GB2628673A

GB2628673A - Manufacture of thermoformed containers

Info

Publication number: GB2628673A
Application number: GB2304896.0A
Authority: GB
Inventors: Reginald Clarke Peter
Original assignee: GR8 Engineering Ltd
Current assignee: GR8 Engineering Ltd
Priority date: 2023-03-31
Filing date: 2023-03-31
Publication date: 2024-10-02
Also published as: GB202304896D0

Abstract

A method of forming a container comprises a first stage of forming an intermediate article by a first process comprising providing a sheet 2 composed of a biaxially orientable thermoplastic material; heating the sheet to a moulding temperature which is above the glass transition temperature (Tg) of the thermoplastic material, wherein the moulding temperature is within a range of from 5 to 25 °C above the glass transition temperature (Tg); and thermoforming the heated sheet using a mould tool to cause at least a portion of the annular sidewall, and optionally at least a portion of the bottom wall, to be stretched from an initial thickness to a final thickness and the final thickness is less than 45% of the initial thickness, thereby to be biaxially oriented with a strain induced crystallinity of at least 27%. A second stage thermally conditions the intermediate article at an annealing temperature of at least 135 °C to anneal the respective portion or portions by relaxing amorphous regions between strain induced crystals and causing the strain induced crystals to grow in dimensions, wherein after the heating step the strain induced crystals having a maximum width dimension of 500 nm. A third stage forms a container from the intermediate article by cooling the intermediate article from the annealing temperature to a quenching temperature of no greater than 90 °C to quench and rigidify the bottom wall and annular sidewall and form the container.

Description

Manufacture of Thermoformed Containers The present invention relates to a method of forming a thermoformed container. In particular the present invention relates to the manufacture of thermoformed containers which are transparent and are dimensionally stable at elevated temperatures, for example above 150°C, and therefore can be used to hold food to be cooked or reheated in a cooking oven or a microwave oven.

In the packaging industry, the process of blow moulding is often used in the manufacture of containers, particularly bottles for carbonated beverages. This process involves the initial formation of a preform, typically by injection moulding, which preforms are subsequently blow moulded to form the containers. Such preforms are typically formed of thermoplastic material, particularly polyethylene terephth al ate (PET).

For the manufacture of containers in the form of wide-mouth containers such as trays, cups or tubs, typically thermoforming is used. A sheet of thermoplastic material, Otypically a polyolefin, is heated and then urged, by a movable mould member and a blowing pressure, into a moulding cavity. Such trays, cups or tubs often suffer from the Oproblem of poor mechanical properties, in particular poor impact resistance, particularly at low temperatures. This is because the thermoformed thermoplastic tends to exhibit poor molecular alignment or orientation, which may be monoaxial orientation or only a low degree of biaxial orientation. Furthermore, such trays, cups or tubs typically have low dimensional stability when subjected to elevated temperature, for example when placed in an oven to cook or reheat food. Also, such trays or tubs are typically opaque.

It is well known that biaxial orientation increases polymer toughness in thermoplastic packaging. However, conventional thermoforming processes tend to produce no or only low biaxial orientation, particularly in regions of the packaging which may be subjected to the greatest impact stresses during use, and so which require the greatest toughness or impact resistance.

It is known to produce trays, cups or tubs from polyethylene terephthalate (PET). However, these products typically have a low thermal stability, for example when subjected to elevated temperature, typically when placed in an oven to cook or reheat food, and/or low transparency or translucency.

Also, known trays, cups or tubs produced from polyethylene terephthalate (PET) may have a shape and configuration in which the sidewall of the container is inclined at an angle of at least 10 degrees to the longitudinal axis of the container. This particularly applies to containers which have been subjected to a multi-stage moulding process to increase the crystallinity of the PET by a heat setting process. Therefore the opening is significantly larger than the base, and correspondingly the volume of the container is relatively low for a given outer dimension of the upper edge of the container.

The Applicant's earlier WO-A-2018/007604 discloses a method of forming a container in which an injection moulded preform is stretch blow moulded to form a container. Although this method can produce a container which has good transparency, nevertheless there is still a need to for an improved method to form a highly transparent container composed of a biaxially oriented polymer, such as PET, which can have very Othin walls, and therefore low material costs and can be manufactured at high production rates and low cost, and which can exhibit high mechanical strength at elevated temperatures.

The present invention aims at least partially to overcome these problems of known containers and corresponding container manufacturing methods. There is a need in the art for a container, and a corresponding method of manufacture, which provides cost-effective containers having dimensions to enable them to be used as pots, jars, trays, cups or tubs, and which have good mechanical properties, for example impact resistance, thermal stability at elevated temperatures, for example at oven temperatures, and transparency/translucency, for example a transmissivity through at least a sidewall of the container of at least 90% in visible light.

There is a particular need in the art for transparent thermoplastic containers, which are "wide-mouth" containers such as pots, jars, trays, cups or tubs, which can be subjected to elevated temperatures of typically above 150 °C, even above 200 °C, with minimal thermal distortion so that such containers are "dual ovenable" i.e. can be placed in a microwave oven or a conventional oven to cook or reheat food without degradation of the tray. Such containers have particular application in the packaging of "cook-chill" food which is sold by many supermarkets and grocery stores, and is precooked (at least partially) and refrigerated and sold to the consumer for reheating/finish cooking at home. Also, such containers can be used to package products which are conventionally packed at elevated temperatures in glass containers, such as jam, with minimal distortion or loss of transparency of the container.

The present invention provides a method of forming a container according to claim 1.

Preferred features are defined in the dependent claims.

The present invention is at least partly predicated on the finding by the present inventor that a container having excellent mechanical properties, in particular impact resistance as Owell as thermal stability at high temperatures and transparency in visible light, can be obtained by using a three-stage method comprising the combination of, in the first stage, an initial thermoforming step which forms an intermediate article, in the second stage, a subsequent thermal conditioning of the intermediate article, and, in the third stage, a subsequent forming of a container from the intermediate article by cooling the intermediate article.

