HIGH-FREQUENCY HEATING DEVICE

Abstract: To prevent malfunction of switch due to sliding and abrasion of door key and door hook by opening and closing of door of a microwave oven without grease application. A microwave oven comprises a heating compartment 3, a door 4 having a door key 6 for closing the front opening of the heating compartment 3, a hook spacer 13 manipulated by the door 4, a door hook 9 to be engaged with the door key 6, and switches 10, 11, 12 turned on or off by the door key 6 and hook spacer 13, wherein the door key 6 is made of a resin filled with at least one of acicular crystals of potassium titanate, calcium metasilicate, magnesium pyroborate, barium titanate, and zinc oxide of which Mohs's hardness is lower than that of glass fiber, with the diameter of acicular crystal being 0.1 µm or more and 50 µm or less, and the door hook 9 to be engaged with the door key 6 is made of a resin material filled with glass fiber, with the resin surface roughness of the door key 6 being smaller than the resin surface roughness of the door hook 9, thereby not required to apply grease on the sliding parts of the door key 6 and door hook 9.

Description: TECHNICAL FIELD The present invention relates to a high frequency heating apparatus having a door of a microwave oven or microwave oven-range provided with a switch function, so as to turn off the switch and stop oscillation of microwaves when the door is opened, and to turn on the switch and start oscillation of microwaves when the door is closed. BACKGROUND ART In the conventional microwave oven having a laterally opening door, the role of the switch for stopping oscillation of microwaves for cooking and heating is provided by combination of a door key formed in the door and the door hook formed in the main body, and the latch switch is cut off when the door is opened and the latch switch is turned on when the door is closed, and by turning on the operation switch of the main body, oscillation of microwaves is started. The door key and door hook are composed so as to be engaged with each other as the mutual contact surfaces slide to be engaged with each other, and the sliding surfaces were coated with grease to have lubricity, and the sliding surfaces were prevented from being abraded and worn out during use so that, finally, the latch switch may not fail to work. For grease application on the sliding surfaces, however, uneven coating tends to occur, and the sliding becomes poor when coating is insufficient, and abrasion of the sliding surfaces may increase, and the microwave oven may fail to work if the door is closed. Besides, as the grease is applied on the sliding surfaces, it may be discoloured due to contamination or deterioration in a long course of use, and the appearance becomes poor. Reference is also made to JP-A-6160008 relative to which the present invention is characterised. DISCLOSURE OF THE INVENTION In the light of the above background, it would be desirable to present a high frequency heating apparatus characterised by specifying the material used in the door key, so as be free from switch trouble due to abrasion of sliding parts in a long course of use without applying grease on the sliding parts of the door key and door hook. It would also be desirable to eliminate defective appearance due to contamination or deterioration of grease by not applying grease on the sliding parts. The present invention provides a high frequency heating apparatus having a door with a door key, characterised in that the door key is made of a resin filed with acicular crystal of which Mohs' hardness is lower than that of glass fiber. Preferably, the acicular crystal is at least one of potassium titanate, calcium metascilicate, magnesium pyroborate, barium titante, and zinc oxide. Preferably, the diameter of the acicular crystal is 0.1µm or more and 50µm or less. Preferably, the apparatus further comprises a heating compartment arranged to be closed by the door, a hook spacer manipulated by the door, a door hook to be engaged with the door key, and a switch turned on or off by the door key and hook spacer, wherein the door hook to be engaged with the door key is made of a resin filled with glass fiber. Preferably, the surface roughness of the door key is smaller than the resin surface roughness of the door hook. The invention, being thus constituted, can prevent malfunction due to abrasion by sliding of the door key and door hook by opening and closing of the door without applying grease on the sliding parts of the door key and door hook. It can also be free from defective appearance due to contamination by application of grease on the sliding parts of the door key and door hook and discoloration of the grease as experienced in the prior art. Brief Description of the Drawings Fig. 1 is a perspective appearance view of open door state of a microwave oven in an embodiment of the invention; Fig. 2 is an essential sectional view showing door key and door hook of the microwave oven; Fig. 3 is an essential sectional view showing the door key of the microwave oven; Fig. 4 is an essential sectional view showing the door hook of the microwave oven; and Fig. 5 is a diagram showing the relation between fiber diameter and