Compression moulding punch

Abstract

A punch is disclosed for forming an annular gasket in a cap for closing containers; the punch comprises at least one internal annular portion, an intermediate annular portion and an external annular portion that bound a forming cavity; the internal and 5 external annular portions are coupled slidingly with the intermediate annular portion; between the intermediate annular portion and the internal annular portion an internal annular gap communicating with the forming cavity is defined; between the intermediate annular portion and the external annular portion an external annular gap communicating with the forming 10 cavity is defined; during compression moulding, a part of the moulding material enters the internal annular gap, forming an internal annular protrusion and another part enters the external annular gap forming an external annular protrusion; this enables the risk of formation of irregularities or inconsistencies in the annular gasket to be reduced.

Description

Compression moulding punchBackground of the invention

[0001] The invention relates to compression moulding punch. The invention further relates to a use of this punch, and also a moulding apparatus that comprises this punch and a moulding method that uses this punch.

[0002] Specifically but not exclusively, the invention in question can be used to mould plastics to form an annular gasket, in particular an annular gasket inside a closure of a container, for example an annular gasket of a crown cap, or of an aluminium cap, or of still other types of closure for containers.Prior art

[0003] Compression moulding plastics to form an annular gasket in a closure for containers is known.

[0004] Various aspects of the prior art are improvable.

[0005] It is first desirable to improve moulding reliability ensuing the quality of the moulded gasket, in particular the regularity of the shape thereof, constantly and repeatedly, avoiding or reducing the performance of complicated and delicate finishing tasks after moulding.

[0006] Further, it is desirable to ensure moulding of gaskets that are as compact and consistent as possible, with a shape that is well-defined and acceptable both aesthetically and functionally.Summary of the invention

[0007] One object of the invention is to provide a solution that is able to solve one or more of the aforesaid limits and drawbacks of the prior art.

[0008] One object is to provide a punch that is suitable for compression moulding plastics to form annular gaskets and which is alternative to those of the prior art.

[0009] One object is to make a punch that is usable to form gaskets in container closures like, for example, crown caps.

[0010] One advantage is allowing an annular product (gasket) to be formed that is compression moulded and devoid of irregularities or defects, so as to avoid the performance of laborious and complicated post-moulding finishing tasks on the product, or even discarding the entire moulded product.

[0011] One advantage is permitting moulding of an annular gasket with a particularly compact and consistent shape.

[0012] One advantage is making a constructionally simple and cheap compression moulding punch available.

[0013] These objects and advantages, and still others, are achieved by a moulding punch according to one or more of the claims set out below.

[0014] In one embodiment, a punch comprises at least one internal annular portion, an intermediate annular portion and an external annular portion that bound a forming cavity, in cui the internal and external annular portions are coupled slidingly with the intermediate annular portion, in which between the intermediate annular portion and the internal annular portion an internal annular gap communicating with the forming cavity is defined and / or in which between the intermediate annular portion and the external annular portion an external annular gap communicating with the forming cavity is defined.

[0015] It is provided that a part of the moulding material that is compressed in the forming cavity can enter the internal annular gap forming an internal annular protrusion and / or a part of the moulding material that is compressed in the forming cavity can enter the external annular gap forming an external annular protrusion.

[0016] The internal annular gap is sized so as to permit, as mentioned above, the entry of a quantity of moulding material that is compressed by the punch of the forming cavity.

[0017] Accordingly, the surfaces facing one another that define the internal annular gap are not designed to define an axially sliding geometric coupling between the internal annular portion and the intermediate annular portion or, to put it better, it is provided that such function of axially sliding geometric coupling is performed by from other surfaces of the internal annular portion and of the intermediate annular portion.

[0018] This function (axially sliding geometric coupling) is performed (in a substantially known manner) by a sliding geometric coupling between an outer coupling surface of the internal annular portion and an inner coupling surface of the intermediate annular portion.

[0019] In practice, a dimension in a radial direction of the internal annular gap will be greater than a dimension in a radial direction (i.e. than the radial clearance of the sliding geometric coupling) between the aforesaid coupling surfaces of the internal and intermediate annular portions.