The initial thermoforming step, in the first stage of forming an intermediate article from a sheet, causes at least a portion of the annular sidewall, and optionally at least a portion of the bottom wall, in the intermediate article to be stretched from an initial thickness to a final thickness and the final thickness is less than 45% of the initial thickness, thereby to be biaxially oriented with a strain induced crystallinity of at least 27%. The initial thermoforming step typically introduces biaxial orientation and such a high degree, i.e.at least 27%, of strain induced crystallinity throughout the intermediate article, for example including at any corners of the intermediate article; the high strain induced crystallinity forms a high number density of very small crystals, which are not visible to the naked eye since they preferably have a maximum dimension of 500 nm. The thermoforming step can be carried out quickly on a thin sheet of the biaxially orientable polymer, and this can avoid excessive crystal growth, which would otherwise cause the thermoformed wall to appear translucent.

Thereafter, the thermoformed intermediate article is subjected to a high temperature annealing process, in this context "high temperature" meaning an annealing temperature of at least 135 °C in a second thermal conditioning stage. The high temperature annealing process anneals the portion or portions of the intermediate article which have been stretched from an initial thickness to a final thickness and the final thickness is less than 45% of the initial thickness, thereby to be biaxially oriented with a strain induced crystallinity of at least 27% by relaxing amorphous regions between strain induced crystals and causing the strain induced crystals to grow in dimensions, wherein after the heating step the strain induced crystals having a maximum width dimension of 500 nm.

Otypically, the high temperature annealing process is carried out for a very short time period, and therefore the crystals of the high strain induced crystallinity do not Osignificantly grow in dimensions. Also, the high number density of crystals provides that adjacent crystals are very closely packed, which minimizes or prevents significant crystal growth. The high number density of small crystals forms a crystal network which is reduced in stress during the annealing process and any shape memory effect within the crystal network is substantially eliminated.

Thereafter, in a third stage, which forms a container from the intermediate article. The annealed intermediate article is subjected to cooling from the annealing temperature to a quenching temperature of no greater than 90 °C to quench and rigidify the bottom wall and annular sidewall and form the container. The quenching prevents any further crystal growth, and produces the final container. In the final container, there is maintained the high strain induced crystallinity which forms a high number density of very small crystals, which are not visible to the naked eye since they preferably have a maximum dimension of 500 nm, and so the container is transparent. The high number density of crystals is very high, which provides thermal stability. The annealing and quenching steps have mechanically and thermally stabilised the crystal network formed as a result of the high strain induced crystallinity.

The result is a container with high impact resistance and thermal stability, coupled with high transparency. The container is robust at low, for example below freezing, and high, for example above 150 °C, temperatures. Also, the container is preferably a wide mouth which, as a result of the thermoforming step, can have a high volume and capacity for a given peripheral dimension.

In this specification the container may be in the form of any wide mouth container which may be in the form of a tray, tub, pot, jar optionally a threaded jar, cup, etc. The wide mouth of the container has an opening which has substantially the same or greater dimensions and area as compared to the body and base of the container. The container may have a variety of different shapes, dimensions and aspect ratios.

OEmbodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figures 1 to 5 are schematic cross-sections showing sequential steps in a method of forming a container in the form of a pot from a sheet of polymer in accordance with an embodiment of the present invention.

Referring to Figures 1 to 5, as shown in particular in Figure 1 a sheet 2 composed of a biaxially-orientable thermoplastic material is provided. The sheet 2 is an elongate sheet which is typically fed out from a roll (not shown). In preferred embodiments, the thermoplastic material comprises polyester, typically at least one polyalkylene polyester or a blend of polyalkylene polyesters. Preferably the polyester comprises at least one polyester selected from polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate and polybutylene naphthalate. Preferably, the biaxiallyorientable thermoplastic material is polyethylene terephthalate.

N

The sheet 2 has typically been formed by blowing, rolling or extruding the thermoplastic material into an elongate sheet material which is stored in roll form for subsequent use in a thermoforming step as described below. Preferably, the sheet 2 is an extruded sheet.

The material of the sheet 2 is preferably substantially unoriented and amorphous, which preferably has been achieved by extrusion to form the sheet 2. Alternatively, the material of the sheet 2 may be semi-crystalline and have some orientation resulting from the sheet forming process. Typically, the material of the preform 2 has less than 10% crystallinity.

The sheet 2 typically has an overall general shape and configuration which is planar, although in a modified embodiment the sheet 2 may have some localised three-dimensional shaping. However, the sheet 2 is preferably planar, and two-dimensionally shaped, with no three-dimensional shaping.

The sheet 2 typically has a thickness of from 0.3 to 1.2 mm, optionally from 0.4 to 0.6 mm, for example about 0.5 mm. Preferably, the sheet 2 has a constant thickness. However, in some embodiments the sheet 2 may vary in thickness, for example across the width of the elongate sheet 2.

As shown in Figure 1, the sheet 2 is disposed between opposed parts of a thermoforming mould tool 4.

The sheet 2 is heated to a thermoforming temperature, in particular to a moulding temperature which is above the glass transition temperature (Tg) of the thermoplastic material. In accordance with the present invention, the moulding temperature is within a range of from 5 to 25 °C above the glass transition temperature (Tg). Preferably, the moulding temperature is within a range of from 5 to 20 °C, more preferably from 5 to W °C, above the glass transition temperature (Tg).

Any suitable heating method known in the art for heating sheets, in particular elongate sheets, may be used.

The sheet 2 is heated with at least one heated element 3 in an oven 5. The heating may be conductive by contact with the at least one heated element or alternatively may be heated by infrared or near-infrared radiation or may be preheated using infrared or near-infrared radiation and then conditioned conductively using contact with the at least one heated element.

