Manufacture of thermoplastic tubular articles

Abstract: A polyethylene terephthalate (PET) body for a food container has open ends and is made by stretch blow moulding a hollow elongate preform of heat-softened PET. The axial elongation and radial expansion of the preform which occurs in the formation of the body are initiated at a sole initiation site which is located at one end of the preform. From there the elongation and expansion are propagated progressively along the preform until substantially the whole preform has been elongated and expanded. The position of the initiation site is determined by profiling the temperature of the preform along its length by use of a specially designed oven, initiation occurring where the preform temperature is highest.

Description: This invention relates to the manufacture of thermoplastic tubular articles, particularly but not exclusively to the manufacture of tubular articles of saturated linear polyester material such as polyethylene terepthalate (PET), intended to form the bodies of processable food and beverage containers. It is known in the manufacture of such articles that a high degree of orientation of the container body is desirable for improving its mechanical properties. This can be induced by mechanical deformation such as drawing, stretching or stretch-blow-moulding of the thermoplastic material. A method and apparatus for forming biaxially oriented tubular articles by stretch-blow-moulding are described in our British Publication No. 2089276A. In that method continuously extruded hollow tube is drawn and blown to orient it, a can-sized length at a time and the oriented tube is chopped off. The present invention is concerned with the manufacture of biaxially oriented articles from discrete lengths of plastic tube. The present inventors have found that in the manufacture of long (approx. 2m) biaxially oriented articles from PET by stretch-blow-moulding several initiation sites may occur along the length of the article, each initiation site being a zone from which radial and axial expansion of the material propagates. This results in the distribution of polymer material in the finished article being uneven. The inventors were particularly concerned with the problem of producing long articles which could then be severed to form the bodies of can-like containers. For this purpose the articles had to be produced repeatably with consistent characteristics along their lengths, particularly as regards their diameter, wall thickness, orientation and crystalline properties. Hereto, there has not been available a process which would enable the necessary quality control conditions to be met consistently and repeatably in mass production. According to one aspect of the present invention there is provided a method of manufacturing a thermoplastic tubular article from a hollow elongate parison of an orientable polymer material in the manufacture of open-ended container bodies, which comprises gripping end portions of the preform to define between them a central portion from which the article is to be formed, and moving the end portions apart whilst admitting gas at a controlled pressure to the interior of the preform, such movement and admission of gas causing the central portion to be biaxially oriented by simultaneous elongation and radial expansion of its polymer material, characterised in that prior to its biaxial orientation the polymer material of the central portion is heated to above its glass transition temperature and so as to produce in the central portion a temperature gradient which extends, without discontinuities, between the ends of the central portion, the elongation and the radial expansion of the central portion to achieve biaxial orientation being thereby each initiated at the higher temperature end of the central portion and proceeding progressively and simultaneously as an expansion front which moves along the central portion from its higher temperature end to its lower temperature end. In the preferred embodiment, the preform is made of PET and the tubular article to be made from it is approximately 2 metres in length. The temperature difference between the ends of the central portion is then at least 3 o C and preferably about 5 o C. The inventors have discovered that by providing a controlled temperature differential between the ends of the preform and by controlling air pressure (in dependence on the polymer), long articles of a very uniform wall thickness and with consistent material qualities can be produced repeatably. It is preferred that both the elongation and the radial expansion of the central portion should exceed the "natural stretch ratio" of the polymer material, that is to say, the stretch ratio at which substantial strain hardening with rapid crystallisation and formation of network chains occurs in the polymer material. By achieving high levels of biaxial orientation such high stretch ratios confer desirable