Abstract:
A heat retaining dish includes a pressure relief mechanism and has a heat retention material capable of being heated by microwave or other thermal radiation in order to maintain any food placed on the dish at an elevated temperature. The heat retention material is capable of accommodating expansion during heating of the device, and when an overpressure condition occurs as a result of inadvertent overheating, the pressure relief mechanism vents the pressure to the ambient environment. The pressure relief mechanism is an integral part of the wall construction of at least one of the portions making up the housing of the device, and deformation due to overpressure directly causes the opening of the pressure relief mechanism as soon as the housing is deformed sufficiently to open a fluid communication path through an aperture in the wall.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a heat retention device for containers and more specifically to heat storage and retention devices capable of absorbing thermal or microwave energy, storing it as latent heat energy in a material disposed in a chamber of the device that is isolated from the food stuffs, thereby maintaining the temperature of the food stuffs at an elevated temperature. 
     2. Background Art 
     Keeping food warm after its preparation and prior to its consumption has long been a desirable goal of food preparers. Especially in more recent times, following the recognition that to be safe, food must be free of bacteria and other unhealthy contaminants, food is required to be kept in a temperature range of less than about 38° F. (for which occasion assignees of the present invention have developed a corresponding construction, see commonly owned U.S. Pat. No. 4,989,419) and above about 140° F., the subject of this invention. Special attention to this problem is required for foods served in restaurants, to patients in hospitals, and other instances when a relatively long period of time elapses between the food preparation and the time the food is served and consumed. Additionally, it is also desirable to maintain the temperature of food that requires delivery over long distances, for example, pizza or take out food. 
     Another instance in which food should be kept warm is when it has been prepared for self service, for example, on an appetizer tray. Here, as the food is consumed over a period of time by persons serving themselves, there is normally a lapse of time between the food being ready and its actual consumption. 
     Microwave ovens have become standard appliances in most kitchens and food preparation areas. They use electromagnetic radiation to heat, in most instances, water molecules contained within food stuffs, and so to cook foods or warm them up for serving. A microwave oven utilizes very short radio waves, the so called microwaves, which are also commonly employed in other standard uses, such as radar and satellite communications. When concentrated and focused into a small volume, microwaves can efficiently heat water and other substances contained in that volume, such as foods. Microwaves generally cook food rapidly and efficiently because, unlike conventional ovens, they only heat, for example, water contained in the food, and no need exists to heat the air or the oven walls. Heat energy then disperses within the food by conduction from the heated water molecules. 
     Microwaves can easily pass through many types of materials, including heat insulating materials, for example, glass, paper, ceramics, and plastics. Containers made of these materials are thus usable for containing food. Various types of dishes are currently available in the marketplace and are adequate for the uses to which these items are required. One drawback to these types of dishes is that the materials from which they are made do not normally retain heat and nothing but the internal latent heat of the food exists to maintain the proper food temperature. After the initial heating in a microwave oven, a relatively large thermal gradient exists between the heated food and the environment, including the container material. Therefore, upon removal of the heated food from the microwave, the heat quickly dissipates from the food and transfers to the ambient environment and to the container, thereby reducing food temperature to below acceptable levels. 
     Past attempts to counteract the tendency of the food in a container to quickly cool include the use of materials that are able to retain some heat energy after the container and food is removed from the microwave oven. These materials are capable of absorbing and retaining the microwave radiation energy and then reradiating or conducting the energy as heat from the heat retention material to the food or to the walls of the food container that are in contact with the food. Such materials have included, for example, quarried soapstone (McCarton et al.; U.S. Pat. No. 4,258,695), wet sand (Sepahpur; U.S. Pat. No. 4,567,877), silicone rubber with entrained ferrite particles (U.S. Pat. No. 5,107,087), earthenware with entrained small iron filings or particles (Ramirez; U.S. Pat. No. 7,176,426), etc. While these and other materials are adequate for retaining heat energy that can be transferred to the food, the materials may not be palatable, and may indeed be unsafe for human consumption. Thus, many of the known food containers enclose the heat absorbing material in a sealed portion of the food warming container, mainly to isolate the food from the heat retaining material as a safety feature, and also to maintain the heat retaining material in place for future reuse. 
     Johnson, U.S. Pat. No. 5,052,369 teaches a heat retaining food container having a cover and a bottom portion, each one of the cover and bottom portion including a heat storage system comprised of a non-metallic heat storing mass enclosed within a sealed chamber. The walls forming the chamber are formed from a polymeric material, such as hard plastic, which is transparent to microwave radiation, is also physically and chemically stable up to approximately 400° F., is chemically stable to detergents and other rinsing agents, and is resistant to staining and discoloration. 
     One disadvantage of the heat retaining containers that are hermetically sealed, for example, such as that taught by Johnson, is that neither the cover nor the bottom portion include a safety valve for escape of gases that may be generated by overpressure during excessive heating of the material in the container. Such temperatures may be far in excess of those to which the container and material would be exposed in normal usage, and may even exceed those that might occur through accidental overheating. 
