Stackable containers

Abstract

A stackable container 10 for liquids or flowable solids formed of a plastics material, the container comprises side walls 16 with circumferentially spaced foam regions of increased wall thickness that

Description

Field of the invention The present invention relates to stackable containers for liquids or flowable solids and to a method of manufacturing such containers. The containers may be conical cups for beverages, or larger tapering containers, such as pails, for products such as paints. Background Though the invention is applicable to containers of different shapes and sizes, it will be considered first in relation to single use 12 oz (fluid ounce) drinking cups, i.e. a conical cup having a volume of about 360 ml. For environmental reasons, fast food outlets have endeavoured to switch from plastics cups to paper cups. Ironically, this is believed to be a retrograde step, as plastics cups can be recycled more efficiently than paper cups. As paper is not water impermeable, paper cups need to be coated with a plastics film. The film cannot itself be recycled and it makes the paper difficult to recycle. The present invention is therefore instead concerned with cups made from a plastics material suitable for recycling, such as polypropylene or polyethylene. It is important for beverage cups to provide efficient thermal insulation, so that a consumer may safely hold a hot beverage. It is known to provide a plastics cup with a corrugated paper sleeve to achieve the desired degree of insulation, but this has its own disadvantages and it is preferred to provide a plastics cup that can itself provide a sufficient degree of thermal insulation to enable the cup to be handled when filled with a hot beverage at a temperature approaching 100°C. Object of the invention The invention seeks to provide a stackable plastics container that offers thermal insulation and structural strength to resist radial and / or axial compression, while minimising the weight of plastics material required to manufacture the container and optimising its stacking height. Summary of the invention In accordance with a first aspect of the invention, there is provided a stackable container for liquids or flowable solids formed of a plastics material, wherein side walls of the container have circumferentially spaced foamed regions of increased wall thickness that form ribs projecting from the outer surface of the container and extending over at least part of the axial length of the container. As the projecting ribs in the present invention are formed of foamed plastics material, as well as being thicker than the regions between them, they have poorer thermal conductivity on account of the trapped gas. They can therefore provide a sufficient thermal barrier to enable a cup filled with a hot liquid to be handheld comfortably. In an embodiment of the invention, the regions of increased wall thickness additionally form ribs projecting from the inner surface of the container. In such an embodiment, it is desirable for the width of the ribs to be less than their circumferential separation, to allow two containers to be stacked one within the other, with the ribs on the outer surface of the upper container locating in between the ribs on the inner surface of the lower container. For this to be possible, the ribs on the inner and outer surfaces must, of course, have the same separation and inclination as one another. Such a configuration offers the advantage that the containers may be stacked more closely together, for any given maximum thickness of the foamed regions of the container. Ribs can be used not only to provide thermal insulation but also to improve structural rigidity. A container requires structural strength to resist radial compression, this being referred to as the hoop strength, and to resist axial compression, referred to as the top load. In embodiments of the invention, the inclination of the ribs is selected to achieve desired hoop strength and top load. The term “inclination” is used herein to refer to the angle measured in the plane of the wall of the container between the length of the rib and the line of intersection of the wall with a plane containing the central longitudinal axis of the container. If the inclination is 0°, i.e. the ribs run parallel to the axis of the container, then the top load is maximised, but the ribs do not improve the hoop strength. On the other hand, if the inclination is 90°, i.e. the ribs are circumferentially extending rings, then the hoop strength is maximised but the ribs do not improve the top load. In order that the ribs should increase both hoop strength and top load, it is desirable for them to have an inclination greater than 0°. However, the presence of such inclined ribs could interfere with the stackability of the containers if each container has both internally and externally projecting ribs. If the containers are of circular cross section and all the ribs are evenly spaced and inclined in the same direction, then the containers remain stackable, as the ribs on adjacent cups can then interdigitate with one another. In such an embodiment, the containers would rotate slightly relative to one another as they are being stacked. Such a configuration is preferred for a drinking cup because the distance between adjacent ribs would be constant over the height of the cup, ensuring that the user will only contact the ribs, rather than the hotter regionals between the ribs. In such embodiments, the inclination is preferably such as to ensure that any line on the wall of the container lying in an axial plane will intersect two or three ribs over the height of the container. For this to be achieved in a 360 ml cup, the inclination may typically be in the range of 5° to 7°. Such a configuration offers adequate hoop strength for a drinking cup. When the container is of a non-circular cross section, such as a square, then it is possible for alternate ribs to be inclined in opposite directions to one another, to form V or W shapes that allow containers to be stacked into one another. With ribs of this configuration, which can also be used on circular containers, the separation between ribs is not constant over the height of the container, but this is of lesser significance when the purpose of the ribs is to improve structural strength rather than provide thermal insulation. In such an embodiment, the inclination of the ribs may be steeper, possibly in the range of 10° to 15° or even more. The mouth of the container need not be strengthened by foaming but may instead be strengthened by a conventional curled lip or flange surrounding the mouth. According to as second aspect of the invention, there is provided a method of injection moulding a container which comprises providing a mould having a first mould part defining the outer surface of a mould cavity and a core movable relative to the first mould part and defining an inner surface of the mould cavity, wherein at least one of the inner surface and