US3808391A

US3808391A - Microwave oven door assembly

Info

Publication number: US3808391A
Application number: US00327834A
Authority: US
Inventors: D Graff; S Athey
Original assignee: Hobart Manfacturing Co
Current assignee: Premark FEG Corp
Priority date: 1973-01-29
Filing date: 1973-01-29
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30

Abstract

A door assembly for microwave ovens includes a seal plate having an integral viewing screen which may be used for venting vapors and fumes from the heating cavity. The holes in the viewing screen are extremely small and are arranged to prevent objects from being inserted through the screen. A protective transparent covering is placed over the perforated area of the screen to provide additional protection and is hinged for easy cleaning of the covering as well as the viewing-venting screen.

Description

United States Patent Graff et al.

[ Apr. 30, 1974 MICROWAVE OVEN DOOR ASSENEBLY Inventors: David F. Graff, New Iberia, La.;

Stuart E. Athey, Troy, Ohio Assignee: The Hobart Manufacturing Company, Troy, Ohio Filed: Jan. 29, 1973 Appl. No.: 327,834

US. c1. 219710.55 1111. cu. H05b 9/06 Field of Search 219/1055 References Cited UNITED STATES PATENTS 5/1973 Jarvis et al 2'l9/l0.55 7/1972 Binzer 219/1055 3,544,751 12/1970 Valles 219/1055 Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney, Agent, or Firm-Biebel, French & Eng

[57] ABSTRACT A door assembly for microwave ovens includes a seal plate having an integral viewing screen which may be used for venting vapors and fumes from the heating cavity. The holes in the viewing screen are extremely small and are arranged to prevent objects from being 7 inserted through the screen. A protective transparent covering is placed over the perforated area of the screen to provide additional protection and is hinged for easy cleaning of the covering as well as the viewing-venting screen.

9 Claims, 9 Drawing Figures PAFENTEB APR 30 $974 SHEET 1 BF 2 n a I? lllftl l l lwnwhlill FIG-1 FIG-4 MICROWAVE OVEN DOOR ASSEMBLY BACKGROUND .OF THE INVENTION The present invention relates to a microwave oven venting system and more particularly to a door assembly for microwave ovens designed to permit direct venting through the door viewingscreen.

In microwave heating appliances, the nature of the heating phenomenon is that of stressing certain of the molecules of the product to be heated by using an electromagnetic field, commonly in the heating frequency range of 2,450 MHz. The resultant heat generated within the product causes water vapor and steam to be emitted from the product into the cavity of the appliance which then must be vented to the exterior of the cavity to prevent the condensation and retention of moisture within the cavity.

In the case of food products being heated in commercial or home type cooking appliances, certain amounts of fat are also heated and carried out of the product along with the water vapor and steam. These fats and moisture to some extent condense in the venting area of the appliance. As a result of this condensation of water vapor and fats, it is essential that the venting system be capable of being cleaned by the user without undue difficulty. If the fats and moisture are permitted to accumulate and are not cleaned on a regular basis, such accumulation will permit the growth of bacteria, resulting in a potential health hazard. Likewise, the accumulation of fats presents a potential fire hazard because of their flammable nature. This is particularly true around other cooking equipment such as found in a commercial kitchen.

For these reasons, the common practice of venting indirectly through the other walls of the cavity is not practical since this requires some sort of ducting and baffling to avoid radiation hazards. Because of such ducting and baffling it is not possible to obtain practical cleanability of the venting system.

As a solution to this problem, it has been suggested that vapors be vented directly through the door viewing screen. For example, in U.S. Pat. No. 2,860,026 there is disclosed a door assembly having openings with screens therein. The screens are of a sufficiently small mesh to prevent the escape of electromagnetic radiation but permit the escape of vapors and fumes, and

permit viewing the food load as it is cooked. Other disclosures of such multifunctional door viewing screens can be found in U.S. Pat. Nos. 3,484,573, 3,544,751 and 3,576,417.

