Method of forming an article

Abstract

A method of forming an article of articles, the method comprising: (a) providing a mold having a first mold component and a second mold component, the first mold component and the second mold component having a first cavity forming surface and a second cavity forming surface, respectively; (b) closing the mold to define a cavity between the first cavity forming surface and the second cavity forming surface; (c) injecting a molten plastic composition comprising a polymer and a physical foaming agent into the cavity, wherein during or after the injection step (c), the injected plastic composition in contact with the first cavity forming surface and the second cavity forming surface is cooled to form a first solid skin and a second solid skin, respectively adjacent to and in contact with the first cavity forming surface and the second cavity forming surface, whereby at least a portion of the plastic composition is positioned in at least one region of the cavity, wherein at least some of the plastic composition between the first solid skin and the second solid skin remains molten, wherein the thickness of the respective portion is constant within a tolerance of + / - 0.5%, preferably + / - 0.2%, based on the nominal thickness of the respective portion; and (d) opening the mold before the molten plastic composition between the first solid skin and the second solid skin solidifies in at least a portion. The tool is configured to expose the molten plastic composition of the corresponding portion to an external pressure lower than the injection pressure, thereby allowing the molten plastic composition between the first solid skin and the second solid skin of the corresponding portion to expand by foaming to form an expanded porous foam, as the molten plastic composition below the first solid skin expands outward away from the second solid skin, thereby stretching the first solid skin in the corresponding portion. The opening step includes removing the first mold component so that the first solid skin no longer forms surface contact with the first cavity; and (e) cooling the expanded porous foam to solidify the molten plastic composition between the first solid skin and the second solid skin of the corresponding portion and form at least one first portion in the article, the first portion comprising a porous foam core layer between the first solid skin and the second solid skin, wherein after the cooling step (e), the length of the first solid skin in the corresponding portion is stretched, with a stretch ratio from 0.5 to up to 3% compared to the first solid skin present before the opening step (d), wherein the stretch ratio is based on the ratio of the length of the first solid skin before the opening step (d) to the increase in the length of the first solid skin after the cooling step (e).

Description

Technical Field

[0001] This invention relates to the formation of foamed plastic articles (e.g., cups or containers) for use with liquids and / or food. Background Technology

[0002] In the packaging industry, a common type of disposable cup (such as takeaway coffee cups) is the paper cup, which has a plastic liner, such as low-density polyethylene (LDPE). Because these cups are made of two different materials, separating them is difficult and / or costly, making their recycling challenging. Furthermore, due to a seam along one side of the cup where the paper material holds together, liquid may leak from the seam area at the rim when the cup is tilted to drink from it (especially when the cup is used with a lid that has a spout through which the beverage is drunk).

[0003] The industry has been working to provide disposable polypropylene cups. In conventional injection molding, thicker walls are required to create some thermal insulation within the cup. Typically, porous structures provide thermal insulation, and foaming agents are added to the thermoplastic polymer to create a foam structure to further improve thermal insulation and reduce the density of the foam walls.

[0004] WO-A-2017 / 134181 discloses an article and a method of forming the article, wherein the article is a polypropylene cup having an expanded region comprising porous foam. This production method requires providing a corrugated surface on at least one molding surface defining a mold cavity to control the expansion of intermediate molten polypropylene between opposing solid skins after removal of an outer mold component with an already molded outer solid skin. The corrugated surface effectively divides the annular injection preform into a series of individually expanded arcuate zones to avoid uncontrolled circumferential expansion around the preform, resulting in an article with controlled shape and dimensions. However, the resulting ribbed structure is heterogeneous and does not have a uniform circular cross-section in the sidewalls because the outer surface of the sidewalls tends to exhibit some residual corrugation, thus the outer surface is not very smooth or precisely shaped. This known method provides a hollow article with high wall strength and low mass, however, it is not a cup with the desired very smooth surface and the desired precisely shaped geometry.

[0005] Despite the prior art disclosed above, there remains a need to produce foamed plastic articles, such as cups or containers, for liquids and / or food, having very small wall thickness and very small mass, and a very smooth surface corresponding to the desired precise geometry, while still having high wall strength, and preferably having walls that exhibit good thermal insulation properties, and consisting of a single recyclable material. Summary of the Invention

[0006] The present invention aims to at least partially overcome the problem of achieving very small wall thickness and very small mass, as well as a completely smooth surface corresponding to the desired precise geometry, in articles made of a single recyclable material, while the articles also have high wall strength and preferably have walls that exhibit good thermal insulation properties.

[0007] This invention provides a method for forming a plastic article, wherein the plastic article is a hollow container, the method comprising:

[0008] Step (a) is provided, in which a mold (20) is provided, the mold having a first mold component (22) and a second mold component (24), the first mold component (22) and the second mold component (24) having a first cavity forming surface (26) and a second cavity forming surface (28), respectively;

[0009] Closing step (b), in which the mold (20) is closed, thereby defining an annular cavity (32) between the first cavity forming surface (26) and the second cavity forming surface (28);

[0010] In injection step (c), a molten plastic composition comprising a polymer (40) and a physical foaming agent is injected into the annular cavity (32), wherein the physical foaming agent is a gas dissolved in the polymer, the molten plastic composition (40) is injected under injection pressure, wherein during or after injection step (c), the injected plastic composition in contact with the first cavity forming surface (26) and the second cavity forming surface (28) is cooled to form a first cavity forming surface adjacent to and in contact with the first cavity forming surface (26) and the second cavity forming surface (28), respectively. A solid skin (14) and a second solid skin (16) are provided, thereby correspondingly positioning at least one annular portion (52) of the plastic composition in at least one annular region (30) of the annular cavity (32), in which at least some of the plastic composition between the first solid skin (14) and the second solid skin (16) remains molten, wherein the thickness of the corresponding portion of the at least one annular portion (52) of the plastic composition is constant within a tolerance of + / - 0.5% based on the nominal thickness of the corresponding portion of the at least one annular portion (52) of the plastic composition, and wherein the first solid skin (14) and the second solid skin (16) are curved and parallel in the corresponding portion of the at least one annular portion (52) of the plastic composition;

[0011] In the opening step (d), the mold (20) is opened before the molten plastic composition between the first solid skin (14) and the second solid skin (16) solidifies in the corresponding portion of the at least one annular portion (52) of the plastic composition, so as to expose the molten plastic composition (40) of the corresponding portion to an external pressure lower than the injection pressure, thereby allowing the molten plastic composition (40) between the first solid skin (14) and the second solid skin (16) of the at least one annular portion (52) of the plastic composition to solidify. The molten plastic composition (40) expands by bubbling to form an expanded porous foam (21) as the molten plastic composition (40) below the first solid skin (14) expands outward away from the second solid skin (16), thereby stretching the first solid skin (14) in a corresponding portion of the at least one annular portion (52) of the plastic composition, wherein the opening step (d) includes removing the first mold component (22) so that the first solid skin (14) is no longer in contact with the first cavity forming surface (26); and

[0012] In a cooling step (e), the expanded porous foam (21) is cooled to solidify the molten plastic composition (40) between the first solid skin (14) and the second solid skin (16) of the corresponding portion of the at least one annular portion (52) of the plastic composition, and at least one first wall portion (10) is formed in the hollow container, the first wall portion comprising a porous foam core layer (12) between the first solid skin (14) and the second solid skin (16), wherein the hollow container has an annular sidewall (4), and the wall portion (10) is included in the annular sidewall (4) and is both continuous and annular.