outer surfaces of the mould cavity is formed with circumferentially spaced flats or cut-aways to increase the thickness of the mould cavity, applying pressure to close the mould cavity, injecting a molten plastics material containing a blowing agent into the closed mould cavity, releasing the pressure acting to close the mould cavity after formation of a skin on the injected plastics material but prior to complete solidification of the plastics material, and allowing foaming of the plastics material to take place after opening of the mould to form a container having projecting ribs. Though it is possible to form cut-aways in the first (female) mould part, in some embodiments, flats or cut-aways are formed only on the surface of the core. Aside from the fact that such a design simplifies manufacture, it is desirable for the female mould part to have a smooth conical wall as this is better to suited to in-mould labelling. During the injection moulding process, the temperatures of the first mould part and of the core are set such that, when the plastics material is injected into the mould cavity, the regions of the molten plastics material in contact with the mould cavity are cooled and form inner and outer skins. The depth of the skins is determined by temperatures of the surfaces of the mould cavity and the time during which the molten plastics material remains in contact with the mould before it is opened. By appropriate setting of the temperatures and the timing, it is possible to ensure that the plastics material hardens within the regions of the mould cavity that are not thickened so that on release of the pressure in the mould cavity foaming will only occur within the thickened regions of the cavity to cause the skin to stretch and thereby produce the desired ribs. The temperatures of the surfaces of the cavity will affect the ability of the inner and outer skins to stretch, so that ribs can be formed on the outer surface of the container even though prior to foaming the thickening is only present on the inner surface of the moulded plastics material. Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure lisa front view of a drinking cup of a first embodiment of the invention, Figure 2 shows a section taken in the plane II-II in Figure 1, Figure 3 show a simplified exploded view of a mould and a moulded cup, Figure 4 is a perspective view of the core shown in Figure 3, Figures 5A and 5B are diagrams illustrating how forming ribs on both the interior and exterior of cups reduces their stacking height, for a given degree of thermal insulation, Figure 6 is a perspective view of a pail of a second embodiment of the invention, and Figures 7 and 8 show schematically a section and a plan view of a further embodiment of the invention. Detailed description of the drawings Figure 1 shows a front view of a drinking cup of the invention and Figure 2 shows a section through the same cup in the plane designated II-II in Figure 1. The cup 10 has a base 12, a top rim or mouth 14 and side walls 16. The side walls 16 have ribs 18 that project from the outer surface and aligned ribs 20 projecting from the inner surface. The ribs 18 and 20 are evenly spaced around the circumference of the cup 10 and the spacing between adjacent ribs is greater than the width of the ribs. Furthermore, the ribs 18 should be sufficiently close to one another to ensure that the cup can be handheld while only contacting the ribs 18. The broken line 24 represents the intersection of the side wall of the cup 10 with a plane of symmetry of the cup, i.e. a plane passing through the central axis of the cup. It will be seen from Figure 1 that the ribs 18 and 20 are inclined at an angle to the line 24, this angle being referred to herein as the inclination of the ribs 18 and 20. As well as providing thermal insulation, the ribs 18 and 20 act to stiffen the side wall 16 of the cup. Because the inclination is greater than 0°, they serve to improve not only the top load but also the hoop strength of the cup 10. As the inclination is increased, the hoop strength is increased and the top load decreased. It has been found that a suitable compromise for a drinking cup is for the inclination to be such that the line 24 intersects two or three ribs 18 over the height of the container. For a 360 ml container, the inclination lies in the range of 5° to 7°. It will be noted that cups 10 as shown in Figures 1 and 2 are stackable within one another. The ribs 18 on the outer surface of each upper cup can lie between the ribs 20 on the inner surface of the next lower cup. The inclination of the ribs 18 and 20 will act in the same way as rifling to cause the upper cup to rotate as the ribs 18 intermesh with the ribs 20 of the lower cup. The effectiveness of the thermal insulation provided by the ribs 18 and 20 will depend on the combined thickness of the ribs on the inner and outer surfaces. While it would be possible to provide ribs of a desired thickness on only one of the inner and outer surfaces, by providing ribs on both surfaces the stacking height of the cups is reduced. This is illustrated in Figures 5A and 5B. In Figure 5A, a detail is shown of the side walls of three stacked cups in which ribs of a desired height are provided on only one side, while Figure 5B the cups have ribs on their inner and outer side with a combined height equal to the height of the rubs in Figure 5A. As the thickness of the ribs on each surface in Figure 5B is only half of the thickness of the ribs in Figure 5A, the cups can to be stacked more closely to each other. The size of the container in the present invention is not restricted to small drinking cups. Figure 6 shows a pail 50, for example for paint, which is provided with strengthening ribs 52. In this case, the additional thermal insulation provided by the ribs offers no benefit, but the ribs 52 allow a pail of the desired hoop strength and top load to be manufactured using less plastics material. Instead of all the ribs being inclined in the same direction, Figure 6 shows that alternative ribs may be inclined in opposite directions, allowing the pails 50 to be stacked without them rotating relative to one another. This also demonstrates that the invention is not restricted to containers of circular section, but can be used to produce square containers 60, as shown schematically in Figures 7 and 8, that are provided with ribs 62. The mouths of all the containers described above can be manufactured in a conventional manner. Thus, the cup 10 in Figure 1 may have a curled lip 22 and the pails of Figures 6 to 8 may have a flange 54 of L-shaped cross section surrounding the mouth and reinforced, if necessary, by stiffening ribs 56. As such stiffening of the mouth of containers is well known, it need not be described herein in detail. While it would be possible to use foaming to provide hoop strength around the mouth of containers, it is not currently preferred as it is difficult to ensure foaming uniformity, shape and size control being of importance if the