It should be apparent that such screens prevent accidental or inadvertent exposure to microwave radiation and at the same time make it possible for food soil accumulation to be easily cleaned away. An additional advantage in venting through the door viewing screen is that any accumulation of food vapors such as fats are readily visible to the user, thereby reminding the user that they need to be cleaned away. Obviously, this advantage is not available in the duct type or indirect venting systems.

Venting through the door viewing screen has raised an additional problem, however. In the past such screens have contained holes of a size and type which made it possible to insert metal articles, such as the tip of a knife or fork, or a paper clip, or hairpin, into one of the holes. For example, U.S. Pat. No. 3,544,751 discloses that the holes should have a diameter of 1/10 the wavelength of the frequency of the microwave energy used. At 2,450 MHz, such hole sizes would be approximately 0.47 inch. Likewise, U.S. Pat. 3,484,573 discloses holes having a diameter of 0.10 inch or less, the size of the holes being selected to provide for viewing while at the same time presenting a high impedance to the microwave field within the oven cavity.

If a metal wire is inadvertently inserted through one of the holes in the screen while the oven is operating and contacts the screen, an electric spark might be struck between the wire and the metal screen. Similarly, inserting a metal wire into the holes, if insulated from the screen, will provide a conductive path for microwave energy to be radiated outside the cavity. The danger of this happening has necessitated placing a protective covering over the screen. In U.S. Pat. No. 3,700,846, there is disclosed such a system with a removable transparent plate installed over the perforated metal plate of the door viewing screen. The transparent plate is arranged so that when it is removed, the microswitch cannot be closed and the oven cannot be operated. U.S. Pat. No. 3,679,855 also shows a transparent cover which may be slidably removed for ease in cleaning the viewing-venting screen. In contrast, U.S. Pat. No. 3,681,557 discloses a transparent plate that is permanently installed in front of the door viewing screen. In both U.S. Pat. Nos. 3,679,855 and 3,681,557 the door assembly has a plurality of air-vent holes or slots, which present an additional cleaning problem, located around the edges of the transparent plate.

SUMMARY OF THE INVENTION In accordance with the present invention, a door assembly is provided which has a seal plate with an integral perforated screen having holes of a sufficiently small diameter that it is not possible to insert most commonly found metal objects through any of the holes. The screen provides both a means for-viewing objects inside the heating cavity and for venting vapors and fumes from the cavity.

It has been found that the seal plate can be made of a thin metal which is flexible enough to provide sealing pressure when the door is closed. The holes in the perforatedarea of the seal plate are much smaller than heretofore used. Preferably they are in the order of 0.015 inch in diameter or about the thickness of the plate, and are arranged in a staggered pattern on approximately 0.031 inch centers. Such an arrangement results in approximately 900 holes per square inch of venting area, and provides a high degree of visibility into the oven cavity. Since the hole sizes are less than 1/100, actually less than 1/300, the wavelength of the frequency commonly used (2,450 MHz), such a screen effectively seals the microwave energy in the oven cavity to well below the limit of one milliwatt per square centimeter (1 mw/cm) of radiation leakage which is set by government standards, and presents to the user an easily cleanable surface on which food soils accumulate during the venting process. The holes also bar the passage of all but very small flexible wires, particularly insulated wires which will not readily burn away.

It is also a feature of the present invention to provide a hinged transparent covering plate which fits into an opening in the door assembly which is somewhat larger than the screen. The covering plate is slightly larger than the perforated areas of the screen, but slightly smaller than the opening in the door assembly. Since screen there are air passages between the screen and the transparent plate. Thus, the areas around the edge of the transparent plate serve as a substantially continuous air vent for the vapors and fumes which vent through the screen.

