TECHNICAL FIELD The present disclosure relates to the two-stage injection blow moulding of containers using polyolefins such as high density polyethylene (HDPE) and polypropylene (PP). Aspects of the invention relate to a method of forming a preform, to a preform, to a method of forming a container, and to a container. BACKGROUND

Polyolefins such as high density polyethylene (HDPE) and polypropylene (PP) are frequently used in the manufacture of moulded plastics containers. Containers formed of HDPE or PP are generally manufactured by extrusion blow moulding. In extrusion blow moulding the HDPE or PP material is melted down and extruded into a hollow tube or parison, before being clamped into a mould and subjected to internal pressure to form the shape of the finished container. Extrusion blow moulding provides a cheap, simple and rapid process for forming plastics containers, including those with complex shapes. HDPE and PP are both particularly well suited to extrusion blow moulding.

An alternative method of manufacturing plastics containers such as bottles is two- stage injection stretch blow moulding. In this process the plastic material is melted down and then injection moulded to form a preform including a finished neck or mouth and a body portion arranged to be subsequently expanded and moulded into a desired shape. The preform is then cooled (or allowed to cool) to ambient temperature, and may be transported to a different location, before being reheated, stretched by a stretch rod, and subjected to internal pressure while clamped in a container mould to form the shape of the finished container.

Two-stage injection stretch blow moulding provides the advantage of allowing finished containers to be blown at or close to a site at which they are to be filled or otherwise used. It is therefore possible for a user of the finished containers, such as a beverage manufacturer, to be supplied with the comparatively small preforms, and then blow the full size finished containers on-site. It is therefore not necessary to transport the full size finished containers all the way from a first facility at which the containers are manufactured to the location at which they are required, or to store the containers in their full size finished form. Some plastic materials are particularly well suited to two-stage injection stretch blow moulding, especially those that are strain hardening and that tend to even out in thickness during stretching and blow moulding. However, polyolefins such as HDPE and PP are not strain hardening materials, and tend to become thin at certain points and then break at the thinnest point rather than evening out if reheated and stretched, and can suffer from visual defects including poor clarity and inconsistent clarity.

It is an aim of the present invention to address disadvantages associated with the prior art. SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method of forming a preform for use in the production of a container, the method comprising: providing stock material formed predominantly of polyolefin material;

melting the stock material;

introducing the melted material into a mould cavity of a preform mould;

moulding the melted material into the shape of a preform within the mould cavity; and

setting the preform.

It will be appreciated that the method may be used to form a plurality of preforms simultaneously where the preform mould comprises a plurality of mould cavities.

The stock material may be provided in pellet form, although other forms are also possible. The stock material may be formed of virgin material or alternatively recycled material.

The stock material may comprise polyethylene (PE), optionally high density polyethylene (HDPE). The HDPE may have a density in the range 0.951 to 0.966g/cm ³ , or 0.955 to 0.963g/cm ³ , or 0.957 to 0.961g/cm ³ . The HDPE may have a melt flow index in the range 0.3 to 5.8g/10min, or 0.65 to 4g/10min, or 0.7 to 3.5g/10min, or 0.8 to 2g/10min. Alternatively the stock material may comprise low density polyethylene (LDPE) or medium density polyethylene (MDPE). The stock material may comprise a blend of two or more of HDPE, LDPE and MDPE.

Alternatively, the stock material may comprise polypropylene (PP), for example clarified polypropylene (that is polypropylene having a relatively high clarity or translucency compared to other materials). The PP may have a density in the range 0.895 to 0.912 g/cm ³ , or 0.900 to 0.905g/cm ³ . The PP may have a melt flow index in the range 6 to 12 g/10min, or 8 to 10g/10min.

Using PE or PP stock material with the density and melt flow index values indicated above has been found to provide maximised stretch possibilities and optimised material distribution in subsequent blow moulding of the preforms.

