IMPROVEMENTS IN CLOSURES FOR SCREW-THREADED CONTAINERS

Abstract: A closure cap for a bottle or other container having an externally screw threaded neck is moulded in a tough thermoplastics material and has an internally threaded skirt (22) and a top panel (21). An inwardly directed lip (26) is provided on the skirt (22) between the top panel and the thread. The internal diameter of the lip is less than the nominal diameter of a co-operating sealing surface on the container neck and serves to retain an essentially annular lining (25) of more compressible material within the shell. The shell is also provided with an annular rib (27) on the underside of the top panel (21) and is dimensioned to fit within the mouth of the container. The axial extent of the rib (27) is preferably less than the space between the lip (26) and the top of the shell (21). The undersurface (28) of the lip (26) preferably slopes downwardly and inwardly. The lip (26) may have an intermediate portion of reduced thickness to promote flexing of the rib in the upward direction.

Description: "IMPROVEMENTS IN CLOSURES FOR SCREW-THREΛDΞD CONTAINERS" The present invention relates to closure caps for screw-threaded containers and in particular to internally screw-threaded caps for glass or plastic bottles, the caps being moulded from a thermoplastic material. The caps of the invention are primarily intended for retaining substantial gas pressure within the bottle, for example, pressures up to 160 p.s.i. as are sometimes encountered in carbonated beverages and pasteurised beverages, such as beer. However the caps may also be employed for vacuum retention or non-pressurised containers. Many attempts have been made to produce one-piece thermoplastic pressure-retaining bottle caps, 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 caps do no 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, particularly of glass bottles, is subject to slight ovality, and other imperfections. In order to withstand the internal pressures associated with carbonated beverages, it is necessary to mould a one-piece cap from a tough, somewhat hard material, such as polypropylene or high density polyethylene. However when such screw-threaded 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 sealing ribs, which are determined by the characteristics of the plastic -2- material from which the screw-threaded cap is moulded. It has already been proposed to form a liner within a threaded thermoplastics cap shell by inserting a thermally softened slug of a somewhat softer material on the underside of the top of the shell and moulding such a slug to form a liner in situ in the shell. Such a cap is open to the objection that the liner, which is formed by extruding the lining material in the form of a rod and shearing off a slug length therefrom, necessarily covers the whole of the underside of the top of the closure. When the central area of the liner is made very thin to economise on the material it tends to be rather readily permeated by gas. This can result in "ballooning" of the liner when the cap is removed from a bottle, because the gas between the liner and the shell is at superatmospheric pressure. It is already known to form a liner in situ in an unthreaded metal shell by injecting a quantity of a flowable plastisol composition into the upturned shell, which is then rotated rapidly to spread out . he plastisol material across the surface of the. top of the shell and to form a thickened annulus at the angle between the top and skirt of the shell. The plastisol layer is in the known arrangement rapidly cured by passage of the closure shell through an oven. Because of the ready deformability of the cured 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 open- ing 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 liner material forms, inter alia, a seal with a cylindrical surface on the exterior of the bottle neck and located above the threads on the bottle neck. Since it has been the practice to cure the plastisol composition by stoving at a temperature of at leas 200°C, flowable plastisol compositions would seem to be un¬ suitable materials for lining thermoplastic cap shells because of the probable thermal distortion and/or softening of the shell in the curing operation. It has however been disclosed that such p.v.c. plastisol material can be cured by employing micro-wave 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 cap shells are formed. It has already been propose that a plastic shell containing a shot of deposited p.v.c. plastisol material be spun to distribute the lining material and then be exposed to micro-wave radiation, in conjunction with mild heating, to cure the p.v.c. lining material. It has also been proposed to provide a cap for an externally screw-threaded container, comprising a thermo¬ plastics 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 th top of the shell, and to form in situ a p.v.c. plastisol liner, covering the whole underside of the top of the closur and forming a thickened annulus occupying the annular space between the rib and the top of the shell. However it is found that when the liner is formed with a very thin central portion, as required for maximum economy in the use of plastisol material, the mass of the plastisol in the central area is too small to allow it to be adequately cured by micro-wave radiation. For adequate curing of the plastisol it is necessary for the central portion of the liner to be of somewhat uneconomic thickness. In order to overcome these and other difficulties, the present invention provides a closure cap, having a shell moulded in a tough thermoplastics material having a top panel and an internally threaded skirt, an inwardly directed lip on said skirt between the top panel and the skirt thread and having an internal diameter less