     When the microwave absorbing material contents is overheated, that is, it is heated beyond the time or power level necessary to achieve optimum latent heat retention, the pressure in a sealed chamber may become excessive and must be accommodated by the structure of the chamber in which the heat retaining material is disposed. Accidental, and sometimes malicious, overheating in a microwave oven, a frequent event, thus normally causes the container to crack, rupture or become permanently deformed. On occasion, the deformation is catastrophic because the high pressures developed in the chamber are contained until such a high pressure is reached that renders the container wall material susceptible to rupture or explosive stress fracture, thereby relieving the overpressure, sometimes in a violently destructive manner. 
     To overcome the risk of loss of structural integrity, some devices have a very robust construction so as to be capable of withstanding high temperature and pressure levels. For example, Murdough et al., U.S. Pat. No. 3,734,077, and Lanigan et al., U.S. Pat. No. 3,837,330, both teach that the danger of bursting is avoided by reason of the configuration and construction of the device, which utilizes a secure interconnection between the upper and lower portions of the shell containing the heated material. 
     Many devices provide means to overcome the destructive capacity of overpressurization of the containers due to overheating by including some pressure relief mechanism. For example, Ramirez U.S. Pat. No. 7,176,426, relies on using a solid, rather than fluid, heat retention material and also on minimizing the volume of air within the chamber by sealing the chamber at high temperatures so as to cause a semi vacuum, i.e., negative pressure. Because the volume of fluid material susceptible to expansion upon heating is minimized, gas within the chamber does not cause excessive pressures when overheated to a reasonable level. Others, for example, Wyatt, U.S. Pat. No. 6,005,233 and U.S. Pat. No. 6,188,053, teach an elaborate and complicated pressure relief system using one or more types of check valves that vent excess pressure built up within the heat storage chamber to the environment. These types of complicated and expensive devices, such as dish carriers (with their corresponding covers), thermos bottles or containers having elaborate check valve systems, are not readily suitable for use in restaurants, hospitals or homes. 
     All of the above described methods for accommodating the overpressure caused by overheating of the microwave absorbing material suffer from one or more problems, including, in some cases, the destructive, that is, irreversible, nature of the pressure relief, or the devices themselves are so complicated that both the construction and manufacturing method for making them becomes cumbersome and/or overly expensive. Alternatively, some devices rely on physical principles or robust construction, with the hope that the person heating the microwave absorbing material will not exceed expected parameters. This hope is not always borne out in reality. 
     While the present invention is described at least partially as a process of heating by microwaves, use of other methods of heating are also possible, for example, induction heating of foods. See, for example, U.S. Pat. No. 7,183,525 to Fuchs and U.S. Pat. No. 7,038,179 to Kim et al. While this invention relies as a best mode of heating that includes use of a microwave oven, it is conceivable that other types of indirect, quick heating may be used and or developed in the future. Thus, the source of heat provided in this invention should be understood to include any form of heating that quickly and efficiently heats up a heat absorbing material, as described below. 
     None of the prior art methods known heretofore teach an easy to manufacture, flexible device that can accommodate internal overpressure by opening a relief valve at the moment that the walls of the chamber begin to deform, and which exact pressure and temperature combination does not depend on preselected parameters, but depends directly on the individual characteristics of the particular device that is being heated. What is needed is a non-destructive, overpressure mechanism for use in a chamber holding thermal energy or microwave absorbing materials, that will relieve the pressure only when the structure defining the chamber begins the deformation process, and as a result of the structural characteristics and materials comprising the walls of the chamber, the chamber can return to its previous state to eliminate such deformation once the materials have cooled and the internal pressure has been reduced. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is provided a food service heat retaining device comprising an enclosure member having a preselected shape, including two opposed portions, a top portion and bottom portion, attached to each other to form a chamber that is sealed at the edges of each portion during the manufacturing process, a heat retention material within the sealed chamber, said heat retention material being capable of being heated by thermal or microwave radiation and of retaining heat in a latent state for at least a preselected time, an aperture in at least one of the opposed portions of said enclosure member and a pressure relief mechanism disposed in the aperture that seals the aperture from the ambient environment external to the sealed chamber when the enclosure member is in a normal condition, wherein at least one of the top and bottom portions of said enclosure member are deformable when the heat retention material has been abnormally or excessively heated by thermal or microwave radiation so as to cause excessive temperature or pressure to deform at least one of said portions and cause the pressure relief mechanism to open a fluid communication path through said aperture, thereby to permit excess pressure within the enclosure member to be relieved to the ambient environment. 
     In the discussion below, when describing the walls of the container as being rigid or semi-rigid, it should be understood that the walls are essentially rigid when the temperature and pressure within the container are at normal operating levels. The rigidity factor of the walls of the container may be a function of the temperature and/or pressure. One main feature of the invention is that the walls are deformable upon abnormal conditions that may develop through accidental misuse of the container, as is explained below. Additionally, while the invention is described as being used in the preferred method by heating with microwave energy, other types of energy are also considered to be capable of providing the same effects. 
     The invention is a container, semi-rigid in form when utilized in a normal manner. The container has walls made of a material that is transparent to microwaves or other thermal energy that is emitted through the walls of the container. The walls of the container can be come flexible and deform, as described below, to provide a means for pressure relief. Thus, it should be understood that when described as being rigid or semi-rigid, the walls of the container may become flexible when the contents of the container are under excessive heat and/or pressure. Additionally, for purposes of this description, while microwave energy is described as the preferred form of energy that is imparted to the material contained in the container, other forms of thermal energy are intended to be encompassed by the description, for example, induction heating energy. Thus, where these terms are set forth in the description, they also should be understood to also refer to the alternative forms. 
