CA1204259A - Method and apparatus of producing a biaxially oriented container of thermoplastics material - Google Patents

Abstract

Abstract of the Disclosure A method and apparatus for producing a container made from a tubular blank of polyethylene terephthalate or similar material, according to which the polyethylene terephthalate blank is stretched at least approximately 3-fold to produce a preform. In an axial section through the preform, the contour length substantially corresponds to the contour length in an axial section through the final container. The preform is heated to a temperature above the glass transition temperature (TG) and is converted to the container in a blowmould. In doing so, the contour length of the material is maintained through successive reduction of the axial length of the body which the preform constitutes during the conver-sion to the container. In order to achieve this, the base portion of the blowmould is moved towards the orifice of the final container. The container acquires a neck portion and a container body which are axially oriented, in the case of polyethylene terephthalate to an extent corresponding to an about 3-fold monoaxial stretching, whilst the container body is also trans-versely oriented, in the case of polyethylene tereph-thalate to an extent corresponding to an about 3-fold stretching as a maximum. The wall thickness of the container is inversely proportional to the radius of the container. The container is dimensionally very stable and has excellent strength characteristics.

Description

~ZQ4259 The present invention relates to a container made from a tubular blank of polyethylene terephthalate or similar material, wh~rein, according to the inven-tion, the blank is stretched axially, in the case of polyethylene terephthalate at least 3-fold, in order to produce a preform, in which the material has a profile length which substantially corresponds to or exceeds the profile length of those parts of the container w~lich correspond to the stretched part of the preform, after which the preform is heated to the moulding temperature and converted to the container.
In the production of containers made of a thermoplastic, where the material has a property that its strength and stability increase with orientation of the material, it is desirable that as large a proportion of the container as possible should consist of oriented material. The material is oriented by a stretching pro-cedure, with biaxially stretched material giving the best utilization of the available amount of material. Espec-ially in the case of bottle-type containers, it is, however, difficult to achieve stretching, and thereby create orientation of, for example, the neck and ori-fice parts, and in certain cases even of the centrally located base parts~
Canadian Patent Application 378,503, filed May 28, 1981 in the name of K. M. Jakobson et al, dis-closes how axial orientation of the material in the preform can be achieved by a mechanical procedure ,~

~Z~42S~

wherein a preform passes through one or more draw rings which reduce the wall thickness in the preform and therehy also the outer diameter of the preform. It has been shown that the best effect is achieved when the material in the preform, immediately prior to stretching, has a temperature within or below but near the region of the glass transition temperature (TG) of the material. The axially oriented preform obtained by the process described i-s mechanically stable and the subsequent treatments for converting the stretched pre-form to a container must therefore be appropriate to this fact.
Exceptionally good mechanical properties are achieved with polyethylene terephthalate if the material, during stretching of the preform, is stretched at least about 3-fold. In this connection, an "over-stretching"
of the material is easily achieved, resultin~ in the material in the preform shrinking during the heating of the preform to the moulding temperature, so that the preform reaches a length corresponding to that it would have reached on stretching about 3-fold.
on conversion of a preform, axially stretched as above, to a container, a problem arises in the form of fracture in the material if attempts are made to stretch the preform additionally to the stretching which the material has acquired throu~h the axial orientation of the preform described above. During conversion of the iZ~)42S9 . ~
preform to the container, one therefore seeks to limit the axial stretching of the preform and essentially let the conversion result in a stretching of the mater-ial only in the circumferen-tial direction of the pre-form.
In certain applications, a particular combin-ation of container length and container diameter is sought but it is not possible to produce such a container by known techniques because, during the conversion to a lo container, the material of the preform is stretched far too much in the axial direction of the preform. This problem arises with a ~ontainer having a relatively large diameter in relation to the container length.
Containerswith such dimensional characteristics are normally involved with bottles having a volume of less than 0.5 litres.
The present invention concerns a container~and a method and a device for moulding a container, where the problems mentioned have been eliminated. The invention will be described especially in relation to polyethylene terephthalate, hereafter referred to as PET, but is in principle applicable to many other thermo-plastics of the polyester or polyamide type.
Examples of such other materials are polyhexa-methylene adipamide, polycaprolactam, polyhexamethylene sebacamide, polyethylene Z,6- and 1,5-naphthalate, polytetramethy~ene l,2-dioxybenzoate and copolymers o~ ethylene terephthalate, ethylene iso-phthalate and other similar plastics p~lymers.

