DOOR ASSEMBLY FOR MICROWAVE HEATING APPARATUS

Abstract: A high frequency heating apparatus comprising an oven-defining structure having an access opening in com­ munication with a heating chamber (19) defined therein, and a hingedly supported metallic door for selectively opening and closing the access opening. The door has its peripheral area formed with a groove open towards the oven-defining structure, the bottom of which groove is situated frontwardly of the door with respect of the apparatus. An outer wall (10) of the groove is formed with a plurality of cutouts (7A) and a plurality of generally rectangular openings (13) alternating with the cutouts, each of the cutouts and each of the rectangular openings having one edge positioned on one side of a setup portion, protruding from the bottom of the groove, adjacent the oven-defining structure. The outer wall has a portion bent to protrude transversely into the groove to define a partitioning wall (15).

Description: The invention relates to a high frequency heating apparatus comprising an oven-defining structure having an access opening in communication with a heating chamber defined therein; and a hingedly supported metallic door for selectively opening and closing the access opening; said door having its peripheral area formed with a lateral flange having a groove being open towards the oven-defining structure and defined by an inner, a bottom and a backturned outer wall, the bottom of the groove being situated frontwardly of the door with respect to the apparatus, and the groove containing a partition wall member having a plurality of transverse cutouts alternating with a plurality of rectangular openings. As is well known to those skilled in the art, it is a customary practice to provide the door assembly of a microwave oven with a high frequence attenuator for the prevention of leakage of microwaves out of the enclosure. Many types of attenuators currently utilized are known, and the most popular one is a choke type. As disclosed in, for example, United States Patent No. 3,182,164, the choke type is characteristically constituted by a choke groove which is defined in a generally rectangular metallic frame of the door assembly so as to have its effective depth equal to one fourth of the wavelength of the microwaves used. On the other hand, the use of the choke groove having its effective depth smaller than one fourth of the wavelength is also well known from, for example, PCT International Publication No. WO84/01083 as shown in Fig. 1 of the accompanying drawings. Referring to Fig. 1 the door assembly comprises a perforated metal plate 1 having a plurality of perforations defined at that portion thereof which coincides with the access opening of the enclosure, and also having a non-perforated peripheral portion so bent and so shaped as to have a generally channel-shaped cross-section delimited by an inner wall 1a extending outwardly of the enclosure at right angles to the perforated body of the plate 1, an outer wall 1b parallel to the inner wall 1a, and a front wall 1c connecting the inner and outer walls 1a and 1b together and spaced a predetermined distance from the plane of the perforated body of the plate 1, said walls 1a, 1b and 1c altogether defining a choke cavity or groove 2. The door assembly shown in Fig. 1 also comprises a generally C-sectioned partition wall member 3 accommodated within the choke groove 2 and having a base 3a secured to the front wall 1c, a lateral wall 3b parallel to the outer wall 1b and perpendicular to the base 3a, a rear wall 3c perpendicular to the lateral wall 3b and parallel to and confronting the base 3a, and a parting wall 3d perpendicular to the rear wall 3c and extending a predetermined distance from the rear wall 3c in a direction close towards the base 3a in parallel relation to the inner wall 1a. The partition wall member 3 has a plurality of equally spaced cutouts 4 each traversing the walls 3d, 3c and 3b and terminating at the joint between the base 3a and the lateral wall 3b, and also has a plurality of generally rectangular openings 5 defined in the lateral wall 3b in alternating relationship with the cutouts 4. In this construction, the rear wall 3c of the partition wall member partially closes the opening leading to the choke groove 2 whereas a groove delimited between the inner wall 1a and the parting wall 3d constitutes an inlet line for the introduction of the high frequency into the choke groove 2, which groove between the walls 1a and 3d is widened beyond the free edge of the parting wall 3d opposite to the rear wall 3c with the impedance characteristic of the inlet line consequently varied. By optimizing a combination of the narrow and wide grooves referred to above, it is possible to reduce the depth of the choke groove to a value smaller than the quarter wavelength of the high frequency used. A groove delimited between the partition wall member 3 and the outer wall 1b may be referred to as a second choke groove