Patent Publication Number: US-11040495-B2

Title: Method of making a retort container

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/514,350, filed Jul. 17, 2019, entitled “Method of Making A Retort Container”, which is a continuation of Ser. No. 13/445,162, filed Apr. 12, 2012, which is related to U.S. patent application Ser. No. 13/224,651, filed on Sep. 2, 2011, and U.S. patent application Ser. No. 13/284,056, filed on Oct. 28, 2011, the entire disclosures of said applications being hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present disclosure relates to containers, particularly to containers having one or two metal ends applied to one or both ends of the container body and crimp-seamed or double-seamed onto the container body, and most particularly to such containers used for retort processing. 
     Traditionally, retort containers have been constructed substantially of metal. For many decades the standard retort food containers have been three-piece or two-piece metal cans. In a three-piece metal can, a metal can body is closed by a pair of metal ends that are typically double-seamed onto the ends of the can body. A two-piece metal can eliminates one of the metal ends because the can body is a deep-drawn body with an integral bottom wall. The metal ends of such typical retort containers have an outer peripheral portion forming a “curl” that receives the end of the can body, and after the end is applied the curl and the wall of the can body are rolled up together to form a double seam. This construction has the great advantage that it readily withstands retort processing without the seams being compromised, because the plastically deformed metal of the can body in the seam area tends to hold its deformed shape despite the high pressure and temperature during retort. 
     More recently there has been a desire to construct retort containers that use less metal, motivated by the potential cost reduction and improved aesthetics that such a construction can offer. The development described in the present disclosure at least in some aspects is aimed at addressing this desire. 
     BRIEF SUMMARY OF THE INVENTION 
     In particular, the present disclosure describes a retort container having one or two metal ends attached to a container body in such a way that there is an improvement in blow-off resistance when the inside of the container is pressurized relative to outside ambient pressure for any reason (e.g., during retort processing, or as a result of changes in altitude of the container, such as when a container is filled and sealed at sea level and is subsequently transported to a high-altitude location). The improvement derives from a thermal fusing of the metal end to the container body, as described herein. This could allow the container body to be a thinner metal than typically used for metal retort containers, where the thinner metal would have less resistance to “unrolling” under high internal-pressure conditions, or could allow the container body to be formed of a non-metallic material (e.g., plastic or composite), since blow-off resistance is not dependent primarily upon the ability of the rolled-up container end in the seam being able to hold its deformed shape. 
     In accordance with the invention in one embodiment, a method of making a retort container comprises the steps of:
         providing a container body having a side wall extending about a container body axis, the side wall having a lower end and an upper end, the upper end defining an upper edge that extends about a top opening of the container body, the side wall having an inner surface and an outer surface;   applying a metal end to the upper end of the container body, the metal end having at least a metal layer and comprising a central portion and an outer peripheral portion extending generally radially outwardly from the central portion and extending circumferentially about the central portion, the peripheral portion having a radially outer part and a radially inner part, a first heat-sealable material being present on at least one of (a) a lower surface of at least the peripheral portion of the metal end and (b) the inner surface of the side wall adjacent the upper end thereof, the radially outer part of the peripheral portion defining a curl, the radially inner part of the peripheral portion defining a chuck wall that extends generally downward from the curl and has a radially outer surface forming an interface with the inner surface of the side wall of the container body;   forming a seam connecting the metal end to the upper end of the side wall, the seam having the curl of the metal end and the upper end of the side wall interlocked, the container body and seamed metal end constituting a container assembly;   disposing the container assembly between a lower conveyor and an upper pressure belt which cooperatively engage opposite ends of the container assembly to prevent the metal end from coming off the container body, and which convey the container assembly along a path; and   induction heating the metal end to melt the first heat-sealable material and then cooling the first heat-sealable material so as to fuse the metal end onto the container body, wherein the heating and cooling steps take place during the engagement of the conveyor and pressure belt with the container assembly.       

     Heat-sealable materials useful in the practice of the present invention can comprise any known heat-sealable materials. The metal end can have an interior coating, and optionally an exterior coating as well. 
