CA1090972A - Process for tubular water-bath thermoplastic films - Google Patents

Abstract

ABSTRACT

Substantial and commercially important increases in thermoplastic film production rates without adversely affecting film quality are obtained in a tubular water film process. The improvements comprise modifications in the die lip, in the mandrel air (fluid) slot affecting the cooling gas velocity, and in the upper bubble pressure control.

Description

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This invention relates to an improved process for the production of tubular film from a film-forming thermoplastic.
More particularly, the invention relates to modifications in the die-lip in the mandrel fluid slot affecting the cooling gas velocity and in the upper bubble pressure control.
Tubular water-bath processes are described in U.S.
Patents 3,400,184; 3,450,806; and 3,622,657. The tubular water-bath process described in these patents is an effective technique for producing tubular films of high quality from a film-forming thermoplastic.
Essentially, the thermoplastic is extruded from a die in the form of a tubular molten extrudate in a substantially verti-cal direction from above. The molten tubular extrudate ultimately is rapidly cooled by a combination of a water bath on one side and a cooling mandrel on the other side, the water bath being equipped with a seal at its lower surface acting against the mandrel.
Due to the external water pressure, the film must ultimately touch the surface of the mandrel, and therefore it is very important that the hot, sticky extrudate be processed and treated prior to contact with the mandrel so that it will not adhere to or be marred by the mandrel.
Adhesion to the mandrel results in several extremely undesirable effects. Included in these are that seizing might occur and the process stop aItogether; the process might be slowed because of the difficulty of the film passing over the mandrel; the

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film may become marred or otherwise would acquire imperfections on its surface.
The genius of the original invention was to avoid this adverse effect by a combination of features. These include the technique of directing a high velocity jet of cooling air on a small area of the inside surface portion of the extrudate. This air jet causes preliminary cooling of a small inside portion of the extrudate. Subsequently, the extrudate is conveyed into the water bath.
The inside portion of the extrudate is kept from con-tacting the mandrel by the air gap formed by some of the air from the high velocity jet while the exterior portion of the extrudate is cooled by the water bath.
When the inside portion of the extrudate ultimately -contacts the mandrel, the portion contacting the mandrel has had sufficient preliminary cooling both by the air jet and the water bath portion to have been rigidified and hardened, so that when it contacts the mandrel, no sticking, adhesion, or seizure will occur.
This enables the production of high quality film at relatively high production rates, i.e. about 400 pounds per hour for a line producing 70-inch flat width thermoplastic tubing.
Utilizing improvements such as the external air ring pressure reduction orifices of Canadian Patent 941,811 which controls the expansion of the melt and stabilizes it as it is drawn down, the improved water bath design of U.S. Patent 10909'7'~

3,685,576 which provides a much more uniform quench of the extrudate and improved sealing at the bottom of the external water bath and the improvement of U.S. Patent 3,702,224 which discloses a texturized mandrel having an improved internal design, considerably higher production rates for difficult-to-process resins such as polypropylene, without adversely affecting the quality of the resulting film are obtained. Such rates are in the order of about 500 to 540 pounds per hour.
The above Canadian patent (941,811) discloses a multi-step air ring having a plurality of annular pressure zonesarranges externally to the tubular film in the region between the die and the mandrel, the pressure zones are highest near the die and lowest near the mandrel in order to guide the film from the die to the mandrel, and are defined as an expansion gradient for said film between said die and said mandrel.
These are improved rates compared to those which could be obtained with the original technique described in U.S.
Patent 3,400,184. But, the exigencies of the marketplace are such that rèsin prices are continually rising while film prices are declining and, therefore, in order to remain competitive, a given piece of capital equipment must be able to produce consider-ably more film product in a given time span. That can be accom-plished with the technique of the present invention.
Tubular film (preferably 60 to 80 inches wide) production rates of consistently over 700 pounds per hour from difficult-to-process resins such as polyolefins, polyamides, lV~30~i7;Z

polyesters, PVC and the like can be produced in a process in which the die orifices, external lips of the air slot, and the velocity of the air as well as the pressure in the upper bubble are all controlled within critical parameters to achieve a substantial increase ln production rate without concomitantly adversely affecting film quality.