physical properties on the finished article. By thus causing the central portion to be expanded and elongated to beyond its natural stretch ratio in both the radial and longitudinal directions, as it moves progressively along the central portion beginning at the higher temperature end the expansion front of the central portion is accompanied by a moving boundary of work hardening, referred to as a work hardened front, which is formed in the polymer material. The inventors have discovered that for consistent wall thickness of the biaxially orientated material the extent and speed of the separation of the gripped end portions (and therefore the rate of extension of the central portion) should be so matched to the initial length of the central portion and to the speed of propagation of the work hardened front that the work hardened front reaches the lower temperature end of the central portion at substantially the same time as the movement of the gripped ends is completed and the required stretch ratio of the polymer material in its longitudinal direction is achieved. In the described embodiment a biaxially oriented article of approximately 2m long is formed from a preform approximately 1m in length, the speed of propagation of the work hardened front being approximately 4ms⁻¹ and the speed of separation of the gripped ends being approximately 2ms⁻¹. It is preferred that during the biaxial orientation of the central portion its higher temperature end is held stationary, and its lower temperature end is moved away from the higher temperature end longitudinally of the preform. It is furthermore preferred for the pressurised gas to be admitted through only the higher temperature end of the preform. UK patent Publication No. 2099358 describes a method for making thermoplastics articles from a length of elongate tube, in which a central portion of the tube is stretched by gripping devices engaged with the tube ends and simultaneously or subsequently is radially expanded by pressurised gas. Differential heating of the central portion is discussed, but there is no disclosure in 2099358 of the simultaneous stretching and radial expansion of the central portion when at a temperature above its glass transition temperature and with a continuously (i.e. monotonically) varying temperature difference between its ends. At volume 5, No. 172 (M-95) (844) of Patent Abstracts of Japan there is described, in relation to Japanese patent Publication 56-98123, a method of forming articles from vinyl chloride tubes. The tube is heated to softening temperature and with its ends closed is located within a mould. One end of the mould is then cooled, and pressurised air is admitted to the interior of the tube, so causing the tube to expand radially into conformity with the mould, starting at the uncooled end. It will be understood that the articles produced by the process of this Japanese Patent Abstract will be unelongated and so will be orientated only in the circumferential direction. They will therefore not be suited for use as a container bodies. Furthermore, because it relies upon thermal conduction along the tube, the method of generating the temperature gradient described is wholly unsuited to the manufacture of articles in large quantities and at high rates of production. At Volume 4, No. 108 (M-24) (590) of Patent Abstracts of Japan there is desribed, in relation to Japanese patent Publication 55-067408, a method of intitiating expansion and extension of a thermoplastics pipe by thinning it mechanically at a required initiation point by physical contact with a member such as a pin. Such an arrangement, however, would be unsatisfactory in the production of container bodies because it would leave irregularities or surface blemishes at the area of contact of the thermoplastics material with the member. Moreover, there is no disclosure that a single initiation site is provided at one end of the tube, from which elongation and radial expansion proceed simultaneously to the other end. The thermoplastics materials which are preferred for use in the process of the present invention are those having well-defined natural stretch ratios. The preferred material is a polyester homopolymer, which consists of polymer units derived from ethylene terephthalate, produced from terephthalic acid plus ethylene glycol (PET). Other suitable materials include polyester copolymers which consist of polymer units based on ethylene terephthalate, produced from terephthalic acid, isophthalic acid, plus ethylene glycol, diethylene glycol, cyclohexane dimethanol but could also include other polyesters or copolyesters including poly (ethylene isophthalate) PEI, poly (ethylene napthalene-dicarboxylate) PEN., or polyester or copolyester blends. The present invention is suitable for use with monolayer preforms but may also be used for multilayer preforms, for example those having coextruded structures of PET with a gas barrier material