Note: Descriptions are shown in the official language in which they were submitted.
117734~
Background of the Invention
This invention relates to the coating of narrow
linear material such as strip, and especially wire, with
metal coatings in a molten metal coating bath. More par-
ticularly the invention relates to the combined use of
protective atmospheres and gas wiping in treating linear
material issuing from a molten metal coating bath in order
to establish an accurate and uniform thickness of coating on
the surface of the linear material.
Metallic linear material such as strip and wire
have been economically coated for many years by passing the
linear material through a bath of molten metal, such as
molten zinc or aluminum. Usually the linear material has
been a ferrous material, such as steel or the like. The
coating of aluminum or zinc or sometimes other metals or
alloys, such as tin or terne (an alloy of lead with up to
25% tin), provides corrosion resistance to the underlying
ferrous metal.
Linear material issuing from a molten metal coating
bath usually does not have a satisfactory layer of molten
coating metal on its surface. The molten metal coating is
invariably either too thick, too uneven, or both, or has
some other defect which would prevent the molten metal from
- solidifying into a satisfactory metal coating upon the
; substrate metal. As a consequence, it has been customary to
j wipe the coating in some manner after the linear material
leaves the molten coating bath in order to smooth and/or
reduce the weight, or thickness, of the coating. ~Jarious
~1'77341
wiping devices have been used to wipe the coating while it
is still molten including soft wipers such as asbestos
wipers and the like, rigid wipers such as rolls and scrapers
and occasionally semi-rigid wipers composed of layers of
various materials such a charcoal or gravel through which
the coated linear material passes. More recently gas
wipers, or gas doctors, have been used to blow a gas such as
air, steam or some inert or reducing gas forcibly against
the surface of the molten metal coated linear material to
remove excess metal and smooth the coating of molten metal.
In order to attain good adherence of the coating
metal to the substrate metal it is necessary for the surface
of the substrate to be clean prior to passage through the
molten coating bath. The linear material must, therefore,
be cleaned prior to being coated to provide a suitable
clean, active substrate surface for contact with the molten
coating bath. Once the substrate metal is clean it must be
kept active, i.e. oxide free, until it is submerged in the
molten coating bath. It is therefore necessary to protect
the substrate metal after cleaning either with a coating of
flux or else by immersion or continuous bathing in an inert
or reducing atmosphere. Thus, ferrous linear material
frequently enters the molten bath in a protective or oxygen
excluding atmosphere of some nature. The protective atmos-
phere will usually be composed of either an effectively
inert gas or a reducing gas or gases.
Inert or reducing atmospheres have also been
maintained about the linear material as it exits from the
~7734~
molten bath to prevent excesslve or otherwise detrimental
oxidation of the surface of the coating while it ls still
hot, both before and after the coating solidifles. The
protective atmosphere ls usually contalned ln a protectlve
chamber, or hood, of some form which extends to or into the
surface of the molten bath.
With the more recent frequent use of gas wipers
for smoothing and wiping the molten coating, the use of an
inert or more frequently a reducing gas to wipe the surface
of the linear material has sometimes been adopted to prevent
surface oxidation of the coating metal. In some installations,
- and particularly in wire wiping installations, the wiper has
been enclosed in or attached to a chamber containing a
protective atmosphere so that the molten coating on the wire
is completely protected from exposure to the atmosphere
until it is smoothed and wiped.
The use of a non-oxidizing gas as both a wiping
and a protective gas has been found to be particularly
desirable in the wiping of wire material. Otherwise,
oxidized coating particles on the molten coating surface
tend to increase the viscosity of the molten metal and
result in buildup of a thick viscous oxide coating layer
which seriously interferes with effective gas wiping. The
small circumference of the wire allows viscous rings of
oxide material to form about the wire and break through the
gas barrier resulting in thick rings of coating on the wire,
which rings crack and flake when the wire is bent after
solidification of the coating.
