JPH0357594B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a sheathed heater that is widely used as a heating source for general cooking utensils and industrial equipment. Conventional configurations and their problems Sheathed heaters have traditionally been used in general cooking utensils and home appliances such as toaster ovens, microwave ovens, rice cookers, hot plates, and irons, as well as as a heating source for industrial electric furnaces and molds. Widely used. In general, a sheathed heater has a coiled electric heat source with terminal bars at both ends inserted into a metal pipe, which is then filled with electrically insulating powder made of fused magnesia powder. It has a structure sealed with a synthetic resin. Among these, sheathed heaters used in home appliances such as rice cookers, hot plates, and irons are sometimes used in low-temperature ranges, and from the viewpoint of price and quality, silicone resin is added as an electrical insulating powder. Electrofused magnesia powder is used. As described above, the sheathed heater to which silicone resin is added has excellent characteristics without deterioration in electrical characteristics even when used in a humid atmosphere. However, in recent years, the price of fused magnesia powder, which is the main raw material for electrical insulating powder, has increased with the rise in electricity costs, and cheaper raw materials are desired. For this reason, it is being considered to use sintered magnesia powder before electrofusion instead of electrofused magnesia powder. However, since sintered magnesia powder easily absorbs moisture and exhibits alkalinity, it decomposes and deteriorates silicone resin. As a result, the water-repellent effect of the silicone resin is impaired, resulting in a problem in that the electrical properties are significantly deteriorated. OBJECTS OF THE INVENTION The present invention solves the above conventional problems and provides a sheathed heater that is inexpensive and has excellent electrical characteristics even when used in a humid atmosphere. Structure of the Invention The present invention uses a mixed powder of sintered magnesia powder and fused magnesia powder as an electrical insulating powder,
Silicone resin powder and activated alumina powder are added to the electrical insulating powder. The activated alumina adsorbs moisture that enters the sheathed heater and turns it into stable chemically bonded water, which prevents the decomposition of the silicone resin and makes it inexpensive. This enables a sheathed heater with excellent electrical characteristics even when used in a humid atmosphere. DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in comparison with conventional examples with reference to the drawings. (Example 1) Electrical insulating powder 4 having the composition shown in Table 1 was prepared in advance. 1st table Electro-fused magnesia powder 70 parts Sintered magnesia powder 30 parts Silicone resin powder 2 parts Activated alumina powder 0.5 parts As the metal pipe 3, SUS304 was used. On the other hand, a heating wire 2 made of a coiled nichrome wire (wire diameter 0.32 mm) with terminal rods 1 at both ends is prepared, inserted into the center of a metal pipe 3, and filled with electrical insulating powder 4 prepared in advance. The diameter was reduced by rolling. Thereafter, heat treatment was performed at 250° C. for 1 hour, and water repellency treatment was performed using silicone resin. As shown in the figure, five sheathed heaters with a diameter of 8 mm and a length of 500 mm were completed, with sample numbers 11 to 15. On the other hand, for comparison, using the electrical insulating powder 4 shown in Tables 2 and 3, five sheathed heaters shown in the figure were similarly completed, and sample numbers 1 to 3 were used.
5 and 6 to 10. 2nd table Electrically fused magnesia powder 100 parts Silicone resin powder 2 parts 3rd table Electrically fused magnesia powder 70 parts Sintered magnesia powder 30 parts Silicone resin powder 2 parts In order to examine the characteristics of the completed sheathed heater, It was left in a humid atmosphere at 40°C and relative humidity of 95%, and the electrical insulation resistance values were measured after 7, 14 and 30 days, and the results are shown in Table 4. As is clear from Table 4, the sheathed heaters of sample numbers 1 to 5 using the conventional electrical insulating powder 4 having the composition shown in Table 2 all showed 2000 MΩ or more even after 30 days; As shown in the table, sample number 6 uses electrical insulation powder 4 in which a part of the fused magnesia powder is replaced with sintered magnesia powder.
~10 sheathed heaters have over 2000MΩ after 7 days, but are on the order of 100MΩ after 14 days, and 30
After a day, the resistance decreased to 1 to 10 MΩ, and no water repellency was obtained by the silicone resin. However, as shown in Table 1, samples Nos. 11 to 15, which are the sheathed heaters of the present invention with activated alumina powder added, showed a high temperature after 30 days, similar to samples Nos. 1 to 5, which used the conventional electrical insulating powder 4. Also 2000MΩ
The above values were shown.

【table】

[Table] (Example 2) Using the electrical insulating powder 4 having the composition shown in Table 5, five sheathed heaters were completed in the same manner as in Example 1. Table 5 Electro-fused magnesia powder 50 parts Sintered magnesia powder 50 parts Silicone resin powder 2 parts Activated alumina powder 1 part When the completed sheathed heaters were placed in a humid atmosphere for 30 days in the same way as in Example 1, 5 heaters were used. Both showed values of 2000MΩ or more, and excellent electrical properties were obtained. In this way, by replacing part of the electrofused magnesia powder with sintered magnesia powder and adding activated alumina powder in addition to conventional silicone resin powder, we have achieved low cost and improved electrical properties even when used in a humid atmosphere. An excellent sheathed heater can be obtained. In the present invention, the smaller the amount of sintered magnesia powder used in place of the fused magnesia powder, the better the electrical properties in a humid atmosphere. There is no particular problem if the amount of activated alumina powder added is increased. Effects of the Invention As is clear from the above description, according to the sheathed heater of the present invention, a mixed powder of sintered magnesia powder and fused magnesia powder is used as the electrical insulating powder, and silicone resin powder and activated By adding alumina powder, it is possible to obtain a sheathed heater that is inexpensive and has excellent electrical characteristics even when used in a humid atmosphere, and has great practical value.

[Brief explanation of drawings]

The figure is a sectional view of a sheathed heater according to an embodiment of the present invention. 2... Heating wire, 3... Metal pipe, 4... Electrical insulation powder.

Claims (1)

[Claims] 1 Equipped with a metal pipe and a heating wire inserted into the metal pipe, using a mixed powder of sintered magnesia powder and fused magnesia powder as the electrical insulating powder,
A sheathed heater in which silicone resin powder and activated alumina powder are added to the electrically insulating powder.