The thermoforming mould tool 4 includes a first lower female mould 6 defining a first moulding cavity 8 surrounded by an outer moulding surface 10. In this embodiment, the outer moulding surface 10 defines the outer shape of an intermediate article to be thermoformed from the sheet 2. The first moulding cavity 8 has an upper opening 9. The peripheral edge 11 of the upper opening 9 is defined by an annular ring 13 of a polymer, such as PTFE, which is inserted into the first lower female mould 6 to assist slipping and stretching of the sheet 2 into the first moulding cavity 8 during the thermoforming step.

The thermoforming mould tool 4 also includes an upper housing 12 having a lower Oclamping surface 14 which clamps against an upper surface 16 of the first lower female mould 6. As described hereinafter, the thermoforming forms the intermediate article Ofrom the sheet 2 by the outer moulding surface 10 moulding the outer shape of the intermediate article.

The thermoforming mould tool 4 further includes a first male mould 18 depending downwardly from a shaft 19 which extends through the upper housing 12. The first male mould 18 is disposed within a cavity 17 formed within the upper housing 12. The shaft 19 is slidably disposed in a tubular support 13 of the upper housing 12 which is located upwardly away from the lower clamping surface 14. The shaft 19 mounting the first male mould 18 is slidable within a hole 20 in the upper housing 12 so that the first male mould 18 can be moved between a first position above the lower clamping surface 14 and a second position below the lower clamping surface 14. The first male mould 18 is axially aligned with the lower female mould 6 and the lower clamping surface 14 of the upper housing 12.

The first male mould 18 has an inner moulding surface 21 which matches the shape and configuration of the outer moulding surface 10 of the first lower female mould 6, but the inner moulding surface 21 of the first male mould 18 has smaller dimensions than the dimensions of the outer moulding surface 10 of the first lower female mould 6 so that the first male mould 18 can freely slidingly fit within the moulding cavity 8 of the first lower female mould 6.

As described in greater detail hereinbelow, the thermoforming mould tool 4 forms an intermediate article 54. The sheet 2 is positioned over the upper opening 9 of the first moulding cavity 8. The first male mould 18 functions as a stretching plug and urges the sheet 2 into the first moulding cavity 8 to form the intermediate article 54.

Adjacent to, and downstream of, the thermoforming mould tool 4 is provided an annealing/quenching apparatus 60 which forms the final container from the intermediate article 54.

OThe annealing/quenching apparatus 60 includes a second lower female mould 62 defining a second moulding cavity 63 surrounded by an outer moulding surface 64. The Osecond moulding cavity 63 has an upper opening 99. The outer moulding surface 64 defines the outer shape of the final moulded container.

In the illustrated embodiment, the first and second mould cavities 8, 63 have the same shape and dimensions. However, in an alternative embodiment, the second moulding cavity 63 is larger in shape and dimensions than the first moulding cavity 8.

In some preferred embodiments, the second moulding cavity 63 is configured to mould structural features, by stretching of the intermediate article 54, in at least one or both of the annular sidewall and the bottom wall which are not moulded by the first moulding cavity 8.

The annealing/quenching apparatus 60 also includes a second upper male mould 66 having an inner moulding surface 68. The annealing/quenching apparatus 60 also includes a second upper housing 112 having a lower clamping surface 114 which damps against an upper surface 116 of the second lower female mould 62. As described hereinafter, the annealing/quenching forms the final container 86 from the intermediate article by the second upper male mould 66 annealing, and subsequently the second lower female mould 62 quenching, the intermediate article 54.

The second upper male mould 66 depends downwardly from a shaft 119 which extends through the second upper housing 112. The second male mould 66 is disposed within a cavity 117 formed within the second upper housing 112. The shaft 119 is slidably disposed in a tubular support 113 of the second upper housing 112 which is located upwardly away from the lower clamping surface 114. The shaft 119 mounting the second male mould 66 is slidable within a hole 120 in the second upper housing 112 so that the second male mould 66 can be moved between a first position above the lower clamping surface 114 and a second position below the lower clamping surface 114. The second male mould 66 is axially aligned with the second lower female mould 62 and the lower clamping surface 114 of the second upper housing 112.

O The upper male mould 66 is heated to an annealing temperature and the lower female Omould 62 is maintained or controlled to be at a quenching temperature. 10. Typically, the annealing temperature is within the range of from 160 to 200 °C and the quenching temperature is within the range of from 15 to 90 °C, preferably from 40 to 90 °C, further preferably from 50 to 90 °C, still further preferably from 60 to 80 °C.

In this embodiment, as shown in Figure 1, the sheet 2 is an elongate sheet 2 which is passed along a process flow direction from the oven 5 to the thermoforming mould tool 4 and then to the annealing/quenching apparatus 60. The moulded intermediate article 54 formed by the thermoforming mould tool 4 remains integral with the elongate sheet 2 and then, after removal from the thermoforming mould tool 4, the moulded intermediate article 54 is moved by an indexing operation, to the annealing/quenching apparatus 60 which forms the final container 86 from the intermediate article 54. The intermediate article 54, carried by the elongate sheet 2, is thus disposed in the annealing/quenching apparatus 60 which forms the final container from the intermediate article 54. The thermoforming stage is carried out to form an intermediate article 54 from an upstream portion of the elongate sheet 2 simultaneously with the annealing and quenching stages to form a final container 86 from a downstream intermediate article 54. The final container 86 also remains integral with the elongate sheet 2. In this embodiment, each final container 86 is separated from the elongate sheet 2 after the annealing and quenching steps. In alternative embodiments, each intermediate article 54 may be separated from the elongate sheet either during the thermoforming step or after the thermoforming step and prior to the annealing and quenching steps.

