Container base with optimised stiffness to mass ratio

Abstract

A container (Figure 1, 1) comprising a base 2 and a side wall (Figure 1, 4). The base comprises at least one lower layer 6 at a first elevation, an upper layer 8 at a second elevation, the at least on

Description

FIELD OF THE INVENTION Embodiments of the present invention relate to a container base with an optimised stiffness to mass ratio. In particular, but not exclusively, the container is a moulded plastics container, such as a walled tray. BACKGROUND TO THE INVENTION Containers can be strengthened in the design phase by adding additional reinforcement. However, this increases the mass of the container. Containers can also be lightened by removing material. However, this reduces the stiffness of the container. Furthermore, containers are designed to minimise tooling costs. BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION According to various, but not necessarily all, embodiments of the invention there is provided a container comprising a base and a side wall arrangement peripheral to the base, wherein the base comprises at least one lower layer at a first elevation, an upper layer at a second elevation above the first elevation, and a plurality of ribs extending between the first elevation and the second elevation, wherein the at least one lower layer extends in a first offset region laterally offset to a first side of the upper layer, and extends in a second offset region laterally offset to a second opposite side of the upper layer, such that the upper layer is located between the first and second offset regions of the at least one lower layer, and wherein the plurality of ribs comprises a first set of ribs connected to at least one of the offset regions and extending upwardly to the second elevation in a cantilevered manner. An advantage is a hybrid base with an optimised stiffness to mass ratio. Part of the base has an upper layer extending through a central region of the container, while the adjacent regions have an open cell structure of cantilevered ribs at the second elevation. This hybrid design increases the stiffness particularly of the central region of the base, with a lower overall mass than a full sandwich base. An advantage of having an open cell rib structure when viewed from above is that it enables the base to have a continuous / closed bottom surface. This is better for conveying because there is more surface for the conveyor to grip on, and conveying will be quieter when running. Optionally, the first set of ribs comprises ribs at the first offset region, and ribs at the second offset region. Optionally, the plurality of ribs further comprises a second set of ribs connected to at least the upper layer to stiffen the upper layer. Optionally, the second set of ribs is connected to an underside of the upper layer. Optionally, the base comprises a single-layer structure and a double-layer structure, wherein the single-layer structure comprises the first and second offset regions of the at least one lower layer, and wherein the double-layer structure comprises the upper layer and an underlying layer beneath the upper layer. Optionally, the underlying layer is a portion of the lower layer. Optionally, a single lower layer extends through the first offset region, below the upper layer, and through the second offset region. An advantage is a hybrid base combining both single-layer and double-layer structures. The double-layer structure increases stiffness particularly of the centre of the base, while the single-layer structure has less mass than the double-layer structure. Optionally, the plurality of ribs comprises a set of ribs interconnecting the upper layer with the underlying layer. The set of ribs may be the second set of ribs described above. An advantage is improved stiffness, due to an internally-ribbed double-layer sandwich structure. Optionally, the base comprises side access openings enabling access to one or more cavities between the upper layer and the underlying layer of the double-layer structure, allowing a side pull tool arrangement to form the one or more cavities. Optionally, the at least one lower layer, the upper layer, and the plurality of ribs provide the single-layer structure with a lower mass per unit length than the double-layer structure, in one or both horizontal orthogonal axes. Optionally, the upper layer extends through a centreline of the base, and widens with increasing distance from the centreline of the base towards respective sides of the base. Optionally, the upper layer widens diagonally towards corner regions of the base. Optionally, the upper layer widens diagonally with increasing distance from the centreline of the container towards the respective sides. A diagonal direction is relative to the sides of the base and / or the side wall arrangement of the container which may be rectilinear. Optionally, the upper layer widens symmetrically about the centreline of the container, with increasing distance from the centreline. Optionally, the upper 3 layer defines an hourglass shape. Optionally, the upper