such as an ethylene vinyl alcohol copolymer or a polyamide copolymer. The invention further contemplates apparatus for carrying out the method defined above, and from another aspect accordingly provides an apparatus for producing a thermoplastic tubular article from a hollow elongate preform of an orientable polymer material which comprises:- a) first and second spaced gripping means (13, 14) for gripping respective end portions of the preform so as to define a central portion of the preform from which the article is to be formed; b) heating means (4) for heating the preform; c) drive means (15) operable on the gripping means with the heated preform held thereby to move the gripping means apart longitudinally of the preform; and d) gas pressure supply means (19) effective to supply gas at a controlled pressure to the interior of the heated preform during the operation of the drive means whereby the polymer material of the central portion is biaxially orientated by simultaneous elongation and radial expansion; characterised in that the heating means comprises means for differentially heating the central portion along its length whereby the polymer material of the central portion is heated to above its glass transition temperature, and whereby there is produced in the central portion a temperature gradient which extends, without discontinuities, between the ends of the central portion. Advantageously, the heating means is an oven through which the preform is carried along a predetermined path, the oven having a plurality of parallel elongate nozzles arranged adjacent the predetermined path and by which heated gas is directed onto the preform, the nozzles being differentially closed off so as in combination to create the said temperature gradient on the preform. For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings, in which:- Figure 1 is a block diagram of apparatus for manufacturing thermoplastic articles; Figure 2 is a plan view of the oven (with the top removed) used for heating the preform; Figure 3 is diagrammatic section through a preform showing its relationship to heating nozzles; Figure 4 is a diagram showing the layout of the nozzles and the baffle arrangement; Figures 5(a) to 5(f) are diagrams illustrating steps in the manufacturing process; Figures 6(a) and 6(b) are graphs showing an exemplary temperature profile applied to the preforms; Figure 7 is a graph showing the degree of orientation of articles manufactured using the method described herein; Figure 8 is a graph showing the degree of crystallinity of articles manufactured using the method described herein; Figure 9 is a graph of the diameters of articles manufactured using the method described herein; Figure 10 is a graph of the wall thicknesses of articles manufactured using the method described herein. The apparatus comprises a hot air oven 2 mounted above a stretch/blow station 4. The oven 2 has associated therewith a tube supply device 10 which sequentially supplies preforms in the form of elongate tubes to preform trays 6 of a carrier 8. The preforms are hollow extruded tubes made of PET or another suitable polymer and having a length of about 1m. The carrier 8 has a plurality of vertically spaced preform trays. The carrier 8 moves stepwise to carry preforms vertically downwards through the hot air oven 2 wherein they are heated. An air heater 7 heats the air for the oven while thermocouples 3, 5 monitor the temperature. The construction of the oven 2 will be described in more detail hereinafter. A tube transfer device 12 transfers the tube from the oven to the stretch/blow station 4 where it is supported by a "static" gripper 13 at one end of a mould 17 and a "stretch" gripper 14 carried from the axially opposite end of the mould 17. The corresponding ends of the preform are termed the "static" end and the "stretch" end. The static gripper 13 is so-called because apart from moving a short distance back and forth to collect and grip an incoming tube it remains substantially in the same position. The static gripper 13 has a hollow gripper head permitting the supply of air from an air supply hose 19. The heads of the grippers 13,14 are not visible in Figure 1 as they are within the mould. The mould 17 comprises two halves, one of which can be seen in Figure 1, which are locked together once the preform is in place. The stretch gripper 14 can be moved axially to the right in Figure 1 under the control of a drive unit 16 movable on a track 22 driven by a nylon belt 24 which in turn is driven by the servomotor 15. The servomotor 15 is controlled by a dedicated motion controller (not shown) to move a stretch rod 20 attached to the stretch gripper 14 to a preset position at a preset velocity once blowing has commenced. The dedicated motion controller is preprogrammed with operational conditions depending on the requirements of the stretch/blow process and the specification of the product. Finally, a transfer device 