Closure assembly for pressurized containers

A container closure assembly that includes a container neck having an opening and a closure for the container neck, wherein the closure has a base portion and a skirt portion. A first screw thread on one of the container neck and the closure, the first screw thread comprising one or more first thread segments and a second screw thread on the other of the container neck and the closure, the second screw thread having a plurality of second thread segments, each of the second thread segment including upper and lower thread surfaces. A seal that forms a seal between the container neck and closure when the closure is screwed down on the container neck. Mutually engageable elements on the container neck and closure to block or restrict rotation of the closure in an unscrewing direction beyond an intermediate position when the closure is under an axial pressure in a direction emerging from the container neck. The container neck and closure are constructed and arranged to provide a vent for venting gas from the container neck at least when the closure is in the intermediate position and the vent includes a recess in the other of the container neck and closure. The recess being located between and circumferentially overlapping two of the plurality of second thread segments to increase the cross-sectional area of the vent between the second thread segments.

Applicant claims, under 35 U.S C. .sctn. 119, the benefit of priority of
 the filing date of Oct. 10, 1997 of a United Kingdom patent application,
 copy attached, Serial Number 9721568.5, filed on the aforementioned date,
 the entire contents of which is incorporated herein by reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a container neck and closure assembly for
 use on pressurized containers such as carbonated beverage containers.
 2. Description Of the Related Art
 Current commercially mass-produced carbonated beverage containers use
 threads on the container and closure of the continuous, helical type. The
 threads comprise a single, substantially continuous thread portion on the
 container neck with a low thread pitch angle, typically less than
 5.degree.. The low pitch angle is needed in order to ensure that the
 closure does not unscrew spontaneously under pressure from inside the
 container. The low pitch angle also provides the necessary leverage to
 achieve a gas-tight compressive seal between the closure and the container
 neck when the closure is tightened onto the container neck. The low pitch
 of the helical threads also means that the closure typically needs to be
 rotated through more than 360.degree. to disengage it completely from the
 container neck. Whilst this can be laborious, especially for elderly or
 child users, it also permits some gas venting to take place while the
 closure is being unscrewed, and thereby reduces the risk that the closure
 will blow off uncontrollably once unscrewing of the closure from the
 container neck has commenced. This gas venting is usually assisted by the
 provision of axial gas venting notches extending longitudinally through
 the helical threads.
 Drawbacks of these low pitch helical threads include the laborious rotation
 required to remove and resecure the closure on the neck, excessive use of
 molding material to form the long helical threads, and unreliable
 separation of tamper-evident rings from the closure skirt due to the low
 pitch angle of the threads.
 U.S. Pat. No. 5,135,124 describes a container closure assembly for a
 carbonated beverage container that incorporates a safety feature to
 prevent the closure blowing of off uncontrollably (missiling) as it is
 unscrewed from a container under high pressure. The closure assembly is
 provided with a complex double-bayonet thread arrangement to provide for
 gas venting at an intermediate position of the closure on the container
 neck. The bayonet thread arrangement can be difficult for infirm or very
 young users to assemble and disassemble successfully, since these
 operations involve sequential steps of pressing down and rotating the
 closure. Moreover, to achieve a pressure-tight seal, a strong axial
 sealing force must be applied by the user in the initial pressing-down
 step of securing the closure on the container neck. Furthermore, the
 bayonet-type threads are inherently less suitable for reliable operation
 of a tamper-evident ring that is frangibly attached to the closure skirt,
 but that is retained on the container neck after the assembly is opened
 for the first time.
 The present applicant has described an improved pressure safety cap for
 carbonated beverage containers in International Patent Publication
 WO95/05322. This application describes container closure assemblies having
 substantially continuous threads defining a substantially continuous
 helical thread path, although the pitch of the helix can vary. The closure
 can be moved from a fully disengaged to a fully secured position on the
 container neck by rotation through 360.degree. or less. The threads on the
 neck or the closure are provided with mutually engageable elements to
 block or restrict rotation of the closure in an unscrewing direction
 beyond an intermediate position when the closure is under an axial
 pressure in a direction emerging from the container neck, the neck and
 closure being constructed and arranged to provide a vent for venting gas
 from the container neck at least when the closure is in the intermediate
 position. This pressure safety feature prevents the closure from blowing
 off uncontrollably once unscrewing of the closure from the container neck
 has started. It thus allows the use of shorter, more steeply pitched or
 multiple-start threads in the container and closure assembly, thereby
 rendering the assembly much more elderly- and child-friendly without
 sacrificing pressure safety.