     The inventive container has a semi-rigid rigid hollow base or bottom portion that is joined to a top portion to provide a sealed cavity enclosing a microwave or other thermal energy absorbing and heat retaining material. The material may be one of those described as being known in the prior art above, or it may be specially developed for use with the present invention. Ideally, the heat retention material is not in contact with the food and may be in minimum physical contact with the internal walls defining the rigid container. The rigid container and the microwave and absorbing, heat retaining material may have different predetermined shapes, volumes and masses, according to the desired intended use of the heat retention device, but generally the outer shape of the container may take the form of generally recognized tableware, such as plates, bowls, trays, saucers, mugs, cups, etc. The inventive heat retention device can be used individually as an additional element that can be brought adjacent the tableware to which heat needs to be applied, if a solid, or it may be integrated into the structure of the dishes, bowls, trays, coffee mugs, etc., especially if a liquid or gel is used as the heat retention material. 
     In another aspect the invention is a method of manufacture of a food service heat retaining device comprising: providing a top and bottom portion, each portion having corresponding peripheral edges around the periphery of a central heat retention material container, providing a pressure relief mechanism in one of the top or bottom portions, the pressure relief mechanism being biased to a closed position when the pressure and temperature parameters of the device are in a normal condition of use, and being forced into an open condition when there is an overpressure condition, inserting the pressure relief mechanism into an aperture in one of the top or bottom portions so as to plug said aperture, connecting the pressure relief mechanism to at least one of the top and bottom portions, bringing the portions toward each other so that the peripheral edges come into contact with each other and connecting the peripheral edges to each other to create a sealed chamber between the top and bottom portions. 
     In another aspect, the invention comprises a method of manufacture of the device that is relatively inexpensive, and utilizes frictional heat of two plastic surfaces that are vibrated against each other to produce the joining of the bottom and top portions without use of any chemical adhesives or glue that avoids contamination of food stuffs during use of the device. It has been found that a heat retention device made in accordance with the inventive method is capable of withstanding high temperatures and pressures while maintaining its integrity and sealing properties. This has removed the need present in some prior art devices which have required central welding of the portions of the devices to each other, and has also made for a robust construction that can be easily manufactured. 
     The present invention will now be discussed in further detail below with reference to the accompanying figures as described briefly below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a first, preferred embodiment, in a cross-sectional cutaway view, showing a microwaveable heat retention device according the present invention. 
         FIG. 2  is a detail cross-sectional view of another embodiment showing an alternative pressure relief mechanism according to the present invention; 
         FIG. 3  is a detail cross-sectional view of yet another embodiment of the present invention showing an alternative pressure relief mechanism; 
         FIG. 4  is a detail cross-sectional view of still another embodiment showing an alternative pressure relief mechanism according to the present invention; 
         FIG. 5  is a detail cross-sectional view of yet another embodiment of the present invention showing an alternative pressure relief mechanism; 
         FIGS. 6A and 6B  illustrate in a detail cross-sectional view another embodiment of the present invention showing an alternative pressure relief mechanism and the process for venting the chamber when it is in an overpressure condition; and 
         FIG. 7  is a detail cross-sectional view of still another embodiment of the present invention showing an alternative pressure relief mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , a first, preferred embodiment of the heat storage device  10  is shown. The device  10  comprises two main structural elements, a top portion  20  and a bottom portion  50 , each shown in broken cross-section. The top and bottom portions  20 ,  50  are ideally bonded together to form a sealed container or vessel for containing a heat retention material  12  within a chamber  14  defined by the heat retention device. The bottom portion  50  includes a bottom surface element  51 , a peripheral edge  52  and side wall(s)  53 , and provides for a centrally impressed hollow  54 . 
     The top portion  20  further comprises a peripheral edge  22  provided for bonding the top portion to the bottom portion  50 , a food serving surface  24  and a centrally located post  26  having a protruding end  28  that extends essentially perpendicularly from the body of top portion  22  and toward the bottom portion  50 . The post interacts with elements of the bottom portion  50  to provide a pressure relief mechanism  30 , as will be explained below. As shown in  FIG. 1 , a second food serving element  80 , having a food serving surface  82 , and optionally, an upturned flange  84  that provides for an upwardly concave depression for containing food and liquid sauces, etc. 
     The second food serving element  80  is attached to the surface  24  of the top portion  20 , and is in close contact therewith in order to conduct the heat from the heat retention material  12  to the surface  82  of the second food serving element or tray  80 . The second food serving element  80  includes upwardly turned sides  84  that contain the food and possible fluid food stuffs, for example soups or sauce, within the concave bowl shaped member that surrounds the food serving surface  82 . The second food serving element  80  preferably is adjacent to and in close contact with the surface  24  of the top portion  20 , so as to provide good heat transfer form the material  12  in chamber  14  to the food that is being served. While it is contemplated that the materials comprising the device  10  and the second food serving element  80  may be identical, for example, a hard plastic, i.e., melamine resin or melamine formaldehyde, other materials may also be used for each of the elements. The surface  82  or the element  20  may comprise a material having high heat conductivity, for example, a metal, or other, composite material that is transparent to microwaves. As another alternative, it is possible to include a metal only in a detachable food container  80  to provide for the higher heat conductivity, so that the metal does not disturb the operation of the microwave. 