[0020] This is applicable to a radial dimension of the external annular gap and to the radial clearance of the sliding geometric coupling between coupling surfaces of the intermediate and external annular portions.Short description of the drawings

[0021] The invention can be understood better and implemented with reference to the appended drawings, which illustrate embodiments thereof by way of non-limiting example, in which:figure 1 shows a partial section, according to a vertical section plane, of a first embodiment of a moulding punch according to this invention;figure 2 shows an enlarged detail of the punch of figure 1 in which the internal and external annular gaps are visible;figure 3 shows an enlarged detail of figure 2 at the internal annular gap;figure 4 shows another enlarged detail of figure 2 at the external annular gap; figure 5 shows a detail of a second embodiment of a moulding punch according to this invention;figure 6 shows an enlarged detail of figure 5 at the internal annular gap;figure 7 shows another enlarged detail of figure 5 at the external annular gap.Detailed description

[0022] With reference to the aforesaid figures, for greater simplicity and ease of clarity, identical elements of different embodiments have been indicated by the same numbering.

[0023] With 1, a compression moulding punch has been indicated above all, which is usable in particular for compression moulding plastics to form gaskets in container closures like, for example, crown caps or caps made of aluminium or other materials.

[0024] The punch 1 comprises a compression moulding axis X. In the configuration of use, the compression moulding axis X is vertical.

[0025] The punch 1 comprises an internal annular portion 2 with an external coupling surface 20 arranged around the moulding axis X.

[0026] The punch 1 comprises an intermediate annular portion 3 with an internal coupling surface 30 coupled geometrically with the external coupling surface 20 of the internal annular portion 2 with the possibility of reciprocal axial sliding between the internal annular portion 2 and the intermediate annular portion 3 parallel to the moulding axis X.

[0027] The internal coupling surface 30 of the intermediate annular portion 3 and the external coupling surface 20 of the internal annular portion 2 are geometrically coupled with one another with relatively reduced set radial coupling clearance (the size of which is determinable in function of the various operating conditions and of the desired precision and can be preset in a substantially known manner by a designer), so as to permit axial slidingalong the moulding axis X of significant efficacy and reliability and with relatively high precision, to ensure centred and precise moulding of an object of annular shape (gasket) and the possibility of easy reciprocal axial sliding between the internal annular portion 2 and the intermediate annular portion 3 parallel to the moulding axis X.

[0028] The punch 1 comprises an external annular portion 4 with an internal coupling surface 40 coupled geometrically with an external coupling surface 31 of the intermediate annular portion 3 with the possibility of reciprocal axial sliding between the external annular portion 4 and the intermediate annular portion 3 parallel to the moulding axis X.

[0029] The external coupling surface 31 of the intermediate annular portion 3 and the internal coupling surface 40 of the external annular portion 4 are geometrically coupled with one another with relatively reduced set radial coupling clearance, the size of which can be preset similarly to what has been said previously with regard to the axially sliding radial coupling clearance between the internal coupling surface 30 of the intermediate annular portion 3 and the external coupling surface 20 of the internal annular portion 2.

[0030] It is possible, as in these specific illustrated embodiments, for the intermediate annular portion 3 to be fixed in an axial direction and for the internal and external annular portions 2 and 4 to be axially movable.

[0031] In particular, as in these embodiments, it is possible that the internal coupling surface 30 of the intermediate annular portion 3 and the external coupling surface 20 of the internal portion 2 are cylindrical.

[0032] In particular, as in these embodiments, it is possible that the external coupling surface 31 of the intermediate annular portion 3 and the internal coupling surface 40 of the external annular portion 4 are cylindrical.

[0033] The internal, intermediate and external annular portions 2, 3 and 4 are able to adopt a moulding operating configuration (see in the appended figures) in which they bound and help define a forming cavity F of annular shape that is arranged around the moulding axis X. Compression moulding of a moulding material (in particular, plastics) for forming an annular gasket occurs in this moulding operating configuration.

[0034] The internal annular portion 2 comprises an external contact surface 21 that is intended for contact with the moulding material and is configured to bound the forming cavity F internally.

[0035] The intermediate annular portion 3 comprises a lower contact surface 32 that is intended for contact with the moulding material and that is configured to bound above theforming cavity F.

[0036] The external annular portion 4 comprises an internal contact surface 41 that is intended for contact with the moulding material and is configured to bound the forming cavity F externally.

[0037] In use, the forming cavity F is bound below by an annular wall portion of an object 5 (for example, a cap for closing containers) that is outside the punch 1 (and outside the rest of the mould) and is fed to the mould of which the punch 1 is part. The moulding material for forming the annular gasket is compression-moulded on the object 5. After moulding, the object 5, with the gasket moulded thereupon, will be evacuated from the mould.

