JPH0311775B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a heating method applied to melting and casting of glass materials for casting such as dental alloys, glass, and glass-ceramic base materials, or firing of metal-bonded porcelain dentures. Regarding equipment.

(Prior art) In recent years, dental alloys used as denture materials have changed from conventional precious metals (Au alloys) to semi-precious metals (Pd-Ag and other Pd alloys) due to the rising prices of precious metals. Furthermore, there is a shift to non-precious metals (Ni-Cr, Co-Cr alloys, etc.), but among these, non-precious metals in particular have extremely high melting temperatures (1200 to 1550°C) and are susceptible to oxidation. Conventional heating methods mainly use high-frequency induction heating or arc discharge heating because they are easy to use. However, with the former, heat is generated from the surface of the workpiece due to the skin effect, so the surface is heated to a higher temperature than originally required for melting. As a result, some of the alloy components are vaporized and lost, and the properties of the alloy are impaired. Moreover, since the temperature distribution is not uniform, the temperature of the workpiece cannot be detected accurately, and the temperature control based on the temperature cannot be properly controlled.
Further, in the latter case, only the arc generation point on the workpiece becomes partially high temperature, resulting in the same drawback as the former case.

On the other hand, in the porcelain baking furnace used to obtain a metal-porcelain composite denture by building up porcelain on a denture base made of Pd-Ag or other Ag-containing alloys, Ag increases as the temperature increases.
The alloy oxidizes to form a yellow oxide (Ag ₂ O),
Since this causes yellowing of the built-up porcelain, generally, as a preventive measure, a carbon material is stored in the furnace to create a reducing atmosphere.

(Problems to be Solved by the Invention) In view of the above, the present invention is a heating device for firing, melting, and casting dental materials that require high-temperature heating in a reducing atmosphere, and that eliminates uneven heating. It is an object of the present invention to provide a device which can easily detect and control temperature and which does not form oxides.

(Means for Solving the Problem) The above object is generally achieved by using a SiC resistance heating element that circumferentially surrounds the workpiece as a heating means. This SiC resistance heating element is placed in a sealed chamber and generates heat when energized while oxygen density is reduced due to reduced pressure, resulting in SiC+3/2O ₂
→The oxidation reaction of SiO ₂ +CO occurs to reduce the amount of oxygen, and the generated CO maintains a reducing atmosphere in the chamber to prevent oxidation of the workpiece, and heats the entire surface of the workpiece by Joule heat surrounding the workpiece. By applying it almost uniformly, temperature detection and control become easier. Therefore, the apparatus of the present invention is suitable for melting and casting non-precious metals among dental alloy materials, firing silver-containing alloy-based metals, bonded porcelain, and casting glass (porcelain baking furnace). Of course, the present invention is also applicable to a casting apparatus which combines melting and gas pressure casting, which was provided by the present applicant in Japanese Patent Application No. 58-246548.

The configuration of the present invention will be explained in more detail below with reference to drawings in which the configuration of the present invention is applied to the above-mentioned casting device; In the drawings, FIG. Figure 2 is a perspective view showing an example of the SiC resistance heating element used in the present invention;
FIG. 3 is a perspective view of another example of a SiC resistance heating element.
FIG. 4 is a perspective view of a SiC resistance heating element of yet another embodiment, and FIG. 5 is a perspective view of a SiC resistance heating element of still another embodiment.

First, regarding the SiC resistance heating element 2 used in the present invention, the one in Figure 2 is a helical slot type, the one in Figure 3 is a straight slot type, and the one in Figure 4 is a straight slot type.
The ones in the figure each show a multi-column type, but the one in FIG.
The halves 200, 20 are electrically insulated and separated from each other in the upper part of the main body 20 by forming a series of helical slots 210 in the longitudinal direction.
0' and conductive terminal semicircular body 26,
Insulating chips 260, 260' are inserted between the two halves 26', and the halves 200, 200' are tied together,
On the other hand, in the lower part of the halves 200, 200', a pair of helical rings 201, 201' insulated by the helical slot 210 are arranged alternately in the longitudinal direction, and at the ends of the helical rings 201, 201', respectively. As a result, when one half 200 is energized, the current can be passed through the helical ring 210 from the helical ring 201' to the other half 200'. (Note that the current path is indicated by an arrow). Due to this energization, the Joule heat is concentrated in the narrow helical rings 201, 201', forming the heat generating part 21. The heat generation temperature range is As an example, the temperature ranges from 500 to 1800° C. In FIG.
The conductive terminal rings 26 and 26' are fitted into the upper and lower parts of the main body 20. In this case, the straight path 2 divided by the slot 220 is
02 forms the heat generating part 21. In the embodiment shown in Fig. 4, upper and lower conductive terminal/support rings 26'',
A large number of vertical SiC sintered rods 2 between 26"
03 are implanted and held in the circumferential direction, and these rods 2
03 itself forms the heat generating part 21. In the embodiment shown in FIG. 5, conductive terminals 26 are provided at both ends.
, 26...
The upper and lower rods 204 are stacked one above the other in a grid pattern, and an insulating support seat (not shown) is provided between the upper and lower rods 204 to electrically insulate the space between them. Shape structure part 20
5 surrounds a melting crucible (not shown).