tensile strength. Best Mode of Carrying Out the Invention Referring now to Fig. 1 to Fig. 5, an embodiment of the invention is described in detail below. In Fig. 1 and Fig. 2, reference numeral 1 is a microwave oven which is a high frequency heating apparatus, and the microwave oven 1 consists of a microwave oven main body 2 (hereinafter called main body), a heating compartment 3 for accommodating the food, a door 4 for closing the heating compartment 3, and an operation unit 5, and the door 4 is provided with a door key 6, and in combination with a door hook 9, a latch switch A 10, a latch switch B 11, and a short switch 12 incorporated in the main body 2, a latch switch (switch) is composed for stopping oscillation of microwaves by a magnetron (not shown) for heating the food. The door key 6 is designed to slide as being coupled with a door key cover 8 and a spring 7 shown in Fig. 3, and the door hook 9 has the latch switch A 10, latch switch B 11, and short switch 12 for stopping oscillation of microwaves, and when the door 4 is closed, an upper key 6a of the upper side of the door key 6 pushes the latch switch A 10, and a lower key 6b at the lower side of the door key 6 pushes the latch switch B 11 and short switch 12 through a hook spacer 13, thereby turning on the latch switch A 10, latch switch B 11, and short switch 12, so that it is ready to start oscillation of microwaves. Then the door 4 is opened, on the other hand, the door key 6 is detached from the door hook 9, and the latch switch A 10, latch switch B 11, and short switch 12 are turned off, so that oscillation of microwaves is stopped. In this constitution, when the door 4 is closed, as shown in Fig. 3 and Fig. 4, an upper key leading end portion 17a of the upper key 6a of the door key 6 contacts with an upper key insertion portion 14a of the door hook 9, and slides upward to pass a flat portion 15a inside of the upper key insertion portion, and further passes an end portion 16a inside of the upper key insertion portion and pushes the button 10a of the latch switch A 10 while causing the end portion 16a inside of the upper key insertion portion and the upper key leading end lower portion 18a to rub against each other, thereby turning on the latch switch A 10. Similarly, a lower key leading end portion 17b of the lower key 6b of the door key 6 contacts with a lower key insertion portion 14b of the door hook 9, and slides upward to pass a flat portion 15b inside of the lower key insertion portion, and further passes an end portion 16b inside of the lower key insertion portion and pushes the hook spacer 13 while causing the end portion 16b inside of the lower key insertion portion and the lower key leading end lower portion 18b to rub against each other, and thereby-the hook spacer 13 pushes the button 11a of the latch switch B 11 and button 12a of the short switch 12, thereby turning on the latch switch B11 and short switch 12. When opening the door 4, in reverse operation, the latch switch A 10, latch switch B 11, and short switch 12 are turned off. In this embodiment, the door key 6 is formed of a resin adding 5 wt.% of potassium titanate whisker to PBT (polybutylene terephthalate). Conventionally, using the door key made of resin blending 10 wt.% of glass fiber to PBT resin, and the door hook made of resin blending 10 wt.% of glass fiber to PBT resin, as mentioned above, the lubricity was enhanced by applying grease on the sliding parts, but when the grease application was not enough, the door key and door hook rubber against each other, and the rubbing parts abraded, and the leading end of the key failed to push the button of the latch switch A provided in the door hook, as a result, microwaves were not oscillated, and the microwave oven failed to work. In the conventional glass fiber filled PBT, the skin layer of the PBT resin was peeled and internal glass fibers were exposed, and the exposed glass fibers acted like files to abrade mutually and were worn out. In the invention, however, the glass fiber of the resin of the door key 6 is replaced by acicular crystal lower in hardness than the glass fiber to avoid the file effect to abrade the opposite door hook 9. Besides, the glass fiber is large in diameter, large in fiber diameter, and hence large in surface roughness to form a rough file surface to abrade the opposite surface, but by using the acicular crystal of smaller diameter and smaller fiber diameter, the surface roughness is smaller, and the degree of wear is smaller, so that the opposite door hook 9 may be hardly abraded. Whisker of potassium titanate has Mohs' hardness of 4 which is lower as compared with glass fiber, and the wear is less if rubbing against the glass fiber filled PBT. The thrust wear test data is shown in Table 1. Sample Coefficient kinematic viscosity Specific wear (mm 3 /kgfkm) Fixed side Rotating side µk Fixed side Rotating side PBT/GF PBT/GF 0.44 0.0812 0.0920 PBT/GF PBT/TIK 0.16 0 0.0450 PBT/TIK PBT/GF 0.38 0.0640 0.0330 PBT/TIK PBT/TIK 0.51 0.0394 0.0619 PBT/GF : glass fiber filled PBT PBT/TIK: potassium titanate whisker filled PBT As known from Table 1, by testing in the conditions of surface pressure of 0.6 kgf/cm 2 , peripheral speed of 30 cm/sec, and running distance of 10 km, the smallest wear was achieved by using glass fiber filled PBT at the fixed side, and potassium titanate filled PBT at the rotating side. Moreover, in comparison of the door key 6 of the microwave oven 1 made of PBT containing 10 wt.