than the nominal external -.11- diameter of the sealing surface on the container for which it is designed, a downwardly extending rib on the top panel dimensioned to fit within the mouth of such container and a liner, formed of a more compressible material than said tough thermoplastics material, located in the recess between said lip and said rib, the inner rib preferably has an axial extent less than the distance between the top panel of the shell and the inner edge of the inwardly extending lip. The liner is preferably a cured plastisol. In any event the liner material may be a foamed material to render it more readily compressible. In a lined thermoplastics cap of the present invention there is no liner on the central area of the top panel and accordingly there is no risk of "ballooning". The essentially annular liner can be formed by depositing a flow- able plastisol into the recess between the lip and the rib and, after spinning the upturned cap to cause centrifugal flow of the liner material, the plastisol may be cured in situ by micro-wave radiation. 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 recess in the closure shell and spun out to form a liner which sets on cooling. In a closure of the present invention the plastisol (or other lining material) may be injected into the annular recess between the lip and the rib while the closure is turned slowly relatively to an injection nozzle (thus the nozzle(s) may orbit in relation to a stationary closure shell). The duration of the plastisol injection is such that the shell and nozzle(s) turn relatively through slightly more than a whole number of revolutions during the injection of plastisol, so that there is no local redudction in thick¬ ness of the plastisol " layer. In practice it is not possible to time the plastisol injection precisely to coincide with a whole number of revolutions of the shell and consequently a OMPI slight local thickness of the liner at one position is accepted. However that is not found of much significance. The extent of the local thickening may however be kept small by dispensing the plastisol during two or more revolutions of the shell. Even the relatively slow rotation of the shell leads to some centrifugal flow of the dispensed plastisol and it is possible to charge more plastisol to the annular space between the ribs than would be possible (with¬ out overflow) if the shell was stationary. After dispensin the plastisol the rate of rotation of the shell may be increased to increase the centrifugal action on the plastiso before it is cured. Such centrifugal action increases the amount of plastisol in the space between the inwardly directed lip and the closure top for sealing with the side sealing surface of the container without at the same time unacceptably decreasing the amount of plastisol just out¬ wardly of the inner rib, where the liner will seal with the top end surface of the container. . It is sometimes desired that the liner should turn with the shell - and thus slip on the container sealing surfaces - during closing and opening. A variety of expedients may be employed alone or together for this purpose. Thus it is possible to provide projections or small radial recesses in the skirt between the inwardly directed lip and the top for engaging the liner at its outer periphery. It is also possible to apply the like or similar expedients at the outer surface of the inner rib. A further expedient is to apply some local roughening or patterning to the undersurface of the top panel of the shell between the ribs. On the other hand it is sometimes found to be preferable to allow the liner to turn in the shell so as to avoid skidding ' on the container sealing surface(s). In such case the surface on the shell, contacted by the liner, is made as smooth as possible and may be coated with a slip agent to lower friction between the liner and the shell. In some instances it may be desirable for the oute diameter of the inner rib to be somewhat greater at its free eύge than at its root so as to provide some degree of outwar overhang. This serves not only to assist in the retention of the liner in its space, but also to check the tendency of the cured plastisol to flow in the axial direction between the shell and the inner periphery of the container mouth. It has been found in the production of plastic closures that polypropylene, which is probably the most suitable material on a strength/cost ratio basis, is prone to crack at sharp re-entrant angles and regions where high flow of the plastic material is required. The shell of a closure in accordance with the present invention is therefor preferably designed so that, in section, the surfaces of the lip and the rib are rounded and also the surface at the spac between them. This assists not only the avoidance of the formation of cracks in the moulded shell, but also in the easy spread of the plastisol within the liner space in the shell and in particular it greatly reduces the risk of en- trapment of air between the retaining rib and the panel top during outward spreading of the plastisol. In the production of closure shells from a tough, but somewhat resilient plastic, such as polypropylene, the closure shell should be constructed so that when exposed to the pressures commonly occurring with carbonated beverages, the amount of lifting of the top panel of the closure in the area where the liner lies is minimised. Any lifting of the top of the closure is accompanied by local extension of the shell plastic. It is therefore preferred to reinforce the peripheral region of the shell top against radial stress. This may be achieved by local thickening of the shell in this region (making the skirt wall thickness greater than at the threaded portion of the skirt) or by the provision of shallo radial ribs which also assist in holding the liner from turning in the shell. As compared with a metal closure, in which the top panel of the closure is deformed downwardly at its periphery O PI WIPO when in position on the container for the purpose of pressin the plastisol material against an essentially cylindrical sealing surface surrounding the mouth of the bottle, the liner of a pressure-retaining plastic cap requires to seal 5 both with the top end of the bottle and with a cylindrical or conical side sealing surface surrounding the mouth of the bottle and lying above the neck threads (also with the radiused corner lying between the side sealing surface and t top end surface of the bottle). 