Abstract:
An induction heatable server comprises a base element having top and bottom elements, said bottom element having a peripheral wall defining an upwardly opening cavity in which are disposed a heat retentive disc and a ring member which is bonded to the peripheral wall. A top element extends over the ring member and seals the cavity. The top element and ring member are bonded to the peripheral wall of the bottom element to preclude moisture penetration into the cavity.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part of Applicant&#39;s application Ser. No. 12/010,768, filed Jan. 30, 2008 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to induction heated servers for heating and/or maintaining the temperature of food in serving plates placed thereon. 
     Food must be served at temperatures above 140° F. to maintain good taste and quality. This is a challenge in food service applications where meals are delivered to remote locations such as in hospitals and nursing homes. Meals are served on china plates or serving dishes in a central kitchen and then transported to patients in their rooms. To keep the food warm, the served food is placed on a special heated “base” or server and then covered with an insulated dome. Typically, the food temperature must be held above 140° F. for up to 1 hour. 
     The special base or server is itself heated by an electromagnetic induction process. The base assembly includes an encapsulated “load” or heat retentive disc which is (1) susceptible to electromagnetic excitation and converts electromagnetic energy into equivalent heat energy, and (2) desirably capable of storing both latent and sensible heat energy at temperatures in the range of 200° F. through 350° F. Prior to placing the food plate on the special base or server, the base is electromagnetically charged with a typical 60,000 joules (57 BTU) of energy. Typically, the charging power is 3,300 watts which is maintained over a typical charging time of 18 seconds. As the base is inductively heated, the load temperature increases to as high as 350° F. In the process the load stores latent heat energy within a relatively narrow temperature range, e.g., between 250° and 350° F. 
     The serving plate with the food thereon is placed on the server and the latent energy stored in the load is slowly transferred to the food by thermal conduction to maintain the food at an elevated temperature above 140° F. The driving force for the heat transfer is the difference in temperature between the load temperature and the food temperature. 
     Immediately after charging the server, the food is typically at a served temperature of 180° F., and the load may reach a maximum post-charging internal temperature as high as 350° F. At the end of 1 hour, the food temperature and load temperature fall to about 140° F. Thus the rate of heat transfer into the food zone is greatest within a few minutes after charging the base when the temperature difference between the load and the food is greatest. As heat is gradually transferred to the ambient environment through the base and dome cover, the latent energy in the load becomes depleted and the load temperature begins to fall along with the food temperature. 
     Current server technology is exhibited by the serving system sold by Dinex International Corporation under the designation Model 511 Smart Therm™. The server or base assembly consists of two molded synthetic resin elements—upper and lower. The load and insulation layer are sandwiched between the upper and lower elements which are ultrasonically welded to each other. The insulation on the bottom of the load substantially prevents heat loss through the bottom element and promotes heat flow from the load into the upper surface of the upper element. It is important to have intimate contact between the load and the inner surface of the upper element. The intimate contact promotes good thermal conduction between the load and the food zone above the upper plastic element. A radio frequency identification (RFID) tag can be incorporated to provide information to the induction charger to prevent overheating of the temperature of the server. 
     Aladdin employs technology in which the load is an inductively susceptible metal plate. A disadvantage of the Aladdin technology is that the thermal storage in the metal plate is 100 percent sensible heat and 0 percent latent storage. The Aladdin system is not capable of storing energy to the same level as the Dinex server without using a heavy and costly metal load that operates at temperatures in excess of 500° F. Because of the high temperature, air trapped inside the Aladdin server expands and creates a high internal pressure. High pressure will also be developed if water penetrates into the induction zone (when the server is washed) through a faulty perimeter seal. A pressure relief valve is provided to vent the high pressure air or moisture that, if not otherwise relieved, could create an unsafe condition. The reliability of the Aladdin system is diminished by failure of the pressure relief valve which itself can result in water infiltration during the washing process. The ability of the base to store heat at high temperature is greatly diminished once water enters into the induction zone because the water acts to remove heat and reduce temperature as heated steam escapes through the relief valve. 
