CA1038125A - Balloon blown plastic molding - Google Patents

Abstract

BALLOON BLOW PLASTIC MOLDING

ABSTRACT OF THE DISCLOSURE
This invention covers the core rod or parson pin of a blow mold machine with a rubber tube or balloon so that the plastic is supported internally during a blow molding operation. The fluid that blows the molding inflates the balloon and has contact with the inside of the balloon in-stead of with the plastic of the parison. Cooling fluid circulates inside the balloon so that the molding is cooled from both sides to shorten the cycle. Higher pressure fluid can be used without risk of rupturing the molding and blow-ing can, therefore, be carried out at a lower temperature where the plastic has begun to crystallize and can be orien-ted for greater strength.

Description

- 1a38~2s In injecting molding machines the core pin is first in-serted into the mold cavity of an injection mold where the surfaces of the cavity are spaced from the surfaces of the core pin by a distance equal to the desired thickness of plastic coating which is to be applied to the core pin. The plastic material is then injected into the mold cavity and the core pin thus coated with the material~

The mold is then opened and the core pin is transferred to a blowing mold where the cavity has the shape of the product which is to be blown. Blowing fluid under pressure is then discharged from the core pin against the plastic that surrounds the core pin and the plastic is blown outwardly away from the core pin and into contact with the sides of the mold. After a short cooling time, the blowing mold is opened and the core pin is moved to a stripping station where the blown article is stripped from the core pin.

In extrusion molding machines, a plastic tube is extru-ded downwardly into an open blowing mold and a core pin ex-tends from the extruder downwardly along the axis of the tube.
When the tube has reached a length greater than the length of the blow mold, the mold closes and pinches the bottom of the -tube closed while clamping the upper end of the tube tightly around the core pin. The tube is then blown to the shape of the mold cavity by blowing fluid discharged from the core pin and the mold is opened to permit the extrusion of another length of tubing. The blown molding is cut off from the tube and the mold closes on the new length of tube to repeat the blowing step.

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In both types of machines, the molding must remain in the blowing mold until the plastic coos sufficiently to hold its shape. The blowing mold is cooled by circulation of water or other cooling fluid through cooling passages in the wall of the mold..

The pressure of air or other fluid used to blow the plastic must be limited in pressure. If introduced into the blowing mold at too high pressure, the blowing fluid will blow out the plastic wall at the weakest place or location of greatest stress, and the temperature of the plastic must be controlled so that the plastic is hot enough to blow at the pr~ssure that can be safely used.

By covering the core rod with a balloon and coating the outside of the balloon with the plastic to be blown, a number of new results are obtained. One is that higher pressure can - be used to blow the molding. The balloon prevents the blowing fluid from having contact with the plastic and there is no danger of a blowout of the plastic. Because higher pressure can be used for blowing, plastic can be blown at lower tempera-ture and after it has been cooled to the temperature range at -~hich it begins to crystallize. This permits orientation of the molecules and by having a balloon that is deformed biaxially, the plastic of the molding is stretched both circumferentially and axially (or in other directions at right angles to one another) so that the molding has biaxial orientation with re-sulting greater strength, and in the case of some plastics, clear transparencY

Another new result is obtained if the balloon is made ~ith different wall thickness at different sections of its extent. There is then a sequential expansion of the different -sections of the balloon so that corresponding sections of the parison are blown at different times. For example, the upper or lower portion of the molding might be blown ahead of another portlon.

Blowing fluid, such as liquid, at temperatures substan-tially lower than the plastic can be used and circulated through the balloon during the blowing operation and immediate-ly thereafter so that the molding is cooled from both the in-side and outside to obtain substantially faster cooling and a - shorter cycle for the machine, and increased production.

The invention is suitable for use on either injection blow molding machines or e~trusion blow molding machines.

Other objects, features, and advantages of the invention will appear or be pointed out as the description proceeds.