As shown in Figure 1, at the beginning of each moulding cycle, initially the thermoforming mould tool 4 is in an open configuration and the first lower female mould 6, and the assembly of the upper housing 12 and the first male mould 18, are mutually spaced to provide a vertical gap 28 therebetween. The elongate sheet 2 extends horizontally through the vertical gap 28. Correspondingly, in each moulding cycle, initially the annealing/quenching apparatus 60 is in an open configuration, and the second lower female mould 62, and the second upper male mould 66, are mutually C\I spaced by the vertical gap 28. Figure 1 shows that, at the beginning of a moulding cycle, the elongate sheet 2 has an undeformed region located between the upper and lower parts Oof the thermoforming mould tool 4, an intermediate article 54 carried by the elongate sheet 2, downstream of and adjacent to the undeformed region, and located between the upper and lower parts of the annealing/quenching apparatus 60, and a final container 86 carried by the elongate sheet 2 and located downstream of the annealing/quenching apparatus 60, and downstream of and adjacent to the intermediate article 54.

In the illustrated embodiment, in the thermoforming step caned out by the thermoforming mould tool 4 a series of the intermediate articles is sequentially formed along the elongate sheet and in a final cooling step carried out by the annealing/quenching apparatus 60 a series of the containers is sequentially formed along the elongate sheet, and each container is separated from the elongate sheet after the cooling step. In alternative embodiments, each intermediate article may be separated from the elongate sheet either during the thermoforming step or after the thermoforming step and prior to the annealing and quenching steps.

As shown in Figure 2, at the start of the thermoforming stage, the first lower female mould 6 and the upper housing 12 are, preferably simultaneously, moved towards each other so that the elongate sheet 2 is clamped between the lower clamping surface 14 of the upper housing 12 and the upper surface 16 of the lower female mould 6. The elongate sheet 2 defines a datum surface along which the lower clamping surface 14 of the upper housing 12 and the upper surface 16 of the lower female mould 6 are clamped together, so that the elongate sheet 2 is not moved vertically during the initial clamping operation.

The lower surface 14 of the upper housing 12 and the upper surface 16 of the lower female mould 6 are urged together against the opposite upper and lower surfaces 30, 32 of the sheet 2 under sufficient clamping pressure so that a moulding chamber 34, defined between the upper housing 12 and the lower female mould 6 and containing a central portion 36 of the elongate sheet 2, is sealed, preferably hermetically sealed.

OAs shown in Figure 3, the central portion 36 is then thermoformed. A peripheral flange 38 of the elongate sheet 2 surrounds the central portion 36 is clamped in position around Oand above the first moulding cavity 8, and is not moulded during the thermoforming process. The lower surface 14 of the upper housing 12 and the upper surface 16 of the first lower female mould 6 act to hold the elongate sheet 2 in position during the thermoforming step.

As described above, the central portion 36 of the sheet 2 has been preheated to a moulding temperature within a predetermined range slightly above its glass transition temperature so as to be readily mouldable by thermoforming. The selected temperature range ensures that when the biaxially orientable polymer, such as PET, is stretched, the desired minimum threshold of strain induced crystallinity is induced in at least some portions of the thermoformed article.

The first lower female mould 6 is preferably heated to a temperature which is lower than, or only slightly, higher than a glass transition temperature of the thermoplastic material, and preferably is lower than the temperature of the thermoplastic material during thermoforming. Typically, the outer moulding surface 10 of the first lower female mould 6 has a temperature of from 40 to 90 °C, typically from 50 to 70 °C. The upper housing 12, and the associated first upper male mould 18, may also be heated to the same or similar temperature as the first lower female mould 6. These temperatures are particularly selected for moulding PET, and if other polymers are moulded the skilled person would be readily able to provide suitable alternative temperatures.

Referring to Figure 3, the first upper male mould 18 is then lowered into the first moulding cavity 8. This causes the sheet 2 to be mechanically deformed and stretched downwardly to form a substantially inverted container-like shape 42, which is the same or similar to the shape of the final container, against the moulding surface of the first upper male mould 18. The free lower end 22 and conical side surface 23 of the first upper male mould 18 engages an inner side 44 of the central portion 36 of the sheet 2, and axially stretches the central portion 36. The free lower end 22 forms a bottom wall 75 of the intermediate article 54 and the conical side surface 23 forms an annular sidewall 77 of the intermediate article 54.

OThe first upper male mould 18 is moved a selected downward distance, along an axis of movement of the first upper male mould 18, against the sheet 2 so that the first upper male mould 18 axially stretches the central portion 36 by a distance which is from 75 to 100% of the height of the outer moulding surface 10, and thereby the depth of the moulding cavity 8. Preferably, the first upper male mould 18 is moved downwardly by a distance which is substantially the entire depth of the first moulding cavity 8 defined by the outer moulding surface 10, The first upper male mould 18 and the first lower female mould 6 have moulding surfaces which are shaped and dimensioned so that when the first upper male mould 18 is received within the first lower female mould 6, the moulding surfaces are spaced by a distance with is greater than the thickness of the thermoformed central portion 36. The result is that the thermoformed central portion 36 cannot simultaneously be in contact with both of the opposed moulding surfaces.

During or shortly after closing of the thermoforming mould tool 4, pressurised air is introduced into a lower portion 35 of the first moulding cavity 8 through one or more air ports 37 formed in the bottom part 39 of the first lower female mould 6 as shown by the arrows in Figure 3. The introduction of air at a positive gas pressure between the central portion 36 and outer moulding surface 10 provides that during the initial thermoforming step the thermoformed central portion 36 has a good and uniform contact with the first upper male mould 18 and is prevented from contacting the outer moulding surface 10 of the first lower female mould 6.

After the sheet 2 has been axially stretched by the first upper male mould 18, the introduction of air into the lower portion 15 of the first moulding cavity 8 through the one or more air ports 17 is terminated and, preferably immediately thereafter, as shown in Figure 4 pressurized air is blown against the inner side 44 of the central portion 36 during a blowing step.

Ol'he pressurized air is introduced into the cavity 17 of the upper housing 12 through one or more air ports 47 formed in the upper housing 12, as shown by the arrows in Figure 4.

