Abstract:
The invention concerns a closure for a cylindrical can ( 10 ) The closure includes a flexible member ( 11 ) of the easy-open type secured across the open end of the can ( 10 ). A rigid, screw cap ( 22 ) is screwed over the thus sealed end of the can. An annular, resilient member ( 24 ) depending downwardly from the underside of the end wall of the cap ( 22 ), engages the membrane ( 11 ) in the vicinity of the end flange ( 18 ) of the can body ( 10 ) thereby strengthening the seal between the membrane ( 11 ) and the flange ( 18 ). This advantageously permits cooking of the contents of the can with the flexible membrane ( 11 ) in situ, thereby giving rise to a can having an easy-open end, the contents of which can are cooked and sterilized using conventional processing lines.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 09/445,043, filed Mar. 20, 2000, the entirety of which is hereby incorporated by reference. This application also claims priority to PCT Application No. PCT/IB98/00825, filed May 28, 1998, and United Kingdom Patent Application No. 9711462.3, filed Jun. 4, 1997, the entireties of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to container assemblies in particular the invention concerns such assemblies including closures known as “easy open ends”.  
         [0003]     Easy open ends are typically provided in containers that are elongate and, in the unfilled state, open at at least one end. An example of such a container is a metal can.  
         [0004]     There are two main types of easy open end. One is made from relatively thick and rigid steel or aluminum, which incorporates a “score” or weakened annular region. This weakened region allows the centre part of the end to be removed, but has the disadvantages that the required opening force is relatively high, making it difficult for less dextrous people to open, and that the ruptured edge is sharp and may cause laceration injuries. The process to form the rivet by which an opening tab may be attached requires many drawing and forming steps. Typically the thickness of steel easy open ends is 0.22 mm or greater, even up to 0.30 mm, depending on the diameter of the closure.  
         [0005]     An alternative easy open end typically comprises a flexible, frangible membrane usually of metal foil, or of a laminated material including a layer of metal foil, secured over the open end of a can after filling thereof with e.g. a food product. Since the flexible membrane is easily peeled off the can end, it is easy for a user of the can to tear the membrane to gain access to the food product inside the can. The flexible membrane is then usually torn off the can and discarded. Some types of flexible membrane include pull tabs and weakened lines to assist the opening process.  
         [0006]     Where a flexible membrane is used there are a number of ways to secure it to the can body. It may be sealed to a ring of aluminum or tinplate or electrolytically chromium coated steel (ECCS), which has been coated with either a layer of flexible polymer such as polypropylene or with a layer of a lacquer which incorporates a quantity of fusible polymer such as polypropylene. To effect a seal the foil membrane (also coated with a layer of fusible polypropylene) is placed over the ring and heat is applied through tools above and below the membrane-ring components. This heat melts one or both of the polymer layers which are then sealed together on cooling. The ring is then attached to the can body by a conventional double seam. In this component the opening is achieved by either breaking the polymer layer to metal adhesion or by breaking within the polymer layer.  
         [0007]     An alternative method is to seal the flexible foil membrane directly to the can body, by again heating the membrane and can body until the polymer layers soften sufficiently to melt together and cool to form a homogeneous solid layer, which can then operate as above when opened. It is also possible (but not common) to use an adhesive material to fix the foil on to the can.  
         [0008]     Many food products are packed in cans in an uncooked or partially cooked state. On sealing of the cans in food production factories their contents are heated (e.g. by steam or steam/air heating) to cook the completely and simultaneously sterilise the interiors of the cans. This process, which has been in widespread use for more than 150 years, allows the safe canning of food products at very high rates of production. However, it has been traditional to employ three piece cans for this process. Both ends of a filled three piece can are substantially rigid. Hence it is necessary to use a can opening machine to open such a can. This is generally considerably slower than opening an easy open end. Also, many people find can opening machines difficult or impossible to use.  
         [0009]     It is possible, and indeed is common, to use easy open ends for continuous mass production of canned food products, but these ends are of the more rigid type with relatively high thickness, as described above. What is not currently possible is to use foil sealed cans in a continuous steriliser, without the use of over-pressure to counterbalance the pressure generated inside the can.  
         [0010]     It has not previously been possible to employ the flexible membrane-type easy open ends in the continuous mass production of cans the contents of which require cooking in situ. This is primarily because the heating process causes expansion of gases sealed within the cans, and causes further gases to evaporate from the food products, with the result that the seals between the flexible membranes and the can ends burst or, less desirably, leak in a manner that is difficult to detect. Failures of the flexible membranes themselves (as contrasted with the seals) also occur.  