[0013] Wherein, after the cooling step (e), the length of the first solid skin in the corresponding portion of the at least one annular portion (52) of the plastic composition has been stretched by a stretch ratio of 0.5% to up to 3% compared to the first solid skin (14) present before the opening step (d), wherein the stretch ratio is based on the length of the first solid skin (14) before the opening step (d) and the rate by which the length of the first solid skin (14) increases after the cooling step (e).

[0014] The method of the present invention forms articles having walls, which can have any shape, form, or function. However, the present invention has specific applications in the manufacture of hollow articles for containing liquids or food, such as cups, or any other containers or utensils.

[0015] The technical effects and advantages provided by this invention are that the articles formed by this method are easy to recycle and reuse, have a high level of stiffness provided by (one or more) expanded wall portions, and also have good thermal insulation properties provided by (one or more) expanded wall portions, but are very thin, thus allowing the production of articles of a given size, such as cups with a desired volumetric capacity, using a low-mass plastic composition, and having a very smooth surface corresponding to the desired precise geometry. The article may incorporate unexpanded or less expanded areas that can provide structural and / or aesthetic properties to the article; for example, the unexpanded or less expanded areas may be transparent or visually translucent, while (one or more) expanded wall portions may be translucent or opaque.

[0016] This invention is at least partly based on the inventors' discovery that by controlling the thickness of a plastic composition portion comprising a layer of molten plastic composition between opposing first and second solid skins, such that when this portion is subsequently exposed to depressurization to cause expansion of the molten plastic composition layer, thereby forming an expanded porous foam core layer between the first and second solid skins, the thickness is kept constant with a very low tolerance of + / - 0.5% (preferably + / - 0.2%), and by controlling the stretching of the first solid skin within the range of 0.5% to 3%, the expansion of the plastic composition in the expanded wall portion can be controlled very uniformly. This means no uncontrolled expansion; and the absence of uncontrolled expansion means that the expanded wall portion can provide a thinner, more uniform, and lighter article wall than in the prior art.

[0017] The inventors have also discovered that articles produced by the method of the present invention also possess the required level of stiffness, although the amount of material used is reduced. It has been found that when a porous foam composition (typically a thermoplastic polymer, such as a polyolefin, typically polypropylene) is slowly cooled, the crystallinity of the plastic composition increases due to its thermal insulation properties, which in turn increases the stiffness of the porous foam composition. The molten plastic composition between the first and second skins expands through foaming, also providing good thermal insulation properties to the article.

[0018] Furthermore, since the entire container can be made from a single layer of recyclable material (i.e., without layers of different materials that need to be separated), the container of the present invention is easier to recycle than commonly used plastic-lined paper cups.

[0019] Furthermore, since the article is injection molded using the method of the present invention, there are no joints in the article that could cause leakage of the liquid contained therein. For example, the sidewalls and base of the hollow container can be integrally molded. Attached Figure Description

[0020] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:

[0021] Figure 1 This is a side view of a cup formed by the method of the first embodiment of the present invention;

[0022] Figure 2 yes Figure 1 The first part of the cup is shown in a cross-sectional view along line AA, showing the appearance of the cup's expanded translucent area;

[0023] Figures 3a and 3b show the formation of [something] at different corresponding stages of the method of the present invention. Figure 1 Figure 3a shows the injection step, and Figure 3b shows the cooling step. (The text also includes a cross-sectional view of the mold along line AA for the expanded porous foam area of ​​the cup.)

[0024] Figures 4a and 4b are respectively used to form Figure 1 The cross-sectional view of the mold along line BB for the second part of the cup shows the appearance of the unexpanded transparent area of ​​the cup. Figure 4a shows the injection step, and Figure 4b shows the cooling step.

[0025] Figure 5 This is a bottom view of the base and sidewalls of a food tray formed by the method of the second embodiment of the present invention;

[0026] Figure 6 It is formed before expansion during the opening step. Figure 5 A cross-sectional view of a portion of the base and / or sidewall area of ​​a food tray;

[0027] Figure 7 It expands after the opening step, by Figure 6 The diagram shows a cross-sectional view of a portion of the base and / or sidewall area of ​​the food tray.

[0028] Figure 8 This is a cross-sectional view of a mold used to form a hollow article by in-mold labeling according to the method of the third embodiment of the present invention;

[0029] Figure 9 It is by Figure 8 A side view of a hollow product with an in-mold label produced by the mold;

[0030] Figure 10 This is a partial dashed cross-sectional side view of a third mold component of a mold combined with the second mold component used in the first embodiment, for forming a hollow article according to the method of the fourth embodiment of the present invention;

[0031] Figures 11a and 11b are schematic side cross-sectional views of the walls of the injection-molded portion at different corresponding stages in the fifth embodiment of the method of the present invention. Figure 11a shows the shape and structure of the first solid skin before the mold is opened, and Figure 11b shows the shape and structure of the first solid skin after the mold is opened and the expanded porous foam is cooled.

[0032] Figure 12 This is a schematic perspective view of the wall of the article formed by the injection molding portion of Figure 11a, showing the shape and structure of the first solid skin after the mold is opened and the expanded porous foam is cooled.

[0033] Figure 13 This is a cross-sectional view of a mold used to form a hollow article according to the method of the sixth embodiment of the present invention;

[0034] Figure 14 It is by Figure 13 A cross-sectional view of a hollow product made from a mold. Detailed Implementation

[0035] refer to Figure 1 The image shows a side view of an article formed by the method of the present invention, such as a hollow container, like a cup, or for example a coffee cup.

[0036] The cup 2 has an annular sidewall 4, which has an annular expanded foam region 6 and an annular unexpanded region 8. In this specification, the term "annular" means "generally circular," and is not limited to a geometrically circular shape, but encompasses shapes that can be circular or non-circular, such as ellipses, polygons, etc. The thickness of the expanded foam region 6 is typically 1 to 2 mm, optionally 1 to 1.5 mm. The thickness of the unexpanded region 8 is typically 0.25 to 0.75 mm, optionally 0.25 to 0.5 mm.

[0037] The expanded foam region 6 is typically translucent to the naked eye because the expanded porous foam includes pores with walls that reflect visible light. However, if pigments are incorporated into the thermoplastic polymer at high concentrations, the expanded region 6 will usually appear as an opaque solid color.

