The present invention relates to a heater including an immersion heater for maintaining temperature of a molten metal that is a metal in a liquid state and heating the molten metal.

As a heater, there has been known a heater disclosed in JP2023-84536A (Patent Document 1).

The heater includes a heating element 4 disposed inside a closed distal end portion of a tubular portion 2. A first lead wire portion 6 and a second lead wire portion 8 are connected to the heating element 4. The tubular portion 2 has an open base end portion sealed by a sealing portion in a state where the first lead wire portion 6 and the second lead wire portion 8 pass through the base end portion.

A filler, which is a high thermal conductive powder filler disposed between the tubular portion 2 and the heating element 4, suppresses contact of air containing oxygen gas with the heating element 4, thereby suppressing embrittlement of the heating element 4 due to oxidation in a high-temperature environment.

Patent Document 1: JP2023-84536A

The above-described suppression of the embrittlement of the heating element 4 due to oxidation by the filler is not necessarily sufficient depending on the environment. The oxidation of the heating element 4 caused by the contact with air entering from the sealing portion progresses over a long period of time, and the product lifetime of the heating element 4 may become relatively short due to embrittlement and breakage associated with long-term use. As the amount of heat generated by the heating element 4 increases and the heating temperature by the heating element becomes higher, the oxidation of the heating element 4 tends to progress more readily. Therefore, there is a demand for simply and inexpensively attempting the suppression of the oxidation of the heating element 4 in a high-temperature environment.

On the other hand, details of the sealing portion in the above-described heater are unclear, and there is room for improvement in the sealing performance of the tubular portion 2 by the sealing portion. When the sealing performance is improved, deterioration due to the oxidation inside the tubular portion 2 including the heating element 4 is further suppressed. Since the lead wire passes through the sealing portion, and the sealing portion is exposed to high heat, the sealing portion is required to have insulation properties and heat resistance in addition to the sealing performance, and is desired to be formed simply at low cost while having these properties. In particular, in the heating element 4 made of a material having lower resistance to oxidation in a high-temperature environment than others, improvement in sealing performance is more strongly required. Particularly under a more severe environment than conventional one, such as a high-temperature environment, there may be a limit to suppression of oxidation by the filler alone.

It is a main object of the present invention to provide a heater improved in sealing performance of a tubular portion that houses a heating element.

It is another main object of the present invention to provide a heater improved in sealing performance of a tubular portion in a simple and low-cost manner.

The description discloses a heater. The heater may include a heating element. The heater may include a lead wire portion connected to the heating element. The heater may include a tubular portion that houses the heating element. The heater may include a sealing portion that seals the tubular portion while allowing the lead wire portion to pass therethrough. The sealing portion may be a cured product obtained from a raw material in a liquid, gel, paste, or powder form.

The cured product may be formed from a siloxane material.

Further, the cured product may contain at least one of quartz and alumina.

It is a main effect of the present invention to provide the heater improved in sealing performance of the tubular portion that houses the heating element. Accordingly, the embrittlement and breakage due to oxidation of the heating element in a high-temperature environment are suppressed, and the product lifetime of the heating element is further lengthened.

It is another main effect of the present invention to provide the heater improved in sealing performance of the tubular portion in a simple and low-cost manner.

• Fig. 1 is a schematic diagram of an immersion heater according to the present invention and its surroundings.
• Fig. 2 is a graph illustrating results of an analysis by a thermogravimetric-differential thermal analyzer under Condition T1 in Example 2.
• Fig. 3 is a graph illustrating results of an analysis by the thermogravimetric-differential thermal analyzer under Condition T2 in Example 2.
• Fig. 4 is a graph illustrating results of an analysis by the thermogravimetric-differential thermal analyzer under Condition T3 in Example 2.
• Fig. 5 is a graph illustrating temperatures of a sealing portion in Fig. 1 at each flow rate of air from an air blowing unit in Fig. 1 under Conditions R1 to R3.
• Fig. 6 is a graph illustrating differences of the temperature of the sealing portion at each air flow rate from the temperature of the sealing portion at the air flow rate of 0 L/min in Fig. 5 under Conditions R1 to R3.

The following describes embodiments and modifications thereof according to the present invention based on the drawings as appropriate.

The present invention is not limited to the embodiments and modifications described below.

Fig. 1 is a schematic diagram of an immersion heater 1 as an exemplary heater according to the embodiment and its surroundings.

The immersion heater 1 includes a tubular portion 2, a heating element 4, a first lead wire portion 6, a first connecting portion 7, a second lead wire portion 8, a second connecting portion 9, a sealing portion 10, a heat insulating material 11, a filler (not illustrated), a supporting portion 12, a joint portion 14, and an air blowing unit 16.

The immersion heater 1 is configured to maintain temperature of molten aluminum W that is a molten metal of aluminum and to heat the molten aluminum W. The immersion heater 1 may be configured to heat a molten metal of nonferrous metal such as zinc. The immersion heater 1 may be configured to heat a molten metal of other metals including iron. The immersion heater 1 may be configured to heat other materials. The heater may be a component other than the immersion heater.

The molten aluminum W is for aluminum die-cast products. The molten aluminum W may be used for other applications.

When heating the molten aluminum W, typically, a distal end portion of the immersion heater 1 is inserted into the molten aluminum W from above the molten aluminum W, and the immersion heater 1 is oriented such that a longitudinal direction of the immersion heater 1 extends in an up-down direction and the distal end portion is positioned at a lower side. The immersion heater 1 is inserted into the molten aluminum W passing through a hole H provided at a lid T of a molten metal bath. The immersion heater 1 may be used in an orientation other than the posture oriented in the up-down direction. The molten metal bath does not necessarily include the lid T.

The tubular portion 2 is a cylindrical tube made of ceramics.

The tubular portion 2 has a closed distal end portion 2P that serves as a closed end, that is, a sealed end. The distal end portion 2P of the tubular portion 2 has a hemispherical shape.

The tubular portion 2 has an open base end portion 2B that serves as an open end.

The material of the tubular portion 2 is not limited to ceramics. The shape of the tubular portion 2 is not limited to a shape with a closed end and an open end, and may be, for example, a shape with open ends at both ends. Further, the shape of the distal end portion 2P of the tubular portion 2 is not limited to the hemispherical shape.

The heating element 4 is formed in a single-coil shape as a whole, and generates heat when energized.

The heating element 4 is inserted into the tubular portion 2. The heating element 4 is located at the distal end portion 2P side in the tubular portion 2. The tubular portion 2 is located outside the heating element 4 and covers the heating element 4. The tubular portion 2 houses the heating element 4. The tubular portion 2 protects the heating element 4.

The heating element 4 is made of molybdenum or a molybdenum alloy. Hereinafter, both are referred to as molybdenum without distinction.

The melting point of molybdenum is about 2500°C, which is higher than the melting point of a nichrome wire of about 1400°C and the maximum service temperature of the nichrome wire of 1150°C. Accordingly, the higher heat generation temperature can be obtained and the larger output can be obtained compared with a heating element made of nichrome wire.

The heating element 4 may be in a form of multiple coils or a plurality of coil shapes. The material of the heating element 4 may be any material other than molybdenum.

The first lead wire portion 6 supplies electric power to the heating element 4.

The first connecting portion 7 is made of metal, for example, stainless steel, and connects the heating element 4 and the first lead wire portion 6. The first connecting portion 7 is interposed between the heating element 4 and the first lead wire portion 6.

The first connecting portion 7 is connected to an upper end portion of the heating element 4.

The first lead wire portion 6 is disposed outside the heating element 4. The first lead wire portion 6 and the heating element 4 are arranged in the longitudinal direction.