operable to attenuate that component of the microwave power which has leaked from the above described, first choke groove without having been completely attenuated. The cutouts 4 periodically defined in the partition wall member 3 over the length thereof serve to restrict the propagation of the high frequency in a direction lengthwise of the partition wall member 3. The structure defining the above described choke groove is advantageous in that, in order to realize the choke groove of a depth equal to one n-th of the wavelength of the high frequency used, compactness and lightweight feature can be accomplished by increasing the number n. However, the extent to which the accuracy of the dimensions of the various component parts brings about change in characteristic impedance tends to increase with increase of the number n, and therefore, accuracy of the dimensions of the various component parts is an extremely important factor for achieving an optimum attenuation of the high frequency energy. More specifically, if the number n is of a great value, a slight change in width of the high frequency inlet line brings about a relatively great change in characteristic impedance with the consequent reduction in attenuation. While the partition wall member 3 is often secured to the front wall 1c by the use of a spot welding technique, the positioning is difficult during the welding and, accordingly, it has long been felt difficult to increase the accuracy in dimension. Moreover, since the lateral wall 3b of the partition wall member 3 is continued to the base 3a at local areas left by the cutouts 4 and the rectangular openings 5, the structure as a whole has an insufficient physical strength and is susceptible to bending during the machining and/or transportation, accompanied by detrimental changes in width of the choke groove. In order to compensate for reduction in attenuating power because of the presence of the above discussed problems, numerous methods are contemplated to use a second choke groove of increased width, to add structural elements to a portion adjacent the second choke groove to make it complicated in shape, to employ microwave absorbing elements such as ferrite, and so on. However, all of the contemplated methods tend to increase the dimensions as well as the weight of the door assembly, rendering the microwave oven as a whole to be costly. Furthermore, in order to reduce the size of the choke groove and to simplify the method for the manufacture thereof, Japanese Laid-open Patent Publication No. 59-177893 discloses the choke groove formed by preparing a generally rectangular metal plate having its four side portions so slit inwardly as to leave a plurality of tongues and then bending these tongues inwardly so as to represent a generally G-shaped cross-section. Even in this example, making the choke groove compact results in the reduction of the physical strength of the frame structure for the door assembly as a whole to such an extent that the door assembly may deform or warp during the use thereof and/or the door assembly may fail to tightly contact the front trim panel around the access opening when in the closed position. Further prior art solutions, similar to the solution as disclosed in Fig. 1, are described and shown in DE-A-32 11 528 and DE-A-22 24 664. It is an object of the present invention to avoid the disadvantages of the prior art solutions, i.e. to provide a high frequency apparatus having a door assembly of compact construction, and which can be manufactured easily with good accuracy in order to obtain better attenuation of leaking microwave energy. The high frequency heating apparatus as defined in the preamble of Claim 1 is characterized in that a) the partition wall member is placed at a position flush with the outer wall, b) the outer wall has a reduced height in respect of the height of the inner wall, and c) the lower edges of the transverse cutouts and the lower edges of the rectangular openings are flush with the free edge of the outer wall. Preferable embodiments are defined in the subsidiary claims. The invention will now be described in conjunction with preferred embodiments therefore with reference to the accompanying drawings, in which: Fig. 1 is a fragmentary perspective view of a portion of a prior art door assembly for a microwave oven; Fig. 2 is a view similar to Fig. 1, showing a door assembly according to a preferred embodiment of the present invention; Fig. 3 is a transverse sectional view of a portion of the door assembly shown in Fig. 2; Figs. 4 and 5 are views similar to Fig. 1, showing that portion of a door assembly according to second and third preferred embodiments of the present invention, respectively; Fig. 6 is a transverse sectional view of a portion of a metal plate forming a part of a door assembly according to a fourth preferred embodiment of the present invention; Fig. 7 is a plan view of the metal plate shown in Fig. 6 before a choke is formed; Fig. 8 is a view