     The 511 Dinex base design is inherently less sensitive to the effects of trapped air. Pressure increase is lessened because the Dinex latent heat load does not require very high temperatures as are required with the Aladdin metal plate load. Thus, a pressure relief valve is not required in the Dinex server. 
     However, the reliability of this Dinex server is compromised by the need to ultrasonically weld the upper and lower plastic elements together. Experience has shown that the weld seam is not reliable, and water can infiltrate the induction zone during washing when the seal is not 100 percent hermetic. After inductive charging, steam escapes through any perimeter leak, thereby venting heat to the atmosphere. When this happens, the server loses its ability to keep the food warm. 
     It is an object of the present invention to provide a novel server construction which effectively precludes water infiltration into the server. 
     It is also an object to provide such a server which eliminates the need for welding the two elements. 
     Another object is to eliminate or minimized trapped air inside the server to preclude the creation of high internal pressure and resultant mechanical deformation of the server when inductively heated. 
     Another object of the present invention is to provide such server that reduces heat losses to the atmosphere and maximize heat transfer into the food zone. 
     Another object is to provide such a server which is readily fabricated and long lived. 
     A further object is to provide a novel method for fabricating an improved server which eliminates the need for welding top and bottom elements. 
     SUMMARY OF THE INVENTION 
     It has now been found that the foregoing and related objects may be readily attained in an induction heatable server comprising a bottom element having a bottom wall and a peripheral wall defining an upwardly opening cavity in which are disposed a heat retentive disc or load. A top element having a dependent peripheral flange extends over the heat retentive disc and seals the cavity. The base element is overmolded with resin about the heat retentive disc and has a generally horizontal flange which extends over a peripheral portion of the top element. As a result, the heat retentive disc is encapsulated and the top element is bonded to the peripheral wall of the base element to preclude moisture penetration into the cavity. 
     The preferred assembly includes a pre-molded ring member upon which the heat retentive disc and top element are seated. 
     Desirably, a layer of insulation is provided in the cavity below the heat retentive disc. 
     The ring member has a body portion and a depending leg portion with an inwardly extending flange at its lower end. The top element has a depending peripheral flange abutting the top and outer side surfaces of the ring member. 
     An RFID tag is sealed in a well in the lower surface of the bottom wall, and vermiculite is provided above the insulation to preclude flow of phase change material into the insulation. The ring member has an inwardly extending flange on which the insulation and disc are seated. The peripheral wall of the base element has an inwardly extending flange extending over a peripheral portion of the top element. 
     The heat retentive server is made by molding in a first mold cavity a generally disc-shaped top element with a depending peripheral flange. The ring member is made by molding in a second mold cavity. The top element and ring member along with heat retentive disc, vermiculite, insulation, and film are then mechanically fastened to form a sub-assembly. This sub-assembly is then placed into a third mold cavity into which molten resin is injected and overflows about the outer periphery and lower surface of the ring member and the top element to form an integral overmolded structure or shell with a top wall, bottom wall and sidewall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric top view of a server embodying the present invention; 
         FIG. 2  is an isometric bottom near thereof; 
         FIG. 3  is a top plan view thereof; 
         FIG. 4  is a transverse sectional view thereof; 
         FIG. 5  is a side elevational view thereof; 
         FIG. 6  is an exploded view thereof; 
         FIG. 6A  is a partial by exploded view thereof; 
         FIG. 7  is a fragmentary sectional view thereof; 
         FIG. 8  is an enlarged sectional view of a peripheral portion of the top element drawn to an enlarged scale; 
         FIG. 9  is a fragmentary perspective view of a portion of the periphery of the top element; 
         FIG. 10  is an enlarged fragmentary cross sectional view the server; 
         FIG. 11  is a top perspective view of the top element; 
         FIG. 12  is a bottom perspective view thereof; 
         FIG. 13  is a top perspective view of the perimeter ring; 
         FIG. 14  is an exploded view of an alternate embodiment of a server embodying the present invention; and 
         FIG. 15  is a transverse sectional view of said alternate embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning first to  FIGS. 1 ,  2 ,  6 ,  6 A and  10 , therein illustrated is a server embodying the present invention comprised of a top element generally designated by the numeral  2 , a bottom element generally designated by the numeral  4 , an RFID tag  68  seated in a well  66 , and a cap  70  sealing the tag  68  in the well. 
