Abstract:
A method of making a metal can end with a tear-open plastics closure, for a can intended to be resistant to leakage when subject to internal pressures, such as arise from carbonation of a beverage or thermal processing, wherein the undersurface of the can end is provided with a plastics coating, and the can end is pierced to form an aperture with a peripheral downturned flange at an angle of 80° to 120° to the plane of the can end and having a flat end. A one-piece ring pull closure is injection moulded on to the can end from a plastics material resistant to softening at up to at least 67° C., without additional heating of the metal, so as to fill the aperture, bond with the coating, and enclose the flange, with a residual thickness below the flange of 0.08 to 0.5 mm. The thickness of the plastics material of the closuure is such that it can be sheared against the flange when the ring pull is pulled manually. The residual thickness of the plastics material may be varied around the aperture to increase resistance of the closure to creep, and the aperture may be pear-shaped or may be circular with a separate vent hole.

Description:
This application is a continuation of application Ser. No. 906,887, filed Sept. 15, 1986, now abandoned. 
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     This invention relates to methods of making can ends with plastics closures, particularly tear-open plastics closures of the kind incorporating a ring pull, for cans intended to be resistant to leakage when subject to internal pressures, such as arise from carbonation of a beverage contained in the can and/or from thermal processing of the can contents. Examples of such thermal processes include heating cold-filled carbonated beverage-containing cans to 35° C. to drive off condensation before shrink-wrapping packages of such cans, pasteurising beverage-containing cans at 65° C., and sterilising food-containing cans at 121° C. The closures must accordingly be capable of withstanding considerable internal pressure in the container, sufficient to cause distortion of the can end into a domed shape. 
     The plastics materials used must therefore be resistant to softening or creep at the temperatures envisaged for the thermal processing. The closures must also be resistant to distortion due to creep when the filled cans are stored under internal pressure, at temperatures up to 35° C. Furthermore, in order to enable the can ends to be made rapidly and economically, it is important that the plastics closure should be made in a single piece and attached to the can end in the same operation. Finally, it is essential that the closures should be designed so that they can be torn open manually without difficulty, even though the aperture used for pouring out a beverage needs to be of relatively large size. 
     2. Description of the Prior Art 
     Our British Patent Specification No. 1,393,875 describes metal containers having at least one orifice in a container wall closed by a unitary closure of resilient plastics material, such as nylon or polypropylene, comprising a pull tab integral with a plug portion which projects transversely to the plane of the pull tab, the plug portion extending through a said orifice and having a free end upset behind said wall to define a rivet head having a radial flange sealingly bonded to said wall around the orifice, e.g. by being bonded to an internal coating on the wall by means of heat and pressure applied during the upsetting operation. Examples described in Specification No. 1,393,875 are made of polypropylene and are fitted into relatively small orifices having a downturned flange around the periphery, so that on removal of the plug portion it is severed from the portion of the plastics material outside the downturned flange by the force exerted on the pull tab. The disadvantage of the containers of Specification No. 1,393,875 which renders them impractical in economic terms is that the plastics closure is necessarily moulded in a separate operation from that in which it is secured to the container wall by the upsetting of the free end of the plug portion. Furthermore, no guidance is given as to how to combine resistance to distortion or creep with easy manual opening of the relatively large apertures which are required for beverage-containing cans. 
     In our U.K. Patent Specification No. 2,073,646B, we have described a method and apparatus for forming a plastics moulding on a metal substrate, in particular a plastics closure for a can, in which plastics material is injected into a cavity defined by the apertured metal substrate (e.g. a can end) and a die or dies to form a plug closure filling the aperture, and the metal substrate is locally heated adjacent the plastics material by means of energy from an induction coil to bond the plastics moulding to the metal substrate. Although it is stated that a coating material to which the plastics material will adhere may be applied to the metal wall before the plastics material is moulded therein, it is indicated that the induction heating is necessary t prevent the plastics material chilling before a seal can be effected between the plastics material and either the metal or the coating material thereon. In this way, the closure is formed and located in the can end in a single operation, but the provision of induction heating involves complication and expense. An example illustrated has a flange formed in the metal around the aperture which is curled through more than 180° and the polyethylene material of the closure is formed with a circular groove in its underside as a weakening line to enable the plug closure to be manually torn away. Unfortunately, the tougher plastics which will resist thermal processing, such as nylon and polypropylene, will not tear along such weakening lines because tensile forces orientate the polymer chains to produce a strength which cannot be overcome manually. 