In the Drawings:

Fig. 1 shows the overall process sequence and apparatus therefor;
Fig. 2 shows the detail of a flared die orifice;
Fig. 3 shows the detail of precooling means;
Fig. 4 shows an embodiment for combining air and water precooling; and Fig. 5 shows an embodiment for water only pre-cooling, i.e. inside-film, between-mandrel water (liquid) cooling~
Although the inventive process and inventive apparatus described herein can be used effectively with any film-forming thermoplastic resin, it is especially well suited for preparing transparent tubular films having uniform diameters from crystalline thermoplastic resins, especially polypropylene.
Furthermore, it is particularly well adapted to produce commercial gage thickness film having a 60 to 80 inch width.
For the purposes of the explanation and discussion which follow, polypropylene will be used as a preferred embodiment, but it is to be understood that the invention is not so limited.

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There are several quite different and important considerations in the film-making art, depending upon whether one is making tubular films from a relatively high crystallinity product such as polypropylene or a relatively low-crystallinity product, such as low density polyethylene.

Generally, the highly crystalline thermoplastics cannot be used in conventional tubular film processes because if they are cooled slowly, as by air, relatively large spherulites form; thus resulting in a less transparent film. Rapid cooling of the tubular film can eliminate the formation of such spheru-lites. The processdescribed herein is highly suitable for preparing films from resin such as polypropylene where premature crystal-lization might be a problem, but that is not to preclude the use of other resins in the process.
In order to understand the specific process apparatus, improvements and features of the present invention, a review of the basic process for general background information is useful.
Thus, as a general matter, a film-forming resin is extruded downwardly in the shape of a molten tubular extrudate from a circular die and passes over a sizing mandrel which is in a water bath that surrounds the mandrel.
Cooling gas from an air jet, usually located in the mandrel, both cools a portion of the extrudate when it impacts upon it and also escapes to inflate the tube formed from the lV~3~1~i7;~

process. Preferably the tubular film may be extruded through an annular die to yield a downwardly directed tubular film having an inside diameter of 3 to 48 inches, and a wall thickness (as it issues from the die~ of 10 to 60 mils, preferably 20 to 40 mils.
The downwardly moving molten sheet is preliminarily contacted with air at -70C. to +60C., preferably 10C. to 50C., to precool the film to 100C. to 280C.
Preferably, the air will contact the film both ex-ternally and internally. Thus an air ring, such as that shown in Canadian Patent 941,811 may be employed for external cooling.
The air inside the tube serves to support the film and to expand it to a larger diameter. Precooling is commonly effected to an extent sufficient to achieve maximum polymer viscosity and surface hardening, but insufficiently to generate a "frost-line" on the film prior to further cooling.
Typically, polypropylene film is precooled to 180 C.
to 289C., preferably 190C. to 230C., and polyethylene film is precooled to 160C. to 235C., preferably 160C. to 190C., at this point, prior to further cooling.
The internal precooling gas is usually passed upwardly within (i.e., countercurrently to) the downwardly descending tubular film. Upwardly, as used herein does not refer to the angle of an air (gas) slot, but rather to the relationship of the air (gas) with respect to the direction of film movement. As 1.0~3097f~

