Abstract:
A dual wall cooking vessel is formed by the impact or friction bonding of the an inner to an outer vessel wherein a laminate of aluminum and copper layers is disposed between the outer surface of the bottom of the inner vessel and the inner surface of the bottom of the outer vessel. The aluminum layers are arranged to surround the copper layer of the uppermost aluminum layer being the upper aluminum layer being thinner than the lower aluminum layer and having a slightly smaller diameter than the copper and aluminum layer. The appropriate dimensions of the aluminum layers and sequence of welding and bonding operation results in the co-extrusion of both aluminum layers into a portion of the adjacent sidewall formed by the gap between the walls of the inner and outer vessel. This co-extruded layer s of aluminum within the side walls and the bottom of the vessel improves the heat transfer from the outer vessel to the inner vessel during cooking, but without significantly diminishing the insulating properties of the dual wall vessel that serve to keep the food warm while it is being served.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to and is a division of the U.S. Patent Application for a “Double Wall Cooking Vessel” having Ser. No. 10/766,221, filed on Jan. 28, 2004 now U.S. Pat. No. 7,097,064, which is incorporated herein by reference. 

   BACKGROUND OF INVENTION 
   The present invention relates to improved cooking vessels, particularly to double wall cooking vessels. 
   Double wall cooking vessels have a solid bottom surface and a pair of concentric co-axial sidewalls separated by an air gap there between. The double wall construction provides insulation so that the food stays warm after cooking, permitting the same cookware to be used as serving ware at the table. 
   Also known in the art is “waterless cookware”, that is a cooking vessel with a self-sealing lid so that a minimum of water is used to cook the food, with the steam generated from the added water and the foodstuff itself is retained, rather than lost through the gap between the vessel&#39;s rim and cover. The extreme example of “waterless cookware” is a pressure cooker, in which a pressure containing cooking vessel has a match lid that locks to secure a gasket between the rim and the lid. The lid must have a pressure release valve, lest the internal pressure cause a violent explosion of the vessel. The other form of “waterless cookware” involves a pot or vessel rim that extends outward from the vessel&#39;s perimeter to provide a slightly concave region where steam can condense between the extended rim and the matching lid, thus forming a “water” seal in placed of the rubber gasket in the pressure cooker. The mass of the lid serves as a “release valve” preventing excess pressure within the confined volume that holds the foodstuff. Both forms of “waterless cooking” are popular as they offer a superior method of preserving vitamins, nutrients and natural flavors, creating a more pleasing an uniform texture to the cooking food than microware methods. 
   Double wall cookware however has certain disadvantages. The contained wall must be sealed from water for the expected lifetime of the product, as any water that enters or seeps in during use or washing presents a hazard when covert to steam during cooking. Thus the cookware is difficult to manufacture, as well as costly. 
   Dual wall cookware also suffers in performance relative to single wall cooking vessels, as the outer surface near the bottom of the vessels is easily overheated during cooking, being insulated from the remainder of the vessel. This rapidly leads to discoloration, and distortion under extreme conditions, making the cookware unattractive for use at the table, or display in the kitchen. 
   Accordingly, there is a need for an improved dual wall cooking vessel and method of making the same that overcomes the aforementioned disadvantages, and in particular making the vessel suitable use a “waterless cookware”. 
   It is therefore a first object of the present invention to provide an improved construction for dual wall cookware. 
   It is a further object for providing a reliable and cost effective method of making such an improved construction, that results in a complete an secure seal at the rim where the inner and outer walls meet. 
   It is a further object of the invention that the securely sealing rim be suited shaped so that the vessel may serve as waterless cookware with the appropriate matching lid. 
   SUMMARY OF INVENTION 
   In the present invention, the first object is achieved by constructing the dual wall cookware in a manner that the lower portion of the dual adjacent the bottom of the pan is filled with a thermally conductive material. 
   Another object of the invention is achieved by filling the lower wall portion with aluminum during the forming of the pan and attachment of a thermally conductive bottom deployed for generating a uniform temperature profile over the interior bottoms that serves as the cooking, or foodstuff contact surface. 