:~2;~4ZS9 According -to the invention, the material, in the form of a tub~lar blank of PET, is stretched at about least 3-fold to produce a preform having a length appropriate to the dimensions of the final container.
The length, in the axial direction, of the stretched material of the preform is equal to or greater than the profile length of the parts which the stretched material c~nstitutes in the container after conversion of the preform to a container. The preform is then heated to the moulding temperature and is converted to a container, for example in a bl~mould. As the pre-form has a length greater than that of the final con-tainer~hence greater than that of the internal height of the blowmould, it is not possible, without special measures, to introduce the heated preform into the blowmould. According to the invention, the conver-sion of the preform to a container takes place so as to maintain the contour length of the material in an axial section through the body which is formed during the progressive conversion of the preform to a container, this being achieved through a successive reduction in the axial length of the body in question.
In a preferred embodiment of the invention, the material, immediately prior to the stretching in the axial direction of the blank, is at a temperature in the range of or preferably below the glass transition temperature (TG1 of the material.

The tubular blank is preferably stretched by passing it through one or more draw rings so as to ~LZ04Z,S9 reduce the wall thickness of the material and at the same time diminish the outer circumference of the blank.
In an embodiment where the container has exceptionally hiah dimensional stability at elevated temperature, the blank is so chosen that the preform, on heating to the conversion temperature, suffers a reduction in contour length, in a section in the axial direction of the preform, to a value which is not below that of the contour length in an axial section of the final container.
In one embodiment of the invention, the base of the blowmould is moved, during the conversion of the preform to a container, in the axial direction of the preform and towards the orifice of the preform, in order to assume a position in which the final inner shape of the blowmould is fixed.
In another embodiment of the invention, a central portion of material at the base of the preform is reshaped and/or reduced in thickness through the portion of material being pressed between the base of the blowmould and a mandrel located inside the preform.
In a preferred device according to the inven-tion, at least two and preferably three parts of the mould can be moved to and fro from a position where they cooperatively provide an inner shaping surface in the blowmould. Two of the mouid parts are mould halves, which can be opened and shut in the normal manner, to form the blowmould. The third mould part is the base part of the blowmould and can be moved in the axial direction of the blowmould, so that the inner height of the blowmould can be varied.
Furthermore, a mandrel is used for fixing the preform, at its orifice, against two gripping halves, and for introducing excess pressure into the interior of the preform.
In a particular embodiment of the invention, the inner mandrel ends in a shaping surface which fits the shape of the base portion of the blowmould. When the base portion is in its upper position, the mandrel accordingly cooperates with the base portion so as to reshape a central portion of the base of the preform and/or reduce the thickness of the central portion of the base.
In yet another embodiment of the invention, the base portion of the blowmould is provided with a central mould portion which is thermally insulated from the rest of the base portion. The central mould por-tion possesses channels for the transportation ofliquid in order to control the temperature of the shapin~ surfaces of the central mould portion. The rest of the base portion is also provided with channels for the transportation of liquid and control of the temperature of the other shaping surfaces of the base portion. Furthermore, all'the other mould portions, as well as the mandrel and the gripping halves, have channels which serve a corresponding purpose.