18 is operable to remove the blown container from the mould 17. Referring now to Figures 2 to 4, the oven 2 comprises two chambers 26, 28 each having a plurality of nozzles 30 having an elongate opening 31 through which heated air issues and being spaced apart vertically of the chambers 26, 28 (Figure 4). The nozzles 30 are fed from a common plenum chamber 32 via a recirculating air heating system 34. The nozzles 30 are arranged to direct heated air into respective preforms carried by the preform trays 6. In the described embodiment there are eighteen vertically spaced nozzles 30 in each of the chambers 26, 28. The chambers 26, 28 are hinged to the common chamber 32 of the oven to allow access to the nozzles 30 by an operator of the oven, and can be locked by a locking mechanism 29 when in use. It is important for the preform to lie correctly in its tray 6 both in relation to the flow of heated air and its longitudinal position relative to the nozzle opening. Figure 3 shows a suitable tray with a preform 1 thereon. As can be seen from Figure 3, the nozzles 30 are preferably set at an angle α = 15 o to the horizontal, so that the heated air flows on or above the centre line of the preform 1. The preform 1 is then not forced off the preform tray 6. It is important that each preform is positioned in the same axial position on the preform tray as it moves through the oven, and the air flow must be such that it does not disturb the preforms. Figure 4 shows the layout of the heating nozzles 30 (numbered i to xviii) in the chamber 26 of the oven 2. The nozzles heat the preform as it is indexed through the oven by the carrier 8 so that there is a temperature differential between the opposite end regions of the preform. This is accomplished by selectively blocking parts of some of the elongate nozzles to prevent hot air from being emitted from these parts. This can be achieved by a suitable baffle arrangement. In Figure 4, the lengths illustrated with a thick black line are lengths of the nozzles which are blocked: Nozzles numbered i to vi are completely open; nozzles numbered vii to ix have a length l₁ blocked off; nozzles numbered x to xii have a length l₂ l₁, blocked off; nozzles xiii and xiv have a length l₃ l₂ l₁; nozzles xv, xvi have a length of l₄ l₃ blocked off and nozzles xvii, xviii have a length l₅ where l₃ l₅ l₄. For a preform length L of 810 mm, these lengths are as follows: l₁ = 15 mm l₂ = 20 mm l₃ = 30 mm l₄ = 760 mm l₅ = 735 mm The result of this is to heat the static end of the preform to a higher temperature than the stretch end of the preform, and to impart a temperature profile such as that shown in Figure 6a or 6b. The preform remains in the oven for a sufficient length of time to ensure that an approximately uniform temperature exists through the wall thickness of the tube, and it will be appreciated from Figs 6a and 6b that the whole of the tube is at a temperature above the glass transition temperature of the PET material. Reference will now be made to Figures 5a to 5f to describe a method of manufacturing thermoplastics articles which are biaxially oriented. Although the method will be described in relation to a single preform, it will be apparent that the apparatus is capable of operating with a continuous supply of preforms. That is, the tube supply device 10 sequentially supplies preforms to respective preform trays 6 of the carrier 8. These preform trays 6 are indexed through the oven 2, in time to ensure that a preform exiting from the oven has the correct temperature profile. As an example, the dwell time of a preform in the oven 2 might be about four minutes. The dwell time is controlled by the indexing speed of the carrier 8. The preform trays 6 support the preforms over their entire length. Indexing of the carrier is controlled by a machine controller (not shown) in response to the mechanical and pneumatic movement in the stretch/blow station, as discussed hereinbelow. A preform 1 in the form of a discrete length of tube of a suitable plastics material, for example polyethylene terepthalate (PET) and having a length of about a metre is heated in the hot air oven in such a manner as to differentially heat it along its length to create a temperature profile therealong, for example as illustrated in Figure 6a or 6b, as described above. The preform 1 is transferred from the oven 2 to the stretch/blow station 4 where it is retained by being secured by the grippers 13, 14 at respective ends of the preform (Figure 5a). The part of the preform lying between the grippers is denoted 98 throughout Fig 5. The mould halves of the mould 17 close around the preform 1 and are locked in position. The gripper head 14 is moved by a short distance (to the right in Figures 1 and 5) and applies a pretensioning force of 200N, for example, to the preform (Figure 5b). This prevents the preform from sagging and prepares it for the ensuing stretch/blow process. The end of the preform