         [0011]     One possible solution to these problems lies in the use of an overpressure cooker that is capable of equalising the pressures acting on both sides of the flexible membranes during cooking. This apparatus is disadvantageous, however, since its heating chamber must be sealed and pressurised during the cooking process. Thus the overpressure cooker cannot be used for continuous mass production employing moving conveyor lines.  
         [0012]     Thus there is a need for an easy open closure suitable for use in continuous mass production of food products.  
         [0013]     U.S. Pat. No. 4,683,016 discloses an easy open end the rigid closure of which includes concentric, downwardly depending annular members that tension the flexible membrane. However, this arrangement only serves to promote a good seal between the container end and the flexible membrane before final curing of the adhesive therebetween. This results in a smooth and well sealed membrane, but would be unlikely to prevent bursting of the seal during cooking since by that stage the strength of the seal depends entirely on the properties of the adhesive material securing the flexible membrane on the container end.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     According to a first aspect of the invention there is provided a container assembly comprising an open-ended container and a closure system therefor, including: 
        (i) a flexible membrane closing the open end of the container;     (ii) a seal between the flexible membrane and the container; and     (iii) a rigid closure mounted on the container having a resiliently deformable member juxtaposed to the flexible membrane, the resiliently deformable member pressing the flexible membrane against the container in the vicinity of the seal, thereby reinforcing the seal sufficiently to withstand pressures generated on heating of the contents of the container.        
 
         [0018]     This assembly is advantageous because the resiliently deformable member (reacting against the rigid closure) continuously and evenly reinforces the seal while the rigid closure is mounted on the container. Furthermore, through judicious choice of the material of the resiliently deformable member, the reinforcing pressure applied to the seal may be arranged to increase as the pressure inside the can increases, since this increases the force conferred by the flexible membrane on the resiliently deformable material. This is ideally suited to in situ cooking of the can contents, since the pressure within the can progressively increases for part of the cooking process.  
         [0019]     Preferably a container assembly in accordance with the invention includes a container which is a metal, plastic or composite can.  
         [0020]     This can advantageously allows the mass production of canned food products that are accessible via easy open ends.  
         [0021]     According to a second aspect of the invention, there is provided a method of forming a container assembly in accordance with the invention, comprising the steps of: 
        (i) securing a flexible membrane on the open end of the container by use of adhesives or heat-sealing, thereby forming a seal;     (ii) engaging the cam and follower of a rigid closure and the container with one another; and     (iii) moving the rigid closure and the container relative to one another to cause relative movement between the cam and follower in the predetermined direction, thereby causing the resiliently deformable member to press the flexible membrane against the container in the vicinity of the seal sufficiently to maintain the seal against pressures generated in the container on heating of its contents.        
 
         [0025]     This method is conveniently suited to the mass production of canned food stuffs in existing food factories. The method obviates the need to use pressure cookers to cook food products in cans having easy open ends, and allows production of the filled, sealed cans to occur while the cans move along the conveyor lines of a continuous production apparatus.  
         [0026]     According to a third aspect of the invention, there is provided a method of packaging a food product, comprising the steps of placing the food product in an open ended container; closing the open end of the container with a closure to provide an assembly in accordance with the invention and heating the container assembly and the food product therein, the container closure system; maintaining the seal between the flexible membrane and the container during such heating.  
         [0027]     According to a fourth aspect of the invention, there is provided a method of packaging a food product comprising the steps of closing an open end of a container having two open ends with a closure to provide a container assembly in accordance with the invention placing a food product in the container; closing the other open end of the container by flanging a container end thereto; and heating the container and the food product therein, the container closure system maintaining the seal between the flexible membrane and the container during such heating.  
         [0028]     Further, advantageous features of the invention are defined in the dependent claims hereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     There now follows a description of preferred embodiments of the invention, by way of example, with reference being made to the accompanying drawings in which:  
         [0030]      FIG. 1  is a vertically sectioned view of the end of a container assembly comprising a container and closure according to the invention;  
         [0031]      FIG. 2  is a partly-sectioned view showing the components of the  FIG. 1  container-assembly;  
         [0032]      FIG. 3  shows a step in a preferred method of forming the container assembly; and  
         [0033]      FIG. 4  shows an alternative form of container assembly according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     Referring to the drawings, there is shown an open ended container in the form of cylindrical metal can  10 .  
         [0035]     The open end of can  10  is closed by a flexible membrane  11  and a rigid cap  12 , each of which is described in more detail below.  
