Patent Description:
In the packaging industry, there is a general need to make disposable packaging of recyclable material. Preferably, the packaging is made of a single recyclable material, such as PET or a polyolefin. There is also a general need to reduce the mass of thermoplastic material used for any given packaging article. The reduced mass not only reduces the cost of manufacturing the packaging article, but also reduces the volume of material to be recycled.

Efforts have been made in the industry to provide various disposable packaging articles. For example, <CIT>, <CIT> and <CIT> disclose the manufacture of a lightweight polypropylene cup which has expanded regions comprising a cellular foam.

<CIT> discloses a tray having the features of the pre-characterising portion of claim <NUM>.

However, despite this prior art disclosure there is still a need to produce foamed plastic articles, in particular trays for foodstuffs, which have even smaller wall thickness and even lower mass, yet high stiffness, as well as low cost. The tray is required to have a desired precise geometrical shape, yet nevertheless have high wall strength, and preferably is composed of a single recyclable material.

The present invention aims at least partially to overcome the problem of achieving even smaller wall thickness and even lower mass, a desired precise geometrical shape, yet nevertheless high stiffness and wall strength, in articles composed of a single recyclable material.

The present invention provides a tray according to claim <NUM>.

The present invention provides a method of forming a tray according to claim <NUM>.

Preferred features of the tray and method are defined in the respective dependent claims.

The drawings are not necessarily to scale, and some dimensions may be exaggerated for the purpose of clarity of illustration.

Referring to <FIG>, there is shown a tray in accordance with an embodiment of the present invention.

The tray <NUM> comprises an annular sidewall <NUM> having upper and lower annular peripheral edges <NUM>, <NUM>. In this specification, the term "annular" means "generally ring-like", is not limited to geometrically circular shapes, and encompasses shapes that may be circular or other than circular, for example elliptical, polygonal, etc.. In the illustrated embodiment, the tray <NUM> is rectangular in plan, but any other shape, which may be polygonal or rounded, may be used.

An upper annular peripheral rim <NUM> is integral with the upper annular peripheral edge <NUM>.

A base <NUM> comprises an annular peripheral edge <NUM> which is integral with the lower annular peripheral edge <NUM> of the sidewall <NUM>.

The rim <NUM>, the sidewall <NUM> and the base <NUM> are composed of a thermoplastic polymer.

In the preferred embodiments of the present invention, the thermoplastic polymer may comprise a polyolefin or blend of a plurality of polyolefins, optionally polyethylene or polypropylene; or a polyester, optionally polyethylene terephthalate or polybutylene terephthalate; or polylactic acid. In a particularly preferred embodiment, the polymer comprises polypropylene. Polypropylenes having a Melt Flow Index (MFI) of from <NUM> to <NUM> are most particularly preferred. The Melt Flow Index of a polymer can be measured according to ASTM D1238.

The sidewall <NUM> and the base <NUM> define a central concavity <NUM> for packaging a product, such as a foodstuff (not shown). Typically, when packaging a foodstuff a sealing film (not shown) is sealed to the upper surface <NUM> of the rim <NUM>.

The base <NUM> comprises a lattice structure <NUM> of elongate foam ribs <NUM> interconnected by foam junctions 24a, 24b. Each foam rib <NUM> has opposite ends 26a, 26b. Each end 26a, 26b is integrally connected to either the annular peripheral edge <NUM> of the base <NUM> or one of the foam junctions 24a, 24b.

As shown in <FIG>, each foam rib <NUM> and each foam junction 24a, 24b comprises opposed upper and lower unfoamed solid skins 25a, 25b of the thermoplastic polymer on opposite sides of a central cellular foam core <NUM> composed of an expanded cellular foam <NUM> of the thermoplastic polymer.

Each foam rib <NUM> comprises a convex solid skin which is preferably continuously convexly curved between opposite elongate longitudinal edges <NUM> of the elongate foam rib <NUM>. The convex solid skin may comprise either or both of the upper and lower unfoamed solid skins 25a, 25b. In the illustrated embodiment, the upper solid skin 25a is convex, and the lower solid skin 25b is convex or substantially planar. In alternative embodiments, the lower solid skin 25b may be convex, and the upper solid skin 25a may be convex or substantially planar.

The foam junctions 24a, 24b comprise at least one primary foam junction 24a. Each primary foam junction 24a comprises an annular foam joint <NUM> having outer and inner peripheral edges <NUM>, <NUM>. The foam joint <NUM> comprises the opposed upper and lower unfoamed solid skins 25a, 25b of the thermoplastic polymer on opposite sides of the central cellular foam core <NUM> of the thermoplastic polymer. A plurality of the ends 26a, of respective foam ribs <NUM> intersect with the outer peripheral edge <NUM> of the foam joint <NUM>. The inner peripheral edge <NUM> of the foam joint <NUM> circumferentially surrounds a central area <NUM> of unfoamed thermoplastic polymer which is significantly thinner than the foamed thermoplastic polymer in the foam ribs <NUM> and the foam junctions 24a, 24b.