container is to be fitted with a lid. Furthermore, a better interlock is achieved with a lid by a curled lip 22 having precisely defined deformability. Figure 3 shows some parts of an injection moulding machine that may be used to form the cup of Figures 1 and 2. The machine comprises two platens 30 and 32 that are movable towards and away from one another to close and open the mould cavity. The platen 30, which is stationary holds the first or female part of the mould 31 while the male part or core 33 is carried by the movable platen 32. The mechanism for moving the platens relative to one another is conventional and not shown in the drawing. The mechanism may be operated hydraulically or mechanically and may include arms employing a toggle action to press the platens 30 and 32 firmly against one another to close the mould. Other components of the injection moulding machine have also been omitted from the drawing as they may be generally conventional. These components include a screw in which granules of the plastics material are melted and later injected into the mould cavity, a cooling system for setting the temperatures of the two parts of the moulds defining the mould cavity, and means for ensuring precise alignment of the platens when the mould is closed. The moulding machine also includes known means of adding a blowing agent to the plastics material as it is being compressed in the screw. For the blowing agent, it is preferred to use a combination of nitrogen gas (N2) and chemically generated carbon dioxide (CO2). The reason for using two different types of blowing agent is that the bubbles of nitrogen created by using MuCell® (from www.trexel.com) expand rapidly but then exhaust through the material and can cause some collapse as the rib cools. On the other hand, Endothermic Chemical Blowing Agent (CBA) uses vinegar and baking soda, which produce CO2 and a small amount of water. As long as the material is above its TG (glass transition temperature), the chemical reaction continues applying pressure, keeping a continuous internal pressure in the rib until after it has cooled enough to remain rigid. The first part 31 of the mould merely defines a smooth walled conical cavity, a base, and a protruding ring around the rim (to form the reinforcement curled lip). The core 33 of the mould, which is shown separately in Figure 4, comprises a cone dimensioned to fit in the conical cavity to leave only a gap of 0.35 mm. To produce thickened portions for forming the ribs 18 and 20, material is removed only from the surface of the core 33 to produce the circumferentially spaced flats 35 shown in Figure 4. The width of the mould cavity in the central region of the flats 35 is about 0.55 mm. It should be noted that the spacing between the flats is less than the spacing between the ribs, this being on account of the fact that the skin does stretch uniformly across the width of the flats. An annular recess 37 surrounds the top of the protruding cone to receive the ring projecting from the rim of the first part 31 of the mould. In use, after the mould cavity is closed by the two platens 30 and 32 being brought into contact with one another, plastics material containing the blowing agent is injected under high pressure into the mould cavity, with vents being provided in the normal way to allow air within the cavity to escape. As soon as the plastics material contacts the walls of the mould cavity, it starts to form inner and outer skins. After the plastics material in the walled regions of the cup has hardened, the pressure in the mould cavity is released by commencing to move the platens 30 and 32 apart. At this time, the inner and outer skins are still stretchable and the pressure of the trapped gases causes the regions that initially had a thickness of 0.55 mm to expand both outwards and inwards to form the ribs 18 and 20 which, in combination, have a thickness of 1.5 mm, thereby producing the finished cup, designated 40 in Figure 3. The cup in Figure 1, which has a capacity of 360 ml (12 oz), can have a weight of only 10.5g when made of polypropylene and have a nested height of only 11 mm, the nested height being the distance between corresponding parts of two nested cups. By comparison, a twin-wall paper cup weighs 14g and has a nested height of 20 mm. The thermal insulation of the 1.5 mm thick expanded ribs is adequate forboiling water assuming the cup is held around the tops of the ribs. The ribs are not designed to keep the beverage hot for longer periods. When forming a pail, a greater degree of rigidity is required, making it necessary to form side walls with a greater thickness than a cup. In this case, the mould cavity may have a thickness of 1.1 mm in the regions which are to expand to form the ribs and 0.8 mm in the regions between the ribs that are to set within the mould. The blown thickness of the ribs is however restricted to 1.5 mm as a greater thickness would interfere with stacking. The expanded rib thickness is controlled by the following factors, namely : - • The thickened width. - The cup will resist expanding under 0.45mm, which limits the width. The effective rib width + 2.5% skin stretch dictates the height of the rib on the outer surface. • The extent to which the thickened regions are blown out. - Typically, one aims to increase the thickness of the parts that form the ribs by between 50% and 100%. • The cooling time before releasing the pressure in the mould cavity. - This is set typically at about 0.2 seconds for the illustrated cup but depends on other factors. • The cooling time on the core. - This is the time delay after opening of the mould but before ejecting the cup. The longer the longer the cup remains on the male mould, the less it can deform inwards. • The temperature of the female mould. - This is typically set at 85°C to encourage outward expansion. • Core temperature. - This is typically set at between 40°C and 60°C to limit inward expansion. A cold male mould would create a thick inside skin that would limit the gaseous melt thickness that is needed to separate the skins of the rib for expansion. A suitable skin stretch factor for the cup is 2.5%, but for the pail a lower value of 2% is preferred, as this allows a slightly wider rib to add more stiffness. If a skin stretch factor of 2.5% is used on the pail 50 of Figure 6, the rib 52 would over expand and interfere with stacking. Note: These are two examples of the application of inclined ribs, each application, from vending cups to 25 litre pails, will require configuring to suit the application. It is possible to use narrower ribs with 3% + skin stretch but that would require no cooling time and higher male / female mould temperatures. Even with no cooling time, there is still an adjustable time delay during the unlocking movement of the mould closing mechanism. A small cooling time is preferred to have control over the process. In practice, the optimum percentages of nitrogen N2 and CB A are readily determined empirically, i.e. by experimentation.

