US3781158A

US3781158A - Continuous centrifugal tube casting apparatus using a liquid mold

Info

Publication number: US3781158A
Application number: US00193476A
Authority: US
Inventors: G Leghorn
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-02-11
Filing date: 1971-10-28
Publication date: 1973-12-25
Anticipated expiration: 1990-12-25

Abstract

A molten castable substance (as metals, glass, plastic, etc.) is continuously centrifugally cast to tube on a centrifuged lining of a heavier liquid mold material (examples of which are lead, tin, and Wood''s metal). The molten substance is continuously introduced into the starting end of the centrifugal casting machine and forms a molten, axially moving, cylinder on the liquid lining. The substantially solidified tube floats out of the bore at the opposite end of the casting machine on a lining of the liquid mold material and without contacting the solid portions of the machine''s exit orifice. The unrestricted floating of the tube out of the exit orifice is accomplished by decreasing the diameter of the tube in its molten state by applying a pressure differential between the liquid mold material and the gas internal to the tube being cast. The pressure differential is brought about by various methods as disclosed in the specification. Prior art methods for the continuous centrifugal casting of tube on a liquid mold lining depend on thermal shrinkage of the tube, as it cools in the mold, to permit egress from the machine. Due to the applied pressure differential, such thermal shrinkage to permit unrestricted exit of the tube does not have to be a requirement of the present invention and, as a result, product variability and casting rates can be greatly increased.

Description

United States Patent 1191 Leghorn 1451 Dec. 25, 1973 Filed: Oct. 28, 1971 Appl. No.: 193,476

Related U.S. Application Data Continuation-in-part of Ser, No. 768,983, Oct. 21, 1968, Pat. No. 3,616,842, which is a continuation-in-part of Ser. No. 538,506, Feb. 11, 1966, Pat. No. 3,445,922.

U.S. Cl 425/435, 164/64, 164/84 Int. Cl B22d 11/02 Field of Search 164/81, 82, 84, 64,

[5 6] References Cited UNITED STATES PATENTS 11/1931 Lindemuth 164/81 6/1960 Daubersy et al. 164/84 X 5/1969 Leghorn 164/84 X 3,605,859 9/1971 Leghorn..... 264/311 X 3,430,680 3/1969 Leghorn 164/81 FOREIGN PATENTS OR APPLICATIONS 22,708 0/1895 Great Britain.... 164/81 7,000,792 7/1970 Netherlands 164/81 36 I I 37 aolidified casting Primary Examiner-R. Spencer Annear Attorney-William H. Maxwell [57] ABSTRACT A molten castable substance (as metals, glass, plastic, etc.) is continuously centrifugally cast to tube on a centrifuged lining of a heavier liquid mold material (examples of which are lead, tin, and Woods metal). The molten substance is continuously introduced into the starting and of the centrifugal casting machine and forms a molten, axially moving, cylinder on the liquid lining. The substantially solidified tube floats out of the bore at the opposite end of the casting machine on a lining of the liquid mold material and without contacting the solid portions of the machines exit orifice. The unrestricted floating of the tube out of the exit orifice is accomplished by decreasing the diameter of the tube in its molten state by applying a pressure differential between the liquid mold material and the gas internal to the tube being cast. The pressure differential is brought about by various methods as disclosed in the specification. Prior art methods for the continuous centrifugal casting of tube on a liquid mold lining depend on thermal shrinkage of the tube, as it cools in the mold, to permit egress from the machine. Due to the applied pressure differential, such thermal shrinkage to permit unrestricted exit of the tube does not have to be a requirement of the present invention and, as a result, product variability and casting rates can be greatly increased.

32 Claims, 22 Drawing Figures PATENTEI] 111-382 51975 3.781.158 SHEET 1 01- 200 I 600' 1000 I400 I800 5,

330C, 700%, l fie Temperature C.

bxzru Pan e a 721.66 Diameters 2 {ash/all 77115011115} Provided by H I the Invention j T i 512,. x L

' T g 6 919 5 5 1 1 raaz'us I 'l l 1 1 1 1 1 1 T /0 so so 0 D J) Parzgc of Tube Diameters A Z (vs. Wall Thickness) 1b which p7'0("55C5 Dependent on jllrz'nk age rbr Exlraaion are Pestrz'atcd PAIENTEUBEEZSIE'H snmanra [III I ///7 PATENTEUU 1 saw u as a II/ III [ll/ll PATENTEUUECZS 191s SHEET 7 0F '8 I/IIII/l INVENTOR. GEORGE 4. L EGHO/Z/V BY Pmmeontcz 15 3, 781 158 GEO Q65 1?. LEGHORN CONTINUOUS CENTRIFUGAL TUBE CASTING APPARATUS USING A LiQUllD MOLD RELATED APPLICATIONS This application is a continuation-impart of my prior application, Ser. No. 768,983, filed Oct. 21, 1968, now 5 U.S. Pat. No. 3,616,842, dated Nov. 2, 1971. That application was in turn a continuation-in-part of my prior application Ser. No. 538,506, filed Feb. 1 1, 1966. Said application Ser. No. 538,506 became my U.S. Pat. No. 3,445,922. Continuation-impart applications derived therefrom have become U.S. Pats. No. 3,430,680 and 3,605,859.

BACKGROUND OF THE INVENTION It has been shown, in the above-identified related apl5 plication, that the Maxim process (British Patent No. 22,708 of 1875). is limited to the production of cylinders wherein the relation of the cylinders outside diameter to its wall thickness is expressed by formula 2D=9OT (where D is the OD. of the cylinder and T is its wall thickness) for a low carbon steel exhibiting a phase transformation at about 600C which causes a slight re-expansion of the cylinders diameter. it has also been shown that production is limited by formula 1D -65T for steels not exhibiting such a phase transformation. This is due to the fact that molten steel, cast upon a liquid lead lining, sinks into the liquid lead by about two-thirds of its wall thickness (the Archimedes principle). Therefore, for a steel tube having a wall thickness of one inch, the molten steel will sink into the liquid lead by two-thirds inch and the CD. of such a tube will be four-thirds of an inch greater in diameter than the exit orifice of the Maxim continuous centrifugal casting machine. The diameter of such a cylinder must, therefore, thermally contract by this amount before the casting can exit from the machine. The above formulas were derived on this basis and show that, for a one inch Wall thickness, the cast cylinder would have to be at least 65 40 times the wall thickness, or five feet five inches in ut diam ert One other patent, Daubersy et al (U.S. Pat. No. 2,940,143) concerns the continuous casting of steel tube on a lubricating lining of liquid lead. This process also depends on thermal shrinkage of the cast tube, while cooling in its solid state, to achieve a diameter small enough to permit its passage through the exit orifice of the casting machine. I

The invention disclosed herein is designed to correct the limitation on product output (large diameter cylinders having a fairly thin wall section) of the Maxim process. It also corrects the casting rate limitation of the Daubersy et al process and can, in fact, continuously cast steel tubing at rates of hundreds of tons per hour.

RESUME OF THE INVENTION The present invention provides for introduction of moderate (about 5 to 25 percent by weight of the material being cast to tube) to large (over 25 percent by weight of the tube material) amounts of liquid mold material through the casting machine by way of maintaining the outside diameter of the cast cylinder less than the l.D. of the exit orifice of the machine. By one method of the invention, the liquid mold material is non-flowing and only minor amounts of liquid mold material need be added to produce the same result. By use of other methods of the invention, the amounts of liquid mold material required can be increased or decreased at will. By maintaining the CD. of the cast tube less than the l.D. of the exit orifice, the cast tube can float out of the casting machine on a lining of liquid mold material.

The methods of the present invention can be used in conjunction with thermal shrinkage, if desired. This is accomplished by creating a pressure differential between the liquid mold material, lining the bore (solid container wall) of the centrifugal casting machine, and the gas internal to the tube being cast. By so doing, the level of the liquid mold material is forced inwardly, and the molten steel cylinder at the starting end of the casting machine is also forced inwardly to a smaller diame' ter. The diameter of the molten tube can be equalized at any predetermined size depending on a precalculated pressure differential.

My prior U.S. Pat. No. 3,616,842 shows the means by which such calculations are made and gives specific examples thereof.

it should be noted that the molten tube material will sink into the liquid mold material to a level where it has displaced its own weight of the liquid mold material as required by the Archimedes principle. The 0.1). of the molten tube is decreased to a size that permits unrestricted egress through the exit orifice of the casting machine by a precalculated pressure differential that forces the liquid molt material inwardly, thus constricting the molten tubes diameter. The molten tube thus partially or completely solidifies to a diameter that is less than that of the 1.1). of the exit orifice as required for the tube to float freely out of the bore of the casting machine.

For purposes of clarity, the following specific example, which was originally disclosed in related application Ser. No. 768,983, is presented. in the presented example, the CD. of the tube in its molten state is made equal to the l.D. of the exit orifice. This is for purposes of calculation only. in ordinary casting practice, the OD. of the molten tube can be greater than the l.D. of the exit orifice when allowance is made for thermal shrinkage of the tube, in its solidified state, to reduce its O.D. to less than the l.D. of the exit orifice. in this instance, an external means for slowing down the tubes exit rate is used to make the tubes O.D. smaller than the exit orifice [.D. at a point considerably to the rear of the exit orifice.

1n the example case, the exit orifice l.D. of the casting machine is 10 inches, and this is also the ID. of the centrifuged cylinder of liquid mold material lining the solid wall (container) prior to introduction of the molten steel to be cast to tube. in this specific instance, a molten tube of mild steel,which without an applied pressure differential would sink into the liquid mold material by two-thirds of an inch on the radius, is used. The wall thickness would, therefore, be one inch. Calculations are predicated on the tubes final O.D., after the applied pressure differential has reduced its initial required 10 inches O.D. since the introduced error is relatively small.