In addition, when the transparent plate is closed it is practically impossible to insert anything through the holes in the screen. Wires which are small enough to enter through the vent holes (i.e., those having a diam eter of less than 0.015 inch) are generally not rigid enough to be formed into the tortuous shape necessary to go over the transparent plate, down behind it, and then through one of the holes in the screen. Even when the transparent plate is open, it is safe to operate the oven since objects commonly found in the kitchen such as small paper clips, hairpins, picture wire, bag tie wraps, and the like, have diameters in excess of the holes in the vent screen area and cannot be inserted in any manner into the cavity.

Additionally, uninsulated wires of a diameter small enough to enter the cavity through the vent holes will burn off within the cavity without coupling microwave energy in excess of l mw/cm to the exterior of the cavity for any appreciable time. While insulated wires having a diameter of less than 0.015 inch may under very exacting conditions be inserted into the cavity, this is only possible when the transparent cover is open and an intentional effort is made to stick a wire of this type through the screen. However, even if inserted, wires of this size have a high resistance and will melt or burn or lose their shape in the presence of microwave energy in excess of 1 mw/cm Besides, wire of this size is normally used in the manufacture of small electrical coils for items such as small solenoids, miniature electric motors and the like. It is not a commonly obtainable type of wire and certainly could not be expected to be found in a readily available condition in an area where a'microwave oven would be used.

Accordingly, it is a primary object of the present invention to provide a door viewing screen which also serves as a venting screen and which has holes therein of a sufficiently small size to prevent most objects from being inserted therethrough and thus prevent significant radiation leakage caused by objects which are 'small enough to be inserted; to provide a door viewing screen which is formed integrally in a flexible seal plate; and to provide additional projection against the possibility of inserting an object through the door viewing screen by providing a transparent cover plate for said screen.

Other objects and advantages of the present invention will be apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a front perspective view of a microwave oven having a door assembly constructed in accordance with the present invention;

FIG. 2 is an enlarged front view of the door assembly;

FIG. 3 is a cross-sectional view of the door assembly along the lines 3-3 of FIG. 2;

FIGS. 4 and 5 represent a portional sectional view of the edge seal area of the door assembly along the lines 4-4 and 5 5, respectively, of FIG. 2;

FIG. 6 is an enlarged rear view of the door assembly;

FIG. 7 is a partial sectional view of the door assembly taken along the line 77 of FIG. 5;

FIG. 8 is another partial sectional view of the door assembly taken along lines 8-8 of FIG. 5; and

FIG. 9 is a greatly enlarged view of a segment of the perforated plate making up the door viewing screen.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, which illustrates a preferred embodiment of the invention, there is shown a microwave oven 10 having a door assembly 12 constructed in accordance with the present invention. The door assembly 12 may be used with any number of styles of microwave ovens, the one shown in FIG. 1 being illustrative only. The illustrated microwave oven is equipped with a heavy duty magnetron which delivers microwave energy in the radio frequency band of 2,450 MHz, for rapid reheating, cooking and defrosting food.

The oven 10 has a conventional timing selector 14, the illustrated example being a six push-button type timer which can be preset and reset to the users requirement to program short time operations. A twospeed dial type timer 16 for longer operations may also be provided. A separate start button 18 is used to activate the dial timer 16 for timed operations. A power switch 20 is provided having three positions Off, Standby and Continuous. With switch 20 in the Standby position, a cooling blower for the magnetron comes on automatically when the operator opens the oven door and a ten second warm-upof the tube begins. If the door is then closed and no timing cycle is initiated, the blower will continue to operate for thirty seconds and then automatically shut off. Opening the oven door will restart the cycle. With switch 20 in the Continuous position the blower comes on immediately and a ten second warm-up of the tube begins, after which the oven is continuously ready for operation. Indicator light 22 is illuminated whenever either the push button timer 14 or the dial timer 16 in is operation.