lZ~4ZS9 According to the invention, a container is formed which is of thermoplastic material having a neck portion, a body portion and a bottom portion with a central region. The material of the neck portion has a monoaxial orientation equivalent to a monoaxial stretching of the thermoplastic material by a mul~iple of its original length. The material of the body por-tion has an axial orientation substantially consisting solely of the axial orientation obtained by the multi-fold monoaxial stretching of the thermoplastic material and a transverse orientation independent of and super-imposed on the axial orientation by transverse stretch-ing of the axially oriented thermoplastic material by a multiple of its original length.
In a preferred embodiment of the invention, the central portion of material at the base of the con-tainer and/or the orifice edge of the container consist of thermo-crystallized, opaque and dimensionally stable material.
The-invention is described in greater detail in relation to the four Figures of the drawings, in which:
Figure 1 shows a longitudinal section through a blowmould, with an axially stretched preform placed in the blowmould, and with thé base portion of the blow-mould in the lower position, ~Z~)4;?,S9 Figure 2 shows a corresponding longitudin~
section during conversion of the pre-form, and with the base portion moving towards the orifice portion of the pre~orm;
Figure 3 shows a corresponding longitudinal section, with the base portion in its upper position and with the pre-form converted to the container, and Figure 4 shows a container produced in accor-dance with the invention.
Figures 1 - 3 show two mould halves lOa, b, which are movable, in the direction of the arrows A, B, to and fro from the position as shown in the Figures.
The mould halves cooperate with a third mould portion in order conjointly to form a blowmould 12, in which the third mould portion 11 is the base portion of the blow-mould 11. The base portion can be moved between a lower position (Figure 1) and an upper position (Figure 3) by means of drives (which are not shown in the Figures). In the upper position, the base portion, together with the mould halves, constitutes the assembled blowmould.
The base portion is provided with a central mould portion 17 which is thermally insulated from the rest of the base portion. A number of channels 18 for the transportation of liquid are present in the central shaping portion. Channels 19, serving a corresponding ~2~ZS9 g function, are located in the outer parts of the base portion, similarly to channels 30 in the mould halves.
FurthermoI~e, grippin~ devices 13a, b are connected to the upper parts of the mould halves and cooperate with a mandrel 15 so as to hold a preform 20 at its mouth or orifice 22. The grippin~ devices also possess channels 31 for the transportation of li~uid for temperature control.
The mandrel 1-5 has a length matching the upper position of the base portion 11, so that with this base portion in its upper position a space is created, between a lower shaping surface 16 of the mandrel and the central mould portion 17 of the base portion 11, which space has a shape and thickness deter-mined by the intended shape and thickness of the moulded container.
Furthermore, the mandrel is provided with a main channel 14 for the pressure medium, which via the side channels 33 passes to the surface of the mandrel and hence to the interior of the preform 20.
In Figure 1, the preform is placed in the blowmould and the central portion 24 of material of the base of the preform is in contact with the base portion 11 of the blowmould 12.
Figure 3 shows a moulded container 21, and Figure 2 shows a body 23 which illustrates the preform being converted to the container 21.

, lZ~ 2S9 Figure 4 shows, in detail, a container according to the invention. The container has a neck portion 25 with an upper part which forms the orifice edge 22. The actual container body, 26, has in its base portion a central material portion 24. In the Figure, the radius and material thickness of the neck portion are designated rl and dl respectively, while r and d are the radius and material thickness of the container body in an arbitrary plane at right angles to the axis of the container.
The material thickness d varies in the con-tainer body with the radius r of the container body, so that the ratio of the material thickness d of the container body to the material thickness dl of the neck portion is approximately equal to the ratio of the radius rl of the neck to the radius r of the container body. The neck consists of axially oriented materi~
and the container body of biaxially oriented material.
In a preferred embodiment, the central material portion 24 and/or the orifice edge 22 consist of thermo-crystallized, opaque, dimensionally stable material.
In employing the invention, a tubular blank is stretched in its axial direction by passing it through one or more draw rings, which reduce the wall thickness of the blank and at the same time diminish the outer circumference of the blank. Immediately prior to stretching, the material of the blank is at a 12~4~S~