73~
One problem which has been encountered in such
combined wiping and protective gas installations as, for
example, that illustrated in U.S. Patent 3,707,~00, which
discloses a combination of a closed hood, containing an
inert gas, and a wiping die, that may use the same inert gas
as a wiping gas, has been a tendency of the wiping die to
provide very poor control of the thickness of the final
coating if only the force of the wiping gas is depended upon
to establish the thickness of the coating. This has been so
in spite of the fact that such combined wiping &nd pro-
` tective gas arrangements very efficiently and effectively
wipe excess coating from and smooth linear material such as
wire passing through the die. However, the exact final
thickness of coating has been impossible to control without
varying the parameters of the wiping die itself. In other
words, while the smoothing of the coating is very effective
and a large excess of coating material can be removed from
the coated material, actual dimensional control of the
coating thickness by the wiping gas has not been satis-
factory. It has thus been necessary in many cases to vary
the velocity of passage of the linear material through the
wiping die in order to effectively control the degree of
wiping of molten coating from the surface of the linear
material. If the molten coating layer is too thick, it has
been necessary to decrease the speed of passage of the
linear material through the die orifice in order to decrease
the coating layer. If the coating layer is too thin, on the
other hand, it has been necessary to increase the speed of
~773~1
the linear material through the die orifice in order to
increase the thlckness. Naturally, the necessity to adJust
the speed of the coating line in order to attain a deslred
coating weight is undesirable, because such ad~ustment
interferes with other operational and production consider-
ations.
A further problem with prior wiping apparatus and
methods particularly in the coating of wire material has
been poor concentricity of the final coating with the wire.
In a "concentric coating" the coating thickness is sub-
stantially equal on all sides of the wire or all around the
wire. In a non-concentric coating the thickness of the
coating on one or more sides of the wire is significantly
thicker than the thickness of the coating on the diametrically
opposite side or sides. The coating may be concentric on
, . . .
portions of the wire and non-concentric on adjoining portions
of the wire. Usually the concentricity varies in a more or
less random manner along any given length of wire. In fact
it is substantially impossible to obtain a substantial
length of perfectly concentric hot dip coated wire par-
ticularly with prior known apparatus.
The importance of concentricity is really the
avoidance of thin spots in the coating and it will be
evident that thin spots may occur because of other factors
such as out-of-round or oval coating deposits or the like as
well as true non-concentricity. One measure of concen-
tricity then is the number of thin spots in a coating, it
being realized that complete concentricity or absence of
--6--
~ ~ - ~
11773~
thin spots is substantially impossible in hot dip coatlng of
wire. It has been the experience of the present inventors
that in a hot dipped aluminum-zinc coating, for example, the
best or most concentric wire coating which could be obtained
using prior gas wipers - based upon an aim coating of 1.5
mils, or thousands of an inch, (i.e. 0.038 millimeter) and
with a thin spot defined as any coating area of less than
0.5 mil (0.0127 millimeter) as measured by a commercial type
non-destructive spot coating weight detector - would have
over 2.5% of the measurements on any given length of wire
below 0.5 mil (0.0127 millimeter). In other words, with a
thin spot being defined as approximately one third of the
aimed for or desired coating thickness, an average of over
2.5% of all tested spots on any given length of wire will
turn out to be thin spots, or below minimum in thickness.
While this number of thin spots is quite acceptable for most
purposes, especially in a sacrificial coating, such as, for
example, zinc or aluminum-zinc on a ferrous base material,
it does represent a waste of coating metal since somewhat
thicker coatings must be used on all portions of the wire to
prevent an excess number of unacceptably thin spots.
Summary of the Invention
The disadvantages of prior combinations of gas
wiping dies and protective hoods for wiping wire have now
been obviated by the improvement of the present invention.
It has been discovered that the use of critical die parameters
~L1773~
permits the gas wiper to effectively determine the weight of coating re-
maining on the final coated wire without regard to the speed of passage of
the wire through the wiper die and also without regard to the presence or
absence of a protective chamber or hood in association with the wiping die.
; The present invention provides a gas wiping die for wiping linear
material after initial passage of such material through a molten metal
coating bath, comprising:
(a) a die body having a gas wiping orifice circumferentially
surrounding a central throat through which the linear material passes, the
gas wiping orifice being positioned from 0.5 to 15 inches above the surface
of the molten metal bath, the throat diameter being from 0.50 to 1.50 inches,
and
(b) the gas wiping orifice having
(1) an angle of about 10 to 45 degrees with respect to
; perpendicular to the surface of linear material passing through the die,
(2) an orifice width of about 0.010 to 0.080 inch, and
(3) substantially parallel side walls at least about .25 inch
in length.