     SUMMARY OF THE INVENTION 
     We have now unexpectedly found that, with an appropriate internal coating on the can end and an appropriate formation of the aperture and downturned flange, it is possible to injection mould a tear-open plastics closure into the can end using a thermally resistant plastics material without additional heating of the metal, i.e. without additional heating (such as induction heating) over and above the heating effect on the metal produced by the molten plastics material employed in the injection moulding process, thereby to produce a closure which will resist distortion due to creep when the can end is distorted into a domed shape by internal pressure in the can, but which can be torn open manually when required. 
     According to the invention, a method of making a metal can end with a central panel and a tear-open plastics closure in said panel, for a can intended to resist leakage when subject to internal pressures, comprises the steps of: 
     (a) providing the undersurface of the metal can end with a coating of plastics material, 
     (b) piercing the central panel of the metal can end to form an aperture surrounded by a downturned flange extending around the periphery of the aperture and extending away from the central panel at an angle of between 80° and 120° to the plane of the central panel and having a substantially flat end surface, and 
     (c) injection moulding on to the can end, without additional heating of the metal, a one-piece ring pull closure of a plastics material which is resistant to softening at temperatures up to at least 65° C., in such manner that the plastics material of the closure fills the aperture and surrounds it on both surfaces of the can end and bonds with the plastics material of the coating, the thickness of the plastics material of the closure being chosen so as to enclose the downturned flange totally, with a residual thickness below the flat end surface of the flange in the range from 0.08 to 0.5 mm, so as to enable the closure to resist distortion of the closure resulting from creep at temperatures up to at least 35° C., but to be capable of being sheared against the flat end surface of the flange when the ring pull is pulled manually. 
     In this way, the can ends can be produced rapidly and economically and with a minimum risk of leakage from the closure in the completed can. The bonding of the plastics material of the closure to the plastics material of the coating produces a surprisingly effective and leakproof seal in spite of the absence of additional heating, while the formation of the flange with its flat end surface extending substantially laterally ensures that any burrs formed in the piercing operation will also extend laterally and not through the residual thickness of the plastics material below the flange. The only critical dimension is the residual thickness of the plastics material below the end surface of the flange, which must be in the specified range of 0.08 to 0.5 mm in order to ensure that the closure remains leak-tight but is easy to open manually by shearing against the flange. A shearing action is essential, because the plastics material, if subjected to tensile forces, will become oriented and too strong to be torn manually. The residual thickness is determined by the thickness of the plastics material (set by the mould cavity dimensions) and the extent to which the flange is downturned in the piercing operation, which is itself fixed by the geometry of the press tool used, so the residual thickness can be controlled within close limits during production. The flange stiffens the central panel to compensate for the weakness introduced by the relatively large pouring aperture and avoids bending of the metal around the aperture, which would result in tensile, rather than shearing, forces being applied to the plastics material of the closure on pulling the ring pull. The hot mouldings can be removed from the mould without distortion because they can be handled indirectly via the metal component of the can end and they can complete their cooling outside the mould. This feature, in combination with the absence of a step of additionally heating the metal can end, makes it possible to achieve rapid and economical production, for example with a reduction of the moulding cycle time from typically 10 seconds to around 2 seconds. 
     Where the closure is moulded from a plastics material with a particularly high creep resistance, such as a nylon, or the conditions of use of the can are not arduous, it has been found effective to form the can end with a single relatively large aperture, e.g. an aperture having two straight parallel sides and semi-circular ends, and to provide an equal residual thickness of the plastics material of the closure below the flat end surface of the flange all round the periphery of the aperture. 
     Polypropylenes are generally less expensive than nylons, but have less creep resistance, though this property can be improved by incorporation of appropriate fillers such as talc. 
     When a plastics material with lower creep resistance than nylon is used and the can is subjected to arduous conditions, it has been found that distortion of the above-described single aperture closure and consequent leakage can occur as a result of creep of the plastics material at one end of the aperture. To reduce the risk of such distortion and leakage a method may be adopted in which the can end is pierced to form a single pear-shaped aperture having ends of differing radii and the moulding is effected in such a manner that the residual thickness of the plastics material of the closure below the flat end surface of the flange is varied around the periphery of the aperture, being greatest in the part around the end of the aperture having the larger radius. The greater residual thickness of the plastics material beneath the flange around the larger-radius end of the aperture effectively reduces the risk of the plastics material pulling away from the flange due to creep, while not affecting the capability of the closure of being sheared against the flange for opening at the smaller radius end. 