The molten steel tube, after application of the pressure differential, has an CD. of 10 inches and and l.D. of 8 inches (wall thickness of about 1 inch) and is centrifugally cast at a rotational speed which is equivalent to 50 Gs (gravities). At the solidification temperature of 1500C, the density of the just-solidifying steel is 7.30 g/cc or 0.264 lbs/cu.inch. However. since the casting operation is being carried out at 50 G's, the effective weight of a cubic inch of the steel (at l500C) is 50 X' 0.264 or 13.2 pounds. But, if we project a square inch area of surface on the CD. of the tube onto the tubes axis (see FIG. 3), we have a truncated wedge removed from the tube wall which has an exterior surface area of one sq. in. and an interior surface area of 0.8 sq. in., along with a radial (wall thickness) depth ofone inch. The volume of this truncated wedge is 0.9 cubic inches, and this volume of molten metal bears on the one square inch of outer surface due to the centrifugal action. Since one cubic inch of the metal weighs 13.2 pounds, then 0.9 cubic inches of the truncated wedge will have an effective weight of 0.9 X 13.2 or 11.9 pounds at 50 G's, and this weight is exerted against the one square inch area on the OD. surface and exerts a pressure of 11.9 psi.

Therefore, in order to raise the liquid level of the mold material inwardly and thus bring the CD. of the molten metal tube to ten inches, a pressure differential, between the liquid mold material and the gas internal to the tube, of 11.9 psi must be achieved. The molten tube metal still sinks into the liquid mold material by two-thirds of an inch on the radius, but the level of the liquid mold material is raised inwardly by this amount by the l 1.9 psi pressure differential applied by the various methods of the invention.

It should be noted that the Archimedes principle states that a floating body will sink into the liquid on which it floats to a point where it displaces its own weight of the liquid. At 50 G's, the floating body effectively weighs 50 times as much, but so does the liquid which is displaced. Therefore, the sinkage is the same regardless of the G factor.

The necessary pressure differential can, and is, created by any one or any combination or permutation of the following five species of my methods (which will herein be designated as Method 1, Method 2,...Method 5 b elovv iandwhen referenced in the teachings of this disclosure.

Method 1. By restricting the liquid mold exit orifice (the annular gap between the solidified tube 0. D. and the centrifuge's exit orifice l. D.),

Method 2. By extending the length of the weir (exit orifice) lip to a sufficiently great extent that the required downstream line pressure drop of l 1.9 psi is'experienced.

Method 3. By creating a vacuum within the cast tube that offsets the sinkage ofthe 1 inch thick layer of steel.

Method 4. By raising the atmospheric (gas) pressure (exterior to the tube and the exit orifice or at the entrance end and exterior to any vacuum seal means) by the desired amount over that of the ambient atmospheric pressure (14.7 p.s.i. is the average sea level atmospheric pressure'and 14.7 11.9 or 26.6 p.s.i. would be required under normal conditions).

Method 5. Instead of a gas pressure differential of 1 1.9 p.s.i. (total of 26.6 p.s.i. where the interior of the tube is at 14.7 p.s.i. ambient pressure) being applied at the enclosure at the exit end as in Method 4, a liquid pressure can be exerted on the liquid mold material (as by a suitable pump or a head of liquid mold material supplied by an overhead tundish or reservoir). By exerting this liquid pressure at the exit end, the liquid mold material will be forced inwardly and constrict the molten tube to the desired diameter. In this case, the bed of liquid mold material will be substantially nonflowing. 1n the case where the extra liquid mold material pressure is applied at the exit end and overflows a fixed-diameter weir at the entrance end (the l. D. of such an entrance end overflow weir would be approximately 10 inches minus 4/3 inches or 8 2/3 inches in the example case of a 10 inches O.D. tube and 1 inch wall thickness at 50 GS), the liquid mold material will flow countercurrent to the axial movement of the tube being continuously cast. Such countercurrent flow is optimum for heat extraction purposes.

Method 5 can also be applied at the starting end (tube casting) of the machine in much the same way of Method 4, and with the same advantage of greater throughput of liquid mold material for heat extracting purposes. Also, both

Methods

5 and 4 can be applied simultaneously at both the starting and exit ends of the machine for purposes of maximizing the pressure differential necessary for reducing the OD. ofa very thick wall of heavy tube. Also, Method 5 can be applied at any intermediate point (as at the mid-length) of the casting machine or at a multiplicity of such points if so desired.

Additionally,

Methods

5 and 4 can be applied in series at the ends of the casting machine in order to facilitate the pressure buildup of the liquid mold material.

With respect to

Methods

1 and 2, it can be noted that these methods are entirely feasible. However, a considerable amount of liquid mold material must be introduced into the system to maintain the desired pressure differential (back-pressure in the case where Methods 1 and/or 2 only are used) at a desired equilibrium value, although this can vary over a wide range depending on whether the cast tube's exit is controlled by the fluid friction inherent to

Methods

1 and 2, or by a braking means (as by magnetic field braking or an external mechanical means such as shown in the Maxim process or other, such as the suction of the internal vacuum when

Methods

1 and 2 are used in conjunction with Method 3), which slows down the axial movement of the cast tube.

Method 3, the creation ofa partial vacuum onv the interior of the tube, is a forceful means of accomplishing the desired pressure differential and in-troduces other beneficial effects as well.

This system exhibits the following advantages:

a. After an initial purge with an inert gas, at start-up, and then applying the suction, the gases given off bythe molten metal are of a reducing or inert nature (as carbon-monoxide, hydrogen and nitrogen), and these gases maintain the inner surfaces ofthe tube in a bright oxide-free condition which permits and facilitates the pressure-welding ofthe contiguous interior surfaces of the tube one to the other.

b. The primary advantage is the reduction of the cast tube 0D. to a point where it is less than the exit orifice ID. of the centrifugal caster.

c. The molten material (being cast to tube) is effectively degassed by the internal vacuum during its entrance into the centrifuge via a conduit extending through and sealed to the nonrotating seal plate. As a matter of note, it is preferred to vacuum-degas molten casting metal prior to its introduction into the continuous centrifugal tube casting device so as to cut down the amount of gas given off by the molten metal.

(I. The liquid mold material has less chance of oxidation since no air is internal to the casting chamber.

e. The internal partial vacuum materially aids the collapse forming operation.

f. The internal partial pressure of reducing gases can be maintained interior to the tube, as has been revealed in the teachings related U.S. Pat. No. 3,616,842, for as long as desired.

In the Method 4, the volume external to the centrifuge is enclosed to afford an effective seal which permits the application of a gas pressure which forces the liquid molt material to back up in the centrifugal caster until it attains the desired ID. This pressurization is accomplished with a dry, inert gas, such as nitrogen, argon, helium, or the like.

Method 4 has the further advantage of preventing any oxidation of the liquid mold material (as lead, leadtin, etc.) since the liquid mold material is protected by the inert gas of the external enclosure. Also, the higher than ambient pressure of the inert gas helps to suppress the vaporizing tendency of the liquid mold at the exit or overflow-end of the centrifuge.

Method 5 exhibits the following advantages.

a. Both the non-flowing and countercurrent flow of liquid mold material create a natural hot zone at the starting (tube casting) end of the machine, and such a hot zone greatly facilitates the leveling and smoothing of the molten cylinder and the solidified product.

b. It is the most foreceful means for reducing the CD. of the tube in its molten state to the desired diameter for free exit. Very heavy wall sections (which may encompass the entire radius of the cast tube) can be cast by this means.

c. Countercurrent flow is recognized as the most efficient means of heat extraction.

d. Very large amounts of liquid mold material maybe forced through the machine for heat extraction, if so desired, by the'various systems of the method.

e. By means of the head or liquid level of the liquid mold material in the supplying overhead reservoir, or standpipe which shows the applied head for any pumping system, the exact pressure of the liquid mold material can be determined and adjusted. It affords a good means of liquid level control within the machine.

From the foregoing advantages of the various methods of the invention, it can readily be seen that all methods can be preferred under specific circumstances.

It should be noted that batch-type centrifugal casting is an old and well-established art. Such parameters as the rotational speed necessary to produce a specific G force for a specific mold diameter are well known as, also, are the lower and upper practical limits of G forces (rotational speeds) utilized. It is sufficient to note herein that the supporting action of the liquid mold material on the outer surface of the tube being centrifugally cast (and, also, the use of

Methods

3, 4 and 5, or combinations thereof) permits the use of much higher rotational speeds (G forces) than is permissable with a conventional dry-wall centrifugal mold.

With respect to the

Methods

3, 4 and 5, it is preferred to utilize higher internal vacuums (Method 3) and lower external positive pressures (Methods 4 and 5) where tubes having a smaller diameter and heavier wall thickness are concerned. Conversely, in the production of large diameter tubes of thinner wall section, it'is preferred to utilize a much lower internal vacuum (Method 3) and higher external positive pressures (Methods 4 and 5) in combination. The reason for this preference is that the ambient pressure on the tube which is directly proportional to the cross-sectional area of the tube and, also, to the pressure differential between the ambient atmospheric pressure and the internal vacuum. As an example, a tube having a 10 inch O.D. (cross-sectional area of 78.5 sq. inches) and an internal vacuum of 4.7 psi (pressure differential of 14.7 4.7 10 psi with regards to a standard atmospheric pressure) would experience a backward thrust of 78.5 in. 2 X 10 psi or 78 5 pounds. In other words, it would require a force of 785 pounds on the tube to counteract the internal suction and pull the tube out of the bore of the centrifugal casting machine. On the other hand, a large diameter thin-walled tube (30 inches in outside diameter as an example) would have a cross-sectional area of 709 sq. inches, and, if the pressure differential (between the interior vacuum and the ambient pressure) was 10 psi, a force of 7090 pounds would be required to get the tube out of the bore of the casting machine. If the 30 inch diameter tube had a A inch wall thickness and was centrifugally cast at 50 Gs, the pressure differential necessary to counterbalance the steel would be one-fourth of 13.2 psi or 3.3 psi. In this case, the required 3.3 psi could be made up entirely by application of a positive external pressure (Methods 4 and 5) of 14.7 3.3 or 18 psi, and the internal pressure of the 30 inches diameter tube would be 14.7 psi or the same as the ambient pressure. By this technique, a very small force (supplied by the head of molten steel) would be required to extract the tube from the bore of the casting machine since the external pressure (of Methods 4 and 5) acts on the periphery of the tube to just counterbalance the weight of the steel tube at 50 Gs and does not act on the end (cross-sectional area) of the tube to create a backforce which must be overcome (as in Method 3) to get the tube out of the casters bore.