temperature within or preferably below the range of the glass transition temperature (TG) of the material.
The preform 20, produced by stretching the blank, is then, after having been heated to the mould-ing temperature, introduced into the ~lowmould 12. The mandrel 15 is introduced into its position inside the preform, thereby holding .he orifice of the preform ~2 against the gripping devices 13a and 13b. The base portion 11 of the blowmould 12 is in its lower posi-tion. The heating medium passes through the channels18, 19, 30, 31 so as to bring to temperature the adja-cent shaping surfaces of the blowmould and above all to heat the surfaces adjoining the gripping devices 13a and 13b around the orifice 22 of the preform.
Thereafter, the interior of the preform is placed under pressure by means of a pressure medium at the same time as the base portion 11 of the blowmould 11 is moved towards the orifice of the preform, that is to say, upwards in the Figures. As a result, the preform expands at the same time that its axial length is reduced (see Figure 2), so that the material of the preform is stretched essentially only in the circum-ferential direction of the preform. When the base portion of the blowmould has reached its upper posi-tion (Figure 3), all portions of the material of the prefor~ have also been brought into contact with the inner shaping surfaces of the blowmould by means of the pressure medium, and the preform has been converted lZ~42S9 - lla -to the container 21.
In certain embodiments, the inner pressure in the container is maintained for some time so that reli-able contact with the blowmould is achieved. The shaping surfaces of the blowmould are then at a temperature in the range of between 110 and 180~C, preferably 130 and 150C, as a result of which possible stresses in the material are released through contact and at the same time a certain amount of thermal crystallization takes place in the material. This mechanically stabilizes the shape of the container, and the latter can, without major change of shape, be reheated to the temperature at which the heat-stabilization took place.
When the heat-stabilization of the container is finished, the mould halves are opened, the base por-tion of the blowmould is moved to its lower position and the produced container is taken out of the blo~1-mould.
In certain applications, there occurs cooling of the material of the preform and hence of the centr~
base portion of the container, i.e. of the material which in this case is reshaped, and/or reduced in thickness, between the shaping surface 16 of the mandrel 15 and lZ~)4;~59 the central mould portion 17 of the base portion of the blowmould 11. This gives a container whose central base portion consists of amorphous material.
In other applications, the material is heated as just described in connection with the heat-stabilisation of the container, with the shaping surface 16 of the mandrel 15 having a temperature within the range at which the amorphous material will crystallise, prefer-ably a temperature of 130-160C~ In this way, the lo centra~ base portion of the container is converted to a thermo-crystallised, opaque, dimensionally stable area of material.
In certain applications, where the requirement for dimensional stability of the moulded container at elevated temperatures is not as great, the material in the preform is stretched9 during mouldlng, also in the axial direction of the preform. The stretching is howe~er relatively slight and in the case of PET must not be equivalent to more than about 30% lengthening of the material. The axial stretching ratiois controlled, according to the invention, by the amount of the verti-cal movement of the base portion 11 of the blowmould.
The in~ention has been described above in connection with a blowmould whose base portion travels in *he axial direction of the blowmould during the con-version of the preform to a container. It is obvious that the invention is not restricted to the design -described, but that the concept of the in~ention as such can also be realised, for example, by means of a blow-12~42S9 mould which has a fixed base portion, and in which the gripping halves, together with the mandrel, are moved in the axial direction of the blo~nould during the conversion of the preform.
A container according to the invention has a crystallinity, in the neck portion 25, of the order of 10-30% and, in the container body 26, of the order of ahout 10-40%~ In the embodiment where the central shaping portion 17 keeps the central base portion 2~ at a temperature which does not cause thermally conditione~
crystallization, or causes this crystallization only to a slight extent (such a temperature being, for PET, below about 100C), an amorphous central base portion 24, with a crystallinity of less than 10%, preferably less than 5%, is obtained. In the case where the central shaping portion 17 holds the mat~rial at the crystal-lization temperature ~about 140C for PET), a crystallized, opaque, dimensionally extremely stable central area of material, with a crystallinity in excess of about 10%, is obtained. Similar conditions regarding crystallinity apply to the edge of the orifice 22, depending on whether it has been cooled or heated to the temperature regions just mentioned, by the mandrel 15 and/or gripper devices 13a, b.
The crysta~linity values stated in the present application relate to the theories disclosed in the publication "Die Makromolekulare Chemie" 176, 2459-2465 (1975)-~Z~)4~'5~

The invention will be clear not only from the above description but its scope and extent will become evident from the claims which follow.
This application is a division of application Serial No. 378,493, filed May 28, 1981.

Claims (51)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for converting an axially stretched preform into a container comprising two side mould parts movable between an open, separated position and a closed position in which the mould parts form an inner shaping surface of a blow mould and a bottom mould part forming a bottom for the blow mould and thereby a bottom of said shaping surface, said bottom mould part being movable relative to said side parts from a lower position to an upper position, said blow mould having a central axis and in said lower position of said bottom mould part, the inner height along the central axis of the blow mould corresponds to the axial length of said preform to allow the preform to be inserted into the blow mould, whereas in said upper position the inner height of the blow mould is less than the axial length of the preform, and also in said upper position, said bottom mould part together with the side mould portions form said shaping surface against which the preform is converted to a container.

2. Apparatus for converting an axially stretched tubular preform into a container, comprising at least two mould portions movable to and fro from a position where the mould portions together form the inner shaping surface of a blowmould, one of the mould portions forming the bottom portion of the blowmould and hence the bottom of the shaping surface and being movable between a lower position and an upper posi-tion, wherein, in the lower position, the inner height along the central axis of the blowmould corresponds to the axial length of the preform to allow the pre-form to be inserted into the blowmould, and wherein, in the upper position, the inner height of the blow-mould is less than the axial length of the preform and the profile length of an axial cross-section of the mould surfaces of the blowmould is substantially the same as the profile length of an axial cross-section of the stretched preform, and wherein said bottom portion together with the other mould portions form the shaping surface against which the preform is converted to a container.