similar to Fig. 3, used for the purpose of explanation of the relationship of dimensions; Fig. 9 is a schematic top sectional view of a door assembly showing the position of a magnetron activating switch relative to the door assembly; and Fig. 10 is a perspective view of one corner portion of a door assembly, showing the employment of a reinforcement plate in the door assembly according to the present invention. Referring first to Figs. 2 and 3, a hingedly supported door assembly for a microwave oven comprises a generally rectangular metal door 6 having its central area perforated at 20 for permitting an operator of the microwave oven to look into the heating chamber identified by 19. Its four-sided peripheral area is so shaped by the use of, for example, a metal forming technique, as to have an inner wall 8 perpendicular to the perforated central area of the metal door 6 and protruding frontwardly from the perforated central area. A front wall 9 protrudes laterally outwardly from the inner wall 8 and lies in a plane spaced a predetermined distance from the plane flush with the perforated central area. A backturned flange having an outer wall 10 protrudes a predetermined small distance rearwardly from the front wall 9 in parallel relation to the inner wall 8 so as to define a choke groove in cooperation with the inner and front walls 8 and 9, which groove opens towards a front trim panel 29 situated exteriorly around the access opening (shown by 28 in Fig. 8) leading to the heating chamber 19. The metal door 6 of the above described structure may be prepared from a sheet metal by the use of any known press work, for example, a metal forming or drawing technique. The door assembly also comprises a generally elongated slitted structure as a partition member 7 of generally C-shaped cross-section including a base 12, a lateral wall 11 which serves as an outer wall in relation to the inner wall 8 with the choke groove defined between them and which protrude from and at right angles to the base 12, a rear wall 14 protruding from and at right angles to the lateral wall 11 and confronting the base 12, and a partition wall 15 protruding from and at right angles to the rear wall 14 in a direction towards the base 12 with its free side edge 15a terminating at a position spaced a distance inwardly from the base 12. The partition wall member 7 is fixedly mounted on the metal door 6 with the base 12 held flat against and rigidly secured to the front wall 9, and has a plurality of transverse cutouts 7a defined therein in equally spaced relation to each other. Each of these cutouts 7a extends inwardly from the free side edge 15a of the partition wall 15 to the lateral wall 11 through the rear wall 14 and terminates spaced a predetermined distance inwardly from the joint between the base 12 and the lateral wall 11 leaving a trimmed lower edge 11a which is generally in flush with a free edge of the backturned outer wall 10 remote from the front wall 9. The slitted partition wall member 7 also has a plurality of generally rectangular openings 13 defined in the lateral wall 11 in equally spaced relation to each other and alternating with the cutouts 7a, each of said rectangular openings 13 being so positioned that one of the four sides of the shape of the respective opening 13 which is closest to the base 12 as shown by 13a can be held generally flush with the free edge of the backturned outer wall 10 and also in line with the trimmed lower edge 11a. The four-sided lip region left in the lateral wall 11 by the formation of each rectangular opening 13 has its four corners rounded slightly, and similarly, corners on respective sides of each trimmed edge 11a left in the lateral wall 11 by the formation of the respective cutout 7a are rounded slightly. Likewise, each of the joints between the front wall 9 and the backturned outer wall 10 and between the base 12 and the lateral wall 11 is correspondingly rounded. In the construction described above, a relatively narrow gap between the inner wall 8 and the partition wall 15 constitutes an inlet line for the high frequency, and the width thereof increases, as it passes beyond the free side edge 15a of the partition wall 15, to a value equal to the width between the inner wall 8 and the lateral wall 11. It is to be noted that the free side edge 15a of the partition wall 15 terminated flush with one of the four sides of the shape of each rectangular opening 13 which is farthest from the backturned flange 10 and opposite to the side edge 13a. As best shown in Fig. 3, the door assembly in practice has a generally rectangular transparent covering 17 fitted to the metal door 6 at a location on one side of the metal door 6 opposite to the oven-defining enclosure, and also has an elastic liner 16 made of synthetic resin and fitted to the metal door 6 so as to cover the gap between the inner wall 8 and the partition wall 15 overlaying the rear wall 14. The liner 16 has a plurality of pawls 16a formed integrally