     The container body can be open at both ends that are each closed by a metal end in accordance with the invention, or can be open at only one end such that only one metal end is needed. The container body can be made in various ways. For example, the container body when metal can be formed from sheet metal seamed along a longitudinal seam in the usual way, or can be deep drawn to have an integral bottom wall. When plastic, the container body can be formed by any of blow-molding, thermoforming, or injection-molding so as to have a bottom wall integrally joined to the side wall, or extruded so as be open at both ends. 
     In some embodiments, the metal end is an easy-open end having a severable panel defined by a score line in the metal layer. Alternatively, the metal end can be a sanitary end, or the metal end can comprise a membrane sealed to an annular metal ring. 
     The step of forming a seam can comprise forming a crimp seam, or it can comprise forming a double seam by rolling the curl of the metal end and the upper end of the side wall together so as to form the upper end of the side wall into a body hook and to form the curl into an end hook and to interlock the body hook and the end hook. 
     In some embodiments the method can further comprise providing a second heat-sealable material present on the other of (a) the lower surface of at least the peripheral portion of the metal end and (b) the inner surface of the side wall adjacent the upper end thereof. Thus, the metal end and the side wall both have respective heat-sealable materials thereon. The method entails placing the second heat-sealable material and the first heat-sealable material in contact with each other at the interface between the chuck wall and the side wall, and heating the first and second heat-sealable materials to a temperature sufficient to cause the first and second heat-sealable materials to be softened or melted and to flow together, after which cooling of the first and second heat-sealable materials is allowed to occur so as to fuse the chuck wall to the inner surface of the side wall. 
     The second heat-sealable material and the first heat-sealable material can be thermally fused together in the seam as well. 
     The method can further comprise the steps of filling the container with a food product prior to the step of applying the metal end to the container body, and, after the interface between the chuck wall and the side wall is fused, retorting the container. During the retorting step the thermoplastic container body is radially unconstrained such that the container body is allowed to expand radially as internal pressure is exerted on the side wall. Notably, the container body is free of any special expansion panels, whereby the radial expansion of the container body occurs substantially uniformly about a circumference of the container body. 
     In some embodiments, the chuck wall extends at a non-zero acute angle relative to a longitudinal axis of the container body and is configured such that a lower end of the chuck wall is smaller in diameter than the inner surface of the side wall, while an upper end of the chuck wall is larger in diameter than the inner surface of the side wall. The step of applying the metal end to the container body results in the side wall of the container body moving relatively upward from the lower end to the upper end of the chuck wall such that an interference fit is created between the chuck wall and the side wall, thereby creating the intimately contacting interface therebetween. 
     During the induction heating step there is a substantial absence of external pressure exerted to urge the chuck wall and side wall into intimate contact; rather, pressure urging the chuck wall and side wall together comes from the interference fit that already exists between them when the end is applied and seamed to the side wall. Thus, there is no need for sealing jaws to create pressure during the heating step in order to form a secure thermal bond between the metal end and the container body. Indeed, in some embodiments the heating step can be carried out with induction heating in which there can be an absence of contact between the induction tool and the metal end. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a diagrammatic depiction of several steps of a process for making containers in accordance with one embodiment of the invention; 
         FIG. 2  is a photomicrograph of a sectioned container in the region of the metal end&#39;s seam with the container body, in accordance with an embodiment of the invention; 
         FIG. 3  is a cross-sectional view of an apparatus, along line  3 - 3  in  FIG. 4 , in accordance with an embodiment of the invention, showing one step of a method in accordance with an embodiment of the invention; 
         FIG. 4  is a top view of the apparatus of  FIG. 3 ; 
         FIG. 5  is a view similar to  FIG. 3 , showing a further step of a method in accordance with an embodiment of the invention; 
         FIG. 6  is a view similar to  FIG. 3 , but illustrating an alternative embodiment of the invention; and 
         FIG. 7  is a cross-sectional view of a portion, greatly enlarged, of a container in accordance with an embodiment of the invention, showing the structure of the metal end and its seam to the container body. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings in which some but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. The drawings are not necessarily to scale, and thus the relative proportions of various elements (e.g., thicknesses of layers in multi-layer structures) suggested by the drawings is not necessarily indicative of the actual relative proportions. 
