CA1121956A - Blow molding of plastic articles having barrier properties - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method of and apparatus for blow molding an article having barrier properties from an expandable tube of thermoplastic synthetic resinous material. A mixture of fluorine and inert gas is introduced to the interior of the tube before it is enclosed within a mold, and an inert gas is employed as the blowing medium.

Description

s~

This invention relates to a method of and apparatus for blow molding a plastic article, and more particularly, to such a method and apparatus in which -fluorine is employed to impart barrier properties to the article.
One of the chief disadvantages of many thermoplastic synthetic resinous materials is that they are permeable by nonpolar solvents at substantial rates of transmission.
This is particularly true of a group of resins which, because of their many adv~ntageous properties, are commonly employed in the manufacture of containers and receptacles.
Referring for example to U.S. Patent 2,811,4689 issued October 29, 1957 to S. P. Joffre, it has been known for some time that barrier properties can be imparted to articles formed of certain thermoplastics, including articles which have been blow molded, by exposing a surface of such an article to fluorine. The fluorine reacts with the material of the article to form a relatively impermeable surface barrier.
U.S. Patent 3,862,28~, issued January 21, 1975 to D. D. Dixon et al., discloses a number of methods of blow molding thermoplastic articles in which a fluorine mixture is employed as the blowing medium. More particularly, in one of these methods, a tube or parison is expanded to conform to an interior surface of a mold within which it is enclosed by introducing under pressure to the interior of the parison a mixture of from about 0.1% to about 10% by volume fluorine and the remainder an inert gas.
It has been found, however, that when a fluorine mixture is employed as the blowing medium in high-speed blow molding systems presently in commercial use, the barrier ~ t~ ~

properties imparted to the blown article may be far from uniform over the extent of its wall, with the result that various portions of the wall are permeated by nonpolar solvents at widely differing rates of transmission. In many instances these permeation rates approach or equal those exhibited by wholly untreated articles, whereby the usefulness of the article for containing nonpolar solvents is gravely impaired or destroyed.
It has also been found that the presence of air, or more particularly, oxygen, during exposure of the interior of the tube or parison to fluorine has a similarly deleterious effect on the barrier properties imparted.
In accordance with the present invention, and ~ntrary to the teaching of the aforementioned patent to Dixon et al., an inert gas is employed as the blowing medium, a fluorine mixture being introduced to the interior of the tube or parison before it is enclosed within the mold.
Like the aforementioned patent to Dixon et al., the method according to the present invention begins with an expandable tube or parison of synthetic resinous material which is enclosed within a mold and expanded to conform to an internal surface of the mold. However, in accordance with the invention, the parison is closed to the ambient 9 a mixture of from about 0.1% to about 10~ by volume ~luorine and the remainder an inert gas is introduced to the interior of the parison before it is enclosed within the mold, and the step of expanding the parison within the mold is carried out by introducing an inert gas under pressure to the interior of the parison.
~ The method according to the invention has been found to overcome the deficiencies of the prior art described 5Çi hereinabove. SpeciEically, an article produced by the method of the invention exhibits uniform barrier properties over the extent of its wall to a degree suitable for containment of nonpolar solvents.
Preferably the parison is partially expanded before it is enclosed within the mold by introducing the fluorine mixture under pressure.
The thermoplastic synthetic resinous material is preferably selected from the group consisting of acrylonitrile polymers, styrene polymers, vinyl chloride polymers, and particularly polymers of olefin monomers having 2 to 8 carbon atoms per molecule.
The invention also provides apparatus for successively blow molding articles, comprising extrusion means for continuously extruding an expandable tube of synthetic resinous material, and a plurality of molds arranged in a series thereof. Each mold has an internal surface complementary to the outer surface of an article to be blow molded, the molds heing adapted to successively enclose therewithin successive portions of the extruded tube whereby to maintain the interior of each tube portion closed from the ambient before said tube portion is enclosed within one of the molds and to isolate the interiors of the tube portions from one another.
The apparatus also includes first and second fluid delivery systems adapted to be closed from the ambient, the first fluid delivery system compri,ing first fluid passage means associated with the extrusion means for continuously introduc-ing a first fluid medium to ~he interior of the tube as it is extruded~ and first conduit means communicating at one end thereof with the first fluid passage means and adapted to S~

