Method of forming an article

Abstract

A method of forming an article, comprising: (a) providing a mold having a first mold part and a second mold part, the first and second mold parts having respective first and second cavity-forming surfaces; (b) closing the mold, thereby defining a cavity between the first and second cavity-forming surfaces; and (c) injecting a molten plastic composition into the cavity, the molten plastic composition including a polymer and a physical blowing agent, the physical blowing agent being a gas dissolved in the polymer, the molten plastic composition being injected at an injection pressure, and during the injecting step (c) The injected molten plastic compound in contact with the first and second cavity-forming surfaces is then cooled to form first and second solid skins adjacent to and in contact with the first and second cavity-forming surfaces, respectively, thereby placing at least one portion of the plastic compound within at least one region of the cavity, where at least a portion of the plastic compound between the first and second solid skins remains molten, and the thickness of each portion is within ±0.5% (preferably ±0.5%) of the nominal thickness of the respective portion.(d) before the molten plastic composition between the first and second solid skins has solidified in at least one section, opening the mold to expose the molten plastic composition in each section to an external pressure lower than the injection pressure, thereby allowing the molten plastic composition between the first and second solid skins in each section to expand by foaming to form an expanded porous foam, and the molten plastic composition directly below the first solid skin expands outward, away from the second solid skin, thereby stretching the first solid skin in each section, the opening step comprising removing the first mold part such that the first solid skin is no longer in contact with the first cavity-forming surface; and (e) cooling the expanded porous foam to solidify the molten plastic composition between the first and second solid skins in each section and form at least one first wall in the article comprising a core layer of porous foam between the first and second solid skins. After the cooling step (e), the length of the first solid skin in each section is stretched by a stretch ratio of 0.5 to 3% relative to the length of the first solid skin present before the opening step (d), the stretch ratio being the ratio of the increase in length of the first solid skin after the cooling step (e) relative to the length of the first solid skin before the opening step (d).

Description

[Technical Field]

[0001] The present invention relates to the formation of foamed plastic articles, such as cups and containers for liquids and / or food. [Background technology]

[0002] In the packaging industry, a commonly used type of disposable cup (e.g., take-out coffee cup) is a paper cup lined with a plastic material (e.g., low-density polyethylene (LDPE)). Recycling these cups can be a challenge because they are made of two different materials that can be difficult and / or expensive to separate. Furthermore, because there is a seam down one side of the cup where the paper materials are joined together, when the cup is tilted to consume the beverage inside (especially if the cup is used in conjunction with a lid that has a mouthpiece through which the beverage is passed to consume it), liquid may leak out from the area of ​​the seam at the rim of the cup.

[0003] The industry has been working to provide disposable polypropylene cups. Traditional injection molding requires thick walls to provide some insulation to the cup. Typically, a porous structure provides insulation, but to further improve insulation and reduce the density of the foam wall, a blowing agent is added to the thermoplastic polymer to create a foam structure.

[0004] Patent Document 1 (WO-A-2017 / 134181) discloses an article and a method for forming the article. The article is a polypropylene cup having an expansion region containing a porous foam. The manufacturing method requires a corrugated surface on at least one molding surface defining the mold cavity. This is to control the expansion of the central molten polypropylene between the outer solid skins after the outer mold parts that formed the outer solid skins (surface layers without bubbles) are removed. The corrugated surface effectively divides the annular injection preform into a series of individually expanding arcuate sections. This prevents uncontrolled circumferential expansion around the preform, and the resulting article has a controlled shape and size. However, the sidewalls form a ribbed structure that is not uniform and does not have a uniformly circular cross-section. Because the outer surface of the sidewall tends to exhibit some residual corrugation, the outer surface is not very smooth or has a precise geometric shape. Although this known method produces hollow articles with high wall strength and low mass, it does not produce cups with the desired very smooth surfaces or precise geometric shapes.

[0005] However, despite this prior art disclosure, there remains a need to produce foamed plastic articles (e.g., cups and containers for liquids and / or food) constructed from a single recyclable material, having thinner wall thicknesses, lower masses, and very smooth surfaces that accommodate precise desired geometries, but still having walls that exhibit high wall strength and, possibly, excellent thermal insulation. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 134181 Brochure Summary of the Invention

[0007] The present invention aims to at least partially overcome the problem of achieving thinner wall thicknesses and lower masses in articles made of a single recyclable material, as well as perfectly smooth surfaces corresponding to the precise geometric shapes desired, while still providing walls that exhibit high wall strength and possibly excellent thermal insulation.

[0008] The present invention provides a method for forming an article as set forth in claim 1.

[0009] Preferred features of the method are defined in the respective dependent claims.

[0010] The method of the present invention forms articles having walls that can have any shape, texture, or function, however, the invention has particular applicability to the production of hollow articles such as cups or any other container or vessel for holding liquids or food.

[0011] The present invention provides the technical effects and advantages of an article formed by the method that is easily recyclable or reusable, has a high degree of rigidity provided by the expanded wall(s), has excellent thermal insulation provided by the expanded wall(s), is very thin, and therefore requires a low mass of plastic compound to produce an article of a given size (e.g., a cup of a desired volume), and has a very smooth surface corresponding to a precise desired geometry. The article may incorporate unexpanded (or less expanded) regions that can provide structural and / or aesthetic properties. For example, the unexpanded (or less expanded) regions may be transparent (i.e., visually clear and clear), while the expanded walls may be translucent or opaque.

[0012] The present invention is based, at least in part, on the inventor's discovery that by controlling the thickness of a section of a plastic compound comprising a layer of molten plastic compound between opposing first and second solid skins so that the thickness is constant within an extremely small tolerance of ±0.5% (preferably ±0.2%), when the section is subsequently subjected to reduced pressure to cause expansion of the layer of molten plastic compound to form a core layer of expanded porous foam between the first and second solid skins, by controlling the stretching of the first solid skin within the range of 0.5 to 3%, the expansion of the plastic compound can be controlled to be highly uniform in the expanded wall, meaning there is no uncontrolled expansion, which in turn means that the expanded wall can result in a thinner, more uniform, and lower mass article wall than the prior art.

