Multiple nozzle jet finishing

Apparatus and method for controlling and leaving different thickness coatings on a continuous web. A jet finishing assembly includes a support means and at least two rotatably mounted jet finishing knives. The orifice opening of each knife is a different height corresponding to a different thickness coating to be left on the web. When the coating thickness requirement for the web changes, the finishing knives are rotated until a knife having the appropriate orifice opening for leaving the required coating thickness is adjacent the passing web.

BACKGROUND OF THE INVENTION 
This invention relates to jet finishing of a continuous web passing through 
a coating pot to control the thickness of the coating and to be able to 
leave coatings of different thicknesses. More particularly, the invention 
relates to an assembly having rotatably mounted jet knives having 
different orifice openings for providing different coating thicknesses. 
It is well known to use jet finishing knives for controlling the thickness 
of liquid coatings in hot dip metal coating processes for metals such as 
zinc and aluminum and in the coated paper and film industries. The liquid 
coating remaining on the metal strip, film or paper web, all three herein 
being referred to as webs, must be uniformly controlled across the width 
and along the length of the web to obtain a satisfactory product. The 
major problem when scheduling production on a coating line, particularly 
in the steel industry, is to schedule extended production runs for 
material to receive the same class of coating weights, i.e. same 
thickness. This means much material must remain in inventory for extended 
periods of time because the coating ordered does not match that of the 
current production schedule. This not only increases costs for the 
manufacturer, but also increases costs for the customer because both must 
maintain larger inventories. Furthermore, an extended line stoppage is 
required to change nozzle size when scheduling a different class of 
coating weight. 
A different but related problem occurs when producing two side differential 
coatings. A differentially coated galvanized steel strip typically has a 
thin alloyed zinc coating on one side of the strip and a thick unalloyed 
zinc coating on the other side of the strip. Switching a production 
schedule for producing two side coatings of the same thickness to a 
production schedule for producing differential coatings normally requires 
a line stoppage to change the nozzle size of at least one of the jet 
finishing knives. 
It previously has been proposed that multiple coating weights can be 
obtained using a pair of opposed jet knives for two side hot dip coating. 
U.S. Pat. No. 3,459,587 issued to D. L. Hunter, et al. discloses higher 
strip speeds, reduced gas pressures in the jet knives and a greater 
distance between the jet knives and the strip can produce heavier coating 
weights. Conversely, lower strip speed, higher gas pressure in the jet 
knives and a shorter distance between the strip and the jet knives can 
produce lighter coating weights. This patent further discloses that for 
given strip speed, gas pressure and distance between the strip and jet 
knives, coating weight can be varied using different orifice heights in 
the nozzles of the jet knives. Increased orifice height decreases coating 
weight due to the increase of gas passing through the larger orifice 
opening. 
Varying one or more of the above parameters to produce different coating 
weights has not been very successful. Line speeds generally cannot be 
varied since they are limited by the heating capability of the furnace in 
the coating line. It is difficult to maintain good coating quality if the 
distance between the strip and nozzle is not maintained at the preferred 
distance determined for a given coating line. It is difficult to be able 
to vary and then accurately maintain constant gas pressure passing through 
the jet knives to produce different coating weights. Temperature changes 
to the gas, thermal expansion of the nozzle orifice, coating metal 
splatter into the orifice, etc. may cause the gas pressure to fluctuate 
from time to time. Finally, it is not practical to produce different 
coating weights by changing gas pressure while maintaining a constant 
orifice height. Producing light coatings using a large orifice opening may 
be limited by insufficient supply of the jet finishing gas. Using a small 
orifice opening for producing heavier coatings may result in poor surface 
appearance i.e. "jet lines". 
It previously has been proposed to use multiple jet knives mounted for 
rotation so that either one of the knives could be used for controlling 
the weight of the liquid coating. The knives are identical, the extra one 
serving as a replacement if the main knife becomes damaged or plugged from 
coating splatter. 
Unlike the prior art, my invention utilizes knives having different sized 
orifices or nozzle openings so that by rotating a different knife into 
position, a different coating thickness can be placed onto the web. This 
arrangement solves the production scheduling and inventory problems 
referred to above. It permits a production schedule to include a variety 
of coating weights or differential coatings without any need for shutting 
down the coating lines to apply a different coating weight to the web. 