The second lead wire portion 8 supplies electric power to the heating element 4. The second lead wire portion 8 includes a single-wire group portion 8G and a stranded wire portion 8B. The single-wire group portion 8G consists of a plurality of single wires. The stranded wire portion 8B is disposed at a base end side of the single-wire group portion 8G and connected to the single-wire group portion 8G.

The second connecting portion 9 is made of metal, for example, stainless steel, and connects the heating element 4 and the single-wire group portion 8G of the second lead wire portion 8. The second connecting portion 9 is interposed between the heating element 4 and the second lead wire portion 8.

The second connecting portion 9 is connected a lower end portion of the heating element 4.

The single-wire group portion 8G at the distal end side of the second lead wire portion 8 passes through a radially inside of the heating element 4. The second lead wire portion 8 passes through the inside of the heating element 4.

The center portion of the first lead wire portion 6 and the center portion of the second lead wire portion 8 pass through the sealing portion 10, extend upward from the upper end of the tubular portion 2, enter the supporting portion 12, and pass through the inside of the supporting portion 12.

In the materials of the first lead wire portion 6 and the materials of the second lead wire portion 8, the material of the single-wire group portion 8G in the second lead wire portion 8 is molybdenum and the material of the other portion is steel. The single-wire group portion 8G consists of a plurality of molybdenum single wires. The first lead wire portion 6 and the stranded wire portion 8B of the second lead wire portion 8 are formed by twisting a plurality of steel single wires together.

The material of the first lead wire portion 6 and the material of the second lead wire portion 8 may be entirely molybdenum or may be entirely a material other than molybdenum. Alternatively, the material of the portion inside the heating element 4 may be steel while the material of the portion other than the inside of the heating element 4 may be molybdenum, and three or more materials may be combined. A part or the entirety of the first lead wire portion 6 and the second lead wire portion 8 may be covered with coating by an insulator. Further, the single-wire group portion 8G may be omitted and the second lead wire portion 8 may be formed as a continuous single wire, or the whole of the second lead wire portion 8 may be formed as a continuous single-wire group portion 8G. The first lead wire portion 6 may include a single-wire group portion, or the first lead wire portion 6 may be formed as a continuous single wire. At least one of the first lead wire portion 6 and the second lead wire portion 8 may include a rod-shaped portion formed in a rod shape. Furthermore, the number of the lead wire portions may be one, or may be two or more.

The sealing portion 10 is disposed at the base end portion 2B of the tubular portion 2, and closes the base end portion 2B to perform sealing.

The first lead wire portion 6 and the stranded wire portion 8B of the second lead wire portion 8 pass through the sealing portion 10.

The heat insulating material 11 is disposed inside the tubular portion 2 so as to be adjacent to the sealing portion 10.

The heat insulating material 11 suppresses the transfer of heat from the molten aluminum W and the heated heating element 4 to the sealing portion 10 side and the supporting portion 12 side.

The heat insulating material 11 is, for example, at least one of ceramic fiber and glass fiber wool. The shape of the heat insulating material 11 may be a bracket shape or a fiber shape.

The heat insulating material 11 may be omitted.

The filler is filled in the tubular portion 2.

The main component of the filler is magnesium oxide (magnesia, MgO). The main component is a component that constitutes more than half by weight ratio or volume ratio. Here, the filler contains MgO of 90% or more by volume ratio. MgO is a high thermal conductivity filler having excellent thermal conductivity. The thermal conductivity of MgO is about 60 W/m·K (watts per meter per kelvin). The thermal conductivity of the filler is close to the thermal conductivity of MgO.

The filler covers the heating element 4. The filler is in contact with the inner surface of the distal end portion 2P of the tubular portion 2. The filler is disposed around the heating element 4 and holds the heating element 4. The filler is disposed between the tubular portion 2 and the heating element 4. The filler is filled at the distal end side from, and including, the first connecting portion 7. The filler also enters between adjacent loop portions of the coiled heating element 4. Since the filler holds the heating element 4 and enters the gaps of the heating element 4, contact between adjacent portions of the heating element 4 due to expansion during heat generation or the like is suppressed, thereby protecting the heating element 4 from electrical leakage.

The filler suppresses contact of oxygen with the molybdenum heating element 4, thereby suppressing oxidation of the heating element 4 in a high-temperature environment and suppressing embrittlement due to the oxidation.

The main component of the filler may be other than magnesium oxide. The material of the filler may be MgO alone. The filler may be in a form other than a powder, such as spherical bodies. Further, the filler may be filled at the distal end side with respect to the distal end portion of the first lead wire portion 6 and the distal end portion of the second lead wire portion 8, or may be filled at the distal end side with respect to the sealing portion.

The supporting portion 12 is made of metal and connected to the base end portion 2B of the tubular portion 2.

The supporting portion 12 includes a connecting portion 20, a wire lead-out portion 22, and a plurality of bolts 24.

The connecting portion 20 includes a cylindrical portion 20T and a flange portion 20F. The cylindrical portion 20T has a cylindrical shape extending in the up-down direction. The flange portion 20F has a ring plate shape, is fixed to an upper end portion of the cylindrical portion 20T, and extends laterally.

The wire lead-out portion 22 includes a cylindrical portion 22T and a flange portion 22F. The cylindrical portion 22T has a cylindrical shape extending in the up-down direction. The flange portion 22F has a ring plate shape, is fixed to a lower end portion of the cylindrical portion 22T, and extends laterally.

The connecting portion 20 and the wire lead-out portion 22 are connected to each other by inserting the bolts 24 in the up-down direction into the flange portions 20F and 22F stacked in the up-down direction.

A support member SP is interposed between a lower surface of the flange portion 20F and an upper surface of the lid T.

The support member SP supports the immersion heater 1.

The support member SP is extendable in the up-down direction. By adjusting a length of the support member SP in the up-down direction, the position of the tubular portion 2 with respect to the molten aluminum W is adjusted.

The support member SP may be omitted, and the immersion heater 1 may be supported through the hole H of the lid T or the like. The support member SP may be configured as a component of the immersion heater 1.

The base end of the first lead wire portion 6 and the base end of the second lead wire portion 8 are connected to corresponding terminals (not illustrated) in the wire lead-out portion 22, and lead wires are connected to the respective terminals (not illustrated). The lead wires are each extracted outside from the wire lead-out portion 22 and connected to a power source (not illustrated) via a control device (not illustrated). The wire lead-out portion 22 does not need to house the terminals, and the base end portion of the first lead wire portion 6 and the base end portion of the second lead wire portion 8 may be directly extracted outside from the wire lead-out portion 22.

The power source here is single-phase alternate current.

The control device controls electric power from the power source to the heating element 4 to control heat generation by the heating element 4.

The power source may be direct current, may be three-phase alternate current, or may be any other type. The voltage of the power source may be appropriately selected.

The joint portion 14 joins the tubular portion 2 and the supporting portion 12 by brazing.

The joint portion 14 is provided between a groove portion formed at an outer surface of the base end portion 2B of the tubular portion 2 and a stepped portion formed at an inner surface of the cylindrical portion 20T of the connecting portion 20 of the supporting portion 12.

The tubular portion 2 and the supporting portion 12 may be joined by a method other than brazing.

The sealing portion 10 is disposed inside the cylindrical portion 20T of the connecting portion 20. The connecting portion 20 surrounds the sealing portion 10.

Particularly, under the high-temperature environment during heating of the molten aluminum W or the like and under an environment in which a heated state and a non-heated state are repeated, while the joint portion 14 can maintain the joining between the ceramic tubular portion 2 and the metallic connecting portion 20, inflow of air containing oxygen gas from outside is allowed.