[0013] The inventors have also found that articles produced by the method of the present invention have the required level of rigidity, despite the reduced amount of material used. It has been found that because porous foam plastic compounds (typically thermoplastic polymers such as polyolefins (typically polypropylene)) cool slowly due to their inherent insulating properties, the crystallinity of the plastic compound can be increased, thereby increasing the rigidity of the porous foam plastic compound. The foaming expansion of the molten plastic compound between the first and second skins also provides the article with excellent thermal insulation.

[0014] They are also easier to recycle than commonly used plastic-lined paper cups because the entire container can be made from a single layer of recyclable material (i.e., there are no layers of different materials that need to be separated).

[0015] Furthermore, because the article is injection molded in the inventive method, the article does not have any joints through which liquid contents could leak. For example, the sidewalls and bottom of a hollow container can be molded as a single unit.

[0016] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a side view of a cup formed by the method of the first embodiment of the present invention. [Figure 2] 2 is a cross-section of the first portion of the cup taken along line A-A in FIG. 1, illustrating the appearance of the expanded translucent portion of the cup. [Figure 3a] 2A-2C are cross-sections of a mold for forming the expanded porous foam portion of the cup of FIG. 1 along line A-A, illustrating the injection step, at different stages in the method of the present invention. [Figure 3b] 2A-2C are cross-sections of a mold for forming the expanded porous foam portion of the cup of FIG. 1 along line A-A, illustrating the cooling step, at different stages of the method of the present invention. [Figure 4a] 2 is a cross-section of the mold for forming the second part of the cup of FIG. 1 taken along line B-B, illustrating the appearance of the unexpanded transparent portion of the cup, showing the injection step. FIG. [Figure 4b] 2 is a cross-section of the mold for forming the second part of the cup of FIG. 1 taken along line B-B, illustrating the appearance of the unexpanded transparent portion of the cup, showing the cooling step. FIG. [Figure 5] FIG. 10 is a bottom view of the base and sidewall of a food tray formed by the method of the second embodiment of the present invention. [Figure 6] 6 is a cross-section of a portion of the food tray of FIG. 5 in a region intended to form the bottom and / or side walls, prior to expansion during the opening step. [Figure 7]7 is a cross-section of a portion of a food tray in the region of the bottom and / or side wall formed from the portion shown in FIG. 6 after expansion during the opening step. [Figure 8] 10 is a cross-section of a mold for forming a hollow article using an in-mold labeling method according to a third embodiment of the method of the present invention. [Figure 9] 9 is a side view of a hollow article having an in-mold label produced by the mold of FIG. 8. [Figure 10] FIG. 10 is a side view (partially in phantom cross section) of a third mold part that is combined with a second mold part as used in the first embodiment to form a hollow article by the method of the fourth embodiment of the present invention. [Figure 11a] 5A-5C are schematic side cross-sections of the wall of an injection molded part at different stages of a fifth embodiment of the method of the present invention, showing the shape and configuration of the first solid skin before opening the mold. [Figure 11b] Schematic side cross-sections of the wall of an injection molded part at different stages in a fifth embodiment of the method of the present invention, showing the shape and configuration of the first solid skin after the mold is opened and the expanded porous foam has cooled. [Figure 12] 11b is a schematic perspective view of the wall of an article formed from the injection molded portion of FIG. 11a, showing the shape and configuration of the first solid skin after the mold is opened and the expanded porous foam has cooled. [Figure 13] 10 is a cross section of a mold for forming a hollow article according to a method of a sixth embodiment of the present invention. [Figure 14] 14 is a cross section of a hollow article made by the mold of FIG. 13. DETAILED DESCRIPTION OF THE INVENTION

[0018] Referring to FIG. 1, there is shown a side view of an article formed by the method of the present invention, for example a hollow container such as a cup (e.g., a coffee cup).

[0019] Cup 2 has an annular sidewall 4. Sidewall 4 has an annular expanded foam region 6 and an annular unexpanded region 8. As used herein, the term "annular" means "generally ring-shaped" and is not limited to a circular geometry, but encompasses shapes that may be circular or non-circular (e.g., oval, polygonal, etc.). Expanded foam region 6 typically has a thickness of 1 to 2 mm, optionally 1 to 1.5 mm. Unexpanded region 8 typically has a thickness of 0.25 to 0.75 mm, optionally 0.25 to 0.5 mm.

[0020] The expanded foam region 6 typically appears translucent to the naked eye because the expanded porous foam contains multiple cells with cell walls that reflect visible light, although if the thermoplastic polymer is heavily pigmented, the expanded region 6 may appear opaque, typically of a uniform color.

[0021] In contrast, the unexpanded region 8 has no bubbles, or if any bubbles are present (e.g., at a low density), they are not visible to the naked eye because the bubble size is typically less than 0.5 microns. Thus, the unexpanded region 8 appears transparent to the naked eye. The unexpanded region 8 appears transparent to the naked eye because the blowing agent (in this embodiment, CO gas) remains dissolved in the polymer (in this embodiment, polypropylene) during the manufacture of the article. After the molten polymer solidifies, no bubbles can form as a result of any action of the blowing agent.

[0022] The present invention relates to the reliable manufacture of such articles with expanded foam regions 6 (and, optionally, non-expanded regions 8). In some embodiments, substantially the entire article may be comprised of expanded foam regions 6.

[0023] Referring to Figure 2, there is shown a cross section along line A-A illustrating the structure of a portion of the expansion region 6 of the article of Figure 1, with dimensions exaggerated for clarity of illustration. The expansion region 6 is a wall 10 comprising a core layer 12 of porous foam (comprising hollow cells 11 and a matrix 9) between first and second solid skins 14, 16.