 WO97/21602 describes an improved version of the assemblies of WO95/05322 in
 which the thread on the container neck has a lower surface having a
 variable pitch, such that the pitch of thread is lower in a region near
 the bottom of the thread. This reduces the tendency of the closure to blow
 off when the container is sealed and pressurized. A further region of low
 pitch may be provided on the neck thread adjacent to the intermediate
 position where gas venting takes place. This reduces the tendency of the
 closure to override the blocking means at the intermediate position while
 gas venting is taking place.
 GB-A-2288390 describes container closure assemblies for beverage
 containers. The closure cap screws onto the container neck in less that
 half a turn, with pins carried on the cap engaging between screw threads
 provided on the container neck. The threads are variably pitched to give a
 decreased final angle of pitch in order to reduce the likelihood that
 pressure exerted on the cap will cause the cap to back off the container
 neck. Slots may be provided on the underside of the threads to block
 unscrewing of the cap beyond an intermediate position until venting of
 pressure from inside the container has taken place.
 SUMMARY OF THE INVENTION
 It is an and advantage of the present invention to provide an improved
 pressure venting arrangement for a pressure safety container and closure
 assembly that can permit faster venting of excess pressure from inside the
 container, and thereby enable quicker removal of the closure from the
 container neck.
 One aspect of the present invention regards a container closure assembly
 including:
 a container neck having an opening;
 a closure for the neck, the closure having a base portion and a skirt
 portion;
 a first screw thread on one of the neck and the closure, the first screw
 thread having one or more first thread segments; and
 a second screw thread on the other of the neck and the closure, the second
 screw thread comprising a plurality of second thread segments, each second
 thread segment having upper and lower thread surfaces, and regions of the
 second thread segments being circumferentially overlapping,
 a seal between the neck and the closure when the closure is screwed down
 onto the neck;
 mutually engageable elements on the neck and closure to block or restrict
 rotation of the closure in an unscrewing direction beyond an intermediate
 position when the closure is under an axial pressure in a direction
 emerging from the container neck;
 wherein the neck and closure are constructed and arranged to provide a vent
 for venting gas from the container neck at least when the closure is in
 the intermediate position,
 and wherein the vent includes a recess in the other of the neck and
 closure, the recess being located between and circumferentially
 overlapping two of the plurality of second thread segments to increase the
 cross-sectional area of the vent between the second thread segments.
 The second thread segments are not bayonet-type thread segments. The second
 thread segments extend around the container neck or closure skirt a
 sufficient distance so that a top portion of one thread segment is
 proximate to a bottom portion of another thread segment, and preferably
 overlaps the other thread segment for a finite angular distance around the
 neck or closure skirt. That is to say, preferably adjacent second thread
 segments are circumferentially overlapping. A thread gap is defined
 between the top and bottom portions of the thread segments. One of the
 first thread segments travels through this thread gap as the closure is
 screwed onto or off the container neck. It has been found that this thread
 gap may have a cross-section that is too small for optional gas venting in
 all circumstances. The present invention overcomes this difficulty by
 providing a recess in the container neck or closure skirt to increase the
 cross-section of the thread gap to increase the rate of gas venting
 through the thread gap.
 The increased cross-sectional area of the venting pathway in the
 circumferentially overlapping regions of the second thread permits faster
 venting of pressure from inside the container, and thereby reduces the
 length of time that the closure is blocked at the intermediate position
 while venting takes place, without any loss of pressure safety.
 Preferably, the recess has an elongate groove extending around the
 container neck or the closure skirt between the second thread segments in
 the overlapping regions. Preferably, the elongate groove extends
 substantially parallel to the second thread segments. Preferably, the
 second thread segments are on the container neck, where they project
 outwardly from a substantially cylindrical neck surface. In that case, the
 recess preferably has an elongate groove in the container neck.