When the sealing performance is lost, the heating element 4 is oxidized by air that has flowed into the tubular portion 2 and has penetrated into gaps in the filler, and deteriorates. In particular, in the molybdenum heating element 4, the loss of sealing performance leads to rapid oxidation under a high-temperature oxygen atmosphere, thereby shortening its service life.

In addition, in the immersion heater 1, the first lead wire portion 6 and the second lead wire portion 8 expand during heating of or when approaching the molten aluminum W, and contract when the molten aluminum W is not heated and not in the proximity, thus repeating expansion and contraction. The internal pressure of the sealed tubular portion 2 becomes larger than the external pressure, that is, positive pressure, during heating of or when approaching the molten aluminum W. On the other hand, the internal pressure of the tubular portion 2 becomes smaller than the external pressure, that is, negative pressure when the molten aluminum W is not heated and not in the proximity, thus repeatedly fluctuating between the positive pressure and the negative pressure. Due to the repeated expansion and contraction of the first lead wire portion 6 and the second lead wire portion 8, as well as the repeated change between the positive pressure and the negative pressure of the tubular portion 2, deterioration may occur in the sealing portion 10 depending on the material, gaps between the sealing portion 10 and the first lead wire portion 6 or the second lead wire portion 8, that is, lead wire gaps, may be formed, or gaps between the sealing portion 10 and the tubular portion 2, that is, tubular portion gaps, may be formed. Further, depending on the material, the sealing portion 10 itself deteriorates due to the repeated temperature rise during heating and temperature drop during non-heating, resulting in the formation of at least one of the lead wire gap and the tubular portion gap, and causing the loss of the sealing performance.

Therefore, it is preferable that the sealing portion 10 maintains the sealing performance over a long cumulative service time while withstanding heat generated by the heating element 4 and heat from the molten aluminum W, that is, heat during use.

Accordingly, in order to withstand the heat during use and maintain the sealing performance of the tubular portion 2 without losing adhesion and sealing performance with respect to the first lead wire portion 6, the second lead wire portion 8, and the tubular portion 2 even over the long cumulative service time and without forming the lead wire gap and the tubular portion gap, various materials and physical properties of the sealing portion 10 were tested.

As a result, it was found that when the sealing portion 10 has at least any of the materials and the physical properties described below, the sealing performance of the tubular portion 2 can be maintained even when the cumulative service time is long.

That is, first, the sealing portion 10 may be a cured product obtained from a liquid, gel, paste, or powder raw material. An example of the environmental temperature before curing is 25°C. That is, the sealing portion 10 may be a cured product obtained from a liquid, gel, paste, or powder raw material at 25°C.

The cured product is a solid or a rubber.

Examples of such materials include at least one of silicone and a heat-resistant adhesive. Here, the silicone includes silicone resin and silicone rubber. The silicone also includes organopolysiloxanes.

In this case, for example, the sealing portion 10 is formed by performing curing at 25°C in a state where the raw material in a liquid, gel, paste, or powder form penetrates between the respective single wires of the first lead wire portion 6 that is a stranded wire and the stranded wire portion 8B of the second lead wire portion 8. Accordingly, the sealing portion 10 is in close contact with the first lead wire portion 6 and the second lead wire portion 8 without gaps. The sealing portion 10 is sufficiently in close contact also with the tubular portion 2, and particularly in close contact with the ceramic tubular portion 2 properly. Therefore, the sealing portion 10 suppresses the formation of the lead wire gaps and the tubular portion gaps, thereby suppressing penetration of air into the tubular portion 2. Even when the first lead wire portion 6 and the stranded wire portion 8B of the second lead wire portion 8 are each a single-wire bundle portion, which is a bundle of a plurality of single wires not twisted together, similarly to the case of the stranded wire, the raw material penetrates into the gaps between the respective single wires, and the effect of providing adhesion is exhibited.

While the curing may be performed by natural drying, from the viewpoints of favorable properties when forming the sealing portion 10 and rapid formation, heat curing is preferable. The raw material may be a single substance in a liquid, gel, paste, or powder form (one-component type). The raw material may be two substances that are originally in a liquid, gel, paste, or powder form (two-component type), and in this case, the two substances may be mixed to form a liquid, gel, paste, or powder. The raw material may be three or more substances that are originally in a liquid, gel, paste, or powder form, and in this case, the three or more substances may be mixed to form a liquid, gel, paste, or powder.

Even when the first lead wire portion 6 and the second lead wire portion 8 passing through the sealing portion 10 are rod-shaped, the oxidation of the heating element 4 due to the damage on the sealing portion 10 can be suppressed insofar as the sealing portion 10 is configured as described above.

That is, since the raw material of the sealing portion 10 spreads over the surfaces of the rod-shaped first lead wire portion 6 and second lead wire portion 8, the formation of the lead wire gaps is suppressed by the cured sealing portion 10. Therefore, even when the rod-shaped first lead wire portion 6 and second lead wire portion 8 expand and contract in the longitudinal direction due to the repeated heating and non-heating, the formation of the lead wire gaps is suppressed, the inflow of air containing oxygen gas into the tubular portion 2 is suppressed, and the oxidation of the heating element 4 is suppressed.

The configuration of the sealing portion 10 may be regarded as being defined by a manufacturing method, namely, formation by curing a raw material in a liquid, gel, paste, or powder form before the curing. However, even in such a case, since so-called impossible or impractical circumstances are present, such a definition is considered to be permissible.

In other words, since a wide variety of such materials exist, it is not practically feasible to directly define the sealing portion 10 by its structure or properties by enumerating specific examples in order to distinguish it from other materials.

Furthermore, the specific structure of such cured products is not currently known, and it is considered that investigating the structures for all types would require substantial equipment and time and would be virtually impossible or impractical.

Accordingly, even when the sealing portion 10 is regarded as being defined by the above-described manufacturing method, namely, formation by curing the raw material, such a definition should be considered permissible.

The sealing portion 10 may be a heat-resistant insulating material capable of withstanding a temperature of at least 250°C after curing. In the heating by heat generation of the heating element 4 including the heating of the molten aluminum W, when the heat resistance capable of withstanding a temperature of at least 250°C is provided, the damage on the sealing portion 10 during the heating is suppressed, and occurrence of damage on the immersion heater 1 including oxidation of the heating element 4 is suppressed.

Further, when the sealing portion 10 is an insulator, electric leakage from at least one of the first lead wire portion 6 and the second lead wire portion 8 to the sealing portion 10 is suppressed, and occurrence of short circuit between the first lead wire portion 6 and the second lead wire portion 8 is suppressed.

In addition, when the sealing portion 10 does not expand or is unlikely to expand, that is, hardly expands under an environment of 250°C or higher after curing, the occurrence of cracks due to deterioration caused by expansion or repeated expansion during heating and contraction during non-heating is suppressed. Accordingly, oxidation of the heating element 4 due to inflow of air from cracked portions is suppressed. In a precise sense, not expanding means that the volume remains the same or the volume decreases. On the other hand, in the present invention, not expanding includes slight expansion. However, even when slight expansion occurs in the sealing portion 10, the sealing performance of the tubular portion 2 is maintained. Similarly, even when slight contraction occurs in the sealing portion 10, the sealing performance of the tubular portion 2 is maintained. When the sealing performance of the tubular portion 2 is maintained, the inflow of air into the tubular portion 2 is suppressed, and oxidation of the heating element 4 is suppressed.

The sealing portion 10 after curing may be an elastic body, a non-elastic solid that does not exhibit elasticity, or a material in which an elastic portion and a non-elastic portion are mixed.