The present invention also provides a method of controlling the
coating thickness on wire issuing from a molten coating bath comprising:
(a) passing said wire through a gas wiping die with a gas wiping
orifice;
(i) having a width of about 0.010 to 0.080 inch,
(ii) curved sidewalls substantially parallel to each other
not less than about 0.25 inch in length,
(iii) downwardly inclined at an angle of about 10 to 45 degrees
to perpendicular to the surface of wire passing through the die, and
(iv) surrounding a central opening not less than 0.50 and
not more than 1.50 inches in diameter through which the wire passes,
(b) wiping the molten coating on the wire by blowing a non-oxidiz-
-8-
::L17'7341
ing gas through the gas wiping orifice onto the molten coatlng upon the
wire while the wiping orifice is positioned from 0.5 to 15 inches above
the surface of the molten coating bath, and
(c) controlling the coating thickness by changing the pressure
of the non-oxidizing gas.
The critical die parameters are an orifice angle downwardly of
about 10 to 45 degrees and preferably 15 to 30 degrees with respect to
perpendicular to the surface of the wire material passing through the die,
an orifice width parallel with the direction of movement of the wire
through the die of about 0.010 to 0.080 inch (0.254 to 2.032 millimeters)
and preferably 0.020 to 0.050 inch (0.508 to 1.27 millimeters), substanti-
ally parallel sides on the orifice and a minimum length of the orifice
along the line of passage of the wiping gas of not less than 0.25 inch
(0.635 centimeter). The height of the orifice above the molten coating
bath should be from 0.50 to 15 inches (1.27 to 38.1 centimeters) and
preferably about 0.50 to 10 inches (1.27 to 25.4 centimeters) and most
preferably 0.50 to 4 inches (1.27 to 10.16 centimeters), and the throat of
the die must be not less than 0.50 inch (1.27 centimeters) up to 1.50 inches
(3.81 centimeters) and preferably 0.75 to 1.25 inches (1.9 to 3.175 centi-
meters). A most preferable orifice angle has been found to be about 20
to 25 degrees with a most preferable orifice width of about 0.035 to 0.045
inches (0.889 to 1.043 millimeters). The height above the bath surface
will depend somewhat upon the structure of the wiping die. If the die has
a hood or protective gas chamber or partial
-8a-
117734~
protective chamber, i.e. ln which the chamber walls only
partlally surround a space, at the lower end the die can be
positioned farther from the bath surface whereas if there is
no protective chamber best results are obtained if the die
is closely spaced, for example, 0.50 to 4 inches (1.27 to
~ 10.16 centimeters) with respect to the bath surface.
':
Description of the Figures
....
; FIGURE 1 shows in cross section a wire wiping
arrangement in accordance with the improvement of the
invention.
FIGURE 2 shows in cross section a further form of
gas wiper in accordance with the invention.
FIGURE 3 shows in cross section a further form of
gas wiper in accordance with the invention.
FIGURE 4 is a curve illustrating the general
relationship of gas wiping pressure to coating thickness in
apparatus constructed in accordance with the invention.
Description of the Preferred Embodiment
The present invention provides an improved gas
wiping arrangement for wiping molten metal coated linear
material such as wire to both smooth the coating surface and
determine the coating weight or thickness. In accordance
; with the invention, there is provided a gas wiping die which
is positioned adjacent to the surface of a molten metal
coating bath. The gas wiping die may be mounted either
within or closely adjacent to and connected with a hood or
protective chamber which encloses the linear material as it
'
_9_
1177341
passes from the molten metal coating bath to the gas wiping
die. Alternatively the gas wiping die may be positioned
close to the surface of the molten bath without a protectlve
hood. The protective hood, if used, is supplled wlth an
inert or effectively lnert gas which serves to protect the
surface of the molten coating from oxidation until it reaches
the wiping die. A portion of the surface of the molten
coating bath may also be enclosed within the hood to prevent
or minimize the formation of an oxide film or scum upon the
surface of the molten bath.