The stock material may comprise at least 85% or at least 90% or at least 95% polyolefin material. The stock material may further comprise one or more additives. The additives may include one or more of: a colourant; a light barrier; a gas barrier (for example against oxygen and/or carbon dioxide); and a filler (for example chalk).

The method may further comprise treating the stock material prior to melting to remove solid contaminants from the stock material. It is noted that polyolefin stock material (for example comprising HDPE or PP) does not typically require drying prior to melting because polyolefins such as HDPE and PP are far less hygroscopic and pick up significantly less moisture that other materials such as PET that are conventionally used in two-stage injection stretch blow moulding processes. Therefore the use of a drying system may be considered to be redundant when dealing with polyolefins such as HDPE and PP. However, it has been found, counterintuitively, that running polyolefin stock material through a drying system (with or without operating its heating system) provides an unexpectedly large improvement in the clarity of the preforms and subsequent containers produced from the stock material due to the removal of solid contaminants such as dust. The removal of solid contaminants may therefore be performed by running the stock material through a drying system, which may form part of the injection moulding apparatus. In this case the drying system may be operated without its heating system being operated in order to reduce energy requirements for the preform moulding process.

Alternatively the drying system may be operated with its heating system switched on, which may further improve the clarity of the preforms and the subsequent containers. The heating system of the drying system may also act to pre-heat the stock material, which may reduce the subsequent heating requirements before the stock material is introduced into a preform mould, in which case it may be possible to use a smaller and/or lower power extruder than would otherwise be required for melting the stock material.

The step of melting the stock material may comprise heating the stock material to a temperature in the range 230 to 250°C, or 235 to 245°C and/or the melted material may be injected into the preform mould at a temperature in the range 230 to 250°C, or 235 to 245°C. The stock material may be melted using a melting apparatus at a temperature in the range 230 to 250°C, or 235 to 245°C and/or injected into the preform mould or mould cavity via a nozzle at a temperature in the range 230 to 250°C, or 235 to 245°C. The stock material may be melted using, for example, a heated screw-type extruder. It is noted that there is a general prejudice in the field of injection stretch blow moulding towards the use of higher melting and injection temperatures in order to improve visual clarity. However, the applicant has determined that polyolefin preforms and subsequent containers (for example formed of HDPE or PP) having good visual properties may be produced using a surprisingly low melting and injection temperature in the range 230 to 250°C at a reduced energy cost.

The stock material may be melted and/or passed through at least a portion of a melting apparatus at a pressure in the range 130 to 150bar, or 137 to 144bar, or at a pressure of approximately 140bar. This pressure has been determined to provide a more homogenous melt with minimised air and gas content. The pressure may be applied, for example, by a heated screw-type extruder. (It will be appreciated that all pressures quoted in this specification are gauge pressures experienced by the material and not simply machine pressure settings (which may vary for different machines).)

The melted material may be introduced into the preform mould using a shooting pot delivery system. The use of a shooting pot delivery system allows simultaneous injection of the melted material and preparation of the material to be used in the next cycle, thereby allowing a gentle melting process and minimised molecule chain degradation, to which preforms formed of polyolefins such as HDPE and PP have been found to be particularly susceptible. The melted material may be transferred to the shooting pot (or other delivery system) at a rate in the range 60 to 80cm ³ /sec, or 65 to 75 cm ³ /sec.

The melted material may be introduced into the mould cavity via a gate having a diameter in the range 3.1 to 4.3mm, or 3.7 to 4.1 mm.

The melted material may be introduced into the preform mould at a rate in the range 200 to 400g/sec, or 220 to 350g/sec. The melted material may fill the mould cavity at a rate in the range 6 to 12g/sec, or 7 to 11 g/sec. Injection rates in this range have been determined to minimise visual defects in the preforms and subsequent containers.

The melted material may be introduced into the preform mould at a pressure in the range 490 to 540bar, or 500 to 525bar, or 510 to 515bar. The injection pressure may be increased, for example from Obar up to the maximum injection pressure, during injection.