Examples of the material in which an elastic portion and a non-elastic portion are mixed include a material in which a part of an elastic body, such as silicone containing aluminum oxide (alumina), has become non-elastic. In the sealing portion 10 made of an elastic body, due to heat from the molten aluminum W and the heating element 4, the silicone component responsible for elasticity gradually dissipates from the portion at the tubular portion 2 side including the heating element 4, leaving alumina and causing it to become non-elastic. In this case, although the non-elastic transformation often occurs non-uniformly, the portion of the sealing portion 10 mainly at the tubular portion 2 side becomes a non-elastic portion, while the portion of the sealing portion 10 mainly at the upper side (the ambient air side) becomes an elastic portion.

The sealing portion 10 may be formed from a siloxane material. Also in this case, in the sealing portion 10, penetration into the gaps in the stranded wire or adhesion to the surfaces of the rod-shaped single wires is achieved. Accordingly, the formation of the lead wire gaps is suppressed, and oxidation of the heating element 4 is suppressed. The sealing portion 10 also in this case has heat resistance capable of withstanding the temperature of at least 250°C, and has an insulating property.

The siloxane material is a material having a siloxane structure and is a material for forming silicone. The siloxane material is, for example, a silicone rubber material, an organohydrogen polysiloxane material, a reactive cyclic siloxane material, or a combination of two or more thereof.

Although the siloxane material is not particularly limited, since the siloxane material has non-expansibility while exhibiting adhesion, heat resistance, and insulating properties, the siloxane material is preferably a reactive cyclic siloxane material.

The reactive cyclic siloxane material is a material having a cyclic siloxane structure as a main backbone and having a plurality of reactive functional groups in one molecule.

The number of -Si-O- in the cyclic siloxane structure is not particularly limited. However, from the viewpoints of ease of production and ease of handling of the material itself, as well as favorable reactivity of the material, it is preferably 3 (trimer) or more and 10 (decamer) or less, and more preferably 3 or more and 5 or less.

While the reactive functional group is not particularly limited, from the viewpoints of ease of production and ease of handling of the material itself, as well as favorable reactivity of the material and favorable properties when cured to form the sealing portion 10, the reactive functional group is preferably an organic functional group, more preferably an unsaturated hydrocarbon group, and still more preferably a vinyl group.

Each Si in the cyclic siloxane structure can bond to two groups in addition to bonds with the two adjacent O atoms in the main backbone.

Accordingly, from the viewpoints of favorable reactivity of the material and favorable properties when cured to form the sealing portion 10, it is preferable that all Si atoms in the cyclic siloxane structure each have one reactive functional group.

The reactive cyclic siloxane material is, for example, 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane represented by the following formula (1). The reactive cyclic siloxane material of formula (1) is a tetramer. Each of the four Si atoms in the reactive cyclic siloxane material of formula (1) has one - CH=CH₂ group, which is a vinyl group, as a reactive functional group.

The configuration of the sealing portion 10 may be regarded as being defined by a manufacturing method, namely, formation using the siloxane material. However, even in such a case, since so-called impossible or impractical circumstances are present, such a definition is considered to be permissible.

In other words, there exists a wide variety of the siloxane materials that vary depending on the number of -Si-O- units and the types, arrangements, and the like of reactive functional groups while having the siloxane structure, and therefore, it is not practically feasible to directly define the sealing portion 10 by its structure or properties by enumerating specific examples in order to distinguish it from other polymers.

Furthermore, the specific structure of a reaction product of such a siloxane material is a polymer structure and varies with each curing, and it is considered that investigating the structures for all types would require substantial equipment and time and would be virtually impossible or impractical.

Accordingly, even when the sealing portion 10 is regarded as being defined by the manufacturing method using the siloxane material, such a definition should be considered permissible.

While the sealing portion 10 may be formed solely from the siloxane material, from the viewpoint of favorable properties when forming the sealing portion 10, the sealing portion 10 is preferably formed with the addition of at least one of alumina, quartz, zinc oxide, and carbon black. Alternatively, the sealing portion 10 may be formed from at least one of the siloxane material, alumina, quartz, zinc oxide, and carbon black. The sealing portion 10 has the effect of avoiding inflow of air into the tubular portion 2, which causes promotion of oxidation of the heating element 4, even when exposed to high temperatures.

While the amounts of the materials, in other words, the composition of the composite material, are not particularly limited, from the viewpoints of ease of formation and favorable properties when forming the sealing portion 10, the composite material preferably has a composition of the following two components (agent A and agent B) to be cured after mixing. Here, "x to y" means x or more and y or less, and the same applies hereinafter. At least one of the agent A and the agent B may be adjusted such that the total number of parts by mass is 100 parts by mass.

• Agent A: 40 to 50 parts by mass of alumina, 30 to 40 parts by mass of quartz, 0.25 to 1 part by mass of zinc oxide, and 0.1 to 1 part by mass of carbon black
• Agent B: 30 to 40 parts by mass of alumina, 30 to 40 parts by mass of quartz, and 0.3 to 1.0 parts by mass of a reactive cyclic siloxane material

While the siloxane material may be cured by any method including natural drying, from the viewpoints of favorable properties when forming the sealing portion 10 and rapid formation, heat curing is preferable.

Further, the sealing portion 10 may be formed from a material containing organohydrogen polysiloxane. Examples of the material containing organohydrogen polysiloxane include those described in WO 2019/021824. Also in this case, similarly to the case of the reactive cyclic siloxane material, the sealing portion 10 excellent in gas inflow suppression, heat resistance, insulating property, and non-expansibility is formed.

The air blowing unit 16 blows an air N to the sealing portion 10.

The air blowing unit 16 is a single fan.

The air blowing unit 16 is configured to perform air cooling of the sealing portion 10.

The air blowing unit 16 may be a plurality of fans, may be an air duct, or may be a combination of fan and air duct. The air blowing unit 16 may be omitted.

In order to bring the sealing portion 10 into contact with ambient air, the cylindrical portion 20T is provided with one or more opening portions 20P from the outside to the inside in a radial direction. Therefore, an outer surface in the radially outside of the sealing portion 10 is partially exposed to ambient air through the opening portion 20P. The support member SP is disposed to avoid closing the opening portion 20P. The opening portion 20P is provided at the upper side with respect to the joint portion 14.

Through the opening portion 20P, ambient air comes into contact with the sealing portion 10, thereby cooling the sealing portion 10.

Insofar as the sealing portion 10 can be exposed, the opening portion 20P may be provided in common to the tubular portion 2 and the cylindrical portion 20T. The opening portion 20P may be omitted.

Next, an exemplary manufacturing method for the immersion heater 1 is described.

First, the first connecting portion 7 is connected to the distal end portion of the first lead wire portion 6, and the first connecting portion 7 is connected to the base end portion of the heating element 4.

The second connecting portion 9 is connected to the distal end portion of the second lead wire portion 8, and the second connecting portion 9 is connected to the distal end portion of the heating element 4.

Next, the integrated heating element 4, first lead wire portion 6, first connecting portion 7, second lead wire portion 8, and second connecting portion 9 are inserted into the tubular portion 2.

Next, the filler and the heat insulating material 11 are put in the tubular portion 2.

The supporting portion 12 is located at the outside of the base end portion of the first lead wire portion 6 and the outside of the base end portion of the second lead wire portion 8.

Subsequently, the tubular portion 2 and the supporting portion 12 are joined by forming the joint portion 14.

The base end of the first lead wire portion 6 and the base end of the second lead wire portion 8 are connected to the terminals.