     The components which are internal to the assembled top and bottom elements  2 ,  4  comprise a perimeter ring (“P-ring”)  14 , the disc or load  8 , insulation  10 , high temperature resin film  12 , and a layer of vermiculite  22 . These internal elements (and top element  2 ) are fastened as a sub-assembly which is placed collectively in a mold cavity. Molten resin is injected into the mold cavity to overmold the bottom element  4  and encapsulate the internal elements to bond to the top element  2  and P-ring  14 . 
     The new induction heated server design and assembly process was developed to address the limitations of the current base technology. The new server encapsulates the load in an over molding process that eliminates the need to ultrasonically weld top and bottom pieces together. This results in a strong, consistent and highly reliable server that is impervious to water infiltration into the induction heating zone. 
     During the mold cycle, the mold machine operator assembles and places in a mold core a sub-assembly  6  that consists of the previously molded top element  2 , the load disc  8 , vermiculite  22 , the insulation  10 , the film  12 , and a previously molded perimeter ring (P-ring)  14 . The molten resin introduced in the mold cavity bonds to the top element  2  and the P-ring  14  and thereby forms the bottom element  4 . 
     Molten resin is preferably injected at the center of assembly  6  so that the resin fills the mold cavity behind the insulation  10 . The preferred injection point is indicated as  18  in  FIGS. 4 and 6 . A high temperature film  12  is applied over the insulation  10  to protect the insulation from the severe stress caused at the injection point. The film  12  is preferably a high temperature plastic such as polyethylene terephthalate (Mylar) with a typical thickness of 0.004 inch. The film is required to have a significantly higher melt temperature than that of the injected resin and must be capable of surviving the injection process intact to protect the insulation  10 . The insulation  10  may be any number of materials such as various insulating felts, woven fiberglass and woven high temperature plastic. Typically the insulation  10  is 0.125 inch thick in the uncompressed state and it may be compressed to as little as 0.040 inch after being over molded in the second cavity. An optional layer of vermiculite  22  having typical thickness of 0.030 inch may be applied between the load  8  and insulation  10  to provide a further layer of separation between the load  8  and the insulation  10 . This is to prevent the latent polymer components in the load  8  from flowing into the voids in the insulation. The vermiculite acts as a barrier to improve overall insulation performance. 
     During the fill cycle, the high pressure of the molten resin pushes the resin from the injection point  18 , at the bottom center of molded assembly, radially outwardly along bottom wall  24  and then upwardly over the outer perimeter of the top element  2  and into the side wall  26 . It is essential for a strong and complete overmolded seal to be developed at the interface of the molten resin and the perimeter  28  of top element  2 . The high temperature of the molten resin re-melts the surface of the perimeter  28 . Upon cooling, a “welded” structure is developed at perimeter  28  to provide a strong and reliable hermetic encapsulation of the load  8 , vermiculite  22 , insulation  10  and film  12 . 
     To increase the surface area and strength of the “weld” interface at perimeter  28 , a series of vertical ribs  34  is formed on the circumferential surface  28  of the top element  2 . The preferred rib structure is characterized as having an equilateral triangular cross section approximately 0.030 inch at the base. Both surface area and molten resin penetration into the perimeter  28  are enhanced, thus providing increased over mold seam strength and durability. 
