Partially shielded microwave heating container

Abstract: A paperboard container (10) for heating products placed therein by microwave energy. The container comprises a first region (152) and a second region (154) both being formed from a unitary single sheet of microwave permeable paperboard material. The first and second regions each include a top panel (14) and a bottom surface (16). The bottom surface of each region has a peripheral edge and the bottom surface of the first region being defined by an area shielded from microwave energy commences along the peripheral edge and extends across less than the entire bottom surface. The second region being defined by the area of the bottom surface that is not shielded from microwave energy. A single bottom panel (16) forms the bottom surface for both of the first and second regions with a plurality of side panels extending upwardly from the bottom panel and extending downwardly from the top panel.

Description: BACKGROUND OF THE INVENTION TECHNICAL FIELD The present invention relates to a sectioned carton having a compartment which is shielded from microwave energy and another compartment which is exposed to microwave energy. More particularly, the invention pertains to a carton formed from a single blank of microwave permeable sheet material which is cut, scored and folded to form a multi-compartment container, wherein at least one section is coated with a material substantially impermeable by microwaves. BACKGROUND ART Multi-compartment containers have been manufactured with a variety of constructions which allow portions of the container to be exposed to microwave energy while shielding other portions from the microwave energy. Many of these constructions, however, pertain to pre-packaged food products and do not allow the end user of the container to selectively place the food products into either shielded or non-shielded compartments in the container. Additionally, these constructions are often formed from separate components or materials for each compartment and must be assembled together to form the multi-compartment container. Document Wo 92 19515, on which the preamble of claim 1 is based, discloses a pizza box having a metal layer laminated to the lower and the upper portions of the box. U.S. Patent No. 4,233,325 issued to Slangan et al. discloses one such microwaveable multi-compartment package. This package includes a lower compartment, containing frozen ice cream, which is substantially impermeable to microwave energy and an upper compartment, containing frozen syrup, which is microwave permeable. The package is placed in a microwave oven to melt the frozen syrup in the upper compartment. The two compartments are manufactured from separate cardboard blanks, formed and then fit together, wherein the upper compartment further includes a lip for easy removal of the upper compartment from the food package after microwave heating. A similar multi-compartment packaged food product is disclosed in U.S. Patent 4,794,008 issued to Schmidt et al. As in the above-mentioned patent, an upper frozen food compartment permeable to microwave energy is provided with a lower frozen food compartment which is reflective of microwaves. The compartments are formed into U-shaped plastic compartments and joined together to form a single multi-compartment food package, and the two compartments must be further separated from one another after heating in order to access the food contained within the compartments. Both of the above-mentioned patents further are directed to pre-packaged frozen products, such as ice cream in the lower compartment and syrup in the upper compartment, wherein the upper and lower compartments are arranged vertically. U.S. Patent No. 4,081,646 issued to Goltsos discloses a multi-compartment heating container including a tray and cover which are thermoformed from plastic which is transparent to microwave energy. Furthermore, the outer surfaces of the tray and cover may be shielded against the microwave energy by a thin film of radiation-opaque material, except for the regions which are intended to define microwave-transparent openings Each compartment in this container is shielded against different amounts of microwave energy by leaving different sized windows on each compartment free from radiation-opaque material, wherein these windows are substantially identical to windows formed in the cover so that the windows in the cover will be in alignment with the windows on the tray. Forming the radiation-opaque material on the majority of the already formed curved surfaces of the thermoformed container and then removing portions of the radiation-opaque material to form windows adds significantly to the costs and complicates the manufacturing process entailed in forming the final product. Additionally, the formation of substantially identical windows in the radiation-opaque material must be performed with precision and requires additional steps after the container is thermoformed. Canadian Patent CA-A-1 082 655 issued to Carlino discloses a microwave oven heating container made of a cardboard material. The side walls of the cardboard container are coated with a microwave reflecting material. Top and bottom walls of the container are only partially covered with printings of metallic ink being at least partially opaque to microwave radiation allowing different portions of microwave energy to penetrate to the inner part of the container to heat the food. This is in order to selectively heat different types of food with the proper amount of energy. Document EP-A- 0 336 325 discloses a food package including a container formed of a heat resistent material wich is pervious to microwaves. A metallized ink is deposited on a surface area of the container such as when the metallized ink is exposed to microwaves, the required enhanced heat is produced in the vicinity of the surface area. Therefore, as can be seen from the foregoing, there is clearly a need for a carton of the above-mentioned type which includes a microwave permeable compartment and a microwave shielded compartment, which is formed from a single microwave permeable blank having a portion of the blank laminated with a microwave shielding layer. There is further a need for a partially shielded microwave heating container which can be filled with a product by the end user of the package and does not need to be pre-packaged with a food product. SUMMARY OF THE INVENTION It is a primary object of the present invention to overcome the aforementioned shortcomings associated with the prior art. Another object of the present invention is to provide a paperboard carton having a compartment which is permeable by microwave energy and another compartment which is reflective to microwave energy. Yet another object of the present invention is to provide a multi-compartment microwave heating carton having a compartment shielded against microwaves and a microwave permeable compartment wherein both compartments are formed from a single paperboard blank. It is yet another object of the present invention is to provide a microwave heating carton partially shielded against microwaves which is formed from a single paperboard blank laminated with a microwave reflective material on only a portion of the paperboard blank. Still another object of the present invention is to provide a multi-compartment microwave heating carton having a compartment shielded against microwaves and a microwave permeable compartment which does not need to be pre-packaged with a product and which allows the end user to place the products being used into the two compartments. These as well as additional objects and advantages of the present invention are achieved by producing a paperboard container for heating products placed therein by microwave energy as defined in appended claim 1. Further favorable embodiments thereof are defined in the dependent claims. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a top view of the paperboard blank used in forming the shielded microwave heating device in an alternative embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the clamshell-shaped microwave heating container 10 is illustrated in Fig. 1. One-half of the paperboard blank 12 is laminated with a microwave reflective layer 150, such as aluminum foil or the like, and then cut and scored to form the microwave heating container 10 shown in Fig. 1 having a microwave shielded section 152 and a microwave permeable section 154. The foil layer 150 covers one-half of the length of the inside surfaces of the top panel 14, bottom panel 16, and side walls 26, 28, 30 and 32, while completely covering the inside surfaces of end walls 18 and 22 and extensions 34, 40, 50 and 52. The microwave heating container 10 is assembled exactly as described above, except that all of the sections of clamshell section 100 from Figs. 1 and 2 have been removed from this embodiment. Once the top portion of the microwave heating container 10 has been folded to close the package, the inside surfaces of the top panel 14 and bottom panel oppose one another. The microwave shielded section 152 is enclosed by top panel 14, side walls 26, 28, 30 and 32, end walls 18 and 22, and bottom panel 16. Additionally, extensions 34, 40, 50 and 52 will also serve to enclose the microwave shielded section 152 since they are affixed to the inner laminated surfaces of end wall 18 and side walls 30 and 32, respectively. The microwave permeable section 154 is enclosed by top panel 14, side walls 26, 28, 30 and 32, end walls 20 and 24, bottom panel 16, and extensions 36, 38, 54 and 56. The microwave shielded section 152 will only be enclosed on five sides with the sixth side 156, the side lying in the plane between microwave shielded section 152 and microwave permeable section 154, being open to the microwave permeable section 154. With this embodiment, it is possible for microwaves to enter the microwave shielded section 152 through this open side 156 between the two sections 152 and 154. However, the microwave energy will be applied from above the microwave heating container 10 and directed toward the outer surface of top panel 14, wherein the bottom panel 16 of microwave heating container 10 will rest upon the bottom surface of the microwave heating apparatus and only a small fraction of microwaves emitted will actually pass through the open side 156 of microwave shielded section 152. In order for any microwaves to enter through open side 156, the microwaves must first pass through microwave permeable section 154 where the microwaves will encounter the product placed therein to be heated. Therefore, very few microwaves will actually enter into the microwave shielded section 152 through open side 156. The microwave heating container 10 may thus be sent to the end user in a partially assembled state where both the top panel 14 and bottom panel 16 facing upwards, where the end user need only place the desired products within the microwave shielded section 152 and microwave permeable section 154 and close the top panel 14 over the bottom panel 16. A plurality of microwave heating containers 10 may be stacked together in this partially assembled state and shipped to the end user in this manner. Accordingly, the above-described configurations provide a simple and efficient method of manufacturing a multi-compartment heating container 10 which allows one compartment to be heated by microwave energy while another compartment is substantially shielded from the microwave energy. This configuration allows multiple products to be placed into the heating container 10 and exposing the heating container 10 to microwave energy, while only the products in the non-shielded portion of the heating container 10 are actually heated by the microwave energy.