Here, the sealing portion 10 is formed by putting the raw material in a liquid, gel, paste, or powder form into the base end portion of the tubular portion 2 and curing by natural drying or heating.

In the formation of the sealing portion 10, the sealing portion 10 before curing that is the raw material may be applied with an action of external forces including compression in a state of being put into the base end portion of the tubular portion 2 and located around the first lead wire portion 6 and the second lead wire portion 8. By adding such an external force application step, the raw material further penetrates into the gaps in the first lead wire portion 6 and the second lead wire portion 8, which are stranded wires, or further closely adheres to the surfaces of the rod-shaped first lead wire portion 6 and second lead wire portion 8.

The sealing portion 10 is preferably produced from at least one of the reactive cyclic siloxane material and the material containing organohydrogen polysiloxane.

Particularly, when the sealing portion 10 is produced using the agent A and the agent B described above, a mixture of the agent A and the agent B in a gel from is subjected to heat curing in a state of being located at the base end portion of the tubular portion 2 for a predetermined time, for example, 30 minutes or a longer period under an environment of a predetermined temperature, for example, 100°C or a higher temperature.

An exemplary operation of the immersion heater 1 is described below.

A user turns on the power to cause the heating element 4 of the immersion heater 1 to generate heat. The heat from the heating element 4 is efficiently transferred to the tubular portion 2 by the filler. The heat insulating material 11 suppresses the heat transfer to the base end side.

The user immerses the distal end portion of the tubular portion 2 in the molten aluminum W in the molten metal bath from above, thereby allowing heating of the molten aluminum W by the heat transfer from the heating element 4 through the filler and the tubular portion 2.

The amount of heat generation of the heating element 4 is controlled by the control device. Here, a temperature sensor (not illustrated) electrically connected to the control device senses the temperature of the molten aluminum W and transmits a temperature signal indicating the temperature to the control device, and the control device controls the amount of heat generation of the heating element 4 corresponding to the temperature according to the received temperature signal. The heating element 4 may be driven at a constant output without the temperature control.

The maximum amount of heat generation (maximum output) of the heating element 4 can be further increased in the molybdenum heating element 4 compared with the nichrome heating element 4. The oxidation of molybdenum leading to the embrittlement of the heating element 4 is suppressed by locating the filler around the heating element 4.

Even when the power of the immersion heater 1 is repeatedly turned on and off, and the immersion heater 1 is repeatedly moved close to and away from the molten aluminum W, the above-described sealing portion 10 suppresses formation of at least one of the lead wire gap and the tubular portion gap. Therefore, inflow of air containing oxygen gas into the tubular portion 2 is suppressed, and the oxidation of the heating element 4 is suppressed.

Further, when the sealing portion 10 is air-cooled by the air blowing unit 16, deterioration of the sealing portion 10 due to heat is further suppressed, thereby further extending the service life of the sealing portion 10.

The above-described immersion heater 1 provides the following operational advantages.

That is, the immersion heater 1 includes the heating element 4, the first lead wire portion 6 and the second lead wire portion 8 connected to the heating element 4, the tubular portion 2 that houses the heating element 4, and the sealing portion 10 that seals the tubular portion 2 while allowing the first lead wire portion 6 and the second lead wire portion 8 to pass therethrough. The sealing portion 10 is a cured product obtained from the raw material in a liquid, gel, paste, or powder form.

Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided in a further simple and low-cost manner.

The sealing portion 10 is a heat-resistant insulating material capable of withstanding a temperature of at least 250°C. Accordingly, sealing and the insulating property of the sealing portion 10 are maintained under the high-temperature environment, and the oxidation of the heating element 4 is suppressed while insulating the first lead wire portion 6 and the second lead wire portion 8.

Further, the sealing portion 10 does not expand under the environment of 250°C or higher. Accordingly, the occurrence of cracks in the sealing portion 10 is suppressed under the high-temperature environment, and the oxidation of the heating element 4 due to the inflow air from cracked portions is suppressed.

Furthermore, the sealing portion 10 suppresses the inflow of air into the tubular portion 2. Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided.

The sealing portion 10 is formed from at least one of the siloxane material, alumina, quartz, zinc oxide, and carbon black. Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided in a further simple and low-cost manner.

In addition, the immersion heater 1 includes the heating element 4, the first lead wire portion 6 and the second lead wire portion 8 connected to the heating element 4, the tubular portion 2 that houses the heating element 4, and the sealing portion 10 that is formed from a siloxane material and seals the tubular portion 2 while allowing the first lead wire portion 6 and the second lead wire portion 8 to pass therethrough. Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided in a further simple and low-cost manner.

The siloxane material is a reactive cyclic siloxane material. Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided in a further simple and low-cost manner.

Further, the reactive cyclic siloxane material is 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane. Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided in a further simple and low-cost manner.

Furthermore, the sealing portion 10 is formed from the siloxane material and at least one of alumina, quartz, zinc oxide, and carbon black. Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided in a further simple and low-cost manner.

In addition, the immersion heater 1 includes the heating element 4, the first lead wire portion 6 and the second lead wire portion 8 connected to the heating element 4, the tubular portion 2 that houses the heating element 4, and the sealing portion 10 that contains at least one of quartz and alumina and seals the tubular portion 2 while allowing the first lead wire portion 6 and the second lead wire portion 8 to pass therethrough. Accordingly, the immersion heater 1 improved in sealing performance of the tubular portion 2 that houses the heating element 4 is provided in a further simple and low-cost manner.

At least any of the heating element 4, the first lead wire portion 6, and the second lead wire portion 8 is made of molybdenum. Accordingly, the sealing performance of the tubular portion 2 that houses the heating element 4 made of molybdenum, which is more susceptible to oxidation and embrittlement of the heating element 4 under the high-temperature environment compared with other materials, is improved in a further simple and low-cost manner.

Further, the first lead wire portion 6 and the second lead wire portion 8 include the stranded wire portion 8B. Accordingly, the sealing portion 10 is formed in a state where the raw material in a liquid, gel, paste, or powder form penetrates into the stranded wire portion 8B, and the sealing performance of the tubular portion 2 that houses the heating element 4 is improved in a further simple and low-cost manner. Even in a case where the first lead wire portion 6 and the second lead wire portion 8 include a single-wire bundle portion that is a bundle of a plurality of single wires, the effect similar to that in the case of the stranded wire portion 8B is provided.

Furthermore, even when the first lead wire portion 6 and the second lead wire portion 8 include a rod-shaped portion, the sealing portion 10 is formed in a state where the raw material in a liquid, gel, paste, or powder form is in close contact with the surface of the rod-shaped portion. Accordingly, the sealing performance of the tubular portion 2 that houses the heating element 4 is improved in a further simple and low-cost manner.

Further, the immersion heater 1 includes the air blowing unit 16 that blows the air N to the sealing portion 10.

The tubular portion 2 is provided with the opening portion 20P to expose the sealing portion 10 to ambient air.

Accordingly, the service life of the sealing portion 10 is further extended by cooling.

Further, the immersion heater 1 includes the heating element 4, the first lead wire portion 6 and the second lead wire portion 8 connected to the heating element 4, the tubular portion 2 that houses the heating element 4, the sealing portion 10 that seals the tubular portion 2 while allowing the first lead wire portion 6 and the second lead wire portion 8 to pass therethrough, and the air blowing unit 16 that blows air to the sealing portion 10.

Therefore, the sealing portion 10 is cooled by airflow. Accordingly, the service life of the sealing portion 10 is further extended.