     As the molten resin progresses past the perimeter  28  of top element  2 , it flows into the lower portion of the sidewall  26  and envelopes the top portion  28   a  of perimeter  28 . The top and bottom portion of perimeter  28  of part  2  comprises a ring structure  32  that further enhances the overmold melt zone by creating two additional melt planes ( 28   a  and  28   b , respectively). This geometry further enhances the strength of the final overmolded structure by providing a geometry that effectively locks the molded subassembly  6  in place and creates a sealed structure over three different melt planes ( 28   a  and  28   b  plus vertical melt planes  34   a  on multiple vertical rib structures  34 ). Destructive testing has confirmed that this geometry creates a hermetically sealed structure with strength equal to that of the baseline resin, i.e. the same strength as if no weld seam existed. 
     The main purpose of P-ring  14  is to improve the thermal performance of the base  16  through the selective enhancement of insulation and prevention of heat loss through the bottom and sides of the bottom element  4 . Another purpose of the P-ring  14  is to assist in assembling and positioning of the load  8  and insulation  10  inside of top element  2  through creation of a sub-assembly  6  prior to overmolding. 
     The inside diameter of p-ring  14  is approximately 0.100 inch larger than the outside diameter of the load  8 . The sub-assembly of the load inside the p-ring  14  creates a gap  44   a  on the order of 0.050 inch between the OD of the load  8  and ID of the P-ring  14 . Another air gap  44   b  is formed by 3-sides of the U-feature  76  in the P-ring  14 . The P-ring  14  is also designed with a bottom L-shaped feature  36 . When the P-ring  14  is assembled into the first molded element, it can be locked into position through the use of a cam lock feature  38  that engages against tabs  40  on the inside rim of part  2 . Clearance slots  42  are provided in P-ring  14  to allow tabs  40  to pass as the P-ring  14  is initially engaged in the element  2 . Next, the P-ring  14  is rotated so that tabs  40  compress upwardly against cam lock  38 . Thus, the top surface  36   a  of L-shaped feature  36  presses directly against the film  12  and indirectly against insulation  10  and load  8 , and firmly locks the P-ring into position in the subassembly  6 . The bottom surface  36   b  of L-shaped feature  36  is designed with a leading edge angle  58  to direct molten resin flow below and away from the P-ring  14  and into the foot ring  46  formed in the bottom of base  16 . During injection into the mold cavity, molten plastic is unable to penetrate between the Mylar film  12  and L-shaped feature  36 . The gaps  44   a  and  44   b  remain preferentially unfilled. Gaps  44   a  and  44   b  create a barrier to heat loss through the perimeter of load  8  and through the sidewall of base  16 . Furthermore the thickness of P-ring  14  creates a further barrier to heat transfer through the side wall and bottom of base  16 . A preferred embodiment is to fabricate the P-ring  14  from a resin that has lower thermal conductivity than the overmold resin (with resistance to heat flow being inversely proportional to thermal conductivity and proportional to P-ring material thickness). Without the P-ring  14 , undesirably high temperatures and associated heat loss are increased in the sidewall and the bottom of base  16 . The effect of the P-ring  14  in reducing heat loss has been verified using thermal graphic imaging. Bottom temperatures of base  16  under the perimeter of load  8  were reduced from 160° F. to less than 130° F. through application of the P-ring innovation. 
     The overmolding process has the benefit of forcing the removal of air that would otherwise be retained inside the base since trapped air causes significant pressure increase as the base is inductively heated. As molten resin fills the cavity starting at the injection point  18 , air is displaced by the resin and exhausted to the atmosphere external to the mold cavity through small vent passages that are designed into the mold surfaces. 