   The object of achieving a suitable rim for waterless cooking is to align and weld the bottoms of the inner and outer vessels, that form the dual walls, together before friction bonding them together. This results in the precise alignment of the a previously formed inner and outer rim portions that can be consistently welded together to form the water tight seal between the inner and outer wall. 
   The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1A  is a cross-sectional elevation of a first embodiment of a dual wall cooking vessel.  FIG. 1B  is an expanded view of a portion of  FIG. 1A . 
       FIG. 2A  is a cross-sectional elevation of the bottom of the vessel showing the inner and outer pans as well as the materials used to form the thermally conductive bottom prior to friction bonding.  FIG. 2B  illustrates the same region after friction bonding. 
       FIG. 3A–3G  illustrate via a sequence of cross-sectional elevations the steps used to construct the dual wall cooking vessel of  FIG. 1 . 
       FIG. 4  is a plan view to further illustrate a preferred method of conducting the step of sealing the inner and outer wall rim portions by welding, corresponding to  FIG. 3F . 
   

   DETAILED DESCRIPTION 
   In accordance with the present invention,  FIG. 1  illustrates . . . thermally conductive material is interposed between inner pan  135  and outer pan  125  encompassing the bottom  200  of vessel  100 . However, by fabricating the vessel  100  according to the teachings of this invention the thermally conductive material extends upward to fill the lower portion of the cavity  105  separating the outer surface  130   a  of the upright wall  130  of the inner pan  202  and the inner surface  120   b  of the upward wall  120  of the outer pan  201 . As illustrated in the expanded view in  FIG. 1B , marked as A, the thermally conductive material in this preferred embodiment comprises at least three layers of materials. The first layer  150  is in contact with the outer surface  135   a  of the inner pan, having the opposing side in contact with a middle or second layer  150 . The other side of the middle layer  150  is in contact with the a first surface of the third layer  160 , the other surface of layer  160  being contact with the inner surface  125   b  of the outer pan. As will be further illustrated with reference to  FIG. 2 , the middle layer generally does not extend upward into the cavity  105 , thus layers  150  and  160  are connected over the extent of the cavity  105  which they partially fill, terminating at an edge  210 , having a common interface therein  206 . Layers  150  and  160  are preferably aluminum, or a suitable allow thereof, and surround a middle layer  150  comprising copper or a suitable allow thereof. The middle copper layer, being more thermally conductive than the surrounding aluminum layers transfer heater laterally from layer  160 , such that the temperature across the inside bottom surface  135   b  of the inner pan  202  is uniform for cooking foodstuff, thus accommodating a range of heating methods and burner or flame configures used to heat the vessel  100  from the bottom of surface of the outer pan  125   a.    
   Referring back to  FIG. 1A , the cooking vessel has an upper rim  102  formed at the termination of the edge  103  of the outer upper wall  120 , with edge  104  of the inner upper wall  130 . Edges  104  and  103  are preferably welded together during fabrication to prevent water from seeping in or entering cavity  105 . The heating from cooking would rapidly vaporize a small quantity of water trapped in cavity  105 , which may present a hazard or damage the vessel  100  in escaping rapidly therefrom. Further, edge  104  flairs outward in a substantially horizontal direction before terminating at the contact point with upper end of the inner wall  130 , thus forming a sealable surface for receiving lid  110 . Lid  110  has a domelike central region  112  terminating at its periphery with an edge  115  that conforms to the shape of rim  104 . A slight upward facing concavity in rim  104  provides for the collection of condensed moisture therein, thus providing a sealing liquid between rim  102  and lid  110  to form a so called “waterless” cooking vessel. Lid  110  is illustrated as including an optional handle or knob  166  for ease of placement and removal from vessel  100 . It should be noted that the outward extending flair of rim portion  104  also approximately defines the width of cavity  105 , as wall section  103  extends in the substantially vertical direction where it intersect rim  104  at edge  102 . Dual wall cooking vessel  100  also preferably includes one or more handles (not shown) disposed on the exterior side surface for grasping during cooking or serving. 
