Screw Cap Core Seal Structure

This invention concerns screw caps for containers. More particularly, the invention relates to screw caps provided with a core seal which is expandable to engage the inner surface of a container neck.

Such expandable core seals are known in various forms from GB2280895. In one form shown there, the expandable core has a face sealing ring which in use rests against the rim of the container neck, and a central section which projects above the face sealing ring when the core is in a relaxed (contracted, unsealed) state. The central section of the expandable core therefore rests against the inner surface of the top wall of a cap applied to the container neck. A somewhat flexible outer skirt depends generally vertically from the inner margin of the face sealing ring, to lie against the inner surface (bore) of the container neck in use. A flared (trumpet-shaped) inner skirt joins the lower periphery of the outer skirt to the outer periphery of the central section. At least the lower, outer peripheral part of the inner skirt is somewhat flexible. As the cap is applied to the container neck, the face sealing ring comes to rest against the rim of the container neck. Then as the cap moves axially further onto the container neck, the central section of the core seal continues to move with the cap, axially into the container neck. The outer skirt and the lower periphery of the inner skirt are constrained against axial movement relative to the container neck by the engagement of the face sealing ring against the container neck rim. Continued application of the cap therefore causes compressive collapse and corresponding radial expansion of the lower periphery of the inner skirt. Such radial expansion causes corresponding radial expansion of the outer skirt, particularly at and towards its lower (inner) end. The outer skirt is thereby expanded into sealing engagement with the inner surface of the container neck.

In another form shown in GB2280895, the inner skirt has a shallow frustoconical shape and is apparently connected to the lower periphery of the outer skirt and to the outer periphery of the central section, by respective outer and inner circumferential living hinges. The cap top wall is secured to the central section of the expandable core. In the relaxed, contracted, unsealed state of the core seal, the outer circumferential living hinge lies below the inner circumferential living hinge. As the cap is applied to the container neck, the shallow frustoconical inner skirt begins to flatten, resisting the application of the cap and again expanding at least the lower portion of the outer skirt towards, and then into sealing engagement with, the inner surface of the container neck. When the inner skirt has been distorted to a substantially flat shape, it reaches an “overcentre” condition, in which it snaps into an everted shape, in which the inner circumferential living hinge lies below the outer circumferential living hinge. This pulls the cap top wall towards (or, absent any other cap-to-neck securing means, into) engagement with the face sealing ring and the container neck rim; at the expense of some relaxation of the sealing expansion of the outer skirt.

Such expandable core seals may be used to seal otherwise conventional internally screw-threaded caps to conventionally externally threaded container necks. The outer skirt may have a facing layer made from a relatively yielding material, to take up and seal against any irregularities in the container neck inner surface. A backing structure for the outer skirt, and the entire inner skirt, may be formed from a relatively stiffer, more creep resistant material. A final screwing-on torque limit may be selected (a “target torque”) which ensures that the facing layer is properly deformed into full circumferential sealing engagement with the container neck inner surface. At this torque, there will be considerably more deformation in the inner skirt than in the outer skirt facing layer. Thus expansion of the container neck e.g. due to a rise in temperature, or thinning of the facing layer due to creep, does not result in any significant relaxation of the energizing force pressing the facing layer against, and maintaining it in sealing contact with, the container neck inner surface. If the cap is screwed on to a higher torque than that which is necessary to first achieve a good circumferential seal, the excess strain is largely taken up by further deformation of the inner skirt. The facing layer is thereby protected against overstraining, improper distortion, and extrusion. The expanding core container closure seals disclosed in GB2280895 therefore have good fault tolerance against poor neck inner surface finishes and inaccurate tightening torques.

The amount of radial expansion of the outer skirt which is possible, is limited by the elastic limit of the material(s) from which the outer skirt is formed. The maximum amount that the outer skirt can expand also largely depends upon the distance that the central section projects above the face sealing ring when the core is in the relaxed state, or the axial distance between the inner and outer circumferential living hinges when the core is in the relaxed state, for the two expandable core seal types described above, respectively.