Furthermore, each coating weight can be accurately maintained because 
those parameters affecting coating weight do not have to be changed. To 
change coating weight, the operator observes the tail end of a web 
receiving a first coating weight as it passes through the coating pot. He 
then rotates the jet knives until a nozzle having the appropriate orifice 
height is adjacent the passing web for the next lot of material requiring 
a second coating weight. 
BRIEF SUMMARY OF THE INVENTION 
This invention relates to an apparatus and method for controlling and 
providing different coating thicknesses on at least one side of a 
continuous web. The web passes through a coating pot and adjacent to an 
assembly including a support means and rotatably mounted jet knives for 
discharging pressurized gas against the web to remove excess liquid 
coating. Each knife includes a nozzle for discharging the gas with one of 
the nozzles having a first orifice height for leaving a coating of a 
desired thickness and another of the nozzles having a different orifice 
height for leaving a coating of a different desired thickness. The 
assembly includes a valve for permitting gas flow through only the knife 
adjacent the web for removing excess liquid coating. 
When it is desirable to coat both sides of the web, a pair of opposing 
assemblies on opposite sides of the web may be used. If it is desirable to 
protect the liquid coating from air, a sealed enclosure may be placed 
around the jet knives and at least the exit portion of the coating pot. 
It is a principal object of this invention to provide jet knives which 
permit an operator to control liquid coating on a web to a first desired 
thickness and rapidly change to a second desired thickness without 
interrupting the movement of the web on the coating line. 
An advantage of this invention is the reduction of manufacturing costs. 
Inventories may be reduced because material to be coated requiring 
different classes of coating weights may be scheduled together. 
Furthermore, coating line stoppages to install a knife having a different 
orifice opening can be eliminated. 
The above and other objects, features and advantages of my invention will 
become apparent upon consideration of the detailed description and 
appended drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the reference numeral 10 denotes an apparatus for 
two-side dip coating incorporating one embodiment of the invention. A web 
12 moves along a feedpath by passing into a coating pot 14 containing a 
molten coating bath 16. For hot dip metal coating, the web normally is 
given a surface preparation and heat treatment and maintained in a 
protective atmosphere contained in an entry snout 20 submerged below a 
metal level 18. Web 12 continues along the feedpath by passing around a 
sink roller 22 and then passes vertically between stabilizing rollers 24 
supported by arms 26. Web 12 exits coating bath 16, enters a sealed 
enclosure 28 and passes between a pair of opposed jet finishing assemblies 
38, 40, finally exiting through a chimney 30. Enclosure 28 includes a pair 
of access ports 32 which normally are closed by covers 42, connected by 
hinges 44, fasteners 46, and sealed by seals 48, 50. A finishing gas, such 
as nitrogen, is supplied to plenums 52 and flows into finishing assemblies 
38, 40 through pipes 36 which are sealed with enclosure 28 by rubber 
bellows 54, 55. 
The purpose of access ports 32 is to allow an operator access to the inside 
of enclosure 28 for maintenance or replacement of finishing assemblies 38 
or 40. As illustrated in FIG. 2, bellows 55 is expanded as assembly 38 is 
raised by hand crank 57. Cover 42 is raised and assembly 38 may be 
repaired or replaced if necessary. Of course, assembly 40 could similarly 
be raised by crank 56 and be repaired or replaced through another cover 
42. 
As will be discussed in detail later, each jet finishing assembly includes 
a support means and at least two rotatably mounted knives. FIG. 2 
illustrates the knives being rotated (arrow 58) wherein one knife 
providing a first coating thickness on one side of web 12 is being taken 
out of service and a second knife is about to be put into service to 
provide a coating having a second thickness. Of course, the knives for 
assembly 38 can be similarly rotated. 