The SiC resistance heating element 2 of this embodiment constructed in this way allows the temperature distribution in the vertical direction to be concentrated only in the vicinity of the crucible, compared to the one in which the heating element rods are installed vertically as shown in FIG. This is suitable when it is necessary to maintain the crucible at a relatively low temperature, and it has the added advantage of reducing the distance between the crucible and the mold 431.

In addition, since each rod 204... has no rigid support part and is simply assembled in a stacked relationship, the rods 204... may crack due to thermal stress.
The possibility of cracking and breakage is also significantly reduced. This kind of SiC resistance heating 2 is similar to the general sealed chamber 1.
More preferably, the chamber 1 is housed in a sealed chamber 1 that can be freely depressurized to lower the oxygen density (the example shown is also pressurizable because it is pressure cast with gas), and the heat generating part 21 is effectively heated. A workpiece (in the illustrated example, a dental alloy material 6 on a melting crucible 3) is placed in zone l. The upper opening of the heating element 2 is sealed with a central through hole 191 and a heat insulating stopper lid 19 which holds a thermocouple 23 for detecting the temperature inside the heat generating part 21 through the central through hole 191 .
It opens into the upper chamber lumen 24 of 2. The lower end of the heating element 2 is held by a ring support 25 that holds the heat insulating means 71, and a lid 28 is fixed to the upper end opening of the chamber 1 with a set screw 27. A viewing window hole 281 is provided through the window, and a window edge member 31 holding a viewing window plate 30 with a set screw 29 is attached with a set screw 32.

That is, when replacing the SiC resistance heating element 2, open the sealing lid 28 of the chamber 1, unplug the stopper lid 19 fitted to the top of the heating element 2, and then take the heating element 2 out of the device. Reset the new SiC resistance heating element 2 into the chamber 1 by performing the reverse operation. The conduit 8 is connected to an exhaust device (not shown). Melting crucible 3
If there is no substantial pressure difference between the inside of the closed chamber 1 and the inside of the mold 4,
It is held in the socket 311 by its surface tension, and by pressurizing the chamber 1, the molten material is pressure cast from the sprue 5 into the mold 4. The details of the other constituent members in FIG.
Since it is not directly related to the effect, it is omitted here to avoid duplication. However, among the above-mentioned constituent members, those that overlap with those of the earlier application are discontinuous, but the same reference numerals are used so as to match those of the earlier application.

