US2972330A - Vacuum seal for coating apparatus - Google Patents

Description

Feb. 21, 1961 w. F. BUGBEE VACUUM SEAL FOR COATING APPARATUS 5 Sheets-Sheet 1 Filed Feb. 15, 1956 Feb. 21, 1961 w, BUGBEE 2,972,330

VACUUM SEAL FOR COATING APPARATUS Filed Feb. 15, 1956 3 Sheets-Sheet 2 Feb. 21, 1961 w. F. BUGBEE VACUUM SEAL FOR comma APPARATUS 5 Sheets-Sheet 3 Filed Feb. 15, 1956 lOOO h=0.05 cm 0.020 inch h=0.025 cm =0.0lO inch h=0.0l25 cm =.OO5 Inch O 0000 0 w 5 32 l mu E Onuoo+w bum +c0E0taUo m n-8am Pumping Requiremerd Per S+age Seal a Four S+qge For VS W/L For Various Values of FIG. 4

2,972,330 VACUUM SEAL FOR COATING- APPARATUS Wallace F. Bughee, Brookline, Mass.,'assignor, by mesne ass1gnments,to National SteelCOrpm-ation, a corporation of Delaware Filed Feb. 15, 1956, Ser.'No.-'565,599

6 Claims. (Cl.11849) This inventionrelates to the continuous-coating of a -metallic or-nonmetallic substrate in a vacuum. -Before tained by positioning vacuum pumps so astoreduce the 1 pressure further in each successive chamber. However,

all prior systems developed along these lines have involved objectionable features which rendered the proc esses economically unfeasible. The weightiest objection to these systems was the diificulty encountered: in coping with the excessive leak caused by the throughput apertures mentioned above. These apertures-were either'so large as to cause pumping requirements to become" excessive or so small as to materially interfere with the passage of the substrate. In the instant invention, due to the development of a-novel typeof vacuum seal and a completely novel arrangement of the seals, this problem has been successfully brought to a solution.

Accordingly, it is the primary object of this invention to provide a commercially satisfactory method of vacuum coating a continuously moving substrate, suitable substrates being sheet steel or iron, organic substrates such as the polyethylene terephthalate sold under the name of Mylar and the like.

Another object of the invention is to provide seals for a vacuum chamber in'which a substrate is to be coated efiicient enough to effectively reduce the leakage of air into thecharnber to a relatively low figure.

Another object of the invention is to. provide a throughput aperture into a vacuum chamber small enoughv to keep the pumping requirements at a reduced'figure. yet large enough to allow passage. of a moving substrate without materially affecting the substrate.

Still another object of the inventionisto provide a throughput aperture small enough to keep pumping .requirements at an acceptable level yet large enoughso as not to materially affect a weld of the type'utilized to .join two sheets of steel, iron or the like.

Other objects of the invention will .in partbe, obvious and will in part appear hereinafter.

The invention accordingly comprises. the process involving the several steps and the relation and the order of one or more of such steps with respectto each'of the others and the apparatus possessing the construction, combination of elements and arrangementof parts which are exemplified in the following detailed-disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the:inature:andifobjects of the inventiomreference: should be :had .to theefollowite Stat Patemfl F 'they can be fed to the coater. tiallyincapableofwithstanding a sharp blowand, there- 2,972,336 Patented Feb. 21., 1961 ing detailed description taken in connection with the accompanying drawings wherein:

' Fig. 1 is a diagrammatic, schematic view of*-onepreferred embodiment of the invention;

Fig. 2 -isa diagrammatic, schematieview I of the vacuum seal;

Fig. 3 is adiagrammatic, schematic cutaway-"-view "of Fig. 2 taken alongthe lines 3-3; and

' Fig. 4 is a graphshowing the pumping requirement per stage for a four-stage seal plotted against W/L (width/ length) for various values of clearance (h).

lnthe instant invention, the' vacuum seals are 'comprised of convex surfaces 'with-close-fitting'concave housings. A substrate is passed through the slots or openings between the convex surfaces and the housings. This substrate is kept taut and in contact with the convex surface, damage to the substratexduetoabrasive rubbing against the sides of the slots thus being avoided. In the continuous coating of sheet steel or iron, this-is particularly important as it is necessary to lap Weld-the ends of the individual'sheets of metal together'soithat This lap weldis essen fore, all unnecessary contact between the moving welded sheets, especially that area of the sheets containingthe welded joint, and objects capable of producing vsharp blows or ,large amounts of abrasive rubbing must :be .avoided. The seal in the instant invention has beenso designed as to allow welded sheetsof iron or steelto be introduced and removed from arcoatingchamber without coming in contact With anything except the-convex surface, which convex surface is, in a preferredembodiment, a roller. The substrate is held taut against the roller and away from the housing, thereby insuring complete" lack of contact withxanything but the roller.