   The method and result of friction bonding the inner and outer vessels is illustrated by the schematic expanded view of  FIGS. 2A and 2B , which corresponds to region B in  FIG. 1 . Initially an aluminum plate  160  is disposed on the bottom surface  125   b  of the outer vessels  125 . A copper layer in the form of a sheet or plate  140  is disposed on top of aluminum plate  160 . A second aluminum plate  150  is then disposed on top of copper plate  140 . Finally, the outer surface  135   b  of the bottom of vessel  201  is disposed on top of aluminum plate  150 . As the copper plate  140  has a series perforations or holes to enhance the attachment with the surrounding aluminum plates  150  and  160 , which are illustrated as a series of gaps  145 . 
   As will be further described with respect to  FIG. 3 , upon impact or friction bonding of the assembly in  FIG. 2A  the gaps  145 , caused by perforations in copper plate  140 , are filled as the upper surface of aluminum plate  160  has become bonded or welded to the lower surface of aluminum plate  150  at interface  205 . Both the upper  150  and lower aluminum plate  160  have are essentially welded or fused to the surrounding stainless steel layers  125   b  and  135   b  respectively by the friction bonding process. Both aluminum plates  150  and  160  are reduced in thickness due to the lateral flow caused by the impact bonding, the upper aluminum plate  150  is reduced in thickness more than the lower plate  160 . 
   The preferred sequential steps used to construct a dual wall vessels from the two single wall vessels is illustrated in  FIG. 3A through 3G , inclusive.  FIGS. 3A and 3B  merely illustrate that the inner vessel  201  and outer vessels  202 , which are initially formed of stainless steel by a drawing operation that shapes the inchoate rims  104  and  103  in shaping the upper portions proximal to the open end of each vessel. 
   In  FIG. 3C  the previously described assembly of the lower aluminum plate or layer  150 , copper layer  140  and upper aluminum layer  160  are spot welded via electrodes  301  (disposed on the inside of the vessel  201 , and electrode  302 , contacting bottom of the lower aluminum layer  150 , the assembly of layer being aligned with the center of vessel  201 . Preferably, each of the aluminum plates and copper plate are substantially circular corresponding to the shape of the bottom of vessel s  201  and  202 , however the upper aluminum plate  150  in addition to being about half the thickness of aluminum plate  160  in this preferred embodiment also has a smaller diameter owing to its greater propensity to flow during impact bonding process illustrated by  FIG. 3E . 
   However, prior to impact bonding of the inner and outer vessels to the intervening aluminum copper layers, as shown in  FIG. 3D , it is also preferable that the inner vessel  201  and outer vessel  202  are carefully co-axially aligns such that the inchoate rim  103  of outer vessel  202  is in contact with the inchoate rim  104  of inner vessel  201 . This assembly is then stabilized by spot welding at the center of the bottom of vessels  201  and  202  a shown by the presence of inner electrode  301 ′ and the outer electrode  302 ′. Thus the inner vessel  201  and outer vessel  202  is attached at the centers of their respective bottom portion  135  and  125  to aluminum later or plate  160 , copper sheet  140  and aluminum plate  150 . 
   In the step portrayed by  FIG. 3E  the inner and outer pans are impact or friction bonded to each after first pre-heating the assembly  300   e  to about 500° C. o, after which a forming mandrel contacting the inner bottom surface  135   b  is accelerated by a driven mass downward toward the support under the bottom surface  125   a  of vessel assembly  300 E. As the aluminum layer having the lowest melting point of the material in the assembly and have been preheated to about 80% of its melting point, the friction and heat generated by the sudden impact causes the flow and fusion of the intervening aluminum layers to each other and the remainder of the contacting layers of the vessels not previously welded together to form strong bonds there between. 
   It should be noted in  FIG. 3C  that as upper aluminum layer  150  has a narrower diameter than both the copper layer  140  and the bottom aluminum layer  105  such that the force applied by the friction or impact bonding process results in a proportionately higher compressive stress on layer  150 , thus causing it to extrude laterally and upward into cavity  105 . As lower aluminum layer  160  also flows into cavity  105 , generally surrounding and embedding copper layer  140 , its flow terminates at substantially the same height as extruded aluminum layer  150  about the air-metal interface labeled  210  in  FIG. 11B . Not wishing to be bound by theory, it is believed that the initial flow of layer  150  eventually equalizes the stress on both layers causing them to flow together into cavity  105 . Also not wishing to be bound by theory, it is further believed that the initial and greater extrusion of layer  150  serves another purposes in that it facilitates the initial fusion bonding of layer  160  to the stainless steel bottom  125  at interface  125   b , further stabilizing the friction bonding and flow of the other layers in a uniform and repeatable manner. As the fusion or friction bonding occurs in less than a fraction of a second the actual manner and operation of the invention is not certain, and hence was not readily predictable. 