The method may further comprise applying a hold pressure to the material in the mould cavity after the mould cavity has been filled. Applying a hold pressure may help to ensure stability of the preform, particularly in the gate area, and counteract the effects of shrinkage during cooling. Hold pressure may be applied for a total period in the range 2 to 6 seconds.

The step of applying hold pressure may comprise successively applying a plurality of different (and preferably progressively lower) hold pressures, for example two or three different hold pressures. Each hold pressure may be applied for a time period in the range 1 to 3 seconds, or 1.5 to 2.5 seconds, or 1.7 to 2.3 seconds.

The step of applying a hold pressure may comprise applying a hold pressure in the range 150 to 200bar, or 165 to 185bar, or 170 to 180bar, and/or applying a hold pressure in the range 1 10 to 140bar, or 120 to 130bar. Hold pressures in these ranges may be applied successively.

The preform may be retained within the mould cavity for a cooling period in the range 5 to 9 seconds, or 6 to 8 seconds, or for a cooling period of approximately 7 seconds before being released from the preform mould. The cooling period may be timed from the end of the injection process or from the end of the period during which holding pressure is applied. The preform may be cooled to a temperature in the range 50 to 70°C or 50 to 60°C before being released from the preform mould.

The total moulding cycle time may be in the range 18 to 25 seconds. The overall cycle time for material between entering the heated portion of the apparatus and being ejected from the preform mould may be approximately 120 seconds. The preform mould may be maintained at a temperature in the range 35 to 60°C, or 45 to 57°C, or 50 to 55°C before and/or during injection of the melted material into the preform mould. The preform mould may be cooled with coolant at a temperature in the range 35 to 60°C, or 45 to 57°C, or 50 to 55°C. These temperature ranges are significantly higher than the temperatures of 8 to 10°C conventionally used for injection moulding preforms. However, it has been determined that cooling water temperatures and associated preform mould temperatures in this range result in reduced cycle time and improved clarity of preforms and subsequent containers, especially around the gate area. The preform may comprise a neck portion, a body portion, and a closed end portion. The neck portion may be threaded, or may alternatively comprise a feature such as a circumferential flange for forming a snap-lock engagement with a closure element in use. It will be appreciated that the neck portion is not required to have a diameter smaller than that of the body portion.

The body portion may be in the form of a cylinder, optionally a circular cylinder. The body portion may include a slight taper (for example of less than 1 degree), which may help to facilitate removal of the preform from the mould cavity and/or to facilitate removal of a mould core from the preform during manufacture of the preform. The body portion may have the same diameter or a smaller diameter than the neck portion, which may improve stability of the preform during subsequent reheating.

The body portion may have an at least substantially constant cross-section along its length. Alternatively the body portion may taper inwardly towards the closed end portion, which may further improve stability of the preform during subsequent reheating, especially in the case of large diameter preforms.

The closed end portion may be at least substantially hemispherical.

The body portion of the preform may have a wall thickness in the range 1.2 to 4.5mm, or 2 to 3mm.

The body portion may have a greater wall thickness than the neck portion. Optionally, an increase in wall thickness between the neck portion (that is the portion of the preform that is not stretched during subsequent stretching and blow moulding) and the body portion may begin within the neck portion in order to minimise temperature gradients within the body portion when the preform is subsequently reheated for stretching and blow moulding.

The body portion may have a substantially constant wall thickness along the length of the preform. Alternatively the body portion may include one or more thickened regions and/or one or more changes in wall thickness along the length of the preform, for example to provide increased strength in particular parts of the preform or subsequent container. Thickened regions may be provided at portions of the preform corresponding to corners or other particular design features of the intended container.