However, the great advantage of the instant vacuum seal over seals of the prior art is due to the far. more efiicient reduction of the rate of leak in the present ,invention, than was heretofore possible. The rate of'leak through a narrow slot can be calculated from the following equation which results from a modified derivation of the Poiselle law equation:

where 10 Wh 24L?) Finally converting K to Q and solving for P 0 P 3 P P where; Q, equals'theleak in c.'f:m.- at-pressur'e 'P It should be mentoned .that, forshortzslotsgintiwhich the:velocity ofgas .is high, ,accertainrreduction,ina-through- ".putis observed lduezztoz-turbu-lence' atithemendssof the'slot.

Several sample calculationsuofithisc-type twermmade ahd along a multistage seal can now be determined, as has been done in Fig. 4 for several sets of values. The only restrictions on Fig. 4 are that (1) an equalvolume of gas be pumped from each chamber and (2) the slots between chambers all have the same dimensions.

Referring now to Fig. 4, where pumping requirements per stage for a four-stage seal are plotted against W/L for various values of h, it becomes readily apparent that the length of the slot (L) is of great importance. Using, by way of example, a slot with a clearance (h) of 0.025 cm. (or 0.010 inch), we observe that, if the W/L ratio is 10, that is, if the slot is times as wide as it is long, the pumping requirement per stage is about 75 c.f.m. If, on the other hand, the W/L ratio is equal to 1, the pumping requirement per stage is about 7.5 c.f.m. The seal of the present invention makes maximum use of this relationship.

Referring now to Figs. 1, 2, 3 and 4, wherein like numbers refer to like parts, there is indicated schematically one embodiment of the invention wherein a substrate such as sheet iron or steel can be vacuum coated. This preferred embodiment of the invention consists of four chambers 1, 2, 3 and 4 and a coating chamber 15, all of which are connected by means of vacuum seals 8 set into the Walls 14 of the vacuum chambers. These four chambers can be maintained at a steep pressure gradient, the pressure decreasing from atmospheric pressure outside chamber 1 to well below a millimeter of pressure inside the coating chamber. A substrate 11 is guided by means of a series of vacuum seals through the four vacuum chambers 1, 2, 3 and 4 into the high vacuum chamber 15, over a source of coating vapors 16, and then is guided by means of another series of vacuum seals out of the system.

Each seal is comprised of a narrow slot 9 between a rotatable, cylindrical roller 12 and a stationary housing 10. In a preferred embodiment of the invention, this slot is a groove in the periphery of the roller on the side of the roller facing the housing. The clearance of the slot should not be more than 0.050 inch and the W/L' ratio should be less than 10 if the pumping requirements are to be within the limits of what is commercially practicable. Suitable brushes 13 of bronze, graphite or the like are utilized to prevent excessive leakage of air through the groove 9 in the roller 12 along that part of thehousing 6 opposite which the roller 12 does not come in contact with the substrate 11.

In the preferred embodiment of the invention shown in the drawings, the seal 8 is so'designed as to cause the substrate to follow an arcuate path of at least 120. A rotary seal is preferred to a linear seal because of the above meutoned difiiculties that would otherwise result due to the lap weld and because, if the air leaking from one chamber to another is made to follow an arcuate path, the amount of air reaching the succeeding chamber is still further reduced.

The optimum size of the slot varies in accordance with the particular substrate being utilized. For sheet steel that is 30 inches wide and 0.010 inch thick, the clearance slot should be about 0.030 inch in order to allow for the passing of a weld. If the sheet is kept very close to one wall of the slot (in the instant case, it is held taut against the groove in the roller surface), the actual leak is determined by the clearance between the sheet and the opposite wall. In this particular case, since the slot has a clearance of 0.030 inch and the sheet is 0.010 inch thick, the net clearance would be 0.020 inch (or 0.050 cm.). If the length of the slot'(L), which length is the linear length of the arc which the sheet describes while within the housing, is 30 inches, then W/L equals 1 1 and, therefore, the pumping requirement per chamber is 60 c.f.m.,-as indicated by Fig. 4.

Using a sheet of the same specifications as the sheet in the example above and assuming the length of the slot to be 3 inches instead of 30 inches, the value obtained from the graph for the pumping requirement per stage is about 600 c.f.m. Therein lies the great advantage of the instant system over all prior systems. The 60 c.f.m. is an entirely acceptable figure, whereas the 600 c.f.m. pumping requirement per stage is almost impossible to meet and certainly is entirely impractical from the commercial point of view.