[0038] Conversely, the unexpanded region 8 has no cells, or if any cells are present, they have a cell size typically smaller than 0.5 micrometers at low concentrations, and are therefore invisible to the naked eye. Thus, the unexpanded region 8 appears transparent to the naked eye. The unexpanded region 8 is transparent because, during the manufacturing process, the foaming agent (in this example, CO2 gas) remains in the polymer (in this example, polypropylene) solution. Once the molten polymer has solidified, no action of the foaming agent is possible to form cells.

[0039] This invention relates to the reliable manufacture of an article comprising an expanded foam region 6 and an optional unexpanded region 8. In some embodiments, substantially the entire area of ​​the article may consist of the expanded foam region 6.

[0040] refer to Figure 2 The cross-sectional view along line AA is shown, illustrating... Figure 1 The structure of a segment of the expanded region 6 in the article, wherein the dimensions are exaggerated for clarity of illustration. The expanded foam region 6 is a wall portion 10, which includes a porous foam core layer 12 comprising hollow pores 11 in a matrix 9 between a first solid skin 14 and a second solid skin 16.

[0041] The hollow container also has an integral base 13. At least one region of at least one of the annular sidewalls 4 and the base 13, or at least one region of both, is formed by wall portions 10. Different wall portions 10 with different constructions of the expanded foam can be provided, for example, only in the sidewalls 4, throughout the entire sidewalls 4, or only in a portion of the sidewalls 4, and / or in the base 13, throughout the entire base 13, or only in a portion of the base 13.

[0042] The method for manufacturing article 2 according to the present invention will now be described with reference to Figures 3a and 3b.

[0043] Referring to Figure 3a, which illustrates the injection molding process, an article is formed using a mold 20 having a first outer mold component 22 and a second inner mold component 24. The first mold component 22 and the second mold component 24 have corresponding first cavity forming surfaces 26 and second cavity forming surfaces 28, which are annular in the illustrated embodiment. The first cavity forming surfaces 26 and 28 of the outer mold component 22 are macroscopically smooth and have no undulations in the circumferential direction. In the illustrated embodiment, each of surfaces 26, 28 defines a precise geometric surface that is circular in horizontal cross-section. Because a cup is to be formed, surfaces 26, 28 are also truncated conical in vertical cross-section.

[0044] The mold 20 is closed, thereby defining a region 30 of the cavity 32 between the first cavity forming surface 26 and the second cavity forming surface 28. When the first cavity forming surface 26 and the second cavity forming surface 28 are circular and annular, respectively, the region 30 of the cavity 32 is correspondingly circular and annular.

[0045] In the illustrated embodiment, region 30 is located in the sidewall forming portion 31 of mold 20 and extends to the base forming portion of mold 20 (not shown). Region 30 defines a large portion of the sidewall forming portion 31. However, region 30 may be located at any other location in cavity 32 to define a wall portion in the final article that will include expanded porous foam, as further described below.

[0046] According to the invention, the shape and size of the first cavity forming surface 26 and the second cavity forming surface 28 are very accurate, and any undesirable movement of the first mold component 22 and the second mold component 24 during the molding operation described below is substantially avoided, such that during the molding operation, the width of the region 30 of the cavity 32 between the first cavity forming surface 26 and the second cavity forming surface 28 is constant within a tolerance of + / - 0.5%, preferably + / - 0.2%, based on the nominal (i.e., design) width.

[0047] A molten plastic composition 40, comprising a polymer and a physical foaming agent, is injected into a cavity 32. The physical foaming agent is a gas dissolved in the polymer.

[0048] In embodiments of the invention, the polymer of the plastic composition may include a polyolefin or a blend of multiple polyolefins, optionally polyethylene or polypropylene; or a polyester, optionally polyethylene terephthalate or polybutylene terephthalate; or polylactic acid. In a preferred embodiment, the polymer comprises polypropylene. Polypropylene with a melt flow index (MFI) of 10-120 is particularly preferred. The melt flow index of the polymer can be measured according to ASTM D1238.

[0049] The foaming agents that can be used in embodiments of the present invention include physical foaming agents in gaseous form dissolved in the molten plastic composition. Such gases may include, for example, carbon dioxide. The gases may optionally further include a fragrance composition (i.e., aroma) that remains present in the polymer material after expansion to enhance the consumer experience.

[0050] When carbon dioxide is used as a foaming agent, CO2 gas is generated by the foaming agent in the extruder of the injection molding machine, and due to the relatively high pressure applied to the material, which is greater than the required pressure (usually greater than 80 bar), CO2 then enters the solution during the injection stage (usually 300-500 bar in the mold cavity), forcing CO2 into the solution of molten thermoplastic resin such as polypropylene.

[0051] Molten plastic composition at injection pressure P 注射 Injection. Typically, the injection pressure P... 注射 At least 150 bar. Optionally, at the end of the injection step, a filling pressure P is applied to the cavity 32. 填充Typically, the filling pressure is at least 150 bar.

[0052] During or after the injection step, the injection plastic composition 40 in contact with the first cavity forming surface 26 and the second cavity forming surface 28 is cooled to form a first solid skin 14 and a second solid skin 16 that are adjacent to and in contact with the first cavity forming surface 26 and the second cavity forming surface 28, respectively.

[0053] In region 30 of cavity 32, a corresponding portion 52 of plastic composition 40 is positioned. In portion 52 of plastic composition 40, at least some of the plastic composition in the intermediate layer 54 between the first solid skin 14 and the second solid skin 16 remains in a molten state.

[0054] Since the first mold component 22 and the second mold component 24 are configured such that the width of the region 30 of the cavity 32 between the first cavity forming surface 26 and the second cavity forming surface 28 is constant within a tolerance of + / - 0.5%, preferably + / - 0.2%, based on the nominal, i.e., design width, during the molding operation, correspondingly, in the portion 52 of the plastic composition 40, the thickness of the portion 52 is constant within a tolerance of + / - 0.5%, preferably + / - 0.2%, based on the nominal, i.e., design thickness of the portion 52.

[0055] Typically, before opening mold 20, as further described below, portion 52 has a thickness ranging from greater than 0.5 mm to up to 1 mm.

[0056] During the injection step and any filling process, the injection pressure P 注射 and any filling pressure P 填充 The minimum pressure threshold P in region 30 of cavity 32 is respectively higher than that in region 32. 阈值 Typically, the minimum pressure threshold P 阈值 It is 80 bar. This prevents the physical foaming agent from partially escaping from the polymer solution in region 30, thus preventing the formation of honeycomb-like bubbles in region 30 during the injection step and any filling process.