layer comprises lateral side edges which widen as described above. An advantage is optimising the stiffness to mass ratio of the container. The loads from the centre of the container are distributed widely towards the stiff corners of the container, while minimising the dimensions of the relatively heavy double-layer structure at the centre. Optionally, the first set of ribs comprises ribs extending towards corner regions of the base. Optionally, the ribs of the first set extend diagonally towards the corner regions of the base. Optionally, the first set of ribs comprises parallel rows of ribs which each extend in a parallel direction to the widening lateral edges of the upper layer. Optionally, each row of the first set of ribs at the first offset region defines a chevron pattern complementary to a shape of the widening upper layer. Optionally, each row of the first set of ribs at the second offset region defines a chevron pattern complementary to a shape of the widening upper layer. Optionally, the chevron patterns face opposite directions than each other. Optionally, the / each chevron pattern is generally parallel to corresponding sides of the upper layer. The chevron patterns may be formed from straight or curved lines. An advantage of the chevron patterns is that as articles are slid on or off the floor, the angled rib pattern reduces the likelihood of the article snagging on the ribs. In contrast, perpendicular rows and columns of ribs present a barrier which may stop a load from sliding. Optionally, the upper layer has a first end connected to a first side wall of the side wall arrangement, and a second end connected to a second side wall of the side wall arrangement, opposite the first side wall. Optionally, the upper layer extends continuously from the first side wall to the second side wall. An advantage is improved stiffness. Optionally, the base comprises elongate beams interconnected by the first set of ribs. Optionally, the plurality of ribs comprise a third set of ribs defining the elongate beams. Optionally, each elongate beam comprises a lattice structure, or alternatively a mono-rib. Optionally, the elongate beams are connected to the lower layer. Optionally, the elongate beams extend through the upper layer and are also connected to the upper layer. Optionally, each elongate beam extends continuously through the first offset region, the upper layer, and the second offset region. Optionally, the elongate beams extend to a third elevation above the second elevation, the elongate beams therefore being taller than the first set of ribs and the upper layer. Optionally, the elongate beams comprise cantilevered top edges which extend to the third elevation of the base. The third elevation may define a floor plane on which articles can be placed. Optionally, the upper layer and top edges of the first set of ribs are vertically recessed relative to the elongate beams. Optionally, the upper layer extends between a first pair of opposing side walls of the side wall arrangement, and the elongate beams extend between a second pair of opposing side walls of the side wall arrangement, each different than the first pair of opposing side walls. Optionally, the second pair of opposing side walls extend perpendicular to the first pair of opposing side walls. Optionally, the base comprises lateral side members connecting lateral sides of the upper layer to the at least one lower layer. Optionally, the double-layer 5 structure comprises the lateral side members, the lateral side members interconnecting the upper layer with the underlying layer (e.g., lower layer). Optionally, the lateral side members widen as described earlier. Optionally, the plurality of ribs comprise underside ribs located beneath the upper layer, which extend non-parallel to the lateral side members and connect to inner surfaces of the lateral side members to provide intermediate bracing of the lateral side members. Optionally, the second set of ribs described above comprises the underside ribs. Optionally, the underside ribs are parallel to each other. Optionally, the elongate beams extend non-parallel to the lateral side members and are connected to outer surfaces of the lateral side members to provide intermediate bracing of the lateral side members. This may be in addition to, or instead of the bracing provided by the underside ribs. Optionally, the container is a moulded plastics container. Optionally, the container is a walled tray. According to various, but not necessarily all, embodiments of the invention there is provided a container comprising a base and a side wall arrangement peripheral to the base, wherein the base comprises single-layer structures and a double-layer structure, wherein the double-layer structure is located between the single-layer structures in plan view, wherein the base comprises a lower layer defining the single-layer structure and one of the layers of the double-layer structure, and wherein the double-layer structure further comprises an upper layer above the lower layer. According to various, but not necessarily all, embodiments