In order to have the inert or reducing gas deter-
mine or control the final weight or thickness of the final
coating on the wire it has been discovered that the following
criteria must be adhered to. These are:
(1) the gas wiping orifice must be inclined downwardly
at an angle of about 10 to 45 degrees with respect
to perpendicular to the surface of the wire and
more preferably about 15 to 30 degrees from
perpendicular with respect to the surface of wire
passing through the die, with a most preferable
angle of about 20 to 25 degrees,
(2) the orifice thickness or width parallel to the
wire should be between about 0.010 to o.o80 inch
(0.254 to 2.032 millimeters), preferably 0.020 to
0.050 inch (0.508 to 1.27 millimeters) and most
preferably 0.035 to 0.045 inch (o.889 to 1.043
millimeters),
;
--10--
(
1~773~1
(3) the orifice must have curved sidewalls parallel to
each other and equidistant at all points from the
surface of the material being wiped and at least a
minimum of about 0.25 inch (o.635 centimeter) in
length in the direction of the flow of the gas.
In general, the longer the sidewalls are within
the constraints of the dimensions of the die the
better,
(4) the height of the die orifice must be about 0. 50
to 15 inches (1.27 to 38.1 centimeters), preferably
between about 0. 50 to 10 inches (1. 27 to 25.4
centimeters) and most preferably about 0. 50 to 4
inches (1. 27 to 10.16 centimeters) above the
surface of the molten bath,
(5) the throat diameter of the die should be not less
than 0. 50 inch (1. 27 centimeters) nor more than
1.50 inches (3.81 centimeters) and preferably
between o.75 and 1.25 inches (1.9 to 3.175
centimeters).
Very satisfactory wiping-has been obtained, as an
example, with an orifice angle of 22. 5 degrees, an orifice
width of 0. o40 inch (1.016 millimeters) and a throat diameter
of 1 inch (2. 54 centimeters) at between 1 to 4 inches (2. 54
to 10.16 centimeters) above the bath surface. It is
preferable for best results that the inert or reducing gas
used as the wiping gas be a "heavy" gas. However, other
non-oxidizing gases can be effectively used in most cases.
Suitable heavy gases are nitrogen, argon, propane and the
--11--
1177341
like. The term heavy is used in contradlstinction to
"light" protective gases such as hydrogen (H2), methane
(CH4), natural gas and helium. (A heavy wiping gas may be
defined as a gas having a molecular weight or a density
substantially the same or greater than the average molecular
weight or density of air.)
It has surprisingly been found that when the above
die parameters and conditions are strictly adhered to good
control of the coating thickness or weight can be obtained
merely by varying the pressure of the wiping gas. On the
other hand, when a wiping die is used with a completely
closed protective chamber into which the wiping gas is
discharged, as for example in U.S. Patent No. 3,707,400,
; adequate control of the coating thickness or weight cannot
be attained on wire, although the coating is wiped and
smoothed.
Surprisingly also it has been found that very
excellent concentricity of the coating about the wire, or
stated in other words, a decreased number of thin spots in
the coating, can be obtained with the present invention.
; 20 Using the lmproved wiper and method of the invention the
present inventors have consistently been able to obtain
aluminum-zinc coatings, for example, for which the die
invention has been found to be particularly suitable, having
less than 0.3% of the coating thickness measurements made
with a commercial type non-destructive spot coating weight
detector over any given length of wire less than 0.5 mil
(0.0127 millimeter) when the aim coating was 1.5 mils (0.038
-
1~'7734i
millimeter). This is an order of magnitude greater than the
best previous experience of the inventors with prior gas
; wipers where the best results which could be attained showed
more than 2.5% of the readings less than 0.5 mil (0.0127
millimeter). In other words, where a thin spot is considered
- to be approximately one third or less of the aimed for or
desired coating thickness a wire coated in accordance with
the present invention will show less than 0.3% of the spots
tested over any given length as being thin.