BRIEF ~ESCRIPTION OF DRAWING

In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views:

Figure 1 is a diagrammatic view of an injection blow molding machine with the molds, plastic, and balloon covered core pin shown in section;

Figure 2 is a greatly enlarged sectional view of a blow-ing mold with a differently shaped cavity from that shown in Figure 1 and with the plastic on the core pin not yet blown;

Figure 3 is a sectional view taken on line 3-3 of Figure ~;

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. Figure 4 is a greatly enlarged fragmentary view of a portion of the core pi~ shown in Figure 2;

Figure 5 is a view showing diagrammatically a closed circuit connected with the core pin of Figure 2 for supplying cooling and blowing fluid to the core pin;

Figure 6 is a diagrammatic view of an injection blow molding machine with a balloon covered core pin made in accordance with this invention;

Figure 7 is a view similar to Figure 6 but showing the blowing mold in closed position and the condition of the plastic and balloon at the end of the blowing operation; and FIgure ~ is an enlarged, fragmentary, detail view of vents for the core pin of Figures 6 and 7.

DESCRIPTION OF PREFERRED EMBO~IMENT

Figure 1 shows an injection blow molding machine 10 which includes a turret 12 with four core pins, 14, 15, 16 and 17 projecting from different sides of the turret 12 at 90 angular relation to one another. The turret 12 is ro-tated about an axis 20 wherever the core pins 14-17 are to be transferred from one station to another.

The first station of the machine 10 is an injection station 22 which has a mold 24 to which plastic is supplied from an extruder 26 through a nozzle 2~. The core pin 14 e~tends into a cavity 30 of the mold 24.

The core pin 14 is covered by a balloon 32 which is secured to the core pin in a manner which will be explained in connection with Figures 2 and 4. The core pin 14 with the ~ 1C~38 125 -- -- .
balloon 32 covering it, extends into the cavity 30. With the mold 24 closed, plastic 34 is injected into the cavi~y 30 so as to coat the entire portion of the balloon 32 which is spaced from the surface of the mold cavity 30.

The injection mold 24 is of conventional construction ~ith passages 36 for the circulation of cooling fluid; and the mold 24 opens to release the plastic 34. The mold opens wlde enough so that the turret 12 can rotate and move the core pin 14, balloon 32 and plastic 34 in a counterclockwise direction about the center 20 into the position occupied by the core pin 15 in Figure 1.

During this movement of the turret 12 through an angle of 90~, the core pin 15 is moved into the position occupied by the core pin 16 in Figure 1 and the core pin 16 is moved . into the position occupied by the core pin lZ. The core pin 17 moves into the open mold 24 where it is ready to receive a charge of plast~c 34 from the nozzle 2~ at the beginning of the next cycle of the machine. The operation of the mold 24 in opening and closing; and the operation of the turret 12 in transferring core pins from one station to another are well understood in the injection blo~ molding machine art and no further illustration or description of this structure is necessary for a complete understanding of this invention.

The cecond station of the machine 10 is a conditioning station 3~. The core pin 15 at the conditioning station 3~
has a balloon 32 ~ covered with plastic 34 ~ which was applied to the core pin lS in the mold 24 in the manner already des-cribed for the core pin 14. Each core pin 15 with its plastic 34' remains at the conditioning station 3~ until the next ~038125 mo~rement of the turret 12, and durlng thi~ time the plastlc 34' c0018 to substantially the temperatur~ desired for blowing.
This cooling time ls determinet by the dwell period of the turret 12; but the amount of cooling desired will vary with - ~he amount of plastic 34' which i3 applied at different times depending upon the molds which are on the machine for produc-in~ a particular molding. The amount of coollng also dependq on the temperature at which the blowing ls to be performed.
Thls ln turn depend~ upon the pressure that is to be used and upon the amount of orientation of the molecules that are required or deslred for a particular molding. Although the d~ell time at the conditioning station 3~ ls not ~ariable to suit the desired cooling, the extent of cooling can be controlled at the dwell station by controllin~ the temperature of the atmosphere surrounding the plastic 34' or by subjecting the plastic to radiant heatlng of various degrees while at 'che conditioning station 3~.

The machine can have a blowlng station 40 at which there 18 a blowin~ mold b,2 into which the core pin 16 travelled during the precedlng movement of the turret 12 and while the mold 42 was open. When the mold 42 is closed, and blowing fluid is dlscharged from openlngs 44 ln the core pin 16, a balloon 32a on the core pin 16 expandq and moves plastic 34a lnto contact wlth the s~urfaces of a mold cavlty 46 of the mold 42.
Flgure 1 shows the balloon 32a expanded and the plastic 34a ~haped to the contour of the mold cavity 46.