With respect to the foregoing example, it can be appreciated that a higher than atmospheric pressure can be utilized internal to the tube being cast to aid in forcing the tube out of the bore of the casting machine. Also, a partial vacuum within the tube being cast can be used as a means of controlling the exit rate of the cast tube from the casting machine since, in the case of an internal vacuum, the exit rate can be retarded by the applied suction. In the example of a large diameter thin walled tube (30 inches in OD.) having 709 square inches of cross-sectional area and a A inch wall thickness at 50 G5 (a pressure differential of 3.3 psi to counterbalance the steel), a pressure of 15.7 psi could be used internal to the tube (one pound gage pressure above atmospheric) and this would cause an outward thrust on the end of the tube of 709 pounds. A restraining mechanism of any type would be used to control the exit rate in such a case. The pressure differential of 3.3

psi could then be made up of a positive external pressure (greater than that of the surrounding atmosphere) of 3.3 1 14.7 or 19 psi or 4.3 psi above atmospheric and this can be accomplished by Method 4 and/or Method 5.

lt is readily apparent from the foregoing examples that a very wide range of latitude is available to the operator, in the application of an internal vacuum (Method 3) and an external positive pressure (Methods 4 and S), for ready extraction of a tube from the centrifugal casting machine A judicious (readily calculated) selection of internal and external pressures is available for all practical casting requirements.

All of the foregoing examples have been predicated on the use of an internal vacuum (Method 3), or an external positive pressure (Methods 4 and or a combination thereof, just counterbalancing the centrifugal weight of the layer of metal being cast and, under these circumstances, any slight thermal contraction (as in cooling from l500C solidification temperature of mild steel down to a collapse deforming and roll-welding temperature of about lll5C) or slight back-pressure (as is normally attendant to such a system by Method 1 and/or Method 2) is sufficient for free exit of the tube from the exit orifice.

Actually, by increasing the through-put of liquid mold material for any fixed conditions of Methods 1 and/or 2, such back-pressure quickly asserts itself, and the molten part of the tube being cast is squeezed-in to a decreased equilibrium O.D. Due to this combined action, of Method 1 and/or 2 in combination with

Methods

3, 4 and 5, the action of

Methods

3, 4 and 5 can be considerably less than that necessary to make the CD. of the cast tube less than the [.D. of the exit orifice of the centrifugal casting machine. The action of Methods l and/or 2 can be utilized to further decrease the OD. of the cast tube to the amount desired for purposes of exit from the system.

In the case where the pressure differential of Method 3 and/or 4 is sufficiently great to more than just counterbalance the centrifugal weight of the material being cast, it might be expected that the tube would decrease in diameter (which it does) to the extent that it would lift away from the liquid mold and permit ingress of air or inert gas into the vacuum of the tubes interior via bubbling through the molten zone of the tube. This can and does happen, but not immediately beyond the point where the pressure differential overbalances the zero-point.

A stable-state condition exits for pressure differentials in excess of the zero-point, and this is due to the wetting action (attraction) of the liquid mold material (especially where tin is present) and the surface tension of the molten material being so cast. This operatingarea (pressure differential beyond the zero-point) is not actually used since the stable-state condition is not that broad and can readily be destroyed by any out-ofbalance or other vibration-producing condition of the rotating system. It does, however, afford a usable margin of safety for the condition of exact counterbalance.

However, Method 3 and/or Method 4 may be used by taking advantage of this stable state condition, to permit the tube to exit, from the casting machines exit orifice 9, in a molten state provided that the conditions of exact counterbalance are closely approximated and the molten tubes 0D. is very close to its solidification temperature. In this case, the molten tube, as soon as it departs from the liquid mold material at the exit end of the centrifuge, is immediately congealed by a great multiplicity of cooling spray jets which create an inward pressure on the CD. of the molten tube and rapidly solidify it. Such cooling sprays can be composed of suspensions of hydrocarbons (as an emulsion of oil in water) in water or solutions of water and various alcohols which act as a reductant and prevent substantially the oxidation of the exiting liquid mold material. Such a means for casting the tube is non-preferred since the conditions for successfully carrying out the process are more exacting than the other preferred systems of the invention. However, it has the advantage of permitting use of a shorter casting apparatus or one of a moderate length with increased rate of casting.

The danger of gas being forced into the interior of the tube via bubbling through the molten tube material (as can occur with the gas-induced pressure differential of Method 4) does not exist where Method 5 is used to create the pressure differential. Since Method 5 uses a pressurized liquid mold material at the exit orifice of the casting machine, any excess pressure will merely decrease the diameter of the tube being cast. Due to this safer action, Method 5 is usually preferred over Method 4.

It is one of the important features of this invention to utilize the advantageous system of a vacuum internal to the tube being cast (Method 3) in the instance wherein tube itself is the end-item, instead of a longitudinal structure formed by inwardly collapsing the tube walls over its entire output length as taught in parent patent US. Pat. No. 3,445,922. In the practice of making tube for its own use, a tube (having a capped or crimped vacuum sealed exit end) is used as the starting tube so that the desired vacuum (depending on the wall thickness of the tube, the densities of the molten tube metal and the liquid mold, the G force of the centrifuge, and the ambient pressure of the atmosphere) can be drawn on the tube interior. The machine then continuously produces a long length of solidified rotating tube which exits into an axially aligned cradle which permits such combined egress and rotation. Such a cradle can rotate with the tube by virtue of the same drive mechanism as that which rotates the centrifugal casting machine. A multiplicity of axially aligned rollers supports the periphery of the tube and, at the same time, can either permit or cause the tube to move axially away from the casting machine. In the case where axial movement is permitted, the rollers are mere idlers which are attached to and rotate with the cradle. In the case where they cause the tube to move axially, the rollers are spring or piston loaded onto the outer surface of the tube to give a friction drive contact which pulls the tube from the bore of the centrifuge as is necessary where an internal vacuum (Method 3), which causes a suction, must be opposed. The rollers, in this instance, are suitably driven by sun gears (via a suitable gear cluster system for such power transmission) and are activated or de-activated by a suitable clutch mechanism. Such mechanisms are well known to those practiced in the art of rotary coupling and un-coupling. At the same time, there is an axial gap in this cradle system, near the exit end of the centrifuge, with appropriate torch reheating means and rotating opposed swaging or forging hammers which move in axial synchronization with the exiting tube and swages or pinches a re-heated section of the tube to a vacuum-tight closure after any desired length has been produced. The pinch or swage closing mechanism then returns to the initial starting place where its operation is re-commenced after another appropriate length of vacuum-sealed tube has been produced. Along with the swaging mechanism, and axially further away from the centrifugal caster by any appropriate length (a two-foot long swaged section and a ZOO-foot length of tube between swages would limit the loss of tube due to swaging to one percent), is located an appropriate cutoff device which travels in axial synchronism with the exiting tube and cuts off the tube at the middle of the swaged or forged-down closure so as not to destroy the integrity of the internal vacuum. After cutting the tube in the axial center of the swaged section, the cutoff returns to its starting point for recoupling to the axial travel mechanism and cut-off of the tube section at the appropriate time. By this synchronized and discretely repeatable sequence of swaging-down and subsequently cutting off the exiting tube, the integrity of the internal vacuum (with its manifold advantages) is maintained during and after the tube casting operation.

It is convenient to forge-flatten the exiting tube (just as a soda-straw can be pinch-flattened in a selected area between thum and forefinger) at the separating point. However, even though this serves as a simple means of sealing and maintaining the integrity of the internal vacuum, it is the preferred method of this invention to swage or peripherally hammer forge such separation points to a solid round having its forge welded center-line coincident with the axis of the tube. These end closures (after separation of the tube lengths at the mid-length of the solid swaged-down closure) can be cut from the tube'ends with an integral portion of the tube length as long as desired. Such cut-off closure lengths are conveniently used to fabricate pressure bottles or tanks for oxy-acetylene, propane storage and the like. In this manner, the closure part of the tube is not subject to re-melt, but affords great economy in the manufacture of pressure tanks and storage vehicles.

My preferred means for extracting (pulling the tube out of the bore of the caster in opposition to the suction of the internal vacuum) is to power the rotating swaging apparatus so that, once it has swaged down the tube to a vacuum-tight solid round, the swaging apparatus remains gripped to the solid reduced tube closure and pulls the tube out of the bore. The axial travel of the apparatus can be powered by any convenient means (such as a chain drive, cog-wheel, worm screw, etc.) and can be geared to or be separate from the rotational means as desired. The system utilizes two such swaging down and pull-out mechanisms so that, while one mechanism is pulling out the tube, the second mechanism can be swaging-down a tube closure some 200 feet closer to the centrifugal caster. Once the second mechanism has swaged-down and gripped the tube closure for powered pull-out, the first mechanism (axially further away from the centrifugal caster) then severs the tube lengths from each other at the mid-length of the swaged-down closure so as not to destroy the vacuum seal. The first mechanism is then returned to the starting point to restart as the second mechanism. The two mechanisms thus continually replace each other at the starting point.