3. Apparatus as claimed in claim 1, wherein said blowmould includes an inner mandrel having a lower shaping surface which faces said bottom mould part so that, with said bottom part in its upper position, said shaping surface and said bottom part shape a central bottom portion of the preform.

4. Apparatus as claimed in claim 3, wherein said bottom mould part and said shaping surface of said mandrel cooperate to reduce the thickness of the central bottom portion of the preform.

5. Apparatus as claimed in claim 3, wherein said bottom part of the blowmould includes a central shaping portion which is thermally insulated from the remainder of said bottom part, said central shaping portion being provided with channels for transport of liquid to control the temperature of shaping surfaces of said central shaping portion, said remainder of said bottom part being provided with channels for transport of liquid to control the temperature of remaining shaping surfaces of said bottom part.

6. Apparatus as claimed in claim 1, wherein said mould parts are provided with channels for trans-port of liquid to control the temperature of shaping surfaces of said side mould parts.

7. Apparatus for producing a container of orientable, thermoplastic material comprising a blow-mould for receiving a preform to be formed into a container, said blowmould including means for applying internal pressure into the preform to expand the pre-form, wall means for forming the preform into a container upon expansion of the preform and means for providing relative axial movement of opposite ends of the preform towards one another during the expansion of the preform to maintain constant the contour length of the preform in axial sections therethrough during expansion into the container and thereby substantially prevent axial strain of the preform during expansion.

8. Apparatus as claimed in claim 7, wherein said means for applying internal pressure comprises a blow mandrel.

9. Apparatus as claimed in claim 8, wherein said blow mandrel has a lower surface facing said wall means to shape a bottom portion of the container in said blowmould.

10. Apparatus as claimed in claim 7, comprising means for heating said preform via said blow mandrel.

11. Apparatus as claimed in claim 7, comprising means for heating said mould to heat the expanded preform.

12. Apparatus as claimed in claim 7, wherein said wall means includes a bottom part which supports the preform and is displaced by the means which pro-vides relative axial movement.

13. Apparatus as claimed in claim 7, wherein said orientable, thermoplastic material is selected from the group consisting of polyethylene terephthalate, polyhexamethylene adipamide, polycaprolactam, poly-hexamethylene sebacamide, polyethylene 2,6 and 1,5-naphthalate, polytetramethylene 1,2-dioxybenzoate and copolymers of ethylene terephthalate and ethylene isophthalate.

14. Apparatus for converting a preform into a container comprising two side mould parts movable between an open, separated position and a closed position in which the mould parts form an inner shaping surface of a blowmould and a bottom mould part forming a bottom for the blowmould and having an inner shaping surface forming a bottom for said inner shaping sur-faces of said mould parts, said bottom mould part being movable relative to said side parts from a lower position to an upper position, said blowmould having a central axis and in said lower position of said bottom mould part, the inner height along the central axis of the blowmould is not less than the axial length of said preform to allow the preform to be inserted into the blowmould, whereas in said upper position the inner height of the blowmould is less than the axial length of the preform, and also in said upper position, said bottom mould part together with the side mould portions form said shaping surface against which the preform is converted to a container, said bottom mould part having a position in which the inner height of the mould is equal to the axial length of the preform when the bottom mould part comes into contact with the bottom of the preform, the mould having a profile length measured in a plane passing through the central axis of the mould along the inner shaping surfaces of the mould parts and bottom part when the bottom mould part is in said upper position which is not greater than said inner height of the blowmould when the bottom mould part comes into contact with the bottom of the preform, whereby the stretched portion of the preform undergoes substantially no axial stretching when moulded to the container.

15. Apparatus for producing a container of thermoplastic material comprising a blowmould for receiving a preform to be formed into a container, said blowmould having a central axis and including means for applying internal pressure into the preform to expand the preform, wall means for forming the preform into a container upon expansion of the pre-form and means for providing relative axial movement of opposite ends of the preform towards one another during the expansion of the preform, said blowmould having an inner surface with a profile length measured in a plane passing through said axis which is no greater than the axial height of the blowmould when the blowmould comes into contact with the bottom of the preform before expansion of the preform to prevent increase of the contour length of the preform in a plane through said axis during expansion into the container and thereby substantially prevent axial strain of the preform during expansion.