of the invention there is provided a container comprising a base and a side wall arrangement peripheral to the base, wherein the base comprises at least one lower layer at a first elevation, and an upper layer at a second elevation above the first elevation, wherein the at least one lower layer extends in a first offset region laterally offset to a first side of the upper layer, and extends in a second offset region laterally offset to a second opposite side of the upper layer, such that the upper layer is located between the first and second offset regions of the at least one lower layer, and wherein the upper layer extends through a centreline of the base, and widens with increasing distance from the centreline of the base towards respective sides of the base. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of various examples of embodiments of the present invention reference will now be made by way of example only to the accompanying drawings in which: FIG. 1 illustrates a perspective view of an example container; FIG. 2 illustrates a perspective view with a horizontal cross-section taken at a lower elevation than FIG. 1; FIG. 3 illustrates the perspective view with a horizontal cross-section taken at a lower elevation than FIG. 2; FIG. 4 illustrates the perspective view with a horizontal cross-section taken at a lower elevation than FIG. 3; FIG. 5 illustrates the perspective view with a vertical diagonal cross-section; FIG. 6 illustrates an underside view of the container; FIG. 7 illustrates a transverse vertical cross-section of the container at a first longitudinal position; FIG. 8 illustrates a transverse vertical cross-section of the container at a second longitudinal position; FIG. 9 illustrates a longitudinal vertical cross-section of the container at a first transverse position; and FIG. 10 illustrates a longitudinal vertical cross-section of the container at a second transverse position. DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION The Figures illustrate a container 1 in the form of a tray. Orthogonal X, Y, and Z axes are also defined, herein referred to as longitudinal, transverse, and vertical axes. The container 1 is optionally formed from a plastics material, such as an injection-moulded plastic. The container 1 comprises a base 2 and a side wall arrangement 4 peripheral to the base 2 and connected to the base 2. The side wall arrangement 4 comprises a pair of long side walls 4A extending in the longitudinal axis x, and a pair of short side walls 4B extending in the transverse axis y. The long and short side walls 4A, 4B collectively define a rectangular shape. Alternatively, another shape may be defined. The illustrated base 2 has a partial sandwich construction. Specifically, the base 2 is a hybrid comprising a sandwich, double-layer structure and a single-layer structure. The base 2 starts with a lower layer 6 at a first elevation. The first elevation may be a bottom elevation of the base 2. The lower layer 6 may be a horizontally extending bottom layer of the base, at the bottom elevation. An underside surface of the lower layer 6, illustrated in FIG. 6, may define an underside of the base 2. The underside surface may extend continuously to the periphery of the container 1 at each side of the container 1. The lower layer 6 may provide a continuous layer between the longitudinal ends of the base 2, save for any optional drainage holes (not shown). The top surface of the lower layer 6 is shown in the horizontal cross-section view of FIG. 4. A plurality of ribs 10 are connected to the top surface of the lower layer 6. The ribs 10 stiffen the container 1. The ribs 10 are interconnected to each other, and are also connected to the side wall arrangement 4. The ribs 10 may be integrally-moulded portions of the container 1. As shown in FIG. 3, which is a higher horizontal cross-section than FIG. 4, the base 2 further comprises a horizontally-extending upper layer 8 at a mid-height elevation of the base 2. The mid-height elevation can be referred to as a second elevation relative to the first elevation. The upper layer 8 is above the lower layer 6 and generally parallel to the lower layer 6. This represents the double-layer structure of the base 2. The ribbed structure to either side of the upper layer 8 is the single-layer structure of the base 2. The ribs 10 can be divided generally into three sets or types. A first set of ribs 16 comprises rows of cantilevered free-standing ribs supported by the lower layer 6 and extending upwardly to the mid-height elevation. The rows of first ribs 16 are located to either lateral side of the upper layer 8. A second set of the ribs 18, shown in FIGS. 4, 5, 9, and 10, comprises rows of transversely extending ribs extending beneath the upper layer 8, and interconnecting the lower and upper layers 6, 8. A third set of the ribs 19 comprises columns of cantilevered free-standing ribs which are taller than the first set of ribs 16, and extend the full longitudinal length of the container 1. This third set of ribs 19 define a plurality of elongate beams 28 and are best shown