[0057] Region 30 of the mold cavity 32 has sufficient thickness and / or processing time so short that the molten polymer resin in the interlayer 54 does not solidify during the injection step and any subsequent filling. Furthermore, region 30 can be additionally heated by an external heater to maintain the plastic composition 40 in the interlayer 54 in the molten liquid phase. The first mold component 22 can be cooled by a cooling system, such as by a cooling fluid flow therethrough, to maintain the first mold component 22 at a lower temperature than the second mold component 24. This temperature control allows for control of the absolute and relative thicknesses of the interlayer 54, as well as the first solid skin 14 and the second solid skin 16, thereby achieving the desired expansion of the interlayer 54 and the stretching of the first solid skin 14, as described below.

[0058] Referring to Figure 3b, the mold 20 is then opened before the molten plastic composition 40 in the intermediate layer 54 between the first solid skin 14 and the second solid skin 16 solidifies in region 30 of the cavity 32.

[0059] During the opening step, at least some of the molten plastic composition 40 in the intermediate layer 54 is exposed to external pressure, such as atmospheric pressure, below a minimum pressure threshold, to allow the foaming agent to escape from the solution and form bubbles within the molten plastic composition 40 in the intermediate layer 54. This action forms an expanded foam region 6 in the article 2, which includes an expanded porous foam core layer 12 between a first solid skin 14 and a second solid skin 16 formed from the plastic composition 40.

[0060] The opening step includes removing the first mold component 22 so that the first solid skin 14 is no longer in contact with the first cavity forming surface 26, while maintaining the second solid skin 16 in contact with the second cavity forming surface 28. In the illustrated embodiment, this opening is achieved by removing the outer mold component 22, exposing the first solid skin 14 to atmospheric pressure, and leaving the second skin 16 on the inner component 24.

[0061] However, any other construction that opens the mold can be used. In particular, in an alternative embodiment, at least one or more portions of the inner mold component 24 may be removed from the second solid skin 16, thereby exposing the second solid skin 16 or any portion thereof to atmospheric pressure additionally or alternatively.

[0062] Therefore, this opening step allows the molten plastic composition 40 in the intermediate layer 54 between the first solid skin 14 and the second solid skin 16 to expand by bubbling, forming the core layer 12 of the expanded porous foam 21. As the molten plastic composition 40 in the intermediate layer 54 below the first solid skin 14 expands outward away from the second solid skin 16, the expanded porous foam 21 is formed. This action is controlled to stretch the first solid skin 14 in portion 52 at a desired stretch ratio.

[0063] Subsequently, in a cooling step, the expanded porous foam 21 is cooled to solidify the molten plastic composition 40 in the intermediate layer 54 between the first solid skin 14 and the second solid skin 16 of portion 52, and to form an expanded porous foam region 6 in the article 2, the expanded porous foam region 6 comprising the core layer 12 of the porous foam 21 between the first solid skin 14 and the second solid skin 16. Cooling can be performed passively in the ambient atmosphere or by active cooling, for example by blowing cold air onto the article 2.

[0064] After the cooling step, the wall portion 10 typically has a thickness of 1 to 2 mm. Generally, from the opening step to the cooling step, the portion 52 increases in thickness by 1 to 1.5 mm to form the wall portion 10.

[0065] Before the opening step, in section 52, the first solid skin 14 and the second solid skin 16 are curved and parallel, and after the cooling step, in wall section 10, the first solid skin 14 and the second solid skin 16 are also curved and parallel.

[0066] After the cooling step, the length of the first solid skin 14 in portion 52 is stretched by a stretching ratio of 0.5% to up to 3% compared to the length of the first solid skin 14 present before the opening step. The stretching ratio is the rate at which the length of the first solid skin 14 increases after the cooling step, based on the length of the first solid skin before the opening step. For example, an increase in the length of the first solid skin from an initial value of 100 mm to a final value of 102.5 mm would represent a stretching ratio of 2.5%. Preferably, the stretching ratio is from 2% to 3%, more preferably from 2.25% to 2.75%, and even more preferably from 2.4% to 2.6%, for example, approximately 2.5%.

[0067] According to the invention, due to the expansion of the molten plastic composition 40 in the intermediate layer 54 between the first solid skin 14 and the second solid skin 16, the solid skin 14 is stretched to form a finally cured porous foam core layer 12, comprising hollow cells 11 in the solid matrix 9, between the first solid skin 14 and the second solid skin 16. This stretching is controlled, combined with a very uniform thickness of the portion 52, before the mold 20 is opened, making the stretching measurable, but small and uniform around the circumference of the expanded porous foam region 6.

[0068] The degree of stretching can be controlled by a number of parameters that are easily determined by a technician. For example, the thickness of the first solid skin 14 and the second solid skin 16 can be controlled before the mold 20 is opened, because for a thicker skin, the tendency to stretch it due to the expansion pressure applied to the thicker skin by the expanding molten plastic composition is relatively low. Furthermore, the expansion pressure applied to the skin by the expanding molten plastic composition can also be controlled by changing the concentration of the foaming agent in the molten plastic composition.

[0069] Importantly, the inventors have surprisingly discovered that by controlling the uniformity of shape and size of (one or more) portions to be expanded, and by controlling the degree of stretching of (one or more) skins subjected to stretching due to the expansion of the molten plastic composition, (one or more) corresponding wall portions in the resulting article can exhibit very accurate shapes, such as very accurate concentricity in annular cross-sections, combined with small thickness and small mass. Due to the presence of the expanded porous foam core layer, (one or more) wall portions can also exhibit high strength and thermal insulation properties.

[0070] Conversely, in WO-A-2017 / 134181, a wavy surface on at least one molding surface effectively divides the annular injection preform into a series of individually expanding arcuate zones. The wavy surface of the outer solid skin has "peaks" and "valleys," and during expansion, the valleys are circumferentially "blown out," modifying the shape of the outer skin to become more rounded, but without increasing the length of the outer solid skin. The length does not increase because the outer skin is thick enough and / or the foaming agent concentration is low enough before mold opening that there is insufficient blowing pressure to stretch the outer skin upon opening. However, by providing a wavy surface, the final sidewalls are not uniformly rounded but have a slightly wavy outer surface due to the formation of upright longitudinal ribs in the final sidewalls, and the final sidewalls may also be excessively thick.

[0071] This invention addresses the problems in WO-A-2017 / 134181 by controllably stretching the outer skin and controllably providing a constant and uniform thickness of the portion to be expanded. The constant and uniform thickness of the portion to be expanded ensures that a uniform expansion pressure is applied over the entire area of ​​the corresponding wall or wall section. By providing a uniform expansion pressure to a portion with a constant and uniform thickness, this ensures that the expansion is also constant and uniform, resulting in a final wall or wall section with a very uniform shape, size, and construction. For example, in Figure 1 In cup 2, the outer surface of the sidewall is a geometrically circular shape with high accuracy in the horizontal section, and the sidewall has a very constant wall thickness.