The body portion may have a substantially constant wall thickness around the circumference of the preform. Alternatively the preform may include one or more internal ribs. Ribs may extend longitudinally along the body portion, may have a height in the range 0.3 to 0.8mm, and may have a length in the range 30 to 80mm. Ribs may act to provide reinforcement to the preform and to form stabilising panels in the subsequent container. It has been discovered that internal ribs can be formed more precisely in preforms formed of polyolefins such as HDPE and PP than in preforms formed of other materials commonly used for two-stage injection stretch blow moulding such as PET, and that the stabilising panels formed by the ribs have a greater effect in containers formed of polyolefins such as HDPE and PP than in containers formed of other materials such as PET. The closed end portion of the preform may have a wall thickness in the range 0.6mm to 2.5mm, or 1 to 2mm. The closed end portion of the preform may have a wall thickness that is smaller than the wall thickness of the body portion, for example in the range 50 to 90% or 55 to 80% of the wall thickness of the body portion. Alternatively the closed end portion and the body portion may have substantially the same wall thickness.

The preform may have a length in the range 50 to 300mm.

The preform may have a diameter in the range 18 to 50mm.

The preform may have a weight in the range 15g to 190g, or 15g to 100g.

A second aspect of the present invention provides a preform for use in the production of a container formed by the method of the first aspect of the present invention. The preform may include any of the features described above in relation to the first aspect of the present invention.

A third aspect of the present invention provides a method of forming a container, the method comprising:

providing a preform formed predominantly of polyolefin material;

heating the preform; and

blow moulding the preform within a container mould to produce a container.

The step of providing the preform may comprise forming the preform using the method of the first aspect of the present invention or obtaining a preform formed using such a method. The preform may be a preform according to the second aspect of the present invention. The preform may include any of the features described above in relation to the first aspect of the present invention. The step of heating the preform may comprise heating the preform to a temperature in the range 106 to 1 14°C, or 108 to 1 12°C. By heating the preform to a temperature in this range it is possible to minimise the quantity of material required in the preform and in the subsequent container, particularly compared to a conventional extrusion blow moulded container, for which higher moulding temperatures typically require a significant increase in wall thickness in order to provide the necessary strength during reheating and subsequent stretching and blow moulding. This temperature range also allows optimised polymer chain orientation, thereby providing a container having improved stiffness and load resistance for a given weight.

Heating of the preform may be performed using a plurality of heating elements, for example infrared heating lamps, which may be spaced apart along a length of the preform from a first end of the body portion adjacent to the neck portion to the closed end portion.

It has been determined that a more uniform temperature within the preform may be achieved by operating one or more heating elements corresponding to the end of the body portion closest to the neck portion at a higher input power (or heating output) than the heating elements corresponding to the remainder of the body portion. It has further been determined that a more uniform temperature within the preform may be achieved by operating one or more heating elements corresponding to a radiused part of the closed end portion at a higher input power (or heating output) than the heating elements corresponding to the body portion and the heating element(s) corresponding to the tip of the closed end portion.

The step of heating the preform may comprise operating one or more fans to cool the outer surface of the preform. Fan cooling may help to ensure uniform heating along the length of the preform. Fan cooling may also help to ensure substantially uniform heating through the thickness of the preform wall, optionally with a slightly higher temperature at the inner surface of the preform wall compared with the outer surface of the preform wall, for example by 0.5 to 2°C. Generating a slightly higher temperature at the inner surface of the preform wall has been found to improve material distribution and container properties due to the fact that the inner portion of the preform wall is stretched slightly further during blow moulding than the outer portion of the preform wall. The method may further comprise a step of stretching the preform. Stretching of the preform may be performed using a stretch rod. Stretching of the preform may be performed before and/or during blow moulding of the preform (including during a pre- blow phase).

The preform may be subjected to an axial stretch ratio in the range 1.5 to 5, or 2 to 4, the axial stretch ratio being defined as the ratio between the length of the stretched portion of the preform (from the bottom of the neck portion to the top of the base portion) and the length of the final container corresponding to this portion of the preform. The preform may be subjected to a hoop stretch ratio in the range 1.5 to 5, or 2 to 4, the hoop stretch ratio being defined as the ratio of the circumference of the final container to the circumference of the preform. The preform may be subjected to an overall stretch ratio in the range 4 to 20, the overall stretch ratio being calculated as the axial stretch ratio multiplied by the hoop stretch ratio.