in FIGS. 1,2, 5, 7, 8, and 10. The elongate beams 28 are connected to the lower and upper layers 6, 8, and extend in the longitudinal axis x. Each elongate beam 28 is formed from a lattice 9 rib structure formed from a pair of parallel longitudinal ribs interconnected by cross-bracing ribs. The elongate beams 28 significantly stiffen the container 1 against flexure in the x-z plane. Alternatively, each elongate beam 28 may consist of a longitudinal monorib rather than a lattice. The elongate beams 28 are taller than the ribs 16, 18, and extend to a third elevation higher than the mid-height elevation, the upper layer 8, and the ribs 16, 18. The third elevation may be a top elevation, defining a floor of the base 2 on which articles can be placed. The cantilevered top edges of the longitudinal ribs of the elongate beams 28 collectively define a floor plane on which articles in the container 1 can be placed. Therefore, articles are placed on top of the cantilevered top edges of the longitudinal ribs 19 of the elongate beams 28. Therefore, the top elevation of the base 2 has an open cell rib structure along substantially its whole length and width. FIG. 1 illustrates the elongate beams 28 extending continuously between the longitudinal ends of the base 2, meaning that the elongate beams 28 are connected to the top surface of the upper layer 8 without being discontinued by the upper layer 8. Therefore, the upper layer 8 is in fact recessed below the top elevation of the base 2, which is difficult to see from the perspective of FIG. 1 but can be seen from FIGS. 2, 3, 5, and 7-9. These Figures show the elongate beams 28 being connected to the top surface of the upper layer 8. Therefore, it can be said that the open cell rib structure of the top elevation of the base 2 extends continuously from a first longitudinal end of the base 2 to a second opposite longitudinal end of the base 2. Furthermore, the open cell rib structure extends continuously from a first transverse side of the base 2 to a second opposite transverse side of the base 2. In other examples, the elongate beams 28 are omitted. FIG. 3 shows the lower layer 6 extending in a first offset region 12 laterally offset to a first side of the upper layer 8, and extending in a second offset region 14 laterally offset to a second opposite side of the upper layer 8, such that the upper layer 8 is centrally located between the first and second offset regions 12, 14 of the lower layer 6. The illustrated first offset region 12 is located to a first longitudinal side of the container 1. The second offset region 14 is located to an opposite longitudinal side of the container 1 The illustrated first offset region 12 extends to a first longitudinal end of the container 1. The first offset region 12 may be connected to one of the short side walls 4B. The second offset region 14 extends to an opposite longitudinal end of the container 1. The second offset region 14 may be connected to the other of the short side walls 4B. The x-z plane cross-section of FIG. 9 further illustrates how the offset regions 12,14 are located to either side of the upper layer 8, along the longitudinal axis x. FIGS. 3, 5, 7, and 8 show that the upper layer 8 may be connected at each end to one of the long side walls 4A. The upper layer 8 extends the full width of the container 1 in the transverse axis y, and is connected to both of the long side walls 4A. As shown in FIG. 3, the illustrated lateral side edges 26 of the upper layer 8 are connected to the long side walls 4A close to but spaced from the corners of the 11 base 2. Instead, the first and second offset regions 12, 14 are connected to the corners of the base 2. As shown in FIGS. 3, 7, and 8, the upper layer 8 may extend from the periphery of the container 1 through a centre 22 of the base 2. In examples, extending through the centre 22 of the base 2 means extending through the centroid (centre of mass) of the base 2 when viewed in plan view. In examples, extending through the centre 22 of the container 1 also means extending through the longitudinal centreline 24 (FIG. 1) of the base 2. Furthermore, the upper layer 8 may be generally symmetrical about the longitudinal and transverse centrelines 24, 25 of the base 2. Furthermore, FIG. 3 shows that the lateral sides of the upper layer 8 diagonally widen with increasing distance from the centre 22, towards respective lateral sides of the base 2. The width of the upper layer 8 refers to the width measured along the longitudinal axis x. The overall result is a generally hourglass-shaped upper layer 8, wider at the periphery of the container 1 than at the longitudinal centreline 24 of the container 1. The hourglass-shaped upper layer 8 distributes loads from the centre 22 towards the stiffer comers of the base 2. The term ‘hourglass-shaped’ does not limit to the specific linear diagonal shape shown. The illustrated first and second offset regions 12, 14 are each generally pentagonal shaped, complementary to the hourglass-shaped upper layer 8. Three corners of the pentagonal shape are also comers of a corresponding