     With reference to  FIG. 1 , several steps of a process for making containers in accordance with an embodiment of the invention are schematically depicted. In a first step, an extruder  10  is employed to extrude a substantially thermoplastic tube  12  as a continuous extrusion. The extruder  10  includes a screw  14  or the like that feeds a molten substantially thermoplastic material under pressure through a die  16  such that the continuous tube  12  is extruded through an annular die orifice  18 . The extruded tube  12  can have a monolayer or multi-layer construction. As an example of a multi-layer construction, the tube wall can have the structure (from ID to OD): heat-sealable layer/tie layer/barrier layer/tie layer/heat-sealable layer. 
     The tube  12  is cooled sufficiently (via known cooling means, not illustrated) and is then cut into parent tubes  20  of a convenient length. Typically each parent tube  20  will be of sufficient length to provide a plurality of container bodies  22  cut from the parent tube as shown. 
     Alternatively, in the case of metal container bodies  22 , they are manufactured according to known techniques (not shown). 
     Each container body  22  is then mated with a pair of metal ends  30 . 
     The metal end  30  and container body  22  in some embodiments can be constructed to mate with each other as described in Applicant&#39;s co-pending application Ser. No. 13/161,713 filed on Jun. 16, 2011, the entire disclosure of which is hereby incorporated herein by reference. 
     The metal end  30  includes a central portion  32  and an outer peripheral portion  34  extending generally radially outwardly from the central portion  32  and extending circumferentially about the central portion  32 . The peripheral portion  34  has a radially outer part and a radially inner part. The radially outer part defines a curl  36  having a lower surface that is generally concave downward in an axial direction of the metal end. The radially inner part defines a chuck wall  38  that extends generally downward and radially inward from the curl  36 . The chuck wall  38  can be a compound-angle chuck wall, as described in the above-noted &#39;713 application, having an upper part adjacent the curl  36  and a lower part joined to and positioned below the upper part. The upper part of the chuck wall is substantially linear and oriented relative to the axial direction at a relatively smaller non-zero angle and the lower part of the chuck wall is substantially linear and oriented relative to the axial direction at a relatively larger angle compared to the upper part of the chuck wall. 
     The metal end  30  is configured such that at least a bottom edge of the lower part of the chuck wall has an outside diameter that is smaller than the inside diameter of the container body side wall  24  at the upper edge thereof. Additionally, the chuck wall  38  is configured such that it becomes somewhat larger in diameter than the inside diameter of the container body side wall  24  as the top edge of the side wall progresses up toward the curl  36  during mate-up of the metal end  30  with the container body  22 . In other words, the side wall&#39;s ID is undersized in relation to the OD of the chuck wall adjacent the curl. This has the effect of “wiping” the inner surface of the side wall  24  with the metal end during mate-up, which has the benefit of cleaning the inner surface prior to seaming. This also results in an interference fit between the chuck wall  38  and the side wall  24 . 
     Once the metal end  30  is mated with the container body  22 , a seaming operation is performed in order to seam the metal end onto the container body. In the illustrated embodiment, the container body is a straight-walled (non-flanged) container body, and a crimp seam  40  is formed between the metal end and the container body, in which the side wall  24  remains substantially straight and is compressed between the chuck wall  38  and a deformed portion of the curl  36 . Alternatively, in other embodiments, a double seam can be formed, in which case the container body can be flanged. The crimp seam  40  has the advantage of being usable with non-flanged container bodies and yet providing a positive locking of the metal end  30  onto the container body  22  even before the metal end is heat-sealed to the container body. This can be seen in  FIG. 2 , which is a photomicrograph of a sectioned container in the region of the crimp seam  40 . A “nub” or interlocking portion of the container body side wall is formed by the folded peripheral edge of the curl “biting” into the side wall. The nub and the folded edge effectively interlock, thereby locking the metal end onto the container body. 
     It will be understood, of course, that a second metal end is attached to the opposite end of the container body  22  in the same fashion described above. Alternatively, in the case of a container body having an integral bottom wall (as may be the case with, for example, a blow-molded, thermoformed, or injection-molded plastic container body, or a deep-drawn metal container body), the second metal end is not required. 