FIGS. 1 and 2 illustrate the use of the invention for two side hot dip 
metal coating of steel strip using a protective atmosphere. It will be 
understood by those skilled in the art the invention could be used for one 
side coating. For one side coating, only one jet finishing assembly may be 
required. For hot dip metal coating or coating using other coating 
liquids, the use of a protective atmosphere may not be necessary. In that 
event, the use of sealed enclosure 28 would not be necessary. Other gases 
such as air or gases contaminated with air cold be used to remove the 
excess liquid coating. Nevertheless, the present invention has primary 
utility for hot dip metal coating for steel strip with coating metals such 
as zinc, aluminum or alloys thereof. By using an inert finishing gas and 
maintaining an atmosphere in the enclosure having less than 1000 ppm 
oxygen, preferably less than 300 ppm oxygen, and especially less than 100 
ppm oxygen, my invention will uniformly control coating thickness across 
the width and along the length of steel strip that is free of oxides and 
surface defects. For nonoxidizing coating using zinc or zinc alloys, zinc 
vapors escaping into the work environment through chimney 30 are 
undesirable. Zinc vapor formation will be prevented by introducing a small 
amount of water vapor into enclosure 28 such as through a gas inlet 34. 
Details of one and two side coating with hot dip metals using water vapors 
are provided in U.S. Pat. No. 4,557,952 - Mitch et al. which is 
incorporated herein by reference. 
FIG. 3 illustrates an elevational view of apparatus 10 taken along line 
3--3 in FIG. 1. A pair of gas supply means for furnishing gas to each 
assembly such as pipes 36 and bellows 54, 55 are positioned on opposite 
sides of chimney 30. For nonoxidizing coating, sufficient inert gas passes 
through the gas supply means and the jet finishing assemblies to maintain 
a positive pressure at exit 30 to prevent entry of air. The atmosphere 
within enclosure 28 is further protected by seals 48 and 50 around covers 
42. 
Turning now to FIGS. 4-8, details of my novel finishing assembly will be 
explained. Since finishing assemblies 38 and 40 shown in FIGS. 1 and 2 are 
identical, a detailed explanation of only one will be given. FIG. 4 
illustrates an elevational view of assembly 40 in FIG. 2. Assembly 40 
includes oppositely facing knives 60 and 62 each having nozzles 64 and 66 
respectively. The knives preferably are mounted equi-distantly from each 
other, e.g., two knives would be 180.degree. apart and three knives would 
be 120.degree. apart. Knives 60 and 62 are internally separated by a 
divider 68. Finishing gas is supplied to both ends of knives 60 and 62 
through feed pipes 36 and support housings 70 and 72. Knives 60 and 62 are 
rotated by turning an arbor 88 by a bevel gear 78 which is operated by a 
shaft 74. 
FIG. 5 is similar to FIG. 4 except gas pipes 36, support housings 70, 72 
and gear 78 have been removed. Pipes 36 and housings 70, 72 also support 
jet finishing knives 60 and 62. Each end of knives 60 and 62 are welded to 
a mandrel 82. The interior of each mandrel is divided into finishing gas 
passageways which communicate with the interior of each knife. For 
assembly 40, mandrel 82 would have two passageways with one supplying gas 
to knife 60 and the other for supplying gas to knife 62. Each passageway 
receives finishing gas from pipe 36 through openings 84 and 86. The inside 
cylindrical surface of support housings 70 and 72 are partially lined (not 
shown) with a heat resistant elastomeric material. A valve is formed by 
rotatably mounting mandrels 82 within support housings 70, 72 and coupling 
mandrels 82 to the lining so that finishing gas can pass into one of the 
openings, through a corresponding passageway and into the knife in use 
while blocking gas flow into the remaining knives. For example, FIG. 5 
illustrates a finishing gas 80 flowing into opening 84, through the 
interior of mandrel 82, into knife 60 and discharged through nozzle 64 to 
remove excess coating from web 12. The valve prevents gas 80 from entering 
opening 86 and from flowing into knife 62. When knife 62 is to be placed 
into service, knives 60 and 62 would be rotated 180.degree. by bevel gear 
78. Openings 86 now occupy the positions formerly occupied by openings 84. 
As described above, gas 80 now passes through openings 86 and into knife 
62. The valves inside mandrels 82 prevent gas 80 from entering openings 
84. 