OThe introduction of air at a positive gas pressure between the central portion 36 and inner moulding surface 21 of the first male mould 18 that after the initial thermoforming step, the thermoformed central portion 36 is blown downwardly and outwardly away from contacting the first upper male mould 18 and urged into a good and uniform contact with the moulding surface 10 of the first lower female mould 6. Typically, the pressurized gas has a pressure of from 2 to less than 7 bar ( 2 <7 x105 N/m2).

The pressurized gas urges the central portion 36 radially outwardly against the outer moulding surface 10 of the lower female mould 6. The sheet 2 is urged outwardly so as to contact and assume the shape of the sidewall 50 and the bottom wall 48 of the outer moulding surface 10. The outer moulding surface 10 defines the outer shape 52 of an intermediate moulded article 54 which has been thermoformed from the sheet 2.

Although pressurised air is described above in the illustrated embodiment, any suitable gas (for example nitrogen) may be used.

The thermoforming step, which results in the formation of the intermediate article 54 having a shape and dimensions defined by the outer moulding surface 10 of the first lower female mould 6, causes at least a portion of the annular sidewall 77, and optionally at least a portion of the bottom wall 75, to be stretched from an initial thickness to a final thickness and the final thickness is less than 45% of the initial thickness, thereby to be biaxially oriented with a strain induced crystallinity of at least 27%. Preferably, after the thermoforming step the final thickness is from 30 to 45% of the initial thickness. Typically, after the thermoforming step the final wall thickness of the portion(s) having the required minimum strain induced crystallinity may be as low as 150 to 200 microns (0.15 to 0.2 mm), and in some configurations of containers the minimum final wall thickness may even be as low as 100 microns (0.1 mm).

Typically, the thermoforming step causes the portion of the annular sidewall 77, and C\I optionally the portion of the bottom wall 75, to be biaxially oriented with a strain induced crystallinity of from 32 to 35%. Preferably, the thermoforming step causes the Oannular sidewall 77, and optionally the bottom wall 75, to be biaxially oriented with a strain induced crystallinity of at least 27% throughout the annular sidewall 77, and optionally throughout the bottom wall 75.

Preferably, the thermoforming step is carried out such that the bottom wall 75 and annular sidewall 77 in the intermediate article 54 comprise strain induced crystals having a maximum width dimension of 500 nm. This dimension threshold means that the crystals cannot be seen with the human naked eye and the bottom wall 75 and annular sidewall 77 of the intermediate article 54 appear transparent to the human naked eye.

In the illustrated embodiment, a single upper male mould 18 is used but in alternative embodiments, particularly for sheets for producing containers having a large surface area, for example in the form of trays, a plurality of mutually laterally spaced male mould is provided which engage respective mutually spaced areas on the inner side of the sheet.

In the illustrated embodiment, the upper male mould 18 has a shape and dimensions which are substantially, but slightly smaller as described above, the same as the shape and dimensions of the outer moulding surface 10 of the first lower female mould 6 which defines the shape and dimensions of the intermediate article 54. Fundamentally, the upper male mould 18 functions as a plug to cause mechanical deformation and stretching, as would be understood by those skilled in the art. In alternative embodiments, other shapes and configurations for such a plug may be used. The selected plug typically depends on the shape and dimensions of the container to be moulded. For example, the upper male mould 18 may be in the form of a stretch rod, which typically has an elongate cylindrical shape with a free lower end which engages the thermoplastic sheet. The free lower end may have various different shapes, for example hemispherical, conical etc, and dimensions. Such a stretch rod may include a central conduit for introducing the blowing gas under pressure into the moulding cavity 8 during a blowing Ooperation in the thermoforming stage, which may be provided instead of the one or more air ports 47 formed in the upper housing 12. The outer surface of such a stretch rod Oincludes gas outlet holes communicating with the central conduit.

As described hereinafter, the intermediate article 54 is then subjected to a sequence of annealing and quenching steps in the annealing/quenching apparatus 60 to form the final moulded container having a desired shape and dimensions.

The annealing and quenching steps may be carried out in a number of different embodiments. In each of these embodiments, the intermediate article 54 is subjected to the following steps: a second stage of thermally conditioning the intermediate article 54 by heating at least the portion, or each portion, of the intermediate article 54 which has a strain induced crystallinity of at least 27% to an annealing temperature of at least 135 °C to anneal the respective portion or portions by relaxing amorphous regions between strain induced crystals and causing the strain induced crystals to grow in dimensions, wherein after the heating step the strain induced crystals having a maximum width dimension of 500 nm; followed by a third stage of forming a container 86 from the intermediate article 54 by cooling the intermediate article 54 from the annealing temperature to a quenching temperature of no greater than 90 °C to quench and rigidify the bottom wall 75 and annular sidewall 77 and form the container 86.

Preferably, in the heating step the intermediate article 54 is heated to the annealing temperature for a period of from 0.5 to 1.5 seconds. Preferably, the cooling step is started within a period of from 0.5 to 2 seconds from the end of the heating step. In the cooling step, preferably the intermediate article 54 is cooled to the quenching temperature within a period of from 0.2 to 1.5 seconds from the start of the cooling step.

In the illustrated embodiment, as shown in Figure 5, after the intermediate article 54 has been formed as described above, the thermoforming mould tool 4 is opened and the intermediate article 54, carried on the elongate sheet 2, is removed from the thermoforming mould tool 4. Simultaneously, the adjacent downstream final container 86 is removed from the annealing/quenching apparatus 60. Thereafter, the elongate sheet 2 is indexed forwardly to align the intermediate article 54 with the annealing/quenching apparatus 60 and simultaneously, an undeformed region of elongate sheet 2 is aligned Owith the thermoforming mould tool 4 and the final container 86 is moved downstream of the annealing/quenching apparatus 60 for subsequent severing from the elongate sheet 2 to form an individual container 86.

