WO2011163217A1

WO2011163217A1 - Hermetically encapsulated electric heater

Info

Publication number: WO2011163217A1
Application number: PCT/US2011/041233
Authority: WO
Inventors: Brian C. Jones; Austin Carosello; Brian C. Biller; Jeff Bernthisel
Original assignee: Egc Enterprises, Incorporated; Seitz Corporation
Priority date: 2010-06-21
Filing date: 2011-06-21
Publication date: 2011-12-29

Abstract

A heating apparatus includes a plate having a first surface and a second surface defining for receiving an. object or foodstuffs to be heated. An electrically conductive sheet is hermetically sealed to the first surface of the plate. The conductive sheet is electrically connected to a power source for selectively energizing the conductive sheet to provide direct heat to the second surface of the plate in order to heat the object received on the second surface of the plate.

Description

HERMETICALLY ENCAPSULATED ELECTRIC HEATER

Related Applications

This application, claims priority to U.S. Provisional Application No. 61/356,763, filed June 21 , 2010, the entirety of which is incorporated herein by reference.

Technical Field

The invention relates to electric resistance heating devices and, in particular, relates to a hermetically sealed, flexible graphite heating element for heating objects within a container.

Background

Electric resistance heating technology spans a wide range of applications, methods, and configurations. Resistance heaters are made from various conductive materials including resistance wire, metalized polymer films, conductive fibers, graphite, metal foil, conductive inks, etc. These conductors are often required to be insulated by a dielectric or non-conductive material to protect the heating element and help to prevent unsafe exposure of high voltage presented to the conductive heating element. Whiie many heater types and configurations are available, all resistance heaters have the common element of directing an electric current through a resistive circuit to produce heat. There can be significant differences in geometry and materials depending on the application requirements, resulting in specific advantages and disadvantages of each heater type. Typical application requirements may include heating rate, i.e., time to temperature, maximum temperature, and minimum

temperature, physical requirements such as heater flexibility, abrasion, UV and chemical resistance, heat flux uniformity, overall dimensions, applied voltage, and cost. The heat flux or heat transfer rate per unit of heater surface area is usually custom selected to account for thermal losses, operating environment, control method, and the thermal properties of the media being heated.

Many heating applications require relatively low watt density, e.g., generally less than 5 watts per square inch, in a variety of markets including but not limited to medical, food/beverage service, and commercial appliances. For example, one such application in the food, service field involves the need to keep prepared, food, warm in heated food trays at a desired food temperature maintained between approximately 140° F and 180° F. Specific applications include self service buffet tables, fast food restaurants, and food court kiosks.

In fast food restaurants prepared food is often pre-cooked and then placed in food, trays. The food trays are then placed into portable ovens to keep the food tray in a warm environment. Buffet tables may use portable flames placed under food trays to keep the food warm. The open flame is too hot for direct transfer into the bottom of the food tray and will burn the food. Instead, a portable flame is placed under a second tray that contains a hot water bath. The food tray itself is then immersed in the hot water bath to provide indirect heating and prevent the food from being over heated.

Since it is desirable to uniformly heat the food within the food tray, it is preferable that the portable flame, hot water bath, and food tray cooperate to provide uniform heat flux to the food.

Another potential application exists in potable water heating. High watt density heaters are known to cause elevated heater surface temperatures exposed to the water.

The elevated surface temperatures cause precipitation of minerals contained in the water, thereby causing hard scale to develop on the heater surfaces. The hard scale causes the heater core temperature to rise, resulting in premature failure of the heating elements. Low watt density water heating results in reduced heater surface temperature, reduction in hard scale formation, and improved heater life.

Another example of the need for low watt density heating is in the medical field wherein refrigerated blood bags require warming prior to the blood being infused into the patient. A heated tray can be integrated with a low watt density resistance heater. The blood bag may be placed in the heated toy to accelerate the warming process without overheating the blood, which could otherwise damage the blood.

Another potential application for low watt density heating may reside in sleep apnea devices wherein liquid water must be gently heated and vaporized to add humidity to the patient's air supply. Another related application may involve the vaporizing of anesthetics to keep patients sedated during surgery. From the above, it is clear that there are many existing and potential applications for low watt density heating for the heating of solids, liquids, and gases in a wide array of markets. One particular type of electric resistance heater that uses resistance paths fabricated from thin layers of flexible graphite is the Q-Foil^® heater (Manufactured by EGC Enterprises of Chardon, Ohio). A more detailed explanation of pertinent Q-Foii® technology may be found in U.S. Patent No. 3,719,608 related to an oxidation resistant graphite composition, U.S. Patent No. 4,250,397 related, to heating elements and methods of manufacturing thereof, U.S. Patent No. 4,490,828 related to electric resistance heating elements and an electric resistance furnace using the same as a heat source, and U.S. Patent No. 5, 198,063 related a method and assembly for reinforcing flexible graphite, the entirety of which are incorporated by reference herein.