     Alternatively, the can end may be pierced to form a pouring aperture and a smaller vent aperture and the moulding effected in such a manner that the residual thickness of the plastics material of the closure below the flat end surface of the flange around the periphery of the pouring aperture is varied, being greatest in the part of the periphery remote from the vent aperture. 
     This method provides even greater assurance against distortion and leakage due to creep, because it results in production of a part of the closure joining the parts which fill the two apertures and lying against the outer surface of the can end, which can act as a strap to hold the parts of the closure in position when the can end is distorted into a domed shape under internal pressure. Opening of the closure may be initiated by opening of the vent aperture, which may be quite small, in which case it need not be provided with a downturned flange around its periphery because it does not then substantially affect the stiffness of the central panel. 
     In either of the forms of the invention described in the three preceding paragraphs, the closure may be moulded of polypropylene and the residual thickness may be varied either in a continuous manner or in a stepwise manner. 
     The plastics material of the coating on the under surface of the can end is preferably of a similar polymer to that of the closure, in order to facilitate good bonding. 
     Where the metal can end is formed with an outwardly flared raised edge with a curled periphery for seaming to a can side wall, the plastics closure is preferably injection moulded on to the can end so as to have a total thickness of plastics material no greater than the depth of the peripheral curl on the can end so as not to impede stacking of nested can ends. 
     Preferably the plastics material of the closure is injection moulded so as to have a residual thickness below the flat end surface of the flange in the range from 0.15 mm to 0.40 mm. 
     Preferably the flange is downturned to extend for a distance of 0.125 mm to 2.5 mm below the plane of the can end, the most preferred dimension being substantially 0.75 mm. 
     Preferably the plastics material of the closure is injection moulded so as to extend laterally from the flange on the undersurface of the can end for 0.125 mm to 7.5 m, the most preferred dimension being substantially 1.25 mm. 
     Preferably the plastics material of the closure is injection moulded so that the ring pull lies against the upper surface of the can end and has a thickness of 0.25 mm to 4.3 mm, the most preferred thickness being substantially 1.0 mm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Specific embodiments of the invention will now be described in more detail by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a metal can end, 
     FIG. 2 is a similar view of the can end after it has been pierced and flanged, 
     FIG. 3 is a plan view of the can end of FIG. 2, 
     FIG. 4 is a diagrammatic cross-sectional view illustrating the injection moulding of a closure on to the can end of FIGS. 2 and 3, 
     FIG. 5 is a cross-sectional view of the can end with the plastics closure moulded thereon, 
     FIGS. 6A, 6B and 6C are detail cross-sectional views to a larger scale illustrating alternative forms of the downturned flange around the periphery of the aperture, 
     FIG. 7 is a plan view of the can end of FIG. 5, 
     FIGS. 8A and 8B are cross-sectional views similar to FIG. 5 showing the opening of the closure, 
     FIG. 9 is a view similar to FIG. 8A but illustrating the effect of internal pressure on the can end, 
     FIG. 10 is a plan view similar to FIG. 3 of an alternative form of can end with two apertures pierced therein, 
     FIG. 11 is a plan view of the can end of FIG. 10 with a plastics closure moulded thereon, 
     FIG. 12 is a cross-sectional view of the closure of FIG. 11, 
     FIG. 13 is a cross-sectional view similar to FIG. 12 showing the opening of the closure of FIGS. 10 to 12. 