The immersion heater 1 includes the heating element 4, the first lead wire portion 6 and the second lead wire portion 8 connected to the heating element 4, the tubular portion 2 that houses the heating element 4, the sealing portion 10 that seals the tubular portion 2 while allowing the first lead wire portion 6 and the second lead wire portion 8 to pass therethrough, and the connecting portion 20 that surrounds the sealing portion 10. The connecting portion 20 is provided with the opening portion 20P to expose the sealing portion 10 to ambient air.

Therefore, the sealing portion 10 is cooled by ambient air through the opening portion 20P. Accordingly, the service life of the sealing portion 10 is further extended.

The following describes more specific examples according to the above-described embodiments of the present invention.

The present invention is not limited to the examples below.

As Example 1, the sealing portion 10, which is one-component heat-resistant silicone and formed by curing a single gel-like raw material, was prepared. In more detail, the gel-like raw material was packed into the base end portion of the tubular portion 2 and left for 60 minutes under an environment of 100°C to be heat-cured.

As Example 2, the sealing portion 10, which is two-component heat-resistant silicone and formed from the above-described agent A and agent B, was prepared. In more detail, the raw material obtained by mixing the above-described agent A and agent B and forming the mixture in a paste was packed into the base end portion of the tubular portion 2 and left for 60 minutes under an environment of 100°C to be heat-cured.

The sealing portions 10 according to Examples 1 and 2 were both excellent in gas inflow suppression, heat resistance, insulating property, and non-expansibility after curing.

Comparing these, the sealing portion 10 of Example 2 was more excellent in gas inflow suppression, and moreover, exhibited better durability in a durability test than the sealing portion 10 of Example 1.

For the sealing portions 10 of Examples 1 and 2, samples having the reduced dimensions relative to and the same compositions as the actual sealing portions 10 were prepared in the same manner as the actual sealing portions 10, and composition analysis and hardness measurement were performed under various conditions.

The result of the composition analysis of Example 1 is indicated in Table 1 below, and the result of the composition analysis of Example 2 is indicated in Table 2 below. The units of numerical values in Table 1 and Table 2 are mass percent, and when the total is less than 100 mass%, the remainder may be regarded as carbon atoms.

Condition A is defined as a case in which heating for 41 hours under a temperature environment of 250°C was performed. Condition B is defined as a case in which heating for 8 hours under a temperature environment of 400°C was performed. Condition C is defined as a case in which heating for 8 hours under a temperature environment of 400°C was performed, and then heating for 1 hour under a temperature environment of 700°C was performed as promptly as possible thereafter. Condition D is defined as a case in which a temperature environment of 1000 °C was prepared over 4 hours and then heating for 4 hours was performed under the environment. Condition E is defined as a case in which heating for 8 hours under a temperature environment of 250°C was taken as one set, and one set per day was performed for three consecutive days, for a total of three sets. Condition F is defined as a case in which heating for 8 hours under a temperature environment of 600°C was performed.

The result of the hardness measurement of Example 1 is indicated in Table 3 below, and the result of the hardness measurement of Example 2 is indicated in Table 4 below. As the numerical value of the hardness increases, the hardness increases. The symbol "A" described before the numerical value indicates that the shape of the probe at the measurement is A type. Unheated Condition A Condition B Condition C Condition D Oxygen (O) 34 30 42 48 49 Silicon (Si) 61 62 51 40 44 Aluminum (Al) 4 7 6 11 6 Magnesium (Mg) - - - < 0.5 - Calcium (Ca) < 0.5 - - - - Iron (Fe) < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Zinc (Zn) < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Titanium (Ti) 1 1 1 1 1 Vanadium (V) < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Unheated Condition E Condition B Condition F Condition D Oxygen (O) 53 51 58 61 61 Silicon (Si) 32 33 24 25 23 Aluminum (Al) 15 16 18 14 16 Magnesium (Mg) < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Calcium (Ca) < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Iron (Fe) < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Zinc (Zn) < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 Titanium (Ti) - - - - - Vanadium (V) - - - - - Unheated Condition A Condition B Condition C Condition D Hardness A35 A23 -^*1 -^*1 -^*1 A35 A23 -^*1 -^*1 -^*1 A35 A23 -^*1 -^*1 -^*1 A36 A22 -^*1 -^*1 -^*1 A34 A21 -^*1 -^*1 -^*1 Average Hardness A35 A22 Unmeasurable Unmeasurable Unmeasurable *1: Unmeasurable because sample is in powder form Unheated Condition E Condition B Condition F Condition D Hardness A60 A65 A80 -^*1 -^*1 A62 A64 A90 -^*1 -^*1 A63 A65 A77 -^*1 -^*1 A64 A66 -^*2 -^*1 -^*1 A63 A65 -^*2 -^*1 -^*1 Average Hardness A62 A65 A82 Unmeasurable Unmeasurable *1: Unmeasurable because sample is in powder form
*2: Unmeasurable because indenter penetrated through sample

As shown in Table 1, the main composition of Example 1 was 34 mass% of oxygen atoms (O), 61 mass% of silicon atoms (Si), and 4 mass% of aluminum atoms (Al) before being exposed to heat (unheated). After the heating under Condition A, the main composition of Example 1 was O: 30 mass%, Si: 62 mass%, and Al: 7 mass%. After the heating under Condition B, the main composition of Example 1 was O: 42 mass%, Si: 51 mass%, and Al: 6 mass%. After the heating under Condition C, the main composition of Example 1 was O: 48 mass%, Si: 40 mass%, and Al: 11 mass%. After the heating under Condition D, the main composition of Example 1 was O: 49 mass%, Si: 44 mass%, and Al: 6 mass%.

According to the main composition of Example 1 as described above, as the thermal conditions become more severe, the mass fraction of oxygen atoms increases, while the mass fraction of silicon atoms relatively decreases. On the other hand, it remains to the extent that the oxygen atoms have an upper limit of 49 mass% and the silicon atoms have a lower limit of 40 mass%. Accordingly, while the sealing portion 10 of Example 1 permits oxidation and partially having anelasticity due to heating, the oxidation and the partial anelasticity remain within a predetermined extent. Conventionally, the cross-sectional area of the heating element 4 made of a predetermined material decreased by about 50% in some locations due to oxidation resulting from long-term use, even when a filler was used. In contrast, in Example 1, the cross-sectional area of the same heating element 4 as in the conventional case is expected to be reduced by at most about 10% over a comparable period of use. Accordingly, the sealing portion 10 of Example 1 is capable of maintaining the sealing that suppresses oxidation of the heating element 4.

As shown in Table 2, the main composition of Example 2 was O: 53 mass%, Si: 32 mass%, and Al: 15 mass% in the unheated state. After the heating under Condition E, the main composition of Example 2 was O: 51 mass%, Si: 33 mass%, and Al: 16 mass%. After the heating under Condition B, the main composition of Example 2 was O: 58 mass%, Si: 24 mass%, and Al: 18 mass%. After the heating under Condition F, the main composition of Example 2 was O: 61 mass%, Si: 25 mass%, and Al: 14 mass%. After the heating under Condition D, the main composition of Example 2 was O: 61 mass%, Si: 23 mass%, and Al: 16 mass%.

According to the main composition of Example 2 as described above, as the thermal conditions become more severe, the mass fraction of oxygen atoms increases, while the mass fraction of silicon atoms relatively decreases. On the other hand, it remains to the extent that the oxygen atoms have an upper limit of 61 mass% and the silicon atoms have a lower limit of 23 mass%. Accordingly, while the sealing portion 10 of Example 2 permits oxidation and partially having anelasticity due to heating, the oxidation and the partial anelasticity remain within a predetermined extent. The sealing portion 10 of Example 2 has the mass fraction of aluminum atoms greater than that of the sealing portion 10 of Example 1. Conventionally, the cross-sectional area of the heating element 4 made of a predetermined material decreased by about 50% in some locations due to oxidation resulting from long-term use, even when a filler was used. In contrast, in Example 2, the cross-sectional area of the same heating element 4 as in the conventional case is expected to be reduced by at most about 5% over a comparable period of use. Accordingly, the sealing portion 10 of Example 2 is capable of maintaining the sealing that suppresses oxidation of the heating element 4.