' It should be pointed out that the same restrictions mentioned above as regards Fig. 4 apply to these calculations also. Actually, it is not necessary that the restriction regarding an equal volume of gas to be pumped from each chamber be absolute. There is some evidence tending to indicate that pumps arranged in decreasing capacities, the largest pump in the chamber closest to the atmosphere, would actually result in a greater purnping elliciency than is realized in the above arrangement. However, for the sake of simplicity, the assumption that equal volumes of gas were being pumped from each chamber was adhered to.

Referring now to the above calculated example for the purposes of illustration, it has been experimentally shown that it is extremely difiicult to draw sheet steel in a straight line under high tension through a slot of this length without causing damage to the sheet, the slot and especially the lap weld. But, in the instant application, due to the use of curved rotary seals, the aforesaid difiiculty is avoided.

In still another preferred embodiment, the entrance end of the seal in vacuum chamber 1 is connected directly to an atmosphere furnace used for drying and outgassing the substrate after cleaning. This embodiment of the invention comprises precleaning a sheet of metal such as steel and then, without allowing the sheet to come in contact with the atmosphere, passing it through an oxygen-free barometric leg, subsequently flash-drying it, and finally passing it into the above described vacuum system so that it can be coated. In a slight modification of the above procedure, the sheet, after precleaning and while under a nonoxidizing atmosphere, is passed through a nonoxidizing bath, subsequently flash-dried under a nonoxidizing atmosphere, and then fed into a vacuum-coating system of the type described above without any exposure to an oxidizing atmosphere prior to coating. This procedure has, in many cases, been found to remarkably improve the adherence of the coating to the metal substrate. The nonoxidizing atmosphere mentioned above is comprised of either a reduced pressure or an inert or reducing'atmosphere.

In still another embodiment of the invention, an aspirator is used to preclean a sheet of steel and to reduce the pressure so that the sheet can be easily dried. Subsequently, of course, the sheet is passed into a vacuum system of the type claimed in order to be coated.

Since certain changes may be effected in the above process without departingfrom the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In apparatus for vacuum coating a continuously traveling material, said apparatus consisting of a plurality of vacuum chambers, separate means to reduce the pressure in said chambers, means to provide coating vapors within one of said chambers, and means for guiding said substrate over said coating vapors, the improvement which comprises means for providing vacuum seals in the walls of one of said vacuum chambers to allow passage of said substrate into and out of said vacuum chamber, said vacuum seals comprising a roller and a close-fitting housing for said roller, the opening between said roller and housing being not wider than 0.050 inch,

said substrate following an arcuate path around said roller, said path describing an arc of between 120 and 180 degrees, and means for holding the substrate taut against said roller in order that contact between the substrate and the housing be prevented.

2. In apparatus for vacuum coating a continuously moving substrate, said apparatus consisting of a plurality of vacuum chambers, separate means to reduce the pres sure in said vacuum chambers, means to provide coating vapors within one of said chambers, and means for guiding said substrate over said coating vapors, the improvement which comprises means for providing at least two vacuum seals in the walls of one of said chambers to guide said substrate into and out of said vacuum chamber, each said vacuum seal comprising a roller, a closefitting housing for said roller, said substrate following an arcuate path around said roller, the ratio of the width to the linear length of said path being less than 10, said path describing an arc of at least 120 degrees,'said roller having a groove extending circumferentially about its periphery, means for holding the substrate taut against said groove in said roller in order that contact between the substrate and the housing be prevented, said housing containing brushes along that part of its circumference opposite which the roller does not come in contact with 6 v the substrate in order that leakage of air into the succeeding vacuum chamber be further reduced.

3. The apparatus of claim 2 wherein the groove in said roller is less than 0.050 inch in depth.

4. The apparatus of claim 2 wherein a groove of less than 0.050 inch is placed in the housing instead of the roller.

5. The apparatus of claim 2' wherein said vacuum chamber is a plurality of vacuum chambers interconnected by means of vacuum seals of the type claimed.

6. The apparatus of claim 2 wherein there are four vacuum chambers and a coating chamber all interconnected by means of vacuum Seals of the type claimed.

References fited in the file of this patent UNITED STATES PATENTS 1,833,497 Prouty Nov. 24, 1931 2,384,500 Stoll Sept. 11, 1945 2,508,500 De Lange et al May 23, 1950 2,557,584 Wishart et al June 19, 1951 2,615,822 Huebner Oct. 28, 1952 2,734,830 Hagge et a1. Feb. 14, 1956 2,795,522 Johns June 11, 1957 2,815,307 Beck Dec. 3, 1957

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