In the preferred embodiment, as illustrated the base <NUM> comprises a primary foam junction 24a at a centre C of the base <NUM>.

The base <NUM> preferably comprises an injection moulding sprue <NUM> which comprises unfoamed thermoplastic polymer and is circumferentially surrounded by a region <NUM> of unfoamed thermoplastic polymer adjacent thereto. In the illustrated embodiment this is provided by the central area <NUM> of unfoamed thermoplastic polymer in the primary foam junction 24a. The injection moulding sprue <NUM> may be located at the outer surface <NUM> or inner surface <NUM> of the base <NUM>.

Preferably, the primary foam junction 24a at the centre C of the base <NUM> is aligned with the injection moulding sprue <NUM> in the base <NUM>.

Additionally, as illustrated, the foam junctions 24a, 24b comprise at least one second foam junction 24b. Each second foam junction 24b comprises a plurality of the ends 26a of respective foam ribs <NUM>, wherein the ends 26a intersect and surround a central region <NUM> of unfoamed thermoplastic polymer which is significantly thinner than the foamed thermoplastic polymer in the foam ribs <NUM> and the foam junctions 24a, 24b. In the preferred embodiment, as illustrated the base <NUM> comprises a plurality of the second foam junctions 24b which surround, and are spaced from, the centre C of the base <NUM>.

In alternative embodiments, only primary foam junctions 24a or only second foam junctions 24b are provided. Any combination of primary and second foam junctions 24a, 24b may be provided.

The lattice structure <NUM> is polygonal and the lateral spacings <NUM> between the ribs <NUM> are polygonal. Typically, as illustrated, the lattice structure <NUM> is rectangular and the lateral spacings <NUM> between the ribs <NUM> are rectangular, for example when surrounded only by four ribs <NUM>, or triangular, or example when surrounded by two ribs <NUM> and a portion of the annular peripheral edge <NUM> of the base <NUM>.

In a first preferred embodiment of the present invention as shown in <FIG>, the base <NUM> further comprise a base wall <NUM> of unfoamed thermoplastic polymer which fills the lateral spacings <NUM> between the ribs <NUM> and is integral with the lattice structure <NUM> to form a continuous base surface <NUM> of the tray <NUM>. The base wall <NUM> is significantly thinner than the foamed thermoplastic polymer in the foam ribs <NUM> and the foam junctions 24a, 24b. The lattice structure <NUM> therefore comprises a closed framework <NUM> in which parts of the base wall <NUM> fill the lateral spacings <NUM> between the ribs <NUM> and between the ribs <NUM> and the annular peripheral edge <NUM> of the base <NUM>. In the illustrated embodiment, a lower surface <NUM> of the base wall <NUM> is aligned with a lower surface <NUM> of the lower unfoamed solid skins 25b of the foam ribs <NUM> and foam junctions 24a, 24b.

In some embodiments, the annular peripheral edge <NUM> of the base <NUM> consists of unfoamed thermoplastic polymer, which is significantly thinner than the foamed thermoplastic polymer in the foam ribs <NUM> and the foam junctions 24a, 24b. Alternatively, the annular peripheral edge <NUM> of the base <NUM> may consist of foamed thermoplastic polymer, which foam is produced at the same time and in the same manner as producing the foam ribs <NUM> and foam junctions 24a, 24b.

The annular sidewall <NUM> may consist of unfoamed thermoplastic polymer, which is significantly thinner than the foamed thermoplastic polymer in the foam ribs <NUM> and the foam junctions 24a, 24b. Alternatively, the annular sidewall <NUM> may comprise at least one annular foam hoop reinforcement (not shown) integral with the annular sidewall <NUM>. The foam hoop reinforcement comprises, like the foam ribs, the opposed unfoamed solid skins of the thermoplastic polymer on opposite sides of on opposite sides of a central cellular foam core of the thermoplastic polymer.

In some embodiments, the upper annular peripheral rim <NUM> consists of unfoamed thermoplastic polymer. Alternatively, the upper annular peripheral rim <NUM> comprises an annular flange <NUM> of unfoamed thermoplastic polymer reinforced by an integral annular foam hoop reinforcement (not shown). The foam hoop reinforcement comprises opposed unfoamed solid skins of the thermoplastic polymer on opposite sides of a central cellular foam core of the thermoplastic polymer.

The method of the present invention to make the tray <NUM> of <FIG> will now be described with reference to <FIG>.

