GB2067160A

GB2067160A - Plastics container base

Info

Publication number: GB2067160A
Application number: GB8039995A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-12-13
Filing date: 1980-12-12
Publication date: 1981-07-22
Also published as: ES255009Y; NL8006687A; ES255009U; DE3046977A1; IT1197398B; IT8026653A0; BR8008294A; FR2471921A1

Abstract

A container (11) formed by stretch blow moulding a preform of a plastics material, such as polyethylene terephthalate, has a domed base (12) which has a concave central portion (13) protruding into the container and periphery within a range of diameters able to withstand internal pressure. The periphery of the concave central portion intersects an inwardly curved peripheral wall of the base to form a narrow rim (14) from which extend a plurality of curved legs (15) circumferentially distributed around the base. The legs bulge outwardly and are shaped so that the container is self- supporting on the legs. The legs may be sufficiently long that the rim cannot bulge far enough under the influence of internal pressure to topple the container. <IMAGE>

Description

SPECIFICATION A plastics container having an improved base structure This invention relates to plastics containers produced by stretch blow moulding, and more particularly concerns an improved base structure for such moulded containers intended for use with carbonated beverages.

Containers produced by stretch blow moulding plastics materials, such as polyethylene terephalate, polypropylene, vinylidene chloride or the like are lightweight, have good transparency and high strength and deteriorate less with time than containers produced by normal blow moulding. Stretch blow moulded containers are also safe in terms of food hygiene, especially those made from polyethylene terephthalate. As a result stretch blow moulded containers are being increasingly used.

However, if a container having a flat base is used to contain carbonated beverages, the internal pressure in the container causes the base to bulge outwardly so that the container is not self-supporting without modification.

Accordingly, in the case of containers to which internal pressure is applied, the base is moulded into a convex hemi-spherical configuration to avoid deformation by the internal pressure and a plastics base cup is snugly fitted into the base to make the container selfsupporting.

A container provided with such a base cup is naturally more costly than a normal selfsupporting container. A container having a base cup can be used despite its relatively high cost where the container has a capacity greater than 1 litre. However, in the case of a container with a capacity of less than 1 litre, the cost of the container becomes a significant proportion of the selling price of the contents and therefore it is difficult to use containers having base cups for general carbonated beverages. Even the reclaiming of used plastics material becomes more expensive if containers with base cups are used, because it is then necessary to separate the container body from the base cup during the reclaiming process.

Thus, containers that are self-supporting in the same way as the flat bottomed containers used with non-carbonated beverages are in demand and some have already been produced. The base of one known self-supporting container comprises a concave central portion extending into the container and a domed rim portion formed around the periphery of the concave central portion, the curvature between the concave central portion and the rim portion being selected to withstand the internal pressure.

However, even with the base structure described above, if the inside diameter of the rim portion, that is, the diameter of the concave central portion is enlarged as it is in order to improve stability of the container, the base tends to deform under the internal pressure. This deformation appears inside the rim portion because of the internal pressure applied to the concave central portion which projects under the influence of the internal pressure.

The upper limit for the diameter of the concave central portion before deformation occurs varies to some extent with the diameter of the base. An experiment was conducted using a test piece in which a central portion of a circular base is formed into a concave configuration and a narrow peripheral edge of the convexed central portion intersecting a bottom wall is used as a rim portion by which a container can be stood upright. This experiment demonstrated that, with base diameters of 4.8 cm (200 cc container), 7.3 cm (500 cc container) and 8.0 cm (1000 cc container), the limits for the diameter of the concave central portion were respectively 3.0 cm, 3.6 cm and 4.0 cm. In case of larger diameters, the rim portion becomes deformed under the influence of internal pressure and the concave central portion projects outwards.Although such phenomena would be affected to some extent by distribution of wall thickness and orientation of the base, which naturally varies with the magnitude of the diameter of the base, it is essential in securing the stability of the container that the diameter of the concave central portion, that is, the size of the rim required for the container to be self-supporting should be within the limits mentioned above for each given diameter of the base.

To enable the rim portion to be made larger than the aforesaid limitation and the concave central portion not to project under the influence of internal pressure, the central portion is made higher or formed into a domed configuration such as is provided in the base of a printed wine bottle. Alternatively the bottom may be formed with ribs for reinforcement.

However, in stretch blow moulding, if the distance between a top surface of the concave central portion and a bottom surface of the rim portion becomes poor and as a result the bottom surface will be uneven preventing the container being stood stably upright. Such is the case with the container of U.S. Patent 3,598,270, which discloses moulding a plurality of legs for blocking a circular base.