     Another alternative to the integral construction shown in  FIG. 1  is that the two parts may be separable for purposes of reuse. It is not necessary that the food containing part, that is, the second food serving element  80  need be permanently attached to the heat retention device  10 , but it may be detachable for purposes of cleaning, for example, in a dishwasher. A mechanism, for example, clamps (not shown) may be used to attach the second food serving element  80  to the surface  24 , which after the user is done, can detach the element  80  from the device  10 , to enable the cleaning of only the surface that had come into contact with the food. The heat retention device  10  may then be attached to the same or to another food containing element, such as the element  80  shown, and so enable the reuse of the heat retention by microwaving the material  12  therein with newly served food in a clean second food serving element  80 . Of course, the materials comprising the elements  20 ,  80  can also be robust and capable of withstanding the expected abuse that is normally encountered in the process of cleaning the dishes, for example, in a dishwasher. 
     The outer diameter edges, for example, top edge  22  and bottom edge  52  are attached to each other around the complete periphery of the device so as to seal the chamber  14  from the ambient environment in a leak-proof seal when the container is used in normal conditions. The method of attachment is not critical to the structural aspects of this device, and is more germane to the method of making the device  10 , as will be described in greater detail below. For example, the seal may include an adhesive, or one or more additional structural elements (not shown) which create the necessary seal. However, the preferred method of attaching the edges  22 ,  52  of the top and bottom portions  20 ,  50  is by vibration or ultrasonic welding of the edges after they have been brought into contact with each other. Ideally, the welding process produces a seal having sufficient strength that it can remain integral upon exertion of normal pressure build-up within the chamber  14 , whether through overheating or any other cause. 
     Significantly, the seal provided must be able to withstand extremes of internal pressure that may develop upon heating of the heat retaining material  12  contained in the chamber  14 . However, in the event of an accidental, or otherwise, overheating of the material  12 , a pressure relief mechanism  30  is provided in the device  10  in the form of a centrally disposed vent aperture or opening  32  defined by sides  34  of the bottom portion  50 . The pressure relief mechanism  30  may be frangible, where the breaking of the seal during a pressure relief operation is irreversible, or may be a seal that will reform in the event that the pressure returns to normal. 
     The opening  32  is preferably circular and is sealed by an elastic ring plug  40  that covers the aperture  32  in an elastic manner. The use of an elastic material for the ring plug  40  is not an absolute requirement, but is preferred as an expedient manufacturing technique to forgive tolerance differences and also to permit the preferred manufacturing process, as will be explained below. Moreover, although the shape of the device is preferably cylindrical, and the shape of the apertures are also circular for easy manufacturing, it is possible that the device may take any of a number of shapes, such as square, rectangular, octagonal, ovoid and other shapes, and will work equally as well. 
     The aperture  32  is disposed in centrally impressed hollow  54  of the bottom portion  50  which is dimensioned to fit aperture  32  in an indent removed from the plane at which the device will contact a flat surface, such as a table (not shown). The use of a hollow  54  permits resting of the device  10  on such a flat horizontal surface, while still leaving enough clearance for the operation of the pressure relief mechanism  30 . Additional protection from accidental damage to the pressure relief mechanism  30  is provided by a second indent  56 , within the hollow  54 , which accommodates the elements of the pressure relief mechanism  30  and partially encloses them. 
     As shown in  FIG. 1 , the post  26  of the top portion  20  extends toward the pressure relief mechanism  30  and into the aperture  32  defined by the sides  34 . To provide a fairly good seal in the aperture  32 , there is provided the elastic ring plug  40 , having a central throughhole  42 . A flanged section  44  of the ring plug  40  is formed to seal against the bottom of the wall forming the indent  56  and the sides of throughhole  42  will simultaneously also seal against the post  26  adjacent the end  28 . In order to maintain the seal against the wall of bottom portion  50 , a flanged retainer  46  may be used to capture the flanged section  44  and compress it against the wall. To enable a better seal, or to better calibrate the extent to which deformation must occur before opening of a fluid communication path, tubular extensions  48  may be disposed in the internal terminal of the throughhole  42  causing the end  28  of post  26  to travel a greater extent, thus requiring an extra measure of deformation of the walls before activating the venting capability of the pressure relief mechanism  30 . 
     The flexibility of plastic material comprising the walls of the top and bottom portions  20 ,  50  can provide a predetermined amount of tolerance in the device  10  in response to material expansion and the increased pressure of gas within the device, thereby causing the walls to move apart to a slight extent. This separation of the walls will be most pronounced at central locations by virtue of the peripheral connection of the top and bottom portions  20 ,  50  at their respective edges. However, the design of the device includes some tolerance to permit a slight wall separation, but not so great a separation that it will open the vent provided by the pressure relief mechanism  30 . That is, when threshold values of temperature and pressure are reached, the deformation to the plastic walls will be such as to withdraw the end  28  of the post  26  from the throughhole  42  causing the pressure to be relieved and the walls to again come toward each other to recreate the seal. Of course, and under normal use conditions, it is highly desired to have the heat retention device  10  to perform with no physical deformation, the pressure relief only being activated when there is a severe overheating of the material  12 . 