communicate at the other end thereof with a source of the first fluid medium. The second fluid delivery system comprises second fluid passage means associated with each of the molds and adapted to sequentially introduce a second fluid medium under pressure to the interiors of the respective tube portions enclosed within the molds whereby to expand each tube portion in sequence to conform to the internal surface of the respective mold, and second conduit means communicating at one end thereof with the second fluid passage means and adapted to communicate at the other end thereof with a source of the second fluid medium.
Apparatus in accordance with the invention is admirably suitable for continuously carrying out the method of the invention to produce a succession of aTticles which exhibit the advantageous barrier properties described herein-above.
The drawing is a partial schematic representation of a blow molding system constructed and adapted to operate in accordance with a preferred embodiment of the invention.
As disclosed herein, the invention is applied to a blow molding system of a general type which is otherwise largely conventional. Examples of systems of this type are disclosed with particu]arity in U.S. Patents 2,515,093 and

2,579,390, issued ~uly 11, 1950 and December 18, 1951, respectively, both to E. E. Mills, and in U.S~ Patent 2,784,452, issued March 12, 1957 to H. S. Ruekberg et al. Because blow molding machines of this type of system are now well known and in widespread commercial use, their principal conventional elements can be described bricfly, as follows.

9~6 An endless series of partible blow molds arranged in a continuous circular array is rotatable in vertical planes about a horizontal central axis (not shown) normal to the plane of the drawing. One such mold in the closed position is indicated generally at 10 in the drawing. A
second mold 12 is shown in vertical section in a partially open position. A third mold 14 is represented in fragmentary form in the fully open position. The direction of rotation of the molds is clockwise as viewed in the drawing.
Each mo]d comprises a pair of mold halves 16, 18.
The radially inner mold half 16 of each mold is radially immovable relative to the axis of rotation, whereas each outer mold half 18 is radially reciprocable between the closed and fully open positions shown in the drawing. Re-ciprocation of the outer mold half is normally effected by means of a simple cam-and-follower arrangemen~ or a fluid-actuated piston-cylinder device, neither of which is shown.
Ihe mold halves 16, 18 of each mold are provided with contoured surfaces 20, 22, respectively, which cooperate when the mold is closed to form a continuous internal surface thereof complementary to the outer surface of an article to be blow molded. The internal surface so formed defines the mold cavity. Each mold half is also provided with uppcr and lower pinch-off surfaces 24, 26, respectively, the function ; of which will be pointed out hereinafter~
Each of the inner mold halves 16 carries therewithin a fluid passage in the form of a hollow hlow needle 28 mounted for limited reciprocating movement along its longi-tudinal axis which is normal to the parting plane 30 of the 5~

mold. The blow needle is connected to a source of blowing medium, as will be described with particularity hereinafter.
Also carried by each mold half 16 outwardly thereof is a needle actuating device 32 which acts intermittently in accordance with a predetermined program to move the associ-ated blow needle 28 between the retracted position indicated in the drawing and an advanced position in which the distal end of the needle obtrudes into the mold cavity. Details of the construction and operation of similar blow needles and needle actuating devices are disclosed in U.S. Patent