The mold 42 haq passage-q 36a for cooling fluid snd lt is nece~sary for the plastic34a to cool in the mold 42 to a temperature low enDugh to make the plastic self-sustaining before the artlcle molded from the plastic can be removed from 103~125 the mold 42. This time required for the molded article to cool has been the limiting factor on the output of injectlon molding machines. With the present invention, the balloon 32a can be inflated with water or other liquid at a temperature which will cool the plastic 34a from the inside at the same time that contact with the walls of the mold 42 cool the plastic from the outside. By cooling the plastic from both sides, the cooling time can be greatly reduced. This shortens the cycle of the molding machine and greatly increases the production.

When the article molded from the plastic 34a, which is shown in Figure 1 as a bottle 50, has cooled to a shape sus-taining temperature, the mold 42 is opened and the turret 12:rotates another 90 to move the core pin to a stripping station 52 where the core pin 17 is shown with a bottle 50 held on the core pin by the neck 54 of the bottle. The core pin 17 has a balloon 32b which is shown deflated in Figure 1, the deflation having taken place upon completion of the cooling at the blowing station 40 and while the core pin was travel-ling from the blowing station to the stripping station 52.
At the stripping station 52 a stripper plate 55 mo~es into engagement with the neck S4 of the bottle and pushes the bottle axially along the core pin 17 to disengage the bottle from the core pin. This completes the manufacture of the bot-tle 50 on the molding machine.

Figyre 2 shows the core pin 16 in a mold 42' which is similar to the mold 42 of Figure 1 except that the mold cavity 46' is of different shape and the passages 36' for cooling the mold are also of different shape. The balloon 32a is secured to the core pin 16 by clamping rings 56 which clamp the material of the balloon 32a into circumferential _ ~ _ grooves 57 extending around the outside surface of the core p~n 16. Two rings 56 are shown; one outside the mold 42' and the other inside. The mold 42' closes firmly against the outside surface of the balloon 32a so that none of the plas-tic material 34a can be extruded from the mold cavity between the balloon and the clamping surface of the mold 42' which clamps against the surface of the balloon 32a.

There is a shoulder 5~ on the core pin 16 and there is a clearance 5~c between the core pin and the inside surface -of the balloon 32a for some distance to the left of the shoul-der 5~ in Figure 2. This clearance is maintained, during in-jection of the plastic into the mold at the injection station 22 (Figure 1) by supplying fluid within the balloon 32a sub-stantially equal to the pressure at which the plastic is in-jected into the mold.

A circumferential air manifold 59 ex~ends around one end of the mold 42' and the side of the manifold is closed by a cover 60. Passages 61, at angularly spaced locations around the mold, lead from the air manifold 59 to the outside surface of the balloon 32a at regions where the balloon 32a can col-lapse toward the core pin; i.e. at the region of the balloon surface outside of clearance space 5~c.

Plastic is preventéd from entering the passage 61, at the injection station, by maintaining enough pressure within the balloon to prevent the plastic from collapsing the balloon into the clearance 5~c. Less counterbalancing pressure in the balloon is required when each passage 61 is located near the end of the clearance 5~ as shown in Figure 2.

When the blowing operation is complete, and the blown article has cooled enough to hold its shape, a partial vacuum _9--t' 1038~5 is drawn on the core pin 16 to collapse the balloon. The balloon cannot collapse unless air can gain entrance between the outside surface of the balloon and the inside surface of the blown article. The balloon 32a collapses into the clear-ance 5~ when the vacuum is first drawn on the core pin. This opens a clearance between the balloon and the inside surface of the neck of the blown bottle. Air from the passages 61 rushes into this clearance between the balloon and the blown bottle for the length of the balloon that did not expand during the blowing operation. Beyond this the expanded balloon col-lapses away from the blown bottle as air continues to flow to-ward the left in Figure 2 along the outside surface of the balloon.

Figure 2 shows the feature of blowing different parts of the parison sequentially. In order to obtain expansion of - some parts of the balloon 32a ahead of other parts, the walls of the balloon 32a are thicker around the region 66 than at other parts of the length of the balloon. Thus when the bal-loon 32a is inflated, as indicated by the bro~en lines in Figure 2, the part of the balloon to the left of the region 66 expands before the wall at the region 66 expands. By controlling the stiffness or elasticity of the walls of the balloon 32a at different regions, the plastic 34a can be blown se~uentially as to different portions of its length.