In the foregoing manner, long sections of tube (like straight sausage links) are produced which have an internal vacuum of a partial nature.

A great advantage of continuous centrifugal casting derives from the fact that small producers, who may have only as much as 100 tons of melting capacity, can

effectively compete with the larger steel producers due to the advantage of strategic location. As an example, a ton heat of steel can be continuously centrifugally cast into a tube having a three-foot diameter and a half inch wall thickness, of one continuous length generally exceeding a thousand feet. This is accomplished by capping the end of the starter tube, as by welding, and then producing the thousand foot length of tube by one pour of the one hundred ton melt. Such tube can be converted to pipeline or be collapseformed to a plate one inch thick and about five feet wide. in such a case, the apparatus remains running at the end of pour until the molten steel within the machine has solidified into a solid tube, the external length of produced tube is severed near the exit end of the machine, and the solidifed portion within the machine is utilized as the starter blank for the next one hundred ton heat of steel.

lt is a purpose of this invention to improve the invention of Maxim (Br. Patent No. 22,708) by application thereto of Method 3 (a vacuum internal to the tube being cast) or Methods 4 and 5 (a positive external pressure exterior to the tube and the exit orifice or at the entrance of the centrifuge) and combinations of

Methods

3 and 4 or 5.

In the Maxim process, as improved by the foregoing means, a static (not axially flowing) centrifuged cylinder of liquid mold material has its interior diameter (adjacent to the exit orifice annular weir) substantially equal to the ID. of the exit orifice of the centrifuge. No liquid mold material overflows the exit orifice weir except the dragout that naturally occurs with the Maxim process. Small additions of liquid mold material are added to the system by any convenient means so as to continually make up the liquid level and compensate for any losses due to drag-out, vaporization, etc. The application of

Methods

3, and 4 or 5, as taught in this inventions disclosure, may be utilized to decrease the CD. of the semi-solidifed tube (being cast) to a slight extent, or to its greatest possible extent, or to any inbetween extent as desired. Due to the Maxim process not having available an exiting volume of liquid mold material which can be restricted to build up an aiding back-pressure by the restriction to flow Methods of l and 2, the present invention must depend to a slight or a large extent (depending on the amount of application of the Methods of 3 and/or 4) on the diametrical shrinkage of the tube O.D. as it cools to the desired exit temperature. The exiting rate of the tube is controlled, as in the Maxim process, so that the OD. of the tube thermally shrinks to a less value than the ID. of the exit orifice considerably prior to passage through the annular exit orifice in order to preclude jamming. However, by using Method 5, such a backpressure is established, and the OD. of the cast tube can be readily restricted to one which is less then the ID. of the exit orifice. Method 5 obviates the necessity for thermal shrinkage to aid in decreasing the tubes diameter.

The Methods of 3, 4 and 5, or combinations thereof, are also applied as an improvement to the process of Daubersy et al (U. S. Pat. No. 2,940,143) as a positive and practical means of reducing the CD. of the tube (being cast) to one which is less than the exit orifice ID. The amount of application of the above Methods extends from the minimum to the maximum range as desired. By this means, small amounts of liquid mold material (normally less than 5 percent of the throughput weight of the molten metal being cast to tube on a timed basis) may be continuously circulated through the system in order to maintain a ring of liquid mold material between the outside surface of the tube and the face of the annular exit orifice of the centrifuge. I also apply, as an improvement to the Daubersy et al process, the means elucidated herein for the positive extraction of the centrifugally cast tube so that a controlled rate of output can be effected and thus preclude the danger of jamming the tube into the exit orifice of the casting machines. I also apply, to the teachings of Maxim and Daubersy, the methods of vacuum sealing at the exiting end of the tube so that Method 3 can be effectively applied to these older processes.

Whereas

Methods

1 and 2 are inherent to the Daubersy et al invention, their use is restricted to reducing the CD. of the cast tube in its solidified, but still plastic, state. Also, the CD. of the hot solidified tube is maintained larger than the ID. of the exit orifice so that an interference fit can occur with the lip of the exit orifice by way of preventing the tube s escape from the casting machine until its thermal shrinkage has been accomplished. This is the Daubersy et al means of controlled output. In the present invention,

Methods

1 and 2 are utilized to reduce the CD. of the cast tube in its molten state and to reduce the CD. of the tube to one that is less than the ID. of the exit orifice at a point that is considerably to the rear of said orifice. In this manner, the jamming of the solified tube into the exit orifice (as required in the Daubersy et a] process) is precluded, and the casting machine can be made as long as desired for continued heat extraction and tube solidification. Much higher rates of output are thus permitted.

The liquid mold material, used in conjunction with the continuous centrifugal casting systems herein disclosed, embodies the folowing characteristics: (1) has a solidification temperature lower than that of the material being cast; (2) is substantially immiscible with and non-reactive to the molten material being cast (except where alloying is desired for a corrosionpreventive surface coating such as tin or iron); (3) has a boiling point which is substantially higher than the melting point of the material being cast under the rotational forces involved (high G rotation suppresses the boiling tendency); and (4) has a density which is greater than the material being cast to tube. Many other substances (such as bismuth, indium, silver, etc.) and mixtures thereof can be used as liquid mold materials. The preferred substances are lead, tin, and mixtures thereof.

The novel features which are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the description when read in connection with the accompanying drawings.

IN THE DRAWINGS FIG. 1 is a graphical representation of the change in specific volume of a solidifying and cooling steel;

FIG. 2 is a graphical representation of the example formulas l and 2, respectively, which show the limitations of product output of liquid mold centrifugal tube casting machines which depend on diametrical shrinkage of the solidified tube to accomplish extraction thereof;

FIG. 3 is a diagram of a unit volume section of tubing wall, in the form ofa truncated wedge with radial sides, used in computing the pressure differential required for reducing the CD. of the tube being cast to one that is less than the I.D. of the exit orifice;

FIG. 4 is a partial sectional view of a simplified centrifugal, liquid-mold continuous casting machine wherein no exit orifice lip (reduced diameter annular orifice weir) is used;

FIG. 5 is a more detailed axial sectional view of a liquid-mold continuous centrifugal casting machine adapted to the floating of tube out of the bore;

FIG. 6 is an axial sectional view of one embodiment of this invention depicting vacuum sealing means at the entrance (pouring) end of the centrifuge and a means of vacuum sealing the tube subsequent to the exit end;

FIGS. 6A, 6B and 6C are partial axial sectional views depicting other embodiments of the entrance end vacuum sealing means;

FIG. 7 is an axial sectional view of an embodiment of the exit end of a centrifugal casting machine which depicts means of enclosure to effect a positive pressure (above ambient) external to the exiting tube;

FIGS. 8, 8A and 8B are partial axial sectional views depicting various means of layering the molten metal onto the liquid mold material in a smooth continuous manner;

FIGS. 9, 9A and 9B are partial axial sectional views depicting various means whereby the liquid mold material may be introduced or pressurized at the exit end of the casting machine or at points intermediate between the exit and starting ends (Method 5);

FIGS. 10, 10A and 10B are partial sectional axial views depicting various means at the starting end of the casting machine when the liquid mold material is pressurized by the arrangements shown in FIGS. 9 (Method FIG. 1 l is a partial axial sectional view ofa system for pressurizing the liquid mold material at the starting end of the casting machine by Method 5;

FIG. 12 is an axial sectional view of a simplified casting machine whereby tube is cast vertically downwards on a lining of liquid mold material; and

FIG. 13 is a simplified sectional view of an inverted modification of FIG. 12.

DETAILED DESCRIPTION Referring now to the drawings in detail, and in particular to FIG. 1 (re-drawn from Wulff's Metallurgy for Engineers), l have shown, by way of example, that a centrifugally cast mild steel tube will experience a diametrical shrinkage of about 2.0 percent in cooling from the solidification temperature of about 1500C to a temperature just above that of the melting point of a liquid lead mold material, or 330C. I have also shown that the diametrical shrinkage of a centrifugally cast mild steel tube in cooling from l500C down to 700C is about 1.53 percent.

By using these percent shrinkage values and the densities of the axially flowing centrifuged tube of mild steel and the liquid lead mold at the temperatures involved, I have derived Formulas l and 2, given earlier, which illustrate the minimum diameter of a mild steel tube for any given wall thickness, in order to satisfy the displacement requirements of the Archimedes principle and the diametrical contraction requirements for withdrawal of the tube from the exit orifice of the centrifugal casting machine where such tube shrinkage is the means by which such exit is accomplished.

The limitations of Formulas l and 2 (D=65T and D=9OT, respectively) are graphically illustrated in FIG. 2 wherein, for any wall thickness of mild steel being centrifugally cast to tube on a liquid mold of lead, the tube diameter necessary to permit sufficient contraction of the tube so that it can just escape out of the systems exit orifice, can readily be determined. it should be realized that these are merely example formulas and graphical figures which are applicable to the continuous centrifugal casting of a mild steel tube on a liquid lead mold. Similar formulas and graphs can readily be derived for other systems of casting materials (as aluminum, glass, ceramics, copper, nickel, plastics, etc.) when used in conjunction with other liquid mold materials (as the preferred lead, tin, and lead-tin alloys).