Claims: A paper board container (10) for heating products placed therein by microwave energy, said container comprising: a top panel (16), at least two side panels (30, 32) extending downwardly from said top panel (16), at least two end panels (22, 24) extending downwardly from said top panel (16), a bottom panel (14); at least two side panels (26, 28) extending upwardly from said bottom panel (14), at least two end panels (18, 20) extending upwardly from said bottom panel (14), a unitary, single sheet of microwave shielding material being laminated to a portion of said top panel (16), a portion of said bottom panel (14), characterised in that said sheet of microwave shielding material is laminated to a portion of said side panels (26, 28, 30, 32) and at least two of said end panels (22, 24, 18, 20) thus forming a first region (152) shielded from microwave energy extending from said two of said end panels (22, 24, 18, 20) across less than the entire width of the bottom and top panels (14, 16) and leaving a second region (154) unshielded and thus permeable for microwave energy. The paperboard container (10) of claim 1, wherein said side panels (26, 28, 30, 32) also have first and second regions, with the first and second regions of said side panels (26, 28, 30, 32) corresponding to and being contiguous with the first and second regions of said top panel (14) or said bottom surface (16), said first region of said side panels (26, 28, 30, 32) and the end panels (40, 22, 42, 50, 18, 54) extending from the first regions of the bottom and top panels (14, 16) being shielded from microwave energy. The paperboard container (10) of claim 2, wherein said side panels (26, 28, 30, 32) are also formed form said single sheet of microwave permeable paperboard material and said first region of said side panels (26, 28, 30, 32) has a layer of microwave shielding material affixed to said single sheet of paperboard material. The paperboard container (10) of claim 1 or 2, wherein said first and second regions of said top panel (14), bottom surface (16), side panels (26, 28, 30, 32) and end panels (18, 20, 22, 24, 34, 36, 38, 40, 50, 54, 56) each constitute approximately 50 % of the interior surface area of said paperboard container (10).

Inventors: CAPO JAMES L, LAFFERTY TERRANCE P, CAPO, JAMES L., LAFFERTY, TERRANCE P., Lafferty, Terrance P. Graphic Packaging Corp.
Applicants: GRAPHIC PACKAGING CORP, GRAPHIC PACKAGING CORPORATION
Application year: 1997
Main classification: B65D81
IPC codes: B65D5, B65D81