     FIG. 14 is a plan view similar to FIGS. 3 and 10 of another alternative form of can end, 
     FIG. 15 is a plan view of the can end of FIG. 14 with a plastics closure moulded thereon, 
     FIG. 16 is a cross-sectional view on the line XVI--XVI of FIG. 15, 
     FIG. 17 is a plan view similar to FIGS. 3, 10 and 14 of a further alternative form of can end, 
     FIG. 18 is a plan view of the can end of FIG. 17 with a plastics closure moulded thereon, 
     FIG. 19 is a cross-sectional view on the line XIX--XIX of FIG. 18, 
     FIG. 20 is a similar cross-sectional view of a modification, 
     FIG. 21 is a perspective view of a modified form of the can end of FIG. 17, 
     FIG. 22 is a plan view of the can end of FIG. 21 with a plastics closure moulded thereon, 
     FIG. 23 is a section on the line XXIII--XXIII of FIG. 22, 
     FIG. 24 is a perspective view of a modified form of the can end of FIG. 14, 
     FIG. 25 is a part-section through the can end of FIG. 24 with a closure moulded thereon, 
     FIG. 26 is a perspective view of another modified form of the can end of FIG. 14, and 
     FIG. 27 is a part-section through the can end of FIG. 26 with a closure moulded thereon. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the embodiments of the invention now to be described, one starts with a standard type of can end 10, as illustrated in FIG. 1, which comprises a flat panel portion 11 with an outwardly flared raised edge portion 12 and a curved periphery 13 for seaming to a can side wall (not shown). The undersurface of the panel portion 11 is provided with a coating 14 of plastics material. As shown in FIGS. 2 and 3, the panel portion 11 is pierced so as to form an aperture 15 with a downturned flange 16 around its periphery. The flange 16 extends at an angle of between 80° and 120° to the plane of the panel 11, and preferably at 90° to it as shown. The flange 16 has a substantially flat end 17 which thus extends approximately parallel to the plane of the panel 11. Any burrs 18 (FIGS. 6A, 6B and 6C) produced in the piercing operation will also extend laterally, as described below. 
     FIG. 4 illustrates diagrammatically the step of injection moulding the closure on to the can end. The pierced and flanged can end 10 of FIGS. 2 and 3 is placed between a steel top mould 19 having an injection passage 20 and a steel bottom mould 21. In the closed opposing positions of the moulds 19, 21 of FIG. 4, the moulds 19, 21 cooperatively define a mould cavity 50. The mould cavity 50 is in turn defined by opposing cavity portions 51, 52. Each cavity portion 51, 52 has a respective peripheral wall 53, 54 appreciably outboard of the opening-defining flange 16 and a respective axial wall 55, 56 axially outboard of the outer surface and flat end or terminal edge 17 of the can end 10. Molten plastics material is injected through the passage 20 to form the closure 22 shown more clearly in FIGS. 5 and 6. It is an important and surprising feature of the present invention that no additional heating of the can end 10 is necessary, over and above the heating effect produced in the apparatus by the molten plastics material, before injection of the plastics material. As can be seen in FIG. 4, the plastics material fills the aperture 15 and surrounds it on both surfaces of the can end 10. Furthermore, the plastics material bonds with the plastics material of the coating 14 on the underside of the can end so as to secure the closure in a leakproof manner. 
     FIGS. 6A, 6B and 6C illustrate how the piercing operation produces burrs 18 on the inside edge of the flange 16. FIG. 6A shows the flange 16 formed so as to extend at 90° to the plane of the panel 11, as is preferred, but it may extend at an angle of 80°, as shown in FIG. 6B, or 120°, as shown in FIG. 6C, or at any angle between 80° and 120°. It can be seen that the burrs 18 then extend substantially laterally and do not penetrate the residual thickness 27 of the plastics material. 
     The form of the closure 22 is illustrated in more detail in FIGS. 5 and 7. It will be seen that the closure comprises a plug portion 23 filling the aperture 15 and, on the underside of the can end 10, a surrounding portion 24 outside the flange 16. The thickness of the plastics material is such as to enclose the flange 16 totally but to be capable of shearing against the flat end surface 17 of the flange when the closure is removed as described below. The residual thickness 27 (FIGS. 6A-6C) of the plastics material below the end surface 17 of the flange is in the range from 0.08 to 0.5 mm, preferably from 0.15 to 0.4 mm. The upper surface of the plug portion 23 is recessed below the level of the surrounding portion 25 which extends laterally to form a ring pull 26. 
     The total thickness of the plastics material is also chosen so as to be no greater than the depth of the peripheral curl 13 so that when a number of the can ends are stacked one upon another the plastics material will not impede the stacking or prevent the can ends from nesting one inside another. 