 Preferably, the longitudinal cross-sectional area of the recess is from 5%
 to 50% of the longitudinal cross-sectional area of the second thread
 segments adjacent to the recess.
 Preferably, the first and second screw threads are constructed and arranged
 to permit axial displacement of the closure relative to the neck at least
 when the closure is at the intermediate position, and preferably the
 engageable elements are adapted to engage each other when the closure is
 axially displaced in a direction emerging from the neck, for example by
 axial pressure from inside the pressurized container. More preferably, the
 mutually engageable elements are constructed and arranged not to mutually
 engage each other when the closure is axially displaced in a direction
 inwardly towards the neck at the intermediate position, for example when
 the closure is being screwed down onto the container neck.
 Preferably, the mutually engageable elements have a step or recess formed
 in the lower surface of one of the second screw thread segments to provide
 a first abutment surface against which a second abutment surface on one of
 the first screw thread segments abuts to block or restrict rotation of the
 closure in an unscrewing direction at the intermediate position when the
 closure is under axial pressure in a direction emerging from the container
 neck.
 More Preferably, the second thread segment has a first thread portion
 having a first cross section and a second thread portion having a second
 cross section narrower than the first cross section, whereby a step is
 provided in the lower thread surface of the second thread segment where
 the first and second thread portions meet, and the first abutment surface
 being provided by the step. The relatively broad first cross section is
 preferably adjacent to the circumferentially overlapping region of the
 second thread segments, resulting in a relatively narrow thread gap in
 that region, hence the desirability of the recess provided by the present
 invention to increase the cross-section of the thread gap. More
 preferably, the upper surface of the second thread segment opposite the
 lower surface of the second thread segment is substantially smooth and
 continuous where the first and second thread portions meet.
 Preferably, at least one of the first and second threads has four thread
 starts. This minimizes the amount of rotation of the closure on the
 container neck that is required to achieve initial engagement of the
 threads, thereby making the assembly more elderly- and child-friendly.
 Preferably, the closure can be moved from a fully released to a fully
 engaged position on the neck by a single smooth rotation through
 360.degree. or less, more preferably 180.degree. or less, and most
 preferably about 90.degree. or less.
 Preferably, the first thread segments follow a substantially continuous,
 preferably substantially helical thread path for the whole of the rotation
 as the closure is screwed onto the container neck, although the pitch of
 the helix may vary. The continuous thread path renders the assembly
 especially easy to close by the elderly and infirm, or by children. In
 contrast, bayonet-type threads of the kind described in U.S. Pat. No.
 5,135,124 require a relatively complex, stepped manipulation to secure the
 closure onto the container neck, with the result that the closure is often
 inadequately secured on the container neck. Furthermore, it is extremely
 difficult to devise a tamper-evident ring for the closure that separates
 reliably and easily upon opening of a bayonet-type closure assembly.
 Finally, a continuous thread is easier for physically weak people to screw
 down against pressure from inside the container than a bayonet thread.
 A seal between the neck and the closure is formed if screwed down on the
 neck is preferably a compressible sealing wad inside the base portion of
 the closure for abutting against a lip of the container neck. Preferably,
 the sealing wad is formed from a compressible elastomer. A circumferential
 sealing rib may be provided on the lip of the container neck, or inside
 the base of the closure underneath the sealing wad, in order to optimise
 compression of the elastomer to achieve a pressure-tight seal.
 Preferably, the assembly further has complementary locking devices on the
 container neck and the closure that prevent unscrewing of the closure from
 the fully engaged and sealing position on the container neck until a
 predetermined minimum opening torque is applied. More preferably, the
 locking devices comprise a longitudinal locking rib on one of the
 container neck or on the skirt portion of the closure, and a complementary
 locking ramp on the other of the container neck or the skirt portion of
 the closure, wherein the locking rib abuts against a retaining edge of the
 locking ramp when the closure is fully engaged on the container neck. In
 alternative preferred embodiments, a locking recess such as a longitudinal
 groove may be provided in one or more of the first or second thread
 segments, and a longitudinal locking rib is provided on the other of the
 container neck or on the skirt portion of the closure, whereby the locking
 rib is received in the recess in the thread segments at the fully engaged
 and sealing position of the closure on the container neck. Locking devices
 of this kind are described in detail in WO91/18799 and WO95/05322, the
 entire disclosures of which are expressly incorporated herein by
 reference.