side of the upper layer 8. The other two comers are defined by the comers of the base 2. The lateral side edges 26 of the upper layer 8 are connected to the lower layer 6 by lateral side members 30. The lateral side members 30 and corresponding lateral side edges 26 of the upper layer 8 diagonally widen in the manner 12 described above. They may define opposite-facing chevron shapes, one to each side of the upper layer 8. In each of the first and second offset regions 12, 14 to either side of the upper layer 8, rows of the first set of ribs 16 may extend non-parallel to the side wall arrangement 4, instead extending parallel to the widening sides of the upper layer 8. Therefore, the rows of the first set of ribs 16 are also diagonal. The illustrated rows of the first set of ribs 16 extend parallel to the diagonal lateral side edges 26 of the upper layer 8, and the corresponding lateral side members 30. The first set of ribs 16 therefore also define a plurality of chevron shapes, complementary to the side edges 26 of the upper layer 8. The chevron shapes of the ribs in the first offset region 12 face oppositely to the chevron shapes of the ribs in the second offset region 14, the narrowest point therebetween corresponding to a narrowest point of the upper layer 8, on the longitudinal centreline 24. The first ribs extend towards the corners of the base 2, to distribute loads from the centre 22 of the base 2 to the stiff comers. Furthermore, the rows of the first set of ribs 16 are located in the spaces between the elongate beams 28, and interconnect the sides of the elongate beams 28. The first set of ribs 16 therefore stiffen the container 1 against flexure in the y-z plane. The cross-sections of FIGS. 4 and 5 illustrate the hidden second set of ribs 18, which are underside ribs hidden beneath the upper layer 8. The second set of ribs 18 interconnect the lower and upper layers 6, 8 to stiffen the double-layer structure. The illustrated second set of ribs 18 comprises a plurality of rows of transversely-extending ribs to each side of the longitudinal centreline 24. The transverse ribs 18 optionally extend generally along the transverse axis y. 13 Furthermore, some of the transverse ribs 18 are connected to inner sides of the diagonal lateral side members 30 at intermediate bracing positions. The outer sides of the lateral side members 30 are connected to the elongate beams 28. FIG. 4 also illustrates the second set of ribs 18 further comprising a longitudinal rib between the lower and upper layers 6, 8, the longitudinal rib extending along the longitudinal centreline 24 of the base 2 to interconnect the lateral side members 30 and the lateral side edges 26 of the upper layer 8 at the narrowest point of the upper layer 8. Since the lower and upper layers 6, 8 are continuous layers, tool access may be required during moulding. Therefore, as best shown in the cross-sections of FIGS. 7 and 8, the base 2 comprises side access openings 20 allowing side pull tools to be inserted at least partially in the transverse axis y, into the space between the lower and upper layers 6, 8. The space in between each pair of adjacent transverse ribs 18 defines a pocket into which the side pull tool can be inserted. By contrast, the first and third sets of ribs 16, 19 can be formed using a straight pull tool above the base 2 and moving in the vertical axis z, since they are cantilevered ribs. The overall result is a base 2 with a high stiffness to mass ratio. The base 2 is stiffest in the centre 22 where the deflection would otherwise be greatest, and material has been selectively removed to either side of the centre 22, in particular by limiting the size of the upper layer 8 relative to the lower layer 6 such that it is smaller. Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, the lower layer 6 may be discontinuous and formed from a plurality of lower layers 6. 14 Furthermore, the or each lower layer 6 may terminate at the sides of the upper layer 8, such that the upper layer 8 does not have a layer underlying it and the base 2 is only formed from single-layer structures. 5 Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. 10 Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. 15 Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A container comprising a base and a side wall arrangement peripheral to the base,wherein the base comprises at least one lower layer at a first elevation, an upper layer at a second elevation above the first elevation, and a plurality of ribs extending between the first elevation and the second elevation,wherein the at least one lower layer extends in a first offset region laterally offset to a first side of the upper layer, and extends in a second offset region laterally offset to a second opposite side of the upper layer, such that the upper layer is located between the first and second offset regions of the at least one lower layer, andwherein the plurality of ribs comprises a first set of ribs connected to at least one of the offset regions and extending upwardly to the second elevation in a cantilevered manner.

