Microwave oven using a precoated sheet steel as walls of internal chamber thereof

Abstract: There is provided a microwave oven using as walls of its internal chamber (5) a precoated sheet steel (10) which has no harmful effect on food sanitation. The precoated sheet steel (10) is comprised of a sheet steel (11), a strontium chromate primer (12) comprising strontium chromate dispersion-mixed with a resin and being applied to one side of the sheet steel, a paint (13) applied over the strontium chromate primer, and a paint (15) applied over the other side of the sheet steel (11). The one side of the precoated sheet steel (11) constitutes exterior surfaces of the walls of the internal chamber (5) and the other side of the precoated sheet steel (10) constitutes interior surfaces of the walls of the internal chamber (5). A molybdenum primer (22, 24) comprising molybdenum pigment dispersion-mixed with a resin may be applied to both sides of the sheet steel (21) instead of the strontium chromate primer (12).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microwave oven according to the preamble of claim 1. 2. Description of the Prior Art PCM sheet steel requires no post-working painting, and this provides for simplification of the process for product fabrication and eliminates the need for painting apparatus for post-forming painting. Because of this fact, PCM sheet steel has been receiving considerable attention as a material for use in constructing the internal chamber walls of microwave ovens. Microwave ovens in which PCM sheet steel is used as internal chamber walls of the oven are known including, for example, one described in Japanese Patent Publication No. 3-61094. The PCM sheet steel used in this prior art type of microwave oven has its primer and top coats formed of a polyester resin. A microwave oven will cause vapor to be generated within the internal chamber thereof upon heating a foodstuff. In this case, if the interior of the internal chamber is filled with vapor, the food in the internal chamber may not be seen from the outside. In order to increase the commercial attractiveness of the microwave oven and/or to enable a user to determine the progress of cooking in the internal chamber of the oven, it is required that the condition of cooking within the internal chamber be clearly visible to the user from the outside. Some known type of microwave oven is designed to meet this requirement, being such that the internal chamber of the microwave oven has intake and exhaust vents provided in the walls thereof so that when vapor is produced within the internal chamber the vapor may be exhausted outward in order to enable the food in the internal chamber to be seen clearly from the outside. The intake and exhaust vents generally consist of punch holes each of the order of 3 to 3.5 mm in diameter. The PCM sheet steel used in such prior art microwave oven has a drawback that its corrosion resistance is rather low because the sheet steel has its primer and top coats formed of a polyester resin. Where the PCM sheet steel is formed with such punch holes as noted above, it is required that the punched-out portion be subjected to post-coating with a resin paint in order to provide improved corrosion resistance. Technically, however, it has hitherto been impracticable to use a paint having good rust-preventing characteristics in view of possible unfavorable effect of such paint upon the hygienic quality of the oven interior. JP-A-61 147 025 discloses a high frequency heating device having a layer of steel over which a zinc plating is formed and a painted film formed over the zinc plating. The only specific reason given for such a layered arrangement is due to the fact that the film can be colored and that a thickness of a certain amount is desirable, and further that the films have poor anti-pollution property under a certain thickness and poor painting operability over another thickness due to the generation of foaming. Further, JP-A-1026 438 discloses a chromate layer formed over a steel and zinc plating combination, a primary feature being the addition of an organic resin layer over the chromate layer with a specific size metallic powder formed therein in order to improve a spot welding property while ensuring corrosion resistance. Document GB-A-2,170,428 shows a heat resistant precoated steel sheet and process for the production thereof, having excellent heat resistance, corrosion resistance and formability, and having a primer coat of strontium chromate intimately admixed with resin. SUMMARY OF THE INVENTION The object of the present invention is therefore to provide a microwave oven in which corrosion is prevented. This object