 The complementary locking devices provide a number of important advantages.
 Firstly, they prevent accidental backing off of the closure from the fully
 engaged and sealing position on the container neck due to pressure from
 inside the container. This also permits the use of more steeply pitched
 threads on the container neck and the closure. Furthermore, the locking
 devices provide a positive "click" when the fully engaged and sealing
 position of the closure on the container neck is reached, thereby giving
 the user a positive indication of that position. This helps to ensure that
 exactly the right degree of compression is applied between the container
 and closure to achieve an effective pressure-tight seal.
 Preferably, the container closure assemblies according to the present
 invention further have a first stop on one of the container neck and the
 closure for abutment against a complementary second stop on the other of
 the container neck and the closure to block over-tightening of the closure
 beyond the predetermined fully engaged and sealing position on the
 container neck. More preferably, the first stop has a longitudinal
 shoulder adjacent to the bottom of the second thread segment, and the
 second stop is an end of the first thread segment. In other preferred
 embodiments, the first stop may project from the container neck or the
 closure skirt adjacent to the locking ramp as described above, and the
 second stop is the longitudinal locking rib referred to above, which snaps
 into a recess between the first stop and the locking ramp at the said
 fully engaged and sealing position. Suitable stop devices are described in
 WO91/18799.
 The provision of the stop devices to prevent over-tightening of the closure
 on the container neck is useful to prevent damage to the threads by
 over-tightening. It also ensures that precisely the right degree of
 compression of the sealing wad is achieved at the fully engaged and
 sealing position so that an effective pressure seal is formed.
 Over-compression of elastomeric sealing wads can result in a loss of
 resilience and cracking of the sealing wads, resulting in loss of
 pressure-tightness.
 Preferably, the first and second threads on the container neck and closure
 are variable pitch threads, preferably as described in WO97/21602, the
 entire contents of which are incorporated herein by reference. Preferably,
 the pitch of the lower thread surface of the second thread segments is
 relatively lower in a first region and relatively higher in a second
 region displaced from the first region in an unscrewing direction. The
 pitch of the lower thread surface in the first region is preferably
 substantially constant. Preferably, the first region extends for
 20-40.degree. about the circumference of the container neck or the closure
 skirt. Preferably, the pitch of the lower thread surface in the first
 region is in the range of -5.degree. to 10.degree., more preferably
 1.degree. to 7.degree..
 Preferably, the second region is adjacent to the first region of the lower
 thread surface. Preferably, the pitch of the lower thread surface in the
 second region is substantially constant, and the second region preferably
 extends for 15-35.degree. about the circumference of the container neck or
 the closure skirt. Preferably, the pitch of the lower thread surface in
 the second region is in the range of 15.degree.to 350.degree..
 The use of a variable pitch thread renders it easier to combine fast-turn
 threads having a steep average pitch that are elderly-and child-friendly
 with pressure safety. A problem that could arise with fast-turn threads is
 that they are steeply pitched, which results in a tendency to back off
 from the fully secured position on the container neck when the container
 is pressurized. This problem can be overcome by using bayonet-type
 threads, but the use of bayonet-type threads results in a number of
 different problems, as described above. In contrast, the variable pitch
 threads solve the problem of backing off of the closure under pressure,
 whilst retaining all of the advantages of continuous, fast-turn threads.
 Preferably, the lower thread surface further has a third region adjacent to
 the second region, wherein the third region has a relatively low pitch.
 Preferably, the third region has a relatively constant pitch, preferably
 in the range 1 to 12.degree.. The third region is located to abut against
 the first thread segments of the other of the container neck and the
 closure when the cap is blocked at the intermediate gas venting position.
 The relatively low pitch of the third region reduces the tendency of the
 cap to override the blocking device at high gas venting pressures.