     The heat retention material  12  may be any type of known heat retention material, such as those described above, but in the preferred embodiment, is a fluid, a liquid or a viscous material, such as a gel, under normal ambient temperatures and pressures. It is noted that all matter, including the heat retention material  12 , changes volume when heated, usually expanding with increasing temperature. To accommodate the expansion, a small pocket or gap for air or other gas is allowed within the chamber defined by the walls of the device, which gas is compressible and thus can absorb a slight expansion of the material  12 . It is contemplated that the lack of an air pocket, in other words, the material  12  completely fills the chamber  14 , would exert a greater pressure on the walls and seams bonding the top and bottom portions  20 ,  50  to each other until there is a failure in the seal between them. Even if that were not to occur the first time that the device overheated, the continued cycling between heated and cooled material  12  would eventually force a crack or other opening in the surface of the walls of the device  10 . 
     In a preferred embodiment, the material  12  is a gel made to precise specifications for the particular use with this invention. Ideally, the material  12  has properties that include easy microwavability without damage to the material  12 , an ability to quickly absorb heat from microwaves, and also the ability to retain latent heat absorbed by the material for a predetermined period of time so it can maintain the heat and apply it to the associated chamber and container walls. Since at least one of the walls is in contact with the food stuffs, the food will be kept warm for the duration desired by the user. Moreover, the material  12  cannot be toxic, for if there is a release of the material in an overpressurization event, then the escape of gases that had been in contact with the material that are vented to the ambient environment, for example, the inside of a microwave oven, do not cause irreparable damage. That is, even if the material is not safely consumable, the user may still be able to wipe off the inside of the microwave oven and be able to safely reuse it after an escape of gas form the inside of the chamber  14 . 
     Referring now to  FIG. 2 , a second embodiment of the inventive device is shown and identified by numeral  110 . For purposes of discussion of this and the following embodiments, identical identification numerals will be used for identical elements, and where elements having similar characteristics or functions, the similar numeral, but having a different hundred place number (i.e., the initial digit) will be used. For example, the material  12  in the embodiment of  FIG. 2  is the same, so it has the same identification numeral, but the pressure relief mechanism  130  is different, hence it is designated by the numeral  130 , rather than  30  as in the  FIG. 1  embodiment. Moreover, because the remainder of the alternative devices  110  etc., will be essentially identical, for example in the peripheral connection of the top and bottom portions, the illustrated figures will only show the details of the central portion of each embodiment, including the pressure relief mechanisms, for example, pressure relief mechanism  130 . 
     A similar aperture  132  for venting the overpressure that may arise in the chamber  114  of a second embodiment of the device  110  includes in each of the walls of the top portion  120  and the bottom portion  150  an inwardly concave depression  126 ,  156 , respectively, formed by angled or frustoconical wall sections  128 ,  158 , teach terminating in a horizontally extending terminal wall  127 ,  157 , respectively. The aperture  132  is disposed preferably in the bottom portion terminal wall  157 . 
     By virtue of the device construction, the top and bottom portions  120 ,  150  each cause the respective terminal walls  127 ,  157  to be biased inwardly toward the other terminal wall, so as to cause at least a section of the terminal walls  127 ,  157  to engage and contact each other, thereby forming a seal to inhibit fluid communication between the chamber  114  and the ambient environment. To ensure that a tight seal is formed, an elastic ring  140  may be disposed between the terminal walls or may be attached to the contacting sections of the terminal wall  127 . Although shown in cross-section as an ring to include a throughhole  142 , the elastic seal may take any shape, including a plug that is attached to the bottom of terminal wall  127 , so that if overpressure develops in the chamber  114  that forces the top and bottom portions  120 ,  150  apart, and the terminal walls separate, a fluid communication path is opened to vent the chamber  114  to the ambient environment, for example into the inwardly concave depression  156 . After the overpressure condition is relieved, and the device  110  cools naturally by dissipation of the excess heat, the walls are constructed to once again revert to their previous state, bringing the terminal walls  127 ,  157  toward each other to reform the seal resulting from engagement of the terminal walls. 
     Referring now to  FIG. 3 , another embodiment of the inventive device is shown and identified by numeral  210 . It is in many respects similar to the embodiment of device  110  of  FIG. 2 , including the frustoconical wall  227  and a pressure relief mechanism  230  including the aperture  232 . One significant difference from the second embodiment is the angled walls  258  do not meet the terminal wall  227  of the top portion  220  directly, but angle upwardly toward the surface  224  before angling back to extend from a sharp corner  260  along a second frustoconical wall  262  to end at the terminal wall  257 . The benefits of this construction are two-fold, and include the greater contact area between second frustoconical wall  262  and the wall  228 . In addition, the height of the corner  260  is preselected to be higher than the expected height of the heat retention material  12 , when the device  210  is in a normal horizontal position, as shown. Thus, when the top and bottom portions,  220 ,  250  move apart under an overpressure condition, only gas from the gap  214 , and not fluid from the material  12 , will be vented to the ambient environment. 