10 It is therefore desirable that the thickness of the liner material in the location for sealing with the top end surface should be more controllable than is readily achieved when the liner is formed by placing a shot of plastisol centrally of the closure, distributing it over the 15 top panel of the closure by centrifugal action and curing it When a closure of the present type is screwed down firmly onto a standard container, the mouth portion of the container neck bites into the soft cured plastisol which 20 flows locally to a.limited extent -to enable the formation of a seal between the container and the closure, extending, in the radial direction, over at least a major part of the top end surface of the container, round the radiused corner to the side sealing surface. 25 Since the internal diameter of the lip is slightly smaller than the neck of the bottle, the lip is slightly bent upwards and is restrained from being bent downwardly by reason of its contact with the bottle neck. The lip thus forms a strong barrier to downward flow movement of the liner 30. material when the cap is tightened down. This effect may be further accentuated by shaping the lip so that its under- surface slopes slightly downwardly towards the axis of the shell. Plastic flow of the liner material is thus confined within the space bounded by the relatively rigid top and 35 skirt of the shell and the lip, having its free edge supported by the neck of the bottle. With this arrangement the amount of liner material required to form the desired efficient pressure seal is low. By reason of the confined lower margin of the body of liner material, the liner material will flow in a peripheral direction to make good - any irregularity in the shape of the side sealing on the container neck. It also has the further advantage that the vertica extent of the side seal is controlled and the seal between cap and bottle is vented after a predetermined turn of the cap on the bottle thread. In a cap -of the present invention the vertical distance between the lip and the top, which is a major facto in determining the vertical extent of the side seal between the cap and the bottle, is preferably in the range of 1.5 to 2.5 mm, measured between the inner margin of the lip and the closure top. The upper surface of the liner-retaining lip is preferably also sloped slightly downwardly (towards the open end of the cap) at an angle, for example, of up to 15 but more preferably in the range of 3 - 7 . The inner diameter of the lip is then preferably chosen to be slightly less than the minimum permissible diameter of the sealing surface on the bottle which the cap is designed to fit. There are in fact well recognised ranges of tolerances for the sealing surfaces of standard bottle neck finishes. The external diameter of the inner rib is preferably slightly less than the minimum permissible internal diameter of the container mouth, so as to hold the internal rib clear of contact with the inside of the neck. Thus the liner-retaining lip is free to exert a centering action on the cap with respect to the container without interference by the inner rib. The vertical extent of the inner rib is conveniently in the range of 1.0 - 1.5 mm, its size being essentially controlled by the amount of plastisol to be contained. 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 ( OMPI WIPO the 28 mm closure the minimum quantity of plastisol required for the liner formation is about 150 mgms, whereas the maximum that would be employed without undue waste is about 500 mgms respectively. These values should be increased or decreased approximately pro rata for the 38 mm and 26 mm siz closures. Referring to the accompanying drawings: Figure 1 illustrates one form of closure cap in accordance with the invention, Figure 2 illustrates a further form of closure cap in accordance with the invention, Figure 3 illustrates two forms of a further development of the closure cap of the present invention, and Figure 4 illustrates the two forms of caps, shown in Figure 3, when fitted to appropriate forms of container. The caps of Figures 1 and 2 comprise a moulded polypropylene shell having a top 1 and a skirt 2. The skirt 2 of the cap shell has an internal thread formation 3 and vertically extending external gripping ribs 4. A cured plastisol liner 5 is confined between an inwardly projecting retaining lip 6 and a downwardly projecting inner rib 7. It will be seen that the surfaces of the lip 6 and rib 7 and the portion of the top 1 lying between them is smoothly curved for reasons explained earlier. The rib 7 may be provided with projections 8 at intervals around its outer periphery for engagement with the inner periphery of the liner 5. In addition or alternativel shallow transverse ribs 9 may be provided on the underside of the top 1 extending outwardly from the rib 7 and the ribs 9 may be extended round the radiused corner to meet the lip 6, as shown. As may be seen from the horizontal line 10, the quantity of plastisol charged into the annular space outward of rib 7 somewhat exceeds the amount that could be charged if the then inverted shell had been stationary, instead of ^ ^ turning at a