         [0036]     The body  13  of can  10  is manufactured in a generally conventional manner. Body  13  may be of the one-piece or two-piece types well known in the art of can making. Body  13  is a two-piece body in the embodiment shown.  
         [0037]     A short distance from its open end, body  13  is necked inwardly at  14 . Thus there is defined a parallel sided main body portion  13   a  of maximum diameter; and a further body portion  13   b,  proximate the open end of the can, of reduced diameter.  
         [0038]     The necking (at  14 ) of the body  13  is defined by an inclined shoulder or chamfer extending about the periphery of can  13 . Reduced diameter body portion  13   b  is substantially parallel sided and terminates in a further neck  16  defining a yet further reduced diameter portion  17 .  
         [0039]     Reduced diameter portion  17  is also substantially parallel sided, and terminates in an outwardly turned, annular flange  18  the outer diameter of which is substantially the same as that of body portion  13   b.    
         [0040]     The cylindrical walls of the body portion portions  13   a,    13   b  and  17  are substantially parallel to the longitudinal axis of the can  10 .  
         [0041]     The annular surface of flange  18  remote from body portion  17  faces outwardly at the open end of the can, and is substantially perpendicular to the longitudinal axis of the can. Flexible membrane  11  is adhesively secured to flange  18  by means of e.g. an annular strip of heat seal material that cures on heating (typically up to 180° C. for 1 second) thereof. The heat sealing tools  150 , 151  are shown in  FIG. 3 . The step of securing the flexible membrane to the open end of the container neck may utilize a heat-sealing method such as heat contact, ultra sonic, induction or hot air.  
         [0042]     The radial dimension x of the flange  18  is, typically, 2 to 4 mm in length. The width of the annular band of adhesive material between membranes  11  and flange  18  is of a similar dimension.  
         [0043]     In practice the heat seal lacquer material extends over the entire interior surface of the can, as shown at  160  in  FIG. 3 . The lacquer may be e.g. a polypropylene or polyethylene extrusion coating, or could be a PET film.  
         [0044]     The membrane  11  may be e.g. a metal (eg. aluminum or steel) foil, or a laminated, flexible, composite material such as a layer of metal foil bonded to a layer of paper or a plastic film with a functional barrier layer. In any event, the lower surface  11   a  of flexible member  11  is substantially inert, in the sense that it does not contaminate or react with the contents of container  10 . The upper surface  11   b  of flexible membrane  11  may be printed with advertising material or user instructions.  
         [0045]     Body portion  13   b  has disposed at intervals about its outer periphery a series of cam members in the form of threads  19 . Each thread in the embodiment shown lies at the same angle as the adjacent threads, and extends over the same length. In preferred embodiments this length is a few degrees (e.g. 5-10°) of arc. As illustrated schematically in  FIG. 1 , each thread  19  is formed as an embossment that is slightly proud of the surface of body portion  13   b.  The embossments may be formed in a conventional manner e.g. by means of an expanding, rotatable tool insertable through the open neck of can  10  during manufacture thereof, to deform the material of wall portion  13   b  as desired.  
         [0046]     The closure of the open end of can  10  includes a rigid cap  12  comprising a laminar member  21 . In the embodiment shown, the laminar member is a circular disc having a cylindrical, annular skirt  22  depending downwardly therefrom.  
         [0047]     Annular skirt  22  includes on its outer surface a series of recesses of substantially the same size, angle and length as the threads  19  formed on body portion  13 . The recesses  23  appear as embossments on the inner surface of skirt  22 . Hence they constitute cam followers in the form of threads complementary to the threads  19 . Thus the cap  12 , which may be manufactured e.g. by deep drawing of a slug of metal using a per se known process, may be screwed onto the end of can  10  through cooperation of the threads  19  and recesses  23 .  
         [0048]     When cap  12  is screwed onto the open end of can  10  as aforesaid, the angles of the threads relative to the can  10  cause disc  21  to be driven towards membrane  11  on tightening of cap  12 .  
         [0049]     The underside of disc  21  has adjacent its outermost circumference an annular member  24  secured thereto so as to depend downwardly from the underside of disc  21 .  
         [0050]     Annular member  24  is formed of a resiliently deformable material, such as an expanded foam, a rubber based formulation, a PVC plastisol or a similar material. It is secured to the underside of disc  21  by virtue of its formation there (e.g. by moulding or injection) or, possibly, by adhesive fixing in the cap  12  of a pre-formed sealing ring  24 .  
         [0051]     As cap  12  is tightened onto can  10 , annular member  24  engages membrane  11 .  