     Another difference is the elastic plug  240  between the terminal walls  227 ,  257  has no throughhole, as in the previous two embodiments. Preferably, the plug is in the shape of a disc  240  that is attached to the bottom of terminal wall  227 . In this constriction, separation of the terminal walls  227 ,  257  caused by overpressure will permit venting of the gas in chamber  214  through the fluid communication gap that will open between walls  228  and  262 , past the disc  240  and through the aperture  232  to the ambient environment immediately adjacent comprising the depression  256 . 
     Referring now to  FIG. 4 , yet another embodiment of the inventive device is shown and identified by numeral  310 . It comprises a top portion  320  and a bottom portion  350 . One significant difference from the previous embodiments is that the top portion  320  includes an aperture  338  that is a mirror image of the aperture  332  that is centrally disposed in bottom portion  350 . Each of the portions  320  and  350  include an inwardly extending and overhanging lip, a lip  336  around the aperture  338  and a bottom lip  334  around the aperture  332 . Lips  334 ,  336  face each other and define a gap between them that is plugged by an elastic tubular member  340  extending from inside lip  334  to inside lip  336  The tubular member  340  is sized and dimensioned to seal off the gap by engaging the inside walls of the lips  334 ,  336 . Although the tubular member may be connected to one or the other of the lips  334 ,  336 , such connection is not necessary as the elastic tension of the tubular member may retain the tubular member in place. 
     In the event of an overpressure event, the top and bottom portions  320 ,  350  will be forced to separate from each other, and so cause one or another of the seals at the lips  334  or  336  to open a fluid communication path to the ambient environment. Because of the cantilevered construction with the lips, and to avoid venting of the chamber  314  by accidental opening of the seal provided by the elastic tubular member, it may be advisable to include optional posts  390  that connect the top to the bottom portions  320 ,  350  and extend at discrete points from the central apertures  332 ,  338  to retain the two portions in the desired distance from each other. The posts  390  may have a laterally extending, cantilevered member  392  that will enable the cantilevered portion  392  to snap fit and attach within a receiving enclosure  322  by inserting the cantilevered member  392  into an aperture  324  of the top portion  320  until it locks in place, as shown. 
     Of course, proper lateral placement of the posts  390  within the chamber  314  is essential if the overpressure is to be relieved early in the process. That is, the posts  390  should be far enough away from the aperture  332 ,  338  to permit some flexibility in the walls of top and bottom portions  320 ,  350 , but not so much flexibility as to break the seal between the elastic tubular member  340  and the lips  334 ,  336  during normal use. Another feature provided by this construction, in which a middle portion of the elastic tubular member  340  is unsupported by the rigid walls of the top and bottom portions  320 ,  350 , is that will allow the tubular member  340  to itself deform slightly in response to an overpressure condition and open a fluid path to relieve the overpressure to the ambient environment. 
     Referring now to  FIG. 5 , yet another embodiment of the inventive device is shown and identified by numeral  410 . It is in many respects similar to the embodiment of device  310  of  FIG. 4 , but includes an angled frustoconical wall  428  connected to the top portion  420  as a pressure relief mechanism  430 , including an aperture  432 . 
     The bottom portion  450  includes a flanged side wall  454  that extends from the bottom portion  450  to a terminal point  452  that is adjacent the oppositely facing top portion  420 . Preferably, the height of the terminal point  452  is preselected to be higher than the expected height of the heat retention material  12 , when the device  410  is in a normal horizontal position, as shown. Thus, when the top and bottom portions,  420 ,  450  move apart under an overpressure condition, only gas from the gap  414 , and not fluid from the material  12 , will be vented to the ambient environment. 
     One significant difference of the embodiment of  FIG. 4  is the absence of an elastic plug providing a seal, which is present in the other embodiments. Because the walls  428 ,  454  of the two engaging sections are angled, they form a tight interference fit between the frustoconical surfaces so as to minimize communication of the gas in chamber  414  to the ambient environment in the air hole  432 . 
     Referring now to  FIGS. 6A and 6B , still another embodiment of the inventive device is shown and identified by numeral  510 .  FIG. 6A  shows the device  510  in a normal condition and  FIG. 6B  shows the device  510  in an overpressure condition, during which the pressure is being relieved by venting the chamber  514 . As seen in  FIG. 6B , top portion  520  has been forced to move in the direction of the arrow away from bottom portion  550 . 
     The structural configuration of the device  510  is in many respects similar to the embodiment of device  10  of  FIG. 1 . The bottom portion  550  has a tubular conduit  554  that extends from the bottom portion  550  toward the top portion  520  for a length that is preselected to be higher than the expected height of the heat retention material  12 , when the device  510  is in a normal horizontal position, as shown. The length of the tubular conduit  554  should ideally provide a vertical end  556  that extends to a position between the expected height of the heat retention material  12  and the inner surface of the top portion  520 . It is important to leave a sufficient amount of clearance to permit the operation of the pressure relief mechanism  530  as will be explained below. 
     Device  510  further has a centrally disposed post  526 , including a longitudinal orifice  532  extending through the center of the post  526 , which extends from the top portion  520  toward the bottom portion  550  and in normal assembly is inserted in to the tubular conduit  554 . The orifice  532  opens out of a distal end  528  of the post  526 , that is, from an end that is open to the environment, and has a perpendicular turn to a shorter end  534  extending from the center of post  526  laterally toward the surface of the post ending at a vent outlet hole  536 . 