The step of blow moulding the preform may comprise blow moulding the preform at a pressure in the range 10 to 40bar or 20 to 30 bar. The step of blow moulding the preform may comprise a step of pre-blowing the preform at a pressure that is lower than a final blow moulding pressure. The pre- blowing stage may use a pressure in the range 6 to 14bar or 8 to 12bar. A significant portion of the total expansion of the preform may occur during the pre-blow phase. The container mould may have a wall temperature in the range 14 to 24°C or 16 to 22°C. In particular a mould temperature of approximately 20°C (+/-2°C) may be used for blow moulding a PE preform and a mould temperature of approximately 18°C (+/- 2°C) may be used for blow moulding a PP preform. The container may have a wall thickness in the range 0.18 to 0.65mm in a main body portion of the container.

The container may have a length in the range 100 to 500mm. The container may have a diameter or width in the range 40 to 210mm. The container may have a volume in the range 100ml_ to 8L, or 100ml_ to 5L.

The container may be, for example a bottle or a jar. The container may have a generally circular or generally square cross-section, although other shapes are also possible.

A fourth aspect of the present invention provides a container formed by the method of the third aspect of the present invention. The container may include any of the features described above in relation to the third aspect of the present invention.

The above-described two-stage injection blow moulding process has been found to enable the use of polyolefin preforms having a reduced wall thickness compared to conventional preforms (for example PET preforms) and the production of polyolefin containers having a reduced wall thickness compared to conventional two-stage injection blow moulded containers (for example formed of PET), thereby reducing material usage and production and distribution costs.

The above-described two-stage injection blow moulding process also provides several advantages over the extrusion blow moulding process typically used to form polyolefin containers. For example, the above described process results in containers having a bi-oriented molecule structure compared to longitudinal orientation that results from extrusion blow moulding. This provides improved container performance (for example improved strength, stiffness and barrier properties), more consistent and predictable containers, and a reduction in the required wall thickness for a specific application. Two-stage injection blow moulding is also less wasteful of material than extrusion blow moulding as it is only necessary to use as much material as is required to fill the preform cavity and form the preform. Two-stage injection blow moulding also allows containers to be blown on-site and on-demand from comparatively small preforms, thereby eliminating the need to transport full sized containers to and store full sized containers at the end use location. Two-stage injection blow moulding also allows containers to be manufactured with more precise neck portions as the neck portion is injection moulded as part of a preform instead of being blow moulded. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figures 1 , 2a and 2b illustrate preforms manufactured in accordance with embodiments of the present invention;

Figures 3, 4a and 4b illustrate containers manufactured in accordance with embodiments of the present invention;

Figures 5 to 9 illustrate various alternative preforms and containers that may be manufactured in accordance with other embodiments of the present invention; Figures 10 to 15 schematically illustrate the apparatus used in manufacturing the preform of Figure 1 and the container of Figure 3; and

Figure 16 illustrates the internal pressures applied to a preform during a blow moulding process according to one embodiment of the present invention.

DETAILED DESCRIPTION

The following description relates to the two-stage injection stretch blow moulding of containers according to one possible embodiment of the present invention. Stock material of pellet-form HDPE having a density of approximately 0.960g/cm ³ and a melt flow index of approximately 1g/10min was obtained for use in the manufacture of containers. The HDPE stock material was loaded into a drying system typically used for drying stock material before melting for use in a moulding process, and run through the drying system in order to remove dust from the HDPE stock material. The drying system 100 is schematically illustrated in Figure 10, and included a hopper 101 for receiving the stock material and an air flow system 102 including an electric heater 103 and a dust removal unit 104.