therewith and is fitted to the metal door 6 with the pawls 16a engaged into the rectangular openings 13 that are formed in the outer periphery of the choke groove. The liner 16 also has a projection 16b formed integrally therewith so as to project in a direction opposite to the pawls 16a, which projection 16b is engaged substantially in a shakehand fashion with a mating projection 18 integrally formed with the transparent front covering 17 so as to protrude towards the oven-defining enclosure, thereby permitting the liner 16 to be securely held by the metal door 6. As hereinbefore described, since the slitted partition wall member 7 is so positioned and so mounted on the metal door 6 with the generally right-angled joint between the base 12 and the lateral wall 11 held in abutment against the generally right-angled joint between the front wall 9 and the backturned outer wall 10, the slitted partition wall member 7 can be precisely positioned relative to the metal door 6 when they are connected together by the use of a welding technique or any other connecting method. Moreover, since the free edge of the backturned outer wall 10 and both of the trimmed lower edges 11a and the side edges 13a are flush with each other, it is easy to avoid any possible floatation of the slitted partition wall member 7 during the performance of welding or any other connecting process. Furthermore, because of the rounding at the joint between the base 12 and the lateral wall 11, and because of the employment of the base 12 continued to the lateral wall 11, the strength against the bending starting from the joint is high. Particularly, because the backturned outer wall 10 overlaps the lateral wall 11 thereby to provide a substantially double-walled structure, any possible change in position can hardly occur even when an external force is applied in a direction outwards. It is, however, pointed out that, even though the joint between the lateral wall 11 and the base 12 is reinforced, portions of the lateral wall 11 around the rectangular openings 13 and the cutouts 7a may remain weak, which disadvantage is compensated for by the presence of the slight rounding formed during the machining of the slitted partition wall member 7. It is to be noted that it is a general practice to employ a spot welding technique to secure the slitted partition wall member 7 to the choke groove defined by the walls 8, 9 and 10. Figs. 4 and 5 illustrate different methods for obviating inconveniences which may arise during the performance of a spot welding job. Referring to Fig. 4, a welding electrode can be inserted through the cutout 7a and a welding be carried out between the front wall 9, which forms the bottom of the choke groove, and a point 21 at the partition wall member 7. If at this time the backturned outer wall 10 is in contact with a root portion of the lateral wall 11 adjacent the joint with the base 12, a welding current will be divided at that portion with the consequence that a sufficient amount of welding current will not flow to the point 21, resulting in insufficient welding. Accordingly, in order to avoid the division of the welding current, a small gap d is provided between the root portion of the lateral wall 11 and that portion 10a of the lateral wall which aligns with each trimmed lower edge 11a. Fig. 4 makes it clear, the backturned outer wall 10 shown therein is so shaped and so formed as to have setback portions 10a spaced from the root portion of the lateral wall 11, and the remaining portions in contact with the root portion of the lateral wall 11, said setback portions 10a and said remaining portions alternating with each other. This is advantageous in that not only can the remaining portions of the backturned outer wall 10 facilitate the positioning of the slitted partition wall member 7 relative to the choke groove, but also the corrugation permits the backturned outer wall 10 to have an increased strength. In the example shown in Fig. 5, the backturned outer wall 10 is not corrugated such as shown in Fig. 4, but the gap d is formed between the outer wall 10 and the root portion of the lateral wall 11 over the entire length thereof. Where the positioning of the slitted partition wall member 7 relative to the choke groove is performed by the use of jigs and tools, the arrangement shown in Fig. 5 can advantageously be employed because it does not involve any increase in cost and amount of material used. Shown in Figs. 6 and 7 is another embodiment of the present invention wherein the slitted partition wall member is integrally formed with the metal door 6. As best shown in Fig. 6, the lateral wall 11 of the partition member extends into the outer wall 10 at the choke groove at the side edge of the front wall 9 opposite to the inner wall 8. The metal door 6 including the slitted partition wall member can be prepared by the use of a metal forming or drawing technique from a generally rectangular sheet metal which, as shown in Fig. 7, has a plurality