[0038] The lower contact surface 32 of the intermediate annular portion 3 can comprise, in particular, an internal annular surface portion 320, an intermediate annular surface portion 321 and an external annular surface portion 322.

[0039] The intermediate annular surface portion 321 can be interposed between the internal annular surface portion 320 and the external annular surface portion 322.

[0040] The intermediate annular surface portion 321 can be, in particular, concave so as to define a suitable recess for forming an annular protuberance of the annular gasket.

[0041] The internal, intermediate and external annular portions 2, 3 and 4 are so configured that, in the moulding operating configuration, between the intermediate annular portion 3 and the internal annular portion 2 an internal annular gap 6 communicating with the forming cavity F is defined.

[0042] A maximum dimension in a radial direction of the internal annular gap 6 can be, in particular, greater than 0.025 millimetres.

[0043] A maximum dimension in a radial direction of the internal annular gap 6 can be, in particular, less than 0.20 millimetres.

[0044] A dimension in a radial direction of the internal annular gap 6 is greater than a radial distance (defined as the radial clearance that permits axially sliding geometric coupling) between the external coupling surface 20 of the internal annular portion 2 and the internal coupling surface 30 of the intermediate annular portion 3, where radial is defined with reference to the moulding axis X.

[0045] The internal annular gap 6 can be, in particular, interposed between the forming cavity F and a zone in which the internal coupling surface 30 of the intermediate annular portion 3 is coupled geometrically with the external coupling surface 20 of the internalannular portion 2.

[0046] A maximum dimension in an axial direction of the internal annular gap 6 can be, in particular, greater than 0.5 millimetres.

[0047] A maximum dimension in an axial direction of the internal annular gap 6 can be, in particular, less than 2.0 millimetres.

[0048] A maximum dimension in an axial direction of the internal annular gap 6 can be, in particular, greater for example at least three times greater than the maximum dimension in a radial direction of the internal annular gap 6, where axial is defined with reference to the moulding axis X.

[0049] The internal, intermediate and external annular portions 2, 3 and 4 are so configured that, in the moulding operating configuration, between the intermediate annular portion 3 and the external annular portion 4 an external annular gap 7 communicating with the forming cavity F is defined.

[0050] A dimension in a radial direction of the external annular gap 7 is greater than a radial distance (defined as the radial clearance that permits axially sliding geometric coupling) between the internal coupling surface 40 of the external annular portion 4 and the external coupling surface 31 of the intermediate annular portion 3, where radial is defined with reference to the moulding axis X.

[0051] A maximum dimension in a radial direction of the external annular gap 7 can be, in particular, greater than 0.025 millimetres.

[0052] A maximum dimension in a radial direction of the external annular gap 7 can be, in particular, less than 0.20 millimetres.

[0053] The external annular gap 7 can be, in particular, interposed between la forming cavity F and a zone in which the external coupling surface 31 of the intermediate annular portion 3 is coupled geometrically with the internal coupling surface 40 of the external annular portion 4.

[0054] A maximum dimension in an axial direction of the external annular gap 7 can be, in particular, greater than 0.5 millimetres.

[0055] A maximum dimension in an axial direction of the external annular gap 7 can be, in particular, less than 2.0 millimetres.

[0056] A maximum dimension in an axial direction of the external annular gap 7 can be, in particular, greater, for example at least three times greater than the maximum dimension in a radial direction of the external annular gap 7, where axial is defined withreference to the moulding axis X.

[0057] In particular, the internal annular gap 6 can have an internal annular opening 8 below that places the internal annular gap 6 in communication with the forming cavity F.

[0058] A maximum dimension in a radial direction of the internal annular gap 6 can be arranged, in particular, at the internal annular opening 8.

[0059] In particular, the external annular gap 7 can have an external annular opening 9 below that places the external annular gap 7 in communication with the forming cavity F.

[0060] A maximum dimension in a radial direction of the external annular gap 7 can be arranged, in particular, at the external annular opening 9.

[0061] The internal annular gap 6 can have, in particular, an upwardly diminishing radial dimension (see embodiment of figure 5).

[0062] The external annular gap 7 can have, in particular, an upwardly diminishing radial dimension (see embodiment of figure 5).