In the case of screw-threaded container caps, expansion of the expandable core seal from the relaxed, fully contracted, unsealed condition to the sealed condition, best takes place during the final few turns (or even during a fraction of the final turn) of the screw cap. This ensures that the cap is in a predictable axial position when tightened to the target torque, with sufficient length of the cap and neck threads inter-engaged. A secure interconnection between the cap and container results, so that the cap will not “pop off” under rough handling or under internal pressure, e.g. due to heating or agitation of gas-evolving container contents, or storage or transportation of an at least partly gas-filled container at low ambient pressure, such as in aircraft or at altitude. Accurate final axial positioning of the cap is also often necessary to ensure correct inter-engagement and secure operation of co-operating anti-tamper features provided on the cap and container neck respectively. The face sealing ring is therefore axially positioned relative to the screw cap so that it contacts the container neck rim and begins expansion of the outer skirt only when less than one turn of the screw cap remains (or perhaps a few turns, in the case of screw caps having a fine thread) before the cap is fully screwed on to the container neck. The necessity for such an arrangement therefore further restricts the possible range of expansion of the core seal. On occasion, where the container neck is particularly oversized (out of tolerance, as can occur in the case of blow-moulded containers for example, which are inherently quite variable in neck internal diameter; and/or because nominally standard manufacturing dimensions in fact vary between different manufacturers), the available range of expansion of the core seal is inadequate to form a reliable seal inside the container neck bore. On the other hand, if the core were allowed to begin expanding many turns before the cap is fully screwed on, the possible range of expansion of the core seal is increased, but the final axial position of the screw cap when the target torque is reached then becomes unacceptably unpredictable.

WO2020/200619 discloses a screw cap and core seal arrangement in which a resiliently deformable seal energizing element has a rotational coupling with a cap body end wall. The coupling includes cam surfaces which transform relative rotation between the cap body and the seal energizing element as the cap body is screwed onto the container neck, into axial movement of the seal energizing element. This axial movement is added to the movement of the cap body end wall as the cap is screwed onto the container neck. The cap can therefore provide a highly expandable core seal operated by little rotation as the cap is torqued into its final, fully screwed on position on the container neck.

However the resiliently deformable seal energizing element disclosed in WO2020/200619 again acts primarily, if not exclusively, at a lower end of the sealing element disposed furthest within the container neck. If more even loading of the sealing element is required, according to one embodiment the opposite, upper end of the sealing element (closest to the mouth of the container neck) may be expanded outwardly by a separate annular wall which optionally depends from the inner surface of the cap body end wall. Oblique sections at the upper end of the sealing element are urged to expand radially, as they are urged against the depending annular wall during final tightening of the screw cap body onto the container neck. Effective operation of the core seal of WO2020/200619 may also be hindered if there is rotational slippage between the resiliently deformable seal energizing element and the container neck.

U.S. Pat. No. 3,788,510 relates to a cap liner which can be energized to wrap around the upper marginal zone of the neck of a container. An upper part of an annular wall portion of the liner is energized by a shallow conical web whose apex is forced downwardly by the inner face of the cap end panel. The lower edge of the wall portion has a hook-like form including a bevelled downwardly and outwardly directed face and an upwardly facing internal shoulder. The upwardly facing shoulder sealingly engages a downwardly facing flat shoulder within the bottle neck, essentially perpendicular to the neck principal axis. The base of the conical web is remote from the hooked lower edge of the wall portion and therefore has little or no ability to expand or provide a tighter sealing interengagement between the shoulders. The pliability of the wall portion, necessary for it to conform to and seal against the container neck, also means that the stress and strain at the base of the of the conical web remains localised there.