of equally spaced tongues 23 integral therewith and protruding outwardly from each side of the shape of the sheet metal door 6. Broken lines shown in Fig. 7 represent the lines of bending and, after the bending and drawing job has been worked on the sheet metal, the space between each adjacent tongues 23 constitutes the respective cutouts 7a referred to hereinbefore in connection with the preceding embodiments. The rectangular openings 13 are formed in the tongues 23 beforehand. The door assembly shown in Fig. 6 and 7 is easier to make than that in the preceding embodiments with no welding job required and moreover, the choke groove can advantageously be dimensioned precisely. Thus, since the choke groove can be precisely dimensioned as hereinabove described, the high frequency attenuation can be maintained at a high value with no deviation and, moreover, the door assembly according to the present invention requires no provision of such a second groove as required in the prior art door assembly described with reference to Fig. 1. Therefore, it has now become clear that not only is it possible to reduce the amount of material used to fabricate the door assembly, but also a door assembly compact in size and light in weight can be realized. The depth D of the choke groove in Figs. 2 and 3 can be smaller than a quarter wavelength of the high frequency used, according to the impedance reversion theory discussed in PCT international Publication No. WO84/01083 referred to hereinabove. On the other hand, as discussed with particular reference to Fig. 3, the use of the liner 16 for the choke groove is essential. If the liner 16 is not used, the opening leading to the choke groove opens towards the heating chamber in the oven-defining structure, and a problem will arise that, in the event of the adherence of spills of food items, they will locally absorb the microwave energy, constituting a cause of spark discharge. In view of this, the rear wall 14 of the slitted partition wall member essentially forms between it and the front trim panel 29 around the access opening a gap of a size greater than the thickness of the liner 16. Because of the above, that portion of the microwaves which leaks out of the heating chamber 19 is in part introduced into the attenuator device and in part travels straight out of the attenuator device. In order to obviate this problem, such countermeasures as will now be described with reference to Figs. 8 and 9 are taken. Referring now to Fig. 8, there is shown the front trim panel 29 exteriorly around the access opening 28, which panel 29 confronts the door assembly in the closed position. The metal door 6 has a flat area 30 located between the perforated central area and the inner wall 8, which flat area 30 is spaced a distance B from the plane flush with the rear wall 14 of the slitted partition wall member 7. The liner 16 shown in Fig. 3 has a thickness sufficient for it to be accommodated within this distance B. The front trim panel 29 has a lateral flange 31 integral therewith and protruding frontwardly of the microwave oven so as to encircle the door assembly, when and so long as the latter is in the closed position, in uniformly spaced relation to the lateral wall 11 of the slitted partition wall member 7, the spacing between the lateral wall 11 and the lateral flange 31 being indicated by E. In order to prevent the spacing E from being locally reduced as a result of the local deformation of the door assembly due to a structural defect, a free edge portion of the lateral flange 31 opposite to the front trim panel 29 is crimped together with a front edge of the outer casing at the front of the microwave oven accommodating the oven-defining enclosure. It is to be noted that the distance B is preferred to be as small as possible, but must be greater than zero. A factor limiting the maximum allowable value for the distance B will be described later. That is, the flat area 30 must be spaced from the plane in which the rear wall 14 of the slitted partition wall member 7 lies. Although the spacing represented by the distance B is essentially created by the provision of the liner 16 as hereinbefore described with reference to Fig. 3, the spacing permits a portion of the microwaves, leaking outwardly through between the flat area 30 and the front trim panel 29 and travelling straight without entering the choke groove, to be reflected by the lateral flange 31 towards the inner wall 8 and then to be guided into the choke groove after having been reflected by the inner wall 8, finally attenuating. That portion of the microwave leaking outwards through between the front trim panel 29 and the flat area 30 of the door assembly in part attenuates within the choke groove after having been guided thereinto through the gap between the inner wall 8 of the metal door 6 and the partition wall 15 of the slitted partition wall member, and in part travels towards the lateral flange 31. The leaking microwave component travelling