Referring in particular to <FIG>, the tray <NUM> is formed using a mould <NUM> having a first, inner, mould part <NUM> and a second, outer, mould part <NUM> for respectively moulding inner and outer surfaces of the tray <NUM>. The first and second mould parts <NUM>, <NUM> have respective first and second cavity-forming surfaces <NUM>, <NUM> defining a mould cavity <NUM> therebetween. The mould <NUM> has an injection moulding gate <NUM>, located in the second mould part <NUM> at a geometric centre of the mould <NUM> and aligned with the centre C of the base <NUM> of the resultant tray <NUM>, through which a molten plastic composition is injected into the mould cavity <NUM>. The injection moulding gate <NUM> may alternatively be located in the first mould part <NUM> and/or at any position relative the tray <NUM> to be manufactured.

The mould <NUM> is closed as shown in <FIG> thereby defining the mould cavity <NUM> connected to the gate <NUM>. The mould cavity <NUM> is shaped and dimensioned to mould an intermediate article <NUM> as shown in <FIG>.

The first and second cavity-forming surfaces <NUM>, <NUM> define first portions of the mould cavity <NUM> which are to mould those areas in the tray <NUM> that are to comprise unfoamed thermoplastic polymer throughout the entire wall thickness, such as the base wall <NUM> and all of, or areas of, the sidewall <NUM> and rim <NUM> that are to be formed of unfoamed thermoplastic polymer, as described above. These first portions are defined between the first and second cavity-forming surfaces <NUM>, <NUM> which are spaced to define the final wall thickness, and the final shape and configuration, of the areas of the tray <NUM> that are to be formed of unfoamed thermoplastic polymer throughout the entire wall thickness.

A first portion <NUM> of the mould cavity <NUM> to mould the base wall <NUM>, which is consequently part of the intermediate article <NUM>, is shown in <FIG>.

The first and second cavity-forming surfaces <NUM>, <NUM> define second portions of the mould cavity <NUM> which are to mould those areas in the tray <NUM> that are to comprise a core of foamed thermoplastic polymer between opposed solid skins of foamed thermoplastic polymer, such as the foam ribs <NUM> the foam junctions 24a, 24b that are to comprise a core of foamed thermoplastic polymer, as described above. These second portions are defined between the first and second cavity-forming surfaces <NUM>, <NUM> which are spaced to define the wall thickness of the intermediate article <NUM>, but not the final wall thickness, final shape or final configuration, of the areas of the tray <NUM> that are to comprise a core of foamed thermoplastic polymer.

A second portion 84a of the mould cavity <NUM> to mould a foam rib-forming part <NUM> is shown in <FIG> and a second portion 84b of the mould cavity <NUM> to mould a foam junction-forming part <NUM> is shown in <FIG>.

As shown in <FIG>, the first and second cavity-forming surfaces <NUM>, <NUM> have respective first and second regions <NUM>, <NUM> for moulding the base <NUM> of the tray <NUM>.

As schematically indicated in <FIG> also shows the second, outer, mould <NUM>, the mould cavity <NUM> defines an annular peripheral rim-forming portion <NUM>, an annular sidewall-forming portion <NUM> which is adjacent to the rim-forming portion <NUM>, and a base-forming portion <NUM> which is adjacent to the sidewall-forming portion <NUM>. The first region <NUM> of the first cavity-forming surface <NUM> comprises a lattice-forming portion <NUM> comprising a latticed network <NUM> of concavities <NUM> in the first cavity-forming surface <NUM>.

A molten plastic composition <NUM>, comprising a thermoplastic polymer and a physical blowing agent, is injected into the cavity <NUM> through the gate <NUM> at an injection pressure. The physical blowing agent is a gas dissolved in the polymer.

Blowing agents which can be used in the embodiments of the present invention include physical blowing agents in the form of a gas dissolved in the molten plastic composition. Such a gas may comprise, for example, carbon dioxide. The gas may optionally further include a perfume composition (i.e. a scent) which remains present in the polymer material after expansion, to enhance the consumer experience.

When using carbon dioxide as the blowing agent, CO<NUM> gas is produced by the blowing agent in the extruder of the injection moulding machine, and the CO<NUM> gas then goes into solution during the injection phase (typically from <NUM> to <NUM> bar within the mould cavity) due to the relatively high pressure exerted on the material being greater than the pressure required (typically greater than <NUM> bar) to force CO<NUM> into solution within molten thermoplastic resin, such as polypropylene.

The molten plastic composition is injected at an injection pressure Pinjection. Typically, the injection pressure Pinjection is at least <NUM> bar. At the end of the injecting step, optionally a packing pressure, Ppacking, is applied to the cavity. Typically, packing pressure Ppacking is at least <NUM> bar.

During the injecting step, and any packing, the injection pressure Pinjection, and any packing pressure Ppacking, respectively, are above a minimum pressure threshold, Pthreshold, in the regions of the cavity <NUM> to form unfoamed parts of the tray <NUM>, such as the base wall <NUM>. Typically, the minimum pressure threshold Pthreshold is <NUM> bar. This prevents the physical blowing agent from coming partly out of solution in the polymer so that cellular gas bubbles are not formed in those region during the injecting step, and any packing step.