Attempts have been made in tests to mould containers having legs substantially similar to the legs disclosed in U.S. Patent No.

3,598,270 which are high at the back and have a larger bulge, and it was found that the probability of moulding a plurality of legs all with the same configuration was small, except for 2000 cc containers, and most 500 cc containers were uneven.

Various other shapes of legs have been moulded in tests and it has been found that the degree of difficulty in moulding the legs is dependent upon the shape of the container base. The base shape which most facilitates moulding is that where the peripheral wall of the base has a curved surface and is partially bulged outward. Conversely, in the case where a plurality of legs are formed so as to bulge toward a flat bottom face or where a plurality of depressions are formed from the peripheral wall of the base to the bottom face to form a plurality of independent legs, in U.S. Patent No. 3,935,955, it is often the case that even if the concave central portion of the base is relatively shallow, the legs are difficult to mould and may be uneven.

It has been found difficult to mould not only container bases where the innermost part of the concave central portion is spaced apart from the rim by too great a distance, but also bases having narrow grooves forming ribs as described, for example, in U.S. Patents 3,403,804 and 3,727,783.

The difficulty of moulding the aforesaid legs or ribs and grooves results from the fact that a preform is thinly stretched by blowing in a mould cavity, the preform setting and losing heat during this expansion and ceasing further elongation when in contact with the cavity.

Thus, the greater the distance between the portion initially in contact with the cavity and the portion finally in contact with the cavity the more difficult it becomes to ensure good expansion and orientation.

According to the results obtained from the tests carried out in which the aforesaid legs or ribs and grooves were moulded at a stretch blow moulding temperature for the preform which is higher than the stretch blow moulding temperature at which good orientation is obtained, such moulding was not found to be difficult. However, it was found that the shock resisting and gas barrier properties are reduced, and although self-supporting containers were obtained they lacked the shock resisting and gas barrier properties indispensable to containers for carbonated beverages and could not therefore be put to practical use.

Tests have been carried out to deterrr.ne the difficulty involved in moulding other containers which require no base cups and to determine the pressure resisting properties of such containers. Containers having relatively shallow grooves constituting ribs as disclosed in U.S. Patent 3,871,541 are easy to mould but the central portion thereof often projects outwardly due to internal pressure because of a slight inclination of the ribs. This phenomenon is understood to increase with room temperature and there is therefore room for further improvement.

It is also known to form reinforcing ribs on the concave central portion and on the rim of the container base, said ribs preventing deformation and projection of the concave central portion due to internal pressure. Therefore, the curvature of the base becomes complicated, and as the container decreases in volume, moulding becomes difficult. Even if the container is moulded as designed, the container is not acceptable for other reasons. In view of the ease of moulding, it is preferable that, as in the aforesaid U.S. Patent 3,727,783, leg portions are bulged outward from the peripheral wall of the base before moulding. However, as previously mentioned, the base structure described in U.S. Patent 3,727,783 is extremely difficult to mould, and a base structure as disclosed therein cannot be moulded by stretch blow moulding.

Various of the advantages of producing plastics containers by stretch blow moulding depend upon biaxial orientation. Therefore, a base having good orientation as well as enabling the container to be self-supporting should be provided in order to obtain containers suitable for carbonated beverages.

It is therefore an object of the present invention to provide plastics container having a base which is easily moulded with a low reject rate and the moulding of which is not affected by the volume of the container.

Accordingly, the invention provides a selfsupporting plastics container formed by stretch blow moulding and having a domeshaped base, in which container the base has a concave central portion which protrudes into the container and has a diameter enabling the concave central portion to withstand internal pressure in the container, the periphery of the concave central portion intersecting an inwardly curved peripheral wall of the base to form therewith a rim around the central portion; and a plurality of legs connected to and formed radially outside the rim such that the legs bulge radially from the rim.

In order that the invention may be readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view, partly in longitudinal section, of a container embodying the invention, the base of the container being shown in cross-section; Figure 2 is a plan view of the container base; Figure 3 is a longitudinal sectional view of the base of a second container which embodies the invention and is moulded so that the rim portion and each leg bottom surface lie on the same surface; Figure 4 is a plan view of the base shown in Fig. 3; Figure 5 is a plan view of a third container embodying the invention and which is moulded so that each leg bottom surface extends from the rim portion of the container base; Figure 6 is a sectional view taken along line 6-6 or Fig. 5; ; Figure 7 is a sectional view taken along line 7-7 of Fig. 5; Figure 8 is a partial sectional view showing a modification of the leg portions of the container base shown in the Fig. 3 embodiment; Figure 9 is a partial sectional view showing a modification of the leg portions of the container base shown in the Fig. 5 embodiment; Figure 10 is a longitudinal sectional view of a fourth enbodiment of a container which embodies the invention and has a preferred form of base.