In FIG~RE 1 there is shown diagrammatically in
elevated cross section a gas wiping die in accordance with
one embodiment of the invention. A gas wiping die 11 is
positioned a predetermined distance from the surface 13 of
a molten metal coating bath 15. The die per se is comprised
of an outer cylindrical body 17 having internal threads 19
at the upper end within the hollow interior of the cylindrical
body. The cylindrical body has a lower end 21 in which
there is an orifice 23 leading into a gas passageway 24
confined within side walls 25 of a cylindrical gas directing
member 27. The orifice 23 constitutes the so-called throat
of the wiping die.
The cylindrical member 27 is secured to the bottom
of cylindrical body 17 of the die 11 by means of removable
machine bolts 41. It will be understood, however, that any
other suitable connecting means such as, for example, a
threaded connection or the like could be used. Alterna-
tively, the member 27 may comprise a extension of the bottom
or lower end 21 of the die 11.
-13-
117734~
The outer cylindrical body 17 of the dle 11 has an
inner cylinder 43 threaded into it. The inner cylinder 43
has an extension or nose 45 which, when the two cylindrlcal
members 17 and 43 are correctly positioned with respect to
each other, defines between its surface and the inner
surface of the outer cylindrical body 17 an arcuate circum-
ferential gas passageway 47. The lower portion of this
passageway constitutes a circumferential gas wiping orifice
; 49. The central space about which the circumferential gas
wiping orifice 49 extends may be considered to constitute an
upward extension of the throat 23 of the gas wiping die.
Wiping gas is supplied to the circumferential gas passageway
47 via a gas inlet pipe 51 which, it will be understood, is
connected to a supply of pressurized wiping gas such as a
tank or tanks of pressurized gas or a gas generation plant
or the like through suitable intermediate pipe and an
ad~ustable valve means or, if desired, automatic pressure
control means, not shown, the details of which are known to
; those skilled in the art.
The gas wiping orifice 49 in accordance with the
invention has straight sidewalls 49a and 49b which are
parallel to each other and are inclined downwardly at an
angle of 25 degrees with respect to perpendicular to the
surface of the wire. The thickness of the wiping orifice in
a direction parallel to the surface of the wire, i.e. the
distance between the parallel sidewalls 49a and 49b is 0.040
inch (1.016 millimeters). The length of the parallel
sidewalls 49a and 49b is 0.5 inches (1.27 centimeters). The
-14-
1177341
distance between the inner edges of the orifices ls 1 inch
(2.54 centimeters). This last dimension is also the diameter
of the throat 23 of the dJe. The distance of the gas
wiping orifice 49 where such orifice communicated with the
throat 23 from the surface 13 of the coating bath 15 is 4
inches (10.16 centimeters). The height of the bottom of the
cylindrical member 27 above the bath surface is 1 inch (2.54
centimeters).
In operation of the apparatus shown in FIGURE 1
the wire 37 passes through the molten metal coating bath in
any conventional manner, usually down around a lower sinker
roll, or sheave, not shown, and then up through the bath
surface, up through the gas passageway 24, through the
orifice or throat 23, past the circumferential wiping gas
orifice 49 and finally upwardly through the central passage-
way 53 of the inner cylinder and out of the gas wiper.
, As the wire passes by the circumferential gas
wiping orifice 49 it is wiped by a precisely dimensioned,
, compact curtain of gas which has been shaped by the critical
dimensions of the wiping orifice. This curtain of gas wipes
and smooths the molten coating on the wire. Excess coating
is in effect pushed back into the molten coating bath. The
,~ gas used is preferably a reducing or inert gas and should
preferably, it is presently believed, in order to attain the
best control of coating weight, be a heavy gas, such as, for
example, argon, nitrogen, propane or the like. This gas
~ curtain is directed downwardly and inwardly at an angle of
!~ about 25 degrees plus or minus several degrees, toward the
-15_
.
.~
117';13~1
wire to effect the wiping action. The non-oxidizing gas
passes downwardly toward the surface of the molten bath
where it additionally serves to protect the molten coating
on the wire and the molten surface of the bath from oxidation.
Such oxidation would tend to form a coating of oxide on the
surface of the bath which could then be drawn upwardly with
the molten coating on the wire causing an undesirable
roughness on the wire and interfering with smooth wiping of
; the coating. The reducing or inert gas can, since it protects
the molten metal from oxidation, be referred to broadly as
the protective or non-oxidizing gas.
