact
with the wires of the metering rod. The wrap angle, the angle between the web direction
as it approaches the rod and its direction as it leaves the rod, should be
15"
for
a
heavy
web tension, or up to 25" for a light web tension (Fig.
7).
Web tension is a critical factor in the design of
a
rod coating station. With a wrap
angle of 15-25', the web must be tight enough to ensure intimate contact with the metering
rod, yet not
so
tight that the web is deformed by the wires. Adhesives and some other
liquids can solidify between the wire windings
of
the rod whenever the coater is stopped.
Many coating machines have
a
"throw-off" feature, a mechanical means of separating
the web from the rod automatically. whenever the machine is turned
off.
This allows for
quick removal of the rod for flushing and cleaning before the coating material has
a
chance
to congeal between the wires (Fig.
8).
This automatic releasing feature also simplifies rod changing between production
runs. One method used by coating machine manufacturers is a rocker
arm
throw-off system.
A series
of
idler rollers presses the web against the metering rod while the coating machine
is running. Whenever the coater is stopped, the idlers automatically rise, lifting the web
up, away from the metering rod. At the same time, a water flushing system can be triggered
Figure
7
Wrap
angle.
136
MACLEOD
Figure 8
Automatic
throw-off.
to remove coating material from the rods before it can set up. Other techniques involve
lowering the metering rod and its holder when the coater is turned off.
5.1
Rod
Station Variations
Every coating application is different. Converters have created many unique systems for
rod coating which they developed through experimenting in the lab or in actual production.
A
smoothing rod is a finely polished metering rod without wire, which is used several
different ways. It can be used in the web path after printing with
a
gravure cylinder,
to
eliminate the etched pattern and enhance the coated surface. It also can be used after a
wire-wound metering rod, when the viscosity of the coating liquid prevents it from leveling
through surface tension effect (Fig.
9).
When a large amount of coating liquid must be metered off the web, two wire-
wound rods are used in tandem, spaced
1
or
2
in. apart. The first rod has a larger wire
size for doctoring off most of the excess liquid, and the second, smaller wire rod controls
the finished coating thickness.
A
typical coating station may use one or two metering rods and
also
have a position
for a smoothing rod, which can be engaged for certain applications. The coating roll applies
liquid to the moving web which is metered off by one or two rods; then it can be flattened
by the smoothing bar. The complete coating station design must
also
consider draining,
filtering, and pumping the coating liquid, controlling the wet edge
of
the web and disengag-
ing the metering rods when the machine is stopped.
Figure
9
Smoothing
rod.
WIRE-WOUND ROD
COATING
137
6.0
ADVANTAGES AND DISADVANTAGES
Metering rod coating is the third most popular method in use today, behind gravure and
reverse roll coating.
6.1
Low Cost
Rod coating stations are relatively inexpensive and easy to add to existing coaters. Replac-
ing worn or damaged rods, and changing from one coating weight
to
another, are both
inexpensive and fast. The machine downtime for changing rods or cleaning them can be
measured in minutes instead of hours, using much less labor than is required by other
coating systems.
6.2
Precise Coat Weights
Metering rods can be selected to control the wet coating thickness in 0.1-0.2 mil incre-
ments. Without changing the coating formulation at all.
6.3
Lower Setup Cost
Another important factor in the new popularity of rod coating is the worldwide trend
toward shorter and shorter production runs. The faster setups inherent to rod coating
allows the converter more productive running time, in addition to lower labor costs for
changeovers.
6.4
Less Edge Wear
Rod coating offers the converter other advantages because of the method used
to
control
wet edges. In a rod coating system, the dry edge is controlled by wipers
or
deckle straps
on each edge of the applicator
roll.
Because the wipers are constantly wet with coating
liquid, the tendency to scratch the roller is reduced. Even when scoring eventually occurs,
it is on the applicator roll, not on the metering rod (which controls the final coating
thickness). Also, because the wipers are easily moved, their position can be adjusted while
the coater is operating, with
no
downtime at all.
6.5 Limitations
Standard metering rods work best with low viscosity liquids, which will flow easily be-
tween the wire windings. Two-wire Super-coat rods can be used where viscosities are
higher. however. Rod coating also can be used for a flat web, without tight or baggy
edges. In production rod coating, the actual thickness of the coating can be affected by
web speed, viscosity, and other factors. Depending on the application, rod coating speeds
are usually limited to
1000
ft/min., although some coaters claim web speeds up to 2000
ft/min. The critical factor
in
the web speed of a rod coating system is the time required
for the striations formed by the rod to level. The rod meters liquid by allowing a measured
amount
to
flow through the spaces between the wires. Normal surface tension forces the
raised portions to flow out and form a flat, even coating, but there is a time element
involved, which is different for each coating material. The web speed must be controlled
to
allow time
for
leveling before the web is dried.
This Page Intentionally Left Blank
16
Slot Die Coating
for
Low
Viscosity
Fluids
Harry
G.