During or after the injecting step, the injected plastic composition <NUM> in contact with the first and second cavity-forming surfaces <NUM>, <NUM> is cooled to form first and second solid skins 106a, 106b respectively adjacent to and in contact with the first and second cavity-forming surfaces <NUM>, <NUM>. In at least the latticed network <NUM> of concavities <NUM> is located a lattice-forming part <NUM> of the plastic composition <NUM>. In the lattice-forming part <NUM> at least some of the plastic composition <NUM> between the first and second solid skins 106a, 106b remains molten.

The lattice-forming part <NUM> defines a structure of elongate rib-forming parts <NUM> interconnected by junction-forming parts <NUM>. Each rib-forming part <NUM> has opposite ends 112a, 112b, and each end 112a, 112b is integrally connected to either an annular outer peripheral edge-forming part <NUM> of the base-forming part <NUM> or one of the junction-forming parts <NUM>. Each rib-forming part <NUM> and each junction-forming part <NUM> comprise opposed first and second solid skins 106a, 106b on opposite sides of a central core <NUM> of the molten plastic composition <NUM>.

In the parts of the mould cavity <NUM> that are to mould unfoamed areas in the tray <NUM>, such as the base wall <NUM>, these unfoamed areas are moulded in the intermediate article <NUM>, which is achieved by the molten plastic composition <NUM> solidifying to form a solid wall of unexpended thermoplastic polymer throughout its thickness.

The junction-forming parts <NUM> are configured so that, in the tray <NUM>, the foam junctions comprise at least one primary foam junction 24a and/or second foam junction 24b as described above. The central area <NUM> and the central region <NUM> of unfoamed thermoplastic polymer are formed between the first and second cavity-forming surfaces <NUM>, <NUM> prior to the opening step. The primary foam junction 24a at the centre C of the base <NUM> is aligned with the injection gate <NUM> in the injecting step.

In the injecting step, the molten plastic composition <NUM> is injected through the injection moulding sprue <NUM> in the base-forming portion <NUM>. The base-forming portion <NUM> is configured to space the first and second cavity-forming surfaces <NUM>, <NUM> from each other so that in the tray <NUM> the injection moulding sprue <NUM> comprises unfoamed thermoplastic polymer and is circumferentially surrounded by the region <NUM> of unfoamed thermoplastic polymer adjacent thereto. The first and second cavity-forming surfaces <NUM>, <NUM> are spaced from each other in the base-forming portion <NUM> to form a base wall-forming portion <NUM> of the mould <NUM>, and in the injecting step the solid base wall <NUM> of unfoamed thermoplastic polymer is formed which fills the lateral spacings <NUM> between the ribs <NUM> and is integral with the lattice structure <NUM> to form the continuous base surface <NUM> of the tray <NUM>.

After the unfoamed areas in the tray <NUM> have been formed by solidifying the molten plastic composition <NUM> in those areas, the mould <NUM> is opened before the molten plastic composition <NUM> in the central core <NUM> between the first and second solid skins 106a, 106b has solidified in the lattice-forming part <NUM>.

The opening of the mould <NUM> is achieved by removing the first mould part <NUM> as shown by arrow F in <FIG> so that the first solid skin 106a is no longer in contact with the first cavity-forming surface <NUM>. In the illustrated embodiment the first and second mould parts <NUM>, <NUM> are, respectively, outer and inner mould parts which respectively form outer and inner surfaces of the tray <NUM>, and the inner mould part is removed. Alternatively, the first and second mould parts <NUM>, <NUM> are, respectively, inner and outer mould parts which respectively form inner and outer surfaces of the tray <NUM>, , and the outer mould part is removed.

The opening of the mould <NUM> causes the intermediate article <NUM> to be transitioned into the final tray <NUM>. In particular, foam-forming parts of the intermediate article <NUM> are transitioned into foam parts in the tray <NUM>.

The removal of the first mould part <NUM> exposes the molten plastic composition <NUM> of the lattice-forming part <NUM> to an external pressure lower than the injection pressure, for example atmospheric pressure. Such a lowering of pressure allows the molten plastic composition <NUM> between the first and second solid skins 106a, 106b of the lattice-forming part <NUM> to expand by foaming to form the expanded cellular foam <NUM>, as shown in <FIG>, as a result of the molten plastic composition <NUM> beneath the first solid skin 106a expanding away from the second solid skin 106b. The expansion occurs because the blowing agent comes out of solution in the molten plastic composition at the reduced pressure and forms gas bubbles.

During the opening step, at least some of the molten plastic composition <NUM> in the central core <NUM> is exposed to an external pressure lower than the minimum pressure threshold, for example atmospheric pressure, to permit the blowing agent to come out of solution and form gas bubbles within the molten plastic composition <NUM> in the central core <NUM>. This action forms in the tray <NUM> the expanded cellular foam <NUM> comprising the core layer <NUM> of expanded cellular foam <NUM> between the first and second solid skins 25a, 25b, formed from the plastic composition <NUM>.