The present invention aims to mitigate at least some of these issues, and accordingly provides a screw cap comprising:

The tendency of the backing segments to rotate upwardly and outwardly, when added to the tendency of the annular sealing element to expand radially at the level of the seal energizing element periphery, causes the sealing pressure of the annular sealing element against the inner surface of the container neck to be more evenly distributed across the entire axial extent of the sealing element. This improves sealing performance of the sealing element. If the cap is screwed on to a higher torque than that which is necessary to first achieve a good circumferential seal, the excess strain is largely taken up not only by further deformation of the seal energizing element but also by bending stress in the segments of the backing. The facing is thereby protected against overstraining, improper distortion, creep relaxation, and extrusion. The screw caps of the present invention therefore have excellent sealing performance and fault tolerance against poor neck inner surface finishes and inaccurate tightening torques.

The seal energising element, annular sealing element, detent, backing, and facing may comprise parts of an expandable plug seal subassembly within the screw cap. The plug seal subassembly and cap body may comprise a snap-fit connection by which the plug seal subassembly is held in the cap body even when the screw cap is not screwed onto the container neck. The snap-fit connection may comprise a recess at the central part of the seal energizing element, into which a projection extending from the cap body end wall is snap-fittingly received. The projection may comprise a plurality of axially extending fingers or segments. The projection may comprise an enlarged end which is snap-fittingly received in an undercut portion of the recess at the central part of the seal energizing element. The snap-fit connection may allow relative rotation between the seal energizing element and the cap body, whereby normally there is substantially no relative rotation between the annular sealing element and the container neck as the screw cap is screwed on or unscrewed.

The detent may be integrally formed with the expandable plug seal subassembly, e.g. as an annular flange extending radially from an upper portion of the backing. The backing and seal energizing element may comprise through-going slots extending radially in the seal energizing element and axially in the annular sealing element; thereby allowing easier radial expansion of the plug seal by widening of the slots to take up much of the resulting strain. The axially extending through-going slots may serve to define the plurality of separate, axially extending segments of the backing. These slots may continue into the annular flange which forms the detent. Additionally or alternatively these slots may be united with, run into, or become, the radial through-going slots in the seal energizing element. The facing may cover at least the outer circumference of the annular sealing element and may sealingly cover the through-going slots in it, so that when the plug seal is expanded, the facing is sealingly pressed against the inner surface of the container neck around its entire circumference, to thereby hermetically seal the container. The facing may also cover a lower surface of the annular flange forming the detent, so as to hermetically seal against the rim of the container neck, when the cap body is screwed onto it. A bottom surface of the seal energizing element may have a covering of a softer and more compliant material to seal the slots therein. Such a covering may be continuous with the facing, so as to prevent contact between the packaged material and the other components of the screw cap. PCR materials may therefore be used to make these other components, while still allowing the screw cap to be used in food-grade or similar hygienic applications. The backing and the facing may be formed by insert moulding (bi-injection). The seal energizing element may comprise a domed shape which becomes flatter and expands radially as the detent engages the container neck, the cap body is screwed onto the container neck and the cap body end wall presses down on the central part of the seal energizing element.

The expandable plug seal subassembly may comprise a through-going aperture, over which a gas permeable, liquid impermeable membrane is secured, to provide a gas venting path. For example, the through-going aperture may be formed in the seal energizing element. The through-going aperture may be formed in a recess at the central part of the seal energizing element. This recess may be part of the snap-fit connection as described above. The detent and/or the inside surface of the cap body end wall may be provided with grooves or projections which space the detent away from the inside surface of the cap body end wall when the screw cap is fully tightened, thereby providing a gas flow path connecting the space between the seal energizing element and the cap body end wall to the atmosphere via the cap body internal thread.