   After impact bonding as described with respect to  FIG. 3E , the rim of the pan is formed in the steps illustrated by  FIG. 3F  and  FIG. 4 . In the first of a sequence of two steps, the now aligned and contacting inchoate rims of the inner  104  and outer wall  103  as welded by the electrode assembly and process illustrated further detail in  FIG. 4 . Counter rotating electrodes  410  and  420  substantially conform to the external shape of the inchoate rim surfaces formed during the drawing processes in the internal vessel  210  and external vessel  202  Illustrated in  FIGS. 3A and 3B . Thus, complimentary shaped electrodes  400  and  420  rotating about their respective spindles  411  and  423  grasp the mating rim portion causing the rotation of the bonded assembly (which will form double wall vessel  100  shown in  FIG. 3G ) about its central axis  431 , thus exposing the entire periphery of the rim to the welding electrodes  410  and  420 . Therefore the entire periphery of the contact wall edges that form surface  103  and  104  in  FIG. 1  are welded together. The welding operation thus seals cavity  105 . In the second step, illustrated in  FIG. 3F , the final rim shape of vessel  10  is formed by a circular cutting tool  310  that follows around the upper end of outer wall  120  of vessel  202  trimming an annulus through the weld to form the top edge  102  illustrated in  FIG. 1 . The thus completed double wall vessel  100  is illustrated in  FIG. 3G . 
   It should be appreciated that the aluminum layers  160  and  150  are optionally laminates of multiple layers of thinner aluminum sheet with the outer layers being selected for their ability to adhere to stainless steel, copper, the adjacent aluminum layer encountered between the gaps in the copper sheet, or alternative materials used to formed the inner and outer vessels, or a substitute heat transfer layer for the copper sheet. In a preferred embodiment the lower aluminum sheet  160  is constructed of three layers of aluminum in which aluminum alloy 3003 is surrounded by layers of aluminum alloy 1050 to provide a total thickness of 6 mm. The outer aluminum layers in this laminate preferably have thickness of about 0.2 to 0.3 mm. The upper aluminum layer  150  is similarly of a three layer construction with aluminum alloy 3003 being surrounded by sheets of aluminum alloy 1050, however the initial thickness is preferably less, or about 3.5 mm. This construction is preferred as the 3003 aluminum alloy is harder than the surrounding 1050 aluminum alloys. However, it should be appreciated that the other metals may be substituted for the inner layer of 1003 aluminum layer. The copper layer preferably has a thickness of about 0.6 mm before impact bonding. The holes or gaps in the copper layer are preferably of a diameter of about 2 to 10 mm and cover less than about 30% of the area of the sheet. After impact bonding the upper aluminum layer  150  is reduced in thickness from its initial value of about 3.5 mm to about 1.5 mm. The lower aluminum layer or plate  160  undergoes a more limited reduction of thickness, from the initial value of 6 mm to about 3 mm. The copper layer is only slightly deformed from about 0.6 mm to 0.5 mm. The surrounding inner and outer vessel walls if fabricated from stainless steel do not undergo a substantial change thickness upon impact bonding, retaining their initial thickness of about 0.5 mm. Although the copper layer is preferably of comparable dimensions to the bottom of the inner and outer vessels, it may also extend into the cavity  105  there between, as it can be initially fabricated in a bowl like shape to conform to the intended cavity shape or, being significantly thinner than the surrounding aluminum layers, is readily deformed from a plate into a bowl like shape as the inner and outer vessel are nested together in  FIG. 3D . 
   It should be appreciated that the outer surface of the outer vessel can have cladding or decorative layers outside of the stainless steel, for example one or more layers of external copper cladding optionally extends partly upward corresponding to the portion of the cavity that is filled with the aluminum layers during fusion or impact bonding. Such a contrasting external layer also serves a non-decorative function of alerting the consumer to the distinct thermal characteristics of the bottom portion of the pan, as opposed to prior art dual wall cooking vessels. 
   While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.