the gas contacts the inside of the tubular film, it often, in addition to precooling, serves to expand the film to a diameter slightly larger than that of the cylindrical mandrel used in subsequent steps. The internal precooling gas also serves the important purpose of carrying away undesired volatile products.
Subsequently, the precooled downwardly moving tubular film is passed over and around in slidable contact with a verti-cally extending mandrel which is completely surrounded by the film.
The mandrel, essentially of cylindrical shape, is usually a metal through which heat transfer will readily occur. The outside diameter of the mandrel will be 80% to 300%, preferably 110% to 170%, of the diameter of the annular die. The outside diameter of the mandrel will be substantially the same as the inside diameter of the film whereby as the film passes downwardly thereover, cooling of the film will occur. The purpose of the mandrel is to size the tubular film as well as cool it.
Preferably, the mandrel may be internally cooled with cooling liquid, typically water entering at 2C. to 75C., preferably 2c. to 40C., and leaving at about 4C. to 76C. The major portion of the vertically extending circumferential wall of the mandrel will be cooled by circulating liquid as hereinafter set forth.
The outside surface of the downwardly descending tubular film passing over the mandrel is passed through a body of cooling liquid, preferably water, at 2C. to 90C., maintained in l.V90~7Z
the cooling cell. The hydrostatic pressure of the cooling liquid in the cell assists in maintaining the tubular film in contact with the mandrel.
The mandrel contains a circumferential fluid slot or slots located in the upper portion of the cylinder which provides passageways for the high intensity jet of cooling fluid.
The preferred fluid is gas, e.g. air, although it will be seen that one of the embodiments of the present invention is to use liquid (water) or a combination of a gas and liquid, e.g. air and water. While, the term air is used in connection with the prior art, and is used for convenience herein, it is to be understood that the concept of this invention is to use any suitable fluid, not just gas or air.
One of the subcombination inventions of the present application is the improvement of the air slot design within the mandrel so that a given volume of cooling gas can be delivered to the surface of the extrudate at the preferred angle.
Very generally, the air slot is adjusted to provide approximately 100 to 250, preferably 150 to 200, and most preferably 180 to 220 standard cubic feet per minute tscm) of cooling gas to the surface of the resin extrudate when using a 46 inch mandrel.
The factors which govern the volume are a combination of the slot size and the air velocity. ~7ery generally, the width of the slot should be from about 50 to 100 mils, preferably 60 to 70 mils, and most preferably 60 to 68 mils. The length of the 1090'37Z

land is preferably V 8", most preferably at least 3/16", and most preferably at least l/4". The slot will have an angle measured from the top horizontal surface of the mandrel of about 25 to 45, preferably 27 to 33, and most preferably 29 to 31.
Another point in the process sequence where one of the features of the invention is profitably employed is in the die orifice itself. It has been discovered and forms a feature of this invention that control of die swell prior to the point where the molten extrudate exits from the die orifice is éxtremely beneficial.
Many resins, including polypropylene, comprise - molecules which are very closely and densely packed in an elastic deformation relationship. When these molecules are compressed under heat and pressure and forced through a narrow channel as in an extrusion die, when they exit from the die they have a tendency to pop out or expand elastically from the very tight configuration that they have been forced into from the confines of the die orifice.
It has been discovered that, if this swell occurs nonuniformly, this undesirably results in the formation of film with defects such as surface crazing and alternate thin and thick portions.
The problem has been solved according to a feature of this invention by modifying the flow channel of the die orifice.
Customarily, a flow channel will gradually taper down to a final gap of about 0.025 inch. That gap will be maintained for a total - of about 1 inch. This is customarily referred to as the channel ~090'.~7Z

gap or the gap of the flow channel.
According to the invention, the last half inch of the channel gap is increased by flaring it about 2 to 5 , preferably 2 to 4, and most preferably 3, measured from the parallel sides which define the flow channel about a half of an inch before the end of the channel where the die lips are located.
This will be further illustrated in the detailed discussion of the drawings.
It is also an additional feature of this particular facet of the invention that additional longer internal taper can be provided to supplement the exit flare described above.
The overall effect is to provide a gradual trans-ition so that die swell occurs relatively slowly and within predetermined confined boundaries so that it is controlled and film characterized by lack of uniformity is eliminated.
Another important feature of the present invention resides in the use of constant pressure control of the upper bubble. Previously, before this inventive feature was developed, it was thought that the tubular film in the upper bubble was required to conform to a particular bubble shape. Accordingly, photoelectric tubes were placed at various positions surrounding the bubble so that any variation in the shape of the bubble would be reflected in the instrument circuit and cooling gas would be provided or bled in order to compensate for variations in the tube diameter or shape so that this was kept at a constant value. See ~IV90~'7Z

U.S. Patent 3,400,184, column 6, lines 11-38, for a detailed description of this complex control system.
According to the present inventive feature, that type of control based on maintaining a predetermined shape can be completely eliminated. The controlling criteria of this inven-tive facet is the upper bubble pressure. Thus, it has been discovered that a certain predetermined, defined pressure is critical to good operation and that this particular pressure must be kept constant in order to achieve the desired results.
Very generally, this pressure shouId be maintained at a constant figure of about 0.1 to 1.0 inch of water in the bubble, preferably 0.15 to 0.32 inch of water, and most prefer-ably about 0.25 inch of water.
This pressure is adjusted during operation until a stable operation is achieved early in the run. Thereafter! the pressure is kept at this level and does not have to be changed continuously in response to other conditions, as had been previously thought. This provides a much easier and more effective mode of control than anything heretofore known.
The tubular water bath process known to the art and exemplified by U.S. Patent 3,400,184 has been used commer-cially for the production of relatively narrow polyolefin tubing.
However, when the technique of the prior art is utilized to make wide polypropylene sheeting, it is only possible to have satisfactory operations, particularly in terms of through-put rates with material of a relatively heavy gage thickness, i.e., about 2.5 mils to 10.0 mils.