The oven 10 has a metal cabinet 24, preferably of stainless steel, with a sealed-in high temperature ceramic shelf. The shelf is preferably made of the ceramic material Pyroceram sold by Corning Glass Works, Corning, New York. The oven may be conveniently equipped with such known items as an oven light, an

As shown in FIGS. 3-5, the door assembly 12 has both a sealing ring 32 and a seal plate 34. The material of seal plate 34 is a thin metal, for example 0.015 inch stainless steel, which is capable of flexing for sealing pressure between the seal plate 34 and the metal plate 36 of the face of the cabinet 24 around the oven cavity. The surface of plate 34 has a thin coating 35 of a suitable dielectric organosol, thus

plates

34 and 36 form a capacitive seal. As an additional sealing means conductive sealing ring 32 assures the existence-of a conductive connection around the periphery of the oven cavity 37, thereby preventing leakage of microwave energy from the heating chamber. Area 38 next to the seal plate 34 is filled with a foam material. The rear edge of the door outer shell 40 is slanted at 42 to provide clearance between the door 12 and the plate 36 as the door swings open on its

hinges

26 and 28.

Formed in the seal plate 34, and integral therewith, is a perforated area or screen 44. As best illustrated in FIG. 9, the perforations 50 in the screen 44 are arranged in a staggered pattern on a 60 array. The perforations 50 are 0.015 inch in diameter, +0.000 inch, 0.003 inch on 0.031 inch centers. There are approximately 900 holes per square inch of perforated area. These holes 50 may conveniently be formed by an etching process. I-Ioles of this minute size provide an opening less than one three-hundredth of a wavelength of the frequency of the microwave energy used. Thus the seal of the microwave oven cavity permits no more than one milliwatt per square centimeter (1- mw/cm of radiation leakage.

Likewise, holes 50 are smaller in diameter than most of the metal objects commonly found in the kitchen. Therefore, it is not possible even intentionally to insert most objects into one of the holes in the screen. In addition, tests have shown that conductors, such as uninsulated wires, of a diameter small enough to be inserted through the holes will burn off within the cavity without any radiation leakage.

These tests have also indicated that by insertion, under very exacting conditions, of insulated wires having a diameter of less than 0.015 inch where the insulation was a heavy duty conformable insulating coating, emission of the microwave radiation above 1 mw/cm might temporarily be achieved. Even then, the wire soon burns off at radiation levels above 1 mw/cm and/or the insulation will break down and permit arcing to the screen, thereby stopping the emission. It should be further noted that when such a wire is inserted through the screen, the temporary radiation of microwave energy is from a point source and does not create radiation directly over a large area, but instead it is emitted in a very small region around the point where the wire is passed through the screen.

When a protective transparent covering 52 (FIGS. 1-3 and 7-8) is placed in front of screen 44, the likelihood of inserting an object through one of the holes 50 is even more remote. The transparent covering 52 can be made of glass, plastic, etc. As an example, it may be made of clear ultraviolet stabilized polycarbonate having a thickness of 0.250 inch. Protective covering 52 is swingably attached to the door assembly by hinges 54 and 56 (FIGS. 1-3), hinge 54 being shown enlarged in FIG. 8. As illustrated hinge 54 utilizes a steel cylindrical pin 58 inserted into hole 60 bored into the transparent covering 52. The protective plate 52 can, thus, rotate around pin 58 when it is swung open.

At the outer edge of protective covering 52, opposite hinges 54 and 56, there is a latch 62 (FIGS. 1-3) which is shown in enlarged form in FIG. 7. Latch 62 consists of a captive screw 64 made of chrome plated brass. The screw 64 is locked into attachment with protective covering 52 by washer 66. Screw 64 is also adapted to screw into steel weldnut 68 which is attached to bezel 70 which defines the opening in the face of the door as sembly l2. Bezel 70 is attached to seal plate 34 by drive screws 72 (FIG. 7) which are driven through holes 74 (FIG. 6) in the plate. Pins 76 through holes 78 attach the front of bezel 70 to the outer shell 40 of the door assembly 12. When it is desired to open the protective covering 52 to clean it and the screen underneath, screw 64 is merely turned until it disengages from weldnut 68. It may be reengaged in order to close the protective covering 52. I