In a typical graphite heating element, the graphite element is normally encapsulated between layers of temperature resistant polymers. Adhesive layers may also be added so that the thin-film heating element assembly may be permanently held together and affixed on the surface desired to be heated. Electrically conductive terminations are provided at the beginning and end of the graphite element circuit to facilitate connection to an external voltage source.

The thin-film element heating system is particularly effective in providing gentle, even heating of a surface with relatively low watt density ranging upwards to a practical limit 6 watts per square inch. The thin-film element may be designed for a range of input voltage as needed for good performance and safety. Although a voltage of up to 240V AC may be used, a desirable voltage range may be approximately 24 to 1 10 volts AC or DC, which is low enough to provide a reasonable balance of current and power in the Q-Foil^® element. The wide graphite conductive pathways provide even heating and. uniform heat flux (or watt density) over the entire heated surface area. This results in the transfer of thermal energy at lower element temperatures than heaters with non-uniform heat flux wherein there may be locally high heat flux on the heated surface. A locally high heat flux can result in excessively high surface temperature that may damage materials adjacent to the heater, the media being heated or even the heater itself.

The appl ication of thin-film elements to a heating surface by means of adhesives or mechanical means, e.g., clamping, is well established. Thin-film elements, however, have limited durability and therefore must be protected from the harmful application conditions or environments that may damage the heater. Mechanical damage of a thin-film element heater can expose the conductive element to the environment, which may cause a safety hazard due to high voltage exposure or concerns with cleaning the device to which the thin-film element heater is attached (vis-a-vis the heated food tray). This may limit the number of feasible or practical applications for unprotected thin-film element heaters.

It is possible to encapsulate the thin-film element using epoxy encapsulation or gluing a protective cover over the thin-film element. Current encapsulation methods, however, are costly, time consuming, and limited in their protection capability. For example, in the heated food tray application, conventional encapsulation of the thin- film element heater is deemed unreliable to withstand service conditions. For example, a routine dishwashing cycle can expose the encapsulant to superheated water and to harsh chemicals several times a day. The heated trays may also be impacted or dropped in routine handling. Such environmental exposures may cause failure of the encapsulant and allow water penetration into the heating element and ultimately cause failure of the heater.

The electric heating device 20 of the present invention is advantageous over prior heater devices in that it overcomes the limitations of current heater encapsulation methods by 1) reducing cost, 2) increasing product reliability and durability in extreme environmental conditions, 3) providing increased flexibility to add custom engineered features to the encapsulated heater assembly, 4) improving thermal efficiency and reducing energy usage through more intimate contact with the object or fluid being heated, and 5) improved method and control of said electric heater.

Summary of the Invention

In accordance with an aspect of the present invention a heating apparatus includes a plate having a first surface and a second surface for receiving an object or foodstuffs to be heated. An electrically conductive sheet is hermetically sealed to the first surface of the plate. The conductive sheet is electrically connected to a power source for selectively energizing the conductive sheet to provide direct heat to the second surface of the plate in order to heat the object received on the second surface. in accordance with another aspect of the present invention a heating apparatus includes a container having an outer surface and an inner surface defining a cavity for receiving an object to be heated. A heater assembly includes a thin-film heater, first and second temperature resistant polymer layers secured to the thin-film heater, and a pair of terminals connected to the thin-film heater for electrically connecting the thin- film heater to a power source. A cover is molded over the heater assembly to hermetically seal the heater assembly to the outer surface of the container.

In accordance with another aspect of the present invention a heating apparatus includes a plate having a first surface and an opposing second surface for receiving an object to be heated. An electricaily conductive sheet is hermetically sealed to the first surface of the plate. The conductive sheet is electrically connected to a power source for selectively energizing the conductive sheet to provide direct heat to the second surface of the plate in order to heat the object received on the second surface.

In accordance with another aspect of the present invention a method of forming a heating apparatus includes providing a p!ate having a first surface and a second surface for receiving an object or foodstuffs to be heated. An electrically conductive sheet is positioned on the first surface of the plate, the conductive sheet being eiectrically connected to a power source for selectively energizing the conductive sheet to provide direct heat to the second surface of the plate in order to heat the object received on the second surface. A cover is overmolded onto the plate to hermetically seal the electrically conductive sheet to the first surface of the plate.

In accordance with another aspect of the present invention a heating apparatus includes a cabinet having a plurality of individual slots. Each slot includes one or more exposed terminals and is configured to recei ve corresponding terminals on a food tray to be heated. A power source is electrically connected to the terminals in each slot for selectively energizing the terminals in each slot to provide direct heat to each food tray.

Other objects and advantages and a fuller understanding of the invention will be had from the following detailed description of the preferred embodiments and the accompanying drawings.

