JPS5910555B2 - Manufacturing method of sheathed heater - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a sheathed heater containing silicone resin as a filler, and particularly aims to prevent thermal decomposition of the silicone resin during the heating process and provide a sheathed heater with high insulation resistance. be.

By adding silicone resin to the filler of a sheathed heater, attempts have been made to utilize the water repellency of the silicone resin to suppress the hygroscopicity of the filler, magnesia, and to obtain a sheathed heater with high insulation resistance. .

However, this silicone resin has a low heat resistance limit, and is known to decompose at temperatures above 350° C. in the atmosphere, resulting in a decrease in water repellency. On the other hand, the sheathed heater is heated to a high temperature of 500 to 700° C. in the annealing step of its manufacturing process. For this reason, the silicone resin contained in the filler is thermally decomposed and the water repellency is impaired, so that the magnesia powder absorbs moisture and inevitably deteriorates the high insulation properties.
The present invention solves the drawbacks of the conventional sheathed heater manufacturing method described above and achieves the desired objective by suppressing thermal decomposition of silicone resin.Examples of the present invention will be described below.

First, as a result of studying the thermal decomposition of silicone resin, we found that N
It has been found that in a two-gas atmosphere, this phenomenon hardly occurs at temperatures below 750°C.

A specific example of silicone resin is polysiloxane, which has a high heat resistance, generates little decomposition gas, and is ideal for high-temperature sheathed heaters.It also has an organic group to silicon ratio of 1.0 to 1.1 and contains 80% filler. Using a silicone resin, this silicone resin was heated in an N2 gas atmosphere and in the air, and the reduction in temperature and weight was investigated. The results are shown in FIG. As is clear from FIG. 1, the weight decreases in the temperature range of 100 to 250°C in both case A in the N2 gas atmosphere and case B in the air, and this is because the silicone resin solidifies. In the case of heating in the air, silicone resin gradually thermally decomposes at 250 to 550°C, showing a gradual weight loss, and when the temperature exceeds 550°C, thermal decomposition becomes more intense, resulting in a rapid weight loss, and the silicone resin becomes repellent. It turns out that the aqueous quality is impaired. On the other hand, in the case of heating in an N2 gas atmosphere,
At temperatures below 750°C, the weight hardly decreases and no thermal decomposition occurs. When the temperature exceeds 750°C, a rapid weight loss is observed, and thermal decomposition occurs violently.

In addition to N2 gas, an inert gas such as Ar gas, a reducing gas such as ammonia decomposition gas, or a non-oxidizing gas may be used.

Furthermore, the inert gas atmosphere is preferably 100% N2.
Although the atmosphere is an inert gas such as , Ar, etc., almost the same results can be obtained even if it contains several percent of oxygen.

Furthermore, the reducing atmosphere may be a combustion gas such as propane or a green gas containing H2lO(!). next,
A method for manufacturing a sheathed heater based on the above results will be explained.

In FIG. 2, reference numeral 1 denotes a heating wire having terminals 2 at both ends, which is inserted into a metal pipe 3.

An insulating filler 4 is filled between the heating wire 1 and the metal pipe 3. The insulating filler 4 is made of magnesia powder with the addition of silicone resin which is methyl group-related polysiloxane, has an organic group to silicon ratio of 1.0 to 1.1, and contains 30 to 80% filler. . In this sheathed heater, N2 gas is sealed in the metal pipe 3, then both ends of the metal pipe 3 are caulked and sealed, and then the diameter is reduced by 10% or more by rolling, swaging, or the like. After this, in order to solidify the silicone resin, the temperature is 200-300°C.
Heat for 5 to 3 hours. Furthermore, in order to anneal the metal pipe 3, instantaneous heat treatment is performed at 500 to 700°C,
Thereafter, bending is performed, and finally the end of the metal pipe 3 is cut to expose the terminal 2, thereby obtaining a completed sheathed heater as shown in FIG. 3. The end of the metal pipe 3 can be cut by brazing a sheathed heater to a hot plate, by casting,
It may be cut after welding. In this embodiment, in the heat treatment process for solidifying the silicone resin and annealing, the end of the metal pipe 3 is caulked and sealed, and N2 gas is sealed inside the metal pipe 3. Thermal decomposition of silicone resin hardly occurs and water repellency is maintained.

Note that the same effect can be obtained by using a non-oxidizing gas other than N2 gas.

As is clear from the above explanation, according to the present invention, the ends of the metal pipes are sealed during the heating process, and non-oxidizing gas is sealed inside the metal pipes, so that most silicone resins are thermally decomposed. It is possible to obtain a sheathed heater that maintains water repellency, suppresses the hygroscopicity of magnesia, etc., and has high insulation resistance.

[Brief explanation of the drawing]

Fig. 1 is a characteristic diagram showing the weight change due to temperature of silicone resin, Fig. 2 is a cross-sectional view of the main part of the sheathed heater during the heating process of the sheathed heater manufacturing method showing one embodiment of the present invention, and Fig. 3 is It is a sectional view of the sheathed heater obtained by the same manufacturing method. 1...Heating wire, 3...Metal pipe, 4
...Insulating filling material.

Claims (1)

[Scope of Claims] 1. In a sheathed heater formed by filling an insulating filler containing silicone resin between a heating wire and a metal pipe, after filling the metal pipe with a non-oxidizing gas, the metal pipe is heated. A method for manufacturing a sheathed heater, characterized in that the end portion of the metal pipe is sealed, the end portion of the metal pipe is then subjected to heat treatment, and then the end portion of the metal pipe is cut. 2. The method of manufacturing a sheathed heater according to claim 1, wherein the sealing is performed by caulking the end of the metal pipe.