         [0052]     Annular member  24  is located and dimensioned to sandwich a portion of membrane  11  against flange  18 , in the vicinity of the adhesive material between membrane  11  and flange  18 . Thus on tightening of cap  12 , resilient, annular member  24  presses membrane  11  into tight, sealing contact with flange  18 . This seal is capable of withstanding pressures developed within the can  10  during cooking of food products therein.  
         [0053]     Furthermore, cooking of food products in the can  10  preferably occurs with the cap  12  in the position shown in  FIG. 1 . In this position, the annular member  24  continues to press down on the seal between membrane  11  and flange  18 , thereby providing additional reinforcing of the seal.  
         [0054]     In the position shown in  FIG. 1 , the gap  25  between membrane  11  and disc  21  is of the order of 1-6 mm. Thus the stretching of membrane  11  that occurs during cooking of food products in can  10  is accommodated by expansion of membrane  11  towards disc  21  that is, as indicated, rigid. Thus the gas pressure within the can is reduced compared with that encountered in conventional cans.  
         [0055]     A preferred method of packing a food product in accordance with the invention includes placing food products in an open ended can  10  one end  27  of which is sealed (by virtue of manufacture of the can body as a two-piece body sealed at one end) by a closure to provide a container assembly according to the invention. If appropriate, a suitable modified atmosphere may be added above the level of the food product in the can  10  by a conventional apparatus; and then a conventional can end may be secured in a per se known manner by a “flanger”, i.e. a double seaming machine.  
         [0056]     Before cooking of the food products, and preferably before the food products are placed in the can, a cap  12  is screwed onto the threads  19  of  30  the closure of the invention again by machine or by hand as appropriate and tightened down onto the end of can  10  until annular member  24  presses membrane  11  against flange  18  with a predetermined pressure. The moment prior to contact between the components is shown in  FIG. 3 . The predetermined pressure may be achieved e.g. by sensing the torque necessary to rotate cap  12  onto the threads  19 .  
         [0057]     Thereafter, the can  10  is passed to a suitable cooking apparatus such as a steam, steam/air or water cascade cooker that cooks the food products within the can  10 . As is well known, this process kills bacteria in the can rendering the food products safe for long term storage. It also temporarily increases gas pressure in the can, primarily by virtue of expansion of any gas between the food material and the can body; and also through migration of gas molecules from the food products as the food product temperature increases.  
         [0058]     The action of annular member  24  ensures that the peripheral seal of membrane  11  is strong enough to withstand the additional pressures generated during cooking. The presence of disc  21  prevents rupture of membrane  11  at locations spaced from flange  18 .  
         [0059]     In some embodiments the heating process may cause the material of member  24  to change, thereby allowing easy removal of cap  12 .  
         [0060]     After cooling of the can  10  it may be distributed. A user of the can may then unscrew cap  12  to reveal the membrane  11 . Membrane  11  may then be peeled off in order to gain access to the food product within the can.  
         [0061]     After peeling membrane  11  may be removed and discarded. Subsequent reclosing of can  10  using cap  12  causes the annular member  24  to engage either flange  18  or an annular portion of membrane  11  remaining adhered thereto, to provide a short to medium term resealing facility thereby extending the life of the food products after opening of the can.  FIG. 2  shows an optional pull-off tab  26 , formed integrally with membrane  11 , that may be provided to assist the opening of the membrane  11 . Since the hinge securing the tab  26  is of the same material and thickness as membrane  11 , lifting of tab  26  is facilitated.  
         [0062]     Thus the invention advantageously provides an apparatus and a method by means of which cooked food products may be provided in metal or other cans having easy open ends.  
         [0063]     Furthermore, the process readily lends itself to automation using high speed can making machinery capable of forming cans at rates of perhaps 300 per minute or greater. The quality and integrity of the heat sealing operation can readily be tested and verified.  
         [0064]     The neck  14  in the can body  13  provides a neat appearance to the can when cap  12  is secured thereto, since the skirt  22  depending downwardly from disc  21  is of the same diameter as body portion  13   a.  The neck  14  therefore provides for a generally flush appearance to the can end.  
         [0065]     Alternatively the cap diameter can be made the same as the seam diameter on the opposing end of the can, so that the can will roll satisfactorily during existing processes. This is shown schematically at  130  in  FIG. 4 .  
         [0066]      FIG. 4  also-shows use of an optional, rippled form  121  of the upper wall of cap  12 . This assists in resisting the cooking pressure in a per se known manner.  
         [0067]      FIG. 3  shows the membrane  11  in its preferred form, i.e., an upper, metal foil layer  11   b  having its lower surface coated with e.g., polypropylene  11   a.