 Preferably, the first and/or the second thread segments are interrupted by
 axial gas venting channels, similar to those on existing carbonated
 beverage shallow-pitch threads. The axial gas venting channels assist the
 venting of pressure from inside the container as the closure is unscrewed.
 However, the molding of axial gas venting channels on the container neck
 by blow molding can be difficult using a conventional two-part mold.
 Therefore, more preferably, the container closure assembly according to the
 present invention further includes a transverse gas venting channel
 extending through one or more of the first and/or second thread segments.
 The term "transverse" implies that the gas venting channel extends
 substantially circumferentially around the container neck or the closure
 skirt. Preferably, two transverse gas venting channels extend through the
 thread segments on opposite sides of the container neck and across the
 blow-molding seam of the container neck.
 Preferably, the transverse gas venting channel is tapered, so that the
 channel is narrower on the lower side of the thread segment than on the
 upper side of the thread segment. This is to maximize the area of contact
 between the first and second thread segments when the closure is under
 pressure from inside the container.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
 Referring to FIGS. 1 and 2, this embodiment is a container closure assembly
 especially adapted for a carbonated beverage container The main features
 of this assembly resemble those of the assembly described and claimed in
 our International Patent Publications WO95/05322 and WO97/21602, the
 entire contents of which are expressly incorporated herein by reference.
 The assembly includes a container neck 10 of a container for carbonated
 beverages, and a closure 12. Both the container neck and the closure are
 formed from plastics material. The container is preferably formed by
 injection molding and blow molding of polyethylene terephthalate in the
 manner conventionally known for carbonated beverage containers. The
 closure is preferably formed by injection molding of polypropylene. The
 closure 12 has a base portion 14 and a skirt portion 16.
 On the inside of the skirt portion 16 there is provided a four-start first
 screw thread made up of four first thread segments 18, as shown in phantom
 on the thread developments of FIGS. 3-5. The first thread segments 18 are
 short thread segments having an upper surface 60 with relatively low pitch
 of about 6.degree. and a lower surface 62 with intermediate pitch of about
 13.5.degree..
 The container neck 10 is provided with a second screw thread formed from
 four second thread segments 20, each of which is a substantially
 continuous helical thread having an upper thread surface 22 and a lower
 thread surface 24. The upper and lower second thread surfaces 22, 24 are
 sloped to give the second thread segment a trapezoidal cross-section. A
 substantially continuous, approximately helical thread gap 26 is defined
 between overlapping regions of the said upper and lower surfaces 22, 24 on
 adjacent second thread segments 20.
 It can be seen that the second thread segments 20 are circumferentially
 overlapping over part of their length. A groove 66 is provided in the
 container neck 10 between the second thread segments 20 in the overlapping
 region 26.
 It can also be seen that a transverse groove 68 is provided in alternate
 second thread segments 20, extending from the upper thread surface 22
 through to a first region 28 of the lower thread surface 24. The
 transverse groove is tapered from top to bottom.
 An important feature of this assembly is the profiling of the lower surface
 24 of the second thread segments 20, which is described in more detail in
 our International Patent Publication WO97/21602. The lower thread surface
 24 includes a first, lower region 28 having a substantially constant pitch
 of only about 6.degree.. The lower region 28 adjoins an intermediate
 region 30 having a substantially constant, much higher pitch of about
 25.degree.. The average pitch of the thread segment 20 (i.e. the pitch of
 the straight upper thread surface 22) is 13.5.degree..
 The second thread segments 20 also include a pressure safety feature
 similar to that described and claimed in our International Patent
 Publication WO95/05322. Briefly, a step 32 is provided in the lower
 surface 24 of the second thread segment 20 to abut against an end of the
 first thread segments 18 and block unscrewing of the closure 12 from the
 neck 10 when the first thread segments 18 are in abutment with the lower
 surface 24, i.e. when there is a net force on the closure in an axial
 direction out of the container neck. A third region 34 of the lower
 surface 24 of the second thread segments situated adjacent to the step 32
 also has a low pitch of about 6.degree.. The step is formed by the
 junction between a relatively broad top portion 70 of the second thread
 segment and the relative narrow third region 34 of the second thread
 segment 20.