[0024] The hollow container also has an integral bottom 13. At least one section of the annular side wall 14 and / or bottom 13 is included in the wall 10. Different wall sections 10 with different compositions of expanded foam may be provided, for example, only on the side wall 14 (either throughout the entire side wall 14 or only on portions of the side wall 14) and / or only on the bottom 13 (either throughout the entire bottom 13 or only on portions of the bottom 13).

[0025] The method of the present invention for making the article 2 will now be described with reference to Figures 3a and 3b.

[0026] Referring to FIG. 3a, which illustrates the injection step, an article is formed using a mold 20 having a first (outer) mold part 22 and a second (inner) mold part 24. The first and second mold parts 22, 24 have respective first and second cavity-forming surfaces 26, 28 (which are annular in the illustrated embodiment). The first and second cavity-forming surfaces 26, 28 of the outer mold part 22 are macroscopically smooth and lack any circumferential undulations. In the illustrated embodiment, each surface 26, 28 defines a precise geometric surface that is circular in horizontal cross section. Because a cup is to be formed, each surface 26, 28 is also frustoconical in vertical cross section.

[0027] When mold 20 is closed, it defines a portion 30 of cavity 32 between first and second cavity-forming surfaces 26, 28. If first and second cavity-forming surfaces 26, 28 are circular and annular, then portion 30 of cavity 32 is correspondingly circular and annular.

[0028] In the illustrated embodiment, the region 30 is located within a sidewall-forming portion 31 of the mold 20 and extends to a bottom-forming portion (not shown) of the mold 20. The region 30 defines a majority of the sidewall-forming portion 31. However, as explained further below, the region 30 may be otherwise located at any location(s) of the cavity 32 to define the respective walls within the final article to be provided with the expanded cellular foam.

[0029] In accordance with the present invention, the first and second cavity-forming surfaces 26, 28 are shaped and dimensioned very precisely, and any undesired movement of the first and second mold parts 22, 24 during the molding operation, as described below, is substantially avoided. As a result, during the molding operation, the width of the portion 30 of the cavity 32 between the first and second cavity-forming surfaces 26, 28 remains constant within a tolerance of ±0.5% (preferably ±0.2%) of the nominal (i.e., designed) width.

[0030] A molten plastic compound 40 containing a polymer and a physical blowing agent is injected into the cavity 32. The physical blowing agent is a gas dissolved in the polymer.

[0031] In embodiments of the invention, the polymer of the plastic compound may comprise a polyolefin or a blend of polyolefins (optionally polyethylene and / or polypropylene), or a polyester (optionally polyethylene terephthalate and / or polybutylene terephthalate), or polylactic acid. In preferred embodiments, the polymer comprises polypropylene. Polypropylenes having a melt flow index (MFI) of 10 to 120 are particularly preferred. The melt flow index of a polymer may be measured according to ASTM D1238.

[0032] Blowing agents that can be used in embodiments of the present invention include physical blowing agents in the form of a gas dissolved within the molten plastic compound. Such a gas may include, for example, carbon dioxide. The gas may optionally further include a perfume compound (i.e., a fragrance) that remains present within the polymeric material after expansion to enhance consumer sensory perception.

[0033] When carbon dioxide is used as the blowing agent, CO2 gas is produced by the blowing agent in the extruder of the injection molding machine, and during the injection stage, the CO2 gas begins to dissolve due to the relatively high pressures exerted on the material (typically 300 to 500 bar), which are higher than the pressures required to force dissolution of CO2 in molten thermoplastic resin (e.g., polypropylene) (typically above 80 bar).

[0034] The molten plastic compound is injected at an injection pressure P injection Typically, the injection pressure is P injection At the end of the injection step, the packing pressure (P packing ) is applied to the cavity 32. Typically, a filling pressure P packing is at least 150 bar.

[0035] During or after the injecting step, the injected plastic compound 40 in contact with the first and second cavity-forming surfaces 26, 28 cools to form first and second solid skins 14, 16 adjacent to and in contact with the first and second cavity-forming surfaces 26, 28, respectively.

[0036] Located within the region 30 of the cavity 32 are respective portions 52 of the plastic composition 40. In the portions 52 of the plastic composition 40, at least a portion of the plastic composition remains molten (in a central layer 54 between the first and second solid skins 14, 16).

[0037] Because the first and second mold parts 22, 24 ensure that the width of the portion 30 of the cavity 32 between the first and second cavity-forming surfaces 26, 28 is constant within a tolerance of ±0.5% (preferably ±0.2%) of the nominal (i.e., designed) width during the molding operation, the thickness of the portion 52 of the plastic compound 40 is correspondingly constant within a tolerance of ±0.5% (preferably ±0.2%) of the nominal (i.e., designed) thickness of the portion 52.

[0038] Typically, before opening the mold 20 as further described below, the portion 52 has a thickness of greater than 0.5 mm to 1 mm.

[0039] During the injection step (and optional filling), an injection pressure P injection (and any filling pressure P packing ) are the minimum pressure thresholds (P threshold ) is typically above a minimum pressure threshold P threshold is 80 bar, which prevents the physical blowing agent from partially coming out of solution in the polymer in region 30, so as to prevent the formation of porous cells in region 30 during the injecting step (and any filling).

[0040] The mold cavity 32 section 30 is sufficiently thick and / or the process time is short enough so that the molten polymer resin in the central layer 54 does not solidify during the injecting step (and any subsequent filling). The section 30 can also be additionally heated by an external heater to maintain the plastic compound 40 in the central layer 54 in a molten liquid phase. The first mold part 22 can be cooled by a cooling system (e.g., by flowing a cooling fluid through the mold part 22) to maintain the first mold part 22 at a lower temperature than the second mold part 24. Such temperature control allows for control of the absolute and relative thicknesses of the central layer 54 and the first and second solid skins 14, 16. As a result, the desired expansion of the central layer 54 and stretching of the first solid skin 14 can be achieved, as described below.