As best illustrated in FIGS. 1 and 2, protective covering 52 is smaller in area than the opening in the face of the door assembly 12 asdefined by bezel 70. Thus there is a continuous (except for the areas where latch 62 and hinges 54 and 56 are attached) space 80 around the periphery of covering 52 which acts as an air vent. Since protective covering 52 is spaced a short distance apart from screen 44, an air passage 82 (FIG. 7) is formed between screen 44 and plate 52 so that the vapors and fumes from the heating cavity escape through holes 50 and out the peripheral air vent 80 around the edge of the covering. As pointed out in US. Pat. 3,679,855, such air passages permit currents of air to sweep the inner surface of the protective covering 52 so as to prevent the condensation of vapor on the inner surface of the covering. In addition, however, the air vents of the present invention present no cleaning problem. i

It should be noted, however, that while protective covering 52 is smaller than the opening in the face of the door assembly, it is sufficiently large to cover the perforated area of screen 44. Therefore, it provides additional protection against insertion of objects through holes 50 in the screen. Small diameter insulated wires of the type previously mentioned are generally not capable of retaining the bent shape necessary to permit them to go around the screen and through a hole. Wires which are capable of retaining such a shape are usually of a diameter larger than the hole size and will not fit through holes 50.

Thus, it can be seen that the door assembly of the present invention provides a safe and convenient means for venting fumes and vapors from the heating cavity of microwave ovens and permitting viewing of the interior of the cavity. At the same time, it prevents, to an extent heretofore unobtainable, the leakage of radiation from the cavity and the possibility of inserting objects through the screen. All of this is accomplished with a venting screen which is easily accessible for cleaning.

While the articles herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise articles, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. A door assembly for microwave ovens wherein the oven cabinet has a face of substantial width around the oven cavity, said door assembly having a flexible seal plate which, when the door is closed, provides sealing pressure between said seal plate and the cabinet face area around the oven cavity, said seal plate having formed integrally therein a perforated area serving as both a viewing screen and a venting screen, the holes in said perforated area having a diameter no greater than in the order of 0.015 inch so as to seal against radiation leakage from the oven and to prevent the insertion of most objects through said holes, wherein said conductors which are small enough in diameter to fit through said holes are burned off at radiation levels in excess of l mw/cm thereby preventing harmful radiation leakage.

2. A door assembly as recited in claim 1 further including a conductive rubber sealing ring around the periphery of said seal plate.

3. A door assembly as recited in claim 1 wherein said holes are arranged in a staggered pattern on approximately 0.031 inch centers.

4. A door assembly as recited in claim 1 wherein said holes are etched in said seal plate to form said integral perforated area.

5. A door assembly as recited in claim 1 further including a bezel attached to said seal plate, said bezel defining an opening in the door assembly, said opening being somewhat larger in areathan the perforated area of said seal plate.

6. A door assembly as recited in claim 5 further including an openable transparent plate over said perforated area of said seal plate, said transparent plate being attached to said bezel and spaced a slight dis-.

tance apart from the seal plate, said transparent plate being slightly larger in area than said perforated area but slightly smaller in area than said opening in the door assembly so as to-provide a substantially continuous air vent around the periphery of said transparent plate.

7. A viewing and venting assembly for a microwave oven having a closed cavity and a thin viewing screen which is the only vent for vapors and fumes from the heating cavity of the oven, the improvement comprising holes in said viewing screen having a diameter no greater than in the order of 0.015 inch, wherein conductors which are small enough in diameter to fit through said holes are burned off at radiation levels in excess of 1 mw/cm thereby preventing harmful radiation leakage, said holes being closely spaced to provide a viewing area through the otherwise opaque screen.

8. An assembly as defined in claim 7 wherein the diameter of said holes is approximately equal to the thickness of said screen.