Reference is now made to FIG. 4 which is an axial cross-sectional view of a simplified version of a continuous centrifugal tube caster (casting machine) or centrifuge utilizing a liquid mold and having an exit orifice diameter which is greater than the CD. of the cast tube. In FIG. 4!, the centrifugal caster is rotatable about its axis 1 by means of suitable trunnions and drive mechanisms not shown. At the entrance orifice 2 a liquid mold material 3 is poured upon the rotating annular refractory and thermally insulating part 4 of the centrifuge via spout 5. (It can also be introduced as an addition to the molten tube material.) At the same time, the molten material 6 to be cast to tube, is poured onto the refractory part 4, of the centrifuge by way of spout 7. The refractory part 4 of the centrifuge extends to a point 8 (towards the exit end 9) so as to form a hotzone 10 wherein solidification of the tube is retarded and where the molten tube material 6 and the liwuid mold material 3 have time to layer into over-and-underlying cylindrical shells in the liquid state. The refractory part 4 is enclosed in a structural shell M which supports the refractory part t and then extends to the exit end 9 as the solid wall 12 of the centrifuge. The solid wall )12 is cooled on its exterior surface by multiple peripherally arranged jets of water (not shown) or other cooling material so as to remove heat from the molten tube material 6 through the liquid mold lining 22 and solidify the molten material to a solid tube 14. The solidified tube lid continues out of the centrifuge into an axially aligned and rotating cradle (not shown) and is intennittently cut off to desired lengths by any desired mechanism, such as that shown in the Maxim patent (Br. 22,708). The liquid mold material 3 cascades at from the annular exit end 9 of the centrifuge into an annular trough (not shown), such as that used in U.S. Pat. No. 2,866,703, issued to Gross in 1958, wherein an axially flowing molten metal effluent is spun out of the exit end of a centrifuge into an annular catch basin. The liquid mold material 3 is then recirculated back to the pouring spout 5 by any convenient means, such as that of U.S. Pat. No. 2,617,148, issued to Ryan in 1952, wherein a metallic liquid mold material is recirculated from the exit end of a casting machine back to the entrance end via suitable heat exchangers (coolers) and a suitable pump.

it should be noted that the continuous centrifugal tube casting machine of FIG. 4 utilizes a long bore and a limited amount of liquid mold material addition so as lid to accentuate the shearing action (resistance flow) in the liquid mold material. The bore of the casting ma chine can be extended to the point that heat extraction through the solid walls 12 of the casting machine is sufficient to solidify most of the liquid mold material 3 near the exit end 9. In this manner, the resistance to flow of the liquid mold material is accentuated (Method 1). This simplified type of casting machine (having no exit orifice lip or a built-up lip of solidified liquid mold material) exhibits controlled output of the cast tube by its very long length since such a length resists the movement of the tube axially along its bore. It has the drawback of requiring an inconveniently long mold to permit the tube material to solidify prior to exit. However, it can be used with any convenient other means to restrict the exit rate of the cast tube and permit solidification thereof in a machine of moderate length.

The refractory part iof the centrifuge is preferably made of pyrolytic boron nitride or pyrolytic graphite with the C" planes (the plane of low heat conductivity) being perpendicular to the axis 1 of the bore and the A plane (the plane of greatest heat conductivity) being parallel to the axis of the bore. in this manner, the inside (l.D.) of the hotzone Ml is at a high and uniform heat that prevents solidification in that area. By greatly extending such a hot zone, an extended hot zone results which permits the accentuation of gravity segregation to a useful extent.

The use of an extended hot zone and the benefits which can be made to accrue in certain cases (such as the manufacture of automotive sheet steel) are discussed in my prior U.S. Pat. No. 3,616,842.

This system has the virtue of extreme simplicity; however, due to the high G forces involved, the liquid mold material can have a higher exiting flow than the cast tube with (if used) controlled pull-out. This flow differential can cause wrinkling (shirt-sleeving) of the tube surface at the point of incipient solidification, and this surface roughness anchors the liquid mold material and results in excessive drag-out.

FIG. 5 is illustrative of a more sophisticated system for the continuous centrifugal casting of tube on an axially flowing lining of liquid mold material. A criterion of the apparatus of FIG. 5 is that the OD. of the tube (prior to the point of exit) be less than the exit orifice ll). in FIG. 5, the molten tube material 6 pours into an annular trough 16 which is similar to the annular distributing chamber used by Stravs and J ager in U.S. Pat. No. 777,559 of 1904 and serves to take up the impact of the inpouring molten material 6 and to evenly distribute it, via the refractory annular shelf 117, as a molten cylindrical tube within the bore of the centrifuge. The refractory part 4- of the centrifuge is extended towards the exit end 9, as shown, so as to form a hot zone It) whereon the cylinder of molten tube material 6 becomes leveled or layered into a smooth cylindrical tube 26 on top of a thin cylindrical layer 118 of hot liquid mold material.

The liquid mold material 3 is poured into an annular sump 19 and moves (via a multiplicity of longitudinal holes 20 peripherally spaced around the base of the refractory part 4) downstream in the centrifugal caster via the main series of flow-holes 2b to the main exit 2t whereat the main part of the cooler liquid mold material flows into a heat-extracting ring 22 of liquid mold material which both supports and solidifies the ring of molten material to an exiting solid tube 14. Upstream from the main exit 21 of the liquid mold material is another series of annular liquid mold flow-holes 23 via which a restricted (quite small) amount of liquid mold material forms a thin lining 18 of very hot liquid mold material which extends downstream for the length of the hot-zone l0 and permits rapid and effective cylindrical layering and leveling of the molten 6 and liquid 3 materials. The cylindrically layered ring 26 of molten material substanially solidifies to a solid tube 14 on the ring 22 of heat-conducting liquid mold material which flows axially down the bore of the centrifugal tube caster dowards the exit end 9 and becomes a thin ring 24 of restricted flow (in accordance with Method 1 for creating a back-pressure on the liquid mold material 3) as it passes over the exit orifice weir 25 of axially extended surface area, adjacent to the periphery of the solidified tube 14, which creates a line pressure drop along its length (in accordance with Method 2 detailed in this disclosure) which accentuates the back-pressure on the liquid mold material 3 to the extent that the DB. of the cast tube is maintained less than the ID. of the exit orifice weir. The rotating solid tube 14 exits axially from the centrifuge for cut-off, capping, seal crimping, or continuous collapse deformation as desired, while the liquid mold material 22 spins off as a tangential stream 15 into a suitable annular catch-ring 83 and is recirculated by conventional means not shown. These means, along with the rotational mechanisms and spray cooling method, are indicated but are not detailed since they are a part of the prior art and well understood by those versed in such techniques. l-Iere also, the hot zone, as 10, may be extended in length so that slow cooling of the molten tube material can be accomplished. In this manner, when desired, accentuated gravity segregation results (as delta ferrite being centrifuged towards the outside surface of a mild steel tube which is later to be converted to automotive sheet steel).

In the mechanism of FIG. 5, it is preferred to use a means of any type (not shown) to control the rate of exit of the cast tube out of the exit orifice. In this manner, the casting machine can be of any convenient length, and the axial movement of the exiting tube is slowed down to an extent that guarantees the CD. of the tube being less than the LB. of the exit orifice at a point considerably to the rear of the exit. In this manner, the cast tube floats out of the bore on a lining of liquid mold material and without danger of jamming. However, the length of the machine is not unduly restricted since casting rates can be increased merely by lengthening its bore.

FIG. 6 is illustrative of a vacuum seal means at the entrance end 2 of a liquid mold continuous centrifugal tube casting machine wherein a solid non-rotating disc has its periphery 31 immersed into the liquid mold material 3 which is contained in the annular rotating trough l9. Passing through and vacuum sealed to the non-rotating end plate 30 are the liquid mold conduit 5, the molten tube material conduit 7, a dry inert gas purge tube 32, and a vacuum suction outlet 33. The purge tube 32 (or other sealed entrance conduit) may be used as a plasma torch entrance for the purpose of heating up the refractory part 4 prior to start-up. In this instance, the inert gas (as helium, argon, nitrogen, etc.) from the plasma torch also acts as an initial purge of the centrifuge cavity, and the torch melts down the starter blank which has solidified within the bore of the centrifuge from the prior shut-down operation. The suction tube 33 is fairly large and connects to a vacuum pumping system (not shown) so that the interior cavity of the centrifuge can be continuously pumped down to any desired vacuum.

Exterior to the exit end 9 of the centrifugal casting machine is a set of opposed forging

rolls

34 and 35 which travel axially and in synchronism with exiting tube 14. At the same axial location and at right angles to the plane between the axes of the forging rolls (34 and 35) are two opposed banks of burners (as, not shown, plasma torches) which maintain the heat of the exiting tube 14, or bring it to a desired forge welding temperature. These forging rolls 34 and 35 move synchronously and axially along with the hot tube and gradually come together with sufficient force to collapse a small portion of the tube (as a two-foot length) to a solid round having a forge welded interior 36 which is vacuum tight. Such collapsed sections of the tube can be as far apart as desired (as every 300 feet of solid tube length) and provide the vacuum seal to the tube at the exit end of the centrifugal caster. Further on, and after another seal has been so forge-closed, the solid section 36 can be cut off at its mid-length 37 for removal of the discrete length of vacuum sealed sausage-like tube lengths, for use as previously described. It can be appreciated that other conventional means, as swaging, flat-crimping, etc., can be used to form the discrete collapsed section for vacuum closure, beyond the exit end 9, of the hot tube. Also, the axial travel of the sealing rolls (34 and 35) can be extended (as to 300 feet) so that they act as pull-out grips for the tube so cast.

FIG. 6A is a partial sectional axial view of another configuration of the entrance end 2 vacuum seal means wherein the stationary seal disc 30 is peripherally immersed in an annular trough 40 of a low melting liquid metal, such as Woods metal or molten tin. It has the advantage of permitting the seal to be at a lower temperature and obviates oxidation losses of the seal fluid. In this case, both the molten tube material and the liquid mold material 3 are subjected to the internal vacuum at the entrance end 2.