is achieved by the features indicated in the characterizing portion of claim 1. With the structure according to claim 1, because the primer comprising strontium chromate has high corrosion resistance, the walls of the internal chamber is hard to corrode, thus providing high corrosion resistance for the microwave oven. It is that the one side of the precoated sheet steel constitutes exterior surfaces of the walls of the internal chamber and the other side of the precoated sheet steel constitutes interior surfaces of the walls of the internal chamber. Where the walls of the internal chamber are formed with punch holes for air intake and also with punch holes for air exhaust, the temperature and humidity conditions within the internal chamber, in the vicinity of the intake holes, provide a favorable corrosion resistant atmosphere since the temperature and humidity conditions are closer to those in the outside air, whereas both temperature and humidity conditions in the vicinity of the exhaust holes are unfavorably high and present a corrosive environment. By arranging so that one side of the precoated sheet steel constitutes the exterior surface of the walls of the internal chamber and the other side of the precoated sheet steel constitutes the interior surface of the walls of the internal chamber, it is possible to provide for sufficient health safety even if some trace amount of strontium chromate elution occurs in the vicinity of the exhaust holes, because air streams in that location are outward oriented preventing the eluted strontium chromate from entering the interior of the internal chamber. Further, it is difficult for strontium chromate to elute in the vicinity of the intake holes, because the temperature and humidity conditions therein are closer to those in the outside air. Further, it is preferable that proportion of strontium chromate in composition of the primer after baking of the primer is not less than 20 wt%. Then, it is possible to provide satisfactory corrosion resistance that can be maintained even after a long period of operation. It is preferable that the sheet steel is an iron/zinc alloy plated, surface treated sheet steel. Further it is preferable that the sheet steel is an iron/zinc non-alloy plated, surface treated sheet steel. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: Fig. 1 is a perspective view showing the general appearance of a microwave oven of one embodiment of the invention; Fig. 2 is a view showing an internal chamber of the microwave oven; Fig. 3 is a fragmentary perspective view showing a wall portion formed with punch holes in the internal chamber; Fig. 4 is a view showing the condition of air streams in the vicinity of the punch holes; Fig. 5 is a view showing the material configuration of a PCM sheet steel used in a first embodiment according to the present invention; Fig. 6 is a table showing a post-baking composition of the primer on the back side of the PCM sheet steel; Fig. 7 is a table showing a post-baking composition of top coat paint on the back side of the PCM sheet steel; Fig. 8 is a table showing a post-baking composition of the primer on the front side of the PCM sheet steel; Fig. 9 is a table showing a post-baking composition of the top coat paint on the front side of the PCM sheet steel; Fig. 10 is a table showing anti-corrosion performance test results with respect to PCM sheet steels of various different gauges; Fig. 11 is a table showing anti-corrosion performance test results with respect to PCM sheet steel with its strontium chromate content varied in different ways. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the invention will now be described in further detail with reference to the accompanying drawings. First Embodiment Fig. 1 is a general view of a microwave oven representing the embodiment. In Fig. 1, the microwave oven comprises a turntable 3, a door 4, an internal chamber 5, a control panel 6, and a cabinet 7. The internal chamber 5 has, as shown in Fig. 2, an exhaust vent 1 on one side wall thereof and an intake vent 2 on the other side wall. The internal chamber 5 is press molded from a PCM sheet steel 10 which conforms to the post-drying paint specifications shown in tables given in Figs. 6 to 9. Fig. 6 shows the composition of back-side primer. In the present example, the proportion of strontium chromate is 35 wt%. Fig. 7 shows the composition of back-side top coat. For the top coat is used a paint having a polyester content of 70 wt%. Fig. 8 shows the composition of front-side primer. For this primer is used a paint having a 28 wt% zinc phosphate content. Fig. 9 shows the composition of front-side top coat. This top coat contains 60 