 The container and closure assembly is also provided with complementary
 locking elements on the container neck and the closure to block unscrewing
 of the closure from the fully engaged position on the container neck
 unless a minimum unscrewing torque is applied. These locking elements
 comprise four equally radially spaced locking ribs 36 on the inside of the
 closure skirt 16, and four equally radially spaced retaining ramps 38 on
 the container neck. The ramps 38 have a radially sloped outer face 40 and
 a radially projecting retaining edge 44 against which the rib 36 on the
 closure abuts when the closure is fully engaged on the container neck. The
 complementary locking elements may be as described in our International
 Patent Publication WO91/18799, the entire content of which is hereby
 expressly incorporated by reference.
 The container and closure assembly also includes a gas-tight seal between
 the closure and the container neck. This seal preferably comprises a
 gas-tight elastomeric sealing wad 46 that is compressed against the lip of
 the container neck. Optimum sealing is preferably achieved when the
 elastomeric sealing wad is compressed to between 30% and 70% of its
 original thickness.
 The second thread segments 20 terminate at their lower end in a
 longitudinal shoulder 72 forming a first stop against which a second end
 74 of the first thread segments 18 may abut thereby to block
 overtightening of the closure on the neck.
 The container closure assembly also comprises a tamper-evident safety
 feature. The safety feature includes a tamper-evident ring 50 that is
 initially formed integrally with the skirt 16 of the container closure 12
 and joined thereto by frangible bridges 52. The tamper-evident ring 50
 comprises a plurality of integrally formed, flexible, radially inwardly
 pointing retaining tabs 54. A circumferential retaining lip 56 is provided
 on the container neck 10. Ratchet projections 58 are also provided on the
 container neck below the circumferential retaining lip 56 and radially
 spaced around the container neck to block rotation of the tamper-evident
 ring 50 on the container neck 10 in an unscrewing direction. The structure
 and operation of the tamper-evident ring feature are as described and
 claimed in our International Patent Publication WO94/11267, the entire
 contents of which are expressly incorporated herein by reference.
 In use, the closure 12 is secured onto the container neck 10 by screwing
 down in conventional fashion. The closure 12 can be moved from a fully
 disengaged position to a fully engaged position on the container neck 10
 by rotation through about 90.degree.. When the closure is being screwed
 down, there is normally a net axial force applied by the user on the
 closure into the container neck, and accordingly the first thread segments
 18 abut against and ride along the upper surface 22 of the second thread
 segments 20 on the container neck. It can thus be seen that the first
 thread segments follow a substantially continuous path along a variable
 pitch helix. The first and second threads are free-running, which is to
 say that there is substantially no frictional torque between the thread
 segments until the fully engaged position is neared. These features of a
 90.degree. closure rotation, substantially continuous thread path and
 free-running threads all make the closure extremely easy to secure on the
 container neck, especially for elderly or arthritic persons, or children.
 As the closure nears the fully engaged position on the container neck 10,
 several things happen. Firstly, the tamper-evident ring 50 starts to ride
 over the retaining lip 56 on the container neck. The retaining tabs 54 on
 the tamper-evident ring 50 flex radially outwardly to enable the
 tamper-evident ring to pass over the retaining lip 56 without excessive
 radial stress on the frangible bridges 52. The flexible retaining tabs 54
 subsequently ride over the radial ratchet projections 58 on the container
 neck in similar fashion.
 Secondly, the locking ribs 36 on the closure skirt 16 ride up the outer
 ramped surface 40 of the retaining ramps 38 on the container neck. The
 gentle slope of the ramped surfaces 40, together with the resilience of
 the closure skirt 16, mean that relatively little additional torque is
 required to cause the locking ribs 36 to ride up the ramped surfaces 40.
 Thirdly, the initial abutment between the sealing wad 46 in the container
 closure base and the sealing rib 48 on the container neck results in a net
 axial force on the closure in a direction out of the container neck. This
 pushes the thread segments 18 on the closure skirt out of abutment with
 the upper surface 22 of the second thread segments 20 and into abutment
 with the lower thread surfaces 24 of the second thread segments 20. More
 specifically, it brings the first thread segments 18 into abutment with
 the lower region 28 of the lower thread surfaces 24. Continued rotation of
 the closure cap in a screwing-down direction causes the first thread
 segments 18 to travel along the lower region 28 until the final, fully
 engaged position shown in FIG. 3 is reached. The low pitch of the lower
 surface 28 devices that this further rotation applies powerful leverage
 (camming) to compress the sealing wad 46 against the sealing rib 48 in
 order to achieve an effective gas-tight seal.