3,513,502, issued May 26, 1970 to W. A. Chambers, and U.S.
Patent 3,571,848, issued March 23, 1971 to J. L. Szajna.
~ epresented schematically at 34 in the drawing is a stationary extruder die head which forms the terminus of a conventional plasticizer-extruder, the remaining Eeatures of which are not shown. Die head 34 comprises essentially a hollow outer die 36, normally of cylindrical form, and an inner die or core 38 disposed concentrically within and spaced from the outer die to define therewith an orifice 40 for the extrusion of plastic melt 42 delivered by the plasticizer. Orifice 40 is normally annular or of similar configuration. Core 38 is affixed by any suitable means to a core stem 44, the core and core stem being mounted for limited reciprocating axial movement in a core support 46 fixedly carried by die head 34. This reciprocating movement may be effected by any one of a number of well-known core actuating devices as represented schematically at ~8. Such a device operates in accordance with a predetermined program in synchronism with operation of the molds to vary the dimensions o-f orifice 40 and thus the wall thickness of the extrudate 50, thereby affecting the blowing characteristics of the extrudate and the wall thickness of the resulting blown article at various locations. Because the variations so provided are small relative to the average or nominal wall thickness, they are not represented in the drawing. It will be apparent that extrudate 50 is of generally tubular form owing to the configuration of orifice 40.
Formed in core 38 and core stem 44 is a fluid passage 52 having an inlet 54 and an outlet 56, the latter being located radially inwardly of and concentric with orifice 40 whereby fluid passage 52 is disposed to communicate with the interior of extrudate 50.
The apparatus described thus far is well known and widely employed in the blow molding of hollow plastic articles, particularly containers. Conventionally blow needle 28 and fluid passage 52 are connected to a source or sources of ordinary air under pressure. (The expression "under pressure"
is employed throughout this specification to mean at a pressure greater than the opposing ambient pressure.) In accordance with the present invention, however, blow needle 28 and fluid passage 52 are closed to the ambient and to other sources of air, or more particularly, oxygen; instead they are incorporated in novel fluid delivery systems in which other gases are employed and from which air and oxygen are excluded.
Specifically, a pair of fluid delivery systems, indicated generally at 58 and 60, respectively, are closed to the ambient and comprise well-known elements which in themselves require little description. Delivery systcm 58 begins with a source 62 of inert gas, preferably nitrogen in the liquid state. (It will be apparent from the foregoing statement that the expression "inert gas" as used throughout this specification is intended to include not only the rare gases but also gases such as nitrogen which under expected conditions of operation are virtually inactive chemically.) Source 62 is connected with each blow needle 28 by means of a fluid conduit 64 which includes in series a nitrogen vaporizer 66, a solenoid shutoff valve 68 selectively energized to the open position by a manually controllable switch 70, a rotary union 72 which provides communication between stationary and rotatable portions of fluid conduit 64, and a shu~off valve 74 controlled by a cam-and-follower device 75 in a manner such that flow of gas through each blow needle 28 is synchTonized with operation of the mold with which the blow needle is associated, as will be described with greater particularity hereinafter. Fluid conduit 64 may also be provided with shutoff or isolation valves 76 where appropriate or desirable.
Delivery system 60 begins with a source 78 of a mixture of fluorine and an inert gas. Commercially available in the United States and suitable for the purposes of the invention is a gaseous mixture of 10~ by volume fluorine and the remainder nitrogen, which is shipped and stored in relatively small cylinders at a pressure of 2400 psi. Under this system of containerization source 78 will normally Sfi comprise a manifold of such cylinders or a plurality of manifolds connected in parallel. In any case, source 78 as well as source 62 is readily replaceable when depleted.
Because most applications of the invention do not require the full ten-percent concentration of fluorine, inert gas source 62 is included in delivery system 60 as well as in delivery system 58, though a separate source of inert gas may be employed in system 60 if desired.
Sources 62 and 78 are connected with fluid passage 52 by means of a fluid conduit 80 which comprises a number of segments 80a-80h, some of which are connected in parallel to provide alternative routes, as will become clear from the following description. Like fluid conduit 64, fluid conduit 80 is provided with shutoff or isolation valves 76 where appropriate or desirable.
Under normal conditions of continuous operation conduit segmen~ 80a provides communication, by way of a series of pressure-reducing valves 82 and 84, between fluorine source 78 and a blender 86. A suitable blender for this purpose, or instructions for the construction of SUC}l a blender, can be obtained from Air Products and Chemicals, Inc. of Allentown, Pennsylvania. Similarly, conduit segment 80b provides communication between inert gas source 62 and the blender. Conduit segments 80c, 80e, 80f and 80g, which are connected in series, provide communication between blender 86 and fluid passage 52.
Downstream of blender 86, conduit segment 80c includes a source 88 of a dilute mixture of fluorine in inert gas, source 88 being supplied directly by blender 86 g and acting as a surge chamber. Conduit segment 80e includes in series a check valve 90 and a solenoid shutoff valve 92.
Conduit segment 80f includes a metering valve 94. Conduit segment 80g is in communication with inlet 54 of fluid passage 52 and is preferably formed of flexible material to accommodate the reciprocating movement of core stem 54.
It will be apparent that when the gas is to follow the route just described, valves 76a, 76b and 76c must be open and valves 76d and 76h must be closed.
It should be noted that blender 86 is adjustable to provide any desired concentration of fluorine below 10%
by volume. Concentrations of from about 2% to about 8% by volume are preferred. A gas analyzer 96 of any suitable type may be provided in communication with conduit segment 80e to monitor or sample the fluorine concentration.
If the full concentration of ten-percent fluorine is required, valves 76a, 76b and 76c may be closed and valve 76d opened, whereby conduit segment 80d will interconnect conduit segments 80a and 80e and thereby act as a bypass around blender 86 and dilute gas source 88.
Similarly, if only an inert gas is to be introduced to fluid passage 52, as might be done for reasons of safety and economy upon commencement of operation and continued until a stable operating condition is achieved, valves 76a, 76b, 76c and 76d are maintained closed and valve 76h opened, whereby conduit segment 8~h will interconnect nitrogen vaporizer 66 and conduit segment 8Qf, thus bypassing conduit segments 80c, 80d and 80e and the elements included therein.
Like conduit segment 80e, conduit segment 80h includes in 30 series a check valve 98 and a solenoid shutoff v~lve 100.
Pressure gauges 101 may be provided in fluid conduit 80 where desired.