[0041] Referring to FIG. 3b, the mold 20 is then opened before the molten plastic compound 40 in the central layer 54 between the first and second solid skins 14, 16 has solidified in the area of ​​the cavity 32.

[0042] During the opening step, at least a portion of the molten plastic composition 40 within the central layer 54 is exposed to an external pressure (e.g., atmospheric pressure) below a minimum pressure threshold such that the blowing agent de-melts and forms bubbles within the molten plastic composition 40 within the central layer 54. This action forms an expanded foam region 6 within the article 2, comprising an expanded porous foam core layer 12 (between the first and second solid skins 14, 16) formed from the plastic composition 40.

[0043] The opening step comprises removing the first mold part 22 so that the first solid skin 14 is no longer in contact with the first cavity-forming surface 26, while maintaining contact with the second cavity-forming surface 28 of the second solid skin 16. In the illustrated embodiment, this opening is accomplished by removing the outer mold part 22, exposing the first solid skin 14 to atmospheric pressure, while leaving the second solid skin 16 on the inner part 24.

[0044] However, any other configuration for opening the mold may be used. In particular, in alternative embodiments, at least one or more portions of the inner mold part 24 may be removed from the second solid skin 16 so that the second solid skin 16 (or any portion thereof) is additionally or alternatively exposed to atmospheric pressure.

[0045] This opening step allows the molten plastic composition 40 in the central layer 54 between the first and second solid skins 14, 16 to expand by foaming to form the core layer 12 of expanded porous foam 21. The expanded porous foam 21 is formed as a result of the molten plastic composition 40 in the central layer 54 directly beneath the first solid skin 14 expanding outward and away from the second solid skin 16. This action is controlled to stretch the first solid skin 14 in portion 52 to a desired stretch ratio.

[0046] Thereafter, in a cooling step, the expanded porous foam 21 is cooled and the molten plastic composition 40 in the central layer 54 between the first and second solid skins 14, 16 in the portion 52 is solidified to form an expanded porous foam section 6 in the article 2 with a core layer 12 of porous foam 21 between the first and second solid skins 14, 16. Cooling may be accomplished passively in the surrounding atmosphere or by active cooling (e.g., by blowing cool air onto the article 2).

[0047] After the cooling step, wall 10 typically has a thickness of 1 to 2 mm. Typically, portion 52 increases in thickness by 1 to 1.5 mm from the opening step to the cooling step to form wall 10.

[0048] Prior to the opening step, the first and second solid skins 14, 16 are curved and parallel to one another in the portion 52. After the cooling step, the first and second solid skins 14, 16 are also curved and parallel to one another in the wall portion 10.

[0049] After the cooling step, the length of the first solid skin 14 in portion 52 is stretched by a stretch ratio of 0.5 to 3% compared to the length of the first solid skin 14 present prior to the opening step. The stretch ratio is the ratio of the increase in length of the first solid skin 14 after the cooling step relative to the length of the first solid skin 14 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 stretch ratio of 2.5%. Preferably, the stretch ratio is 2 to 3%, more preferably 2.25 to 2.75%, and even more preferably 2.4 to 2.6%, e.g., about 2.5%.

[0050] In accordance with the present invention, the solid skins 14 are stretched as a result of the expansion of the molten plastic compound 40 in the central layer 54 between the first and second solid skins 14, 16 to ultimately form the solidified porous foam core layer 12 (containing hollow cells 11 within the solid matrix 9) between the first and second solid skins 14, 16. This stretching is controlled so that it is measurably small and uniform around the circumference of the expanded porous foam section 6, relative to the highly uniform thickness of the section 52 prior to opening the mold 20.

[0051] The degree of stretching can be controlled by a number of parameters readily determined by one skilled in the art, such as controlling the thickness of the first and second solid skins 14, 16 before opening the mold 20 (because thicker skins will be less prone to stretching as a result of the expansion pressure exerted on each skin by the expanding molten plastic compound), or by controlling the expansion pressure exerted on each skin by the expanding molten plastic compound by varying the concentration of blowing agent within the molten plastic compound.

[0052] Essentially, the inventors have unexpectedly discovered that by uniformly controlling the shape and size of the section(s) to be expanded, and by controlling the degree of expansion of the skin(s) as a result of the expansion of the molten plastic compound, the corresponding wall(s) in the resulting article can exhibit very precise shapes (e.g., very precise concentricity of the annular cross-section) combined with low thickness and mass, and can also exhibit high strength and excellent thermal insulation resulting from the presence of a core layer of expanded cellular foam.

[0053] In contrast, in the '661 patent, a corrugated surface on at least one molding surface effectively divides the annular injection preform into a series of arcuate sections that expand independently. The corrugated surface of the outer solid skin has "peaks" and "troughs," and each trough is "inflated" circumferentially during expansion, causing the shape of the outer skin to change to a more circular shape but preventing the outer solid skin from increasing in length. Because the outer skin is sufficiently thick before mold opening and / or the blowing agent concentration is low enough, there is insufficient expansion pressure to stretch the outer skin upon opening, preventing the increase in length. However, the inclusion of corrugated surfaces can result in upstanding longitudinal ribs in the final sidewall, resulting in the final sidewall having a slightly corrugated outer surface rather than a uniform circular shape, and potentially resulting in the final sidewall being excessively thick.

[0054] The present invention solves these problems in the '661 patent by controllably stretching the outer skin to controllably produce a constant and uniform thickness in the area to be expanded. A constant and uniform thickness in the area to be expanded ensures that uniform expansion pressure is applied throughout each wall or wall section. Applying uniform expansion pressure to areas of constant and uniform thickness also ensures that the expansion is constant and uniform. As a result, the final wall or wall section has a very uniform shape, size, and configuration. For example, in cup 2 of FIG. 1, the outer surface of the side wall is geometrically circular with high precision (in horizontal cross section), and the side wall has a very uniform wall thickness.

