US3525841A - Door seal for microwave ovens - Google Patents

Description

Allg- 25, 1970 n. B. HAAGENs'L-:N ET Al. 3,525,841

DOOR SEAL FOR MICROWAVE OVENS Filed Nov. 4, 1968 INVENTORS DUANE 5. HAGENSE/V SAMUEL G. SWEET BY-gwu;

ATTOBN E Y United States Patent O 3,525,841 DOOR SEAL FOR MICROWAVE OVENS Duane B. Haagensen, Edina, and Samuel G. Sweet,

Hopkins, Minn., assignors to Litton Precision Products, Inc., Minneapolis, Minn., a corporation of Dela- Ware Filed Nov. 4, 1968, Ser. No. 772,982 Int. Cl. H05b 9/ 06 U.S. Cl. 219-10.55 8 Claims ABSTRACT OF THE DISCLOSURE An improved door seal for microwave ovens wherein a plurality of spacers provides a port of entry for electromagnetic wave energy to enter a combination of an attenuating cavity and an absorption cavity which together provide a volumetric resonant cavity that presents high empedance to electromagnetic wave energy within the attenuating cavity and a terminating impedance to electromagnetic wave energy within the absorption cavity.

FIELD OIF THE INVENTION This invention relates to microwave ovens and more particularly to a novel door seal for preventing the escape of electromagnetic wave energy from such ovens.

DESCRIPTION OF THE PRIOR ART It is well known that electromagnetic Wave energy may be utilized for heating foodstuff or other lossy dielectric materials. The foodstuff or other materials are placed in a cavity of a microwave oven and are exposed to electromagnetic wave energy that is supplied by a suitable source, e.g., a magnetron. In order to facilitate the insertion and the removal of foodstuff to and from the cavity of the microwave oven, a movable door is provided for such ovens. 'I'he interior surface of the movable door completes the cavity to ensure proper heating of the foodstuff and prevents the wholesale escape of electromagnetic wave energy from the oven. In addition, door seals must be provided to prevent any further leakage of electromagnetic wave energy from the oven.

In the past, microwave oven manufacturers have utilized various types of door seals. Originally, microwave ovens featured direct contact between the inside surface of the metallic door and the front surface of the metallic housing, with the addition of spring biasing of the door to maintain the door seal. However, the spring biasing method did not prove satisfactory over a period `of time, in some cases, because of warping of the metallic surfaces or other damage thereto, and in other cases, because of the deterioration of the springs. In either case, arcing between the respective surfaces of the door and the housing and excessive leakage from the oven occurred, unless proper maintenance procedures were strictly observed.

Other approaches attempted by microwave oven manufacturers included the insertion of a number of nonmetallic spacers or a conductive rubber gasket between the door and the microwave oven housing to establish a gap so as to prevent arcing of the electromagnetic wave energy. One of these approaches included the placing of a cavity having a terminating conducting surface located an integral number of one-half wave lengths from the origin of the gap. -In some instances, such cavities have been filled with a dielectric material, which acts to shorten the physical length of such cavities, in order to make the door seal more compact, while still electrically maintaining the one-half wave length between the origin of the gap and the terminating conducting surface of the cavity. In either case, the cavity presented a short circuit which was reflected back as a short circuit between the parallel plate transmission line formed by the interior surface of 3,525,841 Patented Aug. 25, 1970 ICC door and front surface of the housing. Thus, electromagnetic wave energy was blocked from leaving the oven cavity. However, either warping of the metallic surfaces or eventual wearing of the non-metallic spacers caused arcing between the metallic surfaces. In the case of the conductive rubber seal, the burning out of the rubber seal occurred.

Accordingly, it is an object of the present invention to provide an improved door seal f or microwave ovens which eliminates the possibility of arcing of electromagnetic wave energy in proximity of the door seal.

It is a further object of the present invention to provide an improved door seal for preventing the escape of electromagnetic wave energy through the exterior junction of the `door and the housing of microwave ovens.

It is a still further object of the present invention to provide an improved door seal for microwave ovens which includes a first cavity and a second cavity for attenuating and absorbing electromagnetic wave energy, respectively.

\It is another object of the present invention to provide an improved door seal for microwave ovens which includes novel cavity means for preventing the escape of electromagnetic wave energy from such ovens and rcduces the Weight of the door of such ovens.

SUMMARY OF THE INVENTION In accordance with the objects set forth above, the present invention provides an improved `door seal for microwave ovens comprising a plurality of spacers to providea gap for preventing arcing of electromagnetic wave energy between the respective surfaces of the door and the housing of the oven, the gap ensures a port of entry for the electromagnetic wave energy, and a combination of an lattenuating cavity and an absorption cavity provides a volumetric resonant cavity that presents high impedance to electromagnetic wave energy within the attenuating cavity and a terminating impedance to electromagnetic wave energy within the absorption cavity.

BRIEF DESCRIPTION OF THE DRAWINGS i accordance with the present invention;

FIG. 2 isa cross sectional view of the microwave oven taken along the line 2 2 of FIG. l;

FIG. 3 is an alternate embodiment of the present invention in which the cavities have a vaporized metallic coating thereon; and

FIG. 4 is an enlarged view of one of the corners of the door of the microwave oven of FIG. 1 with part of the door cut away to show the elements of a door seal in accordance with the present invention.