     When the device  510  is in a normal condition, as shown in  FIG. 6A , the vent hole remains within the tubular conduit  554  which inhibits venting or egress of any material or gas from chamber  514  through the orifice  532  to the ambient environment. However, when the material  12  has been overheated, which produces the overpressure conditions by which the device begins to deform, the top and bottom portions  520 ,  550  begin to separate at the central area shown by the arrow, which causes the post  526  to be withdrawn from the tubular conduit  555  until the vent outlet hole  536  clears the top end  556 . As the vent outlet hole clears the top end  556 , the pressure within the chamber  514  is relieved by outgassing of the air or gas in the chamber that is above the heat retention material  12 . Thus, when the top and bottom portions,  520 ,  550  move apart under an overpressure condition, as shown by the arrow in  FIG. 6B , the material is below the top  556  of the conduit  554  and so only gas from the gap  514 , and not fluid from the material  12 , will be vented to the ambient environment. The central location of the post in this, as well as the other embodiments, also assists in the proper operation of the device  10 ,  110 , etc., in that the post is less likely to bind in one or another direction if the structure is symmetrical. 
     Referring now to  FIG. 7 , another alternative configuration of a device  610  having a pressure relief mechanism  630  is shown. The bottom portion  620  includes a tubular conduit  654  that extends from the bottom portion  620  most of the way to the top portion  650 . A central orifice  656  within the conduit  654  terminates at an upper end  658 . The length of the tubular conduit  654  is preselected to extend above the expected level of the heat retention material  12 , as shown. 
     The top portion  620  includes a recess  624  that is provided by protruding walls  626 , and the dimensions and location of which are preselected to receive a disc  632  made of an elastomeric material having good sealing properties. The disc is shaped and dimensioned to engage the upper end  658  of the tubular conduit  654  and as the device  610  is constructed so as to bias the top portion  620  toward the bottom portion  650 , the upper end  658  will be pressed into and seal against the disc  632 . If there is an overpressure condition in the device  610 , then the top and bottom portions  620 ,  650  will be forced slightly apart as the walls of the device  610  begin to deform, causing the seal to be broken and the gas in the gap of chamber  614  to be vented to the ambient environment. 
     The strength of the walls predicates the valve operation pressure, thus the valve activates only when it is needed. Sufficient flexibility is provided by the structure and materials of the device that cause the deformation of the walls of the top and/or bottom portions to open the pressure relief valve. For example, the rigidity of the container walls can be varied by the wall thickness and by providing one or more rib structures and other geometric arrangements the function of which is to guide the flexibility of the device. It may be desirable to increase the rigidity of the structure so as to allow greater pressure to build up within the chamber, which in turn allows more thermal energy to be absorbed before the valve operates. This may be especially desirable when an increase in temperature or the latent heat capability of the device is desired so as to prolong the heat retention duration. 
     Another significant feature and significant advantage of the present invention is the method of manufacture thereof. Specifically, the two main portions  20 ,  50 ;  120 ,  150 ; etc., of the specific embodiment  10 ,  110 , etc., respectively, of the invention are first formed by an appropriate method, for example, blow molding, injection molding, etc., so as to form the configuration desired for one of the embodiments described above, or its equivalent. To simplify the following description, the preferred manufacturing methods will be described in relation to the embodiment  10  of  FIG. 1 , it being understood that the description is equally applicable to the other embodiments, with appropriate modifications as necessary. 
     The two portions  20 ,  50  are preferably formed from a hard plastic material that maintains its shape under pressure and tension experienced by a device according to the present invention, and which is also permeable to microwaves. The side walls  53  of the bottom portion  50  are made in accordance with a preselected height, so that the depth of the bottom portion  50 , that is the distance between the top of edge  52  to the bottom of the surface  51  is uniform or is defined to accommodate the corresponding height of the pin or post  26  of the top portion  20 . 
     Referring again to  FIG. 1 , the device has a premolded plastic weld plug assembly including the flanged retainer  46  and elastic ring  40 . As shown, the flange section  44  of ring  40  fits within a recess  48  of the flanged retainer  46  to form the unitary premolded plug assembly. The flanged retainer  46  and elastic ring  40  can be bonded to each other, but a unitary co-molded part is preferred where the height of the flanged section  44  is slightly wider than the height of the recess  48 . 
     In the next step of the manufacturing process, the plastic weld plug assembly is attached within the impressed the central impressed hollow  54  so as to cover the aperture  32 . Ideally, the sides  34  of aperture  32  are concentric with the central throughhole  42  so that the throughhole  42  becomes centrally located with reference to the peripheral edges  52  of the bottom portion  50 . The top of flanged retainer  46  is connected to the second indent  56  of the bottom portion  50 . While any appropriate method may be used, it is preferable that a benign connection be made. For example, vibration welding the top of flanged retainer  46  to the underside of the second indent  56 , taking care that the flange section  44  of the elastic ring  40  is between the second indent and the recess  48  ensures that a fluid seal is provided therebetween. This construction also provides for the throughhole  42  to be the only fluid communication through the bottom portion  50 . 