The lattice-forming part <NUM> in the mould cavity <NUM> has sufficient thickness, and/or the processing time is so short, that the molten polymer resin in the central core <NUM> does not solidify during the injection step, and any subsequent packing. Also, the lattice-forming part <NUM> can be additionally heated by an external heater to maintain the plastic composition <NUM> in the central core <NUM> in a molten liquid phase. The second mould part <NUM> may be cooled by a cooling system, for example by a flow of cooling fluid therethrough, to maintain the second mould part <NUM> at a lower temperature than the first mould part <NUM>. Such temperature control can control the absolute and relative thickness of the central core <NUM> and the first and second solid skins 106a, 106b, so that as described herein the desired expansion of the central core <NUM> and deformation and/or stretching of the first solid skin 106a is achieved.

Prior to the opening step, the rib-forming parts <NUM> and the junction-forming parts <NUM> comprise opposed first and second solid skins 106a, 106b on opposite sides of the central core <NUM> of the molten plastic composition <NUM>. During foam expansion, the first solid skin 106a is deformed away from the second solid skin 106b. This deformation may be achieved by stretching the first solid skin 106a so that in the tray <NUM> the first solid skin 25a is longer than the first solid skin 106a in the intermediate article <NUM>.

In the opening step, in each foam rib <NUM> the first solid skin 25a is expanded to form a convexly curved surface, and preferably the convexly curved surface is continuously convexly curved between opposite elongate longitudinal edges of the elongate foam rib <NUM>.

As shown in <FIG>, <FIG>, the first solid skin 106a of the rib-forming parts <NUM> and the junction-forming parts <NUM> for form the primary foam junctions 24a may be moulded by the first moulding surface <NUM> to provide a concave recess <NUM>, for example a groove which typically extends along the length of the rib-forming parts <NUM> and around the circumference of the junction-forming parts <NUM>, centrally located in the moulded surface <NUM> of the rib-forming parts <NUM> and the junction-forming parts <NUM>. Accordingly, the first solid skin 106a of the rib-forming parts <NUM> and the junction-forming parts <NUM> has a profiled upper moulded surface <NUM>.

When such a profiled upper moulded surface <NUM> incorporating the concave recess <NUM> is provided, additionally or alternatively to stretching, the first solid skin 106a of the rib-forming parts <NUM> and the junction-forming parts <NUM> is popped outwardly by the expanding gas pressure and the concave recess <NUM> in the intermediate article <NUM> is transitioned into a convex surface in the foam ribs <NUM> and foam junctions 24a in the tray <NUM>.

Typically, after the cooling step, the length of the first solid skin 25a in the tray <NUM> has stretched, as compared to the first solid skin 106a present prior to the opening step in the intermediate article <NUM>, by a stretch ratio of from <NUM> to up to <NUM>%. The stretch ratio is the ratio of the increase in the length of the first solid skin 25a after the cooling step based on length of the first solid skin 106a before the opening step. For example an increase in length of the first solid skin from an initial value of <NUM> to a final value of <NUM> would represent a stretch ratio of <NUM>%. Preferably the stretch ratio is from <NUM> to <NUM>%, more preferably from <NUM> to <NUM>%, still more preferably from <NUM> to <NUM>%, for example about <NUM>%. However, alternatively the first solid skin 25a is not stretched.

For the unexpanded regions to be formed in the tray <NUM>, the mould <NUM> is shaped and dimensioned so that a narrow region <NUM> of the cavity <NUM> is formed between the first and second cavity-forming surfaces <NUM>, <NUM>. During the injecting step, and optional packing, the injection pressure, the optional packing pressure, are maintained above the minimum pressure threshold in the narrow region <NUM> of the cavity <NUM> to maintain the physical blowing agent as a gas dissolved in the molten plastic composition <NUM> so that substantially no gas bubbles are formed in the narrow region <NUM> of the cavity <NUM>. Prior to the opening step, the plastic composition <NUM> in the narrow region <NUM> of the cavity <NUM> is cooled so as to be fully solidified, to form in the tray <NUM> at least one the unexpanded region comprising a substantially homogeneous, solid phase, unexpanded thermoplastic polymer.

This narrow region <NUM> of the mould cavity <NUM> is thin, and so the molten polymer resin requires a relatively short time period, shorter than the injection step, and the optional packing, to cool and solidify. Also, this narrow region <NUM> can be additionally cooled by an external cooler to transition the polymer resin from the molten liquid phase into a solid phase. After the opening step, the solid plastic composition cannot expand further by foaming, and cannot form an expanded cellular foam. Therefore the unexpanded region appears transparent to the naked eye.