In Figure 2, for example, the expansion of the balloon from the dotted line position shown in Figure 2 will soon bring the plastic into contact w~th the side of the mold cavity at a region midway between the right and left hand ends of the cavity. Further expansion of the balloon will stretch the plastic in both directions from this mid region as the left hand end of the balloon travels toward the left hand end t;

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of the mold cavity and as the stiffer right hand portion of th~e balloon expands toward the right hand end of the mold cavity. Thus the plastic is stretched both circumferentially, with the increase in diameter of the balloon, and also axially to produce a biaxial orientation of the plastic material if the plastic material is blown at a temperature low enough to permit orientation of the molecules. Since the balloon pre-~ents the blowing fluid from having contact with the plastic 34a, the blowing fluid can be at extremely high pressure with-out danger of brea~ing through a weak portion of the plastic and the blowing at higher pressure permits the plastic material 34a to be blown at lower temperature at which it has begun to crystallize.

The proportion of the core pin 16 which extends into the cavity of the mold has a center partition 70 extending lengthwise of the core pin as shown in dotted lines in Figure

2. Water or other blowing and cooling fluid flows into the space on one side of the partition 70 through an inlet passage 72 and flows out of the space on the other side of the parti-tion 70 through an exhaust passage 74. These passages commu-nicate with tubing 76 and 7~, respectively, which are part of a closed circuit as will be explained in connection with Figure 5. Thus the fluid supplied to the passage 72 flows out through the openings 44 on one side of the core pin and circulates ge-nerally circumferentially around the core pin within the in-flating balloon 32a; and this liquid or other fluid returns to the interior of the core pin on the other side of the ' partition 70 through the openin~s 44 that are on that side of the partition. The blowing and cooling fluid then passes out through the exhaust passage 74, as already explained. Figure 5 shows the core pin 16 in a mold cavity ~0 which tapers to a --11_ larger diameter at the far end of the mold instead of a to a smaller diameter as shown in Figure 2. This difference depends upon the desired shape of the article which i9 being blown.
As in the other molds, the core pin 16 is shorter than the mold so that the plastic being blown is stretched axially as well as circumferentially. Figure 5 shows in dotted lines indicated by the reference characters 32b and 32c the contour of the balloon as it expands in the mold cavity ~0 with differences in the localized expansion as the result of greater resistance to e~pansion toward the right hand end of the balloon.
Figure 5 also shows diagrammatically the closed circuit through which blowing fluid from the exhaust tube 7~ travels through a valve ~2 and cooler ~ to a blowing fluid supply tank ~6 from which the blowing fluid is fed back by a pump 8~ to the supply tube 76.

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- The pump ~ is driven by a motor 90 connected w~th a power supply line 92 and having a speed control 94 for regu-lating the rate of supply of blowing fluid to the core pin.
No attempt is made to show the automatic control for insti-gating and stopping the flow of blowing fluid in accordance with the cycle of the blow molding machine since this is con-ventional. The use of the blowing fluid to supply cooling to the inside of the molded article is one of the novel features of this invention. The valve ~2 is shown with spring loading 96 which is adjustable to change the back pressure in the core pin and this valve g2 is adjusted to make the back pressure as high as desirable in order to obtain the necessary blowing pressure. A relief valve 9~ is lnterposed in the closed system to permit discharge from the pump ~g to flow back to the blowing fluid supply tank ~6 through a by-pass around the -valve ~2.

The cooler ~4 is shown as a tube with heat radiating plns, but this is merely representative of means for cooling fluid passing through the tubing and any other cooling ex-pedient can be used depending upon the amount of heat to be d~ssipated.

Figure 6 and 7 show the application of the invention to an extrusion molding machine. A plastic pipe 102 is extruded downwardly from an extruder 104 through an extruder die 106 located above clamping jaws 10~. A core pin 110 extends downward from theextruder 104 and this core pin 110 is lo-cated within the ex~ruded tube 102 and substantially concen-tric therewith.

A blowing mold 112 is made up of two mold sections 114 and 116 which are shown separated in Figure 6 and closed together in Figure 7. The core pin llO is covered with a balloon 120 in the same way as the core pins described in Figures 1-5. -When the tube 102 has been extruded to a length suffi-cient to reach the bottom of the mold cavity of the mold 112, the jaws 10~ are brought together by clamping forces to pinch the tube 102 tight around the outside surface of the balloon 120 as shown in Figure 7; and the mold sections 114 and 116 are then brought together so that bottom edges 124 pinch the lower end of the tube 102 tightly closed with a lower tail end of the tube projecting below the edges 124 as indicated by the reference character 102a in Figure 7.