Turning first to, the illustrative screw cap comprises a cap bodywithin which an expandable plug seal subassemblyis relatively rotatably housed, as further explained below.also shows a complementary container neckwhich can be closed and sealed by the cap bodyand attached plug seal subassembly. The cap bodycomprises a disc-shaped end walland a generally cylindrical side walldepending from the periphery of the end wall. The bottom of the side wallopposite to the end wallis provided with a radially outwardly extending protective flange, beneath which an anti-tamper ringis frangibly attached, in known manner. The screw cap bodyand anti-tamper ringmay be manufactured as a single component, e.g. by injection moulding from a suitable plastics material such as PE or a PCR plastic. As best shown in, the cap body side wallis formed with an internal screw thread. A projectionformed by a pair of axially extending fingers or segmentsdepends centrally from the inner face of the cap body end wall. The projectionforms a part of a snap-fit connection by which the plug seal subassemblyis rotatably held in the cap bodyeven when the screw cap is not screwed onto the container neck. Each of the axially extending fingers or segmentshas an enlarged end

show the annular sealing elementof the plug seal subassembly. More particularly the sectional views of these Figures show the structure of the annular sealing element, comprising its backingand its facing. The backingis formed of a relatively stiff and resilient material, such a suitable PCR plastic material. which is sufficiently resilient so as to return to or towards its original shape when external stresses are removed. The facingis of a softer, more compliant material, for example a suitable elastomer such as NBR, EPDM, neoprene or a silicone elastomer. For example, the material used to form the facingmay be selected to be compatible with (e.g. inert to) the container contents.

The detent for supporting the annular sealing elementwithin the container neckas the screw cap,is screwed on, may comprise an annular flangeprojecting radially outward from at or near the upper end of the backing.

The seal energizing elementillustrated in the drawings has a shallow, generally frustoconical configuration, although this is not essential to the invention. Other (usually generally dome-shaped) configurations are also possible, which allow radial expansion of the seal energizing element to arise from compression applied axially between its central and peripheral parts as the cap bodyis screwed onto the container neck.

The apex or central partof the seal energizing elementis directed towards the centre of the cap body end wallin use. A pocketof circular cross-section is provided, having an entrance at, and a depth extending axially below, this apex. The pockethas an inwardly extending, peripheral retaining lipwhich forms an undercut or re-entrant portion at the bottom of the pocket, into which the enlarged endsof the axially extending fingers or segmentsare snap-fittable. The plug seal subassemblyis thereby rotatably retained in the cap body. The apex or central portionof the seal energizing element, the pocket, and the projectionalso form a rotary thrust bearing, allowing the cap bodyto rotate relative to the plug seal subassemblyas the cap is screwed onto or unscrewed from the container neckand the plug seal subassemblyis held stationary within the container neck. However the described snap-fit interconnection,between the cap bodyand plug seal subassemblyis not essential to the invention. The apexof the seal energizing elementmay simply bear directly or indirectly against the inner face of the cap body end wallto form a rotary thrust bearing. Other retention mechanisms/rotary thrust bearings disposed between the cap bodyand plug seal subassemblyare also possible, as known to those skilled in the art.

The peripheral partof the resiliently deformable seal energizing elementis joined to the backingof the annular sealing elementat a position at or below the level of the detent (radially projecting annular flange). The backingextends below this junction and is divided by a plurality of circumferentially distributed, axially extending, through-going slots into a corresponding plurality of axially extending, radially distributed segments(). As illustrated in, the part of the backingextending below the junction or seal energizing element peripheryis about twice as deep as the part of the backing extending between this junction up to the level of the flange. However other configurations are also effective in providing the beneficial sealing effects of the invention. For example, the junction or seal energizing element peripherymay be provided at the level of the flange, whereby all of the backingextends below the junction; or the junctionmay be provided at a level about three quarters of the way down the backingbelow the flange; or the junctionmay be provided at a level anywhere within this range.