Claims

1. A stackable container for liquids or flowable solids formed of a plastics material, in which side walls of the container have circumferentially spaced foamed regions of increased wall thickness that form ribs projecting from the outer surface of the container and extending over at least part of the axial length of the container.

2. A stackable container as claimed in Claim 1, wherein the regions of increased wall thickness additionally form ribs projecting from the inner surface of the container.

3. A stackable container as claimed in Claim 2, the width of the ribs is less than their circumferential separation, to allow two containers to be stacked one within the other, with the ribs on the outer surface of the upper container locating in between the ribs on the inner surface of the lower container.

4. A stackable container as claimed in any preceding claim, wherein the ribs are inclined relative to a line of intersection of the side wall with a plane containing the central longitudinal axis of the container.

5. A stackable container as claimed in any preceding claim, wherein the container is of circular cross section and all the ribs are evenly spaced and inclined in the same direction.

6. A stackable container as claimed in Claim 5, wherein the inclination is such as to ensure that any line on the wall of the container lying in an axial plane will intersect two or three ribs over the axial length of the container.

7. A stackable container as claimed in Claim 6, wherein the container is a beverage cup having a capacity of 360 ml, the inclination of the ribs lies in the range of 5° to 7°.

8. A stackable container as claimed in any one of Claim 1 to Claim 4, wherein alternate ribs to be inclined in opposite directions to one another.

9. A stackable container as claimed in Claim 8, wherein the inclination of the ribs lies in the range of 10° to 15° or even more.

10. A method of injection moulding a container as claimed in any preceding claim, which comprises providing a mould having a first mould part defining the outer surface of a mould cavity and a core movable relative to the first mould part and defining 5 an inner surface of the mould cavity, wherein at least one of the inner surface and outer surfaces of the mould cavity is formed with circumferentially spaced flats or cut-aways to increase the thickness of the mould cavity, applying pressure to close the mould cavity, injecting a molten plastics material containing a blowing agent into the closed mould cavity, releasing the pressure acting to close the mould cavity after formation of a skin on io the injected plastics material but prior to complete solidification of the plastics material, and allowing foaming of the plastics material to take place after opening of the mould to form a container having projecting ribs.

11. A method as claimed in claim 10, wherein the flats or cut-aways are formed 15 only on the surface of the core.

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