The inner shell of a double-shell closure is arranged for screw-threaded engagement with the neck of a container such as a medicine or tablet bottle. The outer shell is captive and rotatable on the inner shell, but can be used to drive the inner shell, and thereby unscrew the closure, by simultaneous press and turn operations performed on it by the user. Manipulation of this kind is difficult or impossible for a child to achieve, and the closure accordingly has child-resistance as desired.

A double-shell closure which has found substantial commercial success is manufactured in UK by United Closures and Plastics Plc and sold under the trade name CLICLOC. Such a closure is the subject of European patent specification EP 0440361 B1, to which the reader's attention is drawn.

In that closure the crowns of the two shells are urged resiliently apart axially of the closure to a separated but captive position which is determined by interengaged beads on the skirts or tubular parts of the shells. The biassing is achieved by spring fingers which extend integrally from the crown of the outer shell and into substantially continuous engagement at their free ends with the crown of the inner shell.

If an attempt is made to unscrew the CLICLOC closure by turning the outer shell in the appropriate direction

but without depressing it, the spring fingers on the outer crown ride over the surface of the inner crown and periodically spring into wells of the inner crown, making a distinctive clicking noise as they do so. The varying resistance which is generated between the spring fingers and the inner crown while this movement occurs requires torque to be applied to the outer shell, but the magnitude of the torque is small and insufficient to turn the inner shell on the container for removal. To fit the closure onto the container, however, it is sufficient merely to turn the outer shell in the appropriate direction. The free ends of the spring fingers are thereby brought into firm engagement with upstanding end walls of the wells, so driving the inner shell in the required direction.

For some applications of double-shell closures it is desired that the helix angle of the threads should be relatively large, so that the closures have "quick-rise" characteristics, that is to say, the angular movement required to free them for removal is relatively small, for example, less than half a turn.

However, Applicants have found that to give a double-shell closure quick-rise characteristics can interfere with its child-resistant operation. The use of the relatively large helix angle causes a substantial reduction in the torque which is required to unscrew the inner shell from the container, particularly after a period of time when the plastics material of the inner shell has undergone "cold creep" relaxation in response to the stresses generated when it was fitted. It is found that under some circumstances the required removal torque may fall to a lower value than the torque which is

naturally generated between the shells when the outer shell is rotated in the removal direction without depression. Mere rotation of the outer shell in the appropriate direction may then cause the inner shell to turn, so enabling the closure to be removed. In such circumstances, therefore, the double-shell closure loses its child-resistant characteristics, and behaves essentially as a one-piece closure lacking child- resistance.

A primary object of the present invention is to protect the child-resistant capability of a double-shell closure. Accordingly, therefore, the invention provides a double-shell closure and a container in combination, the closure having inner and outer shells disposed one within the other, the inner shell having thread means with one or more starts engageable with complementary thread means on the container, the outer shell being rotatably captive on the inner shell and movable axially in relation thereto to a limiting position in which the crowns of the shells are separated, the closure including spring means for biassing the shells to the said limiting axial position, and drive means engageable when the outer shell is moved axially from the said limiting position for enabling the inner shell to be driven by the outer shell if rotated in the direction required for unscrewing from the container, characterised in that the inner shell and the container are provided with additional engagement means to increase the resistance to rotation between the inner shell and the container in the unscrewing direction of rotation, the resistance to rotation between the inner shell and the container in that direction of rotation

being greater than that existing between the inner and outer shells.

Preferably the additional engagement means comprise at least one projection on one of the inner shell and the container and, on the other of the inner shell and the container, at least one recess adapted for receiving a said projection.

Advantageously, the or each recess is formed in a respective thread formation of the inner shell, the projection or projections being formed on the container and having on opposite sides thereof relatively steep and relatively gently inclined faces of which the steep face is the trailing face in the direction of screwing-up.

The or each projection may then advantageously merge with a respective start of the thread means at one end, and from there extend axially of the container in the direction away from the closure crowns.

In the described embodiment the inner shell has thread formations individually formed with recesses, the thread formations of the container having associated projections for engagement in those recesses.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which; Fig.l is a schematic sectional drawing of a closure/container combination according to the present invention, showing the closure in section on a container; Fig.2 is a schematic sectional drawing of the closure of Fig.l; Fig.3 is a plan view of the inner shell of the closure of Fig.2;

Fig.4 is a sectional plan view of the closure of Fig.2, taken through the line A-A; and Fig.5 is an enlarged scrap view of part of the view shown in Fig.4 of a modified inner shell.

Referring firstly to Figs. 1 to 4 of the drawings, a double-shell closure 10 is shown fitted to the neck 12 of a container of a suitable material, plastics (e.g. PET) or otherwise. The closure has outer and inner shells 14, 16, the inner shell of which is internally formed with threading formed of three identical thread formations 18.

The container neck 12 is correspondingly formed with three thread formations 20 adapted for screw-threaded engagement by the formations 18. The inner closure has a tamperevident ring 8 snap-engaged into it and arranged for engagement under a peripheral bead 9 on the container neck. In a modification (not illustrated) the tamperevident ring is moulded integrally with the remainder of the inner shell.

Each shell of the closure 10 has a generally plane crown 22, 24 and a generally cylindrical depending skirt 26, 28. The outer shell is held captive on the inner shell by a peripheral bead 30 on its skirt 26 which is snap-engaged behind a shoulder 32 on the inner skirt 28.

The shoulder 32 forms the upper end (as shown) of a peripheral groove 34 which is formed around the inner skirt 28 and within which the bead is free to move. The outer shell is accordingly free to rotate and to move axially to a limited extent on the inner shell.