The opening step comprises removing the first mould part <NUM> so that the first solid skin 106a is no longer in contact with the first cavity-forming surface <NUM>, while maintaining the second solid skin 106b in contact with the second cavity-forming surface <NUM>. In the illustrated embodiment, this opening is achieved by removing the inner mould part <NUM>, exposing the first solid skin 106a to atmospheric pressure and leaving the second skin 106b on the inner mould part <NUM>.

However any other configuration to open the mould may be used. In particular, in an alternative embodiment at least one or more portions of the outer mould part <NUM> may be removed from the second solid skin 106b so that the second solid skin 106b, or any part thereof, is additionally or alternatively exposed to atmospheric pressure. In other words, the foam expansion may be oriented towards the inner surface of the tray <NUM> or towards the outer surface of the tray <NUM>.

Thereafter, the expanded cellular foam <NUM> is cooled to cause the molten plastic composition <NUM> between the first and second solid skins 106a, 106b of the lattice-forming part <NUM> to solidify and to form in the base <NUM> of the tray <NUM> the lattice structure <NUM> of elongate foam ribs <NUM> interconnected by foam junctions 24a, 24b. The cooling may be carried out passively in the ambient atmosphere, or by active cooling, for example by blowing cool air onto the tray <NUM>.

In the intermediate article <NUM> the rib-forming parts <NUM> and the junction-forming parts <NUM> typically have a thickness of from <NUM> to <NUM>, optionally from <NUM> to <NUM>. In the tray <NUM> the expanded foam ribs <NUM> and foam junctions 24a, 24b typically have a thickness of from <NUM> to <NUM>, optionally from <NUM> to <NUM>. Typically, rib-forming parts <NUM> and the junction-forming parts <NUM> have increased in thickness by from <NUM> to <NUM> to form the expanded foam ribs <NUM> and foam junctions 24a, 24b respectively.

The unexpanded regions typically have a thickness of from <NUM> to <NUM>, optionally from <NUM> to <NUM>.

The expanded foam ribs <NUM> and foam junctions 24a, 24b typically appear translucent to the naked eye because the expanded cellular foam includes cells that have cellular walls that reflect visible light. However, if a pigment is incorporated into the thermoplastic polymer at a high concentration, the foam ribs <NUM> and foam junctions 24a, 24b may typically appear opaque, with a solid colour. In contrast, the unexpanded regions has no cells, or if any cells are present, for example at a low concentration, they have a cell size of typically less than <NUM> microns and therefore are not visible to the naked eye, and consequently the unexpanded regions appears transparent to the naked eye. The unexpanded regions appear transparent to the naked eye, since the blowing agent, in this embodiment CO<NUM> gas, has stayed in solution in the polymer, in this embodiment polypropylene, during manufacture of the article. After the molten polymer has solidified, it is not possible for cells to form as a result of any action of the blowing agent.

Referring to <FIG>, there is shown a second preferred embodiment of the present invention. In the second embodiment, there are two essential differences as compared to the first embodiment: (i) instead of the lattice structure of the base being a closed framework with a base wall connecting the ribs and junctions of the lattice to form a continuous base surface of the tray, instead the lattice structure <NUM> is an open framework <NUM> with a plurality of openings <NUM>, with each opening <NUM> being surrounded by a plurality of the elongate foam ribs <NUM>, or by at least one of the elongate foam ribs <NUM> and a portion of the lower annular peripheral edge <NUM> of the base <NUM>, and (ii) the base <NUM> further comprises a flexible sheet <NUM> which is bonded to an upper or lower surface <NUM> of the open framework lattice structure <NUM> and to the annular peripheral edge <NUM> of the base <NUM> to cover the openings <NUM> and form a sealed base surface <NUM> of the tray, and the flexible sheet <NUM> can be incorporated as an in-mould label (IML) using IML technology.

Typically, the flexible sheet <NUM> is composed of an unfoamed thermoplastic polymer comprising a single polymer layer or a multilayer laminate. Alternatively, the flexible sheet <NUM> is composed of a foamed thermoplastic polymer comprising opposed unfoamed solid skins of the thermoplastic polymer on opposite sides of a central cellular foam core of the thermoplastic polymer. The flexible sheet <NUM> may alternatively be composed of any suitable sheet material for use in packaging, for example cardboard, metal or metallised plastics material.

In one arrangement, as illustrated, the flexible sheet <NUM> is bonded to a lower surface <NUM> of the lattice structure <NUM> and to a lower surface <NUM> of the annular peripheral edge <NUM> of the base <NUM>. The sprue <NUM> may pass through a hole <NUM> in the flexible sheet <NUM> when the sprue <NUM>, and during injection moulding the injection gate <NUM>, and the flexible sheet <NUM> are on the same side of the tray <NUM>, i.e. on the inner side <NUM> or the outer side <NUM> of the base <NUM> of the tray <NUM>. Alternatively, the sprue <NUM>, and during injection moulding the injection gate <NUM>, may be located on the outer or inner side of the tray <NUM> and the flexible sheet <NUM> may be located on the other side of the tray <NUM>.