(Function) When using this device, connect chamber 1 to conduit 8.
With the air removed from the chamber 1 to reduce the oxygen density within the chamber 1 and with the supply of fresh oxygen (that is, air here) cut off, electricity is applied to the SiC resistance heating element 2 to generate heat. When the surface of the heating element 2 is heated, CO gas is released into the chamber 1 through a reaction of oxide SiC+3/2O ₂ →SiO ₂ +CO. However, since there is no fresh air supplied to chamber 1, as the above reaction progresses, the oxygen density in chamber 1 further decreases, while the SiC in the heat generating area changes to SiO ₂ and the amount of SiC gradually decreases. However, this phenomenon can be confirmed by the formation of a white SiO ₂ film on the surface of the SiC resistance heating element 2. When the pressure inside the sealed chamber 1 can be reduced as shown in the example shown in the figure, it is desirable to reduce the oxygen density prior to energization and heat generation. In the case of the apparatus of the embodiment, after the workpiece 6 in the crucible 3 is melted, air is introduced into the chamber 1 from the conduit 8 and the molten material is pressure cast into the mold 4 below by the pressure, so that the casting is performed. At the final stage, the oxygen density in the chamber 1 increases extremely, but at that time the heat generation of the heating element 2 has stopped and the molten material has been poured into the mold 4 which has not been heated, so oxidation does not occur. The degree to which this occurs is small. In the case of the example shown in the figure, after each heating and casting, the inside of the chamber 1 is reset to the degree of vacuum within the above range, and is used for the next heating and casting. In the case of mere heating without casting, after the heating process is completed, the inside of the chamber 1 is brought into communication with the atmosphere to create an atmospheric atmosphere, and then the chamber 1 is reset to a reduced pressure in the same manner. For example, the SiC resistance heating element 2 can be used as a heating means in a conventional porcelain furnace to perform composite denture firing, staining, and glazing of Ag-based alloy-based porcelain (build-up) crowns.
When subjected to day gearing, no formation of Ag ₂ O was observed, and therefore no yellowing occurred in the porcelain crown. In all of the SiC resistance heating elements 2 having the three embodiment structures shown in FIGS. 2 to 4, the workpiece is surrounded by the heating section 2 in the circumferential direction.
1, the entire workpiece 6 is heated almost uniformly and efficiently because it is located within the effective heating zone l that exhibits a particularly high and uniform value in the heating temperature distribution. Detection using the thermoelectric material 23 shown in FIG. 1 is easily and accurately performed,
Therefore, temperature control associated with this detection is also appropriately performed. In the embodiment shown in FIG. 1, an example was given in which the resistance heating elements 2 were arranged in the vertical direction, but it goes without saying that the resistance heating elements 2 are not limited to this and may be arranged in the horizontal direction as appropriate.

(Example) (Example 1) (i) Heating element: Helical slot type SiC resistance heating element shown in Fig. 2 (ii) Degree of vacuum in the chamber: 5 × 10 ^-1 torr (iii) Workpiece: 92% Ni -3 g of non-precious metal of 6% Cr alloy (iv) Heating temperature: 1380°C, 3 minutes (v) Properties of melted workpiece: The workpiece melted in one beat in the socket.

(vi) Application of heating: Melting and pressure casting using gas as shown in Figure 1 (Example 2) (i) Heating element: Straight slot type SiC resistance heating element as shown in Figure 3 (ii) Degree of vacuum in the chamber: 100 torr (iii) Workpiece: Composite denture with porcelain built up to a thickness of 1.5mm on a coping base cast from 28%Ag-60%Pd alloy (iv) Firing process: 600℃→950℃, 50℃/min (v) Workpiece Properties: There was no yellowing of the porcelain due to oxidation of the Ag-Pd alloy of the denture base.

(vi) Applications of heating: firing, staining, glazing, day gearing (effects of the invention) As already understood from the above description, the device of the present invention uses a SiC resistance heating element to create a hermetic seal that can freely reduce pressure. Because the workpiece is heat-treated in the chamber, it is possible to substantially prevent oxidation of the material (workpiece) even though the material (workpiece) is heated at a high temperature due to the reducing atmosphere created by the CO generated during the heat treatment, and separate oxidation prevention measures (e.g. Yellowing, strength deterioration, and composition changes caused by oxidation at high temperatures, especially dental materials such as non-precious metals, semi-precious metals, porcelain, and glass ceramics, without using carbon materials (reducing agents) , no residual air bubbles, no incomplete filling into the mold, etc. In addition, it eliminates the need to introduce inert gas, which helps simplify the equipment, and since the workpiece is heated from the entire surrounding area, temperature detection and control become easier. be effective.

Note that if the SiC resistance heating element becomes worn out with use, it may be replaced with a new one at a timely manner.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal sectional front view showing an embodiment of a casting device including the device of the present invention, and Fig. 2 is a SiC used in the present invention.
FIG. 3 is a perspective view of an SiC resistance heating element according to another embodiment; FIG. 4 is a perspective view of a SiC resistance heating element according to another embodiment; FIG. 5 FIG. 2 is a perspective view of another example of a SiC resistance heating element. Explanation of symbols, 1... Sealed chamber, 2... SiC
Resistance heating element, 6...Work, l...Effective heating zone.

Claims (1)

[Claims] 1 SiC stored in a sealed chamber that can be depressurized
The SiC resistance heating element includes a resistance heating element, a melting crucible installed in the heating element, and a workpiece installed in the melting crucible within the effective heating zone of the heating element, and the SiC resistance heating element has a heat generating part as described above. The melting crucible is surrounded in the circumferential direction, and CO gas is generated from the heating element by energizing the heating element while the sealed chamber is maintained at a reduced pressure and the oxygen density is reduced. A dental heating device configured to heat the work while maintaining a reducing atmosphere in the chamber.