FIG. 6B is representative of another such configuration wherein the annular seal trough 40 is intermediate between the molten tube material trough 16 and the liquid mold material trough 19. By this means, the liquid mold material is not subject to the internal vacuum, but to the ambient atmospheric pressure, and this helps to raise the level (decrease the CD. of the molten tube 26) of the liquid mold material within the bore of the caster.

FIG. 6C is another variation of the vacuum seal means at the entrance end 2 wherein the method of FIG. 6B is further enhanced by use of another end plate 41, exterior to the end plate 30, which is peripherally immersed into an annular rotating trough of liquid sealing metal 42. This system permits the liquid mold material 3 in annular trough 19 to be pressurized via inert gas tube 38 while the interior cavity of the centrifuge is subjected to vacuum. The system of FIG. 6C is even more effective in reducing the CD. of the molten tube 26 to the desired size.

FIG. 7 is illustrative of a means for applying a positive pressure of inert gas to the outside of the solidified tube 14 at the exit end 9 of the continuous centrifugal tube caster. The inert gas 50 is intoduced into the end closure 51 via the high pressure gas tube 52, and the pressurized gas 50 acts on the liquid mold material 3 at the point of tangential spin-off so as to produce a greater than normal back-pressure on. the liquid mold lining 22 within the centrifuge. This back-pressure (Method 4) causes the heat-extracting ring of liquid mold material at 22 to push inwardly and decrease the CD. of the molten metal tube to any desired limit.

The end closure 51 is sealed at the annular area 53 (exterior to the exit end 9 of the centrifugal tube caster) by means of an iris ring of carbon or graphite blocks 54 of layered blades (other structural materials, such as ceramics or metals, can be used, but are nonpreferred), which are contained within the annular holding rings 55. An annular pressure cavity 57 is behind the iris blocks 54 so that, by pressurizing this annular cavity 57 by means of the high pressure inert gas line 58, the iris blocks 54 are forced against the OD. area 53 of the centrifuge to form a pressure seal. Alternately, the seal at the area 53 can be of the liquid metal type as designated by trough 40 of FIG. 6A.

A similar inert gas pressure seal exists at area 60 on the opposite. side of the end-closure 51 so as to prevent undue gas leakage around the tube periphery. This iris of carbon 'ploughs or blocks 61 also act as scrapers to remove any excess liquid mold material from the periphery of the tube. Alternately, a carbon iris block 62 can be used which has a multiplicity of small radial holes 63 leading from the annular pressure cavity 64 to the ID. of the blocks 62 at the area 60. Passage of high pressure inert gas (as nitrogen) through the holes 63 onto the periphery of the tube 14 at area 60 causes a gas bearing action which wipes back any excess liquid mold material into the closure 51 and, at the same time, maintains the desired inert gas pressure therein. As a still further alternate, the pressure cavity 64 may be pressurized with relatively cool liquid mold material 3 so that a liquid bearing seal is formed. This alternate would only be used where a maximum amount of liquid rnold material was desired as an exterior coating to the tube so produced.

The iris ring of layered and overlapping carbon blades act as a variable diameter pressure containing seal whichwill conform to any inadvertent changes in the tube diameter. More importantly, at startup, the solidified starter tube is at a relatively low temperature and has a correspondingly smaller diameter than when in the hot casting state. In this instance, the iris seal permits startup without requiring a substantial increase in the flow of liquid mold material to maintain the melted tube at the desired diameter.

Referring to FIG. 8, this partial axial sectional view of the entrance end 2 of the centrifugal continuous tube caster illustrates s simplified means of sluicing the molten tube material 6 onto the ID. surface of the axially flowing ring 22 of liquid mold material 3. In FIG. 8, the peripheral flow-holes for the liquid mold material 3 terminate downstream at a point 27, and the refractory part 4 continues downstream and tapers to an annular feather edge at point 28. At point 28, the axial flowing annular rings of molten tube material 26 and of liquid mold material 22 come into heat exchange contact with a layered laminar flow. The shelf 17 of the refractory part 4 acts as a hot zone for layering and leveling of the molten metal 26. This is the simplest technique, but not the preferred one, for introducing the I6 molten tube material layer 26 onto the liquid mold layer 22.

FIG. 8A represents an improvement of the method for sluicing the molten ring of axially flowing tube material 26 onto the axially flowing ring of cool liquid mold material 22 via an interposed thin ring I8 of axially flowing hot liquid mold material which is introduced onto the shelf 17 of the refractory part 4 by way of the small inclined flow-holes 23 to produce a hot zone or extended hot zone 10 as desired.

The preferred technique for producing a hot or extended hot zone 10, and for bringing the axially flowing annular streams of molten tube material and hot and cool liquid mold materials into laminar contact, is illustrated in FIG. 8B. In this technique, an annular trough 16 is filled with a small flow of liquid mold material 3 by way of the small ducts 23 which lead from the liquid mold trough 19 to the bottom of the molten metal trough 16 from whence it flows internal to the ledge 17 of the refractory part 4 as a hot, relatively thin lining which supports the molten metal ring 26. The molten tube material 6 pours onto the surface of the liquid mold material, which fills the trough l6, and heats the liquid mold material to a temperature above the melting point of the tube material. The annular trough 16 serves the purpose of decreasing the impact of the mo]- ten tube material input 6 and of creating an effective layering and leveling zone even prior to the downstream hot zone represented by the relatively thin hot liquid mold lining 18. At the downstream sharp edge 28 of the refractory part 4, the hot liquid mold lining l8 continues downstream for a short distance and acts as a buffer between the axially flowing cool liquid mold ring 22 and the molten tube material 26 and prevents too rapid chilling of the tube. It is preferred that all three annular rings (the molten tube material ring 26, the hot liquid mold 18, and the cooler liquid mold 22) have an approximately synchronized axial flow rate at the point 29 where solidifcation of the molten tube metal begins.

All of the systems illustrated in FIGS. 8 to 88 can be used in conjunction with the entrance end vacuum seal means of FIGS. 6 to 6C.

FIGS. 9, 9A and B illustrate various means by which the pressurized (as by any means such as a pump or overhead reservoir, not shown) liquid mold material 3 can be introduced at the exit end 9, or at any point intermediate between the exit and starting ends, of the casting machine.

FIG. 9 shows an annular trough assembly 151 which is similar to assembly 51 shown in FIG. 7, with the exception that, instead of a pressurized gas being introduced into the assembly 51 to create a back-prssure on the liquid mold material 22 lining the bore of the casting machine, the liquid mold material itself is introduced under pressure and fills the assembly trough 151. The walls of the trough assembly 151 are sealed to the exterior surface of the rotating mold wall 13 at point 53 and to the exterior surface of the rotating solid tube 14 at point by means of iris rings 157 as shown in FIG. 7. The makeup of these iris seal rings are shown in greater detail in FIG. 4 of my US. Pat. No. 3,605,859 issued Sept. 20, 1971.

' Whereas the non-rotating iris seal rings at 53 and 60 are shown as being forced against the rotating

members

13 and 14 by means of a liquid or gas pressure at their back faces, it is preferred to spring-load the iris rings since the liquid mold material at the point of introduction into the casting machine is relatively cool. FIG. 9 shows the basic assembly 151.

FIG. 9A shows a series of such assemblies 152, 153, etc. on either side of the basic assembly 151. The other contiguous assemblies 152 and 154, which is not shown, may be pressurized with liquid mold material 3 at a lower pressure than that in assembly 151 by way of maintaining the pressure therein. A multiplicity of such sequential assemblies extended towards either the exit or entrance end of the apparatus can be used if desired.

FIG. 9B shows a pressurizing assembly 159 which can be located at any convenient point or points between the exit and entrance ends of the casting machine. The assembly 159 is similar to the basic pressurizing assembly 151, except that the iris seal rings 157 are forced against the solid mold wall 13 at points 161 and 162 of either side of an annular ring of apertures 160 which provide entrance for the pressurized liquid mold material in assembly 159, to form the liquid mold lining 22.

In the arrangements of FIGS. 9-98, the length of the basic assembly 151 can be greatly extended to give greater heat extracting contact between the cast tube 14 and the liquid mold material 3. More than this, an overflow standpipe or weir, not shown, can be located at the top of assembly 151 to create an overflow head of liquid mold material. In this manner, the heated liquid mold material in assembly 151 is continually replaced with cooler liquid mold material for more efficient heat extraction. Likewise, the assemblies 152, 153, etc. can be extended or multiplied for the same purpose.

In the case where such an assembly (as 151) partly encompasses the outgoing tube 26 in its still molten state, it is preferred to introduce the liquid mold material tangentially and in the same direction as the rotation of the molten tube and in general synchronization with its rotational speed to create as little disturbance to the molten material as possible.

FIG. 10 shows the design of the casting machine at the entrance or tube casting end, in its simplest form, when the liquid mold material 3 is back-pressured (Method by any of the arrangements shown in FIGS. 9. In this case, the liquid mold material has no gross axial flow, but is merely pressurized to a liquid level that will float the cast tube out of the bore of the casting machine.

FIG. A shows a similar, non-axially flowing, entrance end design whereby the level of the liquid mold material may be observed in annular trough 19.

FIG. 10B is similar to FIG. 10A except that the trough 19 has an overflow weir 1 that permits the liquid mold material to overflow into an annular collecting trough, not shown. By this arrangement, the lining 22 of liquid mold material 3 can flow axially countercurrent to the movement of the cast tube for better heat extraction. The arrangement of FIGS. 10 and 10A provide a natural hot zone of liquid mold material at the starting (tube casting) end of the casting machine. However, the arrangement of FIG. 10B is greatly preferred since, with a non-flowing lining 22, the hot zone can extend axially towards the exit end of the casting machine to a greater extent than desired due to centrifugal layering of the very hot (and therefore lighter) liquid mold material adjacent to the periphery of the tube material.