It is noted that polyolefin stock material (for example comprising HDPE or PP) does not typically require drying prior to melting because polyolefins such as HDPE and PP are far less hygroscopic and pick up significantly less moisture that other materials such as PET that are conventionally used in two-stage injection stretch blow moulding processes. Therefore the use of a drying system may be considered to be redundant when dealing with polyolefins such as HDPE and PP. However, it has been found that running polyolefin stock material through a drying system (with or without operating its heating system) provides an unexpectedly large improvement in the clarity of the preforms and subsequent containers produced from the stock material due to the removal of solid contaminants such as dust. It is noted that the dust removing effect of the drying system can be achieved whether or not its heating system is also operated, and so the stock material may be run through the drying system without its heating system being switched on in order to reduce energy consumption, although the heating system may optionally also be operated in order to pre-heat the stock material.

The HDPE stock material was then loaded into an injection moulding machine. The injection moulding machine 200 is schematically illustrated in Figure 1 1 , and included a hopper 201 for receiving the stock material, a screw type extruder 202 with a diameter of 85mm and a length of 2125mm provided with a plurality of heating elements 203, and a shooting pot delivery system 204 comprising a shooting pot 205 and a shooting pot piston 206. The stock material was introduced into the extruder 202, which was operated at a speed of 45rpm and a temperature of approximately 240°C to melt the stock material and heat it to a temperature of approximately 240°C. It is noted that there is a general prejudice in the field of injection stretch blow moulding towards the use of higher melting and injection temperatures in order to improve visual clarity. However, the applicant has determined that polyolefin preforms and subsequent containers (for example formed of HDPE and PP) having good visual properties may be produced using a melting and injection temperature of approximately 240°C at a reduced energy cost. Melting and homogenisation of the stock material occurred within approximately 8.5 seconds.

The HDPE material was passed along the extruder 202 at a pressure of approximately 140bar in order to ensure homogeneous melting with substantially no gas included in the melted material.

The melted material was introduced into the shooting pot delivery system 204 at an average rate of 70cm ³ /sec to prepare a charge of 544g. The shooting pot delivery system 204 was then operated to inject the charge of melted material through an injection moulding machine nozzle 207 at a temperature of 240°C into a preform mould via a hot runner system. The use of a shooting-pot delivery system 204 allowed simultaneous injection of the melted material and preparation of the material to be used in the next cycle, thereby allowing a gentle melting process with minimised molecule chain degradation, to which polyolefins such as HDPE and PP have been found to be particularly susceptible.

The hot runner system 300, schematically illustrated in part in Figure 12, included a network of channels 301 configured to deliver melted material from the injection moulding machine 200 to individual mould cavities of the preform mould, and a plurality of injection nozzles 302 including valves 303 configured to control the flow of melted material into each individual mould cavity. The hot runner system injection nozzles 302 were maintained at a temperature of 240°C. The preform mould 400, schematically illustrated in part in Figure 13, included 32 mould cavities 401 for moulding preforms, each mould cavity 401 being defined by a plurality of outer moulding elements 402 in combination with a retractable core 403. Each mould cavity 401 had a gate 404 with a diameter of approximately 4mm, and was cooled by a cooling system 405 with water at 50°C to maintain a mould temperature of approximately 50°C. Surprisingly, it has been determined that a higher cooling water temperature and associated mould temperature result in a reduced overall cycle time and improved clarity of the preforms (and subsequent containers), particularly around the gate area.