Lippert
E.rtrusioI1
Dies,
Inc.,
Chippewo
Folls,
Wisconsir!
1
.O
INTRODUCTION
Slot head coating has spawned
a
wide range of designs. some quite radical in their concept.
This chapter discusses conservative manufacturing experience along with the experience
of
a
wide variety
of
processors currently utilizing the proximity or wipe-on method.
2.0
MANIFOLD
THEORY
AND DESIGN
The primary purpose of
a
die is to define
a
width and provide an even coating in terms
of cross-sectional thickness and smoothing. The manifold and coathanger section
of
the
die
is
the main component in accomplishing uniform distribution. Smoothing will be
addressed in
a
later section.
There are two basic styles of manifold design in use today: coathanger shaped, with
a volumetrically reducing cross section (Fig. l), and T-shaped, with a constant cross section
(Fig.
2).
In either style, flow through the manifold is analogous to flow through
a
pipe in
that there is an increasing resistance to material flow as the length increases. The wider
the die (the longer the pipe), the greater the resistance to flow. It follows then that the
primary criterion in a good die design is
to
ensure adequate flow to the ends of the die
as
the width requirements increase.
The coathanger-style die utilizes
a
slot section (preland) with a varying length down-
stream of the manifold to compensate for this pressure increase (see area
B,
Fig. 1). The
Adapted from
1987
Polymers, Laminations and Coatmg Conference
Book
1,
December 1987. Copyright, TAPPI,
Technology Park/Atlanta, Atlanta. GA
30348,
1987.
139
140
LIPPERT
Figure 1
Coathanger volumetrically diminishing manifold.
Figure
2
Constant
cross
section manifold.
SLOT
DIE
COATING
FOR
LOW VISCOSITY
FLUIDS
141
pressure drop
in
the preland section must decrease at the same rate it increases in the
manifold section. If the sum of these two components is equal at any point
in
the overall
flow stream, the result is an even flow.
It can be seen that while the generic coathanger style design is fixed, the overall
dimensions may vary greatly, depending on
a
given die width, flow rate, or general coating
material requirements. Generally
as
the die gets wider, the length of the preland section
(BI)
must get longer and the manifold larger;
as
the flow rate increases, the manifold
must get larger
as
well
as
the height
of
the gap at
El.
The compensating preland section
downstream of the manifold allows the die design to be varied greatly to suit
a
given
application.
These large internal designs are used for applications characterized by coating mate-
rials that vary greatly
in
viscosity levels or call for an extreme range of flow rates. Larger
flow channels are less sensitive to rate and viscosity changes than small channels. Small
internal designs are used for materials that require
a
low residence time in the die because
of
thermal degradation, or high shear rates
to
prevent gelation (thixotropic materials).
In analyzing the coathanger manifold, it must be emphasized that the manifold
decreases in cross section
as
it approaches the ends of the die (dimensions
A
in
Fig.
I);
this rate
of
reduction may
also
be changed to suit
a
specific application. Because material
is flowing out of the front of the die along its entire width, less material is presented to
the manifold
as
it approaches the ends
of
the die. The reduction in manifold volume is
an attempt to keep the velocity
of
the material at the ends of the die to
a
maximum, to
compensate for the lower flow rate, and to prevent carbonization
of
the resin or changes
in viscosity in
a
thixotropic or dilatant adhesive.
In summary, it can be seen that the coathanger manifold design can be modified to
suit an application and still accomplish the primary criterion of even flow distribution.
To adequately design
a
coathanger die, the following information is required
1.
Rheology curve (see Fig.
3).
A
rheology curve,
a
fingerprint of
a
particular
resin, predicts its viscosity level at
a
given shear rate. This is required for all
non-Newtonian or shear thinning tluids.
2.
Flow rate or range of rates.
3.
Material density at processing temperatures.
4.
General material characteristics, such
as
heat degradability or thixotropicity.
The T-shaped manifold in the constant cross section style (see Fig.
2)
has
no
compen-
sating preland section; this is because
of
its inherent design. Rather, this style of die design
relies on
a
larger manifold section to reduce the resistance to flow to the ends of the die;
the larger the manifold, the less the resistance and the better the flow distribution. In
theory, there can never be an even distribution, because
no
matter how large the manifold
is, there will always be some pressure drop across it, and therefore less flow to the ends
of the die when compared to the center.
The larger manifold has some drawbacks in that the residence time is greatly in-
creased and flow at the ends of the die is nearly stagnant. The overall internal flow channel
design cannot be increased or decreased to suit
a
given application,
as
it is restricted by
the need for manifold size to achieve some flow distribution.
If the flow presented to the die is not constant over time, if the fluid is not homogene-
ous in terms
of
temperature and mix, and because there are inherent errors
in
viscosity
measurement and theoretical flow calculations, thickness variations will sometimes occur.