     In order to remove the closure, as shown in FIGS. 8A and 8B, the ring pull 26 is lifted to pull the plug portion 23 upwards, whereupon it shears away from the surrounding portion 24 against the flange 16 along line 161 (FIG. 8A) adjacent to the ring pull 26. Shearing continues down the sides of the aperture 15 until the plug portion 23 can be completely removed from the can end. 
     When the can is subject to internal pressure, e.g. when it is used to contain a carbonated beverage, the panel 11 assumes a convex shape as illustrated in FIG. 9. This causes tension in the closure, tending to pull the portion 24 away from the flange 16, but does not affect the seal of the closure in the can end, provided that the plastics material is sufficiently resistant to creep and consequent distortion of the closure. Nylon is a suitably creep-resistant material. 
     FIGS. 10 to 13 illustrate a modified embodiment in which a can end 10 is pierced with two circular apertures, namely a pouring aperture 30 and a smaller vent aperture 31, each surrounded by a downturned flange 32,33, respectively, extending at 90° to the plane of the panel portion 11 and having a flat end surface 34. 
     The plastics closure 35 is injection moulded as described above with reference to FIG. 4 so as to have two plug portions 36,37 fitting respectively into the apertures 30,31 and surrounded on the underside of the can end 10 by surrounding portions 38. The plastics material encloses each flange 32,33 totally, with a residual thickness below the flat end surface 34 in the range from 0.08 to 0.5 mm, preferably from 0.15 to 0.40 mm. The closure 35 also incorporates a ring pull 39 which in this case lies on the right hand side of the plug portions (as seen in FIGS. 11, 12 and 13) but is connected to them at the left hand end, i.e. close to the pouring aperture plug 36. The opening procedure accordingly starts at the left-hand side, as shown in FIG. 13, with shearing of the plug 36 from its surrounding portion 38 to open the pouring aperture 30, followed by shearing of the plug 37 from its surrounding portion 38 to open the vent aperture 31. The use of two apertures enables one to use apertures of smaller area and smaller radii of curvature than in the embodiment of FIGS. 2 to 9, and thereby reduces the tendency to distort as a result of creep under tension when subject to internal pressure in the can, so that a material with a slightly lower creep resistance than nylon can be used, such as a polypropylene. 
     A modified form of single-aperture can end is illustrated in FIGS. 14 to 16. This form differs from that described above with reference to FIGS. 2 to 9 in that it has a pear-shaped aperture 151, having a pouring end 152 of larger radius than the opposite or venting end 153, and a downturned flange 161 around its periphery. The residual thickness 127 of the plastics material of the closure below the flat end of the flange 161 is varied around the periphery of the aperture, while remaining within the range of 0.08 to 0.5 mm. As can be seen in FIG. 16, the residual thickness 127 is varied in a continuous manner around the left-hand half of the aperture 151, being greatest at the pouring end 152 which has the greater radius of curvature and where the tendency to distort is greatest under tension resulting from internal pressure in the can. The increased residual thickness 127 at the pouring end 152 improves the resistance to creep and distortion of the closure, so that a material with a lesser creep resistance than nylon may be used, such as a polypropylene. In a specific example, the residual thickness 127 may vary from 0.4 mm at the pouring end 152 to 0.2 mm in the right-hand half of the aperture 151. If desired, an intermediate portion may have a constant residual thickness of 0.3 mm along the straight convergent sides of the aperture. The transition from the thicker to the thinner portion may be gradual, as shown in FIG. 16, or may be by way of a step in the lower surface of the plastics material. 
     FIGS. 17 to 19 illustrate a further embodiment with two apertures, namely a pouring aperture 30 and vent aperture 31 with respective downturned flanges 32,33, as in FIGS. 10 to 13. The pull ring 261 is similar to that of FIGS. 7 to 9, so as to ensure that the vent aperture 31 is opened first, but is provided with an additional thin lip 262 to facilitate engagement by a finger. The residual thickness 127 of the plastics material below the flat end of the flange 32 is continuously varied around the periphery of the pouring aperture 30, as can be seen in FIG. 19, while remaining within the range 0.08 to 0.5 mm being greatest in the part of the periphery remote from the vent aperture 31, so as to provide improved resistance to creep where the tendency to distort is greatest. It is not necessary to vary the residual thickness around the vent aperture 31. The portions of plastics material enclosing the flanges 32,33 are joined on the underside of the can end by a straight feeder portion 34 to facilitate flow of plastics material during the moulding operation. 