towards the lateral flange 31 has its course of travel disturbed by the lateral flange 31, some reflected thereby and some leaking frontwardly of the microwave oven after having been deflected by 90°. In other words, the path of travel of the microwave component is lengthened by the presence of barriers constituted by the metal walls upon which it is reflected and, therefore, the microwave component can be greatly attenuated. Also, the microwave component reflected back by the lateral flange 31 attenuates by the interference with the microwave component travelling straight. With respect to the relationship among the spacing E, the distance B and the width W of the high frequency inlet line, the width W must be greater than any one of the spacing E and the distance B, in order for a great portion of the microwaves leaking to be introduced into the choke groove. If this relationship is reversed, the amount of the leaking microwave component travelling straight will become greater than that introduced into the choke groove and the lateral flange 31 will serve no purpose. The width of the lateral flange 31, indicated by A, is determined as will now be described with reference to Fig. 9. A switch 32 for interrupting the oscillation of high frequency is operated by a key 33 provided in a portion of the door assembly. In general, the operating position of the switch has a predetermined width, and the switch remains operated during the progressive separation of the key 33 from the switch 32 at the time the door assembly is pivoted about the hinge 34 from the closed position towards the opened position. In other words, the operating point of the switch 32 exists during the progressive separation of the key 33 from the switch 32. Considering the accuracy in positioning of the switch as well as the accuracy of the dimensions of other associated component parts, and assuming that the rear wall 14 of the slitted partition wall member in the door assembly is, as shown by the phantom line in Fig. 9, located spaced a distance C from the front trim panel, the width A should be determined to be greater than the distance C while the distance C must be greater than the distance B. In other words, the width A of the lateral flange 31 is so selected that the lateral flange 31 projects frontwardly of the microwave oven from the front trim panel 29 a distance sufficient to permit the lateral flange 31 to encircle the outer periphery of the door assembly then being pivoted from the closed position towards the opened position, before the key 33 rigid with the door assembly separates from an actuator of the switch 32 to deenergize a magnetron. In addition, the distance L represented by the sum of the width of the front trim panel 29 and the width A of the lateral flange A is selected to be of a value equal to one quarter wavelength of the high frequency used. By so selecting, an infinite impedance acts on the microwave component, which leaks outside after having travelled straight and then deflected by 90°, at a position adjacent the crimped joint of the lateral flange 31, the inversion effect of which is that an approximately zero impedance is attained at a position adjacent the free side of the front trim panel 29 opposite to the lateral flange 31, with the consequence that the microwave leakage is essentially minimized. The door assembly of the construction described hereinbefore, is of a type wherein the slitted partition wall member having both the cutouts 7a and the rectangular openings 13 defined in the lateral wall 14 which forms an outer wall for the metal door 6 is mounted on the metal door 6. Therefore, the door assembly as a whole has a strength lower than that according to the prior art and, therefore, requires countermeasures to be taken for eliminating a problem associated with warping and/or twisting without deteriorating the performance of the high frequency attenuator device. Fig. 10 illustrates, therefore, one method to increase the strength of the door assembly as a whole. The fact that the outer wall 10 protrudes a small distance from the front wall 9 as hereinbefore described constitutes a major cause of reduction in strength of the door assembly as a whole. On the other hand, in terms of the capability of attenuating the microwaves, the width of the lateral outer wall 10, that is, the distance over which it protrudes transversely from the front wall 9, cannot be selected to be of a greater value than necessary. While the strength of the door assembly as a whole is somewhat increased because of the presence of a step between the perforated central area 35 and the flat area 30 which, when the door assembly is in the closed position, contacts the front trim panel 29, a generally rectangular reinforcing plate 36 having an opening is secured by, for example, welding to the central area 35 with the opening aligned with the multiplicity of the perforations 20. This reinforcing plate 36 has a flange 36a integral therewith and protruding