The edges of the slotsand the corresponding edges of the axially extending segmentsare indicated in dotted lines in, being still covered by the facingin this part-sectional view. In, the left-hand side of the plug seal subassemblyis shown sectioned through the material of the backingand of the seal energizing element. However, the right-hand side is shown sectioned through one of the slotsin the backing. Normally this slot may be filled with the facing material. (The slotwidens during the required deformation of the annular sealing element, allowing separation from any filler material, whereby interference from compression resistance of such material is not an issue). For clarity of illustration, the right-hand side ofshows this slotempty. To make the frustoconical or dome shaped seal energizing elementmore easily radially expandable under axial compression, it may be provided with a plurality of radially distributed, radially extending, through-going slotsThe slotsin the backingmay run into the seal energizing element, e.g. so as to be united with, run into, or become, the radial through-going slotsas shown in. The upper endsof the slotsterminate near to the apex or central partof the seal energizing element, e.g. at or near to the pocket. The slotsin the backingmay also have upwardly extending, through going continuationswhich may terminate in the radially projecting annular flange (detent), so as to leave a series of relatively small bridging pieces() holding the flangetogether. This increases the out-of-plane flexibility of the flange, allowing it to more easily accommodate corresponding irregularities in the upper end surface of the container neck. The slotsandalso increase the radial flexibility of the annular sealing element, allowing it to better accommodate out-of-roundness of the container neck bore. The facingmay continue radially outwards onto the lower surface of the annular flangeso as to help to form a seal with the upper end surface of the container neck. The backingtogether with the detent flangeand the seal energizing elementmay be formed as a one-piece component, e.g. by injection moulding.

In its relaxed state, the annular sealing elementtapers slightly in diameter in the axial direction away from the flange. This enables the expandable plug seal subassemblyto be guided and more easily pushed into the bore of the container neckas the cap bodyis screwed into place on the container neck. As a result, the flange (detent)eventually comes to rest on the upper end surface of the container neck. When the plug seal subassemblyin its relaxed state is first secured to/fully inserted within the cap body, a gapexists between the upper surface of the flangeand the adjacent part of the cap body end wall(). After the flange (detent)comes to rest on the upper end surface of the container neck, continued screwing on of the cap bodycauses the gapto diminish and the domed seal energizing elementto become flatter. Such flattening causes the seal energizing elementto expand radially, the flattening and radial expansion being aided by the presence of the radial through-going slotsThe radial expansion of the seal energizing elementforces the adjacent upper part of the annular sealing elementmore tightly into engagement within the bore of the container neck. Because they are joined to its periphery, flattening of the seal energizing elementalso causes the axially extending segmentsof the backingto pivot upwardly and outwardly. The unslotted part of the flange (detent)and/or the adjacent rim of the container neck may assist such pivoting movement by acting as a fulcrum. The taper of the annular sealing elementtherefore diminishes as the cap body continues to be screwed on. The annular sealing elementis therefore pressed against the bore of the container neckwith a more even pressure across its axial extent (as well as evenly around its circumference). A point is reached at which further expansion of annular sealing elementis substantially fully constrained by the container neck. Further screwing on of the cap bodybeyond this point (e.g. to fully eliminate the gap) mainly results in further bending of, and hence additional locked-in bending stresses within, the segmentsand seal energizing element. These locked in stresses (prestresses) are available to maintain the even sealing pressure between the annular sealing elementand the bore of the container neck, even if there is creep in the facingand/or in the container neck. A low and predictable tightening torque (to fully eliminate the gapand cause the anti-tamper ringto engage one-way teethon the container neck,) therefore results in a reliable and long-lasting sealed closure, able to accommodate wide variations in the container neck bore size and profile. With the gapeliminated, when the final tightening torque is applied to the screw cap body, the facingon the underside of the flangeis forced into tighter sealing engagement of the corresponding rim surface of the container neck.

A coveringof a softer and more compliant material (such as, but not limited to, the material of the facing) may be applied to the bottom surface of the seal energizing elementto seal the slotswithout significantly affecting the ability of the seal energizing elementto expand radially under axial compression. This coveringmay be continuous with the facingand therefore also cover the radially inner surfaces of the axially extending segments, the slotsand the lower edge of the backing. For example the facingand coveringmay be insert moulded (bi-injected) onto the underside of the flange, onto both sides of the backing, and onto the bottom surface of the seal energizing element, in a single operation. The facingand coveringtherefore may seal not only the slotsand the slotsbut also the slots(where present). The facingand coveringmay also provide a continuous, uninterrupted layer which seals around the entire circumference of the bore of the container neck, and spans this bore so as to isolate the container contents from the other components of the screw cap. These other components may therefore be made from PCR plastics, without risk of contaminating the container contents, even when the screw cap is used in pharmaceutical, food, or other similar hygienically demanding applications.