16. Apparatus as claimed in claim 15, comprising means controlling the relative axial movement of the opposite ends of the preform in relation with the expansion of the preform to prevent the axial strain of the side wall of the preform during expansion.

17. Apparatus for forming a container from orientable thermoplastic material comprising:
a mould, means for holding a hollow prefoxm of thermo-plastic material in said mould, said preform having one end held by the holding means and including a side wall and a closed end opposite said one end, the material of said side wall having axial orientation and being capable of circumferential expansion and circumferen-tial orientation, said mould including a movable mould part facing said closed end of the preform, means for expanding the preform in the mould to form the container such that said preform, prior to its formation to the finished shape of said container, comes into contact with said movable mould part without any axial stretching of said side wall, means for displacing the closed end portion of the preform by said movable mould part, and means for controlling the displacing of said movable mould part in correspondence with the expanding of the preform such that the material of the side wall is not stretched in a plane perpendicular to the direc-tion of expanding and thereby the material does not undergo any axial strain due to elongation and said expanding produces circumferential stretching and circumferential orientation of the material in said side wall independent of and superimposed on said axial orientation.

18. Apparatus as claimed in claim 17, wherein said means for controlling the displacing of said movable mould part is operative to produce displace-ment of said movable mould part during the expanding of the preform.

19. Apparatus as claimed in claim 17, wherein said means which controls the displacement of the movable mould part is operatively associated with the means which expands the preform to cause the closed end portion of the preform to be displaced in relation to said expanding of the preform such that the side wall of the preform is reduced in thickness in relation to its expansion and the resultant thickness of the side wall of the formed container is inversely related to its diameter.

20. A method of forming a container of thermoplastics material comprising providing a blank of material which has been stretched in the axial direction of the blank to produce a pre-form whose length in relation to the dimensions of the final container is such that the contour length of a section, in the axial direction, through said preform is at least equal to the contour length in an axial section of the container, heating said preform to a moulding temperature, and expanding said preform to form the container while supporting the container in at least the final phase of the moulding and axially dis-placing the bottom of the preform to maintain the con-tour length of said preform, in axial sections through the container while the container is being formed.

21. A method as claimed in claim 20, wherein said preform is progressively expanded in corres-pondence with progressive axial displacement of the bottom of the preform.

22. A method as claimed in claim 21, wherein the moulding temperature is above the glass transition temperature (TG) of the material.

23. A method as claimed in claim 22, wherein the material of the blank, immediately prior to stretching the blank in the axial direction, is at a temperature within or below the region of the glass transition temperature (TG) of the material.

24. A method as claimed in claim 23, wherein said blank is tubular and is stretched in its axial direction to reduce its wall thickness while concur-rently diminishing the outer circumference of the blank.

25. A method as claimed in claim 24, wherein the inner diameter of the blank is substantially main-tained during the stretching of the blank.

26. A method as claimed in claim 23, wherein said blank is tubular and has inner and outer faces, said blank being stretched in its axial direction to reduce the wall thickness while substantially main-taining the diameter of one of said faces and changing the diameter of the other of said faces.

27. A method as claimed in claim 26, wherein said preform is formed into said container in a blow-mould, the preform, prior to being placed in the blow-mould being heated to a temperature above the glass transition temperature (TG), as a result of which the contour length of the preform in axial sections of the preform is reduced to a value not less than the con-tour length in axial sections of the final container.

28. A method as claimed in claim 27, wherein during the converting of the preform to said container, a bottom of the blowmould is moved in the axial direc-tion of the preform and towards the mouth of the pre-form, to assume an upper position corresponding to the final shape of the blowmould and thereby also to the final shape of the container produced in the blowmould.

29. A method as claimed in claim 28, wherein a central portion of the base of the preform is reduced in thickness by compressing the material in said central portion between the bottom of the blowmould and a mandrel located inside the preform.

30. A method as claimed in claim 29, wherein said preform is converted to said container by applying pressure inside the preform to expand the same against the walls of the blowmould, said method further compris-ing supplying heat to the expanded preform from said walls so that the material of the container, through heat transmission from the blowmould, acquires an addi-tional crystallinity over and above that which results from the orientation of the material while additionally internal stresses of the material originating from the orientation are released.