~l~lL21~ri6 Solenoid shuto-ff valves 92 and 100 are selectively energized to the open position by means of a manual selector switch 102, which is so arranged that flow cannot occur simultaneously in conduit segments 80e and 80h. The selector switch also includes an OFF position to prevent flow alto-gether in fluid passage 52 if desired, even when the molds are being rotated. Selector switch 102 is in turn energized by a machine automatic mode relay 104 which also energizes mold and plasticizer operation during the normal continuous operating condition. Thus when relay 104 is not energized, flow in fluid passage 52 is precluded.
In the ensuing description it is assumed that the blow molding system of the invention is in continuous, stable, automatic operation; that is to say, the normal operating condition. In this condition, relay 104 and thus selector switch 102 are energized. Switch 70 is also energized.
Plastic melt 42 is delivered at a constant pre-determined rate to extruder die head 34 where it is continuously formed into tubular extrudate 50 by way of orifice 40. The temperature of the melt is maintained at a level high enough that the extrudate is readily expandable but not so high as to imperil the integrity of the wall of the extrudate. The plastic is preferably a polymer of ethylene, particularly such a polymer having a density of at least 0.94.
The molds are rotated at a speed synchronous with the extrusion rate. In succession the molds approach the extruder die head in the fully open position represented by mold 14. From the open position the outer halves 18 of the 112~ 6 molds are successively directed towards the closed position represented by mold 10 to close upon successive portions of the extrudate at a location some distance below the extruder die head. As each mold is closed its pinch-off sllr-faces 24 and 26 act to close to the ambient the interior o-E the enc].osed portion of the extrudate and to isolate it from the interiors of other portions of the extrudate. Thus the interior of that porti.on of the extrudate between the extruder die head and the most recently closed mold is maintained closed to the ambient and provides a tubular parison to be enclosed by the next succeeding mold.
Because extrudatc SO is hollow as it emerges from orifice ~0, it will be obvious that it will collapse upon itself if its internal pressure does not at least equal ambient pressure. In conventional systems, therefore, provision is made to i.ntroduce air continuously to the interior of the extrudate, as by a fluid passage similar to passage 52. Further, the air is customarily introduced at a constant rate under pressure to partially expand each successive portion of extrudate 50 before it is enclosed within one of the molds. This partial pre-expansion imparts improved blowing characteristics to the extrudate and permits closer control of the wall thickness of the blown article at various critical locations.
In accordance with the present invention, however, in place of the air a mixture of fluorine and an inert gas is continuously introduced at a constant rate, and preferably under pressure, to the interior of the extrudate, where the fluorine reacts with the inner surface of the extrudate to provide the barrier properties mentioned previously. The fluorine mixture is delivered by way of delivery system 60 as follows.
The ten-percent concentration by volume o-f fluorine in inert gas is conducted at 2400 psi from source 78 thereof to pressure reducing valve 82 where its pressure is brought down to, say, 200 psi. From there it is led to pressure reducing valve 84 where its pressure is again reduced, in this instance to, say~ 50 psi, and whence it is introduced to blender 86. In the blender the mixture is diluted by inert gas introduced from source 62 thereof, or more precisely, from vaporizer 66, to a predetermined concentration of fluorine preferably between about 2% and about 8% by volume.
Blender 86 delivers the dilute gas mixture ~o dilute gas source 88, from which it is conducted to the interior of extrudate 50 by way of check valve 90, solenoid shutoff valve 92 and metering valve 94. Selector switch 102 is in the condition represented in the drawing whereby to maintain solenoid valve 92 in the open posi~ion and solenoid valve 100 in the closed position. The rate of flow in -fluid passage 56 may be established, and adjusted as necessary, by means of metering valve 94.
When each mold is closed in succession upon the corresponding portion of extrudate 50, the blow needle 28 carried by that mold is moved to its advanced position by the corresponding needle actuating device 32, whereby the needle pierces the wall of the enclosed parison; that is, the portion of the extrudate enclosed within the mold; the distal end of the needle thus obtruding into the interior of the parison. Simultaneously with movement o-f the needle to the aclvanced position, the associated valve 74 is moved to the open position by its cam-and-follower device 75 to admit blowing gas under pressure to the interior of the parison by way of the blow needle, thereby expanding the parison to conform to the internal surface of the mold formed by surfaces 20 and 22 thereof; thus is formed a hollow article having an outer surface complementary to the internal mold surface.
If desired, cam-and-follower device 75 may be timed to hold valve 74 in the open position for a predetermined period after the parison has been fully expanded, thereby maintaining a positive pressure within the hollow article long enough to alleviate shrinkage as the article begins to cool within the mold. In any event, blow needle 28 is retracted at some point after the article has been blownS and the pressure within the article is relieved, for example in the manner disclosed in the previously mentioned U.S. Patent 3,571,848.
The excess gas thus released is conducted to a remote location in a manner similar to ~hat described hereinbelow. The mold is opened when the article has cooled to a degree such that it is self-supporting, and the article is removed from the mold, whereupon it may be subjected to any number of well-known finishing steps.
As indicated hereinbefore, the blowing gas conven~ion-ally comprises ordinary air. In the aforementioned patent to Dixon et al. it comprises a mixture of -fluorine and inert gas. In accordance with the present invention, on the other hand, the blowing gas comprises an inert gas alone, preferably nitrogen. Specifically, liquid nitrogen is directed from source 62 thereof to vaporizer 66, where it is converted to the gaseous state. From the vaporizer the nitrogen gas is conducted to rotary union 72 by way of solenoid valve 68 which is maintained in the open position by the energized switch 70~ The gas is distributed from the rotary union to the various blow needles by way of valves 74, each of which is operated intermittently in the manner described hereinbefore.
I'he mixture of fluorine and inert gas contained in each parison when it is enclosed within one of the molds is substantially diluted upon the introduction of inert gas by way of the blow needle; that is to say, the fluorine concentration is substantially reduced. Nevertheless, for the protection of personnel and property the blown article should be purged of rluorine. This is most conveniently done after the article has been remo~ed from the mold and trimmed to provide an opening (the neck opening in the case of a container) for reception of a purging wand. It may be necessary to conduct large articles to two or three purging stations in succession in order to ensure thorough purging.
The purging is carried out using ordinary air in a shrouded and vented environment. The gas so removed is conducted to a remote location, if necessary by way of suitable treating stations, and exhausted to the atmosphere in innocuous form or in harmless concentrations.
Operation of the system at the full ten-percent concentration of fluorine is carried out simply by closing valves 76a, 76b and 76c and opening valve 76d.