     The above-described interlocking of the metal end  30  and container body  22  alone, however, may not be sufficient to enable the container to withstand a retort process in some container configurations, such as when the container body  22  is plastic or is a thin metal or a composite material. In order to be able to withstand retort intact in those instances, the container is subjected to a heat-sealing operation to fuse portions of the metal end  30  to the container body side wall  24 . In this regard, at least one of the respective surfaces of the metal end and side wall that are intimately contacting each other in the region of the crimp seam  40  is formed by a heat-sealable material, and the two surfaces are such that heating of the crimp seam to soften or melt this heat-sealable material, followed by cooling of the material, causes the two surfaces to be “thermally fused” to each other. More specifically, it is important to the attainment of adequate “blow-off resistance” during retort (or other high-internal-pressure condition of the container) that at least the chuck wall  38  of the metal end  30  be thermally fused to the inner surface of the side wall  24  of the container body, and preferably both the chuck wall  38  should be thermally fused at the ID and a portion of the curl  36  (or, more accurately, what was the curl prior to the seaming operation) should be thermally fused at the OD of the container body side wall  24 . 
     The thermal fusing operation is diagrammatically depicted in  FIG. 3 , illustrating an induction sealer  50  that can be used in the practice of the invention, depicted in a diagrammatic cross-sectional view. The sealer  50  includes a conveyor  60  comprising an endless belt  62  looped about a pair of spaced parallel rolls  64 ,  66 . At least one of the rolls  64 ,  66  is rotatably driven about its axis and in turn drives the belt  62 . As shown in  FIG. 3 , the rolls  64 ,  66  rotate clockwise and the belt  62  thus travels clockwise such that its upper flight moves from left to right in the figure. The belt  62  supports a series of container assemblies (each consisting of a container body  22  and a metal end  30 ) on the upper flight of the belt. 
     The container assemblies are loaded (by suitable means, not shown) onto the conveyor  60 . There is a gap between adjacent container assemblies in the conveyance direction (i.e., the left-to-right direction in  FIG. 3 ), as illustrated. This gap may be maintained in a uniform fashion by, for example, configuring the belt  62  to have a series of uniformly spaced recesses each of which receives a container assembly. The belt  62  can be a “single-lane” or “multiple-lane” belt. A single-lane belt has a single row of such recesses extending in the longitudinal (conveyance) direction. A multiple-lane belt has two or more such rows spaced apart widthwise on the belt so that multiple series of container assemblies can be conveyed simultaneously through the induction sealing process. The open ends of the container bodies  22  are against the belt  62  and the metal ends  30  are located at the upper ends of the container bodies. 
     The apparatus  50  further includes a pressure belt  70  comprising an endless belt  72  looped about a pair of spaced parallel rolls  74 ,  76 . At least one of the rolls  74 ,  76  is rotatably driven about its axis and in turn drives the belt  72 . As shown in  FIG. 3 , the rolls  74 ,  76  rotate counterclockwise and the belt  72  thus travels counterclockwise such that its lower flight moves from left to right in the figure. The pressure belt  72  is driven to travel at the same linear speed as that of the conveyor belt  62 , and the pressure belt is arranged so that its lower flight presses down on the metal ends  30  in a downward direction toward the conveyor  60 . 
     Disposed within the loop of (as illustrated), or adjacent to a lower flight of (not shown), the pressure belt  72  is one or more induction heads  80  (only one being illustrated). Each induction head comprises a wire coil  84  and a ferrous core  86 , depicted schematically in the figures. The wire coil is wound in a particular configuration so as to produce the desired electromagnetic field when an alternating electrical current is passed through the wire. In particular, as known to those skilled in the art, the coil configuration dictates the pattern and strength of the electromagnetic field for a given AC current. More particularly, the magnetic axis A 1  of the induction head  80  is schematically illustrated in  FIG. 4  as being parallel to the conveyance direction, but of course the magnetic axis can be oriented with a different orientation with respect to the conveyance direction. The magnetic axis can be parallel to the plane defined by the lower flight of the pressure belt  70 , or can lie in a different plane. The illustrated orientation of the magnetic axis A 1  is exemplary only, and other orientations can be used. 