As shown in Table 3, the hardness of Example 1 was A35, as an average of five measurements (average hardness), in the unheated state. After the heating under Condition A, the hardness of Example 1 was A22 as the average hardness. After the heating under Conditions B, C, and D, the hardness of Example 1 was unmeasurable because the samples were in powder form in the hardness measurement. The symbol *1 in Table 3 and Table 4 indicates that the hardness was unmeasurable because the sample was in powder form.

According to the hardness of Example 1 as described above, as the thermal conditions become more severe, the sealing portion 10 of Example 1 becomes softer. Accordingly, it can be said that the sealing portion 10 of Example 1 gradually softens while maintaining the sealing.

On the other hand, as shown in Table 4, the hardness of Example 2 was A62, as an average of five measurements (average hardness), in the unheated state. After the heating under Condition E, the hardness of Example 2 was A65 as the average hardness. After the heating under Condition B, the hardness of Example 2 was A82 as the average hardness. However, in Example 2 after the heating under Condition B, in two of the five hardness measurements, since the durometer indenter penetrated through the surface of the sample, the measurement was impossible. Therefore, the average hardness of Example 2 after the heating under Condition B is an average value of the three measurements. After the heating under Conditions F and D, the hardness of Example 2 was unmeasurable because the samples were in powder form in the hardness measurement. The symbol *2 in Table 4 indicates that the measurement was impossible because the durometer indenter penetrated through the surface of the sample in the hardness measurement.

According to the hardness of Example 2 as described above, as the thermal conditions become more severe, the sealing portion 10 of Example 2 becomes harder. Accordingly, it can be said that the sealing portion 10 of Example 2 gradually hardens while maintaining the sealing.

For the sample of Example 2, those that had not been subjected to any tests including the above-described composition analysis and hardness measurement were each analyzed by thermogravimetric-differential thermal analyzer (TG-GTA) after heating under the following conditions. The weights of the respective samples here are about 10 mg. The samples are each placed in an open alumina container.

Condition T1 is defined as a case in which heating is performed in air by increasing temperature from 40°C to 300°C and then maintaining the temperature environment of 300°C for 8 hours. Condition T2 is defined as a case in which heating is performed in air by increasing temperature from 40°C to 350°C and then maintaining the temperature environment of 350°C for 8 hours. Condition T3 is defined as a case in which heating is performed in air by increasing temperature from 40°C to 500°C and then maintaining the temperature environment of 500°C for 8 hours.

In each of Conditions T1 to T3, the rate of temperature rise is 5 °C/min. In each of Conditions T1 to T3, air at a flow rate of 200 mL/min is introduced into the heating furnace during heating.

Fig. 2 is a graph illustrating the result of the thermogravimetric-differential thermal analysis under Condition T1. Fig. 3 is a graph illustrating the result of the thermogravimetric-differential thermal analysis under Condition T2. Fig. 4 is a graph illustrating the result of the thermogravimetric-differential thermal analysis under Condition T3.

In Fig. 2 to Fig. 4, the horizontal axis indicates an elapsed time (min) from the start of sample heating. The vertical axis indicates the temperature (°C), TG (%), and DTA (µV). TG indicates the rate of decrease in thermal mass from an initial value. DTA indicates the differential thermal.

In Fig. 2 to Fig. 4, solid lines indicate the temperature. One-dot chain lines indicate TG. Further, dashed lines indicate DTA.

First, the behavior of weight change temperature obtained from the results of thermogravimetric-differential thermal analysis is described.

Under Condition T1, the DTA curve exhibits a small peak immediately after the start of analysis, and then continues to decrease uniformly until about 60 minutes, when the temperature reaches 300°C. Thereafter, the DTA curve remains substantially constant until the end of the measurement, and no peak is observed.

Under Condition T2, the DTA curve exhibits a small peak immediately after the start of analysis, similarly to Condition T1, and then exhibits one large peak at 70 to 75 minutes corresponding to a temperature range from slightly above 300°C to 350°C. A temperature CR at a time indicated by an intersection of an extrapolated line U1 of the DTA curve before a local minimal value of the DTA curve immediately preceding the peak and an extrapolated line U2 of the DTA curve after the local minimal value was 329°C. Here, referring to the TG curve under Condition T2, the weight loss is observed at approximately the same temperature as the temperature CR. Therefore, a weight change temperature in Example 2 can be regarded as 329°C. Under Condition T2, after reaching 350°C, the DTA curve remains substantially constant until the end of the measurement, and no peak is observed.

Under Condition T3, the DTA curve exhibits a small heat generation peak immediately after the start of analysis, similarly to Conditions T1 and T2, and then exhibits a large peak that broadens toward the high-temperature side at 70 to 75 minutes corresponding to a temperature range from slightly above 300°C to 350°C. The peak is considered to be the same as the peak observed under Condition T2, and the temperature CS at the time to which the intersection of the two extrapolated lines V1 and V2 belongs was 329°C, which is the same as the temperature CR under Condition T2. Further, the DTA curve under Condition T3 exhibited a plurality of peaks from 90 to 100 minutes corresponding to a temperature range of approximately 450°C to 500°C. Among the plurality of peaks in the DTA curve here, a temperature CT at a time indicated by an intersection of an extrapolated line W1 of the DTA curve before a local minimal value immediately preceding the first peak and an extrapolated line W2 of the DTA curve after the local minimal value was 450°C. Referring to the TG curve under Condition T3, the weight loss is observed at the time corresponding to the temperature CT. Therefore, it can be said that, in Example 2, at least two weight change temperatures are present, the weight change temperature of the first stage is 329°C corresponding to the temperature CS, and the weight change temperature of the second stage is 450°C corresponding to the temperature CT.

Table 5 below summarizes the weight change temperatures under Conditions T1 to T3. Condition Holding Temperature [°C] Weight Change Temperature [°C] First Stage Second Stage T1 300 - - T2 350 329 - T3 500 329 450

According to the behavior of weight change temperature as described above, it can be said that, in Example 2, the first significant compositional change occurs at 329°C, exceeding 250°C.

Accordingly, Example 2 can withstand a temperature of at least 250°C. Example 2 does not expand under the environment of 250°C or higher.

Next, the behavior of the rate of decrease of TG obtained from the results of thermogravimetric-differential thermal analysis is described.

An average value of DTA in a period starting at a time point 420 minutes after reaching a holding temperature and ending at a time point 480 minutes after reaching the holding temperature is defined as a DTA stable average value. Under Condition T1, a time point at which the DTA first falls within a range from a lower limit corresponding to 99% of the DTA stable average value to an upper limit corresponding to 101% of the DTA stable average value is defined as a baseline stabilization start point BS1 under Condition T1. Similarly, under Condition T2, a time point at which the DTA first falls within the range relative to the DTA stable average value is defined as a baseline stabilization start point BS2 under Condition T2. Further, under Condition T3, a time point at which the DTA first falls within the range relative to the DTA stable average value is defined as a baseline stabilization start point BS3 under Condition T3.

