GB2092999A - Closure for screw-threaded container - Google Patents

Abstract

A moulded plastics closure for an externally screw-threaded bottle has an outer shell of a relatively rigid plastics such as polypropylene and a softer liner gasket 15, formed in situ and retained in position by an inwardly directed rib 19 near the top end of the skirt 12. The groove 20 between the rib 19 and the top 11 of the closure shell may have a continuous curved profile, in section, and the liner gasket, which covers the whole top of the closure, is thickened in this region 16. The internal diameter of the rib 19 may be close to the nominal diameter of the sealing surface on the side of the bottle neck. The inwardly directed rib may slope somewhat downwardly towards the open end of the shell and usually there is a small gasket-free area 19a adjacent the lip of the top surface of the rib. <IMAGE>

Description

SPECIFICATION Improvements in closures for screw-threaded containers The present invention relates to closures for screwthreaded containers and in particular to internally screw-threaded closures for glass or plastic bottles, the closures being made from a thermoplastic material and intended for retaining substantial gas pressure within the bottle, for example pressures up to 160 p.s.i. as are sometimes encountered in the pasteurisation of carbonated beverages, such as beer.

Many attempts have been made to produce onepiece thermoplastic pressure-retaining bottle closures, comprising an internally screw-threaded body and an integral gasket, composed of one or more flexible ribs which bear against the surface of the neck of the bottle.

Existing one-piece thermoplastic bottle closures do not operate satisfactorily because of the imperfections found in commercial glass or plastic bottles.

Not only does the diameter of the inner and outer surfaces of the bottle neck adjacent the mouth of the bottle vary within allowable tolerances, but also the neck is subject to slight ovality, and other imperfections.

In order to withstand the internal pressures associated with carbonated beverages, it is necessary to make a one-piece closure from a tough, somewhat hard material, such as polypropylene or high density polyethylene. However when such screwthreaded plastic caps, having one or more integral sealing ribs, have been employed for closing bottles containing carbonated beverages, it has not been found possible to achieve a reliably acceptable combination of pressure retention and opening torque characteristics on commercially available bottles. This difficulty arises because of the stiffness and hardness of the ribs, which is determined by the characteristics of the plastic material from which the screw-threaded closure shell is moulded.

It has already been proposed to form a gasket within a threaded thermoplastics shell by inserting a thermally softened slug of a somewhat softer material and moulding such a slug into a gasket in situ in the shell. Such a closure is open to the objection that the gasket, which is formed by extruding a rod and shearing off a slug length therefrom, is still rather harder than is desirable to permit a good seal to be made with a bottle, without at the same time giving rise to unacceptably high opening torques.

It is already known to form a gasket in situ in a metal sheel by injecting a measured quantity of a flowable plastisol composition into an upturned shell, which is then rotated rapidly to spread out the plastisol material across the surface of the top of the shell and forming a thickened annulus at the angle between the top and skirt of the shell. The plastisol layer is in the known arrangement rapidly gelled by passage of the closure shell through an oven.

Because of the ready deformability of gelled plastisol, coupled with its relatively low friction characteristics in relation to glass or plastic containers, it has been found possible to achieve a good pressure retaining seal between the screw-threaded neck of a bottle or other container and the closure shell, while at the same time achieving acceptably low opening torques. In the case of closures formed with a metal shell, it has been usual to decrease the diameter of the top of the closure shell during application of the closure to a bottle with the result that the gasket material forms, inter alia, a seal with a cylindrical surface of the exterior of the bottle neck and located above the threads on the bottle neck.

Since it has been the practice to gel the plastisol composition by stoving, flowable plastisol compositions would seem to be unsuitable materials for lining thermoplastic shells because of the probable damage to the shell in the gelling operation. It has however been disclosed that such p.v.c. plastisol material can be gelled by employing microwave radiation of a selected wave length which excites the components of the plastisol but which does not excite the materials from which pressure-resistant plastic closure shells are formed. Thus the plastic shell with deposited p.v.c. plastisol material may be spun to distribute the lining material, and then exposed to micro-wave radiation to gel the p.v.c.

lining material. Air convection heating may be employed to assist micro-wave heating to raise the plastisol temperature for gelling.

The closure of the present invention may be lined by the technique outlined above. The present invention provides a closure for an externally screwthreaded container comprising a thermoplastic outer shell, having a top and a skirt, formed with internal thread formations and an inwardly directed rib located between the upper end of the thread formation and the top of the shell, the inner diameter of said rib being equal to or close to (less or more) the nominal diameter of the cylindrical external sealing surface on the closure neck lying between the thread and the upper end of the neck and an in situ formed gasket of a material more yielding than the material of the shell, covering the whole underside of the top of the closure and forming a thickened annulus in the peripherial region between the top and the rib and occupying the annular space between the rib and the top.