DESCRIPTION OF THE PREFERRED` EMBODIMENTS provided to enable viewingL of the cooking of any foodstuff which may be within the microwave oven 10. Various controls are shown above the door 12; however, since such controls are conventional and do not relate to the present invention, the controls are not described or illustrated further herein.

Referring now to FIG. 2, there is shown a cross sectional view of the microwave oven 10 taken along the line 2-2 of FIG. 1. FIG. 2 illustrates a door seal for preventing the escape of electromagnetic wave energy from the microwave yoven 10. As further illustrated by FIG. 4, the door seal is located completely around the perimeter of the interior surface of the door 12. The door 12 is constructed of metal castings '116 and 18, preferably aluminum to maintain a light weight door which may be held together by a number of conventional means, such as, a screw 19. The spaces 27 located between metal castings 16 and :18 house the mechanism, not shown, for operating the handle 13, or are empty so as to reduce the weight of the door 12. Also shown in FIG. 2 is the housing 11 having three surfaces 11b, 11C and 11d. The surface 11b and the window 14 dene the location of a microwave oven cavity 111a, or the interior of the microwave oven 10. The numeral 17 designates the exterior panel of the door 12.

Within the metal casting 18, as defined by the surfaces 24a, 24b, 24C and 24d, is an attenuating cavity 24 which is filled with a dielectric, preferably polypyroleyne. Utilization of the polypyroleyne filler in the attenuating cavity 24 allows the cavity to be physically smaller than if the cavity only contained air and further provides for attenuating any electromagnetic wave energy entering the attenuating cavity 24. Also within the metal casting 18 is an absorption cavity 22 which is filled with an electromagnetic wave energy absorbing material, for example, ferrite. The numeral 23 defines the surface of the absorption cavity 22 that is adjacent the attenuating cavity 24. A retaining strip 20 is affixed between the metal castings 16 and 18 and retains the ferrite in the cavity 22, as shown. 'Ihe retaining strip 20 includes spacers 21 at the four corners of the door 12, as illustrated in FIGS. 2 and 4. The four spacers 21 ensure that a proper gap is maintained between the interior surface of the door 12 and the surface 11c of the housing 11.

In combination, the attenuating cavity 24 and the absorption cavity 22 provide a volumetric resonant cavity such that there is a high impedance to electromagnetic wave energy within the attenuating cavity 24 for attenuating such electromagnetic wave energy, and there is a terminating impedance to electromagnetic wave energy within the absorption cavity 22 for absorbing such electromagnetic wave energy. Furthermore, a port of entry 26, which has been established by the four spacers 21, ensures that a physical opening of such a capacitance to provide minimum impedance to electromagnetic wave energy is maintained. Thus, the establishment of a transmission line into the volumetric resonant cavity, or resonant cavity choke, is such, that it allows purposeful capacity transmission into the volumetric resonant cavity.

In the operation of door seal illustrated in FIG. 2, the surface 11C of the .housing 11 and the interior surface of the window 14 define a gap which establishes the port f entry 26. Since an electromagnetic wave energy pattern is established within the cavity 11a, a portion of the. electromagnetic wave energy will travel to the port of entry 26. Any electromagnetic wave energy within the cavity 11a that is directed thereto is allowed to enter. Upon entering the attenuating cavity 22, the electromagnetic wave energy therein will be attenuated. A major portion of this attenuated electromagnetic wave energy will then enter the absorption cavity 24 which maintains a terminating impedance to absorb such electromagnetic energy. Thus, instead of providing a short circuit at the port of entry 26 which facilitates possible arcing, the present invention provides a port of entry 26 which allows electromagnetic wave energy to enter; therefore, eliminating the possibility of arcing. It was found that if the absorption cavity 22 is placed as near as possible to a port of exit 25, maximum absorption of any attenuated electromagnetic wave energy attempting to leave said attenuating cavity 24 is accomplished by the absorption cavity 22.

In the practice of this invention, it was found that an effective seal was constructed which maintained a radiation leakage level of less than 1 rnw./cm.2 under all operating conditions. The constructed door seal, which was maintained entirely around the interior perimeter of the door 12, included the cavity 24 having a polyproleyne filler therein, with the polyproleyne filler 'being of the following dimensions at the following respective surfaces: 2411:.480, +1100, .005 inch; 240:.900, -[-.000, 005 inch; 24d:.590; and 25:.240 inch. The ferrite material in cavity 22 had a width of .1251.015 inch at surface 23 and a depth of .30811020 inch. In addition, it was found that if the opening is .0153005 at the port of entry 26, no arcng is experienced and the port of entry 26 thereby established allows the electromagnetic wave energy to enter the attenuating cavity 24. As is well known in the art, one of the standard operating frequencies for magnetrons employed in microwave ovens is 2450 megacycles per second. The door seal in accordance with the present invention provides a volumetric resonant cavity exhibiting broad 'band characteristics that effectively prevents the escape of electromagnetic wave energy from the microwave oven 10.