     As each container assembly is carried on the conveyor  60  along the conveyance direction, the electromagnetic field of the induction head  80 , schematically illustrated by field lines EF 1  in  FIG. 4 , induces eddy currents through the entirety of a metal end  30  when it comes beneath the head  80 . These eddy currents heat the metal layer of the metal end and this heat is conducted to the heat-sealable thermoplastic material, causing the metal end  30  to become thermally fused to the container body  22 . 
     The container assemblies thus have the metal ends  30  sealed to the one end of the container bodies  22  by the action of the induction head  40 . As the container assemblies are conveyed beyond the induction head on the conveyor  60 , a cooling device  90  ( FIG. 4 ) such as an air knife, water sprayer, or the like, provides cooling to the metal ends while the pressure belt  70  is still applying pressure on the metal ends to keep them from coming off the container bodies. As noted, the pressure belt  70  can be vertically adjustable, as denoted by arrow A in  FIG. 3 , to accommodate containers of different heights. The pressure belt  70  acts to prevent the containers from growing in height, and thus effectively clamps the metal ends on the container bodies during the cooling process. The cooling of the ends by the cooling device  90  ensures that the heat-sealable thermoplastic material is solidified before the containers are discharged from the conveyor  60 . 
     Container assemblies produced by the process explained above and depicted in  FIG. 3  are of course only partially completed, but are in condition to be filled with the desired contents and sealed closed.  FIG. 5  depicts the process of sealing the filled containers closed. After the containers are filled and metal ends are applied and seamed onto the open ends, the containers are again loaded onto the conveyor  60  for a second pass through the apparatus  50 . It will be understood, of course, that the apparatus  50  of  FIG. 5  may be a duplicate of the apparatus  50  of  FIG. 3 , and may be located at a different site from that of  FIG. 3 . For example, at a first site, partial container assemblies, consisting of container bodies  22  closed at only one end by metal ends  30  may be produced as in  FIG. 3 . These assemblies may be transported to a second site at which another apparatus  50  is located. At this second site, the containers may be filled, metal ends may be seamed onto the open ends to close them, and then these filled containers may be loaded onto the conveyor  60  of the apparatus  50 . 
     In the second pass through the apparatus  50  illustrated in  FIG. 5 , the process of  FIG. 3  is essentially repeated to seal the metal ends  30  onto the container bodies  22 . Again, the induction head  40  heats the metal ends  30  to seal them onto the container bodies, and the cooling device  90  cools the metal ends before the containers are discharged from the conveyor  60 . 
       FIG. 6  depicts an apparatus  50 ′ in accordance with an alternative embodiment of the invention. The apparatus  50 ′ differs from the apparatus  50  previously described in that the induction head  40  is located within the loop of (as illustrated), or adjacent to a lower flight of (not shown), the lower belt  62  rather than the upper pressure belt  72 . With this arrangement, the product within the containers acts as a heat sink when the second metal end is induction-sealed to each container body. 
     In the embodiments illustrated and described above, the conveyance path for the workpieces is linear. The present invention, however, is not limited to any particular conveyor configuration. For example, a rotary conveyor (turntable, turret, etc.) can be used for conveying workpieces and multiple induction heads can be disposed adjacent the resulting circular conveyance path for exposing the workpieces to a plurality of differently oriented electromagnetic fields, in a manner closely analogous to that described herein. 