     While overmolding is very effective in displacing trapped air during the injection step, there remains the challenge of air that remains trapped inside the laminated structure of and is inherent to the construction of load  8 . When load  8  is inductively heated, this trapped air may expand and move to the radial perimeter and escape load  8 . Thermal expansion of solid components of load  8 , combined with air expansion inside the load, causes the load to grow radially when the load is inductively heated. A further benefit of gap  44   a  is to provide 1) room for the radial growth of load  8  and 2) an incremental volume into which air that escapes from load  8  can accumulate. This limits air pressure increase in load  8  that may otherwise cause load  8  to grow in thickness and thereby impart mechanical stresses into and cause top element  2  to crown. As escaped air vents into gap  44   a , the ballast air pressure rises significantly. The mechanical structure provided by the P-ring  14  is strong and can easily withstand the pressure increase without undue mechanical stress. 
     Foot ring  46 , conformally formed as a result of the application of p-ring  14 , provides the further benefit of creating an enclosed air gap  50  between the bottom surface  48  of the base  16  and the surface  52  upon which the base is resting. The gap  50  creates a further barrier to heat transfer from the bottom of base  16 . Foot ring  46  also provides means to stack multiple bases one on top of the other. Without foot ring  46 , the lower outside perimeter  54  at bottom of side wall will interfere with inside edge ring  56  and cause unstable stacking. Foot ring  46  solves this by elevating the perimeter  54  above the edge ring  56 , thereby providing stable stacking support. Foot ring  46  also allows room for convenient placement of RFID well  66  below insulation  10  and inside the space created by air gap  50 . 
     The high pressure of the overmolding process causes the top surface  60  of the load  8  to be pushed against bottom surface  62  of top element  2 . After the assembly is cooled and cured, there is intimate contact and increased contact pressure between the surfaces  60  and  62 . This has the preferential effect of increasing thermal conduction and heat transfer from the load  8  through the wall section  64  into the food warming zone above the top surface  20 . 
     The overmolded assembly  4  is removed from the mold cavity upon completion of the cycle. The assembly  4  is a durable, hermetically sealed, over molded assembly that encapsulates the load, vermiculite, insulation, Mylar film and P-ring. The final assembly process step is to install a radio frequency identification (RFID) tag  68  inside the well  66  on which the RFID tag  68  is sealed by adhering the cap  70  over the well  66 . The RFID tag  68  is used to provide information to the machine that provides the induction charge into the base  16 . The information includes: (1) time duration since last charge and (2) the amount of energy that must be delivered in a full charge. The preferred geometry for the new server includes the encapsulation of the tag  68  in the well  66  through the process of ultrasonically welding the cap  70  over the well  66 . This process is accomplished after molding to protect the RFID tag from the heat and pressure of the molding process that could otherwise damage the tag  68 . The inside well  66  conforms to the geometry of tag  68  so as to centrally locate tag  68  in the well  66  prior to welding the cap  70 . The ultrasonic weld joint  72  is formed using conventional welding technology and provides a very reliable and hermetic seal to protect the tag  68  since the weld joint  72  is impervious to water infiltration from the commercial dish washing cycle. 
     An alternate embodiment of the present invention may be achieved by an assembly in which the bottom L-shaped feature  36  of P-ring  14   a  is lengthened to form a continuous wall section that spans across the inner diametric section of the P-ring, thus changing its geometry from a ring structure with a hollow center to a disc structure with solid center and as seen in  FIGS. 14 and 15 . The lengthened wall section  36   c  of the P-ring is configured to include a foot ring  46   a  which is integral with the P-ring  14   a . The wall section  36   c  is configured to include the RFID well  66  into which the RFID tag  68  can be placed and then encapsulated by welding cap  70  over the well. Vertical ribs  34   c  are also added to the exterior perimeter of the P-ring  14  to facilitate a good bond with the bottom element  4  during the overmolding process. 
     A sub-assembly  6   a  is formed by press fitting the top element  2   a  and P-ring element  14   a  using a using a, mating tongue-and-groove joint  82  provided between the round elements the air gap  44   b . Load  8   a  and vermiculite  22  are encapsulated between elements  2   a  and  14   a  to complete the sub-assembly  6   a . A boss feature  78  is alternatively provided at the center of top element  2   a  to mate with center the hole  80  provided in load  8   a . This helps to precisely center the load  8  inside the sub-assembly  6   a.    