In another arrangement, the flexible sheet <NUM> is bonded to an upper surface of the lattice structure and to an upper surface of the annular peripheral edge of the base.

Typically, the flexible sheet <NUM> is additionally bonded to a surface of the sidewall <NUM> which surrounds the base <NUM> of the tray <NUM>.

In the second preferred embodiment, each opening <NUM> is covered by a respective portion <NUM> of the flexible sheet <NUM>. Each portion <NUM> is typically slack and thereby not under tension, which is because the lattice structure <NUM> tends to shrink upon cooling whereas the flexible sheet <NUM> tends to exhibit lower shrinkage than the lattice structure <NUM>.

In a preferred tray structure, the flexible sheet <NUM> has an outer peripheral edge (not shown) which is sealed within the thickness of the sidewall <NUM> or base <NUM>. Preferably, the outer peripheral edge of the flexible sheet <NUM> is sealed within a central cellular foam core of the sidewall <NUM> or base <NUM>. Such a sealing structure means that if the flexible sheet <NUM> comprises a foam layer, or a recycled polymer layer, the foam or recycled polymer layer is prevented from coming into direct contact with the contents, e.g. foodstuffs, which may be packed within the tray <NUM>.

Preferably, at least one or both of an outer surface <NUM> and an inner surface <NUM> of the flexible sheet <NUM> is printed whereby the sheet <NUM> functions as a label. The flexible sheet <NUM> can be applied using an in-mould label (IML) process to provide structure to the tray <NUM> and the function of a label, providing printed information on either or both of the outer and inner surfaces of the tray <NUM>.

In the second embodiment, the injecting and opening steps are carried out as for the first embodiment.

However, the mould <NUM> is configured to form the openings <NUM> and to incorporate an in-mould labelling function to bond the flexible sheet <NUM> to the open lattice structure <NUM> of the tray <NUM>.

The method is modified as compared to the method described with respect to the first embodiment by further comprising the step, before the injecting step from an injector nozzle, of providing an in-mould label <NUM> in the form of the flexible sheet <NUM> at least partly surrounding the cavity <NUM>. The in-mould label <NUM> is located adjacent to the second cavity-forming surface <NUM>.

In the mould closing step, the first and second cavity-forming surfaces <NUM>, <NUM> are compressed together in the base-forming portion <NUM> to form an opening-forming portion <NUM> of the mould. Before the injecting step, the in-mould flexible sheet <NUM> is provided in the cavity <NUM>. The in-mould flexible sheet <NUM> is located, as illustrated, adjacent to the second cavity-forming surface <NUM> at a location opposite the lattice-forming portion <NUM> of the first cavity-forming surface <NUM>. Alternatively, the in-mould flexible sheet <NUM> may be located adjacent to the lattice-forming portion <NUM> of the first cavity-forming surface <NUM>.

Thus the in-mould flexible sheet <NUM> is located in the opening-forming portion <NUM> of the mould <NUM> and compressed between the first and second cavity-forming surfaces <NUM>, <NUM>.

During the injecting step, in the regions of the mould cavity <NUM> to form the openings <NUM> of the open framework <NUM> of the lattice structure <NUM>, the flexible film <NUM> is squeezed between the first and second cavity-forming surfaces <NUM>, <NUM> so that the molten plastic composition is not injected between the first and second cavity-forming surfaces <NUM>, <NUM> so that the openings <NUM> are formed, with the portion <NUM> of the flexible sheet <NUM> covering the opening <NUM>. During the injecting step, the flexible sheet <NUM> is bonded to the first or second solid skin 25a, 25b of the lattice structure <NUM>, and bonded to the annular peripheral edge <NUM> of the base <NUM> to cover the openings <NUM> and form a sealed base surface <NUM> of the tray <NUM>.

Prior to the mould closing step, the in-mould flexible sheet <NUM> may be located adjacent to the outer mould part <NUM> of the first and second mould parts <NUM>, <NUM> which forms an outer surface <NUM> of the tray <NUM>. In the resultant tray <NUM>, the flexible sheet <NUM> is bonded to the lower surface <NUM> of the lattice structure <NUM> and to the lower surface <NUM> of the annular peripheral edge <NUM> of the base <NUM> of the tray <NUM>. The flexible sheet <NUM> may be additionally bonded to an outer peripheral surface <NUM> of the sidewall <NUM> which surrounds the base <NUM> of the tray <NUM>.

In an alternative embodiment, prior to the closing step the in-mould flexible sheet <NUM> may be located adjacent to the inner mould part <NUM> of the first and second mould parts <NUM>, <NUM> which forms an inner surface <NUM> of the tray <NUM>. The flexible sheet <NUM> is bonded to an upper surface of the lattice structure <NUM> and to an upper surface of the annular peripheral edge <NUM> of the base <NUM> of the tray <NUM>.