When a closure of this type is screwed down firmly onto a standard bottle, the mouth portion of the bottle neck bites into the thickened annulus of gelled plastisol to form a seal extending from the top surface of the bottle neck round the radiused corner to the external, essentially cylindrical side sealing surface of the bottle.

Preferably the internal diameter of the rib is less than 0.4 mm greater than the nominal diameter of the sealing surface on the standard container for which it is designed and in many instances is less than the diameter of the sealing surface. The vertical distance between the rib and the top, which is a major factor in determining the vertical extent of the side seal between the closure and the bottle, is preferably in the range of 1.5 to 2.5 mm, measured between the inner lip of the rib and the closure top.

The annular gasket-receiving groove between the top and the inwardly directed rib may advantageous ly be provided with a continuously curved profile, so that the liner material, flowing outwardly during the spinning operation, washes out any air from the groove and avoids entrapment of air which can lead to non-uniformity and local difference in compressibility of the gasket material in the groove. For manufacturing convenience the groove would be provided with an essentially semi-circular profile, but could in fact have other profiles, such as a part-elliptical or parabolic profile.

The inwardly projecting rib is preferably also sloped slightly downwardly (towards the open end of the closure) and an angle, for example, of up to 15 but more preferably in the range of 3 - 7". The inner diameter of the rib is then preferably chosen to be slightly less than the minimum permissible diameter of the sealing surface on the bottle which the closure is designed to fit. There are in fact well recognised ranges of tolerances for the sealing surfaces of standard bottle neck finishes.When the inwardly directed rib is dimensioned so as to be an interference fit on the sealing surface of the neck, it becomes compressed or slightly upward deflected as the closure is screwed on to the bottle and this may result in a small degree of radial compression on the relatively soft liner material lying within the annular groove defined above the inwardly projecting rib. Conversely when the closure is unscrewed the rib tends to return towards its initial position and to decrease the frictional resistance of the compressed liner material to the unscrewing of the closure. A further result arising from the slight reduction in the diameter of the inwardly directed rib is that it permits the rib to effect a centering action on the liner material in relation to the sealing surface on the neck of the bottle when the closure is being applied initially in a bottling machine.

Instead of employing a plastisol the liner material may be introduced in molten form and may in addition be a foamed material. Such molten material may be injected into the closure and spun out to form a gasket which sets on cooling. This avoids the necessity of employing a heated oven structure or the use of micro-wave radiation for gelling plastisol material in the gasket. Appropriate grades of ethylene-vinyl acetate, polyvinylchoride, polyethylene or polypropylene may be employed for forming a melt-spun gasket. These materials are preferably foamed with carbon dioxide or other inert gases to give a foam-like resilient structure. The foams employed require to have a closed cell structure to avoid forming a leakage path and should have a void space of 30 - 70% to ensure the desired compressibility coupled with good pressure retention in the bottle.Where a plastisol is employed to form the gasket, it also may be foamed.

Screw-threaded pressure-holding closures in accordance with the invention are primarily intended for application to standard bottles for carbonated beverages. Well known examples of such standard bottles have external neck diameters of 26 mm, 28 mm, and 38 mm respectively. For the 28 mm closure the minimum quantity of plastisol required for the gasket formation is about 200 mgms, whereas the maximum that would be employed without undue waste is about 500 mgms respectively. These values should be increased or decreasd approximately pro rata forthe 38 mm and 26 mm size closures.

One form of closure made in accordance with the invention is shown in the accompanying drawing in underneath plan in Figure 1 and in longitudinal section in Figure 2.

An alternative form of closure of is shown in Figure 3 in vertical section.

The closure of Figures 1 and 2 comprises a moulded polypropylene shell having a top 1 and a skirt 2. The whole of the flat portion of the underside of the top 1 is covered with a slightly raised cross-hatched pattern 3 to grip with a flowed-in and gelled plastisol gasket 4 (not shown in Figure 1), which has an enlarged annular peripheral portion 5.

The skirt 2 of the closure has an internal thread formation 7 and vertically extending external gripping ribs 8. The peripheral portion 5 of the gasket is ideally confined between the top of the closure and an inwardly projecting rib 9. The cross-hatched pattern 3 may be replaced by other formations such as dimples for engagement with the gasket and/or may extend over only part of the top 1 or may be omitted in some cases.

It is to be noted that in the illustrated closure the top of the closure shell is free of any downwardly dependent formation in the region of the periphery which might impede the outward flow of the plastisol into the annular recess between the top and the inwardly projecting rib 9. This ensures that in the lined cap this space is essentially filled with gelled piastisol material and essentially free of void space into which such material might flow when the cap is screwed down onto the bottle, thus ensuring a more reliable seal than could be achieved where a downwardly depending rib is provided on the underside of the top and may act as a baffle to impede the outward flow of the ungelled plastisol in the lining operation and to act as a trap for localised air bubbles.