Referring now to the drawings, Figs. 1 and 2 show a plastics container 11 in the form of a bottle produced from a plastics preform such as an injection moulded polyethylene terephthalate by axially stretched in a blow cavity, the cavity being fully supplied with air for expansion. A base 1 2 of the container 11 is moulded into a generally domed shape, while a circular central part of the base wall protrudes into the container to form a concave central portion 1 3 of the base. The diameter of the periphery of the concave central portion 1 3 is limited by the overall diameter of the base, and if greater than the limit, the concave surface will extend downwardly.

The periphery of the concave central portion 1 3 intersects a lower end of an inclined rim 14.

Externally of the rim 14 the peripheral wall 1 2 of the base is moulded and expanded partially outwardly to form six curved equally spaced legs 1 5. The inner end of each leg 1 5 is connected to the rim 14 and, when the legs are moulded apart from the rim 14, the rim portion 1 4 tends to deform under internal pressure within the container.However, a bottom 15a of each leg 1 5 extends slightly downwardly from the rim 14 so that, even if the rim 14 is deformed to some extent by the stresses created when the concave central portion 1 3 is pressed outward by internal pressure that is, by stress possibly concentrated on the periphery of the concave central portion 13, the base is supported so that the deformation does not impair self-supporting nature of the container 11.

In such a base structure, the plurality of legs 1 5 used to support the container 11 mean that, even if the peripheral wall of the base 1 2 is dome-shaped, the container can be stood upright directly without use of a base cup. Further, the provision of the concave central portion extending into the container means that the domed base is effectively truncated such that there is clearance between the centre of the base and a supporting surface when the legs 1 5 rest on the supporting surface.Thus, even if the rim portion 1 4 is forced outwardly by internal pressure on the concave central portion 13, the rim 1 4 is rarely deformed outward further than the bottoms 1 5a of the legs; as a consequence the base 1 2 is never sufficiently deformed by internal pressure to prevent the container from being self-supporting.

The downward extension of the legs 1 5 can be reduced, and the distance between the top of the concave central portion 1 3 and the bottom of the legs 1 5 is also within a range capable of being moulded, so that moulding of the container base is not difficult compared to the prior art. Orientation of the container base is sufficiently provided to increase the shock resisting properties as well as the pressure resisting properties of the base, so that, even if the bottle should be dropped when filled with carbonated beverage, the bottom thereof would not be destroyed.

The container base structure shown in Figs.

3 and 4 is moulded such that a rim 24 and a bottom 25a of each leg 25 are level. A bottom peripheral wall 22 of the container 21 is given an overall convex shape though the curvature is different from that of Fig. 1, and a central concave portion 23 is formed in the base. In this case, the rim 24 is moulded into a web-like configuration.

The diameter of the rim 24 is determined according to the shape, height and diameter of the container 11. As the diameter of the rim portion 24 increases, the container becomes more stable. However, the size of the rim 24 is limited by the diameter of the periphery of the base 22, and when moulding a web-like rim having a diameter larger than said limitation, it is suggested that the weblike rim be moulded so that the base 22 is extended outwardly from the body of the container 21 to have a large peripheral diameter. Also, legs 25 are formed in which six sides of the peripheral wall 22 of the base are partially extended to adjacent web-like rim 24, and a bottom surface 25a of each of the legs 25 is formed so as to be level with the weblike rim 24.

In such base, the legs 25 moulded radially outside the web-like rim 24 are made continuous with the web-like rim portion 24 and moulding is effected such that the web-like rim 24 and the bottom 25a of each leg 25 are level. Therefore, the diameter of the weblike rim 24 is limited by the pressure resisting properties of the container and even where it is difficult for the container 21 to be selfsupporting within a given rim diameter, the area of contact with a supporting surface is enlarged by the legs 25 positioned radially outside the web-like rim and therefore the container is more stable so that the bottle never falls over under the influence of minor vibrations.

Incidentally, the number of the leg portions 25 is suitably determined according to the size of the container 21 and is not limited to six as shown. Also, any suitable shape for the legs can be employed as long as the legs can be moulded simultaneously with the container 21. The web-like rim 24 may be polygonal in shape rather than circular.