As described above, the flexible sheet <NUM> has an outer peripheral edge which may be sealed within the thickness of the sidewall <NUM> or base <NUM>.

In some embodiments of the present invention, the mechanical properties and dimensions of the in-mould label <NUM> can be selected to control the stretch ratio of the solid skin of the tray <NUM> to which the in-mould label <NUM> is bonded. In this way, the in-mould label <NUM> can function to control the shape, dimensions and properties of the tray <NUM>.

During the expansion of the first solid skin 106a to form first solid skin 25a in the tray <NUM>, the second solid skin 106b may remain fully in contact with the second cavity-forming surface <NUM> with the result that the shape and configuration of the second solid skin 25b in the tray <NUM> corresponds to the shape and configuration of the second cavity-forming surface <NUM>. This shaping of a planar second skin 25b is shown in <FIG>, and in <FIG>.

However, in many embodiments the expansion force from the blowing agent causes the second solid skin 106b in the foam ribs <NUM> and foam junctions 24a, 24b to be urged away from the second cavity-forming surface <NUM>, in particular at the edges of the foam ribs <NUM> and foam junctions 24a, 24b.

Therefore, although the surface of the foam ribs <NUM> and junctions 24a, 24b formed on the second mould side are illustrated as being planar in <FIG>, and <FIG>, corresponding to the second cavity-forming surface <NUM>, in practice the surface of the second skin 25b tends to exhibit some convexity as a result of the edges of the surface being urged away from the mould surface under the action of the expansion force from the blowing agent. <FIG> shown an embodiment in which both opposed surfaces of the rib <NUM> are convex. In <FIG>, the upper surface 223a comprises the first solid skin 225a and the lower surface 223b comprises the second solid skin 225b.

Moreover, although the expanded cellular foam <NUM> shown in <FIG>, and <FIG>, has a continuous composition throughout the cross-section of the rib <NUM> and junctions 24a, 24b, in many embodiments the expansion force from the blowing agent causes at least one enlarged central cavity to be formed within the rib <NUM> and junctions 24a, 24b. Such a morphology is shown in <FIG>. It may be seen in <FIG> that the rib <NUM> has two central cavities <NUM>, <NUM> separated by a central web <NUM>. The two central cavities <NUM>, <NUM> are surrounded by a layer of expanded cellular foam <NUM> and the central web <NUM> is composed of the expanded cellular foam <NUM>. The two central cavities <NUM>, <NUM> are formed on opposite sides of the concave recess <NUM>, for example the groove, which extends along the length of the rib-forming part <NUM>. The expanded cellular foam <NUM> therefore has a cross-section similar in shape to a figure-of-<NUM>. It is believed that any enlarged cavity formed in the foam rib or foam junction is a result of expanded cells coalescing during the expansion or cooling processes while the thermoplastic polymer is still flowable at an elevated temperature and the blowing agent exerts an expansion pressure on the thermoplastic polymer. The presence of a hollow rib or hollow junction can increase the mechanical properties of the rib or junction while minimizing material weight.

In the preferred embodiments of the present invention, the tray may be designed or configured to package foodstuffs, but the tray may be used for any purpose. The tray may have heat resistance and may be suitable for warming foodstuff in an oven or microwave oven. The tray may be disposable or reusable, and in either case is recyclable since the tray is preferably composed of a single polymer, for example polypropylene.

Claim 1:
A tray (<NUM>) comprising: an annular sidewall (<NUM>) having upper and lower annular peripheral edges (<NUM>, <NUM>), an upper annular peripheral rim (<NUM>) which is integral with the upper annular peripheral edge (<NUM>), and a base (<NUM>) which comprises an annular peripheral edge (<NUM>) which is integral with the lower annular peripheral edge (<NUM>) of the sidewall (<NUM>), wherein the rim (<NUM>), the sidewall (<NUM>) and the base (<NUM>) are composed of a thermoplastic polymer and the sidewall (<NUM>) and the base (<NUM>) define a central concavity (<NUM>) for packaging a product, wherein the base (<NUM>) comprises a lattice structure (<NUM>, <NUM>) of elongate foam ribs (<NUM>) interconnected by foam junctions (24a, 24b), wherein each foam rib (<NUM>) has opposite ends (26a, 26b), and each end (26a, 26b) is integrally connected to either the annular peripheral edge (<NUM>) of the base (<NUM>) or one of the foam junctions (24a, 24b), characterised in that each foam rib (<NUM>) and each foam junction (24a, 24b) comprise opposed upper and lower unfoamed solid skins (25a, 25b) of the thermoplastic polymer on opposite sides of a central cellular foam core (<NUM>) of the thermoplastic polymer.