     At this point, a temporary plug may be inserted into the throughhole  42  and a heat retention material  12  is inserted in the bottom portion  50 . If heat retention material  12  comprises a fluid, the bottom surface  51  and side walls  53  contain the fluid, and the temporary plug (not shown) does not permit the material from flowing out through the throughhole  42 . Of course, if the material  12  is a solid or semi-solid at room temperature, no temporary plug may be needed. As shown in  FIG. 1 , the heat retention material  12  is a gel, which may be inserted in the bottom section  50  at this time. 
     The top section  20  is then brought down and the end  28  of the post  26  is inserted into the throughhole  42  to seal the throughhole  42 , simultaneously pushing out the temporary plug, which may then be reused in the manufacture of the next device  10 . The elastic ring throughhole  42  preferably is slightly smaller in diameter than the post  26 , thereby creating a compression seal to be formed between them. 
     The edges  22 ,  52  of the two portions are then brought together until they are engaged around the complete periphery of the portions  20 ,  50 , and the edges are then attached together to create a complete seal to the chamber  14 . The preferred method of attaching the edges  22 ,  52  of the top and bottom portions  20 ,  50  is by vibration or ultrasonic welding of the edges after they have been brought into contact with each other. Vibration welding is a process by which the edges of the top and bottom portions are pressed together and then subjected to vibration at a high frequency so that the friction of the edges in contact and rubbing against each other causes the plastic material to melt locally and weld to each other. This operation uses high frequency, low displacement vibration, which permits the positioning of the parts relative to each other to be more precise. Ideally, the welding process produces a seal having sufficient strength that it can remain integral upon exertion of normal pressure build-up within the chamber  14 , whether through overheating or any other cause. 
     The vertical position or height of the portions is precisely known and repeatable. When the edges  22 ,  52  are brought together, the end  28  of the post  26  preferably extends through and just outside of the aperture  32  clearing the sides  34  of the aperture  32 , but not so far as to protrude beyond the plane of the bottom surface  51 . Thus, the device can be placed on a table or other surface without mishap to the seal. The vibration weld process results in a slightly random final horizontal positioning of the top and bottom portions  20 ,  50 , but the tolerances can be reduced to within +/−0.030 inches (+/−1.0 mm) of axial alignment. Here, the design of the elastic weld plug ring  40  provides a secondary feature of this invention in that the slight variance in the horizontal imprecise positioning of the post  26  relative to the throughhole  42  can be accommodated. The elastic characteristics of the ring  40  permit the use of the vibration welding as a connection operation because the elastic can vibrate with the vibration of the flanged retainer  46  without causing the ring to adhere or otherwise bond to the post  26 . 
     The safety features of the plug assembly is provided by a combination of elements that are each associated to the walls of both the top and bottom portions  20 ,  50  of device  10 . The seal is optimally placed in the center of the device  10 , so that it is centrally located and is susceptible to the greatest amount of deflection in the event of an overpressure event. This location will provide the initial and greatest outward deflection when internal pressure builds up within chamber  14 . If internal pressure is created, the flat walls will move away from each other and withdraw the end of the post  26 , thereby opening the plug to expel fluid from within the chamber  14 , thereby reducing the pressure therein. The fluid expelled will depend upon the position of the external fluid opening that provides communication from the chamber  14 . It is, of course, desirable that the expulsion of the internal contents will prevent the rupture of the primary vibration weld joint at the edges  22 ,  52 , or the secondary plug sonic weld joint between the flanged connector  46  and the second indent  56 . The design described also allows a greater amount of fluid pressure to escape with greater deformation of the device  10 . The rigidity of the device  10  walls dictate the operating pressure of the plug assembly, and when the pressure relief mechanism will open, therefore it is reasonable to strengthen the materials and structure of the walls to provide maximum performance. 
     The preferred embodiment for simplifying the manufacturing process may utilize a temporary plug (not shown) providing the holding capacity of the fluid used as the heat retention material  12 . However, if necessary, an additional fill aperture, not shown, may be disposed in the wall of the upper portion  20  so as to permit more fluid heat retention material  12  to be inserted into the chamber  14 . 
     The invention herein has been described and illustrated with reference to the embodiments of  FIGS. 1-7 , but it should be understood that the features and method of making and of use of the invention is susceptible to substitution, change, modification, or alteration without departing significantly from the spirit of the invention. For example, the dimensions, size and shape of the various elements may be altered to fit specific applications. Similarly, the use of different materials may permit variations in the structure. Since other modifications and changes may be varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the embodiments chosen for purposes of disclosure. Other additional features may be included, for example, a holding chamber or other collection area to receive any expelled gas or thermal retention material so as to prevent the requirement of excessive clean up by the consumer in the event of an overpressure accident. Other features may include a valve having an alert that overpressure conditions are encountered. For example, the valve may be formed in the shape of a whistle, much like a teapot, so that as the pressure approaches a predetermined level, the device emits a sound or other indicator to alert the consumer to shut off the microwave or other heat or thermal energy imparting element. 
     Accordingly, it is intended that this invention include all changes and modifications which do not constitute departures from the true spirit and scope of this invention. Accordingly, the specific embodiments illustrated and described herein are for illustrative purposes only and the invention is not limited except by the following claims.