[0063] The internal annular gap 6 can have, in particular, a radial section of triangular shape.

[0064] The internal annular gap 6 can have, in particular, a radial section with an angle, defined between two contiguous surfaces of the intermediate annular portion 3 and of the internal annular portion 2, which is greater than 0.5° and / or less than 10°.

[0065] Figure 5 shows, by way of example, some possible tilts that the internal annular gap 6 could adopt. The enlargement of figure 6 shows, for the sake of simplicity and clarity, only one tilt.

[0066] The external annular gap 7 can have, in particular, a radial section of triangular shape.

[0067] The external annular gap 7 can have, in particular, a radial section with an angle, defined between two contiguous surfaces of the intermediate annular portion 3 and of the external annular portion 4, which is greater than 0.5° and / or less than 10°.

[0068] Figure 5 shows, by way of example, some possible tilts that the external annular gap 7 could adopt. The enlargement of figure 7 shows, for the sake of simplicity and clarity, only one tilt.

[0069] The internal annular gap 6 can have, in particular, a constant radial dimension (see embodiment of figure 2).

[0070] The external annular gap 7 can have, in particular, a constant radial dimension (see embodiment of figure 2).

[0071] The internal annular gap 6 is so designed that a small part of the moulding material, which is compressed in the forming cavity F by the punch 1, can enter inside the internal annular gap 6 and thus cause a protuberance to be formed on the moulded annular object.

[0072] The quantity of moulding material introduced into the forming cavity F at each moulding task (i.e. the dose of material) is preset so as to cause this phenomenon, i.e. the entry of a part of material into the internal annular gap 6 (anyway a part that is percentually very reduced with respect to the total of the dose of material introduced into the forming cavity F).

[0073] The same applies to the external annular gap 7.

[0074] Embodiments of the punch have been disclosed in which there is both the internal annular gap 6, and the external annular gap 7. Embodiments are possible, which are not illustrated, in which there is only the internal annular gap 6, or only the external annular gap 7.

[0075] A combination is disclosed comprising the punch 1 and the object 5 on which the annular gasket is to be moulded.

[0076] The object 5 can be, in particular, a cap for closing containers. The object 5 can be, in particular, a shell of a crown cap or another type of closing cap.

[0077] The object 5 comprises a wall that, during compression moulding of the annular gasket, is below the moulding punch 1 and has a wall portion that bounds the forming cavity F below.

[0078] The internal annular surface portion 320 and the external annular surface portion 322 are shaped so as to correspond to a shape of the wall portion of the object 5 so that the aforesaid internal and external annular surface portions 320 and 322 are, in the aforesaid moulding operating configuration, arranged at the same distance D in an axial direction (in a direction parallel to the moulding axis X, i.e. in a vertical direction) from the wall portion of the object 5.

[0079] The moulding punch 1 can be used to form an annular gasket inside a cap for closing containers.

[0080] The moulding punch 1 can be used, in particular, to form a gasket inside a crown cap.

[0081] A moulding apparatus is disclosed (which is not illustrated) for moulding annular gaskets inside caps for closing containers.

[0082] The moulding apparatus comprises a plurality of moulding punches arranged spaced angularly apart from one another on a carousel rotating with a vertical axis.

[0083] Each of the aforesaid moulding punches can be made, in particular like one of the punches disclosed above.

[0084] In use, the operation of the punch implements a moulding method that comprises the step of feeding an object 5 to a moulding punch 1 made as disclosed previously.

[0085] As said, the object 5 can be a cap for closing containers, for example a shell of a crown cap.

[0086] The moulding method comprises the step of depositing moulding material on the object 5 and the step of compression moulding this moulding material by the moulding punch 1 to form an annular gasket on the object 5.

[0087] During the step of compression moulding, part of the moulding material can be discharged inside the internal annular gap 6 so as to form an internal annular protrusion that protrudes from the annular gasket.

[0088] During the step of compression moulding, part of the moulding material can be discharged inside the external annular gap 7 so as to form an external annular protrusion that protrudes from the annular gasket.

[0089] It has been established that the possibility of formation of an internal annular protrusion and / or the possibility of formation of an external annular protrusion promotes moulding of an annular gasket with a compact and consistent construction, reducing the risk of generating irregularities, defects or imperfections (burrs, fraying, bloating, etc) in the moulded product due to melting and running of material in the forming cavity. This enables delicate, laborious and complicated gasket finishing tasks to be avoided.