FIG. 11 shows an arrangement whereby the liquid mold material 3 can be forced into the casting machine at its starting end by means of a pressurizing assembly 151, the walls of which locate against the solid mold wall 11 and an inward cylindrical extension thereof 93. The arrangement of FIG. 11 shows a small amount of liquid mold material flowing into the hot zone 10 by way of a plurality of small vents 23, while the preponderance of the cooling liquid mold material 3 comes into contact with the molten tube at a downstream point 21. The small vents 23 can be dispensed with, if desired, to create a non-axially flowing layer 18 of liquid mold material in the hot zone 10. A higher temperature hot zone will result.

FIG. 12 shows a simplified apparatus for the continuous centrifugal casting of tube vertically downwards on a lining of liquid mold material. The apparatus of FIG. 12 is illustrative only since all of the designed machines shown for continuous horizontal tube casting can be adapted to cast vertically downwards or upwards by simple design changes, such as those shown in FIG. 1].

Horizontal casting has by far the greater advantage inasmuch as the molten tube material and the solid tube produced can be handled at floor level, and much longer tubes can be produced with far greater facility.

In FIG. 12, a relatively small amount of liquid mold material 3 is introduced into the apparatus by way of spout 5 and annular trough 19. The liquid mold material then flows, by way ofa plurality of small holes 10], onto the bore of the casting machine and forms a thin layer 22 of centrifuged liquid mold material between the solid mold wall 13 and the centrifuged cylinder of molten tube material 6. The molten tube material 6 is introduced through the entrance opening 2 by way of spout 7 and flows tangentially, and in the same direction as the rotating apparatus, to create a smoothing action. Layering and smoothing of the molten tube material into a cylinder is also facilitated by use of a hot zone 10 which is maintained as short as possible for said layering in order to congeal the molten cylinder to tube prior to gravity accelerating the molten layer of tube material downwards. The apparatus can use an internal offset and tapered rotating core mold, now shown, as illustrated in British Patent No. 984,053 issued in 1963, to accomplish this same purpose. External to the solid walls 13 of the apparatus are a plurality of spray nozzles, not shown, similar to those of FIG. 5, for heat extraction. At the exit end 9 of the casting apparatus is an annular trough which may be similar to annular trough 83 of FIG. 5, but is shown in FIG. 12 as having annular iris seals as depicted in FIGS. 9. External to the exit end 9 of the apparatus are a plurality of rolls, 102 and 103 being shown, which control the exit rate of the cast tube and prevent it from falling out of the bore of the casting machine. The plurality of rolls, 102, etc., are attached to a frame, not shown, which rotates in unison with the rotating cast tube, and the rolls are powered, by means of a suitable gear train (not shown) to give the desired rate of controlled exit of the cast tube 14.

FIG. 13 shows a simplified apparatus for the continuous centrifugal casting of tube in a vertically upwards direction using a lining of liquid mold material 3. In FIG. 13, the external casing 11 of the rotating mold may be supported on suitable bearings, not shown, and be rotated by a powered gear drive, not shown, about its vertical axis 1. The liquid mold material 3 is introduced into an annular trough, which faces upwards, by means of the inverted .I-shaped spout 5. The molten casting material 6 is introduced into the same trough by way of inverted .I-shaped spout 7 and is fed from a holding reservoir 105 having a controlled liquid level which assures a constant rate of introduction of the molten material 6. Similarly, the liquid mold material 3 has a controlled rate of introduction from a similar reservoir, not shown. Both the liquid mold material 3 and the molten tube material 6 form an accentuated parabolic curve upwardly within the solid cylindrical encasement 13 and the distance these materials move upwardly is a function of the mold diameter and the rotational speed of the casting apparatus. It is preferred to operate at high G (centrifugal acceleration) values since this forces the liquid and molten materials further up the bore and increases the length of wall over which solidification of the molten tube material can take place. External to this length of wall, where solidification takes place, is a plurality of peripherally spaced spray nozzles 104 for heat extraction through theencasing solid wall 13 and the lining of liquid mold material 22.

At the extreme upper point to where the liquid mold material 3 (which is introduced at the bottom or starting end of the apparatus) ascends, is a plurality of annularly spaced holes 171 through which'the liquid mold material can overflow. An annular trough assembly, not shown, having iris seals, not shown, which locate at

points

172 and 173, collects the overflow and returns it, via a conventional heat exchanger, pump and liquid mold material rejuvenation system, to the starting reservoir. Upwards from the overflow point 171, may be located other annular rings of holes, as at 174, for further introduction of cooler liquid mold material by way of system 159 as shown in FIG. 9B; and upwardly from 159 would be located another annular overflow trough system at 83. Such an extended system adds to the heat extraction capabilities of the apparatus and permits a faster casting rate. Beyond the upper end 9 of the solid wall encasement is a plurality of peripherally spaced pull-out rolls, 102 and 103 of which are shown, which rotate'on a suitable frame, not shown, in synchronism with the exiting solid cast tube 14 and which are suitably geared to give a controlled axial rate of extraction to the tube. Upwardly from the pull-out rolls such as 102, 103 is a traveling cut-off assembly 180 which severs the tube after a desired length has been produced. The severed lengths of cast tube are handled and removed by any suitable known type of mechanism, not shown.

The above apparatus is a simplified version and is readily extended to encompass the more complex means of the overall invention. For example, the liquid mold material may be introduced through the molten casting material in the form of solid shot, or it may be introduced underneath the molten casting material by way of an upwardly'facing annular trough immediately inwards (diametrically) from the shown main trough. Such an inward trough, similar to trough 19 of FIG. 12, would connect to the main trough by means of a plurality of small holes through the refractory body 4. Also, the stationary end plate 30 may be saucer-shaped with the periphery thereof immersed in a centrifugally maintained annular trough of liquid sealant. Such an apparatus is shown in FIG. of my US. Pat. No. 3,605,859, except that the rotating seal trough would be attached to the bottom end of the casting machine. By such a non-rotating bottom plate seal and by suitably sealing the upper end of the cast tube 14, a partial vacuum can be drawn, if desired, on the tubes interior, with attendant advantages.

It can be seen that a great many variations of design can be adapted to use the basic principles of the present invention which are, primarily, the adjustment of the cast tube s diameter in its readily changeable molten state, the floating of the molten tube inwardly to a desired decreased diameter, and the floating of the tube out of the exit orifice without restrictive contact therewith for more ready egress from the casting machine. As an illustrative example, the design of Bathom, US. Pat. No. 3,367,400, could be made more effective by introducing a liquid mold material, such as liquid lead, through the machine instead of his encompassing centrifuged cylinder of water coolant. Centrifuged liquid lead does not exhibit boiling with its attendant problems. Further, a small amount of liquid mold material could be added to the molten casting material to form a lining on the dry wall portions of the Hathorn casting apparatus; or, the dry wall mold portions could be made of a refractory material and the cylinder of casting material could come into contact with the main body of liquid mold material downstream and in a molten state. All such design variations are within the province of the apparatus of this invention.

Further, whereas point 9 (as shown in FIGS. 4, 5 and 6) is the end of the solid cylinderical shell encasing the liquid mold material and (in the simplified versions of the apparatus) generally indicates the exit end of the apparatus, the invention pertains to a liquid wall mold. As such, the exit end of the apparatus is where the tube makes its final departure from the liquid mold material. In the more sophisticated versions of the apparatus, (as shown in FIGS. 9, 9A and 13) this departure occurs at the end of the multiple non-rotating assemblies 151, 152, etc. and exit port assemblies 83 (which may be utilized in any series such as 151, 83, 152, 83 etc. if so desired) and, in such cases, the exit end of the apparatus would be considerably further away from point 9 than in the simplified apparatus cases.

It should also be noted that the iris seals 61, 62 and 157, as shown in FIGS. 7, 9 and 9A are of a variable [.D. since they can expand or contract against the CD. of the solid (or solidified on the OD.) exiting cast tube 14. As such, the CD. of the cast tube is always not greater than the ID. of the exit orifice.

Rejuvenation of the Liquid Mold Material:

Regardless of the use of internal and external inert atmospheres, the liquid mold (whether of lead, tin, or leadtin alloys) will gradually build up an oxide content which, being lighter than the liquid mold material, will centrifuge to the 1D. surface of the liquid mold and adhere to the CD. of the metal tube being cast. Normally, this concentration is not large enough to cause problems but, at heavier concentrations, it can cause excessive drag-out of the liquid mold material and, in extreme cases, clogging of the conduits and flow-holes of the system. This can be corrected either by continuous or occasional passage of the liquid mold material through abath of molten cyanides (as those of sodium, potassium or barium, or mixtures thereof). By such treatment, an oxidation-reduction reaction takes place that produces a reduced liquid mold material that is completely rejuvenated (oxide-free). Carbon or graphite mold linings also rejuvenate the liquid mold mate-

Claims

1. Apparatus for continuously casting tubing, comprising: a substantially cylindrical mold container having an inlet at one end and an outlet at its other end of greater inside diameter than the outside diameter of the said tubing when at said outlet; means for rotating said container generally about its longitudinal axis to produce substantial centrifugal force; means to introduce a liquid lining substance into said container to an inside diameter when at said outlet less than the aforementioned inside diameter of said outlet; means to inject molten casting material into said inlet, said lining substance having a lower melting point and greater density than said casting material, said lining substance assuming the form of a liquid lining and said molten casting material assuming the form of a congealed tube of smaller diameter than and flOating through said outlet on said lining in response to said centrifugal force, the wall thickness of said tube being determined by the relation between the rate of injection of said material and the rate of exit of said tube floating from said outlet; diameter control means to control the outside diameter of said tube while molten so that said tube congeals substantially back of said outlet to a diameter no greater than the diameter of said outlet, and means external of said outlet and for controlling the rate of exit of the congealed tube in an unrestricted manner from said outlet.