Regardless of the flworine concentration to be employed, upon commencement of operation valves 76a, 76b, 76c and 76d are preferably maintained closed, valve 76h is opened, and selector switch 102 is positioned to open solenoid valve 100 and maintain solenoid valve 92 closed. The parisons and blown articles are thereby subjected only to nitrogen until stable operation is achieved and acceptable articles are being produced. At this point valves 76a, 76b, and 76c are opened, valve 76h is closed, and selector switch 102 is positioned to close valve 100 and open valve 92 to commence the fluorine tr~atment. The few untreated articles produced by this procedure may be treated as scrap and reground or otherwise disposed of.
~ ile the invention has been particularly described in connection with a certain specific embodiment thereof, it is to be understood that this is by way of illustration and not of limitation~ and that the scope of the appended claims should be construed as broadly as the prior art will permit.

Claims (14)

C L A I M S
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of blow mo lding an article from an expandable tube of thermoplastic synthetic resinous material, wherein the interior of the tube is closed to the ambient, and the tube is enclosed within a mold and expanded to conform to an internal surface of the mold, the improvement wherein a mixture of from about 0.1% to about 10% by volume fluorine and the remainder an inert gas is introduced to the interior of the tube before the tube is enclosed within the mold, and wherein the step of expanding the tube within the mold is carried out by introducing an inert gas under pressure to the interior of the tube.

2. In a method of blow molding an article from an expandable hollow parison formed of thermoplastic synthetic resinous material, wherein the interior of the parison is closed to the ambient and the parison is partially expanded, then enclosed within a mold, and further expanded to conform to an internal surface of the mold, the improvement wherein the step of partially expanding the parison is carried out by introducing under pressure to the interior of the parison a mixture of from about 0.1% to about 10% by volume fluorine and the remainder an inert gas, and wherein the step of further expanding the partially expanded parison is carried out by introducing an inert gas under pressure to the interior of the parison.