[0072] As a result of these improvements, the wall thickness and mass of the article can be reduced while maintaining the desired porous foam layer properties to provide stiffness and thermal insulation to the wall. The present invention can provide finished articles with a substantially constant sidewall thickness around the circumference, resulting in articles with controlled shapes and dimensions, such as uniform circular cross-sections in the sidewalls, and hollow articles with high wall strength and low mass, exhibiting improved properties compared to the articles disclosed in WO-A-2017 / 134181.

[0073] Referring to Figures 4a and 4b, for the unexpanded region 8, the shape and size of the mold 20 are such that a narrow region 35 of cavity 32 is formed between the first cavity forming surface 26 and the second cavity forming surface 28. During the injection step and optional filling, the injection pressure, and optional filling pressure, are maintained above a minimum pressure threshold in the narrow region 35 of cavity 32 to maintain the physical foaming agent as a gas dissolved in the molten plastic composition 40, such that substantially no bubbles are formed in the narrow region 35 of cavity 32. Prior to the opening step, the plastic composition 40 in the narrow region 35 of cavity 32 is cooled to fully solidify, thereby forming at least one unexpanded region 8 in the article 2, which comprises a substantially homogeneous solid unexpanded plastic composition 37.

[0074] The narrow region 35 of the mold cavity 32 is relatively thin, thus requiring a relatively short time (shorter than the injection and optional filling steps) for the molten polymer resin to cool and solidify. Furthermore, this narrow region 35 can be additionally cooled by an external cooler to transform the polymer resin from a molten liquid phase to a solid phase. After the opening step, the solid plastic composition cannot expand further through bubbling and cannot form expanded porous foam. Therefore, the unexpanded region 8 appears transparent to the naked eye.

[0075] refer to Figures 5 to 7 , Figure 5This is a bottom view of the base 70 of the food tray 72 formed by the method of the second embodiment of the present invention. The adjacent portions 78a, 78b, 78c, 78d of the base 70 and the sidewall 80 include four different regular arrays 74a, 74b, 74c, 74d of a plurality of expanded wall portions 76a, 76b, 76c, 76d.

[0076] Four different arrays 74a, 74b, 74c, and 74d are shown in a common base 70 and sidewall 80, mainly to illustrate the different constructions of the corresponding arrays 74a, 74b, 74c, and 74d according to different embodiments of the present invention.

[0077] However, typically only one such array 74a, 74b, 74c, 74d exists in the base 70 and the sidewall 80, and one of the arrays 74a, 74b, 74c, 74d extends substantially over the entire area of ​​the base 70, and at least over the middle region of the corresponding adjacent portions 78a, 78b, 78c, 78d of the sidewall 80.

[0078] In the illustrated embodiment, arrays 74a, 74b, 74c, and 74d are present in the substrate 70 and the sidewall 80. However, in an alternative embodiment, arrays 74a, 74b, 74c, and 74d may be included in either or both of the annular sidewall 80 and the integral substrate 70.

[0079] Each of the expanded wall portions 76a, 76b, 76c, 76d passes through regions 82a, 82b, 82c, 82d of the plastic composition and is separated from one or more adjacent expanded wall portions 76a, 76b, 76c, 76d. Regions 82a, 82b, 82c, 82d comprise an expanded foam core layer between an inner solid skin and an outer solid skin, the core layer having a lower degree of expansion than the expanded wall portions 76a, 76b, 76c, 76d, or comprising unexpanded plastic composition over its entire thickness.

[0080] Figure 6 It is formed before expansion during the opening step. Figure 5 A cross-sectional view of part 152 of the base or sidewall area of ​​the food tray. Before opening mold 120, Figure 6 The portion 152 shown is located within the mold cavity 132 defined by mold components 122, 124. The portion 152 includes a first outer solid skin 114 and a second inner solid skin 116 on opposite sides of the intermediate layer 154 of the molten plastic composition 140.

[0081] Prior to the opening step, in section 152, the first outer solid skin 114 and the second inner solid skin 116 are planar and parallel. Between adjacent sections 152, the plastic composition 140 is in the form of a thin layer 190, which is either fully cured or has a thinner intermediate layer of molten plastic composition.

[0082] Figure 7 It expands after the opening step, by Figure 6 The diagram shows a cross-sectional view of the wall portion 110 of the base 70 of the partially formed food tray 72. The wall portion 110 includes a first outer solid skin 114 and a second inner solid skin 116 located on opposite sides of a core layer 112 of expanded porous foam made of a plastic composition. After the opening step, in the wall portion 110, the second inner solid skin 116 remains planar, but the first outer solid skin 114 has been stretched into a curved shape and is not parallel to the second inner solid skin 116.

[0083] Between adjacent wall portions 110, in region 82, a thin layer 190 is retained, or an intermediate layer of a thinner molten plastic composition is slightly expanded, such that region 82 serves as a series of anchor points 115 to prevent excessive or uncontrolled stretching of the first outer solid skin 114.

[0084] Figure 8 This is a cross-sectional view of a mold used to form a hollow product by in-mold labeling according to the method of the third embodiment of the present invention. Figure 9 It is by Figure 8 A side view of a hollow article with an in-mold label produced by the mold. Similarly, some dimensions have been exaggerated for clarity.

[0085] refer to Figure 8 and 9 The first mold component 222 and the second mold component 224 are respectively the outer mold component 222 and the inner mold component 224 forming the outer surface 278 and the inner surface 280 of the article 202. A modification to the method described with respect to the first embodiment is that, prior to the injection step from the syringe nozzle 221, a step of providing an in-mold label 290 that at least partially surrounds the cavity 232 is included. The in-mold label 290 is located near the first cavity forming surface 246 and radially outside the second cavity forming surface 248. Typically, the in-mold label 290 is annular and completely surrounds the cavity 232. Preferably, the in-mold label 290 consists of a polymer layer 292 having opposing ends 294, 296, which are joined together at a seam 298 to form an annular shape, such as... Figure 9 As shown.

[0086] In the third embodiment, the injection and opening steps are performed as in the first embodiment. However, after the opening step, the in-mold label 290 restricts the stretching of the first solid skin formed adjacent to it. The in-mold label 290 is in a stretched state due to the expansion of the molten intermediate layer to form a solidified core layer of expanded porous foam and due to the stretching of the first outer solid skin.

[0087] The mechanical properties and dimensions of the in-mold label 290 can be selected to control the stretch ratio of the first outer solid skin of the hollow article 202. Thus, the in-mold label 290 is used to control the shape, size, and properties of the wall portion 210 of the hollow article 202.

[0088] Figure 10 This is a partial dashed sectional side view of a third mold component used to form a hollow article according to the method of the fourth embodiment of the present invention, combined with the second mold component used in the first embodiment.