The plug sealshown indiffers slightly from the version shown in, in that ina central region of the seal energizing element(e.g. the bottom wall() of the pocketenclosure, where present) is not covered by the coveringThis exposed portionis provided with a number of through-going apertures. Three such apertures may be used, although other numbers are also suitable, including one. In, only two of the three aperturesare visible; the third being cut away by the sectioning. A microporous, gas permeable, liquid impermeable membraneis sealingly secured at its periphery to the exposed central regionof the seal energizing elementso as to cover and surround the apertures.

For the purposes of illustration rather than any technical necessity, inthe microporous membraneis shown as being partially transparent, so that the aperturescan be seen through it. For “venting out” applications, the membranemay be secured to the exposed region of the seal energizing elementimmediately surrounded by the covering(e.g. to the lower surface of the bottom wallof the pocketenclosure, where present). For “venting in” applications (not illustrated), the membrane may be secured to the opposite side of the seal energizing element, e.g. on the upper surface of the bottom wallof the pocketenclosure, where present. In each case, the seal energizing elementthereby supports and protects the membraneagainst bursting by overpressure.

Further gas venting channels may be provided in the form of shallow grooves(or other appropriately shaped channels) extending across the radial width of the flange/detenton its side facing away from the container. These channels therefore lead from the space between the plug sealand the cap body end wall, to the annular region occupied by the container neck. They therefore provide a gas venting path between the inside of the container and the atmosphere, via the screw cap bodyinternal thread. Additionally or alternatively, to complete the gas venting pathway, similar gas venting channels, conduits, spacing ridges, or the like may be provided in or on: the projection, the interior surfaces of the pocket, the apexof the seal energizing element, and/or the interior surface of the cap body end wall. Optionally, as shown on, in non-gas venting applications, the aperturesmay be present in the seal energizing element, but are covered and sealed e.g. by the coveringIn this way the same moulding tools may be used to form the seal energizing elementin both gas venting and non-gas venting versions of the screw cap.

Referring to, the radially outer surface of the annular sealing elementtapers substantially constantly from the detent (radially projecting annular flange), to a rounded bottom edge formed where the facingand coveringenfolds the distal tips of the segments. This profile allows the plug seal assemblyto be easily guided into the bore of the container neck as the screw cap is screwed onto it. Other profiles are also possible. A further non-limiting example is shown in, in which a lower part of the annular sealing element's radially outer surface is tapered, and an upper part of this surface (adjacent to the detent/flangeand adjacent to the junction) is substantially cylindrical. Expansion of the plug seal assemblyas the cap is screwed onto the container neck may lead to a substantially even pressure on this cylindrical portion of the facingas it is pressed against the bore of the container neck.

Tests have ben conducted using core seal subassembliesas shown in, in each case fitted to a 60 mm screw cap (a modified 60 mm “Plasticap” (RTM) cap from the applicants). The test caps were screwed onto a simulated jerry can neck to a torque of 5 Nm. The simulated jerry can neck was machined internally to mimic a 3 mm bore ovalisation—i.e. a difference in major and minor diameters of the bore of 3 mm, the major and minor diameters being at right angles to each other. No leakage was observed when the caps/neck were submerged in a water tank and pressurised to 318 mbar using compressed air. In another test, the caps were screwed onto ordinary sample jerrycans to a 5 Nm torque. The jerry cans were filled with water at room temperature and held upside down for a 24-hour period. Again no leakage was observed. In a third test, two ordinary empty jerry cans fitted with the test caps according torespectively (again at the 5 Nm closing torque) were pressurised with air until visibly distended (“blown up”). The air pressure required to do this was recorded (5.5 and 6.0 bar respectively). The jerry cans were then submerged in a water tank. Again no leakage was observed. The test caps therefore performed well at low closing torques, even with a substantial deformity in the container neck bore.