[0055] These improvements result in reduced wall thickness and mass for the article while maintaining the properties of the porous foam layer necessary to provide rigidity and thermal insulation to the wall. The invention can produce a finished article with a substantially constant sidewall thickness around its periphery. The resulting article has a controlled shape and size (e.g., a uniform circular cross-section in the sidewall). The hollow article has high wall strength and low mass, which are improved properties compared to the article disclosed in U.S. Patent No. 5,629,499.

[0056] 4a and 4b, the mold 20 is shaped and sized to form the narrow portion 35 of the cavity 32 between the first and second cavity-forming surfaces 26, 28 for the unexpanded portion 8. During the injecting step (and optional filling), the injection pressure (and optional filling pressure) is maintained above a minimum pressure threshold within the narrow portion 35 of the cavity 32 to maintain the gaseous physical blowing agent dissolved in the molten plastic compound 40 so that substantially no bubbles are formed within the narrow portion 35 of the cavity 32. Prior to the opening step, the molten plastic compound 40 within the narrow portion 35 of the cavity 32 is cooled to completely solidify, to form at least one unexpanded portion 8 within the article 2, comprising a substantially homogeneous, solid-phase unexpanded plastic compound 37.

[0057] Because this narrow section 35 of the mold cavity 32 is thin, the molten polymer resin requires a relatively short time (shorter than the injecting step (and optional filling)) to cool and solidify. This narrow section 35 can also be additionally cooled by an external cooler to convert the polymer resin from a molten liquid phase to a solid phase. After the opening step, the solid plastic compound cannot further expand by foaming to form an expanded porous foam. Therefore, the unexpanded section 8 appears transparent to the naked eye.

[0058] 5-7, Figure 5 is a bottom view of the base 70 of a food tray 72 formed by the method of a second embodiment of the present invention. The base 70 (and adjacent portions 78a, 78b, 78c, 78d of the side wall 80) includes four different regular arrangements 74a, 74b, 74c, 74d of a plurality of expanded wall portions 76a, 76b, 76c, 76d.

[0059] Four different arrangements 74a, 74b, 74c, 74d are shown on a common bottom 70 and sidewall 80, primarily to illustrate each arrangement 74a, 74b, 74c, 74d according to different embodiments of the present invention.

[0060] Typically, however, only one such array 74a, 74b, 74c, 74d will be present in the bottom 70 and sidewall 80, with one of the illustrated arrays 74a, 74b, 74c, 74d extending across substantially the entire bottom 70 and at least the central region of each adjacent portion 78a, 78b, 78c, 78d of the sidewall 80.

[0061] In the illustrated embodiment, arrays 74a, 74b, 74c, and 74d are present on bottom 70 and sidewall 80. However, in alternative embodiments, arrays 74a, 74b, 74c, and 74d may be included on either or both annular sidewall 80 and unitary bottom 70.

[0062] Each of the inflated walls 76a, 76b, 76c, 76d is spaced from one or more adjacent inflated walls 76a, 76b, 76c, 76d by a region 82a, 82b, 82c, 82d of plastic compound, which region 82a, 82b, 82c, 82d has a core layer of expanded porous foam between its inner and outer solid skins that has a lower degree of expansion than the inflated walls 76a, 76b, 76c, 76d, or comprises an unexpanded plastic compound throughout its thickness.

[0063] Figure 6 is a cross-sectional view of portion 152 in the area to form the bottom or sidewall of the food tray of Figure 5 prior to expansion during the opening step. Part 152 shown in Figure 6 is in mold cavity 132 (defined by mold parts 122, 124) prior to opening of mold 120. Part 152 includes first outer and second inner solid skins 114, 116 on either side of a central layer 154 of molten plastic compound 140.

[0064] Prior to the opening step, the first outer and second inner solid skins 114, 116 are flat and parallel to one another in the sections 152. Between adjacent sections 152, the plastic compound 140 is in the form of a thin layer 190 that is either completely solidified or has a thinner central layer of molten plastic compound.

[0065] Figure 7 is a cross-sectional view of a wall 110 of a portion of the bottom 70 of a food tray 72 formed from the section shown in Figure 6 after expansion during the opening step. The wall 110 includes first outer and second inner solid skins 114, 116 on either side of an expanded porous foam core layer 112 comprised of a plastic compound. After the opening step, the first outer solid skin 114 of the wall 110 is stretched so that it is curved and not parallel to the second inner solid skin 116, while the second inner solid skin 116 remains flat.

[0066] Between adjacent walls 110, a thin layer 190 remains, or a thinner layer of slightly expanded molten plastic compound remains, in each region 82. As a result, each region 82 acts as a series of anchor points 115 to prevent excessive or uncontrolled stretching of the first outer solid skin 114.

[0067] Figure 8 is a cross-section of a mold for forming a hollow article using an in-mold labeling process according to a third embodiment of the method of the present invention. Figure 9 is a side view of a hollow article with an in-mold label produced by the mold of Figure 8. Again, some dimensions have been exaggerated for clarity of illustration.

[0068] 8 and 9, the first and second mold parts 222, 224 are the outer and inner mold parts 222, 224 that respectively define the outer and inner surfaces 278, 280 of the article 202. The method is modified compared to the method described with respect to the first embodiment by further comprising, prior to the step of ejecting from the injection nozzle 221, the step of providing an in-mold label 290 that at least partially surrounds the cavity 232. The in-mold label 290 is positioned adjacent to the first cavity-forming surface 246 and radially outward from the second cavity-forming surface 248. Typically, the in-mold label 290 is annular and completely surrounds the cavity 232. As shown in FIG. 9, the in-mold label 290 preferably comprises an end-to-end polymer layer 292 having opposite ends 294, 296 joined together at a seam 298 to form an annular shape.

[0069] In the third embodiment, the injecting and opening steps are performed as in the first embodiment, except that after the opening step, the in-mold label 290 limits the stretching of the first solid skin formed adjacent to it. The in-mold label 290 is subjected to tension as a result of the expansion of the molten central layer to form a core layer of solidified expanded porous foam, and as a result of the stretching of the first (outer) solid skin.