When the intermediate article 54 is aligned with the annealing/quenching apparatus 60, the annealing/quenching apparatus 60 is then closed, with the result that the intermediate article 54 is located within the second moulding cavity 63 of the second lower female mould 62, as shown in Figure 2.

As shown in Figure 1, initially the annealing/quenching apparatus 60 is in an open configuration and the second lower female mould 62 and the second upper male mould 66 are mutually spaced to provide a vertical gap 69 therebetween. The intermediate article 54, carried by the elongate sheet 2, is located in the vertical gap 69.

The second lower female mould 62 and the second upper male mould 66 are simultaneously moved towards each other so that the intermediate article 54 is located between second lower female mould 62 and the second upper male mould 66, as shown in Figure 2. The elongate sheet 2 defines a datum surface along which the opposite peripheral lower surface 70 of the second upper male mould 66 and the opposite peripheral upper surface 72 of the second lower female mould 62 are clamped together, so that the elongate sheet 2 is not moved vertically during the initial mould closing operation.

The lower surface 70 and the upper surface 72 are urged together against the opposite upper and lower surfaces 30, 32 of the sheet 2 under sufficient clamping pressure so that a moulding chamber 74, defined between the second upper male mould 66 and the second lower female mould 62 and containing the intermediate article 54, is sealed, preferably hermetically sealed.

After the second lower female mould 62 and the second upper male mould 66 have been closed as shown in Figure 3 so that the intermediate article 54 is located within a second moulding cavity 76 of the second female mould 62, initially the intermediate article 54 is Oshrunk onto the heated upper male mould 66 to anneal the bottom wall 75 and annular sidewall 77 of the intermediate article 54, in particular the portion(s) thereof which have been provided with the required strain induced crystallisation of at least 27 % by the thermoforming step, as described above. The degree of shrinkage is small. Typically, the intermediate article 54 is shrunk by a shrinkage, defined by reduction in area of the respective portion or portions of the intermediate article 54, of from 0.2 to 0.5%, typically about 0.3%, Accordingly, during the heating step, in the illustrated embodiment the intermediate article 54 is disposed on the second male mould 66 and is located within the second moulding cavity 76 of the second female mould 62, the intermediate article 54 being spaced inwardly from the second moulding surface 64 of the second female mould 62.

During or shortly after closing of the annealing/quenching apparatus 60, pressurised air is introduced into a lower portion 35 of the second moulding cavity 76 through one or more air ports 87 formed in the bottom part 89 of the second female mould 62, as shown by the arrows in Figure 3. The introduction of air at a positive gas pressure between the intermediate article 54 and outer moulding surface 64 provides that during the annealing step the intermediate article 54 has a good and uniform contact with the heated upper male mould 66, to ensure that the desired controlled annealing is achieved, and is prevented from contacting the second outer moulding surface 64 of the second lower female mould 62.

Accordingly, during the heating step, in the illustrated embodiment in the heating step the intermediate article 54 is shrunk by passing pressurised gas through the second female mould 62 to apply a positive gas pressure against a surface of the intermediate article 54 spaced inwardly from the second outer moulding surface 64 of the second lower female mould 62.

In an alternative embodiment, in the heating step the intermediate article 54 is shrunk by applying a negative vacuum pressure to a surface of the intermediate article 54 spaced Ofrom and facing the second male mould 66.

In the illustrated embodiment, the second male mould 66 may optionally be configured to mould structural features, by stretching of the intermediate article 54, in at least one or both of the annular sidewall 77 and the bottom wall 75 which are not moulded by the first moulding cavity. The second male mould 66 in preferably provided, as shown in the Figures, with an upper annular ring 73 which is configured to mould an upper rim 81 of the container 86.

Thereafter, in the subsequent cooling step as shown in Figure 4, the annealed intermediate article 54 is expanded so as to be placed in contact with the cooler second lower female mould 62 and removed from contact with the second upper male mould 66.

Accordingly, in the illustrated embodiment, in the cooling step, the second female mould 62 is provided which is at the quenching temperature and the intermediate article 54 contacts the second moulding surface 64 of the second female mould 62, the intermediate article 54 being located within the second moulding cavity 63 of the second female mould 62, to quench the bottom wall 75 and annular sidewall 77.

The annealed intermediate article 54 is expanded by subjected the portion of the annealed intermediate article 54 between the upper male mould 66 and the lower female mould 62 to at least one or both of (i) a positive gas pressure by passing pressurised gas past or through the upper male mould 66 to apply a positive gas pressure against an internal surface 82 of the intermediate article 54 in contact with the upper male mould 66, and/or (ii) a vacuum pressure by applying a negative gas pressure to a series of vacuum ports which are located in the outer moulding surface 64.

In the illustrated embodiment, in the cooling step the intermediate article 54 is expanded against the second female mould 62 by passing pressurised gas, such as air, to apply a C\I positive gas pressure against the internal surface 82 of the intermediate article 54 in contact with the second male mould 66. As illustrated, the pressurized air is introduced Ointo the cavity 117 of the second upper housing 112 through one or more air ports 147 formed in the second upper housing 112, as shown by the arrows in Figure 4. The pressurized air may be blown past the second male mould 66, or alternatively through the second male mould 66.

As shown in Figure 4, the second male mould 66 may be internally hollow and the upper end of the second male mould 66 may be provided with a manifold plate 123 incorporating through-holes 125. When the pressurized air is introduced into the cavity 117 of the second upper housing 112, the pressurized air is passed into the hollow second male mould 66 via the through-holes 125, and then the pressurised air passes through an array of vent-holes 127 in the moulding surface 68 of second male mould 66, so that the air is blown against the internal surface 82 of the intermediate article 54.

The introduction of air at a positive gas pressure between the intermediate article 54 and inner moulding surface 68 of the second male mould 66 provides that after the annealing step, the annealed intermediate article 54 is blown downwardly and outwardly away from contacting the second male mould 66 and urged into a good and uniform contact with the moulding surface 64 of the second lower female mould 62. Typically, the pressurized gas has a pressure of from 2 to less than 7 bar ( 2 < 7 x105 N/m2).