Blowing fluid is then supplied to the core pin 110 and the balloon 120 is expanded to force- the tube 102 out-ward into contact with the surfaces of the mold cavity formed by the closed sections 114 and 116.

~038125 When the plastic is fully blown in the mold 112, the mold cools the plastic to a shape-sustaining temperature and ~he mold is then opened, the balloon ccllapsed, and the molded bottle removed from the core pin.

Figure ~ shows the core pin 110 with vent tubes 126 opening through holes 12~ in the sides of the core pin 110 and through the sides of the balloon 120 for admitting air between the outside of the balloon and the inside of the blown article for collapse of the balloon.

Vent openings for permitting escape of air from molds asethe plastic is blown, and for admitting air to break ~acuums when articles are to be removed from the molds are provided by using openings or channels of small cross-section that does not permit entrance of plastic into the opening.
Such vents are well-known and used in conventional blow molding apparatus and a description of the vents-is not necessary for a complete understanding of this invention.

The prefered embodiments of the invention have been illustrated and described, but changes and modifications can be made and some features can be used in different combina--tions without departing from the invention as defined in the claims.

Claims (18)

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

1. An injection blow molding apparatus including an injection mold, a core rod that extends into the injection mold, a support for the core rod at one end thereof, an elastic balloon at the support end to the core rod and that covers and hugs the core rod, means for injecting molten material into the injection mold to apply a parison over the balloon, a blowing mold enclosing a cavity, means for moving the support to transfer the core rod, balloon and parison from the injection mold to the blowing mold, and for locating the core rod in a position extend-ing into the blowing mold and supporting the parison therein, the parison being smaller than the blowing mold cavity, and the core rod having a passage through which fluid is sup-plied to expand the balloon and by the pressure on the fluid from the core rod, transmitted through the balloon to the parison, to expand the pairson into contact with the sur-face of the mold cavity as the balloon is blown up by said fluid from the core rod.

2. The blow molding apparatus described in claim 1 characterized by the balloon being made of elastic material capable of repeatedly expanding successive parisons and then contracting back to its original shape after each expanding operation at the temperature of the material in-jected into the injection mold.

3. The blow molding apparatus described in claim 1 characterized by the balloon having an open end that fits over the core rod at a location beyond the mold cavity and adjacent said support, and means securing the balloon to the core rod near the open end of the balloon.

4. The blow molding apparatus described in claim 3 characterized by the means securing the balloon to the core rod being a clamping ring, the core rod completely filling the open end of the balloon and having a circumferential groove into which the balloonis clamped by said ring, a shoulder on the core rod beyond the groove, and a clearance between the core rod and balloon near said shoulder and into which the balloon can collapse to move the balloon away from the inside surface of a blown parison to admit air into the parison as the balloon collapses after each blowing operation.

5. The blow molding apparatus described in claim 1 characterized by means for circulating fluid simultaneously into and out of the balloon to cool the parison from the inside while building up pressure in the balloon to expand the balloon and the parison.

6. The blow molding apparatus described in claim 5 characterized by the means for circulating fluid including a pump, a source of liquid at a substantially lower temperature than the parison for cooling the balloon and cooling the parison from the inside by conduction of heat through the wall of the balloon from the inside surface of the parison in contact with the balloon, the blowing mold having cooling chambers therein around the cavity for cooling the mold and cooling the parison by contact with the outside surface of the parison with the surface of the cavity.

7. The blow molding apparatus described in claim 6 characterized by a closed circuit in which the blowing fluid for the core rod flows, a partition in the core rod for controlling the flow, means adjustable to control the rate of exhaust of liquid from the core rod independently of the rate of supply of liquid for building up pressure in the balloon, and means for cooling the liquid as it travels from the core rod exhaust back to the source of liquid.

8. The blow molding apparatus described in claim 1 characterized by means for supplying fluid to the core rod to expand the balloon and to expand the parison into contact with the wall of the mold cavity, the fluid supply means being capable of delivering fluid at a pressure that expands the balloon and the parison when the plastic of the parison is at a temperature at which crystallization of the plastic begins.