FIG. 3 illustrates another embodiment in accordance with this invention wherein the attenuating cavity 24 and the absorption cavity 22 may be provided with a vaporized coating of conductive material 28, for example, aluminum, that the volumetric resonant cavity will be effective and at the same time will enable the elimination of a substantial portion of the metal casting 18 so as to provide a relatively light weight door.

Thus, although the lpresent invention has been shown and described with reference to particular embodiments, for example, an attenuating cavity of a defined shape having polyproleyne therein, or a microwave oven providing a seal having its major components within the door, nevertheless, various changes and modifications obvious to a person skilled in the art to which the invention pertains, for example, attenuating cavities of a different physical size having other dielectric material therein, or a microwave oven providing a seal having its major cornponents within the housing are deemed to lie within the spirit, scope, and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. Microwave oven apparatus comprising:

a housing;

a heating cavity located within said housing, said heating cavity having an opening therein;

means for supplying electromagnetic wave energy to said heating cavity;

a movable door mounted to said housing, said movable door in the closed position completing with said heating cavity an effective heating cavity for said electromagnetic wave energy supplied to said heating cavity;

spacing means for providing a physical gap between said movable door and the surface immediately adjacent said movable door, said gap having a port of entry to allow electromagnetic wave energy to enter said gap; and

volumetric resonant cavity means located adjacent said gap and in proximity of the entire perimeter of the interior surface of said movable door for ensuring an electrical opening of such a capacitance that minimum impedance to said electromagnetic wave energy occurs at said port of entry, said volumetric resonant cavity means includes attenuation cavity means for attenuating said electromagnetic wave energy entering said gap, said attenuating cavity means having a port of exit, said volumetric resonant cavity means further includes absorption cavity means having a surface abutting said attenuating cavity means for absorbing said attenuated electromagnetic wave energy, said absorption cavity means being located in proximity of said port of exit of said attenuating means. L

2. Microwave oven apparatus as recited in claim 1 wherein said volumetric resonant cavity means comprises:

a first cavity filled with a dielectric material for attenuating said electromagnetic wave energy; and

a second cavity filled with an absorption material for absorbing said electromagnetic wave energy.

3. Microwave oven apparatus as recited in claim 2 wherein said dielectric materaial is polyproleyne and said absorption material is ferrite.

4. Microwave oven apparatus as recited in claim 1 wherein said volumetric resonant cavity means is located within said movable door.

5. Microwave oven apparatus as recited in claim 1 wherein said volumetric resonant cavity means comprises in combination:

a dielectric material for attenuating said electromagnetic wave energy;

an absorption material for absorbing said electromagnetic wave energy, said absorption material having a surface abutting said dielectric material; and

a vaporized coating of conductive material enclosing said dielectric material and said absorption material except at the surface of said dielectric that is exposed to said gap.

6. Microwave oven apparatus as recited in claim 5 wherein said dielectric material is polyproleyne, said absorption material is ferrite, and said conductive material is aluminum.

7. Microwave oven apparatus as recited in claim 1 wherein the width of said port of entry is approximately .015 inch.

8. Microwave oven apparatus comprising:

a housing;

a heating cavity located within said housing, said heating cavity having an opening therein;

means for supplying electromagnetic wave energy at a frequency of approximately 2450 megacycles to said heating cavity;

a movable door mounted to said housing, said movable door in the closed position completing with said heating cavity an effective heating cavity for said electromagnetic wave energy supplied to said heating cavity;

spacing means for providing a gap between said mofvable door and the surface immediately adjacent said movable door, said gap having a port of entry ofI a width of approximately .015 inch to allow electromagnetic wave energy to enter said gap; and

volumetric resonant cavity means, located adjacent said gap and including polyproleyne filled attenuating cavity means and ferrite filled absorption cavity means, for attenuating and absorbing said electromagnetic wave energy traveling through said gap and that enters said volumetric lresonant cavity, said ferrite being located in close proximity of said gap and having a surface abutting said polyproleyne.

References Cited UNITED STATES PATENTS 2,958,754 ll/1960 Hahn 2l9-10.55 3,182,164 5/1965 Ironfield 2l9-l0.55 3,196,242 7/1965 De Vries et al 2l9l0.55 3,249,731 5/1966 Johnson 2l9-l0.55 3,351,730 11/1967 Pahlman 2l9l0.55

JOSEPH V. TRUNE, Primary Examiner L. H. BENDER, Assistant Examiner img? UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 315251841 Dated August 2751 1970 Invencods) Duane B. Hclagensen und Samuel G. Sweet It is certified that error appears in the above-idetified patent and that said Letters Patent are hereby corrected as shown below:

r-Col. l, line 18, "empedanceI should be --impedcmce j Col. 3, line 22, lpolypyrolene" should be polyproleyne Col. 3, line 23, "polypyrolene" should be --polyproleyne Col. 4, line ll, the figure ".900" should be .990

SIGNED @man m5 1971 Ism) Attest: 2

Edward M. Hawker. Ii'- m *usgang n. mum om omissioner at Pam

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