[0070] 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. In this manner, the in-mold label 290 functions to control the shape, dimensions, and properties of the wall 210 of the hollow article 202.

[0071] FIG. 10 is a side view (partially in phantom cross section) of a third mold part that is combined with a second mold part as used in the first embodiment to form a hollow article by the method of the fourth embodiment of the invention.

[0072] In the fourth embodiment, the injecting and opening steps are performed as in the first embodiment. After the first mold part (not shown) has been removed, the intermediate molded article 302 with wall 310 having first (outer) solid skin 314 is on second mold part 324. However, after the opening step and before the cooling step, the molded article is placed into a second mold 320 having a third mold part 322. The third mold part 322 has a molding surface 324. The first (outer) solid skin 314 is placed in contact with the molding surface 324.

[0073] Thus, after the opening step, but before cooling to solidify the molten plastic compound in the central layer and form a core layer of solidified expanded cellular foam, the molding surface 324 limits the extension of the first (outer) solid skin 314 that is in contact with it.

[0074] Typically, during the placing step, the molding surface 324 compresses the wall 310, reducing the thickness of the central layer of molten plastic compound, thereby reducing the thickness of the core layer of porous foam radially inward of the first solid skin 314. Thus, the molding surface 324 can function to control (optionally, shrink) the length of the first (outer) solid skin 314 in the wall 110.

[0075] In a preferred embodiment, the third mold part 322 is hollow, and the molding surface 324 defines a hollow cavity 380 with a tapered surface 382. During the placing step, the molding surface 324 progressively presses against the first (outer) solid skin 314, thereby causing the compression described above.

[0076] Typically, the third mold part 322 is at a lower temperature than the first mold part that has been removed, so that the first solid skin 314 is cooled by contact with the third mold part 322. The third mold part 322 may be cooled by a cooling system (e.g., a flow of a cooling fluid through the mold part 322). While the hollow article 302 is placed in contact with the third mold part 322, the third mold part 322 is supported on the second mold part 324 to maintain the third mold part 322 at a lower temperature than the second mold part 324.

[0077] In some embodiments of the present invention, the method may further include pressing an embossing tool against the first solid skin prior to solidifying the molten plastic compound between the first and second solid skins during the cooling step to stretch the first solid skin inward toward the expanded porous foam, thereby imprinting an inward-facing image into the first solid skin. In embodiments in which a third mold part 322 is pressed against the first solid skin 14, the third mold part 322 may function as the embossing tool. The embossed image may be a pattern, logo, name, or the like.

[0078] Figures 11a and 11b are schematic side cross-sections of the wall of an injection molded part at different stages of a further embodiment of the method of the present invention, with Figure 11a showing the shape and configuration of the first solid skin before the mold is opened and Figure 11b showing the shape and configuration of the first solid skin after the mold is opened and the expanded porous foam has cooled.

[0079] After the injecting step (c) but before the opening step (d), an ordered array 400 of wall portions 402 in the intermediate article 404 is formed. Each wall portion 402 includes a central layer 406 of molten plastic compound between opposing first and second solid skins 408, 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 the substantially smooth portion 418 of the second solid skin 410 is formed by a second cavity-forming surface 420.

[0080] Prior to the opening step (d), each wall portion 402 of the regular array 400 includes a central convex apex 422. Also prior to the opening step (d), the outwardly extending convex surface 414 of each wall portion 402 is spaced from the outwardly extending convex surface 414 of each adjacent wall portion 402 by a plastic composition area 424 that is a solid layer (a layer containing no air bubbles) 426 of plastic composition extending between and in contact with the first and second cavity-forming surfaces 416, 420.

[0081] FIG. 12 is a schematic perspective view of a wall 430 of an article formed from the injection molded portion of FIG. 11, showing the shape and configuration of the first solid skin 408 after the mold is opened and the expanded porous foam has cooled.

[0082] During the opening step (d), each wall portion 402 in the regular array 400 of wall portions 402 expands to stretch the first solid skin 408, thereby forming a regular array 432 of wall sections 434, each wall section 434 formed from a respective wall section 402, after a cooling step (e). The regular array 432 of wall sections 434 comprises an array of convex walls 434 that are substantially pyramidal, conical, or part-sphere shaped. Each convex wall section 434 comprises a core layer 436 of expanded porous foam between opposing skin layers 438, 440. The skin layers 438, 440 comprise a first solid skin portion 438 having a three-dimensional shape that partially defines the convex surface 442 of the convex wall portion 434, and a substantially smooth second solid skin portion 444. Convex walls 434 are spaced from adjacent convex walls 434 by regions 446 of unexpanded plastic compound. In this embodiment, unexpanded regions 446 have distinct sections (typically circular) in plan view, although in other embodiments, unexpanded regions 446 may have other shapes or dimensions, and multiple unexpanded regions 446 may be interconnected.

[0083] Figure 13 is a cross-section of a mold for forming a hollow article according to the method of a sixth embodiment of the present invention. Figure 14 is a cross-section of a hollow article produced by the mold of Figure 13. The hollow article is a cup in the form of final expanded molded article 550 shown in Figure 14. Article 550 has an upper open end 552, a lower closed end 554, and a sidewall 556.

[0084] 13 and 14 , after the injecting step (c) and before the opening step (d), the plastic compound 500 forms a molded intermediate article 502 within the cavity 504. During the initial stages of the opening step (d), the first mold part 506 is removed, as indicated by the arrow in FIG. 13 , 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 part 506 exposes a first end 512 of the intermediate article 502, while an opposite second end 514 of the intermediate article 502 remains in contact with the first and second cavity-forming surfaces 510, 516. The first mold part 506 is then completely removed, as described above for the previous embodiments, so that the first solid skin 508 is fully exposed and extends from the first end 512 to the opposite second end 514.