The melted material was introduced into the preform mould 400 at a rate of approximately 280g/sec during an injection period of 1.9sec, such that each mould cavity 401 was filled at a rate of approximately 8.7g/sec. Injection rates in this range have been determined to minimise visual defects in the preforms and subsequent containers. The injection pressure was raised from Obar up to a maximum injection pressure of 515bar during the injection process. After the mould cavities had been filled the material in the mould cavities was maintained at a hold pressure of approximately 175bar for an additional 2 seconds and then at a subsequent hold pressure of approximately 125bar for an additional 2 seconds by the shooting pot piston 206 in order to stabilise the gate area/injection point of the preforms and counteract the effects of shrinkage. After releasing the hold pressure, the moulded preforms were held in the mould cavities 401 for an additional 7 seconds in order to allow the moulded preforms to cool to a temperature of approximately 60°C. The preform mould 400 was then opened and the moulded preforms released. The total moulding cycle time was approximately 19.4 seconds. The moulded preforms were removed from the injection moulding apparatus by mechanical grippers at a speed of 5m/sec to avoid deformation. The contact portions of the grippers were polished to prevent surface damage to the preforms, and the grippers were cooled to a temperature of approximately 7°C. Figure 1 illustrates one of the preforms 1 produced in accordance with the above- described injection moulding process. The preform comprises a threaded neck portion 2, a (circular) cylindrical body portion 3, and a generally hemispherical closed end portion 4. The overall length of the preform 1 is approximately 88.5mm. The body portion 3 has an outside diameter of 33.5mm, which is slightly smaller than the outside diameter of the neck portion 2 at 35.5mm (excluding the threads). The preform 1 has a constant wall thickness of 2.5mm along the majority of its body portion 3, with a reduced wall thickness at the neck portion 2. The increase in wall thickness between the neck portion 2 and the body portion 3 begins in the neck portion 2 (that is the portion of the preform 1 that is not stretched during subsequent stretching and blow moulding). The preform 1 also has a reduced wall thickness of 1.5mm at its end portion 4. The total weight of the preform 1 is 16.6g. The material of the moulded preforms 1 had a density within 0.009g/cm ³ compared to virgin material. Figures 2a and 2b show, respectively, photographs of one of the HDPE preforms 1 produced in accordance with the above-described injection moulding process and an equivalent PP preform 1 a produced by the same process but using a stock material of PP.

In other embodiments the preform 1 may have one or more thickened regions and/or one or more changes in wall thickness along the length of the body portion 3, for example to provide increased strength in particular parts of the preform 1 or subsequent container. Thickened regions may be provided at portions of the preform corresponding to corners or other particular design features of the intended container. In other embodiments the preform may alternatively or additionally include one or more internal ribs, for example longitudinal ribs with a height in the range 0.3 to 0.8mm and a length in the range 30 to 80mm. Ribs may act to provide reinforcement to the preform and to form stabilising panels in the subsequent container. It has been discovered that internal ribs can be formed more precisely in preforms formed of polyolefins such as HDPE and PP than in preforms formed of other materials commonly used for two-stage injection stretch blow moulding such as PET, and that the stabilising panels formed by the ribs have a greater effect in containers formed of polyolefins such as HDPE and PP than in containers formed of other materials such as PET.

In other embodiments the wall thickness of the preform 1 at the end portion 4 may be the same as the wall thickness through the body portion 3, which can help to reduce temperature gradients within the preform during reheating for subsequent stretch blow moulding, and may therefore lead to more reliable blow moulding of the preform 1.

The preforms 1 were transferred to a stretch blow moulding machine after having cooled to ambient temperature. It will be appreciated that the stretch blow moulding machine could have been at a separate location to the location in which the preforms 1 were injection moulded, and that the preforms 1 could have been stored at another separate storage location until finished containers were required before insertion into a stretch blow moulding machine. In a first portion of the stretch blow moulding machine, the preforms 1 were heated using a set of infrared lamps 500 arranged along the length of the preforms from the top of the body portion to the tip of the closed end portion (as schematically illustrated in Figure 14) to a temperature of 108 to 112°C. By heating the preforms 1 to a temperature in this range it is possible to minimise the quantity of material required in the preforms 1 and in the subsequent containers, particularly compared to conventional extrusion blow moulded containers, for which higher moulding temperatures typically require a significant increase in wall thickness in order to provide the necessary strength during reheating and subsequent stretching and blow moulding. This temperature range also allows optimised polymer chain orientation, thereby providing a container having improved stiffness and load resistance for a given weight.