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And, if this wide material is subjected to draw-down to the commercial prerequisite of about 1 mil to 2 mils at rates which are commercially feasible, serious difficulties are encountered.
Fundamentally, these difficulties mean that at high rates, film of inferior quality is made because of surface imperfections, and if film is made of commercially acceptable quality, it must be done at such low throughput rates that the operation is grossly noncompetitive.
The improvements of Canadian Patent No. 941,811, U.S. Patent 3,685,576 and U.S. Patent 3,702,224 all help to some extent to overcome the problem, but the problem was still not completely solved even with these innovations since the throughput was still unsatisfactory, i.e. not at commercial rates.
Only when the techniques of the above-mentioned patents were combined with the techniques of the present applica-tion were production rates of over 700/lbs./hr. actually accomplished with no surface imperfections.
It is to be noted that the extrudate melt exiting initially from the die has a thickness of ahout 35 mils as contrasted to the final, desired film thickness of 1 to 2 mils. Thus it is plain that the film not only requires extrusion at high rates throuqh tlle apparatus, but at the same time very high draw-down rates are expected to take plaoe. This is an extremely difficult feat to achieve.

Althou~h, in general, any conventional fi~forming - 13 ~

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grade of thermoplastic can be used in the process of the invention, it has been found that polypropylene meeting certain specifications gives optimum results in the process of the invention.
Very generally, polypropylene resins with a shear stress between 18.0 and 22.0, preferably between 19.3 and 21.3 and with a die swell, i.e. swelling ratio between 2.2 and 3.6 but preferably between 2.5 and 3.5 is preferred.
The term "shear stress" means the measurement made at a constant shear rate of 1,280 reciprocal seconds through a capillary die having a diameter of 0.060 inch and a length of about 1 inch at a temperature of 400F. The "swelling ratio" or the "die swell" is the ratio of an extrudate cross-sectional area to a die cross-sectional area under the same conditions.
Referring now to the Drawings:

.
In Fig. 1 a typical operation of the invention is shown where film-forming thermoplastic resin is introduced into extruder 10 through hopper 11 from which molten extrudate passes through conduit 12 into die 13 to annular die orifice 14 which is initially tapered and finally flared at the die lip.
(Fig. 2 is a detailed blow-up of this feature.
This illustrates the last inch of the extrudate flow channel prior to the extrusion.) In operation, resin is extruded from orifice 14 as a tubular shaped extrudate 15.

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Extrudate 15 is passed over mandrel 17 as the external surface of the extrudate 15 is cooled in the water bath in cooling cell 16. The water in the water bath flows in counter-current fashion against the direction of film flow and the water level is maintained in the cell 16 by the seal labelled in Figure 1. The cell 16 includes a water input conduit and an overflow outlet conduit bath as labelled in Figure 1. The film 15 in the lower bubble resulting from extrudate 15 is collapsed by rolls 18 and thereafter subject to further treatment, as may be required for particular uses.
Cooling fluid, usually and conventionally gas, i.e. air (but not necessarily so limited according to this inven-tion), is introduced into conduit 19 and conveyed under compression and pressure to the interior of mandrel 17 where it is permitted to exit from annular circumferential slot 20.
The fluid exiting from slot 20 serves to precool the interior surface of the molten extrudate 15 prior to its initial contact with mandrel 17. When the fluid from slot 20 is gaseous, e.g. air, it is then permitted to expand upward, where it is maintained at a constant predetermined pressure, as described above, and any excess air is allowed to exit from the system through conduits 21 and 22. These can be further regulated with remote-controlled check valves.
When the fluid is liquid, e.g. water, air is suppl-ied directly to the upper bubble to control the pressure.