Brief Description of the Drawings

Fig. 1 is a schematic illustration of a heating device in accordance with an embodiment of the present invention;

Fig. 2A is an exploded, assembly view of a portion of the heating device of

Fig. 2B is a partial assembly view of the heating device of Fig. 1 ; Fig. 3 is a schematic illustration of a heater assembly of the heating device of

Fig. l ;

Fig. 4A is a schematic illustration of a portion of the heater assembly of Fig. 3; Fig. 4B is a side view of the heater assembly of Fig. 4A;

Fig. 4C is a bottom view of the heater assembly of Fig. 4 A;

Fig. 4D is an enlarged view of a portion of Fig. 4B;

Fig. 5A is a schematic illustration of the heating device of Fig. 1 ;

Fig. 5B is a bottom view of the heating device of Fig. 5 A;

Fig. 5C is a sectional view of Fig. 5B taken along line 5C-5C;

Fig. 5 D is an enlarged view of a portion of Fig. 5C;

Fig. 5E is an enlarged view of a portion of Fig. 5C;

Fig. 6A is a heating device in accordance with another aspect of the invention; Fig. 6B is a top view of the heating device of Fig. 6A;

Fig. 6C is a sectional view of Fig. 6B taken along line 6C-6C;

Fig. 6D is an enlarged view of a portion of Fig. 6C;

Fig. 6E is a sectional view of Fig. 6B taken along line 6E-6E;

Fig. 6 F is an enlarged view of a portion of Fig. 6E;

Fig. 7A is a heating device in accordance with another aspect of the invention;

Fig. 7B is a top view of the heating device of Fig. 7A;

Fig. 7C is a sectional view of Fig. 7B taken along line 7C-7C;

Fig. 7D is an enlarged view of a portion of Fig. 7C;

Fig. 7E is a sectional view of Fig. 7B taken along line 7E-7E;

Fig. 8A is a heating device according to another aspect of the present invention;

Fig. 8B is an enlarged view of a portion of Fig. 8A;

Fig. 8C is a sectional view of Fig, 8A taken along line 8C-8C;

Fig. 8D is a section view of Fig. 8A taken along line 8D-8D;

Fig. 9A is a heating device in accordance with another aspect of the invention;

Fig. 9B is an enlarged view of a portion of Fig. 9A;

Fig. 9C is a sectional view of Fig. 9A taken along line 9C-9C; and

Fig. 10 is a holding cabinet for using multiple heating devices in accordance with the present invention. Detailed Description

The invention relates to electric resistance heating devices and, in particular, relates to a hermetically sealed, flexible graphite heating element for heating objects within a container, on or beneath a panel, or otherwise in contact with a heating surface having any desired shape, size, and construction. Figs. 1 -5E illustrate a heating device 20 in accordance with an embodiment of the present invention. The heating device 20 includes a container 22 into which objects are placed in order to be heated. The container 22 in Fig. 1 constitutes a food tray for heating solid or liquid foods placed therein. Those having ordinary skill in the art, however, will recognize that the container 22 may constitute any container in which it is desirable to heat objects, such as any tray, cup or bucket structure or containers used in medical applications such as blood bags or sleep apnea devices. The objects to be heated in such containers 22 may include solids, liquids, gasses, and combinations thereof.

Referring to Fig. 1 , the container 22 includes an outer side wall 24, a handle 26 that extends from the side wall, and a bottom wall or plate 28 integral with the side wall. The handle 26 has an elongated shaped or otherwise is suitable for grasping and manipulating the container 22. The side wall 24 has a generally rectangular shape. Alternatively, the side wall 24 may be triangular, square, circular or elliptical or have any other shape. The side wall 24 and the bottom wall 28 define a cavity 30 for receiving objects to be heated. The side wall 24 may be provided with a cover (not shown) for closing and/or sealing the cavity 30. The bottom wall 28 includes a heating surface 29 positioned within the cavity 30 and defining a surface for receiving and heating the objects in the cavity 30. The heating surface 29 may be the same size or smaller than the bottom wail 28. The bottom wall 28 further includes an outer surface 32 (see Fig. 2A) positioned outside of the cavity 30 and facing away from the heating surface 29. The outer surface 32 includes one or more bosses 34 that extend away from the outer surface. In Figs. 1-5E, the outer surface 32 constitutes the bottom of the container 22.

It will be appreciated that the heating device 20 is not limited to use as a container 22 with an inner cavity 30 and bottom surface 29. For example, the heating device 20 may be constructed in a planar manner, i.e., formed only as the bottom wall or plate 28 without a side wall 24, with the opposing first and second surfaces 29, 32 without the need to construct a receiving cavity 30. In other words, the heating device 20 may, for example, be formed only as a plate 28 that defines a surface for receiving objects to be heated or may include the plate and the side wall 24 to define the cavity 30 in which the objects are heated. The planar construction is beneficial in applications where a planar heating device 20 is immersed in fluids or affixed to curved or planar surfaces.