% of glass fiber and the door hook 9 made of PBT containing 10% of glass fiber, and the door key 6 made of PBT containing 5 wt.% of potassium titanate whisker and the door hook made of PBT containing 10 wt.% of glass fiber, the door opening and closing test was conducted 150,000 strokes without application of grease, and in the case of the door key 6 of the microwave oven 1 made of PBT containing 10 wt.% of glass fiber and the door hook 9 made of PBT containing 10% of glass fiber, the door key 6 and door hook 9 abraded, and the latch switch failed or nearly failed, whereas in the case of the door key 6 made of PBT containing 5 wt.% of potassium titanate whisker and the door hook 9 made of PBT containing 10 wt.% of glass fiber, there was almost no abrasion, and the latch switch worked normally. Fig. 5 shows the relation between fiber diameter and tensile strength. As known from the diagram, the tensile strength of fiber increased when the fiber diameter was 50 µm or less. Preferably, the tensile strength of fiber is higher at 0.1 µm or more and 5 µm or less. In the acicular crystal, the smaller the fiber diameter, the higher strength closer to the theoretical strength of the substance is realized, and it is known that the reinforcing effect is greater by using acicular crystal of smaller fiber diameter. In this embodiment, as the acicular crystal of whisker of potassium titanate, the material with mean fiber diameter of about 0.5 µm and mean fiber length of about 15 µm was used. Herein, the crystal with a short fiber is expressed as acicular crystal, but it also includes column and bar shape. The reason of definition of diameter of acicular crystal of 0.1 µm or more is that the crystal length is not enough if the diameter of acicular single crystal is less than 0.1 µm, and therefore the reinforcing effect is smaller and the strength of the resin is lowered. As the diameter of the acicular single crystal is larger, the crystal length becomes longer and the reinforcing effect increases and the resin strength is enhanced, but the surface roughness of the resin also increases and the lubricity drops. Therefore, the diameter of the acicular single crystal is preferred to be somewhere between 0.5 µm and 5 µm. Besides, calcium metasilicate (wollastonite), magnesium pyroborate, barium titanate, and zinc oxide are also lower in Mohs' hardness than glass fiber, and are expected to enhance the lubricity, and also obtain reinforcing effects, so that the same effects as potassium titanate are obtained. As the acicular crystal of zinc oxide, whether in single fiber or in tetrapod, same effects are expected. The embodiment relates to the microwave oven or microwave oven-range of laterally opening type, and in the case of microwave oven or microwave oven-range of vertically opening type, when the door key and door hook are engaged and the latch switch mechanism of friction structure is composed, by using the same resin material, abrasion is prevented, and hence it is equally effective. INDUSTRIAL APPLICABILITY According to the invention, as described herein, by using the door key made of a resin filled with acicular crystal of which Mohs' hardness is lower than that of glass fiber, malfunction of switch due to abrasion by sliding of the door key and door hook by opening and closing of the door can be prevented without applying grease on the sliding parts of the door key and door hook (or by applying very sparingly). It hence presents the microwave oven free from defective appearance due to contamination by application of grease on the sliding parts of the door key and door hook and discoloration of greases.

Claims: A high frequency heating apparatus (1) having a door (4) with a door key (6), characterised in that the door key (6) is made of a resin filed with acicular crystal of which Mohs' hardness is lower than that of glass fiber. A high frequency heating apparatus (1) according to claim 1, wherein the acicular crystal is at least one of potassium titanate, calcium metascilicate, magnesium pyroborate, barium titante, and zinc oxide. A high frequency heating apparatus (1) according to claim 2, wherein the diameter of the acicular crystal is 0.1µm or more and 50µm or less. A high frequency heating apparatus (1) according to claim 1, 2 or 3, wherein the apparatus further comprises a heating compartment (3) arranged to be closed by the door (4) , a hook spacer (13) manipulated by the door, a door hook (9) to be engaged with the door key, and a switch (10, 11, 12) turned on or off by the door key (6) and hook spacer (13), wherein the door hook (9) to be engaged with the door key (6) is made of a resin filled with glass fiber. A high frequency heating apparatus according to claim 4, wherein the surface roughness of the door key (6) is smaller than the resin surface roughness of the door hook (9).

Inventors: ISOGAI MAMORU, KAI TOSHIO, ISOGAI, MAMORU, KAI, TOSHIO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: F24C7
IPC codes: H05B6