[0089] In the fourth embodiment, the injection and opening steps are performed as in the first embodiment. After removing the first mold component (not shown), the intermediate molded article 302 is positioned on the second mold component 324. The intermediate molded article 302 has a wall portion 310 having a first outer solid skin 314. However, after the opening step and before the cooling step, the molded article is placed in a second mold 320 having a third mold component 322. The third mold component 322 has a molding surface 324. The first outer solid skin 314 is positioned to contact the molding surface 324.

[0090] Therefore, after the opening step but before the molten plastic composition in the intermediate layer is cooled to solidify to form the solidified core layer of the expanded porous foam, the molding surface 324 restricts the stretching of the first outer solid skin 314 in contact with it.

[0091] Typically, during the setting step, the molding surface 324 compresses the wall portion 310 to reduce the thickness of the intermediate layer of the molten plastic composition, thereby reducing the thickness of the porous foam core layer radially inward of the first solid skin 314. Therefore, the molding surface 324 can be used to control and optionally shorten the length of the first outer solid skin 314 in the wall portion 310.

[0092] In a preferred embodiment, the third mold component 322 is hollow, and the molding surface 324 defines a hollow cavity 380 having a tapered surface 382. During the setting step, the molding surface 324 is gradually pushed against the first outer solid skin 314. This results in compression as described above.

[0093] Typically, the third mold component 322 is at a lower temperature than the first mold component that has been removed, so that the first solid skin 314 is cooled when it comes into contact with the third mold component 322. The third mold component 322 can be cooled by a cooling system, for example by a cooling fluid flowing there, to maintain the third mold component 322 at a lower temperature than the second mold component 324 that supports the hollow article 302 during the process of setting the hollow article 302 into contact with the third mold component 322.

[0094] In some embodiments of the invention, the method may further include pressing an embossing tool onto the first solid skin before the molten plastic composition between the first and second solid skins solidifies during the cooling step, to imprint an inward-facing image onto the first solid skin by stretching it inward against the expanded porous foam. In embodiments where the third mold component 322 presses against the first solid skin 314, the third mold component 322 may serve as the embossing tool. The embossed image may be a pattern, logo, name, etc.

[0095] Figures 11a and 11b are schematic side cross-sectional views of the walls of the injection-molded portion at different corresponding stages in another embodiment of the method of the present invention. Figure 11a shows the shape and structure of the first solid skin before the mold is opened, and Figure 11b shows the shape and structure of the first solid skin after the mold is opened and the expanded porous foam is cooled.

[0096] Following injection step (c) but before opening step (d), a regular array 400 of a plurality of wall portions 402 of intermediate article 404 is formed. Each of the individual wall portions 402 comprises an intermediate layer 406 of the molten plastic composition between opposing first solid skins 408 and second solid skins 410. The three-dimensionally shaped portion 412 of the first solid skin 408 defines an outwardly extending convex surface 414 formed by a first cavity forming surface 416 and a substantially smooth portion 418 of the second solid skin 410 formed by a second cavity forming surface 420.

[0097] Prior to the opening step (d), each wall portion 402 of the regular array 400 includes an intermediate peak 422. Furthermore, prior to the opening step (d), the outwardly extending convex surface 414 of the wall portion 402 is separated from the adjacent outwardly extending convex surface 414 of the wall portion 402 by a region 424 of the plastic composition extending between and in contact with the first cavity forming surface 416 and the second cavity forming surface 420, which is a solid layer 426 of the plastic composition extending between and in contact with the first cavity forming surface 416 and the second cavity forming surface 420.

[0098] Figure 12This is a schematic perspective view of the wall 430 of the article formed by the injection molding portion of Figure 11, showing the shape and structure of the first solid skin 408 after the mold is opened and the expanded porous foam is cooled.

[0099] During the opening step (d), each wall portion 402 of the regular array 400 of multiple wall portions 402 expands to stretch the first solid skin 408, thereby forming, after the cooling step (e), a regular array 432 of multiple wall portions 434, each wall portion 434 formed by a corresponding wall portion 402. The regular array 432 of multiple wall portions 434 comprises an array of generally pyramidal, conical, or partially spherical convex wall portions 434. Each of the individual convex wall portions 434 includes an expanded porous foam core layer 436 between opposing skin layers 438, 440, the skin layer consisting of a three-dimensionally shaped first solid skin portion 438 defining the convex surface 442 of the convex wall portion 434 and a substantially smooth second solid skin portion 444. The convex wall portions 434 are separated from adjacent convex wall portions 434 by unexpanded regions 446 of the plastic composition. In this embodiment, the plan view of the unexpanded region 446 includes individual points, typically in the form of a circle, although in other embodiments the unexpanded region 446 may have other shapes and sizes, and multiple unexpanded regions 446 may be connected to each other.

[0100] Figure 13 This is a cross-sectional view of a mold used to form a hollow article according to the method of the sixth embodiment of the present invention. Figure 14 It is by Figure 13 A cross-sectional view of a hollow product made using a mold. Hollow products are... Figure 14 The cup shown is in the form of a final expanded and molded article 550, having an upper open end 552 and a lower closed end 554, as well as side walls 556.

[0101] refer to Figure 13 and 14 After injection step (c) and before opening step (d), the plastic composition 500 forms a molded intermediate article 502 within the cavity 504. In the initial stage of opening step (d), as... Figure 13 As indicated by the arrow, the first mold component 506 is removed so that the first solid skin 508 is no longer in contact with the first cavity forming surface 510. The initial removal of the first mold component 506 exposes the first end 512 of the intermediate article 502, while the opposing second end 514 of the intermediate article 502 remains in contact with the first cavity forming surface 510 and the second cavity forming surface 516. Subsequently, the first mold component 506 is completely removed, so that the first solid skin 508 is fully exposed, extending from the first end 512 to the opposing second end 514, as described above with respect to the previous embodiment.

[0102] After the initial stage begins, the first solid skin 508 at the first end 512 of the intermediate article 502 is exposed, and therefore, for the entire opening step (d), the first solid skin 508 at the first end 512 of the intermediate article 502 is exposed for a longer period of time than the first solid skin 508 at the second end 514 of the intermediate article 502. Furthermore, the plastic composition is hotter at the first end 512 than at the second end 514 because the exposure of the first end 512 before the second end 514 results in a shorter period of contact between the plastic composition at the first end 512 and the relatively cooler first mold component 506 than the plastic composition at the second end 514.

[0103] Therefore, after cooling step (e), Figure 14 The length of the first solid skin 508 at the first end 512 of the final expanded and molded article 550 shown is stretched at a greater stretch ratio than the length of the first solid skin 508 at the second end 514 of the final expanded and molded article 550. This is because the first end 512 has a longer stretching time before cooling than the second end 514.