in any case, the container 22 is formed from molded plastic. For example, the container 22 may be formed from an injection molded grade plastic that can withstand hot ambient oven temperatures, dishwasher detergents, rinse agents, and hot water cleaning cycles. Alternatively, the container 22 may be formed entirely or partially from corrosion and oven heat resistant stainless steel or other metals. The bottom wall 28 may be formed from the same material as the side wall 24 or from a different material. In any case, the bottom wall 28 is from a material that readily allows for the transfer of heat from the outer surface 32 to the heating surface 29 within the cavity 30. Additionally or alternatively, the side wall 24 is formed from a material that readily allows the transfer of heat therethrough. The side wall 24 may also be made of an insulating material in order to retain heat within the cavity 30. Moreover, non-organic fillers may be added to the material of the bottom wall 28 to improve heat transfer, i.e., conductivity, of the bottom wall to the cavity 30 while using a different material having reduced conductivity to form the side wall 24 and reduce ambient heat losses.

The heating device 20 further includes a heater assembly 40 that is hermetically sealed to the container 22 via an over molded cover 1.00 (Fig. 2A). As shown in Figs. 3-4, the heater assembly 40 includes an electrically conductive material, such as a graphite element 42, that is secured to a temperature resistant first polymer layer 62 via an adhesive layer 60. The heater assembly 40 is also secured to a temperature resistant second polymer layer 64 via an adhesive layer 60. Due to this construction, the graphite element 42 becomes encapsulated between the layers of temperature resistant polymer 62, 64. The adhesive layers 60 may be formed from a polyester adhesive. The polymer layers 62, 64 may each constitute one or more dielectric films and/or one or more dielectric fabrics. The polymer layers 62, 64 may be formed, for example, from a dielectric material such as polyethylene terephthaiate (PET) or fiberglass. The graphite element 42 may include a nickel or copper foil (not shown). Each temperature resistant polymer layer 62, 64 includes a series of notches 63 and 66, respectively that correspond with the bosses 34 on the container 22.

The adhesive layers 60 permanently hold the heater assembly 40 together and allow the heater assembly to be affixed on a surface desired to be heated, such as the outer surface 32 of the bottom wall 28 of the container 22 before the cover 100 is over molded over the heater assembly. In such case an additional adhesive layer 60 not shown in Fig. 3 may be applied to the bottom of the first polymer layer 62.

Alternatively or additionally, the heater assembly 40 may be secured to the side wall 24 of the container 22 (not shown).

Those having ordinary skill will appreciate that additional components can be added and alternative materials can be used in the heater assembly 40 to suit the requirements of a given application. For example, one particular type of heater assembly 40 that may be used with the present invention is the aforementioned thin- film element heater although those skilled in the art will recognize that alternative graphite or even non-graphite heating elements may be used.

As shown in Fig. 4C, the graphite element 42 may be formed into a geometric pattern that includes a series of first portions 44 that extend in a first direction and a series of second portions 46 that extend in a second direction transverse to the first direction to connect the first portions together. Collectively, the first portions 44 and the second portions 46 may form a serpentine pattern. Alternatively, the graphite element 42 may have any desirable geometric pattern, e.g., concentric, to suit particular heating requirements within, the cavity 30. The graphite element 42 is mechanically cut into a geometry which can give each portion 44, 46 of the element trace either a constant or variable resistance along its length depending on the desired heating application. In other words, each of the portions 44, 46 may be specifically sized and shaped to provide a particular heating profile for the heating element 42. The geometry of the element trace is governed by several factors, e.g., the heater laminate geometry requirement, the type of flexible graphite selected, the required element resistance, and other performance related requirements. In any case, the portions 44, 46 of the graphite element 42 extend between a first terminal 48 and a second terminal 50.

The adhesive layers 60 bond the first and second polymer layers 62, 64 to each side of the graphite e lement 42 in order to laminate the graphite element. After laminating, portions of the second polymer layer 64 covering the first and second terminals 48, 50 are removed, which allows for the connection of a lead wire and/or terminals. For example, electrically conductive termination points 51 such as metal foil, may be affixed to each terminal 48, 50 of the graphite element 42. The termination points 51 provide a secure point to weld, solder or mechanically affix terminals or wires to the graphite element 42 in order to electrically connect the graphite element to a power supply (not shown).

The construction of the heater assembly 40 provides versatility to accommodate a range of desired heating profiles. The heater assembly 40 may provide up to 100 W/in² with variable heat flux or watt densities within a single heater circuit. The heater assembly 40 may operate over a large range of temperatures, e.g., about -238°F to 575°F in air or higher temperatures in non-oxidizing atmospheres, and across a wide range of operating voltages, e.g., 0-480 V. The use of a non-metallic and graphite conductor within the heater assembly 40 ensures very little thermal expansion of the heater assembly during operation, and the compliance of the material prevents differential thermal expansion problems and allows the heater to move with the part. The non-metallic conductor also will not work harden and break unlike stiff metallic conductors.