secured by the gripper head 14 is sealed by a closure within the gripper head 14. Pressurised air is introduced into the other end of the preform via the hose 19 (Figure 5c). The mould can be at room temperature or, preferably, cooled to around 15 o C. Introduction of the pressurised air into the preform causes the static, higher temperature end of the part 98 of the preform to begin to expand radially and simultaneously to undergo axial elongation, as manifest by an expansion front 100 (Figs 5c to 5e). A work hardened front of expanded material develops in the polymer on this expansion front and moves with it. The axial elongation characteristics of the preform depend on the temperature profile achieved in the oven 4, and on the air pressure, and will be described in more detail hereinafter. The axial and hoop stretch induced by the pressurised air serves to orient the PET. To aid the axial elongation the gripper head 14 is moved very quickly to the right (Figure 5d) by the stretch rod mechanism 15, i.e. at a velocity at least in excess of 1ms⁻¹, and generally between 2 and 4ms⁻¹. The stretch rod mechanism can be activated with introduction of the preblow air, or a short time thereafter to allow for initiation of the work hardened front. The delay can lie between 0.01 and 0.15 sec. The actual stretch rate depends on the process conditions, the material of the preform and the respective sizes of the preform and finished article. The relationship between the stretch rate, the stretch ratio and the preblow period is given by the following equation: λ1 is the desired axial stretch ratio in the finished article. Δ = dλ / dt is the stretch rate. t = preblow period L o is the initial length of the preform. (1) An example using typical values for these parameters is given below: t = 0.3 λ₁ = 2.2 Δ = 4 ms⁻¹ L o = 1m The expansion front and work hardened front move progressively along the preform until they reach the lower temperature stretch end of the central part 98. The pressurised air is turned off just before stretching is completed and, after a preset delay, mainblow air at a higher pressure is introduced (Figure 5d). The high pressure air used to blow a component is thus achieved in two stages, identified as "pre-blow" which lies in the range from 60 psi to 135 psi (4 to 9 bar) and "main-blow" which lies in the range from 150 psi to 200 psi (10 to 14 bar) for straight walled articles and up to 600 psi (40 bar) for shaped articles. Close control of the preblow pressure, preferably to within 1 psi, is required to achieve a good blown component. The mainblow air achieves accurate conformity of the article with the mould. Finally, the finished article is released from the mould and passed for cutting and heatsetting if required. The dwell time in the oven and the precise temperature of the tube varies depending on the material grade and dimensions of the tube. In one example the dwell time is 4.16 minutes and the average temperature of the tube lies between 95 o C and 105 o C with a difference of 6 o C between the ends of the tube as it emerges from the oven. The thickness and diameter of the blown article can be calculated from the following: In one example: A preform has a mean diameter = 21mm and is blown into a finished article with an outer diameter = 84mm. The transverse stretch ratio is λ₂=4. The transverse stretch ratio λ₂ of the finished article is given by where a is the inner preform radius, b is the outer preform radius, A is the inner radius of the blown article and B in the outer radius of the blown article. In another example: a preform with a thickness 4mm is blown into a finished article with a mean thickness 0.36mm and has a thickness reduction ratio λ t = 11.11. The thickness reduction ratio λ t of the finished article is given by where b-a is the thickness of the preform, B-A is the mean thickness of the finished article. The repeatability of the stretch/blow process is dependent upon close control of the section of the preform at which propagation of the plastics material begins when pressurised air is introduced. Inflation of the preform here is encouraged in the described embodiment by the creation of a hot area. The present inventors have found that by creating this hot area close to the end of the tube into which pressurised air is introduced, advance of the work hardening front within the PET from that end to the opposite end occurs at a controlled and repeatable rate. The present inventors have found that by permitting the material itself to adopt its natural stretch dynamics, a more consistent product of high quality can be produced. The natural stretch rate of the material is far faster than could be applied by mechanical means alone, and can exceed 5 metres per second. Examples of the characteristics of products produced by the described process are shown in Figures 7 to 10. Figure 7 shows the orientation characteristics as the birefringence in the axial and hoop directions. These are calculated from the refractive indices in the respective directions. Figure 8 shows the crystallinity as calculated from density measurements at 20 o C. where P = measured density pa = amorphous density pc = crystalline density