9. The blow molding apparatus described in claim 1 characterized by the balloon having side walls of different degrees of elasticity at different regions of the balloon so that regions of greater eleasticity expand ahead of those of lesser elasticity to obtain a selective sequence of expansion of the parison.

10. Blow molding apparatus including a blowing mold enclosing a cavity, a core rod supported at one end and that extends into a parison that is smaller than the blowing mold cavity and through which fluid is supplied to expand the parison into contact with the surface of the mold cavity, and an elastic balloon connected to the supported end of the core rod and within the parison and through which pressure of the fluid from the core rod is transmitted to the parison to ex-pand the parison into contact with the surface of the mold cavity as the balloon is blown up by said fluid from the core rod, characterized by the apparatus being an injection molding machine with an injection chamber in which the core rod is originally located for receiving a coating of plastic on the outside surface of the balloon, means for effecting transfer of the core rod and balloon from the injection mold to the blowing mold cavity, means for obtaining the orientation temperature for the parison prior to the blowing operation, the balloon being of shorter length and less cross section than the blowing mold cavity thereby the plastic deposited on the balloon in the injection mold is stretched both circum-ferentially and axially during expansion of the balloon in the mold cavity to effect biaxial orientation of the molecules of the plastic material.

11. The blow molding apparatus described in claim 10 characterized by the injection chamber being in a mold at an injection station of the machine, a turret to which the core rod is secured and by which the core rod is moved from one station to another, a conditioning station to which the coated balloon is transferred after leaving the injection station, a blowing station at which the blow mold cavity is located and to which the core pin and balloon are transferred after leaving the conditioning station, and a stripping sta-tion beyond the blow molding station and to which the core pin and blown parison are transferred by the turret after blowing at the blowing station.

12. The injection blow molding apparatus described in claim 1 characterized by the core rod having openings therein for the discharge of blowing fluid from the core rod at dif-ferent locations along the length thereof to expand the balloon, the material of the balloon, when contracted into contact hugging the core rod being sufficiently stiff to pre-vent material of the balloon from being extruded through said openings and into the core rod by the pressure in the injection mold when applying a molten parison to the outside of the balloon in the injection mold.

13. The injection blow molding apparatus described in claim 1 characterized by the balloon hugging the core rod for substantially the full length of the portion of the core rod that extends into the blowing mold cavity, and the balloon having a closed end beyond the end of the core rod that is remote from said core rod support.

14. The method of injection blow molding which com-prises surrounding a core rod with a deflated balloon, coat-ing the balloon with molten plastic material in an injection mold, removing the coated core rod from the injection mold, and inflating the balloon to expand the plastic material into contact with a restraining surface characterized by applying the balloon over a core pin of a blow molding machine, placing the core pin and the covering balloon in an injection mold, injecting the molding material into the cavity of the injection mold to coat the balloon with mold-ing material of a predetermined thickness, moving the pin, balloon and coating plastic to the cavity of the mold in which the balloon is expanded to bring the plastic into con-tact with the surfaces of the blowing mold cavity, cooling the molded article in the blowing mold cavity from both sides by circulating cooling fluid through the walls of the blowing mold and by flowing cooling fluid through the core pin and into the balloon.

15. The method of blow molding described in claim 14 characterized by circulating cooling fluid into the core rod, from localized regions of the core rod, into the balloon, and from the balloon into the core rod at other localized re-gions of the core rod, and then out through an exhaust passage of the core rod.

16. The method of blow molding described in claim 14 characterized by passing the plastic coated balloon on the core rod through a conditioning station between the injec-tion mold and a blowing mold to control the temperature of the plastic coating on the balloon, bringing the core rod, balloon and coating into a blowing mold cavity that is longer than the core rod and balloon in the direction of the axial extent of the core rod and balloon, inflating the balloon with the plastic molding material at a reduced temperature at which the material of the plastic has begun to crystallize, and biaxially orienting the plastic material by stretching it both circumferentially and axially by the inflating of the balloon.

17. The method of blow molding described in claim 14 characterized by applying the molding material to the balloon in a liquid state, delaying the inflating of the balloon until after the molding material has begun to harden, and then stretching the molded material by inflating the balloon.

18. The method of blow molding described in claim 17 characterized by expanding the balloon in both diameter and length to stretch the molding material both circumferentially and axially to obtain biaxial orientation of molecules of the plastic material.