The configuration of the tortuous graphite element 42 path and the resistivity of the graphite permit the element to cover a higher percentage of the heated area, allowing the element to operate with reduced watt density for a given level of total heating power and with correspondingly lower temperature than competing heaters. Additionally, the anisotropic thermal conductivity of the graphite element 42 spreads heat better throughout the heater assembly 40 to prevent formation of localized hot spots.

As shown in Figs. 4A-4B and 4D, a series of conductive fasteners 87, e.g., rivets, are used to secure the graphite element 42 and second polymer layer 64 to conductive terminals 80. In particular, a conductive terminal 80 is placed on the second polymer layer 64 and secured to the terminal 48 of the graphite element 42 and to an opening 52 in the second polymer layer via fasteners 87 such that the terminal 80 spans the notch 66 in the second polymer layer. A terminal 80 is also placed on the second polymer layer 64 and secured to the terminal 50 of the graphite element 42 and to an opening 54 in the second polymer layer via fasteners 87. The openings 52, 54 in the second polymer layer 64 allow both ends of the terminals 80 to be securely fastened to the second polymer layer to provide stability.

Each terminal 80 has a generally U-shaped construction and includes a body portion 82 and a pair of legs 84 that are offset from the body portion. The legs 84 include holes 88 through which the fasteners 87 extend. The terminals 80 may be formed from strips of conductive material that include corrosion resistant metals such as stainless steel, brass, zinc coated copper, nickel, and nickel plated copper or steel.

As shown in Fig. 2A, the heater assembly 40 is secured to the bottom surface 32 of the bottom wall 28 of the container 22 using an adhesive or the like such that the notches 63, 66 in the first and second polymer layers 62, 64, respectively, are aligned with the bosses 34 on the bottom wall 28. The contour of the terminals 80 is configured to compliment the contour of the raised bosses 34 such that the terminals abut the bosses when the heater assembly 40 abuts the bottom surface. In this configuration, the graphite element 42 is positioned between the second polymer layer 64 and the bottom wall 28 of the container 22.

Once the heater assembly 40 is secured to the container 22 the sub-assembled heating device is inserted into an injection mold cavity. Pressurized and heated molten plastic is then injected around the heater assembly 40, thereby forming the plastic cover 100. The over molded plastic resin used to form the cover 100 may constitute a thermoplastic resin but may also include other moldable materials including thermoplastic elastomers. Such a construction would render the entire heating assembly 40 flexible and would thereby provide benefits in applications where the heating assembly is affixed to curved surfaces that require external heating. In one aspect of the present invention, thermally conductive additives such as graphite and metal particles are added to the plastic that encapsulates the heater assembly 40 to enhance thermal conduction of heat away from the heater assembly and to increase the watt density capability of the system.

In any case, the plastic cover 100 is molded such that the entire heater assembly 40 becomes completely and hermetically encapsulated against the bottom wall 28 of the container 22 (Fig. 5A) in order to prevent water and other foreign bodies from contacting the heater assembly. In the configuration in which the heating device 20 does not include the side wall 24, i.e., only the plate 28 is formed, the cover 100 is overmoided in the same manner. The cover ] 00 is also molded to include openings 102 to allow the terminals 80 secured to the graphite element 42 to remain exposed through the cover. In other words, the conductive element of the heater assembly 40 - the body portions 82 of the terminals 80 - remains exposed after the cover 100 is molded over the heater assembly. The exposed body portions 82 allow an external power source to be electrically connected to the terminals 80 and, thus, to the graphite element 42 in the heater assembly 40 in an efficient and cost-effective manner.

Referring to Fig. 5E, the bottom wall 28 of the container 22 may include one or more protrusions 1 1 ί that extend from the bottom surface 32 of the bottom wall and away from the cavity 30. When the plastic cover 100 is over molded onto the bottom wall 28 of the container 22 the molten plastic flows around and bonds to the protrusions 11 1 on the bottom wall. The bonding between the over molded plastic cover 100 and the protrusions 1 1 1 greatly increases the strength of the joint or connection formed between the plastic cover and the container 22. Bonding of the over molded plastic cover 100 to the protrusions 111 in the bottom wall 28 also helps to ensure that there is a hermetic seal at the joint between the plastic cover and the bottom wall. Maintaining a hermetic seal at the joint is important to prevent dishwasher water penetration between the over molded plastic cover 100 and the bottom wall 28 of the container 22 that could otherwise damage the heater assembly 40.

The cover 1.00 is configured such that an. outer surface 104 of the cover lies flush with the exposed surface of the body portion 82 of each terminal 80. In other words, the surface 104 of the cover 100 and the body portion 82 of each terminal 80 are co-planar with one another. AH other surfaces of the terminals 80, including the entirety of both legs 84, are encapsulated by the plastic cover 100 during the over molding process. There is also no gap between the edge of the terminals 80 and the over molded plastic cover 1.00, i.e., the openings 102 in the cover are the same size as the periphery of the body portion 82 of the terminals. This relationship between the over molded plastic cover 100 and the terminals 80 helps to prevent damage to the terminals during use of the heating device 20 in the prescribed environment, e.g., food service kitchen or hospital. The configuration of the exposed portions of the terminals 80 provides an expanded target area to facilitate engagement of the terminals with mating power terminals (not shown) to provide power to the terminals and, thus, to provide power to the heater assembly 40 in order to supply heat to the heating surface 29 of the container 22.