The base shown in Figs. 3 and 4 allows a container to withstand internal pressures at room temperature and remain self-supporting.

However, when the internal pressure increases more than expected when, for example, the temperature rises abnormally for some reason, the web-like rim 24 sometimes deforms as shown in Fig. 8. This results from concentration of stress due to internal pressure on the concave central portion 23 of the base. The stress on the concave central portion 23 causes the container rim 34 to be deformed from a flat surface and as a result the container tends to fall over.

In order to prevent such unexpected occurences a portion of each leg 35 may be extended beyond the rim 34 before moulding.

As shown in Figs. 5 to 7, a third embodiment of a container in accordance with the invention is formed such that the bottom 35a of each leg extends slightly beyond the rim 34.

Also, in this case, a peripheral wall 32 of the base of the container 31 is moulded into a convex shape, a central portion 33 of the base being formed into a concave ridged shape extending inwardly of the container.

The web-like rim portion 34 formed by the intersection of the peripheral edge of the concave central portion 33 and the peripheral wall 32 of the base is flattened during moulding and six legs 35 are moulded in the base radially outside the rim 34. Further, the legs 35 are formed in such a manner that the bottom of each leg is partially outwardly bulged adjacent the web-like rim 34 and the bottom 35a of each leg comes into contact with the radially outer edge of the rim 34 and is gently radially outwardly inclined such that a radially outer end portion 35b of each leg 35 extends slightly beyond the rim 34 and the whole container is supported by the outer end portions 35b of the legs 35.

The outer end portion 35b of the bottom of each leg 35 extends beyond the rim 34 to ensure that the self-supporting nature of the container is not impaired when the bottcrns 35a of the legs are deformed and a relatively small extension of the legs beyond the rim will suffice, thus preventing problems occurring in moulding and ensuring correct orientation of the base. For a container of 500 cc capacity having a rim diameter of 3.5 cm, a 1.4 mm extension of the outer end portion 35b of the bottom 35a of each leg beyond the rim 34 will be acceptable.

As a result, as shown in Fig. 9, the deformed portion 35c produced by an internal pressure of 6.4 kg/cm2 is mostly at the periphery of the concave central portion 33.

However, since the outer end portions 35d extend beyond the rim 34, the deformed portion 35c does not contact the surface on which the container is supported and the container therefore remains self-supporting in a manner similar to that before the deformation ocurred.

Fig. 10 shows a fourth embodiment of a base in accordance with the invention. Preferably, the concave central portion 43 is moulded with a radially outwardly extending curved lip 44a at the periphery thereof, that is, where the central portion 43 and the rim 44 intersect.

A radially inner surface 45a of the bottom of each leg of the base is connected to the rim and is moulded to form an inclined surface extending beyond the rim and a radially outer end portion of the bottom of each leg is moulded to form curved surface and a base surface therebetween is moulded as a flat surface which contacts the ground.

Moulding of a base in accordance with the invention is facilitated and orientation by stretch blowing may be obtained as desired, allowing the formation of a container having a good gas barrier in addition to pressure resisting and shock resisting properties.

In any of the above-mentioned embodiments, the diameter of the concave central portion should be below the limit at which the concave central portion is deformed under the influence of internal pressures and impairs the self-supporting nature of the container by such deformation.

Claims

1. A self-supporting plastics container formed by stretch blow moulding and having a dome-shaped base, in which container the base has a concave central portion which protrudes into the container and has a diameter enabling the concave central portion to withstand internal pressure in the container, the periphery of the concave central portion intersecting an inwardly curved peripheral wall of the base to form therewith a rim around the central portion; and a plurality of legs connected to and formed radially outside the rim such that the legs bulge radially from the rim.

2. A container according to claim 1, wherein a bottom of each leg extends radially from, and is connected to the rim, the bottom of each leg being level with the rim.

3. A container according to claim 1, wherein the bottom of each leg is formed radially outside the rim, the bottom of the legs projecting downwardly below the rim so as to maintain the self-supporting nature of the container notwithstanding deformation of the rim under the influence of internal pressure.

4. A container according to claim 1, wherein a radially inner end portion of the bottom of each leg inclines downwardly from the rim to a flat standing surface of the leg formed between the radially inner and radially outer end portions of the bottom of the leg.

5. A self-supporting plastics container sub stantially as hereinbefore described with refer ence to, and as illustrated in, the accompanying drawings.

6. Any novel feature or combination of features herein disclosed.