[0090] Legend:1 compression moulding punch2 internal annular portion3 intermediate annular portion4 external annular portion5 annular moulded object (gasket)6 internal annular gap7 external annular gap8 internal annular opening9 external annular opening20 external coupling surface21 external contact surface30 internal coupling surface31 external coupling surface32 lower contact surface40 internal coupling surface41 internal contact surface320 internal annular surface portion 321 intermediate annular surface portion 322 external annular surface portion D distanceF forming cavityX coupling axis

Claims

CLAIMS1. Moulding punch (1), comprising a vertical compression moulding axis (X), an internal annular portion (2) with an external mating surface (20) arranged around said moulding axis (X), an intermediate annular portion (3) with an internal mating surface (30) geometrically coupled with said external mating surface (20) of said internal annular portion (2) with the possibility of mutual axial sliding between said internal annular portion (2) and said intermediate annular portion (3) parallel to said moulding axis (X), an external annular portion (4) with an internal mating surface (40) geometrically coupled with an external mating surface (31) of said intermediate annular portion (3) with the possibility of mutual axial sliding between said external annular portion (4) and said intermediate annular portion (3) parallel to said moulding axis (X), said internal, intermediate and external annular portions (2; 3; 4) being capable of assuming an operational moulding configuration in which they define an annular-shaped forming cavity (F) which is arranged around said moulding axis (X) and in which compression moulding of a moulding material takes place for forming an annular gasket, said internal annular portion (2) comprising an external contact surface (21) which is intended for contact with the moulding material and which is configured to internally delimit said forming cavity (F), said intermediate annular portion (3) comprising a lower contact surface (32) which is intended for contact with the moulding material and which is configured to externally delimit said forming cavity (F), it being provided that, in said operational moulding configuration, said forming cavity (F) is delimited below by an object (5) which is external to said punch (1) and on which the moulding material is compression moulded to form the annular gasket, said internal, intermediate and external annular portions (2; 3; 4) being configured in such a way that, in said operational moulding configuration:between said intermediate annular portion (3) and said internal annular portion (2) an internal annular gap (6) communicating with said forming cavity (F) is defined, wherein a dimension in radial direction of said internal annular gap (6) is greater than a radial distance between said external mating surface (20) of said internal annular portion (2) and said internal mating surface (30) of said intermediate annular portion (3), where radial is understood with reference to said moulding axis (X); and / orbetween said intermediate annular portion (3) and said external annular portion(4) an external annular gap (7) communicating with said forming cavity (F) is defined, wherein a dimension in radial direction of said external annular gap (7) is greater than a radial distance between said internal mating surface (40) of said external annular portion (4) and said external mating surface (31) of said intermediate annular portion (3), where radial is understood with reference to said moulding axis (X).

2. Punch according to claim 1, wherein:said dimension in radial direction of said internal annular gap (6) is greater than 0.025 millimetres; and / orsaid dimension in radial direction of said external annular gap (7) is greater than 0.025 millimetres.

3. Punch according to claim 1, wherein:a maximum dimension in radial direction of said internal annular gap (6) is less than 0.20 millimetres; and / ora maximum dimension in radial direction of said external annular gap (7) is less than 0.20 millimetres.

4. Punch according to any one of the preceding claims, wherein:said internal annular gap (6) is interposed between said forming cavity (F) and a zone in which said internal mating surface (30) of said intermediate annular portion (3) is geometrically coupled with said external mating surface (20) of said internal annular portion (2); and / orsaid external annular gap (7) is interposed between said forming cavity (F) and a zone in which said external mating surface (31) of said intermediate annular portion (3) is geometrically coupled with said internal mating surface (40) of said external annular portion (4).

5. Punch according to any one of the preceding claims, wherein:a maximum dimension in axial direction of said internal annular gap (6) is greater, in particular at least three times greater, than said maximum dimension in radial direction of said internal annular gap (6), where axial is understood with reference to said moulding axis (X); and / ora maximum dimension in axial direction of said external annular gap (7) is greater, in particular at least three times greater, than said maximum dimension in radial direction of said external annular space (7), where axial is understoodwith reference to said moulding axis (X).