     With reference to  FIG. 7 , the metal end  30  can have a metal layer  42  and an interior layer or coating of a heat-sealable material  44 . Any suitable heat-sealable material can be used for the layer  44 , non-limiting examples of which include: acrylonitrile butadiene styrene (ABS), acrylic (PMMA), celluloid, cellulose acetate, cyclic olefin copolymer (COC), ethylene-vinyl acetate (EVA), ethylene vinyl alcohol (EVOH), fluoroplastics (PTFE, alongside with FEP, PFA, CTFE, ECTFE, ETFE), ionomers, liquid crystal polymer (LCP), polyoxymethylene (POM or acetal), polyacrylates (acrylic), polyacrylonitrile (PAN or acrylonitrile), polyamide (PA or Nylon), polyamide-imide (PAI), polyaryletherketone (PAEK or Ketone), polybutadiene (PBD), polybutylene (PB), polybutylene terephthalate (PBT), polycaprolactone (PCL), polychlorotrifluoroethylene (PCTFE), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polycyclohexylene dimethylene terephthalate (PCT), polycarbonate (PC), polyhydroxyalkanoates (PHAs), polyketone (PK), polyester, polyethylene (PE), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherimide (PEI), polyethersulfone (PES), chlorinated polyethylene (CPE), polyimide (PI), polylactic acid (PLA), polymethylpentene (PMP), polyphenylene oxide (PPO), polyphenylene sulfide (PPS), polyphthalamide (PPA), polypropylene (PP), polystyrene (PS), polysulfone (PSU), polytrimethylene terephthalate (PTT), polyurethane (PU), polyvinyl acetate (PVA), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), styrene-acrylonitrile (SAN). Where the container is to be retort-processed, a suitable heat-sealable material able to withstand the retort-processing conditions should be selected. 
     When the metal layer  42  is heated by induction heating, the heat-sealable layer  44  is heated by conduction, which causes the heat-sealable material to be softened or melted. Because the electromagnetic field&#39;s strength obeys the inverse square law, Joule heating of the metal end is greatest in the parts of the end closest to the coil of the induction heater and decreases proportional to the inverse square of the distance from the coil. Thus, only localized heating of the metal end occurs with a great enough magnitude to cause melting of the heat-sealable layer  44 . More particularly, the melting of the heat-sealable layer  44  is confined essentially to the region of the seam  40 . 
     As  FIG. 7  indicates, the induction heating of the seam  40 , followed by cooling (which occurs rapidly upon cessation of the electromagnetic field or movement of the container away from the coil), results in two areas of thermal fusing between the metal end  30  and container body side wall  24 : there is an inner seal S i  between the inner surface of the side wall  24  and a portion of the chuck wall  38  that lies parallel to and intimately contacts the side wall, and there is an outer seal S o  between the outer surface of the side wall  24  and a portion of what was the curl of the metal end prior to seaming. The seals S i  and S o  in  FIG. 7  are depicted for illustrative purposes as if they were each a distinct layer between the metal end  30  and the side wall  24 , but it is to be understood that in reality the seals are formed by a melding of the heat-sealable layer  44  of the metal end and the thermoplastic material on the surface of the side wall  24 . 
     It is important to the attainment of adequate blow-off resistance that the chuck wall  38  include a portion that is parallel to and intimately contacting the inner surface of the side wall  24 , and that this portion be thermally fused as described above. This results in the interface between the chuck wall  38  and the side wall  24  being oriented along a direction substantially parallel to the axis of the container, such that stress on the interface caused by internal pressure inside the container exerted on the metal end  30  is predominantly shear stress in the plane of the interface (as opposed to out-of-plane stress tending to peel one part from the other). 
     It is also a feature of the present invention that during the heating step for thermally fusing the end  30  to the side wall  24 , there is a substantial absence of external pressure exerted on the chuck wall  38  and side wall  24  for urging them together. Rather, pressure urging the chuck wall and side wall together comes from the interference fit that exists between them, as previously described. Indeed, when an induction heating apparatus  50  is employed as described above, there is no contact between the induction head and the metal end. 
     Various constructions of the metal end  30  and container body side wall  24  can be employed in the practice of the present invention. As noted with respect to  FIG. 7 , in one embodiment the metal end  30  can have an interior heat-sealable layer  44 . The container body can be metal as illustrated. 