[0085] After the initial stage begins, the first solid skin 508 at the first end 512 of the intermediate article 502 is exposed. Therefore, throughout 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. Additionally, the plastic compound at the first end 512 is hotter than the plastic compound at the second end 514 because, as a result of the first end 512 being exposed before the second end 514, the plastic compound at the first end 512 is in contact with the relatively cooler first mold part 506 for a shorter period of time than the plastic compound at the second end 514.

[0086] Thus, after the cooling step (e), the length of the first solid skin 508 at the first end 512 of the final expanded molded article 550 (shown in FIG. 14 ) has been stretched to a higher stretch ratio than the length of the first solid skin 508 at the second end 514 of the final expanded molded article 550. This is because the time to achieve stretch before cooling is longer at the first end 512 than at the second end 514.

[0087] The first end 512 of the intermediate article 502 is injection molded to have a thinner wall thickness than the second end 514 of the intermediate article 502. This thickness difference is greatly exaggerated in FIG. 13 . Typically, the wall thickness of the intermediate article 502 gradually tapers, increasing in thickness from the first end 512 of the intermediate article 502 to the second end 514 of the intermediate article 502. This thickness difference at least partially compensates for the higher stretch ratio for the length of the first solid skin 508 at the first end 512 of the intermediate article 502 compared to the second end 514 of the intermediate article 502. Typically, the wall thickness of the first end 512 of the intermediate article 502 is 5 to 15% thinner than the wall thickness of the second end 514 of the intermediate article 502. This thinner wall thickness at the top end of the intermediate article 502 reduces excessive skin stretching at the top end of the resulting article. As an example, for a typical container having a second end 514 thickness of 0.7 mm with sidewalls (e.g., having a height of 135 mm) inclined at an angle of 6 degrees to the longitudinal axis of the container, the first end 512 would be approximately 0.05 mm thinner than the second end to avoid excessive stretching at the top end of the container.

[0088] The first and second ends 512, 514 of the intermediate article 502 form the respective first and second wall portions 520, 522 in the final expanded molded article 550 shown in Figure 14. The first and second wall portions 520, 522 typically have the same thickness within a tolerance of ±0.5% based on the nominal thickness of the respective wall portions 520, 522.

[0089] In this embodiment of the invention, initial removal of the first mold part 506 exposes the first end 512 of the intermediate article 502 (typically to the atmosphere). This exposure allows the first solid skin at the first end 512 to stretch as a result of the expansion of the molten plastic compound in the central layer. In contrast, the opposite second end 514 of the intermediate article 502 remains in contact with the first and second cavity-forming surfaces 510, 516 and is therefore initially unable to expand or stretch. Subsequently, once the opposite second end 514 has been exposed (after complete removal of the first mold part 506), which takes only a limited time, the first solid skin 508 at the second end 514 can subsequently stretch. The first solid skin 508 at the first end 512 has more time to stretch than the second end 514. Thus, if it is desired to maintain a substantially constant wall thickness in the final expanded molded article 550, the cavity thickness at the first end 512 is made thinner than at the second end 514 to compensate for the increased stretch ratio (associated with the extended stretch time) at the first end 512 compared to the second end 514. This allows, for example, for a constant wall thickness to be formed despite a higher stretch ratio of the skin at the upper open end 552 of the hollow article compared to the lower closed end 554 of the hollow article.

[0090] In the illustrated embodiments of the present invention, the annular sidewall is straight in longitudinal cross section, and therefore may be frustoconical or frustopyramidal. In other embodiments of the present invention, the annular sidewall has an upper annular end distal to the bottom and a lower annular end adjacent the bottom, the upper end having a longer perimeter than the lower end, and the sidewall is curved in longitudinal cross section, e.g., the article may be shaped like a bowl with a larger opening. In such embodiments, the outer first solid skin is stretched to a greater extent at the upper annular end than at the lower annular end. This is because the much larger perimeter causes increased stretching of the outer first solid skin during formation of the expanded porous foam core layer. In preferred embodiments of the present invention, the article may be a cup, mug, bottle, pot, bowl, tray, receptacle, or container for holding food liquids, such as a coffee cup or food tray. The receptacle may be heat-resistant and suitable for heating beverages or food in a microwave oven. The articles may be disposable or reusable, and in either case are recyclable because the articles are composed of a single polymer (eg, polypropylene).

[0091] Various modifications to the illustrated embodiments will be apparent to those skilled in the art and are intended to be encompassed within the scope of the invention as defined by the appended claims.

Claims

1. 1. A method of forming an article, said article being a hollow container made of a single polymer, said method comprising: (a) providing a mold having a first mold part and a second mold part, the first and second mold parts having respective first and second annular cavity-forming surfaces; (b) closing the mold, thereby defining an annular cavity between the first and second cavity-forming surfaces; (c) injecting a molten plastic composition into the cavity, the molten plastic composition including a polymer and a physical blowing agent, the physical blowing agent being a gas dissolved in the polymer, at an injection pressure, and cooling the plastic composition in contact with the first and second cavity-forming surfaces during or after the injecting step (c) to form first and second solid skins adjacent to and in contact with the first and second cavity-forming surfaces, respectively, thereby depositing at least one annular portion of the plastic composition within at least one annular section of the annular cavity, wherein at least a portion of the plastic composition between the first and second solid skins remains molten in the annular portion, the thickness of each annular portion being constant within a tolerance of ±0.5% based on the nominal thickness of the respective annular portion, and the first and second solid skins being curved and parallel to each other within the at least one annular section; (d) opening the mold so that the molten plastic compound between the first and second solid skins in each of the annular portions is exposed to an external pressure lower than the injection pressure before the molten plastic compound between the first and second solid skins solidifies in the at least one annular portion, thereby allowing the molten plastic compound between the first and second solid skins in each of the annular portions to expand by foaming to form a porous foam, and stretching the first solid skin in each of the annular portions as a result of the molten plastic compound directly below the first solid skin expanding outward away from the second solid skin, and the opening step includes removing the first mold part so that the first solid skin is no longer in contact with the first cavity-forming surface; (e) cooling the porous foam to solidify the molten plastic compound between the first and second solid skins in each of the annular portions to form at least one first wall within the article with a core layer of expanded porous foam between the first and second solid skins, the hollow container having an annular sidewall, the first wall being contained within the annular sidewall and being continuous and annular; Equipped with After the cooling step (e), the length of the first solid skin in each annular portion is stretched by a stretch ratio of 0.5 to 3% compared to the length of the first solid skin present before the opening step (d), wherein the stretch ratio is the ratio of the increase in length of the first solid skin after the cooling step (e) to the length of the first solid skin before the opening step (d).