6. Punch according to any one of the preceding claims, wherein:a maximum dimension in axial direction of said internal annular gap (6) is greater than 0.5 millimetres; and / ora maximum dimension in axial direction of said external annular gap (7) is greater than 0.5 millimetres.

7. Punch according to any one of the preceding claims, wherein:a maximum dimension in axial direction of said internal annular gap (6) is less than 2.0 millimetres; and / ora maximum dimension in axial direction of said external annular gap (7) is less than 2.0 millimetres.

8. Punch according to any one of the preceding claims, wherein:said internal annular gap (6) has an internal annular opening (8) at the bottom which puts said internal annular gap (6) in communication with said forming cavity (F), a maximum dimension in radial direction of said internal annular gap (6) being arranged at said internal annular opening (8); and / orsaid external annular gap (7) has an external annular opening (9) at the bottom which puts said external annular gap (7) in communication with said forming cavity (F), a maximum dimension in radial direction of said external annular gap (7) being arranged at said external annular opening (9).

9. Punch according to any one of the preceding claims, wherein said internal annular gap (6) has a radial dimension decreasing upward and / or wherein said external annular gap (7) has a radial dimension decreasing upward.

10. Punch according to any one of the preceding claims, wherein:said internal annular gap (6) has a radial section of triangular shape, in particular with an angle defined between two surfaces facing each other and inclined with respect to each other, said two surfaces being of said intermediate annular portion (3) and of said internal annular portion (2), which is greater than 0.5° and / or less than 10°; and / orsaid external annular cavity (7) has a radial section of triangular shape, in particular with an angle defined between two surfaces facing each other and inclined with respect to each other, said two surfaces being of said intermediate annular portion (3) and of said external annular portion (4), which is greater than0.5° and / or less than 10°.

11. Punch according to any one of claims 1 to 8, wherein said internal annular gap (6) has a constant radial dimension and / or wherein said external annular gap (7) has a constant radial dimension.

12. Punch according to any one of the preceding claims, wherein said intermediate annular portion (3) is fixed in the axial direction and said internal and external annular portions (2; 4) are axially movable and / or wherein said internal mating surface (30) of said intermediate annular portion (3) and said external mating surface (20) of said internal portion (2) are cylindrical and / or wherein said external mating surface (31) of said intermediate annular portion (3) and said internal mating surface (40) of said external annular portion (4) are cylindrical.

13. Punch according to any one of the preceding claims, wherein said lower contact surface (32) of said intermediate annular portion (3) comprises an internal annular surface portion (320), an intermediate annular surface portion (321) and an external annular surface portion (322), said intermediate annular surface portion (321) being interposed between said internal annular surface portion (320) and said external annular surface portion (322), said intermediate annular surface portion (321) being concave so as to define a recess for forming an annular protuberance of the annular gasket.

14. Combination of a moulding punch (1) and an object (5) on which to form an annular gasket, wherein said moulding punch (1) is made according to any one of the preceding claims.

15. Combination according to claim 14, said moulding punch (1) being made according to claim 13, wherein said object (5) comprises a wall which, during compression moulding of the annular gasket, is located below said moulding punch (1) and has a wall portion which defines said forming cavity (F) at the bottom, said internal and external annular surface portions (320 and 322) being shaped so as to correspond to a shape of said wall portion of said object (5) so that said internal and external annular surface portions (320 and 322) in said moulding operating configuration are arranged at the same distance (D) vertically from said wall portion of said object (5).

16. Use of a moulding punch (1) according to any one of claims 1 to 13 to form an annular gasket inside a capsule for closing a container, in particular inside a crown cap.

17. Moulding apparatus for moulding annular gaskets inside caps for closing containers,said moulding apparatus comprising a plurality of moulding punches arranged angularly spaced from each other on a rotating carousel with a vertical axis, each of said moulding punches being made according to any one of claims 1 to 13.

18. Moulding method comprising the steps of: feeding an object (5), said object being, in particular, a crown cap shell, to a moulding punch (1) made according to any one of claims 1 to 13, depositing moulding material on said object (5), compression moulding said moulding material by means of said moulding punch (1) to form an annular gasket on said object (5), wherein, during said compression moulding:a portion of said moulding material is discharged inside said internal annular gap (6) so as to form an internal annular protrusion protruding from said annular gasket; and / ora portion of said moulding material is discharged into said external annular gap (7) so as to form an external annular protrusion protruding from said annular gasket.

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