9. A door assembly for microwave ovens wherein the oven cabinet has a face of substantial width around the oven cavity, comprising:

a door frame attached by hinges to the body of said oven cabinet,

the inner face of said door assembly, adjacent said door frame, being a thin flexible metal seal plate which when the door is closed provides sealing pressure between said seal plate and the cabinet face area around the oven cavity,

said seal plate having an integral perforated area consisting of a large number of holes etched in said seal plate, said holes having a diameter no greater than in the order of 0.015 inch arranged at a 60 array on approximately 0.031 inch centers, wherein conductors which are small enough in diameter to fit through said holes are burned off at radiation levels in excess of l mw/cm thereby preventing harmful radiation leakage,

a further sealing means in the form of a conductive rubber sealing ring around the periphery of said seal plate,

a bezel attached to said seal plate and defining an opening in the door assembly, said opening being somewhat larger in area than the perforated area of said seal plate, and

an openable transparent plastic rated area of said seal plate,

said transparent plate being attached to said bezel and spaced a slight distance apart from the seal plate, said transparent plate also being slightly larger in area than said perforated area but slightly smaller in area than said opening in the door assembly to provide a substantially continuous air vent around the periphery of said transparent plate while preventing frontal access to said perforated area.

plate over said perfo-

Claims

1. A door assembly for microwave ovens wherein the oven cabinet has a face of substantial width around the oven cavity, said door assembly having a flexible seal plate which, when the door is closed, provides sealing pressure between said seal plate and the cabinet face area around the oven cavity, said seal plate having formed integrally therein a perforated area serving as both a viewing screen and a venting screen, the holes in said perforated area having a diameter no greater than in the order of 0.015 inch so as to seal against radiation leakage from the oven and to prevent the insertion of most objects through said holes, wherein said conductors which are small enough in diameter to fit through said holes are burned off at radiation levels in excess of 1 mw/cm2 thereby preventing harmful radiation leakage.

5. A door assembly as recited in claim 1 further including a bezel attached to said seal plate, said bezel defining an opening in the door assembly, said opening being somewhat larger in area than the perforated area of said seal plate.

6. A door assembly as recited in claim 5 further including an openable transparent plate over said perforated area of said seal plate, said transparent plate being attached to said bezel and spaced a slight distance apart from the seal plate, said transparent plate being slightly larger in area than said perforated area but slightly smaller in area than said opening in the door assembly so as to provide a substantially continuous air vent around the periphery of said transparent plate.

7. A viewing and venting assembly for a microwave oven having a closed cavity and a thin viewing screen which is the only vent for vapors and fumes from the heating cavity of the oven, the improvement comprising holes in said viewing screen having a diameter no greater than in the order of 0.015 inch, wherein conductors which are small enough in diameter to fit through said holes are burned off at radiation levels in excess of 1 mw/cm2 thereby preventing harmful radiation leakage, said holes being closely spaced to provide a viewing area through the otherwise opaque screen.

9. A dooR assembly for microwave ovens wherein the oven cabinet has a face of substantial width around the oven cavity, comprising: a door frame attached by hinges to the body of said oven cabinet, the inner face of said door assembly, adjacent said door frame, being a thin flexible metal seal plate which when the door is closed provides sealing pressure between said seal plate and the cabinet face area around the oven cavity, said seal plate having an integral perforated area consisting of a large number of holes etched in said seal plate, said holes having a diameter no greater than in the order of 0.015 inch arranged at a 60* array on approximately 0.031 inch centers, wherein conductors which are small enough in diameter to fit through said holes are burned off at radiation levels in excess of 1 mw/cm2 thereby preventing harmful radiation leakage, a further sealing means in the form of a conductive rubber sealing ring around the periphery of said seal plate, a bezel attached to said seal plate and defining an opening in the door assembly, said opening being somewhat larger in area than the perforated area of said seal plate, and an openable transparent plastic plate over said perforated area of said seal plate, said transparent plate being attached to said bezel and spaced a slight distance apart from the seal plate, said transparent plate also being slightly larger in area than said perforated area but slightly smaller in area than said opening in the door assembly to provide a substantially continuous air vent around the periphery of said transparent plate while preventing frontal access to said perforated area.