109(~'72 As noted above, Fig. 2 illustrates the last inch of the extrudate flow channel prior to the extrusion. In the initial part of the inch it is seen that the flow channel 23 is of constant width. In the last half inch of the flow channel, it can be seen that the constant gap is followed by flare 24 which is preferably at an angle of about 3 measured from the parallel sides of the constant taper portion of the channel.
Figure 3 shows, in cross-section, the constrNction of the mandrel circumferential slot 20 which is in mandrel 17, and which controls gas velocity and direction when gas, usually air, is the precooling medium. The preferred size and angle of this slot are described in detail elsewhere herein.
Fig. 4 illustrates an embodiment where bath liquid (H2O) and gaseous (air) precooling fluids are used together. With this combination, outstanding results can be achieved, since the precooling is more effective and the water inside the film acts as an internal lubricant on the mandrel.
The added cooling efficiency and capacity supplied by the water on the inner surface of the film also prevents the - curling noticed with some thick films made from resins such as high density polyethylene, etc. Such curling is apparently caused by the uneven cooling rates between the outer film surface cooled by the water bath and the inner film surface, which is primarily cooled by contact with the metal surface of mandrel 17.
Observation indicates that the cooled metal mandrel 1090~7~

does not cool at as fast a rate as direct contact with water in a cooling bath.
The net effect is unequal cooling which causes the undesirable curling or warping. It is avoided with the use of water on the inside of the film.
Fig. 5 illustrates an embodiment where liquid pre-cooling, i.e. water alone in the absence of gas, is used.
Both precooling and inner film/mandrel, water cooling are accomplished by this arrangement. The location of water orifice slot 25 as shown in both Figs. 4 and 5 is chosen so that extrudate/mandrel contact without cooling, lubricating liquid, preferably water, is avoided. Thus, when the extrudate makes initial contact with mandrel 17, there is at least a boundary layer of liquid between it and mandrel 17.
Some of the water will be carried into water bath 16 by the moving extrudate. A portion of the water will build up from the point of contact of extrudate with mandrel. To prevent overflowing, it is desirable to provide in both the Fig. 4 and Fig. 5 embodiments, liquid overflow slots 2Ob. In the Fig. 5 embodiment slot 2Ob is located in a separate top mandrel plate assembly 17b.
Moreover, in Fig. 4 air slot 20a is contiguous to liquid overflow slot 20b. The overflow slots can either drain back into the internal mandrel bath 26 or to any other convenient reservoir, through conduit means tnot shown).

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Although the tubular water bath technique of the art was especially useful to obtain fast rates because of rapid cooling, the technique of the present invention without l'inside-film liquid," i.e. water, permits much better rates and the additional technique which employes "inside-film liquid" enables the attainment of outstanding throughputs.
The invention will be further illustrated by the following example.

.

The working example was carried out on a 36-inch diameter die orifice and a 46-inch diameter mandrel. Other tech-niques than those specifically described in this example and application as features of the invent~ion were systematically tested, such as various air ring orifice plate settings, dle-to-mandrel spacings, and various internal bubble inflation pressures and external air flow rates, but every effort resulted-in the production of nonacceptable film at high throughput rates.
Generally, an attempted increase in production rates resulted-in having molten plastic hitting the top of the mandrel which caused unacceptable marks on the film.
But with the technique of this example, such deleter-ious results were avoided. In general, except as stated below, the example utilized the process and apparatus of the drawings and the detaile~ background conditions of Canadian patent 941,811 which illustrates the use of external air rings for external cooling of the upper bubble.

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Polypropylene with a shear stress of about 20 and a die swell of about 3 was extruded at a melt temperature of about 430F. at a rate of ilO lbs./hr. from the die onto a mandrel, whose upper air slot was located 6.000" + 0.250" vertically below the extrudate die orifice in the die. Air was supplied to the external lips at constant flow rate between 500 and 1500 SCFM
and air was supplied to the upper slot on top of the mandrel at a flow rate of about 180 SCFM. The air exited from the mandrel air slot at a velocity between 2,800 and 3l200 FPr~.