perpendicular to the plate 36 in a direction close towards the inner wall 8, said flange 36a being held generally flush with the front wall 9. The door assembly of the construction as hereinbefore detailed has the following advantages. (1) Since a periodic structure having the cutouts and the rectangular openings alternating with each other is employed as an outer wall defining the choke groove and, at the same time, a continuous portion is provided at a root portion thereof, the dimensions and positions of various parts of the groove can be accurately maintained with no possibility of the high frequency attenuating performance being reduced as a result of the machining accuracy, making it possible to manufacture the door assembly light-weight and compact. (2) In the case where the slitted partition wall member having the cutouts and the rectangular opening is formed from a member separate from the door assembly and is secured by spot-welding or any other method to the metal plate door of the door assembly, any possible occurrence of welding defects can be avoided by providing a small gap between the outer peripheral face of the slitted partition wall member and the outer wall of the choke groove and, therefore, no high frequency attenuating performance will be reduced. (3) By the provision of the metallic outer wall laterally of the front trim panel so as to encircle the door assembly, particularly, the high frequency attenuating portion of the door assembly, an obstruction can be provided to the path of travel of that portion of the microwaves which has not been introduced into the high frequency attenuating portion, with the result that the attenuating performance can be increased. Moreover, by selecting the width of the metallic lateral flange in reference to the operating point of the switch for stopping the high frequency generator, the leakage of the microwaves which would occur during the initial stage of opening of the door assembly can also be avoided. (4) By the provision of the reinforcing plate to the door assembly at such a location where the high frequency attenuating device will not be adversely affected, any possible reduction in strength resulting from the decreased width of the outermost wall of the choke groove can be prevented. (5) Since there is provided a step of predetermined size between the flat area of the metal plate door exterior around the perforated area thereof and the rear wall of the slitted partition wall member, the leaking microwave component which travels straight and is subsequently reflected by the lateral flange can easily be guided into the choke groove. (6) By selecting the sum of the width of the front trim panel and that of the lateral outer wall to be equal to one quarter wavelength of the high frequency used, the microwave component leaking exteriorly can further be reduced.

Claims: A high frequency heating apparatus comprising an oven-defining structure having an access opening in communication with a heating chamber (19) defined therein; and a hingedly supported metallic door (6) for selectively opening and closing the access opening; said door (6) having its peripheral area formed with a lateral flange having a groove being open towards the oven-defining structure and defined by an inner (8), a bottom (9) and a backturned outer wall (10), the bottom of the groove being situated frontwardly of the door with respect to the apparatus, and the groove containing a slitted partition wall member (7) having a plurality of transverse cutouts (7a) alternating with a plurality of rectangular openings (13), characterized in that a) the partition wall member (7) is placed at a position flush with the outer wall (10), b) the outer wall (10) has a reduced height in respect of the height of the inner wall (8), and c) the lower edges (11a) of the transverse cutouts (7a) and the lower edges (13a) of the rectangular openings (13) are flush with the free edge of the outer wall (10). The apparatus as claimed in claim 1, wherein there is a small gap (d) between the outer peripheral face of the partition wall member (7) and the outer wall (10) at locations where a spot welding (21) is to be effected. The apparatus as claimed in claim 1, wherein the partition member (14, 15) is integrally formed with the outer wall (10) of the door (6). The apparatus as claimed in any of the preceding claims, wherein the peripheral area of the door (6) has a flat area directed towards the oven-defining structure and is set back at a distance sufficient to accomodate the thickness of a liner (16) for covering the cutouts and the openings of the lateral flange. The apparatus as claimed in any of the preceding claims, comprising a reinforcing plate (36) secured to the door (6) without closing a plurality of observatory perforations (20) defined in a central area of the door (6).

Inventors: ISHIMURA, YOHZOH, NAKANO, OKIHIKO
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1985
Main classification: H05B6
IPC codes: H05B6