Then, the TG decrease rates (%) at the following three time points are summarized in Table 6 below. The three time points are, under Condition T1, a time point TA1 at which the holding temperature is reached, the baseline stabilization start point BS1, and a time point after 8 hours from when the holding temperature is reached. The three time points are, under Condition T2, a time point TA2 at which the holding temperature is reached, the baseline stabilization start point BS2, and a time point after 8 hours from when the holding temperature is reached. Further, the three time points are, under Condition T3, a time point TA3 at which the holding temperature is reached, the baseline stabilization start point BS3, and a time point after 8 hours from when the holding temperature is reached. Condition Holding Temperature [°C] Decrease Rate [%] Time Point When Set Temperature Is Reached Baseline Stabilization Start Point Time Point after 8 Hours from Reaching Holding Temperature T1 300 0.6 1.6 8.0 T2 350 2.2 13.1 16.4 T3 500 17.3 19.9 20.0

According to the behavior of the TG decrease rate as described above, it can be said that, in Example 2, the decrease rate is low at 300°C, exceeding 250°C, and under Condition T1 at the holding temperature of 300°C, no significant changes such as combustion, sublimation, or evaporation occurred in Example 2. Under Condition T2 at the holding temperature of 350°C, it can be said that significant changes such as combustion, sublimation, or evaporation gradually occurred after reaching the holding temperature of 350°C in Example 2. Further, under Condition T1 at the holding temperature of 500°C, it can be said that significant changes such as combustion, sublimation, or evaporation occurred relatively soon after reaching the holding temperature of 500°C in Example 2.

Accordingly, Example 2 can withstand a temperature of at least 250°C. Example 2 does not expand under the environment of 250°C or higher.

On the other hand, in the immersion heater 1 in which the sealing portion 10 was actually formed according to Example 2, the temperature of the sealing portion 10 was measured under specific conditions.

The temperature measurements were performed under four types of conditions broadly classified according to the presence or absence of the opening portion 20P communicating with the sealing portion 10 and the presence or absence of the lid T.

The temperature measurements were performed for each type of condition with the output of the immersion heater 1 set to five levels of 0 kW, that is, off, 1 kW, 2 kW, 3 kW, and 4 kW.

Further, the temperature measurements were performed one hour after completion of storage of the molten aluminum W.

As the specific condition, the length in the up-down direction of the heating element 4, which corresponds to an arrow LH in Fig. 1, is 200 mm. The length from the lower surface of the heat insulating material 11 to the distal end portion 2P of the tubular portion 2 in the up-down direction, which corresponds to an arrow LF in Fig. 1, is 550 mm. The filler is filled from the lower surface of the heat insulating material 11 to the distal end portion 2P of the tubular portion 2. Further, the length in the up-down direction of the sealing portion 10, which corresponds to an arrow LS in Fig. 1, is 30 mm. The length in the up-down direction of the heat insulating material 11, which corresponds to an arrow LI in Fig. 1, is 550 mm. Further, a distance from the lower surface of the lid T to the upper surface of the molten aluminum W, which corresponds to an arrow LP in Fig. 1, is 80 mm. When the lid T is not provided, the arrow LP corresponds to the distance from the upper surface of the wall of the storage portion of the molten aluminum W to the upper surface of the molten aluminum W. The depth from the upper surface of the molten aluminum W to the upper end of the heating element 4, which corresponds to an arrow LD in Fig. 1, is 150 mm.

Further, the temperature of the molten aluminum W is 680°C. The depth in the up-down direction of the stored molten aluminum W is 457 mm. The length in the up-down direction of the tubular portion 2 is 600 mm. One opening portion 20P is provided in a circular shape, and its diameter is 5 mm. Further, the temperature measurement of the sealing portion 10 in Example 2 was performed using a thermocouple. The temperature measurement point of the sealing portion 10 in Example 2 is located 10 mm below the upper surface of the sealing portion 10 and is inside the sealing portion 10.

The temperature measurement results are indicated in Table 7 below. Output (Kw) Temperature of Sealing Portion 10 (°C) Lid T Provided Lid T Not Provided Opening Portion OP Provided Opening Portion OP Not Provided Difference Opening Portion OP Provided Opening Portion OP Not Provided Difference 0 145 164 19 123 158 35 1 142 169 17 129 163 34 2 156 171 15 132 169 37 3 162 176 14 139 173 34 4 163 179 16 144 177 33

According to Table 7, when the lid T is provided, the temperature of the sealing portion 10 in the case where the opening portion 20P is provided is lower than in the case where the opening portion 20P is not provided at any output, and the difference therebetween is approximately 15°C. When the lid T is not provided, the temperature of the sealing portion 10 in the case where the opening portion 20P is provided is lower than in the case where the opening portion 20P is not provided at any output, and the difference therebetween is approximately 35°C.

Thus, when the opening portion 20P is provided, the temperature rise of the sealing portion 10 is more suppressed compared with the case where the opening portion 20P is not provided. The temperature suppressing effect appears more significantly in the case where the lid T is not provided.

The conditions in the temperature measurement are frequently observed when handling the molten aluminum W. Then, even at the output of 4 kW, the temperature of the sealing portion 10 is 179°C in the case where the lid T is provided and the opening portion 20P is not provided, and is 177°C in the case where the lid T is not provided and the opening portion 20P is not provided. Accordingly, when the sealing portion 10 has heat resistance capable of withstanding heat of at least 250°C, that is, when the heat-resistant temperature of the sealing portion 10 is 250°C or higher, the sealing portion 10 has sufficient durability.

On the other hand, a relationship between the airflow volume from the air blowing unit 16, that is, the air flow rate and the temperature of the sealing portion 10 was measured under the above-described specific conditions. However, in the measurement of the air flow rate, the lid T was provided, but the opening portion 20P was not provided.

The measurement of the air flow rate was performed under three Conditions R1 to R3 below. That is, first, Condition R1 is defined such that the distance from the lid T to the lower end of the sealing portion 10, which corresponds to the arrow LA in Fig. 1, is 80 mm and the output of the immersion heater 1 is 6 kW. Next, Condition R2 is defined such that the distance from the lid T to the lower end of the sealing portion 10 is 80 mm and the voltage of the immersion heater 1 is 200 V. Subsequently, Condition R3 is defined such that the distance from the lid T to the lower end of the sealing portion 10 is 5 mm and the voltage of the immersion heater 1 is 200 V.

Fig. 5 is a graph illustrating the temperature of the sealing portion 10 at each air flow rate under the three Conditions R1 to R3. Fig. 6 is a graph illustrating a temperature difference at each air flow rate relative to the temperature of the sealing portion 10 when the air flow rate is 0 L/min under the three Conditions R1 to R3. The air flow rate of 0 L/min corresponds to a state in which no air is supplied from the air blowing unit 16.

According to Fig. 5 and Fig. 6, under any of the three Conditions R1 to R3, as the air flow rate increases, the temperature of the sealing portion 10 decreases more significantly. Under each of Conditions R1 and R2, the temperature of the sealing portion 10 decreases by 100°C or more at the air flow rate of 50 L/min or more. Under Condition R3, the temperature of the sealing portion 10 decreases by 150°C or more at the air flow rate of 60 L/min.

Therefore, the sealing portion 10 is sufficiently cooled by the air from the air blowing unit 16. Accordingly, the service life of the sealing portion 10 is further extended.

1:

Immersion heater (heater)

2:

Tubular portion

4:

Heating element

6:

First lead wire portion

7:

First connecting portion

8:

Second lead wire portion

9:

Second connecting portion

10:

Sealing portion

11:

Heat insulating material

12:

Supporting portion

14:

Joint portion

16:

Air blowing unit

N:

Air

20P:

Opening portion