It has been determined that a more uniform temperature within the preform 1 may be achieved by operating one or more heating elements corresponding to an end of the body portion 3 closest to the neck portion 2 at a higher input power (or heating output) than the heating elements corresponding to the remainder of the body portion 3. It has further been determined that a more uniform temperature within the preform may be achieved by operating one or more heating elements corresponding to a radiused part of the closed end portion 4 at a higher input power (or heating output) than the heating elements corresponding to the body portion 3 and the heating element(s) corresponding to the tip of the closed end portion 4. Therefore the lamp arrangement 500 illustrated in Figure 14 was operated with the lamp 501 closest to the neck portion 2 at 90% of maximum output, the lamp 503 corresponding to the radiused part of the closed end portion 4 at 50% of maximum output, and the intermediate lamps 502 and lower-most lamp 504 at 30% of maximum output.

Fans were operated to cool the outer surfaces of the preforms 1 to ensure that the preforms 1 were heated substantially uniformly but with a slightly high temperature at their inner surfaces compared to their outer surfaces, which has been found to improve material distribution and mechanical properties of the finished container after stretch blow moulding.

The preforms 1 were then inserted into and clamped in a mould 600 for forming final containers, as schematically illustrated in Figures 15a and 15b. The container mould 600 was maintained at a temperature of 20°C. The preforms 1 were stretched using a stretch rod 601 , as schematically illustrated in Figure 15c, before being subjected to an elevated internal pressure to blow mould the preforms 1 into their final shape. The step of blow moulding the preforms 1 included a pre-blow stage lasting for approximately 0.9 seconds during which an internal pressure of approximately 10bar was applied, followed by a second blowing stage lasting for a further 0.9seconds during which an internal pressure of approximately 40bar was applied, as illustrated in Figure 16. During the stretch blow moulding process the preforms 1 were subjected to a hoop stretch ratio of approximately 2 and an axial stretch ratio of approximately 3, giving an overall stretch ratio of approximately 6. The finished containers were then released from the mould 600, as schematically illustrated in Figure 15d, and removed from the stretch blow moulding machine.

Figure 3 illustrates one of the containers 10 (in this case a bottle) produced in accordance with the above-described stretch blow moulding process. The final containers 10 had a height of 162mm, including the threaded neck portion, a square cross-sectional profile with a width of 58mm, an average wall thickness of approximately 0.6mm in their main body portions, and a fill-point volume of 330ml.

Figures 4a and 4b show, respectively, photographs of one of the HDPE bottles 10 produced in accordance with the above-described stretch blow moulding process and an equivalent PP bottle 10a produced by the same process but using a preform 1 a formed of PP.

It will be appreciated that the above-described preform 1 and the above-described method may equally be used to manufacture other containers with different design features and different volumes in other embodiments of the present invention by selecting an appropriately shaped final container mould. For example, Figure 5 illustrates an alternative container 10b (also in the form of a bottle) having a circular cross section and a smaller volume of 250ml that may be manufactured from the above-described preform 1 using the above-described method in combination with an appropriately shaped final container mould.

It will further be appreciated that the method of the present invention may equally be used to manufacture other preforms for use in the manufacture of still further containers. For example, Figures 6 and 7 respectively illustrate an alternative preform 1 c with a weight of 190g that may be manufactured in accordance with another embodiment of the present invention and a bottle 10c having a capacity of 6L that may be manufactured from the preform 1 c. As a further example, Figures 8 and 9 respectively illustrate a further alternative preform 1 d with a weight of 190g that may be manufactured in accordance with another embodiment of the present invention and a jar 10d having a capacity of 6L that may be manufactured from the preform 1 d. The preform 1 d illustrated in Figure 8 has a comparatively wide neck portion and body portion with a maximum diameter of approximately 120mm. In order to improve stability of the comparatively wide preform 1 d during reheating, the body portion is tapered inwardly towards the closed end portion, as illustrated in Figure 8.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims. The above-described two-stage injection stretch blow moulding process has been extensively optimised in order to overcome the many disadvantages associated with using polyolefins such as HDPE and PP in a two-stage injection stretch blow moulding process. The above described method allows polyolefin containers with good mechanical and visual properties and a low weight and material cost per unit to be reliably manufactured using a two-stage injection stretch blow moulding process, which was not possible using methods previously known in the art.