2. The method of claim 1 , wherein the thickness of each of the annular portions is constant within a tolerance of ±0.2% relative to the nominal thickness of each of the annular portions.

3. 3. The method of claim 2, wherein after the cooling step (e), the length of the first solid skin in each annular portion is stretched by a stretch ratio of 2 to 3%, alternatively 2.25 to 2.75%, alternatively 2.4 to 2.6%, alternatively 2.5%, relative to the length of the first solid skin present before the opening step (d).

4. 4. The method of claim 1, wherein prior to the opening step (d), the at least one annular portion has a thickness of greater than 0.5 mm to 1 mm.

5. 5. The method of claim 1, wherein after the cooling step (e), the at least one first wall has a thickness of 1 to 2 mm.

6. 6. The method of claim 1, wherein from the opening step (d) to the cooling step (e), the at least one annular portion increases in thickness by 1 to 1.5 mm to form the at least one first wall portion.

7. 7. The method of claim 1, wherein the first and second mold parts are outer and inner mold parts that form the outer and inner surfaces of the hollow container, respectively.

8. 8. The method of claim 1, wherein after the cooling step (e), the first and second solid skins of the at least one first wall are curved and parallel to each other.

9. 9. The method of claim 1, wherein the article is a hollow container having an annular side wall and an integral bottom, and at least one portion of at least one or both of the annular side wall and the bottom is respectively defined by the at least one first wall portion.

10. 10. The method of claim 9, wherein the annular sidewall has an upper annular end distal to the bottom and a lower annular end adjacent the bottom, the upper annular end having a greater perimeter than the lower annular end, the sidewall being curved in a longitudinal cross section such that the stretch ratio is greater at the upper annular end than at the lower annular end.

11. 11. The method of claim 1, wherein after the injecting step (c) and before the opening step (d), the plastic compound forms a molded intermediate article in the cavity, and during the initial stage of the opening step (d), a first end of the molded intermediate article is exposed by removing the first mold part so that the first solid skin is no longer in contact with the first cavity-forming surface, while an opposite second end of the molded intermediate article remains in contact with the first and second cavity-forming surfaces.

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

13. The method of claim 12, wherein the first end of the molded intermediate article has a thinner wall thickness than the second end of the molded intermediate article.

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

15. 15. The method of claim 13 or claim 14, wherein the first and second ends of the molded intermediate article form respective first and second wall portions in the final expanded molded article, the first and second wall portions having the same thickness within a tolerance of ±0.5% based on the nominal thickness of the respective wall portion.

16. 16. The method of any one of claims 13 to 15, wherein the wall thickness of the molded intermediate article gradually tapers with an increasing thickness in a direction from the first end of the molded intermediate article to the second end of the molded intermediate article.

17. 17. The method of any one of claims 1 to 16, wherein the polymer is polypropylene.

18. 18. The method of any one of claims 1 to 17, wherein the physical blowing agent is carbon dioxide.

19. 19. The method of any one of claims 1 to 18, wherein during the injecting step (c), the injection pressure is at least 150 bar.

20. 20. The method of any one of claims 1 to 19, wherein at the end of the injecting step (c), a packing pressure of at least 150 bar is applied to the plastic compound in the cavity.

21. 21. The method of any one of claims 1 to 20, wherein during the opening step (d), the first solid skin is exposed to the atmosphere and the external pressure is atmospheric pressure.

22. 22. The method of claim 1, wherein the first and second mold parts are outer and inner mold parts that respectively form the outer and inner surfaces of the article, and further comprising, prior to the injecting step (c), providing an in-mold label that at least partially surrounds the cavity, the in-mold label being positioned adjacent to the first cavity-forming surface, and wherein, after the opening step (d), the in-mold label limits extension of the first solid skin formed adjacent to it.

23. 23. The method of claim 22, wherein the in-mold label is annular and completely surrounds the cavity.

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

25. 25. The method of claim 1, further comprising, between the opening step (d) and the cooling step (e), placing the molded article into a second mold having a third mold part, the third mold part having a molding surface with which the first solid skin is in contact, and wherein the molding surface limits extension of the first solid skin in contact with it after the opening step (d).

26. 26. The method of claim 25, wherein during the placing step, the molding surface compresses the at least one first wall to reduce a thickness of the expanded porous foam core layer between the first and second solid skins and to shorten a length of the first solid skin of the at least one first wall.

27. 27. The method of claim 25 or claim 26, wherein the third mold part is hollow and the molding surface defines a hollow cavity with a tapered surface, and wherein the molding surface presses against the first solid skin during the placing step.

28. 28. The method of any one of claims 25 to 27, wherein the third mold part is at a lower temperature than the first mold part such that the first solid skin is cooled by contact with the third mold part.

29. 29. The method of any one of claims 1 to 28, wherein the first mold part is at a lower temperature than the second mold part.

30. 30. The method of any one of claims 1 to 29, further comprising the step of imprinting an inward-facing image into the first solid skin by pressing an embossing tool against the first solid skin to stretch the first solid skin inward toward the expanded porous foam before the foamed molten plastic compound between the first and second solid skins solidifies in the cooling step (e).

31. 31. The method of any one of claims 1 to 30, wherein the article is a cup, mug, bottle, pot, bowl, tray, receptacle, or container for holding food or liquid.

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