     The sub-assembly  6   a  is inserted into the previously described third mold cavity about which molten resin is injected to form alternate embodiment bottom element  4   a . In this alternate embodiment the injection point is moved to position  18   a  located at the exterior radial perimeter of P-ring  14   a . Multiple injection points  18   a  are preferable about the perimeter of P-ring  14   a  to ensure good filling of the mold cavity. As molten resin enters the injection points  18   a , the resin generally flows upwardly about the exterior perimeter of the P-ring  14   a  and top element  2   a , and it encapsulates the top element  2   a  through the formation of edge ring  56 . This alternate embodiment provides the same benefits of internal air pressure management and heat loss reduction through the side walls provided by the formation of air gaps  44   a  and  44   b  as previously described. It also provides for the creation of melt planes about the perimeter  28  of top element  2   a  and P-ring  14   a  and in combination with vertical ribs  34   b  and  34   c  to create a strong, monolithic and hermetically sealed structure. An incrementally attractive benefit of this alternate embodiment arises from the fact that the injection point is moved to  18   a , and thereby precluding the compression of insulation  10  due to exposure to the high molding pressure. Thus the insulation effectiveness is increased and heat loss through wall section  36   c  is greatly reduced. 
     As is well known, the load may comprise, but is not limited to, a metallic disc, a composite structure consisting of 1) electromagnetically inductive materials such as metal foil, metal particles, and graphite, and 2) materials that undergo phase change in the desired temperature range of 160 F to 200 F such as wax or low melting point plastic such as polyethylene. The composite structure may be homogeneously structured or laminated into a bonded, monolithic assembly. 
     As is well known, the RFID tag can provide information concerning the inductive energy charge required and the inductive heating history of the server to a microprocessor which can actuate or terminate the operation of an associated charger to bring and/or maintain the server in a desired temperature range. 
     The P-ring provides a platform upon which the previously molded top element, load, insulating material and film may be supported in the mold cavity prior to and during the overmolding process which encapsulates the various components to provide a monolithic structure. The toothed configuration in the perimeter of the top element strengthens the bond between the perimeter of the top element and the body of the resin which is molded thereover. 
     Upon removal of the assembly from the mold, the RFID chip or tag is inserted into the cavity molded in the bottom surface and a cap is secured thereover by ultrasonic welding, or a high temperature adhesive. 
     The plastic resins employed in the molded bottom element  4  must be suitable for withstanding up to three daily hot water wash cycles and to withstand various detergents and rinse agents. Suitable resins include various grades of thermoplastics including polypropylene, nylon, polycarbonate, and polyethylene terephthalate. The resins may be blended with fillers such as glass beads, fibers and talc to enhance durability. Also secondary resins may be added as an alloy or in a blended material to enhance strength and resistance to the hot dish water, detergents and rinse agents. For example a blend of polycarbonate and polybutyl terephthalate (PBT) is deemed a desired alternative to enhance the properties of polycarbonate. 
     In the preferred embodiment, the P-ring resin should have a melt temperature above the melt temperature of the overmold resin to ensure that the P-ring does not soften and maintains physical integrity during the high temperature and pressures experienced in the overmolding process. For example suitable materials for the P-ring are filled and unfilled grades of polypropylene, polycarbonate, and nylon. Higher end engineering resins such as polysulfone and polyetherimide are also suitable candidates. In the alternate embodiment, the resin used to mold the P-ring and the bottom elements is preferably the same. This is to ensure that the P-ring perimeter surface melts during the overmolding process to create a strong bond between the P-ring and the overmolded bottom element. 
     Thus, it can be seen that the structure and method of the present invention provides a novel, long-lived server which can maintain food in a dish placed thereon within a desired temperature range for an extended period of time.