     A further feature of this embodiment is that a portion 40 of the closure joining plug portions 41,42 filling the two apertures 30,31 acts as a strap to hold the parts of the closure in position when the can end is distorted under internal pressure. 
     As mentioned above, the residual thickness of the plastics material below the downturned flange may be varied in a stepwise, rather than a continuous manner. This is illustrated in FIG. 20, which represents a modification of the embodiment of FIGS. 17 to 19 and shows a plug portion 43 in the pouring aperture 30 formed with a transverse step 44 so as to result in a stepwise increase in the residual thickness at the left-hand side of the pouring aperture 30, remote from the vent aperture 31. 
     As mentioned above, if the vent aperture is small enough it does not need to be provided with a downturned flange. FIGS. 21 to 23 illustrate a modification of the embodiment of FIGS. 17 to 19 in which the vent aperture 311 is of substantially smaller dimensions than the vent aperture 31 of FIGS. 17 to 19, having straight sides joined by curved ends 45,46 of small radius of curvature. No flange is provided around the vent aperture 311. On pulling on the pull tab 261, the closure shears easily against the end 46 of the vent aperture because the stiffness of the can end remains sufficient to prevent it from bending, due to the small area and sharp curvature of the vent aperture. The residual thickness 127 below the flange 32 around the pouring aperture 30 is varied as in the embodiment of FIGS. 17 to 19. The closure again shears easily at the right hand side of the pouring aperture 30 due to the small residual thickness at this point, while the increased residual thickness at the side remote from the vent aperture provides the necessary resistance to creep. 
     FIGS. 24 and 25 illustrate a modification of the embodiment of FIGS. 14 to 16, in which the pear-shaped aperture 151 has a flange 162 which extends around the broader end 152 but is discontinued at the narrow end 153 of the aperture, where the need for resistance to creep is minimal. The plug portion 154 of the closure can be seen from FIG. 25 to provide an increased residual thickness at the broader end 152 of the aperture, as in FIGS. 14 to 16. 
     FIGS. 26 and 27 illustrate a further modification of the embodiment of FIGS. 14 to 16, in which the pear-shaped aperture 151 has a flange 163 which varies continuously in depth from a maximum at the broader end 152 to zero at the narrow end 153 of the aperture. The plug portion 155 in this case is thickened in a stepwise fashion at 156 but otherwise varies in thickness in proportion with the depth of the flange 163, so that the residual thickness below the flange is greater around the broader end 152 but is constant around the narrow end 153 of the aperture. The greater depth of the flange 163 at the broader end 152 of the aperture assists the increased residual thickness to provide the necessary resistance to creep. 
     The can ends illustrated and described are intended for use on cans for beverages which require thermal processing, i.e. heating to 35° C. to drive off condensation before shrink-wrapping and/or pasteurisation at 65° C., so it is necessary that the plastics material used for the closure should be resistant to softening at temperatures up to 65° C. The plastics material used for the coating 14 on the underside of the can end is preferably a similar polymer in order to ensure adequate bonding. 
     It is essential that the thickness of the plastics material of the closure is such as to enclose the flange 16 totally but to be capable of shearing against the flat end surface 17 of the flange when the ring pull is pulled to open the closure. To achieve this, we have found the optimum dimension for the downturn of the flange to be 0.75 mm, though it may vary from 0.125 mm to 2.5 mm. The lateral extent of the surrounding portions 24 and 38 is also important. The preferred lateral extent from the flange of these portions is 1.25 mm though it may vary from 0.125 mm to 7.5 mm. 
     The residual thickness 27,127 of the plastics material of the closure below the flat surface 17 of the flange 16 is of considerable importance in order to ensure that leakage is prevented but that the plug portion can be sheared off against the flange when the closure is to be opened. The optimum residual thickness for shearing is between 0.15 mm and 0.40 mm, though other considerations may make it preferable to use a thickness from 0.08 mm to 0.50 mm. 
     The preferred thickness of the ring pull is substantially 1.0 mm, though it may vary from 0.25 mm to 4.3 mm. 
     Alternative forms of the closure according to the invention may be provided with plugs adapted for fitting into apertures of larger diameter than those illustrated in the drawings, e.g. for use on cans for solid or semi-solid food products.