31. A method of moulding a container having a neck portion, a body portion and a bottom portion from a parison of polyethylene terephthalate having a body which is monoaxially oriented, said method comprising introducing said parison into a mould for the container, heating said parison to a moulding temperature above the glass transition temperature (TG) and applying internal pressure into the parison while neck and bottom portions of the parison move relatively towards one another to keep constant the contour length of the material, in an axial section through the parison as it is being formed into the container without axial strain of the material of the parison.

32. A method as claimed in claim 27, wherein said parison is converted to said container in a blowmould, the parison, prior to being placed in the blowmould being heated to said temperature above the glass trans-ition temperature (TG), as a result of which the con-tour length of the parison in a section in the axial direction of the parison is reduced to a value not less than the contour length in an axial section of the final container.

33. A method for forming a container from a blank of polyethylene terephthalate or similar material which is monoaxially oriented comprising introducing said blank into a mould, and expanding said preform in the mould to form the container while maintaining the contour length of the material in axial sections through the preform as it is being expanded into the container such that the preform during expansion is substantially without axial strain.

34. A method as claimed in claim 33, wherein the expansion of the preform is such to produce circum-ferential orientation in the formed container super-imposed on and independent of the monoaxial orientation.

35. A method of forming a container from orien-table thermoplastic material comprising:
providing a hollow preform of thermoplastic material including a side wall and a closed end, the material of said side wall having axial orientation and being capable of circumferential expansion and circumferential orientation, positioning the preform in a mould having a movable mould part, expanding the preform in the mould to form the container, said preform, prior to its formation to the finished shape of said container, coming into contact with said movable mould part without any axial stretching of said side wall, and displacing the closed end portion of the preform by said movable mould part in correspondence with the expanding of the preform such that the material of the side wall is not stretched in a plane perpendicular to the direction of expanding and thereby the material does not undergo any axial strain due to elongation and said expanding produces circum-ferential stretching and circumferential orientation of the material in said side wall independent of and superimposed on said axial orientation.

36. A method as claimed in claim 35, wherein said preform is progressively expanded in corres-pondence with progressive displacement of the closed end of the preform.

37. A method as claimed in claim 35, comprising effecting said expanding with the temperature of the material in the vicinity of the glass transition temperature (TG).

38. A method as claimed in claim 37, wherein the axial orientation of the material of the preform is effected by stretching the material in the axial direction at a temperature within or below the region of the glass transition temperature (TG) of the material.

39. A method as claimed in claim 38, wherein said preform is tubular and is stretched in its axial direction to reduce its wall thickness while concur-rently diminishing the outer circumference of the preform.

40. A method as claimed in claim 39, wherein the inner diameter of the preform is substantially main-tained during the stretching of the blank.

41. A method as claimed in claim 38, wherein said preform is tubular and has inner and outer sur-faces, said preform being stretched in its axial direction to reduce the wall thickness while substan-tially maintaining the diameter of one of said sur-faces and changing the diameter of the other of said surfaces.

42. A method as claimed in claim 35, comprising heating said preform, prior to placement in the mould, to a temperature above the glass transition temperature (TG).

43. A method as claimed in claim 35, wherein the preform is at a moulding temperature prior to expanding.

44. A method as claimed in claim 35, wherein the side wall of the preform is out of contact with the mould when the closed end of the preform contacts said movable mould part.

45. A method as claimed in claim 35, wherein the preform comes into contact with said movable mould part before the preform is expanded.

46. A method as claimed in claim 45, wherein said expanding is effected by blowing under pressure.

47. A method as claimed in claim 35, wherein the circumferential stretching of the side wall causes reduction in the thickness thereof such that the resultant thickness of the side wall is inversely related to its diameter.

48. A method as claimed in claim 35, wherein said closed end of said preform is compressed against said movable mould part to undergo shaping and thickness change.

49. A method as claimed in claim 35, wherein said mould is heated and said container undergoes heat transfer with the mould and consequent heat setting.

50. A method as defined in claim 20, wherein the thermoplastics material is polyethylene terephthalate and has been stretched at least about three fold.

51. A method as defined in claim 20, wherein the bottom is heated to be converted to a thermo-crystallized, opaque, dimensionally stable area of material.