2. The container of claim 1, wherein the first set of ribs comprises ribs at the first offset region, and ribs at the second offset region.

3. The container of claim 1 or 2, wherein the plurality of ribs further comprises a second set of ribs connected to at least the upper layer to stiffen the upper layer, and optionally wherein the second set of ribs is connected to an underside of the upper layer.

4. The container of any preceding claim, wherein the base comprises a single-layer structure and a double-layer structure, wherein the single-layer structure comprises the first and second offset regions of the at least one lower layer, and wherein the double-layer structure comprises the upper layer and an underlying layer beneath the upper layer, optionally wherein the underlying layer is a portion of the lower layer, and optionally wherein a set of the plurality of ribs interconnects the upper layer with the underlying layer.

5. The container of any preceding claim, wherein the base comprises side access openings enabling access to one or more cavities between the upper layer and the underlying layer of the double-layer structure, allowing a side pull tool arrangement to form the one or more cavities.

6. The container of any preceding claim, wherein the at least one lower layer, the upper layer, and the plurality of ribs provide the single-layer structure with a lower mass per unit length than the double-layer structure, in one or both horizontal orthogonal axes.

7. The container of any preceding claim, wherein the upper layer extends through a centreline of the base, and widens with increasing distance from the centreline of the base towards respective sides of the base.

8. The container of claim 7, wherein the upper layer widens symmetrically about the centreline of the container, with increasing distance from the centreline.

9. The container of any preceding claim, wherein the first set of ribs comprises ribs extending towards corner regions of the base.

10. The container of claim 7 and claim 8 or 9, wherein the first set of ribs comprises parallel rows of ribs which each extend in a parallel direction to the widening lateral edges of the upper layer.

11. The container of claim 10, wherein each row of the first set of ribs at the first offset region defines a chevron pattern complementary to a shape of the widening upper layer.

12. The container of claim 11, wherein each row of the first set of ribs at the second offset region defines a chevron pattern complementary to a shape of 17the widening upper layer, optionally wherein the chevron patterns at the first and second offset regions face opposite directions than each other.

13. The container of any preceding claim, wherein the upper layer has a first end connected to a first side wall of the side wall arrangement, and a second end connected to a second side wall of the side wall arrangement, opposite the first side wall.

14. The container of claim 13, wherein the upper layer extends continuously from the first side wall to the second side wall.

15. The container of any preceding claim, wherein the base comprises elongate beams interconnected by the first set of ribs.

16. The container of claim 15, wherein the elongate beams are connected to the lower layer, and optionally wherein the elongate beams are also connected to a top surface of the upper layer.

17. The container of claim 15 or 16, wherein the elongate beams extend to a third elevation above the second elevation, the elongate beams therefore being taller than the first set of ribs and the upper layer.

18. The container of claim 17, wherein the elongate beams comprise cantilevered top edges which extend to the third elevation of the base.

19. The container of claim 17 or 18, wherein the third elevation defines a floor plane on which articles can be placed, optionally wherein the upper layer and top edges of the first set of ribs are vertically recessed relative to the elongate beams.

20. The container of any one of claims 15 to 19, wherein the upper layer extends between a first pair of opposing side walls of the side wall arrangement, and the elongate beams extend between a second pair of opposing side walls of the side wall arrangement, each different than the first pair of opposing side walls.

21. The container of any preceding claim, wherein the base comprises lateral side members connecting lateral sides of the upper layer to the at least one lower layer.

22. The container of claim 21, wherein the plurality of ribs comprise underside ribs located beneath the upper layer, which extend non-parallel to the lateral side members and connect to inner surfaces of the lateral side members to provide intermediate bracing of the lateral side members.

23. A container as claimed in each of claims 1 to 22, further wherein the upper layer defines an hourglass shape, wherein the first set of ribs at the first offset region consists of four rows of ribs, wherein the first set of ribs at the second offset region consists of four rows of ribs, and wherein the second set of ribs consists of six rows of transversely-extending ribs per side of the centreline of the base.

24. According to various, but not necessarily all, embodiments of the invention there is provided a container comprising a base and a side wall arrangement peripheral to the base, wherein the base comprises single-layer structures and a double-layer structure, wherein the double-layer structure is located between the single-layer structures in plan view, wherein the base comprises a lower layer defining the single-layer structure and one of the layers of the double-layer structure, and wherein the double-layer structure further comprises an upper layer above the lower layer.

25. According to various, but not necessarily all, embodiments of the invention there is provided a container comprising a base and a side wall arrangement peripheral to the base,wherein the base comprises at least one lower layer at a first elevation,5 and an upper layer at a second elevation above the first elevation,wherein the at least one lower layer extends in a first offset region laterally offset to a first side of the upper layer, and extends in a second offset region laterally offset to a second opposite side of the upper layer, such that the upper layer is located between the first and second offset regions of the at least10 one lower layer, andwherein the upper layer extends through a centreline of the base, and widens with increasing distance from the centreline of the base towards respective sides of the base.A

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