In the alternative construction shown in Figure 3 the closure comprises a moulded polypropylene shell having atop 11 and a skirt 12 having internal thread 17. A cross-hatched pattern of essentially square section projections 14 may cover substantialliy the whole of the under surface of the top 11 to grip the flowed-in gasket liner material as indicated at the left hand side of Figure 3. Alternatively the undersurface of the top 11 may be left plain as shown at the right hand side of the drawing. In such case conventional techniques may be employed to bond the liner material to the top of the closure shell, as explained below.

The liner gasket 15 has an enlarged annular peripheral portion 16 as a result of the spinning operation performed in the course of its production.

The skirt 12 of the closure has an internal thread formation which extends through approximately 360 so that the closure is released from the bottle thread by approximately a single turn of the closure in the unscrewing direction.

The principal feature of the alternative closure lies in the form of the inwardly projecting rib 19 and the annular groove 20, which lies between it and the top 11. As will be seen the groove 20 is provided with a substantially semi-circular profile. Both the curved upper face of the rib 19, forming part of the surface of the groove 20, and the lower face 21 of the rib 19 are inclined somewhat downwardly in relation to the axis of the closure. As already indicated the internal diameter of the rib 19 may be slightly less than the minimum permitted value of the diameter of the sealing surface of the standard neck finish of the bottle, for which the closure is designed. Thus the internal diameter of the rib 19 may be about 1% less than the permitted minimum value of the diameter of the sealing surface.

As already explained above the interference fit of the rib 19 on the bottle sealing surface results in slight compression or upward deflection of the rib during screwing onto the bottle with resultant compression of the gasket material lying within the groove 20 with consequent slight flow to form a uniform seal around the whole periphery of the sealing surface on the bottle neck.

The amount of liner material placed in the closure shell and rate of rotation employed in the lining operation are preferably closely related so that, as shown, the liner material does not completely fill the groove 20. On the contrary in this construction there is preferably a small annular area of the top surface of the rib 19, adjacent the lip of the rib as indicated at 19a, which is free of liner material.

These measures, in conjunction with the adoption of the curved-profile groove 20 (which avoids air entrapment in the course of the lining operation) leads to the production of closures which achieve rather uniform pressure-retention when employed with commercially available bottles having necks which vary within the permissible limits set for commercial glassware or other commercially available containers.

The closures shown both in Figures 1 and 2 and in Figure 3 may be modified by the inclusion of a security band, moulded integrally with the skirt of the closure, and adapted for engagement with a shoulder on the neck of a container either by heat-shrinking or by means of a heated roller which tucks its periphery under the shoulder or by means of an internal rib which may be jumped over the shoulder to clip under the same.

As already indicated the undersurface of the top 11 may be left flat and this may be somewhat advantageous in that it is then possible to save some of the material required for the downward projections 14 on the shell and at least some of the liner material lying between the projections.

Where the liner material is a p.v.c. plastisol or like flowable material adhesion to the shell may be achieved by first subjecting the polypropylene to flame oxidation or corona discharge as is commonly practised to render polypropylene articles receptive to printing inks.

As an alternative to the above, liner material, supplied in molten form, may include an adhesive to secure direct bonding of the liner material to the polypropylene surface, which may in such circumstances require no pretreatment.

It will be understood that the described caps, in addition to the stated purpose of acting as a pressure retaining closure, are equally useful for nonpressure holding- and vacuum-retaining applications.

Claims (10)

1. A closure for an externally screw-threaded container, comprising a thermoplastics outer shell, having a top and a skirt, formed with internal thread formations and an inwardly directed rib located between the upper end of the thread formation and the top of the shell, the inner diameter of said rib being equal to or close to (less or more) the nominal diameter of the cylindrical external sealing surface on the closure neck lying between the thread and the upper end of the neck and an in situ formed gasket of a material more yielding than the material of the shell, covering the whole underside of the top of the closure and forming a thickened annulus in the peripheral region between the top and the rib and occupying the annular space between the rib and the top.

2. A closure according to claim 1 further characterised in that the space lying between the top and the inwardly directed rib is in the form of a groove having a continuously curved profile.

3. A closure according to claim 1 or claim 2 further characterised in that the inwardly directed rib sloped downwardly towards the open end of the closure.

4. A closure according to claim 3 further characterised in that said rib slopes downwardly at an angle in the range of 3 - 7 .

5. A closure according to any preceding claim in which the internal diameter of the rib is less than 0.4 mm greater than the nominal diameter of the sealing surface of the standard container for which it is designed.

6. A closure according to any preceding claim further characterised in that the inner diameter of said rib is less than the nominal diameter of the sealing surface on the container neck.

7. A closure according to claim 6 further characterised in that there is a small gasket-free annular area of the top surface of the rib adjacent its lip.

8. A closure according to any preceding claim further characterised in that the gasket is formed of a gelled p.v.c. plastisol.

9. A closure according to any preceding claim further characterised in that the gasket is formed of a foamed plastics material.

10. A closure according to any preceding claim further characterised in that the undersurface of the top of the shell is formed with a pattern of small projections to grip the gasket liner material.