In an alternative configuration shown in Figs. 6A-6F, the over molded cover 100 may also include a raised portion 1 1 0 that extends away from the bottom wall 28 of the container 22 and beyond the terminals 80 in order to recess the body portion 82 of each terminal from the bottom of the container. Features in Figs. 6A-6F that are identical to features in Figs. 1 -5E are given the same reference number whereas features in Figs. 6A-6F that are unique or different from features in Figs. 1 -5E are given different reference numbers. In this configuration, part of the outer surface 104 of the cover 100 remains flush or co-planar with the body portion 82 of the terminals 80 while the raised portion 1 30 of the cover provides additional protection to the body portions from normal physical wear and tear associ ated with the bottom of the container 22.

In a further configuration shown in Figs. 7A-7E, the graphite element 42 remains positioned along the bottom surface 32 of the bottom wail 28 of the container 22 but the terminals 80 abut the side wall 24 of the container 22 as opposed to the bottom wall. The terminals 80 may be positioned on opposing or adjacent wall segments of the side wall 24 and along any number of side wall segments. In any case, the cover 100 is over molded onto the container 22 such that the cover includes a flange portion 120 that extends around the bottom wall 28 of the container and along a portion of the side wall 24 sufficient to surround the peri phery of the body portion 82 of each terminals 80. Similar to previous embodiments, the surface 104 of the cover 100 is configured to be flush or co-planar with the body portion 82 of each terminal 80 while the remainder of each terminal is completely covered by the over molded cover.

In all of the aforementioned configurations and orientations of the terminals 80, in order to ensure that the molded over plastic cover 100 does not cover the entire terminal, the mold cavity used to form the cover must "shut-off on the exposed body portion 82 of the terminal. During over molding of the cover 100, the tool steel mold cavity presses against the body portion 82 with a high degree of pressure that could deform or damage the terminal 80. In the embodiments shown in Figs. 1-6E, due to presence of the cooperating raised bosses 34 on the container 22, any force applied to the body portion 82 of the terminals 80 is borne by the bosses (see Fig. 5D). in this way the terminals 80 are strengthened and securely held in place during the over mold process. In effect the terminals 80 are sandwiched between the mold cavity steel, the bosses 34 on the container 22, and the mold core steel to ensure that the heater assembly 40 is hermetically sealed while providing an exposed portion of each terminal 80 to facilitate contact with an external power supply. In the embodiment shown in Figs. 7A-7E the terminals 80 abut the side wail 24 (Fig. 7E) during over molding of the cover 100 and the flange portion 120 to prevent deformation of the terminals. Therefore, in each embodiment the terminals 80 are supported to prevent damage and ensure a hermetic seal around the exposed body portion 82 of the terminal.

An alternative embodiment of the present invention in which the terminals 80 are omitted is illustrated in Figs. 8A-8D and Figs. 9A-9C. Instead, in Figs. 8A-8D the conductive graphite element 42 of the heater assembly 40 is connected to one or more conductive, pronged male terminals 130 that extend away from the side wall 24 of the container 22. The cover 100a is then over molded onto the container 22 such that the cover includes a male plug 132 formed around a portion of the terminals 130 adjacent to the side wall 24 while leaving a portion of the terminals spaced from the outer wall exposed. The exposed portions of the terminals 130 are configured to cooperate with a mating female socket assembly (not shown) for electrically connecting the terminals and, thus, for electrically connecting the heater assembly 40 to an external power source. In Figs. 9A-9C, the terminals 1.30a are configured as an alternative male plug 132a in which the terminals are recessed to provide better protection for the terminals.

Whether the electrical connection between the heater assembly 40 and the external power source includes the terminals 80 or 130, the operation of the heating device is the same. Once the terminals 80 or 130 are connected to the power source, the conductive graphite element 42 is supplied with electricity, thereby increasing the temperature of the graphite element. Heat is then transferred through the heated graphite element 42, the second polymer layer 64, the bottom surface 32 of the bottom wall 28 of the container 22, through the bottom wall, and finally to the heating surface 29 within the cavity 30. Accordingly, any objects on the heating surface 29 and/or within the cavity 30 are directly heated, i.e., the transferred heat directly irapinges upon the objects in the cavity without first transferring through another medium, such as water.

In any case, a heating device 10 having the over molded plastic cover of the present invention is advantageous over other heating devices in that over molded cover reduces manufacturing and assembly costs and increases product reliability and durability in extreme environmental conditions due to the hermetically sealed heater assembly. The heating device 10 of the present invention is also advantageous in that it improves thermal transfer between the heater and the object to be heated. The configuration of the heating device of present in vention also provides increased flexibility to add custom engineered features to the encapsulated heater assembly, has improved thermal efficiency, and reduces energy usage through more intimate contact with the object or fluid being heated in the cavity 30.