2. The apparatus of claim 1, wherein the gas inside said tube is substantially at atmospheric pressure and said diameter control means comprises pressurizing means to apply a higher pressure to said liquid lining to constrict said tube.

3. The apparatus of claim 2, wherein said pressurizing means comprises a restricted annular gap between said tube and said outlet, said gap being dimensioned to create a back pressure in said liquid lining to constrict said tube.

4. The apparatus of claim 2, wherein said pressurizing means comprises an axial extension of the outlet bore end portion of said mold container, said extension being dimensioned to create sufficient fluid resistance to the outward flow of said liquid lining substance to create a substantial back pressure therein, said pressure constricting said tube.

5. The apparatus of claim 2, wherein said pressurizing means comprises means to apply a gas pressure to said liquid mold material, said pressure being transmitted by said material to said tube to constrict it.

6. Apparatus for centrifugal casting of tubing from molten casting material, comprising: a substantially cylindrical mold container having an inlet adjacent one end and an outlet exit orifice adjacent its other end; means for rotating said mold container about its longitudinal axis to produce substantial centrifugal force therein; means for introducing a liquid lining substance of lower melting point and higher density than said casting material into said mold, said substance assuming the form of a liquid lining in response to said centrifugal force; means for injecting said molten casting material into said inlet, said material assuming the form of a tube in response to said centrifugal force and floating on said lining and being cooled and congealed thereby to a state in the range of solid to semi-solid; means for controlling the rate of exit of said tube from said outlet; and control means to control the diameter of said tube in its molten state to restrict its outside diameter, after congealing, to a value no greater than the inside diameter of said outlet, to permit free exit therethrough, said control means comprising a seal at said inlet, a closure of said tube after its exit from said outlet, and means for maintaining a pressure differential between the interior and exterior of said tube, said pressure differential being of sufficient magnitude to constrict the molten tube to a diameter substantially below the normal limit imposed by Archimedes'' Principle.

7. The apparatus of claim 6, wherein said control means further comprises: internal pressure means for creating an internal gas pressure lower than atmospheric inside said tube in said mold, and external pressure means to apply an external pressure greater than said internal pressure to the exterior of said tube.

8. The apparatus of claim 7, wherein said external pressure is substantially atmospheric.

9. The apparatus of claim 7, wherein said external pressure means comprises a restricted annular gap between the outside of the exiting tube in its congealed state and the inside of said exit orifice, said gap creating a back pressure in said liquid lining to constrict said tube.

10. The apparatus of claim 7, wherein said external pressure means comprises an axial extension of the bore of the apparatus to a length suffIcient to create substantial frictional resistance to flow to produce a line pressure drop in said liquid mold lining, said drop creating a back pressure in said lining to constrict said tube.

11. The apparatus of claim 7, wherein said external pressure means comprises means to apply a pressure greater than atmospheric to the exterior of said tube after its exit, to create a back pressure on said liquid lining to constrict said tube inside said mold.

12. The apparatus of claim 7, wherein said external pressure means comprises means to apply a pressure greater than atmospheric at the inlet end of said mold container.

13. Apparatus for continuous centrifugal casting of tubing from molten casting material, comprising: a mold container having a bulbous cavity, an inlet at one end, and an exit orifice at its opposite end; means for rotating said mold container to produce substantial centrifugal force; a partial filling in said cavity of a liquid lining substance of greater density and lower melting temperature than said casting material, said filling extending radially inward to a diameter substantially equal to that of said exit orifice, but not overflowing it, and forming a liquid mold in said container in response to said centrifugal force; means for injecting said molten casting material into said inlet inside said lining, said material assuming the form of a tube under the influence of said centrifugal force, said material being cooled toward solidification by said lining; diameter control means applying pressure to said liquid lining to reduce the diameter of said tube in its molten state substantially below the normal limit imposed by Archimedes'' principle, to aid in its clearance, when congealed, through said exit orifice; and means for controlled exit of said tube when congealed to make its diameter less than that of said orifice to permit free passage therethrough.

14. Apparatus as in claim 13, wherein: said diameter control means comprises means to apply a gas pressure greater than atmospheric to the exterior of said tube after its exit and to said exit orifice, said pressure being transmitted to said liquid lining to constrict said tube in said mold.

15. Apparatus for the continuous centrifugal casting of tube, comprising: a rotatable mold container having an entrance end and an exit orifice; means to introduce a molten tube material into said entrance end; means to introduce a liquid mold material into said mold container to act as a liquid mold therein; means to create a positive gas pressure differential between the exterior of the molten tube at said exit orifice and the interior of the tube being cast, said differential being sufficient to substantially constrict the diameter of said molten tube to permit it to exit from said orifice while molten in a stable state condition without contacting the solid portion of said orifice; means for rapidly cooling said molten tube after exit and prior to any substantial disturbance of said stable state condition, to at least a semi-solidified state; and means for causing controlled exit of said tube.

16. Apparatus for the continuous contrifugal casting of tubing from selected molten casting material, comprising: a generally cylindrical mold container having an inlet at one end and an outlet at its other end, both disposed on its longitudinal axis; means for rotating said container about said axis to produce substantial centrifugal force therein; means for injecting into said container a liquid lining substance of higher density and lower melting point than said casting material, said substance assuming the form of a liquid lining in response to said centrifugal force; means for injecting said molten casting material into said inlet internally of said liquid lining, said material assuming under said force the form of a molten tube floating on the inner surface of said lining; said molten casting material bEing cooled toward solidification and progressing axially toward said outlet; an axially-extending cylindrical sleeve of refractory, thermally-insulating material in said container coaxial therewith and extending from the vicinity of said inlet toward said outlet and forming an extended hot zone within said mold, said casting material remaining substantially molten while in con-tact with said sleeve, said hot zone being extended, beyond the length required merely for layering and levelling of said molten casting material, far enough to permit gravity segregation in said material sufficient to substantially modify the physical properties of said tubing; and means for controlling the rate of exit of said tubing from said outlet.

17. Apparatus as in claim 16, wherein: said axially-extending cylindrical sleeve is made of pyrolitically-deposited material so oriented that it is relatively thermally-insulating in the radial direction and a relatively good heat conductor in the axial direction.

18. The apparatus of claim 16, wherein: the inner surface of said sleeve has a relatively thin layer of said liquid lining substance to provide a surface on which said molten casting material floats.

19. Apparatus for the continuous centrifugal casting of tubing from selected molten casting material, comprising: a tubular mold container having an inlet at one end and an outlet at the other end, both disposed on a longitudinal axis of said container; means to inject into said container a liquid lining substance of higher density and lower melting temperature than said casting material; means for rotating said container to cause said lining substance to assume the form of a tubular liquid mold lining in response to centrifugal force; means for injecting said molten casting material into said container through said inlet and internally of said lining, said material assuming the form of a tube under centrifugal force and being buoyantly supported and cooled toward solidification by said lining; means for maintaining a desired pressure on the exterior of said tubing after its exit from said outlet, comprising a non-rotating annular pressurized enclosure enclosing both said tube and said container; encircling seal rings composed of seal-forming material sealing said enclosure rotatably to said tubing and said container; and means for controlling the rate of exit of said tubing from said outlet.

20. Apparatus as in claim 19, wherein said enclosure is substantially filled with said liquid lining substance, said substance being pressurized.

21. The apparatus of claim 19, wherein: said seal rings are composed of a plurality of sickle-shaped blades pivoted at one end, pressure means forcing the inner surfaces of said blades in sealing contact with the rotating surfaces; said blades being disposed in overlapping relation around the circumference of said ring in the manner of an iris diaphragm, whereby said blades maintain sealing contact over a range of inner diameters.

22. The apparatus of claim 6, wherein: the gas inside said tube is at a pressure greater than atmospheric, and said diameter control means comprises differential pressure means for creating a pressure differential between said lining and the inside of said tube.

23. The apparatus of claim 22, wherein: said differential pressure means comprises a restricted annular gap between said tube and said outlet, said gap being dimensioned to create a back pressure in said liquid lining to constrict said tube sufficiently to permit its free exit from said outlet.

24. The apparatus of claim 22, wherein said differential pressure means comprises an axial extension of an outlet bore end portion of said mold container, said extension being dimensioned to create sufficient resistance to the outward flow of said liquid lining substance to create a substantial back pressure therein, said back pressure constricting said tube sufficiently to permit its free exit through said outlet.

25. The apparatus of claim 22, wherein said differential pressure means comprises means to apply a pressure greater than atmospheric to the exterior of the exiting tube to transmit a back pressure of said liquid lining sufficient to constrict said tube in said mold to permit its free exit from said outlet.

26. The apparatus of claim 22, wherein said differential pressure means comprises means to apply a pressure greater than atmospheric at the entrance end of said mold container and a still greater pressure to said liquid lining, sufficient to constrict said tube enough to permit its free exit from said outlet.

27. The apparatus of claim 2, wherein said pressure means is hydraulic.

28. The apparatus of claim 6, wherein: the axis of said mold is inclined at an angle to the horizontal, and further comprising an annular trough at the inlet end of said mold and having a central opening, and means to introduce said liquid lining substance and said molten casting material into said container, the hydrostatic pressure in said liquid lining substance aiding to control the diameter of said tube to facilitate its exit through said outlet.

29. The apparatus of claim 28, wherein said angle is substantially 90*.

30. Apparatus as in claim 6, wherein: said seal at said inlet comprises an annular trough with its opening facing inwardly at the inlet end of said mold container and rotating therewith, and containing a liquid sealing substance, and a fixed seal disc with its periphery in said trough and sealed by immersion in said sealing substance.

31. The apparatus of claim 1, further comprising means for rejuvenating said liquid lining substance.

32. The apparatus of claim 6, further comprising means for rejuvenating said liquid lining substance.