CA1092658A - Microwave oven with corrosion-resistant oven cavity walls - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A microwave oven equipped with a rotary table is driven by magnetic coupling. The oven has a heating chamber at least the bottom wall or base plate of which is made of non-magnetic metallic material, while its other walls are formed of another and cheaper material. This reduces the cost of the microwave oven. Junctions between the walls of the non-magnetic plates and the other plates are coated with a film to improve corrosion resistance. Furthermore, the heating chamber and the rotary table are so arranged as to facilitate the insertion and withdrawal of a container for food material into and from the heating chamber. This helps to keep the oven clean and is convenient to use.

Description

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The present invention relates to a high frequency heating apparatus and more particularly to a microwave oven or electronic oven. In one example, the oven is equipped with a rotary table or turntable for receiving an object to be heated.
A microwave oven which is now widely used essentially includes an oven-defining structure preferably of double wall construction provided therein with a heating cavity or heating chamber having a hingedly supported door which is adapted to selectively open and close an access opening formed at one side of the oven defining structure. A magnetron assembly is provided for generating high fre~uency energy to heat an object ox food material placed in the heating cavity based `~ -on the principle of dielectric heating. Some microwave ovens are further provided with rotary tables or turntables within the heating chambers for rotation together with the objects or food material mounted thereon, so that the degree of heating of such objects is more uniform.
Conventionally, in a microwave oven equipped with a rotary table of the above described type, the rotational force of the motor employed as the driving source is mechanically transmitted via reduction gears to a rotary shaft which extends into the heating chamber through its bottom plate and is rotatably journalled to be fitted, for example, into a corres-; ponding opening formed in the rotary table for engagement therebetween. A plurality of rollers are mounted between the ;~
lower surface of the rotary table and the bottom plate of the heating chamber for smooth rotation of the rotary table.
The conventional microwave ovens of the above ~
30 described type, however, have various diciadvantages in that; -(i) Since the rota~y shaft extends into the -' ~ '`
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heating chamber through the bottom pla-te thereof, excessive heat generation, electric wave leakage and the like tend to take place during operation of the microwave oven, at the portion oE the rotary shaft pro~ecting into the heating chamber. Prevention of such inconveniences causes the construc-tion of the microwave oven to become undesirably complicated, with consequent high cost.
(ii) The projection of the rotary shaft into the heating cavity and provision of the rollers undetachably disposed within the heating chamber for smooth rotation of the rotary table make it difficult to clean the interior of the heating cavity in an efficient manner.
(iii) Oil drops, juice and the like produced by the food material during cooking in the heating chamber, or water used for cleaning the interior of the heating chamber, tend to leak out of the heating chamber through a clearance between the shaft and the bottom plate of the heating chamber to find their way toward the motor, reduction gears and the like, ~hus not only giving rise to various troubles, ~0 but emitting odor in the course of time.
(iv) Since the rotary table is mechanically ; coupled to the shaft, it is difficult to readily remove the rotary table when necessary.
(v) Although non-magnetic metallic material is most suitable for the walls of the heating chamber from various aspects, such non-magnetic metallic material is expensive, resulting in a high cost for the oven. On the other ~ -hand, if a magnetic metallic material which is less expensive is employed, corrosion of such material comes into question, and (vi) When a vessel or container of glass, ceramic -:

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material and the like is mounted on the rotary table, with the food material placed therein, such a vessel of hard material tends to damage the walls of the heating chamber upon coming into contact therewith during rotation of the rotary table or insertion and withdrawal of the vessel into and from the heating chamber.
Accordingly, an essential object of the present i invention is to provide a microwave oven equipped with a rotary table in which a magnetic driving system is employed to eliminate the rotary shaft extending into the heating chamber for efficient use of the microwave oven, with interior of the heating cavity coated to impart a clean appearance and also an economical and durable structure to the heating chamber.
Another important feature of an embodiment of the present invention is to provide a microwave oven of the above described type in which a non-magnetic metallic material of high magnetic permeability is employed for a particular wall of the heating chamber to allow magnetic flux to pass there-20 through for the magnetic driving, while the other walls of -the same heating chamber are formed with a less expensive metallic material other than the non-magnetic metallic material, whereby to reduce the cost for the oven.
A further feature of an embodiment of the present invention is to provide a microwave oven of the above described type in which the interior of the heating chamber is coated to prevent corrosion, especially at the junctions between the walls of the non-magnetic metallic material and the the metallic material other than the non-magnetic metallic 30 material, due to battery action or the like. ~ -A still further feature of an embodiment of the -.
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present invention is to provide a microwave oven of the above described type in which a vessel or container to be mounted on the rotary table for placing food material to be heated therein is unlikely to come into contact with or rub against the coated walls of the heating cavity to avoid damage to the coating and consequent corrosion.
Another feature of an embodiment of the present invention is to provide a microwave oven of the above described type in which an upper surface of the rotary table is arranged . 10 to be higher than that of the heating chamber bottom wall or base plate for improved processability during manufacture and efficient cleaning in use, with simultaneous facilitation of insertion and withdrawal of the object to be heated into and from the heating chamber.
Having regard to the foregoing the invention can :
be defined as a microwave oven comprising: an oven .
defining structure; a heating cavîty within said oven ~;
defining structurer said heating cavity being defined by a .
base plate formed oE a non-magnetic metallic material and by a top wall, a front wall having an access opening therein, a pair of side walls, and a rear wall, said walls : being formed of magnetic metallic material, said base ; plate being joined àt junctions to said front wall, said : -:
side walls and said rear wall; door means for selectively ~' opening and closing said access opening in said front ; wall; means for supplying microwave energy into said heating cavity: rotary table means, selectively and ::
readily removably positioned to rotate in contact with a ~.
top surface of said base plate within said heating cavity, for receiving an object to be heated, said rotary table ~ 4 ~
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means being free of any fixed attachment to said base plate; magnetic means, mounted below said base plate and exterior of said heating cavity, for rotating said rotary table means within said heating cavity; and coating means, covering said junctions and said top surface of said base plate, except a portion thereof contacted by said rotary table means, for preventing corrosion of said heating cavity.
These and other features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the attached drawings in which; -Fig. 1 is a perspective view of a microwave oven to which the present invention is applicable, with a door thereof in an opened state, Fig. 2 is a similar view to Fig. 1, but an outer casting and the door thereof are removed for clarity, Fig~ 3 is a schematic sectional view of the microwave oven of Fig. 1, Fig. 4 is an exploded view showing construction of the heating chamber employed in the microwave oven of Fig. 1, and Fig. 5 is a similar view to Fig. 3, but showing a modification thereof.
Referring now to the drawings, there is shown in Figs. 1 and 2 a m:icrowave oven M to which the present inven~
tion is applicable. The microwave oven M heat-treats objects -or food material based on the principle of dielectric heating by utilizing high frequency energy, for example, of the order ':
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of about 2450 MHz~ and generally includes an outer casing 1 of cubical box-like shape open at the front side thereof.
The outer casing 1 forms a double wall structure together with inner walls 2 and defines a heating cavity or heating chamber H. The inner walls 2 include a horizontal bottom wall or base plate 2a, vertical side walls 2b, a top wall 2c and a rear wall 2d and also a front wall 2e which defines an access opening O at the front of the oven M. In the heating chamber . H immediately above the horizontal base plate 2a, there is rotatably disposed a rotary table 8 in a manner described later, on which table 3, a vessel or dish for accommodating an object or food material to be heated is detachably mountable.
Outer surfaces of the walls 2a, 2b, 2c and 2d are spaced from the corresponding walls of the outer casing 1 for providing spaces therebetween. The outer casing 1 further includes an outside front wall portion la immediately above the opening O. On the front wall portion la, there is mounted a control panel 7 for controlling the functioning of a high frequency oscillator. The control panel 7 carries a rotatable drum D
having a scale Ds of heating times for a plurality of foods imprinted on its peripheral surface in positions correspon- .
ding to a timer scale ts, a pointer needle Da for the drum D, .:
a drum rotating ring Db, a timer operating knob t, a function ' indication lamp Q to be illuminated during emission of high frequency energy and a cooking start button c for initiating the high frequency heating. For setting the heating time for : optimum cooking, the drum D is rotated by the ring Db to find the heating time required for a particular food to be cooked, to which the pointed needle Da is aligned by the timer operating knob t.
Further included in the microwave oven M is a ~ ~

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door 4 provided with a handle 6 adjacent one edge thereof remote from a hinge through which the door 4 is supported at the lower edge thereof to the lower front edge of the casing 1 in a position corresponding to the access openiny 0 for pivotal upward and downward movements about the hinge to open and close the opening O. The door 4 has a rectangular observation window 5 to allow the object in the heating cavity ~ to be observed and also for preventing the high frequency energy from leaking out of the heating chamber H during operation. The outer casing l further includes an interior portion which provides sliding accommodation for a pair of door arms 3, each of which is pivotally connected to a lower side portion of the door 4 and passes through a portion between the ~ide walls of the casing 1 and the corresponding side walls 2h of the heating cavity H.
Referring particularly to Fig. 2, on the top wall 2c of the heating chamber H in the space defined by said top wall 2c and the corresponding top wall of the outer casing 1, there are mounted the high frequency oscillator or magnetron assembly 10 for radiating high frequency energy into the heating chamber H, a fan motor 11 for cooling the magnetron assembly 10, an air guide 12 for leading the air after cooling the magnetron assembly 10 into the heating chamber H, a high voltage transformer 13 for supplying high voltage to the magnetron assembly 10, and worm gears 15 for ~-transmitting the rotation of the fan motor 11 toward the rotary table 8 in a manner described in detail later, while a stirrer fan 14 is disposed to protrude into the upper portion `
of the heating chamber H for stirring the high frequency energy therein.
Referring also to Fig. 3, the mechanism for driving ., -" ~.:

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the rotary table 8 in the heating chamber H will be described hereinbelow.
A first pulley lla secured to a rota$irlg shaft of the fan motor 11 is connected to a second pulley 15a fixed to a worm shaft of the worm gears 15 through a belt 16, while a wheel shaft of the worm gears 15 extends downwardly to a bearing (not shown) secured to the bottom wall of the outer casing 1 to form a conducting rod 17, with a third pulley 18 being secured adjacent to the lower end of the rod 17. The pulley 18 is connected through a belt 20 to a pulley 19 which is rotatably disposed at approximately the ~:
same height as the pulley 18 under the lower surface of a circular recess 9 formed in a generally central portion of the base plate 2a of t.he heating chamber H for accommodating :
therein the rotary table 8. The pulley 19 is mounted on a shaft l9a, being secured, at one end thereof, in approximately the :
central portion of the lower surface of the recess 9, for example, by welding and with the support of a pin l9b. On the upper surface of the pulley 19 there are disposed a plurality of magnets 21, and rollers 22, for example, of synthetic resinous material, which rotatingly contact the lower surface of the circular recess 9. The rotary table 8 rotatably supported within the recess 9 through rollers 23, for example, of similar synthetic resinous material, is also provided with a plurality of magnets 24 fixed at its lower surface in positions corresponding to the magnets 21 of the pulley 19. Accordingly, upon rotation of the pulley 19, the magnets 24 of the rotary table 8 are attracted by the magnets of the pulley 19, with consequent rotation of the rotary 30 table 8. The rollers 23 for the rotary table 8 are more than three in number and are hPld in place, for example, by : - 8 -~z~

a coupling ring 25. The vessel or container V in which the object or food material F is to be placed is detachably mounted on the rotary table 8 for insertion in-to or withdrawal from the heating chamber H as required.
Referring also to Fig. 4, the side walls 2b and the rear wall 2d are preferably integrally formed into a single wall of approximately U-shaped cross section. The top wall 2c, the main portions of the front wall 2e, side walls 2b, rear wall 2d and top wall 2c are made of a metallic material other than a non-magnetic metallic material, while the base plate 2a is constituted by a non-magnetic metallic material having high permeability to magnetic flux, for example, aluminum, aluminum alloy, stainless steel of the austenite group or the like.
In general, although a non-magnetic metallic material having a large elongation and readily subject to a drawing process during manufacture while retaining sufficient strength is suitable for magnetic driving of the rotary table 8 due to its good permeability to magnetic flux, such material is very expensive, resulting in an extremely high cost for the microwave oven, if the heating chamber H is constructed totally from non-magnetic metallic material. Accordingly, in a microwave oven M according to the present invention, use of this expensive material is limited to the portion directly utilized for the magnetic driving, while other portions of ; the heating chamber H can be made of any other suitable material, such as steel, iron or plastic. This enables a marked reduction of manufacturing cost which is achieved with-out sacrificing the performance of the microwave oven.
Furthermore, after the walls 2a to 2e have been assembled, for example, by welding to form the heating chamber _ 9 _ .~ :
- , - H, at least junctions iY (Fig. 3) between the walls of the non-magnetic material and the other material are covered with a suitable coating, for example, of a synthetic resinous material for protection against air, water, juice or the like from the food. The ~unctions are thus free from wet corrosion, and the heating chamber has high durability. The formation of the coating at the junctions j is also advantageous to avoid accumulation of crumbs or chips of food material thereat for providing a clean heating chamber free from an unsanitary appearance.! Additionally, a sufficiently durable coating may also be formed on the portion on the upper surface of the circular recess 9 contacted by the rollers 23 suppor-ting the rotary table 8. Such coating should resist soiling by pigments or the like from crumbs or chips of food material depressed by the rollers 23 to which all the weight of the rotary table 8, the vessel V and the food material F
is applied. The coating should also resist being peeled off.
By this arrangement, not only is the coating kept clean for a long period of time, but the slippage loss in the rotation of the rollers 23 can be reduced, because the frictional resistance upon contact between synthetic resinous materials is generally larger than in the case where metallic material -contacts synthetic resinous material.
The vessel V for accommodating the food material F is made of glass or ceramic material so that the food material F is heated from all sides by the high frequency ` energy, while preventing damage to the vessel V even if the ; food material F is scorched or burnt. Therefore, if a coating or paint is applied to the surface of the rotary table 8, such coating may readily be spoiled and give rise to corrosion, if the vessel V is repeatedly placed on or - 10 - : ' : ~ ' , - , . . . .