Claims: A method of manufacturing a thermoplastic tubular article from a hollow elongate parison of an orientable polymer material in the manufacture of open-ended container bodies, which comprises gripping end portions of the preform to define between them a central portion from which the article is to be formed, and moving the end portions apart whilst admitting gas at a controlled pressure to the interior of the preform, such movement and admission of gas causing the central portion to be biaxially oriented by simultaneous elongation and radial expansion of its polymer material, characterised in that prior to its biaxial orientation the polymer material of the central portion is heated to above its glass transition temperature and so as to produce in the central portion a temperature gradient which extends, without discontinuities, between the ends of the central portion, the elongation and the radial expansion of the central portion to achieve biaxial orientation being thereby each initiated at the higher temperature end of the central portion and proceeding progressively and simultaneously as an expansion front which moves along the central portion from its higher temperature end to its lower temperature end. A method according to claim 1, in which a work hardened front is formed in the polymer material and accompanies the expansion front as it moves along the central portion. A method according to claim 1 or claim 2, characterised in that the preform is heated prior to the gripping of the end portions and to the elongation and radial expansion of the central portion. A method according to any preceding claim, characterised in that during biaxial orientation of the central portion its higher temperature end is held stationary, and its lower temperature end is moved away from the higher temperature end longitudinally of the preform. A method according to any preceding claim, characterised in that during the biaxial orientation of the central portion the pressurised gas is admitted through its higher temperature end only. A method according to any preceding claim, arranged to produce long biaxially orientated articles using polyethylene terephthalate as the polymer material, characterised in that the temperature difference betweeen the higher and lower temperature ends of the central portion is at least 3 o C. A method according to any preceding claim, characterised in that the central portion is elongated and radially expanded within a chilled mould, the pressurised gas being admitted at a first, pressure for biaxially orienting said polymer, and, subsequent to expansion of most but not all of the central portion into contact with the mould, at a second, higher pressure for conforming said article with the mould. Apparatus for producing a thermoplastic tubular article from a hollow elongate preform of an orientable polymer material which comprises:- a) first and second spaced gripping means (13, 14) for gripping respective end portions of the preform so as to define a central portion of the preform from which the article is to be formed; b) heating means (4) for heating the preform; c) drive means (15) operable on the gripping means with the heated preform held thereby to move the gripping means apart longitudinally of the preform; and d) gas pressure supply means (19) effective to supply gas at a controlled pressure to the interior of the heated preform during the operation of the drive means whereby the polymer material of the central portion is biaxially orientated by simultaneous elongation and radial expansion; characterised in that the heating means comprises means for differentially heating the central portion along its length thereby the polymer material of the central portion is heated to above its glass transition temperature, and whereby there is produced in the central portion a temperature gradient which extends, without discontinuities, between the ends of the central portion. Apparatus according to claim 8, characterised in that the heating means is an oven (4) through which the preform (1) is carried along a predetermined path, the oven having a plurality of parallel elongate nozzles (30) arranged adjacent the predetermined path and by which heated gas is directed onto the preform (1), the nozzles being differentially closed off so as in combination to create the said temperature gradient on the preform.

Inventors: BROOKS DAVID WILLIAMS, CALLAN MAURICE SIMON, CHAPMAN ROGER CHARLES, WATSON MARTIN JOHN, BROOKS, DAVID WILLIAMS, CALLAN, MAURICE SIMON, CHAPMAN, ROGER CHARLES, WATSON, MARTIN JOHN
Applicants: METAL BOX PLC, CARNAUDMETALBOX PLC
Application year: 1990
Main classification: B29C49
IPC codes: B29C49, B29L22, B29K67