In an alternative embodiment, in the cooling step the intermediate article 54 is expanded against the second female mould 62 by applying a negative vacuum pressure to a surface of the intermediate article 54 spaced from and facing the second female mould 62.

The positive gas pressure pushes, and/or the vacuum pressure sucks, the external surface 84 of the annealed intermediate article 54 against the cooler lower female mould 62 and spaces the annealed intermediate article 54 from the hotter upper male mould 66, thereby quenching the intermediate article 54 by contact with the lower female mould 62. Typically, in the cooling step the intermediate article 54 is spaced from the upper male mould 66 by a distance of from 0.5 to 2 mm, for example from 0.5 to 1.2 mm.

OThe cooling step forms the final container 86.

As shown in Figure 5, the lower female mould 62 and the upper male mould 66 are then opened so that the final container 86 can be removed, and subsequently separated from the remainder of the elongate sheet 2.

The heating step and the cooling step do not significantly change the shape and dimensions of the intermediate article 54 to form the final container 86. The heating step and the cooling step are carried out such that the bottom wall 88 and annular sidewall 90 in the final container 86 comprise strain induced crystals having a maximum width dimension of 500 nm. Accordingly, the final container appears transparent to the naked eye.

The thermoforming step creates significant radial stretch and axial stretch in the intermediate article 54, so that throughout the intermediate article 54 there is a very high level of strain induced crystallinity in the polymer, and the crystals are invisible to the naked eye. The subsequent annealing and quenching steps are carried out to remove stress and shape memory in the crystalline network formed by the high number density of strain-induced crystals, but the annealing and quenching steps are carried out using temperature and time parameters which do not cause the strain induced crystals to grow significantly in dimensions. Consequently, the resultant polymer in the final container has a very high level of transparency, and also high thermal stability and mechanical strength.

The illustrated embodiment of present invention uses a continuous "one step" moulding process in which the sheet is thermoformed to form the intermediate article, and then the intermediate article is annealed and quenched.

Alternatively, other embodiments of the present invention may use a discontinuous "two Ostep" moulding process, which may be called a reheat moulding process, in which the sheet is thermoformed to form the intermediate article and then cooled to ambient Otemperature, and optionally stored. Subsequently, the cooled thermoformed intermediate article is reheated and then is annealed and quenched.

In the illustrated embodiments, the resultant thermoformed container has a polygonal, e.g. square plan. However, the container may have any other desired shape. For example, the container may have a horizontal cross-section which is substantially shaped as follows: circular, oval, elliptical or polygonal, optionally square or rectangular. Furthermore, the vertical cross-section may have any desired shape or configuration.

This method can produce a container which has a wide mouth, for example a tray, cup, jar, for example a threaded jar, or tub, composed of biaxially oriented polymer, such as a polyester, typically PET, having high visual clarity and high heat stability as a result of achieving high strain-induced crystallization during the thermoforming process. The container can provide a highly transparent thermoplastic container, such as a tray, which can be subjected to elevated temperatures of typically above 150 °C, even above 200 °C, without thermal distortion so that such a container is "dual ovenable" i.e. can be placed in a microwave oven or a conventional oven to cook or reheat food without degradation of the tray.

Additionally, by having high biaxial orientation, the container is adapted to contain a frozen product at a temperature of less than 0°C, for example ice-cream or sorbet, without fracturing.

Various modifications to the illustrated embodiments will be apparent to those skilled in the art and are intended to be included within the scope of the present invention. C\I O