FIG. 6 illustrates a sectional view of knives 60 and 62 along line 6--6 in 
FIG. 4. Knife 60 includes nozzle 64 having a first orifice height 90 and 
knife 62 includes nozzle 66 having a second orifice height 92. Knives 60 
and 62 are separated by divider 68. The interior of knives 60 and 62 are 
supported by a longitudinally extending support 98 and laterally extending 
supports 94 and 96. Holes 100 and 102 allow finishing gas to pass through 
supports 94 and 96 respectively when nozzle 64 of knife 60 is being used 
to control coating weight. Holes 104 and 106 allow finishing gas to pass 
through supports 94 and 96 respectively when nozzle 66 of knife 62 is 
being used to control coating weight. 
As illustrated in FIG. 6, each nozzle 64 and 66 includes orifices 90 and 
92, respectively. The orifice height is defined by a pair of lips 108 and 
110. The orifice height will be different for each nozzle and correspond 
to a different thickness of coating. The pressure of the gas flowing 
through orifices 90 and 92 is decreased by increasing the height between 
lips 108 and 110 respectively thereby increasing the thickness of coating 
left on the web. For example, the height between lips 108 for orifice 
opening 90 could be 0.040 inch (1 mm) to provide a first thickness of 
coating and the height between lips 110 for orifice opening 92 could be 
0.080 inch (2 mm) to provide a second thickness of coating. 
Further explanation is now provided for using my invention. For 
galvanizing, typical weights of zinc coatings specified by the customer 
are 0.08 oz/ft.sup.2 (24.6 gm/m.sup.2) or 0.20 oz/ft.sup.2 (61.5 
gm/m.sup.2) per side. Therefore, most of the coating thickness 
requirements can be met by using a finishing assembly having only two 
knives. Orifice openings providing gas flows to leave the above two 
coating weights would vary for each manufacturer depending on finishing 
gas flow rate and distance between the nozzle and web. For processing a 
customer order for 0.08 oz/ft.sup.2 (24.6 gm/m.sup.2) zinc coating, the 
knives positioned adjacent the steel strip would be those having a first 
orifice height. If a succeeding order to be processed required 0.20 
oz/ft.sup.2 (61.5 gm/m.sup.2) coating weight, the operator could manually 
rotate the jet knives 180.degree.. For example, as the tail end of the 
strip receiving the light weight coating is observed by the operator, he 
immediately rotates knives 60 and 62 on assembly 40 until nozzle 66 on 
knife 60 having large orifice 92 is adjacent passing web 12. Similarly, 
the opposing knives on assembly 38 also would be rotated. The heavy weight 
of coating remains on the succeeding strip. For coatings requiring a 
broader range of thicknesses, three or more knives can be provided on the 
assembly with the orifice opening of each nozzle corresponding to a 
different thickness of coating. 
Two side differential coatings can also be produced without interrupting 
the flow of material through the coating line. In the above example, one 
of the assemblies can be rotated 180.degree. so that nozzle 64 on knife 60 
having small orifice 90 for one assembly is adjacent one side of the web 
and nozzle 66 on knife 62 having large orifice 92 for the other assembly 
is adjacent the other side of the web. Small orifice 90 will leave a thin 
coating on one side of the web and large orifice 92 would leave a thick 
coating on the other side of the web. After producing a differential two 
side coating on one or more coils of material, either one of the 
assemblies could be rotated again to begin producing two side coating 
wherein the coating on both sides of the strip would have the same 
thickness. 
The ability to produce different coating weights can be expanded by using 
an assembly having three or more finishing knives. For one or two side 
coating, it is possible to produce three or more different coating 
weights. Of course, it now becomes possible to produce three or more 
different two side differential coatings. If a pair of jet finishing 
assemblies having three or more knives are being used to produce two-side 
coating on a web wherein the coating thickness on both sides of the web 
are the same, one or both of the assemblies can be rotated to produce 
various differential coatings. 
While only two embodiments of my invention have been described, it will be 
understood various modifications may be made to it without departing from 
the spirit and scope of it. For example, one or two finishing assemblies 
may be used. Each assembly will have two or more knives with the orifice 
opening of each nozzle corresponding to a different thickness of coating. 
The actual orifice height used will depend on the liquid coating and 
coating weights required. The assembly may be enclosed in a sealed 
enclosure. Therefore, the limits of my invention should be determined from 
the appended claims.