It has been found, contrary to the situation with
previous combined wiping and protective chamber gas arrange-
ments, that when the critical parameters of the present
invention are adhered to, very effective control of the
coating thickness on the wire can be obtained merely by
varying the pressure of the wiping gas. If the critical
parameters of the die are not adhered to, however, effective
wiping control by gas pressure alone is not obtained unless
a critically sized exhaust orifice in the side of the
protective chamber is used as disclosed in an application
filed substantially concurrently with the present application.
In FIGURE 2 there is shown an alternative arrange-
ment of a die and hood for the coating of wire. In the
FIGURE is shown a cylindrical hood 111. The hood 111 has an
exit orifice 117 in the center of the top of the hood. The
hood also has a circumferential bracket 119 in the center
having a central opening in which there is mounted a gas
-16-
117'~3~1
wiping die 121 comprised of an outer cylindrlcal ~ody 123
having internal threads 125 into which is threaded an inner
cylindrical member 126 having a central conical throat 127.
A cylindrical throat member 12~ having an interior passage
129 in the shape of two opposed interior conical ~ections
129a and 129b connected by a central cylindrical section
129c is positioned in the bottom of the outer cylindrical
body 123 and secured in place by machine bolts 131. The
cylindrical throat member is preferably made from a wear
resistant material such as a ha,rd stainless steel. An
annular pasageway 133 between the outer cylindrical body 123
and the inner cylindrical member 126 is connected to a
circumferential gas wiping orifice 134 which leads to the
upper portion of the interior passage 129.
The inner cylindrical member 126 has a short nose
128 which has an outer conical surface 128a. The conical
surface 128a of the nose 128 is parallel to or equidistant
at points from the interior conical section 129a and when
the inner cylindrical member is threaded into the outer
cylindrical body 123 the surfaces 128a and 129a form a
downwardly inclined orifice having substantially parallel
curved surfaces approximately 0.030 inch (0.762 centimeter)
apart, the parallel or equidistant portions of each surface
128a and 129a being approximately 0.25 inch (0.635 centimeter)
in length. This provides a gas wiping orifice 134 having a
thickness of 0.030 inch (0.762 centimeter) and a uni~orm
length of 0.25 inch (0.635 centimeter). The wiping orifice
is inclined downwardly at an angle of 30 degrees with
117'73~1
respect to perpendicular to the surface of the wire and the
opening of the gas wiping orifice is approximately 5 inches
(12.7 centimeters) above the surface 144 of the molten
coating bath 145.
The circumferential bracket 119 which supports the
wiping die 121 divides the cylindrical hood lll into an
upper chamber 135 and a lower chamber 137. The upper and
lower chambers 135 and 137 are not in direct communication
with each other. A gas inlet pipe 141 passes thrcugh the
side of the hood lll and is threaded into an opening 142 in
the outer cylindrical body 123 leading into the annular
passageway 133 . Alternatively there may, in order to obtain
more uniform gas pressure in the annular passageway 133, be
several gas inlets 141. The lower edge 112 of the hood lll
is preferably spaced a small distance above the bath surface
144. This distance may be about . 25 to . 50 inches (0. 635 to
1.27 centimeters) but can be significantly more or less.
In operation a wire 143 passes up through a molten
coating bath 145 exiting from the bath surface 144 into the
lower chamber 137, thence through the wiping die 121, past
the gas wiping orifice 134 where it is wiped by a curtain of
inert or reducing gas issuing from the circumferential gas
wiping orifice, and into the upper chamber 137 from which
the wire 143 exits through the orifice 117.
At least a portion of the wiping gas after wiping
and smoothing the coating on the wire as it passes through
the circumferential orifice 134 passes downwardly through
the interior passage 129 of the throat member 128 into the
-18-
~7'^~34~
confined space of the lower chamber 137 of the hood 111
where the gas shields the molten coating on the wlre and the
molten surface 144 of the coatlng bath 145 directly under
the chamber 137 from oxidation. Excess gas escapes from the
chamber 137 around the lower edge. Alternatively the lower
portion of chamber 137 could be submerged in the molten
coating bath ~orming a substantially completely confined
space within the chamber 137 and excess accumulated pro-
tective gas could pass back through the interior passage 129
and conical throat 127 into the upper chamber 135 where it
would continue to shield the wire and finally be exhausted
through the orifice 117 in the top of the hood. If the
wiping and shielding gas, i.e. protective gas, is a com-
bustible reducing gas, it is preferably burned as it passes
through the orifice 117.