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Claims

Claims 1. A method of forming a container, the method comprising the steps of: A. in a first stage, forming an intermediate article by a first process comprising toe steps of: 0 providing a sheet composed of a biaxially-orientable thermoplastic material; ii) heating the sheet to a moulding temperature which is above the glass transition temperature (Tg) of the thermoplastic material, wherein the moulding temperature is within a range of from 5 to 25 °C above the glass transition temperature (Tg); iii) thermoforming the heated sheet using a mould tool to form an intermediate article having a bottom wall and an annular sidewall, wherein the mould tool comprises a first female mould, defining a first moulding cavity, and a further mould part, comprising a first male mould, which is lowered into the first moulding cavity to mechanically deform and stretch the heated sheet by a.distance, along an axis of movement of the further mould part, which is from 75 to 100% of a depth of the first moulding cavity, and a pressurised gas is blown against an inner surface of the mechanically deformed and stretched heated sheet to urge an outer surface of the mechanically deformed and stretched heated sheet against a first moulding surface of the first female mould, wherein the thermoforming step causes at least a portion of the annular sidewall, and optionally at least a portion of the bottom wall, to be stretched from an initial thickness to a final thickness and the final thickness is less than 45% of the initial thickness, thereby to be biaxially oriented with a strain induced crystallinity of at least 27%; B. in a second stage, thermally conditioning the intermediate article by a second process comprising the step of: iv) heating at least the portion, or each portion, of the intermediate article which has a strain induced crystallinity of at least 27% to an annealing temperature of at least 135 °C to anneal the respective portion or portions by relaxing amorphous regions between strain induced crystals and causing the strain induced crystals to grow in dimensions, wherein after the heating step the strain induced crystals having a maximum width dimension of 500 nm; and C. in a third stage. forming a container from the intermediate article by a third process comprising the step of: v) cooling the intermediate article from the annealing temperature to a quenching temperature of no greater than 90 °C to quench and rigidify the bottom wall and annular sidewall and form the container.
2. A method according to claim 1 wherein the moulding temperature is within a range of from 5 to 20 °C, optionally from 5 to 10 °C, above the glass transition temperature (Tg).
3. A method according to claim 1 or claim 2 wherein the thermoforming step (iii) causes the annular sidewall, and optionally the bottom wall, to be biaxially oriented with a strain induced crystallinky of at least 27% throughout the annular sidewall, and optionally throughout the bottom wall.
4. A method according to any one of claims = to 3 wherein the thermoforming step (hi) causes the portion of the annular sidewall, and optionally the portion of the bottom wall, to be biaxially oriented with a strain induced crystallinity of from 32 to 35%.
5. A method aocording to any foregoing claim wherein in the thermoforming step (iii) a moulding surface of the first female mould defining the first moulding cavity is heated to a temperature of from 40 to 90 °C, optionally from 50 to 70 °C.
6. A method according to any foregoing claim wherein after the thermoforming step (iii) the final thickness is from 30 to 45% of the initial thickness.
7. A method according to any foregoing claim wherein in heating step (iv) the intermediate article is heated to the annealing temperature for a period of from 0.5 to 1.5 seconds.
8. A method according to any foregoing claim wherein cooling step (v) is started within E period of from 0.5 to 2 seconds from the end of heating step. (iv).
9. A method according to any foregoing claim wherein in cooling step (v) the intermediate article is cooled to the quenching temperature within a period of from 0.2 to 1.5 seconds from the start of cooling step (v).
10. A method according o any foregoing claim wherein in heating step (iv) the annealing temperature is withiP the range of from 160 to 200 °C.
11. A method according to any foregoing claim wherein in cooling step (IC the quenching temperature is within the range of from 15 to 90 °C, optionally from 40:o 90 °C, further optionally from 50:o 90 °C, still further optionally from 60 to 80 °C.
12. A method according to any foregoing claim wherein in heating step (iv) a second male mould is provided which is heated to the annealing temperature, and the intermediate article is shrunk onto the second male mould to anneal the respective portion or portions.
13. A method according to claim 12 wherein in heating step (iv) the intermediate article is shrunk by a shrinkage, defined by reduction in area of the respective portion or portions of the intermediate arti:le, of from 0.2 to 0.5%; optionally about 0.3%.
14. A method according to claim 12 or claim 13 wherein the second male mould is 2onfigured to mould structural features, by stretching of the intermediate article, is at least one or both of the annular sidewall and the tottom wall which are not moulder by the first moulding cavity.
15. A method according to any one of claims 12 to 14 wherein in heating step (iv) the intermediate article is shrunk by applying a negative vacuum pressure to a surface of the intermediate article spaced from and facing the second male mould.
16. A method according to any foregoing claim wherein in cooling step (v) a second female mould is provided which is at the quenching temperature and the intermediate article contacts a second moulding surface of the second female mould, the intermediate article Deing located within a second moulding cavity of the second female mould, to quench the bottom wall and annular sidewall.
17. A method according to claim 16 wherein In cooling step (v) the intermediate article is expanded against the second female mould by applying a negative vacuum pressure to a surface of the intermediate article spaced from and facing the second female mould.
18. A method according to claim 16 or claim 17 when dependent on claim 12 or any claim dependent thereon, wherein in heating step (iv) the intermediate article is disposed on the second male mould and is located within the second moulding cavity of the second female mould, the intermediate article being spaced inwardly from the second moulding surface of the second female mould.
19. A method according to claim 18 wherein in heating step (iv) the intermediate article is shrunk by,passing pressurised gas through the second female mould to apply a positive gas pressure against a surface of the intermediate article spaced inwardly from the second moulding surface of the second female mould.
20. A method according to any one of claims 16 lo 19 wherein in cooling step (v) the intermediate article is expanded against the second female mould by passing pressurised gas through or past the second male mould to apply a positive gas pressure against a surface cf the intermediate article in contact with the second male mould.
21. A method according to any one of claims 16 to 20 wherein in cooling step (v) the intermediate article is spaced from the second male mould by a distance of from 0.5 to 2 IIII11.
22. A method according to claim 21 wherein it cooling step (v) the intermediate article is spaced from the second male mould by a distance of from 0.5 to 1.2 mm.
23. A method according to any one of claims 16 to 22 wherein the first and second moulding, cavities have the same shape and dimensions.
24. A method according to any one of claims 16 to 23 'wherein the second moulding cavity is larger in shape and dimensions than the first moulding cavity.
25. A method according to any one of Cairns 16 to 24 wherein the second moulding cavity is configured to mould structural features, by stretching of the intermediate article, in at least one or both of the annular sidewail and the bottom wall which are not moulded by the first moulding cavity.
26. A method according to any foregoing claim wherein the sheet is an extruded sheet, and optionally wherein in the sheet the thermoplastic material is substantially unoriented and amorphpas.
27. A method according to any foregoing claim wherein the sheet has a thickness of from 0.3 to 1.2 mm.
28. A method according to any foregoing claim wherein the sheet has an overall general shape and configuration which is planar, and optionally the sheet has some localised three-dimensional shaping.
29. A method according to any foregoing claim wherein the thermoplastic material comprises polyester, optionally wherein the polyester comprises at least one polyalkylene polyester cr a blend of polyalkylene polyesters, further optionally the polyester comprising at least one polyester selected from polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate and polybutylene naphthalate, yet further optionally the polyester comprising polyethylene terephthalate.
30. A method according to any foregoing claim wherein the thermoforming step (iii) is carried out such that the bottom wall and annular sidewall in the intermediate article comprise strain induced crystals having a maximum width dimension of 500 nm.
31. A method according to claim 30 wherein the heating step (iv) and the cooling step (v) are carried out such that the bottom *wall and annular sidewall in the container comprise strain induced crystals having a maximum width dimer_sion of 500 run.
32. A method according to any foregoing claim wherein the sheet provided in -itep (i) is an elongate sheet, in the thermoforming step (iii) a series of the intermediate articles is sequentially formed along the elongate sheet, in the cooling step (v) a series of the containers is sequentially formed along the elongate sheet, and each container is separated from the elongate sheet after the cooling step (v).