The outer shell is biassed upwardly in relation to the inner shell by a plurality of integral spring fingers 36 of which one only is shown for clarity. These fingers bridge the gap which exists between the crowns 22, 24,

and except momentarily when they make a clicking noise to indicate an unsuccessful attempt to unscrew the closure, they make continuous engagement with the top surface of the inner crown.

It will accordingly be understood that if an attempt is made to unscrew the closure by merely turning the outer shell, the outer shell will turn on the inner shell but the spring fingers will generate a small but potentially significant torque on the inner shell by friction at their engagement with it. This frictional resistance is increased by unevenness of the upper surface created by a plurality of formations 38 formed in the upper surface of the inner crown 22 and by means of which (a) the clicking noises referred to above are made, and (b) the spring fingers are able to engage and drive the inner shell for rotation when the closure is screwed onto the container. One formation 38 is provided for each spring finger 36.

As can be understood from Fig.3 which shows the inner crown 22 as seen from above, each formation 38 has a raised part 40 and a well 42. The raised part is approached on one side by a ramp 44, and on its other side it falls away at a wall 46 to the adjacent well 42.

Thus, when the outer shell is turned on the inner shell in the anticlockwise sense in an attempt to unscrew the closure, the fingers move across each formation 38 in turn, and as they do so they move up the ramp 44 and make a clicking noise as they are subsequently freed to spring into the adjacent well. On the other hand, when the outer shell is turned in the clockwise sense, the free end of each spring finger enters a well 42 and by engagement with the associated wall 46 establishes a

drive between the two shells to screw the closure onto the container.

In order to unscrew the closure it is necessary for the user to depress the outer shell against the resilience of the spring fingers 36. This causes castellations (not shown) which are formed around the undersurface of the outer crown 24 to enter gaps 50 (Fig.3) formed between castellations 52 which are formed around the inner crown 22. Torque applied to the outer shell in the appropriate direction can then drive the inner shell to unscrew the closure by edge-to-edge engagement of the castellations.

The closure 10 insofar as it has been described above is conventional, and substantially as disclosed in European patent specificaton EP 0440361 mentioned above.

However, in the present embodiment of the invention the neck 12 of the container is provided with projections 60 spaced around the neck as shown in Figs. 1 and 4. Each projection 60 has sloping opposite faces 61, 62, the first face 61 being relatively steep, and the second face 62 being relatively gently inclined. The face 61 is the trailing face (in the direction of screwing-up). In the embodiment shown there are three projections 60, to correspond with the three internal thread formations 18 of the inner shell 16.

The thread formations 18 are each provided with a discontinuity forming a recess 63 in which the projections 60 on the container neck can be received.

When the closure 10 is being applied to the container neck 12, (e.g. by the manufacturer of the product within the container,) the closure is screwed on to the container neck with the thread formations 18 on the inner

shell engaging the thread formations 20 on the container neck. As the closure is rotated, the projections easily ride over the thread formations 18 and into the recesses 63, with the relatively shallow second faces 62 of the projections 60 passing over the trailing edges 64 of the thread formations. The length of the recesses 63 is such that the projections 60 are always accommodated in the recesses when the closure is fully applied. In this way the projections 60 do not form a significant hindrance to the application of the closure 10 to the container neck 12.

If the outer shell 14 is subsequently rotated in the unscrewing direction of the closure some of the torque applied to the outer shell 14 will be transferred to the inner shell 16. If the frictional resistance of the inner shell to rotation on the container neck is less than that of the outer shell rotating with respect to the inner shell, the closure will start to rotate on the container neck. However, before the inner shell can be unscrewed the leading edges 65 of its thread formations must first pass over the relatively steep first faces 61 of the projections 60, after which the projections must ride continuously along the thread formations of the inner shell, in frictional engagement with them. This serves to significantly increase the resistance to turning of the inner shell on the bottle neck, and ensures that if the outer shell is rotated without being depressed, the outer shell 14 will rotate with respect to the inner

shell 16, rather than the inner shell 14 becoming unscrewed from the container neck 12.

In this way, the closure 10 can be prevented from 'backing off' the container neck 12, even if, as shown, the thread formations 18 and 20 are provided with a relatively large helix angle so as to be removable with only a relatively small angular movement.

Fig.5 shows a modification of the inner shell 16, in which an elongate inward projection 70 is moulded on the inside of the inner shell below each recess 63 in its thread formations 18. These projections have a sufficient length so as to bridge their associated recesses circumferentially of the closure. Their radial depth is small, typically about 0.15mm, but sufficient for them to be engaged by the projections 60 on the container neck when those projections are located in the recesses.

The three projections 70 accordingly act as rubbing strips to generate frictional engagement between the inner skirt 16 and the container 12 which is in addition, and complementary, to the frictional resistance which is generated between the container projections 60 and the thread formations 18. The projections 70 therefore ensure that some frictional resistance to relative movement of the inner shell on the container is present at all times, thereby preventing any "looseness" of the closure when fitted and further ensuring that the child- resistance of the closure will be effective.

From the foregoing it will be understood that in the embodiments which have been particularly described with reference to the accompanying drawings the ramped

projections 60 of the container 12 and the further projections 70 (if provided) of the closure cooperate respectively with the thread formations 18 of the inner shell 16 and with one another to form additional engagement means by which the resistance to rotation of the inner shell on the container in the unscrewing direction of the closure is increased. However, other forms of additional engagement means are possible; for example, the projections 60 may be in the form of isolated lands, ribs (preferably vertical) or pimples which are spaced from the associated thread formations 20 of the container.

In a non-illustrated embodiment of the invention the projections 70 of the Fig.5 are replaced by an inwardly projecting shallow annular bead which extends continuously and with uniform depth around the inner circumference of the inner shell for engagement by the projections 60 of the container.

Whilst the invention has particular value for closure/container combinations having multi-start threads; it may be applied to such combinations having a single start or continuous thread on each component.