In one aspect of the invention illustrated in Fig. 10, the heating device 20 constitutes a food tray 22 integrated with a custom designed holding cabinet 300. The food tray 22 may constitute a flat, planar tray or the food tray may be formed to include the cavity 30 for receiving objects to be heated and terminals 80 for electrically connecting the heating assembly 40 (not shown) in each tray to a power supply 326. The holding cabinet 300 may accommodate multiple food trays 22 in individual positions or slots 310 with selective heating controls for each slot provided by a heating control unit 312. in use, the operator slides the food tray 22 filled with food into the slot 310. The slot 310 provides top, bottom, and edge guides to direct the tray 22 into the proper orientation relative to positive and negative power terminals 320 permanently affixed in each slot and connected to the power supply 326 and the heating control unit 312. The power terminals 320 engage the exposed portions of the terminals 80 on the bottom or sidewall of the tray 22 to electrically connect the terminals 80 to the power supply 326. The area of the exposed terminals 80 provides a large target to engage the power terminals 320 even if the tray 22 is not fully inserted into the slot 310.

The heating control unit 312 selectively controls power to the power terminals 320 in each slot 310 and may use "smart" programmable logic to accomplish this objective. This control logic is envisioned to be within the scope of the present invention. In one instance, safety may be enhanced through an interlock to prevent application of high voltage, e.g., as high as 120 volts AC, to the exposed power terminals 320 inside the slots 310. Voltage is normally not presented to the power terminals 320 when the tray slot 310 is empty. After the tray 22 is inserted into the slot 310, a proximity sensor or switch (not shown) connected to the heating control unit 312 senses the presence of the food tray through a continuity check. Voltage is routed to the power terminals 320 only if continuity is detected.

Additionally, heater resistance may be determined by the heating control unit 312 through measuring of the resistance across the power terminals 320.

Measurement of the heater resistance provides the heating control unit 312 with the ability to 1 ) verify that the correct tray resistance is detected and the heater assembly 40 is not defective, and/or 2) identify a particular food tray type or power capability based its specified resistance. With this knowledge, the control unit 312 may implement a custom heating profile, e.g., watts vs. time, using preset software or firmware programs associated with the control until and/or optionally programmed by the user. Resistance detection may also negate the need, to use a proximity sensor to verify that the tray 22 is correctly engaged.

The same sensing logic within the heating control unit 312 may be used to provide an alarm 330, e.g., visual or audible, to the operator indicating whether the food tray 22 is being heated. For example, if the food tray 22 is fully inserted into the slot 310 such that continuity is verified, the tray is in the correct position and the power supply 326 may be switched ON to provide power to the power terminals 320 and, thus, provide power to the terminals 80 to start the heating cycle. A red light on the heating control unit 312 may be illuminated to alert the kitchen staff that the food tray 22 is being heated,

A further variation in the sensing capability may be to encapsulate into the tray 22 a Radio Frequency Identification (RF1D) tag and/or a small magnet (not shown). The RFID tag may be used to help the control determine the type of food tray 22 being registered within the slot 310 to provide custom heating control. A. magnet can be used also in conjunction with a Hall Effect sensor positioned in the cabinet 300 to establish that the tray 22 has been properly inserted into the food tray slot 310, If the heating control unit 310 senses the presence of the tray 22 via the Hall Effect sensor, but does not register continuity across the terminals 80, then an audible or visual alarm 330 may be sounded to indicate that the tray is not inserted into the correct position in the slot and is not being heated.

Voltage and current to each zone or slot 312 may also be monitored. This may make it possible for the heating control unit to calculate the heater assembly 40 resistance and heater power which may vary with changes in heater temperature. The system of the present invention may therefore control the level of voltage and/or provide ON/OFF control to maintain a desired internal heater temperature and/or heating power versus time transient. Each zone or slot 310 may be programmed with algorithms unique for different food types, thereby providing a custom heat setting and improved food quality.

As compared to conventional food tray methods (vis-a-vis heated using open flames or ovens), the electric heated food tray 22 of the present invention with individual "smart" zone control can be much more energy efficient. There is virtually no wasted heat as the heat is directly transmitted through the bottom wall 28 of the tray 22 and into the food via the heating surface 29. In other words, due to the configuration of the present invention it is not necessary to heat the environment around the food, which results in reduced heat loss and energy cost to keep the food warm. In the summer time, use of the present invention when the kitchen must be air conditioned provides additional, significant energy savings that are realized through a reduction in air conditioning loads.

The preferred embodiments of the invention have been illustrated and described in detail. However, the present invention is not to be considered limited to the precise construction disclosed. Various adaptations, modifications and uses of the invention may occur to those skilled in the art to which the invention relates and the intention is to cover hereby all such adaptations, modifications, and uses which fall within the spirit or scope of the appended claims.