removed from the surface of the rotary table 8. Accordingly, a corrosion resistant metallic plate, for example, of stainless steel or synthetic resin such as a polyester, is employed for the rotary table 8. A microwave oven having a clean appearance and free from corrosion is thus obtained with improved durability. Moreover, since the surface of the rotary table 8 is arranged to be higher than the surface of the base plate 2a by a distance h (Fig. 3), it is possible to increase the diameter of the rollers 23 to reduce the frictional resistance during their rotation, and also to make it easy for the rollers 23 to override minor obstacles such as crumbs or chips of food material which may be present. The arrange-ment described above is also effective for reducing the depth of the circular recess 9 which tends to be large in conven-tional microwave ovens wherein the distance h is zero or negative. This facilitates the drawing process in he manu-facture of the ovens, with simultaneous elimination of the development of cracks at the corners during such process, and an improvement of yield. Also the mounting and detaching of the rotary table 8 in the heating chamber H are facilitated, with improved accessibility to the recess 9 for cleaning.
Furthermore, according to the arrangement of the microwave oven described above, the bottom rim Va of the vessel V is advantageously prevented from touching the surface of the base plate 2a during insertion and withdrawal of the vessel V into and from the heating chamber H, so that even when this surface of the base plate 2a is coated, such coating is free from damage whereby to provide the heating chamber with improved durability. Additionally, when the microwave oven is provided with the door 4 which can be opened toward the user as shown in Fig. 3, the height of the inner surface c-c' ~ . . , , ~ :' of the door 4 when fully opened is arranged to be approximately the same as that of the upper surface of the rotary table 8.
Therefore the vessel V can be held horizontal at all times during insertion or withdrawal of such vessels for preventing any food materlal from spilling therefrom, thus making the microwave oven very convenient to use.
Referring to Fig. 5, there is shown a modification of the microwave oven of Figs. 1 to 4. In this modification, the insides of the walls 2b, 2c, 2d and 2e of the heating chamber H are coated with a coating film P, except for the inner surface of the circular recess 9 of the base plate 2a, by which arrangement the corrosion resistance of the heating chamber H is further increased, with improved appearance and cleanliness. Since the other functions and construction of the microwave oven of Fig. 5 are similar to that of Figs.
1 to 4, the detailed description thereof is abbreviated for brevity.
It is to be noted here that, although the present invention is mainly described with reference to a microwave oven equipped with the rotary table, the concept of the present invention is not limited to such microwave ovens alone~ but may readily be applicable to other heating apparatus without rotary tables which utilize high frequency energy for dielectric heating.
Although the present invention has been fully described by way of example with reference to the attached drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. For example, the rear wall 2d described as integrally formed -with the side walls ~b to form the single wall of U-shaped cross section may be modified to be separately formed for :~ , , - . .

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rigid connection with said side walls 2b. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as lncluded therein.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A microwave oven comprising:
an oven defining structure;
a heating cavity within said oven defining structure, said heating cavity being defined by a base plate formed of a non-magnetic metallic material and by a top wall, a front wall having an access opening therein, a pair of side walls, and a rear wall, said walls being formed of magnetic metallic material, said base plate being joined at junctions to said front wall, said side walls and said rear wall:
door means for selectively opening and closing said access opening in said front wall;
means for supplying microwave energy into said heating cavity;
rotary table means, selectively and readily removably positioned to rotate in contact with a top surface of said base plate within said heating cavity, for receiving an object to be heated, said rotary table means being free of any fixed attachment to said base plate;
magnetic means, mounted below said base plate and exterior of said heating cavity, for rotating said rotary table means within said heating cavity; and coating means, covering said junctions and said top surface of said base plate, except a portion thereof contacted by said rotary table means, for preventing corrosion of said heating cavity.

2. A microwave oven as claimed in claim 1, wherein said base plate has therein a substantially circular recess, said rotary table means being rotatably mounted within said recess, which recess constitutes said portion of the base plate not covered with the coating means.

3. A microwave oven as claimed in claim 2, wherein the upper surface of said rotary table means is located at a level higher than the level of the uppermost portion of said base plate.

4. A microwave oven as claimed in claim 3, wherein said door means is hingedly supported for pivotal rotation at one edge thereof to said oven defining structure, so that when said door means is in a position opening said access opening, an upper surface of said door means extends substantially horizontally at a position approximately level with said upper surface of said rotary table means.

5. A microwave oven as claimed in claim 1, wherein said rotary table means comprises a rotary table member supported on said base plate solely by a plurality of roller members positioned substantially peripherally of said table member, said roller members being formed of a synthetic resin material.

6. A microwave oven as claimed in claim 5, wherein that portion of said base plate which is contacted by said roller members during rotation of said rotary table member is covered with a durable coating of synthetic resin material.