[0104] The first end 512 of the intermediate article 502 is injection molded to have a smaller wall thickness than the second end 514 of the intermediate article 512, and this thickness difference is... Figure 13 The thickness of the intermediate article 502 is often exaggerated. Typically, the wall thickness of the intermediate article 502 tapers gradually in a tapered manner from the first end 512 to the second end 514 of the intermediate article 502. This thickness difference at least partially compensates for the greater stretch ratio of the length of the first solid skin 508 at the first end 512 of the intermediate article 502 compared to the second end 514. Typically, the wall thickness of the first end 512 of the intermediate article 502 is 5% to 15% smaller than the wall thickness of the second end 514. This reduced wall thickness at the upper end of the intermediate article 502 reduces excessive skin stretching in the resulting article. As an example, for a typical container with sidewalls, such as one with a height of 135 mm and tilted at a 6-degree angle relative to the longitudinal axis of the container, with a second end 514 thickness of 0.7 mm, the thickness of the first end 512 would be approximately 0.05 mm thinner than the second end to avoid excessive stretching at the upper end of the container.

[0105] The first end 512 and the second end 514 of the intermediate product 502 are respectively formed Figure 14 The first wall portion 520 and the second wall portion 522 of the final expanded and molded article 550 are shown. The first wall portion 520 and the second wall portion 522 generally have the same thickness within a tolerance of + / - 0.5% based on the nominal thickness of the respective wall portions 520, 522.

[0106] In this embodiment of the invention, the initial removal of the first mold component 506 exposes the first end 512 of the intermediate article 502 to the atmosphere. This exposure causes the first solid skin at the first end 512 to stretch due to the expansion of the molten plastic composition in the intermediate layer. Conversely, the opposing second end 514 of the intermediate article 502 remains in contact with the first cavity forming surface 510 and the second cavity forming surface 516, and therefore cannot expand or stretch in the initial stage. After the opposing second end 514 is subsequently exposed (which requires a limited time period), and after the first mold component 506 has been completely removed, the first solid skin 508 at the second end 514 can subsequently be stretched. Since the first solid skin 508 at the first end 512 has a longer stretching time than at the second end 514, if it is desired to maintain a substantially constant wall thickness in the finally expanded and molded article 550, the cavity thickness at the first end 512 is made smaller than that at the second end 514 to compensate for the increased stretch ratio associated with the increased stretching time at the first end 512 compared to the second end 514. This allows, for example, the formation of a constant wall thickness, although the upper open end 552 of the hollow article has a higher skin stretch ratio compared to the lower closed end 554 of the hollow article.

[0107] In the illustrated embodiments of the invention, the annular sidewall is linear in longitudinal section, and therefore can be truncated conical or truncated pyramidal. In other embodiments of the invention, the annular sidewall has an upper annular end away from the substrate and a lower annular end adjacent to the substrate, the upper end having a larger circumference than the lower end, and the sidewall is curved in longitudinal section, and the article can, for example, be in the shape of a bowl with a large-diameter opening. In such embodiments, the stretch ratio of the outer first solid skin is higher at the upper annular end than at the lower annular end because the significantly larger circumference during the formation of the expanded porous foam core layer results in increased stretching of the outer first solid skin. In a preferred embodiment of the invention, the article can be a cup, mug, bottle, basin, bowl, tray, container, or vessel for containing liquid food, such as a coffee cup or food tray. The container can be heat-resistant and suitable for heating beverages or food in a microwave oven. The article can be disposable or reusable, and in either case recyclable, because the article is composed of a single polymer such as polypropylene.

[0108] 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 invention as defined by the appended claims.

Claims

1. A method for forming a plastic article, wherein the plastic article is a hollow container, the method comprising: A step (a) is provided in which a mold (20) is provided, the mold having a first mold component (22) and a second mold component (24), the first mold component (22) and the second mold component (24) having a first cavity forming surface (26) and a second cavity forming surface (28), the first cavity forming surface (26) and the second cavity forming surface (28) being annular, macroscopically smooth, and without any waveform in the circumferential direction; Closing step (b), in which the mold (20) is closed, thereby defining an annular cavity (32) between the first cavity forming surface (26) and the second cavity forming surface (28); In injection step (c), a molten plastic composition comprising a polymer (40) and a physical foaming agent is injected into the annular cavity (32), wherein the physical foaming agent is a gas dissolved in the polymer, the molten plastic composition (40) is injected under injection pressure, wherein during or after injection step (c), the injected plastic composition in contact with the first cavity forming surface (26) and the second cavity forming surface (28) is cooled to form a first cavity forming surface adjacent to and in contact with the first cavity forming surface (26) and the second cavity forming surface (28), respectively. A solid skin (14) and a second solid skin (16) are provided, thereby correspondingly positioning at least one annular portion (52) of the plastic composition in at least one annular region (30) of the annular cavity (32), in which at least some of the plastic composition between the first solid skin (14) and the second solid skin (16) remains molten, wherein the thickness of the corresponding portion of the at least one annular portion (52) of the plastic composition is constant within a tolerance of + / - 0.5% based on the nominal thickness of the corresponding portion of the at least one annular portion (52) of the plastic composition, and wherein the first solid skin (14) and the second solid skin (16) are curved and parallel in the corresponding portion of the at least one annular portion (52) of the plastic composition; In the opening step (d), the mold (20) is opened before the molten plastic composition between the first solid skin (14) and the second solid skin (16) solidifies in the corresponding portion of the at least one annular portion (52) of the plastic composition, so as to expose the molten plastic composition (40) of the corresponding portion of the at least one annular portion (52) of the plastic composition to an external pressure lower than the injection pressure, thereby allowing the first solid skin (14) of the corresponding portion of the at least one annular portion (52) of the plastic composition to solidify in the corresponding portion of the second solid skin. The molten plastic composition (40) between the first solid skins (16) expands by bubbling to form an expanded porous foam (21) as the molten plastic composition (40) below the first solid skin (14) expands outward away from the second solid skin (16), thereby stretching the first solid skin (14) in a corresponding portion of the at least one annular portion (52) of the plastic composition, wherein the opening step (d) includes removing the first mold component (22) so that the first solid skin (14) is no longer in contact with the first cavity forming surface (26); and In a cooling step (e), the expanded porous foam (21) is cooled to solidify the molten plastic composition (40) between the first solid skin (14) and the second solid skin (16) of the corresponding portion of the at least one annular portion (52) of the plastic composition, and at least one wall portion (10) is formed in the hollow container, the wall portion comprising a porous foam core layer (12) between the first solid skin (14) and the second solid skin (16), wherein the hollow container has an annular sidewall (4), and the wall portion (10) is included in the annular sidewall (4) and is both continuous and annular. Wherein, after the cooling step (e), the length of the first solid skin in the corresponding portion of the at least one annular portion (52) of the plastic composition has been stretched by a stretch ratio of 0.5% to up to 3% compared to the first solid skin (14) present before the opening step (d), wherein the stretch ratio is based on the length of the first solid skin (14) before the opening step (d) and the rate by which the length of the first solid skin (14) increases after the cooling step (e).