Claims

Having described the invention, the following is claimed:

1. A heating apparatus comprising:

a plate having a first surface and a second surface for receiving an object or foodstuffs to be heated; and

an electrically conductive sheet hermetically sealed to the first surface of the plate, the conductive sheet being electrically connected to a power source for selectively energizing the conductive sheet to provide direct heat to the second surface of the plate in order to heat the object received on the second surface.

2. The heating apparatus of claim 1 , wherein the conductive sheet comprises flexible graphite.

3. The heating apparatus of claim 1 further comprising a side wall that cooperates with the plate to define a container having a cavity for receiving an object or foodstuffs to be heated.

4. The heating apparatus of claim 3, wherein the conductive sheet includes a pair of terminals for electrically connecting the conductive sheet to the power source.

5. The heating apparatus of claim 4, wherein a pair of bosses extend from the outer surface of the container for supporting the pair of terminals.

6. The heating apparatus of claim 4, wherein the pair of terminals are positioned along at least one of an outer side wall or bottom wall of the container.

7. The heating apparatus of claim 4, wherein the pair of terminals is recessed from a cover over molded onto the container to hermetically seal the graphite sheet to the container.

8. The heating apparatus of claim 4, wherein the pair of terminals comprises a male plug.

9. The heating apparatus of claim 1 , wherein a plastic cover is over molded onto the plate to hermetically seal the conductive sheet to the plate.

10. A heating apparatus comprising:

a container having an outer surface and an inner surface defining a cavity for receiving an object to be heated;

a heater assembly comprising:

a thin-film heater;

first and second temperature resistant polymer layers secured to the thin-film heater; and

a pair of terminals connected to the thin-film heater for electrically connecting the thin-film heater to a power source; and

a cover molded over the heater assembly to hermetically seal the heater assembly to the outer surface of the container.

1 1. The heating apparatus of claim 10, wherein the thin-film heater comprises flexible graphite.

12. The heating apparatus of claim 10, wherein a pair of bosses extend from the outer surface of the container for supporting the pair of terminals.

13. The heating apparatus of claim 10, wherein the pair of terminals are positioned along an outer side wall of the container.

14. The heating apparatus of claim 10, wherein the pair of terminals is recessed from the over molded cover to hermetically seal the thin-film heater to the container.

15. The heating apparatus of claim 10, wherein the pair of terminals comprises a male plug.

16. A heating apparatus comprising:

a plate having a first surface and an opposing second surface for receiving an object to be heated; and

an electrically conductive sheet hermetically sealed to the first surface of the plate, the conductive sheet being electricaily connected to a power source for selectively energizing the conductive sheet to provide direct heat to the second surface of the plate in order to heat the object received on the second surface.

17. A method of forming a heating apparatus comprising the steps of:

providing a plate having a first surface and a second surface for receiving an object or foodstuffs to be heated;

positioning an electrically conductive sheet on the first surface of the plate, the conductive sheet being electrically connected to a power source for selectively energizing the conductive sheet to provide direct heat to the second surface of the plate in order to heat the object received on the second surface; and

overmolding a cover onto the plate to hermetically seal the electrically conductive sheet to the first surface of the plate.

18. The method recited in claim 17 further comprising the step of providing a pair of terminals for electrically connecting the conductive sheet to the power source, the cover being overmolded onto the plate such that at least a portion of each terminal is exposed once the electrically conductive sheet is hermetically sealed to the first surface of the plate.

1.9. The method recited in claim 17 wherein a mold for overmolding the cover onto the plate includes a tool that is selectively shut off on the terminals to provide the exposed terminals through the overmolded. cover.

20. A heating apparatus comprising:

a cabinet having a plurality of individual slots, each slot including one or more exposed terminals and being configured to receive corresponding terminals on a tray to be heated; and a power source electrically connected to the terminals in each slot for selectively energizing the terminals in each slot to provide direct heat to each tray.

21. The heating apparatus of claim 20, wherein each tray comprises a plate having a heater assembly and a cover overmo!ded over the heater assembly to hermetically seal the heater assembly to the plate, wherein the tray terminals are electrically connected to the heater assembly and are at least partially exposed through the overmolded cover.

22. The heating apparatus of claim 20 further comprising a controller for individually controlling electrical power to the terminals in each slot such that each slot has a desired heating profile.

23. The heating apparatus of claim 22 further controlling a sensor for sensing the electrical resistance across each slot terminal, the controller controlling electrical power to the slot terminals based on the sensed electrical resistance of each slot terminal.

24. The heating apparatus of claim 22, wherein the controller removes electrical power to the slot terminals if infinite electrical resistance is sensed.

25. The heating apparatus of claim 20, wherein the tray and the slot include cooperating structure for verifying that the tray is properly positioned within the slot.

26. The heating apparatus of claim 22, wherein the tray and the slot include RFID structure for identifying the contents of the food tray and relaying the content information to the controller.