3. A method of blow molding articles in succession, comprising the steps of continuously extruding an expandable tube of synthetic resinous material, enclosing successive portions of the extruded tube within a succession of molds, thereby maintaining the interior of each tube portion closed from the ambient before said tube portion is enclosed within one of the molds and isolating the interiors of the tube portions from one another, continuously introducing to the interior of the tube as it is extruded a mixture of from about 0.1% to about 10% by volume fluorine and the remainder an inert gas, and introducing an inert gas under pressure to the interiors of the tube portions when enclosed within the molds to expand each of the tube portions to conform to an internal surface of its respective mold.

4. The method according to any one of claims 1 to 3, wherein the thermoplastic synthetic resinous material is a material selected from the group consisting of acrylonitrile polymers, styrene polymers, vinyl chloride polymers, and polymers of olefin monomers having 2 to 8 carbon atoms per molecule.

5. The method according to any one of claims 1 to 3, wherein the thermoplastic synthetic resinous material is .alpha. polymer of an olefin monomer having 2 to 8 carbon atoms per molecule.

6. The method according to any one of claims 1 to 3, wherein the thermoplastic synthetic resinous material is .alpha. polymer of ethylene.

7. The method according to any one of claims 1 to 3, wherein the thermoplastic synthetic resinous material is a polymer of ethylene having a density of at least 0.94.

8. The method according to any one of claims 1 to 3, wherein said remainder of the mixture consists essentially of nitrogen.

9. The method according to any one of claims 1 to 3, wherein the last-mentioned inert gas consists essentially of nitrogen.

10. The method according to any one of claims 1 to 3, wherein the mixture comprises from about 1% to about 10% by volume fluorine, and from about 99% to about 90% by volume nitrogen.

11. The method according to any one of claims 1 to 3, wherein the mixture comprises from about 2% to about 8% by volume fluorine, and from about 98% to about 92% by volume nitrogen.

12. The method according to any one of claims 1 to 3, wherein the thermoplastic synthetic resinous material is a polymer of ethylene having a density of at least 0.94, wherein the mixture comprises from about 2% to about 8% by volume fluorine, and from about 98% to about 92% by volume nitrogen, and wherein the last-mentioned inert gas consists essentially of nitrogen.

13. Apparatus for successively blow molding articles, comprising extrusion means for continuously extruding an expandable tube of synthetic resinous material, a plurality of molds arranged in a series thereof, each mold having an internal surface complementary to the outer surface of an article to be blow molded, the molds being adapted to successively enclose therewithin successive portions of the extruded tube whereby to maintain the interior of each tube portion closed from the ambient before said tube portion is enclosed within one of the molds and to isolate the interiors of the tube portions from one another, and first and second fluid delivery systems adapted to be closed from the ambient, the first fluid delivery system comprising first fluid passage means associated with the extrusion means for continuously introducing a first fluid medium to the interior of the tube as it is extruded, and first conduit means communicating at one end thereof with the first fluid passage means and adapted to communicate at the other end thereof with a source of the first fluid medium, the second fluid delivery system comprising second fluid passage means associated with each of the molds and adapted to sequentially introduce a second fluid medium under pressure to the interiors of the respective tube portions enclosed within the molds whereby to expand each tube portion in sequence to conform to the internal surface of the respective mold, and second conduit means communicating at one end thereof with the second fluid passage means and adapted to communicate at the other end thereof with a source of the second fluid medium.

14. Apparatus for blow molding articles in succession, comprising an extruder die head, means defining a substantially annular orifice in the die head for continuously extruding an expandable tube of synthetic resinous material, a plurality of partible molds arranged in an endless series thereof, each of the molds having an internal surface complementary to the outer surface of an article to be blow molded, the molds being adapted to successively close upon successive portions of the extruded tube to maintain the interiors of the tube portions closed from the ambient and to isolate the interiors of the tube portions from one another, means in the die head defining a fluid passage having an outlet disposed radially inwardly of the orifice whereby to communicate with the interior of the extruded tube, a source of gas under pressure comprising a mixture of from about 0.1% to about 10% by volume fluorine and the remainder an inert gas, first conduit means interconnecting the fluid passage with the mixture source, a hollow blow needle carried by each mold, a source of inert gas under pressure, second conduit means interconnecting the blow needle with the source of inert gas, the blow needle being adapted to pierce a tube portion enclosed within the mold, and means in the second conduit means for controlling flow of the inert gas there-within.