wt% tetrafluoroplastic; therefore, it is not liable to staining by food and has good cleanability. Fig. 5 shows the material configuration of the PCM sheet steel 10. The metal body of this PCM sheet steel 10 consists of an iron/zinc alloyed, surface treated sheet steel (GA or galvanized alloy material) 11. On the back side of the metal body sheet are applied a strontium chromate primer 12 of the composition shown in Fig. 6 and a polyester resin top coat 13 of the composition shown in Fig. 7. On the front side of the metal body sheet 11 are applied a zinc phosphate primer 14 of the composition shown in Fig. 8 and a fluoroplastic or polyester resin top coat 15 of the composition shown in Fig. 9. The corrosion resistant characteristics of the PCM sheet steel 10 are shown in Figs. 10 and 11. Fig. 10 shows the results of corrosion resistance tests carried out with pieces of 0.4 mm to 2.0 mm in thickness for time periods of 240 to 1000 hrs. Fig. 11 shows the results of corrosion resistance tests carried out with pieces, 4 mm thick, having strontium contents of from zero to 35 wt%, for time periods of 240 to 1000 hrs. In Figs. 10 and 11, mark ○ denotes "normal," ▵ denotes "slightly swollen," and × denotes "occurrence of red rust." It may be appreciated from Fig. 10 that the thickness of PCM sheet steel 10 is preferably not more than 0.8 mm. Also, it can be seen from Fig. 11 that a strontium chromate content of not less than 18 wt% is preferred. The internal chamber 5 has an intake vent 2 and an exhaust vent 1 as already stated. These vents 1 and 2 consist of sets of punch holes of 3 to 3.5 mm in diameter each as shown in Fig. 3. When food heating is effected in the microwave oven, flow of air streams within the microwave oven is as shown in Fig. 4. Outside air passes through punch holes 8 from the back side of the PCM sheet steel 10 toward the front side, while vapor produced from the foodstuff passes through punch holes 8 from the front side of the PCM sheet steel 10 toward the back side. A location adjacent to the intake vent 2 is under the same condition as the atmospheric condition and, therefore, is under a condition close to the atmospheric condition in both temperature and humidity; thus, the punch holes 8 are kept in a satisfactory corrosion-resisting atmosphere. While, on the other hand, a location adjacent to the exhaust vent 1 (more particularly, the inner side thereof) is exposed to hot and humid conditions and is in a corrosive environment, by virtue of the strontium chromate primer 12 applied on the back side, the neighborhood of the punch holes 8 are prevented from rusting and, even if a trace amount of strontium chromate elution should occur at the inner end of the vent, it will not enter the interior of the internal chamber 5 because air streams are outward oriented; thus, the interior of the internal chamber 5 is kept in sanitarily safe condition. As is apparent from the foregoing description, the microwave oven according to the present invention involves no problem whatever from the standpoints of corrosion resistance and food sanitation. Further, the arrangement of the present invention permits the internal chamber to be formed only by pressing and without post-painting being required. This provides for simplification of fabrication process thereof and considerable cost reduction, with an added advantage of stable quality production. On the equipment side, the invention eliminates the need for equipment for post-molding painting, which in turn, eliminates the need for measures for environmental protection, with the result of considerable indirect cost saving.

Claims: A microwave oven in which walls of an internal chamber (5) of the microwave oven are comprised of a precoated sheet steel (10), wherein the precoated sheet steel (10) is comprised of a sheet steel (11), a primer (12) being applied to one side of the sheet steel (11), a paint (13) applied over the primer (12), and a paint (14, 15) applied over the other side of the sheet steel (11), characterized in that the primer (12) comprises strontium chromate dispersion-mixed with a resin, wherein the one side of the precoated sheet steel (10) constitutes exterior surfaces of the walls of the internal chamber (5) and the other side of the precoated sheet steel (10) constitutes interior surfaces of the walls of the internal chamber (5). A microwave oven as set forth in claim 1, wherein a proportion of strontium chromate in a composition of the primer (12) after baking of the primer (12) is not less than 20 wt%.

Inventors: INOUE MASAHIRO, UENO YOSHIHARU, INOUE, MASAHIRO, UENO, YOSHIHARU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: H05B6
IPC codes: F24C7, H05B6