     When plastic, the container body side wall  24  can be a mono-layer construction, and the substantially thermoplastic side wall  24  can be heat-sealable to the heat-sealable layer  44  of the metal end. Alternatively, in other embodiments, the side wall  24  can be a multi-layer plastic construction. For example, the side wall  24  can comprise at least two layers including an interior heat-sealable layer and a barrier layer providing moisture and gas barrier properties for the container body. The metal end  30  furthermore does not necessarily have to have an interior heat-sealable layer, as long as the interior surface is fusible to the heat-sealable layer of the side wall  24 . The metal end  30  can have a bare metal surface on its interior side. It can have a metal layer of homogeneous construction, but it is also possible for the metal end to be, for example, ETP (electrolytic tin plate steel) consisting of a layer of steel to which an ultra-thin coating of tin is electrolytically deposited, for example on the interior product-facing surface. As an unillustrated example, the container body side wall  24  can consist of five layers, in order from ID to OD: an interior heat-sealable layer, a tie layer, a barrier layer, a tie layer, and an exterior heat-sealable layer. Any of the previously described heat-sealable materials can be used for the heat-sealable layers. The barrier layer can comprise any suitable material providing the necessary barrier properties for the particular application to which the container will be put. Non-limiting examples of such barrier materials include ethylene vinyl alcohol (EVOH), polyvinyl alcohol (PVOH), polyvinylidene chloride copolymer (PVDC), polyacrylonitrile (PAN), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), liquid crystal polymers (LCP), amorphous nylon, nylon 6, nylon 66, nylon-MXD6, and the like. The tie layers can be any suitable adhesive materials for adhering the heat-sealable layers to the barrier layer. 
     When the metal end  30  does not include a heat-sealable layer, the heat-sealable layers of the container body wall can be designed to thermally fuse to the bare metal surface so as to form the seals S i  and S o  depicted in  FIG. 7 . For example, an ionomer (e.g., SURLYN® or the like) will thermally fuse to a bare metal such as ETP. 
     The above-described embodiments are not limiting in terms of the particular construction of the metal end  30  and side wall  24 . The present invention is applicable to and includes any combination of metal end and side wall constructions in which at least one of their respective surfaces that are intimately contacting each other in the region of the seam  40  is formed by a heat-sealable material, and the two surfaces are such that heating of the seam to soften or melt this heat-sealable material, followed by cooling of the material, causes the two surfaces to be “thermally fused” to each other. Additionally, as previously noted, it is important for at least part of the chuck wall  38  to be parallel to and intimately contacting the side wall  24  so that an interior seal S i  is created that is placed predominantly in shear by internal pressure in the container such as during retort. 
     A further advantage of the container of the invention is its ability to undergo elastic expansion during high internal-pressure conditions such as retort, and then return substantially to its original configuration when the high internal pressure is relieved. This helps alleviate internal pressure and, consequently, the stresses exerted on the chuck wall/side wall interface and the seam. To realize this advantage, of course, the container body must be relatively unconstrained so that it is able to expand radially. 
     The foregoing description focuses on containers having crimp-seamed and sealed metal ends. As noted, however, the invention is not limited to crimp seaming. Alternatively, the metal ends can be double seamed and then sealed via an induction heating or other process. part from the different seam configuration, the double-seamed containers are similar to the previously described crimp-seamed containers. The double seam is characterized by the upper end of the side wall  24  forming a body hook and the curl of the metal end forming an end hook that is interlocked with the body hook. 
     In typical double-seamed containers, a seaming compound is often applied to the metal end in the region of the curl. The seaming compound flows during double seaming so as to fill up any gaps that may exist between the metal end and container body wall in the seam area. Containers in accordance with the invention can be made either with our without conventional seaming compounds. 
     In the foregoing description and the appended claims, references to the container body being “substantially thermoplastic” or the like mean that thermoplastic is the majority ingredient of the container body on a volume basis, and furthermore that any non-thermoplastic ingredient(s) does (do) not impair the ability of the container body to be heat-sealed to a metal end or to expand elastically during retort processing as previously described. For example, a substantially thermoplastic container body can include non-thermoplastic ingredients such as pigments (e.g., titanium dioxide), dyes, or other additives for imparting visual characteristics (e.g., coloration, opacity, etc.) or other properties not provided by the thermoplastic itself. As another example, a container body of composite construction such as paper/thermoplastic or metal/thermoplastic would not be “substantially thermoplastic” (even if the thermoplastic were the majority ingredient by volume) if the paper or metal component impaired the ability of the container body to be heat-sealed to a metal end and/or to expand elastically during retort processing. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.