In FIGURE 3 there is shown a further embodiment of
the invention. In FIGURE 3 there is shown a gas wiping die
which is substantially identical to the wiping die shown in
FIGURE 1 with the exception that the cylindrical gas directing
member 27 is not used on the bottom of the die. Since all
the parts of the die are substantially the same as shown in
FIGURE 1, the same designating numerals have been used to
identify the various parts in FIGURE 3 as were used in
FIGURE 1. The die has a throat diameter of 1 inch (2.54
centimeters) and the gas wiping orifice is located 1.5
inches (3.81 centimeters) above the surface of the molten
bath. The other parameters of the die are the same as the
die shown in FIGURE 1. ~ flow of nitrogen gas from the
--19--
11773~1
; wiping orifices combined with the large throat diameter and
the close positioning of the die to the surface of the
molten coating bath very effectively wipes the coatlng on
the wire and allows the coating weight to be determined
merely by adJustment of the pressure of the wiping gas. The
closeness of the wiping die to the surface of the molten
bath and the large volumes of gas which pass through the
large throat diameter allow the surface of the coating on
the portion of wire between the surface of the coating bath
and the wiping die and part of the surface of the molten
coating bath about the emerging wire to be effectively
flooded with the non-oxidizing wiping gas and temporarily
protected from oxidation. In general the shorter any
extension is on the lower portion of the wiping die or if
there is no extension the more desirable it is to have the
die closer to the coating bath surface.
In FIGURE 4 there is shown a diagrammatic graph of
the pressure effects upon wiping efficiency with the coating
thickness obtained plotted against the gas pressure applied
in the wiping die. The plot is approximate only and no
~` precise or numerical relationship are intended to be shown.
The horizontal sections of the curve designated "Low Pressure
Region" (A) and "High Pressure Region" (C) respectively show
: regions in which the linear material is wiped and smoothed,
but the weight of the applied coating is not effectively
controlled by varying the pressure. The "Transition Region"
(B) on the other hand, is a region in which variation of gas
pressure results in varying thicknesses or weights of
.,
-20-
. .
.;
, . ~
1~773~1
coating remaining upon the wire or other linear material.
The exact slope and contour depends upon various factors.
However, so long as the crltical parameters of the present
application are adhered to the general slope of the curve is
maintained and the entire "Transition Region" or "Inter-
mediate Region" can be used for control of the coating
weight by adjustment only of the pressure of the wiping gas.
The wiped coating also has very excellent concentricity with
respect to the surface of the wire. While the exact reason
for the improvement in wiping provided by the use of the
critical parameters of the invention is not well understood
at this time, it appears that the provision of the proper
parameters and conditions decreases the slope of the
transition region curve and thus in effect lengthens the
region in which a change in gas pressure will result in a
change in thickness. The relationship between the coating
thickness and weight and the wiping gas pressure used is
thus improved or made more controllable by decreasing the
amount of change in the coating weight for any given change
in wiping gas pressure. It is thought also that the
relatively large throat and wiping orifices of the wiping
die with respect to the other parameters of the die provides
a relatively lower velocity gas wipe or a "softer" wipe than
comparable wiping with other dies using the same relative
gas pressures and that this may account for the excellent
` wiping control and good concentricity of the coating. At
the present time this is only conjecture, however, and the
Applicants do not wish to be held to any particular theory
-21-
,
11773~
with respect to their excellent results which allow wire to
be effectively wiped at speeds of from 100 to 600 or more
feet per minute (33 to 200 meters per minute or more) with
very excellent results.
: While this invention has been illustrated and
explained with reference to specific gas wiping equipment it
should be understood that the critical parameters of the
invention could also be used with other types of properly
designed wiping equipment and that various wiping gases in
~ 10 addition to those specifically disclosed can be used.
:
:
.
;
~ -22-