The size of the openings in the top of the die were adjusted to allow this air to exhaust from the upper bubble at a pressure between .16 and .25 in water. The air slot on the mandrel directed the air upward at an angle to the horizontal of about 30.
The film of this example was made using air ring lips described in the example given in Canadian Patent 941,811 and was adjusted so that the inner lip was 1.531" + 0~062" vertically below the die, the middle lip was 2.938" + 0.125" vertically below the die and the outer lip was 4.000" + 0.250" vertically below the die.
The air ring lips are not illustrated in this application since they are, except for the specific settings used herein, actually the separate invention of said Canadian Patent.
Although the invention has been described with particular emphasis on polypropylene, other film-forming resins, particularly highly crystalline or crystallizable ones, can also 1090~'7Z

benefit from the utilization of the features of this invention.
Examples of such resins include polyolefins such as polyethylene, both high density and low density, polymethylbutene-l, poly-methylpentene, polystyrene, polyamides (nylons), polyesters (dacron) , PVC, and the like.
Furthermore an orientation step can be appended to the process of the invention so that the film, after passing over the mandrel, can be pulled and oriented in the lower bubble portion of the overall process.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the formation of tubular polypropylene film having a width of 60 to 80 inches and a thickness between 1 and 2.5 mils com-prising extruding a polypropylene resin having a shear stress between 18.0 and 22.0 and a die swell between 2.2 and 3.6 generally downwardly through a die with an annular orifice in the form of a molten substantially tubular film body, maintaining the pressure in said substantially tubular film body at a constant figure at about 0.1 to 1.0 inch of water, preliminarily cooling the internal surface of said tubular film body with a directed stream of gaseous material to partially solidify said film subsequently, downwardly passing said film to engage in sequence a cooling mandrel comprising an air slot at the point of film entry over said mandrel, said mandrel having a texturized surface next to the film and being internally water cooled, a water bath for cooling the exterior of said film comprising a cell designed to control the distribution and flow of cooling water in countercurrent fashion against the direction of film flow, said cell having a resilient seal at its lower end to maintain water level within the cell and to minimize water carryover by the film passing through the cell and an overflow weir at the flow entry point to maintain outflow water level and utilizing a multistep air ring comprising a means for supplying a pressurized air stream from a primary orifice, said stream expanding in con-trolled fashion through successively larger miltiple orifices thereby guiding and controlling the expansion of the film between the die and mandrel, said successive orifices being independently adjustable in axial position relative to the primary orifice, whereby a plurality of annular pressure zones are arranged externally to said tubular film body in the region between said die and said mandrel, said pressure zones being highest near said die and lowest near said mandrel, to define an expansion gradient for said film between said die and said mandrel and thereafter recovering said tubular film at throughput rates of about 700 lbs./hr. which comprises the following steps in combination:
(a) effecting controlled shaped pre-expansion of molten polyproplyene from said annular orifice by providing a gradual taper of said orifice to a gap of about 0.025 inch which is maintained for about 1 inch and subsequently flared at an angle of 2° to 5° within said die at the last 0.5 inch of said orifice just prior to extrusion; and (b) effecting cooling along the internal surface of the film with water supplied to the exterior of said cooling mandrel at a point above the level of said water bath for cooling the exterior of the film, said water for cooling the internal surface additionally forming a liquid boundary between the interior of the film and the mandrel.

2. A process according to claim 1 wherein said preliminary cooling is effected with a high velocity jet of air of a rate of approximately 100 to 250 scm from an annular slot in said cooling mandrel, the angle of said slot from horizontal being about 25° to 45° with the slot being from about 50 to 100 mils.

3. The process according to claim 1 wherein said die orifice is 36 inches in diameter and said mandrel is 46 inches in diameter.

4. A method according to claim 2 wherein said die has an orifice 36 inches in diameter and said mandrel is 46 inches in diameter, and there are three annular pressure zones, the first of which is located 1.531?
0.062 inch below the die, the middle one is located 2.938 ? 0.125 inch verti-cally below the die and the third one is located 4.000 ? 0.250 inch vertically below the die.

5. A process according to claim 1 wherein the angle of said slot from the horizontal is from about 25° to about 35 .

6. A process according to claim 2 wherein said cooling gas is omitted and said water boundary layer is the primary means of preliminarily cooling said film, although the air necessary to maintain the constant tube pressure exerts some minor preliminary cooling effect.