2. The method according to claim 1, wherein, Based on the nominal thickness of the corresponding portion of the at least one annular portion (52) of the plastic composition, the thickness of the corresponding portion of the at least one annular portion (52) of the plastic composition is constant within a tolerance of + / - 0.2%.

3. The method according to claim 2, wherein, After the cooling step (e), the length of the first solid skin (14) in the corresponding portion of the at least one annular portion (52) of the plastic composition has been stretched by a certain stretch ratio of 2% to 3%, compared to the first solid skin (14) that existed before the opening step (d).

4. The method according to any one of claims 1 to 3, wherein, Prior to the opening step (d), the corresponding portion of the at least one annular portion (52) of the plastic composition has a thickness greater than 0.5 mm to up to 1 mm.

5. The method according to any one of claims 1 to 3, wherein after the cooling step (e), the at least one wall portion (10) has a thickness of 1 mm to 2 mm.

6. The method according to any one of claims 1 to 3, wherein, From the opening step (d) to the cooling step (e), the thickness of the corresponding portion of the at least one annular portion (52) of the plastic composition is increased by 1 mm to 1.5 mm to form the at least one wall portion (10).

7. The method according to any one of claims 1-3, wherein the first mold component (22) and the second mold component (24) are respectively the outer mold component and the inner mold component forming the outer surface and the inner surface of the hollow container.

8. The method according to any one of claims 1-3, wherein after the cooling step (e), in the at least one wall portion, the first solid skin (14) and the second solid skin (16) are curved and parallel.

9. The method according to any one of claims 1-3, wherein the hollow container has an integral base (13), wherein at least one region of the base (13) is formed by the at least one wall portion (10).

10. The method according to claim 9, wherein the annular sidewall (4) has an upper annular end away from the substrate (13) and a lower annular end adjacent to the substrate (13), the upper annular end having a larger circumference than the lower annular end, and the sidewall (4) is curved in a longitudinal section, wherein the stretch ratio at the upper annular end is higher than the stretch ratio at the lower annular end.

11. The method of claim 1, wherein after the injection step (c) and before the opening step (d), the plastic composition forms a molded intermediate article within the cavity, and in the initial stage of the opening step (d), the first mold component is removed such that the first solid skin is no longer in contact with the first cavity forming surface, thereby exposing a first end of the intermediate article, while the opposite second end of the intermediate article remains in contact with the first cavity forming surface and the second cavity forming surface.

12. The method of claim 11, wherein after the start of the initial stage, the first solid skin at the first end of the intermediate article is exposed for a longer period of time than the first solid skin at the second end of the intermediate article, thereby, after the cooling step (e), the length of the first solid skin at the first end of the final expanded and molded article is stretched at a greater stretch ratio than the length of the first solid skin at the second end of the final expanded and molded article.

13. The method of claim 12, wherein the first end of the intermediate article has a smaller wall thickness than the second end of the intermediate article to at least partially compensate for the greater stretch ratio of the length of the first solid skin at the first end of the intermediate article compared to the second end of the intermediate article.

14. The method of claim 13, wherein the wall thickness of the first end of the intermediate article is 5% to 15% smaller than the wall thickness of the second end of the intermediate article.

15. The method of claim 13 or claim 14, wherein the first end and the second end of the intermediate article form respective first wall portions and second wall portions of the final expanded and molded article, and wherein the first wall portions and the second wall portions have the same thickness within a tolerance of + / - 0.5% based on the nominal thickness of the respective wall portions.

16. The method according to claim 13, wherein, The wall thickness of the intermediate product gradually tapers in a direction of increasing thickness from the first end of the intermediate product to the second end of the intermediate product.

17. The method according to any one of claims 1 to 3 and 11 to 14, wherein the polymer is polypropylene.

18. The method according to any one of claims 1 to 3 and claims 11 to 14, wherein the foaming agent is carbon dioxide.

19. The method according to any one of claims 1 to 3 and 11 to 14, wherein during the injection step (c), the injection pressure is at least 150 bar.

20. The method according to any one of claims 1 to 3 and 11 to 14, wherein at the end of the injection step (c), a filling pressure of at least 150 bar is applied to the plastic composition in the cavity.

21. The method according to any one of claims 1 to 3 and 11 to 14, wherein during the opening step (d), the first solid skin is exposed to the atmosphere, and the external pressure is atmospheric pressure.

22. The method according to any one of claims 1 to 3 and 11 to 14, wherein the first mold component and the second mold component are respectively an outer mold component and an inner mold component forming an outer surface and an inner surface of the article, and the method further comprises the step of providing an in-mold label at least partially surrounding the cavity before the injection step (c), the in-mold label being located near the first cavity forming surface, and wherein after the opening step (d), the in-mold label restricts the stretching of a first solid skin formed adjacent to it.

23. The method according to claim 22, wherein, The label inside the mold is ring-shaped and completely surrounds the cavity.

24. The method of claim 23, wherein the in-mold label comprises a polymer layer having opposite ends joined together at a seam to form an annular shape.

25. The method according to any one of claims 1 to 3 and 11 to 14, further comprising a setting step between the opening step (d) and the cooling step (e), wherein in the setting step, the molded article is set in a second mold having a third mold component having a molding surface, wherein the first solid skin is in contact with the molding surface, and wherein, after the opening step (d), the molding surface restricts the stretching of the first solid skin in contact with it.

26. The method of claim 25, wherein, During the setting step, the molding surface compresses at least one wall portion to reduce the thickness of the porous foam core layer between the first solid skin and the second solid skin, and shortens the length of the first solid skin in the at least one wall portion.

27. The method according to claim 25, wherein, The third mold component is hollow, and the molding surface defines a hollow cavity with a tapered surface, and during the setting step, the molding surface is gradually pushed against the first solid skin.

28. The method according to claim 25, wherein, The temperature of the third mold component is lower than that of the first mold component, so that the first solid skin is cooled when it comes into contact with the third mold component.

29. The method according to any one of claims 1 to 3 and claims 11 to 14, wherein the temperature of the first mold component is lower than the temperature of the second mold component.

30. The method according to any one of claims 1 to 3 and 11 to 14, further comprising the step of: pressing an embossing tool onto the first solid skin in the cooling step (e) prior to the curing of the foamed molten plastic composition between the first solid skin and the second solid skin, to imprint an inward-facing image onto the first solid skin by stretching the first solid skin inward against the expanded porous foam.

31. The method according to any one of claims 1 to 3 and 11 to 14, wherein the article is a container or vessel for containing food or liquid.

32. The method according to any one of claims 1 to 3 and 11 to 14, wherein the article is a cup, bottle, basin, bowl or tray for containing food or liquid.

33. The method according to any one of claims 1 to 3 and 11 to 14, wherein the article is a mug.

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