relatively slow rate. Thus it is ensured that the quantity of liner material provides a satisfactorily thick portion for engaging the top end sealing surface of the container and for becoming properly cured by micro-wave curing. In the construction of Figure 2 like parts are indicated by the same reference numerals as in Figure 1. The construction of Figure 2 differs from the con¬ struction of Figure .1 principally in that the inner rib 17 is arranged to overhang and retain the inner edge portion of the liner 5. It will also be seen that the top of the shell is locally thickened at 1 ' over the inner thinner portion of the liner 5. Although not shown in Figure 2 the projections 8 and/or shallow ribs 9 might be incorporated with advantage in this construction. It will also be seen that the portion 2 1 of skirt 2 lying radially outwardly of the liner 5, is thicker than the wall of the threaded lower portion, so as to form a strong hoop-like backing to the liner. The closure cap illustrated in Figures 3 and 4 is in fact two separate caps, respectively designed to fit (left half) a standard moulded plastic P.E.T. bottle for holding pressurised beverages and to fit (right half) a standard glass bottle. In these two closures the shell is injection- moulded through a gate at the centre of the top panel 21, which is slightly thickened centrally and decreases in thick¬ ness outwardly towards its periphery to allow better flow of plastic material in the mould outwardly to the skirt 22. This local thickness also improves the resistance of the top panel 21 to "doming" under upward pressure. As in the constructions of Figures 1 and 2 a moulded p.v.c. plastisol gasket 25 is retained between an inner rib 27 and a retaining lip 26. The lip 26 at the left hand side is essentially the same as the lip 6 in Figure 1 , and has a downwardly and inwardly facing under-surf ce 28. Lip 26^ at the right hand side has a similar undεr-surface OMPI 28a. The lip 26a has a rounded head portion 29 and is reduced in thickness at 30, so as to render it more flexible in the upward direction. This is necessary because the range of tolerances on the neck diameter of a glass bottle is necessarily wider than for a moulded plastics bottle. In both constructions the internal diameter of the lip is set at the minimum permitted diameter of the container neck. In Figure 4 the caps are shown screwed onto bottle which have necks having diameters towards the maximum permitted limit. In the construction of these two caps there are no radial ribs or similar formations provided to prevent turnin movement of the liner 25 in the shell. According to whether it is desired to reduce such movement or to promote it, the surface 32 of the shell may be subjected to appropriate post- treatment before the liner 25 is formed. The caps of Figures 3 and 4 may be used with bottles having vertical vent slots cut in the thread in known way (not illustrated in Figure 4). The distance between ribs 26 and 26a_ and the top panel are dimensioned in such a way that the seal between liner 25 and the container is dis¬ engaged before the cap thread 34 is released from the con¬ tainer thread to allow gas to vent before such disengagement. It would be possible to provide vent slots in the cap thread 34, but it is preferred not to do so, because of the weaken¬ ing effect of such slots on the thread and consequently of the hoop strength of the skirt 22. It will be seen that the vertical extent of the thread 34 greatly exceeds the distance between the top panel 21 and the lip 26. Both of the forms of closure shown in Figure 3 may be provided with an integral security band of any known construction, as shown in Figure 4. The security band 40, shown in dotted lines, is connected ' to the cap skirt 22 by a series of spaced bridges 41, in the well known way. The security band 40 has an integral internal rib 42, adapted to be sprung over and re¬ tained under the ring R on the bottle. It is one of the advantages of the liner-forming method employed in the present invention that it renders it ' possible to incorporate a liner having a maximum outer diameter equal to or greater than the internal diameter of a security band, having a mechanical clip-under retaining rib, such as 42. OMPI

Claims: CLAIMS : 1. A closure cap, having a shell moulded in a tough thermoplastics material having a top panel and an internally threaded skirt, an inwardly directed .lip on said skirt between the top panel and the skirt thread and having an internal diameter less than the nominal external diameter of the sealing surface on the container for which it is designed, a downwardly extending rib on the top panel dimensioned to fit within the mouth of such container and a liner, formed of a more compressible material than said toug thermoplastics material, located in the recess between said lip and said rib. 2. A closure cap according to claim 1 in which the axial extent of the inner rib is less than the distance between the lip and the top of the closure. 3. A closure cap according to claim-1 or 2 in which the under-surf ce of the lip slopes downwardly and inwardly. 4. A closure cap according to any preceding claim in which the inside surface of the closure between said lip and said rib is rounded and free of sharp angles. 5. A closure cap according to any preceding claim in which the lip, in section, has a rounded edge portion and an intermediate portion of reduced thickness to promote flexing of the rib in the upward direction. 6. A closure cap according to any preceding claim in which the thickness of the top panel of the shell decreases outwardly from the centre.

Inventors: GUEST JOHN HARRY, HAINES PETER REGINALD, GUEST, JOHN, HARRY, HAINES, PETER, REGINALD
Applicants: METAL CLOSURES GROUP PLC
Application year: 1983
Main classification: B65D41
IPC codes: B65D41, B65D53