 When the fully engaged position of the closure 12 on the container neck 10
 is reduced, the locking ribs 36 click over the top of the respective
 ramped surfaces 40 and into abutment with the steep retaining surfaces of
 the ratchet 30 ramps 38. At the same position, the second ends 74 of the
 first thread segments 18 may come into abutment with the stop shoulders 72
 at the bottom of the second thread segments, thereby blocking further
 tightening of the closure than could damage the threads and/or
 over-compress the sealing wad.
 When the closure 12 is in the fully engaged position on the container neck
 10, the upper surfaces 60 of the first thread segments 18 abut against the
 lower region 28 of the lower thread surfaces 24 of the second thread
 segment 20 on the container neck, as shown in FIG. 3. The upper surface of
 the first thread segments has a low pitch to match that of the lower
 region 28, so as to maximize the contact area between the thread segments
 in this region 28, and thereby distribute the axial force exerted by the
 closure as evenly as possible around the container neck. Because of the
 low pitch in the region 28, relatively little of the axial force emerging
 from the container neck due to pressure inside the container is cammed
 into unscrewing rotational force by the abutment between the thread
 surfaces in this position. This greatly reduces the tendency of the
 closure to unscrew spontaneously under pressure. Spontaneous unscrewing is
 also prevented by the abutment between the locking ribs 36 and the
 retaining edge 44 on the locking ramps 38. An important advantage of the
 assembly is that the reduced tendency to unscrew spontaneously due to the
 low pitch of the lower thread surfaces in the lower regions 28 devices
 that the minimum opening torque of the locking elements 36, 38 can be
 reduced without risk of the closure blowing off spontaneously. This makes
 the closure easier to remove by elderly or arthritic people, or by
 children, without reducing the pressure safety of the closure.
 In use, the closure is removed from the container neck by simple
 unscrewing. An initial, minimum unscrewing torque is required to overcome
 the resistance of the locking elements 36, 38. Once this resistance has
 been overcome, essentially no torque needs to be applied by the user to
 unscrew the closure. The internal pressure inside the container exerts an
 axial force on the closure in a direction emerging from the mouth of the
 container, as a result of which the first thread segments 18 ride along
 the lower surface 28 of the second thread segments 20 as the closure is
 unscrewed. The first thread segments initially ride along the lower region
 28, and then along the steeply pitched intermediate region 30 of the lower
 surface of the second thread segments 20. The first thread segments 18
 then come into abutment with the step 32 of the second thread segments 20,
 as shown in FIG. 4. In this position, further unscrewing of the closure is
 blocked while gas venting takes place along the thread paths 26. It should
 also be noted that, in this intermediate gas venting position, the first
 thread segments 18 abut primarily against the third region 34 of the lower
 surface of the second thread segments 20. The low pitch of this region 34
 results in relatively little of the axial force on the closure being
 cammed into unscrewing rotational torque, thereby reducing the tendency of
 the closure to override the pressure safety feature and blow off.
 It will be appreciated that the groove 66 in the container neck enables
 faster gas venting along a helical gas venting path 26 between the
 overlapping regions of the second thread segments 20. In addition, the
 transverse vents 68 through the second thread segments 20 provide further
 gas venting pathways at the intermediate position of the closure on the
 container neck.
 Once gas venting from inside the container neck is complete so that there
 is no longer axial upward force on the closure, the closure can drop down
 so as to bring the thread segments 18 into abutment with the upper
 surfaces 22 of the second thread segments 20. In this position, unscrewing
 can be continued to disengage the closure completely from the container
 neck as shown in FIG. 5.
 The above embodiment has been described by way of example only. Many other
 embodiments of the present invention falling within the scope of the
 accompanying claims will be apparent to the skilled reader.